Platinum Monolayer Electrocatalysts [1st ed.] 9783030495657, 9783030495664

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Radoslav Adzic Nebojsa Marinkovic

Platinum Monolayer Electrocatalysts

Platinum Monolayer Electrocatalysts

Radoslav Adzic • Nebojsa Marinkovic

Platinum Monolayer Electrocatalysts

Radoslav Adzic Chemistry Dept Bldg 555 Brookhaven National Laboratory Upton, NY, USA

Nebojsa Marinkovic Synchrotron Catalysis Consortium and Department of Chemical Engineering Columbia University New York, NY, USA

ISBN 978-3-030-49565-7    ISBN 978-3-030-49566-4 (eBook) https://doi.org/10.1007/978-3-030-49566-4 © Springer Nature Switzerland AG 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Electrochemistry, and fuel cell electrocatalysis in particular, has undergone exceptional progress in the last 20 years. This has been facilitated by the development of several experimental techniques that can characterize electrochemical systems in situ with an atomic level resolution and molecular specificity. A number of surface science techniques applied ex situ provided additional structural characterization. These techniques have enabled the exploration of the role of surface structure in determining reaction kinetics using well-ordered single crystal surfaces and well-­ characterized nanoparticle catalysts. This work established the importance of surface crystallography on reaction kinetics, provided novel information on the structure of active sites, and elucidated the structure of adlayers and adsorbate-­ substrate interactions. In parallel to these advances in research, strong theoretical developments helped in understanding the new information and providing guidelines for further work. In this book, we describe the creation of platinum monolayer electrocatalysts which constitute an extraordinary breakthrough that will profoundly impact the science and technology of electrocatalysis. They promise to decrease the platinum content in catalysts to the ultimately low levels of a single atomic layer on a suitable core and to maximize platinum utilization to close to one hundred percent. We discuss how platinum activity and stability can be increased and controlled by the tuning of catalyst properties, which can be effectuated by the supporting core-platinum shell interaction. In the introductory part of the book, we discuss basic notions of the science of electrocatalysis: the formation of the electrical double layer, the potential distribution at the electrode–electrolyte interface, and the charge transfer in electrode reactions. The electrode potential, charge transfer reactions, and current–potential curves and basic formal electrode kinetics are presented. The differences between catalytic and non-catalytic reactions are explained. The role of electrocatalysis in

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Preface

electrochemical energy conversion in fuel cells is reviewed and novel methodologies that can help novices in electrochemical research are briefly described. In addition, seven electrocatalytic reactions of major significance for energy conversion applications are presented. The concepts of platinum monolayer electrocatalysts and core-shell catalysts are explored. Further discussions of platinum monolayer electrocatalysts including their types, structure, preparation, activity, stability, use in fuel cells for automotive applications, and scale up syntheses provide an essential review on this new catalyst that will affect the future of electrocatalysis. East Setauket, NY, USA December  2019

Radoslav Adzic Nebojsa Marinkovic

Contents

1 Short Introduction to the Science of Electrocatalysis��������������������������    1 1.1 Potential at Electrode–Electrolyte Interfaces������������������������������������    2 1.2 Electrical Double Layer��������������������������������������������������������������������    3 1.3 Charge-Transfer Reactions����������������������������������������������������������������    6 1.3.1 Current-Potential Curves������������������������������������������������������    8 1.3.2 Cyclic Voltammetry��������������������������������������������������������������   10 1.3.3 Theories of Charge Transfers������������������������������������������������   12 References��������������������������������������������������������������������������������������������������   13 2 Electrocatalytic Reactions ����������������������������������������������������������������������   15 2.1 Effect of Electrode Material: Volcano Plots��������������������������������������   15 References��������������������������������������������������������������������������������������������������   17 3 Electrochemical Energy Conversion in Fuel Cells��������������������������������   19 3.1 Types of Fuel Cells ��������������������������������������������������������������������������   21 3.1.1 Proton Exchange Membrane Fuel Cell (PEMFC)����������������   22 3.1.2 Alkaline Fuel Cells ��������������������������������������������������������������   23 3.1.3 Phosphoric Acid Fuel Cell����������������������������������������������������   24 3.1.4 Solid Oxide Fuel Cells����������������������������������������������������������   24 3.1.5 Molten Carbonate Fuel Cells������������������������������������������������   25 References��������������������������������������������������������������������������������������������������   25 4 Studies of Electrocatalytic Reactions ����������������������������������������������������   27 4.1 Structural Effects in Electrocatalysis������������������������������������������������   27 4.2 Single-Crystal Electrodes������������������������������������������������������������������   27 4.3 Preparation of Well-Ordered Single-Crystal Electrodes ������������������   31 4.4 Density Functional Theory Calculations in Electrocatalysis������������   32 References��������������������������������������������������������������������������������������������������   33 5 Important Electrosorption Reactions����������������������������������������������������   35 5.1 Hydrogen Adsorption on Platinum Metals ��������������������������������������   35 5.2 Hydroxyl Adsorption and Oxide Formation ������������������������������������   40

