Niobium: Properties, Production and Applications: Properties, Production and Applications [1 ed.] 9781613247570, 9781611228953

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Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Niobium: Properties, Production and Applications : Properties, Production and Applications, Nova Science Publishers, Incorporated, 2011. ProQuest

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Niobium: Properties, Production and Applications : Properties, Production and Applications, Nova Science Publishers, Incorporated, 2011. ProQuest

CHEMICAL ENGINEERING METHODS AND TECHNOLOGY

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NIOBIUM: PROPERTIES, PRODUCTION AND APPLICATIONS

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CHEMICAL ENGINEERING METHODS AND TECHNOLOGY

NIOBIUM: PROPERTIES, PRODUCTION AND APPLICATIONS

THOMAS M. WONG

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EDITOR

Nova Science Publishers, Inc. New York

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Copyright © 2011 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers‟ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works.

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Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book.

Library of Congress Cataloging-in-Publication Data Niobium : properties, production, and applications / [edited by] Thomas M. Wong. p. cm. Includes index. ISBN H%RRN 1. Niobium. I. Wong, Thomas M. QD181.N3N536 2010 546'.524--dc22 2010043944

Published by Nova Science Publishers, Inc. † New York Niobium: Properties, Production and Applications : Properties, Production and Applications, Nova Science Publishers, Incorporated, 2011. ProQuest

CONTENTS Preface Chapter 1

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Chapter 2

vii The Role of Niobium Species in Heterogeneous Catalysis - Selected Aspects Maria Ziolek, Izabela Sobczak and Maciej Trejda Studies on Novel Solid Acid Catalysts of Niobium-based Oxides with Mesoporous, Layered and Nanosheet Structures Caio Tagusagawa, Atsushi Takagaki, Junko N. Kondo and Kazunari Domen

Chapter 3

Niobium Oxide Compounds: Synthesis, Properties and Applications Maria Lucia Caetano Jardim Pinto da Silva and Rafael Caetano Jardim Pinto da Silva

Chapter 4

Geochemistry of Nb and Variations of Nb/Ta Ratio in Geological Systems Yuanyuan Zhang and Jaroslav Dostal

Chapter 5

Chapter 6

Chapter 7

Densification of in situ Cu-NbC Composite Using Uniaxial and Cold Isostatic Pressing Zuhailawati Hussain, Indra Putra Almanar and Nurfateen Fakhariah Ahmad Niobium Powders: Uses and Process Developments for their Production Stefan Luidold Improvement of the Electrochemical Behavior of AISI 4140 Steel Substrate Using [TiCN/TiNbCN]n Multilayers System J. C. Caicedo, C. Amaya, L. Yate, W. Aperador, M. E. Gómez and P. Prieto

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vi Chapter 8

Chapter 9

Chapter 10

Niobium Distribution Pattern in Rare-metal-bearing Palaeoproterozoic Co-genetic Felsic Rocks and Minerals in Central India Yamuna Singh and Anjan Chaki

191

A Novel Amperometric Transducer Electrode with Iridium-Niobium Binary Alloys Toru Matsumoto, Naoaki Sata and Yoko Yamabe-Mitarai

205

Niobate Crystals Inserted in a Glass Matrix Manuel Pedro Graça, Manuel Almeida Valente and Maria G. Ferreira da Silva

Chapter 12

Use of Niobium for Fabricating Superconducting Radio Frequency Cavities A. T. Wu

Chapter 14

179

Niobium in Oxalate Complexes: From Discrete Molecules to Heterometallic Supramolecular Structures Marijana Jurić, Pavica Planinić and Nevenka Brničević

Chapter 11

Chapter 13

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Contents

Investigation of Surface Treatments of Niobium Flat Samples and SRF Cavities by Gas Cluster Ion Beam Technique for Particle Accelerators A. T. Wu, D. R. Swenson, P. Kneisel, G. Wu, Z. Insepov, J. Saunders, R. Manus, B. Golden, S. Castagnola, W. Sommer, E. Harms, T. Khabiboulline, W. Murayi and H. Edwards Electrodeposition of Niobium from Molten Salts Alain Robin and Antonio Fernando Sartori

Index

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PREFACE This new book presents current research in the study of niobium including the role of niobium species in heterogeneous catalysis; the geochemistry of Nb and variations of Nb/Ta ration in geological systems; niobium distribution pattern in rare-metal-bearing Palaeoproterozoic co-genetic Felsic rocks and minerals; niobium oxalate complexes and the electrodeposition of niobium from molten salts. Chapter 1 - This review article is devoted to the discoveries and/or knowledge developed in the past years in the area of the application of heterogeneous catalysts containing niobium for selective oxidation in gas and liquid phases by the use of different oxidants (oxygen and hydrogen peroxide). The idea of this chapter is to give the readers a general overview of the role of niobium species located in crystalline and amorphous solids in catalytic oxidation processes. The important ability of niobium to combination with many elements towards formation of new compounds determines the unique properties of niobium containing catalysts. Two main roles of niobium species in heterogeneous catalysis are considered: as catalytic active centers and as promoters. In some cases niobium containing materials are also considered as the supports. The chapter is divided into four parts. After a short introduction (first section), the most important aspects of the heterogeneous catalysis and solids used as catalysts are drawn in second section. The most detailed is the third section summarizing recent advances on the use of catalysts containing niobium species in various industrially important oxidation processes: i) gas phase oxidative dehydrogenation (ODH) of propane, ii) gas phase ammoxidation of propane, iii) gas phase selective oxidation of methanol, iv) liquid phase oxidation of olefins (with H2O2), v) liquid phase oxidation and ammoxidation of glycerol (with oxygen and H2O2, respectively). The choice of the reactions discussed is determined by the questions which we would like to answer that is, what is the role of niobium species in the catalyst when oxidation proceeds with the use of gas oxygen as oxidant (both in gas and liquid phases) and when hydrogen peroxide oxidizes organic compounds in the liquid phase. These aspects are considered on the basis of different types of catalysts containing niobium described in the literature: bulky niobium(V) oxides (amorphous and crystalline), mono- and multicomponent metal oxides, ordered mesoporous metalosilicate materials and microporous zeolites, also used as supports for noble metals and different mono and multi metal oxides. The last section summarizes the role of niobium in heterogeneous catalysts. Chapter 2 - The acidities of hydrated niobium oxide (Nb2O5·nH2O) and niobium mixed oxides have been widely studied as a promising substitute for liquid acid. Nb2O5·nH2O which

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is usually called niobic acid, exhibits a remarkably high acid strength corresponding to the acid strength of 70 % H2SO4. In the view of catalytic activity and stability, niobic acid exhibited excellent stabilities for acid-catalyzed reactions activities in water solution reactions including hydrolysis, hydration and esterification. In this chapter, niobium-containing mixed metal oxides with different nanostructures (nanosheet aggregates, layered and mesoporous) are presented to study the effects caused by the structure and metal combination for the acid sites and acid-catalyzed reactions. Nanosheet aggregates and protonated layered niobates (HTiNbO5, HNb3O8 and HNbWO6) are examined as potential solid acid catalysts. However, as the high charge density of the oxide sheets prevents reactants from penetrating into the interlayer region, unmodified layered transition-metal oxides are generally ineffective as solid acid catalysts. Exfoliation and aggregation of layered HTiNbO5, HNb3O8 and HNbWO6 using soft chemical processing form aggregates of nanosheets with high surface areas, making possible the access of reactants to acid sites formed by the bridged hydroxyl groups, M(OH)M‟ (M=Ti, Nb; M‟=Nb, W). The catalytic activity for the Friedel-Crafts alkylation of anisole in the presence of benzyl alcohol increased in the order HTiNbO5 < HNb3O8 < HNbWO6, consistent with the acid strengths determined by desorption measurements and nuclear magnetic resonance spectroscopy. Layered HNbMoO6 is a very unique layered metal oxide able to intercalate different organic reactants (alcohols, saccharides, ketones, alkenes, hydroxyl acids) during the catalytic reactions. Owing to the intercalation ability and strong acidity in the interlayer, layered HNbMoO6 functioned as a highly-active solid acid catalyst. The catalytic activity for the Friedel-Crafts alkylation, acetalization and hydrolysis of saccharides exceeded the activity of zeolites and ion-exchange resins. Mesoporous NbxW(10-x) mixed oxides with different Nb and W concentrations are examined as potential solid acid catalysts. Amorphous wormhole-type mesopores are observed for samples from x = 3 to 10 whereas W-rich samples (x = 0 to 2) formed a nonmesoporous structure with presence of crystallized tungsten oxide (WO3). The acid-catalytic activity, acid strength and mesopore structure of mesoporous Nb-W oxides changed in order of W concentrations, exhibiting a very high activity for both Friedel-Crafts alkylation and hydrolysis. The results were compared with those for non-porous Nb2O5-WO3 and a range of conventional solid acids. Mesoporous Nb-W oxides obtained higher turnover rate than that of non-porous Nb2O5-WO3 led to the strong acid sites and a mesoporous structure with a high surface area and easy reactant accessibility. Chapter 3 - In the past decade an increasing interest in niobium-containing materials was observed and its related to their multiple uses in the production of high-tech materials. Among niobium compounds, the niobium oxides are by far the most relevant and important ones in terms of industrial and research applications. Niobium oxides shaped as thin films, porous nanoparticles and nanowires have important application in various fields including catalysis, ion-exchange, adsorption, enzyme immobilization, highly sensitive sensors, photo/electrochemical luminescent devices, optical fibers and electro-optical device, piezoelectric materials, high-power batteries, biocompatible etc. It‟s well-know that unique physicochemical properties and consequently their applications depend upon the way they are tailored. Thus, to attend these quite different demands, many methods have been used for synthesis of niobium oxide materials like sol-gel,

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Preface

ix

molten salts, co-precipitation, hydrothermal process, precipitation in homogeneous solution, water-in-oil micro emulsion precipitation and so on. Once there are many variables playing the role in defining niobium oxides characteristics, the aim of this chapter is to provide the first oriented review of the main niobium oxide characteristics and reported synthesis methods described in the literature linking them to the desired properties and the envisaged materials application. Chapter 4 - Niobium (Nb) is one of the high-field-strength elements, i.e. its ions are relatively small and highly charged. In geological systems, Nb does not readily substitute into most common rock-forming minerals and behaves incompatibly during the crystallization of silicate melts. This element is concentrated in silicate melts by small degrees of partial melting or by high degrees of fractional crystallization. In the Earth, it is enriched in the crust, particularly in the upper continental crust, relative to mid-ocean ridge basalts or chondritic meteorites. In the natural materials, Nb and tantalum (Ta) have almost identical ionic radii, very similar chemical characteristics and are pentivalent. Thus, following Goldschmidt‟s' rules, Nb and Ta can be expected to remain tightly coupled in geological processes, implying that the Nb/Ta ratio of rocks and minerals should remain rather constant and close to the chondritic ratio of ~ 17.5. Many mantle-derived rocks including mid-ocean ridge basalts and ocean island basalts have such ratios suggesting that partial melting of the mantle does not fractionate the Nb/Ta ratio. However, the ratios exhibit significant variations in certain hydrothermal systems and crustal igneous rock suites including fractionated granitic rocks. Even typical continental crust rocks have significantly lower values (Nb/Ta in continental crust ~11) with some peraluminous granitic rocks having values as low as ~ 2. An explanation for the deviation of this ratio from the chondritic values is still under debate. The data presented here suggest that fluid fractionation leads to an enrichment of Ta relative to Nb and hydrothermal fluids appear to be an efficient medium for the fractionation of this ratio. Chapter 5 - A new method to improve mechanical properties of copper metal matrix reinforced with niobium carbide particles involving in-situ reaction of niobium with graphite powder is presented. Mechanical milling process was performed on a mixture of copper, niobium and graphite powders with composition of Cu-24.11%Nb-3.11%C for various milling time in order to promote in situ reaction to produce mechanically alloyed copper composite powder containing niobium carbide precipitates. To study the effect of powder consolidation technique on the composite powder densification and finally their bulk properties, the as-milled powders were consolidated by cold uniaxial pressing and cold isostatic pressing followed by sintering in argon atmosphere at 900˚C. The obtained nanostructured Cu-NbC composite were characterized for its density, microhardness, electrical conductivity, phase presence and microstructure. In the early stage of mechanical milling, according to X-ray diffraction analysis and scanning electron micrograph, niobium particles were refined and mechanically alloyed in copper matrix since niobium is brittle and fractured easily compared to soft copper. After 8 hours, the mechanical milling process was found sufficient to promote reaction between niobium and graphite to form niobium carbide precipitates without additional high temperature heat treatment. After sintering, all the niobium peaks were no longer detectable because all niobium elements have already reacted with graphite to form niobium carbide with small traces of NbO and NbO2 phases due to oxidation of niobium. In the case of uniaxial pressing, the oxides disappeared when the composite powder was pressed at 400 MPa while in the case of cold isostatic pressing (CIP), it occured at 200 MPa. CIP composite produced better hardness and electrical conductivity

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properties as the result of less porosity due to homogenous pressure distribution in the green body that produced higher densification in sintering. Microhardness of the Cu-NbC composite was increased slightly after prolonged milling time because fine niobium carbide particles were precipitated in the copper matrix due to refinement of niobium and graphite particles before a chemical reaction took place between them. However, due to insulation property of niobium carbide particles together with refined grain size in copper, the electrical conductivity of the composite was reduced with the increased in milling time. Chapter 6 - Besides its use in superconducting materials, niobium has several other areas of application in the form of pure metal, alloys, intermetallics and other compounds. Some of them require niobium powders as a raw material, such as the fabrication of electrolytic capacitors. Because the element niobium has all desired properties as an anode material of capacitors for electronic appliances, thus it is spotlighted as a substitute to tantalum. This application was furthermore investigated for several years due to a shortage of tantalum raw materials at the beginning of this decade causing a rapid price increase for tantalite concentrate in the year 2000. This trend resulted in various alternative approaches to produce niobium powders. Therefore this chapter gives an overview about techniques and process developments for the production of niobium powders as well as their utilization in different fields of applications. Thereby the main focus is set to the process development for capacitor grade niobium powders as well as to the use of niobium powders for the production of intermetallics. An example for the latter is Nb3Sn, which is applied as superconducting material in high energy physics, fusion energy applications, in nuclear magnetic resonance systems and in standard high field laboratory magnets. Chapter 7 - The aim of this work is the improvement of the electrochemical behavior of 4140 steel substrate using TiCN/TiNbCN multilayered system as a protective coating. We have grown [TiCN/TiNbCN]n multilayered via reactive r.f. magnetron sputtering technique in which was varied systematically the bilayer period (Λ), and the bilayer number (n), maintaining constant the total thickness of the coatings. The coatings were characterized by (XRD), optical microscopy, electron microscopy (SEM) and transmission electron microscopy assisted with SAED-TEM. The electrochemical properties were studied by electrochemical impedance spectroscopy (EIS) and Tafel curves. XRD results showed a preferential growth in the FCC (111) crystal structure for [TiCN/TiNbCN]n multilayered coatings [1]. In this work was obtained the maximum corrosion resistance for the coating with (Λ) equal to 15 nm, corresponding to n = 200 bilayers. The polarization resistance and corrosion rate were around 8.6 kOhm·cm2 and 7.5910-4 mmy, these values were 8.6 and 0.001 times higher, respectively, than that uncoated 4140 steel substrate (1.0 KOhm and 0.57 mmy). The improvement of the electrochemical behavior of the 4140 covered with this TiCN/TiNbCN multilayered system can be attributed to the presence of several interfaces that offers resistance of Cl- ions diffusion from the electrolyte toward the steel surface. Chapter 8 - In this study, co-genetic rare-metal-bearing granites (n=35) and pegmatites (n=10) and related perthites (n=8), muscovites (n=7), and biotite (n=1) have been investigated for their Nb abundances. Niobium was analysed by wavelength-dispersive x-ray fluorescence (WDXRF) spectrometry. Granites have the lowest content of niobium, but it is widely varying and anomalous (11-364, av. 56, ppm Nb), whereas, the co-genetic pegmatites have still higher and more anomalous abundances of niobium (36-475, av. 132, ppm Nb). Among the co-genetic pegmatitic minerals, perthites record the lowest and almost normal content of

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5-26, av. 13, ppm Nb, whereas, the biotite reveals the highest absolute content of 687 ppm Nb. An intermediate, yet anomalously high value of 203-485, av. 380, ppm Nb is observed in the case of muscovites. With compared to 21 ppm Nb in „average‟ low-Ca granite, the investigated granite shows anomalously high (2.7 times) enrichment of niobium, whereas, still higher (6.3 times) enrichment of Nb is noted in the case of co-genetic pegmatites. Such a behaviour of Nb in cogenetic felsic rocks is due to progressive fractionation leading to gradual enrichment of niobium in successively evolved melts, with maximum Nb concentration being in geochemically most evolved pegmatites. Still higher enrichments of Nb are observed in micas, that is, 18 and 33 times in muscovite and biotite, respectively. Preferential entry of Nb in mica lattice might have taken place due to extended ionic substitution during pegmatite crystallization and geochemical evolution. Because of more geochemical affinity of Nb with Fe, Nb concentration is more in biotite. Significantly, with compared to average content of 238 ppm Nb in muscovites from mineralized pegmatites of Central Nigeria, the enrichment of Nb in investigated pegmatite muscovite (1.6 times) and biotite (~3 times) is up to 3 times more. The observed pattern of Nb distribution suggests that the felsic rocks are suitable for hosting economic concentrations of rare-metals. Chapter 9 - The oxalate ion, C2O42– is well known in the chemistry of niobium. Oxalic acid was among the first agents used for the dissolution of the melt obtained by the fusion of Nb2O5 and KHSO4. The first niobium(V) oxalate-containing species of the composition M3[NbO(C2O4)3]·nH2O (M = NH4+, Na+, K+, Rb+) were isolated at the beginning of the last century. As found later, niobium atom in these compounds is heptacoordinated by oxygen atoms forming a distorted pentagonal bipyramid. The same coordination polyhedron was found in the ammonium and caesium salts of the diaquabis(oxalato)oxoniobates(V) M[NbO(C2O4)2(H2O)2]·nH2O (M = NH4+, K+, Rb+, Cs+, ½Mg2+), being formed by two bidentate oxalate groups, two water molecules and one oxo-oxygen atom. Specific supramolecular motifs and solvatomorphism have been found within the series of compounds [M(bpy)3]2[NbO(C2O4)3]Cl·nH2O (M = Fe2+, Co2+, Ni2+, Cu2+, Zn2+; bpy = 2,2'-bipyridine) crystallizing in the monoclinic (n = 11) and orthorhombic (n = 12) crystal systems. In the oxalate-bridged heterometallic complexes [{M(bpy)2}2(µ-C2O4)][M(bpy)2(µC2O4)NbO(C2O4)2]2·0.5bpy·7H2O (M = Cu2+, Zn2+) and [{Cu(phen)2}2(µ-C2O4)] [Cu(phen)2(µ-C2O4)NbO(C2O4)2]2·8H2O (phen = 1,10-phenanthroline) one-dimensional motifs in the structures are generated through the ligand stacking interactions. The square-antiprismatic coordination of niobium(IV) atoms was found in the compounds K4[Nb(C2O4)4]·3H2O, K4[Nb(C2O4)4]·4H2O·1/2H2C2O4 and K2(H3NCH2CH2 NH3)[Nb(C2O4)4]∙4H2O. Chapter 10 - The use of iridium (Ir) as a material for amperometric electrodes, especially in biosensors, has been reported in several studies [1-5]. Research results have also been reported on the use of other materials [6], including binary alloys [7], for this purpose. However, using such materials is associated with inadequate hydrogen peroxide detection, susceptibility to interference species (e.g., ascorbic acid, uric acid, and acetaminophen), a high base current, and increased complexity in fabricating electrodes, among other problems [1-5]. Hydrogen peroxide is produced by oxidases, and the interference species exist in body fluids. Adequate hydrogen peroxide detection and insusceptibility to interference species are needed in amperometric biosensors. Accordingly, Platinum has almost always been used in the materials of amperometric biosensor electrodes; carbon is an exception [8-12]. Certain

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binary Ir alloys, i.e., Ir-Nb, Ir-Zr, and Ir-Nb-Zr, have been noted as having high temperature materials [13, 14]. However, the prices of rare metals including Ir have been fluctuating widely because of the global economic crisis and/or because they are mined in limited quantities. This has created a need to use alternative materials in place of Ir in various fields, including biosensor electrode fabrication. In our work, we found that an Ir-23 at.%Nb (Ir23Nb) binary alloy can oxidize hydrogen peroxide better than any other Ir-Nb binary alloy. Chapter 11 - Niobate-based materials have found important technological applications due to their physical characteristics, such as: stable electrical parameters, high dielectric constant, and reduced ignition. Their wide availability and lower cost is also an advantage. Lithium niobate (LiNbO3) crystals present, at room temperature, ferroelectric behavior. This crystal has excellent piezoelectric, electro, electroacoustic, pyroelectric and photorefractive characteristics. The usual preparation of niobate crystals is by an expensive Czochralski process. The preparation of glass-ceramics, containing a ferroelectric crystalline phase, seems to be a good alternative to the Czochralski method due to its relatively low preparation time and costs. On the other hand, the preparation, structural, electrical and optical analysis of glass-ceramics showing ferroelectric properties is a topic with large actuality. The melt-quenching method was used to prepare silicate glasses and glass-ceramics containing LiNbO3 crystals. The glass-ceramics were obtained by controlled heat-treatments with and without the application of an electric field. SiO2 was used as a glass matrix because it does not modify the LiNbO3 crystalline lattice. The microstructure of the samples, with different compositions and different thermal conditions, was characterized by differential thermal analysis, X-ray diffraction (XRD), scanning electronic microscopy (SEM) and Raman spectroscopy. The dc and ac conductivities and complex impedance (Z*) in function of temperature and frequency were measured. In the niobate glasses prepared by melt quenching, the amount of LiNbO3 increases with the increase of the heat-treatment temperature. The presence of an electric field during the heat-treatment promotes a decrease in the temperature needed for the LiNbO3 formation and favors a superficial crystallization. The impedance spectra was adjusted through an equivalent circuit model consisting of a serial circuit between a resistance (R1) and the parallel between another resistance (R2) and a constant phase element (CPE). Chapter 12 - Since the pioneer work done by the High-Energy Physics Lab at Stanford University in 1965, superconducting radio frequency (SRF) technology has been developing steadily up to now. Demanding on niobium (Nb) has been increasing constantly, since more and more particle accelerators select Nb based SRF technology as a key part of their accelerator constructions. For example, the proposed International Linear Collider (ILC) that will probe new physics using TeV collisions of electron and positron beams will need approximately 17,000 1-meter-long Nb SRF cavities. Others such as x-ray free electron laser (XFEL) at DESY in Germany, energy recovery linac (ERL) at Cornell University in USA, the new Spiral 2 facility in France, the isotope separation and acceleration (ISAC) II in Canada, and the 12 GeV upgrade of CEBAF at Jefferson Lab in USA will all require Nb. This popularity in Nb can be, at least partially, attributable to the unique physical and mechanical properties that Nb possesses --- the highest superconducting transition temperature of 9.25 K and the highest superheating field of 0.23 T among all available pure metals with excellent ductility that enables machining to be done relatively easily. In this chapter, the use of Nb for

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fabricating SRF cavities is reviewed, giving particular attention to some examples of important new developments in the past decade on reducing the production costs and increasing the throughput of high quality Nb SRF cavities. Some R&D examples on the study of the requirements in the physical, chemical, metallurgical, and mechanical properties of Nb for the applications in particle accelerators based on Nb SRF technology are updated and reviewed. This chapter also includes some unpublished experimental results from my own research. Hopefully this review can be served as a useful reference for new researchers who want to use Nb for their various R&D projects in particle accelerators and for Nb suppliers and manufacturers who want to provide the best and the most economic products to be used in particle accelerators. Chapter 13 - More and more particle accelerators are using Nb Superconducting Radio Frequency (SRF) technology due to the steady progress made during the last few decades in the SRF field. Improvement of the surface treatments of Nb SRF cavities is an indispensable part of the evolution of SRF technology. In this chapter, a study of the surface treatments of Nb flat samples and SRF single cell cavities via Gas Cluster Ion Beam (GCIB) technique will be reported. Beams of Ar, O2, N2, and NF3 clusters with accelerating voltages up to 35 kV were employed in the treatments. The treated surfaces of Nb flat samples were examined by a scanning field emission microscope, a scanning electron microscope equipped with an energy dispersive x-ray analyzer, a secondary ion mass spectrometry, an atomic force microscope, and a 3-D profilometer. The experiments revealed that GCIB technique could not only modify surface morphology of Nb, but also change the surface oxide layer structure of Nb and reduce the number of field emission sites on the surface dramatically. Computer simulation via atomistic molecular dynamics and a phenomenological surface dynamics was employed to help understand the experimental results. Due to its effectiveness at changing the depth and composition of the surface oxide layer structure of Nb, GCIB might be a key to understanding and overcoming the limitations of the high-field Q-slope. Based on the encouraging experimental results obtained from flat sample study, a novel setup was constructed to allow GCIB treatments on Nb single cell cavities. First results of RF tests on the GCIB treated Nb single cell cavities showed that the quality factor Q of the cavity could be improved substantially at 4.5 K and the superconducting gap value, extracted from RF measurements at different temperatures below superconducting transition temperature, was enhanced by oxygen GCIB treatments. This study indicates that GCIB is a promising surface treatment technique for Nb SRF cavities to be used in particle accelerators. Chapter 14 - Niobium as well as the other refractory metals (4th, 5th and 6th columns of periodic table of elements), except chromium, cannot be electrodeposited from aqueous medium. Nevertheless, these metals can be obtained from alkali halides due to the high decomposition voltage of these solvents. Electrodeposition can be divided into electrowinning, electroplating, electrorefining and electroforming. The aim of this work is to present our results on the three latter topics for niobium in alkali fluoride melts. The effect of electrolysis parameters, such as cathodic current density and temperature on current efficiency, deposit morphology, microstructure, grain size and niobium purity is discussed. Niobium electroplating was performed on wires, tubes, plates and cathodes of complex shapes in order to study the influence of the cathode shape and the anode-cathode distance on coating distribution. Corrosion data of niobium electroplates in mineral acid media are also shown. Some Nb electroforms are also presented.

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In: Niobium: Properties, Production and Applications Editor: Thomas M. Wong

ISBN: 978-1-61122-895-3 © 2011 Nova Science Publishers, Inc.

Chapter 1

THE ROLE OF NIOBIUM SPECIES IN HETEROGENEOUS CATALYSIS - SELECTED ASPECTS Maria Ziolek*, Izabela Sobczak and Maciej Trejda Adam Mickiewicz University, Faculty of Chemistry Grunwaldzka 6, 60-780 Poznan, Poland

ABSTRACT

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This review article is devoted to the discoveries and/or knowledge developed in the past years in the area of the application of heterogeneous catalysts containing niobium for selective oxidation in gas and liquid phases by the use of different oxidants (oxygen and hydrogen peroxide). The idea of this chapter is to give the readers a general overview of the role of niobium species located in crystalline and amorphous solids in catalytic oxidation processes. The important ability of niobium to combination with many elements towards formation of new compounds determines the unique properties of niobium containing catalysts. Two main roles of niobium species in heterogeneous catalysis are considered: as catalytic active centers and as promoters. In some cases niobium containing materials are also considered as the supports. The chapter is divided into four parts. After a short introduction (first section), the most important aspects of the heterogeneous catalysis and solids used as catalysts are drawn in second section. The most detailed is the third section summarizing recent advances on the use of catalysts containing niobium species in various industrially important oxidation processes: i) gas phase oxidative dehydrogenation (ODH) of propane, ii) gas phase ammoxidation of propane, iii) gas phase selective oxidation of methanol, iv) liquid phase oxidation of olefins (with H2O2), v) liquid phase oxidation and ammoxidation of glycerol (with oxygen and H2O2, respectively). The choice of the reactions discussed is determined by the questions which we would like to answer that is, what is the role of niobium species in the catalyst when oxidation proceeds with the use of gas oxygen as oxidant (both in gas and liquid phases) and when hydrogen peroxide oxidizes organic compounds in the liquid phase. These aspects are considered on the basis of different types of catalysts containing niobium described in the literature: bulky *

Corresponding author: e-mail: [email protected]

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Maria Ziolek, Izabela Sobczak and Maciej Trejda niobium(V) oxides (amorphous and crystalline), mono- and multicomponent metal oxides, ordered mesoporous metalosilicate materials and microporous zeolites, also used as supports for noble metals and different mono and multi metal oxides. The last section summarizes the role of niobium in heterogeneous catalysts.

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1. INTRODUCTION Niobium compounds exhibit certain unique properties not shared by the compounds of neighboring elements in the periodic table. Some of them, like strong metal support interaction (SMSI) or unique reversible interaction with several reagents are very important for design of catalysts. The last decades have brought increasing interest in the catalysts containing niobium [1,2]. In the 1990s the first international symposia devoted to niobium compounds, especially those applied in catalysis, were organized. They were preceded by the conference organized in 1989 by I.E. Wachs [3] at which the first application of niobium compounds in catalysis was discussed. Soon the scope of these meetings was extended to all the elements of Group Five. At present, International Symposia on Group Five Elements are organized every third year, which reflects the importance of V, Nb and Ta compounds in catalysis. Firstly, the catalysis on niobium oxide was concentrated on its acidic properties. Later the focus was on oxidative properties of niobium–containing catalysts. The pioneer works in the field of niobium oxide structure and application of niobium species in catalysis have been reported by Tanabe et al. [4] and I.E. Wachs and co-workers [5-12] and were summarized in [13]. The function of niobium compounds in catalysis can be that of a promoter or active phase, support, solid acid catalyst, or redox material. Niobium can be present in niobium oxide used in its bulk form as a catalyst, in oxide support for metals or metal oxides and in mixed oxides. Of course, its activity is determined by the surrounding of niobium species and by the type of catalytic reaction. All these features were widely discussed in the above mentioned papers. The beginning of the 21st century has brought intense development of the use of niobium species as the main component or an additive to the heterogeneous catalysts applied in various processes. This chapter is devoted to the discoveries and/or knowledge developed over the last few years in the area of application of heterogeneous catalysts containing niobium for selective oxidation in gas and liquid phases by the use of different oxidants (oxygen and hydrogen peroxide). The idea of this paper is to give the readers a general overview of the role of niobium species, located in crystalline and amorphous solids, in catalytic oxidation processes. The important ability of niobium to combine with many elements towards formation of new compounds determines the unique properties of niobium containing catalysts. Two main roles of niobium species in heterogeneous catalysis are considered: as catalytic active centers and as promoters. In some cases niobium containing materials are also considered as the supports. The chapter is divided into four sections. After a short introduction (first section), the most important aspects of the heterogeneous catalysis and solids used as catalysts are drawn in second section. The most detailed is the third section summarizing recent advances on the use of catalysts containing niobium species in various industrially important oxidation processes. Each subsection in this section is devoted to a different oxidation process and starts with general remarks related to this process followed by a review of the possible use of niobium containing catalysts in the desired reaction. In each of these oxidation processes

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niobium is considered as the major active component as well as the promoter in the catalysts applied in gas and liquid phase oxidations. In the liquid phase oxidation a great importance of amorphous phase vs. crystalline one is widely discussed. This section covers the following fundamental oxidation reactions in which niobium species plays an important role: i) gas phase oxidative dehydrogenation (ODH) of propane, ii) gas phase ammoxidation of propane, iii) gas phase selective oxidation of methanol, iv) liquid phase oxidation of olefins (with H2O2), v) liquid phase oxidation and ammoxidation of glycerol (with oxygen and H2O2, respectively). The choice of the reactions discussed is determined by the questions which we would like to answer that is, what is the role of niobium species in the catalyst when oxidation proceeds with the use of gas oxygen as oxidant (both in gas and liquid phases) and when hydrogen peroxide oxidizes organic compounds in the liquid phase. These aspects are considered on the basis of different types of catalysts containing niobium described in the literature: bulky niobium(V) oxides (amorphous and crystalline), mono- and multicomponent metal oxides, ordered mesoporous metalosilicate materials and microporous zeolites, also used as supports for noble metals and different mono and multi metal oxides. Various parameters of the preparation procedure and niobium content affecting the catalytic activity are considered. The last section summarizes the role of niobium in heterogeneous catalysts. We hope that this chapter will be of value to chemists involved in the catalytic oxidation reactions in both academic and industrial research and that it will allow them to understand the fascinating role of niobium species in heterogeneous catalysis and stimulate further development in the use of niobium as the active phase or promoter in catalytic oxidation reactions.

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2. HETEROGENEOUS CATALYSIS – GENERAL REMARKS The term „catalysis‟ (meaning „loosening down‟ in Greek) was introduced by Berzelius in 1836 [14]. Nowadays it is interpreted as the phenomenon of reducing the activation energy for transformation of substrates to products thanks to the interaction with material called the catalyst. The occurrence of catalysis has been extensively studied since the early decades of the 19th century and the most intense development in the area of catalytic processes application in industry and various aspects of human life was noted in the 20th century. At present catalysts are applied e.g. in the production of foods, clothes, energy, chemicals used for fabrication of materials used in and around our homes and offices, pharmaceuticals, or for purification of air. Catalysts are at the heart of almost all biological and many chemical transformations of molecules and mixtures into useful products [15]. In terms of chemical conversions, catalysts are responsible for the production of over 60 % of all chemicals that are made and are used in over 90 % of all chemical processes worldwide [16,17]. It is clear that catalysis plays an important role in society today and will be a critical technology for advancing our future especially in terms of the use of renewable sources of energy, purification of air and production of many chemicals. Let‟s consider the meaning of catalysis in detail. Catalysis deals with changes on the path towards equilibrium. It concerns the reaction kinetics, not thermodynamics. A very important point is that catalysts do not and cannot change the thermodynamic equilibrium [18] but they permit reaching the equilibrium faster by reducing the energetic barrier for the reaction route.

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Two types of catalysis are distinguished: the homogeneous catalysis in which the reagents and the catalyst are in the same phase and the heterogeneous one in which the catalyst and the reagents are in different phases. Usually the catalysts are in solid phase and when the reagents are in gas phase the reaction is called the gas phase one, while when the reagents are in liquid phase it is the liquid phase reaction. This chapter deals with the application of niobium species in heterogeneous catalysis in which solids containing Nb are applied. We will not focus on biocatalysis which is subjective. A typical enzyme used as catalyst is so much larger than its substrate, and the reaction environment it presents is so different from the surrounding solvent, that it qualifies as a „heterogeneous catalyst‟. Moreover, there are soluble catalysts which are on the borderline between „homogeneous‟ and „heterogeneous‟ systems [18]. Typically in the classic gas/solid heterogeneous catalysis, the gaseous reactants are fed over the catalyst bed, usually at high temperatures, and sometimes at high pressures. Reactants must diffuse through the catalyst pores, get adsorbed on its surface, travel to the active sites, react there, and get desorbed back to the gas phase as illustrated in Figure 1.

Figure 1. The mechanisms of catalytic reactions.

Two distinct mechanistic situations can arise in the surface-catalyzed transformation of gas-phase species X and Y to product Z; either both species are attached to the surface, and atomic reorganization takes place in the resulting adsorbed layer (the so-called LangmuirHinshelwood mechanism), or only one of them is bound and is converted to product when the other impinges upon it from the gas phase (the Eley-Rideal mechanism) [16]. The first of these mechanisms is met far more frequently than the second. The surface interactions hold the key to the catalyst activity, selectivity and stability. For a given reaction A→B, the conversion of A is the number of molecules of A that have reacted up to a time t. It is often expressed as fraction or percentage of [A]0, the initial concentration

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of A. For comparing the catalyst efficiency the turnover frequency (TOF) is often used. TOF is articulated by the number of A molecules that can be converted to B molecules by one molecule of the catalyst (more precisely one active center – the meaning of active center will be explained below) in one second, minute or hour [18]. A good catalyst must show both high activity and long-term stability. But its single most important attribute is its selectivity, S, which reflects its ability to direct the conversion of reactant(s) along one specific pathway [16]. When a reactant A can be transformed to either B or C at rates R1 and R2 (Scheme 1), respectively, the selectivity S is calculated as S = R1 – R3 / R1 + R2. To obtain the desired selectivity the proper chemical composition of the catalysts giving rise to the appropriate active centers must be designed. Lewis and BrØnsted acid and base sites as well as redox centers can be present on the catalyst surfaces depending on their composition. The „construction‟ of the catalysts (particle sizes, shapes, porosity) determines the access to the active centers and in a consequence their activity and selectivity.

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Scheme 1.

In the liquid/solid catalytic system, the reaction occurs on the surface of the catalyst particles which are suspended in the reagent mixture. In the liquid phase reactions an important parameter is hydrophobicity of the catalyst surface, because water (the reaction medium or product of the decomposition of oxidant – hydrogen peroxide) easily poisons active acidic sites on the hydrophilic surface. The other features which should be considered are the porosity of the solid catalyst which should allow easy diffusion of reactants. The stability of the active species which should not leach into the solution by the forming of complexes with oxidant (e.g. H2O2) seems to be a key point for catalysis of this type [19]. The fact that heterogeneous catalysis occurs at a surface makes it relatively difficult to understand the reaction pathways and determine the mechanism of the catalytic process. The surface of solids is not uniform, e.g. metal crystal surfaces show various steps and kinks. Often catalysis is realized at these irregular points because the surface atoms at these spots are not fully coordinated and thus have more options for interacting with substrate molecules [18]. The chemical interaction of reagents (chemisorption) with the surface is required before the subsequent catalytic steps. The catalytic active sites are often isolated on the inactive surface and often are not the energetically preferred sites for adsorption (i.e., there are other sites on the surface where adsorption would be more exothermic). The most favored site for adsorption may be „too good‟ and hold molecules too strongly, which means that any species that adsorbed on it would simply stay there. The center is called the active site when the reactant is able to be adsorbed there, but after the reaction it is also able to leave it (desorb). To get an ideal catalyst of this type, there should be a number of identical active sites on the

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catalyst surface well isolated from each other. Commercial heterogeneous catalysts, except for zeolites, are often amorphous and multiphase solids containing many types of active sites. In fact, industrial catalysts consist of several phases containing a number of species. These species originate from either impurities or to a low extent promoters or modifiers, which are added intentionally during the catalyst synthesis.

Figure 2. The components of heterogeneous catalysts [according to ref. 20].

The classical scheme of the catalyst composition proposed by Richardson [20] is shown in Figure 2. Of course, the main component of each catalyst is its active phase which can be in the bulk (e.g. single oxides, zeolites) or can be loaded on the support. The role of the support is to give high surface area for the well dispersed active phase (protection from agglomeration) and mechanical stability. There are also other functions like protection from thermal growth of the support crystals and/or thermal decomposition (thanks to high melting point of the support) or chemical interaction with the active phase which modifies the properties of active centers (e.g. decreasing the strength of active center which allows desorption of the product). The latter function combines both, support and promoter behavior and such support is called „active support‟. Promoters can modify the support (e.g. increase in stability or accessibility) or can affect the active phases (e.g. electronic promoters). An example could be the addition of alkali promoters to the transition metal active phase, which makes the dissociative chemisorption of such molecules like N2 or CO much easier through electron donation. The electropositive alkali cation donates electron density to the transition metal. This increases the back-donation into the * antibonding orbital of the substrate, and facilitates the substrate decomposition [18]. It is important to stress that the promoter

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activates the surrounding sites and it is not an active species itself. Usually transition metals are the active components of the heterogeneous catalysts applied in redox processes, whereas alkali metals and earth alkali metals are used as promoters. However, it often happens that depending on the location in the catalyst structure transition metal can play the role of an active site or a promoter. It is just the case for niobium.

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Figure 3. Different possible constructions of supported catalysts.

Figure 3 shows schemes of different possible constructions of supported heterogeneous catalysts which can be related to niobium containing catalysts. In constructions (a) and (d) the inactive or active support is accessible for the reagents, whereas in (b) and (c) the inactive support is totally covered either by active support (b) or by both active support and active phase (c) [21]. Active supports often contain promoters and/or their own active centers which enhance the activity and/or selectivity of the active components. Niobium species can play the roles of an active phase or an additive to the active support or a promoter. Taking into account the chemical nature of the materials, niobium containing heterogeneous catalysts can be classified as carbides, sulfides, oxides, nitrides (oxynitrides) and phosphates [22]. The largest group of catalysts includes those based on niobium oxides which can be used as bulk niobia – active phase, bulk niobia – support, mixed oxides, niobia supported on various oxides and niobium species as a promoter. The catalysts considered in this chapter are based on niobium oxides and mixed oxides.

3. NIOBIUM SPECIES IN CATALYSTS APPLIED FOR HETEROGENEOUS OXIDATION PROCESSES Transition metal oxides are active materials for conversion and selective oxidation of a wide range of different reagents. They can be used as acidic, basic, reducible, non-reducible and bifunctional catalysts to carry out specific reactions. The mechanisms of these reactions can be quite different depending on the nature of the catalyst and the oxidant (e.g. oxygen or hydrogen peroxide) as well as the type of oxygen species on the catalyst surface. Oxygen on

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the surface of metal oxides can exist in various different forms ranging from adsorbed O2, to fully oxidized atomic oxygen (O2-), O2*, O2-, O- and O2- [15]. The first three types are electrophilic and they tend to activate C-H bond in organic compounds and lead to total oxidation of the reactants. The form O2- is more nucleophilic and can insert into the C-C bonds of the reactants molecules, thus favoring more selective oxidation paths. The reaction path is different on reducible and non-reducible metal oxides. Niobium oxides belong to nonreducible metal oxides and oxidative catalysis on such oxides does not involve change in the oxidation state of the metal. Instead, it requires the presence of promoters, defect sites or sites that do not require changes in the redox character of the cations [23] to help activate hydrocarbon reagents. The structure, composition and electronic properties of catalysts containing niobium oxide species control the properties of the catalytic system used in the oxidation processes described in the subsections below (3.1 – 3.5).

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3.1. Gas phase oxidative dehydrogenation (ODH) of propane 3.1.1 General information about the catalytic process The oxidative dehydrogenation (ODH) of lower alkanes leads to formation of unsaturated hydrocarbons as feedstock for many industrially significant processes. Especially, ODH of propane to propene is currently a very active area of research. The demand for propene continues to increase because of the rapid growth of the market for polypropylene, acrolein, acrylic acid and methyl-t-butylether (MTBE – the key ingredient of reformulated gasoline) [24]. Low alkanes are very unreactive, their dehydrogenation that gives the corresponding olefins and H2 is a highly endothermic process and has to be carried out at temperatures above 873 K [24, 25]. The introduction of a hydrogen acceptor into the reaction medium shifts the thermodynamic equilibrium making the reaction irreversible even at lower temperatures [24]. When oxygen is used as an acceptor, the oxidative dehydrogenation of alkanes becomes exothermic, and the thermodynamic constraints of non-oxidative routes are avoided by forming water as a byproduct according to the reaction: CnH2n+2 + O2  CnH2n + H2O

Scheme 2.

Moreover, another advantage of ODH is the elimination of carbon deposition. The presence of oxygen prevents the accumulation of carbonaceous deposits on the catalyst surface, leading to stable catalytic activity. Thus ODH is an attractive alternative to

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dehydrogenation. Propane ODH occurs via the parallel and sequential oxidation steps shown in Scheme 2 [25]. Propene is a primary product and carbon oxides (COx) form directly from propane and by subsequent oxidation of propene. The greatest challenge is to find catalyst active and selective towards alkenes without causing complete oxidation of alkene to carbon oxides. The most active and selective catalysts for propane ODH are vanadium or molybdenum supported metal oxides also containing some other additives. It was evidenced that the oxidative dehydrogenation of lower alkanes on metal oxide catalyst systems proceeds via a Mars-van-Krevelen redox mechanism [26, 27]. According to this mechanism hydrocarbons are adsorbed onto the catalyst surface and react with lattice oxygen forming vacancies. Oxygen atoms abstract hydrogen from alkane molecules, finally making olefins and water. In the next step the oxygenated product gets desorbed from the reduced surface and molecular oxygen is adsorbed onto the surface of the oxide as O-, O2-, or it can be incorporated into the lattice as O2-. At this step, the metal oxide is oxidized and the electrons transferred to the adsorbed oxygen could come from surface cation or from anion vacancies with trapped electrons. If the lattice oxygen, O2-, is transferred, then the sites for oxygen adsorption and for oxygen incorporation may be different, and migration of O2- ions in the solid between two sites would occur. Detailed steps of the ODH mechanism on VOx/SiO2 and VOx/ZrO2 are shown in ref. [28, 29]. They are as follows: 1) weak adsorption of propane by interaction with lattice oxygen (O*) C3H8 + O*  C3H8O*

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2) C–H bond activation by abstraction of an H atom from adsorbed propane using a neighboring oxygen atom C3H8O* + O*  C3H7O* + OH* 3) desorption of propene by hydride elimination C3H7O*  C3H6 + OH* 4) recombination of OH groups to form water and a reduced V center (*) OH* + OH*  H2O + O* + * 5) reoxidation of V centers via dissociative chemisorption of O2 O2 + * + *  O* + O* . In this scheme, O* is the lattice oxygen in V=O or V–O–V structures, OH* is a hydroxyl group in V–O–H, C3H7O* represents the adsorbed propoxide bonded to V through an O atom (e.g., V–O–C3H7), and * is the surface vacancy associated with either one V3+ or two V4+ atoms in the VOx lattice. It has been proved that the lattice oxygen participated in the activation of C-H bond in C3H8 and that the surface oxygen, O–H groups and, especially, oxygen vacancies are the most abundant reactive intermediates during ODH on the active VOx domains [27, 29-31]. Literature data suggest that Nb-containing catalysts should be also attractive for the oxidative dehydrogenation reaction of lower hydrocarbons [e.g. 23, 32-42]. An example is the

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VNbO system [32, 33], which exhibits good efficiency in propene formation (90 % selectivity) and apparently does not yield any oxygenated products. It was found that the activity and selectivity of a vanadium site depends on the nature of its neighboring atoms. Vanadium neighbors promote the activity, while niobium neighbors promote the selectivity. However, the precise role of niobium in ODH process is still not clear. The effect of niobium is discussed in the subsection below (3.1.2.).

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3.1.2 Role of Niobium in ODH of Propane Bulk niobium(V) oxide, also in the form of supermicroporous or mesoporous materials is well known as strong acidic catalyst [e.g. 43-45] and in this form it is almost not active (very low activity of bulk Nb2O5 was reported in [32]) in oxidative dehydrogenation of propane but it is highly selective to propene. To induce ODH activity, the dispersion of niobium oxide species in the other oxide, e.g. silica, was one of the strategies described in literature for inducing oxidative dehydrogenation activity. For this purpose Ziolek and co-workers [35-38] synthesized niobiosilica hexagonally ordered mesoporous molecular sieves of MCM-41 and SBA-3 types and applied in oxidative dehydrogenation of propane. Niobium was incorporated into the material during the synthesis and therefore it occupies a position in the skeleton between tetrahedral silica forming Nb-O-Si bridges and generating new hydroxyls on the surface [2, 19, 46, 47]. After dehydroxylation of such material very attractive active species are formed, NbO- oxidative species and Nb+ Lewis acid sites (Scheme 3) [19, 46, 47]. The centers of both types can be considered active in the oxidative dehydrogenation of propane [35, 37]. However, there is no linear relationship between the number of Lewis acid sites and the activity in ODH of propane, showing that some other factors are important.

Scheme 3. Active centers formed after dehydroxylation of ordered mesoporous niobosilicates [19, 46, 47].

For better understanding of the role of niobium in the catalysts applied in ODH of propane and for design of new catalysts, MCM-41 materials containing three transition metal elements (niobium, vanadium, and molybdenum introduced during the synthesis) were studied [35]. It was shown that the presence of niobium in the NbVMoMCM-41 samples

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promotes the selectivity to propene, which is the highest (above 60 % at 793-819 K) for the materials containing a low amount of niobium; i.e., when niobium species are isolated. NbOspecies located in the surrounding of V5+ promotes the electron transfer thus increasing the selectivity to propene. The same is true for monometallic NbMCM-41 samples. If Nb concentration is higher, niobium dimers are formed on the surface and they activate the total combustion of propane which is accompanied by the increase of activity. Moreover, it was found that the selectivity to propene strongly depends on the presence of defects generated by niobium in MCM-41. These defect holes facilitate the accessibility of reagents to the active species and diffusion, which means that niobium species in multimetallosilicate MCM-41 materials play the role of electronic promoter and/or structural promoter (generating defect holes) in ODH of propane. Vanadium in VMCM-41 is responsible for high propane conversion. However, the products of total oxidation (CO and CO2) predominate on this catalyst. Isolation of niobium species in a mesoporous silica matrix, important in ODH of propane, can be achieved when Si/Nb ratio is relatively high (Si/Nb > 50) and when all Nb species are located in the framework. Unfortunately, in NbMCM-41 materials the synthesis procedure in the basic medium induces the location of some niobium in the form of niobium oxides in extra framework positions [38]. Thus, the active niobium species are heterogeneous which is obstacle in determination of the reaction pathway. The synthesis of analogous hexagonally ordered mesoporous NbSBA-3 materials in acidic medium gave hope to obtain more homogeneous niobium species [37]. Preparation of SBA-3 in acidic media instead of basic ones increased the number of Nb species in the walls of the solid without formation of niobium species at the extra framework positions. Moreover, contrary to the other niobosilicate mesoporous solids, in NbSBA-3 materials the diffusion in pores (not blocked by extra framework species) does not limit the reaction rate. The selectivity to propene in ODH of propane on monometallic NbSBA-3 (with Si/Nb > 50) materials was above 80 % at 763-803 K [37]. Because of the homogeneity of Nb species in these materials, one can correlate the activity and selectivity with the number and strength of active sites as well as with the isolation of Nb species. A linear relationship between the real Si/Nb atomic ratio and the number of LAS in NbSBA-3 was found. The selectivity to propene in ODH of propane over NbSBA-3 catalysts changed in a way different than that of the estimated acidity, showing that in this reaction much more important is isolation of the active metal species. Analysis of ODH results over NbSBA-3 and NbMCM-41 types of mesostructured materials has shown [35, 37] that niobium tetrahedrally coordinated and connected with siliceous-oxygen species in the walls is responsible for the oxidative dehydrogenation of propane towards propene, whereas niobio-oxide species located in the extra framework sites are responsible for the oxidation of hydrocarbons towards CO and CO2. In the catalysts described above, in which niobium species are isolated in the mesoporous silica matrix, the niobium species play the role of active and selective centers in propane ODH process because they exist in bifunctional forms [2, 19, 46, 47]. The results of propane ODH obtained on various ordered mesoporous niobiosilicas (NbMCM-41 [35], NbSBA-3 [37] and NbSBA-15 [38]) shown in Table 1 indicate that the most attractive catalysts are NbSBA-3-128 and NbSBA-15-128, which contain niobium in the skeleton only. If niobium is partially in the extra framework position as bulk niobium(V) oxide it decreases the activity and selectivity in the ODH reaction. When another transition metal (vanadium and/or molybdenum) is added to this kind of catalysts the niobium species act mainly as the agents

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increasing the selectivity of the reaction. The active sites involved in the activation of various alkanes require some degree of bifunctionality; therefore the best materials appear to couple redox properties together with acid or base properties [15, 23, 48-50]. Such bifunctionality is achieved in ordered mesoporous niobiosilicas by generation of NbO- oxidative species and Nb+ Lewis acid sites (as shown in Scheme 3) after activation. Bifunctionality can also be obtained when niobium is added to mixed and supported metal oxides, which in fact are the most commonly studied catalysts in ODH of propane. Table 1. The catalytic activity of NbMCM-41, NbSBA-3 and NbSBA-15 materials in ODH of propane [35, 37, 38] Catalyst*

Treact. (K)

Conv. C3H8 (%)

Selectivity (%) C3H6

CO

CO2

CxHy

Yield C3H6

Ref.

