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]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
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
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[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
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
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
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[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]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
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
Landolt-Börnstein New Series III/42A5
8
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[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 << M) this phonon coupling mechanism is not as effective as it is for heavier adsorbate species [76Bre, 79Mue, 79Bre, 80Sed, 82Bre]. A principally much more complicated situation arises, if a molecule (such as the H2 molecule) can or will dissociate as it hits the surface. Sophisticated multi-dimensional quantum chemical calculations are required to describe this dissociation event properly. Recent theoretical treatments revealed that dynamical steering processes (i.e. the time-dependent orientation of the incoming molecule with respect to the configuration of the substrate atoms) can decisively govern the hydrogen dissociation reaction [95Gro1, 95Gro2, 98Gro].
3.4.1.3.1.3 The coverage dependence of the sticking probability
Sticking or trapping requires at least a single empty adsorption site. As the filling of the adsorption sites proceeds, s usually decreases with coverage as accounted for by the function f (Θ ) in Eq. (4). For nondissociative adsorption (H2 molecules at very low temperatures) and negligible lateral interactions, f (Θ ) simply equals 1 – Θ ; for dissociative adsorption (H2 on TM surfaces) and vanishing mutual interactions, f (Θ ) = (1 – Θ )2. Formation of phases with long-range order or laterally inhomogeneous phases (islands etc.) require more complicated expressions. Furthermore, one has to take into account that impinging molecules can become transiently trapped in a weak van-der-Waals potential in which they can ‘live’ for a few microseconds and diffuse across the surface, until they find an empty adsorption site. This wellknown precursor kinetics strongly affects the coverage dependence of the sticking probability and various s(Θ ) relations have been derived and reported in the literature, among others by Kisliuk [57Kis, 58Kis]. More details on precursor kinetics, terms and definitions can be found in the review by King, Bowker, and Morris [84Mor]. Since trapped hydrogen molecules are usually only very weakly bound, precursor kinetics are not often observed in hydrogen adsorption experiments performed in the temperature range from 77 to 300 K, in contrast to carbon monoxide adsorption on transition metal surfaces where Kisliuktype adsorption kinetics is almost the rule. Generally, the hydrogen sticking coefficient drops markedly
Landolt-Börnstein New Series III/42A5
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3.4.1 Adsorbate properties of hydrogen on solid surfaces
9
with increasing coverage. As an example, we present the s (Θ ) relation for the system hydrogen on Rh(110) in Fig. 4, which shows in addition the influence of the formation of ordered H phases and a marked isotope effect [88Ehs]. 1.0
Sticking probability s
0.8 D 0.6 H 0.4
0.2
0
0.5
1.0 Coverage Q
1.5
2.0
Fig. 4: Coverage dependence of the sticking probability of hydrogen and deuterium interacting with a clean rhodium(110) surface at T = 100 K. On this surface, hydrogen forms various phases with long-range order which apparently has some influence of the shape of the curve s(Θ ) [88Ehs].
3.4.1.3.1.4 The experimental determination of sticking coefficients
Convenient means to measure initial sticking probabilities use i) direct gravimetry, e.g., by a microbalance [78Rob] or a quartz oscillator [74Bry], ii) thermal desorption [91Chr] or iii) flow methods (which have the advantage of a continuous measurement of the rate of adsorption) using calibrated capillaries or molecular beam (MB) techniques [78Eng]. A widely used method was proposed by King and Wells [72Kin], which is especially suited for single crystal samples: The gas is introduced to the system by means of a sharp molecular beam. The recipient contains two gauges, only one of which can receive molecules directly from the gas source. The other gauge measures the ambient pressure, and by comparison a simple expression for s results. In the following tables, we have listed most of the accessible sticking coefficient data reported for single crystalline surfaces. The data are organized as follows: s0 for metals (listed according to the groups of the periodic table and subdivided with respect to the crystallographic orientation), and, in a separate short section, for semi- and non-conducting chemical elements. Note, however, that for these latter systems there is only a very scarce base of reliable data.
3.4.1.3.1.5 The sticking probability of hydrogen on metal surfaces
Note that most of the measurements of Table 3.4.1.3.1.5 refer to data that have been measured in the temperature range from ~80 K (liquid N2) to room temperature. One can safely assume that hydrogen adsorbs dissociatively on most transition metal (TM) surfaces in this temperature range, however, a couple of smooth, low-index TM surfaces such as Ni(100) [85Ham, 93All], Ni(111) [89Ren1], or Pt(111) [85Poe], exhibit surprisingly low hydrogen sticking coefficients, provided these surface are free of crystallographic defects or impurity atoms. Slight activation barriers of adsorption of hydrogen on these surfaces have thus been concluded; this is indicated in the table. A somewhat different situation is encountered if the samples are exposed to H2 gas at temperatures far below 80 K, i.e., around 10...50 K. In this case, molecular hydrogen adsorption competes with dissociative (atomic) adsorption (the latter may be weakly thermally activated). The residence time of the hydrogen molecules in the shallow van-der-Waals adsorption potentials usually increases with falling
Landolt-Börnstein New Series III/42A5
10
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
temperature and may, in addition, sensitively depend on coverage, surface roughness and quantummechanical effects, leading to the interesting situation that the sticking probability s (Ĭ) can indeed increase with coverage in the low-coverage regime, because molecule - molecule collisions on the surface help to provide the energy accommodation [88Wil]. Only at medium coverages a constant, Ĭ-independent sticking due to condensation is observed [95Fri]. However, not too many measurements have been performed under these low-temperature conditions, experiments with Cu(100) at 10 K [83And, 88Wil, 93And], Pd(210) and Ni(210) at 40...50 K are examples [01Sch2]. For the latter TM surfaces indeed a competition between molecular and atomic adsorption has been observed and theoretically explained [98Mus, 01Sch1]. Metal surfaces
Surface
Initial sticking coefficient s0
Remarks
Reference
V(111)
Surface temperature [K] 223
0.60
MB experiments, performed with deuterium
00Beu
Fe(100)
300
0.03
Fe(100) Fe(110) Fe(110) Fe(110) Fe(111) Fe(211) Co(0001) Co(10−10) Ni(100) Ni(100) Ni(100) Ni(110) Ni(110) Ni(110) Ni(110) Ni(110)
300 150 200...450 300 300 40…200 300 100…200 120 120 250 300 100 140 100 155
Ni(110)
300
Ni(111)
200
Ni(111) Ni(111)
140 155
Ni(111)
300
Ni(111)-(2×2)-2H
220
80Ben −3
1.5 × 10 0.16 0.15...0.18 1.2 × 10−2 1.6 × 10−2 1.0 0.045 >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 <0.1; s0 increases MB studies with nozzle with beam energy beams of different energies; activated adsorption, barrier height ~5...10 kJ/mol 0.45 TOF-low energy recoil spectrometry
91Ber2; 92Ber1 77Boz 88Kur 91Ber2 91Ber2 95Sch1 79Bri 94Ern 85Ham 79Chr1 88Zhu 73McC 89Chr 82Win 85Chr 85Rob 89Ren1
74Rin 82Win 85Rob
89Ren1
99Ito
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Ni(210) Cu(100) Cu(100)
Surface temperature [K] 100 15 15
Cu(100)
10
Cu(100)
218...258
Cu(100) Cu(110)
300 15
2.5 × 10−13 (H2) 7.8 × 10−14 (D2) ~10−5 0.38 (D2)
Cu(110) Cu(110)
140 200
~0 0.18 ± 6%
Cu(110)
90
0.05 ± 0.04
Cu(111)
10
0.25
Cu(111) Cu(111) Cu(111) Nb(100)
5...300 K 300 300 90
Nb(110)
300...550
0 ~10−5 <10−7 0.23 (state 1) 0.056 (state 2) 0.91 ± 0.01
Mo(110) Mo(110) Mo(110) Mo(100) Mo(100) Mo(111) Mo(211) Ru(0001) Ru(0001) Ru(0001) Ru(0001 Ru(0001)(1×1)H
78 300 350 300 210 300 100…320 95 173 100 250 100 5
0.10 0.35 0.06 0.7 1 0.7 0.76 ± 0.08 0.58 0.03 0.25 ± 0.1 0.1 0.4 0.03
Ru(10−10) Rh(100) Rh(100) Rh(110) Rh(111)
100 100 90 80 100
1.0 0.53 ± 0.05 High 0.97 ± 0.1 0.65
Landolt-Börnstein New Series III/42A5
Initial sticking coefficient s0 ~1 0.05 at Θ ĺ 0 0.03 at Θ ĺ 0; ~0.5 at Θ = 0.8 0.27 (D2)
Remarks
MB expts. with H2 and D2 MB expts. with D2 at very low T (physisorption) MB studies of H2 (D2) physisorption H2 pressures up to 5 bar were used MB expts. MB expts. with D2 at very low T (physisorption) MB expts TDS expts., surface exposed to H atoms TDS expts.; surface exposed to H atoms MB expts. With D2 at very low T (physisorption) MB expts. MB expts. with D2
T-dependent ; competing absorption MB expts. ESD expts. MB expts. TDS MB expts. N species P species precursor kinetics
11 Reference
01Sch2 85And 88Wil 93And 93Ras 89Ang1 93And 89Hay 93Bis 94Roh 93And 83Gre 89Ang1 92Ret1 74Hag 81Pic 72Mad 72Cha 89Ern 72Cha 92Baf 72Cha 94Lop 85Feu
80Shi 79Sch 78Dan physisorption; sticking coeff. 95Fri increases linearly with Θ up to 0.6 at Θ ≈ 0.6 89Lau expts. performed with D2 82Kim 88Ric1 88Ehs 79Yat
12 Surface
3.4.1 Adsorbate properties of hydrogen on solid surfaces Surface temperature [K]
Rh(111)
[Ref. p. 111
Initial sticking coefficient s0
Remarks
Reference
MB expts.
99Beu
MB expts.; activated adsorption
96Col
Rh(111)
180...305
0.33 (H2) 0.28 (D2) 0.01 ± 0.005
Rh(311) Rh(311) Pd(100) Pd(110) Pd(111) Pd(111)
90 35 170 130 200 100
0.25 0.30 0.5 0.7 0.15 0.5...0.6
Pd(111) Pd(111)
80 423
0.5 0.76 ± 0.05
Pd(210)
90
~1
Pd(311) Ag(111) Ag(111)
40...200 10 110
1 ~1 near Ĭ = 1 ML ~0
W(100)
78...300
W(100) W(100) W(100)
190...900 300 300
W(100) W(100) W(100) W(110) W(110) W(111) W(111) W(211)
300 170...400 80...1000 300 90 300 80...1000 150
Re(10−10) Ir(100)-(1×1) Ir(100)-(5×1) Ir(110)-(1×2) Ir(110)-(1×2) Ir(111) Pt(100)-(1×1)
100 200 200 130 130 130 100 35
Pt(100)-(1×1) Pt(100)-hex
300 78
0.077 ± 0.008 0.133 ± 0.013 0.50 ± 0.03 0.65 0.51 ± 0.03 (H2) 0.57 ± 0.03 (D2) 0.56 0.60 ± 0.03 0.90 0.22 0.12 1 0.85 0.57 (β2 state) 0.05 (β1 state) 0.7 0.18 0.10 7 × 10−3 1 0.007 7 × 10−3 0.1 a2 state 0.03 a1 state 0.17 0.15
theoretical analysis precursor kinetics
MB studies with nozzle beams of different energies MB studies quantum-state resolved
HREELS data expts. performed with D2 thermal beams ȕ1 state ȕ2 state MB expts. calibrated capillary
MB study TDS and ∆Φ study MB study
D2 MB study D2 MB study ȕ1 state ȕ2 state ȕ1 state activated adsorption
91Nic1 99Pay 80Beh 83Cat1, 88He1 76Con1 95Beu1 87Gdo 97Gos 98Mus 01Sch1 98Fri 82Avo 95Hea 69Tam 70Tam 89Aln 66Est 73Mad 74Bar 80Kin 92Ber2 74Bar 97Nah2 74Bar 92Ber2 73Rye 95Mus 98Ali 98Ali 80Ibb, 81Ibb 81Ibb 87Eng 91Pen2 95Dix 77McC2 Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Pt(100)-hex Pt(100)-hex Pt(100)-hex Pt(100)-hex Pt(100)-hex Pt(110)-(1×2) Pt(110)-(1×2) Pt(110)-(1×2) Pt(110)-(1×2) Pt(110)-(1×2) Pt(110)-(1×2) Pt(110)-(1×2) Pt(110)-(1×2)
Surface temperature [K] 160 150 122...150 190 300 190 78 100 170 300 300 100 285
Pt(111) Pt(111) Pt(111) Pt(111) Pt(111) Pt(111) Pt(111)
190 78 150 300 300 300 100...200
Pt(111) Pt(210) Pt(211) Pt(997) Au(110)-(1×2)
600...800 78 190 120 150
Initial sticking coefficient s0 0.1 0.06 0.06 0.07 0.17 0.33 0.2 0.46 0.8 0.31 0.33 0.45...0.48 0.6 0.016 0.1 0.1 0.07 0.2 0.016 6 × 10−2 at 240 K; 4.5 × 10−2 at 90 K 2 × 10−1 0.4 0.14 0.34 0
Remarks
MB studies using D2
TDS
MB study MB study, rotational state resolution activated adsorption MB studies
increases with T; activated adsorption s increases with T
13 Reference
93Klo 95Dix 95Pas 74Lu 75Net 74Lu 77McC2 87Eng 92She 76McC 72Cha 89Ang2 96Beu 74Lu 77McC2 75Chr 77Sal 77McC2 77Sch 85Poe 79Sal 77McC1 74Lu 76Chr 86Sau
3.4.1.3.1.6 Sticking and adsorption of hydrogen on bimetallic and alloy surfaces
From the viewpoint of heterogeneous catalysis, the sticking behavior of hydrogen on alloy and bimetallic surfaces deserves particular attention, because these materials are often catalytically more active than the individual metals. Hydrogen adsorption studies with both crystallographically well-defined alloy single crystal surfaces and samples consisting of epitactic films grown on single crystalline substrates have been reported, and the researchers have been interested in kinetic coefficients, adsorption/desorption energies, hydrogen vibrations, work function changes and hydrogen surface phases. Regarding the thin-film systems, so-called surface alloys are sometimes formed, i.e., interdiffusion of the components leads to more or less well-defined alloy phases which are restricted to the surface region. However, there is a class of metallic elements which are immiscible in the bulk and do not show noticeable surface alloying effects either. Nevertheless even marginal chemical interaction between the constituents can (and often will) modify the surface sites for adsorbing hydrogen. These systems are well-known as bimetallic surfaces and play a significant role in heterogeneous catalysis [83Sin, 90Cam]. Unfortunately, systematic and quantitative investigations concerning hydrogen adsorption have not been performed, due to the complexity of these materials regarding their variable surface morphology and surface chemical composition. Often, the standard surface-analytical techniques do not provide
Landolt-Börnstein New Series III/42A5
14
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
sufficiently accurate information, although some progress has been made in the recent decade by using scanning tunneling microscopy techniques in conjunction with LEED [93Sch, 98Gau]. Quite generally, reliable kinetic (and energetic) data concerning H adsorption are rare. The respective quantities often depend in a very complex manner on surface morphology and composition. Since interdiffusion of the alloy constituents is a thermally activated process, the aforementioned properties depend sensitively on temperature and require a steady experimental control. This is particularly awkward in the course of TDS experiments (which have been carried out by many researchers), since even a single TD run may irreversibly alter both morphology and surface composition. Moreover, the hydrogen adsorbate itself (as well as other reactive adsorbates such as carbon monoxide or oxygen) may cause de-mixing and segregation of one component of the alloy, usually that component, which provides the higher interaction energy with the adsorbed H atom. This component is enriched at the surface (corrosive chemisorption). An example is provided by the Ge-on-Si(001) system [98Rud]. Regarding the ‘chemistry’ of the systems in question, the following distinction may be useful: Alloys which consist of two metals, both of which adsorb hydrogen readily (for example, Ni and Fe, or Pd and Pt), have to be distinguished from alloys where one of the constituents does not spontaneously adsorb hydrogen, examples being Cu-Ni or Cu-Pd or Au-Pt alloys. In the first case hydrogen will probably not differentiate between the two kinds of metal atoms (unless the local charge distribution will be greatly modified and differ for the two kinds of atoms so that one kind of site is favored over the other). In the second case the alloy system (with its ‘active‘ and ‘inert‘ atoms) can behave in three different ways: 1) Hydrogen will strictly only adsorb at and near the ‘active’ TM atom – then the system follows a coverage dependence which resembles the one of the pure metal, but the coverage is reduced to the fraction of the ‘active’ atoms. 2) Hydrogen will again only adsorb at and near the active substrate atoms, but after dissociation the H atoms can freely diffuse across the surface and adsorb also in sites provided by inactive atoms a process well-known as spill-over effect [88Chr, 88Con]. 3) The chemical nature of all sites is modified compared to the clean constituents, depending on the (surface) mole fraction and the mutual distribution of the atoms: There exist local areas (patches) consisting predominantly of active metal atoms (which exhibit almost the adsorptive properties of the clean active metal) and patches, in which ensembles of active atoms are more or less diluted by inert atoms. Here, the sticking behavior (and/or the adsorption energy) may be greatly reduced and impaired, depending on how many inert atoms are included in the respective ensemble. As mentioned above, problems of this kind are crucial in heterogeneous catalysis [69Bal]: Not only do they influence the rate of the hydrogen uptake (adsorption kinetics), but can also affect the entire energetics and electronic interaction involved in the hydrogen chemisorption reaction. The dependence of the kind and number of hydrogen binding states on the alloy composition and/or the lateral distribution of the different kinds of atoms are well known as ensemble and ligand effect [79Ert], both of which can decisively govern the activity and selectivity of surface reactions involving hydrogen. Since there exist only quite few systematic and reliable numbers for sticking coefficients and adsorption energies (which often depend in a complicated manner on preparation and annealing conditions as mentioned above) we refrain from trying to list these numbers systematically in tables. Rather, we provide some miscellaneous information on the interaction of hydrogen with a variety of alloy or bimetallic systems for which relevant adsorptive properties have been determined. In view of the many possible binary alloy systems [58Han] and the fact that just in recent years the respective reports on binary and ternary systems have increased almost exponentially, this list is by no means complete and provides only a small (but hopefully representative) survey over the principal features. Alloy and bimetallic surfaces
Alloy/bimetallic system
T [K] Measured properties
Remarks
Ag - Pt(111) Au - Pt(111)
100
TDS state at 160 K TDS state at 120 K
Ag - Ru(10−10)
80... 100
LEED, ∆Φ, TDS; ĬAg=0.2ĺEdes=73kJ/mol; ĬAg=0.5ĺEdes=50kJ/mol
Ag and Au films on Pt(111); H 04Ogu adsorption is thermally activated Ag(111) films on Ru (10−10); 93Len 0.1 < ĬAg < 1.0
Ref.
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Alloy/bimetallic system
T [K] Measured properties
Al - Pd(100) p(2×2)-p4g
Au - Ir(111)
100
Bi - Pt(111)
150
Cs - Al(111) • p(2×2) ĺ ĬCs = 0.25 • (¥3×¥3)R30° ĺ ĬCs = 0.33 complete Cs layer Cu0.85Pd0.15(110) - (2×1) Cu0.85Pd0.15(110) - disordered Cu - Ru(0001)
85
Cu - Ru(0001)
100
Cu - Ru(0001)
170
Cu3Pt(111) - (2×2)Pt sublattice Cu - W(100) Ag - W(100) Au - W(100) Fe - Al
100 300
Fe - Ru(0001)
220
Fe - Ti
300
Fe - W(110) Fe - W(100)
100
Fe - W(110)
90
Fe - W(100) Mo1-xRex(110); x=0.05, 0.15, 0.25
140 <130
Mo0.75Re0.25(100), (110), (111)
130
Ni - Cu(111)
140
Landolt-Börnstein New Series III/42A5
150
300
TDS į-state at 360 K 1st-order, Ȟ = 4 × 1020 s−1, Edes = 144 kJ/mol STM + HREELS Ȟsym = 77 meV (H in 4fold hollow) TDS states at 160 and 240 K 2 TDS states; Bi acts as a site blocker for H ads. TDS, HREELS, work function. Formation of CsAlH2 surface complex MB study; activated adsorption of D2 TDS states at ~360 K
TDS: Cu-induced H states at • 190 K • 220 K TDS, ǻĭ measurement; ĬCu = 0.15 ĺ Edes= 166, s0 = 0.29 ĬCu=0.70ĺEdes= 72, s0 =0.43 TDS 2nd-order state H adsorption linearly blocked with Cu (Ag, Au) coverage D2-TDS states at 500 and 820 K; desorption energy 1.57 eV D2-TDS; sticking coeff. for 1 ML Fe = 0.05 TDS state at 710 K; desorption energy 2.10 eV TDS, LEED, for 0.12 < ĬFe < 1.1 ML 3 TD states between 250 and 500 K TDS, ∆Φ, equilibrium measurements; ∆Hdes = 81.6 kJ/mol for Θ ≤ 0.3 and 125 kJ/mol at Θ > 0.3; Θmax = 0.6; s0 = 0.3 at 90 K 3 TDS states LEED, TDS, HREELS; losses at 48, 70, 97 (110), 155 meV H (D) for sat’n on x = 0.15 alloy LEED, nuclear reaction analysis; D coverages • (100) ĺ ĬD= 2.0 • (110) ĺ ĬD= 0.99 • (111) ĺ ĬD= 2.86 UV photoemission states, electronic band structure, H-induced ∆Φ (50 meV)
15
Remarks
Ref.
Al films on Pd(100) form a p(2×2)-p4g surface alloy
93Oni
99Kis
Au films on Ir(111); H2 adsorbs dissociatively on Au Bi films on Pt(111); 0 < ĬBi < 1ML Cs thin films on Al(111); annealing to T> 200 K ĺ Cs-Al surface alloys with several ordered phases sticking strongly influenced by annealing thin Cu films on Ru(0001) (bimetallic system); Cu strongly suppresses H uptake thin Cu films (1 - 8 ML) on Ru(0001) with preadsorbed atomic hydrogen thin Cu films on Ru(0001) depos. at 1080 K; expts. performed with D2 H adsorption at Pt sites coinage metal films deposited on W(100) up to ~2 ML alloy films deposited on Si sample Fe films on Ru(0001) (bimetallic system) alloy films deposited on (400) Si Fe films on W(110) and (100) (bimetallic system)
03Oka 89Paf 96Kon
95Cot 80Shi
85Goo
82Vic
94Lin 89Att
02Che 88Ega 03Che 90Ber
Fe monolayer films on W(110) 97Nah1 annealed to 600 K prior to H2 97Nah2 adsorption
1 ML Fe on a W(100) surface alloy single crystals; (2×2)-H, c(2×2) and (1×1) H phases depending on compos.
88Zho 98Oka1
alloy single crystals
98Oka2
Ni monolayer film on Cu(111)
89Frau
16
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
Alloy/bimetallic system
T [K] Measured properties
Remarks
Ref.
NiAl(110)
115
alloy single crystal; exposure to H atoms
95Han
NiAl(110)
190
alloy single crystal
95Beu2
Pd - Ta(110)
100
epitactic Pd(111) films on Ta single crystal
93Hei
Pt80Fe20(111) p(2×2)
120
alloy single crystal, strong Pt surface segregation upon annealing
92Atl
Pt0.5Ni0.5(111)
115
strong Pt surface segregation
94Atl
Re - Pt(111)
150
thin Re films on a Pt(111) single crystal
88God
Sn films form a surface alloy with Pt; H binds on Pt sites only Sn films form surface alloys; no spontan. D2 dissociation ĺ exposure to D atoms
96Jan
Sn - Pt(111) (¥3×¥3)R30°
Sn - Pt(111) p(2×2) Sn - Pt(111) (¥3×¥3)R30°
300
Ti - Pd(100) p(2×2)-p4g
300
LEED, TDS, HREELS; single TD state 300...250 K with 2nd order kinetics; Edes = 52.2 kJ/mol at low ĬH; 33.8 kJ/mol at high ĬH TDS, LEED, MB techniques; H sat’n = 0.49, H-induced ∆Φ (550 meV), activation barrier for H2 dissociation 0.72 eV LEED, TDS; s0 = 0.58, TD states at 150, 385, 615 and 1100 K UPS, TDS; 2 TDS states shifting with H coverage: • 358...304 K • 440...406 K H-induced ∆Φ = −400 meV LEIS, TDS: 2nd-order desorption, Edes = 50 kJ/mol; s0 = 5 × 10−3; H-induced ∆Φ = −100 meV, nuclear reaction analysis: ĬH = 2.7 × 1018 (= 0.2 ML) activation energy for desorption decreases with ĬRe, H uptake increases with ĬRe XPS • ĬSn = 0.25 ĺ ĬD= 0.68, s0 = 0.33 • ĬSn = 0.33 ĺ ĬD= 0.51, s0 = 0.18; reduced Pt - D bond energy 232 kJ/mol 2 TDS states • 355...345 K due to desorption from c(2×2) domains • 380 K due to desorption from p(2×2)-p4g patches
98Vos
04Tsu Ti films on Pd(100) form a Pd3Ti p(2×2)-p4g surface alloy
3.4.1.3.1.7 The sticking on semiconducting and insulating surfaces
Compared with hydrogen-on-metal systems the available data for sticking of H2 on non-metallic elements is scarce, for several reasons. For very shallow surface - H2 interaction potentials the equilibrium concentration of trapped hydrogen molecules at non-cryogenic temperatures and pressures usually accessible in UHV model experiments is very small and so is the probability of H2 dissociation under these conditions [83Sch2]. Therefore, pressures in the mbar or bar range are necessary to populate these potentials with hydrogen molecules at common temperatures. The high pressures in turn prevent the use of the standard surface (electron) spectroscopies and cause serious purity problems, since the H adsorption properties are especially altered by co-adsorption of contaminating species [90Chr]. Parallel to
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
17
‘chemical’ impurities, even traces of crystallographic defects (steps, kinks, holes etc.) may dramatically affect the dissociation probability and, hence, the hydrogen uptake. This is particularly true for semiconductor surfaces, where structural defects play a decisive role in hydrogen sticking: On vicinal Si surfaces, for example, the sticking probability (i.e., the hydrogen uptake) increases dramatically for slightly misoriented samples in that defect sites serve as adsorption conduits from which H atoms diffuse onto the rest of the surface [96Han]. In other words: H2 molecules may become dissociated at edge atoms, kinks, steps or holes. Sticking (in conjunction with the formation of a covalent Si - H bond) is the first step towards a chemical hydrogenation reaction. On the Si(111)-(7×7) surface, for example, two main channels for this reaction have been proposed: i) direct adsorption of a H atom on the Si adatom dangling bonds and ii) breaking by H of the Si adatom back bond [91Mor, 93Bol, 96Nog]. Thus, a progressive hydrogenation of the entire surface is initiated which first leads to monomers Si - H, then to dimers Si - H2 or trimers Si - H3 until finally silane-like species are formed. A somewhat more detailed description of these various interaction steps of hydrogen (and other molecules) with several Si surfaces is given by Joyce and Foxon [84Joy]. Various mechanisms as to how the H2 dissociation takes place are discussed in the literature: By means of an STM investigation a so-called ‘Si dimer’ mechanism has recently been confirmed [02Due]. Another interesting reaction path which can make molecular hydrogen dissociate on a semiconductor surface at elevated temperatures is the so-called phonon-assisted sticking, which has been deduced for H2 on Si(111)-(7×7) [95Bra] and on Si(100)-(2×1) [95Kol1] from the increase of the sticking probability with temperature. Of course, the shallow molecular adsorption potentials of H2 on non-metallic surfaces can be filled with molecules by lowering the temperatures into the 5...10 K regime, and actually several studies have been carried out with physisorbed hydrogen, especially on graphite surfaces, aiming at formation of ordered phases and two-dimensional phase transitions [82Seg, 85Fre2, 87Fre, 88Cui, 89Cui] or at the mechanism of the ortho – para hydrogen conversion [83And, 97Sve]. The sticking probability under these conditions is difficult to estimate, since the phonon-assisted mechanism may not be very effective. Rather, particle scattering effects within the adsorbed H2 layer are essential, leading to an increase of s with coverage, but no systematic studies of the sticking behavior of physisorbed hydrogen have been performed to our knowledge. All in all, spontaneous dissociative hydrogen adsorption is not a very common feature on semiconductor and insulator surfaces, at least not as long as defect-free surfaces are considered. An entirely different situation exists, if H2 molecules are pre-dissociated in the gas phase into reactive H atoms [84But]. In most of the experimental studies this is accomplished by means of a hot W filament: By choosing the temperature of the filament, the yield of H atoms can be sensitively controlled [80Sch2, 83Sch2]; the H atoms formed then react vigorously with the Si (C, Ge, GaAs, InP etc.) surfaces to monoor dihydride compounds. Oxides exposed to H atoms may easily become reduced, leading to the formation of water and the loss of lattice oxygen, whereby the lattice is often destabilized, oxygen vacancies are being formed which provide additional reaction sites for hydrogen. However, these complex chemical processes (which are of great relevance in heterogeneous catalysis though) cannot be covered here. In the following table, we put together both the molecular and the atomic hydrogen data obtained so far for a variety of common semiconductor and insulator surfaces with emphasis on silicon and carbon (in the form of diamond). Note that many (if not all) of the clean surfaces are reconstructed, since a surface with unsaturated covalent (“dangling”) bonds is energetically inherently unstable. Wellknown examples are the silicon (111) surface which is, in the clean state, (7×7) reconstructed [83Bin2], and the Si(100) surface, which reconstructs to (2×1), c(4×2), and p(2×2) surface phases [86Ham]. Much interest has been (and still is) spent to diamond single crystal surfaces and their interaction with hydrogen, owing to the technological interest in diamond thin films [02Big]. In some respect, diamond surfaces resemble silicon, for example, as far as surface reconstruction and reactivity with respect to hydrogen is concerned. In other respects, there are, of course, important differences (conductivity, hardness etc.). For more details, we refer to the exhaustive article by Pate [85Pat]. In the last decade, the interest in H2 adsorption dynamics especially on Si single crystal surfaces aroused both experimentally and theoretically, and since then, a wealth of articles appeared on that subject. One particular issue is the correct theoretical description of the dissociative adsorption of H2 at elevated temperatures and the recombinative hydrogen desorption from partially or totally hydrided
Landolt-Börnstein New Series III/42A5
18
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
silicon single crystal surfaces, without violating the principle of detailed balancing. Due to space limitations, we cannot enter this interesting topic and refer instead to some selected references [93Wu], [94Bre], [94Kra], 95Kol2], [95Jin], [00Peh], [00Zim]. A somewhat more general overview of the Si + H interaction has been given by Oura et al. [99Our]. Comparatively few studies have been performed with Ge, but its behavior with respect to hydrogen appears to be quite similar to Si; again, mono and dihydrides are formed upon exposure to H atoms. To some extent, this is also true for the diamond surfaces. Semiconductor and insulator surfaces
Surface C(111) -1×1 diamond C(0001) -1×1 (HOPG) graphite Si single crystal surfaces in general
T [K] 300
Sticking coefficient ~0
150 300
0.4 (H) 0.25...0.5 (D) < 10−6...10−8
Si(111)7×7
580 1050
2 × 10−9 5 × 10−6
Si(111)
300
<10−6
Si(100)2×1
300...900
Si(100)2×1
440...670
Si(100)2×1
300
<10−10 strongly T-dependent; s0 increases to ~10−4 as T approaches 750...800 K
Si(100) Si(111) vicinal surfaces Si(111)
>50
~0.1
300 1000
GaAs(110) GaAs(110)
300 300
10−10 10−4 ; activation barrier = 0.94±0.1 eV = 90.7 kJ/mol ~1.0 up to 1 ML coverage initially high, but decreases strongly with H coverage
1.0 5 × 10−6 at 300 K 1.5 × 10−5 at 630 K 10−8...10−4
Remarks no adsorption of molecular hydrogen exposure to H (D) atoms
Ref. 85Pat 02Zec
activation barrier ca. 0.5 eV
59Law 83Sch2 90Lie1 95Bra 96Bra
phonon-assisted sticking; adsorption activation barrier 0.9 eV exposure to H2 molecules
83Sch2
exposure to H atoms sticking rises with temperature
95Kol1
adsorption at terraces; 99Due activation energy of 0.8 eV can be lowered by dynamical distortions of Si surface atoms phonon-assisted sticking only 00Zim at small H coverages; at medium and larger coverages a barrier-free autocatalytic pathway exists second harmonic generation 96Han (SHG) sum frequency vibrational spectroscopy and SHG
01Mao
exposure to H atoms 87Mha exposure to H atoms. Upon H 99Gay adsorption, surface changes from relaxed to unrelaxed form
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
19
3.4.1.3.2 Kinetics of hydrogen desorption
3.4.1.3.2.1 General remarks
The rate of a desorption reaction (we consider here only thermal desorption processes occurring from a homogeneous surface and neglect any activation barriers in the adsorption channel) can be usually described by the Polanyi-Wigner equation (Eq. 6)
rdes = −
dΘ dt
§ E · x −1 exp¨ − des ¸ = ν ( x) g (Θ ) Nmax © kT ¹
(6)
where Ȟ = frequency (pre-exponential) factor, x = reaction order, Edes = desorption energy, Nmax = maximum number of adsorbed particles. The function g(Ĭ) describes the concentration dependence of the desorption reaction. Since the ongoing desorption reduces the surface concentration of the adsorbate, rdes is defined as a negative quantity, for convenience, we consider only its absolute value here. Multiplying (t ) Eq. (6) with the inverse heating rate ȕ−1 (ȕ = dT/dt) transforms the time-dependent rate rdes to a (T ) . Note that the standard procedure in a thermal desorption experiment is temperature-dependent rate rdes to apply a linear temperature program (T (t ) = T0 + ȕ t; with ȕ = const) which transforms Eq. (6) to
(T ) rdes =−
dΘ ν (x) § E · x −1 = g (Θ ) Nmax exp¨ − des ¸ ⋅ dT β © kT ¹
(7)
This is the standard form of the Polanyi-Wigner equation and mostly used to evaluate TPD data. The pre-exponential factor and the desorption energy are system-specific quantities. Often, especially on highindex surfaces, several energetically different hydrogen binding sites may be populated; the desorption of the respective adsorbed species from the various types of sites is then governed by individual relations of the type of Eq. (7). A series of typical hydrogen thermal desorption spectra is reproduced in Fig. 5, taken from the H-on-Co(10−10) system, which shows (in addition to the 2nd order β state) a relatively sharp α state indicating H atoms bound in different surface sites [94Ern]. The coverage function g(Ĭ) simply equals Ĭ x, with the desorption order x mostly taking the value 1 or 2. A first-order (x = 1) hydrogen desorption process results, if the rate-limiting step is the removal of the H2 entity from the surface (associative desorption), which certainly applies to physisorbed H2. A secondorder (x = 2) desorption mechanism implies a surface migration of the separated H atoms, a subsequent collision event of two atoms, their recombination to the molecule, and, finally, the desorption of the H2 molecular entity. The overall reaction rate is primarily dependent on how fast the dispersed, migrating H atoms collide and recombine, whereas the desorptive removal of the molecule to the gas phase is not ratelimiting. Hence, the coverage function in its simplest form equals Ĭ 2 in this case.
3.4.1.3.2.2 The frequency factor and the order of the desorption reaction
For the physical interpretation of the pre-exponential factor Ȟ we must again delineate between 1st and 2nd order desorption processes. In terms of transition-state theory, Ȟ can be identified with the universal frequency factor kTh (§ 6 × 1012 s−1 at 300 K), modified with the ratio of the partition functions of the transition state and the adsorbed molecule, if necessary. If the activated complex (the H2 entity „ready“ for desorption) and the adsorbed species have the same degrees of translational and rovibrational freedom, these partition functions cancel each other. Usually however, the transition state complex is less strongly bound to the surface and may have higher degrees of freedom; then Ȟ may significantly exceed the value 6 × 1012 s−1. In a naive view, Ȟ can be regarded as representing the attempt frequency to move along the direction of the desorption trajectory, i.e., away from the surface; it would then correspond to the frequency of vibration of the adsorbed particle perpendicular to the surface. Depending on the
Landolt-Börnstein New Series III/42A5
20
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
physical state of the adsorbate prior to the desorption reaction, two extreme cases can be distinguished. i) The adsorbed atoms are completely mobile in two dimensions, then the (second-order) frequency factor Ȟ2 simply equals the collision frequency in the two-dimensional gas. ii) The adsorbed atoms are completely immobile, then the frequency factor corresponds to the number of available adsorption sites. Note the different units of the 1st and 2nd order frequency factor: Ȟ1 ĺ [s−1]; but Ȟ2 ĺ [cm2ǜs−1atom−1]. Since the configuration of the adsorbed particles changes with coverage, the frequency factor usually is coveragedependent; this Ĭ dependence often resembles the one encountered with the adsorption energy, the reason being the well known compensation effect [55Cre, 81Aln]. 4 750 × 10- Pa×s 375
150 α
75
H2 partial pressure pH 2 [a.u.]
37.5 β 15 7.5 3.75 1.13
0.84 0.75 0.6 0.45 0.3 0.15 0.075 200
300
400
500
Fig. 5: A typical series of hydrogen thermal desorption spectra, obtained from a cobalt(10−10) surface exposed to increasing amounts of hydrogen gas at T = 200 K. Two binding states are formed: At low and medium coverages, a single β state develops with a 2nd order desorption kinetics, while at elevated coverages a sharp α shoulder state appears at lower temperatures. After [94Ern].
Temperature T [K]
In the following table we have listed the available first and second-order frequency factors for hydrogen desorption from single crystal surfaces. Since H atoms often adsorb in several different binding states (denoted as α, β, γ, δ etc.) having different kinetic and energetic properties, we have also listed the respective states and their desorption temperature maximum Tmax at the saturation coverage. Unfortunately, reliable Ȟ data are scarce, for the same reasons mentioned in the foregoing section. In many cases it is not quite clear whether the desorption occurs via 1st or 2nd order; mostly, this information is taken from series of thermal desorption spectra, i.e. from the shift of the TD maximum with coverage, whereby a judgement of the peak position can be obscured by lateral interactions between the adsorbed particles (which can cause similar peak shifts [74Ada]). Relatively accurate frequency factors for hydrogen desorption can be determined by a so-called line shape analysis of TD spectra, according to evaluation ‘recipes’ developed by Bauer et al. [75Bau1] or King [75Kin]; both procedures (which are entirely equivalent) are solely based on the validity of the Polanyi-Wigner equation (Eq. 7) and explicitly consider coverage dependencies of Ȟ and Edes. Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
21
We also recall from sect. 3.4.1.3.1 that there is a number of systems (including the noble metals Cu, Ag, Au, but also other ‘free electron’ metals like Be or Al) which do not spontaneously adsorb and dissociate hydrogen molecules. Accordingly, they exhibit vanishingly small sticking coefficients and practically no H uptake. Nevertheless, the interaction with hydrogen can be studied by simply exposing the respective surfaces to fluxes of H atoms or supersonic H2 molecular beams. The respective data for the frequency factors and reaction orders etc. are included in our list, together with a brief comment.
3.4.1.3.2.3 The desorption energy as a kinetic quantity
In the context of adsorption and desorption kinetics, it is justified to consider the desorption energy simply as a kinetic quantity: As mentioned above, the particle residing at the bottom of the adsorption potential has to be supplied with this energy to be able to leave the surface; it may therefore be regarded as kind of an activation energy to run the desorption reaction. Fortunately, since most of the hydrogen adsorption cases involve no or only vanishingly small activation barriers, the thermal desorption experiment is well suited to map out the depths of the adsorption potential wells under equilibrium conditions; therefore the quantity ‘desorption energy’ can serve as a measure of the hydrogen - surface interaction energy, and actually many of the values listed in the compilation of sect. 3.4.1.3.3 stem from TD experiments. To illustrate this internal correlation, we have, in the following table, listed the number of desorption states and the temperature position of the TD maxima, together with the respective frequency factors. Note that the desorption energy Edes refers to the actual energy that a particle must overcome to leave the potential energy well. For non-activated adsorption Edes actually equals the adsorption energy Ead. Only in the case of activated adsorption, Edes consists of the adsorption energy Ead, plus the height of the activation barrier, E*ad. Particles leaving the surface first have to surmount this ‘extra’ barrier and, accordingly, carry a certain amount of excess energy and are not necessarily in thermal equilibrium with the surface. This may significantly alter the shape of the thermal desorption curves as was proven, e.g., for hydrogen molecules desorbing from Cu(100) by Anger et al. [89Ang1]. Furthermore, the flux of the desorbing particles can exhibit directional dependencies. This shows up in a cosn θ distribution (n > 1) with a lobe perpendicular to the surface, θ being a polar angle relative to the surface normal. Measurements of these deviations from thermal equilibrium requires fairly sophisticated experimental setups, especially, if quantum-state resolution is attempted (combined molecular beam and/or laser spectroscopy). A brief overview is given by Zacharias [88Zac]. In some cases, parallel entrance channels can exist for dissociative adsorption, activated and non-activated ones. How an impinging H2 molecule finds the easiest pathway to dissociate and chemisorb as atoms is called dynamical steering effect [03Gro]. Most of the common transition metal single crystal surfaces, however, provide non-activated adsorption pathways resulting in spontaneous dissociation of the incident hydrogen molecules. 3.4.1.3.2.3.1 Metal surfaces Surface
Be(0001)
H adsorption states + desorption temperature at sat’n [K] ȕ 470 (low Θ )
Frequency factor 1st order [s−1]
Al(100) Al(100) Al(110) Al(110)
ȕ ȕ ȕ ȕ
345 330 338 315…335
close to 0 order fractional order fractional order
Al(111)
ȕ
335
close to 0 order
Landolt-Börnstein New Series III/42A5
Ref.
surface exposed to H atoms at 80 K
90Ray
activation barrier for adsorption = 0.95 eV surface exposed to H atoms surface exposed to H atoms surface exposed to H atoms surface exposed to H atoms; H-induced surface reconstruction surface exposed to H atoms
88Pau 91Win 91Win 92Kon
2nd order [cm2/s] 2nd order with attractive H - H interaction
ȕ 510 (high Θ ) fractional-order
Remarks
91Win
22 Surface
Al(111) Al(111) V(111) Fe(100) Fe(100) Fe(100) Fe(100) Fe(110) Fe(110)
Fe(111)
Fe(111) Fe(211)
Co(0001) Co(0001)
Co(10−10) Ni(100) Ni(100) Ni(100) Ni(100)
Ni(100) Ni(110)
Ni(110)
Ni(110)
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
H adsorption Frequency factor Remarks states + desorption temperature nd 2 1st order [s−1] 2 order [cm /s] at sat’n [K] ȕ 323…334 zero order; surface exposed to H atoms 2 × 1013 ȕ 320…340 fractional surface exposed to H atoms; desorption of Al-hydride surface 330 MB expts. performed with D2, 2nd order bulk 1200 1st order competing bulk dissolution TDS expts.; H saturation ȕ1 ~300 ȕ2 380 coverage 5.6 × 1018 m−2 1.5 × 10−2 ȕ1 290 400 2nd order ȕ2 ȕ1 265 ȕ2 355 pseudo-1storder ȕ1 ȕ2 ȕ1 ~340 ~430 2nd order ȕ2 Į 300 MB expts. ȕ1 ~330 ~400 2nd order ȕ2 ȕ1 ~240 ~300 ȕ2 ȕ3 ~370 2nd order 2nd order MB expts. ȕ1 ~260 ~350 ȕ2 2nd order T-dependent reconstruction : Į1 ~210 Į states on unreconstructed, Į2 ~230 ȕ state on reconstructed surface ~265 Į3 ȕ ~340 ~2nd order ȕ ~400 2nd order ȕ1 273 preliminary study ȕ2 308 333 ȕ3 T-dependent reconstruction Į 260 1st order 2nd order ȕ 300 ȕ1 318 ȕ2 348 3 × 100 2.5 × 10−1 ȕ1 330 370 ȕ2 8 × 10−2 Į2 200 Į1 270 Į (=ȕ2) 350 5 × 10−2 0.4 laser-induced thermal desorption expts. Į 220 fractional-order T-dependent reconstruction 290 ȕ1 first order ȕ2 340 Į 225 ȕ1 290 340 initial 2nd order ȕ2 Į expts. performed with H2 and D2 ȕ1 molecular beams 2nd order ȕ2
Ref.
88Mun 91Har 00Beu 80Ben 77Boz 91Ber2 96Mer 77Boz 91Ber2
77Boz
91Ber2 95Sch1
79Bri 86Gre
94Ern 79Chr1 72Lap1 74Chr 81Joh1
84Hal 89Chr
82Win
85Rob
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
Ni(111) Ni(111) Ni(111) Ni(111) Ni(111) Ni(111) Ni(111)
Cu(100) Cu(100)
Cu(100) Cu(100) Cu(110) Cu(110) Cu(110)
3.4.1 Adsorbate properties of hydrogen on solid surfaces H adsorption Frequency factor states + desorption temperature nd 2 1st order [s−1] 2 order [cm /s] at sat’n [K] ȕ 2 × 10−1 ȕ1 ȕ2 360 8 × 10−2 ȕ1 ca. 300 360 ȕ2 0.15 ± 0.5 ȕ1 355 2 × 10−4 383 ȕ2 10−3 ȕ1 290 370 ȕ2 ȕ1 325 ȕ2 365 Į 250 1st order ȕ1 355 383 ȕ2 10...15 218...258 590...650 (bulk spec.) ~270 1st order 2nd order 6 × 10−6...0.1
285; 340...290 330
10−6 α ȕ1 ȕ2
200 290 335 1.6 × 1013
Cu(110)
Cu(111) Zn(0001)
ȕ 240 α 316 ȕ1 235 ± 10 292 ± 5 ȕ2 sgl. state 310 ȕ 319
Nb(100)
state 1
Nb(110)
state 2 175 two surface states
Cu(110) Cu(111)
Nb(110) Mo(110) Mo(100)
Landolt-Börnstein New Series III/42A5
1st order due to deconstruction
3.3 × 10−4 order n=0.5 due to island formation; Ȟ = 6 × 106 s−1
120
10−2 10−2 290 350 450
Ref.
72Lap2 74Chr absolute (volumetric) measurements 74Rin 79Chr1 ȕ1 peak position angle-dependent due to activated adsorption H2 exposure at 170 K
86Rus
Į state due to desorption from subsurface sites, filled by implantation physisorption of H2 molecules after exposure to H2 at 4 K pressures up to 5 bar were used; after extended exposures bulk desorption occurs non-equilibrium effects prevent reliable data analysis exposure to H atoms surface reconstructs exposure to H atoms; both Edes and Ȟ increase with coverage exposure to H atoms
90Gol
86Gre
82Ebe 93Ras
89Ang1 91Cho 89Ang1 79Wac
exposure to H atoms
93Bis
data derived from H uptake curves (He scattering) exposure to H atoms; activation barrier for deconstruction 1.02 eV exposure to H atoms
93Goe 94Roh
Edes decreases with coverage exposure to H atoms
89Ang1 84Cha2
83Gre
molecular precursor; combined TDS 74Hag and work function study
2.75 × 1014 ȕ1 ȕ2 ȕ1 ȕ2 ȕ3
Remarks
23
~1013
~5 × 10−2 5 × 10−2
foils with (110) orientation; data taken from T-dependent UV photoemission expts. H uptake measurements precursor kinetics
80Smi2
81Pic 72Mah 71Han
24 Surface
Mo(100)
Mo(211)
Ru(0001) Ru(0001) Ru(0001) Ru(0001)
3.4.1 Adsorbate properties of hydrogen on solid surfaces H adsorption Frequency factor states + desorption temperature nd 2 1st order [s−1] 2 order [cm /s] at sat’n [K] ȕ1 325 ȕ2 ~390 450 ȕ3 ȕ1 330 ȕ2 400 480 1...10 ȕ3 ~1010 β1 320 ~1017 ~360 β2 β1 330 1 × 10−3 400 β2 β1 ~200 2nd order β2 ~350 β1 1 × 10−2 330 3 390 β2
st Ru(0001)(1×1)H 15 1 layer 7.5 2nd layer 5.8 3rd layer Ru(10−10) α 220 258 β1 287 β2 350 β3
Ru(11−21)
Rh(100) Rh(100) Rh(100) Rh(100) Rh(110)
Rh(110)
Rh(111) Rh(111) Rh(311)
Pd(100)
Pd(100)
zero order zero order 1014
[Ref. p. 111 Ref.
Remarks
92Baf
94Lop strong decrease of Ed and ν with coverage
79Sch 80Shi 78Dan
P state N state
85Feu
physisorption of D2, exposure at 4.8 K
95Fri
89Lau
9 × 10−3
α1 120 α2 150...200 γ 350 β ~310 α ~140 β 290 α 150 β1 ~330 β2 ~130 β 326
01Fan
1.0 × 10−2
expts. performed with deuterium
82Kim 84Ho 85Heg 87Ric1
0.04
88Ric1 1013 1013
α1 138 α2 216 β 245 α 140 β 210 1011 γ 250 α ~140 β 260 300 (HD signal) α1 120 170 α2 γ1 210 255 γ2 δ 305 shoulder state 250 β 350 1st order β 350
88Ehs 2nd order
1.2 × 10−3 2 × 10−2
theoretical analysis
94Kre2
strong decrease of Ed and ν with coverage MB experiments; activated adsorpt. expt. and theoretical modelling
79Yat 96Col 99Pay
4 × 1011 80Beh 1 × 10−2 3.2 × 10−3
adsorption performed at T = 110 K; 1st order precursor model
90Bur
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
Pd(100)
Pd(110)
Pd(110)
Pd(110)
Pd(110)
Pd(111)
Pd(111) Pd(111) Pd(210)
Pd(311)
Pd(311)
Ag(100)
Ag(110) Ag(111)
Ag(111)
Ag(111) Ag(111)
Landolt-Börnstein New Series III/42A5
3.4.1 Adsorbate properties of hydrogen on solid surfaces H adsorption states + desorption temperature at sat’n [K] α 155 β 350
Frequency factor 1st order [s−1]
170 285 310
v (α) ss rs (β1) s (β2)
170 212 280 315
α 110 (subsurface) β 150 β2 175 155 β1 β2 191 173 β1 (shoulder) β2 191 173 β1 (shoulder) β 180 β 160
Remarks
Ref.
α state (caused by subsurface H) only appears at 100 K ads. temperature; H absorption likely room temperature study shoulder due to H desorption from bulk
98Oku
2nd order [cm2/s]
8 × 10−2 α 368 high T shoulder state ~680 1st order α1 165 α2 220 ~270 β1 2nd order 295 β2 α1 165 230 α2 fractional-order ~270 β1 316 β2 α1 165 230 α2 ~270 β1 β2 310 2nd order 300 8 × 10−2 shoulder state at ~250 β 380 fractional order α 170 2nd order β 310 α 150 β1 200 240 β2 300 β3 2nd order α β1 β2
25
α states due to population of subsurface sites
83Beh 83Cat1 83Cat2
isothermal desorption expts. performed with deuterium
88He2
subsurface state population
88He1
76Con1
exposure at 300 K
83Ebe
at Tad = 80 K only β state; for 90 < T < 140 K α + β present expts. performed with deuterium α state due to D sorption in subsurface and bulk states
87Gdo
α state due to H sorption in subsurface states
1013
74Con
98Mus
99Far
3.88 × 107s−1
2nd order
H adsorption strongly dependent on 98Fri adsorption temperature (s = surface, rs = surface after lifting of (1×2) reconstruction, ss = subsurface, v = bulk) exposure to H atoms 04Kol
2nd order
TD expts after exposure to H atoms
93Spr
TD expts after exposure to H atoms
89Zho
fract. order 1st order
st
1 order fractional order
2nd order
TD expts. after exposure to H atoms. 90Par2 Data of [89Zho] re-examined expts. performed with H atoms expts. performed with H atoms
95Lee 04Kol
26 Surface
Ag(111)
Ta(110)
W(100) W(100) W(100) W(110) W(110) W(111)
W(111)
W(211) Re(0001) Re(0001) Re(10−10)
Ir(100)-(1×1) Ir(100)-(5×1)
Ir(100)-(5×1)
Ir(110)-(1×2) Ir(110)-(1×2) Ir(111)
3.4.1 Adsorbate properties of hydrogen on solid surfaces H adsorption states + desorption temperature at sat’n [K] α 170 175 β1 200 β2 state 1 150 state 2 385 state 3 615 state 4 1100 β1 450 550 β2 β1 450 550 β2 β1 450 550 β2 β1 440 510 β2 β1 440 550 β2 α 130 β1 230 340 β2 490 β3 ~640 β4 α 135...145 β1 230 340 β2 490 β3 ~640 β4 β1 330 675 β2 shoulder 350 420 β2 shoulder 420 490 β2 α 240 300 β1 470 β2 α 240 β 400 β 400
A 125 B 240 C 365 β1 200 390 β2 β1 200 380 β2 β1 ~200 (shoulder only) β2 290
Frequency factor 1st order [s−1]
[Ref. p. 111
Remarks
Ref.
subsurface species expts. performed with D atoms
95Hea
surface state surface state
93Hei
2nd order [cm2/s]
0th order fractional order (0.5)
½ ¾ ¿
13
10
bulk absorption
surface reconstructs under hydrogen
69Tam
4.2 × 10−2 70Ada 103 ...10−5
84Hor
1 × 10−2 1.4 × 10−2
71Tam 81Hol
1st order
71Tam 1 × 10−2 1 × 10−2 1 × 10−2 1 × 10−2
1st order
72Mad 2nd order 2nd order 2nd order 2nd order
½ ° 10−2 ° ¾ ° °¿
ratio of peak integrals: β1: β2: β3: β4 = 1.5 : 1.5 : 1.5 : 1
73Rye
1 × 10−2
81Duc 2nd order 90He 2nd order 95Mus 2nd order 1st order
zero or fractional-order 8 × 1012
MB study
98Ali
MB study
98Ali
1st order 2nd order 2nd order 2 × 10−7 1.5 × 10−2
00Mor
80Ibb 88Cha TDS
87Eng
2 × 10−6 Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
Ir(111) Ir(111) Pt(100)-hex
Pt(100)-hex
Pt(100)-hex
Pt(100)-hex Pt(100)-hex
Pt(100)-hex
Pt(100)-hex
Pt(110)-(1×2) Pt(110)-(1×2) Pt(110)-(1×2)
Pt(110)-(1×2) Pt(111) Pt(111) Pt(111) Pt(111) Pt(111)
Landolt-Börnstein New Series III/42A5
3.4.1 Adsorbate properties of hydrogen on solid surfaces H adsorption states + desorption temperature at sat’n [K] β1 240 380 β2 β1 190 270 β2 α1 170 245 α2 270 α3 β1 shoulder 430 β2 β1 ~350 β2 ~410 ~240 α1 ~260 α2 state 1 390 state 2 350 (shoulder) peak “2” 330 peak “1” 360 γ1 200 γ2 218 γ3 240 α3 366 a1 195 a2 225 330 a3 b 375 γ1 194 216 γ2 237 γ3 246 α1 336 α2 377 α3 β1 170 230 β2 β1 175 300 β2 α 180 ~220 β1 β2 290 β1 220 β2 310 β β1 β2 β1 β2
Frequency factor 1st order [s−1]
Remarks
27 Ref.
2nd order [cm2/s] 88Cha 99Hag 2nd order 74Lu
2nd order adsorption T-dependent: at Tad = 300 K, two (not well resolved) TD states (β1+β2); at Tad = 200 K, four TD states (β1+β2+α1+α2); only at 200 K deconstruction to (1×1) TDS + theoretical analysis, effects of reconstruction on H desorption considered
1013
81Bar3
87Sob
93Klo
1.3 × 10−5
95Dix
4 × 1013
10−4...0.3
1 × 10−2
TPD features complex because of T and H coverage dependent reconstruction
91Pen1
4 × 1013
TPD features complex because of T and H coverage dependent reconstruction
95Pas
1 × 10−2 3 × 10−4
76McC strong coverage dependence
87Eng 89Ang2
2nd order
92She
2nd order
74Lu 75Chr
st
1 order
330 ~220 310 <250 310
3 × 10−9
77Col1 2.7 × 105 10−1...10−3
MB study; activated adsorption, barrier height 2 - 6 kJ/mol determined by He scattering expts.
79Sal 81Poe
28 Surface
3.4.1 Adsorbate properties of hydrogen on solid surfaces H adsorption states + desorption temperature at sat’n [K]
Frequency factor 1st order [s−1]
β1 β2 Pt(211) β1 β2 Au(100)-(5×20) β Au(110)-(1×2) β Au(110)-(1×2) β α Pt(111)
310 390 295 430 170 216 177 121
Remarks
Ref.
D2 study; coverages determined by nuclear reaction analysis (NRA) β1 shifting to lower T with increasing Ĭ H
82Nor1
2nd order [cm2/s] 10−2...10−5
Pt(111)
[Ref. p. 111
88God 74Lu
2nd order 0.1...0.001 1st order
exposure to H atoms at 100 K exposure to H atoms at 100 K exposure to D atoms at 96 K
96Iwa 86Sau 97Luh
3.4.1.3.2.3.2 Semiconductor and insulator surfaces
Experimental investigations of hydrogen desorption from semiconducting and non-conducting surfaces including the determination of desorption energies and frequency factors are practically restricted to C, Si and Ge surfaces. As pointed out before, there is no spontaneous adsorption of hydrogen atoms due to the extremely low sticking probabilities; in order to accumulate H atoms on the surfaces the H2 molecules have to be pre-dissociated by a hot W filament or by using the translationally or vibrationally excited molecules of a supersonic molecular beam. Many efforts have been made to determine the activation energy for adsorption (which includes the activation energy for H2 dissociation) and the reaction pathway for desorption, especially for clean Si single crystal surfaces [94Kol, 00Hil]. To determine the activation energy for dissociation, often MB studies have been performed, but also kinetic studies following changes in the surface reflectivity [97Bei]. In some cases, the order of the thermal desorption reaction can provide valuable hints to the adsorption and desorption mechanism [89Sin, 94Kol]. Generally, however, the data base is again scarce. One reason here is certainly the experimental difficulty to mount and prepare these materials and to reliably measure temperatures and hydrogen atom fluxes. Nevertheless, a quite general scenario can be drawn: In the course of the interaction of H atoms with the respective surfaces, the first step is very often the lifting of the inherent surface reconstruction(s); as the H exposure continues, hydride compounds (monoand/or dihydrides) are formed which thermally decompose at elevated temperatures thereby chemically etching the respective surfaces as mentioned before. We recall that C, Si, and Ge surfaces can adsorb at least a full monolayer of H atoms so as to form a (1×1)-H phase which practically passivates the surfaces and gives them interesting chemical and physical properties. In many recent studies, these passivated surfaces have served as convenient templates for depositing thin films of all kinds of materials.
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
29
Semiconductor and insulator surfaces Surface
C(100)-2×1 diamond C(100)-2×1 diamond C(111)-1×1 diamond C(0001) graphite (HOPG)
Si(100)-2×1
Si(100)-2×1 Si(111)-7×7
Si(111)
Si(111)-7×7
H adsorption Frequency factor states + desorption temperature 1st order [s-1] 2nd order [cm2/s] at sat’n [K] 5 1200 (low ΘH)... 3 × 10 1050 (high ΘH) 1173 1st order 9.5 ±4.0 ×1013 at Θ = 0.2 ML
1310 H: 445 (flat surface) 500 + 560 (step sites) D: 490 (flat surface) 540 + 580 (step sites) 795 (monohydride)
680 (dihydride) β1 790 ~670 β2 β1 870 ~695 β2 β1 870 (small Θ) β1 810 (Θ ≈ 1.0) ~695 β2 ~650 β3 (shoulder state) β1 820 (ΘH=0.12) shifting to 785 (ΘH=0.76)
1st order
2.2 × 1011 (H2) 1.3 × 1011 (D2)
2 × 1015 (D2)
3 × 1015 1.2 × 101 (± 1.3) 136
9.7 × 1012
91 ± 10
2.2 × 1012
Si(100)-2×1
β1
5.5 × 1015
Ge(100) Ge(111)
650 (low Θ) 620 (high Θ) 493 (monohydride)
Landolt-Börnstein New Series III/42A5
expts. performed with D atoms
90Ham
atomic H inefficient to break C C dimer bonds expts. performed with D atoms
92Tho
exposure to H (D) atoms; max. coverages ~0.4...0.5
02Zec
97Su
89Sin isothermal desorption expts., H coverages of 0.06, 0.34, and 1.0 90Sin ML; TD expts. after exposure to H (D) atoms at 123 K. Desorption energy 188.3 kJ/mol for both H2 and D2 exposure to D atoms 93Flo
5.6 × 1011 (D2)
β1 795 (monohydride) β2 ~680 (shoulder only, dihydride)
Ge(100)-2×1
Ref.
1st order
Si(100)-2×1
800
Remarks
2nd order 1st order
exposure to H and D atoms, 88Koe Laser-induced thermal desorption expts. exposure to H atoms; H coverages 83Sch2 β1 state ~ 8 × 1018 H atoms m−2 β2,3 states ~ 3 × 1018 H atoms m−2
TDS and isothermal desorption measurements; desorption activation energy 266 kJ/mol; derived Si - H bond strength 343 kJ/mol TDS and isothermal desorption measurements using D atoms; desorption activation energy 210 kJ/mol derived Si - H bond strength 376 kJ/mol TDS and isothermal desorption measurements; desorption energy 243 kJ/mol TDS after exposure to H atoms at 300 K
91Wis
90Sin
91Wis
84Sur 86Pap
30 Surface
Ge(100)-2×1
3.4.1 Adsorbate properties of hydrogen on solid surfaces H adsorption Frequency factor states + desorption temperature 1st order [s-1] 2nd order [cm2/s] at sat’n [K] 4 × 1013 570 (0.4< Θ < 1.1) 2.7 × 1013 (H2) 1.2 × 1013 (D2)
Ge(100)-2×1
500...550 K
Ge(100)- 2×1
645...620 (0 < Θ < 1) 645...620 (0 < Θ < 1) ~380...420 (0 < Θ < 1)
2nd order
460 (small Θ ) 415 (high Θ ) 480...500 K (high Θ ) 303 - 423
2nd order
Ge(111) GaAs(110)
GaAs(110)
GaAs(001)
2nd order 2 × 10−7
1.0 × 108
[Ref. p. 111
Remarks
Ref.
desorption energy 176 kJ/mol; pairing energy of 22 kJ/mol of Ge surface dimers desorption energy 160 kJ/mol for both H2 and D2 exposure to H atoms; formation of dihydride phase exposure to H atoms; formation of dihydride phase exposure to H atoms, adatom adatom interactions important desorption monitored by intensity decrease of vibrational bands with temperature exposure to H atoms at 250 K, desorption energy between 0.5 and 0.7 eV = 48...68 kJ/mol H coverage 3× 1018 m−2 exposure to H atoms
93Dev
03Lee 84Sur 84Sur 81Lue
84Mok
95Qi
3.4.1.3.3 The energetics of hydrogen adsorption and desorption 3.4.1.3.3.1 General remarks
A quantity that largely characterizes a given hydrogen adsorption complex is the strength of the bond(s) formed between the adsorbed hydrogen atom (or molecule) and the surface atoms of the substrate, EB. We have seen that in the case of non-activated associative adsorption EB equals the adsorption energy, Ead. Thermodynamically, Ead simply is the energy released by the system upon adsorption and counted negatively. We recall that in the case of activated adsorption an additional activation energy of adsorption, E*ad comes into play. When considering the adsorption energetics in general, impinging H2 molecules can encounter two principally different situations: i) They find a clean, H free, surface and can occupy sites that are not influenced by any neighboring adsorbed particles: In this case, the initial adsorption energy, Ead,0 is released by the very first molecules that dissociate and adsorb on the bare surface. ii) A H2 molecule hits a surface which contains already adsorbed H atoms (H coverage Θ ), competes for an empty dissociation site and, once the atoms have become adsorbed, they interact with the already existing neighbors. The resulting adsorption energy EΘ usually deviates from Ead,0. In case of attractive interactions EΘ > Ead,0; for repulsive interactions (which are the rule) EΘ < Ead,0. The respective difference depends, of course, on the number of interacting neighbors leading to a (more or less pronounced) coverage dependence of the adsorption energy, E(Θ ). This kind of coverage dependence described above is induced only by the adsorbed particles; the respective effect is called induced (a posteriori) energetic heterogeneity. As an example, we present in Fig. 6 a typical curve which has been measured for H adsorbing on Ni(111) under extremely clean and well-defined conditions in a glass apparatus [74Rin]. However, there also exist surfaces with inherent (a priori) structural heterogeneity, characterized by energetically different adsorption sites even in the bare state. These are, for example, high-index surfaces with differently coordinated adsorption sites, steps or foreign (impurity) atoms. Exposing such a surface to hydrogen will result in a more or less subsequent filling of the energetically inequivalent sites and, hence, also in a coverage dependence of the adsorption energy.
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
31
Isosteric heat of adsorption Ead [kJ/mol]
90 80 70 60 50 40 30 0
0.2
0.4 Coverage Q
0.6
0.8
Fig. 6: Coverage dependence of the isosteric heat of adsorption (here denoted as Ead) of hydrogen on a nickel(111) surface. Ead has been measured volumetrically [74Rin]. Note that the coverage is given on an absolute scale. Only at lower coverages the heat of adsorption remains constant; it drops at elevated coverages due to repulsive H - H interactions.
In the following table we will mostly focus on values of the initial adsorption energy measured at vanishing or at least small coverages. We caution, however, that the respective measurements and data can suffer from small concentrations of impurities or inherent surface defects. It is known that especially carbon atoms can modify the hydrogen adsorption energetics to a large extent [90Chr]. Therefore it is quite important for the reliability of the tabulated energy data to carefully consider the chemical and crystallographic state of the surface under consideration as well as the vacuum conditions at which the experiments have been carried out. A few words should be added concerning the measurement of hydrogen binding states and their adsorption energies and coverage dependencies. There exist thermodynamic and kinetic means to experimentally determine the adsorption energy Ead. In a thermodynamic experiment, often the isosteric heat of adsorption is measured, either as an integral or a differential quantity. In this case equilibrium conditions are required (which are not always easy to establish), but then the data provide quite reliable thermodynamic information [70Cla]. For more details and definitions, we refer to the chapters 1 and 2 of this Landolt-Börnstein subvolume III/42A (which you can find in parts 1 and 2, respectively) and to the specific literature [77Hie, 69Tra]. Very convenient and relatively simple to perform are Ead measurements based on thermal desorption spectroscopy (TDS, c.f., [91Chr]). Since the exponential term of the PolanyiWigner equation (Eq. 6) contains the desorption energy, it is straightforward to deduce this quantity from an appropriate evaluation (Arrhenius plot) of a TD data set [75Bau1, 75Kin, 83Hab]. It is also possible to measure heats released by adsorption directly by calorimetry. This has been employed in the early days of surface science mainly for powders [55Tra], later for gas adsorption on thin metallic (polycrystalline) films vapor-deposited from a central cathode filament consisting of the film material in question onto the inner surface of glass bulbs [70Wed]. The calorimetrically determined heats of adsorption are integral quantities, unless one admits small gas pulses and follows the development of the released heats, a procedure which requires an utmost sensitivity. Only recently direct calorimetric measurements were carried out also with metal single crystal samples, using a microcalorimeter in conjunction with a molecular beam set-up [92AlS]. In the following section we will examine the kind and number of hydrogen binding states and present data on the initial heat of adsorption as defined above. The cases of associative (molecular) and dissociative adsorption and, for the latter, in addition non-activated and activated adsorption will be distinguished. We will start with a presentation of the (scarce) data on the energetics of genuine hydrogen physisorption (requiring adsorption temperatures <10 K), sect. 3.4.1.3.3.2. Thereafter, the (numerous) scattering experiments using supersonic molecular hydrogen/deuterium beams will be dwelled upon (with emphasis on the noble metals Cu, Ag, and Au), sect. 3.4.1.3.3.3, until we will expand on the much more important atomic (dissociative) interaction of H2 with (predominantly metal) surfaces, sect. 3.4.1.3.3.4. In the column “remarks” a comment can be found on how the energy has actually been measured.
Landolt-Börnstein New Series III/42A5
32
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
3.4.1.3.3.2 Molecular hydrogen adsorption and physisorption phenomena
D2 desorption rate [ML /s]
Turning to physisorption of H2 first, it is agreed upon that physisorption is the ‘normal’ process to occur if chemically inert (insulating and/or semiconducting) surfaces are exposed to and interact with hydrogen gas at sufficiently low temperatures. We recall that free electron (alkali metal, alkaline earth and/or noble metal (Cu, Ag, Au) as well as most semiconductor) surfaces exhibit a surprisingly small activity to dissociate H atoms, in contrast to transition metal surfaces as was already pointed out in sect. 3.4.1.2. Consequently, the dissociation reaction on these latter materials is so effective that the hydrogen molecule breaks instantaneously apart even below 10 K. Hence, a normal TM surface will be immediately covered by a layer of H atoms which saturate the chemical valencies and sort of passivate the surface. Once this layer of chemisorbed H atoms is formed, H2 molecules can merely condense on top, with enthalpies ranging below 5 kJ/mol. This was shown by Frieß et al. for H2 interacting with a (1×1)-H layer on top of a Ru(0001) surface [95Fri]; a series of H2 thermal desorption spectra from their work is reproduced in Fig. 7. The strictly molecular nature of this adsorption bond can be established either by H2/D2 isotope exchange experiments (absence of isotopic scrambling) or by measurements of the H-H stretching vibration which should yield a frequency close to the gas phase value 4153 cm−1 [86Mar].
1.0
0.5
0 5
6
7 10 Temperature T [K ]
15
Fig. 7: Thermal desorption spectra for molecular deuterium multilayers on (1×1)D/Ru(0001). At least five layers can be distinguished from the various TD maxima. The features right to the perpendicular dashed line belong to the first condensed D2 layer, while the peak at 7 K stems from the 2nd layer, the one at 5.8 K to the 3rd, and the maxima below 5.5 K to the 4th and 5th layer, respectively. Note the zero order kinetics of the multilayer peaks. After Frieß et al [95Fri].
On the non-TM surfaces mentioned above and non-metallic materials such as graphite, silicon etc. hydrogen molecules merely physisorb. This means that low, in some cases very low, temperatures are required to capture the physisorbed H2 species and keep it on the surface; even lower temperatures are needed if one wishes to detect two-dimensional molecular hydrogen phases with long-range order or the build-up of hydrogen multilayers (c.f., sect. 3.4.1.3.4). Without going into too much detail here it is simply referred to a review article [04Ptu] and some exemplary papers on H2 adsorption on graphite(0001) covering these low-temperature phenomena [85Fre2, 88Cui].
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
33
Physisorption of hydrogen molecules
Surface
Ni(111)
Initial adsorption energy [kJ/mol] 7.53
Cu(111) Cu(100) Cu(110) Mo(110) + H2; D2 between 1.5..30 K Ru(0001)(1×1)H
2.89 2.99 2.99
Ag(111)
3.1
Au(111)
1.72
2.5 1st layer 1.3 2nd layer
States and their coverage dependence E(Θ )
extrinsic precursor states, Eden clusters 5...6 states
Remarks
Ref.
Physisorption energy of tritium (T2) determined by radiotracer method between 5...30 K MB expts., partial monolayer desorption expts.
73Ren
93And
magneto-resistance expts.
92Lut
physisorption of D2; TDS expts. at 4.8 K; at least three layers can be distinguished diffractive selective adsorption of H2 physisorption potential HD physisorption potential as det’d by MB translational spectroscopy
95Fri
85Yu 86Har
3.4.1.3.3.3 Hydrogen adsorption dynamics
A special field of interest is the interaction of H2 molecules with the free-electron metal surfaces Cu, Ag, Au. These systems have frequently been studied by means of molecular beam techniques which allow, among others, the determination of the H2 - Cu (Ag, Au) scattering potential(s) (‘selective scattering’) and (by measuring the sticking probability as a function of the kinetic energy of the MB) the activation energy for dissociative adsorption. Also, the degree of vibrational and rotational excitation of the H2 molecules and their kinetic energy is often considered and related with the dissociation/adsorption probability [94Ren, 94Hol, 00Din]. Sometimes, molecular dynamics (MD) simulations have been carried out [94Tul]. An important issue in these studies is the energy distribution among the rotational and vibrational states before and after the collision of the H2 molecules with the surface. In some cases, ‘rotational cooling’ has been reported, i.e., the rotational energy distribution of the scattered molecules was only 80...90 % of the surface temperature [85Kub]. Although the dynamical behavior of hydrogen molecules interacting with surfaces is in the focus of scientific interest since many years and a wealth of investigations both experimentally and theoretically have been performed accordingly, we can only touch this topic here for the sake of space limitations. Some special remarks may be devoted to the adsorption dynamics of hydrogen on semiconductor (silicon and germanium) surfaces. Scientists have been interested in energetic quantities, viz., the activation energies for adsorption and desorption and in the actual reaction paths, preferentially the dissociation mechanism. This topic has already been touched in sect. 3.4.1.3.1.7. and we recall the respective remarks made there. The experimental and theoretical progress, along with the technological interest in these materials, has led to an enormous increase in the number of the respective investigations. For example, the availability of scanning tunnelling microscopy made it possible to directly monitor the H2 dissociation process on a Si surface, i.e., disentangle the individual reaction steps [91Bol2, 99Bie]. Dürr et al. directed a heated H2 beam (temperature T ) onto the surface and found with STM that the dissociated H atoms occupy Si atoms of adjacent dimers, and thus pairs of adjacent doubly occupied dimers are readily formed as the reaction proceeds [02Due].
Landolt-Börnstein New Series III/42A5
34
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
Activated adsorption: Metal surfaces and elemental semiconductors Surface
Ni(100)
Height of barrier Depth of for adsorption repulsive [kJ/mol] (scattering) potential [kJ/mol] 4.0 (H2) (v=0) 2.24 (D2) (v = 2) 67 87 14.8 (ĬD=0.6) 12.0 (ĬD=1.0) ĬD= 0.2
Cu(100)
21
Cu(100)
66
Cu(100)
19.3
Cu(100)
56.1 (v=0) 25.0 (v=1)
Cu(110)
13
Cu(110)
67
C(111) graphite
Si(100)2×1 Si(111)7×7 Si(100)2×1
Cu(110)
2.12
Cu(110) Cu(110)
Cu(110)
55.0 (v=0) 25.1 (v=1)
Cu(110)
59.8 ± 6
Cu(110)
57.8 (H2)
Exponent n of cosΘ distribution
Experimental technique
Remarks
Ref.
MB expts
expts. performed at 100 K with H2 and D2
80Mat
SHG expts.
expts. performed with D2 96Bra
5.17 (ĬD=0.6) angle-resolved 3.90 (ĬD=1.0) TDS 2.5...3 TDS; angular distribution + TOF mass spectrometry 5 angular distribution + TOF mass spectrometry of desorption 8 angular distribution + TOF mass spectrometry of desorption 10 supersonic MB expts. angular dep. of adsorption; model fitting 2.5 angular distribution of desorption; supersonic MB expts. seeded MB expts. MB expts; selective adsorption 15.8 supersonic MB expts. 7 angular distribution of desorption angular dep. of adsorption; model fitting H2 reactivity measurements supersonic MB expts. with H2 and D2
expts. performed with D2 93Par1 93Par2 93All expts. performed with HD and D2
74Bal1 74Bal2
H2 and D2 permeation expts.
82Com
89Ang1 barrier height depends on H2 vibrational excitation
91Mic
74Bal1 74Bal2
91Ber1 six bound H2 states
82Per
89Ang1 89Ang1
barrier height depends on H2 vibrational excitation
91Mic
91Cam translational and/or vibrational excitation facilitate dissociation; late barrier found
91Hay
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
3.4.1 Adsorbate properties of hydrogen on solid surfaces Height of barrier Depth of for adsorption repulsive [kJ/mol] (scattering) potential [kJ/mol]
Cu(111)
Cu(111)
66
Cu(111)
6
angular distribution of desorption angular distribution + TOF mass spectrometry of desorption MB expts.+ TOF
2.14
13
60.0 (v=0) 22.1 (v=1)
Cu(111)
Cu(111)
Experimental technique
8
Cu(111)
Cu(111)
Exponent n of cosΘ distribution
8
48...86 (H2, v=0) 19...38 (H2, v=1)
Cu(310)
21
supersonic MB expts REMPI + TOF mass spectrometry
Cu(110)
Cu(111)
REMPI + TOF mass spectrometry
Cu(111)
14
Ag(110)
3.03
Ag(111)
3.09
Ag(111)
angular distribution of desorption angular dep. of adsorption; model fitting angular distribution + TOF mass spectrometry of desorption supersonic MB expts. with H2, D2, and HD + TOF expts.
26.8 ± 0.6
Au(111)
Landolt-Börnstein New Series III/42A5
8 ≥2.02
supersonic MB expts. supersonic MB expts. MB scattering + TOF expts. angle-resolved TPD using D atoms MB expts.+ TOF
Remarks
35 Ref.
74Bal1
H2 and D2 permeation expts.
82Com
scattering of D2 and HD; 86Har selective adsorption potentials 89Ang1
barrier height depends on H2 vibrational excitation H2 permeation expts;
91Mic
82Com
strong increase of 92Ret2 sticking probability with 93Aue gas temperature indicates that both translational and vibrational excitation is important for passing the barrier 74Bal2 H2 permeation expts; 85Kub rotat. and vibrat. stateresolved; vibrational heating observed H2 permeation expts. 85Kub rotat. and vibrat. stateresolved; vibrational heating observed expts. performed with D2 92Ret1 91Can scattering of H2, D2 and 83Yu HD; selective adsorption potentials determined 95Hea
scattering of D2 and HD; 86Har selective adsorption potentials determined
36
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
3.4.1.3.3.4 Dissociative (atomic) hydrogen adsorption
The values listed in the table below were obtained by various methods at different adsorption temperatures; usually, the crystallographic structure of the surfaces in question was determined by LEED, the chemical cleanliness was controlled by Auger electron spectroscopy, XPS, or other techniques. The table includes adsorption energy data also for the inherently inactive noble metal surfaces Cu, Ag and Au. In this case, the data were mostly taken by exposing the respective surfaces to a flux of H atoms (formed by pre-dissociation in the gas phase or by supersonic molecular beams having sufficient thermal energy to overcome the activation barrier for dissociation). This holds for other ‘free electron’ metals, too, such as Be, Mg, Al, Ca, Zn etc., which have recently gained some interest, especially in conjunction with hydrogen storage materials and heterogeneous catalysis. Some of the respective data are also included in the following table. Although one will realize that some fields in the table do not contain numbers, the respective investigations have nevertheless not been omitted, because other important binding staterelated properties were communicated therein, such as the number of states, the temperature position of TPD peaks or the absolute coverage, which may be of interest for the reader and help to get access to the H adsorption energy. Surface
Initial adsorption energy [kJ/mol]
Be(0001)
92 (= activation energy to remove H from the potential well)
Al(100)
States and their coverage dependence E(Θ ) β
Maximum number of H atoms adsorbed [H at/m2] or [ML]
Remarks
1 ML at T = 77 K
TD expts. after exposure 90Ray to H atoms; no adsorption of H2 molecules for T > 90K
β
1 ML at T< 90 K
TD expts. after exposure to H atoms TD expts. after exposure to H atoms TD expts. after exposure to H atoms TD expts. after exposure to H atoms TD expts. after exposure to H atoms MB expts. performed with H2 and D2, competing bulk dissolution
Al(100)
72.85
β
1.6 ML = 1.95 × 1019
Al(110)
69.1
β
24 ML = 2.07 × 1019
Al(111)
67.6
β
1 ML = 1.41 × 1019
Al(111)
76.20
β
1.3 ML = 1.83 × 1019
V(111)
~63
surface 1 ML bulk
Fe(100)
86.6
Fe(100)
100.4 75.4
Fe(100) Fe(100) Fe(110)
Fe(110)
59.0 109 ± 5
101.3 (H) 103.4 (D)
β2 β1 β2 β1
2…3 × 1018
sat’n 1.1 ML β2 β1 β2 β1 19 β2 indep. of Θ; 1.7 × 10 = 1 ML β1 decreases with Θ ~0.8 ML at 200 K
Ref.
88Pau 91Win 91Win 88Mun 91Win 00Beu
80Ben 77Boz 91Ber2 96Mer determined by TDS (Redhead analysis)
77Boz
isosteric heats determined by He scattering
88Kur
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
3.4.1 Adsorbate properties of hydrogen on solid surfaces Initial adsorption energy [kJ/mol]
Maximum number of H atoms adsorbed [H at/m2] or [ML]
Remarks
Ref.
0.48 ML sat’n
MB expts.
91Ber2
determined by TDS (Redhead analysis)
77Boz
1.52 ML sat’n
MB expts.
91Ber2
1.75 ML at 40 K
TDS analysis; surface reconstructs under hydrogen
95Sch1
TDS analysis TDS (isosteric heat)
79Bri 94Ern
isosteric heat (equilibrium) measurements TDS Redhead analysis
74Chr
TDS analysis
81Joh1
Laser-induced thermal desorption TDS analysis; T dependent reconstruction
84Hal
Fe(211)
101.3 82
Co(0001)
73 ± 7
States and their coverage dependence E(Θ ) β2 β1 α β3 β2 β1 β2 β1 β α3 α2 α1 β
Co(10−10)
75.5 (80) 62 96.3
β α β2
96 84 89.2 69.5 49.4 96.2
β2 β1 ‘α’ state (=β2) 5.5 × 1018 at 137 K α1 α2 smaller coverages (0.1 L exposure) 1 ML β2 β1 1.5 ML total at 100 K α
Fe(110)
Fe(111)
88 75 54
Fe(111)
Ni(100) Ni(100) Ni(100)
Ni(100) Ni(110)
Ni(110) Ni(110)
88 75 30 81.6 90.0 (E0) ; 98 (at Θ ≠ 0)
β2
β2
Ni(110)
β2 β1 α
Ni(110)
β2 β1 α
Ni(110)
β2 β1 α β2 β1 α β2 β1
Ni(110)
71
Ni(111) Ni(111)
Landolt-Börnstein New Series III/42A5
95
37
0.13 at 300 K 18
1 ML ( = 9.8 × 10 ) 1.5 ML at sat’n at 85 K
isosteric heat (equilibrium) measurements isosteric heat (equilibrium) measurements; heat increases with Θ due to attractive H-H interactions 1 ML 0.9 ML 0.4 ML total at 140 K = 2.3 ML 1 ML = 1.14 ×1019 D at./m2 coverages determined 0.5 ML by nuclear reaction total (α+β1+ β2) at 175 K analysis (NRA) using D2 = 1.5 ML coverages determined 1.7 ± 0.05 ML upon by nuclear reaction cooling in D2 analysis using D2 expts. performed with H2 and D2 molecular beams 1.9 ± 0.2 × 1019 at 140 K = TDS measurements 1.0 ± 0.1 ML total TDS analysis 2.7 × 1018 at 293 K
79Chr1
89Chr
71Ert 74Chr
82Win
84Jac 85Gri
87Jac1
85Rob
82Win 72Lap2
38
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
Initial adsorption energy [kJ/mol]
Ni(111)
96.3
Ni(111)
85 ± 2
Ni(111)
90.0 79.5
Ni(111) Cu(100)
States and their coverage dependence E(Θ ) β2
4.7 × 1018 total at 100 K β2 β1 β2 β1
0.5 ML 1 ML
48 ± 6 (H2) 56 ± 8 (D2)
Cu(100)
single state
Cu(100)
two states
Cu(110)
Cu(110)
45...96
surface state subsurface state single state
Cu(110) Cu(110)
84.9 ± 1
single state
Cu(110)
59.9 ± 6 (H2) 65.3 ± 7 (D2)
Cu(110)
77.1 ± 10
β2 β1 α
Cu(111) Cu(111)
Maximum number of H atoms adsorbed [H at/m2] or [ML]
~84 ... ~64
β2 β1 single state
0.5 ML (exposure to H atoms) 1.03 ML 0.67 ML
0.5 ML
0.45 (± 0.05) ML at 140 K
1 ML at sat’n (1×1) LEED pattern 0.7 ML total at 83 K after exposure to H atoms (exposure to supersonic MB) 0.7 ML total at 150 K
Cu(111) Cu(111)
64.3 ± 1.3
Zn(0001)
46.5 ± 1.3
β
1 ML
Nb(100)
110.8 ± 12
9 × 1018 3.2 × 1018
Nb(110)
105.8
state 2 state 1 (molecular precursor) two-state chemisorption
Nb(110)
61.9
Mo(110)
142.3 117.1
β2 β1
~7 × 1018 ~7 × 1018 1.3 × 1019 total at 78 K
[Ref. p. 111
Remarks
Ref.
isosteric heat (equilibri- 74Chr um) measurements isosteric heat (equilibri- 74Rin um) measurements TDS analysis (Redhead) 79Chr1 TDS analysis
86Rus
pressures up to 5 bar were used; after extended exposures bulk uptake non-equilibrium effects prevent a data analysis surface reconstructs (exposure to H atoms) exposure to H atoms
93Ras
89Ang1 91Cho 86Rie
exposure to H atoms Edes increases with coverage He scattering expts.
89Ang1
exposure to H atoms; population of subsurface sites beyond 0.5 ML exposure to H atoms; activation energy for dissociative adsorption exposure to H atoms; TDS analysis TDS
93San1 93Bis
Edes decreases slightly with coverage IRAS and LEED study isothermal desorption expts. exposure to H atoms; TDS analysis TDS analysis
89Ang1
90Spi
91Cam
93Goe 83Gre
89McC 05Luo 84Cha2 74Hag
foils with (110) 80Smi2 orientation: data derived from T dependence of UV photoemission kinetic uptake 81Pic measurements TDS analysis 72Mah
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
Initial adsorption energy [kJ/mol]
Mo(100)
113 83.7 66.9
Maximum number of H atoms adsorbed [H at/m2] or [ML]
Remarks
Ref.
5 × 1018 at 78 K
TDS analysis
71Han
Mo(211)
101.3 85.8 71.6 145… 65
Ru(0001)
109
States and their coverage dependence E(Θ ) β3 β2 β1 β3 β2 β1 β3 β2 β1 β3 β2 β1 β2
Ru(0001)
44 70 43.9
β1 β2 β1
Ru(0001)
92
1 ML (= 1.58 × 1019 at 100 K)
Ru(0001)
125....90 <90
β2 β1 β2 (N state) β1 (P state) β2 β1
1.02 ± 0.05 at 200 K
β3 β2 β1 α β α β1 β2 β α2 α1 β α β δ γ2 γ1 β α
1 ML 1.2 1.5 2.0 total (1.73 × 1019)
Mo(100)
Mo(100)
Ru(0001)
Ru(10í10)
82
Rh(100)
50 79.9 ± 1.7
Rh(100)
108.8 ± 3
Rh(110)
Rh(111)
90 (80) 53 33 77.8
Rh(111) Rh(311)
71.2 ± 8 67.5
Rh(311) Pd(100)
Landolt-Börnstein New Series III/42A5
102.6
β shoulder state
39
2 ML total at 210 K
TDS; combined LEED and IR expts. total 2.1 ML ± 0.2 at 100 K combined LEED, HREELS and TDS measurements 2 ML total at 180 K data extracted from isobars; strong decrease of Ed with coverage TDS data plotted as 1 ML total at 250 K; isotherms β2: strong decrease of Ed with coverage 0.85 ML (=1.3 × 1019) at 150 K
87Pry 92Baf 86Zae
94Lop
79Sch
strong decrease of E with coverage; TDS analysis TDS analysis
80Shi
TDS data plotted as isosters
85Feu
78Dan
TD comparison with CO 89Sun (via ads. of formaldehyde) TDS (full line shape 89Lau analysis)
1 ML (= 1.39 × 1019) total at 100 K >0.5 ML
expts. performed with D2; TDS analysis TDS analysis
82Kim
2 ML total at 80 K
TDS (isosteric equilibrium data)
88Ehs 86Chr
1 ML at 100 K (= 1.58 × 1019) 1 ML at 140 K
strong decrease of Ed with coverage TDS and MB expts.
79Yat
less than 1 ML
expt. + theoretical modelling
91Nic1 99Pay
3...4 ML
He scattering
1.35 ML at 170 K
isosteric heat measurements
95Ape1 95Ape2 80Beh
87Ric1
96Col
40 Surface
3.4.1 Adsorbate properties of hydrogen on solid surfaces Initial adsorption energy [kJ/mol]
Pd(100)
Pd(110)
Pd(110)
Pd(110)
Pd(110)
States and their coverage dependence E(Θ ) β α
96.3 (TDS) β 102.15 high T (isosteric heat) shoulder state ~680 K 87.9 ± 8 β2 β1 α2 α1 71 β2 β1 α2 α1 β2 β1 α2 α1
Pd(111) Pd(111)
87.9 ± 5
Pd(111)
89.5 ~77
β shoulder state
75 ± 10 (TDS) (90 ± 10 isosteric heat) 63 (TDS) 45 (TDS) 38 (TDS) 82.9 76.2 22.2
β3
Pd(111) Pd(210)
Pd(311)
Maximum number of H atoms adsorbed [H at/m2] or [ML]
Pd(311)
82.9 76.2 22.2
Ag(100)
~58
β1 150 K α 110 K (sub-surface)
Remarks
Ref.
2.5 ML at 105 K; including α state (due to some bulk absorption subsurface H) only contribution appears at 100 K ads. temp.; H bulk absorption likely; activation energy for bulk absorption 4.6 kJ/mol room temperature study 0.5 ML at 300 K shoulder due to (= 4.7 × 1018) H desorption from bulk 1.5 ML at 130 K; Ĭ > 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]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
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)
Landolt-Börnstein New Series III/42A5
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
3.4.1 Adsorbate properties of hydrogen on solid surfaces
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
Maximum number of H atoms adsorbed [H at/m2] or [ML]
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
Landolt-Börnstein New Series III/42A5
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
Maximum number of H atoms adsorbed [H at/m2] or [ML]
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)
Landolt-Börnstein New Series III/42A5
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
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[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]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
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
Landolt-Börnstein New Series III/42A5
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 <140 (tunneling)
H: 6.5 × 10−3at Ĭ = 0.3 5.9 × 10−4at Ĭ = 0.8 D: 5.7 × 10−4at Ĭ = 0.4 7.1 × 10−4 at Ĭ = 0.8 10−5...10−7
W(110) (tip)
Diffusion energy Ediff [kJ/mol] 14.6 (H) 20.9 (D) 5.0 (H) 4.39 (D)
180...300
10−9...10−10
15.5 ± 2 Ĭ-independent
[Ref. p. 111
Experimental method and remarks
Reference
linear optical diffraction technique. At 170 K, transition from activated tunneling to classical thermal diffusion physisorbed tritium (T2) molecules; radiotracer method field emission fluctuation technique; below ~140 K quantum tunneling sets in
92Lee 92Zhu
73Ren
91Lin
optical grating method plus laser-induced thermal desorption
97Cao
direct STM observations in conjunction with inelastic tunneling spectroscopy:
00Lau
quantum tunneling of single H atoms laser-induced thermal desorption 0.15 < Ĭ < 0.78 strong decrease of D0 for surface contamination with sulfur or carbon laser-induced thermal desorption of H and D
15.5 (low Ĭ) (H) 18.0 (high Ĭ) 18.2 (low Ĭ) (D) 20.5 (high Ĭ) (D) H: laser-induced thermal 13.4 ± 2 at Ĭ = 0.3 desorption 11 ± 2 at Ĭ = 0.8 D: 13.8 ± 2 (0.3 < Ĭ < 0.9) 16.7...29.3 field emission fluctuation technique; for T < 200 K 4.2...8.4 coexistence of two diffusion regimes due to fluctuations caused by single atoms and by collective modes 21.8..24.7± 4 field electron emission, front diffusion 39.7 at very low coverage
86Mak1 87Mak1 87Mak2 88Bra
88See
92Man
95Dan
57Gom
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
3.4.1 Adsorbate properties of hydrogen on solid surfaces Temperature range [K] 143...200
Diffusion coefficient D0 [cm2/s] 5 × 10−5 (H)
<140
7.83 × 10−8
<130
4 × 10−13...10−12, depending on Ĭ.
W(110)
> ~ 100 (thermal regime)
W(211)
80...250
W(110)
<167...190
Pt tip (111), (110), (100) orientation Pt(111)
4.2...320
200...250
47
Diffusion energy Ediff [kJ/mol] 20.0 ± 1.5 (H) 20.3 ± 1.5 (D) 0; (tunnel probability P = 8.69 × 10−5)
Experimental method and remarks
Reference
field emission fluctuation technique; tunneling dominates below 140 K
80DiF
coverage Θ = 0.1: 1.7 × 10−7 (H) 3.5 × 10−5 (D) 3.3 × 10−3 (T); coverage Θ = 0.3: 1.7 × 10−7 (H) 3.5 × 10−5 (D) 3.3 × 10−3 (T); coverage Θ = 0.6: 1.7 × 10−7 (H) 3.5 × 10−5 (D) 3.3 × 10−3 (T); coverage Θ = 0.9: 1.7 × 10−7 (H) 3.5 × 10−5 (D) 3.3 × 10−3 (T) 9 × 10−5 (H) 5 × 10−6 (D) (along channels); 8 × 10−6 (H) 2 × 10−6 (D) (across channels) 6 × 10−14 (H) 5 × 10−14 (D) (along channels) 4 × 10−14 (H) 3 × 10−14 (H) (across channels) 9 × 104
coverage Θ = 0.1: 17.1 (H) 16.5 (D) 20.1 (T) coverage Θ = 0.3: 19.2 (H) 19.6 (D) 20.7 (T) coverage Θ = 0.6: 19.7 (H) 20.3 (D) 22.3 (T) coverage Θ = 0.9: 21.5 (H) 22.4 (D) 24.4 (T) 31 (H) 27.6 (D) (along channels); 27.2 (H) 25.5 (D) (across channels) 0 0
field emission fluctuation technique
85Wan
field emission fluctuation technique; H coverage Ĭ = 0.5
88Dan
3 × 104 at Ĭ = 10−3 0.5 at Ĭ = 0.33
50.2 at Ĭ = 0.001 29.3 at Ĭ = 0.33
82DiF
non thermally activated tunneling dominates
0 0 18.8 (H)
field emission – direct observation of moving H front laser-induced thermal desorption
69Lew
86See
3.4.1.3.5 The structure of adsorbed hydrogen phases
In the following section, the wealth of data which has so far been accumulated for hydrogen phases with long-range order will be reviewed. The vast majority of these phases consists of atomically adsorbed hydrogen and the quantities of interest are i) the coordination number of the local site (terminal, long or short bridge, threefold hollow (octahedral or tetrahedral)) of the adsorbed H atom, and ii) the Me-H bond length (chemical bonding to the adjacent substrate atoms). Often, also layer distances have been determined, which are, of course, geometrically related to the Me-H bond lengths; for the sake of brevity, these data are not included in the following tables, but easily available from the respective references. If necessary, these data (as all surface structural information) can be taken from the NIST structural compilation, vers. 5.0 (NIST = National Institute of Standards and Technology), accessible from the internet under the address http://www.nist.gov/srd/nist42.htm. Since mostly diffraction methods were Landolt-Börnstein New Series III/42A5
48
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
used for structure determination, in particular low-energy electron diffraction (LEED), to some extent also He atom scattering (HAS), the prerequisite for such a structural analysis is the formation of a hydrogen phase with long-range order, which, in turn, is governed by attractive or repulsive mutual H-H interaction forces. The respective ‘extra’ diffraction intensities reflect the degree of order, and the scattering amplitude depends, as usual, on the atomic scattering factor and the position of the H atom(s) in the surface unit mesh. Since a hydrogen atom is a very weak scatterer for electrons and, furthermore, exhibits forward scattering, i.e., large scattering amplitudes in the direction towards the bulk, low LEED intensities of true H superstructures are expected [74Pen, 77Ton]. More details on the specifics of lowenergy electron diffraction at H layers can be found elsewhere [79Chr2, 88Oed, 93Mue, 96Hei]. It is worth to mention that in many cases the ‘extra’ LEED intensities of H adsorbed layers are larger than predicted from kinematic theory. The chemical interaction between atomic hydrogen and the surface atoms causes systematic displacements of the latter leading to phenomena of multilayer relaxation (a modification of the layer distances perpendicular to the surface) or surface reconstruction (the substrate atoms move laterally to new periodic sites giving rise to new diffraction features). Local buckling effects of the substrate atoms varying with the periodicity of the adsorbed H lattice can substantially reinforce the diffraction features. Careful surface-structure analyses performed over the last decades have shown that H-induced alterations of the substrate atom positions are the rule rather than the exception. The following table therefore contains a column in which H-induced surface reconstructions will be listed and commented. Another remark is worthwhile in this context: Some of the 5d electron metals of the third row of the periodic table reconstruct even in the clean state, for example the (100) and (110) surfaces of Ir, Pt, and Au. Since the respective reconstructed surfaces exhibit a complicated structure (which is nevertheless seen by the adsorbing H atoms), we have cited, in a separate line, the respective clean surface structure analysis. Note that under certain conditions the exposure of these reconstructed surfaces to hydrogen can lead to a lifting of the reconstruction, a process referred to as (H-induced) ‘deconstruction’. As mentioned above, He atom diffraction (HAS) is used in a variety of cases for structure determination. Although this atom beam diffraction is the most sensitive method it actually probes only the integral surface corrugation. From this, the H adsorption site and possible first-layer reconstruction(s) must be extracted. Note that subsurface structural changes also enter the corrugation function. Problems and benefits of atom diffraction are covered in various articles [80Rie, 82Eng, 94Rie]. In many cases, valuable information about the local coordination of an adsorbed H atom can arise from analyses of vibrational properties of the H - substrate complex. Especially for metal and semiconductor surfaces, high-resolution electron energy loss spectroscopy (HREELS) has often been employed for this purpose, since both the number and frequencies of the vibrational bands, in conjunction with the electron scattering geometry, provides hints to the local symmetry of the adsorption site. Therefore, the following table also contains – where available – information on the local H - metal geometry obtained by HREELS. Note that a separate section (3.4.1.3.5) is devoted to H-induced vibrations at surfaces. A solid data basis exists concerning both the formation of (H) adsorbate phases with long-range order and phase transitions occurring between the respective H phases. As pointed out in sect. 3.4.1.3.3.1, the long-range order within these phases is determined by the mutual interaction forces between the adsorbed particles (H atoms or, in case of physisorption, H2 molecules), in relation to their thermal energy content, kT. Accordingly, low temperatures are always beneficial to establish long-range order and to render an observation of this order possible. Indeed, several ordered H phases could only be detected at liquid N2 temperature or below (Pd(111)/H [86Fel] and Ru(0001)/H [91Sok] being prominent examples). The formation of ordered H overlayers on metals has also been dealt with theoretically, whereby calculations regarding kind and magnitude of the H - H interaction forces are in the focus of the interest [86Mus]. An exciting field in this context is, of course, the experimental determination and theoretical modelling of phase diagrams (e.g., in the temperature - coverage plane) of ordered H adsorbed layers using the tools of statistical mechanics [78Dan, 79Dom2] and Monte Carlo simulations [83Roe]. This includes the determination of phase boundaries, critical temperatures, critical exponents, and the classification of the order - disorder or order - order phase transitions. Almost a textbook example here is the (2×2)-2H superstructure (with honeycomb symmetry) formed by the Ni(111)+H system at half a monolayer H coverage, which has stimulated much experimental [79Chr2] and theoretical work [79Dom1, 82Kin, Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
49
84Nag1, 84Nag2, 86Mus, 86Roe, 90Ein]. The structure and the respective phase diagram are reproduced as Fig. 8. However, in the context of this data collection we must refrain from any further attempt to expand on all the phase diagrams (which are often quite complicated) and to list the lateral interaction energies (which may be attractive or repulsive, strictly pairwise or three-body related, long-range or oscillatory, depending on the nature of the metal and ist crystallographic orientation). Just with hydrogen, these lateral interaction forces have often quantum-chemical origin and exhibit oscillatory character, as pointed out by Koutecký [58Kou], Grimley [67Gri1, 67Gri2, 73Gri] and Einstein et al. [73Ein, 79Ein]. 300
r
Temperature T [K]
Tc = 270 K
200
c(2×2) -2H 100
0
a Fig. 8a: The famous ‘honeycomb’ structure formed by H atoms chemisorbed on a Ni(111) surface below T = 273 K giving rise to a c(2×2) LEED pattern. The a distance between two H neighbors equals r = 0 6 3 = 2.87 Å.
b
0.25
0.50 Coverage Q
0.75
1.0
Fig. 8b: The formation of the c(2×2) structure requires a H coverage of 0.5 monolayers; lower or larger H surface concentrations impair or even prevent its formation, leading to the phase diagram shown.
Fig. 9: Photograph of the (√3×√3)R30° structure formed by molecular hydrogen adsorbed on a graphite surface. The H-induced ‘extra’ spots have a much lower intensity than the graphite spots and are hardly visible as an internal hexagon. After Seguine and Suzanne [82Seg].
Landolt-Börnstein New Series III/42A5
50
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
Concerning ordered phases of molecular hydrogen, only very few (although quite rich and interesting) data are available in the literature, mainly concerning H2 physisorption on graphite surfaces [82Seg, 89Cui, 87Fre]. Besides LEED, heat capacity measurements and neutron diffraction techniques were used for long-range order detection and analysis. A comprehensive overview is given by Suzanne [03Suz] in sect. 3.6.1 and Wiechert [03Wie] in sect. 3.6.2 of part 3 of this Landolt-Börstein volume III/42A. So far no complete structure determination of the bond geometry of physisorbed H2 molecules has been performed. It is worth to mention that the LEED intensities arising from the ordered H2 phases are as low as expected from kinematic theory [82Seg]. An example is provided in Fig. 9 taken from Seguine’s and Suzanne’s work on H on graphite(0001): The LEED pattern gives a convincing impression of the diffraction intensity differences between the graphite substrate and the ordered H2 overlayer.
3.4.1.3.5.1 Metal surfaces Experimental method
H-subCoordistrate nation bond distance [Å] LEED I,V 1.53 ± 3-fold analysis 0.2
Ordered H phase
Be(0001)
(√3×√3) R30° honeycomb at T = 130 K
Mg(0001)
no H superstructure at T > 100 K
LEED HREELS
Al(100)
no H superstructure at T <280 K; p(1×1)
LEED HREELS
Al(110)
Coverage [ML] and /or [H atoms/m2] 0.67 - 1.0
Critical temp. [K] <270
Surface
1.0
(1×2)rec after annealing at 150 K
Al(111)
ca. 330
LEED, HREELS
HREELS
Ti(0001)
(1×1)H
Cr(110)
p(2×2) streaky (1×1) ~1.0 no H superstructure at 140 K
Fe(100)
1-fold (terminal) H species; 2-fold (bridge) species 2-fold (short bridge) and 1-fold (terminal) 1-fold (terminal) below 150K; 2-fold site for T>150K;
LEED, ARUPS LEED, ARUPS LEED
Remarks
Reference
exposure to H atoms; Be ‘vacancy’ structure in a honeycomb array: each vacancy is decorated by 3 tilted bridge-bonded H atoms exposure to H atoms; surface hydride phase is formed below 425 K; surface does not reconstruct under hydrogen exposure to H atoms
96Stu; 99Poh
surface reconstruction of PR or MR type; exposure to H atoms
92Kon
88Pau
formation of AlH3 at T 91Kon > 150K
1.8...1.9 1.6...1.7
91Spr 94Spr
80Fei2 adsorption at 80 K
88Kom 77Boz
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces Criti- Experical mental temp. method [K] HREELS
Surface
Ordered H phase
Fe(100)
no H superstructure at 140 K p(2×1) =c(2×2)
0.5 0.5
245
(2×2)-2H
0.5
80
LEED
(3×1)-2H =(3×3)-6H (1×1)
0.67
265
LEED
1.84
HREELS
1.94
Fe(110)
Coverage [ML] and /or [H atoms/m2]
Fe(211)
no H superstructure at 135 K c(2×6)-8H (2×1)p1g1 c(2×6)-16H c(2×4)-12H
0.67 1.00 1.33 1.50
140 180 180 80
LEED LEED LEED LEED
(1×2)-3H
1.50
170
LEED
c(2×6)-20H (1×3)-5H (1×1)-2H
1.67 1.67 2.00
<40 >40 60
LEED LEED LEED
(1×2)-rec
0.2 - 1.2
no H superstructure at 300 K Co(10−10) c(2×4)-4H (2×1)-2H p2mg (1×2)
Ni(100)
Ni(100)
Ni(100)
no H superstructure at 300 K; none at 120 K quasiordered p(2×2)-H p(1×1) no H superstructure at 120 K (1×1)
Landolt-Börnstein New Series III/42A5
2-fold longbridge 3-fold, H atom radius 0.58 Å
Reference
96Mer
local buckling reconstruction; expts. at 40 K
2-fold (short bridge)
expts. performed at 130 K
LEED
77Boz 85Mor1 82Imb; 92Nic 85Mor1 93Ham1 82Imb
unknown
metastable metastable metastable annealing reqd. T-dept. PT annealing reqd. T-dept. PT
270 290
LEED HREELS
missing-row (1×2) reconstruction of all phases for T > 280 K
3-fold 3-fold
90Sch 95Has
94Ern
missing-row or pairing-row reconstruction LEED
LEED
1.0
74Chr
4-fold
HREELS
78And2
4-fold
HREELS
86Kar 79Chr1
expts. performed at 100...200 K
83Rie1
LEED
1.0
95Sch1
79Bri
1.50
0.25
90Sch
95Sch1 95Sch1
LEED
0.50 1.00
81Bar1
77Boz
LEED
Co(0001)
Ni(100)
1.75 ± 0.05
Remarks
1.0 = 1.7 19 −2 × 10 m
Fe(110)
Fe(111)
LEED LEED
H-subCoordistrate nation bond distance [Å] 4-fold
51
He 1.95..2.0 diffraction
52
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
Ordered H phase
Ni(100)
(1×1)
Ni(110)
(2×1)= (1×2) streak (2×1) (1×2) (1×2) streak (1×2) streak (2×3)-1D c(2×6) c(2×4) c(2×6) c(2×6) (2×1)-2H p2mg
Ni(110)
Ni(110) Ni(110)
(1×2)-3H
Ni(110)
Ni(110)
Ni(110)
Coverage [ML] and /or [H atoms/m2] 1.0
Criti- Experical mental temp. method [K] He transmission ion channeling LEED
Reference
expts. performed with deuterium
85Ste
H-induced reconstruction
62Ger 74Tay
>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
<175 <175 <220
>220
missing-row reconstr. expts. performed at 84Jon 150 K, missing-row reconstruction assumed expts. performed with 87Jac1 deuterium at T = 175 K
LEED analysis
1.0 1.5
LEED 220
0.6 0.8 1.0 1.2-1.6
(1×2)
He diffraction
metastable metastable metastable metastable
>220 0.33 0.5 0.67 0.83 1.0 1.5 1.00
<220
Ni(110)
c(2×6) c(2×4) c(2×6) c(2×6) (2×1)-2H (1×2)-3H (2×1)-2H
Ni(110)
(1×2)-3H
1.5
<220
Ni(110)
(2×1)-2H (1×2)-3H (1×2)-streak
1.0 1.5
<220
Ni(110)
Remarks
120 K LEED
(1×2)-streak (1×2)-3H
(2×1)-2H (1×2)-3H (1×2) streak (2×6) (2×6) (2×1)-2H (1×2)-3H
H-subCoordistrate nation bond distance [Å] 4-fold; D-Ni layer distance = 0.5 Å
[Ref. p. 111
He diffraction
LEED analysis
LEED analysis HREELS
1.72 ± 0.05
3-fold; H atom radius = 0.47 ± 0.05 Å
reconstructed disordered phase metastable metastable metastable metastable metastable metastable metastable
pairing-row reconstruction 3-fold
81Eng 82Eng
83Rie2 85Rie
87Rei1
87Kle1 85DiN
>220
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces Critical temp. [K] <220 <220
Experimental method
(1×2)-streak
>220
Ni(110)
(1×2)-streak
Ni(110)
(1×2)-3H
300 ≤1 (5L exposure) 1.5 <220
LEED, time-offlight + Ne recoil scattering STM
Ni(110)
(1×2)-streak
Ni(110)
(2×1)-2H (1×2)-3H (1×2)-streak
Ni(111) Ni(111) Ni(111)
(2×2) (2×1)=(2×2) (2×2)-2H honeycomb
0.3-0.6 0.5
Ni(111)
(2×2)-2H
0.5
(1×1)-H
1.0 ± 0.1
Ni(111)
(2×2)-2H honeycomb
0.5
He 1.86 ± diffraction 0.07
Ni(111)
(2×2)-2H
0.5
LEED analysis
Ni(210)
no H superstructure at 90 K p(2×2)p4g /pgg (4√2×4√2) R45°
Surface
Ordered H phase
Ni(110)
(2×1)-2H (1×2)-3H
Ni(110)
Cu(100)
Landolt-Börnstein New Series III/42A5
Coverage [ML] and /or [H atoms/m2] 1.0 1.5
>220
270
HREELS
H-subCoordistrate nation bond distance [Å] 3-fold two types of 3-fold sites
X-ray photoelectron diffraction He diffraction STM
LEED LEED LEED 1.84 ± analysis at 0.06 T = 120 K
transmission ion channeling
3-fold; bcc and hcp sites H atom radius = 0.59 ± 0.06 Å two kinds of 3-fold sites
Remarks
Reference
3-fold modified row-pairing
89Voi
missing-row reconstruction
91Rou
missing/added-row reconstruction pairing-row reconstruction
91Nic2
missing-row reconstruction Ni-H strings; mechanism of Hinduced missing row reconstruction elucidated
93Far
protonic band structure discussed
expts. performed with D2
LEED LEED
92Kna
04Ale
69Ber 78Beh 79Chr2
88Mor
exposure to D atoms
1.73 ± 0.08
both types of 3-fold sites 3-fold; additional local H atom surface reconstruction radius = (local buckling) 0.49 ± 0.08Å
LEED
1.03
53
91Gro
93Ham1
01Sch2
H-induced reconstruction obs. with D at 83 K; exposure to H(D) atoms
91Cho
54
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
Ordered H phase
Cu(110)
(1×2)*
Coverage [ML] and /or [H atoms/m2]
Criti- Experical mental temp. method [K] LEED, He diffraction
H-subCoordistrate nation bond distance [Å]
(1×2)
Cu(110)
(1×3)
0.2....0.8
100
Neutral impact collision ion spectroscopy (NICISS) LEED, HREELS
Cu(110)
(1×4) (1×3) (1×2) (1×1) (1×2)rec (1×2)
0.25 0.33 0.50 0.5 0.5
80 120 100 120 >140
92Mor
tilted reconstructed 3-fold sites exposure to H atoms exposure to H atoms
He diffraction
missing-row reconstr. exposure to H atoms
LEED, TDS, UPS
(1×2)
93Goe
93San1
missing or added row reconstruction
(1×3)rec
0.12 - 0.4
(1×2)rec
0.5
Cu(110)
(1×2)rec
0.5
Cu(111)
(2×2) (3×3)
0.5 0.67 ?
186
Cu(111)
(2×2)
0.5 0.67
100
Mo(100)
IC(4×2) c(2×2) (4×2) IC c(2×2) (3×2) IC(2×2) (2×2) (1×1)-2H
~0.1 0.2-0.3 0.12-0.28 0.3-0.28 0.3-0.4 0.4-0.55 0.5-0.75 >0.8
~200 ~210 >200 <200
Cu(110)
Reference
reconstructed
(1×2) Cu(110)
Remarks
subsurface 86Rie reconstruction at 300 K transforms to (1×2)* at 240 K exposure to H atoms 90Spi missing-row reconstruction; (1×2)* phase [ref. 86Rie] could not be confirmed
(1×2)
Cu(110)
[Ref. p. 111
LEED
LEIS
>200
LEED, RAIRS, HREELS LEED
LEED
94Roh
1.54 ± 0.09
missing or added row reconstruction missing-row reconstruction
3-fold (2 1st layer + 1 2nd layer atoms) 2-fold exposure to H atoms bridge site
modified 2-fold
bridge sites
exposure to D atoms Θ = 0.67 max. H coverage determined by nuclear reaction analysis (NRA) complicated phase diagram; H-driven surface reconstructions (periodic distortions) partially involved ; several T-dependent phase transitions
98Mij
89McC
96Lee
79Est 80Bar 87Pry 80Ben
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces Criti- Experical mental temp. method [K] LEED, HREELS
Surface
Ordered H phase
Mo(100)
c(2×2)
Coverage [ML] and /or [H atoms/m2] <0.2
(4×2)
<0.2
(3×2)
(2×2)
sat’n = 2.1 ± 0.2 H:Mo = 2 ML ~0.5
(1×1)
>0.5
(2×2)-2H
0.5
(1×1)-H
1.0
LEED
Mo(111)
(1×1)-3H
3.0
LEED analysis
Mo(211)
Ru(0001)
(1×1) (1×2) (√3×√3)R30° p(2×1) (2×2)-3H p(2×1)
<1.0 >1.0 0.33 0.50 0.75 0.50
Ru(0001)
disordered
1.0
Mo(110)
Mo(110)
Ru(0001)
H-subCoordistrate nation bond distance [Å] modified 2-fold bridge sites
LEED, HREELS
200
360 74.5 68.0 71.0 68
Ru(0001)
LEED
1.93
1.90 ± 0.03
LEED, HREELS LEED
Ru(10−10) c(2×2)-3H
(1×1)-2H
Landolt-Börnstein New Series III/42A5
1.0
150
220 1.2 200 1.5 2.0 = 1.73 150 19 × 10 1.5
2.0 =1.73 19 × 10
3-fold hollow 3-fold hollow H atom radius = 0.51 ± 0.03 Å 2-fold 3-fold
LEED analysis
2.0 ± 0.2
3-fold fcc site
VLEED analysis
1.91 ± 0.06
3-fold fcc site; H atom radius = 0.56 Å 3-fold hollow
HREELS
Ru(10−10) c(8×2) = “(1×2)split” (1×2) c(2×2)-3H (1×1)-2H
quasi3fold, intermediate longbridge site 3-fold hollow
LEED, HREELS
LEED analysis
2.01 ± 0.40 1.95 ± 0.60 1.91
H atom radius = 0.65... 0.70 Å
55
Remarks
Reference
H-induced reconstr. below Θ = 0.2; T-dependent phase transitions
86Zae
H-induced surface reconstr.
97Oka
H-induced buckling; honeycomb struct.; H atom radius = 0.57 Å (2×2) and 0.65 Å (1×1) phase
87Alt 97Arn1
99Arn
93Lop H driven reconstr. phase diagram determined
91Sok 93San2
+ buckling and pairing 92Hel reconstruction; H atom radius = 0.75 ± 0.2 Å expts. performed at 87Lin 170 K; no subsurface H 86Feu adsorption expts. performed at 170 K expts. performed at 100 K
83Bar
quasi-3-fold site; slight buckling and lateral displacement effects
98Doe
89Lau
56 Surface
3.4.1 Adsorbate properties of hydrogen on solid surfaces Ordered H phase
Coverage [ML] and /or [H atoms/m2]
Ru(11−21) no Hinduced superstructure (1×1)? Rh(100) (1×1) = no H 1.0 = 1.39 19 super× 10 structure at 100 K Rh(100) (1×1) 1.0
Criti- Experical mental temp. method [K] HREELS LEED
H-subCoordistrate nation bond distance [Å] pseudo-3fold sites + 4-fold site
[Ref. p. 111
Remarks
Reference
01Fan
LEED
82Kim
HREELS
Rh(100)
(1×1)
1.0
HREELS
Rh(110)
(1×3)-H (1×2)-H (1×3)-2H (1×2)-2H
0.33 0.50 0.67 1.0
LEED
1.64
4-fold hollow 4-fold hollow
87Ric1 CO coadsorption study 88Ric2 86Chr 88Ehs
Θ (1×2)-2H has been corrected to contain 3 H atoms/unit cell: → Θ (1×2)-3H = 1.5
Rh(110)
(1×1)-2H (1×3)-H (1×2)-H (1×3)-2H (1×2)-2H
(1×1)-2H
2.0 0.33 0.50 0.67 1.0 (1.5)
130 140 >180 >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
Landolt-Börnstein New Series III/42A5
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)
Landolt-Börnstein New Series III/42A5
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
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
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
<100
HREELS
0.25 0.50 0.75 1.0
170 170 170 170
He diffraction, HREELS
Ag(111)
4-fold 4-fold 3-fold
<170
He diffraction
He diffraction (1×4) (1×3) (2x6)
0.20 <0.5 0.5-0.75
(2×2) = mixture of (2×1)2H + (1×2) (1×2) rec
1.0
~110
Ag(111)
(2×2) mixed (2×2) and (3×3)
0.5 >0.5
Ta(100)
(1×1)-H
1.0
Landolt-Börnstein New Series III/42A5
LEED, HREELS
tilted 3fold
1.5
(2×2) mixed (2×2) and (3×3) (4×4)
140
Reference
expt. + theory population of surface + subsurface sites
01Sch1 02Lis
(co)adsorption of 01Sch1 molecular H2 thermally activated 99Far transition surface H → subsurface H
LEED, HREELS
3-fold
1.92... 1.94
3-fold hollow
04Kol
89Can
determination of 91Can interaction potentials; no H2 chemisorption H lattice gas structures 93Spr accompanied by weak Ag reconstruction
row-pairing reconstruction expts. performed with D atoms
LEED
LEED analysis, HREELS
Remarks
subsurface H in octahedral sites surface exposed to H atoms; H-induced reconstruction expts. performed with D atoms
LEED
Ag(110)
Ag(110)
H-subCoordistrate nation bond distance [Å] sites with three different coordinations: A, B, C
59
surface reconstruction likely expts. performed with H and D atoms; (3×3) phase induced by LEED beam. H phase diagram det’d. (2×2) phase accompanied by Ag surface reconstruction H atom radius = 0.5..0.6 Å
95Hea
94Lee 95Lee
04Yam
60
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
Ordered H phase
W(100)
c(2×2) = (√2×√2)R45° c(2×2)-H followed by complex series of LEED patterns indicating IC phases
Coverage [ML] and /or [H atoms/m2]
Criti- Experical mental temp. method [K]
H-subCoordistrate nation bond distance [Å]
2-fold (bridge) sites
0.25
1.0 = 1.002 × 1019
≥ 1.7 × 1019 ≥ 1.5 ML (√2×√2)R45° 0
[Ref. p. 111
Remarks
Reference
clean surface (√2×√2) R 45° reconstructed (zig-zag chains)
88Alt 81Bar2
reconstruction in a dimer-like arrangement H/W(100) phase diagram involving C/IC transitions
77Deb 79Deb
(1×1)
W(100)
78Bar1 66Est LEED
clean surface reconstructed; dimer-surface reconstruction, expts. at 300 K
c(2×2)
0.04 0.22 0.22 c(2×2)-H split ½-order 0.36 streaked ½ order
0.3-0.48
streaks + 1/5-order beams
0.8-1.0
86Wil 81Bar1
80Kin
bridge complexes are formed + W surface atom displacements occur
1.0-<2.0 disordered (1×1) (1×1)-H W(100)
2.0 0 ...1.0
(1×1)
2.0
W(100)
(1×1)-H
2.0 ± 0.3
W(100)
(√2×√2) = c(2×2)-H (1×1) (1×1)-2H
<0.5
W(100)
2.0 2.0
Ĭ-dependent occupation of two sites
HREELS
76Fro
2-fold (bridge) LEED electronstimulated desorption (ESD) HREELS
LEED analysis
66Est 73Mad
bridge sites 1.97 ± 0.04
2-fold (bridge)
two-state reconstruction (tilted dimer model)
83Did
85Pas
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
W(100) clean
W(100)
W(100)
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Ordered H phase
Coverage [ML] and /or [H atoms/m2] (√2×√2)R45° 0 = c(2×2)-H
Criti- Experical mental temp. method [K] LEED analysis
<0.5 c(2×2)-H phase= (√2×√2)R45° c(2×2)H 0.4 = 4 × 1018
c(2×2)
HREELS
0.3
H-subCoordistrate nation bond distance [Å]
1.95 ± 0.05
c(2×2)-H 0.3-0.5 split ½-order
W(100) W(100)
W(110)
streaked ½ order; streaks + 1/5-order beams; disordered (1×1)
0.6-1.9
(1×1)-2H c(2×2)-H rec p(1×1)-H c(2×2)
2.0 < 0.5 1.0 <0.5
p(2×1) (2×2) p(1×1)
0.5 0.75 1.0
VLEED
Reference
precise crystallography of clean reconstructed surface. Clean: zigzag rows; with H: dimer model. position of H atoms not determined
92Sch1
78Bar2
accurate and detailed vibrational investigation
86Arr
reconstructed surface relaxed surface
86Her
bridge sites (pinched and antipinched)
89Rif
78Gon
W(110)
angle-resolved UV photoemission (ARUPS) HREELS LEED
W(110)
(1×2) (2×2)
<0.5 >0.5
W(110)
p(2×1)
1.0
VLEED
W(110)
(1×1)
1.0
LEED analysis
Landolt-Börnstein New Series III/42A5
1.73 1.96
RAIRS (reflectionabsorption IR)
Remarks
bridge sites on a reconstructed surface
RAIRS, LEED
61
>200 >250
bridge or distorted bridge
2.09
3-fold
3-fold ; H atom radius = 0.66 Å
82Bla
clean surface unreconstructed; H-induced surface reconstruction (lateral shift = registry shift) two inequivalent H sites sequentially occupied high quality data; no H-induced registry shift!
