摘要:

Contents 4259 4269 4277 4287 4295 431 1 4321 4335 Protonation Constant of Caffeine in Aqueous Solution M. Spiro, D. M. Grandoso and W. E. Price Ionic Equilibria in Acetonitrile Solutions of 2-, 3- and 4-Picoline N-oxide Perchlorates, studied by Potentiometry and Conductometry L. Chmurzynski, A. Wawrzyn6w and Z. Pawlak Liquid-phase Adsorption of Binary Ethanol-Water Mixtures on NaZSM-5 Zeolites with Different Silicon/Aluminium Ratios W-D. Einicke, M. Heuchel, M. v.Szombathely, P. Brauer, R. Schollner and 0. Rademacher Influence of Oxidation/Reduction Pretreatment on Hydrogen Adsorption on Rh/TiO, Catalysts. An lH Nuclear Magnetic Resonance Study J. P. Belzunegui, J. M. Rojo and J. Sanz Volumetric Properties of Mixtures of Simple Molecular Fluids A. C. Colin, E. G. Lezcano, A.Compostizo, R. G. Rubio and M. D. Peiia Study of Ultramicroporous Carbons by High-pressure Sorption. Part 4.-Iso- thems and Kinetic Transport in Activated Carbons J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros Kinetic and Equilibrium Studies associated with the Solubilisation of n- Pentanol in Micellar Surfactants G. Kelly, N. Takisawa, D. M. Bloor, D. G. Hall and E. Wyn-Jones The effect of Carboxylic Acids on the Dissolution of Calcite in Aqueous Solution. Part 1 .-Maleic and Fumaric Acids R. G. Compton, K. L. Pritchard, P. R. Unwin, G. Grigg, P. Silvester, M. Lees and W. A. House 130-2Contents 4259 4269 4277 4287 4295 431 1 4321 4335 Protonation Constant of Caffeine in Aqueous Solution M. Spiro, D. M. Grandoso and W. E. Price Ionic Equilibria in Acetonitrile Solutions of 2-, 3- and 4-Picoline N-oxide Perchlorates, studied by Potentiometry and Conductometry L.Chmurzynski, A. Wawrzyn6w and Z. Pawlak Liquid-phase Adsorption of Binary Ethanol-Water Mixtures on NaZSM-5 Zeolites with Different Silicon/Aluminium Ratios W-D. Einicke, M. Heuchel, M. v.Szombathely, P. Brauer, R. Schollner and 0. Rademacher Influence of Oxidation/Reduction Pretreatment on Hydrogen Adsorption on Rh/TiO, Catalysts. An lH Nuclear Magnetic Resonance Study J. P. Belzunegui, J. M. Rojo and J. Sanz Volumetric Properties of Mixtures of Simple Molecular Fluids A. C. Colin, E. G. Lezcano, A. Compostizo, R. G. Rubio and M. D. Peiia Study of Ultramicroporous Carbons by High-pressure Sorption. Part 4.-Iso- thems and Kinetic Transport in Activated Carbons J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros Kinetic and Equilibrium Studies associated with the Solubilisation of n- Pentanol in Micellar Surfactants G. Kelly, N. Takisawa, D. M. Bloor, D. G. Hall and E. Wyn-Jones The effect of Carboxylic Acids on the Dissolution of Calcite in Aqueous Solution. Part 1 .-Maleic and Fumaric Acids R. G. Compton, K. L. Pritchard, P. R. Unwin, G. Grigg, P. Silvester, M. Lees and W. A. House 130-2

ISSN:0300-9599    

DOI:10.1039/F198985FX013

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA DEUTSCHE BUNSEN-GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE KONINKLIJKE NEDERLANDS CHEMISCHE VERElNlGlNG SOCIETE FRANGAISE DE CHIMIE, DIVISION DE CHlMlE PHYSIQUE FARADAY DIVISION GENERAL DISCUSSION No. 90 Colloidal Dispersions University of Bristol, 10-12 September 1990 Orga nising Com mitte e Professor R. H. Ottewill (Chairman) Professor P. Botherol Professor E. Ferroni Or J. W. Goodwin Professor H. Hoff mann Professor A.L. Smith Professor P. Stenius Dr Th. F. Tadros Professor A. Vrij Dr D. A. Young The joint meeting of the Societies will be directed towards examining current understanding of the behaviour of colloidal dispersions. In particular, stability and instability, short range interactions, dynamic effects, non-equilibrium interaction, concentrated dispersions and order-disorder phenomena will form topics for discussion.The preliminary programme is now availablemay 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. 26 Molecular Transport in Confined Regions and Membranes Oxford, 17-18 December 1990 Experimental, theoretical and simulation studies which address fundamental aspects of molecular transport will be discussed in the following main areas: a) Transport of atoms and molecules in pores, zeolite networks and other cavities; time-dependent statistical mechanics of small systems in confined geometries b) Molecular transport through synthetic membranes, biological membranes, smectic liquid crystalline phases and Langmuir Blodgett films; the dynamics of the molecules forming the membrane c) Diffusion, reorientation, conformational dynamics, viscosity and conductivity of polymer melts, to include papers dealing with bulk systems since the segments of the polymer will move in the anisotropic environment of the complete chain d) Applications of Brownian dynamics to the study of diffusion in porous media and across membranes including the transport of flexible aggregates such as microemulsions e ) The growth of crystals, colloidal aggregates and droplets on irregular surfaces and in pores Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 31 December 1989 to: Dr D.J. Tildesley, Department of Chemistry, The University, Southampton SO9 SNH. Full papers for publication in the Symposium Volume will be required by August 1990.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA DEUTSCHE BUNSEN-GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE KONINKLIJKE NEDERLANDS CHEMISCHE VERElNlGlNG SOCIETE FRANGAISE DE CHIMIE, DIVISION DE CHlMlE PHYSIQUE FARADAY DIVISION GENERAL DISCUSSION No. 90 Colloidal Dispersions University of Bristol, 10-12 September 1990 Orga nising Com mitte e Professor R. H. Ottewill (Chairman) Professor P. Botherol Professor E. Ferroni Or J. W. Goodwin Professor H. Hoff mann Professor A.L. Smith Professor P. Stenius Dr Th.F. Tadros Professor A. Vrij Dr D. A. Young The joint meeting of the Societies will be directed towards examining current understanding of the behaviour of colloidal dispersions. In particular, stability and instability, short range interactions, dynamic effects, non-equilibrium interaction, concentrated dispersions and order-disorder phenomena will form topics for discussion. The preliminary programme is now availablemay 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. 26 Molecular Transport in Confined Regions and Membranes Oxford, 17-18 December 1990 Experimental, theoretical and simulation studies which address fundamental aspects of molecular transport will be discussed in the following main areas: a) Transport of atoms and molecules in pores, zeolite networks and other cavities; time-dependent statistical mechanics of small systems in confined geometries b) Molecular transport through synthetic membranes, biological membranes, smectic liquid crystalline phases and Langmuir Blodgett films; the dynamics of the molecules forming the membrane c) Diffusion, reorientation, conformational dynamics, viscosity and conductivity of polymer melts, to include papers dealing with bulk systems since the segments of the polymer will move in the anisotropic environment of the complete chain d) Applications of Brownian dynamics to the study of diffusion in porous media and across membranes including the transport of flexible aggregates such as microemulsions e ) The growth of crystals, colloidal aggregates and droplets on irregular surfaces and in pores Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 31 December 1989 to: Dr D.J. Tildesley, Department of Chemistry, The University, Southampton SO9 SNH. Full papers for publication in the Symposium Volume will be required by August 1990.

ISSN:0300-9599    

DOI:10.1039/F198985BX015

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

Special Issues of Faraday Transactions Readen of theFaraday Transactions will be aware that we have now established Special Issues which fall broadly into two categories (a) Collections of the ref& p a p that have been presented at a Scientific conference, normally run by a Subject Gmup of the RSC, and approved in advance by the FaractqY Editorial Board. We insist, with varying degrees of success, that the papers published under this scheme shall d d b e original work which fully meets the n o d requirements for submitted papers. (b) Keynote Issues which are opened by apaper Written by an acknowledged authority in a particular field of interest 'Ihe Keynote author then invites colleagues known to him to contribute original papers on cognate topics, to appear in the same Keynote Issue.These papers are r e f d and, as intended, have led to issues of an exceptionally high scientific standard The initial stage of this development having been completed, Faraday Editorial B o d has diFected that wider invitations should be issued to the physicochemical and chemical physics Community to send us original work which they would like to appear alongside papers on similar topics. Special issues in the planning stage include the topics listedbelow. lf you think that you have some good work coming along which could be ready for submission around the indicated date, please do write to me and let me know, when I will make the necessary arrangements. For my part, I see no objection to adding such papers to Special Issues in both categories.DavidYmg Scientific Editor special Issue Reactive and Inelastic Scattering Concentrated Colloidal Dispersions Structure and Activity of Adsorbed Species (with special emphasis on surface science) Closing Date 15thNovember 1989 30th September 1989 Tobeannounced 21 FAR 1Special Issues of Faraday Transactions Readen of theFaraday Transactions will be aware that we have now established Special Issues which fall broadly into two categories (a) Collections of the ref& p a p that have been presented at a Scientific conference, normally run by a Subject Gmup of the RSC, and approved in advance by the FaractqY Editorial Board. We insist, with varying degrees of success, that the papers published under this scheme shall d d b e original work which fully meets the n o d requirements for submitted papers.(b) Keynote Issues which are opened by apaper Written by an acknowledged authority in a particular field of interest 'Ihe Keynote author then invites colleagues known to him to contribute original papers on cognate topics, to appear in the same Keynote Issue. These papers are r e f d and, as intended, have led to issues of an exceptionally high scientific standard The initial stage of this development having been completed, Faraday Editorial B o d has diFected that wider invitations should be issued to the physicochemical and chemical physics Community to send us original work which they would like to appear alongside papers on similar topics. Special issues in the planning stage include the topics listedbelow. lf you think that you have some good work coming along which could be ready for submission around the indicated date, please do write to me and let me know, when I will make the necessary arrangements.For my part, I see no objection to adding such papers to Special Issues in both categories. DavidYmg Scientific Editor special Issue Reactive and Inelastic Scattering Concentrated Colloidal Dispersions Structure and Activity of Adsorbed Species (with special emphasis on surface science) Closing Date 15thNovember 1989 30th September 1989 Tobeannounced 21 FAR 1ISSN 0300-9599 JCFTAR 85(4) 783-1 01 8 (1 989) 783 80 1 813 829 837 843 855 869 883 895 907 917 929 945 957 969 977 99 I JOURNAL OF THE CHEMICAL SOCIETY Faraday Transact ions Physical Chemistry in Condensed Phases CONTENTS Tin Dioxide Gas Sensors.Part 3 .-Solid-state Electrochemical Investigations of Reactions taking place at the Oxide Surface J. F. McAleer, A. Maignan, P. T. Moseley and D. E. Williams Electron Spin Resonance Investigation of 21-0,-supported Ruthenium. Evi- dence of Strong Metal-Support Interaction M. G. Cattania, A. Gervasini, F. Morazzoni, R. Scotti and D. Strumolo Photoelectrochemical Investigations of Phenosafranine Dye bound to some Macromolecules Electron Spin Resonance Studies in an Irradiated Single Crystal of Hexakis- (ammonium) Diformylated Octamolybdate Dihydrate S. Han, J. Chen and X. You OH Groups in Boralites Determination of the Kinetics of Facilitated Ion Transfer Reactions across the Micro Interface between Two Immiscible Electrolyte Solutions J.A. Campbell, A. A. Stewart and H. H. Girault Effect of Form on the Surface Reactivity of Differently Prepared Zinc Oxides V. Bolis, B. Fubini, E. Giamello and A. Reller Characterization of Iron Oxide-dispersed Activated Carbon Fibres with Fe K- Edge XANES and EXAFS and with Water Adsorption K. Kaneko, N. Kosugi and H. Kuroda Fonnate Oxidation induced by a Copper Peroxo Complex produced in Fenton- like Reactions H. C. Sutton Structural Investigation on a Spinel-related Zn/Cr = 1 Mixed-oxide System C. Cristiani, P. Forzatti and M. Bellotto Characterization of the Mixed Perovskite BaSn,-,Sb,O, by Electrolyte Electroreflectance, Diffuse Reflectance, and X-Ray Photoelectron Spectroscopy G. Larramona, C. GutiCrrez, I. Pereira, M. R. Nunes and F. M. A. da Costa 'H and 13C Longitudinal and Transverse Relaxation in Aerosol OT in Methanol Solution and Inverted Microemulsions in Benzene F.Heatley Carbon Monoxide and Carbon Dioxide Adsorption on Cerium Oxide studied by Fourier-transform Infrared Spectroscopy. Part 1 .-Formation of Carbonate Species on Dehydroxylated CeO, at Room Temperature C. Li, Y. Sakata, T. Arai, K. Domen, K-i. Maruya and T. Onishi Isopiestic Measurement of Salt Imbibition in Zeolites Na-X and Na-Y B. M. Lowe and C. G. Pope Study of the Conformational Equilibrium between Rotational Isomers using Ultrasonic Relaxation and Raman Spectroscopy. Part 3 . 4 -Bromo-2- chloroethane S. Koda, H. Matsui and H. Nomura Electrochemical Behaviour of Polyaniline in Weak Acid Solutions T. Hirai, S. Kuwabata and H. Yoneyama Simultaneous Alternating Current Impedance/Electron Spin Resonance Study of Electrochemical Doping in Polypyrrole A. M. Waller and R. G. Compton The Dark and Radiation-induced Microwave Conductivity of Frozen Aqueous Gels J. Eden, D. van Lith, J. M. Warman and A. Hummel R. Ramaraj and P. Natarajan J. Datka and Z. Piwowarska 21-2Contents 999 Supported Palladium Catalyst Prepared from Amorphous Palladium-Zirco- nium. Structural Properties and Catalytic Behaviour in the Hydrogenation of Carbon Dioxide A. Baiker and D. Gasser Estimation of Group Dipole Moments from Surface Potential Measurements on Langmuir Monolayers 0. N. Oliveira Jr, D. M. Taylor, T. J. Lewis, S. Salvagno and C. J. M. Stirling 1009

