Principles and Practice of Heterogeneous Catalysis

Transcription

1 J. M. Thomas, W. J. Thomas Principles and Practice of Heterogeneous Catalysis VCH Weinheim New York Basel Cambridge Tokyo

2 Preface V 1 Setting the Scene Introduction The Selectivity of Catalysts Perspectives in Catalysis: Past, Present, Future Applied Catalysis since the 1940s Current Trends in Applied Catalysis Auto-Exhaust Catalysts Catalysts in Electrochemistry and Photoelectrochemistry Asymmetric Sites on Heterogeneous Catalysts Immobilized Transition Metals Immobilized Enzymes and Cells: Present and Future Catalytic Antibodies Ribozymes Catalytic Oxidation of Methane: the Centrepiece of Future Power Sources Definition of Catalytic Activity Magnitude of Turnover Frequencies and Active Site Concentrations Volcano Plots The Evolution of Important Concepts and Techniques in Heterogeneous Catalysis Mechanistic Insights from Isotopic Labelling Concepts from Organometallic Chemistry Contributions from Theoretical and Computational Chemistry Some Intellectual and Practical Challenges for Catalysis in the 21st Century Problems Further Reading General Background 60 2 The Fundamentals of Adsorption: Structural and Dynamical Considerations, Isotherms, and Energetics Catalysis Must Always be Preceded by Adsorption Physical Adsorption, Chemisorption and Precursor States 65

3 X 2.2 The Surfaces of Clean Solids are Sometimes Reconstructed There are Many Weil-Defined Kinds of Ordered Adlayers Adsorption Isotherms and Isobars The Empirical Facts Information that can be Gleaned from Isotherms Adsorption is Almost Invariably Exothermic Dynamical Considerations Residence Times Rates of Adsorption Applying Statistical Mechanics to Adsorption Adsorption Kinetics Can Often be Represented by the Elovich Equation Rates of Desorption Applying Statistical Mechanics to Desorption The Influence of a Precursor State on the Kinetics of Desorption Deriving Adsorption Isotherms from Kinetic Principles Using the Langmuir Isotherm to Estimate the Proportions of Non-dissociative and Associative Adsorption Other Adsorption Isotherms Henry's Adsorption Isotherm The Freundlich Isotherm The Temkin Isotherm The Brunauer-Emmett-Teller Isotherm Developments from Polanyi's Adsorption Theory Kaganer's Isotherm and the DKR Equation The Virial Equation of State Energetics of Adsorption Estimating the Binding Energies of Physically Adsorbed Species The Binding Energies of Chemisorbed Species Estimating Heats of Adsorption From Thermodynamic Data Decline of the Heat of Adsorption with Increasing Coverage Mobility at Surfaces Kinetics of Surface Reactions The Influences of Precursor States on the Kinetics and Energy Distribution of Catalysed Reactions Comparing the Rates of Heterogeneous and Homogeneous Reactions Autocatalytic, Oscillatory, and Complex Heterogeneous Reactions An Outline of Autocatalysis Background to Oscillating Reactions Instabilities and Transient Phenomena in Heterogeneous Catalysis Multiple Steady States Transient Phenomena Problems Further Reading 142