86Chu
86Her
97Arn2 97Arn3
62
3.4.1 Adsorbate properties of hydrogen on solid surfaces Criti- Experical mental temp. method [K] HREELS
Surface
Ordered H phase
W(110)
(2×1) (2×2)
<0.75
(1×1)
1.0
p(2×1) p(2×2) p(1×1)
0.4...0.5 0.4...0.8 1.0
LEED
8 × 1018 = 1 ML
Time-offlight and recoiling spectrometry LEED
W(110)
W(211)
Coverage [ML] and /or [H atoms/m2]
Re(0001)
no H superstructure Re(0001) (2×2) with 0.25 ~300 missing spots 1.5 = Re(10−10) c(2×2)-3H 1.22×1019 2.0 (1×1)-2H 1.5 = Re(10−10) c(2×2)-3H 1.22×1019 (1×1)-2H
LEED analysis
2.0
LEED
Ir(100)(1×5)
(1×3) (1×3)+(1×1)
sat’n
LEED
Ir(100)(1×5)
(1×3) (1×3)streaky
sat’n
LEED analysis
Ir(110)(1×2)
clean surface 0
LEED LEED analysis
Ir(110)(1×2)
Remarks
Reference
quasi-3-fold in 94Bal between the long bridge and shortbridge site H liquid like with 1-D ordering phase diagram: careful 97Nah1 TDS and LEED Θ calibrations reveal systematic Θ differences 89Gri
expts. performed at 300 K LEED pattern visible between 110...300 K
LEED
clean surface 0
no H superstructure for T >130 K
3-fold trough site
LEED
Ir(100)(1×5)
Ir(110)(1×2)
H-subCoordistrate nation bond distance [Å]
[Ref. p. 111
LEED
HREELS
81Duc 90He 95Mus
1.85 ± 0.4
2 H atoms H atom radius = 0.47± in quasi 3- 0.4 Å fold + 1 H atom in a 2-fold (bridge) site clean surface hexagonally reconstructed the reconstruction is affected by H. (1×3) phase is metastable (1×3) phase is stabilized by H; (1×3) phase reconstr.; it contains 3 Ir atoms + layer rumpling clean surface (1×2) reconstructed reconstruction of missing-row type with layer relaxation (paired rows in 2nd and buckled rows in 3rd layer) clean surface MRreconstructed 3-fold hollow
clean surface MRreconstructed
98Doe
85Bic
00Mor
01Sau
73Chr2 86Cha
80Ibb
88Cha
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
Ordered H phase
Ir(111)
no H superstructure for T >130 K
3.4.1 Adsorbate properties of hydrogen on solid surfaces Coverage [ML] and /or [H atoms/m2]
Ir(111)
Criti- Experical mental temp. method [K] LEED
HREELS
Ir(111)
no H superstructure
HREELS
Pt(100)(5×20)
clean surface 0.00
LEED
LEED analysis Pt(100)(5×20)
no H superstructure
Pt(100)(5×20)
no H superstructure
Pt(100)(5×20)
no H superstructure
LEED
Pt(100)(5×20)
no H superstructure at 35 K
LEED
Pt(100)(5×20)
no H superstructure at 300 K (1×1)
LEED
no H superstructure at and below 250 K
He diffraction
Pt(100)(5×20)
Pt(110)(1×2)
LEED
1.54 ± 0.1 × 1019 = 1.20 ± 0.08 ML
1.0 at 120 K = 1.28 × 1019 H atoms/m2 clean surface 0
Rutherford backscattering
LEED analysis
LEED Field ion microscopy
Landolt-Börnstein New Series III/42A5
H-subCoordistrate nation bond distance [Å]
Remarks
63 Reference
87Eng
3-fold hollow 1-fold (terminal site bonding) at larger coverages hexagonal arrangement of top Pt layer
88Cha quantum delocalized H motion at low coverages
99Hag
clean surface hex. and hex.-rot reconstructed, respectively, depending on T
79Hei
81Van1 81Van2
hex reconstruction can 81Bar3 be lifted to (1×1) phase by H adsorption at T < 200 K reconstruction only 81Nor2 partially lifted by H adsorption hex-rot reconstruction only partially lifted by H adsorption; critical coverage = 2.8 × 1018 H atoms/m2 at 35 K no removal of (5×20) reconstruction by H, heating to 100 K necessary to lift reconstruction no structural change after H2 dosing. (1×1) phase obtained after dosing to H atoms at 300 K. (1×1) = disordered Pt, H atoms neglected hex reconstruction is not removed below 250 K; H atoms cause charge redistribution at the (1×1) surface clean surface (1×2) reconstructed reconstruction of missing-row type with layer relaxation
93Klo
91Pen1 91Pen2
95Hu
95Rom
81Ada 85Kel
64 Surface
Pt(110)(1×2)
3.4.1 Adsorbate properties of hydrogen on solid surfaces Ordered H phase
Coverage [ML] and /or [H atoms/m2] clean surface 0
Criti- Experical mental temp. method [K] LEED analysis
H-subCoordistrate nation bond distance [Å]
Remarks
Reference
surface (1×2)- and/or (1×3) reconstructed. Reconstruction of missing-row type H adsorption into (111) microfacets
88Fer1 88Fer2
Pt(110)(1×2)
no H superstructure
LEED, TDS
Pt(110)(1×2) Pt(110)(1×2)
no H superstructure no H superstructure
MB expts.
Pt(110)(1×2)
no H superstructure
Pt(110)(1×2)
no H superstructure
TensorLEED (+ DFT calc.)
Pt(111)
no H superstructure at T>150 K (1×1)-H phase likely no H superstructure
LEED
slight lattice expansion 75Chr due to H deduced from LEED (I,V) data
Rutherford backscattering + nuclear reaction analysis HREELS
slight outward relaxation under hydrogen
Pt(111)
Pt(111) Pt(111) Pt(111)
no H superstructure no H superstructure (1×1)-H at 160 K
Pt(111)
(1×1)-H
Pt(111)
(1×1)-H
deep trough sites populated first
[Ref. p. 111
LEED, W.F. measurements He 1.8 diffraction (HAS)
0.8 ± 0.4 at 150 K 1.1 × 1019 = 0.733 ML
HREELS 1.0
1.93
He diffraction
Low1.78 ± energy ion 0.08 recoil Scattering (LERS) Ion chan- 1.9 ± 0.1 neling
2-fold (bridge) sites; H atoms below the topmost Pt rows 2-fold (bridge) sites; confirmation of HAS data [90Kir]
3-fold hollow 2-fold + 3-fold 3-fold (C3v) (hcp-type)
3-fold fcc type
3-fold fcc type
87Eng 88Duc
H adsorption into (111) microfacets two different sites (H sits in diatomic Pt clusters)
89Ang2
Pt atom relaxation changes by H adsorption
90Kir
relaxation of Pt - Pt layer distance changes with H coverage
04Zha
92She
80Dav 82Nor1
79Bar (theory to HREELS)
84Say1
corrugation changes 83Lee slightly with (1×1)-H layer; confirmed by calculations of [84Bat] kind of site differs 86Koe from Lee’s work [83Lee]
99Lui
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
Ordered H phase
Pt(111)
no H superstructure at 85 K
Pt(111)
(1×1)-H
Pt(111)
no H superstructure
3.4.1 Adsorbate properties of hydrogen on solid surfaces Criti- Experical mental temp. method [K] HREELS
H-subCoordistrate nation bond distance [Å]
1 ± 0.05
HREELS, TPD
3-fold hollow fcc-site
0.25
DFT theory
Coverage [ML] and /or [H atoms/m2] <0.75
Au(100)- clean surface 0 c(26×68); phase often incorrectly described as (5×20) Au(100)- incomplete 0.3 (5×20) (1×1)
LEED analysis
LEED
Au(110)(1×2)
clean surface 0
STM
Au(110)(1×2)
clean surface 0
LEED analysis
Au(110)(1×2)
no H-indu0.5 ced ordered phase clean surface 0
LEED
Au(111) Au(111) (thin film grown on Ir(111)
STM LEED, TDS
65
Remarks
Reference
H atoms quantum delocalized, several vibrational bands resolved H is quantum delocalized; expts. performed at 85 K 3-fold fcc site; H is quantum delocalized complicated hexagonal reconstruction of top layers
02Bad
exposure to H atoms; after large H exposures incomplete (1×1) + faint remanent (5×20) phase: Hinduced deconstr. missing-row reconstruction with (111) facets clean surface missingrow reconstructed (2nd layer row-pairing + 3rd layer buckling) exposure to H atoms. (1×2) reconstruction not lifted clean surface reconstructed dissociative H2 adsorption claimed
96Iwa
03Bad
01Kae
81Van1
83Bin1
85Mor2
86Sau
90Bar 03Oka
3.4.1.3.5.2 Semiconductor and insulator surfaces
The surface structure of semiconductors (especially of the elemental semiconductors Si and Ge) has been of interest since the early days of Surface Science. It has been found that practically all of these materials reconstruct spontaneously, the reason being the covalent bonding chemistry which forces cleaved surface bonds to bend and to form dimers thus enabling a lowering of the surface free energy. The most famous example is certainly the silicon(111) surface which usually reconstructs to a complicated (7×7) surface phase whose structure could unequivocally be resolved only by combined electron diffraction [85Tak] and STM studies in the eighties and nineties [83Bin2, 91Bol1, 91Bol2, 91Mor]. For the same chemical reason, also the Si(100) surface is (2×1) reconstructed in the clean state, the driving force being the formation of both a ı bond and a weak ʌ bond between the two silicon atoms in each dimer [85Tro].
Landolt-Börnstein New Series III/42A5
66
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
Less was known at this time about how adsorbed hydrogen could modify these structures until the possibility of chemically ‘etching’ Si surfaces by H containing solutions (ammonium fluoride NH4F etc.) was discovered leading to almost perfect (1×1) surface phases in which each cleaved Si - Si bond was saturated by a H atom. The same perfect surfaces could be obtained, if the Si surfaces were exposed to molecular hydrogen at elevated temperatures (T § 900 K), because the sticking probability increases very much with temperature [99Mao]. Since then a feverish activity to prepare, characterize and technologically use these smooth and chemically inert surfaces began which led to an enormously distended literature. About the same time, the chemical origin of this ‘deconstruction’ behavior was acknowledged, and related phenomena were reported also for Ge and C and some compound semiconductor surfaces. However, here we can only briefly review some of the exemplary articles. Semiconductor and insulator surfaces Surface
C(100)diamond C(100)diamond
C(100)diamond C(111) graphite
Ordered H phase or H-induced reconstructed phase clean surface
Coverage [ML] and /or [H atoms/m2]
Experimental method
Remarks
Ref.
0
LEED
reconstruction with two (2×1) domains
77Lur
LEED, electronstimulated desorption LEED I,V analysis LEED
thermally driven reconstruction from 1×1 to 2×1; H-C-H surface dimers are present
90Ham
1×1 and 2×1
2×1-H
monohydride (¥3×¥3)R30° <1ML
incommen>1ML surate phases C(111)-1×1 clean surface 0 diamond C(111)≤1 ML 1×1 to 2×1 transition C(111)-2×1 clean surface 0 diamond Si(100)2×1 clean surface 0 c(2×4) p(2×2) Si(100) (3×1) Hsaturation, H:Si atom ratio 3:2 Si(111)7×7 clean surface 0
Si(111)7×7
(7×7)-H
(Tensor-LEED) dimer model, dimer length = 1.60 Å 99Wan physisorbed H2, HD, D2 on graphite single crystals; phases and phase transitions
89Cui
LEED-I,V surface atoms have bulk position with small relaxation LEED thermally induced (1×1)-to-(2×1) phase transition in ESDIAD the presence of hydrogen
82Yan 88Ham
LEED-I,V reconstruction with two (2×1) domains LEED STM dimer-type reconstruction with buckled and nonbuckled dimers, stabilized by defects
82Yan 90Ham 85Tro 86Ham
LEED, highresolution IR STM
alternating monohydride and dihydride unit cells responsible for (3×1) phase
85Cha1
clean surface reconstructed in a complex manner with corner holes and adatoms to yield a (7×7) repetition unit
83Bin2 91Mor
TED (transmission electron diffraction) fully H-co- kinematic vered sur- LEED face (1 H /Si atom)
85Tak
adsorbed H atoms form triangular Si islands with [−1−12] step boundaries
81McR
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
Ge(111) (2×1) c(2×8) Ge(111)
GaAs(001) (2×4) c(8×2) c(4×4) GaAs(001) c(4×4)
GaAs(110)
3.4.1 Adsorbate properties of hydrogen on solid surfaces Ordered H phase or H-induced reconstructed phase clean surface
Coverage [ML] and /or [H atoms/m2]
(1×1)-H
fully Hcovered surface 0
clean surface
0
disordered H covered (1×1)at 323 K surface c(2×2) + c(4×2) at T > 423 K clean surface
Experimental method
0
LEED I,V analysis LEED, STM
reflection highenergy electron diffraction (RHEED) TOF-ion scattering spectroscopy
67
Remarks
Ref.
clean surface reconstructed either in (2×1) or in c(2×8) phase
81Cha
Negligible influence of adsorbed H atoms on Ge LEED intensities. Slight contraction of 1st -to -2nd layer distance surface As terminated. Dimer vacancy structure responsible for 4-fold periodicity in [110] direction top layer is based on rectangular units.
87Imb
reconstruction still not fully understood disordered (1×1) phase = AsHx clusters
87Cha 88Pas 90Bie 95Ave 01Nag 04Kha
centered phases: H adsorbed on surface Ga atoms, H-induced loss of As clean surface relaxed, 1st As layer located above the Ga layer
97Gay
3.4.1.3.6 Vibrational modes of adsorbed hydrogen
Vibrational spectroscopy is extremely helpful in determining both the structure and coordination of a given H - substrate adsorbate complex, but also in gaining information about the strength of the respective H - substrate bond potential [87Ham]. In contrast to LEED or other diffraction techniques it has the advantage to be applicable also to systems which do not possess long-range order. As far as experimental means to measure vibrational frequencies is concerned, classical (reflection - absorption) infrared techniques (IRAS or RAIRS) and high-resolution electron energy loss spectroscopy (HREELS) are common techniques, whereby especially the latter method has been frequently employed for H-induced surface vibrations. An example concerning the system H on Pt(111) taken from the work of Badescu et al. [03Bad] is presented in Fig. 10. For theoretical and experimental details we refer to review articles and monographs on this subject [85Hol; 86Ueb; 80Wil, 77Fro, 82Iba]. Another experimental technique to probe surface vibrations is (inelastic) helium atom scattering (HAS), which has, for example, extensively been used in the laboratory of J.-P. Toennies (Göttingen) to study phonon dispersion curves, but also for probing vibrations of adsorbed hydrogen [94Ben]. Although HAS gives very high resolution, the magnitude of the possible momentum transfer is small, in contrast to the aforementioned HREELS technique. Internal vibrations of adsorbed molecules can also be excited by a method called Inelastic Electron Tunneling Spectroscopy (IETS). This is known since 1966 when vibrational spectra were obtained for the first time from molecules adsorbed at the buried metal - oxide interface of a metal - oxide - metal tunneling junction [66Jak]. However, this ‘buried state’ usually causes a complex local environment of the probed molecule and restricted the applicability of IETS for quite a while. Yet, recent developments of STM technology allowed the monitoring of vibrational properties even of single molecules under very well defined local conditions – these molecules are adsorbed on metal surfaces and reside at the tunneling gap between the STM tip and the surface. In this way, Ho and coworkers succeeded in obtaining IET spectra, e.g., from single acetylene molecules adsorbed on a Cu(100) surface [98Sti] and more recently also from H atoms adsorbed on the same surface [00Lau]. In a different laboratory, the electrical conductivity of a single H2 molecule trapped in a tunnel junction between Pt electrodes held at 4.2 K could even be measured [02Smi]. Landolt-Börnstein New Series III/42A5
68
3.4.1 Adsorbate properties of hydrogen on solid surfaces
× 1500
E0 = 2 eV
84 111
D/Pt (111)
Intensity [arb.units]
50
× 1500
68
113 153 H/Pt (111)
0
50
[Ref. p. 111
100 150 Energy [meV]
200
250
Fig. 10: Vibrational loss spectra obtained from a full monolayer of H (D) atoms adsorbed on a platinum(111) surface at 85 K, obtained with a primary electron beam energy of 2 eV and a resolution better than 1 meV. Note the shift of the vibrational bands between the H and D spectra, due to the isotope mass difference. After Badescu et al. [02Bad].
Nevertheless, the classical ‘integral’ vibrational spectroscopies have been and still are widely applied, and most of the data collected here refer to either IRAS or HREELS experiments. It is, perhaps, worth to mention that a real breakthrough in the performance of low-energy electron spectrometers has been reached during the past two decades in that the energy resolution along with the signal-to-noise ratio could be improved almost by a factor of ten [91Iba]. In the data listing below, the vibrational energies are given in both wave numbers [cm−1] and millielectronvolts [meV]. In the context of our compilation, only a few peculiarities of HREELS concerning its application to hydrogen adsorbed layers [90Ste] are to be mentioned: A frequent problem is the comparatively small cross section for dipole scattering of adsorbed H atoms leading to weak loss signals and unsatisfactory signal-to-noise ratio. Often, off-specular measurements allow to discriminate between dipole-excited and impact-excited loss features, because the cross section for dipole excitations drops sharply away from the specular reflection. Especially H parallel modes cannot be dipole-excited due to the surface selection rule. If they are, however, impact-active, they can well be detected in off-specular measurements. In addition, the H-substrate bond lengths are usually quite short, in other words, the H atom is often more or less ‘immersed’ in the surface (sometimes below the image-charge plane) making any dipole excitation difficult. On the other hand, vibrational loss studies of adsorbed hydrogen can benefit from the large mass difference between the H and the D isotope in that isotopic shifts of the vibrational frequencies are large (downshift by a factor of √2 § 1.42) and relatively easy to determine. In this respect, true H substrate vibrations (which always exhibit this downshift, if the experiments are carried out with deuterium) can be distinguished from substrate phonons and/or protonic band excitations whose band positions do not shift under deuterium. There is still another interesting peculiarity of electron energy loss spectroscopy performed with adsorbed H: Electronic excitations of surface states of metal substrates can be quite sensitively modified by adsorbed hydrogen as demonstrated by strong changes of the energy dependence of the low-energy electron reflectivity due to hydrogen. By adjusting the primary electron beam energy accordingly (to fall together with a reflectivity minimum), the electron beam can be kind of coupled into the surface quite effectively, resulting in a largely enhance sensitivity for detecting H vibrations [84Con, 86Con1]. A different (but also H-specific) phenomenon is the occurrence of a so-called giant phonon anomaly for the H-on-W(110) (and Mo(110)) adsorption system, first detected by Hulpke and Lüdecke by means of HAS in 1992 [92Hul1, 92Hul2, 93Hul], which since then has stimulated a lot of experimental and theoretical work on surface phonons and the effect of adsorbed (and absorbed) H on the lattice dynamics of the respective substrates [96Bal and references cited therein]. Not only in that respect, one will find that the adsorption system ‘H on tungsten’ represents (besides, perhaps H on nickel) the most frequently and most thoroughly studied subject, not only concerning H - surface vibrational excitations, but also regarding structural, kinetic, and thermodynamic properties.
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
69
Before listing the data material available for H - surface vibrational frequencies, some other short remarks may be useful. H atoms bound to a single atom are excited in the 2200...1600 wave number range; edge-bridging H atoms lead to absorption bands between 1400 and 800 cm−1; triply coordinated hydrogen should yield absorption features at even lower wave numbers [72Kae]. The energetic position of a H-induced vibrational band, the number of detectable modes and their angular and azimuthal dependence are important properties, since they provide (via symmetry selection rules) conclusive information about the local symmetry properties of a given H adsorption complex. For details, we refer to the monograph by Ibach [82Iba]. Finally, there is an issue which comes up from time to time when dealing with adsorbed hydrogen and electron energy loss spectroscopy – the concept, whereupon adsorbed H atoms can (or even must) be considered as quantum particles which are delocalized as atomic bands. In other words, the H atoms can perform quantum motion. This idea was first proposed and discussed in 1979 via an atomic band model, in which the motion of the H atoms is represented by wave functions localized normal to the surface, but delocalized along the surface [79Chr2]. Some years later Puska et al. calculated the band energy levels and wave functions of the chemisorbed H atoms on Ni using the effective medium theory [83Pus, 85Pus]. Since then, evidence of quantum delocalized chemisorbed hydrogen has been reported several times, e.g., for H on Rh(111) [86Mat], H on Pd(111) [91Hsu], H on Pd(110) [96Tak], H on Cu(110) [92Ast], and, most recently, H on Pt(111) [02Bad], whereby HREELS and helium diffraction, respectively, were used for detecting and confirming the respective evidence. The state of the art has been reviewed by Nishijima et al. [05Nis].
3.4.1.3.6.1 Metal surfaces Surface
Be(0001)
H coverage [ML] at T [K] 0 - 0.38
Observed frequencies or loss bands [meV] [cm−1] 1492 185
0.4...0.74
532 1274 1371 1468 887
66 more trigonal (tilted bridge) 158 site 170 182 107...121 bridge site of Mg - hydride phase assumed;
742 1315
92 163
T <90 K
750 1125 1750
93 139 217
280 K
1825 1900 831 1653 1895 A 800 B 1700 C 1200
226 236 103 205 235 99 211 149
Mg(0001)
Al(100)
Al(110) Al(111)
Al(111)
1.0 at 85 K
Landolt-Börnstein New Series III/42A5
Vibrational mode assignment and coordination
Remarks
Refe.
Be - H stretch (bridge site)
exposure to H atoms 90Ray at 80 K single site occupation at low coverages exposure to H atoms 91Spr at 110 K; Mg surface 94Spr hydride phase metastable up to 425 K
perpendicular Mg - H mode parallel Mg - H mode (H adsorption in a single fcc or hcp 3-fold site) dipole active losses Al-H bending mode exposure to H atoms bridge-bonded H terminal Al-H stretching mode
88Pau
ordered islands of terminal H occupation of ‘multiple’ sites
exposure to H atoms exposure to H atoms at 100 K
85Thi 88Mun
bending mode (terminal H) stretching mode (terminal H) stretching mode ( bridged H)
exposure to H atoms AlH3 molecules preformed at surface
91Kon
70 Surface
Fe(100)
3.4.1 Adsorbate properties of hydrogen on solid surfaces H coverage [ML] at T [K] low coverage (β2 state) high coverage (β1 state)
Fe(110)
Co(10−10)
Ni(100)
Ni(110)
1000
asymmetric site within the 4fold hollow
124
Remarks
Refe.
stretching mode 4-fold hollow site
Fe - H stretching mode Fe - H asymmetric stretching mode H located in the short bridge two types of 3-fold coordinated sites
96Mer
adsorption at 300 K
81Bar1
expts. performed at 100 K; measurements performed in two perpendicular azimuthal directions exploiting surface symmetry selection rules
94Ern
expts. performed at 200 K
78And2
468 605 1089 1169 580 1122 904
58 75 135 145 72 = ν 1 143 = ν 2 112 = ν 3 ν 3 only visible along [0001] azimuth
609 984 1205 581 ± 8
76 = ν 1 122 = ν 2 150 = ν 3 72 ± 1
Ni - H symmetric stretching vibration
0.5 ML
597 ± 8
74 ± 1
<0.1 <0.5
629 532+629
78 66 +78
H in a 4-fold hollow (‘center’) site expts. performed at adsorption into defects 80 K vibr. motion in dilute disordered phase Ni - H stretch in (1×1) H phase
< 0.5 (c(2×4) phase)
(1×2)-rec
Ni(100)
Vibrational mode assignment and coordination
1060 spe- 132 cular 880 non- 109 specular
1.0 ML (2×1)p2mg phase
Ni(100)
Observed frequencies or loss bands [meV] [cm−1] 700 87
[Ref. p. 111
quasiordered p(2×2)H at Ĭ=0.25 ML
sat’n (~1 629 78 ML) at 80 K 1.0 ML 645 80 (= (1×1)-H) 700...850 87...105 low Θ medium Θ after heating to >300 K
650 1060 610 940 640 930
81 131 76 117 79 115
quasi-3-fold site (hcp-type)
ν 1 = perpendicular Co - H stretching mode ν 2 = parallel Co - H mode ν 3 = non-dipole active (impact) parallel mode
symmetric stretching mode asymmetric stretching mode low-symmetry bridge site short bridge site(s)
collective asymmetric mode with large dispersion expts. performed at 100 K dipole active modes
86Kar
02Oku
84Oll
modes change due to (1×2)streak reconstruction
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
Ni(110)
3.4.1 Adsorbate properties of hydrogen on solid surfaces H coverage [ML] at T [K] Θ < 0.4 0.4<Θ <1 Θ = 1.0
Θ > 1.1 Θ = 1.5 (T = 150 K)
Θ < 0.4 at 230 K
Observed frequencies or loss bands [meV] [cm−1] 573 71 1057 131
573 1057 613 944 613 944 1129 573 1057
1.0 at 230 K 1129 Ni(110)
1 ML = (2×1)-2H
637 1049
613 944 1210 (1×2)streak ~645 for T >220 944 1129 560...640 1 ML = 870...910 (2×1)-2H (offphase specular) 1100 ([1-10] azimuth) 620...640 1100 ([001] azimuth) 1.5 ML = (1×2)3H
Ni(110)
1.5 ML = (1×2)-3H
Landolt-Börnstein New Series III/42A5
610...635 560...710 (offspecular) 1035-40 890...930 1210-40 ([1−10] azimuth) 635 1035 450 (offspecular) 930 1210-40
mixed 71 131 76 117 76 117 140 (weak) 71 131
71
Vibrational mode assignment and coordination
Remarks
Refe.
symmetric Ni - H stretch asymmetric Ni - H stretch H atoms in quasi 3-fold site
expts. performed either at 150 K to study the metastable lattice gas phases or at 230 K to induce the H-induced (1×2)streak reconstruction;
82DiN 85DiN
symmetric stretching mode asymmetric stretching mode symmetric stretching mode asymmetric stretching of H in a 3-fold site symmetric stretching mode asymmetric stretching of H in a 3-fold site with distortion
modes dipole-active
mixed 140 (weak) 79 130
H in a lower coordinated site
76 117 150 ~80 117 140 69...79 108...113
low-symmetry short bridge sites + high-symmetry shortbridge sites distorted + undistorted surface patches with local structures as (2×1) + (1×2) 89Voi species 1 exposure at 100 K; 87Leh species 1; ν polarized parallel HREELS measurements to Ni rows momentum-resolved. species 1 Two H sites each with Cs symmetry in the (2×1)-2H species 1 phase identical sites species 1 with quasi-3-fold symmetry occupied (species 1)
136
77...79 136
76..79 69...88
low-symmetry short bridge sites
species 1 species 1
128...129 species 1 110...115 species 2 150...154 species 2
79 128 56
species 1 species 1 species 2
species 2 115 150...154 species 2
expts. performed between 100..300 K modes dipole-active
in the (1×2)-pairingrow reconstructed surface likewise sites with local quasi 3fold symmetry are occupied (species 2)
85Jo
72 Surface
Ni(111)
3.4.1 Adsorbate properties of hydrogen on solid surfaces H coverage [ML] at T [K] all coverages
Ni(111)
1 ML surf.
Ni(111)
1 ML surf. + 1 ML bulk 0.05…0.5
>0.5
1.0 Ni(111)
0.5 (= c(2×2) honeycomb
1.0 (= 1×1 phase) Ni(311)
Observed frequencies or loss bands [meV] [cm−1] 710
88
1121
139
955 1170 800 955 1170
118 145 99 118 145
726 1048
90 130
726 927 1129 927 1129 732 774 1054 1094 1250 1390 2180 954 1170 1744
90 115 140 115 140 91 96 131 136 155 172 270 118 145 216
very low Θ 444
55
1202
149
low and medium Θ
347 452 726 1202
43 56 90 149
high Θ
282 524 686 887 1000 1250
35 65 85 110 124 155
Vibrational mode assignment and coordination
Remarks
Refe.
original interpretation : perpendicular Ni - H stretch (site « A » = hcp) perpendicular Ni - H stretch (site « B » = fcc)) later interpretation : 88 = symmetric Ni - H stretch (perpendicular mode) ; 139 = asymmetric Ni - H stretch (parallel mode) symmetric Ni - H stretch antisymmetric Ni - H stretch vibration due to embedded H symmetric Ni - H stretch antisymmetric Ni - H stretch
expts. performed at 170 K; both modes nondipole active
80Ho
asymmetric Ni - H stretch symmetric Ni - H stretch associated with (2×2) honeycomb structure
½ ¾ ¿
[Ref. p. 111
correlated with (1×1)-H structure antisymmetric Ni - H stretch in two different 3-fold sites symmetric Ni - H stretch in two different 3-fold sites indicate delocalization of excited vibrational states
exposure to atomic H 91Joh at 80...100 K; after prolonged exposure crystal contains embedded (bulk) H H2 exposure at 100 K 97Yan
H2 exposure at 90 K
03Oku
exposure to H2 at 100 K
96Sch
overtone of Ȟs asymmetric Ni - H stretch symmetric Ni - H stretch bending (parallel) mode (H species 1) stretching (perpendicular) mode (H species 1) (species 1 = H in a 3-fold site =(111)-facet) bending mode (H species 2) bending mode (H species 1) stretching mode (H species 2) stretching mode (H species 1) (species 2 = H in a 4-fold site =(100)-facet) bending mode (H species 2) bending mode (H species 1) stretching mode (H species 2) bending mode (H species 3) stretching mode (H species 3) stretching mode (H species 3) (H species 3 = 3-fold sites in the grooves of (110)-surface)
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
Ni(510)
3.4.1 Adsorbate properties of hydrogen on solid surfaces H coverage [ML] at T [K] low and medium Θ
Cu(100)
Cu(110)
Cu(110)
Cu(110)
Vibrational mode assignment and coordination
48
589
73
766 + 1242 363 581 4178
95 + 154 45 72 518
4516
560
4734
587
low Θ
565
70
high Θ (1.03 ML)
468 565 948
58 70 117.5
modified 4-fold hollow site
68 81 93 117+108 140 77
long-bridge site 4-fold hollow site
1 ML of physisorbed H2 molecules
548 653 T > 140 K 750 (=(1×2) rec) 944+871 1129 620 Θ < 0.2 =(1×1) T < 140 K Θ > 0.2 620 + =(1×3) shoulder T < 140 K at 505 950 0.1< Θ <1 765 (1×2) MR- 950 recon1150 structed surface 637 ≤ 0.33 952 =(1×3) phase
Landolt-Börnstein New Series III/42A5
77 + shoulder at 63 118 95 118 143 79 118
Remarks
wagging mode expts. performed at asymmetric Ni - H stretch 100 K symmetric Ni - H stretch of low-symmetry short-bridge site located at the terrace steps (3-fold symmetry) component of the asymm. (parallel) mode (4-fold symmetry site on the terrace) symmetric (perpendicular) stretch of H in 4-fold (terrace) sites
387
high Θ Cu(100) + H2
Observed frequencies or loss bands [meV] [cm−1] 645 80 847 105 138 1113
H adsorbed at step sites (lowsymmetry long-bridge site) rot. trans. J = 0 → J = 2 rot. trans. J = 1 → J = 3 rot.+vibr. trans. v = 0→v = 1 + J=0→J=2 rot.+vibr. trans. v = 0→v = 1 + J=1→J=3 rot.+vibr. trans. v = 0→v = 1 + J=1→J=3 H in a 4-fold hollow site
bridge-bonded H
73 Refe.
88Mar
surface reconstr. cannot be excluded 83And expts. performed with H2, D2, and HD 97Sve molecules condensed at 15 K
expts. performed with 91Cho atomic H at 83 K high Θ leads to surface reconstr. of p4g or pgg type; losses dipole-active exposure to H atoms 87Bad disordered H adsorption
symmetric stretch (species 1) symmetric stretch (species 2) in long-bridge + 4-fold sites alternative assignment: H in quasi-3-fold sites
symmetric mode asymmetric mode: H in a tilted trigonal site with mirror plane parallel to [001] azimuth parallel mode perpendicular mode H in quasi-3-fold site, ν polarized in [1−10] mirror plane
exposure to H atoms at 100 K
90Hay
exposure to H (D) atoms at 110 K
92Ast
delocalization of low-density H atoms (protonic band structure)
74 Surface
Cu(111)
Cu(111)
Nb(100)
3.4.1 Adsorbate properties of hydrogen on solid surfaces H coverage [ML] at T [K] 0.5...0.67 ?
1150-70
143-145
0.67 = (3×3) phase
770
95
1040
129
sat’n
944 1049
117 130
555 (at 270 K) 1025 1220 1260
69
low Θ (0.08) = (5×2)at 120 K low Θ (0.35)= (4×2) at 250 K high Θ (2.0 ML) = (1×1)-2H phase at 100 K sat’n = (1×1)-2H
1220
151
1205
149
1020
126
<0.5 beginning (2×2)
710
Mo(100)
Mo(100)
Mo(100)
Mo(110)
Observed frequencies or loss bands [meV] [cm−1] 1040 129
1300
1016 1302
1097
intermedi- 798 ate Θ (2×2) 871
Θ > 0.5
(1×1) phase; sat’n = 1 ML
1226 798 1226
127 151 156
Vibrational mode assignment and coordination
[Ref. p. 111
Remarks
Refe.
symmetric stretching of H in 2-fold (bridge) site 1st overtone of deformation mode of H in a 2-fold site frustrated parallel translational modes of H in 3-fold sites
exposure to H atoms at 150 K. Use of RAIRS + HREELS technique RAIRS measurements; 1151 cm−1 [89McC] mode not observed H in tetrahedral sites just expts. performed at below the surface 300 K ? H-induced losses exhibit inhomogeneous broadening; hint to occupation of inequivalent subsurface sites Mo - H bending mode expts. performed at 80 K and at 270 K; losses between ½ ¾ Mo - H stretching modes 400...600 and ¿ 700...900 cm−1 due to Mo phonons ν 1 mode of H in a short bridge RAIRS expts. site
89McC
95Lam
86Li
86Zae
87Pry
H on a shorter bridge site on reconstructed Mo surface
symmetric stretch (ν 1): all H atoms in bridge sites asymmetric Fano line to the 161 overtone of the wagging mode of Mo - H - Mo structure 126 symmetric stretch Surface infrared 161 overtone of wagging mode spectroscopy (SIRS) expts 88 antisymmetric stretching low Θ losses due to mode reconstructed (2×2) 136 symmetric stretching mode of (rippled) surface H atoms in quasi trigonal (3fold) sites 99 108 meV loss due to 108 (only (1×1) island in [1−10] nucleation azimuth) 152 high Θ losses due to H-saturated (1×1) 99 phase; 152 fully H-covered (110) surface exhibits a giant phonon anomaly, see, e.g., [92Hul1]
88Reu
97Oka
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
Mo(211)
Ru(0001)
Ru(0001)
Ru(0001)
Ru(0001)
Ru(10−10)
3.4.1 Adsorbate properties of hydrogen on solid surfaces H coverage [ML] at T [K] Θ <1 ML: no H superstr. Θ >1 ML: (1×2) phase exists up to sat’n (= 2.0 ML) sat’n = 1 ML
Observed frequencies or loss bands [meV] [cm−1]
sat’n (~ 1 ML)
Vibrational mode assignment and coordination
75
Remarks
Refe.
93Lop
12501323
two types of binding sites 155...164 symmetric stretch of 2-fold (bridge-bonded) H
expts. performed at 200 K
750
93
1323
164
847 1113
105 138
823 1137
1549 1960 2274
102 = ν 1 141 = ν 2 three weak additional losses at 192 = ν 3 243 = ν 4 282 = ν 5
(1×2) phase due to H-induced PR reconstruction; no protonic bands mainly impact 83Bar scattering; expts. performed at 170 K, strong dependence of electron reflectivity on H coverage expts. performed at 84Con 115 K strong dependence of electron reflectivity on H coverage
< 0.3
686
85
1.0
726 823 1137 820
90 102 141 102
1137
141
0.7...1.0 (= c(8×2) phase)
847 1145
105 142
1.2...1.3 (= (1×2) phase)
(355) 847 1210
(44) 105 150
0...1 ML
621 1.5 (c(2×2)-3H 847 phase) 12901331 2.0 ((1×1)- 621 2H phase) 774 847 shoulder 1057 11451226 1355
Landolt-Börnstein New Series III/42A5
symmetric stretch of H in 3fold coordinated site symmetric stretch of 2-fold (bridge-bonded) H symmetric Ru-H stretch asymmetric Ru-H stretch
Ru - H parallel mode Ru - H perpendicular mode
½ ¾ overtones + combination ¿ modes perpendicular Ru - H stretching mode ; H in 3-fold hollow site 3-fold site with reduced symmetry doubly degenerate parallel Ru - H mode perpendicular Ru - H stretching mode parallel Ru - H mode perpendicular Ru - H mode (site A) phonon ? (ν H = ν D) parallel mode (site A) perpendicular mode (site A)
expts. performed at 90 K; two kinds of adsorbed H species, depending on coverage
expts. performed at 04Kos 170 K; only one kind of adsorbed H species reported expts. performed at 89Lau 100 K azimuthal dependences of losses suggest Cs site symmetry of quasi 3fold site(s)”A” and “B”
77 105 160...165 77 96 105 shoulder 131 142...152
parallel mode (site B) parallel mode (site B) parallel mode (site A)
168
perpendicular mode (site B)
perpendicular mode (site B) perpendicular mode (site A)
94Shi
additional occupation of quasi-3-fold site “B” in the highcoverage phase
76 Surface
Ru(10−10)
Ru(11−21)
Rh(100)
Rh(100)
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
H coverage Observed frequen[ML] at T cies or loss bands [K] [meV] [cm−1] 169 21 1.0 ML 218 27 323 40 1210 150 78 2.0 ML 629 807 100 1032 128
Vibrational mode assignment and coordination
Remarks
Ru phonon mode parallel mode (site A) parallel mode (site A) perpendicular mode (site A) parallel mode (site B) parallel mode (site B) perpendicular mode (site B)
500 742 1169
62 92 145
parallel mode (site 1) parallel mode (site 1) perpendicular mode (site 1) (site 1 = pseudo-3-fold “A”)
expts. performed at 93Gru 90 K; excitation cross section strongly dependent on primary electron energy. Quantum motion at small coverages due to wide protonic band expts. performed at 01Fan 90 K all modes dipoleactive
516 927 1290
64 115 160
parallel mode (site 2) parallel mode (site 2) perpendicular mode (site 2) (site 2 = pseudo-3-fold “B”)
339 742
42 92
parallel mode (site 3) perpendicular mode (site 3) (site 3 = quasi-4-fold)
565 661
70 82
1226
152
1113
138
528 ± 4
65.5 ± 0.5 82.0 ± 0.3 125 ± 2 137 ± 4 154 ± 2 173 ± 2 193 ± 2
small coverages < 0.12 L (γ - TD state) medium coverages < 0.17 L (β TD state) high coverages (α-TD state) 0.4 ML sat’n = 1 ML
sat’n = 1 ML = 1.39 19 × 10
661 ± 2.5 1008 ±16 1105 ±32 1242 ±16 1395 ±16 1557 ±16
ν 1 = Rh - H perpendicular
Refe.
expts. performed at 95 K
87Ric1
expts. performed at 90 K
88Ric1
stretch 1st overtone of ν 1 H in a 4-fold hollow site doubly degenerate parallel mode (impact-active)
νasym νsym
½ ° ° ¾ overtones ° °¿
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
Rh(110)
3.4.1 Adsorbate properties of hydrogen on solid surfaces H coverage [ML] at T [K] 0.3...0.5 (=(1×3) + (1×2) phases)
Θ > 0.5 (nonprimitive H phases: (1×2)-2H + (1×3)-2H)
Rh(111)
0.4...1 ML
Observed frequencies or loss bands [meV] [cm−1] 444.. 460 55...57 532 66 694 86
984 1024 1145 1234 1355
122 127 142 153 168
403 871 694 1048 1153 1420 1532
50 108 86 130 143 176 190
450
56
660 750 1050
82 93 130
Vibrational mode assignment and coordination
expts. performed at ν 1 (A’ symmetry) ν 2 (A” symmetry (impact m.)) 100 K in the two azimuthal directions ν 3 (A’ symmetry) = perpendicular Rh - H mode (quasi-3fold hollow sites) combin. mode (=ν 1 + ν 2) overtone (2 ν 2) combin. mode (=ν 1 + ν 3) combin. mode (=ν 2 + ν 3) overtone (2 ν 3)
Rh(311)
0.05 0.1...0.5
0.5...0.8
0.8...1.0
1...>1.3
Landolt-Börnstein New Series III/42A5
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<Θ <1
J”=0 → J”=2 transition v = 0→ v = 1 + J”=0 → J’= 0 transition v= 0 → v = 1 + J”=0→ J’ = 2 transition parallel mode (impact active)
395 4187
49 518
4533
562
702
87 (offspecular) 106 perpendicular mode (dipole active) Ag - H stretch 140 Ta - H stretching vibration (superposition of symmetric stretch (= A1) and antisymmetric stretch (= E) with ∆E = 15 meV)
855 Ta(100)
Vibrational mode assignment and coordination
parallel Pd - H mode perpendicular Pd - H stretch mode of H located in quasi-3790 98 fold sites 968 (line 120 reconstruction leads to broadeenhanced linewidth (only slightly modified H-Pd sites) ning) low 702...718 87...89 in the low-coverage region the coverages (weak) observed bands can reflect (0.04..0.4) 774...807 96...100 transitions from protonic 976...984 121...122 ground-state to excited bands medium H 774 96 ν 1 = parallel Pd - H stretch coverages 1000 124 ν 2 = perpendicular Pd - H (2 L stretch of H bound in 3-fold exposure) hollow site low...high 444 55 limited resolution prevents a coverages 694...726 86...90 precise assignment. Broad losses indicate various Pd - H coordinations 0.25 ML 452 56 parallel mode (4-fold site) (= (2×1)-H 686 85 perpendicular mode (4-fold s.) phase) 758 94 parallel mode (3-fold site) 968 120 perpendicular mode (3-fold s.) 0.5 ML 403 50 ~686 ~85 slight Θ -dependent band shifts ~726 ~90 1016 126 0.1…1 ML 484 60 parallel mode 847 105 perpendiclar mode (H in tilted trigonal sites with Cs symmetry)
Ag(111)+ H2 1 ML
sat’n at 300 1129 K (~1 ML)
[Ref. p. 111
exposure to H atoms at 100 K H atoms localized; no evidence of quantum motion physisorption of molecular hydrogen at T = 10 K
delocalized (protonic band) motion ruled out
86Con1 86Con2
98Mus1
98Sch 99Far
93Spr
82Avo
95Lee
broad single loss at 04Yam all coverages assumed to consist of two peaks; no H-induced reconstruction
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
W(100)
3.4.1 Adsorbate properties of hydrogen on solid surfaces Vibrational mode assignment and coordination
Remarks
on-top site (β2-hydrogen)
Θ=2
Observed frequencies or loss bands [meV] [cm−1] 1251 155 1049 130 1251 155 1049 130 1251 155 weak 1049 130
expts. performed at 76Fro 300 K in specular direction; occupation equilibrium of two different sites, depending on H coverage
Θ < 0.5 Θ = 1.2
1282 ± 8 637
159 ± 1 79 weak
Θ=2
1065 ± 8
132 ± 1
Θ=2
1049
130 spec.
645 1049 1290 2097
80 o-sp. 130 o-sp. 160 o-sp. 260 o-sp
H coverage [ML] at T [K] Θ < 0.5 Θ ≥ 0.5
Θ > 0.7
W(100)
W(100)
(=2 × 1019 at/m2 (sat’n))
W(100)
W(100)
79
0.5 484 (= c(2×2)H 1008 phase) 1250
bridge sites exclusively occupied
125 150...160
expts. performed at T W - H bending mode (β2 -TD- < 350 K in specular direction; evidence state) of atomic nature of adsorbed H species symmetric W - H stretch (A1) expts. performed at in 2-fold (bridge) site 300 K in both ν 2 = parallel (bending) mode specular and offν 1 = symm. stretch (2-fold s.) specular direction ν 3 = asym. stretch (2-fold s.) overtone of symm. stretch (2ν 1) ν 2 = bending mode (2-fold s.) expts. performed at ν 1 = symm. stretch (2-fold s.) 300 K; consideration of H-induced surface ν 3 = asym. stretch (2-fold reconstruction: site) detection-angle (local site geometry affected dependent by surface reconstruction) measurements ν 2 = bending mode (2-fold s.) ν 1 = symm. stretch (2-fold s.) ν 3 = asym. stretch (2-fold site) only bridge-bonded H species ν 1 = symm. stretch (2-fold s.) expts. performed at 150 K and at 300 K; moves from 155 to 130 meV consideration of as Θ increases from 0.1 to 1. displacive W surface ν 2 = νwag wagging mode shifts reconstruction; from 55 to 80 meV for Θ →1 confirmation of tilted ν 1 = symm. stretch (2-fold s.) dimer model ν 3 = asym. stretch (2-fold site)
133 157
symmetric stretch overtone of wagging mode
60 125 155 + weak overt.
2.0 (= (1×1)-2H phase)
645 1049 1290
80 130 160
0.1...1.0
1250 1049
155... 130 (specular)
444...645 55...80
W(100)
sat’n = (1×1)-2H
1008 1210...90 1070 1270
W(100)
sat’n = (1×1)-2H
1069 1269
133 157
symmetric stretch overtone of wagging mode
W(100)
0...2.0 ML at 100 K
1100 136 (Θ = 1.4) 1070 133 1260 156 (Θ = 1.65..2.0)
ν 1 = W2 - H symm. stretch
Landolt-Börnstein New Series III/42A5
ν 1 = W2 - H symm. stretch 2 ν 2 = overtone wagging md.
Refe.
77Adn
78Ho
78Bar2 80Will
83Did
high-resolution 85Cha2 infrared spectroscopy expts. Surface infrared 88Reu spectroscopy (SIRS) expts. RAIRS expts. 89Rif
80 Surface
3.4.1 Adsorbate properties of hydrogen on solid surfaces
W(110)
H coverage [ML] at T [K] 0.1...1 ML
W(110)
p(2×1)
(2×2)
Observed frequencies or loss bands [meV] [cm−1] 767 95
Re(10−10)
197 continuum below 850 1300 Θ > 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
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
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)
<0.4 ML
525
65
>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
Vibrational mode assignment and coordination
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
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[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
Vibrational mode assignment and coordination
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
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface
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
Landolt-Börnstein New Series III/42A5
83
Observed frequencies or loss bands [meV] [cm−1] 645 80 2097 260
Vibrational mode assignment and coordination
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
Vibrational mode assignment and coordination
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
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
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
Landolt-Börnstein New Series III/42A5
86
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[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.
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
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)
Landolt-Börnstein New Series III/42A5
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
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
H coverage, H phases
Temperature [K]
Photon energy of incident light hν [eV]
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)
Band dispersion; band width (1s-derived band(s))
Remarks, mode of measurement etc.
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
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
H coverage, H phases
Temperature [K]
Photon energy of incident light hν [eV]
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)
Landolt-Börnstein New Series III/42A5
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)
Remarks, mode of measurement etc.
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
<200
21.2 16.8
Band dispersion; band width (1s-derived band(s))
89
H-induced band structure determined in ΓΚ- and ΓΜ direction IPE
88Rei
exposure to H2; physisorption of H2 molecules
82Ebe
unoccupied surface state shifts with Θ to 1.3 eV above EF occupied surface state
exposure to H atoms 93San1 IPE + UPS (angleresolved) H-induced (1×2) reconstruction produces shifts of Cu surface states
H 1s-derived bonding state normal emission
ARUPS
H derived state, disappears for T > 700 K
83Gre
ARUPS at 88Fan synchrotron. From T dependence of H states H subsurface/bulk absorption processes inferred angle-integrated 80Smi2 UPS
90
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
H coverage, H phases
Temperature [K]
Photon energy of incident light hν [eV]
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
Band dispersion; band width (1s-derived band(s))
[Ref. p. 111
Remarks, mode of measurement etc.
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
angle-integrated UPS
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
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
H coverage, H phases
Temperature [K]
Photon energy of incident light hν [eV]
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)
Landolt-Börnstein New Series III/42A5
high (recon structed surface) high Θ
Band dispersion; band width (1s-derived band(s))
Remarks, mode of measurement etc.
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
angle-integrated UPS
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
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
H coverage, H phases
Temperature [K]
Photon energy of incident light hν [eV]
W(111)
0.39
Pt(111)
medium Θ
80
Pt(111)
medium Θ
<200 21.2
Pt(111)
medium Θ
140
18...20
21.2
40
Position of Hinduced photoemission state(s) below the Fermi energy EF [eV] at normal emission ~6 (broad) at Γ point
7.3 no distinguishable H-induced state 9.4 at Γ point
[Ref. p. 111
Band dispersion; band width (1s-derived band(s))
Remarks, mode of measurement etc.
Ref.
H split-off state, dispersion from 4.5 to 6 eV width 1.5 eV
ARUPS at synchrotron, polarization dependent, momentum resolved angle-integrated UPS angle-integrated UPS
82Cer
77Dem
strong suppression of d band emission H 1s split-off angle-integrated state UPS
77Col2
Band dispersion; band width (1s-derived band(s))
Ref.
81Ebe
3.4.1.3.7.2 Semiconductor and insulator surfaces Surface
H coverage, H phases
Temperature [K]
Photon energy of incident light hν [eV]
C(111)
(1×1) terminated
300
21.22
C(100) - 2×1 reconstructed (diamond)
likely monohydride terminated + CH2 and CH3 groups
C(100)-1×1
C(100)-2×1 (diamond)
monohydride terminated surface
Si(100)-2×1 Si(111)-2×1 Si(100)-2×1 Si(100)-1×1
monohydride + dihydride phase
21.22 40.8
Position of Hinduced photoemission state(s) below the Fermi level [eV] at normal emission no distinct ‘extra’ signals; as unreconstructed surface no distinct ‘extra’ signals; the H-terminated surfaces show negative electron affinity (NEA)ĺ vacuum level lies below the conduction band minimum the H-terminated surfaces show NEA
300
21.22
300
21.22
11.1 below Evac
300
21.22
10 below Evac 12 below Evac near 10 12 below Evac
Remarks, mode of measurement etc.
85Pat
96Die
XPS, UPS
exposure to H atoms necessary to form (1×1) phase
broad featuUPS + ab-initio 94Wei res between 5 theory and 15 eV below EF ads. H removes the 74Iba ‘dangling-bond’ surface state low H exposures: 76Sak (2×1) persists; at larger H exposures: (1×1) phase forms
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
H coverage, H phases
Temperature [K]
Si(100)-2×1
monohydride + dihydride phase
300.. 21.22 600 40.8
H saturated
300
Si(111)-1×1
Si(100)-2×1 Si(111)-7×7
mono - to dihydride coverage
300
Photon energy of incident light hν [eV]
21.22
40.8
Position of Hinduced photoemission state(s) below the Fermi level [eV] at normal emission 10.0 below Evac 14.7 below Evac 8.0 below Evac 9.2 below Evac 10.0 below Evac 11.5 below Evac 13.9 below Evac 10 below Evac 12 below Evac (after gentle exposure) 11 below Evac 15 below Evac (excessive H exposure) 10 below Evac 12 below Evac 10 below Evac
Si(111)
Si(111)-7×7
Si(111)-1×1
mono300 hydride (low Ĭ); mono + dihydride (larger Ĭ) low exposure (monohydride ?) H300 terminated
5.3 7.3 6.2
32
4.3 (sharp)
Ge(100)-2×1
exposed to H2 plasma
430.. 21.22 460
5.6
Ge(100)-2×1
fully H covered
300
14, 17
4.4...5.5 4.5...5.5
Ge(111)-2×1
fully H covered (1×1)
300
21.22
300
21.22 16.85
4.9 12.8 4
(1×1)-H
300
29...60
Ge(111)-c2×8
Landolt-Börnstein New Series III/42A5
no specific Hinduced feature
Remarks, mode of measurement etc.
exposure at 300 K
exposure to H atoms; UPS + tightbinding and monohydride extended Hückel features, after calculations exposure at 523 K
Ref.
84Cir
exposure to H atoms 75Pan
new trihydride state (questioned, however, by [82But] who ascribed the states to oxygen contaminations) monohydride exposure to H 84But enhanced satoms; little like bulk state dependence on enhanced ĺ crystallographic dihydride orientation monohydride exposure to H 90Kou monohydride atoms; all surface states disappear dihydride with H coverage
5.6 7.6
9 (broad)
Ge(111)-2×1 Ge(111)c(2×8)
Band dispersion; band width (1s-derived band(s))
93
combined exptl. and 95Sta theor. study
surface resonance contrib, from bulk sp band (2×1) H phase, no dispersion dispersion of ~1 eV. H-induced surface states
4...5 (width = 1eV)
ARUPS at synchrotron
00Gal
atomic H through RF plasma; ARUPS
92Cho
ARUPS at synchrotron
94Lan
UPS
75Row
ARUPS; H deconstructs surface to (1×1); two surface states disappear with H ARUPS at synchrotron
82Bri
85Wac
94
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
H coverage, H phases
Temperature [K]
Ge(113)
(1×1)
300
GaAs(110)
Hterminated (1 ML) Hterminated (1 ML)
300
GaAs(110)
300
Position of Hinduced photoemission state(s) below the Fermi level [eV] at normal emission 11.8 (Ar) 5.5 21.22 (HeI) 16.67 (NeI) 21.22 0.12 below valence band edge 21.22 3.7...4 below 40.8 upper valence band edge Photon energy of incident light hν [eV]
6.8
Band dispersion; band width (1s-derived band(s))
[Ref. p. 111
Remarks, mode of measurement etc.
Ref.
exposure to H atoms; ARUPS
95Sch2
photon emission yield spectroscopy
87Mha
exposure to H atoms, ARUPS
94Ple
H atoms on top of As sites
3.4.1.3.8 Hydrogen-induced work function changes
Another property which reflects the electronic interaction between the hydrogen adsorbate (atoms or molecules) and the solid surface is the change of the work function as the adsorption proceeds. Since it cannot be the intention of this data compilation to provide a fundamental theory of the work function – the reader is referred to textbooks or review articles [49Her; 58Sim; 69Riv; 79Hoe] – we only make some short introductory remarks and will add comments on special features characteristic of hydrogen adsorption, such as H-induced structural phase transformations and their influence on the work function. Furthermore, we refer to the contribution of K. Jacobi about work function changes in in section 4.2 in part 2 of this Landolt-Börnstein volume III/42A [02Jac]. In a very general and simple view, any chemical surface complex formed between an adsorbed H atom and the underlying atom(s) of the substrate can be thought of as a tiny dipole carrying a dipole moment µ with either the negative or the positive end pointing away from the surface. The amount of charge located on each dipole thereby depends on the polarization of the chemical bond(s) within the respective adsorption complex and, hence, reflects the differences in electron affinity of the adsorbate (H or H2 in our case) and the substrate atom(s) involved in the bonding. The overall work function change ∆φ produced by a surface layer of adsorbed dipoles (density σ [particles/m2]) each having a dipole moment µ 0 [A s m] can simply be modelled by a plate capacitor carrying the charge q+ and q− separated by the plate distance d. This leads to the well-known Helmholtz equation 1 ∆Φ = 4π Θ N max µ 0 f * ; f * = (12) 4πε 0 .
µ 0 is thereby the dipole moment of an individually adsorbed particle at vanishing coverage, ε 0 the
dielectric constant (8.85 × 10−12 As V−1m−1), f * a conversion factor from cgs to SI system, Nmax the maximum number of adsorption sites per unit area, and Θ the coverage. It is assumed that the (imaginary) plane of electroneutrality is within the adsorbate layer; if it is defined with respect to the image charge plane within the substrate surface, an emerging electron must do work only against half of the adlayer potential, and the factor 4 reduces to a factor of 2. In principle, this equation allows the determination of the dipole moment of an individual adsorption complex, if the absolute coverage is known. From µ 0, in turn, the dipole length l or the charge separation between the two ends of the dipole could be derived. Note that the Helmholtz equation in this simple form does not consider depolarization effects which
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
95
usually come into play as the coverage increases and the adsorbate dipoles interact with each other. Then, a more refined relation has to be used, for example, the Topping equation [27Top] which contains a coverage dependence of µ and the polarizability of the adsorbed molecule [86Woo, 91Chr]. Experimentally, depolarization effects show up in the coverage dependence of the work function change, ǻĭ (Ĭ), which is linear only at small coverages. If defect (for example, step) sites with a different dipole moment are involved and preferentially occupied, the ǻĭ (Ĭ) relation may even run through a minimum or maximum as it was found for a stepped Pt(111) surface, c.f., Fig. 12 [76Chr]. The same is true, if in the course of the adsorption sites of different local charge transfer become populated, or the surface undergoes reconstruction at a critical hydrogen coverage.
0 B
b
a
Df
Work function change D f [mV]
20 -100
A
10 0
0
0.1 0.2 0.3 Q
-200
Pt (111) Pt (997)
-300
-400
0
0.2
0.4 0.6 0.8 H coverage Q
B
1.0
0.4 C
Fig. 12: The H-induced work function change, ∆Φ, as a monitor of adsorption into geometrically different surface sites. Shown is the coverage dependence of H adsorption on a flat (curve a) and on a stepped (curve b) Pt(111) surface (sketch of the structure in the bottom with different types of sites indicated as A, B, and C). While ∆Φ decreases monotonically on the flat Pt(111) surface (occupation of terrace sites), an initial ∆Φ increase observed with the stepped surface is attributed to preferential adsorption into step sites. After Christmann and Ertl [76Chr].
C
A
For atomic H chemisorption on most of the TM surfaces the negative end of the dipole is pointing away from the surface causing generally a work function increase. Only few metal surfaces (Fe(110), W(110) and Pt(111)) show the inverse behavior. According to the following table, the H-induced ǻĭ increase ranges from some 10 meV (for Ru(0001)) to almost 1 eV for H on various ‘open’ TM surfaces (Rh(110)). The amount of charge transferred from the metal to the H atom is usually less than 1/10 of the elementary charge, e0. In cases where the adsorption of hydrogen is accompanied by a surface reconstruction, some care has to be taken in interpreting H-induced work function changes, since the displacement or shifts of substrate atoms can (and usually will) alter the electric surface situation and contribute to ∆φ as well. Turning to the few cases where the work function change due to molecular hydrogen adsorption was determined one can safely state that adsorbed H2 molecules are positively polarized, i.e., a net charge seems to flow from the adsorbed molecule to the surface, giving rise to a decrease of ǻĭ. Examples are the (210) faces of Ni and Pd, where the work function decreases between 300 and 500 meV upon H2 adsorption [01Sch1; 01Sch2]. As far as the experimental tools to measure H-induced work function changes are concerned, we simply note that most of the standard methods can be employed, photoemission spectroscopy, Kelvin probe or diode methods being the most frequently used ones. For more details, we refer to the respective literature [49Her, 59Cul, 86Woo, 91Chr].
Landolt-Börnstein New Series III/42A5
96
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
Finally, it should be emphasized that the work function change caused by hydrogen is probably one of the most sensitive monitors of surface cleanliness. In his scientific life, the author of this contribution has measured a whole variety of H-on-metal systems and has always recognized that even traces of impurities (carbon, sulfur, phosphorus etc.) can cause a dramatic loss in the H-induced ǻĭ; as a rule of thumb, one could even state that a surface which gives the larger work function change upon adsorbing a given amount of H is the cleaner surface. Work function changes caused by adsorption of hydrogen Surface
H coverage, H phases
Temperature [K]
Be(0001)
0...1 ML
90
Mg(0001) Ti(0001) Fe(100)
sat’d hydr- 110 ide phase (1×1) 140
(200) +75
Fe(110)
(1×1)
140
−85 ± 5
140
+310 (annealing required) +225 (negatively polar. species)
Fe(111) Co polycryst. films
5.5 × 1019m−2 8.0 × 1019m−2
Co(10−10) c(2×4) (Θ = 0.5)
78
Work function change at saturation [meV] (values in parentheses calculated) −440 −950 ± 70
Method (CPD = contact potential difference)
Initial dipole moment [D]
HREELS analyser in RF mode HREELS analyser in RF mode photoemission CPD (Kelvin probe) CPD (Kelvin −0.032 probe) CPD (Kelvin probe) CPD (static capacitor)
Remarks
Ref.
∆Φ exhibits a minimum 90Ray at −560 meV at Θ = 0.4 ML exposure to H atoms; 91Spr surface hydride is formed 80Fei2 77Boz ∆Φ exhibits a small mini- 77Boz mum (−55 meV) at completion of β2 TD state 77Boz 76Dus
−200 (ca.) (positively polarized species) 85
CPD (Kelvin probe)
(2×1)-2H (Θ = 1.0)
§ +200
(1×2)-3H (Θ = 1.5)
−122
Ni(100)
0<Θ <0.5
273
total +85 at sat’n +170
Ni(100)
0<Θ <0.8
150
+96
Ni(110) (1×2)MR phase Ni(110)
Θ <1.5
<170 +580
Θ <1.5
273
+530
CPD (Kelvin probe) CPD (Kelvin probe)
CPD (Kelvin probe)
0.0137
0.049
∆Φ (Θ ) exhibits a maximum (207 meV) at 1.1 ML, then drops to 85 meV at saturation
94Ern
74Chr ∆Φ linear with Θ up to Θ = 0.5 79Chr1 linear increase of ∆Φ with Θ; maximum at Θ = 0.5, followed by decrease to 40 meV at Θ = 0.8 74Tay ∆Φ (Θ ) linear with break at Θ = 0.5 (2×1)phase 74Chr
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
H coverage, H phases
Temperature [K]
Ni(110), (1×2) PR reconstr. Phase Ni(111)
0<Θ <1.5
120
0<Θ <0.5
273
+ 195
Ni(111)
0<Θ <0.9 110 up to Θ = 0.5 adsorption into c(2×2) honeycomb structure 10
+165
Cu(100)
Cu(100)
Cu(100)
Cu(110) Cu(110)
0<Θ <1 p4greconstr. phase (1×3) (1×2) (1×2)
Work function change at saturation [meV] (values in parentheses calculated) +510 (Θ = 1: ∆Φ = 250 meV)
Initial dipole moment [D]
CPD (Kelvin probe)
0.06 ∆Φ (Θ ) linear with (0<Θ<1) positive break at Θ =1 0.12 due to reconstruction (1<Θ<1.5)
−200 (H2)
CPD (Kelvin probe) CPD (Kelvin probe)
electron beam retardation mode electron beam retardation mode
10
−120 (H2) −145 (D2)
170
−250
CPD (Kelvin probe)
90
<150
100
−150 at 100 K (Θ H < 0.4); +250 at 200 K
CPD (Kelvin probe) photoemission
Cu(110)
(1×3) (1×2)
115
+120
Nb(100)
sat’n = 2 × 1019 /m2
90
+540
Nb(110)
sat’n Θ
300
<100
Mo(100)
(4×2) c(2×2) sat’n (1.0)
240 +500 step <160 +600 plateau 160- > +1200 240
Landolt-Börnstein New Series III/42A5
Method (CPD = contact potential difference)
CPD (Kelvin probe)
UPS (width of spectrum)
0.029
Remarks
97 Ref.
89Chr
∆Φ linear with Θ
74Chr
linear increase of ∆Φ with Θ; maximum at Θ = 0.5, followed by decrease to 80 meV at Θ = 0.9
79Chr2
linear decrease of ∆Φ with Θ H
83And
surface exposed to mole- 88Wil cular hydrogen; study of physisorbed H2 molecules (1 ML = 0.65 × 1019 molec./m2) exposure to H atoms 91Cho
exposure to H atoms
87Bad
93San1 exposure to H atoms; ∆Φ exhibits strong T dependence ∆Φ > 0 includes MR reconstruction. 94Roh exposure to H atoms ∆Φ (Θ ) exhibits an initial increase of 25 meV, followed by a shallow minimum and final increase to 120 meV 74Hag − 0.0022 state 1 = molecular (state 1) precursor) − 0.141 state 2 = atomic H, (state 2) coverage associated with state 2 = 9 × 1018 /m2 80Smi2 79Est ∆Φ (Θ ) exhibits a plateau due to (4×4) or c(2×2) phase, but continuous increase otherwise
98
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
H coverage, H phases
Temperature [K]
Mo(110)
Θ < 0.1 Θ = 0.5 Θ > 0.6
350
Mo(211)
0<Θ<2
Ru(0001)
small Θ
+960 at Θ = 1 decrease by 280 to 680 at Θ = 1.75, rise to ~1000 near sat’n 100.. +25 (negative 150 “N” species) 100
medium and high Θ (~1 ML)
− 10...− 30 (positive “P” species) + 420 + 360 + 390 + 250
Ru(10−10) 0 < Θ < 1 1 <Θ< 1.2 Θ = 1.5 Θ=2 (sat’n) Rh(111)
Work function change at saturation [meV] (values in parentheses calculated) −20 +100 +110 (sat’n)
Θ=1=
Method (CPD = contact potential difference) CPD (retarding field diode method)
CPD (Kelvin probe)
Θ c(2×2) =
170
+200
CPD (Kelvin probe)
UV photoemission CPD (Kelvin probe)
300
+360
CPD (Kelvin probe)
130
+325
CPD (Kelvin probe)
138
+300
CPD (Kelvin probe)
300
+180
120
+160
CPD (Kelvin probe); UPS cut-off CPD (Kelvin probe)
0.027 (p(1×2)) 0.12 (1×3)3H 0.13 (1×1)2H 0.021
Θmax = 1.4
Pd(110)
Θmax = 1.5 + subsurface states Θmax = 1.5 + subsurface states
Pd(111)
Pd(210)
Θmax > 3 ML
89Ern
85Feu 0.006..... ∆Φ (Θ ) first increases, 0.01 forms a maximum of ~30 meV, then decreases to negative values between – 10 and – 30 meV, depending on T 0.12 ∆Φ (Θ ) linear up to Θ = 89Lau 1, subsequent decrease by ~50 meV, minimum at Θ = 1.25, increase by −0.08 20 meV to second maximum at Θ = 1.5 95Wit
Pd(100)
Pd(110)
∆Φ (Θ) exhibits a small initial minimum (20 meV), then a rise up to 0.5 ML. ∆Φ (Θ ) linear with H coverage up to Θ = 1
CPD (Kelvin probe)
Rh(110)
Pd(110)
Ref.
0.17 ± 0.01
+50
0.5;
Remarks
CPD (diode method using LEED electron gun)
130 1.58 × 1019 100 Θ=2
+930
Initial dipole moment [D]
[Ref. p. 111
0.0714
94Lop
linear increase of ∆Φ (Θ ) 88Ehs through form’n of 1×3, 1×2, 1×3-2H phases; stronger increase with 1×2-3H
linear increase of ∆Φ (Θ ) with positive break at Θ = 1.0 ∆Φ (Θ ) includes contributions due to (1×2) reconstruction ∆Φ (Θ ) includes contributions due to (1×2) reconstruction ∆Φ (Θ ) includes contributions due to (1×2) reconstruction
80Beh
74Con
83Cat1, 83Cat2 88He1
74Con ∆Φ (Θ ) strongly T dependent: Tad < 120 K causes lower ∆Φmax, competition of a molecularly adsorbed H2 species with reverse dipole moment
83Ebe 98Mus
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Surface
H coverage, H phases
Temperature [K]
Work function change at saturation [meV] (values in parentheses calculated) +280 ± 10
Method (CPD = contact potential difference)
(1×4) (1×3) (2×6) (2×2)
100
+220 at sat’n (1 ML)
EELS analyzer in RF mode
exposure to H atoms
93Spr
Ag(111)
100
−170
exposure to H atoms
89Zho
Ag(111)
+320 −200
W(100)
sat’n
+880
exposure to H atoms, at high Θ, ∆Φ ~ Θ n (n<1) ∆Φ (Θ ) linear with H coverage ∆Φ (Θ ) linear with H coverage
95Lee
Gd(0001)
100 (2×2) (3×3) (Θ = 0.5-0.6) 300 Θ=1
UV photoemission electron beam retardation (HREELS) UV photoemission
W(100)
sat’n = 1019/m2
W(100)
Θ=2
330
+900
W(100)
Θ=1
130
+940
W(100)
sat’n
300
+900
W(100)
150 0<Θ<sat’n 300
+960
W(110) W(110)
Θ=1 Θ=1
300 130
−500 −500
W(110)
Θ=1
90
−500
W(110)
Θ=1
Pd(311) Ag(110)
W(110)
300
+900
300 (sat’n) 0<Θ<sat’n 300
W(211)
130 Θ = 0.2 Θ= 0.6...0.8 Θ = 1 (sat’n) 110 Θ=1
W(211)
Θ=1
W(111)
Landolt-Börnstein New Series III/42A5
130
CPD (retarding field diode method) CPD (retarding field method) CPD (Kelvin probe)
(CPD) electron reflection
CPD (Kelvin probe) CPD (Kelvin probe)
UPS
−480
(CPD) electron reflection CPD (Kelvin probe)
+150(+β3+2 state) +280 (+β1 state) 300 at sat’n CPD (Kelvin probe) 300 at sat’n
Remarks
0.093
−450
−20 (β4 state)
Initial dipole moment [D]
CPD (Kelvin probe)
0.18 −0.062
99 Ref.
99Far
93Li 66Arm 66Est
0.21
73Mad 74Bar inflection at 160, knee at 78Bar1 270 meV no inflection initial linear increase, 86Her shoulder at 270 meV, discontinuity at 350 meV, linear increase up to sat’n 72Plu 74Bar careful study; exposure 97Nah1 dependence exhibits a 97Nah2 small irregularity after 2 L; TDS- and LEED derived coverages differ slightly 82Wen 86Her 74Bar
∆Φ (Θ ) exhibits a maximum (600 meV) at 0.5 ML ∆Φ (Θ ) exhibits a maximum (600 meV) at 0.5 ML
73Rye
74Bar
100 Surface
3.4.1 Adsorbate properties of hydrogen on solid surfaces H coverage, H phases
Re(10−10) Θ = 0.9 Θ > 0.9
Temperature [K] 120
Work function change at saturation [meV] (values in parentheses calculated) +370 −220, final value at sat’n = +150 meV
Method (CPD = contact potential difference)
Initial dipole moment [D]
Remarks
Ref.
CPD (Kelvin probe)
0.1 ± 0.04
∆Φ (Θ ) exhibits a maximum (370 meV) at 0.9 ML
95Mus
Pt(100)(5×20)
0<Θ<1
100
+70 (state b) −370 (states a1 + a2)
CPD (Kelvin probe)
Pt(110)(1×2)
Θ = 0.9
120
+ 300 at Θ=0.3
CPD (retarding field using electron gun)
–500 at sat’n (Θ = 0.9) 0<Θ<1
170
+150 at Θ = 0.1; –600 at sat’n (Θ=1)
MEM-LEED
Pt(111)
Θ < 1.0
150
−230
CPD (Kelvin probe)
Ir(110) (1×2)
Θ = 0.3
140
+300 in ȕ2 sites
CPD (retarding field using electron gun)
Au(100)
Θ = 0.3
−300 in ȕ1 sites 100
−200
+0.12 −0.17
Pt(110)(1×2)
Θ > 0.85
[Ref. p. 111
91Pen1 ∆Φ (Θ ) runs through a maximum. Complex T dependence due to activated reconstruction/deconstruction phenomena 87Eng ∆Φ (Θ ) exhibits a maximum (300 meV) at Θ = 0.3 ∆Φ (Θ ) exhibits a maximum (150 meV) at Θ = 0.1
92She
−0.036
∆Φ ~ Θ n (n= 1.33)
75Chr
+0.14
∆Φ increases monotonously up to Θ = 0.3, then decreases again to ∆Φfinal = 0 exposure to H atoms
80Ibb
CPD (retarding field method)
96Iwa
3.4.1.4 The interaction of hydrogen with solid surfaces: theory 3.4.1.4.1 General remarks
In the following section, a brief survey shall be given over theoretical descriptions of hydrogen interaction with solid surfaces. It is thereby not attempted to draw detailed comparisons between the experimentally determined and the theoretically derived quantities and judge on the quality or benefits of a given theoretical modelling; rather a list of H - surface interaction systems will be compiled for which theoretical calculations exist. In a coarse distinction, these theoretical treatments may be regarded to focus 1) on the H2 - surface interaction dynamics (prediction of dissociation pathways and rates, kinetic coefficients (sticking probabilities, frequency factors), activation energies etc.) and potential energy surfaces (PES), and 2) on the (equilibrium) binding energies, adsorption site geometries, long-range order phases (lateral interaction energies) as well as H-induced electronic states, vibrational frequencies and work function changes. For each of these two theoretical goals we will present a listing of available theoretical reports, and as in the tables before, the H interaction systems will be compiled according to the position of the element in the periodic table. In a separate column comments, for example concerning the kind of the theoretical treatment applied, will be added. For this purpose it is necessary to supply the reader with a short list of the currently used abbreviations, together with references from which basic information concerning the details and procedures of the respective treatments can be obtained. Beginning with the early work by Koutecký [65Kou], Grimley [71Gri] and Schrieffer and Gomer [71Sch], the theoretical description of the H chemisorption has seen huge progress in the past twenty
Landolt-Börnstein New Series III/42A5
Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
101
years in that the initially relatively crude models and approximations were step by step improved to a much more sophisticated level. This was also very much supported by the unbelievable development of the computing facilities and available program codes. A real breakthrough was attained by applying the density functional theory, and many of the modern theoretical treatments are actually based on DFT methods. We emphasize again that it cannot be the intention of this chapter to recall all the theoretical concepts, instead, the reader is referred to the extended special literature in that field [60Gri, 78Mus, 79Gri, 79Mes, 80Smi1, 83Sch1, 83Gri, 85Mus, 03Gro]. The available reports can be further subdivided into two basic groups: There are many articles which focus on one or two distinct H adsorption systems only and calculate their respective specific properties. In the other category a certain theoretical model is applied to H chemisorption on whole classes of surfaces; in these articles, usually many adsorption systems are covered to demonstrate the merits of the respective theoretical approach. A brief separate table is devoted to these latter reports, again subdivided into general reports dealing with H2 adsorption dynamics and those rather focussing on equilibrium properties. Listing of abbreviations used in theoretical treatments of H - surface interaction Abbreviation ASED-MO BEBO TBA FPLAPW DFT GGA GGC LDA LSD DOS RHF EHT EMT TST CT CHAIN HF LEPS FP-LMTO CI CEM MC PW91 CVT MINDO PP LCGTO Car-Parinello VASP
Landolt-Börnstein New Series III/42A5
Full name (semi-empirical) atom superposition and electron delocalization molecular orbital theory Bond Energy Bond Order Tight-binding approach Full-potential linear augmented plane wave Density functional theory Generalized gradient approximation (often in conjunction with PW91 density functional) Generalized gradient correction Local-density functional approximation Local spin density Density of states Restricted Hartree-Fock Extended Hückel theory Effective medium theory Transition state theory Classical trajectories CHAIN method Hartree-Fock Valence-bond method based on work by London, Eyring, Polanyi, and Sato Full-potential linear muffin-tin orbital Configurational interaction Corrected effective medium theory Monte Carlo calculations Density functional developed by Perdew and Wang Canonical variational theory Modified intermediate neglect of differential overlap Pseudopotential method Linear combination of Gaussian type orbitals combination of molecular dynamics and density functional theory Vienna ab-initio simulation package
Reference 97Iri 91Ben 71Gri; 94Har 95Bla; 96Koh 64Hoh; 65Koh; 75Yin; 90Dre; 89Par 92Per; 93Ham; 95Ham1; 96Per 64Hoh, 65Koh
80Nor2; 82Nor2 66Joh 92Lio 29Lon; 31Eyr; 55Sat
87Kre 91Per 86Lau; 88Tru 85Cla 82Coh 79Dun 85Car 93Kre1; 93Kre2; 94Kre1; 98Eic
102
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
3.4.1.4.2 General theories for hydrogen adsorption 3.4.1.4.2.1 Theories covering the adsorption dynamics (more general reports)
Applied theory DFT CI-SCF Band model EMT Model PES Model PES EMT SCF / jellium Overlap expansion of unperturbed wave functions
Remarks Electron-hole pair excitation and dissociative sticking Electron-hole pair excitation and dissociative sticking Potential energy surfaces of MH2 clusters(M = Co, Fe, Cu) Quantum diffusion of H on metal surfaces Adiabatic PES for H2 interaction with Ni and Cu surfaces Dissociation of a H2 and vibrational - translational energy transfer 2D-PES (combination of Morse fct. + Gaussian barrier) influence of PES topology on H2 dissociation on metals 6D-PES for H2 molecules interacting with (111) and (110) surfaces of Cu and Ni H2 molecules in interaction with Al, Mg, Na. Calculation of PES H2 physisorption on noble metal surfaces
Reference 97Men 82Sch 84Sie 86Wha 89Nor 89Har 90Hal 90Nie 92Eng 81Joh2 85Nor
3.4.1.4.2.2 Theories covering equilibrium properties (more general reports)
Applied theory EMT Spin-density functional formalism Cluster + muffin-tin Anisotropic EMT SC-LAPW Various (review) Various (review) TBA and others Various (review) self-consistent DFT scheme according to Zaremba & Kohn [77Zar] Spin-unrestricted screened HartreeFock method
Remarks H interaction with various 3d, 4d, and 5d metals; trends in H chemisorption energies H chemisorption on simple (sp electron) metals
Reference 84Nor
Ordering of H layers on various metal surfaces H2 interaction with Cu, Ag, Au and Al surfaces H bond distances and vibrational frequencies on close-packed metal surfaces Chemisorption on metals (preference on hydrogen) Chemisorption on metal surfaces Indirect interactions between two H atoms
86Mus 90Kar 87Fei3
Order-disorder phase transitions in adsorbed H layers H2 dissociation on Au, Cu, Ni, Pt H2 physisorption potential on simple metals with focus on Al surfaces location of H atoms and adsorption energies for (110) faces of Al, Cu, Ni, and NiAl
78Hje
78Mus 90Nor 67Gri1; 67Gri2 58Kou; 73Ein 78Dom 95Ham2 86Nor
96Cas
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Ref. p. 111]
3.4.1 Adsorbate properties of hydrogen on solid surfaces
103
3.4.1.4.3 Theories covering specific interaction systems 3.4.1.4.3.1 Equilibrium properties Surface
Li2 Li(110) Li(100) Li(111)
Li(100) Be(0001) Be(0001) Be(0001) Be(0001)
Be(0001) C(100) C(100) C(110) C(111) C(100) C(111)-1×1-H C(111)
C(111) graphite Na(100) Na(110) Mg(0001)
Mg(0001)
Al(100)
Landolt-Börnstein New Series III/42A5
Theory describing surface equilibrium properties model ab-initio study Many-body perturbation theory SCF cluster calculations; many-body perturbation theory DFT SC ab initio cluster calc. SC ab initio cluster calc. ab initio HF SCF calcul. ab initio PP method + DFT supercell geometry cluster calc. RHF Slab-MINDO parametrized tigh-binding model DFT + GGA plane-wave pseudopotential calc’s ab-initio LDF calculations, MD calc’ns MC calculations Kohn-Sham calc. + 1st-order perturbation Kohn-Sham calc. + 1st-order perturbation SCF LDA + exchange and correlation SCF jellium + pseudopotentials
Remarks
Reference
reaction of hydrogen with Li2 clusters, activation energies, transition 86Rag states, reaction energies location and binding energy of H on Lin clusters, n = 2...10 88Ray
location and binding energies of H on Lin clusters (n = 3...10)
90Hir
location of H atoms, electronic structure of H monolayer location and bond energies of H atoms on a Be10 cluster
95Bir 75Bau2
location of adsorbed H atoms on Be36 clusters
83Bag
location of H atoms, Be - H bond strength, electronic structure for Be slabs location of adsorbed H atoms, H - Be vibrational frequencies, H - Be bond energies
84Ang
location of H atom, bond distances, binding energies
89Mar
H adsorption geometry, total energy (H chemisorption energies) band structure, total energies, relaxed surface geometries, densities of states, C - H vibrations; extensive calculation
91Zhe 94Dav
H phase formation as a function of chemical potential (coverage), vibrational frequencies of C - H bonds calculation of surface relaxations due to H adsorptive bonding
02Ste 93Stu
geometry of hydrogenated diamond surfaces
98Ker
simulation of adsorption isotherms for H2 adsorbed on graphite at 77 K H - Na binding energies, H - Na bond lengths, H - Na vibrational frequencies
99Dar
H - Mg binding energies, H - Mg bond lengths, H - Mg vibrational frequencies
79Hje
PES for H2; activation barriers, electronic states
81Nor1
location of adsorbed H atoms, bond distances, charge densities
76Gun
89Lam
79Hje
104 Surface
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Al(110) Ti(0001)
Theory describing surface equilibrium properties Kohn-Sham calc. + 1st-order perturbation tight-binding and EHT calc. EHT, DOS calculations Cluster calc’s: DFT + ab initio pseudopotentials realistic tightbinding calcul. self-consistent LDA self-consistent pseudopotential ab initio slab + local DFT first principles local density total-energy + atomic force calculations first-principles total-energy pseudopotential calc’ns ab-initio LCAO HF (slab geometry) DFT + GGA SC-LCAO
Fe(100)
MINDO
Fe(100)
CEM
Fe(100) Fe(110)
SC-LAPW ab initio CI on embedded cluster surface model CEM
Al(100) Al(110) Al(111) Si(100) Si(111)-7×7 Si(100)
Si(111) Ge(111) Si(100)-1×1 Si(111) Si(111), Ge(111) Si(111)-7×7
Si(111)-7×7
Si(111)-(1×1)H
Fe(110) Fe(110)
Fe(110)
EHT + ASEDMO cluster method Spin-polarized DFT, GGA, PAW
[Ref. p. 111
Remarks
Reference
H - Al binding energies, H - Al bond lengths, H - Al vibrational frequencies
79Hje
H - Si surface states, H - Si bonding configuration
84Cir
H - Si bonding configuration, dangling bonds, surface states
95Sta
energetics of SixHy clusters (x = 9, 15, 21; y = 12, 16, 20)
99Pen
surface energy bands and local density of states of H atoms bound to 76Pan Si (Ge) surface atoms H - Si bonding geometry; surface charge distributions 77App H - Si bonding configuration, formation of mono-, di-, and trihydride H - Si bond energies, substrate geometry, H - Si vibrational frequencies H-on-Si adsorption geometry; surface electronic structure
77Ho
Si - H binding energies for H adsorbed in different locations; H-induced charge transfers
95Lim
H - Si bonding nature, atomic energies, polarization of the Si charge densities, charge transfers, surface geometry and relaxation
00Car
potential-energy curves, H dipole moment location of H atoms in (1×1) layer, H-induced electronic states and work function effects location of H atoms, binding energy, charge densities and vibrational frequencies on a Fe12 cluster location of adsorbed H atoms, binding energies, Fe-H bond lengths, binding energies, adsorbate-induced surface relaxation location and bond length of H atoms in a (1×1)-2H geometry location of adsorbed H atoms, adsorption energies, H - Fe bond length, vibrational frequencies
93Ham2 80Fei1 80Fei2 83Bly
88Kax 93Ye
location of adsorbed H atoms, binding energies, Fe-H bond lengths, binding energies, adsorbate-induced surface relaxation bonding situation of H in a Fe vacancy, Fe - H bond strength, decohesion phenomena induced by H location of adsorbed H atoms (coverage-dependent), charge densities, long-range order, binding energies, diffusion barriers, work function changes ǻĭ. The authors predict a ǻĭ increase, while experimentally a decrease was observed [77Boz]
90Rae 87Fer 95Cre
90Rae 99Jua
03Jia
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Ref. p. 111] Surface
Fe(110)
Fe(100) Fe(110) Fe(111) Co(0001) Co(0001)
Ni2 Ni(100) Ni(100) Ni(100) Ni(100) Ni(100)
Ni(100)
Ni(100) Ni(100)
Ni(100); Ni(110); Ni(111) Ni(111) Ni(111)
Ni(111)
Ni(111)
Ni(111) Ni(111) Ni(111)
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3.4.1 Adsorbate properties of hydrogen on solid surfaces Theory describing surface equilibrium properties lattice gas model: transfer matrix scaling and MC methods CFSO-BEBO
105
Remarks
Reference
phase diagrams, correlation function and critical exponents of ordered H phases
82Kin
heats of molecular hydrogen desorption, radius of adsorbed H atoms
83Pas
DFT and FPlocation of adsorbed H atoms, 3-fold site predicted LAPW Semi-empirical role of H chemisorption in surface magnetism SC tightbinding model Ab initio dissociation pathway Tight-binding location of H atoms, adsorption energy, electronic density of states LCAO HF binding energies, bond distance, vibrational frequencies for H on Ni20 cluster EMT location of H atoms, binding energies, vibrational frequencies Cluster dissociation path and energy barriers calculations Self-consistent location of H atoms, bond distances, vibrational frequencies of spin-polarized seven-layer Ni(100) film with p(1×1)-H monolayer calculations first-principles location of H atoms, Ni - H bond distance, vibrational frequencies, calc’s using a Ni - H binding energy model potential Total-energy location of H atoms, Ni - H bond length, adsorption energy of DFT p(1×1) H layer All electron location of H atom, binding energies of Ni4 and Ni5 cluster cluster calculations with ECP Spin-polarized H adsorption energies, H - H interactions, diffusion barriers; gradientlocation of adsorbed H atoms including consideration of (2×2)-2H corrected DFT phase EMT location of H atoms, binding energies, vibrational frequencies Lattice gas phase diagram of c(2×2) honeycomb H phase model with 6 NN interactions Lattice gas phase diagram of c(2×2) honeycomb H phase model, transfer matrix finite scaling Cluster location of adsorbed H atoms, H - Ni binding energies and bond calculation; ab lengths, work function changes initio valence orbital CI DFT, GGA location of adsorbed H atoms, adsorption energies, electronic states DFT and FPlocation of adsorbed H atoms, H-induced electronic states LAPW DFT-GGAlocation of adsorbed H atoms including subsurface sites, H binding PW91 energy, vibrational frequencies
99Kli 00Pic
76Mel 76Fas1 76Fas2 79Upt 82Nor3 84Sie 85Wei
85Umr 87Pan
00Kre2
82Nor3 84Nag2
86Roe
88Yan
96Pau 99Kli 03Gre
106 Surface
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Mo(100) Mo(100)
Theory describing surface equilibrium properties Classical trajectory and quantal calcul. DFT + GGA, VASP DFT + GGA, VASP ab-initio CI embedding ab initio slab + local DFT ab-initio LCAO HF (slab geometry) SC PP EMT
Mo(111)
DFT
Cu(100)
Cu(110) Cu(111) Zn(0001) Ge(111) Ge(111)-(1×1)H
Ru(0001)(1×1)H Ru(0001) Rh(100) Rh(100)
FP-LMTO
Rh(111)
DFT-GGAPW91 CAS-MCSCF
Pd(100) Pd(110) Pd(111) Pd(100) Pd(100)
Pd(100) Pd(100) Pd(100) Pd(110)
Pd(110)
Remarks
Reference
physisorption of H2
86DeP
PES, location of adsorbed H atoms, adsorption energy, Cu - H bond lengths PES, location of adsorbed H atoms, adsorption energy, Cu - H bond lengths location of adsorbed H atoms on a Zn12 cluster, binding energies
00Bae
H - Ge bond energies, substrate geometry, H - Ge vibrational frequencies H - Ge bonding nature, atomic energies, polarization of the Ge charge densities, charge transfers, surface geometry and relaxation
88Kax
electronic structure of H layer, energy levels, work function effects equilibrium geometry; 3D adiabatic PES, vibrational modes
location of adsorbed H atoms, on 5-, 7-, and 9-layer Mo slabs; H - Mo distances LAPW structure, bond lengths, vibrational frequencies PP + LDA location of H atoms, H - Ru adsorption energy, vibrational frequencies, work function changes, H-induced electronic states DFT, LDA, H - Rh adsorption energies, location of H atoms, work function FP-LMTO changes ab-initio LDF + H2 orientation during dissociation; H adsorption site, H - Rh bond GGC distances, 6D-PES
Rh(111)
Pd clusters
[Ref. p. 111
DFT + GGA
EAM
location of adsorbed H atoms, adsorption energy, work function change PES , location of H atoms, vibrational properties PES for H2 dissociation; dissociation barriers; H bonding geometry for Pd3 clusters H adsorption sites, H phases, work function changes, diffusion barriers
H - H lateral interactions, location of H atoms, H - Pd binding energy Tight-binding + electronic structure of interacting H atoms, pairwise and trio generalized interactions, location of H atoms phase-shift DFT, LDA, H - Pd adsorption energies, location of H atoms, work function FP-LMTO changes DFT, FPPES for H2 dissociation, location of H atom LAPW, GGA DFT+GGA, H2 orientation during dissociation; H adsorption energy, H - Pd bond VASP distances, 6D-PES LCGTO-MCP- 2D-PES, location of adsorbed H atoms, H - Pd binding energy, H LSD cluster Pd bond lengths, H-induced changes of the electronic structure calc’ns DFT + LDA; H-induced reconstruction, location of adsorbed H atoms DFT + GGA + pseudopotentials
00Bae 90Cre
00Car
79Ker 90Lou 99Arn 87Fei1 89Cho 94Wil1 94Wil2 96Eic 97Eic 98Eic 97Löb 04Lai 95Dai 98Don
90Ein 90Sta
94Wil1 94Wil2 96Will 97Eic 98Eic 90Pap
97Don2 98Led
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Ref. p. 111] Surface
Pd(111)
Pd(111) Pd(111) Pd(111)
3.4.1 Adsorbate properties of hydrogen on solid surfaces Theory describing surface equilibrium properties SC-pseudopotential mixed-basis method SC-pseudopotential c’lns EAM
Pd(111) Pd(111)
Quantummechanical transition state theory DFT, GGA FP-LMTO
Pd(111)
DFT + GGA
Pd(111)
DFT + GGA
Pd(111)
W(100) W(110)
DFT + LDA; DFT + GGA periodic band structure calc. + DFT + GGA DFT + GGA, plane waves, ultrasoft pseudopotentials DFT+GGA, VASP DFT – LCGTO-LSD Classical trajectory and quantal calcul. Extended Hückel MO theory LCAO Ab initio LAPW Ab initio plane wave pseudopotential, GGA EMT CEM
W(211) Re(0001)
EMT DFT-GGA
Pt clusters
CAS-MCSCF
Pd(111)
Pd(210)
Ag(100) Ag(111) Ag(111)
W(100)
W(100) W(100) W(100)
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107
Remarks
Reference
location of H atoms, H - Pd bond lengths, surface electronic structure, bonding and antibonding electronic states
79Lou1 79Lou2
density of states, electronic character of Pd - H bond, location of subsurface H atoms location of surface and subsurface H atoms, H phases, critical temperatures surface diffusion constant for H and D
84Cha1
location of adsorbed H atoms, adsorption energies, electronic states location of adsorbed H atoms, adsorption energy, work function change dissociation pathways of H2; location of H atoms, H - Pd binding energies binding energies and geometric locations of adsorbed H atoms, bond distances 2D - PES, dissociation pathway, location of H atoms, subsurface hydrogen probabilities 3D - PES; Pd - H adsorption energies, location of adsorbed H atoms, vibrational frequencies, ordered H phases and H - H interactions
87Daw 93Ric
96Pau 97Löb 97Don1 99Pal 97Ols 98Lov
location of H and H2 , H – Pd bond lengths, H and H2 – Pd binding energies, PES
02Lis
H2 orientation during dissociation; H adsorption energy, H – Ag bond distances, 6D-PES, activation barrier physisorption of H2 on Agn clusters, n = 5, 7, 10, 12; dissociation of H2 physisorption of H2
97Eic 98Eic 91Mij 86DeP
relative H - W bonding energies, preferred H adsorption sites; energy barriers for surface diffusion
73And
electronic states of 16-layer W slab with (1×1) H phase H bonding geometry, vibrational frequencies, dipole moments
76Smi 86Bis
PES for H2 dissociation, location of H atoms, H - W binding energy
96Whi
equilibrium geometry; 3D adiabatic PES location of adsorbed H atoms, binding energies, bond lengths of atoms, adsorbate-induced surface relaxation location of adsorbed H atoms in equilibrium binding energies and geometric locations of adsorbed H atoms, bond distances PES for H2 dissociation; dissociation barriers; H bonding geometry for Pt3 clusters
90Lou 90Rae 89Gri 99Pal 95Dai
108 Surface
Pt(111) Pt(111) Pt(111) Pt(111)
3.4.1 Adsorbate properties of hydrogen on solid surfaces Theory describing surface equilibrium properties LAPW DFT DFT-GGAPW91 pseudopotential planewave DFT
[Ref. p. 111
Remarks
Reference
structure, bonding location, vibrational frequencies of H monolayer six-dimensional PES for H2 interaction H binding site, binding energy, diffusion barriers (comparative study of various atomic adsorbates) H binding energy, H adsorption geometry
87Fei2 02Ols 05For 00Pap
3.4.1.4.3.2 Theories covering dynamic properties of specific systems Surface Mg(0001) Si(100) Si(100) Si(100) Si(100)-2×1 Si Si(111) Ni(100) Ni(100) Ni(100) Ni(100) Ni(100) Ni(100) Ni(110) Ni(110)
Ni(110) Ni(111) Ni(111) Ni(111) Cu(100) Cu(100) Cu”(100)” Cu(100)
Applied theory or theoretical concept Remarks SCF LDA + exchange and correlation PES for H2; activation barriers, electronic states, dissociation path ab initio calculations PES, transition state calculations ab-initio quantum dynamics calc. dynamics of coupled H2 + Si surface system; sticking coefficients, adsorption barriers DFT + LDA PES of H2 interacting with Si(100), calc. of activation energy for adsorption and desorption DFT using 1-dimer and 3-dimer adsorption/desorption energetics; activation cluster models barriers 5 D quantum reaction dynamics study H2 dissociation dynamics, energy-dependent sticking coefficients, activation barriers MC transition-state theory methods activation barriers for H2 adsorption, preexponential factors, PES quantum-mechanical study (restricted H2 dissociation dynamics 2D model) LEPS ; TST with multidimensional reaction rates and kinetic isotope effects for H2 semiclassical transmission coefficients and D2 dissociation S-matrix Kohn technique 6D PES (with consideration of cartwheel and helicopter degree of freedom) Surrogate Hamiltonian method quantum diffusion of H and D atoms [97Bae] delocalized EMT PES of H2 spin-polarized gradient-corrected DFT six-dimensional PES ; dissociation and sticking of H2 molecules delocalized EMT PES of H2 LEPS; CVT-TST with multidimensio- reaction rates and kinetic isotope effects for H2 nal semiclassical transmission and D2 dissociation coefficients spin-polarized gradient-corrected DFT six-dimensional PES; dissociation and sticking of H2 molecules delocalized EMT PES of H2 LEPS; TST with multidimensional Reaction rates and kinetic isotope effects for H2 semiclassical transmission coefficients and D2 dissociation Spin-polarized gradient-corrected DFT six-dimensional PES; dissociation and sticking of H2 molecules LEPS approach PES for H2 Delocalized EMT PES for H2 accurate PES from CI calcul. of H2 – relaxation dynamics of vibrationally excited H2 on Cu cluster 4-fold Cu sites Wave packet calc. + ab initio PES PES for H2
Ref. 81Nor1 93Wu 96Kra 95Peh 01Tok 00Hil 85Noo 87Jac2 89Tru 93Saa 98Bae 86Lee 00Kre1 86Lee 89Tru
00Kre1 86Lee 89Tru 00Kre1 78Gre 86Lee 89Cac 89Han
Landolt-Börnstein New Series III/42A5
Ref. p. 111] Surface Cu(100)
3.4.1 Adsorbate properties of hydrogen on solid surfaces
Cu(111) Cu(111)
Applied theory or theoretical concept first principles total energy calc.; CarParinello approach, LDA + GGA DFT LEPS approach Delocalized EMT Mixed quantum-classical approach Reaction path description + model Hamiltonian LEPS approach Quantum dynamic study
Cu(111) Cu(311) Zn(0001)
DFT – GGA LEPS approach LEPS approach
Mo(100) Rh(100)
EMT ab-initio LDF + GGC
Pd(100) Pd(100)
DFT-GGA-LAPW DFT
Ag(111)
DFT - LCGTO-LSD
W(100) Pt(111) Pt(111)
EMT CT; Gaussian weighting, TST, CHAIN method DFT
physisorption of H2 on Agn clusters, n = 5, 7, 10, 12; PES for dissociation of H2 3D adiabatic PES ab initio PES, dynamics of H2 desorption; late barrier position six-dimensional PES for H2
Pt(111)
DFT + GGA
six-dimensional PES for H2 and D2
Cu(100) Cu(110) Cu(110) Cu(110) Cu(111)
Remarks PES for H2
Ref. 94Whi
six-dimensional PES for H2 PES for H2 PES for H2 H2 dissociation quantum-state-resolved, PES PES for H2
02Ols 78Gre 86Lee 94Kum 91Küc
PES for H2 PES for H2, sticking probability as a function of quantum state numbers 5D-PES for H2 PES for H2 quasi-classical trajectories for H2 interaction with Zn; sticking probability 3D adiabatic PES H2 orientation during dissociation; H adsorption site 6D-PES, H2 dissociation 6D-PES, H2 sticking
78Gre 94Dai
3.4.1.5 List of acronyms
ARUPS BIS CAS-MCSCF CFSO-BEBO
CPD EAM ECP EDC EELS EHT ESD GGA HAS hex HOPG HREELS IC IETS Landolt-Börnstein New Series III/42A5
109
angle-resolved UV photoelectron spectroscopy Bremsstrahlung isochromat spectroscopy complete active space multiconfiguration self-consistent field crystal field surface orbital-bond energy bond order contact potential difference embedded atom method effective core potential energy distribution curve electron energy loss spectroscopy extended Hückel theory electron-stimulated desorption generalized gradient approximation He atom scattering hexagonal highly-oriented pyrolytic graphite high-resolution electron energy loss spectroscopy incommensurate inelastic electron tunneling spectroscopy
94Gro 78Gre 90Cre 90Lou 96Eic 96Will 95Gro1 97Gro 91Mij 90Lou 05Per 99Ols 02Pij 02Ols 04Vin
110 IPE IR IRAS LCAO LCGTO LDF LEED LEIS LERS MB MCP MD MEM-LEED MO MR NEA NICIS NIST NN NRA PAW PES PR PT PW91 RAIRS REMPI RF RHEED SBZ SC SCF SES SFG SHG SIMS SIRS STM TD(S) TED TM TOF TPD UHV UPS UV VASP VLEED WF XPS
3.4.1 Adsorbate properties of hydrogen on solid surfaces
[Ref. p. 111
inverse photoemission Infrared Infrared absorption spectroscopy Linear combination of atomic orbitals linear combination of Gaussian-type orbital local density functional low energy electron diffraction low energy ion scattering low-energy ion recoil scattering molecular beam model core potential molecular dynamics mirror electron miscroscope LEED molecular orbital missing row (reconstruction) negative electron affinity neutral impact collision ion spectroscopy National Institute of Standards and Technology next neighbor nuclear reaction analysis projector-augmented wave (formalism) photoelectron spectroscopy pairing row (reconstruction) phase transition Perdew-Wang code from 1991 reflection-absorption infrared spectroscopy resonance-enhanced multi-photon ionization retarding field reflected high energy electron diffraction surface Brillouin zone self consistent self-consistent field secondary electron emission study Infrared-visible sum frequency generation second harmonic generation secondary ion mass spectroscopy surface infrared spectroscopy scanning tunneling microscopy thermal desorption (spectroscopy) transmission electron diffraction transition metal time-of-flight temperature-programmed desorption ultra high vacuum ultraviolet photoemission spectroscopy ultraviolet Vienna ab-initio simulation package very low energy electron diffraction work function X-ray photoelectron spectroscopy
Landolt-Börnstein New Series III/42A5
3.4.1 Adsorbate properties of hydrogen on solid surfaces 3.4.1.6 References
12Lan 14Bau 14Lan 15Lan 27Top 29Lon 31Eyr 49Her 55Cre 55Sat 55Tra 57Gom 57Kis 57Wor 58Han 58Kis 58Kou 58Sim 59Cul 59Law 60Gri 60Jos 61Gom 62Ger 64Hoh 65Jos 65Koh 65Kou 66Arm 66Est 66Jak 66Joh 67Gri1 67Gri2 67Lew 68Sha 69Bal 69Ber 69Lew 69Riv 69Tam 69Tra 70Ada 70Cla 70Tam 70Wed 71Ert 71Gri 71Han
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3.4.1 Adsorbate properties of hydrogen on solid surfaces Eberhardt, W., Cantor, R., Greuter, F., Plummer, E.W.: Solid State Commun. 42 (1982) 799. Engel, T., and Rieder, K.H: in: Structural Studies of Surfaces, Springer Tracts in Modern Physics, Vol 91 (1982), p.55 Ibach, H., Mills, D. L: Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, New York 1982 Imbihl, R., Behm, R.J., Christmann, K., Ertl, G., Matsushima, T.: Surf. Sci. 117 (1982) 257. Kim, Y., Peebles, H.C., White, J.M.: Surf. Sci. 114 (1982) 363. Kinzel, W., Selke, W., Binder, K.: Surf. Sci. 121 (1982) 13. Knor, Z: in: Catalysis Vol.3, (J.R. Anderson and M. Boudart eds.), Springer, Berlin 1982, p.231 Norton, P.R., Davies, J.A., Jackman, T.E.: Surf. Sci. 121 (1982) 103. Nørskov, J.K.: Phys. Rev. B 26 (1982) 2875. Nørskov, J.K.: Phys. Rev. Lett. 48 (1982) 1620. Nyberg, C., Tengstål, C.G.: Solid State Commun. 44 (1982) 251. Perreau, J., Lapujoulade, J.: Surf. Sci. 122 (1982) 341. Schönhammer, K., Gunnarsson, O.: Surf. Sci. 117 (1982) 53. Seguine, J.L., Suzanne, J.: Surf. Sci. 118 (1982) L241. Vickerman, J.C., Christmann, K.: Surf. Sci. 120 (1982) 1. Weng, S.-L.: Phys. Rev. B 25 (1982) 6188. Winkler, A., Rendulic, K.D: Surf. Sci. 118 (1982) 19. Yang, W.S., Sokolov, J., Jona, F., Marcus, P.M.: Solid State Commun. 41 (1982) 191. Andersson, S., Harris, J.: Phys. Rev. B 27 (1983) 9. Bagus, P.S., Schaefer III, H.F., Bauschlicher Jr., C.W.: J. Chem. Phys. 78 (1983) 1390. Barteau, M.A., Broughton, J.Q., Menzel, D.: Surf. Sci. 133 (1983) 443. Behm, R.J., Penka, V., Cattania, M.G., Christmann, K., Ertl, G.: J. Chem. Phys. 78 (1983) 7486. Binnig, G., Rohrer, R., Gerber, Ch., Weibel, E.: Surf. Sci. 131 (1983) L379. Binnig, G., Rohrer, R., Gerber, Ch., Weibel, E.: Phys. Rev. Lett. 50 (1983) 120. Blyholder, G., Head, J., Ruette, F.: Surf. Sci. 131 (1983) 403. Cattania, M.G., Christmann, K., Penka, V., Ertl, G.: Gazz. Chim. Ital. 113 (1983) 433. Cattania, M.G., Penka, V., Behm, R.J., Christmann, K., Ertl, G.: Surf. Sci. 126 (1983) 382. Didham, E.F.J., Allison, W., Willis, R.F.: Surf. Sci. 126 (1983) 219. Dose, V.: Prog. Surf. Sci. 13 (1983) 225. Eberhardt, W., Louie, S.G., Plummer, E.W.: Phys. Rev. B 28 (1983) 465. Froitzheim, H., Lammering, H., Günter, H.-L.: Phys. Rev. B 27 (1983) 2278. Greuter, F., Plummer, E.W.: Solid State Commun. 48 (1983) 37. Grimley, T. B: in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 2, Elsevier, Amsterdam 1983, Ch. 5, p.333 ff Habenschaden, E., Küppers, J.: Surf. Sci. 138 (1983) L147. Kobayashi, H., Edamoto, K., Onchi, M., Nishijima, M.: J. Chem. Phys. 78 (1983) 7429. Lee, J., Cowin, J.P., Wharton, L.: Surf. Sci. 130 (1983) 1. Murgai, V., Wenig, S.-L., Strongin, M., Ruckman, M.W.: Phys. Rev. B 28 (1983) 6116. Nyberg, C., Tengstål, C.G.: Phys. Rev. Lett. 50 (1983) 1680. Pasco, R.W., Ficalora, P.J.: Surf. Sci. 134 (1983) 476. Puska, M.J., Nieminen, R.M., Chakraborty, B., Holloway, S., Nørskov, J.K.: Phys. Rev. Lett. 51 (1983) 1083. Rieder, K.H., Wilsch, H.: Surf. Sci. 131 (1983) 245. Rieder, K.H.: Phys. Rev. B 27 (1983) 7799. Rieder, K.H., Baumberger, M., Stocker, W.: Phys. Rev. Lett. 51 (1983) 1799. Roelofs, L.D., Hu, G.Y., Ying, S.C.: Phys. Rev. B 28 (1983) 6369. Scheffler, M., Bradshaw, A. M: in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 2, Elsevier, Amsterdam 1983, Ch. 3, p.252 ff Schulze, G., Henzler, M.: Surf. Sci. 124 (1983) 336. Sinfelt, J. H: Bimetallic Catalysis, Wiley & Sons, New York 1983 Landolt-Börnstein New Series III/42A5
3.4.1 Adsorbate properties of hydrogen on solid surfaces 83Yu 84Ang 84Bat 84But 84Cha1 84Cha2 84Cha3 84Cir 84Con 84Fro 84Hal 84Ho 84Hor 84Jac 84Jon 84Joy
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84Nag1 84Nag2 84Nau 84Nor 84Oll 84Pen 84Rie 84Say1 84Say2 84Sie 84Sur 84Wan 85And 85Bic 85Car 85Cha1 85Cha2 85Chr 85Cla 85DiN 85Fel 85Feu 85Fre1 85Fre2 85Geo 85Goo 85Gri 85Ham 85Heg
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Yu, C.-F., Whaley, K.B., Hogg, C.S., Sibener, S.J.: Phys. Rev. Lett. 51 (1983) 2210. Angonoa, G., Koutecký, J., Ermoshkin, A.N., Pisani, C.: Surf. Sci. 138 (1984) 51. Batra, I.P., Barker, J.A., Auerbach, D.J.: J. Vac. Sci. Technol. A 2 (1984) 943. Butz, R., Oellig, E.M., Ibach, H., Wagner, H.: Surf. Sci. 147 (1984) 343. Chan, C.T., Louie, S.G.: Phys. Rev. B 30 (1984) 4153. Chan, L., Griffin, G.L.: Surf. Sci. 145 (1984) 165. Chabal, Y.J., Raghavachari, K.: Phys. Rev. Lett. 53 (1984) 282. Ciraci, S., Butz, R., Oellig, E.M., Wagner, H.: Phys. Rev. B 30 (1984) 711. Conrad, H., Scala, R., Stenzel, W., Unwin, R.: J. Chem. Phys. 81 (1984) 6371. Froitzheim, H., Köhler, U., Lammering, H.: Surf. Sci. 149 (1984) 537. Hall, R.B., DeSantolo, A.M.: Surf. Sci. 137 (1984) 421. Ho, P., White, J.M.: Surf. Sci. 137 (1984) 117. Horlacher Smith, A., Barker, R.A., Estrup, P.J.: Surf. Sci. 136 (1984) 327. Jackman, T.E., Davies, J.A., Norton, P.R., Unertl, W.N., Griffiths, K.: Surf. Sci. 141 (1984) L313. Jones, G.J.R., Onuferko, Julia H., Woodruff, D.P., Holland, B.W.: Surf. Sci. 147 (1984) 1. Joyce, B. A., Foxon, C. T., in: Chemical Kinetics – Vol. 19 ‘Simple Processes at the Gas – Solid Interface’ (Bamford, C.H., Tipper, C.F.H., Compton, R.G: eds.), Elsevier Amsterdam 1984, Ch. II, p. 181 Mokwa, W., Kohl, D., Heiland, G.: Phys. Rev. B 29 (1984) 6709. Morris, M.A., Bowker, M., King, D.A., in: Chemical Kinetics – Vol. 19 ‘Simple Processes at the Gas – Solid Interface’ (Bamford, C.H., Tipper, C.F.H., Compton, R.G: eds.), Elsevier Amsterdam 1984, Ch. I Nagai, K.: Surf. Sci. 136 (1984) L14. Nagai, K., Ohno, Y., Nakamura, T.: Phys. Rev. B 30 (1984) 1461. Naumovets, A.G., Vedula, Y.S.: Surf. Sci. Rep. 4 (1984) 365. Nordlander, P., Holloway, S., Nørskov, J.K.: Surf. Sci. 136 (1984) 59. Ollé, L., Baró, A.M.: Surf. Sci. 137 (1984) 607. Penka, V., Christmann, K., Ertl, G.: Surf. Sci. 136 (1984) 307. Rieder, K.H., Stocker, W.: Surf. Sci. 148 (1984) 139. Sayers, C.M.: Surf. Sci. 143 (1984) 411. Sayers, C.M., Wright, C.J: J. Chem. Soc. Faraday Trans. I 80 (1984) 1217. Siegbahn, P.E.M., Blomberg, M.R.A.: J. Chem. Phys. 81 (1984) 1373. Surnev, L., Tikhov, M.: Surf. Sci. 138 (1984) 40. Wang, S.C., Gomer, R.: Surf. Sci. 141 (1984) L304. Andersson, S., Wilzén, L., Harris, J.: Phys. Rev. Lett. 55 (1985) 2591. Bickel, N., Heinz, K.: Surf. Sci. 163 (1985) 435. Car, R., Parrinello, M.: Phys. Rev. Lett. 55 (1985) 2471. Chabal, Y., Raghavachari, K.: Phys. Rev. Lett. 54 (1985) 1055. Chabal, Y.J.: Phys. Rev. Lett. 55 (1985) 845. Christmann, K., Chehab, F., Penka, V., Ertl, G.: Surf. Sci. 152/153 (1985) 356. Clark, T: A Handbook of Computational Chemistry: A Practical Guide to Chemical Structure and Energy Calculations (Wiley, New York, 1985) DiNardo, N.J., Plummer, E.W.: Surf. Sci. 150 (1985) 89. Felter, T.E., Stulen, R.H.: J. Vac. Sci. Technol. 3 (1985) 1566. Feulner, P., Menzel, D.: Surf. Sci. 154 (1985) 465. Freed, K.F.: J. Chem. Phys. 82 (1985) 5264. Freimuth, H., Wiechert, H: Surf. Sci. 162 (1985) 432; 178 (1986) 716. George, S.M., DeSantolo, A.M., Hall, R.B.: Surf. Sci. 159 (1985) L425. Goodman, D.W., Yates Jr., J.T., Peden, C.H.F.: Surf. Sci. 164 (1985) 417. Griffiths, K., Norton, P.R., Davies, J.A., Unertl, W.N., Jackman, T.E.: Surf. Sci. 152/153 (1985) 374. Hamza, A.V., Madix, R.J.: J. Phys. Chem. 89 (1985) 5381. Hegde, R.I., White, J.M.: Surf. Sci. 157 (1985) 17.
