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Landolt-Börnstein Numerical Data and Functional Relationships in Science and Technology New Series / Editor in Chief: W. Martienssen

Group III: Condensed Matter Volume 42

Physics of Covered Solid Surfaces Subvolume A Adsorbed Layers on Surfaces Part 5 Adsorption of Molecules on Metal, Semiconductor and Oxide Surfaces Editor H.P. Bonzel Authors K. Christmann, H.J. Freund, J. Kim, B. Koel, H. Kuhlenbeck, M. Morgenstern, C. Panja, G. Pirug, G. Rupprechter, E. Samano, G.A. Somorjai

ISSN 1615-1925 (Condensed Matter) ISBN-10: 3-540-25848-5 Springer Berlin Heidelberg New York ISBN-13: 978-3-540-25484-3 Library of Congress Cataloging in Publication Data Zahlenwerte und Funktionen aus Naturwissenschaften und Technik, Neue Serie Editor in Chief: W. Martienssen Vol. III/42A5: Editor: H.P. Bonzel At head of title: Landolt-Börnstein. Added t.p.: Numerical data and functional relationships in science and technology. Tables chiefly in English. Intended to supersede the Physikalisch-chemische Tabellen by H. Landolt and R. Börnstein of which the 6th ed. began publication in 1950 under title: Zahlenwerte und Funktionen aus Physik, Chemie, Astronomie, Geophysik und Technik. Vols. published after v. 1 of group I have imprint: Berlin, New York, Springer-Verlag Includes bibliographies. 1. Physics--Tables. 2. Chemistry--Tables. 3. Engineering--Tables. I. Börnstein, R. (Richard), 1852-1913. II. Landolt, H. (Hans), 1831-1910. III. Physikalisch-chemische Tabellen. IV. Title: Numerical data and functional relationships in science and technology. QC61.23 502'.12 62-53136 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in other ways, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution act under German Copyright Law. Springer-Verlag Berlin Heidelberg New York Springer is a member of Springer Science+Business Media springeronline.com © Springer-Verlag Berlin Heidelberg 2006 Printed in Germany The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Product Liability: The data and other information in this handbook have been carefully extracted and evaluated by experts from the original literature. Furthermore, they have been checked for correctness by authors and the editorial staff before printing. Nevertheless, the publisher can give no guarantee for the correctness of the data and information provided. In any individual case of application, the respective user must check the correctness by consulting other relevant sources of information. Cover layout: Erich Kirchner, Heidelberg Typesetting: Authors and SciCaster - Wissen kompakt (Dr. Christian Meier), Darmstadt Printing and Binding: AZ Druck, Kempten

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Preface Surface Science is understood as a relatively young scientific discipline, concerned with the physical and chemical properties of and phenomena on clean and covered solid surfaces, studied under a variety of conditions. The adsorption of atoms and molecules on solid surfaces is, for example, such a condition, connected with more or less drastic changes of all surface properties. An adsorption event is frequently observed in nature and found to be of technical importance in many industrial processes. For this reason, Surface Science is interdisciplinary by its very nature, and as such an important intermediary between fundamental and applied research. Intense world-wide research in this field over the last 50 years has lead to a considerable degree of maturity, such that a documentation of quantitative results in a single source seems desirable. Tribute is being paid to this effect by the renowned Series of LANDOLT-BÖRNSTEIN whose editor-in-chief Werner Martienssen, Frankfurt/ Main, has initiated several volumes of collected scientific data in the field of Surface Science. The beginning has been made with LANDOLT-BÖRNSTEIN volume III/24, entitled Physics of Solid Surfaces. This volume, consisting of four subvolumes, appeared in 1993-96 and covers the properties of clean solid surfaces. The current volume III/42 is devoted to Physics of Covered Solid Surfaces and, in particular, to Adsorbed Layers on Surfaces. It is as such a collection of data obtained for adsorbates on well-defined crystalline surfaces. "Well-defined" means surfaces of known crystallographic structure and chemical composition. It was almost clear at the beginning, that the amount of general information and quantitative data on Adsorbed Layers on Surfaces is enormous, too large to fit into a single book. Hence several subvolumes had to be planned. Unfortunately, the chapters anticipated for each of the subvolumes did not arrive synchronously with the production schedule, such that the sequence of chapters actually printed in the subvolumes deviates from that in the original outline of the whole volume. We apoligize for this inconvenience, but in the age of electronic information distribution this problem will be solved, once all volumes are available electronically. Search routines will guide the reader to the data of his/her desire. Until that time, the index of each subvolume will have to do. Four subvolumes A1 to A4 of volume III/42 have already appeared in the years 2001-2005. The present subvolume A5 entitled Adsorbed Molecules on Metal, Semiconductor and Oxide Surfaces is the final one in this sequence. Finally, it is my great pleasure to thank all authors of this volume for their excellent contributions, and the editing and production offices of the Landolt-Börnstein Section of the Springer-Verlag for efficient cooperation and excellent support. Jülich, May 2006

Hans P. Bonzel

VI

Contributors

Editor H.P. Bonzel Forschungszentrum Jülich Institut für Schichten und Grenzflächen (ISG 3) 52425 Jülich Germany

Authors E.I. Altman Department of Chemical Engineering Yale University New Haven, CT 06520 USA 3.4.3 Halogens on metals and semiconductors M. Bienfait CRMC2/CNRS Faculté de Luminy Physique - Case 910 F-13288 Marseille Cedex 9 France 3.1.2 Noble gases on graphite, lamellar halides, MgO, NaCl H.P. Bonzel Forschungszentrum Jülich Institut für Schichten und Grenzflächen (ISG 3) 52425 Jülich Germany 1 Introduction to physical and chemical properties of adlayer/substrate systems 3.7.1 CO and N2 on metals W.A. Brown Department of Chemistry University College London London WC1H 0AJ U.K. 3.7.2 NO, CN, O2 on metals H. Brune Institut de Physique Expérimentale (IPE) École Polytechnique Fédérale de Lausanne (EPFL) PHB-Ecublens CH-1015 Lausanne 3.3.1 Metals on metals

Contributors K. Christmann Institut für Chemie und Biochemie Bereich Physikalische und Theoretische Chemie Freie Universität Berlin 14195 Berlin Germany 3.4.1 Adsorbate properties of hydrogen on solid surfaces R. Denecke Universität Erlangen-Nürnberg Lehrstuhl für Physikalische Chemie II Egerlandstraße 3 91058 Erlangen Germany 4.3 Adsorbate induced surface core level shifts of metals R.D. Diehl Department of Physics Pennsylvania State University University Park, PA 16802 USA 3.2.1 Alkali metals on metals W. Eck Universität Heidelberg Angewandte Physikalische Chemie Abteilung Materialchemie Im Neuenheimer Feld 253 69120 Heidelberg Germany 3.8.10 Polyatomic chain-like hydrocarbons on metals and semiconductors M. Enachescu Candescent Technologies 6320 San Ignacio Ave. San José, CA 95119 USA 3.4.4 Adsorption of S, P, As, Sb and Se on metals, alloys and semiconductors J.E. Fieberg Department of Chemistry Georgetown College Georgetown, KY 40324 USA 3.8.9 Halogen-substituted hydrocarbons on metals and semiconductors A. Föhlisch Institut für Experimentalphysik Universität Hamburg Luruper Chaussee 149 D-22761 Hamburg Germany 3.7.1 CO and N2 on metals

VII

VIII

Contributors

H.-J. Freund Fritz-Haber-Institut der Max Planck Gesellschaft (MPG) D-14195 Berlin Germany 3.9 Adsorption on oxides H.J. Grabke Max-Planck Institut (MPI) für Eisenforschung GmbH D-40074 Düsseldorf Germany 3.5 Surface segregation of atomic species E. Hasselbrink Institut für Physikalische und Theoretische Chemie Universität Essen D-45117 Essen Germany 3.8.3 NH3 and PF3 on metals and semiconductors G. Held University of Cambridge Department of Chemistry Lensfield Road Cambridge CB2 1EW United Kingdom 3.8.7 Cyclic hydrocarbons K. Hermann Fritz-Haber-Institut der Max-Planck Gesellschaft (MPG) Abteilung Theorie D-14195 Berlin Germany 4.1 Surface structure on metals and semiconductors H. Ibach Institut für Schichten und Grenzflächen (ISG 3) Forschungszentrum Jülich D-52425 Jülich Germany 4.4 Surface free energy and surface stress K. Jacobi Fritz-Haber-Institut der Max-Planck Gesellschaft (MPG) D-14195 Berlin Germany 4.2 Electron work function of metals and semiconductors

Contributors W. Jaegermann Fachbereich Materialwissenschaft Fachgebiet Oberflächenforschung Technische Universität Darmstadt D-64287 Darmstadt Germany 3.8.2 H2O and OH on semiconductors J. Kim Department of Chemistry Lehigh University 6 E. Packer Ave. Bethlehem, PA 18015-3172 3.8.4 CO2, NO2, SO2, OCS, N2O, O3 on metal surfaces B.E. Koel Department of Chemistry, and Center for Advanced Materials and Nanotechnology (CAMN) 6 E. Packer Ave. Lehigh University Bethlehem, PA 18015-3172 USA 3.8.4 CO2, NO2, SO2, OCS, N2O, O3 on metal surfaces H. Kuhlenbeck Fritz-Haber-Institut der Max-Planck-Gesellschaft (MPG) Abteilung Chemische Physik D-14195 Berlin Germany 3.9 Adsorption on oxides V.G. Lifshits Institute of Automation and Control Processes 690041 Vladivostok Russia 3.3.2 Metals on semiconductors N. Mårtensson Department of Physics Uppsala University S-751 21 Uppsala Sweden 4.3 Adsorbate induced surface core level shifts of metals T. Mayer Fachbereich Materialwissenschaft Fachgebiet Oberflächenforschung Technische Universität Darmstadt D-64287 Darmstadt Germany 3.8.2 H2O and OH on semiconductors

IX

X

Contributors

R. McGrath Surface Science Research Centre and Department of Physics The University of Liverpool Liverpool L69 3BX U.K. 3.2.1 Alkali metals on metals E.G. Michel Departimento Fisica de la Materia Condensada C-III Instituto Universitario de Ciencia de Materiales "Nicolas Cabrera" Universidad Autonoma de Madrid 28049 Madrid Spain 3.2.2 Alkali metals on semiconductors R. Miranda Departimento Fisica de la Materia Condensada C-III Instituto Universitario de Ciencia de Materiales "Nicolas Cabrera" Universidad Autonoma de Madrid 28049 Madrid Spain 3.2.2 Alkali metals on semiconductors M. Morgenstern II. Institute of Physics B Rheinisch-Westfälische Technische Hochschule Aachen D-52056 Aachen Germany 3.8.1 H2O on metals D.R. Mullins Oak Ridge National Laboratory Oak Ridge, TN 37831-6201 USA 3.8.5 Substituted hydrocarbons on metals B.E. Nieuwenhuys Gorlaeus Laboratory Leiden University NL 2300 Ra Leiden The Netherlands 3.7.3 Adsorption of diatomic molecules on alloy surfaces K. Oura Department of Electronic Engineering Faculty of Engineering Osaka University Osaka 565-0871 Japan 3.3.2 Metals on semiconductors

Contributors H. Over Physikalisch-Chemisches Institut Justus Liebig Universität Gießen Heinrich-Buff Ring 58 D-35392 Gießen Germany 3.4.2 Adsorption of C, N, and O on metal surfaces C. Panja Apic Corporation 5800 Uplander Way Culver City, CA 90230-6608 USA 3.8.4 CO2, NO2, SO2, OCS, N2O, O3 on metal surfaces G. Pirug Institut für Schichten und Grenzflächen (ISG 3) Forschungszentrum Jülich D-52425 Jülich Germany 3.8.1 H2O on metals M.A. Rocca Centro di Fisica delle Superfici e Basse Temperature del CNR Istituto Nazionale di Fisica della Materia I-16146 Genova Italy 4.5 Surface phonon dispersion G. Rupprechter Institut für Materialchemie Technische Universität Wien Veterinärplatz 1 A-1210 Wien Austria 3.8.6 Adsorbate properties of linear hydrocarbons M. Salmeron Lawrence Berkeley Laboratory Materials Science Bldg. 66/208 Berkeley, CA 94720 USA 3.4.4 Adsorption of S, P, As, Sb and Se on metals, alloys and semiconductors E. Samano Centro de Ciencias de la Materia Condensada-UNAM 22820 Ensenada B.C., Mexico 3.8.4 CO2, NO2, SO2, OCS, N2O, O3 on metal surfaces

XI

XII

Contributors

D. Sander Max-Planck Institut (MPI) für Strukturphysik D-06120 Halle Germany 4.4 Surface free energy and surface stress A.A. Saranin Institute of Automation and Control Processes 690041 Vladivostok Faculty of Physics and Engineering Far Eastern State University 690000 Vladivostok Russia 3.3.2 Metals on semiconductors G.A. Somorjai Department of Chemistry University of California at Berkeley Berkeley, CA 94720 USA 3.8.6 Adsorbate properties of linear hydrocarbons H.-P. Steinrück Lehrstuhl für Physikalische Chemie II Universität Erlangen-Nürnberg D-91058 Erlangen Germany 3.8.7 Cyclic hydrocarbons J. Suzanne Departement de Physique CRMC2 - Centre National de la Recherche Scientifique (CNRS) Faculte des Sciences de Luminy F-13288 Marseille, Cedex 9 France 3.6 Molecules on graphite, BN, MgO (except noble gases) W.T. Tysoe Department of Chemistry and Laboratory for Surface Studies University of Wisconsin - Milwaukee Milwaukee, WI 53211 USA 3.8.5 Substituted hydrocarbons on metals Ch. Uebing Department of Physics and Astronomy Rutgers, The State University of New Jersey Piscataway, NJ 08854-8019 USA 3.5 Surface segregation of atomic species

Contributors H. Viefhaus Max-Planck Institut (MPI) für Eisenforschung GmbH D-40074 Düsseldorf Germany 3.5 Surface segregation of atomic species J.M. Vohs Department of Chemical Engineering University of Pennsylvania Philadelphia, PA 19104-6315 USA 3.8.8 Oxygenated hydrocarbons on metals and semiconductors M.A. Van Hove Lawrence Berkeley National Laboratory Materials Science 66 Berkeley, CA 94720 and Department of Physics University of California-Davis Davis, CA 95616 USA 4.1 Surface structure on metals and semiconductors P.R. Watson Department of Chemistry Oregon State University Corvallis, OR 97331 USA 4.1 Surface structure on metals and semiconductors J.M. White Department of Chemistry and Biochemistry University of Texas at Austin Austin, TX 78712 USA 3.8.9 Halogen-substituted hydrocarbons on metals and semiconductors H. Wiechert Institut für Physik der Johann Gutenberg-Universität D-55099 Mainz Germany 3.6.2 Adsorption of molecular hydrogen isotopes on graphite and BN Ch. Wöll Lehrstuhl für Physikalische Chemie I Ruhr-Universität Bochum D-44801 Bochum Germany 2 Characterization of adsorbate overlayers: Measuring techniques

XIII

XIV

Contributors

P. Zeppenfeld Institut für Experimentalphysik Atom- und Oberflächenphysik Johannes-Kepler-Universität Linz A-4040 Linz, Austria 3.1.1 Noble gases on metals and semiconductors A.V. Zotov Faculty of Electronics Vladivostok State University of Economics and Service 690600 Vladivostok, Russia Institute of Automation and Control Processes 690041 Vladivostok , Russia 3.3.2 Metals on semiconductors

Landolt-Börnstein Editorial Office Gagernstr. 8, D-64283 Darmstadt, Germany fax: +49 (6151) 171760 e-mail: [email protected] Internet http://www.landolt-boernstein.com

Contents

XV

Contents III/42 Physics of Covered Solid Surfaces A: Adsorbed Layers on Surfaces

Part 5: Adsorption of molecules on metal, semiconductor and oxide surfaces 1

Introduction to physical and chemical properties of adlayer/substrate systems (H.P. BONZEL) ............................................................................................................................................ see subvolume III/42A1

2

Characterization of adsorbate overlayers: measuring techniques (CH. WÖLL).................................................................................................................................................... see subvolume III/42A2

3

Data: Adsorbate properties

3.1 3.1.1 3.1.2

Adsorption of noble gases Noble gases on metals and semiconductors (P. ZEPPENFELD)................... see subvolume III/42A1 Noble gases on graphite, lamellar halides, MgO and NaCl (M. BIENFAIT).............................................................................................................................................. see subvolume III/42A1

3.2 3.2.1 3.2.2 3.3 3.3.1 3.3.2

Adsorption of alkali metals Alkali metals on metals (R.D. DIEHL, R. McGRATH) ......................................... see subvolume III/42A1 Alkali metals on semiconductors (E.G. MICHEL, R. MIRANDA) ..... see subvolume III/42A1 Adsorption of metals Metals on metals (H. BRUNE)...................................................................................................... see subvolume III/42A1 Metals on semiconductors (V.G. LIFSHITS, K.OURA, A.A. SARANIN, A.V. ZOTOV) .................................. see subvolume III/42A1