4 XI 3 Characterizing Catalysts and their Surfaces Model Systems and Real-Life Catalysts A Portfolio of Modern Methods: Introducing the Acronyms Which Elements and Which Phases are Present? X-Ray Fluorescence (XRF), X-Ray Emission (XRE) and Proton- Induced X-Ray Emission (PIXE) Developing Techniques: ICPMS X-Ray Diffraction (XRD) and Electron Diffraction (ED) Mean Size, Surface Area and Particle-Size Distribution from SAXS In-Situ Studies by X-Ray Diffraction Detecting, Identifying, and Counting the Atoms at Solid Surfaces: Sub-monolayer Amounts can be Measured Ion-Scattering Spectroscopy (ISS): A Detection Limit of 10 4 Monolayers Nuclear Microanalysis (NMI): A Sensitive Means of Detecting Specific Elements, Including Hydrogen, at Surfaces Rutherford Back-Scattering (RBS) Identifying the Atoms at a Surface and Probing their Immediate Environment: SIMS, IR, HREELS, AES and XPS Secondary-Ion Mass Spectroscopy (SIMS): A Detection Limit of 10" 5 Monolayers Infrared Spectroscopy (IR): A Non-destructive Technique Usable on Catalysts Exposed to High Pressure High-Resolution Electron-Energy-Loss Spectroscopy (HREELS): The Most Sensitive Tool for Identifying Surface Vibrational Modes Electron Spectroscopy: The Ability to Probe Composition and Bonding at Surfaces The Realization that Electron Spectroscopy is Sufficiently Sensitive to Detect Fractions of a Monolayer Auger Electron Spectroscopy (AES) and Scanning Auger Microscopy (SAM) X-Ray Photoelectron Spectroscopy (XPS) UV-Induced Photoelectron Spectroscopy (UPS) Inverse Photoemission: A Means of Probing Unoccupied States The Structure and Crystallography of Surfaces: Measuring the Symmetries, Order and Disorder, and Deducing Bond Lengths and Bond Angles in the Adsorbed State Two- and Three-Dimensional Surface Crystallography Notations for Describing Ordered Structures at Surfaces How do Bond Distances at Surfaces Compare with Those of Bulk Solids? What of Displacive Reconstructions? Atomic Scattering and Diffraction EXAFS, SEXAFS, XANES and NEXAFS: Probing Bond

5 XII Distances and Site Environments even when There is no Long- Range Order The Origin of EXAFS and How it is Used Applications of EXAFS to the Study of Catalysts SEXAFS XANES and Pre-edge Structure: Deducing Site Symmetry and Oxidation States NEXAFS Other Structural Techniques for Characterizing Bulk and Surfaces of Catalysts Electron Spin Resonance (ESR): Probing the Nature of Catalytically Active Sites and the Concentration of Paramagnetic Intermediates on Surfaces and in the Gas Phase Examples of the Use of ESR in Heterogeneous Catalysis Electron Spin-Echo Modulation Spectroscopy (ESEMS): Probing the Environment of Paramagnetic Species Nuclear Magnetic Resonance (NMR): A Technique Applicable, at High Resolution, to Solids and their Surfaces Basic Principles NMR Spectra of Solids Applications of NMR to the Study of Catalysts, Adsorbents and Adsorbates Future Prospects for the Study of Catalysts by Solid-State NMR Mössbauer Spectroscopy: A Means of Determining Valence, Spin States, and Site Environments of Ions Specific Applications Conversion-Electron Mössbauer Spectroscopy (CEMS): A Double- Resonance Technique of an Unusual Kind Electron Microscopy Scanning Probe Microscopy: STM and AFM Optical Microscopy Ellipsometry: A Non-invasive Technique Neutron Scattering: A Technique of Growing Importance in the Study of Catalysts Determining the Atomic Structure and Texture of Microcrystalline Catalysts, the Nature of the Active Sites and the Disposition of Bound Reactants Determining the Structure of, and Identifying Functional Groups in, Chemisorbed Layers at Catalyst Surfaces A Miscellany of Other Procedures Determining the Strength of Surface Bonds: Thermal and Other Temperature-Programmed Methods Temperature-Programmed Desorption (TPD) or Flash Desorption Spectroscopy (FDS) Temperature-Programmed Reaction Spectroscopy (TPRS) Magnitude of the Heat and Entropy of Adsorption 229

