ISSN:0300-9599    

DOI:10.1039/F198783BX013

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

Contents 3663 3669 3675 3683 3693 370 1 3709 3717 3725 3737 Normal and Abnormal Electron Spin Resonance Spectra of Low-spin Cobalt(r1) IN,]-Macrocyclic Complexes. A Means of Breaking the Co-C Bond in B12 Co-enzyme M. Green, J. Daniels and L. M. Engelhardt The Interaction between Superoxide Dismutase and Doxorubicin. An Electron Spin Resonance Approach V. Malatesta, F. Morazzoni, L. Pellicciari-Bollini and R. Scotti Biomolecular Dynamics and Electron Spin Resonance Spectra of Copper Complexes of Antitumour Agents in Solution. Part 2.-Rifamycins R. Basosi, R. Pogni, E. Tiezzi, W. E. Antholine and L. C. Moscinsky An Electron Spin Resonance Investigation of the Nature of the Complexes formed between Copper(I1) and Glycylhistidine D. B. McPhail and B. A. Goodman A Vibronic Coupling Approach for the Interpretation of the g-Value Temperature Dependence in Type-I Copper Proteins M.Bacci and S. Cannistr aro The Electron Spin Resonance Spectrum of Al[C,H,] in Hydrocarbon Matrices J. A. Howard, B. Mile, J. S. Tse and H. Morris N; and (CN); Spin-Lattice Relaxation in KCN Crystals H. J. Kalinowski and L. C. Scavarda do Carmo Single-crystal Proton ENDOR of the SO, Centre in y-Irradiated Sulphamic Acid N. M. Atherton, C. Oliva, E. J. Oliver and D. M. Wylie Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice 1,. Part 1.-The 0- Radicals Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice I,. Part 2.-The HO, Radicals J. Bednarek and A. Plonka J. Bednarek and A. PlonkaContents 3663 3669 3675 3683 3693 370 1 3709 3717 3725 3737 Normal and Abnormal Electron Spin Resonance Spectra of Low-spin Cobalt(r1) IN,]-Macrocyclic Complexes.A Means of Breaking the Co-C Bond in B12 Co-enzyme M. Green, J. Daniels and L. M. Engelhardt The Interaction between Superoxide Dismutase and Doxorubicin. An Electron Spin Resonance Approach V. Malatesta, F. Morazzoni, L. Pellicciari-Bollini and R. Scotti Biomolecular Dynamics and Electron Spin Resonance Spectra of Copper Complexes of Antitumour Agents in Solution. Part 2.-Rifamycins R. Basosi, R. Pogni, E. Tiezzi, W. E. Antholine and L. C. Moscinsky An Electron Spin Resonance Investigation of the Nature of the Complexes formed between Copper(I1) and Glycylhistidine D. B. McPhail and B. A. Goodman A Vibronic Coupling Approach for the Interpretation of the g-Value Temperature Dependence in Type-I Copper Proteins M. Bacci and S. Cannistr aro The Electron Spin Resonance Spectrum of Al[C,H,] in Hydrocarbon Matrices J. A. Howard, B. Mile, J. S. Tse and H. Morris N; and (CN); Spin-Lattice Relaxation in KCN Crystals H. J. Kalinowski and L. C. Scavarda do Carmo Single-crystal Proton ENDOR of the SO, Centre in y-Irradiated Sulphamic Acid N. M. Atherton, C. Oliva, E. J. Oliver and D. M. Wylie Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice 1,. Part 1.-The 0- Radicals Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice I,. Part 2.-The HO, Radicals J. Bednarek and A. Plonka J. Bednarek and A. Plonka

ISSN:0300-9599    

DOI:10.1039/F198783FX017

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

ISSN 0300-9599 JCFTAR 83( 5) 1 347-1 649 (1 987) 1347 355 363 369 38 1 1395 1405 1417 1427 1437 1449 1469 1477 1485 1493 507 515 53 1 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases CONTENTS Nature of Oxide-supported Copper(I1) Ions and Copper derived from Copper@) Chloride P. A. Sermon, K. Rollins, P. N. Reyes, S. A. Lawrence, M. A. Martin Luengo and M. J. Davies Study of the Electronic Structure of UBr, using X-Ray Photoelectron Spectro- scopy The Role of Electron Transfer Processes in Determining Desorption Kinetics P. J. Pomonis Aspects of Temperature-programmed Analysis of some Gas-Solid Reactions. Part 2.-Hydrogen Temperature-programmed Desorption from Silica- supported Platinum M. s. W. Vong and P. A. Sermon Potential-energy Calculations of the Mechanisms of Self-diffusion in Molecular Crystals. Part 2.-Naphthalene D.H. Smith Photogeneration of Hydrogen from Water over an Alumina-supported ZnS-CdS Catalyst J. Kobayashi, K. Kitaguchi, H. Tanaka, H. Tsuiki and A. Ueno Monolayer Adsorption of Non-spherical Molecules on Solid Surfaces. Part 1 .-Adsorption of Hard Dumb-bells on Flat Surfaces L. Lajtar, J. Penar and S. Sokolowski A Study of the Adsorption Sites on Thoria by Scanning Transmission Electron Microscopy and Fourier- t ransform Infrared Spectroscopy . Adsorption and Desorption of Water and Methanol X. Montagne, J. Lynch, E. Freund, J. Lamotte and J-C. Lavalley Infrared Spectroscopic Study of Catalytic Activity of Rh/TiO, for the CO + H, Reaction I. Mochida, H. Fujitsu and N. Ikeyama Effect of 1,4-Dioxane on the Conductance and Ion-pairing of Hydrogen Chloride in Wet and Dry Methanol Mixtures at 25 "C M.Goffredi Emulsion Polymerization of Butyl Acrylate. Kinetics of Particle Growth I. A. Maxwell, D. H. Napper and R. G. Gilbert Adsorption of Benzene on Acidified Alumina E. Garbowski and M. Primet Effect of Diphosphonates on the Precipitation of Calcium Carbonate in Aqueous Solutions A. G. Xyla and P. G. Koutsoukos Electrical and Catalytic Properties of some Oxides with the Fluorite or Pyrochlore Structure. CO Oxidation on some Compounds derived from Gd,Zr,O, S. J. Korf, H. J. A. Koopmans, B. C. Lippens Jr, A. J. Burggraaf and P. J. Gellings Characterisation of Aerosol OT-stabilised Oil-in-water Microemulsions using a Time-resolved Fluorescence Method P.D. I. Fletcher Kinetics of Pyrrole Polymerisation in Aqueous Iron Chloride Solution R. B. Bjorklund 2H and 13C Nuclear Magnetic Relaxation Studies of the Cubic Liquid-crystalline Phase I, in the Sodium Octanoate-Octane-Water System 0. Soderman and U. Henriksson Dealumination of Sodium Y Zeolite with Hydrochloric Acid E. F. T. Lee and L. V. C. Rees G. C. Allen and J. W. Tyler1539 1553 1559 1569 1579 1591 1601 1609 1621 1631 1641 Con tents Oxygen Evolution by Water Oxidation with Polynuclear Ruthenium Com- plexes R. Ramaraj, A. Kira and M. Kaneko Thermodynamic Stability of Solid Intermolecular Compounds S. R. Goates, J. Boerio-Goates, J. R. Goates and J. B. Ott Photochemical Formation of Colloidal Metals Y. Yonezawa, T. Sato, M. Ohno and H.Hada Single-ion Free Energies for Transfers to Benzonitrile derived from Solubility and Complexing Data A. F. Danil de Namor and H. Berroa de Ponce Nature of Radical Reactivity of Organic Plasma-exposed Glass Surface studied by the Electron Spin Resonance Spin-trapping Technique M. Kuzuya, S. Nakai, T. Okuda, T. Kawaguchi and Y. Yanagihara Structural Effects on the Adsorption of Alcohols on Titanium Dioxides G. Ramis, G. Busca and V. Lorenzelli The Nature of Supported-molybdena Catalysts. Evidence from a Raman and X-Ray Diffraction Investigation of Pyridine Absorption H. M. Ismail, C. R. Theocharis, D. N. Waters, M. I. Zaki and R. B. Fahim Fluorescence of Ethidium in Alcohol-Water Solutions G. Varani, G. Baldini and M. Manfredi Self-consistency of the Percolation Model as applied to a Macrofluid-like Water-in-Oil Microemulsion Photocatalytic Dehydrogenation of Liquid Alcohols by Platinized Anatase F. H. Hussein and R. Rudham Basicity of Water in Dipolar Aprotic Solvents using the 1,4,8,ll-Tetramethyl- 1,4,8,11 -tetra-azacyclotetradecane Nickel(r1) Cation as a Probe of Electron-pair Acceptance S. Yamasaki, Y. Yanai, E. Iwamoto, T. Kumamaru and Y. Yamamoto R. Hilfiker and H-F. Eicke