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5.3 Underpotential Deposition (UPD) of Metals and Catalytic Properties of Surfaces Modified by Metal Adlayers ������������������������   43 5.3.1 Oxidation of Small Organic Molecules on Surfaces Modified by UPD Adlayers������������������������������   49 5.3.2 Hydrogen Evolution on Surfaces Modified by UPD Adlayers������������������������������������������������������������������   51 5.3.3 Oxygen Reduction Reaction on Surfaces Modified by UPD Adlayers��������������������������������������������������   52 References��������������������������������������������������������������������������������������������������   53 6 Important Electrocatalytic Reactions����������������������������������������������������   57 6.1 Oxygen Reduction Reaction ������������������������������������������������������������   57 6.1.1 The d-Band Theory of Surface Reactivity����������������������������   63 6.2 Hydrogen Evolution Reaction����������������������������������������������������������   64 6.3 Hydrogen Oxidation Reaction����������������������������������������������������������   67 6.3.1 CO Tolerance������������������������������������������������������������������������   68 6.4 Methanol Oxidation Reaction ����������������������������������������������������������   69 6.5 Ethanol Oxidation Reaction��������������������������������������������������������������   71 6.6 Formic Acid Oxidation Reaction������������������������������������������������������   74 6.7 Oxygen Evolution Reaction��������������������������������������������������������������   76 References��������������������������������������������������������������������������������������������������   78 7 Platinum Monolayer Electrocatalysts����������������������������������������������������   83 7.1 Concept of Pt Monolayer Electrocatalysts����������������������������������������   83 7.2 Synthesis of Pt Monolayer Catalysts by Galvanic Displacement������������������������������������������������������������������������������������   85 7.3 Other Syntheses of Pt Monolayer Electrocatalysts��������������������������   87 7.3.1 Pd-Pt Catalyst Synthesized in Ethanol����������������������������������   88 7.3.2 Synthesis of PtML by Surface Mediated Growth ������������������   89 7.3.3 Characterization of Pt Monolayer Electrocatalysts��������������   90 7.4 In Situ FTIR and Synchrotron X-Ray Absorption Spectroscopies, and Hydrodynamic Rotating Disk Electrode Techniques������������������������������������������������������������������������   93 7.4.1 In situ FTIR Spectroscopy����������������������������������������������������   93 7.4.2 X-Ray Absorption Spectroscopy������������������������������������������   94 7.4.3 Rotating Disk Electrode��������������������������������������������������������   95 References��������������������������������������������������������������������������������������������������   98 8 Catalytic Properties of Pt Monolayer Electrocatalysts������������������������  101 8.1 Oxygen Reduction Reaction (ORR) ������������������������������������������������  101 8.1.1 Pt Monolayer on Extended Areas Single Crystals����������������  101 8.1.2 Pt Monolayer Shell on Nanoparticle Cores��������������������������  105 8.1.3 Effects of Composition, Shape, and Size of Cores ��������������  107 8.1.4 Modifications of Pt Monolayer Surfaces������������������������������  126

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8.1.5 Modified Alloy Surfaces as Support for Pt Monolayers ����������������������������������������������������������������  128 8.1.6 Nitride-Stabilized Nonnoble Metal Core Components�����������������������������������������������������������������  129 8.2 Hydrogen Oxidation Reaction on Pt Monolayer Electrocatalysts and CO Tolerance ��������������������������������������������������  135 8.3 Methanol Oxidation on Platinum Monolayer Electrocatalysts ��������������������������������������������������������������������������������  135 8.4 Ethanol Electrooxidation on Platinum Monolayer Electrocatalysts ��������������������������������������������������������������������������������  141 8.4.1 Oxidation of Methanol and Ethanol on Modified Pt Monolayer Electrocatalysts Surfaces��������������������������������  144 8.4.2 Subsurface Modification of Cores����������������������������������������  146 8.4.3 Reducing the Number of Low-Coordination Sites on Cores ����������������������������������������������������������������������  148 References��������������������������������������������������������������������������������������������������  150 9 Performance Stability and Scale-Up Syntheses of Pt Monolayer Electrocatalysts������������������������������������������������������������  153 Reference ��������������������������������������������������������������������������������������������������  155 10 Palladium Monolayer Electrocatalysts��������������������������������������������������  157 References��������������������������������������������������������������������������������������������������  160 11 Prospects for Platinum and Platinum Group Metal Monolayer Electrocatalysts����������������������������������������������������������  161 References��������������������������������������������������������������������������������������������������  162 Index������������������������������������������������������������������������������������������������������������������  163