(%)

NbMCM-41-128

819

6.6

66.3

14.9

18.8

0

4.4

35

NbSBA-3-128

763

6.5

86.2

1.8

10.2

1.8

5.6

37

NbSBA-15-128

803

6.0

87.4

0

8.9

3.7

5.2

38

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* The last number in the catalyst symbol stands for Si/Nb molar ratio

The effect of niobium addition on the activity and properties of VOx/SiO2 and VOx/MgO catalysts [39, 51] as well as VOx/TiO2 samples [40] in oxidative dehydrogenation of propane has been the interest of many authors. It has been shown that the addition of Nb species influences both the activity and selectivity to propene and the extent of this effect depends on the support. The activity of VOx/SiO2 with Nb addition is higher than that of the undoped sample, whereas the addition of Nb to VOx/MgO results in a decrease in its activity. However, for both these types of catalysts the Nb addition increases the selectivity to propene (from 27 to 33 % for VOx/SiO2 and from 31 to 36 % for VOx/MgO at 723 K). CO2 is the main product of total combustion, CO is formed only in minor quantities. As the formation of mixed Nb–V bulk oxide compounds was avoided, the observed activity and selectivity of VOx/SiO2 and VOx/MgO catalysts after modification with niobium in propane ODH were related to the acido-basic properties of these catalysts [51]. Addition of Nb to VOx/SiO2 catalyst decreased the amount of strong LAS, whereas to VOx/MgO it significantly increased the number of Lewis acid sites. The VOx/SiO2 catalyst containing Nb revealed acidic character whereas VOx/MgO modified with niobium addition exhibited basicity, well observed in dehydrogenation of 2-propanol to acetone. However, a correlation of the catalytic performance in ODH of ethane with acido-basic properties of the catalysts was not evident. As to the selectivity, it was concluded that for VOx/SiO2 catalysts (with and without niobium), in which V2O5 was the main phase and acidic properties dominated over dehydrogenating one, the selectivity to propene increased with decreasing acidity. It was related to increase in the electron density (i.e. higher basicity and lower acidity) of the V–O centers. The role of acidic V cations from the V–Ox center in activation of the C–H bonds in alkanes was stressed in ref. [52]. For VOx/MgO catalysts, containing magnesium orthovanadate, in which dehydrogenating properties dominate over acidic ones, the selectivity

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to propene does not depend clearly on the acido-basic properties, but appears to be influenced by the properties of the catalysts‟ oxygen.

(1)

CnH2n+2

(2)

CnH2n

CO

(3)

CO2

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Scheme 4.

As established for V/Nb/TiO2 catalysts [40, 53], the niobium addition influences the catalytic properties of vanadium ions (active site V*) and favors the selectivity to propene. It has been shown that the activity of V/Nb/TiO2 in ODH of propane at 673 K increases with increasing vanadium content, whereas the selectivity to propene decreases (Table 2). It indicates that vanadium oxide species must be involved in the paraffin activation. It is noteworthy that in the range from 5 to 20 % of propane conversion, the addition of Nb to V/TiO2 catalysts has a promoting effect on the olefin selectivity at low V/Nb ratio, whilst it depresses the activity at higher V/Nb ratios. The presence of Nb at low vanadium content enhances the selectivity to propene more than 15 % in comparison with that obtained for V/TiO2. Increase in the vanadium loading results in a slight lowering of the propene selectivity, mainly as a consequence of consecutive CO formation (Table 2) according to Scheme 4. The test of 2-propanol decomposition has shown that Nb/TiO2 catalysts have a typical acidity, which is probably associated with niobium sites. The acidity was correlated to high selectivity to propene. The presence of high concentration of vanadium in the V/Nb/TiO2 catalysts seemed to diminish this acidic character, which corresponded to a decrease in selectivity to propene (Table 2). Table 2. The catalytic activity of V/Nb/TiO2 catalysts in ODH of propane at 673 K [53] Catalyst

Conv. C3H8 (%)

Selectivity (%) C3H6

CO

CO2

C1+C2

6Nb/TiO2

20.9

36.6

41.8

21.1

0.5

1V/TiO2

18.7

25.5

55.2

18.3

0.012

6V/TiO2

19.9

23.1

57.4

19.4

0.0

1V6Nb/TiO2

20.4

30.6

47.4

21.7

0.3

2V6Nb/TiO2

21.0

23.6

52.9

23.5

0.0

6V6Nb/TiO2

19.4

19.9

58.6

21.6

0.0

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The redox mechanism for ODH reaction over Nb/VOx/TiO2 was proposed [53] with the consumption of surface O2- anions and their replacement by gas oxygen:

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C3H8 + ZO C3H6 + Z + H2O Z + 1/2O2 ZO where ZO and Z are the oxidized and the reduced sites, respectively. In this purely heterogeneous mechanism, the rate-limiting step is the breaking of the first C-H bond, with the formation of C3H7 radical adsorbed on the catalyst surface. The activity of a series of pure or silica-dispersed Nb-V, V-Sb and Nb-V-Sb systems prepared by sol-gel method in ODH of propane was studied in the group of Moggi and Devillers [41, 54, 55]. The crystalline, practically pure phases NbVO5 and SbVO4 were identified by XRD in the silica-free Nb-V and Sb-V catalysts (atomic ratio 1:1). Mixed oxide phases were detected in the Nb-V-Sb (1:2:1) and Nb-V-Mo (1:1:1) catalysts. The systems containing silica (Nb-V-Si) were amorphous and characterized by quite high surface areas (130-140 m2/g). At 773 K all the prepared catalysts revealed about the same level of propane conversion and propene selectivity of about 30 % and above 20 %, respectively. The Nb-V (1:1) sample prepared via the hydrolytic method and Sb-V (1:1) catalysts were the most selective ones (almost 40 % selectivity to propene). Moreover, CO2 was the other main product formed. The increase in reagents partial pressures and contact time caused a decrease in the propene selectivity, but acrolein was detected for almost all the catalysts. Analysis of the propene productivity data from both sets of experiments normalized per unit of catalyst mass, proved Sb-V and Nb-V-Si catalysts to be the most efficient ones. For Nb-V systems, the influence of various Nb precursors (alkoxide or chloride), acid promoters and the Nb/V ratio (Nb/V = 1:1, 4.5:1 and 9:1) was also investigated [41]. From among the 1:1 Nb/V samples, the one obtained in the presence of HCl acid as the sol–gel promoter, gave the highest conversions of C3H8 at 723 K. It is consistent with the presence of segregated crystalline V2O5 in their structure (XRD results) and specific activity of V–O bonds in the C–H bond activation of alkanes, which is the rate-limiting step of the reaction. Moreover, it was found that the 1:1 Nb/V samples prepared from NbCl5 gave a lower conversion of C3H8 than the sample prepared from alkoxide (respectively 3 and 10 %), most probably because of the lowest value of surface area. The effect of Nb/V ratio was also observed. It should be stressed that while in the 1:1 Nb/V sample the crystalline phases NbVO5 and V2O5 were detected by XRD, in both 9:1 Nb/V and 4.5:1 Nb/V samples, there was no evidence of V-based crystalline phases and crystalline Nb2O5 was the only phase detected. Additionally, in the 4.5:1 Nb/V sample, the Raman spectra evidenced the presence of microcrystalline Nb18V4O55 (as a result of some diffusion of V5+ ions into the structure of orthorhombic Nb2O5 phase). A Nb–V „synergistic effect‟ in ODH of propane seems to characterize the 4.5:1 Nb/V sample, which shows both higher activity (21 %) and selectivity to propene (35 %) than the 1:1 Nb/V sample (respectively, 10 and 25 %). The 9:1 Nb/V material containing the lowest amount of V was found to behave similarly as pure Nb2O5, showing rather poor activity (1 %) and higher selectivity to propene (52 %). It was hypothesized that a peculiar V–O–Nb functionality existed in the 4.5:1 Nb/V sample, more reducible than the V–O–Nb functionality in the NbVO5 phase, and less reducible than the V– O–V one.

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Besides the binary mixed oxides, the ternary systems containing Nb, V, Mo and Sb (Mo/Nb/Sb, Mo/Nb/V and Nb/Sb/V) have been tested as catalysts for the selective oxidation of propane [56]. All the materials synthesized have shown a very similar catalytic activity to that obtained for binary systems. From among the ternary systems, the Mo/Nb/V 3:1:1 catalyst exhibited higher activity than Mo/Nb/Sb 3:1:1 (~50 % and 30 % at 673-773 K, respectively). However, the catalysts based on Mo, Nb and Sb oxides showed the best selectivity to propene at lower temperatures and their selectivity increased with increasing Sb content. For these ternary systems the oxygenated products were also observed, but the highest selectivity values did not exceed 6 %; the products observed in higher amounts were acetic acid and acrylic acid at lower temperatures, while acrolein was the principal oxygenated product at higher temperatures, probably due to the higher consumption of oxygen in combustion reactions. The Mo/Nb/V 3:1:1 catalyst showed the highest selectivity to propene at the highest temperature (823 K) and to acrylic acid at the lowest temperature (623 K). The crystalline phase which probably favors the formation of acrylic acid is the mixed oxide (Nb0.09Mo0.91)O2.80. It has also been found [54] that -Sb2O4, physically blended with the Nb-V oxide materials, could act as an appropriate promoter improving the catalytic performances of these systems. The addition of -Sb2O4 resulted in a general improvement in the conversion and selectivity to propene and the propene yield. For the V:Nb 1:1 catalyst the propene yield, at about the same conversion, showed a threefold increase when mixed with -Sb2O4 (2.6 instead of 0.9 % at 773 K). The possible explanation of the synergetic effect between V-Nb catalyst and -Sb2O4 is the formation of new more active oxide phases (V-Sb, Nb-Sb or VNb-Sb), or a co-operation between separate phases as it has been proposed previously [57, 58]. Very recently, H. Wan et al. [42] studied nanosized CeNbNiO catalysts prepared by a sol–gel method in oxidative dehydrogenation of propane to propene. The catalytic results demonstrated that for the doped NiO samples Ce and Nb largely improve the catalytic activity and propene selectivity, respectively. It was observed that in comparison with pure NiO, CeNbNiO gave a higher propane conversion (increase from 6 to 21 %) and higher propene selectivity (from 20 to 48 %) at a relatively low temperature (523 K) (Table 3). These results clearly indicate a synergetic effect between the dopants and Ni species in the catalysts studied. The strong interaction of Ce/Nb with Ni oxide species, confirmed by TPR technique, may be indicative of the formation of Ce–O–Ni and Nb–O–Ni linkage that diminish the reducibility of the doped NiO. Moreover, it was found by O2-TPD, that Nb doping leads to a significant decrease in the nonstoichiometric oxygen present on the NiO surface mainly in the form of O2- and O- electrophilic species. It can be one of the reasons why NbNiO and CeNbNiO catalysts show high propene selectivity. However, the electrophilic oxygen species in NiO have also been proved to be active in ODH of propane at low temperature. This conclusion was based on the very low activity of NbNiO at 523 K due to very small amount and low reducibility of nonstoichiometric oxygen in the sample. In CeNbNiO, although the nonstoichiometric oxygen is reduced to some extent, the reducibility of bulk nonstoichiometric oxygen is increased due to Ce doping. It indicates that Ce is a key element that improves the reducibility of bulk nonstoichiometric oxygen in the sample and, consequently, enhances the catalyst activity for propane conversion at low temperature. Thus,

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it is most likely that the highly reactive electrophilic oxygen species in CeNbNiO sample are responsible for the high activity in the low-temperature ODH reaction. Table 3. The catalytic activity of CeNbNiO mixed oxide in ODH of propane at 523 K [42]

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Catalyst

Conv. C3H8 (%)

Selectivity (%) C3H6

C2H4

COx

NiO

6.4

20.3

0

79.7

1.5CeNiO

18.3

30.7

0.3

68.9

3NbNiO

2.8

48.4

0

51.6

1.5Ce3NbNiO

21.4

48.5

0.4

51.1

It has been claimed that also the acid-base character of Ce/Nb-doped NiO catalysts plays a role in determining the selectivity of the oxidative dehydrogenation of propane. NH3-TPD experiments showed that especially Nb introduction generates new weaker acid sites on the surface of the NbNiO and CeNbNiO catalysts, while the number of stronger acid sites decreases. The formation of weak acid sites is probably correlated with generation of Nb(Ce)–O–Ni interacted species. The doped Nb5+ and/or Ce4+ fill the cationic vacancies and reduce the amount of electrophilic oxygen species, which then decreases the surface acidity of the catalysts. The higher the strength and the number of the acid sites on the catalyst surface, the lower the selectivity to olefin, which can be justified by the strong retention of the olefin intermediate on these acid sites and its deep oxidation to CO and CO2. Taking the above into account, the significant decrease of the strength of surface acidity caused by Nb introduction was assumed to be one of reasons for the higher propene selectivity of Nb-containing samples. In addition to being used as a promoter, niobium oxide (Nb2O5) has also been successfully used as a support for preparing supported metal oxide catalysts [32, 33, 59-61]. The studies of Smits et al. [32] indicated that niobium oxide is a selective catalyst for the oxidative dehydrogenation of propane (but with very low activity). The activity of this material is improved considerably by addition of molybdenum, chromium and especially vanadium, while the selectivity to propene is maintained at the same high level. The addition of only 1 mol% of V2O5 is sufficient to get an active and selective catalyst (77 % propene selectivity). The results obtained indicated that the active site is a vanadium ion at the surface, and that the activity and selectivity of this active site depend on the number of neighboring vanadium and niobium ions. Neighboring vanadium ions provide additional activity, while neighboring niobium ions improve the selectivity. The optimal activity and selectivity are given by a site having both vanadium and niobium as neighbors (V-O-V*-O-Nb). When the density of vanadium atoms at the surface is too low, the chance of one vanadium site having a vanadium ion as a neighbor is small and so the catalyst will be selective but not very active.

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The application of niobium oxide (Nb2O5) as a support for preparing surface metal oxide (Cr2O3) species active for ODH of propane was the focus of the study by Deo et al. [59]. The Cr2O3/Nb2O5 samples were prepared by the incipient wetness impregnation with several Cr loadings (1-10 wt. %). ODH of propane was carried out over the Cr2O3/Nb2O5 samples at 673, 683, 703 and 723 K. It was found that with increasing Cr loading the conversion initially increases and after reaching a maximum for the 3% Cr2O3/Nb2O5 sample the conversion decreases for higher chromium oxide loading at all temperatures (Table 4 – for 673 K). Similar behavior was found by Zhao and Wachs [60] for the V2O5/Nb2O5 and MoO3/Nb2O5 catalysts. Table 4. The catalytic activity of x% CrNb catalysts in ODH of propane at 673 K [59]

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Catalyst

Activity × 106 (mol/g cat. s)

C3H6 Selectivity (%)

TOF (s-1)

1% CrNb

0.41

85

3.1

2% CrNb

1.14

85

4.4

3% CrNb

1.6

86

4.1

4% CrNb

1.25

82

2.3

7% CrNb

0.83

85

0.9

The characterization techniques of Cr2O3 samples [59] including Raman, EPR and XPS revealed the presence of molecularly dispersed surface chromium oxide (+6) species below monolayer coverage, which was achieved for the 3 wt.% CrNb sample. Above the monolayer loadings (chromium oxide loadings above 3 %) Cr2O3 crystals are formed, known as less active than the surface chromium oxide species. In addition to the surface chromium oxide (6+) species, which dominate on the Nb2O5 support, Cr5+ and dispersed Cr3+ species were also detected. Comparison of the TOF values in ODH reaction reveals that TOF is relatively constant for the surface chromium oxide species (1–3 % CrNb samples – Table 4) i.e. up to monolayer coverage is obtained. Similar observations were reported earlier for the monolayer V2O5/Nb2O5 catalysts [61]. It suggests that monomeric or polymeric chromium oxide species are equally active for ODH reaction. Comparison of the TOF values of the surface chromium oxide species on the different oxide supports shows that the values decrease in the sequence CrNb ~ CrTi ~ CrSi > CrAl > CrSiAl [62]. Thus, the efficiency of the surface chromium oxide species on Nb2O5 in converting propane is comparable to that obtained on the best support, TiO2. On the other hand, the propene selectivity was not significantly affected by chromium oxide loading. A relatively constant propene selectivity of 82–88 % was observed for all the CrNb samples. The relatively high and constant propene selectivities suggest that the Nb2O5 support does not degrade propene, in contrast to what was shown for Cr2O3/Al2O3 and Cr2O3/TiO2 catalysts where the increase in selectivity is due to decrease in the exposed Al2O3 and TiO2 surface area which degrades propene [62].

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The effect of additives, tungsten (4 wt.%) and molybdenum (2 wt.%) oxides, on 1% Cr2O3/Nb2O5 catalyst was also studied. Whereas for the tungsten oxide modified sample, the activity and propene yield decreased with the addition of tungsten, with addition of molybdenum oxide, the catalyst activity and propene selectivity and yield increased. It was shown that the addition of surface tungsten oxide modifies the acid-base properties of the catalyst decreasing the Lewis acidity, and, consequently, decreases the adsorption of propane, which is reflected by a decrease in activity. Earlier results have suggested that the Lewis acidity plays an important role in the ODH activity [63]. Addition of surface molybdenum oxide results in an increase in activity, which is primarily due to the redox activity of the surface molybdenum oxide species. The catalysts effective in the oxidative dehydrogenation of propane to propene are complex materials in which the chemical composition and the catalyst structure must be precisely designed. The results presented by Wachs [64] indicate that the activity in propane ODH is sensitive to the metal oxide and the oxidation state. The redox activity of supported Nb, Te, V and Mo oxides and mixed metal oxides decreases in the following sequence [15, 64]: V5+ > Mo6+ >> Nb5+, Te4+. From among these metals, Nb5+ demonstrates more Lewis acid character, whereas the other cations demonstrate redox behavior. For the mixed metal oxides of the following composition Mo1.0 V3.0 Te0.16 Nb0.12 Ox , used for the conversion of propane, Nb5+ and Te4+ are supposed to act as ligands promoting the activity at the V5+ and Mo6+ sites. The role of niobium in the catalysts used for oxidative dehydrogenation of propane has to be considered individually for each type of catalysts because it is strongly influenced by the surrounding of niobium species.

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3.2 Gas Phase Ammoxidation of Propane 3.2.1 General Information about the Catalytic Process Ammoxidation refers to the interaction of ammonia with reducible organic compounds (e.g. alkene or alkane) in the presence of oxygen or another oxidant and a suitable catalyst [65]. The catalysts are solids, most commonly mixed metal oxides containing variablevalence elements. They are readily reduced by ammonia and hydrocarbons and readily reoxidized by oxygen or other oxidants. Actually, the catalysts are solid reactants continuously reduced and reoxidized and therefore they are called the redox catalysts. The main application of catalytic ammoxidation reaction is the production of acrylonitrile. Nowadays acrylonitrile is produced by ammoxidation of propene using promoted Fe-Bi-O (BP America) or promoted Fe-Sb-O (Nitto) catalysts [66]. There has been significant effort towards direct conversion of propane into acrylonitrile by reaction with ammonia and oxygen as an alternative to the conventional propene ammoxidation, since propene is more expensive than propane. However, the reaction conditions to activate the C-H bond in propane are more energy demanding, requiring higher temperatures of the reaction, which has a negative effect on selectivity [67, 68] as it was described in subsection 3.1. Therefore, the catalysts used in ammoxidation of propene cannot be simply adapted to the reaction with propane. In ammoxidation of propane acrylonitrile can be formed directly from propane or with formation of propene as the intermediate (Scheme 5 [69 - 71]). If this latter reaction route takes place the catalyst must be active in oxidative dehydrogenation of propane (ODH catalysts described in the previous section), but the propene formed cannot be easily desorbed to the gas phase

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but has to be held at the active center of the catalyst to be able to react with ammonia towards acrylonitrile. If the catalyst contains centers able to dissociate the C-C bond, acetonitrile can be formed directly from propane or from propene with COx, C2 and HCN as the side reaction products.

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Scheme 5. Reaction network for ammoxidation of propane (ACN-acrylonitrile, AcCN-acetonitrile) [according to ref. 69-71].

Two main classes of catalysts have been proposed to be active and selective in propane ammoxidation; i) vanadium antimonate with rutile structure [69, 70, 72-74] and ii) molybdates with sheelite structure [69, 75-80]. Recently the third class of catalysts has been proposed, V/Al mixed oxynitrides [81, 82]. In all of them vanadium is incorporated as the key element. The family of antimonates is represented by VSbxMy, where M stands for many different elements, most frequently for W, Te, Nb, Sn, Bi, Al and Ti [71, 73, 74]. Sb-V-O catalysts with an excess of vanadium are highly active and selective in propane oxidative dehydrogenation to propene, while the excess of antimony makes the catalysts more efficient in propane ammoxidation. According to Grasselli [71] propane is activated by V-O* species on the catalyst surface by the abstraction of hydrogen from the methylene group like in ODH process (the process described detailed in 3.1.). The desorbed propene is then adsorbed on the adjacent Sb3+-O-Sb5+ surface species, in which the Sb3+-O site is responsible for abstraction of hydrogen from propene and the Sb5+=NH site for the nitrogen insertion into the chemisorbed allylic surface complex. The latter, after rearrangement and additional hydrogen loss, is desorbed as acrylonitrile. The reoxidation of the catalysts which occurs on V4+ sites involves the lattice oxygen (O2-) migration from the reoxidation site to the active site (typical Mars van Krevelen mechanism mentioned in 3.1.). This short description of the reaction path shows that the proper composition of the catalysts content and structure is a key point of the effective catalyst. Antimonates and doped antimonates can act as catalysts in their bulk form or can be supported on various porous solids. The nature of support modifies the catalyst properties.

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Alumina supported catalysts have been intensively studied by Guero-Perez et al. [83-87] who found that alumina support is covered by antimony oxide, all vanadium species are located as lattice species in the rutile vanadium-antimonates or as surface vanadium species on antimony. By the choice of the support one can modify the activity and selectivity of Sb-V-O phases. Recently, ordered mesoporous molecular sieves of MCM-41 and MCM-48 types with various chemical compositions were applied as supports of vanadium-antimony phases [88]. The molybdate family of ammoxidation catalysts has been also intensively studied [69, 71, 75-80]. It is represented by VMoxMyOz where M is most often Bi or Te. These catalysts are also multiphase in nature. Among molybdates, one of the highest acrylonitrile yields are claimed to have been obtained for catalysts of the following chemical composition: V0.3Te0.23Nb0.12MoOx supported on SiO2 [89]. In molybdate family catalysts also V-O* moiety of VMoxOy phase is responsible for the propane activation and propene is the primary intermediate product. Propene reacts further on the co-phase such as TeMoxOy or BiMoxOy towards acrylonitrile [90, 91]. In both families of ammoxidation catalysts, antimonate and molybdate, niobium additive is frequently used for the improvement of catalytic performance. The role of niobium in these catalysts will be shown in the next subsection (3.2.2.).

3.2.2 Role of Niobium in Ammoxidation of Propane In ammoxidation of propane niobium species play mainly a role of promoter, often it is a key promoter in the multicomponents antimonate or molybdate catalysts. Recently, its promoting effect has been comprehensively described by Guerro-Perez and Banares [48]. Considering antimonate family of catalysts the multifunctional role of vanadium ions in the mechanism of synthesis of acrylonitrile from propane on V-Sb-oxides should be stressed. It allows understanding of the role of doped niobium species. Vanadium ions in rutile VSbO4 act as active sites for the oxidative dehydrogenation of propane to propene and catalyze the reoxidation of Sb-sites reduced in the propene to acrylonitrile conversion [69]. The active sites for reoxidation are supra-surface Sb-oxides on vanadium antimonate, but the nonstoichiometric characteristics of vanadium antimonate and thus the state of vanadium ions influence the rate of reoxidation. The third effect of vanadium is associated with the influence on the nature and amount of ammonia adspecies. If the catalysts contain the V2O5 phase, this phase catalyses oxidation of ammonia to nitrogen, thus decreasing the ammoxidation activity. Therefore, the supra-surface V5+-oxide should be limited and such a convenient situation can be achieved by the use of excess of Sb-oxide. It is clear that the formulation of effective antimonate catalysts requires the choice of the specific preparation method and the proper Sb/V ratio. The main aim of niobium admission to these V-Sb-oxides systems is to induce the interactions between Nb and V species and to enable generation of cationic vacancies. The formation of Nb-V-O bonds improves the selectivity to acrylonitrile and enhances the catalyst activity [48, 92, 93] because it prevents the formation of V2O5 and enhances both functions of vanadium ions located in rutile VSbO4 phase, namely their activity in ODH of propane to propene and their catalytic effect on reoxidation of the catalysts supported on antimony species. However, the drawback of Nb doping is the possible formation of SbNbO4 phase, which consumes the antimony species, thus decreasing the selectivity of the reaction. The interplay between vanadium, niobium and antimony is particularly complex and depends considerably on the catalyst preparation procedure. When Nb-Sb-oxides interaction prevails over the V-Sb or V-Nb interactions, the catalyst exhibits low activity and selectivity to

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21

acrylonitrile [48]. The type of interactions is determined not only by the preparation procedure but also by the amounts of each metal oxide species. For Sb-V-Nb-O/Al system, the addition of Nb in the preparation of the catalyst enhances the yield of acrylonitrile when the Sb/V=1 ratio is used, but decreases the acrylonitrile formation when the Sb/V ratio is ca. three [48, 79]. In Cr/V/Sb/Nb mixed oxides low content of Sb is not advantageous because niobium doping results in lowering the catalyst activity, but the effect on activity is positive in samples with higher Sb content [94]. The positive effect is due to the modification of the properties of Sb sites at the surface of rutile crystallites, with generation of species improving catalytic properties in allylic ammoxidation. For these catalysts niobium species also increase the concentration of cationic vacancies which would play an additional role in increasing the selectivity to acrylonitrile. Similar results were obtained also when Nb was added to Sn/V/Sb catalyst [95, 96]. Niobium incorporated into the antimonate rutile structure forms defects (cationic vacancies) whose nature is further changed by the effect of tin oxides. The cationic vacancies in rutile oxides originate from the charge unbalance created by the dissolution of altervalent metal cations like Nb5+ cations replacing of V3+ and V4+. However, when the large amount of niobium is used in the preparation of the catalyst, the Nb2O5 phase is formed and it decreases the selectivity to acrylonitrile. The highest selectivity to acrylonitrile under conditions of total oxygen conversion was obtained with the catalyst of the composition Sn/V/Nb/Sb 1/0.2/1/3/ [97]. Incorporation of Nb5+ into the Sb oxide considerably improves the selectivity to acrylonitrile and the structural stability of the catalyst, limiting generation of the unselective -Sb2O4. Moreover, the development of cationic vacancies in the rutile V/Sb/Nb mixed oxides is the reason for enhancement of activity of Nb-containing catalysts. The use of ordered mesoporous supports containing niobium (NbMCM-41 and NbMCM48) for loading with rutile vanadium antimonate led to promising catalysts for ammoxidation of propane [98]. The effect of niobium in these catalysts seems to be higher than when alumina is used as a support of the Sb–V–O phase [92]. When niobium is introduced during the synthesis of NbMCM-41, the resulting acidity in the support influences the activity and selectivity in ammoxidation processes. In both, Sb–V–Nb–O supported on MCM-48 and Sb– V–O on NbMCM-41 materials, Nb sites are able to interact with V and to stabilize the rutile active phase, increasing the selectivity to nitrile formation (acrylonitrile and acetonitrile). Interestingly, niobium in NbMCM-41 material is selective towards acetonitrile formation by cracking the C-C bond and nitrogen insertion. The selectivity is almost 100 % at a very low propane conversion on pristine NbMCM-41 support. The increase in the conversion causes a decrease in acetonitrile formation and increases the parallel selectivity to propene and acrylonitrile. This feature proves that on NbMCM-41 acetonitrile is formed directly from propane, so the route via propene intermediate is excluded. In general, it has been established that the possible roles of niobium in the catalysts based on vanadium antimonate (rutile structure) can be as follows: i) limitation of formation of undesired -Sb2O4 and V2O5 phases, ii) modification of the Sb properties (towards increased selectivity) through the interaction between vanadium and niobium species in rutile structure, iii) generation of cationic vacancies by replacement of lower state vanadium cations by Nb5+ to improve selectivity, and iv) increase in structural stability of rutile structure. The molibdate based catalysts, usually multicomponent (Mo-V-Te(Sb)-O), can also be doped with niobium (Mo-V-Te(Sb)-Nb-O) and the role of niobium species is slightly different than that in the antimonate family of catalysts. These mixed-oxide catalysts contain the so-called „M1‟ and „M2‟ phases of orthorhombic and pseudohexoganal structures,

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respectively, proposed to be active and selective in ammoxidation of propane to acrylonitrile [48, 98-102]. If niobium is added during the synthesis of the catalysts it is preferentially located in M1 phase and only its small amount is present in M2 phase [103-106]. Incorporation of Nb in the Mo-V-O framework results in separation of active Mo and V components. Niobium dopant additionally stabilizes the crystal structure, under the reaction conditions. The active site separation appeared to be a prerequisite for selective ammoxidation. Nb5+ replaces V4+ like in antimonite catalysts, which leads to formation of VO-Nb and Mo-O-Nb surface bonds giving rise to the advantageous separation of vanadium – molybdenum sites. Moreover, the well defined bulk M1 phase can act as a support of NbOx species, modulating the properties of the surface by niobium-support interaction. Such interaction generates new properties of the catalysts moderating the adsorption of propene, the reaction intermediate, and the products preventing over-oxidation of acrylonitrile. The advantageous role of niobium species in both families of catalysts applied in ammoxidation of propane, that is multifunctional materials based on antimonates and molibdates is summarized in Table 5. This table does not include the possible drawbacks of niobium admission resulting from the undesired interaction of niobium with antimony or the formation of Nb2O5 phase. Table 5. Advantageous effects of niobium in the catalysts applied in ammoxidation of propane Catalytic system

Forms of niobium interaction and their advantageous role

References

in propane ammoxidation *

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Sb-V-Nb-Ox

Nb-V-O bonding prevents the formation of V2O5, (V2O5 catalyzes oxidation of ammonia to nitrogen); enhances activity of vanadium species in rutile antimonite structure and selectivity of Sb species Cr-V-Sb-Nb-Ox Nb dopant modifies Sb properties increasing ACN selectivity Nb5+ creates cationic vacancies by replacing of V3+ and V4+; this increases ACN selectivity Sn-V-Sb-Nb-Ox Nb dopant creates cationic vacancies increasing ACN selectivity Nb located in the rutile structure increases structural stability and limits generation of unselective –Sb2O4 SbVOx/NbMCM-41 Nb interaction with V stabilizes vanadium antimonate SbVOx/NbMCM-48 rutile phase increasing ACN formation Nb creates acidity in NbMCM supports which enhances selectivity to AcCN Mo-V-Te(Sb)-Nb-Ox V-O-Nb and Mo-O-Nb bonds leads to separation of Mo and V active components increasing ACN selectivity Nb located in M1 phase prevents over oxidation of ACN * ACN - acrylonitrile; AcCN -acetonitrile

3.3. Gas Phase Oxidation of Methanol

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48, 92, 93

94

95-97

98

103-106

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23

3.3.1. General Information about the Catalytic Process Oxidative dehydrogenation of methanol leads mainly to formation of formaldehyde, a desirable intermediate in synthesis of different organic compounds. The first commercial production began in the end of the 19th century in Germany using Cu catalyst, which was later replaced by Ag catalyst [107]. The process was carried out at high methanol concentration of above 40 % at a temperature from the range 823-1023 K. In the middle of the 20th century a procedure was developed in which a lower methanol concentration as well as lower temperature could be applied. This effect was achieved with the iron molybdate catalyst. Nowadays, both catalytic routes leading to obtaining formaldehyde are used. The formation of formaldehyde (and of course also side-products) proceeds according to different mechanisms depending on the catalyst type. Although the silver catalyst has been applied for the production of formaldehyde for ca. 100 years, the mechanisms of this process is still under debate [107]. Two possible routes are discussed: 1) dehydrogenation of methanol followed by partial oxidation of hydrogen formed CH3OH  HCHO + H2 H2 + ½ O2  H2O 2) oxidative dehydrogenation of methanol

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2 CH3OH + ½ O2  2 HCHO + H2O + H2. As proposed by Wachs and Madix [108], on the basis of the single-crystal studies, the first step of the mechanism involves the dissociative adsorption of dioxygen on silver particles. Such oxygen has nucleophilic character thus it allows abstraction of hydrogen from methanol to form methoxy surface species. It was also found that methanol was not adsorbed on vacant Ag sites. In the next step, the dehydrogenation of adsorbed methoxy species to formaldehyde and hydrogen atoms occurred. Then the hydrogen atoms can form dihydrogen species, which was observed in the reaction products. Although the above described formation of formaldehyde involves the dehydrogenation step, the overall mechanism included the oxidative dehydrogenation process. The scheme of this mechanism can be illustrated as follows (Scheme 6): O2(g) + 2 *  2 O-* CH3OH(g) + O-* + *  CH3O-* + OH-* CH3OH(g) + OH-*  CH3O-* + H2O(g) 2 CH3O-*  2 CH2O(g) + 2 H-* 2 H-*  H2(g) + 2 *

where „*‟ is a site on Ag surface

Scheme 6.

The process over iron molybdate as a catalyst proceeds via a different mechanism than that described above for Ag catalysts. In general it can be treated as a mechanism

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characteristic of metal oxide catalysts, known from literature devoted to the application of different transition metal oxides as catalysts [107, 109-116]. As summarized in ref. [2], the catalytic oxidation of methanol requires the formation of chemisorbed methoxy groups, which takes place on nucleophilic centers such as Lewis or Brønsted acid sites with the formation of water or proton chemisorbed on the catalyst surface as shown in Figure 4. The methoxy species are further transformed to formaldehyde species as a result of hydrogen extraction from activated oxygen or at a high concentration of methoxy species, they react with another methanol molecule and form dimethyl ether and water. In fact the oxidation of methanol proceeds by different consecutive and side reaction routes well illustrated by the scheme proposed by Tatibouet [109] (Scheme 7).

+ CH3OH

CH3OH

CH3OCH3 + H2O

½ O2 + CH3OH

CH2O

(CH3O)2CH2 + H2O - CH3OH

½ O2

+ CH3OH

(CHOOH)

HCOOCH3 + H2O

½ O2

CO + H2O

CO2 + H2O

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Scheme 7 [according to ref. 109].

CH3 OH

H2O

O

CH3OH

M

M

CH3 O M+ M+

CH3OH OM+

O

O M+ M+

H OM+

Figure 4. Formation of methoxy species on Brønsted and Lewis acid sites.

The adsorbed formaldehyde can either desorb or if it is chemisorbed on nucleophilic species strongly enough, it can interact with the surface oxygen to form adsorbed Niobium: Properties, Production and Applications : Properties, Production and Applications, Nova Science Publishers, Incorporated, 2011. ProQuest

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dioxymethylene, which in turn can desorb as dimethoxymethane. Adsorbed formate species are formed by hydration of dioxymethylene and it can interact with water to produce formic acid or interact with another methanol molecule to make methyl formate. Formate species can also decompose to CO2 and finally, the total oxidation of methanol to carbon dioxide occurs. The observed reaction products depend on the possibility of formation of the intermediates desired and the strength of their chemisorption. The types of intermediates and strength of chemisorption are determined by the surface properties of the catalysts. The total oxidation of methanol to carbon dioxide can proceed not only by the consecutive reactions mentioned above but also in the direct oxidation of methanol involving electrophilic oxygen species (radical species) [115] or by the readsorption of HCHO and its secondary reaction as proposed by Wachs and Kim [114]. In terms of the Haber‟s classification [24, 115] of the oxidation reactions involving nucleophilic or electrophilic oxygen, the formation of alkoxy species followed by hydrogen abstraction needs nucleophilic oxygen species, whereas electrophilic (radical) oxygen species give rise to unselective total oxidation of methanol. As follows from kinetic studies, the rate determining step of the selective oxidation of methanol is the abstraction of hydrogen from methyl group of methanol adsorbed. It is determined by the surface properties. Methoxy species formed in the first step can be adsorbed on oxygen vacancies in metal oxides (Lewis acid centers). The methoxy species adsorbed on a terminal M=O vacancy reacts towards formaldehyde by a transfer of a methyl H atom to the neighbouring M=O bond [107, 117, 118]. The methoxy species adsorbed on bridged oxygen (M-O-M) vacancy sites are likely to form dimethyl ether, dimethoxy metane, and methyl formate, which requires the presence of additional Lewis acid sites [107]. Thus, most authors working on the selective oxidation of methanol considered the significant role of acidity and basic oxygen species in the metal oxide catalysts. Interestingly, Wachs and Kim [114] studying CH3OH oxidation over V2O5/Al2O3 have found that the CH3OH oxidative dehydrogenation kinetic parameters are independent of the surface bridging V-O-V concentration, surface V=O bond length/strength, vanadium surface density on the support, surface acidity, and surface vanadia reduction. Surface VO4 species were the catalytically active sites. Such species can be easily generated when the transition metal is located in the zeolite structure or in the ordered mesoporous silica. Considering the role of niobium in the catalysts applied in oxidation of methanol (next subsection 3.3.2.) one should refer to the mechanism of the reaction proposed for metal oxides catalysts because niobium is not present in metallic form on the catalyst surface.

3.3.2. Role of Niobium in Methanol Oxidation Process Catalytic activity of niobium species in methanol oxidation process is strongly affected by the type of niobium species present in the catalyst. One should consider at least three different types: i) niobium species in niobia used as catalyst or as support of other metals, ii) niobium active centers diluted on another oxide support or iii) niobium as promoter for other metals loaded on oxide support. Niobium(V) oxide shows Lewis acidity [12], therefore it should be active in methanol dehydration process, which leads to the formation of dimethyl ether. Indeed, this product was dominant in methanol oxidation process performed using niobia catalyst [13, 119]. However, in spite of the high activity in dehydration process, niobia shows also redox behavior as proved by the formation of some amount of formaldehyde (5 % of formaldehyde selectivity). The redox performance of niobia was also documented in [2]. Nevertheless, the catalyst

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applied in the reaction carried out at 523 and 573 K was not selective, leading at lower temperatures mainly to the formation of CO2, i.e. product of total oxidation of methanol. The increase in the reaction temperature caused the preference of dehydration of methanol towards dimethyl ether. It is in line with the observation of Wachs et al. [5, 6] indicating that amorphous niobia possesses slightly distorted NbO6 and NbO7 and NbO8 groups which only contain Nb-O bonds associated with Brønsted acid sites. High concentration of acid sites gives rise to a high concentration of methoxy species which react with another methanol molecule and form dimethyl ether. To obtain a catalyst having stronger redox properties niobia can be applied as a support for other metals. Incorporation of the active phases on niobium(V) oxide is usually realized by the interaction of surface hydroxyls of the support with the metal precursor, most often salt or oxide. The participation of acid groups (hydroxyls) of niobia in formation of metal-support bonds changes the matrix properties. Moreover, the supported species also affect the catalytic performance of the catalyst obtained. As should be expected, the catalyst obtained by the incorporation of metal precursor onto niobia exhibits lower activity in methanol dehydration process than pristine niobium(V) oxide [13, 119]. The introduction of 1 % of CrO3, Re2O7, MoO3 and V2O5 onto Nb2O5 leads to a decrease in the dimethyl ether selectivity, meanwhile the other products of methanol oxidation appear (dimethoxymethane, CO and CO2). Although, all the supported metal species affect the catalytic behavior of niobia, the activity in methanol oxidation and the selectivity towards formaldehyde strongly depend on the nature of metal supported on the niobia matrix. From among the catalysts mentioned, the most active and selective towards formation of formaldehyde is vanadia supported on Nb2O5 [119]. Moreover, it should be mentioned that both, Nb2O5 and V2O5 used separately as catalysts show much lower activity in methanol oxidation process than vanadia supported on niobia. This fact could be explained by the metal-support interaction. The high reactivity of this catalytic system was associated with the ease of reducing the bridging V-O-Nb bond [10, 13]. Niobia was also applied as a support for gold species [2], which are known as very active centers for the oxidation processes [125-127]. When niobium(V) oxide is applied as a support for gold species, the activity and selectivity in the oxidation of methanol strongly depend on the nature of the oxide, i.e. whether it is amorphous or crystalline. Introduction of metallic gold species onto amorphous Nb2O5 significantly increases the activity of the support but reduces the selectivity activating mainly the total oxidation of methanol. Gold supported on crystalline Nb2O5 appeared to be a selective oxidation catalyst although its activity is much lower. The influence of crystalline niobium phase on the activity and selectivity in methanol oxidation process was also postulated by Wachs et al. [13] for Bi-Nb-O catalyst. The catalytic test reactions were performed after calcination of Bi-Nb-O samples at different temperatures. It was demonstrated that the higher the temperature of catalyst calcination, the higher the selectivity to formaldehyde was observed. The change in selectivity towards formaldehyde was attributed to the formation of crystalline niobia-bismuth mixed oxide phase. Niobium species in niobium(V) oxide form polyhedral metal links having hydroxyls, responsible for the acidity of the niobium oxide surface. As discussed above, the presence of hydroxyls has a dominant role in the methanol dehydration process leading to the formation of dimethyl ether. The dehydration activity can be reduced by the interaction of the niobia – OH groups with the metal source. However, in the catalysts supported on Nb2O5 the niobium polyhedral species are still present on the surface and they are accessible for reagents as

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shown in Figure 3a. Therefore, another idea to improve the niobia performance in the oxidation of methanol is to disperse Nb-Ox species on/in another solid. Dispersed niobium species can be obtained by the Nb dilution in another metal support. For this purpose different supports have been tested [5, 8, 13, 123]. It was found that such metal oxides like Al2O3, TiO2 or ZrO2 allow formation of niobium coverage similar to a monolayer on the support surface, which results in the formation of niobia polyhedral species. These catalysts were examined in methanol oxidation process. The obtained results show that the catalysts possess mainly the acidic character demonstrated by the high selectivity to dimethyl ether. The formation of niobia polyhedral species on the oxide supports mentioned was explained by a high number and reactivity of surface hydroxyls of the support, which made the high concentration of surface niobium species possible. Those Nb species can later recombine to form a surface niobia phase. Therefore, to obtain higher dispersion of niobium species a support (not metal oxide) having different surface properties should be applied. For this purpose different silicas (amorphous or mesoporous ordered ones) were applied [5, 8, 13, 113, 124-127]. The incorporation of small amount of niobium species into amorphous silica (1 wt%) in order to isolate Nb species, leads to the formation of mainly NbO4 isolated species in dehydrated sample as evidenced by Raman spectroscopy by the presence of a band at 982 cm-1 [113]. However, the increase in the number of niobium species up to 10 wt. % causes the appearance of a Raman band at 673 cm-1, which is assigned to bulk Nb2O5. The presence of different types of niobium species on SiO2 surface, depending on niobium concentration, affects the catalytic performance of these catalysts in methanol oxidation process. Although, for all samples the oxidation products are dominant, the selectivity changes with increasing niobium content. For 1 wt. % of Nb the main product is methyl formate (67 % of selectivity). The growth of niobium content diminishes the selectivity to methyl formate, meanwhile the yield of formaldehyde is increasing because the strength of formaldehyde adsorption decreases with increasing Nb content which allows the desorption of formaldehyde. Moreover, for lower content of niobium, dimethyl ether is not observed as a reaction product, contrary to the case of higher Nb loadings for which the selectivity to dimethyl ether is enhanced. These results are in agreement with the presence of polyhedral niobia species estimated for higher Nb loadings on SiO2. The mesoporous silicas give a unique opportunity to achieve a high isolation of niobium species due to its high surface area (c.a. 1000 m2g-1), especially when niobium is incorporated during the hydrothermal synthesis of mesoporous solid. Such a procedure could also lead to the incorporation of niobium species into the material walls. This possibility was broadly discussed by Ziolek and co-workers in ref. [2, 22, 35 - 37, 46, 47, 125, 126, 128, 130, 140, 143]. Different mesoporous silicas containing niobium were examined in methanol oxidation: NbMCM-41 [113, 124, 127], NbSBA-3 [2], NbSBA-15 [125] or NbMCF [126]. For all these samples, especially with low Nb content, niobium was estimated mainly as isolated NbO4 species located in the mesoporous walls. This has an impact on high selectivity towards oxidation products and the lack or very low selectivity to dimethyl ether observed in methanol oxidation process. However, the procedure applied for the preparation of niobium containing mesoporous silicas has an impact on catalytic properties of the materials obtained. One of the important factors is the pH of the medium applied in the preparation route. As described in subsection 3.1.2., the use of acidic pH in the synthesis of SBA-3 type materials allows the introduction of Nb preferentially into the skeleton of silica [37] contrary to the MCM-41 samples prepared

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under basic conditions with a higher Nb content, where part of Nb species are located in the extra framework positions as niobium oxides [35]. The location of niobium in the framework or extra framework positions has a significant impact on the activity in methanol oxidation process. A considerable increase in the number of Nb species in the skeleton of mesoporous material was possible to obtain in SBA-15 applying a special preparation procedure in which niobium(V) pentachloride was the source of Nb and no hydrochloric acid was added [125]. The Si/Nb atomic ratios of 15, 7.5 and 5 were obtained and all niobium atoms were located in the skeleton of the mesoporous material. For all these samples almost the same methanol conversion (28-29 % at 523 K) and selectivity to formaldehyde (80-89 % at 523 K) was observed. Both these values are higher than those noted for SBA-3-128 containing much lower amount of Nb species (Si/Nb=128) [2]. The results observed for NbSBA-15 indicate that not all niobium species are available for the reagent on the surface of mesoporous walls because part of them is hidden inside the amorphous walls. Therefore, methanol conversion for all NbSBA-15 used was almost the same, irrespective of Nb content. Another factor that has to be considered in the preparation of niobium containing catalysts active in the oxidation of methanol is the source of niobium [2]. Two different niobium sources were applied for the preparation of NbSBA-3 material – niobium(V) pentachloride (denoted as „Cl‟) and ammonium niobate(V) oxalate hydrate (denoted as „Co‟). Both samples have the same Si/Nb ratio of 128. The XPS measurements showed that the binding energy (BE) of niobium in ordered mesoporous NbSBA-3-128 is much higher than that of bulk Nb2O5 (207.1 eV) indicating the incorporation of niobium atoms into the mesoporous silica skeleton. As mentioned before, the inclusion of niobium into the silica skeleton allows the generation of NbO4 species, whereas location of niobium in the extra framework positions gives rise to the formation of Nb2O5 species. A little higher BE observed for NbSBA-3-128 (Cl) (208.47 eV) than that of NbSBA-3-128 (Co) (208.23 eV) suggests different surroundings of niobium species in the two samples. Most probably chloride ions are located in the neighborhood of niobium species in NbSBA-3-128 (Cl). Such neighborhood causes a decrease in the number of Lewis acid centers (Nb+), which was evidenced from the pyridine adsorption/desorption study. This difference was found to be responsible for lower methanol conversion observed for NbSBA-3-128(Cl) (2 % vs. 10 % of conversion at 523 K) resulting in a higher selectivity to formaldehyde (26 % vs. 15 % of selectivity at 523 K) caused by the easier abstraction of hydrogen from methoxy species by chloride ions on the surface. As mentioned at the beginning of this chapter, niobium species can be introduced on the oxide support together with other active species. If so, niobium species often act as promoting agents for selective oxidation reactions. It has been recently described in detail by O. Guerrero-Perez and M. Banares [48] in relation to the oxide systems. For vanadia systems supported on titania [11] the introduction of Nb species did not change the structure of the surface vanadia species, as evidenced by Raman spectroscopy. Moreover, the addition of Nb species also did not impact on the redox properties of vanadia supported on TiO2 in methanol oxidation process. However, a huge impact of niobium species on the catalytic behavior of mesoporous silicas supported with gold species was observed [2]. Gold impregnated samples contain metallic gold species [129]. Metallic gold determines the catalytic behavior and influences the oxidative properties of mesoporous supports containing niobium. The results of methanol oxidation at 523 K clearly indicate that the introduction of Au by impregnation enhances the activity of MCM-41 and NbMCM-41-128 (Si/Nb=128) matrices. This effect is

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much higher when Nb is located in the silicate MCM-41, because the interaction between gold precursor (AuCl(OH)3- at pH=7) and Nb in the NbMCM-41-128 stabilizes gold species near positively charged niobium in the skeleton of the mesoporous material. Therefore, this Nb species act as a structural promoter of metallic gold. Important is a significant decrease in the total oxidation towards CO2 after Au loading by impregnation, much higher when niobium is present in the support (from 35 % on MCM-41 to 29 % on Au/MCM-41 and from 46 % on NbMCM-41-128 to 8 % on Au/NbMCM-41-128). Again, gold located near Nb species significantly lowers the catalyst activity towards total oxidation and moreover, shifts the oxidation towards formation of methyl formate. Such a change in selectivity is possible when formaldehyde species is stronger held on the catalyst surface and can interact with another molecule of methanol. Interestingly, the opposite effect was observed for the samples prepared by co-precipitation of gold and niobium species in one-pot synthesis of MCM-41 [127]. The bimetallic system enhanced the total oxidation of methanol (from 20 % of CO2 selectivity for AuMCM-41 to 44 % of CO2 selectivity of AuNbMCM-41-128). As shown above, the activity and selectivity in methanol oxidation process with the use of niobium containing catalysts strongly depend on the type and concentration of niobium species. Both are determined by many factors, among them the nature of the support, catalyst preparation technique and niobium source seem to be of great importance.

