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INTRODUCTION TO

SURFACE CHEMISTRY

AND CATALYSIS

GABOR A. SOMORJAI

Department of Chemistry

University of California

Berkeley, California

A Wiley-Interscience Publication

JOHN WILEY & SONS, INC.

New York Chichester Brisbane Toronto Singapore

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CONTENTS

Preface xiii

General Introduction xv

Lists of Constants xvii

List of Symbols xix

1 SurfacesAn Introduction 1

1.1 Historical Perspective, 1

1.2 Surfaces and InterfacesClassification of Properties, 3

1.3 External Surfaces, 5

1.3.1 Surface Concentration, 5

1 3 1 1 Clusters and Small Particles, 6

1 3 1 2 Thin Films, 8

1.3.2 Internal SurfacesMicroporous Solids, 10

1.4 Clean Surfaces, 12

1.5 Interfaces, 13

1.5.1 Adsorption, 13

1.5.2 Thickness of Surface Layers, 15

1.6 The Techniques of Surface Science, 15

1.7 Summary and Concepts, 17

1.8 Problems, 17

References, 18

2 The Structure of Surfaces 36

2.1 Introduction, 36

2.2 Surface Diffraction, 42

2.3 Notation of Surface Structures, 43

2.3.1 Abbreviated Notation of Simple Surface Structures, 45

2.3.2 Notation of High-Miller-Index, Stepped Surfaces, 47

VII

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viii CONTENTS

2.4 Th e Structure of Clean Surfaces, 48

2.4 .1 Bond -Length Contraction or Relaxation , 48

2.5 Reconstruction, 50

2.5 .1 Atom ic Steps and Kink s, 52

2.6 The Structure of Adsorbed Mo nolay ers, 54

2.6.1 Ordered Monolayers and the Reasons for Ordering, 54

2.6 .2 Adsorbate-Induced Restructuring, 55

2.6 .3 Atom ic Adsorption and Penetration into Substrates, 58

2.6 .4 Metals on M etals: Epitaxial Grow th, 60

2.6 .5 Growth Modes at Metal Surfaces, 60

2.6 .6 M olecular Adso rption, 6 0

2.6.6.1 Ethylene, 62

2.6.6.2 Benzene, 66

2.6.7 Coadsorbed M onolayers , 67

2.6.8 Physisorbed M onolayers , 7 2

2.7 Summ ary and Con cepts, 74

2.8 Problems, 74

References, 75

3 Therm odyn am ics of Surfaces 271

3.1 Introduction, 271

3.2 Definition of Surface Therm odyn am ic Fun ctions, 272

3.3 W ork Need ed to Create a Surface of a One -Com ponen t System:

Surface Ten sion, 273

3.3.1 The Surface Free Energy Is AIways Positive, 275

3.3.2 Tem perature Depen dence of the Specific Surface Free

Energy, 277

3.3.3 Surface Heat Cap acity, 277

3.3.3.1 Exper imenta l Est imates, 277

3.3 .3.2 Theoretical Estim ates, 278

3.4 The Surface Energy and Surface Com position of Tw o-

Component Systems, 282

3.4.1 The Wagn er Exper iment, 284

3.4 .2 Surface Segrega tion in Binary Alloy System s, 285

3.4.3 Surface Com position of Alloys from Mod el

Calculations, 289

3.5 Surfaces Wh en No Bulk Phase Exists: Two-D imensio nal

Phases, 291

3.5.1 Mon omolecular Fi lms, 291

3.6 Metastable Surface Phas es, 292

3.7 Curved Surfaces, 293

3.7.1 Capillary Rise, 294

3.7.2 The Vapo r Pressure of Curved Surfaces, 295

3.7.3 The Contact Angle and Adh esion, 296

3.7.4 Nucleation, 298

3.8 Therm odynam ics of Adsorbed M onolay ers, 301

3.8.1 Heat of Ad sorption, 301

3.8.2 Two-D imensional Phase Approximation, 302

3.8.3 Adsorption Isotherm s, 302

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CONTENTS ix

3.8.4 Integral and Differential Hea ts of Adsorption, 307

3.8.5 Molecular and Dissociative Adsorption, 308

3.9 Summary and Concepts, 313

3.10 Problems, 314

References, 315

4 Dynam ics at Surfaces 319

4.1 Introduction, 319

4.2 Surface Atom Vibrations, 319

4.2.1 The Harmonie Oscillator Model, 319