ISSN:0300-9599    

DOI:10.1039/F198985FP045

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

The following papers were accepted for publication in Faraday Transactions I during January 1989. 6/00008H 8/0 1 33 1 D 8/02209G 8/02540A 8/2644K 8~028635 8J02864H 8/02954G 8/02994F 8/030605 81032636 8/0328 1E 8/03291B 8D3577F 8103655A 8/03658F 8/03735C 8D3795G 8/03941K 8/04064H 8/04069I Modelling the Effect of Pressure on the Rates of Ionic and Polar Reactions Gavish, B. The Interaction between Amine Oxide Surfactant Layers Adsorbed on MICA Herder, C. E., Claesson, P. M. and Herdeq P. C. An Alternative Interpretation of Electron Spin Resonance Data for RhNa-X Effects of Carbon Monoxide, Hydrogen and Oxygen Sayari, A., Morton, J. R. and Preston, K. E. Characterization of Supported N~ICO(TAP)~ ( ~ 3 ) Complexes by the Co Stretching Vibration: Electron Donating Effect of Trialkylphosphine Ligands (TAP) Kermarec, M., Lepetit, C., Cai, F.X. and Olivier, D. Birhythmicity in B-Z System with Ascorbic Acid + Acetone/Cyclohexanone as Mixed Organic Substrate Rastogi, R. P., Das, I. and Sharma, A. Redox Catalysis: Theory for a Nerstian Reaction Coupled to an Irreversible Reaction Mills, A. and McMurray, N. Kinetic Study of the Oxidation of Water by Ce4' Ions Mediated by Activated Ruthenium Dioxide Hydrate Mills, A. and McMurray, N. Densimetric and Viscoimetric Investigations of NaI in HMPA-H20 Mixtures at 293.15,298.15 and 303.15 K Taniewska-Osinska, S. and Jozwiak, M. Temperature Dependent Conformational Analysis of Gen tiobiose Octa-acetate in Solution. Proton and Carbon Nuclear Magnetic Relaxation Study Rossi, C., Ulgiati, S. and Marchettini, N.The Vapour Pressure of Butane from 173 to 280 K Machin, W. D. and Golding, P. D. Transference Number Measurements of Silver Nitrate in Pure and Mixed Solvents Using the Electromotive Force Method. Part 2 Gill, D. S. and Bakshi, M. S. Electronic Structures of para-Benzoquinone and Cyclohexane- 1,4-dione. An E.S.R. Study Symons, M. C. R., Chandra, H. and Portwood, L. Formation of Hydrocarbons in the Electrochemical Reduction of Carbon Dioxide at Copper Electrodes in Aqueous Solution Hori, Y., Murata, A. and Takahashi, R. I.R. Spectroscopic Studies of Amination of OH Groups in ZSM-5 Zeolites Fink, P. and Datka, J. Effect of Solvent on the Reactions of Coordination Complexes. Part 8.-Diamine Cobalt(1II) in Methanol + Water, Propan-2-01 + Water and t-Butanol + Water Dash, A.C. and Das, P. K. Effect of Solvent on the Reactions of Coordination Complexes. Part 81.-Kinetics of Solvolysis of cis(Chloro)(cyclohexylamine) + Water, Ethyleneglycol + Water and t-Butanol + Water Media Dash, A. C. and Pradhan, J. Electron Transfer between Alpha and Beta Haeme Groups in Haemoglobin: An Electron Spin Resonance Study Symons, M. C. R. and Taiwo, F. A. A Theoretical Model of the EthaneDeuterium Exchange Reaction Catalysed by Platinum. The Nature of the Alpha Beta Process Hegarty, B. F. and Rooney, J. J. Transference Number, Conductance and Viscosity Studies of Some 1: 1 Electrolytes in Pyridine-Methanol Mixtures at 25°C Gill, D. S. and Bakshi, M. S. Quasi-elastic Neutron Scattering Study of Benzene Absorbed in ZSM-5 Jobic, H., Bee, M.and Dianoux, Albert J. Application of Piker's Equations of Dissociation Constants of Ammonium Ions in Lithium Chloride-Sodium Chloride Mixture Solutions Madea, M., Hisada, 0. and Ito, K.8/04162H Dielectric Properties of Water in the Coexisting Phases of Aqueous Polymeric Two-phase Systems Zaslavsky, B. Yu., Miheeva, L. M., Rodnikova, M. N., Spivak, G. V., Harkin, V. S. andManmudov, A. U, 8 ” ) C Solvation and Complexation of Copper@) and Chloride Ions in 2,2,2-Trifluoroethanol-Dimethyl Sulphoxide Mixtures Suzuki, H., Ishiguro, S . 4 . and Ohtaki, H. 8/04386H Quenching of the Luminescence of the Aqueous Gadolinium Ion by Nitrate and Thiocyanate Vuilleumier, J.-J., Deschaux, M. and Marcantonatos, M, D. 8/14512J Cationic Leado Halide Complexes in Molten Alkali-metal Nitrate.Part 3.-The Structure of Pb2X3+ and the Solvated Pb(II) Ion, Determined by Liquid X-Ray Scattering and Raman Spectroscopy Holmberg, B. and Bengtsson, L. Photophysics of the Excited Uranyl Ion in Aqueous Solutions. Part 6.--Quenching Effects of Aliphatic Alcohols Burrows, H. D., Emilia, M., Azenha, D. G., Formosinho, S. J. andMigue1, M. da G. M. 8/104578J (ii)Cumulative Author Index 1989 Aguilella, V. M., 223 Akitt, J. W., 121 Albuquerque, L. M. P. C., 207 Allen, G. C., 55 Amodeo, P., 621 Anpo, M., 609 Apelblat, A., 373 Arai, T., 929 Asakura, K., 441 Baiker, A., 999 Bald, A., 479 Barone, G., 621 Barone, V., 621 Beckett, M. A., 727 Bellotto, M., 895 Bengtsson, L., 305, 317 Berry, F. J., 467 Bertoldi, M., 237 Blandamer, M. J., 735 Bolis, V., 855 Bond, G.C., 168 Borowko, M., 343 Boss, R. D., I I Bowker, M., 165 Brimblecome, P., I57 Burgess, J., 735 Busca, G., 137, 237 Campbell, J. A., 843 Cattania, M. G., 801 Chadwick, A. V., 166 Che, M., 609 Chen, J., 829 Chen, L-f., 33 Clegg, S. L., 157 Coluccia, S., 609 Comninos, H., 633 Compton, R. G., 761, 773, 977 Conway, S. J., 71, 79 Copperthwaite, R. G., 633 Cox, B. G., 187 Cristiani, C . , 895 Cristinziano, P., 621 da Costa, M. A., 907 Datka, J., 47, 837 Dawber, J. G., 727 De Giglio, A., 23 Dell’Atti, A., 23 Domen, K., 929 Donini, J. C., 91 Drummond, C. J., 521, 537. 551, Eden, J., 991 el Torki, F. M., 349 Falconer, J. W., 71, 79 Finch, J. A., 91 Fletcher, P. D. I., 147 56 1 Foo, C. H., 65 Forzatti, P., 895 Frey, H. M., 167 Fubini, B., 237, 855 Gabriel, C.J., 11 Gabrys, B., 168 Garrone, E., 585 Gasser, D., 999 Gervasini, A., 801 Geus, J. W., 269, 279, 293 Giamello, E., 237, 855 Gilbert, P. J., 147 Girault, H. H., 843 Gottschalk, F., 363 Grieser, F., 521, 537, 551, 561 Guardado, P., 735 Gutiirrez, C., 907 Hampton, S., 773 Han, S., 829 Handreck, (3. P., 645 Harland, R. G., 761 Hasted, J. B.. 99 Hatano, M., 199 Healy, T. W., 521, 537, 551, 561 Heatley, F., 917 Hesselink, W. H., 389 Hester, R. E., 171 Higgins, J. S., 170 Higuchi, A., 127 Hill, W., 691 Hirai, T., 969 Holmberg, B., 305, 317 Hong, C. T., 65 Howarth, 0. W., 121 Hubbard, C. D., 735 Hummel, A., 991 Hunter, R., 363, 633 Hutchings, G. J., 363, 633 Ichikawa, K., 175 Ikeda, R., 1 1 1 Ishida, H., 1 1 1 Itoh, N., 493 Iwasawa, Y., 441 Jin, T., 175 Johnson, G.R. A., 677 Jonkers, G., 389 Jutson, J. A,, 55 Kaneko, K., 869 Kanno, T., 579 Katoh, T., 127 Keeler, J. H., 163 Kelebek, fj., 91 Kishi, R., 655 Knijff, L. M., 269, 293 Kobayashi, M., 579 Koda, S., 957 Kosugi, N., 869 Kozlowski, Z., 479 Kuroda, H., 869 Kuwabata, S., 969 Larramona, G., 907 Laschi, F., 601 Lawrence, K. G., 23 Lelj, F., 621 Levy, O., 373 Lewis, T. J., 1009 Leyendekkers, J. V., 663 Li, C., 929 Lorenzelli, V., 137 Loudon, R., 169 Lowe, B. M., 945 Lund, A., 421 Mafe, S., 223 Maignan, A., 783 Manzurola, E., 373 Marcus, Y., 381 Markovits, G., 373 Maruya, K., 929 Masiakowski, J. T., 421 Matsuhashi, N., 1 1 1 Matsui, H., 957 Matsumoto, T., 175 McAleer, J. F., 783 Meima, G. R., 269, 279, 293 Miessner, H., 691 Mills, A., 503 Morazzoni, F., 801 Moseley, P. T., 783 Mosier-Boss, P.A., 11 Nakagawa, T., 127 Nakamura, D., I 1 1 Nakamura, T., 493 Natarajan, P., 813 Nazhat, N. B., 677 Neagle, W., 429, 719 Newman, K. E., 485 Nicholas, A., 773 Nomura, H.. 957 Nowak, R. J., 11 Nowicka, B., 479 Nunes, M. R., 907 Ohlmann, G., 691 Ohyama, Y., 749 Okubo, T., 455, 749 Oliveira Jr, 0. N., 1009 Onishi, T., 929 Orchard, S. W., 363 Osada, Y., 655 Otsuka, K., 199 Pandey, J. D., 331 Pellicer, J., 223 Pereira, I., 907 (iii)AUTHOR INDEX Schneider, H., 187 Schneider, I., 187 Scotti, R., 801 Selvaraj, U., 251 Shido, T., 441 Shukla, R. K., 331 Smith, G. W., 91 Smith, J. J., 11 Smith, M. R., 467 Smith, T. D., 645 Stewart, A. A., 843 Stirling, C. J. M., 1009 Stroka, J., 187 Strumolo, D., 801 Sundar, H. G. K., 251 Sutton, H. C., 883 Symons, M.C. R., 711 Szejgis, A., 479 Szpak, S., 11 Takagi, Y., 493 Taniewska-Osinska, S., 479 Taylor, D. M., 1009 Thamm, H., 1 Themistocleous, T., 633 Piwowarska, Z., 47, 837 Pope, C. G., 945 Portwood, L., 711 Price, W. E., 415 Rai, R. D., 331 Ramaraj, R., 813 Ramis, G., 137 Rao, K. J., 251 Reed, W. F., 349 Rees, L. V. C., 33 Reis, J. C. R., 207 Reller, A., 855 Rhodes, C. J., 711 Rochester, C. H., 71, 79, 429, Rosen, D., 99 Rossi, C., 601 .Rowlinson, J. S., 171, 172 Saadalla-Nazhat, R. A., 677 Sacco, A., 23 Said, M., 99 Sakata, Y., 929 Salvagno, S., 1009 Schmehl, R. H., 349 719 Ugliengo, P., 585 Vaccari, A., 237 van Buren, F. R., 269, 279, 293 van Dillen, A. J., 269. 279, 293 van Leur, M. G. J., 279 van Lith, D., 991 van Rensburg, L. J., 633 van Veen, J. A. R., 389 Vink, H., 699 Vis, R.J., 269, 279 Wacker, T., 33 Waller, A. M., 773, 977 Warman, J. M., 991 Waugh, K. C., 163 Weale, K. E., 165 Williams, D. E., 783 Williams, G., 503 Yamada, Y., 609 Yeh, C-t., 65 Yoneyama, H., 969 You, X., 829 Young, D. A., 173 Zecchina, A., 609THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 88 Charge Transfer in Polymeric Systems University of Oxford, 11-13 September 1989 This Discussion aims to bring together physicists and chemists interested in the mechanism of electron and ion transport in polymeric systems. The systems include conducting polymers, redox polymers, ion exchange membranes and modified electrodes. Discussion topics will cover experimental evidence from spectroscopy, electrochemistry and new techniques such as the quartz microbalance.Theoretical models ranging from band theory through polarons to localised chemical structures will be critically evaluated and compared with experiment. The following have agreed to participate in the Discussion: R. Murray W. J. Albery M. B. Armand D. Bloor P. G. Bruce R. Friend A. J. Heeger A. R. Hillman A. G. MacDiarmid M. Ratner S. Roth W. Salaneck G. Tourillon C. Vincent G. Wegner The preliminary programme 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 GENERAL DISCUSSION No. 89 Structure of Surfaces and Interfaces as Studied using Synchroton Radiation University of Manchester, 4-6 April 1990 Organising Committee: Professor J.N. Shewood (Chairman) Professor D. A. King Dr G. King Dr C. Nods Dr R. Oldman Dr G. Thornton The Discussion will focus on the wealth of novel information which can be obtained on the nature and structure of surfaces using the full spectral range of synchroton radiation. Emphasis will be placed on the scientific results of recent investigations rather than on technical aspects of experimentation. Papers will be welcome which shed new light on the structure of the complete range of interfaces: solid/solid, solid/gas, solidAiquid, gasniquid and "dean" surfaces including both static and dynamic in sifu examinations. It is hoped that the discussion will define the utility of synchroton radiation examinations in surface science studies at a time of expansion of the availability of such sources and the inauguration of new and more powerful sources.Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 31 May 1989 to: Professor J. N. Sherwood, Department of Pure and Applied Chemistry, University of Strath- Clyde, Thomas Graham Building, 295 Cathedral Street, Glasgow G1 1 X L Full papers for publication in the Discussion Volume will be required by December 1989.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA DEUTSCHE BUNSEN-GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE KONIKNLIJKE NEDERLANDS CHEMISCHE VERElNlGlNG SOCIETE FRANGAISE DE CHIMIE, DIVISION DE CHlMlE PHYSIQUE FARADAY DIVISION GENERAL DISCUSSION No.90 Colloidal Dispersions University of Bristol, 10-12 September 1990 Organising Committee Professor R. H. Ottewill (Chairman) Professor P. Botherol Professor E. Ferroni Dr J. W. Goodwin Professor H. Hoffmann Professor A.L. Smith Professor P. Stenius Dr Th. F. Tadros Professor A. Vrij Dr D. A. Young The joint meeting of the Societies will be directed towards examining current understanding of the behaviour of colloidal dispersions. In particular, stability and instability, short range interactions, dynamic effects, non-equilibrium interaction, concentrated dispersions and orderdisorder phenomena will form topics for discussion. Contributions for consideration by the Organising Committee are invited. Titles and abstracts of about 300 words should be submitted by 30 September 1989 to: Professor R.H. Ottewill, School of Chemistry, University of Bristol, Bristol BS8 lTS, England. Full papers for publication in the Discussion Volume will be required by May 1990. DEUTSCHE BUNSEN-GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SOCIETE FRANGAISE DE CHIMIE, DIVISION DE CHlMlE PHYSIQUE JOINT DISCUSSION MEETING 1989 Transport Processes in Fluids and in Mobile Phases Aachen, 25-27 September 1989 Organised by: H.Versmold (F.R.G.) Al. Weiss (F.R.G.) M. Zeidler (F.R.G.) G. R. Luckhurst (U.K.) P. Turq (France) The purpose of the meeting is to bring together scientists working on transport and related phenomena in simple and complex fluids, colloidal and micellar systems, and surface phases.Experimental techniques considered include classical methods, optical spectroscopy, light scattering, nuclear magnetic resonance, and neutron scattering. The following persons have accepted invitations to present talks: D. Evans, Canberra; B. U. Felderhof, Aachen; D. Frenkel, Amsterdam; A. Geiger, Dortmund; W. Gliiser, Grenoble; H. G. Hertz, Karlsruhe; S. Hess, Berlin; J. Jonas, Urbana; R. Klein, Konstanz; K. Lucas, Duisburg; H.-D. Ludermann, Regensburg; H. Posch, Men; P. Pusey, Malvern; J. P. Ryckaert, Brussels; W. A. Steele, Penn State; D. J. Tildesley, Southampton; H. Weingartner, Karlsruhe. Further details may be obtained from: Professor H. Versmold, lnstitut fur Physikalische Chemie, RWTH Aachen, Templergraben 59, D-5100 Aachen, Federal Republic of Germany.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 25 Large Gas Phase Clusters University of Warwick, 12-14 December 1989 Organising Committee: Professor K.R. Jennings (Chairman) Professor P. J. Derrick Professor D. Phillips Dr N. Quirk Dr R. P. H. Rettschnick Dr A. J. Stace The Symposium will focus on recent developments in the rapidly expanding field of large gas phase clusters, including the preparation, structure and reaction of both neutral and ionic dusters. It is hoped that the meeting will bring together scientists working on many different types of duster, e.g. rare gas atoms, metals, inorganic and organic species, and biomolecules, to discuss the chemistry and physics of clusters from different viewpoints.The preliminary programme may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. (vii)FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division jointly with the Colloid and Interface Science Group Annual Congress: Surfactant Interactions in Colloidal Systems To be held at the University of Hull on 4-7 April 1989 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1 V OBN Molecular Beams Group Surfaces, Ions and Clusters To be held at the University of Liverpod on 9-1 1 Apnll989 Further information from Dr J. M. Hutson, Department of Chemistry, University of Durham, South Road, Durham DH13LE ~~ Electrochemistry Group Spring Informal Meeting To be held at the Uniwrsity of Warwick on 1 @12 April 1989 Further information from Dr S.P. Tyfield, CEGB, Berkeley Nudear Laboratories, Berkeley, Gkwcestershire GL13 9PB Electrochemistry Group with the Electroanalytical Group Electroanalysis To be held at Lwghborough University of Technology on 12-14 April 1989 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nudear Laboratories, Berkeley, Gloucestershire GL13 9PB Gas Kinetics Group Developments in Gas Kinetics: New Techniques, Results and their Interpretation To be held at the University of York on 3-4 July 1989 Further information from Professor R. J. Donovan, Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ Industrial Physical Chemistry Group with the Thin Films and Surfaces Group of the IOP Materials for Non-linear and Electro-optics To be held at Girton College, Cambridge on 4-7 July 1989 Further information from The Meetings Officer, Institute of Physics, 47 Belgrave Square, London SWlX 8QX Polymer Physics Group Biologically Engineered Polymers 89 To be held at Churddl College, Cambnclge, on 31 July to 2 August 1989 Further information from Dr M.J. Richardson, Division of Materials, National Physical Laboratory, Queens Road, Teddington, Middlesex lW11 OLW Polymer Physics Group Biennial Meeting To be held at the University of Reading on 131 5 September 1989 Further information from Dr M. J. Rirdson, Division of Materials, National Physical Laboratoory, Queens Road, Teddington, Middlesex lW11 OLW Colloid and Interface Science Group Inorganic Particulates To be held at Chester Coltege on 19-21 September 1989 Further information from Dr R.Buscall, ICI plc, Corporate Colloid Science Group, Po Box 11, The Heath, Runcom, Cheshire WA7 4QE Division with the Institute of Physics Sensors and their Applications To be held at the University of Kent at Canterbury on 1422 September 1989 Furher information from The Meetings officer, Institute of Physics, 47 Belgrave Square, London SWlX 8QX (viii)Division with the Deutsche Bunsen Gesellschaft, Division de Chimie Physique of the Soci&e Franpise de Chimie and Associazione ltaliana di Chimica Fisica Transport Processes in Fluids and Mobile Phases To be held at the Physikalische Institiit, Aachen, West Germany on 2528 September 1989 Further information from Professor G.Luckhurst, Department of Chemistry, University of Southampton, Southampton so9 5NH Division Autumn Meeting: Chemistry at Interfaces To be held at Loughborough University of Technology on 26-28 September 1989 Further information from Professor F. Wilkinson, Department of Chemistry, Loughborough University of Technology, Loughborough LE11 3TUJOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistry or chemical physics which appear currently in J. Chem. Research, The Royal Society of Chemistry's synopsis + microform journal, include the following: Structural and Magnetic Properties of the Radical-cation Salt BBDTA"FeC14- (BBDTA = Benzo-bisdithiazole) Gotthelf Wolmershausser, Gerhard Wortmann and Martin Schnauber (1 988, Issue 11 ) Frank Hibbert and Rowena J.Sellens (1988, Issue 11) Species in Aqueous Perchlorate Solution at Different Temperatures and Ionic Strengths Concetta De Stefano; Carmelo Rigano, Sihrio Sammartano and Rosario Scarcella (1988, Issue 11) Jaroslav Pecka (1 988, Issue 12) Christopher J. Rhodes (1989, Issue 1) Electrolyte Effects on the Reactions of Hydroxide Ion in 70% (vh) Dimethyl Sulphoxide-Water Studies on Sulphate Complexes. Literature Data Analysis of the Stability of HS04- and NaS04- Contribution of an Intramolecular Hydrogen Bond to the Dipole Moment Otto Exner and An Electron Spin Resonance Investigation of the Trimethylsilyl Azide Radical Cation: Me3SiN3+' A Fast and Accurate Estimation of Amidelminol Tautomerization Energies by the AM1 Method Andrzej Sygula (1 989, Issue, 1 )NOMENCLATURE AND SYMBOLISM For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers.Nomenclature. The following publications provide the IUPAC nomenclature rules and guidance on their use: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn.) Nomenclature of Inorganic Chemistry (Butterworths, London, 1971, now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff. Units and Symbols.A detailed treatment of units and symbols with specific application to chemistry, based on the Systbme lnternationale d'Unites (SI), is given in Quantities, Units and Symbols in Physical Chemistry, published for IUPAC by Blackwell Scientific Publications, Oxford (1 988 edn.). A comprehensive list of IUPAC publications on nomenclature and symbolism appears in the January issue of J. Chem. SOC., Faraday Transactions.NOMENCLATURE AND SYMBOLISM For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. Nomenclature. The following publications provide the IUPAC nomenclature rules and guidance on their use: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn.) Nomenclature of Inorganic Chemistry (Butterworths, London, 1971, now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff. Units and Symbols. A detailed treatment of units and symbols with specific application to chemistry, based on the Systbme lnternationale d'Unites (SI), is given in Quantities, Units and Symbols in Physical Chemistry, published for IUPAC by Blackwell Scientific Publications, Oxford (1 988 edn.). A comprehensive list of IUPAC publications on nomenclature and symbolism appears in the January issue of J. Chem. SOC., Faraday Transactions.