ISSN:0300-9599    

DOI:10.1039/F198783FP057

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

693 707 723 73 1 74 1 747 759 767 775 783 61428 61728 61907 611681 61 1850 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions II, Issue 5,1987 Molecular and Chemical Physics For the benefit of readers of Faraday Transactions I, the contents list of Faraday Transactions II, Issue 5 , is reproduced below. Concentration-modulated Absorption Spectroscopy. Part 3.-The Effect of Finite Spectral Linewidths W. J. Jones Concentration-modulated Absorption Spectroscopy. Part 4.-The Use of Con- tinuous-wave Lasers W. Mallawaarachchi, A. N. Davies, R. A. Beaman, A. J. Langley and W. J. Jones Site-selected Fluorescence Spectra of (n-C,H,),NUO,(NO,), in Poly(methy1 methacrylate) P. Brint and A. J. McCaffery Rate Constant for the Reaction OH+H,S in the Range 243-463 K by Dis- charge-flow Laser-induced Fluorescence c.Lafage, J-F. Pauwels, M. Carlier and P. Devolder Photoelectron Spectra of Benzazulenes M. Higashi, H. Yamaguchi and W. Schmidt Dynamical Scattering and the Quantification of Electron Energy-loss Spectra of Solids R. D. Brydson, J. M. Thomas and B. G. Williams Rate Constants for the Addition of Methyl Radicals to Propene R. R. Bald- win, A. Keen and R. W. Walker Chemiluminescent Reactions of Fluorine Atoms with Inorganic Iodides in the Gas Phase D. Raybone, T. M. Watkinson and J. C. Whitehead Introductory Remarks on the Concept of Interaction Pressure B. Blaive and J. Metzger Cusped-Gaussian Molecular Wavefunctions. Part 5.-Basis Sets for the Second-row Atoms E. Steiner The following papers were accepted for publication in Faraday Transactions I during February.Ionization Equilibria in Solution of Cobalt(r1) Thiocyanate in N,N-Dimethyl- formamide M. Pilarczyk, W Grzybkowski and L. Klinszporn Electron Spin Resonance Investigations of Ruthenium supported on Alumina M. Grazia, C. Sabbadini, A. Gervasini, F. Morazzoni and D. Strumolo Thermodynamic Properties of the PMMA-Acetonitrile-l,4Dioxane System I. Katime, J. Escobal and J. R. Ochoa Refractive Index and Excess Volume for Binary Liquid Mixtures. Part 1 .-An- alyses of New and Old Data for Binary Mixtures M. Nakata and M. Sakurai Structure-sensitive Chemisorption and Disproportionation of Carbon Mon- oxide on Palladium/Silica Catalysts C. L. M. Joyal and J. B. Butt6/1958 Characterization of CO-Rh Species formed on Rh-Y Zeolite by T.P.D and I.R. Techniques No Takahashi, A.Mijin, T. Ishikawa, K. Nebuka and H. Suematsu 6/2205 Electrochemical Properties of Cation Exchange Membranes A. D. Dimov and I. Alexandrova 6/2264 Hydrogen Bonding. Part 2.-Equilibrium Constants and Enthalpies of Com- plexation for 72 Monomeric Hydrogen-bond Acids with N-Methylpyrrolidin- one in 1,1,l-Trichloroethane M. H. Abraham, P. P. Duce, J. J. Morris and P. J. Taylor 6/2349 Excited States Reactivity of Photoinitiators based on the Acetophenone Nucleus J. P. Fouassier and D. J. Bougnot 6/2359 Formation of Hydrocarbons from CO+H using a Cobalt Manganese Oxide Catalyst. A 132C Isotopic Study M. van der Riet, R. G. Copperthwaite and G. J. Hutchings 6/2390 Redox Reactions with Colloidal Metal Oxides A. Harriman, M-C. Richoux, P.A. Christensen, S. Mosseri and P. Neta 6/2429 Non-isothermal Reduction Kinetics and Reducibilities of Nickel and Cobalt Faujasites G. Schulz-Ekloff, L. Czarnetzki and A. Zukal 6/2462 The Conductivity of Dilute Solutions of Mixed Electrolytes. Part 1 .-The System NaCl+BaCl,+H,O at 298.2 K H. Bianchi, H. R. Corti and R. Fernandez-Prini 6/2486 Control of Ni Metal Particle Size in Ni/SiO, Catalyst by Calcination and Reduction Temperatures H. Tamagawa, K. Oyama, T. Yamaguchi, H. Tanaka, H. Tusiki and A. Ueno Reactions of Alkenes and the Equilibration of Hydrogen and Deuterium on Zirconia R. Bird, C. Kemball and H. F. Leach lH N.M.R. of Solid Propylammonium Chloride and Bromide S. Fukada, H. Yamamoto, R. Ikeda and D. Nakamura 7/036 7/056 (ii)Cumulative Author Index 1987 Agnel, J-P. L., 225 Akalay, I., 1137 Alberti, A., 91 Allen, G.C., 925, 1355 Anderson, A. B., 463 Anderson, J. B. F., 913 Antholine, W. E., 151 Ardizzone, S., 1159 Atherton, N. M., 37, 941 Axelsen, V., 107 Baldini, G., 1609 Balbn, M., 1029 Barratt, M. D., 135 Barrer, R. M., 779 Basosi, R., 151 Bastl, Z., 51 1 Bateman, J. B., 841 Battesti, C. M., 225 Becker, K. A., 535 Berclaz, T., 401 Berleur, F., 177 Berroa de Ponce, H., 1569 Berry, F. J., 615 Bertagnolli, H., 687 Berthelot, J., 231 Beyer, H. K., 511 Bianconi, A., 289 Bjorklund, R. B., 1507 Blandamer, M. J., 559, 865 Blyth, G., 751 Boerio-Goates, J., 1553 BorbCly, G., 511 Boucher, E. A., 1269 Braquet, P., 177 Brazdil, J. F., 463 Briscoe, B. J., 938 Bruce, J. M., 85 Brustolon, M., 69 Budil, D. E., 13 Burch, R., 913 Burgess, J., 559, 865 Burggraaf, A.J., 1485 Burke, L. D., 299 Busca, G., 853, 1591 Buscall, R., 873 Cairns, J. A., 913 Carley, A. F., 351 Cassidy, J. F., 231 Celalyan-Berthier, A., 40 1 Chalker, P. R., 351 Chandra, H., 759 Chieux, P., 687 Chittofrati, A., 11 59 Clark, B., 865 Clifford, A. A., 751 Chu, D-Y., 635 Colin, A. C., 819 Coller, B. A. W., 645, 657 Coluccia, S., 477 Compostizo, A., 819 Compton, R. G., 1261 Conway, B. E., 1063 Corvaja, C., 57 Couillard, C., 125 Courbon, H., 697 Craven, J. B., 779 Crossland, W. A., 37 D’Alba, F., 267 Danil de Namor, A. F., 1569 Dash, A. C., 1307 Daverio, D., 705 Davies, M. J., 1347 Davoli, I., 289 Dawber, J. G., 771 De Doncker, J., 125 De Laet, M., 125 De Ranter, C. J., 257 Declerck, P. J., 257 Delafosse, D., 1137 Delahanty, J.N., 135 Di Lorenzo, S., 267 Diaz Peiia, M., 819 DimitrijeviC, N. M., 1193 Dodd, N. J. F., 85 Ducret, F., 141 Dudikova, L., 511 Dusaucy, A-C., 125 Eicke, H-F., 1621 Elbing, E., 657, 645 Empis, J. M. A., 43 Endoh, A., 41 1 Engberts, J. B. F. N., 865 Evans, J. C., 43, 135 Fahim, R. B., 1601 Fan, G., 323 Fatome, M., 177 Fejes, P., 1109 Fletcher, P. D. I., 985, 1493 Flint, N. J., 167 Formaro, L., 1159 Forniosinho, S. J., 431 Forrester, A. R., 21 1 Forster, H., 1109 Fraissard, J., 451 Freund, E., 1417 Fricke, R., 1041 Fujii, K., 675 Fujitsu, H., 1427 Galli, P., 853 Garbowski, E., 1469 Garrido, J., 1081 Garrone, E., 1237 (iii) Gellings, P. J., 1485 Geoffroy, M., 401 Gervasini, A., 705 Gilbert, B. C., 77 Gilbert, R. G., 1449 Goates, J. R., 1553 Goates, S.R., 1553 Goffredi, M., 1437 Golding, P. D., 1203 Gottschalk, F., 571 Gozzi, D., 289 Grampp, G., 161 Gratzel, M., 1101 Gray, P., 751 Greci, L., 69 Grieser, F., 591 Grigorian, K. R., 1189 Grossi, L., 77 Groves, G. S., 1119, 1281 Grzybkowski, W., 281, 1253 Guardado, P., 559 Guilleux, M.-F., 1137 Hada, H., 1559 Hagele, G., 1055 Hakin, A. W., 559, 865 Halawani, K. H., 1281 Hall, D. G., 967 Hall, M. V. M., 571 Halpern, A,, 219 Hamada, K., 527 Harland, R. G., 1261 Harrer, W., 161 Harris, R. K., 1055 Hartland, G. V., 591 Hasegawa, A., 759 Hatayama, F., 675 Heatley, F., 517 Hemminga, M. A., 203 Henriksson, U., 1515 Herold, B. J., 43 Hertz, H. G., 687 Hidalgo, J., 1029 Higgins, J. S., 939 Hilfiker, R., 1621 Holden, J. G., 615 Howe, A. M., 985, 1007 Howe, R. F., 813 Hudson, A., 91 Hunter, R., 571 Hussein, F.H., 1631 Hutchings, G. J., 571 Ikeyama, N., 1427 Imamura, H., 743 Imanaka, T., 665 Ismail, H. M., 1601 Ito, T., 451AUTHOR INDEX Iwaki, T., 943, 957 Iwamoto, E., 1641 Jackson, S. D., 905 Jaenicke, W., 161 Janes, R., 383 Juszczyk, W., 1293 Kakuta, N., 1227 Kaneko, M., 1539 Kanno, T., 721 Kariv-Miller, E., 1 169 Karpinski, Z., 1293 Kawaguchi, T., 1579 Kazusaka, A., 1227 Kerr, C W., 85 Kira, A., 1539 Kiricsi, I., 1109 Kitaguchi, K., 1395 Kiwi, J., 1101 Kobayashi, J., 1395 Kobayashi, M., 721 Koda, S., 527 Kondo, Y., 1089 Konishi, Y., 721 Koopmans, H. J. A., 1485 Kordulis, C., 627 Korf, S. J., 1485 Koutsoukos, P. G., 1477 Kowalak, S., 535 Kubelkova, L., 511 Kubokawa, Y., 675 Kumamaru, T., 1641 Kusabayashi, S., 1089 Kuzuya, M., 1579 La Ginestra, A., 853 tajtar, L., 1405 Lambelet, P., 141 Lamotte, J., 1417 Lavagnino, S., 477 Lavalley, J-C., 1417 Lawin, P.B., 1169 Lawrence, S. A., 1347 Leaist, D. G., 829 Lecomte, C., 177 Lee, E. F. T., 1531 Lin, C. P., 13 Linares-Solano, A., 108 1 Lindgren, M., 893 Lippens, B. C., Jr, 1485 Liu, R-L., 635 Liu, T., 1063 Loliger, J., 141 Lorenzelli, V., 853, 1591 Loretto, M. H., 615 Lund, A., 893 Lycourghiotis, A., 627, 1 179 Lynch, J., 1417 Lyons, C. J., 645 Lyons, M. E. G., 299 McAleer, J. F., 1323 McDonald, J. A., 1007 Machin, W. D., 1203 Maestre, A., 1029 Maezawa, A., 665 Korth, H-G., 95 Makela, R., 51 Manfredi, M., 1609 Maniero, A. L., 69, 57 Marchese, L., 477 Marcus, Y., 339 Mari, C. M., 705 Markarian, S. A., 1189 Martin Luengo, M. A., 1347 Martin-Martinez, J.M., 1081 Masiakowski, J. T., 893 Masliyah, J. H., 547 Matralis, H., 1179 Matsuura, H., 789 Maxwell, I. A., 1449 McCarthy, S. J., 657 McLauchlan, K. A., 29 Mehandru, S. P., 463 Merwin, L. H., 1055 Micic, 0. I., 1127 Miyahara, K., 1227 Miyata, H., 675 Mochida, I., 1427 Molina-Sabio, M., 1081 Monk, C. B., 425 Montagne, X., 1417 Morazzoni, F., 705 Morimoto, T., 943, 957 Moseley, P. T., 1323 Moyes, R. B., 905 Mozzanega, M-N., 697 Muiioz, M. A,, 1029 Nair, V., 487 Nakai, S., 1579 Nakajima, T., 13 15 Napper, D. H., 1449 Narayanan, S., 733 Narducci, D., 705 Nayak, R. C., 1307 Nazer, A. F. M., 11 19 Nedeljkovic, J. M., 1127 Nenadovic, M. T., 1127 Nomura, H., 527 Nomura, M., 1227 Norris, J. 0. W., 1323 Norris, J. R., 13 Nukui, K., 743 Nuttall, S., 559 O’Brien, A.B., 371 Ohno, M., 1559 Ohno, T., 675 Ohshima, K., 789 Okabayashi, H., 789 Okamoto, Y., 665 Okuda, T., 1579 Okuhara, T., 1213 Ono, T., 675 Otsuka, K., 1315 Ott, J. B., 1553 Pallas, N. R., 585 Parry, D. J., 77 Patrono, P., 853 Pedersen, J. A., 107 Pedulli, G. F., 91 Penar, J., 1405 Perez-Tejeda, P., 1029 Pethica, B. A., 585 Pethrick, R. A., 938 Pichat, P., 697 Pielaszek, J., 1293 Pilarczyk, M., 281 Pizzini, S., 705 Pogni, R., 151 Pomonis, P., 627 Pomonis, P. J., 1363 Primet, M., 1469 Priolisi O., 57 Puchalska, D., 1253 Purushotham, V., 21 1 Radulovic, S., 559 Raffi, J. J., 225 Ramaraj, R., 1539 Ramis, G., 1591 Rees, L. V. C., 1531 Reyes, P. N., 1347 Ritschl, F., 1041 Riviere, J. C., 351 Roberts, M. W., 351 Robinson, B. H., 985, 1007 Rodriguez-Reinoso, F., 1081 Rollins, K., 1347 Roman, V., 177 Romso, M.J., 43 Rosseinsky, D. R., 23 1, 245 Rowlands, C. C., 43, 135 Rubio, R. G., 819 Rudham, R., 1631 Sakai, T., 743 Sanchez, M., 1029 Sangster, D. F., 657 Saraby-Reintjes, A., 271 Sato, T., 1559 Saucy, F., 141 Savoy, M-C., 141 Sayed, M. B., 1149 Seebode, J., 1109 Segal, M. G., 371 Segre, U., 69 Sermon, P. A,, 1347, 1369 Seyedmonir, S., 813 Sidahmed, I. M., 439 Simonian, L. K., 1189 Smith, D. H., 1381 Soderman, O., 151 5 Sokolowski, S., 1405 Steenken, S., 113 Stevens, D. G., 29 Stone, F. S., 1237 Suppan, P., 495 Sustmann, R., 95 Suzuki, T., 1213 SvetliEid, V., 1169 Swartz, H. M., 191 Symons, M. C. R., 1, 383, 759 Szostak, R., 487 Tabner, B. J., 167 Taga, K., 789 Takaishi, T., 41 1 Tan, W. K., 645AUTHOR INDEX Tanaka, H., 1395 Tanaka, K., 1213, 1213 Tempkre, J-F., 1137 Tempest, P.A., 925 Theocharis, C. R., 1601 Thitry, C. L., 225 Thomas, T. L., 487 Thurai, M., 841 Tilquin, B., 125 Tomellini, M., 289 Tonge, J. S., 231, 245 Torregrosa, R., 1081 Toyoshima, I., 1213 Trabalzini, L., 151 Tsuchiya, S., 743 Tsuiki, H., 1395 Tsukamoto, K., 789 Turner, J. C. R., 937 Tyler, J. W., 925, 1355 Ueno, A., 1395 Ukisu, Y., 1227 Uma, K., 733 Unwin, P. R., 1261 Vachon, A., 177 van de Ven, T. G. M., 547 Varani, G., 1609 Vattis, D., 1179 Vincent, P. B., 225 Vink, H., 801, 941 Vong, M. S. W., 1369 Vordonis, L., 627 Vuolle, M., 51 Waddicor, J. I., 751 Waller, A. M., 1261 Waters, D. N., 1601 Wells, C. F., 439, 939, 1119, Wells, P. B., 905 1281 White, L. R., 591, 873 Whyman, R., 905 Williams, D. E., 1323 Williams, J. O., 323 Williams, W.J., 371 Wilson, 1. R., 645, 657 Wbjcik, D., 1253 Xyla, A. G., 1477 Yamada, K., 743 Yamamoto, Y., 1641 Yamasaki, S., 1641 Yanagihara, Y., 1579 Yanai, Y., 1641 Yonezawa, Y., 1559 Zaki, M. I., 1601 Zhang, Q., 635THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 84 Dynamics of Elementary Gas-phase React ions University of Birmingham, 14-16 September 1987 Organising Committee: Professor R. Grice (Chairman) Dr M. S. Child Dr J. N. L. Connor Dr M. J. Pilling Professor I. W. M. Smith Professor J. P. Simons The Discussion will focus on the development of experimental and theoretical approaches to the detailed description of elementary gas-phase reaction dynamics. Studies of reactions at high collision energy, state-to-state kinetics, non-adiabatic processes and thermal energy reactions will be included. Emphasis will be placed on systems exhibiting kinetic and dynamical behaviour which can be related to the structure of the reaction potential- energy surface or surfaces.The final programme and application form may be obtained from: Mrs. Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 23 Molecular Vibrations University of Reading, 15-16 December 1987 Organising Committee: Professor I. M. Mills (Chairman) Dr J. E. Baggott Professor A. D. Buckingham Dr M. S. Child Dr N. C. Handy Dr B. J. Howard The Symposium will focus on recent advances in our understanding of the vibrations of polyatomic molecules. The topics to be discussed will include force field determinations by both ab initio and experimental methods, anharmonic effects in overtone spectroscopy, local modes and anharmonic resonances, intramolecular vibrational relaxation, and the frontier with molecular dynamics and reaction kinetics.The final programme and application form may be obtained from: Mrs. Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBNTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 85 Solvation University of Durham, 28-30 March 1988 Organising Committee: Professor M. C. R. Symons (Chairman) Professor J. S. Rowlinson Professor A. K. Covington Dr I. R. McDonald The purpose of the Discussion is to compare solvation of ionic and non-ionic species in the gas phase and in matrices with corresponding solvation in the bulk liquid phase.The aim will be to confront theory with experiment and to consider the application of these concepts to relaxation and solvolytic processes. Contributions for consideration by the organising Committee are invited in the following areas: (a) Gas phase non-ionic clusters (b) Liquid phase non-ionic clusters (c) Gas phase ionic clusters (d) Liquid phase ionic solutions (e) Dynamic processes including solvolysis Abstracts of about 300 words should be sent by 31 May 1987 to: Professor M. C. R. Symons, Department of Chemistry, The University, Leicester LE17RH. Dr J. Yarwood Dr A. D. Pethybridge Professor W. A. P. Luck Dr D. A.Young THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 86 Spectroscopy at Low Temperatures University of Exeter, 13-15 September 1988 Orga nising Com m ittee: Professor A. C. Legon (Chairman) Dr P. B. Davies Dr B. J. Howard Dr P. R. R. Langridge-Smith Dr R. N. Perutz Dr M. Poliakoff The Discussion will focus on recent developments in spectroscopy of transient species (ions, radicals, clusters and complexes) in matrices or free jet expansions. The aim of the meeting is to bring together scientists interested in similar problems but viewed from the perspective of different environments. Contributions for consideration by the Organising Committee are invited. Titles should be submitted as soon as possible and abstracts of about 300 words by 30 September 1987 to: Professor A.C. Legon, Department of Chemistry, University of Exeter, Exeter EX4 4QD. Full papers for publication in the Discussion volume will be required by May 1988. (vii)~ JOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistrykhemical physics which have appeared recently in J. Chem. Research, The Royal Society of Chemistry's synopsis + microform journal, include the following: Polarizability and Conformation of Mono- and Bis-(trialky1silyl)-, -(trialkylgermyl)-, and -(trialkylstannyl)-acetylenes Krystyna Kamienska-Trela, Hanna Ilcewicz, Maria Rospenk, Maria Pajdowska, and Lucjan Sobczyk (1 987, Issue 4) Tetraphenylcyclopentadienylidene Anion Radical: Factors influencing the Ease of Formation Donald Bethell and Vernon D.of Carbene Anion Radicals from the Diazo Compounds Parker (1 987, Issue 4) Photochemical Reduction of Chromium(V1) over Cadmium Sulphide, Zinc Sulphide, and Tungsten(V1) Oxide Javier Domenech and Javier Muiioz (1987, Issue 4) lnterstructural and Structure - Reactivity Correlations in Cycloalkane Derivatives and their Hans- Solvolysis Reactions: the Limitation of Geometry - Reactivity Relationships Jorg Schneider, Ulrich Buchheit, and Gunther Schmidt (1 987, Issue 3) E.s.r. Studies on Carboxylic Esters, Part 8. E.s.r. Studies on Thioamides, Part 7. Radical Anions of 1 ,I -Dithio- and 1,1,2-Trithio-oxaIic Esters and Amides Horst Gunther and Jurgen Voss (1987, Issue 3) Is the PhCH2(N20)+ Cation likely to be a Reaction Intermediate? A Theoretical Study Peter M.W. Gill, Howard Maskill, Dieter Poppinger, and Leo Radom (1 987, Issue 2) Conformational Structure of Bipyridine Radical Cations Hans-Jorg Hofmann, Renzo Cimiraglia, and Jacopi Tomasi (1 987, Issue 2) Solid-Liquid Equilibria in the Quarternary System Monomethylhydrazine - Sodium Alexandrina Salas-Padron and Chloride - Sodium Hydroxide - Water a t 298.1 K Marie-Th6rdse Saugier Cohen-Adad (1 987, Issue 2) Gas-phase Pyrolysis of Nitroethene. Surface-promoted Formation of Nitrosoethene Helge Egsgaard and Lars Carlsen (1987, Issue 1 ) Unified Theory of Metal-ion-complex Formation Constants Paul L. Brown and Ronald N. Sylva (1987, Issue 1 ) (viii)FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division Full-day Endowed Lecture Symposium on Intramolecular Dynamics and Chemical Reactivity including the Centenary Lecture by S.A. Rice and the Tilden Lecture by M. S. Child To be held at the Scientific Societies Lecture Theatre, London on 6 May 1987 Further information from Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN Polymer Physics Group Electroactive Polymers To be held at the Geological Society, London on 14 May 1987 Further information from Dr G. C. Stevens, CERL, Kelvin Avenue, Leatherhead KT22 7SE Electrochemistry Group with the Electrochemical Technology Group of the SCI Graduate Students' Meeting To be held at imperial College, London on 10 June 1987 Further information from: Dr G. Kelsall, Department of Mineral Resources Engineering, Imperial College, London SW7 2AZ Electrochemistry Group with Macro Group UK Polymer Electrolytes To be held at the University of St.Andrews on 18-19 June 1987 Further information from Dr C. A. Vincentor Dr J. I?. MacCallum, Department of Chemistry, Universityof St. Andrews, St. Andrews KY16 9ST Gas Kinetics Group Thermally and Photochemically Activated Reactions To be held at the University of Edinburgh on 9-10 July 1987 Further information from Professor R. Donovan, Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ Division Xlth International Symposium on Molecular Beams To be held a t the Universityof Edinburgh on 13-17 July 1987 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN Industrial Physical Chemistry Group The Physical Chemistry of Small Carbohydrates (as part of the International Symposium on Solute-Solute-Solvent Interactions) To be held at the University of Regensburg, West Germany on 10-14August 1987 Further information from Dr F.Franks, Pafra Ltd, 150 Science Park, Milton Road, Cambridge CB4 4GG Industrial Physical Chemistry Group The Interaction of Biologically Active Molecules and Membranes To be held at Girton College, Cambridge on 8-10 September 1987 Further information from Dr T. G. Ryan, ICI New Science Group, PO Box 11, The Heath, Runcorn WA7 4QE Polymer Physics Group Biennial Meeting To be held at University of Reading on 9-1 1 September 1987 Further information from Dr D. Bassett, Department of Physics, Universityof Reading, Reading RG7 2AD Neutron Scattering Group Applications of Neutron and X-Ray Optics To be held at the University of Oxford on 14-15 September 1987 Further information from Dr R, K.Thomas, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 302Surface Reactivity and Catalysis Group New Methods of Catalyst Preparation and Characterization To be held at Brunel University on 14-16 September 1987 Further information from: Dr M. Bowker, ICI New Science Group, PO Box 11, The Heath, Runcorn, Cheshire WA7 4QE Colloid and Interface Science Group Polydispersity in Colloid Science To be held at the University of Nottingham on 15-16 September 1987 Further information from Dr. R. Buscall, ICI plc, Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 4QE Polymer Physics Group New Materials To be held at the University of Warwick on 22-25 September 1987 Further information from Dr M.J. Richardson, Division of Materials Applications, National Physical Laboratory, Queens Road, Teddington, Middlesex M I 1 1 OLW Division Autumn Meeting Spectroscopy of Gas-phase Molecular Ions and Clusters To be held at the University of Nottingham on 22-24 September 1987 Further information from Professor J. P. Simons, Department of Chemistry, University of Nottingham, Nottingham NG7 2RD Polymer Ph ysics Group with the Institute of Marine Engineers Polymers in a Marine Environment To be held in London on 14-16 October 1987 Dr G. J. Lake, MRPRA, Brickendonbury, Herts SG13 8NL Electrochemistry Group with the SCI Electrosynthesis To be held at the University of York on 15-17 December 1987 Further information from Dr G.Kelsall, Department of Mineral Resources Engineering, Imperial College, London SW7 2AZ Neutron Scattering Group Scattering from Disordered Systems To be held at the University of Bristol on 16-18 December 1987 Further information from: Dr R. J. Newport, Physics Laboratory, The University, Canterbury, Kent CT2 7NRFARADAY TRANSACTIONS: I SPECIAL ISSUES I from the Royal Society of Chemistry The following Faraday Transactions contain original papers from meetings and symposia organised by the Faradoy Division of the RSC. As special issues they are available as single copies and may be ordered from the addresses indicated below. Further special issues will be published in Faraday Transactions later in 1987.Faraday Transactions I Special Issue - January 1987 Electron Spin Resonance This issue contains papers presented at the 19th Annual E.S.R. Conference at Leeds in April 1986. including the first Bruker Lecture by Professor M. C. R. Symons. Price: Nan-RSC members €21.00 ($42.00) RSC members €5 00 Faraday Transactions Ii Special issue - January 1987 Structure and Reactivity of Gas-phase Ions The first eleven papers of Faraday Transaction II. 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ServicesFARADAY TRANSACTIONS: I SPECIAL ISSUES I from the Royal Society of Chemistry The following Faraday Transactions contain original papers from meetings and symposia organised by the Faradoy Division of the RSC.As special issues they are available as single copies and may be ordered from the addresses indicated below. Further special issues will be published in Faraday Transactions later in 1987. Faraday Transactions I Special Issue - January 1987 Electron Spin Resonance This issue contains papers presented at the 19th Annual E.S.R. Conference at Leeds in April 1986. including the first Bruker Lecture by Professor M. C. R. Symons. Price: Nan-RSC members €21.00 ($42.00) RSC members €5 00 Faraday Transactions Ii Special issue - January 1987 Structure and Reactivity of Gas-phase Ions The first eleven papers of Faraday Transaction II. 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Price: Non-RSC members €14.30 ($27 70) RSC members €4.00 Faraday Transactions I Special Issue - April 1986 New Electroc hemica I Sensors This special Issue of fffroday Transactions I contains papers on electrochemical sensors, the result of a symposium held at the 30th International Congress of Pure and Applied Chemistry, Manchester, September 1985. The symposium covered five main areas - fundamentals, gas sensors, ion-selective electrodes, integrated devices and biosensors. Prito: Non-RSC members €14.30 ($27.70) RSC members €4.00 Faraday Transactions II Special issue - August 1986 Molecular Dynamics and Spectroscopy Professor Alan Carrington delivered the 1985 Faraday Lecture at the Royal Institution on 10th December, 1985. 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ISSN:0300-9599    