3.4

Non-metallic atomic adsorbates on metals and semiconductors

3.4.1 3.4.1.1 3.4.1.2 3.4.1.3 3.4.1.3.1 3.4.1.3.2 3.4.1.3.3 3.4.1.3.4 3.4.1.3.5 3.4.1.3.6 3.4.1.3.7 3.4.1.3.8 3.4.1.4 3.4.1.4.1 3.4.1.4.2 3.4.1.4.3 3.4.1.5 3.4.1.6

Adsorbate properties of hydrogen on solid surfaces (K. CHRISTMANN) ...................................................... 2 Introduction ................................................................................................................................................................................................................ 2 Some general principles of the hydrogen – surface interaction ............................................................................ 4 The interaction of hydrogen with solid surfaces: experimental data ............................................................... 7 Adsorption kinetics ............................................................................................................................................................................................ 7 Kinetics of hydrogen desorption........................................................................................................................................................ 19 The energetics of hydrogen adsorption and desorption ............................................................................................. 30 The diffusion of adsorbed hydrogen.............................................................................................................................................. 44 The structure of adsorbed hydrogen phases ........................................................................................................................... 47 Vibrational modes of adsorbed hydrogen ................................................................................................................................ 67 Electronic states of adsorbed hydrogen and photoemission spectroscopy ............................................. 85 Hydrogen-induced work function changes ............................................................................................................................. 94 The interaction of hydrogen with solid surfaces: theory........................................................................................100 General remarks...............................................................................................................................................................................................100 General theories for hydrogen adsorption.............................................................................................................................102 Theories covering specific interaction systems ...............................................................................................................103 List of acronyms ..............................................................................................................................................................................................109 References .............................................................................................................................................................................................................111

XVI

Contents

3.4.2 3.4.3 3.4.4

Adsorption of C, N, and O on metal surfaces (H. OVER) ............................... see subvolume III/42A4 Halogens on metals and semiconductors (E.I. ALTMAN) ............................... see subvolume III/42A1 Adsorption of S, P, As, Sb and Se on metals, alloys and semiconductors (M. ENACHESCU, M. SALMERON) ............................................................................................ see subvolume III/42A3

3.5

Surface segregation of atomic species (H. VIEFHAUS, H.-J. GRABKE, CH. UEBING) ................................................................. see subvolume III/42A3

3.6 3.6.1 3.6.2

Molecules on graphite, BN, MgO (except noble gases) Adsorption of molecules on MgO (J. SUZANNE) .................................................... see subvolume III/42A3 Adsorption of molecular hydrogen isotopes on graphite and BN (H. WIECHERT) ........................................................................................................................................... see subvolume III/42A3

3.7 3.7.1

Molecular diatomic adsorbates on metals and semiconductors CO and N2 adsorption on metal surfaces (A. FÖHLISCH, H.P. BONZEL) ........................................... .................................................................................................................................................................................... see subvolume III/42A4

3.7.2 3.7.3

NO, CN, O2 on metals (W.A. BROWN) ............................................................................. see subvolume III/42A3 Adsorption of diatomic molecules on alloy surfaces (B. E. NIEUWENHUYS) ......................................................................................................................... see subvolume III/42A3

3.8

Molecular polyatomic adsorbates on metals and semiconductors

3.8.1 3.8.1.1 3.8.1.2 3.8.1.2.1 3.8.1.2.2 3.8.1.2.3 3.8.1.3 3.8.1.4 3.8.1.4.1 3.8.1.4.2 3.8.1.4.3 3.8.1.4.4 3.8.1.4.5 3.8.1.5 3.8.1.6 3.8.1.7

H2O on metals (G. PIRUG, M. MORGENSTERN) ................................................................................................................133 Introduction ..........................................................................................................................................................................................................133 Electronic structure ......................................................................................................................................................................................135 Valence band orbitals and core levels .......................................................................................................................................135 Molecular vibrations ...................................................................................................................................................................................136 Work function changes ............................................................................................................................................................................137 Dissociative versus molecular adsorption .............................................................................................................................138 Geometric structure of molecularly adsorbed ice..........................................................................................................138 Adsorption geometry ..................................................................................................................................................................................138 Binding energy and desorption temperatures ....................................................................................................................139 Trapping and sticking ................................................................................................................................................................................140 Diffusion and formation of small clusters.............................................................................................................................141 Ice bilayer...............................................................................................................................................................................................................141 Tables for 3.8.1.................................................................................................................................................................................................143 Figures for 3.8.1 ..............................................................................................................................................................................................157 References for 3.8.1.....................................................................................................................................................................................162

3.8.2 3.8.3

H2O and OH on semiconductors (W. JAEGERMANN, T. MAYER) ......... see subvolume III/42A4 Adsorbate properties of NH3 and PF3 on metals and semiconductors (E. HASSELBRINK) ................................................................................................................................... see subvolume III/42A3

3.8.4

CO2, NO2, SO2, OCS, N2O, O3 on metal surfaces (B.E. KOEL, C. PANJA, J. KIM, E. SAMANO) .......................................................................................................................170 Introduction ..........................................................................................................................................................................................................170 CO2................................................................................................................................................................................................................................171 Structure and bonding of CO2 ...........................................................................................................................................................171 CO2 adsorption on metal surfaces .................................................................................................................................................172 CO2 adsorption on chemically modified metal surfaces.........................................................................................174 CO2 adsorption on alloy surfaces...................................................................................................................................................176 NO2 ...............................................................................................................................................................................................................................176

3.8.4.1 3.8.4.2 3.8.4.2.1 3.8.4.2.2 3.8.4.2.3 3.8.4.2.4 3.8.4.3

Contents

XVII

3.8.4.3.1 3.8.4.3.2 3.8.4.3.3 3.8.4.4 3.8.4.4.1 3.8.4.4.2 3.8.4.4.3 3.8.4.3.4 3.8.4.5 3.8.4.5.1 3.8.4.5.2 3.8.4.6 3.8.4.6.1 3.8.4.6.2 3.8.4.6.3 3.8.4.7 3.8.4.7.1 3.8.4.7.2 3.8.4.7.3 3.8.4.7.4 3.8.4.8 3.8.4.9 3.8.4.10

Structure and bonding of NO2...........................................................................................................................................................176 NO2 adsorption on metal surfaces .................................................................................................................................................177 NO2 adsorption on alloy surfaces ..................................................................................................................................................179 SO2 ................................................................................................................................................................................................................................180 Structure and bonding of SO2 ............................................................................................................................................................180 SO2 adsorption on metal surfaces ..................................................................................................................................................181 SO2 adsorption on metals with coadsorbed alkali metals......................................................................................185 SO2 adsorption on alloy surfaces ...................................................................................................................................................186 OCS ..............................................................................................................................................................................................................................186 Structure and bonding of OCS .........................................................................................................................................................186 OCS adsorption on metal surfaces................................................................................................................................................186 N2O ...............................................................................................................................................................................................................................187 Structure and bonding of N2O...........................................................................................................................................................187 Adsorption of N2O on metal surfaces........................................................................................................................................187 N2O adsorption on alloy surfaces ..................................................................................................................................................190 O3 ....................................................................................................................................................................................................................................190 Structure and bonding of O3 ...............................................................................................................................................................190 O3 adsorption on metal surfaces......................................................................................................................................................191 O3 adsorption on alloy surfaces .......................................................................................................................................................191 O3 adsorption on metal oxide surfaces .....................................................................................................................................192 Tables for 3.8.4.................................................................................................................................................................................................192 Figures for 3.8.4 ..............................................................................................................................................................................................217 References for 3.8.4.....................................................................................................................................................................................235

3.8.5

Substituted hydrocarbons on metals (W.T. TYSOE, D.R. MULLINS) ........................................................................... see subvolume III/42A3

3.8.6

Adsorbate properties of linear hydrocarbons (G. RUPPRECHTER, G.A. SOMORJAI)............................................................................................................................................243 Introduction ..........................................................................................................................................................................................................243 General considerations..............................................................................................................................................................................243 Experimental aspects ..................................................................................................................................................................................244 List of symbols and abbreviations ................................................................................................................................................245 Reviews ....................................................................................................................................................................................................................246 Alkanes .....................................................................................................................................................................................................................248 Methane CH4.......................................................................................................................................................................................................248 Ethane C2H6 .........................................................................................................................................................................................................254 Propane C3H8 ......................................................................................................................................................................................................256 Butane C4H10.......................................................................................................................................................................................................257 Pentanes C5H12 and higher alkanes ..............................................................................................................................................258 Various (Hydrocarbon fragments, Radicals, etc) ..........................................................................................................259 Alkenes .....................................................................................................................................................................................................................262 Ethylene C2H4 and Ethylidyne C2H3 ..........................................................................................................................................264 Propene C3H6 ......................................................................................................................................................................................................276 Butenes C4H10 ....................................................................................................................................................................................................278 Pentenes C5H10 and Hexenes C6H12 ............................................................................................................................................282 Dienes ........................................................................................................................................................................................................................283 Propadiene C3H4..............................................................................................................................................................................................283 Butadiene C4H6.................................................................................................................................................................................................283 Pentadiene C5H8, Hexadiene C6H10 .............................................................................................................................................285 Alkynes.....................................................................................................................................................................................................................285 Acetylene C2H2.................................................................................................................................................................................................286 Propyne C3H4 .....................................................................................................................................................................................................292

3.8.6.1 3.8.6.1.1 3.8.6.1.2 3.8.6.1.3 3.8.6.2 3.8.6.3 3.8.6.3.1 3.8.6.3.2 3.8.6.3.3 3.8.6.3.4 3.8.6.3.5 3.8.6.3.6 3.8.6.4 3.8.6.4.1 3.8.6.4.2 3.8.6.4.3 3.8.6.4.4 3.8.6.5 3.8.6.5.1 3.8.6.5.2 3.8.6.5.3 3.8.6.6 3.8.6.6.1 3.8.6.6.2

XVIII

Contents

3.8.6.7 3.8.6.8

Tables for 3.8.6.................................................................................................................................................................................................295 References for 3.8.6.....................................................................................................................................................................................320

3.8.7 3.8.8 3.8.9

Cyclic hydrocarbons (G. HELD, H.P. STEINRÜCK) ................................................. see subvolume III/42A4 Oxygenated hydrocarbons on metals and semiconductors (J. VOHS) ... see subvolume III/42A3 Halogen-substituted hydrocarbons on metals and semiconductors (J. FIEBERG, J.W. WHITE) ................................................................................... see subvolume III/42A3 Polyatomic chain-like hydrocarbons on metals and semiconductors (W. ECK)....................................... .................................................................................................................................................................................... see subvolume III/42A4

3.8.10

3.9 3.9.1 3.9.2 3.9.3 3.9.3.1 3.9.3.2 3.9.4 3.9.4.1 3.9.4.2 3.9.4.3 3.9.5 3.9.5.1 3.9.5.2 3.9.6 3.9.6.1 3.9.6.2 3.9.6.3 3.9.6.4 3.9.6.5 3.9.7 3.9.7.1 3.9.7.2 3.9.7.3 3.9.8 3.9.8.1 3.9.8.2 3.9.8.3 3.9.8.4 3.9.9 3.9.9.1 3.9.10 3.9.10.1 3.9.10.2 3.9.10.3 3.9.11 3.9.11.1 3.9.11.2 3.9.11.3 3.9.11.4 3.9.11.5 3.9.11.6 3.9.11.7 3.9.12

Adsorption on oxides (H. KUHLENBECK, H.J. FREUND) .........................................................................................332 Introduction ..........................................................................................................................................................................................................332 Abbreviations used in the text...........................................................................................................................................................332 Al2O3 ...........................................................................................................................................................................................................................334 CO adsorption ....................................................................................................................................................................................................335 H2O adsorption .................................................................................................................................................................................................335 CaO ...............................................................................................................................................................................................................................335 CO2 adsorption..................................................................................................................................................................................................336 H2O adsorption .................................................................................................................................................................................................336 SO2 adsorption ..................................................................................................................................................................................................336 CeO2.............................................................................................................................................................................................................................336 CO adsorption on CeO2(111) .............................................................................................................................................................337 H2O and D2O adsorption on CeO2(001) and CeO2(111) .......................................................................................337 α-Cr2O3.....................................................................................................................................................................................................................338 CO adsorption ....................................................................................................................................................................................................339 NO adsorption ...................................................................................................................................................................................................340 CO2 adsorption..................................................................................................................................................................................................340 O2 adsorption ......................................................................................................................................................................................................340 H2O adsorption .................................................................................................................................................................................................341 CoO ..............................................................................................................................................................................................................................341 CO adsorption ....................................................................................................................................................................................................342 NO adsorption ...................................................................................................................................................................................................342 H2O adsorption .................................................................................................................................................................................................342 Cu2O ............................................................................................................................................................................................................................343 CO adsorption ....................................................................................................................................................................................................344 H2O adsorption .................................................................................................................................................................................................344 CH3OH adsorption ........................................................................................................................................................................................344 O2 adsorption ......................................................................................................................................................................................................344 FeO, Fe3O4 and α-Fe2O3 .........................................................................................................................................................................345 Ethylbenzene, water and styrene adsorption ......................................................................................................................347 MgO .............................................................................................................................................................................................................................350 H2O adsorption .................................................................................................................................................................................................353 CO adsorption ....................................................................................................................................................................................................354 CO2 adsorption..................................................................................................................................................................................................354 NiO ...............................................................................................................................................................................................................................355 CO adsorption ....................................................................................................................................................................................................359 NO adsorption ...................................................................................................................................................................................................359 H2O adsorption .................................................................................................................................................................................................361 HCOOH adsorption on NiO(111) .................................................................................................................................................362 H2 adsorption on NiO(100) ..................................................................................................................................................................363 H2S adsorption on NiO(100) ..............................................................................................................................................................363 CO2 adsorption on NiO(111) .............................................................................................................................................................363 RuO2 ............................................................................................................................................................................................................................363

Contents

XIX

3.9.12.1 3.9.13 3.9.13.1 3.9.13.2 3.9.13.3 3.9.13.4 3.9.14 3.9.14.1 3.9.14.2 3.9.14.3 3.9.14.4 3.9.15 3.9.15.1 3.9.15.2 3.9.16 3.9.16.1 3.9.16.2 3.9.17 3.9.17.1 3.9.17.2 3.9.17.3 3.9.17.4 3.9.18 3.9.19

CO adsorption ....................................................................................................................................................................................................364 SnO2 .............................................................................................................................................................................................................................365 O2 adsorption ......................................................................................................................................................................................................366 H2O adsorption .................................................................................................................................................................................................366 CH3OH adsorption ........................................................................................................................................................................................366 HCOOH adsorption .....................................................................................................................................................................................366 TiO2 ..............................................................................................................................................................................................................................366 CO adsorption ....................................................................................................................................................................................................370 H2O adsorption .................................................................................................................................................................................................372 HCOOH adsorption .....................................................................................................................................................................................372 CH3COOH adsorption...............................................................................................................................................................................375 V2O3 .............................................................................................................................................................................................................................376 O2 adsorption ......................................................................................................................................................................................................376 H2O adsorption .................................................................................................................................................................................................376 V2O5 .............................................................................................................................................................................................................................377 CO and SO2 adsorption ............................................................................................................................................................................377 H2 and H adsorption ....................................................................................................................................................................................377 ZnO ...............................................................................................................................................................................................................................378 CO adsorption ....................................................................................................................................................................................................380 CO2 adsorption..................................................................................................................................................................................................381 CH3OH adsorption ........................................................................................................................................................................................381 HCOOH adsorption .....................................................................................................................................................................................382 Tables of selected adsorbate properties ...................................................................................................................................382 References for 3.9..........................................................................................................................................................................................389

3.10

Surface diffusion on metals, semiconductors, and insulators (E.G. SEEBAUER, M.Y.L. JUNG) ............................................................................................... see subvolume III/42A1

4 4.1

Data: Adsorbate-induced changes of substrate properties Surface structure on metals and semiconductors (M.A. VAN HOVE, K. HERMANN, P.R. WATSON) ................................................. see subvolume III/42A2 Electron work function of metals and semiconductors (K. JAKOBI) ... see subvolume III/42A2 Adsorbate induced surface core level shifts of metals (R. DENECKE, N. MǖRTENSSON) ............................................................................................. see subvolume III/42A4 Surface free energy and surface stress (D. SANDER, H. IBACH) ............... see subvolume III/42A2 Surface phonon dispersion (M.A. ROCCA) ........................ see subvolume III/42A2