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85Hof 85Hol 85Jo 85Kel 85Kno
Hofmann, P., Menzel, D.: Surf. Sci. 152/153 (1985) 382. Hollins, P., Pritchard, J.: Prog. Surf. Sci. 19 (1985) 275. Jo, M., Onchi, M., Nishijima, M.: Surf. Sci. 154 (1985) 417. Kellog, G.L.: Phys. Rev. Lett. 55 (1985) 2168. Knor, Z., in: Physics of Solid Surfaces 1984, Proceedings of the 3rd Symposium on Surface Physics, Smolenice Castle, Czechoslovakia, J.Koukal, ed: Elsevier Amsterdam 1985, p.781 85Kub Kubiak, G.D., Sitz, G.O., Zare, R.N.: J. Chem. Phys. 83 (1985) 2538. 85Mor1 Moritz, W., Imbihl, R., Behm, R.J., Ertl, G., Matsushima, T.: J. Chem. Phys. 83 (1985) 1959. 85Mor2 Moritz, W., Wolf, D.: Surf. Sci. 163 (1985) L655. 85Mus Muscat, J.P.: Prog. Surf. Sci. 18 (1985) 59. 85Noo NoorBatcha, I., Raff, L.M., Thompson, D.L.: J. Chem. Phys. 83 (1985) 1382. 85Nor Nordlander, P., Holmberg, C., Harris, J.: Surf. Sci. 152/153 (1985) 702. 85Pas Passler, M.A., Lee, B.W., Ignatiev, A.: Surf. Sci. 150 (1985) 263. 85Pat Pate, B.B.: Surf. Sci. 165 (1985) 81. 85Poe Poelsema, B., Verheij, L.K., Comsa, G.: Surf. Sci. 152/153 (1985) 496. 85Pus Puska, M.J., Nieminen, R.M.: Surf. Sci. 157 (1985) 413. 85Rie Rieder, K.H., Stocker, W.: Surf. Sci. 164 (1985) 55. 85Rob Robota, H.J., Vielhaber, W., Lin, M.C., Segner, J., Ertl, G.: Surf. Sci. 155 (1985) 101. 85Ste Stensgaard, I., Jakobsen, F.: Phys. Rev. Lett. 54 (1985) 711. 85Tak Takayanagi, K., Tanishiro, Y., Takahashi, S., Takahashi, M.: Surf. Sci. 164 (1985) 367. 85Thi Thiry, P.A., Pireaux, J.J., Liehr, M., Caudano, R.: J. Vac. Sci. Technol. A 3 (1990) 1439. 85Tri Tringides, M., Gomer, R.: Surf. Sci. 155 (1985) 254. 85Tro Tromp, R.M., Hamers, R.J., Demuth, J.E.: Phys. Rev. Lett. 55 (1985) 1303. 85Umr Umrigar, C., Wilkins, J.W.: Phys. Rev. Lett. 54 (1985) 1551. 85Wac Wachs, A.L., Miller, T., Hsieh, T.C., Shapiro, A.P., Chiang, T.-C.: Phys. Rev. B 32 (1985) 2326. 85Wan Wang, S.C., Gomer, R.: J. Chem. Phys. 83 (1985) 4193. 85Wei Weinert, M., Davenport, J.W.: Phys. Rev. Lett. 54 (1985) 1547. 85Yu Yu, C.F., Whaley, K.B., Hogg, C.S., Sibener, S.J.: J. Chem. Phys. 83 (1985) 4217. 86Arr Arrecis, J.J., Chabal, Y.J., Christman, S.B.: Phys. Rev. B 33 (1986) 7906. 86Bis Biswas, R, Hamann, D.R.: Phys. Rev. Lett. 56 (1986) 2291. 86Cha Chan, C.-M., Van Hove, M.A.: Surf. Sci. 171 (1986) 226. 86Chr Christmann, K., Ehsasi, M., Block, J.H., Hirschwald, W.: Chem. Phys. Lett. 131 (1986) 192. 86Chu Chung, J.W., Ying, S.C., Estrup, P.J.: Phys. Rev. Lett. 56 (1986) 749. 86Con1 Conrad, H., Kordesch, M.E., Stenzel, R.W., Sunjiü, U., Trninic-Radja, B.: Surf. Sci. 178 (1986) 578. 86Con2 Conrad, H., Kordesch, M.E., Scala, R., Stenzel, W.: J. Electron Spectrosc. Relat. Phenom. 38 (1986) 289. 86DeP DePristo, A. E., Lee, C.-Y., Hutson, J. M.: Surf. Sci. 169 (1986) 451. 86Fel Felter, T.E., Foiles, S.M., Daw, M.S., Stulen, R.H.: Surf. Sci. Lett. 171 (1986) L379. 86Feu Feulner, P., Pfnür, H., Hofmann, P., Menzel, D.: Surf. Sci. 173 (1986) L576. 86Gre Greuter, F., Strathy, I., Plummer, E.W., Eberhardt, W.: Phys. Rev. B 33 (1986) 736. 86Ham Hamers, R.J., Tromp, R.M., Demuth, J.E.: Phys. Rev. B 34 (1986) 5343. 86Har Harten, U., Toennies, J.P., Wöll, Ch.: J. Chem. Phys. 85 (1986) 2249. 86Her Herlt, H.-J., Bauer, E.: Surf. Sci. 175 (1986) 336. 86Kar Karlsson, P.-A., Mårtensson, A.-S., Andersson, S., Nordlander, P.: Surf. Sci. 175 (1986) L759. 86Koe Koeleman, B.J.J., de Zwart, S.T., Boers, A.L., Poelsema, B., Verheij, L.K.: Phys. Rev. Lett. 56 (1986) 1152. 86Lau Lauderdale, J.G., Truhlar, D.G.: J. Chem. Phys. 84 (1986) 1843. 86Lee Lee, C.-Y., DePristo, A.E.: J. Chem. Phys. 85 (1986) 4161. 86Li Li, Y., Erskine, J.L., Diebold, A.C.: Phys. Rev. B 34 (1986) 5951. 86Mak1 Mak, C.H., Brand, J.L., Deckert, A.A., George, S.M.: J. Chem. Phys. 85 (1986) 1676. 86Mak2 Mak, C.H., George, S.M.: Surf. Sci. 172 (1986) 509. Landolt-Börnstein New Series III/42A5
3.4.1 Adsorbate properties of hydrogen on solid surfaces 86Mar 86Mat 86Mul 86Mus 86Nie 86Nor 86Pap 86Rag 86Rie 86Roe 86Ros 86Rus 86Sau 86Sch 86See 86Tri1 86Tri2 86Ueb 86Wha 86Wil 86Woo 86Zae 87Alt 87Aue 87Bad 87Bes 87Cha 87Cho 87Daw 87Eng 87Fei1 87Fei2 87Fei3 87Fer 87Fre 87Gdo 87Ham 87Imb 87Jac1 87Jac2 87Kle1 87Kle2 87Kom 87Kre 87Leh
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3.4.1 Adsorbate properties of hydrogen on solid surfaces Tok, E.S., Chuan Kang, H.: J. Chem. Phys. 115 (2001) 6550. Wetzig, D., Rutkowski, M., Zacharias, H., Groß, A.: Phys. Rev. B 63 (2001) 205412. Badescu, ù.C., Salo, P., Ala-Nissila, T., Ying, S.C., Jacobi, K., Wang, Y., Bedürftig, K., Ertl, G.: Phys. Rev. Lett. 88 (2002) 136101. Bigelow, L.K., D’Evelyn, M.P.: Surf. Sci. 500 (2002) 986. Caudano, Y., Thiry, P.A., Chabal, Y.J.: Surf. Sci. 502-503 (2002) 91. Checchetto, R., Miotello, A., Tosello, C., Principi, G., Mengucci, P.: J. Phys. Condens. Matter 14 (2002) 6307. Dürr, M., Hu, Z., Biedermann, A., Höfer, U., Heinz, T.F.: Phys. Rev. Lett. 88 (2002) 0461. Jacobi, K.: “Electron work function of metals and semiconductors” in Landolt-Börnstein, New Series, Vol. 42, subvol. A, part 2 (2002) Lischka, M., Groß, A.: Phys. Rev. B 65 (2002) 075420. Naumovets, A.G., Zhang, Z.: Surf. Sci 500 (2002) 414. Okuyama, H., Hossain, M.Z., Aruga, T., Nishijima, M.: Phys. Rev. B 66 (2002) 235411. Olsen, R.A., Busnengo, H.F., Salin, A., Somers, M.F., Kroes, G.J., Baerends, E.J.: J. Chem. Phys. 116 (2002) 3841. Pijper, E., Kroes, G.J., Olsen, R.A., Baerends, E.J.: J. Chem. Phys. 117 (2202) 5885. Rosei, F., Rosei, R.: Surf. Sci. 500 (2002) 395. Smit, R.H.M., Noat, Y., Untiedt, C., Lang, N.D., van Hemert, M.C., van Ruitenbeek, J.M.: Nature (London) 419 (2002) 906. Steckel, J.A., Kresse, G., Hafner, J.: Phys. Rev. B 66 (2002) 155406. Zecho, Th., Güttler, A., Sha, X., Jackson, B., Küppers, J.: J. Chem. Phys. 117 (2002) 8486. Badescu, ù.C., Jacobi, K., Wang, Y., Bedürftig, K., Ertl, G., Salo, P., Ala-Nissila, T., Ying, S.C.: Phys. Rev. B 68 (2003) 205401 and 209903. Checchetto, R., Bazzanella, N., Miotello, A., Principi, G.: J. Alloys Compounds 356 (2003) 521. Greeley, J., Mavrikakis, M.: Surf. Sci. 540 (2003) 215. Groß, A: Theoretical Surface Science, Springer Berlin-Heidelberg 2003, p. 171 Jiang, D.E., Carter, E.A.: Surf. Sci. 547 (2003) 85. Lee, J.Y., Maeng, J.Y., Kim, A., Cho, Y.E., Kim, A.: J. Chem. Phys. 118 (2003) 1929. Mitsui, T., Rose, M.K., Fomin, E., Ogletree, D.F., Salmeron, M.: Surf. Sci. 540 (2003) 5. Mitsui, T., Rose, M.K., Fomin, E., Ogletree, D.F., Salmeron, M.: Nature (London) 422 (2003) 705. Okada, M., Nakamura, M., Moritani, K., Kasai, T.: Surf. Sci. 523 (2003) 218. Okuyama, H., Ueda, T., Aruga, T., Nishijima, M: Phys. Rev. B 63 (2003) 233403; 233404. Suzanne, J.: “Adsorption of molecules on MgO” in: Landolt-Börnstein, New Series, Vol. 42, subvol. A, part 3 (2003) Thachepan, S., Okuyama, H., Aruga, T., Nishijima, M., Ando, T., Mazur, A., Pollmann, J: Phys. Rev. B 68 (2003) 041401R. Thachepan, S., Okuyama, H., Aruga, T., Nishijima, M., Ando, T., Bagci, S., Tütüncü, H.M., Scrivastava, G.P.: Phys. Rev. B 68 (2003) 033310. Wiechert, H.: „Adsorption of molecular hydrogen isotopes on graphite and BN“ in: LandoltBörnstein, New Series, Vol. 42, subvol. A, part 3 (2003) Alemozafar, A.R., Madix, R.J.: Surf. Sci. 557 (2004) 231. Khatiri, A., Ripalda, J.M., Krzyzewski, T.J., Jones, T.S.: Surf. Sci. 549 (2004) 143. Kolovos-Vellianitis, D., Küppers, J.: Surf. Sci. 548 (2004) 67. Kostov, K.L., Widdra, W., Menzel, D.: Surf. Sci. 560 (2004) 130. Lai, W., Xie, D.: Surf. Sci. 550 (2004) 15. Ogura, S., Fukutani, K., Wilde, M., Matsumoto, M., Okano, T., Okada, M., Kasai, T., Diño, W.A: Surf. Sci. 566-568 (2004) 755. Ptushinskii, Y.G.: Low Temp. Phys. 30 (2004) 1 {Review}. Tsuboi, N., Okuyama, H., Aruga, T: Surf. Sci. 566 - 568 (2004) 777 Vincent, J.K., Olsen, R.A., Kroes, G.J., Baerends, E.J.: Surf. Sci. 573 (2004) 433. Yamazaki, H., Sakamoto, K., Fujii, A., Kamisawa, T.: Surf. Sci. 563 (2004) 41. Landolt-Börnstein New Series III/42A5
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3.8.1 H2O on metals List of acronyms and symbols used in this chapter ∆Φ
Θ
(hkl) 2D 3D AES DFT ESDIAD FTIR-RAS HAS HREELS IWB IRAS IV-LEED LEED ML MO NEXAFS NRA PDB RAIRS S SE SERS SEXAFS SFG SHG STM TPD UHV UPS XPS
work function change coverage in fractions of a bilayer specific lattice plane two-dimensional three-dimensional Auger electron spectroscopy density functional theory electron stimulated desorption ion angular detection fast fourier transform infrared-absorption spectroscopy Helium atom scattering high resolution electron energy loss spectroscopy intact water bilayer infrared reflection absorption spectroscopy intensity vs. voltage LEED measurements low energy electron diffraction monolayer molecular orbital near-edge X-ray absorption fine structure nuclear reaction analysis partially dissociated bilayer reflection absorption infrared spectroscopy sticking probability or sticking coefficient secondary emission surface enhanced Raman spectroscopy surface enhanced X-ray absorption fine structure sum frequency generation second harmonic generation scanning tunneling microscopy temperature programmed desorption ultrahigh vacuum ultraviolet photoelectron spectroscopy X-ray photoelectron spectroscopy
3.8.1.1 Introduction Water as the most abundant chemical species on earth plays an important role in many fields. Three quarters of the globe are covered by the oceans. The atmosphere contains water in significant amounts up to 4 vol %. The influence of water on the climate is obvious and manifested in different appearances such as fog, clouds or rain and even more solid consistencies as snow, hail or ice dependent on the atmospheric pressure and temperature. Also organic materials, including the human body consist mainly of water. Finally, minerals may contain substantial amounts of water, e.g. as crystal water. Hence knowledge about the chemical and physical behaviour of water is of fundamental importance. Besides this natural abundance, water exhibits an enormous importance for various technical processes. First of all, it is widely used as a solvent for different mainly chemical applications. Especially electrochemistry makes use of the unique properties of water. Water is also involved in etching, galvanic and corrosion processes. Common to the latter is that water gets into contact with solid often metallic surfaces. Hence, the characterisation of the chemical interaction of water with metal surfaces is important Landolt-Börnstein New Series III/42A5
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for the understanding and optimization of these reactions. Consequently, adsorption studies under UHV conditions are performed. Although certain objections exist with regard to the transferability of the results, these measurements allow the characterisation of adsorbed layers under well defined conditions. The whole arsenal of surface sensitive methods has been applied to get information about the thermodynamic, structural, electronic, vibrational or chemical properties of the adsorbed water layers. The physical properties of an isolated water molecule are listed in Table 1 together with a schematic geometric model of the molecule, shown in Fig.1 [87Thi]. The shape of the molecule is determined by the sp3 hybridization between the three 2p oxygen orbitals and the 1s orbitals of the two H atoms. The resulting H-O-H bonding angle of 104.5° is close to the ideal tetrahedral angle of 109.5°. The redistribution of charge from the hydrogen atoms to the oxygen leads to a relatively large dipole moment of 1.83×10−18 esu cm (1.8 D, 6.14×10−30 Cm) pointing from the oxygen to the hydrogen. This dipole moment is responsible for the solvation capability of water for ions in solutions, which has important consequences in electrochemistry. The water molecule is chemically relatively stable as indicated by the dissociation energy of 498 kJ/mol (5.18 eV). Two types of intermolecular bonding mechanisms can be distinguished, H bonding and covalent bonding via the two oxygen lone pair orbitals. Both have to be taken into account upon water adsorption on surfaces. Intermolecular bonding happens by H-bonding in the liquid and the solid state. While in liquid water only irregular H bonded networks of water molecules exist, crystalline ice is constructed by a periodic arrangement of water molecules connected by H bonds to each other. The resulting crystal structure is characterised by a net of buckled hexagons within a bilayer of two hexagonal layers shifted into the threefold hollow site with respect to each other and separated by 0.48 Å in height [76Wha]. Successive bilayers follow with a unit cell edge of 6.35 Å corresponding to an O-O distance of 2.76 Å. H-bonding does not only occur among water molecules but also between water and other chemical species providing the acidic and basic group, respectively. Covalent chemical bonding through the H atom is rare. Instead, bonding of water molecules to surfaces is mostly accomplished via the two oxygen lone pair orbitals. This results in a redistribution of charge with a net transfer of charge to the surface. Consistently a work function decrease is observed. The strength of H bonding within ice and covalent bonding of water molecules to metallic substrates is comparable with the consequence that clustering of H bonded water molecules often occurs even at very low coverage. Water monomers are only observed at low adsorption temperatures due to the low diffusion barriers for water molecules on metal surfaces. Andersson et al. were the first who identified isolated water molecules on Cu(100) and Pd(100) at 10K [84And]. On other surfaces, water monomers have been found at defects such as steps or coadsorbed with alkali metals [95Bau]. At higher temperature, the molecules usually produce a bilayer on the surface with a similar structure as crystalline ice. This is followed by three-dimensional ice condensation. If the H-bonds are stronger than the metal-water-bonds, three-dimensional growth occurs even before the surface is completely covered by an ice bilayer. Besides molecular adsorption of water, dissociation into OH or H can be triggered by the metal. In some cases, one even finds that dissociation products and molecularly adsorbed water coexist on the surface. In 1987 a comprehensive review about the interaction of water with solid surfaces has been published by Thiel and Madey [87Thi] revisited by Henderson [02Hen] who added the experimental results found until 2001. The present chapter summarizes the data for water adsorption on single crystal metal surfaces, based on these two review papers and complemented by results published since then. A collection of data for water adsorption on semiconductors is given by Jaegermann and Mayer in section 3.8.2 of this Landolt-Börstein subvolume III/42A, part 4 [04Jae]. According to the general scheme of the current subvolume “Adsorbed Layers on surfaces” only data for adsorbed water in the monolayer regime on structurally and chemically well-defined crystalline metal surfaces will be presented. The importance of H-bonding between adsorbed water molecules requires a modified definition of a monolayer, i.e. the saturation coverage of the first adsorbed bilayer is regarded as a monolayer. Surface reactions between water and other adsorbates will not be considered here. Only the reactivity of the individual metal surface upon water adsorption will be dealt with. Besides molecularly physisorbed or chemisorbed water molecules, reaction products such as OH or H have to be taken into account. The interaction of these species, formed either directly via dissociative adsorption or by consecutive heating of the adsorbed layer, with coadsorbed molecular water will be described. Landolt-Börnstein New Series III/42A5
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Within this chapter, the collected experimental data are ordered in paragraphs describing different physical and chemical quantities. This allows a convenient comparison of the properties of different metal substrates.
3.8.1.2 Electronic structure The electronic structure of adsorbed water molecules is characterized by their resulting molecular orbitals, vibrations and work function changes.
3.8.1.2.1 Valence band orbitals and core levels The 1s1 hydrogen and the 2s2 and 2p4 oxygen valence orbitals participate in the chemical binding within the water molecule. Together with the oxygen 1s2 derived orbital, five occupied molecular orbitals can be classified by symmetry: (1a1)2, (2a1)2, (1b2)2, (3a1)2, (1b1)2. Contour plots of the latter four molecular orbitals together with the first two unoccupied orbitals, (4a1)0 and (2b2)0 are shown in Fig. 2 as given by Jorgensen and Salem [73Jor]. While the ‘a’ denoted MO’s reflect the c2v symmetry of the water molecule, the ‘b’ MO’s exhibit the orbital plane as the only symmetry element. Whereas the (1a1)2 and (1b1)2 are nonbonding, the (1b2)2 is of antibonding character. In contrast the (3a1)2 and (2a1)2 are partly bonding and partly antibonding. Energetically these MO’s can be devided into valence orbitals ((1b2)2, (3a1)2 and (1b1)2) and core levels ((1a1)2 and (2a1)2). The valence orbitals 3a1 and 1b1 are directly involved in the chemical bonding resulting in the so called ‘chemical shift’ of the core levels. The energetically well separated first unoccupied 4a1 orbital plays a major role for the dissociation of water. Fig. 3 shows photoelectron spectra for all five occupied molecular H2O orbitals adsorbed on a Pt(111) surface as measured with X-rays. The excitation energies are hȞ = 1253.6 eV and hȞ = 120 eV for the core level and the valence band region, respectively. The binding energies of the three valence MO’s should be characteristic for the chemical interaction of water with metal surfaces and are listed in Table 2. Since these values are typically referred to the Fermi level, they can not be directly compared to the gas phase ionization potentials of 18.5 eV (1b2), 14.7 eV (3a1) and 12.6 eV (1b1) which are related to the vacuum level [74Rab]. Instead, one has to correct these values by adding the work function and possible relaxation energies. Nevertheless relative shifts may indicate which orbital is mainly involved in the chemical bonding. However, Henderson showed that a clear correlation does not exist for the observed energy spacings between 1b2 and 3a1 or 1b1 orbital [02Hen]. While for water monomers the bonding via the oxygen lone pair orbital dominates, hydrogen bonding to the surface has been observed for water molecules at the rims of molecular clusters, in addition. More recently, structure models with alternating metal-oxygen and metal-hydrogen bonds have been proposed for water bilayers on Pt(111) [02Oga] and Ru(0001) [03Den] (Fig. 15). Although the molecular structure reflects the bonding configuration, changes in the binding energies do not directly allow conclusions on the exact bonding mechanism. On the other hand, a distinction between molecularly adsorbed water and possible reaction products, such as OH can be made in a more straightforward manner. In the valence band regime OH can be distinguished from molecular water by its two orbital structure built up by the 1ʌ and 3ı orbital with ionization energies of 8.3 eV and 12.3 eV although overlapping may complicate the identification [77Con, 85Kis]. In general the O1s core level spectroscopy does not only allow the identification of the adsorbed species but also a quantification of the amount of adsorbed H2O, as seen in Fig. 3 for increasing water exposure. Table 3 summarizes O1s binding energies for molecularly adsorbed H2O on the metal single crystal surfaces studied so far. While the identification of a single peak in the binding energy range above 532.2 eV as molecularly adsorbed water is relatively straightforward, double peak structures as observed for Ru(0001) are discussed controversially in the literature [04And, 04Wei, 91Pir]. Previously, Pirug et al. proposed a layer dependent binding energy of 531.3 eV and 532.7 eV for molecular H2O in the first and the second layer of a bilayer [91Pir]. Lateron, Andersson showed that molecularly adsorbed water gives rise to only one O1s peak at 533.0 eV for H2O and D2O as shown in Fig. 4 a and c. The second peak at Landolt-Börnstein New Series III/42A5
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530.8 eV results from X-ray induced dissociation of water leaving hydodroxyl species on the surface (Fig. 4 b and d) [04And]. At the same time, Weisenrieder published O1s spectra for water (H2O or D2O) clearly showing a double peak structure with peaks at 531.0 - 530.8 eV and 532.7 - 532.3 eV explained by the formation of a stable wetting layer consisting of OH and H2O (Fig. 5 a) in a 3:5 proportion (Fig. 5 b) [04Wei]. Both interpretations are based on experimental and theoretical findings. Whereas the “intact water bilayer” (IWB) is supported by recent findings in second harmonic sum frequency (SFG) vibrational spectroscopy by Denzler et al. [03Den] and density functional theory (DFT) performed by Michaelides et al. [03Mic], the “partially dissociated bilayer” (PDB) agrees with a low energy electron diffraction (LEED)-IV structure analysis by Held and Menzel for D2O on Ru(0001) [94Hel1] and ‘ab initio’ calculations based on DFT by Feibelman [02Fei, 04Fei, 05Fei]. Although the theoretical studies report consistently that the PDB is thermodynamically more stable than the IWB, the calculated activation energy for dissociation of about 0.5 eV is of the same order as the activation energy for desorption favoring the IWB model at low enough temperatures. Concerning the application of electron spectroscopies, special care has to be taken in order to exclude beam induced changes in the adsorbed layer. Another controversy exists for water adsorption on Ni(110) [94Pir]. Pirug et al. resolved the broad O1s peak for a saturated H2O layer with a c(2×2) LEED pattern into almost equal contributions at 533.2 eV and 531.6 eV originating from molecular H2O and OH, respectively. Together with a saturation coverage of one O atom per surface Ni atom a bilayer structure consisting of an oxygen bonded first OH layer and hydrogen bonded second H2O layer has been proposed. This interpretation is supported by photoelectron diffraction [94Pir] and X-ray absorption finestructure measurements in the extended (SEXAFS) and near-edge region (NEXAFS) [94Pan] but in contradiction to a single molecular layer of H2O with a saturation coverage of 0.5 O atom per Ni surface atom proposed by Callen et al. based on a coverage calibration by nuclear reaction analysis (NRA) [90Cal, 92Cal2] and the missing OH stretch frequency using fast Fourier transform ir-absorption spectroscopy (FTIR) [91Cal].
3.8.1.2.2 Molecular vibrations The intra- and intermolecular bonding, as well as the bonding to the substrate upon adsorption of water can be studied applying vibrational spectroscopy. The isolated H2O molecule belongs to the c2v symmetry group and exhibits 3 internal vibrations as shown in Fig. 6a. Adsorbed on surfaces the restriction of the 3 translational and 3 rotational degrees of freedom leads to 3 additional librational and 3 frustrated translational modes (Fig. 6b). Besides conventional methods such as infrared absorption spectroscopy (IRAS) [94Oga], fast Fourier transform infrared spectroscopy (FTIR), reflection absorption infrared spectroscopy (RAIRS) [02Haq], surface enhanced Raman spectroscopy (SERS), second harmonic generation (SHG), sum frequency generation (SFG), elastic and inelastic He atom scattering (HAS) [99Gle], high resolution electron energy loss spectroscopy (HREELS) and more recently scanning tunneling spectroscopy [02Kom] can be applied under UHV conditions. Compared to all other methods which have specific restrictions, HREELS can be applied in the most universal manner [01Jac1]. The accessible frequency range is practically not restricted ranging from 20 cm-1 to more than 4000 cm−1 covering the frustrated molecule-substrate vibrations, inner layer lattice vibrations and all intramolecular vibrations. Making use of different excitation mechanisms, such as dipole and impact scattering, and surface selection rules, water layers can be characterized with respect to their clustering behaviour. The characterisation ranges from monomers over dimers to clusters and well ordered bilayers, local and long-range bonding configurations and the chemical composition [82Iba]. Table 4 contains data gained with all methods listed above. Fig. 7 shows a typical sequence of HREEL spectra recorded at increasing coverage of molecular water on Pt(111) [95Bau]. The vibrational signature with losses at 3380 cm−1 (O-H stretch), 1630 cm−1 (H-O-H symmetric bending, “scissor”) and a broad peak at 500 - 800 cm−1 (frustrated librations) indicates that H2O is molecularly adsorbed. The appearance of a loss at 240 cm−1 was assigned by Sexton to a translational mode parallel to the surface (TŒ) representing an oscillation of the whole molecule in an icelike layer [80Sex]. Lateron, the presence of this mode was taken as an indication for the presence of iceLandolt-Börnstein New Series III/42A5
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like layers [87Thi]. However, Jacobi et al. showed recently that this mode belongs to a perpendicularly hindered translation (Tŏ) [01Jac1] in agreement with recent HAS findings [99Gle]. Glebov et al. detected frustrated translational vibrations at even lower energy of 22 cm−1, 28 cm−1 and 47 cm−1 for H2O monomers, dimers and bilayer clusters, respectively. Due to the observed isoptope shift between H2O and D2O these modes could be assigned to vibrations of the water molecules parallel to the surface. The higher resolution and sensitivity of modern HREELS spectrometers allow the identification of water monomers adsorbed in chains at the upper step edges on Pt(111) at very low coverages even at 85 K. As shown in Fig. 8, monomers are characterized by a very soft frustrated-translational mode Tŏ at 15 meV (121 cm−1) and two frustrated-rotational modes L1 L2 at 28 meV (226 cm−1) and 36 meV (290 cm−1). In addition, two bending modes įam and įbm can be observed, which can only be dipole active for tilted H2O molecules. The chemical interaction between adsorbed water molecules and the substrate due to H-bonding is reflected by the observed OH-stretch vibrations. As shown in Fig. 9 Ogasawara et al. reported significant changes in this frequency range with increasing coverage [94Oga]. D2O monomers can be clearly distinguished by their reduced D-H stretch frequency of 2477 cm−1 from H-bonded water clusters and icelike layers with an O-H stretch frequency of about 2556 cm−1 and 2539 cm−1, respectively. The additional peak observed at 2731 cm−1 is ascribed to free OD bonds in ice in agreement with earlier findings by Ibach et al. for the hexagonally reconstructed Pt(100) surface [80Iba].
3.8.1.2.3 Work function changes As listed in Table 1 the isolated water molecule exhibits a static dipole moment µ = 1.83×10−18 esu cm (1.8 D, 6.14×10−30 Cm) pointing from the oxygen to the hydrogens. Therefore characteristic changes in the work function are expected upon adsorption of molecular water depending on the orientation of this dipole. In addition the transfer and polarization of charge due to the chemical bonding of the water molecule to the metal surface should influence the observed work function change. In general a decrease in work function is observed for molecularly adsorbed water as shown for Pt(111) in Fig. 10 [85Kis]. From the linear slope at small coverages an initial dipole moment of µ = 0.38 D has been determined. This decrease in work function is consistent with a dipole orientation with a perpendicular component to the surface. In principal this should indicate, that the water molecules are adsorbed via the oxygen atoms upright standing or tilted within or with the molecular plane by less than 90°. This simple explanation does not hold in view of structure models assuming more or less flat lying water molecules [94Hel1]. Hence a more refined interpretation of the work function changes is required including charge transfer and polarization effects as shown theoretically by Bonzel et al. [87Bon]. As shown in Fig. 11 for D2O adsorption on Ni(110), the work function change depends on the adsorption temperature [90Cal]. This behaviour can be explained by the condensation of water at 130 K. At 180 K condensation is no longer possible. Instead the formation of a first chemisorbed layer with a sharp c(2×2) LEED pattern is observed. The minimum in the ǻĭ curve of 970 mV corresponds to a coverage of Ĭ = 0.38, while a saturation value of 780 mV is reached at Ĭ = 0.48 according to the given coverage calibration. Discrete changes during desorption upon annealing as shown by a comparison of work function measurements with thermal desorption spectroscopy in Fig. 12 allow a detailed structural interpretation. The two desorption states A2 and A1 corresponds to molecularly adsorbed D2O with different dipole moments of 1.93 D and 0.58 D, respectively. While the A1 peak has been related to the desorption of molecularly adsorbed D2O from c(2×2) structure, the A2 peak has been explained by a decomposition of OD-D2O complexes, which results from D2O dissociation at temperatures above 205 K. Worthwhile to note, that this interpretation is at variance with structure models published by Benndorf and Madey [88Ben] and Pirug et al. [94Pir], as mentioned above. Work function changes have been measured for a number of single crystal metal surfaces as listed in Table 5, based on the data collection presented by Jacobi in section 4.2 of this Landolt-Börstein subvolume III/42A, part 2 [01Jac2] and completed by some new results published since then.
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The work function of water covered surfaces has also some importance with respect to electrochemistry [95Stu, 95Tra]. The equivalence between the work function measured in UHV and the potential of zero charge under electrochemical conditions justifies UHV model experiments concerning the electrochemical double layer [86Stu, 98Pir].
3.8.1.3 Dissociative versus molecular adsorption A further important question concerning H2O adsorption on metal surfaces is, whether it adsorbs as a molecule or whether it dissociates. This topic has been discussed in detail by Thiel and Madey [87Thi] and new results have been added by Henderson [02Hen]. Mostly, TPD, vibrational spectroscopy and photoemission have been used to deduce the chemical species present on the surface [87Thi]. But also other techniques such as isotope exchange [80Bow, 81Net, 82Ben, 84Nyb, 84Stu] or work function measurements [01Jac2, 82Her1, 82Her2, 88Her] provide information on the chemical species. Thiel and Madey have outlined that a first approach based on thermodynamic properties, in particular the enthalpy gain of the reaction, is rather successfull in predicting dissociation. Thus, in many cases, kinetic barriers appear to be of minor importance. Most surfaces prefer a dissociative adsorption, but some group VIII and group IB metals lead to molecular adsorption. The use of thermodynamic parameters appears to be safe as long as the enthalpy difference between molecular and dissociative state is larger than 100 kJ/mol with the exception of Zn. This results in some borderline cases such as Co, Ni, Re, Ru and Cu, where the dissociation depends critically on the particular surface and/or the step density. Since minute amounts of oxygen, other adsorbates or intrinsic defects, e.g. step edges, can trigger the dissociation, some of these cases are still controversial as discussed in the previous section. Interestingly, the structure of the H2O-layer itself can lead to dissociation. Held et al. concluded from detailed measurements that domain boundaries of the ice bilayer on Ru(0001), which exhibit a different H2O-density, are stabilized by dissociation [95Hel1, 95Hel2]. Based on DFT calculations, Feibelmann even proposed that the ice bilayer itself on Ru(0001) is stabilized by dissociating 2/5 of the molecules [02Fei, 03Fei, 05Fei]. An activated dissociation has been shown to occur on Ni(110) [89Gri, 94Pir, 96Kas] and Pt(111) [97Wan, 99Kin], in the latter case induced by adsorption of hyperthermal H2O. The dissociation can also be favored by the use of a high pressure (> 10−6 mbar) atmosphere of H2O as has been shown on Au(111) [75Che] and C(0001) (graphite) [92Chu, 93Chu]. Moreover, the application of an electric field of the order of 0.5 V/Å can lead to dissociation as has been shown on Pt tips [00Sco, 98Sco, 99Pin]. Thus, a direct transfer of dissociation data obtained in UHV to environments involving higher pressure, liquids or applied fields has to be taken with care. Table 6 summarizes the results concerning dissociation of H2O on crystalline metal surfaces in UHV. For comparison the differences in enthalpy taken from Thiel and Madey are indicated [87Thi]. The first number compares the cases of complete dissociation with molecular adsorption assuming an adsorption energy of 50 kJ/mol independent of the substrate. The second number compares partial dissociation, i.e. formation of OH and O with molecular adsorption. The minus sign means that dissociation is favored. Recently, the dissociation of H2O on metal surfaces by electrons, photons and ions has been investigated in more detail. This is reviewed by Henderson, where the additional dissociation channels either triggered by ionization or by activation of the adsorbed molecule are described [02Hen].
3.8.1.4 Geometric structure of molecularly adsorbed ice 3.8.1.4.1 Adsorption geometry It is generally believed that H2O prefers an on-top adsorption site with the oxygen binding to the surface atom [87Thi]. This is largely concluded from DFT calculations and has been confirmed by experiments on Ru(0001) [94Hel1], Pd(111) [04Cer] and Ni(110) [94Pan] by LEED-IV measurements, STM and SEXAFS measurements, respectively. As outlined by Thiel and Madey, this can be understood by Landolt-Börnstein New Series III/42A5
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regarding H2O as an electron donor with respect to the metal, which prefers to sit on sites of relatively low electron density on the metal surface [87Thi]. This idea is in accordance with the preferential adsorption of H2O at the upper edge of steps [96Mor] and the fact, that desorption temperatures of molecular H2O are enhanced on the rougher single crystal surfaces [87Thi] having stronger corrugation in electron density for H2O adsorpion and, thus, providing positions of lower electron density. DFT calculations give a more detailed picture. Basically, the occupied orbitals 3a1, 1b1, and 1b2 of H2O hybridize with unoccupied d-orbitals of the metal including a charge transfer from H2O to the metal. Since H2O gets closer to the d-orbitals at positions of low sp-electron density, the binding is stronger at these positions [85Bau]. The orientation of the water molecule with respect to the surface normal has been studied for several metals as Ru(0001) [77Mad, 82Doe, 94Hel2, 95Hel2], Cu(110) [83Mar], Cu(100) [86Nyb], Pd(100) [86Nyb], Ni(110) [85Nöb], Ni(111) [94Pan] and Ni(665) [87Nöb]. The orientation has been concluded from work function changes presuming a certain dipole moment of the water molecule, from electron stimulated desorption patterns (ESDIAD), from NEXAFS, from polarization dependence of photoemission yields and from the angular distribution of HREELS yields. Generally, it has been found that the H2O-molecule is tilted significantly with respect to the surface normal. The molecule is even parallel to the surface for Ni(111) [94Pan] and Ru(0001) [94Hel2, 95Hel2], and has a tilt angle of 57°-58° on Pd(100) and Cu(100) [86Nyb]. ESDIAD studies on Ru(0001) have shown that the azimuthal angle of the tilted molecule is random at low coverage presumably due to the presence of monomer H2O and gets ordered with six-fold symmetry at higher coverage presumably due to the formation of a hydrogenbonded network [77Mad, 82Doe]. Recent XPS results and DFT calculations indicated that besides the oxygen bonding to the metal an additional hydrogen-metal bond is formed in bilayers on Pt(111) [02Oga] and Ru(0001) [02Fei, 04And, 04Wei]. In the latter case, DFT calculations even found that a partial dissociation of the H2O takes place leaving 3/5 of the molecules intact [02Fei, 05Fei]. However, this topic is still controversial.
3.8.1.4.2 Binding energy and desorption temperatures The strength of the H2O-metal bond is difficult to determine. It is mostly concluded from TPD. Generally, two peaks originating from the desorption of ice multilayers and the ice bilayer on top of the metal are observed. However, since ice on metal surfaces forms islands including hydrogen bonds, the identification of the desorption temperature of the bilayer with binding energy is not straightforward. A detailed picture of the desorption kinetics is required, which has only partly been settled. Nevertheless, typical values for the deduced oxygen-metal binding energy are about 50±15 kJ/mol [87Thi]. A more detailed analysis performed on Ag(110) results in a binding energy of 60±1 kJ/mol [89Wu]. Besides the bilayer and multilayer peak, additional peaks can be found at higher temperature. They are assigned to H2O bound to special defects as e.g. step edges, and to recombinative desorption of dissociation products, respectively [02Hen, 87Thi]. The latter assignment requires information from other techniques as e.g. UPS in order to proof that dissociation takes place on the surface. On the most noble surfaces as Au and Ag as well as on Cu(111), only a single peak is found which shifts to higher temperatures at higher coverage (see references in Table 7). This indicates that the binding to the metal is weaker than the binding within the ice layer. The result makes sense, since the d-shell of Ag, Au and Cu is largely filled reducing the density of empty d-levels for binding. On these surfaces, the ice grows in three-dimensional hydrogen-bonded clusters on the surface. Fig. 13 shows the typical cases of TPD spectra [87Thi]. Only a multilayer TPD peak is observed on Ag(110). A multilayer and a bilayer peak are observed on Pt(111). Ru(0001) exhibits the exceptional behaviour, that two bilayer peaks are observed in addition to the multilayer peak as discussed below. Finally, Ni(110) shows multilayer and bilayer peak and, in addition, two peaks originating from desorption of molecules presumably stabilized by dissociation products and from recombinative desorption, respectively. Table 7 lists measured TPD peak temperatures of ice adsorbed on different metal surfaces. They are ordered with respect to their assignment. The slight deviations of peak temperature in different Landolt-Börnstein New Series III/42A5
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experiments on the same substrate can be due to different heating rates, partly different initial coverages and different adsorption temperatures. An uncertainty in the temperature calibration cannot be ruled out either. With regard to these uncertainties, the original values have been partly rounded to a 5 K scale. Since it is usually assumed that the ice multilayer desorbs at the same temperature on each surface, it might be helpful to relate the other peaks to the multilayer peak. Note that the TPD peaks often shift with coverage and, thus, the given desorption temperatures are typical values only. If possible, they are given at the same H2O coverage, i.e. the bilayer peak is given at a coverage of about half a bilayer and the multilayer peak at a coverage of about 2 bilayers. Generally, the desorption temperatures are higher for the rougher surfaces of the same material, which fits with the idea that water is an electron donor bound most tightly to positions of low electron density of the metal. Interestingly, a relation between desorption temperature and lattice mismatch between ice(0001) and the hexagonal metal surfaces has been found by Thiel and Madey (Fig. 32 of [87Thi]). The highest desorption temperature exists on Ru(0001), where the ice lattice is expanded by 3% with respect to pure ice. The desorption temperatures get increasingly smaller if the lattice mismatch deviates from the 3% value in both directions. However, since the H2O bilayer desorption on Ru(0001) is unusual exhibiting two TPD bilayer peaks, while the D2O bilayer on Ru(0001) shows only one bilayer peak, the conclusion has to be taken with care [95Hel1, 95Hel2]. In addition, more recent TPD spectra e.g. from Pt(111) do not fit into this simple picture [02Haq]. Recently, the two TPD peaks of H2O on Ru(0001) have been correlated with a domain structure of the bilayer, which exists for H2O but not for D2O [82Doe, 95Hel1, 95Hel2]. The H2O domain boundaries transform from being H2O enriched to being H2O deficient at the temperature of the first TPD peak. Held et al. suppose that the domain boundaries may be stabilized by fragments of dissociated H2O. An influence of dissociation on this surface has also been found by DFT calculations of Feibelman, who, however, proposes a homogeneous dissociation of 2/5 of the molecules in the D2O bilayer [02Fei, 05Fei]. Indeed, recent XPS studies have shown that 3/8 of the D2O and the H2O molecules are dissociated after adsorption at 145 K [04Wei]. In contrast, Anderson found no dissociation using a slightly different preparation at 150 K, but a clear onset of dissociation at 180 K [04And]. Thus, the topic of the isotope effect on Ru(0001) is still controversial. Interestingly, the isotope effect has only been studied on Ru(0001), although Jo et al. found two bilayer peaks after D2O adsorption on Pt(111) [91Jo] while other authors studying H2O adsorption on Pt(111) find only one peak [02Haq].
3.8.1.4.3 Trapping and sticking As pointed out by Henderson, trapping and sticking is determined from different measurements [02Hen]. The trapping coefficient corresponds to the part of molecules which is not reflected by the surface, while the sticking coefficient S excludes also the molecules which are trapped by the surface but desorb prior to the measurement. The trapping coefficient is measured by molecular beam techniques, while the sticking coefficient is deduced from measurements performed after the adsorption as TPD, photoemission, AES, STM or others. Molecular beam measurements of H2O trapping on metals are rare, but as outlined by Thiel and Madey the sticking coefficient is already close to unity, if adsorption temperatures well below the desorption temperature of H2O are used [87Thi]. Moreover, the sticking coefficient at low temperature does not depend on coverage implying that trapping and sticking coefficients are similar. Exact values for the sticking coefficient are difficult to deduce due to uncertainties in the determination of the exposure and the coverage and will not be listed here. The published values are between 0.6 and 1, i.e. close to unity [87Thi]. Measurements using a well defined molecular beam for exposure (Au(111) [89Kay], Ru(0001) [97Smi], Ni(110) [90Cal], Pt(111) [02Haq]) and a well calibrated measure for coverage always give a value of S > 0.95. Also in the other measurements, the sticking is found to be independent of coverage in the temperature range between 100 K and 140 K, i.e. below the onset of significant multilayer desorption [87Thi]. Table 8 of Thiel and Madey summarizes some of the results concerning sticking. Landolt-Börnstein New Series III/42A5
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3.8.1.4.4 Diffusion and formation of small clusters Adsorption of H2O partly leads to vibrational frequencies, which have been interpreted in terms of H2O monomers. The assignement is not always unambigous, i.e. the features are partly interpreted as due to very small clusters as dimers or tetramers. Fig. 9 shows vibrational spectra (IRAS) observing the transition from isolated monomers to clusters as a function of D2O coverage on Pt(111) (T = 84 K) [94Oga]. The features at around 1200 cm−1 and 2500 cm−1 are assigned to the D-O-D scissor mode and the O-D stretch mode, respectively. Both modes consist of two peaks. The lower frequency peak dominates at low coverage, while the higher frequency peak dominates at higher coverage. One deduces that the low frequency peaks correspond to the unperturbed vibrations, while the high frequency peaks are caused by stiffer vibrations due to O-D bridge bonds within ice clusters. The exact peak frequencies, thus, can be used to assign the species present on the surface. However, the assignment becomes more intricate, if one has to decide, whether monomers or dimers are present. Additional information can be gained by using ESDIAD or UPS as reviewed by Thiel and Madey and Henderson [02Hen, 87Thi]. Recently, also He scattering [97Gle, 99Gle] and STM [02Mit, 02Mor2, 04Cer, 96Mor, 97Ike] have been used to deduce the size of the adsorbed clusters. Figures 14 a-c show STM images of H2O monomers, small H2O clusters and the complete ice bilayer adsorbed on Pd(111) [02Mit], Ag(111) [03Mor] and Pt(111) [97Mor], respectively. Here, the assignment is rather direct. Table 8 summarizes the assignments with respect to measurement temperature and coverage for the different surfaces. The existence of monomers has been deduced for Al(111), Cu(100), Cu(110), Cu(111), Ni(110), Pd(100), Pd(110), Pt(111), Re(0001) and Ru(0001). Some results on Ag(111), Ni(100), Ni(110), Pd(110), Pt(111), Re(0001) and Ru(0001) have been interpreted as due to dimers, tetramers or hexamers. Generally, there are three different cases, where monomers, dimers, tetramers and hexamers have been found. First, they exist at low temperatures (10-40 K), where certain diffusion processes are blocked and disappear at higher temperatures at the onset of diffusion [02Mit, 02Mor2, 97Gle, 99Gle]. Second, they exist at very low coverage probably due to sticking at defects, e.g. at step edges (e.g. [91Jac, 92Xu]). Third, they exist up to the coverage of half a bilayer, in particular on the more open surfaces as Cu(110) [82Spi] and Ni(110) [85Nöb, 94Pir], but disappear at coverages close to one bilayer. In the latter case, the most reasonable cause for the existence of monomers is the competition between hydrogen-metal bonds and H-bridge bonds within the ice. Monomers are stabilized by tilting the molecule towards the surface giving rise to H-metal interaction [94Pan]. The H-metal bonds might also be relevant in H-bridge bonded clusters as they appear on Pt(111) [02Oga] or Ru(0001) [02Fei] at higher temperature. There is only one direct measurement of the diffusion of isolated H2O molecules performed on Pd(111) by STM (see inset of Fig. 14 a) [02Mit]. It results in a monomer diffusion barrier of 126 meV (8 kJ/mol) with a prefactor of 1012/s. Similar results are expected on other close-packed surfaces. For example, the onset of monomer diffusion on Pt(111) takes place at 40 K requiring a diffusion barrier of about 10 kJ/mol [99Gle]. On more open surfaces, the diffusion barriers could be significantly higher, e.g. recent DFT calculations on Al(100) give a value of about 320 meV (20 kJ/mol) [04Mic]. Diffusion measurements of multilayer water on a field-emission tip have been done, but can only provide an upper limit, giving e.g. an upper bound of 25 kJ/mol for the diffusion barrier on Pt(111) [95Bry, 97Bry, 99Bry].
3.8.1.4.5 Ice bilayer If two requirements are fulfilled, a stable bilayer of ice can be prepared on a metal surface. First, it requires the absence of dissociation at least as a major pathway (see Table 6) and second, H2O within the ice bilayer on the surface must be stronger bound than H2O within the ice multilayer, i.e. a distinct TPD bilayer peak must exist (see Fig. 13). Then, the H2O exposure at temperatures higher than the multilayer and lower than the bilayer TPD peak results in a well defined bilayer. The bilayer basically is a layer of Ih-ice(0001) with half of the oxygen lone pair orbitals bound to the metal. It consists of hexagonal ice rings as depicted in Fig. 15a.
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Half of the molecules bind to the metal via their oxygen lone-pair orbital and the other half is bound by H-bridge bonds to three neighbouring H2O molecules. The exact geometry of the bilayer can be different from Ih-ice. In particular, it has been shown that the vertical O-O distance can be much smaller in the bilayer than in ice e.g. on Ru(0001) [94Hel1], Pd(111) [04Cer] or Pt(111) [01Jac1, 02Oga]. Reasons are the electron deficiency within the ice layer due to electron donation to the metal, but also the competition between H-bridge bonds within the ice and H-metal bonds. The latter details have only recently been realized by detailed analysis of IV-LEED [94Hel1], STM [04Cer] and XPS [02Oga] results, respectively, and have been confirmed by DFT calculations. Fig. 15 b shows the structure of the ice bilayer on Pt(111) deduced from XPS [02Oga]. In contrast to usual Ih-ice, where the remaining dangling H-atom points away from the surface, the upper molecules are rotated by 180° giving rise to an additional H-metal bond. The structure of the D2O bilayer on Ru(0001) as calculated by DFT is shown in Fig. 15 c [02Fei]. Here, the dangling H-atom is abstracted giving rise to an additional O-metal bond of the resulting OH and to an additional H-metal bond at a different surface atom. However, the symmetry of the ice bilayer is still intact and the number of H-bridge bonds is the same as in a bilayer of Ih-ice. The registry of the bilayer with the metal surface is guided by the interplay between optimal adsorption site on the metal (i.e. on-top) and optimal length of the H-bridge bonds within Ih-ice. Early LEED studies have mostly concluded that the periodicity of the bilayer is given by the substrate resulting in a (√3×√3)R30° superstructure on the close-packed surfaces and a c(2×2) superstructure on the more open fcc(110)-surfaces (see Table 9). The basic idea was that the resulting stress within the ice bilayer can be relieved by buckling the H-bridge bonds [87Thi]. However, more recent, detailed studies on Pt(111) and Ru(0001) using He-scattering [97Gle], LEED [02Haq, 82Doe, 95Hel1, 95Hel2] and STM [97Mor] have shown that the structure can be more complex. On Ru(0001), the stress within the H2O bilayer resulting from the (√3×√3)R30° superstructure is additionally relaxed by forming a regular array of stripe domains separated by domain walls every 6.5 lattice spacings of the substrate [95Hel1, 95Hel2]. Two types of domain walls exist depending on the preparation. They have either additional or missing H2O molecules with respect to the perfect (√3×√3)R30° superstructure and are probably responsible for the two TPD peaks of the H2O-bilayer on Ru(0001). These domain walls do not exist for D2O on Ru(0001), where only one TPD peak is found. A structure with domain walls may also exist on Rh(111) as concluded from weak He diffraction spots appearing in addition to the (√3×√3)R30° spots [00Gib]. For the bilayer on Pt(111), which exhibits a lower desorption temperature than the bilayer on Ru(0001), the H-bridge bonds seem to be of larger importance. Three different phases have been detected by STM depending on the preparation [97Mor]. One of these phases (I) realizes the optimal bridge bond length and is rotated with respect to the substrate in order to optimize the bonding positions of the O-atoms on the metal (Fig. 14c). The other two phases appear to have a similar local structure, but are less dense than phase I. They are most likely identical to the (√37×√37)R25.3° superstructure detected by He scattering [97Gle]. Interestingly, the transformation between the different phases is mediated by a highly mobile, disordered phase which coexists with the ordered phases at higher temperature. He-scattering [97Gle] and LEED [02Haq] measurements have found a fourth superstructure, i.e. (√39×√39)R16.1°, by preparing the bilayer at slightly lower temperature. Due to the complexity of the superstructures, the long-range order of the ice bilayer on Ru(0001) and Pt(111) is not completely settled. However, it might even be that more complex structures also exist on other surfaces, which have been less investigated so far. Table 9 summarizes the available results on the superstructure of the ice bilayer on different surfaces. Partly, the ice layers are prepared at temperatures below the onset of multilayer desorption, but still exhibit a (√3×√3)R30° and a c(2×2) superstructure. However, no superstructure spots are observed on a number of surfaces including all fcc(001) surfaces. Note that the H2O adsorption on Co, Al and Zr leads to the coexistence of dissociated and molecular H2O on the surface (Table 6, Table 7), which might prohibit the long-range ordering. Acknowledgements: We gratefully appreciate that M.A. Henderson provided the text of his recent review.
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143
3.8.1.5 Tables for 3.8.1 Organisation of the tables Table 1: Physical properties of the isolated H2O molecule Table 2: Valence band peak positions (in eV relative to the Fermi level, unless otherwise noted) for molecularly adsorbed water on metal surfaces Table 3: O 1s XPS peak positions (in eV) for molecularly adsorbed water on metal surfaces Table 4: Vibrational assignments and frequencies (in cm−1) for molecularly adsorbed H2O and D2O on metal surfaces Table 5: Work function changes for molecularly adsorbed water on metal surfaces given with respect to the clean surface Table 6: Assignments of molecular versus dissociative adsorption of water on crystalline metal surfaces Table 7: Desorption temperatures of water from different crystalline metal surfaces ordered with respect to their assigned adsorption geometry and desorption paths, respectively Table 8: Adsorption structure of water on metal surfaces Table 9: Measured superstructure periodicity of ordered 2D ice bilayers on solid surfaces Table 1. Physical properties of the isolated H2O molecule Parameter HOH bond angle, ș HOH OH bond length, r OH van der Waals (hard-sphere) radius, rVdW moments of inertia, I: Iy Iz Ix Dipole moment, µz mean polarizability, J O-H bond dissociation energy, D 0
Value 104.5° 0.957 Å 1.45 Å 1.0220 × 10−40 g−1 cm−2 1.9187 × 10−40 g−1 cm−2 2.9376 × 10−40 g−1 cm−2 1.83 × 10−18 esu cm (6.14 × 10−30 Cm) 1.444 × 10−24 cm3 498 kJ/mol (5.18 eV)
Ref. 69Eis 69Eis 71Cot 69Eis 69Eis 69Eis 69Eis 69Eis 69Eis 71Cot
Table 2. Valence band peak positions (in eV relative to the Fermi level, unless otherwise noted) for molecularly adsorbed water on metal surfaces Substrate
1b2
3a1
1b1
Al(100) Ag(110) Ag(111) Co(0001) Co(11 2 0)
a
11.6 8.9 10.0 b 9.3 9.9 9.4
7.3
Landolt-Börnstein New Series III/42A5
12.0 13.7 b 13.0 13.5 13.0
a
7.1 b 6.9 7.3 7.4
Temperature [K] 100 155 110 100 100 100
Reference 82Sza 84Bar 90Bla 82Her2 82Her2 94Gre
144 Cu(100) Cu(110)
Ni(100) Ni(110)
Ni(111) Ni(221) Ni(665) Ni(775) Pt(110)-(1×2) Pt(111)
3.8.1 H2O on metals 13.1 b 11.9 13.0 13.2 12.6 13.0 12.7 13.2 12.8 12.3 11.7 12.3 12.7 11.5 11.9 11.3 12.0 18.6 e
9.1 b 9.9 8.8 9.0 8.8 9.3 9.4 9.3 8.6 9.5, 8.3 d 8.9 8.5 8.2 8.5 8.2 7.6 8.1 14.7 e
6.9 b 6.3 7.2 7.1 6.8 7.0 6.8 7.6 6.8 6.3 6.7 6.2 6.3 5.5 5.6 5.4 6.0 12.6 e
11.7 12.2 12.4
8.4 8.7 8.2
5.8 6.4 6.4
c
Pt6(111)×(100) Rh(100) Rh(111) Ru(0001) c
[Ref. p. 162 90 80 90 100 90 100 150 137 150 120 157 120 145 100 100 90 100 100 120 120 100 90 95 120 90
12.5 8.7 7.1 Ti(0001) c a. Peak position not given or not resolved. b. Estimated from the spectra in the paper. c. Spectrum not shown; paper only refers to ‘three peak spectrum’. d. The 3a1 is split due to the bilayer water structure. e. Relative to the vacuum level.
85Spi2 80Sas 82Spi 83Ban1, 84Ban 83Mar 84Pee 81Ben 94Pir 85Nöb 89Pac, 91Bor 98Mun 87Nöb 98Mun 90Fus 80Fis1, 80Fis2 89Ran1 85Kis 84Lan 84Lan 81Mil 85Gre 87Wag1 80Thi, 81Thi, 82Thi2 91Pir 82Sto
Table 3. O 1s XPS peak positions (in eV) for molecularly adsorbed water on metal surfaces Surface Al(100) Al(110) Ag(110) Au(111) Co(110) Cu(110)
Cu(111) Ni(100) Ni(110) Ni(111) NiAl(110) Ni3(Al,Ti)(100) Ni3(Al,Ti)(111)
O 1s peak energy [eV] 533.3 535.0 533.5 533.2 532.6 533.6 533.4 533.4 533.5 533.5 533.5 533.4 533.2 533.2 533.9 533.4 533.3
Temperature [K] 100 105 80 110 100 90 95 110 80 100 100 180 120 123 140 140
Reference 82Sza 94Mil 83Au 93Laz 94Coe 94Gre 85Spi2 03Amm 89Cle 79Au, 80Au 84Pee 82Ben 94Pir 93Kuc1, 93Kuc2, 94Kuc, 95Sch 95Gle 95Chi1, 95Chi2 95Chi1, 95Chi2 Landolt-Börnstein New Series III/42A5
Ref. p. 162] Surface
3.8.1 H2O on metals O 1s peak energy [eV] 534.0 532.2 532.2-532.9 532.2-531.8 532.2 533.2 and 531.3 532.7 - 532.3 (H2O) 531.0 - 530.8 (OH) 533.0 (D2O) 532.8 (H2O) 530.8 (OH)
Pb(110) Pt(111)
Rh(111) Ru(0001)
145
Temperature [K] 80 100 100 90 90 120 145
Reference 93Au 80Fis1 85Kis 89Ran1, 89Ran2 87Wag1, 88Wag1 91Pir 04Wei
150
04And
Table 4. Vibrational assignments and frequencies (in cm−1) for molecularly adsorbed H2O on metal surfaces, values in parentheses are for D2O Substrate
O-H HOH Libration stretching bending (rocking)
Al(100)
(2720, 2560) 3695, 3510 3450 3450 (2590) 3390 3410 3452 (2549) 3452 (2549) 3295
Al(110) Al(111) Ag(100) Ag(110)
Au(111) Cu(100) Cu(110)
Fe(100)
3600, 3375, 3180 3350 3600, 3375, 3180 3350 3380, 3070 (2595, 2315) 3470, 3000 (2595, 2315)
Landolt-Börnstein New Series III/42A5
Frustrated Adsorption Libration M-OH2 (wagging) stretching translation configuration of water
(1200)
Refs
93Bus
1655
685 a
1640 1650 (1210) 1610 1660 1645 (1170) 1645 (1170) 1650 1589 (1178) 1600
1600 1600
685 a
265
82Sza
780 775 (645)
220
870 730 740 766 (581)
240 200
94Mil 87Cro, 88Che 86Din 81Stu 89Wu
cluster
766 (581) 835
89Wu
monomer
98Pir 84And
low coverage
89Lac
780 750
cluster
90Sch 91Sas
1600 1630 (1220)
780 690 (545)
cluster
90Lac 82Bar
1630 (1220)
690 (545)
cluster
230 (198) 745
460
91Hun, 93Hun
146
3.8.1 H2O on metals
Substrate
O-H HOH Libration stretching bending (rocking)
Ni(110)
3350 3660, 3350, 3030 (2670, 2450, 2140)
[Ref. p. 162
1610 (1110)
Libration M-OH2 Frustrated Adsorption Refs (wagging) stretching translation configuration of water 290 cluster − low 85Oll coverage 760 (530) 530 230 (275) cluster − high 85Oll coverage
1600
780
800 (588) 667 (493) 3700 (3330) NiAl(110) 3620 (2660) 3460 (2550) Pb(110) 3420 Pd(100) 3404
Pd(110)
1620 (1200) 1650 (1220) 1660 1597 (1186) 1605
cluster
760 (600)
240 (240)
cluster − low coverage multilayer
95Gle
750
230 335 (282)
momomer
93Au 84And
613
218
cluster
480
235 215
cluster cluster – low coverage cluster – high 90Bro1, coverages 91Bro
560 (560)
807
3380 3600, 3440 3500, 3300 (2600, 2470)
1640 1610
810 750
1630, 1580 (1200, 1160)
745 (570) 660 (510) 500
3400 (2500) 3430 3670, 3380, 2850 (2710, 2550) 3670, 3410 3400 (2530), 3445 (2543) 3680, 3430 (2700, 2560)
1616 (1188) 1645 1630 (1210)
812 (730) 642 (585)
660
215 (215)
403 Pd(111) Pt(100)
Pt(110) Pt(111)
98Kov 86Hoc
270
192
95Gle
84Nyb, 86Nyb 84Stu 90Bro2
monomer cluster
02Kom 91Zhu
920 520 920 (680) 560 (470) 460 (470) 240 (240)
92Kiz 80Iba
1620
730
99Che
1625 (1200) 1625 (1195) 1630 (1195)
700 (550700) 688 (750)
550 (500) 250 (240)
80Sex
530 (510) 260 (230)
96Gil
695 (510580)
560
620
270
240 (240)
cluster
95Bau
Landolt-Börnstein New Series III/42A5
Ref. p. 162] Substrate
3.8.1 H2O on metals
O-H HOH Libration stretching bending (rocking)
Pt(111) cont. (2706, 2465) (2641, 2577, 2492) 3440
Rh(111) Ru(0001)
Zn(0001)
Libration M-OH2 Frustrated Adsorption Refs (wagging) stretching translation configuration of water 22 (21)b monomer 99Gle b dimer 28 (27) bilayer cluster 47 (46)b monomer 99Oga dimer
99Oga
bilayer
87Wag2, 88Wag1, 88Wag2 91Sas 80Fis2 95Vil
bilayer
01Jac1
(1199)
sub-bilayer
02Haq
(2355, 2472, 2721) 3318
(1199)
bilayer
02Haq
3370 (2480) 3490 3565, 3400, 2935 3350
1620 (1190) 1615 1520
800-400 a (600-300 a) 740 920 700
sub-bilayer, 04Gre mol. chains at steps cluster 87Wag1, 88Wag1 cluster 94Shi cluster 84Thi
1650
890-700
(2531)
(1157)
monomer
(2690, 2611, 2442) 3445 (2729, 2290, 2550) 3670, 3532, 3290
(1172)
tetramer
1562
monomer
3350 3420 (2730, 2535) 3670, 3420 (2552)
Pt(553)
147
Landolt-Börnstein New Series III/42A5
(1179, 1166) 1610
670
530
1600 1630
780 700
550
1621
1605
1040, 928 678, 524
850
460-310 a
266
240
129
230 (230)
390
cluster
80Thi, 81Thi, 82Thi1 00Nak, 01Nak 00Nak, 01Nak 02Nak 03Den
86Sen
148
3.8.1 H2O on metals
Substrate
O-H HOH Libration stretching bending (rocking)
Zr(0001)
(2543, 2732)
[Ref. p. 162
Libration M-OH2 Frustrated Adsorption (wagging) stretching translation configuration of water cluster
Refs
97Li2
a. poorly resolved. b. frustrated translational vibrations parallel to the surface
Table 5. Work function changes for molecularly adsorbed water on metal surfaces given with respect to the clean surface. Additionally applied experimental methods are commented together with an interpretation of the adsorption behaviour H2O, D2O/ Substrate Ag(110)
T [K] 80
Method
Type
(Kelvin)
III
Al(100)
100 100
Kelvin Kelvin
III IV
100 100
Kelvin Kelvin
10 80
¨ĭ [eV] −0.65 −0.95
at ș
Comments, interpretation
Ref.
2L
LEED, ESDIAD, TDS no long-range order at 80 K
87Ban1
I I
−1.2 −0.9 −1.1 −1.2
12 L 2.5 L 16 L 10 L 10 L
SE edge (UPS) EELS SE edge (UPS) Kelvin
III
−1.3
0.7 L
−0.06
III
−0.9
0.25 L saturation 10 L
III
−0.9
1L
350 110
SE edge (UPS) * Kelvin
III
−0.09 −0.95
4L
Cu(111)
110
Kelvin
III
−0.85
4L
Ir(110)-(1×2) Ni(100)
140 100
III
−0.7 −1.05
1L sat.
Ni(110)
150 130
Diode SE edge (UPS) Kelvin Kelvin
III III
−0.7 −1.15
180
Kelvin
III
−0.6
1.2 L 1.2 L ș ≈ 0.5
100
(Kelvin)
III
−0.65
2.5 L
120
SE edge (UPS)
III
−1.1
ș= 0.66
Co(0001) Co(11 2 1) Co(11 2 0) Cu(100)
120 Cu(110)
Ni(111)
90
−0.8
89Mem2 93Bus
FT IR-RAS, NRA at 100 K no dissociation UPS, TDS reversible adsorption below 300 K; ¨ĭ>0 XPS, UPS
94Gre
EELS, LEED UPS
84And 85Spi1
HREELS, TDS defects stabilize H2O clusters UPS, LEED, ELS
93Bro2
*mirror electron microscopy LEED, UPS, TDS µ0= 0.85 D
83Phu 83Ban1, 84Ban 83Ban1
82Her2
82Spi
LEED, UPS, TDS µ0= 0.5 D TDS, XPS UPS, XPS, TDS
81Wit 84Pee
UPS, TDS, ELS LEED, TDS, NRA c(2 × 2) structure LEED, ESDIAD, TDS, FTIRRAS c(2×2), H2O plane highly inclined to surface normal LEED, UPS disordered layer ARUPS, LEED, XPS, TDS authors propose bilayer model
81Ben 90Cal 92Cal1
87Nöb 89Pac
Landolt-Börnstein New Series III/42A5
Ref. p. 162]
3.8.1 H2O on metals at ș
Comments, interpretation
Ref.
III
¨ĭ [eV] −0.7
ș= 0.75
LEED, TDS H2O decorates the steps:
92Ben
III
−1.4
ș= 1.1
11(111) terraces + 1(11 1 )step LEED, TDS H2O decorates the steps:
92Ben
3(111) terraces + 1(11 1 )step LEED, TDS H2O decorates the steps:
93Mun
H2O, D2O/ Substrate Ni(665)
T [K] 150
Method
Type
Kelvin
Ni(221)
150
Kelvin
Ni(11 11 9)
140
Kelvin
Ni(775)
150
Kelvin
Pd(100) Pd(110) Pt(110) Pt(111)
10 100 100 90
EELS Kelvin Kelvin SE edge (UPS) Kelvin Kelvin
90 137
III
−1.05
III
−0.75
1.5 L
0.07 III III III
−0.73
0.15 L ș = 0.5 5L
III
(−1.1) −0.6
−1.0 −1.15
5L
10.5 (111) terraces +1(11 1 )step µ0= 1.2 D LEED, AES also: co-adsorption with Na EELS, LEED LEED, TDS ESDIAD, TDS UPS LEED, AES, IRAS LEED
( Ru(0001)
95
Diode
III
−0.6
2L
400
SE edge (UPS)
5L
III
−1.28
ș = 0.66 LEED, TDS, bilayer p( 3× 3 )
+0.20 +0.75
Kelvin
III
−1.3 −1.7
Landolt-Börnstein New Series III/42A5
0.25 ML 0.5 ML 1.6 L 6L bilayer at 1.3 L 3.5 L
84And 90He 86Fus 89Ruc 97Vil 03Har 81Thi
3 × 3 )R 30° structure
−1.2
Kelvin
94Mun
) R16.1°
III
+0.20 +0.77 82
39 × 39
LEED, HREELS, TDS hydrogen bonded bilayer
( 120
149
LEED, UPS, XPS
µ0= 0.34 D µ0= 0.65 D; depolarization included with two A states for the H2O bilayer one A state for the D2O bilayer LEED, HREELS, TDS similar result for Ru(12(001)×(010))
91Pir 95Hel2
96Koc
similar result for Ru(12(001)×(010)) MDS, TDS
96Liv
150
3.8.1 H2O on metals
H2O, D2O/ Substrate Ru(0001) cont.
T [K] 120
Method
Type
Kelvin
III
Zr(0001)
80
(Kelvin)
III
[Ref. p. 162
¨ĭ [eV] −1.34
at ș
Comments, interpretation
Ref.
2L
97Hof
−1.1 −1.3
1. layer 2. layer (ș=0.75)
TDS 2 states A1, A2 for the H2O bilayer only one A state for D2O LEED, TDS, NRA, FTIR-RAS
97Li2
Table 6. Assignments of molecular versus dissociative adsorption of water on crystalline metal surfaces; ∆H : enthalpy difference between molecularly adsorbed water (50 kJ/mol) and dissociated water deduced from gas phase reactions (Minus sign favors dissociation). First number: complete dissociation, second number: OH + O. Substrate
Dissociation either as minor or major pathway
Dissociation only at defect sites
Ag(100) Ag(110)
Ag(111) Ag(112) Ag(113) Al(100) Al(110) Al(111)
Co(0001) Cu(100) Cu(110)
Cu(111) Cu(113) Cu/Ru(0001) Fe(100)
Fe(110)
∆H [kJ/mol]
84Kla, 86Din 80Bow, 81Stu, 83Au, 85Ban1, 85Ban2, 86Stu, 87Ban1, 87Ban2, 87Mad, 89Wu, 90Kiz, 91Kiz 84Kla 84Kla 83Yat
180/120 180/120
87Out 89Kay, 93Laz, 94Coe, 96Smi, 97Ike, 98Pir 00Ske
220/100 220/100
180/120 180/120 180/120
82Sza, 89Mem1, 93Bus, 93Sha 78Ebe, 94Mil 83Net, 87Cro, 88Che, 89Smi, 99Pöl
Au(110) Au(111) Au/Pt(335) Be(0001) Co(11 2 0)
No dissociation detected or indicated
03Zal 82Her2, 94Gre, 95Gre 82Her2
03Amm, 82Spi, 85Spi2, 86Pra, 94Pol 79Au
−60/−20 −60/−20
88Sti (at oxide impurities) 88Sti (at oxide impurities)
00Dvo, 80Sas, 84And, 85Spi1, 93Bro2, 97Sue 83Ban1, 83Ban2, 83Mar, 84Ban, 86Stu, 89Lac
88Sti (at oxide impurities)
83Ban1, 91Hin, 92Hin
5/60
91Rod
5/60 5/60 −110/−10
93Xu 01Suz, 77Dwy, 82Bar, 91Hun, 93Hun 82Dwy
220/100
5/60 5/60
−110/−10 Landolt-Börnstein New Series III/42A5
Ref. p. 162] Substrate
Fe(111) γ-Fe(111) film FeAl(100) Ir(110) Nb(110) Ni(100) Ni(110)
3.8.1 H2O on metals Dissociation either as minor or major pathway 96Jia 97Rub 96Gle
Ni3(Al,Ti)(110) Ni3(Al,Ti)(111) Pb(110) Pd(100) Pd(110)
Landolt-Börnstein New Series III/42A5
∆H [kJ/mol] −110/−10 −110/−10
93Bro1
80/−240/−40/−5
82Ben (at oxygen impurities)
−40/−5
92Ben, 93Mun, 81Net, 82Mad, 85Stu, 87Mad, 94Mun, 96Rei, 87Nöb, 89Pac, 93Kuc1, 98Mun 93Kuc2, 94Kuc, 95Sch
−40/−5 −40/−5 −40/−5
96Kas 83Car 95Gle 95Chi1, 95Chi2, 96Chi 95Chi1
Pd(111) Pt(100) Pt(110)-(1×2) Pt(111)
Pt(111), stepped PtxSny(111) Re(0001) Rh(111) Rh(100) Ru(1000)
No dissociation detected or indicated
81Wit 93Col 77Nor, 84Pee, 97Kam 78Fal, 79Hop, 85Oll, 86Hoc, 88Ben, 89Gri, 90Cal, 91Cal, 92Cal1, 92Cal2, 94Pir, 98Kov
Ni(111)
Ni(760) Ni(210) NiAl(110) Ni3(Al,Ti)(100)
Dissociation only at defect sites
151
96Gri (at Si impurities)
95Chi1, 95Chi2 93Au 84Nyb, 86Kis, 86Nyb 90Bro1, 90Bro2, 90He, 91Bro, 92Xu, 95Ben 91Wol, 91Zhu 92Kiz, 94Gri 86Fus, 99Che 01Jac1, 80Fis1, 80Fis2, 80Iba, 80Sex, 82Cre, 84Lan, 85Kis, 87Wag2, 88Wag1, 88Wag2, 89Ran1, 89Ran2, 91Jo, 91Lac, 91Mau, 91Sas, 92Sta, 93Sta, 94Oga, 95Bau, 95Kiz, 95Vil, 96Gil, 96Mor, 97Gle, 97Mor, 97Wan, 98Löf, 98Oga, 98Su, 99Gle, 99Nak, 99Oga 00Ske, 81Mil
110/45 110/45 110/45 170/75 170/75 170/75
170/75
98Pan 84Jup 80Zin, 87Wag1, 88Wag1 85Heg 89Lea1, 89Lea2
−60/100/100/60/-
152 Substrate
Ru(0001)
Sr/Si(111) Ti(0001) W(100) W(110) Y(0001) Zn(0001) Zr(0001)
3.8.1 H2O on metals Dissociation either as minor or major pathway 91Pir, 95Hel2
Dissociation only at defect sites 82Kre (at oxide impurities), 77Mad, 84Thi
[Ref. p. 162
No dissociation detected or indicated
∆H [kJ/mol]
01Lil, 01Nak, 03Den, 03Pui, 80Thi, 81Kre, 81Thi, 82Doe, 83Doe, 83Wil, 86Pol, 86Sem, 87Pol, 87Sch, 89Lea1, 90Cou, 94Shi, 95Rom, 96Sch, 97Hof, 98Ras, 99Liv
60/-
01Mau 82Sto 67Pro, 88Mue 90Mue 03Bly
−350/−150/−150/−150/−70 −430/-
84Au, 86Sen 83Zwi
00Kan, 02Kan, 03Ank, 97Li1, 97Li2
C(0001)
93Cha, 95Cha1, 95Cha2, 95Phe
Table 7. Desorption temperatures of water from different crystalline metal surfaces ordered with respect to their assigned adsorption geometry and desorption paths, respectively Substrate
Multilayer
Bilayer
Ag(100) Ag(110)
170 K 170 K 140 K 160 K 155 K 170 K
-
Ag(111) Al(100) Al(111) Au(110) Au(111) Au/Pt(335) C(0001) Co(0001) Co(1120) Cu(100)
Cu(110)
At steps/ defects/ disociation products
Recombination
210 K 155 K 160 K 185 K 160 K 161 K 150 K 140 K 155 K 150 K 162 K 170 K 165 K 160 K 165 K 170 K 175 K
160-200 K 320 /650 K 190 K
180 K 170 K
270
Reference
84Kla 86Stu, 81Stu 90Kiz 85Ban2, 85Ban1 85Ban1, 87Mad 84Kla 89Mem1* 87Cro 88Che, 99Pöl 87Out 89Kay, 93Laz 00Ske 95Cha1 82Her21 82Her21 00Dvo, 93Bro2, 97Sue
170 K 175 K 175 K 170 K 175 K
89Lac, 94Pol, 90Lac, 91Lac 83Ban1 84Ban 86Stu 89Cle
Landolt-Börnstein New Series III/42A5
Ref. p. 162]
3.8.1 H2O on metals
Substrate
Multilayer
Cu(111)
155 K 175 K
Cu/Ru(0001) Fe(100) γ-Fe(111) film FeAl(110) Ir(110) Ni(100) Stepped Ni(100)
165 K 180 K 150 K 160 K 170 K 174 K 160 K
Stepped Ni(110)
160 K
Ni(111)
150-155 K 153 K 160 K 160 K 250 K
Stepped Ni(111) Ni3(Al,Ti)(110) Ni3(Al,Ti)(111) Pb(110) Pd(100) Pd(110) Pd(111) Pt(100) Pt(111)
Re(0001) Rh(100) Rh(111)
Ru(1010)
Landolt-Börnstein New Series III/42A5
167 K 160 K 155 K 155 K 150 K 170 K 160 K 165 K 160 K 165 K 150 K 155 K 155 K 164 K 165 K 153 K 140 K 170 K 150 K 158 K 165 K 160 K 170 K
Bilayer
At steps/ defects/ disociation products
190 K 220 K 188 K 200 K 182 K 185 K 210 K 210 K 212 K 170 K 165 K 175 K 190 K
Recombination
310 K (300 K)2 270/300 K
260 K
350 K
250 K 260 K 250 K
370 K 360 K 340 K
225 K
260-325 K 500 K
185 K 168 K 150 K 190 K 170 K 183 K 191 K 190-220 K 180-240 K
Reference
92Hin 83Ban1 91Rod 91Hun 97Rub 96Gle 81Wit 97Kam 84Pee* + 88Ben, 92Cal1 , 96Kas* 89Gri* 78Fal 96Kas 82Mad, 85Stu, 87Mad, 89Pac, 93Kuc2, 95Sch 87Nöb, 92Ben, 95Jac 00Wan 95Chi1, 95Chi2 93Au +
175 K 170 K 195 K 167 K 165 K 165 K 170 K 175 K 180 K 180 K 165 K 168 K 171/177 K
153
84Nyb , 84Stu +
90He, 90Bro1, 91Bro , 92Xu
200 K 195 K 200 K 180 K 200 K
250-450 K
91Wol, 91Zhu 80Iba 92Kiz 80Fis1, 91Mau, 96Gri*, 98Pan, 98Su + 96Gil 88Wag1, 97Wan* 94Oga*, 98Oga* 02Haq 00Ske 91Jo* 04Gre 02Haq 84Jup 85Heg 00Gib 86Kis 80Zin + 89Lea2 89Lea1
154
3.8.1 H2O on metals
Substrate
Multilayer
Bilayer
Ru(0001)/H2O
150 K 155 K 155 K 155 K 155 K 160 K 160 K 165 K 170 K 170 K 185 K 150 K 155 K 160 K 165 K 170 K 160 K 163 K
170/212 K 170/210 K 175/220 K 180/215 K 190/212 K 185/220 K 175/215 K 185/215 K 180/220 K 200/220 K 200/230 K 185/220 K 180-190 K 185 K 185 K 185 K 185 K 166 K 178 K
Ru(0001)/D2O
Sn-Pt(111) Zr(0001)
At steps/ defects/ disociation products
[Ref. p. 162
Recombination
230/260 K
Reference
94Hel2, 95Hel2, 95Rom 90Cou 86Pol 82Doe, 83Doe 97Hof 81Thi, 84Thi, 99Liv 01Lil 82Kre, 87Sch, 96Liv 89Lea1 87Pol 77Mad 80Thi, 86Sem 83Doe*, 94Hel2*, 95Hel2* 97Hof* 01Lil* 87Sch* 89Lea1* 98Pan 97Li2
1
deduced from temperature dependent UPS; 2parantheses indicate that dissociation leads to H2 desorption only
* D2O;
+
D2O = H2O
Table 8. Adsorption structure of water on metal surfaces; temperature and coverages in fraction of a
bilayer partly estimated from data given in Langmuir are given; D: Dimer, T: Tetramer, H: Hexamer Substrate
Monomere
Di-/Tetra/Hexamere
Ag(100) Ag(110) Ag(111) Al(100) Al(111)
70 K/0.5 H 20 K/<0.05
Au(111) C(0001) Cu(100)
Larger cluster 150 K 160 K 100 K/>0.2 70 K/0.5 130 K/>0.05 20 K/>0.05 127 K/>0.5 90 K/>0.5 120 K 100 K/ ∼1.0 85 K/>0.2 90-140 K/>0.3
10 K/0.1 10 K/0.1 Cu(110) 90-190 K/>0.05
Cu(111)
130-170 K/>0.1 90 -175 K/>0.05 100 K/0.1
90-140 K/<0.5 <16 K/0.03 150 K/>0.4
Dissociation
130 K >90 K
Ref 86Din 89Wu 81Stu 02Mor1 94Gri 91Jac 87Cro 88Che 98Pir 97Ike 95Cha1, 95Phe 85Spi1 86Nyb 84And 89Lac 94Pol 91Sas 82Spi 02Mor2 91Hin Landolt-Börnstein New Series III/42A5
Ref. p. 162] Substrate
Monomere
3.8.1 H2O on metals Di-/Tetra/Hexamere
Fe(100) Ni(100)
140 K/<0.1 D 130 K/<0.2
Ni(110)
155
Larger cluster
Dissociation
Ref
130 K/>0.1 100 K/>0.5 140 K/>0.1 130 K/>0.01
130 K/<0.1 200 K/>0.5
82Bar 91Hun 93Bro1 92Ell
>110 K/<0.1
100 K/>0.1 >40 K 150 K/>0.1 100 K/>0.05 130 K/>0.1 100-130 K
86Hoc 85Oll 94Pir 90Cal, 91Cal, 92Cal1, 94Gri, 94Pir, 98Kov 85Nöb, 88Ben, 91Cal, 94Pir 84And, 86Nyb 84Nyb, 86Nyb 84Stu 95Ben 92Xu 90Bro1 93Bro1 90Bro2, 93Bro1 02Mit 80Iba 86Fus 94Gri 97Gle, 99Gle
85 K/ >0.01 >130 K/0.4
01Jac1 99Nak
84 K/>0.13 25 K/>0.27 90-140 K/>0.1
94Oga, 98Oga 99Oga 80Sex, 84Lan, 87Wag1, 89Ran1, 95Bau, 96Mor, 97Mor 84Jup 87Wag1 84Thi 80Thi, 81Kre, 81Thi, 82Kre, 82Thi1, 84Thi, 91Pir, 94Shi 00Nak, 01Nak 00Nak, 01Nak, 02Nak 00Nak 81Thi
150 K/>0.01 H >120 K 80-200 K/<0.5
80-200 K/<0.5 D Pd(100)
80-200 K, >0.5
10 K/<0.05 110 K/0.1-1 80 K/>0.1 120 K/>0.02 120 K/>0.1 100 K/>0.15
Pd(110) 120 K/<0.1 100 K/<0.1 100 K/<0.1 T Pd(111) Pt(100) Pt(110)
<40 K/<0.01
Pt(111)
40 K/<0.05 85 K/<0.01 20 K/<0.1
40-100 K /<0.15 D/T 20 K/0.2-0.5 D/T
84 K/ <0.13 25-40 K/<0.18
Re(0001) Rh(111) Ru(0001)
80K/<0.07
80 K/0.07-0.3
80 K/>0.3 90 K/>0.1
80K/<0.02 80-200 K/>0.1
20 K/>0.2 T
> 70 K/>0.1
20 K/0.1 50 K/0.1 T 95 K/>0.01 Landolt-Börnstein New Series III/42A5
156 Substrate
3.8.1 H2O on metals Monomere
Di-/Tetra/Hexamere
Zn(0001) Zr(0001)
[Ref. p. 162
Larger cluster
Dissociation
Ref
80-120 K/>0.1 80 K/>0.23
>120 K 80 K/<0.23
86Sen 97Li2
Table 9. Measured superstructure periodicity of ordered 2D ice bilayers on solid surfaces (no=no longrange order observed). Substrate Ag(100) Ag(110) Ag(111) Ag(112) Al(100) Al(111) Au(111) Co(0001) Co(11 2 0) Cu(100) Cu(110)
Ir(110) Ni(110) Ni(111)
Pd(100) Pd(110) Pd(111) Pt(110) Pt(111)
Re(0001) Rh(111) Ru(0001)
Long range structure no no no hexagonal along 112/110 no no no (√3×√3)R30° no no
Temperature 80 K 80-160 K 80 K 155 K 80 K 130 K 80 K 120 K 100 K 100 K
Reference 84Kla 85Ban2, 86Stu 84Kla 79Fir 84Kla 94Gri 83Net 98Pir 82Her2 82Her2
no c(2×2) c(2×2) c(2×2) no c(2×2) c(2×2) (√3×√3)R30° (√3×√3)R30° (√3×√3)R30° no c(2×2) (√3×√3)R30° lace structure no (√3×√3)R30° (√37×√37)R25.3° and (√39×√39)R16.1° domains a hexagonal
90 K 90 K 100-130 K 90 K 130 K 180 K 80-150 K 80 K 120 K 150 K 110 K 100 K 100 K 100 K 100 K 130-140 K 130-135 K 160 K 80 K 150 K
85Spi1 94Pol 83Ban2, 84Ban, 86Stu, 89Lac 82Spi 81Wit 90Cal, 92Cal1 85Nöb, 88Ben, 94Gri 81Net, 82Mad 89Pac 93Mun 84Nyb 90He, 92Xu 04Cerc 86Fus 80Fis1, 87Wag1 97Glea, 97Morc 02Haq, 03Har, 97Glea 92Sta, 93Sta, 95Mat, 97Mor 84Jup 84Jup
80-100 K 80 K 95 K
80Zin, 87Wag1, 88Wag1 00Giba 81Thi
80-175 K 100-150 K
82Doe, 83Wil, 91Pir 03Pui, 94Hel2, 95Hel1, 95Hel2b
(√3×√3)R30° (2×2) (√3×√3)R30° (√3×√3)R30°+superstructure (√3×√3)R30°H2O (√3×√3)R30°H2O e (√3×√3)R30°D2O 1 2· H2O-domains: §¨¨ ¸¸ phases ©0 N ¹
(N=13 / 19-20) Landolt-Börnstein New Series III/42A5
Ref. p. 162]
3.8.1 H2O on metals
Substrate Zr(0001)
Long range structure no
157
Temperature 80 K
Reference 97Li2
a. Detected with He atom scattering, b. Detected with both He atom scattering and LEED, c. detected by STM
3.8.1.6
Figures for 3.8.1 H
H
h n= 1253.6 eV
q
z
m
r OH
x
O
Fig. 1. Schematic geometric model of H2O [87Thi]. → Fig. 3. Photoelectron spectra for the O1s, O2s and the valence band region of H2O at different H2O coverages on Pt(111); coverages are indicated in monolayer equivalent: 1 MLE = 1.5 × 1015 cm−2. In addition, the peak energies related to the 1b1 peak are shown as lines for condensed water (SJK, hȞ =40 eV [80Sch]) and water gas (S, hȞ =1253.6 eV [69Sie]; K, hȞ = 21.2 eV [81Kim]) [89Ran1].
01s 1a1
Intensity [rel.units]
y
Pt (111) + H2O
1.8MLE 90K H
C
0.4MLE G 90K
B
0.2MLE F 90K
-540
clean -530
1b2
[K] [S] [SJK] 1b1
E -40 -30 -20 Energy below EF [eV]
XPS O 1s O
Symmetry: C2v
3a1
02s 2a1
D
A
h n = 120 eV
-10
EF
h n = 785 eV D2O/H2O
H H
H
2B2 E = 0.5812 π* OH2
4A1 E = 0.4056 σ* OH2
H O
H
Intensity [arb.units]
O H
OD /OH 150 K
a b c d
O
H
1B1 E = − 0.4294 n 180 K
H 3A1 E = − 0.4833 n
f
O H H
H
O
1B2 E = − 0.6313 π OH2
H 2A1 E = − 1.3049 σ OH2
Fig. 2. Delocalized molecular orbitals of H2O [73Jor]. Landolt-Börnstein New Series III/42A5
e
538
536
534 532 Binding energy [eV ]
530
Fig. 4. O 1s photoelectron spectra for water mono-layers on Ru(0001) adsorbed at 150 K (a-d) and after an additional water exposure of 1013 mole-cules cm−2 s−1 at 180 K (e-f): D2O/OD (a, b, e) H2O/OH (c, d, f). Spectra b and d were taken after a X-ray irradiation 60 times larger than for spectra a and b [04And].
158
3.8.1 H2O on metals 2.96 L (0.61 ML) 2.11 L (0.59 ML) 1.79 L (0.58 ML) 1.48 L (0.56 ML) 1.17 L (0.52 ML) 0.96 L (0.44 ML) 0.70 L (0.39 ML) 0.54 L (0.30 ML) 0.37 L (0.21 ML) 0.21 L (0.11 ML) 0.00 L (0.00 ML)
Intensity [arb.units]
OH OD H2O D2O Ratio OH:H2O Ratio OD:D2O
0.8
Rel. coverage [ML] / ratio
O1 s 145 K hn = 625 eV
[Ref. p. 162
0.6
0.4
0.2
0 536
a
534
532 530 Binding energy [eV]
0
528
0.1
0.2 0.3 0.4 Total coverage [ML]
b
0.5
0.6
Fig. 5. a) O 1s spectra during H2O adsorption on Ru(0001) at 145 K and 1.6×10−9 mbar with a collection time of 140 s per spectrum, corresponding coverages are indicated. b) coverage dependence of OH (OD) and H2O (D2O) peak intensities (ż, Ɣ, Ƒ, Ŷ) and their ratios (∆, Ÿ) at 145 K [04Wei].
240 a
ν (O - Hbr)
ν (O - H)
Pt (111) + H2O
760
δ (HOH)
T = 110 K
695
3380
1630 255 560
-
+
+
-
1050 Intensity
+
× 50
1630
3430
q H2 O 0.74
× 50
0.52
× 100
0.22
Hydrogen
× 250
0.15
Fig. 6. Sketch of a) intramolecular and b) extramolecular vibrational modes for H2O [01Jac1].
× 500
0.07
L r (HOH) rocking
L w (HOH) wagging
L t (HOH) twisting
Oxygen TII (H2O)
T (H2O) T
b
ĺ Fig. 7. HREEL spectra of H2O on a Pt(111) surface for increasing coverages as indicated [95Bau].
0
500
1000 1500 3000 3500 4000 -1 Energy loss [cm ]
Landolt-Börnstein New Series III/42A5
Ref. p. 162]
3.8.1 H2O on metals qD2O / ML O-D stretch 0.13 × 8
H2O / Pt (111) T 1L
E0 = 2.0 eV
1
159
D-O-D scissor
T
1184 0.27
L2
δam δbm
2706 2556 0.67
0.025
C
1195
×3
D R/R
Intensity [arb.units]
H2O Dose (L) ×500
×5
2562
2477
1.33 ×500
0.01
B
2545 2 2731
0.005
×1000
1201
A 2539
0
50
100 Energy [meV ]
150
3000
200
H2O / Pt (111)
1000
D2O / Ni (110)
0
T = 105 K
Work function change D f [mV]
Work function change D f [eV]
2500 2000 1500 -1 Wavenumber [cm ]
Fig. 9 IRAS spectra of D2O on Pt(111) adsorbed at 84 K at different coverages as indicated on the left. The origin of the vibrational lines is marked. O-D-stretch corresponds to the second mode displayed in Fig. 6a and D-O-D-scissor to the third mode in Fig. 6a. Note the two lines of the O-D-stretch mode which are assigned to D2O monomers and D2O bound in clusters (courtesy of H. Ogasawara) [94Oga].
Fig. 8. HREEL spectra for small H2O exposures on Pt(111); the assignement of the peaks is indicated and described within the text [01Jac1].
0
0.001
-0.5
- 400
180 K
- 800
- 1200
-1.0
130 K - 1600
0
2
4 6 8 -4 Exposure [10 Pa×s]
10
12
Fig. 10. Work function change versus H2O exposure for H2O on Pt(111) at 105 K [85Kis]
Landolt-Börnstein New Series III/42A5
0
1
2 Exposure [L]
3
Fig. 11. Work function change versus D2O exposure for D2O on Ni (110) at 180 K and 130 K [90Cal]
3.8.1 H2O on metals
8
200 A2
6
Tads = 180 K
- 200
Df minimum - 600
4 A1 B
2
- 1000
- 1400
0 150 190
230 270 310 350 Temperature T [K ]
[Ref. p. 162
Recombination
Partial pressure of H2O [arb.units]
600
D2O pressure [arb.units]
10
Work function change D f [mV]
160
Ni (110)
d
Domain boundaries?
c
Ru (001) Bilayer
Pt (111)
b
Multilayer
390
Fig. 12. Thermal desorption spectra (full line) and work function changes (broken line) for D2O dosed onto Ni(110) at 180 K for an exposure corresponding to the minimum of ǻφ in Fig. 11 [90Cal].
Ag (110)
a 100
200 300 Temperature T [K ]
400
Fig. 13. TPD spectra of H2O from different substrates as indicated; coverage: Ag(111) 3.0 L, Pt(111) 3.8 L, Ru(0001) 2.0 L, Ni (110) not known. The origin of the different desorption peaks is marked (courtesy of P. A. Thiel) [87Thi].
Fig. 14. STM images of H2O adsorbed on different surfaces at different temperatures as indicated. (a) H2O momomers on Pd(111); inset: two consecutive images showing the diffusion of individual monomers e.g. of the monomer marked by a white line in the upper image (courtesy of E. Fomin and M. Salmeron) [02Mit]. (b) Small H2O clusters of different size (1-10 atoms) on Ag(111), coverage: 0.1 molecules/nm2 (courtesy of K. Morgenstern) [02Mor1, 02Mor2]. (c) Complete H2O bilayer on Pt(111); the visible network is due to a superstructure explained by the misfit between the H2O bilayer and Pt(111) (phase I); inset: larger resolution image showing the arrangement of the H2O molecules within the bilayer [97Mor].
Landolt-Börnstein New Series III/42A5
Ref. p. 162]
3.8.1 H2O on metals
161
Fig. 15. Structural models of the ice bilayer on close-packed metal surfaces: upper row: side view, lower row: top view. (a) Usually assumed √3×√3 superstructure of a perfect H2O bilayer with only oxygen atoms binding to the metal surface (courtesy of H. Ogasawara). (b) Model for the H2O bilayer on Pt(111) with oxygen and hydrogen atoms binding to the metal surface; the model is based on the analysis of X-ray-absorption, X-ray emission and X-ray photoelectron spectroscopy experiments (courtesy of H. Ogasawara) [02Oga]. (c) Model for the H2O bilayer on Ru(0001) involving partial dissociation of H2O into OH and H; the model is concluded from ab initio calculations based on density functional theory (courtesy of P. Feibelman) [02Fei].
Landolt-Börnstein New Series III/42A5
162
3.8.1 H2O on metals
3.8.1.7 References for 3.8.1 67Pro 69Eis 69Sie
71Cot 73Jor 74Rab 75Che 76Wha 77Con 77Dwy 77Mad 77Nor 78Ebe 78Fal 79Au 79Fir 79Hop 80Au 80Bow 80Fis1 80Fis2 80Iba 80Sas 80Sch
80Sex 80Thi 80Zin 81Ben 81Kim
81Kre 81Mil 81Net 81Stu 81Thi 81Wit 82Bar 82Ben 82Cre
Propst, F.M., Piper, T.C.: J. Vac. Sci. Technol. 4 (1967) 53. Eisenberg, D., Kauzmann, W.: The structure and properties of water, New York: Oxford University Press, 1969. Siegbahn, K., Nordling, C., Johansson, G., Hedman, J., Hedén, J., Hamrin, K., Gelius, U., Bergmark, T., Werme, l.O., Manne, R., Baer, Y.: ESCA applied to free molecules, Amsterdam: North Holland, 1969. Cotton, F.A.: Chemical application of group theory, 2nd ed., New York: Wiley-Intersience, 1971. Jorgensen, W.L., Salem, L.: The organic chemist's book of orbitals, New York, San Francisco, London: Academic Press, 1973. Rabalais, J.W., Debies, T.P., Berkosky, J.L., Huang, J.T.J., Ellison, F.O.: J. Chem. Phys. 61 (1974) 516. Chesters, M.A., Somorjai, G.A.: Surf. Sci. 52 (1975) 21. Schuster, P., Zundel, G., Sandorfy, C.: Dynamics, thermodynamics and special systems, in: The Hydrogen Bond,Vol. 3, Whalley, E. (ed.), Amsterdam: North-Holland, 1976. Connor, J.A., Considine, M., Hillier, I.H.: J. Electron Spectrosc. Relat. Phenom. 12 (1977) 143. Dwyer, D.J., Simmons, G.W., Wei, R.P.: Surf. Sci. 64 (1977) 617. Madey, T.E., Yates, J.T.: Chem. Phys. Lett. 51 (1977) 77. Norton, P.R., Tapping, R.L., Goodale, J.W.: Surf. Sci. 65 (1977) 13. Eberhardt, W., Kunz, C.: Surf. Sci. 75 (1978) 709. Falconer, J.L., Madix, R.J.: J. Catal. 51 (1978) 47. Au, C.T., Breza, J., Roberts, M.W.: Chem. Phys. Lett. 66 (1979) 340. Firment, L.E., Somorjai, G.A.: Surf. Sci. 84 (1979) 275. Hopster, H., Brundle, C.R.: J. Vac. Sci. Technol. 16 (1979) 548. Au, C.T., Roberts, M.W.: Chem. Phys. Lett. 74 (1980) 472. Bowker, M., Barteau, M.A., Madix, R.J.: Surf. Sci. 92 (1980) 528. Fisher, G.B., Gland, J.L.: Surf. Sci. 94 (1980) 446. Fisher, G.B., Sexton, B.A.: Phys. Rev. Lett. 44 (1980) 683. Ibach, H., Lehwald, S.: Surf. Sci. 91 (1980) 187. Sass, J.K., Richardson, N.V., Neff, H., Roe, D.K.: Chem. Phys. Lett. 73 (1980) 209. Schmeisser, D., Jacobi, K., Kolb, D., in: Proc. 4th Int. Conf. on Solid Surfaces and 3rd European Conf. on Surface Science, Les Couches Minces 201, Le Vide, Suppl., Vol. I, 1980, p. 256. Sexton, B.: Surf. Sci. 94 (1980) 435. Thiel, P.A., Hoffmann, F.M., Weinberg, W.H.: Vide Couches Minces 201 (1980) 307. Zinck, J.J., Weinberg, W.H.: J. Vac. Sci. Technol. 17 (1980) 188. Benndorf, C., Nöbl, C., Rüsenberg, M., Thieme, F.: Surf. Sci. 111 (1981) 87. Kimura, K., Katsumata, S., Achiba, Y., Yamazaki, T., Iwata, S.: Handbook of HeI photoelectron spectra of fundamental organic molecules, Tokyo: Japan Science Societies Press, 1981. Kretzschmar, K., Sass, J.K., Hofmann, P., Ortega, A., Bradshaw, A.M., Holloway, S.: Chem. Phys. Lett. 78 (1981) 410. Miller, J.N., Lindau, I., Spicer, W.E.: Surf. Sci. 111 (1981) 595. Netzer, F.P., Madey, T.E.: Phys. Rev. Lett. 47 (1981) 928. Stuve, E.M., Madix, R.J., Sexton, B.: Surf. Sci. 111 (1981) 11. Thiel, P.A., Hoffmann, F.M., Weinberg, W.H.: J. Chem. Phys. 75 (1981) 5556. Wittrig, T.S., Ibbotson, D.E., Weinberg, W.H.: Surf. Sci. 102 (1981) 506. Baró, A.M., Erley, W.: J. Vac. Sci. Technol. 20 (1982) 580. Benndorf, C., Nöbl, C., Thieme, F.: Surf. Sci. 121 (1982) 249. Creighton, J.R., White, J.M.: Chem. Phys. Lett. 92 (1982) 435. Landolt-Börnstein New Series III/42A5
3.8.1 H2O on metals 82Doe 82Dwy 82Her1 82Her2 82Iba
82Kre 82Mad 82Spi 82Sto 82Sza 82Thi1 82Thi2 83Au 83Ban1 83Ban2 83Car 83Doe 83Mar 83Net 83Phu 83Wil 83Yat 83Zwi 84And 84Au 84Ban 84Bar 84Jup 84Kla 84Lan 84Nyb 84Pee 84Stu 84Thi 85Ban1 85Ban2 85Bau 85Gre 85Heg 85Kis 85Nöb 85Oll 85Spi1 85Spi2 85Stu 86Din 86Fus
163
Doering, D.L., Madey, T.E.: Surf. Sci. 132 (1982) 305. Dwyer, D.J., Kelemen, S.R., Kaldor, A.: J. Chem. Phys. 76 (1982) 1832. Heras, J.M., Albano, E.V.: Z. Phys. Chem. N. F. 129 (1982) 11. Heras, J.M., Papp, H., Spiess, W.: Surf. Sci. 117 (1982) 590. Ibach, H., Mills, D.L.: Electron energy loss spectroscopy and surface vibrations, New York, London, Paris, San Diego, San Francisco, Sao Paulo, Sydney, Tokyo, Toronto: Academic Press, 1982. Kretzschmar, K., Sass, J.K., Bradshaw, A.M., Holloway, S.: Surf. Sci. 115 (1982) 183. Madey, T.E., Netzer, F.P.: Surf. Sci. 117 (1982) 549. Spitzer, A., Lüth, H.: Surf. Sci. 120 (1982) 376. Stockbauer, R.L., Hanson, D.M., Flodström, S.A., Madey, T.E.: J. Vac. Sci. Technol. 20 (1982) 562. Szalkowski, F.J.: J. Chem. Phys. 77 (1982) 5224. Thiel, P.A., Hoffmann, F.M., Weinberg, W.H.: Proc. 2nd. Int. Conf. on Vibrations and Surfaces, 1982, p. 201. Thiel, P.A., Hoffmann, F.M., Weinberg, W.H.: Phys. Rev. Lett. 49 (1982) 501. Au, C.T., Singh-Boparai, S., Roberts, M.W., Joyner, R.W.: J. Chem. Society Faraday Trans. 179 (1983) 1779. Bange, K., Döhl, R., Grider, D.E., Sass, J.K.: Vacuum 33 (1983) 757. Bange, K., Grider, D.E., Sass, J.K.: Surf. Sci. 126 (1983) 437. Carley, A.F., Rassias, S., Roberts, M.W.: Surf. Sci. 135 (1983) 35. Doering, D.L., Semancik, S., Madey, T.E.: Surf. Sci. 133 (1983) 49. Mariani, C., Horn, K.: Surf. Sci. 126 (1983) 279. Netzer, F.P., Madey, T.E.: Surf. Sci. 127 (1983) L102. Phu, S.U., Bardolle, J., Bujor, M.: Surf. Sci. 129 (1983) 219. Williams, E.D., Doering, D.L.: J. Vac. Sci. Technol. A 1 (1983) 1188. Yates, J.T., Ceyer, S.T.: J. Electroanal. Chem. 150 (1983) 17. Zwicker, G., Jacobi, K.: Surf. Sci. 131 (1983) 179. Andersson, S., Nyberg, C., Tengstal, C.G.: Chem. Phys. Lett. 104 (1984) 305. Au, C.T., Roberts, M.W.: Proc. R. Soc. (London) A 396 (1984) 165. Bange, K., Grider, D.E., Madey, T.E., Sass, J.K.: Surf. Sci. 136 (1984) 38. Barteau, M.A., Madix, R.J.: Surf. Sci. 140 (1984) 108. Jupille, J., Pareja, P., Fusy, J.: Surf. Sci. 139 (1984) 505. Klaua, M., Madey, T.E.: Surf. Sci. (Lett.) 136 (1984) L52. Langenbach, E., Spitzer, A., Lüth, H.: Surf. Sci. 147 (1984) 179. Nyberg, C., Tengstal, C.G.: J. Chem. Phys. 80 (1984) 3463. Peebles, D.E., White, J.M.: Surf. Sci. 144 (1984) 512. Stuve, E.M., Jorgensen, S.W., Madix, R.J.: Surf. Sci. 146 (1984) 179. Thiel, P.A., DePaola, R.A., Hoffmann, F.M.: J. Chem. Phys. 80 (1984) 5326. Bange, K., Madey, T.E., Sass, J.K.: Surf. Sci. 162 (1985) 252. Bange, K., Madey, T.E., Sass, J.K.: Surf. Sci. 152/153 (1985) 550. Bauschlicher, C.W.: J. Chem. Phys. 83 (1985) 3129. Greenlief, C.M., Hegde, R.I., White, J.M.: J. Phys. Chem. 89 (1985) 5681. Hegde, R.I., White, J.M.: Surf. Sci. 157 (1985) 17. Kiskinova, M., Pirug, G., Bonzel, H.P.: Surf. Sci. 150 (1985) 319. Nöbl, C., Benndorf, C., Madey, T.E.: Surf. Sci. 157 (1985) 29. Ollé, L., Salmeron, M., Baró, A.M.: J. Vac. Sci. Technol. A 3 (1985) 1866. Spitzer, A., Ritz, A., Lüth, H.: Surf. Sci. 152/153 (1985) 543. Spitzer, A., Lüth, H.: Surf. Sci. 160 (1985) 353. Stulen, R.H., Thiel, P.A.: Surf. Sci. 157 (1985) 99. Ding, X., Garfunkel, E., Dong, G., Yang, S., Hou, X., Wang, X.: J. Vac. Sci. Technol. A 4 (1986) 1468. Fusy, J., Ducros, R.: Surf. Sci. 176 (1986) 157.
Landolt-Börnstein New Series III/42A5
164
3.8.1 H2O on metals
86Hoc
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3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
[Ref. p. 235
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces List of abbreviations AES ARUPS BE E Ed Ediss ELS ESD FTIR HAS HREELS HR-PES IRAS L LEED MBRS ML NEXAFS Sdiss SEXAFS STM TPD TPRS UPS XAES XAFS XPS
Auger electron spectroscopy angle-resolved ultraviolet photoelectron spectroscopy binding energy electric field vector activation energy for desorption activation energy for dissociation energy loss spectroscopy electron simulated desorption Fourier transform infrared helium atom scattering high-resolution electron-energy loss spectroscopy (EELS) high-resolution photoelectron spectroscopy (PES) infrared reflection-absorption spectroscopy (RAIRS) Langmuir low-energy electron diffraction molecular beam relaxation spectroscopy monolayer near-edge X-ray absorption fine structure (XANES) dissociative sticking coefficient surface extended X-ray absorption fine structure scanning tunneling microscopy temperature-programmed desorption spectroscopy (TDS) temperature-programmed reaction spectroscopy ultraviolet photoelectron spectroscopy X-ray excited Auger electron spectroscopy X-ray absorption fine structure (EXAFS, NEXAFS, SEXAFS) X-ray photoelectron spectroscopy
3.8.4.1 Introduction This chapter discusses adsorption and reaction of six triatomic molecules on metal surfaces: carbon dioxide (CO2), nitrogen dioxide (NO2), sulfur dioxide (SO2), carbonyl sulfide (OCS), nitrous oxide (N2O) and ozone (O3). All of these molecules have a wide range of interactions with metals and undergo a number of chemical transformations. CO2, NO2 and SO2 adsorption has been characterized extensively. Only a handful of studies of OCS adsorption are available. Ozone is the most reactive molecule of this collection and its facile dissociative adsorption has been utilized to oxidize several metal and alloy surfaces under mild conditions. However, no information is available about O3 molecular adsorption. We have included data on adsorption of these molecules on chemically modified metal surfaces, i.e., surfaces containing preadsorbed or coadsorbed species. Specifically, alkali metal adatoms were considered and reactions of CO2 and SO2 with coadsorbed oxygen to form surface carbonate and sulfite or sulfate species, respectively, are discussed. Alkali metal adatoms often serve as promoters, enhancing the reactivity of these molecules at the modified metal surface. Landolt-Börnstein New Series III/42A5
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In addition, when available, we have included results from adsorption studies on alloy surfaces. Such chemistry gives insight into reactions on bimetallic heterogeneous catalysts that are ubiquitous in the chemical and petroleum industries. Alloying metal in a catalyst with a second metal can strongly change the activity and selectivity of the catalyst. Even when alloy formation does not occur, the catalytic activity and selectivity are often changed by deposition of one metal on another. For example, adding Cu on Ru supported on silica to form bimetallic clusters gave a catalyst with a dramatically lower capacity for hydrogen chemisorption and improved selectivity for dehydrogenation [76Sin]. In several cases, we have included some information about adsorption of these triatomic molecules on non-metal surfaces for comparison or in order to anticipate what may be found in later studies on metals. We begin the discussion in each section by a few comments on structure and bonding in the gas-phase molecule [66Her], followed by brief remarks on the coordination chemistry of these molecules as ligands in metal complexes. The literature describing coordination complexes or organometallic cluster compounds provides excellent references for understanding adsorption and reactions on metal surfaces and we refer the interested reader specifically to this material (e.g., see [87Alb, 87Col]). In summary, carbon dioxide (O=C=O), carbonyl sulfide (O=C=S) and sulfur dioxide can be considered to belong to a class of ligands called cumulenes. CO2 and OCS are linear triatomic molecules in which an sp-hybridized central atom is bonded to both outer atoms via double bonds. Along with other molecules like allene (CH2=C=CH2), carbon disulfide (S=C=S) and ketene (CH2=C=O) that are not discussed herein, these have coordination bonding in metal complexes and at metal surfaces that is analogous to that of the well-known Dewar-Chatt-Duncanson model of ethylene coordination. In this model, electron density from a filled molecular π bond is delocalized into empty orbitals on the metal and electron density from an occupied d orbital on the metal is “back donated” into an empty molecular π* orbital on the molecule. The extent of this back bonding affects rehybridization of atoms in the molecule and distortion of the molecule from linearity. These molecules can also more weakly bond to metal atoms at the surface through an oxygen or sulfur lone pair and maintain linearity. Nitrous oxide (N≡N+–O−) is also a linear triatomic molecule, but its coordination and surface chemistry is quite different, behaving as a pseudohalogen. NO2, SO2 and O3 are bent triatomic molecules in the gas phase and are highly reactive molecules that often easily dissociate on metal surfaces to form an oxygen adatom along with a nascent coadsorbed molecular product. No study has yet reported on the molecular chemisorption of O3 on a metal surface, due to its facile dissociation, but there is a rich literature on NO2 chemisorption on metals. NO2 is a radical in the gas phase, and as expected, it exhibits a diversity of bonding modes analogous to the linkage isomerism observed in the coordination of the nitrite anion (NO2−). NO2 can bond to metal surfaces through either one (monodentate) or both (bidentate in a chelating structure) of the oxygen atoms or through the central N atom, and in bidentate bridging structures involving bonding to N and O atoms.
3.8.4.2 CO2 3.8.4.2.1 Structure and bonding of CO2 CO2 in its ground state is a closed-shell, nonpolar, linear molecule, as shown in Scheme I.
Scheme I. Lewis structure for CO2. Undistorted, linear CO2 molecules can exhibit two bonding modes: (i) dative bond formation via donation of a lone pair on one oxygen atom to an empty orbital on the metal with the molecule in an upright orientation with its axis nearly perpendicular to the surface (as in Scheme II c); or (ii) via π bonds of the CO2 molecule analogous to ethylene and bonding with the molecular axis oriented parallel to the surface. Both of these are expected to be weak, physisorption interactions. However, charge transfer of an electron from the substrate to CO2 forms a CO2− anion, which is “bent” and isoelectronic with NO2. This additional electronic charge is accommodated by breaking one of the C=O double bonds and forming an Landolt-Börnstein New Series III/42A5
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additional oxygen lone-pair orbital. This bond breaking process “opens” the carbon atom and occupies a previously unoccupied 2πu orbital of linear CO2, making the molecule a 17-electron system, which tends to avoid a linear geometry [77Are, 83Cal]. Such highly distorted, “bent” CO2 molecules have several bonding modes possible, and Scheme II illustrates two types: (i) an η1 C-bonded complex (II a); and (ii) an η2 C,O-bonded complex (II b) (the superscript-η notation indicates the number of atoms in the molecule that are bonded to the metal). Both linearly adsorbed and bent-CO2− chemisorbed states have been assigned on metal surfaces upon adsorption. Several other molecular orientations and bonding modes, in addition to those given in Scheme II, are feasible.
Scheme II. Types of coordination for CO2.
3.8.4.2.2 CO2 adsorption on metal surfaces CO2 adsorption on metal surfaces has been extensively studied, and several reviews are available [86Fre, 91Sol1, 96Fre]. The thermodynamics for CO2 adsorption are given in Table 1. On most metal surfaces, CO2 adsorbs only weakly. CO2 does not appear to chemisorb on Pt, Pd, Cu and Ag, although physisorption occurs [91Sol1, 96Fre, 74Nor, 75Nor, 84Seg, 86Ber, 86Sol, 89Rod, 87Sak, 83Bac, 82Stu]. It has been asserted that the metal work function and surface structure, including defects, controls whether or not physisorbed CO2 can be readily converted to a chemisorbed species. On Fe, Ni, Rh and Re single crystal surfaces, CO2 dissociates into coadsorbed CO and oxygen adatoms [87Lin, 87Bar1, 88Ill, 87Pir, 86Beh, 87Fre, 87Beh, 87Bau, 91Wam, 89Pau, 93Nas, 95Hes, 85Sol, 87Pel, 88Ass, 91Rod, 85Beh, 94Mey1, 94Mey2, 98Sey]. Table 2 provides dissociation parameters for adsorbed CO2. Vibrational spectroscopy has played a key role in characterizing such surface reactions and adsorption configurations and vibrational data for adsorbed CO2 is collected in Table 3. CO2 interacts on metal surfaces in two states: as a physisorbed, linear CO2 molecule and as a chemisorbed, non-linear CO2 molecule, which is bent as in a CO2− species. The chemisorbed bent species is often considered an intrinsic precursor for CO2 dissociation into CO and O adatoms. Fig. 1 illustrates how a CO2 molecule might physisorb in a linear configuration as it approaches the surface and, under certain conditions, transform into a bent CO2− species. This molecule resides in a new adsorption well from where it might scatter back into the gas phase or dissociate into two chemisorbed species, coadsorbed CO and O [87Bar1]. Many studies have addressed the adsorption of CO2 on Fe surfaces and concluded that the adsorption mechanism is face-sensitive [87Pir, 86Beh, 87Fre, 87Beh, 87Bau]. The open Fe(111) surface was much more reactive for CO2 dissociation than the Fe(110) surface, which was inactive towards CO2 chemisorption [87Pir, 86Beh, 87Fre, 87Beh, 87Bau]. Introduction of steps and defects into the Fe(110) surface lead to CO2 chemisorption. Steps on the surface lower the work function and this favors electron transfer from the metal to the adsorbate. This distorts the molecule toward CO2− and chemisorption of CO2 occurs. The adsorption and reactivity of CO2 increases for the sequence: Fe(110) < stepped-Fe(110) < Fe(111) < evaporated Fe films [87Pir, 86Beh, 87Fre, 87Beh, 87Bau]. Molecular CO2 adsorption was observed on an Fe(111) surface at 85 K [87Pir, 86Beh, 87Fre, 87Beh, 87Bau]. Fig. 2 shows UPS spectra of adsorbed CO2 on Fe(111) at 85 and 140 K, along with calculated one-electron orbital energies of CO2 and CO2−. The UPS spectra reflected that most of the adsorbed molecules were only slightly distorted upon adsorption at 85 K, but there was a small fraction of the Landolt-Börnstein New Series III/42A5
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molecules that were identified as bent, anionic CO2− species [87Pir, 86Beh, 87Fre, 87Beh, 87Bau]. These anionic species were stable up to 160-180 K. Further heating the substrate to room temperature lead to the formation of coadsorbed CO and O on the surface. Thus, on (111) and polycrystalline Fe surfaces, it was concluded that CO2− was a precursor for dissociative adsorption [87Pir, 86Beh, 87Fre, 87Beh, 87Bau]. These data are presented along with other information on the valence electronic structure for adsorbed CO2 in Table 4. Related electronic excitations for adsorbed CO2 are summarized in Table 5. Work function change (∆φ ) measurements following CO2 adsorption on Fe(111) and stepped-Fe(110) surfaces [86Beh] are shown in Fig. 3. These data supported the presence of two different species at 77 K. One species caused the work function to increase while the other one lead to a decrease in work function with increasing CO2 exposures. HREELS was used to probe the molecular adsorption state of CO2, its dissociation to yield coadsorbed CO and O, and possible formation of carbonate (CO3−) species on Fe(111) between 100 and 300 K [95Hes]. A carbonate intermediate was formed from CO2 exposures on an oxygen-precovered Ag surface [82Stu, 83Bac]. Figs. 4a and b show a series of HREELS spectra probing the effect of pressure and temperature on CO2 adsorption on Fe(111) [95Hes]. In Fig. 4a, two different groups of energy loss peaks dominate the spectra. Losses at 50, 133 and 145 meV were attributed to a strongly bound CO2 species because the intensities of these peaks did not increase with increasing CO2 pressure, while losses at 80 and 290 meV were associated with a weakly bound species. In Fig. 4b, appearance of a new peak at 225 meV, due to a νCO stretching mode, from heating the layer at 150 K indicates dissociation. In addition, the strong loss peak at 133 meV at 150 K indicates the presence of two different states of strongly bound CO2 on Fe(111). The behavior and intensity changes of the losses at 145 and 133 meV as the temperature was raised suggested that the two bent CO2 states were strongly coupled and had two different activation energies for dissociation Ediss. At 240 K, a new feature appeared at 160 meV, which was associated with a linear CO2 adsorbed in an upright orientation based on off-specular HREELS spectra, as presented in Fig. 5. The 290 meV loss peak was associated with the νas(OCO) asymmetric stretching mode of a weakly adsorbed CO2 molecule in a linear configuration. The appearance of two losses near 160 meV was explained by splitting of the νs(OCO) symmetric stretching mode due to a Fermi resonance with the first harmonic of the 80 meV mode. The 80 meV loss peak was assigned to the δ(OCO) bending mode of an adsorbed linear CO2 molecule. A vertical geometry of the CO2 molecules was suggested because the two split modes were excited by impact scattering and the νas(OCO) mode by dipole scattering. The two modes at 133 and 145 meV and two other modes at 170 and 200 meV were assigned as corresponding to νs and νas modes of CO2 adsorbed in a bent configuration. A definite analysis of the geometry of the bent CO2− on Fe(111) was difficult because the number of observed modes agreed with both Cs and C2v symmetry. According to UPS, XPS, HREELS and NEXAFS measurements, CO2 adsorption on Ni(110) [87Lin, 87Bar1, 88Ill] was very similar to that on Fe(111). Information on core-level binding energies for adsorbed CO2 in this system and others is given in Table 6. ARUPS and HREELS studies revealed that two species were present on the Ni(110) surface at low temperature: a bent, chemisorbed CO2δ− species and a linear physisorbed CO2 species. All physisorbed CO2 desorbed from the surface by heating to 180 K, and CO2δ− species dissociated into coadsorbed CO and O upon heating above 190 K. ARUPS and HREELS results were compatible with C2v symmetry of the CO2δ− adsorbate complex. Two possible coordinations satisfy this constraint, C-atom down or O-atoms down, as shown in Fig. 6. An η2 O-coordination of CO2δ− species was proposed by comparison with the bonding of formate ions (HCOO−) on Cu(110) [87Lin, 87Bar1], where the molecular plane was aligned along the [110] azimuth and the O atoms were bound to the metal in an η2 configuration. Further information about the local geometry of CO2 and CO2δ− species on Ni(110) was available from NEXAFS [88Ill]. Fig. 7 shows spectra at the oxygen K-edge for a θE of 90º (normal incidence) and 20º for adsorbed CO2 at 100 and 180 K. Spectra at 100 K were composed of a mixed phase of linear CO2 and bent CO2−, whereas the spectra at 180 K only contained the bent CO2− phase. A spectrum for physisorbed CO2 was derived by subtraction of the bent CO2− spectrum at 180 K from the spectrum of the mixed phase, and this is included in Fig. 7. The sharp features were assigned to a π resonance caused by excitation from the O 1s core level to the 2πu valence level of linear CO2 or 2b2 valence level of bent Landolt-Börnstein New Series III/42A5
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CO2−. Because the polarization dependence of the π resonance did not vary strongly with the angle of incidence, it was proposed that the physisorbed species adopts a “lying-down” configuration. If the molecular axis of the physisorbed CO2 molecule was oriented perpendicular to the surface, then a strong attenuation of the π* resonance intensity would be expected. This is in contrast to the vertical geometry proposed for physisorbed linear CO2 on the Fe(111) surface. Evidence about the orientation of these species from the polarization dependence of NEXAFS spectra has been obtained [88Ill]. The π resonance was strongly attenuated at θE = 20º in both azimuth angles, while the intensity of the σ resonance remained almost constant. The interpretation was that the molecular plane of a bent chemisorbed species was most likely oriented perpendicular to the surface plane, but that there was no preferential orientation of the molecular plane along the <110> or <001> azimuth. The spectra were also compatible with the presence of two different species oriented along each azimuth. A diffuse LEED analysis gave equal numbers of bent molecules adsorbed on atop sites and oriented along the <100> and <110> directions [82Stu]. The adsorption geometry and dissociation of CO2 molecules on Re(0001) at 85-135 K was studied by HREELS [88Ass] with results similar to those found on Ni(110). Below 120 K, CO2 adsorbs as a linear molecule with its axis parallel to the surface. A fraction of the adsorbed molecules desorbed at 120-135 K and the rest transformed into an intermediate identified as bent CO2δ−, which then decomposed into coadsorbed CO and O at 135 K. Initially, CO2 was thought to dissociate on Rh(111) [79Dub], but this was later attributed to B impurities [85Sol]. This latter result was consistent with a calculation of the dissociative sticking coefficient Sdiss for CO2 on Rh(111) to be about 10−15 [83Wei].
3.8.4.2.3 CO2 adsorption on chemically modified metal surfaces 3.8.4.2.3.1 CO2 adsorption and reaction on metals with coadsorbed O and H adatoms CO2 does not adsorb on the Ag(110) surface at 100 K [83Bac, 82Stu]. However, preadsorbed oxygen stabilized a molecular CO2 state and induced reaction of CO2 to form surface carbonate (CO32−) anionic species, as characterized by LEED, TPRS, UPS, XPS and HREELS [83Bac, 83Bar, 82Stu, 83Beh, 91Beh]. Upon heating, CO32− decomposed into coadsorbed CO2 and O with a dissociation activation energy of Ediss = 27 kcal/mol. UPS showed that the valence levels of adsorbed CO32− agreed well with those found in bulk K2CO3 and CaCO3. This information, coupled with the increase in work function upon formation of adsorbed CO32−, helped to establish the anionic nature of these species. C and O K-edge NEXAFS were performed to determine the orientation of the surface carbonate species on Ag(110) at room temperature [88Mad], as shown in Fig. 8. From the dependence of the spectra on the polar and azimuthal angles, it was concluded that the C-O bonds of CO32− were oriented parallel to the surface within 10º. A single σ* resonance at 300.5 eV for both azimuths also suggested that all C-O bonds were equivalent. Excellent agreement between the NEXAFS spectra for both azimuths to that from bulk CdCO3 led to the conclusion that the C-O bond length was 1.29 Å, i.e. the same as that in CdCO3 [88Mad]. The assignment from NEXAFS of the orientation of the CO32− species as parallel to the Ag(110) surface contradicted the tilted geometry that was assigned previously based on ARUPS and HREELS [87Sak, 83Bac, 82Stu]. However, it was possible to reinterpret the vibrational spectrum to be consistent with the NEXAFS results and to indicate the Cs symmetry of the surface-adsorbate complex [88Mad]. CO2 interacts more weakly with an O-precovered Ni(110) surface than with a clean Ni(110) surface. ARUPS was used to determine that the orientation of CO2 was parallel (±20º) to the surface on O-precovered Ni(110) at 85 K, similar to that on clean Ni(110) at 85 K [87Bar1], as shown in Fig. 9a. The spectrum at 293 K, shown in Fig. 9b, was different from that obtained from the oxygen-free Ni(110) surface. The spectrum could be explained either by the presence of a CO/O coadsorbed phase or by the presence of carbonate species, but these two possibilities were not distinguished. The reactivity of adsorbed CO2 with coadsorbed hydrogen on Ni(110) was investigated by using HREELS and high-resolution XPS [91Wam]. HREELS spectra shown in Fig. 10 indicate both linear CO2 Landolt-Börnstein New Series III/42A5
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and bent CO2− species by a δ(OCO) peak at 636 cm−1 and losses at 727, 1103 and 403 cm−1 attributed to δ(OCO), νs(OCO) and νNi-CO2 modes of a bent CO2− species, respectively. Upon heating, the band for linear CO2 disappeared and the losses corresponding to bent CO2− species increased in intensity. Linear CO2 was apparently converted to bent CO2− species. Above 200 K, the spectra were characterized by three loss peaks at 403, 727, 1353 and a weak peak at 2904 cm−1. These vibrational energies compared well with those obtained for adsorbed formate (HCOO−) and it was concluded that formate was formed on the surface and was stable to 300 K. Formation of formate species was also supported by XPS [91Wam], as shown in Fig. 11. At low temperatures, peaks due to coadsorbed linear CO2 (291.2 eV BE) and bent CO2δ− (286.6 eV BE) were found. At 120 K, physisorbed CO2 desorbed, CO2δ− remained on the surface, and a small peak grew at 285.6 eV BE that corresponded to CO(a). At 200 K, a shift of the 286.6 eV BE peak to 287.0 eV BE indicated formation of formate species along with coadsorbed CO. The inset of the figure provides a spectrum of formate species that were formed by heating formic acid to 200 K, in good agreement with the spectra from CO2 + H2/Ni(110).