3.4. Liquid Phase Epoxidation of Olefins

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3.4.1. General Information about the Catalytic Process Epoxides are versatile intermediates in many organic syntheses especially in the production of fine chemicals and pharmaceuticals [130]. The most general reagents for conversion of alkenes to epoxides in homogeneous conditions are peroxycarboxilic acids or hypochlorous acid and the reaction proceeds according to the equations shown in Scheme 8.

+

R

R R’CO3H +

O

R R’CO2H R +

HOCl O R

OH Cl

Scheme 8.

For the environmental reasons traditional methods are welcome to be replaced by the processes using „greener‟ oxidants like tert-butyl hydroperoxide or in particular hydrogen proxide. The latter decomposes to water and therefore it is considered an environmentally friendly oxidant. Moreover, there is a tendency to replace homogeneous systems by heterogeneous ones. There is rising demand for solid materials that would catalyze

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epoxidations, therefore heterogeneous epoxidation remains a very active field of research [130, 131]. A breakthrough in catalytic heterogeneous liquid phase oxidation was synthesis of titanosilicate – TS-1 zeolite – made in 1983 by researchers from ENI [132, 133]. The process of selective phenol oxidation (to form catechol and hydroquinone) with the use of TS-1 catalyst and hydrogen peroxide as oxidant, has been already commercialized. The oxidation process using peroxides (e.g. tert-butyl hydroperoxide or hydrogen peroxide) as oxidants is illustrated in Scheme 9.

+

base

R

+

where: R‟ = H, t-Bu Scheme 9.

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The types of oxidation products and the rates of their formation in these processes strongly depend on the reaction mechanism, determined by the type of catalyst and the type of O-O bond cleavage in R‟O-OH [19, 130, 134]. There are two possibilities of the O-O bond cleavage, i.e. homolytic and heterolytic. The first one leads to the formation of R‟O radicals, which initiate the radical pathway of the oxidation process as shown in Scheme 10 [130].

RO2H + Mn-1 RO2H + Mn 2 RO2 RO + RH R + O2 RO2 + RH

     

RO + MnOH RO2 + Mn-1 + H+ 2 RO + O2 R + ROH RO2 R + RO2H

Scheme 10. [according to ref. 130].

The heterolytic cleavage of R‟O-OH initiates two other possible pathways of the reaction which depend on the catalyst used. The reaction pathway can proceed through oxometal or peroxometal active intermediate complexes and the same oxidation products can be formed via both reaction routes. These two mechanisms are illustrated in Scheme 11. As it can be deduced from the Scheme 11, the main difference between both reaction pathways is the change or no change in the metal oxidation state. The formation of peroxometal species does not involve a change in the metal oxidation state in contrast to the oxometal pathway, which proceeds through the reduction of metal species followed by reoxidation of metal by oxygen donor. One of the olefins most frequently examined in liquid phase oxidation is cyclohexene [19, 135-142]. Because of the known mechanism of this reaction it is often used as a test for both redox and acidic centers. Different catalytic systems have been investigated in this reaction, among them: porphyrins containing Mn and Fe [135-137]; phthalocyanines

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containing Fe, Co [138]; Ti, Fe and V incorporated on amorphous silica [139] or mesoporous silicas [19, 140]. The most often oxidants used were: hydrogen peroxide [136, 141, 142] and tert-butyl hydroperoxide [19, 138], however iodosylbenzene [135, 141] or O3 [137] were also used. The most important products of cyclohexene oxidation are presented in Scheme 12 [35].

O C C

OMn

ROOH

peroxometal

C

C

O C

OOMn

ROH

Mn

ROOH

C oxometal

C

C

OMn+2

ROH

Scheme 11. OH

O

OH

OH

OH

OR

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O

OH

O

Scheme 12. [according to ref. 35].

As mentioned at the beginning of this section, microporous titanosilicalite (TS-1) of the MFI zeolite structure is the one of the most effective catalyst which can be applied for liquid phase oxidation processes. However, it has important limitations, e.g. it can be used only for relatively small olefins because of small pore size of the MFI structure. Therefore, much efforts has been made to introduce titanium to more open structures such as ordered mesoporous silica. However, titanium is not stable (easy to reduce) in environment of tetrahedral silica and it is also easily leached to the reaction solution. To overcome these problems attempts have been made to replace Ti by other metals [134], among them niobium appeared to be attractive, because it is difficult to reduce and therefore its stability in the mesoporous amorphous silica is higher. The next subsection deals with the niobium containing catalysts applied in olefin epoxidation.

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3.4.2 Role of Niobium in Epoxidation of Olefins As mentioned in the previous subsection (3.4.1.), to overcome the limitations of titanium containing catalysts and therefore to achieve better performance in catalytic liquid phase epoxidation of olefins, much attention has been directed to niobium containing materials. To obtain suitable catalysts capable to activate epoxidation process of larger olefin molecules, the ordered mesoporous metallosilicates have been investigated in details. Different structures and types of these solids have been applied as hosts of niobium species and they were tested in the olefins epoxidation. For example, niobium was introduced to MCM-41 [143-148], MCM-48 [147], SBA-15 [38, 149], MCF [126], MSU [150], aerogels [151-153]. Niobium(V) bulk oxide was also tested as a catalyst in epoxidation of cyclohexene [2, 154]. The above mentioned catalysts were examined in epoxidation of different types of olefins. The greatest attention has been put on epoxidation of cyclohexene [19, 38, 126, 143, 146, 149, 154, 155], cyclooctene [144, 145, 148, 151-153], geraniol [150, 151, 153], nerol [151], -pinene [147] and trans-2-pentene-1-ol [151]. The distribution of products formed in olefin epoxidation strongly depends on certain factors. The most important are: the size of mesopores (influences diffusion effects), isolation of active metal species, hydrophobicity of catalysts, nature of solvent, reducibility of metal species, leaching of active component from the solid and the number of Lewis acid centers. The influence of these factors on oxidation processes has been examined in details and described in ref. [19]. It was indicated that most of the abovementioned factors depend on the kind of support applied for metal species and on the metal itself. Therefore, the choice of niobium species used as active centers for the epoxidation processes was particularly carefully analyzed. The interaction of niobium species with the support and hydrogen peroxide as one of more explored oxidants was also discussed. The cyclohexene oxidation pathway is presented in subsection 3.4.1 (Scheme 12) according to which to obtain epoxide from olefin only one reaction step is required. Moreover, the catalytic process should be stopped to secure against hydrolysis of the epoxide formed. The hydrolysis of epoxide converts this product into diol. The key factor that should be taken into account in prevention against hydrolysis of epoxide is the acidity of the catalyst surface [19]. It is well known that acidic centers are responsible for the opening of the epoxide ring. Therefore, the samples of acidic surface character are not selective towards epoxide but catalyze the consecutive reaction to diols. Bulk niobium(V) oxide can be classified as such a material, because it has Lewis acid centers on its surface [12]. The epoxidation of cyclohexene using hydrogen peroxide was carried out on different niobia, i.e. amorphous hydrated exhibiting higher acidity and dehydrated crystalline niobium(V) oxide [2, 154]. As expected 1,2-cyclohexenediol was the dominant product. The niobia acidic centers transform the epoxide to diol very quickly, as concluded from in-situ FTIR measurements [154]. Epoxide was not observed as a product of the reaction in these conditions. This phenomenon was explained by very fast hydrolysis that occurred on niobia. Moreover, in addition to the pathways of cyclohexene transformation presented in Scheme 12, a possibility of direct transformation of cyclohexene into 1,2-cyclohexenediol was postulated. The desired performance of niobium species in olefin epoxidation processes can be enhanced by the dilution of Nb species in another oxide, usually silica, i.e. by formation of isolated NbO4 forms. This strategy has at least two advantages. Firstly, isolated niobium species behave more as redox centers. Secondly, the lack of other Nb species (creating Lewis

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acidity) in the neighborhood diminishes the possibility of hydrolysis of epoxide formed. The role of niobium isolation has been demonstrated e.g. in ref. [35]. For the samples with the Nb content in mesoporous silica varied in the range of Si/Nb ratio form 128 to 32 a significant amount of epoxide was produced. Moreover, it was shown that the increase in the amount of Nb species in the sample enhances both the conversion of cyclohexene and selectivity to epoxide. However, at a higher niobium content, depending on the procedure of the synthesis of ordered mesoporous niobiosilicas, part of Nb can be located in the extra framework positions in the form of Nb2O5 or niobiates (see section 3.1.2.) which directs the reaction pathway towards diols formation. Another feature of niobium, which indirectly influences the epoxidation process, is its ability to form defects in the mesoporous silica structure (in particular in NbMCM-41), which was evidenced by TEM with EDX analysis [35]. These defect holes are generated during the hydrothermal synthesis of mesoporous solid. Their presence creates the interconnections between linear channels in MCM-41 structure, which improves interchannel diffusion. Such a catalyst structure enhances its activity in the cyclohexene epoxidation process. It is important to add that the defect holes mentioned were not observed in other mesoporous silicas, e.g. NbSBA-15. The difference between these two solids, NbMCM-41 and NbSBA-15, was explained by the difference in the synthesis conditions; NbMCM-41 sample was prepared in a basic medium, whereas NbSBA-15 was synthesized at a low pH value. For catalytic liquid phase processes, usually it is difficult to distinguish whether the reaction is carried out in the homogeneous or heterogeneous system. The liquid phase oxidation processes using titanium containing mesoporous silicas, especially when hydrogen peroxide is applied as oxidant, suffer from leaching of the active phase into the reaction media. In that context niobium containing catalysts appear as very promising materials. To check for the occurrence of leaching the activities of the reused catalysts are compared. However, it is difficult to avoid the loss of material upon recycling. Another strategy was applied for NbMCM-41 sample [140]. Firstly, the process of cyclohexene epoxidation was carried out in the presence of niobium in a homogeneous system (the content of niobium in the reaction solution was the same as in the NbMCM-41 sample). It was found that this process proceeds with very low activity. Moreover, 1,2-cycohexenediol was detected as the dominant product in contrast to the results obtained for NbMCM-41 (cyclohexene expoxide was the main product). Additionally, to identify the liquid responsible for the leaching of Nb species from NbMCM-41 sample, the catalysts were treated by the solvent and hydrogen peroxide or by the solvent and cyclohexene mixtures for 5 h upon stirring. The latter solution did not show the activity after addition of hydrogen peroxide. Residual activity was only detected for the first solution after adding of cyclohexene. These results compared with a high activity and selectivity to epoxide in oxidation of cyclohexene performed on NbMCM-41 samples clearly indicated that niobium in the structure of MCM-41 plays a crucial role in the reaction discussed. Another interesting aspect of niobium applied for olefin epoxidation is the difference in the interaction of niobium species located in the amorphous (e.g. NbMCM-41, NbSBA-15) and crystalline (e.g. anhydrous Nb2O5, Nb-Y zeolite) materials with hydrogen peroxide. For this reason the oxidation of cyclohexene with hydrogen peroxide in acetonitrile as the reaction medium was used as the test reaction [2]. The activity of the catalysts is significantly higher when niobium is dispersed in amorphous ordered silica (NbMCM-41 and SBA-15). For the same SBA-15 structure, similar to that of NbMCM-41, increasing number of niobium

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species slightly enhances the cyclohexene conversion and changes the selectivity to become higher towards diol formation. Once again this phenomenon is related to the increase in the number of Lewis acid centers with increasing niobium content. Moreover, it is also clear that niobium located in crystalline NbY zeolite is much less active in epoxidation process than that incorporated into amorphous mesoporous NbMCM-41 or NbSBA-15 samples. Commercial amorphous, hydrous Nb2O5 exhibits much lower activity because niobium species are not isolated like in the ordered mesoporous silica. Dimers or oligomers of niobium centers in Nb2O5 lead to too fast decomposition of hydrogen peroxide. However, the conversion of cyclohexene on amorphous Nb2O5 is relatively high in comparison to that of anhydrous crystalline Nb2O5. This observation prompted the application of zeolite Y containing niobium species in the crystalline framework and isolated by silica and tetrahedral alumina [156]. This zeolite is a very well crystallized solid and its activity in the oxidation of cyclohexene is negligible. This means that not only isolation of niobium species by dissolution in silica matrix is important for achieving effectiveness in epoxidation of cyclohexene but also the phase state of the material, amorphous vs. crystalline one. In the amorphous phase niobium species are more mobile than in the crystalline materials and therefore their interaction with the oxidant (hydrogen peroxide) is different. The treatment of niobium containing amorphous samples with hydrogen peroxide leads to the change in color from white to yellow, typical of metal peroxo species [157]. It is not the case of interactions of hydrogen peroxide with NbY zeolite or crystalline Nb2O5. The species formed as a result of the interaction of the samples containing niobium with H2O2 were identified by ESR [2]. The spectrum showed the characteristic signal assigned to O=Nb(V)O2 radical species, according to ref. [156, 158]. The highest intensity of this signal was observed for bulk amorphous niobium(V) oxides in which the metal peroxy radicals occurred in the highest concentration. The intensity of this signal for mesoporous materials depends on the Si/Nb ratio. The higher the ratio, the lower the ESR signal intensity because of the lower content of Nb species. The crystalline anhydrous niobium(V) oxide and crystalline NbY zeolite treated with hydrogen peroxide do not show the presence of O=Nb(V)O2 radical species. The interaction of amorphous niobia with hydrogen peroxide was also confirmed by the changes in the UV-Vis spectra. According to [157], O=Nb(V)O2 radical takes part in the formation of the epoxy product after interaction with olefins. Summarizing the liquid phase oxidation on Nb-containing catalysts described above, one can conclude that the location of Nb in amorphous materials: MCM-41, SBA-15, and amorphous niobia is responsible for its unique properties in the interaction with H2O2. Crystalline Nb2O5 and NbY zeolite do not exhibit these properties. This phenomenon is caused by the weaker Nb-O-Nb bond in amorphous phase than in the crystalline one. It is also the reason why the defect holes are easily created in amorphous ordered mesoporous materials (e.g. NbMCM-41) and they play an important role in the liquid phase oxidation with hydrogen peroxide, as shown in [35]. The indicated role of the amorphous phase allows answering the question why the catalysts containing crystalline niobium oxide phases dispersed on the supports were less active in epoxidation processes than those containing amorphous niobium species [19, 46, 47]. However, it is important to stress that not only the amorphous phase of catalysts containing niobium is needed to ensure effectiveness in the epoxidation of olefins but also isolation of niobium species is important. That is why the

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activity of amorphous hydrous Nb2O5 is much lower than that of amorphous mesoporous niobiosilicas (NbMCM-41, NbSBA-15) [35].

3.5 Liquid Phase Oxidation and Ammoxidation of Glycerol

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3.5.1 General information about the Catalytic Process Glycerol (1,2,3-propanetriol) is an unavoidable by-product of producing fatty acids and biofuels [159]. For every 9 kg of biodiesel produced, about 1 kg of crude glycerol by-product is formed [160]. Most of the glycerol used today is a highly refined product (97 % purity). Because of the cost of purification, glycerol price is high and as a result most glycerol product markets are small and fragmented. If the production of biodiesel increases as predicted, the supply of glycerol will be in excess of demand [160-163]. Therefore, new outlets should be found of glycerol to optimize the economy of biodiesel production and to rebalance supply and demand. There is a tremendous potential to develop a variety of new processes and product lines with the use of glycerol, taking advantage of its unique structure and properties [159]. Glycerol is a nontoxic, edible, biodegradable compound. These characteristics provide important environmental benefits to new platform products. Many valuable commodity chemicals can be formed from glycerol via oxidation, reduction, halogenation, etherification [160, 161]. One of the most intensively studied pathways of glycerol transformation into useful chemicals is the oxidation process [160, 161, 164, 165]. Glycerol‟s structure lends itself to catalytic oxidative processes using inexpensive oxidizing agents such as air, oxygen or hydrogen peroxide [159]. Combination of the use of these inexpensive oxidizing agents with the inexpensive source of glycerol will allow production of a number of new derivatives. Selective oxidation of glycerol leads to various valuable oxygenates (glyceric, tartronic, glycolic, hydroxypyruvic acids, dihydroxyacetone – Scheme 13). OH HO

OH

glycerol OH HO

O O

HO

glyceraldehyde

dihydroxyacetone OH

HO

OH OH

O glyceric acid

OH OH

OH

O

HO

OH

O O tartronic acid

O

HO

O hydroxypyruvic acid

OH OH

O

OH

O

HO

OH

O O glycolic acid

O glyoxylic acid

OH oxalic acid

O O mesoxalic acid

Scheme 13. General reaction pathways of the glycerol oxidation [according to ref. 168].

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It is widely shown in literature [160, 161], that these oxygenates can be obtained by oxidation with oxygen in the presence of mono and bimetallic catalysts based on Au, Pd, and Pt metals. The aqueous phase oxidation occurs under mild conditions (333 K, water as a solvent), but it is highly dependent on the pH of the reaction medium. The presence of strong base was found required to obtain good activity [166-170]. As to the mechanism of the reaction, the important role of NaOH for initiation of the catalytic glycerol oxidation was confirmed [166-170]. It was noted that for supported Pd and Pt catalysts, the addition of OH-, as NaOH, increases the conversion of glycerol. However, the presence of OH- is essential to observe any glycerol oxidation using gold containing catalysts [167]. In the presence of the base, the proton is readily abstracted from one of the primary hydroxyl groups of glycerol. The initial dehydrogenation in the process of formation of glycerolate species is proposed as the first step in the oxidation process [166, 169]. Two major routes whose primary oxidation products are hydroxyacetone and glyceric aldehyde and the end-products are glycolic and oxalic acid, respectively, have been firmly established. The rapid oxidation of glyceraldehydes favors glyceric acid rather than hydroxyacetone formation [171]. Moreover, it was found that at high NaOH/glycerol molar ratio (NaOH/Gly = 4) further oxidation of glyceric acid to tartronic acid is strongly favored [168]. The tartronic acid would be expected to undergo facile decarboxylation to give glycolic acid [172]. Nano-sized gold particles supported on different carbons have been found as the most attractive for glycerol oxidation [166-176]. Carbon supports (i.e. black carbon, activated carbon and graphite) studied by Claus and co-workers [168, 174] showed higher activity than supported oxides (TiO2, MgO and Al2O3) under the same reaction conditions and with a load of comparable size gold particles. The 100 % selectivity of glyceric acid was obtained by the group of Hutchings in the oxidation of glycerol under mild conditions (333 K, 3 h, under 0.3– 0.6 MPa of O2, water as solvent) over 1% Au/activated carbon or 1% Au/graphite catalysts [166, 167]. For the monometallic catalysts similar activities were observed for Au and Pd containing catalysts, but again carbon-supported catalysts were more active than those based on TiO2 [166, 172]. Compared with monometallic catalysts such as Au/C, which generally were more active and stable than Pd/C or Pt/C, a combination of the gold and palladium as supported nanocrystals (Pd-Au/C and Pt-Au/C) leads to significant enhancement in catalysts activity, which indicates a synergetic effect between the two metals [172, 177, 178]. In terms of selectivity, Pd in Pd-Au system was found to promote tartronic acid formation, while the presence of Pt as a promoter increases the selectivity to glycolic acid [178] and DHA [175]. The new idea in the oxidation of glycerol is the use of niobium containing catalysts. Very recent results in this field are discussed in the subsection below (3.5.2.).

3.5.1 Role of Niobium in Oxidation and Ammoxidation of Glycerol Nb2O5 catalysts are active for the transformation of glycerol to acrolein [179, 180]. However, more attractive seems to be the use of niobia as support for gold and its application in the oxidation of glycerol. The effect of Nb2O5 as a new support for gold on the efficiency of the oxidation of glycerol in liquid phase by the use of oxygen as oxidant is discussed in ref. [2, 181, 182]. The oxidation of liquid glycerol with gas oxygen was performed in the autoclave on niobium(V) oxides (amorphous and crystalline). The results displayed in Table 6 clearly show that the crystalline Nb2O5 exhibits much higher activity and selectivity to glyceric acid than amorphous Nb2O5. Taking into account the difference in the surface area of amorphous hydrated and crystalline Nb2O5 (30 and 7 m2/g, respectively) the amount of

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glycerol molecules reacted on one Nb atom on 1 nm2 of the surface was calculated (Table 6). It is clear that crystalline Nb2O5 exhibits higher activity normalized to 1 Nb/nm2. It is important to note that amorphous hydrated niobium(V) oxide reveals much higher BrØnsted acidity than the crystalline one (it is dehydroxylated form), established on the basis of the activity in acetonyl acetone cyclization to dimethylfurane. BrØnsted acid sites diminish the redox activity of the catalysts. Table 6. Liquid phase oxidation of glycerol at 333 K using Au-supported catalysts [2, 181]. Catalyst

Gly. conv. (%)

Glycerol [molecules] reacted on 1Nb/nm2

Selectivity (%) Glyceric acid

Tartronic acid

Glycolic acid

Formic acid

Oxalic acid

Nb2O5 crystalline Au/Nb2O5 crystalline Nb2O5 amorphous Au/Nb2O5 amorphous Au/Al2O3

11

0.64

22

traces

traces

0

0

67

-

47

5

5

4

0

5

0.29

2

6

traces

0

0

31

-

55

2

1

1

0

28

-

28

1

6

4

0

Au/C

76

-

36

9

7

4

1

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Reaction conditions: glycerol 100 mmol/kg, NaOH/glycerol molar ratio = 2, pO2= 6 bar, stirring = 1500 rpm, time = 5 h.

When niobium containing materials are used as supports for gold (amorphous and crystalline Au/Nb2O5 [2, 181]) the oxidation of glycerol is enhanced. However, the amorphous state of the support containing niobium does not play an important role in the catalytic properties of the gold modified catalysts in the oxidation of glycerol with oxygen in the liquid phase. The crystalline Au/Nb2O5 is much more active than gold loaded on amorphous niobia in the oxidative dehydrogenation to aldehyde which is the first step in oxidation of glycerol. Although amorphous Nb2O5 modified with Au is less active than its crystalline form, its activity is higher than that of gold catalysts based on the other group five metal oxides (Au/Ta2O5, Au/V2O5 oxides) and Au/Al2O3 [181]. It indicates the role of Au-Nb interactions in Au/Nb2O5 catalysts (SMSI between gold and niobium). The Au-Nb interaction in the crystalline Nb2O5 is stronger than in amorphous Nb2O5. The crystalline Nb2O5 contains niobium atoms at the corners and edges of the crystallites and their coordination is not fully saturated, whereas inside the amorphous niobia the coordination of Nb atoms is saturated. Unsaturated niobium species interact stronger with metallic gold leading to electron transfer to oxygen making it more basic. This phenomenon increases the ease of abstraction of proton from alcohol, which occurs on basic sites of the catalyst. An additional reason for different behavior of alumina and niobia in the oxidation process can be a definitely lower O1s binding

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energy in Nb2O5 and Au/Nb2O5 than in Al2O3 and Au/Al2O3 as indicated from XPS experiments [182]. This difference should be related to higher ionicity of the Nb-O bond than the Al-O one and thus better dispersing ability of niobia (like titania, zirconia and ceria) than alumina and to generation of active oxygen on the interface between gold particles and the support (niobia). Moreover, it was found that in the crystalline Au/Nb2O5 the atomic ratio Nb/O is lower than 0.4 (the stoichiometric value for Nb2O5). A significant excess of oxygen in comparison with Nb2O5 suggests the storage of oxygen on the catalyst surface. Because gold in Au/Nb2O5 sample exists in metallic form (BE = 83.7 eV) the storage of oxygen occurs on not fully covered niobia support (as in Figure 3a), most probably on the interface with gold. Thus, one can expect that oxygen stored on niobia surface after gold introduction can become a catalytically active species. OH O O OH oxalic acid OH

OH HO

OH HO

HO

glycerol

O glycolic acid

O O tartronic acid

OH OH

OH

OH

OH O

glyceric acid

OH OH

+

HCHO

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O OH formic acid

Scheme 14. [according to ref. 181].

Gold containing Nb2O5 oxides activate the reaction mainly towards glyceric acid (Table 6). Other products of oxidation formed besides glyceric acid include: glycolic, tartronic, oxalic and formic acids. However, the yields of these products are low. Analysis of possible mechanisms of glycerol oxidation process presented in literature [166, 167, 169, 171, 172] has shown that the oxidation of glycerol to glyceric acid on gold supported Nb2O5 catalyst most probably proceeds via initial formation of glyceraldehyde, which is rapidly oxidized to glyceric acid. The presence of tartronic acid and C2 or C1 by-products among the reaction products indicates that further oxidation of glyceric acid leads to tartronic acid which would be expected to undergo facile decarboxylation to give glycolic acid [172]. The origin of formic acid is less obvious. However, as suggested in ref. [172], it is possible that the initially formed glyceric acid could undergo a reverse-aldol fragmentation to give directly glycolic acid and formaldehyde whose oxidation would then account for the formic acid presence. The possible reaction pathways for the partial oxidation of glycerol with gold loaded on Nb2O5 oxides are shown in Scheme 14 [181]. The oxidation of glycerol is a typical consecutive

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reaction proceeding according to the rake model. In such a process the selectivity is strongly determined by the adsorption and desorption rate of each product. It is important to note, that under the same conditions the conversion of glycerol on crystalline Au/Nb2O5 catalyst prepared by gold-sol method is comparable to the conversion reached for gold supported on carbon (Table 6) [181], nowadays considered as the most attractive catalysts in glycerol oxidation [166-176]. Moreover, one should especially stress that the selectivity to glyceric acid obtained with Au loaded on niobia samples is better than that of the Au/C sample. This makes the Au/Nb2O5 catalyst quite promising for this reaction. A new challenge for the use of niobium containing catalysts is their application in ammoxidation of glycerol performed in the liquid phase with the use of hydrogen peroxide as oxidant and microwave radiation activation [48, 88, 183-185]. Of course, as for ammoxidation reactions, the active phase is based on V-Sb-O system. However, the addition of niobium to V-Sb-O supported on alumina or mesoporous MCM-41 or MCM-48 materials improves significantly their catalytic behavior in the ammoxidation of glycerol to acrylonitrile. The presence of niobium is critical to maximize the activity and selectivity and its role is to increase acidic properties of the catalysts.

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4. FINAL REMARKS The unique chemical properties of niobium make it an invaluable promoter, active phase or support in catalysts applied in several oxidation reactions. Niobium promoting effect is related to its ability to coordinate with many elements. This behavior results in the promoting entanglement of other elements in mixed oxide catalysts and prevents from appearance of segregated pure metal oxide phases. Such a phenomenon is observed in vanadium antimonate and molibdate mixed oxides applied in ammoxidation of propane and glycerol. The replacement of vanadium (+3 or +4) or molybdenum (+6) with niobium at a different oxidation state (+5) leads to generation of defects (cationic vacancies) in the structure of mixed oxides. The cationic vacancies in rutile oxides originate from the charge imbalance created by dissolution of altervalent metal cations, Nb5+, replacing of V3+ and V4+. They play a crucial role in the reoxidation of the catalyst surface when the oxidation reaction takes place according to the Mars van Krevelen mechanism. The defects are also formed when niobium species are introduced into mesoporous silicas (e.g. MCM-41). The promoting effect of niobium additive is limited to low loadings, and usually it is lost when well defined niobium containing phases are formed (e.g. Nb2O5, SbNbO4 phases). The presence of niobium species in mixed oxides or mesoporous silicas enhances also the acidity of the catalysts which is especially important in the oxidative dehydrogenation of propane to propene and methanol to formaldehyde. Acidic character of niobium(V) oxide is advantageous for the use of Nb2O5 as supports for metals and metal oxides. Thanks to strong metal support interaction in Au/Nb2O5 the catalytic behavior of these catalysts in selective liquid phase oxidation of glycerol to acids is comparable with the properties of the best gold catalysts based on activated carbons. Dilution of niobium species in silica matrix (especially ordered mesoporous silica of MCM-41, SBA3, SBA-15 types) leads to the unique properties for both applications, as a support and as the active catalyst. Nb-containing mesoporous silicas are very attractive supports for metallic

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Maria Ziolek, Izabela Sobczak and Maciej Trejda

gold and cationic copper species applied in oxidation of methanol. Niobium located in the support gives rise to higher dispersion of metals and protects against their agglomeration. Niobium species play an important role as the active centers in the liquid phase oxidation with the use of hydrogen peroxide as the oxidant (e.g. epoxidation of olefins). The amorphous materials containing niobium species, like amorphous Nb2O5xH2O or NbMCM-41, are highly effective catalysts because Nb in such catalysts strongly interacts with H2O2 resulting in the formation of active O=Nb(V)O2 radicals. It is not the case for the crystalline catalysts containing niobium in which Nb is more stable and does not interact so easily with hydrogen peroxide. On the other hand, the crystalline oxide catalysts containing niobium are attractive in transformation of methanol because niobium species playing the role of Lewis acid centers are active in formation of methoxy species from methanol, which is the first step of its oxidation. The strong interaction of niobium with other elements and its hard reducibility make the niobium containing catalysts more stable in the reactions performed in the gas phase (no changes in the crystalline structures under the reaction conditions) and in the liquid phase (no leaching of niobium to the solution). Finally, the important message is that the properties of the niobium containing catalysts strongly depend on the preparation procedure and on the niobium sources used for their synthesis or modification.

ACKNOWLEDGMENTS COST action D36, WG No D36/0006/06 and the Polish Ministry of Science (Grants No. 118/COS/2007/03 and N N204 032536) are acknowledged for the financial support.

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Nowak, I.; Ziolek, M. Chem. Rev. 1999, 99, 3603–3624. Ziolek, M.; Decyk, P.; Sobczak, I.; Trejda, M.; Florek, J.; Golinska, H.; Klimas, W.; Wojtaszek, A. Appl. Catal. A General 2011, 391, 194-204. Wachs, I.E. Proc. Intern. Conf. Niobium Tantalum 1989, 679. Iizuka, T.; Ogasawara, K.; Tanabe, K. Bull. Chem. Soc. Japan 1983, 56, 2927-2931. Jehng, J.-M.; Wachs, I.E. Catal. Today 1990, 8, 37-55. Jehng, J.-M.; Wachs, I.E. J. Phys. Chem. 1991, 95, 7373-7379. Jehng, J.-M.; Wachs, I.E. Chem. Mater. 1991, 3, 100-107. Jehng, J.-M.; Wachs, I.E. Catal. Toda, 1993, 16, 417-426. Deo, G.; Wachs, I.E. J. Catal. 1991, 129, 307-312. Deo, G.; Wachs, I.E. J. Catal. 1994, 146, 323-334. Deo, G., Wachs, I.E. J. Catal. 1994, 146, 335-345. Datka, J.; Turek, A.M.; Jehng, J.-M.; Wachs, I.E. J. Catal. 1992, 135, 186-199. Wachs, I.E.; Jehng, J.-M.; Deo, G.; Hu, H.; Arora, N. Catal. Today 1996, 28, 199-205. Berzelius; J. J. Annals Chim.Phys. 1836, 61, 146.

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In: Niobium: Properties, Production and Applications Editor: Thomas M. Wong

ISBN: 978-1-61122-895-3 © 2011 Nova Science Publishers, Inc.

Chapter 2

STUDIES ON NOVEL SOLID ACID CATALYSTS OF NIOBIUM-BASED OXIDES WITH MESOPOROUS, LAYERED AND NANOSHEET STRUCTURES Caio Tagusagawa,† Atsushi Takagaki,† Junko N. Kondo‡ and Kazunari Domen* †,* †

Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-7656, Japan, ‡ Chemical Resources Laboratory, Tokyo Institute of Technology 4259 Nagatsuta Midori-ku, Yokohama, 226-8503, Japan

ABSTRACT

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The acidities of hydrated niobium oxide (Nb2O5·nH2O) and niobium mixed oxides have been widely studied as a promising substitute for liquid acid. Nb2O5·nH2O which is usually called niobic acid, exhibits a remarkably high acid strength corresponding to the acid strength of 70 % H2SO4. In the view of catalytic activity and stability, niobic acid exhibited excellent stabilities for acid-catalyzed reactions activities in water solution reactions including hydrolysis, hydration and esterification. In this chapter, niobiumcontaining mixed metal oxides with different nanostructures (nanosheet aggregates, layered and mesoporous) are presented to study the effects caused by the structure and metal combination for the acid sites and acid-catalyzed reactions. Nanosheet aggregates and protonated layered niobates (HTiNbO5, HNb3O8 and HNbWO6) are examined as potential solid acid catalysts. However, as the high charge density of the oxide sheets prevents reactants from penetrating into the interlayer region, unmodified layered transition-metal oxides are generally ineffective as solid acid catalysts. Exfoliation and aggregation of layered HTiNbO5, HNb3O8 and HNbWO6 using soft chemical processing form aggregates of nanosheets with high surface areas, making possible the access of reactants to acid sites formed by the bridged hydroxyl groups, M(OH)M‟ (M=Ti, Nb; M‟=Nb, W). The catalytic activity for the Friedel-Crafts alkylation of anisole in the presence of benzyl alcohol increased in the order HTiNbO5 < HNb3O8 < HNbWO6, consistent with the acid strengths determined by desorption measurements and nuclear magnetic resonance spectroscopy.

*

Corresponding Author: TEL: +81-3-5841-1148, FAX: +81-3-5841-8838, E-mail: [email protected] (K. Domen).

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Caio Tagusagawa, Atsushi Takagaki, Junko N. Kondo et al. Layered HNbMoO6 is a very unique layered metal oxide able to intercalate different organic reactants (alcohols, saccharides, ketones, alkenes, hydroxyl acids) during the catalytic reactions. Owing to the intercalation ability and strong acidity in the interlayer, layered HNbMoO6 functioned as a highly-active solid acid catalyst. The catalytic activity for the Friedel-Crafts alkylation, acetalization and hydrolysis of saccharides exceeded the activity of zeolites and ion-exchange resins. Mesoporous NbxW(10-x) mixed oxides with different Nb and W concentrations are examined as potential solid acid catalysts. Amorphous wormhole-type mesopores are observed for samples from x = 3 to 10 whereas W-rich samples (x = 0 to 2) formed a non-mesoporous structure with presence of crystallized tungsten oxide (WO3). The acidcatalytic activity, acid strength and mesopore structure of mesoporous Nb-W oxides changed in order of W concentrations, exhibiting a very high activity for both FriedelCrafts alkylation and hydrolysis. The results were compared with those for non-porous Nb2O5-WO3 and a range of conventional solid acids. Mesoporous Nb-W oxides obtained higher turnover rate than that of non-porous Nb2O5-WO3 led to the strong acid sites and a mesoporous structure with a high surface area and easy reactant accessibility.

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1. INTRODUCTION Bulk metal oxides and mixed oxides have been widely studied as promising substitutes for liquid acids. Initially discovered single metal oxides exhibited weak acidities such as H0 < -3 for TiO2 and Al2O3 [1] and H0 = +1.5 to +3.3 for ZrO2 [2]. However, the discovery of the strong single metal oxide solid acids niobium oxide and tantalum oxide led to in-depth studies of the characteristics of their acid sites, catalytic activities [3-10] and other single metal oxide like WO3. Since then several review articles about Nb-based solid acids have been published by Tanabe et al. [4, 11-13] and Ziolek et al. [3, 14-15]. Hydrated niobium oxide (Nb2O5·nH2O), which is usually called niobic acid, exhibits a remarkably high acid strength (H0 = -5.6) as a single metal oxide, that corresponds to the acid strength of 70% H2SO4. With regard to its catalytic activity and stability, niobic acid exhibits excellent stability in acid catalyzed reactions and high activity for aqueous based reactions including hydrolysis, hydration and esterification. The acidic properties of niobic acid widely depend on the calcination temperature. The catalyst exhibits strong acidity when calcined between 373 and 573 K and becomes neutral when calcined at temperatures above 773 K. The ratio of Brønsted and Lewis acid sites also changes with the calcination temperature (373-573 K). The highest number of Brønsted acid sites is observed for samples calcined at 373 K, while the number of Lewis acid sites increases for samples calcined at 573 K. Niobic acid is used for industrial production of agricultural drug intermediates such as methyl tertbutyl ether, methyl methacrylate and 2,5-dimethyl-2,4-hexadiene. Niobium phosphate (NbPO4), which has a higher acid strength of H0 = -8.2 (corresponding to the acid strength of 90% H2SO4), shows a high catalytic activity for particular reactions such as dehydration of alcohols [4]. Niobium pentafluoride, obtained by direct fluorination of niobium metal or niobium pentachlorides with HF, is a strong Lewis acid, which coordinate with fluoride ion to form anions of the type (NbF6)-. It complexes a wide variety of donors such as ethers, sulfides, amines, halides, etc. [16,17] Mixed metal oxides are known to exhibit higher reaction activities and stronger acidities than single metal oxides. To explain the acid generation capability of binary oxides, Tanabe et

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al. proposed a hypothesis involving acid site generation caused by an excess of negative or positive charges in the model structure of a binary oxide [18]. While the hypothesis agrees with the experimental results, it does not explain the acid strength of binary oxides. It has been determined that the acid strength of binary oxides is related to the averaged eletronegativities of the metals [19]. Alumina supported niobia (Nb2O5/Al2O3) calcined at 1173 K is highly active in the Friedel-Crafts alkylation reaction of benzyl alcohol and anisole [20]. Unlike niobic acid, the Brønsted acid sites still remain on the Nb2O5/Al2O3 calcined at 1173 K. Nb2O5-MoO3 and Nb2O5-WO3, reported by Niwa et al. exhibit very strong acid strength and catalytic activity for the Friedel-Crafts alkylation of anisole with benzyl alcohol [21]. Their activities are much higher than that of other niobium containing metal oxides (Nb2O5/SiO2, Nb2O5/TiO2, Nb2O5/MgO and Nb2O5). The researchers suggested two hypotheses for the formation of the active species: an isomorphous replacement model [22], which is known in zeolites, and formation of heteropoly compounds like H3PO4-WO3-Nb2O5 [23, 24]. In this chapter, a number of novel Nb mixed metal oxide solid acids including exfoliated nanosheets oxides, a layered oxide and mesoporous oxide are introduced in order to study the acid catalytic activity of solid acids with different structures and compositions.

2. NANOSHEETS AND LAYERED METAL OXIDE AS SOLID ACID CATALYSTS

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2.1 Exfoliated nanosheets Ion-exchangeable layered metal oxides [25-27] offer several attractive features from the viewpoint of heterogeneous catalysis. Metal oxide polyanion sheets have a negative charge, and the high negative charge density of the metal oxide sheets functions as a host structure for ion-exchange reactions with various cations. Although proton-exchange layered metal oxides are expected to function as solid acid catalysts, as are clay minerals such as montmorillonite, these layered metal oxides do not exhibit solid acid catalytic activity because of the difficulties associated with the intercalation of reactant molecules. This resistance is attributed to the high density of the sheets. Several classes of layered materials can be exfoliated into single sheets by soft chemical procedures [28-29]. Typically, exfoliation of layered transition metal oxides is carried out by addition of bulky quaternary alkylammonium ions (TBA+, tetrabutylammonium) to aqueous solutions containing protonated layered transition metal oxides. The resulting exfoliated nanosheets are determined to be inorganic macromolecules having a two dimensional crystalline lattice. Exfoliated nanosheet colloids are then precipitated under acidic conditions and the resultant aggregates examined as solid acid catalysts (Figure 1) [30-32]. Acid properties of exfoliated transition metal oxide nanosheets depend strongly on the surface OH groups on the two dimensional sheets. The crystal structure of the parent layered metal oxides (Figure 2) affects the formation of acidic OH groups on exfoliated nanosheets, resulting in different acid properties. In this chapter, aggregated nanosheets HTiNbO5, HNb3O8, HNbWO6 are described to show the difference between the bridging hydroxyl, Nb(OH)M (M=Ti, Nb, W) formed between the niobium and elements from group 4, 5 and 6.

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Figure 1. Soft-chemical procedures exfoliating the layered materials into single sheets to obtain the aggregated nanosheets.

Figure 2. Schematic structures of layered KTiNbO5, KNb3O8 and LiNbWO6.

2.1.1 Structures of Aggregated Nanosheets SEM images of layered HTiNbO5, HNb3O8, HNbWO6, and the corresponding aggregated-nanosheet catalysts can be observed at Figure 3. The as-prepared layered metal oxides consist of tabular particles of 3–20 m in size. The BET surface areas of the aggregated-nanosheet precipitates are 153 m2 g-1 for HTiNbO5, 101 m2 g-1 for HNb3O8 and 66 m2 g-1 for HNbWO6, increasing remarkably from the layered metal oxides with 1 m2 g-1. The precipitate is free of the tabular particles observed in the original layered compounds. [30-32]

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Figure 3. SEM images of (a) layered HTiNbO5, (b) HTiNbO5 nanosheets aggregate, (c) layered HNb3O8, (d) HNb3O8 nanosheets aggregate, (e) layered HNbWO6 and (f) HNbWO6 nanosheets aggregate.

Figure 4. XRD patterns for (a) layered HTiNbO5, (b) HTiNbO5 nanosheets aggregate, (c) layered HNb3O8, (d) HNb3O8 nanosheets aggregate, (e) layered HNbWO6 and (f) HNbWO6 nanosheets aggregate.

The XRD patterns for the layered metal oxides and the corresponding aggregatednanosheet precipitates are shown in Figure 4. The XRD pattern for the HTiNbO5 nanosheet precipitate has a relatively weak diffraction peak at low angle (2 < 10°), indicating that the periodic layered structure of the original HTiNbO5 is partially destroyed. However, the presence of diffraction peaks due to in-plane diffractions (e.g., (011) and (200)) for the HTiNbO5 nanosheet catalyst indicates that the two-dimensional structure is preserved upon precipitation. Both HNb3O8 and HNbWO6 precipitate also have weaker diffraction peaks at

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low angle (2 < 10°), and the in-plane diffraction peaks (e.g., (101) for HNb3O8 and (110) and (200) for HNbWO6) are similarly retained upon exfoliation-aggregation.

2.1.2. Acid Properties of Exfoliated Niobium Oxide Nanosheets

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NH3-TPD NH3 temperature-programmed desorption is one of basic methods to measure the acid amount and acid strength from the desorbed NH3 quantity and desorbed temperature. Figure 5 shows the NH3-TPD (m/e = 16) results for the metal-oxide aggregated-nanosheet precipitates. The NH3-TPD profiles exhibit two distinct peaks for the aggregated-nanosheets, at 430 K and 535 K for HTiNbO5, at 440 K and 550 K for HNb3O8 and at 455 K and 560 K for HNbWO6. The peaks at 535–560 K are attributable to strong acid sites present on nanosheets. The order of acid strength in these solid acids increases in the order HTiNbO5 < HNb3O8 < HNbWO6. The acid density of the aggregated-nanosheet precipitates are 0.24 mmol/g for HTiNbO5, 0.28 mmol/g for HNb3O8 and 0.34 mmol/g for HNbWO6.

Figure 5. NH3-TPD (m/e = 16) curves for (a) HTiNbO5 nanosheets aggregate, (b) HNb3O8 nanosheets aggregate and (c) HNbWO6 nanosheets aggregate. 1

H MAS NMR The properties of hydrogen species in the interlayer space and on the nanosheets are examined in detail by 1H MAS NMR spectroscopy. The 1H chemical shift reflects acidity, and hydrogen species with a large chemical shift are expected to possess strong acidity. In the case of HTiNbO5 nanosheets, four peaks are observed at 12, 7.6, 4.9 and 1.6 ppm (Figure 6). The peak at 12 ppm is ascribed to OH groups in an environment similar to that of layered HTiNbO5, suggesting that some of the exfoliated sheets aggregate in a regular stacking pattern. Figure 6 also schematizes the structure of the HTiNbO5 nanosheets. In [TiNbO5]– sheets obtained by the exfoliation of layered HTiNbO5, a negative charge is localized on the TiO6 octahedra. There are two types of OH groups on the nanosheets: isolated Ti–OH and bridged Ti(OH)Nb. The peaks at 1.6 and 4.9 ppm are assigned to isolated Ti–OH and

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hydrogen-bonded Ti–OH, respectively. The peak at 7.6 ppm is assigned to the bridged Ti(OH)Nb, regarded as strong Brønsted acid sites in HTiNbO5 nanosheets responsible for the acid catalytic activity of HTiNbO5 nanosheets.

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Figure 6. 1H MAS NMR spectra for layered HTiNbO5 and HTiNbO5 nanosheets aggregate, measured at room temperature after removal of adsorbed water. The spinning rate of the sample was 8 kHz.

Strong Brønsted acid sites such as Ti4+(OH)Nb5+ appear only on nanosheets prepared from layered compounds. [31] Figure 7 shows the amount of ethyl acetate formed by esterification of acetic acid for 6 h in the presence of two kinds of metal oxides containing Ti4+ and Nb5+ (Ti4+ : Nb5+ 1 : 1) (an amorphous oxide and a mixture of crystalline TiO2 (rutile), Nb2O5 and TiNb2O7). Neither sample exhibit catalytic activity for the reaction, whereas both titanium niobate nanosheets (HTiNbO5 and HTi2NbO7) gave high activity. The peak due to Ti(OH)Nb is absent in the 1H MAS NMR spectrum for the bulk samples. Thus, it appears that Ti(OH)Nb cannot be formed on bulky metal oxides prepared simply by mixing Ti4+ and Nb5+, suggesting that nanosheets of TiO6 and NbO6 octahedra mixed at an atomic level are essential for the generation of such strong acid sites. The formation of this kind of strong Brønsted acid site on pristine niobium oxide nanosheets is helpful for understanding the unique acid properties of niobic acid. However, the properties of Nb2O5·nH2O have yet to be examined thoroughly, and although it has been reported that Nb2O5·nH2O hosts several types of acid sites, [3, 33–36] the details remain unclear, presenting an obstacle to the further development of active Nb2O5·nH2O catalysts. The lack of investigation can largely be attributed to the complexity of amorphous Nb2O5·nH2O. [34–38] Niobium oxide nanosheets (HNb3O8) can be readily synthesized from layered KNb3O8 through exfoliation and aggregation. The two-dimensional single-crystal nanosheets enable a simpler characterization of the surface structure and surface functional groups. The hydrolysis of ethyl acetate in the presence of a large amount of water shows that the HNb3O8 nanosheets exhibit high catalytic performance, reaching a level twice that of niobic acid. The HNb3O8 nanosheets are also superior to niobic acid for other acid-catalyzed

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reactions, including esterification and Friedel–Crafts alkylation. In contrast, HSr2Nb3O10 nanosheets exhibit no or slight acid catalytic activity for these reactions. These nanosheets host only isolated and hydrogen-bonded OH groups assigned to 1.6 and 5.1 ppm respectively, which do not have strong acidity.

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Figure 7. Production of ethyl acetate on titanium niobate nanosheets and Ti–Nb mixed oxides (343 K, catalyst: 0.2 g, acetic acid: 0.10 mol, ethanol: 0.10 mol, reaction time: 6 h).

Although Nb2O5·nH2O, HNb3O8 nanosheets and HSr2Nb3O10 nanosheets are all Nb5+ containing oxides, there appears to be a definite distinction in catalysis and acidity between these materials, suggesting that the appearance of strong Brønsted acid sites is largely structure-dependent. Figure 8 shows 1H MAS NMR spectra for HSr2Nb3O10 nanosheets, HNb3O8 nanosheets and niobic acid, along with the schematic structures of the sheets. On HSr2Nb3O10 sheets, which consist of corner-sharing NbO6 octahedra, [39–41] H+ is located only on oxygen atoms at the vertices of the NbO6 octahedra. The two peaks at 5.1 and 1.6 ppm in the NMR spectrum for the HSr2Nb3O10 nanosheets are assigned to Nb–OH groups with and without hydrogen bonds, respectively. [42] In the NMR spectrum for HNb3O8 nanosheets, three peaks appear at 1.3, 5.5 and 8.5 ppm. As the HNb3O8 sheets are composed of edge-sharing NbO6 octahedra, [43–45] there are OH groups shared by two Nb5+ (Nb(OH)Nb) in addition to isolated Nb–OH groups. As a result, the chemical shift at 8.5 ppm are attributed to Nb(OH)Nb. This peak has a large chemical shift and is not observed in the 1 H MAS NMR spectrum of HSr2Nb3O10 nanosheets (not an acid catalyst), indicating that Nb(OH)Nb functions as a strong Brønsted acid site. The 1H MAS NMR spectrum for dehydrated Nb2O5·nH2O reveals a broad peak centered around 6.9 ppm with a shoulder peak at 1.7 ppm. Nb2O5·nH2O is formed by the random condensation of H8Nb6O19 clusters, [35,36] resulting in a large variety of hydroxyl groups; hence the broad peak in the 1H MAS NMR spectrum. Thus, Nb(OH)Nb functions as a strong Brønsted acid site in Nb2O5·nH2O as well.

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Figure 8. 1H MAS NMR spectra for HSr2Nb3O10 nanosheets, HNb3O8 nanosheets and niobic acid.

The 1H MAS NMR spectra for HNbWO6 nanosheet precipitate is shown in Figure 9. The spectrum for HNbWO6 nanosheets consists of three peaks, at 0.9, 5.5, and 7.2 ppm. The peaks at 0.9, 5.5, and 7.2 ppm for the HNbWO6 nanosheet precipitate are similarly assigned to the weak acid peaks of isolated Nb–OH, hydrogen-bonded Nb–OH, and strong acid sites of Nb(OH)W, respectively. The 1H MAS NMR chemical shifts of M(OH)M‟ (M = Ti, Nb; M‟ = Nb, W) for nanosheet materials composed of transition-metal octahedra appear at 6.9– 8.5 ppm, representing bridging hydroxyl groups that function as strong Brønsted acid sites.[30-32]

Figure 9. 1H MAS NMR spectra for HnbWO6 nanosheets.