DOI:10.1039/F198783BP059

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Furaduy Trans. I , 1987,83, 1347-1353 Nature of Oxide-supported Copper@) Ions and Copper derived from Copper@) Chloride Paul A. Sermon,* Keith Rollins, Patricio N. Reyes, Stephen A. Lawrence, Maria A. Martin Luengo and Michael J. Davies Department of Chemistry, Brunel University, Uxbridge, Middlesex UB8 3PH Cu2+ derived from aqueous CuC1, solutions and supported upon silica and anatase, but not alumina, shows asymmetric e.s.r. peaks and an absence of X.P.S. shake-up satellite peaks. Consideration is given to whether this may be attributed in part to its higher dispersion and different constrained symmetry. Divalent copper is stabilised by these oxide supports but in two different forms : one highly dispersed (with properties detectable by X.P.S. and e.s.r.) and the other poorly dispersed [as Cu,(OH),CI-type species on alumina and CuCl, on silica and anatase] characterisable by X.r.d.In respect to its effect upon the X.P.S. and reduction (but not e.s.r.) properties of supported Cu2+ species, rutile is different from anatase, a phenomenon which may be of some value in optimising activity of heterogeneous catalysts. The results cast doubt upon X.P.S. diagnosis of divalent copper. Resistivity measurements for titania-supported copper after reduction at 700 K suggest that very little of the copper may have been intercalated into the anatase support, but that on rutile such an intercalation may have been significant. This may be relevant to SMSI effects, especially at even higher reduction temperatures. -~ Studies' of many Cuz+ compounds have observed intense X.P.S.shake-up satellite peaks at kinetic energies 8-9 eV lower than the Cu 2p,,, and Cu 2p,,, core level peaks; this was not the case for Cu+ compounds and doubt2 was cast upon sample purities in earlier observations3 of copper(1) shake-up satellites. Subsequently, the intensity of X.P.S. shake-up satellites, caused by simultaneous 2p and 3d excitation,* has been deemed a useful analytical tool for differentiating Cu2+ and Cu+ species in solid^.^ The properties of Cu2+ derived from aqueous solutions of copper(I1) chloride upon several oxide supports has now been considered, together with the properties of the zero-valent Cu produced therefrom by reduction. Experimental Materials CuCl, - 2H20 (F'isons, purity 98 % ) was used unsupported and also after impregnation from aqueous solution onto y-alumina, boehmite, silica, anatase and rutile.The predried supports defined in table 1 were wetted with aqueous CuCl, solution of sufficient volume to just fill the support pore volume and were then dried in air at 393 K. Samples of supported Cu2+ so prepared are listed in table 1. Methods Temperature-programmed bulk reduction (t.p.b.r.) was carried out on pre-dried samples (0.2-0.3 g) using 6% H, in N, (BOC) as described previously.s X.P.S., e m . and X.r.d. were carried out using Kratos ES300 and ES200 spectrometers with incident 1347 45-21348 Oxide-supported Copper(r1) Ions Table 1. Samples of Cu2+ derived from CuCl, t.p.b.r. results mol H,/mol Cu; (T/K) sample wt % Cu supporta a P CUCI,.~H,O - CUl'/S- 1 3.8 Cu"/S-2 5.0 Cu"/A- 1 4.2 Cu"/A-2 5.0 CuI'/TA- 1 4.2 Cu"/TA-2 5.0 Cu1'/TA-3 5.0 Cu"/TR- 1 5.0 0.51 (705) 923 0.50 (583) 200 0.43 (598) 6373 0.48 (535) C 0.47 (588) P25 0.52 (533) - P25 0.44 (53.8) A 0.29(513j R 0.33 (573) 0.57 (838) 0.55 (641) 0.48 (733) 0.47 (678) 0.56 (753) 0.54 (623) 0.44 (643) 0.84 (603) 0.38 (633) a 923 and 200 denote Davison 923 and Degussa aerosil200 silica supports.6373 and C denote Norton SA6373 boehmite AlOOH and Degussa Alumin Oxid C y-alumina. P25, A and R denote Degussa P25 predominantly anatase titania, Tioxide anatase and Tioxide rutile supports. radiation at 1253.6 eV and 1486.6 eV and calibration with C 1s peaks, Varian E4 and Bruker ER-200-SRC e.s.r. spectrometers with samples in air, vacuum or nitrogen, and Philips diffractometers were used.Resistivity measurements on some samples of titania- supported copper were measured as described. Results T.p.b.r. results in table 1 suggest that all of the samples prepared as described above and shown in this table contain copper essentially in a divalent state, but that the precise temperature and contribution of the a and /? reduction steps vary with the nature of the support. Given the fact that all unreduced samples must, therefore, contain Cu2+, it is surprising that fig. 1 shows that, unlike unsupported CuC12-2H,0 and Cuz+ upon alumina (y-alumina or boehmite), divalent copper upon silica (CuII/S- 1) and anatase (Cu*I/TA-l) does not show significant shake-up satellites adjacent to the 2p core-level peaks. However, it is clear that Cu binding energies are lower upon silica and anatase.Nevertheless, this may arise from ligand effects as seen previously rather than a real reduction. E.s.r. responses of Cu2+ are often broad isotropic symmetrical peaks at ca. g = 2.18 [attributed to a mobile species with a distorted octahedral configuration, such as Cu(H,O);+] or asymmetric peaks with gll > g , (attributed to Cu2+ species with greater octahedral distortion), although in addition very broad signals for Cuz+ in tetrahedral states and asymmetric resonances for the trigonal-bipyramidal state have also been reported. E.s.r. spectra for these same samples are shown in fig. 2. First, these e.s.r. signals confirm the presence of divalent d9 (t&:eg) Cu2+. Secondly, the e.s.r.spectrum for Cu2+ upon y-alumina is different from thqt upon silica and anatase, with the former being similar to symmetrical peaks for mobile hydrated Cu2+, while the latter are asymmetric. This correlates with differences seen in X.p. spectra; it might suggest a greater distortion of Cu2+ octahedral symmetry on anatase and silica than upon y-alumina. Thirdly, the e.s.r. responses do not seem to be dependent upon the extent of vacuum treatment. Furthermore, fig. 3 reveals that the e.s.r. spectrum for one sampleP. A . Sermon et al. 1349 A 1 binding energy Fig. 1. Normalised X-ray photoelectron spectra of (a) unsupported CuO, (b) unsupported CuC1,. 2H,O, (c) Cu"/A-l, (d) Cu"/TA-l, and (e) CuI1/S-1 plotted as intensity us. binding energy relevant to the Cu 2p,,, core-level peak.A1 radiation at 1486.6 eV was used. Absolute binding energies for Cu 2p,,, peaks (936.1 eV for CuCI2.2H,O; 935.4 eV for Cu'I/A-l; 932.9 eV for Cu"/TA-l and 933.4eV for Cu'I/S-l) were lower upon silica and titania supports than on alumina, but for the Cu (Auger) peaks (328 eV for CuC1,.2H2O and 337.8 eV for CuII/A-l; 338.3 eV for CuI1/TA-l and 338.4 eV for CuI1/S-l) were higher for Cu2+ upon silica and titania supports. of alumina-supported Cu2+ before t.p.b.r. is unaffected significantly by programmed reduction to the point between a and B t.p.b.r. peaks. This suggests that the two t.p.b.r. peaks do not reflect reduction of the Cu2+ to Cu+ and Cu+ to Cuo, respectively, but to reduction of Cu2+ in two different states to essentially the zero-valent metal.Discussion Motschis suggested recently that the interaction of hydrated Cu2+ ions in aqueous solution with silica and titania surfaces involves the formation of a square-pyramidal surface complex, while the interaction with alumina surfaces produces an octahedrally coordinated surface complex. The differences might be judged to arise from the higher metal-oxygen bond strength in alumina or the greater solubility of alumina in the acidic aqueous CuC1, solution (with the formation of a Cu-A1 complex in solution which then1350 Oxide-supported Copper(q) Ions 7 Fig. 2. E.s.r. results obtained for Cu2+ samples [(a) CuT1/S-2, (b) Cu11/A-2, (c) Cu"/TA-2] derived from aqueous solutions of CuC1, measured in air at atmospheric pressure (-), in vacuum (----) and in air at atmospheric pressure after release of vacuum ( - .a ) at ambient temperature and constant e.s.r. conditions (0.1 G modulation amplitude; 0.5 s time constant; 500 s scanning time; 3150 G field; 2500 G scan; 13 dB power; 9.78-9.77 GHz frequency) apart from variable gain (e.g. for Cu11/S-2; 10000 in air, 63000 in uacuo, and 6300 in air after). E.s.r. responses were unchanged in shape by vacuum treatment, but the intensity of the derivative peaks for silica- and titania-supported CuII were increased by a factor of 1.6 by vacuum treatment. This increase was not subsequently lost on release of vacuum to air at ambient temperature. Width of spectra = 2500 G. absorbs). However, were this to be so, Cu2+ would be expected to absorb in the same state upon anatase and rutile.Fig. 4 shows that this is by no means the case. Thus, with respect to its effect upon X.P.S. responses of supported Cu2+, anatase appears to behave like silica, but rutile appears to behave like alumina. The surface geometries of anatase and rutile might dictate different spatial geometries for Cu2+ supported thereon which change the location of the three e, electrons by greater octahedral distortion (although CuCl, is itself greatly distorted Cu-Cl, = 298 pm; Cu-C1, = 231 pm) or might induce a tetrahedral configuration with a resultant 3d-electron rearrangement, prohibiting intense shake-up satellites. Certainly studies with CuCr,Fe,-,O, (x < 2) suggest that the shake-up satellite intensity increases as the percentage of Cu2+ in octahedral sitesP.A . Sermon et al. 1351 Fig. 3. E.s.r. results for alumina-supported Cu2+ in CulI/A-l before reduction (-), after reduction in t.p.b.r. to a temperature between its u and p peaks (----) and after reduction in t.p.b.r. to a temperature above its fi peak ( * - a ) . Results were obtained with a Varian E3 instrument operating at a sample temperature of 298 K, scan range 2500 G and microwave power 10 mW. Width of spectra = 3650 G. increases and the percentage in tetrahedral sites decreases. However, fig. 4(b) shows distorted e.s.r. responses for Cu2+ upon both anatase and rutile. The validity of this geometric model therefore remains somewhat uncertain. Nevertheless, it is clear from fig. 4(c) that anatase and rutile constrain Cu2+ in states which exhibit different reducibilities, thus the temperatures and areas of the a and B peaks are very different upon these titania supports.Conclusions First, the use of X.p.s. shake-up satellite intensity as a diagnostic tool for differentiating divalerit (from mono- and zero-valent copper) may not be always valid. Secondly, although SMSIlO may be thought of as a contamination effect with titania (and other reducible supports), it appears that there are strong support-transition-metal ion interactions in the precursor stages before reduction which may be relevant to the preparation of heterogeneous catalysts. Thirdly, an absence of X.P.S. shake-up satellite peaks for Cu2+ supported upon silica and anatase may be associated with the spatial geometry of hydroxy groups on oxide surfaces rather than the support solubility, acidity or reducibility.Such effects could only be induced in highly dispersed supported Cu2+ species. It must be noted that X-ray diffraction also revealed rather poorly dispersed copper species in all unreduced samples (e.g. average particle sizes of 69.0 nm in CuI1/A-l and 68.6 nm in CuI1/TA-l) so the molecular description of X.p.s. and e.s.r. results presented here can only apply to a fraction of unreduced supported copper in a highly dispersed form. It may be that the two t.p.b.r. peaks relate to the reduction of a monodispersed positive oxidation state copper and the reduction of a poorly dispersed copper species X.r.d. suggests that on silica and titania (both anatase and rutile) this may be closer to CuCl, but closer to Cu,(OH),Cl on aluminas.Othersll have also suggested the strongest interaction of CuCl, phases is with alumina. Impregnation is likely to produce both highly and poorly dispersed Cu2+ species simultaneously.1352 0 xide-suppo r ted Copper (11) Ions Fig. 4. (a) Normalised X-ray photoelectron spectra, (b) e.s.r. spectra and (c) t.p.b.1. of CuZf supported upon titania as anatase A and rutile R by impregnation of CuCl, from aqueous solution giving 5 wt% Cu on drying. On anatase, unlike rutile, Cu2+ shows no shake-up satellites of measurable intensity. However, e.s.r. responses reveal Cu2+ on both titania supports, which are both asymmetric as a result of distortion of the octahedral state. The areas, ratios and temperatures of a and p t.p.b.r. peaks for Cu2+ on anatase and rutile are different, suggesting a different ease of reduction of copper thereon. Samples upon anatase and rutile are Cu*'/TA-3 and Cu"/TR-l, respectively. Width of e.s.r.spectra = 5860 G.P. A . Sermon et al. 1353 Finally, it is important to note that resistivity measurements on Cu11/TA-3 and Cu"/TR-l before reduction in t.p.b.r. to 700 K and then afterwards revealed no major increase in conductivity for the the Cu-anatase system (i.e. p was 34.0 MR cm before reduction and 29.0 M a cm after reduction), but for the Cu-rutile system the conductivity of the support was increased substantially by reduction (i.e. p was above 1 GR cm before reduction and decreased to 38.1 MR cm after reduction). Titania is normally an n-type semiconductor and it is interesting to speculate whether the reduction of Cu2+ to Cuo on rutile at 70 K allows partial electron transfer (producing Cud+ with a change in the Ti3+:Ti4+ ratio and simultaneous intercalation of the copper to form the bronze Cu,TiO,).Certainly, Cu and Ag vanadium bronzes X,V,O, are known, as are other bronzes of Ti.12 This throws a new light on titania-supported catalysts and their ability to hold metals in a partially monodispersed and positive oxidation state. It also highlights a further profound difference between Cu-anatase and Cu-rutile interactions. Reduction or heating in an inert atmosphere at higher temperatures may have increased these differences still further. There is much uncertainty and interest in the locations and nature of Cu reaction centres in, for example, CH30H synthesis catalysts;13 the parameters controlling the state of catalyst precursors and their reactivity seen here may be of value in optimising such catalysts and deducing their state in situ.They are the subject of further study.14 The authors gratefully acknowledge the support of K . R. by the S.E.R.C. and ICI Mond Division, the support of S. A. L. by Mobil, the provision of a fellowship for P. N. R. by the British Council and the provision of study leave for M. A. M. L. by the Consejo in Madrid. References 1 D. C. Frost, A. Ishitani and A. C. McDowell, Mol. Phys., 1972,24,861; S . I. Vetchinkin, S. L. Zimont, M. S. Loffe and Yu. Borodko, Khimiya, 1984, 3, 635; P. G. Hall, R. A. Hann, P. Heaton and D. R. Rosseinsky, J.Chem. Soc., Faraday Trans. I, 1985,81, 69. 2 S. Evans and M. W. Roberts, Faraday Discuss. Chem. Soc., 1974, 58, 126. 3 T. Novakov, Phys. Rev., 1971,38, 2693; G. Schon, Sur- Sci., 1973, 35, 96. 4 A. Rosencwaig, G. K. Wertheim and H. J. Guggenheim, Phys. Rev. Lett., 1971, 27, 479. 5 T. L. Barr, J. Phys. Chem., 1978, 82, 1801. 6 J. L. Falconer and J. A. Schwarz, Catal. Rev. Sci. Eng., 1983, 25, 141; N. D. Hoyle, P. H. Newbatt, 7 S. A. Lawrence, K. Mavadia, P. A. Sermon and S. Stevenson, Proc. R. SOC. London, Ser. A , in press. 8 T. Ichikawa and L. Kevan, J. Phys. Chem., 1983,87,4433. 9 H. Motschi, Colloids Surf., 1984, 9, 333. 10 S. J. Tauster, S. C. Fung and R. L. Garten, J. Am. Chem. Soc., 1978, 100, 170; Studies in Surfuce Science and Catalysis 12, ed. B. Imelik, C.Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G. A. Martin and J. C. Vedrine (Elsevier, Amsterdam, 1982); G. C. Bond and R. Burch, in Catalysis (Specialist Periodical Report, Royal Society of Chemistry, London, 1983), vol. 6, p. 27; M. S. Spencer, J. Phys. Chem., 1984, 88, 1046; R. W. Joyner, J. B. Pendry, D. K. Saldin and S. R. Tennison, Surf. Sci., 1984, 138, 84; J. B. F. Anderson, J. D. Bracey, R. Burch and A. R. Flambard, 8th Znt. Congr. Catal. (Verlag Chemie, Berlin, 1984), vol. V, p. 11 1 ; M. A. Vannice and P. Chou, 8th Znt. Congr. Catal. (Verlag Chemie, Berlin, 1984), vol. V, p. 99. K. Rollins, P. A. Sermon and A. T. Wurie, J. Chem. Soc., Faraday Trans. 1, 1985,81, 2605. 11 E. M. Forlini, C. L. Garcia and D. E. Resasco, J. Catal., 1986, 99, 12. 12 J. Van den Berg, J. H. L. M. Brans-Brabant, A. J. Van Dillen, J. C. Flach and J. W. Geus, Ber. Bunsenges. Phys. Chem., 1982, 86, 43; J. Van den Berg, J. H. L. M. Brans-Brabant, A. J. Van Dillen, J. W. Geus and M. J. J. Lammers, Ber. Bunsenges. Phys. Chem., 1983, 87, 1204; P. A. Sermon, in Chemical Reactions in Organic and Inorganic Constrained Systems, ed. R. Setton (Reidel, Dordrecht, 1986), p. 341. 13 G. C. Chinchen and K. C. Waugh, J. Catal., 1986,97,280; T. H. Fleisch and R. L. Mieville, J. Catal., 1986, 9, 284. 14 S. A. Lawrence and P. A. Sermon, Spectrochim. Acta, Part B, submitted for publication. Paper 61126; Received 18th January, 1986

ISSN:0300-9599    

DOI:10.1039/F19878301347

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

.I. Chem. SOC., Faraday Trans. 1, 1987, 83, 1355-1361 Study of the Electronic Structure of UBr, using X-Ray Photoelectron Spectroscopy Geoffrey C. Allen"? Laboratoire de Radiochimie, Institut de Physique Nuclkaire, BPI, 91406 Orsay, France Jonathan W. Tyler Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB X-Ray photoelectron spectra of uranium and bromine core levels and valence bands have been recorded from UBr, using a Kratos ES300 electron spectrometer. The spectra are compared to their counterparts recorded from UF,, UCl, and UO,. The results obtained are discussed as a function of metal ionicity, covalency of the metal-ligand bond and the participation of 5f electrons in bonding. The satellite structure associated with the principal core levels in the early lanthanide and actinide halides are compared to the equivalent satellites observed in transition-metal halides and their origins are discussed.The uranium halides are of commercial importance for nuclear fuel fabrication and reprocessing cycles. In recent years the electronic structure of these compounds as elucidated by ultraviolet and X-ray photoelectron spectroscopy has attracted consid- erable attention.l-lO Interest is centred on the role of the U 5felectrons in compound formation and the nature of the uranium-halogen bond. Binding energies recorded for core and valence-band photoelectron peaks vary over the range k0.5 eV. This may be attributed to the sensitivity of the solids to atmospheric attack and the variety of techniques used in handling samples and calibrating the energy levels measured. Here we report core and valence-band spectra recorded from UBr, and compare the results with those obtained from UF,, UCl, and U0,.8v lo, l1 Experiment a1 X-Ray photoelectron spectra were obtained from a Kratos ES300 spectrometer using A1 Ka radiation at 300 W.All spectra were recorded in the fixed analyser transmission mode with a pass energy of 65 eV and source and collector slit widths set to 1.8 and 3.0 mm, respectively. Measurements were made using a Digital micro PDPl 1 computer and Kratos DS800 software. The UBr, sample was prepared at the Laboratoire de Radiochimie, Institut de Physique Nucleaire, Universite Paris Sud, Orsay, France. The polycrystalline sample was cut to generate a fresh surface suitable for analysis and was mounted on a specimen probe using double-sided adhesive tape.Samples were handled and prepared under argon in a polythene glove box attached to the preparation chamber of the spectrometer in order to prevent atmospheric contamination and to provide radiolytic containment. The spectra recorded showed no significant oxygen contamination. Pure copper foil and the known 233.0 eV difference between A1 Ka and Mg Ka radiation was used to calibrate the binding-energy scale.l2* l3 During the measurements the sample acquired a surface Permanent address: Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucester GL I3 9PB. 13551356 Electronic Structure of UBr, Table 1. Binding energies (eV) of core and valence electrons in UBr," level binding energy/eV 782.2 739.5 391.5 380.7 105.2 97.4 29.1 19.2 4.7 2.7 256.2 189.4 282.8 69.5 16.2 a Values are calibrated against the C 1s peak at 285.0 eV (E Au 4f7,2 at 84.0 eV).charge of 6-7 eV and therefore the energy scale was referenced to traces of adsorbed carbon. The binding energies presented in table 1 are averaged best estimates from several experiments using a C 1s value of 285.0 eV. The problem of selecting a suitable reference energy has always been a difficult one. Two methods are commonly used.', The first involves the use of adventitious carbon contamination within the spectrometer which we have chosen here. The second involves the evaporation of a thin layer of gold onto the sample so that binding energies may be referenced to the Au 4f712 line.Both techniques have disadvantages. In the case of carbon, the peak is often small and its position may vary with the substrate in question. Values have been recorded ranging from 284.7eV on noble metals to 285.6eV on oxidised magnesium.15 For gold the binding energy for the deposited layer may vary with coverage owing to extra-atomic relaxation effects between the layer and the substrate16-18 and chemical interaction with substrates such as halides and ~yanides.l~-~l Here we favour the use of adventitious carbon and have referenced the C 1s binding energy to the value 285.0 0.15 eV recorded from a clean UO, surface which had been left in the spectrometer for periods up to several days. The reproducibility of the present results is exemplified by the fact that even after 4 days exposure to the vacuum within the spectrometer, the spectrum recorded was virtually identical to that obtained immediately after insertion into the apparatus, the only difference being a very small increase in the very weak peaks associated with the 0 1s and C 1s levels. Results and Discussion Photoelectron spectra for the U 4fand valence-band regions from UBr, are shown in fig.1 and 2, respectively. These regions are compared with the corresponding spectra recorded from UF,, UCl, and UO, in fig. 3 and 4. Binding energies for the uranium and bromine photoelectron peaks are presented in table 1. Through a comparison with previous X-ray photoelectron studies of uranium halides1° and oxides,ll it is evident from fig.1 and 3 that a single 'shake-up' satellite is present situated at 6.1 eV to the high-binding-energy side of each uranium core level peak. The intensity of this satelliteG. C. Allen and J . W. Tyler 1357 380.7 391.7 1 c U L - 3 6.1 eV 6.leV 1 I I I 41 0 400 390 380 binding energy/eV Fig. 1. The U 4fphotoelectron spectrum of UBr, recorded using A1 Ka X-rays. F.w.h.m. U 4f5,2, 7,2 = 2.3 eV. u- B r 4 p I I I I I 40 30 20 10 0 binding energy/eV Fig. 2. The valence band photoelectron spectrum of UBr, recorded using A1 Ka X-rays. in the UBr, spectrum is ca. 45% of the parent peak. In contrast, however, the very much weaker but broader satellite situated ca. 15 eV to the high-binding-energy side of the U 4f;,, core peak is barely discernible in the UBr, spectrum, but is more easily observed in the corresponding fluoride compound.These higher-energy satellites were thought to be due largely to energy loss of the photoelectron on its way through the solid.3> Five main features are identified in the valence-band spectra of UBr, (fig. 2). The most intense band, centred at 4.7 eV, may be assigned to photoemission from orbitals which1358 Electronic Structure of UBr, " O L I I I I I I 420 410 400 390 380 37 binding energy/eV Fig. 3. The U 4fphotoelectron region of UF,, UCl,, UBr, and UO, recorded using A1 Ka X-rays. are mainly Br 4p in character (with some U 5 f , 6d and 7s admixture). We will refer to this entire structure as the 'bonding band' but identify separately the well defined shoulder to the low-energy side at ca.2.7 eV which we attribute to ionisation from the U 5flevel. At higher energy in the region of 20 eV, the broad band is in fact considered to be a poorly resolved doublet comprising photoionisations at 19.2 and 16.2 eV from the U 6p3,2 and Br 4s levels, respectively. The weak peak at 29.1 eV is associated with the U 6p1,2 level. Previously it was argued that X-ray photoelectron binding energies and peak intensities for uranium halides and oxides could be divided into two main categories.1° The first included binding-energy measurements from the U 4f and U 5f levels, which may be influenced by the effect of both covalency and Madelung potential. The second included data derived from peak separations and intensities such as the satellite-metal 4f peak separation and the corresponding ratio of their intensities, the U 5f 'bonding band' separation and intensity ratio and the intensity of the U Sfpeak itself.These should be independent of lattice effects and might be expected to reflect the covalency of the metal-ligand bond directly. These values for UBr, are presented in table 2 . For comparison, data from UF,, UCI, and UO, are also included.G. C. Allen and J . W. Tyler 1359 1 1 I I 15 10 5 0 ‘binding energy/eV Fig. 4. The valence band region of UF,, UCI,, UBr, and UO, recorded using A1 Ka X-rays. It is evident that the results for UBr, follow the trends observed previ~usly.~. lo As the electronegativity of the ligand is diminished the U 5fpeak height decreases with respect to the U 4f core levels and the U 5f ‘bonding band’ separation also decreases as does the binding energy of the U 4f core levels.In addition the satellites associated with the U 4f levels approach the parent peaks more closely in UBr, and their relative intensity is increased. All of these phenomena may be taken as an indication of increasing covalent character in the metal-ligand bond along the series UF,-UBr, and a progressively greater participation of the metal 5f level in bonding orbitals. The results are in good agreement with those of Thibaut et ~ l . , ~ who studied many different uranium halide and oxyhalide compounds. However, considerable variation exists in the U 4f,,2 binding energy values reported for the uranium tetrahalides. These are summarised in table 3.As noted earlier, this variation reflects the different sample handling and energy reference methods employed. During the past decade much attention has been paid to the observation of satellite structure in photoelectron spectra, particularly in compounds of the transition metals. However, the actinides are equally fascinating from this point of view. One important manifestation of the incorporation of oxygen in the fluoride lattice during the oxidation of UO, was the change in satellite structure and we have therefore paid special attention to the origin of these features in compounds of the actinide metals. Satellite behaviour appears to be very similar in the early lanthanide and actinide elements. Just as we have noted a progressive increase in the metal 4fcore-level satellite1360 Electronic Structur,? of UBr, Table 2.Summary of main peak binding energies and intensities in UF,, UCl,, UBr, and UO, compound UF, UCl, UBr, UO, U 4f7/2 binding energy/eV 382.8 381.5 380.7 380.2 U 5f binding energy/eV 3.2 2.7 2.7 1.5 ‘bonding band’ to U 5f peak 5.3 2.9 2.0 4.3 U 4f satellite-to-main peak 0.24 0.26 0.45 0.25 U 4f satellite separation 6.8 5.9 6.1 6.8 U 5f to U 4f intensity ratio 0.017 0.015 0.010 0.014 separationlev intensity ratio from main peak/eV Table 3. Binding energies (eV) of U 4f77/2 core electrons in UF,, UC1, and UBr, 382.8 684.5 381.5 380.7 10, this work 382.2 684.7 380.2 379.9 7b 1 382.7 684.8 3b 382.2 684.7 5c 382.5 (5) 684.6 - gd 382.3 684.6 2 - - - - - - - - - 381.5 - aValues are calibrated against the C 1s peak at 285.0eV (=Au 4f77/2 at 84.0 eV).Referenced to Au 4fiI2 = 83.8 eV, values increased by 0.2 eV. Referenced to C 1s = 284.8 eV, values increased by 0.2 eV. Referenced to F 1s = 684.6 eV. intensity along the series UF, to UBr,, so Weber et aZ.22 have reported the same effect in 3d photoelectron spectra from the compounds LaF, and LaBr,. Their molecular orbital calculations using the multiple scattering Xa method also indicate a greater covalency between the ligand and central metal ion for LaBr,. This is especially interesting when the recent results of de Boer et ~ 1 . ~ ~ are taken into account. For a similar series of scandium compounds these authors noted a general decrease in intensity from ScF, to ScI, and a similar trend for TiF, to TiI,. These satellites were assigned to exciton satellites which arise when the vacancy in the core level following photoelectron emission is screened by polarisation of the ligands. Such polarisation in turn induces an interaction between ligand p and s levels and satellites may arise from intra-atomic excitations on the ligand from a ligand p to an s orbital.In contrast, the opposite behaviour observed in our study may be attributed to screening of the core hole by the transfer of charge towards the metal atom. Thus the charge-transfer satellites observed may be described by transfer of valence electrons from a neighbouring atom to low-lying localised levels on the cation. Thus it would seem that though the exciton model may be appropriate for the early 3d metal compounds the strong satellites in many other transition-metal compounds, the compounds of the rare earths and those of the actinides, where a higher degree of covalency is present in the metal-ligand bond, are best described by the charge-transfer model.G.C. Allen and J . W. Tyler 1361 Conclusions For the X-ray photoelectron spectrum of UBr, a detailed study has been made of the U 5fpeak intensity, U 5f bonding band separation, U 4f binding energies and satellite- metal 4f peak separations and intensities. The measurements confirm that these parameters serve as a sensitive indicator of the nature of the metal-ligand bond in uranium compounds. They indicate a progressively greater participation of the outermost U 5J’levels in bonding orbitals along the series: UF, > UO, > UCl, > UBr,. The ‘ shake-up’ satellite structure associated with the U 4f photoelectron levels in uranium compounds has proved a valuable aid in the characterisation of oxide fuel.The present study identifies the importance of this feature in halide compounds. For the actinide elements such satellites are best described using a model in which screening of the core hole occurs by the transfer of anion electrons to the metal ion. This work was carried out at the Laboratoire de Radiochimie, Institut de Physique Nucleaire, Orsay, France, and at the Berkeley Nuclear Laboratories of the Technology Planning and Research Division and the paper is published with permission of the Central Electricity Generating Board. One of us (G. C. A.) thanks Professors Genet and Guillaumont and their colleagues at Orsay for their hospitality and the privilege of discussion with them during the preparation of this paper.References 1 D. Chadwick, Chem. Phys. Lett., 1973, 21, 291. 2 C. Miyake, H. Sakurai and S. Imoto, Chem. Phys. Lett., 1975, 36, 158. 3 J. J. Pireaux, N. Mirtensson, R. Didriksson, K. Siegbahn, J. Riga and J . J. Verbist, Chem. Phys. Lett., 4 J. J. Pireaux, J. Riga, E. Thibaut, D. Tenret-Noel, R. Caudano and J. J. Verbist, Chem. Phys., 1977, 5 W-Y. Howng and R. J. Thorn, Chem. Phys. Lett., 1979, 62, 57. 6 G. Thornton, N. Edelstein, N. Rosch, R. G. Egdell and D. R. Woodwark, J. Chem. Phys.. 1979, 70, 7 E. Thibaut, J-P. Boutique, J . J. Verbist, J-C. Levet and H. Noel, J. Am. Chem. Soc., 1982, 104, 5266. 8 J. M.Dyke, G. D. Josland, A. Morris, P. M. Tucker and J. W. Tyler, J. Chem. Soc.. Faraday Trans. 9 L. D. Trowbridge and H. L. Richards, Surf. Interface Anal., 1982, 4, 89. 10 G . C. Allen, P. M. Tucker and J . W. Tyler, Phifos. Mag. B, 1983,48, 63. I 1 G. C. Allen, I. R. Trickle and P. M. Tucker, Philos. Mag. B, 1981, 43, 689. 12 S. Evans, in Handbook of X-ray and Ultrariolet Photoelectron Spectroscopy, ed. D. Briggs (Heyden, 13 M. T. Anthony and M. P. Seah, Surf. Interface Anal., 1984, 6, 95. 14 P. Swift, D. Shuttleworth and M. P. Seah, in Practical Surface Analysis by Auger and X-Ra? Photo- 15 P. Swift, Surf. Interface Anal., 1982. 4, 47. 16 D. S. Urch and M. Weber, J . Electron Spectrosc. Relat. Phenom., 1974, 5, 791. 17 Y. Uwamino and T. Ishizuka, J. Electron Spectrosc. Relat. Phenom., 1981, 23, 55. 18 S. Hohiki and K. Oki, J . Electron Spectrosc. Relat. Phenom., 1985, 36, 105. 19 D. Betteridge, J. C. Carver and D. M. Hercules, J. Electron Spectrosc. Relar. Phenom.. 1973, 2, 327. 20 L. J. Matienzo and S. 0. Grim, Anal. Chem., 1974, 46, 2052. 21 V. 1. Nefedov, Ya. V. Salyn, G. Leonhardt and R. Scheibe, J. Electron Spectrosc. Relat. Phenom., 1977, 10, 121. 22 J. Weber, H. Berthou and C. K. Jargensen, Chem. Phys., 1977, 26, 69. 23 D K. G. de Boer, C. Haas and G. A. Sawatzky, Phys. Rev. B, 1984, 29,4401. 1977, 46, 2 15. 22, 113. 52 18. 2, 1981, 77, 1273. London, 1977), p. 130. electron Spectroscopy, ed. D. Briggs and M. P. Seah (John Wiley, Chichester, 1983), p. 437. Paper 6/ 1440; Received 18th July, I986

