PRINCIPLES OF ADSORPTION AND REACTION ON SOLID SURFACES

Transcription

1 PRINCIPLES OF ADSORPTION AND REACTION ON SOLID SURFACES Richard I. Masel Department of Chemical Engineering University of Illinois at Urbana-Champaign Urbana, Illinois A WILEY-INTERSCIENCE PUBLICATION JOHN WILEY & SONS, INC. New York Chichester Brisbane Toronto Singapore

2 CONTENTS Preface xiii 1 Introduction Historical Introduction Overview of the Approach Taken in This Book Techniques of Surface Analysis 5 2 The Structure of Solid Surfaces and Adsorbate Overlayers Historical Introduction Qualitative Description of Surface Structure Quantitative Description of the Structure of Solids and Surfaces Two-Dimensional Bravais Lattices Extension to Slabs and Surfaces Three-Dimensional Bravais Lattices Miller Indices The Structure of Solid Surfaces Models of Ideal Single-Crystal Surfaces The Stereographic Triangle The Structure of Stepped Metal Surfaces The Surface Structure of Semiconductor Crystals Relaxations and Surface Reconstructions Rare Gas Crystals Metals Elemental Semiconductor Surfaces Ionic Crystal Compound Semiconductors Descriptive Notation for Surface Reconstructions Site Notation Roughening and Surface Melting Solved Examples Supplemental Material Proof That There Can Only Be Two-, Three-, Four-, and Sixfold Axes in an Infinite Periodic Two- Dimensional Lattice The Reciprocal Lattice Somorjai Step Notation 94

3 vi CONTENTS The Correspondence of Planes in BCC and FCC Lattices 94 Problems 95 3 Adsorption I: The Binding of Molecules to Surfaces Historical Overview Chemisorption of Gases on Surfaces The Nature of the Adsorbed Layer More Complex Behavior The Geometry of the Adsorbed Layer Modeling Molecule-Surface Binding: Physisorption Simple Models of Chemisorption Some Results from Density Functional Theory Electronegativity Equalization Quantum Mechanical Methods Adsorption on Metals Cluster Models of Adsorption on Surfaces The Surface Electronic Structure The Jellium Model of Metal Surfaces Quantification of the Jellium Model Adsorption on Jellium The Newns-Anderson Model Lang-Williams Theory Qualitative Results The Need for an Improved Model The Effective Medium Model Summary of Predictions of the Effective Medium Model for the Adsorption on Metal Surfaces More Advanced Computational Methods Slab Calculations Embedded Cluster Method Predictions of the Advanced Computational Methods Summary of Adsorption on Metals Adsorption on Semiconductor Surfaces The Surface Electronic Structure of Semiconductors Bonding to Semiconductor Surfaces A Two-State Model for Adsorption on Semiconductors Adsorption on Ionic Surfaces The Surface Electronic Structure of Insulating Oxides Adsorption on Insulating Surfaces Synopsis and Plans for the Next Several Chapters Solved Examples 210 Problems 215

4 CONTENTS vii 4 Adsorption II: Adsorption Isotherms Introduction Analytical Models for Reversible Monolayer Adsorption Langmuir Adsorption Model Kinetic Derivation of the Langmuir Adsorption Isotherm Comparison to Data Modifications of the Langmuir Adsorption Isotherm: Inequivalent Sites The Freundlich Adsorption Isotherm The Multisite Model Adsorbate/Adsorbate Interactions The Tempkin Adsorption Isotherm The Fowler Adsorption Isotherm Other Analytical Adsorption Isotherms Summary of Analytical Models Adsorbate Phase Behavior Lattice Gas Models Derivation of the Basic Equations Conversion to the Ising Model Overview of the Solution Methods Exact Solution for the One-Dimensional Ising Model: F = Exact Solution for the One-Dimensional Ising Model: F * Exact Solution: Adsorption an an Infinite Strip Monte Carlo Solution Qualitative Results Approximation Methods: The Bragg-Williams Approximation Bethe-Peierls Approximate Solution A Modified Bethe Approximation The Kukuchi Cluster Approximation Quasichemical Approximation Incommensurate Adsorption Two-Dimensional Equations of State: Ideal Lattice Gas Other Two-Dimensional Equations of State Monte Carlo Solution Perspective Multilayer Adsorption The BET Adsorption Isotherm Wetting, Capillary Condensation, and Other Advanced Topics Solved Examples Supplemental Material 330 Problems 336

5 viii CONTENTS 5 Adsorption Kinetics of Adsorption Historical Overview Scattering, Trapping, and Sticking Trapping Baule's Hard Sphere Model' The Baule-Weinberg-Merrill Approximation for Trapping Probabilities An Improved Baule Hard Sphere Model: Ion Cores in Jellium Cube Models Zwanzig's Lattice Model Molecular Dynamics Simulations of Lattice Models Sticking Models for the Variation in the Sticking Probability with Energy and Coverage Langmuir's Model Relationship to the Langmuir Adsorption Isotherm Limitations of Langmuir's Analysis Models for Precursor-Moderated Adsorption Immobile Adsorption Exact Solution for Adsorption on a Line of Sites Series Solution: Adsorption on a Two-Dimensional Array of Sites The Role of Active Centers in Adsorption Macroscopic Reversibility and Time-Reversal Symmetry Perspective Solved Examples 408 Problems Introduction to Surface Reactions Introduction General Mechanisms of Surface Reactions Systematic Trends in the Rates and Mechanisms of Surface Reactions Reactions on Metals Structure-Sensitive Reactions "Models" of Structure Sensitivity The Effects of Varying Composition The Effects of Alloys, Promoters, and Poisons Reactions on Insulators Reactions on Semiconductors Summary and Plans for the Remainder of This Book 476 Problems 476

