FUNDAMENTAL CONCEPTS IN HETEROGENEOUS CATALYSIS FUNDAMENTAL CONCEPTS IN HETEROGENEOUS CATALYSIS JENS K. NØRSKOV FELIX STUDT FRANK ABILD-PEDERSEN THOMAS BLIGAARD Cover Image: iStockphoto © olgaaltunina Cover illustration by Johannes Voss, Jens Strabo Hummelshøj, and Aleksandra Vojvodic Copyright © 2014 by John Wiley & Sons, Inc. 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QD505.N674 2015 541′.395–dc23 2014028859 Printed in the United States of America 10 9 8 7 6 5 4 3 2 1 CONTENTS Preface viii 1 Heterogeneous Catalysis and a Sustainable Future 1 2 The Potential Energy Diagram 6 2.1 Adsorption, 7 2.2 Surface Reactions, 11 2.3 Diffusion, 13 2.4 Adsorbate–Adsorbate Interactions, 15 2.5 Structure Dependence, 17 2.6 Quantum and Thermal Corrections to the Ground-State Potential Energy, 20 3 Surface Equilibria 26 3.1 Chemical Equilibria in Gases, Solids, and Solutions, 26 3.2 The Adsorption Entropy, 31 3.3 Adsorption Equilibria: Adsorption Isotherms, 34 3.4 Free Energy Diagrams for Surface Chemical Reactions, 40 Appendix 3.1 The Law of Mass Action and the Equilibrium Constant, 42 Appendix 3.2 Counting the Number of Adsorbate Configurations, 44 Appendix 3.3 Configurational Entropy of Adsorbates, 44 vi CONTENTS 4 Rate Constants 47 4.1 The Timescale Problem in Simulating Rare Events, 48 4.2 Transition State Theory, 49 4.3 Recrossings and Variational Transition State Theory, 59 4.4 Harmonic Transition State Theory, 61 5 Kinetics 68 5.1 Microkinetic Modeling, 68 5.2 Microkinetics of Elementary Surface Processes, 69 5.3 The Microkinetics of Several Coupled Elementary Surface Processes, 74 5.4 Ammonia Synthesis, 79 6 Energy Trends in Catalysis 85 6.1 Energy Correlations for Physisorbed Systems, 85 6.2 Chemisorption Energy Scaling Relations, 87 6.3 Transition State Energy Scaling Relations in Heterogeneous Catalysis, 90 6.4 Universality of Transition State Scaling Relations, 93 7 Activity and Selectivity Maps 97 7.1 Dissociation Rate-Determined Model, 97 7.2 Variations in the Activity Maximum with Reaction Conditions, 101 7.3 Sabatier Analysis, 103 7.4 Examples of Activity Maps for Important Catalytic Reactions, 105 7.4.1 Ammonia Synthesis, 105 7.4.2 The Methanation Reaction, 107 7.5 Selectivity Maps, 112 8 The Electronic Factor in Heterogeneous Catalysis 114 8.1 The d-Band Model of Chemical Bonding at Transition Metal Surfaces, 114 8.2 Changing the d-Band Center: Ligand Effects, 125 8.3 Ensemble Effects in Adsorption, 130 8.4 Trends in Activation Energies, 131 8.5 Ligand Effects for Transition Metal Oxides, 134 9 Catalyst Structure: Nature of the Active Site 138 9.1 Structure of Real Catalysts, 138 9.2 Intrinsic Structure Dependence, 139 9.3 The Active Site in High Surface Area Catalysts, 143 9.4 Support and Structural Promoter Effects, 146 CONTENTS vii 10 Poisoning and Promotion of Catalysts 150 11 Surface Electrocatalysis 155 11.1 The Electrified Solid–Electrolyte Interface, 156 11.2 Electron Transfer Processes at Surfaces, 158 11.3 The Hydrogen Electrode, 161 11.4 Adsorption Equilibria at the Electrified Surface–Electrolyte Interface, 161 11.5 Activation Energies in Surface Electron Transfer Reactions, 162 11.6 The Potential Dependence of the Rate, 164 11.7 The Overpotential in Electrocatalytic Processes, 167 11.8 Trends in Electrocatalytic Activity: The Limiting Potential Map, 169 12 Relation of Activity to Surface Electronic Structure 175 12.1 Electronic Structure of Solids, 175 12.2 The Band Structure of Solids, 179 12.3 The Newns–Anderson Model, 184 12.4 Bond-Energy Trends, 186 12.5 Binding Energies Using the Newns–Anderson Model, 193 Index 195 PREFACE The discovery and development of efficient chemical reactions and processes converting fossil resources into a broad range of fuels and chemicals is one of the most significant scientific developments in chemistry so far. The key to efficient chemical processes is the control of the rates of reaction. This control is usually provided by a catalyst—a substance that can facilitate a chemical reaction and determine the product distribution. The science of catalysis is the science of controlling chemical reactions. There are many challenges to the science of catalysis that need to be met over the coming years. A sustainable future calls for the development of catalytic processes that do not rely on a net input of fossil resources. This can only be achieved if we discover new catalysts that can efficiently utilize the energy input from the sun or other sustainable sources to synthesize fuels as well as base chemicals for the production of everything from plastics to fertilizers. It also requires more selective processes with fewer waste products and catalysts made from Earth-abundant elements. This represents a formidable challenge. This textbook describes some of the fundamental concepts that will be needed to address this challenge. Our basic assumption is that the discovery of new catalysts can be accelerated by developing a framework for understanding catalysis as a phenomenon and by pinpointing what are the most important parameters characterizing the chemical properties of the catalyst. We will concentrate in this book on heterogeneous catalysis, that is, catalysts where the processes take place at the surface of the solid. We will develop a systematic picture of the surface-catalyzed processes from the fundamental link to surface geometry and electronic structure to the kinetics of the network of elementary reactions that constitute a real catalytic process. The end result is a theory of variations in catalytic activity and selectivity from one catalyst to the next that will PREFACE ix allow the reader to understand the present literature and to make predictions of new catalysts. The latter is aided by the consistent involvement of public databases of surface chemical processes. The text is aimed at senior undergraduate and graduate students but should be a good guide for any researcher interested in the science and technology of heteroge- neous catalysis. We are grateful to a large number of colleagues for providing material for the book and for providing important feedback during the writing: Rasmus Brogaard, Karen Chan, Søren Dahl, Lars Grabow, Jeff Greeley, Heine Hansen, Anders Hellmann, Jens Strabo Hummelshøj, Karoliina Honkala, Tom Jaramillo, Hannes Jonsson, John Kitchin, Adam Lausche, Nuria Lopez, Nenad Markovic (for providing the unpublished results in Figure 11.10), Andrew Medford, Poul-Georg Moses, Anders Nilsson, Hirohito Ogasawara, Andrew Peterson, Jan Rossmeisl, Jens Sehested, Venkat Viswanathan, Aleksandra Vojvodic, and Johannes Voss. Our thoughts and ideas of this book has been influenced by an additional number of people including Flemming Besenbacher, Charles Campbell, Ib Chorkendorff, Bjerne Clausen, Claus Hviid Christensen, Bjørk Hammer, Karsten Jacobsen, Bengt Kasemo, Norton Lang, Bengt Lundqvist, Alan Luntz, Manos Mavrikakis, Horia Metiu, Yoshitada Morikawa, Matt Neurock, Lars Pettersson, Jens Rostrup-Nielsen, Robert Schlögl, Henrik Topsøe, and Art Williams. We are also grateful to the students of the early versions of