ISSN:0300-9599    

DOI:10.1039/F19878301355

出版商:RSC    

年代:1987

数据来源: RSC

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J. Chem. SOC., Faraday Trans. I, 1987,83, 1363-1367 The Role of Electron Transfer Processes in Determining Desorp tion Kinetics Philip J. Pomonis Department of Chemistry, University of Ioannina, Ioannina 45332, Greece The desorption step of heterogeneous processes taking place on solid-gas interfaces may be influenced by the bulk conductivity of the solid. This might be seen in systems where, during adsorption, negative charge is transferred to adsorbed species, leaving a positive hole. If the n-type conduction of the solid is high, then this hole may be filled by an electron, not from the adsorbed species but from the bulk of the solid. Desorption is thus shown to be a second-order process necessitating, apart from the adsorbed species, electronically vacant sites. Application of transition-state theory shows that the second-order desorption of monomolecular processes should appear at sufficiently low pressures, while at higher ones normal first-order kinetics applies.In a typical catalytic process, taking place on a solid surface, the net reaction contains the three necessary and distinct steps of adsorption, reaction and finally desorption. For a monomolecular decomposition reaction this consequence can be represented as follows. Adsorption H eac tio n Desorption A I I I A(&+-VS--+-VS- A X -vs- +- -vs- X I I I I I -vs- +X(g) + -vs- I I where VS stands for a vacant surface site. The sequence of reactions (1 a), (1 b) and (1 c ) does not tell us anything about the charges transferred to, or from, the solid. A more detailed representation of the same process should take these effects into account.Thus for a reaction where electrons are transferred from the solid to the adsorbed molecule during adsorption, reactions (1 a), (1 b) and (1 c ) can be written as follows. Adsorption A- t I I A(g) +-VS- + -EVS- 1363Electron Transfer Processes in Desorption Kinetics 1364 Reaction Deso rp t ion A- X- -EVS- -+ -EVS- I I 1 I X- I -EVS-+X(g)+-VS- I I A- T X- T Fig. 1. The species A adsorbed on the surface as A- remains adsorbed on the surface as X- (11). Then (I) may yield the product X, the last of which X- may desorb as X(g) by the return of the electron to the electronically vacant site (111) if this site has not been filled’by-electrons from the bulk of the solid (IV). In the above scheme EVS stands for an electronically vacant surface site and is a hole left behind by an electron transferred to adsorbed molecule A.Step (2c) involves the transfer of an electron from X-, which represents the product of the reaction, to the EVS, which is then transformed to a VS. However, if the n-type conductivity of the solid is high the EVS might be filled by an electron from the bulk of the solid. As a result step (2c) will be prohibited (fig. 1) and species X will be trapped on the surface.l The purpose of this paper is to examine the consequences of this effect. The discussion which follows is not affected significantly if the reaction is diatomic, as the majority of heterogeneous catalytic reactions usually are, and the charge is transferred to one or both molecules.However, for simplicity we shall refer to a typical monomolecular reaction, e.g. the decomposition of N,O, which has been studied extensively.2yP. J. Pomonis 1365 Discussion Many studies of the decomposition of N,O on different solid catalysts have shown that the rate-determining step is the desorption of oxygen,’. written as (3) 20-(ads) + 2EVS -+ O,(g) + 2VS or 0-(ads) + EVS + N,O(g) --+ VS + (N, . . . O,)(g) -+ VS + N,(g) + O,(g). (4) The desorption can be written in a general way so as to contain the ‘transition complex ’ : where X# stands for the intermediate (transition) complex transformed without con- sumption of energy to the final product X(g). Then, according to transition-state theory (TST),4 we obtain the following expression for the rate of desorption Rd from reaction A-(ads) + EVS -+ X # (g) + VS ( 5 ) ( 5 ) : (6) R d = CA(ads) cEVS f i f V S exp (- E;/RT) cvs h f*(ads)fEVS where Cvs and C,,, refer to the concentration of vacant and electronically vacant surface sites while the f are partition functions having their customary significance.Eqn (6) may be compared with the expression describing, according to TST, the rate of desorption, which equals the rate of formation of the transition complex for monomolecular processes :4 A(ads) --+ Xf(ads). (8) Eqn (6) and (7) are similar, apart from the terms C,, and C,,, and the corresponding partition functionsf,, andf,,,. However, eqn (6) should be nearer to reality since it takes into account the necessity that desorption requires EVS. The question arising now is, on what does the CEVS depend? We believe that the main factor affecting it is the conductivity of the solid.Namely, for the system described by reactions 2(a)-(c) and shown in fig. 1, if the n-type conducting on of the solid tends to zero, as in a perfect insulator, then step (IV) in fig. 1 is totally avoided and the EVS created by step (I) in the same figure has the unique possibility of being filled by the electron returning to it from the adsorbed species X-. This will be done to an extent which, among other things, depends on the activation energy of desorption E:. If on the other hand the n-type conductivity CT, of the solid is high, step (IV) should prevail and therefore the rate of desorption, which is equal to the rate of the process, should tend to zero.These arguments can be put formally into theory by setting cEVS = CA(ads) d o n ) (9) where g(an) is a conductivity function such that for small an, g(on) -+ 1 while for large on, d o n ) -+ 0. Substitution of eqn (9) into eqn (6) yields exp (- E:/RT). ‘k(ads) d o n ) kT - f # f V S R, = cVS fA(ads)fEVS1366 Now we can consider that Electron Transfer Processes in Desorption Kinetics CA(ads) = GSO and c,, = C&( 1 - 0) (12) where 0 is the fractional coverage and Cvs is the initial concentration of vacant sites. Substitution of eqn (1 1) and (12) into eqn (10) provides kT fifvs exp (- E:/RT). o2 Rd = CVS -don) ' fA(ads1fEVS Then by using the Langmuir isotherm we obtain For sufficiently small values of pressure KP 6 1, and eqn (14) is written as Rd = C&(KP)'g(a,) kT f i f v s exp (- Eg/RT).fA(ads)fEVS Conversely, for high values of pressure KP + 1, and eqn (14) is reduced to the form In other words we observe that the general equation (14) describes kinetics of second order at sufficiently low pressures, but kinetics of first order at relatively higher pressures. This result is similar to that described in the theory of homogeneous monomolecular reactions of Lindeman.5 However, in order to derive it for surface reactions we have introduced the term for conductivity of the solid, and especially the EVS which corresponds to the second atom, necessary for collision, in Lindemann's theory. In spite of our efforts we have not been able to find in the relative literature clear experimental evidence corresponding to the transition from first- to second-order kinetics, as the pressure decreases, in monomolecular heterogeneous reactions.It might be the case that the observation of such a transition could be restricted by factors similar to those operating in homogeneous This matter requires further attention. A second point about the general equation (14) is that under constant pressure, P, and temperature, T, and considering that the values of K and f are approximately constant, it is reduced to the form Eqn (1 7) indicates that, according to the definition given above, as the n-type conductivity 0, of the system increases, then g(a,) decreases and the rate of desorption should drop. In this respect the observation* that p-type semiconductors are good catalysts for the N20 decomposition, whereas n-type materials are not, fits well with the above discussion.The holes created by step (I) in fig. 1 should be filled much more easily in an n-type material than in a p-type one, although their net conductivity due to n- and p-type carriers might be of the same magnitude. We also believe that the results described in ref. (I) may also be explained according to this model. Namely, in this work some doped insulators of the form Al,-,Cr,O, and MgAI,-,Cr,O, were examined with regard to different catalytic reactions. For small values of x (x 5 0.2) the material showed a low conduction (1 0-lo R-l cm-l), while the catalytic activity increased almost linearly with the degree of doping, x. This means that g(a,) z 1, so that eqn (1 7) is written as Rd = kc& eXp ( - E : / R T ) .(18)P. J . Pomonis 1367 This shows that the activity depends only on C&, which corresponds to doped active centres (x). For large values of x(x 2 0.4) the material showed a stabilized conductivity of ca. W1 cm-l, while the activity again increased almost linearly with doping. This may also show that the term g(a,) was stabilized between its two extreme values (zero and unity) and therefore that an equation similar to eqn (18) was again operating. Finally, in the region 0.2 < x < 0.4 the conduction of the solid showed a sharp drop by ca. five orders of magnitude, while the activity dropped by ca. one order of magnitude. This phenomenon could also be explained according to eqn (17). A similar explanation may also be proposed for the system La,Cu,-,Ni,O, examined recentlyg in the decomposition of N,O. References 1 P. Pomonis and J. C. Vickerman, J. Catal., 1978, 55, 88. 2 A. Cimino, Chim. Ind. (Milan), 1974, 56, 27. 3 J. C. Vickerman, in Catalysis, ed. C. Kemball and D. A. Dowden (Specialist Periodical Reports, the 4 J. M. Thomas and W. J. Thomas, Introduction to the Principles of Heterogeneous Catalysis (Academic 5 F. A. Lindeman, Trans. Faraday SOC., 1922, 17, 597. 6 Theory of Kinetics, ed. C. H. Bamford and C. F. Ripper, vol. 2 of Comprehensive Chemical Kinetics 7 K. J. Laidler, Reaction Kinetics (Pergamon Press, Oxford, 1963), vol. 1. 8 R. P. De, F. S. Stone and R. F. Tilley, Trans. Faraday SOC., 1953,49, 201. 9 K. V. Ramanujachary and C. S. Swamy, J. Catal., 1985, 93, 279. Chemical Society, London, 1979), vol. 2, p. 107. Press, London, 1967). (Elsevier, Amsterdam, 1969). Paper 6/289; Received 10th February, 1986

