Mechanical Catalysis

Gerhard F. Swiegers, PhD, earned his doctorate at the University of Connecticut in 1991 and then worked at the Australian National University and the University of Wollongong, Australia. In 1998, he joined the Commonwealth Scientific and Industrial Research Organization (CSIRO), the major government laboratory in Australia. From 1998 to 2006, he was involved with designing anti-counterfeiting devices for bank notes. In 2005, one of his inventions was commercialized as a spin-off company known as Datatrace DNA Pty Ltd, and in 2006, Dr. Swiegers joined the firm as Vice President, Strategic Research. Several of Dr. Swiegers's inventions are currently used by national governments and major companies around the world.

?This book is a useful addition to the library of any individual working with functionalized catalysts, especially those that may undergo conformational changes during the reaction process. This text is especially informative for those working with enzymes, biomimetic, and organometallic-based catalysts. It also unifies many of the kinetic models that have been put forth to describe heterogeneous, homogeneous, and enzymatic catalysis.? (Journal of the American Chemical Society, October 2009)

• Preface xxiContributors xxvGlossary xxvii1 Introduction to Thermodynamic (Energy-Dependent) and Mechanical (Time-Dependent) Processes: What Are They and How Are They Manifested in Chemistry and Catalysis?Gerhard F. Swiegers1.1 Thermodynamic (Energy-Dependent) and Mechanical (Time-Dependent) Processes 11.2 What Is a Thermodynamic Process? 51.3 What Is a Mechanical Process? 71.4 The Difference between Energy-Dependent (Thermodynamic) and Time-Dependent (Mechanical) Processes 91.4.1 Time-Dependent (Mechanical) Processes Are Path-Reliant and Spatiotemporal in Character 91.4.2 Time-Dependent (Mechanical) Processes Have a Flat Underlying Energy Landscape (or Are Unaffected by the Energy Landscape) 101.4.3 Time-Dependent (Mechanical) Processes Display Deterministic Chaos; This Causes Them to be Stochastic and Complex 111.4.4 Time-Dependent (Mechanical) Processes Often Involve Synergies of Action 141.4.5 Time-Dependent (Mechanical) Processes Characterize Numerous Aspects of Human Experience 151.5 Time- and Energy-Dependence in Chemistry and Catalysis 171.5.1 The Origin of Time- and Energy-Dependent Processes in Chemistry 171.5.2 Examples of Time-Dependent Processes in Chemistry 191.5.3 Time- and Energy-Dependent Processes in Catalysis 211.5.4 Is There Such a Thing as a Time-Dependent Process in Catalysis? 231.6 The Aims, Structure, and Major Findings of this Series 241.6.1 Summary of the Key Finding: Many Enzymes Seem to be Time-Dependent Catalysts 251.6.2 The Aims and Structure of this Series. Summary: Other Major Findings of this Series 28References 342 Heterogeneous, Homogeneous, and Enzymatic Catalysis. A Shared Terminology and Conceptual Platform. The Alternative of Time-Dependence in Catalysis 37Gerhard F. Swiegers2.1 Introduction: The Problem of Conceptually Unifying Heterogeneous, Homogeneous, and Enzymatic Catalysis? Trends in Catalysis Science 372.2 Background: What Is Heterogeneous, Homogeneous, and Enzymatic Catalysis 382.2.1 Homogeneous and Heterogeneous Catalysis 382.2.2 Hybrid Homogeneous–Heterogeneous Catalysts 402.2.3 Enzymatic Catalysis 412.2.4 Theories and Mimicry of Enzymatic Catalysis 422.3 Distinctions Within Homogeneous Catalysis: Single-Centered and Multicentered Homogeneous