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P MAS NMR The acid properties of the aggregated-nanosheet precipitates are also evaluated by 31P MAS NMR using TMPO as a probe molecule. As the 31P chemical shifts of protonated TMPO (i.e., TMPOH+) tend to move downfield, higher chemical shifts indicate higher protonic acid strength. Figure 10 shows 31P MAS NMR spectra for 50 mol% TMPO adsorbed aggregated nanosheets. The peaks below 50 ppm are ascribed to physisorbed or crystalline TMPO, which are excluded from the following discussion. The HNb3O8 nanosheet catalyst exhibits a broad 31P peak centered at ca. 70 ppm, while the HTiNbO5 nanosheet catalyst exhibits a peak at ca. 63 ppm with a shoulder peak at 67 ppm. HNbWO6 nanosheets catalyst exhibits a broad peak at ca. 71 ppm, with a shoulder peak at 65 ppm, stronger than those in HY zeolite at 65 ppm [46]. The stronger acidic sites are present in the following order; HNbWO6 > HNb3O8 > HTiNbO5.

Figure 10. 31P MAS NMR spectra for TMPO adsorbed- (a) HTiNbO5 nanosheets aggregate, (b) HNb3O8 nanosheets aggregate, (c) HNbWO6 nanosheets aggregate, and (d) HTaWO6 nanosheets aggregate, measured at room temperature. The spinning rate of the sample was 10 kHz.

2.1.3. Acid Catalytic Activity The acid catalytic activities of aggregated nanosheets are compared by Friedel-Crafts alkylation of anisole in the presence of benzyl alcohol (Table 1). The original layered metal oxides (HTiNbO5, HNb3O8, HNbWO6) do not catalyze these reactions, [30-32] with alkylated products absent or afforded at yields of less than 1 %. In contrast, the exfoliated and aggregated materials, other than HTiNbO5, exhibit significant activity for this reaction. The HNb3O8 aggregated-nanosheet precipitate has relatively low activity for this reaction at 373 K, but exhibits remarkable activity at 423 K, as previously reported. [28] The catalytic activity of the nanosheet precipitates increases in the order HTiNbO5 < HNb3O8 < HNbWO6,

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consistent with the acid strength determined by NH3-TPD and 31P-MAS NMR. The nanosheet materials consisting of group-5 elements (Nb5+) and the group-6 element (W6+) afford higher benzylanisole yields, 31P NMR peaks and NH3 desorption temperatures than the nanosheet precipitate bearing the group-4 element (Ti4+). Reactions are also performed using niobic acid (Nb2O5·nH2O), and Nb2O5–WO3. The turnover frequency (TOF), which is the molar ratio of converted substrate to catalyst (acid amount) per unit time (hour), of the HNbWO6 nanosheet precipitate is comparable to that for Nb2O5–WO3 and higher than that of Nb2O5·nH2O. Table 1. Friedel–Crafts Alkylation over Several Solid Acid Catalystsa SBET / m2 g-1

Acid amount / mmol g-1

Yield / %

Rate of benzylanisole / mol g-1 min-1

TOF / h-1

Layered HTiNbO5

1

2.0

N. D.

0

0

Layered HNb3O8

1

1.3

N. D.

0

0

Layered HNbWO6

1

0.9

0.5

2.1

0.1

HTiNbO5 nanosheets

153

0.24

N. D. (0.3)b

0 (0.6)b

0 (0.1)b

HNb3O8 nanosheets

101

0.28

1.8 (93.7)b

7.5 (195)b

0.6 (16.7)b

HNbWO6 nanosheets

66

0.34

8.6

62

4.1

Nb2O5·nH2O

128

0.3

0.5

2.1

0.4

Nb2O5–WO3

36

1.0

18.9

79

4.7

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a

Reaction conditions: anisole (100 mmol), benzyl alcohol (10 mmol), catalyst (0.2 g), 373 K, 2 h. Values in parentheses are data which were carried out at 423 K for 4h. N.D. – not detected.

Table 2. The results obtained for the acid catalytic activity and acid properties of the present exfoliated-aggregated nanosheet catalysts

Catalyst

Acid amount / mmol g-1

NH3–TPD peaks / K

HTiNbO5 nanosheets

0.24

535

HNb3O8 nanosheets

0.28

HNbWO6 nanosheets

0.34

31

P MAS NMR peaks / ppm

Friedel–Crafts alkylation a Yield / %

TOF / h-1

63

N. D.

0

550

70

1.8

0.6

560

71

14.9

4.1

a Reaction conditions: anisole (100 mmol), benzyl alcohol (10 mmol), catalyst (0.2 g), 373 K, 2 h.

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Table 2 summarizes the results obtained for the acid catalytic activity and acid properties of the present exfoliated-aggregated nanosheet catalysts. The nanosheet catalysts with higher activity for Friedel-Crafts alkylation possess stronger acid sites, as confirmed by NH3-TPD and 31P MAS NMR spectroscopy. The strength of Brønsted acid sites in the nanosheet aggregates is closely related to the combination of metal cations, increasing in the order Ti4+– (OH)–Nb5+ < Nb5+–(OH)–Nb5+ < Nb5+–(OH)–W6+.

2.2. Layered HNbMoO6 Very recently, a layered metal oxide, protonated niobium molybdate, HNbMoO6, was found to exhibit a significantly high catalytic activity without exfoliation. [47–51] Layered HNbMoO6·nH2O are obtained easily from the Li form by a proton-exchange reaction and possess a high intercalation behavior. The resultant material consists of layers formed of randomly sited MO6 (M = Nb and Mo) octahedra with H2O in the interlayer. (Figure 2)

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2.2.1 Structures of Layered HNbMoO6 Figure 11 shows electron microscope images of layered LiNbMoO6, layered HNbMoO6, exfoliated [NbMoO6]-, and aggregated HNbMoO6 nanosheets. The as-prepared HNbMoO6 consists of tabular particles of 4–10 m in size, with a surface of ca. 5 m2g-1. The nanosheets aggregates consist of random aggregates formed by the addition of H+ to yield the expected composition of Nb:Mo = 50.9:49.1, identical to that prior to exfoliation. The surface area of aggregate is 14 m2g-1, which is much lower than that of other aggregated-nanosheet metal oxides such as HTiNbO5 and HNb3O8, which typically have a surface area exceeding 100 m2g-1. This result indicates that most of the exfoliated [NbMoO6]- sheets are restacked in the original layered structure, due to the high charge density of the two-dimensional sheets.

Figure 11. Electron microscope images of (a) layered LiNbMoO6 (SEM), (b) layered HNbMoO6 (SEM), (c) exfoliated [NbMoO6]- (TEM) and (d) aggregated HNbMoO6 nanosheets (SEM).

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2.2.2. Acid Properties of Layered HNbMoO6 31

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P MAS NMR The acid properties of layered and nanosheet HNbMoO6, Nb2O5-MoO3, and niobic acid are determined by 31P MAS NMR using TMPO as a probe molecule. Layered HNbMoO6 exhibits distinct peaks at 86 and 81 ppm, attributable to strong acid sites (Figure 12). The main peak of nanosheet HNbMoO6 occurs at ca. 60 ppm, and peaks due to strong acid sites appear at 65–83 ppm. The formation of two peaks at nanosheet HNbMoO6 (60 ppm and 6583 ppm) could be attributed for the different structures formed after exfoliation. Nb2O5-MoO3 exhibits a higher chemical shift (ca. 70 ppm) than niobic acid (ca. 65 ppm) and both solid acids display a sharp peak at ca. 40 ppm. The peak at 42 ppm assigns to crystallized TMPO [46] and the 40 ppm peak is ascribed to TMPO complexed to a very weak acid site [52]. The peak positions indicate that the acid sites of layered HNbMoO6 are stronger than those of both H-type zeolites (65 ppm for HY [46], 78 ppm for H-Beta [53]) and ion-exchange resin (81 ppm for Amberlyst-15), and comparable in strength to those on strongly acidic zeolites (86 ppm for HZSM-5 [54] and HMOR [53]).

Figure 12. 31P MAS NMR spectra for TMPO-adsorbed (a) Nb2O5•nH2O, (b) Nb2O5-MoO3, (c) HNbMoO6 nanosheets and (d) layered HNbMoO6 after exposure to 0.8mmol/g-oxide TMPO.

It is also confirmed by XRD that TMPO intercalates into HNbMoO6, indicating that the strong acid sites of HNbMoO6, detectable in the NMR spectrum, are present in the interlayer. The basal spacing of the sample exposed to 0.8 mmol/g TMPO increases from 10.8 Å to 14.8 Å, indicating that TMPO had penetrated into the interlayer to form a monolayer configuration. Exposure to higher concentrations of TMPO results in the formation of bilayer structures with basal spacings of 18.6 Å (1.6 mmol/g) and 18.8 Å (3.2 mmol/g). Figure 13 shows the NMR spectra for TMPO-intercalated HNbMoO6. The main peaks shift to ca.

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68 ppm with increasing TMPO intercalation. Although a small peak at 80–86 ppm is present in the sample exposed to 1.6 mmol/g TMPO, the peak is negligible compared with the spectrum of the sample exposed to 0.8 mmol/g TMPO. The sample exposed to 3.2 mmol/g TMPO exhibits peaks at 45 and 42 ppm accompanying the main peak at 68 ppm, attributable to physisorbed TMPO [46] (45 ppm) and crystallized TMPO (unreacted TMPO) [46] (42 ppm). From the peak area of the spectrum, the maximum density of TMPO adsorption to HNbMoO6 is estimated to be ca. 1.9 mmol/g, consistent with carbon analysis and thermogravimetric (TG) analysis. XRD and 31P NMR analyses of the adsorbed TMPO indicate that TMPO is preferentially adsorbed to strong interlayer acid sites of HNbMoO6 in the monolayer intercalation.

Figure 13. 31P MAS NMR spectra for TMPO-adsorbed layered HNbMoO6 after exposure to (a) 0.8, (b) 1.6 or (c) 3.2 mmol/g-oxide TMPO

2.2.3. Acid Catalytic Activity This layered protonated niobium molybdate is highly active for several acid-catalyzed reactions, including Friedel–Crafts alkylation, acetalization, esterification, hydrolysis, and hydration. Figure 14 shows the catalytic performance results for the Friedel–Crafts alkylation of toluene with benzyl alcohol. Layered HNbMoO6 exhibits remarkable activity for the reaction, much higher than that of the other solid acids, including HNbMoO6 nanosheets, tested. Layered HNbMoO6 also catalyzes the alkylation of anisole or benzene, achieving high yields: the yield of benzyl anisole reached 99% after 30 min, whereas those for ion-exchange resins only reached ca. 42% even after 1 h. The results of acetalization of cyclohexanone, esterification of lactic acid, hydrolysis of ethyl lactate, and hydration of 2,3-dimethyl-2butene over HNbMoO6 are summarized in Table 3. For these reactions, HNbMoO6 shows remarkable activity, comparable or superior to conventional ion-exchange resins (Nafion NR50 and Amberlyst-15). Even in the presence of water (hydrolysis and hydration), HNbMoO6 functioned as an efficient catalyst. The HNbMoO6 is recoverable by filtration and

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washing with water and acetone to remove the residue within the interlayer with no change in crystal structure or activity after three reuse cycles. This high catalytic activity of layered HNbMoO6 is attributed to the efficient intercalation of substrates within the interlayer with strong acidity. Substrates such as benzyl alcohol are readily intercalated into HNbMoO6.

Figure 14. Yield of benzyltoluene for Friedel–Crafts alkylation over layered HNbMoO6. Reaction conditions: toluene (100 mmol), benzyl alcohol (10 mmol), catalyst (0.2 g), 353 K, 4 h.

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Table 3. The results for acetalization, esterification, hydrolysis and hydration over layered HNbMoO6 Acetalizationa reaction rate /mmol h-1

Esterificationb reaction rate /mmol h-1

Hydrolysisc reaction rate /mmol h-1

Hydrationd Yield (%)

Layered HNbMoO6

158

5.7

0.9.

3.4

Amberlyst-15

66

4.5

1.22

3.7

Nafion NR50

58

6.0

1.06

2.2

Blank

0

1.4

0

0.4

a

Cyclohexanone (20 mmol), methanol (5 mL), catalyst (0.1 g), 323 K. b Lactic acid (100 mmol), ethanol (1 mol), catalyst (0.2 g), 343 K. c Ethyl lactate (50 mmol), H2O (9 mL), catalyst (0.1 g), 343 K. d 2,3-dimethyl-2-butene (12.5 mmol), H2O (7.6 mL), catalyst (0.2 g), 343 K, 5 h.

In order to validate this process, the adsorption of benzyl alcohol onto HNbMoO6 is investigated in isolation by immersing HNbMoO6·nH2O (n = 1.23) in benzyl alcohol solution under constant shaking for 30 min. Figure 15 shows the XRD pattern of the resultant material after drying at room temperature. Immersion in benzyl alcohol shifts the (001) peak due to HNbMoO6 to lower angles, corresponding to an increase in basal spacing from 10.8 [27] to 16.3 Å, which confirms the intercalation of benzyl alcohol into the layered HNbMoO6. In the Friedel–Crafts alkylation of toluene in the presence of benzyl alcohol, the interlayer spacing of the layered HNbMoO6 increases to 16.6 Å. A preliminary benzyl alcohol-intercalated sample, placed in anisole solution, then heated at 373 K for 1 h, indicates that alkylated

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products (benzyl anisole) are detected, and the basal spacing of HNbMoO6 decreases to 14.1 Å. These results indicate that intercalated benzyl alcohol is consumed in the reaction and that the interlayer sites of HNbMoO6 function as active sites. Other substrates, including n-alkyl alcohols and ketones, could also be successfully intercalated into the interlayer gallery of HNbMoO6.

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Figure 15. XRD patterns of HNbMoO6: (a) dehydrated, (b) after immersion in benzyl alcohol for 30 min, (c) after reaction of anisole, (d) during Friedel–Crafts alkylation.

The intercalation mechanism greatly affects the catalytic activity of this layered catalyst. A difference in intercalation behavior was observed between carboxylic acid (propionic acid) and hydroxycarboxylic acid (lactic acid). The latter could easily intercalate, whereas the former did not, resulting in a unique behavior of acid catalysis for esterification. [48] Layered HNbMoO6 exhibits negligible activity for the esterification of carboxylic acid, and remarkable activity for the esterification of hydroxycarboxylic acid. Figure 16 shows the results of the esterification of three carboxylic acids (acetic, propionic, butyric) and lactic acid with n-butanol. Amberlyst-15, Nafion NR50 and layered HNbMoO6 were compared. For the three carboxylic acids, the ester yield over the three catalysts examined decreases in the order Amberlyst-15 > Nafion NR50 > HNbMoO6, and the activity decreases with increasing carbon number of the carboxylic acid. The two ion-exchange resins exhibit similar yields for propionic acid and lactic acid, which have the same carbon number. However, layered HNbMoO6 exhibits a very high activity for lactic acid, with a yield of 35.1%, as compared to the 6.2% for propionic acid. The catalytic activity of ion-exchange resins is influenced only by the carbon number of the carboxylic acid, and is unaffected by the presence of OH groups in lactic acid. In contrast, the OH groups of lactic acid appear essential for allowing intercalation into the layered HNbMoO6. Given that activation of carboxylic acid by a proton is necessary for the reaction, esterification of the three carboxylic acids did not occur due to the difficulty of intercalation. However, lactic acid, which has an OH group adjacent to the carboxyl group, could intercalate into the oxide, resulting in a high catalytic activity for the reaction.

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Figure 16. The results of esterification of carboxylic acids (acetic, propionic, butyric) and lactic acid with n-butanol over HNbMoO6, Amberlyst-15 and Nafion NR50. Reaction condition: carboxylic acid (0.05 mol), n-butanol (0.25 mol), catalyst (0.1 g), 343 K, 5 h.

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2.3. Biorefinery Applications Exfoliated nanosheets and layered HNbMoO6 function as efficient solid acid catalysts even in the presence of water. This remarkable feature can be exploited in biorefinery applications. Valente and co-workers reported that exfoliated metal oxide nanosheets could dehydrate D-xylose to produce furfural in a water-toluene solvent mixture. [55] HTiNbO5 nanosheets catalyze this reaction to achieve a furfural yield of 55% at 92% conversion, whereas the furfural yield in the presence of niobic acid (Nb2O5·nH2O) is 12%. Other nanosheets, including HTi2NbO7, H2Ti3O7, HNb3O8, and H4Nb6O17, also give a high furfural yield. These nanosheets can be reused at least three times without loss of activity. Layered HNbMoO6 exhibits remarkable performance for hydrolysis of saccharides. [50] Polysaccharides such as starch and cellulose are converted into monosaccharides such as glucose, a major platform for the synthesis of a variety of chemicals, by a homogeneous acidcatalyzed reaction using sulfuric acid and/ or by enzymatic reactions. The development of a reusable and readily separable solid acid catalyst for the hydrolysis of saccharides is considered essential for converting biomass into bioethanol and useful chemicals with the lowest environmental impact. Figure 17 shows the results of the hydrolysis of sucrose and cellobiose. The hydrolysis of cellobiose was carried out at 373 K using 0.2 g of catalyst, 1.0 g of cellobiose, and 10 mL of water. The catalytic performance of these catalysts are compared with that of a range of conventional solid acids, including ion-exchange resins, H-type zeolite and niobic acid. For the hydrolysis of sucrose, an equivalent amount of glucose and fructose

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is produced. H-ZSM5 zeolite is inactive for this reaction, and niobic acid, although known to be water-tolerant, do not achieve appreciable activity for this reaction. The activity of layered HNbMoO6, however, substantially exceeded the maximum performance of any of the other solid materials tested. This high activity is also found for hydrolysis of cellobiose. Cellobiose, a subunit of cellulose, consists of β-1,4-glycosidic bonds, which are much more stable than the α-1,2-glycosidic bonds comprising sucrose, and thus is more resistant to hydrolysis. The layered HNbMoO6 catalyst also exhibits the highest performance in this hydrolysis reaction, producing glucose at twice the rate of the ion-exchange resins and achieving a turnover rate comparable to that of Nafion NR50. The turnover rate of HNbMoO6 is much higher than that of Amberlyst-15 and twice that of sulfuric acid. This layered oxide can produce glucose from starch and cellulose in the presence of water.

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Figure 17. Hydrolysis of disaccharides (sucrose and cellobiose) over solid acid catalysts. Sucrose hydrolysis: sucrose (2.92 mmol), H2O (20 mL), catalyst (0.2 g), 353 K. Cellobiose hydrolysis: cellobiose (2.92 mmol), H2O (10 mL), 373 K.

2.4. Conclusion Strong solid acid catalysts based on nanosheets obtained through protonation, exfoliation and aggregation of cation-exchangeable layered metal oxides have been described. Exfoliation-aggregation of protonated layered oxides does not always result in strong solid acids, and the formation of bridging hydroxyl groups, M(OH)M‟, which are intrinsic to the crystal structure of the two-dimensional sheets, is essential for the preparation of nanosheet catalysts with strong acidity. The acid strength of the strong Brønsted acid sites is significantly influenced by metal cations M and M‟ in the order (Ti4+, Nb5+) < (Nb5+, Nb5+) < (Nb5+, W6+). Considering the acidic properties of binary metal oxides proposed by Prof. Tanabe, the increase in the acid strength is caused mainly by an increase in the electronegativities of the incorporated metals (Ti = 1.5, Nb = 1.6, W = 2.4). In addition, a unique solid acid catalysis was found for layered niobium molybdate, exploiting its facile intercalation availability for substrate within the interlayer gallery with strong acidity. Layered HNbMoO6 can readily intercalate alcohols, ketones, alkenes and saccharides even at room temperature, resulting in a remarkable activity for alkylation, hydrolysis, esterification, acetalization and hydration reactions. Exfoliated nanosheets and

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layered HNbMoO6 function as efficient solid acid catalysts even in the presence of water, and are exploited in biorefinery applications, including the formation of monosaccharides by hydrolysis and furan-derivatives by dehydration.

3. MESOPOROUS METAL OXIDES AS SOLID ACID CATALYST Due to their wide range of potential applications, the synthesis of mesoporous transition metal oxides has been extensively studied [56-61]. Examples include mesoporous TiO2 [6272], ZrO2 [62-63,73-80], Nb2O5 [62-63,65,81-84], Ta2O5 [62-63,85-92], (Nb,Ta)2O5 [6263,85-97], SnO2 [62-63,98-100] and WO3 [62-63], which are used as heterogeneous solid acid catalysts [74,76,77,79,84,90,91], photocatalysts [56,69-71,86,92], oxidation catalysts [83] and catalyst supports [76,79]. The use of mesoporous transition metal oxides is an interesting approach for the development of solid acid catalysts with enhanced activity. The mesopores in the oxide enable the reactants to access additional active acid sites, resulting in improved rates of acid catalysis.

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3.1 Mesoporous Nb2O5·nH2O Oxides Different from bulk Nb2O5, hydrated niobium oxide (Nb2O5·nH2O; niobic acid) possesses both Lewis acid sites and relatively strong Brønsted acid sites, stable and reusable even in the presence of water, on its surface. Kondo et al. have reported the synthesis of supermicroporous Nb2O5·nH2O (ca. 1.5 nm) [101,102] using an amphiphilic block copolymer templating route and subsequent acid catalysis reactions. A water washing treatment for template removal instead of removal by calcination afforded a highly hydrated supermicroporous material suitable for gas-phase esterification and dehydration reactions. Recently, Nakajima et al. [103] reported that mesoporous Nb2O5·nH2O function as a recyclable and highly active solid acid catalyst for hydrolysis of cellobiose. To obtain the porous Nb2O5·nH2O samples, the template were removed from the aged gel by solvent-extraction treatment. After washing with hot water repeated four times, the samples were dried in air overnight at 373 K. [62,63] Supermicroporous (1.5 nm) and mesoporous (1.8-10.0 nm) Nb2O5·nH2O with different pore sizes were successfully prepared using different amphiphilic block copolymers as structure-directing agents (P85 for microporous oxide and L64, P103 and P123 for mesoporous oxide). FT-IR analyses using CO as a basic probe molecule indicated that the acid strength of both Lewis and Brønsted acid site for mesoporous and supermicroporous Nb2O5·nH2O are identical to that for bulk Nb2O5·nH2O. Mesoporous Nb2O5·nH2O with large pores exhibits higher catalytic performance for the hydrolysis of cellobiose (a hydrophilic reaction) than bulk Nb2O5·nH2O because of facile incorporation and diffusion of the hydrophilic reactants in the hydrophilic mesopores.

3.2 Mesoporous Nb Sulfated Oxides

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The pure mesoporous Nb oxide (H0 = -6.6) [61] exhibits a higher Hammett acidity and has about 10 times of the number of acid sites compared to bulk niobia (H0 = +3.3), resulting in an activity ca. 8 times higher than that of the sulfated bulk material for benzylation of anisole with benzyl alcohol in the liquid phase at reflux temperature. Since phosphonation and sulfonation of bulk Nb oxides are known to enhance the acid properties, same treatments have been studied for the applications of mesoporous Nb oxide as solid acid catalysts. [61,84] Pure mesoporous Nb samples obtained by using a ligand-assisted liquid crystal templating method treated with 1 M sulfuric acid or phosphoric acid methanol solution show considerable enhance on catalytic activity for benzylation of anisole. The Hammett acidity of the both treated mesoporous Nb oxide increased to H0 = -8.2, and the catalytic activity of the sulfated mesoporous Nb oxide increased remarkably, with 100% conversion obtained in only 30 min for the benzylation of anisole with benzyl alcohol in the liquid phase at reflux temperature. This activity is ca. 200 times greater than that of the sulfated bulk Nb oxide. The use of recycled catalyst remains a challenge due to leaching of sulfate species, similarly to a problem also reported for mesoporous silica and organosilicas bearing sulfonic acid groups. Unlike mesoporous silica and alumina, the acid treated mesoporous Nb oxide demonstrated retention of its mesoporous structure under low pH conditions. The FT-IR analysis of the spectra of pyridine adsorption exhibited a strong dominance of Brønsted acid sites for sulfated and phosphated mesoporous Nb oxide. In addition, the n-butylamine titration experiments showed that sulfated mesoporous Nb possesses 10 times more total Lewis and Brønsted acid sites (31.78 mmol g-1) than the phosphated (3.086 mmol g-1) mesoporous Nb oxides, and almost 100 times more acid sites than the bulk samples (0.024– 0.338 mmol g-1).

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3.3 Nb-incorporated Mesoporous Silicas Mesoporous silica, MCM-41, is known to generate various active sites by substitution of silica by other metals. Many efforts have been devoted to the introduction of Al into the MCM-41 framework, which gives rise to the formation of Brønsted acid sites. Ziolek et al. [104] reported study about substituting Si for niobium to obtain a mesoporous Nbincorporated solid acids. The strength of Brønsted acid sites was higher on the protonated (H) Al-sample than on the protonated Nb-material, which was concluded on the basis of the FT-IR analysis using pyridine and the activity in the test reactions of cumene cracking. H, Nb-MCM-41 materials were inactive in the cumene cracking, which requires the presence of strong Brønsted acid sites, whereas the H, Al-MCM-41 catalyst was active in this reaction. H, Nb-MCM-41 mesoporous materials exhibited lower acidity and relatively higher Lewis acid sites than that of commonly used H, Al-samples. These characters are important in the application of these materials as a support or catalyst. The low acidity with high concentration of Lewis acid sites allowed it to obtain a higher selectivity in the catalytic synthesis of methanethiol from CH3OH and H2S on H, Nb-MCM-41 than that of H, Al-MCM-41.

3.4 Mesoporous NbxW(10-x) Oxides

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Different from previous commented mesoporous Nb solid acids, mesoporous Nb-W mixed oxides afford very high catalytic performance in Friedel-Crafts alkylation, hydrolysis, and esterification, which originated from the mesoporous structure and different acid properties formed by specific Nb and W concentrations. It is obtained by simple ligandassisted liquid crystal templating method, using a 1-propanol as solution, P-123 as a SDA and Nb and W chlorides.

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Structures of Mesoporous NbxW(10-x) Oxides SEM and TEM images of the prepared porous oxides are shown in Figure 18. The mesoporous Nb oxide has a hexagonally structured mesopores, which is consistent with previous studies[81-84]. The mesoporous Nb7W3, Nb5W5, and Nb3W7 oxides have wormholetype mesopores, and no mesoporous structure is observed in Nb1W9 or W oxides.

Figure 18. SEM images of mesoporous a) Nb, b) Nb7W3, c) Nb5W5, and d) Nb3W7 oxides and nonmesoporous e) Nb1W9 and f) W oxides and TEM images of mesoporous h) Nb, i) Nb7W3, j) Nb5W5, and k) Nb3W7 oxides.

The presence of mesopores is also observed by the N2 sorption isotherms (Figure 19) for the same samples (x = 3 to 10). The surface areas are estimated using the Brunauer-EmmettTeller (BET) method, and pore volumes are obtained by the Barrett-Joyner-Halenda (BJH) method. Although the surface area decreases gradually from 200 (mesoporous Nb oxide) to 52 m2 g1 (non-mesoporous W oxide) with increasing addition of W, up to x = 0, the pore volume decreases up to x = 3 (Figure 20). Then, the pore volumes increase in the non-

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mesoporous W-rich oxides (x = 0 to 2) due to the formation of void spaces between particles. The pore diameter obtained by the BJH method decrease from 7 (mesoporous Nb oxide) to 4.2 nm (mesoporous Nb3W7 oxide) with increasing W content.

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Figure 19. N2 sorption isotherms of a) Nb, b) Nb9W1, c) Nb8W2, d) Nb7W3, e) Nb6W4, f) Nb5W5, g) Nb4W6, h) Nb3W7, i) Nb2W8 oxides and non-mesoporous j) Nb1W9 and k) W oxides.

Figure 20. BET surface areas and mesopore volumes of mesoporous NbxW1-x oxides.

Correlation between the initial stoichiometry and the resulting composition of the products measured by energy dispersive x-ray (EDX) spectroscopy analysis of Nb and W

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were very close to the initial stoichiometry, within 1 % of differences for almost all samples. However, considerable standard deviation are observed for non-mesoporous W rich Nb2W8 (4 %) and Nb1W9 (5 %) oxides. The lack of uniformity is observed for samples with excess of W led to the formation of non-uniform structure by the calcination of the material at 673 K for template removal, which induce the aggregation and crystallization of pure WO3. The aggregation and crystallization result in the destruction of the original mesoporous structure and the development of larger pores (between 5.4 and 21.5 nm) for W rich NbxW(10-x) oxides (x = 0 to 2) as inter-particle voids. The addition of transition metal Nb to the W oxide improve the thermal stability of the material in the amorphous phase by elevating the crystallization temperature beyond that required to completely remove the mesoporous template (673 K). Same process are observed for mesoporous TiO2 oxides.[72]

Acid Properties of Mesoporous Metal Oxides

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FT-IR The acid properties of mesoporous NbxW(10-x) oxides are evaluated by probing the vibrational frequencies of adsorbed pyridine using Fourier transform infrared (FT-IR) spectroscopy. FT-IR spectra for pyridine adsorbed mesoporous NbxW(10-x) oxides are shown in Figure 21. The W-containing samples have both of Brønsted acid sites and Lewis acid sites whereas Nb oxide sample has negligible Brønsted acid sites. The FT-IR spectra indicate that the peak intensity at 1532 cm-1, attributed to pyridinium ions formed on strong Bronsted acid sites,[23,24,105-107] enhances by increasing the W content. The Brønsted acid sites peak intensities increase twice from mesoporous Nb7W3 (2.5 %) to Nb5W5 (5.0 %) oxide and more than twice from mesoporous Nb5W5 (5.0 %) to Nb3W7 oxide (13.0 %).

Figure 21. FT-IR spectra for pyridine adsorbed mesoporous a) Nb, b) Nb 7W3, c) Nb5W5, and d) Nb3W7 oxides. (B = Brønsted acid site; L = Lewis acid site) Assignments: 1590 cm1 (strong Lewis acid site), 1535 cm1 (strong Brønsted acid site), 1485 cm1 (very strong Brønsted acid site or strong Lewis acid site), 1440 cm1 (very strong Lewis acid site).

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P MAS NMR The acid properties of mesoporous NbxW(10-x) oxides are also evaluated by 31P MAS NMR, using trimethylphosphine oxide (TMPO) as a probe molecule. As the 31P chemical shifts of protonated TMPO (i.e., TMPOH+) tended to move downfield, higher chemical shifts indicated higher protonic acid strength. The 31P NMR spectra of mesoporous NbW oxides are shown in Figure 22. A total of 0.8 mmol of TMPO was adsorbed per gram of mesoporous NbxW(10-x) oxide. Mesoporous Nb, Nb7W3, and Nb5W5 oxides have two principal peaks: a broad peak at 65–70 ppm and a sharp peak at 39 ppm. The latter is ascribed to physisorbed TMPO.[46] Mesoporous Nb3W7 has three peaks indicating acid strength: a main peak at 75 ppm comparable in strength to H-Beta zeolite (78 ppm),[53] another peak at 63 ppm comparable in strength to HY zeolite, (65 ppm),[46] and a distinct small sharp peak at 86 ppm stronger than that of ion-exchange resin (81 ppm for Amberlyst-15) and comparable in strength to those of strongly acidic zeolites (86 ppm for HZSM-5 [54] and HMOR [53]) and sulfated zirconia.[108] From the 31P MAS NMR results, an enhancement of acid strength is observed in mesoporous NbxW(10-x) oxides, with shifts of the main peaks from 67 ppm (x = 7) to 70 ppm (x = 5) or 75 ppm (x = 3).

Figure 22. 31P MAS-NMR spectra for TMPO-adsorbed (TMPO/catalyst: 0.8 mmol/g) mesoporous a) Nb, b) Nb7W3, c) Nb5W5, and d) Nb3W7 oxides, measured at room temperature. The spinning rate of the sample was 10 kHz.

The acid strength of HY zeolite (65 ppm) is evaluated to H0 = -6.6.[109] Zheng et al. proposed that 31P chemical shift of adsorbed TMPO above 86 ppm is attributed to superacidity of solid acid (H0 < -12) by theoretical calculation.[110] Therefore, mesoporous NbW oxides have a range of acid strength between -12 ≤ H0 < -6.6. The total acid amounts are also estimated from the NMR peaks assigned to adsorbed TMPO. The obtained acid amounts are 0.30 mmol/g for Nb3W7, 0.36 mmol/g for Nb5W5, and 0.39 mmol/g for Nb7W3. The increase in acid amount corresponds to the increase in surface area, indicating that the acid density of mesoporous NbxW(10-x) oxides is constant.

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Acid Catalytic Activity The acid catalyzed reactions are first tested on the mesoporous NbxW(10-x) oxides using liquid-phase Friedel-Crafts alkylation of anisole with benzyl alcohol, and hydrolysis of sucrose (a disaccharide composed of glucose and fructose) in water. A plot of the product yield of mesoporous NbxW(10-x) oxides with different Nb and W content in the Friedel-Crafts alkylation of anisole and the hydrolysis of sucrose, reacted for 1h at 373 and 353 K, respectively, is shown in Figure 23. Variation of Nb and W content results in remarkably different reaction rates of benzylanisole formation in the alkylation. The reaction rates increase gradually with increasing W content, starting from a 0% yield for mesoporous Nb oxide and reaching the highest yield (94%) for mesoporous Nb3W7 oxide. The yield decreases drastically for non-mesoporous oxide samples with x from 2 to 0, reaching 0% for W oxide. The same curve is plotted for the hydrolysis of sucrose, obtaining the highest yield (65%) for mesoporous Nb3W7 oxide. These results indicate the importance of the mesoporous structure to the reaction, and demonstrate the drastic changes in the nature of the acid sites.

Figure 23. a) Friedel-Crafts alkylation of anisole and b) hydrolysis of sucrose results for mesoporous NbxW(10-x) oxides. (Reaction conditions: anisole (100 mmol), benzyl alcohol (10 mmol), catalyst (0.2 g), 373 K, 1 h, and sucrose (0.5 g, 1.46 mmol), H 2O (10 mL, 556 mmol), catalyst (0.1 g), 353 K, 1 h.).

The acid catalytic activity of mesoporous Nb-W oxides is compared to that of conventional solid acids. The rate of glucose production and turnover frequency (TOF) for hydrolysis of sucrose over several solid acid catalysts are shown in Table 4. The hydrolysis of saccharides requires sufficient acid strength, and is an important class of reaction, used to convert biomass into bioethanol and other useful chemicals with minimal environmental impact.[50,111-116] Ion-exchange resins like Amberlyst-15[116] are powerful catalysts for the hydrolysis of saccharides due to strong sulfonic acid sites, and niobic acid is a unique solid acid resistant in water solution.[5,7,8,17,117] The activity of mesoporous Nb3W7 oxide, however, substantially exceeds the maximum performance of any of the other solid acid, achieving a glucose formation rate of 11.9 mmol g1 h1. This reaction rate is significantly higher than that of niobic acid or H-ZSM5, and six times that of Nafion NR50 and 2 times

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that of Amberlyst-15. Moreover, the turnover frequency of mesoporous Nb3W7 is over 15 times that of Nafion NR50 and Amberlyst-15. The mesoporous Nb3W7 is recoverable by filtration and washing with water to remove residue, and the material is confirmed to be reusable with no change in activity and structure investigated by XRD analysis and SEM after three reuse cycles. The catalyst used in the first and third runs has glucose yields at 2 h of 85.9 and 84.1%, respectively. The mesoporous Nb3W7 oxide also exhibits a reaction rate and turnover frequency 2 times that of bulk Nb3W7 oxide, indicating that the mesoporous structure enhances the reaction rate of the accessible acid sites. Table 4. Hydrolysis of sucrose and cellobiose over several solid acid catalysts

Catalysts

Hydrolysis of cellobiose b

Rate of glucose production / mmol g-1 h-1

TOF / h-1

Rate of glucose production / mmol g-1 h-1

TOF / h-1

mesoporous Nb3W7

0.30e

11.9

39.7

0.42

1.40

mesoporous Nb5W5

0.36e

6.7

18.6

0.10

0.28

mesoporous Nb7W3

0.39e

4.9

12.6

0.06

0.14

0.4

0.3

0.6

0

0

1.0

0.1

0.1

0

0

H-ZSM5 d

0.2

0.1

0.7

0

0

Amberlyst-15

4.8

6.1

1.4

0.22

0.05

Nafion NR50

0.9

2.0

2.4

0.16

0.18

H2SO4

20.4

45.6

2.2

6.73

0.33

Nb2O5•nH2O H-Beta

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

Acid amount /mmol g-1

Hydrolysis of sucrose a

c

Blank 0 0 0 0 a Reaction conditions: sucrose (0.5 g, 1.46 mmol), H2O (10 mL, 556 mmol), catalyst (0.1 g), 353 K. b Reaction conditions: cellobiose (0.5 g, 1.46 mmol), water (5 ml, 278 mmol), catalyst (0.1 g), 368 K. c SiO2/Al2O3 = 25, JRC-Z-HB25. d SiO2/Al2O3 = 90, JRC-Z-5-90H. e Determined by 31P NMR.

The acid amount and surface area of the tested solid acids, and the results of the FriedelCrafts alkylation of anisole are summarized in Table 4. Non-porous Nb2O5-WO3 [23] and HNbWO6 nanosheet aggregates, obtained by exfoliation of layered HNbWO6,[118] are also used for comparison. The mesoporous Nb3W7 oxide exhibits the highest performance in this Friedel-Crafts alkylation reaction, obtaining the highest yield and turnover frequency. After the reaction, ortho-benzylanisole, para-benzylanisole and dibenzylether are formed. The selectivity of ortho-benzylanisole via para-benzylanisole observed for mesoporous NbxW(10-x) oxides (Table 4) at Friedel-Crafts alkylation gradually increases with increasing the W content (36.3, 40.3 and 42.4 % for mesoporous Nb7W3, Nb5W5 and Nb3W7 oxides, respectively). Same behavior on selectivity of o-benzylanisole are observed for nonporous NbxW(10-x) oxides (Nb7W3, Nb5W5 and Nb3W7 oxides). The variation on selectivity is caused not by mesoporous structures but by the variation on acid property (Brønsted acid and Lewis acid sites) of Nb and W concentrations. The selectivity of dibenzylether, a by-product of

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73

benzyl alcohol, is 18, 13 and 11% for mesoporous Nb7W3, Nb5W5 and Nb3W7 oxides respectively. These results are consistent with the results obtained by FT-IR, which increase on Brønsted acid sites 1532 cm-1 promotes decrease on dibenzyl ether selectivity. However, dibenzyl ether is also a good alkylating agent and its concentration decreases as it is all consumed together with the benzyl alcohol at the end of the alkylation reaction. Mesoporous NbxW(10-x) oxides are also tested for hydrolysis of cellobiose (Table 5). The subunit of cellulose consists of β-1,4-glycosidic bonds, which are much more stable than the α-1,2-glycosidic bonds comprising sucrose, and are thus more resistant to hydrolysis. [50,113-116] Accordingly, the rate of glucose production by hydrolysis of cellobiose is much lower than that of sucrose over all of the acid catalysts tested. Nevertheless, mesoporous Nb3W7 oxide exhibits the highest rate of glucose production among the solid acids tested. Table 5. Friedel-Crafts alkylation of anisole over several solid acid catalysts. a Catalyst mesoporous Nb3W7 mesoporous Nb5W5

Acid amount / mmol g-1

Yield / %

Selectivity of obenzylanisole /% b

TOF / h-1

132

0.30 e

94, 66g

42.4

221

0.36

e

51

40.4

71

e

7

36.3

9

166

mesoporous Nb7W3

174

0.39

mesoporous Nb

193

0.54e

Non-porous Nb3W7 Non-porous Nb5W5

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SBET / m2 g-1

51 38

N.D.

N.D.

N.D.

0.27

f

58

42.2

107

0.17

f

12

40.3

35

f

9

38.9

30

1

39.6

2

5

28.4

7

Non-porous Nb7W3

62

0.15

Nb2O5•nH2O

128

0.3

HNbWO6 nanosheets

e

66

0.34

326

0.2

9

40.4

22

420

1.0

31

42.1

15

Amberlyst-15

50

4.8

42

45.0

4

Nafion NR50

0.02

0.9

42

49.2

24

H-ZSM5 H-Beta

c

d

Blank 0 0 0 0 a Reaction conditions: anisole (100 mmol), benzyl alcohol (10 mmol), catalyst (0.2 g), 373 K, 1 h. b selectivity = (o-benzylanisole)/(o-benzylanisole+p-benzylanisole)*100 c SiO2/Al2O3 = 90, JRC-Z5-90H. d SiO2/Al2O3 = 25, JRC-Z-HB25. e Determined by 31P NMR. f Determined by NH3-TPD g Reaction time 30 min. N.D. – not detected.

3.5 Conclusion A variety of mesoporous Nb based solid acid catalysts have been described. The pure mesoporous Nb oxide (H0 = -6.6) exhibits a higher Hammett acidity and has about 10 times the number of acid sites compared to bulk niobia (H0 = +3.3). By substituting the template

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removal method from calcination treatment to a simple water washing treatment, a mesoporous Nb2O5·nH2O oxide with strong water tolerant acid sites and high catalytic activity for hydrolysis is obtained. The mesoporous Nb oxide treated with sulfuric acid and phosphoric acid showed remarkable activities for benzylation led to the strong dominance of Brønsted acid sites for both samples. The use of recycled catalyst remains a challenge for acid treated samples due to leaching of sulfate and phosphate species, a problem also reported for mesoporous silica and organosilicas bearing sulfonic acid groups. Although Nb-incorporated mesoporous silica presented lower activity and acidity than Al-incorporated mesoporous silica, the high concentration of Lewis acid sites allowed to obtain a high selectivity for synthesis of methanethiol. Worm-hole type mixed metal oxide solid acid catalysts, mesoporous NbxW(10-x) oxides, were found to function as recyclable, highly-active for Friedel-Crafts alkylation, hydrolysis, and esterification. The reaction rate and acid strength increased gradually with the addition of W, reaching the highest reaction rate with mesoporous Nb3W7 oxide, which exceeded the reaction rate of ion-exchange resins, zeolites, and non-mesoporous metal oxides. The very high catalytic performance of mesoporous Nb3W7 oxide was attributed to a high surface area mesoporous structure, strong acid sites comparable in strength to those of strongly acidic zeolites (HZSM-5 and HMOR), and the formation of strong Brønsted acid sites by the isomorphous replacement of Nb5+ by higher-valence W6+ cations in W-enriched samples Higher concentrations of W oxide deformed the mesoporous structure, decreasing the reaction rate. Mesoporous Nb3W7 oxide enabled both a high reaction rate and reusability, two essential characteristics of solid acids for industrial application.

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ISBN: 978-1-61122-895-3 © 2011 Nova Science Publishers, Inc.

Chapter 3

NIOBIUM OXIDE COMPOUNDS: SYNTHESIS, PROPERTIES AND APPLICATIONS Maria Lucia Caetano Jardim Pinto da Silva* and Rafael Caetano Jardim Pinto da Silva‡ Universidade de São Paulo - Escola de Engenharia de Lorena, São Paulo, Brazil

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ABSTRACT In the past decade an increasing interest in niobium-containing materials was observed and its related to their multiple uses in the production of high-tech materials. Among niobium compounds, the niobium oxides are by far the most relevant and important ones in terms of industrial and research applications. Niobium oxides shaped as thin films, porous nanoparticles and nanowires have important application in various fields including catalysis, ion-exchange, adsorption, enzyme immobilization, highly sensitive sensors, photo/electrochemical luminescent devices, optical fibers and electro-optical device, piezoelectric materials, high-power batteries, biocompatible etc. It‟s well-know that unique physicochemical properties and consequently their applications depend upon the way they are tailored. Thus, to attend these quite different demands, many methods have been used for synthesis of niobium oxide materials like sol-gel, molten salts, co-precipitation, hydrothermal process, precipitation in homogeneous solution, water-in-oil micro emulsion precipitation and so on. Once there are many variables playing the role in defining niobium oxides characteristics, the aim of this chapter is to provide the first oriented review of the main niobium oxide characteristics and reported synthesis methods described in the literature linking them to the desired properties and the envisaged materials application.

* Universidade de São Paulo - Escola de Engenharia de Lorena, Departamento de Engenharia Química - Campus I, P.O. Box 116, Lorena, São Paulo, Brazil, [email protected] , +55 (12) 31595079 ‡ Universidade de São Paulo - Escola de Engenharia de Lorena, Departamento de Materiais - Campus II, P.O. box 116, Lorena, São Paulo, Brazil, [email protected] , +55 (12) 31595111

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80 Maria Lucia Caetano Jardim Pinto da Silva and Rafael Caetano Jardim Pinto da Silva

INTRODUCTION The past two decades have a brought an increasing interest in niobium-containing materials, which can be applied within many fields. The development of studies within synthesis, characterization and application of Nb-containing materials in high-tech applications was possible thanks to the great progress of the spectroscopic as well as the other physical techniques which allow the analysis of Nb state in the solids. As Niobium compounds exhibit special properties not shown by the compounds of neighboring elements of niobium in the periodic table in this chapter it will be reviewed the in part A, the niobium oxides general physicochemical properties and polymorphism, and in part B, the summary of niobium oxide synthesis methods related to envisaged applications.

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PART A – OCURRENCE, STRUCTURES AND PHYSICOCHEMICAL PROPERTIES Niobium has small relative abundance in Earth‟s crust occupying the 32nd with 20ppm and as his periodic table neighbor‟s, presents many oxidation states ranging from (-I) to (+V), being the last on the most stable and probable one [1]. Moreover, niobium does not occur in a free state and has strong geochemical affinity with tantalum, they are restrictly associated and encountered together in the majority of rocks and minerals were they are found. In nature there are more than 90 known mineral species for niobium and tantalum, among then we can highlight the following chemical compositions: columbite-tantalite (Fe, Mn) (Nb, Ta2)O6, with up to 76% of Nb2O5 content; pyrochlore (Na3, Ca)2 (Nb, Ti)(O, F)7, with up to 71% of Nb2O5; bariumpyrochlore (Ba, Sr)2 (Nb, Ti)2 (O, OH)7, with up to 67% of Nb2O5; loparite(Ce, Na, Ca)2 (Ti, Nb)2 O6, with up to 20% of Nb2O5 and pandeite (Ba, Sr)2 (Nb, Ti, Ta)2 (O, OH, F)7 [2]. Columbite-tantalite and pyrochlore are the main source of niobium in the world, and are frequently referred as Nb2O5 in the market records for simplification [2]. Columbite-tantalite occurs associated to pegmatites, the final terms of granitic magma that do not enter initially in the crystallized rock. Moreover, it is plutonic rock, cooled slowly below the surface (the volcanic magmas such as basalts, spill and cool quickly), giving rise to crystals that can reach meters in length and can exceed 20 mm. In pegmatites a range of minerals can be found such as quartz, feldspar, mica, rare earths, gems, i.e. topaz and aquamarine, in addition to minerals such tin as cassiterite and tungsten as wolframite, besides the columbite-tantalite. The pyrochlore occurs in carbonatite associated with alkaline intrusions from the upper cretaceous of the Mesozoic era. The carbonatites often contain one or more of the following ores: niobium, nickel, copper, titanium, vermiculite, apatite (phosphate), rare earth, barite, fluorite, in addition to nuclear mineral thorium and uranium. There is some disagreement among experts on the subject of the origin of carbonatites, but the most accepted version is that they are related to alkaline rocks, including kimberlites (associated with the occurrence of diamonds) [1-2] Besides the relative poor abundance, niobium ores are found in few regions, being Brazil the leader in the niobium world‟s mine reserves with 98,19%, followed by Canada with 1,35% and Austrália 0,46% [3].

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Once the niobium oxide physicochemical properties as well as it‟s polymorphisms were well and extensively disclosed in part of the Nowak & Ziolek [4] work, the following texts of the remaining part A will consist in reprints and adaptations with permission from ref.[4] (copyright 1999 American Chemical Society) with some complementary and updated information.

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Niobium Oxides and it’s Physicochemical Properties Niobium pentoxide is a white, air-stable, water-insoluble solid. Nb2O5 is attacked by concentrated HF and dissolves in fused alkali. It may be described as amphoteric but is more characteristically inert. Its structure is extremely complicated and displays extensive polymorphism, which will be discussed further in this chapter. Nb2O5 is probably comprised of NbO6 octahedra connected by edges and corners. Since dissociation becomes discernible even at 1150°C , oxygen defects may be produced upon heating the Nb2O5 in a direct flame. This results in a yellow form that reverts to white Nb2O5 upon cooling in air [4]. Hightemperature reduction (800-1300°C) of Nb2O5 with hydrogen gives the bluish-black dioxide NbO2 that has a distorted rutile structure , show diamagnetic characteristics and reversible. The NbO2 structure only exists when the oxygen ratio is maintained close to 2. Thus, an oxide of the composition NbO2.09 shows X-ray diffraction lines that are characteristic of the pentoxide, even though it contains only a small excess of oxygen. NbO2 is insoluble in water and is a very strong reducing agent in the dry state [5]. Between Nb2O5 and NbO2, there exists a homologous series of structurally related niobium oxide phases having the general formula Nb3n+1O8n-2 where n = 5, 6, 7, 8. In addition, oxides of formula Nb12O29 (12Nb2O5-2O) and Nb94O232 (47Nb2O5-3O), which are stoichiometrically related to Nb2O5, have been reported. These materials possess paramagnetic properties [6]. Further reduction of Nb2O5 (13001700°C) produces the gray monoxide NbO that has a cubic structure and metallic conductivity but differs markedly from its vanadium analogue in that its composition ranges only from NbO0.982 to NbO1.008. The diffraction lines of NbO2 begin to appear in NbO1.04, while the oxides NbO0.94 and NbO0.87 show diffraction lines that are characteristics of the metal. The structure of NbO is a unique variant of the rock salt NaCl structure in which there are Nb vacancies at the eight corners of the unit cell and an O vacancy at its center as shown in Figure 1.

Figure 1. NbO Crystal structure, where Nb and O are represented by gray and red balls, respectively.