6 XIII 3.10 In-Situ Methods of Studying Catalysts: The Current Scene and Future Prospects Isotopic Labelling and Transient Response, Infrared, Raman, NMR, Mössbauer and X-Ray Absorption Spectroscopy for In-Situ Studies In-Situ X-Ray and Neutron Diffraction Studies Combined X-Ray Absorption and X-Ray Diffraction for In-Situ Studies of Catalysts Problems Further Reading The Significance of Pore Structure and Surface Area in Heterogeneous Catalysis The Importance of Pore Structure and Surface Area Experimental Methods of Estimating Surface Areas The Volumetric Method The Gravimetric Method The Dynamic Method Experimental Methods of Estimating Pore Volume and Diameter Gas Adsorption Method of Estimating Pore Volume and Diameter Mercury Porosimeter Method of Estimating Pore Volume and Diameter Models of the Pore Structure of Catalyst Materials Hysteresis and the Shapes of Capillaries Type A Hysteresis Loops Type В Hysteresis Loops Type С Hysteresis Loops Type D Hysteresis Loops Type E Hysteresis Loops Other Adsorption Desorption Characteristics Geometric Models of Pores Wheeler's Semi-empirical Pore Model Mathematical Models of Porous Structures The Dusty Gas Model The Random Pore Model Stochastic Pore Networks and Fractals Diffusion in Porous Catalysts The Effective Diffusivity Molecular (Maxwellian) Diffusion or Bulk Diffusion Knudsen Diffusion The Transition Region of Diffusion Forced Flow in Pores Chemical Reaction in Porous Catalyst Pellets Effect of Intraparticle Diffusion on Experimental Parameters 303

7 4.6.2 Non-isothermal Reactions in Porous Catalyst Pellets Criteria for Diffusion Control Experimental Methods of Assessing the Effect of Diffusion on Reaction Problems Further Reading The Solid-State and Surface Chemistry of Catalysts Classification of Heterogeneous Catalysts Structures Metals and Alloys Miller Indices and Miller-Bravais Indices Transition-Metal Alloys and Bimetallic Clusters Highly-Dispersed Metals Interstitial Phases Simple Metallic Oxides and their Non-stoichiometric Variants Shear and Block Structures Based on Re More Complicated Metallic Oxides Even More Complicated Oxides Perovskites and their Defective Variants Perovskites as the Prototypes of New Homologous Series Spinels, Scheelites and the Bismuth Molybdates Heteropolyions (Keggin Structures) as Catalytically Significant Entities Clays, Zeolites and Related Structures Pillared Clays are Effectively Two-Dimensional Zeolites A Synoptic Guide to the Structure of Zeolitic and Related Solid Catalysts Computational Approaches A Resume of Available Methodologies Selected Applications A Chemist's Guide to the Electronic Structure of Solids and Their Surfaces Energy Bands Bands in ID and 3D Crystals Energy Bands in Ionic Solids Energy Bands in Transition-Metal Oxides: Understanding the Electronic Structure of the Monoxides of Titanium, Vanadium, Manganese and Nickel Energy Bands in Structures Related to Re Fermi Levels in Insulators and Semiconductors Surface Electronic States and the Occurrence of Energy Levels Within the Band Gap Band Bending and Metal-Semiconductor Junctions: Schottky Barriers 392

8 XV Depletive Chemisorption on Semiconductors The Bending of Bands when Semiconductors are Immersed in Electrolytes Quantum Chemical Approaches to the Electronic Properties of Solids The Cluster and Thin-Slab Approach Hückel and Extended Hückel Calculations 'Ab-initio' Methods A Brief Selection of Quantum Chemical Studies Band Widths, DOS and Fermi Levels of the Transition Metals Heats of Chemisorption from EHT Calculations The Adsorption of CO on Nickel Dissociative Chemisorption of CO Insight from Ab-initio Computations: Methanol Synthesis and Olefin Metathesis Epilogue Problems Further Reading Poisoning, Promotion, Deactivation and Selectivity of Catalysts Background Effect of Mass Transfer on Catalytic Selectivity Effect of Intraparticle Diffusion Non-isothermal Conditions Effect of Interparticle Mass and Heat Transfer Bifunctional Catalysts Catalyst Deactivation Deactivation Processes Deactivation Models Steady-State Model A Dynamic Model Operational Consequences of Poisoning Modern Theories of Poisoning and Promotion General Theoretical Considerations Theoretical Interpretation of Poisoning and Promotion The Electronegativity of a Poison Seems to be of Secondary Importance Other Factors Responsible for Promotion and Poisoning Problems Further Reading Catalytic Process Engineering Statement of the Problem Kinetics of Heterogeneous Catalytic Reactions 455