4.2.2 Vibrational Modes of Surface Atoms, 322

4.2.3 Surface Mean-Square Displacements, 322

4.2 .4 Vibrations of Adsorbed Atoms and Molecules, 324

4.3 Elementary Processes of Gas-Surface Interaction, 329

4.3 .1 Adsorption. Energy Accommodation Coefficients, 331

4.3 .2 Sticking Probability, 333

4.3 .3 Models of Energy Transfer and Adsorption, 336

4.3.4 Surface Diffusion, 340

4.3 .5 Mechanisms of Surface Diffusion, 343

4.3.6 Desorption, 347

4.3 .7 Surface-Structure Sensitivity of Thermal Desorption, 349

4.3 .8 Collision-Induced Desorption, 350

4.3 .9 Electron-Beam-Induced Desorption, 351

4.3.10 Photon-Stimulated Desorption, 351

4.3.11 Ion-Beam-Induced Desorption and Sputtering, 351

4.4 Summary and Concepts, 352

4.5 Problems, 352

References, 354

5 Electrica Properties of Surfaces 362

5.1 The Surface Electron Potential, 362

5.2 The Surface Space Charge, 363

5.2.1 The Surface Space Charge at the Solid-Vacuum

Interface, 363

5.2.2 Surface Space Charge at the Solid-Liquid Interface, 365

5.3 The Work Function, 366

5.3.1 Effect of Surface Roughness on Work Function, 367

5.3.2 Change of Work Function with Particle Size, 367

5.4 Adsorption-Induced Charge Transfer at Surfaces: Metals and

Insulators, 369

5.4.1 Charge Transfer at the Solid-Solid Interface, 375

5.4.2 Gas-Phase Ion Production by Surface Ionization: Emission

of Positive and Negative Ions, 376

5.5 Surface Electron Density of States, 378

5.6 Electron Excitation at Surfaces, 379

5.6.1 Thermal Emission of Electrons from Surfaces, 380

5.7 Electron Emission from Surfaces by Incident Electron or Photon

Beams, 382

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x CONTENTS

5.7.1 High-Resolution Electron-Energy-Loss Spectroscopy

HREELS) , 384

5.7.2 X -Ray Photoelectron Spectroscopy XP S), 386

5.7.3 Aug er Electron Spectroscopy AE S), 389

5.8 Field Electron Em ission, 39 0

5.9 Field Ioniz ation , 391

5.10 Electron Tun neling, 392

5 1 1 The Scanning Tunneling Microscope STM ), 392

5.11 Sum mary and Co ncepts, 394

5.12 Problems, 394

References, 395

6 The Surface Chem ical Bo nd 400

6.1 Introduction, 400

6.2 Bonding Trends Across the Periodic Tab le, 401

6.3 Cluster-like Bonding of M olecular Adso rbates, 402

6.4 The Carbon Mo noxide Chemisorption Bond, 409

6.5 Adsorbate-Induced Restructuring. The Flexible Surface, 412

6.6 Therm al Activation of Bond Breaking, 417

6.7 Surface-Structure Sensitivity of Bond Break ing, 41 9

6.8 Coverage Dep endence of Bonding and Coad sorption, 425

6.8.1 Coadsorption, 426

6.9 Weak Surface Bon ds, 430

6.9.1 Phase Transformations in the Weakly Adsorbed

Layer, 433

6.10 Summ ary and Con cepts, 436

6.11 Problems, 436

References, 438

7 Catalysis by Surfaces 442

7.1 Introduction, 443

7.1.1 Brief History of Surface Catalysis, 444

7.2 Catalytic Action, 445

7.2.1 Kinetic Exp ressions, 446

7.2.2 Selective Catalysis, 448

7.2.3 Tabulated Kinetic Parameters for Catalytic Reactions, 449

7.3 Catalyst Preparation, Deactivation, and Regeneration, 450

7.3.1 Catalyst Preparation, 450

7.3.2 Catalyst Deactivation, 451

7.3 .3 Catalyst Regen eration, 4 52

7.4 M etal Catalysis, 452

7.4.1 Trends Across the Periodic Tab le, 452

7.4.2 Some Frequently Used Concepts of Metal Catalysis, 454

7.5 Catalysis by Ions at Surfaces. Ac id-B ase Catalysis, 456

7.5.1 Acid Catalysis in Solutions, 457

7.5 .2 Solid Acid s, 458

7.5.3 Carbenium Ion Reactions, 459

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Most Frequently Used Catalyst Matenals, 459

