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/F198985FX029

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/F198985BX031

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

JSSN 0300-9599 JCFTAR 85(8) 1881-2647 (1 989) 1881 1897 1907 1921 1933 1945 1963 1979 199 200 20 1 202 1 2035 2047 2055 207 1 2079 2087 JOURNAL OF THE CHEMICAL SOCIETY FARADAY TRANSACTIONS I Physical Chemistry in Condensed Phases Thermodynamics of the Transition State and the Application to Interfacial Reactions D. G. Hall Tin Oxide Surfaces. Part 18.-Infrared Study of the Adsorption of very low Levels (20-50 ppm) of Carbon Monoxide in Air on to Tin(1v) Oxide Gel P. G. Harrison and A. Guest Tin Oxide Surfaces. Part 19.-Electron Microscopy, X-Ray Diffraction, Auger Electron and Electrical Conductance Studies of Tin(1v) Oxide Gel P. G. Harrison and M. J. Willett Tin Oxide Surfaces. Part 20.-Electrical Properties of Tin(rv) Oxide Gel : Nature of the Surface Species Controlling the Electrical Conductance in Air as a Function of Temperature Interaction between Amine Oxide Surfactant Layers Adsorbed on Mica C.E. Herder, P. M. Claesson and P. C. Herder Zeolites treated with Silicon Tetrachloride Vapour. Part 5.4atalytic Cracking of n-Hexane M. W. Anderson, J. Klinowski, J. M. Thomas and M. T. Barlow Further Electron Spin Resonance Studies of RhNa-X. Effects of Carbon Monoxide, Hydrogen and Oxygen A. Sayari, J. R. Morton and K. F. Preston Photocurrent Kinetics in Metal Phthalocyanine Crystals, Films and Pellets G. S. Bahra, A. V. Chadwick, J. W. Couves and J. D. Wright Characterization of Supported Ni'CO(TAP), (n < 3) Complexes by the CO Stretching Vibration. Electron-donating Effect of Trialkylphosphine Ligands M. Kermarec, C. Lepetit, F.X. Cai and D. Olivier Role of Solvent Dynamics on Kinetics of Electro-oxidation of Ferrocene at Platinum Electrode Sequential Oscillations in the Belousov-Zhabotinski System with Ascorbic Acid-Acetone/Cyclohexanone as the Mixed Organic Substrate R. P. Rastogi, I. Das and A. Sharma Chemical Environments around Active Sites and Reaction Mechanisms for Deuterium-Acrolein Reaction over Ir/Nb,O, in Normal and SMSI States H. Yoshitake, K. Asakura and Y. Iwasawa Longitudinal Relaxation of Protons in Partially Deuterated [A1(H,0),I3+ J. W. Akitt, J. M. Elders and 0. W. Howarth Redox Catalysis. Theory for a Nernstian Reaction Coupled to an Irreversible Reaction Kinetic Study of the Oxidation of Water by CerV Ions Mediated by Activated Ruthenium Dioxide Hydrate Mechanistic Studies of Capillary Processes in Porous Media.Part 1 .-Prob- abilistic Description of Porous Media V. Mayagoitia, M. J. Cruz and F. Rojas Mechanistic Studies of Capillary Processes in Porous Media. Part 2.-Con- struction of Porous Networks by Monte-Carlo Methods M. J. Cruz, V. Mayagoitia and F. Rojas The Peptide-Urea Interaction. Excess Enthalpies of Aqueous Solutions of N- Acetylamides of Amino Acids and Urea at 298.15 K G. Barone, G. Castronuovo, P. Del Vecchio and C. Giancola P. G. Harrison and M. J. Willett S. U. M. Khan A. Mills and N. McMurray A. Mills and N. McMurray2099 21 13 2127 2141 2149 2159 2173 2185 2199 221 1 2229 224 1 2249 2255 2273 2285 2297 2309 2327 2335 2347 Contents Chemical Relaxation and Equilibrium Studies associated with the Binding of Anionic Surfactants to Neutral Polymers N.Takisawa, P. Brown, D. Bloor, D. G. Hall and E. Wyn-Jones The Formation of a Well Defined Rhodium Dicarbonyl in Highly Dealuminated Rhodium-exchanged Zeolite Y by Interaction with CO H. Miessner, I. Burkhardt, D. Gutschick, A. Zecchina, C. Morterra and G. Spoto Physico-chemical Characterization and Framework Topology of Zeolite ZSM- 20 V. Fulop, G. BorbCly, H. K. Beyer, S. Ernst and J. Weitkamp Densimetric and Viscosimetric Investigations of NaI in Hexamethylphos- phoramide-Water Mixtures at 293.15, 298.15 and 303.15 K S. Taniewska- Osinska and M. Joiwiak Temperature-dependent Conformational Analysis of Gentiobiose Octa-acetate in Solution. Proton and Carbon Nuclear Magnetic Relaxation Study C. Rossi, S.Ulgiati and N. Marchettini Exchange Reactions of Alkanes containing Quaternary Carbon Atoms over Supported Metal Catalysts Reactions of Unsaturated Hydrocarbons on Rutile and Anatase B. I. Brookes, R. Bird, C. Kemball and H. F. Leach Ionic Solvation in Water-Co-solvent Mixtures. Part I 8.-Free Energies of Transfer of Single Ions from Water into Water-2-Methoxyethanol Mixtures K. H. Halawani and C. F. Wells Electron Spin Resonance Study of the Interaction of Oxygen with Ag/SiO, Y-P. Wang, C-t. Yeh and S-H. Chien Complex Formation and Self-association in Ternary Mixtures. Apparent Heat Capacities for Alcohol-Methyl Acetate-Hydrocarbon and Acetone-Chloro- form-Cyclohexane M. Costas, Z. Yao and D. Patterson Vapour Pressure of Butane from 173 to 280 K W. D. Machin and P.D. Golding Kinetic Salt Effects on the Reaction between the [ 1,3,6,8,10,13,16,19-Octa- azabicyclo(666)icosane]Cobalt( 111) Ion, C~(sep)~+, and the Hexa-aquochrom- ium(r1) Ion Redox and Acidic Properties of the Borate Radical B(0H);. A Flash Photolysis Study S. Padmaja, V. Ramakrishnan, J. Rajaram and J. C. Kuriacose Channel-elec t rode Vol tammet ry . Waves hape Analysis of the Curren t-Vol tage Curves of EC, and DISP2 Processes R. G. Compton and M. B. G. Pilkington Flat - Vertical Transitions of Fluoranthene Molecules adsorbed on a Mercury Electrode Transference Number Measurements of Silver Nitrate in Pure and Mixed Solvents using the Electromotive Force Method D. S. Gill and M. S. Bakshi Transference Number, Conductance and Viscosity Studies of some 1 : 1 Electrolytes in Pyridine-Methanol Mixtures at 25 "C D.S. Gill and M. S. Bakshi Formation of Hydrocarbons in the Electrochemical Reduction of Carbon Dioxide at a Copper Electrode in Aqueous Solution Y. Hori, A. Murata and R. Takahashi Structure of Germanium Oxide on CVD Zeolites by Extended X-Ray Absorption Fine Structure and X-Ray Photoelectron Spectroscopy T. Hibino, M. Niwa, Y. Murakami and M. Sano Nuclear Magnetic Resonance Relaxation Studies on the Preferential Solvation of tris-Acetylacetonato-Chromium(rr1) in Bromofom-Carbon Tetrachloride Solutions Diffusion Coefficients for a Series of n-Alkanes in two Polar Solvents K. D. Bartle, A. A. Clifford, D. Mills and R. Moulder R. Brown and C. Kemball F. Ferranti and A. Indelli R. G. Compton, R. G. Harland and R.J. Northing P-L. Wang and L-P. HwangContents 2355 2369 238 1 2397 2405 2417 2427 2435 2445 2453 2465 2473 2481 2499 2507 2525 2535 2555 2563 2575 258 1 Comparison of Entropic and Enthalpic Components of the Barrier Symmetry Factor, b, for Proton Discharge at Liquid and Solid Hg in Relation to the Variation of Tafel Slopes and /3 with Temperature B. E. Conway and D. P. Wikinson Photolysis of Cyclotetrasilanes. Remarkable Dependence on Molecular Structure H. Shizuka, K. Murata, Y. Arai, K. Tonokura, H. Tanaka, H. Matsumoto, Y. Nagai, G. Gillette and R. West Reaction of Preadsorbed Methane with Oxygen over Magnesium Oxide at Low Temperatures T. Ito, T. Watanabe, T. Tashiro and K. Toi Bending Vibrations of OH Groups resulting from H, Dissociation on ZnO A. A.Tsyganenko, J. Lamotte, J. Saussey and J. C. Lavalley Effect of Solvent on the Reactions of Coordination Complexes. Part 8.-Kinetics of Solvolysis of cis-(Chloro)(ethanolamine)bis(ethylenediamine)- cobalt(II1) in Methanol-Water, Propan-2-01-Water and t-Butyl Alcohol-Water A. C. Dash and P. K. Das Isomeric Equilibria of Monosaccharides in Solution. Influence of Solvent and Temperature F. Franks, (in part) P. J. Lillford and G. Robinson Electron Transfer between a: and /3 Haem Groups in Haemoglobin. An Electron Spin Resonance Study M. C. R. Symons and F. A. Taiwo Application of Radiation and Electron Spin Resonance Spectroscopy to the Study of Ferry1 Myoglobin R. L. Petersen, M. C. R. Symons and F. A. Taiwo Silver(1)-Polyamine Systems in Dimethyl Sulphoxide. A Thermodynamic and Spectroscopic Investigation A.Cassol, P. Di Bernardo, P. Zanonato, R. Portanova, M. Tolazzi, G. Tomat, V. Cucinotta and D. Sciotto The Effect of Pressure on the Electrical Conductivity of Liquid Iodine, Iodine Chloride, Iodine Bromide and Bromine Trifluoride B. Cleaver and D. H. Condl yffe The Effect of Glucose on the Crystallization of Hydroxyapatite in Aqueous Solutions Electrochemical Characterisation of BaSn,-,Sb,O, Perovskites M. I. da Silva Pereira, M-J. B. V. Melo, F. M. A. da Costa, M. R. Nunes and L. M. Peter The Second Emission of the Uranyl Ion in Aqueous Solution M. D. Marcantonatos and M. M. Pawlowska Intermicellar Interactions and Micelle Size Distribution in Aqueous Solutions of Polyoxyethylene Surfactants T. Kato, S-i. Anzai, S. Takano and T.Seimiya Partial Oxidation of Methane over Oxide Catalysts. Comments on the Reaction Mechanism G. J. Hutchings, J. R. Woodhouse and M. S. Scurrell Quasi-elastic Neutron Scattering Study of Benzene Adsorbed in ZSM-5 H. Jobic, M. We and A. J. Dianoux AlPOJTiO, Catalysts. Part 2.