9 XVI The Overall Rate of Reaction The Rate of Chemical Reaction Fundamental Kinetic Models The Effect of Intraparticle Diffusion The Effect of Interparticle (Fluid-to-Solid) Transport The Effect of Catalyst Deactivation Catalytic Reactors Experimental Laboratory Reactors Batch Reactors Tubular Reactors Continuous Stirred-Tank Reactor Recycle Reactor Flowing-Solids Reactors Slurry Reactors Industrial Chemical Reactors Batch Reactors Continuous Tubular Reactors Fluidised-Bed Reactor The Trickle-Bed Reactor Metal Gauze Reactors Thermal Characteristics of a Catalytic Reactor Problems Further Reading Heterogeneous Catalysis: Examples and Case Histories The Synthesis of Methanol Proof that CO is not Dissociated During Methanol Synthesis The Role of C0 2 : Evidence that it is the Main Source of CH 3 OH The State of Copper in the Working Catalyst The Role of Oxide Supports: Is There Anything Special About ZnO? Views on the Mechanism of the Reaction Process Conditions, Reaction Configurations and Kinetics Fischer-Tropsch Catalysis Mechanistic Considerations Does Synthesis Proceed via Hydroxymethylene Intermediates? What of the CO Insertion Mechanism? Synthesis by the Fischer-Tropsch Process First Requires Dissociation of CO Schultz-Flory Statistics Other Possible Mechanisms Fine-Tuning the Fischer-Tropsch Process Practical Fischer-Tropsch Catalysts and Process Conditions Reductive Coupling of Two CO Ligands Forming Acetylene from Syn-Gas 538

10 XVII Methanation, Steam Reforming and Water-Gas Shift Reactions Methanation Steam Reforming Water-Gas Shift Reaction The Synthesis of Ammonia Catalyst Promoters are of Two Kinds Kinetics of the Overall Reaction: The Temkin Pyzhev Description The Surface of Iron Catalysts for Ammonia Synthesis Contain Several Other Elements: But is the Iron Crystalline? Does Ammonia Synthesis Proceed via Atomically or Molecularly Adsorbed Nitrogen? How and Where are the Reactant Gases Adsorbed at the Catalyst Surface? A Potential-Energy Diagram Illustrating How the Overall Reaction Leading to Ammonia Synthesis can be Constructed How Potassium Serves as an Electronic Promoter The Technology of Ammonia Synthesis Reactor Configurations are Important Industrially Oxidation of Ammonia: Stepping Towards the Fertilizer Industry In-Situ Catalytic Reaction and Separation Catalytic Distillation Pressure Swing Reaction Catalytic Membrane Processes Automobile Exhaust Catalysts and the Catalytic Monolith The Three-Way Catalyst (TWC) Why is Rhodium in the Auto-Exhaust Catalyst? The Catalytic Monolith Catalytic Monoliths may be Used in Several Applications Rate Characteristics of Catalytic Combustion Processes Combustion Reactions in a Catalytic Monolith Differ from Those Occurring in a Homogeneously Operated Combustor Simulation of the Behaviour of a Catalytic Monolith is Important for Design Purposes Photocatalytic Breakdown of Water and the Harnessing of Solar Energy Oxygen Generation by Photo-induced Oxidation of Water Hydrogen Generation by Photo-induced Reduction of Water Simultaneous Generation of Hydrogen and Oxygen by Catalysed Photolysis of Water Other Photochemical Methods of Harnessing Solar Energy Catalysis and Photoelectrochemistry: Photocatalysis and Photoelectrosynthesis The Principles Practical Examples The Prospects 604

11 XVIII 8.8 Catalysis Using Microporous or Mesoporous Solids and Modified Clays: Its Growing Role in the Petroleum Industry and Clean Technology Activity of Zeolitic Catalysts Shape-Selective Zeolitic Catalysts New Microporous Crystalline Catalysts Some Case Studies of In-Situ Monitoring of Catalysis with ZSM A Rationally Chosen Zeolitic Catalyst New Mesoporous Catalysts Clays and Other Solid Acid Catalysts Clays and their Possible Role in Replication, Evolution and the Origin of Life Catalytic Processes in the Petroleum Industry Catalytic Reforming Catalytic Cracking Cracking Reactions Cracking Catalysts The Catalytic Cracking Reactor Hydrotreating The Role of Catalysis in Energy-Related Environmental Technology Problems Further Reading 650 Index 659