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Therefore, the structure could be described as a vacancy-defect NaCl structure Nb0.75+0.25O0.75-0.25. However, since all the vacancies are ordered, it is better to consider NbO as having a new structure type in which both the Nb and O form four coplanar bonds. The central feature of this structure is a 3D framework of Nb6 octahedralclusters (Nb-Nb 298 pm, cf. Nb-Nb 285 pm in Nb metal), which accounts for the metallic conductivity of the compound. The topological relationship between the NbO structure and SOD-type (zeolite) nets has been recognized [7]. The hydrated pentoxide (niobic acid) is obtained as a white precipitate with indeterminate water content when water-soluble complexes of the metal are hydrolyzed or when a solution of niobate is acidified. It is most commonly prepared by fusing the anhydrous pentoxide with 5-10 times its weight of alkalimetal pyrosulfate or hydrogen sulfate. This forms a soluble sulfato complex that is then leached with sulfuric acid or with a solution of another complexing agent such as oxalic acid. The hydrous oxide is then precipitated by diluting and boiling the solution in sulfuric acid or by adding ammonia to the oxalic acid solution [5]. Niobic acid is an insoluble polymeric oxide, and it seems probable that polymerization takes place through the intermolecular elimination of water between units such as Nb(OH)5 or NbO(OH)3. Different types of niobium oxide ionic exist in aqueous solutions and the solution pH as well as the niobium oxide concentration and tempereture determines the specific niobium ionic species present, as can be seen in the Figure 2 [8].

Figure 2. Pourbaix diagram for the Nb–H2O system at 25, 75 and 95°C. [ Reprinted from “Corrosion of niobium in sulphuric and hydrochloric acid solutions at 75 and 95°C”, E. Asselin et al. / Corrosion Science 49 (2007) 694–710, with permission from Elsevier] .

Niobium oxide compounds generally possess an octahedrally coordinated NbO6 structure that is distorted to different extents depending on whether its polyhedra are corner- or edgeshared. Occasionally, NbO7 and NbO8 structures can also be found in niobium oxide phases.

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After the Nb2O5.nH2O precipitate is dried at 120°C for 2 h, it possesses features very similar to amorphous Nb2O5 in which there are slightly distorted NbO6, NbO7, and NbO8 sites [4]. In addition, Nb2O5.nH2O also possesses a small number of highly distorted NbO6 sites. The Nb5+ cation is very large and has difficulty fitting into an oxygen anion tetrahedron. Thus, only a few niobium oxide compounds like YNbO4, YbNbO4, LaNbO4, and SmNbO4 possess a tetrahedrally coordinated NbO4 structure that is similar to the scheelite-like structure. Jehng and Wachs [9] also reaveled that the interaction of niobium oxide with basic surfaces of a support results in the formation of highly distorted NbO6 octahedra, while its interaction with acidic surfaces result in the formation of slightly distorted NbO6, NbO7, and NbO8 groups. The dehydration process further distorts the already highly distorted NbO6 octahedra due to the removal of coordinated water but does not perturb the only slightly distorted NbO6 octahedra. The highly distorted NbO6 octahedra possess Nb=O bonds and are associated with Lewis acid sites [10]. In contrast, the slightly distorted NbO6 octahedra as well as the NbO7 and NbO8 groups only possess Nb-O bonds and are associated with Brønsted acid sites. Lewis acid sites are present in all supported niobium oxide systems, but the Brønsted acid sites are limited to the Nb2O5/Al2O3 and Nb2O5/SiO2 systems [9]. Hydrated niobium oxide (Nb2O5.nH2O; niobic acid) has a high acid strength (Ho equal to -5.6 up to approx. -8.2). It remains bound to the support surface even in the presence of water and acts as an effective catalyst for reactions in which water molecules participate or are liberated [11]. It also has been reported [12,13] that the acidity of niobic acid increases upon treatment with sulfuric or phosphoric acid. Generally, hydrated niobium oxide crystallizes at 580°C, and its strong acid property disappears when it is heated to temperatures higher than 530°C [14-16]. To sum up, Nb2O5.nH2O possesses both Lewis acid sites (which increase with increasing pretreatment temperatures up to 500°C and then decrease at higher temperatures) and Brønsted acid sites (which are most abundant at 100°C and decrease at higher temperatures) on its surface [17]. Fraissard et al. [18] reported that the theoretical number of protons per gram of hydrated niobium pentoxide at 300 °C (H2Nb6O16) is 1.6 x 1021, which indicates that niobic acid contains two hydroxyl groups per unit cell. As the number of coordinated water molecules increases, the number of Brønsted acid sites decreases, the concentration of hydronium ions (H3O+) increases, and the formation of H2O∙∙∙HO species is observed. In materials containing more than 2 water molecules per H2Nb6O16, there are no longer any Brønsted acid sites, and the water molecules do not interact with the surface of the niobic acid. However, there is a linear correlation between the concentration of hydronium ions and the number of the adsorbed water molecules [4]. The H3O+ concentration becomes constant when the number of adsorbed water molecules per Brønsted acid sites (nH2O/ Brønsted acid sites) is qual or higher than 5.3. The ionization coefficient is ca. 0.2 when nH2O/ Brønsted acid sites equal to 1, so at this water content the acidity of niobic acid is comparable to that of NH4Y (Si/Al equal to 2.4) and the dealuminated form of ammonium Ytype zeolite (Si/Al equal to 4.4). At saturated hydration, 50% of niobic acid‟s acid sites are ionized, revealing that only one of the two acid sites is particularly strong [4].

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Niobium Pentoxide an it’S Polymorphism

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Nb2O5 possesses a quite complex crystalline morphology and about 20 different structures have been identified [19]. The phases most often found have been labeled and an extensive review of the 20 Nb2O5 structures with their corresponding production methods, structural parameters and density have been done recently by Reznichenko et al [20]. However, the most common structures will be reviewed below in order to facilitate the understanding of changes in physicochemical properties among different Nb2O5 structures. The amorphous Nb2O5 begins to crystallize in a “low-temperature” form called the T form, from the German “tief” for low, at about 500°C. Crystallization occurs more rapidly at higher temperatures until about 830°C, where upon a transition to a “moderate-temperature” (M) form becomes apparent [6]. This transition continues more rapidly at higher temperatures and heating for 4 h at 1000°C brings about a complete conversion. At even higher temperatures, a third transformation to a “high-temperature” (H) form has been reported [6,21]. These polymorphic transitions take place slowly, at temperatures which are not welldefined, and are irreversible. Reisman & Holtzberg [22] have suggested that, although these three crystal modifications, (which have been re-designated as γ = T, β = M, α = H) exist, the β- and α-phases are essentially the same, and the transition therefore is to be regarded as occurring from γ to α. A further high-temperature metastable ε-phase has been observed to crystallize from molten Nb2O5 without supercooling and to transform spontaneously and exothermically to the α-modification at any temperature between 830 and 1200°C. In fact, the only stable phase between 830 and about 1400°C is the α-form. A low temperature δ-phase, which was described in Frevel and Rinn work [23], was later shown to be a poorly crystalline variety of the γ-form. H-niobia has a shear structure consisting of blocks of NbO6 octahedra (3 x 4 and 3 x 5) that share corners with octahedra in their own block and edges with octahedra in other blocks [24,25], as shown in Figure 3.

Figure 3. Projection of the H-Nb2O5 structure along the b axis. Squares represent the projections of NbO6 octahedra linked by the corners. Note the upper blocks containing 3 × 4 = 12 (white gray) octahedra, and the lower blocks containing 3×5 = 15 (dark gray) octahedra. The black dots represent Nb atoms in tetrahedral sites along channels parallel to the b axis.

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One of the 28 Nb atoms in each unit cell is present in a tetrahedral site, which occurs at some block junctions, as depicted by the black circles in Figure 3. The T phase has a completely different structure, as shown in Figure 4.

Figure 4. Projection of the T-Nb2O5 structure parallel to the [001] plane. Black and dark gray small circles represents Nb atoms and large white gray circles represents O atoms [24].

The unit cell contains 42 oxygen atom positions (large open circles). Eight of the Nb ions are present in distorted octahedra, while another eight Nb ions occupy pentagonal bipyramids [25,26]. The remaining 0.8 Nb ion per unit cell is located in interstitial 9-coordinated sites in the unit cell (small open circles). The principal polymorphs of stoichiometric Nb2O5 described above, T(γ), M(β), and H(α), are obtained as polycrystalline products from the thermal treatment of pentoxide in contact with air or oxygen at varying temperatures. Single crystals of these polymorphs cannot be obtained from pure Nb2O5. However, they can be formed by chemical transport reactions in the presence of a halogen, usually chlorine [6]:

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Nb2O5 (s) + 3NbCl5 (g) = 5NbOCl3 (g)

(Equation I)

After this initial step, the pentoxide is enclosed with a suitable amount of pentachloride and chlorine gas in a sealed tube that is maintained in a temperature gradient. By varying the upper and lower temperatures at the respective ends of the tube, it is possible to obtain wellformed single crystals of not only the previously known polymorphs, T(γ), M(β), and H(α), but also of other forms. The primary new forms obtained possess a characteristic and welldefined X-ray powder diffraction pattern and have been designated B (from “Blätter” that means leaflets in german or δ), N (from needles), and P ( from prism or ε) in accordance with their crystal habit [27]. Ko and Weissman reported [28] that the many structures of bulk niobium pentoxide can be grouped into low-temperature and high-temperature forms, with the latter being more ordered. The crystallization behavior of niobium pentoxide, however, is influenced by the starting materials used, impurities that may be present, and any interactions with other components. These interactions are known to affect both the physical (mobility) and chemical (reducibility, acidity) properties of catalytic systems containing niobium pentoxide. Despite this variability, Nb2O5 crystallizes into a low-temperature form (TT or T) at 500°C, a medium-temperature form (M or B) at 800°C, and a high-temperature form (H) at 1000°C [28] when starting from an amorphous phase, as shown in Figure 4.

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Figure 4. Modifications of Nb2O5 as a function of temperature. [ Reprinted from “Structures of niobium pentoxide and their implications on chemical behavior”, E.I. Ko & J.G . Weissman / Catalysis Today, 8 (1990) 27-36, with permission from Elsevier] .

The two low-temperature forms of Nb2O5, TT or T, have long been thought to be the same, because (I) they have similar X-ray diffraction patterns, once some reflections that are split in T niobia-orthorhombic-occur as one peak in TT niobia-monoclinic, and (II) the TT phase does not always form from pure components as starting material. These observations suggest that TT may simply be a less crystalline form of T, stabilized by impurities. The main differences are that some of the oxygen atoms in T are replaced by monovalent species such as F- or Cl- or vacancies these being reported as impurities in TT while the Nb atoms occupy a range of positions between two crystallographically similar sites. The structure of H-Nb2O5 is highly ordered and consists of a sequence of blocks with (5 x 3) or (4 x 3) groups of cornersharing NbO6 octahedra within each block. M-Nb2O5 has a related structure, while the structure of B-Nb2O5 consists of rutile-like ribbons of edge-sharing NbO6 octahedra [29]. All these crystal differences induce to important changes in the physicochemical properties and some of them will be discussed in this chapter. Stoichiometric Nb2O5 is an insulator [30] with, for example, a conductivity of ζ=3x10-6 S/cm for a H-type Nb2O5 single crystal. However, it becomes an n-type semiconductor at lower oxygen content and Nb2O4.978 has a conductivity ζ=3x10+3 S/cm. The conduction band is built from the 3d orbitals of Nb atoms and the valence band from the 2p orbitals of oxygen. The band gap observed from optical measurements varies between 3.2 and 4 eV. The index of refraction n and density ρ both depend on the crystalline phase: for amorphous niobia n was found to vary from 2.0 to 2.26 and ρ from 4.36 to 5.12 g/cm3. For the TT and T phase ρ takes the values of 4.99 and

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5.00 g/ cm3, respectively [1]. Traditionally, niobium pentoxide is used in metallurgy for the production of hard materials or in optics as additive to prevent the devitrification and to control the refractive index of special glasses. In electronics it is used for the preparation of electroacoustic or eletrooptical components such as LiNbO3 and KNbO3 or relaxor ferroelectric ceramics such as Pb(Mg1/3Nb2/3)O3 [31]. However, in the last 15 years, it‟s interesting semiconducting properties and the advent of more sophisticated methods of preparation allowed to obtain in a controlled way of innovative systems such as highly porous materials, very "ne powders and coatings. These new materials have found further important application in the fields of electrochromism, batteries, solar cells and catalysis [32].

PART B – SYNTHETIC STRATEGIES Worldwide studies on ceramics, polymers and metals during this century have resulted in the establishment of materials science as a scientific discipline. A feature of these studies, particularly for ceramics, is their interdisciplinary nature and at the present time chemistry is making an increasingly important contribution to the research, development and manufacture of inorganic materials. Solid-state chemistry has two major roles when applied to ceramics. The first is the synthesis of novel materials and the second is to develop synthesis routes and techniques for their fabrication of these novel materials into useful and controlled shapes with for a wide variety of applications. In this sense, some important techniques being used nowadays to the synthesize niobium oxide materials can be found below.

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Direct Solid-State Reaction Synthesis This classic method of preparation remains the bulwark of direct solid-state synthesis where fine powdered reactants are mixed/grounded togheter and than heated up to procede the chemical conversion. Most ceramic preparations require relatively high temperatures which are generally attained by resistance heating. Electric arc and skull techniques give temperatures up to 3100°C while highpower CO2 lasers give temperatures up to 4100°C. The ceramic method suffers from several disadvantages. When no melt is formed during the reaction, the entire reaction has to occur in the solid state, initially by a phase boundary reaction at the points of contact between the components and later by diffusion of the constituents through the product phase. As the reaction progresses, diffusion paths become increasingly longer and the reaction rate slower. The product interface between the reacting particles acts as a barrier [33]. This type of traditional solid-state chemical synthesis is characterized by high temperatures and long reaction times, being the fundamental reason for the use of high reaction temperatures it‟s diffusion coefficients. In solution, where organic synthesis typically occurs, the diffusion coefficient for species is typically 10-5 cm2/s. Hence, species in their Brownian movement bump into each other frequently. In the solid-state, however are orders of magnitude smaller than this, typically being 10-10 to 10-15 cm2/s [34]. From a practical point of view one is faced with the frustration that the temperature at which most devices operate is significantly lower than the high preparative temperatures. Some caution is necessary in

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88 Maria Lucia Caetano Jardim Pinto da Silva and Rafael Caetano Jardim Pinto da Silva choosing the container; platinum, silica and alumina containers are generally used for the synthesis of metal oxides. The reaction can be speeded up to some extent by intermittent grinding between heating cycles. Serafim et al [35] described a route to obtain amorphous hydrous niobium oxide by sintering niobium pentoxide with a fivefold excess by weight of potassium carbonate at 1000 °C for 6 h followed by addition of hot water and nitric acid (1 mol/dm−3). After drying at 50 °C for 24 h the authors obtained particle sizes in the range of 0.10 - 0.18 mm with a surface area about 42.15 m2 g−1 (BET method). Hydrous niobium oxide obtained from the hydrolysis of an intermediary product from alkaline fusion of Nb2O5 with an excess of K2CO3 have been applied in the adsorption process and enzyme immobilization for biochemical applications [35]. Miranda et al [36] evaluated and optimized the immobilization of Candida Rugosa lipase in hydrous niobium oxide obtained by the Serafim et al method. Under lipase loading of 450Ug−1 matrix , niobium oxide activation with glutaraldehyde at a concentration of 2.5% and pH 8., high activity recovery of 47.21% and esterification yield 86.90% were observed, indicating that hydrous niobium oxide can be a valid alternative to replace high-cost, commercially available inorganic matrices such as controlled pore silica. There is no simple way of monitoring the progress of the reaction in the direct solid-state synthesis method. It is only by trial and error (by carrying out X-ray diffraction and other measurements periodically) that one decides on appropriate conditions that lead to completion of the reaction. Moreover, the products formed at these necessarily high temperatures represent the thermodynamically stable phase. Because of this difficulty, one frequently ends up with mixtures of reactants and products. Separation of the desired product from these mixtures is generally difficult, if not impossible. It is sometimes difficult to obtain a compositionally homogeneous product by the direct solid-state synthesis technique, even when the reaction proceeds almost to completion. In spite of such limitations, ceramic techniques have been widely used for the synthesis of solid materials and classical pathways to overcome these problems with reaction time and temperature have been proposed. The use of Molten salts seems to be the most successful employed one. The term molten salt refers to the liquid state of compounds which melt to give liquids displaying a degree of ionic properties [37]. For this purpose, alkali metal halides and alkali metal nitrates that has relatively lower melting points than metal oxides can behave as a solvent and even as reactant for the last one. Chang et al [38], compared the obtention of NaNbO3 through different direct solid-state methods and investigated the effects of preparation methods on the phase structure, morphology and Raman characteristics of the NaNbO3 particles. The authors compared the conventional mixed-oxide (CMO) and molten salt synthesis (MSS), starting from Na2CO3 and Nb2O5 powders in stoichometric ratio, mixed by ball-milling in ethanol for 12h and heated after dried in 975°C for 6h, differentiating them by adding NaCl to the system in a salt-to-oxide weight ratio of 3:2 for the MSS. The particles synthesized by the MSS method have a similar orthorhombic structure and shape as those prepared by the CMO method, as shown in Figure 5, but they are much larger in size which may result from the small diffusion distances and high mobility of oxides in the NaCl flux.

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Figure 5. Scanning Electronic Microscopy (SEM) images of NaNbO3 particles prepared by conventional mixed-oxides (CMO) and molten salts synthesis (MSS), respectivey [ Reprinted from “Phase structure, morphology, and Raman characteristics of NaNbO3 particles synthesized by different methods”, Y. Chang et al / Materials Research Bulletin 44 (2009) 538–542, with permission from Elsevier] .

Besides the energetic and time-saving advantages offered by reactions in molten salts, readly useful unaggregated powders are more likely to be achieved by using them [33].

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Conventional and Homogeneous Solution Precipitation The main objective of a precipitation reaction is the phase separation of a pure solid phase in a dense and compacted way that could be easily filtered. For a long time, conventional precipitation is used by taking advantage of a small solution supersaturation degree [39] . For this reason, the addition of the precipitating solution agent is made by slow and smooth pouring under stirring. However, in the precipitation in homogeneous solution, the precipitating agent isn‟t physically directly added to the reaction media, but generated through chemical reactions in the media. By this way, the precipitated is formed under conditions that the undesirable side effects of high concentration observed in conventional precipitation are eliminated and more uniform and dense particles are obtained. Besides that, by changing the speed reaction it‟s possible to change the precipitated physical appearance, morphology and size. In simple words, more slowly the reaction is carried out more large particles are obtained [40]. Generally, the chemical reactions employed in this type of synthesis are the thermal decomposition of ammonium carbonate and urea at low temperatures, respectively presented in Equations II and III: 2(NH4)+ + CO3 –2 + H2O CO(NH2)2 + 3H2O

2NH4 + + HCO3 - + OH-

2NH4 + + CO2 + 2 OH-

(Equation II) (Equation III)

Silva et al [41], evaluated the obtention Nb2O5.nH2O by the conventional precipitation (PC) of a NbOF5- aqueous solution having NH4OH as precipitating agent and by Niobium: Properties, Production and Applications : Properties, Production and Applications, Nova Science Publishers, Incorporated, 2011. ProQuest

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homogeneous solution precipitation method (PSH) using the ammonium carbonate decomposition. They reported that Nb2O5.nH2O obtained by the PSH method induced to large particles more likely to go under filtration easily and larger specific surface area ( 91 m2/g for PC and 199 m2/g for PSH method). Both conventional and homogeneous solution precipitation method has been used to synthesize hydrous niobium oxide for ion-exchange and adsorption applications [41-44]. Rodrigues & Silva [43] investigated the phosphate adsorption from aqueous solution onto hydrous niobium oxide prepared by conventional precipitation (PC) of a NbOF5- aqueous solution having NH4OH as precipitating agent. They reported that adsorption equilibrium time was found at 7.0 h, the phosphate adsorption process onto Nb2O5·4H2O obtained after drying at 50°C is thermodinamically favorable and could be best described by the pseudo second-order model. The phosphate removal tended to increase with a decrease of pH and the adsorption behavior followed the Langmuir adsorption isotherm with a maximum adsorption capacity of 13 mg-P g−1. Additionally, phosphate desorbability of approximately 60%was observed with water at pH 12, which indicated a relatively strong bonding between the adsorbed phosphate and the sorptive sites on the surface of the adsorbent. Tagliaferro et al [44] evaluated the cadmium, lead and silver adsorption process in hydrous niobium oxide prepared by homogeneous solution method. In this approach, the Nb2O5.nH2O shown in the Figure 6 was obtained by homogeneous solution precipitation method (PSH) of a NbOF5- aqueous solution using the urea decomposition at 85-90°C in the presence of boric acid.

Figure 6. SEM images of Nb2O5.nH2O obtained by homogeneous precipitation of NbOF5- in urea under 85-90°C [ Reprinted from “Cadmium, lead and silver adsorption in hydrous niobium oxide prepared by homogeneous solution method”, Tagliaferro, G. V. et al. (2011) in press, with permission from Química Nova] .

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They reported an Nb2O5. 3,4H2O well organized crystalline structure with a favorable Langmuir Adsorption isotherms with high values of maximum adsorption capacity of 452.5, 188.68 and 8.85 mg. g-1 for Pb2 +, Ag+ and Cd2 + respectively.

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Coprecipitation Methods The aim in coprecipitation is to prepare multicomponent ceramic oxides through formation of intermediate precipitates, usually hydrous oxides or oxalates, so that an intimate mixture of components is formed during precipitation, and chemical homogeneity is maintained on calcinations, if the ceramic obtaining is desired. In coprecipitation careful control of solution conditions is required to precipitate all cations and thus maintain chemical homogeneity on the molecular scale. This fine-grained ceramic with potential improved mechanical properties due to the increased percentage of atoms in the grain boundary region cannot be obtained by conventional synthesis owing to the high reaction temperature. Several reports can be found in literature to Nb2O5 obtention by this method for catalisys applications due to possibility of obtain highly dispersed niobium oxides sites in supports used in heterogeneous catalisys before calcinations at lower temperatures. According to Wachs et al. [45], the preparation method and niobium precursors do not affect the molecular structures of surface niobia species, but they determine their dispersion. Different niobium oxide precursors and preparation methods have been used for the preparation of supported niobium oxide catalysts such as: (I) aqueous impregnation with niobium oxalate [46]; (II) impregnation of niobium ethoxide using organic solvents [47 and 48]; and (III) chemical vapor deposition of niobium ethoxide or niobium pentachloride [49]. The niobium oxalate possesses low solubility in aqueous solutions, which can be increased by the addition of oxalic acid. However, the niobium oxalate and the oxalic acid precipitate from solution at high oxalic acid concentrations. The use of niobium ethoxide or niobium pentachloride requires a controlled environment and special procedures to avoid the decomposition of the precursor in the presence of water and to control the density of surface hydroxyl groups. Mendes et al [50] also described a coprecipitation route to Ammonium oxalate complex of niobium was investigated as an aqueous precursor for the preparation of x% Nb2O5/Al2O3 (x=5, 10, 20 and 30 wt.%) samples. Catalysts with the same Nb2O5 contents were also prepared from the traditional niobium oxalate/oxalic acid aqueous solution. They related the presence of multilayers of niobium oxide on the Nb2O5/Al2O3 samples prepared using niobium ammonium oxalate complex and formation of islands of Nb2O5 particles even at low loadings. Consequently, the nature of the precursor affected significantly the reduction of niobium oxide species. The Nb2O5/Al2O3 samples exhibited a high reducibility due to the lower interaction between the niobium oxide species of the islands and the support.

Hydrothermal Hydrothermal syntheses of ceramic powders have interesting potential in view of increasing demand for environmentally benign materials and manufacturing methods.

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92 Maria Lucia Caetano Jardim Pinto da Silva and Rafael Caetano Jardim Pinto da Silva Hydrothermal reactions are usually performed in moderate conditions, do not require expensive precursors or equipment and may yield homogeneous crystalline powders [33,51]. These methods may offer, thus, a low temperature (up to 300 °C) direct route to fine oxide powders. The commonest and cheapest starting material used in niobium chemistry is niobium(V) oxide, Nb2O5. Niobium(V) alkoxides and halides are also commercially available, among other reagents, but they need to be handled in special controlled conditions and some of them are rather expensive. Nb2O5 is fairly unreactive. Aqueous solutions of Nb(V) of high pH are usually obtaining by fusing Nb2O5 with alkali metal (usually potassium) hydroxides or carbonates followed by dissolution of the melt. Soluble polyoxoniobates are formed in the process [53, 54]. Addition of acid to those solutions yields hydrated oxides, Nb2O5·nH2O, also called „niobic acids‟. Complexing agents, like oxalate, citrate or tartrate, prevent the precipitation of niobic acids, due to the formation of Nb(V) species soluble in neutral or acid aqueous solution [53]. The hydrated niobium oxides, usually obtained as white precipitates with indeterminate water content [53,4,54], are more reactive than the anhydrous Nb2O5, and they dissolve in aqueous solutions of NaOH, oxalic acid, tartaric acid and others. This could make niobic acids more convenient as Nb(V) starting material for studies or synthesis involving aqueous solutions, but, besides having variable niobium contents, they have the drawback of being unstable and losing reactivity as time passes. They have generally to be freshly prepared prior to use, as they may be kept in reactive conditions only for about a month [54]. Kinomura et al. have prepared NaNbO3 by hydrothermal reaction of Na8[Nb6O19]·13H2O with NaOH and found a new metastable phase with ilmenite-type structure, that was obtained pure in a very narrow temperature and concentration region [55]. Also, NaNbO3 has been obtained by evaporation of a solution of Na7H[Nb6O19] under a CO2 atmosphere [56]. An old report states that crystals of NaNbO3 were obtained by boiling a suspension of Nb2O5 in NaOH for about 20 min, but no synthetic details were provided [57]. To our knowledge, no report referring the synthesis of NaNbO3 by hydrothermal reaction of Nb2O5 with NaOH has been published. The synthesis of KNbO3 through the hydrothermal reaction of Nb2O5 with KOH has already been reported [58,59]. Depending on the KOH concentration and temperature, that reaction may provide KNbO3, K4Nb16O17·3H2O or complete dissolution of the initial niobium oxide [58,59]. These studies were mainly concerned with the synthesis of the oxides and the dissolution of Nb2O5 in the alkaline solutions was only briefly mentioned. Santos et al [60] described the hydrothermal synthesis of microcrystalline, single phase NaNbO3 through the reaction of commercial Nb2O5 with NaOH, at 200 °C. Different concentrations and NaOH:Nb2O5 molar relations were examined. In the conditions used for the synthesis of NaNbO3, the use of KOH in place of NaOH leads to solubilization of the initial Nb2O5. From these solutions, solids like K6H2[Nb6O19]·13H2O or niobic acid were obtained. This procedure allows, thus, preparation of alkaline solutions of Nb(V) without need of the cumbersome technique of fusion of mixtures of Nb2O5 with alkali. The authors concluded that the hydrothermal reaction of Nb2O5 with NaOH at 200 °C can be used in the synthesis of NaNbO3 under milder conditions than by the ceramic method. A low temperature form, described as stable up to 365 °C was obtained. Under similar conditions, reactions of Nb2O5 with KOH allowed the preparation of solutions containing soluble potassium

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hexaniobates by a more convenient process than the traditional method of fusion of mixtures of the niobium oxide with KOH or other potassium salts. Lin et al [61] described a simple hydrothermal method to prepare hydrated sodium hexaniobate nanorods in an alkaline solution with Nb2O5 powder as the niobium source. They obtained crystalline sodium hexaniobate nanorods of high purity in gram quantity by treating Nb2O5 powder in 10 M NaOH at 150 °C without utilizing template molecules, as shown inf Figure 7.

Figure 7. SEM images of Nb2O5 powder conversion into sodium hexaniobate nanorods at (a) 0h, (b) 16h, (c) 32h and (d)) 48h of reaction times with 10 M NaOH solution [ Reprinted from “A simple preparation procedure for the synthesis of sodium hexaniobate nanorods”, C.-H. Lin et al. / Materials Chemistry and Physics 92 (2005) 128–133, with permission from Elsevier] .

The molecular composition of such material was identified as Na6H2Nb6O19·2H2O and SEM images presented in Figure 8, revealed that the majority of these hexaniobate rods had a diameter distribution of 200–450 nm and a length distribution of 40–60 μm, though finer rods with a diameter of 50–100 nm could also be found in the product mixture. Furthmore, the preparation procedure was straightforward and requires no complicate instrumentation, and could be operated in the absence of a template molecule. Therefore, it seems to us that this method has a potential for use in large-scale industrial production of oxidic nanotubes and nanorods in an aqueous solution.

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94 Maria Lucia Caetano Jardim Pinto da Silva and Rafael Caetano Jardim Pinto da Silva

Figure 8. SEM images of sodium hexaniobate nanorods at (a) lower magnification and (b) higher magnification; SEM image revealing a single nanorod (c) and the smaller outer diameter one (d) [ Reprinted from “A simple preparation procedure for the synthesis of sodium hexaniobate nanorods”, C.-H. Lin et al. / Materials Chemistry and Physics 92 (2005) 128–133, with permission from Elsevier] .

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Sol-Gel Processing The sol-gel method has emerged to become an important means of preparing inorganic oxides in recent years. It is a wet chemical method and a multistep process involving both chemical and physical processes such as hydrolysis, polymerization, drying and densification. The name "sol-gel" is given to the process because of the distinctive increase in viscosity which occurs at a particular point in the sequence of steps. A sudden increase in viscosity is the common feature in sol-gel processing, indicating the onset of gel formation. In the sol-gel process, synthesis of inorganic oxides is achieved from inorganic or organometallic precursors (generally metal alkoxides) [62] Most of the sol-gel literature deals with synthesis from alkoxides. The important features of the sol-gel techniques are better homogeneity compared with the traditional ceramic method, high purity, lower processing temperature, more uniform phase distribution in multicomponent systems, better size and morphological control, the possibility of preparing new crystalline and noncrystalline materials, and lastly easy preparation of thin films and coatings. The important steps in sol-gel synthesis are as follows. Hydrolysis. The process of hydrolysis may start with a mixture of a metal alkoxide and water in a solvent (usually alcohol) at the ambient or a slightly elevated temperature. Acid or base catalysts are added to speed up the reaction.

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Polymerization. This step involves condensation of adjacent molecules wherein H20 and ROH are eliminated and metal oxide linkages are formed. Polymeric networks grow to colloidal dimensions in the liquid (sol) state. Gelation. In this step, the polymeric networks link up to form a three-dimensional network throughout the liquid. The system becomes somewhat rigid, characteristic of a gel. The solvent as well as water and alcohol remain inside the pores of the gel. Aggregation of smaller polymeric units to the main network continues progressively on aging the gel. Drying. Here, water and alcohol are removed at a moderate temperature (less than 200°C), leaving a hydroxylated metal oxide with residual organic content. If the objective is to prepare a high surface area of aerogel powder with low bulk density, the solvent is removed supercritically. Dehydration. This step is carried out between 400 and 800°C to drive off the organic residues and chemically bound water, yielding a glassy metal oxide with up to 20%-30% microporosity. Densification. High temperatures up to 1000°C are used to form the dense oxide product. The various steps in the sol-gel technique described above may or may not be strictly followed in practice. Thus, many complex metal oxides are prepared by a modified sol-gel route without actually preparing metal alkoxides. The sol-gel method was recognized as a promising route for the preparation of niobium oxide gels, powders and coatings only in 1986 by Alquier et al. [63] who presented four different routes for the synthesis of sols and gels: the dissolution of NbCl5 in water with or without addition of hydrogen peroxide, the “classical” but expensive way using alkoxides like Nb(OEt)5 and a cheaper synthesis using chloroalkoxides. Griesmar et al. [64] from the same work group later showed the possibility to obtain Nb2O5 xerogels by reactively modifying (with acetic acid) Nb(OPentn)5, previously synthesized by reacting Nb(OEt)5 with n-pentanol. These authors only mentioned that such chemical routes could also be useful to prepare coatings but only amorphous phases (gels) and, upon heating in air at 500°C, pure tetragonal niobium pentoxide powders have been obtained. To sum up, pure or doped niobium pentoxide made by sol-gel processes can lead to thin or thick coatings, powders, xerogels or aerogels an extensive review of the state of the art in the development of electrochromic (EC) coatings and devices, batteries, nanocrystalline solar cells and in the "eld of catalysis achieved during the last decade using this technique an be found in literature [65] Çopuroğlu et al [66], described recently the sol–gel synthesis and preparation of a nearmorphotropic phase boundary lead–magnesium–niobium titanate (PMNT) material system. Using a Nb(C2H5O)5 as niobium source and 2-methoxyethanol / formamide as sol-gel, they obtained a Pb(Mg0.33Nb0.67)0.65Ti0.35O3+15% PbO xerogel film and investigated the influences of room temperature-UV-irradiation and rapid thermal annealing processes on the structural, crystallographical, morphological, and electrical properties of its thin films, which were deposited by the spin-coating technique, generating the structures shown in Figure 9.

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96 Maria Lucia Caetano Jardim Pinto da Silva and Rafael Caetano Jardim Pinto da Silva

Figure 9. Representative SEM images of the films: (a) UV-exposed and annealed at 550 °C; (b) UVexposed and annealed at 700 °C; (c) unexposed and annealed at 750 °C; and (d) UV-exposed and annealed at 750 °C. [ Reprinted from “UV-/thermal processing of sol–gel-derived lead–magnesium– niobium titanate thin films”, M. Çopuroğlu et al. / Thin Solid Films 518 (2010) 4503–4507, with permission from Elsevier] .

Ferroelectric ceramics typically demonstrate very high and tunable electrical permittivities, and are appealing candidates to enhance the performance of devices such as high-value dielectric constant (k) capacitors, piezo sensors and actuators, non-volatile memories, etc. [67,68,69] and thin films of PMNT have particularly been attracting attention owing to its high-value k with low-loss [68,70]. The authors postulated that the PMNT thin film system described (particularly those annealed at higher temperatures) appeared to be potentially suitable for ultrahigh-value capacitor applications once the unexposed film annealed at 750 °C exhibited the highest high k value (1425) with a strongly pronounced perovskite phase (97%) with crack-free films with well-developed grain structure .

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Microemulsion Technique

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Synthesis of nanoparticles by microemulsion method is an area of considerable current interest. Since the discovery of microemulsions, they have attained increasing significance both in basic research and in different industrial fields. Due to their unique properties, namely, ultralow interfacial tension, large interfacial area, thermodynamic stability and the ability to solubilize otherwise immiscible liquids. The uses and applications of microemulsions are numerous in chemical and biological fields. The nanoparticles not only are of basic scientific interest, but also have resulted in important technological applications, such as catalysts, high-performance ceramic materials, microelectronic devices, high-density magnetic recording and drug delivery. The microemulsion technique promises to be one of the most versatile preparation method which enables to control the particle properties such as mechanisms of particle size control, geometry, morphology, homogeneity and surface area [71].

Figure 10. Hypothetical phase regions of microemulsion systems. [ Reprinted from “Microemulsion method: A novel route to synthesize organic and inorganic nanomaterials”. M.A. Malik et al., Arabian Journal of Chemistry (2010), doi:10.1016/j.arabjc.2010.09.027with permission from Elsevier] .

Microemulsions are isotropic, macroscopically homogeneous, and thermodynamically stable solutions containing at least three components, namely a polar phase (usually water), a nonpolar phase (usually oil) and a surfactant. On a microscopic level the surfactant molecules

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form an interfacial film separating the polar and the non-polar domains. This interfacial layer forms different microstructures ranging from droplets of oil dispersed in a continuous water phase (O/W microemulsion) over a bicontinuous phase to water droplets dispersed in a continuous oil phase (W/O microemulsion), as shown in Figure 10 [71 ].

Figure 11. Mechanism of the formation of nanoparticles in a two-microemulsion method. [ Reprinted from “Microemulsion method: A novel route to synthesize organic and inorganic nanomaterials”. M.A. Malik et al., Arabian Journal of Chemistry (2010), doi:10.1016/j.arabjc.2010.09.027with permission from Elsevier] .

The Nb2O5.3H2O nanoparticles obtained by them, showed crystallite size in nanometer scale , as can be observed at TEM and SEM images in Figures 12 and 13, respectively .

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Figure 12. TEM image of Nb2O5.nH2O synthesized in water-in-oil microemulsion of aqueous solution of NbOF5-/n-heptane with CTAB and n-butanol as surfactant and co-stabilizer respectively [ Reprinted from “Synthesis of Nb2O5.nH2O nanoparticles by water-in-oil microemulsion”, L. A. Rodrigues & M.L.C.P. da Silva / Journal of Non-Crystalline Solids 356 (2010) 125–128, with permission from Elsevier] .

Figure 13. SEM image of Nb2O5.nH2O synthesized in water-in-oil microemulsion of aqueous solution of NbOF5-/n-heptane with CTAB and n-butanol as surfactant and co-stabilizer respectively [ Reprinted from “Synthesis of Nb2O5.nH2O nanoparticles by water-in-oil microemulsion”, L. A. Rodrigues & M.L.C.P. da Silva / Journal of Non-Crystalline Solids 356 (2010) 125–128, with permission from Elsevier] .

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100 Maria Lucia Caetano Jardim Pinto da Silva and Rafael Caetano Jardim Pinto da Silva They also reported that a TT-phase of Nb2O5 was obtained when the sample is annealed at 550 °C and the crystallinity as well as the crystallity size increased with increasing annealing temperature from 1 to 28nm, according to Scherrer‟s equation of the XRD measurements. Recently, Rodrigues & da Silva [80] also reported the successful application to this nanomaterial in the removal of phosphate ions through a rapid and thermodynamic favorable adsorption process.

Templated/Surface Derivatized Nanoparticles

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The use of organic compounds as templates for the generation of inorganic structures and materials has received increasing attention over the last decades [81]. Inorganic materials, although diverse in composition, generally lack the structural variety characteristic of supramolecular and other organic structures [82]. Although inorganic materials are easily molded on the macroscopic level, our ability to shape them on the microscopic level is, at best, deficient. However, the transfer of the structure from a variety of organic templates to the inorganic product allows for the formation of otherwise unattainable inorganic structures. The formation of these inorganic structures is challenging not only from a pure scientific point of view, but these materials also have great potential for application in a large variety of fields. One can envision the use of hollow, inorganic spheres for the controlled release of a variety of substances, as well as their use as light, but strong filler material or as microreactors. Chiral inorganic materials might one day be applied in chiral catalysis, or ultrathin inorganic fibers used as nanowires in nanotechnological devices. The application of these materials in useful processes and devices is assured as soon as their production is accomplished in a precise, reproducible manner, and if possible at reasonable costs [83] .

Figure 14. SEM images very-ordered PS colloidal compacted bed to be used as a template for the three dimensionally ordered macroporous Nb 2O5 [ Reprinted from “Fabrication and characterization of threedimensionally ordered macroporous niobium oxide”, L. Yao et al. / Solid State Sciences 11 (2009) 1625–1630, with permission from Elsevier] .

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In this way Yao et al [84] described a route to obtain three dimensional highly ordered macroporous Nb2O5 using an aqueous organic gel of water-soluble Nb–citric complex and a compacted bed of polystyrene (PS) spheres as template. Their mentioned fabrication followed three stages: first, monodisperse polystyrene (PS) spheres thin films are formed on glass substrates by capillary force during the evaporation of the dispersion medium to prepare colloidal templates (Figure 14); second, interstices between PS spheres are infiltrated with desired materials; third, removal of the colloidal templates by calcinations or chemical etching to obtain three-dimensionally ordered macroporous frameworks (Figure 15).

Figure 15. SEM images of three dimensionally ordered macroporous Nb2O5 at (a) a top-view and (b) side-view [ Reprinted from “Fabrication and characterization of three-dimensionally ordered macroporous niobium oxide”, L. Yao et al. / Solid State Sciences 11 (2009) 1625–1630, with permission from Elsevier] .

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They verified that well-ordered macroporous niobium oxide materials were successfully prepared by using aqueous organic gel method with polystyrene templates revealing both hexagonal and square packing arrays, influenced by different observation planes of PS templates. They also demonstrated that the concentration of Nb–citric precursor had a great effect on the 3-DOM structure and the desirable monoclinic phase has been only obtained after calcinations. Finally, they observed that the temperature of 600°C used to complete the reaction to crystallize Nb2O5 is certainly sufficient to remove the template and obtain inverse opal, but small cracks in the three dimensionally ordered macroporous structure are still observed as shown in Figure 16.

Figure 16. SEM images showing the cracks on the three dimensionally ordered macroporous after calcination at 500°C [ Reprinted from “Fabrication and characterization of three-dimensionally ordered macroporous niobium oxide”, L. Yao et al. / Solid State Sciences 11 (2009) 1625–1630, with permission from Elsevier] .

Bizeto & Constantino [85] also reported the use of triammonium trioxalato(oxido)niobate (NH4)3[NbO(C2O4)3] as an inorganic precursor to assemble organized hybrid niobium oxide mesophases using n-octylamine (OCT) and cetyltrimethylammonium (CTA) bromide as structure-directing agents is described for the first time. In this route water-soluble niobium complex goes under hydrolysis at controlled pH and in the presence of the organic templates leading to the formation of hybrid mesophases. A lamellar mesophase is produced by a neutral route based on the interaction of hydrolyzed units with the octylamine micelles. On the other hand, a hexagonal (p6m) mesophase is obtained when hydrolyzed species interact with CTA micelles through a charge-matching route (ionic route). In concluded that it is possible to use triammonium trioxalato(oxido)niobate as an inorganic precursor to synthesize niobium oxide mesostructures by means of a neutral or an ionic route. The advantage of using this niobium compound instead of metal alkoxides or chloride precursors is related to its low cost, stability in water, easy manipulation, and possibility of recycling. They also verified tha triammonium trioxalato(oxido)niobate can interact with neutral and cationic templates to

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produce organized mesophases under controlled pH conditions. However, to obtain mesoporous niobium oxides from the synthesized mesophases, further studies are necessary to define experimental conditions that promote a better condensation and consolidation of the skeletal inorganic units, since attempts at template removal by calcinations or solvent extraction lead to the collapse of the mesostructure.

CONCLUSION

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In this chapter, we have reviewed niobium oxides properties, some synthetic routs and applications. Nb2O5 is the most important niobium compound being employed in high-tech applications directly or as a starting reactant for commercial, industrial and R&D uses and. Nb2O5 possesses a quite complex crystalline morphology, its physicochemical properties changes significantly among them. In this scenario, the synthetic strategy adopted is crucial in order to tailor the material to obtain the desired properties for envisaged application. As materials synthesis is an intellectually vast and economically important area, all the available techniques used in the niobium oxide strategic synthesis cannot be covered in a single text. So, the purpose of this chapter was to review, and maybe introduce, some of the important techniques being used to the obtention of niobium oxide materials and the references cited along the chapter were meant to provide a representative glimpse of this extensive and expanding field as opposed to being an exhaustive list. In spite of some interesting upcoming methods like the electrochemical approaches, and coupled intensification process such as ionic liquid media, microwave and sonochemistryassisted synthesis couldn‟t be covered in this work, it was possible to observe that are currently two global themes being pursued in the niobium oxide synthetic strategies: lower temperature reactions and couple well-defined methods .

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[39] Tagliaferro, G.V., Estudo das variáveis de preparação do óxido de nióbio (V) hidratado para utilização em troca iônica. MSc Dissertation, Faculdade de Engenharia Química de Lorena (2003). [40] Vogel, A.I. “Análise Química Quantitativa”, 5°ed. Rio de Janeiro, Guanabara Koogan, 1992. [41] Silva, M.L.C.P., et al., An. Assoc. Bras. Quim., 50(2), p. 83 - 87, 2001. [42] Tagliaferro, G. V. et al. Quim. Nova, Vol. 28, No. 2, 250-254, 2005 [43] L.A. Rodrigues, M.L.C.P. da Silva Colloids and Surfaces A: Physicochem. Eng. Aspects 334 (2009) 191–196 [44] Tagliaferro, G. V. et al. Quim. Nova, 2011, in press. [45] I.E. Wachs, L.E. Briand, J.M. Jehng, L. Burcham and X. Gao. Catal. Today 57 (2000), p. 323. Article | PDF (77 K) | View Record in Scopus | Cited By in Scopus (71) [46] J.M. Jehng, I.E. Wachs, Symposium on New Catalytic Materials and Techniques, Miami Beach, 1989, p. 546. [47] K. Asakura and Y. Iwasawa. J. Phys. Chem. 95 (1991), p. 1711. [48] N. Ichikuili and Y. Iwasawa. J. Phys. Chem. 98 (1994), p. 11576. [49] C.L.T. da Silva, V.L.L. Camorim, J.L. Zotin, M.L.R.D. Pereira and A.C. Faro, Jr.. Catal. Today 57 (2000), p. 209. [50] F.T.M. Mendes et al. Catalysis Today 78, 1-4, (2003), p. 449-458. [51] D. Segal, J. Mat. Chem. 7 (1997) 1297. [52] W.J. Dawson, Ceram. Bull. 67 (1988) 1673. [53] F. Fairbrother, The Chemistry of Niobium and Tantalum, Elsevier, Amsterdam, 1967. [54] D. Brown, in: J.C. Bailar, H.J. Emeleus, R. Nyholm, A.F. Trotman-Dickerson (Eds.), Comprehensive Inorganic Chemistry, vol. 3, Pergamon, Oxford, 1973, p. 553. [55] S. Lanfredi, L. Dessemond, A.C. Martins Rodrigues, J. Eur. Ceram. Soc. 20 (2000) 983. [56] N. Kinomura, N. Kumata, F. Muto, Mat. Res. Bull. 19 (1984) 299. [57] P. Sue, Ann. Chim. 7 (1937) 493. [58] P. Vousden, Acta Crystallogr. 4 (1951) 373. [59] C.H. Lu, S.Y. Lo, H.C. Lin, Mater. Lett. 34 (1998) 172. [60] I.C.M.S. Santos et al. Polyhedron 21 (2002) 2009-2015. [61] C.H. Lin et al. Materials Chemistry and Physics 92 (2005) 128–133. [62] C.N.R. Rao, Materials Science and Engineering, BI8 (1993) 1-21 [63] C. Alquier, M.T. Vandenborre, M. Henry, J. Non-Cryst Solids 79 (1986) 383. [64] P. Griesmar, G. Papin, C. Sanchez, J. Livage, Chem. Mater. 3 (1991) 335. [65] M.A. Aegerter Solar Energy Materials & Solar Cells 68 (2001) 401}422 [66] M. Çopuroğlu et al. Thin Solid Films 518 (2010) 4503–4507 [67] P. Muralt, Integr. Ferroelectr. 17 (1997) 297. [68] S. Nagakari, K. Kamigaki, S. Nambu, Jpn. J. Appl. Phys. 35 (1996) 4933. [69] H. Fan, H.-E. Kim, Jpn. J. Appl. Phys. 41 (2002) 6768. [70] P. Kumar, O.P. Thakur, C. Prakash, T.C. Goel, Physica B 357 (2005) 241. [71] Malik, M.A. et al., Microemulsion method: A novel route to synthesize organic and inorganic nanomaterials. Arabian Journal of Chemistry (2010), doi:10.1016/ j.arabjc.2010.09.027 [72] Julian, E., Martin, J., Hollamby, Laura, H., 2006. Recent advances in nanoparticle synthesis with reversed micelles. Adv. Colloid. Interf. Sci. 128, 5–15.

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[73] Destree, C. et al., 2008. J complexes of retinol formed within the nanoparticles prepared from microemulsions. Colloid. Polym. Sci. 286, 1463–1470. [74] E. Goikolea, M. Insausti, L. Lezama, I.G. de Muro, J.S. Garitaonandia, J. Non- Cryst. Solids 354 (2008) 5216. [75] J. Wu, H. Yan, X. Zhang, L. Wei, X. Liu, B. Xu, J. Colloid Interf. Sci. 324 (2008) 167. [76] H. Zhou, C. Peng, S. Jiao, W. Zeng, J. Chen, Y. Kuang, Electrochem. Commun. 8 (2006) 1142. [77] S.A.P. Gonçalves, S.H. Pauli, A.C. Tedesco, F.H. Quina, L.T. Okano, J.B.S. Bonilha, J. Colloid Interf. Sci. 267 (2003) 494. [78] Luisi, P.L., Majid, L.J., Fendler, J.H., 1986. Solubilization of enzymes and nucleic acids in hydrocarbon micellar solution. Crit. Rev. Biochem. 20, 409–474. [79] L.A. Rodrigues, M.L.C.P. da Silva. Journal of Non-Crystalline Solids 356 (2010) 125– 128 [80] L.A. Rodrigues, M.L.C.P. da Silva. Adsorption (2010) 16: 173–181 [81] N. K. Raman, M. T. Anderson, C. J. Brinker, Chem. Mater. 1996, 8, 1682 – 1701 [82] J. H. van Esch, B. L. Feringa, Angew. Chem. 2000, 112, 2351 – 2354 [83] S. Shinkai et al. Angew. Chem. Int. Ed. 2003, 42, 980 – 999 [84] L. Yao et al. Solid State Sciences 11 (2009) 1625–1630 [85] M. A. Bizeto, V. R. L. Constantino Eur. J. Inorg. Chem. 2007, 579–584

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Chapter 4

GEOCHEMISTRY OF NB AND VARIATIONS OF NB/TA RATIO IN GEOLOGICAL SYSTEMS Yuanyuan Zhang 1,2and Jaroslav Dostal2 1

School of Earth and Space Sciences, Peking University, Beijing, China Department of Geology, Saint Mary‟s University, Halifax, Nova Scotia, Canada

2

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ABSTRACT Niobium (Nb) is one of the high-field-strength elements, i.e. its ions are relatively small and highly charged. In geological systems, Nb does not readily substitute into most common rock-forming minerals and behaves incompatibly during the crystallization of silicate melts. This element is concentrated in silicate melts by small degrees of partial melting or by high degrees of fractional crystallization. In the Earth, it is enriched in the crust, particularly in the upper continental crust, relative to mid-ocean ridge basalts or chondritic meteorites. In the natural materials, Nb and tantalum (Ta) have almost identical ionic radii, very similar chemical characteristics and are pentivalent. Thus, following Goldschmidt‟s' rules, Nb and Ta can be expected to remain tightly coupled in geological processes, implying that the Nb/Ta ratio of rocks and minerals should remain rather constant and close to the chondritic ratio of ~ 17.5. Many mantle-derived rocks including mid-ocean ridge basalts and ocean island basalts have such ratios suggesting that partial melting of the mantle does not fractionate the Nb/Ta ratio. However, the ratios exhibit significant variations in certain hydrothermal systems and crustal igneous rock suites including fractionated granitic rocks. Even typical continental crust rocks have significantly lower values (Nb/Ta in continental crust ~11) with some peraluminous granitic rocks having values as low as ~ 2. An explanation for the deviation of this ratio from the chondritic values is still under debate. The data presented here suggest that fluid fractionation leads to an enrichment of Ta relative to Nb and hydrothermal fluids appear to be an efficient medium for the fractionation of this ratio.