3.8.4.2.3.2 CO2 adsorption and reaction on metals with coadsorbed alkali metals Alkali metals function as promoters for the adsorption and reactivity of CO2 on metal surfaces [89Pau, 93Nas, 95Hes, 85Sol, 87Pel, 88Ass, 91Rod, 85Beh, 94Mey1, 94Mey2, 98Sey, 96Kra, 89Liu, 91Liu, 87Sol, 88Kis, 87Mat, 89Wam, 94Hof, 94Sol, 90Ehr]. Several examples exist where alkali metals dramatically increased the strength of the interaction between CO2 and metal surfaces. This was often explained as arising from either direct interaction with the alkali metal or the lowering of the work function of the substrate by adsorption of the alkali metal, because electron transfer to CO2 to form a more stable CO2− species should be favored in either case. Different species, such as a CO2 dimer, carbonate and oxalate, were proposed to form on these alkali-precovered metal surfaces depending on the nature of the alkali metals and metal substrates. Spectroscopic evidence for the presence of CO2− and CO32− species was presented for the system CO2/K/Rh(111) [87Sol, 88Kis, 94Sol] and CO2/K/Pt(111) [89Liu, 91Liu]. Interactions of CO2 with Fe(110) surfaces precovered with K and Cs was studied by UPS, XPS and work function measurements [94Mey1, 94Mey2, 98Sey]. In contrast to the inactivity of CO2 on Fe(110), a strong interaction between CO2 and alkali-promoted Fe(110) was found. UPS spectra of CO2 on Fe(110) at 85 K for two different K precoverages are shown in Fig. 12. At low K coverages and low CO2 coverages, dissociation occurs to form coadsorbed CO and oxidic oxygen. At higher CO2 coverages, disproportionation reactions form coadsorbed CO and CO32− species. Linear CO2 molecules were found at high CO2 exposures. Though no stable bent CO2− was identified, its formation as an intermediate was suggested. At higher K coverages (θ K 0.16), a new adsorbate “species A” was formed. This was proposed to be an oxalate (C2O4m−) species, although a C-O-C bridged iso-oxalate or a CO2−· CO2 adduct could be ruled out. HREELS was used to identify formation of oxalate species upon CO2 adsorption on K-modified Fe(100) at 85 K [89Pau]. The stability of CO32− on Fe(110) depends on the K coverage [94Mey2]: small to medium (0 < θ K 0.20) or high coverages (0.26 θ K < 0.3). Fig. 13 shows that CO32− is unstable and decomposes between 200 and 300 K at small to medium θ K values, whereas CO32− decomposition does not occur until 500 K at high θ K values. Reaction schemes for the CO2/K/Fe(110) coadsorbate system for the two different θ K regimes have been proposed [94Mey2]. UPS and XPS were used to determine that the chemistry of CO2 on Cs/Fe(110) and K/Fe(110) surfaces was similar, despite the larger size and induced work function change of Cs compared to K [98Sey]. Similarity of the chemistry of the CO2/K/Cu(110) system to that of CO2/K/Fe(110) led to the conclusion that the adsorption behavior of CO2 was dominated by the adsorbed K and not by the underlying transition metal [96Kra]. IRAS and TPD investigations of the interaction of CO2 with a (¥3×¥3)R30º-K monolayer and K multilayer on Ru(0001) identified CO2−, CO22−, oxalate and carbonate formation [94Hof]. On a clean Ru(0001) surface, CO2 physisorbs in a monolayer that desorbs at 100 K, but interaction of CO2 with a K Landolt-Börnstein New Series III/42A5
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monolayer readily forms oxalate at 85 K. Fig. 14a compares IR spectra after CO2 adsorption on a K monolayer and multilayer on Ru(0001) with that of bulk K2C2O4. Three bands at 1716, 1342 and 806 cm−1 were assigned to νa(OCO), νs(OCO) and δ(OCO) modes, respectively, of an oxalate species. Possible structures of this species are shown in Fig. 14b. A study of matrix isolated Li2+C2O42− species and theoretical cluster calculations supported structure AII in Fig. 14b with D2h symmetry for the observed oxalate species [98Zhu]. Application of the surface dipole selection rule determined that the oxalate species was preferentially oriented with its molecular plane perpendicular to the surface and the C-C axis parallel to the surface, because the νa(OCO) asymmetric stretching mode was much stronger than the other two modes, in contrast to that found in the bulk IR spectrum. After heating above 150 K, oxalate (C2O42−) species on K/Ru(0001) disproportionated into coadsorbed CO32− and CO. FTIR spectra of the carbonate species formed from CO2 adsorption on multilayer, bilayer and monolayer K-covered Ru(0001) are shown in Fig. 15a. For the K monolayer, only one intense band was present at 1467 cm−1, which was assigned to νa(OCO) of CO32− with D3h symmetry. Possible structures for this species are shown in Fig. 15b. The vibrational spectra suggested that structure CIV in Fig. 15b, with its molecular plane perpendicular to the surface and C-O bond of the inequivalent, single oxygen atom parallel to the surface, was the most probable structure. Structures CII and CV were ruled out because these structures with inequivalent oxygen atoms should have given more than one CO stretching mode. Decomposition of CO32− starts at 700 K and results in simultaneous desorption of K and CO2.
3.8.4.2.4 CO2 adsorption on alloy surfaces There are two reports concerning adsorption of CO2 on bimetallic surfaces. One is on the c(2×2) Mn/Pd(100) surface alloy [99San]. No CO2 adsorption was found on this alloy at any temperature exceeding that required for condensation. Preadsorbed oxygen stabilized adsorbed CO2, but oxygen adsorption actually destroys the alloy structure, disorders the surface, and forms MnOx complexes before CO2 adsorption. Fig. 16 shows C(1s) XPS spectra after 20 L CO2 was dosed on an O2-pretreated surface. In addition to the peak at 280 eV BE corresponding to atomic carbon, four other peaks were found and assigned to CO(a) on Pd, CO(a) on Mn, CO2(a) and CO32−(a). Valence photoemission spectra for the same conditions were used to identify adsorbed CO2 at 110 K and CO32− after heating the adlayer to 500 K based on comparisons to angle-resolved spectra for CO2/Ni(110) [91Wam] and CO32–/Na/Pd(111) [89Wam] measured at hν=38 eV. It was concluded that both of these two latter species were bound to MnOx complexes at the surface by comparing the results to those for CO2 adsorption on oxide surfaces. In studies on bimetallic Cu/Re(0001) surfaces, it was proposed the observed enhanced activity for CO2 dissociation over that of pure Cu arose from Cu-Re interactions that enhanced the electron donor capability of Cu atoms supported on Re(0001) [91Rod].
3.8.4.3 NO2 3.8.4.3.1 Structure and bonding of NO2 NO2 is isoelectronic with CO2−, and as such, it is a bent triatomic molecule, as shown in Scheme III. In the gas phase, it has a 2A1 ground electronic state, i.e., it is a radical, with the unpaired electron residing on the N atom, and it has an O-N-O angle of 134° and an O-N bond length of 1.2 Å.
Scheme III. Lewis structures for NO2.
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As expected, adsorbed NO2 exhibits a diversity of bonding modes analogous to the “linkage isomerism” observed in inorganic transition metal complexes [82Hit, 91Aln]. In these complexes, NO2 coordinates in the form of nitrite (NO2−) anion. NO2 and NO2− can bond through either one (monodentate) or both (bidentate in a chelating structure) of the O atoms or through the central N atom, and in bidentate bridging structures involving bonding to N and O atoms. Scheme IV gives some of the possible linkage isomers of NO2− that have been characterized in complexes that are relevant to adsorption on surfaces. There are four adsorption isomers expected for NO2 that is bonded upright at the surface, with the molecular plane aligned near the surface normal: nitro or N-bonded (a in Scheme IV); O-nitrito (b and c in Scheme IV); O,O'-nitrito (d in Scheme IV); and µ-N,O-nitrito (the prefix-µ notation indicates the number of metal atoms to which the molecule is bonded) (e in Scheme IV). In complexes, O-nitrito ligands (b and c in Scheme IV) are known to be thermodynamically unstable and usually rearrange to the nitro isomer, so these should be only weakly bound at the surface. In addition, one may find weakly bound adsorption isomers with the molecular plane parallel or slightly inclined to the surface.
Scheme IV. Bonding modes for NO2.
3.8.4.3.2 NO2 adsorption on metal surfaces Thermodynamic values for NO2 adsorption are given in Table 7. NO2 is a very reactive molecule and dissociative adsorption occurs on most metal surfaces to form coadsorbed NO and O products at 300 K. Table 8 provides information on dissociation parameters for adsorbed NO2. Dissociative adsorption of NO2 was reported on W(110), Mo(110), Ru(0001), Ag(110) and Ag(111) surfaces even at low temperatures. On W(110), NO2 almost completely dissociated at 100 K to form coadsorbed N and O adatoms [79Fug, 87Bab]. On Ru(0001) at 80 K, NO2 initially dissociated to produce coadsorbed NO and O, but higher exposures lead to molecular chemisorption of NO2 [86Sch1, 86Sch2]. This molecularly adsorbed NO2 was weakly chemisorbed with an energy of 9 kcal/mol. HREELS was used to conclude that NO2 bonds through the N atom to Ru(0001) with C2v symmetry and the molecular axis oriented essentially perpendicular to the Ru(0001) surface. Vibrational data for adsorbed NO2 on Ru(0001) and other surfaces is summarized in Table 9. TPD, LEED, AES and ESD were used to investigate adsorption and decomposition of NO2 on Pt(100) [85Sch]. It was proposed that molecular adsorption occurred on the reconstructed, hexagonal (5×20) Pt(100) structure of the clean surface at 200 K and the reconstruction was not lifted by NO2 adsorption at this temperature. Thermal dissociation of NO2 occurred in a narrow temperature range (<10 K) near 295 K, and a surface structural phase transition occurred to form the unreconstructed (1×1) Pt(100) structure within the same temperature interval. This phase transition lead to highly reactive domains that accelerated NO2 decomposition and caused an autocatalytic reaction. NO2 adsorption on Pt(111) has been investigated by TPD, UPS and HREELS [82Dah, 82Seg, 87Bar2]. Early on, there were some contradictory results concerning the dissociative adsorption of NO2 on Pt(111) at 120 K. Based on cited unpublished UPS work, it was reported that NO2 adsorption was completely dissociative on Pt(111) [82Seg], while TPD studies indicated that NO2 adsorbed molecularly on Pt(111) at 120 K and dissociated at 240 K [82Dah]. Later, it was confirmed that NO2 adsorption was molecular at all coverages at 100 K [87Bar2]. Fig. 17 summarizes NO2 adsorption kinetics on Pt(111) at 100 K. At low coverages (<0.25 ML), adsorption was irreversible, and NO2 dissociated completely during heating in TPD to produce coadsorbed NO and O. At higher coverages, adsorption was partially reversible and desorption of NO2 occurred with Ed = 19 kcal/mol. Saturation coverage in the chemisorbed Landolt-Börnstein New Series III/42A5
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layer at 100 K was θ NO2 ≈ 0.5 ML, with about 20 % of the monolayer desorbed as NO2. At large exposures, dimerization occurred to form the N-N bonded dimer in a condensed multilayer of N2O4 that desorbed in TPD with an onset near 120 K. Fig. 18 shows a series of HREELS spectra as a function of NO2 exposure on Pt(111) at 100 K. Mode assignments were made by comparison to IR data on inorganic cluster compounds that contained NO2 ligands. The surface-dipole selection rule for specular scattering in HREELS was used to assign Cs symmetry for the NO2 surface complex. The large difference in energies (380 cm−1) between the νs(ONO) and νa(ONO) stretching modes was consistent with a geometry in which NO2 was strongly bound in a low-symmetry structure. Three isomers could have such a symmetry: µ-N,O-nitrito (bridge-bonded NO2), O-nitrito and a three-coordinate nitrite (flat-lying). A µ-N,O-nitrito species in an upright position (e in Scheme IV) was proposed as the most probable configuration. With the lengthening and weakening of the N-O bond that is nearly parallel to the surface, this species was proposed as a logical precursor for the dissociative adsorption of NO2 on Pt(111). HREELS spectra taken after annealing small NO2 coverages (θ NO2 <0.25 ML) on Pt(111) showed that NO2 dissociated completely to coadsorbed bridge-bonded NO and O from 170-240 K. At higher NO2 coverages (θ NO2 >0.25 ML), adsorbed NO2 could be stabilized to above 300 K and NO formed from NO2 dissociation was adsorbed at atop sites. NO2 adsorption in the presence of coadsorbed O adatoms on Pt(111) at 100 K was probed by TPD and HREELS [88Bar]. NO2 was adsorbed molecularly at all O precoverages, and coadsorption with O decreased the amount of irreversible NO2 adsorption. Coadsorption inhibited formation of the µ-N,O-nitrito species and caused the formation of a new, nitro species that is N-bonded in an upright geometry with C2v symmetry. This is shown in Fig. 19. At θ O = 0.75 ML, a coverage of 0.15 ML nitrobonded NO2 could be formed. All of this NO2 was reversibly adsorbed, and desorbed in a TPD peak at 155 K, corresponding to a chemisorption bond energy of 11 kcal/mol. An NO3− species was not observed under any conditions. NO2 adsorption on Pd(111) was similar to that on Pt(111), as probed by TPD and HREELS [91Wic]. NO2 chemisorbed as a µ-N,O-nitrito isomer with a saturation coverage of 0.5 ML on Pd(111) at 110 K. Decomposition into coadsorbed NO and O occurred upon heating to 180 K, and this initially has a lower activation energy than desorption. NO2 desorption became competitive with decomposition at higher coverages. NO2 is quite reactive at Ag surfaces. The chemistry of NO2 on Ag(110) and Ag(111) surfaces is complex [87Out, 90Pol1, 90Pol2, 95Bro1, 95Bar]. Many surface species were identified depending upon the adsorption temperature and coverage. Fig. 20 provides IRAS spectra obtained following NO2 exposure on Ag(111) at 86 K [95Bro1]. The vibrational band assignments were made as follows: 1940 cm−1, NO; 1860-1880 cm−1, (NO)2, N2O3; 1717/1738/1745/1766 cm−1, N2O4; 1590 cm−1, NO3, N2O3; 1265/1285/1305 cm−1, NO3, N2O3, N2O4; 1045 cm−1, NO3; and 761/774 cm−1, NO3, N2O3, N2O4. Initially, adsorption of NO2 on Ag(111) at 86 K was dissociative, producing the dimer (NO)2 , as assigned by the loss peak at 1859 cm−1, and adsorbed O. This conclusion was also reached in UPS and XPS studies. Adsorbed NO3 and N2O3 were formed on Ag(111) by reaction of incoming NO2 with adsorbed O and (NO)2. All of these species coexisted on the surface. A proposed reaction scheme is given in Fig. 21. Available photoelectron spectroscopic data for adsorbed NO2 is given in Table 10. Au(111) is the least reactive metal surface that has been studied, and NO2 adsorption on Au(111) at 100 K was molecular and completely reversible as determined by HREELS, IRAS, AES and TPD [89Bar, 98Wan1]. Fig. 22 shows that chemisorbed NO2 desorbs from Au(111) in a peak at 230 K, indicating Ed = 14 kcal/mol. After saturation of the NO2 monolayer (θ NO2 = 0.4 ML), an N2O4 multilayer was formed at higher NO2 exposures and NO2 from this phase desorbs near 150 K. In the TPD scans shown in Fig. 22, the NO signal reproduced the NO2 signal as expected for a cracking fragment ion, except for the peak at 170 K which then must be attributed to NO desorption. Because NO does not adsorb on Au(111) under these conditions, this must be NO evolution from the decomposition of an NOcontaining surface species, which was shown by HREELS and IRAS to be adsorbed N2O3. Fig. 23 gives relevant IRAS spectra from Au(111) [98Wan1]. The spectrum obtained by dosing NO2 at 175 K (to avoid contaminant NO(g) from forming N2O3) was used to determine the geometry of chemisorbed NO2 on Au(111). Only two bands were observed because of the C2v symmetry of the adsorbed complex. These Landolt-Börnstein New Series III/42A5
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were due to the δ(ONO) bending mode at 805 cm−1 and νs(ONO) stretching mode at 1178 cm−1. There are two possible NO2 geometries with C2v symmetry: O,O'-nitrito (chelating) and nitro (N-bonded) NO2. It was concluded that adsorbed NO2 formed an O,O′-nitro surface chelate with C2v symmetry based on the stretching mode vibrations in coordination compounds containing the chelating isomer (1171-1225 cm−1) and the nitro isomer (1306-1392 cm−1). Deliberate NO exposure to these species forms adsorbed N2O3 on the surface, as shown in Fig. 23, in an intriguing radical-radical reaction. The IRAS spectrum indicates N2O3 species are adsorbed in an upright geometry with a symmetry of CS or lower, in a monodentate O-bonded configuration with the free N=O group oriented nearly perpendicular to the surface. This is a reversible reaction that liberates NO(g) at 170 K in TPD, indicating an N-N bond strength of 10 kcal/mol. While it is not clear whether NO2 chemisorbs on Au(111) as a radical, its reactivity towards gas-phase NO to produce adsorbed N2O3 shows its capability of undergoing radical-radical type of reactions. Difficulties exist in ESR studies of paramagnetic monolayers on metal surfaces and so information from these experiments is limited [93Bec]. In coadsorption studies with H2O on Au(111), NO2 was shown to react to produce nitrous acid (HONO) and nitric acid (HNO3) upon being heated [98Wan1, 98Wan2, 99Wan]. Another reaction channel was observed by exposure of NO2 on amorphous, condensed H2O (ice) films at 85 K (containing free “OH” groups). Heating to 130 K formed the nitrito isomer of N2O4 (ONO-NO2), which reacted at 183 K to form nitrosonium nitrate (NO+NO3−). This species decomposed at 275 K ultimately to deposit a large concentration (θ O = 0.4 ML) of O adatoms on the Au(111) surface. This facile, low-temperature reaction between condensed-phase NO2 and H2O represents a novel route to generate surface oxygen on Au and perhaps on other unreactive surfaces. One application of the high reactivity of NO2 with metal surfaces is the use of NO2 to cleanly, conveniently achieve high, effective O2-pressure conditions that enabled new studies of the interaction of oxygen with several transition metal surfaces. This was first utilized on Pt(111) where θ O = 0.75 ML could be achieved by NO2 dosing with the substrate at 400 K [88Bar, 89Par, 90Par], in contrast to the value of θ O = 0.25 ML that is produced by exposures to O2 in UHV. NO2 exposures on Pd(111) at 530 K was later used to achieve oxygen coverages of up to 3.1 ML under UHV conditions [90Ban, 90Par]. NO2 dosing has been used extensively on Ru(0001) to create high atomic oxygen coverages, novel highdensity ordered structures, subsurface oxygen and RuOx layers [91Mal, 92Mal, 95Hrb, 96Mit, 97He, 97Kos, 99Böt]. Decomposition of NO2 was used for the oxidation of Ag, Zn and Cu films [94Rod] and to study the O/Rh(111) system [95Pet]. At high temperatures (~1000 K), pure films of polycrystalline MoO2 that were 20-Å thick were produced from NO2 exposures on Mo(110) [00Jir].
3.8.4.3.3 NO2 adsorption on alloy surfaces There are a few reports on the interaction of NO2 with bimetallic surfaces. NO2 was found to dissociate on a supported Zn monolayer on Ru(0001) similar to that on polycrystalline Zn [93Rod]. On Zn1.0/Ru(0001) at 80 K, initial NO2 adsorption was dissociative to produce adsorbed O and NO(g), but subsequent NO2 adsorption was molecular due to passivation of the surface by adsorbed O. This chemisorbed NO2 decomposed at 160-250 K. At 300 K, the dissociation probability for NO2 on Zn surfaces was 100 times that of O2. On Pd/Rh(111) bimetallic surfaces (θ Pd = 0.6-1.2 ML) formed by deposition of Pd overlayers on Rh(111), NO2 exhibited a reactivity that was substantially lower than on Rh(111) or Pd(111) [99Jir1]. Pd atoms supported on Rh(111) were able to dissociate NO2 at low temperature, but the NO and O products desorbed or diffused away from the Pd islands below 400 K, spilling over to bare Rh(111) patches. Adsorption of NO2 on a (¥3×¥3)R30°-Sn/Pt(111) surface alloy was investigated using TPD, AES, HREELS and LEED [04Vos]. At 120 K, NO2 dimerizes at the surface to form the N,N-bonded dimer, N2O4, in the chemisorbed monolayer. This species bonds to the surface through a single O atom in an upright but tilted geometry. However, no N2O4 or NO2 desorbs molecularly from the monolayer. The dimer completely dissociates at 300 K, leaving coadsorbed NO2, NO and O on the surface. Adsorbed NO2 further dissociates to coadsorbed NO and O at 300-400 K. The maximum oxygen atom coverage obtained by heating the N2O4 monolayer was θ O = 0.4 ML, but this could be increased to θ O = 1.1 ML by NO2 Landolt-Börnstein New Series III/42A5
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dosing on the alloy surface at 600 K to remove inhibition by coadsorbed NO. Under these latter conditions, adsorbed oxygen desorbs as O2 in three clear desorption states, the lowest of which is associated with O2 desorption from Pt sites and the other two are from decomposition of reduced tin oxide phase(s), SnOx.
3.8.4.4 SO2 3.8.4.4.1 Structure and bonding of SO2 SO2 is a bent, polar triatomic molecule, as shown in Scheme V. It is isoelectronic with NO2−, one of the most versatile oxyanion ligands in metal complexes, as explained above. As usual, the coordination chemistry of SO2 is determined by the nature of its highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO) that act as electron donor and acceptor levels, respectively. For SO2, the HOMO is the 8a1 orbital and the LUMO is the 3b1 orbital, which lies about 2 eV above the 8a1 level [71Roo]. These orbitals are shown in Fig. 24. The empty d orbitals do not participate in the bonding and SO2 can act as both a σ-donor and π-acceptor [81Rya]. When the π-acceptor aspect dominates, π bonds between the SO2 and metal are formed and the molecule lies flat on the surface. On the other hand, in cases where σ-bonding is preferred, the molecule adsorbs with its molecular plane perpendicular to the surface.
Scheme V. Lewis structures for SO2. The structure and bonding of SO2 containing metal complexes has been reviewed [78Min]. In complexes with terminal SO2 ligands, there are three distinct bonding modes, as shown in Scheme VI: η1-planar (a), η1-pyramidal (b) and η2 (c), each having a metal-S bond [74Ang, 85Sak].
Scheme VI. Bonding modes for terminal SO2. Bridging SO2 ligands are also found in polynuclear compounds [79Kub]. The angle between the SO2 plane and metal-S axis is close to 180° in η1-planar complexes and close to 120° in the η1-pyramidal complexes. The infrared active S-O stretching frequencies are diagnostic of different bonding modes in SO2 complexes [81Rya, 85Sak]. Various bonding modes of SO2 can be distinguished by frequency shifts of ν (SO) as shown in Fig. 25. The η2 mode can be easily distinguished because the νs(OSO) stretching mode drops to a value of 850-950 cm−1, far below that for any other modes, and this results in a large splitting of (νas−νs) ~230 cm−1. For η1-planar complexes, (νas − νa) is always less than 180 cm−1 and this characterizes SO2 bonding through the S atom only. Use of the surface-cluster analogy aids identification of the coordination of SO2 on surfaces using the known structures that are available for SO2 ligand in complexes, but one should always proceed with caution because of differences in bonding of the molecules in the two cases. Adsorption behavior of SO2 on metal surfaces in some ways resembles the adsorption of CO on metals because σ donor, π acceptor interactions are important for both molecules. However, the orbital Landolt-Börnstein New Series III/42A5
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energy of the π* orbital in SO2 is much lower than that in CO, suggesting that SO2 should be a stronger π acceptor in metal-SO2 complexes. Charge donation from the substrate into the π* orbital of SO2 is expected if the substrate orbitals have proper symmetry, energy and location to overlap the π* orbital.
3.8.4.4.2 SO2 adsorption on metal surfaces SO2 adsorption on metal surfaces has been extensively studied. Some of this literature has been reviewed [97Haa]. On Ag surfaces, SO2 was molecularly and reversibly adsorbed; it was desorbed with no evidence of decomposition [72Las, 81Rov, 84Out, 86Out1, 86Out2, 93Pre, 90Ahn, 90Rod, 91Sol2, 92Höf, 93Höf, 93Was, 96Gut2, 91Cas]. SO2 decomposed on Fe, Mo, Ni, Cu, Ru, Pd and Pt, surfaces [78Fur, 88Hor, 81Ku, 82Köh, 82Kat, 82Ast, 83Köh, 90Höf, 94Sun1, 97Wil, 97Pol, 95Wil, 85Köl, 88Bur1, 88Bur2, 97Ter, 99Lee, 89Leu, 92Ahn, 93Ahn, 93Pan, 96Pol, 97Nak1, 97Pan, 97Nak2, 98Pol, 93Zeb, 95Yok1, 95Yok2, 95Ter, 96Yok, 97Jac, 98Wil, 99Jir2]. However, the S-O bond in SO2 was weakened more on Ag(110) than on Pd(100), as indicated by a comparison of the νs(OSO) frequencies for adsorbed SO2 on the two surfaces, i.e., 985 cm−1 on Ag(110) and 1035 cm−1 on Pd(100). If so, bond strength arguments alone cannot explain the decomposition behavior of SO2 on these surfaces. SO2 was molecularly adsorbed on Ni surfaces at 150 K, which places a lower limit on the barrier to decomposition Ediss of SO2 on Ni of 10-12 kcal/mol. Ediss for SO2 on Ag(110) was larger than that on Pd(100), Pt(111) or Ni surfaces. In addition, on Ag(110), SO2 is bonded through the S atom only at atop sites. It has been suggested that the participation of more directional d orbitals in bonding leads to lowering of Ediss. SO2 adsorption thermodynamics data and dissociation parameters are summarized in Tables 11 and 12, respectively. At low temperatures, SO2 adsorbed molecularly on most transition metals. SO2 was more reactive on Ru(0001) than on Pt(111) or Pd(100) surfaces [98Jir, 97Wil, 88Bur2], and SO2 dissociatively adsorbed on Ru(0001) at 100 K to form sulfite SO3− and sulfate SO42− species [98Jir]. Decomposition of SO3− and SO42− occurred below 380 K on Ru(0001), but at temperatures higher than 400 K on Pt(111) or Pd(100) surfaces. Several different adsorption geometries for adsorbed molecular SO2 have been observed, with the configuration and site for SO2 adsorption depending on the electronic structure and geometry of the substrate. These species have been characterized by photoelectron spectroscopy, NEXAFS, SEXAFS and vibrational spectroscopy. Tables 13-16 summarize the available measurements for SO2 adsorption, respectively. On Pt(111), Pd(100) and Cu surfaces, SO2 adopted a geometry in which the molecular plane was perpendicular to the surface and the S atom and one O atom directly interacted with the surface [82Köh, 82Kat, 82Ast, 83Köh, 90Höf, 94Sun1, 97Wil, 97Pol, 95Wil, 85Köl, 88Bur1, 88Bur2, 97Pan]. On the other hand, SO2 adsorbed on Ni surfaces with its molecular plane parallel to the surface [93Zeb, 95Yok1, 95Yok2, 95Ter, 96Yok, 97Jac, 98Wil]. A bonding geometry involving only the S atom was found on Ag(110) [86Out2, 93Pre, 90Ahn, 90Rod, 91Sol2, 92Höf, 93Höf, 93Was, 96Gut]. It was suggested that the different behavior of SO2 may be derived from different energies of the metal d bands. For Ag, the metal d bands lie nearly 4 eV below the Fermi level and are too low in energy to bond effectively to SO2. Instead, the s and p orbitals are involved in making a Ag-SO2 σ bond. For Ni, the d orbitals lie at Fermi level and this leads to strong interactions with the π* orbitals in SO2 and results in a flat-lying orientation. It has been argued that the deeper d bands for Cu, Pd and Pt interact less significantly with the SO2 π* orbital and allow a SO2 geometry with the molecular plane perpendicular to the surface.
3.8.4.4.2.1 Molecular adsorption and desorption of SO2 SO2 adsorbs and desorbs molecularly on Ag surfaces [72Las, 81Rov, 84Out, 86Out1, 86Out2, 93Pre, 90Ahn, 90Rod, 91Sol2, 92Höf, 93Höf, 93Was, 96Gut, 91Cas]. The adsorption of SO2 on Ag(110) has been characterized by several techniques, including TPD, UPS , HREELS, XPS and NEXAFS [72Las, 81Rov, 84Out, 86Out1, 86Out2, 93Pre, 90Ahn, 90Rod, 91Sol2, 92Höf, 93Höf, 93Was, 96Gut]. Landolt-Börnstein New Series III/42A5
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Molecular adsorption and desorption of SO2 appear to occur without decomposition or dissociation on the clean Ag(110) surface. At a surface temperature of 100 K, multilayers of SO2 are formed, and this leads to desorption in a sharp peak near 120 K in TPD. Three different desorption states labeled as α1, α2 and α3 were observed at 170, 225 and 275 K, respectively, and these have been assigned to desorption from bilayer, monolayer and submonolayer coverages of SO2. Desorption energies in these peaks are 41, 53 and 64 kJ/mol, respectively, indicating that the binding energy of SO2 molecules at the Ag(110) surface is energetically in between that expected for physisorption and chemisorption. UPS, HREELS and NEXAFS provide evidence that the bonding geometry was unchanged between two chemisorbed phases on Ag(110) [72Las, 81Rov, 86Out2]. HREELS measurements suggested that chemisorbed SO2 was bound via a Ag-S bond, with the O atoms not involved in surface bonding, based on the similarity in the frequencies of the νs(OSO) mode for SO2 bound to the surface and in S-bonded SO2 complexes. Based on the presence of a SO2-Ag out-of-plane wagging mode, a geometry in which SO2 was tilted towards the surface with the O atoms parallel to the surface was proposed. Sulfur K-edge NEXAFS measurements provided evidence that the SO2 molecular plane on Ag(110) was not strongly tilted, but nearly (within 15º) perpendicular to the plane of the surface [96Gut]. The apparent contradiction between the two assignments might be explained by the uncertainty (~10º) in the NEXAFS measurements that could accommodate a small tilt that would allow the wagging modes to be dipoleactive in HREELS measurements, or by an important influence of impact scattering in HREELS that could make it appear that the out-of-plane wagging mode of surface bound SO2 was dipole active. The coverage dependence of the adsorption geometry of SO2 on Ag(110) was probed with S K-edge NEXAFS measurements [96Gut2]. Figs. 26a and b show NEXAFS spectra for coverages of 0.3 and 0.6 ML SO2 adsorbed on Ag(110). The insets give the azimuthal dependence of the π*-resonance intensity. The bonding geometry of SO2 on Ag(110) was obtained from this data using the dipole selection rule and group theory. The π*(b1*) resonance is excited by a component of the E vector perpendicular to the plane of the molecule while the σ*(a1*+b1*) resonance is excited by a component of the E vector parallel to the plane of the molecule. In the low coverage spectra, the π* resonance has a maximum intensity at normal incidence of the X-rays with the E vector along the [110] azimuth. The π*-resonance intensity is very weak with the E vector along the [001] azimuth and independent of the X-ray incident angle. This, along with the polarization dependence of the σ* resonance, suggested that SO2 molecules at low coverage adopt an upright geometry with the molecular plane along the [001] azimuth. The best fit to the data, based on an azimuthally aligned molecules, corresponded to the molecular plane tilting only 2±5º from the surface normal. In the 0.6 ML SO2 layer, Fig. 26b shows that the maximum-to-minimum π*-intensity ratio is dependent on the azimuthal angle, indicating an azimuthal alignment at higher coverage. Best-fit results correspond to a tilt angle of 13±5º for the molecular plane with respect to the surface normal and a twist angle of 55±5º from [001]. This allows a higher packing density on the surface. Using the surface-cluster analogy, an on-top adsorption site was favored [88Hor] and this lead to the structural model shown in Fig. 27.
3.8.4.4.2.2 Molecular adsorption and decomposition of SO2 SO2 adsorption on Pt and Pd surfaces was studied in several laboratories, and it was found that dissociation occurs to a small extent [81Ku, 82Köh, 82Kat, 82Ast, 83Köh, 90Höf, 94Sun1, 97Wil, 97Pol, 95Wil, 85Köl, 88Bur1, 88Bur2, 97Ter]. TPD, AES, work function measurements, UPS, XPS and HREELS were used to characterize adsorption of SO2 on Pt(111) [82Kat, 82Ast, 83Köh, 90Höf, 94Sun1, 97Wil, 97Pol, 95Wil]. Typically, it was observed that SO2 adsorbed molecularly on Pt(111) below 120 K. SO2 desorption from the multilayer occurred at 130 K and from the chemisorbed layer at 285 K in TPD. Following SO2 adsorption at room temperature, both SO and SO4 species were identified on the surface. Studies using TPD have reported molecular and dissociative adsorption states of SO2 on Pt(111) at 160 K at low coverages, along with recombinative SO2 desorption at 280 K [83Köh, 90Höf]. The adsorption geometry and reaction paths of adsorbed SO2 on Pt(111) have been examined by HREELS and XPS [94Sun1, 97Pol]. HREELS spectra are shown in Fig. 28. The inset shows TPD spectra Landolt-Börnstein New Series III/42A5
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with peaks at 127 and 285 K corresponding to SO2 desorption from the multilayer and monolayer, respectively. The HREELS spectra indicate that SO2 adsorbs molecularly on Pt(111) at 110 K. An adsorption geometry in which SO2 bonds to Pt in the monolayer through the S atom and at least one O atom was proposed based on the ν(Pt-SO2) and ν(Pt-O) stretching loss peaks at 266 and 430 cm−1, respectively. The relatively low energy νs(OSO) peak at 940 cm−1 and the large difference of 312 cm−1 between the νs(OSO) and νa(OSO) modes support an η2-SO2 geometry, bonding through both S and O atoms. In addition, both XPS and UPS results reveal electron back-donation from the substrate to the SO2 antibonding orbital [94Sun1, 97Wil, 97Pol]. The absence of a δ(OSO) wagging mode, typically near 680 cm−1, suggested that the SO2 molecular plane was normal to the surface and that SO2 was bonded through one of the O atoms. In addition, it was suggested that the O-O axis of SO2 was tilted with respect to the surface normal because of the presence of dipole-active loss peaks for both νs(OSO) and νa(OSO) modes of SO2. In a similar study on Pd(100), a 9º in-plane tilt was proposed based on a νa/νs intensity ratio of 0.44 [88Bur1]. A larger in-plane tilt for SO2 on Pt(111) compared to Pd(100) was suggested because νa/νs = 2 on Pt(111). The adsorption geometry for SO2 on Pt(111) is shown in Fig. 29. For the η2-SO2 configuration, both S and O are bonded to the surface, and both π and σ bonds can be formed. Formation of a π bond appears to occur between the 3b1 orbital of SO2 and the dxz or dyz orbitals of Pt, whereas σ bonds are formed between the 8a1 orbital of SO2 and dz2 orbital of Pt. It was suggested that atop (Fig. 29a) and two-fold sites (Fig. 29c and d) were energetically accessible for SO2 adsorption, but that two-fold sites were more favorable. SO and SO42− species were identified after SO2 adsorption on Pt(111) at room temperature [83Köh, 90Höf, 94Sun1, 97Wil, 97Pol, 95Wil]. The formation of SO42− is of interest because it was reported to be responsible for the poisoning of Pt-catalyzed oxidation of CO and alkenes, and on the other hand, responsible for the promotion of alkane oxidation [99Lee]. XPS indicated some Pt sulfide formation, but no atomic oxygen was found. The following surface reactions were proposed: SO2(g) → SO2(a) → SO(a) + O(a) SO(a) → S(a) + O(a) SO2(a) + 2O(a) → SO42−(a)
(1) (2) (3)
The characteristic S (2p) doublet and corresponding O (1s) spectra from high-resolution core-level photoemission are shown in Fig. 30 for adsorption and decomposition of SO2 on Pt(111) [97Pol]. Two different kinds of SO2 species forming the monolayer phase at 150 K were indicated by different binding energies. NEXAFS also indicates different molecular orientations for these two species [97Pol]. Quantitative analysis of this data yields an orientation for the molecular plane of 31±10° with respect to the surface normal for the SO2 phase at 212 K. The phase consisting of both SO2 species in the monolayer at 148 K was characterized by a tilt angle of 42º, which indicates that the second SO2 species was more flat lying. At 300 K, an SO42− species was formed. The adsorption and the surface geometry of SO2 on Pd(100) as probed by TPD, HREELS and NEXAFS [85Köl, 88Bur1, 88Bur2, 97Ter], was similar to that on Pt(111). SO2 was molecularly adsorbed on Pd(100) below 120 K and the SO2 multilayer could be desorbed at 135 K to leave behind the chemisorbed monolayer. Fig. 31 shows that decomposition of SO2 occurred above 420 K to leave coadsorbed O and SO, which in turn dissociated at higher temperature. An SO42− species was formed above 300 K by reaction of SO2 with O adatoms from SO2 dissociation. The (νs − νa) difference of 215 cm−1 indicated some bonding occurred through the O atom in addition to S bonding, and so, based on the HREELS data, an η2-SO2 bonding geometry was proposed with the molecular plane perpendicular to the surface. The S K-edge NEXAFS spectra and the Fourier transform of the S K-edge SEXAFS functions k 2χ (k ) are shown in Fig. 32 for SO2 adsorbed on Pd(100). Analysis of the S-O shell indicates that the S-O distance was elongated to 1.48 Å from 1.43 Å in SO2(g), due to charge transfer into the antibonding π* orbital. The polarization dependence of the S-O effective coordination number also lead to the conclusion that the C2 axis of the molecule was tilted by 34° from the surface normal. Thus, one O atom directly interacts with the surface. Comparison of the ratio of the S-Pd effective coordination number,
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N *(θ = 90º)/N *(θ = 55º) = 0.52 ± 0.06, with the calculated values of 0.57 for bridge, 1.12 for hollow, and 0.00 for atop sites suggested that the S atom in SO2 was in a bridge site, as shown in Fig. 32c. Adsorption of SO2 on Cu single crystal surfaces [89Leu, 92Ahn, 93Ahn, 93Pan, 96Pol, 97Nak1, 97Pan, 97Nak2, 98Pol] has been studied extensively. TPD and fast, high-resolution XPS [93Ahn, 96Pol] revealed that SO2 decomposition on Cu(100) results in a coadsorbed SO2 + O phase, with a negligible amount of additional coadsorbed SO and S on the surface. Upon heating to room temperature, a coadsorbed SO + 2O phase formed that exhibited a weak (2×2) LEED pattern. Below 400 K, SO + O recombination appeared to occur to form SO2 that then desorbed from the surface, leaving S and O adatoms on the surface. An earlier HREELS study [89Leu] had reported the presence of SO3−, suggesting a disproportionation reaction on the surface at room temperature, but this was inconsistent with this conclusion. NEXAFS and SEXAFS measurements [97Nak1, 97Pan, 97Nak2] provided evidence that SO2 on Cu(100) was adsorbed with its molecular plane parallel to the surface normal in the coadsorbed SO2 + O phase formed after heating a condensed SO2 layer to 180 K. The coadsorbed O adatoms were located in bridge sites. SO species were lying flat on the surface and bound to Cu through both S and O atoms for the coadsorbed SO + 2O phase formed by heating a SO2 multilayer to 280 K. The S atom in SO was located in hollow sites and the coadsorbed O adatoms were located in bridge sites. Decomposition of SO2 during TPD resulted in the formation of SO. If no additional SO2 was delivered from the gas phase during heating of adsorbed SO2, the decomposition product SO remained on the surface until it recombined with coadsorbed O adatoms and desorbed as SO2; no SO3− was formed. But, in the presence of SO2(g), such as occurs for room-temperature SO2 adsorption, SO3− formation occurs according to SO(a) + SO2(g) → SO3−(a) + S(a) O(a) + SO2(a) → SO3−(a)
(4) (5)
A consistent picture of formation of adsorbed SO3− from adsorption and reaction of SO2 on Cu(100) at room temperature has now been given [97Nak1]. An example of polarization-dependent S K-edge NEXAFS spectra of SO3− on Cu(100) is presented in Fig. 33. Peaks at 2477.4 and 2479.3 eV were assigned to adsorbed SO3− species by comparing the data to S K-edge spectra of SO3− salts. Curve-fitting analysis of the e* and a1* peaks determined that the orientation of SO3− species was nearly perpendicular to the surface. In addition, Fig. 33 shows that the calculated spectrum for Model 1 with SO3− in an upright orientation with the S aimed away from the surface was in good agreement with the Fourier transforms of the SEXAFS spectra at θ = 15° and 90°. The configuration in Model 1 with the S atoms located above the bridge site and two O atoms occupying the adjacent bridge sites was judged the most probable one. STM images of SO2 on Cu(100) revealed two domains, one forming a (2×2) square lattice and the other forming a rhombic c(4×6) structure [97Nak1]. AES, XPS and NEXAFS studies lead to the conclusion that only two types of species, SO3− and S, were present on the surface in a stoichiometric ratio of 2:1 [97Nak1]. All of the bright spots in the (2×2) and c(4×6) domains were assigned to SO3− species and the dim spots appearing in the c(4×6) domain were assigned to S adatoms. Because only SO3− existed in the (2×2) domains, the c(4×6) domains must have a S/SO3− population ratio equal to two. There are some discrepancies about molecular versus dissociative chemisorption in reports on the adsorption of SO2 on Ni surfaces and about the orientation of adsorbed SO2 on Ni(110) [93Zeb, 95Yok1, 95Yok2, 95Ter, 96Yok, 97Jac, 98Wil]. ARUPS and XPS results for SO2 on Ni(110) suggested that SO2 adsorption was molecular for temperatures below 150 K, independent of coverage [93Zeb]. The saturated monolayer formed a c(2×2) structure corresponding to an absolute coverage of 0.5 ML. Initial coverages below 0.25 ML dissociated completely upon heating. For higher initial coverages, SO2 desorption occurred in a single peak between 300 and 400 K. ARUPS was used to assign an orientation of SO2 such that the molecular plane was tilted and perpendicular to the densely packed substrate rows. This was attributed to Ni-O interactions, as was reported on Pt(111) and Pd(100) surfaces. A later study of SO2 chemisorption on Ni(110) by XPS and XAFS indicated partial decomposition of adsorbed SO2 at 160 K to leave SO3(a) and S(a) on the surface [98Wil]. Fig. 34 presents S(2p) and O(1s) spectra obtained at 160 K during annealing a saturation exposure of SO2 on the Ni(110) surface at 160 K [98Wil]. Two different SO2 species are apparently formed upon SO2 adsorption on Ni(110) at l60 K that might correspond to two different adsorption sites. In addition, even at 160 K, SO2 partially decomposed Landolt-Börnstein New Series III/42A5
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to form coadsorbed SO3− and S. Heating the surface at 800 K leaves only S adatoms on the surface, which gives rise to a c(2×2) LEED pattern. Fig. 35 shows S K-edge NEXAFS spectra obtained after a saturation exposure of SO2 on Ni(110) at 160 K and after subsequent heating to room temperature and 800 K. The structures in the spectra obtained at 160 K consisted predominately of overlapping π* and σ* resonances of adsorbed SO2, along with e* and a1* resonances of SO3−. This was more evident after heating to room temperature where XPS showed only the presence of coadsorbed SO3− and S. The polarization dependence of the spectra suggests that SO3− exists as a trigonal pyramid with the C3 axis oriented perpendicular to the surface, because the electric dipoles associated with the e* and a1* transitions are oriented perpendicular and parallel to the molecular axis, respectively. This orientation is like that on Cu(100) [97Pan]. The orientation of SO2 in the coadsorbed SO2 + SO3− phase formed by adsorption at 160 K was approximately flat lying because the π* resonance nearly vanished at normal incidence. Comparison of these experiments with values calculated assuming a two-fold substrate symmetry yielded a tilt angle α of 13 ± 15° between the molecular plane of SO3− and the substrate. A flat-lying geometry of adsorbed SO2 was also found by a similar NEXAFS study on a dilute SO2 layer on Ni(110) [95Ter]. SEXAFS studies of the phase obtained after SO2 exposure on Ni(110) at 160 K were performed to determine the adsorption sites of SO2 and its decomposition products [98Wil]. XPS showed that the phase consisted of coadsorbed SO2, SO3− and S with relative coverages of 1, 0.37 and 0.21, respectively. Adsorption sites for different species were determined by comparing measured SEXAFS amplitude ratios with those calculated for different high symmetry sites assuming a S-Ni bond length of 2.30Å. The simulations also used a SO2 tilt-angle of 13º between the molecular plane and the surface, as determined by NEXAFS, as well as an S-O-S bond angle of 106° and an upright orientation of the C3 axis. The proposed model shown in Fig. 36 includes two different SO2 species adsorbed in both short- and long-bridged sites, SO3− species that have the S atom located on the short-bridge sites, and S adatoms that occupy two-fold hollow sites [98Wil]. After determining the S sites of the SO2 and SO3− species by SEXAFS, the corresponding O sites were guessed from the fact that SO2 lies flat on the surface, as shown in Fig. 36a. For SO3−, two possible symmetric structures were suggested, as shown in Fig. 36b. NEXAFS was used to show that SO2 on Ni(111) and Ni(100) at 170 K adopts a very similar geometry as on Ni(110) with the molecular plane parallel to the surface [95Yok1].
3.8.4.4.2.3 Dissociative adsorption of SO2 SO2 adsorption and surface chemistry on Ru(0001) at 100 and 300 K was probed, and S(2p) XPS spectra after sequential exposure of SO2 on Ru(0001) at 100 K are presented in Fig. 37a [98Jir]. Even at low coverages, multiple S-containing species were present. The final dose of 1 L SO2 produced a strong peak at 167.9 eV that corresponded to a physisorbed SO2 multilayer. As shown in Fig. 37b, heating to 160 K caused peaks corresponding to adsorbed S, SO2, SO3− and SO42−. SO42− was the dominant species above 260 K and annealing to 350 K lead to complete decomposition of SO2 to leave S adatoms. Small SO2 exposures on Ru(0001) at 300 K [98Jir] lead to complete decomposition and produced coadsorbed O and two different kinds of S adatoms. SO3− and SO42− species eventually appeared after high exposures when the number of empty sites was limited. INDO/S and BOC-MP methods predicted that O,O- or S,O-coordinated SO2 species were the most probable precursors for dissociation [98Jir].
3.8.4.4.3 SO2 adsorption on metals with coadsorbed alkali metals Adsorption of SO2 on a Cs-precovered Ag(100) surface at 80 K was investigated using and AES, LEED, TPD, work function measurements and molecular beam back scattering [92Höf, 93Höf]. SO2 TPD spectra from such a surface are reproduced in Fig. 38a. SO2 was molecularly adsorbed and desorbed intact from both the clean and modified Ag(100) surface, but coadsorbed Cs shifted the SO2 desorption peak to higher temperature. This indicated a short-range interaction between SO2 and Cs that increased the SO2 Landolt-Börnstein New Series III/42A5
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adsorption energy. SO2 adsorption caused a sharp increase in the work function, as shown in Fig. 38b. This was thought to be consistent with a strong interaction between SO2 and Cs that would cause neutralization of the Cs-induced charge transfer with the substrate. High-resolution, synchrotron-based XPS was used to investigate the chemistry of SO2 on Cs/Mo(110) surfaces, along with that on Mo(110) and MoO2/Mo(110), specifically probing the effects of Cs and O on the formation of SO3− and SO42− species [99Jir2]. Cs addition to the Mo(110) surface enhanced substantially its reactivity with SO2, forming SO3− and SO42− on the surface at 300 K, in contrast to the chemistry on Mo(110). Cs coadsorption also increased the thermal stability of SO42− compared to that on Mo(110).
3.8.4.3.4 SO2 adsorption on alloy surfaces The interaction of SO2 with several bimetallic surfaces has been studied: (¥3×¥3)R30°-Sn/Pt(111) surface alloy [98Rod], c(2×2)-Mn/Cu(100) surface alloy [98Lu] and Pd/Rh(111) bimetallic surface formed by deposition of a Pd adlayer on Rh(111) [99Rod]. XPS studies of SO2 on the (¥3×¥3)R30º-Sn/Pt(111) surface alloy revealed that this surface was much less reactive towards SO2 decomposition than either pure, metallic Sn or the clean Pt(111) surface [98Rod]. On this alloy, SO2 adsorbed molecularly at 100 K and most of the adsorbed SO2 desorbed molecularly between 250 and 300 K. Only a small amount of decomposition occurred during TPD to form S and O adatoms. A pure Sn film and the clean Pt(111) surface decomposed SO2 to form many sulfur-containing species, such as SO42−, SO3−, SO2 and S on the surface. The large reduction in the reactivity of Sn in the alloy surface was explained by a combination of ensemble and electronic effects, but it was proposed that electronic effects were the main contributors for the decreased reactivity of Pt in the alloy. This was suggested to arise from a reduction in the electron donor ability of Pt atoms due to rehybridization. High-resolution XPS indicated that adsorption of SO2 on the c(2×2)-Mn/Cu(100) surface alloy at room temperature was dissociative, oxidized alloyed Mn atoms to Mn2+, and formed adsorbed O and S adatoms on the surface [98Lu]. High exposures also lead to the formation of SO3− and SO42− complexes on the surface. For the Pd/Rh(111) bimetallic surface, the Pd1.0/Rh(111) surface was less reactive than either pure Pd or Rh(111) [99Rod].
3.8.4.5 OCS 3.8.4.5.1 Structure and bonding of OCS OCS is isoelectronic with CO2, and like CO2, it is a linear triatomic molecule in which the central C atom is sp-hybridized and bonded by double bonds to both outer atoms that each have sp2-hybridization. However, the presence of O and S atoms breaks the symmetry inherent in CO2 so that OCS has a dipole moment and unequal bonding in the C=O and C=S bonds. The coordination chemistry literature reveals that OCS is much more reactive than CO2, almost exclusively cleaving the C=S bond in dissociation reactions to yield a CO product.
Scheme VII. Lewis structure for OCS.
3.8.4.5.2 OCS adsorption on metal surfaces There are only a few reports about the interaction of OCS with metal surfaces. OCS adsorption was explored on evaporated Ni and W films over the temperature range of 195-450 K [85Sal]. OCS was Landolt-Börnstein New Series III/42A5
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reported to adsorb molecularly at 195 K on both Ni and W films, but to dissociate at higher temperatures into CO and S adatoms. However, a later study on polycrystalline Ni foil using XPS, UPS and AES concluded that chemisorption of OCS was dissociative at all temperatures in the range 77-293 K, yielding adsorbed S and gaseous CO [88Sas]. Tables 17-19 provide the available data on OCS adsorption on metals. The thermal and photochemical behavior of adsorbed OCS on Ag(111) was investigated [90Zho]. OCS TPD curves after adsorption of OCS on Ag(111) at 128 K are presented in Fig. 39. OCS adsorbed completely reversibly and only desorption of OCS was found in TPD for all exposures. AES spectra recorded after desorption gave supporting evidence and showed no detectable O, C or S left on the surface. The absolute coverage of OCS on Ag(111) was determined by XPS to be 5.3 ± 0.5 × 1014 molecules/cm2. The UPS spectrum shown in Fig. 40 indicates that OCS adsorbs on Ag(111) with very little distortion from its gas-phase structure. There are a few surface photochemistry studies of OCS adsorbed on Ag(111) [90Zho, 99Kid, 00Kid] and on non-metal surfaces such as LiF(001) [90Leg, 90Dix, 90Pol3], NaCl(001) [97Pic] and GaAs(100) [97Hua]. However, not much is known in this work about the structure and bonding of chemisorbed OCS species or thermal reaction pathways and energetics on these surfaces. The structure and dynamics of an OCS monolayer on NaCl(001) has been investigated by IRAS, helium atom scattering (HAS) and LEED [96Gle, 96Doh].
3.8.4.6 N2O 3.8.4.6.1 Structure and bonding of N2O Nitrous oxide (N2O) is a linear triatomic molecule, like CO2 and OCS, but its bonding at metal surfaces and surface chemistry is different. The Lewis structure shown in Scheme VIII which schematically shows bonding in the gas-phase molecule indicates why this is so, and also helps to explain the large differences in barriers to cleavage of the two bonds in the molecule; the N≡N bond energy is 113.7 kcal/mol, while the much weaker N–O bond has a bond energy of only 38.7 kcal/mol.
Scheme VIII. Lewis structure for N2O. Knowledge about the coordination chemistry of N2O is relatively limited, but N2O in coordination compounds behaves as a pseudohalogen, i.e., forming dimers and strong acids with hydrogen. In this respect, its surface chemistry should resemble that of the isoelectronic anion ligands such as cyanate (OCN−) and thiocyanate (SCN−).
3.8.4.6.2 Adsorption of N2O on metal surfaces On most transition metal surfaces, N2O adsorbs via the terminal N atom to form a weak, donor bond through the lone-pair electrons in the 7σ orbital. Such an assignment was made in many studies on the basis that the N-N stretching vibrational band was blue-shifted (shifted to higher energy) while the energy of the N-O stretching band was nearly unchanged from the respective gas-phase values. Metals can be divided into two groups based on their reactivity toward N2O. For example, molecular adsorption of N2O occurred on Pt(111), Ir(111) and Ag(111) surfaces [72Wes, 73Wei, 83Ave, 91Kis, 92Saw, 90Cor, 90Gri, 96Sch1], which are less reactive and located in the lower right-hand side of the periodic table, and dissociative adsorption of N2O occurred on Ni(100), Ni(110), Ru(0001) and Al(100), desorbing N2 and leaving O on the surface [81Umb, 82Kim, 83Mad, 96Hua, 81Sau, 87Hof, 89Pas]. While charge transfer from the substrate to N2O obviously affects the chemisorption bond strength and reactivity, several papers have suggested that the reactivity of metal surfaces can be qualitatively Landolt-Börnstein New Series III/42A5
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determined by the work function (φ ) of the substrate: surfaces are more reactive if they have φ < 5.4 eV and are less reactive if φ > 5.4 eV. Ag(111) is an exception to such a simple idea, where φ = 4.74 eV predicts a high reactivity of the surface, but only molecular adsorption and desorption was found. Table 20 presents thermodynamic data for N2O adsorption and dissociation parameters are given in Table 21.
3.8.4.6.2.1 Molecular adsorption of N2O Although N2O is often considered an effective oxidant under high-pressure conditions, kinetic limitations often inhibit this reaction under UHV conditions. Thus, N2O was not observed to dissociate under UHV conditions on Pt(111) [73Wei, 83Ave, 91Kis, 92Saw], Pt(100) [72Wes], Ir(111) [90Cor] and Ag(111) [96Sch1]. Only molecular desorption of N2O was seen in TPD. As a useful reference point, N2O desorbed at 30 K from condensed N2O multilayers on Ag(111) [90Gri]. Several studies have addressed the adsorption of N2O on Pt(111) [83Ave, 91Kis, 92Saw]. N2O from the multilayer and monolayer phases on Pt(111) desorbs at very low temperatures of 86 and 90-100 K, respectively [83Ave]. Leading edge analysis of the zero order desorption peak from the monolayer gave an initial value for Ed = 5.6 kcal/mol that increased to 6.05 kcal/mol at saturation coverage. This indicates a small adsorption energy and weak bonding interactions between N2O and Pt. A similar shift of the N2O desorption peak to higher temperatures with increasing coverage was found in later work, along with a (3×3) LEED pattern for the N2O monolayer on Pt(111) [92Saw]. Opposite behavior of the N2O TPD peak with coverage on Pt(111) was observed too [91Kis], and a similar shift of the N2O desorption peak to lower temperatures with increasing coverage was reported on Ir(111) [90Cor]. No evidence of decomposition was found in HREELS. On a Pt(100) surface, N2O was reported to decompose at temperatures greater than 1000 K [72Wes]. HREELS was used to determine the nature and geometry of N2O adsorption in a monolayer and multilayer on Pt(111) at 78 K, as shown in Fig. 41 [83Ave]. A blue shift of the νN-N stretching mode by 80 cm−1 was used to conclude that N2O bonded to the surface via the terminal N atom. This blue shift arises from kinematic coupling between the molecule and a massive metal atom. This may seem surprising, because in adsorbates such as CO, νCO is red-shifted upon adsorption. For CO, this kinematic blue shift is overshadowed by the reduction in the CO bond strength due to back donation into the CO 2π* orbital. However, since the corresponding unfilled orbital in the N2O molecule, i.e., the 3π*, is so strongly antibonding, no back donation occurred. N2O is a soft Lewis acid and can bond to the surface by donating charge from either (or both) of the filled non-bonding 7σ or bonding 2π molecular orbitals. Donation from the 7σ orbital is favored because it is located on the terminal N atom, while the 2π orbital is evenly distributed between the terminal N and O atoms, which would imply some oxygen bonding. Because all of the molecular modes, ¦+(νN-N, νN-O) and Π(δNNO), have large intensities in specular scattering in HREELS spectra of chemisorbed N2O, configurations with the molecular axis parallel or perpendicular to the surface can be ruled out. This reflects that the molecule must be inclined at the surface. This angle was estimated to be ~35° based on a comparison of the measured, screened intensities of the ¦+ and Π modes in the adsorbate relative to their gas phase IR intensities. Furthermore, it was assumed that the N-N-O axis of the N2O molecule remained linear in the adsorbed species because neither the 7σ or 2π molecular orbital has central N character and so the weak donor bond formed to the surface was unlikely to affect the N-N or N-O bond strength. Two models were proposed for the structure of N2O on Pt(111), as shown in Fig. 42, with configuration (b) in Fig. 42 preferred because the more tightly bound 7σ orbital should lead to a linear configuration. Additional evidence that N2O bonds via the 7σ orbital on the terminal N atom in the molecule comes from He II UPS spectra for N2O on Pt(111) at 50 K. The strong attenuation of the 7σ orbital intensity in spectra from submonolayer and monolayer coverages indicated that the 7σ orbital of N2O is more strongly coupled to the surface than other N2O valence orbitals.
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The strong intensity of the δNNO bending peak at 590 cm−1 in HREELS spectra from an N2O multilayer, as shown in the bottom trace of Fig. 41, indicated that the physisorbed N2O in the multilayer has its molecular axis oriented parallel to the surface. In a XPS and SIMS study on Ag(111), N2O was reported to thermally decompose to form coadsorbed NO3−, NO and O, which were stable up to 426 K [90Gri]. A reinvestigation of the same system later by TPD and AES found no N2O dissociation [96Sch1]. These authors proposed that the previous results of N2O dissociation on Ag(111) may have been due to the effects of secondary electrons produced by the X-rays used in XPS. They showed that N2O in the multilayer was dissociated by 50 and 2500 eV electrons to produce mainly coadsorbed N2 and O and a small amount of unidentified NxOy species. Tables 22 and 23 provide the available data for N2O adsorption from UPS and XPS and from vibrational spectroscopy, respectively.
3.8.4.6.2.2 Dissociative adsorption of N2O N2O dissociatively adsorbs on reactive metal surfaces such as Ru(0001) [81Umb, 82Kim, 83Mad, 96Hua], Ni(110) [81Sau], Ni(100) [87Hof] and Al(100) [89Pas] to deposit O adatoms on the surface, and on W(110) [81Umb, 79Fug], Rh(100) [96Li], Rh(110) [81Dan] and Ru(10 1 0) [80Kle] to deposit both O and N adatoms. Adsorption and decomposition of N2O on Ru(0001) was studied with TPD, XPS, UPS and HREELS [81Umb, 82Kim, 83Mad, 96Hua]. N2O adsorption was partly dissociative initially on Ru(0001) at 100 K, but molecular adsorption predominated with increasing exposures. Upon heating, some adsorbed N2O dissociated to form N2(g) and adsorbed O, and some N2O desorbed molecularly in TPD peaks at 120 and 160 K [82Kim, 83Mad]. In another study, three desorption peaks were identified at 116-123, 145 and 160-165 K [96Hua]. HREELS, UPS and ARUPS studies were performed to determine the N2O bonding mode on Ru(0001) [81Umb, 82Kim, 83Mad, 96Hua]. HREELS spectra indicated N2O coordination to the surface through the terminal N atom in the molecule because there was a blue shift of the νN-N band from 2224 to 2290 cm−1 and no change in the νN-O band, similar to results on Pt(111). At low coverage, very weak intensity of the δNNO peak at 470 cm−1 indicated that N2O was terminally bonded in a nearly vertical position [83Mad]. At higher coverage, an inclined linear configuration dominated, as observed at all coverages on Pt(111). UPS and ARUPS gave a consistent picture for N2O bonding on Ru(0001) via the 7σ orbital localized on the terminal N atom in N2O [81Umb, 82Kim]. A physisorbed N2O state oriented parallel to the surface was reported to coexist with vertically chemisorbed N2O in the same adlayer at high coverages [81Umb]. In HREELS data at high coverages, the intensity of the bending mode at 560 cm−1 increased, indicating increased tilting of the N2O molecular axis away from the surface normal or an additional flat-lying state. Only adsorbed O was found from decomposition of N2O on Ru(0001), but both O and N adatoms were observed from decomposition of N2O on the more open Ru(10 1 0) surface [80Kle]. On Cu single crystals, the more open Cu(110) surface [84Spi] was much more active for N2O dissociation than Cu(100) or Cu(111) surfaces. No adsorption or dissociation of N2O was observed on Cu(100) or Cu(111) over the temperature range of 90 to 300 K. On Cu(110) at 90 K, adsorption of N2O was dissociative at low coverages to produce O adatoms on the surface. Molecular adsorption of N2O was observed after an initial coverage of 0.25 ML. ELS of N2O on Cu(100) showed a 9.5 eV peak associated with the π-π* transition. Because the dipole moment associated with this transition is oriented parallel to the molecular axis, it was suggested that adsorbed N2O was oriented nearly perpendicular to the surface. On most of the transition metal single crystal surfaces discussed above, N2O decomposes to give N2 and leave O adatoms on the surface. On W(110), both O and N adatoms were found following N2O adsorption and dissociation, as investigated using XPS, UPS and XAES at 100 and 300 K [81Umb, 79Fug]. N2O adsorption on W(110) at 300 K was completely dissociative, while on W(110) at 100 K, initial dissociation of N2O was followed by condensed layers of N2O. Heating of the condensed N2O layers lead to further dissociation as well as desorption of molecular N2O. XPS spectra of adsorbed N2O on W(110) at 100 K [79Fug] are shown in Fig. 43. At low coverage, only one N(1s) peak at 396.8 eV BE and one O(1s) peak at 530.3 eV BE appeared from N2O dissociation, Landolt-Börnstein New Series III/42A5
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3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
[Ref. p. 235
corresponding to N and O adatoms, respectively. With increasing coverages, two new N(1s) peaks at 401.8 and 405.9 eV BE were observed with a splitting (separation) close to that in N2O gas. This molecularly adsorbed N2O can be readily desorbed by heating to 100 K and can be recondensed on top of the dissociated adlayer produced by heating to 160 K. Valence band photoemission was used to determine the orientation of adsorbed N2O molecules coadsorbed with O and N adatoms from N2O dissociation on W(110) [81Umb], as shown in Fig. 44. Spectra taken at polar angles of 83° and 62° with respect to the surface normal were similar, but spectra taken at 41° showed decreased intensities of the two σ orbitals relative to the π orbitals. Because σ states couple strongly with E parallel to the molecular axis, it was suggested that N2O was lying flat, i.e. oriented parallel to the surface.
3.8.4.6.3 N2O adsorption on alloy surfaces Adsorption of N2O on a Cu/Ru(0001) bimetallic surface obtained by depositing Cu on a Ru(0001) substrate was investigated by TPD and AES [81Shi]. A small amount of Cu (<0.02 ML) deposited on Ru(0001) lowered the dissociative sticking probability Sdiss of N2O by 40%, but only slightly decreased the monolayer saturation coverage. Two possible origins of the large drop in Sdiss were suggested [81Shi]. First, Cu could decrease the precursor-state lifetime. Because adsorption of N2O on Ru(100) is controlled by precursor kinetics, if Cu atoms at the Ru(100) surface promoted the desorption of N2O from this molecular precursor state, this would decrease the precursor lifetime and decrease the dissociation probability. The second possibility was that a restrictive geometric or energetic requirement existed for N2O dissociation. It was suggested that a configuration of N2O with the molecular axis parallel to the surface was required for dissociation. If there was only a weak interaction of N2O on Cu, as predicted from the very small sticking coefficient (10−5) of N2O on Cu(111), then an N2O configuration with the molecular axis parallel to the surface would not occur easily on a Cu-covered Ru(0001) surface. However, no data exists at present to test these molecular level details. In studies of N2O on a Ag/Rh(100) bimetallic surface obtained by depositing Ag on a Rh(100) substrate, the effect of Ag on N2O chemisorption was similar to that found for Cu in the Cu/Ru(0001) system [81Dan]. A simple site-blocking model was used to adequately describe the influence of Ag on N2O chemisorption on Rh(100).
3.8.4.7 O3 3.8.4.7.1 Structure and bonding of O3 Ozone is a polar triatomic molecule that is isoelectronic with NO2−. It has equal O-O bond distances of 1.28 Å and is bent with an angle of 116° [00Oya]. It has a singlet ground state (no unpaired electrons), with partial double-bond character between each of the oxygen atoms. Scheme IX shows a localized electron picture of the bonding in O3 along with the resonance structures that are needed. A valence bond view would more clearly illustrate that there are four electrons in the π orbitals of O3. A final state effect that arises because of the stability of O2 creates a very small bond dissociation energy in O3 with D(O2-O) = 25.5 kcal/mol. This causes ozone to be a very reactive oxidant.
Scheme IX. Lewis structures for O3.
Landolt-Börnstein New Series III/42A5
Ref. p. 235]
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
191
3.8.4.7.2 O3 adsorption on metal surfaces No study has yet reported on the molecular chemisorption of O3 on a well-defined metal surface. The facile dissociation of this molecule, even on a Au(111) surface at 90 K [98Sal], is responsible for this situation, but low-temperature adsorption experiments in the future may be able to trap the molecular precursor to dissociation. Studies of ozone adsorption on metals that have been reported find dissociative adsorption. Indeed, the high reactivity of ozone compared to O2 [78Ele, 80Leg, 84Pir, 84Can, 86Sau, 82Nor] or even NO2, has been utilized to produce high concentrations of oxygen adatoms on metal surfaces and even to oxidize the surface under mild, UHV conditions. An Ag2O film up to 20 µm thick can be created by exposing O3 to a polycrystalline silver film [01Wat]. In addition, while a number of studies on Au surfaces determined that no dissociative adsorption of O2 occurred under UHV conditions [78Ele, 80Leg, 84Pir, 84Can], O3 can be used as a clean source (with no H2O or OH contamination) of atomic oxygen on the Au(111) surface [98Sal]. TPD spectra for O2 desorption after O3 exposures on Au(111) at 300 K are presented in Fig. 45. Similar results were obtained on Au(111) at 100 K, no lower temperature O2 or O3 desorption was observed. The saturation coverage of oxygen on the surface under these conditions corresponds to θ O = 1.2 ML. Such experiments give information on the O-Au bond strength and temperatures required for oxygen desorption from the surface. The O2 desorption activation energy was determined to be 23 kcal/mol, increasing rapidly at low coverages up to 30 kcal/mol near saturation coverage. Similarly, surface oxygen concentrations of up to θ O = 2.4 ML were obtained by exposing O3 on Pt(111) [99Sal1]. For θ O 1 ML, the O2 TPD peak is due to desorption from chemisorbed oxygen adatoms and shifts to lower temperature with increasing θ O due to lateral repulsions in the adlayer. For θ O 1 ML, O2 desorption is from decomposition of oxidic species in a sharp and narrow TPD peak and the peak temperature shifts higher with increasing θ O. It may be useful to comment on some related studies of ozone condensation. O3 deposition on a gold cube substrate at various temperatures utilizing direct deposition or using helium as a carrier gas have been reported [00Cha1]. Ozone forms an amorphous phase for deposition below 11 K, a crystalline phase above 55 K and a mixed phase over the range 11 - 55 K. IRAS was employed to study these phases of solid ozone. The νas (ν3) stretching and δ (ν2) bending modes are often used to characterize ozone phases. Fig. 46 shows the νas and δ regions in IRAS spectra of ozone deposited on the substrate under various condition. The spectrum of O3 deposited at 11 K showed a broad νas band at 1037 cm−1. This band became broader with deposition at 20 and 40 K, which indicated mixed phases. For deposition at 55 K, the νas band developed into a very narrow line at 1026.9 cm–1 independent of deposition methods. Ozone desorbed (sublimed) at higher temperatures between 61 and 68 K, with Ed = 23 ± 2 kJ mol–1. The available thermodynamic data for O3 adsorption is given in Table 24. In addition, Table 25 provides a summary of the vibrational data for O3 adsorption. In addition, ozone adsorption was investigated on amorphous and crystalline phases of water (ice) [00Cha1, 01Bor]. If ozone is dosed on amorphous ice at 55 K, it exists as crystalline phase. Two bands appear at 1027.8 and 1033.7 cm–1 in the region of the νas band. The band at 1027.8 cm–1 corresponds to ozone physisorbed on ice and the band at 1033.7 cm–1 to ozone chemisorbed to free OH in ice pores. Chemisorbed ozone desorbed at slightly higher temperature than physisorbed one indicating that additional activation energy is needed to break the O3-HO bond.
3.8.4.7.3 O3 adsorption on alloy surfaces Dissociative ozone adsorption was found on two alloy surfaces, the (2×2)-Sn/Pt(111) and (¥3×¥3)R30ºSn/Pt(111) surface alloys [99Sal2]. Both of these two alloys were less reactive towards O3 than clean Pt(111) at 300 K, but dissociative O3 adsorption still caused extensive oxygen uptake and oxidation of these alloys. The saturation oxygen coverage after modest O3 exposures on these surfaces at 300 K was 1.2 and 0.87 ML on the (2×2)- and (¥3×¥3)R30º-Sn/Pt(111) alloys, respectively. This was decreased from θ O = 2.4 ML on Pt(111). Dissociative ozone adsorption disorders the surface, dealloying and Landolt-Börnstein New Series III/42A5
192
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
[Ref. p. 235
extracting Sn into the adlayer. Fig. 47 shows the O2 TPD spectra obtained after O3 exposure on the (¥3×¥3)R30º-Sn/Pt(111) surface alloy at 300 K, for example. Initially, only a high temperature peak at 1000 - 1100 K occurs due to O2 desorption from SnOx decomposition. At higher oxygen coverages, a low temperature desorption peak at 700 - 834 K occurs that is similar to that on Pt(111), which is due to due to O2 desorption from PtOx decomposition.
3.8.4.7.4 O3 adsorption on metal oxide surfaces While no studies have been reported on molecular adsorption of ozone on metal surfaces, molecular adsorption of O3 has been observed on metal oxide surfaces. This work is briefly discussed here to provide some insight into the nature of ozone molecular adsorption. Ozone adsorption has been studied on CaO [97Bul], CeO2 [98Bul], MgO(poly) [02Ber], SiO2 [94Bul] and TiO2(anatase) [95Bul] powders pressed into pellets. For TiO2 (anatase) at 77 K with water present on the surface, vibrational bands of O3 measured by FTIR spectroscopy were at ν s = 1108 and ν as = 1034 cm–1 indicating that ozone was mostly weakly bound on the surface [95Bul]. Removing the water caused shifts such that ν s = 1145 (∆ν = + 27 cm–1) and ν as = 990 cm–1 (∆ν = − 44 cm–1). These relatively large band shifts indicated a large distortion of the O3 molecule adsorbed on TiO2 dehydrated at 773 K. O3 adsorbs on SiO2 through hydrogen bonding interactions with Si-OH groups at 77 K [94Bul]. In contrast to TiO2, most of vibrational modes of O3 adsorbed on SiO2 were close to those in solid O3, indicating less distortion of the adsorbed O3 molecules. While O3 adsorbed on hydrated CaO was only a weak complex, bonding via surface OH groups, FTIR studies of O3 on dehydrated CaO showed a more distorted geometry of chemisorbed O3, suggested to be ozonide ion, O3– [97Bul]. This ozonide species, along with superoxide, O2−, was also detected when O3 was adsorbed on dehydrated CeO2 at 973 K [98Bul]. These species appeared to be intermediates of ozone decomposition. FTIR spectra from O3 deposited on polycrystalline MgO showed two different types of ozone bonding geometries, attributed to two different bonding sties [02Ber]. Fig 48a shows FTIR spectra in the region of the νs and νas stretching bands. Schematic drawings of adsorbed O3 geometries are shown in Fig. 48b. Bands A1 and A3 in Fig. 48a were assigned to O3 adsorbed on 5-coordinated Mg2+ on MgO, as shown in Scheme 1 in Fig. 48b and bands B1 and B3 in Fig. 48a were assigned to O3 adsorbed on defect sites on MgO, as shown in Scheme 2 in Fig. 48b. Acknowledgments We acknowledge support by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.
3.8.4.8 Tables for 3.8.4 Organisation of tables Table 1. Thermodynamics of CO2 adsorption Table 2. Dissociation parameters for adsorbed CO2 Table 3. Vibrational data for adsorbed CO2 Table 4. Valence electronic structure for adsorbed CO2 Table 5. Electronic excitations for adsorbed CO2 Table 6. Core-level binding energies for adsorbed CO2 Table 7. Thermodynamics of NO2 adsorption Table 8. Dissociation parameters for adsorbed NO2 Table 9. Vibrational data for adsorbed NO2 Table 10. Photoelectron spectroscopic data for adsorbed NO2 Table 11. Thermodynamics of SO2 adsorption Table 12. Dissociation parameters for adsorbed SO2 Table 13. Photoelectron spectroscopic data for adsorbed SO2 Landolt-Börnstein New Series III/42A5
Ref. p. 235]
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
193
Table 14. NEXAFS data for adsorbed SO2 Table 15. K-edge SEXAFS spectroscopic data for adsorbed SO2 Table 16. Vibrational data for adsorbed SO2 Table 17. Thermodynamics of OCS adsorption Table 18. Photoelectron spectroscopic data for adsorbed OCS Table 19. Vibrational data for adsorbed OCS Table 20. Thermodynamics of N2O adsorption Table 21. Dissociation parameters for adsorbed N2O Table 22. Photoelectron spectroscopic data for adsorbed N2O Table 23. Vibrational data for adsorbed N2O Table 24. Thermodynamics of O3 adsorption Table 25. Vibrational data for adsorbed O3 Table 1. Thermodynamics of CO2 adsorption Substrate
Ts
Chemi-
Ag(111) Ag(110)
[K] 40 300 100
sorption No No No
100 100
Yes Yes
480 420
80 100 100 80 90 80 130 100
Yes No Yes No Yes (weak) No No No
<295
80 80
Yes Yes
0.25 0.25
100 130
Yes Yes
Satn.
80 80 80 77
O/Ag(110) θ O= 0.1 θ O= 0.25 Al foil Al(100) Na/Al(100) Bi(0001) Cu film Cu(110) Cu(100) Cs/Cu(110) θ Cs= 1 K/Cu(110) θ K= 0.5 θ K= 0.75 O/Cu(110) O/Cu(211) Fe(poly) Fe(111)
Fe(110) Fe(100) K/Fe(100) θ K= 0.83 Mg(0001) Ni(110)
Landolt-Börnstein New Series III/42A5
Coverage
Tdes
Ed
[K]
[kJ/mol]
Technique
Ref.
TPD TPD TPD
91Sol1, 87Sak 83Bac 82Stu
TPD TPD
91Sol1, 82Stu 83Bac
XPS TPD TPD XPS, HREELS XPS TPD TPD XPS, HREELS
87Car 91Sol1 91Sol1 91Bro 98Poh 89Rod 96Kra 91Bro
500-600 370, 500
TPD TPD
94Car, 01God 89Rod
<175 <160
HREELS UPS, XPS
95Ons 96Kra
Yes Yes (weak) Yes (weak) Yes (weak)
<295 <130 <110 <160
XPS, HREELS XPS XPS, UPS ARUPS. HREELS
100 77 110 100
Yes (weak) No Yes Yes
<130
88 135 150 80 300
Yes Yes (weak) No Yes (weak) No
0.25
84-102 113
285 <200
1
162 590
0.25
<130 <220 <230
40
94Car 88Cop 87Pir 86Beh, 87Fre, 87Beh, 87Bau HREELS 95Hes ARUPS. HREELS 86Beh, 87Beh, 87Bau TPD 93Nas TPD 89Pau XPS, HREELS HREELS NEXAFS, XPS ARUPS, HREELS TPD
91Sol1, 86Cam 87Lin 88Ill 87Bar1 73McC
194
Substrate Ni(100) O/Ni(110) θ O= 0.3 Pd(100) Pd(111) K/Pd(100) θ K = 0.05 θ K = 0.10 θ K = 0.26 θ K = 0.42 Mn-Pd(100) O/Mn-Pd(100) Na/Pd(100) θ Na= 1 ML Na/Pd(111) θ Na = 0.25 θ Na = 0.25 θ Na = 0.25 Pt(111) K/Pt(111) θ K= 0.5 Re(0001) Rh(111)
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
Ts
Chemi-
[K] 150 80
sorption Yes (weak) Yes (weak)
100 130 85 100
No
110 110
No Yes
130
Yes
No Yes
Yes (weak) 85 90 85 112
Coverage
Tdes
Ed
[K] 250 <300
[kJ/mol]
Monolayer 135 200 Monolayer 135 0.03 185 215 432 556
Technique
[Ref. p. 235
Ref.
TPD 79Ben ARUPS, HREELS 87Bar1 34
TPD
34
ARUPS TPD
86Ber 86Ega 89Wam, 90Ehr 86Ber
UPS UPS
99San 99San
TPD
86Ega
ARUPS HREELS HREELS
89Wam 90Ehr 89Woh
TPD TPD
89Liu, 91Liu 89Liu, 91Liu
TPD TPD TPD TPD TPD
87Pel 78Cas,79Dub, 80Dub 85Sol 94Sol 84Sol
170 Monolayer 380, 550, 650 Monolayer <125 <120 <120
No
112
Yes
80 300 110 90 300
Yes Yes (weak) Yes (weak) Yes (weak) Yes
0.5 130 Monolayer 490 244 250 502
90 90
Yes Yes
Monolayer 724 Monolayer 740
TPD TPD
87Sol 94Sol
85
Yes
1
100
TPD
94Hof
85
Yes
1
434, 710
TPD
94Hof
CeO2
323
Yes
413, 713
TPD
97Luo
Cr2O3(0001)
90
Yes
180, 330
TPD
99Sei
B/Rh(111) K/Rh(111) θK = 1 θ K = 0.36 Ru(0001) K/Ru(0001) θ K= 0.33 ML
674 35.5 42.7
Landolt-Börnstein New Series III/42A5
Ref. p. 235]
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
Table 2. Dissociation parameters for adsorbed CO2 Substrate
Ts [K]
Fe(111)
77 100 110 85 85 88 100
Fe(100) K/Fe(110) Cs/Fe(110) Mg(0001) K/Pd(100) θ K= 0.26 θ K= 0.42
Coverage
Tdis [K]
Reference
140 150 300 140 140 100
87Fre 95Hes 93Nas 94Mey2 98Sey 86Cam 86Ber
573 663 153
90Ehr
Na/Pd(111) θ Na= 0.25
85
K/Pt(111) θ K =0.16 θ K =1 Re(0001)
112
Rh(111)
300
B/Rh(111) θ B=0.36 K/Rh(111) θ K=0.36
110
500
84Sol
90
130
87Sol, 94Sol
Landolt-Börnstein New Series III/42A5
80
7 7 × R10.9°-Na; θ CO2=0.5 3 3
Monolayer
Sdiss =0.85 at θ ≤0.1 Sdiss =0.35 at θ =1 (2×2) structure Sdiss = 10−15 at 10−6 torr and 300 K Sdiss = 10−11 at 1 atm. and 300 K
89Liu 200 640 135
91Rod
83Wei
195
Table 3. Vibrational data for adsorbed CO2 Substrate Ts [K] Coverage
Vibrational frequencies [cm−1]
ν (M-O) δ (OCO) CO2 (gas) Ag(111) O/Ag(110) θ O=0.1
θ O=0.25 K/Ag(111) θ K=0.26 Cs/Ag(111) θ Cs=0.13 Na/Al(foil) Bi(0001) Cu(100) K/Cu(110) θ K=0.4 Cs/Cu(110) Fe(111)
Landolt-Börnstein New Series III/42A5
Fe(100) K/Fe(100) θ K=0.83 Mg(0001)
40 50
Satn.
270 660 260 653
νs (OCO)
νa (OCO)
672
1351
2396
670 645
1320 1307
2360 2380
850, 1050 830, 1050 830, 1050 847, 1057
1280 1360 1360 1355
100 200 100 300
Satn. (2×1) Satn. (4×1)
50
Satn.
760
1260
1600
50 100 80 160
Satn.
770
1220 1310 1360
1600 1480
107 173 233 110 298 100 100 150 110 300 100 200 131
Satn.
460 340
362 295 350 Satn. 403 403
Satn.
Adsorption site / Configuration
670 800 627 1045 1045 660 1060 645 766 766
linear
bent 2350
bent bent
2360 1500
1375, 1440 1512 1512 1460 1510 1250 1170 1072, 1170 1232 1246
650 800, 950 1200 850, 1090 1390
1704 1704 1660 1660 1370 1370 1634 1639 1450 1630
2356 2050
2340
linear bent linear bent bent
2258 2325
linear bent bent
Landolt-Börnstein New Series III/42A5
Substrate
Ni(110)
H/Ni(110) θ H=0.1 O/Ni(110) Na/Pd(111) θ Na=0.25 Pt(111) K/Pt(111) θ K=0.15 θ K=0.5 K/Rh(111) θ K=0.09 θ K=0.36 θ K=1.0 Re(0001) O/Re(0001) Ru(001) K/Ru(001) θ K=0.33 Cr2O3(0001) NiO(111)
Ts [K]
135 140 200 270 90 90 200 145 220 100
Satn.
112
Satn.
85 85 85 85
90 200 123
ν (M-O) δ (OCO)
νs (OCO)
387 410 405 470
1113 1130, 1390 1620 1130 1895
403 403
112 112 90
Vibrational frequencies [cm−1]
Coverage
282
430
726 670, 750 745
636 727 727 653 831 645 744
Adsorption site / Configuration
νa (OCO)
bent 2350 2015 2337
1103 1353 1274 1307 1298 1210
640, 780 660, 870
1220 1240
640 646, 840 640, 840 650 650 653 666
1490 1440 1340
1342
910
1289 1301, 1342 1263
linear bent
2355 1564 1452 1530 1855
2100
1520 1610
2350 2350
bent bent
2350 2350 2350 2350 2350 2352 2357
linear bent
2340
linear bent
Satn.
Satn. Satn. Satn. (√3×√3)R30° Satn. Satn.
1290
1630 1630 1600 1650
2375
linear linear linear
linear bent bent
198
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
[Ref. p. 235
Table 4. Valence electronic structure for adsorbed CO2 1 Substrate
Ts [K]
Valence levels [eV] Structure Coverage/ adsorbed state 1πg 1πu / 3σu 4σg
CO2 (gas) CO2 (solid) Cu(poly) Cs/Cu(110) θ Cs=1.0 H/Cs/Cu(110) θ Cs=1.0 K/Cu(110) θ K=0.5 θ K=0.75 θ K=1.1 Fe(poly) Fe(111) Cs/Fe(110) θ Cs=0.13 K/Fe(110) θ K=0.06 θ K=0.30 Ni(110) O/Ni(110) θ O=0.3 Na/Pd(111) θ Na=0.25 θ Na=2 Pt(poly) Rh(111) K/Rh(111) θ K=0.33 K/Rh(111) θ K=0.1 θ K=0.3
Reference
13.8 17.6 / 18.1 19.4 13.0 16.7 / 17.6 18.8
UPS UPS
87Bar1 87Bar1, 84Foc
77 160
Satn. Satn.
7.4 9.2
11.6 13.4
12.9 15.0
UPS UPS
76Nor 01God
160
Satn.
8.9
13.1
14.7
UPS
01God
8.5 9.0 9.0
12.6 13.3 11.5
14.2 14.8 18.0
UPS UPS UPS
95Ons 96Kra 95Ons
7.9 5.8 7.3
12.1 9.2 11.2
13.6 11.0 12.8
UPS
87Pir
UPS
8.8
12.6 / 13.2 14.8
UPS
86Beh, 87Fre, 87Beh 98Sey
8.8 9.3 7.3 7.0 6.8 7.1
13.0 13.5 11.1 / 11.7 10.9 / 11.4 10.2 / 10.6 9.0/10.7
14.5 15.0 13.0 12.8 12.4 12.4
UPS UPS ARUPS ARUPS ARUPS
94Mey1 98Sey 87Bar1
ARUPS
89Wam
7.8 10.1 6.9 6.6 5.2
11.6 14.1 10.9 10.7 8.7
13.5 15.8 12.5 12.1 10.9
UPS UPS UPS
75Nor, 76Nor 91Sol1, 88Kis 91Sol1, 88Kis
UPS
88Kis
8.0 8.8
12.0 13.0
13.6 14.5
7.6
12.2
14.5
UPS
98Och
Satn. 107 130 107 80 110 77
Satn.
85 85
Satn.
80 293 85 253 85
Submono. Monolayer Monolayer Monolayer
77 110 90 bent 90
Ca/Si(111) 300 θ Ca=thick film 1
Technique
Satn.
linear
Monolayer
87Bar1
Electron binding energies for adsorbed layer are referenced to the Fermi level, with EF = 0 eV BE.
Landolt-Börnstein New Series III/42A5
Ref. p. 235]
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
199
Table 5. Electronic excitations for adsorbed CO2 Substrate Fe(111) Fe(100)
Ts [K] 77 110 220 350 100 100
Pd(100) K/Pd(100) θ K=0.21 θ K=0.42 Rh(111) 100 B/Rh(111) 300 K/Rh(111) 90
Coverage Satn. Satn.
Satn. Satn.
Energy loss peaks [eV] Configuration 6.0 8.5 12.0 15.3 8.1 11.9 3.2 8.7 11.3 3.2 7.9 11.6 4.0 6.6 14.7 22.3 via O, ⊥ surface bent, || surface 3.7 11.8 15.6 19.3 24.6 3.7 11.9 15.4 18.8 24.7
Satn. Satn.
9.4 8.1
14.0 13.0 11.9 15.0
linear, || surface
Ts [K]
Coverage
170 80 90
Multilayer bent
CO2 (gas) O/Ag(110) Bi(0001) Cu(film)
Cs/Cu(110) 110 θ Cs=0.48 K/Cu(110) 139 θ K=0.4 139 O/Cu(211) 130 Fe(poly) 80 Fe(100)
110
Fe(111)
77
Cs/Fe(110) θ Cs=0.11 θ Cs=0.30 K/Fe(110) θ K=0.08 θ K=0.34 Al(poly) Mg(0001) Ni(110)
85
120 88 150
O/Ni(110)
85
85
Satn.
Reference Core level Adsorption configuration energies [eV] C 1s O 1s 297.5 541.1 72All
linear linear
287.7 291.5 289.8 291.8
529.9 535.0 532.6 535.4
linear
292.5
536.3
bent bent
293.2 286.4 293.0 291.5 286.0
291.5 286.0
531.0 531.0 533.0 534.8 531.0 534.9 531.1 534.8 531.0
291.9 292.3
535.6 535.9
292.3 292.3 291.3 292.0 291.2 286.4 283.0
535.6 535.9 531.8 533.0 534.7 531.1 531.2
Satn.
Satn. Satn. Satn.
linear bent Monolayer linear bent linear bent Monolayer
Landolt-Börnstein New Series III/42A5
Satn.
83Bar 91Bro 91Sol1, 98Poh 76Nor 89Rod
98Llo 96Kra 88Cop 88Ill, 87Pir 93Nas 87Bau 98Sey
Monolayer
98Sey linear linear linear linear bent
86Sol 86Sol
85Sol, 94Sol 84Sol bent, bidentate (C,O) 91Sol1, 94Sol
Table 6. Core-level binding energies for adsorbed CO2 1 Substrate
Reference 87Beh 93Nas
87Car 86Cam 88Ill 91Sol1
200
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces Ts [K]
[Ref. p. 235
Reference Core level Adsorption configuration energies [eV] C 1s O 1s Pt(poly) 110 0.5 ML linear 291.1 534.5 74Nor 80 linear 291.1 534.5 76Nor Rh(111) 90 Satn. 91Sol1, 88Kis linear 292.0 534.7 linear 292.0 534.0 91Sol1, 88Kis K/Rh(111) 90 bent 290.5 532.8 θ K=0.33 Re(0001) 120 Multilayer 530.5 91Rod Ca/Si(111) 300 Monolayer bent 532.0 98Och 1 Electron binding energies for adsorbed layer are referenced to the Fermi level, with EF = 0 eV BE. Substrate
Coverage
Table 7. Thermodynamics of NO2 adsorption Dissociative Coverage Substrate Ts [K] adsorption Ag(111) 300 Yes Satn. 500 Yes Ag(110) 298 Yes Satn. 95 No 140 No O/Ag(110) θ O= 0.5 ML Au(111) 100 No 0.4 35 No 1.0 85 No ∼1 Pd(111) 530 Yes < 1.0 110 No 1.0 Pt(111) 300 Yes 120 No 100 No 400 Yes Pt(100) 200 Yes O/Pt(111) 300 Yes θ O= 0.25 100 No Satn. θ O= 0.75 300 Yes θ O= 1.0 120 Sn/Pt(111) 120 Yes 1 (√3×√3)R30° Rh(111) 300 Yes 0.5 Ru(0001) 80 No 1 400 Yes 600 Yes 800 Yes 600 Yes Ag/Ru(0001) 600 Yes θ Ag=10 ML Zn/Ru(0001) 80 Yes Monolayer θ Zn=1 ML HOPG 90 No 1
Technique
Reference
TPD TPD TPD TPD TPD
90Pol1 95Bar 95Bar 87Out 87Out
TPD TPD TPD TPD TPD TPD TPD TPD TPD
89Bar 93Bec 98Wan1 90Ban 91Wic 82Seg 82Dah 87Bar2 89Par 85Sch
TPD TPD TPD TPD TPD
82Seg 88Bar 90Par 82Dah 04Vos
TPD TPD TPD TPD TPD TPD TPD
95Pet, 99Jir 86Sch1, 86Sch2 91Mal 91Mal, 92Mal 91Mal, 95Hrb 96Mit 94Rod
245
TPD
93Rod
150
TPD
90Sjö
Ed Tdes [kJ/mol] [K]
75.3
280
58.5
230 230 220
58
79.5
320 285
155
37.7
38.5
140
Landolt-Börnstein New Series III/42A5
Ref. p. 235]
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
201
Table 8. Dissociation parameters for adsorbed NO2 Substrate Ag(111) Ag(110) Mo(110) Pd(111) Pt(111) Pt(100) Rh(111) Ru(0001) Zn/Ru(0001) θ Zn=1 ML Sn/Pt(111) (√3×√3)R30°
Ts [K] 300 95 100 110 120 100 200 110 80 80
Monolayer Monolayer
Tdis [K] 300 280 300 180 240 285 290 150 140 80
120
(4×1)
400
Coverage/ Adsorbed state Satn. Monolayer 0.5 ML-(2×2) Monolayer 0.5 (Satn.) (5×20)
Ediss [kJ/mol]
79.5
Reference 90Pol1 87Out 00Jir 91Wic 82Dah 87Bar2 85Sch 99Jir1 86Sch1 93Rod 04Vos
Table 9. Vibrational data for adsorbed NO2 Substrate Ts Coverage Vibrational frequency [cm−1] [K] ν δ ν ν
Technique
Reference
NO2 (gas phase)
M-mol
Ag(111) Ag(110) Au(111) Pd(111) Pt(111)
86 95 100 85 110 230 100 100 240 170
O/Pt(111) θ O=0.75 Sn/Pt(111) 120 (√3×√3)R30° 300 Ru(0001) 80 280 O/Ru(0001) 80 θ O=0.5 600 θ O=1.0 300 HOPG GaAs(100) O/GaAs(100) θ O=thick film
Landolt-Börnstein New Series III/42A5
90 110 110
0.4 1.0
270 250
600 465 Low coverage 300 High coverage 460 High coverage Monolayer 460 Multilayer 460 1.0 280
(ONO)
sym
asym
750
1318
1610
FTIR
65Stl
780 795 800 805 790
1291 1280 1180 1178 1170
1742 1390
IRAS HREELS HREELS IRAS HREELS
95Bro1 87Out 89Bar 98Wan1 91Wic
1180 1180 1190 1270 1270 1273 1278 1300
HREELS
87Bar2
HREELS
88Bar
HREELS
04Vos
HREELS
86Sch1
1545 1575 1550 1560
1.0
440 440
780 780 795 795 795 780 588 780 580
1.0 Multilayer Satn.
420 530 280
810 820 675
1220
1580 1795
HREELS
86Sch2 96Mit 97He
782 782 850
1290 1255 1670
1760 1760 2235
HREELS IRAS IRAS
90Sjö 90vom 90vom
1.0
1750 1792 1648 1600 1600
202
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
[Ref. p. 235
Table 10. Photoelectron spectroscopic data for adsorbed NO2 1 Substrate Ts Valence levels [eV] Coverage [K] 2πg 4σg 1πg 3σg
N (1s)
O (1s)
NO2 (gas)
11.2
Ag (111)
Mo(110) Ni (poly) Pd(111) O/Pt(111) θ O=0.25 θ O=0.75 Rh(111) O/Rh(111) θ O=0.6 Ru(0001) O/Ru(0001) θ O=0.4 θ O=0.5 Zn/Ru(0001) θ Zn=1 ML W(110) 1
300 25 145 80 100 77 530 400
Saturation Saturation Multilayer Saturation Saturation Multilayer 1.0 Saturation
3.2
Core levels [eV]
Technique
Ref.
14.0
17.6
19.0
412.4
541.3
UPS, XPS
76Bru
5.5
10.0
14.0
401.6 401.6 405.8 404.0 398.0 400.8
531.0 531.0 531.4
UPS, XPS
90Pol1 90Pol2
XPS
00Jir
UPS XPS
76Bru 90Ban 89Par
5.2
9.6
11.3
530.5 531.9 529.6
6.8 6.8 300 300
0.5 0.5
800
Multilayer
600 800 80
Multilayer Saturation 1.0
100 300
Saturation Saturation
402.0 401.3
532.0 531.7
7.0
5.8
99Jir1 99Jir1 UPS
10.2
15.0
403.8
529.8 529.9 530.0
398.0 397.6
530.4 530.5
95Hrb 92Mal 95Hrb 93Rod
UPS
79Fug
Electron binding energies for adsorbed layer are referenced to the Fermi level, with EF = 0 eV BE.
Table 17. Thermodynamics of OCS adsorption Substrate Ag(111)
Adsorbate Coverage/ Adsorbed state structure Molecular
Ni film
Ni(poly) W film
OCSads: associative at 195 K OCSg Æ OCSads Æ Sads+ COads Æ Sinc+ COg at ≥293 K OCSg Æ OCSads Æ Sads+ COg at 77 ~ 293K OCSads: associative at 195 K
Tdes Ed [K] [kJ/mol] 128 115 97
150
NaCl(001) (2×1)
95
GaAs(100)
25 160 391
Technique
Ref.
TPD TPD TPD
90Zho 00Kid 85Sal
UPS, XPS TPD
88Sas 85Sal
HAS, LEED 97Pic HAS 96Gle TPD 97Hua
Calculated value from Redhead analysis assuming first order desorption and 1013 s−1 for preexponential factor. 1
Landolt-Börnstein New Series III/42A5
Landolt-Börnstein New Series III/42A5
Table 11. Thermodynamics of SO2 adsorption Substrate
Ts [K]
Ag(111)
105
Adsorbate Structure
Coverage/adsorbed state Multilayer Compressed Monolayer Monolayer Multilayer α α1 α2 α3 α4
77
Tdes [K] (heating rate) 130 145 (2.1 K/s) 180 130 145 170 220 245 275 170 225 275 122 170 277 110 146 240 330 (2 K/s) 380 660 780 860
Preexponential factor [s−1] 1013
371 461 1013
Bilayer / α1 1013 Monolayer / α2 0.50(±0.05) / α3 77 Multilayer 10(16±0.5) α1 10(12.6±0.2) α4 10(12.6±0.2) Ag(100) 80 Multilayer (3×1) α1 1015 c(2¥2×3¥2)R45º α2 1013 Cs/Ag(100) 80 c(2¥2×4¥2)R45º α3 θ Cs=0.3 α4 (θCs≥1.5 ML) α5 α6 (θCs1.5 ML) α7 130 Cu(111) 80 Multilayer /α1 280 0.34 ML /α2 Ni(110) 100 Multilayer 112 Pd(100) 120 Multilayer 135 Pt(111) 110 ~3 ML 127 (5 K/s) 1013 Recombinative desorption 285 1 Calculated value from Redhead analysis assuming first order desorption and 1013 s-1 for preexponential factor. Ag(110)
130
p(1×1) c(4×2) p(1×2)
Ed [kJ/mol
41 54 61 69 41 53 64 36 41 64 42 67 861 1001 1761 2091 2311 61.7~64.6
72
Table 12. Dissociation parameters for adsorbed SO2 Substrate Ts [K]
Adsorbate Coverage/adsorbed state structure
Cu(111) Ni(110)
80 100
Pd(111)
160
Dec. on defect sites 0.5 ML dec. < 0.25 ML disordered θ =0.3 at Satn.
Pd(100)
120
Pt(111)
160 160
c(2×2)
Tdes [K] Ed [kJ/mol] Technique Reference Recombinative Recombinative desorption desorption 480 TPD 93Ahn 360 TPD, LEED 93Zeb, 95Ter, 98W
240 330-370 Molecular & dissociative ads. 240 270 465 Molecular + dissociative ads. 285 0.15 ML 275-290
59.6 55.9 59 66 116
TPD, LEED 85Köl TPD
88Bur1, 88Bur2
TPD TDMS
97Wil 83Köh
Landolt-Börnstein New Series III/42A5
Table 13. Photoelectron spectroscopic data for adsorbed SO2 1 Substrate Ts [K] Coverage Valence levels [eV] 1a2+5b2 2b1+7a1 8a1 Gas phase 12.5 13.4 16.5
Core levels [eV] S (2p3/2) O (1s) 174.8 539.6
Adsorption geometry / Adsorbed species
Ag(110)
125
Tilted by 13(±5)°, rota [001] at high θ Upright (within 2°±5°) the [001] at low θ
Au(111) Cd(poly) Cu(100)
77
Cu(111)
170 285 450
178 303
Multilayer Satn. Satn.
0.24 ML
3.8
-
7.7, 9.0
165.4
530.6
5.6
6.7 6.7
10.1 9.1
168.4
532.5
164.3, 165.3 165.3, 160.9
531.6, 530.9 SO2, ads Æ SOads+Oads+ 531.6, 530.9, 530.2 530.2 Less SOads formed than 530.2 530.2
164.1, 165.3 165.2, 166.4 160.2, 161.3
Landolt-Börnstein New Series III/42A5
Substrate
Ts [K]
Coverage 8a1
Mn/Cu(100) c(2×2)
300
Valence levels [eV] 1a2+5b2 2b1+7a1
½-satn. Multilayer
Fe(poly) FeS FeSO4 Ni(poly) Ni(110)
Pd(poly)
80
Satn.
77 100 160
Cond. 0.24 ML 0.25 ML
5.4
90 170 100
Multilayer 0.4 ML Monolayer Multilayer Multilayer
7.3 6.0
300 Pt(111)
Sn(poly)/ Pt(111)
100 130 60 160 160 90 148 212 100
300
6.4
9.8
10.6 8.9
5 ML 3 ML Satn. Multilayer 1 ML 0.43 ML 0.05 ML 1.5 ML Multilayer Multilayer
4.7
5.8
Core levels [eV] S (2p3/2) O (1s) 161.5 529.7, 531.9 529.7, 531.8 161.1, 161.5, 167.3, 166.9, 166.4, 165.9 166.7 531.2 161.3 168.9 532.5 168.0 532.5 163.8, 165.0 163.4, 164.1, 531.2, 533 165.2 169.0 169.0 165.7 165.7 165.8 165.8, 168.6 162.7, 165.7, 167.0 167.6 532.3 165.3 530.1
Adsorption geometry / Adsorbed species SO2,adsÆ 2Oads + Sads ( SO2,ads+2Oads Æ SO4,ad SO2,ads+Oads Æ SO3,ads
171 166.4, 167.5, 164.0, 164.8 164,164.8,166 164.8, 166.0 161.6, 164.7 + add’l 166.2 166.2,168.8 162.2, 166.9, 165.8
η2-SO2 (S,O)
SO2 adsorbed molecula No molecular adsorptio 160 K, formed SO3, S, SO2 adsorbed molecula 0.03 ML Sads SO2 adsorbed molecula 100 K, no dissociation SO2,adsÆ 2Oads + Sads o SO2,ads+2OadsÆSO4,ads Non-dissoc. at 100 K Dissoc. at 300 K
9.0 8.3 532.4 530.8 530.8 530.5 ~531 533 530, 531.6
Tilted by 31±10° 2SO2,adsÆ SOg + SO3,a
SO2,ads Æ SO4,ads+Sads+ annealed to 300 K
Substrate
Ts [K]
Coverage 8a1
Sn/Pt(111) (¥3×¥3)R30°
Rh(111)
100
150 350 100 300
Pd/Rh(111) θ Pd=0.5 Ru(0001)
100
Zn(poly)
300 100
Half satn.
100
300
300 1
0.1 ML 2.5 ML Multilayer Monolayer Satn. Submono Multilayer Multilayer Submono Multilayer Multilayer
300
Valence levels [eV] 1a2+5b2 2b1+7a1
162.5, 165.8 165.8, 168.2 162.5, 165.5, 166.0, 167.0 161.8, 162.4, 164.9, 165.4 168.2 161.8 , 162.4 + add’l 166 165.5 , 166.9 166.9 166.4 , 169.2 161.5, 163.5 162.5, 165.8, 166.8, 167.7
Satn. 1/4 satn. Satn. 1/5 ML Monolayer Multilayer 1/3 ML Multilayer Multilayer
Core levels [eV] S (2p3/2) O (1s) 164.9, 165.7 + add’l 168.0 168 533 165.7 530.5 162.2 530 162, 165.5 165.5, 168.5 162, 165.5, 166.3
7.9
530.8 530.8, 533 531.5, 533.4
Adsorption geometry / Adsorbed species SO2 adsorbed molecula 100 K, little dissociatio No SO3 or SO4 formed
SO2 adsorbed molecula 100 K, little dissociatio SO2,adsÆ 2Oads + Sads o & SO4,ads at 300 K SO2 adsorbed molecula 100 K, little dissociatio SO2,adsÆ 2Oads + Sads o & SO4,ads at 300 K SO2 adsorbed molecula 100 K, little dissociatio
SO2,gÆ Sads+Oads at 30 SO3, SO4 formed at h SO2,adsÆ SOg+SO3,ads
~532
9.9
Electron binding energies for adsorbed layer are referenced to the Fermi level, with EF = 0 eV BE.
SO2,gÆ SOg+Oads & Sa
Landolt-Börnstein New Series III/42A5
Landolt-Börnstein New Series III/42A5
Table 14. NEXAFS data for adsorbed SO2 Substrate
Ts [K]
Coverage
Ag(110)
100
Multilayer
185
Mono L (~0.3 ML)
Cu(111)
170
0.24 ML
Cu(100)
300
0.15 ML
100 180
Cond. 0.25 ML
92
Cond. 0.35 ML
Ni(111)
Ni(100)
150
0.5 ML
92 170
Cond. 0.41 ML
92 170
Multilayer ~0.4 ML
X-ray incident angle [°]
Azimuth of substrate
O K-edge Position/Area a1*, b2* b1* 530.7/5.0 535.8/1.6
b1*
S K-edge Position/Area a1*, b2*
90
[110]
531.9/3.6
535.2/0.7
2473.6/1.0
2477.6
90 20
[001]
531.9/0.6 531.9/0.4
535.2/3.9 534.3/4.1
2473.6/0.05 2473.6/0.1
2477.6 2477.6
531.9/0.6 532.5 -
534.3/4.0 536
2473.6/0.01 2473.5/1.03
2477.6 2478/0.23
2473.5/0.4
2473.4 2474.2/1.03 2474.2/0.1
2478/0.4 2477.4 2477.4 2479.3 2478.6 2470.0/0.43 2470.0/1.0
2473.2 2473.3/1.03 2473.3/0.4 2473.3/0.03 2473/1.03 2473/0.4 2473/0.03 2473.2 2473.3/1.03 2473.3/0.4 2473.3/0.03
2478.4 2477/0.13 2477/0.5 2477/1.0 2477/0.13 2477/0.6 2477/1.0 2478.4 2476.7/0.13 2476.7/0.5 2476.7/1.0
20 90 20 30 90 55 15 90 20 5
[110] [001]
531.5 532.0/1.0
536.5 535.6/0.6
535.6/0.3
535.6/1.0
90 55 15
90 55 15 55 90 55 15
530.7 530.7/1.0 530.7/0.4 530.7/0.1
535.9 534.9/0.2 534.9/0.5 534.9/0.5
Ads Ads
For surf &σ For surf &σ [001
C2 a (wit
3SO 2SO SO3 SO2 C2 ( SO2 at 2
Flat
Flat S&
Flat
Flat
Substrate
Ts [K]
Coverage
Ni(110)
92 170
Cond. 0.24 ML
90 160
Multilayer 0.25 ML
Pd(100)
170
0.35 ML
Pt(111)
160-270 148
1 ML
212
0.43 ML
X-ray incident angle [°]
Azimuth of substrate
O K-edge Position/Area a1*, b2* b1*
90 55 15 <110> 90 20
90 55 15 90 90 10 90 10
530.8 531.6 530.8 531.6
σ denotes the molecular S-O-S plane that contains all three atoms. Trigonal pyramid structure is labeled by t.p. 3 Calculation based on figures contained in cited references. 1 2
535.3 536.0 535.3 536.0
S K-edge Position/Area b1* a1*, b2* 2473.2 2478.4 2473.3/1.03 ~2477/0.23 2473.3/0.4 ~2477/0.7 2473.3/0.03 ~2477/1.0 2473.2 2473.2/0.013 2473.2/1.0
2478.4 2477.5/2.03 2477.5/0.4
2473.5/1.03 2473.5/0.4 2473.5/0.1 2473.7
2479 2478 2478 2478.2
Ads Ads
SO2 orie SO2 long site SO2 SO2 Two flatin sh brid C2 a
Tilt Tilt
Landolt-Börnstein New Series III/42A5
Landolt-Börnstein New Series III/42A5
Table 15. K-edge SEXAFS spectroscopic data for adsorbed SO2 Substrate Ts [K] Coverage/ X-ray Azimuth Adsorbate incident of state angle [º] substrate Gas phase Cu(100) 100 Multilayer 300 180
Cu(111) Ni(111) Ni(100) Ni(110)
Pd(100)
0.15 ML
90, 15 90 20
280 170
0.24 ML
90, 40, 20
170 170 170 160
0.35 ML 0.41 ML 0.2 ML 0.25 ML
90 90 90 90 20 90 20
170
0.35 ML
1.1 1.1 1.1 <100> <100> <110> <110> 1.0
Table 16. Vibrational data for adsorbed SO2 Substrate Ts Coverage/ Adsorbed [K] state ν (M-SO2) Gas
100 100 100
Multilayer Monolayer Monolayer
S-O distance [Å] 1.43 1.43±0.02 1.48±0.03 1.49±0.03 1.48 1.42 1.43±0.03 1.43±0.03 1.48±0.03 1.48±0.03 1.51±0.03 1.49±0.03 1.50±0.05
M-S [Å]
M-O [Å]
2.30±0.02 2.3
2.0
2.34±0.05 2.33±0.05
2.15±0.05 2.14±0.05
2.16±0.05 2.18±0.05 2.20±0.03 2.30±0.04 2.28±0.04 2.31±0.04 2.29±0.04
1.48
Vibrational frequencies [cm−1] ν (M-O) δ (SO2) νs (SO) 517.7 1151.4
Solid Ag(110)
FT peak of S-O bond [Å] 1.03 1.03 1.0 1.0
Elong of bo [Å] 0.05 0.06 0.06
0.05 0.05 0.08 0.06
0.05
νas (SO) 1361.8
Adsorption site/ Configuration
521
1147
1308, 1330
1145 1005 985
1320
Monolayer: on-top o fold bridging site/ til
-
C2 axis ⊥ surface
200
-
530
210
360
505, 660
Ts [K] O/Ag(110) 98 169 θ O=0.25 241 418 500 815 Cu(100) 100 300 Pd(100) 115
Coverage/ Adsorbed state Multilayer Monolayer /SO3 /SO3 /SO2(g)+SO4(ad)+Osub /SO4 Satn. Satn. Multilayer Monolayer Submonolayer
Pt(111)
Multilayer Monolayer
Substrate
110 190
ν (M-SO2) 220
Vibrational frequencies [cm−1] ν (M-O) δ (SO2) νs (SO) 535 1145 565, 670 1010
1
1 ML at 100 K
290 296
266
6.0
Adsorption site/ Configuration
Unidentate O-bonde Bidentate O-bonded Disproportionated Bidentate O-bonded 346 340
430
515 482, 613 540 535 520
978 859 1155 1147 1040
1290 1355 1275 1245
533 540
1140 940
1333 1252
Table 18. Photoelectron spectroscopic data for adsorbed OCS 1 Substrate Coverage Valence levels [eV] 3π 2π 9σ 8σ Gas phase 11.17 15.07 16.04 17.96 Ag(111)
νas (SO) 1315 -
10.6
13.3
C (1s)
Core levels [eV] O (1s) S (2p3/2)
O-bonded SO3 in py S-bonded on 4-fold binding sites C2 axis ⊥ surface wi plane tilting
η2-SO2 (S,O) with ti O axis
∆φ [eV]
Tech
UPS 288.5
533.7
164.6
Electron binding energies for adsorbed layer are referenced to the Fermi level, with EF = 0 eV BE.
−0.6
XPS
Landolt-Börnstein New Series III/42A5
Ref. p. 235]
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
Table 19. Vibrational data for adsorbed OCS Substrate Ts [K] Vibrational Frequencies [cm−1]
Gas phase In Ar liquid
84.4
NaCl(001)
77
Technique
Reference
δ (OCS)
ν (CS)
ν (CO)
520 519.77
859 858.93
2062 2052.50
IR IR
77Shi 91Zit
2058.3 2065.2 2070.1 2071.3 2074.2 2076.3
s FTIR m sh s sh s
96Doh
211
Table 20. Thermodynamics of N2O adsorption Substrate Ts [K] Coverage/ adsorbed state Ag(111) 83 Multilayer 0.44 ML / Monolayer 45 Multilayer Monolayer Cu(111) 90 No ads. & dec. at 100 K Cu(100) 90 No ads. & dec. at 100 K Ir(111) 84 No dissoc. Pt(111)
50
Ru(0001) 75
Multilayer 0.44 ML - (3×3) Multilayer
Tdes [K] 86 (1 K/s) 94-102 78-83 83-86
Ed [kJ/mol] 1 Technique TPD
93 (1 K/s) 102 86 (0.8 K/s) 21.9 90-100 23.4-25.3 95
IRAS
Reference 96Sch1 95Bro2
UPS, ELS, XPS 84Spi UPS, ELS, XPS 84Spi TPD 90Cor TPD TPD
83Ave, 91Kis, 92Saw 96Hua
Calculated value from Redhead analysis assuming first order desorption and 1013 s−1 for preexponential factor. 1
Landolt-Börnstein New Series III/42A5
Table 21. Dissociation parameters for adsorbed N2O Substrate Ts [K] Coverage/ Adsorbed state Al(100) 80 0.11 (±0.03) ML Cu(110) 90 θ > 0.25 ML: N2O ads θ < 0.25 ML: Dec. Ni(110) 323-873 Ni(100) 170-500 Rh(111) 200-573 Inert to N2O Ads. & dec. on defects Rh(110) 200-573 Dec. Rh(100) 530-680 Rh(100) + 0.5 530-680 Ag block sites for N2O ML Ag dissociation Ru(0001) 75 Multilayer Molecular and dissociative ads.
Landolt-Börnstein New Series III/42A5
Ru( 10 1 0 ) Ru(0001) + 0.14 ML Cu
150 300-900
dec. at 150 K
W(110)
100
Si(100)
90
Condensed layer at high expos. Dec. at low expos. Multilayer α-N2O β-N2O 1.3 ML
60
Tdis [K]
Ed [kJ/mol]
400
26.1±1.5
Products by dissociation N2(g) + Oads N2(g) + Oads N2(g) + Oads N2(g) + Oads N2,ads + Oads N2,ads + Oads
95 116-123 145 160-165 150
Initial reaction probability, P 0.0016 at 80 K ~0.7 at 100K
N2(g) + Oads
0.55 at 198 K 0.48 at 530 K ~ 0 at 530 K 0.46
15
N2,ads + Oads
Decreased initial reaction probability by 75% at 520 K N2,ads + Oads
N2(g) + Oads 110 150 80
26.4 36.4 20
N2(g) + Oads
Landolt-Börnstein New Series III/42A5
Table 22. Photoelectron spectroscopic data for adsorbed N2O 1 Substrate Ts [K] Coverage/ Adsorbed ∆φ [eV] Valence levels [eV] state 2π 7σ 1π Gas phase Ag(111) 32
6σ
N(1s)
O(1s)
20.1
Multilayer 1 ML
408.5, 412.5 402.5, 406.3 402.3, 406.3
541.2 535.1 535.1
403.5, 407.1
532.0
12.9
Al(100)
80
Satn.
Au(111)
77
condensed
Cu(111) Cu(100) Cu(110)
80 90
0.08 ML Monolayer
Ni film
77 300 50
Condensed
Pt(111)
Ru(0001) 100
W(110) Si(100) 1
100 60
Core levels [eV]
2 ML 1 ML 0.15 ML γ, θ low, Flat lying α, θ high, Flat lying v, θ high, Vertical
0.27 at 0.5 L N2O
−0.8 at 3 ML
16.4
18.2
6.1
9.5
11.3
13.3
401.6, 405.6
534.5
6
9.5
11.5
13.3
402, 406
535
402, 406
534.9
6.2
9.6
11.6
13.5
6.3
9.7
11.5
13.4
402.0, 406.0
6.7 5.9 ~5.9 6.3 5.2 6.0
10.0 9.3
11.8 11.0 ~11.0 11.6 10.6 11.0
13.6 12.8 ~12.8 13.6 12.6 13.2
401.4, 405.6 401.0, 405.0 401.6, 405.0 401.6, 405.6 400.5, 404.5 400.9, 403.6
534.6, 53 530.5 534.1 533.9 ~533.9 534.3 533.2 533.5
9.9 8.9 7.7
θ low
399-409 (broad)
~534
θ high
401.8, 405.7
534.2
Flat lying at θ low At saturation 1.2 ML
400.6, 404.4 401.8, 405.9
533.6 534.3
403, 407
536
6.1
9.6
11.4
13.4
Electron binding energies for adsorbed layer are referenced to the Fermi level, with EF = 0 eV BE.
Table 23. Vibrational data for adsorbed N2O Coverage/ Adsorbed Substrate Ts [K] state 14
Gas phase
14
Vibrational frequencies [cm−1]
ν (M-N2O)
16
N N O N15N16O 15 15 16 N N O 14
Solid phase 65 [Ru(NH3)5N2O]Br2 298 Ag(111)
67
Cu(110)
85
Ir(111) Pt(111)
84 78
Ru(0001)
Si(100)
75
90
δ (NNO) 2δ (NNO) ν (NO)
ν (NN)
588.7 585.3 571.9
1284.9 1269.9 1265.3
2223.7 2201.6 2154.7
589
1293 1157
2235 2236
1296 1279 1277 1273, 1308 1308 1310 1280 1300 1300
2223, 2249 258 2227 2228 2225, 2267 2250 2249 2320 2230 259 2310
1290
2290
Linear NNO Multilayer Monolayer Submonolayer Multilayer Monolayer Submonolayer Multilayer Multilayer: || to surf. Monolayer: end-on, inclined 35° to surf.
Landolt-Börnstein New Series III/42A5
θ high =coexisted with horizontal physisorbed θ low =vertical chemisorbed Multilayer α-N2O: unclear β-N2O: bonded via t-N
330 325
570 590 575
230
540~560
1180
2224
470 589 363
1282 1411
2242 2323 1637
2ν (
231
257
Landolt-Börnstein New Series III/42A5
Table 24: Thermodynamics of O3 adsorption Substrate Au cube H2O: Ice Au(111) Pt(111) Sn/Pt(111) (2×2) Sn/Pt(111) (¥3×¥3)R30°
Ts [K] 11 50 50
Coverage/ Adsorbed state Amorphous Crystalline
300 300 300
decomposed to give 1.2 ML O decomposed to give 2.4 ML O decomposed to give 1.2-ML O
TPD, UPS, LEED TPD, LEED AES, TPD
300
decomposed to give 0.87-ML O
AES, TPD
Table 25: Vibrational data for adsorbed O3 Substrate Ts [K] Adsorbed state
Gas phase O3 (Solid)
Tdes [K] 61 - 68
Ed [kJ/mol] 23 ±2
Technique FTIR
70 - 90
20 ±3
FTIR, TPD
Vibrational frequencies [cm−1] δ (O3) ν as νs
Ar matrix
4 5 20
Kr matrix Ne matrix
16 30 5
700.9 w 704 703.9 703.6, 704.2 701 701.66 699.63
N2 matrix Xe matrix
17 35
704.2 699.1
1042.1 s 1050 1037 1039.7, 1041.2 1038 1036.05 1039.87, 1038.7 1042.8 1043.14
Au plated Cube
11 50
703.5 707.6
1037.1 1026.9
Amorphous Crystalline
1103.1 vw 1120 1108.8 1105.1
1106.1 1107.1
1104 1103.2 1104.4 1108.3 1097.31
Techni
ν as + ν s
3ν as
2110.7 2110 2108.1
3046.1 3060 3033.6
2103.7 2109.7, 2107.6 2117.1 2090.3, 2091.1 2110.1 2108.6
3041.5, 3041.0 3051.4
3033.9
IR IR IR FTIR
Raman FTIR FTIR FTIR FTIR
Substrate
CaO powder
Ts [K]
77
CeO2 powder 77 H2O: amorphous 55 ice 25 45 H2O: crystalline 55 ice MgO(poly) SiO2 80 80 TiO2 (anatase) 77 1
Predicted value.
Adsorbed state
Vibrational frequencies [cm−1] δ (O3) ν as νs
Physisorbed Chemisorbed 705 Physisorbed + Chemisorbed Solid aggregates Physisorbed
18
O3 O3 on hydrated O3 on dehydrated
703 664
Techni
ν as + ν s
1035 1034.5 1035 1027.8 1033.7 1029 1036 1027.8
1106 1109 1104
1024, 1038 1037 978.8 1034 990
1105, 1140 11041 2106.4 1992.1 1044.71 1108 1145
3ν as
2106 2113
FTIR FTIR IRAS FTIR IRAS
3027 2867
FTIR FTIR FTIR
Landolt-Börnstein New Series III/42A5
Ref. p. 235]
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
217
3.8.4.9 Figures for 3.8.4 δθ-
CO ad +O ad
CO2ad
CO − oxidation
Fe (111) / CO2 5L p - pol., hw:36 eV T = 140 K
CO 2ad
CO2 - dissociation
q = 0° q = 20°, f = 90°
O-C-O
q-
O
CO2ad
GMe - CO2
O
C
O O C
δθ-
6a1 1a2 4b2 (a’) (a’’) (a’) (2πu)
CO ad +O ad
CO2ad
5a1 1b1 3b2 (a’) (a’) (a’’) 1π g
2
EF
O
C
1200
1200
1000
1000
800 600 400 77 K
4 6 8 10 12 Binding energy E Bin [eV]
14
16
Fig. 2. Assignment of the photoelectron spectra at 85 K (lower curve) and 140 K (normal emission θ = 0°, and off normal emission θ = 20°, φ = 90°) (upper curve) to – undisturbed molecular CO2 and adsorbed CO2 based on ab initio calculations [87Fre].