4.2 4.3 4.4 4.5

2

3.4

3.4.1 Adsorbate properties of hydrogen on solid surfaces

[Ref. p. 111

Non-metallic atomic adsorbates on metals and semiconductors

3.4.1 Adsorbate properties of hydrogen on solid surfaces 3.4.1.1 Introduction Hydrogen is the most abundant chemical element in the universe. On earth and at temperatures below ~ 2000 K the thermodynamic stable form of hydrogen is dihydrogen H2. This simplest homonuclear diatomic molecule exhibits a strong chemical bond (bond dissociation energy 432 kJ/mol) which may be considered the prototype of covalent bonding. A survey of the physical and chemical properties of H2 and hydrides has been given by Silvera [80Sil]. A full potential energy surface (PES) for the H2 molecule has been calculated by Shavitt et al. [68Sha]. For a variety of reasons (hydrogen´s role in heterogeneous catalysis, battery and fuel cell technology, materials science, plasma physics), its interaction with solid surfaces (preferentially metallic surfaces) has attracted and still attracts much attention in science and technology. It is useful, for chemical and energetic reasons, to distinguish the interaction of H atoms and that of H2 molecules (which is by far more important) with these surfaces. A fairly consistent understanding of the underlying general chemical scenario has arisen from the numerous surface studies performed hitherto: The thermal H2 molecule approaches the surface and is transiently trapped in a weakly bound precursor state. It can either reside in this state for some time or move on towards a more strongly bound state, dissociate into atoms, which then become adsorbed in a deep chemisorption potential. Depending on the thermal energy of the physisorbed molecule or chemisorbed atom compared to the depth of the adsorption potential well the hydrogen particle may be able or unable to migrate across the surface whereby also tunnelling processes can play a role. With a given layer of adsorbed hydrogen atoms formed by dissociation, also the reverse process, namely, the recombination of two individual H atoms to dihydrogen and its desorption is a likely event, especially at elevated temperatures. The hydrogen - surface interaction may be thermally activated and will then be governed by kinetics rather than by thermodynamics. Accordingly, the field of hydrogen reaction dynamics and kinetics represents a topic of greatest scientific interest, experimentally as well as theoretically. Modern (quantumstate selective) spectroscopic techniques render the investigation and (in many cases state-selective) characterization even of ultra-short particle - surface interaction steps possible [88Zac, 90Zac]. However, once the adsorbed hydrogens (be it molecules or atoms) have been accommodated on the solid surface, the system has usually reached thermodynamic equilibrium. Depending on temperature the particles either reside in distinct adsorption sites and often form ordered two-dimensional phases (localized or immobile adsorption, preferred at low temperatures) or they can freely migrate across the surface (delocalized or mobile adsorption dominating at elevated temperatures) and then be considered within the framework of a lattice gas system. Of course, various transitions between these two extreme cases are conceivable, for example, particle hopping and site exchange processes. In addition, due to the light mass of H, delocalization by quantum-mechanical tunnelling may contribute to H surface diffusion, especially at very low temperatures [86Wha]. Generally, the hydrogen adsorption process can be considered a transition from an initial state (= clean, i.e., uncovered, solid surface plus an ensemble of gaseous hydrogen molecules) to a final state (= surface covered with a two-dimensional layer of adsorbed hydrogen molecules or atoms). In this scheme the solid surface must not be considered a rigid lattice of periodic adsorption sites; many studies revealed that the solid surface is a dynamic, flexible and ‘soft’ system that will instantaneously respond (geometrically and electronically) to the presence of hydrogen. Consequently, surface restructuring effects are the rule rather than the exception affecting both surface energetics and geometry, c.f., sects. 3.4.1.3.3. and 3.4.1.3.4. Another noteworthy property of adsorbing hydrogen is its ability to migrate through the topmost surface layer, enter the surface-near crystal region and accommodate in subsurface and bulk sites, respectively, a process referred to as sorption or absorption. A variety of metals is known which dissolve hydrogen gas quite readily, among others Ti, V, Zr, Nb, Hf, Ta, and, most notably, Pd [67Lew, 78Wic, 79Bur]. Both subsurface and bulk absorption sites can be distinguished from the (surface) location of

Landolt-Börnstein New Series III/42A5

Ref. p. 111]


3

adsorbed H atoms in a simple one-dimensional potential energy diagram (Fig. 1). The various potential energy wells are separated by (usually H concentration-dependent) activation barriers making the H uptake temperature dependent. A wealth of literature exists concerning the sorption properties of these metals (which cannot be covered here for the sake of space limitation); overviews are given, for example, in the monograph ‘Hydrogen in Metals’ [78Ale] and other review articles [92Sch2, 01Kir].

»

Ediss

Ehydr EH 2

Ess Esol

Ediff

EH

surface

Fig. 1: One-dimensional diagram illustrating the change in potential energy of a hydrogen molecule approaching a metal surface (indicated by the hatched area). It describes the energetic situation during the following processes • Physisorption of the H2 molecule into a shallow potential energy minimum of depth EH 2 • Dissociation of the H2 molecule and the formation of a stable chemisorptive bond with adsorption energy EH. • Transport of H atoms into subsurface sites, with a coverage-dependent sorption energy ESS. • (Possible) absorption of H atoms in interstitial sites with heat of solution Esol. Indicated is also the activation energy of diffusion of the respective H atoms, Ediff.

Probably one of the most important quantities which governs adsorption and desorption phenomena is the hydrogen surface concentration (number of atoms or molecules per m2). The related quantity mostly used in experimental work is the hydrogen coverage Ĭ, which is usually defined as a dimensionless quantity between 0 and 1, relating the number of actually adsorbed particles (H atoms or H2 molecules in the first layer) with the maximum number of adsorption sites (or sometimes substrate surface atoms per unit area):

ΘH =

N ad N max

(1)

Multiplying Θ H with the number of substrate surface atoms, Nmax, yields the number of the adsorbed H atoms per m2. Since in most cases the H-related system properties (listed in the upcoming tables) will depend on Θ H, this quantity will be given where necessary. This survey of hydrogen - surface interaction is organized in the following manner: We will accompany a hydrogen molecule on its way towards the surface and make a coarse distinction between (time-dependent) kinetic properties on the one hand and equilibrium phenomena consisting of (static) Landolt-Börnstein New Series III/42A5

4


[Ref. p. 111

energetic, structural (geometric and electronic) and vibrational properties on the other hand. In the first category (dealing with kinetic phenomena) we will list the available data for the initial sticking probability, the frequency factor and the reaction-order for desorption. Where the respective data exist we will also include activation energies for adsorption, because (as mentioned above) the adsorption dynamics of hydrogen has received considerable interest during the last decade, motivated by the increasingly more sophisticated (molecular beam and/or quantum-state resolved short time) experiments and calculations. In the second part we will expand on the equilibrium properties, namely, kind and number of hydrogen adsorption states, (coverage-dependent) adsorption/ desorption energy, surface structure (long-range ordered phases with and without reconstruction), structural phase transitions, local adsorption site geometry (bond length, coordination number), vibrational frequencies, and electronic structure (adsorbate-induced electronic-bonding states and work function changes). In a final chapter, a brief overview over the theoretical attempts to describe the H2 dissociation and H chemisorption will be given. Quite generally, the position of the respective substrate element in the periodic table will be the ordering principle of the various interaction systems, whereby the available data for elemental semiconductors and non-metallic elemental solids will be listed separately. For the sake of reliability, only those data will be considered that have been measured with clean and structurally well-defined single crystal surfaces. In this sense, we have tried to include most of the relevant data available in the literature, and it is possible, by comparing older with more recent data, to directly follow the progress that has been made in the respective research area during the past twenty or thirty years. Not in all instances a complete data base will be found in the columns of the tables – despite the relatively many “white spots” on the map of hydrogen properties it is deemed useful to give the citation which offers the possibility to at least look up the reference and judge on the kind and qualitiy of the respective scientific work.

3.4.1.2 Some general principles of the hydrogen – surface interaction Our current understanding of the interaction of hydrogen with solid surfaces is documented in several review articles [82Kno, 88Chr, 90Dav]. Most of these reviews are concerned with the interaction of hydrogen with metal surfaces, only in some cases also its interaction with semiconducting or insulating surfaces is addressed in sections 3.6.1 and 3.6.2 in part 3 of this Landolt-Börnstein volume III/42A, because interest in the interaction of hydrogen with these materials has arisen only during the past decade [90Hig, 90Cha, 96Hoe, 99Bal], especially in conjunction with the discovery that H atoms can lift the semiconductor’s inherent surface reconstruction by selectively saturating its dangling bonds. As a consequence, the respective reconstructions are replaced by (1×1) surface phases with H termination, prominent examples being the Si(111)-(1×1)-H [91Cha] or the diamond C(111)-(1×1)-H surfaces [91Mit]. A well-known property of the hydrogen molecule is its ability to dissociate into H atoms when getting into contact with surfaces. The H atoms then can interact quite strongly with the solid leading to the process of chemisorption with typical binding energies ranging between 50 and ~150 kJ/mol. Further reactions that affect the chemical state of the solid may consist of dissolution, absorption and compound (hydride) formation: Hydrogen often exhibits a peculiar reactivity here because of its small size. Note that the dissociation reaction itself can proceed as a homolytic process, according to the scheme: H2 ļ H• + H•, or heterolytic, along the path: H2 ļ H+ + H−. On metals, the homolytic step is certainly the rule, while on (polar) semiconductor and insulating surfaces (oxides, in particular) also the heterolytic mechanism has been reported. An electrically neutral hydrogen molecule arriving from the gas phase and getting in contact with a solid surface will first experience a (weak) van-der-Waals potential in which it is physisorbed at sufficiently low temperatures. Generally, the binding forces acting between the trapped H2 molecule and the surface are quite small, due to the closed-shell character of H2. The respective adsorption energies lie well below ~10 kJ/mol and resemble the condensation enthalpy of elemental hydrogen. Accordingly, temperatures around or below 20 K are required to stabilize H2 molecules on the surface [82Avo]. However, it was shown that on otherwise active metal surfaces with special geometry, containing steps


Ref. p. 111]


5

and/or holes such as Ni(510) or Pd(210), H2 dissociation may be kinetically hindered resulting in a chemisorbed molecular hydrogen species which can be stable up to 100…130 K [86Mar, 93Nyb, 01Sch1]. Physisorption is the only interaction if chemically inert (insulating and/or semiconducting) surfaces are exposed to hydrogen gas at low temperatures. A completely different situation is encountered when surfaces are exposed to H atoms. Then a strong chemical interaction (often leading to compound, i.e. hydride, formation) is the rule. It is worth to mention here that semiconducting or insulating surfaces as well as free electron and/or noble metal surfaces (Cu, Ag, Au) [95Ham2] with their deep-lying d electron states exhibit a surprisingly small activity to dissociate H2 molecules, in contrast to transition metal (TM) surfaces with their high density of d electron states at the Fermi level (EF) [98Chr]. Typical transition metals (e.g., Ni, Ru, or Pt) effectively catalyze the spontaneous homolytic dissociation of dihydrogen, especially in the presence of active (defect) sites [88Ren]. It is thought that the existence of empty d electron states right at EF allows the filled molecular orbitals of the H2 molecule to effectively circumvent the Pauli repulsion barrier by rehybridization [88Har, 89Har]. This process is not possible with free-electron metals such as Cu, Ag, or Au because of their lack of empty d electron states right at EF. [This matter will be taken up again in sect. 3.4.1.3.5 which is devoted to the electronic interaction between a hydrogen molecule and a metal surface]. Accordingly, the activation barriers for dissociation and chemisorption of hydrogen on these surfaces are relatively large [81Nor1]. This has motivated a whole number of studies, especially during the last two decades, to expose the respective materials either to thermally excited H2 beams (using supersonic molecular beam techniques, often with well-defined translational and/or rovibrational quantum-states) to overcome the dissociation barrier, or to dissociate the H2 molecules prior to adsorption in the gas phase (either by the thermal energy of a hot tungsten filament (following Langmuir’s early recipe [12Lan, 14Lan, 15Lan])) or by a hydrogen RF discharge operating at a frequency of 2.450 GHz [88Bas]). By exposure to a reactive beam of H atoms, many surfaces which are inert with respect to ‘normal’ H2 gas exposure can be forced to build up atomic hydrogen layers, e.g., Cu, Ag, Au, but also diamond, silicon and various alkaline, alkaline earth, and earth metals. Particularly these latter materials quite easily form salt-like (in some cases volatile) hydrides, AlH3 being a good example [91Kon]. Within simple transition-state theory, the process of dissociation and subsequent atomic adsorption can be visualized by a two-dimensional potential hyperface [85Kno] either with an ‘early’ or with a ‘late’ activation barrier; in other words, the dissociation reaction may be supported by translational (early barrier) or vibrational excitation (late barrier) of the incoming molecule [87Pol]. The situation is illustrated by means of Fig. 2. Quantum-chemical calculations performed, e.g., with the Pd(100)/H2 system clearly revealed the quite complex nature of the dissociation reaction as a multi-dimensional process [95Gro1, 98Gro, 99Eic]. Generally, up to 6 dimensions are considered when calculating the potential energy surfaces (PES) for the H2 dissociation reaction. Latest experimental developments in low-temperature scanning tunnelling microscopy (STM) have made it possible to directly watch hydrogen molecules dissociating on a Pd(111) surface [03Mit2], with the interesting result that the dissociation event requires – at least for the Pd(111)/H2 system – more than two adjacent empty adsorption sites, namely, at least three such sites, a conclusion that had been indirectly deduced from H adsorption studies on bimetallic Ru surfaces more than twenty years ago [80Shi]. After H2 dissociation a layer of H atoms is readily built up, a process referred to as hydrogen chemisorption, with appreciable adsorption energies involved: The trapped H atoms reside at the bottom of a deep chemisorption potential; neighboring sites can be reached by hopping or tunnelling. While the dynamics of dissociation may be fairly complicated [03Gro] – the overall energetics can be relatively simply visualized in terms of the one-dimensional Lennard-Jones potential model (Fig. 3). It can easily be seen that the following energy balance holds EMe − H =

1 2

(Ead + Ediss ) ,

(2)

in which Ead stands for the adsorption energy (i.e., depth of the adsorption potential) and Ediss denotes the H - H bond dissociation energy (= 432 kJ/mol). EMe-H is the binding energy of a single H - metal adsorptive bond. At room temperature, chemisorbed H atoms may stay for quite a while in the respective potential, since the energies involved (Ead) can easily reach 100 kJ/mol or more for typical transition metals. Frequent structural consequences of these strong interaction forces are relaxation or Landolt-Börnstein New Series III/42A5

6


[Ref. p. 111

reconstruction phenomena of the substrate either locally, i.e., in the direct vicinity of an adsorbed atom, or by H-induced perturbations of the surface electronic structure with long-range character. Further interaction steps can include occupation of subsurface (between the topmost and second substrate layer) or bulk sites (H solution or absorption processes), or compound (hydride) formation, Pd-H being a wellknown example. y

y 1 x P

1 y 2

2 P

x

x

Fig. 2: Two-dimensional representation (so-called elbow plots) of the potential energy surface of the H2 molecule interacting with an active metal surface. The coordinate x denotes the internuclear H - H distance, y the distance of the molecular entity to the surface. Several trajectories are indicated: (1) denotes a reflection trajectory (unsuccessful event) with no chemisorption, (2) a successful approach leading to dissociation. Note that the saddle point P can be located either in the ‘entrance’ channel relatively far away from the surface (left-hand side) or in the ‘exit’ channel (right-hand side). In the first case, mainly translational energy of the H2 molecule is required for a successful passage across the barrier, while vibrational excitation is advantageous if P is located closer to the surface.

2H

Potential energy U(z)

»» »

»

»

»

» Ediss

E*

0

Eph Ead

H2

EMe -H

z

Fig. 3: One-dimensional potential energy (LennardJones) diagram of a H2 molecule interacting with an active (full line) and an inactive metal surface (dotted line). The dashed line indicates the potential energy U(z) if the H2 molecule is pre-dissociated in the gas phase (dissociation energy Ediss) and the two isolated, reactive, H atoms approach the surface. The deep potential energy well (EMe - H) represents the energy of the metal - H bond formed (which is gained twice). While the shallow physisorption minimum Ephys characteristic of inactive surfaces causes the intersection between the dotted and dashed line to occur at positive energies (above zero) and, hence, leads to an activation barrier of height E *, a deeper physisorption well pushes the respective intersection to negative energies (below zero) thus enabling a nonactivated (spontaneous) dissociation (cross-over between the full and the dashed line). Accordingly, the heat of adsorption, Ead, is released.

At higher temperatures, two chemisorbed H atoms will diffuse, meet each other, recombine and desorb as a H2 molecule, leaving behind the empty surface. This process of desorption can be considered the time reverse of the adsorption process and is usually described in terms of simple kinetic models based on transition-state theory (TST). However, a correct description of the rate of desorption becomes difficult, if the adsorption and dissociation are activated processes [01Wet]. Possible accompanying Landolt-Börnstein New Series III/42A5

Ref. p. 111]


7

processes such as adsorbate-induced structural phase transformations, occupation of subsurface sites or bulk (sorption) phenomena can and will introduce even more severe obstacles in the endeavor to quantify the complete interaction scenario.