Catalysis 442.3.1 Single-Centered Homogeneous Catalysts. Most Manmade Homogeneous Catalysts Are Single-Centered Catalysts 442.3.2 Multicentered Homogeneous Catalysts: Most Enzymes Are Multicentered Homogeneous Catalysts 462.4 The Distinction between Single-Site/Multisite Catalysts and Single-Centered/MultiCentered Catalysts in Heterogeneous Catalysis: An Important Convention Used in This Series 482.4.1 A Key Convention Used in This Series: A Catalytic Site Is a Collection of Atoms about Which a Reaction Is Catalyzed. A Catalytic Center Is an Atom Within that Site Which Binds and Facilitates the Transformation of a Reactant 482.5 The Alternative of Time-Dependence in Catalysis 48References 523 A Conceptual Description of Energy-Dependent (“Thermodynamic”) and Time-Dependent (“Mechanical”) Processes in Chemistry and Catalysis 55Gerhard F. Swiegers3.1 Introduction 553.2 Theoretical Considerations: Common Processes in Uncatalyzed Reactions 563.2.1 Reactions as Collisions Between Molecules 563.2.2 The Fundamental Origin of Energy-Dependent and Time-Dependent Reactions 573.2.3 Time-Dependent and Energy-Dependent Domains Were First Observed in Unimolecular Gas-Phase Reactions 583.2.4 The Pathway of the Reaction Is also Controlled by the Least-Likely Step in the Sequence 593.2.5 Transition State Theory (TST) Describes the Pathway and Rate of Energy-Dependent Reactions. Transition State Theory Corresponds to the High-Pressure Limit of Hinshelwood–RRK Theory 613.2.6 Time-Dependent Reactions in the Liquid Phase: Some Examples 633.2.7 The Transition between Energy-Dependence and Time-Dependence as a Function of Temperature. Curvature in Arrhenius Plots 653.2.8 Methods of Creating Time-Dependent Reactions 673.2.9 Summary: The Key Properties of Time-Dependent and Energy-Dependent Reactions 683.3 Theoretical Considerations: Common Processes in Catalyzed Reactions 683.3.1 Catalyzed Reactions Are More Likely to be Time-Dependent than Are Uncatalyzed Reactions 683.3.2 Catalysis Changes the Reaction Processes 693.3.3 Physical Manifestation of Time- and Energy-Dependence in Catalysts 723.3.4 The Distinction Between Time-Dependent Catalysis and Diffusion-Controlled Catalysis 723.3.5 Energy-Dependent and Time-Dependent Control of Catalysis 733.3.6 The Influence of the Product Release Step 743.4 Conclusions: Energy- and Time-Dependent Catalysis 75Acknowledgments 75References 764 Time-Dependence in Heterogeneous Catalysis. Sabatier’s Principle Describes Two Independent Catalytic Realms: Time-Dependent (“Mechanical”) Catalysis and Energy-Dependent (“Thermodynamic”) Catalysis 77Gerhard F. Swiegers4.1 Introduction 774.2 Sabatier’s Principle in Heterogeneous Catalysis 794.2.1 Volcano Plots 794.2.2 Some Important Points about Volcano Plots 824.2.3 Time-Dependent Catalysis in Volcano Plots 824.2.3.1 How Is Time-Dependence Created on the Left-Hand Side of the Volcano Plot? 824.2.3.2 Why Do Volcano Plots Slope Upward on the Left 844.2.3.3 The Rate-Determining Step in a Time-Dependent Catalyst 864.2.3.4 The Physical Manifestation of Time-Dependent Catalysis. “Saturation” of a Time-Dependent Catalyst 874.2.4 Energy-Dependent Catalysis in Volcano Plots 884.2.4.1 How Is Energy-Dependence Created on the Right-Hand Side of the Volcano Plot? 884.2.4.2 Why Do Volcano Plots Slope Downward on the Right? 