-Structure, Texture and Catalytic Activity of Systems Precipitated with Ammonia or Ethene Oxide J. M. Campelo, A. Garcia, D. Luna, J. M. Marinas and M. S. Moreno Applicaton of Pitzer's Equations to Dissociation Constants of Ammonium Ion in Lithium Chloride-Sodium Chloride Mixtures M. Maeda, 0. Hisada, K. Ito and Y. Kinjo Enthalpic Interaction Coefficients of some Dipeptides Dissolved in N,N- Dimethylfonnamide Activity Coefficients for Aqueous Solutions of Potassium Succinate (K,Succ) at 25 "C M.A. Esteso, L. Fernandez-Merida, 0. M. Gonzalez-Diaz and F. F. Hernandez-Luis Perhydrotriphenylene Radical. An Initiator for Inclusion Polymerization as Characterized by Electron Spin Resonance Spectroscopy P. Sozzani, R. Scotti and F. Morazzoni E. Dalas and P. G. Koutsoukos A. H. Sijpkes and G. Somsen2587 2597 2605 2615 2625 2635 2641 2647 Con tents Solvation and Complexation of Copper(n) and Chloride Ions in 2,2,2- Trifluoroethanol-Dimethyl Sulphoxide Mixtures H. Suzuki, S. Ishiguro and H. Ohtaki Active Structures and Electronic States for Adsorption of CO, and NO on an Na/TiO,(l 10) Surface H. Onishi, T. Aruga, C. Egawa and Y. Iwasawa Quenching of the Luminescence of the Aqueous Gadolinium Ion by Nitrate and Thiocyanate J-J. Vuilleumier, M. Deschaux and M. D. Marcantonatos Detection and Quantitative Determination of the Composition of Bismuth Molybdate Phases by Various Spectroscopic Techniques R. Olier, G. Coudurier, M. El Jamal, M. Forissier and J. C. Vedrine Photophysics of the Excited Uranyl Ion in Aqueous Solutions. Part 6.- Quenching Effects of Aliphatic Alcohols M. E. D. G. Azenha, H. D. Burrows, S. J. Formosinho and M. de Graga M. Miguel Oxygen Adsorption on Ag Powder M. Bowker, P. Pudney and G. Roberts Induction of Mesoporosity in ALPO-5. Treatment with Silicon Tetrachloride C. R. Theocharis, M. R. Gelsthorpe and D. Yeates Corrigendum to Successive Addition of Electrons to Sodium Quinizarin-2- and -6-Sulphonate in Aqueous Solution. A Pulse and y-Radiolysis Study T. Mukherjee, E. J. Land, A. J. Swallow, P. M. Guyan and J. M. Bruce

ISSN:0300-9599    

DOI:10.1039/F198985FP095

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions II, Issue 8,1989 Molecular and Chemical Physics For the benefit of readers of Faraday Transactions I, the contents list of Faraday Transactions II, issue 8, is reproduced below. This issue contains the proceedings of Faraday Symposium 24 on Orientation and Polariaztion Effects in Reactive Collisions. 925 Alignment Measurements in Orbitally Selective Collision-induced Huorescence D. M. Segal and K. Burnett 939 Fignment Effects involving Multiple Pathways. Electronic Energy Transfer of Sr 5s6p 955 %5 975 983 1003 1017 1027 1059 1081 1097 1115 1133 1141 1155 1169 1185 Pi with Rare Gases L. J. Kovalenko, R. L. Robinson and S. R. Leone Orientational and Spin-orbital Dependence of Interatomic Forces V. Aquilanti, G. Liuti, F.Pirani and F. Vecchiocattivi Mj Selectivity in Collisional Energy Transfer between Alkali-metal Atoms M. Baba and H. Kat6 Orbital Polarization and Fine-structure Effects in Time-resolved Cooperative Fluorescence from Dissociating Alkali-metal Diatoms G. Kurizki, G. Hose and A. Ben-Reuven Coherence Effects in the Polarization of Photofragments M. Glass-Maujean and J. A. Beswick Stereochemical Influences in Atom-Triatomic Collisions 2. T. Alawahabi, C. G. Harkin, A. J. McCaffery and B. J. Whitaker Differential Scattering of Na(3P) from HF. Reactive and Non-reactive Processes R. Diiren, S. MiloSevi4 U. Lackschewitz and H. J. Waldapfel General Discussion Products' Angular Distribution for Stemselective Reactions. Simple Optical and Kinematic Considerations I, Schechter and R.D. Levine Quasiclassical Trajectory Styudy of the F+I2 Potential-energy Surface N. W. Keane, J. C. Whitehead and R. Grice Determination of Molecular Orientation and Alignment from Polarized Laser Photo- fragmentation Measurements. Oriented CH3I Molecular Beams R. B. Bernstein, S. E. Choi and S. Stolte Reactant Orientation-Product Pola@ition Correlations. Collision Energy Dependence in the Ba + N20 + BaO + N2 Reaction H. Jalink, G. Nicolasen, S. Stolte and D. H. Parker Orbital Orientation in van der Waals Reactions C. Jouvet, M. C. Duval, B. Soep, W. H-Breckenridge, C. Whitham and J. P. Visticot Photoinitiated H + N20 Reactions. N20-HBr Complexes and Gas-phase, Single-collision, Arrested-relaxation Conditions G. Hoffmann, D. Oh and C. Wittig Vector and Product Quantum-state Correlations for Photohgmentation of Formaldehyde T.J. Butenhoff, K. L. Carleton, M-C. Chuang and C. B. Moore Vector Correlations from Doppler-broadened Lineshapes. State-selected Dissociation of Methyl Nitrite M, P. Docker, A. Ticktin, U. Briihlmann and J. R. Huber Vector Correlations in the 157 nm Photodissociation of OCS and the 266 nm Photodissociation of Methyl Iodide R. Ogorzalek Loo, C. E. Strauss, H-P. Haerri, G. E. Hall, P. L. Houston, I. Burak and J. W. Hepburn 1 (91207 1221 1243 1325 1337 1347 1357 1371 Photohgment Vector Correlations in Vibrationally Mediated Photodissociation. A New Angle on Intamolecular Vibrational Redistribution M. Brouard, M. T. Martinez, J. O'Mahony and J. P. Simons Femtochemistry: The Role of Alignment and Orientation A.H. Zewail General Discussion Rotational Alignment of NO from Pt (111). Inelastic Scattering and Molecular Demption D. C. Jacobs, K. W. Kolasinski, R. J. Madix and R. N. Zare Steric Effects in Scattering and Adsorption of NO at Ag (111) A. W. Kleyn, E. W. Kuipers, M. G. Tenner and S. Stolte Application of Translational Spectroscopy to the Study of Reactive Collisions of Molecules with Surfaces K. J. Snowdon General Discussion Summarizing Remarks R. N. Dixon Index of Names List of Posters JOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistry or chemical physics which appear currently in J. Chem. Research, The Royal Society of Chemistty's synopsis + microform journal, include the folkwing: Effects of Substituents on the Kinetics and Mechanism of the Silver Ion-promoted Hydrolysis of Di(ethyLhio)(phenyl)methane in Aqueous Solution Derek P.N. Satchell and Rosemary Satchell (1 989, Issue 4) Catalysis by Surfactants of Nucleophilic Displacement Reactions of Primary Alkyl Halides with Solid-phase Nucleophile Salts Branko JurgiC (1 989, Issue 4) The Kinetics of the Diazotization of Anilines in the Presence of Thiosulphate b n Lluls Abla, Albino Castro, Emilia Igleslas, J. Rarn6n Leb and M. Elena PeAa (1 989, Issue 4) Kinetics and Mechanism of Oxidation of Sulphite by Vanadium(v) Kalyan Kall Sen Gupta, Sankar Das and Shipra Sen Gupta (1 989, Issue 4) An Electron Spin Resonance Study of Radicals formed from Tetrolic Acid by Radiotysis in a Freon Matrix Christopher J. Rhodes (1 989, Issue 6) On the Existence of the Bicycb[l.l .llpentyl Cation Ernest W.Della and Carl H. Schiesser (1 989, Issue 6) The Bonnet Transformation in Organic Chemistry: Conformatinal Changes in Hydrocarbons ZoRan Blum and Stephen T. Hyde (1 989, Issue 6) Protonated Sulphurous Acid Lars Carlsen and Helge Egsgaard (1 989, Issue 6) An Electron Spin Resonance Study of the 1,2,3-Triphenylcycbpropene Radical Cation Radical Cations from STwylaziridine and N-Tosylazetidine Christopher J. Rhodes (1 989, The Influence of Kinetic Factors on the Products of the Decomposition of Thionitriles Derived from Christopher J. Rhodes (1989, Issue 7) Issue 7) N,N'-Dialkytthioureas Francisco Meijide and Geoffrey Stedman (1 989, Issue 7) (ii)The following papers were accepted for publication in Faraday Transactions I during May, 1989. 