3.4.1.3 The interaction of hydrogen with solid surfaces: experimental data 3.4.1.3.1 Adsorption kinetics

3.4.1.3.1.1 Introductory remarks The quantity that governs the hydrogen uptake of a given surface is the sticking coefficient s. It is understood as the probability that a gaseous particle hitting the surface will become adsorbed for a finite time rather than be immediately back-reflected into the gas phase. ‘Finite’ time means that the particle must have accommodated on the surface and lost the memory from which direction it has impinged; it varies with the depth of the interaction potential and depends on the surface temperature. Following Groß [03Gro], the decisive condition for sticking or trapping is that the particle can transfer its kinetic energy to the substrate. He defines a function PE(ε ) which is the probability that an incoming particle with kinetic energy E will transfer the energy ε to the surface. If the particle transfers at least its entire gas phase kinetic energy to the surface it safely remains trapped in the adsorption potential, and the (energydependent) sticking probability can be expressed as

s (E ) =

∞

³ P (ε )dε

(3)

E

E

For more details on definitions of s, its coverage and temperature dependencies, precursor kinetics etc., we refer to the special literature [84Mor, 92Ren] and to the introductory chapters 1 and 2 of this Landolt-Börnstein subvolume III/42A (which you can find in parts 1 and 2, respectively). Generally, the (coverage-dependent) sticking probability can be regarded as a product of an initial, coverageindependent, but system-immanent, factor, s0, called the initial sticking coefficient, and a function f (Θ ) which contains the coverage dependence. Then, using simple kinetic theory, the rate of adsorption rad becomes rad =

dΘ dt

= s0 ⋅ f (Θ )N −max1

PH 2 2πmkT

⋅ exp§¨ − ©

E *ad kT

· , ¸ ¹

(4)

in which PH 2 stands for the hydrogen gas pressure, Nmax for the maximum number of adsorbed particles, * m for the absolute mass of the molecule, and Ead for a (possibly important) activation barrier in the adsorption process to account for a temperature dependence of the sticking probability. It is important to note here that especially hydrogen sticking depends quite sensitively on the physical state of the solid surface (impurities, structural features, structural defects, foreign atoms) as pointed out by Poelsema et al. [85Poe] for the system H on Pt(111) and in a comprehensive article by Rendulic and Winkler [89Ren2]. Usually, surfaces rich in defects show a much larger activity in trapping (and subsequently dissociating) H2 molecules than smooth surfaces (smooth on the microscopic scale), the difference sometimes amounting to several orders of magnitude. In the catalytic chemist’s language these defects are known as ‘active sites’ and can strongly influence the general chemical reactivity of a given system. In the same sense, even traces of foreign atoms can hinder or enhance hydrogen sticking quite effectively. It is not trivial to detect and characterize traces of surface impurities (in the order of a few percent of a monolayer) and it is even more difficult to ascertain the short-range crystallographic order of a given surface. Therefore, the literature data available for s0 have to be considered with some care, especially data that were obtained from surfaces whose crystallographic order and chemical cleanliness


8


[Ref. p. 111

were not properly controlled. Only in a few cases the surfaces were sufficiently well characterized, e.g., by parallel scanning tunnelling microscopy (STM) or LEED measurements. Another problem concerns temperature dependencies of the sticking – especially when the adsorption is activated. In these cases, the sticking coefficient depends sensitively on the kinetic and internal energy of the incoming hydrogen molecule, and only quantum-state selective molecular beam and coupled laser experiments can provide the necessary physical information. Using standard Boltzmann formalism, the height of the activation barrier for adsorption can nevertheless be estimated from the temperature dependence of the hydrogen uptake.

3.4.1.3.1.2 The initial sticking probability

The initial sticking coefficient of hydrogen s0 reflects the specific energy accommodation and dissipation properties of a given hydrogen - surface interaction system. In the zero-coverage limit (Θ → 0) one actually considers the sticking probability of the first impinging hydrogen molecule. The energy accommodation can occur either by direct coupling of the respective molecule to the surface phonons of the heat bath of the solid or (in case of metals) by excitation of electron - hole pairs right at the Fermi level (electronic friction) [77Kno, 80Sch1, 82Sch, 97Men, 99Nie]. In a simple approximation, the impinging event is considered as a binary elastic collision between a gas particle of mass m and initial energy Ei and a fixed surface atom of mass M. Applying the rules of energy and momentum conservation, the amount of transferred energy, ∆E = Ei − Ef is described by the classical Baule expression [14Bau] (µ being the mass ratio m /M):

∆E =

4µ

(1 + µ )2

Ei

(5)

The respective energy is then used to heat up the phonon bath of the solid. It has been argued that with hydrogen as the lightest molecule (m 0.8 0.06...0.1 0.06 0.19 0.01 0.9…1.0 0.96 0.87 0.1

T-dependent reconstruction activated adsorption T-dependent reconstruction activated adsorption SIMS study

T-dependent reconstruction T-dependent reconstruction T-dependent reconstruction expts. performed with deuterium ~0.55 MB studies with nozzle beams of different energies; mixture of activated and non-activated sites absolute (volumetric) 0.15 ± 0.05 measurement 0.05 activated adsorption 0.1 expts. performed with deuterium ; activated adsorption 1.5 ML including subsurface H

β2+β1 = surface H ≈1.5 ML ; α2 = 0.5 ML subsurface, α1 = absorbed H, >1 ML 1 ML

98Oku

74Con

83Cat1 83Cat2 population of subsurface 83Beh sites isothermal desorption expts. performed with deuterium

1 ML total at 37 K

β2 β1 α (subsurf.) β2 β1 α (subsurface) s (β2) 315 K rs (β1) 280 K ss 212 K v (α) 170 K

[Ref. p. 111

88He2

88He1 subsurface state population LEED expts. under stat. H2 pressure isosteric heat (equilibrium) measurements TDS analysis; data taken at 190 K STM study

73Chr1 74Con 76Con1 03Mit1

> 2 ML total at 90 K

expts. performed with deuterium; TDS and equilibrium expts.

98Mus

1 ML total at 120 K

TDS analysis

99Far

H adsorption strongly 98Fri dependent on adsorption temperature (s = surface, rs = surface after lifting of (1×2) reconstr. ss = subsurface, v = bulk) TD expts. after exposure 04Kol to H (D) atoms; D2 - TD spectra more reproducible Landolt-Börnstein New Series III/42A5

Ref. p. 111]


Surface

Initial adsorption energy [kJ/mol]

Ag(110)

41.7 ± 7 28.9 ± 3 23.0 ± 1.2 (H2) β2 28.0 ± 1.2 (D2) β1 43.6

Ag(111) Ag(111)

States and their coverage dependence E(Θ ) β2 β1

Ag(111) Ag(111)

26.8 ± 0.6 (D2)

W(100) 135.2 (H) 136.5 (D) 110.0 ± 1 (H) 111.4 (D)

β2 β1

W(100) W(100) W(100)

167.5 83.7

β2 β1

W(100) W(110)

136.1 113 ± 5

β2 β1

W(110) W(110) W(110)

146.4

W(111)

153.2 ± 6 127.3 ± 5 90.85 ± 3 59.0 ± 2

W(111)

129.7 ± 10 104.6 ± 10 79.5 ± 7 50.2 ± 5

W(111) W(211)


146.5 67

β2 β1 β2 β4 β3 β2 β1 γ β4 β3 β2 β1 β2 β1

Remarks

Ref.

TD expts. after exposure 93Spr to H atoms TD expts. after exposure 89Zho to H atoms

0.6 (± 0.1) ML at 100 K

Ta(110)

W(100)

Maximum number of H atoms adsorbed [H at/m2] or [ML]

41

low coverages sat’n = 1.0 ± 0.3 ML ca. 1 ML surface species; > 1 ML bulk uptake total 2.0 × 1019 H at./m2 at 300 K (= 2 ML) 2.5 × 1018 molec./m2 (0.85 ± 0.08 at 77 K) 5 × 1018 molec./m2 total 2.0 × 1019 H at./m2 at 300 K (= 2 ML) total 1.9 (± 0.3) × 1019 H at./m2 0.5 1.5; total 2 ML 2 × 1019 H at./m2 (2ML) at 100 K total 0.60 ± 0.09 at 77 K total 9.1 × 1018 H at./m2 at 135 K total 1.5 × 1019 H at./m2

expts. performed with D atoms (data of [89Zho] re-examined) TD expts. after exposure to H atoms TD after exposure to D atoms TDS expts.

90Par2

95Lee 95Hea 93Hei 66Est

TDS analysis

69Tam

King&Wells method [72Kin] LEED analysis

73Mad 80Kin

TDS analysis

84Hor

Infrared expts.

89Rif

TDS analysis

71Tam

TDS analysis

74Bar 81Hol

TDS analysis; ∆Φ measurements; isothermal desorption total 1.65 ± 0.2 ML at 77 K TDS analysis

97Nah2

total coverage = 6 × 1018 H TDS analysis at./m2

72Mad 75Sch

total 9.4 × 1018 H at./m2 4.0 × 1018 4.6 × 1018; 8.6 × 1018 H at./m2 total at 110 K

74Bar 73Rye 73Car

total coverage = 1 ML = 1.42 × 1019 H atoms/m2

TDS analysis TDS analysis

71Tam

42


Surface

Initial adsorption energy [kJ/mol]

Re(0001)

83.7

Re(0001)

134

Re(10−10)

125 80 60 111.5 (TDS) 98 ± 14 (equil. data) 85

Ir(100)-(1×1)

Ir(100)-(5×1) Ir(100)-(5×1) Ir(110)-(1×2) Ir(111)

91 ± 12; 75 ± 12 96.3 71.2 52.8

Pt tip (111), (110), (100) orientation Pt(100)-hex

67

Pt(100)-hex

62.7...66.9

Pt(100)-hex Pt(100)-hex

63.2

Pt(100)-hex

49

Pt(100)-hex

49.5 57.3 82.4

Pt(100)-hex

49


Remarks

Ref.

4 × 1018

TDS analysis

81Duc

19

1.63 × 10 (= 2 ML)

total at 120 K

TDS analysis

90He

TDS analysis

95Mus

β α

1 ML total at 200 K

MB study using D2

98Ali

β α state “C”

1.22 ML at 200 K

MB expts. performed with D2 [98Ali]

98Ali 80Ibb 00Mor

β2 β1 β2 with lowT shoulder

2.2 (± 0.2) × 1019 total at 130 K

TDS analysis; E(Θ ) decreasing with coverage TDS analysis TDS lineshape analysis; expts. performed with D2 TDS analysis

87Eng

β2 β1

Ir(111)

Pt(110)-(1×2)

States and their coverage dependence E(Θ ) β with low-T shoulder β β2 β1 α

[Ref. p. 111

1.57 × 1019 total at 100 K 1 ML total at 90 K

field emission measurements β2 4.1 × 1018 total at 135 K (= 0.63 ML) β1 shoulder α3 α2 α1 one state + 4.6 × 1018 total two substates 1.2 × 1019 total at 120 K peak “1” 1.5 × 1019 total peak “2” 1.55 × 1019 total at 150 K α3 γ3 γ2 γ1 a1 a2 a3 b γ1 γ2 γ3 α1 α2 α3 β2 β1

1.2 × 1019 total (= 1ML)

4.2 × 1018 total at 125 K (= 0.47 ML)

80Ibb

99Hag; 96Lau 69Lew

TDS analysis

74Lu

TDS analysis

75Net

nuclear reaction analysis 80Nor1 93Klo

TDS analysis

MB study TPD features complex because of T- and H coverage dependent reconstruction TPD features complex because of T- and H coverage dependent reconstruction TPD features complex because of T- and H coverage dependent reconstruction

95Dix

TDS analysis

74Lu

91Pen1

95Pas


Ref. p. 111] Surface

3.4.1 Adsorbate properties of hydrogen on solid surfaces Initial adsorption energy [kJ/mol]

Pt(110)-(1×2)

Pt(110)-(1×2) Pt(111)

73.3

Pt(111)

39.3 26.8

Pt(111) Pt(111)

65.3 ± 2

Pt(111)

71

Pt(111)

67 ± 7

Pt(111) Pt(111)

79.5 ± 8

Pt(211) Au(100)-(5×20)

States and their coverage dependence E(Θ ) β2 β1 α β2 β1 β β2 β1 β2 β1


Remarks

43 Ref.

89Ang2

1.7 ML (± 10%)

92She 4.1 × 1018 total at 125 K (= 0.55 ML) ~1 ML

TDS analysis

74Lu

isosteric heat data; TDS analysis

75Chr 77Col1

1 ML

MB study; activated adsorption, barrier height 6.3 kJ/mol determined by He diffraction (isosteric heat) isosteric heat data using D2; coverages determined by nuclear reaction analysis He diffraction study

β2 β1 β2 β1 β

1 ML (= 1.5 × 1019)

TDS analysis

88God

3.3 × 1018 total at 125 K (= 0.43 ML) 0.3 ML at 100 K

TDS analysis

74Lu

exposure to H atoms at 100 K exposure to H atoms at 100 K exposure to H atoms at 96 K

96Iwa

Au(110)-(1×2)

51 ± 4

β

0.5 ML

Au(110)-(1×2)

45 ± 4 31 ± 2

β α

>0.5 ML

79Sal

80Poe 82Nor1

83Lee

86Sau 97Luh

Semiconductor and Insulator Surfaces Surface

states and their coverage dependence E(Θ ) terrace site C(0001) 57.9 (H2) terrace site graphite (HOPG) 91.6 (D2) Si(100) 238.5 β1 196.6 β2 257 ± 20 Si(111)-7×7 β1 state at (D2 = 247 ± 13) 870 K

Ge(100) Ge(111) GaAs(001)


desorption energy [kJ/mol]

145 ± 10 60 ± 8

β state

maximum number of H atoms adsorbed [H at/m2] or [ML]

Remarks

Ref.

~0.5 ML ~0.5 ML 1.5 ML

TDS analysis

02Zec

TDS analysis

93Flo

Si - H (D) bond energies of exposure to H and D 346 (341) kJ/mol; H sat’n atoms, Laser-induced thermal desorption coverage = 1 × 1019 m−2 expts. TDS As - H bond energy 289, Ga - H bond energy 259 kJ/mol

88Koe

84Sur 95Qi

44


[Ref. p. 111

3.4.1.3.4 The diffusion of adsorbed hydrogen

Surface diffusion is an important process in the interaction of hydrogen with solid surfaces in that it often governs the rate of adsorption and desorption, determines the formation of phases with long-range order and, of course, decisively affects the rate of catalytic reactions involving transfer of H atoms or H2 molecules. Due to the limited space we will exclude bulk diffusion phenomena from our considerations, although certain metals such as Pd, V, Ti, Zr, Nb, Ta etc. can under appropriate thermodynamic conditions absorb large quantities of hydrogen which makes these materials interesting for hydrogen storage. For details on this subject as well as on a general formal description of diffusion phenomena, the reader is referred to the respective monographs and textbooks [65Jos, 78Ale]. We recall that especially Pd surfaces exhibit a variety of phenomena which involve diffusion steps, overlayer - underlayer (surface subsurface) transitions and absorption/hydride formation processes. Pioneering field emission work, focussing to a large extent on H surface diffusion, was performed in Gomer’s laboratory [57Wor, 61Gom, 90Gom]. Morris et al. [84Mor] and Naumovets and Vedula [84Nau] reviewed the state of surface diffusion until the mid-eighties (including a description of experimental methods). More recent compilations deal with single adatom diffusion phenomena [94Ehr] or with the mechanisms of surface diffusion processes in general [02Nau, 02Ros]. A historical review is provided by Antczak and Ehrlich [05Ant]. Until the nineties, direct observation of surface diffusion by field emission techniques (either by watching the propagation of diffusion fronts or by an analysis of field emission fluctuations [82DiF]) was by far the most frequently applied and effective technique. Only in recent decades additional powerful methods were developed. In 1972 Ertl and Neumann introduced the laserinduced thermal desorption technique [72Ert], which was then further improved [86See, 86Mak1, 87Mak1, 87Mak2]: This method is based on the ‘hole refilling’ phenomenon: A focused laser beam illuminates a well-defined patch on the surface with an energy just sufficient to thermally desorb all the particles in that area. The refilling of the hole from the unperturbed surrounding is then followed as a function of time by subsequently fired laser pulses. The refilling signal is then fitted to expressions derived from Fick’s second law. However, this technique in its simple form bears some disadvantages; among others, it is difficult to deduce directional and coverage dependencies [92Man]. Mak and George have published a simplified method to determine the coverage dependence of surface diffusion coefficients [86Mak2]. In the nineties, optical diffraction of laser beams [92Zhu, 97Cao], He atom scattering [99Gra], or scanning tunneling microscopy [96Zam, 96Tro, 97Win] were used to follow surface diffusion. A real breakthrough was achieved by applying the STM techniques: A direct counting and subsequent statistical analysis of the number of migrating N (O) atoms on a Ru surface as a function of time revealed much insight into the principal surface hopping, diffusion, and lateral ordering phenomena at and around room temperature. However, in order to watch diffusing hydrogen atoms with their much larger diffusion rate, considerably lower temperatures are necessary; a possible solution is provided by performing STM observations in combination with inelastic electron tunneling (IETS) in a 4 K-STM [97Sti, 98Sti]. For more details about this exciting technique and its application to hydrogen adsorption systems, the internet site http://www.physics.uci.edu/~wilsonho/stm-iets.html is recommended for reading. As was first convincingly shown by Gomer, the diffusion of H (D, T) atoms can be subdivided into ‘classical’ diffusion (with discrete thermally activated hopping events) and quantum diffusion in which the light H (D, T) atoms behave as wave-like quantum particles and propagate by tunneling processes, without any thermal activation barrier [80DiF, 82DiF]. This latter behavior becomes immediately evident, if one follows the temperature dependence of the diffusion coefficient D(T ) [87Aue]. The temperature dependence of the classical surface diffusion is commonly described in the form of an Arrhenius equation § Ediff D(T ) = D0 exp ¨¨ − © kT