ISSN:0300-9599    

DOI:10.1039/F19878301363

出版商:RSC    

年代:1987

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J . Chem. SOC., Faraday Trans. I , 1987,83, 1369-1 380 Aspects of Temperature-programmed Analysis of some Gas-Solid Reactions Part 2.-Hydrogen Temperature-programmed Desorption from Silica-supported Platinum Mariana S. W. Vong and Paul A. Sermon" Department of Chemistry, Brunel University, Uxbridge, Middlesex UB8 3PH Well resolved temperature-programmed desorption spectra have been ob- tained for hydrogen preadsorbed upon silica-supported platinum; these reveal three maximum rates of desorption (B1 at Tmaxl, p2 at Tmaxz and p3 at Tmax3). The total extent of desorption of hydrogen exceeds the amount of hydrogen observed to chemisorb in extrapolation of adsorption data at ambient temperature to zero pressure. As the Pt dispersion increases, the amount of hydrogen associated with the p3 peak becomes smaller than the sum of the (D1+p2) peaks.This may contradict suggestions that integral enthalpies of hydrogen chemisorption on supported platinium are indepen- dent of its average particle (Sen et al., J. CataE., 1986, 101, 517). Therefore the temperature required for desorption of a given fraction of adsorbed hydrogen is observed to decrease as the Pt surface area increases and the average Pt particle size decreases, possibly indicating an increase in the concentration of sites binding hydrogen with lower energy. The p, peak may arise from desorption from distinct spillover sites upon the support, but the and p2 peaks appear associated with Pt-held hydrogen, and interdiffusion between these means that and p2 peaks cannot be taken to indicate the populations of different surface Pt sites; indeed Pt surface areas estimated from the sum of the (D, +p2) peaks can exceed those determined by hydrogen chemisorption.However, Pt surface areas estimated from hydrogen t.p.d. to the temperature (here 573 K) to which catalysts are evacuated prior to volumetric measurement of chemisorption do agree well with chemisorption measurements. There is no consistent trend of desorption activation energies with Pt dispersion (as reported by Takasu et al., J . Chem. SOC., Chem. Commun., 1983, 1329) or t.p.d. peak temperature (as reported by Foger and Anderson, J. Catal., 1978, 54, 318 and Anderson et al., J. Catal., 1979, 57, 458). A recent consideration' of temperature-programmed bulk reduction (t.p. b.r.) of oxide- supported hexachloroplatinic acid and subsequent temperature-programmed desorption (t.p.d.) of chemisorbed hydrogen has suggested that diffusion may be important in defining the profiles and maxima seen.Thus in t.p.b.r. the rate of anion diffusion in the bulk solid particles to be reduced may be too slow to allow good resolution of t.p.b.r. peaks arising from the reduction of distinct homogeneous phases. In t.p.d., surface diffusion of adsorbate prior to desorption may allow significant randomization and make it difficult to associate particular t.p.d. peaks with the occupancy of distinct surface sites. This point is considered using detailed analysis of temperature-programmed desorption of hydrogen preadsorbed upon silica-supported platinum. In addition attention is given to the relationship between the extents of hydrogen desorption in different t.p.d. peaks and the amounts of hydrogen found to chemisorb upon these catalysts, and how the size of such t.p.d.peaks varies with the Pt dispersion. 13691370 Hydrogen T.P.D. from Supported Platinum Table 1. Properties of silica supports used most pore volume frequent /cm3 g-l pore SN2 radius supporta /m2 g-l Hg H2O /nm catalysts SiO, (D70) 321.9 1.1 1.28 4.4 A, E, H, J SiO, (D923) 534.9, 517.0 out of 1.10 3.75 D range 6 K Sorbsil AQU30 307.0 0.41 - ~~ ~~~~~ a D70 and D923 denote Davison 70 and 923 silicas; Sorbsil AQU30 denotes a silica support. Table 2. Supported adsorbents and catalysts wt % method of conditions of samples metal preparationa drying/reduction A 5.0 (4.4) IMP 393 K, 24 h/373 K, 2 h D 5.0 (4.3) IMP 393 K, 24 h/373 K, 2 h E 5.0 (3.9) IMP 393 K, 24 h/373 K, 2 h H 2.4 (1.5) IE sintered J at 873 K/6 h in H, J 2.4 IE 295 K (vac)/573 K, 3 h K 6.2 (2.4) CE 378 K, 16 h/673 K, 0.5 h a IMP, IE and CE denote samples prepared by impregnation and ion exchange methods and loaned by the Council of Europe group on catalysis.wt% Pt are those predicted from the preparation, and data in brackets are concentrations estimated by atomic absorption. Experimental Materials Flow desorption measurements used hydrogen (99.9 % purity, BOC) and nitrogen (white spot 99.9 % purity, BOC). The hydrogen stream was further purified by passage through 1% Pd/H,WO, (5 g) and then a trap at 195 K, thereby removing both 0, and H,O. The nitrogen stream was similarly purified by passage through 44.8% MnOJCelite (10 g) and then a trap at 195 K, leaving residual concentrations of < 1 ppb 0, and 0.87 ppm H,O.Table 1 indicates the supports used together with their principal properties. Pt was introduced to these (except H, J and K) as indicated in table 2 using aqueous solutions of H,PtCl, (Specpure, Johnson Matthey) to produce supported catalysts which have been studied here., Methods Total surface areas of supports and catalysts were estimated by application of B.E.T. theory to N, adsorption measured at 77 K. Pore volumes and pore size distributions were measured using a Carlo Erba porosimeter. Absorption spectrophotometry (Perkin-Elmer 303) was used with an air-ethyne flame to analyse for platinum at 265.9 nm after calibration with standard solutions (0.1-100 ppm) prepared by dissolving platinum black (98.84% purity) in aqua regia, filtering and making up to 0.1 dm3.Oxide supports alone before and after adsorptionM. S. W. Vong and P. A . Sermon 1371 Table 3. Platinum surface areas and average particle sizes estimated by chemisorption and physical methods2 sample -_____ 8.27 27.56 27.60 44.70 54.40 166.0 average of all chemsn data2 S /m2 g,: ____ 16.93 52.50 53.60 203.6 249.5 256.1 dpt/nm by TEMa dp,/nm by XRDb d, 16.62 5.38 5.33 1.35 1.15 1.09 7.30 11.20 12.35 5.37 8.37 10.10 5.02 8.14 9.45 ND ND ND ND ND ND ND ND ND 10.5 9.4 5.8 6.8 5.3 5.9 ND ND ND ND ND ND a TEM yielded mean number, mean surface and mean volume diameters of Pt particles dn, ds and dv, X-Ray camera and diffractometer measurements of 11 1 reflection line broadening.ND indicates that Pt particles in this sample were not detected by this technique. and t.p.d. experiments showed no significant Pt concentration detectable by AA (i.e. their platinum content was < 2 ppm). Transmission electron microscopy was applied to the silica supports and catalysts using a Jeol lOOC with microtomed sections of samples embedded in epoxy resin. The sizes of several hundred Pt particles were estimated in each sample. X-Ray diffraction line broadening (XRDLB) was followed by camera-microdensitometer (PW 1024/ 10 camera, Cu K, radiation, PW 1008 generator, Mk3CS microdensitometer) and horizon- tal diffractometer methods. The extents of H,, CO and 0, chemisorption were measured at 295 K volumetrically.2 Temperature-programmed desorption (t.p.d.) of H, was followed with samples (0.5 g) of catalysts flushed with N, (101 kPa, 60 cm3 min-l, 15 min, 293 K) and then H,.T'ae sample temperature was then raised (LMVS 100 Stanton Redcroft low-mass vertical furnace and LVPCA4R Stanton Redcroft temperature programmer) from 295 to 423 K, where it was held in H, for 1 h. The sample was then purged with N, (60 cm3 min-l, 1 h, 423-573 K). It was then allowed to adsorb H, at 101 kPa and 423 K and finally 273 K, and the sample was then flushed with N, (60 cm3 min-l, 30 min) at 273 K until no gaseous H, was detected when samples of the gas stream were injected into an HWD gas chromatograph (Perkin-Elmer F17 calibrated with samples of 6% H, in N,, fitted with a molecular-sieve column at 323 K through which flowed an N, carrier gas using a 5 mm3 gas-tight syringe Hamilton 1750 RN).The N, flow rate was then set and held at 15 cm3 min-l and t.p.d. commenced by raising the temperature of the samples at 4 K min-l from 273 to 900 K while the concentration of H, in the exit gas stream was monitored and plotted as a function of time and temperature. Sample temperatures were measured using an adjacent chromel- alumel thermocouple. Results Characterisation of Supported Platinum Table 2 shows that the Pt concentrations extracted from catalysts were less then the amounts introduced during preparation, especially in samples H and K. However, even after extraction such samples were greyish, indicating the retention of traces of platinum.In all calculations the platinum concentration estimated from the support impregnation1372 Hydrogen T.P.D. from Supported Platinum Fig. 1. Histograms of the sizes of Pt particles supported upon silica in samples A (-), D(----) and E (--.--.--. ) measured from transmission electron micrographs. (catalysts A, D and E) or the concentration supplied by others (catalysts H and I) or the concentration determined2 by atomic absorption (catalyst K) was used. The extents of H, chemisorption on each sample estimated by volumetric methods2 are given in table 3, together with average surface areas and average particle sizes of Pt calculated2 from CO, 0, and H, chemisorption. The adsorption stoichiometry H : Pt, = 1 and the average number of surface Pt atoms adsorbing an H, molecule irrespective of the extent of adsorbate dissociation nH2 = 2 were assumed.Catalysts were assumed to contain spherical crystallites of Pt of uniform size dpt, where the Pt surface area is given by S = 6000/dptppt bPt = density of platinum). Platinum catalysts H, J and K (with an average platinum crystallite size dpt < 2 nm estimated by chemisorption) did not show any X-ray diffraction patterns as a result of excessive line broadening (XRDLB). However, a platinum crystallite size of 1.34 nm determined by XRDLB in catalyst K has been rep~rted.~ The width of the x.r.d. peaks corrected for instrumental broadening using the Warren correction and an aluminium standard with > 200 nm crystallites were converted to average platinum crystallite sizes dpt, using the Scherrer equation (d = KA/pcos8).Table 3 shows that for catalyst A Pt particle sizes obtained by XRDLB are in reasonable agreement with those from electron microscopy. However, for samples D and E the Pt particle sizes obtained by XRDLB agree well with those from hydrogen chemisorption. It appears that gas adsorption and XRDLB (where applicable) give estimates of the average crystallite size of supported Pt which are in moderate agreement for the series of supported Pt catalysts studied here. However, it must be remembered that most catalysts contain a relatively broad particle-size distribution, revealed by transmission electron micrographs of all catalysts except those of greatest Pt dispersion (i.e. H, J and K). Although electron microscopy of catalyst K has shown3 that 78 % of Pt particles were < 2 nm, micrographs of the other catalysts showed the Pt particles to be almost spherical or hemispherical and of a wide range of sizes (see fig.1). The calculated number-average Pt diameter d, (= Xnidi/ni>, the surface-average Pt diameter d, (= Cnidf/nidf) and the volume-average diameter dv (= Cnidf/nid:, where ni is the number of Pt crystallites with diameter di) were calculated from micrographs and are given in table 3. Average diameters of Pt crystallites determined by chemisorption and XRDLB are also tabulated. The values of as and &, from electron microscopy were higher than the d values determined by hydrogen chemisorption and XRDLB.A 1373 H ri D P2 00 0 12 ri 4 0 0 0 0 8 2 0 0 P3 E 0 0 0 0 "a 0 0 0 0 ; 0 I I 0 0 0 0 0 ri 0 0 0 0 0 I 01 I 40 80 120 160 tlmin 0 p1 z (32 0 0 0 0 0 0 O O O O b 0 ' 0 O 1 0 O 0 0 J 0 ?J% 0 1 1 1 1 3 2 1 ,8.P1 K tlmin Fig. 2. Profiles of temperature-programmed desorption for hydrogen preadsorbed on different silica-supported Pt samples (A, D, E, H, J and K) as their temperature is raised at a constant rate dT/dt (4 K min-l) from 273 to 900 K. The rate of desorption is given as dn/dt or ri in lo1' H atom per g catalyst per min.1374 Hydrogen T.P.D. from Supported Platinum Table 4. Results of temperature-programmed desorption of H, preadsorbed on silica-supported Pt samples H atoms ( x desorbed per g catalyst sample 6 573 K PI P2 P 3 total A 2.2 1.04 2.39 5.79 9.22 D 2.5 1.87 2.13 1.61 5.02 E 3.4 2.80 2.39 7.68 12.78 H 4.9 4.49 0.91 0.80 6.20 J 7.6 7.10 3.10 4.80 15.00 K 14.33 10.80 4.60 14.13 2.10 4.35 20.60 H atoms ( x per g Pt S/m2 per g Pt chemi- t.p.d.t.p.d. chemi- sample (Dp,+P,) d 573 K sorption (D1+P2) d 573 K sorption A 68.6 44.0 19.8 54.88 35.20 15.94 D 80.0 50.0 66.1 64.00 40.00 53.14 E 102.0 66.8 66.7 8 1.60 53.46 53.60 H 220.0 199.6 223.5 176.0 159.7 179.6 J 425.0 316.7 272.0 340.0 253.3 218.5 K 261.8 231.1 321.6 209.4 209.4 261.1 198.7 Overestimation by TEM could arise from aggregation of small metal particles into larger ‘apparent’ crystallites and also because Pt particles smaller than 2 nm which chemisorb hydrogen may not be visible in TEM. For these catalysts hydrogen chemi- sorption probably measures the total platinum surface areas satisfactorily.Temperature-programmed Desorption of Hydrogen Temperature-programmed desorption (t.p.d.) profiles (273-900 K) obtained for hydro- gen preadsorbed at 273 K and 101 kPa for 30 min on silica-supported platinum (see fig. 2) show the rate of hydrogen dn/dt or n desorbed as a function of time and hence temperature. In general, three different maximum rates of chemisorbed hydrogen on platinum were indicated by peaks appearing in the t.p.d. spectra at specific temperatures (Tmax) at ca. 347-397, 573-633 and 693-786K. These peaks were designated as /I1, /Iz and /I3, respectively. The quantity of hydrogen desorbed was estimated by integrating the area under each of the peaks, and the results so obtained are given in table 4. Overlapping peaks were separated by dividing these vertically at minimum rates of desorption; uncertainties in t.p.d.peak areas were & 15%. In the measurements of the extent of hydrogen chemisorption,2 samples (see tables 3 and 4) were pretreated in that they were reduced in H, at 423 K and 13.33 kPa and then evacuated at 573 K and 1.33 mPa before the isotherms were measured at room temperature. By assuming that the hydrogen desorbing above 573 K in t.p.d. (i.e. associated with /Iz and /I3 peaks) was not removed by evacuation in adsorption experiments, the amount of hydrogen taken up by the sample in the subsequent adsorption process should be comparable to the amount of hydrogen desorbed from the catalyst between 273 and 573 K (i.e. associated in part with the /II t.p.d. peak). Table 4M .S. W. Vong and P. A . Sermon 1375 Table 5. Extents of hydrogen chemisorption at ambient temperature and desorption" per g platinum H atoms ( x 1019) desorbed per g Pt % (D1+82) adsorbed per g Pt H atoms ( x of total desorbed 0 kPae 10kPa 100kPa sampleb +P2 total - - 22.58 (22.58) 22.58 30.15 112.55 28.00 110.40 A 68.60 184.40 37.2 D 80.00 100.40 79.7 66.40 85.68 259:15 66.50 156.37 965.21 (67.46 157.33 966.1 7 E 102.00 255.6 39.9 - - 74.95 110.73 432.75 F 33.49 L 30.37 G 64.55 51.97) - 60.14 H 220.00 258.30 J 425.0 625.00 K 248.39 322.3 261.80 - 215.64 - 228.38 263.20 - - 89.60 - 87.1 223.35 225.40 272.02 68.0 (274.07 74.8 325.38 78.8 323.60 - 330.25 - 314.79 - 322.62 305.87 117.67 323.70 325.75 419.1 1 421.16 - 433.33 441.41 424.41 370.30 1226.87 1228.92 1742.92 1744.97 - 1500.22 1508.00 1491.00 a 'T.p.d. results for samples L, F and G have been reported.' previously.2 Determined by extrapolation of chemisorption data.Samples have been described shows that the agreement with chemisorption is fair and far better than any other t.p.d. assessment previously used (i.e. areas, p1 +p2 areas etc.). This is novel, important and potentially very useful. Discussion The hydrogen t.p.d. results obtained here are more detailed and cover a wider range of Pt particle sizes than previously. Here /I1 and p2 in the present work may be assumed to result from hydrogen atoms chemisorbed on the platinum because (i) extents of chemisorption (after evacuation at 573 K) at ambient temperature and 'zero pressure' and the fraction of the @,+p2) t.p.d.peak areas desorbed up to 573 K are in fair agreement, and (ii) the B1 and Bz t.p.d. peaks appeared at the same temperatures as the y and 6 t.p.d. peaks detected4 on Pt black, and are also in reasonable agreement with t.p.d. on Pt single crystals5 and B2 peaks from Pt film.6 Although strict comparison of the present t.p.d. results with those in the literature is difficult because of differences in methods of preparation, Pt dispersion, supports and the adsorption-t.p.d. conditions, the present p3 t.p.d. results also agree well with those reported on supported platinum catalyst^.^? Thus T,,, for the p3 peak was very close1376 Hydrogen T.P.D. from Supported Platinum to that for the desorption peak (at 753 K)g of hydrogen from platinum-free alumina exposed to dissociated hydrogen atoms in a high-frequency discharge.It is also close to that identified for hydrogen on platinum supported upon silica, alumina and zeolites.l'-12 Furthermore, table 5 shows that the total size of the (al+p,+/3,) peaks is too large to be accounted for in terms of desorption of Pt-held hydrogen alone, although this quantity does vary significantly with adsorption pressure. However, no desorption peak was detected in the t.p.d. of Pt-free supports alone, indicating that the b3 peak was not dehydroxylation or dehydration of the support surface. The concentration of OH groups on the silica surface has been estimated13 to be 0.2 OH per nm2 or 0.6 x 1020 OH per g Davison 70 silica. If significant dehydroxylation had occurred the amount of hydrogen desorbed would have been insignificant.Hence it is concluded that the p3 peak detected in the present work is associated with spilt-over hydrogen? which migrates back from the silica support to platinum and then desorbs therefrom. Differences in the p3 size with various samples might be due to the varying extent of interaction between Pt and the oxide support. Future studies on the effect of adsorption parameters on the t.p.d. profiles may provide more information on the effect of metal and support interaction. T.p.d. profiles of catalysts E, A and D, which had relatively large Pt particles (i.e. 5.3, 16.6 and 5.4 nm, respectively) exhibited a singlet B1 peak (between the doublet p1-p2 states of catalysts H, J and K) and then a larger peak (or peaks) at higher temperature.T.p.d. profiles of highly dispersed catalysts K, J and H (dpt of 1 . 1 ? 1.2 and 1.4 nm, respectively) showed the peak to be split into a doublet with the first maxima at 373,363 and 347 K and the second maxima at 443,432 and 410 K, respectively. As dpt decreases, the fraction of Pt-held hydrogen released as PI increased. The results show a less clear trend for the temperature of the profile maxima ( Tmax) with dpt, but certainly do not show an increase in Tmax with decreasing dpt as lo Furthermore, the relative contribution of the second peak in the doublet became more significant as dp, decreased. With t.p.d. from these silica-supported Pt samples at the low heating rates used it is not believed that mass-transfer 1imitationsl4 are significant. Nor is there any evidence that desorption is from a non-metallic active phase (e.g.formed uiu Pt-Si-0 linkages15), especially since only very modest temperatures of pretreatment and desorption have been used; indeed the porous supports may have reduced contamination of the supported Pt surfaces.16 Nevertheless, with silica at high temperatures or with other supports interactions might be far more significant. Conclusions The decrease in the temperature of the desorption maxima as the platinum particle size decreased could be due to a decreasing presence of higher-energy binding sites for hydrogen. However, an increase in the proportion of relatively low coordination number platinum atoms as the size of the particle becomes smaller might be lo There is evidence that platinum atoms at corner, edge and kink1' sites are relatively electron- deficient.In addition, theoretical calculationsls of the hydrogen-binding energy have shown that these sites adsorb hydrogen more strongly. This suggests an increase in Tmax with decreasing dpt should be expected (and seen8, lo for supported Pt); this is the reverse of the t.p.d. results here. Nevertheless, for these samples the enthalpy of chemisorption of hydrogen at 5-6 kPa has been found1 to decrease from 91.99 to 60.60 kJ mol-1 H, as dpt decreased from 3.59 to 1.09 nm. The causes of these different trends with dpt remain uncertain, but it is clear that the extent of adsorption-desorption measured here for sample K agrees with previous estimatesll and that the fraction of preadsorbed hydrogen which is subsequently desorbed at a particular temperature increases as the average dpt decreases.Possibly as a result of the use of lower rates of temperature-programming than previously (i.e. 20 K min-1)8, lo, l9 rather than stepped-temperature desorption, theM. S. W. Vong and P . A . Sermon 2.0 3.0 4.0 1 1 1 1 1 1 1 ~ 1 ~ ' ~ ~ - "? - PP i: '0 - \ 5 - - t Oe * 3.0 ~ j - - \ $$P* '> 00 4 " 0 3, - -Og$"Q: " 0 d\$ - 0; 0 Q 0 e i 0 o f - p o r! or: ip? " 0 @,, CI 2.0- ,! ""t, , Q 0 ' - .: - 9 0 ' :a 0: - 9 '4 8 'e - e: '>QQ 0 \ ""0 O b '\ 0 Gb '\,O B 0 QB? '\ O 8 - - , - - Q 0 ' " 8 - I , \ - %i - 9 - O'\\, - l o"',, @a - *4 O\'$, "\ Q "b 1.0 - I - "0 - I @? Oo \\ - 0 ) 4 1 1 9 1 I 1 1 I I I I I I . - 2.0 3.0 4.0 1377 3.0 'L -2.0 E 1.0 resolution of the hydrogen desorption profiles shows more detail and not a rather broad t.p.d. peak8? lo, lS with poorly resolved shoulders which are difficult to separate even by deconvolution.1° Instead, at least three different moderately well resolved t.p.d.peaks are seen ; and b2 tentatively associated with dissociated hydrogen atoms chemisorbed on platinum and P3 with hydrogen spilt over on to the silica oxide support. Insufficient evidence exists to conclude whether the B1 and b2 peaks arise from hydrogen on binding states whose energetics are defined by surface sites of different local crystallographic properties.12 Nevertheless, it must be remembered that none of the silica-supported Pt samples studied here contained a narrow range of Pt particle size, hence to look for particle-size effects of their t.p.d.profiles cannot strictly be warranted. There can be many binding states of adsorbed molecules on a given surface and AH,,, may differ by as much as 82 kJ mol-l. The existence of several binding states indicates that even a single crystal plane is heterogeneous when viewed by the adsorbed species. These sites of different adsorptive strengths are filled with increasing coverage or with changes in other experimental variables. Clearly the choice of programming rate is very important, as suggested.l T.p.d. peak breadths did not appear dependent upon dpt as predicted elsew here. Consider for a moment whether B1, B2 and P3 peak areas really correspond to the 46 FAR 11378 Hydrogen T.P.D. from Supported Platinum Table 6.Activation energiesa of desorption of hydrogen from supported platinum measured from plots in fig. 3 when dt+O and [S] or hydrogen coverage is essentially constant E,/kJ mol-1 (at T,,,/K) sample El for Dl E2 for 8 2 E3 for 83 A 19.38/408.2 77.321582.7 36371692.2 D 13.761393.2 32.111594.2 - 1739.2 E 21.63/388.2 71.501597.2 26.581708.5 H 38.45/347.2 - /410.2 - /786.2 J 20.11/363.7 50.471432.2 - 1694.2 K 21.901373.2 - 1443.2 - 1755.2 a Desorption activation energies for H2 from Pt/Si02 were previously measured19 over a wide range of temperature. populations of energetically different surface sites. The rate of hydrogen desorption from i surface sites will be given by if readsorption is negligible and the rate constant of desorption does not vary with surface-site occupancy20 at small dt.Hence i can be 1, 2 or 3 (i.e. for #I1, p2 or p3 here) and [S], is the concentration of the particular discernible occupied surface sites with specific energetics of adsorption corresponding to one t.p.d. peak; s is the kinetic order with respect to this surface site concentration. Therefore for any one t.p.d. peak i a plot of In (dnldt,) (or Inn,) us. l / T at essentially constant site occupancy and [S], for a particular surface site [i.e. for a small increment of time (dt + 0) or temperature] should allow Ei for desorption of that species to be deduced. Fig. 3 shows such plots for the hydrogen desorption peaks #I,, #Iz and p3 for Pt/SiO, used here. Activation energies El, E2 and E3 so derived are given in table 6.The average activation energy (35.80 kJ mol-l) for hydrogen desorption in B1, #I2 and p3 peaks from silica-supported platinum shown in table 6 is fortuitously close to Edesorption (37.66 kJ mol-l) seen previously for H, from Pt.,l As expected, these are far greater than the activation energy for diffusion of hydrogen on the Pt surface (18.85 kJ mo1-1).22 It is clear that such activation energies do not decrease in the sequence p3 > a, > #I1 (or its reverse) for all samples, nor do they change consistently with dpt for one t.p.d. peak for samples of different Pt dispersions. Previously it has been reported19 that Ei decreases from 50 to 34 kJ mol-' as dpt decreases from 4.3 to 1.6 nm; here samples have values of dpt lower and higher than previously19 and also have equally wide particle-size distributions as previo~s1y.l~ This underlines the difficulty of associating particular t.p.d.peaks with the populations of distinct surface sites i and therefore that surface interdiffusion may be important.', 22? 23 With such complex and overlapping t .p.d. peaks complete profile analysis20 is not generally possible. Thus surface site occupancies deduced from t.p.d. profiles of hydrogen12 and subsequent correlations with catalytic activity* may not be useful. Compensation effects24 between A, and Etpd, upon n have not been considered here. If temperature-programmed desorption of hydrogen on silica-supported platinum releases more hydrogen in total than measured in chemisorption from the zero-pressure intercept (with part of this arising from the support and part from the Pt, and only partial differentiation of hydrogen adsorbed in sites with different adsorption energies), it is possible that temperature-programmed titration of adsorbed hydrogenz5 could differen-M.S. W. Vong and P. A . Sermon 1379 tiate hydrogen readsorbed in these different Pt, surface sites at lower temperatures and with slower and less extensive interdiffusion than is involved in t.p.d. at higher temperatures. Nevertheless, it has been shown here for the first time that the fraction of the hydrogen @,+p,) t.p.d. area up to the temperature used to evacuate samples in chemisorption experiments (here 573 K) measured under the present t.p.d. conditions (i.e. a low heating rate dT/dt of 4 K min-l, giving better t.p.d.peak resolution than previously reported for oxide-supported Pt) can be used to estimate the Pt dispersion with an accuracy comparable with chemisorption of hydrogen. This finding is useful; it could allow the characterisation of the surface coverage of hydrogen at any selected reaction or chemisorptive-pretreatment temperature. There is also much uncertainty about the effect of average Pt particle size on the stoichiometry26 and enthalpy2' of hydrogen chemisorption. Recent results27 suggest no effect of Pt dispersion on integral enthalpies of hydrogen chemisorption (although the values are lower than many reported2*) and this would not be expected from the present t.p.d. results. Clearly temperature-programmed such as t.p.d. complement direct enthalpy measurement~~~t 28 and this discrepancy must be resolved.Nevertheless, for the present the summed (P,+P2) t.p.d. peak areas for hydrogen measured under present conditions do appear to estimate supported Pt surface areas and dispersions moderately well. The (P, +/I2) results in table 4 for sample K agree with those recently reported30 from laboratory A1 on the sample catalyst after adsorption of H, (101 kPa, flowing, ambient temperature) during desorption at 300-700 K [i.e. (1 1.5-1 1.9) x 1019 H atoms desorbed per g catalyst in B'+C' t.p.d. peaks] with only a small fraction remaining to be desorbed at higher temperature [i.e. (0.8-1.5) x 1019 H atoms per g catalyst] and possibly arising from spilt-over hydrogen. The advantages of temperature-programmed titrations of preadsorbed hydrogen over desorption will be reported.The provision of a studentship for M. S. W. V. by S.E.R.C. is gratefully acknowledged. References 1 N. D. Hoyle, P. H. Newbatt, K. Rollins, P. A. Sermon and A. T. Wurie, J. Chem. Soc., Faraday Trans. 2 A. R. Berzins, M. S. W. Vong, P. A. Sermon and A. T. Wurie, Adsorption Sci. Technol., 1984, 1, 51. 3 Council of Europe (unpublished data). 4 S. Tsuchiya, Y. Amenomiya and R. J. Cvetanovic, J. Catal., 1970, 19, 245; 1971, 20, 1 . 5 K. Christmann and G. Ertl, Surf. Sci., 1976, 60, 365; K. E. Lu and R. R. Rye, Surf Sci., 1974, 45, 6 J. J. Stephan, V. Ponec and W. M. H. Sachtler, J . Catal., 1975, 37, 81. 7 R. A. Dalla Betta and M. Boudart, Proc. 5th Znt. Congr. Catal. 1973, vol. 2, p. 1329; J. C. Vedrine, 8 K . Foger and J.R. Anderson, J. Catal., 1978, 54, 318. 9 R. Krarner, Naturwissenschaften, 1977, 64, 269; Originalmitteilungen Physikalische Chemie der 1, 1985, 81, 2605. 677. M. Dufaux, C. Naccache and B. Imelik, Proc. 7th Int. Conf. Solid Surfaces 1977, 1, 481. Universitat Innsbruck 1977, 64, 269; R. Kramer and M. Andre, J . Catal., 1979, 58, 287. 10 J. R. Anderson, K. Foger and R. J. Breakespeare, J . Catal., 1979, 57, 458. 1 1 Y. Amenomiya, J . Catal., 1971, 22, 109. 12 J. P. Candy, P. Fouilloux and A. J. Renouprez, J. Chem. Soc., Faraday Trans. I , 1980,76, 616. 13 P. A. Sermon, J . Chem. Soc., Faraday Trans. I , 1980, 76, 885. 14 E. Tronconi and P. Forzatti, J . Catal., 1985, 93, 197; Y. J. Huang and J. A. Schwarz, J. Catal., 1986, 99, 249; J. S. Rieck and A. T. Bell, J . Catal., 1984, 85, 143. 15 S. M. Levine and S. H. Garofalini, SurJ Sci., 1986, 177, 157; K. E. Keck and B. Kasemo, Surf. Sci., 1986, 167, 313. 16 S. Ichikawa, Surf. Sci., 1986, 176, L853. 17 Y . W. Tsang and L. M. Falicov, J . Phys. C, 1976,9, 51 ; K. Besocke, B. Krahlurban and H. Wagner, 18 D. J. M. Fassaert and A. V. D. Avoird, Surf. Sci., 1976, 55, 313. 19 Y. Takasu, M. Teramoto and Y. Matsuda, J . Chem. Soc., Chem. Commun., 1983, 1329. Surf. Sci., 1977, 68, 39. 46-21380 Hydrogen T.P.D. from Supported Platinum 20 R. J. Cvetanovic and Y. Amenomiya, Adv. Catal., 1967, 17, 103; E. Habenschaden and J. Kuppers, 21 K. Christmann, G. Ertl and T. Pignet, Surf. Sci., 1976, 54, 365. 22 G. Ehrlich and D. 0. Hayward, Discuss. Faraday SOC., 1966, 41, 102; R. Lewis and R. Gomer, Surf. 23 D. D. Beck and J. M. White, J. Phys. Chem., 1984, 88, 174. 24 P. J. Estrup, E. F. Greene, M. J. Cardillo and J. C. Tully, J. Phys. Chem., 1986, 90, 4099. 25 M. S. W. Vong and P. A. Sermon, J. Chem. Soc., Chem. Commun., 1983,660. 26 S. Ladas, Surf. Sci., 1986, 175, L681; A. Coma, M. A. Martin and J. Perez Pariente, Surf. Sci., 1984, 27 B. Sen, P. Chou and M. A. Vannice, J. Catal., 1986, 101, 517; M. A. Vannice, L. C. Hasselbnng and 28 E. R. A. Mills, P. A. Sermon and A. T. Wurie, Proc. 8th Znt. Congr. Catal. (Verlag, Berlin, 1984), Paper 29 A. Jones and B. McNicol, Temperature-Programmed Reduction for Solid Materials Characterization 30 A. Frennet and P. B. Wells, Appl. Catal., 1985, 18, 243. Surf. Sci., 1984, 138, L147. Sci., 1969, 17, 333. 136, L31. B. Sen, J. Catal., 1985, 95, 57. 111-131 and ref. (23H25) therein. (Marcel Dekker, New York, 1986). Paper 6/451; Received 5th March, 1986