8/03684E 8/045731 8/04726T 8/04786C 8/04879G 9/00129H 9W05G 9/00211A 9100214F 9J00251K 8X)5054F 9/00306A 9/00366E 9/00367C 9/00434c 9m14H Membrane Potential and Ion Transport in Inhomogeneous Ion-exchange Membranes Higuchi, A.and Nakagawa, T. Activity Enhancement for the Hydrogenation of Adsorbed CO on Pt/A1203 and Pt/Al203 Induced by High-temperature Reduction Taniguchi, S., Mori, T., Mori, Y., Hattori, T. and Murakami, Y. Zeolite Synthesis in the Si02-A1203-Na2~Pyndine-H20 System Smith, W. J., Dewing, J. and Dwyer, J. The Influence of Potassium on the Catalytic Properties of V205/ri@ Catalysts for Toluene Oxidation Andersson, S. L. T. and Zhu, J. 35Cl Nuclear Quadropole Resonance and Infrared Studies of Hydrogen-bonded Adducts of 2-Chloro4-nitrobenzoic Acid Kalenick, J., Malerz, I.and Sobczyk, L. PAX (Photoelectron and X-Ray Emission) Spectroscopy and Electronic Structure: Thiourea Urch, D. S., Luck, S. and Foerch, R. Solute-Solvent Interaction in NaI-Glycerol-Diol Systems Taniewska-Osinska, S., Woznicka, J. and Bald, A. Mutual Diffusion Coefficients of NiCl~H20 at 298.15 K from Rayleigh Interferometry Rard, J. A., Miller, D. G . and Lee, C. M. Particle-Metal Interactions: A Raman and Electrochemical Study of a Compacted rode of Copper Phthalocyanine and Silver Metal Smith, W, E. and Bovill, A. J. hfluence of Phosphorus on the Catalytic Properties of V205/Ti@ Catalysts for 'loluene Oxidation Andersson, S. L. T., Zhu, J. and Rebenstorf, B. Structure of Vanadium Oxides on 2302 and Oxidation of Butene Miyata,H., Kohno, M., Ono, T., Ohno, T.and Hatayama, F. The Photochemical Reduction of 2,1,3-Benzothiadiazole-4,7dicarbonitrile in the presence of Cationic Micelles, and Onward Electron-transfer Reactions Robinson, J. N., Cole-Hamilton, D. J., Camilleri, P., Dainty, C. and Maxwell, V. TPD Study of Activated Chemisorption involving a Precursor State: Desorption of Water from Ti02 Malet, P. and Munuera, G. Coordination States of Co(NCS)2 and Ni(NCS)2 in Dimethyl Sulphoxide Pilarczyk, M., Grzybkowski, W. and Klinszporn, L. Kinetics of Aquation of [(5-Br-Phen)3I2' Ions in Aqueous Solutions as a Function of Temperature and Pressure: The Isochoric Controversy and Analysis of Equilibrium and Kinetic Data Blandamer, M. J., Burgess, J., Cowles, H.J., Horn, I. M., Engberts, J. B. F. N., Galema, S. A. and Hubbard, C. D. Pore Structures of Electrochemicallly Pretreated Glassy Carbon and Uptake of Lithium Ions into Micropores Nagaoka, T., Uchida, Y. and Ogura, K. A Study of Block Copolymer Adsorption Kinetics via Internal Reflection Interferometry Gast, A. P. and Munch, M. R. Excess Volumes of Binary Mixtures of an Octane Isomer and a Polar Component Ramon, G. R., Alanon Lopez, R. M., Caceres, M. and Nuiiez, J. Hydrogenolysis of Alkanes. Part 4.-Hydrogenolysis of Propane, n-Butane and Isobutane over Pt/A1203 and Pt-Re/&03 Catalysts Bond, G. C. and Gelsthorpe, M. R. Quadratic Autocatalysis and Self-heating in Hydrocarbon Oxidation Griffiths, J. F. and Phillips, C. H. (iii)9 n m m 9n1263J 9p1333D 9/01349K 9n16OoG 9m1597C 9101634A Infrared Study of the Effects of Oxychlorination on Pt Dispersion in F't/AI203 Catalysts Rochester, C.H. and Mordente, M. G. V. Spectroscopic Studies of Molybdate Species deposited on Nt&j Support Vedrine, J. C., Jin, Y. S, and Aurox, A. Salt Effects in the Kinetics of the Formation of the Iron@) "himyanate Complex Capitan, M. J., Munoz, E, Graciana, M.M., Jimenez, R., Tejera, I. and Sanchez, F. Non-steady-state and Transient Isotope Tracer Studies in Methanol Synthesis Jackson, S , D. and Brandreth, B. J. Haem Peptide Protein Interactions. Part 3.--Kinetics and Mechanism of the Interaction of Mimperoxidase-8 with Apo Myoglobin Adams, P. A., Goold, R. D. and Thumser, A. E. Thermodynamic Properties of Electrolyte Solutions: An E.M.F.Study of the System NaCl-Na2S04-HzO at 25,35 and 45 'C Ananthaswamy, J. and Sarada, S. Droplet Formation and Contact Angles of Liquids on Mammalian Hair Fibres Carroll, B. J. Surface Chamcterization of the Active Ru02-xHzO Catalyst Supported on Teflon Sabbatini, L, Morea, G. Tangari, N., Tortorella, V. and Zambonin, P. G. Infrared Study of the Adsorption of 1,2-Dimethoxyethane on Silica Rochester, C. H. and Anderson, J. A. ESR, ENDOR and TRIPLE Resonance of some 9,lO-Anthraquinone Radicals in Solution. Part 2.-hthraquinone Sulphonates Makela, R. and Vuolle, M. Stabilization of Biochemically Interestin Intermediates by Metal Coordination. Part 5.4omplexes of Znn, Cu', Re' and Ru with Singly Reduced 2,5-DiacetyIpyrazine Kaim, W., Bessenbacher, C, Emst, S., Kohlmann, S, Kasack, V., Roth, E.and Jordanov, J. An ESR Study of the Reaction of Group 11 Atoms with Ketene Genin, F., Howard, J. A, Hampson, C. A. and Mile, B. ESR Investigation of the Electronic Structure of Hopping Centres and the Polaronic Conduction in Iron-containing Phosphate Glasses Morazzoni, F., Scotti, R., Narducci, D., Lucca, M. and Pimini, S. Structural Information from Powder ENDOR Spectroscopy: Possibilities and Limitations Attanasio, D. IfCumulative Author Index 1989 Abe, M., 1493 Adachi, K., 1065, 1075, 1083 Agathonos, P., 1357 Aguilella, V. M., 223 Akitt, J. W., 121, 2035 Albery, W. J . , 1181, 1189 Al-Bizreh, N., 1303 Albuquerque, L. M. P. C., 207 Allen, G . C.. 55 Almeida, B. S.. 1217 Amodeo, P., 621 Anderson, J. A., 11 17, 1129 Anderson. M.W., 1945 Anpo, M.. 609 Anzai, S., 2499 Apelblat, A., 373 Arai, T., 929, 1451 Arai, Y., 2369 Archer, M. D., 1027 Aruga, T., 2597 Asakura, K., 441, 2021 Austin, J. C., I159 Azenha, M. E. D. G., 2625 Bahra, G. S., 1979 Baiker, A., 999 Bakshi, M. S., 2285, 2297 Bald, A., 479 Barlow, M. T., 1945 Barone, G., 62 1, 2087 Barone, V., 621 Bartle, K. D., 2347 Beckett, M. A,, 727 Bee, M., 2525 Bellotto, M., 895 Bengtsson, L., 305, 317 Berry, F. J., 467 Bertoldi, M., 237 Bertran, J., 1207 Beyer, H. K., 2127 Bicelli, L. P., 1685 Bird, R., 2173 Black, S. N., 1795 Blandamer, M. J., 735, 1809 Bloor, D., 2099 Bolis, V . , 855, 1383 Bolton, J. R., 1027 Bond, G. C., 168 Borbely, G., 2127 Borowko, M., 343 Bosch, H., 1425 Boss, R. D., I I Bowker, M., 165, 2635 Brimblecome, P., 157 Brookes, B.I . , 2173 Brown, P., 2099 Brown, R., 2159 Bruce, J. M.. 2647 Biilow, M.. 1501 Burgess, J.. 735. 1809 Burkhardt, I., 2 1 1 3 Burrows, H . D., 2625 Busca, G.. 137. 237 Cai, F. X.. 1991 Calado. J. C. G., 1217 Campbell, J. A.. 843 Campelo. J. M., 2535 Carbonara, M.. 1257 Carlstrom. G., 1049 Caro, J., 1501 Cassol, A., 2445 Castronuovo. G.. 2087 Cattania, M. G.. 801 Chadwick, A. V.. 166, 1979 Chandra, H., 1801 Che, M., 609 Chen, J.. 829 Chen, L-f., 33 Chien, S-H.. 2199 Claesson, P. M.. 1933 Cleaver, B.. 2453 Clegg, S. L . , 157 Clifford, A. A., 2347 Cohen, H., 1169 Colling, C. N.. 1303 Coluccia, S., 609, 1655 Comninos. H.. 633 Compton, R. G., 761, 773, 97 1821, 2255, 2273 Condlyffe, D. H., 2453 Conway, B. E.. 2355 Conway, S. J., 71. 79, 1841 Cooper, J.. 1365 Copperthwaite.R . G., 633 Costas, M., 221 1 Cottrell, M. R., 1809 Coudurier, G., 1607, 2615 Couves, J. W., 1979 Cox, B. G., 187 Cristiani, C., 895 Cristinziano, P., 621 Cruz; M. J., 2071, 2079 Cucinotta, V.. 3445 da Silva Pereira, M. I., 2473 da Costa, F. M. A,, 2473 da Costa, M. A,. 907 Dalas, E., 2465 Das, I., 2011 Das, P. K.. 2405 Das, S., 1531 Dash, A. C., 2405 Datka, J., 47, 837 Davey, R. J., 1795 ( v ) Dawber. J . G., 727 De Giglio, A,, 23 Dell’Atti, A., 23 Del Vecchio. P., 2087 Deschaux, M., 2605 Di Bernardo, P., 2445 Dianoux, A. J., 2525 Dickel, G., 1463, 1671 Ding, J.. 1599 Domen, K., 929, 1451 Dong, S.. 1575, 1585, 1599 Donini, J. C., 91 Downs, G. W., 1841 Drummond, C. J.. 521, 537, 551, Easteal, A. J., 1091 Eden, J., 991 Egawa, C.. 2597 Elders, J. M..2035 Elisei, F., 1469 El Jamal, M.. 2615 el Torki, F. M., 349 Endoh, A., 1327 Ernst, S.. 2127 Espinos, J. P., 1279 Esteso, M. A., 2575 Fahim, R. B., 1723 Falconer, J. W., 71, 79, 1841 Fernandez, A., 1279 Fernandez-Merida, L., 2575 Fernandez-Pineda, C., 10 19 Ferranti, F., 2241 Finch, J. A., 91 Flanagan, T. B., 1787 Fletcher, P. D. I., 147 Foerch, R., 1139 Foo, C. H., 65 Forissier, M., 1607, 2615 Formosinho, S. J., 2625 Forster, H., 1149 Forzatti, P., 895 Franchini, G., 1697 Franks, F., 2417 Frey, H. M., 167 Fubini, B., 237, 855. 