Chemical Kinetics and Catalysis

Chemical Kinetics and Catalysis

FUNDAMENTAL AND APPLIED CATALYSIS Series Editors: M. V. Twigg Johnson Matthey Catalytic Systems Division Royston, Hertfordshire, United Kingdom M. S. Spencer School of Chemistry and Applied Chemistry University of Wales College of Cardiff Cardiff, United Kingdom CATALYST CHARACTERIZATION: Physical Techniques for Solid Materials Edited by Boris Imelik and Jacques C. Vedrine CATALYTIC AMMONIA SYNTHESIS: Fundamentals and Practice Edited by J. R. Jennings CHEMICAL KINETICS AND CAT AL YSIS R. A. van Santen and J. W. Niemantsverdriet DYNAMIC PROCESSES ON SOLID SURFACES Edited by Kenzi Tamaru ELEMENTARY PHYSIOCHEMICAL PROCESSES ON SOLID SURFACES V. P. Zhdanov PRINCIPLES OF CAT AL YST DEVELOPMENT James T. Richardson A Continuation Order Plan is available for this series. A continuation order will bring delivery of each new volume immediately upon publication. Volumes are billed only upon actual shipment. For further information please contact the publisher.

Chemical Kinetics and Catalysis R. A. van Santen and J. W. Niemantsverdriet Schuit Institute at Catalysis Eindhoven University at Technology Eindhoven, The Netherlands Springer Science+Business Media, LLC

Library of Congress Cataloging-in-Publication Data On file ISBN 978-1-4757-9645-2 DOI 10.1007/978-1-4757-9643-8 ISBN 978-1-4757-9643-8 (ebook) 1995 Springer Science+Business Media New York Originally published by Plenum Press, New York in 1995 Softcover reprint of the hardcover 1st edition 1995 109 8 76543 2 1 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, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher

FOREWORD to the Fundamental and Applied Catalysis Series Catalysis is important academically and industrially. It plays an essential role in the manufacture of a wide range of products, from gasoline and plastics to fertilizers and herbicides, which would otherwise be unobtainable or prohibitively expensive. There are few chemical- or oil-based material items in modem society that do not depend in some way on a catalytic stage in their manufacture. Apart from manufacturing processes, catalysis is finding other important and ever-increasing uses; for example, successful applications of catalysis in the control of pollution and its use in environmental control are certain to increase in the future. The commercial importance of catalysis and the diverse intellectual challenges of catalytic phenomena have stimulated study by a broad spectrum of scientists, including chemists, physicists, chemical engineers, and material scientists. Increasing research activity over the years has brought deeper levels of understanding, and these have been associated with a continually growing amount of published material. As recently as sixty years ago, Rideal and Taylor could still treat the subject comprehensively in a single volume, but by the 1950s Emmett required six volumes, and no conventional multivolume text could now cover the whole of catalysis in any depth. In view of this situation, we felt there was a need for a collection of monographs, each one of which would deal at an advanced level with a selected topic, so as to build a catalysis reference library. This is the aim of the present series, Fundamental and Applied Catalysis. Some books in the series deal with particular techniques used in the study of catalysts and catalysis: these cover the scientific basis of the technique, details of its practical applications, and examples of its usefulness. An industrial process or a class of catalysts forms the v

vi FOREWORD basis of other books, with information on the fundamental science of the topic, the use of the process or catalysts, and engineering aspects. Single topics in catalysis are also treated in the series, with books giving the theory of the underlying science, and relating it to catalytic practice. We believe that this approach provides a collection that is of value to both academic and industrial workers. The series editors welcome comments on the series and suggestions of topics for future volumes. Royston and Cardiff Martyn Twigg Michael Spencer