· ¸ ¸ ¹

(8)

with D0 = pre-exponential factor [cm2 s−1], and Ediff = activation energy for diffusion [kJ/mol] which corresponds to the lateral hopping barriers between adjacent adsorption sites. For stationary diffusion, D(T ) can be expressed from Fick’s first law as the ratio of the particle flux through the concentration front and the actual concentration gradient at time t. Likewise, the diffusion progress is described by the Landolt-Börnstein New Series III/42A5

Ref. p. 111]


45

mean square displacement of a particle, according to Einstein’s equation, which also contains the diffusion coefficient: x2 =

2 Dt

(9)

Another frequently used expression is based on random walk events between fixed sites and combines the pre-exponential factor D0, the jump length a and the vibrational frequency parallel to the surface, ν, via D0 =

1 2 aν. 4

(10)

Of course, depending on the surface structure and corrugation, there may exist ‘easy’ and ‘difficult’ pathways for diffusion; hence, the diffusion coefficient is usually strongly direction-dependent. Most of the experiments focus on a determination of the activation energies for diffusion and the diffusion coefficients, whereby, as mentioned above, the ‘classical’ regime must be delineated from quantum diffusion. As a rule of thumb, the activation energy of diffusion is between one fifth and one tenth of the depth of the chemisorption potential. One can define the lifetime Ĳs of a particle adsorbed in a specific site s on the surface; it is related to the diffusion energy by the expression §E

·

τ s = τ s ,0 exp ¨¨ diff ¸¸ © kT ¹

(11)

Apparently, the particle’s residence time in a certain site depends sensitively on the thermal energy (temperature) of the surface; Ediff 10 kT means actually immobile particles, whereas the case Ediff < kT enables a free motion of the adatoms across the surface. The space limitations do not allow us to further expand on both experimental and theoretical investigations on hydrogen diffusion. There exist numerous theoretical articles dealing with diffusive H motion on surfaces, most of them focusing on the interesting non-thermally activated quantum tunneling processes [85Fre1, 98Bae]. In the following table we have compiled some diffusion coefficients and diffusion energies for a variety of hydrogen adsorption systems. As can be seen, there are not too many hydrogen adsorption systems that have been investigated; a strong preference exists for tungsten(110) which has been scrutinized in Gomer’s laboratory [57Gom, 80DiF, 82DiF, 84Wan, 85Tri, 85Wan, 86Tri1, 86Tri2, 87Aue]. Surface

Ni(100) (tip)

Temperature Diffusion range coefficient D0 [cm2/s] [K] 240...300

Ni(100)

223...283

Ni(100)

211 236 263 140…250

Ni(100)

< 140


3 × 1013 s−1 preexponential factor for hopping frequency 2.1 (± 0.2) × 10−7 7 (± 0.2) × 10−7 1.5 (± 0.3) × 10−6 8 × 10−6 (Θ-indep.) (H) 2 × 10−5 (low Θ )(D) 2 × 10−4 (high Θ )(D) 10−12 (H, D)

Experimental method and remarks

Reference

field electron emission, front diffusion laser-induced thermal desorption

57Wor

17.6

laser-induced thermal desorption of D atoms

86Mul

13.4 (H)

field emission fluctuation technique

91Lin

Diffusion energy Ediff [kJ/mol] 29.3 ± 4 16.7 ± 2

15.1 (D), with little Θ dependence quantum tunneling ~0

85Geo

46 Surface

Ni(100)

3.4.1 Adsorbate properties of hydrogen on solid surfaces Temperature range [K] 170...200 120...170

Ni(111)

13...20

Ni(111)

140...250

Ni(111)

< 140 110…240 65…110

Cu(100)

Ru(0001)

65...80 (classical thermal diffusion)

Diffusion coefficient D0 [cm2/s] 1.1 × 10−6 (H) 5 × 10−5 (D) 1.5 × 10−9 (H) 9 × 10−10 (D)

0.84 2.8 × 10−4; hopping frequency =3 × 1012 s−1 3 × 10−4 (low Θ ) (H) 12.5 (low Θ ) (H) 7 × 10−2 (mid Θ ) (H) 16.7 (high Θ ) (H) 14.2 (low Θ ) (D) 18.4 (high Θ ) (D) ~10−10 0 18.9 (H) 2.8 × 10−3 (H); 21.0 (D) 3.4 × 10−3 (D); 10.1 (H) 2.4 × 10−7 (H); 1.6 × 10−8 (D) 10.1 (D) hopping frequency 19.0 ± 0.4 (H) 18.7 ± 0.4 (D) ν = 1012.9 s−1 (H) ν = 1012.7 s−1 (D)

9 < T < 60

10−19

260...330

6.3 × 10−4at Ĭ = low 16.7 ± 2

230...270

7.9 × 10−4 Ĭ-independent

Rh(111)

150...300

8 × 10−2 for 0.02 < Ĭ < 0.4

Rh(111)

186...216

W(100)

>220 (activ. regime) 140...220 220 >1.00

LEED

at 300 K

74Chr

0.33 0.33 0.50 0.67 0.83 1.00

LEED

metastable metastable metastable metastable metastable metastable

84Pen

LEED

pairing-row reconstr. below T = 220 K

1.5

160

90Nic 91Nic2

LEED

Θ (1×2)-2H has been

2.0 = 1.96 19 × 10 >100

Rh(110)

(1×n); n = 1, 2, 3) (1×3)-H

0.33

LEED analysis

Rh(110)

(1×3)-H

0.33

Rh(110)

(1×2)-H

0.50

He 1.76 diffraction LEED 1.87 ± analysis 0.1

Rh(110)

(1×2)-H

0.5

Rh(110)

(1×2)-3H

1.0

Rh(110)

(1×1)-2H

2.0

Rh(110)

89Mic

corrected to contain 3 H atoms/unit cell: → Θ (1×2)-3H = 1.5 HREELS 1.86 ± 0.1

He diffraction LEED 1.87… analysis 1.93

LEED analysis

quasi-3fold 3-fold (two 1st and 1 2nd layer Rh atom) quasi-3fold quasi 3fold; H atom radius = 0.53±0.1 Å 3-fold 3 non-equivalent quasi-3fold sites

94Mue slight local reconstruction (shiftbuckling); H atom radius = 0.52 ± 0.1 Å

89Leh

91Par H-induced shiftbuckling reconstruction

89Puc

H - Rh top layer 90Par1 distance = 0.82±0.1 Å H-induced 89Mic reconstruction; H atom radius = 0.5 ± 0.1 Å Rh’s multilayer relaxation almost entirely removed by adsorbed H

87Nic


Ref. p. 111]

3.4.1 Adsorbate properties of hydrogen on solid surfaces Criti- Experical mental temp. method [K] LEED analysis He diffraction LEED HREELS

Surface

Ordered H phase

Rh(110)

(1×1)-2H

Coverage [ML] and /or [H atoms/m2] 2.0

Rh(110)

(1×1)-2H

2.0

Rh(111) Rh(111)

(1×1)-H (1×1)-H

1.0 1.0

Rh(111)

(1×1)-H

1.0

Rh(111)

(1×1)-H

1.0

Rh(311)

(1×3)-H (1×2)-H (1×3)-2H (1×2)

0.33 0.50 0.67 ~1.0 = 8.34 × 1018

90 250

(1×3)-H (1×2)-H (1×3)-2H (1×2)-2H rec c(1×1) c(1×3) cp(1×1) c(1×3) p(1×1)rec p(1×1)rec c(1×1)rec

0.33 0.50 0.67 ~1.0 1.0

185 ± LEED 5

Rh(311)

Rh(311)

Rh(311)

c(2×2) (1×1)-H

Pd(100)

c(2×2) (1×1)-H

Pd(100)

c(2×2) p(1×1) c(2x2) p(1x1) (2×1)-2Hp2mg (1×2)-3H (1×2)-streak

0.5 1.0 0.5 1.0 ± 0.1 1.0 1.5 >1.5

Pd(110)

(1×2)-3H

1.5

Pd(110)

(1×2)-3H

1.5

Pd(110)

(2×1)-2Hp2mg (1×2)-3H (1×2)-streak

1.0 1.5 >1.5

Pd(100) Pd(110)


260 0.5 1.0= 1.32 19 × 10 0.5 1.0 ± 0.1

180

120

Coordination

Remarks

Reference

quasi-3fold quasi-3fold

H atom radius = 0.50 ± 0.2 Å

88Oed

quantum motion of H atoms assumed phonon dispersion curves measured Tad = 160 K

He diffraction He diffraction LEED

quasi 3fold

He diffraction LEED

transmission channeling He diffraction HREELS

quasi 3fold hollow

4-fold hollow 1.97 2.00

H-induced reconstr.; (1×2) cannot be saturated; population of subsurface sites complete phase diagram for H/Rh(311) det’d c(1×1) = 1st ordered phase → s0 = 1

phase diagram determined expts. performed with deuterium

2.05 2.00

95Wit 96Col

93Ham2

95Ape1

95Ape1 95Ape2 95Ape2 96Ape 80Beh

87Bes

84Rie 4-fold hollow

82Nor1 83Nyb 83Cat2

LEED

Ion scattering LEED analysis LEED

91Par 91Kir 78Cas 86Mat

91Nic1 90Lie2

He 1.92 ± diffraction 0.1

3...4 2...3 1.0

Pd(100)

H-substrate bond distance [Å] 1.84 ± 0.2 1.85 ± 0.1

57

H-induced reconst. H-induced reconst. formation of subsurface H H-induced pairing-row reconstruction row-pairing reconstruction

H-induced recon. formation of subsurface H

83Beh 86Nie 87Kle1 87Kle2 88He1

58

3.4.1 Adsorbate properties of hydrogen on solid surfaces Criti- Experical mental temp. method [K] He diffraction

Surface

Ordered H phase

Pd(110)

(2×1)-2H

Coverage [ML] and /or [H atoms/m2] 1.0

(1×2)-3H

1.5

Pd(110)

(2×1)-2H

1.0

LEED analysis

Pd(110)

(2×1)-2H

1.0

HREELS

(1×2)-3H

1.5

Pd(110)

(2×1)-2H

Pd(110)

(2×1) + (1×2)-3H (1×2)streak

1...1.5 ML

(1×3) (1×2)-MR (1×1)-H

lower higher Θ 1.0

Pd(110) Pd(111) Pd(111)

Pd(111)

Pd(111)

Pd(111)

Pd(210)

H-subCoordistrate nation bond distance [Å] quasi 3fold

2.00 ± 0.1

HREELS 150

STM

300

(√3×√3)R30° 0.33 -H (√3×√3)R30° 0.67 -2H (√3×√3)R30° -H (√3×√3)R30° -2H (1×1)-H (√3×√3)R30° -2H

0.33

(√3×√3)R30° -H (√3×√3)R30° -2H+ (1×1)-H no H superstructure

0.33

300

STM LEED

85

LEED

105

He diffraction

0.67 1.0 0.67

82

LEED analysis

1.78 1.80

Remarks

Reference

H-induced recon. formation of subsurface H PR H-induced rec. + subsurface H H atom radius = 0.6 ± 0.1 Å

83Rie3

quasi 3fold: two 1st layer atoms, one 2nd layer atom quasi-3fold quasi 3fold quasi-3quantum-delocalized H fold pairing-row reconstruction missing/added-row reconstruction evidence of H-induced missing-row reconstr’s expts. performed at 300 K 3-fold + phase diagram octadetermined; hedral subsurface site sub-surpopulation face sites (theory) 3-fold quantum delocalization of H assumed

3-fold fcc

STM

3-fold

LEED

sites with three different coordinations: A, B, C

0.67 1.0 3...4

[Ref. p. 111

partial occupation of single 3-fold hollow fcc site + up to 60% subsurface (octahedral) sites H adsorption, diffusion and ordering followed by direct observation population of surface + subsurface sites

87Sko

89Ell

96Tak 95Yos

96Kam 73Chr1 85Fel 86Fel 87Daw

91Hsu

89Fel

03Mit1 03Mit2

98Mus1



Ordered H phase

Pd(210)

no H superstructure

Pd(311)

no H superstructure (2×1)H (2×1)2H (2×1)3H c(1×1)2H

Ag(100)

diffuse (2×2)

Ag(110)

sequence of ordered phases; c(4×4) at sat’n

3.4.1 Adsorbate properties of hydrogen on solid surfaces Coverage [ML] and /or [H atoms/m2] 3...4

Criti- Experical mental temp. method [K] LEED

1 ML

1.3


470 750 1100 1450 452 1226...50 444 (565) 887 1250 1613 258 444 (508) 887 1250 1613 290 (218) 444. 492, 1250 1613 323 484 710 927 1290 1613

58 93 136 180 56 152 (155) 55 (70) 110 155 200 32 55(63,70) 110 155 200 36 (27) 55, 61, 72 155 200 40 60 88 115 160 200

Refe.

94Mue

[1−10] and [001]; excitation strongly dependent on impact energy due to surface resonances

ν 1 (A1+B1 symmetry) ν 2 (B2 + A2 symmetry) ν 3 (A1+B1 symmetry)

combin. mode (=ν1 + ν3) combin. mode (=ν 1 + ν 2) overtone (2 ν 3) combin. mode (=ν 2 + ν 3) trans. from ground-state band to 1st excited (E) band trans. from ground-state band to A11 (750 cm−1) and

A12 (1050 cm−1) bands 0.6...1.0

Remarks

77

450 cm−1 equivalent mode not observed with D! strong evidence of protonic band motion: hydrogen ‘fog’

86Mat

due to the ‘open‘ geometry, a complicated vibrational scenario arises; three H species I, II and III can be distinguished with increasing exposure

97Far

transitions from ground-state to excited protonic bands

species I species I + II

species I + II + III

species I + II + III

78 Surface

Pd(100)

Pd(110)

Pd(110)

Pd(111)

Pd(210)

Pd(311)

Ag(110)

3.4.1 Adsorbate properties of hydrogen on solid surfaces H coverage [ML] at T [K] 0.08 ML

Observed frequencies or loss bands [meV] [cm−1] 486 60.2

0.59 ML (c(2×2)) 0.96 ML ((1×1)-H) 1 ML (= (2×1)-2H phase) 1 ML ((1×2)-3H phase)

502

62.2

512

63.5

790 968

98 120

Ag(111)

Remarks

Refe.

symmetric Pd - H stretching vibration (H in a 4-fold hollow site)

expts. performed at 80 K

82Nyb 83Nyb

expts. performed at 100 K

89Ell

expts. performed at 90 K; loss peak positions shift with coverage expts. performed at 120 K; evidence of strong H-induced surface resonances expts. performed at 120 K HREELS insensitive to subsurface H expts. performed at 120 K; activated population of subsurface sites. Octahedral subsurface sites identified

96Tak

0.1 0.1 ML 1291 (broad)

24

Θ < 0.9

379...484 645...669 1347 379...484 669 936 1347 645 903 1000 1347 590

47...60 80...83 167 47...60 83 116 167 80 112 124 167 73

(2025)

(251)

1.5 (c(2×2)3H) 2.0 (1×1)-2H Ir(110)-(1×2)

Remarks

Refe.

symmetric stretch of H in 2fold (bridging) site 1267 157 symmetric stretch of H in atop sites (?). A later reconsideration suggested H in sites with higher coordination (2-fold and/or quasi-3-fold) 113 14 substrate phonon specular and off213 26 specular experiments ½ 539...550 67...68 at 110 K with ¾ 3-fold hollow site likely ¿ 768...774 95...96 momentum 1252...56 155...156 perpendicular W - H mode resolution; emphasis on the phonon substrate phonon 214 27 dispersion curves. 621 77 W - H vibrational ½ 736 91 modes for three ¾ 3-fold hollow site(s) ¿ 884 110 coverages were perpendicular W - H mode 1222 152 followed through k perpendicular W - H mode 1327 165 space. (two kinds of H species)

p(1×1)

W(111)

Vibrational mode assignment and coordination

[Ref. p. 111

105 161 160

substrate phonon

sharp W - H mode H in sites with higher coordination (2-fold and/or quasi-3fold) Re phonons H species 1 (C2v symmetry) H species 1 + species 2 (Cs symm.)