Work function change ∆ f [mV ]
Work function change ∆ f [mV ]
3su 4sg CO2
Fe (111) / CO2 5L p - pol., q = 0, T = 85 K hw:36 eV
Fig. 1. Schematic two-dimensional potential energy diagram for metal-CO2 interaction (vertical axis) and CO-O-dissociation (horizontal axis). [87Bar1]
0
4a1 (a’)
O
GCO - O
140 K
1π u
O
Reaction barrier
200
O
C
O
O C
CO2
77 K
800 140 K
600
77 K
77 K
400 200 0
0
1.0
a
2.0 3.0 Exposure [L]
4.0
0
1.0 2.0 Exposure [L]
0
b
1.0
2.0 3.0 Exposure [L]
4.0
0
1.0 2.0 Exposure [L]
Fig. 3 Change in work function due to adsorption of CO2 on Fe(111) (a) and stepped Fe(110) (b) at 140 and 77 K [86Beh].
Landolt-Börnstein New Series III/42A5
218
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
[Ref. p. 235
Fe (111) + 5L CO2
×200
T = 370 K ×250
Fe (111) /CO2
T = 100 K
T = 300 K
-7
×100
×350
pCO2 = 10 mbar
T = 240 K
Intensity [a.u.]
×400
T = 210 K ×400 ×300
T = 150 K
5 meV -9
pCO2 = 10 mbar
5 meV
×350
T = 100 K 50
0
a
100 150 200 Loss Energy [meV]
250
300
50
0
b
100 150 200 Loss Energy [meV]
250
300
Fig. 4 (a) HREELS spectra taken from Fe(111) under permanent CO2 pressure (10−9 mbar, lower spectrum), (10–7 mbar, upper spectrum) and 100 K surface temperature. The shaded areas denote loss peaks that grew with increased pressure. (b) Series of HREELS spectra taken from Fe(111) under permanent CO2 pressure of 10–9 mbar and different temperatures as indicated. The shaded areas correspond to loss peaks of the CO2 dissociation products of CO, O and C [95Hes].
Fe (111) /CO2
δ-
CO2 − possible coordination sites
T = 110 K -7
×4500
pCO2 = 10 mbar
C
Ni
Ni
Ni Ni
Ni
O
O O
O
C
Ni
Ni
C2v
off specular j = 20°
Intensity [a.u.]
C O
O
A1/A’
O
C
Ni
Ni Ni
O Ni
Ni
O
C
O
Ni
Ni
Ni
C2v
Cs A1/A’
B2/A’
C2v /C s
×1500
vibrations j = 6°
Symmetric stretch ns
×300
5 meV
in specular
50
100 150 200 Energy - loss [meV]
250
nb
Asymmetric stretch nas
Fig. 6. Schematic representation of the stretching and bending vibrations in bent CO2− for the three types of CO2− coordination. The irreducible representations of the vibrations are given for the sites of C2v and Cs symmetries [96Fre].
j = 0°
0
Bending
300
Fig. 5 Series of HREELS spectra taken from – Fe(111) under permanent CO2 pressure of 10 7 mbar and different off-specular angles as indicated. [95Hes].
Landolt-Börnstein New Series III/42A5
Ref. p. 235]
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces qE = 90°
219
qE = 20°
110 T [K]
100
180
Diff. 530
540
550
560 570 530 Photon energy [eV ]
540
550
560
570
π*C-O
π*C-O
b → E [001]
c
→ E
Intensity [a.u.]
a → E [110]
a
→ E [110]
b
→ E [001]
c
→ E [110]
d
→ E [001]
→ E [110]
d
→ E [001] 280
290 300 310 Photon energy [eV ]
320
→ E
σ *C-O
→ E
σ *C-O
Intensity [a.u.]
Fig. 7. NEXAFS spectra at the oxygen edge. The spectrum at 180 K has been subtracted from that at 100 K. θ E = 90° (left panel), θ E = 20° (right panel) [88Ill].
520
530 540 550 Photon energy [eV ]
→ E
560
Fig. 8. Carbon (left) and oxygen (right) K-edge NEXAFS spectra for surface carbonate on Ag(110) at room temperature are shown as a function of polar and azimuthal orientations. The π* resonance is predominate at glancing incidence (θ = 20°) and the σ* resonance is predominate at normal incidence indicating that the C-O bonds of the carbonate species lie in the plane parallel to the surface [88Mad].
Landolt-Börnstein New Series III/42A5
220
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces z
hw A
e
q
_
z
hw:36 eV q = 0° p - pol.
f y
hw A
q
e
[Ref. p. 235
_
f y
x
x
2L CO2 T = 293 K 0.3L O2, 2L CO2 T = 253 K 0.3L O2, 2L CO2 T = 165 K
?
0.4L O2+ 1L CO2 0.4L O2
0.3L O2, 2L CO2 T = 135 K 0.3L O2, 2L CO2 T = 114 K
2L CO2
0.3L O2, 2L CO2 T = 85 K
Ni (110) / CO2 ,O hw:36 eV; q = 0°; p - pol.
Ni (110) / CO2 , O T = 85 K 2
EF
a
4
6 8 10 12 14 Binding energy E Bin [eV]
16
EF
2
b
4
6 8 10 12 14 Binding energy E Bin [eV]
Fig. 9. (a) Comparison of a pure Ni(110)/CO2 adsorbate with a CO2 adsorbate on an oxygen precovered surface. For reference the spectrum of the oxygen covered surface is shown. (b) Photoelectron spectra recorded at normal emission of the CO2, O/Ni(110) coadsorbate as a function of surface temperature. For comparison, the spectrum of a dissociated CO2/Ni(110) adsorbate is shown at the top [87Bar1]. 1L CO2+0.1L H2 /Ni (110)
HREELS Ep = 5.0 eV specular
5L CO2 / Ni (110) XPS
a T = 200 K
T = 360 K
b T = 180 K
T = 300 K 295
Relative intensity
4.5° off-specular
285 290 E Bin [eV]
T = 200 K
T = 170 K
T = 88 K
T = 150 K
120 K 52
T = 125 K
180 K T = 100 K T = 90 K 0
1000
2000 3000 4000 -1 Loss energy [cm ]
320K 295
290 Binding energy E Bin [eV]
285
Fig. 11. Highresolution C (1s) XPS spectra of 5 L CO2 + as a H2/Ni(110) function of temperature. The inset com-pares formate species formed (a) (in the top spectrum) out of formic acid by heating to 200 K with formic acid as solvent, and (b) after reaction of CO2 and H2 at 180 K solvated with readsorbed CO2 [91Wam].
Fig. 10. HREELS spectra obtained after heating a Ni(110) surface that had been dosed with 1 L CO2 + 0.1 L H2. The spectrum at T = 200 K was also measured for a 4.5° offspecular scattering geo-metry [91Wam]. Landolt-Börnstein New Series III/42A5
Ref. p. 235]
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
221
700
qK »0.05 600
Temp. of CO3 decomp.[K]
a
Intensity [a.u.]
CO2
n-
2.0L 0.7L 0.4L 0.2L 0L
500 400 300 200
b qK »0.31
100 0
0.8L 0.4L 0.2L 0L 2
0
(i) 1510 1342
O
O
806
C
C
IR absorption
O
1625
1345 1133 1240
2000
a
1500 1000 -1 Wavenumbers [cm ]
C
C O
M O
A II
M O
1308
O
AI
(iii) 1625
O
765
×0.01
1%
2-
M O
(ii) 1708
0.4
← Fig. 12. HeI UPS spectra of a K-covered Fe(110) surface at 85 K for (a) θ K § 0.05 and (b) θ K § 0.31 after subsequent exposures to CO2 as indicated. In (a) vertical lines denote positions of molecular orbital levels of different adsorbed species: ( · __ · __ · __ ): CO2; (········· ): CO; (___ ___ ___) : CO3n–; (· · í · · í · ·): species A (see text); (_________): Oox [94Mey1].
×2 1716
0.2 0.3 Potassium coverage qK
Fig. 13. Decomposition temperature of the CO3n– species as a function of potassium coverage. The lower symbols correspond to the temperature just before, the upper symbols to the temperature after complete decomposition ((¨): HeI UPS; (¸): HeII UPS; (Ÿ): XPS) [94Mey2].
2.2L 1.2L
20 18 16 14 12 10 8 6 4 Binding energy E Bin [eV]
0.1
C O
766
O
O
O
O
C
O C
C
O
M
M
A III
A IV
b
Fig. 14. (a) Vibrational spectra of oxalate species: (i) (¥3×¥3)R30°-K-Ru(0001); (ii) K-multilayer; (iii) bulk K2C2O4. (b) Schematic representation of oxalate structures. [94Hof]
Landolt-Börnstein New Series III/42A5
222
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces ×2
[Ref. p. 235
(i)
1467 (ii) 1040
762
IR absorption
1316 1610 1414
O
O O
884
O
O
C
C
C O
(iii) 1623 1490
1%
O
1071 884 764 1121 1228
2000
a
M C III
C II O
2-
O C
O
K+
K+
O
1447 1500 1000 -1 Wavenumbers [cm ]
O
M
CI
K+
O
_
C O
O
_
K+
CV
C IV
b
12
Intensity [a.u.]
500K
300K
170K
8
O + NO + NO2 at 250K
0.8 total NO2 assuming S=1 NO2 chemisorbed 0.6 at 100K 0.4
4 0.2
0 0 110K 296
292 288 284 Binding energy E Bin [eV]
280
1.0
Coverage [ ML]
NO2 reversibly adsorbed NO2 multilayer O atoms NO
14
C 1s Al K α 1487 eV
1 LO2 + 20 L CO2 @ 110K
-2
O2 + CO2 /Pd (100)-Mn-c (2×2)
Integrated thermal desorption [10 cm ]
Fig. 15. (a) Vibrational spectra of carbonate species: (i) (¥3×¥3)R30°-K-Ru(0001); (ii) K-bilayer; (iii) K-multilayer. (b) Schematic representation of carbonate structures. [94Hof]
1.0
2.0 3.0 NO2 exposure [L]
4.0
0 5.0
Fig. 17. NO2 uptake curve showing adsorption kinetics on Pt(111) at 100 K as a function of NO2 exposure. Surface species concentrations were calculated by integration of TPD spectra [87Bar2].
Fig. 16. C (1s) XPS spectra for CO2 on a O2-pretreated Pd(100)-Mn-c(2×2) surface alloy before and after subsequent heating [99San].
Landolt-Börnstein New Series III/42A5
Ref. p. 235]
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
NO2/Pt(111)
795
265
1770 1545 2055
460
1270
×70
1300
Intensity [counts]
Intensity
2.5L 1560 1765
×300 1180
0.4 ML
N
O
O
O O
O N
O
295
O 1550
O
N
O
780
N
0.21L
0
clean 2000 1000 -1 Loss energy [cm ]
0
O
0.25 ML
0.83L
×300
O
×250
×1000
CO 484
O
1180 1560
×200
Multilayer begins
2.1L
O
NO 0.75 ML
1565 1750
1210
O N
460 ×200
300 ×300
O
q0
N2O4 Multilayer
17L
×300
NO2 /O/Pt(111)
795
T = 100 K
1290
×10
223
O
clean 1000 2000 -1 Loss energy [cm ]
Fig. 19. HREELS spectra for NO2 adsorbed with increasing amounts of preadsorbed O adatoms on Pt(111) at 170 K [88Bar].
Fig. 18. HREELS spectra for increasing NO2 exposures on Pt(111) at 100 K [87Bar2]. O
O NO2
a 1859
% Transmittance
b
1261 1745 1590 1940
O
N N Ag(111)
Ag(111) 1300
1868 1856
774
1045
NO2 O
1285
N
O
c
1590
0.2%
1878 1869
N 1738
2300
1900 1500 -1 Wavenumbers [cm ]
1265
700
Fig. 20. IRAS spectra following the adsorption of NO2 on Ag(111) at 86 K. The exposures for the spectra are (a) 0.5, (b) 1 and (c) 2 L [95Bro1].
Landolt-Börnstein New Series III/42A5
N
N
O
O
N O
O
761
1281
1100
O
Ag(111)
767 1305
O
O
774
1766
2700
O
1045
1717
N
1975
O
Fig. 21. A reaction scheme for adsorption of NO2 on Ag(111) at 86 K. Initial adsorption is dissociative and subsequent adsorption leads to formation of NO3 and N2O3. Further adsorption leads to multilayers of N2O4 [95Bro1].
224
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
[Ref. p. 235
IRAS
NO2 / Au(111)
12
Mass 46
NO2
0.05%
8
0
c 1897
Absorbance
4 Ion current [nA]
(b) heated to 185 K
6.5 L 3.3 2.2 1.1 0.5
60
1272 1182
Mass 30
806
NO
40 6.5 L 3.3 2.2 1.1 0.5
20 0 100
200 300 Temperature T [K]
800
400
Fig. 22. Desorption spectra after NO2 exposures on Au(111) at 100 K. Except for a peak at 170 K, the NO signal followed the NO2 signal with the expected cracking ratio [89Bar].
b
1ML NO2 dosed at 185 K
a
1000 1200 1400 1600 1800 -1 Wavenumbers [cm ]
2000 2200
Fig. 23. IRAS spectra obtained after (a) NO2 dosed on Au(111) at 185 K to give a pure monolayer of chelating, chemisorbed NO2, (b) NO exposure on a chelating NO2 monolayer on Au(111) at 120 K to form a pure N2O3 monolayer and (c) annealing the surface in (b) to 185 K for 30 s. All of the spectra were collected at 86 K [98Wan1].
-
+
+
-
O
(a) + 0.06 L NO at 120 K
O
+
S
-
-
+
S
-
+
-
O
+
O
8a1
3b1
Electron and SO2 bonding configuration
Observed ranges for n (SO)
6
1
1300....1225
1140.....1065
8
1
1275...1245
1125...1085
8
1
{MSO2} η − planar {MSO2} η − planar
1225....1150
{MSO2} η − pyramidal 10
1
10
1
{MSO2} η − planar
1290.........1190
1240.........1135
N
{MSO2} bridging SO2 N
1120...1045 1060...1005
1160...1100
2
{MSO2} η − SO2
{MSO2} ligand-bound
1065.....990
1220..........1115
{MSO2} η − pyramidal N
Fig. 24. Schematic drawings of the HOMO (8a1) and LUMO (3b1) for SO2 [81Rya].
1325..............1210
1300
1200
950.............850 1085.............975
1145......1060
1100
1000
−1
900 cm
Fig. 25. Diagnostic features of SO2 coordination geometries [81Rya] Landolt-Börnstein New Series III/42A5
[110]
π*( b1*)
hn
q
π*( b1*)
[110] [110]
σ*(a1* + b2* )
[001] 80 60 40 20 q°
q° 90 80 70 60 50 40
2460
2470 2480 Photon energy [eV ]
a
π * intensity [a.u.]
1.0 0.8 0.6 0.4 0.2 0
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
225
1.0 [001]
0.8
0.2
hn
q [110]
0.6
[110]
0.4
σ*(a1* + b2* ) 0
Normalized S LMM intensity [a.u.]
Normalized S LMM intensity [a.u.]
π * intensity [a.u.]
Ref. p. 235]
[110] 80 60 40 20 q°
q° 90 80 70 60 50
30
40
20
30 20
2490
2470
2480 2490 Photon energy [eV ]
b
2500
Fig. 26. Sulfur K-edge NEXAFS spectra of Ag(110) (a) after exposure to 500 L SO2 at 95 K to form a multilayer and subsequent heating to the measurement temperature of 180 K, yielding a coverage of 0.3±0.1 ML and (b) then recooling the sample to 100 K and readsorbing SO2 to give a coverage of 0.6 ±0.1 ML. The X-ray E vector is along the [110] azimuth. The inset shows the absorption-edge-step normalized Gaussian area of the π* resonance recorded in the two azimuths. The best fit to the data points is for (a) α = 88±5°, φ = 0±5° and (b) α = 77±5°, φ = 55±5°, for the [110] azimuth data in the function I ∝ (cos2θ cos2α + sin2α cos2θ ), where θ is the angle of incidence, α is the angle between the molecular plane and the surface, and φ is the angle between the molecular plane normal and the measurement azimuth [96Gut2]. [001]
[110]
a
b
Fig. 27. Models of the Ag(110) surface with the suggested bond geometries of SO2 at coverages of (a) 1/3 ML and (b) 1/2 ML. S and O atoms are shown as dark circles and are scaled to the Van der Waals radii. The gas-phase geometry of SO2 has been assumed [96Gut2].
Landolt-Börnstein New Series III/42A5
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces 127 K
QMS intensity [a.u.]
226
520
SO2 /Pd (100) 285 K
525 d(SO2) 1035 nS(SO2)
Dose at
d 300 K 1250 na(SO2)
1252
940
430
Intensity
Intensity [a.u.]
925
c 190 K
×300
1140
905
1333
a 500 1000 -1 Loss Energy [cm ]
SO2 190 K
×1000
520 d(SO2) 1040 nS(SO2)
b 130 K
533
0
SO 420 K
×330 ×1000
540
266
510 n(Pd-H) 1130 n(SO)
100 200 300 400 500 Temperature T [K] 1235 1060
620
[Ref. p. 235
1245 na(SO2)
110 K
+0.1 L SO2 115 K
2000
1500
65 ×300
×1000
514 n(Pd-H)
Fig. 28. HREELS spectra of (a) multilayer SO2 adsorbed at 110 K and (b) flashed to 130 K, (c) 190 K, and (d) dosed at 300 K. [94Sun1] → Fig. 31. HREELS spectra following an SO2 exposure of 3.0×1013 molecules⋅cm−2 on Pd(100) at 115 K. The dotted line denotes the baseline subtracted to find loss intensities [88Bur1].
clean 115 K
×100
0 1000 2000 -1 Energy - loss [cm ]
O
O O S
S
S
O
z x
O
O
y
a
b O
O S
O
c
S
S O
O
d
Fig. 29. SO2 bonding configurations on Pt(111). (a) SO2 on an atop site with its HOMO interacting with a dxz or dyz orbital, (b) SO2 in a three-fold site with its HOMO interacting with the dxz bonding combination (left) and antibonding combination (right), (c) SO2 on a two-fold site with its HOMO interacting with the dxz bonding combination (left) and antibonding combination (right) and (d) SO2 on a two-fold site with its LUMO to d x 2 antibonding combination [94Sun1]. Landolt-Börnstein New Series III/42A5
Ref. p. 235]
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
227
Multilayer SO2
95 K Multilayer SO2
XPS intensity
Monolayer SO2
XPS intensity
148 K 212 K
95 K
Monolayer SO2
149 K
Atomic S SO4
333 K 170
168
a
166 164 162 Binding energy [eV]
160
158
SO4
212 K
Atomic O
333 K
536
534 532 530 Binding energy [eV]
b
528
Fig. 30. (a) S (2p) data for an SO2 multilayer (top), two-monolayer phase (middle) formed from SO2 adsorption at 148 K and subsequent heating to 212 K, respectively, and a phase formed on heating a multilayer to 333 K (bottom). (b) O (1s) data corresponding to the S (2p) spectra of (a). For comparison, the O (1s) spectrum of Pt(111)-(2×2)-O (bottom) is also shown [97Pol]. 4 π*
4
E
* a1* b2
hn
3
2 1
Intensity [a.u.]
σ* 0
2 S-Pd 1
SO2 /Pd(100) 0
3
q = 55 deg q = 90 deg
S-O q
úF ( R )ú [a.u.]
3
q = 90° q = 55° q = 15°
π*
1
0
2 Distance R [Å]
b
34°
2
4
C2
O
1 σ* 0 2460
3
2470
a
S
SO2 /Ni(100)
2480 2490 Photon energy [eV ]
2500
O
Pd 2510
c
1.48 Å
2.24 Å
Fig. 32. (a) Sulfur K-edge NEXAFS spectra of SO2/Pd(100) (top) and SO2/Ni(100) (bottom). (b) Fourier transforms of the sulfur K-edge EXAFS functions k2χ(k) of SO2/Pd(100). (c) Schematic view of the surface structure of SO2/Pd(100) [97Ter].
Landolt-Börnstein New Series III/42A5
228
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
[Ref. p. 235
C3 O
S O
O
O O
model 2
model 1 4
peak 3 SO32- :a1 2479.3 eV
peak 2 SO32- :e* 2477.4 eV
q = 90° q = 55° q = 15°
Intensity [a.u.]
q = 15° exp. model 1 model 2
3
peak 1 S 2469.8 eV
2 úF(R)ú [a.u.]
exp.
1 0 4
q = 90° exp. model 1 model 2
3 2 SO42- :t2 2481.7 eV
SO2 : π* 2473.2 eV
O S
1 0
2465
2470
a
2475 2480 Photon energy [eV ]
2485
0
2490
1
2
b
4 3 Distance R [Å]
5
6
Fig. 33. (a) S K-edge NEXAFS spectra after 5 L SO2 on Cu(100), compared with those of some references. (b) Fourier transforms of the SEXAFS spectra at θ =15° and 90°, compared with the FEFF6-simulated curves for models 1 and 2 [97Nak1]. SO3 SOx
SO2 at.S
160 K SOx
210 K
SO3+ SO2
160 K 290 K 370 K 800 K
Intensity [a.u.]
Intensity [a.u.]
255 K 210 K 255 K at.O
290 K 370 K 800 K
c(2×2) - S
(2×1) - O
536 534 532 530 528 166 164 162 160 158 Binding energy [eV] Binding energy [eV] Fig. 34. S(2p) (left) and O(1s) (right) core-level photoemission data for Ni(110)-SO2 with the sample temperature as a parameter during continuous heating (~0.1 K/s). For comparison, corresponding spectra of reference structures are included [98Wil]. 170
168
Landolt-Börnstein New Series III/42A5
Ref. p. 235]
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces π*
σ*
229
<110> multilayer T = 90 K
σ* + e*
Intensity [a.u.]
π*
σ* + a1*
e* a * 1
SO2 + SO3 T = 160 K 20° 90° SO3 + S RT 20° 90° c(2×2) − S T = 800 K
2460
2470
a
20° 90°
Fig. 35. S K-edge NEXAFS spectra of SO2 condensed on Ni(110) (top), adsorbed at 160 K (second from top) and subsequently heated to room temperature (second from bottom) and 800 K (bottom) taken along the <110> azimuth at polar angles of 20º and 90º [98Wil].
2480 2490 2500 Photon energy [eV]
b
Fig. 36. (a) Suggested local SO2 structures with S atoms in long-bridge and short-bridge sites and atomic S in hollow sites as determined with SEXAFS. (b) Suggested local SO3− structures with S atoms in short-bridge sites and atomic S in hollow sites as determined with SEXAFS [98Wil].
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3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
SO4
SO4
anneal 350 K
SO3 SO2 +1 L SO2
PE intensity [a.u.]
Sn /S+O
PE intensity [a.u.]
S
S 2p SO2 /Ru(001) 100 K h n = 260 eV
S 2p SO2 /Ru(001) 100 K h n = 260 eV
S
+0.5 L SO2
[Ref. p. 235
Sn /S+O SO3 SO2
qS 0.25
anneal 260 K
0.30
anneal 160 K
0.33
+0.3 L SO2 +0.1 L SO2
172
multilayer 100 K
170
168 166 164 Binding energy [eV]
a
162
160
172
170
168 166 164 Binding energy [eV]
b
162
160
Fig. 37. (a) S(2p) photoelectron spectra for different amounts of SO2 adsorbed on clean Ru(0001) at 100 K (b) S(2p) photoelectron spectra taken after annealing a Ru(0001) surface saturated with SO2 from 100 to 350 K [98Jir]. α
1
SO2 / Ag(100) + Cs
1 ... 5 : SO2
Tads = 80 K, b = 2 K/s
1.5 0.5 0.3
80
a
200
0
2
3
0
5
4 -1
-2
Cs /Ag (100)
0.1
α2 400 600 Temperature T [K ]
Cs cov erage
O2
jS des
α1
Work function change ∆ f [eV]
α5
α6
qCs [ M
α3 α4
×10
L]
×1
/ Ag(100) + Cs
1
Tads = 80 K -3
800
0
b
10
20 Time t [min]
40
50
Fig. 38. (a) TPD spectra of SO2 adsorbed on clean and Cs-precovered Ag(100) at 80 K with θ Cs as a parameter. The desorption signal is magnified by a factor of 10 on the right side of the curves. SO2 was always exposed until the beginning of condensation, indicated by the saturation of ∆φ (t) (see below). (b) Change of the work function ∆φ during Cs exposure on the clean Ag(100) surface (bold curve with points) and during SO2 exposures of clean (1) and Cs-precovered (2-5) Ag(100) at 80 K. Cs precoverages for curves (2)-(5) are 0.07, 0.13, 0.38 and 1.5 ML, respectively [92Höf].
Landolt-Börnstein New Series III/42A5
Ref. p. 235]
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
231
He (II) UPS of OCS /Ag (111)
OCS /Ag(111) 128 K
e d
Intensity [a.u.]
TPD area [a.u.]
Mass 60 QMS intensity [a.u.]
f
c 0
b
80 160 Dosing time [s]
240
c
B C A
b
a 100
120
140 160 Temperature T [K ]
180
200
a
Fig. 39. TPD spectra of OCS after dosing at 100 K for (a) 15, (b) 30, (c) 50, (d) 75, (e) 100 and (f) 240 s. The inset shows the OCS TPD area versus dosing time. The temperature ramp rate was 2.5 K/s [90Zho].
21.3
17.3
13.3 9.3 Binding energy [eV]
5.3
1.3
Fig. 40. He(II) UPS spectra at 100 K of (a) clean Ag(111), (b) saturation OCS-covered Ag(111) with the clean Ag(111) spectrum subtracted and (c) gasphase OCS [90Zho].
monolayer N2O - Pt(111)
0.6
N2O - Pt(111)
0.8
78 K
78 K 590
0.2
Intensity [% elastic]
325
Intensity [% elastic]
1300
0.4
0.6
2310
575
1300
2230 2310
0.4
0.2
1180
2590
0
0 0
1000 2000 -1 Loss Energy [cm ]
0
1000
2000
3000
-1
Loss Energy [cm ]
Fig. 41. HREELS spectrum (left) of a saturated monolayer of N2O on Pt(111) at 78 K and (right) after an N2O exposure on Pt(111) at 78 K that produced a multilayer that did not quite screen the monolayer spectrum. The beam energy was 4.1 eV and angle of incidence (to the surface normal) ~60° [83Ave].
Landolt-Börnstein New Series III/42A5
232
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces O
O
O 1s
N 1s
N
N
N
N
Pt
1000 c/s
1000 c/s
Pt
a
Intensity
Run in 5×10 Pa N2O
b
~25 Ex N2O
→ Fig. 43. N(1s) and O(1s) XPS spectra from W(110) at 100 K after the pretreatments with N2O given in the figure (the temporal sequence runs from bottom to top) [79Fug].
~4.5 Ex N2O clean 405
6σ
2π
1π
-5
heated to 160 K
Fig. 42. Bond configurations of inclined N2O on Pt(111) in (a) bent configuration and (b) linear configuration [83Ave].
Sat
[Ref. p. 235
400
He II
395 Binding energy [eV ]
O(a) / Au(111) O2 TPD
7σ
535
530
550 K
W (110) [qrel (N2O) = 1]
62°
83°
Ru (001)
QMS intensity at 32 amu
Intensity
41°
qO (ML) 1.2 1.1 0.9 0.83 0.74 0.50 0.22 0.10 0.06 0.03
O +N O D ad 2 (qrel = 1) D O +N O ad 2 (qrel = 0.45)
520 K
Oad 16
4 12 8 Binding energy [eV ]
0
Fig. 44. He(II) UPS spectra of molecular N2O coadsorbed with dissociated O and N adatoms on W(110) (upper part) and readsorbed on an O-predosed Ru(0001) surface (lower part). The upper spectra are direct curves taken at saturation coverage and at different polar angles as indicated. The lower part contains difference spectra (labeled D) at one-half and full coverages obtained by subtraction of the direct spectrum after oxygen exposure [81Umb].
300
400
500 600 Temperature T [K]
700
Fig. 45. O2 TPD curves following increasing exposures of O3 on Au(111) at 300 K. The oxygen coverages indicated were determined from the ratio of the integrated area under the peaks to that of the largest curve for which θ O = 1.2 ML [98Sal].
Landolt-Börnstein New Series III/42A5
Ref. p. 235]
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
233
0.6 n2
n3 0.100
0.5
0.4
0.3
c 0.2
Absorbance units
Absorbance units
d 0.075
d c b
b 0.1
a
0.050
a 0 1100
1075
1050 1025 Wavenumber [cm -1 ]
1000
740
730
720 710 700 Wavenumber [cm -1 ]
690
680
Fig. 46. IRAS spectra in the νas (ν3) stretching and δ (ν2) bending regions of condensed ozone on a gold cube. Spectra were taken after (a) Ozone deposition at 11 K, (b) O3/Ar deposition at the ratio of O3:Ar = 1:50 at 20 K and annealing to 40 K to evaporate Ar, (c) O3/He deposition at 30 K at the ratio of O3:He = 1:20 and (d) ozone deposition at 55 K [00Cha1]. O3 / (Ö3×Ö3) R30° Sn/Pt (111) Texp = 300 K 834
32 amu intensity (O2 )
760 730
O2 TPD qO (ML) 0.87 0.83 0.73 0.5 0.15 0.1 0.04 0.002 1078 1002
Fig. 47. O2 TPD curves following O3 exposures on the (√3×√3)R30°-Sn/Pt(111) alloy at 300 K [99Sal2]. 400
600
Landolt-Börnstein New Series III/42A5
800 1000 Temperature T [K]
1200
234
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
[Ref. p. 235
A3
Absorbance
0.2
O
0.1
O
O
O
O
O
O
O
O
2-
2+ 2 -
2+ 2 -
2+ 2 -
O Mg O Mg O Mg O Scheme 1
B1
A1
B3
O O Mg
0 1150
a
1100 1050 -1 Wavenumber [cm ]
2+
O
O O Mg
2+
Mg
1000
b
O 2+
Scheme 2
Fig. 48. (a) FTIR spectra of ozone adsorbed at 77 K on polycrystalline MgO. Spectra were taken between PO3= 5 Torr (dashed line, top) and PO3= 10−3 Torr (dotted line, bottom). (b) Schematic drawings of ozone adsorbed on MgO [02Ber].
Landolt-Börnstein New Series III/42A5
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces
235
3.8.4.10 References for 3.8.4 54Pol 57Eis 58Har 64Yam 65StL 66Her 66Sid 68Mac 70Tur 71Roo 72All 72And 72Bot 72Goo 72Las 72Shi 72Wes 73McC 73Wei 74Ang 74Dia 74Nor 74Rob 75Bru 75Nor 76Bru 76Nor 76Sin 77Are 77Bot 77Sex 77Shi 78Cas 78Ele 78Fur 78Min 79Ben 79Cas 79Dub 79Fug 79Joh 79Kub 80Cam 80Del 80Dub
Polo, S.R., Wilson, M.K.: J. Chem. Phys. 22 (1954) 900. Eischens, R.P., Pliskin , W.A.: Adv. Catal. 12 (1957) 662. Harvey, K.B., Bass, A.M.: J. Mol. Spec. 2 (1958) 405. Yamada, H., Person, W.B.: J. Chem. Phys. 41 (1964) 2478. St. Louis, R.V., Crawford, B.L.: J. Chem. Phys. 42 (1965) 857. Herzberg, G.: Electronic spectra of polyatomic molecules, New York: Van Nostrand, 1966. Sidhu, K.S., Csizmadia, I.G., Strausz, O.P., Gunning, H.E.: J. Am. Chem. Soc. 88 (1966) 2412. MaCaa, D.J., Shaw, J.H.: J. Mol. Spec. 25 (1968) 374. Turner, D.W., Baker, A.D., Baker, C., Brundle, C.R.: Molecular photoelectron spectroscopy, London: Wiley and Sons, 1970. Roos, B., Siegbahn, P.: Theor. Chim. Acta 21 (1971) 368. Allan, C.J., Gelius, U., Allison, D.A., Johansson, G., Siegbahn, H., Siegbahn, K.: J. Electron Spectrosc. Relat. Phenom. 1 (1972/73) 131. Andrews, L. Spiker, Jr. R.C.: J. Phys. Chem. 76 (1972) 3208. Bottomley, F., Crawford, J.R.: J. Am. Chem. Soc. 94 (1972) 9092. Goodsel, A.J., Blyholder, G.: J. Catal. 26 (1972) 11. Lassiter, W.S.: J. Phys. Chem. 76 (1972) 1289. Shimanouchi, T., Tables of molecular vibrational frequencies consolidated, Volume I, Washington: National Bureau of Standards, 1972, p. 1 - 160. West, L.A., Somorjai, G.A.: J. Vac. Sci. Technol. 9 (1972) 668. McCarty, I., Falconer, J., Madix, R.J.: J. Catal. 30 (1973) 235. Weinberg, W.H.: J. Catal. 28 (1973) 459. Angoletta, M., Bellon, P.L. Manasero, M., Sansoni, M.: J. Organomet. Chem. 81 (1974) C40. Diamantis, A.A., Sparrow, G.J.: J. Colloid Interface Sci. 47 (1974) 455. Norton, P.R.: Surf. Sci. 44 (1974) 624. Handbook of Spectroscopy, Vol. 1: Robinson, J.W. (ed.), CRC Press, Cleveland 1974. Brundle, C.R., Carley, A.F.: Faraday Discuss. Chem. Soc. 60 (1975) 51. Norton, P.R., Richard, P.J.: Surf. Sci. 49 (1975) 567. Brundle, C.R.: J. Vac. Sci. Technol. 13 (1976) 301. Norton, P.R., Tapping, R.L.: Chem. Phys. Lett. 38 (1976) 207. Sinfelt, J.H., Lam, Y.L., Cusumano, J.A., Barnett, A.E.: J. Catal. 42 (1976) 227. Aresta, M., Nobile, C.F.: J. Chem Soc. Dalton Trans. 7 (1977) 708. Bottomley, F., Brooks, W.V.F.: Inorg. Chem. 16 (1977) 501. Sexton, B.A., Somorjai, G.A.: J. Catal. 46 (1977) 167. Shimanouchi, T.: Tables of Molecular Vibrational Frequencies Consolidated Volume II, J. Phys. Chem. Rev. Data 6(3) (1977) 993. Castner, D.G., Sexton, B.A., Somorjai, G.A.: Surf. Sci. 71 (1978) 519. Eley, D.D., Moore, P.B.: Surf. Sci. 76 (1978) L599. Furuyama, M., Kishi, K., Ikeda, S.: J. Electron Spectrosc. Relat. Phenom. 13 (1978) 59. Mingos, D.M.P.: Transition Met. Chem. (London) 3 (1978) 1. Benziger, J.B., Madix, R.J.: Surf. Sci. 79 (1979) 394. Castner, D.G., Somorjai, G.A.: Surf. Sci. 83 (1979) 60. Dubois, L.H., Somorjai, G.A.: Surf. Sci. 88 (1979) L13. Fuggle, J.C., Menzel, D.: Surf. Sci. 79 (1979) 1. Johnson, D.W., Matloob, M.H., Roberts, M.W.: J. Chem. Soc. Faraday Trans. 75 (1979) 2143. Kubas, G.J.: Inorg. Chem. 18 (1979) 182. Campbell, C.T., Ertl, G., Kuipers, H., Segner, J.: J. Chem. Phys. 73 (1980) 5862. Delwiche, J., Hubin-Franskin, M.-J., Caprace, G., Natalis, P.: J. Electron. Spectrosc. Relat. Phenom. 21 (1980) 205. Dubois, L.H., Somorjai, G.A.: Surf. Sci. 91 (1980) 514.
Landolt-Börnstein New Series III/42A5
236 80Kle 80Leg 81Bar 81Dan 81Ku 81Rov 81Rya 81Sal 81Sau 81Shi 81Umb 82Ast 82Dah 82Hit 82Kat 82Kim 82Köh 82Nor 82Seg 82Stu 83Ave 83Bac 83Bar 83Beh 83Cal 83Gai 83Köh 83Mad 83Wei 84Can 84Foc 84Out 84Pir 84Seg 84Sol 84Spi 85Beh 85Köl 85Sak 85Sal 85Sch 85Sol 86Beh 86Ber 86Cam 86Ega 86Fre 86Out1 86Out2 86Sau 86Sch1
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces Klein, R., Siegel, R.: Surf. Sci. 92 (1980) 337. LeGaré, P., Hilaire, L., Sotto, M., Maire, G.: Surf. Sci. 91 (1980) 175. Barteau, M.A., Madix, R.J.: J. Chem. Phys. 74 (1981) 4144. Daniel, W.M., Kim, Y., Peebles, H.C., White, J.M.: Surf. Sci. 111 (1981) 189. Ku, R.C., Wynblatt, P: Appl. Surf. Sci. 8 (1981) 250. Rovida, R., Pratesi, F.: Surf. Sci. 104 (1981) 609. Ryan, R.R., Kubas, G.J., Moody, D.C., Eller, P.G.: Struct. Bonding (Berlin) 46 (1981) 47. Sales, B.C., Turner, J.E., Maple, M.B.: Surf. Sci. 112 (1981) 272. Sau, R., Hudson, J.B.: J. Vac. Sci. Technol. 18 (1981) 607. Shi, S.-K., Lee, H.-I., White, J.M.: Surf. Sci. 102 (1981) 56. Umbach, E., Menzel, D.: Chem. Phys. Lett. 84 (1981) 491. Astegger, St., Bechtold, E.: Surf. Sci. 122 (1982) 491. Dahlgren, D., Hemminger, J.C.: Surf. Sci. 123 (1982) L739. Hitchman, M.A., Rowbottom, G.L.: Coord. Chem. Rev. 42 (1982) 55. Katekaru, R.C., Garwood, G.A., Hershberger, J.F., Hubbard, A.T.: Surf. Sci. 121 (1982) 396. Kim, Y., Schreifels, J.A., White, J.M.: Surf. Sci. 114 (1982) 349. Köhler, U., Wassmuth, H.-W.: Surf. Sci. 117 (1982) 668. Norton, P.R., Davies, J.A., Jackman, T.E.: Surf. Sci. 122 (1982) L593. Segner, J., Vielhaber, W., Ertl, G.: Isr. J. Chem. 22 (1982) 375. Stuve, E.M., Madix, R.J., Sexton, B.A.: Chem. Phys. Lett. 89 (1982) 48. Avery, N.R.: Surf. Sci. 131 (1983) 501. Backx, C., De Groot, C.P.M., Biloen, P., Sachtler, W.M.H.: Surf. Sci. 128 (1983) 81. Barteau, M.A., Madix, R.J.: J. Electron Spectrosc. Relat. Phenom. 31 (1983) 101. Behm, R.J., Brundle, C.R.: J. Vac. Sci. Technol. A 1 (1983) 1223. Calabrese, J.C., Herskovitz, T., Kinney, J.B.: J. Am. Chem. Soc. 105 (1983) 5914. Gainey, T.C., Hopkins, B.J.: J. Phys. C 16 (1983) 975. Köhler, U., Wassmuth, H.-W.: Surf. Sci. 126 (1983) 448. Madey, T.E., Avery, N.R., Anton, A.B., Toby, B.H., Weinberg, W.H.: J. Vac. Sci. Technol. A 1 (1983) 1220. Weinberg, W.H.: Surf. Sci. 128 (1983) L224. Canning, N.D.S., Outka, D., Madix, R.J.: Surf. Sci. 141 (1984) 240. Fock, J.-H., Lau, H.-J., Koch, E.E.: Chem. Phys. 83 (1984) 377. Outka, D.A., Madix, R.J.: Surf. Sci. 137 (1984) 242. Pireaux, J.J., Chtaïb, M., Delrue, J.P., Thiry, P.A., Liehr, M., Caudano, R.: Surf. Sci. 141 (1984) 211. Segner, J., Campbell, C.T., Doyen, G., Ertl, G.: Surf. Sci. 138 (1984) 505. Solymosi, F., Kiss, J.: Chem. Phys. Lett. 110 (1984) 639. Spitzer, A., Lüth, H.: Phys. Rev. B 30 (1984) 3098. Behner, H., Wedler, G.: Surf. Sci. 160 (1985) 271. Kölzer, J.G., Wassmuth, H.-W.: Ann. Phys. (Leipzig) 42 (1985) 265. Sakaki, S., Sato, H., Imai, Y., Morokuma, K., Ohbuko, K.: Inorg. Chem. 24 (1985) 4538. Saleh, J.M., Nasser, F.A.K.: J. Phys. Chem. 89 (1985) 3392. Schwalke, U., Niehus, H., Comsa, G.: Surf. Sci. 152/153 (1985) 596. Solymosi, F., Kiss, J.: Surf. Sci. 149 (1985) 17. Behner, H., Spiess, W., Wedler, G., Borgmann, D.: Surf. Sci. 175 (1986) 276. Berkó, A., Solymosi, F.: Surf. Sci. 171 (1986) L498. Campbell, S., Hollins, P., McCash, E., Roberts, M.W.: J. Electron Spectrosc. Relat. Phenom. 39 (1986) 145. Egawa, C., Doi, I., Naito, S., Tamaru, K.: Surf. Sci. 176 (1986) 491. Freund, H.-J., Messmer, R.P.: Surf. Sci. 172 (1986) 1. Outka, D.A., Madix, R.J.: Langmuir 2 (1986) 406. Outka, D.A., Madix, R.J., Fisher, G.B., DiMaggio, C.: J. Phys. Chem. 90 (1986) 4051. Sault, A.G., Madix, R.J.: Surf. Sci. 169 (1986) 347. Schwalke, U., Parmeter, J.E., Weinberg, W.H.: J. Chem. Phys. 84 (1986) 4036. Landolt-Börnstein New Series III/42A5
3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces 86Sch2 86Sol 86Zom 87Alb 87Bab 87Bar1 87Bar2 87Bau 87Beh 87Car 87Col 87Fre 87Hof 87Lin 87Mat 87Out 87Pel 87Pir 87Sak 87Sol 87Zom 88Ass 88Bar 88Beh 88Bur1 88Bur2 88Cop 88Hor 88Ill
88Kis 88Mad 88Sas 89Bar 89Leu 89Liu 89Par 89Pas 89Pau 89Rob 89Rod 89Wam 89Woh
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Schwalke, U., Parmeter, J.E., Weinberg, W.H.: Surf. Sci. 178 (1986) 625. Solymosi, F., Berkó, A.: J. Catal. 101 (1986) 458. Zomack, M., Baberschke, K.: Surf. Sci. 178 (1986) 618. Albert, M.R., Yates, J.T.: The surface scientist’s guide to organometallic chemistry, Washington, DC: ACS,1987. Baberschke, K., Farle, M., Zomack, M.: Appl. Phys. A 44 (1987) 13. Bartos, B., Freund, H.-J., Kuhlenbeck, H., Neumann, M., Lindner, H., Müller, K.: Surf. Sci. 179 (1987) 59. Bartram, M.E., Windham, R.G., Koel, B.E.: Surf. Sci. 184 (1987) 57. Bauer, R., Behner, H., Borgmann, D., Pirner, M., Spiess, W., Wedler, G.: J. Vac. Sci. Technol. A 5 (1987) 1110. Behner, H., Spieess, W., Wedler, G., Borgmann, D., Freund, H.-J.: Surf. Sci. 184 (1987) 335. Carley, A.F., Gallagher, D.E., Roberts, M.W.: Surf. Sci. 183 (1987) L263. Collman, J.P., Hegedus, L.S., Norton, J.R., Finke, R.G.: Principles and applications of organotransition metal chemistry, University Science Books, Mill Valley, CA, 1987. Freund, H.-J., Behner, H., Bartos, B., Wedler, G., Kuhlenbeck, H., Neumann, M.: Surf. Sci. 180 (1987) 550. Hoffman, D., Hudson, J.B.: Surf. Sci. 180 (1987) 77. Lindner, H., Rupprecht, D., Hammer, L., Müller, K.: J. Electron Spectrosc. Relat. Phenom. 44 (1987) 141. Matsushima, T.: J. Phys. Chem. 91 (1987) 6192. Outka, D.A., Madix, R.J., Fisher, G.B., Dimaggio, C.: Surf. Sci. 179 (1987) 1. Peled, H., Asscher, M.: Surf. Sci. 183 (1987) 201. Pirner, M., Bauer, R., Borgmann, D., Wedler, G.: Surf. Sci. 189/190 (1987) 147. Sakurai, M., Okano, T., Tuzi, Y.: J. Vac. Sci. Technol. A 5 (1987) 431. Solymosi, F., Bugyi, L.: J. Chem. Soc. Faraday Trans. I 83 (1987) 2015. Zomack, M., Baberschke, K.: Phys. Rev. B 36 (1987) 5756. Asscher, M., Kao, C.-T., Somorjai, G.A.: J. Phys. Chem. 92 (1988) 2711. Bartram, M.E., Windham, R.G., Koel, B.E.: Langmuir 4 (1988) 240. Behr, A.: Angew. Chem. 27 (1988) 661. Burke, M.L., Madix, R.J.: Surf. Sci. 194 (1988) 223. Burke, M.L., Madix, R.J.: J. Vac. Sci. Technol. A 6 (1988) 789. Copperthwaite, R.G., Davies, P.R., Morris, M.A., Roberts, M.W., Ryder, R.A.: Catal. Lett. 1 (1988) 11. Horsley, J.A., in: Chemistry and physics of solid surfaces VII, Springer Series in Surface Science 10, Vanselow, R., Howe, R.F. (eds.), Berlin: Springer-Verlag, 1988. Illing, G., Heskett, D., Plummer, E.W., Freund, H.-J., Somers, J., Lindner, T., Bradshaw, A.M., Buskotte, U., Neumann, M., Starke, U., Heinz, K., deAnders, P.L., Saldin, D., Pendry, J.B.: Surf. Sci. 206 (1988) 1. Kiss, J., Révész, K., Solymosi, F.: Surf. Sci. 207 (1988) 36. Madix, R.J., Solomon, J.L., Stöhr, J.: Surf. Sci. 197 (1988) L253. Sass, C.S., Rabalais, J.W.: Surf. Sci. Lett. 194 (1988) L95. Bartram, M.E., Koel, B.E.: Surf. Sci. 213 (1989) 137. Leung, K.T., Zhang, X.S., Shirley, D.A: J. Phys. Chem. 93 (1989) 6164. Liu, Z.M., Zhou, Y., Solymosi, F., White, J.M.: J. Phys. Chem. 93 (1989) 4383. Parker, D.H., Bartram, M.E., Koel, B.E.: Surf. Sci. 217 (1989) 489. Pashutaski, A., Folman, M.: Surf. Sci. 216 (1989) 395. Paul, J.: Surf. Sci. 224 (1989) 348. Roberts, M.W.: Chem. Soc. Rev. 18 (1989) 451. Rodriguez, J.A. Clendening, W.D., Campbell, C.T.: J. Phys. Chem. 93 (1989) 5238. Wambach, J., Odörfer, G., Freund, H.-J., Kuhlenbeck, H., Neumann, M.: Surf. Sci. 209 (1989) 159. Wohlrab, S., Ehrlich, D., Wambach, J., Kuhlenbeck, H., Freund, H.-J.: Surf. Sci. 220 (1989) 243.
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3.8.4 CO2, NO2, SO2, OCS, N2O and O3 on metal surfaces Ahner, J., Effendy, A., Vajen, K., Wassmuth, H.-W.: Vacuum 41 (1990) 98. Banse, B.A., Koel, B.E.: Surf. Sci. 232 (1990) 275. Cornish, J.C.L., Avery, N.R.: Surf. Sci. 235 (1990) 209. Dixon-Warren, St.J., Leggett, K., Matyjaszczyk, M.S., Polanyi, J.C., Young, P.A.: J. Chem. Phys. 93 (1990) 3659. Ehrlich, D., Wohlrab, S., Wambach, J., Kuhlenbeck, H., Freund, H.-J.: Vacuum 41 (1990) 157. Grimblot, J., Alnot, P., Behm, R.J., Brundle, C.R.: J. Electron Spectrosc. Relat. Phenom. 52 (1990) 175. Höfer, M., Hilling, S., Wassmuth, H.-W.: Vacuum 41 (1990) 102. Leggett, K., Polanyi, J.C., Young, P.A.: J. Chem. Phys. 93 (1990) 3645. Maynard, K.J., Moskovitz, M.: Surf. Sci. 225 (1990) 40. Parker, D.H., Koel, B.E.: J. Vac. Sci. Technol. A 8 (1990) 2585. Polzonetti, G., Alnot, P., Brundle, C.R.: Surf. Sci. 238 (1990) 226. Polzonetti, G., Alnot, P., Brundle, C.R.: Surf. Sci. 238 (1990) 237. Polanyi, J.C., Young, P.A.: J. Chem. Phys. 93 (1990) 3673. Rodriguez, J.A.: Surf. Sci. 226 (1990) 101. Sjövall, P., So, S.K., Kasemo, B., Franchy, R., Ho, W.: Chem. Phys. Lett. 171 (1990) 125. vom Felde, A., Kern, K., Higashi, G.S., Chabal, Y.J., Christman, S.B., Bahr, C.C., Cardillo, M.J.: Phys. Rev. B 42 (1990) 5240. Zhou, X.-L., White, J.M.: Surf. Sci. 235 (1990) 259. Alnaji, O., Dartiguenave, M., Dartiguenave, Y., Simard, M., Beauchamp, A.L.: Inorg. Chim. Acta 187 (1991) 31. Behm, R.J., Brundle, C.R.: Surf. Sci. 255 (1991) 327. Browne, V.M., Carley, A.F., Copperthwaite, R.G., Davies, P.R., Moser, E.M., Roberts, M.W.: Appl. Surf. Sci. 47 (1991) 375. Castro, M.E., White, J.M.: J. Chem. Phys. 95 (1991) 6057. Kiss, J., Lennon, D., Jo, S.K., White, J.M.: J. Phys. Chem. 95 (1991) 8054. Liu, Z.M., Zhou, Y., Solymosi, F., White, J.M.: Surf. Sci. 245 (1991) 289. Malik, I.J., Hrbek, J.: J. Phys. Chem. 95 (1991) 10188. Rodriguez, J.A., Campbell, R.A., Goodman, D.W.: Surf. Sci. 244 (1991) 211. Solymosi, F.: J. Mol. Catal. 65 (1991) 337. Solomon, J.L., Madix, R.J., Wurth, W., Stöhr, J.: J. Phys. Chem. 95 (1991) 3687. Wambach, J., Illing, G., Freund, H.-J.: Chem. Phys. Lett. 184 (1991) 239. Wickham, D.T., Banse, B.A., Koel, B.E.: Surf. Sci. 243 (1991) 83. Zittel, P.F.: J. Phys. Chem. 95 (1991) 6802. Ahner, J., Effendy, A., Wassmuth, H.-W.: Surf. Sci. 269/270 (1992) 372. Höfer, M., Stolz, H., Wassmuth, H.-W.: Surf. Sci. 272 (1992) 342. Malik, I.J., Hrbek, J.: J. Vac. Sci. Technol. A 10 (1992) 2565. Sawabe, K., Matsumoto, Y.: Chem. Phys. Lett. 194 (1992) 45. Ahner, J., Wassmuth, H.-W.: Surf. Sci. 287/288 (1993) 125. Beckendorf, M., Katter, U.J., Schlienz, H., Freund, H.-J.: J. Phys. Condens. Matter 5 (1993) 5471. Brosset, P., Dahoo, R., Gauthier-Roy, B., Abouaf-Marguin, L.: Chem. Phys. 172 (1993) 315. Höfer, M., Stolz, H., Wassmuth, H.-W.: Surf. Sci. 287/288 (1993) 130. Nassir, M.H., Dwyer, D.J.: J. Vac. Sci. Technol. A 11 (1993) 2104. Pangher, N. Köppe, H.M., Feldhaus, J., Haase, J.: Phys. Rev. Lett. 71 (1993) 4365. Pressley, L.A., Kiss, J., White, J.M., Castro, M.E.: J. Phys. Chem. 97 (1993) 902. Rodriguez, J.A.: J. Phys. Chem. 97 (1993) 6509. Wassmuth, H.W., Ahner, J., Höfer, M., Stolz, H.: Prog. Surf. Sci. 42 (1993) 257. Zebisch, P., Weinelt, M., Steinrück, H.-P.: Surf. Sci. 295 (1993) 295. Bulanin, K.M., Alexeev, A.V., Bystrov, D.S., Lavalley, J.C., Tsyganenko, A.A.: J. Phys. Chem. 98 (1994) 5100. Carley, A.F., Roberts, M.W., Strutt, A.J.: J. Phys. Chem. 98 (1994) 9175. Landolt-Börnstein New Series III/42A5
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3.8.6 Adsorbate properties of linear hydrocarbons
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3.8.6 Adsorbate properties of linear hydrocarbons 3.8.6.1 Introduction 3.8.6.1.1 General considerations The adsorption of organic molecules on surfaces plays a central role in our life on this planet. We live in a world full of organic molecules; all exposed surfaces are covered with them ranging from the car body and the top of the dining room table to the human skin. Thus, the properties of most surfaces we come in contact with are modified by the presence of this adsorbed organic layer that may contain small molecules such as acetylene or butadiene or large ones like proteins. During the past 30-35 years a large number of techniques were developed that permit investigation of adsorbed organic layers on the molecular level. The studies that were performed indicated great complexity. Adsorption bond strength, the structure and reactivity of the adsorbed organic layer depends on: 1) the structure of the substrate on which adsorption occurs; 2) the coverage of the adsorbate. Usually the first layer (monolayer) adsorbs more strongly than subsequent layers. Even within the first monolayer, the heat of adsorption often declines rapidly with coverage; 3) the temperature of the adsorbate-substrate system. Elevated temperatures lead to bond breaking and sequential decomposition of the adsorbed organic molecules. Another important feature has been the pressure range that was experimentally available for most of these molecular adsorption studies. Because of the predominance of electron scattering to determine surface structure by diffraction or vibrational spectroscopy most adsorption studies had to be performed at low enough pressures to satisfy the electron mean free path requirement of the signal detection. Only recently have photon in-photon out techniques gained popularity for these studies that can be performed at higher ambient pressures where there is equilibrium between the adsorbed organic molecules and the organic vapor over the surface. In order to avoid experimental pitfalls and simplify studies of the adsorbed organic layer, most molecular adsorption studies were carried out using single crystal surfaces with well-defined, close packed surface structures. The use of powders and highly reactive surfaces with large concentrations of low coordination sites (steps and kinks) have been avoided. The studies were usually restricted to low coverages. The temperature range was limited to below 400 K to avoid excessive bond breaking and decomposition. Not surprisingly, most investigations were carried out at low pressures to take advantage of the available surface analytical techniques that can be employed only in this circumstance. In this chapter, we summarize the investigations that reported studies of adsorbed short chain linear C1-C12 hydrocarbons; their structure and bonding on metal and semiconductor single crystal surfaces. It is our hope that these studies provide the foundation to investigate organic monolayers of greater complexity and higher molecular weights and organic molecules adsorbed on more reactive, more structurally complex surfaces. In future studies, a larger temperature and pressure range of adsorption must be explored to correlate adsorption behavior with reaction intermediates that are produced in high turnover catalytic reactions. The adsorption studies of the small C1-C12 molecules on metal and semiconductor surfaces reveal several important findings. The adsorption bond is surface structure sensitive. Adsorbate-adsorbate repulsive interactions lead to weakening of the surface chemisorption bonds at higher coverages. The metal or semiconductor substrates restructure when adsorption occurs. This adsorbate induced restructuring may be local, occurring in the neighborhood of the adsorption site. It may also lead to complete restructuring of the substrate, especially for higher Miller-Index, more open surfaces. Increasing temperatures lead to sequential bond scission so that the molecular structure of the organic adsorbate markedly changes as the temperature is increased. Hydrogen coadsorption and coverage could have a large effect on the structure of the organic adsorbates. Unfortunately, hydrogen coadsorption has been investigated only in a few studies. New techniques that may be used to investigate the adsorbed organic monolayers such as optical spectroscopies and the scanning probes will permit high pressure and high temperature in situ studies and also dynamical investigations to explore the roles of surface motions, diffusion and molecular rotation Landolt-Börnstein New Series III/42A5
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and rearrangements in the adsorption process. The structure and bonding of adsorbed organic monolayers is an exciting field of surface science with impact on our understanding of the surface chemical bond, catalysis, friction and lubrication and in the case of adsorption of proteins at the polymer-water interface, biocompatibility. Indeed, adsorption of organic monolayers at the buried interfaces, solid-high pressure gas, solid-liquid, and solid-solid has yet to be explored.
3.8.6.1.2 Experimental aspects Our knowledge of hydrocarbon adsorption is certainly still far from complete and this collection has inherent deficiencies. The majority of studies focused on small molecules (C2H4, C2H2) on close packed surfaces of Pt and Ni. This is illustrated in Fig. 1a and 1b showing which hydrocarbons and which substrates were studied most frequently. Furthermore, only a limited number of studies deal with quantitative parameters of hydrocarbon adsorption such as sticking and accommodation coefficients or adsorption energies as function of coverage. Many adsorption studies were motivated by heterogeneous catalysis since the initial step of every heterogeneous catalytic reaction is the same: the adsorption of the reacting gases on the surface of the catalyst (which may already strongly predetermine the catalytic properties). Consequently, geometrical (structural) information was mostly collected to determine adsorption sites and the molecular orientation of the hydrocarbon layers. 40
30
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25
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Percent [%]
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15 10
10 5
5 C2H3 C3H6 C4H8 C5H10+ C3H4 diene C4H6 diene C5H8+diene C2H2 C3H4 C4H6
CH4 C2H6 C3H8 C4H10 C5H12+ radicals etc C2H4
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Pt Ni Pd Cu Rh Mo Ru W Ir Ag semicond. alloys Fe Re Au Co V Ta Cr
0
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Percent [%]
20
Fig. 1a Relative frequency of most often studied hydrocarbons (papers published until early 2005).
15
b Relative frequency of substrate surfaces used for hydrocarbon adsorption studies (papers published until early 2005).
10 5
c Relative frequency of surface analysis techniques used for studies of hydrocarbons (papers published until early 2005).
c
LEED TDS(TPD)/TPRS HREELS IRAS XPS DFT AES microcaloriometry NEXAFS UPS molec.beams STM isotope exch. ARUPS PED EHT work function SFG SIMS HAS ISS/MEIS/ESDIAD NMR
0
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For experimental reasons, most studies report the adsorption of various hydrocarbons on one or two surfaces – experimentally, it is easier to change the adsorbate molecule than the substrate surface. Some authors report 3 or 4 hydrocarbons on one or two surfaces, sometimes including O-, N-, or C-precovered, i.e. there may be 8 or more adsorbate/substrate combinations appearing in a single article. Adsorption data of a specific hydrocarbon on various substrates are therefore typically “spread out” over many articles. By contrast, this chapter is organized differently. To facilitate the comparison of the adsorption properties of a specific hydrocarbon on different substrates, the collected data are arranged by the adsorbate molecule first, and by the material (alphabetically) second (transition metals are discussed before alloys and semiconductors, though). For a given metal the order of surface geometries is (100), (110), (111), followed by higher-index or stepped surfaces. The most important findings are also summarized in the Tables 3.8.6.7.1 - 3.8.6.7.5. Adsorption studies on heterogeneous catalysts (supported metals) and modified single crystal surfaces (e.g. by oxide overlayers etc.) were not included due to the structural complexity of these systems. However, references to these studies can be typically found in the reviews and original articles summarized here. It is our hope that the collection in this chapter will assist to further evaluate hydrocarbon adsorption on different substrates.
3.8.6.1.3 List of symbols and abbreviations The listings below collect symbols and abbreviations and the techniques most frequently used to study adsorbate properties of hydrocarbons on metal and semiconductor surfaces. A brief explanation of these methods and references to more detailed descriptions can be found e.g. in [93Nie, 94Som, 96She, 97Tho, 98Som]. Fig. 1c graphically displays how often the different methods were utilized, showing that for structure investigations LEED, HREELS and IRAS dominate, and that adsorption energies were mostly determined by thermal desorption spectroscopy/temperature programmed techniques, (micro)calorimetry and molecular beam methods (based on ca. 450 selected studies published until early 2005). Symbols and abbreviations bridge site 2-fold coordinated adsorption site CVD chemical vapor deposition fcc site 3-fold coordinated hollow site with a substrate atom in the 3rd layer underneath Eproc activation energy for various processes (“proc” may be desorption (des), dissociation (diss), diffusion (diff) or reaction (rcn)) ET or Ei incident translational energy of gas particles hcp site 3-fold coordinated hollow site with a substrate atom in the 2nd layer underneath L Langmuir; one Langmuir is equivalent to a gas exposure of 10−6 Torr for 1 second ML “monolayer”, unit for absolute coverage (absolute coverage, i.e. ratio of adsorbed molecules to substrate surface atoms) on-top terminally adsorbed on a single atom S0 initial sticking coefficient (for zero coverage) Τdes desorption temperature (usually peak maximum in TDS) θ absolute coverage (adsorbed molecules per substrate surface atom) UHV ultrahigh vacuum Most frequently applied surface analysis techniques (alphabetically) AES Auger Electron Spectroscopy ARUPS Angle Resolved Ultraviolet Photoelectron Spectroscopy DFT Density Functional Theory (ab initio) EHT Extended Hückel Theory Calculations (semi-empirical) ESDIAD Electron Stimulated Desorption Ion Angular Distribution (HR)EELS (High-Resolution) Electron Energy Loss Spectroscopy HAS Helium Atom Diffraction/Scattering Landolt-Börnstein New Series III/42A5
246 IETS ISS Isot. Exch. LEED MEIS MB NEXAFS NMR PED IRAS SFG SIMS STM TDS TPD TPRS UPS ∆Φ XAS XPS
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[Ref. p. 320
Inelastic Electron Tunneling Spectroscopy Ion Scattering Spectroscopy Isotope Exchange Low-Energy Electron Diffraction Medium Energy Ion Scattering Molecular Beam Techniques (sticking coefficient measurements) Near-Edge X-Ray Absorption Fine Structure Nuclear Magnetic Resonance Photoelectron Diffraction Infrared Reflection-Absorption Spectroscopy (=RAIRS) Sum Frequency Generation Secondary Ion Mass Spectrometry Scanning Tunneling Microscopy Thermal Desorption Spectroscopy Temperature-Programmed Desorption Temperature-Programmed Reaction Spectroscopy Ultraviolet Photoelectron Spectroscopy Work Function Change X-Ray Absorption Spectroscopy X-Ray Photoelectron Spectroscopy
3.8.6.2 Reviews The determination of quantitative adsorption parameters such as adsorption energies is not always part of adsorption studies. In many cases studies have rather focused on the adsorbate structure determined by electron diffraction, vibrational spectroscopy and other techniques. Adsorption energies were mainly determined by temperature programmed techniques, molecular beam methods or by calorimetry. Microcalorimetry is a useful tool for studies of heterogeneous catalysts, because it provides a direct measurement of the strength with which molecules interact with solid surfaces. Hydrocarbon adsorption on low and high Miller-Index surfaces of Pt, Ir and Au as determined by lowenergy electron diffraction surface crystallography was reviewed in [77Som,79Som]. Cerny [96Cer] and Spiewak and Dumesic [98Spi1, 98Spi2] have reviewed calorimetric methods for the determination of the heat of adsorption of gases on single crystals of metals but also for less defined surfaces such as filaments and vacuum-evaporated films. The development of the technique and advanced microcalorimetric techniques and their applications to the study of low surface area metal single crystals are discussed. The results on hydrocarbon adsorption were added to the Tables, for a complete list of the investigated systems and the measured heats of adsorption were refer to [96Cer] and Spiewak and Dumesic [98Spi1, 98Spi2]. Microcalorimetric measurements were also successfully applied by King and coworkers [95Stu, 99Bro1, 99Bro2] who measured the sticking probability and heat of adsorption of C2H4 on various lowindex and stepped Pt and Ni surfaces. Depending on coverage several different adsorbate species were identified. As an example, Fig. 2 presents C2H4 adsorption on Pt(110)-(1×2) indicating that the heat of interaction (reaction) drops stepwise from 205 kJ/mol at low coverage to 125 kJ/mol with increasing coverage. Several stable surface species were identified (see section 3.8.6.4.1.10).
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210 C CH2
Ethylene /(2×1) Pt(110) 300 K
Heat of reaction [kJ/mol]
190
CH3 C
170 H2C CH2 150 H2C = CH2 130
110
0
0.5
2.5 1.0 1.5 2.0 Apparent coverage [ML]
3.0
3.5
Fig. 2: Heat of interaction of ethylene on Pt(110)-(1×2) vs. coverage as determined by microcalorimetry; adapted from [95Stu].
Madix and coworkers [87Ham, 03Wea, 04Kao] performed extensive studies on the adsorption dynamics of methane, ethane, propane, n-butane, etc. on e.g. Ni(111), Ni(100), Pt(111) and Pd(111), utilizing molecular beam techniques and stochastic trajectory simulations. Typically, for each alkane the initial adsorption probability was measured as a function of incident energy and incident angle. In general, at a fixed incident energy and angle the trapping probability was highest on Pd(111), followed by Pt(111) and Ni(111). Three-dimensional stochastic trajectory simulations for alkane trapping on the three metals indicated that incoming molecules lose considerable energy to Pd lattice vibrations, resulting in a high trapping probability while the stiffer Ni lattice prevents the excitation of surface phonons. Infrared reflection absorption spectroscopy has established itself as a powerful and versatile technique for monitoring molecular adsorption at well-defined single crystal metal surfaces. The technique itself and its application to hydrocarbon adsorption, both on single crystal surfaces and supported metals, was described in excellent and comprehensive reviews by Sheppard and De La Cruz [78She, 88She, 96She, 97She], including tables of vibrational frequencies, and by Hoffman [83Hof] and others [95Rav]. The technique provides a wide range of information, including the identification of hydrocarbon species and their structure, and of adsorbate-induced surface reconstructions. An important breakthrough was to recognize the analogy of surface-adsorbed hydrocarbons with organometallic complexes [78Kes, 79Kes, 99Ans, 04Ans]. The use of IR spectroscopy in determining the exact nature of chemisorbed species was, however, sometimes questioned [97Bra], in particular the validity of vibrational “fingerprints”. Implications of the surface infrared selection rule for structure determinations were discussed in [76Pea, 94Fan]. For an extensive description of vibrational spectra of hydrocarbon surface species we refer to [97She]. RAIRS spectra of C2 to C6 normal alkanes on Pt(111) were reported in [89Che]. Bradshaw [97Bra] described case studies determining structural parameters of molecular fragments created in simple heterogeneous reactions and of hydrocarbons adsorbed on single crystal metal surfaces using electron and X-ray diffraction techniques. Scanned energy mode photoelectron diffraction and diffuse low energy electron diffraction provide access also to adsorption layers that do not show long range order. Dumas et al. [99Dum] reviewed recent developments in the major experimental vibrational spectroscopies (i.e. infrared absorption, Raman scattering, high resolution electron loss, helium atom scattering and sum frequency generation) and illustrated them with selected results. Particular emphasis was given to two important topics which have attracted much attention: complex surface reactions taking place on technologically relevant surfaces and interfaces, and vibrational dynamics with emphasis on energy dissipation at surfaces. High resolution electron energy loss spectroscopy (HREELS) [85Koe, 94Iba], is another sensitive and versatile surface analysis technique that can be used to study surface vibrations at an unprecedented level. The most important application of this technique has been to characterize atoms and molecules adsorbed on single crystal metal surfaces. Landolt-Börnstein New Series III/42A5
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Steinrück et al. [96Ste, 01Whe, 02Whe] examined the decomposition of unsaturated hydrocarbons on Ni(100) by temperature-programmed X-ray photoelectron spectroscopy using synchrotron radiation. The use of a third generation light source allowed to follow the thermal chemistry of acetylene, ethylene and propene by acquiring high resolution photoemission spectra within a few seconds, approaching “realtime” analysis. The evolution of the C1s core level spectra was monitored in situ between 90 and 530 K. Analysis of binding energies and intensity changes allowed to distinguish various surface intermediates formed during thermal decomposition. Steinrück [96Ste] also reviewed angle-resolved UPS studies employed to investigate the electronic structure and bonding of adsorbed hydrocarbons, the orientation and symmetry of the adsorbate on the surface, the influence of lateral interactions, and the formation of two-dimensional adsorbate band structures. Several examples were presented, including ethylene and acetylene, adsorbed on Ni(110), Ni(111), Ru(001) and the reconstructed Pt(110)1×2 surface. Solymosi [98Sol] summarized X-ray and ultraviolet photoelectron spectroscopy, high resolution electron energy loss spectroscopy and temperature programmed desorption studies of reactions of hydrocarbon fragments (methylene, methyl, ethyl, propyl and butyl) (with adsorbed oxygen) on various single crystal metal surfaces. The hydrocarbon species were generated by the thermal and photo-induced dissociation of the corresponding iodo-compounds or by azomethane pyrolysis. CxHy fragments readily combine with adsorbed oxygen atoms above 150 K but the exact oxidation pathways sensitively depend on the nature of the metals. A comprehensive review of the coordination, structure and reactivity of hydrocarbon ligands on metal cluster compounds, including structural conclusions for adsorbates on single-crystal metal surfaces, was given by Zaera [95Zae]. This topic was also discussed by Bradshaw [95Bra]. Ceyer summarized surface studies of hydrogenation reactions of ethylene, acetylene and hydrocarbon fragments on Ni surfaces, focusing in particular on the reactivity of bulk (dissolved) hydrogen vs. those of surface hydrogen [01Cey]. STM is a very versatile tool that even allows to probe individual adsorbed atoms and molecules to reveal properties which otherwise would be hidden in the study of an ensemble of atoms and molecules. Imaging, atom manipulation and chemical modification, as well as spectroscopic characterization by inelastic electron tunneling spectroscopy are fascinating new approaches to study e.g. electronic and vibrational properties of single adsorbed hydrocarbon molecules, as discussed by Ho [02Ho].
3.8.6.3 Alkanes Alkanes are chemically saturated molecules which tempers their bond breaking activity even on transition metal surfaces. In the absence of C-H or C-C bond breaking these molecules prefer to adsorb with their carbon chain parallel to the surface since their heat of adsorption increases by ~9 kJ/mol per -CH2- group. Thus at low temperatures and on relatively chemically inactive low Miller-index metal surfaces the molecules adsorb intact and in a flat-lying configuration. As the temperature is increased and/or the metal substrate becomes more corrugated sequential C-H bond breaking occurs to produce organic fragments that become increasingly dehydrogenated as the temperature is raised. The presence of hydrogen would slow down the extent of dehydrogenation. However, few of these alkane adsorption studies have been carried out in the presence of hydrogen. Adsorption, surface structure and dehydrogenation studies were mostly carried out at low pressures (10−7 Torr). Studies of these parameters at high reactant pressures (Torr range or higher) are very much needed.
3.8.6.3.1 Methane CH4 3.8.6.3.1.1 Co Burghgraef et al. [95Bur] studied the adsorption of CH3, CH2, CH, C and H on a one-layer 7-atom cluster and a spherical 13-atom cluster model of cobalt. Starting from gas phase CH4, the formation of adsorbed Landolt-Börnstein New Series III/42A5
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CH3 and adsorbed H was endothermic on all clusters, but the endothermicity was strongly reduced on the 13-atom cluster (135 kJ/mol on Co-7, 8 kJ/mol on Co-13). The formation of adsorbed CH2 and H from CH3 was endothermic by 25-40 kJ/mol on Co-13, but exothermic on Co-7 (3 kJ/mol), mainly because of the much stronger adsorption of CH2 on this cluster. The formation of adsorbed CH and H from CH2 was exothermic on all clusters, but the exothermicity differs by a factor of two between the 7- and 13-atom clusters (60 kJ/mol on Co-7, 32 kJ/mol on Co-13). Finally, the formation of adsorbed C and H from CH was strongly endothermic on the 7 atom clusters, but the endothermicity was again strongly reduced on the 13-atom clusters (77 kJ/mol on Co-7, 14 kJ/mol on Co-13).
3.8.6.3.1.2 Cu Adsorption of methane on Cu(100) at 24 K was studied by Camplin et al. [95Cam] using RAIRS. Physisorbed mono- and multi-layers were observed. The photochemistry of CH4 physisorbed on Cu(111) at 35 K was investigated by Watanabe and Matsumoto using TPD [00Wat1]. Methane was photodissociated into hydrogen, methylene, and methyl by 6.4 eV photon irradiation. Post-irradiation TPD showed desorption peaks of ethylene at 115, 380 and 430 K. The peaks were attributed to molecular desorption of ethylene photochemically formed from methane at 35 K, associative thermal recombination of two methylene groups, and thermal disproportional reactions of four methyl groups, respectively. The photoreaction cross-section of methane depletion was estimated to be 2.0 × 10−20 cm2.
3.8.6.3.1.3 Ir Verhoef et al. [95Ver] measured the initial probability of dissociative chemisorption of CD4 on the reconstructed Ir(110) surface as a function of polar angle of incidence using vibrationally hot supersonic molecular beams (beam temperature 290 -745 K). The probability of chemisorption scaled approximately with the component of translational energy normal to the surface (Ei cos2θ i).
3.8.6.3.1.4 Mo CH4 adsorption and subsequent decomposition on Mo(100) produced c(4×4)-C, c(2×2)-C, c(6¥2 × 2¥2)R45°-C and (1×1)-C structures [76Gui].
3.8.6.3.1.5 Ni Using LEED to study CH4 adsorption on Ni(100), Maire et al. [70Mai] observed c(2×2) and (2×2) structures. On Ni(110) (2×2), (4×3), (4×5)-C and (2×3)-C were detected, the latter in the temperature range 473-579 K [70Mai, 77Sch, 78Sch]. Above 600 K carbon diffuses into the Ni bulk and forms (4×5)-C superstructures. On Ni(111) (2×2), (2×2)-C, (2×¥3), (16¥3 × 16¥3)R30°-C, (4×5)-C and graphite overlayers were reported at 298 - 660 K [70Mai, 79Sch, 84Ben]. Yoshinobu and Kawai [96Yos1] studied CH4 adsorption on Ni(100) at 20 K using IRAS and TPD. For the first CH4 layer bands at 3000, 2884 and 1298 cm−1 were observed, while only two bands at 3017 and 1304 cm−1 were observed for the second CH4 layer. The bands between 3000 and 3017 cm−1 were assigned to the degenerate CH stretching mode, the bands between 1294 and 1304 cm−1 to the degenerate deformation mode, and the 2884 cm−1 band to the symmetric CH stretching mode, respectively. Desorption temperatures of the first and second CH4 layer were 51 and 34 K, respectively. Interlayer mixing between the first and second methane layers during adsorption was observed. For dissociative chemisorption of CH4 on Ni(100) Nielsen et al. [95Nie] measured the initial sticking coefficient between 0.010-7.0 mbar and 375-500 K. A strong pressure dependence was observed, Landolt-Börnstein New Series III/42A5
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consistent with a direct dissociation mechanism under these thermal conditions. This was confirmed by experiments where the gas at a low pressure was heated by a thermal finger facing the crystal surface. With the thermal finger at the same temperature as the surface, it was possible to ensure that the methane was fully equilibrated to the crystal and an activation energy of 59±1.5 kJ/mol was determined under isothermal conditions. McCabe et al. [00McC] described eigenstate-resolved measurements of the dissociative chemisorption of CH4 on Ni(100) using a supersonic molecular beam-surface scattering apparatus. Infrared light from a narrow-bandwidth tunable laser intersecting a supersonic molecular beam was employed to prepare an ensemble of molecules in a single rotational and vibrational quantum state. Schmid et al. [02Sch] reported state resolved sticking coefficients for highly vibrationally excited CH4 on Ni(100) at well-defined kinetic energies in the range of 12-72 kJ/mol. Incident CH4 molecules were prepared by pulsed laser radiation in single rovibrational levels of the first overtone of the antisymmetric stretch 2ν3 at 6004.69 cm−1 and collided at normal incidence with clean Ni(100). The vibrational excitation enhanced the reaction probability by a factor 100 at an incident translational energy of 72 kJ/mol, but this enhancement increased to more than 4 orders of magnitude at low kinetic energy (from ~2 × 10−8 to 5 × 10−4 (12 kJ/mol)). Despite this large increase in the sticking coefficient, vibrational energy in 2ν3 appeared to be about 80% as effective as an equivalent amount of translational energy in promoting the chemisorption reaction. Denecke et al. [05Den, 05Fuh2] reported high resolution XPS spectra of activated CH4 adsorption on Ni(111) at 120 K, followed by thermal decomposition between 120 and 450 K (Fig. 3). C-H bond breaking occurred already upon adsorption producing adsorbed CH3. Species observed during thermal treatment encompass CH, C2H2 and carbon. Fig. 3a shows a series of C 1s spectra taken during annealing of a methyl layer, produced by CH4 molecules from a molecular beam impinging on Ni(111) with a kinetic energy of 0.54 eV. The spectra, acquired every 10 K (linear heating ramp of 0.4 K/s), showed distinct binding energy changes, characterisitc of the occurrence of the different hydrocarbon species. In particular, a vibrational fine structure was observed for the methyl species at low temperature, caused by excitation of the C-H stretching mode in the photoemission process [05Fuh2, 05Den]. The fine structure was then utilized to identify different hydrocarbons and a quantitative analysis was performed employing a fitting routine (Fig. 3b). Interestingly, the CH species could only be observed in a very narrow temperature range. Kratzer et al. [96Kra] carried out a DFT study of the first step of CH4 adsorption on Ni(111), i.e. dissociation into adsorbed CH3 and H. The rupture of the C-H bond occurs preferentially on top of a Ni atom, with a dissociation barrier of ~100 kJ/mol (Fig. 4). The transition state involves considerable internal excitation of the molecule. The active C-H bond was both stretched to 1.6 Å and tilted relative to the methyl group. Alloying the surface with gold also affects the reactivity of the Ni atoms on adjacent surface sites. The dissociation barrier was increased by 16 and 38 kJ/mol for a Ni atom with one or two gold neighbors, respectively. These changes were attributed to a shift in the local density of d states at the nickel atoms in the neighborhood of gold.
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CH4 /Ni (111) 450 K
280 K
229 K
Peak area [a.u.]
Intensity [a.u.]
370 K
CH3 CH C2H2 C CO
200 K
110 K 285
284
a
283 282 Binding energy [eV ]
200
281
300 400 Temperature [K ]
b
500
600
Fig. 3 (a) Thermal annealing of a CH3 layer on Ni(111), produced at a surface temperature of 120 K by a molecular beam of CH4 molecules with a kinetic energy of 0.54 eV. XPS C1s spectra were taken approximately every 10 K (heating ramp 0.4 K/s). Important spectra are highlighted by thick lines and labeled with the respective temperatures. (b) Quantitative analysis of the spectra in (a), shown are total intensities of the marked species (small amounts of CO result from background adsorption); adapted from [05Den]. 2.0
CH4 /Ni (111) C3 n b
pure Ni 1 Au neighbor 2 Au neighbors
H
1.0 H
Ni
0.5
H
9 1.5 q 2.22
Energy [eV ]
1.5
Ni
a Ni
0 − 7.5
− 2.5
2.5 Reaction coordinate
7.5
Fig. 4. The energy along the reaction path for CH4 dissociation over a Ni atom in the Ni(111) surface, as calculated by DFT. The rightmost data points (dashed curves) refer to infinite separation of the dissociated H and CH3 group on the surface. The geometry of the transition state is shown on the right. The angles α ~ 80° and θ =55° denote the orientation of the C-Ni and C-H bond. The C3v symmetry axis of the CH3 group (note that one H atom is hidden) forms an angle β ~30° with the active C-H bond; adapted from [96Kra].
Burghgraef et al. [95Bur] studied the adsorption of CH3, CH2, CH, C and H on a one-layer 7-atom nickel cluster and a spherical 13-atom cluster. Starting from gas phase CH4, the formation of adsorbed Landolt-Börnstein New Series III/42A5
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CH3 and adsorbed H was endothermic on all clusters, but the endothermicity was strongly reduced on the 13-atom clusters (142 kJ/mol on Ni-7, 30 kJ/mol on Ni-13). The formation of adsorbed CH2 and H from CH3 was endothermic by 25-40 kJ/mol on all clusters. The formation of adsorbed CH and H from CH2 was exothermic on all clusters, but the exothermicity differs a factor two between the 7- and 13-atom clusters (61 kJ/mol on Ni-7, 27 kJ/mol on Ni-13). Finally, the formation of adsorbed C and H from CH was strongly endothermic on the 7 atom clusters, but the endothermicity was strongly reduced on the 13atom clusters (92 kJ/mol on Ni-7, 27 kJ/mol on Ni-13).
3.8.6.3.1.6 Pd Activated adsorption of CH4 on clean and oxygen modified Pd(110) was studied by Valden et al. [97Val] using molecular beam methods. The absolute dissociation probability of CH4 was measured as a function of the incident normal energy and the surface temperature and a direct dissociation mechanism was suggested. The dissociation probability decreased linearly with increasing oxygen coverage. Klier et al. [97Kli] studied C-H bond dissociation of CH4 at 400-600 K on Pd(111) and Pd(311) surfaces, and compared it to Pd(679). C-H bond dissociation was found to be structure sensitive, in the order Pd(111)
3.8.6.3.1.7 Pt The dynamics of CH4 trapping on clean Pt(111) was investigated by Carlsson, Madix [00Car, 01Car] and others [89Aru] using molecular beam techniques at 50 K, well below the desorption temperature of 67 K. The initial trapping probability for CH4 scales with normal incident energy (ET cos2θ ), indicating a smooth gas-surface potential. The trapping probability decreased from 0.7 to zero as the incident normal energy was increased from 3 to 20 kJ mol−1 (Fig. 5). Trapping on the methane-saturated surface was greatly enhanced compared to the clean surface at all incident energies and angles, and exhibited near total energy scaling (ET cos0.3θ ), indicating a corrugated gas-surface potential. The trapping probability increased with coverage, indicating that trapping into an extrinsic precursor state is more efficient than trapping onto the bare Pt(111) surface. Molecular beam experiments on Pt(111) were also reported by Yagyu et al. [99Yag]. Fuhrmann et al. [04Fuh, 05Fuh1] studied the activated adsorption of methane on Pt(111) by combining a supersonic molecular beam and in situ high-resolution X-ray photoelectron spectroscopy. Exposing the surface at 120 K to a CH4 beam with kinetic energies between 0.30 and 0.83 eV produced CH3. The spectra show a unique fine structure, caused by vibrational excitations of C-H stretching modes in the photoemission process. Upon heating to ~260 K adsorbed methyl partly dehydrogenated to CH and partly recombined to methane, which desorbed. Adsorption at 300 K yielded CH as surface species.
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1.0
CH4 /Pt (111)
Initial trapping probability
0.8
Expt Ts = 50 K, n = 2 Simulation Ts = 95 K, n =0.6 Expt Ts = 100 K, n = 2
0.6
0.4
Fig. 5 The adsorption probability of CH4 on Pt(111) versus the scaling function ETcosnθҏ for molecular beam measurementsҏ at 50 K and 100 K, compared with molecular dynamics simultations. Adsorption probabilities are plotted with the optimum scaling parameter. Points at normal incidence are indicated; adapted from [00Car].
0.2
0 0
5
10
15 n E cos ( q )
20
25
30
Yoshinobu et al. [96Yos2] studied CH4 adsorption on Pt(111) and processes photoinduced by ArF laser irradiation (193 nm) using IRAS. The symmetry of the first layer methane was degraded from Td to C3v (or lower symmetry), but the symmetry of subsequent methane layers maintained Td. The CH4 multilayer and first layer molecules desorbed from Pt(111) at ~40 and ~70 K, respectively. The first layer C3v CH4 photodissociated into CH3 and H species. These reaction products modified the surface such that the remaining first-layer CH4 molecules became photochemically inactive, and thus the reaction became self-limiting. The photoreaction kinetics was studied with time-resolved IRAS, and the total crosssections were estimated to be σ = 2.6 × 10−19 cm2 for CH4 and σ = 1.7 × 10−19 cm2 for CD4. Activated adsorption of CH4 on clean and oxygen modified Pt(111) was studied by Valden et al. [97Val] with molecular beams. The results from clean Pt(111) were consistent with a direct dissociation mechanism and the dissociation probability decreased linearly with increasing oxygen coverage.
3.8.6.3.1.8 Rh Liu and Hu [03Liu] reported a DFT study of CH4 dissociation to CH3 and H and the reverse reaction on flat, stepped and kinked Rh surfaces. For the CH4 to CH3 + H reaction, the dissociation barrier was reduced by ~0.3 eV on steps and kinks as compared to that on flat surfaces. On the other hand, there was essentially no difference in the barrier for the association reaction of CH3 + H on the flat surfaces and the defects. The DFT calculations show that surface defects such as steps and kinks can largely facilitate bond breaking. However, whether surface defects promote bond formation or not rather depends on the individual reaction as well as on the particular metal.
3.8.6.3.1.9 Ru Goodman and coworkers [94Wu, 02Cho] studied the surface species formed during CH4 decomposition on Ru(0001) and Ru(1120) by HREELS and TPD. Methane dissociated on Ru(0001) to methylidyne (CH) and vinylidene (CCH2), while on Ru( 11 2 0 ) CH, CCH2 and ethylidyne (CCH3) were found. Above 700 K graphitic phases formed. Landolt-Börnstein New Series III/42A5
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3.8.6.3.1.10 Si Adsorption of hot filament activated CH4 on Si(100) was reported in [93Jac, 94Chu1]. Thermal dehydrogenation to carbon competes with thermal desorption and polymerization to volatile species.
3.8.6.3.1.11 W Nahm and Gomer [97Nah] investigated the adsorption of CH4 and CD4 on W(110). A monolayer of methane, probably containing 7 × 1014 molecules cm−2 was adsorbed at 25 K and desorbed near 50 K. A desorption activation energy of ca. 8 kJ mol−1 was determined. (5×1)-C carbon overlayers were detected upon CH4 adsorption on W(100) and (6×6)-C on W(111) [69Bou].
3.8.6.3.1.12 Various Au et al. [98Au] presented a comprehensive theoretical treatment of the partial oxidation of CH4 to syngas on Ni, Pd, Pt and Cu catalysts. Using cluster models of 7-13 atoms, the adsorption energies for a number of intermediates in the dissociation of methane on the metals were calculated and reaction energies for methane dissociation were determined. Larsen and Chorkendorff [98Lar] investigated the reactivity of Co films deposited on Cu(111) towards CH4 dissociation using a molecular beam source. A few layers of Co on Cu were found to be more active than pure Co itself.
3.8.6.3.2 Ethane C2H6 3.8.6.3.2.1 Ir The initial probability of dissociative chemisorption of C2H6 and C2D6 on the reconstructed Ir(110) surface was measured as a function of polar angle of incidence using vibrationally hot supersonic molecular beams [95Ver, 95Sou]. The chemisorption probability scaled approximately with the component of translational energy normal to the surface (Ei cos2θi), for C2D6 (Fig. 6) [95Ver]. Steinrück et al. [86Ste] reported that S0 is constant at ~0.03 at kinetic energies <62 kJ/mol and increased nearly linear to a value of 0.40 at an energy near 165 kJ/mol. 0.3
Initial chemisorption probability
C2D6 /Ir (110)
0.2
Fig. 6 The initial probability of dissociative chemisorption as a function of normal (Ei cos2θi ) translational energy of C2D6 on Ir(110) for a beam temperature of 745 K (molecular beam measurements). Data are for polar angles of incidence θ i with respect to the surface normal of 0°(Ŷ), 22.5° (Ɣ), 30° (ź) and 45° (Ÿ); adapted from [95Ver].
0.1
0 0
10 20 Normal energy [kcal/ mol]
30
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3.8.6.3.2.2 Ni LEED studies of C2H6 adsorption detected c(2×2) and (2×2) structures on Ni(100) [70Mai], (2×2) on Ni(110) [70Mai], and (2×2), (2×¥3), (¥7 × ¥7)R19°-C, (2×2)-C and disordered graphite on Ni(111) [69Ber, 70Mai, 84Ben].
3.8.6.3.2.3 Pd Kao et al. [02Kao] investigated the C2H6 trapping on Pd(111) by molecular beam techniques and stochastic trajectory simulations. The initial trapping probability was measured over the range of incident energy, ET, from 10 to 34 kJ/mol and incident angles, θ, from 0-45° at 95 K. The trapping probability scales with ET cos0.9θ, indicating a corrugated gas-surface potential. Simulations predicted the experimental values of the initial trapping probability within 30%. Calculations of energy transfer for C2H6 after the first bounce on Pd(111) clearly indicated that vibrational excitation of the lattice phonons account primarily for the increase in trapping probabilities of C2H6 on Pd(111) compared with Pt(111).
3.8.6.3.2.4 Pt The molecular adsorption probability of C2H6 on clean Pt(110)-(1×2) at 95 K was measured by Stinnett et al. [96Sti] using molecular beams. At normal incidence the adsorption probability decreased with incident translational energy from near unity (ET 10 kJ/mol) to 0.5 (40 kJ/mol). For molecules incident with the tangential velocity component directed along the [ 1 1 0 ] (smooth) direction, the initial adsorption probability increased with increasing θ i, scaling with ET cos0.6θi ; however, the adsorption probability decreased with θ i for molecular beams directed along the [100] (rough) direction. Stochastic trajectory simulations illustrated that collisions on the ridges of Pt(110)-(1×2) mitigate against trapping of ethane while collisions within the troughs facilitate trapping. Madix and coworkers [90Aru, 92McM, 93McM1, 94Sou, 97Sti, 98Sti1, 00Kao] studied C2H6 trapping on Pt(111) and Pt(111)-p(2×2)-O by supersonic molecular beams. The initial trapping probability at 100 K was measured in the range of incident energy from 10 to 45 kJ/mol and incident angles from 0° to 60°, yielding e.g. 0.85 for Ecosnθ =10 and 0.06 for Ecosnθ =35. A broad angular distribution of scattered ethane and total energy scaling with ET cos0.2θ for ethane trapping indicated a corrugated gas-surface potential. Calculations of energy transfer for ethane after the first bounce on Pt(111) and Pt(111)-p(2×2)O clearly indicate that interconversion of parallel and perpendicular momentum and energy transfer to lattice vibrations account primarily for the differences in trapping probabilities between ethane on the two surfaces. At glancing incidence trapping is not significantly reduced on the oxygen-covered Pt(111) because the parallel momentum appears to be transferred partially to phonons. Newell et al. [98New] reported the IRAS spectrum of adsorbed ethyl (C2H5) on Pt(111) produced by the dissociative adsorption of C2H6 from a supersonic molecular beam at 150 K. The thermal chemistry of the C2H5 fragment was followed by IRAS and TPD, indicating the presence of ethylidene (=CHCH3) and ethylidyne (ŁCCH3) at 250 and 350 K, respectively. This implies the stepwise loss of one and two hydrogen atoms from the ethyl moiety during the heating process. TPD measurements showed that hydrogen desorption was not accompanied by desorption of either saturated or unsaturated C-2 hydrocarbons or methane. RAIRS spectra of C2H6 on Pt(111) were also reported by Chesters et al. [89Che]. The spectra indicated that the orientation of the plane containing the carbon chains was almost exclusively parallel to the metal surface in the case of monolayers. Multilayers were consistent with liquid-like films.
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3.8.6 Adsorbate properties of linear hydrocarbons
[Ref. p. 320
3.8.6.3.2.5 Ru Disordered C2H6 layers were reported for Ru(0001) at 80 K [78Mad].
3.8.6.3.2.6 W (1×1) C2H6 layers were reported for W(111) [78Win].
3.8.6.3.3 Propane C3H8 3.8.6.3.3.1 Ir Precursor-mediated dissociative adsorption of C3H8 on Ir(110)-(1×2) dominates over direct dissociation at low incident kinetic energies for surface temperatures at least up to 1000 K [95Sou].
3.8.6.3.3.2 Mo Microcalorimetric studies of C3H8 adsorption on polycrystalline Mo film reported a heat of adsorption of 558 kJ mol−1 at 295 K (θ →0) [77Smu].
3.8.6.3.3.3 Ni Hamza and Madix [87Ham] used supersonic molecular beam techniques to examine the dissociative chemisorption of C3H8 (and other saturated C1 to C4 hydrocarbons) on Ni(100) using molecular beams. For incident translational energies less than 10 kJ/mol no measurable adsorption was observed at 500 K. The initial sticking probability for propane at 120 kJ/mol incident beam energy was independent of surface temperature from 300 to 700 K (~0.25), suggesting a direct mechanism for dissociation.
3.8.6.3.3.4 Pt McMaster et al. [93McM2] measured the molecular adsorption probability of C3H8 on clean and propanecovered Pt(110)-(1×2) at 95 K using molecular beams. Adsorbed propane facilitated trapping; S0 increased linearly with propane coverage up to 0.55 ML saturation coverage. IRAS spectra of C3H8 on Pt(111) were reported by Chesters et al. [89Che], both for monolayers and multilayers. The spectra indicated that the orientation of the plane containing the carbon chains was almost exclusively parallel to the metal surface in the case of monolayers. Propane multilayers were consistent with liquid-like films. Microcalorimetric studies of C3H8 adsorption on polycrystalline Pt film report a heat of adsorption of 248 kJ mol−1 at 295 K (θ→0) [84Pal]. Wang [03Wan] studied the effect of surface steps on Pt(655) on the molecular adsorption of C3H8 by molecular-dynamics simulations. Incidences along the step edge and perpendicular to the step edge with upstairs and downstairs momentum were considered. In general, the surface step enhanced the initial trapping probability of C3H8 except for the downstairs incidences. The most efficient adsorption zone was near the bottom of the surface step on the lower terrace. The least efficient zone was the top of the surface step on the upper terrace. The surface steps also created a steric effect, i.e. more incident molecules along the upstairs azimuth impact the step-bottom zone but significantly less molecules along the downstairs azimuth. This “shadowing effect” reduces the high trapping efficiency of the step-bottom zone and causes the downstairs incidences to have the lowest trapping probabilities. However, the influence of surface Landolt-Börnstein New Series III/42A5
Ref. p. 320]
3.8.6 Adsorbate properties of linear hydrocarbons
257
steps diminished at low incident energies and large incident angles because longer contact times and less normal momenta result in a high trapping probability across the entire stepped surface.
3.8.6.3.4 Butane C4H10 3.8.6.3.4.1 Ag Pawela-Crew and Madix [95Paw1, 95Paw2] applied TPD, XPS and NEXAFS to study the desorption kinetics of butanes on Ag(110) which shows evidence of weak attractive intermolecular interactions. Activation energies of desorption at zero coverage were 44±1.2 and 41±1.2 kJ/mol for n-butane and isobutane, respectively.
3.8.6.3.4.2 Mo Kelly et al. [86Kel] investigated the chemisorption and reactions of n-butane on clean Mo(100), and with sulfur or carbon overlayers, using TDS. At low additive coverage (0-0.2 monolayers of S or C), and at low ambient pressure (10−10 Torr) a large fraction (95%) of butane desorbed molecularly at all additive coverages. As additive (S or C) coverage increased the amount of decomposition decreased, enabling hydrogenation, partial dehydrogenation, and isomerization to become more probable. Molecular binding on the additive overlayers was also found to be very different. On S overlayers the binding of the hydrocarbon was weak (physisorption), usually on the order of the heat of sublimation (38-42 kJ/mol). However, molecular binding on C overlayers was stronger: the heat of desorption was 46 kJ/mol.
3.8.6.3.4.3 Pt Using LEED Firment and Somorjai reported various superstructures and order-disorder transitions of butane on Pt(111) at 100-220 K (Fig. 7) [77Fir]. The chain axis is aligned parallel to the Pt surface and to Pt[ 1 1 0 ].
n − butane /Pt(111)
4.80 Å
8.43 Å
a
b 2.77 Å
Fig. 7 (a) Schematic LEED pattern at 19.5 eV of an n-butane monolayer on Pt(111) (below 105 K). (b) Real space unit mesh of the structure with the proposed arrangements of molecules; adapted from [77Fir].
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[Ref. p. 320
Raut et al. [97Rau] used molecular-dynamics simulations to study the structure and mobility of n-butane adlayers on Pt(111), for submonolayers and the multilayer regime. At submonolayer coverages n-butane molecules adsorb with their molecular plane parallel to the surface. Upon increasing coverage close to a monolayer, some of the molecules form a tilted structure with their long axes oriented away from the surface. At monolayer saturation, the molecules exhibit temperature-dependent ordering similar to previous LEED studies [77Fir]. At moderate submonolayer coverages, the diffusion activation energy is coverage-independent. In the multilayer regime, molecules in the top layer were an order of magnitude more mobile than those in the layer adjacent to the surface. Vibrational characteristics of the adsorbed butane molecules were also discussed. IRAS spectra of C4H10 on Pt(111) were reported by Chesters et al. [89Che], both for monolayers and multilayers. The spectra indicated a parallel orientation of the plane containing the carbon chains with respect to the metal surface for monolayers. The multilayers appeared to be crystalline again with molecules aligned so that the plane containing the carbon atoms was parallel to the substrate. Weaver et al. [01Wea] studied n-butane adsorption on Pt(111) using molecular beam techniques, TPD and LEED. Below 0.14 ML coverage a disordered monolayer formed, from 0.14 to 0.20 ML ordered regions developed, with the C4H10 molecules preferrentially oriented parallel to the surface. After the lowcoverage ordered phase saturates at 0.20 ML, a more densely-packed ordered phase formed at 98 K, with the n-butane molecules probably tilted away from the surface. After the high coverage ordered phase saturated at 0.35 ML, a disordered second layer was observed.
3.8.6.3.4.4 V Chen [95Che] investigated the adsorption and decomposition of n-butane on clean and carbide-modified V(110) by HREELS and TDS. The formation of carbide significantly modified the reactivity of vanadium. n-butane interacted very weakly and reversibly with the clean surface but the reactivity was enhanced on carbide-modified surfaces.
3.8.6.3.5 Pentanes C5H12 and higher alkanes 3.8.6.3.5.1 Au Chesters and Somorjai observed no n-heptane C7H16 adsorption on Au(111) and Au(S)-[6(111) × (100)] under low pressure (~10−6 Torr) at 300 K [75Che]. Scoles and coworkers [98Wet, 00Lib] used helium atom reflectivity to study the adsorption of n-alkanes on Au(111). For the long-chain n-alkanes studied (C6H14 - C12H26), the physisorption energy increased linearly with the chain length by 6.2±0.2 kJ/mol per additional methylene unit. Collision-induced desorption (CID) of n-alkanes of various chainlengths (CnH2n + 2, n = 5, 7, 10, 12) physisorbed on Au(111) was reported in [00Lib]. The adsorbed layers were exposed to a beam of hyperthermal Xe generated in a supersonic expansion (translational energies of 1.65.8 eV), with adsorbate coverages detected by He atom reflectivity. The n-alkanes show a strong chainlength-dependent reduction of the CID cross sections.
3.8.6.3.5.2 Cu Fuhrmann and Wöll [97Fuh] studied monolayers of saturated long-chain hydrocarbons (n-octane, perdeuterated n-octane, n-nonane and n-decane) adsorbed on a Cu(111) surface by high-resolution elastic and inelastic He atom scattering. At low temperatures (<160 K) the alkane chains form an ordered, welldefined two-dimensional lattice with the molecular C-C-C planes being oriented parallel to the surface. At higher temperatures a phase transition to a disordered, liquid-like state with the transition temperature dependent on the length of the hydrocarbon chain was observed.
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Ref. p. 320]
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259
3.8.6.3.5.3 Pt Adsorption of n-hexane C6H14 on Pt(111) at 100-220K was reported in [77Fir]. n-heptane C7H16 adsorption on Pt(111) produced (2×2) overlayers [74Bar, 77Fir]. On the stepped Pt(S)-[4(111) × (100)] surface, (4×2) and (4×2)-C structures were observed while on Pt(S)-[6(111) × (100)] (2×2) and (9×9)-C appeared [74Bar]. On Pt(S)-[7(111) × (310)] C7H16 adsorbed disordered, on Pt(S)-[9(111) × (100)] (2×2), (5×5)-C, (2×2)-C and 2 (one-dimensional order)-C was detected [74Bar]. n-octane adsorption was reported in [77Fir]. Using supersonic molecular-beam techniques Weaver et al. [97Wea, 98Wea] measured adsorption probabilities for neopentane on Pt(111) for coverages from zero to monolayer saturation (for incident translational energies up to 110 kJ mol−1 and incident angles between 0° and 60° at 105 K). The adsorption probability increased with coverage up to near monolayer saturation and an enhanced trapping into the second layer was suggested. Dissociative chemisorption occurs by both direct collisionally activated and trapping-mediated mechanisms. Direct dissociation dominates at translational energies > ~110 kJ mol−1 while the trapping-mediated pathway occurred at translational energies <110 kJ mol−1. Trapping-mediated dissociation seemed facilitated by surface defects. IRAS spectra of pentane and hexane on Pt(111) were reported by Chesters et al. [89Che], both for monolayers and multilayers. The spectra indicated that the orientation of the plane containing the carbon chains was parallel to the metal surface in the case of monolayers. The multilayers appeared to be crystalline with molecules aligned so that the plane containing the carbon atoms was parallel to the substrate in the case of hexane but with this plane at an angle to the surface in the case of pentane.
3.8.6.3.6 Various (Hydrocarbon fragments, Radicals, etc) For an extensive description of vibrational spectra of hydrocarbon surface species, obtained from the dissociative adsorption of halogen- or nitrogen-substituted alkanes and olefins see [97She].
3.8.6.3.6.1 Cu Chuang et al. [99Chu] investigated the interactions of methyl and methylene radicals on Cu(111) with XPS, AES and HREELS. The CH2 and CH3 radicals were generated through a hot nozzle source with ketene and azomethane gases. It was shown that at 300 K the impinging CH3 radicals were trapped mainly as CH3, while a part decomposed to CH2 and H. The hydrocarbon adspecies desorbed at ~420 K. Exposing the clean Cu surface to methylene radicals resulted not only in the trapping of CH2, but also in the formation of complex aromatic species. The adlayer was sensitive to annealing and desorption and partial conversion to methylidyne took place at ~420 K. The CH species survived up to 700 K and then decomposed to form residual carbon above 800 K. In both radical-Cu(111) systems, surface coverage appeared to saturate near one monolayer.
3.8.6.3.6.2 Mo Wu et al. [99Wu] measured IRAS spectra (700-2300 cm−1) of CH3I, CD3I, CH2I2 and CD2I2 adsorbed on Mo(100) at 80 K. Initially, the strongest infrared absorption was the δs(CH3) mode of CH3I at 1236 cm−1, which shifts on heating to 135 K, yielding a new peak at 1106 cm−1 indicating the formation of a surfaceCH3 species. CH3 species dominated the spectrum after annealing to 160 K and disappeared at 235 K, where TPD showed methane desorption. When CH2I2 was adsorbed on Mo(100), the ω(CH2) mode at 1107 cm−1 was the strongest feature, and upon heating to 135 K, a new peak appeared at 1061 cm−1, which was ascribed to a surface-CH2I species. This peak disappeared on heating to about 200 K, where UPS data showed the formation of a surface C-1 species. No IRAS data were detected for adsorbedLandolt-Börnstein New Series III/42A5
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3.8.6 Adsorbate properties of linear hydrocarbons
[Ref. p. 320
methylene species, because of either a low-adsorption cross section or the lack of appropriate symmetry of these modes. Parker et al. [97Par] used HREELS and LEED to examine the adsorption of methyl radicals on two different oxygen-modified Mo(100) surfaces at 300 K. CH3 radicals produced CH4, H2 and CO as reaction products. No surface methoxy but rather a metal alkyl analog was observed.
3.8.6.3.6.3 Ni Dickens and Stair [98Dic] studied the adsorption of CH3 radicals on clean Ni(100), on Ni(100) with a chemisorbed oxygen overlayer, and on Ni(100) with a NiO(111) oxide overlayer by XPS and TPD. Methyl radical dosing at surface temperatures of 120-170 K produced carbon coverages in excess of 5 monolayers with a C1s peak indicative of an adsorbed hydrocarbon species. The carbon coverage never saturated on any surface. TPD indicated C2-C4 formation following very large methyl radical exposures. The results were indicative of the formation of surface hydrocarbon chains. The methyl radical gas temperature had no effect on the surface chemistry. However, hydrocarbon chains formed with higher selectivity on the oxygen-modified surfaces compared to the clean surface suggesting that a direct reaction between incoming methyl radicals and adsorbed hydrocarbon groups is not involved. The chains are likely produced by polymerization of surface methylene species (-CH2-) produced by dehydrogenation of chemisorbed methyl groups via a mechanism similar to the Fischer-Tropsch synthesis reaction. On NiO, TPD indicated that surface alkoxy groups were formed based on the low-temperature desorption of CO. The chemistry of various alkyl fragments on Ni(100), produced from iodo-precursors, was described by Tjandra and Zaera [95Tja]. 1-iodopropane, 1-iodobutane, 2-iodobutane, 1-iodo-2-methylpropane, 2iodo-2-methylpropane, 1-iodopentane and 1-iodohexane on Ni(100) surfaces were studied by TPD and XPS. Below 100 K, all compounds adsorbed molecularly through the iodine atom. The hydrocarbon chain oriented parallel to the surface at first, but flipped as the coverage increased, and became perpendicular to the surface at saturation. The C-I bond dissociated between 120 and 180 K to yield the corresponding alkyl fragment on the surface. At higher temperatures the alkyl groups decomposed further, directly to carbon and hydrogen at low coverages (below half-saturation), but mainly to a mixture of alkanes and alkenes at saturation. Yang et al. [95Yan] reported HREELS spectra of CH3, CH2D and CD3 on Ni(111) (and of products of their reactions). Adsorbed methyl radicals originated from CH4, CH3D, or CD4. The CH3 radical was adsorbed with C3v symmetry on a threefold hollow site and dissociated to form adsorbed CH above 150 K. The CH species adsorbed on a threefold hollow site with the geometry of Ni3-C-H being pyramidal. Above 250 K, carbon-carbon bond formation between CH species produced C2H2. Low coverages of C2H2 dehydrogenated at 400 K while high coverages of C2H2 trimerized to adsorbed benzene (no C2H2 dissociation to adsorbed CH). The relative stabilities of the hydrocarbon species on Ni(111) were suggested to be CH3
Ref. p. 320]
3.8.6 Adsorbate properties of linear hydrocarbons
261
two different temperatures (150 and 250 K) in course of TPD. Methyl groups adsorbed on 3-fold sites react with adsorbed hydrogen atoms with an activation energy of 58 kJ/mol to form gaseous methane desorbing at 250 K. The methane peak at 150 K in TPD was attributed to mobile methyl groups (at high coverage) reacting with hydrogen with zero activation energy. These CH3 groups traverse over on-top sites and have higher energy than the methyl groups adsorbed on 3-fold sites. The cleavage of C-I bond with an activation energy of 12 kJ/mol constituted the rate determining step.
3.8.6.3.6.4 Pt Minot et al. [83Min] performed a molecular orbital study of the location of CHn and C-CHn (n=1-3) species on Pt(111) by using both a cluster model and a band structure calculation within the framework of the Extended Hückel Theory (EHT). The reported dependence of the adsorption site on the number of hydrogens in the CHn fragments suggested that any C-H bond breaking in CHn species must involve a change of the adsorbate bonding site. The carbon was found to be located on the surface in such a way as to complete its tetra-valency. Thus, CH occupies a three-fold coordinated hollow site, CH2 a two-fold coordinated bridge site and CH3 a one-fold coordinated top site, C-CH3 (ethylidyne) was found to be perpendicular to the surface in a three-fold hollow site in agreement with experimental observations. It was also found that a displacement of -C-CH2-R to a top site makes a β C-R cleavage easier. Henderson et al. [87Hen, 91Hen] used HREELS, TPD, SIMS and AES to study methyl iodide on Pt(111). CH3I decomposes to CH3 and I at ~250 K. CH4 is formed at 290 K by hydrogenation of CH3 groups with H being supplied from the decomposition of other CH3 groups. Newell et al. [98New] reported the IRAS spectrum of adsorbed ethyl (C2H5) on Pt(111) produced by the dissociative adsorption of C2H6 from a molecular beam at 150 K. IRAS indicated the presence of ethylidene ([77Fir] =CHCH3) and ethylidyne (ŁCCH3) at 250 and 350 K, respectively. This implies the stepwise loss of one and two hydrogen atoms from the ethyl moiety during the heating process. TPD measurements showed that hydrogen desorption was not accompanied by desorption of either saturated or unsaturated C-2 hydrocarbons or methane. Zaera [99Zae, 02Zae1, 02Zae2] reviewed the surface chemistry of hydrocarbons on transition metal surfaces, focusing on the clean production of alkyl surface moieties on well characterized metals and the thermal chemistry of those moieties. Alpha, beta and gamma hydride eliminations, reductive elimination with hydrogen or other alkyl groups, methylene insertions, and C-C bond breaking reactions were all treated.
3.8.6.3.6.5 Rh Klivenyi and Solymosi [95Kli] examined the thermal and photochemistry of methylene iodide (CH2I2) on Rh(111) by HREELS, XPS, AES and TPD. CH2I2 adsorbs dissociatively at submonolayer coverage at 90 K and molecularly at higher coverages. The dissociation of a monolayer starts above 170 K and is complete below 250 K. The primary products of thermal dissociation were adsorbed CH2 and I. The CH2 species was stable up to 300 K. A larger fraction undergoes self-hydrogenation to CH4 at 200-300 K, and a much smaller fraction dimerizes into C2H4. Coadsorbed O atoms inhibited the C-I bond breaking and, in addition, reacted with adsorbed CH2 to give formaldehyde at 170-340 K, and CO2 and H2O at 340-460 K. No coupling between CH2 and CH3, and CH2 and C2H5 was observed.
3.8.6.3.6.6 Ru Kis and coworkers [00Kis] investigated the adsorption and dissociation of CH2I2 on Ru(001) at 110 K using XPS, UPS, TPD, AES and work function measurements, with the aim of generating CH2 species. Adsorption of CH2I2 was characterized by a work function decrease (0.96 eV at 1 ML), indicating that adsorbed CH2I2 has a positive outward dipole moment. Three adsorption states were distinguished: a Landolt-Börnstein New Series III/42A5
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[Ref. p. 320
multilayer (200 K), a weakly bonded state (220 K) and an irreversibly adsorbed state. A new observation was the formation of CH3I, which desorbs at 160 K. The adsorption of CH2I2 at 110 K was dissociative at submonolayer, but molecular at higher coverages. Dissociation of the monolayer to CH2 and I proceeded at 198-230 K, as indicated by a shift in the 3d5/2 binding energy from 620.6 eV to 619.9 eV. A fraction of adsorbed CH2 was self-hydrogenated into CH4 (220 K), and another one was coupled to di-σ-bonded ethylene, which − instead of desorption − was converted to ethylidyne at 220-300 K. Zhou et al. [89Zho] investigated methyl iodide on Ru(001) by TPD, AES and HREELS. CH3I adsorbs dissociatively at 110 K. During TPD methane and C-C bonds were formed with CH2, CH3, CCH3 and CH being identified by HREELS. Goodman and coworkers [94Wu, 02Cho] used HREELS and TPD to examine surface species formed from CH4 decomposition on Ru(0001) and Ru( 11 2 0 ). Methane dissociated on Ru(0001) to methylidyne (ŁCH) and vinylidene (=CCH2), while for Ru( 112 0 ) CH, CCH2 and ethylidyne (ŁCCH3) were reported.
3.8.6.3.6.7 Various Au et al. [98Au] presented a theoretical treatment of CHx fragments on Ni, Pd, Pt and Cu catalysts (see 3.8.6.3.1.12).
3.8.6.4 Alkenes Because of the availability of π-electrons that can readily be donated to the metal, the bonding of alkenes to the metal surface is much stronger than the bonding of alkanes. There are two consequences: 1) the reactant alkene undergoes molecular rearrangements depending on the substrate surface structure and the temperature; 2) the metal substrate may restructure during alkene adsorption to optimise bonding to the adsorbed species. The various adsorbate geometries of ethylene represented a controversial topic for some time (see below). However, based on many studies using a variety of methods the different adsorbed species could be eventually unambiguously identified. At low enough temperatures di-σ bonding of alkenes is often preferred. An example of this is shown in Fig. 8 for ethylene adsorption on Pt(111). The center of the molecule occupies a three-fold site and the molecular axis is at 23° with respect to the surface plane. As the temperature is increased, a hydrogen shift occurs from one carbon atom to the other to form ethylidene (=CHCH3) (Fig. 9). As the temperature rises, another hydrogen atom splits off from the carbon nearest to the metal and the molecule assumes an up-right position to become ethylidyne, C2H3, at the same three-fold metal site [95Cre1, 96Cre2, 97Döl]. The metal surface restructures around the adsorption site with movements of the metal atoms in the proximity of the site. These rearrangements are shown for the Pt(111) and Rh(111) crystal faces in Fig. 10. It should also be noted that it is too simplistic to consider that only the nearest-neighbor metal atoms of the substrate participate in the bonding. There is evidence that the atoms at next-nearestneighbor sites change their location when chemisorption occurs, moving either closer or further away from the chemisorption bonds. The nature of the chemical bonds of alkenes is similar to that in organo-metallic clusters, and this analogy holds for a large number of alkene-transition metal surface adsorption systems [88Van]. Not surprisingly the vibrational spectra of the adsorption and the cluster systems are very similar. As the temperature is increased the adsorbed alkenes undergo sequential decomposition just as the adsorbed alkanes do, as evident from sequential hydrogen evolution during temperature programmed desorption (Fig. 11).
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Ref. p. 320]
3.8.6 Adsorbate properties of linear hydrocarbons
Di − σ bonded ethylene /Pt(111) Side view bCC
b1
bu
b2
fcc site
0.5 L Alkene on Pt (111) a
d00’ d01
Partial H2 pressure p H2
Top view
d12 dp
b1 hcp site
b2
C3H6 C2H4 C4H8
bCC
d00’ a d01 bu d12
200
dp
fcc site hcp site
b1 [Å] 1.92 ±0.15 1.96 ±0.15
b2 [Å] 2.07 ±0.30 2.05 ±0.30
600 400 Temperature [K ]
800
Fig. 11 Temperature programmed desorption (TPD) of C2-C4 alkenes on Pt(111) (exposure 0.5 Langmuir at 150 K). Hydrogen evolution indicates the sequential decomposition/dehydrogenation of the alkenes and carbon deposition; adapted from [77Fir].
Fig. 8 The structure of di-σ bonded ethylene on Pt(111) as determined by LEED; adapted from [97Som].
a [° degr] 23 +21/−11 22 +21/−11
263
bCC [Å] 1.56 ±0.50 1.53 ±0.40
bu [Å] 0.03 ±0.07 0.02 ±0.07
dp [Å] 0.83 ±0.25 0.74 ±0.25
d00` [Å] 0.61 ±0.07 0.57 ±0.07
d01 [Å] 1.32 ±0.12 1.35 ±0.12
d12 [Å] 2.27 ±0.05 2.27 ±0.05
Ethylene decomposition on Pt (111)
di − σ ethylene /Pt(111) H
H H
C
C
ethylidyne /Pt(111) H
H
di − σ ethylene /Pt(111)
Landolt-Börnstein New Series III/42A5
H
H H
C
H C ethylidene /Pt(111)
H
C
H
C ethylidyne /Pt(111)
Fig. 9 Schematic of ethylene decomposition on Pt(111); adapted from [95Cre1, 97Som]
264
3.8.6 Adsorbate properties of linear hydrocarbons
Rh (111) − (2×2) − C2H3
Pt (111) − (2×2) − C2H3
H H H
H H H Side view
dCC
[Ref. p. 320
Side view
dCC d01
d01 b1
b1 d12
d12 b2
b2 d23
d23
Top view
b
c
a
c
a r1 = 0.1 + 0.08 Å r2 = 0.0 + 0.09 Å
Top view
d
r1 = 0.01 + 0.06 Å r2 = 0.00 + 0.07 Å
r1
b
d
r1 r2
Fig. 10 The structure of ethylidyne adsorbed on Pt(111) and Rh(111) as determined by LEED. On Pt(111), ethylene forms an ethylidyne molecule which binds to an fcc threefold hollow site. That is, there is no metal atom directly underneath the carbon in the second metal layer. In this circumstance, the surface metal atoms move inward to presumably provide a bond to the carbon as strong as possible and metal-metal distances are altered on the surface as well. The second metal atom next to the chemisorption bond moves downward to produce a corrugated surface. On Rh(111), ethylidyne is bonded to the hcp 3-fold hollow site. This site has a metal atom right underneath the carbon bonding site in the second layer. The metal atoms move away from the carbon atom bound to the hollow site to allow the carbon to bind to the Rh atom directly underneath the carbon in the second layer. The adsorption-induced distortion in the top metal layers pulls the nearest neighbor metal atoms up out of the surface plane; adapted from [97Som].
3.8.6.4.1 Ethylene C2H4 and Ethylidyne C2H3 3.8.6.4.1.1 Ag Slater et al. [94Sla] used IRAS, LEED and electron spectroscopies to examine low-temperature adsorption of C2H4 on Ag(100) (as well as the effect of preadsorbed chlorine). On Ag(100) ethene was found weakly π-bonded at all coverages with dynamic pressures being required to saturate the monolayer at 100 K. At low coverages, the molecule lies parallel to the surface plane, but further adsorption induced a reorientation involving rotation about the C-C axis. The influence of preadsorbed Cl depended critically on the Cl coverage. At low coverage adsorption was enhanced producing ordered ethene-chlorine structures. High Cl coverage progressively passivated the surface. Rovida et al. observed no (ordered) C2H4 adsorption on Ag(110) at 293-423 K [80Ron]. IRAS spectra of C2H4 adsorbed on oxygen-activated Ag(111) at 300 K at a pressure of 1 Torr were reported by Stacchiola et al. [01Sta2].
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3.8.6.4.1.2 Au Chesters and Somorjai observed no C2H4 adsorption on Au(111) and Au(S)-[6(111) × (100)] at 300 K [75Che].
3.8.6.4.1.3 Cr Upon adsorption of C2H4 Gewinner et al. observed c(2×2)-C and (¥2 × 3¥2)R±45°-C structures on Cr(100) [82Gew].