PREFACE The reaction rate expressions of a catalytic process provide the chemical engineer with essential information on the performance of a catalyst. The parameters in the kinetic expressions, such as the order of the reaction and the activation energy, are determined by the chemistry of the process. Understanding the full meaning of these parameters requires a thorough knowledge of the molecular basis of reaction kinetics. Insight at this level is essential for the development of new catalytic reactions or the improvement of existing catalytic technology. During the past ten years it has become clear that there is not a large gap between industrial processes, which operate at high pressures, utilizing complex catalysts and high conversion rates, and laboratory experiments, which use pressures down to the ultrahigh vacuum range, with well-defined single-crystal surfaces as the catalysts and differential conversion levels. For several catalytic processes, such as the ammonia synthesis and the oxidation of carbon monoxide, it has been possible to predict kinetic behavior under process conditions from laboratory data obtained in surface science studies. Another important development in this field has been that people are beginning to understand the chemical kinetics of oscillating, exploding, and chaotic reactions. The thermodynamics of reversible processes can be considered a scientifically mature field. Kinetics, however, which belongs to the discipline of irreversible thermodynamics, is still developing. The same is true for the dynamics of reacting molecules, which is a field of intensive research. As described in Chapter 1, the historical development of kinetics and catalysis as scientific disciplines started in the nineteenth century. Both fields experienced a major breakthrough in the beginning of this century. The kineticist likes to reduce his problem so that optimum use can be made of thermodynamics by treating the difficult time-dependent part separately. This requires approximations which tum out to be very useful and widely applicable. Concepts such as the steady-state approximation, as well as sequences of elemenvii

viii PREFACE tary reaction steps in which one detennines the rate of the entire sequence, are introduced in Chapter 2 and illustrated in Chapter 3. In this book we intend to make a connection between molecular properties of species involved in catalytic reactions, their reactivity, and the expression for the reaction rate. Relations between the vibrational and rotational properties of molecules and their propensity to adsorb and react on a surface and to desorb into the gas phase, are derived from the field of statistical thennodynarnics. The latter forms a substantial part of Chapter 4, where we also introduce the transition-state reactionrate theory. Although it is convenient to describe a reaction as if it occurs in isolation, interactions of the reacting species with other molecules are essential for letting the reaction event occur. Medium effects and energy exchange between reacting molecules and their surroundings are the subject of Chapter 5. The knowledge gained in the first five chapters is applied in Chapter 6, where we present an integrated microscopic description. This book represents the third refinement of material that has been used over the past three years for the course "Chemical Kinetics and Catalysis" in the Schuit Institute of Catalysis at the Eindhoven University of Technology. We thank the students who participated in the course during the past three years; their comments, questions, and suggestions have played an important role in the revision of the previous versions. We are particularly grateful to Hannie Muijsers, Joop van Grondelle, Ton Janssens, and Tiny Verhoeven for their assistance in the production of text and figures. Eindhoven Rutger van Santen Hans Niemantsverdriet

CONTENTS CHAPfER 1. THE SCIENCE OF CATALYSIS 1.1. Introduction.... 1.2. The Early Days of Catalysis........ 1.3. Kinetics and Chemical Thermodynamics. 1.4. The Ammonia Synthesis.......... 1.5. Catalysis and the Growth of the Chemical Industry.. 1.6. The Scientific Disciplines of Catalysis 1.7. The Scope of This Book.... 3 6 9 10 14 18 CHAPfER 2. THE RATE EQUATION 2.1. The Reaction Equation................. 2.2. The Second Law of Thermodynamics.... 2.3. Coupled Reactions and the Steady-State Assumption. 2.4. Chain Reactions.......... 2.5. Parallel and Consecutive Reactions.... 2.6. Principle of Catalysis................ 2.7. Steady States Far from Equilibrium; Autocatalysis 2.7.1. Autocatalysis and Oscillating Reactions 2.7.2. Oscillating Surface Reactions......... 21.26. 28. 35. 38.43. 58.59.66 CHAPfER 3. INTRODUCTION TO CATALYTIC REACTIONS 3.1. Catalysis by Metals.... 3.1.1. Chemisorption on Metal Surfaces.. 3.1.2. Hydrogenation and Related Reactions 3.1.2.1. Ammonia Synthesis....74.74.79.79 ix