ISSN:0300-9599    

DOI:10.1039/F19878301369

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I , 1987’83, 1381-1394 Potential-energy Calculations of the Mechanisms of Self-diffusion in Molecular Crystals Part 2 .-Naphthalene D. Harold Smith? Department of Chemistry, Northwestern University, Evanston, Illinois, U.S.A. Self-diffusion in crystalline naphthalene has been investigated by calculation of the activation energies for various postulated mechanisms. A first-order ‘rigid-lattice’ calculation is first performed in which only the diffusing molecule is allowed to move. This yields several conclusions: (1) self- diffusion normal to the ab plane occurs only by vacancy exchange between sixth-nearest-neighbour sites; (2) diffusion along the a-direction occurs by the same mechanism and also by vacancy exchange between nearest- neighbour sites; (3) diffusion along the b-direction occurs by the nearest- neighbour mechanism; and (4) the molecular trajectories in both mechanisms apparently do not cross saddle points on their respective potential surfaces.It is then postulated that diffusion occurs without rotation of the diffusing molecule when its initial and final lattice sites belong to the same sublattice (the sixth-nearest neighbour mechanism) and with a single, direct rotational flip of the diffusing molecule while it is translating between lattice sites when the sites belong to different sublattices (the nearest- neighbour mechanism). The theoretical activation energies for these trajectories in the rigid lattice model are twice as large as the experimental values, but most of the barrier comes from just four (sixth-nearest neighbour mechanism) or two (nearest-neighbouring mechanism) nearby molecules in the lattice.A second-order ‘ relaxed-lattice ’ calculation is then performed in which these ‘blocking’ molecules are allowed to rotate, and good agreement with experiment is found for both mechanisms when the blocking molecules rotate ca. 10-15” to their ‘equilibrium’ orientations in the ‘transition state’. The tentative conclusion that the operative trajectories in naphthalene do not cross saddle points is of considerable theoretical interest. Since the introduction of the radiotracer technique,’ self-diffusion coefficients for atomic and ionic crystals have been extensively measured.2 The development of interatomic potentials coupled with high-speed computing machinery has made it possible to simulate lattice defects in these solid^,^ and self-diffusion in crystalline rare gases has been studied in this Compared to molecular crystals the theoretical interpretation of diffusion in atomic systems has several major advantages.(1) The lattice symmetry is very high, often cubic; this greatly reduces the number of possible diffusion pathways as well as the number of calculations required for any one pathway. (2) The number of atomic species present is small, usually only one; in this case only one atomic pair potential is required. ( 3 ) The atoms can be treated as point masses; the three angular coordinates which would be needed to describe the orientation of a rigid body are not required, and the motion of each species in the crystal can be completely described by just three spatial coordinates.Present address : Morgantown Energy Technology Center, Morgantown, West Virginia 26507-880, U.S.A. 13811382 Self-difusion in Naphthalene In spite of the relative complexities, however, there exist many more molecular crystals than atomic ones. And molecular crystals possess many of the most interesting and useful properties found in the crystalline state. In recent years, primarily due to the careful radiotracer measurements of Sherwood and coworkers,8 reliable data for the temperature dependence of the intrinsic self- diffusion have become available for several aromatic compounds. Isotopic substitution experiments have shown that self-diffusion in benzene9 (and presumably other) crystals occurs by a vacancy mechanism.However, no experimental methods for obtaining detailed mechanical descriptions of diffusion mechanisms have been reported. For example, for no aromatic crystal has it been shown which pair(s) of lattice sites participate in the vacancy interchange or what role in the diffusion is played by molecular rotation. The lack of this type of information makes it impossible to apply modern dynamic theories of diffusion to real crystals and invariably necessitates the use of transition-state theory for the interpretation of experimental data. Detailed inter- pretations of the diffusion mechanism in molecular crystals have been considered impractical,8 however, owing to the difficulties in calculating the intermolecular forces.With the availability of accurate semi-empirical atomic pair potentials for carbon and hydrogen,l* it is now possible to obtain the intermolecular potential for any specified distance and relative orientation between two hydrocarbon m o l e c ~ l e s ; ~ ~ - ~ ~ this makes it feasible to attempt calculations of the self-diffusion pathways in these molecular cry~ta1s.l~ For all the compounds so far examined (adamantane, naphthalene, anthra- cene), the value of AU, calculated in the rigid-lattice approximation is almost exactly twice the experimental value, presumably owing to neglect of lattice relaxations. In eqn (3) of ref. (14) the factor 0.5 was introduced to correct for this effect; in the present paper the 0.5 factor is omitted, and we attempt to calculate the effect of lattice relaxation on A Uf directly.Naphthalene is one of the most studied molecular crystals; its monoclinic (P2,la) structure15~ l6 represents a typical molecular packing for aromatic c o m p o ~ n d s . ~ ~ ~ l8 Furthermore, this system has three unique axes hence, in principle, three different rates of self-diffusion. Radiotracer measurements of the self-diffusion normal to ab basal plane19 and of diffusion along sub-grain boundaries20 have been reported, as well as the results of creep flow studies.21 Sherwood has also made unpublished studies of the self-diffusion parallel to the a and b axes.22 The purposes of the present study were to (I) confirm the assumption that self-diffusion in naphthalene occurs by a vacancy mechanism or mechanisms, (2) discover which pair(s) of lattice sites participate in vacancy exchange, (3) determine the role of molecular rotation in the mechanism(s) and (4) investigate activation volume effects.A treatment similar to the present one has previously been reported by this author of the mechanisms of self-diffusion in plastic-crystalline adamantane.14 Transition-state Theory of Naphthalene Self-diffusion The experimental data8 indicate that self-diffusion in aromatic crystals takes place by activated processes in which a neighbouring molecule jumps into a single-vacancy hole in an otherwise ‘perfect’ region of the crystal. In the usual ‘transition-state’ theory the diffusion along direction q due to each type of neighbour (nearest, next-nearest etc.) may be represented2 by an equation of the form 0% = r,(a%)2 V, exp (ASJR) exp (- PAV,/RT) exp (- AU,/RT) in which vm is a vibrational frequency of the neighbour rn; a% is a component of its jump distance; y m depends on the number of rn neighbours; and ASm, AVm and AU, are theD.H. Smith I383 net entropy, volume and potential-energy changes associated with formation of the vacancy and the transition state. In contrast to the common assumption for atomic self-diffusion in cubic crystals, there is no a priori reason to restrict the jumps to nearest neighbours; nor is it reasonable to expect that -(AU,+PAV,)/RT should be equal for all m. In general, the measured diffusion coefficient for a given crystalline direction should be DQ = CDS, (2) in which each Dq, has a different temperature dependence and a different weighting factor v,(aS,)2vmexp (ASm/R).Empirically, however, it is found8 for each of the crystalline directions so far examined that the intrinsic or lattice self-diffusion can be expressed by the simple form D = Do exp(-AH,,,/RT). (3) Moreover, separate values of Do and AHact for non-equivalent lattice directions have not yet been experimentally resolved for any aromatic molecular crystal. Hence, in addition to obtaining an accurate value for AH,,,, the calculated potentials for each compound must show either (1) that AHm is prohibitively large for the diffusion of all but one type of neighbour, (2) that within experimental resolution AH, is the same for all important mechanisms and/or (3) conclude that Do is exceedingly small for any mechanism whose theoretical AH, is significantly smaller than the experimental AHact.Assuming, for the moment, that only one mechanism contributes significantly and dropping the subscript m, we may write the coefficient of self-diffusion as Dq = y ( ~ q ) ~ v exp [(AS,+ASf)/R] exp[-P(AVf+AV$)/Ra exp[-(AU,+AU)/RTJ where the subscript f and the superscript $ refer to the vacancy and the transition state, respectively.2 In this case the temperature dependence of the diffusion would be identical for all three directions. The absolute rates would be in the ratios of the ( ~ q ) ~ , but in general these rations are 5 2 and thus too small for accurate measurement. (4) From eqn (3) and (4) it follows that for the variable X (= S, U or V) AXact = AXf + AH.( 5 ) The enthalpy required to form a vacancy, AHf, is (neglecting small corrections) just the heat of sublimation,11’23 so that an experimental value for AH$ can be obtained by subtracting the heat of sublimation from the activation enthalpy obtained from the temperature dependence of D according to eqn (3).t Further experimental deconvolution of eqn (4) is more difficult. In principle, AV,,, (but not A& or AYI separately) can be obtained from the isothermal pressure dependence of D if the pressure dependence of v can be estimated:24 No pressure studies of the self-diffusion in these compounds have yet been reported. The value of AS,,, can be calculated with fair accuracy from Do = ya2v exp (AS,,,/R) (7) and reasonable estimates of 7, a2 and v. This has been done by S h e r ~ o o d , ~ ~ and the results were taken as further evidence for diffusion by the single-vacancy mechanism.It was t More exactly, AU, = AHsubl+AU0- RT, where W , is the zero-point energy, the usual approximations involving ideality of the gas and neglect of the volume of the solid are made, and temperature corrections involving the difference between the heat capacities of the solid and gas are neglected. Not only are these terms small, but they partially cancel.1384 Self-difksion in Naphthalene Table 1. Values of the parameters used in the atomic pair potentialsloU atomic pair A,/kJ nm6 mo1-I B,,/kJ mo1-I C,/ nm- * carbon-carbon 2238 x lop6 carbon-hydrogen 581.6 x hydrogen4 ydrogen 150.6 x lop6 31 1540 0.360 39 376 0.367 16 736 0.374 suggested that ASf N AS1 e ;AS act, but the bases for an accurate analysis are not available. The value of v was calculated from the Debye temperature, although it has been objected that the lattice vibrations of a molecule close to a vacancy will be much different from those of a molecule far from any defect.26 A more serious problem than the experimental determination of the transition-state parameters is the question of their physical meanings.Some of the objections to eqn (1) and (4) have been discussed elsewhere in connection with the formulation of alternate and much of the apparent success of the theory may be due to the lack of sufficiently stringent tests. The present treatment provides opportunities for a more critical evaluation of the theory. Diffusion Model and Calculation of Potentials for Vacancy Formation and Diffusion The potential energy between two molecules in the crystal is assumed to be the sum of all the atomic pair potentials between their constituent atoms.The atomic pair potentials are assumed to depend only on the distance between the two atoms and on their atomic species, i.e. to be unaffected by the presence of third atoms on either other molecules or on the molecules of which the two given atoms form a part. The exponential-6 function Vij = A i j P + Bij exp (- C,r) is used for the potentials between atoms i andj, with Williams’ values for the parameters A,, Bij and Cij.l0 The empirical justification for these assumptions and evidence for the accuracy of the potentials chosen may be found el~ewhere.~O-~~ The values of the parameters used are listed in table 1.The potential of a molecule M surrounded by any given configuration of other molecules can be obtained by calculation of each intermolecular potential and summation of the molecular pair potentials. In particular, if the positions of M and the surrounding molecules correspond to the equilibrium crystal structure (as determined by X-ray or neutron diffraction measurements), the calculated potential corresponds to - AUf, where AU, is the energy required to create a vacancy. Once AUf has been calculated, M may be moved successively through six-dimensional translational-rotational space to other points and orientations in the lattice and the potential calculated for each point. When the potential surface for a molecular trajectory between two lattice sites has been obtained in this way, AUt is equated to the maximum in the potential encountered by the molecule along the jump path actually taken, minus its initial potential.No attempt is made here to link the position of maximum potential with a thermodynamically definable transition state. The initial potential is just -AUf, less the molecular pair potential term within -AUf which exists between the diffusing molecule at its initial site and a molecule at the vacancy site when the latter site is actually filled. The origin is chosen to be at M, and the potential barriers for diffusion of molecule M into single vacancies at various other lattice sites are calculated and compared. In theD. H . Smith 1385 0 1.0 nm I Fig.1. The unit cell and molecular packing of naphthalene (hydrogens are omitted for clarity). Among the nearest neighbours to molecule M are : A(first), B(second), C(third), D(fourth), E(fifth), F(sixth). Table 2. Unit-cell dimensions of naphthalene (space group P2 1 /a)'6 a/nm b/nm c/nm a/O D/" Y/" 0.8266 0.5968 0.8669 90 122.92 90 initial calculation, all molecules except M are held fixed in the positions and orientations indicated by X-ray and neutron-diffraction studies. Hence, at this level of approximation, A vf and A Vx are zero and corresponding energy and enthalpy terms are equal. If desired, these conditions (i.e. the lattice) can be relaxed later, after the initial 'screening' calculation to determine which of the neighbouring molecules can actually jump into a vacancy.The unit cell and molecular packing of naphthalenel59 l6 are illustrated in Fig. 1, and the cell dimensions are listed in table 2. At ambient pressure the compound crystallizes in the P2Ja space group with two molecules per unit cell. It is convenient to think of the lattice as being composed of two equivalent sublattices, here designated as s and t , respectively. (The coordinates of the s and t molecule in the unit cell are interchangeable by a screw diad pera at ion.^^) A molecule can, in principle, jump into a vacancy in its own sublattice by pure translation. However, the exchange of molecules between sublattices must be accompanied by molecular rotation, or else leave the diffused molecule in a non-equilibrium orientation.In the computation of the potentials from eqn (8) carbon positions were taken from the published cell dimensions and fractional coordinate~,~~~ l6 and hydrogen positions were either taken from the neutron-diffraction study16 or were calculated from the carbon coordinates and an assumed C-H bond length of 1.08 A. In his calculations to obtain the parameters in table 1, Williams used C-H bond lengths of 1.027 A.loa However, the difference in length is small, and empirical justification has been found for using these potentials with the true C-H intemuclear distance.11-14 For the enthalpy of sublimation of naphthalene there is little difference among the value used by Williams (72.4 kJ mol-l), .1386 Self-diflusion in Naphthalene Table 3. Calculated potential barriers for molecular jumps into neighbouring vacancies: AUX, barrier of operative mechanism; A U*, ‘ Saddle-point ’ Barriera relaxed rigid lattice lattice distance no.of neighbour /nm neighbours sublattice AU/kJ mol-1 AU*/kJ rno1-I Ui/kJ mol - - - (0 - 0th 0.0 1 st 0.51 4 S 230 50 84 2nd 0.60 2 t 470 30 3rd 0.79 4 3 480 480 4th 0.81 2 t > 1000 > 1000 5th 0.83 2 t > 1000 > 1000 6th 0.87 2 t 210 40 95 - S - - - a Experimental barrier height AH$ z 106 kJ mol-l. Table 4 Comparison of relaxed-lattice model and experimental values (in kJ mol-I) of self-diffusion parameters for the three crystalline directions theoretical experimental direction mechanism AUf AUt AUact AHsubla AHJb AHact 11 to b axis nearest 71 84 155 73 ca.106 ca. 179c 1) to a axis nearest 71 84 155 73 ca.106 ca. 179c neighbour neighbour neighbour neighbour sixth-nearest 71 95 166 I to ab plane sixth-nearest 71 95 166 73 106 1 79d a Average of 5 experimental values.32* 36 directly. Ref. (22). Ref. (19). Difference between AHac+, and AH,,,,, not measurable the value calculated in the present work (71 .O kJ mol-l) and the average experimental value (72.8 kJ mol-1)33-37 (see below). The diffusing ‘molecule was assumed in all cases to move along the shortest transla- tional path between lattice sites. This assumption is justified for naphthalene by the molecular and crystal symmetries. 14-16 Early in the calculations evidence arose that the molecular trajectories might not cross saddle points in the six-dimensional potential surface. Hence, procedures for starting at the origin to seek the path of minimum potential between the initial and final sites were rejected in favour of the following method, which covers all of the reasonable possibilities in a computationally efficient manner.The complete potential surface in rotational space was calculated with the diffusing molecule midway between its initial and final translational coordinates. In some cases the potentials were sufficiently large at all molecular orientations to eliminate the possibility of diffusion between the given pair of neighbouring sites. When areas of reasonably low potential were found, the surfaces were refined by calculation of the potentials for additional orientations. Only after this winnowing procedure was it efficient to calculate the molecular potentials of M for other spatial coordinates and confirm that the barrier maximum did indeed occur at the jump mid-point.After it had been found between which pairs of lattice sites vacancy exchange actually occurs, the rigid-lattice approximation was also discarded, and neighbouringD. H . Smith 1387 molecules in the lattice were allowed to move as part of the diffusion mechanism. The results for potentials calculated in this way are summarized in tables 3 and 4: Energy of Vacancy Formation, AUf The calculated energy required to form vacancies in naphthalene is shown in table 4, along with the experimental enthalpy of sublimation. The average of five experimental determinations of AH,,,,, which range from 65.6 to 82.0 kJ mol-l, is 72.8 kJ mol-1.33-37 The computed value of AU, is 7 1 .O kJ mol-l.The quantities AU, and AH,,,, are not strictly comparable, since the enthalpy includes the additional term PAV,.23 In the present model, however, the change of volume of the crystal due to vacancy formation is identically zero. Temperature correction terms also occur in comparisons of calculated and measured values in table 4,23 since the heats of ~ u b l i m a t i o n , ~ ~ - ~ ~ crystal s t r u ~ t u r e s ~ ~ ~ l6 and rates of self-diff~sion~~-~~ were measured at different temperatures. However, the corrections are ca. 5% (or approximately equal to the uncertainties in the measured and computed values), so that in numerical comparisons they can be neglected. The agreement between the calculated AU, and the measured heat of s ~ b l i m a t i o n ~ ~ - ~ ~ is essentially an indication of the validity of the assumptions made in treating the lattice energy as the sum of atomic pair potentials, since the heat of sublimation of naphthalene was one of the 81 fitting equations used by Williamsloa to obtain the nine parameters listed in table 1.Heights of Potential Barriers, AU, in the Rigid Lattice Model Nearest Neigh bows It is generally assumed that self-diffusion in atomic solids takes place between nearest- neighbour sites. In naphthalene this diffusion mechanism is complicated by the fact that it requires molecular rotation. The four nearest-neighbour sites to a molecule at the origin lie 0.51 nm distant in the ab basal plane at (k a/2, b/2,0). Hence this mechanism would produce virtually identical rates of diffusion parallel to the a and b axes, but would not contribute to diffusion normal to the ab plane.[In terms of eqn (3), AH,,, would be identical for the two directions, and the ratio D t / D i would be a2/b2 = 1.9; given the accuracy with which Do can be measured, these values are essentially equal.] It appears from the calculations that there must be no significant nearest-neighbour diffusion in which the molecular trajectory crosses a saddle point in the potential surface: achieving the saddle-point orientation requires a rather complicated set of rotations by the diffusing molecule, and the saddle-point potential yields a barrier height (labeled AU* in table 3) which is ca. 60 kJ mol-l smaller than the experimental value (AH1 z 106 kJ mol-’, table 4).Because of this latter fact we are forced to conclude that the diffusing molecule must choose, instead, a dynamically simple trajectory which does not cross the saddle point. Since nearest-neighbour sites belong to different sublattices, a molecule diffusing between such sites must rotate while translating if it is to arrive at its new site with the ‘correct’ orientation.? Geometrically, there are two ways in which a molecule can 1 It is well known from theory and n.m.r. experiments that naphthalene molecule in an equilibrium orientation at a lattice site can undergo an in-plane rotation with an activation energy of CQ. 100 kJ mol-l.ll This motion occurs in the absence of any nearby vacancy, and a vacancy slightly reduces the activation energy.However, the barriers to out-of-plane rotations are extremely large, and the molecular planes of s- and t- molecules are not parallel. Thus the requirement of rotation-with-translation as stated above is not affected, except that it might not be necessary for the diffusing molecule to arrive with exactly the ‘correct’ orientation within the required plane. When the statistical aspects of the jumps are omitted, as here, the effect of the latter possibility is to increase slightly the uncertainty in the calculated value of AUS. If the diffusing molecule exchanged sublattices with no rotation whatsoever, its potential at the new site would be 550 kJ mol-l larger than the equilibrium potential - a very unlikely event. Hence, rotation-with-translation is a quite rigid requirement for any mechanism in which the molecu!e changes sublattices.1388 Self-dflusion in Naphthalene exchange sublattice orientations with a simple uniaxial rotation: it can either rotate 53" in one direction or 127" in the opposite sense, about a 'diagonal' molecular axis passing approximately through the numbers 2 and 6 carbons (IUPAC numbering).