About the Authors

Dr.  Radoslav  Adzic  is a Senior Chemist Emeritus at Brookhaven National Laboratory and  Adjunct Professor at Stony Brook University.  After graduating from the University of Belgrade in 1974 with a Ph.D. in chemistry, he held various positions at the University of Belgrade, becoming professor in 1990 and served as director of the Institute of Electrochemistry from 1983 to 1992. From 1988 to 1990 he was Visiting Professor at Case Western Reserve University. He is a Fellow of the Electrochemical Society and a Fellow of the International Society of Electrochemistry. Dr. Adzic is a correspondent member of the Serbian Academy of Sciences and Arts, a Foreign Member of the Academy of Engineering Sciences of Serbia, and a member of the International Academy of Electrochemical Energy Science. He has authored more than 300 scientific publications in the fields of surface electrochemistry and electrocatalysis and has 20 U.S. patents. Some of his awards and recognitions include the Annual Award of Belgrade for Natural Sciences 1983, the Science and Technology Award from Brookhaven National Laboratory in 2005, the SciAm 50 Award in 2007, the U.S. Department of Energy’s Hydrogen R&D Award in 2008 and in 2012, and the “R&D 100” Award in 2012. The Symposium Electrocatalysis 9 at the 333rd Electrochemical Society Meeting was held in his honor. According to Thomson Reuters he was a highly cited researcher in 2016, 2017, and 2018. Nebojsa  Marinkovic  obtained his Ph.D. in Physical chemistry in 1992 at the University of Belgrade. After postdoctoral experiences at the University of Louisville and the University of California, Davis, he moved to Brookhaven National Laboratory to study electrocatalytic processes on noble metals. Currently, he is an associate scientist at Columbia University and a member of the Synchrotron Catalysis Consortium, a contributing user group that supports research at the National Synchrotron Light Source II at BNL. He is also an adjunct associate professor of chemistry at the State University of New York at Riverhead.

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About the Authors

Nebojsa Marinkovic’s research involves the promoting and utilization of synchrotron-­based methods to nano-scale metal and metal oxide materials of interest for catalytic reactions, particularly those that can be integrated with complementary benchtop techniques for in-situ experiments. He has authored over 100 scientific papers and book chapters, served as section editor for the Encyclopedia of Applied Electrochemistry, and has given numerous invited lectures and presentations.

Chapter 1

Short Introduction to the Science of Electrocatalysis

In general terms, electrochemistry is concerned principally with chemical reactions that are associated with the transfer of electrons or ions across interfaces. The most typical electrochemical interface is between a solid metal and a liquid solution of an electrolyte (ionic conductor), but any interface involving a predominantly electronic conductor and a predominantly ionic conductor is considered to be an electrochemical interface. Interfacial reactions are governed by the potential distribution across the interface in a so-called electric double layer. The electric double layer is formed by the accumulation of excess charges of equal amount at the interface with a very small distance between such formed layers, which are similar to condenser plates. As a consequence of small separation of charges, the electrochemical interfaces have very high capacitance. Another consequence of the small inter-plate distance and of a typical voltage drop of 1 V between them is an extremely high electric field of 109 Vm−1 in the double layer. This is another characteristic of electrochemical interfaces that cannot be achieved at the metal vacuum counterparts. The capacitance, in addition to ohmic resistance, determines the rate with which we can change the electrode potentials. Since the double-layer capacity is large, the time required to change electrode potential from one value to another is about 1 microsecond with the best potentiostats. The interfacial regions can be modified by various adsorbates to produce very different electrode surface properties that are particularly important for the area of electrocatalysis, which addresses the effect of nature and structure of electrode surfaces on reaction kinetics. Electrochemistry has important applications in various areas of technology such as energy storage and conversion, large-scale production and refining of almost all nonferrous metals, production of inorganic chemicals, organic synthesis, corrosion prevention, electrochemical sensors, and biomedical applications.

© Springer Nature Switzerland AG 2020 R. Adzic, N. Marinkovic, Platinum Monolayer Electrocatalysts, https://doi.org/10.1007/978-3-030-49566-4_1

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1  Short Introduction to the Science of Electrocatalysis

1.1  Potential at Electrode–Electrolyte Interfaces Consider a metal electrode in equilibrium with an electrolytic solution contains the ion M1z1+ resulting from the dissolution of the metal; e.g.,

M1 = M1z1+ + z1e −

(1.1)

The electrons accumulate on the metal M1. On the basis of thermodynamics, the electrical work We available from this process is:

We = z1 EF = −∆G

(1.2)

where ΔG is the Gibbs free energy change per mole of M1 converted to M1z1+, F is the Faraday, z1 is the number of electrons per metal ion in Eq. (1.1), and E is the thermodynamic electrode potential [1]. The cationic charge must be compensated by the generation of an equal amount of charge of opposite sign, or the consumption of an equal charge of the same sign in a second reaction must attend reaction (1.1); e.g.:

M 2 z 2 + + z2 e − = M

(1.3)

The difference in the electrode potentials for reactions (1.1) and (1.3) is determined with the voltage measuring devices with two identical metal leads, M and M', that can be shown to correspond to the difference in the thermodynamic electrode potentials E1 and E2 (see, for example, Trasatti, [2]). The individual values of E1 and E2 cannot be determined by direct measurement. The potential difference between the bulk of the metal and the bulk of the solution does not correspond to the absolute value of the electrode potential, as some assumed. This potential difference is known as the Galvani potential and corresponds to the electric work to transport a unit positive test charge from the bulk of the solution phase into the metal. It is not a measurable quantity because one of its components, the surface potential, cannot be determined. The other notion here is the outer or Volta potential of the phase, which is the work required to bring a unit point charge from infinity to a point just outside the surface of the phase is a measurable quantity. For the two metal leads of the same composition, the inner potential difference can be measured since their surface potentials are the same. In such a case, the Galvani potential difference reduces to the Volta potential [3]. The difference in the thermodynamic electrode potentials (E1 – E2) of two electrodes in a galvanic cell is equal to the difference in inner Galvani potentials. Therefore, it is possible to take as zero the potential of one phase as a relative scale thermodynamic electrode potential [4]. For protic solvent systems such as water, the reference electrode establishing the zero of the electrode potential scale is the reversible hydrogen electrode involving the process occurring on a metal surface such as platinum which catalyzes this reaction:

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1.2 Electrical Double Layer



H 2 = 2 H + +2e −

(1.4)

Thus, a conventional electrochemical scale has ESHE = E0 H2/H+ = 0.