884.2.4.3 The Rate-Determining Step in an Energy-Dependent Catalyst 894.2.4.4 The Physical Manifestation of Energy- Dependence. Saturation in an Energy-Dependent Catalyst 894.2.5 The Physical Origin of Sabatier’s Principle 894.2.6 Other Plots Illustrating Sabatier’s Principle 904.2.7 Modeling of Volcano Plots 914.2.8 Reaction Pathway as a Function of the Most-Favored Transition State 924.3 Exceptions to Sabatier’s Principle 934.4 Sabatier’s Principle in Homogeneous Catalysis 934.5 Conclusions. Sabatier’s Principle Describes Two Independent Catalytic Domains: Energy- and Time-Dependent Catalysis 94Acknowledgments 95References 955 Time-Dependence in Homogeneous Catalysis. 1. Many Enzymes Display the Hallmarks of Time-Dependent (“Mechanical”) Catalysis. Nonbiological Homogeneous Catalysts Are Typically Energy-Dependent (“Thermodynamic”) Catalysts 97Robin Brimblecombe, Jun Chen, Junhua Huang, Ulrich T. Mueller-Westerhoff, and Gerhard F. Swiegers5.1 Introduction 975.2 Historical Background: Are Enzymes Generally Energy-Dependent or Time-Dependent Catalysts? 995.3 The Methodology of This Chapter: Identify, Contrast, and Rationalize the Common Processes Present in Biological and Nonbiological Homogeneous Catalysts 1005.4 Does Michaelis–Menten Kinetics in Enzymes Indicate that They Are Time-Dependent Catalysts? 1025.4.1 Michaelis–Menten Kinetics 1025.4.2 Kinetics in Most Nonbiological Catalysts 1035.4.3 The Contradiction of Saturation Kinetics in Enzymes 1035.4.4 Saturation in Time- and Energy-Dependent Catalysts. Saturation Kinetics Is Necessarily an Indication of Time-Dependence 1045.4.5 Physical Studies of the Rate Processes in Enzymes Are Consistent with a Time-Dependent Action 1065.4.6 A Time-Dependent Catalyst Cannot Become an Energy-Dependent Catalyst, or vice versa, Without Changing the Temperature or Chemically Altering the Reactivity of the Reactants 1075.4.7 The Current View of Michaelis–Menten Kinetics Is Flawed by an Unwarranted Assumption 1075.4.8 Summary: Michaelis–Menten Kinetics Is Characteristic of Time-Dependent Catalysis. Time-Dependent Catalysis Provides an Explanation for Michaelis–Menten Kinetics in Enzymes 1095.5 Other General Characteristics of Catalysis by Enzymes and Comparable Nonbiological Homogeneous Catalysts 1105.5.1 Enzymes Employ Weak and Dynamic Individual Binding Interactions with Their Substrates. Nonbiological Catalysts Do Not 1105.5.2 Enzymes Display Transition State Complementarity. Nonbiological Catalysts Do Not 1115.5.3 Enzymatic Catalysis Is “Structure-Sensitive.” Nonbiological Catalysis Is “Structure-Insensitive” 1125.5.4 Enzymes Transform Catalytically Unconventional Groups into Potent Catalysts. Nonbiological Catalysts Use Only Conventional Catalytic Groups 1135.5.5 Enzymes Catalyze Forward and Reverse Reactions. Nonbiological Catalysts Do Not 1135.5.6 Enzymes Display High Selectivity and Activity. Nonbiological Catalysts Do Not 1155.5.7 Enzymes Display Convergent Synergies. Nonbiological Catalysts Display Complementary Synergies 1155.5.8 Summary 1165.6 Rationalization of the Underlying Processes. The Mechanism of Action in Time-Dependent and Energy-Dependent Catalysts 1175.6.1 Common Processes in Multicentered Homogeneous Catalysts 1175.6.2 The Influence of the Strength of the Individual Catalyst–Reactant Binding Interactions 1195.6.3 The Coexistence of Transition State Complementarity, Structure-Sensitive Catalysis, and Unconventional Catalytic Groups in Enzymes Is Caused by their Weak Individual Binding Interactions 1225.6.4 The Origin of the Time-Dependence and the Synergies of Enzymes 1235.6.5 The Mechanism of Time-Dependence in