Surface-Science Approach to Catalytic Chem istry, 461

7.7 .1 Tech niques to Characterize and Study the Reactivity of

Small-Area Catalyst Surfaces, 463

7 7 1 1 High-P ressure Reacto rs, 463

7 7 1 2

Com parison of the Reactivities of Small- and

Large-Surface-Area Catalysts, 464

Case Histories of Surface Catalysts, 465

7.8.1 Am monia Synthesis , 465

7 8 1 1 Thermod ynamics and Kinetics , 46 5

7 8 1 1 1

Kinetics, 466

7 8 1 2 C atalyst Preparation, 466

7 8 1 3 Activity for Ammonia Synthesis Using Transition

M etals Across the Periodic Tab le, 467

7 8 1 4

Surface Science of Am mo nia Synthesis, 467

7 8 1 4 1

Structure Sensitivity of Ammonia

Synthesis, 467

7 8 1 4 2 Kinetics of Dissociative Nitrogen

Adsorption, 470

7 8 1 4 3 Effects of Aluminum Oxide in

Restructuring Iron Single-Crystal

Surfaces for Ammonia Synthesis, 471

7 8 1 4 4 Characterization of the Restructured

Surfaces, 472

7 8 1 4 5 Effect of Potassium on the Dissociative

Chemisorption of Nitrogen on Iron

Single-Crystal Surfaces in U H V , 475

7 8 1 4 6 Temperature-Programmed Desorption

Studies of Ammonia from Iron Surfaces

in the Presence of Potassiu m , 477

7 8 1 4 7 Effects of Potassium on Ammonia

Synthesis Kinetics, 478

7 8 1 4 8 Effects of Potassium on the Adsorption

of Ammonia on I ron Under Ammonia

Synthesis Con ditions, 48 0

7 8 1 5 Mechanism and Kinetics of Ammonia

Synthesis, 482

7.8.2 Hydrogenation of Carbon M onoxide , 483

7 8 2 1 Thermodynamics , 483

7 8 2 2 C atalyst Preparation, 48 8

7 8 2 3

Methanation. Kinetics, Surface Science,

Mechanisms, 489

7 8 2 4 Promotion of the Rates of CO Bond

Hyd rogenation by the Ox ide-M etal Interface, 491

7 8 2 5 Methanol Production. Kinetics, Surface Science,

and M echanism s, 494

7 8 2 6

Production of Higher-Molecular-Weight

Hydrocarbons. Kinetics, Surface Science, and

Mechanisms, 495

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xii CONTENTS

7.8.2 .7 Formation of Oxygen ated Hydrocarbo ns from C O

and H

2

and Organic Molecules , 500

7.8.3 Hyd rocarbon Conversion on Platinum, 500

7.8.3.1 Introduction, 500

7.8 .3.2 Structure Sensitivity of Hydrocarbo n C onversion

Reactions on Platinum Surfaces, 503

7.8 .3.3 C arbonaceous Overlayers, 505

7.8 .3.4 Catalysis in the Presence of a Strongly Adsorbed

Overlayer, 507

7.8.3 .5 Structure Mo difiers, 509

7.8 .3.5 .1 Site Blocking by Sulfur, 509

7. 8.3 .5 .2 Ens em ble Effect in Alloy Catalysis and

the Creation of New Sites by

Alloys, 510

7.8.3 .6 The Building of Improved Platinum and Other

Metal Catalysts, 512

7.9 Summ ary and Con cepts, 513

7.10 Problems, 513

References, 515

8 Mech anical Prop erties of Surfaces 596

8.1 Introductio n, 596

8.2 Historical Pers pect ive, 597

8.3 Hardness, 597

8.4 M echanical Forces Requ ired to Break a Chem ical Bon d, 600

8.5 Adhesion, 602

8.6 Surfaces in Relative M otion . Trib olog y, 603

8.6.1 Friction and Slidin g, 603

8.6.2 Hea ting by Frictio n, 605

8.6.3 A pplication s of Frictio n, 605

8.6.4 Lubrication, 606

8.7 Solid Lu brican ts. Co atin gs, 609

8.7.1 C oatings : Mech anical Protection, 610

8.7.2 Coating s: Chem ical Protection, 611

8.8 Crack Formation and Propagation . Fracture, 611

8.9 Summary and Con cepts, 613

8.10 Problems, 613

References, 614

Answers to the Prob lems 619

Index

643