1383 Fiilop, V., 2127 Gabriel, C. J., 1 1 Gabrys, B., 168 Gadzekpo, V. P. Y., 1027 Gans, P., 1835 Garcia, A., 2535 Garrone, E., 585, 1373, 1383 Garst, J. F., 1245 Gasser, D., 999 Gavish, B.. 1199 Gelsthorpe, M. R., 2641 56 IAUTHOR INDEX Gervasini, A., 801 Geus, J.W., 269, 279, 293, 1267 Giamello, E., 237, 855, 1373 Giancola, C., 2087 Gilbert, P. J., 147 Gill, D. S., 2285, 2297 Gill, J. B., 1835 Gillette, G., 2369 Girault, H. H., 843 Golding, P. D., 2229 Gonzalez-Diaz, 0. M., 2575 Gonzilez-Elipe, A. R., 1279 Gonzalez-Lafont, A., 1207 Gorner, H., 1469 Gottschalk, F., 363 Grieser, F., 521, 537, 551, 561 Guardado, P., 735 Guest, A., 1897 Guil, J. M., 1775 GutiCrrez, C., 907 Gutschick, D., 2113 Guy, P. D., 1795 Guyan, P. M., 2647 Hagele, G., 1409 Hakin, A. W., 1809 Halawani, K. H., 2185 Hall, D. G., 1881, 2099 Halle, B., 1049 Hampton, S., 773 Han, S., 829 Handreck, G. P., 645 Harland, R. G., 761, 2273 Harris, R. K., 1409, 1853 Harrison, P. G., 1897, 1907, Hasselaar, M., 1267 Hasted, J. B., 99 Hatano, M., 199 Hazra, D.K., 1531 Healy, T. W., 521, 537, 551, 561 Heatley, F., 917 Hegarty, B. F., 1861 Herder, C. E., 1933 Herder, P. C., 1933 Hernandez-Luis, F. F., 2575 Hesselink, W. H., 389 Hester, R. E., 171, 1159 Hey, M. J., 1743 Hibino, T., 2327 Higgins, J. S., 170 Higuchi, A., 127 Hill, W., 691 Hirai, T., 969 Hisada, O., 2555 Holmberg, B., 305, 317 Holz, M., 1257 Hong, C. T., 65 Hori, Y., 2309 Horn, I. M., 1809 Howard, J., 1233 Howarth, 0. W., 121, 2035 Hubbard, C. D., 735 Hummel, A., 991 Hunger, M., 1501 1921 Hunter, R., 363, 633 Hussein, G. A. M., 1723 Hutchings, G. J., 363, 633, 2507 Hwang, L-P., 2335 Ichikawa, K., 175 Ikeda, R., 111 Ikeda, S., 1619 Ikeda, Y., 1099 Imamura, H., 1647 Imanishi, Y., 1065, 1075, 1083 Indelli, A., 2241 Inoue, Y., 1765 Ishida, H., 111 Ishiguro, S., 2587 Itaya, K., 1351 Ito, K., 2555 Ito, T., 2381 Itoh, N., 493 Iwasawa, Y., 441, 2021, 2597 Jiang, R., 1575, 1585 Jin, T., 175 Jobic, H., 2525 Johnson, G.R. A., 677 Johnston, C., 11 11 Jonkers, G., 389 Jorgensen, N., 11 11 Jbiwiak, M., 2141 Juillard, J., 1337, 1709 Jutson, J. A., 55 Kaneko, K., 869 Kanno, T., 579 Karaiskakis, G., 1357 Karger, J., 1501 Kato, S., 1619 Kato, T., 2499 Katoh, T., 127 Keeler, J. H., 163 Kelebek, $., 91 Kemball, C., 2159, 2173 Kermarec, M., 1991 Khan, S. U. M., 2001 Kim, T. H., 1537, 1545, 1557 Kinjo, K., 2555 Kishi, R., 655 Kishimoto, S., 1787 Kitajima, K., 1647 Kiwi, J., 1043 Klinowski, J., 1945 Knijff, L. M., 269, 293 Kobayashi, M., 579 Koda, S., 957 Koresh, J. E., 1537, 1545, 1557 Koros, W. J., 1537, 1545, 1557 Kosugi, N., 869 Kotaka, T., 1065, 1075, 1083 Koutsoukos, P.G., 2465 Kozlowski, Z., 479 Kuriacose, J. C., 2249 Kuroda, H., 869 Kuwabata, S., 969 Lamotte, J., 2397 Lancaster, N. M., 1303, 1315 Land, E. J., 2647 Larramona, G., 907 (vi) Laschi, F., 601 Lavalley, J. C., 2397 Lawrence, D. G., 1365 Lawrence, K. G., 23 Leach, H. F., 2173 Lelj, F., 621 Lepetit, C., 1991 Levy, o., 373 Lewis, T. J., 1009 Leyendekkers, J. V., 663 Lhermet, C., 1709 Li, C., 929, 1451 Lillford, P. J., 2417 Liu, J.-Y., 1027 Liu, T., 1607 Lluch, J. M., 1207 Longdon, P. J., 1835 Lorenzelli, V., 137 Loudon, R., 169 Louis, C., 1655 Lowe, B. M., 945 Lubetkin, S. D., 1753 Luna, D., 2535 Lund, A., 421 Machin, W. D., 2229 Maeda, M., 2555 MafC, S., 223 Maffi, S., 1685 Maignan, A., 783 Malet, P., 1279 Malitesta, C., 1685 Manchado, M.C., 1775 Manzurola, E., 373 Marcantonatos, M. D., 248 1, Marchese, L., 1655 Marchetti, A., 1697 Marchettini, N., 2149 Marcus, Y., 381 Marinas, J. M., 2535 Markovits, G., 373 Maruya, K., 929, 1451 Masiakowski, J. T., 421 Matsuhashi, N., 111 Matsui, H., 957 Matsumoto, H., 2369 Matsumoto, T., 175 Matthews, R. W., 1291 Mayagoitia, V., 2071, 2079 Mazzucato, U., 1469 McAleer, J. F., 783 McLure, I. A., 1217 McMurray, N., 2047, 2055 Meima, G. R., 269, 279, 293, Melo, M-J. B. V., 2473 Merwin, L. H., 1409 Meyerstein, D., 1169 Miessner, H., 691, 2113 Miguel, M. de G. M., 2625 Mills, A., 503, 2047, 2055 Mills, D., 2347 Mitani, T., 1485 Mitchell, B., 1795 2605 1267AUTHOR INDEX Miyoshi, H., 1873 Mizoe, K., 1327 Mizuno, K., 1099 Morazzoni, F., 801, 258 1 Morel, J-P., 1709 Morel-Desrosiers, N., 1709 Moreno, M.S., 2535 Morrison, C., 1043 Morterra, C . , 1383, 21 13 Morton, J. R., 1963 Moseley, P. T., 783 Mosier-Boss, P. A., 11 Moulder, R., 2347 Mount, A. R., 1181, 1189 Mousset, G., 1337 Mukherjee, T., 2647 Munuera, G., 1279 Murakami, Y., 2327 Murata, A., 2309 Murata, K., 2369 Nagai, Y., 2369 Nakagawa, T., 127 Nakamura, D., 111 Nakamura, T., 493 Nandi, D., 1531 Natarajan, P., 813 Nazhat, N. B., 677 Neagle, W., 429, 719 Neilson, G. W., 1365 Newman, K. E., 485 Nicholas, A., 773 Nicol, J. M., 1233 Niwa, M., 2327 Nomura, H., 957, 1619 Northing, R. J., 2273 Nowak, R. J., 11 Nowicka, B., 479 Nunes, M. R., 907, 2473 Ohlmann, G., 691 Ohtaki, H., 2587 Ohyama, Y., 749 Okubo, T., 455, 749 Olier, R., 2615 Oliva, A., 1207 Oliveira Jr, 0.N., 1009 Olivier, D., 1991 Onishi, H., 2597 Onishi, T., 929, 1451 Orchard, S. W., 363 Osada, Y., 655 Otsuka, K., 199 Pacynko, W. F., 1397 Padmaja, S., 2249 Pandey, J. D., 331 Paniego, A. R., 1775 Patterson, D., 221 1 Pawlowska, M. M., 2481 Pellicer, J., 223 Pereira, I., 907 Peter, L. M., 2473 Petersen, R. L., 2435 Pilkington, M. B. G., 2255 Piwowarska, Z., 47, 837 Pope, C. G., 945 Portanova, R., 2445 Portwood, L., 711, 1801 Preston, K. F., 1963 Preti, C . . 1697 Price, W. E., 415, 1091 Pritchard, T. N., 1853 Pudney, P, 2635 Rai, R. D., 331 Rajaram, J., 2249 Ramakrishnan, V., 2249 Ramaraj, R., 813 Ramis, G., 137 Rao, K. J., 251 Rastogi, R. P., 201 1 Raven, C. I., 1743 Reed, W. F., 349 Rees, L. V. C.. 33, 1501 Reis, J. C. R., 207 Reller, A., 855 Rhodes, C.J., 71 1 Roberts, G., 2635 Robinson, G., 2417 Rochester, C. H., 71, 79, 429, 719, 1 1 1 1 , 1117, 1129, 1841 Rodriguez-Mellado, J. M., 1567 Rojas, F., 2071, 2079 Rooney, J. J., 1861 Rosen, D., 99 Rossi, C., 601. 2149 Rowlinson, J. S., 171, 172 Ruddick, A. J., 1795 Ruiz, J. J., 1567 Saadalla-Nazhat, R. A., 677 Sabbatini, L., 1685 Sacco, A., 23, 1257 Said, M., 99 Sakata, Y.. 929, 1451 Salvagno, S., 1009 Sanchez, F., 1809 Sarkany, A., 151 1, 1523 Sato, Kazunori, 1765 Sato, Kiyoshi, 1765 Saussey, J.. 2397 Sayari, A,, 1963 Schmehl, R. H., 349 Schmidt, J. A.. 1027 Schneider, H., 187 Schneider, I.. 187 Schumann, M.. 1149 Sciotto, D.. 2445 Scotti, R., 801, 2581 Scurrell, M. S., 2507 Seimiya, T., 2499 Selvaraj, U.. 251 Sharma, A., 2011 Sheppard, N., 1723 Shido, T., 441 Shindo, Y ., 1099 Shizuka, H., 2369 Shukla, R. K . , 331 Sijpkes, A. H., 2563 Sinot, P. J., 1425 Smith, E. G., 1853 Smith, G. W., 91 (vii) Smith, J. J., 1 1 Smith, M. R., 467 Smith, T. D., 645 Soares, V. A. M., 1217 Somsen, G., 2563 Song, S.. 1575 Sorek, Y . , 1169 Sozzani, P., 2581 Spoto, G., 21 13 Stevens. A. D.. 1439 Stewart, A. A,. 843 Stirling, C. J. M., 1009 Stroka, J., 187 Strumolo, D., 801 Sugawara, S., 1351 Sundar. H. G. K., 251 Sutton, H. C., 883 Suzuki, H., 2587 Swallow, A. J., 2647 Symons, M. C. R., 71 1 , 1439, 1801, 2427, 2435 Szejgis, A., 479 Szpak, S., 11 Taiwo, F. A., 2427, 2435 Takagi, T., 1099 Takagi, Y., 493 Takahashi, R., 2309 Takaishi, T., 1327 Takano, S., 2499 Takisawa, N., 2099 Tamura, K., 1493 Tanabe, T., 1787 Tanaka, H., 2369 Taniewska-Osinska, S., 479, Tashiro, T., 2381 Tassi, L., 1697 Taylor, D.M., 1009 Terai, M., 1493 Thamm, H., 1 Themistocleous, T., 633 Theocharis, C. R., 2641 Thomas, J. M., 1945 Tiddy, G. J. T., 1397 Timmermann, E. 0.. 