ISSN:0300-9599    

DOI:10.1039/F198985BP049

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. SOC., Furaday Trans. I, 1989, 85(4), 783-799 Tin Dioxide Gas Sensors Part 3.-Solid-state Electrochemical Investigations of Reactions taking place at the Oxide Surface Jerome F. McAleer, Antoine Maignan, Patrick T. Moseley* and David E. Williams Materials Development Division, Harwell Laboratory, Didcot, Oxfordshire OX1 1 O R A The use of standard electrochemical measurements effected through solid oxide electrolytes has proved to be a useful method for observing the surface chemistry of semiconducting oxides in gaseous atmospheres. Open-circuit potential measurements across a stabilized zirconia ceramic with electrodes of tin dioxide and platinum, respectively, yield non-zero values over most of the temperature range between ambient and ca. 400 "C. Such measurements are made without the separation of a reference atmosphere and finite potential differences are attributable to non-equilibrium processes by which differences in surface oxygen potential may arise on the two electrode materials. Open-circuit potentials measured under these circumstances are extremely sensitive to the presence of reducing gases in an air ambient.Current-potential measurements with a relatively small contact area between tin dioxide and the electrolyte allow the derivation of exchange current densities at this interface, and these too are sensitively affected by the presence of reducing gases in air. Measurement of the exchange current density over a range of temperatures has enabled the activation energy for the surface oxygen reaction to be derived (1.4 eV).This value is in tolerable agreement with the activation energy obtained from conductivity measure- ments (1.1 eV) as described in part 1 of this series. The principal mechanism by which tin dioxide gas sensors, operating at temperatures above ambient, offer a resistance modulation in response to the introduction of a small concentration of a reducing gas in air is thought to depend' on the availability of reactive oxygen ions chemisorbed at the oxide surface. The mechanism proceeds via an oxidation mechanism, consuming oxygen atoms from the surface to form neutral product gas species so that electrons that had been trapped at the surface return to the oxide conduction band to augment the concentration of charge carriers. Thus the temperature envelope within which tin dioxide sensors achieve a significant response coincides with the temperature range (ca.30WOO "C) over which the predominant oxygen species on the surface is a chemisorbed ion such as 0;. Under some circumstances other mechanisms2 can contribute changes to the surface resistance of tin dioxide. Water, arising as a product of the surface oxidation of reducing gases, causes a reversible resistance decrease over a wide temperature range. Also, at temperatures near ambient the decoration of the oxide surface with fine particles of precious metals allows the detection of carbon monoxide and hydrogen via the transfer of charge between the oxide and the metal. In general, a resistance change only results from a process that alters the distribution of charge at the oxide surface and at elevated temperatures the key surface species is likely to be an oxygen ion.Oxygen concentration in gas atmospheres can be readily evaluated by the classical Nernst probe in which the analyte is isolated from a reference oxygen pressure by an 783784 Tin Dioxide Gas Sensors oxide-ion conductor and the potential difference is measured between electrodes sampling the respective oxygen/oxide equilibria in the two volumes. It has been shown that this technique can be extended to follow the progress of catalytic oxidation reactions taking place at the electrode in the test v01urne~~~ and further it has been found that it is not always necessary to isolate a separate reference atmosphere in order to obtain useful information.A mixture of a reducing gas such as carbon monoxide at relatively low concentrations with air does not reach equilibrium on a platinum electrode unless the temperature exceeds a critical value of ca. 400 0C.5 Below this temperature a mixed potential is measured which has contributions from the reactions 0, (gas) + 4e- 20,- (electrolyte) (1) CO + 0,- (electrolyte) + CO, + e- (2) and the electrolyte (usually stabilized zirconia) can be used as a high-impedance probe of the oxygen species at the triple point electr~de/electrolyte/gas.~ Mixed potentials of this sort can also be measured with both electrodes exposed to the analyte gas (i.e. no reference volume) if the two electrode materials are invested with a significant difference in catalytic activity for the reaction taking place on them.6 Under these circumstances potential responses can be measured on the introduction of reducing gases to an air ambient at temperatures in the range 20-400 OC.' A recent extension of this type of potentiometric investigation has been the use of stabilized zirconia as a high-impedance probe of the oxygen surface states on a group of semiconducting oxides.' E.m.f.measurements were made across the electrolyte with electrodes (one made of platinum the other of a semiconducting oxide) exposed to the same atmosphere. Finite potentials were generally measured at temperatures below ca. 400 "C with the specimens in air and large temperature-dependent relaxations in potential were measured when small concentrations (< 1 %) of various reducing gases were introduced.These data were interpreted as manifestations of the differences in surface oxygen potential between the platinum and the oxide electrodes and this, of course, was disturbed to a different extent on the two electrodes by the introduction of a gas with which the surface oxygen could react. The present paper is the third in a series on tin dioxide gas sensors and describes the use of a zirconia electrolyte to investigate the reactions taking place on the surface of tin dioxide. The paper seeks to relate the results of these experiments to the response mechanisms, involving surface barriers, emerging from the surface conductivity studies described in parts 1 and 2. Cells of the form (-)Au, Pt/yttria.stabilized zirconia (YSZ)/SnO,, (M), Au( +), where M was either gold or platinum, were used to probe properties of the SnO,/gas/solid electrolyte interface.In such cells oxygen can be consumed or evolved (depending on the polarity of any voltage applied to the cell) at the tin dioxide and platinum electrodes, supported by migration of oxygen vacancies through the electrolyte. Two sorts of measurements have been made, namely open-circuit potential measurements, in which the YSZ is used as a probe to explore variations in band- bending at the tin dioxide surface, and d.c. current-potential measurements to reveal some information about the kinetics of the surface reactions. Comparison has been made with the behaviour of zinc oxide as the semiconductor electrode. Experimental Tin dioxide was prepared from metastannic acid as described previously.' Measurements of open-circuit potential of the cell Pt/YSZ/SnO, were made by preparing composite pellets in which a layer of SnO, powder was pressed onto a layer of yttria-stabilized zirconia powder and the resulting artefact fired at 1000 "C for 2 h.785 J.F. McAleer et al. Pt or A u e l e c t rode P t e l e c t r o d e Au e l e c t r o d e s V o l t a g e measure -ment Fig. 1. Schematic representation of the experimental arrangement for e.m.f. measurement. The composite pellet specimen was held in a silica-glass tube inside a horizontal tube furnace. For current-potential measurements the tin dioxide component of the specimen was replaced by a pellet in the shape of a truncated cone with its smaller end in contact with the stabilized zirconia and the voltage measuring equipment was replaced by a potentiostat and ammeter.Platinum was sputtered onto the zirconia side of the composite and gold onto the tin dioxide side except in one series of experiments in which platinum was applied to the tin dioxide also. The cell assembly was then clamped between gold electrodes and the cell potential measured using a high-input-impedance electrometer (1 014 R, Keithley Instruments). A digital voltmeter (Sangamo-Weston) served to connect the electrometer output to the IEEE 488 interface of a microcomputer. The experimental arrangement is shown schematically in fig. 1. The ceramic components and electrodes were enclosed in a silica-glass chamber held axially in a horizontal tube furnace.During experiments the microcomputer was used to control the atmosphere in the chamber by switching a system of solenoid valves. For the experiments in which current-voltage characteristics of the SnOJYSZ interface were investigated two separate pellets were used. The pellet of yttria-stabilized zirconia was cylindrical, ca. 1 cm in diameter and 3 mm in height. The tin dioxide pellet took the shape of a truncated cone with a base diameter of 1 cm and an apical diameter of ca. 1 mm. The YSZ pellet was brought into contact with the small diameter surface of the tin dioxide pellet. It was assumed that, since the tin dioxide contact area onto the stabilized-zirconia pellet was much less than the platinum contact area, the dominant interfacial effects on the current-potential curves would be those at the SnOJYSZ interface.Identical methods were used to explore the behaviour of other semiconducting oxides. Results Open-circuit Potential Measurements As we note in the Discussion (later), in the configuration adopted for these experiments, it is necessary to distinguish effects at the SnO,/YSZ contact from effects at the Pt/YSZ contact. To this end, we compared the behaviour of different semiconducting oxides786 500 F Tin Dioxide Gas Sensors -500 t \ in dry air 500 r [i 200 300 -500 Fig. 2. E.m.f. of a (-)Pt/YSZ/SnO,( +) cell as a function of temperature. (a) An initially undried pellet in dry air and (b) the behaviour of a dried pellet in dry air. (ZnO and SnO,) and the behaviour of tin dioxides from different sources.We also studied the effect of moisture content of the pellets (we have previously shown the profound effect that this has upon the electrical conductivity in the temperature range up to 450 "C) and the effect of reducing gases present at low concentration. Preliminary studies of cells involving other semiconducting oxides (BaSnO,, BaTiO,) as electrodes have been presented elsewhere.' Initially the open-circuit potential from a Pt/YSZ/SnO, cell was measured as a function of temperature in air with moisture content as the only atmospheric variable. A dry specimen, in dry air, has the SnO, electrode positive with respect to the platinum electrode by ca. 300mV at room temperature (fig. 2). As the temperature is raised the potential difference falls until it reaches zero at just over 100 "C.Above this temperature a potential difference develops which is opposite in sign to that of the low-temperature regime : the tin dioxide electrode becomes negative with respect to the platinum electrode. This polarity reaches a maximum value of ca. 200 mV at ca. 200 "C. At higher temperatures the potential falls once more, reaching zero at ca. 400 "C. The open-circuit potential in air in all the experiments covered here was always zero at temperatures significantly above 400 "C. When the composite pellet specimen was undried at the start of the run [fig. 2(a)] the initial stage, during which the potential of the tin dioxide electrode was positive with respect to the platinum, persisted to almost 200 "C and the peak negative value for the tin dioxide electrode was shifted up to 300 "C.After the specimen had been heated to 600 "C it behaved as the dry material had done. Changing the metal contact onto SnO, from Au to Pt had no effect whatsoever on the measured potential-temperature trace. It was shown previously4 that tin dioxides from different sources had slightly different electrical behaviour. Fig. 3 shows that a difference was evident in the cell potential also. The use of a different tin dioxide resulted in little change to the e.m.f. in dry air, but during the temperature-increasing phase of the experiment with an undried specimen the e.m.f. rose to a much higher level (400 mV) than that indicated above (150 mV, fig. 2). The use of ZnO rather than SnO, gave qualitatively similar results (fig.4), again showing a strong effect of water on the oxide surface: the potentials obtained were larger than those observed with an SnO, contact, and the curves showed evidence of two maxima in the variation of cell potential with temperature.J . F. McAleer et al. 787 \ - 500 in dry air - 500 Fig. 3. E.m.f. of a cell utilizing tin dioxide from a different source as a function of temperature. (a) Undried pellet in dry air, (b) dried pellet in dry air. 400 r 200 0 > < -200 -400 -600 -800 L Fig. 4. E.m.f. of the cell (-)Pt/YSZ/ZnO( +) in dried air as a function of temperature: (-) first temperature cycle; (---) second temperature cycle; arrows indicate direction of temperature change. With temperature increasing on the first cycle the semiconducting oxide was initially wet, and the first increase of temperature dried it.In our previous work, we noted an irreversible effect of hydrogen upon the electrical behaviour of a dried pellet of SnO,, which we attributed to hydrogenation of the surface. In this work, some particularly interesting effects of hydrogen were observed also, with behaviour dependent on both the temperature and the condition, wet or dry, of the SnO, electrode in a Pt/YSZ/SnO, cell before the exposure to hydrogen. With a wet pellet and with temperature increasing, both the sign and the magnitude of the change in potential were temperature-dependent (fig. 5). In a lower temperature range the introduction of llJ drogen was accompanied by a positive shift in potential (i.e. towards zero e.m.f.), which reached a maximum value of ca.150 mV at ca. 100 "C. As the temperature was increased above this value so the size of this response decreased until, at ca. 280 "C it was practically zero. As the temperature was increased still further a set md type of response, in the opposite sense to the first, developed and this reached a maximum value of ca. 100 mV at ca. 400 "C.788 Tin Dioxide Gas Sensors l h - I - 300 1 I I I I I 100 2 (Yo 300 L O O 500 TIOC Fig. 5. E.m.f. response of (-)Pt/YSZ/SnO, cell to pulses of 1 YO of hydrogen in air as a function of temperature, with temperature increasing from ambient. In the lower part of the temperature range the introduction of the hydrogen results in a positive move in potential (towards zero potential difference). In the higher part of the temperature range the introduction of hydrogen results in a negative move in potential.Fig. 6. E.m.f. changes of a (-)Pt/YSZ/SnO,( +) cell at 300 "C, which had been previously dried at high temperature, in response to the introduction of small concentrations of hydrogen. With temperature decreasing from above 400 "C in a dried air stream (i.e. with an initially dry pellet of SnO,) responses to the introduction of hydrogen were not stable with time (fig. 6). Particularly interesting effects were noted when a dried Pt/YSZ/SnO, cell was first exposed to hydrogen at a temperature below ca. 200 "C (fig. 7). First, even a very brief exposure to a dilute hydrogen stream resulted in a shift in the potential subsequently measured in air from a value characteristic of a dry cell to one characteristic of a cell in which the semiconducting oxide was wet.Secondly, three distinct phases could be distinguished in the response to hydrogen of a dried cell : first, there was an initial transient which changed depending on any previous exposure to hydrogen, but was typically in the form of a step with a potential change of ca.J . I;. McAfeer et al. 789 T/OC 195 280 260 367 205 1.0 190 250 214 193 192 212 1191 188 180 194 197 2021 1 230 200 197 11 195 192 A 1 1 1 I I I l l I I I I I l l I - I I l l I I Ill I I I I 1 1 1 I I I l l I I - I 0.8 - I I 0.6 - 0-4 - > 0.2 - I l l I I I I I I I I I 1 1 I I I 1 I I I 1%H2/air Air 1% HZ/air Air - Fig. 7. Transient e.m.f. changes shown at a control temperature of 180-190 "C by a Pt/YSZ/SnO, cell which had been previously dried at high temperature, in response to the introduction of 0.1 % H,.(A) Very large e.m.f. changes occur, accompanied by temperature transients upon introduction and interruption of the hydrogen supply. (B) Three phases of the response in the presence of hydrogen, labelled (a), (b) and (c). + 100 mV; secondly, there was a very large potential shift, typically ca. + 1 V; thirdly, the potential dropped back abruptly to a much lower value. After this third phase of the response, the system behaved stably, and in a fashion characteristic of a cell with a wet SnO, contact. When the system was in the 'partly dry' state which gave the large potential transients, large temperature transients also accompanied the initial intro- duction, and the interruption, of the hydrogen supply. These points are all illustrated in fig.7. The responses to 1 % CO of a similar Pt/YSZ/SnO, cell are shown in fig. 8, both as the temperature was raised to 600 "C and as it was lowered again. The response was in the same sense as that of hydrogen in the higher temperature range (i.e. negative) and at a maximum (ca. 150 mV) after the potential in air had fallen to near zero (450 "C). The potential responses generated as the temperature was lowered were similar to those that had been measured while the temperature was raised. Fig. 9 compares the temperature dependence of e.m.f. response to CO for Pt/YSZ/ZnO and Pt/YSZ/SnO, cells. The ZnO cell response peaked at lower temperature, and changed sign with further reduction of temperature.The concentration dependence of the potential response of the Pt/YSZ/SnO, at 400 "C to carbon monoxide in air is shown in fig. 10. Over the range790 I o l m l h Tin Dioxide Gas Sensors 0 > E --. -100 -200 L .- a 0 .: 0 o a u $ $ i L .L 0 .: 0 0 2 0 .- a V Q O o a o a I I I I LOO 500 600 500 T/OC Fig. 8. E.m.f. responses of Pt/YSZ/SnO, cell to pulses of 1 'YO carbon monoxide in air as the temperature was increased from 350 to 600 "C and lowered again. Fig. 9. Comparison of the temperature dependence of e.m.f. response to 0.1 YO CO, in air, of Pt/YSZ/SnO, (@) and Pt/YSZ/ZnO (0) cells. up to 1000 ppm the size of the voltage response appeared to be dependent upon the logarithm of the gas concentration, although the data range was not wide enough to distinguish accurately such a dependence from, for example, a power-law dependence.Fig. 1 1 shows an interesting observation made with cells having either wet SnO, or wet ZnO as the semiconducting oxide contact when they were exposed to a low concentrationJ. I;. McAleer et al. -0.4 -0.6 -0.8 79 1 / / - \ \ \ - / / c I I - '.& -100 r t '210 2!2 2!4 216 218 3:O 312 log ( W I / P P d Fig. 10. Dependence of the e.m.f. response of a Pt/YSZ/SnO, cell on the concentration of carbon monoxide in air at 400 "C. -0-41 I , I , 100 200 300 400 T/"C Fig. 11. E.m.f. change of Pt/YSZ/oxide cells in 1 % CO in air: comparison with behaviour in the absence of CO. Arrows give direction of temperature change, starting at room temperature. The cells were initially undried; the gases were dried.(-) air+ 1 YO CO, (---) air. (a) ZnO, (b) SnO, 'FEM'. of CO in air as the temperature was increased from room temperature. An abrupt and large voltage shift occurred as the temperature approached 100 "C. Furthermore, at temperatures greater than 100 "C the presence of a small concentration of CO eliminated the hysteresis in the e.m.f.-temperature curves caused by the initial presence of water on the semiconducting oxide surface. In fig. 12 the response to low concentrations of methane is compared with the response792 Tin Dioxide Gas Sensors 200 300 400 500 -100 TIOC Fig. 12. Comparison of the e.m.f. response to the presence of methane (---) and carbon monoxide (-) of Pt/YSZ/SnO, cell. (a) 200 ppm, (b) 1000 ppm.- 1 00 -80 -6 0 > E 1 w - L 0 -20 0 CH4 composition (ppm) 0 1000 0 190 1000 l h t I Fig. 13. Dependence of the e.m.f. response of a Pt/YSZ/SnO, cell on the concentration of methane in air between 0 and 1000 ppm at 380 "C. to CO, for a Pt/YSZ/SnO, cell. At sufficiently high temperature, the responses were the same. The methane response peaked at a higher temperature than the CO response and diminished rapidly with further decrease of temperature; there was no change of sign of response at low temperature, however. The response to methane was also approximately logarithmic at the temperature of maximum response. Fig. 13 shows the variation of the e.m.f. response between 0 and 1000 ppm of methane at 380 "C.J. F. McAZeer et al. 793 1000 -600 -LOO &ZOO 0 200 d LOO 600 n n q/mV Fig.14. Semilogarithmic current, i, us. overpotential, q, plots at several temperatures for a cell of the tvnp Pt/V97/Cnn in i i r ThP C n n / V C 7 rnntart i c wprv miirh cmaller than the VC7fPt contact. The overpotential q = E-E,, where E is the applied voltage and E , is the open-circuit cell e.m.f. 0, 478 "C; A, 540 "C; 0, 646 "C; x , 737 "C. Current-Voltage Measurements Current-voltage measurements were made for a cell incorporating the truncated conical tin dioxide pellet at temperatures in the range 45&750 "C. The data (e.g. fig. 14) follow a Tafel relationship, i = io exp (aFv/RT) (1) (where denotes the overpotential) over a significant range of applied voltage, with a rather small value for the slope (ca. 1.4 V per decade of current, i).Extrapolation of the anodic and cathodic lines to their intersection at the open-circuit potential on a semilogarithmic plot (fig. 14) gives the exchange current density, io, proportional to the surface reaction rate with no applied potential. Taking the values of the exchange current density at different temperatures and presenting them in the form of an Arrhenius plot (fig. 15) allows the derivation of an194 Tin Dioxide Gas Sensors 0.1 1 0 1 1 1 2 1 3 lo3 K/T Fig. 15. Arrhenius plot of logarithm of exchange current densities derived from plots such as that shown in fig. 14, against reciprocal temperature. The activation energy for the process of oxygen reduction at the SnO,/YSZ interface is 1.4 eV. activation energy for the oxygen-reduction process at the SnOJYSZ interface. The slope of the plot in fig.15 corresponds to an activation energy of ca. 1.4 eV. The effect of the presence of combustible gases was to displace the anodic branch of the current-potential plot to higher currents (fig. 16), i.e. the open-circuit potential moved to more negative values, as in the equivalent temperature range of fig. 5 and 8, and the exchange current density increased. Discussion Open-circuit Cell Potential In order to simplify construction, and also because one objective of the work was the development of new gas-sensor concepts, the whole cell was bathed in a common gaseous atmosphere : no separation of test and reference atmospheres was made. This in some ways also simplifies aspects of the interpretation, but makes other aspects more complex; it distinguishes this type of construction from the usual form of Nernst sensor.There were a number of interfaces in the cell, across each of which a potential difference would have been generated. In such cells, at the interfaces with the solid electrolyte, a reaction must take place to exchange charge across those interfaces. The complete circuit of the type of cell studied can be written ( -) (Cu) (Au) Pt/YSZ/SnO, (M) (Au) (Cu) ( + )J . F. McAleer et al. 795 X - -600 -LOO -200 0 200 LOO 600 qlmV Fig. 16. Comparison of current-overvoltage relationship for a Pt/YSZ/SnO, cell in air ( x ) with that in air containing 1 YO H, (0). Overvoltage, 7 = E-E,, where E, is the open-circuit cell e.m.f. in air and E is the applied voltage.The SnO,/YSZ contact was much smaller than the YSZ/Pt contact; T = 445 "C. where M was Pt or Au, the contacts to the cell were made with gold electrodes which were at the same elevated temperature and the contacts between the gold connection leads and the external circuit (Cu) were both at room temperature. Since the net cell reaction was the transport of oxygen gas from one side of the solid electrolyte to the other, and since the oxygen partial pressure was the same at both electrodes, the equilibrium cell e.m.f. should have been zero. As is shown in the Results section, this indeed pertained at sufficiently high temperature. However, at lower temperatures, non-zero cell e.m. f.s were developed in an atmosphere containing air only, and these obviously have their origin in non-equilibrium phenomena at the electrodes. Any potential differences generated at other interfaces cancel out as one moves from left to right through the cell.This is one consequence of ensuring that the gaseous atmosphere is uniform over the whole cell. The asymmetry which gives rise to the cell e.m.f. must arise as a consequence of an asymmetry in the chemical reactions which transport charge across the electrode/electrolyte interfaces. As was noted in the Introduction, one way of describing non-equilibrium potentials is as mixed potentials in which the potential is determined by balancing the rates of various anodic and cathodic processes. In the case of the tin dioxide electrode, processes in addition to the oxidation and reduction of oxygen gas could be the oxidation and reduction of the non-stoichiometric tin dioxide electrode material, and electrode796 Tin Dioxide Gas Sensors I I I I Au Pt I ' t I I I I I I I I I I I I I Z r 021 YzO 3 Fig. 17.Schematic diagram of electrical potential variation across the cell : A& denotes the band- bending at the semiconducting oxide/gas interface. reactions of water. It is these additional reactions which could generate an asymmetry giving a non-zero cell potential in an atmosphere of uniform oxygen partial pressure. The potential changes across a cell of the type studied here can be sketched as in fig. 17. One element in the cell potential is the band-bending, inside the SnO,, at the SnO,/gas/electrolyte interface which is established as a consequence of the surface reactions mentioned in the previous paragraph. We may therefore now argue that the non-zero cell potentials observed give precisely the change in band-bending at the SnO,/gas/electrolyte interface caused by non-equilibrium processes at that interface.In effect, we argue that the Pt/YSZ contact can be used as a reference probe to explore variations in the band-bending at the SnO,/gas interface. Unfortunately, in the configuration which we have chosen to study, there may be ambiguity in the interpretation of the data at lower temperature as a consequence of the development of a mixed potential at the Pt/YSZ contact, especially in the presence of small concentrations of CO. These mixed potentials generally disappear at temperatures above ca. 300 "C.We have previously argued,' in common with others, that the conductance effects on SnO, are caused by non-equilibrium changes in the band-bending at the semi- conductor/gas interface ; evidence has been presented supporting the view that the conductivity of tin dioxide gas sensors is controlled to a large degree by Schottky barriers at grain boundaries, that the barriers are established by chemisorption of oxygen and of water and that the activation energy for conduction is equal to the barrier height. Therefore, according to the argument just advanced, the cell e.m.f, changes should be equal to the changes in activation energy of electrical conductivity of tin dioxide pellets in a non-equilibrium gaseous atmosphere. Furthermore, since many semiconducting oxides show similar responses in conductivity to the presence of reducing gases, caused by the same surface mechanism,s similar cell e.m.f.changes should be observed using materials other than SnO, as one contact. This has been demonstrated here for ZnO and in other work for other oxide~.~ Interpretations of specific observations can now be offered. (a) At temperatures above ca. 400 "C, cells of type Pt/YSZ/semiconducting oxide show zero cell potential in air, as expected, but show a substantial negative cell potential in response to the introduction of a small concentration of a reducing gas. In the presence of the non-equilibrium gas mixture, the semiconductor/gas interface is displaced from an equilibrium state; the band-bending at the interface is reduced as negative charge is transferred from the semiconductor surface to the bulk as a consequence of the lowering of the surface coverage of adsorbed ionized oxygen species.l A negative cell potential numerically equal to the change in band-bending is expected.Both the conductance and conductance activation energy of a semiconducting oxideJ . F. McAleer et al. 797 are lowered in the presence of a reducing gas,' and the diminution of activation energy has been assumed to be equal to a change in surface barrier height. The changes observed (0.14.2 eV) were consistent with the changes in cell potential reported here (e.g. fig. 8) so the interpretation appears to be sustained. (b) For most of the temperature range between ambient and 400 "C the open-circuit potential of the cell in air was non-zero.At the bottom end of this temperature range the semiconducting oxide electrode was positive with respect to the platinum. At intermediate temperatures there was a switch and at the top end of the range the oxide was negative with respect to the platinum. The cell potentials in the intermediate temperature range were dependent on whether or not the oxide had been heated to high temperature in dried air in order to dry it thoroughly, were different for different oxides and furthermore (e.g. for ZnO) showed some evidence of two distinguishable levels at different temperatures. We therefore conclude again that this cell potential, at least in the intermediate temperature range (100-400 "C), was a characteristic of the semiconducting oxide, and that the values observed represent changes in band-bending caused by the presence on the oxide surface of various ionized oxygen species and hydroxyl species, which are not at equilibrium with the atmosphere.Over this intermediate temperature range, water is lost from the oxide surface and conductance measurements both on SnO,' and Zn09 have been interpreted in terms of a change in the majority surface species from an ionized oxygen to a hydroxyl one. Furthermore, experiments in which the oxygen partial pressure was changed' showed that equilibrium with oxygen in a dry atmosphere was not established or was only established very slowly at temperatures below ca. 200-300 "C. The argument implies that the cell e.m.f. gives the change in band-bending from a hypothetical equilibrium value. The differences in cell potentials between that for a wet and that for a dry oxide pellet should be equal to the difference in surface barrier height and hence conductance activation energy for 'wet ' and ' dry ' tin dioxide pellets. Comparison with earlier work' shows that this appears to be approximately true. The interpretation would therefore seem to be sustained.An interesting point is then that the observation of two peaks on the temperature-potential curve for cells with ZnO as the electrode implies for this oxide the presence of two different important surface species in distinguishable temperature ranges. (c) The cell potential in the intermediate potential range (100-400 "C) clearly distinguishes circumstances in which the semiconducting oxide surface is hydroxylated from those in which it is not, The introduction of hydrogen into the air causes hydroxylation of a dry oxide surface.This observation and interpretation is consistent with the irreversible change in conductivity of a dried pellet of SnO, at room temperature caused by the introduction of hydrogen.' We can furthermore state that with a 'wet' oxide surface the potential shift induced by hydrogen is positive, whilst with a 'dry' oxide surface it is negative : these two circumstances are quite clearly distinguished in the experimental data, although the interpretation of a positive potential shift is ambiguous. A positive potential shift could be caused either by an increase in band-bending at the semiconducting oxide/gas interface or by a mixed potential at the platinum 'reference' electrode.( d ) The cell potential transients trace the kinetics of the surface reaction of hydrogen with the dried semiconducting oxide. It is of course an assumption that the effect is due to the semiconducting oxide and not to effects at the Pt/zirconia interface : what is clear is that the system has to be dried before the effects can be observed, and that the temperature range over which they are observed corresponds with that over which water is lost from the semiconducting oxide. We assume that the transition to ca. + 1 V marks the start of the surface reaction and the transition back to ca. -0.3 V marks its completion. We do not have an explanation for the origin of these rather large cell potentials but make two points : first, temperature198 Tin Dioxide Gas Sensors increases occur momentarily upon the introduction and removal of hydrogen, but only when the surface reaction is part-completed (cell potential w + 1 V, fig.7). Although these temperature transients might be simply a consequence of some momentary reduction of the gas flow rate, the point is that there must be some particularly reactive species present on the surface when the reaction is only part completed which is not there when the reaction is fully completed. Secondly, if the effects are assumed to be due to the oxide electrode, then the change in cell potential implies an increase of band- bending, contrary to expectations of the effect of a reducing gas on a semiconducting oxide.If the effects are assumed due to the platinum electrode, then the sign of the potential change can be understood, but the origin of the effect and its dependence upon previous drying cannot; an effect of hydroxylation of zirconia at the Pt/zirconia interface could perhaps be postulated. Another effect which occurred only in a 'wet' system was the potential transition observed at ca. 100 "C in the presence of CO (fig. 11). Again, no definite explanation can be given, but various phenomena may be postulated as the cause; for example, an effect of interaction of surface hydroxyl and CO on the semiconducting oxide' or competitive adsorption of water and CO on Pt. Finally, we remark, as illustrated by fig. 1 I also, the fact that the presence of CO eliminates the effect of surface hydroxylation on the cell potential in the temperature range 200-400 "C.It is precisely over this temperature range that a conductance response to CO is observed in the presence of moisture but not in its absence.'. These experiments suggest that the effect of the introduction of CO is effectively to dry the semiconducting oxide surface, by a reaction with surface hydroxyls. Current-Potential Measurements The electrode reaction of oxygen can be written : At a tin dioxide electrode, the electrode reaction (2) could be formulated as an exchange of oxygen between surface states on the tin dioxide and oxygen vacancies in the stabilized zirconia : physisorption of 0, on SnO, 02(g) op(ads) (3) carrier-defect equilibrium in bulk SnO, Snk, n ' (4) establishment of chemisorbed oxygen species on SnO, n' + 0; e 20- (6) n' + 0-e 0,- (7) (8) Reactions (3)-(7) establish a Schottky barrier at the tin dioxide/gas interface,' which charge must pass across in order to accomplish the electrode reaction (2).If the activation energy for reaction (8) is less than the barrier height, then the activation energy for the whole sequence will be equal to the barrier height. Image forces across the SnO,/gas/YSZ contact will make the barrier height different in magnitude from that of SnO,/gas/SnO, interface barrier which controls conduction in porous tin dioxide pellets. The difference will depend upon the difference in dielectric constants of YSZ and exchange of surface species on SnO, with oxygen vacancies in Y S Z VO(,SZ) + 02- * Ot(YSZ) t Kroger-Vink notation is used: subscripts denote lattice sites and supercripts charge with respect to the lattice (' is positive and ' negative); n denotes a conduction electron.J.I;. McAleer et al. 799 SnO, and the assumption made here is that such differences are not great. The observed activation energy for the electrode process (1.4 eV) is indeed not dissimilar to the activation energy at higher temperature for conductance in porous SnO, (1.1 eV). According to this argument, the increase of exchange current density in the presence of a reducing gas (fig. 16) directly reflects the decrease in surface barrier height induced by the gas. Conclusion The use of standard electrochemical techniques effected through a solid oxide electrolyte has proved useful for observing the surface chemistry of semiconducting oxides in gaseous atmospheres. Open-circuit potential measurements reveal changes in band- bending at the semiconducting oxide/gas interface, caused by the presence of reducing gases at low concentration in air, or by hydroxylation of the surface. The simple cell configuration can easily be adapted to give a new type of voltage-generating gas sensor.1o Measurement of the energy of activation of the exchange current density gives the surface barrier height. Financial support from the Electronics and Avionics Requirement Board of the Department of Trade and Industry is gratefully acknowledged. We are grateful to Miss P. Bourke for valuable experimental assistance. References 1 J. F. McAleer, P. T. Moseley, J. 0. W. Norris and D. E. Williams, J. Chem. SOC., Faraday Trans.1, 2 J. F. McAleer, P. T. Moseley, J. 0. W. Norris, D. E. Williams and B. C. Tofield, 1. Chem. Sac., 3 S . Pancharatnam, R. A. Huggins and D. M. Mason, J. Electrochem. SOC., 1975, 122, 869. 4 H. Okamoto, G. Kawamura and T. Kudo, J . Catal., 1983, 82, 332. 5 H. Okamoto, H. Obayashi and T. Kudo, Solid State Ionics, 1980, 1, 319. 6 D. E. Williams, P. McGeehin and B. C. Tofield, 1983, Proc. 2nd European Conf. Solid State Chem., ed. R. Metselaar, H. J. M. Heijligers and J. Schoonman (Elsevier, Amsterdam) vol. 3, p. 275; D. E. Williams, CiK Patent GB 21 19933B-(1982). 7 J. F. McAleer, A. Maignan, P. T. Moseley and D. E. Williams, Proc. Br. Ceram. SOC., 1988, in press. 8 D. E. Williams, in Solid State Gas Sensors, ed. P. T. Moseley and B. C. Tofield (Adam Hilger, Bristol, 9 M. Nakagawa and H. Mitsudo, Surf. Sci., 1986, 175, 157. 1987,83, 1323. Faraday Trans. 1, 1988, 84, 441. 1987), pp. 71-123. 10 P. T. Moseley, D. E. Williams, J. F. McAleer and A. Maignan, UK Patent Application GB 21856090A. Paper 8/00873F; Received 4th March, 1988