77Bac

80Jay 94Bal 96Bal

In the (1×1) phase, H is adsorbed in a twodimensional quasi liquid-like phase

77Bac 80Jay expts. performed at 120 K in two perpendicular directions of scattering plane

98Mus2

H species 3 (Cs symmetry)

H in a quasi-3-fold site

expts. performed at 88Cha 170 K note: a strong vibration around 2025 cm−1 – compare Hagedorn’s study [99Hag]) – was observed, but assigned as Ir - C=O stretching mode caused by slight CO impurities



Ir(111)

Ir(111)

Pt(110)(1×2)

Pt(111)

3.4.1 Adsorbate properties of hydrogen on solid surfaces H coverage [ML] at T [K] medium Θ

Observed frequencies or loss bands [meV] [cm−1] 560 69 (2025)

(251)

0.44 ML low Θ

2030 790 1137 1549

252 98 141 192

high Θ (sat’n)

540 669 1202

67 83 149

550 1230

68 153

low Θ and high Θ (up to 0.7 ML)

Pt(111)

1 ML = 540 1.49 × 1019; 903 well1234 ordered (1×1) phase

67 112 153

Pt(111)

Θ ≤ 0.75

31 49 broad 68

ML

Pt(111)

250 395 548

Θ =0.8...1.0 548 ML

911

68 113

1234

153

81


Remarks

H in a 3-fold site

expts. performed at 88Cha 170 K note: the observed strong vibration around 2025 cm−1 was assigned as Ir - C=O stretching mode expts. performed at 99Hag 90 K

excitation into delocalized protonic bands Ir - H mode (terminal site) ? symmetric stretch parallel mode perpendicular to rows (H in 2-fold bridge at bottom of missing-row trough)

symmetric mode locally 3-fold sites at edges of (1×2) rows symmetric Pt - H stretching asymmetric Pt - H stretching (vibration perpendicular and parallel for H in a 3-fold coordinated site) asymmetric Pt - H stretching symmetric Pt - H stretching overtone + combination loss 0→2 ν asy and (ν asy + ν sym ) (unresolved) bands are due to transitions between protonic bands of delocalized H atoms

symmetric stretching mode hybride mode with both inplane and dipole character symmetric stretching mode (H in the 3-fold hollow (fcc) site)

Refe.

91Ste besides the (1×2)MR reconstruction a (1×4) reconstructed surface was observed

expts. performed at ≥90 K; specular and off-specular to distinguish dipolar and impact contributions expts. performed at 85 K and 170 K; evidence for soft parallel modes

79Bar 84Say1 84Say2 87Ric2

expts. performed at 02Bad 85 K. At lower Θ, the bands at 112 and 153 [87Ric2] are not detected. Strong evidence of H atomic band structure (delocalized H motion) expts. performed at 03Bad 85 K

3.4.1.3.6.2 Semiconductor and insulator surfaces

The vibrational properties of H-covered semiconductor surfaces, silicon in particular, have been extensively studied since the early days of high-resolution electron-energy loss spectroscopy. An overview of this early work is given by Froitzheim [77Fro]; further aspects of the HREELS technique and its application to semiconductor studies (excitation of phonons etc.) can be taken from Ibach’s monograph [82Iba]. The status of hydrogen interaction with elemental and compound semiconductors up to 1986, mainly in the view of vibrational loss spectroscopy, is given in a report by Schaefer [86Sch]. In more Landolt-Börnstein New Series III/42A5

82


[Ref. p. 111

recent years, also other techniques, mainly infrared spectroscopy, second harmonic generation or sum frequency generation spectroscopy have increasingly been exploited to determine the vibrational bands of interest. In the nineties, the upcoming technological interest in diamond films, coatings and tools has motivated a whole wealth of studies into the properties of H-covered Cdia surfaces, and the degree of hydrogenation, surface roughnesses etc. were frequently investigated also by HREELS. For space limitations, however, only some of the more important data are listed in the subsequent table. Surface

C(111) (diamond) C(111) (diamond) C(111)-1×1 (diamond)

C(100)-2×1 (diamond)

C(100)-1×1 C(111) (diamond) C(100)-2×1 (diamond)

C(100)-2×1 (diamond)

C(0001) HOPG graphite

Si(100) Si(111)-7×7

Si(100)-2×1

H coverage [ML] at T [K] 1 ML at 300 K

Observed frequencies or loss bands [cm−1] [meV] 1290 160 2903 360


Remarks

Reference

C - H rocking mode C - H stretching mode

85Pat

1 ML (1×1) H terminated terminated with methyl (CH3) groups

2830

351

stretching vibration of C - H bonds with top C atoms

1000 to 1450

124 to 180

2839

352

1000 to 1450

124 to 180

mixed modes of C - H bending vibrations and/or substrate phonons C - H stretching mode of sp3 hybridized bonding mixed modes of C - H bending vibrations and/or substrate phonons

atomic hydrogen exposure on as polished C crystals Infrared-visible sum frequency generation (SFG) exposure to H plasma; impact scattering important

2928 650 to 1690

363 C - H stretching vibration 80 to 210 phonon modes and C - H bending vibrations

terminated with C - H bond (monohydride) H terminated

2930 1250 2440 2920 3600 H 823 terminated 968 1097 1202 1258 2903 2919 sat’n 1210 coverage at 2650 300 K 640 (0.5 ML) 1950 850 1580 H630 terminated 2080 H630 terminated 900 1 ML (H saturated)

1 ML (monohydride)

2080 2087.5 2098.8

363 155 303 362 446 102 120 136 149 156 360 362 150 329 79 242 105 196 78 258 41 112 258 259 260.2

C - H stretching vibration C - H bending mode overtone of 1250 cm−1 mode C - H stretching mode overtone, multiple losses phonons off-specular only phonons off-specular only phonons phonons symmetric and antisymmetric C - H stretching modes C - H bending mode C - H stretching mode C - D bending mode C - D stretching mode surface phonon surface phonon monohydride bending (scissor) mode monohydride stretching vibr. dihydride wagging mode dihydride bending (scissor) mode monohydride stretching vibr. Si - H stretching modes

92Chi

93Aiz

exposure to H plasma; impact scattering important

93Aiz

exposure to H plasma

93Lee

exposure to H plasma

94Tho

exposure to H atoms, HREELS study

03Tha1 03Tha2

exposure to H (D) atoms

02Zec

exposure to H atoms 84But at 500 K exposure to H atoms 84But at 140 K

exposure to H (D) atoms; surface IR study

84Cha3



Si(100)-2×1 Si(100)-1×1

Si(100)

Si(100)-2×1

Si(111)

Si(111)-2×1 (7×7)

Si(111)-7×7

3.4.1 Adsorbate properties of hydrogen on solid surfaces H coverage [ML] at T [K] monohydride phase dihydride phase 1 ML (1×1) H terminated

ca. 1 ML

less than 1 ML

ca. 1 ML

0 ...sat’n

Si(111)-7×7

H sat’n

Si(111)-2×1

H sat’n ĺ (1×1) LEED phase


83

Observed frequencies or loss bands [meV] [cm−1] 645 80 2097 260


Remarks

Si - H wagging mode Si - H stretching mode

exposure to H atoms 86Sch

645 915 2097 400 490 520 650 910 2105 2084 2094

80 113.5 260 50 61 64.5 80 113 261.5 258.4 259.6

Si - H wagging mode Si - H3 scissor mode Si - H stretching mode

2104 2114 2127

261 262 263.7

2089

259

630

78

900

112

2100 460; 919 968 2258

260 57; 114 120 280

637 879 2080 (smaller expos.) 637 879 2077 (medium exp’s) 2097 (large exp’s) 637 887 2089 613 806.5 2073

79 109 258

Si - H bending mode Si - H2 scissor mode Si - H stretching mode

79 109 257.5

Si - H bending mode Si - H2 scissor mode Si - H stretching mode

260

Si - H stretching mode

79 110 259 76 100 257

Si - H bending mode Si - H2 scissor mode Si - H stretching mode Si - H bending mode Si - H2 scissor mode Si - H stretching mode

possibly several phonon bands

scissor mode of SiH2 Si - H2 stretching vibration parallel component vertical component = symm. stretching mode of monohydride (lower H exposures) dihydride stretching modes trihydride (larger exposures) of occupied dimer phase monohydride stretching mode (only feature at very large exposures) Si - H bending mode SiH2, SiH3 wagging and rocking modes Si - H2 bending mode with bond angle changes Si - H stretching mode phonon + phonon overtone Si - H stretching vibration

Reference

surface prepared by etching in a 40% ammonium fluoride solution

91Dum

exposure to atomic hydrogen (high-resolution surface IR spectroscopy)

99Niw

exposure to atomic hydrogen

81Wag

freshly cleaved surf. 83Fro exposure to H atoms after annealing to 623 K exposure to H atoms 83Kob at 300 K; heating to 650...750 K causes minor shifts in frequency and intensity of the three characteristic vibrational bands

exposure to H atoms 84Fro

exposure to H atoms 86Sch at 300 K. Quenching of Si phonon modes

84 Surface

Si(111)

Si(111)

Ge(100) GaAs(110)

GaAs(001) c(2×8) (1×6) reconstr.

3.4.1 Adsorbate properties of hydrogen on solid surfaces H coverage [ML] at T [K] 1 ML (1×1) H terminated

1 ML (1×1) H terminated (2×1) H terminated 1 ML

1 ML

GaAs(001) • c(4×4)

(1×1)-H

• (2×4)

ĺ (1×4)

• (2×6)

• (4×2)

InP(110)

(1×1)H

InP(110)

~0.5 ML H

Observed frequencies or loss bands [meV] [cm−1] 520 64.5 636 79 795 2085 626.7

98 258.5 77.7

2083.7

258.4

532 1976 1890 (H) 1380 (D) 2150 (H) 1660 (D) 1950 2150 1835 1875

66 245 234 171 267 206 242 267 228 232

1980 2080 1900 2200 1835 1880 2020 2050 2100 2140 1000 1190 1620 1740 2020 2050 2100 2140 1480 1605 1730 1875 1710 2282 339 1678 2266 2339

246 258 236 273 228 233 250 254 260 265 124 148 201 216 250 254 260 265 183.5 199 214.5 232.5 212 283 42 208 281 290

[Ref. p. 111


Remarks

Reference

substrate phonon bending mode of the monohydride

all losses disappear at 750 K due to H2 desorption

91Dum

Si - H stretch mode doubly degenerate bending mode stretching vibration of the surface Si - H bond Ge - H bending mode Ge - H stretching mode Ga - H stretching mode Ga - D stretching mode As - H stretching mode As - D stretching mode arsenic hydride vibrations vibrations due to two terminal Ga hydrides

As-hydride stretching vibrations, As-hydride stretching modes Ga-hydride stretching modes ½ ° ¾ As - H stretching modes ° ¿

transmission IR 02Cau spectroscopy with vicinal H/Si(111) surfaces exposure to H atoms 86Pap exposure to H (D) atoms

81Lue

exposure to H atoms 95Qi between 303 and 433 K. IR reflectance spectroscopy H atom + four 99Hic reconstructed GaAs phases. Profound internal –reflection IR spectroscopy , polarizationdependent; LEED, XPS + STM study

½ ° ¾ Ga - H stretching modes ° ¿ ½ ° ¾ As - H stretching modes ° ¿ ½ ° ¾ Ga - H stretching modes ° ¿ In - H stretching mode P - H stretching mode Fuchs-Kliewer phonon In - H stretching mode P - H stretching mode P - H stretching mode of PH2

exposure to H atoms 86Sch exposure to H atoms 93Pen


Ref. p. 111]


85

3.4.1.3.7 Electronic states of adsorbed hydrogen and photoemission spectroscopy

The geometrical structures of H adsorbate layers listed in sect. 3.4.1.3.3 reflect nothing but the consequences of the electronic (quantum-chemical) interaction of a hydrogen molecule with the respective substrate. The following paragraph(s) will be devoted to this interaction and finally lead up to a (short) presentation of current theories and models to adequately describe this interaction. In section 3.4.1.3.7 we will review data that have basically been obtained by (UV) photoelectron spectroscopy (UPS). UPS actually maps the electronic states of a solid surface and their adsorbate-induced changes occurring in the substrate’s conduction and valence band energy region (0 < E < 50 eV), whereby the interaction of hydrogen with conducting (metallic) surfaces is in the focus of the scientific interest. Section 3.4.1.3.8 then samples the H-induced work function data. A few remarks may be helpful to understand the principles of the quantum-chemical interaction of a H2 molecule with a solid surface; for the sake of simplicity we will consider again metallic surfaces here. In this description, we will largely follow the informative exposition given by Harris [88Har]. When an unperturbed H2 molecule with its large covalent binding energy of 4.7 eV and the comparatively quite close H - H bond distance of 0.74 Å is approaching a metal surface, say, a nickel (100) surface, it is first attracted by a weak van-der-Waals interaction potential. Since the energetically more favorable situation consists in a cleaved H - H bond and the formation of two stable Ni - H bonds (c.f., Eq. (2)) the system faces the difficulty that the typical distances between two metal (Ni) atoms are a factor of ~3 larger (Ni Ni distance = 2.49 Å) than the internuclear spacing in H2, in other words, the proton - proton distance has to be stretched quite considerably to match the Ni - Ni distance and make the two Ni - H bonds. The decisive quantity to be considered here is the (multi-dimensional) potential-energy surface (PES) which determines the transition (and the trajectories) of the incoming H2 molecule to the final equilibrium situation where two adsorbed H atoms exist on the metal surface. The accurate calculation of both the PES and the hydrogen trajectories is among the most prominent tasks of hydrogen chemisorption theory. For the H2/Mg(0001) system, Nørskov et al. [81Nor1, 87Nor] have calculated the one-electron density of states and the total binding energy for a H2 molecule approaching this surface using the effective medium theory [80Nor2, 82Nor3]. As repeatedly mentioned in the previous sections, the shallow van-der-Waals potential is separated from the deep chemisorption well by a more or less pronounced activation energy barrier which must be overcome to reach the chemisorbed state and to minimize the total energy of the combined system. According to Harris [88Har], this activation barrier has the following origin: The unperturbed H2 molecule is a closed-shell entity possessing a filled and very compact 1σg molecular orbital (MO). As it approaches the surface, interference between this 1σg MO and the metallic (sp) wave functions results in the well known Pauli repulsion: The necessary orthogonalization of the involved MOs pulls up their energies. As the molecule gets closer to the surface, this rise in energy continues until, for energetic reasons, a dramatic change in the system’s configuration can take place, namely, the dissociation of the H2 molecule. Since only with transition metals the sp and d wave functions share a common Fermi level, the far-reaching and diffuse metallic sp orbitals (which are especially affected) can avoid the Pauli repulsion by ‘escaping’ to unfilled (but energetically equivalent) d orbitals and thus offer the H2 molecule a trajectory of minimum potential energy and a much smaller activation barrier for dissociation [88Har]. Of course, in a more detailed view the spatial orientation of the H2 molecule relative to the surface must be considered on its way into its adsorbed state, which requires a full dynamical treatment (i.e., inclusion of time-dependent phenomena). Accordingly, an increasing number of calculations taking care of dynamical quantum processes have been performed in recent years; for more details, which are beyond the scope of this presentation, it is referred to the special literature [95Dar, 96Gro]. We simply point to the respective calculations performed by Wilke and Gross who successfully treated the adsorption dynamics of the H2/Pd(100) system as a six-dimensional problem and introduced the dynamical steering effect already mentioned in the introduction [96Wil]. Returning to Nørskov’s treatment of the H2/Mg(0001) system [81Nor1], which gives an idea about possible broadenings and downshifts of the involved hydrogenic MO’s along the reaction coordinate, c.f., Fig. 11, one immediately realizes that the adsorptive bonding of a H atom on a metal usually causes redistribution(s) of metallic electron density and can even lead to the formation of discrete electronic states especially in the


86


[Ref. p. 111

conduction band region: These can be viewed at as localized quantum levels formed by the overlap and, hence, chemical bond, of metallic and hydrogenic (1s) electron states. -1

Density of states [eV ] (B) 0.1

(D) 0.1

(M2)

(M1)

0.1

0.1

(A) 0.1

0

Energy [eV]

vacuum level

Su

eF S u + Sg

0.0 eV

-10

bottom of band

-0.9 eV

Eintra = -3.1 eV Sg

Fig. 11: One-electron density of states (1/eV) for a hydrogen molecule approaching a magnesium(0001) surface along the reaction coordinate. The capital letters in the top indicate extrema on the potential energy surface. From right to left, the practically unperturbed gaseous H2 molecule (P) feels a slight activation barrier (A) for entering the physisorbed state (outer part of the well M1, inner part of the well M2). At the same time, the molecular orbitals (MO) of the H2 molecule 1σg and 2 σ u* begin to downshift and broaden. As the molecule further approaches the Mg surface, a large barrier for dissociation occurs (D), and only after passing this barrier two separated H atoms can exist on the surface in a bridge position (B). After Nørskov et al. [81Nor1].