Enzymes Resolves the Contradiction of a Kinetically Observed Rapidly Forming and Dissociating Intermediate in the Face of Strong Overall Substrate Binding 1255.6.6 Catalysis in Enzymes Involves Synchronization of Enzyme Binding and Enzyme Flexing 1255.6.7 Summary: The Origin of the General Properties of Enzymes 1275.6.8 Catalysis in Nonbiological Analogues Depends on the Activation Energy E a 1275.6.9 Enzymatic Selectivity and Synergies Derive from Time-Dependence 1285.6.10 Enzymatic Activity Is Consistent with Time-Dependence 1295.7 All Generalizations Support Time-Dependence in Enzymes 1295.8 Time-Dependence in a Nonbiological Catalyst Generates the Distinctive Properties of Enzymes 1305.9 Conclusion: Many Enzymes Are Time-Dependent Catalysts 133Acknowledgments 134References 1346 Time-Dependence in Homogeneous Catalysis. 2. The General Actions of Time-Dependent (“Mechanical”) and Energy-Dependent (“Thermodynamic”) Catalysts 137Robin Brimblecombe, Jun Chen, Junhua Huang, Ulrich T. Mueller-Westerhoff, and Gerhard F. Swiegers6.1 Introduction 1376.2 Time- and Energy-Dependent, Multicentered Homogeneous Catalysts 1396.3 The Action of Energy-Dependent, Multicentered Homogeneous Catalysts 1416.4 The Action of Time-Dependent, Multicentered Homogeneous Catalysts 1466.4.1 The Activation Energy E a Does Not Provide a True Measure of the Threshold Energy in Time-Dependent Catalysts 1486.4.2 Weak and Dynamic Binding and Activation Is Sufficient to Fulfill the Threshold Energy in Time-Dependent Catalysts 1496.4.3 Transition State Formation in a Time-Dependent Catalyst Can Be Thought of as a Coordinated Mechanical Process 1506.4.4 Time-Dependent Catalysts Are Machine-Like (Mechanical) in Their Catalytic Action 1506.4.5 The Origin of Michaelis–Menten Kinetics in Time-Dependent Catalysts 1516.4.6 Time-Dependent Catalysts like Many Enzymes Display All of the Characteristic Hallmarks of Mechanical Processes 1536.4.7 Additional Insights into Enzymatic Catalysis: The Bidirectionality of Enzymatic Catalysis Originates from the Mechanical Nature of the Catalytic Action 1546.4.8 Additional Insights into Enzymatic Catalysis: Many Enzymes Select the First-Encountered Transition State, Rather than the Lowest Energy Transition State 1556.5 The Importance of Recognizing Time-Dependent Catalysis 1556.6 Time-Dependent Catalysis Is Very Different to Energy-Dependent Catalysis and Therefore Seems Unfamiliar 1566.7 Conclusions for Biology 1576.8 Conclusions for Homogeneous Catalysis 1576.9 The “Ideal” Homogeneous Catalyst 1586.10 Conclusions for the Conceptual Unity of the Field of Catalysis 158Acknowledgments 159References 1597 Unifying the Many Theories of Enzymatic Catalysis. Theories of Enzymatic Catalysis Fall into Two Camps: Energy-Dependent (“Thermodynamic”) and Time-Dependent (“Mechanical”) Catalysis 161Gerhard F. Swiegers7.1 Introduction 1617.2 Theories of Enzymatic Catalysis 1637.2.1 Adsorption Theory 1637.2.2 “Lock-and-Key” Theory 1637.2.3 Haldane’s Strain Theory 1647.2.4 Pauling’s Theory of Transition State Complementarity 1657.2.5 Koshland’s Induced Fit Theory. Fersht’s Concept of Stress and Strain 1657.2.6 Intramolecularity 1657.2.7 Orbital Steering 1677.2.8 Entropy Traps 1687.2.9 The Proximity (Propinquity) Effect 1687.2.10 “Coupled” Protein Motions 1687.2.11 The Spatiotemporal Hypothesis 1697.3 Theories Explaining Enzymatic Catalysis Fall into Two Camps: Energy-Dependent and Time-Dependent Catalysis 1697.3.1 Haldane’s Strain Theory and