ISSN:0300-9599    

DOI:10.1039/F19898500783

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Truns. I, 1989, 85(4), 801-812 Electron Spin Resonance Investigation of ZrO ,-supported Ruthenium Evidence of Strong Metal-Support Interaction Maria Grazia Cattania Centro CNR per lo Studio sulle Relazioni tra Struttura e Reattiuita chimica, Via Golgi 19, 20133 Milano, Italy Antonella Gervasini, Franca Morazzoni", Roberto Scotti and Donatella Strumolo Dipartimento di Chimica Inorganica e Metallorganica Centro CNR, Via Venezian 21, 20133 Milano, Italy An e.s.r. investigation of Ru/ZrO, has shown that Ru centres are present in different oxidation states, two of which are paramagnetic: Ru"' and Rut. The assignment of the electronic configuration follows from the results of the interaction of Ru/ZrO, with CO, for the identification of RuTTT and RuI, and with NO for the identification of Ru".Magnetic tensor components allow recognition of an Ru'II-CO adduct (gll = 2.039, g, = 2.003) and an RuI-CO adduct (g = 2.05). An Ru"-NO species was also identified [8, = 2.00, gll = 1.91, A,(N) = 40 GI. The results of the interaction with 0, also have been discussed in order to give a fuller description of the electronic configuration of the surface ruthenium centres and of the interaction effects between metal and support centres. It appears that at the Ru-ZrO, interface occurs an interaction stronger than in the Ru-y-Al,O, case, the strength depending also on the temperature of the pyrolysis used to obtain Ru/ZrO, from the Ru,(CO),,/ZrO, metal precursor. Also the analogous Rh/ZrO, system was considered with respect to interaction with NO in order to compare the variation in the metal-support interaction owing to differences in the supported metal.Previously reported investigation^^-^ on y-Al,O,-supported rhodium and ruthenium have led to the conclusion that positively charged metal centres arise from the interaction of the metal centres with the acidic centres of the support. The number of positive e.s.r.-active Ru centres is two orders of magnitude higher than that of the positive Rh centres. This was first explained as an increase in the metal-support interaction on changing the metal and accords with the deep location of Ru centres, found in R ~ / y - A 1 ~ 0 ~ . ~ ~ Modifications in the relative amounts of the paramagnetic centres of the same metal, but in a different oxidation state, were observed if different pyrolysis temperatures of the Rh,(CO),, and Ru,(CO),,/y-Al,O, metal precursor were used.,' The variations were explained as a consequence of the sintering processes, induced by an increase in temperature; these are related to modifications in the metal-support interaction.E.s.r. investigations have provided a means of studying the influence of the support on the electronic state of the supported metal, when the metal and pyrolysis conditions were kept constant. The interaction of Rh with Zr0,1,2 gives rise to a smaller number of paramagnetic centres with respect to y-Al,O, and, if the number of e.s.r. active centres had been the only measure of metal-support interaction, we would have concluded that the interaction decreases from y-Al,03 to ZrO,.In contrast, temperature-programmed 80 1802 Strong Metal-Support Interaction desorption studies and parallel experiments on the hydrogenation of carbon mon- oxide5 suggested the opposite order for the Rh interaction effects for the two supported oxides. These arguments are to be found in other showing that the metal-support interaction effect is still under investigation. This paper reports an e.s.r. investigation of Ru on ZrO,, which was carried out to gain further insight into the electronic and steric effects which control the metal-support interaction of Ru and Rh, when y-Al,O, and ZrO, are used as supports. Experimental y-Al,O, was from Akzo-Chemie CK-300, 50-150 mesh. Conventional treatment, 1 h at 200 "C, 1 h at 300 "C, 1 h at 400 "C and 6 h at 550 "C, under an inert dry atmosphere was performed (surface area 200 m2 g-I).The y-Al,O, was then stored under an inert dry atmosphere. Ru/y-Al,O, (1.5 g Ru/ 100 g y-A1203) and Rh/y-Al,O, (1.5 g Rh/ 100 g y-Al,O,) were obtained by pyrolysis in vacuo of the appr~priatel-~ Ru3(C0),,/y-A1,0, and Rh,(CO),,/ y-Al,O, systems (low3 Pa), at 673 and 523 K, respectively, if not specified. Pyrolysis was carried out for 2 h in a small flask connected to an e.s.r. tube. ZrO, was obtained by precipitation of the hydroxide from ZrOCl,, with a 28 % aqueous ammonia solution. The filtered compound was treated for 2 h at 550 "C in an air stream (surface area 70 m2 g-l), then 7 h at 550 "C in N,. Rh/ZrO, (I .5 g Rh/ 100 g ZrO,) and Ru/ZrO, (1.5 g Ru/ 100 g ZrO,) were obtained by pyrolysis in vacuo of the appropriate Ru,(CO),,/ZrO, and Rh,(CO),,/ZrO, systems.Contact with gases (O,, C0,NO) at controlled pressures was carried out on a gas-vacuum line. Pure 0, was dried over molecular sieves. Samples were contacted with 26.6 kPa 0, at room temperature for 5 min; the 0, was then pumped off to 13.3 kPa to eliminate the major part of the paramagnetic physisorbed gas and the e.s.r. spectrum was then recorded. CO was used as received from SIO Blu Gas. All samples, unless otherwise specified, were contacted with 26.6 kPa CO at room temperature and the e.s.r. spectra recorded under the same CO pressure. NO was used as received from SIO Blu Gas. NO was introduced into the samples at room temperature at pressures of 9.3 and 26.6 kPa.Samples were then cooled to liquid-nitrogen temperature for 30 min4" and pumped for 1 min at room temperature at 13.3 kPa, before recording the spectrum. The spin concentration of the magnetically diluted species were determined by double integration of the area of the resonance lines. The reference area was that of the signal from the Varian weak pitch (lo1, spin cm-l). Results and Discussion The e.s.r. investigation was performed on samples before and after contact with interacting gases (O,, CO, NO). The use of these gases allows improved resolution of the signals attributed to Run+ centres (n = 1,3) and makes the otherwise e.s.r.-inactive metal oxidation states evident. ZrO, The e.s.r. spectrum of ZrO, under an argon atmosphere, pyrolysed at 673 K, has the following features [fig.1 (a)]: (i) resonance lines of a radical species (g = 2.00) already seen with lower intensity before thermal-vacuum treatment. These signals could result' from electrons in thermally generated (during preparation and pyrolysis) oxygen vacancies. (ii) An anisotropic resonance (g,, = 2.028, g , = 2.00), recently1? attributed to 0; fixed at Zr4+ centres. (iii) An anisotropic resonance of a dl centre having axialM. G. Cattania et al. 803 ~DPPH 1' Fig. 1. E.s.r. spectra at 123 K of ZrO, recorded under (a) argon atmosphere, (b) 0, atmosphere (13.3 kPa). (c) E.s.r. spectrum at 123 K of Ru/ZrO, recorded under 0, (13.3 kPa). Peaks marked with an asterisk are due to electrons in oxygen vacancies. symmetry (8, = 1.97, g,i = 1.95). These are most likely due to the reduction of Zr4+ centres close to oxide-ion lattice vacancies.Similar effects were observed previously7 and, although support impurities could not be excluded, Zr3+ centres were suggested as being responsible for these resonances. Treatment with CO did not affect the spectrum observed under argon, while 0, contact increased the intensity of the Zr4+-O; signal, leaving that of Zr3+ almost unaffected [fig. 1 (b)]. The formation of 0; could be interpreted as being due to electron transfer from the surface electrons, located at the surface oxide-anion lattice vacancies, Zr3+ centres do not reduce the surface oxygen, probably because they are located in the bulk. Treatment with NO leads to adsorption of gas on ZrO,. The values of the g to 0,.804 Strong Metal-Support Interaction tensor components (gl = 1.96, gll = 1.92) are similar to those reported for A13+-N0,, obtained by the same procedure on y-Al,O,.Consequently the formation of a Zr4+-NO species can be postulated. No hyperfine interaction due to the N nucleus was observed and the paramagnetic species could be removed by degassing for 1 min to 13.3 Pa at room temperature. A slight increase in the Zr3+ signal was observed, probably due to some electron transfer within the Zr4+-N0 moiety, at the contact point between NO and the surface centres. Ru/ZrO, The pyrolysis of pale-yellow Ru,(CO),,/ZrO, at 673 K led to a dark-grey dispersed metal system Ru/ZrO,, whose e.s.r. spectrum, under an argon atmosphere, shows broad, unresolved, but strong lines, centred at g = 2.13 [fig.2 (a)]. The lines of the radical species, observed on ZrO,, are still visible (shown by an asterisk in fig. 2). The morphology of the broad lines is not too different from that observed on Ru/y- Al,0,,4 and the same doubts about the attribution of the signal on y-Al,O, still exist. Their assignment to the supported transition-metal centres is appropriate ; similarly it is acceptable that these metal centres are oxidized Ru centres, the oxidative process being an electron transfer from Ru to ZrO,. However, the possible paramagnetic configurations of ruthenium, RuI and RuIII, cannot be distinguished in this spectrum. The total number of paramagnetic centres is less than 1 YO of the total supported metal. Compared to Ru/y-Al,O, it seems to be lower (2-2.5 %) in this case (see tables 1 and 2).We suggest that the lower acidity of ZrO, with respect to y-Al,O, is responsible for a decrease in electron transfer to the support and consequently for a decrease in the number of oxidized paramagnetic centres. So as to observe this behaviour in greater detail, contact with CO was carried out. Dramatic changes occurred and at least two carbonyl derivatives were distinguishable easily [fig. 3(b)]: one peak (A) has axial symmetry (g,, = 2.039, g, = 2.003); the other (B) has only one g-tensor component detectable (g = 2.05). Owing to the overlap of the resonance lines no quantitative evaluation of the individual paramagnetic carbonyl species can be made; their total sum is 1/5 of the paramagnetic Ru centres seen before CO contact (table 1).The remaining unaffected centres are still observable in the spectrum. The two carbonyl derivatives show different stability. A remains unaffected, even under thermal and vacuum treatment (lo-, Pa, 373 K, 1 h), while B is removed at room temperature under vacuum (10-1 Pa, 10 min). After 1 h treatment a loss of intensity of A was observed above 473 K in vacuo ( Pa), followed by disappearance at 573 K in vacuo (lo-, Pa), The contact experiments with CO were also performed on Ru/ZrO,, pyrolysed at 573, 723 and 773 K. Under an argon atmosphere, all the samples showed the same signal attributed to oxidized Ru centres. After CO contact small amounts of carbonyl B were observed, with the maximum amount at 673 K, while carbonyl A was always present in larger amounts (fig.3). Both systems A and B probably contain the metal in different oxidation states, analogous to Ru/y-A120,.* Although the small amount of B prevents an independent and unambiguous assignment of this carbonyl adduct, the similarity of its g-tensor value to that of the Ru' carbonyl on Ru/y-Al,Oi and the low stability analogous to the RuI carbonyl on Ru/y-Al,O: suggest that Ru+l is most likely present in B. For A, on the basis of the same comparison with R~/y-A1,0,,~ the + 3 oxidation state seems to be more probable. Contact with 0, followed by CO stabilizes only the A adduct, thus confirming the higher oxidation state of the Ru centres contained in A. The differences observed between the carbonyl compounds on Ru/ZrO, and those on Ru/y-Al,O, can be rationalized in terms of the different ability of the two supports to be reduced and to form oxygen vacancies.In fact both Zr3+ and 0; which we have observed in thermally treated ZrO, samples indicate that oxygen vacancies have beenM. G. Cattania et al. 805 t 9 2.13 n 9,,= 1.92 r Fig. 2. E.s.r. spectra at 123 K of Ru/ZrO, recorded (a) under an argon atmosphere, (b) under an NO atmosphere (13.3 x lop3 kPa). Peaks marked with an asterisk are due to the support. The Ru centres are unaffected by NO chemisorption. formed in ZrO,, and the metal atoms can be located there. The strong metal-support interaction thus obtained also promotes dispersion of the metal centres and leads to small metal particles. It has been demonstrated previously4 that the Ru' carbonyl system becomes stable only as the metal particle size increases.This explanation accounts for the stabilization of a higher oxidation state of Ru on a less acidic support (ZrO, vs. y-Al,O,). Furthermore it explains why on Ru/ZrO, an increase in the pyrolysis temperature does not lead to the disappearance of the oxidized Ru centres observed before contact with CO on Ru/y-Al,O,. We cannot exclude the possibility that carbonyl adduct A contains CO as a bridging group between the Ru centres;' in fact A is stable at low CO pressures (lo-, Pa) and in increasing amounts when B is removed underTable 1. Amounts of paramagnetic species observed on Ru/ZrO, and Ru/y-Al,O, relative to the total amount of supported metal ~- ~ ~ __-_______ ~ ~- ~ ~- ~ ~ ~ CO(26.6 kPA) NO(13.3 x kPa) ____ -~ ~- _______ ~- ~~~~~ Ar Run+ Run+ + Run+-CO Ru"+<O Run+ + Ru2+-NO Ru2+-N0 (Yo) ~ ("") Ru,.-(n = 1,3)(%) ( O h ) -~ ( O h / . ) RUT RUT ~ ~ _______-- RUT RUT -~ ~ ~ _ _ _ _ ~ - ~ ~ ~ ~ _ _ _ _ Ru/ZrO, 0.2" 0.2 0.045 - - 0.7" 0.7 0.15 2.5" 2.5 1.65 2.2" 2.2 0.9 - - Ru/~-AI,O, - - - - ~~ ~~ - ~ _ _ _ _ - -~ ~ - - s z s " Two different Ru/ZrO, and two different Ru/y-Al,O, samples were studied. Table 2. E.s.r. data for paramagnetic centres in Ru/ZrO, (y-Al,O,), Rh/y-Al,O, i5 type of z 2 2 - 2.13 2.13 2.13 0.2-0.7 Ru+ + Ru3+ 2 - - RuX+-O, 7 zr4+-02 s .- ___-- paramagnetic centres us. P(O,)/kPA P(CO)/kPa P(NO)lkPa g,, g,, gZz total metal centres (Yo) A/G Paramagnetic centre Ru/ZrO, ~- - - - 1.72 0.2-0.7 F' 3 Ru"'<O Zr4+-NO 26.6 13.3 x 10-3 2.00 2.00 1.91 0.2-0.7 Ru2+-N0 RuI-CO 2.061 2.04 1 13.3 x 10-3 2.00 2.00 26.6 2.00 2.00 2.00 2.00 2.03 unidentified g = 2.05 9.3 1.96 1.96 1.92 Ru"+-NO 13.3 x lo-, 2.00 2.00 1.91 0.2-0.7 Ru/y-Al,O," - - 2.1 2.1 2.1 2-2.5 Ru' + Ru"' 9.3 1.99 1.99 1.95 AIJ+-NO 13.3 x 2.00 2.00 1.92 2.2 36 Ru2+-N0 13.3~ 2.00 2.00 1.92 2.4 34 Ru2+-NC0 26.6 Rh/y-Al,O," 13.3 x 10-3 g, = 2.08 g, = 2.05 g, = 2.03 Rh"-NO ~ ~- ~- - -~ ~ a From Akzo-Chemie y-A1,03CK-300 40 40M.G . Cattania et al. 807 I DPPH Fig. 3. E.s.r. spectra at 123 K of Ru/ZrO, recorded under a CO atmosphere (26.6 kPa). Peaks marked with an asterisk are due to the support. Pyrolysis temperatures (K) were: (a) 573, (b) 673, (c) 723 and (d) 773. vacuum (10-1 Pa, 10 min, at room temperature). This parallels what is observed on R~/y-A1,0,.~ By analogy with Ru/y-Al,O, species B should contain a terminal CO.Contact with 0, gives rise to a series of Ru derivatives which, in principle, could help to identify the oxidation states assumed by Ru on 25-0,. Molecular oxygen is a ‘non- innocent ’ probe because of the special reactivity of its unpaired electrons in n* orbitals. This valence-electronic structure ensures that when 0, lies in an oxide crystal field the808 Strong Me tal-Suppor t Interact ion r L i A1(40 G) 5 = 2.00 L 80G Fig. 4. E.s.r. spectra at 123 K of Ru/ZrO, recorded under (a) NO (13.3 x kPa), (b) NO (13.3 x low3 kPa) and CO (26.6 kPa). The Ru centres unaffected by NO chemisorption. orbital degeneracy is removed and the molecule is activated so as to accept electrons.The original metal oxidation state can thus be varied by interaction with oxygen. Ru/ZrO,, pyrolysed at 673 K and contacted with O,, shows an e.s.r. signal of 0; fixed to Zr4+ centres. An additional resonance is also visible in the g region peculiar to 0; derivative^,^ at g,, = 2.064 [fig. 1 (c)]. Since this value is the same as that of a species seen in an analogous experiment in R~/y-A1,0,,~ it is probable that it belongs to 0, fixed on oxidized Ru centres, rather than on the support centres. Compared with the indications given by CO about the electronic structure of Ru/ZrO,, oxygen is a very poor probe. Indeed the amount of 0, fixed on the metal centres is low compared with that on the Zr4+ support centres; as expected, the total number of paramagnetic centres involved inM .G. Cattania et al. 809 the 0, chemisorption is higher than in the case of CO because of the additional interaction with the support centres (see table 1). It is difficult to assign the Ru oxidation state of the superoxide derivative in the absence of isolated model complexes containing the Ru"+-O; moiety. The most probable possibilities should be Ru2+ or Ru4+, obtained by the transfer of one metal electron, respectively, from Ru' and Ru3+ to ZrO,, but this distinction cannot be made using the 0, probe molecule. The last 'probe' molecule used was nitrogen monoxide which, like 0,, behaves as a 'non-innocent ' ligand. Ru/ZrO,, contacted with NO, as described in the experimental section, shows well resolved resonance lines of a magnetically anisotropic species.Two g components are distinguishable (gl = 2.00, g,, = 1.91) and hyperfine coupling with the 14N nucleus is visible in the perpendicular region of resonances (AI = 40 G) [fig. 4(a)]. The values of the magnetic tensor components indicate that the unpaired electron of the nitrosyl derivative is mainly located on the NO moiety, not affecting the metal oxidation state. The species is stable to vacuum thermal treatment (1 OP2 Pa, 373 K) and is removed only at 473 K in vacuo (lop2 Pa). The different stability with respect to the nitrosyl derivative observed on the ZrO, support confirms that a bonding interaction with Ru occurs on Ru/ZrO, samples. Ruthenium most probably interacts as Ru2+, the diamagnetic electronic state not being visible in the absence of NO.The spectrum reveals that most paramagnetic Ru+ and Ru3+ centres are not affected by the nitrosyl interaction [see table 1 and fig. 2(b) A]. The results of the NO interaction with Ru/ZrO, exactly mirror those observed on R~/y-A1,0,.~ However, since the Ru/y-Al,O, sample, whose interaction with NO we described in a previous paper, was made with a different y-Al,O, sample from that used in our last paper on the Ru/y-Al,O, interaction with CO, we report here the spectra of the Ru nitrosyl derivative obtained on Ru/y-Al,O, made with CK-300 (see Experimental) Ketjen y-Al,O,. This catalyst shows a larger amount of the paramagnetic NO species discussed and better resolved spectra than that made by grade A Ketjen Two g components (gl = 2.00, g,, = 1.92) are clearly visible and confirm the attribution of the signal to Ru2+-N0 species.Similar adducts .were observed by Kasai lo on bivalent y-Al,O,-supported Zn centres; the hyperfine coupling ( A = 36 G) is also in agreement with the assignment. A comparison of the results of the NO interaction on Ru/y-Al,O, with those on Ru/ ZrO, gives evidence of small differences in the Ru-NO derivatives obtained in the two cases, i.e. a slight increase in the hyperfine coupling constant with 14N nucleus, attributable to a slightly different transfer of the unpaired electron from NO to Ru, could be observed. To improve the interpretation of the results obtained through the NO interaction it is useful to make further comparison between Ru/ y-Al,O,(ZrO,) and the analogous rhodium samples studied in some of our previous papers.1.2 Rh/y-Al,O, and Rh/ ZrO,, prepared as already described'.from supported Rh,(CO),, and pyrolysed at 523 K were contacted with NO by the conventional method. On the Rh sample the resonance lines attributed to Rh2+ centres totally disappear, being replaced by those of a nitrosyl Rh derivative (fig. 6). No unaffected Rh2+ centres were observed to remain. The anisotropy of the g tensor and the values of the three components (8, = 2.08, g , = 2.05, g, = 2.034) lead to the conclusion that the unpaired electron of the Rh-NO moiety is mainly located on the Rh centre. No hyperfine interaction with 14N was observed. The spectrum indicates that the unpaired electron predominantly lies in the d,2-y2 orbital and could be interpreted either as due to a d7 metal electronic configuration in a compressed octahedral symmetry or a d9 electronic state of an elongated octahedron.Since the process producing the nitrosyl species is presumably coupled to a reduction of the Rh centres, it seems more likely to be the reduction of starting ds Rh+ centres than of y-Ai203 [fig. 5(a)i.810 Strong Me tal-Suppor t Interaction ~ D P P H A,(36 GI - ’ g, = 2.00 + Ia I Fig. 5. E.s.r. spectra at 123 K of Ru/y-Al,O, recorded under (a) NO (13.3 x kPa), (b) NO (13.3 x kPa) and CO (26.6 kPa). d6Rh3+. The latter would seem to be too oxidized to be formed in significant amounts on the metal particles. No changes were observed with respect to the spectrum in an argon atmosphere when Rh/ZrO, was contacted with NO.This behaviour does not agree with that of Ru/ZrO, samples. The different behaviour between Rh and Ru, with respect to their reactivity towards NO, outlines the distinction between the two aspects of the metal-support interaction which we refer to, depending both on the metal and on the support : while A1,03 is acidic enough to allow nitrogen monoxide to interact both with Rh and with Ru oxidized centres, ZrO, is not able to activate Rh centres to adsorb NO, because the ZrO, supported metal centres become positive in large amounts only at high pyrolysis temperatures. In the latter case the formation of positive Ru centres is promoted by the partial reduction of the lattice oxide centres. Residual oxidized metal centres were observed on Ru/ y-Al,O,(ZrO,) samples, but not in Rh/y-Al,O,.This could mean that some metal centres are not accessible in Ru catalysts, both on y-Al,O, and on ZrO, and indicates that the ability of Ru to occupyM. G. Cattania et al. 81 1 Fig. 6. E.s.r. spectra at 123 K of Rh/y-Al,O, recorded under NO (13.3 x low3 kPa). cages of the support is higher than that of Rh and occurs, though it does not prevail, also on Ru/y-Al,O,. In the Ru/ZrO, samples described, if contact with NO is followed by contact with CO (26.6 kPa), no substantial modifications are observed in the nitrosyl spectrum [fig. 4(b)]. By reversing the contact order, NO removes CO from metal coordination to stabilize the nitrosyl adduct already described ; even 0, is removed by NO. On the other hand, results from the same experiments performed on Ru/y-Al,O, made by using A1,0, CK-300, call for further comment with respect to the previously published data.3 Changes of the e.s.r.lines with respect to an essentially nitrosyl Ru adduct become evident. There is also a slight change in hyperfine coupling ( A = 34 G) [fig. 5(b)]. Based on the reported behaviour, the following hypotheses can be made: (i) CO and NO interact with two different Ru2+ centres; (ii) CO and NO interact with the same Ru centre but not with each other; (iii) CO and NO interact with the Ru centre and with each other forming In case (i) we expect a paramagnetic nitrosyl derivative slightly different from that observed after contact with NO alone; however, it seems improbable that, as a consequence of the CO interaction, no paramagnetic Ru carbonyl species can be observed.Hypothesis (ii) is excluded by the results of the experiment performed in the opposite order. The most probable hypothesis remains (iii), where, as expected, the e.s.r. spectrum is not very different from that of an Ru nitrosyl derivative. If we assume the mechanism of NCO formation proposed by Davydov and Bell," ZrO, is evidently unable to active the NO dissociation. In the same way the centres on Rh/y-Al,O, do not promote NO dissociation. Ru2+-NC0. 28 FAR 1812 Strong Metal-Support Interaction Conclusion If we assume that the behaviour of the paramagnetic species described in this paper, though limited to 1-2 % of the total supported metal, gives an indication of the chemical behaviour of the supported metal as a whole, the comparative discussion on the chemical behaviour of Ru(Rh)/ y-Al,O,(ZrO,) allows the distinction of two main effects responsible for the metal-support interaction : the acidic properties of the support and the lattice oxide vacancies of the reducible supports.The relative influence of these depends on the metal, the vacuum-thermal treatment of the catalysts and on the support. Ruthenium seems to stabilize a stronger interaction than Rhodium. On the same support, at the same pyrolysis temperature, the number of oxidized ruthenium centres is higher than that of rhodium centres. The 'probe' molecules leave a large number of oxidized Ru centres unaffected, while all Rh centres are involved in the chemisorption of CO and NO. Ru thus appears to be enclosed by the supports more than Rh.y-Al,O, stabilizes a stronger interaction than ZrO, in the temperature range where the oxide lattice vacancies are not present in ZrO,: Rh/ZrO, does not interact with NO because the metal centres are not sufficiently electron deficient to stabilize bonding with NO, while Ru/ZrO,, pyrolysed at 673 K, does because of the interaction with the reduced support. On y-Al,O,, where the acid-base interaction mechanism prevails, the increase in temperature leads to a decrease in the metal-support interaction and the Ru3+ carbonyl species transforms into Ru+, while on ZrO, from 673 K the electronic and acid-base effects combine with a steric effect (due to formation of oxide vacancies) and the Ru+ carbonyl adduct resonances have a maximum at the equilibrium (673 K) between the two effects. The metal-support interaction on y-Al,O, is dominated by an acid-base interaction mechanism; the increase in temperature leads to sintering of the metal particles and to a consequent decrease of the average charge of the positive centres (Ru"'-CO transforms into RuI-CO).The metal-support interaction on ZrO, results from the combination of the acid-base effect and the steric effect, which locates the metal in the oxide vacancies and avoids the sintering effect responsible for the Ru~~~--CO + Ru'-CO conversion. The RuI-CO resonances have a maximum at the equilibrium (673 K) between the two effects. References 1 T. Beringhelli, A. Gervasini, F. Morazzoni, D. Strumolo, S. Martinengo and L. Zanderighi, J. Chem. 2 A. Gervasini, F. Morazzoni, D. Strumolo, F. Pinna, G. Strukul and L. Zanderighi, J. Chem. SOC., 3 M. G. Cattania, A. Gervasini, F. Morazzoni, R. Scotti and D. Strumolo, J. Chem. Soc., Faraday 4 (a) M. G. Cattania, A. Gervasini, F. Morazzoni and D. Strumolo, J. Chem. SOC., Faraday Trans. 1, 5 C. Dall'Agnol, A. Gervasini, F. Morazzoni, F. Pinna, G. Strukul and L. Zanderighi, J. Catal., 1985, 6 M. G. Sanchez and J. L. Gasquez, J. Catal., 1986, 104, 12. 7 M. J. Torralvo, M. A. Alario and J. Soria, J. Catal., 1984, 86, 473. 8 E. Guglielminotti, Lungmuir, 1986, 2, 812. 9 J. H. Lunsford, Catal. Rev., 1973, 8, 135. SOC., Faraday Trans. 1, 1984, 80, 1479. Faraday Trans. 1 , 1986, 82, 1795. Trans. I, 1987, 83, 3619. 1987, 83, 2271. (6) V. Ragaini and M. G. Cattania, J . Catal., 1985, 93, 161. 96, 106. 10 P. H. Kasai and R. M. Gaura, J. Phys. Chem., 1982, 4257; P. H. Kasai and R. J. Bishop Jr, J. Am. 1 1 A. A. Davydov and A. T. Bell, J . Catal., 1977, 49, 332; 1977, 49, 345. Chem. SOC., 1972, 94, 5560. Paper 8/00573G; Received 16th February, 1988