1631 Tissier, M., 1337 Toi, K., 2381 Tolazzi, M., 2445 Tomat, G., 2445 Tonokura, K., 2369 Tosi, G., 1697 Tsuchiya, S., 1647 Tsutsumi, K., 1327 Tsyganenko, A. A., 2397 Ugliengo, P., 585, 1373 Ulgiati, S., 2149 Unwin, P. R., 1821 Urch, D. S., 1139 Vaccari, A., 237 van Buren, F. R., 269, 279, 293, Van-Den-Begin, N., 1501 van Dillen, A. J . , 269, 279, 293, 2141 1267 1267AUTHOR INDEX van Leur, M. G. J., 279 van Lith, D., 991 van Rensburg, L. J., 633 van Veen, J. A. R., 389 Vazquez-Gondlez, M. I., 1019 Vedrine, J.C., 1607 Vedrine, J.C., 2615 Vink, H., 699 Vis, R. J., 269, 279 Vuilleumier, J-J., 2605 Wacker, T., 33 Walker, D. R. B., 1545, 1557 Walker, P. A. M., 1365 Waller, A. M., 773, 977 Wang, P-L., 2335 Wang, Y-P., 2199 Warman, J. M., 991 Watanabe, T., 2381 Waugh, K. C., 163 Weale, K. E., 165 Weitkamp, J., 2127 Wells, C. F., 2185 West, R., 2369 Wilkinson, D. P., 2355 Willett, M. J., 1907, 1921 Williams, D. E., 783 Williams, G., 503 Woodhouse, J. R., 2507 Woolf, L. A., 1091 Wormald, C. J., 1303, 1315 Woinicka, J., 1709 Wright, J. D., 1979 Wyn-Jones, E., 2099 Yamada, Y., 609 Yao, Z., 2211 Yarwood, J., 1397 Yeates, D., 2641 Yeh, C., 2199 Yeh, C-t., 65 Yoneyama, H., 969, 1873 Yoshida, N., 1787 Yoshioka, H., 1485 Yoshitake, H., 2021 You, X., 829 Young, D. A., 173 Zaki, M. I., 1723 Zambonin, P.G., 1685 Zanonato, P., 2445 Zecchina, A., 609, 1655, 21 13 Zhan, R., 1599 Zielinski, R., 1619 THE 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 physidsts and chemists interested in the mechanism of electron and ion transport in polymeric systems. The sysbms indude conducting polymers, redox polymers, ion exchange membranes and modified electrodes. Discussion topics will cover experimental evidence from spedrosoopy, electrochemisby and new techniques such as the quartz microbalance. Theoretical models ranging from band theoty through polarons to localised chemical structures will be u i t d y evaluated and compared with experiment.The following haw agreed to participate in the Discussion: R. Murray W. J. Albety M. D. lngram M. 8. Armand 0. Bloor H. Cheradame P. G. Bruce R. Friend R. J. Latham A. J. Heeger A. R. Hillman P. V. Wright A. G. MacDmid M. Ratner M. E. G. Lyons S. Roth 0. Haas T. J. Lewis G. Tourillon C. Vincent P. G. Pickup G. Wegner A. Hamnett L. M. Peter K. Doblhofer The final programme and application form may be obtained from: Mrs Y. A Fish, The Royal Society of Chemistry, Burllngton Housq London W1V OBN. (viii)D EUTSCH E BU NSE N-GESELLSCHAFT FU R PHY SI KALlSC H E CH E MI E ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SOCICTC FRANCAISE DE CHIMIE, DIVISION DE CHlMlE PHYSIQUE JOINT DISCUSSION MEETING 1989 Transport Processes in Fluids and in Mobile Phases Aachen, Federal Republlc of Germany, 25-27 September 1989 Organised by: H. Versmold (F.R.G.) Al.M i s s (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; 6. U. Felderhof, Aachen; D. Frenkel, Amsterdam; A. Geiger, Dortmund; W. Glgser, Grenoble; H. G. Hertz, Karlsruhe; S. Hess, Berlin; J.Jonas, Urbana; R. Win, Konstanz; K. Lucas, Duisburg; H.-D. Uldermann, Regensburg; H. Posh, Wien; P. Pusey, Malvem; J. P. Ryckaert, Brussels; W. A. Steele, Penn State; D. J. Tildesley, Southampton; H. Weingilrtner, Karlsruhe. Fumer details may be obtained from: Professor H. Versmold, lnstitut fiir Physikalische Chemie, RWTH Aachen, Templergraben 59, D-5100 Aachen. Federal ReDublic of Germanv. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 25 Large Gas Phase Clusters University of Wamick, 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 dusters, induding 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 bmmolecules, to discuss the chemistry and physics of clusters from different viewpoints. The preliminary programme is now available and may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN.~~ FARADAY DIVISION INFORMAL AND GROUP MEETINGS Gas ffinetks Group Developments in Gas Kinetics: New Techniques, Resub and their Interpretation To be heidathe UnksitydYorkon3-4 Jdy 1989 Furtherinfonnabon . from Professor R. J. Donovan, Deparbnentofchemislry, UniwmitydEdinburgh,West Mains Road, Ecrinbugh EH93JJ Industrial Physical Chemistry Group with the Thin films and Swfaoes Grwp of the iOP Material8 for Non-linear and Electro-opticr To be held at Girlon cdege, Cambridge on 67July 1989 Furlherinfomakn - from The Meetings Ofk0r, InSliMQ of physics, 47Belgm3sqJan3, Locldon sWlX8Qx Electrochemistry Group with Uectroanalyt~kal Grwp Graduate Students' Meeting To be held at Imperial College, London on 12 July 1989 Further information aWn DrG.H. Wl, Depatmmtof Mineral Fbsoums Engineering, imperial College, landon sw7 2Bp Polymer Physics Group Biologically Engineered Polymers 89 To be heidatchurchl College, Cambridge, on 31 Julyto2August 1989 Furtherinfomalion 6wn Dr M. J. W, AFRC lnstituceof Food Research, cdney Lane, Nomich NR4 7UA Carbon Group with surface Readivity and Catalysis &up Carbons and Catalysis hrrtheridomabon ' from Dr J.W. Patrick, Dimcbr, Carbon Fleseach Grorrp, Loughborwgh Consuttants M., Uniwsitydkdmobgy,~LMlghboroughLE113TF Polymer Physics Grwp Biennial Meeting: Physical Aspects of Polymer Science 25th Annfvmary To be heldatthe Unksityof Reading on lSl5September 1989 Fwlherinfwmabon . from DrG. R MWl, Polymer Physics Labomtwy, UniwsitydReadng, MMnights, Readii RG6 2AF. Colloid and Interface sdenae Gmp Inorganic Particulates To be Mat Chestercdlegeon 19-21 September 1989 Furtherinfwmabon - frwn Dr R Buscail, ICI plc, corporale CdIoidSchce GForrp, PO Box 11, The Heath, RuKxxn, Cheshire WA7 4QE TO be Mat w w h University OfTechnolOgy~n 11-13sept~~nber 1989 Polar M i & Grwp w-h Low Temperature Group of he IOP and the Ins&& of Ceramics Hlg h Temperature Semiconductors To be he# at the Uniuwsityd Birmingham on 19-21 Septtmber 1969 Furdwinbmwbon - B k m i 615 2 l l from Dr c.Gmim3s, Deparbnent of chemistry, Un'NerSily of Birmingham, P.O. Box 363, Division with the lnsfitute of Physks Sensors and their Applications To be held at Ihe Un'hmity d Kent at canterkrry on 1922 September 1989 Furlherinfwmabon ' h r n The MeetingSOfbr, InstiMeof Physics, 47BelgmsquaFe, LondonsWlX8QXDivision with the Demche Bunsen Gesellschafz DiWon de Chimie PhysigUe of the Societd Fmpise de Chimie and Associazbne Italr‘ana di Chimica f i s h Transport Procetssea In Fluids and Mobile Phaser To be heldath PhysiMSche InslitiR, Aachen, Wst Gemranyon 2S28seplembec 1989 Furtheridmnabon .fromprdessorGLud<hwst.DeparBnentofchemistry,~ofsouthampbm, soulhampton SO9 5NH Division Autumn Meeting: Chemistry at Interfaces To be held at hghborough UnivwSity of Techrrdogyon 26-28 Seplember 1969 Furlher inlonnalion from Professor F. Whson, Departmeclt of Chddryl Loughborough Uni\Eersity of Teckbgy, Loughborough E l 1 3Tu Potymer Physics Group Polymers in Motion To be heldatthe Uniwrsityof Cam- on 14 Deoembw 1989 Furtherinfcmabm * fromDrM.J.Fkha&on , NationalPhysicd LaboraBMy,,Queen’sRoad,T~, hAiddesex w11 OLW ___ ~ ~ _ _ ~ ~~~ ~ Theoretical Chemistry Group Electronic Structure Calculations on Large Molecules: Novel Methods and Applications TobeheldhCambridgeon1517December1989 Furlherinoomlaban - frwn Dr P.Fowk, Deprmntof chemisby, UIlhmilyOf Exeber, sbck3fRoad, ExererEx44Qo H&h Resdutbn Sp8czrvscopy Group with the Mdeaular h m s Group Spectroscopy and Molecular Beams To be heldatihe Unik&yof Nottingham on 1819 December 1989 F u M infomation from Dr P. G. Sam, Department 0s chemistry, Uniwxsity d Notbjngham, Universi3, Park, Nottingham NG7 2RD Neutron Scattering Group Applied Neutron Scattering To be heidat the Universityof WaMlidron 21 Deoember 1989 Further information from Dr C. Windsor, B 521 .l, ERE, IiimnA, Didcat, Oxkxkhim ~ colloid and Interface science Group with Protein and Peptide Group, Biotechnology Group and Gesellsahaft fir Biologische Chemie Stability of Proteins: Theory and Practice To be Mat GirlM1 College, Camkidgem 26-29 March 1990 Further information from Pmkssor F.Franks, PAFRAM, 150 Cambridge Science park, cambridge CB44GG Division Annual Congress: The Solid State: Reactivity and Electrical Properties To be heid at Queen’s UnWsity, Belfast an 912 April 1990 Furtheridomatm . from DrJ. F. Gibson, The FbyaIltydChemistry, Buhgkm House, London WlVoBN ~~~ Polymer Physics Group New Materials and their Applications To be held at the Uni\Fersity of warnidcan l(112ApriI 1990 Furtherinfonnaborr * fromDrM.Flkhahm , National Phw hboratoty, Qmn’s Road,Teddngkm, Middesex w11 OLWN e m n Scattering Group Neutron Scattering Data Analysis To be held at he Rutherlwd Appleoon IAmakwy, C h h on 1 May 1990 Furtheridmmbon - from Dr M. W. Johnsun, Nwtm scienoe Division, R u l h e r f o r d m Iaborawy, Chifton, Didoof- EAectrochennisby Group with the SCI flectrosynthe8is TobeheldatIheUnivemitydSouthamptononQ-11 Jurv1990 Furtherinfomaba * from Dr 0. Pkdw, Department of chemisby, Unkmity ofswthampton, Southamp~~l SO9 5NH. Gas