x 3.1.2.2. Synthesis Gas Conversion. 3.1.2.3. Hydrocarbon Activation 3.1.2.4. Oxidation.... 3.2. Catalysis by Oxides.... 3.2.1. Chemisorption on Oxides.... 3.2.2. Catalytic Reactions on Oxides. 3.2.3. Solid Acid Catalysis. 3.3. Catalysis by Sulfides.......... CONTENTS 81 83 87 89 89 93 97.102 CHAPTER 4. COLLISION AND REACTION-RATE THEORY 4.1. Microscopic Theory and Thermodynamics........ 105 4.2. Partition Functions.................... 112 4.2.1. Partition Function of an Ideal Monoatomic Gas. 112 4.2.2. Classical Partition Function of a Diatomic Molecule... 114 4.2.3. The Quantum-Mechanical Partition Function of the Diatomic Molecule.................. 117 4.3. Microscopic Expressions for the Rate Constant. 128 4.3.1. The Rate Expression............ 128 4.3.2. Collision Theory of Reaction Rates.... 131 4.3.3. Transition-State or Activated Complex Theory.. 139 4.4. Association and Dissociation Reactions: Elementary Reactions on Surfaces.......................... 148 4.4.1. Dissociation and Desorption Reactions...... 149 4.4.2. The Rate of Atomic Adsorption and Desorption. 153 4.4.3. Rate of Molecular Desorption... 159 4.4.4. Rate of Dissociation.. 162 4.4.5. Surface Diffusion......... 166 CHAPTER 5. MEDIUM EFFECTS ON REACTION RATES 5.1. Introduction.......................... 169 5.1.1. The Chemical Potential of the Activated Complex.. 170 5.2. Energy Exchange and Transition-State Theory........ 172 5.3. Kramers' Reaction-Rate Theory.... 179 5.4. Rate of Vibrational Energy Transfer between Gas Molecules.. 187 5.5. The Surface Thermal Accommodation Coefficient...... 194 5.6. Marcus Theory of Proton and Electron Transfer in Liquids. 197 5.7. Surface Overlayers and Inhomogeneity.... 201 5.7.1. Long-Range Coadsorbate Effects.... 201 5.7.2. Short-Range Coadsorbate Interaction.. 206

CONTENTS xi CHAPTER 6. MICROSCOPIC THEORY OF HETEROGENEOUS CATALYSIS 6.1. Prediction of the Overall Rate of a Catalytic Reaction 213 6.2. The Elementary Rate of Adsorption 217 6.3. Dissociative Adsorption................ 223 6.3.1. Introduction.................. 223 6.3.2. Molecular Orbital Picture of Chemisorption. 224 6.3.3. Chemistry of Surface Dissociation...... 228 6.3.4. Chemical Precursor-Assisted Dissociation.. 231 6.3.5. Dynamics of Dissociation; Quantum-Mechanical Tunneling 233 6.3.5.1. The Dissociation of Hydrogen by Copper... 233 6.3.5.2. The Tunneling Correction to the Rate Constant in the Transition-State Theory.............. 235 6.4. Transition States of Surface Reactions................. 238 6.4.1. The Transition State for Methane Dissociation on a Metal.. 238 6.4.2. The Transition State for Methane Activation by Bfl2Insted-Acid Protons........... 243 6.5. Kinetics of Dissociative Adsorption.. 247 6.6. Catalytic Reactions........... 253 6.6.1. The Hougen-Watson Approach 253 6.6.2. Competitive Adsorption in Hydrocarbon Catalysis 256 6.6.3. Volcano Curve for the Synthesis of Ammonia. 260 6.6.4. Selectivity Control by Competitive Adsorption 261 6.6.5. Selectivity in the Fischer-Tropsch Synthesis. 264 REFERENCES.... 269 Related Reading: General Texts on Kinetics and Catalysis. 271 INDEX... 273