The respective changes in the electron density of states in the valence band region of the solid surface can be probed particularly well with the experimental technique of photoelectron spectroscopy. This has been demonstrated many times especially for carbon monoxide chemisorption, but also for the adsorption of hydrogen on metal surfaces. Without entering any details of the experimental set-up and theory (which can be obtained from the literature [78Feu, 86Woo]) the standard experiment is performed having vacuum UV light (generated by means of a noble gas discharge lamp (He, Ne)) incident on the (H covered) sample and collecting the emitted photoelectrons in an energy-dispersive analyzer. One then obtains the so-called energy distribution curves (EDCs) of the photoelectrons which mirror the electron density of states of the probed surface region. Much more detailed information on the surface electronic band structure is, of course, available, if these measurements are performed in a momentum-resolved manner (‘angle-resolved’ UV photoelectron spectroscopy, ARUPS [75Plu, 92Kev]): Then the surface Brillouin zone (SBZ) can be mapped and band dispersion curves of the H-induced state (which mostly is derived from the hydrogenic 1s orbital) be followed in k-space. This yields most valuable information on the energy dispersion and symmetry of the electronic states involved in the H adsorptive binding, but sheds also light on the proximity of the adsorbed H atoms in a given structure: Phases with high density should result in a larger band dispersion than H phases with wide mutual H - H distances. Furthermore, due to the often significant energy dependence of the cross sections for the respective electronic excitations it is advantageous to perform the experiments at a synchrotron storage facility, where a high flux of photons with continuously variable energy is available. While UPS measurements only probe the occupied states in the region at and below the Fermi level, Bremsstrahlung isochromat spectroscopy (BIS), better known as ‘inverse photoemission’ (IPE) provides access also to the density of unoccupied states in the energy region between the Fermi and the vacuum level [83Dos], but the literature data base concerning unoccupied H-induced electronic states is still small. Furthermore, it is remarkable that photoemission measurements (especially when performed at a synchrotron storage ring) are seldom combined with other surface spectroscopic techniques. This may sometimes introduce uncertainty as far as surface cleanliness, accuracy of H coverages and population of binding states is concerned. In the following table, the H-induced electronic levels found in UPS measurements performed with metal and semiconductor surfaces are listed. Most of the data were obtained by angle-integrated measurements; in those cases, where angle-resolution is provided, we will add a respective comment.


Ref. p. 111]


87

3.4.1.3.7.1 Metal surfaces Surface

H coverage, H phases

Ti(0001)

(1×1)H (all coverages)

Cr(110)

Cr(110)

Temperature [K]

Photon energy of incident light hν [eV]

22

300

Θ = 0.25

80

25

140

40.8

(p(2×2) phase)

Θ = 0.88 and Θ = 1

Fe(100) Fe(110)

((1×1)H str phase) Θ≈1 (sat’n) (2×1)-H = c(2×2) (Θ = 0.5)

80

Fe(111)

sat’n

140

40.8

Co(0001)

0.6 ML

170.. 21.2 .300 40.8

Band dispersion; band width (1s-derived band(s))

H-induced surface state (no dispersion) 4.5...7 (6.9 eV at H 1s derived band Γ point) new energy loss H-induced at 16 eV below emission Ep (Ep = energy peak of primary electron beam) H-induced 2.5 at Γ point surface state only weak and 5.5 broad 7.8 at Γ point dispersion of ~2.8 eV in [001] azimuth 5.6 broad 1sderived band H 1s-derived 7.9 at Γ point state, upward dispersion by ~ 1.5 eV in [1−10] and by ~1 eV in [001] azimuth 8.2 at Γ point

(3×1)-2H = (3×3-6H (Θ = 0.67)


Position of Hinduced photoemission state(s) below the Fermi energy EF [eV] at normal emission 1.3 at Γ point

5.6 1.7 (sp-like surface state) 7.2 at Γ point

Remarks, mode of measurement etc.

Ref.

angle-integrated and angle-resolved UPS measurements using synchrotron radiation secondary electron emission study (SES), in combination with electron loss spectroscopy ARUPS; at low Θ H-induced levels with very small dispersion only

80Fei2

angle-integrated UPS ARUPS measurements using synchrotron radiation

77Boz

H 1s-derived state, upward dispersion by ~0.9 eV in [1−10] and by ~1.6 eV in [001] azimuth broad 1sangle-integrated derived band UPS angle-integrated UPS H 1s split-off H 1s state shows state ‘some’ dispersion

80Sak 81Kat

88Kom

91Mar

77Boz 86Gre

88


Surface


Temperature [K]


Co(10−10)

(2×1)-2H (Θ = 1)

100

45

Position of Hinduced photoemission state(s) below the Fermi energy EF [eV] at normal emission ~6

~8 (1×2)-3H

100

19.7

~ 5.6

~ 9.0

Ni(100)

Θ = 1.0

160

Ni(110)

?

78

1.3 above EF fixed photon energy of 9.3 eV (CaF2 photon detector) 21.2 ~ 5.8

Ni(110)

(1×2)-3H (Θ = 1.50) (1×2)-3H (Θ = 1.50)

80

30 eV

Ni(110)

Ni(110)

(1×2)-3H (Θ = 1.50)

Ni(110)

(1×2)-3H (Θ = 1.50)

Ni(111)

(2×2)-2H (Θ = 0.50) medium Θ larger Θ

Ni(111)



Ref.

downward dispersion by ~ 0.3 eV upward dispersion by ~ 1 eV downward dispersion by ~ 0.5 eV upward dispersion by ~ 3 eV formation of sp-derived surface resonance

ARUPS

90Ern

IPE, angleintegrated UPS

87Rei2

width ~ 3 eV

angle-integrated UPS ARUPS at synchrotron ARUPS

77Dem

ARUPS

87Kle1

9.0 at Γ point

7.5...8 at Γ point upward dispersion by ~ 4 eV in [1−10] dir.; upward dispersion by ~5 eV in [001] dir. 80 30 upward 9 at Γ point dispersion by 3.1 eV in [1−10] dir., almost no dispersion in [001] dir. 90 fixed pho- set of new formation of ton ener- surface states sp-derived gy of 9.4 (A, B1, B2, B3) surface eV (SrF2 ca. 1 eV, 0.2 eV, resonances 2.1 eV, 4.5 eV photon detector) above EF 200 21.2 5.9 width ~2.5 eV 26.9 78.... 21.2 5.8 width ~3 eV 300 100

19

[Ref. p. 111

87Kom 88Chr 90Ern

IPE; angle-resolved 90Ran measurements provide the dispersion of unoccupied bands in the SBZ angle-integrated 76Con2 UPS angle-integrated 77Dem UPS


Ref. p. 111]


Surface


Temperature [K]


Ni(111)

small coverages (~ 0.1 ML)

300

13...22

Position of Hinduced photoemission state(s) below the Fermi energy EF [eV] at normal emission 0.25

sat’n

22

6.2

0.5...0.6 ML 140

24 35

4.9 (vanishes at Θ < 0.5)

Ni(111)

1 ML (sat’n) ~100 40

9.0 at Γ point

Ni(111)

medium

160

no H-induced extra emission above EF

Cu(100)

from submonolayers to multilayers 0.45 ML

4

Cu(110)

Cu(111)

medium

Nb(100)

Nb(110)


0.1...sat’n

100

fixed photon energy 9.3 eV 30, 35, 40 eV at synchrotron IPE: fixed photon energy 9.4 eV (SrF2); UPS: hν = 16.85 eV

9.2

10.2 multilayer H2 film −1.9...1.3

0.5

6.2

150.. 21 754

5.1...4.6 (states B+C) ~7 (state A, weak)

300

5.4 (large)


Ref.

surface state ARUPS 79Him of Ni; bluesp-orbitals dominate shifted with H in the H-Ni(111) bonding H surface resonance between 1...7 eV below EF 86Gre position of ARUPS, split-off state momentum-resolved 81Ebe strongly Θ dependent H split-off state disperses between 5 and 9 eV; hints to admixture of Ni 3d states

1st layer

700 K

83Gre

ARUPS at 88Fan synchrotron. From T dependence of H states H subsurface/bulk absorption processes inferred angle-integrated 80Smi2 UPS

90


Surface


Temperature [K]


Ru(0001)

medium Θ

200

11...50

Position of Hinduced photoemission state(s) below the Fermi energy EF [eV] at normal emission 3.7 8.1 at Γ point

Pd(111)

medium Θ

300 200

Pd(111)

medium Θ

80

Pd(111)

1 ML (1×1)- ~100 30, 40, H phase 50

26.9 40.8

21.2

‘invisible’ 6.5

6.4

1.2 3.1 7.9 at Γ point 6.4 at M point 5.9 at K point

300 100

saturation

Ag(111)

100 0.5 ML (2×2) LEED phase

Ce(0001)

100 - 500 L

300

>500 L

300

3.4

Gd(0001) 0...20 L [epitactic films grown on W(110)] Gd(0001) 0.4 L [epitactic films grown on W(110)]

120; 300

3.8

300

40.8

no visible state 7.3

Ag(111)

2.5 8.5 broad

21.2

16.85

4.2

4.0


[Ref. p. 111


Ref.

ARUPS, extrinsic surface state spectra contain final state bands weak and broad, H 1sderived splitoff state from Ru d-band, disperging from 5.9 (K point) to 8.1 eV (Γ point)

85Hof

decrease in the d band emission intensity width 1.5 eV; dramatic loss of Pd d-band intensity normal emission

angle-integrated UPS

76Chr


77Dem

ARUPS at synchrotron

83Ebe 81Ebe 86Gre

H 1s split-off state disperses between 5.9 and 7.9 eV. H 1s split-off state only observable in off-normal directions Ce - H solid solution phase Ce-dihydride formation formation of one H monolayer after 3 L normal emission

angle-integrated UPS 89Zho exposure to D atoms angle-resolved UPS 00Lee at 100 K after exposure to H atoms at synchrotron angle-resolved UPS Ros86

ARUPS using synchrotron rad.

93Li

ARUPS study using NeI rad.

98Get


Ref. p. 111]


Surface


Temperature [K]


Ta(110)

exposures up to 7 L (mediumΘ )

300 ?

14...30

W(100)

0 < Θ < sat

W(100)

sat’n (Θ = 2)

W(110) W(110)

W(110)

Position of Hinduced photoemission state(s) below the Fermi energy EF [eV] at normal emission 2.2 6.5

300 ?

21.2 16.8

1.2...1.5 3.6 5.7

13...20

4.3

Θ > 0.5 ML

6.5 (+ H-induced doublet between 0 and 2 eV below EF) 2.8 4.0 2.0 4.0, further H-induced resonances at 0.5; 1; 3; 6; 7 eV below EF clean W states (1, 1.5, 3.5 eV below EF) affected 3.5

1 ML (sat’n)

6...9

low Θ (β2) high Θ (β1) low Θ (β2) high Θ (β1)

300

10.2

300

21.2 16.8

low Θ

42

W(110)

sat’n

300

21.2

2.0 3.8 6.0

W(110)

low (unreconstructed surface)

80

42 60

300

10.2

~2.5 3.8...4 4.8 9.0 (at ī point) ~2.5 ~4.0 9.3...7.0 1.7 2.8 strong

W(111)


high (recon structed surface) high Θ



at normal angle-integrated emission UPS at synchrotron bonding state between H 1s orbital and metal d orbitals angle-integrated considerable UPS coverage dependence of peak positions H band with polarizationodd parity dependent ARUPS H band with even parity

normal emission normal emission; H-induced bands show only little dispersion normal emission

angle-integrated UPS ARUPS band dispersion curves measured along two azimuths

normal emission


91 Ref.

83Mur

75Plu

78And1

73Feu 81Hol

polarization82Bla dependent ARUPS at synchrotron. Band dispersion only excited (between 6 and 9 eV) determined. for E-vector Reduction in surface parallel to [1−10] direct. symmetry for Θ > H split-off 0.5. state peak positangle-integrated 82Wen ions indepen- UPS dent of H coverage 9 eV state ARUPS at 93Aiu disperses to synchrotron 6.8 eV at N point

73Feu

92


Surface


Temperature [K]


W(111)

0.39

Pt(111)

medium Θ

80

Pt(111)

medium Θ

0.1 100 K/>0.15

Pd(110) 120 K/105 L). O2 was found to partially re-oxidize the reduced surface and molecular as well as dissociative adsorption were observed for SO2 on the reduced surface. The reduced surface appeared to be inert with respect to interaction with CO at room temperature.

3.9.16.2 H2 and H adsorption The interaction of molecular and atomic hydrogen with UHV-cleaved V2O5(001) was studied by Tepper et al using HREELS, ARUPS and XPS [02Tep1]. Both adsorbates led to a reduction of the surface: while a few Langmuirs of atomic hydrogen were sufficient to induce a considerable surface reduction, ten thousands of Langmuirs of molecular hydrogen were needed to induce significant effects. Formation of Landolt-Börnstein New Series III/42A5

378

3.9 Adsorption on oxide surfaces

[Ref. p. 389

hydroxyl groups was not observed in these experiments. From vibrational data of the reduced surface and from a comparison of an ARUPS spectrum of the reduced surface with a calculated density of states [99Her3] indications could be found that preferentially twofold bridging oxygen atoms are removed from the V2O5(001) surface during the first stage of reduction by hydrogen atoms. O(1) O(2)

O(3)

V2O5 unit cell

Fig. 22. Left: structure of the V2O5(001) surface. Right: unit cell of V2O5.

3.9.17 ZnO Zinc oxide crystallizes in the hexagonal wurtzite structure. Since this structure does not exhibit an inversion center, a disk cut from a single crystal along the hexagonal basal plane has two structurally different surfaces. The hexagonal surfaces ZnO(0001)-O (often also called ZnO( 000 1 )-O) and ZnO(0001)-Zn are the most often studied ones (see Fig. 23a and b). Some studies have also been performed for the ZnO( 10 1 0 ) surface. The ZnO(0001)-Zn and the ZnO( 000 1 )-O surface are terminated by zinc and oxygen layers, respectively, and exhibit different chemical properties. A special point to note is that these surfaces are polar which means that they are energetically unstable if not special surface conditions like adsorption, reconstruction, charge-rearrangement or similar stabilizes them. There are reports that under typical UHV conditions the non-reconstructed ZnO( 000 1 )-O surface may be terminated by a layer of hydrogen atoms which stabilizes it [02Kun1, 03Sta1, 03Kun2]. The hydrogen-free surface was found to exhibit a (1×3) reconstruction. For the zinc terminated surface STM revealed the presence of nanosized islands with triangular holes exhibiting oxygen terminated step edges [03Dul1, 02Dul1]. It was suggested that the oxygen terminated step edges provide the necessary stabilization for the ZnO(0001)-Zn surface. Usually disks cut off from a single crystal rod are used as samples. These are prepared by polishing followed by sputtering and annealing cycles as well as oxygen treatment after introduction into the UHV chamber. Since the oxygen and the zinc terminated surfaces behave chemically different they may be differentiated by chemical methods. Chemical etching with HCl may be employed [65Kle1]. ZnO is one of the most often studied oxides which is due to its importance in the field of catalysis. Cu/ZnO catalysts are widely used for the synthesis of methanol via CO hydrogenation and for the watergas shift reaction. In the following we give an overview of results for some adsorption systems. For the remaining systems the reader may consult the references listed in Table 20. (a) ZnO(0001)-Zn

(b) ZnO(0001)-O

(c) hexagonal unit cell

oxygen zinc

Fig. 23. Structure of ZnO. (a): zinc terminated ZnO(0001)-Zn. (b): oxygen terminated ZnO( 000 1 )-O. (c): hexagonal unit cell of ZnO. Landolt-Börnstein New Series III/42A5

Ref. p. 389]