6 CONTENTS ix 7 Rate Laws for Reactions on Surfaces I: Kinetic Models Introduction Kinetics of Surface Reactions The Langmuir Rate Equation Langmuir, Hougen, and Watson Rate Laws Comparison to Data Mars-Van Klevan Kinetics Tempkin Kinetics Relationship to Mechanisms of Reactions Evidence for More Complex Kinetics Temperature Programmed Desorption Redhead's Analysis of TPD Comparison to Data Improved Analysis of TPD Data Bragg-Williams Approximation Monte Carlo Simulation of TPD First-Order Desorption Results for Ising Universality Class: First Nearest Neighbor Interactions Qualitative Picture of the cp' s Effects of Ordering on TPD Spectra Precursor-Moderated Desorption Comparison of the Methods Monte Carlo Simulation of Surface Reactions Dissociative Adsorption Formation of Pairs of Molecules on the Surface The Immobile Limit Implications for TPD The Completely Mobile Limit Bethe Approximation Implications for TPD Changes in the Order of Steady-State Reactions A + B Reactions The Mobile Limit The Immobile Limit Multiple Steady States and Oscillations Precursor Reactions Dynamic Corrections Summary Solved Examples 568 Problems A Review of Reaction-Rate Theory Historical Introduction Collision Theory 582

7 X CONTENTS 8.3 Reactions as Motion on Potential Energy Surfaces Molecular Dynamics Simulation of Reactive Collisions: Linear A + BC --+ AB + C The Nonlinear Case Derivation of the Angular Momentum Barrier to Reaction The Angular Momentum Barriers to Reaction Influence on the Overall Rate Energy Transfer Barrier to Unimolecular Reactions MD/MC Calculations of Reaction-Rate Constants Transition-State Theory The Role of the Transmission Coefficient Tunneling Unimolecular Reactions Application to Surface Reactions Solved Examples Supplemental Material Tunneling 608 Problems Models of Potential Energy Surfaces: Reactions as Curve Crossings and Electron Transfer Processes Introduction Empirical Correlations for Reaction Rates: The Polayni Relationship and Br$nsted Catalysis Law Reactions as Bond Extensions Plus Curve Crossings The Polayni Relationship The Marcus Equation An Alternative Derivation: Bond Energy-Bond Order Relationship Qualitative Features of the Marcus Equation The Hammond Postulate Application of the Marcus Equation: Tafel Kinetics Nonlinear Behavior Applications of Linear Free-Energy Relationships and the Marcus Equation in Organic Chemistry Group Contribution Methods Relationship between the Various Transfer Coefficients Applications of the Marcus Equation in Inorganic Chemistry Reactions as Electron Transfer Processes Perturbation Theory Some Results from Density Functional Theory Limitations of the Simple Model The Configuration Mixing Model Symmetry Forbidden Reactions Forbidden Crossings 666

8 CONTENTS xi Reactions with Negligible Coupling Conservation of Orbital Symmetry Some Results from Group Theory Examples of Symmetry Allowed and Symmetry Forbidden Reactions Prediction of Mechanisms of Gas Phase Reactions Applications: Leaming about Transition States for Reactions Summary Supplemental Examples 683 Problems Rates and Mechanisms of Surface Reactions Historical Introduction Radical Reactions in the Gas Phase Introduction to the Role of the Surface: Stabilization of Intermediates Thermodynamic Effects Fundamental Limitations of the Analysis in Section BOC-MP Modeling Derivation of the BOC-MP Method Fundamental Limitations of the BOC-MP and Polayni Methods Surface Reactions as Chain-Propagation Reactions Variation in the Intrinsic Barriers to Reaction with Changing Reaction Types An Example of the Prediction of Reaction Mechanisms A Modified BOC-MP Method The Proximity Effect Effects' of Surface Geometry on the Intrinsic Barriers to Reaction Holding the Reactants in a Position Conducive to Reaction The Role of Surface Electronic Structure on the Intrinsic Barriers to Reaction The Influence of Interstitial Electron Density on the Intrinsic Barriers to Reaction The Role of d-bands in Modifying the Intrinsic Barriers to Reaction 731 ' The Role of Antibonding Orbitals in the H2 Dissociation Process A Comparison of s- and d-orbitals for Bond Scission: Quantum Derivation A Pictorial Representation of the Results from Section Implications to the Intrinsic Barrier to H2 Bond Scission 740

9 xii CONTENTS Analogy to Symmetry Forbidden Reactions Extension to Other Bond Scission Processes The Role of Net Charges Electronic Effects in the Binding of Molecules on Surfaces Summary of the Factors that Influence the Intrinsic Barriers to Bond Scission The Effects of Surface Structure on Reactions: Metals Models for the Overall Trends in the Structural Sensitivity of Elementary Bond Scission Reactions on Metals Detailed Models of the Structure Sensitivity of Reactions on Metal Surfaces Geometrie Effects Variations in the Interactions with the d-bands Models for the Influence of the d-bands on the Structure Sensitivity of Surface Reactions Putting It All Together: Metals Copper, Silver, and Gold Platinum, Palladium, and Nickel Iridium, Rhodium, Cobalt, Osmium, Ruthenium, hon, Rhenium, Tungsten, Molybdenum, and Chromium Rare Earths and Early Transition Metals Alkali Metals and Alkaline Earths Aluminum Putting It All Together: Insulating Oxides General Principles for Reactions on Insulating Oxides Trends with Changing Composition and Surface Structure Putting It All Together: Semiconducting Oxides Putting It All Together: Other Systems Surnmary 780 Problems 780 Index 793