3.8.6.4.1.4 Cu C2H4 adsorption on Cu(100) at 80 K was investigated by Nyberg and coworkers [82Nyb] using EELS and LEED. Ethylene was found adsorbed in a configuration parallel to the Cu(100) surface with the moleculemetal interaction of π-character. Ertl observed a (2×2) C2H4 structure on Cu(100), one-dimensional order on Cu(110), while no ordered structure was reported on Cu(111) [77Ert]. On Cu(111) a weakly physisorbed adsorbate state without noticeable structural changes with respect to the gas phase molecule is typically observed experimentally. Witko and Hermann [98Wit] have examined C2H4 on Cu(111) by ab-initio DFT cluster studies, yielding potential energy curves E(z) which exhibited two minima. One minimum referred to an undistorted (physisorbed) adsorbate while the other minimum pointed to a strongly distorted adsorbate (suggesting a competitive binding state). The E(z) curves also explained experiments on C2H4-Cu(111) which have observed only one of the two adsorbate states so far. Schaff et al. [95Sch] examined the adsorption geometry of C2H4 on Cu(110) by PED in the scanned energy mode as well as by angle-resolved valence level photoemission. The molecule adsorbed either in an atop site on the close-packed Cu rows at a perpendicular height of 2.08±0.02 Å with a C-C bond length of 1.32±0.09 Å, or in a short bridge site on the Cu rows at a perpendicular height of 2.09±0.02 Å with a C-C bond length of 1.53±0.13 Å. In each case the molecular plane was parallel to the surface, but the C-C axis can be twisted azimuthally out of the [110] direction by as much as 24°. STM at 4 K was used by Buisset et al. [96Bui] to directly determine the binding site of C2H4 on Cu(110) by simultaneously imaging the adsorbate and the underlying lattice. The molecule was found to bond in the short bridge site on the close-packed rows with its C-C axis oriented in the [110] direction. Ethylene may show strong geometrical distortions when adsorbed on transition metals with the bonding normally described in terms of a π-donation-π*-backdonation process. Triguero et al. [98Tri] demonstrated the importance of considering the available excited states of the free molecule in analyzing the bonding scheme of the adsorbate on cluster models of Cu(100), (110), and (111). By comparison to the structures of the triplet excited states in the gas phase it was shown that these must be considered as the states actually involved in the bonding. Yu and Leung [98Yu] investigated the effect of irradiating Cu(100) (and also O-precovered and Nprecovered Cu(100) surfaces) with low-energy (<200 eV) ethylene ions at room temperature using HREELS and TPD. Ion irradiation of a clean Cu(100) surface at 200 eV impact energy was found to produce hydrocarbon fragments that adsorbed readily on the surface at 300 K and decomposed completely after annealing to >600 K. For an O-precovered Cu(100) surface at 300 K, the hydrocarbon species produced appeared to react with the pre-deposited O atoms to form CO. In addition to a redshifted CO stretch observed at a low ethylene ion dose, a blue-shifted CO stretch was also found at a higher ethylene ion dosage. The observed red and blue shifts were attributed to adsorption of CO stabilized by a proposed direct interaction mechanism involving neighbouring surface O atoms and C-containing species. In the case of low-energy ion irradiation of a N-precovered Cu(100) surface in ethylene, EELS features attributable to C=N and N-H stretching vibrations were observed, giving support to the formation of CN and NH radicals as a result of surface reactions between the hydrocarbon species and the pre-deposited N atoms. Landolt-Börnstein New Series III/42A5
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3.8.6.4.1.5 Fe Hung and Bernasek [95Hun] studied the adsorption of C2H4 on Fe(100), and on the C- and O-covered surface using HREELS, TPD, AES and LEED. On the clean surface, C2H4 adsorbed molecularly as di-σ-bonded C2H4 which desorbed at 240 K or decomposed to form methylidyne (ŁCH) and ethynyl (-CŁCH) below 260 K. Hydrogen was the only desorption product of C2H4 decomposition. Heating the adsorbate to 523 K produced a c(2×2)-C surface. Preadsorbed carbon and oxygen blocked the chemisorption of di-ҏσ-bonded C2H4 and induced physisorption of C2H4 at 100 K. Preadsorbed O appeared to inhibit the dehydrogenation of di-ҏσ-bonded C2H4 and induced the adsorption of π-bonded C2H4. A c(2×2)-C structure on Fe(100) was also reported in [77Bru, 85Vin]. Using LEED on Fe(111), (1×1), (5×5) and (3×3) structures were observed [78Yos].
3.8.6.4.1.6 Ir C2H4 adsorption on Ir(100) produced disordered and c(2×2)-C structures [76Bro] while on Ir(110) disordered and (1×1)-C [78Nie] and on Ir(111) (¥3×¥3)R30° and (9×9)-C were observed [76Nie]. On the stepped surface Ir(S)-[6(111) × (100)] a (2×2) appeared [76Nie]. HREELS, SIMS and XPS studies of ethylidyne on Ir(111) were reported in [87Mar2, 87Mar1, 88Mar].
3.8.6.4.1.7 Mo C2H4 adsorption on Mo(100) produced c(2×2)-carbide, (1×1) and “streaked” (1×1) [76Gui, 80Ko, 81Ko, 81Oya, 82Ove]. Wang and Tysoe [90Wan] applied ARUPS, AES and TDS to monitor C2H4 on Mo(100). At 80 K ethylene adsorbed in the four-fold hollow site on Mo(100) and was substantially rehybridized to ~sp3 (θ C2H4 = 0.8±0.1). At 220 K chemisorbed C2H4 dehydrogenated to distorted C2H2. Further heating to 300 K led to carbon-carbon bond scission producing CHx with, according to TDS, an average surface stoichiometry of C1H1. C2H4 interaction with Mo(100) and oxygen-covered Mo(100) was desribed in [97Wu]. Ethylene either thermally decomposed to hydrogen and adsorbed carbon, desorbed molecularly, self-hydrogenated to produce ethane or dissociated to form adsorbed C1 species which may hydrogenate to form methane. Thermal decomposition was proposed to take place on the four-fold sites of Mo(100) since the hydrogen yield decreased linearly with the oxygen coverage. The ethylene desorption activation energy increased with increasing oxygen coverage suggesting that ethylene bonds to Mo(100) predominantly by donation of π-electrons to the surface. Accordingly, the C2H4 hydrogenation activation energy also increased as a function of oxygen coverage. The CH4 yield also varied with oxygen coverage so that no methane desorption was detected for clean Mo(100) but the yield increased with oxygen coverage reaching a maximum at ~0.6 ML and decreasing at higher coverages. XPS spectroscopy suggested that adsorbed oxygen increased the dissociation probability of ethylene. Experiments in which carbenes were grafted onto the surface by decomposing methylene iodide showed that carbenes were stabilized by oxygen on the surface.
3.8.6.4.1.8 Ni Steinrück and coworkers [02Whe, 03Neu] used temperature-programmed XPS to monitor C2H4 on Ni(100) between 90 and 530 K. The use of a third generation synchrotron light source allowed measurements of high-resolution C1s XPS spectra in “real-time” (within a few seconds), enabling to follow the thermal dehydrogenation in a quantitative and quasi-continuous manner. For C2H4 dehydrogenation, a vinyl species (-CH=CH2) was observed, with acetylene (C2H2) being the subsequent dehydrogenation product. Upon further heating, acetylide (-CŁCH) and methylidyne (ŁCH) were successively formed. Carbidic carbon was the final dehydrogenation product. Landolt-Börnstein New Series III/42A5
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Zarea et al. [86Hall, 87Zae1, 87Zae2] studied C2H4 adsorption and decomposition on Ni(100) by TPD and HREELS. Molecular adsorption of C2H4 at 90 K was observed, with little rehybridisation. Vinyl was observed at higher temperature. Using LEED to study C2H4 adsorption on Ni(110), (2×1)-C, (4×5)-C, c(2×4)-C2H4, c(2×2)-CCH and graphite overlayers were reported [75McC, 77Abb, 77McC, 84Str]. HREELS, LEED and TDS were used to study the adsorption and decomposition of ethylene on Ni(110) at 80-500 K [79Leh, 84Str, 86Ban]. Ethylene adsorbed molecularly at 80 K, but showed rehybridization to ~sp3. Its site group symmetry at 80 K is lower than C2v. A complex ordered LEED pattern was formed on adsorption at low temperatures. Above 200 K decomposition produced CCH intermediates, yielding CH upon further heating. At 500 K atomic carbon forms a (4×5) ordered overlayer. Weinelt et al. [92Wei1] studied the electronic structure of a dilute C2H4 layer (θ =0.5 θsat) on Ni(110) by ARUPS, TPD, LEED and model cluster calculations. Ethylene adsorbed with the molecular plane coplanar to the surface and the C-C axis was preferentially aligned along the [ 1 1 0 ] (C1 symmetry). Similar bonding was predicted for π-and di-σ coordination, for both cases the π-donation to the substrate being stronger than the π* backdonation. The optimized geometry parameters for the π-bonded species were: C-C: 1.42 Å; Ni-C: 2.01 Å; tilting of CH2 relative to the (110) crystal plane: 23°. The same authors also investigated the saturated C2H4 layer on Ni(110) [92Wei2]. The layer exhibited a c(2×4) LEED pattern corresponding to a structure containing two adsorbates per primitive unit cell. The C2H4 molecules were again adsorbed with the molecular plane parallel to the surface and the C-C axis preferentially aligned along [ 1 1 0 ]. Brown et al. [99Bro1] measured the heats of adsorption and sticking probabilities for C2H4 on Ni(110) at 300 K. The initial sticking probability and heat of adsorption are 0.78 and 120 kJ mol−1. CCH species were formed on the surface initially with a Ni-C bond strength of 191 kJ mol−1. This is in excellent agreement with the average calculated value of 204 kJ mol−1 for hydrocarbon adsorption on Ni(100). Giessel et al. [99Gie] used scanned-energy mode PED to determine the adsorption geometry of the c(2×4) phase of C2H4 on Ni(110) at 0.5 ML (saturation coverage), and at 0.2 ML when no long-range order was observed. For the c(2×4) phase the two molecules per primitive unit mesh occupied slightly different low-symmetry sites approximately midway between short-bridge and atop. The C-C axis was tilted with respect to the surface plane by ~10°. The C-C axes of the molecules were preferentially aligned along the close-packed Ni rows, but were offset in [100] directions away from the ridges by about 0.2 Å. At low coverage the local adsorption site was also one of low symmetry sites between atop and bridge. Steinrück and coworkers [96Ste, 01Whe, 02Whe] examined the adsorption on C2H4 on Ni(110) by angle-resolved UPS. For a “dilute” 0.25 ML layer (0.8 L exposure below 120 K) and for a saturated c(2×4) layer (0.5 ML) the C-C axis was parallel to surface, oriented along [ 1 1 0 ]. Gutdeutsch et al. [96Gut] investigated the adsorption of C2H4 on Ni(110) by angle resolved inverse photoemission (ARIPE) and DFT model cluster and slab model band structure calculations. Cluster calculations gave a slight preference for the di-σ over the π-coordinated geometry on top of the ridges, but only a very weak binding over the troughs. The half-bridge position on top of the ridges, intermediate between the short bridge (di-σ) and the top site (π), was identified as the adsorption site in the densely packed c(2×4) C2H4 /Ni(110) adsorption system. Cooper and Raval [95Coo] studied C2H4 on Ni(111) at 110 K using IRAS. Adsorption at low temperatures produced the di-σ complex, with the spectra showing good agreement with the analogous vibrational data for the model organometallic compound [Os2(CO)8(u2-η2-(C2H4)]. IR data obtained at coverages θ < 0.25 were interpreted in terms of a local C2v site symmetry. No evidence for significant non-planarity of the C2M2 skeleton was found. Using EELS Lehwald and Ibach [79Leh] studied C2H4 adsorption at 150 K and C2H4 decomposition upon annealing to higher temperature. On Ni(111) ethylene rehybridized upon adsorption and dehydrogenerated to acetylene above 230 K. Ethylene partially dehydrogenated on the stepped Ni [5 (111) × (110)] surface. Further annealing produced coadsorbed H and CH. Hammer et al. [86Ham] performed combined LEED/EELS measurements of C2H4 on Ni(111). Long-range order phases were c(4×2) and (2×2) with bridge positions being suggested as adsorption sites for the carbon atoms. DalmaiLandolt-Börnstein New Series III/42A5
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Imelik and Bertolini [74Dal] reported a change in work function upon adsorption of C2H4 on Ni(111) of −0.3 eV for adsorbed ethylene at 3×10−9 and 10−8 Torr. Bao et al. [95Bao] performed a quantitative evaluation of the local geometry of C2H4 on Ni(111) using C1s scanned-energy PED. At 120 K C2H4 had its C-C axis parallel to the surface, adsorbed on an “aligned” bridge site such that the C atoms were approximately atop Ni atoms. Heating led to dehydrogenation to acetylene and, while the C-C axis remained parallel to the surface, the C-C bond length and C-Ni layer spacing were reduced. C2H2 occupied a “cross-bridge site” with the C atoms above inequivalent hollow sites on the surface. Both adsorbed species had C-C bond lengths larger than those of the associated gas-phase molecules indicating a significant reduction of C-C bond order. Khan and Chen [03Kha] examined the reactivity of monolayer Ni films on Pt(111), W(110), and Ru(0001) single crystal surfaces. The bonding and decomposition of C2H4 was studied by TPD. The 1 ML Ni/Pt(111) surface was relatively inactive toward C2H4 decomposition, while the 1 ML Ni/Ru(0001) surface remained active toward C2H4 dissociation.
3.8.6.4.1.9 Pd Among the platinum metals, Pd is considered the most selective for hydrocarbon hydrogenation. The characterization of hydrocarbon intermediates is therefore of great interest. Stuve and Madix [85Stu1] studied C2H4 adsorption and reaction on Pd(100) by TPRS and HREELS, detecting both di-σ and π-bonded C2H4 at 80 K. IRAS spectra were reported in [97Cam]. Madix and coworkers [85Stu2, 95Guo1] investigated C2H4 adsorption and reaction on an atomic oxygen-covered Pd(100)-p(2×2)-O surface using TPRS. Compared to the dehydrogenation reaction on clean Pd(100), O inhibits both C2H4 adsorption and reaction. Combustion products were H2O, CO and CO2 but no partial oxidation products were observed. Isotope experiments showed that initial reactions occur predominantly with the vinylic C-H bonds. The same group [95Guo2] also investigated the adsorption and reactions of C2H4 (and of propene and 1-butene) on the Pd(100)-p(1×1)-H and Pd(100)p(1×1)-D surfaces by TPRS. Efficient H-D exchange reactions below 300 K occurred for all C-H bonds for ethylene, propene and 1-butene, whereas no hydrogenation products (alkanes) were observed. The exchange reaction was proposed to occur via reversible hydrogenation to a half-hydrogenated intermediate. The absence of alkene hydrogenation may be due to stronger metal-hydrogen bonds on Pd(100) than on other metals such as Pt and Rh (or may be influenced by hydrogen dissolution [04Rup]). The adsorption structure of C2H4 on Pd(110) was analyzed by NEXAFS and STM by Ogasawara et al. [01Oga] pointing to the C-C bond aligned along [ 1 1 0 ]. Interaction of C2H4 with Pd(110)-(2×1)-H, studied by TDS and HREELS, was reported in [92Sek]. On the same surface, Ichihara et al. [98Ich, 00Ich] observed one-dimensional (3×1) and c(2×2) domains by STM and HREELS. π-bonded ethylene adsorbed at on-top Pd sites. On Pd(111), Gates and Kesmodel [81Kes, 83Gat] concluded from angle resolved EELS that the dominant species for 300 K adsorption is ethylidyne with the C-C axis perpendicular to the surface while low temperature spectra showed essentially undistorded π-ethylene. ARUPS results by Lloyd and Netzer supported this [83Llo]. The exact structure of ethylidyne had been already resolved for the Pt(111) system (see 3.8.6.4.1.10) by LEED, EELS and by comparisons to analogous organometallic complexes. Using a combination of TPD and IRAS, Tysoe and coworkers [97Kal, 01Sta1, 02Stac1] reported that C2H4 adsorbed on clean Pd(111) in a di-σ configuration but converted to π-bonded species when the surface was presaturated by hydrogen. Subsurface hydrogen was made responsible for the formation of π-bonded ethylene on hydrogen-covered Pd(111) [03Stac1]. Ethane was formed with an activation energy of 12.5±1.2 kJ/mol only when Pd(111) was pre-covered with hydrogen and not when ethylene and hydrogen were co-dosed, indicating that ethylene blocks hydrogen adsorption [01Sta1]. Upon C2H4 adsorption on Pd(111) at 300 K an ethylidyne species was observed by IRAS (methyl mode at 1329 cm−1 [97Kal]). Upon increasing the pressure up to 1 mbar a loss in the order was reported probably caused by coadsorption of C2H4 [97Kal]. Ethylidyne spectra could still be observed at elevated gas pressure because the methyl bending mode falls into a window of the gas phase C2H4 features. Coadsorption studies with CO indicated that ethylidyne adsorbed in fcc threefold hollow sites of Pd(111) [00Sta, 01Sta1]. Ethylene Landolt-Börnstein New Series III/42A5
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adsorption on ethylidyne-saturated Pd(111) at 80 K indicated the presence of di-σ-bonded ethylene [02Stac2]. TPD revealed that the saturation ethylene coverage on ethylidyne-covered Pd(111) was 0.25 ML compared to 0.33 ML for ethylene on clean Pd(111). TPD indicated a desorption activation energy of 28 kJ/mol for π-bonded ethylene on Pd(111) [84Tys, 01Sta1]. Sock et al. [03Soc] investigated C2H4 adsorption and its thermal evolution on clean and oxygen precovered Pd(111) by HREELS, high-resolution XPS, TDS and by ab initio DFT. On clean Pd(111) at 100 K C2H4 was found in a di-σ bonding state, whereas on Pd(111)-2×2-O the π-bonded configuration was more stable. Upon adsorption at 300 K ethylidyne was formed on both surfaces, but neither di-σ nor π-bonded ethylene were observed to transform into ethylidyne on heating from low temperature up to 450 K. Complete molecular desorption was observed in both cases, with no signs of dehydrogenation. Ethylidyne C2H3 on Pd(111) was also characterized by high-resolution C1s core level photoemission spectra by Sandell et al. [98San]. C2H4 adsorption and hydrogenation on Pd(111) was studied by SFG spectroscopy, both under UHV and atmospheric pressure [02Rup, 03Fre, 05Mor]. Fig. 12 shows SFG spectra after adsorption at different temperatures. Under UHV and at 100-200 K, ethylene adsorbed in a di-σ-configuration with a characteristic peak at 2910 cm−1 (νS(CH2); Fig. 12a(1)). The second weak peak around 2960 cm−1 was attributed to the νS(CH2) of π-bonded ethylene. Its low intensity may be due to a small surface concentration and/or the orientation of π-bonded C2H4. On single crystal surfaces the νS(CH2) signal for π-bonded C2H4 (with the C-H bonds nearly parallel to the metal surface [02Ge]) is supposed to be weak due to the surface-dipole selection rule for metal surfaces (dynamic dipoles parallel to the surface plane are cancelled by image dipoles inside the metal [83Hof]). A surface covered by π-bonded C2H4 was produced by adsorbing hydrogen first (which blocks threefold hollow sites), followed by C2H4 adsorption at 100 K (Fig. 12a(2)), in agreement with corresponding IRAS measurements [01Sta1, 02Stac1]. When the C2H4 layer was heated to 300 K nearly all of the ethylene desorbed and only a small amount was dehydrogenated to ethylidyne [03Soc]. Compared to Pt(111) (see below), on Pd ethylene has a smaller tendency to produce ethylidyne. Only after adsorbing ethylene at 300 K [97Kal], a signal of ethylidyne (M≡C-CH3) could be observed at ~2875 cm−1 (νS(CH3); Fig. 12a(3)) due to ethylene decomposition. Under mbar pressure of ethylene/hydrogen mixtures no distinct signals were observed providing indirect evidence that both di-σ-bonded ethylene and ethylidyne are not reacting and that rather π-bonded ethylene is the active species in C2H4 hydrogenation (similar as for Pt(111)). As mentioned above, C2H4 adsorption on Pd(111) and its coadsorption with H was examined by a number of groups using TPD [01Sta3, 03Doy, 04Doy, 04Rup] (Fig. 12b). TDS spectra of C2H4 adsorption (Fig. 12b(2)) show a broad desorption peak around 260 K and a small desorption feature at 195 K. While the first can be attributed to di-σ-bonded ethylene, the origin of the low temperature desorption is unclear. It may be due to π-bonded ethylene but may also originate from rearrangements in a di-σ bonded ethylene layer [03Mor, 05Mor]. When 1 L hydrogen was adsorbed before 1 L C2H4 was dosed at 95 K (Fig. 12b(4)), C2H4 was (mainly) bonded in ҟa π-configuration which desorbed at lower temperature. Preadsorbed H occupied threefold hollow sites but π-bonded C2H4 could still adsorb at on-top sites (while di-σ-bonded ethylene was (partly) blocked). An influence of subsurface hydrogen on the C2H4 binding state was also suggested [03Stac1]. A small amount of ethylene decomposed into ethylidyne C2H3 around 300 K and its further decomposition led to the small H2 peak at ~425 K (Fig. 12b(3)).
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Pd (111) 3
298
2870
1
π 1 2910
2800
a
2960 2900 3000 -1 Wavenumber [cm ]
+
QMS intensity [a.u.]
SFG intensity [a.u.]
2975
H2 260
195
ethylidyne 2
[Ref. p. 320
di-s
2
C2H4 298 H2
3 4
C2H4 H2 + C2H4
π
200
b
400 Temperature [K]
600
Fig. 12 (a) SFG spectra of C2H4 species on Pd(111). Exposures were 2.5 L C2H4 at 100 K (1), 1 L H2 followed by 2.5 L C2H4 at 100 K (2). Trace (3) was acquired at 300 K after annealing in 5×10−7 mbar C2H4 from 100 to 300 K; adapted from [05Mor]. (b) Thermal desorption spectra of hydrogen (mass 2) and C2H4 (mass 27) on Pd(111). TDS spectra of the individual components are shown in (1, 2), those of coadsorption in (3, 4). Exposures were: (1) 1 L H2 at 95 K; (2) 1 L C2H4 at 95 K; (3) and (4) display the desorption traces after exposing Pd(111) to 1 L H2 and subsequently to 1 L C2H4 at 95 K; adapted from [04Rup].
Although vibrational and photoelectron spectroscopy of C2H4 adsorption at ~100 K indicated di-σ bonded C2H4 as stable species (e.g. [01Sta1, 03Fre, 03Soc, 05Mor]), theoretical studies rather suggest a combination of di-σ bonded C2H4 at bridge sites and π-bonded C2H4 at on-top sites [00Neu1, 02Ge]. Neurock et al. [00Neu2, 02Ge] used first principles DFT calculations to study C2H4 chemisorption on the (111), (100) and (110) surfaces of Pd. On all the three low-index surfaces the most stable site and geometry for C2H4 was that where the C-C bond axis is parallel to the surface along the bridge site. The calculated binding energies Eads followed the trend 111<100<110. Ethylene dehydrogenation paths over Pd(111) [02Pal] and C2H4 hydrogenation on Pd(100) [00Han, 00Neu1] were also examined. The DFTcalculated binding energies for ethylene (π), ethylene (di-σ), ethyl, vinyl, ethylidyne, atomic oxygen, and atomic carbon on a Pd-19 cluster (and a Pd(111) slab) were found to be −30 (−27), −60 (−62), −130 (−140), −237 (−254), −620 (−636), −375 (−400) and −610 (−635) kJ/mol. Mittendorfer et al. [03Mit] also carried out a comparative DFT study of the adsorption of ethylene, 1-butene, acetylene, and 1,3-butadiene on Pd(111) and Pt(111), analyzing structural, electronic, energetic, and spectroscopic properties. Chemisorption of C2H4 on ultrathin (mono-, bi-, and trilayer) Pd films on Mo(100) was studied by Heitzinger et al. [93Hei] using AES, TPD and HREELS. C2H4 was weakly chemisorbed on the Pd monolayer, and the adsorbed species was much less rehybridized from sp2 in the gas phase toward sp3 as compared to C2H4 chemisorbed on Pd(100). Reversible C2H4 adsorption was increased on the Pd monolayer, i.e. a smaller fraction dehydrogenated during TPD.
3.8.6.4.1.10 Pt Ethylene adsorption, particularly on Pt, has received much attention due to its importance for C2H4 hydrogenation. The reaction takes place already at ~300 K presumably by stepwise hydrogenation of ethylene, as proposed already by Horiuti and Polanyi [34Hor]. LEED studies of C2H4 adsorption on Pt(100) revealed a c(2×2) structure [68Mor, 69Mor, 75Lan1, 75Lan2, 77Fis1, 78Fis]. C2H4 adsorption and dehydrogenation on the hexagonally reconstructed Pt(100)-hex-R0.7° surface was examined by Ronning et al. [01Ron] using STM and LEED. The reconstruction was lifted upon ethylene adsorption and heterogeneous nucleation of highly anisotropic (1×1) domains was observed. Landolt-Börnstein New Series III/42A5
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C2H4 adsorption and C2H4-H coadsorption on Pt(110) at 93 K was examined by Yagasaki and Masel using EELS and TPD [89Yag, 90Yag2]. Adsorbed H substantially modified the adsorption of ethylene. While C2H4 adsorption on clean (2×1) Pt(110) resulted in di-σ and π-bound ethylene, little di-σ ethylene was observed if the sample was preexposed to 100 L H2. Hydrogen acts as a site blocker and prevents di-σ ethylene formation, and also inhibits C2H4 decomposition. King and coworkers [95Stu] reported the calorimetric heat of interaction for ethylene on Pt(110)-(1×2) (Fig. 2). Being ~205 kJ/mol at low coverages, it drops in several steps to 120 kJ/mol with increasing coverage. Three stable species were identified on the surface, and the average bond dissociation energy of a Pt-C single bond was extracted for each. The mean value was ~223 kJ/mol, systematically decreasing from 235 to 214 kJ/mol as the number of single Pt-C bonds per adsorbate molecule increased from 2 to 4. On Pt(111) it was shown by UPS and EELS that below 50 K C2H4 molecules interact weakly with the surface via π-coordination [91Cas]. LEED surface crystallography was able to reveal the detailed atomic structure of the different adsorbed ethylene species [94Sta,97Döl]. Below 50 K ethylene is physisorbed with its C-C bond parallel to the surface, and the interatomic distance between the two carbon atoms is almost unchanged with respect to the gas-phase molecule [91Cas,94Sta,97Döl]. The molecule’s π orbital bonds directly with the platinum surface with π-bonded C2H4 bound to on-top sites. Stöhr et al. [84Stö] studied C2H4 on Pt(111) with NEXAFS and also found it parallel to the surface, with a C-C bond length of 1.49±0.03 Å. Albert et al. [82Alb] used ARUPS and reported the carbon-carbon bond axis to be parallel to the surface at low temperature. Kubota et al. reported a desorption activation energy of 40±10 kJ/mol for π-bonded ethylene [94Kub]. At temperatures between 60 K and 240 K, the (“gas phase like”) carbon-carbon double bond of the adsorbed molecule is broken and the carbon atoms attain nearly sp3 hybridization (Fig. 8). Two σ-bonds are formed with the underlying platinum surface atoms and this species is usually referred to as di-σ bonded ethylene [82Iba1]. Di-σ-ethylene occupies three-fold, face centered cubic (fcc) adsorption sites (Fig. 8). This species was characterized in detail by EELS (Fig. 13) [77Iba2, 78Iba]. When the temperature was further increased di-ҏσ-ethylene was dehydrogenated to form ethylidyne ŁC-CH3 (by transferring a hydrogen atom to the other carbon and by losing one hydrogen) [95Cre1] (Figs. 9-11). As shown by LEED, both ethylidyne and di-σ-ethylene are located in fcc threefold hollow sites with the molecular axis of di-σ-ethylene tilted 23° away from the surface plane [93Sta, 97Döl]. Ethylidyne is stable up to 450 K and is further dehydrogenated at higher temperature (ŁC-H species).
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Pt (111)
470
140 - 260 K ×1500 990
Intensity [a.u.]
790
1230 1420
2940
×1400 445
2 L C2H4
895 740 1150
-1
80 cm
2160
Fig. 13 Electron energy loss spectra of C2H4 adsorption on Pt(111) at high coverage. The observed vibrations indicated sp3 hybridization and are consistent with di-σ bonding to the surface. A spectrum of C2D4 is shown for comparison; adapted from [78Iba].
2275 2 L C2D4 0
1000 2000 -1 Energy loss [cm ]
3000
4000
A detailed description of the initial studies that led to the discovery of ethylidyne C2H3 can be found e.g. in [85Bee]. The earliest structural studies were carried out by Kesmodel et al. on C2H2 on Pt(111) using LEED [77Kes] concluding that a stable di-σ C2H2 species was formed. EELS studies of C2H4 adsorption by Ibach [77Iba2, 77Iba1] proposed an ethylidene (=CHCH3) species. After a LEED analysis and reinterpretation of vibrational spectra by Kesmodel et al. [78Kes, 79Kes] ethylidyne was proposed as stable species with the C-C bond normal to the surface and a bond length of 1.5 Å (slightly less than the single carbon-carbon bond length of 1.54 Å). It was suggested that three equivalent Pt-C bonds form in a threefold hollow site on Pt(111) (Fig. 10). Based on vibrational data from transition metal clusters with ethylidyne ligands it was shown that Ibach´s EELS data were consistent with the ethylidyne structure. A crucial proposal from Kesmodel et al. [78Kes, 79Kes] was the analogy to the organometallic clusters CH3CCo3(CO)9 and CH3CRu3(CO)9 which was used as model compound to interpret EELS data (an analogy that is also valid for other hydrocarbons [99Ans, 04Ans]). Albert et al. confirmed the upright orientation using ARUPS [82Alb]. Horsley et al. [85Hor] studied ethylidyne on Pt(111) using NEXAFS and found it disordered but with a C-C bond of 1.47±0.03 Å perpendicular to the surface. Wang et al. [85Wan] used NMR to study ethylidyne on Pt(111) and found it disordered but perpendicular to the surface, with a C-C bond of 1.49±0.02 Å. Using TPD methods, a carbon to hydrogen ratio of 2:3 was determined, further confirming the ethylidyne C2H3 picture [79Dem1, 82Ste]. Convincing evidence came also from a normal mode analysis for the ethylidyne nanocarbonyl tricobalt complex CH3CCo3(CO)9 by Skinner et al. [81Ski] showing an excellent agreement with EELS spectra. It is now generally agreed that ethylidyne is the stable product of C2H4 decomposition on Pt(111) (and Pd and Rh(111)). The different C2H4 adsorbate species could also be identified by vibrational spectroscopy. Mainly infrared spectroscopy [88Moh, 92Rek] was applied but sum frequency generation (SFG) data were also obtained recently, including mbar pressure ranges [95Cre2, 95Cre1, 96Cre2, 96Cre3, 96Cre4, 99Som, 03Fre]. Three prominent features at 2880 cm−1, 2910 cm−1 and 3000 cm−1 were observed. The peak 2880 cm−1 was the νS(CH3) of ethylidyne (M≡CCH3), the feature at 2910 cm−1 resulted from the νS(CH2) of chemisorbed di-σ bonded ethylene, and the peak just below 3000 cm−1 was the νS(CH2) of (weakly bonded) physisorbed π-bonded ethylene. On single crystal surfaces the νS(CH2) signal for π-bonded molecules was weak due to the surface-dipole selection rule for metal surfaces (dynamic dipoles parallel to the surface plane are canceled by image dipoles inside the metal) [83Hof]. Ethylidyne and the di-σ ethylene compete for the same (hollow) adsorption sites. The transition from di-σ ethylene to ethylidyne via ethylidene (=CHCH3) (Fig. 9) was monitored by SFG and a vibrational peak at 2957 cm−1 (νas(CH3)) Landolt-Börnstein New Series III/42A5
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provided strong evidence for the ethylidene intemediate [95Cre1]. A mixture of π-bonded and di-σ-bonded ethylene can be obtained by exposing the Pt(111) surface to a near-saturation coverage of oxygen at room temperature, followed by exposure to ethylene at 120 K [82Ste]. IRAS spectra from the deuterium substituted ethenes, H2C=CD2 and D2C=CD2 on Pt(111) at 360 K were reported by Chesters et al. [90Che4]. The spectra from H2C=CD2 indicated isotopic scrambling between the adsorbed ethylidyne species, possibly mediated by the coexisting Pt-H/Pt-D species. Variable temperature STM was applied by Land et al. [92Lan] to image C2H4 adsorption and decomposition on Pt(111). The conversion of C2H4 to C2H3 was directly observed, occurring in a ”patchy” manner across the surface. As the reaction proceeded, well-defined islands of unreacted C2H4 continued to be observed. Further dehydrogenation of ethylidyne up to 500 K produced randomly dispersed carbon containing particles. Annealing the carbon particle covered surface to above 700 K resulted in the formation of monolayer thick graphite islands, eventually accumulating at Pt steps. Zaera et al. [96Zae] applied IRAS and isothermal kinetic measurements to study the simultaneous occurrence of several reactions, namely molecular desorption, dehydrogenation to ethylidyne, H-D exchange within the adsorbed molecules, and hydrogenation to ethane. The vinyl (-CH=CH2), ethyl (-CH2CH3) and ethylidene (=CHCH3) species were prepared by thermal decomposition of their corresponding iodides and characterized by IRAS. The formation of ethylidene as an intermediate in the conversion of ethylene to ethylidyne was suggested but the complexity of the kinetics of that reaction (depending strongly on surface coverage) made a final proof difficult. A side ethylene-ethyl equilibrium at temperatures below those required for the formation of ethylidyne was responsible for H-D exchange in ethylene. The hydrogenation of ethylene to ethane also involved an ethyl intermediate, but only occurred at high ethylene coverage which was needed for the transition of the strongly bonded di-σ species to a weak π-configuration. Domen et al. [01Oht] reported the suppression of ethylidyne formation on Pt(111) by reversible adsorption of di-σ-bonded ethylene in the presence of 1.3×102 Pa C2H4 using IRAS. Di-ҏσ-bonded ethylene was converted to ethylidyne at 260-300 K in 1.3×102 Pa of ethylene, while it was converted already at 240-260 K in UHV. The vacant sites adjacent to di-σ-bonded ethylene were suggested to be required for ethylidyne formation. These sites were occupied by di-σ-bonded ethylene when the adsorbed molecules were in equilibrium with gaseous ethylene. The reversible adsorption of di-σ-bonded ethylene was indicated by an IRAS peak at 2906 cm−1 even at 300 K, which was confirmed by the substitution of isotopically labeled ethylene at 200 K. The same group also studied C2H4 hydrogenation on Pt(111) by DFT [00Miu] and IRAS [99Oht] and found that the rate did not depend on the coverage of di-σ-bonded ethylene or ethylidyne, in agreement with SFG studies [96Cre2, 99Som, 03Fre]. IRAS studies of ethylidene (=CH-CH3) and ethylidyne (ŁC-CH3) were also reported by Newell et al. [98New]. These species appeared during the decomposition of adsorbed ethyl (C2H5) on Pt(111) produced by dissociative adsorption of ethane from a supersonic molecular beam at 150 K. Lee and Wilson [03Lee] investigated the adsorption and decomposition of C2H4 on Pt(111) by fast XPS. At 100 K ethylene exhibited a precursor-mediated adsorption kinetics, adopting a single environment with a saturation C2H4 coverage of 0.25 ML and a binding energy of 283.2 eV. Thermal decomposition proceeded above 240 K via dehydrogenation to ethylidyne with an activation barrier of 57±3 kJ mol−1 and a preexponential factor ν = 1×1010±0.5 s−1. Site-blocking by preadsorbed SO4 reduced the ethylene saturation coverage but induced a new, less reactive π-bonded ethylene species centered around 283.9 eV, which in turn decomposed to ethylidyne at 350 K. Sautet and Paul [91Sau] compared the different low temperature coordination modes of ethylene on Pt(111) and (110) by extended Hückel calculations, using a 49 or a 44 atoms cluster as model. The di-σ coordination was found more stable on Pt(111) but on Pt(110) the π-coordination yielded the same adsorption energy. The π-mode was favoured by a decrease of the four electron repulsion caused by a smaller number of metal neighbours for the surface atom on Pt(110). Mittendorfer et al. [03Mit] carried out a comparative DFT study of the adsorption of ethylene, 1-butene, acetylene, and 1,3-butadiene on Pt(111), analyzing structural, electronic, energetic and spectroscopic properties. In another theoretical study Cortright et al. [99Cor] have used quantum chemical methods to estimate energetics for interactions of various C2Hx species with platinum. It was suggested that the primary reaction pathways for cleavage Landolt-Börnstein New Series III/42A5
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of the C-C bond take place through activated complexes based on ethyl (C2H5) and ethylidene (=CH-CH3) species. Brown et al. [99Bro2] measured the sticking probability and heat of adsorption for C2H4 on the stepped Pt(211) and Pt(311)-(1×2) surfaces using microcalorimetry. Adsorption on Pt(211) leads to the formation of several different species as a function of coverage, whereas adsorption on Pt(311)-(1×2) produced only one species on the surface over the whole coverage range. The initial heats of adsorption for C2H4 on Pt(211) and Pt(311)-(1×2) were 180 and 220 kJ/mol, respectively. The initial sticking probabilities were 0.84 for both surfaces. The most likely species to be formed on Pt(211) as a function of coverage were quad-σ acetylene followed by ethylidyne whereas on Pt(311)-(1×2) ethylidyne forms at all coverages. LEED studies on a variety of stepped Pt surfaces, Pt(S)-[4(111) × (100)], Pt(S)-[6(111) × (100)], Pt(S)-[7(111) × (310)], Pt(S)-[9(111) × (111)] and Pt(S)-[9(111) × (100)] revealed (2×2), (¥19×¥19)R22.4°-C, disordered structures and graphite overlayers [72Lan, 74Bar, 75Lan1, 78Net1, 78Net2].
3.8.6.4.1.11 Re C2H4 adsorption on Re(0001) produced disordered and (2×¥3)R30°-C structures [78Duc, 81Duc]. On various stepped Re surfaces, Re(S)-[14(0001) × ( 10 1 0 )], Re(S)-[6(0001) × ( 16 7 1 )], (2 × ¥3)R30°-C and disordered structures were observed [81Duc].
3.8.6.4.1.12 Rh LEED studies of C2H4 on Rh(100) revealed c(2×2), c(2×2)-C2H+C2H3, (2×2)-C2H, c(2×2)-C and graphite overlayer structures [78Cas, 82Dub]. Slavin et al. [88Sla1, 88Sla2] demonstrated by HREELS that ethylidyne (ŁC-CH3) was formed on Rh(100). Although the exact binding site was not determined, ethylidyne was identified for the first time on a surface which lacks three-fold hollow sites. The ethylidyne species was produced by preadsorbing ~0.5 ML CO followed by adsorption of C2H4 at 300 K. Adsorption of C2H4 alone produced a mixture of hydrocarbon fragments which included ethylidyne. Ethylidyne was thermally stable up to 350 K. According to HREELS, the ethylidyne species stands approximately upright on the surface with its carbon-carbon bond along the surface normal, similar to the situation on Rh(111). Kose et al. [99Kos] determined coverage dependent heats of adsorption and sticking probabilities for C2H4 on Rh(100) by microcalorimetry. For C2H4, the initial heat of adsorption was 175±10 kJ mol−1, and the initial sticking probability was 0.88±0.01. Ethylene adsorption on Rh(100) at 300 K produced σ-bonded CCH and the corresponding Rh-C bond energy was estimated as ~268 kJ/mol. Ethylidyne (ŁC-CH3) on surfaces other than Pt(111) was first reported by Dubois et al. [80Dub], studying C2H4 on Rh(111) by LEED, TPD and EELS. When ethylene adsorbs on Rh(111) [91Wan], it looses a hydrogen and rearranges (like on Pt) but on Rh ethylidyne occupies an hcp hollow site (i.e. a hexagonal close packed hollow site with a metal atom directly underneath the chemisorption site in the second metal layer) (Fig. 10). Contrary to Pt(111), the Rh metal atoms move away from the carbon atom bound to the hollow site to allow the carbon to better bond to the Rh atom directly underneath the carbon in the second layer. The adsorption-induced distortion in the top metal layers pulls the nearest neighbor metal atoms up out of the surface plane. Koestner et al. [82Koe2] determined by LEED crystallography that on Rh(111) the C-C bond of ethylidyne was anomalously short for an sp3 carbon (1.45±0.10 Å vs. 1.54 Å) and that the terminal carbon was located 1.31±0.10 Å above a 3-fold hcp site. A σ-π hyperconjugation of the ethylidyne orbitals was proposed as reason for bond shortening. Koel et al. [84Koe] combined HREELS with a UHV-high-pressure system to study the structure of stable hydrocarbon species that form during catalytic reactions at atmospheric pressure. A monolayer of adsorbed ethylidyne (ŁC-CH3) on Rh(111) at 310 K did not hydrogenate to ethylene or ethane in one Landolt-Börnstein New Series III/42A5
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atmosphere D2. The exchange of H in the methyl group with deuterium was slow with the amount of exchange strongly dependent on the amount of uncovered, bare-metal surface (with little effect of the hydrogen pressure). A mechanism for H-D exchange involving an ethylidene (=CHCH3) intermediate was proposed. C2H4 and C2H3 on Rh(111) were also characterized via their vibrational fine structure in C1s core level XPS spectra by Andersen et al. [97And]. The chemisorption of C2H4 was studied on stepped Rh(755) and (331) surfaces by LEED, AES and TDS [79Cas]. Several ordered surface structures were observed. The LEED patterns seen on the (755) surface were due to the formation of surface structures on the (111) terraces, while on the (331) surface the step periodicity played an important role in the determination of the unit cells of the observed structures. When heated in a low pressure of C2H4 the (331) surface was more stable than the (755) surface which readily formed (111) and (100) facets. On the stepped Rh(S)-[6(111) × (100)] a disordered layer was observed [79Cas].
3.8.6.4.1.13 Ru Henderson et al. [88Hen] studied the adsorption and decomposition of C2H4 on Ru(001) with HREELS, SIMS and TPD. Ethylene adsorbed molecularly on Ru(100) in a di-σ bonded structure and decomposed to ethylidyne (CCH3) above 150 K. IRAS studies were reported in [93Ran].
3.8.6.4.1.14 Si Jackman et al. [94Chu2, 95Jac] studied reactions of C2H4 with atomic hydrogen on Si(100). Such reactions may be important for CVD diamond nucleation and growth. Without atomic hydrogen, C2H4 displayed a simple surface chemistry, adsorbing molecularly at low temperatures and dissociating irreversibly to gaseous hydrogen and surface carbon as the temperature was raised. Atomic hydrogen stimulated C-H and C-C bond making/breaking processes. It was observed that adsorbed C2H4 was converted to C2H2 and ultimately to adsorbed CH2 species in the presence of an atomic hydrogen flux. LEED studies of C2H4 on Si(331) exhibited (1×1), (2×1) and (3×1) structures [70Hec].
3.8.6.4.1.15 Ta C2H4 adsorption on Ta(100) was examined using LEED and AES [74Che3]. Carbon dissolution into the crystal was observed.
3.8.6.4.1.16 W The interaction of C2H4 with W(100) was studied from 80-500 K by monitoring changes in the carbon Auger peak shape [74Che2, 74Che1, 79Che]. At 80 K decomposition to C2H2 occurred followed by nondissociative adsorption. Heating the adsorbate to 300 K resulted in further decomposition to C2H2. Decomposition to chemisorbed C atoms was detected above 300 K. (15×3)Rα-C and (15×12)Rα-C structures were observed for C2H4 adsorption on W(110), and (1×1) for adsorption on W(111) [69Bou, 78Win].
3.8.6.4.1.17 Alloys Koel and coworkers [89Paf, 97Tsa] investigated the adsorption and decomposition of C2H4 on Pt(111) and the (2×2) and (¥3×¥3)R30° Sn/Pt(111) surface alloys with TPD, LEED and sticking coefficient measurements. A (2¥3×2¥3)R30º ordered structure for C2H4 chemisorbed on the (2×2)Sn/Pt(111) surface Landolt-Börnstein New Series III/42A5
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alloy was reported. Chemisorption energies of C2H4 over Pd and PdAu alloys were calculated from firstprinciples DFT calculations by Neurock et al. [02Neu, 03Mei]. Alloying the surface with Au reduced the activation barrier for hydrogenation (63 kJ/mol on Pd to 29-33 kJ/mol on PdAu) but increased the barriers for H2 dissociation and ethylidyne formation (Au reduces the number of sites that can activate hydrogen which decreases the rate of ethylene hydrogenation). These two effects balance each other out so that the primary influence of Au is to decrease ethylene decomposition that leads to ethylidyne and CHx and deactivates the catalysts.
3.8.6.4.2 Propene C3H6 3.8.6.4.2.1 Mo Vu and Tysoe [97Vu] reported that propene (propylene) C3H6 adsorbed on Mo(100) and oxygen-covered Mo(100) either thermally decomposed to hydrogen and surface carbon, desorbed molecularly, selfhydrogenated to form propane, or ultimately decomposed to adsorbed C1 species, which hydrogenated to CH4. Because the amount of hydrogen desorption decreased linearly with increasing oxygen coverage, it was proposed that propylene decomposition proceeded on the four-fold hollow sites of Mo(100). Predosing the surface with hydrogen increased the yield of self-hydrogenation to propane indicating that propylene reacted with surface hydrogen. It was proposed that propylene adsorbs on molybdenum by π-donation.
3.8.6.4.2.2 Ni Whelan et al. [01Whe, 02Whe] examined adsorption and decomposition of propene (C3H6) on Ni(100) between 90 and 530 K using temperature-programmed C1s core level XPS with synchrotron radiation. At 105 K, C1s spectra indicated precursor mediated occupation of a single adsorption state from submonolayer to monolayer coverage with evidence of adsorbate-adsorbate interactions. High exposures lead to the formation of multilayers which desorbed above 105 K leaving a chemisorbed monolayer. Between 105 and 150 K, a shift of the binding energies in the C1s spectra was attributed to the transition from π- to di-σ-bonded propene. The conversion of di-ҏσ-bonded propene to a C3 intermediate containing a methyl group occurred at 200 K. Formation of this -C2HxCH3 surface species was complete at 300 K and was followed by dehydrogenation to carbidic carbon which was the final decomposition product above 370 K.
3.8.6.4.2.3 Pd Reactions of propene on atomic oxygen-covered Pd(100)-p(2×2)-O were carried out by Guo and Madix [95Guo1] using TPRS. No partial oxidation products were observed, “combustion” yielded H2O, CO and CO2. Adsorbed O did not inhibit propene adsorption (while O inhibited both adsorption and reaction of ethylene). Isotope experiments showed that initial reactions occurred predominantly with the vinylic C-H bond. The same authors [95Guo2] also investigated the adsorption and reactions of C3H6 on Pd(100)p(1×1)-H and Pd(100)-p(1×1)-D. It was found that propene underwent efficient H-D exchange reactions below 300 K for all C-H bonds, whereas no hydrogenation products (alkanes) were observed. The exchange reaction was proposed to occur via reversible hydrogenation to a half-hydrogenated intermediate. The absence of alkene hydrogenation may be due to stronger metal-hydrogen bonds on Pd(100) than on other metals such as Pt and Rh. A possible effect of hydrogen dissolution in the Pd bulk on olefin hydrogenation was discussed by Rupprechter et al. [04Rup]. The adsorption of C3H6 on clean and hydrogen-covered Pd(111) was studied by Stacchiola et al. [03Stac2] using TPD and IRAS. Propylene adsorbs in a di-σ configuration on clean Pd(111) while preadsorption of hydrogen induced some π-bonded propylene (steric effects caused by the methyl group Landolt-Börnstein New Series III/42A5
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seemed to limit the π-configuration). Upon heating, propylene desorbed molecularly at approx. 200 and 280 K and was subject to significant dehydrogenation to propylidyne and η1-allyl species. Formation of propane was observed by reaction of hydrogen with propylene in TPD, with an activation energy of 14±1.7 kJ/mol.
3.8.6.4.2.4 Pt Koestner et al. [82Koe1] studied C3H6 adsorption on Pt(111) by LEED, measuring intensity versus voltage (I-V) spectra. Two phases were detected. At low temperatures, the unsaturated C-C group formed a di-σ bond to two Pt atoms. Upon warming to ~300 K, a conversion took place to an alkylidyne species that was bonded to three Pt atoms and had its C-C bond nearest to the metal substrate oriented perpendicularly to the surface. At 300 K propylene formed ordered (2×2) structures and disordered structures of propylidyne (ŁC-CH2-CH3) [82Koe1, 83Koe, 86Ave1, 86Ogl]. Cremer et al. [96Cre1] studied C3H6 hydrogenation on Pt(111) using SFG. Under UHV (in the absence of hydrogen) propylene adsorbed as di-σ bonded propylene which dehydrogenated to propylidyne (ŁC-CH2-CH3) just below room temperature and to vinylmethylidyne (ŁC-CH=CH2) at 450 K (and forming graphite at higher temperatures). In the presence of hydrogen at 295 K, C3H6 hydrogenation proceeded from π-bonded propylene via a 2-propyl species (Pt-CH(CH3)2) to propane. Koel et al. [97Tsa] studied the adsorption and decomposition of propylene on Pt(111) with TPD, LEED and sticking coefficient measurements. An adsorption energy of 73 kJ/mol was reported. Comparison with the (2×2) and (¥3×¥3)R30° Sn/Pt(111) surface alloys (see 3.8.6.4.2.8) suggested that the Pt three-fold hollow sites were important for strong alkene chemisorption. Zaera and Chrysostomou [00Zae] studied the thermal chemistry of propylene on Pt(111) by TPD. Besides molecular desorption and hydrogen production from propylene dehydrogenation (first to propylidyne and eventually to surface carbon), a small amount of propane from self-hydrogenation of the olefin was detected at ~280 K. Hydrogen coadsorption weakened the adsorption of propylene on the surface, and enhanced the production of propane. Deuterium coadsorption with propylene lead to H-D exchange, in addition to deuteration to propane. Multiple H-D exchange was evidenced by the formation of all possible deuterium-substituted propylenes and propanes, including propylene-d6 and propane-d8. Most of the propylene that remained on the surface above 350 K dehydrogenated to propylidyne (Pt3C-CH2-CH3). A small fraction of that species rehydrogenated to propane at 430 K, while the rest stepwise dehydrogenated to surface carbon.
3.8.6.4.2.5 Rh C3H6 adsorption on Rh(111) produced (2×2) and (2¥3 × 2¥3)R30° structures [82Van]. Hydrocarbon phases originating from propene adsorption on Rh(111) and subsequent annealing and those from direct adsorption at higher temperatures were characterized by HREELS [89Wan]. CxH species, ethylidyne, propylidyne and di-σ adsorbed propene were observed.
3.8.6.4.2.6 Ru Propene adsorption on Ru(0001) [93Ran], studied by IRAS at 130 K, produced a physisorbed layer with a weak spectrum attributed to a di-σ adsorbed species. C3H6 converted to propylidyne at higher temperature and decomposed via the ethylidyne species. EELS and SIMS were used to investigate the adsorption and decomposition of propene on Ru(0001) [92Sak]. Propene was found to adsorb molecularly at 153 K but annealing to 203 K produced adsorbed propylidyne. At 233 K ethylidyne and propylidyne were found to coexist but by 293 K the propylidyne species had decomposed completely to ethylidyne and CxH.
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3.8.6.4.2.7 W C3H6 adsorption on W(100) and W(221) reported (5×1)-C and c(6×4)-C structures, respectively [84Ben].
3.8.6.4.2.8 Alloys Tsai et al. [97Tsa] studied propylene adsorption and decomposition on Pt(111) and the (2×2) and (¥3×¥3)R30° Sn/Pt(111) surface alloys with TPD, LEED and sticking coefficient measurements. Little or no effect of alloyed Sn on either the initial sticking coefficient or the saturation coverage on the two Sn/Pt(111) ordered surface alloys was observed when compared with the clean Pt(111) surface at 100 K. Based on TPD peak temperatures, the propylene adsorption energy decreased from 73 to 62 and then to 49 kJ/mol as the substrate was changed from Pt(111) to the (2×2) and the (¥3×¥3)R30° alloys. This implicates that the “Pt-only” three-fold hollow sites were important for strong alkene chemisorption, since removal of these sites on the (¥3×¥3)R30° alloy caused a sharper decrease in the adsorption energy than expected based upon the changes observed for the (2×2) alloy. Even though propylene contains allylic C-H bonds that are much weaker than the vinylic C-H bonds in ethylene, only ca. 5-7% as much propylene decomposed on the (2×2) alloy compared to Pt(111) and no decomposition occurred on the (¥3×¥3)R30° alloy. This shows the importance of adjacent “Pt-only” three-fold hollow sites for alkene decomposition.
3.8.6.4.3 Butenes C4H10 3.8.6.4.3.1 Ag IRAS and TPD studies of cis- and trans-2-butenes adsorbed on Ag(111) were reported by Wu et al. [00Wu]. The surface infrared spectra of cis- and trans-2-butene were distinguishable, a monolayer of cis2-butene/Ag(111) exhibited peaks at 1445, 1434 and 1030 cm−1 whereas the trans isomer had features at 1429, 973 and 959 cm−1. Pawela-Crew and Madix [95Paw1, 95Paw2] applied TPD, XPS, NEXAFS and IRAS to examine the desorption kinetics of butenes on Ag(110) (Fig. 14). The origin of repulsive intermolecular forces was revealed by NEXAFS, which showed a strong orientation of the double bond axis parallel to the surface, indicative of weak directional bonding between the surface and the alkene. It appears that this preferred orientation caused by the surface altered the intermolecular forces between colliding pairs of butenes, leading to the repulsive interactions that produced a decrease in the activation energy for desorption with increasing alkene coverage. Activation energies of desorption at zero coverage were 56±3 kJ/mol both for 1-butene and isobutylene. According to the observed vibrational modes (making use of the dipole selection rule) isobutylene lies flat on the surface. Unlike isobutylene, only three of the carbons in 1-butene are in a plane parallel to the surface; the methyl group was found tilted 110±10° away from the surface plane.
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1-butene / Ag (110) α
Mass signal ( m/q = 41)
0.83 ML
β
0.29 ML
Fig. 14 TPD spectra of 1-butene on Ag(110) for a broad coverage range exhibiting multiple state desorption for θ >0.8 ML and repulsive lateral interaction over the entire coverage range observed, adapted from [95Paw1].
0.06 ML
100
150
200 250 Temperature [K]
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3.8.6.4.3.2 Mo 2-butene adsorption on Mo(100) and oxygen-covered Mo(100) was studied by Wu and Tysoe [98Wu1]. The 2-butenes either thermally decomposed to yield hydrogen and adsorbed carbon, desorbed molecularly, self-hydrogenated to butane, or dissociated to form a C2 species which further decomposed to methane. It was proposed that 2-butenes thermally decomposed on the four-fold sites on Mo(100) since the hydrogen yield decreased linearly with oxygen coverage (oxygen blocks the four-fold sites). The butene desorption and self-hydrogenation activation energy increased with increasing oxygen coverage suggesting that 2-butenes bind to Mo(100) predominantly via donation of π-electrons to molybdenum. Methane formation was proposed to occur via the formation of methylene carbenes formed by direct carbon-carbon double-bond cleavage. The methane yield was much larger than that found following both ethylene and propylene adsorption on oxygen-modified molybdenum presumably due to differences in π-donation. Kelly et al. [86Kel] investigated the chemisorption and reactions of 1-butene on clean Mo(100), and with sulfur or carbon overlayers, using TDS. The predominant reaction at low additive coverage (0-0.2 monolayers of S or C), and at low ambient pressure (10−10 Torr) was decomposition. As additive (S or C) coverage increased the amount of decomposition decreased, enabling other reaction pathways to become more probable. Hydrogenation, partial dehydrogenation, and isomerization reactions were detected. On sulfur overlayers the binding of the hydrocarbon was weak (physisorption), usually on the order of the heat of sublimation (~40 kJ/mol). However, molecular binding on carbon overlayers was stronger: the heat of desorption was 50-63 kJ/mol. In addition, isomerization of 1-butene to 2-butene occurred on the carbon overlayer. It was suggested that the metal sites control the reactions observed (except for isomerization). This explains why the difference in molecular hydrocarbon binding between the sulfur covered and carbon covered surface plays a minor role in these reactions. Eng et al. [98Eng] studied the reaction pathways of cis- and trans-2-butene on clean Mo(110) and carbide-modified Mo(110) using TPD and HREELS. The vibrational data revealed that the decomposition pathways of cis- and trans-2-butene were different on clean Mo(110). In the case of cis-2-butene, the olefinic α(C-H) bonds were cleaved at 80 K to produce surface hydrogen and 2-butyne. In contrast, the initial decomposition of trans-2-butene also involved β(C-H) bond scission, such that at least one of the methyl groups of the molecule was converted to a CH2 group at temperatures below 150 K. On carbideLandolt-Börnstein New Series III/42A5
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modified Mo(110), the initial decomposition step for both cis- and trans-2-butene involved α(C-H) bond cleavage to form 2-butyne. The difference in reactivity of Mo(110) and carbide-modified Mo(110) towards the 2-butenes provided evidence that the formation of the carbide overlayer enhanced the selective activation of α(C-H) bonds.
3.8.6.4.3.3 Ni Isobutene chemisorbed molecularly on Ni(111) in di-σ configuration (below 150 K) [90Ham]. Both hydrogen atoms of the methylene group were bridge-bonded to the metal. These two weakly bound hydrogen atoms split off and the residual molecular fragment rehybridized towards sp2. The new surface species was presumably a tilted di-σ/π isobutenylidene complex. The adsorption and decomposition of 2,3-dimethyl-2-butene on Ni(111) was examined by HREELS and LEED between 80 and 400 K [92Fri]. Non-dissociated 2,3-dimethyl-2-butene showed no ordered superstructure. Partial decomposition around 170 K was accompanied by the development of a (2×2)-LEED pattern, due to cleaved hydrogen atoms.
3.8.6.4.3.4 Pd 1-butene adsorption on Pd(111), Pd(110) and Pd50Cu50(111) was investigated by NEXAFS, UPS and HREELS by Tourillon et al. [96Tou]. At 95 K, 1-butene was physisorbed on the different Pd single crystals (while being di-σ-bonded on Pt(111)). The NEXAFS experiments revealed a decrease of the hydrocarbon-substrate interaction according to the sequence: Pd(111)>Pd(110) Pd50Cu50(111). Katano et al. [02Kat1] applied STM and NEXAFS to examine the chemisorption of trans-2-butene on Pd(110). In STM trans-2-butene appeared as a dumbbell-shaped protrusion and the C=C bond was at the on-top site of the substrate atom. With increasing coverage, a short-range (3×1)-1D ordered structure at 0.1 ML was formed and a c(4×2) structure was observed at saturation coverage. NEXAFS experiments revealed the adsorption structure of trans-2-butene on Pd(110), where the direction in which the two methyl groups are connected is parallel to [001] at low coverage and the C=C double bond is parallel to [110] at high coverage. Reactions of 1-butene on atomic oxygen-covered Pd(100)-p(2×2)-O were studied by Guo and Madix [95Guo1] using TPRS. No partial oxidation products were observed, combustion yielded H2O, CO and CO2. Adsorbed O did not inhibit 1-butene adsorption and reaction. Isotope experiments showed that initial reactions occurred predominantly with the vinylic C-H bond. The same authors [95Guo2] also investigated the adsorption and reactions of 1-butene on the Pd(100)-p(1×1)-H and Pd(100)-p(1×1)-D using TPRS. It was found that 1-butene underwent efficient H-D exchange below 300 K for all C-H bonds, whereas no hydrogenation products (alkanes) were observed. The exchange reaction was proposed to occur via reversible hydrogenation to a half-hydrogenated intermediate. The absence of alkene hydrogenation may be due to stronger metal-hydrogen bonds on Pd(100) than on other metals such as Pt and Rh, or to the effect of hydrogen bulk dissolution [04Rup]. Mittendorfer et al. [03Mit] carried out a DFT study of the adsorption of 1-butene on Pd(111) and Pt(111).
3.8.6.4.3.5 Pt Using LEED, different superstructures were observed for C4H8 isomers on Pt(111), i.e. (2¥3 × 2¥3)R30° for cis-2-C4H8 and (8×8) for trans-2-C4H8 [82Koe1]. At low temperature, the unsaturated C-C group formed a di-σ bond to two Pt atoms. Upon warming to ~300 K a conversion took place to a butylidyne species that was bonded to three Pt atoms and had its C-C bond nearest to the metal substrate oriented perpendicular to the surface. The butylidyne species was shown to order its ethyl group into an (8×8) or (2¥3 × 2¥3)R30° superlattice when the hydrocarbon exposure was increased; this ordering was probably a natural consequence of the steric hindrance among neighbouring ethyl groups as the hydrocarbon Landolt-Börnstein New Series III/42A5
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coverage increased slightly with larger exposures. Isobutene adsorption and hydrogenation on Pt(111) at 300 K was also studied by SFG by Cremer et al. [96Cre4]. Tourillon and coworkers [96Tou] characterized the chemisorption of 1-butene on Pt(111) by NEXAFS, UPS and HREELS. At 95 K, 1-butene was found to be di-σ-bonded. Avery and Sheppard [86Ave3] applied TDS and EELS to study the adsorption of 1-butene and cis- and trans-2-butenes at 170 K on Pt(111). Each of the chemisorbed butenes produced a different EEL spectrum, corresponding to η2 di-σ adsorbed species. At 300 K, TDS and EELS suggested an n-butylidyne structure. Comparison with the spectrum of 2-butyne and the model cluster compound Os3(CO)10(CH3CCCH3) lead to the assignment of a µ3-η2 CH3C:CCH3 structure to the adsorbed species involving the central C:C bond in two σ-bonds and one π-bond to the metal surface. Koel et al. [97Tsa] examined the adsorption and decomposition of isobutene on Pt(111) with TPD, LEED and sticking coefficient measurements. The isobutene adsorption energy was 72 kJ/mol, as estimated from TPD peak temperatures. Comparison with (2×2) and (¥3 × ¥3)R30° Sn/Pt(111) surface alloys indicated that the Pt three-fold hollow sites were very important for strong alkene chemisorption and decomposition. Mittendorfer et al. [03Mit] reported a DFT study of the adsorption of 1-butene on Pt(111).
3.8.6.4.3.6 Ru Adsorption and decomposition of trans-2-butene on Ru(0001) were studied by Chesters et al. using IRAS [91Che]. IR spectra of submonolayer coverages at 90 K pointed to molecular adsorption as di-σ species. Further exposure produced spectra assigned to a π-adsorbed species. Heating the monolayer to ~200 K produced a spectrum similar to that of 2-butyne bonded to the surface as a di- σ/π complex. This behaviour of trans-2-butene contrasted with that of 1-butene and isobutene, which formed alkylidyne rather than alkyne species [90Che3]. At 90 K isobutene adsorbed as di-σ bonded state. Annealing to 180 K decomposed the di-σ bonded isobutene to isobutylidyne. Heating of the substrate to 300 K produced ethylidyne as the only IR-detectable entity on the surface. 1-butene followed a similar decomposition pathway, with the corresponding alkylidyne, butylidyne, being formed at ~150 K which was replaced by ethylidyne at 300 K.
3.8.6.4.3.7 Si Kiskinova and Yates [95Kis] studied the adsorption, desorption and decomposition of the isomeric hydrocarbons, cis- and trans-2-butene, on Si(100)-(2×1) by means of a kinetic uptake method, TPD and AES. Both 2-butene molecules adsorbed molecularly on Si(100)-(2×1) at 120 K with an initial sticking coefficient of near unity. Measurements of the dependence of the adsorption rate on coverage indicated that for the precursor mediated adsorption process the conformational difference between the isomers was of minor importance. Di-σ bonding was suggested for the chemisorption state where the molecule preserves its cis- or trans-structure. Upon heating of saturated 2-butene layers, 25% of trans-butene and 13% of cis-butene undergo dissociation, with steric conformational effects being presumably responsible for differences with respect to both desorption and decomposition. It was postulated that the intermediate surface species responsible for dissociation was a di-σ (1,3) bonded butane which involved coupling of one methyl group to the substrate. The lack of a significant effect of conformation on the adsorption kinetics indicated that adsorption and desorption occurred by independent pathways.
3.8.6.4.3.8 Alloys 1-butene adsorption on Pd(111), Pd(110) and Pd50Cu50(111) was investigated by NEXAFS, UPS and HREELS by Bertolini and coworkers [96Tou]. At 95 K, 1-butene was physisorbed on the different Landolt-Börnstein New Series III/42A5
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Pd-based single crystals (while being di-σ-bonded on Pt(111)). NEXAFS revealed a decrease of the hydrocarbon-substrate interaction according to the sequence: Pd(111)>Pd(110) Pd50Cu50(111). Tsai et al. [97Tsa] examined the adsorption and decomposition of isobutene on Pt(111) and the (2×2) and (¥3 × ¥3)R30° Sn/Pt(111) surface alloys with TPD, LEED and sticking coefficient measurements. Little or no effect of alloyed Sn on either the initial sticking coefficient or the saturation coverage on the two Sn/Pt(111) ordered surface alloys was observed, as compared to Pt(111) at 100 K. The isobutene adsorption energy decreased from 72 to 62 and then to 45 kJ/mol as the substrate was changed from Pt(111) to the (2×2) and the (¥3 × ¥3)R30° alloys, as estimated by TPD peak temperatures. This implicates that the “Pt-only” three-fold hollow sites are important for strong alkene chemisorption. Even though isobutene contains allylic C-H bonds that are much weaker than the vinylic C-H bonds in ethylene, only ca. 5-7% as much isobutene decomposed on the (2×2) alloy compared to the Pt(111) surface and no decomposition occurred on the (¥3 × ¥3)R30° alloy.
3.8.6.4.4 Pentenes C5H10 and Hexenes C6H12 3.8.6.4.4.1 Au Scoles and coworkers [98Wet] used helium atom reflectivity to study the adsorption of 1-alkenes (C6H12C11H22) on Au(111). The physisorption energies increased linearly with the chain length (~5-6 kJ/mol per additional methylene unit).
3.8.6.4.4.2 Pd The adsorption of trans-2-pentene, cis-2-pentene and 1-pentene on Pd(111) was studied by TPD by Doyle et al. [03Doy, 04Doy]. For all molecules, three distinct molecular desorption states were observed (130, 175 and 260 K), which were assigned to a multilayer, π-bonded pentene and interchanging di-σ-bonded pentene/pentyl groups, with the latter species undergoing stepwise dehydrogenation. For trans-2-pentene on D2 preadsorbed Pd(111), H-D exchange was observed, resulting in D-substituted pentene, which molecularly desorbed or dehydrogenated on heating. Teschner et al. [05Tes] applied “high-pressure” XPS to study trans-2-pentene on Pd(111) and polycrystalline Pd foil. During hydrogenation a huge amount of carbon (up to 73%) was observed. Mainly graphite was present on Pd(111), whereas other components, C-H and C-Pd, were also formed on the foil to a greater extent. Differences found in the valence and the C1s region between Pd(111) and polycrystalline Pd foil were interpreted as indicators of different electronic structures. From UPS it was concluded that trans-2-pentene was hydrogenated in σ-bonded configuration. Vasquez and Madix [98Vas] investigated the adsorption and reactivity of 1-hexene on clean Pd(111) and hydrogen (deuterium)-saturated Pd(111) using TPRS. The low-temperature adsorption configuration for the linear C6 hydrocarbon was proposed to be a weakly π-bonded species. The adsorbed molecules first desorb molecularly with a fraction converting to a more tightly bonded half-hydrogenated state, which undergoes either β-hydride elimination to release the alkene or dehydrogenation to adsorbed carbon and hydrogen. Dehydrocyclization to benzene was observed on Pd(111), whereas it did not occur on Pd(100). H-D exchange was also observed which occurred through the reversible C-H bond formation via a half-hydrogenated intermediate.
3.8.6.4.4.3 Pt Using IRAS Chesters and coworkers [00Ilh] studied low temperature 1-hexene adsorption and thermal decomposition on Pt(111). A mixture of rotational conformers of hexylidyne was detected at 250 K. The decomposition path previously proposed for hexylidyne on Ru(0001), which involved the formation of metallocycles, was confirmed on Pt(111) by the observation of two types of metallocycle. One was Landolt-Börnstein New Series III/42A5
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hydrogenated in C6 [Pt3C-(CH2)5-Pt], forms at lower temperatures (270 K) and was identified by the symmetric stretching mode of the methylene group attached to the surface (at 2911 cm−1). The other, completely dehydrogenated [Pt3C-(CH2)4-CPt3], started to form around 370 K, and was identified by the antisymmetric stretching mode of methylenes on C3 and C4 and by the symmetric stretch of methylenes on C2 and C5. The thermal decomposition of 1-hexene does not involve the formation of ethylidyne (ŁC-CH3), in contrast with shorter 1-alkenes.
3.8.6.4.4.4 Ru 1-Hexene adsorption and its thermal decomposition was studied by IRAS by Ilharco et al. [00Ilh]. A mixture of rotational conformers of hexylidyne was detected at 100 K on Ru(0001). The decomposition path for hexylidyne on Ru(0001) involved the formation of metallocycles, as confirmed by the observation of two types of metallocycle ([Ru3C-(CH2)5-Ru] and [RuC-(CH2)4-CRu]). The thermal decomposition of 1-hexene did not involve the formation of ethylidyne (ŁC-CH3), in contrast with shorter 1-alkene chains on Ru(0001).
3.8.6.5 Dienes Dienes are even richer in π-electrons than alkenes and thus their bonding to the metal surface is stronger. Work function of the metal decreases upon adsorption of all of these organic molecules. However, the magnitude of the decline is in the order of alkanes < alkenes < dienes < alkynes. The dienes may dissociate/decompose at lower temperature than the alkenes and sequential dehydrogenation occurs as the temperature is increased. At low temperature (~100 K), the binding of 1,3-butadiene depends on the type of metal and the crystallographic orientation of the substrate, being either di-π bonded (e.g. on Pd(111)) or di-σ bonded (e.g. on Pt(111); connected with a rehybridization from sp2 to sp3 of half of the carbon atoms). At ~300 K, 1,3-butadiene may dehydrogenate and/or transform into butylidyne, similar to butene. Hexadienes adsorb as weakly π-bonded species at low temperature and may undergo dehydrocyclization to benzene above 300 K. At high temperature benzene is a thermodynamically stable species that makes it an important product in free radical surface reactions.
3.8.6.5.1 Propadiene C3H4 EELS and IRAS studies of propadiene adsorption (H2C=C=CH2, allene) were reported for Cu(110) [96Sho], Ni(111) [96Sho], Rh(111) [87Ben] and evaporated Ag films (transmission infrared spectroscopy) [96Sho]. Adsorption on Cu(110) and Ag films at low temperature produced a molecularly adsorbed species in which the orthogonal π-system of the molecule was retained and oriented so that one of the CH2 units was oriented with its plane parallel to the surface, and the other with its plane perpendicular to the surface. The molecules adopted a preferential orientation with the C=C=C skeleton parallel to the surface. Adsorption of propadiene on Ni(111) was different, leading to hydrogen-transfer induced isomerisation in which the two original C=C double bonds were replaced by one single and one triple bond, resulting in the formation of a di-σ/di-π propyne species.
3.8.6.5.2 Butadiene C4H6 3.8.6.5.2.1 Ag The orientation of butadiene on Ag(110) at submonolayer coverage was determined by NEXAFS [91Cou]. Butadiene was found to chemisorb with its σ-h plane parallel to the surface. The C=C and C-C Landolt-Börnstein New Series III/42A5
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bond lengths were found to be identical to the gas phase values of 1.34 and 1.46 Å, respectively. There was ordering of the molecules such that the π-orbitals were perpendicular to the surface.
3.8.6.5.2.2 Mo Kelly et al. [86Kel] investigated the chemisorption and reactions of 1,3-butadiene on clean Mo(100), and with sulfur or carbon overlayers, using TDS. The predominant process at low additive coverage (0-0.2 monolayers of S or C), and at low ambient pressure (10−10 Torr) was decomposition. As additive (S or C) coverage increased the amount of decomposition decreased and other reaction pathways such as hydrogenation, partial dehydrogenation and isomerization became more probable. Molecular binding on the additive overlayers was found to be very different. On sulfur overlayers the binding of the hydrocarbon was weak (physisorption), usually on the order of the heat of sublimation (~40 kJ/mol). The binding on carbon overlayers was stronger: the heat of desorption was 71-97 kJ/mol.
3.8.6.5.2.3 Pd Reactions of 1,3-butadiene on atomic oxygen-covered Pd(100)-p(2×2)-O were studied by Guo and Madix [95Guo1] using TPRS. No partial oxidation products were observed, combustion yielded H2O, CO and CO2. Adsorbed O did not inhibit 1,3-butadiene adsorption. Isotope experiments showed that initial reactions occurred predominantly with the vinylic C-H bond. The same authors [95Guo2] also investigated the adsorption and reactions of 1,3-butadiene on the Pd(100)-p(1×1)-H and Pd(100)-p(1×1)D surfaces using TPRS. It is found that 1,3-butadiene underwent selective hydrogenation to corresponding alkenes below or around 300 K, whereas no H-D exchange reaction was observed to occur. Strong bonding of a half-hydrogenated intermediate to the surface may be the reason for the irreversible hydrogenation of conjugated polyenes to alkenes. The chemisorption of 1,3-butadiene on Pd(111), Pd(110) and Pd50Cu50(111) samples was studied by Bertolini et al. [96Ber, 96Tou, 02Ber] using NEXAFS, UPS and HREELS. Different chemisorption modes of 1,3-butadiene were observed on the various surfaces: At 95 K, 1,3-butadiene was physisorbed (di-π mode) on the different Pd single crystals (while it was di-σ bonded to Pt(111); see 3.8.6.5.2.4). At 300 K, 1,3-butadiene either dehydrogenated on Pd(110) and Pd50Cu50(111) or very probably transformed into butylidyne on Pd(111), similar to butene adsorbed on the (111) surface. NEXAFS revealed a decrease of the hydrocarbon-substrate interaction according to the sequence: Pd(111)>Pd(110)>Pd50Cu50(111). The activity for 1,3-butadiene hydrogenation should therefore obey the reverse sequence, what has been actually observed in reactivity measurements. 1,3-butadiene hydrogenation on Pd(111), Pd(110) and Pd50Cu50(111) displayed a very good selectivity in butenes and a higher activity as compared to Pt(111). Katano et al. [02Kat2] studied the adsorption of 1,3-butadiene on Pd(110) by HREELS, NEXAFS and STM. 1,3-butadiene was found π-bonded and the molecular plane was parallel to the surface, with the C-C single bond aligned toward [ 1 1 0 ]. Sautet and Paul [91Sau] studied the adsorption modes of butadiene on Pd(111) on the basis of extended Hückel calculations. For the Pd(111) face the π-coordination yielded about the same adsorption energy as the di-σ one. This may provide a qualitative explanation of the better selectivity for butadiene partial hydrogenation on Pd(111) compared with Pt(111) (where the di-σ coordination was found more stable). Mittendorfer et al. [03Mit] also reported a DFT study of the adsorption of 1,3-butadiene on Pd(111) and Pt(111), analyzing structural, electronic, energetic, and spectroscopic properties.
3.8.6.5.2.4 Pt Bertolini and coworkers [96Ber, 96Tou] studied the adsorption of 1,3-butadiene on Pt(111) by NEXAFS, UPS and HREELS. It was found that at 95 K 1-butene was di-σ bonded to Pt(111) (while being π-bonded Landolt-Börnstein New Series III/42A5
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to Pd(111)). At 300 K, a di-σ interaction keeping one central carbon-carbon double bond was proposed on Pt(111) (while partial dehydrogenation was observed on Pd(111)). In the butadiene molecule half of the carbon atoms undergo a large rehybridization from sp2 to sp3, as evidenced by HREELS. At 300 K, butadiene transformed into butylidyne on Pt(111). The differences between 1,3-butadiene adsorption on Pt(111) and Pd(111) could be related to differences in catalytic activities and selectivities in 1,3-butadiene hydrogenation. Sautet and Paul [91Sau] applied extended Hückel calculations to compare the different low temperature coordination modes of butadiene on Pt(111) and (110). The di-σ coordination was more stable on Pt(111) but on the Pt(110) face the π-coordination yielded the same adsorption energy as the di-σ one. A DFT study of 1,3-butadiene adsorption on Pd(111) and Pt(111) was reported in [03Mit].
3.8.6.5.2.5 V Chen [95Che] investigated the adsorption and decomposition of 1,3-butadiene on clean and carbidemodified V(110). By using HREELS and TDS it was observed that the formation of carbide significantly modified the reactivity of vanadium. While 1,3-butadiene strongly interacted with clean V(110) via the interaction between the d-band of vanadium and the π-orbitals of the adsorbate, on the carbide-modified surfaces the interaction was much weaker.
3.8.6.5.3 Pentadiene C5H8, Hexadiene C6H10 3.8.6.5.3.1 Pd Vasquez and Madix [98Vas] investigated the adsorption and reactivity of 1,5-hexadiene, 1,3-hexadiene, and 1,3,5-hexatriene on clean Pd(111) and hydrogen (deuterium)-saturated Pd(111) using TPRS. The low-temperature adsorption configuration for the linear C6 hydrocarbons was proposed to be a weakly π-bonded species. The adsorbed molecules desorbed molecularly with a fraction converting to a more tightly bonded half-hydrogenated state, which either β-hydride eliminated to release the alkene or dehydrogenated completely to adsorbed carbon and hydrogen. Dehydrocyclization to benzene was observed on Pd(111), whereas it did not occur on Pd(100). Cyclization of 1,3,5-hexatriene to benzene occurred at temperatures as low as 333 K on Pd(111). Hydrogenation of 1,3-hexadiene to hexene (selfhydrogenation) was observed on Pd(111). On H/Pd(111) 1,3-hexadiene and 1,5-hexadiene were hydrogenated to hexene, and hexatriene was hydrogenated to hexadiene and hexene. However, (total) hydrogenation to hexane did not occur for any of the unsaturated species. H-D exchange into all the adsorbed alkenes was observed though. The exchange reaction was proposed to occur via reversible C-H bond formation in a half-hydrogenated intermediate.
3.8.6.6 Alkynes Acetylene HCŁCH generally shows strong structural distortions when adsorbed on transition metals and the bonding is often described in terms of a π-donation-π*-backdonation process. At low temperature (200-300 K) acetylene forms a strong chemisorption bond to the substrate, creating a state of hybridization close to sp3. On (111) planes, C2H2 often stabilizes with its C-C axis parallel to the surface over a bridge site with the two C-centers pointing towards adjacent 3-fold hollow sites. The C-C distance of adsorbed C2H2 is stretched by ~20% with respect to that of the free molecule. The C-H axes are tilted by 60° with respect to the C-C axis, pointing away from the surface. Both ordered and disordered acetylene structures, as well as ordered carbon structures (resulting from acetylene decomposition), were observed by LEED. In the temperature region 200-300 K dehydrogenated fragments, possibly acetylide (-CŁCH) and methylidyne (ŁCH), were found to co-exist with molecular acetylene. Further heating typically leads to Landolt-Börnstein New Series III/42A5
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decomposition producing graphitic and carbidic carbon as final dehydrogenation products. Upon increasing the temperature, acetylene may also undergo cyclotrimerization to form benzene. Propyne C3H4 is also significantly perturbed upon adsorption, with the gas phase CŁC triple bond suffering a reduction in bond order to ~1 and formation of a di-σ/di-π-bonded species. The strong correlation between the adsorption complexes of propyne and acetylene suggests that the surface chemistry is largely determined by the CŁC triple bond functionality. At higher temperature, propyne may trimerize to trimethylbenzene.
3.8.6.6.1 Acetylene C2H2 3.8.6.6.1.1 Ag Stuve and Madix studied C2H2 adsorption on Ag(110) by EELS [82Stu]. At 100 K C2H2 adsorbed without rehybridization and desorbed molecularly between 100 and 160 K. In contrast to interactions between acetylene and other metals the carbon-carbon triple bond was preserved, as determined by the CH stretching frequency of 3270 cm−1.
3.8.6.6.1.2 Co Chemisorption of C2H2 on Co(0001) was studied by Ramsvik et al. [02Ram1] by high resolution XPS, XAS and LEED. Below 300 K, C2H2 forms a strong chemisorption bond to the substrate, creating a hybridization state close to sp3 with the C-C axis of the molecules oriented parallel to the surface. The vibrational splitting in the XPS spectra due to excitation of the C-H stretch was determined to be 389±8 meV, which is ~6% lower than the C-H stretch frequency for gaseous acetylene. The same group studied C2H2 chemisorption and dissociation on Co( 11 2 0 ) [02Ram2] using XPS, NEXAFS, LEED and STM. Adsorbed C2H2 dissociated at ~200 K, which was significantly lower than the dissociation onset for C2H2/Co(0001). NEXAFS showed that C2H2 hybridized strongly on the Co( 11 2 0 ) surface, forming antibonding states below the ionisation limit, which were not present in the gas-phase. In the temperature region 200-300 K a dehydrogenated fragment, possibly C2H or C2, was found to co-exist with molecular C2H2. Heating to 450 K produced graphitic carbon, forming an ordered (5×2) carbon overlayer at the expense of molecular C2H2 (carbon overlayer was fully developed at 570 K). Above ~600 K, the amount of ordered carbon atoms decreased, leaving behing mainly graphitic carbon on Co( 11 2 0 ).
3.8.6.6.1.3 Cu Ho and coworkers [98Sti2, 99Lau, 00Lau2, 00Lau1, 02Ho, 02Ols] used a variable temperature STM to monitor the thermally induced rotation of single C2H2 (C2D2) molecules between two equivalent orientations on Cu(100) above 68 K. Acetylene adsorbed on a fourfold hollow site, with the C-H bonds bent away from the surface and the molecular plane perpendicular to the surface. Studies of the rotation rate at various temperatures indicated an energy barrier of 169±3 meV and a preexponential factor of 1011.8 ± 0.2 s–1. By tracking single molecules above 178 K, measuring the hopping rate as a function of temperature, the thermal diffusion barrier of individual acetylene molecules was determined to be 0.53 ± 0.01 eV with a preexponential factor of 1013.6 ± 0.2 s–1. Molecule rotation could also be induced at 9 K with the help of electrons from the STM (Fig. 15). Furthermore, electrons were also used to produce ethynyl (CCH) by breaking a C-H bond in a single C2H2 molecule with both C2H2 and CCH being characterized via inelastic electron tunneling spectroscopy. HCCH dissociation was accompanied by significant changes in the vibrational spectra and bonding geometry. Electrons of 0-5 eV were also used to induce desorption, diffusion, and vibrational excitation of molecules.
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Fig. 15 Constant current STM images of C2D2 and CCD on Cu(100) at 9 K (image size 38 Å × 38 Å). The color scheme was adjusted to emphasize the structure above the substrate surface. C2D2 is imaged as elongated depression (a, b) and adsorbs above fourfold hollow sites in two equivalent orientations (the schematic in (c) represents image (b)). (d-h) show the four possible orientations of a CCD fragment, produced by breaking a single C-D bond by lowenergy electrons from the STM tip. (f) shows the suggested orientation of CCD in image (e); adapted from [00Lau2].
PED [94Bao2] indicated that C2H2 adsorption on Cu(111) was accompanied by major structural changes in the C2H2 layer even though the overall adsorbate-substrate binding was weak (by contrast, experiments on ethylene-Cu(111) had identified a weakly physisorbed adsorbate without noticeable structural changes). These experimental findings were confirmed by Hermann and Witko [95Her] using cluster model calculations. Extended studies on a small Cu7C2H2 cluster (Fig. 16) revealed that C2H2 stabilizes with its C-C axis parallel to the Cu(111) surface over a bridge site, with the two C centers pointing towards adjacent 3-fold hollow sites, as suggested PED. The calculated C-C distance of adsorbed C2H2 increased by 0.16 Å with respect to that of the free molecule which is close to the experimentally observed increase (0.28±0.10 Å). The cluster model indicated that the C-H axes were pointing away from the surface, being tilted by 60° with respect to the C-C axis. The overall weak C2H2-Cu(111) interaction is determined by a competition between the energy required to change the geometry in the adsorbate molecule and the energy gained due to local bond formation of the distorted molecule. The same authors [98Wit] also reported potential energy curves E(z) which exhibited two minima (one referring to an undistorted physisorbed adsorbate and another yielding a strongly distorted adsorbate). It was rationalized why experiments for Cu(111)-C2H2 have reported only one adsorbate state so far. As mentioned, C2H2 shows strong geometrical distortions when adsorbed on transition metals in a π-donation-π*backdonation process. Triguero et al. [98Tri] demonstrated the importance of considering the available excited states of the free molecule in analyzing the bonding scheme of C2H2 on cluster models on Cu(100), (110), and (111). By comparison to the structures of the triplet excited states in the gas phase it was shown that these must be considered as the states actually involved in the bonding. Bandy et al. [84Ban] reported vibrational EELS spectra of C2H2 on Cu(111), Ni(110) and Pd(110) at 110-120 K. Taking into account corresponding C2D2 spectra the loss peaks were assigned to vibrational modes of the non-dissociatively adsorbed, but significantly rehybridized C2H2 molecules.
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C H Cu
Fig. 16 Structure model of the Cu7(4,3)C2H2 cluster used in [95Her]. The C2H2 adsorbate is assumed to bend over a Cu(111) bridge site with the two C-centers pointing towards adjacent fcc and hcp hollow sites.
3.8.6.6.1.4 Fe Hung and Bernasek [95Hun] studied the adsorption of C2H2 on clean, C- and O-covered Fe(100) using HREELS, TPD, AES and LEED. C2H2 adsorbed on Fe(100) in a structure with hybridization similar to sp3. At low exposure (<0.2 L), C2H2 decomposed to form ŁCH and -CŁCH at 253 K. At higher exposure (>0.2 L), C2H2 partially dehydrogenated and hydrogenated to form ŁCH, -CŁCH and -CH=CH2 at the adsorption temperature of 100 K. When the surface was heated to 393 K, a =C=CH2 species was formed by dehydrogenation of -CH=CH2. An ordered carbon overlayer with 0.81 ML coverage was obtained by heating the adsorbed C2H2 layer to 553 K. Carbon overlayers hindered C2H2 adsorption. Anderson and Mehandru performed full structure determinations of C2H2 on small and large cluster models of Fe(100), (110), and (111) surfaces using the atom superposition and electron delocalization molecular orbital theory. Four-fold sites were favored on the (100) and (110) surfaces and the di-σ bridging site was favored on Fe(111). Using LEED, Yoshida et al. observed (2×2), (2×3) and coincidence structures of C2H2 on Fe(110) [78Yos]. On Fe(111), (1×1), (5×5) and (3×3) was reported [78Yos].
3.8.6.6.1.5 Ir C2H2 adsorption on Ir(100) produced disordered structures or c(2×2)-C structures upon decomposition of C2H2 [76Bro, 76Rho]. On Ir(111), Nieuwenhuys et al. [76Nie] reported (¥3 × ¥3)R30° and (9×9)-C structures. On the stepped surface Ir(S)-[6(111) × (100)], a (2×2) structure was observed.
3.8.6.6.1.6 Ni LEED studies of C2H2 on Ni(100) reported c(2×2), (2×2), c(4×2) and (2×2)-C structures [77Cas, 78Hor, 81Cas]. Steinrück and coworkers [02Whe, 03Neu] studied the thermal chemistry of C2H2 on Ni(100) in the temperature range 90-530 K by temperature-programmed XPS. The use of a third generation synchrotron light source allowed measurements of high-resolution C1s XPS spectra in “real-time” (i.e. within a few seconds), enabling to follow the thermal dehydrogenation in a quantitative and quasicontinuous manner. For C2H2 decomposition, acetylide (-CŁCH) and methylidyne (ŁCH) were observed as intermediates. Carbidic carbon was formed as final dehydrogenation product. On Ni(110), a c(2×2) structure was observed by LEED [84Str]. King et al. [99Bro1] measured the heats of adsorption and sticking probabilities for C2H2 on Ni(110) at 300 K. The initial sticking probability and heat of adsorption for C2H2 were 0.8 and 190 kJ mol−1. CCH species were formed on the surface initially, and at higher exposure =CH2 and ŁCH were produced. A value of the Ni-C bond strength of 191 kJ mol−1 was reported (calculated value 204 kJ mol−1). HREELS, LEED and TDS were Landolt-Börnstein New Series III/42A5
Ref. p. 320]
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used to study the adsorption and decomposition of C2H2 on Ni(110) [84Ban, 84Str]. C2H2 adsorbed molecularly at 80 K, but showed rehybridization to ~sp2.5. An ordered c(2×2) LEED pattern was formed. Steinrück and coworkers [95Wei, 96Ste, 01Whe, 02Whe] studied C2H2 adorption on Ni(110) by angleresolved UPS. For a saturated c(2×2) layer (0.5 ML) the C-C axis was oriented parallel to surface, oriented along the substrate throughs ([ 1 1 0 ] azimuth). C2H2 on Ni(111) was frequently studied by LEED revealing (2×2), (¥3×¥3)R30° and disordered layers [77Dem, 78Ber, 79Cat, 82Cas, 84Kob]. Using the same technique, Casalone et al. [82Cas] found that the C2H2 molecules were adsorbed with the C-C bond parallel to the surface, with the center of the C-C bond over a bridge site. The C-C bond was perpendicular to the Ni-Ni bridge. The C-C bond length was 1.50 Å, the carbon atoms were 2.1±0.10 Å above the surface. Hammer et al. [86Ham] performed combined LEED/EELS measurements to study the ordering of C2H2 on Ni(111). Three different phases of long-range order structures were reported: (2×2), (¥3 × ¥3)R30° and (2¥3 × ¥3)R30°. Bridge positions were suggested as adsorption sites for the carbon atoms. Using EELS Lehwald and Ibach [79Leh] studied the C2H2 adsorption at 150 K and its decomposition upon annealing to higher temperature. On Ni(111) acetylene formed a sp3-type configuration which was stable up to 400 K. On the stepped Ni [5 (111) × (110)] surface, even at 150 K, C2H2 instantaneously dehydrogenated to C2 which further decomposed into carbon atoms. Dalmai-Imelik and Bertolini [74Dal] measured the work function change upon adsorption of C2H2 on Ni(111) by the retarding potential method. On the clean surface −0.6 eV was obtained at 3×10−9 Torr. For a pressure of 10−8 Torr the change of work function was −1 eV suggesting the polymerization of the hydrocarbon. Bao et al. [95Bao] examined the local geometry of adsorbed ethylene on Ni(111) and its dehydrogenation to adsorbed C2H2 using C1s scanned-energy mode PED. At 120 K C2H4 adsorbed with its C-C axis parallel to the surface in an aligned bridge site such that the C atoms were approximately atop Ni atoms. Heating this surface lead to dehydrogenation of the adsorbed C2H4 to adsorbed C2H2, and while the C-C axis remained parallel to the surface, the C-C bond length and C-Ni layer spacing were reduced, and the C2H2 now occupied a cross-bridge site with the C atoms directly above fcc and hcp hollow sites on the surface. In a theoretical study, Zhao et al. [97Zha] investigated C2H2 adsorption on Ni(111), as well as its migration pathway from the aligned bridge site (the favoured adsorption site for C2H4) to the cross-bridge site (the favoured adsorption site for C2H2). The C-C bond length of C2H2 was stretched by 20% compared with that of the gas phase, which is in good agreement with PED results [95Bao], and the corresponding bond order is much less than that of the gas phase. It was suggested that acetylene will migrate from the aligned bridge site to the cross-bridge site with a very small energy barrier, 0.02 eV. The favourite pathway of the migration was a translation to the nearest cross-bridge site, with a simultaneous rotation through 30°. In another theoretical study, Medlin and Allendorf [03Med] used plane-wave DFT and extended Hückel calculations to study the adsorption of C2H2 and hydrogen on Ni(111). Atomic hydrogen was found to preferentially adsorb in a 3-fold hollow site, although the potential-energy surface for hydrogen binding was rather flat. C2H2 was found to strongly adsorb above two contiguous hollow sites, with its molecular plane perpendicular to the surface and bisecting a Ni-Ni bond (“cross-bridge” configuration).
3.8.6.6.1.7 Pd IRAS studies of C2H2 adsorption on Pd(100) by Camplin et al. [97Cam, 00Cam] indicated an adsorption geometry with the CŁC bond parallel to the surface, through a di-σ/di-π interaction. The rehybridization of the C-C bond caused the C-H bonds to tilt away from the surface plane. A configuration was suggested in which the HCCH plane was normal to the surface. Guo and Madix [95Guo2] investigated the adsorption and reactions of C2H2 on Pd(100)-p(1×1)-H and Pd(100)-p(1×1)-D using TPRS. C2H2 undergoes selective hydrogenation to the corresponding alkene below or around 300 K, whereas no H-D exchange reaction was observed. Bandy et al. [84Ban] reported EELS spectra of C2H2 chemisorbed on Pd(110) at 110 K indicating non-dissociative adsorption (but significant rehybridization). Yoshinobu et al. [90Yos] studied the Landolt-Börnstein New Series III/42A5
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adsorption and thermal evolution of C2H2 on Pd(110) by HREELS, TDS and LEED. At 90 K C2H2 chemisorbed molecularly and was located in the µ2-site with its C-C bond axis inclined to the surface plane; one of the CH groups was hydrogen-bonded to the surface. LEED studies of C2H2 on Pd(111) indicated (¥3 × ¥3)R30°-diffuse, (¥3 × ¥3)R30°-C2H2, disordered and (¥3 × ¥3)R30°-C2H3 structures [82Gat1, 83Gat, 83Tys]. High-resolution XPS and LEED were applied by Sandell et al. [98San] to study C2H2 (+H) on Pd(111). Adsorbing C2H2 at 125 K produced two ordered phases, a (2×2) and a (¥3 × ¥3)R30° structure, with C2H2 occupying hollow sites. When H was preadsorbed at 110-150 K, only the latter structure was observed. Heating the (¥3 × ¥3)R30° C2H2 +H structure to 350 K produced a well-ordered (¥3×¥3)R30° ethylidyne overlayer with C2H3 in hollow sites. Baddeley et al. [98Bad] performed a scanned-energy mode PED study of the (2×2) adsorption phase of C2H2 on Pd(111). The carbon atoms in C2H2 were found located almost over bridge sites with a C-C bond length of 1.34±0.10 Å. The center of the molecule was almost over a hollow site with the hcp site being favoured but the fcc site could not be excluded. The adsorption site for the (¥3×¥3)R30° phase was basically identical. Tysoe and coworkers [96Tys, 99Kal, 01Sta4, 01Sta3] described C2H2 trimerization on Pd(111). Benzene was formed by reaction between adsorbed acetylene and a surface C4 metallocycle. The addition of hydrogen was found to increase the rate of cyclotrimerization even though this reaction does not involve hydrogen directly. This effect is presumably due to the removal of carbonaceous species from the Pd surface. The same group also investigated the hydrogenation of adsorbed C2H2 and vinyl intermediates (formed by adsorbing vinyl iodide) on Pd(111) by TPD and IRAS [00Aza]. It was reported that vinyl species hydrogenated more rapidly than adsorbed acetylene, indicating that the rate-limiting step in acetylene hydrogenation was the addition of the first hydrogen to acetylene to form a vinyl species. IRAS revealed that vinyl species converted to ethylidyne above ~160 K. Sheth et al. [03She] examined the hydrogenation of C2H2 on Pd(111) by DFT. The binding energies of C2H2, atomic hydrogen, vinyl, and C2H4 at 25% (33%) coverage were computed to be −172 (−136), −260 (−248), −274 (−235), and −82 (−62) kJ/mol, respectively. Another DFT study of the acetylene adsorption on Pd(111) is described in [03Mit]. Medlin and Allendorf [03Med] used plane-wave DFT and extended Hückel calculations to study the adsorption of C2H2 and hydrogen on Pd(111). Atomic hydrogen preferentially adsorbed in a 3-fold hollow site and the most stable adsorption structure was C2H2 oriented above a 3-fold hollow site, with its axis parallel to the surface but tilted away from a metal-metal bond. Chemisorption of C2H2 on ultrathin (mono-, bi-, and trilayer) Pd films on Mo(100) was studied by Heitzinger et al. [93Hei] using a combination of AES, TPD and HREELS. C2H2 was strongly rehybridized from sp in the gas phase toward sp3 on the Pd monolayer (as it is on Pd(100)). Chemisorption of C2H2 on ultrathin Pd films (monolayer, bilayer, trilayer, etc.) deposited on Ta(110) [01Hei] was weaker than on bulk-terminated Pd surfaces, but was not as strongly perturbed as was seen for C2H4. C2H2 was reversibly adsorbed on the 1st monolayer at 91 K, with thermal desorption peaks at 180 and 265 K. C6H6 was formed via cyclotrimerization of C2H2 and desorbed in a single peak at 407 K (yield ~1% of an adsorbed benzene monolayer). On a thick Pd film (θ Pd = 5) C2H2 adsorption at 175 K lead to C2H2 desorption in a very broad peak near 330 K, and to benzene desorption at 250 and 500 K (yield twice of that on the Pd monolayer).
3.8.6.6.1.8 Pt LEED studies on Pt(100) reported c(2×2) structures for C2H2 adsorption [68Mor, 69Mor, 77Fis1, 77Fis2, 78Fis]. Panja et al. [01Pan] investigated the adsorption and reaction of C2H2 on a hexagonally reconstructed (5×20)-Pt(100) surface using TPD, AES, LEED and XPS, and compared them to those on two ordered Sn/Pt(100) alloy surfaces (see 3.8.6.6.1.14). C2H2 nearly completely decomposed during TPD on Pt(100), forming hydrogen, which desorbed as H2, and surface carbon. LEED studies of C2H2 on Pt(111) reported (2×1) and (2×2) structures [69Mor, 74Wei, 77Kes, 77Sta, 82Koe1, 83Fre]. Stöhr et al. [84Stö] studied (disordered) C2H2 on Pt(111) using NEXAFS and observed a C-C bond parallel to the surface with a bond length of 1.45±0.03 Å. Albert et al. [82Alb] used ARUPS to study C2H2 on Pt(111). At low temperature acetylene adsorbed with the carbon-carbon bond parallel to the surface. In the high Landolt-Börnstein New Series III/42A5
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291
temperature phase the carbon-carbon bond axis was normal or nearly normal to the surface, favoring the ethylidyne structure. DFT studies of C2H2 and hydrogen adsorption on Pt(111) [03Med, 03Mit] reported that the most stable adsorption structure was C2H2 oriented above a 3-fold hollow site, with its axis parallel to the surface but tilted away from a metal-metal bond.
3.8.6.6.1.9 Re LEED studies of C2H2 on Re(0001) revealed disordered and (2 × ¥3)R30°-C structures [78Duc, 81Duc]. On stepped surfaces Re(S)-[14(0001) × ( 10 1 1 )] and Re(S)-[6(0001) × ( 16 7 1 )] disordered structures were reported [81Duc].
3.8.6.6.1.10 Rh Using LEED, c(2×2) and c(2×2)-C2H+C2H3 structures were reported for Rh(100) and c(4×2) and (2×2) for Rh(111) [78Cas, 80Dub]. Studies on Rh(331) were described in [79Cas]. On the stepped Rh(S)-[6(111) × (100)] only a disordered layer was observed [79Cas]. Kose et al. [99Kos] determined coverage dependent heats of adsorption and sticking probabilities for C2H2 on Rh(100) by microcalorimetry. For C2H2, the initial heat of adsorption was 210±10 kJ mol−1, and the initial sticking probability was 0.86 ± 0.01. According to a theoretical DFT study by Medlin and Allendorf [03Med] C2H2 adsorbed above a 3-fold hollow site, with its C-C axis parallel to the surface but tilted away from a metal-metal bond.
3.8.6.6.1.11 Ru Adorption of C2H2 on Ru(001) and its coadsorption with CO were investigated by LEED, TPD and HREELS [86Par, 88Par, 92Sas]. On Ru(0001)-p(2×2)O and Ru(0001)-p(1×2)O decomposition to CCH3, CCH, CH and CCH2 was observed between 200 and 350 K. CO and C2H2 form an ordered mixed adlayer.
3.8.6.6.1.12 Si Jackman et al. [95Jac] studied the reactions of atomic hydrogen with C2H2 adsorbed on Si(100). In the absence of atomic hydrogen, C2H2 adsorbed molecularly at low temperature and dissociated irreversibly to gaseous hydrogen and surface carbon as the temperature was raised. Atomic hydrogen stimulated C-H and C-C bond making/breaking processes. CH2 species were found to react with acetylene to produce volatile C3 hydrocarbons. Dyson and Smith [97Dys] simulated the chemisorption of C2H2 and CH3 on the dimerized Si(100) surface using the extended Brenner potential, Hartree-Fock and DFT. Various chemisorption sites were identified. Optimal C2H2 chemisorption was found to occur in a cross-dimer configuration, parallel to the dimer rows. Optimal CH3 chemisorption occurred with the CH3 bonding directly to the surface dangling bonds. A second-layer chemisorption site for CH3 was also identified, which may be important in the formation of diamond films on a silicon substrate. On Si(111), C2H2 adsorbed in a disordered fashion [76Chu] while on Si(311), c(1×1), (2×1) and (3×1) layers were found [70Hec].
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3.8.6 Adsorbate properties of linear hydrocarbons
[Ref. p. 320
3.8.6.6.1.13 W Disordered as well as (5×1)-C, c(3×2)-C and c(2×2)-C structures were described upon C2H2 adsorption on W(100) in [78Raw, 85Ste]. On W(110), (2×2)-C2H2, c(2×2)-C2H2 and (15×3)R14°-C structures were observed [83Fou].
3.8.6.6.1.14 Alloys Panja et al. [01Pan] examined the adsorption and reaction of C2H2 on two ordered Sn/Pt(100) alloy surfaces using TPD, AES, LEED and XPS, and compared them to a hexagonally reconstructed (5×20)Pt(100) surface. Vapor deposition of Sn onto a Pt(100) single-crystal substrate was used to prepare two Pt-Sn alloys, the c(2×2) and (3¥2 × ¥2)R45° Sn/Pt(100) structures with θ Sn = 0.5 and 0.67 ML, respectively. C2H2 nearly completely decomposed during TPD on Pt(100) in the absence of Sn, forming hydrogen, which desorbed as H2, and surface carbon. The decomposition was strongly suppressed on the two Pt-Sn alloy surfaces, and a large C2H2 desorption peak in TPD was observed. Additionally, 15% of the adsorbed acetylene monolayer was converted to gaseous benzene during TPD on the (3¥2×¥2)R45° Sn/Pt(100) alloy. No benzene desorption occurred from the c(2×2) alloy. Alloyed Sn in the c(2×2) alloy decreased the initial sticking coefficient of acetylene on Pt(100) at 100 K by ~ 40%, but additional Sn in the other alloy had no additional effect. The saturation coverage of C2D2 in the chemisorbed monolayer at 100 K decreased from that on Pt(100) by 35% on the c(2×2) alloy and by 50% on the (3¥2×¥2)R45° Sn/Pt(100) alloy. The effectiveness of Sn to “block” sites apparently depends on the location of Sn adatoms or alloyed Sn atoms on the Pt surface. The acetylene chemisorption bond energy, estimated by the acetylene desorption activation energy measured in TPD, also decreased (45-65%) as the alloyed Sn concentration increased. Multiple TPD peaks for C2D2 desorption from both the c(2×2) and the (3¥2×¥2)R45° Sn/Pt(100) alloy surfaces indicated either several energetically distinguishable adsorption sites for C2H2 or the rate-limiting influence of more complex surface reactions on these surfaces.
3.8.6.6.2 Propyne C3H4 3.8.6.6.2.1 Cu Roberts et al. [96Rob] reported IRAS data for propyne C3H4 adsorbed on Cu(110) at low temperature, using molecules with targeted isotopic substitution, e.g. the partially deuteriated molecule CD3-CŁCH, which facilitated band assignments. The molecule was significantly perturbed upon adsorption, with the gas phase CŁC triple bond suffering a reduction in bond order to ca. 1 upon adsorption. Propyne was shown to be molecularly adsorbed with the formation of a di-σ/di-π-bonded species. A strong correlation between the adsorption complexes formed by propyne and those formed by acetylene on these surfaces was found and, therefore, concluded that the surface chemistry of these molecules was largely determined by the CŁC triple bond functionality. The di-σ/di-π-bonded surface species formed at low temperature was shown to be stable up to 300 K. At higher temperature, propyne trimerized to form trimethylbenzene on Cu(110). Chesters and McCash [87Che] examined propyne adsorption on Cu(110) by IRAS and reported strongly rehybridized species (Fig. 17). Deuterium substitution of the acetylenic hydrogen allowed to assign bands at 1361 and 1353 cm−1 to the CŁC stretch of CH3CCH and CH3CCD, respectively. Vibrational bands in the 2800-2950 cm−1 range originated from acetylenic and methyl C-H stretches. Toomes et al. [00Too] applied scanned-energy PED to determine the adsorption site and internal structure of propyne (CH3-CŁCH) on Cu(111). Propyne binds to the surface via the acetylenic unit in a site analogous to that for acetylene on Cu(111). The acetylenic unit is parallel to the surface in a crossbridging position such that one of the C atoms is above a fcc hollow site while the other is above a hcp hollow site, giving a C-C bond length of 1.47 Å. The methyl group was strongly tilted away from the Landolt-Börnstein New Series III/42A5
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surface and was attached with equal probability to the C atoms in the fcc and hcp hollow sites. The molecular plane was found perpendicular to the surface.
2922
2827
1353
CH3CCD 2883 2923
0.1 % ∆ R / R
CH3CCH
1361
2828 2855 2883
3000
2800
Cu (111) 2600 1400 -1 Wavenumber [cm ]
1200
Fig. 17: IRAS spectra of propyne (CH3-CŁCH and CH3-CŁCD) adsorption on Cu(111) at 150 K. With the help of isotopic substitution the bands at 1361 and 1353 cm−1 were assigned to the CŁC stretch and the peaks in the 2800 -2950 cm−1 range to acetylenic and methyl C-H stretch vibrations; adapted from Chesters and McCash [87Che].
Propyne adsorption on Cu(111) was studied by periodic and cluster model density functional theory by Valcarcel et al. [02Val]. A highly distorted propyne with C-1 and C-2 in nearly sp2 hybridization was suggested. Catalytic coupling of C3H4 on Cu(111) was examined by Middleton and Lambert [99Mid]. Instead of trimerizing like acetylene, propyne undergoes coupling reactions in which two molecules react to yield either benzene, with elimination of hydrogen, or C-6 dienes. Propyne trimerization is most likely sterically inhibited by the methyl group.
3.8.6.6.2.2 Ni IRAS spectroscopy of C3H4 adsorbed on Ni(111) at low temperatures was reported in [96Rob]. To facilitate band assignments molecules with targeted isotopic substitution, e.g. CD3-CŁCH were employed. C3H4 was significantly perturbed upon adsorption, with the gas phase CŁC triple bond suffering a reduction in bond order to ca. 1, and CD3-CŁCH was molecularly adsorbed forming a di-σ/di-π-complex. The similarity of adsorption complexes formed by propyne and acetylene suggests that the adsorption properties of these molecules are largely determined by the CŁC triple bond. The di-σ/di-π-bonded surface species formed at low temperature was stable up to room temperature.
3.8.6.6.2.3 Pd IRAS studies of C3H4 adsorption on Pd(100) by Camplin et al. [97Cam, 00Cam] indicated an adsorption geometry with the CŁC bond parallel to the surface (similar to acetylene). The rehybridization of the CŁC bond caused the H and CH3 groups to tilt away from the surface plane. For the propyne adsorption geometry a configuration was suggested in which the HCCC plane was normal or tilted with respect to the surface.
3.8.6.6.2.4 Pt Adsorption of C3H4 on Pt(111) yielded (2×2) structures at 100 K, as shown by LEED [82Koe1]. Koestner et al. [82Koe1] studied H3C-CŁCH adsorption on Pt(111) by LEED I-V analysis. At low temperature, the unsaturated C-C group forms a di-σ bond to two Pt atoms. Upon warming to about room temperature and in the presence of hydrogen a conversion takes place to an alkylidyne species that is bonded to three Pt Landolt-Börnstein New Series III/42A5
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atoms and has its C-C bond nearest to the metal substrate oriented perpendicularly to the surface. Peck et al. [98Pec] investigated the adsorption and reaction of C3H4 on Pt(111) and Sn/Pt(111) surface alloys (see 3.8.6.6.2.5) with TPD, AES and LEED. Hydrogenation of C3H4 to form propylene was the most favoured reaction pathway. No reversibly adsorbed C3H4 was detected on Pt(111). Propyne adsorption and oxidation on Pt(111) at oxygen pressures up to 0.009 Torr were investigated by Gabelnick et al. [01Gab] using fluorescence yield ultrasoft X-ray adsorption methods which indicated that propyne adsorbed with its π-system nearly parallel to the surface and with a saturation coverage of 1.45 × 1015 C atoms/cm2.
3.8.6.6.2.5 Alloys Peck et al. [98Pec] investigated the adsorption and reaction of C3H4 (H3C-CŁCH) on Pt(111) and the p(2×2) and (¥3 × ¥3)R30°-Sn/Pt(111) surface alloys with TPD, AES and LEED. Hydrogenation of C3H4 to form propylene (H3C-CH=CH2) was the most favored reaction pathway on all three surfaces. Addition of Sn to the Pt(111) surface to form two ordered surface alloys suppressed the decomposition of C3H4 to surface carbon. The alloy surfaces also greatly increased the amount of reversibly adsorbed C3H4, from zero on Pt(111) to 60% of the adsorbed layer on the (¥3 × ¥3)R30°-Sn/Pt(111) surface alloy. C3H4 reaction also lead to a small amount of benzene desorption, along with butane, butene, isobutylene and ethylene, depending the Sn concentration, with the (2×2)-Sn/Pt(111) surface alloy having the highest selectivity. Despite previous experiments showing cyclotrimerization of acetylene to form benzene on the Pt-Sn surface alloys, the analogous reaction of C3H4 to form trimethylbenzene was not observed on the alloy surfaces. It was proposed that this and the high yield of propylene was due to facile dehydrogenation of C3H4 because of the relatively weak H-CH2-CŁCH bond compared to acetylene. Desorption of several C4 hydrocarbon products at low (<170 K) temperature indicated some minor pathway involving C-C bond breaking.
3.8.6.6.3 Butyne C4H6 EELS and IRAS studies of 2-butyne adsorption were reported for Cu(111) [87Che, 88Che1], Ni(111) [92Fri] and Pt(111) [86Ave2]. The adsorption of 2-butyne on Pd(100) at 80 K was studied by IRAS [00Cam]. 2-butyne chemisorbed directly via the CŁC bond at 80 K, and further addition of 2-butyne formed a disordered, physisorbed multilayer above the chemisorbed layer. For the 2-butyne adsorption geometry a configuration was suggested in which the CCCC plane was either normal or tilted with respect to the surface.