Table 20. Overview of investigations of the interaction of gases with well ordered ZnO surfaces Adsorbates Method References Substrate: ZnO(0001), ZnO( 000 1 ) CH3, H-CŁC, Cl, PF3 Theory: INDO/S 89Rod1 90Voh1 C2H2, methylacetylene, allene UPS methylacetylene, allene HREELS 93Pet1 98Jon1 CH3OH XPS, NEXAFS, CFS, Theory: SCF-Xα-SW Waveguide CARS 94Wij1 CH3OH, OH HCOOH HREELS 97Cro1, 98Tho1 HCOOH NEXAFS 01Gut2 HCOOH, HCOOD TDS, XPS 94Lud1 TDS, XPS, NEXAFS 00Hov1 C5H5N NEXAFS 01Gut1 C6H6, phenol 76Hop1 Cl LEED, AES, ∆Φ Cl, HCOOH ISS, XPS, work function, TDS 00Gra1 Crystal violet Photocurrent measurements 84Cla1 CO molecular beam 00Bec1 CO HAS, molecular beam 00Bec2 HAS, molecular beam, XPS 00Bec3 CO, C4H10 CO Theory: Monte Carlo 01Bur1 CO EELS, TCS 94Mol1, 95Mol1 NEXAFS, IRAS 96Gut1 CO, CO2 Theory: DFT 94Cas1, 95Cas2 CO, NH3 CO, HCOOH TDS 98Yos1 STM, XPS 02Lin1 CO, CO2, HCOOH CO ARUPS 81McC1 UPS, XPS 88Au2 CO, CO2 CO Theory: INDO 87Rod1 CO Theory: MNDO, AM1, PM3 96Mar3 CO NEXAFS 99Lin2 CO 98Jon2 Theory: SCF-Xα-SW Theory: INDO/S 88Rod1 NH3, C5H5N, H2CO, HCOO, H3CO Theory: DFT 95Cas1 H2O, H2S, HCN Theory: LCAO-LDF 96Cas1 H2O, H2S, HCN, CH3OH, CH3SH Theory: DFT 97Cas2 H2O, H2S Theory: DFT 97Cas2 H2O, H2S, HCN, CH3OH, CH3SH TDS, ARUPS 83Zwi1 H2O, D2O Theory: INDO/S 88Rod2 H2O Theory: DFT 01Wan1 H2O Theory: ab initio cluster calculations 96Nyb1 H2 H2 LEED, HAS 01Bec1 XPS, NEXAFS 99Rod2 SO2 XPS, NEXAFS 01Rod1 SO2, NO2 Xe LEED, TDS, ARUPS 84Gut1 Substrate: ZnO( 10 1 0 ) HCOOH HREELS 97Cro1 UPS, XPS 88Au1 HCOOH, CO2, H+CO2 Landolt-Börnstein New Series III/42A5

379

380 Adsorbates C6H6 C6H6, C5H5N CH3OH benzotriazole, Indazole, benzimidazole, 1methylbenzotriazole Cl CO CO CO, H, CO+H CO, CO+H CO, H2 CO, CO2 CO, CO2, O2, H2, H CO, H2 CO CO2 H2 H2 H2O, D2O O2, CO, CO2 NO O2 NH3 OH+ Rh(CO)2(π-C3H5) S2 Xe Substrate: ZnO( 11 2 0 ) H2


[Ref. p. 389

Method TDS, LEED, UPS NEXAFS XPS, NEXAFS, CFS, Theory: SCF-Xα-SW NEXAFS

References 81Pos1 93Wal1 98Jon1 95Wal1

Theory: INDO/S ARUPS TDS ARUPS, UPS HREELS, AES Theory: DFT surface conductivity, surface potential, TDS, LEED surface conductivity, charge transfer, Theory: SINDO Theory: DFT Theory: MNDO, AM1, PM3 UPS Theory: ab initio cluster calculations Theory: periodic Hartree-Fock calculations TDS, ARUPS TDS, adsorption isotherms, UPS, XPS, ESR, conductivity TDS, UPS TDS, LEED, ESR, AES, ∆Φ, surface conductivity Theory HREELS XPS, Theory: SCF cluster calculations LEED, TDS, ARUPS

89Rod1 80Say1 94Ge1 80DAm1 97Guo3 97Cas2 79Hot1

Theory: ab initio cluster calculations

82Gop1 99Cas3, 98Cas1 96Mar3 80Gop2 96Nyb1 99Zap1 83Zwi1 80Gop1, 85Gop1 84Zwi1 77Gop1, 78Gop1, 76Gop1 99Cas2 90Yam1 97Cha2 84Gut1 96Nyb1

3.9.17.1 CO adsorption CO adsorption on the basal surfaces of ZnO as well as on Zn( 10 1 0 ) has been studied employing different methods. CO adsorbs weakly on ZnO( 000 1 )-O and ZnO(0001)-Zn with the heat of adsorption being (7−2θ CO) kcal/mol (θ CO = CO coverage) on both surfaces as revealed by molecular beam studies employing the King and Wells method [00Bec2, 00Bec1, 00Bec3]. From He reflectivity measurements it was concluded that CO prefers defect sites, but with increasing coverage also regular sites are occupied. Precursor mediated adsorption was found to occur for both surfaces as concluded from the coverage dependence of the sticking coefficient. A sticking coefficient which increases with coverage was observed for both surfaces, but the effect was found to be especially pronounced for ZnO(0001)-Zn. This observation was interpreted as an indication of adsorbate-assisted adsorption. For ZnO(0001)-Zn the angular dependence of ARUPS intensities has been employed to study the molecular orientation of molecules adsorbed at 80 K [81McC1]. It was found that the molecules are standing upright on the surface. With XPS C1s and O1s binding energies of 291.8 eV and of 537.9 eV, respectively, were determined for CO on ZnO(0001)-Zn for an adsorption temperature of 73 K [00Bec3]. Landolt-Börnstein New Series III/42A5

Ref. p. 389]


381

Carbonate formation following CO dosage onto ZnO( 000 1 )-O was observed at 120 K [02Lin1, 88Au2] and 130 K [99Lin2, 96Gut1]. The surface coverage is small at 130 K. Using XPS the maximum CO coverage was determined to be 0.04 ML [99Lin2]. For carbonate resulting from CO dosage the coverage was studied as a function of substrate annealing temperature and oxygen treatment, leading to the result that carbonate formation from adsorbed CO mainly occurs on defect sites [02Lin1]. The coverage varied from 0.2 ML for a surface annealed at 1070 K to nearly zero for 1370 K annealing temperature. With angular dependent NEXAFS it was shown that the CO molecular axis is tilted by 17±10° with respect to the surface normal for CO adsorbed at 130 K [99Lin2]. With NEXAFS the C1s →π*.excitation energy for CO was foundto be 287.7±0.2 eV and for carbonate an energy of 290.4±0.2 eV has been reported [99Lin2]. Reported corelevel binding energies as obtained with XPS are 288.6 eV for the C1s level of adsorbed CO and 290 eV and 532.5 eV for the C1s and O1s level of carbonate, respectively [88Au2]. Less studies have been performed for CO adsorption onto ZnO( 10 1 0 ). For low temperature adsorption (T ∼77 K) at an ambient CO pressure of 1×10−6 Torr the formation of a dense layer with nearmonolayer coverage was reported [80Say1]. The heat of adsorption was reported to be ∼12 kcal/mol [80Say1, 80DAm1]. The adsorption geometry of the CO molecules was determined via the angular dependence of the CO 4σ intensity in angular resolved photoelectron spectra which gave a tilting angle of about 30° with respect to the surface normal [80Say1, 80DAm1]. The adsorption of CO on ZnO( 10 1 0 ) has also been studied at room temperature. After exposing the surface to 100 Pa of CO for 15 min a CO desorption peak was detected around 360 K [94Ge1]. In contrast to the results of Ge and Møller [94Ge1] who only found small amounts of desorbing CO2, Hotan, Göpel and Gaul [79Hot1] detected exclusively CO2 with TDS. However, in the latter case the applied CO pressure was much smaller (1.3×10−5 Pa). Coverage and chemical identity of the adsorbed species were not studied.

3.9.17.2 CO2 adsorption The adsorption of carbon dioxide on ZnO( 000 1 )-O was studied with XPS and NEXAFS [02Lin1, 88Au2, 96Gut1]. CO2 was found to be transformed into carbonate at the oxygen vacancies at step edges [02Lin1]. Above 150 K all physisorbed CO2 is desorbed and at temperatures above 400 K the carbonate signal vanishes [88Au2]. The carbonate molecules stand upright on the surface with an angle of about 30° between the surface normal and the molecular plane as concluded from NEXAFS data obtained after exposing ZnO( 000 1 )-O to CO2 at 130 K [96Gut1]. The C1s →π* resonance was found at 290±0.2 eV. With XPS the carbonate C1s binding energy was determined to be 290.3 eV [88Au2]. The C1s binding energy of physisorbed CO2 was found to be 291.8 eV. CO2 adsorption on ZnO( 10 1 0 ) was studied with XPS and UPS [88Au1, 80Gop2]. Formation of a surface carbonate occurs already at 100 K. Physisorbed CO2 was observed up to about 150 K and the carbonate was found to disappear until 400 K. As determined from XPS intensities the carbonate coverage was θ = 0.1 ML. C1s binding energies of 290.4 and 291.8 eV were measured for the carbonate and the physisorbed CO2, respectively.

3.9.17.3 CH3OH adsorption Methanol adsorption on ZnO(0001)-Zn and ZnO( 10 1 0 ) was studied using NEXAFS and XPS. On both surfaces a methoxide species characterized by a C1s binding energy of 290.2 eV was observed [98Jon1]. Formate forms on ZnO(0001)-Zn after annealing above 220 K. This species was found to be stable even at 523 K which is the methanol synthesis temperature. No formate formation was observed on ZnO( 10 1 0 ). From the energy of the σ shape resonance (295.5 eV) as determined with NEXAFS a C-O bond length of the methoxy groups of 1.39 Å was estimated [98Jon1].


382


[Ref. p. 389

3.9.17.4 HCOOH adsorption HCOOH adsorption was studied on ZnO(0001)-Zn, ZnO( 000 1 )-O and ZnO( 10 1 0 ). On ZnO( 000 1 )-O HCOOH was found to adsorb dissociatively (HCOOH → [HCOO]− + H+) on surface defects [02Lin1, 01Gut2]. With XPS the saturation coverage was studied as a function of the annealing temperature of the substrate and oxygen treatment [02Lin1]. For an annealing temperature of 1070 K a surface coverage of about 0.3 was found which dropped to 0.1 for an annealing temperature of 1370 K. This observation was explained as to result from the decreasing number of surface defects with increasing substrate annealing temperature. From STM results the authors concluded that adsorption preferably occurs on cus zinc cations at step edges. The C1s corelevel of the surface formate was detected at 289.6±0.3 eV. NEXAFS was used to study the geometry of the adsorbed formate ions [01Gut2] on ZnO( 000 1 )-O. From the dependence of the intensity of the C1s 2b2 resonance at 288.3 eV on the light incidence angle a tilting angle of 55±5° with respect to the surface normal was estimated. Other (weaker) C1s resonances were identified at 291.8 eV (7a1), 297.8 eV (8a1) and 301.4 eV (5b1). Ludviksson et al investigated the adsorption of formic acid on ZnO( 000 1 )-O with thermal desorption spectroscopy [94Lud1]. Desorption of molecularly adsorbed HCOOH was found to occur below 200 K with a small tail extending to higher temperatures. CO and CO2 formation due to the decomposition of adsorbed formate (HCOO → CO2 + H and HCOO → CO+OH) was found at 550 K. A large part of the hydrogen resulting from the formic acid decomposition was assumed to dissolve into the bulk. HREELS data for HCOOH adsorption onto ZnO( 000 1 )-O at 300 K have been obtained by Crook et al [97Cro1] and Thornton et al [98Tho1]. Vibrational modes of formate were observed at ∼750 cm–1 (δ(OCO)), 1080 cm–1 (π(CH)), 1371 cm–1 (νs(OCO)), 1605 cm–1 (νa(OCO)) and 2928 cm–1 (ν(CH)). A hydroxyl vibration was not observed which was supposed to result from hydrogen dissolution into the bulk. For the zinc terminated ZnO(0001)-Zn surface HCOOH adsorption was studied with TDS by Yoshihara et al [98Yos1] and Grant et al [00Gra1]. HCOOH desorption occurs at 200 K (multilayer) and 370 K (molecularly chemisorbed formic acid) [98Yos1]. Between ∼350 K and 450 K also H2 adsorption was observed which was attributed to desorption of hydrogen originating from the decomposition of formic acid on the surface (HCOOH → HCOO+H). At about 575 K desorption peaks of CO, H2O, CO2 and H2 showed up which was attributed to the dissociation of formate via the reactions HCOO → CO2 + H and HCOO → CO+OH. The HCOOH adsorption on ZnO( 10 1 0 ) at 300 K was studied with HREELS by Crook et al [97Cro1]. Again formate formation was observed. Vibrational losses of the adsorbed formate were found at 1040 cm–1 (π(CH)), 1363 cm–1 (νs(OCO)), 1573 cm–1 (νa(OCO)) and 2895 cm–1 (ν(CH)). The fate of hydrogen atoms originating from the formic acid decomposition was not clear. An increase of the OHinduced IR absorption-intensity was observed after dosage of HCOOH but no comparably strong OD vibration was found in the spectra after exposure to DCOOD. The authors argued that this observation may be due to isotopic exchange effects and to the fact that the OD vibration would be partially hidden by the νs(OCO) overtone. XPS spectra for HCOOH adsorption onto ZnO( 10 1 0 ) were published by Au et al [88Au1]. Upon adsorption a species with a C1s binding energy of 289.9 eV was observed. The position of the C1s peak did not depend on the dose nor on the annealing temperature and was visible even at 590 K, but with significantly reduced intensity.

3.9.18 Tables of selected adsorbate properties Selected results of the studies discussed in the previous sections are summarized in the following tables. Table 21 gives an overview of desorption temperatures and adsorbate-substrate binding energies, Table 22 lists sticking coefficients and coverages, Table 23 collects vibrational data and Table 24 lists corelevel binding energies and NEXAFS excitation energies.


Ref. p. 389]


Table 21. Desorption temperatures and adsorbate-substrate binding energies. Substrate Desorption Activation energy Notes (method, etc) temperature [K] [eV] Adsorbate: CO Al2O3/NiAl(110) 55, 67 0.14, 0.17 TDS 120, 318, 395 TDS θ-Al2O3/NiAl(100) 120, 375 TDS α-Al2O3/NiAl(100) 0.47 (175 K) TDS, Cr term. surf. α-Cr2O3(0001)/Cr(110) 105, 175 Cu2O(100) 120- 320 (compli- ≤0.36-0.72 TDS cated pattern) MgO(100) 57 0.14 TDS MgO(100)/Mo(100) 0.17 (60 K) TDS ∼60, ∼80 and ∼100 (defect ads.) NiO(100) 115-137 0.30 (low coverage), TDS 0.1 (high coverage) NiO(111)/Mo(111) broad structures TDS between 100 and 250 TDS, see Fig. 14 415 (CO on asym. reduced RuO2(110)/Ru(0001) Rubridge), 470 (CO on sym. Rubridge) reduced TDS, see Fig. 14 ∼300 (CO on RuO2(110)/Ru(0001) Rucus), ∼350 (asym. bridging CO on Rubridge), ∼560 (sym. bridging CO on Rubridge) TDS, see Fig. 14 RuO2(110)/Ru(0001) 270, 320 (CO on Rucus), 470 (CO on Rubridge) rutile TiO2(110) 135-170 0.43 (lateral TDS interactions: ∼0.1 at θ CO = 0.68) rutile TiO2(110) molecular beam 0.31-0.07×θ CO (θ CO =CO coverage) molecular beam 0.3-0.087×θ CO ZnO( 000 1 )-O, θ =CO coverage) ( ZnO(0001)- Zn CO Adsorbate: CO2 TDS, Cr term. surf. α-Cr2O3(0001)/Cr(110) 120 (CO2), 180 (CO2), 330 (CO2δ−) NiO(111)/Ni(111) 395, 645 TDS Adsorbate: D2O TDS CeO2(001)/SrTiO3(001) 152 (mult. D2O), 200 (first layer D2O), 275 (OD) Adsorbate: DCOOD TDS, DCOO+OD rutile TiO2(110) 350 (DCOOD, D2O), 400 (D2), 570 (CO, CO2, D2O, D2, DCOOD) Adsorbate: H2O 300-500 0.99- 1.78 TDS, OH groups α-Al2O3(0001) Landolt-Börnstein New Series III/42A5

383

References

93Jae1, 93Jae2 98Hsi1 98Hsi1 01Pyk1 91Cox1 99Wic1, 99Wic2 01Doh1 99Wic1, 99Wic2 96Xu1 03Kim1

02Sei1

03Kim1

95Lin1

03Kun1 00Bec2, 00Bec1, 00Bec 99Sei1 93Gor1 99Her1

01Iwa1

98Ela1

384 Substrate CeO2(111)/Ru(0001) CeO2(111)/Ru(0001) α-Cr2O3(0001)/ Al2O3(0001)

Cu2O(100) FeO(111)/Pt(111) Fe3O4(111)/Pt(111) Fe2O3(0001)/Pt(111) MgO(100)

NiO(100)/Ag(100)

SnO2(110) rutile TiO2(110) Adsorbate: HCOOH ZnO( 000 1 )-O ZnO(0001)-Zn

NiO(111)/Ni(111)

Adsorbate: NO α-Cr2O3(0001)/Cr(110) NiO(100) (NiO( 100) /Ni( 100) similar) Adsorbate: O2 α-Cr2O3(0001)/Cr(110) SnO2(110)

3.9 Adsorption on oxide surfaces Desorption temperature [K]

Condensed Matter)

Soft Condensed Matter

Condensed matter physics