1.2  Electrical Double Layer To a first approximation, the electrode–electrolyte interface can be represented as a parallel plate condenser with one plate through the centers of the ions separated by a monolayer of solvent molecules from the electrode surface and the second plate at the metal surface. The solvated ions, whose centers define the outer Helmholtz plane (OHP), interact with the electrode surface through electrostatic forces only. Most of the cations, e.g., Na+, K+, and Li+, and anions, e.g., F− and ClO4−, which have solvation shells, generally do not approach the electrode surface closer than the OHP. Some weakly solvated anions, such as C1−, Br−, and I−, are chemisorbed on the electrode surface, undergoing a chemical bonding to the surface. A partial charge transfer takes place in this interaction. The coverage of these “specifically” adsorbed ions can be substantial if a large charge transfer takes place and strong chemical bonding occurs. Their centers define the inner Helmholtz plane (IHP). The coverage of electrostatically adsorbed ions does not usually exceed 0.1–0.2 of a full monolayer. Figure 1.1 gives a schematic representation of the double layer. The potential at which the net charge of the metal is zero is termed the potential of zero charge (PZC) and is characteristic of the metal as well as the electrolytic solution. The field

Fig. 1.1  Schematic representation of the double layer

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across the double layer at the PZC is dependent on the orientation of dipoles, including water molecules at the surface. The model of an electrical double layer as a condenser with a fixed plane of charge in the electrolyte (inner layer) is due to Helmholtz [5]. Gouy [6] and Chapman [7] took into account the ion concentration gradient extending from the electrode surface into the solution (diffuse layer) caused by the charge in the double layer. Stern [8] modified this model by taking into account that the finite size of ions results in a limiting distance of closest approach to the electrode surface. The double-layer capacitance is a series combination of the compact and diffuse layers, and the total capacitance of the metal–solution interface C is given by:

1 / C = 1 / C M − 2 + 1 / C2 − s

(1.5)

where CM−2 and C2−S are the capacitances between the metal and the outher Helmholtz plane and for the diffuse ionic layer, respectively. C2−S passes through the minimum at the PZC value. The measurement of the differential double-layer capacitance is used for the determination of EPZC. At high positive charges, anions lose their solvation. This causes a decrease of the distance of closest approach and consequently a large increase of CM−2. At high negative charges, the corresponding effect is small. The potential of zero charge is related to the modified (electrochemical) work function for electrons in the electrode since zero charge conditions are required for determination of both quantities. (The Kelvin probe measures the disappearance of excess charge.) The Fermi level of an electrode at EPZC correlates with the work function, Φ. The relationship:

EPZC = −Φ + const

(1.6)

holds for transition metals. For sp metals, the slope is not unity because of the metal-dependent water orientation. Deviations from Eq. (1.6) may arise from different dipole structure and ion adsorption at the metal–electrolyte interface. The work function as well as the PZC depends on the crystal orientation. Early studies were done using Hg electrode, which provided a smooth, clean surface. Figure 1.2 shows the double-layer capacitance of Hg in NaF at 25 °C, as a function of E − EPZC. Recent double-layer studies have been done with single crystal surfaces [9]. The coverage of specifically adsorbed anions, such as halides, can be large, and their effects on adsorption of other species and the kinetics of reactions can cause considerable decrease and, in some cases, a complete inhibition (See Chap. 4). Figure 1.3a shows the differential capacitance of Au(100) surface in KBr solution [9], Fig. 1.3b voltammetry curve for Au(100) in KBr, and Fig. 1.3c a scanning tunneling microscopy (STM) image of Br adlayer at E = 0.4 V on Au(100) [10]. Differential capacitance of Au(100) as a function of potential shows a strong potential dependence in agreement with voltammetry curve showing sharp spikes indicating phase transitions in the Br adlayer. Figure 1.3c shows the STM image of a high-coverage quasi-hexagonal c(2 × 2)R45° commensurate structure at potentials

1.2 Electrical Double Layer

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Fig. 1.2  The double-layer capacitance of Hg in NaF at 25 °C, as a function of E − EPZC. (Reproduced from Conway [1] with permission of Ronald Press)

Fig. 1.3 (a) Differential capacitance of Au(100) surface as a function of E in 0.02 M KBr solution [9]; (b) voltammetry curve for Au(100) in KBr; (c) STM image of Br adlayer at E = 0.4 V on Au(100) [10]. (Adapted from Refs. 9 and 10 with permission)

between the first and second spike in the voltammogram of Au(100) [10]. Such specifically adsorbed anions (e.g., halides) at high coverage can block the surface for many reactions. The theoretical analysis of the metal/liquid–electrolyte interface has been the subject of much study over the last two decades. In general, the models avoid some of the oversimplifications of earlier treatments. They take into account the ion size, the effect of solvent molecule size and its dipole moment, and the contribution of the metal valence electron orbitals to the double-layer behavior. The full role of the metal in this context was not realized before recent calculations using the methods

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of surface physics. The penetration of the field into the metal electrode (the Thomas– Fermi screening distance) appears important.