ISSN:0300-9599    

DOI:10.1039/F19898500801

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1989, 85(4), 813-827 Photoelectrochemical Investigations of Phenosafranine Dye bound to some Macromolecules R. Ramaraj and P. Natarajan" Department of Inorganic Chemistry, University of Madras, Guindy Campus, Madras 600 025, Tamil Nadu, India Macromolecular- bound phenosafranine dyes have been synthesized and characterized. The absorption spectra of the polymer-bound dyes show a bathochromic shift while the emission spectra shift slightly to higher frequency. Cyclic voltammetry of polymeric phenosafranines adsorbed at the carbon electrode show the subsequent two-electron redox reaction of the dye. Flash photolysis of the monomeric and polymeric dyes in the presence of EDTA produce the one-electron reduced dye which disproportionates ; the disproportionation rate constant was measured to be lo9 and lo8 dm3 mol-' s-l, respectively.Photoelectrochemical studies of the macromolecular phenosafranine indicate different behaviour depending upon the macro- molecule. In the case of an electrode coated with a film of poly(acry1- amidomethylphenosafranine-co-methylolacrylamide) cathodic behaviour was observed with reference to an inert electrode, while an electrode coated with a film of poly(acrylamidomethylphenosafranine-co-methylolacryl- amide-co-vinylp yridine) exhibited anodic polarity . A wa ter-spli tting regenerative cell was shown to operate using the polymeric phenosafranine- coated electrodes. The photoredox systems are receiving attention to utilize the solar energy as electricity and fuels by chemical methods.' The photochemistry in photosynthesis is known to involve one-electron transfer reactions from the excited state of chlorophyll species to an acceptor. In recent times many workers" are developing dye-sensitized systems as models for the primary processes of photosynthesis and as devices to convert and store solar energy.Among the systems studied so far, the photopotential values reported are 250 mV for the iron-thionine ~ystem,~ 476 mV for the proflavin-EDTA system,' 844 mV for the tolusafranine-EDTA system6 and 870 mV for the phenosafranine-EDTA ~ystem.~ Among these redox systems, the iron-thionine system is reversible and in the other systems the oxidation of EDTA is irreversible. The abovementioned photogalvanic systems are reported in homogeneous conditions, and in these systems the energy- wasting back reactions are found to reduce the efficiency of the photoenergy conversion.On the other hand a microheterogeneous reaction environment such as in micelles,' monolayer as~emblies,~ polymerslO and chemically modified electrode systems," facilitates unidirectional electron flow, and light absorbers show interesting charac- teristics, different from the corresponding monomeric light absorbers. '' In this paper we report the preparation and photoelectrochemistry of the different macromolecular- bound phenosafranines. We have constructed a photoelectrochemical cell by coating the polymer-dyes onto electrodes which show regenerative pho toelectrochemical behaviour. 813 28-2814 Macromolecular- bound Phenosafranine Dyes / H NH NH I CH20H Q / H Fig.1. Structure of polymer-bound phenosafranine dyes. Experiment a1 Materials Phenosafranine dye (3,7-diamino-5-phenylphenazinium chloride) was recrystallized from methanol. Acrylamide (B.D.H.), ethylenediaminetetra-acetic acid disodium salt, EDTA (B.D.H.), potassium chloride (B.D.H.) and sodium perchlorate (Merck) were analytical-grade chemicals and were used as such. Water was distilled twice adding alkaline KMnO,. Preparation of Polymer-dye Samples Poly(N-methylolacrylamide), P(MAAM), and poly(N-methylolacrylamide-co-4-vinyl- pyridine), P(MAAM-co-VP), were prepared by the following procedure as described earlier. l3 N-Methylolacrylamide was prepared by the reaction of paraformaldehyde with acrylamide. N-Methylolacrylamide was polymerized in aqueous solution using potassium peroxodisulphate as the initiator to obtain P(MAAM). The copolymer P(MAAM-co-VP) was prepared by copolymerizing N-methylolacrylamide with 4- vinylpyridine.The homopolymer P(MAAM) and poly(4-vinylpyridine) formed in the preparation of the copolymer P(MAAM-co-VP) were removed as described earlier. l3 Phenosafranine was condensed with P(MAAM) to give poly(acrylamidomethy1- phenosafranine-co-methylolacrylamide). Purified phenosafranine was added to an aqueous solution of the polymer P(MAAM) in the desired molar ratio and the mixture was kept at 90-95 "C in a water bath for 6 h. Hydroquinone was added to this mixture to prevent crosslinking of the polymer. The uncondensed phenosafranine was removed by dialysing the solution for 15-20 days in water.Dialysis was continued until the solution outside the sack did not show any absorption of phenosafranine (Amax = 522 nm). Poly(acrylamidomethylphenosafranine-co-methylolacrylamide-co-vinylpyridine) wasR. Rarnaraj and P. Natarajan 815 prepared by following the same procedure as in the case of polymer dye P(AMPS+-co- MAAM). The structures of the macromolecular dyes used are shown in fig. 1. The phenosafranine dye condensed to the macromolecular chain was stable even under rigorous conditions. The polymer-dye solutions, on standing for months and subsequent dialysis against water, did not show any trace of dyes passing through the membrane, indicating the stability of the polymer-bound phenosafranine dye. Estimation of Phenosafranine A titrimetric method was used to estimate the phenosafranine dye bound to the macr0mo1ec~le.'~ A solution containing a known amount of titanous chloride in 4 mol dm-3 hydrochloric acid was titrated against the polymeric phenosafranine under a nitrogen atmosphere at 60 "C.The colour of the phenosafranine solution disappears owing to the reduction of phenosafranine by titanous ions in the presence of hydrochloric acid to leucophenosafranine. The end point is the appearance of a light brown colour. From the titration data the amount of phenosafranine present in the solution taken was calculated. A known volume of the original polymer-dye solution was evaporated and the weight of the residue was taken as the amount of polymer-dye complex present in the original solution.The number of phenosafranine units bound to the polymer chain consisting of a given number of monomer units (the m/d ratio) was calculated knowing the amount of polymer and the phenosafranine present per unit volume of the solution. Analytical Methods Absorption spectra of the polymer samples in aqueous solutions and in film state (coated onto a glass plate) were recorded using a Beckman 25 double-beam spectrophotometer. The emission spectra of the polymer-dye sample were recorded in a Perkin-Elmer MPF 44B fluorescence spectrophotometer. Flash-photolysis studies were carried out using Applied Photophysics Ltd, KN-020 model flash kinetic spectrometer. The experi- mental methods used for the flash-photolysis studies were described earlier." In the flash-photolysis studies the dye concentration in aqueous solution was maintained at ca.lo-' mol dmP3 and the dye solution in the flash cell was freed of oxygen by passing purified nitrogen through it. Transients were monitored using a tungsten-halogen lamp in the region 600-730 nm. In the outer jacket of the sample cell acetone was taken to filter off radiations below 300 nm. The monitoring beam was first passed through a filter cell containing phenosafranine solution to prevent any steady photolysis of pheno- safranine in the reaction mixture. The absorption spectra of semi-dye species were recorded in the region 60&730 nm, where the absorbance due to phenosafranine and polymer-bound phenosafranine are negligible. The absorbance of the transient was recorded after 0.5ms of the flash at different wavelengths and the spectrum of the transient species was obtained.The electrochemical data were obtained using PAR modules 173 potentiostat, 175 universal programmer and 176 current follower. The reference electrode (SCE) was connected to the potentiostat through a PAR 178 electrometer. The photopotential was measured using an Aplab digital voltmeter and the photocurrent was measured using a Radelkis- 105 universal polarograph. The cell consists of two compartments interconnected by a salt bridge or a single- compartment cell. A plain platinum electrode and polymer-dye-coated platinum or carbon electrode or both platinum electrodes coated with different polymer-dyes were employed. The distance between the two electrodes was maintained at 4+ 1 mm in the single-compartment cell.The polymer-bound phenosafranine was coated onto a platinum or carbon electrode by placing a solution of the macromolecular dye on the surface of the electrically cleaned and dried electrode surface by blowing hot air over the816 Macromolecular- bound Phenosafranine Dyes 0.8 550 650 450 a/nm 350 Fig. 2. Absorption spectra of phenosafranine and P(AMPS+-co-MAAM) : (a) P(AMPS+- m/d = 41 ; ( d ) phenosafranine. CO-MAAM), m/d = 16 ; (b) P(AMPS+-co-MAAM), m/d = 38 ; (c) P(AMPS+-co-MAAM), 0.8 0.6 8 B 0.4 4 0.2 0 400 500 600 A/nm Fig. 3. Absorption spectra of phenosafranine and P(AMPS+-co-MAAM-co-VP) : (a) P(AMPS+- co-MAAM-co-VP), m/d = 21 ; (b) P(AMPS+-co-MAAM-co-VP), m/d = 32; ( c ) phenosafranine.R. Ramaraj and P .Natarajan 817 605 625 645 665W 625 645 665 685 (b), (c) Llnm Fig. 4. Luminescence spectra of (a) phenosafranine, (b) P(AMPS+-co-MAAM) and (c) P(AMPS+-co-MAAM-co-VP). surface. The evaporation of the solvent leaves behind a film on the electrode, In the case of polymer-dye P(AMPS+-co-MAAM-co-VP) the electrodes were immersed in the cell containing 10 cm3 of 6:4 CH,CN:H,O (v/v) and potassium chloride or sodium perchlorate as supporting electrolyte. The pH of the solution was adjusted with perchloric acid or sodium hydroxide. The solutions were deaerated by passing oxygen- free nitrogen for 30 min. The irradiation source was 1000 W tungsten lamp and a 5 cm water filter cell was used to cut off infrared radiations; the design of the lamp is such that the light output used for irradiating the electrodes is more like a 300 W lamp. Results The absorption spectra of poly(acrylamidomethylphenosafranine-co-methylolacryla- mide) with different m/d ratios obtained in solution are shown in fig.2. The absorption spectra of poly(acrylamidomethyIphenosafranine-co-methylolacrylamide-co-vinyl- pyridine) with different m/d ratios are shown in fig. 3. The emission spectra of phenosafranine dyes are shown in fig. 4. Typical cyclic voltammograms of phenosafranine and polymer-bound phenosafranine systems are shown in fig. 5 . The cathodic peak potentials (E& anodic peak potentials (E,& separation of peak potentials (AE,) and the cathodic peak current (I,,) at various scan rates are given in table 1. The absorption spectra of the transients were obtained following flash photolysis of phenosafranine under various conditions and the results are shown in fig.6. The absorbance of semiphenosafranine and polymer bound semiphenosafranine with different m/d ratios at different time intervals after the flash are measured and the corresponding plots for the decay kinetics of the transients are shown in fig. 7 and the rate constants are given in table 2.818 Macromolecular-bound Phenosafranine Dyes 0 -0.2 -0.4 -0.6 EIVvs. SCE -1.2 -1.0 -0.8 - 0.6 - 0.4 - 0.2 0 0.2 0.4 0.6 0.2 -0.2 -0.6 -1.0 EIV us. SCE 0 -0.2 -0.4 -0.6 -0.8 EIVvs. SCE EIVvs. SCE Fig. 5. Cyclic voltammograms of phenosafranine and polymer-bound phenosafranines. Working electrode, carbon plate ; supporting electrolyte, KCl.(a) Phenosafranine in homogeneous solution, (b) phenosafranine adsorbed on carbon, (c) P(AMPS+-co-MAAM) coated onto carbon and ( d ) P(AMPS+-co-MAAM-co-VP) coated onto carbon. Potential scan rate, v/mV s-' : (I) 10, (11) 20, (111) 50, (IV) 100 and (V) 200. Table 1. Cyclic-voltammetric data of phenosafranine and polymer-bound phenosafranine" (A) PS+ (present in solution) 10 -0.36 -0.320 40 125 20 -0.36 -0.315 45 220 50 -0.37 -0.310 60 520 100 -0.38 -0.305 75 970 (B) PS+ (adsorbed on carbon) 10 -0.44 -0.32 120 20 -0.46 -0.28 180 - 50 -0.48 -0.24 240 100 -0.54 -0.62 380 200 -0.62 -0.10 520 - (C) P(AMPS+-co-MAAM) (coated on carbon), m/d = 41 - - - 10 -0.35 -0.44 -0.33 -0.43 20 10 20 -0.35 -0.44 -0.32 -0.43 30 10 50 -0.36 -0.46 -0.31 -0.43 50 30 100 -0.38 -0.47 -0.30 -0.42 80 50 (D) P(AMPS+-co-MAAM-co-VP) (coated on carbon), 10 -0.36 -0.60 -0.31 -0.56 50 40 20 -0.38 -0.61 -0.31 -0.56 70 50 50 -0.40 -0.63 -0.30 -0.54 100 90 m/d = 21 100 -0.43 -0.66 -0.27 -0.53 160 130 a [KCl] = 0.1 mol dm-3, solvent = 8 : 2 = CH3CN : H,O (v/v), potential range = + 0.0 to - 0.8 V, working electrode = carbon.R. Ramaraj and P.Natarajan 819 7.0 6.4 5.8 5.2 4.6 5 4.0 3 3.4 2.8 2.2 I .6 P 3 P 1.8 I .7 1.6 1.5 a 4 X 1.4 8 1.3 4 1.2 1. I 1.0 0.9 P 1 I I 1 1 1 ' 0.8 600 620 640 660 680 700 720 740 Alnm 1.0 1 ' Fig. 6. Absorption spectra of semiphenosafranine formed from A, phenosafranine with EDTA ; ., phenosafranine without EDTA; 0, P(AMPS+-co-MAAM), m / d = 16 with EDTA; A, P(AMPS+-co-MAAM), m/d = 16 without EDTA; 0, P(AMPS+-co-MAAM), m/d = 41 with EDTA; 0, P(AMPS+-co-MAAM-co-VP), m/d = 21 with EDTA.0 : 160 140 I20 too 2 80 60 40 20 a I I I I I I 0 2 4 6 8 10 12 14 m o o 0 0' tlms Fig. 7. Kinetics of the decay of macromolecular semiphenosafranine dyes. A, Phenosafranine with EDTA; 0, phenosafranine without EDTA; 0, P(AMPS+-co-MAAM), m/d = 16 with EDTA; ., P(AMPS+-co-MAAM), m/d = 16 without EDTA; A, P(AMPS+-co-MAAM), m/d = 41 without EDTA; 0, P(AMPS+-co-MAAM-co-VP), m/d = 21 with EDTA.820 Macromolecular- bound Phen osafran ine Dyes Table 2. The disproportionation rate constant of semipheno- safranine and polymer-bound semiphenosafranines sample k,/ los dm3 m/d EDTA mo1-'s-l phenosafranine (PS') - absent 10.10 phenosafranine (PS') present 3.19 P(AMPS+-co-MAAM) 16 absent 1.98 P(AMPS+-co-MAAM) 41 absent 1.63 P(AMPS+-co-MAAM) 16 present 1.40 - P(AMPS+-co-MAAM-co-VP) 2 1 present 1.02 Table 3.Electrodic behaviour of polymer-bound phenosafranine coated onto platinum electrodes in a single-compartment cell in the presence of light sample AqJmV i/pA electrolyte P(AMPS+-CO-MAAM)~ + 22 + 121 + 24 + 120 + 40" P(AMPS+-CO-MAAM-CO-VP)~ -25 -111 - 33 - 125 - 37" +0.16 + 0.66 +0.16 + 0.63 + 0.50 -0.10 - 6.00 -0.16 - 6.00 - 0.40 0.1 mol dm-3 KC1 0.1 mol dm-3 KCl + 0.1 mol dm-3 NaClO, 0.1 mol dmT3 NaClO, + lop3 mol dmP3 0.1 mol dm-3 KCI + 0.1 mol dm-3 KCl 0.1 mol dm-3 KC1 + lop3 mol dmP3 0.1 mol dm-3 NaC10, 0.1 mol dm-3 NaClO, + 0.1 mol dm-3 KCl+ lop3 mol dmP3 mol dmP3 HCl HClO, mol dm-3 HCl NaOH mol dm-3 NaOH NaOH a Air-equilibrated solution. 6 : 4 CH3CN : H,O (v/v) medium.Coated onto carbon electrodes. 7 : 3 CH3CN : H,O (v/v) medium. Table 4. Effect of acid concentration on the electrodic behaviour of polymer- bound pheno- safranine, P(AMPS+-co-MAAM) coated onto a platinum electrode in a single-compartment cell in the presence of light HCl/mol dm-3 AE:,/mV i/PA 0 73 0.06 10-4 89 0.13 10-3 135 0.27 1 o-, 82 0.14 a Electrolyte = 0.1 mol dm-3 KCl. 6 : 4 CH3CN : H,O (v/v) medium.R. Ramaraj and P. Natarajan 82 1 Table 5. Effect of sodium hydroxide on the electrodic behaviour of polymer-bound phenosafranine, P(AMPS+-co-MAAM-co-VP) coated onto platinum electrode in a single- compartment cell in the presence of light 0 - 25 -0.10 10-4 - 40 - 1.00 10-3 - 1 1 1 - 6.00 1 o-, - 70 -2.10 0.6 0.4 0.2 a Electrolyte = 0.1 mol dm-3 KCI.7: 3 CH3CN: H,O (v/v) medium. 1.8 1.2 3 > 0.6 Fig. 8. (a) Photocurrent for a P(AMPS+-co-MAAM)-coated electrode without EDTA ; (b) photocurrent for a P(AMPS+-co-MAAM-co-VP)-coated electrode without EDTA ; (c) photocurrent for one electrode coated with P(AMPS+-co-MAAM) and the other coated with P(AMPS+-co-MAAM-co-VP) coupled and kept in a two-compartment cell without EDTA ; ( d ) photocurrent for the P(AMPS+-co-MAAM jEDTA system: L,, ‘light on’ condition for a P(AMPS+-co-MAAM)-coated electrode ; D,, ‘ light off’ condition for a P(AMPS+-co-MAAM)- coated electrode; L,, ‘light on ’ condition for a P(AMPS+-co-MAAM-co-VP)-coated electrode; D,, ‘ light off’ condition for a P(AMPS+-co-MAAM-co-VP)-coated electrode. The open-circuit photopotential and photocurrent for polymer-bound phenosafranine coated onto platinum and carbon electrode systems measured in single- and two- compartment cells are given in table 3.The photopotential and photocurrent of the electrodes measured at different concentrations of hydrochloric acid and sodium hydroxide are given in tables 4 and 5, respectively. The photocurrent was measured at different times of irradiation of the polymer-coated electrodes and the results are shown in fig. 8. The photocurrent was measured in the photoelectrochemical cell with polymer- bound phenosafranine-coated carbon electrode systems with and without EDTA and plain platinum electrode at different applied potential with respect to saturated calomel electrode (SCE) and are shown in fig. 9. Discussion Interest in the redox behaviour of phenazine-based compounds stems from their use as dyestuffs as well as from the recognition of the biological importance of some phenazine derivatives.Prior to any recorded voltammetric measurements of phenazine, it has been shown that a stable, one-electron reduction product (semiquinone) is formed during822 Macromolecular- bound Phenosafranine Dyes 20 1 I I 1 I I 1 1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 a 0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 o 80 60 - 40 - 20 - 3n 1 I I 1 I I 1 1 1 I LV 0.2-0.1 o 7 6 5 4 3 2 I 0 - I 4 3 EIVvs. SCE 70 60 3 50 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 EIVvs. SCE Fig. 9. A (a) Photocurrent plotted against applied voltage (us. SCE) across the working- electrode-reference-electrode for a P(AMPS+-co-MAAM)-coated electrode-EDTA system.