1. INTRODUCTION Niobium (Nb) is one of the high-field-strength elements (HFSE) with high valence and small size of ionic radius. Nb is a refractory and lithophile element. During geological

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processes, Nb and tantalum (Ta) have long been regarded as behaving comparably because these two elements have very similar characteristics and are pentivalent. According to Goldschmidt‟s' rules, Nb and Ta can be expected to remain tightly coupled in geological processes, implying that the Nb/Ta ratio of rocks and minerals should remain rather constant and close to the chondritic ratio of ~ 17.5 (Hofmann 1988,Green 1995). The Earth's mantle, midocean ridge basalts, ocean island basalts and alkaline mafic magmas typically have a chondritic Nb/Ta ratio of 17.5 (Hofmann et al. 1986; Green 1995). The variation of the Nb/Ta ratio in mantle-derived rocks was regarded as an analytical effect (uncertainties/ analytical errors). However, an improvement of analytical techniques and a significant increase of amount of analytical data provide evidences that the Nb/Ta ratios in geological materials are variable. Typical continental crust rocks have significantly lower values (continental crust ~11, Taylor and McLennan 1985, Green 1995) with some peraluminous granitic rocks having values as low as ~ 2 (Dostal and Chatterjee 1995, 2000). It is important to explore the processes that could modify the Nb/Ta ratio as these processes might have played a significant role during the evolution of continental crust. However, the explanation for the deviation of this ratio from the chondritic values is still under debate. Sweeney et al. (1995) and Schmidt et al. (2004) reported that the major rock forming minerals which all have very low partition coefficients for these elements cannot explain the distinct fractionation of this ratio. They also argued that these elements are mainly concentrated in minor/accessory minerals most likely rutile, cassiterite and columbite-tantalite (Brenan et al. 1994; Zack et al. 2002), crystallization of which can modify this ratio. Therefore, the presence of accessory minerals is often invoked to explain the Nb and Ta fractionation. Some recent experimental studies have shown that residual Ti-rich minerals could fractionate Nb over Ta in silicate melts or fluids (Green 1995; Münker 1998; Linnen and Keppler 1997). Linnen and Keppler (1997) argued that crystallization of accessory phases from magma would lead to a decrease of Nb/Ta ratios in residual peraluminous granitic melts without the involvement of fluid. However, Green (1995) and Dostal and Chatterjee (2000) inferred that fluid fractionation leads to an enrichment of Ta relative to Nb. In this paper we explore a role of the hydrothermal fluids, which appear to be an efficient medium for the fractionation of Nb/Ta ratio.

2. GEOCHEMISTRY OF NIOBIUM Niobium (from Niobe, daughter of Tantalus) also called Columbium, symbol Nb or Cb, is a rare metallic element discovered in 1801 by English chemist Charles Hatchett. Its atomic number is 41 and has the electron configuration with valences of 3+, 4+, 5+, although only 5+ occurs naturally. In the Earth‟s lithosphere, Nb is a lithophile element and therefore it is enriched in the crust (average crustal content ~ 20 ppm; Taylor and McLennan 1985), particularly in the upper continental crust, relative to mid-ocean ridge basalts or chondrites. Minerals of niobium are all complex oxides of several metallic elements. The major ore is the isomorphous solid solution series - columbite-tantalite (Fe,Mn)(Nb,Ta)2O6) as well as the pyrochlore (Ca,Na,Ce)(Nb,Ti,Ta)2(O,OH,F)7 - microlite (Ca,Na)2(Ta,Nb)2(O,OH,F)7 group. Nb also occurs in several minerals containing Ta, U, Th, and rare earths, such as euxenite, samarskite, fergusonite, and eschynite. The element is also found in the crystal lattices of Ti-bearing

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minerals (Ti4+ radius 0.64 Å) such as ilmenite, FeTiO3; rutite, TiO2; brookite, TiO2; titanite, CaTiSiO5; perovskite, CaTiO3; and also wolframite (FeMn)WO4. In geological systems Nb is considered to be one of the most immobile elements. Nb does not readily substitute into most common rock-forming minerals and thus behaves incompatibly during the crystallization of silicate melts. This element is concentrated in silicate melts by small degrees of partial melting or by high degrees of fractional crystallization. The Nb contents generally increase from early to later stages of the evolution of magma. In early magmatic series Nb occurs chiefly camouflaged in rock-forming biotite, amphibole, Fe-Ti oxides, Ti-rich minerals and only rarely as Nb-mineral accessories. In later stages of the magma evolution, Nb occurs both as isomorphic substitution and as discrete accessories; in the latest stage rocks camouflaged Nb may be accompanied by abundant Nb minerals. Niobium becomes concentrated in the late stages of magmatic crystallization in granite pegmatites and especially nepheline syenites where it attains a concentration of 100300ppm. The earliest magmatic minerals (e.g. ilmenite in norite) contain less Nb than late stage titanite from syenites or rutile from granite. Among sedimentary rocks, Nb tends to accumulate in shale and clay rather than limestone and sandstone. The relatively immobile and refractory nature of Niobium enables the use in deciphering and tracing the sources and conditions necessary for the genesis of igneous rocks. Assumed immobility has also been used to determine the amount of mass transfer of mobile elements that may have occurred during hydrothermal alteration accompanying the formation of various types of ore deposits (MacLean and Barrett 1993). Numerous niobium determinations in rocks and minerals are results particularly of optical spectrography, spectrophotometry and X-ray fluorescence (XRF). With the advent of inductively coupled plasma-mass spectrometry (ICPMS) analysis, Nb can be determined routinely and precisely in geological materials at even very low concentration levels (ppmppb). Historically, most of the niobium produced has come from granitic sources, but by far the largest resources of niobium are contained in carbonatites (intrusive carbonate rocks) and associated alkaline igneous complexes. Other significant resources are contained in bauxites resulting from the weathering of nepheline syenite. It was not until the early 20th century that niobium was first used commercially. Brazil is the leading producer of niobium and ferroniobium, an alloy of niobium.

3. RELATION OF NIOBIUM AND TANTALUM Niobium has strong chemical relations to tantalum. Their association steams mainly from the close similarity of the sizes of the ionic radii (Nb5+=0.69Å and Ta5+=0.68Å; Wedepohl 1969), the identical valance states of their ions and the chemical properties of niobium and tantalum, which can account for their isomorphic substitution. In the crystal lattices of minerals, they are typically in six-fold coordination. According to the Goldschmidt‟s Rule, niobium and tantalum can be regarded as geochemical “identical twins” which remain tightly coupled in geological process. Therefore, Nb and Ta have long been regarded as behaving identically during geochemical fractionation processes and thus the Nb/Ta ratio of rocks and minerals remain constant and comparable to the chondritic ratio of ~17.5 (Sun and

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McDonough 1989). Previous studies of Nb/Ta ratios in mid-ocean ridge basalts, ocean island basalts, komatiite and depleted mantle xenoliths (Hofmann et al. 1986; Jochum et al. 1986, 1989) have shown that the ratio is rather constant and similar to the values of carbonaceous chondrites. This suggested that the Nb/Ta of the Earth‟s mantle has a chondritic value of about 17.5 and that there is no significant fractionation. Any deviation from these values determined in mantle derived melts has been considered to reflect analytical uncertainty (Jochum et al. 1986), not as a natural process. Niobium and tantalum are closely associated in nature and geological materials including igneous rocks. In the process of differentiation of parental magmas, the contents of tantalum and niobium increase from the ultrabasic and basic igneous rocks to the acid and alkali rocks. Although various discrete niobium and/or tantalum minerals may occur as accessory species in a wide variety of ordinary igneous rocks, including those of both alkali and calc-alkalic series, relatively high abundances of niobium-tantalum minerals are found primarily in alkalic granites, metasomatically altered granites, granite pegmatities, carbonatites and late hydrothermal veins of alkali complexes for the formation of independent deposits. In the past, niobium and tantalum in geological samples were usually analyzed by two different methods. Nb values were usually obtained by XRF techniques, while Ta values were done by an instrumental neutron activation analysis (INAA). In the last decade, new analytical techniques such as ICP-MS can determine Nb and Ta simultaneously and are capable of high quality analysis of Nb and Ta in low abundances (e.g., Münker 1998). Thus the recent development of high quality analytical methods reduced the analytical difficulties, improved quality of the data and confirmed the subtle variations of Nb/Ta ratios in geological materials including experimental (e.g. Linnen and Keppler, 1997; Schmidt et al., 2004) and natural systems (e.g. Münker 1998; Münker et al. 2004; Dostal et al. 2009; Green 1995; Plank and White 1995; Stoltz et al. 1995, 1996; Dostal and Chatterjee, 2000). Recent results show that compared to the mantle rocks, typical continental crust rocks have significantly lower values (continent crust ~11, Taylor and McLennan 1985, Green 1995) with some peraluminous granitic rocks having values as low as ~1 (Dostal and Chatterjee 1995, 2000). Estimates of the bulk composition of the post-Archaean continental crust give subchondritic Nb/Ta values of ~11 (Taylor and McLennan 1985), which were once recognized as a random (analytical) effect. The mineral-melt partition coefficients (D) for Nb and Ta of the major rock forming minerals are very low and cannot explain the fraction of this ratio observed in geological materials (Green 1995; Sweeney et al. 1995; Schmidt et al. 2004). Since the Ti-bearing minerals are primarily responsible for the distribution and budget of Nb and Ta especially in subduction zone settings, the presence of Ti-bearing phases (e.g. rutile or titanite) is often invoked to explain the Nb and Ta fractionation and depletion. The substitution of Ti4+ in rutile by Nb5+ and Ta5+ has been well documented (Foley et al. 2000) and is usually accompanied by the enrichment of Fe, which suggests a coupled substitution such as 2Ti4+=Fe3++Nb5+ (or Ta5+). Horng and Hess (2000) and Klemme et al. (2005) suggests a coupled substitution such as 2Ti4+=Al3++Nb5+ (or Ta5+). Some recent experimental studies have shown that residual Ti-rich minerals could fractionate Nb over Ta in silicate melt or fluid (e.g. Green 1995; Münker 1998). Rutile could fractionate Nb from Ta in silicate melts or fluids but in opposite directions, as rt/meltDNb/DTa=1. In this case, the fluid strongly favors Ta over Nb, leading to a decrease of the Nb/Ta ratio in rocks affected by the fluid. However, Linnen and Keppler

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(1997) argued that crystallization of accessory phases from magma would lead to a decrease of Nb/Ta ratios in residual peraluminous granitic melts. Their study demonstrated the DNb/DTa between accessory phases enriched in HFSE and a melt should increase by a factor of four to five when going from intermediate to subaluminous or peraluminous felsic melts. In andesitic melts, the accessory minerals prefer Ta over Nb, but in granitic melts, the relative fractionation between Nb and Ta will be inverted, leading to an enrichment of Ta in residual liquids. In subaluminous or peraluminous granites, the solubility of accessory minerals such as rutile is very low and partition coefficients are high so that they can significantly fractionate Nb from Ta. Therefore, as Linnen and Keppler (1997) suggested, small amounts of accessory phases in peraluminous granitic systems will cause a strong decrease of Nb/Ta in residual liquids with no need to invoke a fluid phase to explain the low Nb/Ta values. However, experimental studies (Schmidt et al. 2004) on the partitioning of Nb and Ta between various melt compositions and potential residual phases in the mantle and subducted oceanic lithosphere suggest that rt/meltDNb/DTa varies little and rutile in the residue during partial melting or dehydration of subducting crust is not capable of significantly enriching Nb over Ta in the residue. Furthermore, mantle peridotites commonly do not contain rutile due to a reaction with olivine to form ilmenite and orthopyroxene (Green and Ringwood 1967), and Ti is highly soluble in mafic melts, so that Ti-bearing minerals are unlikely to remain stable after the onset of the mafic melting (Ryerson and Watson 1987). Alternatively, slab dehydration, highly evolved magma or fluid fractionation have been invoked to explain the Nb/Ta variations (Bau 1996; Stolz et al. 1996; Irber 1999; Foley et al. 2000; Dostal et al. 2009). Reasons for Nb and Ta fractionation are still under debate. In this paper we explore the potential fluid influence on Nb and Ta fractionation.

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3.1 Fractionation of Nb and Ta by Scheelite and Rutile from Metaturbiditehosted Quartz Vein Gold Deposits The lode gold deposits of southern Nova Scotia are located within the Meguma Terrane of the Canadian Appalachians (Figure 1) and are considered to be one of the classic occurrences of turbidite-hosted gold mineralization (Malcolm 1929; Kontak et al. 1990). The Meguma Terrane, the most easterly terrane of the northern Appalachians, extends across mainland Nova Scotia for a distance of about 750km and was accreted to North America (Laurentia) during the Acadian Orogeny, The terrane is made up mainly of ~10 km thick sequence of Cambro-Ordovician flyshoid metasedimentary rocks (Meguma Group) which was intruded by Late Devonian-Early Carboniferous granitoids rocks - South Mountain Batholith, which is exposed over an area of 7,300km2. The Meguma Group sediments were deformed and metamorphosed to greenschist to amphibolites grade during the Acadian Orogeny that took place at 405-390 Ma (Keppie and Dallmeyer 1987). The Meguma gold-bearing quartz veins range in thickness from about 1cm up to ~2m. They contain quartz, carbonates, sulphides (pyrite, arsenopyrite and pyrrhotite) and trace amounts of gold, Bi-Ag tellurides, tourmaline, rutile and scheelite (Kontak and Smith 1993a, b). Geochronological studies indicate the gold-bearing quartz vein deposits emplaced at two periods, 408 and 380Ma (Kontak and Archibald 2002; Morelli et al. 2005).

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Figure 1. Geological map of southern Nova Scotia showing the location of the investigated gold deposits (solid circles) and Davis Lake pluton (DLP) (modified after Dostal et al. 2009). Inset map shows the lithotectonic terranes of the mainland Canadian Appalachians (terranes: M-Meguma; AAvalon; G-Gander, D- Dunnage, H-Humber). CCFS- Cobequid-Chedabucto Fault System that represents a tectonic boundary between the Meguma and Avalon terranes.

Scheelite is not a common mineral in the Meguma gold-bearing quartz veins, but it can be abundant locally. It occurs as coarse grains or grain aggregates of multi-centimeter size. Scheelite is a late stage phase, as it is present in cavities in the veins which are lined with euhedral quartz crystals (Kontak and Smith 1993a). Seventy four specimens of quartz veins containing scheelite were collected. Trace elements and sulphur in scheelite were determined using laser-ablation ICP-MS (Jackson et al. 1992). The instrument and data were described by Dostal et al. (2009). In scheelite, niobium and tantalum substitute for W which has a similar size of the ionic radii (for W4+ 0.68 Å, for W6+ 0.65 Å). Rutile is a rare mineral which forms isolated grains in massive quartz. Euhedral and acicular crystals are up to 1cm long, although mm size is more common. It often appeared close to slivers of wall rock or the wall rock itself. Rutile is also a late stage phase, which also occupies vug-like features in the quartz or cavities lined with calcite-quartz. Eleven additional specimens containing rutile were sampled and the geochemical analysis was performed by a JEOL Superprobe 8200 in Department of Earth Sciences, Dalhousie University, Halifax (detailed in Dostal et al. 2009). Relative to gold, both scheelite and rutile are late phases and postdate much of the gold. Since the three mineral phases are not in contact with each other, more refinement of their relative paragenesis is not possible. The scheelites are characterized by bell-shaped chondrite-normalized REE patterns with (La/Sm)n 1(n-chondrite-normalized), which reflect the preferential partitioning of the middle REE into scheelite. They also have high concentration of Sr, Nb, Y and high Nb/Ta ratios (80-300). However, in detail, they can be subdivided into three groups on the basis of the REE patterns and abundances (Dostal et al. 2009) (Figure 2A). Scheelites of type 1 and some of type 2 with (La/Lu)n>1 have a positive Eu anomaly, accompanied with Nb/Ta of 80-200 and 78-87 respectively, whereas type 3 scheelites, with low (La/Lu)n, have lower absolute REE abundances and either do not have a Eu anomaly or have a negative one and the Nb/Ta ratio is typically between 80-300 (Figure 2B). Element pairs in scheelites, such

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as Eu-Sr, Pb-Sr, Nb-Ta and Y-Ho, are positively correlated which implies that element behaviour in hydrothermal fluids is a function of charge and radius (Bau 1991, 1996; Dostal et al. 2009). However, some element ratios, such as Nb/Ta (80-300) and Y/Ho (14-25), are highly fractionated, therefore, indicating crystal/fluid and fluid speciation control the behaviour of the HFSE and REE (Bau 1996). Since the scheelites collectively cover a broad geographic area across the Meguma Terrane, the homogeneous specimens in terms of the shape of the REE patterns indicate that the REE contents of scheelite were not modified as a consequence of the cryptic fluid-rock interaction but probably reflect the primary signature of the vein-forming fluids. The three types of scheelites recording subtle and distinct differences in chemistry probably represent distinct pulses of hydrothermal fluids that differed slightly in their compositions as a result of different pH/redox/ligand compositions.

Figure 2. Variations of La (ppm) versus (La/Lu)n (A) and Nb/Ta versus Nb(ppm) (B) in the scheelite of Nova Scotia gold-bearing quartz vein deposits.

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Figure 3. Variations of Nb (ppm) versus TiO2 (wt%)(A) and Ta (ppm) versus (B) Nb/Ta and in the rutile of Nova Scotia gold-bearing quartz vein deposits.

The rutiles are compositionally zoned and back scattered electron images indicate that the crystal cores are enriched in W, Nb, Ta and Fe and depleted in Ti compared to the grain margins (Figure 3A). Hydrothermal rutiles with high contents of W (up to 4.2 wt.% WO3) are rich in Ta compared to Nb and have a very low Nb/Ta ratio (~0.3) (Figure 3B). The origin of rutile in the quartz veins might be related to the release of W-bearing hydrothermal fluids possibly in pulses from crystallizing granitic magma (Halls 1987). The compositional differences between the core and outer rim of rutile probably reflect changes in fluid chemistry and/or abrupt changes in pressure or temperature which would affect solid solution between Ti and W and the other cations (Rice et al. 1998). Compared to Nb/Ta ratios of the chondritic (~17.5) and continental crustal values (~11), both scheelite and rutile have anomalous fractionated Nb/Ta. The geochemical characteristics of scheelites and rutiles presented here show that hydrothermal fluids efficiently fractionate this ratio, resulting in scheelite with high Nb/Ta and rutile with low Nb/Ta ratio. Three distinct trace element types of the scheelites reflect the chemical differences in the pulses of hydrothermal fluids and the compositional differences between core and rim of rutile also

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record the changes of the hydrothermal fluid. Meanwhile, scheelite has remarkably high Nb/Ta ratios while rutile posses extraordinary low Nb/Ta ratios which indicates that these minerals also play an important role in fractionating the ratio of Nb and Ta in fluid.

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3.2 Fractionation of Nb and Ta in Peraluminous Granitic Pluton The Davis Lake pluton (DLP) is one of the granitoid plutons in South Mountain Batholith within the Meguma Terrane of the Canadian Appalachians (Figure 1). The Davis Lake pluton covers an area of 816 km2 (MacDonald et al. 1992) and is composed mostly of biotite and muscovite-biotite leucomonzogranite. Minor topaz-muscovite leucogranite hosts greisens and a primary tin deposit. The pluton is zoned with less evolved rocks outcropping at the north whereas highly differentiated types occur in the southwestern part. The Pb-Pb isotope data for leucomonzogranite, leucogranite, greisen and mineralization in the DLP plot on the same linear array (Kontak and Chatterjee 1992), implying a close temporal and genetic relationship among the various rock-types and mineralization. The leucomonzogranite is composed of megacrysts(5-15%) of orthoclase-microperthite (up to 10 cm long) enclosed in a medium grained groundmass of sodic plagioclase (An20-10), K-feldspar and quartz with 6-8% biotite and muscovite. Accessory minerals include cordierite, garnet, apatite, zircon, monazite and Fe-Ti oxides. The leucogranite is a fine to medium grained equigranular rock, composed of quartz, plagioclase (frequently zoned An012), K-feldspar and subordinate muscovite and topaz. Accessory minerals include biotite, zircon, monazite, apatite, uraninite, Nb-Ta oxides, triplite, andalusite and ore minerals such as cassiterite, sphalerite, chalcopyrite, pyrite and pyrrhotite (Kontak and Dostal 1992; Dostal and Chatterjee 1995, 2000). Fluorite occurs as euhedral crystals in groundmass and as veinlets along fractures. The leucogranite is variably sericitized and greisenized; it contains massive quartz-topaz-cassiterite greisen zones 10 cm to 25 m thick and 50 to 800 m long. The greisens, hydrothermal alteration assemblages containing quartz-muscovite±fluorite±topaz, formed during a late magmatic-early hydrothermal stage (Halter et al. 1996, 1998; Dostal and Chatterjee 1995). The DLP rocks are peraluminous granitoids with SiO2 contents ranging from 68 to 77wt.%. These three rock types- leucomonzogranite, leucogranite and greisen hosted in leucogranite have distinct geochemical characteristics. Compared to leucogranites and greisens, leucomonzogranites have lower SiO2, higher K2O with K>Na. With respect to the mineral assemblage (biotite or muscovite-biotite bearing) and K/Rb ratio (>150 or 50) than the other two (leucogranites and greisens) types ( 0.9999, for all samples and temperatures of measurement in the frequency range between 100 Hz and 1 MHz), is similar to the Curie-Von Schweidler model, that for periods longer than two time decades follow the i(t) α t-s mathematical function [jon83, cos95]. The inverse Fourier transform of the above expression leads to expressions for dielectric impedance of the following type (eq. 3.2.1) [jon83, cos95]:

Z ´´  k. f n

3.21 where Z'' represents the imaginary part of the complex impedance, k is a constant factor, f is the linear frequency and n a fitting parameter that, according to Jonsher [jon83], can be associated with the type of the polarization system (example: dipoles, charges, etc.). The k and n parameters values, obtained by the linearization of the expression 3.2.1, are summarized in Table 3.2.2 for the samples TT at 600 and 650 ºC and TTE at 575 ºC with the electric field of 100 kV/m (sample 575B). The obtained results (Table 3.2.2), revealed that the k parameter decreases with the increase of temperature. The n parameter is almost constant for all temperatures in samples TT at 600 and 650 ºC but in the sample TET at 575 ºC, it decreases with the increase of the measurement temperature.

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Table 3.2.2. k and n parameters of the TT samples at 600 and 650 ºC and sample 575B for several measurement temperatures Temperature [K] 315 310 305 300 295 290 285 280 275 270

TT600 k (x108) 7.45 7.65 7.82 7.97 8.10 8.28 8.42 8.65 8.77 8.88

n 0.98 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99

TT650 k (x108) 9.89 9.98 10.01 10.11 10.30 10.40 10.53 10.71 10.74 10.82

n 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

575B k (x108) 2.26 2.56 2.87 3.18 3.50 3.81 4.14 4.48 4.81 5.09

n 0.89 0.90 0.91 0.92 0.92 0.93 0.94 0.94 0.95 0.95

Table 3.2.3. Real (‘) and imaginary (‘‘) parts of the dielectric permittivity and the dielectric loss (tan ), measured at 1 kHz and 300 K (44Si composition)

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Sample as-prepared 550 575 600 650 575A 575B

‟ 69.7  2.3 62.9  1.9 62.3  1.6 21.8  0.7 18.72  0.04 74.1  2.5 33.3  1.0

‟‟ 136.4  4.5 165.9  4.9 164.5  4.3 1.1  0.04 0.035  0.001 106.3  3.5 9.9  0.3

tan 1.96  0.09 2.6  0.1 2.6  0.1 0.052  0.003 0.0019  0.0001 1.44  0.07 0.30  0.01

Table 3.2.3 shows the complex dielectric permittivity values for all samples at room temperature and 1 kHz. It can be observed that with the increase of the thermal treatment temperature and with the increase of the amplitude of the applied electric field the value of the dielectric constant („) decreases. The sample TTE at 575 ºC with a field of 50 kV/m (sample 575A) presents a higher value of „ than the sample treated at the same temperature but without the presence of an electric field. The inverse occurs for the 575B sample (Table 3.2.3). In all samples, with the increase of the measurement temperature, the „value increases. The same behavior is observed for the dielectric loss (tan ).

3.3. 34sio2-33Li2O-33Nb2O5 (%Mole) Composition Sample Appearance Figure 3.3.1 shows the macroscopic appearance of the 34Si composition samples before and after thermal treatments (TT). It can be observed that for treatment temperatures above 600ºC, the samples become opaque.

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TT at 575ºC

As-prepared

TT at 600ºC

Figure 3.3.1. Photographs of the 34Si glass samples (the lowest division of the scale = 1mm).

550A

575A

575B

600A Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

550B

600B

-

550C

+

-

575C

+

-

600C

+

Figure 3.3.2. Photographs of the 34Si samples TTE at 550, 575 and 600 ºC with electric fields of 100 kV/m (samples named with “A”), 250 kV/m (samples named with “B”) and 500 kV/m (samples named with “C”). The signs "-" and "+" refer to the sample surface in contact with the electrode negative and positive, respectively (the lowest division of the scale = 1mm).

The macroscopic aspect of the samples submitted to TTE is shown in Figure 3.3.2. For the TTE with an electric field of 500 kV/m, a non-homogeneous white layer on the surface that was in contact with the positive electrode, is formed.

XRD, Raman And SEM Figure 3.2.3 shows the XRD spectra of the 34Si samples heat-treated without the presence of an external electric field. It was observed in these samples that the TT at temperatures above 600 °C favors the formation of the LiNbO3 crystalline phase. The increase of the thermal-treatment temperature promotes the formation of secondary phases, namelly Li2Si2O5, detected in the sample TT at 700 ºC. Figures 3.3.4, 3.3.5 and 3.3.6 show the XRD patterns of the samples TTE at 550, 575 and 600 ºC, respectively.

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The TTE samples with an electric field of 500 kV/m present different aspects in the opposite surfaces. Thus, the XRD was performed in both surfaces. However, the XRD result did not show major significant differences. The sample TTE at 550 ºC (Fig. 3.3.4) presents the LiNbO3 and Li3NbO8 crystal phases, also detected in the samples TTE at 600 ºC with the electric field of 500 kV/m. It must be pointed out that in the sample 600C, the Li3NbO8 phase was only detected on the surface that was in contact with the positive electrode. It is worthy to note that the samples 500A, 500B, 600A and 600B only present X-ray diffraction peaks indexed to the LiNbO3 crystalline phase.

x x

Intensity (arb. units)

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x

x

x

x

xx

x x o o

o

o

o

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x

o

o

TT700 x x

xx x

x

x

x

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x

TT600

TT575 TT550 10

20

30

40

50

60

70

2

Figure 3.3.3. XRD patterns of the 34Si composition samples: as-prepared and heat-treated at temperatures between 550 and 700 ºC (x LiNbO3; o Li2Si2O5). Niobium: Properties, Production and Applications : Properties, Production and Applications, Nova Science Publishers, Incorporated, 2011. ProQuest

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Figures 3.3.7 to 3.3.10 shows the Raman spectra of the samples thermal-treated with and without applying an external electric field applied. x

+ x x

Intensity (arb. units)

550C

+

+

x +

+

550B

550A

TT550

10

20

30

40

50

60

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2

Figure 3.3.4. XRD patterns of the 34Si samples TTE at 550 ºC (x LiNbO3; + Li3NbO8). x x x

575C x

Intensity (arb. units)

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+

x

x

x x

+

x

x x

575B

x

x x

575A x

x

TT575

10

20

30

x

40

50

60

2

Figure 3.3.5. XRD patterns of the 34Si samples TTE at 575 ºC (x LiNbO3; + Li3NbO8).

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The Raman results shows, for the samples TT at temperatures below 575 °C (Fig. 3.3.7) and in the TTE samples 550A, 550B, 575A and 575B (Figs. 3.3.8 and 3.3.9), the presence of broad bands centered on 860-820, 640 and 240 cm-1. In the TT samples, the increase of the thermal treatment temperature at 600 ºC (Fig. 3.3.7) shifts the band centered at 640 to 630 cm-1. The Raman spectrum of this sample also shows bands centered at 433, 365, 325, 275 and 157 cm-1. In the samples TTE with the electric field of 500 kV/m (samples “C”), the Raman spectrum of the surface in contact with the positive electrode is different from the spectrum of the opposing surface. Moreover, the Raman spectrum of the TTE samples 550C(), 575C(-) and 600C(-) are very similar to the spectrum of the samples treated with fields of amplitude less than 500 kV/m. The Raman spectrum of the samples 550C(+) and 575C(+) are similar to the spectrum of commercial LiNbO3. In the sample 600C(+), besides the bands associated with LiNbO3, the presence of other bands centered at 818, 698, 676, 540, 510, 470, 220, 205, 186, 135 and 120 cm-1 was also observed(Fig. 3.3.10). x +

x x x

Intensity (arb. units)

600C (+)

x x

+

+

600C (-)

x

x

x

x

x

x

x

x

+

x

x

x

x

x

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x

x

x

x x

x x

x

x x

TT600

10

20

30

x x

40

x

x

x

50

x x

60

x

70

2

Figure 3.3.6. XRD patterns of the 34Si samples TTE at 600 ºC (x LiNbO3; Li3NbO8 +) (the symbol "+" refers to the sample surface in contact with the electrode positive).

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860

433

240

365

Intensity (arb. units)

325

157

275

630

240

262

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820

640

TT600

TT575

TT500

as-prepared 100

300

500 D (cm-1)

700

900

Figure 3.3.7. Raman spectra of the 34Si as-prepared glass and the glasses treated at 550, 575, 600 ºC.

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625

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676

870 820

640

432

360

240

Intensidade (u.a.)

325

155

260 272

LiNbO3

550C(+)

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550C(-)

550B 550A TT550 100

200

300

400

500

600

700

800

900

1000

D (cm ) -1

Figure 3.3.8. Raman spectra of the 34Si samples TTE at 550ºC (in the graph 550C(-) and 550C(+) indicate the sample surface that was in contact with the negative and positive electrode, respectively).

Figures 3.3.11 and 3.3.12 present the SEM micrographs of the free surface and fracture surface of the 34Si samples heat-treated with and without the presence of an external electric field, respectively. The micrographs of the TT samples show that the increase of the treatment temperature promotes an increase in the number and size of particles. In the samples TT at 575 and 600 ºC, particles in the surface and in the bulk (inside) sample zone were observed.

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240

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860

635

438

860

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300 327

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860

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575C(-)

575B 575A

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240

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263 283

630

LiNbO3

TT575 100

200

300

400

500

600

700

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1000

D (cm ) -1

Figure 3.3.9. Raman spectra of the 34Si samples TTE at 575 ºC (in the graph 575C(-) and 575C(+) indicate the sample surface that was in contact with the negative and positive electrode, respectively).

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676 698

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Niobate Crystals Inserted in a Glass Matrix

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600C(-)

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600B

600A TT600

100

200

300

400

500 600 D (cm-1)

700

800

900

1000

Figure 3.3.10. Raman spectra of the 34Si samples TTE at 600 ºC (in the graph 600C(-) and 600C(+) indicate the sample surface that was in contact with the negative and positive electrode, respectively).

The presence of the 500 kV/m dc field during the heat treatment at 575 °C promotes the formation of a white layer in the surface that was in contact with the positive electrode (Fig. 3.3.2), with a thickness of 100 m, approximately (Fig. 3.3.12.e). On the opposite sample surface, 575C(-), particles were observed but in an amount lower than that observed in the opposing surface (figs. 3.3.12.c and 3.3.12.d). The SEM micrographs revealed that the

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increase of the amplitude of the electric field promotes the increase in the amount of particles. In the TTE samples, particles were also observed in the fracture zones.

(a)

(b)

(c)

(d)

(e) Figure 3.3.11. SEM micrographs of the 34Si composition samples: (a) as-prepared; (b) TT at 550 ºC; (c) TT at 575 ºC; (d) TT at 600 ºC (e) fracture of the TT 600 ºC.

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(b)

(a)

(c)

(d)

(e)

(f)

267

Figure 3.3.12. SEM micrographs of the TTE 34Si composition samples: (a) 575A; (b) 575B; (c) 575C(+) - surface; (d) 575C(-) - surface; (e) 575C(+) – transversal section; (f) 575C(-) – transversal section.

Electric and Dielectric Measurements This section presents the results of the electric measurements (dc and ac conductivity, complex impedance in function of the frequency and temperature) of the 34Si as-prepared glass, TT at 550, 575 and 600ºC and TTE at 575 ºC (575A, 575B and 575C) samples. The dc conductivity (dc) behavior in function of the temperature is showed in Figures 3.3.13 and 3.3.14 for the TT and TTE samples, respectively. It is observed, in all samples, that the increase of the measurement temperature promotes the increase of the dc. The dc decreases with the increase of the heat-treatment temperature. At room temperature (Table 3.3.1) and for the TTE samples treated at 575 ºC, the dc decreases from the sample 575A to the sample 575B and increases for the sample 575C. The sample treated at 575 ºC without

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electric field presents a dc higher than the samples TTE. From the dc conductivity logarithmic representation in function of 1000/T (Figs. 3.3.13 and 3.3.14), the activation energy was estimated. For these calculations, only the experimental points of higher temperatures were used in order to provide a correlation coefficient higher than 0.999. The obtained values are listed in Table 3.3.1. It can be observed that in the samples TT, the Ea(dc) is between 49 and 57 kJ/mol and the value obtained for the samples TT at 575 ºC, 575A and 575B are similar, decreasing to the sample TTE with the higher field amplitude (575C). -11

vidro base TT550

-13

TT575 TT600

-15

ln(dc) (Sm -1)

Série2 -17

Série6 Série7

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Série8 Linear (Série2) Linear (Série6) Linear (Série7) Linear (Série8)

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-1

1000/T (K )

-10

TT575 -12

575A

-14

575B 575C

-16

Série5

-1

ln(dc) (Sm )

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Figure 3.3.13. Logarithm of the dc in function of 1000/T for the 34Si samples: as-prepared and TT at 550, 575 and 600 ºC.

-18

Série6 Série7

-20

Série8

-22

Linear (Série5) Linear (Série6) Linear (Série7) Linear (Série8)

-24 -26 -28 2.6

2.8

3

3.2

3.4

3.6

3.8

1000/T (K-1)

Figure 3.3.14. Logarithm of the dc in function of 1000/T for the TTE 34Si samples at 575 ºC.

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vidro base

-11

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ln(ac) [Sm-1]

TT600 -13

-14

-15

-16 3.1

3.2

3.3

3.4

3.5

3.6

3.7

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1000/T [K-1]

Figure 3.3.15. Logarithm of the ac in function of 1000/T for the 34Si samples: as-prepared and TT at 550, 575 and 600 ºC. -11 TT575 -11.5

575A 575B

ln (ac) [Sm-1]

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-12

575C Linear (575A) Linear (575C) Linear (TT575)

-12.5

-13

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-14

-14.5 3.1

3.2

3.3

3.4

3.5

3.6

3.7

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1000/T [K-1]

Figure 3.3.16. Logarithm of the ac in function of 1000/T for the TTE 34Si samples at 575 ºC.

The dependence of the ac conductivity (ac) with the increase of the measurement temperature and frequency is similar to that observed in the samples of composition 60Si and 44Si, i.e., increasing with the rise of the measurement temperature and for a fixed temperature, it increases with the rise of the frequency. Figures 3.3.15 and 3.3.16 show the variation of the ac, at 1 kHz, with the increase of the measurement temperature for the samples treated without and with the presence of an external electric field, respectively. When analyzing Figure 3.3.15, it is clear that at room temperature, the rise of the thermal treatment temperature promotes an increase of the ac conductivity (Table 3.3.1). In the TTE samples

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treated at 575 ºC, it was verified that the ac decreases from the sample 575A to the sample 575B increasing for the sample 575C (Table 3.3.1). The ac activation energy (Ea(ac)) values, calculated using the Arrhenius equation (Figures. 3.3.15 and 3.3.16), of the as-prepared glass and of the TT at 550 and 575 ºC are similar, decreasing for the sample TT at 600 ºC. The highest value of Ea(ac) was observed for the sample 575A. The samples 575B and 575C present values of Ea(ac) very similar (Table 3.3.1). Table 3.3.1. dc conductivity (dc) at 300 K, dc activation energy (Ea(dc)), ac conductivity (ac), at 1 kHz and 300 K and ac activation energy (Ea(ac))

Sample as-prepared 550 575 600 575A 575B 575C

dc (x10-8) [Sm-1] (300 K) 37,50  0,04 11,61  0,01 7,1  0,8 0,179  0,002 1,00  0,01 2,22  0,03 0,0032  0,0004

ac (x10-7) [Sm-1] (300 K; 1kHz) 64,6  1,6 40,5  1,3 37,6  1,4 6,6  0,2 51,6  1,6 41,9  1,8 44,9  1,1

Ea(dc) [kJ/mol] 53,1  0,6 49,2  1,1 57,1  1,4 51,0  3,3 58,4 2,4 61,9  2,0 51,7  2,1

Ea(ac) [kJ/mol] 39,6  1,1 36,0  0,7 37,1  1,0 27,5  0,5 41,2  1,5 36,1  0,9 36,5  1,0

2.5E+05

2.0E+05

vidro base TT550

1.5E+05

Z´´

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TT575 TT600 1.0E+05

575A 575B

5.0E+04

575C 0.0E+00 0.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

2.5E+05

3.0E+05

3.5E+05

4.0E+05

4.5E+05

5.0E+05

Z´

Figure 3.3.17. Z‟‟ versus Z‟, at 300 K, for all 34Si composition samples.

Figure 3.3.17 shows the Z‟‟ versus Z‟ spectra of all samples at room temperature (300 K) and in the frequency range of 10 mHz to 10 MHz. A quantitative characterization of those spectra was performed using the CNLLS algorithm associated with the electrical equivalent circuit model presented in Figure 2.4. The results of those fits are illustrated by the lines in Figures 3.3.17 to 3.3.24. The values of the electric circuit parameters are registered in Table 34Si – annex. The dielectric permittivity characteristics for all samples and temperatures are also registered in that table. Semi-arcs are observed in the high frequency range, for all TT

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samples (Figure 3.3.17). It is also observed that with the increase of the thermal treatment temperature, the value of the parameter R increases (table 34Si-annex). In the samples TTE at 575 ºC, the sample 575B presents the highest value of R. The Q0 parameter shows a maximum for the sample TT at 575 ºC. This parameter decreases with the increase of the amplitude of the applied electric field. The n parameter presents small fluctuations (0.79 0.84), with the different treatment conditions. The value of the CCPE capacitor decreases with the increase of the thermal treatment temperature and increases with the increase of the amplitude of the applied electric field. Among all of the TT samples, the one treated at 575 ºC and at room temperature conditions presents the maximum relaxation time (). For the samples TTE, the same behavior is observed on the 575B sample.

Z´´

4.0E+05

315K

3.5E+05

310K

3.0E+05

305K

2.5E+05

300K 295K

2.0E+05

290K 1.5E+05

285K

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280K

5.0E+04 0.0E+00 0.0E+00

1.0E+05

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Z´

5.0E+05 4.5E+05 4.0E+05 3.5E+05

290K 285K Theoric Theoric Theoric Theoric Theoric Theoric Theoric

3.0E+05

Z´´

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Figure 3.3.18. Z‟‟ versus Z‟ for the as-prepared glass (34Si composition).

Theori c Theori 8.0E+05 c Theori c Theori c Theori c Theori 315K c 310K Theori c305K Theori 300K c295K

2.5E+05 2.0E+05 1.5E+05 1.0E+05 5.0E+04 0.0E+00 0.0E+00

1.0E+05

2.0E+05

3.0E+05

4.0E+05

5.0E+05

6.0E+05

7.0E+05

8.0E+05

9.0E+05

1.0E+06

Z´

Figure 3.3.19. Z‟‟ versus Z‟ for the TT sample at 550ºC (34Si composition).

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Figures 3.3.17 to 3.3.24 show the Z‟‟ versus Z‟ spectra, for all samples at several measurement temperatures, with the exception of Figure 3.3.21, which is related with the sample TT at 600 ºC that represents the logarithmic dependence of Z‟‟ with the frequency. It is verified that, in all samples, the increase of the measurement temperature promotes a decrease of the R parameter, the increase of the Q0 parameter and the n parameter remains, approximately, constant (Table 34Si-annex). The value of the CCPE is practically constant and the value of Z decreases with the increase of the measurement temperature, for all samples. 6.0E+05

315K 310K 305K 300K 295K 290K 285K Theoric Theoric Theoric Theoric Theoric Theoric Theoric

5.0E+05

Z´´

4.0E+05

3.0E+05

2.0E+05

1.0E+05

0.0E+00 0.0E+00

2.0E+05

4.0E+05

6.0E+05

8.0E+05

1.0E+06

1.2E+06

Z´

Figure 3.3.20. Z‟‟ versus Z‟ for the TT sample at 575ºC (34Si composition). 1.0E+07

315K 310K 305K 300K

1.0E+05

295K 290K 1.0E+04

285K

Z´´

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1.0E+06

280K 1.0E+03

1.0E+02

1.0E+01

1.0E+00 10

100

1000

10000

100000

1000000

Frequência [Hz]

Figure 3.3.21. Z‟‟, in logarithmic scale, versus frequency for the sample TT at 600 ºC.

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4.0E+05 315K 310K 305K 300K 295K 290K 285K Theoric Theoric Theoric Theoric Theoric Theoric Theoric

3.5E+05 3.0E+05

Z´´

2.5E+05 2.0E+05 1.5E+05 1.0E+05 5.0E+04 0.0E+00 0.0E+00

1.0E+05

2.0E+05

3.0E+05

4.0E+05

5.0E+05

6.0E+05

7.0E+05

8.0E+05

Z´

Figure 3.3.22. Z‟‟ versus Z‟ for the TTE sample at 575 ºC with a field of 50 kV/m (575A - 34Si composition). 5.0E+05 315K 310K 305K 300K 295K 290K 285K Theoric Theoric Theoric Theoric Theoric Theoric Theoric

4.5E+05 4.0E+05 3.5E+05

Z´´

3.0E+05 2.5E+05 2.0E+05 1.5E+05

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1.0E+05 5.0E+04 0.0E+00 0.0E+00

1.0E+05

2.0E+05

3.0E+05

4.0E+05

5.0E+05

6.0E+05

7.0E+05

8.0E+05

9.0E+05

1.0E+06

Z´

Figure 3.3.23. Z‟‟ versus Z‟ for the TTE sample at 575 ºC with a field of 100 kV/m (575B - 34Si composition).

The sample TT at 600 ºC shows a dielectric behavior (Fig. 3.3.21) similar to that already observed in the 44Si composition samples, treated at 600 and 650 °C. The dependence of Z'', in logarithmic scale with the frequency, also in logarithmic scale, is approximately linear and therefore can be adjusted by the Curie-Von Schweidler model. Table 3.3.2 summarizes the results of these adjustments. It can be observed that both parameters decrease with the increase of the measurement temperature

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Manuel Pedro Graça, Manuel Almeida Valente and Maria G. Ferreira da Silva 5.0E+05 315K 310k 305K 300K 295K 290K 285K Theoric Theoric Theoric Theoric Theoric Theoric Theoric

4.5E+05 4.0E+05 3.5E+05

Z´´

3.0E+05 2.5E+05 2.0E+05 1.5E+05 1.0E+05 5.0E+04 0.0E+00 0.0E+00

1.0E+05

2.0E+05

3.0E+05

4.0E+05

5.0E+05

6.0E+05

7.0E+05

8.0E+05

9.0E+05

1.0E+06

Z´

Figure 3.3.24. Z‟‟ versus Z‟ for the TTE sample at 575 ºC with a field of 250 kV/m (575C - 34Si composition).

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Table 3.3.2. Curie-Von Schweidler model parameters (k and n) for the TT sample at 600 ºC, at several temperatures Temperature [K] 315 310 305 300 295 290 285 280 275 270

k (x108)

n

2,340 2,717 3,084 3,446 3,781 4,108 4,416 4,702 4,948 5,162

0,9263 0,9379 0,9475 0,9560 0,9630 0,9692 0,9745 0,9792 0,9829 0,9860

Table 3.3.3. real and imaginary part of the dielectric permittivity and dielectric loss, measured at 1 kHz and 300 K Sample As-prepared 550 575 600 575A 575B 575C

‟ 69,0  1,7 60,95  1,9 54,1  2,0 39,9  1,2 62,5  1,9 60,3  2,6 65,6  1,7

‟‟ 116,1  2,8 72,8  2,3 67,6  2,5 11,9  0,4 92,9  2,9 75,3  3,2 80,7  2,1

tan 1,68  0,06 1,19  0,05 1,25  0,06 0,30  0,01 1,49  0,06 1,25  0,07 1,23  0,04

It was observed that the dielectric constant value (Table 34Si-annex) increases with the increase of the measurement temperature. The same behavior was observed for the dielectric

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loss. Table 3.3.3 presents the dielectric permittivity values of all samples, measured at 300 K. The increase of the thermal treatment temperature induces an increase of ‟. In the samples TTE, the dielectric constant is between 60 (sample 575B) and 65 (sample 575C). Sample 575A shows intermediate value. However, the dielectric constant of any one of these samples is larger than the one obtained in sample TT at 575ºC (~ 54).