1.3  Charge-Transfer Reactions Electrochemical reactions are heterogeneous chemical reactions in which electrons are exchanged between the electrode and the molecules or ions in the electrolyte. The electrode is metal or other electronic conductive materials, while the electrolyte is purely ionic conductor which includes water and nonaqueous solvents, melt or solid electrolytes. In the course of an electrochemical reaction, the electron transfer occurs through the electrode/electrolyte interface. Electrons can be transferred through the interface in both directions. Particle in the electrolyte becomes either reduced when it accepts an electron from the electrode or oxidized when it gives an electron to the electrode. Thus, the electrochemical reaction involves the passage of electrical current. When the electrode potential is equal to the equilibrium potential, partial anodic and partial cathodic currents are equal, so that the total current is zero. However, when the imposed electrode potential is more positive or more negative than the equilibrium potential, the total current that passes through the electrode is the anodic or cathodic current, respectively. The simplest electrochemical reactions are those in which the electron transfer causes only the change of the oxidation state of a reactant, and no bond formation or splitting takes place. Much more common are cases in which the electron transfer is followed by or occurs simultaneously with the adsorption and/or chemical changes of a reactant, reaction intermediates, or products. Thus, the electrochemical reactions are divided into two classes: (i) outer-sphere one-electron transfer with the solution-phase electron donors or acceptors in the Helmholtz plane (OHP) of the electrical double layer where the electron transfer occurs and (ii) more complex processes where more than one electron may be transferred. Class 1 of electrochemical reactions involves a simple ionic redox process in which only the change of oxidation state of reactants positioned in the OHP is involved. Class 2 reactions often involve multiple steps, some can be chemical. When a reaction occurs in a series of consecutive steps, the overall reaction rate is determined by the rate of the slowest step, called the rate-determining step. All other preceding and following steps can be considered to be in equilibrium. If the slowest step in the reaction mechanism is the exchange of electrons, then the electrochemical reaction takes place under electrochemical or activation control. Many electrochemical reactions of organic molecules and reactions accompanied by gas evolution or dissolution are in a class 2. Just as for chemical reactions in general, the charge transfer is controlled by the existence of the energy barrier between oxidized and reduced states. A unique feature of the electrode reactions is that the height of this barrier can be decreased or increased by changing the potential across the interface.

1.3 Charge-Transfer Reactions

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The rate of charge transfer across the electrochemical interface depends not only on the potential but also on the double-layer structure and the adsorption of reactants, intermediates, and products and other eventual solution-phase species. Mass transport limitations are not considered here. The expression relating current to the electrode potential can be obtained from the absolute rate theory applied to the electrochemical interface. For that electrochemical reaction case, the heights of the free energy barriers are functions of the potentials drop across the interface in accordance with the absolute rate theory. In the simplest case of one electron-transfer reaction:

O + e− ↔ R

(1.7)

A shift in the electrode potential from 0 to a value E causes the changes depicted in Fig. 1.4. The barrier for the oxidation ΔG≠ is decreased by a fraction α of the energy change nFE, while the barrier for reduction is increased by (1− α) nFE. Rate constants for the reduction and oxidation are kred and kox, respectively. Assuming that there are arbitrary amounts of oxidant (O) and reductant (R) species in the solution, the total current flowing j is the sum of the partial cathodic jc and partial anodic ja currents:

j = jc + ja = nFAkred [O ]0 − nFAkox [ R ]0



(1.8)

where A is the electrode area, F is the Faraday constant, n is the number of electrons transferred, and [O]0 and [R]0 are the surface concentration of (O) and (R),

Fig. 1.4  Effect of electrode potential on the free energy versus coordinate curves for an electron reactant at two electrode potentials: E = Ee and E   Pt/C.  From the (111) diffraction profiles, the changes in lattice constant a relative to bulk Pt aPt can be estimated according to (a  – aPt)/aPt; these were  −  0.5, −2.3, and  −  6.4% for PtNiN/C, PtFeN/C, and PtCoN/C, respectively. The lattice parameters for the catalysts are smaller than that of Pt, and so Pt atoms are compressively strained. An intriguing feature is that the order of the decrease in Pt-Pt distance observed is PtNiN/C  PtNi/C, but adding nitride metal in the core increases the ORR activity in the order of PtNiN/C > PtFeN/C > PtCoN/C. By stabilizing nonprecious cores by bonding with nitrogen changes the geometric and electronic structure of these PtMN/C catalysts compared to their nonnitrided counterparts. DFT calculations have shown a volcano-type behavior with PtNiN/C at the top of the curve, revealing the fact that among the catalysts investigated, it has the best combination of both the surface strain and d-band center shifts.

8.3  Methanol Oxidation on Platinum Monolayer Electrocatalysts

135

Tian et  al. reported a class of core-shell electrocatalysts with well-dispersed inexpensive titanium nitrite nanoparticle cores and platinum layer shells [41]. The optimized Ti0.9Cu0.1N@Pt/NCNT has a Pt mass activity 5 times higher than commercial Pt/C. These authors used titanium nickel binary nitrate as a core and placed several layers of Pt on them. Both activity and stability of this catalyst outperformed commercial Pt/C.