Kinetics Group Xith intenrational Symposium on Gas Kinetics Furtherinsormahon . frwn Rolessor R J. Donovan, -0s Chemistry, Unksityof Edinburgh, West Mains Road, Edhkrgh EH93JJ TO be held h AssiSi, WOn 2 - 7 s e p t ~ d ~ 1990 ~~ Collokiand lnterfaw Scienw Group Confwenco of the Europem Colloid and Interface Science Society (El=) To be Matthe Unksityd k B 0 l on lSl4-k 1990 Furtherinfwmab'on from Professor R H. mil, School of chemistry, Uni\rersity of Btistd. CanWs Close, Bristd BS8 1TS THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 89 Structure of Surfaces and Interfaces as Studied using Synchrotron Radiation University of Manchester, 4 6 April 1990 organisins Cwnmif?ee: Professor J. N. Shemood (Chairman) Professor D. A. King Dr G. King Dr C. Nonis 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 specbal 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 w h i i shed new light on the structure of the complete range of intwfaces: soliisolii, soliigas, solid/lquid, gadliquid and "dean" surfaces including both static and dynamic in situ 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 mom powerful sources. The preliminary programme is now available and may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burllngton House, London W1V OBN. (xii)

ISSN:0300-9599    

DOI:10.1039/F198985BP099

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chern. SOC., Faraday Trans. I, 1989, 85(8), 1881-1895 Thermodynamics of the Transition State and the Application to Interfacial Reactions Denver G. Hall Department of Chemistry and Applied Chemistry, University of Salford, Salford A45 4WT According to transition-state theory, when the equilibrium hypothesis is valid, activated complexes formed from reactants may be treated thermo- dynamically in the same way as other species present in very small amounts. Inclusion of these complexes in the Gibbs-Duhem equation for the solution enables an expression to be derived for the dependence of the equilibrium concentration of activated complexes on the solution com- position. When the transmission coefficient of the reaction is insensitive to the reaction environment this expression leads directly to the effect of solution composition on the reaction rate.For bulk solutions, this approach is entirely equivalent to that published recently based on the Kirkwood-Buff theory of solutions. For reactions at interfaces the Gibbs adsorption equation plays the same role as the Gibbs-Duhem equation for reactions in bulk and similar arguments apply. These arguments are used to obtain a general expression for the transfer coefficient p in the Butler-Volmer equation of electrode kinetics and for the dependence of /3 on solution composition. A similar treatment can also be applied to the effects on reaction rates of macromolecules which interact with reactants. Finally, the situation in which adsorbed material is not in equilibrium with that in the bulk solution immediately adjacent to the interface is considered and general guidelines for dealing with this type of situation are forwarded.A key element of transition-state theory is the equilibrium hypothesis. In a recent paper' it was argued that when this hypothesis is valid the Kirkwood-Buff theory of s01utions~~~ can be used to provide an expression for the effect of solution composition on the concentration of activated complexes. This in turn leads to the corresponding change in reaction rate provided that any accompanying changes in the kinetic behaviour of the complexes are small. It is believed that this is usually so. Although it is general, the Kirkwood-Buff theory is also somewhat formal. For solutions in which molecules associate to form well defined aggregates alternative thermodynamic treatments which allow for aggregation explicitly are more informa- The application of these theories to calculate changes in rate via changes in the concentration of activated complexes has also been described recently.This new treatment appears to account better for the effects of micellar aggregates on reaction rates than its predecessors. It is fairly straightforward to show that this approach, and that based on the Kirkwood-Buff theory, are entirely consistent with each other and that the former is a special case of the latter. The philosophy underlying both is essentially the same as that used in the successful application of transition-state theory to calculate ionic strength effects on the rates of reactions involving ions.'0*'' In this paper an approach equivalent to that based on the Kirkwood-Buff theory but cast in more traditional thermodynamic terms is developed and applied to reactions at interfaces.The development of the argument provides a justification for some points of T Also at: Unilever Research Laboratory, Quarry Road East, Bebington, Wirral, Merseyside L63 3JW. 18811882 Transition-state Theory at Interfaces reasoning in the work on micellar catalysis cited above. A particular application is to eletron-transfer reactions at the metal/solution interface where an expression is derived for the transfer coefficient in the well known Butler-Volmer equation. Reactions in solution We consider the simple reaction A + B + products in a multicomponent solvent and suppose that the reaction proceeds via a transition state.According to transition-state theory" we may write for the rate per unit volume of the system rate = k*n* (1) where n* is the number density of activated complexes which have formed from reactants and k* is a term which involves quatitites such as (i) the rate at which complexes cross the top of the energy barrier whose height is the activation energy and (ii) the probability that a complex crossing in one direction recrosses in the other. The important features of eqn (1) are that n* is a number density not an activity and that k* is insensitive to the environment in which the reaction takes place so that changes in rate are mainly attributable to changes in n*. Let p denote chemical potential, let the subscripts ijk etc.denote solvent components and let += refer to the transition state. We state the equilibrium hypothesis in thermodynamic language by writing p* = pe * ( T, pi9 PA9 P B ) + RT1n * = + pB (2) and supposing that the activated complexes can be treated thermodynamically in the same way as any normal component which is present in very small amounts. Eqn (2) relates changes in n* to changes in pressure and solution composition. The quantity pe * is a standard chemical potential and, as has been supposed previously,' changes in p** are given by the Kirkwood-Buff theory of solutions. At constant T we write the Gibbs-Duhem equation of our reacting system as dp = ~nidpi+nAdpA+nBdpB+n*dp* i where the n, are number densities. Subtracting d(p*n*) from both sides of eqn (3) we obtain d [p - n*p*] = C ni dpi nA dpA 4- nB dpu, - p*dn* i (3) (4) which gives on cross differentiation ( a ~ * /api)pj, pA, pB, n+ = - (ani/an* ) p i , PA, pB Ian * >pi, pA, pB ( 5 a) (5 b) By writing eqn (5a) and (5b) we are in effect supposing that n* can be varied at constant ninA and nB.This appears to contradict eqn (2). However, when dealing with variations at equilibrium it is not uncommon to use quantities that are defined in terms of variations which do not maintain equilibrium. For example, partial molar enthalpies and volumes of species which participate in equilibria are defined in terms of adding a small amount of the species concerned with the amounts of all other species held constant. ('p * / ' p A ) p i , pB, n = -D.G. Hall 1883 Changes in rate are governed by changes in n*. In particular A In (rate) = A Inn* (C71nn*/~,ui)iij.p~,pB. eq and (2 In n* / Z P . A ) i i i . / i B . eq (6) Hence the quantities we wish to evaluate are derivatives such as where eq denotes that the derivatives are evaluated under the condition that eqn (2) is valid. The first derivative reflects solvent composition effects on the rate, whereas the second derivative gives the influence of changes in activity of reactants. We proceed by regarding n* as a function of pi,,uA,,uR and ,u*. This entitles us to write We note that and that according to eqn (2) We now use eqn (5a) and (5b) to substitute for derivatives of p* in eqn (8a) and (8b), feed the results into eqn (7) and then multiply both sides by (8,u*/&z*)pi,pA,pB, we obtain Since, under the conditions of interest eqn (2) and (6) apply, we find that eqn (10) gives (1 1) Eqn (1 1) is a general result expressing the dependence of the reaction rate of pressure and solution composition at constant T.The derivatives on the right-hand side are easily written in terms of integrals of pair distribution functions. For example, (3) = N? * = ni J: (g'(r) - 1) 4 nr2 dr (12) an+ PtVpAvpB where g'(r) is the pair distribution of i molecules around an activated complex. It is apparent from eqn (12) and (13) that the above treatment is entirely equivalent to that given in ref. (1). Indeed the final expressions differ only in notation. The application to electrolyte solutions and to reactions involving ions is straightforward and can be done in such a way that all thermodynamic quantities refer1884 Transition-state Theory at Interfaces to electrically neutral combinations of ions.All that is required is to replace the pi by the quantities gi and proceed as above. The 9, are defined by where v denotes ionic valency including the sign and c is some ionic species which can be chosen in any way that is convenient. It is usually best not to choose species c as a reactant or a product. Evidently 9, = 0. Also, for uncharged species it is obvious from eqn (13) that Bi = pi. When i and c have opposite signs, gi refers to the chemical potential of the neutral salt formed from i and c where the unit amount is that containing one mole of i. When i and c have the same sign, gi can be expressed as the chemical potential difference between a neutral salt of i and a neutral salt of c, where the units whose chemical potentials are compared contain the same amount of a common ion and again refer to one mole of i.When one or both reactants are ionic we write instead of eqn (2) 9' = Se*(T, gi, 9,, 9,) + RTln n* = 9, + 9,. (14) An equation of this form can be written for any normal species present in small quantities. The set of independent variables we have used so far, namely T and the pi or gi, have the advantage that they enable the relationship between the present approach and that based on the Kirkwood-Buff theory of solutions to be displayed in the simplest way. For many purposes, however, it is more convenient to work with the set of variables T, p and the solute chemical potentials.Thus at constant T and p we may write instead of eqn (3) dpo = - C Ci d9, - C* d9* i + O where Ci = ni/no, C* = n*/no and subscript 0 denotes solvent. We may now proceed exactly as above to obtain the expression where in this case the rate refers to that amount of solution containing one mole of solvent. The various derivatives with respect to C* are at constant T and p , and are related to the molecular distribution functions by In a recent treatment of micellar effects on reaction ratesg similar reasoning to that given above was used to discuss change in the micellar concentration of activated complexes. The treatment given here is more detailed and more thorough and provides the justification for some of the procedures used in the earlier work.Reactions at Interfaces The importance of reactions at interfaces can hardly be overstated. The simplest case to consider is that in which the reaction is sufficiently slow compared with mass transportD. G. Hall 1885 effects that the chemical potentials of all components in the system may be regarded as uniform throughout the system. Let N* be the total number of activated complexes in a system consisting of two bulk phases a and B separated by an interface of area A . For the overall rate in the system we write We note that N * , like the amount of any other species in the system, can be written as rate = k*N*. (18) where Va and V, are the volumes of the two phases, T* is the surface excess of activated complexes and n z and n$ are the bulk number densities.When k* is uniform throughout the system we may also think of an excess surface rate given by surface rate = k*T*A. (20) To formulate expressions for changes in r* at constant T we base our discussion on the Gibbs adsorption equation which we write in the form do =-CTid8i-rAd8A-rBd8B-T'd8* a where ci denotes surface tension and the Ti are chosen in accordance with an appropriate convention such as that used by Parsons12 and Hansen13 which at constant T and p eliminates the dp terms of the principal components of the two phases in contact. In analogy with eqn (2), (8) and (15) we suppose that This expression is entirely in accord with the assumption stated above that the activated complexes can be treated thermodynamically like any other normal species present in very small amounts.For such species the two-dimensional analogue of Henry's law is and it is readily shown that eqn (22) follows from this expression. is quite straightforward. The results are The derivation of expressions analogous to eqn (9) and (1 1) from eqn (22) and (23) and Eqn (24) shows that increasing Oi at constant 8,,8, and 8, increases the surface rate if the adsorption of activated complexes leads to an increase in Ti. Eqn (25) shows how increasing 8, affects the surface rate. In analogy with eqn (12) we have RTd In (surface rate) For reactions which proceed via a transition state, and which conform to the equilibrium hypotheses, eqn (24H26) provide a general, but not exact, treatment of how1886 Transition-state Theory at Interfaces bulk phase composition affects their rates at surfaces.These equations can be used in conjunction with any equilibrium theory or model which enables one to estimate the effect of adsorbing an activated complex on the amounts adsorbed of the other species present. So far we have assumed implicitly that I?* is positive. This covers most situations of practical interest. However, the case where r* is negative may also be treated. For a negatively adsorbed species a present throughout the system in very small amounts we expect k / h , to be independent of Inn, in the limit that n, + 0. Hence if activated complexes can be treated in the same way as normal species eqn (22) should still apply. However, since In T* is not now defined we must replace it instead by In (- r*).The left- hand side of eqn (26) now becomes RTdln(-surface rate) but the right-hand side remains unaltered. When r* is positive, the derivative (Xt/X*)T,ei,e ,e, may be regarded as the amount of i associated with an 'adsorbed' activated compqex in much the same way as the corresponding bulk quantity. In this context association may be used in a positive or negative sense. Thus when represents the amount of adsorbed i displaced by an activated complex. When I'* is negative the interpretation of (Wt/X*) in terms of association or displacement of i is less easily visualised. Comparison with Brensted-Bjerrum Theory According to the Brarnsted-Bjerrum theory,'O the rate of the reaction A + B + products, occurring in solution is given by 'A YA 'B YB rate = k Y* where yA, yB and y* denote activity coefficients defined with respect to an infinite dilution standard state and where k is the rate constant at infinite dilution.It follows from eqn (27) that However, by application of the Kirkwood-Buff theory to a mixture of uncharged species it is readily shown that RTdln(rate) = dpA+dpB-RTd1ny*. (28) RTdlny* = dp*-RTcilnC* When eqn (29) is substituted into eqn (28) we recover the analogue of eqn (16) for a mixture of uncharged species. When one or both reactants are charged, the y terms in eqn (27) refer to individual ionic species but the combination refers to a set of species which is electrically netural. However, eqn (29) no longer applies in a simple way. To recover eqn (16) in this case we proceed instead as follows.We define the quantities f A and J y by writing V* V C 8* = p*( T, p ) + RT In C* - - RTln Cc + RTlnfYwhere D. G. Hall lnfA = In yA --In V A Y c V , 1887 (31 4 (31 b) V* VC l n p = In?