Fersht’s Concept of Stress and Strain Are Valid Explanations for Rate Accelerations but Do Not Seem to be Responsible for the Rate Accelerations of Many Enzymes 1717.3.2 Theories Based on Reaction Entropy Are Valid Explanations for Rate Accelerations but Do Not Seem to be Behind the Rate Accelerations of Many Enzymes 1727.3.3 Experiments Studying Intramolecular Reaction Rates Were Probably Often Conceptually Contradictory 1727.3.4 Theories of “Coupled” Protein Motions and Machine-Like Catalytic Actions Seem to Be Generally Accurate Descriptions of Enzymatic Catalysis 1737.4 Studies Verifying Pauling’s Theory in Model Systems Are Correct, but Describe Energy-Dependent and not Time-Dependent Catalysis 1747.5 The Anomaly Described in the Spatiotemporal Hypothesis Originates, in Part, from the Onset of Time-Dependence 176Acknowledgments 177!References 1778 Synergy in Heterogeneous, Homogeneous, and Enzymatic Catalysis. The “Ideal” Catalyst 181Gerhard F. Swiegers8.1 Introduction 1818.2 Synergy in Heterogeneous Catalysts 1838.3 Single-Centered Nonbiological Homogeneous Catalysts and Their ‘Mutually Enhancing’ Synergies 1848.3.1 Facial Selectivity in Single-Centered Catalysts 1848.3.2 Energy-Dependent, Single-Centered Homogeneous Catalysts Display ‘Mutually Enhancing’ Synergies 1878.3.3 The Synergies in Time-Dependent, Single-Centered Homogeneous Catalysts 1888.3.4 The Selectivity of Single-Centered Catalysts 1898.4 Multicentered, Energy-Dependent Homogeneous Catalysts and Their Functionally Complementary Synergies 1908.5 Enzymes and Their Functionally Convergent Synergies 1948.6 Biomimetic Chemistry and Its Pseudo-Convergent Synergies 1978.6.1 Cyclodextrin-Appended Epoxidation Catalysts: Pseudo-Convergence in a Nonbiological, Multicentered Catalyst 1988.7 The Spectrum of Synergistic Action in Homogeneous Catalysis 2008.7.1 The Relationship Between Complementary and Convergent Synergies 2028.7.2 The Ideal Catalyst 2038.8 Synergy in Catalysis Is Conceptually Related to Other Synergistic Processes in Human Experience 205References 2069 A Conceptual Unification of Heterogeneous, Homogeneous, and Enzymatic Catalysis 209Gerhard F. Swiegers9.1 Introduction 2099.2 Diffusion-Controlled and Reaction-Controlled Catalysis 2109.3 The Diversity of Catalytic Action in Heterogeneous Catalysts 2119.4 The Diversity of Catalytic Action in Nonbiological Homogeneous Catalysts 2129.5 The Diversity of Catalytic Action in Enzymes 2149.6 Heterogeneous Catalysis and Enzymatic Catalysis Has, Effectively, Involved Combinatorial Experiments that Have Produced Time-Dependent Catalysts. Nonbiological Homogeneous Catalysis Has Not 2149.7 Homogeneous and Enzymatic Catalysts Are the 3-D Equivalent of 2-D Heterogeneous Catalysts 2159.8 A Conceptual Unification of Heterogeneous, Homogeneous, and Enzymatic Catalysis 216References 21810 The Rational Design of Time-Dependent (“Mechanical”) Homogeneous Catalysts. A Literature Survey of Multicentered Homogeneous Catalysis 219Junhua Huang and Gerhard F. Swiegers10.1 Introduction 21910.2 The Rational Design of Time-Dependent Homogeneous Catalysts 22110.2.1 Design Criteria for a Time-Dependent Homogeneous Catalyst 22110.2.2 The Problem of Simultaneously Identifying Suitable Catalytic Groups and Their Active Spatial Arrangement 22310.2.3 Time-Dependent Homogeneous Catalysis May Conceivably Be Achieved by Mimicry of a Natural Time-Dependent Catalyst 22510.2.4 Time-Dependent Homogeneous Catalysis May Conceivably Be Achieved in the form of a Combinatorial Experiment Involving a “Statistical Proximity” Effect 22610.2.4.1 A Time-Dependent Combinatorial Catalyst May Display Unique Kinetics 22810.2.4.2 Previous Attempts at Concentration-Based Biomimetic Catalysis Involved Energy-Dependent Systems 22910.2.5 Time-Dependent Catalysis May Be Useful in Transformations of Small Gaseous Molecules 23010.2.6 Why Do We Need New Time-Dependent Catalysts? 