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Table 3.8.6.7.1 Alkanes Hydrocarbon Substrate
Properties/remarks
Methods
Methane CH4 CH4
Cu(100)
physisorbed mono- and multi-layers at 24 K
IRAS
CH4
Cu(111)
TPD
CH4
DFT
more CH4 dissociation than on (bulk) Co
molecular beams
CH4 CH4 CH4 CH4 CH4
Cu ,Ni, Pd, Pt cluster models (7-13 atoms) Co clusters (7- and 13-atoms) Co film on Cu(111) Ir(110) Mo(100) Mo polycrystalline film Ni(100) Ni(100)
photodissociation into H, CH2, and CH3 by 6.4 eV photons; photoreaction cross-section ~2.0 × 10−20 cm2 Eads(CH4) [eV] at top-sites for Cu10 −0.09, Ni7 0.09, Pd7 0.08, Pt7 0.18 adsorption of CH3, CH2, CH, C and H
molecular beams LEED microcalorimetry LEED IRAS, TPD, sticking coefficient masureme
CH4
Ni(100)
CH4
Ni(110)
CH4
Ni(111)
CH4
Ni(111)
dissociative chemisorption probability scaled approx. with Ei cos2θ i c(4×4)-C, c(2×2)-C, c(6¥2 × 2¥2)R45°-C, (1×1)-C heat of adsorption 273 kJ mol−1 at 295 K (θ →0) c(2×2), (2×2) first CH4 layer: 3000, 2884 and 1298 cm−1; second CH4 layer: 3017 and 1304 cm−1; Tdes of the first layer 51 K, of the second layer ~34 K enhanced reaction probability (×100-10000) for highly vibrationally excited CH4; S02ν3 5 ×10−4 (12 kJ/mol); 2 ×10−2 (70 kJ/mol) (2×2), (4×3), (4×5)-C, (2×3)-C no adsorption detected at 300 K carbon dissolution above 600 K (2×2), (2×¥3), (4×5), (2×2)-C, (16¥3 × 16¥3)R30°-C graphite overlayer dissociation to CH3, CH, formation of C2H2 and C6H6
CH4
Ni(111)
CH4
Ni(111)
CH4 CH4
DFT
laser excitation, eigen resolved measuremen molecular beams LEED, AES, ellipsom
LEED, AES, TPD HREELS
CH3 upon adsorption at 120 K; thermal decomposition to CH, C2H2 XPS, molecular beam and C between 120 and 450 K CH4 dissociation into CH3 and H preferentially on top of Ni atoms; DFT dissociation barrier ~100 kJ/mol; C-H bond in transition state stretched to 1.6 Å and tilted relative to the CH3 group
Hydrocarbon Substrate CH4 Ni clusters (7- and 13-atoms) CH4 Pd(110) clean and oxygen modified CH4 Pd(111) Pd(311) Pd(679) CH4 Pd flat, stepped, kinked
Properties/remarks adsorption of CH3, CH2, CH, C and H
Methods DFT
direct dissociation; probability decreases linearly with increasing O coverage structure sensitive C-H bond dissociation: Pd(111)
molecular beams TPD, TPO
DFT
CH4
Pt(111)
CH4
Pt(111)
symmetry of first layer degraded from Td to C3v, symmetry of subsequent layers Td; Tdes multilayer ~40 K, Tdes first layer ~70 K
IRAS
CH4
Pt polycrystalline film
CH4
heat of adsorption 151 kJ mol−1 at 295 K (θ →0) CH4 dissociation to CH3 and H dissociation barrier reduced by ~0.3 eV on steps and kinks; association reaction structure-insensitive on Ru(0001) dissociation to CH, CCH2; on Ru(11−20) CH, CCH2 and CCH3
microcalorimetry DFT
CH4
Rh flat, stepped, kinked Ru(0001) Ru( 112 0 )
CH4 CH4 CH4 CH4
Si(100) W(100) W(110) W(111)
C1 and C2 hydrocarbons; dehydrogenation to C (5×1)-C Tdes ~50 K, activation energy ~8 kJ/mol (6×6)-C
HREELS, TPD, neutr vibrational spectrosco (NVS) TPD, AES LEED TPD, ∆Φ LEED
C-C bond parallel to surface
IRAS
C2H6
Cu(110) Cu(111) Ir(110)
molecular beams
C2H6
Mo polycrystalline film
C2H6
Ni(100)
dissociative chemisorption probability scaled approx. ET; S0 ~0.03 at kinetic energies <62 kJ/mol; increases nearly linear to 0.40 at ~165 kJ/mol heat of adsorption 419 kJ mol−1 at 295 K (θ →0) c(2×2), (2×2)
IRAS, molecular beam XPS, molecular dynam simulations
Ethane C2H6 C2H6
Landolt-Börnstein New Series III/42A5
microcalorimetry
LEED, molecular bea
Landolt-Börnstein New Series III/42A5
Hydrocarbon Substrate C2H6 Ni(110) C2H6 Ni(111)
Properties/remarks (2×2) (2×2), (2×¥3), (¥7 × ¥7)R19°-C, (2×2)-C disordered graphite trapping probability scales with ET cos0.9θ
Methods LEED LEED
C2H6
Pd(111)
C2H6
Pt(110)-(1×2)
C2H6
Pt(111)
C2H6
Pt(111) Pt(111)-p(2×2)-O
C2H6
Pt polycrystalline film
molecular beams, stoc trajectory simulations adsorption probability decreased from near unity (ET 10 kJ/mol) to molecular beams, stoc trajectory simulations 0.5 (ET 40 kJ/mol) for normal incidence non-dissociative adsorption at low T; C-C bond parallel to surface; IRAS, TDS dissociative adsorption at 150 K to C2H5, decomposition to ethylidene (CHCH3) at 250 K, to ethylidyne (CCH3) at 350 K molecular beams, stoc initial trapping probability at 100 K e.g. 0.85 for Ecosnθ=10 and trajectory simulations 0.06 for Ecosnθ=35; total energy scaling ET cos0.2θ microcalorimetry heat of adsorption 218 kJ mol−1 at 295 K (θ →0)
C2H6 C2H6
Ru(0001) W(111)
disordered at 80 K (1×1)
LEED, TPD, ESDIAD LEED
C3H8
Ir(110)-(1×2)
C3H8
Mo polycrystalline film
molecular beams, LEE AES microcalorimetry
C3H8 C3H8
Ni(100) Pt(110)
C3H8
Pt(111)
C3H8
Pt(111) Pt(655) Pt polycrystalline film
precursor-mediated dissociative adsorption dominates over direct dissociation heat of adsorption 558 kJ mol−1 at 295 K (θ →0) dissociative chemisorption only for ET >10 kJ/mol at 95 K S0 increased linearly with coverage up to saturation (0.55 ML) plane containing carbon chains aligned parallel to the surface; liquid-like multilayers initial trapping probability e.g. 0.28 for Pt(111), 0.38 for Pt(665), at 95 K for Ei=37.5 kJ/mol and normal incidence
Propane C3H8
C3H8
heat of adsorption 248 kJ mol−1 at 295 K (θ →0)
molecular beams molecular beams, LEE AES IRAS molecular-dynamics simulations microcalorimetry
Hydrocarbon Substrate Butane C4H10
Properties/remarks
Methods
Ag(110)
weak attractive intermolecular interactions; Edes (zero coverage): n-butane 44±1.2 kJ/mol, isobutane 41±1.2 kJ/mol
TPD, XPS, NEXAFS
95% molecular desorption; on S overlayers weak binding 38-42 kJ/mol; on C overlayers 46 kJ/mol
TDS
C4H10
Mo(100) clean and with sulfur or carbon overlayers Pt(111)
LEED
C4H10 C4H10
Pt(111) Pt(111)
C4H10
Pt(111)
C4H10
V(110) clean and carbide-modified
chain axis aligned parallel to the surface and to Pt[ 1 1 0 ]; order-disorder transitions plane containing the carbon chains parallel to the surface <0.14 ML: disordered monolayer; 0.14-0.20 ML: ordered regions; C4H10 preferrentially oriented parallel to the surface; >0.20 ML: densely-packed ordered phase, C4H10 probably tilted away from surface molecular plane parallel to the surface for submonolayers; close to 1 ML tilted structure with long axes oriented away from the surface weak bonding; monolayer desorption at 145 K (Edes 39.3 kJ/mol), multilayer at 101 K (Edes 27.6 kJ/mol); reactivity enhanced on carbide-modified surfaces
C4H10 n- and isobutane C4H10
IRAS molecular beams, TPD LEED
molecular-dynamics simulations HREELS, TDS
Pentane C5H12 and higher (CnH2n + 2, n = 5-12) C5H12 C5H12
Au(111) Ni(110) Pt(111)
Landolt-Börnstein New Series III/42A5
C5H12 neopentane
Pt(111)
C5H12 n- and neopentane C6H14
Pt(111)
physisorption energy increased linearly chain length by 6.2±0.2 kJ/mol per methylene unit (4×3), (4×5)
HAS
LEED LEED chain axis aligned parallel to the surface and to Pt[ 1 1 0 ]; order-disorder transitions adsorption probability increased with coverage; enhanced trapping molecular beams into second layer; dissociative chemisorption by direct collision and trapping-mediated mechanisms plane of carbon chains parallel to the surface for monolayers; for IRAS multilayers plane of carbon atoms parallel to the substrate for hexane but inclined for pentane
Landolt-Börnstein New Series III/42A5
Hydrocarbon Substrate C7H16 Au(111) Au(S)-[6(111) × (100)] C7H16 Pt(111) C7H16 Pt(S)-[4(111) × (100)] C7H16 Pt(S)-[6(111) × (100)] C7H16 Pt(S)-[7(111) × (310)] C7H16 Pt(S)-[9(111) × (100)] C8H18 Cu(111) C9H20 C10H22 C8H18 Pt(111)
Properties/remarks not adsorbed
Methods LEED, AES
(2×2) (4×2), (4×2)-C (2×2), (9×9)-C disordered (2×2), (5×5)-C, (2×2)-C, 2(one-dimensional order)-C ordered, well-defined two-dimensional lattice with the molecular C-C-C planes parallel to the surface below 160 K; disordered liquid-like state at higher temperature
LEED LEED LEED LEED LEED HAS
chain axis aligned parallel to the surface and to Pt[ 1 1 0 ]; order-disorder transitions
LEED
Properties/remarks adsorption energies Eads(CH3) [eV] at top-sites for Cu10 0.65, Ni7 2.21, Pd7 1.31, Pt7 1.75 impinging CH3 trapped as CH3 at 300 K, partial decomposition to CH2; impinging CH2 trapped as CH2 and formation of complex aromatic species surface-CH3 species 1106 cm−1 surface-CH2I 1061 cm−1
Methods DFT
Table 3.8.6.7.2 Fragments Hydrocarbon CH4, CH3, CH2, CH CH3, CH2
Substrate Cu, Ni, Pd, Pt cluster models (7-13 atoms) Cu(111)
CH3, CH2
Mo(100)
IRAS, TPD, HREELS LEED TPD, XPS
iodo-precursors; C-I bond dissociated between 120 and 180 K to yield alkyl fragments; at higher temperatures decomposition to carbon and hydrogen at low coverage, to a mixture of alkanes and alkenes at saturation CH3 Ni(100) CH3 dosing at 120-170 K produced C coverages >5 ML (adsorbed XPS, TPD clean, with oxygen, and with CHx); C2-C4 formation after very large CH3 exposure NiO(111) overlayer CH3 (C3v) and CH (pyramidal Ni3-C-H) adsorbed on threefold CH3, CH2D, Ni(111) HREELS CD3 hollow sites; >250 K C2H2 formation and trimerization to benzene; relative stabilities CH3
Ni(100)
XPS, AES, HREELS
Hydrocarbon Substrate CH3 Ni(111)
CHx (x = 1, 2, 3) CHn and C-CHn (n=1, 2, 3) CH3 C1-C6 alkyl fragments C2H5
Ni(111)
CH2
Rh(111)
CH2
Ru(001)
Pt(111)
Pt(111) Pt(111) Pt(111)
CH Ru(0001) CCH2 Ru( 112 0 ) CCH3 hydrocarbon various metal surfaces fragments (C1-C4)
Properties/remarks adsorbed on 3-fold sites; activation energy for reaction with H to CH4 59 kJ/mol; activation energy of cleavage of C-I bond 12 kJ/mol all CHx intermediates prefer threefold sites; calculated activation energy to form C-H bond 70-85 kJ/mol carbon in CHn tries to complete tetra-valency; CH occupies threefold hollow, CH2 two-fold bridge and CH3 a (one-fold coordinated) top site CH3I decomposes to CH3 and I at ~250 K alkyl surface moieties and their thermal chemistry decomposition to ethylidene (CHCH3) at 250 K and to ethylidyne (CCH3) at 350 K dissociative adsorption of CH2I2 at 90 K for submonolayer coverage, molecular adsorption on multilayers; CH2 species stable up to 300 K dissociative adsorption of CH2I2 at 110 K; adsorbed CH2 selfhydrogenates to CH4 or couples to produce di-σ-bonded ethylene methane dissociation on Ru(0001) to CH, CCH2; on Ru(11−20) CH, CCH2 and CCH3
Methods bond order conservati Morse potential (BOC analysis DFT
cluster models, band structure calculations,
HREELS, TPD, SIMS TPD, IRAS, ISS, XPS IRAS, TDS
HREELS, XPS, AES,
XPS, UPS, TPD, AES HREELS, TPD, AES
CxHy fragments
UPS, XPS, HREELS, IRAS
Properties/remarks
Methods
Table 3.8.6.7.3 Alkenes Hydrocarbon Substrate Ethylene C2H4
Landolt-Börnstein New Series III/42A5
C2H4
Ag(100)
C2H4 C2H4
Ag(110) Ag(111)
IRAS, LEED weak π-bonding at 100 K; oriented parallel to the surface at low coverage, reorientation involving rotation about the C-C axis with increasing coverage no ordered adsorption at 293-423 K LEED, AES on oxygen-activated Ag(111) reaction to acetaldehyde and ethylene IRAS oxide
Landolt-Börnstein New Series III/42A5
Hydrocarbon Substrate C2H4 Ag(410) C2H4 C2H4 C2H4 C2H4 C2H4 C2H4 C2H4
C2H4 C2H4
Au(111) Au(S)-[6(111) × (100)] Co(0001) Cr(100) Cr polycrystalline film Cu(100) Cu(100) clean, O-precovered, N-precovered Cu(110) Cu(110)
C2H4 C2H4 C2H4
Cu(111) Cu (100), (110), (111) Cu(111)
C2H4 C2H4 C2H4
Fe(100) Fe(111) Fe(100) clean, C- and O-covered
C2H4 C2H4 C2H4
Fe polycrystalline film
C2H4 C2H4 C2H4
Properties/remarks adsorption in a π-bonded state at the step edge; steps remove the translational barrier for adsorption not adsorbed not adsorbed complete decomposition below 300 K c(2×2)-C, (¥2 × 3¥2)R45°-C heat of adsorption ~422 kJ/mol at 296 K (θ →0) (2×2) at 80 K parallel to the surface; π-bonded; irradiation with low-energy (<200 eV) ethylene ions at 300 K produced hydrocarbon fragments or CN and NH one-dimensional order adsorbed atop on close-packed Cu rows (perpendicular height 2.08±0.02 Å; C-C bond length 1.32±0.09 Å) or in a short bridge site on the Cu rows (perpendicular height 2.09±0.02 Å; C-C bond length 1.53±0.13 Å); molecular plane parallel to the surface, but C-C axis may be out of the [ 1 1 0 ] direction π-bonded at 91 K excited states involved in bonding undistorted physisorbed adsorbate and a strongly distorted adsorbate state c(2×2)-C (1×1), (5×5), (3×3) molecularly adsorbed as di-σ-bonded C2H4; decomposed to methylidyne (ŁCH) and ethynyl (-CŁCH) upon heating to 523 K; finally c(2×2)-C; preadsorbed O blocked di-ҏσ-bonded C2H4 and induced physisorption of π-bonded C2H4 at 100 K
Methods molecular beam meth LEED
UPS LEED microcalorimetry LEED HREELS, LEED, TPD
LEED, EELS PED, STM
LEED, EELS cluster models DFT cluster studies
LEED LEED HREELS, TPD, AES,
microcalorimetry
Ir(111)
heat of adsorption 284 kJ mol−1 at 296 K (θ →0) disordered, decomposition, c(2×2)-C disordered (1×1)-C (¥3 × ¥3)R 30°, (9×9)-C, ethylidyne formation at 180 K
Ir(S)-[6(111) × (100)] Mo(100)
(2×2) c(2×2)-carbide, (1×1), (1×1) with streaks, c(2×2)-C
LEED LEED
Ir(100) Ir(110)
UPS, LEED, AES LEED, EELS LEED, EELS, SIMS,
Hydrocarbon Substrate C2H4 Mo(100)
Properties/remarks at 80 K adsorption in four-fold hollow sites; substantial rehybridization to ~sp3; at 220 K dehydrogenation to distorted C2H2; at 300 K C-C bond scission producing CH binding predominantly via donation of π-electrons; decomposition to H and adsorbed C on Mo(100) four-fold sites; adsorbed oxygen increases dissociation heat of adsorption 290 kJ mol−1 at 296 K (θ →0) c(2×2), (2×2), (2×2)-C(p4g), c(4×2), (¥7 × ¥7)R 19°-C
Methods ARUPS, AES, TDS
Landolt-Börnstein New Series III/42A5
C2H4
Mo(100) clean and oxygen-covered
C2H4
Mo polycrystalline film
C2H4
Ni(100)
C2H4
Ni(100)
C2H4
Ni(100)
C2H4
Ni(110)
C2H4
Ni(110)
C2H4
Ni(110)
C2H4
Ni(110)
C2H4
Ni(110)
(2×1)-C, (4×5)-C, c(2×4)-C2H4, c(2×2)-CCH, graphite overlayer
C2H4
Ni(110)
C2H4
Ni(110)
ARUPS, TPD, LEED for θ =0.25: molecular plane coplanar to the surface, C-C axis preferentially aligned along the [ 1 1 0 ]; similar bonding predicted for π-and di-σ coordination; optimized geometry parameters for π-bonded species: C-C: 1.42 Å; Ni-C: 2.01 Å; tilting of CH2 relative to the (110) crystal plane: 23° c(2×4) (0.5 ML): C-C axis parallel to surface, oriented along [1−10] ARUPS, LEED, NEX DFT
dehydrogenation species: vinyl (C2H3), acetylene (C2H2), acetylide (CCH), methylidyne (CH), carbidic carbon molecular adsorption at 90 K, little rehybridization; at higher temperature decomposition to vinyl C2H3 initial sticking probability 0.78 heat of adsorption 120 kJ mol−1. Ni-C bond strength 191 kJ/mol molecular adsorption at 80 K, rehybridization to ~sp3; ordered complex LEED pattern; >200 K decomposition to CCH and CH; at 500 K (4×5)-C c(2×4) phase at 0.5 ML and at 0.2 ML (no long-range order): the two molecules per unit cell occupy low-symmetry sites approx. midway between short-bridge and atop; C-C axis tilted ~10° with respect to surface plane; C-C axes preferentially aligned along close-packed Ni rows di-σ or π-coordination on top of ridges, weak binding over the troughs
XPS, TPD
microcalorimetry LEED
XPS EELS, TPD
microcalorimetry, stic coefficient measureme HREELS, LEED, TD
PED
angle resolved inverse photoemission (ARIP DFT LEED
Landolt-Börnstein New Series III/42A5
Hydrocarbon Substrate C2H4 Ni(111) C2H4 Ni(111) C2H4
Ni(111) Ni[5(111) × (−110)]
C2H4 C2H4
Ni(111) Ni(111)
C2H4
Ni polycrystalline film
C2H4
monolayer Ni films on Pt(111), W(110), Ru(0001) Pd(100) Pd(100)-p(2×2)-O Pd(100)-p(1×1)-H Pd(100)-p(1×1)-D Pd(110)
C2H4
C2H4 C2H4
Pd(110) Pd(110)(2×1)-H
C2H4
Pd(100), (110), (111)
C2H4
Pd(111), clean and O- precovered
C2H4 C2H4
Pd(111) Pd(111)
Properties/remarks (2×2) di-σ complex, comparison to vibrational data of [Os2(CO)8(u2-η2(C2H4)] c(4×2), (2×2); carbon atoms adsorb on bridge positions; dehydrogenation to C2H2 >230 K; on the stepped surface partial dehydrogenation and decomposition to H and CH work function change −0.3 eV at 3×10−9 and 10−8 Torr C-C axis parallel to the surface at 120 K, in an aligned bridge site, C atoms approximately atop Ni atoms heat of adsorption 251 kJ mol−1 at 296 K (θ →0) 1 ML Ni/Pt(111) inactive for ethylene decomposition ; 1 ML Ni/Ru(0001) active di-σ and π-bonded C2H4 at 80 K, reactions predominantly with vinylic C-H bonds
Methods LEED, TPD IRAS LEED, EELS
∆φ PED microcalorimetry TPD
TPRS, HREELS, isoto exchange
C-C bond aligned along [ 1 1 0 ] one-dimensional (3×1) and c(2×2) domains; π-bonded at on-top
NEXAFS, STM
binding energies for ethylene (π), ethylene (di-σ), ethyl, vinyl, ethylidyne, atomic oxygen, and atomic carbon on Pd-19 cluster (and Pd(111) slab): −30 (−27), −60 (−62), −130 (−140), −237 (−254), −620 (−636), −375 (−400), and −610 (−635) kJ/mol, respectively at 100 K di-σ bonded ethylene; on Pd(111)2×2-O π-bonded; HREELS vibrational frequencies: di-σ bonded : ~363, 177, 136, 108 meV; πҏ-bonded: ~364, 177, 136, 108 meV; C2H3: ~366, 167, 135, 46 meV; C1s binding energies: di-σ : 283.08 eV; C2H3: 283.70, 284.05 eV; DFT adsorption energies: di-σ bonded: −0.84 eV; π-bonded:ҏ −0.65 eV at 300 K ethylidyne CCH3 formation
DFT
1329 cm−1, 1089 cm−1 Edes 28 kJ/mol for π-bonded ethylene
TDS, HREELS, STM
EELS, HREELS, XPS DFT
ARUPS IRAS, TPD, LEED
Hydrocarbon Substrate C2H4 Pd(111) C2H4 C2H4
C2H4
PdAu Pd films mono-, bi-, and trilayers on Mo(100) Pt(100)
C2H4
Pt(100)
C2H4
Pt(100)
C2H4
Pt(100) hex
C2H4
Pt(100)-hex-R0.7°
C2H4
Pt(110)
C2H4 C2H4
Pt(110) Pt(110)-(1×2)
C2H4
Properties/remarks di-σ bonded at 90 K, ethylidyne at 300 K; hydrogen preadsorption favors π-bonding Au reduces ethylene decomposition to ethylidyne and CHx weakly chemisorbed; less rehybridized toward sp3 as compared to Pd(100) c(2×2) graphite overlayer heat of adsorption 200 kJ mol−1 at 300 K (θ →0) molecular adsorption at 120 K; vibrational bands at 3000 (w), 1468 (w), 1162 (s), 879 (w) and 403 cmí1 (s) heat of adsorption 250 kJ mol−1 at 300 K (θ →0) reconstruction lifted upon ethylene adsorption; heterogeneous nucleation of highly anisotropic (1×1) domains di-σ and π-bound ethylene; little di-σ ethylene on surface preexposed to 100 L H2 same adsorption energy for di-ı- and π-coordination
Methods SFG DFT AES, TPD, HREELS
LEED microcalorimetry IRAS, EELS microcalorimetry STM, LEED EELS, TDS
Landolt-Börnstein New Series III/42A5
heat of adsorption 202-205 kJ mol−1 at 300 K (θ →0) (see also Fig. 2 and 3.8.6.4.1.10)
EHT microcalorimetry, EE TPD
Pt(111)
di-σ bonded C2H4, sp3 hybridization (see also Fig. 13)
EELS, UPS
C2H4
Pt(111)
(2×2), (2×2)-C2H3, (2×1) one-dimensional order-C disordered graphite overlayer
LEED
C2H4 C2H4
Pt(111) Pt(111)
NEXAFS ARUPS
C2H4
Pt(111)
disordered, parallel to surface, C-C bond 1.49±0.03 Å carbon-carbon bond parallel to the surface at low temperature; at ~250 K ethylidyne with carbon-carbon bond normal or nearly normal to the surface at 100 K 0.25 ML C2H4 saturation coverage; binding energy 283.2 eV; decomposition above 240 K to ethylidyne with an activation barrier of 57±3 kJ mol−1 and a preexponential factor ν = 1×1010±0.5 s−1; π-bonded ethylene 283.9 eV
XPS
Landolt-Börnstein New Series III/42A5
Hydrocarbon H2C=CD2 and D2C=CD2 C2H4 C2H4
Substrate Pt(111)
Properties/remarks at 360 K isotopic scrambling between ethylidyne species
Methods IRAS
Pt(111) Pt(111)
carbon to hydrogen ratio 2:3 for ethylidyne 2880 cm−1 νS(CH3) of ethylidyne (M≡CCH3), 2910 cm−1 νS(CH2) of di-σ bonded ethylene, ~3000 cm−1 νS(CH2) of π-bonded ethylene, 2957 cm−1 νas(CH3) of ethylidene (=CHCH3)
TPD IRAS, SFG
C2H4
Pt(111)
STM
C2H4
Pt(111) Sn/Pt(111)
C2H4 C2H4
Pt(111) Pt(111)
adsorption and decomposition: ethylene, ethylidyne, carbon particles, graphite di-σ-bonded below 150 K on ordered Sn/Pt(111) surface alloys; increasing the Sn concentration decreases Edes (C2H4) from 285 K on Pt(111) to 240 K and to 184 K on the alloys di-σ coordination more stable than ʌ-coordination various C2Hx species, activated complexes of ethyl (C2H5) and ethylidene (=CH-CH3)
C2H4
Pt(111)
C2H4
Pt(210)
C2H4
Pt(211)
C2H4
Pt(311)-(1×2)
C2H4
Pt(S)-[4(111) × (100)]
C2H4
Pt(S)-[6(111) × (100)]
C2H4
Pt(S)-[7(111) × (310)]
C2H4 C2H4
Pt(S)-[9(111) × (100)] Pt(S)-[9(111) × (111)]
heat of adsorption 174 kJ mol−1 at 300 K (θ →0) molecular adsorption at 100 K; π-bonded; desorption at 250 K and decomposition to various hydrocarbon species initial heat of adsorption 180 kJ/mol; initial sticking probability 0.84 initial heat of adsorption 220 kJ mol−1; initial sticking probability 0.84; ethylylidyne at all coverages disordered graphite overlayer facets (2×2) ordered graphite overlayer (¥19 × ¥19)R22.4°-C disordered graphite overlayer adsorbed disordered graphite overlayer (2×2)
TPD, UPS, EELS, LE sticking coefficient measurements EHT, DFT quantum chemical me
microcalorimetry TPD, EELS microcalorimetry microcalorimetry LEED
LEED
LEED LEED LEED
Landolt-Börnstein New Series III/42A5
Hydrocarbon Substrate C2H4 Pt(S)-[9(111) × (100)]
Properties/remarks graphite overlayer (511), (311), (731) facetting
Methods LEED
C2H4
Pt polycrystalline film Re(0001)
C2H4 C2H4 C2H4
Re(S)-[14(0001) × (10−10)] Re(S)-[6(0001) × (16−71)] Rh(100)
heat of adsorption 148 kJ mol−1 at 296 K (θ →0) disordered (2 × ¥3)R30°-C (2 × ¥3)R30o disordered ethylidyne species (ŁC-CH3) formed by preadsorbing ~0.5 ML CO followed by C2H4 at 300 K; oriented upright with carbon-carbon bond approx. along surface normal
microcalorimetry
C2H4
C2H4
Rh(100)
C2H4
Rh(100)
C2H4
Rh(111)
C2H4
Rh(111)
C2H4 C2H4
Rh(111) Rh(755) Rh(331) Rh(S)-[6(111) × (100)]
C2H4
Rh polycrystalline film
C2H4
Ru(0001)
C2H4
Si(100)
C2H4 C2H4 C2H4
Si(331) Ta(100) Ta polycrystalline film
C2H4
W(100)
LEED LEED HREELS
initial heat of adsorption 175±10 kJ mol−1, initial sticking probability 0.88 ± 0.01 c(2×2), c(2×2)-C2H + C2H3, (2×2)-C2H, c(2×2)-C graphite overlayer c(4×2), (2×2)-C2H3, (8×8)-C, (2×2)R30°-C, (¥19 × ¥19) R23.4°-C (2¥3 x 2¥3) R30o-C, (12x12)-C ethylidyne (ŁC-CH3) on hcp hollow sites; adsorbate-induced restructuring; C-C bond 1.45±0.10 Å; terminal carbon located 1.31±0.10 Å above hcp site analysis of vibrational fine structure in C1s core level spectra several ordered surface structures; LEED patterns on (755) due to formation of surface structures on (111) terraces; when heated in C2H4 (331) more stable than (755) (which formed (111) and (100)); disordered layer on Rh(S)-[6(111) × (100)]
microcalorimetry
heat of adsorption 205 kJ mol−1 at 296 K (θ →0) C2H4 adsorbed molecularly in a di-σ structure and decomposed to ethylidyne (CCH3) above 150 K an atomic hydrogen flux converted adsorbed C2H4 to C2H2 and CH2; H stimulates C-H and C-C bond making/breaking c(1×1), (2×1), (3×1) decomposition and carbon solution heat of adsorption 577 kJ mol−1 at 296 K (θ →0) at 80 K decomposition to C2H2, followed by non-dissociative adsorption; decomposition to C2H2 around 300 K and chemisorbed C atoms above 300 K
microcalorimetry
LEED, AES, TDS LEED, TPD, EELS
LEED, TPD, EELS, is exchange high-resolution XPS LEED, AES, TDS
HREELS, SSIMS, TP IRAS TDS LEED LEED, AES microcalorimetry AES
Landolt-Börnstein New Series III/42A5
Hydrocarbon C2H4 C2H4 C2H4
Substrate W(110) W(111) W polycrystalline film
Properties/remarks (15 × 3)Rα-C, (15 × 12)Rα-C (1×1) heat of adsorption 422 kJ mol−1 at 296 K (θ →0)
Methods LEED LEED microcalorimetry
C2H3
Pd(111)
EELS, HREELS, high resolution XPS, TDS,
C2H3 C2H3 C2H3 C2H3
Pd(111) Pt(111) Pt(111) Pt(111)
C2H3
Pt(111)
C2H3
Pt(111)
C2H3
Rh(111)
C2H3
Rh(111)
(¥3×¥3)R30° overlayer with C2H3 in hollow sites; HREELS vibrational frequencies: C2H3: ~366, 167, 135, 46 meV C1s binding energies: C2H3: 283.70, 284.05 eV di-σ bonded C2H4 at 90 K, ethylidyne at 300 K disordered, perpendicular to surface, C-C bond 1.47±0.03 Å disordered; perpendicular to surface; C-C bond 1.49±0.02 Å (2×2) bonded perpendicular to surface in fcc sites; C-C bond 1.50±0.05 Å; C-surface distance 1.20±0.05 Å C2H5 decomposition into ethylidene (CHCH3) at 250 K and into ethylidyne (CCH3) at 350 K ethylidyne formation near 300 K with rate constants of 3×1016 exp(−22.4(kcal/mol)/RT) s−1 for C2H4 and 2×1016 exp(−23.0(kcal/mol)/RT) s−1 for C2D4 (2×2) C-C bond perpendicular to surface with 1.45±0.10 Å; terminal carbon 1.31±0.10 Å above hcp site analysis of vibrational fine structure in C1s core level spectra
Ethylidyne C2H3
SFG NEXAFS NMR LEED IRAS, TPD TPRS
LEED
high-resolution XPS
Propene C3H6 C3H6
Mo(100) clean and O-covered
molecular desorption; decomposition to C and H, selfhydrogenation to propane, CH4 formation; decomposition on fourfold hollow sites
UPS, TPD
C3H6
Mo polycrystalline film
heat of adsorption 328 kJ mol−1 at 296 K (θ →0)
microcalorimetry
Hydrocarbon Substrate C3H6 Ni(100)
C3H6
Pd(100)-p(2×2)-O, Pd(100)p(1×1)-H, Pd(100)-p(1×1)-D
C3H6
Pd(111), clean and H-covered
C3H6
Pt polycrystalline film
C3H6
Pt(111)
C3H6
Pt(111)
C3H6
Pt(111)
C3H6 C3H6 C3H6
Pt(111) Sn/Pt(111) Rh(111) Rh(111)
C3H6
Ru(0001)
C3H6 C3H6
W(100) W(221)
Properties/remarks precursor mediated adsorption at 105 K,; transition from π- to di-σ-bonded between 105 and 150 K; at 200 K conversion of di-ҏσ-bonded to C2HxCH3; dehydrogenation to carbidic carbon no partial oxidation; H2O, CO and CO2 as products; adsorbed O does not inhibit propene adsorption; efficient H-D exchange; no hydrogenation di-σ on Pd(111); some π-bonded C3H6on hydrogen-precovered Pd(111); molecular desorption at approx. 200 and 280 K; dehydrogenation to propylidyne and allyl species
Methods high-resolution XPS
heat of adsorption 176 kJ mol−1 at 296 K (θ →0) disordered (2×2) di-σ bonded at low T; around 300 K conversion to propylidyne (ŁC-CH2-CH3); at 300 K (2×2) and disordered structures of propylidyne di-σ bonded at 90 K; dehydrogenation to propylidyne (ŁC-CH2CH3) at ~300 K and to vinylmethylidyne (ŁC-CH=CH2) at 450 K; hydrogenation at 295 K from π-bonded C3H6 via 2-propyl species (Pt-CH(CH3)2) to C3H8 molecular desorption; dehydrogenation to propylidyne and surface C; self-hydrogenation to propane at ~280 K; H-D exchange adsorption energy 73 kJ/mol
microcalorimetry LEED, STM, EELS
(2×2) + (2¥3 × 2¥3)R30°, (2¥3 × 2¥3)R30° molecular di-σ adsorption at 80 K; above 200 K propylidyne ŁCCH2CH3 p(2×2); at 300 K ethylidyne CCH3 and CHx fragments physisorbed layer at 130 K, weak spectrum of di-σ adsorbed species; propylidyne upon annealing to 203 K (5×1)-C c(6×4)-C
TPRS, isotope exchan
TPD, IRAS
SFG, IRAS
TPD
TPD, LEED, sticking coefficient measureme LEED HREELS, TDS, LEED
EELS, IRAS LEED LEED
Landolt-Börnstein New Series III/42A5
Landolt-Börnstein New Series III/42A5
Hydrocarbon Substrate
Properties/remarks
Methods
Butenes C 4H 8 C4H8
Ag(110)
π-bonded, repulsive intermolecular, double bond axis parallel to the surface; Edes (zero coverge) 65±3 kJ/mol for 1-butene and isobutylene
TPD, XPS, NEXAFS,
C4H8 cis- and trans-2butenes C4H8 2-butene
Ag(111)
cis-2-butene/Ag(111): 1445, 1434, 1030 cm−1 trans-isomer: 1429, 973, 959 cm−1
IRAS, TPD
Mo(100) clean and O-covered
UPS, TPD binding via π-electrons; thermal decomposition to H and C on four-fold sites; molecular desorption; self-hydrogenation to butane, dissociation to C2 species and methane TPD, HREELS decomposition of cis-2-butene via cleavage of olefinic α(C-H) bonds to surface H and 2-butyne; decomposition of trans-2-butene via β(C-H) bond scission, one of the CH3 groups converts to CH2
C4H8 cis- and trans-2butene C4H8 C4H8 isobutene
Mo(110) clean and carbide-modified
Mo(100) clean and with S or C overlayers Ni(111)
C4H8
Pd(111)
C4H8
Pd(110)
trans-2-C4H8 Pd(110)
C4H8 1-butene
Pd(100)-p(2×2)-O Pd(100)-p(1×1)-H Pd(100)-p(1×1)-D
weak binding on sulfur overlayers (physisorption) 38-42 kJ/mol; molecular binding on C overlayers, heat of desorption 50-63 kJ/mol isobutene: molecularly adsorbed in di-sigma configuration below 150 K; hydrogen atoms of the methylene group bridge-bonded to the metal 2,3-dimethyl-2-butene: no ordered superstructure; partial decomposition at ~170 K physisorbed at 95 K; π1*-ҏπ2* splitting variations 2.0 eV physisorbed at 95 K π1*-ҏπ2* splitting variations ca. 2.4 eV C=C bond located at on-top sites; short-range (3×1)-1D ordered structure; c(4×2); C=C double bond parallel to [001] at low coverage and parallel to [110] at high coverage adsorbed O does not inhibit 1-butene adsorption; initial reactions occurred with the vinylic C-H bond; efficient H-D exchange below 300 K for all C-H bonds; no hydrogenation (alkanes) observed
TDS
HREELS, LEED
NEXAFS, UPS HREE DFT NEXAFS, UPS HREE STM, NEXAFS
TPRS, isotope exchan
Hydrocarbon Substrate C4H8 Pd50Cu50(111) cis-2-C4H8 Pt(111) trans-2-C4H8 Pt(111) C4H8 Pt(111) C4H8 Pt(111) 1-butene and cis- and trans-2butenes C4H8 Pt(111)
C4H8 2-butene C4H8 trans-2butene 1-butene isobutene) cis- and trans-2butene
Pt(111) Sn/Pt(111) Ru(0001)
Properties/remarks physisorbed at 95 K π1*-ҏπ2* splitting variations ~2.4 eV (2¥3 × 2¥3)R30° (8×8) di-σ-bonded at 95 K η2 di-σ adsorbed species: at 300 K µ3íη2 CH3C:CCH3 structure; central CC bond in two σ-bonds and one π-bond to the surface
Methods NEXAFS, UPS, HRE
LEED LEED NEXAFS, UPS HREE DFT IRAS, EELS, SFG, TP
LEED (2¥3 × 2¥3)R30° for cis-2-C4H8 (8×8) for trans-2-C4H8 at low T di-σ bonding; around 300 K butylidyne formation; bonded to 3 Pt atoms, C-C bond nearest to the metal oriented perpendicular adsorption energy 72 kJ/mol TPD, LEED, sticking coefficient measureme IRAS at 90 K molecular adsorption as di-σ species; decomposition pathways different
Si(100)-(2×1)
molecular adsorption at 120 K; initial sticking near unity, saturation coverages at 120 K 1.4 × 1014 molecules cm−2 for transbutene, 1.6 × 1014 molecules cm−2 for cis-butene; Edes ~142 kJ/mol for trans-2-butene and ~125 kJ/mol for cis-2-butene
kinetic uptake, TPD, A
Au(111)
physisorption energies increased linearly with chain length (~5-6 kJ/mol per methylene unit)
HAS
Pentenes C5H10 and Hexenes C6H12 and higher Landolt-Börnstein New Series III/42A5
1-alkenes (C6H12C11H22)
Landolt-Börnstein New Series III/42A5
Hydrocarbon C5H10 trans-2pentene, cis2-pentene, and 1pentene C5H10 trans-2pentene C5H10 C6H12
Substrate Pd(111)
Properties/remarks three distinct molecular desorption states at 130, 175 and 260 K, due to a multilayer, π-bonded pentene and interchanging di-σ-bonded pentene/pentyl groups
Methods TPD
Pd(111)
up to 73% carbon observed during hydrogenation (graphite, C-H and C-Pd components); trans-2-pentene hydrogenated via σ-bonded configuration
XPS, UPS
Pt(111) Ni(111)
C6H12 1-hexene C6H12
Pd(111) clean and H(D)- saturated Pt(111)
C6H12
Ru(0001)
2,3-dimethyl-2-butene: no ordered superstructure; partial decomposition at ~170 K weak π-bonded species, dehydrocyclization
EELS HREELS, LEED TPRS
mixture of rotational conformers of hexylidyne; IRAS thermal decomposition of 1-hexene does not involve formation of ethylidyne (µ(3)-CCH3) mixture of rotational conformers of hexylidyne; IRAS thermal decomposition of 1-hexene does not involve the formation of ethylidyne (µ(3)-CCH3)
Table 3.8.6.7.4 Dienes Hydrocarbon Substrate
Properties/remarks
Methods
Propadiene C3H4 C3H4
Ag films Cu(110)
C3H4
Ni(111)
C3H4
Rh(111)
orthogonal π-system retained; one CH2 group with its plane IRAS parallel to the surface, the other with its plane perpendicular to the surface; preferential orientation with the C=C=C skeleton parallel to the surface isomerization, original C=C double bonds replaced by one single IRAS and one triple bond; formation of a di-σ/di-π propyne species molecular adsorption at 80 K; above 200 K p(2×2) structure; EELS at 300 K decomposition to CCH3 and CxH
Hydrocarbon Substrate Butadiene C4H6
Properties/remarks
Methods
C4H6
Ag(110)
NEXAFS
C4H6
Mo(100) clean and with S or C overlayers Pd(100)-p(2×2)-O
sigma-h plane parallel to the surface; C=C and C-C bond lengths identical to the gas phase values of 1.34 and 1.46 Å decomposition on clean surface, on S overlayers weak physisorption 40 kJ/mol; on C overlayers 71-97 kJ/mol Oads did not inhibit 1,3-butadiene adsorption; initial reactions with the vinylic C-H bond selective hydrogenation to alkenes 300 K, no H-D exchange
C4H6
C4H6
Pd(100)-p(1×1)-H Pd(100)-p(1×1)-D Pd(110)
C4H6 C4H6
Pd(110) Pd(111)
C4H6 C4H6 C4H6 C4H6 C4H6
Pd(111) Pd50Cu50(111) Pt(110) Pt(111) Pt(111)
C4H6
V(110) clean and carbide-modified
C4H6
TDS
TPRS, isotope exchan
TPRS, isotope exchan
π-bonding with the molecular plane parallel to the surface; C-C single bond aligned towards [ 1 1 0 ] physisorbed (di-π mode) at 95 K, at 300 K dehydrogenation at 95 K di-σ bonded; at 300 K di-σ interaction, keeping one central carbon-carbon double bond (probably transformed into butylidyne) π-bonded and di-σ species have about the same adsorption energy physisorbed (di-π mode) at 95 K, at 300 K dehydrogenation same adsorption energy for di-σҏ andҏπ-coordination di-σ coordination more stable at 95 K π-bonded; at 300 K di-ҏπ bonded monolayer desorption at ~150 K, multilayer at 110 K
HREELS, NEXAFS,
formation of benzene during TPD
XPS, TPD
NEXAFS, UPS, HRE NEXAFS, UPS, HRE
EHT, DFT NEXAFS, UPS, HRE EHT EHT, DFT NEXAFS, UPS, HRE IRAS HREELS, TPD
Pentadiene C5H8 Hexadiene C6H10 Landolt-Börnstein New Series III/42A5
1,5hexadiene
Ni(100)
Landolt-Börnstein New Series III/42A5
Hydrocarbon 1,5-hexadiene 1,3-hexadiene (1,3,5hexatriene)
Substrate Pd(111) clean and H(D)- saturated
Properties/remarks weak π-bonded adsorption configuration at low-temperature; dehydrocyclization to benzene; H-D exchange
Methods TPRS, isotope exchan
Hydrocarbon Substrate Acetylene C2H2
Properties/remarks
Methods
C2H2
Ag(110)
LEED, EELS
C2H2
Co(0001)
C2H2
Co(11−20)
C2H2
Cu(100)
C2H2
Cu(100)
C2H2
Cu(110)
C2H2
Cu(111)
C2H2
Cu(111) Cu(110)
at 100 K molecular adsorption; carbon-carbon triple bond preserved; CH stretching frequency of 3270 cm−1; desorption between 100 and 160 K strong chemisorption bond 300 K, hybridization close to sp3; C-C axis parallel to the surface; vibrational splitting in XPS due to excitation of C-H stretch 389 ± 8 meV C2H2 dissociates at ~200 K; at 200-300 K C2H or C2, coexisting with molecular C2H2; at 450 K decomposition to graphitic carbon and formation of a (5×2) C overlayer; mainly graphitic carbon 600 K pre-adsorbed oxygen facilitates CH bond scission to form CCH species adsorbed on fourfold hollow site, C-H bond bent away from surface, molecular plane perpendicular to surface; energy barrier for molecule rotation 169±3 meV, preexponential factor 1011.8 ± 0.2 s–1; thermal diffusion barrier of individual molecules 0.53±0.01 eV, preexponential factor 1013.6 ± 0.2 s–1 molecular adsorption <200 K; trimerization to benzene which desorbs at ~325 K non-dissociative adsorption; C-C axis parallel to Cu(111), over fcc (111) bridge site; C-C 1.48+0.10 Å C-C axis parallel to Cu(111); on bridge site with the two C centers point towards adjacent 3-fold hollow sites; C-C distance increased by 0.16 Å with respect to free molecule; C-H axes tilted by 60º with respect to the C-C axis, pointing away from the surface
Table 3.8.6.7.5 Alkynes
XPS, XAS, LEED, UP
XPS, NEXAFS, LEED
EELS, XPS STM-IETS
EELS, TPD PED, EELS, IRAS
cluster model DFT
Hydrocarbon Substrate C2H2 Fe(100) clean, C- and O-covered
C2H2
Fe(110)
C2H2 C2H2 C2H2
Fe(111) Fe(100), (110), and (111) cluster models Ir(100)
C2H2
Ir(111)
C2H2 C2H2
Ir(S)-[6(111) × (100)] Mo polycrystalline film
C2H2
Ni(100)
C2H2
Ni(100)
C2H2
Ni(110)
C2H2
Ni(110)
C2H2
Ni(110)
C2H2
Ni(111)
Properties/remarks hybridization ~sp3; <0.2 L: decomposition to ŁCH and -CŁCH at 253 K; >0.2 L: C2H2 partial dehydrogenation/hydrogenation to ŁCH, -CŁCH and -CH=CH2 at 100 K; =C=CH2 species at 393 K via dehydrogenation of -CH=CH2 (2×2), (2×3) coincidence (1×1), (5×5), (3×3) four-fold sites favored on (100) and (110); di-σ bridging favored on (111) disordered c(2×2)-C (¥3 × ¥3)R30°, (9×9)-C at 180 K adsorbed CCH and ethylidyne species (2×2) heat of adsorption 261 kJ mol−1 at 295 K (θ →0) c(2×2), (2×2), c(4×2), (2×2)-C
Landolt-Börnstein New Series III/42A5
dehydrogenation species: acetylide (CCH), methylidyne (CH), carbidic carbon C-C bond parallel to surface; bonded to two meal atoms on adjacent ridges; for c(2×2) C-C axis oriented along the substrate throughs ([ 1 1 0 ] azimuth) adsorbed molecularly at 80 K, rehybridization to ~sp2.5; c(2×2); C-C bond parallel to surface c(2×2) initial sticking probability 0.8; heat of adsorption 190 kJ mol−1; Ni-C bond strength 191 kJ/mol (2×2), (¥3×¥3)R30° disordered C-C bond parallel to the surface, center of the C-C bond over bridge site; C-C bond perpendicular to Ni-Ni bridge; C-C bond length 1.50 Å, carbon atoms 2.1±0.10 Å above surface
Methods HREELS, TPD, AES,
LEED, EELS
LEED, EELS molecular orbital theo LEED
LEED, HREELS, SIM XPS LEED microcalorimetry LEED, EELS high-resolution XPS
(AR)UPS, LEED, NE TPD, DFT
HREELS, LEED, TD LEED, microcalorimetry molecular beams LEED, AES
Landolt-Börnstein New Series III/42A5
Hydrocarbon Substrate C2H2 Ni(111)
Properties/remarks (2×2), (¥3×¥3)R30° and (2¥3 ×¥3)R30°; bridge positions suggested as adsorption sites for the C atoms
Methods LEED, EELS, IRAS
C2H2
Ni (111)
∆Φ
C2H2
Ni(111)
C2H2
Ni (111)
C2H2
Ni(111) Ni [5 (111) × (110)]
C2H2
Pd(100)
C2H2 C2H2
Pd(100)-p(1×1)-H Pd(100)-p(1×1)-D Pd(110)
C2H2
Pd(111)
C2H2
Pd(111)
C2H2
Pd(111)
C2H2 C2H2
Pd(111) Pd(111)
change of work function −0.6 eV was obtained at 3×10−9 Torr; −1 eV at 10−8 Torr; possibly polymerization preferably adsorbed in cross-bridge site; C-C bond length 20% stretched compared with that of gas phase; energy barrier for migration from aligned bridge site to crossbridge site 0.02 eV C-C axis parallel to the surface; on cross-bridge site with C atoms directly above inequivalent hollow sites; C-C bondlength larger than gas-phase molecule, indicating a significant reduction of C-C bond order on Ni(111) sp3-type configuration at 150 K; on the stepped surface C2H2 dehydrogenated to C2 which further decomposed into C atoms adsorption geometry with CŁC bond parallel to the surface; C-H bonds tilted away from surface plane selective hydrogenation to alkene around 300 K; no H-D exchange non-dissociative adsorption; significant rehybridisation; at 90 K molecular adsorption in µ2-site with C-C bond inclined to the surface (¥3 × ¥3)R30°-diffuse, (¥3 × ¥3)R30°- C2H2 disordered (¥3 x ¥3)R30°- C2H3 at 125 K two ordered phases, (2×2) and (¥3×¥3)R30°; C2H2 in hollow sites; (¥3×¥3)R30° with preadsorbed H; (¥3×¥3)R30° ethylidyne overlayer upon heating (¥3×¥3)R30° C2H2+H to 350 K; C2H3 in hollow sites (2×2): C atoms located almost over bridge sites; C-C bond length 1.34±0.10 Å; molecule center over (presumably) hcp hollow site; adsorption site for the (¥3×¥3)R30° identical C=CH2 species, C-C bond length of 0.146 nm, 50º tilted C2H2 trimerization to benzene via reaction of adsorbed C2H2and a surface C4 metallocycle
DFT, EHT
PED
EELS
IRAS, TPD, EELS
TPRS, isotope exchan
HREELS, TDS, LEED
LEED
high-resolution XPS,
PED
UPS, NEXAFS TPD, IRAS, NMR
Hydrocarbon Substrate C2H2 Pd(111)
Landolt-Börnstein New Series III/42A5
C2H2
Pd(111)
C2H2
Pd(111)
C2H2 C2H2
Pd films on Mo(100) Pd films on Ta(110)
C2H2
Pt(100)
C2H2
Pt(100), (110), (100)
C2H2 C2H2
(5×20)-Pt(100), hexagonally reconstructed Pt(111)
C2H2
Pt(111)
C2H2 C2H2
Pt(111) Pt(111)
C2H2 C2H2
Pt(111) Pt polycrystalline film
Properties/remarks adsorbed acetylene, vinyl intermediates (from vinyl iodide), vinylidene binding energy at 25% (33%) coverage −172 (−136) kJ/mol; C2H2 oriented above a 3-fold hollow site, with its axis parallel to the surface but tilted away from a metal-metal bond at 4.7 K three equivalent rotational states on (both fcc and hcp) 3fold hollow adsorption sites, with the fcc adsorption site being more stable C2H2 strongly rehybridized toward sp3 weaker chemisorption than on bulk-terminated Pd; not as strongly perturbed as for C2H4; C2H2 reversibly adsorbed on 1st monolayer (Tdes at 180 and 265 K); on a thick Pd film (θ (Pd) = 5) C2H2 desorption in a broad peak near 330 K c(2×2)
Methods TPD, IRAS, EELS DFT
STM
AES, TPD, HREELS TPD
LEED
most stabe adsorption sites are three-fold hollow on (111), fourfold-hollow on (100); on (110) four-fold and di-σ bridging comparable decomposition into hydrogen and surface C
molecular orbital theo
at 140 K non-dissociative adsorption; rehybridization to ~sp2; di-σ bonded with additional π contribution; bridging ethylidyne and C2H formed between 330 and 400 K (2×1), (2×2)
EELS, TDS
carbon-carbon bond axis parallel to the surface C2H2 oriented above a 3-fold hollow site, with its axis parallel to the surface but tilted away from a metal-metal bond disordered; bond parallel to surface; C-C bond length 1.45±0.03 Å
ARUPS quantum chemical me DFT NEXAFS microcalorimetry
heat of adsorption 188 kJ mol−1 at 295 K (θ →0)
TPD, AES, LEED, XP
LEED
Landolt-Börnstein New Series III/42A5
Hydrocarbon Substrate C2H2 Sn/Pt(100) Pt-Sn alloys: c(2×2) θ (Sn) = 0.5; (3¥2×¥2)R45º θ (Sn) = 0.67 ML C2H2 Re(0001)
C2H2 C2H2
Re(S)-[14(0001) × (10−11)] Re(S)-[6(0001) × (16−71)] Rh(100) Rh(100)
C2H2 C2H2
Rh(111) Rh(111)
C2H2
C2H2 C2H2
Rh(755) Rh(331) Rh(S)-[6(111) × (100)] Ru(0001) Ru(0001)-p(2×2)O Ru(0001)-p(1×2)O Si(111) Si(100)
C2H2
dimerized Si(001)
C2H2 C2H2
Si(111) W(100)
C2H2 C2H2
W(110) W(100), (110), (111)
C2H2
C2H2
Properties/remarks alloyed Sn decreased initial sticking coefficient at 100 K by ~ 40%; saturation coverage of C2D2 at 100 K decreased by 35-50% on the alloys; C2H2 decomposition into H and surface C strongly suppressed on alloys disordered (2 × ¥3)R30°-C disordered
Methods TPD, AES, LEED, XP
c(2×2), c(2×2)-C2H + C2H3
LEED microcalorimetry
−1
initial heat of adsorption 210±10 kJ mol ; initial sticking probability 0.86±0.01 c(4×2), (2×2) oriented above a 3-fold hollow site, with C-C axis parallel to the surface but tilted away from a metal-metal bond several ordered and disordered structures; refacetting into (111) and (100) facets
LEED LEED
LEED, EELS DFT LEED, AES, TDS
molecularly adsorbed; nearly sp3 hybridized; between 200 and 350 K decomposition to CCH3, CCH, CH, CCH2
LEED, TPD, HREEL
disordered molecular adsorption at low temperature; dissociation as temperature is raised; C2H2 converted to CH2, which reacts with C2H2 to C3 hydrocarbons optimal C2H2 chemisorption in a cross-dimer configuration, parallel to the dimer rows c(1×1), (2×1), (3×1) disordered (5×1)-C, c(3×2)-C c(2×2)-C (2×2)-C2H2, c(2×2)-C2H2, (15×3)R14°-C distorted rehybridized molecular complexes; dissociative chemisorption
EELS TDS, reactive beam scattering DFT LEED LEED, EELS
LEED, EELS EELS
Hydrocarbon Substrate
Properties/remarks
Methods
significant perturbation upon adsorption; molecularly adsorbed as di-σ/di-π-bonded species, stable up to 300 K; at higher T propyne trimerized to trimethylbenzene strong rehybridization; ν CŁC 1361 cm−1
IRAS
Propyne C 3H 4 CD3CCH
Cu(110)
C3H4
Cu(111)
C3H4
Cu(111)
C3H4
Cu(111)
C3H4
Mo polycrystalline film
C3H4
Ni(111)
C3H4
Pd(100)
C3H4 Pt(111) Pt(111) C3H4 methylacetylene (H3C-CŁCH) C3H4 Pt(111) C3H4
Pt polycrystalline film
C3H4 methylacetylene (H3C-CŁCH) C3H4
Sn/Pt(111) Pt-Sn alloys: p(2×2) and (¥3×¥3)R30º Rh(111)
adsorption via the acetylenic unit, parallel to the surface in a crossbridging position, one C atom above a fcc hollow site, the other above a hcp hollow site; C-C bond length 1.47 Å, methyl group tilted away from surface highly distorted propyne with C-1 and C-2 in nearly sp2 hybridization heat of adsorption 293 kJ mol−1 at 295 K (θ →0) significant perturbation upon adsorption; molecularly adsorbed as di-σ/di-π-bonded species, stable up to 300 K adsorption geometry with the CŁC bond parallel to the surface, H and CH3 groups tilted away from surface plane (2×2) hydrogenation of methyl-acetylene to form propylene favoured
Landolt-Börnstein New Series III/42A5
adsorbed with π-system nearly parallel to the surface; saturation coverage 1.45 × 1015 C atoms/cm2 heat of adsorption 186 kJ mol−1 at 295 K (θ →0) hydrogenation to propylene favored; alloys suppress decomposition to carbon; small amount of benzene desorption, no cyclotrimerization to trimethylbenzene; C-C bond scission minor pathway molecular adsorption at 80 K; p(2×2); at 300 K decomposition to CCH3 and CxH
IRAS PED
DFT microcalorimetry IRAS IRAS, TPD LEED, IRAS TPD, AES, LEED
XAS microcalorimetry TPD, AES, LEED
HREELS, LEED, TD
Landolt-Börnstein New Series III/42A5
Hydrocarbon Substrate
Properties/remarks
Methods
strong rehybridization; ν CŁC 1392 cm−1 (3×2¥3)rect.; stable up to 300 K; 2-butyne with two flat lying molecules per unit cell chemisorbed via the CŁC bond; CCCC plane either normal or tilted to the surface adsorption via CŁC bond
IRAS
Butyne C4H6 C4H6 C4H6
Cu(111)
C4H6
Pd(100)
C4H6
Pt(111)
Ni(111)
EELS IRAS EELS, IRAS
320
3.8.6 Adsorbate properties of linear hydrocarbons
3.8.6.8 References for 3.8.6 34Hor 50Bee 68Mor 69Ber 69Bou 69Mor 70Dal 70Hec 70Mai 70Smi 72Lan 74Bar 74Che1 74Che2 74Che3 74Dal 74Wei 75Che 75Hor 75Lan1 75Lan2 75McC 76Bro 76Chu 76Gui 76Nie 76Pea 76Rho 77Abb 77Bac1 77Bac2 77Ber 77Bru 77Cas 77Cer 77Dem 77Ert 77Fir 77Fis1 77Fis2 77Iba1 77Iba2 77Kes 77McC 77Sch 77Smu 77Som 77Sta 78Bac
Horiuti, I., Polanyi, M.: Trans. Faraday Soc. 30 (1934) 1164. Beeck, O., Cole, W.A., Wheeler, A.: Discuss. Faraday Soc. 8 (1950) 314. Morgan, A.E., Somorjai, G.A.: Surf. Sci. 12 (1968) 405. Bertolini, J.C., Dalmai-Imelik, G.: Rapport Institute de Recherche sur la Catalyse, Villeurbanne, 1969. Boudart, M., Ollis, D.F., in: The structure and chemistry of solid surfaces, Somorjai, G.A. (ed.), New York: John Wiley & Sons, 1969. Morgan, A.E., Somorjai, G.A.: J. Chem. Phys. 51 (1969) 3309. Dalmai-Imelik, G., Bertolini, J.C: C. R. Acad. Sci. (Paris) 270 (1970) 1079. Heckingbottom, R., Wood, P.R.: Surf. Sci. 23 (1970) 437. Maire, G., Anderson, J.R., Johnson, B.B.: Proc. R. Soc. (London) A 320 (1970) 227. Smith, D,L., Merrill, R.F.: J. Chem. Phys. 52 (1970) 5861. Lang, B., Joyner, R.W., Somorjai, G.A.: Surf. Sci. 30 (1972) 454. Baron, K., Blakely, D.W., Somorjai, G.A.: Surf. Sci. 41 (1974) 45. Chesters, M.A., Hopkins, B.J., Jones, A.R., Nathan, R.: J. Phys. C 7 (1974) 4486. Chesters, M.A., Hopkins, B.J., Jones, A.R., Nathan, R.: Surf. Sci. 45 (1974) 740. Chesters, M.A., Hopkins, B.J., Leggett, M.R.: Surf. Sci. 43 (1974) 1. Dalmai-Imelik, G., Bertolini, J.C.: Jpn. J. Appl. Phys. Suppl. Vol. 2, Pt. 2 2 (1974) 205. Weinberg, W.H., Deans, H.A., Merill, R.P.: Surf. Sci. 41 (1974) 312. Chesters, M.A., Somorjai, G.A.: Surf. Sci. 52 (1975) 21. Horn, K., Pritchard, J.: Surf. Sci. 52 (1975) 437. Lang, B.: Surf. Sci. 53 (1975) 317. Lang, B., Legare, P., Maire, G.: Surf. Sci. 47 (1975) 89. Mccarty, J., Madix, R.J.: J. Catal. 38 (1975) 402. Broden, G., Rhodin, T., Capehart, W.: Surf. Sci. 61 (1976) 143. Chung, Y., Siekhaus, W., Somorjai, G.A.: Surf. Sci. 58 (1976) 341. Guillot, G., Riwan, R., Lecante, J.: Surf. Sci. 59 (1976) 581. Nieuwenhuys, B.E., Hagen, D.I., Rovida, G., Somorjai, G.A.: Surf. Sci. 59 (1976) 155. Pearce, H.A., Sheppard, N.: Surf. Sci. 59 (1976) 205. Rhodin, T.N., Broden, G.: Surf. Sci. 60 (1976) 466. Abbas, N., Madix, R.J.: Surf. Sci. 62 (1977) 739. Backx, C., Fuerbacher, B.F., Fitton, B., Willis, R.F.: Surf Sci. 63 (1977) 193. Backx, C., Willis, R.F., Fuerbacher, B.F., Fitton, B.: Surf Sci. 68 (1977) 516. Bertolini, J.C., Dalmai-Imelik, G., Rousseau, J.: Surf. Sci. 67 (1977) 478. Bruckner, C., Rhodin, T.: J. Catal. 47 (1977) 214. Casalone, C., Cattania, M.G., Simonetta, M., Tescari, M.: Surf. Sci. 62 (1977) 321. Cerny, S., Smutek, M., Buzek, F.: J. Catal. 47 (1977) 159. Demuth, J.E.: Surf. Sci. 69 (1977) 365. Ertl, G.: Surf. Sci. 7 (1977) 309. Firmet, L.E., Somorjai, G.A.: J. Chem. Phys. 66 (1977) 2901. Fischer, T.E., Kelemen, S.R.: Surf. Sci. 69 (1977) 485. Fischer, T.E., Kelemen, S.R., Bonzel, H.P.: Surf. Sci. 64 (1977) 85. Ibach, H., Hopster, H., Sexton, B.: Appl. Phys. 14 (1977) 21. Ibach, H., Hopster, H., Sexton, B.: Appl. Surf. Sci. 1 (1977) 1. Kesmodel, L.L., Dubois, L.H., Somorjai, G.A.: Surf. Sci. 66 (1977) 299. Mccarty, J., Madix, R.J.: J. Catal. 48 (1977) 422. Schouten, F., Kaleveld, E., Bootsma, G.: Surf. Sci. 63 (1977) 460. Smutek, M., Cerny, S.: J. Catal. 47 (1977) 178. Somorjai, G.A.: Adv. Catal. 26 (1977) 1. Stair, P.C., Somorjai, G.A.: J. Chem. Phys. 66 (1977) 573. Backx, C., Willis, R.F.: Chem. Phys. Lett. 53 (1978) 471. Landolt-Börnstein New Series III/42A5
3.8.6 Adsorbate properties of linear hydrocarbons 78Ber 78Cas 78Dem 78Duc 78Fis 78Hor 78Iba 78Kes 78Mad 78Net1 78Net2 78Nie 78Raw 78Sch 78She 78Win 78Yos 79Ber 79Cas 79Cat 79Che 79Dem1 79Dem2 79Kes 79Leh 79Onu 79Sch 79Som
80Dub 80Hub 80Ko 80Ron 81Bar 81Cas 81Duc 81Ham 81Iba 81Kes 81Kis 81Ko 81Oya 81Pas 81Ski 82Alb 82Cas 82Dub 82Erl 82Gar 82Gat1
321
Bertolini, J.C., Massardier, J., Dalmai-Imelik, G.: J. Chem. Soc. Farday Trans. I 74 (1978) 1720. Castner, D.G., Sexton, B.A., Somorjai, G.A.: Surf. Sci. 71 (1978) 519. Demuth, J.E., Ibach, H.: Surf. Sci. 78 (1978) L238. Ducros, R., Housley, M., Alnot, M., Cassuot, A.: Surf. Sci. 71 (1978) 433. Fischer, T.E., Kelemen, S.R.: J. Vac. Sci. Technol. 15 (1978) 607. Horn, K., Bradshaw, A.M., Jacobi, K.: J. Vac. Sci. Technol. 15 (1978) 575. Ibach, H., Lehwald, S.: J. Vac. Sci. Technol. 15 (1978) 407. Kesmodel, L.L., Dubois, L.H., Somorjai, G.A.: Chem. Phys. Lett. 56 (1978) 267. Madey, T.E., Yates, J.T.: Surf. Sci. 76 (1978) 397. Netzer, F.P., Willie, R.: J. Catal. 51 (1978) 18. Netzer, F.P., Willie, R.: Surf. Sci. 74 (1978) 547. Nieuwenhuys, B.E., Somorjai, G.A.: Surf. Sci. 72 (1978) 8. Rawlings, K.J., Hopkins, B., Foulias, S.: Surf. Sci. 77 (1978) 561. Schouten, F., Brake, E., Gijzeman, O.L.J., Bootsma, G.: Surf. Sci. 74 (1978) 1. Sheppard, N., Nguyen, T.T.: Adv. Infrared Raman Spectrosc. 5 (1978) 67. Winters, H.: IBM J. Res. Dev. 22 (1978) 260. Yoshida, K., Somorjai, G.A.: Surf. Sci. 75 (1978) 46. Bertolini, J.C., Rousseau, J.: Surf. Sci. 83 (1979) 531. Castner, D.G., Somorjai, G.A.: Surf. Sci. 83 (1979) 60. Cattania, M.G., Simonetta, M., Tescari, M.: Surf. Sci. 82 (1979) L615. Chesters, M.A., Hopkins, B.J., Taylor, P.A., Winton, R.I.: Surf. sci. 83 (1979) 181. Demuth, J.E.: Surf. Sci. 80 (1979) 367. Demuth, J.E., Ibach, H.: Surf. Sci. 85 (1979) 365. Kesmodel, L.L., Dubois, L.H., Somorjai, G.A.: J. Chem. Phys. 70 (1979) 2180. Lehwald, S., Ibach, H.: Surf. Sci. 89 (1979) 425. Onuferko, J.H., Woodruff, D.P., Holland, B.: Surf. Sci. 87 (1979) 357. Schouten, F., Gijzeman, O.L.J., Bootsma, G.: Surf. Sci. 87 (1979) 1. Somorjai, G.A., Hove, M.A.V., in: Structure and bonding, Dunitz, J.D., Goodenough, J.B., Hemmerich, P., Albers, J.A., Jørgensen, C.K., Neilands, J.B., Reinen, D., Williams, R.J.P. (eds.), Springer-Verlag, Berlin, 1979. Dubois, L.H., Castner, D.G., Somorjai, G.A.: J. Chem. Phys. 72 (1980) 5234. Hubbard, A.T.: J. Vac. Sci. Technol. 17 (1980) 49. Ko, E., Madix, R.J.: Surf. Sci. 100 (1980) L449. Rovida, G., Pratesi, F., Ferroni, E.: Appl. Surf. Sci. 5 (1980) 121. Barteau, M.A., Madix, R.J.: Surf. Sci. 103 (1981) L171. Casalone, G., Cattania, M.G., Simonetta, M.: Surf. Sci. 103 (1981) L121. Ducros, R., Housley, M., Piquard, G., Alnot, M.: Surf. Sci. 108 (1981) 235. Hamilton, J.C., Swanson, N., Waelawski, B.J., Cellota, R.J.: J. Chem. Phys. 74 (1981) 4156. Ibach, H., Lehwald, S.: J. Vac. Sci. Technol. 18 (1981) 625. Kesmodel, L.L., Gates, J.A.: Surf. Sci. 111 (1981) L747. Kiskinova, M.P., Goodman, D.W.: Surf. Sci. 109 (1981) L555. Ko, E., Madix, R.J.: Surf. Sci. 109 (1981) 221. Oyama, T., Ohi, S., Kawazu, A., Tominaga, G.: Surf. Sci. 109 (1981) 82. Passler, M.A., Lin, T.H., Ignatiev, A.: J. Vac. Sci. Technol. 18 (1981) 481. Skinner, P., Howard, M.W., Oxton, I.A., Kettle, S.F.A., Powell, D.B., Sheppard, N.: J. Chem. Soc. Faraday Trans. II 77 (1981) 1203. Albert, M.R., Sneddon, L.G., Eberhardt, W., Greuter, F., Gustafsson, T., Plummer, E.W.: Surf. Sci. 120 (1982) 19. Casalone, G., Cattania, M.G., Merati, F., Simonetta, M.: Surf. Sci. 120 (1982) 171. Dubois, L.H.: J. Chem. Phys. 77 (1982) 5228. Erley, W., Baro, A.M., Ibach, H.: Surf Sci. 120 (1982) 273. Garwood, G., Hubbard, A.: Surf. Sci. 118 (1982) 223. Gates, J.A., Kesmodel, L.L.: Surf. Sci. 120 (1982) L461.
Landolt-Börnstein New Series III/42A5
322 82Gat2 82Gew 82Iba1 82Iba2 82Koe1 82Koe2 82Mad 82Nyb 82Ove 82Ste 82Stu 82Van 83Ans1 83Ans2 83Din1 83Din2 83Fou 83Fre 83Gat 83Hof 83Kes 83Koe 83Lab 83Llo 83Min 83Rie 83Tys 84Alb 84And 84Ban 84Ben 84Kob 84Koe 84Meh 84Pal 84Ram 84Ste 84Stö 84Str 84Tys 85Ave 85Bee 85Che 85Hor 85Koe 85Mar 85Ste
3.8.6 Adsorbate properties of linear hydrocarbons Gates, J.A., Kesmodel, L.L.: J. Chem. Phys. 76 (1982) 4281. Gewinner, G., Peruchetti, J.C., Jaegle, A.: Surf. Sci. 122 (1982) 383. Ibach, H.: Surf. Sci. 117 (1982) 685. Ibach, H., Mills, D.L.: Electron energy loss spectrscopy and surface vibration, New York: Academic Press, 1982. Koestner, R.J., Frost, J.C., Stair, P.C., Van Hove, M.A., Somorjai, G.A.: Surf. Sci. 116 (1982) 85. Koestner, R.J., Van Hove, M.A., Somorjai, G.A.: Surf. Sci. 121 (1982) 321. Madix, R.J.: Appl. Surf. Sci. 14 (1982) 41. Nyberg, C., Tengstal, C.G., Andersson, S., Holmes, M.W.: Phys. Chem. Lett. 87 (1982) 87. Overbury, S., Stair, P.C.: J. Vac. Sci. Technol. A 1 (1982) 1055. Steiniger, H., Ibach, H., Lehwald, S.: Surf. Sci. 117 (1982) 685. Stuve, E.M., Madix, R.J., Sexton, B.A.: Surf Sci. 123 (1982) 491. Van Hove, M.A., Koestner, R.J., Somorjai, G.A.: J. Vac. Sci. Technol. 20 (1982) 886. Anson, C.E., Keiller, B.T., Oxton, I.A., Powell, D.B., Sheppard, N.: J. Chem. Soc. Chem. Commun. 8 (1983) 470. Anson, C.E., Bandy, B.J., Chesters, M.A., Keiller, B., Oxtona, I.A., Sheppard, N.: J. Electron Spectrosc. Relat. Phenom. 29 (1983) 315. Dinardo, N.J., Demuth, J.E., Avouris, P.: J. Vac. Sci. Technol. A 1 (1983) 1244. Dinardo, N.J., Demuth, J.E., Avouris, P.: Phys. Rev. B 27 (1983) 5832. Foulias, S., Rawlings, K.J., Hopkins, B.: Surf. Sci. 133 (1983) 377. Freyer, N., Pirug, G., Bonzel, H.P.: Surf. Sci. 125 (1983) 327. Gates, J.A., Kesmodel, L.L.: Surf. Sci. 124 (1983) 68. Hoffmann, F.M.: Surf. Sci. Rep. 3 (1983) 103. Kesmodel, L.L.: J. Chem. Phys. 79 (1983) 4646. Koestner, R.J., Van Hove, M.A., Somorjai, G.A.: J. Phys. Chem. 87 (1983) 203. Labohm, F., Engelen, C., Gijzeman, O.L.J., Geus, J.W., Bootsma, G.: Surf. Sci. 126 (1983) 429. Lloyd, D.R., Netzer, F.P.: Surf. Sci. 129 (1983) 1249. Minot, C., Van Hove, M.A., Somorjai, G.A.: Surf. Sci. 127 (1983) 441. Rieder, K., Wilsch, H.: Surf. Sci. 131 (1983) 245. Tysoe, W.T., Nyberg, G.L., Lambert, R.M.: Surf. Sci. 135 (1983) 128. Albert, M.R., Sneddon, L.G., Plummer, E.W.: Surf. Sci. 147 (1984) 127. Anderson, A.B., Mehandru, S.P.: Surf. Sci. 136 (1984) 398. Bandy, B., Chesters, M.A., Pemble, M., McDougall, G., Sheppard, N.: Surf Sci. 139 (1984) 87. Benziger, J.B., Preston, R.E.: Surf. Sci. 141 (1984) 567. Kobayashi, H., Teramae, H., Yamabe, T., Yamaguchi, M.: Surf. Sci. 141 (1984) 580. Koel, B.E., Bent, B.E., Somorjai, G.A.: Surf. Sci. 146 (1984) 211. Mehandru, S.P., Anderson, A.B.: Appl. Surf. Sci. 19 (1984) 116. Palfi, S., Lisowski, W., Smutek, M., Cerny, S.: J. Catal. 88 (1984) 300. Ramanathan, R., Quinlan, M., Wise, H.: Chem. Phys. Lett. 106 (1984) 87. Steip, U., Tsai, M.-C., Küppers, J., Ertl, G.: Surf Sci. 147 (1984) 65. Stöhr, J., Sette, F., Johnson, A.L.: Phys. Rev. Lett. 53 (1984) 1684. Stroscio, J.A., Bare, S.R., Ho, W.: Surf. Sci. 148 (1984) 499. Tysoe, W.T., Nyberg, G.L., Lambert, R.M.: J. Phys. Chem. 88 (1984) 1960. Avery, N.R.: J. Am. Chem. Soc. 107 (1985) 6711. Beebe, T.P., Albert, M.R., Yates, J.T.: J. Catal. 96 (1985) 1. Chesters, M.A., McDougall, G., Pemble, M., Sheppard, N.: Appl. Surf. Sci. 22/23 (1985) 369. Horsley, J.A., Stöhr, J., Koestner, R.J.: J. Chem. Phys. 83 (1985) 3146. Koel, B.E.: Scanning Electron Microsc. 4 (1985) 1421. Marchon, B.: Surf Sci. 162 (1985) 382. Stefan, P.M., Shek, M.L., Spicer, W.E.: Surf. Sci. 149 (1985) 423. Landolt-Börnstein New Series III/42A5
3.8.6 Adsorbate properties of linear hydrocarbons 85Stu1 85Stu2 85Vin 85Wan 86Ave1 86Ave2 86Ave3 86Ban 86Hall1 86Ham 86Hil 86Kel 86Lee 86Ogl 86Par 86Ste 87Ben 87Che 87Ham 87Hat 87Hen 87Hil1 87Hil2 87Jak 87Kos 87Lee 87Mar1 87Mar2 87Mar3 87Sak 87Zae1 87Zae2 88Ave 88Cey 88Che1 88Che2 88Hen 88Mar 88Mat 88Moh 88Par 88She 88Sla1 88Sla2 88Van 88Win 89Aru 89Che
323
Stuve, E.M., Madix, R.J.: J. Phys. Chem. 89 (1985) 105. Stuve, E.M., Madix, R.J.: Surf. Sci. 160 (1985) 293. Vink, T.J., Gijzeman, O.L.J., Geus, J.W.: Surf. Sci. 150 (1985) 14. Wang, P.-K., Slichter, C.P., Sinfelt, J.J.: J. Phys. Chem. 89 (1985) 3606. Avery, N.R., Sheppard, N.: Proc. R. Soc. (London) A 405 (1986) 1. Avery, N.R., Sheppard, N.: Proc. R. Soc. (London) A 405 (1986) 27. Avery, N.R., Sheppard, N.: Surf. Sci. 169 (1986) L367. Bandy, B., Chesters, M.A., James, D.I., McDougall, G., Pemble, M., Sheppard, N.: Philos. Trans. R. Soc. (London) A 318 (1986) 141. Hall, R., Bare, S., Desantolo, A., Zaera, F.: J. Vac. Sci. Technol. A 4 (1986) 1493. Hammer, L., Hertlein, T., Müller, K.: Surf. Sci. 178 (1986) 693. Hills, M.M., Parmeter, J.E., Mullins, C.B., Weinberg, W.H.: J. Am. Chem. Soc. 108 (1986) 3554. Kelly, D.G., Salmeron, M., Somorjai, G.A.: Surf. Sci. 175 (1986) 465. Lee, M.B., Yang, Q.Y., Tang, S.L., Ceyer, S.T.: J. Chem. Phys. 85 (1986) 1693. Ogle, K.M., Creighton, J.R., Akter, S., White, J.M.: Surf. Sci. 169 (1986) 246. Parmeter, J.E., Hills, M.M., Weinberg, W.H.: J. Am. Chem. Soc. 108 (1986) 3563. Steinrück, H.-P., Hamza, A.V., Madix, R.J.: Surf. Sci. 173 (1986) L571. Bent, B.E., Mate, C.M., Crowell, J.E., Koel, B.E., Somorjai, G.A.: J. Phys. Chem. 91 (1987) 1493. Chesters, M.A., McCash, E.: J. Electron Spectrosc. Relat. Phenom. 44 (1987) 99. Hamza, A.V., Madix, R.J.: Surf. Sci. 179 (1987) 25. Hatzikos, G.H., Masel, R.J.: Surf. Sci. 185 (1987) 479. Henderson, M.A., Mitchell, G.E., White, J.M.: Surf. Sci. Lett. 184 (1987) L325. Hills, M.M., Parmeter, J.E., Weinberg, W.H.: J. Am. Chem. Soc. 109 (1987) 4224. Hills, M.M, Parmeter, J.E., Weinberg, W.H.: J. Am. Chem. Soc. 109 (1987) 597. Jakob, P., Cassuto, A., Menzel, D.: Surf. Sci. 187 (1987) 407. Kostov, K.L., Marinova, T.S.: Surf. Sci. 184 (1987) 359. Lee, M.B., Yang, Q.Y., Ceyer, S.T.: J. Chem. Phys. 87 (1987) 2724. Marinova, T.S., Kostov, K.L.: Surf. Sci. 181 (1987) 573. Marinova, T.S., Chakarov, D.V.: Surf. Sci. 192 (1987) 275. Marinova, T.S., Stefanov, P.K.: Surf. Sci. 191 (1987) 66. Sakakini, B., Swift, A.J., Vickermann, J.C., Harendt, C., Christmann, K.: J. Chem. Soc. Faraday Trans. 83 (1987) 1975. Zaera, F., Hall, R.B.: J. Phys. Chem. 91 (1987) 4318. Zaera, F., Hall, R.: Surf. Sci. 180 (1987) 1. Avery, N.R.: Langmuir 4 (1988) 445. Ceyer, S.T.: Annu. Rev. Phys. Chem. 39 (1988) 479. Chesters, M.A.: J. Molec. Struct. 173 (1988) 405. Chesters, M.A., in: Analytical applications of spectroscopy, Creaser, C.S., Davies, A.M.C. (eds.), London: Royal Soc. Chem., 1988, p. 201. Henderson, M.A., Mitchell, G.E., White, J.M.: Surf. Sci. 203 (1988) 378. Marinova, T.S., Chakarov, D.V.: Surf. Sci. 200 (1988) 309. Mate, C.M., Kao, C.T., Bent, B., Somorjai, G.A.: Surf. Sci. 197 (1988) 183. Mohsin, S., Trenary, M., Robota, H.: J. Phys. Chem. 92 (1988) 5229. Parmeter, J.E., Hills, M.M., Weinberg, W.H.: J. Am. Chem. Soc. 110 (1988) 7952. Sheppard, N.: Annu. Rev. Phys. Chem. 39 (1988) 589. Slavin, A.J., Bent, B.E., Kao, C.T., Somorjai, G.A.: Surf. Sci. 202 (1988) 388. Slavin, A.J., Bent, B.E., Kao, C.T., Somorjai, G.A.: Surf. Sci. 206 (1988) 124. Van Hove, M.A., Bent, B.E., Somorjai, G.A.: J. Phys. Chem. 92 (1988) 973. Windham, R.G., Bartram, M.E., Koel, B.E.: J. Phys. Chem. 92 (1988) 2862. Arumainayagam, C.R., McMaster, M.C., Schoofs, G.R., Madix, R.J.: Surf. Sci. 222 (1989) 213. Chesters, M.A., Gardner, P., McCash, E.M.: Surf. Sci. 209 (1989) 89.
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98Dic 98Eng 98Ich 98Lar 98New 98Pec 98San 98Sol 98Som 98Spi1 98Spi2 98Sti1 98Sti2 98Tri 98Vas 98Wea 98Wet 98Wit 98Wu1 98Wu1 98Yu 99Ans 99Bro1 99Bro2 99Chu 99Cor 99Dum 99Gie
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00Wat1 00Wat2 00Wu 00Zae 01Car 01Cey 01Gab 01Hei 01Oga 01Oht 01Pan 01Ron 01Sta1 01Sta2 01Sta3 01Sta4 01Wea 01Whe 02Ber 02Cho 02Ge
3.8.6 Adsorbate properties of linear hydrocarbons Ohtani, T., Kubota, J., Kondo, J.N., Hirose, C., Domen, K.: J. Phys. Chem. B 103 (1999) 4562. Somorjai, G.A., Rupprechter, G.: J. Phys. Chem. B 103 (1999) 1623. Tjandra, S., Zaera, F.: J. Phys.Chem. 103 (1999) 2312. Wu, G.F., Kaltchev, M., Tysoe, W.T.: Surf. Rev. Lett. 6 (1999) 13. Yagyu, S., Kino, Y., Ozeki, K., Yamamoto, S.: Surf. Sci. 435 (1999) 779. Zaera, F., Gleason, N.R., Klingenberg, B., Ali, A.H.: J. Mol. Catal. A-Chem. 146 (1999) 13. Azad, S., Kaltchev, M., Stacchiola, D., Wu, G., Tysoe, W.T.: J. Phys. Chem. B 104 (2000) 3107. Azizian, S., Gobal, F.: J. Mol. Catal. A-Chem. 153 (2000) 191. Camplin, J.P., Eve, J.K., McCash, E.M.: Phys. Chem. Chem. Phys. 2 (2000) 4433. Carlsson, A.F., Madix, R.J.: Surf. Sci. 458 (2000) 91. Hansen, E.W., Neurock, M.: J. Catal. 196 (2000) 241. Ichihara, S., Okuyama, H., Kato, H., Kawai, M., Domen, K.: Chem. Lett. 2 (2000) 112. Ilharco, L.M., Garcia, A.R., Hargreaves, E.C., Chesters, M.A.: Surf. Sci. 459 (2000) 115. Kao, C.-L., Carlsson, A.F., Madix, R.J.: Top. Catal. 14 (2000) 63. Kis, A., Smith, K.C., Kiss, J., Solymosi, F.: Surf. Sci. 460 (2000) 190. Lauhon, L.J., Ho, W.: Phys. Rev. Lett. 84 (2000) 1527. Lauhon, L.J., Ho, W.: Surf. Sci. 451 (2000) 219. Libuda, J., Scoles, G.: J. Chem. Phys. 112 (2000) 1522. McCabe, P.R., Juurlink, L.B.F., Utz, A.L.: Rev. Sci. Instrum. 71 (2000) 42. Miura, T., Kobayashi, H., Domen, K.: J. Phys. Chem. B 104 (2000) 6809. Neurock, M., van Santen, R.A.: J. Phys. Chem. B 104 (2000) 11127. Neurock, M., Pallassana, V., van Santen, R.A.: J. Am. Chem. Soc. 122 (2000) 1150. Stacchiola, D., Katchev, G., Wu, G., Tysoe, W.T.: Surf. Sci. 470 (2000) L32. Toomes, R.L., Lindsay, R., Baumgärtel, P., Terborg, R., Hoeft, J.T., Koebbel, A., Schaff, O., Polcik, M., Robinson, J., Woodruff, D.P., Bradshaw, A.M., Lambert, R.M.: J. Chem. Phys. 112 (2000) 7591. Watanabe, K., Matsumoto, Y.: Surf. Sci. 454 (2000) 262. Watwe, R.M., Bengaard, H.S., Rostrup-Nielsen, J.R., Dumesic, J.A., Norskov, J.K.: J. Catal. 189 (2000) 16. Wu, G., Stacchiola, D., Kaltchev, M., Tysoe, W.T.: J. Am. Chem. Soc. 122 (2000) 8232. Zaera, F., Chrysostomou, D.: Surf. Sci. 457 (2000) 89. Carlsson, A.F., Madix, R.J.: Surf. Sci. 479 (2001) 98. Ceyer, S.T.: Acc. Chem. Res. 34 (2001) 737. Gabelnick, A.M., Burnett, D.J., Gland, J.L., Fischer, D.A,: J. Phys. Chem. B 105 (2001) 7748. Heitzinger, J., Beck, D.E., Koel, B.E.: Surf. Sci. 491 (2001) 63. Ogasawara, H., Ichihara, S., Okuyama, H., Domen, K., Kawai, M.: J. Electron Spectrosc. Relat. Phenom. 114 (2001) 339. Ohtani, T., Kubota, J., Kondo, J.N., Hirose, C., Domen, K.: Surf. Sci. 415 (2001) L983. Panja, C., Saliba, N.A., Koel, B.E.: J. Phys. Chem. B 105 (2001) 3786. Ronning, M., Bergene, E., Borg, A., Ausen, S., Holmen, A.: Surf. Sci. 477 (2001) 191. Stacchiola, D., Azad, S., Burkholder, L., Tysoe, W.T.: J. Phys. Chem. B. 105 (2001) 11233. Stacchiola, D., Wu, G., Kaltchev, M., Tysoe, W.T.: J. Mol. Catal. A-Chem. 167 (2001) 13. Stacchiola, D., Wu, G., Molero, H., Tysoe, W.T.: Catal. Lett. 71 (2001) 1. Stacchiola, D., Molero, H., Tysoe, W.T.: Catal. Today 65 (2001) 3. Weaver, J.F., Ikai, M., Carlsson, A.F., Madix, R.J.: Surf. Sci. 470 (2001) 226. Whelan, C.M., Neubauer, R., Borgmann, D., Denecke, R., Steinrück, H.P.: J. Chem. Phys. 115 (2001) 8133. Bertolini, J.C., Jugnet, Y.: in: The chemical physics of solid surfaces, Vol. 10: Surface alloys and alloy surfaces, Woodruff, D.P. (ed.), Elsevier, 2002. Choudhary, T.V., Goodman, D.W.: Top. Catal. 20 (2002) 35. Ge, Q., Neurock, M.: Chem. Phys. Lett. 358 (2002) 377. Landolt-Börnstein New Series III/42A5
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Ho, W.: J. Chem. Phys. 117 (2002) 11033. Kao, C.L., Weaver, J.F., Madix, R.J.: Surf. Sci. 505 (2002) 115. Katano, S., Kim, Y., Furukawa, M., Ogasawara, H., Komeda, T., Kato, H., Nilsson, A., Kawai, M., Domen, K.: Jpn. J. Appl. Phys. Part 1 41 (2002) 4911. Katano, S., Ichihara, S., Ogasawara, H., Kato, H., Komeda, T., Kawai, M., Domen, K.: Surf. Sci. 502 (2002) 164. Neurock, M., Mei, D.: Top. Catal. 20 (2002) 5. Olsson, F., E. Persson, M., Lorente, N., Lauhon, L.J., Ho, W.: J. Phys. Chem. B. 106 (2002) 8161. Pallassana, V., Neurock, M., Lusvardi, V.S., Lerou, J.J., Kragten, D.D., van Santen, R.A.: J. Phys. Chem. B 106 (2002) 1656. Ramsvik, T., Borg, A., Worren, T., Kildemo, M.: Surf. Sci. 511 (2002) 351. Ramsvik, T., Borg, A., Venvik, H.J., Hansteen, F., Kildemo, M., Worren, T.: Surf. Sci. 499 (2002) 183. Rupprechter, G., Unterhalt, H., Morkel, M., Galletto, P., Hu, L., Freund, H.-J.: Surf. Sci. 502-503 (2002) 109. Schmid, M.P., Maroni, P., Beck, R.D., Rizzo, T.R.: J. Chem. Phys. 117 (2002) 8603. Stacchiola, D., Burkholder, L., Tysoe, W.T.: Surf. Sci. 511 (2002) 215. Stacchiola, D., Tysoe, W.T.: Surf. Sci. 513 (2002) L431. Valcarcel, A., Ricart, J.M., Clotet, A., Markovits, A., Minot, C., Illas, F.: J. Chem. Phys. 116 (2002) 1165. Whelan, C.M., Neubauer, R., Denecke, R., Steinrück, H.-P.: Surf. Rev. Lett. 9 (2002) 789. Zaera, F.: Mol. Phys. 100 (2002) 3065. Zaera, F.: J. Phys. Chem. B 106 (2002) 4043. Doyle, A.M., Shaikhutdinov, S.K., Jackson, S.D., Freund, H.-J.: Angew. Chem. Int. Ed. Engl. 42 (2003) 5240. Freund, H.-J., Bäumer, M., Libuda, J., Risse, T., Rupprechter, G., Shaikhutdinov, S.: J. Catal. 216 (2003) 223. Khan, N.A., Chen, J.G.: J. Vac. Sci. Technol. A 21 (2003) 1302. Lee, A.F., Wilson, K.: J. Vac. Sci. Technol. A 21 (2003) 563. Liu, Z.P., Hu, P.: J. Am. Chem. Soc. 125 (2003) 1958. Medlin, J.W., Allendorf, M.D.: J. Phys. Chem. B 107 (2003) 217. Mei, D.H., Hansen, E.W., Neurock, M.: J. Phys. Chem. B 107 (2003) 798. Mittendorfer, F., Thomazeau, C., Raybaud, P., Toulhoat, H.: J. Phys. Chem. B 107 (2003) 12287. Morkel, M., Rupprechter, G., Freund, H.-J.: J. Chem. Phys. 119 (2003) 10853. Neubauer, R., Whelan, C.M., Denecke, R., Steinrück, H.-P.: J. Chem. Phys. 119 (2003) 1710. Sheth, P.A., Neurock, M., Smith, C.M.: J. Phys. Chem. B 107 (2003) 2009. Sock, M., Eichler, A., Surnev, S., Andersen, J.N., Klötzer, B., Hayek, K., Ramsey, M.G., Netzer, F.P.: Surf. Sci. 545 (2003) 122. Stacchiola, D., Tysoe, W.T.: Surf. Sci. 540 (2003) L600. Stacchiola, D., Burkholder, L., Tysoe, W.T.: Surf. Sci. 542 (2003) 129. Wang, J.: Surf. Sci. 540 (2003) 326. Weaver, J.F., Carlsson, A.F., Madix, R.J.: Surf. Sci. Rep. 50 (2003) 107. Zheng, T., Tysoe, W.T., Poon, H.C., Saldin, D.K.: Surf. Sci. 543 (2003) 19. Anson, C.E., Sheppard, N., Pearman, R., Moss, J.R., Stossel, P., Koch, S., Norton, J.R.: Phys. Chem. Chem. Phys. 6 (2004) 1070. Doyle, A.M., Shaikhutdinov, S.K., Freund, H.-J.: J. Catal. 223 (2004) 444. Fuhrmann, T., Kinne, M., Whelan, C.M., Zhu, J.F., Denecke, R., Steinrück, H.-P.: Chem. Phys. Lett. 390 (2004) 208. Kao, C.L., Madix, R.J.: Surf. Sci. 557 (2004) 215. Rupprechter, G., Morkel, M., Freund, H.-J., Hirschl, R.: Surf. Sci. 554 (2004) 43. Valcarcel, A., Clotet, A., Ricart, J.M., Delbecq, F., Sautet, P.: Surf. Sci. 549 (2004) 121.
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3.8.6 Adsorbate properties of linear hydrocarbons Zheng, T., Stacchiola, D., Poon, H.C., Saldin, D.K., Tysoe, W.T.: Surf. Sci. 564 (2004) 71. Denecke, R.: Appl. Phys. A 80 (2005) 977. Fuhrmann, T., Kinne, M., Tränkenschuh, B., Papp, C., Zhu, J.F., Denecke, R., Steinrück, H.P.: New J. Phys. 7 (2005) 107. Fuhrmann, T., Kinne, M., Zhu, J.F., Tränkenschuh, B., Denecke, R., Steinrück, H.-P.: to be published (2005). Jugnet, Y., Sedrati, R., Bertolini, J.C.: J. Catal. 229 (2005) 252. Matsumoto, C., Kim, Y., Okawa, T., Sainoo, Y., Kawai, M.: Surf. Sci. 587 (2005) 19. Morkel, M., Rupprechter, G., Freund, H.-J.: Surf. Sci. 588 (2005) L209. Savio, L., Vattuone, L., Rocca, M.: Surf. Sci. 587 (2005) 110. Teschner, D., Pestryakov, A., Kleimenov, E., Havecker, M., Bluhm, H., Sauer, H., KnopGericke, A., Schlögl, R.: J. Catal. 230 (2005) 195.
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3.9 Adsorption on oxide surfaces 3.9.1 Introduction The interaction of gases with oxide surfaces is important for many all-day processes like corrosion as well as for large-scale industrial fabrication of chemicals with catalytic processes. In order to understand the microscopic processes occurring when gases contact oxide surfaces, two different approaches may be followed. One approach is to study real world systems with their full complexity. Due to this complexity, hard-to-interpret experimental results are to be expected. If the systems are to be investigated in-situ under working pressures which may be in the bar range or higher, a number of powerful techniques like electron spectroscopy which works only under high or ultra-high vacuum conditions can not be applied. Another approach is to study idealized systems (“model systems”) under high or ultra-high vacuum conditions. These systems are usually well ordered and their composition is not too complex. This permits to apply many powerful surface science techniques and it is often possible to understand in detail the microscopic processes occurring during gas-substrate contact. However, the systems are simpler than the real world systems and the pressure is usually lower than the pressure of gases interacting with real systems. This means that it is not sure that the processes occurring during gas-surface interaction are the same as the ones occurring in the non-idealized real systems under working pressure conditions. Nevertheless, this approach has generated a number of important results and ideas concerning gas-surface interactions which are surely also important for processes occurring in “real” systems. In the context of a discussion of these two approaches key phrases like “material gap”, “complexity gap”, or “pressure gap” may be heard. These phrases refer to differences between the two approaches concerning the degree of realism. One of the topics of current surface science research is to increase the degree of realism by going towards increasingly complex systems and by a development of more powerful experimental techniques which extend the usable pressure regime. This chapter tries to give an overview of results obtained for one type of idealized systems, i.e. molecular adsorbates on ordered oxide surfaces which may be single crystal surfaces or the surfaces of thin ordered oxide films.
3.9.2 Abbreviations used in the text ∆φ AES AFM AM1 ARUPS B3LYP BEG CARS CFS DFT DV-Xα ELS ESD ESDIAD ESR EXAFS FLAPW FTIR GC HAS
workfunction change measurements Auger electron spectroscopy atomic force microscopy Austin Model 1 (semi-empirical theoretical method) angular resolved ultra-violet photoelectron spectroscopy a density functional method due to Lee, Yang and Parr which incorporates a 3-parameter functional due to Axel Becke Blume-Emery-Griffiths (theoretical model) coherent anti-Stokes Raman spectroscopy constant final state (spectroscopy) density functional theory discrete variational Xα (theoretical method) energy loss spectroscopy electron stimulated desorption (spectroscopy) electron stimulated desorption ion angular distribution (spectroscopy) electron spin resonance (spectroscopy) extended X-ray absorption fine structure (spectroscopy) full potential linearized augmented plane wave (theoretical method) Fourier transform infrared (spectroscopy) gas chromatography helium atom scattering (spectroscopy) Landolt-Börnstein New Series III/42A5
Ref. p. 389] HF HREELS IMPT INDO IR IRAS ISS LCAO LDA LEED LID LITD MD MIES MNDO MP2 MSINDO MSRI NC-AFM NEXAFS PEEM PES PID PIRSS PM3 PSD PhD REMPI-TOFMS REMPI RHEED RT SCC-DV-Xα SCF-Xα-SW SCF SEXAFS SFG SIMS SINDO SPA-LEED SSIMS STM TCS TDS TOF UHV UPS UV XPS XRD XSW ZINDO
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3.9 Adsorption on oxide surfaces Hartree-Fock (theoretical method) high resolution electron energy loss spectroscopy inter-molecular perturbation theory semi-empirical theoretical method infrared (spectroscopy) infrared absorption spectroscopy ion scattering spectroscopy linear combination of atomic orbitals (theoretical method) local density approximation (theoretical method) low energy electron diffraction laser induced desorption (spectroscopy) laser induced thermal desorption (spectroscopy) molecular dynamics (theoretical method) metastable impact electron spectroscopy modified neglect of diatomic overlap (semi-empirical theoretical method) Møller-Plesset theory truncated at 2nd order semi-empirical theoretical method mass spectroscopy of recoiled ions non-contact atomic force microscopy near edge X-ray absorption spectroscopy photoemission electron microscopy photoelectron spectroscopy photon induced desorption (spectroscopy) polarization infrared surface spectroscopy third parametrization of MNDO (semi-empirical theoretical method) photon stimulated desorption (spectroscopy) photoelectron diffraction resonantly enhanced multi-photon ionization - time of flight mass spectrometry resonantly enhanced multi-photon ionization (spectroscopy) reflection high energy electron diffraction room temperature self consistent charge discrete variational Xα (theoretical method) Xα denotes a certain form of the exchange term, SW means scattered wave (theoretical method) self-consistent field (theoretical method) surface extended X-ray absorption fine structure (spectroscopy) sum frequency generation (spectroscopy) secondary ion mass spectrometry semi-empirical theoretical method spot profile analysis - low energy electron diffraction static secondary ion mass spectrometry scanning tunneling microscopy total (target) current spectroscopy thermal desorption spectroscopy time of flight (spectroscopy) ultra-high vacuum ultra-violet photoelectron spectroscopy ultra-violet X-ray photoelectron spectroscopy X-ray diffraction X-ray standing wave (spectroscopy) semi-empirical theoretical method
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3.9.3 Al2O3 Al2O3 surfaces have been prepared in different ways. α-Al2O3 surfaces may be prepared by cutting a αAl2O3 single crystal along the desired surface (usually [0001]) with subsequent chemical cleaning. Preparation in UHV usually comprises annealing at high temperature and re-oxidation of the surface. Details of the procedure vary. It is also possible to prepare ordered Al2O3 surfaces as thin films on different substrates. NiAl(100) and NiAl(110) are often used substrates. Annealing in oxygen produces thin Al2O3 layers. On NiAl(110) formation of an incommensurate Al2O3 type layer with a well defined LEED pattern has been reported [91Jae1, 97Kuh1]. STM and surface X-ray diffraction studies [04Sti1, 00Sti2, 00Sti1, 03Kul1] led to the conclusion that the structure of the film is similar to that of θ-Al2O3, κ−Al2O3, or γ−Al2O3(111). On NiAl(100) formation of a-Al2O3, θ-Al2O3, α-Al2O3 and γ-Al2O3 has been observed [98Hsi1, 94Gas1]. On Mo(110) an aluminum oxide film with γ−Al2O3 or α-Al2O3 structure may be grown by evaporation of aluminum in an oxygen atmosphere [96Str1]. An overview of studies of adsorption on these systems is given in Table 2. Table 2. Overview of investigations of the interaction of gases with well ordered Al2O3 surfaces. Adsorbates Method References Substrate: Single crystal α-Al2O3(0001) C8H18, CH4 Theory: DFT, united atom, explicit atom 99Bol1 Theory: molecular dynamics 00Jin1 C8H18, C16H34, C32H66, hydroxylated substrate TDS, isothermal desorption 98Nis1 CH3OH phenanthrene electronic absorption spectroscopy 91Hay1 XPS, TDS, laser irradiation 98Slo1 Iodobenzene (C6H5I) Laser induced photochemistry, TDS 99Nis1 CH3I CO Theory: DFT 00Cas1 electronic absorption spectroscopy, TDS, 91Tok1 C60 isothermal desorption Theory: molecular dynamics 97deS1 C4H10, C8H18, C12H26+H2O LITD, TDS 98Nel1 H2O XPS, thermodynamic calculations 98Liu1 H2O, OH molecular dynamics 98Has1, 00Has1 H2O, OH LITD, TDS 98Ela1 H2O, OH HREELS 94Cou2, 97Cou1 H2O, OH LITD, TDS 01Nel1 H2O, HCl OH Theory: DFT 99DiF1 OH Theory 97Nyg1 Substrate: Al2O3(111)/NiAl(110) CO autoionization spectroscopy 96Kli1 CO ELS, TDS 93Jae1, 94Kuh1, 93Jae2, 93Fre2, 96Fre2 ARUPS, TDS, ELS 93Jae2 O2 OH SPA-LEED, XPS 97Lib1 Pd-carbonyl IRAS 97Wol1 glycidyl isopropyl ether, IRAS 99Woo2 epoxyhexane Substrate: a-Al2O3, γ-Al2O3, θ-Al2O3, α-Al2O3 on NiAl(100) CO IRAS 98Hsi1 Substrate: Al2O3/Mo(110) C6H6 TDS, HREELS 96Str1 Landolt-Börnstein New Series III/42A5
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3.9.3.1 CO adsorption Several publications are found for CO on Al2O3/NiAl(110). Using thermal desorption spectroscopy and electron energy loss spectroscopy two adsorption states could be identified with adsorption enthalpies of í170 meV and í140 meV [93Jae1, 93Jae2]. The corresponding desorption temperatures are 67 and 55 K. Also lifetimes, excitation energies and vibrational energies for the a3Π excited state are reported. Angular ∗ dependent autoionization spectra of the C1s → 2π excitation are reported in [96Kli1]. For CO adsorption at 85 K onto a-Al2O3, γ-Al2O3, θ-Al2O3 and α-Al2O3 on NiAl(100) IRAS studies have been performed [98Hsi1]. On γ-Al2O3 no adsorption was observed. For the other three oxide surfaces CO desorption occurred at T = 120 K. Several C-O vibrational states have been observed depending on the type of oxide and the CO dose, with frequencies ranging from 1994 cmí1 to 2074 cmí1. The highest frequencies were found for a-Al2O3 and the smallest ones for α-Al2O3.
3.9.3.2 H2O adsorption The interaction of H2O with α-Al2O3(0001) was studied by several groups due to the importance of Al2O3 in catalysis. Special attention was paid to the formation and the properties of OH groups. These groups are formed by dissociative adsorption of water. The initial water sticking coefficient at T = 300 K is S0 ≈0.1. It decreases exponentially with increasing OH coverage [98Ela1]. Saturation coverage is 0.5×1015 OH groups/cm2 at a dose of 1010 L [98Ela1]. H2O plasma hydroxylation leads to significantly higher coverages but roughens the surface and destroys the LEED pattern [98Ela1]. H2O desorption from hydroxylated α-Al2O3 was observed at temperatures ranging from 300 to 500 K, corresponding to desorption energies between 23 and 41 kcal/molí1 [98Nel1]. The O-H vibrational energy is 3720 cmí1 and the O1s binding energy is 533.1±0.2 eV [97Cou1]. According to molecular dynamics calculations the ideal Al-terminated (0001) surface of α-Al2O3 is very reactive with respect to dissociation of water [00Has1]. The strong relaxation of the clean surface is partly removed by the OH adsorbate. According to the calculations spontaneous unimolecular dissociations as well as dissociation mediated by another H2O molecule should occur [00Has1].
3.9.4 CaO CaO exhibits rocksalt structure (like NiO, see Fig. 10) with a lattice constant of 4.8105 Å [65Wyc1]. CaO may be cleaved along the (001) plane leading to high-quality surfaces, especially if the cleavage is performed in situ in vacuum. Another method comprises cutting the crystal along the desired plane and polishing it. After introduction into the vacuum system the sample is prepared by sputtering and annealing [83Sti1, 84Lee1]. CaO is an electrically insulating material. Therefore charge compensation may be needed when methods involving charged particles are applied. There is also a report of the epitaxial growth of CaO(100) on NiO(100) [00Xu1]. CaO(100) surfaces have a high affinity towards formation of surface hydroxyls and carbonates [99Doy1, 97Kan1] upon interaction with H2O and CO2, respectively. An overview of adsorption studies for this oxide is given in Table 3. Table 3. Overview of investigations of the interaction of gases with well ordered CaO surfaces. Adsorbates Method References Substrate: Single crystal CaO(100) CO2, SO2 Theory: ab initio cluster calculations 94Pac1 XPS, NEXAFS 99Doy1 CO2 Theory 97Kan1 N2O Theory 98Sni2 N2O2, NO XPS 98Liu3 H2O Landolt-Börnstein New Series III/42A5
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Adsorbates Method H2O Theory: DFT O, O+CO Theory: ab initio cluster calculations O Theory: ab initio cluster calculations Theory: ab initio cluster calculations O2 Theory: DFT O2,O OH Theory: semiempirical slab calculations Theory: ab initio cluster calculations NO2CH3 XPS SO2 XPS SO2 Substrate: CaO(100)/NiO(100)/Mo(100) NO TDS
[Ref. p. 389 References 00deL1 94Nyg1 92Str1 95Sni1 97Kan2 95Gon1, 93Nog1 96All1 84Lee1 83Sti1 00Xu1
3.9.4.1 CO2 adsorption CO2 adsorption was studied with synchrotron based XPS and NEXAFS [99Doy1]. It was shown that carbonate forms on the surface at pressures 10í6 Torr applied for 15 min. In [94Pac1] the higher reactivity of CaO(100) as compared to MgO(100) is explained by the smaller Madelung potential of CaO which leads to a smaller stabilization of the O2í ions on the CaO(100) surface.
3.9.4.2 H2O adsorption H2O induces surface hydroxylation with an apparent sticking coefficient of §0.9 at room temperature for surface coverages below 0.8 monolayers [98Liu3]. At higher coverages the sticking coefficient is dramatically reduced to ∼3×10í5. At H2O pressures greater than 1×10í4 Torr bulk hydroxylation was observed [98Liu3].
3.9.4.3 SO2 adsorption SO2 adsorption at room temperature leads to the formation of strongly bound sulfate (SO42í) with a desorption temperature beyond 673 K [84Lee1, 83Sti1]. It was shown that the initial sticking coefficient is about 0.4 and that the adsorption is of first order in surface coverage [83Sti1]. Similar to the case of CO2 adsorption Pacchioni and coworkers [94Pac1] explain the higher reactivity of CaO(100) as compared to MgO(100) by the smaller Madelung potential of CaO which leads to a smaller stabilization of the O2í ions on the CaO(100) surface.
3.9.5 CeO2 CeO2 exhibits fluorite structure with a lattice constant of 5.411 Å. The (111) and the (001) surface have been used for adsorption studies. The (001) surface is polar and thus energetically unstable if not stabilized by geometrical rearrangement at the surface, charge rearrangement, or adsorption. The stabilization mechanism for the (001) surface appears to be an open question. Termination of the surface by oxygen and cerium terminated patches, oxygen termination with a few percent of defects and oxygen termination with 50% of the oxygen atoms removed are named in [99Her1]. The two ideal terminations of the (100) surface and the (111) surface are displayed in Fig. 1. Ceria is a component of automotive emission control catalysts where it acts as a component for oxygen storage. It is also known to be active for the water-gas-shift reaction. CeO2(111) is often prepared as a thin film on Ru(0001) whereas CeO2(001) may be grown on SrTiO3(001). Sub-stoichiometric oxide films may be grown by using smaller Landolt-Börnstein New Series III/42A5
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oxygen pressures during oxidation or by annealing. CeO2 is also commercially available in single crystal form. There is only a limited number of adsorption studies for ordered CeO2 surfaces. An overview is given in Table 4 (a) ideally terminated CeO2(001) CeO2(001)-Ce
(b) CeO2(111)
CeO2(001)-O
(c) CeO2 unit cell
oxygen cerium
Fig. 1. Structure of the two polar CeO2(100) surfaces and the CeO2(111) surface.
3.9.5.1 CO adsorption on CeO2(111) For CeO2(111)/Ru(0001) the adsorption of CO was studied with XPS [99Mul1]. A state with a C1s binding energy of 290.5 eV was found to result from CO interacting with the CeO2(111) surface. This state vanishes between 600 and 700 K. It was suggested that the observed state corresponds to carbonate or carboxylate bonding to cerium sites. Table 4. Overview of investigations of the interaction of gases with well ordered CeO2 surfaces Adsorbates Method References Substrate: CeO2(001)/SrTiO3(001) NO TDS, XPS, ion bombardment 97Ove1 TDS, XPS 99Her1 D2O Substrate: CeO2(111)/Ru(0001) CO XPS 99Mul1 CO, H2O XPS, TDS 00Kun1 TDS 96Put1 O2 TDS, XPS 99Ove1 SO2
3.9.5.2 H2O and D2O adsorption on CeO2(001) and CeO2(111) The adsorption of H2O on CeO2(111)/Ru(0001) was investigated using TDS and XPS [00Kun1].Water was found to desorb fully below 300 K from the fully oxidized surface. On reduced surfaces also H2 desorption occurred at around 580 K. In this case additional H2O desorption states at 250 K and 600 K were observed. The amount of desorbing H2 was found to depend on the degree of reduction of the ceria substrate. Water adsorbed on the fully oxidized surface exhibits a O1s level with a binding energy of 531.8 eV which vanishes above 300 K. On reduced CeO2(111) H2O adsorption leads to a O1s state at Landolt-Börnstein New Series III/42A5
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532.5 eV at 200 K which shifts to 533 eV at 400 K. It was assumed that the O1s peak is composed of two peaks with binding energies of 532.4 and 533.1 eV resulting from chemisorbed water and hydroxyl groups, respectively. Hydrogen desorption as observed with TDS coincides with the disappearance of the O1s level of the hydroxyl groups. D2O adsorption on CeO2(001)/SrTiO3(001) was also investigated using XPS and TDS [99Her1]. With TDS desorption maxima at 152, 200 and 275 K were identified and assigned to desorption of multilayer water (152 K), first layer water (200 K) and recombination of hydroxyl groups (275 K). The O1s peak corresponding to the hydroxyl groups was found at 531.6 eV using XPS. From the intensity of the O1s peak a hydroxyl coverage of 0.9 ML was estimated. It was suggested that the hydroxyl groups might help to stabilize the polar CeO2(001) surface. The O1s signal of the surface water could not be identified in the XPS data which was attributed to non-wetting adsorption of D2O on CeO2(001). This would lead to the formation of large clusters which would give a small O1s XPS intensity.
3.9.6 α-Cr2O3 Of all different chromium oxides α-Cr2O3 appears to be the most easily accessible oxide in well ordered form under UHV conditions. α-Cr2O3 exhibits corundum structure like α-Al2O3 with the hexagonal lattice constants a=4.7628 and c=13.003 Å[65Wyc1]. The (0001) surface is often prepared as a thin film by oxidation of Cr(110) [92Kuh2]. It has been shown that the surface may be terminated by a half layer of chromium atoms after annealing in UHV [97Roh2, 97Roh1] (see Fig. 2C) or by chromyl groups after treatment with oxygen [96Dil1]. We note that the surface shown in Fig. 2C is the only ideal surface of corundum(0001) which is non-polar and thus electrically stable. Figs. 2A and 2B are polar surfaces. Growth of α-Cr2O3(0001) by MBE onto α-Al2O3(0001) and Fe2O3(0001)/α-Al2O3(0001) has also been reported [00Hen1]. Other authors describe the growth on Pt(111) [01Rod5, 97Rod1]. One adsorption study has also been carried out for a Cr3O4(111) film on Pt(111) [97Rod1]. In order to carry out adsorption studies on Cr2O3( 10 1 2 ) a single crystal surface has been prepared by cutting a Cr2O3 single crystal along the ( 10 1 2 ) plane, polishing it and sputtering and annealing it in vacuo after insertion into the vacuum chamber [99Yor1]. Adsorption experiments performed for these surfaces are listed in Table 5. (A)
(C)
(B)
oxygen chromium
Fig. 2. (A), (B) and (C): three different ideal terminations of a corundum structure. (A): termination by an oxygen layer, (B): termination by a metal double layer, (C): termination by a single metal layer. In the lower right panel the non-primitive hexamolecular unit cell is displayed together with the primitive rhombohedral unit cell.
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Table 5. Overview of investigations of the interaction of gases with well ordered Cr2O3 surfaces. Adsorbates Method References Substrate: Cr2O3(0001)/Cr(110) CO TDS, IRAS, Theory: ab initio cluster 01Pyk1 calculations CO TDS, REMPI, ELS 96Bea1 CO autoionization spectroscopy 96Kli1 CO ARUPS 91Xu1, 96Fre2 CO HREELS 00Wol1 CO LID 96AlS1, 96Bea2, 94AlS1, 95Bea1 CO Theory: ab initio quantum dynamics 01Thi1 ELS, HREELS, NEXAFS 95Ben1, 92Kuh1 CO, NO, CO2 ELS, HREELS, TDS, ARUPS, NEXAFS, 92Kuh2, 94Kuh1, CO, NO, CO2, NO2, O2 XPS 93Kuh1, 93Fre2 IRAS 99Sei1 CO2 TDS, ELS, XPS 97Hem1 C2H4 NO LID 98Wil1 NO LID, TDS, IRAS 99Wil1 NO Theory: wavepacket calculations 98Thi1 TDS, IRAS, ELS 96Dil1, 96Fre1 O2, C2H4 ELS, ARUPS, HREELS, NEXAFS 91Xu2 O2, NO, NO2 TDS, XPS 86Foo1 O2, O2+Cl2 ARUPS 93Cap1 OH, H2O, O+H2O Theory 98Bre1 H2O Substrate: Cr2O3(0001)/Al2O3(0001), Cr2O3(0001)/Fe2O3(0001)/Al2O3(0001) H2O, OH TDS, XPS, HREELS 00Hen1 Substrate: Cr2O3(0001)/Pt(111) NO, N2O, NO2 XPS, UPS, Theory: DFT 01Rod5 Substrate: Cr2O3(0001)/Pt(111), Cr3O4(111)/Pt(111) H2S XPS, ARUPS 97Rod1 Substrate: single crystal Cr2O3( 10 1 2 ) O2 LEED, XPS, AES 99Yor1
3.9.6.1 CO adsorption The interaction of α-Cr2O3(0001)/Cr(110) with CO has been intensively studied both experimentally and theoretically. CO interacts only weakly with the chromyl-terminated surface but exhibits strong interaction with the chromium-terminated surface. For the chromium-terminated surface TDS exhibits a desorption maximum at 105 K which shows up after exposing the surface to doses of more than 4 Langmuirs and another maximum at 175-180 K which is visible already at low doses [01Pyk1]. According to infrared absorption spectroscopy the corresponding vibrational energies are 2132-2136 cmí1, and 2170-2178 cmí1 [01Pyk1]. Calculations suggest that CO molecules desorbing at T = 175-180 K adsorb on oxygen threefold hollow sites with an angle of 55° between the molecular axis and the surface normal. For the more weakly bound CO molecules desorbing at 105 K it was suggested that they adsorb on oxygen on-top sites [01Pyk1]. In agreement with these results NEXAFS and ARUPS reveal the existence of a strongly tilted CO species on chromium-terminated α-Cr2O3(0001) [91Xu1, 92Kuh2]. Photoelectron spectroscopy reveals unusually high binding energies for the CO valence levels and the C1s core level which represents a still unexplained topic [91Xu1, 92Kuh2]. Landolt-Börnstein New Series III/42A5
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The photodesorption of CO from chromium-terminated α-Cr2O3 was studied by laser induced desorption with REMPI detection. In such an experiment the surface is exposed to short-time laser pulses to induce photodesorption. The desorbed molecules are detected by a second laser using resonantly enhanced multiphoton ionization via the B1Σ+ state. This type of detection can be carried out fully stateselective. Time-of-flight measurements may be performed by varying the time-delay between the desorption laser pulse and the detection laser pulse. With experiments of this type it was shown that for rotationally hot molecules cartwheel rotation (J vector perpendicular to the surface normal) is the preferred rotational state after desorption whereas for rotationally cold molecules helicopter rotation (J vector parallel to the surface normal) is preferred. Using wave packet calculations based on threedimensional potential energy surfaces it was shown that the corrugation of the excited and ground-state potential energy surfaces in the angular degrees of freedom are responsible for the experimental observation [01Thi1]. 3.9.6.2 NO adsorption NO is chemisorbed on chromium-terminated α-Cr2O3/Cr(100) with a desorption temperature of 340 K [91Xu2, 99Wil1] corresponding to a binding energy of about 1 eV according to the Redhead formula [62Red1]. At higher doses also a weakly bound species desorbing at 105 K (binding energy: 0.35 eV according to the Redhead formula) is detected which is attributed to the formation of NO dimers [99Wil1]. Desorption of the latter species is accompanied by the formation of N2O, possibly forming a bilayer structure. According to IRAS experiments the N-O vibrational energy of the species desorbing at 340 K is 1759-1794 cmí1 and the corresponding N-O symmetric stretching frequency of the NO dimers is at 1847-1857 cmí1 [99Wil1]. Results of laser induced desorption studies of NO with REMPI detection are published in several papers [98Wil1, 99Wil1, 98Thi1]. For the high coverage regime two desorption channels are observed: a direct one where desorption occurs immediately after excitation of the adsorbate-substrate complex and a slow one where the NO molecules desorb after diffusion on the surface [99Wil1]. In the low-coverage regime the NO molecules desorb rotationally and vibrationally highly excited after irradiation with UVlaser pulses. The velocity distributions are non-Boltzmann like and bimodal with a strong dependence on the internal degrees of freedom. 3.9.6.3 CO2 adsorption CO2 interacts strongly with the chromium-terminated α-Cr2O3(0001)/Cr(110) surface. At T = 90 K physisorbed as well as chemisorbed species are observed on the surface [99Sei1]. The physisorbed molecular species desorbs at 120 K and 180 K whereas the chemisorbed species desorbs at 330 K. IRAS spectra identify the chemisorbed species as a negatively charged bent CO2 species (CO2δí) bound to the metal ions of the surface. On the chromyl-terminated surface formation of the CO2δí chemisorbate is strongly attenuated due to blocking of the metal ions by the oxygen atoms of the chromyl groups. 3.9.6.4 O2 adsorption O2 adsorbs molecularly below room temperature onto α-Cr2O3(0001)/Cr(110) as a negatively charged species (O2í) [96Dil1]. Upon annealing part of the molecules desorb at temperatures between 290 K and 330 K. As evidenced by infrared spectroscopy the remaining O2 molecules transform into a strongly bound chromyl species with an infrared absorption peak at 1005 cmí1 which is still detected after annealing at 780 K. TDS data indicate that the chromyl species is stable up to about 1000 K [86Foo1]. As already noted in the previous paragraphs such a chromyl-terminated surface exhibits properties which are significantly different from those of the chromium-terminated surface. Due to the blocking of the chromium atoms on the chromyl-terminated surface, this surface is usually significantly less active. In the case of a α-Cr2O3( 10 1 2 ) substrate O2 adsorption was also found to lead to the formation of chromyl groups [99Yor1]. Landolt-Börnstein New Series III/42A5
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3.9.6.5 H2O adsorption H2O adsorption has been investigated for α-Cr2O3(0001)/Cr(110) [93Cap1], α-Cr2O3(0001)/αAl2O3(0001) and α-Cr2O3(0001)/Fe2O3(0001)/α-Al2O3(0001) [00Hen1]. Molecular as well as dissociative adsorption is reported for all three substrates. According to [00Hen1] molecular water desorbs at 295 K from α-Cr2O3(0001)/α-Al2O3(0001) and from α-Cr2O3(0001)/Fe2O3(0001)/α-Al2O3(0001) (see Fig. 3). Dissociated water desorbs at 345 K. TDS and XPS data suggest that every surface chromium atom has the capacity to bond one molecular and one dissociated water molecule [00Hen1]. The authors observe two distinct O-H vibrations of the hydroxyl groups which they attribute to terminal bonding onto a chromium atom (ν(OH)=3600 cmí1) and to a bridging species with a O-H vibrational energy of ν(OH)=2885 cmí1 [00Hen1].
H2O exposure 14
165
2
(× 10 molecules/cm ) 2.3 6.7 8.9 10 12 14 17 20 21 27
5
m /e = 18 QMS signal [×10 cps]
1.5
1.0
345 185 295
0.5 210
0 100
200
300 Temperature [K]
400
500
Fig. 3. TPD spectra of H2O (m/e = 18) from various exposures at 120 K on the strained α-Cr2O3(0001)/αFe2O3(0001)/α-Al2O3(0001) surface. The dashed and solid line traces correspond to exposures above and below 1.3×1015 molecules/cm2, respectively; [00Hen1]
3.9.7 CoO CoO exhibits rocksalt structure (like NiO, see Fig. 10) with a lattice constant of 4.27 Å [65Wyc1]. Highquality CoO(100) surfaces may be prepared by cleavage of CoO single crystals [95Has2, 89Mac1, 91Jen1]. At room temperature the electrical conductivity of cleaved single crystal surfaces may be high enough to allow application of electron spectroscopy without charging effects. Adsorption has also been studied on thin films of CoO. CoO(100) thin films may be prepared by oxidation of Co( 112 0 ) [96Sch1, 95Has2] and the polar CoO(111) surface by oxidation of Co(0001) [96Sch1, 95Cap1, 95Has1]. Since ideal polar surfaces lead to diverging Madelung potentials [79Tas1] they must be stabilized. For CoO(111)/Co(0001) stabilization by a layer of hydroxyl groups has been reported [96Sch1, 95Cap1]. Due to the strong bond of the hydroxyl groups to the substrate they can not be removed by thermal treatment since this would significantly deteriorate the oxide film due to the high annealing temperature needed. Clean CoO(111) is stabilized by a layer of Co3O4 according to [00Moc1]. In the latter case the CoO(111) surface has been prepared from a CoO single crystal. Table 6 gives an overview of adsorption studies for ordered CoO surfaces.
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Table 6. Overview of investigations of the interaction of gases with well ordered CoO surfaces Adsorbates Method References Substrate: Single crystal CoO(100) CO ELS, HREELS 95Has2 UPS, ion bombardment 89Mac1 CO, O2, H2O UPS, ion bombardment 91Jen1 O2 Substrate: CoO(100)/Co( 11 2 0 ) CO, NO, OH HREELS 96Sch1 CO ELS, HREELS 95Has2 Substrate: CoO(111)/Co(0001) CO, NO, OH HREELS 96Sch1 HREELS 95Cap1 D2O, OH HREELS, XPS 95Has1 D2O, OH, NO
3.9.7.1 CO adsorption CO adsorbs molecularly on CoO(100)/Co( 112 0 ) and CoO(111)/Co(0001) at 80 K [95Has2, 96Sch1] whereas at room temperature no adsorption is observed on regular surfaces [89Mac1]. The C-O vibrational energy as determined with HREELS is ∼2142 cmí1 for CO on CoO(100)/Co( 11 2 0 ) and ∼2168 cmí1 for CO on CoO(111)/Co(0001). The latter value is higher than the gas phase value which is attributed to the so-called wall effect [95Cap2]. This is an increase of the C-O vibrational frequency due to a repulsive interaction with the electron density of the substrate during the vibrational movement. In the case of the CoO(111)/Co(0001) surface there was always a OH co-adsorbate which could not be removed. With electron energy loss spectroscopy electronic excitations within the manifold of 3d electrons of CoO may be studied. The interaction of the surface with CO modifies the electronic surface excitation spectrum. Results of a study of this effect are published in [95Has2] together with theoretical calculations.
3.9.7.2 NO adsorption At 80 K NO adsorbs molecularly on CoO(100)/Co( 11 20 ) and CoO(111)/Co(0001) [96Sch1]. The N-O vibrational energies as determined with HREELS are ∼1813 cmí1 for CoO(100)/Co( 11 2 0 ), and ∼1789 cmí1 for NO on CoO(111)/Co(0001) [96Sch1]. In the latter case a second feature is observed at ∼1650 cmí1 which is even more intense than the loss at ∼1789 cmí1. This vibration is attributed to NO molecules feeling the influence of neighboring hydroxyl groups [96Sch1, 95Has1].
3.9.7.3 H2O adsorption As already noted, the polar CoO(111) surface may be energetically stabilized by hydroxyl groups. Due to this the non-hydroxylated surface has a high affinity towards interaction with water, forming a layer of hydroxyl groups. The O-H vibrational energy is ∼3670 cmí1 [95Has1]. As shown with HREELS, OH groups may be exchanged for OD groups upon dosing D2O at 450 K [95Has1].
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3.9.8 Cu2O Cu2O exhibits the cuprite structure with a lattice constant of 4.27 Å. The two polar (100) and the one nonpolar (111) surfaces are shown in Fig. 4. The two polar (100) surfaces are terminated by an oxygen layer or a copper layer, respectively. One of the (111) surfaces is non-polar (there are also other ones which are polar). Cu2O(100) as well as Cu2O(111) surfaces have been studied in the past with respect to adsorption. The surfaces have been prepared from single crystals by cutting along the desired plane, polishing the surface, and sputtering and annealing in vacuum [98Jon1, 91Sch1]. For the case of Cu2O(100) this preparation method leads to a copper terminated surface which exhibits a (3√2×√2)R45° LEED pattern with missing spots [91Sch1]. Some details may be found in [91Sch1]. Adsorption experiments performed for these surfaces are listed in Table 7.
(a) ideally terminated Cu2O(001) Cu2O(001)-Cu
(b) ideally terminated non-polar Cu2O(111)
Cu2O(001)-O
(c) Cu2O unit cell
oxygen copper
Fig. 4. Structure of the polar Cu2O(100) surface and the non-polar Cu2O(111) surface.
Table 7. Overview of investigations of the interaction of gases with well ordered Cu2O surfaces Adsorbates Method References Substrate: Cu2O(111) CO Theory: DFT 99Bre1 CO Theory: Hartree-Fock SCF, DFT 97Bre1 CO 98Jon2 UPS, Theory: SCF-Xα-SW CO, NO Theory: DFT 97Cas3 CO, NO Theory: LCAO-LDA 97Cas1 CO, NO Theory: Hartree-Fock SCF 96Fer1 98Jon1 CH3OH XPS, NEXAFS, UPS, Theory: SCF-Xα-SW Theory: DFT 99Cas1 H2O, H2S Theory: DFT 99Cas2 NH3 NO Theory: ab initio cluster calculations 97Fer1, 94Fer1 UPS, XPS, LEED 91Sch1 O2 Landolt-Börnstein New Series III/42A5
344 Adsorbates Substrate: Cu2O(100) CO H2 H2O H2O O2 Substrate: Cu2O(110) CO, NO
3.9 Adsorption on oxide surfaces
[Ref. p. 389
Method
References
TDS, UPS Theory: ab initio cluster calculations Theory: ab initio cluster calculations TDS, UPS UPS, XPS, LEED
91Cox1 96Nyg3 96Nyg1 91Cox2 91Sch1
Theory: SCC-DV-Xα
94Dua1
3.9.8.1 CO adsorption The interaction of Cu2O(100) with CO has been studied using TDS and UPS [91Cox1]. After adsorption of CO at 120 K a complicated desorption pattern with desorption temperatures ranging from about 120 K to 320 K was observed. No CO2 and no residual carbon were detected, indicating non-dissociative adsorption. Activation energies for desorption range from 8.4 kcal/mol to 16.7 kcal/mol as calculated with the Redhead equation [62Red1]. It was shown that doses in excess of 1000 Langmuirs are required to saturate the surface. For the case of CO adsorption on Cu2O(111) a set of HeII UPS spectra is shown in [98Jon2]. The structures found in the spectra exhibit a shape typical for molecularly adsorbed CO which is indicative of non-dissociative adsorption. Most calculations for CO/Cu2O listed in Table 7 agree that there are significant covalent contributions to the CO-substrate interactions and that the CO molecules bond to the Cu2O surface with the carbon end. 3.9.8.2 H2O adsorption H2O adsorption has been investigated experimentally using TDS and UPS [91Cox2] at 110 K and 300 K. At 110 K the adsorption was found to be molecular and dissociative with about 10% of a monolayer dissociated. At 300 K only dissociative adsorption occurs. The authors conclude that hydroxyl groups form on the surface [91Cox2]. In addition to the water multilayer desorption peak, TDS exhibits states at 300 K and 465 K which are assigned to recombination processes. Dissociation of H2O is also proposed by theoretical calculations [96Nyg1]. 3.9.8.3 CH3OH adsorption CH3OH adsorption on Cu2O(111) has been studied experimentally using XPS, UPS, and NEXAFS [98Jon1]. It was shown that low coverages of CH3OH (dose 0.6 Langmuirs) are deprotonated at 140 K, forming chemisorbed methoxide. No other species is observed up to a temperature of 523 K. 3.9.8.4 O2 adsorption The adsorption of O2 has been studied on the (100) and (111) surfaces of Cu2O using UPS, XPS, and LEED [91Sch1]. On the non-polar non-reconstructed (111) surface an exposure of 104 Langmuirs of oxygen at 300 K leads to a two-peak structure in UPS which was assigned to a molecular, possibly negatively charged oxygen species (O22−). For Cu2O(100)-(3√2×√2)R45° and Cu2O(100)-(√2×√2)R45° obtained after annealing at 900 K in vacuum the adsorption of oxygen was found to be atomic. For very high O2 exposures (109 Langmuirs) the (3√2×√2)R45° reconstruction is lifted and a (1×1) oxygen terminated surface is observed. Annealing of the latter surface to 400-450 K leads to formation of a surface with (√2×√2)R45° periodicity. Annealing at temperatures above 500 K re-establishes the (3√2×√2)R45° reconstruction.