(b) Photocurrent plotted against applied voltage (us. SCE) across the working-electrode-reference- electrode for the PS+-EDTA system. B (a) Photocurrent plotted against applied voltage (us. SCE) across the working-electrode-reference-electrode for a P(AMPS+-co-MAAM-co-VP)- coated electrode without EDTA. (b) Photocurrent plotted against applied voltage (us. SCE) across the working-electrode-reference-electrode for a P(AMPS+-co-MAAM)-coated electrode without EDTA.R. Ramaraj and P . Natarajan 823 potentiometric titrations in different solvents. l6 Muller l7 has shown that the information on the formal potentials and semiquinone intermediate of a number of compounds could be obtained from polarograms. Previous studies on phenazine16* l7 indicate the following reaction sequence for solutions, in which single cathodic and anodic waves were observed : +Hf -Hf PS+ + e- e PSH'+ +Hf -Hf PSH'+ + e- G= PSH;.In aqueous solution, phenazine (PS+) undergoes two one-electron reduction to give leucophenazine (PSH;) via a one-electron intermediate semiphenazine (PSH'+) which is a protonated species. For the phenosafranine dye at ca. pH 6.5 only one cathodic and one anodic wave are observed [fig. 5 (a)] as established for phenazine derivatives in neutral solutions. 179 la Cyclic-voltammograms run for the dye adsorbed on carbon electrodes [fig. 5(b)] show one cathodic and one anodic wave. However, the peaks are broader and the peak separation (AE,) is large (table l), indicating that the electrodic process is not very reversible at the carbon electrode.It is observed that with the increase in the scan rate of potential, the cathodic curve shifts to more negative potentials (table 1). It is also observed that the separation of peak potentials increases with an increase in the potential scan rate. This observation is explained on the basis of an electron-transfer-electron- transfer mechanism (E-E mechanism) coupled with a fast protonation reaction. l9 In contrast to the behaviour of the monomeric dye, polymer-bound phenosafranine shows [fig. 5(c) and (d)] two anodic and two cathodic waves. The peaks are broader and the peak separation is larger for polymeric phenosafranine as compared to that of monomeric phenosafranine (table 1). When the dye is attached to the macromolecule, a shift in the peak potentials is observed.The cyclic voltammograms of phenosafranine and polymer-bound phenosafranine are not found to be similar, and the two-electron reduction of semiphenosafranine could be separately observed when polymer chain is attached to the dye [table 1 and fig. 5(c) and (d)]. During the reduction of polymer bound phenosafranine, protonation takes place indicating a slow chemical reaction and the process can be described in terms of an electron-transfer-chemical-reaction-electron- transfer (ECE) mechanism. Absorption Spectra of Semiphenosafranine attached to Macromolecular Chains Flash-photolysis studies of the safranine-0,-EDTA system have been 2o The dye phenosafranine is easily photoreduced by electron donors such as EDTA.Semiphenosafranine radicals were produced upon irradiation of a solution containing phenosafranine and EDTA. 21 The photoreduction involves electron transfer from EDTA to the excited triplet state of the phenosafranine molecule. In the present investigation the flash-photolysis experiments of phenosafranine-EDTA and polymer- bound phenosafranine-EDTA systems and the monomeric and polymeric dye in the absence of EDTA were carried out to investigate the effect of polymer backbone on the properties of the semiphenosafranine radical. The absorption spectrum of the free semiphenosafranine radical produced on flash photolysis of the phenosafranine-EDTA system shows an absorption maximum around 640nm as shown in fig. 6. The absorption spectra of semiphenosafranines produced on flash irradiation of polymer- bound phenosafranine systems with EDTA as shown in fig.6 indicate absorption maxima at 680 nm. The polymer-dye P(AMPS+-co-MAAM-co-VP) does not give any transient absorption in the absence of EDTA in solution. The shift in the absorption maxima for the polymer-bound semiphenosafranines compared to that for free824 Macromolecular- bound Phenosafranine Dyes semiphenosafranine is presumably due to the -CH,- group present in the polymer chain. The absorption maxima of the transient obtained for phenosafranine in the presence and in the absence of EDTA during flash photolysis are the same (640 nm). The absorption spectra of the polymer-bound semiphenosafranines obtained for macromolecular-bound phenosafranines P(AMPS+-co-MAAM) and P(AMPS+-co- MAAM-VP) in the presence and absence of EDTA are the same (680 nm).It is concluded that the transient formed from P(AMPS+-co-MAAM) in the absence of EDTA is polymer-bound semiphenosafranine. The absorption maxima of semi- phenosafranine attached to two different polymer chains seen around the same wavelength (680 nm) suggest that the absorption spectrum of the semiphenosafranine radical attached to a polymer chain is not significantly influenced by the nature of the polymer backbone. Failure to see any transient species on flash photolysis of P(AMPS+-co-MAAM-co-VP) may be due to the formation of an adduct between the excited dye centre and the pyridine groups present in the polymer chain, indicating that the photochemical behaviour of P(AMPS+-co-MAAM-co-VP) is different from that of P(AMPS+-co-MAAM).Kinetics of the Decay of the Polymer-bound Semiphenosafranines The decay of semiphenosafranine radical produced by electron transfer from EDTA to excited phenosafranine observed on flash excitation is represented by the equations PS+ + H+ + EDTA -+ PSH'+ + EDTA,, 2PSH'+ -+ PSHg + PS+ (3) (4) where PSH'+ = semiphenosafranine and PSH; = leucophenosafranine. Semipheno- safranine is also produced on flash photolysis of phenosafranine without EDTA present in solution. The second-order rate constant k, for the disproportionation reaction of semiphenosafranine (PSH'+) produced with and without EDTA present in the solution was found to be 1.01 x lo9 s-l. The decay of the transient formed from the polymer-dye also followed equal concentration second-order kinetics.The formation of semireduced dye from excited phenosafranine and polymer-bound phenosafranine in the absence of EDTA is attributed to the reaction *PSH2+ + PS+ -+ 'PSH+ + 'PS2+. ( 5 ) This observation is supported by earlier studies Kosui et al. and Baumgartner et al. carried out for monomeric phenosafranine.209 22 The latter group20 was unable to detect the transient absorption which can be attributed to the semioxidized dye (PS2+) even in the presence of Fe3+ or H202. The failure to detect the semioxidized dye may reflect the rapid reduction by the solvent. The rate constants for the decay of the monomeric semi- reduced dye and polymer-bound semireduced dye are found to be ca. lo9 dm3 mol-' and los dm3 mol-l, respectively (table 2).The rate constant for the disproportionation reaction of macromolecular semiphenosafranine are ca. 10 times slower than that of the monomeric semidye. A similar observation is reported in the case of thionine and polymer-bound thionine ~ystems.'~ This is due to the difference in the diffusional rate of the monomeric phenosafranine and polymer-bound phenosafranine. The polymer-bound dye P(AMPS+-co-MAAM-co-VP) does not give any transient on flash photolysis in the absence of EDTA, whereas in the presence of EDTA a transient similar to that for polymer dye P(AMPS+-co-MAAM) is observed. The flash-photolysis results also show that addition of 0.1 mol dm-3 pyridine to the experimental polymeric dye solution quenches the excited phenosafranine dye.This may be due to the formation of an adduct between the excited dye centre and the pyridine units. In the polymer-dye P(AMPS+-co-MAAM-co-VP) the adduct formation may be enhanced by the polymerR. Ramaraj and P. Natarajan 825 backbone, which brings the dye centre and pyridine units to a closer and more rigid environment. The flash-photolysis studies of polymer-dye P(AMPS+-co-MA AM) and P(AMPS+-co-MAAM-co-VP) in the absence of EDTA show a difference in photochemical reaction. It is suggested that this difference in photochemical behaviour is due to the presence of pyridine units in the polymer chain along with the dye centre. Photoelectrochemical Cell to Produce Fuels like Hydrogen using Macromolecular- bound PhenosafranineEDTA Systems The phenosafranine-EDTA system was suggested as a possible pho toelectrochemical cell to convert photoenergy into electricity or to evolve hydrogen from water.21 The dye phenosafranine is easily photoreduced by electron donors such as EDTA.Its longest- wavelength absorption is centred at 522 nm. Anaerobic photolysis in the presence of EDTA leads to disappearance of the absorption peak at 522 nm, with a new peak appearing around 640 nm for monomeric phenosafranine and 680 nm for polymeric phenosafranine. The current-potential curves presented in fig. 9 show that the photogenerated reductant is easily oxidized at the carbon anode at potentials around 0.1 V in 0.5 mol dmP3 H,SO, catholyte. The cell is given as carbonlphenosafranine, EDTA, pH 71 (0.5 mol dm-3 H,SO,IPt. Cyclic-voltammetric studies of phenosafranine in water showed that the midpoint potential for the first electron reduction is located at -0.3 V us.SCE. The current-potential curve of P(AMPS+-co-MAAM) coated onto the carbon electrode (fig. 9) shows that the photochemically produced semi-dye is easily oxidized at the carbon anode at potentials around -0.1 V vs. SCE. The cyclic-voltammetric data for P(AMPS+-co-MAAM) or P(AMPS+-co-MAAM-co-VP) coated onto carbon electrodes indicate the occurrence of a two-electron redox process which corresponds to the formation of semi-reduced dye at -0.54 V us. NHE and leucodye at -0.69 V us. NHE. In the cathode compartment, protons are getting reduced at ca. pH 1. Photoelectrochemical Reactions at Macromolecular-bound Phenosafranine-coated Electrode : Example of a Regenerative-type Water-splitting Cell In this section certain properties of platinum or carbon electrodes coated with polymer- bound phenosafranine dye on irradiation with visible radiation are discussed.The electrode coated with P(AMPS+-co-MAAM) shows a photopotential in air-equilibrated solutions either in the single compartment or in the two-compartment cell with an uncoated platinum plate as the counter-electrode. Under the same conditions an increase in the acid concentration leads to an increase in the open-circuit photopotential and photocurrent [fig. 8 (a)]. As far as the P(AMPS+-co-VP) coated electrode is concerned, it shows opposite polarity compared to an analogous cell with P(AMPS+-co-MAAM) [fig. 8 ( b ) ] . In this case, however, in alkaline (pH 10) solution, an enhancement in the photocurrent and photopotential is observed.In experiments with two compartment cells where one of the electrodes is coated with P(AMPS+-co-MAAM) and the other coated with P(AMPS+-co-MAAM-VP), coupled and kept in different compartments connected by a salt bridge, the observed current is a sum of that observed for the individual electrodes kept separately along with a plain platinum electrode in a single- compartment cell [fig. s(~)]; in this case both the electrodes were exposed to light. The current-voltage diagram for the electrodes coated with P(AMPS+-co-MAAM) and P(AMPS+-co-MAAM-co-VP) under illumination are given in fig. 9 B. Cyclic-voltammetric data for the macromolecular phenosafranines P(AMPS+-co- MAAM) and P(AMPS+-co-MAAM-co-VP) coated onto a carbon electrode indicate a two-electron redox process in both the cathodic and anodic waves which corresponds to826 Macromolecular-bound Phenosafranine Dyes the formation of semireduced dye at a potential of -0.54 V us.NHE and the leucodye at a potential of -0.69 V vs. NHE. Photoexcitation of the monomeric dye in homogeneous solution leads to the oxidation of EDTA or ascorbic acid present in solution.'^ 22, 23 The semireduced dye undergoes a disproportionation reaction to give back phenosafranine and leucophenosafranine. The results in the present investigation indicate the excitation of the macromolecular dye in the film, which produces either a cathodic or anodic reaction at the electrode, depending upon the nature of the macromolecule. Poly(acrylamidomethylphenosafranine-co-methylolacrylamide) acts as a photocathode whereas poly(acrylamidomethylphenosafranine-co-methylolacrylam- ide-co-vinylpyridine) behaves as an anode with respect to a plain platinum electrode with the electrodes dipped in an aqueous electrolyte solution.The photoelectrochemical reactions are proposed to involve water and oxygen in the redox processes at the electrodes. The photopotential depends upon the extent of oxygen present in the solution, which suggests that the electrode coated with macromolecular dye reduces oxygen to water at the cathode and oxidizes water to oxygen at the anode. The direction of the current is reversed depending upon the nature of the polymer-dye film. The redox potentials for the processes 0, + 4H+ + 4e- -+ 2H,O (6) 0, + e- -+ 0; (7) O;+e--+O;- (8) are, respectively, 0.82, -0.50 and - 1.80 V us.NHE.,* The redox chemistry of oxygen-water systems has been of interest for a long time, and the mechanism of electron-transfer processes depends upon a variety of factors including the nature of the medium. Thus, the reaction (9) is catalysed in the forward and reverse directions by the polymer films of P(AMPS+-co- MAAM-co-VP) and P(AMPS+-co-MAAM), respectively. The methylolacrylamide- bound phenosafranine photocatalyses the reduction reaction, and the same polymeric dye in the presence of pyridine in the copolymer units catalyses the oxidation reaction in the presence of light. The different photochemical behaviour observed for P(AMPS+-co-MAAM) and P(AMPS+-co-MAAM-co-V) by flash-photolysis experi- ment supports the results observed in this photoelectrochemical cell.The fact that the polymer-dye film itself is stable for several cycles indicates that organic molecules in the system are not consumed. Negligible photopotential or photocurrent observed in deaerated solutions shows that oxygen is involved in the photoelectrochemical reaction. In a single-compartment cell with both the electrodes coated by P(AMPS+-co-MAAM) and P(AMPS+-co-MAAM-co-VP) the cyclic reaction catalysed by light occurs at the electrode. In a two-compartment cell, one electrode coated with P(AMPS+-co-MAAM) and dipped in a solution containing supporting electrolyte at pH 3 and the other electrode coated with P(AMPS+-co-MAAM-co-VP) dipped in a solution containing supporting electrolyte at pH 10 shows the additive response of maximum photopotential and photocurrent under irradiated conditions [fig.8(c)]. This cell is perhaps one of the first examples of a regenerative-type photocell using water. In the case of the electrode coated with the polymer film P(AMPS+-co-MAAM), acrylamide becomes swollen because it is a hydrophilic polymer, and the swollen electrode acts as a cathode, whereas because the vinylpyrodine copolymer is hydrophobic, the electrode coated with the polymer film P(AMPS+-co-MAAM-co-VP) shows anodic polarity on irradiation. This type of chemically modified electrode has potential to devise new electrodes to bring about specifically a desired reaction at the electrode.2H,O + 0, + 4H+ + 4e-R. Ramaraj and P. Natarajan 827 The investigations reported here are partially supported by the D.S.T. Thrust Area S.E.R.C. programme and by the U.G.C. COSIST programme. References 1 Energy Resources Through Photochemistr,v and Catalysis, ed. M . Gratzel (Academic Press, New York, 2 K. Kalyanasundaram, Coord. Chem. Rev., 1982, 46, 159. 3 J. Kiwi, K. Kalyasundaram and M. Gratzel, Struct. Bonding, 1982, 49, 37. 4 E. Rabinowitch, J. Chem. Phys., 1940, 8, 551; 560. 5 M. Eisenberg and H. P. Silverman, Electrochim. Acta, 1961, 5, 1. 6 M. Kaneko and A. Yamada, J. Phys. Chem., 1977,81, 1213. 7 K. K. Rohatgi-Mukherjee, M. Bagchi and B. B. Bhowmick, Electrochim. Acta, 1983, 28, 293. 8 J. H. Fendler and E. J. Fendler, Catalysis in Micellar and Macromolecular Systems (Academic Press, 9 D. G. Whitten, Acc. Chem. Res., 1980, 13, 83. 1983). New York, 1975). 10 M. Kaneko and A. Yamada, Adu. Polym. Sci., 1984, 55, 1. 11 R. W. Murray, in Electroanalytical Chemistry, ed. A. J . Bard (Marcel Dekker, New York, 1984), 12 R. Tamilarasan and P. Natarajan, Nature (London), 1981, 292, 224. 13 R. Tamilarasan, R. Ramaraj, R. Subramanian and P. Natarajan, J. Chem. SOC., Faraday Trans. I , 14 A. I. Vogel, Quantitative Inorganic Analysis (ELBS, London, 1975), p. 329. 15 R. Ramaraj, R. Tamilarasan and P. Natarajan, J. Chem. SOC., Faraday Trans. I , 1985, 81, 2763. 16 L. Michaelis, Chem. Rev., 1935, 16, 243. 17 0. H. Muller and J. P. Baumberger, Trans. Electrochem. Soc., 1937, 71, 181. 18 D. N. Baile, D. M. Hercules and D. K. Roe, J . Electrochem. SOC., 1969, 116, 190. 19 R. Nichlolson and I. Shain, Anal. Chem., 1965, 37, 178. 20 C. E. Baumgartner, H. H. Richtol and D. A. Aikens, Photochem. Photobiol., 1981, 34, 17. 21 M. Neumann-Spallart and K. Kalyanasundaram, J. Phys. Chem., 1982, 86, 268. 22 N. K. Kosui, K. Uchida and Koizumi, Bull. Chem. SOC. Jpn, 1965, 38, 1958. 23 R. Bhardwaj, R. C. Pan and E. L. Grass, Photochem. Photobiol., 1981, 34, 215. 24 J. Wiltshire and D. T. Sawyer, Acc. Chem. Res., 1979, 12, 105. p. 191. 1984, 80, 2405. Paper 8/01079J; Received 15th March, 1988