8.2  H  ydrogen Oxidation Reaction on Pt Monolayer Electrocatalysts and CO Tolerance Hydrogen oxidation/reduction on Pt is one of the fastest electrochemical reactions. For its use in fuel cells for energy conversion there is no problems with Pt catalysts in acid solutions. In alkaline solutions, the reaction is considerably slower; the exchange current density is lower by two orders of magnitude. Thus, a considerable interest exists for it. In acid solutions, if hydrogen is not very clean, but contains small amounts of CO, poisoning is a difficult problem. The reformate, obtained by the reforming of ethanol, methanol, or gasoline, contains small amounts of CO, which is very difficult to remove. A concentration as low as 10 ppm is sufficient to poison the Pt sites and block the surface for the hydrogen oxidation reaction (HOR). For such fuels, the Pt catalysts have to have a good CO tolerance. This means that it has to oxidize a certain amount of CO and tolerate the rest. A submonolayer of Pt deposited on Ru nanoparticles by galvanic displacement has excellent CO tolerance in H2 oxidation. In the other class of nanocatalysts, coreshell-structured PdAuM (M  =  Co, Fe, Ni) nanoparticles served as substrates for PtML, and an enhancement in activity and reduction in cost were successfully achieved. These findings can be applied in designing practical nanoparticle catalysts, new catalysts for alcohol, and other organic oxidation at low Pt content, high efficiency, and reduced costs by using supported Pt monolayers and other core-sell structures. Figure 8.24 show rotating disk measurements of CO tolerance of PtRu20 catalyst for H2 oxidation compared with the best commercial Pt2Ru3 electrocatalyst in 1000 ppm of CO in H2, at 60 C. Much larger currents are maintained with PtRu20 electrocatalyst.

8.3  M  ethanol Oxidation on Platinum Monolayer Electrocatalysts Liquid fuels, especially methanol and ethanol, are considered as potential alternatives to hydrogen fuel in PEMFCs due to their high energy density, likely production from renewable sources, and the ease of their storage and transportation.

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8  Catalytic Properties of Pt Monolayer Electrocatalysts

Fig. 8.24  CO tolerance on Pt-submonolayer on Ru nanoparticle, compared with PtRu alloy in 0.5 M H2SO4 with 1000 ppm in H2, rotating disk measurement at 2500 rpm, at 0.05 V [42]. Reproduced with permission of Elsevier

800 600 I, µA

Pt

PtRu20 1 µg Pt/cm2 10 µg Ru/cm2

400

Ru

Pt2Ru3 4 µg Pt/cm2 3 µg Ru/cm2

200 0

997 ppm CO/H2 60 oC, 0.5 M H2SO4 2500 rpm, 0.05V (RHE)

0

2000

4000

6000

8000

10000

t, s

Methanol, containing only one carbon atom, is the simplest alcohol, and its electrocatalysis is also the simplest; therefore, there is a rising interest in direct methanol fuel cells (DMFCs) as potential power sources for portable electronic devices and for transportation applications. Introduction to the MOR and fuel cell anodic oxidation of this fuel is given in Sect. 6.4. Recently, Lee et al. reported a study of methanol oxidation on Pt monolayers on several single-crystal surfaces and on some nanoparticle supports [43]. PtML was deposited on different substrates via the galvanic displacement of a Cu UPD monolayer employing five single-crystal surfaces with hexagonal surface symmetry, which include (Au(111), Pd(111), Ir(111), Rh(111), and Ru(0001)) as substrates. For a PtML/Au(111) surface, where Au exerts on Pt a tensile strain, and a PtML/ Pd(111) surface, where Pt is under compressive strain, a significant enhancement in the catalytic activity associated with the tensile strain and decreased activity associates with the compressive strain were observed. During methanol oxidation (Fig. 8.25a), PtML/Au(111) exhibited a negatively shifted potential at the onset of the reaction and over sevenfold enhancement in peak current density with respect to Pt(111) (the most active low-index plane of Pt). Along with more electrochemical studies, a trend is observable, indicating that increased lattice compression lowers reactivity. In situ infrared reflection absorption spectroscopy (IRRAS) study was carried out to identify the reaction intermediates and products during methanol oxidation on PtML/Au(111), to gain insights into the substrate-induced change in the selectivity of PtML and into the mechanism of the greatly enhanced reaction kinetics. Figure 8.26 displays the in situ IRRAS spectra recorded on PtML/Au(111) during methanol oxidation; the enhanced MOR activity in PtML/Au(111) was due to the formation of COHads, instead of poisoning COads.