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3.9.9 FeO, Fe3O4 and α-Fe2O3 FeO (wüstite) exhibits rocksalt structure (like NiO, see Fig. 10) with lattice constants ranging from 4.28 to 4.32 Å, depending on the iron content [67Kat1], with the higher value being valid for stoichiometric FeO. The FeO phase is thermally not stable at room temperature and tends to disproportionate into Fe and Fe3O4 [96Cor1]. The (111) surface may be grown as a thin film on Pt(111). Typically one monolayer of iron is deposited onto the Pt(111) surface and subsequently oxidized at 1000 K for some minutes in an atmosphere of 10−6 mbar of oxygen, resulting in an oxygen-terminated double layer [02Wei1, 03Lei1]. According to Weiss and Ranke [02Wei1] ordered layers with a thickness of up to 2.5 ML can be grown for an annealing temperature of 870 K. The LEED pattern of the oxide film is characterized by a sixfold ring around each substrate spot which is due to a significant mismatch of the lattice constants of Pt and FeO (about 12%). This leads to a Moiré superstructure with a lattice constant of ∼25 Å [02Wei1, 03Mey1]. The film is polar, which is energetically unfavorable in general, but stabilization of polar surfaces may be possible by charge redistribution and/or modified interlayer spacings in the surface region. Ranke and Weiss have published a review paper on iron oxide films [02Wei1] where an overview of the properties of thin iron oxide films on Pt(111) and ethylbenzene, water and styrene adsorption hereon is given. Growth of FeO(111) films has also been reported for Fe(111) [95Cap1] and Ag(111) [05Wad1] substrates. Deposition and oxidation (P∼10−6 mbar) or more iron on Pt(111) leads to Fe3O4(111) (magnetite) layers [02Wei1]. Fe3O4 exhibits the inverse spinel structure with a lattice constant of 8.396 Å [65Wyc1]. Layer thicknesses of 100 Å may easily be reached on Pt(111). At annealing temperatures of up to 920 K closed layers covering the whole surface form whereas annealing at 1000 K does not lead to closed layers. It was shown that an inward relaxed quarter layer of iron atoms on a close packed hexagonal oxygen layer [99Rit1] terminates films annealed at 1000 K (see Fig. 5). Layers prepared at lower annealing temperature may exhibit mixed termination (biphase). Here a mixture of Fe3O4(111) and FeO(111), and a rearranged oxygen terminated Fe3O4(111) surface were discussed [97Con1, 01Ket1]. Apart from preparation of Fe3O4(111) on Pt(111) also studies of polished natural (100) and (111) surfaces have been reported [00Ken1]. Also, thin films of Fe3O4(111) on single crystal α-Fe2O3(0001) [03Rim1] and Fe3O4(001) on MgO(100) [99Ped1] were prepared. α-Fe2O3 (hematite) exhibits corundum structure (like Cr2O3, see Fig. 2) with hexagonal lattice constants of a =5.035 Å and c =13.72 Å [65Wyc1]. It can be prepared on Pt(111) by oxidizing at oxygen pressures of P∼10−1 mbar or higher [02Wei1]. A termination by a layer of oxygen atoms (see Fig. 2c) or hydroxyl groups was proposed [02Wei1]. Oxidation at lower pressure (P∼10−6 or 10−5 mbar) may lead to a mixed Fe2O3(0001) and FeO(111) termination [95Con1]. Apart from layers on Pt(111) also single crystal surfaces with (0001) and (012) orientation were studied. These surfaces were usually prepared by cutting a single crystal along the desired plane, followed by preparation in UHV using ion sputtering and annealing (see for instance [03Hen1, 01Tol1]). Adsorption experiments performed on these iron oxide surfaces are listed in Table 8. Fe3O4(111)
oxygen iron
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Fig. 5. Surface structure of Fe3O4(111)/Pt(111) according to reference; [99Rit1].
346
3.9 Adsorption on oxide surfaces
[Ref. p. 389
Table 8. Overview of investigations of the interaction of gases with well ordered α-Fe2O3 surfaces Adsorbates Method References Substrate: FeO(100) CO Theory 99Che1 Substrate: FeO(111)/Pt(100) H2O TDS 92Vur1 Substrate: FeO(111)/Pt(111) ethylbenzene UPS 98Ran1 ethylbenzene UPS, TDS 98Zsc1 ethylbenzene TDS, mass spectrometry 97Zsc1 ethylbenzene, styrene NEXAFS 00Jos2 UPS, adsorption isobars 02Ran1 ethylbenzene, H2O,styrene styrene NEXAFS, XPS, Theory: Hartree-Fock SCF 00Wuh1 TDS, UPS 99Jos1, 99Sha1, 00Jos1 H2O TDS 92Vur1 H2O IRAS, TDS 03Lei1 D2O Substrate: FeO(111)/Fe(111) H2O XPS 95Cap1 Substrate: Fe3O4(100) H2O XPS, NEXAFS 00Ken1 Substrate: Fe3O4(111) H2O XPS, NEXAFS 00Ken1 Substrate: Fe3O4(111)/Fe2O3(0001) D2O XPS, MSRI 99Her2 STM 03Rim1 CCl4 TDS, XPS 03Adi1 CCl4 AES 02Cam1 CCl4 Substrate: Fe3O4(001)/MgO(100) H2O TDS 99Ped1 Substrate: Fe3O4(111)/Pt(111) ethylbenzene UPS 98Ran1 ethylbenzene UPS, TDS 98Zsc1 ethylbenzene TDS, mass spectrometry 97Zsc1 ethylbenzene LEED, PEEM, TDS, mass spectrometry 98Wei1 ethylbenzene, styrene TDS, GC 01Kuh1 ethylbenzene, styrene NEXAFS 00Jos2 TDS, UPS 99Sha1 ethylbenzene, H2O, styrene UPS, adsorption isobars 02Ran1 ethylbenzene, H2O, styrene styrene NEXAFS, XPS, Theory: Hartree-Fock SCF 00Wuh1 TDS, UPS 99Jos1, 99Jos2 H2O TDS, UPS, XPS 00Jos1 H2O IRAS, TDS 03Lei1 D2O STM 00Sha1 unidentified (H2O) Substrate: Fe2O3(0001) CH3OH HREELS 99Guo1 H2O XPS, thermodynamic calculations 98Liu1 Theory: molecular modelling 00Jon1 H2O TDS, UPS 86Hen1 H2O LID 98Laz1 O2, O Landolt-Börnstein New Series III/42A5
Ref. p. 389]
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Adsorbates Method O2, H2O, H2, SO2 UPS, ion bombardment OH, H Theory UPS, XPS, UV irradiation SO2 UPS, XPS, AES, UV irradiation SO2 Substrate: Fe2O3(012) CH3OH TDS, HREELS TDS, SSIMS, LEED, HREELS H2O OH, H Theory Substrate: Fe2O3(0001)/Pt(111) ethylbenzene TDS, mass spectrometry ethylbenzene LEED, PEEM, TDS, mass spectrometry ethylbenzene, styrene TDS ethylbenzene, styrene TDS, AES, LEED, STM ethylbenzene, styrene NEXAFS Substrate: Fe2O3(0001)+FeO(111) biphase/Pt(111) D2O IRAS, TDS Substrate: Fe2O3(0001)+Fe1-xO(111) biphase/Fe2O3(0001) CCl4 AES
347 References 87Kur1 97Was1 98Tol1 01Tol1 03Hen1 98Hen2 97Was1 97Zsc1 98Wei1 99Sha1 01Kuh1 00Jos2 03Lei1 02Cam1
3.9.9.1 Ethylbenzene, water and styrene adsorption Ethylbenzene may be dehydrogenated over iron oxide catalysts to form styrene via the process [94Elv1] C6H5CH2CH3 ↔ C6H5CH=CH2 +H2
(1)
This process is endothermic and therefore usually carried out at elevated temperature (T ∼600 K). Water is added for different reasons, one of them being lowering of the educt partial pressure and another one is removal of coke. This reaction triggered a number of studies of styrene, ethylbenzene, and water adsorption on iron oxide, mainly performed by W. Weiss, W. Ranke and coworkers at the Fritz Haber Institute in Berlin. Adsorption was found to be molecular in all cases except in the case of water on Fe3O4(111)/Pt(111) which was found to dissociate, forming hydroxyl groups. This was correlated with the co-existence of iron and oxygen atoms on the Fe3O4(111) surface [02Wei1]. For ethylbenzene on Fe3O4(111) and α-Fe2O3(0001) a physisorbed state and a chemisorbed state were observed whereas for FeO(111) only physisorption was detected [see Fig. 6(top)]. Ethylbenzene was found to adsorb with the aromatic ring parallel to the surface plane for low coverages and with some tilt at higher coverage [02Wei1]. We note that reaction studies were undertaken where it was shown that the defective α-Fe2O3(0001) surface is able to catalyze the formation of styrene from ethylbenzene in the presence of steam [02Wei1] whereas the FeO(111) and the Fe3O4(111) surfaces are inactive. Water was found to adsorb only molecularly on FeO(111) and α-Fe2O3(0001) and dissociatively on Fe3O4(111). A set of thermal desorption spectra is shown in Fig. 6 (second row). From the measurement of kinetic isobars with UPS a number of thermodynamic data was obtained for water on the three oxide surfaces. These data are listed in Table 9.
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3.9 Adsorption on oxide surfaces 0.5
EB on FeO (111)
35 L
β
14 L 7L 3.5 L
0.15
150
200
2.1 L 1.5 L 1.1 L 0.4 L 0.2 L 0.1 L
0.10 0.05
EB on α - Fe2O3 (0001)
β
0.4
γ
4.0 L 3.0 L 2.0 L 1.6 L 1.2 L 1.0 L 0.8 L 0.6 L 0.5 L 0.3 L 0.2 L 0.15 L 0.1 L
0.3 β 400
200
0.2
0.1 γ
β
α
0.4 EB desorption rate [ML/s]
α
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α
EB desorption rate [ML/s]
71 L
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400
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0.2
2.90 L 1.93 L 1.29 L 0.65 L 0.22 L 0.11 L
0.1
γ1 γ2 0 100 200 300 400 500 600 Temperature [K]
100 200 300 400 500 600 Temperature [K]
100 200 300 400 500 600 Temperature [K]
H2O on FeO (111)
H2O on Fe3O4 (111)
H2O on Fe2O3 (0001)
α
α
α, β
2.71 L 1.80 L 1.35 L 0.60 L 0.30 L 0.15 L 100 α
200 300 400 Temperature [K]
β
2.70 L 1.50 L 1.40 L 0.90 L 0.60 L 0.25 L
γ
500
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200 300 400 Temperature [K]
α
ST on FeO (111)
H2O desorption signal
0
H2O desorption signal
0
H2O desorption signal
EB desorption rate [ML/s]
0.25
0.5
EB on Fe3O4 (111) α
[Ref. p. 389
2.90 L 2.71 L 2.03 L 1.35 L 0.90 L 0.60 L 0.30 L 0.15 L
β
γ
500
100
ST on Fe3O4 (111)
α
200 300 400 Temperature [K]
500
EB on α - Fe2O3 (0001) β
β
100 200 300 400 500 600 Temperature [K]
β
γ1
2.26 L 1.93 L 1.61 L 1.29 L 0.86 L 0.43 L 0.21 L 0.10 L
ST desorption signal
6.44 L 4.83 L 3.22 L 1.93 L 1.29 L 0.86 L 0.43 L 0.21 L 0.11 L
ST desorption signal
ST desorption signal
γ1
γ2 200
β
γ1 100 200
300 400 500 600 Temperature [K]
400
γ2
3.76 L 3.23 L 2.93 L 2.56 L 1.96 L 1.28 L 0.83 L 0.45 L 0.23 L 0.08 L
100 200 300 400 500 Temperature [K]
600
Fig. 6. TDS spectra of ethylbenzene (top), water (second row) and styrene (bottom) on FeO(111)/Pt(111) (1-2 ML thick), Fe3O4(111)/Pt(111) and α-Fe2O3(0001)/Pt(111); [02Wei1]. Exposures are given in Langmuirs. Landolt-Börnstein New Series III/42A5
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Table 9. Saturation coverage θsat, isosteric heat of adsorption qst , frequency factors υn and desorption order n of different water species adsorbed on epitaxial Fe-oxide films obtained from the kinetic fit of adsorption isobars measured by UPS. Edes is the desorption energy determined from TDS data. Data from [02Wei1]. Substrate Adsorbate n from kinetic Edes θsat in unit of qst υn from [kJ/mol] [kJ/mol] species fit d /le kinetic fit a FeO(111) 52±2 3±2×1015 s−1 1, mobile prec. 52 β1 (monom.) 0.55±0.03 K = 0.13±0.04 1.3±0.2 47±2 3×10(15±1) s−1 1, mobile prec. β2 (bilayer) K = 0.08±0.05 48c >1.3 40b α (ice) −6 Fe3O4(111) 2 γ (OH + H) 0.43±0.03 65±2 (2.4±1)×10 50±10d 2 −1 cm s 50c 109-1014 1 49f β1 (monom.) 0.86±0.03 e (θ dep.) >0.86 38b 48c α (ice) g 63h α-Fe2O3(0001) γ ∼0.5 g 52h ∼1 β g 48c >1 α (ice) Assuming first order desorption and υ 1 = 3×1015 s−1 , the value deduced from the kinetic fit. b Lower limit. c From direct analysis of the Polanyi-Wigner equation using υ 1 = 4×1015 s−1 from [96Spe1]. d From TDS peak shift analysis assuming second order which also yields υ 2 =10−(5±2) cm2s−1. e Assuming first order desorption with constant qst and fitting the frequency factor υ 1 . f Assuming first order desorption and υ 1 = 1013 s −1 , the mean value deduced from the kinetic fit. g Estimated values. h Redhead analysis assuming υ 1 =1013 s−1. a
TDS spectra for styrene adsorption on the three iron oxides are shown in Fig. 6 (bottom). Similar to what was observed for water and ethylbenzene, the interaction is weakest for FeO(111), intermediate for α-Fe2O3(0001), and strongest for Fe3O4(111). With NEXAFS the orientation of the styrene molecules was determined leading to the conclusion that the phenyl group of styrene is approximately parallel to the surface plane whereas there is a significant tilting angle for FeO(111) [02Wei1]. An overview of the thermodynamic data of water, ethylbenzene and styrene on the three oxides is given in Table 10. Table 10. qst : Isosteric heat of adsorption for ethylbenzene (EB), water and styrene (ST) on FeO(111)/Pt(111), Fe3O4(111)/Pt(111) and α-Fe2O3(0001)/Pt(111). Energies in kJ/mol. υ in s−1, if not stated otherwise. (CC): from Clausius-Clapeyron analysis of isobars. (kin. fit): from a kinetic fit of isobars; reaction order n for adsorption and desorption is given. (TDS): from analysis of thermal desorption data; reaction order for desorption is given. (R): Redhead method, first order, υ assumed. (LE): Leading edge method, large uncertainty in υ. (NE): Ads.-des. equilibrium not established; data uncertain. (Frag): observation of partial adsorbate fragmentation and coke formation; measurement in ads.-des. equilibrium not possible. Data from [02Wei1]. Edes (TDS) Subst. Ads. qst (CC) υ,n (kin. fit) υ, n (TDS) 15 52 (R) 3×1015 (A), 1 FeO(111) H2O(phys.) 3×10 (β1), 1/1 52 (β1) H2O(phys.) 47 (β2) 14 55 (LE) 5×1012 , 1 EB (phys.) 58 4.8×10 , 1/1 10 ST (phys.) 55 3×10 , 1/1 50±10 Fe3O4(111) H2O (chem.) 65 10(−5±2) cm2s−1 , 2 2.4×10−6 cm2s−1 , 2/2 9 14 H2O (phys.) 50 10 - 10 , 1/1 49 (R) 1013 12 10 EB (chem.) 94-74 5×10 - 2×10 , 1/1 86 (LE) 1012 ,1 13 15 EB (phys.) 65-52 10 - 5×10 , 1/1 (NE) 47 (LE) (NE) 8×1011 ,1 ST (chem.) (Frag) 118 (LE) 3×1011 ST (phys.) 46 (LE) 6×1010 ,1
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350 Subst. Ads. α-Fe2O3(0001) H2O (chem.) H2O (phys.) EB (chem.) EB (phys.) ST (chem.) ST (phys.)
3.9 Adsorption on oxide surfaces qst (CC) -
υ,n (kin. fit) -
[Ref. p. 389 Edes (TDS) 63 (R) 52 (R) 64 (LE) 50 (LE) 73 (LE) 48 (LE)
υ, n (TDS)
1013 1013 1×1012 1×1011 ,1 5×1012 4×1010 ,1
3.9.10 MgO MgO(100) is one of the most often studied oxide surfaces. MgO exhibits rocksalt structure (like NiO, see Fig. 10) with a lattice constant of 4.2112 Å [65Wyc1]. The usually studied surface is MgO(100) which may be prepared by cleavage of a MgO single crystal. Another common way to produce a MgO(100) surface is the evaporation of magnesium onto Mo(100) in the presence of oxygen [92Wu2]. MgO(100) is a non-polar surface with equal numbers of cations and anions in the surface layer. Table 11 lists adsorption studies performed for ordered MgO surfaces. Table 11. Overview of investigations of the interaction of gases with well ordered MgO surfaces Adsorbates Method References Substrate: MgO(100) C2H2 LEED structure analysis 98Van1, 97Fer4 LEED, isosteric heat, phase diagram 96Fer3 C2H2 LEED, thermodynamics, Theory: potential 97Fer5 C2H2 energy calculations Theory: DFT cluster calculations 01Cai1 C2H2 Adsorption isotherms 03Sai1 C(CH3)4 Theory: ab initio cluster calculations 99Tod1 CH4, CH3 Theory: CH4 hindered rotor motion, He bound 99Pic1 CH4, He on CH4 state analysis HCOOH Theory: ab initio cluster calculations 95Nak1 HCOOH Theory: DFT-pseudopotential 96Szy1 HCOOH TDS, SFG, AFM 97Yam1 IR, TDS 95Xu2 CH3COOH, CD3COOD acetone, keto-enol Theory: ab initio cluster calculations 98Ovi2 TDS 94Hol1 CH3I UV photodissociation, REMPI-TOFMS 95Fai1 CH3I Theory: MD study of photodissociation 95Set1 CH3I Theory: photodissociation, time-dependent 95Fan1 CH3I Hartree TDS, 193 nm photodissociation, TOF 97Gar1 CH3Br CO Theory: periodic Hartree-Fock 96Min1 CO Theory: periodic Hartree-Fock, B3LYP 01Dam1 CO Theory: FLAPW 98Che1 CO Theory: interaction potential calculations, 96Gir1 structure optimization CO Theory: ab initio cluster calculations 95Mej1, 96Nyg2 CO Theory: Monte Carlo simulations 00Sal1 CO polarization dependent FTIR 95Hei1 CO Theory: ab initio cluster calculations 96Nyg2 CO Theory: ab initio cluster calculations: IR 91Pac1, 92Pac1 shifts Landolt-Börnstein New Series III/42A5
Ref. p. 389]
3.9 Adsorption on oxide surfaces
Adsorbates CO CO CO CO CO CO CO CO, NH3 CO, NO CO, H2, O2 CO– , O2− CO2 CO2 CO2 CO2 CO2 CO2, CO, H2O CO2, SO2 CO2, N2, HCl, HOCl CO2, N2O CO2, NH3 D2O, OD H2O, OH H2O, OH H2O, OH H2O, OH H2O, OH H2O, OH H2O, OH H2O, OH H2O, OH, defects H2O, OH, defects H2O, OH, defects H2O, OH, defects H2O, OH H2O, OH H2O, OH, H+ H2O H2O H2O H2O, D2O H2O, D2O H2O H2O H2O H2O H2O H2O
Method Overview: theory and experiment Theory: BEG spin-lattice model Theory: SINDO1 Theory: DFT Theory: DFT cluster calculations IR, Theory: model calculations HAS Theory: IR profiles TDS Theory: ab initio cluster calculations Theory: ab initio cluster calculations LEED XPS, NEXAFS Theory: ab initio cluster calculations IR, Theory: interaction potential calculations polarization FTIR spectroscopy, SPA-LEED Theory: SINDO1 Theory: ab initio cluster calculations Theory: electric field calculations PIRSS, LEED Theory: IR profile ESD LEED, PES XSW XPS, AFM PES, defects Theory: molecular dynamics Theory: DFT MIES, Theory: Hartree-Fock, MP2 Theory: MP2, Hartree-Fock Theory: Car-Parrinello molecular dynamics Theory: periodic Hartree-Fock Theory: MSINDO Theory: DFT, molecular dynamics PES, defects Theory: SINDO1 Theory: tight binding calculations tensor LEED Theory: MP2, Hartree-Fock Theory: MP2, Hartree-Fock LEED, PIR HAS UPS TDS Theory: HF slab calculations LEED, HAS FTIR Theory: molecular dynamics
H2O
Theory: IMPT
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351 References 00Pac1 98Bur1 96Jug1 00Sny1 95Ney1 96Hoa1 95Ger2 98Gir2 99Wic1, 99Wic2 99Pac1 97Fer2 93Suz1, 94Pan1 99Car1 93Pac1 95Pic1, 95Bri1 93Hei1 97Jug1 94Pac1 99Tou1 96Hei1 96Lak1 97Sor1 98Liu2 98Liu5 99Abr1 98Liu4 99Ode1 98Gio1 99Joh2 96Anc1 95Lan1, 94Lan1 94Sca1 00Ahl1 01Fin1 98Liu4 97Tik1 93Gon1 98Fer1 98Joh1 03Sha1 95Hei2 97Fer3 99Bro1 00Ahm1 98Ahd1 96Fer2 04Fos1 96McC1, 98Gir1, 98Soe1, 98Mar1 99Eng1
352 Adsorbates H2O, CH3OH, CO2, CH3COOH, HCOOH H2O, H2O+ N2, CO2, HCl, HOCL D2O D2O H2 H2 HCl
3.9 Adsorption on oxide surfaces Method XPS, UPS
References 87Oni1
Theory: electric field
99Tou1
TDS, defects TDS, FTIR Theory: DFT Theory: ab initio embedded cluster Theory: mixed quantum/classical timedependent SCF (photolysis) HCl photoexcited molecular beam HCl Theory: photodissociation XPS, Theory: DFT H2S liquid water Theory: molecular dynamics Theory: ab initio cluster calculations NH3 Theory: DFT-pseudopotential NH3 Theory: SFG calculation NH3 Theory: DFT NH3 Theory: Car-Parrinello NH3 Theory: Hartree-Fock, vibrational IR NH3 spectrum Theory: interatomic potential, vibrational IR NH3 spectrum Theory: ab initio cluster calculations NO2CH3 Powder samples: mass spectrometry; Theory: N2O dissociation ab initio cluster calculations Theory: DFT N2O XPS, UPS, Theory: DFT NO, N2O, NO2 NO Theory: DFT OH Theory: slab calculations OH Theory: Hartree-Fock, molecular dynamics Theory: DFT O2,O Theory: ab initio cluster calculations O2− O, O+CO Theory: ab initio cluster calculations XPS, NEXAFS, Theory: DFT SO2 Theory: DFT SO2, SO3 Substrate: MgO powder (preferentially MgO(100) surfaces) C2D2 neutron diffraction CH4 neutron diffraction and spectroscopy neutron diffraction N2 neutron diffraction ND3 volumetric adsorption isotherms NH3 neutron spectroscopy NH3 Substrate: MgO(110) OH Substrate: MgO(111) H2O H2O, CH3OH, CO2, CH3COOH, HCOOH
[Ref. p. 389
96Sti1 05Haw1 97And1 95Ste1 96Hin1 00Kor1 95Sei1, 95Hin1 99Rod1 98deL1 95Fer1 94Pug1 98Pou1 96Nak1 96Lan1 95All1 95Lak1 96All1 98Sni1 99Lu1 01Rod5 99Lu2 95Gon1, 93Nog1 98Ovi1 97Kan3, 97Kan2 96Pac2 94Nyg1 01Rod4 01Sch1 94Cou1 98Lar1 97Tra1 96Pan1 99Joh1 96Hav1, 97Pra1, 97Pra2, 98Pra1
Theory: slab calculations
95Gon1
Theory: ab initio cluster calculations XPS, UPS
95Ref1 87Oni1
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Ref. p. 389]
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Adsorbates Method H2 Theory: molecular dynamics, Hartree-Fock OH Theory: slab calculations Substrate: MgO(100)/Mo(100) C6H6 MIES, UPS TDS, HREELS C6H6 HREELS CH4+O2 HREELS CH3OH HREELS HCOOH, CH3COOH, H2O, CH3OH, C2H4, C2H6 CO IRAS, TDS, Clausius-Clapeyron CO HREELS, XPS, TDS CO TDS CO UPS, TDS, Theory: DFT MIES, UPS D2O, CH3OH MIES, UPS D2O, D2O+Na TDS, IRAS, LEED D2O HREELS H2O, D2O, CH3OH HREELS, UPS H2O, OH TDS, MIES, UPS D2O, CH3OH Laser irradiation, mass spectrometry, IRAS, Mn2(CO)10 TDS IRAS, TDS Mn2(CO)10 XPS, NEXAFS, Theory: DFT SO2 Substrate: MgO(100)/NiO(100)/Mo(100) NO TDS
353 References 98Her1 95Gon1 98Gun1 96Str1 92Wu4, 93Wu1 96Goo1, 92Wu1 92Wu3 92He2 92He1 01Doh1 01Rod3 99Gun1 98Gun2 97Xu1 91Wu1, 92Wu2 03Yu1 00Gun1 98Van2 97Van1 01Rod4 96Xu2
3.9.10.1 H2O adsorption Water is the most often studied adsorbate on MgO(100). The main reason for this is likely the unclear situation with respect to water dissociation and surface hydroxylation. Most of the theoretical studies agree that water does not dissociate on regular surface sites whereas many experimental studies find that water dissociates (see references in Table 11). However, one problem concerning the experimental studies is that several different methods for preparation of the MgO(100) were employed: cleavage in UHV, cleavage in air, cutting and polishing of a single crystal disk, and thin film growth. This influences the defect density and impedes comparison of the results. Currently there appears to be some kind of agreement that water may easily hydroxylate defect sites whereas hydroxylation of regular sites requires higher water pressures (see for instance [04Fos1, 98Liu2]). Liu et al [98Liu2] report that significant hydroxylation of regular sites starts at a pressure of ∼1×10−4 mbar. Using helium atoms scattering (HAS) two commensurate superstructures of molecular water on regular MgO(100) were identified: a c(4×2) phase at temperatures below 185 K and a (3×2) phase at higher temperatures [97Fer3, 96Fer2]. The transition from the c(4×2) phase to the (3×2) phase was accompanied by partial desorption of water which means that the c(4×2) phase is more dense than the (3×2) phase. For the (3×2) phase an isosteric heat of adsorption of 85.3±2.1 kJ/mol and a lateral interaction energy of 35.1±9.6 kJ/mol were determined via LEED spot intensity analysis [97Fer3, 96Fer2]. Heidberg et al [95Hei2] report vibrational data for the c(4×2) phase. The authors observe a very broad absorption band in the range from 3050 cm−1 to 3500 cm−1, pointing towards a significant contribution of hydrogen bonds. The dependence of the IR intensities on the light polarization led the authors to the conclusion that the molecular plane of the water molecules should be oriented essentially parallel to the surface plane. A model of the c(4×2) phase as proposed by Heidberg et al [95Hei2] is displayed in Fig. 7. Landolt-Börnstein New Series III/42A5
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[Ref. p. 389
Fig. 7. Model of the c(4×2) structure of water on MgO(100) as proposed by Heidberg et al [95Hei2]. The primitive and a non-primitive unit cell are indicated.
3.9.10.2 CO adsorption The adsorption of carbon monoxide on MgO(100) has been studied by a number of groups with different methods. Thermal desorption spectroscopy has been used to study the binding energy of CO adsorbed on MgO(100). Wichtendahl et al [99Wic1, 99Wic2] investigated MgO(100) surfaces cleaved in UHV and obtained a binding energy of 0.14 eV by using the Redhead equation [62Red1] for a frequency factor of 1013 s−1. The maximum of the desorption peak was found to be at 57 K (see Fig. 8). Additional structures in the thermal desorption spectra point towards the existence of a detectable number of defects even on the cleaved surface. For CO on a MgO(100) film on Mo(100) Dohnhálek and coworkers reported similar results (17 kJ/mol for a frequency factor of 1015 s−1) [01Doh1]. From the intensities of the desorption peaks attributed to CO adsorbed on surface defects they estimated the density of surface defects to be ∼0.25 monolayers for MgO(100)/Mo(100) and ∼0.15 monolayers for the UHV-cleaved surface using the data of Wichtendahl et al [99Wic1, 99Wic2]. Gerlach et al investigated CO adsorption on UHV-cleaved MgO(100) at temperatures between 36 and 59 K with helium atom scattering [95Ger2]. At temperatures below 40 K a c(4×2) phase was identified on the surface which transformed into a (1×1) phase after warming up to 51 K. Since the phase transition was only reversible when CO was offered from the gas phase, the authors concluded that the coverage of the (1×1) phase was below that of the c(4×2) phase. Time of flight measurements revealed only a dispersion-free mode at 9 meV for the (1×1) phase whereas for the c(4×2) phase an additional dispersionfree mode at 10.5 meV and some dispersing modes were found. It was suggested that the c(4×2) unit cell contains three molecules, one on-top and two occupying bridging sites. The (1×1) phase was attributed to a lattice gas. These phases were studied with polarization dependent infrared spectroscopy by Heidberg et al [95Hei1]. The authors observe three different C-O vibrational modes for the c(4×2) phase at 2152.2 cm–1, 2137.2 cm–1 and 2132.5 cm–1. The latter two modes were attributed to Davydov splitting of the modes of the two tilted CO molecules and the first mode was assigned to CO molecules adsorbed on-top. For the (1×1) structure only a single mode at 2150.5 cm–1 was observed.
3.9.10.3 CO2 adsorption Similar to H2O, CO2 seems to react with MgO(100), forming carbonate (CO32–) on the surface. Carrier et al [99Car1] investigated carbon dioxide adsorption with NEXAFS and XPS. Based on thermodynamic considerations (a calculation of the pressure dependence of the equilibrium MgO+CO2 ↔ MgCO3) they suggest that carbonate formation on regular surface sites occurs at pressures P >2.3×10−6 mbar or P ≥3.3 ×10−9 mbar, depending on the input data. Below the calculated pressure (i.e. under typical UHV conditions) CO32− formation is expected to occur on defect sites only. Landolt-Börnstein New Series III/42A5
Ref. p. 389]
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355
Molecularly adsorbed CO2 forms a (2√2×√2)R45° structure at low temperature on MgO(100) as determined with LEED and SPA-LEED [96Hei1, 94Pan1, 93Suz1]. The structure has at least one glide plane and two molecules in the unit cell. From this one would expect two asymmetric stretching vibrations split by vibrational correlation forces, but three vibrations at 2334 cm–1, 2308 cm–1 , and 2306 cm–1 were observed by Heidberg et al [96Hei1, 93Hei1] (see Fig. 9), one of them possibly being due to adsorption on defect sites. p - polarization
TDS:CO/MgO (100) heating rate 0.2 K /s
1% Transmittance
q = 1.8
s - polarization
0.8
C
A 1.5
0.7
2350
2340
B 2330
2320
2310
2300
-1
Wavenumber [cm ]
0.3
Fig. 9. Polarized infrared spectra in the region of the asymmetric stretching vibration of a monolayer of CO2 on MgO(100). T = 80 K; [96Hei1].
76 K
0.5
← Fig. 8. Thermal desorption spectra of CO on MgO(100) cleaved in UHV. The mass spectrometer was set to mass 28 (CO). The coverages are given relative to the coverage of a full monolayer; [99Wic1].
20
30
40
50
57 K
45 K
29 K
0
60 70 80 90 100 110 120 Temperature [K]
3.9.11 NiO NiO exhibits rocksalt structure with a lattice constant of 4.1684 Å [65Wyc1]. The NiO(100) and NiO(111) surfaces belong to the most often used substrates for adsorption studies among the oxides. NiO(100) surfaces may be prepared with high quality by in-situ cleavage of single crystals in vacuum [99Wic1]. Also methods for the preparation of thin films have been established. A standard method is growth by oxidation of a Ni(100) single crystal surface [91Kuh1]. It has been shown that films grown this way consist of crystallites (∼50 Å diameter) which are tilted by some degrees with respect to the surface plane [91Bau1]. This effect was attributed to the large mismatch between the Ni(100) and NiO(100) lattice constants (∼18%). The defective area between the crystallites was estimated to cover about 20% of the surface. To avoid these problems Ag(100) substrates have been used [98Rei1, 00Rei1, 01Sch2]. The lattice constant of silver is 4.0853 Å [73Liu1] which is only by about 2% different from that of NiO which means that the oxide layer should be less strained. NiO(100) films on Ag(100) exhibit a better order than those grown on Ni(100) [01Sch2, 96Mar2, 96Ber1, 99Seb1]. Films with thicknesses of up to some ten monolayers have been studied. Several adsorption experiments have also been performed for NiO(100) films grown on Mo(100) [93Wu2, 94Ves1, 92Tru1, 96Xu1, 93Tru1, 93Wu3]. The usual film
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[Ref. p. 389
thickness was around 20 to 30 monolayers. It was reported that the films exhibit excellent LEED patterns indicative of long-range order [93Wu3]. NiO(111) is a polar surface and therefore intrinsically unstable [79Tas1]. An unstable ideal surface may be terminated either by a nickel or an oxygen ion layer (see Fig. 10). A NiO(111) surface may be stabilized by a so-called octopolar reconstruction [92Wol1] (see Fig. 11) which has experimentally been verified with grazing incidence X-ray scattering (GIXS) [00Bar2, 00Bar1]. A non-reconstructed NiO(111) surface may be stabilized by a layer of charged adsorbates like hydroxyl groups as reported in references [94Roh1, 98Kit1, 98Bar1]. NiO(111) surfaces may be prepared from single crystals by cutting along the (111) plane, polishing and annealing in an oxygen atmosphere [00Bar2], but adsorption experiments are reported for thin film substrates only. Most adsorption experiments have been performed for NiO(111) grown on Ni(111). These films exhibit rather broad LEED spots with significant background, indicative of only moderate crystalline quality [94Roh1, 98Kit1] which may at least partly be due to the significant mismatch of the lattice constants of the oxide film and the substrate. Some details of the film properties may be found in [98Kit1]. NiO(111) films may also be grown by oxidation of Ni(100) at room temperature. The films are hydroxyl-terminated and exhibit a LEED pattern with significant background, indicative of an appreciable number of defects [94Lan2, 91Kuh1]. Preparation of NiO(111) on Mo(110) has also been reported [96Xu1, 95Xu1]. Here the order of the films may be somewhat better as judged from the LEED pattern [95Xu1]. There is also one adsorption study for NiO(111) grown on Au(111) [97Kat1]. Due to the small lattice mismatch such films may exhibit high crystalline quality as concluded from surface sensitive X-ray scattering experiments [00Bar1]. Table 12 gives an overview of adsorption studies for ordered NiO surfaces.
(a) NiO(100) and NiO(111) surfaces
(b) NiO unit cell
NiO(111)-O
NiO(111)-Ni
NiO(100)
oxygen nickel
Fig. 10. (a): structure of NiO(100), oxygen terminated NiO(111) and nickel terminated NiO(111). (b) unit cell of NiO.
(2x2) reconstructed NiO(111)
Fig. 11. (2×2) reconstruction of NiO(111).
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Table 12. Overview of investigations of the interaction of gases with well ordered NiO surfaces Adsorbates Method References Substrate: NiO(100) C2H4 AES, XPS, hydrogen reduction 85Fur1 HREELS 92Wul2 CH3OH CO Theory: ab initio cluster calculations 92Poh1 CO Theory: ab initio cluster calculations: IR 91Pac1 shifts CO Theory: DFT, Hartree-Fock, hybrid 02Bre1 methods CO Theory: DV-X 95Xu3 AES, XPS, hydrogen reduction 86Lan1 CO, C2H4, H2O CO, NO TDS 99Wic1, 99Wic2 CO, NO Theory: multiple scattering NEXAFS 00Zhu1 calculations PhD, Theory: DFT 01Hoe1, 02Kit1 CO, NO, NH3 AES, XPS, LEED 85Fur2 H2 UPS 92Wul1 H2 H2 XRD, NEXAFS, EXAFS, Theory: DFT 02Rod1 H Theory: ab initio cluster calculations 80Wep1 SEXAFS 99Woo1, 89Tho1 H2S RHEED, LEED, AES 78Ste1 H2S XPS, UPS, ion bombardment 93Li1 SO2 NO TDS, XPS, Theory: ab initio cluster 91Kuh1 calculations NO TDS, XPS 92Bau1 NO PhD 99Lin1 NO, OH ELS 93Cap1 NO Theory: ab initio cluster calculations 94Pet1 NO LID, Theory 98Klu1, 97Klu1, 96Klu1, 98Klu2, 03Bac1 NO Theory: DFT, wave function based 02DiV1 methods UPS, XPS 85McK1 O2, H2O Substrate: NiO(100)/Ni(100) CO TDS, NEXAFS, ARUPS 95Cap2 CO Autoionization 96Kli1 CO, NO TDS 99Wic1, 99Wic2 SFG 98Yuz1 C2H5COOH, HCOOH azomethane (methyl groups) TDS, XPS 98Dic1 ELS, ARUPS, XPS, TDS 93Cap1 D2O, OH, OD, NO HREELS 91Che1 H2 NO TDS, XPS, HREELS, NEXAFS, Theory: 91Kuh1 ab initio cluster calculations NO PhD 99Pol1 NO HREELS, XPS, TDS 92Bau1 NO HREELS, TDS 93Kuh1 NO LID 96Eic1, 98Eic1, 99Eic1, 96AlS1, 90Mul1, 94Men1, 99Zac1
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358 Adsorbates OH, NO perfluorodiethoxymethane Substrate: NiO(100)/Mo(100) CH3OH, C2H5OH, CH3OD CO HCOOH HCOOH, CO H2CO NH3 Substrate: NiO(100)/Ag(100) H2O H2O Substrate: NiO(111) HCOOH Substrate: NiO(111)/Ni(111) C2H2 HCOOH, CO, D2O, OD HCOOH HCOOH HCOOH HCOOH HCOOH C5H5N+ CO CO CO CO, NO CO, NO, OH CO2 CO2, OD H2O, OH H2O, OH H2O, OH D2O, OH, OD, NO NO, OH NO, OH NO Substrate: NiO(111)/Ni(100) CH3COOH, OH NO H2O, OH Substrate: NiO(111)/Mo(110) HCOOH, CO HCOOH Substrate: NiO(111)/Au(111) Di-tert-butylnitroxide
3.9 Adsorption on oxide surfaces
[Ref. p. 389
Method ELS, Theory: ab initio cluster calculations TDS, HREELS
References 93Fre1
HREELS, TDS IRAS, Clausius-Clapeyron TDS, HREELS TDS, HREELS TDS, HREELS HREELS, TDS
93Wu2 94Ves1 92Tru1 96Xu1 93Tru1 93Wu3
TDS, UPS TDS, XPS, UPS
98Rei1, 00Rei1 01Sch2
Theory: DFT, cluster model
01Miu1
UPS IRAS, TDS IRAS SFG, pulsed laser irradiation TDS TDS, IRAS IRAS, mass spectrometry ion scattering LID SFG, pulsed laser irradiation Autoionization SFG, IRAS HREELS, NEXAFS TDS, XPS, UPS IRAS SPA-LEED, ELS STM, LEED, thermal decomposition STM, XPS ELS, ARUPS, XPS, TDS ELS, TDS TDS, XPS LID
77Dem1 98Mat1 96Kub1 99Dom1 96Ban1 96Ban2 97Ban2 95Wai1 92Ass1 99Ban1 96Kli1 97Ban1 96Sch1 93Gor1 99Mat1 94Roh1, 95Cap1 98Kit2 98Kit1 93Cap1 94Cap1 94Win1 96AlS1, 94Men1, 94Men2
HREELS, XPS LID HREELS
94Lan2 90Mul1 78And1
TDS, HREELS TDS, ELS, HREELS
96Xu1 95Xu1
ESR, TDS
97Kat1
94Jen1
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3.9.11.1 CO adsorption NiO(100) The binding energy of CO on NiO(100) has been studied for NiO(100) single crystals cleaved in UHV [99Wic1, 99Wic2], NiO(100)/Ni(100) [99Wic1, 99Wic2, 95Cap2] and NiO(100)/Mo(100) [96Xu1, 94Ves1] with TDS and IRAS. For the high-quality single crystal surfaces a binding energy of 0.30 eV was obtained for low coverage [99Wic1, 99Wic2], decreasing with increasing coverage due to lateral interactions of the CO molecules. A set of data for the cleaved single crystal surface is shown in Fig. 12. There are no significant differences between the results for UHV-cleaved single crystal surfaces and the thin films although in the thin film case the TDS peaks are somewhat broader and exhibit additional structures which may be attributed to the influence of surface defects. In [94Ves1] Vesecky et al report a C-O vibrational energy of 2156 cm–1 for small CO coverages on NiO(100)/Mo(100). This value is higher than the CO gas phase frequency (2143 cm–1) which the authors attribute to the so-called “wall effect” and/or donation of CO 5σ charge to the substrate. The “wall effect” arises from the repulsion which the CO molecules feel when they stretch in the presence of the rigid surface as discussed by Pacchioni and Cogliandro [91Pac1]. Angular dependent NEXAFS investigations revealed that the C-O molecular axis is oriented perpendicular to the surface plane and from angular resolved valence band photoemission data it was concluded that the CO molecules bond to the substrate via their carbon lone pair [95Cap2]. Later on the CO molecular geometry was determined in much more detail with C1s scanned-energy mode photoelectron diffraction [01Hoe1, 02Kit1]. It was found that the molecules adsorb in an essentially perpendicular geometry (12±12° with respect to the surface normal) on top of the nickel surface atoms, interacting with the surface via their carbon atoms. The determined C-Ni distance was 2.07±0.02 Å. ARUPS valence band spectra of CO/NiO(100)/Ni(100) are reported in [95Cap2]. These data reveal rather high binding energies of the CO valence levels: ∼10.6 eV, ∼11.2 eV and ∼13.9 eV for the 5σ, 1π and 4σ ionizations, respectively. The angular dependence of the intensity of the levels is consistent with a perpendicular orientation of the molecular axis. According to NEXAFS data reported in [95Cap2] the energy of the C1s → 2π ionization is about 287.4 eV. NiO(111) Some data also exist for CO adsorption on NiO(111). Thermal desorption spectra for NiO(111)/Mo(111) reveal broad structures between 100 and 250 K [96Xu1]. The maximum shifts from 205 K for low coverage to 155 K for saturation coverage which is somewhat larger than the corresponding values for NiO(100) (137 K and 115 K) [99Wic1, 99Wic2]. Infrared absorption spectra of CO on NiO(111)/Ni(111) are reported in [97Ban1, 98Mat1]. Matsumoto et al [98Mat1] find a doublet at 2146 cm–1 and 2079 cm–1. The two peaks are assigned by the authors to CO adsorption on fully (2146 cm–1) and on less oxidized (2079 cm–1) Ni cation sites. The SFG spectra reported in [97Ban1, 99Ban1] reproduce the high-energy vibration observed in the infrared spectra whereas the low energy vibration could not be observed due to a small cross section. NEXAFS data reported in [96Sch1] indicate that the C-O molecular axis of CO on NiO(111)/Ni(111) is tilted by ∼46° which was attributed to the octopolar reconstruction of the NiO(111) surface. The energy of the C1s → 2π resonance was found to be ∼287.8 eV. 3.9.11.2 NO adsorption NiO(100) NO adsorption has been studied with several methods by a number of authors for single crystals as well as for thin film substrates. The NO-NiO(100) binding energy has been determined with TDS [99Wic1, 99Wic2, 91Kuh1] for a NiO(100) single crystal surface cleaved in UHV and for NiO(100) thin films grown on Ni(100). The data for the cleaved single crystal surface are shown in Fig. 13. The results for both substrates are similar, but the desorption peaks of NO on the thin film oxide are broader and exhibit significant additional intensity due to adsorption on defects. The maximum shifts from 220 to 216 K with Landolt-Börnstein New Series III/42A5
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increasing coverage [99Wic1, 99Wic2]. Evaluation of the TDS data with the leading edge method and complete analysis gave a NO-NiO(100) binding energy of 0.57 eV for low coverage which dropped to ∼0.12 eV at a coverage near to 1. 0.40
TDS:CO /NiO (100) Adsorption energy [eV]
cleaved NiO (100) single crystal
2.17
heating rate 1K/s q = 0.83 0.50
0.17
1.38
0.12 0.08
1.17
100 150 Temperature [K ]
0.25 0.20 0.15
complete analysis 1K /s complete analysis 0.2 K /s leading edge 1 K /s leading edge 0.2 K /s leading edge 2 K /s leading edge 0.1 K /s
0.05
137 K
115 K
30 K 34 K 45 K 65 K
50
0.30 ± 0.03 eV
0.30
0.10
0
0
CO on NiO(100) cleaved in UHV:TDS analysis
0.35
0 200
250
0
0.5 1.0 Relative coverage [ q/ qmonolayer ]
1.5
Fig. 12. Thermal desorption spectroscopy of CO on vacuum-cleaved NiO(100). Left: raw data. Right: CO-NiO binding energy as a function of coverage as obtained with the evaluation methods complete analysis and leading edge; [99Wic1, 99Wic2].
N1s XPS data are published in [92Bau1, 91Kuh1, 99Lin1, 99Pol1]. The N1s feature consists of two peaks at 402.8 and 407.2 eV for UHV-cleaved NiO(100) and at 403.1 and 407.5 eV for NiO(100) on Ni(100) [91Kuh1]. It was suggested that the two peaks are not due to different species but to a final state effect, i.e. to the distribution of the intensity between a screened and a non-screened final state [91Kuh1]. Angular dependent N1s NEXAFS data revealed that the NO molecular axis is tilted by an angle between 20 and 45° with respect to the surface normal [91Kuh1]. The tilting was attributed to be due to an interaction between the NO 2π electron and the Ni 3dx2−y2 level which is only possible in the reduced symmetry of a tilted geometry. A more detailed investigation of the adsorption geometry has been performed with photoelectron diffraction (PhD) [99Lin1, 99Pol1, 01Hoe1, 02Kit1]. From this investigation it was concluded that the NO molecules bond via their nitrogen end to the nickel surface atoms. The Ni-N distance was determined to be 1.88±0.02 Å and for the tilt angle a value of 59° (+31°/−17°) was obtained. The bonding of NO to NiO(100) leads to characteristic electronic surface excitations at 0.9 eV and ∼1.8 eV as observed with medium resolution electron energy loss spectroscopy and modelled with ab initio cluster calculations [93Fre1]. For NO on NiO(100)/Ni(100) the N-O vibrational energy has been determined to be 1797 cm–1 with HREELS [91Kuh1, 92Bau1, 93Kuh1, 96Sch1]. A number of laser induced desorption studies has been performed for NO on NiO(100)/Ni(100) with resolution of the vibrational and rotational states as well as the kinetic energy of the desorbing molecules [96Eic1, 98Eic1, 99Eic1, 96AlS1, 90Mul1, 94Men1, 99Zac1]. It was assumed that the primary excitation step occurs in the substrate. One electron may be captured by a NO molecule which starts moving away from the ground state minimum. After the electron has returned into the substrate the molecule eventually has gained enough energy to desorb. A characteristic feature of the system is that it exhibits bimodal velocity distributions with a distinct dependence on vibration, rotation, and the spin-orbit state. A number of theoretical publications [98Klu1, 97Klu1, 96Klu1, 98Klu2, 03Bac1] have dealt with this problem.
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3.9 Adsorption on oxide surfaces 0.7
q = 0.80 0.42 0.28 0.17 0.11
cleaved NiO (100) single crystal
0.6
heating rate 1K/s
0.5
Adsorption energy [eV]
216 K 220 K
TDS:NO /NiO (100)
1.05 0 56 K
NO on NiO(100) cleaved in UHV:TDS analysis 0.57 ± 0.04 eV
0.4 0.3 0.2 0.1
115 K
361
complete analysis 1K /s leading edge 1 K /s leading edge 0.2 K /s
0 0
50
100
150 200 250 Temperature [K ]
300
350
400
0
0.5 1.0 Relative coverage [ q/ qmonolayer ]
1.5
Fig. 13. Thermal desorption spectroscopy of NO on vacuum-cleaved NiO(100). Left: raw data. Right: NO-NiO binding energy as a function of coverage as obtained with the evaluation methods complete analysis and leading edge; [99Wic1, 99Wic2].
NiO(111) Less data exist for NO adsorption on NiO(111). HREELS spectra of NO on NiO(111)/Ni(111) reveal that the energy of the N-O stretching vibration is 1772 cm–1 for the hydroxylated surface. This value shifts to 1805 cm–1 after dehydroxylation of the oxide film [96Sch1]. For dehydroxylated NiO(111) films on Ni(111) Bandara et al studied the N-O vibration with SFG and IRAS [97Ban1] and found a vibrational energy of 1800 cm–1 with SFG and 1805 cm–1 with IRAS. These energies are near to that found for NO on NiO(100) which was attributed to the octopolar reconstruction of the dehydroxylated surface: the reconstructed surface exhibits microfacets with (100)-type termination. From the polarization dependence of the SFG spectra the authors concluded that the molecular axis is oriented more or less perpendicularly to the surface which would be the case for tilted molecules on the microfacets if all molecules would be bent towards the (111) direction. This result is somewhat at variance with a NEXAFS investigation where it was shown that the NEXAFS spectra of the system are nearly independent of the light incidence angle which was attributed to rotation of the molecules around the surface normal of the microfacets [96Sch1]. The energy of the N1s ĺ 2π resonance is 406.5 eV and the N1s ĺ 6σ is centered at about 421 eV. For NO on NiO(111)/Ni(111) also UV-laser induced desorption experiments have been performed [96AlS1, 94Men1, 94Men2]. Similar to the case of NO on NiO(100) the velocity distributions are bimodal, but the intensity distribution between the two modes and the dependence on the internal degrees of freedom is different.
3.9.11.3 H2O adsorption NiO(100) First studies of water adsorption on NiO(100) have been reported in the eighties. McKay and Henrich [85McK1] report that water adsorbs dissociatively at room temperature on ion-bombarded NiO(100) covered with pre-adsorbed oxygen, forming hydroxyl groups. Langell and Furstenau [86Lan1] conclude that water does not interact with stoichiometric NiO(100) at temperatures between 200 and 300 K whereas at 500 K a not clearly identified layer formed on the surface. Water adsorption on thin NiO(100) films on Ag(100) was studied using XPS, TDS and UPS by R. Reissner, M. Schulze and coworkers [98Rei1, 00Rei1, 01Sch2]. TDS revealed three desorption states at 140 K, 200 K and 210-270 K. The lowtemperature peak was attributed to multilayer desorption and the other two states to the monolayer. For Landolt-Börnstein New Series III/42A5
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the state at 200 K a desorption energy of 52 kJ/mol and a pre-exponential factor of 1014 s−1 was determined using the leading edge method and for the state ranging from 210 to 270 K the calculated desorption energy is 65 kJ/mol assuming a frequency factor of 1013 s−1. The state at 200 K was attributed to desorption from well-ordered flat NiO(100) whereas the state between 210 and 270 K was identified as originating from an ensemble of energetically different adsorption sites. Hydroxyl groups could not be observed for this substrate. For the case of NiO(100) films on Ni(100) Cappus and coworkers [93Cap1] conclude that hydroxyl groups form only on defect sites whereas vacuum-cleaved NiO(100) surfaces seem to be inert with respect to hydroxyl formation. NiO(111) The interaction of NiO(111) with H2O is strong since NiO(111) is a polar surface which may be stabilized by a layer of hydroxyl groups. For NiO(111) grown by oxidation of Ni(100) references [78And1, 94Lan2] report that the oxide films are covered by hydroxyl groups directly after oxide film preparation. Rohr and coworkers [94Roh1] report a SPA-LEED study for the interaction of water with NiO(111)/Ni(111). They find that removal of the OH groups from the NiO(111) surface leads to the formation of a (2×2) superstructure in the LEED pattern which was attributed to a octopolar reconstruction as proposed by Wolf [92Wol1]. This reconstruction stabilizes the hydroxyl-free NiO(111) surface which would not be stable without this reconstruction due to its polar nature. Since a negatively charged layer of hydroxyl groups may also stabilize the surface the reconstruction is not observed for the hydroxyl-covered surface. The O-H vibrational energy is 460 meV as reported by [78And1, 95Cap1]. Kitakatsu and coworkers investigated the interaction of NiO(111)/Ni(111) with H2O using XPS, AES and STM [98Kit1]. They observe that the film is covered by 0.85±0.1 monolayers of OH− after preparation at 300 K. The O1s binding energy of the hydroxyl groups was determined to be 531.4±0.1 eV. Exposing the hydroxylated surface to 150 L of H2O leads to the formation of a second layer of hydroxyl groups with the sequence OH-Ni-OH−, i.e. to a surface β-Ni(OH)2 film. Oxidation of the Ni(111) surface at 500 K leads to a mixture of NiO(100) (93±3%) and NiO(111) crystallites (7±3%). Here exposure to water induces a lateral extension of the NiO(111) crystallites at the expense of the NiO(100) crystallites.
3.9.11.4 HCOOH adsorption on NiO(111) Formic acid adsorption on NiO(111)/Ni(111) was studied with different methods. Domen, Hirose and coworkers published results of TDS, IRAS and SFG studies on this system [96Ban1, 96Ban2, 97Ban2, 96Kub1, 99Dom1, 98Mat1]. Formic acid is transformed into a tilted bidentate formate species under UHV conditions. This occurs already at 163 K, and at 250 K all molecular formic acid has disappeared. Further heating leads to decomposition of the surface formate groups into H2+CO2 at 340, 390 and 520 K and CO+H2O (water undetected) at 415 and 520 K [96Ban2]. The reactions at temperatures between 340 and 415 K and at 520 K are attributed to interaction with surface nickel atoms in different oxidation states [98Mat1]. At higher formic acid pressure (P ≥5×10−5 Pa) also monodentate formate forms under steady state conditions and the bidentate species was found to be non-tilted. The latter result was attributed to the transformation of the reconstructed NiO(111)(2×2) surface into a non-reconstructed one due to the formation of surface hydroxyl groups. Again, two formate dissociation paths are observed: formation of H2 and CO2 starting at 373 K with an activation energy of 22±2 kJ/mol and formation of CO and H2O above 423 K with an activation energy of 16±2 kJ/mol. In both cases the reaction order is 0.5 with respect to the pressure of HCOOH. From pressure dependent IRAS spectra it was concluded that the monodentate formate species acts as an intermediate for dissociation [97Ban2]. This was later substantiated by picosecond SFG spectroscopy in combination with transient laser induced heating [99Dom1]. An overview of vibrational energies observed for formate on NiO(111) and NiO(100) is given in Table 13.
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Table 13. Vibrational energies of HCOOH on NiO(100) and NiO(111) in cm−1. For monodentate formate υa(OCO) and υs(OCO) are the stretching modes of C=O not interacting with the surface and C-O bound to the surface, respectively. The table has been adapted from [96Kub1]. Vibrational mode NiO(111) in HCOOH flow (IRAS) NiO(111) in NiO(100) vacuum (IRAS) (HREELS) 2940 2850 2860 2901 υ(CH) 1570 1594 υ a(OCO) 1253 1355 1360 1377 υ s(OCO) Assignment monodentate bidentate bidentate monodentate Reference 96Ban1 96Ban1 96Ban1 92Tru1
3.9.11.5 H2 adsorption on NiO(100) Hydrogen causes reduction of NiO(100) [85Fur2, 92Wul1, 02Rod1, 80Wep1]. It was shown that there is an induction period during which oxygen vacancies are created which act as hydrogen dissociation sites [02Rod1, 85Fur2]. High-quality NiO(100) crystals exhibit only negligible reactivity towards H2 [02Rod1]. The reduction process was shown to lead to the formation of metallic nickel [85Fur2, 92Wul1, 02Rod1].
3.9.11.6 H2S adsorption on NiO(100) H2S reacts with a cleaved NiO(100) single crystal surface, causing the formation of Ni rafts with a sulfur overlayer. Using EXAFS, the in-plane Ni-Ni distance was determined to be 2.77±0.09 Å [99Woo1], representing a 6±4% contraction with respect to the distance in NiO(100) which is also visible in LEED [78Ste1]. For the S-Ni bond length a value of 2.21±0.02 Å was obtained with the S atoms occupying fourfold hollow sites in a c(2×2) structure.
3.9.11.7 CO2 adsorption on NiO(111) The interaction of CO2 with NiO(111)/Ni(111) has been studied using IRAS, TDS, XPS and UPS [99Mat1, 93Gor1]. For the hydroxyl-free (2×2)NiO(111)/Ni(111) surface adsorption of CO2 at 123 K leads to vibrations at 1263 and 910 cm–1 whereas for the OD covered surface a vibration at 1267 cm–1 was observed with IRAS [99Mat1]. The vibrations at 1263 and 1267 cm–1 were assigned to the symmetric OC-O vibration and the one at 910 cm–1 to the out-of-plane deformation of monodentate carbonate. On the hydroxylated surface the latter vibration was much weaker which was attributed to a tilted configuration on (2×2)NiO(111)/Ni(111) in contrast to an upright geometry on OD/NiO(111)/Ni(111). The different molecular orientations were attributed to the different surface structures: the hydroxyl-covered surface is flat whereas the hydroxyl-free surface exhibits the micro-facets of the octopolar reconstruction. For both surfaces the vibrational signals vanished around 248 K [99Mat1]. Results for adsorption at room temperature are reported in [93Gor1]. CO32− and CO3− were identified on the surface. The TDS spectra exhibit desorption peaks at 395 and 645 K [93Gor1].
3.9.12 RuO2 RuO2 exhibits rutile structure like TiO2 with lattice parameters a = 4.51 Å and c = 3.11 Å [65Wyc1]. The unit cell is displayed in Fig. 14. Ruthenium oxide exhibits high catalytic activity for oxidation reactions like CO oxidation to CO2 or methanol oxidation to formaldehyde which is one of the reasons why this oxide has been studied intensively in recent years. Other reasons are the high electric conductivity which Landolt-Börnstein New Series III/42A5
References for this document 62Red1 65Wyc1 67Kat1 73Liu1 77Dem1 78And1 78Ste1 79Tas1 80Wep1 83Sti1 84Lee1 85Fur1 85Fur2 85McK1 86Foo1 86Hen1 86Lan1 87Kur1 87Oni1 89Mac1 89Tho1 90Mul1
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92Str1 92Tru1 92Vur1 92Wol1 92Wu1 92Wu2 92Wu3 92Wu4 92Wul1 92Wul2 93Cap1 93Fre1 93Fre2
93Gon1 93Gor1 93Hei1 93Jae1 93Jae2 93Kuh1 93Li1 93Nog1 93Pac1 93Suz1 93Tru1 93Wu1 93Wu2 93Wu3 94AlS1 94Cap1
94Cou1 94Cou2 94Dua1 94Elv1 94Fer1 94Gas1 94Hol1 94Jen1 94Kuh1 94Lan1 94Lan2 94Men1 94Men2 94Nyg1 94Pac1 94Pan1 94Pet1
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98Joh1 98Jon1 98Jon2 98Kit1 98Kit2 98Klu1 98Klu2 98Lar1 98Laz1 98Liu1 98Liu2 98Liu3 98Liu4 98Liu5 98Mar1 98Mat1 98Nel1 98Nis1 98Ovi1 98Ovi2 98Pou1 98Pra1 98Ran1 98Rei1 98Slo1 98Sni1 98Sni2 98Soe1 98Thi1 98Tol1 98Van1 98Van2 98Wei1 98Wil1 98Yuz1 98Zsc1 99Abr1 99Ban1 99Bol1 99Bre1 99Bro1
99Car1 99Cas1 99Cas2 99Che1 99DiF1 99Dom1 99Doy1 99Eic1 99Eng1 99Gun1 99Guo1
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Table 13. Vibrational energies of HCOOH on NiO(100) and NiO(111) in cm−1. For monodentate formate υa(OCO) and υs(OCO) are the stretching modes of C=O not interacting with the surface and C-O bound to the surface, respectively. The table has been adapted from [96Kub1]. Vibrational mode NiO(111) in HCOOH flow (IRAS) NiO(111) in NiO(100) vacuum (IRAS) (HREELS) 2940 2850 2860 2901 υ(CH) 1570 1594 υ a(OCO) 1253 1355 1360 1377 υ s(OCO) Assignment monodentate bidentate bidentate monodentate Reference 96Ban1 96Ban1 96Ban1 92Tru1
3.9.11.5 H2 adsorption on NiO(100) Hydrogen causes reduction of NiO(100) [85Fur2, 92Wul1, 02Rod1, 80Wep1]. It was shown that there is an induction period during which oxygen vacancies are created which act as hydrogen dissociation sites [02Rod1, 85Fur2]. High-quality NiO(100) crystals exhibit only negligible reactivity towards H2 [02Rod1]. The reduction process was shown to lead to the formation of metallic nickel [85Fur2, 92Wul1, 02Rod1].
3.9.11.6 H2S adsorption on NiO(100) H2S reacts with a cleaved NiO(100) single crystal surface, causing the formation of Ni rafts with a sulfur overlayer. Using EXAFS, the in-plane Ni-Ni distance was determined to be 2.77±0.09 Å [99Woo1], representing a 6±4% contraction with respect to the distance in NiO(100) which is also visible in LEED [78Ste1]. For the S-Ni bond length a value of 2.21±0.02 Å was obtained with the S atoms occupying fourfold hollow sites in a c(2×2) structure.
3.9.11.7 CO2 adsorption on NiO(111) The interaction of CO2 with NiO(111)/Ni(111) has been studied using IRAS, TDS, XPS and UPS [99Mat1, 93Gor1]. For the hydroxyl-free (2×2)NiO(111)/Ni(111) surface adsorption of CO2 at 123 K leads to vibrations at 1263 and 910 cm–1 whereas for the OD covered surface a vibration at 1267 cm–1 was observed with IRAS [99Mat1]. The vibrations at 1263 and 1267 cm–1 were assigned to the symmetric OC-O vibration and the one at 910 cm–1 to the out-of-plane deformation of monodentate carbonate. On the hydroxylated surface the latter vibration was much weaker which was attributed to a tilted configuration on (2×2)NiO(111)/Ni(111) in contrast to an upright geometry on OD/NiO(111)/Ni(111). The different molecular orientations were attributed to the different surface structures: the hydroxyl-covered surface is flat whereas the hydroxyl-free surface exhibits the micro-facets of the octopolar reconstruction. For both surfaces the vibrational signals vanished around 248 K [99Mat1]. Results for adsorption at room temperature are reported in [93Gor1]. CO32− and CO3− were identified on the surface. The TDS spectra exhibit desorption peaks at 395 and 645 K [93Gor1].
3.9.12 RuO2 RuO2 exhibits rutile structure like TiO2 with lattice parameters a = 4.51 Å and c = 3.11 Å [65Wyc1]. The unit cell is displayed in Fig. 14. Ruthenium oxide exhibits high catalytic activity for oxidation reactions like CO oxidation to CO2 or methanol oxidation to formaldehyde which is one of the reasons why this oxide has been studied intensively in recent years. Other reasons are the high electric conductivity which Landolt-Börnstein New Series III/42A5
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enables the application of electron spectroscopy and the simplicity of preparation of a RuO2(110) surface by oxidation of a Ru(0001) single crystal surface. Exposure of Ru(0001) to several 106 L of O2 at 760 K leads to formation of a stoichiometric RuO2(110) surface [02Wen1] which is structurally similar to TiO2(110) (see Fig. 14a). Reduction with CO at 410 K leads to a slightly different surface where the oxygen rows at the surface are missing [02Wen1]. A structural model is displayed in Fig. 14b. Obridge
Rucus
Rubridge
oxygen ruthenium
a c a
(a) Stoichiometric RuO2(110) surface
(b) reduced surface
(c) RuO2 unit cell
Fig. 14. Structure of the stoichiometric (a) and the mildly reduced (b) RuO2(110) surface. Panel (c) displays the rutile type unit cell of RuO2.
Table 14. Overview of investigations of the interaction of gases with well ordered RuO2 surfaces Adsorbates Method References Substrate: RuO2(110) CO, O2 Theory: DFT 03Reu1 Substrate: RuO2(110)/Ru(0001) C2H4 TDS, HREELS, isotopic labeling 04Pau1 CO STM 00Ove1 CO STM, TDS, HREELS 03Kim1 CO HREELS, TDS 01Wan2, 01Fan1, 03Pau1 CO TDS 03Ove1, 02Wen1 CO TDS, LEED IV analysis, Theory: DFT 02Sei1 LEED IV analysis, Theory: DFT 01Kim1 CO, N2 LEED, TDS 02Ove1, 01Mad1 CO, CH3OH, O2 HREELS, TDS 02Wan1, 02Laf1 CO2 NO HREELS, TDS 03Wan2 TDS, HREELS 03Lob1 H2O TDS, HREELS, LEED IV analysis, Theory: 01Kim2 O2 DFT For RuO2 mainly the adsorption of CO was studied which will also be the system discussed in the following. For the remaining systems the reader may consult the references listed in Table 14.
3.9.12.1 CO adsorption Most of the adsorption studies performed for RuO2(110) dealt with CO [03Reu1, 00Ove1, 03Kim1, 01Wan2, 03Ove1, 02Wen1, 01Kim1, 02Ove1, 01Mad1, 02Sei1]. Other adsorbates have also been studied, but the CO adsorption system was surely in the focus which may be due to the high catalytic activity of RuO2(110) for CO oxidation. For the RuO2(110) surface it could be shown that CO interacts already at room temperature with the weakly bound oxygen atoms of the oxygen rows on the surface (Obridge in Fig. 14), forming CO2. After removal of the oxygen atoms CO molecules may adsorb directly on the underlying ruthenium atoms (Rubridge in Fig. 14). With HREELS two C-O vibrations at 234.5 meV and 248.5 meV are observed for this species and a CO-Ru vibration at 53.5 meV [01Wan2]. The two C-O vibrations have been attributed to CO molecules adsorbed on a symmetric bridge site and to CO bonding Landolt-Börnstein New Series III/42A5
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to one Ru atom in a bent geometry [02Sei1]. The corresponding TDS spectrum exhibits two peaks at 415 and 470 K [03Kim1]. If the stoichiometric RuO2(110) surface is exposed to CO at 85 K CO molecules may bond to the Rucus sites (see Fig. 14). With HREELS vibrations at 39 meV and 262 meV are detected [01Wan2]. In the TDS spectra desorption peaks at 270, 320 and 470 K show up [03Kim1]. The first two of them are attributed to desorption of CO molecules from Rucus sites whereas the third state is attributed to desorption from a Rubridge site which was formed by reduction of the surface due to the interaction with the CO molecules [03Kim1]. With STM (2×1) and c(2×2) structures were observed for CO bonded to Rucus sites which may explain the two different desorption peaks observed with TDS [03Kim1]. Reuter and Scheffler calculated the binding energies for CO on Rucus and Rubridge and obtained values of 1.26 eV and 1.58 eV, respectively [03Reu1]. An experimental value of 0.9 - 1.0 eV was given for the CO molecules on the Rucus sites [03Kim1]. We note that a somewhat different TDS spectrum of CO on a reduced surface has been published by Seitsonen et al [02Sei1]. Desorption peaks were identified at ∼300 K, ∼350 K and ∼560 K. These peaks were attributed to CO molecules on Rucus sites, asymmetrically bridging CO molecules on Rubridge sites, and symmetrically bridging CO molecules on Rubridge sites, respectively. The differential heats of adsorption as calculated with density functional theory are reported to be 1.00 eV, 1.33 eV and 1.85 eV, respectively [02Sei1].
3.9.13 SnO2 SnO2 (cassiterite) exhibits rutile structure (like TiO2, see Fig. 15) with lattice parameters of a = 4.59373 Å and c = 3.186383 Å [65Wyc1]. Usually the (110) surface is studied. A single crystal surface may be prepared by cutting off a piece from a single crystal needle and polishing it followed by sputtering and annealing in combination with O2 or N2O treatment. Depending on the preparation conditions stoichiometric as well as reduced surfaces may be prepared [95Ger1]. With increasing surface reduction a sequence of 4×1, 1×1 and 1×2 surface LEED patterns may be observed. Due to the tendency of this oxide to undergo gas-induced changes of the electrical conductivity it has important applications in gas-sensing applications. Table 15 gives an overview of adsorption studies for ordered SnO2 surfaces. Table 15. Overview of investigations of the interaction of gases with well ordered SnO2 surfaces Adsorbates Method References Substrate: SnO2(110) CH3OH Theory: MNDO 94Mar Theory: DFT, HF 99Cal1 CH3OH XPS 00Kaw1 CH3OH TDS, XPS 94Ger1 CH3OH HCOOH ARUPS, AES, LEED 96Irw1 Theory: MNDO, AM1, PM3 95Mar1 H2 Theory: DFT 96Gon1, 02Bat1, 00Lin1 H2O TDS, UPS 95Ger1 H2O TDS, UPS, band bending 87Sem1 H2O, O2 PYS 97Szu1, 94Szu1 O2 92She1 O2 ARUPS, TDS, ∆Φ O2 surface conductivity 87Eri1 Theory: DFT 01Ovi1, 00Yam1 O2 Theory: LDA cluster calculations 95Ran1 O2, CO CO TDS, UPS 95Ger1 CO Theory: DFT 00Mel1 Theory: DFT 01Mel1 CO2
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[Ref. p. 389
3.9.13.1 O2 adsorption The interaction of SnO2(110) with O2 has been studied by several authors due to the oxygen-induced conductivity changes of SnO2. Results of surface conductivity investigations have been published by Erickson and Semancik [87Eri1]. The authors concentrate on the influence of the surface preparation, i.e. oxygen exposure and annealing and find variations of more than two orders of magnitude of the surface sheet conductivity which was mainly attributed to variations of the concentration of oxygen vacancies. Oxygen exposure changes the concentration of vacancies and thus the conductivity. ARUPS and TDS studies indicated that oxygen adsorption at low temperature occurs molecularly. O2 thermal desorption peaks were found at about 200 and 250 K [92She1].
3.9.13.2 H2O adsorption Water adsorption on stoichiometric and defective SnO2(110) was experimentally studied using TDS and UPS [95Ger1]. Molecular desorption of water was found at 200 and 300 K and a desorption state at 435 K was attributed to OH disproportionation. It was shown that the water dissociation probability was highest on a moderately defective surface. For this surface it was assumed that all bridging oxygen atoms at the surface were removed while 80% of the surface in-plane oxygen anions did still exist.
3.9.13.3 CH3OH adsorption The interaction of SnO2(110) with methanol depends also on the surface structure. Methanol may be oxidized to form formaldehyde on SnO2(110) [94Ger1]. The conversion of methanol was found to exhibit a maximum for intermediate surface reduction. Using XPS it was shown that on the pre-oxidized surface methanol decomposition occurred via the abstraction of a hydrogen atom while on the reduced surface the methanol C-O bond was cleaved [00Kaw1]. For a list of XPS binding energies see [00Kaw1].
3.9.13.4 HCOOH adsorption Formic acid adsorption on reduced SnO2(110) exhibiting (1×1) and (1×2) LEED patterns was studied with ARUPS, AES and LEED [96Irw1]. At 105 K formic acid adsorbs molecularly and after annealing at 375 K it is fully desorbed, leaving no carbon residue behind. While the (1×1) LEED pattern was unaffected by this process, the (1×2) pattern was transformed into a (1×1) pattern. It was assumed that oxygen atoms from the HCOOH molecules re-oxidize the surface.
3.9.14 TiO2 Three different modifications of TiO2 may be found at ambient conditions: rutile, anatase and brookite. Rutile and anatase are the technically more important ones and have thus been employed for surface science studies. Both are electrically insulating in pure form. Rutile exhibits the tetragonal cassiterite structure with lattice constants a = 4.59 and c = 2.96 Å [65Wyc1]. The lattice of anatase is tetragonal; here the lattice parameters are a = 3.88 and c =9.51 Å [65Wyc1]. The (110) surface of rutile is the most stable one of this TiO2 modification. An image of this surface and the rutile unit cell are shown in Fig. 15. Together with MgO(100), the rutile TiO2(110) surface is probably the most often studied oxide surface in surface science. It has been characterized extensively and well established methods for its preparation do exist. In order to establish a sufficiently high electrical conductivity for STM or electron spectroscopy the sample is usually annealed at elevated temperature and bombarded with ions which leads to a bulk reduction of the oxide as documented by a color change from colorless transparent to yellowish or bluish or even black [00Die1]. A common defect occurring upon annealing is the removal of oxygen atoms from Landolt-Börnstein New Series III/42A5
Ref. p. 389]
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the rows of bridging oxygen atoms, leaving behind vacancies in these rows. Especially for the strongly reduced samples, high defect densities at the surface have to be expected. The stoichiometric (110) surface is not reconstructed, but under reducing conditions a (1×2) superstructure may form. Exposing the reduced surface to oxygen leads to re-oxidation. An overview of the properties may be found in U. Diebold’s review article [03Die1]. Much less studies have been performed for TiO2(100) and TiO2(001). Both surfaces are less stable than the (110) surface. For TiO2(100) (1×1) and (1×3) surface structures have been observed with the latter one corresponding to a reduced, micro-facetted surface [93Har1]. Due to its high surface energy the (001) surface also exhibits a tendency to form microfacets as revealed by STM and LEED studies [82Fir1, 03Ter1]. Large high-quality rutile single crystals are readily commercially available which is not the case for anatase. Therefore, most studies have been performed on the rutile modification of TiO2 although anatase appears to exhibit higher catalytic activity. Investigations have been performed for the (101) and (001) surfaces of anatase. Usually natural single crystals or thin films (grown on natural crystals) were used for the studies [03Die1].
(a) stoichiometric rutile TiO2(110)
(b) TiO2 unit cell
a c a
oxygen titanium
Fig. 15. (a) structure of the rutile TiO2(110) surface. (b) unit cell of rutile TiO2.
Table 16. Overview of investigations of the interaction of gases with well ordered TiO2 surfaces Adsorbates Method References Substrate: anatase TiO2(001) HCOOH STM, NC-AFM 02Tan2 STM, TDS 02Tan1 HCOOH, CH3COOH Theory: periodic Hartree-Fock calculations 95Fah1 HOOC-COOH, HOOC-COO–, C2O42− H2O Theory: SINDO1 95Bre1 Theory: DFT 98Vit1 H2O Substrate: anatase TiO2(010) HOOC-COOH, HOOC-COO– , Theory: periodic Hartree-Fock calculations 95Fah1 C2O42− Substrate: anatase TiO2(101) HCOOH, HCOOH+OH Theory: DFT 00Vit1 Theory: molecular dynamics 98Sel1 H2O, H2S, HI TDS, XPS 03Her1 H2O, CH3OH Theory: DFT 98Vit1, 03Til1 H2O 02Hau1 cis(CO)-trans(I)-Ru-(4,4’-dicar- Theory: DFT boxylate-2,2’-bipyridine)(CO)2I2 Substrate: rutile TiO2(100) C4H4S AES, UPS, XPS, electron and X-ray 97Raz1 irradiation
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Adsorbates C6H6
Method AES, UPS, XPS, electron and X-ray irradiation UPS, MIES UPS, XPS Theory: periodic Hartree-Fock calculations Theory: periodic Hartree-Fock calculations
01Bra1 99Wan1 96Fah1 95Fah1
TDS, Theory: PM3 Theory: periodic Hartree-Fock calculations XPS, TDS NEXAFS
00Wil1 96Mar1 03Far1 96Raz1
TDS, XPS, scanning kinetic spectroscopy photoreaction TDS TDS TDS, XPS TDS, NEXAFS NEXAFS, TDS
01Tit1 03Wil1 96Hen1 95Pie1 02She1 01She1 00She1
XPS, NEXAFS, Theory: INDO
99Pat1
NEXAFS, Theory: ZINDO
00Per1
NEXAFS, Theory: DFT
03Ode1
XAS, STM
03Sch2, 03Sch1
photocatalysis, molecular beam, XPS photon irradiation, REMPI TDS
98Bri3, 98Bri1, 00Bri1 96Hol1 96Gam1
UV photodesorption, mass spectrometry TDS, UV photodesorption TDS, ESD, electron irradiation TDS, XPS Theory: DFT, pseudopotential TDS, HREELS, LEED
98Kim1 00Kim1 98Hen3 03Far2 98Bat1 99Hen2
STM XPS XPD XPD, XPS, LEED, TDS, HREELS TDS, SSIMS, HREELS XPS, IRAS, LEED HREELS molecular beam, STM, LEED STM UPS, XPS, Theory: ab initio cluster calculations
03Ter1 96Idr1 98The1, 97Cha1 98Cha1 97Hen1 99Hay1 00Cha1 02Bow1 00Ben1 97Wan1
CO2 H2O H2S, MeSH HOOC-COOH, HOOC-COO–, C2O42− maleic anhydride NH3 NH3, (CH3)2NH, C2H5NH2 SO2 Substrate: rutile TiO2(001) acrylic acid CH3COOH H2O tert-butylacetylene trimethylsilyl acetylene cyclooctatetraene benzaldehyde Substrate: rutile TiO2(110) 2,2’-bipyridine-4,4’dicarbocyclic acid 2,2’-bipyridine-4,4’dicarbocyclic acid 2,2’-bipyridine-4,4’dicarbocyclic acid isonicotinic acid, nicotinic acid, picolinic acid 2-propanol+oxygen CD3I C2H5OD, tetraoxysilane, pre and post-adsorption of H2O CH3I, CH3Br CH3I CH3OH CH3OH CH3OH CH3OH, CH3OH+H2O, CH3OH+O2 CH3OH, HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH HCOOH
[Ref. p. 389 References 98Raz1
Landolt-Börnstein New Series III/42A5
Ref. p. 389]
3.9 Adsorption on oxide surfaces
Adsorbates HCOOH HCOOH, O2, C5H5N HCOOH, OH HCOOH, CH3COOH, C2H5COOH HCOOH, CH3COOH DCOOD DCOOD CH3COOH, C6H5COOH CH3COOH, CF3COOH CH3COOH CH3COOH CH3COOH, DCOOH CH3COOH CH3COOH C6H5COOH C5H5N C17H35COOH benzene, naphtalene, anthracene glycine C4H4S Cl2 CO
Method Theory: DFT STM STM, NC-AFM NEXAFS
References 00Kac1, 00Kac2 98Iwa1 01Iwa1, 96Oni1 01Gut3
NC-AFM XPS, TDS TDS, LEED, AES, XPS, UPS STM, LEED NC-AFM UPS LEED, ESDIAD STM NC-AFM Temperature jump STM STM, ESDIAD, LEED STM, NC-AFM AFM, photo degradation NEXAFS, XPS, TDS PES, photon damage XPS, TDS, Theory: DFT STM Theory: ab initio cluster calculations, band structure calculations Theory: FLAPW Theory: DFT, pseudopotential Theory: periodic Hartree-Fock ESD, AES TDS molecular beam TDS, PID Theory: DFT Theory: DFT TDS, SSIMS, HREELS TDS TDS, ∆Φ , AES, SSIMS, XPS TDS, XPS, SSIMS
99Fuk1, 01Sas1 03Wan1 94Oni1 99Guo2 01Sas2 97Coc1 97Guo1 98Egd1 00Fuk1 96Oni3 97Guo2 99Suz1 99Saw1 02Rei1 99Sor1, 00Sor2 03Liu1 98Die1 96Pac1
Theory: DFT Theory: DFT, molecular dynamics, slab Theory: SINDO1 Theory: HF slab calculations Theory: FLAPW Theory: Hartree-Fock, DFT Theory: MP2, Hartree-Fock Theory: DFT Theory: Hartree-Fock, MP2 UPS TDS, HREELS TDS
97Lin1, 96Gon1 96Lin1, 98Lin1 95Bre1 98Ahd1 98Vog1 99Ste1 03Sha1 03Zha2 02Sha1 77Hen1 96Hen2 96Hen1
CO CO CO CO CO CO CO+O2 CO, H2O CO, H2O, H2S CO2+H2O CO2 CrO2Cl2 FPTS[(3,3,3trifluoropropyl)trimethoxysilane] H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O Landolt-Börnstein New Series III/42A5
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01Yan1 98Sor1 96Rei1 95Tor1 95Lin1 03Kun1 96Lin2 99Cas4 98Cas2 98Hen1 03Tho1 98Ala1 98Gam1
370
3.9 Adsorption on oxide surfaces
Adsorbates H2O H2O, O2 H2O, CO2, NH3, OH H2O, liquid and vapor H2O H2O, CH3OH, H2O2, HCOOH HOOC-COOH, HOOC-COO– , C2O42− N2 N2 NO
Method TDS, XPS, TDS Theory: Hartree-Fock UPS, XPS TDS, molecular beam scattering Theory: DFT Theory: periodic Hartree-Fock calculations
References 94Hug1 98Epl1 99Ahd1 95Wan1 98Bri2 98Bat2 95Fah1
Theory: ab initio cluster calculations Theory: Monte Carlo simulations UV photochemistry, TOF mass spectrometry Theory: Hartree-Fock, MP2 TDS, Theory: DFT, pseudopotential Theory: periodic Hartree-Fock calculations APECS SHG, XPS XPS, NEXAFS, Theory: DFT PES SHG, XPS STM TDS, ELS, isotopic labeling, sticking coefficient TDS, SSIMS, EELS Theory: DFT XPS Theory: slab calculations ion scattering UPS Theory: periodic Hartree-Fock calculations NEXAFS XPS XPS, UPS, PSD 03Zha1 IRAS, XPS STM XPS, TDS
98Rit1, 99Rit3 99Rit2 00Rus1
NO, CO, H2CO NO NH3 NH3 N2O NO2 NO2 O2 O2 O2 O2, O2+H2O O2 OH OH OH, H H2S H2S, MeSH SO2 SO2 SO2 SO2 Theory: DFT {Rh(CO)2Cl}2+H2 {Rh(CO)2Cl}2 Rh((CH3CO)2CH)2(CO)2, Rh(CO)2Cl merocyanine dye
NEXAFS, STM
[Ref. p. 389
01Li1 00Sor1 96Mar1 00Siu1 97Shu1 01Rod2 02Cha1 95Shu1 96Oni2, 98Die2 99Hen1 01Per1 99Shu1 96Bul1 95Gon1, 93Nog1 01Fuj1 89Smi1 96Fah1 89Tho1 01Say1 97Rom1 98Hay1 01Ben1 00Eva1 02Mat1
For TiO2 a number of adsorbates has been studied, partly in great detail. Some systems will be discussed in the following and for the remaining systems the reader may consult the references listed in Table 16.
3.9.14.1 CO adsorption Only a limited number of adsorption studies has been performed for CO on TiO2. Linsebigler et al [95Lin1] performed a TDS study for CO adsorption on stoichiometric and reduced rutile TiO2(110). CO desorption was found to occur at 170 K for low coverage. With increasing coverage the desorption temperature shifted to about 135 K as shown in Fig 7.16a. From these data an adsorption energy of 9.9 Landolt-Börnstein New Series III/42A5
Ref. p. 389]
3.9 Adsorption on oxide surfaces
371
kcal/mol was determined for the zero coverage limit. CO was found to desorb molecularly and no CO2 formation was observed (if CO is co-adsorbed with O2 on a TiO2(110) surface with oxygen vacancies, CO2 may be formed upon irradiation with light with hν ≥ 3.1 eV [96Lin2]). The shift of the CO desorption peak with increasing coverage was attributed to lateral interactions which were found to have an energy of about 2.2 kcal/mol at a relative coverage of 0.68 ML. The authors estimated that the maximum CO density at the surface is about half of the density of in-plane titanium atoms which were named as the CO adsorption sites. Fig 7.16b demonstrates that defects on the surface lead to more strongly bound CO molecules. The desorption peaks tail up to 350 K. Pre-adsorbed oxygen was found to suppress these desorption states. Since the CO maximum coverage was the same as for the stoichiometric surface the authors of the TDS study [95Lin1] concluded that the CO adsorption sites are the same in both cases and that the additional binding energy observed for the reduced surface is provided by interaction of the oxygen end of the CO molecules with a neighboring vacancy site. A molecular beam study for CO on rutile TiO2(110) was performed by Kunat and Burghaus [03Kun1] as a function of incidence angle, kinetic energy and surface temperature. For an impact energy of 0.05 eV an initial sticking probability (this is the sticking coefficient for vanishing coverage) of S 0 = 0.84±0.05 was obtained. The sticking coefficient was found to decrease with increasing kinetic energy towards a value of S 0 = 0.1±0.05 for a kinetic energy of 0.57 eV. Temperature dependent measurements revealed that the initial sticking coefficient is independent of the temperature which was interpreted as an indication of non-activated adsorption. The dependence of the initial sticking coefficient on the angle is a function of the kinetic energy: for small kinetic energies the initial sticking coefficient is only weakly dependent on the incidence angle whereas for energies above 0.5 eV normal energy scaling takes place. For normal incidence the sticking coefficient S(θ ) is nearly independent of the coverage θ : S(θ )∼S 0 independent of the incidence energy. For grazing incidence and a kinetic energy of 0.52 eV auto-catalytic behavior (increase of S(θ ) with increasing θ ) was observed for incidence along [1 1 0] whereas for incidence along [001] the adsorption probability was found to decrease slightly with increasing incidence angle. From the temperature dependence of the saturation coverage the authors derived a heat of adsorption of Ed = (7.2−1.6θ ) kcal/mol using a frequency factor of νd = 1×10−13 s−1 which is not far from the value determined with thermal desorption spectroscopy [95Lin1]. 18
18
Thermal desorption of C O from the oxidized TiO2(110) surface -9
C O thermal desorption from A pre - annealed or oxidized TiO2(110) surface
Tads = 105 K dT/dt = 0.5 K /s
2 ×10 A
- 10
1 ×10
A
Tads = 105 K dT/dt = 0.5 K /s
f eCO (105 K ) 12
d c 100
150
a
2
a 7.1 × 10 CO/cm 13 2 b 1.4 × 10 CO/cm 13 2 c 2.1 × 10 CO/cm 13 2 d 3.6 × 10 CO/cm 13 2 e 8.6× 10 CO/cm 14 2 f 1.4 × 10 CO/cm 14 2 g 2.1 × 10 CO/cm
e
13
eCO = 8.6 × 10 CO/cm
CO QMS current (30 amu)
CO QMS current (30 amu)
g
2
pre - annealed
oxidized
b a 200 300 250 Temperature [K ]
350
400
100 b
150
200 300 250 Temperature [K ]
350
400
Fig. 16. (a): Thermal desorption spectra of C18O on rutile TiO2(110) after adsorption at 105 K. (b): Comparison of TDS spectra of CO on stoichiometric and annealed TiO2(110); [95Lin1].
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[Ref. p. 389
3.9.14.2 H2O adsorption The interaction of water with TiO2 is technologically interesting since TiO2 is active for water photolysis. The chemical activity of TiO2(110) for water dissociation (without photon irradiation) is low according to most experimental results. It appears that there is some activity for dissociation at defect sites whereas the regular surface is inert with respect to H2O dissociation. Henderson [96Hen2], Brinkley et al [98Bri2] and Hugenschmidt et al [94Hug1] have published TDS data of water on TiO2(110). The data provided by Henderson are shown in Fig. 17. The desorption maxima at 155 K, 174 K and 270 K are attributed to multilayer, bilayer, and monolayer water, respectively. According to [98Bri2] the monolayer coverage is 5.2×1014 cm−2 and the condensation coefficient of water is about 1. Water vibrations are found at 1625 and 3420-3505 cm–1 [96Hen2]. The three publications agree that only a very small concentration of hydroxyl groups forms on the surface. According to [94Hug1] surface hydroxyl groups were detected with a concentration of ∼1%, desorbing at 500 K. With HREELS the O-H vibrational energy was determined to be 3690 cm–1 [96Hen2]. It was suggested that the hydroxyl groups bond to oxygen vacancies. The experimentally observed low activity of TiO2(110) for water dissociations is at variance with many theoretical publications which propose that water should dissociate also on regular TiO2(110) sites (see the references in Table 16).
3.9.14.3 HCOOH adsorption The adsorption of a number of organic molecules on TiO2(110) has been studied and formic acid was surely one of the most often investigated molecules which is at least partially due to the activity of titania for the photo-assisted decomposition of organic molecules. An electron-hole pair may be created in TiO2 upon irradiation with sunlight. The charge carriers may travel to the surface and attack water and oxygen forming radicals which may oxidize adsorbed organic molecules. Such a process may be used for purification, environmental cleaning, etc. On TiO2(110) formic acid decomposition into formate+hydrogen occurs already at low temperature: HCOOHgas + Osurf → HCOOads + HadsOsurf
(2)
TPD of water adsorbed on TiO2 (110) at 135 K 155 22.8 18.3 15.1 12.5 9.4 7.7 6.7 6.3 4.6 3.1 2.3 1.5 0.8 bkgd
174
water exposure (molecules/cm2, ×1014 ) 3.1 2.3 1.5 0.8 bkgd
m/e = 18 QMS signal [a.u.]
m/e = 18 QMS signal [a.u.]
water exposure 2 14 (molecules/cm , ×10 )
270
295
200
320
250 300 Temperature [K ]
350
181
Fig. 17. TDS spectra of H2O on TiO2(110); [96Hen2]. 150
200
250 300 Temperature [K ]
350
400
Landolt-Börnstein New Series III/42A5
Ref. p. 389]
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373
(2×1) overlayer Bi - dentale formate H C O
126°
O 2.1±0.1Å Ti
Ti 2.96 Å
[001]
[110]
Hydroxyls formed upon formic acid dissociation
Formate minority species at oxygen vacancies
Fig. 18. Structure of HCOO ions on TiO2 (110); [03Die1].
At higher temperature further decomposition may occur [97Hen1, 94Oni1, 03Die1, 01Iwa1]: HCOOH → CO2 + H2 (dehydrogenation) and HCOOH → CO + H2O (dehydration)
(3) (4)
Formate forms a (2×1) phase on TiO2(110) with a nominal coverage of 0.5 molecules per TiO2(100) surface unit cell. The geometrical parameters of this layer have been investigated with NEXAFS and XPD [97Cha1, 01Gut3] (for the innermolecular angles and distances see Fig. 18). Apart from molecules adsorbing on regular (2×1) sites also defect adsorption as shown in Fig. 18 has been observed. The latter molecules are adsorbed with their molecular plane parallel to [1 1 0] and bind to oxygen defects. With infrared spectroscopy the vibrational energies of the HCOO groups were determined to be νasym(OCO) = 1566 cm–1 and νsym(OCO) = 1363 cm–1 for the molecules with their molecular plane parallel to the [001] direction and νasym(OCO) = 1535 cm–1 and νsym(OCO) = 1393 cm–1 for the other species [99Hay1]. Deviations in the NEXAFS data [01Gut3] from the expected results for the ideal (2×1) structure were also explained by the existence of the minority species. Co-adsorbed molecular HCOOH which is found at formate coverages above 0.5 was shown to desorb at 164 K [97Hen1]. Formate groups on TiO2(110) were imaged with STM and non-contact-AFM [98Iwa1, 00Ben1, 01Iwa1, 99Fuk1, 01Sas1]. Fig. 19 shows a set of STM images obtained by Onishi et al [96Oni1] at room temperature. Here scanning with high bias voltage was used to remove part of the formate molecules. Fig. 19 shows that the hole is mainly filled up by diffusion of molecules along the titanium rows. The bottom row of images shows the mobilization of a single formate ion by the moving formate ion front.
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[Ref. p. 389
Fig. 19. Serial topographs of a manipulated (2×1) formate layer. Before the first image was recorded the sample was scanned with a high voltage to form a hole in the covered area and the scans (a), (b) and (c) were taken 15, 26, and 35 minutes later. The small area scans show the incorporation of an isolated molecule into the migrating monolayer. (e) and (f) were recorded 296 and 666 s after (d) was recorded. Large areas: (29×28 nm2); small areas: (6.8×6.8 nm2); [96Oni1].
QMS signal [a.u.]
m/e 4 ×4 20 ×2
Table 17. The relative amount of desorbing species observed with TDS (see Fig. 20) of a (2×1) formate overlayer on TiO2(110); [94Oni1].
Temperature [K] 350 400 570
28 44 ×2 48 ×4 200
600 400 Temperature [K ]
Product DCOOD D2O D2 CO CO2 D2O D2 DCOOD
Relative amount 16 10 5 16 11 5 6 7
Fig. 20. Thermal desorption spectra of DCOOD on TiO2(110). m/e=4: D2, 20: D2O, 28: CO, 44: CO2 , 48: DCOOD; [94Oni1].
800
Thermal desorption spectra of DCOOD on TiO2(110) are displayed in Fig. 20 [01Iwa1]. These data show that the formate layer decomposes via the dehydration pathway (equation 4) as well as via dehydrogenation (equation 3) with the main desorption occurring at about 570 K. The relative intensities of the desorbing species are listed in Table 17. Iwasawa et al [01Iwa1] investigated the catalytic decomposition of DCOOD on TiO2(110) by determining turnover frequencies as a function of temperature for different pressures (see Fig. 21). The figure shows that for temperatures below ∼500 K dehydrogenation is dominant whereas at higher temperature dehydration is more important. The dehydration process was assumed to be unimolecular with an activation energy of 120 kJ/mol and a pre-exponential factor of 2×10−9 s−1. Since the dehydrogenation rate depends significantly on the gas pressure this process was assumed to be bimolecular (under the participation of another DCOOD molecule) with an activation energy of 15 kJ/mol. According to Diebold [03Die1] these processes are DCOOads
→ COgas + OformiatDads
DCOODgas + OformiatDads → DCOOads + D2Oads
(5)
for dehydration with the unimolecular decomposition of DCOOD being the rate determining step and for dehydrogenation the following process was proposed: DCOOads + DCOODgas → CO2, gas + D2, gas + DCOOads
(6) Landolt-Börnstein New Series III/42A5
Ref. p. 389]
3.9 Adsorption on oxide surfaces 10
10
CO
D2O 1
] -1 -3
1 × 10 Pa -5 5 × 10 Pa -6 4 × 10 Pa
-2
10
-2
-3
-3
1.0
-1
10
10
10
10
1.5 2.0 -1 -3 -1 T [ 10 K ]
10
1.5 2.0 -1 -3 -1 T [ 10 K ]
1.0
2.5 10
CO2
1
D2
] TOF [ s site
(CO)
-1
10
-1
]
1 × 10 Pa -1
2.5
rate = 2 f q P exp (-E2 / RT ) -1 E2 = 15 kJ mol
1 -3
-1 -1
rate = n q exp (-E1 / RT ) 9 -1 n = 2×10 s -1 E1 = 120 kJ mol
-1 -1
10
TOF [ s site
-1
TOF [ s site
-1
]
1
TOF [ s site
375
-1
10
Fig. 21. Left: Arrhenius plot for the catalytic dehydration (according to equation 4) of DCOOD on TiO2(110) at different pressures. Right: Arrhenius plot for the catalytic dehydrogenation (according to equation 3) of DCOOD on TiO2(110) at different pressures; [01Iwa1].
(D2O)
-5
5 × 10 Pa
-2
10
-2
10
-6
4 × 10 Pa
-3
10
1.0
1.5 2.0 -1 -3 -1 T [ 10 K ]
-3
2.5
10
1.0
1.5 2.0 -1 -3 -1 T [ 10 K ]
2.5
3.9.14.4 CH3COOH adsorption Similar to the case of HCOOH, CH3COOH decomposes on TiO2(110), forming a (2×1) superstructure of acetate ions. Structural properties of the adsorbate were studied with ESDIAD by Guo et al [97Guo1]. In the angular distribution of desorbing H+ ions contributions due to hydrogen bonded to the substrate and to hydrogen resulting from C-H bond rupture could be identified. It was proposed that the acetate ions bond to the fivefold-coordinated surface Ti4+ ions in a bridging geometry (bidentate) with the molecular plane parallel to the surface normal. It was also proposed that the adsorbate induced forces lead to a pairing of surface Ti4+ cations along [001]. NEXAFS investigations performed by Gutiérrez-Sosa and coworkers [01Gut3] indicate that the acetate groups stand upright on the surface with an overall twist angle of 26±5° of the molecular plane with respect to [001]. Onishi et al [96Oni3] monitored the decomposition of acetate with STM as a function of time at different temperatures. From the time dependent decrease of the acetate induced features in the STM images the authors computed a unimolecular reaction rate of (4±1)×10−3 s−1. The authors assumed decomposition via ketene formation according to CH3COOads → CH2COgas +OHads Landolt-Börnstein New Series III/42A5
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[Ref. p. 389
3.9.15 V2O3 V2O3 exhibits corundum structure (like Cr2O3, see Fig. 2) with lattice parameters of a = 5.105 Å and b = 14.449 Å for the non-primitive hexagonal unit cell [65Wyc1]. The oxidation state of the vanadium ions is 3+ which means that formally two 3d electrons are left to the vanadium ions. Thin films grown on Au(111), Pd(111) or W(110) are (0001) oriented and show good crystalline order [03Dup1, 04Sch1]. Cutting a single crystal along (0001) followed by sputtering and annealing in UHV has also been used to prepare a (0001)-oriented surface [01Tol2]. V2O3( 10 1 2 ) has been prepared by cleavage of a V2O3 single crystal in UHV [89Smi1]. A large part of the studies of V2O3 was motivated by its physical properties, especially the phase transition from antiferromagnetic insulating below 150 K to paramagnetic metallic at room temperature. This phase transforms into a paramagnetic insulating phase above 500 K [02DiM1, 70McW1, 69McW1]. Studies of the chemical activity of V2O3 surfaces are largely motivated by the use of vanadium oxide based catalysts for different reactions. Only a few adsorption studies have been performed for V2O3 surfaces. Some of them are discussed in the following. An overview is given in Table 18. Table 18. Overview of investigations of the interaction of gases with well ordered V2O3 surfaces Adsorbates Method References Substrate: V2O3(0001) H2O XPS, UPS, work function 01Tol2 Substrate: V2O3(0001)/W(110) O2 HREELS, IRAS, ARUPS, XPS, NEXAFS 03Dup1 Substrate: V2O3( 10 1 2 ) SO2 UPS, XPS 89Smi1 UPS 83Kur1 H2O, O2
3.9.15.1 O2 adsorption Dupuis et al [03Dup1] have shown that V2O3(0001) is terminated by a layer of vanadyl groups under typical UHV conditions. These groups are strongly bonded and cannot be removed thermally, but by electron irradiation. The vanadyl layer may be re-established by dosing the surface with oxygen followed by annealing. At low temperature a molecular negatively charged oxygen species was found on the surface. V2O3( 10 1 2 ) prepared by in-vacuo cleavage was found to interact strongly with oxygen [83Kur1]. It was reported that oxygen increases the surface oxidation state, possibly by forming O2–ions.
3.9.15.2 H2O adsorption Water adsorption on V2O3(0001) was studied between 180 K and room temperature [01Tol2]. Molecular adsorption was observed at 180 K for doses less than 1000 L whereas at larger doses also OH formation became obvious. At room temperature only hydroxyl formation was observed. Newer results for H2O on V2O3(0001) layers on W(110) and Au(111) [06Abu1] show that the interaction with water depends on the termination of the V2O3(0001) surface: a surface terminated by vanadyl groups does not dissociate water to form hydroxyl groups whereas a surface where the vanadyl groups have been removed prior to water adsorption dissociates water and hydroxyl groups are observed. For V2O3( 10 1 2 ) it was found that water dissociates to form hydroxyl groups on nearly perfect as well as on ion-bombarded surfaces [83Kur1].
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Ref. p. 389]
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377
3.9.16 V2O5 The unit cell of the V2O5 lattice is orthorhombic with lattice constants of a = 11.519 Å, b = 3.564 Å and c = 4.373 Å [65Wyc1]. Three different types of oxygen atoms are found in the V2O5 lattice: singly coordinated vanadyl oxygen atoms [O(1)] twofold [O(2)] and threefold bridging [O(3)] atoms (see Fig. 22). The oxidation state of the vanadium atoms is 5+ which means that formally no 3d electrons are left to the vanadium ions. All adsorption studies discussed here have been performed for the (001) surface. V2O5 cleaves easily along this plane since the lattice consists of weakly interacting planes parallel to (001). Therefore, and because no simple recipe for the preparation of well-ordered V2O5 surfaces under UHV conditions is known, most studies have been performed for cleaved single crystal surfaces. Vanadium oxides are catalytically active for a number of oxidation reactions which was the motivation for most of the performed adsorption studies. Only a few adsorption studies have been performed for V2O5 surfaces. Some of them are discussed in the following. An overview is given in Table 19. Table 19. Overview of investigations of the interaction of gases with well ordered V2O5 surfaces Adsorbates Method References Substrate: V2O5(001) C3H6 XPS, SEM 79Fie1 UPS, XPS 94Zha1 CO, SO2 Theory: extended Hückel 99Sam1 CH3OH Theory: extended Hückel 97Sam1 CH3OH oxidation Theory: molecular mechanics 00Kam1, 00Kam2 C3H8, C2H6 ARUPS, Theory: DFT 99Her3 H2, H ARUPS, HREELS, XPS 02Tep1 H2, H Theory: DFT, ZINDO 99Wit1 H, H+, C3H6, C7H8 Theory: DFT, Hartree-Fock, INDO-type 96Wit1 H2, C3H6 Theory: ZINDO/1 99Ran1 H2O TDS: poly-cristalline V2O5, Theory: 00Ran1 H2O ZINDO/1 Theory: DFT 00Yin1 NH3 Substrate: V2O5(010) CH3OH Theory: extended Hückel 99Sam1
3.9.16.1 CO and SO2 adsorption Zhang and Henrich [94Zha1] used XPS and ARUPS to study the interaction of CO and SO2 with V2O5(001) for UHV-cleaved V2O5(001) with a low density of defects and for reduced V2O5(001). Both adsorbates interact only weakly with the UHV-cleaved surface at room temperature. CO seems to induce some reduction of the surface after dosing large amounts (>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
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[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]
3.9 Adsorption on oxide surfaces
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
3.9 Adsorption on oxide surfaces
[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
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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].
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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.
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Ref. p. 389]
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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
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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] <300 180, 250, 600 (580: H2) 185 (phys. water), 210 (phys. water), 295 (chem. water), 345 (hydroxyl groups) 300 and 465: hydroxyl recombination 165-170 185-215, 265-280 (OH recombination) 240-260
Activation energy [eV]
0.92 (hydroxyl groups)
[Ref. p. 389
Notes (method, etc) References TDS, fully oxidized 00Kun1 surface TDS, reduced 00Kun1 surface TDS 00Hen1
TDS
91Cox2
0.54 TDS 0.51 (185- 215 K), TDS 0.52±0.1 (265- 285K)
02Wei1 02Wei1
0.65 TDS 0.88±0.02 (isosteric LEED heat of adsorption), 0.36±0.1 (lateral interaction energy) 140 (multilayer), 0.54 (200 K), 0.67 TDS 200 (regular sites), (210- 270 K) 210-270 (nonreg. sites) 200, 300, (435: OH TDS disprop.) 155 (multilayer), TDS 174 (bilayer), 270 (monolayer)
02Wei1 97Fer3, 96Fer2
200 (molec. HCOOH), 550 (CO, CO2) 200 (multilayer HCOOH), 370 (mol. ads. chem. HCOOH), 350- 450 (H2), 575 (CO, H2O, CO2, H2) 195 and 210 (molec HCOOH), 340, 390 and 520 (H2, CO2), 415 and 520 (CO)
TDS
94Lud1
TDS
98Yos1, 00Gra1
TDS
96Ban2
105, 340 216-220
290- 330 200, 250
98Rei1, 00Rei1, 01Sch2 95Ger1 96Hen2
0.35, 1.0 TDS, Cr term. surf. 91Xu2, 99Wil1 0.57 (low coverage), TDS 99Wic1, 99Wic2 0.12 (high coverage) TDS, Cr term. surf. 96Dil1 TDS 92She1
Landolt-Börnstein New Series III/42A5
Ref. p. 389] Substrate Adsorbate: ethylbenzene FeO(111)/Pt(111) Fe3O4(111)/Pt(111) Fe2O3(0001)/Pt(111) Adsorbate: styrene FeO(111)/Pt(111) Fe3O4(111)/Pt(111) Fe2O3(0001)/Pt(111)
3.9 Adsorption on oxide surfaces Desorption temperature [K]
Activation energy [eV]
200-210 190-210, 250-400
385
Notes (method, etc) References
0.57 0.49 (190-210 K), 0.89 (250-400 K) 200-210, 250-275, 0.52 (200-210 K), 300- 450 0.66 (250-275 K)
TDS TDS
02Wei1 02Wei1
TDS
02Wei1
∼210 ∼210, ∼300, ∼500
TDS TDS
02Wei1 02Wei1
TDS
02Wei1
∼210, 270-295, ∼400
0.52 0.52 (210 K), 0.73 (300 K), 1.22 (500 K) 0.52 (210 K), 0.76 (270-295 K), 0.98 (400 K)
Table 22. Sticking coefficients and coverages of adsorbates on different ordered oxide substrates. Substrate Sticking Saturation coverage Notes (methods etc.) Ref. coefficient Adsorbate: CO molecular beam 03Kun1 rutile TiO2(110) 0.84±0.05 (Ekin = 0.05 eV), 0.1±0.05 (Ekin = 0.57 eV) Adsorbate: D2O CeO2(001)/SrTiO3(001) 0.9 monolayers (OH XPS, OH formation 99Her1 groups) Adsorbate: H2O α-Al2O3(0001) ∼0.1 (at 300 K) 5×1014 OH groups/cm2 LITD, OH formation 98Ela1 XPS, OH formation 98Liu3 CaO( 100) ∼0.9 (at RT, θ OH≤0.8), ∼3×10−5 at higher coverage rutile TiO2(110) TDS 98Bri2 5.2×1014 H2O molecules/cm2 (monolayer) rutile TiO2(110) 3×1013 OH groups/ cm2 HREELS, TDS 96Hen2 TDS 00Hen1 α-Cr2O3(0001)/ 3-7×1014 OH α-Al2O3(0001) groups/cm2, 1.2×1015 OH groups + chem. H2O molecules/cm2 NiO(111)/Ni(111) 98Kit1 0. 85±0.1 monolayers XPS (OH groups) Adsorbate: HCOOH 0.3 monolayers XPS, HCOO+ OH 02Lin1 ZnO( 000 1 )-O (subst. ann. at 1070 K), 0.1 monolayers (subst. ann. at 1370 K)
Landolt-Börnstein New Series III/42A5
386
3.9 Adsorption on oxide surfaces
Table 23. Vibrational energies of adsorbates. Substrate Vibrational Energy [cm−1] Adsorbate: CO a-Al2O3/NiAl(100) 2074 (dose 1 L), 2065 and 2117 (dose 1.5 L) 2047 and 2027 (dose 1 L), θ-Al2O3/NiAl(100) 2047 and 2033 (dose 3 L) 1994 (dose 0.5 L), 2003 and α-Al2O3/NiAl(100) 2030 (dose 2 L) 2132-2136, 2170-2178 α-Cr2O3(0001)/Cr(110) ∼2142 CoO(100)/Co( 112 0 ) CoO(111)/Co(0001) ∼2168 MgO(100) 2152.2, 2137.2, 2132.2 [c(4×2) phase], 2150.5 [(1×1)phase] NiO(100)/Mo(100) 2156 NiO(111)/Ni(111) 2079 (not fully oxidized Ni sites), 2146 (fully ox. Ni sites) reduced RuO2(110)/Ru(0001) RuO2(110)/Ru(0001)
[Ref. p. 389
Notes (method, etc)
References
IRAS
98Hsi1
IRAS
98Hsi1
IRAS
98Hsi1
IRAS HREELS HREELS IRAS
01Pyk1 96Sch1 96Sch1 95Hei1
IRAS IRAS, SFG
94Ves1 98Mat1, 97Ban1, 99Ban1 01Wan2, 02Sei1, 03Kim1 01Wan2, 02Sei1, 03Kim1
HREELS, see Fig. 14 53.5 (CO-Ru), 234.5 (symmetric CO-Rubridge), 248.5 (asym. CO-Rubridge) HREELS, see Fig. 14 39 (CO-Ru), 262 (CO on Rucus)
Adsorbate: CO2 α-Cr2O3(0001)/Cr(110) MgO(100) NiO(111)/Ni(111)
2346- 2353 (Tdes =180 K), 2375 (Tdes =120 K), 12771289 (CO2δ−) 2334, 2308, 2306 1263 (dehydrox. surf.), (910: carbonate), 1267 (OD covered surf.)
IRAS, Cr term. surf.
99Sei1
IRAS [(2√2×2√2)R45° phase] IRAS
96Hei1, 93Hei1 99Mat1
Adsorbate: D2O α-Cr2O3(0001)/αAl2O3(0001) Adsorbate: H2O
2645 (terminal OD), 2120 (bridging OD)
HREELS
00Hen1
α-Al2O3(0001) α-Cr2O3(0001)/αAl2O3(0001) CoO(111)/Co(0001) MgO(100) NiO(111)/Ni(100)
3720 (OH groups) 3600 (terminal OH), 2885 (bridging OH) ∼3670 (hydroxyl groups) 3050-3500 (broad band) 3710 (hydroxyl groups)
HREELS HREELS
97Cou1 00Hen1
HREELS IRAS [c(4×2) phase] HREELS
rutile TiO2(110)
3690 (hydroxyl groups), 1625 HREELS and 3420- 3505 (molec. water)
95Has1 95Hei2 78And1, 95Cap1 96Hen2
Adsorbate: HCOOH NiO(111)/Ni(111), NiO(100)/Mo(100)
see Table 13
IRAS, HREELS
96Ban1, 92Tru1
Landolt-Börnstein New Series III/42A5
Ref. p. 389] Substrate rutile TiO2(110)
ZnO( 000 1 )-O
ZnO( 10 1 0 )
3.9 Adsorption on oxide surfaces
387
Vibrational Energy [cm−1] (1566 [νasym(OCO)], 1363 [νsym(OCO)], molec. plane par. [001]), (1535 [νasym(OCO)], 1393 [νsym(OCO)], molec. plane par. [ 1 1 0 ]) ∼750 [δ(OCO)], 1080 [π(CH)], 1371 [ν(OCO)], 1605 [νa(OCO)], 2928[ν(CH)] 1040 [π(CH)], 1363 [νs(OCO)], 1573 [νa(OCO)], 2895 [ν(CH)]
Notes (method, etc) IRAS, adsorbed formate
1759-1794 (Tdes =340 K), 1847- 1857 (NO dimers) ∼1813 ∼1789, (∼1650, interacting with OH groups; ∼1621, interacting with OD groups) 1772 (hydrox. surf.), 1805 (dehydrox. surf.) 1800 (dehydrox. surf) 1805 (dehydrox. surf) 1797
IRAS, Cr term. surf.
99Wil1
HREELS HREELS
96Sch1 96Sch1, 95Has1
HREELS
96Sch1
SFG IRAS HREELS
97Ban1
References 99Hay1
HREELS, adsorbed formate 97Cro1, 98Tho1 HREELS, adsorbed formate 97Cro1
Adsorbate: NO α-Cr2O3(0001)/Cr(110) CoO(100)/Co( 11 2 0 ) CoO(111)/Co(0001)
NiO(111)/Ni(111) NiO(111)/Ni(111) NiO(100)/Ni(100)
91Kuh1, 92Bau1, 93Kuh1, 96Sch1
Adsorbate: O2 α-Cr2O3(0001)/Cr(110)
1005-1012 (chromyl), 990 (O2–) IRAS, Cr term. surf.
96Dil1
Table 24. XPS binding energies and NEXAFS excitation energies. Substrate Energy [eV] Notes(method, etc) Adsorbate: CO CeO2(111)/Ru( 0001) C1s: 290.5 XPS, possibly carbonate or carboxylate NiO(111)/Ni(100) NEXAFS C1s → 2π: ∼287. 8 NiO(100)/Ni(100) NEXAFS C1s → 2π: 287. 4 ZnO(0001)- Zn C1s: 291. 8, O1s: 537. 9 XPS NEXAFS C1s → 2π: 287.7±0.2 ZnO( 000 1 )-O C1s: 288.6 XPS ZnO( 000 1 )-O C1s: 290. 8 (center, broad peak) XPS α-Cr2O3(0001)Cr(110) NEXAFS α-Cr2O3(0001)Cr(110) C1s → 2π: 286. 0, C1s → 6σ: 305 Adsorbate: CO2 MgO(100) NEXAFS C1s → π: 290.2 Adsorbate: carbonate NEXAFS C1s → 2π: 290. 4±0. 2 ZnO( 000 1 )-O C1s: 290, O1s: 532.5 XPS ZnO( 000 1 )-O Landolt-Börnstein New Series III/42A5
Ref. 99Mul1 96Sch1 95Cap2 00Bec3 99Lin2 88Au2 92Kuh2 92Kuh2
99Car1 99Lin2 88Au2
388 Substrate Adsorbate: D2O CeO2(001)/SrTiO3(001) Adsorbate: H2O α-Al2O3(0001) CeO2(111)/Ru(0001) CeO2(111)/Ru(0001) NiO(111)/Ni(111) Adsorbate: CH3OH ZnO(0001)-Zn, ZnO( 10 1 0 ) Cu2O(111)
Adsorbate: HCOOH rutile TiO2(110) ZnO( 000 1 )-O ZnO( 000 1 )-O
ZnO( 10 1 0 ) Adsorbate: CH3COOH rutile TiO2(110) rutile TiO2(110) Adsorbate: C2H5COOH rutile TiO2(110) rutile TiO2(110) Adsorbate: NO NiO(100) NiO(100)/Ni(100) NiO(111)/Ni(111)
3.9 Adsorption on oxide surfaces
[Ref. p. 389
Energy [eV]
Notes(method, etc)
Ref.
O1s: 531.6 (OH)
XPS
99Her1
O1s: 533.1±0.2 (hydroxyl groups) O1s: 531.8 O1s: 532.4 (chem. H2O), 533.1 (OH) O1s: 531. 4±0. 1 (hydroxyl groups)
XPS 97Cou1 XPS, fully hydroxylated surface 00Kun1 XPS, reduced surface 00Kun1 XPS
98Kit1
C1s: 290. 2, C1s → σ: 295.5
XPS, NEXAFS, methoxide
98Jon1
C1s: 289.5 (methoxide), C1s: 290.7 (multilayer), C1s → σ: 294.8±0.2
XPS, NEXAFS, methoxide
98Jon1
C1s resonances: 288.7 (2b2), 292 (7a1), ∼300 (8a1 + 5b1) C1s: 289.6±0.3 C1s resonances: 288.3 (2b2), 291.8 (7a1), 297.8 (8a1), 301.4 (5b1) C1s: 289.9
NEXAFS, HCOO+ OH
01Gut3
XPS, HCOO+OH NEXAFS, HCOO+OH
02Lin1 01Gut2
XPS, HCOO+OH
88Au1
288.8, ∼300 289. 4±0.2, 285.5±0.2
NEXAFS, CH3COO+OH XPS, CH3COO+OH
01Gut3 01Gut3
289, ∼300 289.4±0.2, 285.5±0.2
NEXAFS, CH3CH2COO+OH XPS, CH3CH2COO+OH
01Gut3 01Gut3
N1s: 402.8, 407.2 N1s: 403.1, 407.5 N1s → 2π: ∼406.5, N1s → σ: ∼421
XPS XPS NEXAFS
91Kuh1 91Kuh1 96Sch1
Landolt-Börnstein New Series III/42A5
References for this document 65Kle1 65Wyc1 69McW1 70McW1 76Gop1 76Hop1 77Gop1 77Hen1 78And1 78Gop1 78Ste1 79Fie1 79Hot1 80DAm1 80Gop1 80Gop2 80Say1 80Wep1 81McC1 81Pos1 82Fir1 82Gop1 83Kur1 83Zwi1 84Cla1 84Gut1 84Zwi1 85Fur2 85Gop1 87Eri1 87Rod1 87Sem1 88Au1 88Au2 88Rod1 88Rod2 89Rod1 89Smi1 89Tho1 90Voh1 90Yam1 91Cox1 91Cox2 91Kuh1
91Xu2 92Bau1
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96Fah1 96Fer2
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97Cou1 97Cro1 97Fer3 97Guo1 97Guo2 97Guo3 97Hen1 97Lin1 97Raz1 97Rom1 97Sam1 97Shu1 97Szu1 97Wan1
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99Her3 99Lin2 99Mat1 99Mul1 99Pat1
99Ran1 99Rit2 99Rit3 99Rod2 99Sam1 99Saw1 99Sei1 99Shu1 99Sor1 99Ste1 99Suz1 99Wan1 99Wic1 99Wic2 99Wil1 99Wit1 99Woo1 99Zap1 00Bec1 00Bec2 00Bec3 00Ben1 00Bri1 00Cha1 00Die1 00Eva1 00Fuk1 00Gra1 00Hen1 00Hov1 00Kac1 00Kac2 00Kam1 00Kam2 00Kaw1 00Kim1 00Kun1 00Lin1 00Mel1 00Ove1 00Per1 00Ran1 00Rei1
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00Rus1 00She1
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01Gut3
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01Iwa1 01Kim1 01Kim2 01Li1 01Mad1 01Mel1 01Ovi1 01Per1 01Pyk1 01Rod1 01Rod2 01Sas1 01Sas2 01Say1 01Sch2 01She1 01Tit1 01Tol2 01Wan1 01Wan2 01Yan1 02Bat1 02Bow1 02Cha1 02DiM1 02Dul1 02Hau1 02Kun1 02Laf1
02Lin1
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