ISSN:0300-9599    

DOI:10.1039/F19898500813

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1989, 85(4), 829-835 Electron Spin Resonance Studies in an Irradiated Single Crystal of Hexakis(ammonium) Diformylated Octamolybdate Dihydrate Shiying Han* Centre of MateriaIs Analysis, Nanjing University, Nanjing, China Jie Chen and Xiaozeng You Coordination Chemistry Institute, Nanjing University, Nanjing, China X-Band e.s.r. studies on Mo" centres formed by an u.v.-irradiated single crystal of the title compound, (NH,),[(CHO),Mo,O,,] - 2H,O, have been carried out at room temperature. Analyses of the e.s.r. spectra show that there are two different paramagnetic species in the unit cell. One may be resolved superhyperfine lines due to the interaction of the Mo" centre with a lH atom; the other can only be resolved hyperfine lines due to the Mo" centre.The spin-Hamiltonian parameters have been rigorously calculated using a least-squares-fitting technique, specially adapted to non-coincident g- and A-tensor systems. The principal values of the g, A,, and A, tensors for the two species, as well as the direction cosines of their principal axes with respect to the reference system, are obtained. The spin density and bond nature of the MoV-O bond are estimated according to the principal values of the A tensors. The results indicate the main component of the ground-state molecular orbital is the atomic orbital 44, of the MoV paramagnetic centre. The MoV-0 bond is an ionic bond with a covalent component. Polyoxomolybdates are potential photosensitizers and electron relay species in a redox cycle. The photoredox reaction of alkylammonium molybdates in the solid state and in aqueous solution, leading to the formation of an MoV paramagnetic centre in the molybdate, together with e.s.r.studies of irradiated single crystals of this kind of compound, have been reported by Yarna~e-l-~ In previous paper we reported e.s.r. studies of a u.v.-irradiated (NH3Pri),[(CHO),Mo,0,,] - 2H20 single crystal and found that the -CHO group is a proton donor, in addition to the (NH,Pri)+ ~ a t i o n . ~ In order to confirm the role which the -CHO group plays in the photoreduction and oxidation mechanism of organic molecules in the presence of polymolybdates, the present paper describes X-band e.s.r. studies on a u.v.-irradiated (NH,),[(CHO)2Mo,0,,] * 2H,O single crystal. Experimental Solid (NH,),[(CHO),Mo,O,,] - 2H,O was synthesised using a previously published methodq5 Single crystals were grown by slow evaporation from an aqueous solution; a crystal with a volume of 2x2.5 x4mm3 was chosen for the e.s.r.measurement. Crystallographic studies5 indicate the structure of the [(CHO),MO,O,,]~- anion shown in fig. 1. E.s.r. signals have not been detected for the colourless single crystals of (NH,),[(CHO),Mo,O,,] - 2H,O. When these crystals were irradiated with ultraviolet light for 0.5 h the white crystals became reddish-brown and e.s.r. spectra could be observed. An idealized representation of such a crystal is given in fig. 2. An experimental orthogonal system (x,y,z) is so chosen that the x axis lies along the one of the edges 829830 (B) I 1 I E.S.R.Studies of an MoV Complex 95897M0 I I I @Mo @o Q c Fig. 1. Structure of the centrosymmetric [(CHO),MO,O,,]~- anion. Fig. 2. Sketch of the e.s.r. reference system (x,y, z ) and the crystallographic direction for an ideal crystal of (NH,),[(CHO),Mo,O,,]~ 2H,O. llS. Hun, J. Chen and X . You 83 1 3846 3686 --- --_____-- ---c-.-- 3606 3766 - 3846 ,--I Fig. 4. B in a in the 0 10 20 30 40 50 60 70 80 90 angle/' 3766 1 3 686 3606 I.. I 1 8 I * 1 a 1 I ' ' * I 1 X 0 10 20 30 40 50 60 70 80 90 angle/" , Angular variation of the X-band e.s.r. spectra for two different paramagnetic species A and u,v.-irradiated crystal of (NH,),[(CHO)2Mo,0,,]~ 2H20 for an external magnetic field lying zx, zy and xy planes. The dots represent experimental data points while the smooth curves connect data points for the same transitions.shown, while the y axis is parallel to the flat face, with the z axis then defined by the right- hand rule. The e.s.r. spectra were recorded at room temperature with a Bruker ER200D-SRC X-band spectrometer using 100 kHz modulation. The external magnetic field was aligned along the three orthogonal planes (i.e. the zx, zy and xy planes) in turn.832 E.S.R. Studies of an MoV Complex Table 1. Spin-Hamiltonian parameters of Mo" centre for a u.v.-irradiated crystal of (NH,), [(CHO),Mo,O,,l4H,O direction cosines (or angles) of the principal axes with respect to the reference system principal values X Y Z 1.947 1.919 1.894 1.920 36.3 1 29.34 74.01 46.55 A, A,, 6.40 A,, 4.27 A,, 11.73 A,, 7.47 1.948 1.936 1.893 1.926 33.82 23.67 70.07 42.52 species A 0.1 172 (83.3') -0.3830 (112.5') 0.4619 (62.5") 0.8378 (33.1") 0.3892 (67.1") -0.8791 (1 5 1.5') 0.2656 (74.6') 0.7771 (39.0') - 0.4493 (1 16.7') 0.8536 (3 1.4') -0.4226 (1 15.0") - 0.4648 (1 17.7') -0.8265 (145.7') 0.4250 (64.9') 0.1 176 (83.3') 0.7716 (39.5') -0.5504 (123.4') -0.4733 (118.3) species B -0.8317 (146.3') -0.1823 (100.5') 0.3684 (68.4') 0.5255 (58.3') -0.4154 (114.8') 0.8310 (33.8') 0.2062 (78.1") 0.7034 (45.3') 0.6170 (51.9") 0.4462 (63.5') 0.5534 (56.4') - 0.7547 (1 39.0') 0.9 162 (23.6") 0.291 1 (73.1') 0.2751 (74.0') 0.5693 (55.3') 0.7869 (38.1') 0.2385 (76.2') - 0.3690 (1 1 1.7') 0.6878 (46.5') 0.6251 (51.3") 0.5425 (58.4") 0.7669 (39.9") - 0.3699 (1 1 1.7') 0.6807 (47.1') 0.3256 (71 .O') -0.6481 (130.4') " g = (1/3)(gx,+g,,+g,,). Unit of the A is lop4 cm-'.A, = (1/3)(Axx+A,.,.+A,,). In each case the magnetic field was kept fixed while the sample was rotated with respect to the direction of the magnetic field. E.s.r. spectra were recorded at intervals of 5 or 10". An example of an e.s.r. spectrum is shown in fig. 3. An analysis of fig. 3 reveals that there are two kinds of spectra, derived from two different paramagnetic species labelled A and B in the unit cell for an MoV centre with electronic spin S = 1/2, molybdenum isotopes with nuclear spin I = 5/2 and Z = 0. The spectrum of the paramagnetic species A indicates that there are two sets of lines, a doublet and a sextet of doublets. The first set of two intense lines, with an intensity ratio 1 : 1, is attributed to a superhyperfine interaction between the unpaired electron spins corresponding to 9 6 M ~ with I = 0 and 'H nucleus with IH = 1/2.The second set of lines, of lower intensity, is characterized by a hyperfine sextet structure of two molybdenum isotopes, 9 5 M ~ and 9 7 M ~ , in a natural abundance of 25 and 15 %, which have the same nuclear spin IMo = 5/2 and nearly the same magnetic moment, and is further split by the same superhyperfine interaction as the first set of lines. The spectrum of the paramagnetic species B can be analysed similarly, but the superhyperfine splitting does not appear. This means that the coupling of MoV centre with 'H cannot be detected for species B. The angular variations of the X-band e.s.r. spectra for species A and B in an irradiated single crystal of (NH,),[(CHO),MO,O,,] - 2H20 for the magnetic field lying in the zx, zy and xy planes are shown in fig.4.S. Hun, J. Chen and X . You 833 The single-ion spin Hamiltonian for a paramagnetic species with S = 1/2, IMo = 5 / 2 and IH = 1/2 is given by 2 = pS*g * B + S * A,, * Inlo + S * A , I H (1) where /3 is the Bohr magneton, B is the static external magnetic field, A,, is the hyperfine tensor, A,, is the superhyperfine tensor and g has its usual meaning. In general, the principal axes of g, A,, and A , are non-coincident, but they are symmetric tensors. It is necessary to measure the e.s.r. spectra from three mutually perpendicular planes in order to determine the components of each tensor. An example of the calculation procedure is described as follows for species A.Initially only the first term is kept in eqn (1) for the even isotope 9 6 M ~ with zero nuclear spin, by neglecting the last term corresponding to the superhyperfine interaction. All the data points for the position of the intense centre lines of the degenerate doublet, corresponding to various relative orientations of the external magnetic field lying in the three orthogonal planes, are simultaneously fitted using a rigorous least-squares-fitting subroutine.6 The principal values of the g tensor and the direction cosines of its principal axes with respect to the reference system have been obtained. Then the first and second terms are kept in eqn (l), still neglecting the last term, for both odd isotopes 9 5 M ~ and "Mo with nuclear spin h0.Based on the principal values of the g tensor and the direction cosines of its principal axes obtained above, the same fitting technique is used to fit all the data points for the positions of the six hyperfine lines of the degenerate doublets recorded in the three orthogonal planes in order to obtain the hyperfine tensor AMo. Finally, the first and last terms are kept in eqn (1) for the even isotope 9 6 M ~ ; the superhyperfine tensor AH can be calculated similarly by fitting the two intense centre lines. The spin-Hamiltonian parameters for the paramagnetic species B were also calculated, and the results are given in table 1. Discussion It is shown by our experiment that there is no e.s.r. signal for solid (NH4)6(Mo7024)*4H20 before and after U.V.irradiation, the (NH,)+ cation may not be a proton donor for such an irradiation with U.V. light. The fact that the single crystal of (NH,),[(CHO),Mo,O,,] 2H20 is easily reduced to a reddish-brown paramagnetic species led us to assume that the -CHO group may be the predominant proton donor in the photochemical reaction of this compound. If the spin density in the MoV-OH fragment is distributed in the orbitals of the oxygen, molybdenum and hydrogen atoms, its value on each atom in the paramagnetic species A (or B) can be estimated as follows. The principal value for the A,, tensor should take the same signs, as shown for various oxomolybdenum ions.7 In this case, all the signs are assumed to be positive. The A,, tensor is divided into an isotropic component 4 and an anisotropic hyperfine tensor T, i.e.A = A , I + T (2) where I is the unit tensor. The matrix form of eqn (2) is given by834 E.S.R. Studies of an MoV Complex Table 2. Spin-density and mixing parameters for species A and B paramagnetic species P5s P4d P I S Pt P2 A 0.070 0.957 0.015 1.042 0.8595 1.024 0.8600 B 0.064 0.960 - The matrix elements are given in the following form: 1 A, = 1/3(Axx +A,, + Azz) Txx = Ax, -4 TYY = A y y - 4 (3) T,, = Azz - 4. I The values of A,,, Txx, Tyy and T,, are obtained by inserting the results given in table 1 into eqn (3). They are 46.55, - 10.24, - 17.21 and 27.46 x lop4 cm-l, respectively. For molybdenum the isotropic hyperfine coupling a is predicted to be - 1984 MHz (-662 x cm-') for one electron spin in the 5s orbital, and the anisotropic hyperfine coupling P is - 150.7 MHz (-50.2 x lop4 cm-')' in a 4d orbital.For 'H the isotropic superhyperfine coupling a is 1420 MHz (474 x cm-l) with one electron in the Is orbital.' Thus the spin densities for an unpaired-electron spin distributed in the 5s and 4d orbitals of Mo and the 1s orbital of lH are given as P 5 s = (A/lal)M, = 0*070 = (T,-/2P x $)Mo = 0.957 P l S = (A/lal)EI = 0.015. Note that the total spin density, pt = 1.04, is close to unity. If opposite signs are taken for the principal values of the the AM, tensor, one obtains pt = 2.5; this unacceptably large total spin density strongly implies that the taking the signs of the principal values for the AM, tensor as the same is a reasonable assumption. The distribution of spin density shows the occurrence of direct spin polarization between the paramagnetic-spin 4d orbital of molybdenum and the Is orbital of hydrogen, and the 4d orbital of molybdenum is the main component of the semi-occupied orbital.The site of the MoV ion in this compound is assumed to have approximately qv symmetry with one unpaired electron in a non-bonding or slightly antibonding molecular orbital of symmetry 4. The fact that the e.s.r. spectra are observed also implies that an unpaired electron is in an orbitally non-degenerate ground-state : the ground-state wavefunction can then be expressed aslo 14) = Pldz,) -Probe (4) where q5,,2 represents the oxygen 2p orbital combination having 4 symmetry. One of the mixing coefficients, p, can be determined from the principal values of the A,, and g tensors using the following equations :' 4, = -4 + [ - (4/7)p2 + (gi -2.0023) + (3/7) (gl- 2.0023)] P A, = - 4 + [(2/7)p + (1 1/14) (gl - 2.0023)lP ( 5 ) ( 6 ) where 4 = Azz, Al = (1 /2) (Axx + A,,), gl = gzz and g l = (112) (gxx +GYY); A, and p are defined above.Inserting the results of table 1 into eqn ( 5 ) or (6) produces the value of p2; these results are listed in table 2. It is evident from the o2 values that the MoV-0S. Hun. J . Chen and X . You 835 Z X Fig. 5. Relative orientations between the principal axes ( X , Y , Z ) of A,, and the principal axes ( X ' , Y', 2') of A , for paramagnetic species A. bond is neither purely ionic (p2 = 1) nor purely covalent (p2 = 0.5), but is rather an ionic bond with a covalent component.By analysing the relative orientations between the principal axes of the A,, and A , tensors both principal-axes systems can be drawn approximately, as in fig. 5. The 2 axis of A , lies almost along the X axis of AM,, i.e. the direction of the maximum principal value of A , lies approximately along the direction of the H(0)...MoV bond in the XY plane of the MoVO, octahedron for species A. For species B the H(0)...MoV bond may lie along the Z axis of the molybdenum oxygen octahedron: OH II / 0 - Mo(V) -0 0' I 0 OH 0 I / 0- Mo(V) -0 0' I 0 species A species B so that the 'H nucleus does not interact with the dxy orbital of MoV, and these lines of the e.s.r. spectrum cannot be split further by 'H. However, the true formula of the reddish-brown species remains unknown. The e.s.r. result may provide a great deal of information in understanding its reaction mechanism in the future. References 1 T. Yamase, J. Chem. Soc. Dalton Trans., 1978, 283. 2 T. Yamase, 1. Chern. Soc. Dalton Trans., 1982, 1987. 3 T. Yamase, Polyhedron, 1986, 5, 79. 4 Chen Jie, You Xiaozeng, Han Shiying and Sui Yunxia, Acta Chim. Sin., 1988, 308. 5 R. D. Adams, W. G. Klemperer and R. S. Liu, J . Chem. Soc. Chem. Commun., 1979, 256. 6 S. K. Misre, Phjsica, 1984, 124B, 53. 7 B. R. McGarvey, J . Phys. Chem., 1967, 71, 51. 8 J. R. Morton and K. F. Preston, J . Magn. Reson., 1978, 30, 577. 9 J. R. Morton, Chem. Rea., 1964, 64, 453. 10 K. DeArmond, B. B. Garrett and H. S. Gutowsky, J. Chem. Phys., 1965, 42, 1019. Paper 8/01291A; Received 28th March, 1988

ISSN:0300-9599    

DOI:10.1039/F19898500829

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chern. SOC., Faraday Trans, I, 1989, 85(4), 837-841 OH Groups in Boralites Jerzy Datka and Z. Piwowarska Faculty of Chemistry, Jagiellonian University, Karasia 3, 30-060 Cracow, Poland Four kinds of OH groups (3740, 3720, 3680 and 3460 cm-') are present in H-boralites. The 3740 and 3720 cm-' OH groups were previously assigned to silanol, Si-OH, and acidic B..-HO-Si groups. The results obtained in the present study suggest that both 3680 and 3460 cm-I OH groups were formed by the hydrolysis of B-0-Si bond upon ionic exchange in an acidic NH,C1 solution. The broad 3460 cm-I band was attributed to O-..H-O-Si groups forming the hydrogen bond, and the 3680 cm-l band to B-OH groups in B-O..-H-O-Si fragments. The OH groups in these fragments are unstable and prone to dehydroxylation. H I Isomorphically substituted zeolites have recently been extensively studied because of their interesting chemical properties and potential industrial applications [for a review see ref.(l)]. Boron-substituted zeolites, boralites, which owing to the small size of the boron atom show properties not observed in Al-substituted zeolites, draw the greatest attention. The boron atom is three-coordinated in dehydrated boralites and becomes four-coordinated upon the adsorption of electron-donor molecules. The strength of the acid sites in boralites is much lower than in ~eolites,~-~ even though the electronegativity of boron is higher than that of aluminium (2.93 and 2.22, respectively). It can be explained by a weak interaction between the OH groups and the small boron atom in the B-..HO-Si units.Our previous i.r. study5 showed that only 3740 cm-l silanol Si-OH groups are present in Na-boralite, but three new OH bands (at 3460, 3680 and 3720 crn-l) appear upon Na+/NH,+ ion exchange and decomposition of the NH; ions. The 3720 cm-l OH groups were found to be weak Bronsted acid sites and were assigned to B..-HO-Si groups. The origin of the 3460 and 3680 cm-' OH groups is not known. The present study was undertaken in order to elucidate the nature and to study the properties of both the 3460 and 3680 cm-l OH groups in H-boralites. Experimental The samples of boralites synthesized at the Department of Chemical Technology of the Jagiellonian University were highly crystalline, as shown by X-ray analysis. Magic-angle spinning n.m.r. spectra showed that the boron atoms were in lattice positions.6 The NH,-boralite was obtained from the Na-form by ionic exchange in NH,Cl solution at room temperature.The compositions of the Na- and NH,-boralites were studies the boralites were pressed into thin wafers (4-7 mg cm-2) and activated in situ in an i.r. cell. The cell used in this study was similar to that described in ref. (7). The boralite wafer was heated in dynamic vacuum at various temperatures (580-780 K) for 1 h. The temperature was measured by a thermocouple situated near the wafer. The spectra were recorded using a Specord 75 infrared spectrometer (Zeiss Jena) working on-line with a KSR-4100 minicomputer. ~ ~ 1 . 0 ~ 0 . 1 t ~ ~ ~ , ~ 1 . 1 ~ ~ ~ ~ 2 ~ 9 4 , 1 and respectively.For infrared 837838 OH Groups in Boralites Results and Discussion In order to study the thermal stability of OH groups in boralite wafers Na- and NH,-boralites were activated in situ in the i.r. cell for 1 h at temperatures increasing in a stepwise manner between 580 and 780 K, and the spectra were recorded 320 K after each activation step. The spectra are presented in fig. 1 and 2. Na-boralite activated at 580 K and above shows an Si-OH band at 3740 cm-l; its intensity does not depend on the activation temperature (fig. 1). The spectrum of H- boralite (fig. 2) shows the 3740 cm-' OH band as well as three others, viz. 3460,3680 and 3720 cm-l. The 3460 cm-l band is broad and typical of hydrogen-bonded OH groups. The intensities of both the 3460 and 3680cm-l bands decrease as the activation temperature increases.The 3460 and 3680 cm-' bands could be assigned either to structural OH groups, in which case their decrease at higher temperatures would be due to dehydroxylation, or else to physisorbed water which is desorbed upon heating. The latter assignment would be confirmed by the frequencies and shapes of the bands being analogous to those of water sorbed in However, the characteristic deformation vibration [6(H20)] is missing from the spectra of H-boralites. The observed 1640 cm-l band is independent of the activation temperature and thus is due to a skeletal vibration of boralite. In order to test the 'water hypothesis', water was sorbed into boralite activated at 770 K. This resulted in the appearance of bands at 1630, 3410 and 3670 cm-l (fig.3). However, all these bands disappeared upon heating in vacuo at 380 K (a temperature lower than the activation temperatures previously applied), indicating that the 3460 and 3680 cm-' bands observed in the spectra of boralites activated above 380 K cannot be due to physisorbed water. Thus structural OH groups must be responsible for the 3460 and 3680 cm-' vibrations. The observed dehydroxylation begins at a relatively low temperature of 590 K. As the bands at 3460 and 3680 cm-' were absent in the Na-form of boralite (fig. l), it was of interest to see if they were formed during Na+/NH,+ exchange or during NHZ decomposition. Unfortunately the direct recording of the spectrum following water desorption and before the decomposition of NH; was impossible, because both processes occur simultaneously.An indirect method had to be applied, and thus the number of 3720cm-' OH groups in H-boralite was compared with the amount of NH,+ ions in its NH, precursor. Our previous study5 showed that of the OH groups under discussion, only the 3720 cm-l ones can protonate pyridine, and their concentration could be determined by measuring the concentration of pyridinium ions (PyH+) thus formed. The concentration of PyH+ ions can be calculated from the intensity of the PyH+ band at 1545 cm-l and the extinction coefficient of this band. The following experimental procedure has been applied. Small portions of pyridine (ca. 0.2 pmol) were introduced at 443 K into the cell containing an activated wafer of H-boralite, and the i.r.spectrum was recorded at the same temperature after each adsorption step. The intensity of the 1545 cm-l pyridinium ion band increased with the amount of pyridine, but after the neutralization of all Bronsted-acid sites it attained a constant level, This maximum intensity was used to calculate the concentration of the Bronsted-acid sites (3720 cm-l OH groups) using the extinction coefficient of the 1545 cm-l band (0.058 cm2 pmol-l) determined in a previous study.12 The details of this method of i.r. studies of acid properties of zeolites were described in previous papers. 12-14 The concentration of Bronsted-acid sites in H-boralites activated at 590 and 780 K were found to be the same (0.81 per u.c.) and comparable, within experimental error, to that of NH: ions before activation (1 .O per u.c.), indicating that all the protons liberated upon decomposition of the NH; ions were used for the formation of the 3720 cm-l groups; thus the 3460 and 3680 cm-l OH groups originate from a different source.J .Datka and Z . Piwowarska 839 1 K K K K K r n I I I I 3 I I - l 1300 1500 1700 3200 m 3600 3800 wavenumber/cm-' Fig. 1. Infrared spectra of Na-boralite activated at various temperatures. 1 1 I t o 1 2300 1500 I 17b 3200 34m 3600 I 3dm wavenumber/m-' Fig. 2. Infrared spectra of NH,-boralite activated at various temperatures.840 OH Groups in Boralites 0.11 dm 1500 ' 1700 $00 3ioo ' 3600 ' 3eb wavenumber/cm-' Fig. 3. Water sorption in H-boralite: (a) NH,-boralite activated at 770 K, (b) water adsorption at room temperature and (c) desorption at 380 K.The following scheme is proposed to explain the formation of the 3460 and 3680cm-l OH groups by boralite hydrolysis during ion exchange in acidic NH,C1 solution : Na+(NH:)HzO Na+(NHt)HzO H O---H-O\ I / O 0 \ /O\ /O\ / O H20 0 \ 0 / \oo/ \ 00 / \o H+ o/ \oo/ \o o/ \* /'\ / B- Si Si B- Si - Si One B-0-Si bond is broken in the first hydrolysis step. If the neighbouring B-0-Si bonds were also hydrolysed, this would result in boron extraction from the lattice. This was indeed observed with b~ralites.~$ l5 Similarly, aluminium is extracted from zeolites in acid solutions. The extent to which this process occurs depends strongly on the boron content in b0ra1ites.l~ The broad band at 3460 cm-l can be attributed to Si-OH groups forming a hydrogen bond with a neighbouring oxygen atom, O-.-H-O-Si, and the 3680cm-l band to B-OH groups whose oxygen is engaged in hydrogen-bond formation.These assignments are supported by the results of Ghiotti et all6 and of Morrow et al.,17 who observed a 3540 cm-l band in Si-O...H-O-Si dimers analogous to our B-O---H-O-Si species. Winde et a1.l' assigned an observed 3707 cm-l band to isolated B-OH groups. The involvement of the oxygen atom in the hydrogen bonding results in a weakening of the 0-H bond,13 in agreement with the Gutmann rule1' and in the shift of the B-OH band to lower frequencies.J. Datka and Z . Piwowarska 84 1 H I The B-O--.H-O-Si fragments seem to be unstable and prone to dehydroxylation. The data presented in fig. 2 show that this is truly the case.Both the 3460 and 3680 cm-l bands decrease noticeably with the activation temperature. Terminal Si-OH (3740 cm-l) and acidic (3720 cm-l) OH groups are much more stable. The intensity ratio of the 3720 and 3680 cm-l OH bands depends strongly on the activation temperature. At lower activation temperatures the 3680 cm-' band is the strongest and at higher activation temperatures the 3720 cm-l band becomes dominant. This explains why some authors3 observed only the 3695 cm-' (B-OH) band, while othersz0 only the 3725 cm-' (B.-.HO-Si) band, depending on the activation conditions. Conclusions Four kinds of OH groups (3740, 3720, 3680 and 3460 cm-') are present in H-boralite. The 3740 and 3720 cm-' hydroxyls were previously attributed to silanol Si-OH and acidic B..SHO-Si groups, re~pectively.~ The results obtained in the present study suggest that both 3680 and 3460 cm-l OH groups were formed by the hydrolysis of the B-0-Si bond upon ion exchange in acidic NH,Cl solution. The broad 3460 cm-' barid is assigned to O-..H-O-Si groups forming the hydrogen H I bond, and the 3680 cm-I band to B-OH groups in B-O.-.H-O-Si fragments. H I The OH groups in B-O--.H-O-Si fragments are unstable and prone to the dehydroxylation. We thank Dr A. Cichocki from Jagiellonian University for the samples of boralites and Dr M. Tencer from Lehigh University for helpful discussions. References 1 M. Tielen, M. Gailen and P. A. Jacobs, Proc. Int. Symp. Zeolite Catalysis, Siofok, 1985, p. 1 2 K. F. M. G. J. Scholle, A. P. M. Kentgens, W. S.Veeman, P. Frenken and G. M. P. van der Velden, 3 G. Coudurier and J. C. Vedrine, Pure Appl. Chem., 1986, 58, 1389. 4 P. Ratnasamy, S. G. Hedge and A. J. Chandwalkar, J . Catal., 1986, 102, 467. 5 J. Datka and Z. Piwowarska, J . Chem. Soc., Faraday Trans. I , 1989, 85, 47. 6 A. Cichocki and J. Datka, to be published. 7 A. Bielanski and J. Datka, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1972, 20, 81. 8 G. J. Frohnsdorf and G. L. Kington, Proc. R. SOC. London, Ser. A, 1958, 247, 469. 9 H. A. Szymanski, D. N. Stamires and G. N. Lynch, J. Opt. SOC. Am., 1960, 50, 1223. J. Phys. Chem., 1984, 88, 5. 10 J. W. Ward, J. Phys. Chem., 1968, 72, 421 1. 11 A. Bielanski and J. Datka, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1970, 18, 173. 12 J. Datka and E. Tuznik, J. Catal., 1986, 102, 43. 13 A. Bielanski and J. Datka, Bull. Acad. Polon. Sci., Ser. Sci. Chim., 1974, 22, 341. 14 J. Datka, J . Chem. Soc., Faraday Trans. I , 1980, 76, 2437. 15 A. Cichocki, to be published. 16 G. Ghiotti, E. Garrone, C. Morterra and F. Boccuzzi, J . Phys. Chem., 1979, 83, 2863. 17 B. A. Morrow and A. Cody, J . Phys. Chem., 1975, 79, 761. 18 H. Winde, P. Fink and A. Kohler, Z . Chem. 1977, 17, 41. 19 V. Gutmann, The Donor-Acceptor Approach in Molecular Interactions (Plenum Press, New York, 20 C. T. W. Chu and C. D. Chang, J . Phys. Chem., 1985, 89, 1569. 1978). Paper 8/01295D; Received 29th March, 1988