Current Density / mA*cm

Fig. 8.25  Comparison of methanol oxidation on single-crystal (top), and nanoparticle PtML-covered surfaces (bottom) in 0.1 M HClO4 + 0.5 M CH3OH solution [43]. Reproduced with permission of the American Chemical Society

-2

8.3  Methanol Oxidation on Platinum Monolayer Electrocatalysts

5 4 3 2 1

137

Pt(111) PtML/Au(111) PtML/Pd(111) PtML/Ir(111)

PtML/Rh(111)

PtML/Ru(0001)

MOR

0 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

E / V vs. Ag/AgCl

DFT calculations increased the understanding of the methanol electrooxidation on the surfaces of PtML supported on different substrates. The onset potential for methanol electrooxidation on the PtML/metal surfaces was estimated by calculating the binding energies and free energies of CO and OH (Fig. 8.27). The DFT-predicted trend in reactivity agreed well with the experimental observations, showing in decreasing sequence, PtML/Au(111) > Pt(111) > PtML/Pd(111) > PtML/Ir(111) > PtML/ Rh(111) > PtML/Ru(0001). PtML/Au(111) displays the highest activity, where PtML is stretched by over 4%, and exhibits enhanced reactivity in the dehydrogenative adsorption of alcohol molecules (yielding Pt-CO) and the dissociation of water (Pt-­ OH formation). That is, the strain effect due to the Au support results in a Pt with moderate reactivity, being able to bind the adsorbates strongly enough to activate methanol, yet weakly enough to prevent CO poisoning and allow the formation of CO2. In situ IRRAS study showed the enhanced MOR activity in PtML/Au(111) was due to the formation of COHads, instead of poisoning COads, and the promoted oxidation of COHads directly to CO2. The enhanced MOR activity was attributed to the combined geometric and electronic effect.

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8  Catalytic Properties of Pt Monolayer Electrocatalysts

Fig. 8.26  In situ IRRAS spectra recorded during MOR on the PtML/Au(111) electrode in 0.1 M HClO4 + 0.5 M methanol. One hundred twenty-eight interferograms with a resolution of 8 cm−1 were co-added to each spectrum [43]. Reproduced with permission of the American Chemical Society

Au octahedra nanocrystals (NCs) were synthesized by a seeded approach (15) by injecting 0.60  mL of ice-cold, freshly prepared NaBH4 (10  mM) into a rapidly stirred mixture of HAuCl4 (0.01  M, 0.25  mL) and CTAB (0.1  M, 9.75  mL). A growth solution was prepared by consecutively adding 0.5 mL of 10 mM HAuCl4 and 0.07 mL of 100 mM AA to a 10 mL aqueous solution of 0.1 M CPC. The seed solution was added to the growth solution under stirring and then left undisturbed at room temperature until the reaction completed. Carbon-supported Au nanoparticles (Au/C) were synthesized by wet impregnation of XC-72 carbon by HAuCl4, which was dried, and the mixture reduced using NaBH4 solution. Core-shell-­structured PdAuM (M = Co, Fe, Ni) nanoparticles with PdAu atoms on the shell and M atoms in the core were synthesized by a step-by-step approach. The molar ratio of Pd:Au:M is 50:10:40, and more details in their synthesis and characterization can be found in our earlier work [16, 17]. Commercial Pt/C and PtRu/C (with molar ratio Pt:Ru of 1:1) catalysts were obtained from ETEK. Pt monolayer (ML) was deposited on the Au single-crystal and nanoparticle surfaces by galvanic displacement of a predeposited Cu underpotential deposition (UPD) monolayer. To synthesize the Ru-modified PtML/Au/C catalyst, Ru nanoclu-

8.3  Methanol Oxidation on Platinum Monolayer Electrocatalysts

139

Fig. 8.27  DFT investigations of methanol oxidation on PtML supported on different substrates. The DFT-calculated variation of the lowest potential to proceed methanol electrooxidation on the PtML supported on (111) surfaces of fcc metals of Cu, Ru, Rh, Pd, Ir, Ag and Au, and (0001) surfaces of hcp metals of Ru, Re, and Os is plotted as a function of the surface strain. The surface strain was calculated by [d(PtML/surf)-d(Pt)]/d(Pt), where d is Pt-Pt bond length. The potential and surface strain were expressed with respect to the case of Pt(111) [43]. Reproduced with permission of the American Chemical Society

sters were deposited on PtML/Au/C by galvanic displacement of a partial Cu UPD monolayer. Nanostructured catalysts were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Selected area electron diffraction (SAED) was carried out on the Au octahedra NCs (see more details in Ref. 15). Cyclic voltammetry of PtML/Au and PtML(100) in 0.05 M H2SO4 and methanol electrooxidation in solution containing 0.5  M methanol and 0.1  M HClO4 were carried out at room temperature to determine specific current density by normalizing the current with the electrochemical active surface area (ECSA). The latter was determined by the charge of stripping one Cu UPD monolayer on PtML (assuming 480 μC/cm2) (substrate-induced strain) and ligand effect (the electronic interaction between PtML and the substrate). These findings can be applied in designing practical nanoparticle catalysts. The effectiveness of stretching PtMLwas probed by using Au octahedra NCs enclosed with Au{111} facets and an Au(100) single crystal as substrates for PtML. Then two classes of nanostructured catalysts that hold potential for practical application in DMFCs were synthesized. In one nanocatalyst, a co-catalyst approach was used and Ru nanoclusters were deposited on PtML/Au/C, resulting in the Ru/ PtML/Au/C catalyst. In the other class of nanocatalysts, core-shell-structured PdAuM (M = Co, Fe, Ni) nanoparticles served as substrates for PtML, and an enhancement in activity and reduction in cost were successfully achieved. As shown in Fig.  8.28a, b, the obtained NCs are single-crystal Au octahedra enclosed with Au{111} facets and have an edge length of about 35 nm [15]. High