(The near coincidence of the rotational axis and the line through the two carbons is 'accidental' and not required by symmetry.) When the molecular potentials for nearest-neighbour diffusion incorporating these rotations are calculated, it is found that the potential barrier for the smaller-angle rotation is prohibitively large. AUI for the 127" rotation, however, is 230 kJ mol-l. (It is assumed that the midpoints of the translation and rotation are reached simultaneously.) Hence, from eqn (9, AU,,, for this motion is 301 kJ mol-l.This value is larger than the experimental one, but it neglects possible motions of other nearby molecules which would decrease AUI. Next-nearest Neighbours Translation without Rotation. The next -neares t -neigh bo ur distance is one unit -cell dimension (0.60 nm) along the b axis; intuitively it might be expected that vacancy interchange between these sites would constitute an important self-diffusion mechanism, since molecular rotation is not required. The potential barrier to this pathway is surprising, however: the value of AUI for a simple translational jump along the b axis is 470 kJ mol-l. From comparison with experiment (see below), the rate of self-diffusion by this mechanism must be completely negligible.Translation with Rotation. Curiously, although molecular rotation is not required since the initial and final sites belong to the same sublattice, the molecule can, in principle, greatly reduce the value of AUt by combining rotation with its translational jump. This is discussed as another example of the many molecular pathways having surprisingly low activation energies which exist in the crystal, but which make no appreciable contribution to the self-diffusion. The postulated mechanism involves a 'sideways slipping' motion in which the molecule rotates about its lengthwise axis (which lies approximately in the bc plane) while translating along the b axis. (see fig.1). The molecule must rotate ca. 40" by the time it reaches the jump mid-point at (0, b / 2 , 0 ) to minimize the repulsive potential from the two nearest molecules at (k a / 2 , b/2, 0). On the second half of the jump the molecule must rotate in the reverse direction, making a -40" rotation to its original orientation. [This is true whether the molecule jumps forward into the original vacancy at (0, b/2, 0) or backwards into the vacancy created at the lattice site it left.] This particular rotation produces a value of AU* of only 30 kJ mol-l, which is ca. 80 kJ mol-1 smaller than the potential barriers of the mechanisms which are actually operative in the crystal (AH1 x 106 kJ mol-l). Self-diflusion to Third-, Fourth- and Fifth-nearest-neighbour Sites From the experimental results it might be expected that the sole diffusion mechanism in naphthalene would be vacancy exchange between third-nearest-neighbour sites.This would account for the apparently isotropic diffusion, since the translational vector has approximately equal components in the three different lattice directions. However, the sites belong to separate sublattices, which makes the mechanisms intuitively less plausible. The calculations are in agreement with this latter expectation : the lowest barrier possible for third-nearest-neighbour diffusion is over 370 kJ mol-1 larger than the experimental value. No other reasonably close pair of sites would produce molecular displacement along all three crystal axes. Hence, there must be at least two separate mechanisms involvingD.H. Smith 1389 different pairs of sites, and the apparent isotropy of the diffusion coefficient must be accidental. No rotation is required by the site symmetries for a molecular jump into a fourth- nearest-neighbour vacancy, which is 0.81 nm distant on the short diagonal of the ac plane ; however, the potential barrier is prohibitively large for any molecular orientation. The situation is similar for diffusion into a fifth-nearest-neighbour vacancy : the site symmetries require only a simple translational motion 0.81 nm along the a axis, but AUI is extremely large regardless of the molecule’s orientation. Hence, significant rates of self-diffusion between third-, fourth- and fifth-nearest- neighbour sites are all ruled out by the intermolecular potentials.Self-diflusion Along the c Axis Sixth-nearest neighbours occur 0.867 nm apart (the unit-cell length) along the c axis. The potential with a molecule at (0, 0, 4) when (0, 0, 0) and (0, 0, 1) are unoccupied can be as little as - 100 kJ mol-l, which corresponds to AU* = 40 kJ mol-1 (table 3). This is a third case in which the height of the potential barrier for the saddle-point trajectory is ca. 70_f 10 kJ mol-1 smaller than the experimental value, and the trajectory must therefore be considered inoperable. This U* is produced by a rotation of M. Since the initial and final sites belong to the same sublattice, it is plausible to think that diffusion between them might occur without rotation of the diffusing molecule. For this particular molecular orientation, the potential passes through its maximum midway between the lattice sites, and the value of AUI is 210 kJ mol-l, which is about twice the experimental value.This point is neither the absolute nor a local sub-saddle point in full six-dimensional space. Summary of Results for the Rigid-lattice Model The experimental enthalpy of activation for self-diffusion is the sum of the heat of vacancy formation, AH,, and the enthalpy barrier, AH$. In the present model, however, both A& and AVJ are identically zero. Accordingly, the equations AUf = AH,, AUI = AH$, and thus AU,,, = AH,,,, should obtain. Early attempts to measure intrinsic self-diffusion in molecular crystals were often thwarted by extrinsic diffusion due to sub-grain boundaries, impurity-induced defects or damage produced in the crystal during preparation of the ample.^^-^^? 35-40 Sherwood, however, has obtained excellent data for self-diffusion normal to the ab cleavage plane of naphthalene.l9 Although the theoretical value of the energy of vacancy formation (AUf = 71 kJ mol-l) agrees well with the experimental heat of sublimation (AHsubl = 73 kJ mol-l, none of the heights of the various potential barriers calculated with the rigid lattice model agrees really well with the experimental value of AH$. While one would expect that the theoretical values should be too large owing to neglect of lattice relaxation, surprisingly all of the trajectories which cross saddle points give theoretical values which are either much smaller than the measured barrier height or else very much larger.In principle, the smaller values could simply indicate that the mechanisms are operative but the atomic pair potentials are too ‘ soft ’. However, the same atomic pair potentials have been used to calculate activation energies in the rigid-lattice approximation for rotational diffusion of a large number of compounds, and it has invariably been found that the theoretical activation energies are larger than the experimental values. This contradicts the suggestion that the potential functions are too soft. Furthermore, in all three saddle-point mechanisms for which AU < AH$ the diffusing molecule must make complicated rotatians about its molecular axes in order to attain the saddle-point orientation. For example, in the second-nearest-neighbour mechanism, the simplest to1390 Self-diflusion in Naphthalene describe, the diffusing molecule must reverse the sign of its angular momentum at the jump mid-point. In a dynamic model it would be implausible that the molecule could do so without being reflected back to its initial lattice site.Together, the evidence indicates that the molecular trajectories of the operative mechanisms do not cross saddle-points in the potential surfaces, and that the transition- state entropy cannot be neglected a priori. Additional evidence has been generated by investigation of two postulated non-saddle-point mechanisms with a relaxed lattice model. Potential Barrier Heights in the Relaxed-lattice Model Whenever it is concluded that the saddle point is not crossed in an operative mechanism, a large number of trajectories can be described which yield the experimental value of the barrier height; the only requirement imposed by the condition that the theoretical and experimental barrier heights agree is that the maximum potential encountered along the trajectory fall somewhere on the contour line in the potential surface defined by AUt = AH$.Despite the apparent limitations imposed by the conclusion that AUt = AH1 is not a sufficient condition to define a unique trajectory, it is still possible to test certain diffusion trajectories which for intuitive or other reasons somehow seem particularly likely. If applied to a variety of plausible trajectories for several different compounds, this procedure may make it possible to build up a detailed and reliable picture of the diffusion mechanisms.From table 3 it may be noted that the smallest value of AUt listed which meet the condition AUt 2 AH1 are for first and sixth-nearest-neighbour mechanism. For both of these trajectories the molecule's angular and spatial coordinates pass simultaneously through the midpoints between their initial and final values, and the orientation angles change either monotonically (nearest neighbour) or not at all (sixth-nearest-neighbour). In other words, changes in direction of the naphthalene molecules' angular momenta during these translational jumps do not occur even though such changes would allow the molecules to cross saddle points. In this section it is hypothesized that these apparent requirements are, in fact, necessary conditions; and the values of AUI are recalculated in a 'relaxed lattice' model which allows other nearby molecules to move.Sixth-nearest-neighbour Mechanism For sixth-nearest-neighbour diffusion the initial and final sites belong to the same sub-lattices, and this hypothesis requires that the diffusing molecule not rotate. Examin- ation of the individual molecular-pair interactions between M and the rest of the molecules in the crystal reveals that even at the barrier maximum in the rigid lattice model only four of these potentials are positive, i.e. repulsive. These four pair potentials are with the neighbours at (-4, -4, 0), (4, #, l), (-4, $, 0) and (+, -4, l), and are all equal to 12.6 kJ mol-1 when M is at (0, 0, +). Furthermore, most of the repulsion comes from just one pair of hydrogens per molecular pair.This suggests that rotation of these four molecules will play a major role in the mechanism and that relaxation of other molecules in the lattice may be minimal. When these four neighbouring molecules are allowed to relax it is found that rotations of ca. 12" (10" for one pair, 15" for the other; the four neighbours are not all equivalent) decrease their molecular pair potential with M at the barrier maximum by 27 and 31 kJ mol-l, respectively. The theoretical barrier for the mechanism in which all four neighbours relax in this manner is AUt = 95 kJ mol-l, in excellent agreement with the experimental value of ca. 106 kJ mol-l. (The agreement would be even better if weD.H . Smith 1391 -801 I I I I 1 I I I I I 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Clnm Fig. 2. Potential for self-diffusion of a naphthalene molecule along the c direction to a sixth-nearest-neighbour vacancy (relaxed lattice model). assumed that for some reason only three of the neighbours rotated.) Rotations of this magnitude are physically quite reasonable; P a ~ l e y , ~ ~ for example, found that the root-mean-square amplitudes of rotational vibration of a molecule in a perfect region of the crystal were ca. 5". Fig. 2 shows the potential barrier for the relaxed lattice model. Nearest-neighbour Mechanism For the nearest-neighbour mechanism, in which M attempts to jump from (0, 0, 0) to (i, f, 0), only two of the neighbours repel the jump in the rigid-lattice model.One of these is at (0, 0, - 1) and has the orientation with which M ends its diffusional jump; the other is at (4, i, 1) and has the orientation with which M starts. Their molecular pair potentials with M at the jump mid-point, (f, f, 0), are 61 and 37 kJ mol-l, respectively. Rotations by ca. 15" of these two molecules (for one the axis is nearly orthogonal to the molecular plane; for the other it is colinear with the central carbon<arbon bond) decrease the net barrier height by 102 and 44 kJ mol-l, respectively, however. Thus, relaxation of these neighbours reduces the calculated barrier height to 85 kJ mol-1 and the value of AU,,, to 155 kJ mol-l. Nearest-neighbour vacancy exchange produces diffusion in both the a and b directions. The experimental enthalpies of activation for these directions are approximately equal, i.e.for both AHact = 180 kJ mol-l; however, the uncertainties in these values are significantly greater than they are for the perpendicular to the ab plane.22 Furthermore, diffusion along the a direction also occurs by a sixth-nearest-neighbour mechanism, so that the experimental value for this direction represents an unresolved average from two different mechanisms. Because of these uncertainties plus those in the calculations, the agreement between the relaxed lattice model and experiment must be considered reasonable for the nearest-neighbour mechanism. Activation-volume Eflects In discussions of diffusion mechanisms it is often suggested that there must be some 'collapse' of the surrounding molecules into a vacancy within a crystal.From a molecular potential standpoint, this effect is defined by the shifts in position of the minima of the potential wells in which the nearby molecules vibrate when a vacancy is introduced into the lattice.1392 -50 - -55- - 8 -60 -65 -70 Self-difusion in Naphthalene - - - - 1 1 I I I I I I I I -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 C/nm Fig. 3. Comparison of the potential wells for vibration of a molecule along the c axis when a sixth-nearest-neighbour site is (a) occupied (filled circles) and (b) vacant (open circles). If the degree of relaxation is small, the changes of position of all other molecules can be treated as small perturbations affecting the change in equilibrium position of any given molecules, and cooperative relaxation effects can be neglected.Fig. 3 compares the potentials for a naphthalene molecule vibrating towards a sixth-neighbour site when the site is (a) occupied and (b) vacant. The coordinates of all other molecules were held constant. Clearly the vacancy produces changes in the vibrational levels due to the increased width and shallower depth of the potential well, but the shift in the position of the potential minimum is negligible. The amplitude of the vibration also increases in the direction away from the vacancy. This happens, of course, because the attractive as well as the repulsive forces of a molecule at the vacancy site are missing. The molecular pair potential with a sixth-nearest neighbour is just 3 % of a molecule’s total lattice potential.The largest molecular pair potential, between nearest neighbours, is four times as large. Even for a nearest-neighbour to a vacancy, however, there is no indication of any significant ‘collapse’ of the molecule into the vacant site. In large part Ayf and AVI can be neglected simply because their contribution to the enthalpy must be very small compared to the potential-energy terms. The calculations indicate that the volume of vacancy formation is a real, but small, effect. On the other hand, it is implausible that the whole lattice can ‘breathe’ during a diffusional jump; at the position of the diffusing molecule, where its potential reaches its maximum, intermolecular distances are short and interatomic repulsions are sensitive functions of any changes of interatomic distances associated with AVt.Hence any expansion of the lattice would lead to substantially lower values of A V t than those calculated. However, excellent agreement with experiment is obtained for the theory in which AVI = 0. Summary of Results for the Relaxed-lattice Model The emergent picture of the mechanisms in the relaxed-lattice model is reminiscent of that envisioned by Rice,26 in which out-of-phase translational lattice vibrations of a pair of neighbours located between the vacancy and the diffusing molecule allow the diffusing molecule to jump between the neighbours and into the vacancy. An important motivation for Rice’s treatment of diffusion26 was the lack of any evidence that at its jump mid-point the diffusing molecule can attain thermodynamic equilibrium with the lattice as assumedD.H. Smith 1393 by the transition-state theory; it was hoped that the pseudo-thermodynamic basis of transition-state theory could be replaced by a completely dynamic model. In the mechanism described here the orientations to .which the neighbouring molecules rotate as M approaches the jump mid-point are, in fact, minima in the potential wells in which the neighbours would vibrate indefinitely if M were somehow fixed at the jump mid-point. Thus in a ‘real’ example similar to a dynamic model previously described in general terms for all crystals, we still find that a quasi-thermodynamic description of the mechanism is possible. Conclusion It has been concluded that diffusion normal to the ab plane occurs only by sixth- nearest-neighbour vacancy exchange, that diffusion along the a-direction occurs by this mechanism and also by nearest-neighbour vacancy exchange, and that diffusion along the b direction also occurs by the latter mechanism.Previous use of the rigid lattice model for rotational diffusion has yielded theoretical activation energies which were too large, but for translational diffusion in naphthalene the model gives saddle-point activation energies which are appreciably smaller than the experimental values. Furthermore, the saddle-point trajectories require complicated rotations of the diffusing molecule; and it appears qualitatively that the transition state entropies of the saddle-point trajectories may be unfavourable. For these reasons, it is tentatively concluded that the molecular trajectories do not cross saddle-points.A relaxed-lattice model, in which the diffusing model either undergoes a direct flip between equilibrium orientations or no rotation at all and in which only two to four lattice molecules undergo substantial rotational relaxation (ca. 10-1 5 O ) , gives activation energies which agree well with experiment. This is further evidence for rejecting the saddle-point trajectories The latter model is reminiscent of Rice’s dynamic theory of atomic diffusion, in which an out-of-phase lattice vibration of a pair of intervening atoms allows the diffusing atom to jump between them and into the vacancy. In the present case, however, the relaxed orientations of the intervening molecules would be their equilibrium orientations if the diffusing molecule were fixed at the potential maximum; hence the language of transition state theory may still be useful.The finding that the saddle-point trajectories are not operative is of considerable theoretical interest and should be further examined to determine if it can be substantiated. I thank Dr W. R. Busing for invaluable discussions and Prof. J. N. Sherwood for providing unpublished data which made possible a more detailed investigation of the diffusion in the ab plane. References 1 2 3 G. Hevesy and F. Paneth, Anorg. Chem., 1913,82, 323. P. G. Shewmon, Diffusion in Solids (McGraw-Hill, New York, 1963). Interatomic Potentials and Simulation of Lattice Defects, ed. P. Gehlen, J. R. Beeler Jr and R. I. Jaffee (Plenum Press, New York, 1972).4 R. Fieschi, G. F. Nardelli and A. Repenci-Chiarotti, Phys. Reo., 1961, 123, 141. 5 J. J. Burton and G. Jura, J . Phys. Chem. Solids, 1966,27,961; 1967, 28, 205. 6 J. J. Burton, Phys. Rev., 1969, 182, 885; 1973, B9, 1200. 7 See also references in the chapter by A. V. Chadwick and J. N. Sherwood in Point Defects in Solids, 8 A. V. Chadwick and J. N. Sherwood, in Diflusion Processes, ed. J. N. Sherwood (Gordon and Breach, 9 R. Fox and J. N. Sherwood, Trans. Faraday SOC., 1971,67, 3364. ed. J. H. Crawford and L. M. Slifkin (Plenum Press, New York, 1975), vol. 2. New York, 1971), vol. 2.Self-diflusion in Naphthalene 10 (a) D. E. Williams, J. Chem. Phys., 1966,45, 3770; (6) D. E. Williams, Trans. Am. Crystallogr. ASSOC., 6, 21. 11 R. K. Boyd, C. A. Fyfe and D. A. Wright, J. Phys. Chem. Solids, 1974,35, 1355. 12 C. A. Fyfe and D. H. Smith, Can. J. Chem., 1976,54, 769; J. Chem. SOC., Faraday Trans. 2, 1975,71, 13 W. R. Busing, J. Phys. Chem. Solids, 1978, 39, 691. 14 C. A. Fyfe and D. H. Smith, Can. J. Chem., 1976,54, 783. 15 D. W. J. Cruickshank, Acta Crystallogr., 1957, 10, 504. 16 G. S. Pawley and E. A. Yeats, Acta Crystallogr, Sect. B, 1967, 25, 2009. 17 R. Mason, Acta Crystaiiogr., 1964, 17, 547. 18 G. P. Charbonneau and Y. Delugeard, Acta Crystallogr., 1976, 832, 1420. 19 J. N. Sherwood and D. J. White, Philos. Mag., 1967, 15, 745. 20 J. N. Sherwood and D. J. White, Philos. Mag., 1967, 16, 975. 21 N. T. Corke and J. N. Sherwood, J. Muter. Sci., 1971, 6, 68. 22 .I. N. Sherwood, personal communication. 23 W. Jost, Diflwion in Solids, Liquids, Gases (Academic Press, New York, 1952), p. 110. 24 N. H. Nachtrieb and G. S . Handler, Acta Metall., 1954, 2, 797. 25 G. Bums and J. N. Sherwood, J. Chem. SOC., Faraday Trans. 1, 1972,5, 1036. 26 S. A. Rice, Phys. Rev., 1958, 112, 804. 27 0. P. Manley and S. A. Rice, Phys. Rev., 1960, 117, 632. 28 0. P. Manley, J. Phys. Chem. Solids, 1960, 13, 244. 29 H. R. Glyde, Rev. Mod. Phys., 1967, 39, 373. 30 C. P. Flynn, Phys. Rev., 1968, 171, 682. 31 S. Brawer, Phys. Rev., 1974, B10, 3287. 32 G. S . Pawley, Phys. Stat. Sol., 1967, 20, 347. 33 R. S. Bradley and T. G. Cleasby, J. Chem. Soc., 1953, 1690. 34 A. A. Zil’bennan-Granovskaya, 1962, 2. Fiz. Khim. USSR, 1940, 14, 759. 35 M. R. Andrews, J. Phys. Chem., 1962, 30, 1479. 36 T. H. Swan and E. Mack Jr, J. Am. Chem. SOC., 1925,47, 21 12. 37 H. Von Hoyer and W. Peperie, Z. Electrochem., 1958, 62, 61. 38 C. H. Lee, H. K. Kevorkian, P. J. Reucroft and M. M. Labes, J. Chem. Phys., 1965,42, 1406. 39 P. J. Reucroft, H. K. Kevorkian and M. M. Labes, J. Chem. Phys., 1966,44,4416. 40 A. R. McGhie, A. M. Voshchenkov, P. J. Reucroft and M. M. Labes, J. Chem. Phys., 1968,48, 186. 967. Paper 61533; Received 17th March, 1986