ISSN:0300-9599    

DOI:10.1039/F19898500837

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1989, 85(4), 843-853 Determination of the Kinetics of Facilitated Ion Transfer Reactions across the Micro Interface between Two Immiscible Electrolyte Solutions Jane A. Campbell, Alan A. Stewart and Hubert H. Girault* Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, Scotland The transfer of K' from water to dichloroethane assisted by dibenzo-18- crown-6 has been studied at a liquid-liquid interface supported at the tip of a micropipette. The apparent electrochemical rate constant and apparent charge-transfer coefficient have been determined by analysis of the quasi- reversible steady-state wave observed. A kinetic model is proposed and the true electrochemical rate constant for the complexation reaction is thereby calculated.Since the first investigation of the kinetics of ion transfer across the interface between two immiscible electrolyte solutions (ITIES) by Gavach et al.,' who studied the transfer of tetra-alkylammonium ions from water to nitrobenzene by chronopotentiometry, much work has been dedicated to this topic. The main conclusion of the results hitherto obtained*-' for simple ion transfer and facilitated ion transfer' is that these reactions are fast and therefore difficult to measure. The standard electrochemical methodology for studying quasireversible charge transfer circumvents mass-transport limitations either by rapid variation of the electrode potential (e.g. Chronoamperometry, a.c. polarography, a.c. impedance and fast-sweep voltammetry) or by enhancing the rate of arrival of the reactants (e.g.rotating-disc, wall- jet and microelectrodes). To date the published investigations of the kinetics of ion transfer have relied only on the first category of techniques, and the purpose of the present paper is to show that, in the case of facilitated ion transfer, data can also be obtained by enhancing the diffusion using a microITIES. The main problem with electrochemical measurements at liquid-liquid interfaces resides in the low polarity of the organic phase. The consequently large iR drop is usually either compensated by positive feedback or decreased by the use of concentrated supporting electrolyte. The effect of analogue compensation is to introduce, by definition, a large amount of noise, which limits the accuracy of current acquisition and hence makes techniques such as cyclic voltammetry non-ideal.Furthermore, in this particular case, 100 % analogue compensation is never achieved, and the residual iR drop also contributes to the peak shift observed. Alternatively, the use of a concentrated supporting electrolyte in the organic phase causes significant ion-pairing effects between the transferring ion and the supporting electrolyte counter-ion. Such interactions complicate the determination of the data sought, and investigations in dilute media are to be preferred. The difficulties just described render most of the techniques used so far inherently inaccurate, as reviewed recently by Samec.' According to this author the most reliable method is a.c. impedance, i.e.measurement of the charge-transfer resistance. However, as shown in ref. (9), full semi-circles are difficult to obtain, and the accuracy of the measurement is heavily dependent on the reliability of the curve-fitting algorithm employed. 843 FAR I 29844 Ion Transfer across Interfaces More generally, all transient techniques require relatively sophisticated instru- mentation, including a potentiostat with a fast rise time and high-speed transient recording equipment. However, the accuracy of these techniques at liquid-liquid interfaces is intrinsically limited at short times by the low impedance of the double layer, in parallel with the faradaic processes. An entirely different experimental approach to the study of ion-transfer reaction kinetics is to increase the rate of mass transport of the reactants to the interface.Taylor and Giraultl' showed that micro-liquid-liquid interfaces supported at the tip of the micro-pipette can be used to provide a spherical diffusion pattern similar to that observed at solid microelectrodes. l1 This enhanced mass transport produces a steady- state current when the transferring species enters the pipette, whereas classical linear diffusion behaviour is observed when the ion exits from the pipette. Consequently a theory similar to that employed at solid-state microelectrodes may be applied in the special case of facilitated ion transfer, where both the rate of arrival of the reactants and the rate of departure of the products is enhanced. This can be achieved experimentally by, for example, using an excess of the concentration of the ion to be transferred inside the pipette together with a low concentration of the ionophore outside the pipette.Theory When the concentration of the transferring ion in the aqueous phase is in excess relative to the ionophore in the organic phase, the observed current for facilitated ion transfer is equal to the rate of departure of the complexed ion. In the case of spherical diffusion this reads as where the subscripts C and CI refer to the ionophore and complexed ion, respectively, the superscripts b and 0 refer to bulk and surface concentrations and where F is the Faraday constant, D the diffusion coefficient, A the area of the microITIES and r the radius of the tip of the pipette. The observed limiting current for the case where the initial concentration of the complexed ion is zero in the organic phase is given by Also, the kinetic expression for the current is + i = zi FA(kaPp - iapp GI) (3) -+ 4- where k,,, and k,,, are the apparent forward and reverse electrochemical rate constants.By substituting eqn (1) and (2) into eqn (3) we have -+ t i = zi FACE[k,,,( 1 - i/id) - kapp tilid] (4) where is the ratio Dc/DcI. Note that the concentration of the ion, C;, does not appear in eqn (3). This is because in the present case the ionic concentration in the pipette is taken to be orders of magnitude greater than that of the ionophore. The complexation reaction Corg + Iwater + CIow can be compared to a reduction reaction at an electrode, and is therefore pseudo-first- order.J .A . Campbell, A . A . Stewart and H. H . Girault 845 t c reaction coordinate mi x e d solvent layer u I I ' I , 4 O I I I I I I I I I I I I I I I +- ; I I I (2) (1) I' aqueous d i f f u s e 7 ' organic d i f f u s e *I layer I layer distance Fig. 1. (a) Simple representation of potential energy of Gibbs-energy changes during an ion- ionophore reaction at an ITIES. (b) Schematic Galvani potential distribution across an TTIES. The potential dependence of electrochemical rate constants for charge-transfer reactions across a liquid-liquid interface has been the cause of many discussions.12 However, the present situation is distinct from a classical non-assisted ion transfer in that the reaction coordinate does not correspond to the position of the ion with respect to the plane of the interface.As shown in fig. 1, the reaction can be modelled according to the transition-state theory. However, the initial and final positions (1) and (2) in fig. 1 correspond to positions just outside the back-to-back double layer, and consequently the rate constants kapp and k,,, correspond to a global transfer and not to the elementary complexation-reaction step occurring somewhere in the mixed-solvent layer between positions (1) and (2). -+ t 29-2846 Ion Transfer across Interfaces In the apparent standard case where the activation energy barrier is symmetrical and the concentration of the ionophore is equal to that of the complexed ion (i.e. at the boundary of the space-charge region assumed to be thin compared to the diffusion layer), the equilibrium is described by the equality ( 5 ) -+ t kapp c"c = kapp C"c* Hence the apparent standard rate constant k,Opp can be classically defined as + kip, = zap, = k,,, = Zexp (- AG",RT). (6) In the apparent standard case the Galvani potential difference between the two phases is defined as the formal potential A:@"'.The formal potential for facilitated ion transfer is related to the standard potential for ion transfer of the species i, A@: (= - AGY, J z i F, where AG; is the Gibbs energy of transfer) by where K , is the complexation constant in the organic phase and y is the activity coefficient. When the potential is varied from the formal potential, e.g. by making the oil phase more negative us. water, the Gibbs energy of the uncharged ionophore remains unchanged and that of the complexed ion is lowered by an amount zi F(AZ 0 - A: 0"').The activation energy barrier for the ion transfer to the oil phase, AG,, is itself lowered by a fraction of this Gibbs-energy difference : -+ AG, = AG; - a,,, zi F(A: 0 - A: 0"') (8) + whereas the activation energy barrier for the reverse process, AGA, is increased according to t AG, = AG: + (1 - a,,,) zi F(Ai 0 - A: 0"'). (9) From eqn (8) and (9) it can be shown that the reverse rate constant is related to the forward rate constant by t Consequently eqn (4) can be more generally expressed as i i = z , F A C ~ k . , , [ ( 1 - ~ ) - ~ ~ e x p ( ~ ( A ~ ~ - - A ~ 0 " ' ) -+ from which the potential dependent rate constant for the assisted-transfer reaction can be obtained.In the case of an irreversible facilitated ion-transfer reaction, the reverse transfer can be neglected and consequently eqn (1 1) reduces to + i = zi FACE kapp (1 -t). Experimental The micropipettes were made from Kwik-Fil capillaries (1.5 mm o.d., 0.86 mm i.d., Clark Electromedical) pulled with a vertical pipette puller (Kopf 102, U.S.A.). The puller was adjusted to provide pipettes with a short shank and a fine tip, which were then polished on an optically flat glass of a pipette beveller (K. T. Brown, BV-10, U.S.A.).J. A. Campbell, A. A . Stewart and H . H . Girault 847 During polishing the resistance of the pipette was constantly monitored, as it decreases until a flat polish is obtained. Following this procedure, pipettes with circular sections can be made with external radii comprised between 5 and 50pm.Outside these limits, polishing becomes extremely difficult. Tetrabutylammonium tetraphenylborate (TBATPB) was prepared by mixing equi- molar aqueous solutions of TBABr (Fluka) and NaTPB (Aldrich), the precipitate being dried and recrystallized from acetone. Dibenzo- 18-crown-6 was used as supplied (Aldrich). The solvents consisted of doubly deionized water and AnalaR grade 1,2- dichloroethane. The voltage ramp was produced by a PPRI waveform generator (Hitek) and the current was measured by a home-made battery-powered current follower based on a high-input impedance FET operational amplifier (Burr Brown OAP 104). The electrochemical cell simply consisted of a glass U-tube in which the micropipette was immersed [see ref.(10) for further details]. The separation distance between the tip of pipette and the reference interface was within a 1 mm. During the experiment the tip of the pipette was monitored with a zoom microscope (Olympus, SZH, maximum magnification 384 x ) together with a colour video attachment (Sony, CCD camera, DXC-102). This constant observation was necessary to ensure that the interface remained located at the tip of the pipette. 10-1 mol dm-3 MgSO, I 1 mol dm-3 KC1 mol dmP3 TBATPB Results lop3 mol dm-3 TBACl The potential window containing the Kf ion transfer facilitated by DB18C6 across a water-1,2-dichloroethane interface supported at the tip of a micropipette is shown in fig. 2. The corresponding electrochemical cell is lo-’ mol dm-3 MgSO, mol dmP3 TBATPB I 1 O-* mol dm-3 KCl II 1 OP4 rnol dm-3 DB 18C6 mol (fm-3 TBACl I Steady-state properties are observed for Kf transferring out of and into the pipette.This behaviour is due to spherical-diffusion mass transport for the foward and return charge transfers and contrasts with typical behaviour observed12 for non-assisted ion transfer across micro-ITIES ( i e . peak-shape voltammogram for transfer out, steady-state wave voltammogram for transfer into the pipette). Steady-state voltammograms for this transfer have been analysed using the theory described above, and fig. 3 shows the result obtained when In kapp is plotted us. the applied potential difference A: 0 -A: 0O’. The linear relationship observed indicates that the ion transfer is quasi-reversible, with a standard rate constant, k:pF, equal to ca.2.0 x = 1. The potential-difference scales given in all the figures are real Galvani potential differences calculated after measurement of the potential of zero charge (taken to be equal to the zero Galvani potential difference). The potential of zero charge (P.z.c.) for the following cell -+ cm s-’, assuming that was measured at room temperature by the streaming method as described in ref. (16). The value measured herewith was - 136 mV. The accuracy of the value of kzPp measured depends chiefly on the exact determination of the formal potential, This difficulty is inherent to this type of study, and there are848 500 0 Ion Transfer across Interfaces I I I . I I -300 -1 00 0 100 A: @lmV Fig.2. Cyclic voltammogram for DB18C6-facilitated K' transfer at a micro ITIES for cell I. Scan rate 0.05 V s-l. -3 -4 -5 a T> 5 -6 -7 -8 -80 -40 0 LO 80 (A; @-A: @O')/mV Fig. 3. Plot of In k,,, us. A;@-A;Oo'. +J . A . Campbell, A . A . Stewart and H. H. Girault 849 Fig. 4. Cyclic voltammogram for cell (I) at large planar liquid-liquid interface used to evaluate the formal potential. Scan rates: 0.025, 0.049, 0.064 and 0.1 V s-l. Table 1. Values of kzpp and aaPp obtained for a range of formal potentials A t O0'/V kzpp/cm s-' - 0.050 9.92 x 10-4 0.92 -0.055 1.15 x 10-3 0.92 - 0.060 1.52 x 10-3 0.9 1 - 0.065" 1.96 x 10-3" 0.9 1 " - 0.070 2.46 x 10-3 0.90 -0.075 3.08 x 10-3 0.89 - 0.0080 4.46 x 10-3 0.87 a Values for the formal Galvani potential ob- tained by the mid-peak potential method.basically two ways to evaluate the formal potential. On the one hand, we could relate the potential scale to a real Galvani potential-difference scale using, for example, the TPATPB as~umption'~ or by measuring the potential of zero charge13 and then evaluating the formal potential from the standard potential and the dissociation constant according to eqn (7). In this approach the activity coefficients of the ionophore and complexed ion in the organic phase are required, and their approximation can lead to large inaccuracies. On the other hand, we can take the empirical approach based on8 50 Ion Transfer across Interfaces the measurement of the mid-peak potential of cyclic-voltammetry experiments at a large planar liquid-liquid interface extrapolated to zero sweep rate. The result of this approach is shown in fig.4. The value thus obtained using cell (I) is -201 mV (oil with respect to water) and compares with a value of apparent half-wave potential equal to - 236 mV (oil with respect to water) measured from a plot of In [(id - i)/z] us. potential difference. The formal Galvani potential difference, when related to the absolute Galvani potential scale established by the P.Z.C. measurement, is -65 mV (oil with respect to water). The importance of the determination of an accurate value of the formal potential is highlighted in table 1, where values of k& are calculated from eqn (1 1) for a range of different formal potentials. Discussion Before discussing the present results it may be worth explaining why the theory outlined here is not applicable to non-assisted ion-transfer reactions.In this case the Nernst diffusion layer thickness is not fixed inside the pipette, where linear diffusion takes place. Consequently eqn (1) no longer holds and the kinetic analysis fails. The results presented show that steady-state measurements are a powerful and straightforward route to kinetic information, especially for the determination of the apparent charge-transfer coefficient and the apparent standard rate constant, This, however, is not very informative about the process itself, as the real goal is the obtention of kinetic parameters for the rate-limiting step occurring somewhere in the mixed solvent layer separating the two diffuse layers. As recently shown15 for the case of simple ion transfer, the desired kinetic parameters can be obtained from the apparent measured values.A similar analysis is used here for the case of facilitated ion-transfer reactions. Let us consider that the complexation reaction occurs somewhere in the mixed solvent layer where k, and k, are the rate constants for the local reaction. Assuming again that the complexation reaction is pseudo-first-order (i.e. an excess of transferring ion) the real standard case for this local reaction is defined by -+ c c,* = c;, (13) where the superscript * refers to the local concentration at the plane of the reaction. These concentrations are related to the surface concentrations just outside the diffuse layer by c,* = c; (15) - z i F and c,*, = c;, exp ( RT (@* - Do)) where @* is the Galvani potential at the local plane where the complexation reaction occurs and @O is the Galvani potential in the bulk of the organic phase.Substituting eqn. (14) and (15) into (13) gives where A: @ is the Galvani potential difference between the water phase and the reaction plane. If we neglect mass transport and assume that the outside the diffuse layer obey the Nernst equation, in this surface concentrations case RT CE A:,@ = A","+-In- zi F C& it can be deduced thatJ . A . Campbell, A . A . Stewart and H. H. Girault 85 1 Note that in the real standard case the potential drop in the aqueous phase, A:@', is equal to the formal potential, i.e. the total Galvani potential difference for the apparent standard case when Cg = C&.At this point it is possible to define the real standard rate constant, k,", as and to express the true rate constant, k,, as - + t k," = k, = k, (19) (20) and similarly (21) where a is the classical charge-transfer coefficient defined for the local reaction. Because in the present case the reaction is pseudo-first-order, the local charge-transfer coefficient a is shifted to unity as the potential at which the activated state occurs coincides with the reaction plane defined above as @*. Eqn (20) and (21) thus reduce to e and k, = k,". (23) + These two equations can now be used to express the apparent rate constants kaPP and kapp by combining eqn (14) and (15) with eqn. (22) and (23), respectively: 4- -+ (A: @ -A: @') c k,,, = k," exp ($(A: @)). (24) We can now identify the apparent charge-transfer coefficient, aapp, introduced earlier in eqn (8) and (9) with @*-OW aapp = a o - m w * (26) Using eqn (18), eqn (24) and (25) can be rewritten as (aapp A:.@ - A: O") As in ref. (12), we see that the apparent charge-transfer coefficient refers to the total Galvani potential difference between the two phases and not to the local interfactial potential drop. It represents the ratio of the potential drop between the locus of the activated state and the bulk aqueous phase to the total Galvani potential difference. By definition aapp is dependent on the applied potential difference and will be a function of the concentrations of the supporting electrolytes. The measured rate constant kZPp is defined for the equality852 Ion Transfer across Interfaces and is obtained when A@ = A@"'; it reads The apparent charge- transfer coefficient derived from following equation : was found to be 0.9.(30) fig. 3 by employing the (31) The true standard rate constant kt can therefore be calculated directly from eqn (30) using the value of the apparent standard rate constant computed from eqn (1 1) and inserting the corresponding value of aapp and the formal potential obtained by the graphical method described above. This calculation leads to a true standard rate constant of 2.5 x lop3 cm s-l. Another way to evaluate k," is by use of eqn (27) at the P.Z.C. for which the Galvani potential difference A: @ is defined to be zero. This second approach can only be used when the formal Galvani potential difference is small enough to provide accurate values of kapp at the P.Z.C.The analysis of the values obtained for these interfacial complexation reactions in comparison with the corresponding bulk reactions is beyond the scope of the present paper and will be discussed in a future publication on the mechanism of ion-ionophore reactions in heterogeneous media. To test the applicability of eqn (26) it is interesting to compare the apparent charge- transfer coefficient value thereby defined with the ratio A:@/A:@, where A:@ is the potential drop in the aqueous diffuse layer, which can be calculated for example using the Gouy-Chapman theory. Indeed for the 1 : 1 organic electrolytes used in the present experiment the Gouy charge is given by = - (8RTC0~")i sinh z=O whereas the aqueous charge for the 2:2 aqueous electrolyte is given by The electroneutrality of the interface allows us to calculate the potential drop in the aqueous diffuse layer, A:@.When the Galvani potential drop is equal to the formal potential (i.e. -0.065 V) A:@ was found to be equal to 2.2 mV. This leads to an apparent charge-transfer coefficient of 0.97. Given the inadequacy of the Gouy- Chapman theory in application to 2: 2 electrolytes and also that MgSO, is significantly ion-paired at such a high concentration in water, this calculated value of aapp compares relatively well with the measured value of 0.9 and justifies a posteriori the definition of aapp given by eqn (26). Conclusion The present work shows for the first time that the measurement of steady-state currents occurring during assisted ion transfer across a micro-liquid-liquid interface supported at the tip of a micropipette allows direct access to the apparent kinetic parameters of these processes.Furthermore, the accompanying kinetic theory proposed enables the calculaton of the true kinetic parameters corresponding to the local interfacial complexation reaction, and this without referring to any a priori model of the interface.J. A. Campbell, A . A. Stewart and H. H. Girault 853 We acknowledge the S.E.R.C. for supporting this work and for providing an Information Technology studentship to J. A. C . and a CASE award studentship in collaboration with Genetics International (U.K.) to A. A. S. The support of the Nuffield Foundation is also gratefully acknowledged. References 1 C . Gavach, B. d’Epenoux and E’. Henry, J . Electroanal. Chem., 1975, 64, 107. 2 T. Osakai, T. Kakutani and M. Senda, Bull. Chem. SOC. Jpn, 1984, 57, 370. 3 B. d’Epenoux, P. Seta, G. Amblard and C . Gavach, J. Electroanal. Chern., 1979, 99, 77. 4 Z. Samec, V. Marecek, J. Weber and D. Homolka, J. Electroanal. Chem., 1981, 126, 105. 5 Z. Samec, V. Marecek and J. Weber, J. Electroanal. Chem., 1979, 100, 841. 6 T. Osakai, T. Kakutani and M. Senda, Bull. Chern. Soc. Jpn, 1985, 58, 2626. 7 Z. Samec and V. Marecek, J. Electroanal. Chem., 1986, 200, 17. 8 T. Kakutani, Y. Nishiwaki, T. Osakai and M. Senda, Bull. Chem. Soc. Jpn, 1986, 59, 781. 9 T. Wandlowski, V. Marecek and Z . Samec, J . Electrounal. Chem., 1988, 242, 291. 10 G. Taylor and H. H. Girault, J. Electroanal. Chem., 1986, 208, 179. 11 M. I. Montenegro, Port. Electrochim. Acta, 1985, 3, 165. 12 H. H. Girault and D. J. Schiffrin, J. Electroanal. Chem., 1985, 195, 213. 13 H. H. Girault and D. J. Schiffrin, Electrochim. Acfa, 1986, 31, 1341. 14 H. H. Girault and D. J. Schiffrin, Elecrroanal. Chem., 1988, 15, I . , 15 H. H. Girault, J . Electroanal. Chem., in press. 16 H. H. Girault and D. J. Schiffrin, J. Electroanal. Chem., 1984, 161, 415. Paper 8/01473F; Receied 15th April, 1988

ISSN:0300-9599    

DOI:10.1039/F19898500843

出版商:RSC    

年代:1989

数据来源: RSC