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4. ANALYSIS The ATD results of the as-prepared glasses of the compositions 60Si, 44Si and 34Si revealed the presence of exothermic and endothermic effects. In the 34Si composition sample, the presence of two exothermic peaks centered at 645 and 945 ºC was verified. The effect at 645 °C shifts to higher temperatures with the increase of silicate oxide content (670 °C, for the 44Si sample and 710 ºC for the 60Si sample). This exothermic effect is attributed, according to the XRD results (Figures 3.1.3, 3.2.3, 3.3.3), to the formation of the LiNbO3 crystal phase and the shift to higher temperatures should be associated with the progressive increase of the molar amount of SiO2. In the 34Si composition, the DTA revealed a second exothermic effect at 945 °C, which is attributed to the formation of a SiO2 crystal phase (Figure.3.3.3). The non-detection of this phase in the 44Si and 60Si compositions may be related to the molar ratio between [Li]/[Nb]. In the 34Si composition, this ratio is equal to 1 which suggests that after the formation of the LiNbO3 phase, the amount of lithium and niobium ions structurally inserted in the glass network is minimal or null. In the compositions 44Si and 60Si, the [Li]/[Nb] ratio is always > 1. Thus, in the extreme condition of the entire inserted niobium being incorporated in the crystal structure of LiNbO3, some lithium ions will remain in the voids of the glass structural network, which in our opinion, is the major factor responsible for the non-formation of the cristobalite phase. The DTA results provided the basis for the heat treatments parameters, performed with and without the presence of an external electric field. The yellow and transparent 60Si as-prepared glass becomes translucent with the TT at 650 °C and opaque with the TT at 700 ° C (Fig. 3.1.1). In the 44Si and 34Si glasses, the TT at temperatures above 600 ºC makes them opaque (Figs. 3.2.1, 3.3.1). The translucent (and opaque) appearance is a characteristic that may indicate the presence of particles dispersed in the glass matrix. However, it is important to note that an optically transparent glass may contain particles dispersed in the network. According to Todorovic and colleagues [tod97; kim96], a glass ceramic is transparent when: i) particles dispersed in the matrix have a minimum size such that the scattering of the visible light in the particle-matrix boundaries is minimal; ii) the difference between the value of the refractive index of the particles and of the glass matrix is minimal. The latter case is visible in transparent silicate glasses containing Li2Si2O5 particles, whose refractive index (n ~ 1.5 [fuss03; yag03]) is very near to the refractive index of SiO2 glass (n ~ 1.4 [nav91]). So, knowing that the XRD patterns of the 60Si sample TT at 650 ºC, revealed the presence of LiNbO3 and Li2Si2O5 crystalline phases (Figure. 3.1.3), it is reasonable to assume that the translucent appearance observed in these samples can be related to the presence of the LiNbO3 crystallites, which are characterized by a refractive index of ~ 2.2 [abo89], quite distant from that of SiO2 (~ 1.4). The XRD of the 60Si sample TT at 650 ºC (Figure 3.1.3) shows that the number and intensity of the

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diffraction peaks indexed to the LiNbO3 phase are larger than those related to the Li2Si2O5 phase. This suggests the possible presence of a larger amount of LiNbO3 particles dispersed throughout the glass matrix than Li2Si2O5 particles. However, in the 60Si sample TT at 600 ºC, which appears optically transparent, particles with an average size of 1m, approximately, were observed by SEM(Figure. 3.1.11), but not detected by XRD (Fig. 3.1.3). This phenomenon may indicate that the observed particles present an amorphous nature or have an incipient crystallinity. The same phenomenon was observed in the 44Si composition samples TT at 575 ºC (Figures 3.2.3). The opaque appearance observed in the glass samples of the 44Si and 34Si compositions treated at temperatures above 600 ºC, is attributed to the presence of LiNbO3 crystalline phase detected by XRD (Figures 3.2.3, 3.3.3). However, it must be pointed out that in these two compositions, the heat treatment at 700 °C favors the formation of a second crystalline phase (Li2Si2O5). The 60Si samples TTE at 600 ºC (samples 600B and 600C), are translucent and present an opaque white layer on the surface which was in contact with the positive electrode during the treatment (Figure 3.1.2). Through the XRD patterns of these samples (Figure 3.1.4), the formation of LiNbO3 and Li2Si2O5 crystalline phases was verified, indicating that the application of an electrical field during the thermal treatment promotes a localized crystallization and at lower temperatures. It was found that increasing the amplitude of the applied electric field causes an increase in the particle size (Figure 3.1.4). Analyzing the amplitude of applied external electric field parameter for a fixed heat-treatment temperature, a critical value of electric field amplitude exists. When exceeding this critical value of amplitude, there is an increase in the electrical current which flows in the glass leading to the formation of dark regions (Figure 3.1.2). The formation of these zones, according to the studies of Kusz [kus03] and Zeng [zen97], can be explained by a oxidation-reduction reaction, activated by the electric field between the glass and the electrodes. Kusz [kus03] noted, based on the study of ionic conduction in silicate glasses, that the onset of the dark regions indicate the existence of a mechanism for oxygen ion conduction during TT that comes in line with the suggestion made by Zeng and colleagues [zen97]. Thus, the main redox reaction can be described by the following equations: Reaction at the anode: O 2 glass network 

1 O2 glass - anode interface   2e  2

Reaction at the cathode: Li glass network  e  Liglass - cathode interface  



Based on the analysis of the above equations, it can be suggested, taking into account this glass system, the possibility of reducing the Nb5+ ion to a lower oxidation state (analogous to what occurs with the lithium). However, according to the studies presented by Zeng [zeng97], on the glass composition 50SiO2-25Li2O-25Nb2O5 (mole %), this possibility is unlikely. The reactions schematized in the previous equations were confirmed by electro-chemical studies. [zen97; ger99]. In the 44Si composition, the TTE at 575 ºC changes the sample appearance, initially transparent to opaque, which does not occur with the TT at the same temperature (575 °C) but without the presence of the applied electric field. These macroscopic results suggest, according to the ones already observed on the 60Si composition, that the presence of an

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external electric field during the heat-treatment process favors the crystallization at lower temperatures. The XRD spectra of the samples TTE at 575 °C (Figure 3.2.4) confirm this hypothesis by the detection of the LiNbO3 crystalline phase. The samples TTE at 600 ºC (samples 600A and 600B) presents the LiNbO3 crystal phase but also the Li2Si2O5 and Nb2O5 phases (Figure 3.2.5). The amplitude of the external electric field is a major factor because the use of electric field with high amplitude (> 100 kV/m) results in the formation of dark regions (Figure 3.2.2) as also observed on the 60Si composition samples. The detection by XRD of the Li2PtO3 phase, on the platinum plate which forms the negative electrode, indicates that the application of electric fields of high amplitude can even promote the migration of ions structurally inserted in the glass network. When analyzing the morphology of the 44Si composition samples TTE (575A and 575B), it is suggested that the rise of the electric field amplitude causes an increase in the particle size (Figure 3.2.10). The opposite surface of the 575A sample presents different macroscopic aspects, i.e., the surface which was in contact with the positive electrode has a white color layer which does not occur in the opposing surface. However, the results of XRD and Raman spectroscopy revealed no significant differences between these two surfaces and the scanning electron microscopy only revealed the presence of a larger number of particles on the surface where it is observed the white layer. Contrasting with what occurred in the 44Si and 60Si composition samples, the application of a TTE with an electric field of 500 kV/m to the 34Si samples did not lead to the appearance of dark areas. We believe that one possible justification for the non-formation of these areas is related to the fact that in this composition, the ratio [Li]/[Nb] = 1 and with the formation of LiNbO3 phase, the number of lithium ions structurally inserted in the network decreases, inhibiting (or limiting) the redox reaction responsible for the appearance of those regions. In the 44Si and 60Si compositions, the ratio [Li]/[Nb] > 1, which leads to, in the extreme case of all niobium ions being structurally inserted in the crystalline phase of LiNbO3, the existence of some “free” lithium ions, contributing to the reaction that promotes the darkening aspect. According to the XRD results (Figure 3.3.4), the application of the electric field of maximum amplitude used (500 kV/m) in the treatment at 550 °C induced the formation of the LiNbO3 crystalline phase, not detected in the samples treated with fields of lower amplitudes. This result confirms the hypothesis raised for the 44Si and 60Si composition samples that the presence of an electric field favors the crystallization process at lower temperatures when compared with the heat-treatment process in the absence of the electric field. The detection in the Raman spectra of the 60Si samples surface that during the TTE were in contact with the positive electrode (Figures 3.1.8 to 3.1.10), of the bands centered at 630, 439-437, 370, 335-334, 280, 265, 239 and 180 cm-1, attributed to vibrations of octahedra NbO6 associated with the LiNbO3 crystalline phase [nas78; shi81; fuk88; ume88; hir93; and99; lip03] shows that the presence of a external electric field favors the formation of the LiNbO3 crystalline phase in a localized area. The Raman band at 750 cm-1, observed on the 600C sample surface that was in contact with the positive electrode during the TTE process, is attributed to Si-O-Si vibrations [shi81; efi99] that are related to the Li2Si2O5 crystal phase and the band centered at 119 cm-1, is associated with Si-O-Si bending [shi81]. The major part of the research made about the insertion of niobium ions in glass networks is done by comparison with the properties of a crystal whose structure is known, such as LiNbO3. Most of the crystals containing niobium are formed by NbO6 octahedra with

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different degrees of distortion. The detection in niobium crystals of vibrations associated with NbO4 tetrahedra is rare and justified by the fact that the Nb5+ ion presents a size too large to be inserted in a tetrahedron of oxygen ions [aro05]. Cardinal and colleagues [car97] reported that the progressive introduction of Nb2O5 in boron-phosphate glasses gives rise to a Raman band close to 900 cm-1, attributed to vibrations of free NbO6 octahedra, which shifts to lower wave-numbers with the increase of the Nb2O5 concentration, suggesting the formation of a NbO6 octahedra network linked by the vertices. Thus, most authors indicate that the Raman band between 800 and 940 cm-1, detected in glasses containing niobium, is due to vibrations of isolated NbO6 octahedra [fuk88; car97]. However, this analysis does not consider the possibility of the niobium ion lying structurally inserted in the glass matrix as a network former. The studies of Alekseeva and colleagues [aro05], in glasses of the system K2ONb2O5-SiO2, suggest that for molar quantities of Nb2O5 below 20%, the Raman band between 800-950 cm-1 should be related to vibrations of NbO4 tetrahedra. For higher concentrations of Nb2O5, the band is assigned to vibrations of NbO6 octahedra. The increase of the distortion degree of these octahedra is related to the displacement of this band to higher wave-numbers. This band is also linked to the NbO6 octahedra that contain at least one terminal Nb-O bond which is shorter than the other Nb-O bonds. Lipovski and colleagues [lip01; lip03] relate this band to non-bridging Nb-O vibrations. The same trend was observed in the SiO2:Nb2O5 binary glasses where there is a certain amount of niobium introduced as network formers [aro05]. Thus, the detection by Raman spectroscopy (Figures 3.1.8 to 3.1.10) of the band centered at 870 cm-1 indicates that some niobium ions are probably introduced into the glass matrix as network formers [gra05; shi81; and99; lip03; efi99]. The same behavior is observed in the 44Si composition samples (as-prepared, TT at 550 and 575 ºC and TET at 550 ºC), by the presence of the band centered at 870 cm-1 (Figures 3.2.6-3.2.8). Considering that some niobium ions are in the glass matrix as network modifiers, the increase of the TT temperature results in a reduction of this number due to their inclusion in the LiNbO3 crystal structure. Consequently, the volume ratio between the particles dispersed in glass matrix and the glass matrix increases. The samples of the 44Si composition where the LiNbO3 crystal phase was detected by XRD (Figs. 3.2.3 to 3.2.5), show Raman bands centered at 630, 435, 370, 333, 303, 274, 260, 240 and 157 cm-1 , attributed to vibrations of the NbO6 octahedra associated with the LiNbO3 crystalline phase [nas78; shi81; fuk88; ume88; hir93; and00; and99; lip03]. The Raman spectrum of the samples TT at 600 and 650 ºC and TET at 600 ºC (600A and 600B) also presents a vibration band at 502 cm-1, which according to Shibata and colleagues [shi81,] must be assigned to the longitudinal vibration mode of the Si-O-Si bond. Finally, the Raman spectra of 34Si composition samples (as-prepared, TT at 550 and 575 ºC and TET at 550 (550A, 550B and 550C (-)) and at 575 ºC (575A, 575B and 575C (-)) (Figs.3.3.7 to 3.3.10) shows vibration bands at 860-820, 640-635 and 240 cm-1. The bands at 640-635 and 240 cm-1 can be attributed to vibrations of NbO6 octahedra dispersed in glass matrix [and00, and99; lip03]. The band in the region of 860 to 820 cm-1 shifts to higher wave numbers with the increase of the thermal treatment temperature indicating, according to Cardinal and colleagues [car97], an increase in the degree of the distortion of the NbO6 octahedra. However, according to the results of the 60Si and 44Si composition glasses, it is suggested that the band observed at 820 cm-1 in the as-prepared glass can also be associated with the presence of NbO4 tetrahedral species [aro05]. In the Raman spectrum of the 600C(+)

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sample (Figure 3.3.10), a band at 818 cm-1 is detected, which is attributed to vibrations of NbO4 tetrahedra and a band at 892 cm -1, associated with “free” NbO6 octahedra [aro05]. In the 34Si samples, TT at 600 ºC, 550C(+), 575C(+) and 600C(+), besides the bands at 818-820, 640-630 and 240-235 cm-1, cumulatively, the bands at 433-427, 365-358, 344-300, 275 and 157 cm-1 are also present, which are related to the vibrations of the NbO6 octahedra associated with the LiNbO3 crystal structure (Figure 3.3.8) [nas78; shi81; fuk88; ume88; hir93; and99 ; lip03; des05]. The presence of the LiNbO3 crystalline phase, confirmed by the XRD pattern (Figures 3.3.3 to 3.3.6), corroborates this assignment. In the Raman spectrum of the 600C(+) sample (Figure 3.3.6), the bands centered at 676, 470, 205 and 186 cm-1 are in accordance with the results of Lipovskii and colleagues [lip03; jua99; hua99], for silicate glasses with niobium, and attributed to NbO6 octahedra vibrations. The band centered at 510 cm-1 may be related to the longitudinal vibration mode of the Si-O-Si bond [shi81] and the band at 135 cm-1, consistent with the results of Shibata [shi81], is associated with the Si-O-Si bond curvature. The Raman spectra of the opposing surfaces of the samples 550C, 575C and 600C, are different. The fact that the XRD spectra, also made in both surfaces, does not present differences, unlike Raman, can be justified by the different penetration depth of the incident radiation of the two techniques. For the case of X-rays is in the order of 100 m [jen96], and for the Raman, is only a few nanometers as observed by Zhang and colleagues [zha94] for amorphous silica (with an incidence of 514.5 nm found a penetration depth of 32.9 nm). The decrease of the dc conductivity (dc), in the 60Si, 44Si and 34Si composition samples (Tables 3.1.1, 3.2.1, 3.3.1), with the increase of the thermal treatment temperature can be related to the formation of crystalline phases. The formation of these phases leads to a decrease in the number of the network modifiers ions (Li+ and Nb5+) leading to the decrease of the conductivity. On the other hand, knowing that the conductivity of lithium silicate glasses at room temperature is approximately 10-9 S/m [marc00], knowing that the LiNbO3 crystals presents a high electric resistivity (~1021 cm, at 300 K [abo89]), and the conductivity of the Li2Si2O5 crystallites in the same temperature region is ~ 10-12 S/m [kone97], it is clear that the increase of the amount of crystalline phases, particularly of LiNbO3, will contribute to the observed increase of resistivity. The dc activation energy (Ea(dc)), in the samples of the 60Si composition decreases from the sample treated at 600 ºC to the sample TT at 650 ºC (Table 3.1.1). This reduction indicates a decrease in the height of the glass network potential barriers, making the conduction process by "jumps", less difficult resulting in a higher mobility of the charge carriers [mac72]. It can be seen that the conductivity decreases and thus the dominant factor for this conduction process will not be the mobility but the number of charge carriers. For all the samples of the 44Si composition, the Ea(dc) (Table 3.2.1), is similar with the exception of the one treated at 600 ºC, which has the maximum value of all samples heat-treated in the absence of an external electric field. These results indicate that also in this composition, the parameter number of charge carriers is the one that dominates the dc conduction process. Samples of this composition, TTE at 575 ºC (575A and 575B) present a dc lower than the one of the sample TT at 575 ºC (Table 3.2.1). This difference is justified by the fact that the thermoelectric treatment promotes crystallization and therefore, the amount of network modifier ions present in samples TTE, when compared to the sample TT without external field, is smaller and also the volume ratio between LiNbO3 crystals and glass matrix is higher

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in the samples TTE. The variation of the Ea(dc) with the heat-treatment temperature in samples of 34Si composition is not monotonous, ranging between 51 and 57 kJ/mol (Table 3.3.1). When analyzing the variation of the conductivity with the increase of the thermal treatment temperature, it is expected that the lower value of Ea(dc), related to a lower height of the potential barriers and hence a higher mobility of charge carriers can be observed. Thus, comparing the 34Si samples TT at 575 and 600 ºC, it is verified that the sample TT at 600 ºC has the smaller value of Ea(dc) but also the smaller value of dc indicating that the contribution of the mobility parameter in this conduction process is lower than the number of charge carrier contributions. This phenomenon is also observed in the 575C sample (TTE), which presents a Ea(dc) smaller than all the others (Table 3.3.1), indicating a lower height of potential barriers and thus, the possibility of a larger mobility of the charge carriers. The fact that the sample 575C has the lowest dc of all TTE samples shows that the number of charge carrier parameters is the major factor responsible for this conduction process. In all samples of this composition, the representation of ln(dc) versus 1/T is not linear in all temperature ranges (Figures 3.3.13 and 3.3.14), revealing the existence of at least two conduction mechanisms activated by different energy. The ac conductivity (ac) behavior can be discussed using the potential barriers model [cut98], which assumes that the ions move by jumps in a non-random mode. Thus, in the 60Si samples TTE, the decrease of the ac (Table 3.1.1) with the increase of the amplitude of the electric field must be attributed to the decreased number of network modifier ions and to the increase of the volume ratio between the LiNbO3 crystals, whose dipoles are difficult to depolarize at room temperature, and the glass matrix. The values of ac activation energy (Ea(ac) - Table 3.1.1) for the various samples treated at different temperatures are similar and knowing that the increase of the heat-treatment temperature promotes the formation of crystals (Fig. 3.1.3), suggests that this parameter is not very dependent on the presence and/or quantity of particles dispersed in the glass network. In the 44Si composition samples, the decrease of the ac from the sample TT at 550 ºC to the sample TT at 650 ºC (Table 3.2.1) must be attributed to the decrease of the number of network modifier ions and LiNbO3 volume increase. The Ea(ac), is a parameter that can be related to the mobility or to the ease/difficulty of the dipoles to follow the external AC field, decreases considerably from the sample TT at 575 to the sample TT at 600 ºC and must be related to the presence, observed by XRD, of the LiNbO3 crystals (Fig. 3.2.3). The sample TT at 650 ºC shows a Ea(ac) larger than the one of the sample TT at 600 ºC but lower than that of the one TT at 575 ºC (Table 3.2.1) indicating that the ac is very dependent on the number of network modifier ions and on the volume amount of LiNbO3 crystallites present in the glass matrix. The ac of the 575A and 575B samples is lower than that of the sample TT at 575 ºC (Table 3.2.1). This behavior suggests that with the increase of the amplitude of the electric field, the volume amount of LiNbO3 crystals increases and/or the number of network ions decreases. For the 34Si samples, the observed decrease of ac with the increase of the thermal treatment temperature (Table 3.3.1), is attributed to the decrease of the number of network modifiers ions and to the increase of the LiNbO3 crystalline quantity, whose dipoles at room temperature are difficult to depolarize [abo89]. In the samples treated at temperatures below 600 ºC, the Ea(ac) is approximately 37 kJ/mol, decreasing for the sample TT at 600 ºC (Table 3.3.1), which indicates that the electric dipoles of the sample TT at 600 ºC present a higher

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mobility to follow the external ac field. These results suggest that the ac depends more on the number of dipoles associated with the network modifier ions present in the glass matrix, and consequently, on the amount of LiNbO3 crystals than on the glass matrix characteristics. Micrographs of the 60Si and 44Si composition samples TTE present two zones with different morphology: the surfaces and the bulk. The surfaces present particles but in the inside zone (referred in this text as bulk) were not observed particles, indicating that they do not exist or their size is below the detection limit of SEM system. It must be noted that in 60Si composition samples TTE (600B, 600C), the surface that was in contact with the positive electrode during the treatment has a higher number of crystalline particles than the opposite surface. Thus, the electrical and dielectric analysis was performed considering the equivalent electrical circuit shown in Figure 4.1.

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Figure 4.1. Electric circuit model: i) sample (a - surface, b - inside zone), ii) electric model where Ra and Ca represent the resistance and capacity associated with the surface of the sample, the resistance R b and the capacity Cb are associated with the insider zone of the sample (bulk); iii) simplified model.

In dielectric analysis, the equivalent electric circuit (Figure 4.1) can be resumed to a combination of a series of three capacitors, each one in parallel with a resistance element: two related with the sample surface characteristics and the third with the bulk characteristics. Knowing that the thickness of all samples is between 1.0 and 1.4 mm, that the surface which was in contact with the positive electrode has a thickness of 50 - 100 m, while the opposite side presents a layer with a thickness of about 60 m and if it is assumed that the capacity

 d ), essentially related to the dielectric constant (') of

related with the surfaces ( C   ´ 0 A

LiNbO3, is greater than the capacity of the bulk zone, it is reasonable to assume that the largest contribution to the dielectric behavior is given by the bulk zone by presenting a value of capacity far below the value associated to the surfaces. Briefly, the initial electric circuit can be approach to a simple electric circuit consisting of the parallel between a resistance and a capacitor if we consider Ca> Cb (see Figure 4.1). According to this model, the values of dielectric constant (') of the samples of the 60Si composition that were submitted to TTE (Table 3.1.1), which are similar to the base glass, should be assigned to the characteristics of the bulk zone of the sample. In the samples of this composition TT, the increase of ' (Table 3.1.1), with the increase of the thermal treatment temperature is attributed to the increase in accordance to the results of XRD and SEM, to the increase of the volume ratio between the particles of LiNbO3 and the glass matrix. For the samples of 44Si composition, it was found that the value of ' decreases with the increase of the TT temperature (Table 3.2.1), which can

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be attributed to the decrease of the number of dipoles existent in the bulk sample zone, which are associated with the number of network modifier ions present in that zone. However, we cannot rule out the possibility of the LiNbO3 crystals, detected mostly on the surfaces, grow with random directions which leads to a decrease in the contribution of these units for the dipole moment. The sample 575A (of the 44Si composition), TTE at 575 ºC with an electric field of 50 kV/m, presents a ' greater than any other sample annealed without an electric field, which suggests that in this composition, this TTE should promote the growth of small LiNbO3 crystals in the inside sample zone with a preferential crystal grow direction. However, the sample 575B has a „ value below the value observed for the sample TT at the same temperature (575 ºC), which implies, according to this model, that the contribution of the LiNbO3 crystals, that possibly exist in the bulk zone, decreases the dipole moment. These results indicate that the increase of the electric field amplitude favors the surface crystallization, which justifies the decrease in number of the electric units within the sample. In all samples of this composition, the increase of the measurement temperature promotes the increase of the ' value (Table 44Si-annex). This behavior shows that by increasing the temperature, the mobility of the dipoles is easier. Samples of 34Si composition show a decrease of the dielectric constant value ('- Table 3.3.1) with the increase of the temperature of heat treatment. This behavior is justified, first by the decrease of the number of the network modifier ions, which should be associated with “free” dipoles and secondly, by the possibility of the LiNbO3 crystals, dispersed in the glass matrix, do not present a preferential crystal orientation, implying that its contribution to the overall dipole moment may be of little relevance. The samples TTE present a value of „ higher than the value observed on the sample TT at 575 ºC, which increases with the increase of the amplitude of the external electrical field indicating that the presence of the external field favors an increase in the number of LiNbO3 crystals when compared with the sample treated without the presence of the external field (Figs. 3.3.11 and 3.3.12). Nevertheless, the fact that the ' value decreases with the increase of the heat-treatment temperature in the samples heat-treated without the presence of an external field, must be related with the volume ratio between the particles and the glass matrix. It is suggested that the increase of ' from the sample 575A to the sample 575C (Table 3.3.1) is due to the increase of the volume ratio between the LiNbO3 crystals, which should present a preferential crystal growth orientation and the glass matrix. In all samples, with increasing the temperature of measurement, the ' value increases (Table 34Si-annex). This indicates that increasing the temperature at which the sample is, facilitates the polarization of the dipoles by the external ac electric field. From the dielectric measurements, the Z'' versus Z' spectrum of the 60Si samples (Fig. 3.1.16) as-prepared, TT at 550 and 575 ºC and TTE at 575 ºC with 50 kV/m (the latter only for measuring temperatures above 300 K), of the 44Si samples (Figures 3.2.16 to 3.2.18 and 3.2.21) and all samples of the 34Si composition, with the exception of the one TT at 600 ºC (Figures 3.3.18 to 3.2.20 and 3.3.23), present semi-circles whose centers are below the Z‟ axis, indicating the existence of a relaxation times distribution [mac87; kre02; ngai86], which, in these glasses, should be related to the presence of several dielectric components such as the glass matrix, the crystalline phases and also dipoles from other electric units, such as network modifier ions. In the low frequency range (f 1%). Take copper (Cu) cavities as an example. For cw operation, the power dissipation through the walls of a Cu cavity is huge. This is due to the fact that the dissipated power per unit length of an accelerating structure is given by the following formula: =

(8)

Here ra/Qo is the geometric shunt impedance in Ω/m, and it depends primarily on the geometry of the accelerating structure. For Cu that has a resistance that is typically 5 orders of magnitude higher than that of a microwave surface resistance of a superconductor, Qo is typically 5 orders of magnitude lower. Some simple calculations can show that if CEBAF used Cu cavities and operated at cw mode with an accelerating gradient of 5 MV/m, the dissipated power for each cavity could have been near 450 kW. This already exceeds the 100 kW power dissipation limit for a Cu cavity since above which the surface temperature of a Cu cavity will exceed 100 oC. This will cause a number of unwanted effects such as, for instance,

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vacuum degradation, stresses in the Cu, metal fatigue due to thermal expansion. Therefore for Cu cavities, high accelerating gradients larger than 50 MV/m can only be produced for a period less than a few microseconds before the RF power needed becomes prohibitive. In contrast, the same CEBAF machine based on Nb would need to dissipate power of only a few watts. Of course, one have to consider also the cost of cooling a superconducting material down to a temperature below Tc and normally the efficiency of refrigerators that are used to cool down the material is low. Nevertheless, for CEBAF operated at 2 K a reduction in the operation cost by a factor of 0.01 to 0.001 can be realized. Apart from the general advantages of reduced RF capital and associated operation costs, superconductivity offers certain special advantages that stem from the low cavity wall losses. Because of superconductivity, one can afford to have a relatively larger beam holes in superconducting cavities than for normal ones. This significantly reduces the sensitivity of the accelerator to mechanical tolerances and the excitation of parasitic modes. Also larger beam holes reduce linac component activation due to beam losses. Superconducting cavities are intrinsically more stable than normal conductor cavities. Therefore the energy stability and the energy spread of the beam are better. We know that there are many superconductors in the world. Why do we select Nb as the major material for building SRF cavities? Apart from some historical reasons, the first obvious answer is that Nb has the highest Tc of 9.25 K among the all available elements in the period table. This makes the requirement for cooling the cavities down to a temperature below Tc a relatively easy task. Furthermore, since Eacc is proportional to the peak electric field (Epk) and peak magnetic field (Hpk) on the surface of a cavity, one has to be sure that the material that is used to make the cavity can sustain large surface fields before causing significant increase in surface resistance or a catastrophic breakdown of superconductivity (called quench). The ultimate limit to accelerating gradient is the theoretical RF critical magnetic field that is called the superheating field Hsh. Nb has the highest Hsh of 0.23 T among the all available metal elements. Another advantage of Nb is that it is relatively easy to be shaped into different structures due to its outstanding ductility and the fact that it is relatively soft (see Table 1). Nb can be cold-worked to a degree more than 90% before annealing becomes necessary. This property is responsible for the recent new developments on fabricating seamless Nb SRF cavities by hydroforming and spinning as described in the following section. Although there are other superconducting compounds that have higher Tc or higher Hsh, they either are not having the three mentioned characters in a superconductor or were discovered much later as superconductors than Nb. It is fair to say that so far Nb is the most investigated material for SRF applications and the major material used in particle accelerators based on SRF technology. Only limited research effort has been put on other superconductors such as NbN (Tc=16.2 K), NbTiN (Tc=17.5 K), Nb3Sn (Tc=18.3 K), V3Si (Tc=17 K), Mo3Re (Tc=15 K), and MgB2 (Tc=39 K). Interested readers for this topic are referred to an overview paper15 from ValenteFeliciano. It is worth mentioning that recently some groups [16] have started to revisit the Nb on Cu as an alternative for making SRF cavities by taking the advantages of the good superconducting properties of Nb and good thermal conductivity and cheap Cu substrates.

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Table 1. Some useful information and constants of Nb

Nb, Niobium (Columbium)

Atomic Atomic Weight: Number: 41 92.9064 g.mol-1

Crystal Color at room structure: temperature: steel bodygrey centered cubic (bcc)

Element category: transition metal

Superconducting Electron transition configuration: temperature: 9.25K [Kr]4d45S1

Electrons per Phase: shell: solid 2,8,18,12,1

First Lattice Specific heat at Covalent Melting Boiling ionization constant at Electronegativity: Poisson‟s ratio: o 15 C: 0.268 (J/g); at radius: Temperature: Temperature: o energy: 6.88 20 C: 1.6 0.38 1.64±6Å 2468 oC 4927 oC 1227oC: 0.320(J/g) eV 3.294Å Recrystallization Stress relieving Workability: ductile to temperature: reactor temperature: reactor Hardness (VHN): Atomic volume: 10.8 Refractive Index: Heat of melting: o brittle transition: grade 900-1300 C; grade 800oC; RRR 60-100 (CC/mol) 899-1204 26.4 kJ/mol o 150 C RRR grade 750-850 grade 649-663 oC o C Thermal conductivity Young‟s Modulus J/(m-sec-deg): @ 0oC Tensile strength, Yield strength, GPa: @20oC Modulus of Coefficient of thermal 52.3; room Thermionic o o typical: MPa @20 C typical MPa: @20 C 98.5; @799oC elasticity: 10600 expansion: oC-1: @500oC temperature 53.7; work function: o 172; @799 C 103; 103; @799oC 69; 82.7; @1199oC kg/mm2 7.47; @900oC 77.94 @302oC 53.6; @ 4.01eV o o @1199 C 34 @1199 C 14 14; @1788oC 799oC 57.1; 51.7 @1600oC 69.1 Atomic Electrical conductivity Dielectric constant of Oxidation states: Magnetic ordering: Shear Modulus: Bulk modulus: Debye Temperature: radius: typical @0oC 152 surface oxide: 41 5,4,3,2,1 paramagnetic 38GPa 170GPa 250K 1.46Å nΩ.m Lower critical Thermodynamic Higher critical Heat of vaporization: Magnetic susceptibility: @ room Bulk Nb is type II magnetic field: 0.17 critical magnetic magnetic field: 0.24 -6 2 694 kJ/mol temperature +2.20x10 gauss.cm /g superconductor tesla field: 0.2 tesla tesla Superheating field: 0.23 tesla Mole entropy: 35 J/mol.K

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Density: 8.57 g/cm3

ocID=3018127.

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I am not going to discuss high temperature superconductors such as, for instance, MgB2, YBCO, BSCCO, etc here since they are not related to Nb that is the topic of this book.

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SECTION 3. FABRICATION OF NB CAVITIES

Figure 4. Flow chart of a typical Nb SRF cavity fabrication steps.

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In order to understand and improve the use of Nb in fabricating Nb SRF cavities, we have to know the typical procedure for doing it. Fig.4 shows a flow chart for a typical procedure of Nb cavity fabrication. This procedure has been well established in the past couple of decades and does not various much from one lab to another. However, significant amount of new developments have taken place in the last decade or so on how each step is done in reality. For instance, Step 15 in Fig.4 can be done in many ways, including Buffered Chemical Polishing (BCP), ElectroPolishing (EP), barrel polishing, or Buffered ElectroPolishing (BEP). In this section, I will first give a general description of the fabrication steps with emphasis on some selected examples of new developments at some steps of the typical cavity fabrication process.

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3.1. Typical Fabrication Steps for a Nb SRF Cavity Normally the as-received Nb sheets from suppliers are either 3 or 4 mm thick. To prevent them from damages and contaminations during transportation, the Nb sheets are covered by sticking tapes that can be peered off. The first thing to do after receiving Nb sheets is to check whether they meet the material specifications. The specifications typically cover RRR value, grain size, impurity tolerance, surface finish, yield strength, and flatness. The sheets are then either deep drawn or spun to form half cells. Trimming is then done to the half cells to remove any irregularities and undesirable features. Normally a lathe or a computer controlled milling machine is employed for the trimming. Since trimming may introduce some contaminants on the surfaces of the half cells, degreasing is then needed. Degreasing normally takes place in soap and water in an ultrasonic tank and then a light BCP of 5 µm is done to further remove any undesirable residuals on the surfaces of the half cells from the previous fabrication or handling steps. After cleaning and visual inspection for surface scratches, defects, and rust, electron beam (E-beam) welding on iris can then be performed. It is a good practice to do some grinding on the welded region to make sure that inner surfaces are smooth. Then a light BCP of 5 µm is done again before E-beam welding on equator takes place. Note here that all welding should be done in vacuum at a pressure less than 10-5 torr to avoid significant impurity intake during these steps. After these steps, we obtain Nb cavities. The cavities are then chemically polished again for about 100 µm in order to make sure that it is completely clean. Then cavities are normally baked at a temperature between 1350 to 1400 o C in a titanium enclosure in a high vacuum furnace for a few hours to purify the cavities. Titanium is a good getter for oxygen, nitrogen, and other gases. This process also serves as an annealing process to remove some of the defects such as edge or screw dislocations generated during the previous fabrication steps, especially from depth drawing and spinning. Then the final and the most important surface Nb removal of 150 µm is followed. This step removes the mechanically damaged layer as well as any evaporated niobium scale deposited on the surface during welding. Then tuning to the correct frequency and field flatness is needed, since a thickness of 260 µm of Nb has been removed the inner surface of a Nb cavity. At this stage, additional 5 µm Nb can be removed from the inner surface of the cavity, but is not necessary. Final rinsing and cleaning are then performed before assembling end flanges and couplers in a clean room. Finally the cavity is pumped down and then baked at 120 oC for 48 hours before RF test.

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3.2. Examples of Innovative Techniques for Fabricating Nb SRF Cavities Recently, Nb SRF cavities have been also fabricated by hydroforming [17] and spinning [18] to create seamless cavities.

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Figure 5. Schematic illustrated the working principle of hydroforming [20].

Figure 6. First double cell seamless Nb SRF cavity fabricated from hydroforming [17].

Fig.5 shows the principle of hydroforming technique. This technique starts with Nb tubes with a diameter half way between the iris and equator. The diameter at iris has to be reduced while the diameter at equator has to be expanded. Since the ratio of the diameters between equator and iris for a typical elliptical cavity is ~3, any attempt to form seamless cavities from

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tubes is a significant challenge. One has to balance the hardness and roughness introduced by the diameter expansion at equator and the diameter reduction at iris to the inner surface of a seamless cavity. It was found [17] that a starting tube diameter between 130 and 150 mm is optimal for a 1.3 GHz cavity. This technique can be used to produce single cell and multi-cell cavities. The very first Nb double cell cavity produced by hydroforming is shown in Fig.6. The highest Eacc for a single cell seamless cavity reaches 43 MV/m as shown in Fig.7.

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Figure 7. Excitation curves measured on a single cell seamless cavity after buffered chemical polishing (bcp) and electropolishing (e-pol) [17].

Figure 8. Photos of various steps during the fabrication of a n1.5 GHz copper cavity by spinning. Seamless Nb SRF cavities are fabricated in the same way [18].

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It was demonstrated [18] by Palmieri that single cell and multi-cell Nb and Cu seamless cavities could also be fabricated by spinning. Fig.8 shows the progressive steps during the fabrication of a single cell seamless Cu cavity by this technique. After spinning, cavity has to be tumbled and mechanically ground for at least 100 µm to remove surface fissures before any further chemical treatment. Typical excitation curves at 1.6 K for a spun seamless single cell Nb cavity are shown in Fig.9. Several single cell cavities reach an accelerating gradient of 40 MV/m with a decent Qo. Fig.10 shows the first nine-cell Nb cavity manufactured by the spinning technique. Unfortunately the cavity was damaged before a RF measurement was done. One challenge for this technique is that the cavities can be quite thin after fabrication.

Figure 9. Typical excitation curves for a single cell seamless Nb SRF cavity measured at 1.6 K. Before the measurements, this cavity was mechanically grinded for 100 µm, then barrel polished for 84 hours, vacuum annealed at 750 oC for 3 hours followed by electropolishing for 50 µm and then high pressure water rinsed [18].

Figure 10.Photo of the first 9 cell Nb SRF cavity fabricated by spinning [18].

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Although there are still many technical problems waiting to be resolved for the seamless cavity formation techniques, the exclusion of welding steps from cavity fabrication process is a very significant progress. These new developments also alter the flow chat shown in Fig.4.

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Figure 11.Shaped Al cathode fabricated at JLab a) top view, b) side view.

Figure 12. Nb dumbbells a) before BEP treatment, b) after BEP treatment.

Another interesting idea for fabricating Nb SRF cavities is that after Step 9 in Fig.4 Step 15 is followed. Then the polished Nb dumbbells are E-beam welded to form muilti-cell cavities. Followed either by a light BCP removal of 5 µm + high pressure water rinse (HPWR) or just simply HPWR before being evacuated for RF tests. The attractiveness of this

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idea is that the final chemical treatment on multi-cell cavities can be avoided, which makes the life in this SRF world much simpler and easier. This can be very important for electropolishing if it is employed as the final chemical treatment, since I personally believe strongly [19] that the cathode shape matters during electropolishing on Nb. The size of a cathode is limited by the size of the beam tube of a cavity if electropolishing has to be done on the cavity. This development is on-going through the collaboration between Peking University (PKU) and JLab. Nb dumbbells will be polished by BEP at PUK employing a shaped aluminum cathode fabricated at JLab (see Fig.11). The thus formed cavities will be treated by HPWR and then RF-tested at JLab. It has been demonstrated [20] by PKU that bright and shining Nb dumbbells can be fabricated by BEP via a shaped Al cathode as shown in Fig.12.

3.3. Examples of New Developments in Inspection Technique In my view, there are several key steps in cavity fabrication process that deserve strong attention to ensure the outcome of good cavities with decent excitation curves. The first of such key steps is to make sure that the as-received Nb sheets meet the specifications.

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3.3.1. Eddy Current And SQUID Scanning One significant new development in this respect is the application of eddy current and Superconducting QUantum Interference Device (SQUID) scanning [21,22] to check the defects on the as-received Nb sheets. This allows the removal of some defective starting materials before going into cavity fabrication.

Figure 13. Schematic illustration of the working principle of an eddy current scanning device for SRF applications [21].

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Figure 14. Schematic illustration of the working principle of a SQUID scanning device for SRF applications [24].

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Eddy current scanning is a nondestructive technique. The working principle of this technique is schematically shown in Fig.13. A double-coil sensing probe is used to detect inclusions and defects embedded under the surface by detecting the alternation of the eddy currents. It is important to keep the distance between sensing head and the sample constant during scanning. Typically it takes about 15 minutes to scan a sample size of 300X300 mm2 and a line width of 1 mm. Defects deeper than 0.1 mm from the surface are typically undetectable. This can be improved by replacing the double-coil sensing probe by a SQUID detector. The working principle of the SQUID scanning technique is shown in Fig.14 where the SQUID is used to detect the secondary magnetic field of the eddy current. Probing depth can be improved up to 2.8 mm for a SQUID scanning device operated at 90 kHz with an excellent signal to noise ratio. Latest developments on eddy current scanning can also be found from Ref.25.

3.3.2. Kyoto Camera Checking whether the welding is done satisfactorily and how the welding area and areas close to the welding area look like especially on the inner surface of a cavity are also very critical during cavity fabrication. In the past, this was usually done by a borescope, long distance microscope, or a well-lit angle mirror. All of these were inaccurate and not quantitative. It can become quite a challenge to apply the devices to a multi-cell cavity. Recently a new camera based system [26] was developed jointly by Kyoto University and KEK, which can be used to examine the inner surface quality even for a 9 cell cavity. The schematic diagram of this so called “Kyoto Camera” is shown in Fig15a and the associated key components are shown in Fig.15b. During inspection, the cavity is rotated and moved while the cylinder is kept stationary. The inner surface of the cavity is reflected by the mirror and the image is captured by the camera. A real setup of this system is shown in Fig.16. The resolution of Kyoto Camera can reach 7.5 µm per pixel.

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Figure15. Schematic diagram of Kyoto Camera a) and its main components b). The inset shown in the upper right corner of b) shows the mirror and the pulse motor that drives it. The camera moves for focal adjustment. The electro-luminescence sheets for illumination are shown in the lower part of b). (all from Ref. 26)

By dividing the electro-luminescence sheets into strips to form a strip illuminator and by turning on and off each illumination strip and then following the movement of the bright point across a defect, it is possible to obtain quantitative information regarding the defect structure on the surface via simple geometric consideration. A typical example of this is shown in Fig.17. Further information about Kyoto Camera and examples of real observations can be found also in Refs 25 and 27.

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.

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Figure 16. Overview of Kyoto Camera [26].

Figure17. Example of a quantitative measurement on the defect structure observed at one of the equator region of a 9 cell Nb SRF cavity. a) shows the photo of the defect taken by Kyoto Camera. b) shows the profile of the defect after the calculations based on the movement of the bright point across the defect activated by controlled illumination [26].

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3.3.3. Replica Technique Typically Kyoto Camera cannot detect surface geometric defects with a size smaller than 5 µm. Some surface replica techniques have been developed to visualize surface geometric defects with a better resolution for single cell and in some cases multi-cell cavities. The first reported results from the replica technique are from S. Berry et al (see Refs. 28, 29). They carried out the replica in two steps: a) A negative replica of the inner surface of a cavity was made by a siloxane polymer (vinylpolysiloxane+hydrogenated polysiloxane) that can solidify in 15 minutes. b) Positive replica was done by a bi-compound mixture of polyurethane held under a pressure of 1-2 bar for 15 minutes. A typical result from this replica technique is shown in Fig.18 where the inner surface of a cavity at the quench size was replicated. The resolution of this technique can be less than 1 µm. It was found that the residuals after the replication process on the cavity can be completely cleaned by ultrasound in 65 oC basic bath.

Figure 18. A typical example [28] of the inner surface of a Nb SRF cavity at the quench site replicated by the technique developed by S. Berry et al. Top is 3D view. Bottom is corresponding contour plot.

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Figure 19. A typical example of the inner surface of a Nb SRF cavity near the welding zone replicated by the JLab technique [32]. The replica was coated by palladium for 5 nm. The lower image was taken by an optical microscope on the replica from the heat affected zone. The upper left is taken on the heat unaffected zone. The upper right is an optical image taken on the surface of Nb for comparison.

Similar technique with a different replica material was also developed by Fermi Lab [30,31]. In this case, they used a silicon rubber (two-component translucent silicone RTV compound manufactured by Freeman Mfg. Inc. V3040) to make negative replicas. Then positive replicas were done using a second RTV compound (Momentive RTV630) or an epoxy. The advantage of this technique is that it does not need any releasing agent and it leaves no residuals on the surface [30]. However, the silicone rubber must be evacuated in

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vacuum before pouring into a replicated area. It takes more than 18 hours to cure. The resolution of this technique is comparable to the previous one. JLab recently has applied a simple replica technique [32] to Nb SRF cavities with a resolution of a few nanometers. This technique is done in the following way: Depending on the size of the features that are needed for replicating, either commercial cellulose acetate films of 35µm (for features smaller than 1 µm) or triphan foils of 100µm thick (for features larger than 1 µm) are employed. Negative replica can be obtained by applying a couple of acetone drops on the observed surface and then put a film on the surface. Lift the film in 10 minutes. Then the film is put on a flat surface and coated by palladium for 5 nm (see Fig.19 for an example). Positive replica can be obtained by dissolving the acetate film by acetone. In fact, we really don‟t think that it needs to make positive replicas in all the replica techniques described here, since this can easily be done by some suitable software for image processing. The advantages of this JLab technique are that it is fast and simple and has a resolution basically limited only by the size of the molecule of the film or the foil. It can transform a curved surface to a flat one. The latter point can be extremely useful for subsequent observations or measurements since some instruments (for instance an optical microscope) are very difficult to be applied to a curved surface. The replicas can be examined not only by an optical microscope and a profilometer, but also by a scanning electron microscope (SEM) or a transmission electron microscope (TEM). One challenge for the above mentioned replica techniques is how to effectively apply them to multi-cell cavities when hands are not long enough.

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3.4. Examples of New Developments in Final Chemical Treatment Step 15 in the Fig.4 is perhaps the most critical one in determining the performance of a Nb SRF cavity. It is normally done by either BCP employing the acid mixture of phosphoric, hydrofluoric, and nitric acids with a ratio of 1:1:1 or 2:1:1 or by EP employing the acid mixture [33] of hydrofluoric and sulfuric acids. There are quite some new developments on this topic in the last decade. Most of them are done along the line of modifying or improving the existing techniques. Due to the limited space, here I will discuss only a few new developments that have shown some promising signs. One [34] is BEP. Others include: a) Nb polished [35] by ionic liquids at a temperature higher than 100 oC, b) Plasma Etching [36], and c) FARADAYIC Electropolishing [37].

3.4.1. Buffered Electropolishing (BEP) BEP experiment was initiated at JLab in early 2001. Some preliminary results were published in Ref. 38. BEP uses the acid mixture of hydrofluoric, sulfuric, and lactic acids as the electrolyte at a volume ratio of 4:5:11. Here lactic acid acts as a buffer in a similar way as what H3PO4 does in BCP. By replacing the majority of H2SO4 in the electrolyte of the conventional EP by lactic acid, BEP treatment reduces the aggressiveness of the electrolyte significantly. It has been demonstrated that BEP can produce the smoothest [39-42] Nb surface ever reported in the literature. Smoother inner surface of a Nb SRF cavity is known to have positive effects on its RF performance. Experiments also show [39,43] that Nb removal rate can be as high as 4.67μm/min. This is more than 10 times faster than 0.38 μm/min of EP. Faster Nb removal rate can contribute significantly to the reduction of the costs of the surface

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treatments for Step 15. For instance, conventional EP will take typically 23622 seconds to get Step 15 done. With BEP, it takes only 1931 seconds. Other benefits of BEP as compared with EP include: a) acid mixture is much safer to handle, b) the life of the acid mixture is longer, c) acid mixture is cheaper, and d) less or no sulfur precipitation. Fig.20 shows a quantitative comparison on the surface smoothness for Nb treated by BEP, EP, and BCP as measured by a precision 3-D profilometer [34]. The RMS of the smoothest Nb surface [39] treated by BEP is 20 nm over an area of 200X200 µm2. To the best of my knowledge, this is also the smoothest Nb surface ever reported in the literature.

Figure 20. Typical high resolution 3D profilometer scans on the surfaces of: (a) buffered electropolished Nb, (b) electropolished Nb, and (c) buffered chemical polished Nb. The scans are plotted with the same parameters for comparison [34].

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Early experiments [38] done on Nb half cells have implied that cathode shape plays an important role during BEP treatments on curved surfaces. This is confirmed later by experiments at other labs [20,42,44]. Fig.21 shows a typical example of the surface finish for a Nb half cell treated by BEP for 1800 seconds without electrolyte circulation when the shape of the cathode is changed in such a way to allow a more homogenous electric field distribution inside the half cell.

Figure 21. Nb half cell a) before BEP treatment, b) after BEP treatment for 1800 seconds [34]. Niobium: Properties, Production and Applications : Properties, Production and Applications, Nova Science Publishers, Incorporated, 2011. ProQuest

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At this moment, the polishing mechanism responsible for BEP is not completely clear. It has been suggested [44,45] that apart from HF lactic acid may also participate in the Nb polishing process through the following reactions:

(9) or _

or O

+6H+  2

Nb2O5+4H3C

-

+H2O

(10)

O -

O

(10) or

3-

or O

+6H+  2

Nb2O5+6 H3C

-

+3H2O

(11)

O

O

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-

Nb2O5+6 H3C

O

+6H+  2 -

+3H2O

(11)

O -

O

(11) This may explain why BEP can polish at a much faster way than conventional EP. To investigate whether BEP was governed by diffusion of fluorine ions, experiments [42] with a rotating disc electrode were carried out at CEA Saclay in France. The result is duplicated in Fig.22. Polishing current (I) was found to be not proportional to the square root of the angular speed (ωa) of the rotation disc, implying that the diffusion is not the only process taking place during BEP. This is due to the fact that if the polishing is diffusion limited, I is related to ωa in the following way [46]: I=6.2x10-4nFSD2/3ν-1/6 ωa1/2C

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(12)

320

A. T. Wu I=f(t) for Different Rotation Speeds BEP 4-5-11 Mixture. 5V Vs AgAgCl. 30°C 0,8

4000 rpm 4994 rpm 3000 rpm

0,7

2000 rpm 1200 rpm

0,6 800 rpm

I (A)

0,5 400 rpm

0,4 0 rpm

0,3

200 rpm

0,2 0 rpm 0,1 0 0

100

200

300

400

500

600

Time (s)

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Figure 22. Polishing currents42 obtained at 5V for different rotation speeds at 30 oC.

Where n is the number of electrons in the electrochemical reaction; S is the surface of the electrode (cm2); D is the diffusion coefficient (cm2/s); ν is the viscosity (St); C is the concentration of the active species (mol/l). Further measurements [42] with Electrochemical Impedance Spectroscope (EIS) seemed to indicate the polished surface might be covered by a porous film whose resistance increased with the applied potential. BEP has been applied to treat Nb SRF single cells both vertically in JLab and horizontally at CEA Saclay for comparison. In fact, the first very vertical electropolishing on a Nb SRF cavity was carried out at JLab via BEP in early 2002 as reported in Ref.47. Since then many improvements [48] have been made to the system, including a more reliable acid circulation, more accurate acid flow control, more efficient heat removal from the treated cavity, external cooling, and an automatic data acquisition system. The modern system is shown in Fig.23. The major advantages of vertical EP as compared with the horizontal one are: 1) Easier for draining. 2) Easier for making the electrode contacts, since cavity rotation is not necessary in vertical EP. 3) Easier to incorporate other cavity treatment techniques into a vertical setup such as, for instance, HPWR, BCP, BEP, drying, baking, etc. Of course hydrogen removal is more difficult for a vertical system, which is especially true for treatments on multi-cell cavities. So far the best results were all obtained with the vertical system at JLab and shaped cathodes. Typical excitation curves [49] for a CEBAF shape regular grain single cell cavity are shown in Fig.24. It is noted from Fig.24 that Qo is improved significantly after baking at 120 oC. The best result was achieved on large grain cavity where Eacc reached 32 MV/m. It is worth pointing out here that the post cleaning procedure for all BEP treated cavities so far is identical to that used for conventional EP. Improving on the post BEP cleaning may be one of

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the keys to open the door for a better performance for BEP treated Nb cavities. Optimization process on BEP is now underway.

Figure 23. Modern vertical BEP system at JLab. The small photo shows the old vertical BEP system [47] used at JLab in 2002.

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Figure 24. Typical excitation curves [49] for single cell Nb SRF cavity (regular grain size) of CEBAF shape treated by BEP.

3.4.2. Other New Developments to Remove Nb Currently, Step 15 in Fig.4 is normally done with an acid mixture involving HF. HF is well-known to be very nasty in terms of its effects on the health of human beings. A non-HF electrolyte for polishing on Nb is a very attractive idea. There are many HF-free recipes for polishing Nb as listed in Ref.35. Many of them are even more toxic or dangerous than HF is [35]. Two different ionic liquids have been explored to electropolish Nb at temperatures higher than 100 oC at University of Padua. One [35] consists of choline chloride and urea at a ratio of 4:1 plus sulphamic acid in a concentration of 30 g/l. The other [50] is a mixture of urea and choline chloride at a ratio of 3:1 plus ammonium chloride in a concentration of 10 g/l. Polishing has to take place at 120 oC and 190 oC respectively [35,50]. The highest Nb removal rate can be 12 times quicker than that of the conventional EP. A typical Nb surface produced by this technique is shown in Fig.25. Plasma etching on Nb is a R&D project through the collaboration [51] between University of Old Dominion and JLab. The idea here is to use some reactive gas species [36, 51] such as, for instance, Ar/Cl2 or BF3 to chemically react with Nb under the discharge from a DC or RF source. This will generate some volatile Nb compounds that can be pumped away. An experimental setup employing microwave glow discharge is schematically shown in Fig.26. It was demonstrated [36] that an etching rate of 1.5 µm/min can be reached when Cl2 was used as the reactive agent. The surface finish produced by plasma etching appears to be comparable to that by BCP at the present stage of development. This technique is attractive since it does not employ HF and it can be done with a decent etching rate.

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Figure 25. Typical microscopic photos of Nb a) before polishing, b) after polishing by an ionic liquid [50] (see the text for more details) at 190 oC.

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Figure 26. Schematic illustration of a microwave glow discharge system for Nb sample exposure [36].

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Faradayic Electropolishing was developed [37] by Faraday Technology Inc. The novelty here is that instead of using the chemical mediated method to remove oxides from Nb surfaces, Faradayic Electropolishing employs electrically mediated approach to perform this removal. The electrolyte they used [52] is 20% H2SO4 solution. Here sulfuric acid acts as an oxidation agent. The key here is that the applied electric field is not constant. It is pulsed and modulated in a way to optimize the polishing. For instance, during Faradayic Electriopolishing chemical reactions can take place in the following way: 2Nb+5H2O  Nb2O5+5H2

(13)

Nb2O5+10H++10e-  2Nb+5H2O

(14)

Reaction formula 13 is the same as that of EP and BEP. However, now by controlling the electric potential applied to the surface, before the oxide is formed completely as an insulating layer on the naked Nb surfaces the applied electric field can be either reduced or shut off to allow newly formed oxides (for instance the pent-oxides) to diffuse away from the polished surface. After the oxide concentration is returned to its original value, the electric field will be turned on or ramped up. A typical setup is schematically shown in Fig.27. This technique can produce a polishing rate as high as 5 µm/min. A typical Nb surface treated by Faradayic Electropolishing is shown in Fig.28 at a polishing rate of 2.7 µm/min. It is worth pointing out that any new development is not as simple as it appears to be. Take BEP as an example, by adding only one additional acid into the electrolyte of the conventional EP a new whole set of problems emerge. It takes more than 8 years to reach the stage that it is now. Although the cause of this can always be attributed to external and environmental factors including lack of manpower and the smartness of the persons involved

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in the R&D (easy target), there are perhaps intrinsic reasons due to the nature of any new development. For instance, how many years does it take to develop the conventional EP? We have been still trying up to now to improve it and to understand the mechanism responsible for it [19]. Therefore one has to be extremely careful before taking on a new development, especially on something that is completely new.