ISSN:0300-9599    

DOI:10.1039/F19878301381

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1987, 83, 1395-1404 Photogeneration of Hydrogen from Water over an Alumina-supported ZnS-CdS Catalyst Junya Kobayashi, Kenichi Kitaguchi, Hiroshi Tanaka, Hideyasu Tsuiki and Akifumi Ueno* Department of Materials Science, Toyohashi University of Technology, Tempaku, Toyohashi, Aichi 440, Japan The rate of hydrogen generation from water with an alumina-supported mixed semiconductor catalyst (ZnS-CdS/Al,O,) has been found to be much higher than those from water with singly supported catalysts (ZnS/Al,O, and CdS/Al,O,) under irradiation by both U.V. and visible light. The activity is not improved by the physical mixture of these singly supported catalysts even when the amounts of ZnS and CdS are the same as those present in the ZnS-CdS/Al,O,. The significant improvement in the rate of hydrogen production over ZnS-CdS/Al,O, catalyst is attributed to the intimate contact between ZnS and CdS particles that results from the deposition of fine ZnS particles over the CdS surface. Sites for hydrogen production seem to be newly generated on these contacting regions.Light absorption is an important step in the photogeneration of hydrogen, and it takes place over the surface of a catalyst particle, suggesting that the efficiency of light absorption of the catalyst increases with increasing surface area. Thus attention has been paid to fine semiconductor particles so as to improve the activity for hydrogen production from water. Chalcogenides have been employed as photocatalysts, since their photocor- rosion may be overcome by the presence of a sacrificial reagent such as Na,S.l Among the chalcogenides, ZnS2 and CdS3 have been studied extensively, since much knowledge is available about their electronic and electrochemical properties.Henglein et al.* observed a blue shift of the onset in the absorption spectrum for colloidal ZnS when the colloidal ZnS and CdS were co-precipitated in Na,S solution. These authors also studied the relationship between the electronic properties of ZnS and their activity for hydrogen prod~ction.~ Reber and Meier6 observed the enhancement in the rate of hydrogen generation from water when colloidal ZnS was employed as a photocatalyst in the presence of Na,S. The electronic properties of semiconductor catalysts may be varied not only by changing their particle size but also by the addition of an appropriate material that is in contact with the catalysts.Ueno et al.' observed a significant acceleration in the rate of hydrogen production under illumination by both U.V. and visible light over ZnS- CdS/SiO, catalyst, where the fine ZnS particles coat the surface of the CdS particles deposited over the SiO, support. Electron transfer between ZnS and CdS particles probably occurs at their contacting region and may result in the acceleration of hydrogen generation. Serpone et al.* are the first who proposed an interparticle electron transfer to explain the enhanced rate of hydrogen production with the CdS/TiO,/RuO, mixed semiconductor system. Finlayson et aZ.9 believe that the active site for the photogener- ation of hydrogen over ZnS-CdS/SiO, catalyst might be Cd metal produced during the light irradiation. They also observed a flat-band shift of CdS by 0.7 V to more negative potentials when CdS powder was passivated with Zn2+ ions, resulting in the formation of Cd metal and in the reduction of the number of recombination sites. Recently, Reber and RuseklO reported that a solid solution of zinc and cadmium sulphides exhibited significant activity for hydrogen generation from water even without any noble xnztcils.13951396 Photogeneration of Hydrogen on a Semiconductor Catalyst The purpose of this work was to confirm that the particular structure of the ZnS-CdS system, i.e. a model comprising a CdS core and a ZnS shell, is a key to the enhanced rate of hydrogen generation from water. Alumina powder, instead of silica, was used here as a catalyst support, and similar experiments to previous ones7 were carried out.Experimental In the present work alumina powder was used as a catalyst carrier. The alumina powder was prepared by the hydrolysis of aluminium isopropoxide dissolved in n-butanol, which is one of the best methods to obtain pure alumina.ll The specific surface area of the alumina was varied by controlling the pH value of the solution and the amount of water added for hydrolysis,12 with powders of 260 and 400 m2 g-l being employed in this work. In most of the experiments the alumina powder with 260 m2 g-l surface area was used. The catalysts employed were as follows. (1) ZnS/Al,O, and CdS/Al,O,. These singly supported catalysts were prepared by a conventional impregnation method and the amounts of ZnS and CdS over alumina were 24 and 26 wt % , respectively.(2) ZnS/Al,O,- CdS/Al,O,. This comprised the physical mixture of the singly supported catalysts in (1) with the atomic ratio of Zn/Cd being unity. (3) ZnS-CdS/Al,O,. This catalyst was prepared by a coimpregnation method using an aqueous solution of Zn(NO,), - 6H,O and Cd(NO,), - 4H20 and water saturated with H,S. Then 12 and 13 wt % of ZnS and CdS, respectively, were loaded over alumina, as determined by atomic absorption spectroscopy. This corresponds to a Zn/Cd atomic ratio of unity. (4) ZnS(CdS/Al,O, ) and CdS(ZnS/Al,O, ) : the ZnS(CdS/Al,O, ) catalyst was prepared by sequential depo- sition of CdS over alumina followed by deposition of ZnS over the CdS particles, and the CdS(ZnS/Al,O,) catalyst was made by sequential deposition in the opposite order.In both catalysts the loadings of ZnS and CdS were 12 and 13 wt% , respectively, with a Zn/Cd atomic ratio equal to unity. (5) ZnS-CdS. The unsupported ZnS-CdS catalyst was prepared by coprecipitation of ZnS and CdS using Zn(N03);6H,0 and Cd(NO3),-4H,O and H,S-saturated water as a precipitant. The Zn/Cd atomic ratio was also kept at unity. The catalyst thus prepared was placed (not suspended) on the bottom of a 20 cm3 Pyrex glass vial in which 10 cm3 of water containing 0.1 mol of Na,S was poured, the pH value of this solution being 12.5. Usually, the amount of catalyst employed was 50 mg. The solution was de-aerated by ultrasonication and purged with N,.A 450-W Xe lamp, equipped with a water filter for the removal of i.r. light, was used to irradiate the catalyst from the bottom of the vial. A 440 nm cutoff filter was used for the experiments with visible light. The top of the vial was sealed with a rubber septum, through which the hydrogen produced was transferred to a gas chromatograph by a gas syringe. For analysis a column packed with molecular sieve 13X and N, as a carrier gas were used. The catalysts were examined by X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). XRD (Rigakud- enki, Geigerflex) was operated at 30 kV and with a filament current of 15 mA using an Ni filter for Cu Ka irradiation. AES (JEOL, Jump 10) was operated at an accelerating voltage of 10 kV with a sample current of ca.1 A. The surface composition of the catalyst was measured by the AES sputtering technique using Ar+ ions at an accelerating voltage of 3 kV and a beam current of 25 mA. The particle sizes of ZnS and CdS in ZnS/Al,O, and CdS/Al,O,, respectively, were monitored by TEM (Hitachi, H-800) operated at an accelerating voltage of 200 kV with a magnification of x lo5. The photographs obtained were not clear enough for the ZnS and CdS particles to be hardly distinguished from the shadows of alumina powder. The concentrations of Zn and Cd ions in the catalyst were measured by atomic absorption spectroscopy. The catalyst powder was first ground in an agate mortar and then treated with nitric acid at an elevated temperature to extract the Zn and Cd ionsJ .Chem. SOC., Faraday Trans. I , Vol. 83, part 5 Plate 1 Plate 1. TEM photographs of the unsupported ZnS-CdS (a) and the singly supported ZnS (6) and CdS ( c ) catalysts. J. Kobayashi, K. Kitaguchi, H. Tanaka, H. Tsuiki and A. Ueno (Facing p . 1397)J . Kobayashi, K. Kitaguchi, H. Tanaka, H. Tsuiki and A . Ueno 1397 100 0 . 0 0 I I 20 30 40 50 2ei0 Fig. 1. X-Ray diffraction patterns of (a) unsupported and (b) alumina-supported ZnS-CdS catalysts; Q-ZnS and Q-CdS are denoted and 0, respectively. for analysis. The concentrations of Zn and Cd ions in the impregnating solution were also measured by atomic absorption spectroscopy during the coimpregnation procedure. Thus the rates of precipitation of Zn and'Cd ions over the alumina surface as ZnS and CdS were measured.The power dependence on the rate of hydrogen generation was studied by adjusting the power of the incident light using a few kinds of gauzes made of stainless steel. The power of the incident light was monitored by a photomultiplier. Results The X-ray diffraction patterns of the unsupported ZnS-CdS and alumina-supported ZnS-CdS catalysts are shown in fig. 1. All the peaks observed for the unsupported catalyst were assigned to either P-ZnS (cubic) or P-CdS (cubic), suggesting that the crystallographic structures of ZnS and CdS in the supported catalysts are probably P-ZnS and P-CdS, respectively, although only a small and broad peak appeared at 28 = 28" in the case of the supported catalyst.The activities of the catalysts for the photogeneration of hydrogen under irradiation by u.v.-visible light are shown in fig. 2, where the amount of hydrogen evolved is expressed in units of cm3 per 50mg of each catalyst. TEM photographs of the unsupported ZnS-CdS and the singly supported ZnS/A1,03 and CdS/Al,O, catalysts are shown in plate 1. The size of the particles in the unsupported catalyst was as large as 4 x lo3 A, while CdS particles in CdS/A1,0, were ca. 200 A in size. It was hard to identify the ZnS particles in the ZnS/Al,O, catalyst partly because of their small size. Since the lower limit of TEM under the conditions employed here is ca. 20 A, the size of the ZnS particles was estimated as < 20 A. The change in the concentrations of the Zn and Cd ions in the solution during impregnation is depicted in fig.3. The concentration of Cd ions decreased rapidly when H,S-saturated water was poured into the impregnating solution, while the concentration of Zn ions decreased less rapidly than that of Cd ions. The depth profiles of the ZnS-CdS/AI,O,, ZnS(CdS/Al,O,) and CdS(ZnS/Al,O,) catalysts measured by AES are given in fig. 4. Since the sputtered depth could not be precisely estimated, the sputtering time was employed in fig. 4. The composition was expressed by the peak height ratio of Zn/Cd in the corresponding AES spectrum, the1398 Photogeneration of Hydrogen on a Semiconductor Catalyst 0 1 2 3 4 5 6 7 8 irradiation time/h Fig. 2. Activities of the catalysts for photogeneration of hydrogen under the irradiation of u.v.-visible light.0, ZnS-CdS/Al,O,; (>, ZnS/Al,O,; 0, CdS/Al,O,; 0, ZnS/Al,O,- CdS/Al,O,; 0, CdS-ZnS. The amounts of the catalysts employed were 50 mg. - * - Y v I5 1 2 24 coprecipitat ing time/ h Fig. 3. Changes in the concentrations of Zn (0) and Cd (0) ions in the solution during coprecipitation of ZnS and CdS over alumina powder. peaks due to Zn (LMM) and Cd (MNN) being observed at 997 and 376 eV, respectively. The activities of the designed catalysts, ZnS(CdS/Al,O,) and CdS(ZnS/Al,O,), were compared with that of the ZnS-CdS/Al,O, catalyst under illumination by u.v.-visible light (see fig. 5). The ZnS-CdS/Al,O, catalyst showed a high activity for hydrogen generation even under illumination by visible light (A > 440 nm) and the activity was compared with those of the designed catalysts (see fig.6).J. Kobayashi, K. Kitaguchi, H. Tanaka, H. Tsuiki and A . Ueno 1399 2.0 I- - 0 5 10 15 Fig. 4. Depth profiles of Zn and Cd ions in (a) ZnS(CdS/Al,O,), (b) ZnS-CdS/Al,O, and (c) CdS(ZnS/Al,O,). sputtering t ime/m in P 0 1 2 3 4 5 6 7 8 Fig. 5. Activities of ZnS-CdS/Al,O, (O), ZnS(CdS/Al,O,) (0) and CdS(ZnS/Al,O,) (a) catalysts for photogeneration of hydrogen under irradiation by u.v.-visible light. The amounts of catalysts employed were 50 mg. irradiation time/ h1400 Photogeneration of Hydrogen on a Semiconductor Catalyst 0.10 0.08 \ x - 0 0.06 z (4- 0 c u 2 0.04 m irradiation time/h P 0 2 4 6 a Fig. 6. The activities of ZnS-CdS/A120, (O), ZnS(CdS/Al,O,) (0) and CdS(ZnS/Al,O,) (0) catalysts under irradiation by visible light.The amounts of catalysts employed were 50 mg. Fig. 7. Power dependence of the activity of the ZnS-CdS/Al,O, catalyst for photogeneration of hydrogen. The amount of catalyst employed was 50 mg. Fig. 7 shows the power dependence of the activity of ZnS-CdS/Al,O, catalyst under irradiation by u.v.-visible light. The rate of hydrogen production increased as the first order of the incident light power, indicating that the hydrogen was produced through a one-photon absorption mechanism on the present catalyst. By varying the surface area of the alumina support of the ZnS-CdS/Al,O, catalysts, the activity for photogenerationJ . Kobayashi, K. Kitaguchi, H, Tanaka, H. Tsuiki and A. Ueno 1401 0 1 2 3 4 5 6 7 8 Fig. 8. Change in the activity of the ZnS-CdS/Al,O, catalyst for photogeneration of hydrogen with the change in the surface area of the alumina support.0, 400 m2 g-l; 0, 260 m2 g-l. The amounts of catalysts employed were 50 mg. irradiation time/h of hydrogen was significantly modified (shown in fig. 8). The schematic models for ZnS(CdS/Al,O,) and CdS(ZnS/Al,O,) catalysts are shown in fig. 9, where the area of contact between ZnS and CdS particles in the former is apparently higher than that in the latter. Discussion There have been many works on the photocatalytic properties of ZnS,13 and Kakuta et aZ.14 might be the first who reported the enhanced rate of hydrogen generation under the irradiation of u.v.-visible light over ZnS-CdS/Nafion system. Silica-supported ZnS-CdS catalysts also exhibited a rate much higher than the singly supported ZnS and CdS catalysts.' Similar results were obtained for the alumina-supported ZnS-CdS catalyst in the present work.The rate decreased in the following sequence: ZnS- CdS/Al,O, > unsupported ZnS-CdS > ZnS/Al,O, physical mixture with CdS/Al,O, (see fig. 2). Although the amounts of ZnS and CdS in the unsupported catalyst were four times greater than those in the ZnS-CdS/Al,O, catalyst, the rate with the unsupported .catalyst was four times lower than that with the supported one. This may be due, in part, to the particle sizes of ZnS and CdS in the catalysts. As is shown in plate 1, the particles in the unsupported catalyst are > 4 x lo4 A, while the particles of ZnS and CdS in the singly supported catalysts are ca.20 and 200 A, respectively. Since the preparation procedures and the conditions of the ZnS-CdS/Al,O, catalyst were the same as those of the singly supported catalysts, the particle sizes of ZnS and CdS in the ZnS-CdS/A120,1402 Photogeneration of Hydrogen on a Semiconductor Catalyst sample would also be ca. 20 and 200 A, respectively. The formation of a solid solution between ZnS and CdS in the ZnS-CdS/Al,O, catalyst might be one way of interpreting the enhanced rate of hydrogen generation over ZnS-CdS/Al,O,. Since the X-ray diffraction spectrum of the catalyst showed only a small and broad peak at 28 = 28", the structures of the ZnS and CdS particles could not be precisely determined (see fig. 1). Nevertheless, the formation of a solid solution is unlikely in the ZnS-CdS/Al,O, catalyst, since it was shown by U.V.reflection spectroscopy7 that the solid solution was not observed in the ZnS-CdS/SiO, catalyst. However, the question arises as to why the rates of hydrogen generation with the singly supported catalysts and with their physical mixture were so much lower than that with ZnS-CdS/Al,O,, although the sizes of ZnS and CdS particles are the same in these catalysts. This might be explained by the idea that intimate contact between ZnS and CdS particles is necessary for an acceleration of the rate of hydrogen generation under the illumination of u.v.-visible light and that such intimate contact between the particles is much more likely in the ZnS-CdS/AI,O, catalyst than in the physically mixed sample.As may be seen in fig. 3, Cd ions were deposited more rapidly over the alumina surface than Zn ions during the coprecipitation, suggesting that the ZnS-CdS/Al,O, is composed of fine ZnS particles deposited over CdS particles coating the alumina surface. In order to see if this particular structure causes an enhanced rate of hydrogen production, the activities of the designed catalysts, ZnS(CdS/Al,O,) and CdS(ZnS/Al,O,), were compared with that of ZnS-CdS/Al,O,. The absolute amounts of Zn and Cd ions in the vicinity of the surface of the catalyst powders were hard to determine, but the sequential deposition of Zn and Cd ions in the designed catalysts is clearly shown in fig. 4. The particulate structure of the ZnS-CdS/Al,O, catalyst, i.e. comprising a ZnS shell and a CdS core, was also shown, since the depth profile of the ZnS-CdS/Al,O, catalyst was similar to that of ZnS(CdS/Al,O,).The activity of the ZnS(CdS/Al,O,) catalyst was, as expected, the same as that of ZnS-CdS/Al,O, at the stationary state of the reaction and was higher than that of the CdS(ZnS/Al,O,) catalyst (see fig. 6). It is concluded that in order to enhance the rate of hydrogen production, (1) the ZnS and CdS particles must be fine, (2) the ZnS and CdS particles should be in intimate contact with each other and (3) the ZnS particles should be superimposed on the surface of CdS particles first deposited over the Al,O, surface. Similar conclusions have been obtained for a silica-supported ZnS-CdS ~atalyst,~ indicating that with any kind of support this particular structure exhibits an accelerated rate of hydrogen production from water under the irradiation of u.v.-visible light.Why has this particular structure an advantage for hydrogen generation under illumination? Experiments with visible light (A > 440 nm) were carried out in order to answer the question. Since the band gap between the conduction and valence bands of ZnS is 3.7 eV (320 nm) and that of CdS is 2.4 eV (520 nm),15 the visible light employed in this work can only excite the electrons from the valence band of CdS to the conduction band. As clearly shown in fig. 6 , the ZnS-CdS/Al,O, and ZnS(CdS/Al,O,) catalysts again showed high activities for hydrogen production even under the irradiation of visible light. The activity of ZnS-CdS/Al,O, was much higher than that of the singly supported CdS/Al,O, catalyst, suggesting that the interaction between ZnS and CdS particles at their boundary plays an important role in the hydrogen generation.Part of the interaction may be interpreted by electron transfer from CdS to ZnS particles through impurity levels in the band of the ZnS semiconductor, the levels originating from structural deficiencies such as anion vacancies and interstitial cations.16 The electrons excited to the conduction band of CdS by the irradiation of visible light might be transferred to an impurity level in ZnS if the potential of the impurity level is nearly equal to that of the conduction band of CdS. The electrons thus trapped in the impurity level of ZnS might further absorb another photon to be excited to the conduction band of ZnS to react with H+ in the liquid phase for hydrogen production.According to this electron-transfer model, hydrogen will beJ . Kobayashi, K. Kitaguchi, H . Tanaka, H. Tsuiki and A . Ueno 1403 \ ZnS (CdS/Al2O3) A1203’ Fig. 9. Schematic models for the ZnS(CdS/Al,O,) and CdS(ZnS/Al,O,) catalysts. produced through a two-photon absorption mechanism. This is in contrast to the results given in fig. 7, where a one-photon mechanism is suggested for hydrogen production with the alumina-supported ZnS-CdS catalyst. Consequently, the electron-transfer model seems unable to explain the enhanced rate of hydrogen generation with the ZnS- CdS/Al,O, catalyst. Structural models of ZnS(CdS/Al,O, ) and CdS(ZnS/Al,O,) are presented schemat- ically in fig.8. Since the particle size of ZnS is much smaller than that of CdS, the contacting area between ZnS and CdS particles in the ZnS(CdS/Al,O,) catalyst will be larger than that in the CdS(ZnS/Al,O,) catalyst, resulting in the higher rate of hydrogen production over the ZnS(CdS/Al,O,) catalyst. The area of contact between the particles increases with increasing surface area of the alumina support; hence the activity of the ZnS-CdS/Al,O, catalyst increases with increase in the surface area of the support (see Thus we believe that the key to the enhanced rate of hydrogen generation with the supported ZnS-CdS catalyst systems lies in how large is the area of contact between the ZnS and CdS particles, since at the contacting surface the photocatalytic activity of ZnS(CdS/Al,O,) must be the same as that of the CdS(ZnS/Al,O,) catalyst.Only the particular structure with a ZnS shell and a CdS core supplies the large contact area required between ZnS and CdS particles. The problem to be solved is what kind of interactions are in the contact area and how they contribute to the enhanced rate of hydrogen generation under the illumination of light. fig. 9). References 1 2 3 4 5 6 7 8 9 10 11 12 A. B. Ellis, S. W. Kaiser and M. S. Wrighton, J. Am. Chem. SOC., 1976,98, 6855; T. Inoue, T. Wata- nabe, A. Fujishima, K. Honda and K. Kobayakawa, J. Electrochem. Soc., 1977, 124, 719. R. E. Stephens, B. Ke and D. Trivichi, J. Phys. Chem., 1976, 59, 966; S. Yanagida, T. Azuma and H. Sakurai, Chem. Lett., 1982, 1868. 3.R. Darwent, J. Chem. SOC., Faraday Trans. 2, 1981, 77, 1703; M. Matsumura, Y. Saho and H. Tsubomura, J. Phys. Chem., 1983, 87, 3807; M. Gutierrez and A. Henglein, Ber. Bunsenges. Phys. Chem., 1983,87,474. A. Henglein and M. Gutierrez, Ber. Bunsenges. Phys. Chem., 1983, 87, 852. H. Weller, U. Koch, M. Gutierrez and A. Henglein, Ber. Bunsenges. Phys. Chem., 1984, 88, 649, J-F. Reber and K. Meier, J. Phys. Chem., 1984,88, 5903. A. Ueno, N. Kakuta, K. H. Park, M. F. Finlayson, A. J. Bard, A. Campion, M. A. Fox, S. E. Webber and J. M. White, J. Phys. Chem., 1985, 89, 3828. N. Serpone, E. Borgarello and M. Gratzel, J. Chem. SOC., Chem. Commun., 1984, 342. M. F. Finlayson, B. L. Wheeler, N. Kakuta, K. H. Park, A. J. Bard, A. Campion, M. A. Fox, S. E. Webber and J. M. White, J . Phys. Chem., 1985, 89, 5676. J-F. Reber and M. Rusek, J. Phys. Chem., 1985,90, 824. Y. Takai, A. Ueno and Y. Kotera, Bull. Chem. SOC. Jpn, 1983, 56, 2941. H. Nakabayashi, Y. Hirashima, K. Nishiwaki, E. Tanigawa and A. Ueno, to be published.1 404 Photogeneration of Hydrogen on a Semiconductor Catalyst 13 S. Yanagida, T. Azuma, H. Kawakami, H. Kizumoto and H. Sakurai, J. Chem. SOC., Chem. Commun., 1984, 21; J. Chem. SOC., Perkin Trans. 2, 1985, 1487; S. Yanagida, K. Kizumoto and C. Pac, J. Am. Chem. SOC., in press. 14 N. Kakuta, K. H. Park, M. F. Finlayson, A. J. Bard, A. Campion, M. A. Fox, S. E. Webber and J. M. White, J. Phys. Chem., 1985, 89, 732; A. M. H. Mau, C. B. Huang, N. Kakuta, A. J. Bard, A. Campion, M. A. Fox, J. M. White and S. E. Webber, J. Am. Chem. SOC., 1984, 106, 6537. I5 H. Tsubomura, Photoelectrochemistry and Energy Conversion (in Japanese) (Kagaku Dojin, Tokyo, 1980), pp. 179. 16 H. Tributsch and J. C. Bennet, J. Appl. Chem. Biotechnol., 1981,31,565; N. Riehl, Trans. Faraday SOC., 1939, 35, 135. Paper 61873; Received 6th May, 1986

ISSN:0300-9599    

DOI:10.1039/F19878301395

出版商:RSC    

年代:1987

数据来源: RSC