23010.3 Elements of Rational Design in Multicentered Catalysis 23010.3.1 Modes of Binding in Multicentered Catalysts 23010.3.2 Optimizing the Spatial Arrangement of Catalytic Groups 23110.3.2.1 Intramolecular Catalysts 23110.3.2.2 Intermolecular Catalysts 23310.3.2.3 Unconventional Approaches to Optimizing the Spatial Organization of Catalytic Groups 23310.3.3 Creating Functionally Convergent Catalysts 23410.3.3.1 Practical Approaches to Achieving Functionally Convergent Catalysis 23410.4 A Review of Nonbiological, Multicentered Molecular Catalysts Described in the Chemical Literature 23510.4.1 Intramolecular Catalysts 23510.4.1.1 Functionally Convergent Catalysis (Class A Type): Cofacial and Capped Metalloporphyrins as Oxygen Reduction Catalysts 23510.4.1.2 Functionally Convergent Catalysis (Class B Type): [1.1]Ferrocenophanes and Related Compounds as Hydrogen Generation Catalysts 24110.4.1.3 Pseudoconvergent Catalysis: Supramolecular, Bifunctional Catalysts of Organic Reactions 24510.4.1.4 Probable Functionally Convergent Catalysis: Rhodium-Phosphine Hydroformylation Catalysts 24810.4.1.5 Possible Functionally Convergent Catalysis: Ruthenium-Based Water Oxidation Catalysts 24910.4.1.6 Functionally Complementary Catalysis: Intramolecular Epoxidation Catalysts 25210.4.1.7 Metal Clusters in Multicentered Molecular Catalysis: Triruthenium Dodecacarbonyl Hydrogenation Catalysts 25210.4.1.8 Statistical Approaches to Functionally Convergent Catalysis: Macromolecular Intramolecular Catalysts 25410.4.2 Intermolecular Catalysts 25910.4.2.1 Functionally Complementary Catalysis 25910.4.2.2 Statistical Approaches to Functionally Convergent Catalysis: Concentration Effects in Intermolecular Catalysts 26010.4.2.3 Statistical Approaches to Functionally Convergent Catalysis: Self-Assembled, Supramolecular Catalysts 26110.4.3 Footnote: Unexpected Mechanistic Changes in Multicentered Catalysts 262Acknowledgments 263References 26311 Time-Dependent (“Mechanical”), Nonbiological Catalysis. 1. A Fully Functional Mimic of the Water-Oxidizing Center (WOC) in Photosystem II (PSII) 267Robin Brimblecombe, G. Charles Dismukes, Greg A. Felton, Leone Spiccia, and Gerhard F. Swiegers11.1 Introduction 26711.2 The Physical and Chemical Properties of the Cubanes 1a-b 27311.2.1 Chemical Structures 27311.2.2 Stepwise Hydride Abstraction, Leading to Water Release 27511.2.3 Dioxygen Generation 27511.2.4 A Possible Catalytic Cycle 27711.2.5 Other Reactions 27711.2.6 Summary 27811.3 Nafion Provides a Means of Solubilizing and Immobilizing Hydrophobic Metal Complexes 27811.4 Photoelectrochemical Cells and Dye-Sensitized Solar Cells for Water-Splitting 27911.5 Photocatalytic Water Oxidation by Cubane 1b Doped into a Nafion Support 28211.5.1 Solution Electrochemistry 28211.5.2 Electrochemistry of 1b Doped into a Nafion Membrane 28311.5.3 Electrocatalytic Effects Are Observed Under cv Conditions 28311.5.4 a Photo-electrocatalytic Effect Is Observed at 1.00 v (vs. Ag/AgCl) 28411.5.5 if the Photocurrent Is Caused by Water Oxidation