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Figure 27. Photo (right) and schematic drawing of a low flow channel cell used to electropolish 25.4 mm x 25.4 mm x 3 mm Nb coupons via Faradayic Electropolishing [37].

Figure 28. Typical microscopic photo of the surface of a Nb coupon treated by Faradayic Electropolishing in a H2SO4 electrolyte [37].

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3.5. Examples of New Developments in Final Cleaning Technique HPWR with ultra pure water is the process that is normally done in between Steps 17 and 18 as the final cleaning before the assembly and low temperature baking. This final cleaning is critical since it determines whether there are still contaminants or chemical residuals on the inner surface of a Nb cavity. These contaminants or chemical residuals can have detrimental effects on cavity performance. Here I will focus on two examples of these new developments. One is Gas Cluster Ion Beam (GCIB) technique53. The other is Dry Ice Cleaning (DIC) technique [54].

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3.5.1. GCIB Technique In contrast to HPWR where a mechanical effect is the main cleaning means, as should see in the following GCIB offers both mechanical and chemical effects in the cleaning. The application of GCIB technique to Cu RF cavities and Nb SRF cavities was first done [55,56] by Swenson et al at Epion Corporation to mitigate high voltage breakdown through reducing the surface roughness of oxygen-free Cu via GCIB. Later on, collaboration was established between Epion Corporation, JLab, Fermi National Accelerator Lab, and Argonne National Lab on a R&D project to investigate the application of GCIB technique to Nb SRF cavity both experimentally and theoretically. The results of this collaboration and the current status of the application of this technique to the treatments of Nb SRF cavities are summarized in Ref.53.

Figure 29. Schematic of working principal of GCIB.

The working principal of GCIB is schematically illustrated in Fig.29. Various types of gases can be used for GCIB treatments. The gases can be inert such as Ar, Kr, Xe etc. or chemically reactive such as O2, N2, CO2, NF3, CH4, B2H6 etc. that may react with the surfaces under treatments depending on what the application one has in mind. After selecting an appropriate gas species, the gas is forced through a nozzle that has a typical pressure of 7.6X103 Torr on one side and a vacuum of 7.6X10-3 Torr on the other side. Therefore the gas undergoes a supersonic expansion adiabatically that slows down the relative velocity between the atoms of the gas, leading to the formation of a jet of clusters. A typical cluster contains atomic numbers ranging from 500 to 10,000 that are held together by van der Waals forces. A skimmer is then used to allow only the primary jet core of the clusters to pass through an ionizer. The clusters are ionized by an ionizer via mainly electron impacts and the positively

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charged clusters are electrostatically accelerated via a typical voltage ranging from 2 kV to 35 kV and focused by a beam optics. Monomers and dimers are removed from the beam by a dipole magnet before the beam is neutralized with an electron flood. The aperture in Fig.1 after the neutralizor is used to collect the monometers and dimers. Surface GCIB treatments are done through mechanically scanning an object. Typically, the impact speed of the clusters to the surface of an object under GCIB treatements is 6.5 km/s, and the current of a gas cluster beam can be as high as 1 mA. The selection of an appropriate gas species for doing GCIB treatment is very important. When an inert gas is chosen, the major effects on the treated surfaces are smoothing and asperity removal due to lateral sputtering. Chemical gases, on the other hand, can produce some additional effects such as, for instance, doping, etching, and depositing, etc. depending on the properties of the treated object and the gas species selected. Implantation is only limited to the top several atomic layers during GCIB treatments due to the low individual atomic energy. One can also combine the use of different gas species in a specific order for a particular application, although less work has been done in this research direction so far. Only Ar, O2, N2, and NF3 have been used in the GCIB treatments on Nb. Ar was selected because of its smoothing effect. O2 GCIB is interesting due to the possible chemical reactions between O2 and Nb and so is true also for N2, although in case of using N2 there was a hope that NbN could be formed on the treated surface since the superconducting transition temperature (Tc) is 16.2 K that is much higher than 9.2 K for Nb. NF3 is expected to have a relatively higher etching and removal rates on Nb than those from other chemically reactive gas species. The main objective for the final cleaning is to remove or suppress all sources that can cause field emission and degrade the cavity performance. GCIB was found to be able to suppress field emission significantly [57] as shown typically in Fig.30 through measurements using a scanning field emission microscope (SFEM)47[47]. The cleaning effect of GCIB is done through controlled removing or smashing of any residuals or contaminants through the bombardment using one appropriate gas species or a combination of several gas species. In fact, three effects on Nb surfaces after GCIB treatments have been identified [53]. One is the smoothing effect. GCIB can remove sharp tips or edges as demonstrated through mesoscale modeling [58] shown in Fig.31. This is also verified experimentally [57] through observations by a scanning electron microscope (SEM). The second is the smashing effect. After GCIB treatments, it was found that residuals or contaminants were broken into pieces as if they were stepped on by a heavy sumo wrestler. The third effect is the modification of surface oxide layer structure of Nb [59]. With the selection of a right gas species and treatments parameters, GCIB can increase the thickness of the surface pent-oxide layer. This can lead to the increase in the threshold for field emission [60]. In fact, GCIB can also be used as the final chemical treatment technique in Step 15. It has been demonstrated in Ref.61 that an etching rate larger than 5 nm*cm2/s is possible if NF3+O2 are used as the gas species in GCIB treatments on Nb. Fig.32 shows the measured etching rates on Nb for some selected gas species [61]. GCIB has been applied to the treatments of Nb SRF cavities. Due to the limitation of the space here, interested readers are referred to Ref.53.

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Figure 30. Typical results of measurements on GCIB treated Nb flat samples done by a SFEM. a) A Nb sample that was treated by O2 GCIB on one half while the other half was not. b) A sample that was treated by NF3+O2 on one half while the other half was not. Both Nb samples were polished by BCP prior to GCIB the treatments.

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Figure 31. Mesoscale modeling of a Nb surface irradiated by O 2 GCIB treatments where it shows that the features shown in a) are removed by the treatments as shown in b).

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Figure 32. Measured etch rates of Nb treated by NF3+O2, Ar, and O2 GCIB as a function of acceleration voltage [61].

Figure 33. Photo of the nozzle and CO2/N2 jet during a DIC cleaning of a Nb SRF cavity [54].

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Figure 34. The DIC system at DESY: a) Photo of the setup [54]. b) Schematic diagram of the setup [63].

3.5.2. DIC Technique The cleaning power [54] of DIC is given mainly by a combined thermal and chemical effect. A jet of pure carbon dioxide snow surrounded by supersonic N2 is released from a nozzle as shown in Fig.33 and impacts on the treated surfaces. After leaving the nozzle, liquid CO2 relaxes spontaneously, resulting in a snow/gas mixture with 45% snow and a local temperature [62] of -78.9 oC. The supersonic N2 here functions in the following two ways: a)

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It gives an acceleration and focusing of the jet. b) It prevents condensation of humidity on the cavity surface. The cleaning effect is based on shock-freezing of the contaminants, strong impact of the snow crystals, and an increase of volume by 500 times after sublimation. Contaminants or residuals get brittle and start to flake off from the surface. It is very effective in removing hydrocarbons and silicons, since liquid CO2 is a good solvent for non-polar chemicals. Cleaning is usually done in a clean room of class 10 to minimize air-born contaminants to enter the cavity during treatments. To achieve optimal cleaning, it is important to keep the cavity warm (20~30 oC) during cleaning. Fig.34 shows a photo of the cleaning setup and a schematic diagram of the setup. The DIC cleaning effect was further studied [64] on polycrystalline Cu and Nb and single crystal Nb flat samples by a SFEM and SEM equipped with an energy dispersive x-ray analyzer (EDX). DIC was found to be able to suppress enhanced field emission from metallic surfaces under a DC field up to 250 MV/m. It was found that that emitters down to a size of 400 nm could be effectively removed and DIC also had a smoothing effect on surface protrusions. Fig.35 shows field emission maps of a Nb surface before and after DIC. RF tests have been done on the Nb single cell SRF cavities treated by DIC. In several cases, performance was still limited by field emission [54]. The best was achieved on a cavity that reached 38.2 MV/m before quench. A strong high field Q slope was observed in all excitation curves shown in Ref.54. It is noted here that it takes almost a decade for DIC to reach a stage as described here (any similarities in development between DIC and BEP?).

Figure 35. Filed emission maps of polycrystalline Nb measured by a SFEM over the same 7.5x7.5 mm2 before (left) and after (right) DIC up to a DC field gradient of 120 MV/m.

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SECTION 4. REQUIREMENTS IN PHYSICAL, CHEMICAL, METALLURGICAL, MECHANICAL PROPERTIES OF NB FOR FABRICATING HIGH QUALITY SRF CAVITIES The topic of this section is huge. It would take the space of an entire book in order to cover all the new developments in this field. Here I will try to limit myself to the discussions of some fundamental requirements and some selected topics of the requirements that I feel are important or may have immediate impacts on Nb SRF cavity fabrication.

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4.1. Requirements in Physical Properties Looking at this topic, the first image appears in my mind is how a typical particle accelerator based on Nb SRF technology works. The particle accelerator is typically operated at a temperature below the boiling temperature of liquid helium (abbreviated as LHe2 in the following) that is 4.2 K or -269.0 oC and in a RF field. For instance, CEBAF of JLab is operated at 2 K/-271.2 oC at 1.5 GHz. As stated in Section 1.2, the RF field only penetrates into Nb to a depth of ~50 nm. Therefore surface properties of Nb are extremely important when we discuss the requirements in physical properties of Nb for SRF applications. To study the requirements, we need experimental tools. In the past decade, various experimental tools have been employed to study the physical properties of Nb. Among them, many are surface instruments. For instance, in 2003 a surface science lab [47, 65] (SSL) was established at JLab to study various properties of Nb related to SRF applications. The SSL contains typical instruments often used in the studies of the physical properties of Nb such as, for instance, SEM and EDX, SFEM, a 3-D large scan area profilometer, TEM, secondary ion mass spectrometry (SIMS), scanning Auger microscope (SAM), metallographic optical microscope (MOM), electron backscattered diffraction (EBSD), and a well equipped sample preparation room that can do all sample preparation required by the SSL. Other popular instruments that have been used include x-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), and scanning tunneling microscope (STM). To use experimental tools effectively, it is important that we know the characteristics of each experimental tool. For instance, if one wants to study the general surface roughness of a polycrystalline Nb sample of an average grain size of 50 µm it is extremely unsuitable to use AFM or STM since typically their maximum scanning length is around 50 µm. In this case, one AFM or STM measurement can capture only a couple of grains. Therefore, it is critical to select the right tools for a particular property that one wants to study. For the convenience to the readers, Table 2 summarizes the major characteristics of the popular experimental tools for studying the physical properties of Nb for SRF applications.

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Table 2. Popular Instruments Used in the Studies of the Physical Properties of Nb for SRF Applications: Properties They Probe and Their Characteristics Instrument

SEM

Properties Detected imaging

TEM imaging, atomic structure, defects, dislocations, interstitial atoms, secondary phases

Characteristics

lateral resolution of 1-5 lateral resolution nm down to 0.05 nm

Instrument

STM

EDX

Imaging, surface elemental analysis, topography and Properties Detected composition, morphology and elemental mapping electron density of state

Characteristics

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Instrument

lateral resolution down to 0.1 nm, limited scan size ~50 µm2 at room temperature and smaller at low temperatures, only works on metallic or semiconducting surfaces MOM

Properties Detected imaging

Characteristics

SIMS

AFM

composition, depth profile, elemental mapping

imaging, surface topography and morphology

~ppm down to ppb, lateral resolution can be very surface down to 1nm, limited sensitive (static scan size ~50 µm2 at SIMS) 5% of the room temperature surface top layer, can and smaller at low measure all elements temperatures including H XPS Profilometer composition, electronic structure imagine, surface or chemical state, topography and depth profile, morphology elemental mapping

not surface sensitive, very surface typical probing depth sensitive, typical ~ µm, cannot detect probing depth elements with atomic 200 resolution ~2.5 µm nm

lateral resolution down to 5 nm, extremely larger scan size up to 15x15 cm2 or larger

EBSD crystal orientation, texture, defects, phase identification, morphology, orientation mapping

very surface sensitive, typical probing depth surface sensitive, Qo>6.4x109 could be achieved after post purification at 1250 oC in a Ti box for 12 hours and subsequently held at 1000 oC for 24 hours prior to cool down to room temperature. A typical example of excitation curves [94] measured on a single cell cavity made from a Nb sheet of Ta concentration of 1300 ppm is shown in Fig.44.

Figure 44. Performance [94] of Nb SRF single cell cavity made from high Ta content Nb (1300 wtppm). Shown is Q0 vs. Eacc before heat treatment, after heat treatment and “in situ” baking at 120 oC for 48 hours.

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The impurities in Nb can be reduced substantially by heating Nb to 1250 oC or higher under ultra high vacuum and in a Ti enclosure for a period of time up to 24 hours. At DESY, this is normally done at a temperature between 1350 oC to 1400 oC for 4 hours. Ti serves here as a getter material for N, O, CO2, water vapor, and methane. Even at a temperature as low as 700 oC, Ti can start to absorb N up to 90 at% and O up to 50 at%. The absorption ability of Ti to N increases significantly above 1000 oC due to a phase transition in Ti. One would naturally concern about the possibility of contaminating Nb by Ti during the purification process. As reported in Ref.95, the diffusion (D) of Ti into Nb is given by D = 0.099 exp (-86930/RT) [cm2/sec]

(17)

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when 994 oC ≤ T ≤ 1492 oC. Here R is the gas constant 8.314 J/(mol K). Using Fick‟s law, one can calculate the concentration of Ti as a function of time and temperature. At 1250 oC, Ti concentration has dropped to 10-5 in a depth of 5 µm by assuming 6 hours diffusion time. Such a thickness can be easily removed by, for instance, BCP. From the discussion in this section, we can see that the requirements in chemical properties of Nb depend on the applications that one has in mind. If an application does not need to have an accelerating gradient higher than 30 MV/m with a Qo around 1010 [10], Ta concentration can be higher than 500 wtppm. On the other hand, RRR value does not have too much room to be selected. As discussed above, RRR is related to the effectiveness of the heat transportation between the SRF cavities and their cooling bath. RRR is also important for making the cavities more sustainable to the effects from microscopic defects. Therefore, RRR values ranging from 250 to 300 or better are needed for most applications unless one wants to use the technique of Nb on Cu. For SNS project, typical RRR of as-received Nb sheets ranged from 300 to 400.

4.3. Requirements in Metallurgical Properties In my view, this topic has been a bit overlooked in the SRF field. Defects such as, for instance, dislocations, stack fault, non-fully crystallized regions, non-uniform grain size, etc can degrade cavity performance. One example is the hot spot discussed in Section 4.1.2 which is found to be related to localized misorientations. Normally, it is specified that the Nb sheets should be fully annealed before they are delivered to users. However, the experience from myself shows that this is not always the case. To illustrate the importance of this topic, I will go into some depth in discussing a softening problem that I encountered during SNS project at JLab. All the photos and data discussed here were published only as a JLab technical note [95] and have not been published elsewhere. For example, during SNS project I did cross-section examination using MOM on several samples from different batches of as-received Nb sheets from two different suppliers. On one sample from one supplier, I saw that it was not fully annealed at all. Only about 100 µm thick on the surface could I see crystal structures as shown typically in Fig.45a). From the figure, the trace of rolling process during the fabrication of Nb sheets (see Fig.43) can be easily seen. On another sample from another supplier, I found that the crystal structure was progressively becoming bad from the surface. At the region starting about 1 mm from the surface,

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significant amount of the amorphous phase showed up as shown typically in Fig.45b). Therefore, if cavities are made from such Nb sheets after Step 15, the exposed surfaces can either be amorphous or have a grain size significantly different from the surface before the treatment. This can certainly cause significant scattering in the data of cavity measurements.

Figure 45. Typical MOM cross-section photos of as-received Nb, a) showing that the sample was not crystallized, b) showing a variation in microstructure from the surface to the interior, implying that both samples were not fully annealed after rolling.

At Jlab, we used to do hydrogen outgasing at 800 oC for 1 to 3 hours. However, during SNS project we found that the cavities under such a treatment were significantly softer than they were before the treatment. This created significant microphonic effects among others and

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needed to be remedied. MOM observations [95] clearly showed that grain growth and hydrogen outgassing were responsible for the softening. This conclusion was reached based on the following two considerations: a) MOM observations showed that the grain size could grow from 45 µm to 85 µm after heat treatments at 800 oC for one hour. From any metallurgical text book, we know that the yield strength of a polycrystalline metal is related to its grain size via the following relationship: ζy = ζi + 1.414ζDl1/2d-(1/2)

(18)

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Here ζy is the lower yield stress, ζi is the shear stress resisting the movement of dislocations across a particular slip plane, ζD is the shear stress to unpin a dislocation, l is the distance from the piled-up dislocations at the head of the plastic deformation front held up by a grain boundary to the nearest dislocation source (called Frank-Read source), d is the diameter of the grain. Therefore the larger is the grain size, the lower is its yield stress. b) From an early study of the effect of carbon and silicon concentrations on the stress-strain curves of iron through elongation experiments in 1988, it was found that the elongation curves could be controlled continuously from a continuous yield to a yield with a yield plateau and the width of the yield plateau various regularly with the concentrations of carbon and silicon. TEM observations on a sample during elongation found that carbon and silicon could act as pinning center for dislocations, creating therefore many short Frank-Read sources. Since ζD in equation 18 is inversely proportional to the length of a Frank-Read source, as a result the yield strength is higher. Similarly in Nb, 800 oC heat treatment is known to outgas H and other impurities. These H and other impurity interstitial atoms previously could serve as pinning centers for dislocations were now removed by the outgasing process, leading therefore to a lower yield strength.

Figure 46. An amorphous area was still seen from MOM on one sample cut from a Nb sheet treated at 800 oC for three hours.

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Figure 47. Variation of grain size from surface to the interior observed on one sample cut from a Nb sheet treated at 800 oC for three hours. a) On the surface. b) On the interior adjacent to the surface shown in a). The arrow indicates the area with a smaller grain size as compared with that on the surface.

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Interestingly in some cases, not all the grains were fully grown and grain size could still vary from the surface to the interior even after the heat treatment as showed typically in Figs.46 and 47. Some areas also showed preferential grain growth along the rolling direction as shown typically in Fig.48. Another interesting thing to see is shown in Fig.49 where the arrow indicates a grain with many pits implying that locally the stresses were not completely released by 800 oC annealing for three hours and there were still many dislocations showing up as the pits on the surface. Grain growth was found to be much less serious when heat treated at 600 oC

Figure 48. MOM photo on a sample cut from a Nb sheet that was treated at 800 oC for three hours. The arrow indicates that locally the stresses in this region were not fully released. There were still many dislocations shown as pits on the surface.

From here we can see that 800 oC heat treatment for 3 hours is too high for hydrogen outgasing if the stresses stored in the as-received Nb sheets or in the fabricated cavities are not fully released. This was the base why the 800 oC treatment for 3 hours was changed to 600 oC for 6 hours after SNS project at JLab for hydrogen outgasing. In fact, it is better to outgas H at 650 oC since in this way one can remove unwanted stresses in Nb too while recrystallization can be largely avoided (see Table 1). Based on my experience through doing metallographic observations on Nb, I recommend strongly to use MOM cross-section observation as a quality control tool for both Nb users and manufacturers as I suggested in Ref.95. One should check whether a Nb sheet is fully annealed and whether the grain size is uniform from the surface to the interior. This can save a lot of troubles in cavity production and potentially can save a lot of money wasted in doing cavity fabrications and treatments and testing.

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Figure 49. Typical MOM photo of preferential crystal growth along the rolling direction after heat treatment at 800 oC for three hours.

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Dislocations and other defects can also lower RRR leading to lower thermal conductivity for Nb as pointed out in Ref.96. In my personal view, metallurgical properties of Nb have not been given the attention they deserved in the SRF community. More work is needed in this area. For more information about dislocation movements, slip systems, and other issues related to metallurgical aspects of Nb, please read an excellent recent paper [96] by Bieler et al.

4.4. Requirements in Mechanical Properties This topic is, in fact, strongly coupled with the topic discussed above since grain size, dislocations, interstitial atoms, and other defects can affect mechanical properties of Nb enormously just as they do on any metal. Therefore it is also quite naturally to give specifications on grain size and other mechanical properties of Nb when ordering from Nb supplier. Table 6 shows an example of such specifications for SNS project.

Table 6. Specifications on mechanical properties of Nb for SNS project Yield Tensile Strength Strength (0.2% offset) 7000 psi 14000 psi (48.2 (96.4 N/mm2) N/mm2) minimum minimum

Elongation (1 inch Hardness, gauge length) HV 10

Recrystallization

40% minimum longitudinal, 35% 50 90% minimum maximum transverse

Grain Size (d)

ASTM4 (90µm)>d>ASTM5(64µm)

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Mechanical properties of Nb can change significantly with the impurity contents and therefore RRR for the same reason as discussed in the previous section during hydrogen outgasing process. A study [97] of the variations of mechanical properties with heat treatments at three different temperatures between 1100 oC and 1250 oC was done by Myneni and Umezawa on reactor grade Nb. A significant variation in yield strength was found as reproduced in Table 7. The as-received Nb had a yield strength of 110 MPar that was reduced Table 7. Mechanical properties of reactor grade Nb [97]

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to 56 MPar whereas RRR increased from 68 to 210. Apart from the effect due to RRR, grain growth after the high temperature annealing could contribute to the reduction in yield strength as shown in Equation 18.

Figure 50. Measured yield stress plotted as a function of inverse square root of diameter of grain size form samples cut from Nb sheets supplied by two Nb manufacturers. (see text for more details).

Wu and Myneni measured [95] yield stress of Nb from two different suppliers as a function of the inverse square root of the diameter of grain after heat treatments at 800 oC for 3 hours (one supplier denoted as SNSw1 & SNSw2 RRR~400, the other as SNSt RRR~300) as

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shown in Fig.50. From this figure, following information can be extracted: a) generally speaking, yield strength decreases with increasing grain size, b) regular linear dependence between lower yield stress and diameter of grain as shown in Equation 18 seems to be followed only when the grain growth mechanism is the same.

Figure 51. Typical example of huge grains observed by MOM on a SNSw1 sample after annealing at 800 oC for three hours (see text for more details).

For SNS project, I found that some grains could be huge after heat treatments at 800 oC for three hours as compared with samples from normal sheets where the average grain size was ranging from 45 to 51 µm that was a bit below the specifications (see Table 6). A typical example is shown in Fig.51. Normally exaggerated grain growth is known to occur in one of the following two ways: a) through a critical strain-anneal treatment, or b) through a process of secondary recrystallization. A critical strain-anneal treatment is done by making a critical deformation of a few percent strain to a fully recrystallized sample. Then the sample is subjected to a high temperature annealing in a thermal gradient along the direction of the deformation. This situation is hardly applicable to the present situation. We believe that the process of secondary recrystallization is more likely to occur here, provided that the primary recrystallization treatment was done at a lower temperature than 800oC. This needs to be confirmed. For SNSw2 samples, grain growth proceeded steadily for samples under heat treatments higher than 600oC. However, the difference in grain size between regions close to the surface and the interior still existed even after the heat treatment at 800oC for three hours (see Fig.47), but is much less significant than that of the as-prepared sample. We know that

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the final grain size of a metal after recrystallization depends on the following factors [98]: a) the degree of prior strain hardening, b) the annealing temperature (above or equal to the temperature that is required for a complete recrystallization), c) the duration at the annealing temperature, and d) the rate of heating to reach the annealing temperature. The observation of steady grain growth seemed to indicate that a substantial amount of strain energy was stored in the as-received SNSw2 sample or the primary recrystallization was done at a temperature lower than 600oC. The later possibility is highly unlikely since the as-received sample already had some crystals. For SNSt samples, some grains started to grow and became quite big for all samples under the heat treatment at 800 oC and even for the heat treatment at 600oC for only three hours. However, they were not fully recrystallized until heat treatment at 800oC for three hours. From Fig.50, we can see that the secondary recrystallization tends to change the dependence dramatically for SNSw1 samples. The two data points of SNSw1 samples fall into the line for the SNSw1 samples. This may implies that the primary reason responsible for the grain growth for SNSw2 samples may also be the process of secondary recrystallization. Alternatively, it can also means that the average grain sizes obtained from these two SNSw1 samples and from five SNSw2 samples that their data points form a straight line represent their real microstructures nicely in the same way in terms of their mechanical properties. The one anomalous data point for SNSw1 samples may be due to the fact that the difference in grain size between the regions close to the surface and the interior is too much so that an average grain size cannot be obtained to represent the mechanical properties of the sample. Since there is only one data point for SNSt sample, it is not meaningful to say anything here except that one should not compare it with data from SNSw1 and SNSw2. This is because the mechanisms of grain growth can be different between SNSt and SNSw2/SNSw1. Consequentially, the final state after the heat treatment may not be the same. In STSt samples, the grain growth started from polygonization, primary recrystallization, and then grain growth. Besides, the RRR value is also different between STSt and SNSw2/SNSw1. A careful mechanical study was also done on Nb with RRR~300 to study the relationship between grain size, elongation curve, and annealing temperature as reported in Ref.99. They found nice variation of the grain size as a function of annealing temperature as shown in Fig.52. In Fig.53 the corresponding elongation curves are shown. All the samples were annealed for duration of 4 hours at the different temperatures shown in Fig.53. Similar to what shown in Fig.50, the yield strength also decreases significantly with the increase in grain size as shown in Figs 52 and 53. It is expected that the variation of mechanical properties of Nb at 4.2 K to grain size and RRR can be different from those at room temperature, since there is a transition of the workability of Nb from ductile to brittle at ~126 K (see Table 1). Recently SRF cavities made [100,101] from large/single grain Nb have attracted quite some attentions. This is a very attractive approach since one can skip several steps in Nb production procedure (see Fig.43) for SRF community. Nb manufacturers can deliver Nb ingots instead of sheets, provided that the ingots are large enough for making SRF cavities. In fact, progress in Nb production technology in industry has made it possible to manufacture Nb with ingot sizes ranging from 300 to 500 mm. This is larger enough for making most shapes of high frequency SRF cavities. For instance, a TESLA shape cavity needs only a disk of 265 mm diameter.

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Figuer 52. Grain size measured [99] as a function of annealing temperatures for Nb RRR ~300.

Figure 53. Stress-strain curves [99] for the Nb RRR~300 treated at various temperatures (see the figures) for 4 hours.

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Figure 54. Typical elongation curves [100] measured on polycrystalline and single crystal Nb.

Figure 55. Elongation curves [101] for as-received and high temperature treated Nb single crystals of different RRR and Ta concentration (see the figure).

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Furthermore, it has been demonstrated [100] that an accelerating gradient as high as 45 MV/m is possible for a 2.3 GHz single grain Nb single cell cavity. The mechanical properties of large/single grain Nb are different from those of regular Nb with grain size of ASTM 4 or 5. Fig.54 shows typical elongation curves for single grain and polycrystalline RRR Nb, showing excellent ductility for the Nb single crystal. The stress-strain curve for Nb single crystal even has a yield plateau similar to what unusually found on fine grain metals. It is rather surprising to see this since normally the elongation curves of high purity single crystal metals show a continuous yield since there are not grain boundaries to hinder and not enough pinning sizes to pin the movements of dislocations generated by the plastic deformation as discussed in Section 4.3. The effect of RRR and Ta concentration on the mechanical properties of as-received and high temperature treated Nb single crystals was reported in Ref.101 as reproduced in Fig.55. Here we can see typical elongation curves for single crystal metals with some impurities serving as pinning sites for dislocations. It is not clear that witch curve corresponds to which Ta concentration from Fig.55. I would assume that a higher yield strength comes from a higher Ta concentration. More controlled and characterized studies of the relationship between mechanical properties and grain orientations for Nb single crystals can be found in Refs.96 and 102.

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SECTION 5. SUMMARY AND PERSPECTIVE This chapter gives an overview on the use of Nb for fabricating SRF cavities to be used in particle accelerators. It also review several examples of significant progresses made in this field in the past 10 years from my personal viewpoints. SRF technique based on Nb has been advancing steadily. One example to illustrate this is Fig.56 where it shows a plot of achieved accelerating gradient as a function of year [103]. As one can see, improved RF performance always comes with new ideas and innovations. A significant step jump in gradient can be seen when HPWR and EP were introduced and widely accepted starting in 1995. Efforts on the R&D of Nb based SRF technology have been made mainly in the following two battle grounds: 1) Try to push the performance limit of Nb. 2) Try to reduce the fabrication cost of Nb SRF cavities. Currently, it is possible to obtain Nb SRF cavities with performance close to the limit. For instance, an accelerating gradient of 59 MV/m has been reported [104] on a Nb SRF cavity. However, the production yield for cavities of gradient higher than 35 MV/m with a decent Qo of 1010 is low and often cavity performance is limited by field emission. An example to see about this is a statistic summary of the performance of 9 cells cavities given in Fig.57 where one can see that the data points scatter a lot. One possible way to improve this among others is to improve the surface smoothness. One major difference between BCP and EP is the surface roughness (see, for instance, Fig.58) and so [100] is between cavities made from large/single grain Nb and from the regular grain Nb. Therefore, the ability to produce very smooth Nb surfaces routinely appears to be highly desirable. New development in Nb quality control and characterization techniques will enable us to avoid bad starting materials and can help us understand better the art of cavity fabrication and

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the mechanism behind each fabrication step. This can lead to a significantly improved cavity production yield too and therefore is also highly desirable.

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Figure 56. History [103] of high accelerating gradient of Nb SRF cavities.

Figure 57. Statistics of the performance of 9-cell Nb SRF cavities at DESY.

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Figure58. Typical BCP and EP treated Nb surfaces as seen from a SEM.

Better and more effective ways to remove the chemical residuals and other contaminants on the surfaces of Nb cavities also deserve great attention. These help us pass by the field emission barrier and it is one of the necessary steps that one has to go through in order to achieve routinely the performance limit of Nb SRF cavities. Reduction of cavity production cost can also be possible if we really understand better the metallurgical and mechanical properties of Nb. I feel that efforts in this respect are not enough. Dislocations, interstitial atoms, stacking faults, secondary phases, embedment of foreign materials, and other defects can be introduced to Nb SRF cavities during cavity fabrication. They have to be better understood and controlled. For instance, deep drawing in the cavity fabrication process cause huge plastic deformation and can introduce many edge or screw dislocations. Although some of them are removed by annealing at high temperatures during either hydrogen outgasing process or purification process, many of them still stay (see, for instance, Fig.49). Those defects and imperfections can degrade RF performance seriously. Again, I would recommend here that MOM cross-section observation should be used as a quality control tool for both Nb manufacturers and end users. Looking ahead for the use of Nb in SRF application, one has to turn to Nb alloys that have higher Tc and/or higher Hc such as NbN, NbTiN, and Nb3Sn in order to achieve a higher RF performance limit. One relatively easier way to reach this is perhaps by thin film coating technique to create the SIS structure as suggested by Gurevich [105] or by generating NbN, NbTiN, and Nb3Sn on Nb or Cu surfaces. Regarding the way to reduce Nb cavity fabrication cost, Nb on Cu seems to be promising.

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[74] Chincarini et al, Proceedings of the 10th SRF Workshop, Tsukuba, Japan, (2001) P382 [75] A.T. Wu et al, Proceedings of the 14th SRF Conference, Berlin, Germany, (2009) P300 [76] B. Visentin, Proceedings of the 11th SRF Workshop, Travemunde, Germany (2003) P199 (TUO01) [77] G. Ciovati et al, Phys. Rev. ST Accel. Beams 13, 022002 (2010) [78] P. Bauer et al, Review of Q-slope in SRF Cavities, Fermi Lab Report, TD-05-56, (2006). Can be viewed from Google Search. [79] Romanenko, Proceedings of the 14th SRF Conference, Berlin, Germany, (2009) P95 [80] Visentin et al, Proceedings of the 9th SRF Workshop, Santa Fe, USA, (1999) P198 [81] Visentin et al, Proceedings of the 12th SRF Workshop, Ithaca, USA, (2005) TuP05 [82] H. safa, Proceedings of the 10th SRF Workshop, Tsukuba, Japan, (2001) P279 [83] J. Halbritter, Proceedings of the 11th SRF Workshop, Travemunde, Germany (2003) MOP44 [84] G. Ciovati, Proceedings of the 13th SRF Workshop, Beijing, China, (2007) TU102 [85] Romanenko, Proceedings of the 13th SRF Workshop, Beijing, China, (2007) TU103 [86] J. Knobloch et al, Proceedings of the 9th SRF Workshop, Santa Fe, USA, (1999) P77 [87] K. Saito, Proceedings of the 11th SRF Workshop, Travemunde, Germany (2003) THP15 [88] B. Bonin and R. Roth, Proceedings of the 5th SRF Workshop, Hamburg, Germany (1991) P210 [89] H. Padamsee et al, RF Superconductivity for Accelerators, John Wiley & Sons, (1998) P209 [90] F. Koechlin et al, Superconductors Science and Technology 9 (1996) P453 [91] Reference 89, P206 [92] T. Carneiro et al, Proceedings of the 10th SRF Workshop, Tsukuba, Japan, (2001) P417 [93] K.K. Schulze, Journal of Metals, 33 (1981) P33 [94] P. Kneisel et al, Proceedings of PAC, Knoxville, USA (2005) P3955 [95] A.T. Wu and G. Myneni, JLab-TN-02-027 (2002) [96] T.R. Bieler et al, Phys. Rev. ST Accel. Beams 13, 031002 (2010) [97] G. Myneni and H. Umezawa, Meteriaux and Techniques, 7-8-9 (2003) P19 [98] A.M. Shrager, Elementary metallurgy and metallography, Dover Publications, Inc., New York, (1969) P22 [99] W. Singer, SRF materials workshop, Fermi Lab, (2007) [100] P. Kneisel et al, Proceedings of the 12th SRF Workshop, Ithaca, USA, (2005) MOP09 [101] G. Myneni, Proceedings of the 12th SRF Workshop, Ithaca, USA, (2005) MOP08 [102] D. Baars et al, Proceedings of the 14th SRF Conference, Berlin, Germany, (2009) P144 [103] K. Saito, Proceedings of the 13th SRF Workshop, Beijing, China, (2007) TU202 [104] R.L. Geng et al, Proceedings of PAC2007, Albuquerque, USA (2007) P2337 [105] Gurevish, Applied Physics Letters, 88 012511 (2006)

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ISBN: 978-1-61122-895-3 © 2011 Nova Science Publishers, Inc.

Chapter 13

INVESTIGATION OF SURFACE TREATMENTS OF NIOBIUM FLAT SAMPLES AND SRF CAVITIES BY GAS CLUSTER ION BEAM TECHNIQUE FOR PARTICLE ACCELERATORS *

A.T. Wu, D.R. Swenson1, P. Kneisel, G. Wu2, Z. Insepov3, J. Saunders, R. Manus, B. Golden, S. Castagnola, W. Sommer, E. Harms2, T. Khabiboulline2, W. Murayi2 and H. Edwards2

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Thomas Jefferson National Accelerator Facility, 12000 Jefferson Avenue, Newport News, VA 23606, U.S.A. 1 Epion Corporation, 37 Manning Road, Billerica, MA 01821, U.S.A. 2 Fermi National Accelerator Laboratory, Batavia, IL 60510, U.S.A. 3 Argonne National Laboratory, 9700 South Cass Ave., Argonne, IL 60439, U.S.A.

ABSTRACT More and more particle accelerators are using Nb Superconducting Radio Frequency (SRF) technology due to the steady progress made during the last few decades in the SRF field. Improvement of the surface treatments of Nb SRF cavities is an indispensable part of the evolution of SRF technology. In this chapter, a study of the surface treatments of Nb flat samples and SRF single cell cavities via Gas Cluster Ion Beam (GCIB) technique will be reported. Beams of Ar, O2, N2, and NF3 clusters with accelerating voltages up to 35 kV were employed in the treatments. The treated surfaces of Nb flat samples were examined by a scanning field emission microscope, a scanning electron microscope equipped with an energy dispersive x-ray analyzer, a secondary ion mass spectrometry, an atomic force microscope, and a 3-D profilometer. The experiments revealed that GCIB technique could not only modify surface morphology of Nb, but also change the surface oxide layer structure of Nb and reduce the number of field emission sites on the surface dramatically. Computer simulation via atomistic molecular dynamics and a *

A version of this chapter was also published in Neural Computation and Particle Accelerators, edited by E. Chabot and H. D'Arras, Nova Science Publishers. It was submitted for appropriate modifications in an effort to encourage wider dissemination of research.

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phenomenological surface dynamics was employed to help understand the experimental results. Due to its effectiveness at changing the depth and composition of the surface oxide layer structure of Nb, GCIB might be a key to understanding and overcoming the limitations of the high-field Q-slope. Based on the encouraging experimental results obtained from flat sample study, a novel setup was constructed to allow GCIB treatments on Nb single cell cavities. First results of RF tests on the GCIB treated Nb single cell cavities showed that the quality factor Q of the cavity could be improved substantially at 4.5 K and the superconducting gap value, extracted from RF measurements at different temperatures below superconducting transition temperature, was enhanced by oxygen GCIB treatments. This study indicates that GCIB is a promising surface treatment technique for Nb SRF cavities to be used in particle accelerators.

1. INTRODUCTION Radio frequency superconductivity has matured after several decades of steady progress since the pioneering work began at Stanford University in 1965[1]. Nowadays, superconducting radio frequency technology (SRF) based on niobium (Nb) is a popular choice for particle accelerators under construction or to be built in near future such as, for instance, international linear collider (ILC), x-ray free electron laser (XFEL) at DESY, energy recovery linac (ERL) at Cornell University in USA, the new Spiral 2 facility in France, and the isotope separation and acceleration (ISAC) II in Canada. The popularity can be, at least partially, attributable to the improvement in surface preparation of Nb SRF cavities in the past few decades. Conventionally, buffered chemical polishing (BCP) with hydrofluoric, nitric, and phosphoric acids or electropolishing (EP) with hydrofluoric and sulfuric acids is used as the final step of surface treatments on Nb SRF cavities. However, some limitations on the performance of the Nb SRF cavities prepared by the two techniques have been revealed recently. One limitation is a dramatic reduction in quality factor Q0 (abbreviated as Q-drop) of Nb cavities treated by either BCP or EP starting at a peak surface magnetic field of 90-100 mT without x-ray production[2,3]. Another limitation is that the cavity performance scatters a lot and it is not yet possible to consistently achieve high accelerating gradient and high Q0. Although the Q-drop can be recovered fully or partially by a low temperature baking at 100120 0C, it is highly desirable to have Nb SRF cavities that do not show such limitations. In order to achieve this goal, the mechanism responsible for such limitations has to be understood. It is well known that Nb is a highly reactive metal. When it is in contact with air, an oxide layer of a thickness of about 6 nm is immediately formed on its surface. It is generally believed that a better control over this surface oxide layer structure, chemical composition, smoothness, and defect concentration will contribute to the elimination of the limitations since the typical RF penetration depth for Nb is 50 nm. There are several ways that may eventually lead us to the goal. One promising way in this direction is to fabricate cavities using single or large grain Nb[4]. Others include new surface preparation techniques called buffered electropolishing (BEP)[5,6] and gas cluster ion beam (GCIB) treatments[7] where a smoother and chemically modified surface can be obtained. This chapter will focus on the research and development done on Nb flat samples and SRF cavities treated by GCIB technique.

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The chapter is organized in the following way: Section 2 gives a brief review of GCIB history and its applications to SRF technology based on Nb. The working principal of GCIB is illustrated in Section 3. Section 4 shows the experimental results of suppression of field emission on the surfaces of Nb flat samples after GCIB treatments as revealed by a scanning field emission microscope (SFEM). The shape, microstructure, and composition of the detected emitters were examined by a scanning electron microscope (SEM) coupled with an energy dispersive x-ray (EDX) analyzer. Possible mechanism responsible for the suppression of field emission is discussed. Section 5 deals with experimental and computer simulation investigations of the modifications of the morphology of Nb surfaces by GCIB treatments and discusses the implications of the modifications related to the performance of Nb SRF cavities. The change of surface oxide layer structure of Nb after GCIB treatments from the measurements of a dynamic secondary ion mass spectrometry (SIMS) are addressed in Section 6. The first results of RF measurements on the oxygen GCIB treated Nb SRF cavities will be reported in Section 7. Finally summary and perspective are given in Section 8.

2. BRIEF HISTORY OF GCIB AND ITS APPLICATION TO NB The idea of GCIB first came up in 1988 at the Ion Beam Engineering Experimental Laboratory of Kyoto University by I. Yamada according to Reference [8]. It was demonstrated in 1991 experimentally that an intense gas cluster beam could be formed at room temperature via supersonic expansion through a nozzle[9]. The gas cluster beam was ionized[10] in 1992 so that GCIB was formally born. For detailed information regarding the evolution of GCIB in general, please see reference [8]. The first commercial GCIB system named Ultra-smoother was made by a Boston-based company called Epion Corporation in 1999. Since then, Epion has sold GCIB systems to companies world wide for microelectronics and related manufacturing and for enhancement of critical surfaces in devices used for data storage, optics, and telecommunications[11]. Epion Corporation was purchased by Tokyo Electron (TEL) in 2007. TEL is the world‟s second largest supplier of semiconductor production equipment. The major field for the applications of GCIB is the semiconductor industry. However, development of GCIB technique in Japan and USA in the past decade or so has extended the technology to the applications of many different fields, including surface smoothing of magnetic materials, IC process applications, GCIB-assisted thin film deposition, surface treatments of electrodes, and surface treatments of Nb SRF cavities. These applications are determined by the following two unique properties that GCIB possesses: 1) GCIB has a very low charge to mass ratio. Take Ar GCIB as an example. Typically one Ar cluster contains[12] 10,400 atoms and an average charge of +3.2. Therefore, at a given beam current density GCIB can transport up to thousands of times more atoms than that via a conventional monomer ion beam. 2) The energy that is involved in the interactions between the atoms of a treated surface and the individual atom of a GCIB cluster is low. This is a direct consequence of the first property since the energy of the individual atom of a GCIB cluster is the total energy of the cluster divided by the total number of atoms. For Ar, typical average cluster energy is 64 keV[12]. Therefore, the energy of an atom of the cluster is less than 6.2 eV. Consequently, when a GCIB cluster impacts on a surface the interaction is multiple-atoms low energy collisions instead of the simple conventional binary high energy collisions encountered when a

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monomer ion is employed. These multiple atoms low energy collisions can produce relatively high sputtering, shallow implantation, and other kinds of nano-scale surface modifications[13] with low mechanical damages to the treated surfaces, which are the important characters required by many applications mentioned above and the application discussed in this chapter. Application of GCIB to Cu radio frequency (RF) cavities was initiated by D.R. Swenson and coworkers as evidenced in the reference [14] published in 2005 where efforts were made on mitigating high voltage breakdown by reducing the surface roughness of oxygen-free Cu via GCIB. At the 12th SRF Workshop in July 2005, after I gave a talk on the world‟s only Surface Science Lab (SSL) that was set up at Thomas Jefferson National Accelerator Facility (Jefferson Lab) to study exclusively various surface problems of Nb SRF cavities, I was approached by D.R. Swenson so that a pleasant collaboration on the application of GCIB to Nb was started. At the moment, we were mainly interested in the possible effects of GCIB treatments on the long standing field emission problem on the surfaces of Nb SRF cavities which is still one of the main challenges that our SRF community is facing up to now. As will be shown in the following we found that GCIB treatments could reduce the number of field emitters on the Nb surfaces remarkably. Preliminary results of the study were published in reference 7. Amazed by the results, an effort was made to understand the mechanism of the interactions between various GCIB clusters and Nb via several surface measurement systems[15] available at the SSL and via computer simulation[16] through collaboration between Epion, Jefferson Lab, and Argonne National Lab. Encouraged by the results obtained from flat Nb samples, a system was built at Epion to do GCIB treatments on Nb SRF cavities. RF tests of an O2 GCIB treated Nb SRF single cell cavities were done at Jefferson Lab first and later at Fermi National Lab. A small flat sample was also mounted to the equator of a Nb SRF single cell cavity and then the cavity was treated in exactly the same way as what was done on the first cavity from Jefferson Lab so as to try to understand the results of RF measurements through surface investigations using various surface instruments. All these experimental results will be reported in details in the sections from 4 to 7.

3. WORKING PRINCIPAL OF GCIB The working principal of GCIB is schematically illustrated in figure 1. Various types of gases can be used for GCIB treatments. The gases can be inert such as Ar, Kr, Xe etc. or chemically reactive such as O2, N2, CO2, NF3, CH4, B2H6 etc. that may react with the surfaces under treatments depending on what the application one has in mind. After selecting an appropriate gas species, the gas is forced through a nozzle that has a typical pressure of 7.6X103 Torr on one side and a vacuum of 7.6X10-3 Torr on the other side. Therefore the gas undergoes a supersonic expansion adiabatically that slows down the relative velocity between the atoms of the gas, leading to the formation of a jet of clusters. A typical cluster contains atomic numbers ranging from 500 to 10,000 that are held together by van der Waals forces. A skimmer is then used to allow only the primary jet core of the clusters to pass through an ionizer. The clusters are ionized by an ionizer via mainly electron impacts and the positively charged clusters are electrostatically accelerated via a typical voltage ranging from 2 kV to 35 kV and focused by a beam optics. Monomers and dimers are removed from the beam by a dipole magnet before the beam is neutralized with an electron flood. The aperture in figure 1

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Investigation of Surface Treatments of Niobium Flat Samples and SRF Cavities… 365 after the neutralizor is used to collect the monometers and dimers. Surface GCIB treatments are done through mechanically scanning an object. Typically, the impact speed of the clusters to the surface of an object under GCIB treatements is 6.5 km/s, and the current of a gas cluster beam can be as high as 1 mA.

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Figure 1. Schematic of working principal of GCIB.

The selection of an appropriate gas species for doing GCIB treatment is very important. When an inert gas is chosen, the major effects on the treated surfaces are smoothing and asperity removal due to lateral sputtering. Chemical gases, on the other hand, can produce some additional effects such as, for instance, doping, etching, and depositing, etc. depending on the properties of the treated object and the gas species selected. Implantation is only limited to the top several atomic layers during GCIB treatments due to the low individual atomic energy. One can also combine the use of different gas species in a specific order for a particular application, although less work has been done in this research direction so far. For the study reported in this chapter, only Ar, O2, N2, and NF3 were used in the GCIB treatments on Nb. Ar was selected because of its smoothing effect. O2 GCIB is interesting due to the possible chemical reactions between O2 and Nb and so is true also for N2, although in case of using N2 we were hoping that NbN could be formed on the treated surface since the superconducting transition temperature (Tc) is 16.2 K that is much higher than 9.2 K for Nb. NF3 is expected to have a relatively higher etching and removal rates on Nb than those from other chemically reactive gas species.

4. SUPPRESSION OF FIELD EMISSION BY GCIB TREATMENTS Field emission[17] from the surfaces of Nb SRF cavities has been a limiting factor for particle accelerators operated at high accelerating gradients of 10-30 MV/m. To overcome this problem, the surfaces of Nb SRF cavities must be free from field emission at surface electric fields that are roughly two times the accelerating gradients. This goal is not easy to achieve since in regular accelerating structures field emission often limits the cavity performance starting at a surface field of 20 MV/m. Heating from field emission increases exponentially with the surface field, leading to a dramatic decrease in the quality factor Qo of the excitation curves of Nb SRF cavities. The key here is to produce and maintain a clean surface that does not have micron or submicron particulates, chemical residues, and scratches or other sharp surface features. Various techniques[18] such as, for instance, clean room assembly, high pressure water rinse,

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ultrasonic cleaning with detergent, and recently dry ice cleaning[19] has been employed to mitigate particulates on the surfaces of Nb cavities. However up to now, field emission is still one of the major issues that the SRF community is facing. In this section, it will be shown that field emission on Nb surface can be significantly suppressed by GCIB treatments. To investigate the effect of GCIB treatments on field emission on Nb surface, a unique home-made system named SFEM[15,20] was employed. This system also allows the emitters detected on the Nb surfaces by SFEM to be analyzed by SEM and EDX so that information about the dimensions and compositions of the emitters can be obtained. The experiment was done in the following way: First standard Nb coupons ( a typical sample is shown in figure 2) were fabricated from the same Nb sheet followed by the standard BCP 112 removal of 150 µm. The arrows in figure 2 indicate the markings for coordinate identification so that the coordinates of SFEM system can be transformed into the coordinates of our SEM and EDX systems for locating the emitters. Then the samples were cleaned by ultrasonic degreasing and DI water rinsing. The coupons surfaces were blown by dry nitrogen gas afterwards. Then the samples were covered partially via a 25 µm thick stainless steel for GCIB treatments employing O2, Ar, and NF3. After appropriate GCIB treatments, samples are transferred into SFEM measurement chamber via a load-lock entrance purged with flow nitrogen to prevent contamination on the surfaces of the samples.

Figure 2. Standard Nb flat coupon used for the study described in this chapter. The arrows indicate the markings for coordinate identification.

Experimental results are shown in figure 3, figure 4, and figure 5. The sample used in figure 3 was masked into quadrants as shown in the figure. No GCIB treatment was done on the region marked “Unprocessed”. “P1” region was treated by Ar. “P1+P2” region was treated by Ar first followed by O2. O2 GCIB treatment was done on “P2” region. The locations of the triangles in these figures show where the emitters are and the height of a triangle indicates how strong the emitter is. The taller a triangle corresponds to the lower onset field gradient the emitter has. All treated regions showed fewer emitters than the

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unprocessed quadrant. The number of emitters in each region shows the following trend: P2