140

8  Catalytic Properties of Pt Monolayer Electrocatalysts

yield and uniformity of products allow for the octahedral NCs self-assembling into ordered and well-packed two-dimensional structure [44]. Cyclic voltammogram in 0.1 M HClO4 and Cu UPD curves of Au octahedra (not shown here) also resembled the features of Au(111) and indicated that the exposed facets on NCs were indeed {111}. These NCs were used as substrates for PtML to verify findings with PtML/Au(111). As shown in Fig. 8.28c, a significantly enhanced methanol electrooxidation activity with lower reaction onset potential and higher oxidation current was observed on PtML/Au octahedra, compared to that on the commercial Pt/C catalyst. The result again demonstrates that a stretched PtML on the Au{111} substrate provides enhanced methanol oxidation activity and points out that such an approach can be used in making nanoscale catalysts. Au(100), another single crystal, was used as the substrate for PtML to study the effect of crystallographic orientation of the substrate. Cyclic voltammogram curves of PtML/Au(100) and PtML/Au(111) (Fig. 8.29a) showed a clear difference, and their shapes resembled features of Pt(100) and Pt(111). This, combined with earlier scanning tunneling microscopy (STM) studies [18], proves that one can obtain a continuous PtML film, instead of segregated Pt clusters [20–21]. No activity towards methanol oxidation is observed on Pt(100) until the potential reached approximately 0.72 V vs RHE. However, PtML/Au(100) showed an enhanced activity, and about 200  mV lower onset potential proved that on the PtML/Au(100) there was a destabilization of poisoning species.

Current Density / mA*cm-2

2.5

PtML/Au Octahedra

2

C

Pt/C (ETEK)

1.5 1 0.5 0 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

E / V vs. RHE

Fig. 8.28 (a) SEM image and (b) scheme of Au octahedral nanocrystals. (c) Methanol electrooxidation on PtML/Au octahedra and Pt/C in solution containing 0.5 M methanol and 0.1 M HClO4 with a scan rate of 10 mV/s [44]. Open access at the Electrochemical Society

8.4  Ethanol Electrooxidation on Platinum Monolayer Electrocatalysts

(a)

Current / µA

Fig. 8.29 (a) Voltammetry curves of PtML/Au(100) and PtML/Au(111) in 0.05 M H2SO4 solution with a scan rate of 50 mV/s; (b) Methanol electrooxidation on PtML/ Au(100) in solution containing 0.5 M methanol and 0.1 M HClO4 with a scan rate of 10 mV/s [44]. Open access at Electrochemical Society

141

10 0 –10 PtML/Au(100) PtML/Au(111)

–20 30

0

0.2

0.4 E / V vs. RHE

0.6

0.

(b) Current Density / mA* cm–2

5 4

PtML/Au(100)

3 2 1 0 0.2

0.4

0.6

0.6

1

E / V vs. RHE

8.4  E  thanol Electrooxidation on Platinum Monolayer Electrocatalysts Li et al. reported particularly exciting results with ethanol oxidation to CO2 on a Pt monolayer under tensile strain [43]. This example showed the role of the substrate lattice on the catalytic properties of PtML, which is enhanced when PtML is under tensile strain on Au(111) electrode and decreased when the Pt monolayer is compressed as on (111) electrodes of Pd, Ir, Rh, and Ru (Fig. 8.30). Similar enhancement of methanol oxidation is described in Sect. 8.3. These PtML electrocatalysts can also be synthesized by depositing a Pt monolayer on different substrates via the galvanic displacement of a UPD Cu monolayer. Scanning tunneling microscopy (STM) study of the resulting PtML/Ru(1010) surface revealed that Pt was deposited as a small three-dimensional (3D) island on Ru [17]. In some other Pt-M binary systems, for instance Pt on Pd, a pseudomorphic monolayer of Pt was formed by the displacement of a Cu UPD layer. For example, a PtML/Au(111) surface, where Au exerts on Pt a tensile strain, and a PtML/Pd(111) surface, where Pt is under compressive strain, a significant enhancement in the catalytic activity associated with the tensile strain, and decreased activity associates with the compressive strain, during the ethanol oxidation, the stretched

142

8  Catalytic Properties of Pt Monolayer Electrocatalysts

Fig. 8.30  Positive-going voltammetric scans for Pt(111) and PtML supported on five different substrates (Au(111), Pd(111), Ir(111), Rh(111), and Ru(0001)) in 0.1 M HClO4 containing 0.5 M ethanol with scan rate 10 mV/s [43]. Reproduced with permission of the American Chemical Society

PtML supported on Au(111) demonstrates slightly negatively shifted reaction-onset potential and over fourfold increase in peak current density. Along with more electrochemical studies, a trend is observable, indicating that increased lattice compression lowers reactivity. Several PtML nanocatalysts composed of PtML supported on mono- or bimetallic nanoparticle cores were studied (Fig. 8.31), and the activity for both methanol and ethanol electrooxidation reactions increased in the order of PtML/Pd/C