Catalysis, This Involves a Decrease in the Overpotential of 0.4 V 28511.5.6 The Photocurrent Is Observed only in the Presence of Water. The System Saturates at Low Water Content, Consistent with a Time-Dependent Catalytic Action 28611.5.7 The pH Dependence of the Photocurrent Is Consistent with Water Oxidation 28711.5.8 Bulk Water Is a Reactant and Oxygen Is Generated 28711.5.9 The Quantity of Gas Generated Matches the Current Obtained. Notable Turnover Frequencies Are Implied 28711.5.10 Photocurrent as a Function of the Illumination Wavelength 29011.5.11 The Photoaction Spectrum of the Catalysis Corresponds to the Main LMCT Absorption Peak of 1b 29011.6 The Challenge of Dye-Sensitized Water-Splitting 29111.7 The Mechanism of the Catalysis 29211.8 Conclusions 293References 29412 Time-Dependent (“Mechanical”), Nonbiological Catalysis. 2. Highly Efficient, “Biomimetic” Hydrogen-Generating Electrocatalysts 297Jun Chen, Junhua Huang, Gerhard F. Swiegers, Chee O. Too, and Gordon G. Wallace12.1 Introduction 29712.2 Monomer and Polymer Preparation 30112.3 Catalytic Experiments 30212.3.1 PPy-9 and PPy-12 Display Anodic Shifts in the Most Positive Potential for Hydrogen Generation 30212.3.2 PPy-9 and PPy-12 Increase the Rate of Hydrogen Generation on Pt by ca. 7-Fold after 12 Hat20.44 V 30412.3.3 PPy-9 and PPy-12 Increase the Rate of Hydrogen Generation on Pt per Unit Area by ca. 3.5-Fold 30712.3.4 The Mechanism of Catalysis in PPy-9. Is PPy-9 a Combinatorial (“Statistical Proximity”) Catalyst? 30812.3.5 Polypyrrole Is Likely Involved in the Catalytic Cycle 30912.3.6 Other Evidence for the Involvement of Polypyrrole in the Catalytic Cycle 31112.3.7 The Pyrrole in Polypyrrole Is a Powerful, Time-Dependent, Combinatorial, “Statistical Proximity” Catalyst 31312.4 Conclusions: A Combinatorial “Statistical Proximity” Catalyst Was Obtained as a Bulk, Hybrid Homogeneous–Heterogeneous Catalyst 316Acknowledgments 317References 31713 Time-Dependent (“Mechanical”), Nonbiological Catalysis. 3. A Readily Prepared, Convergent, Oxygen-Reduction Electrocatalyst 319Jun Chen, Gerhard F. Swiegers, Gordon G. Wallace, and Weimin Zhang13.1 Introduction 31913.2 Cofacial Diporphyrin Oxygen-Reduction Catalysts 32113.3 Vapor-Phase Polymerization of Pyrrole as a Means of Immobilizing High Concentrations of Monomeric Catalytic Groups at an Electrode Surface 32313.4 Preparation and Catalytic Properties of PPy- 3 32413.4.1 Vapor-Phase Preparation of Polypyrrolle-Co Tetraphenylporphyrin, PPy- 3 32413.4.2 Electrochemistry of, and Oxygen Reduction by, Polypyrrolle-Co Tetraphenylporphyrin, PPy- 3 32413.4.3 Rotating Disk Electrochemistry (RDE) of Polypyrrolle-Co Tetraphenylporphyrin, PPy- 3 32613.4.4 Rotating Ring Disk Electrochemistry (RRDE) of Polypyrrolle-Co Tetraphenylporphyrin, PPy- 3 32813.4.5 The Product Distribution Relative to the Proportion of 3 in the Polypyrrolle-Co Tetraphenylporphyrin, PPy- 3 32913.5 PPy-3 as a Fuel Cell Catalyst 33013.5.1 PPy-3 on Carbon Fiber Paper 33013.5.2 Electrochemical Characterization of PPy-3 on Carbon Fiber Paper 33013.5.3 Morphology of the PPy-3 Carbon Fiber Composite Film 33013.5.4 Oxygen-Reduction Catalysis by the PPy- 3 Carbon Fiber Composite Film in Simple Fuel Cell Test Apparatus 33113.6 Conclusions 334References 335Appendix A Why Is Saturation Not Observed in Catalysts that Display Conventional Kinetics? 337Appendix B Graphical Illustration of the Processes Involved in the Saturation of Molecular Catalysts 341Index 347