* --ln y,. WhenvA/vc is negative, lnfA is (1 - v,/v,) times the mean ionic activity coefficient of the neutral salt formed by A and C. Since v* = V, + v,, it is clear from eqn (31) that we may replace the y terms in eqn (27) by the corresponding f terms. Thus at constant T and p we may write RTd In (rate) = RTd In CAY, + RTd In CBfB -RTd lnf+ (32) but with eqn (30) and (31) this equation becomes V* VC RTd In (rate) = do, +do, +- RTd In C, - RTd lnf* = doA +do, -(do* - RTd In C*). (33) On substituting for the final term of this expression, which according to the Kirkwood-Buff theory is given by dB*-RTdInC* =-F(s) doi-(%) doA-(%) do, (34) ac* ei,eA,eB ac* e,,e,,e, ac* e,,o,,e, we recover eqn (1 6).The analogous expression to eqh (27) at surfaces is where yA is defined by writing and stipulating that yA --+ 1 as rA and all Ti + 0. However, eqn (35) is of limited value unless we have a way of handling changes in the y terms. For bulk solutions this is provided formally by Kirkwood-Buff solution theory. There is, however, no equally convenient formalism for surfaces. To provide such a formalism we may proceed as follows. Suppose we have a species a present which is identical to A in all of its interactions (e.g. a may be a radiolabelled form of A). For such a species we may write (2) dpa = dpA + RTd In (3 7) but pa may be regarded as a function of the variables pipA and r,.Hence we may write Now when a is present in very small amounts1888 Transition-state Theory at Interfaces also we have in general Eqn (40a) and (406). follow straightforwardly from the Gibbs adsorption isotherm. Eqn (37)-(40) enable us to write dpA-RTdInrA = dp,-RTdInr, which after some rearrangement gives The derivatives with respect to r, in this expression have exactly the same significance as the derivatives with respect to T* in eqn (24) and (26). Thus (WA/aTn)pA,p may be particular A molecule in the surface. [Note that the summations in eqn (38)-(42) include B.] For bulk solutions a derivation parallel to that of eqn (42) has been given previously.2 It is apparent from eqn (42) that Similarly for the transition state For charged species we may switch variables from the pi to the Oi in exactly the same way as outlined above for bulk solutions.When this is done eqn (26) is recovered from eqn (35) in exactly the same way as eqn (16) was recovered from eqn (27). A difficulty with eqn (35) which does not arise with eqn (27) is that some of the r terms may be negative. Examples of such species include co-ions adjacent to a charged surface, potential determining ions such as I- at the AgI/solution interface when the crystal is positively charged and electrons at the metal/solution interface when the metal is positively charged. A further difficulty is that activity coefficients for the latter two species cannot be defined with respect to an infinite dilution standard state.In contrast the more general approach described in the preceding section copes readily with such species and circumvents the above problems. When discussing reactions in bulk solution, one is usually interested in the dependence of rates on reactant concentrations. In this context eqn (27) is particularly useful and it is arguable that all the present treatment adds is a formal route to the various bulk activity coefficients. In contrast for reactions at surfaces, variables other than amounts adsorbed are often of interest. Hence, even when all terms in it are well defined and positive, eqn (35) may be less useful than its bulk solution counterpart. The approach developed in the preceding section has greater flexibility. It is not confined to particular sets of variables because these can easily be altered by standard thermodynamicD. G.Hall 1889 manipulations Clearly it amounts to much more than a formal prescription for calculating surface activity coefficients and in some cases it may actually be more convenient to use than more conventional approaches. Electrode Kinetics Typically the reactions involved in kinetics are redox reactions such as which involves the transfer of a single electron. 0 and R, respectively, denote the oxidised and reduced forms of a redox couple such as Fe3+ and Fe2+. Since reactions which involve the transfer of more than one electron usually occur as a sequence of elementary steps involving one or no electrons, and transition-state theory applies to elementary reactions only, the single-electron case is sufficiently general for our present purposes. It is sometimes found in practice that the net rate of reactions such as the above can be described by the well known Butler-Volmer equation.l4 (45) where the current density i is a measure of the net rate per unit area, q is the overpotential, io is the exchange current density which is in effect the equilibrium forward or backward rate when q = 0 and /? is the so-called symmetry coefficient which is expected to be between 0 and 1 and is typically of the order of 0.5. Eqn (45) is the difference between forward and backward rates where the forward rate is given by In pracitice it is sometimes found that p does not depend significantly on q, solution composition or temperature.However, there is no obvious reason why this must be the case. For the reaction 0 + e + R it is straightforward to relate eqn (46) and (26). We identify if with the rate, species A with 0 and species B with electrons. Thus 8, is given by However at constant T and p we may write do, = - F d E where E is the e.m.f. of a cell in which one electrode is the metal of interest and the other is an electrode reversible to species c. Also, since free electrons are not present in significant amounts in the bulk solution, the bulk composition remains constant when 6, is varied. In this case (49) where (ym- vS) is the inner potential difference between the bulk metal and the bulk solution. Eqn (49) relates changes in the overpotential q to changes in 8, at constant T, p and solution composition. do, = - FdE = - Fd(vrn - vS) = - Fdq The surface excess of electrons re is given by re = -q/F where q is the charge per unit area on the metal.1890 Transition-state Theory at Interfaces From eqn (26), (48) and (49) it is apparent that Likewise from eqn (46) we find that When p is independent of q we obtain on comparing eqn (51) and (52) Eqn (35) shows that surface excess of electrons. On electrostatic grounds one might expect is simply related to the effect of the activated complexes on the to be positive for a positively charged transition state in which case we should have p > 1.However, for reactions such as Fe3+ + e -, Fe2+ this turns out not to be the case in practice.14 We conclude therefore that if the assumptions underlying transition-state theory apply to reactions of this kind, then the activated complexes tend to repel electrons in the metal.A possible explanation is that the transition state has an asymmetric charge distribution with the negative part closer to the metal and that this negative part repels electrons to a greater extent than the more highly charged positive part attracts them with its greater net positive charge. The effect of the concentration of reactant A on the rate is given by R T ( Y ) , , = [ 1 + (s) 1. ar* eA,ei,E (54) In the common case where the contribution of A to the overall ionic strength of the system is small, we will have ( 5 5 ) do, = RTd In C, also will usually be very small unless there is either strong specific adsorption of A or, alternatively, association between A and the transition state.The reason for this is that (X,/X*) may be expressed as a volume integral which includes the potential of mean force between the transition state and A together with the number density of A throughout the region of influence of the transition state. If this number density is small, as is the case in the absence of specific adsorption, then the integral is also small unless the potential of mean force is strongly attractive, but this corresponds to association between A and the transition state. Such association should perhaps be included explicitly in the mechanistic equation for the reaction. When (X,/i3T*) is small, eqn (54) predicts that at constant T,p, Oi and E the rate will be proportional to C, as expected.However, eqn (54) also caters for situations in which this simple expectation is not realised.D. G. Hall I891 The effects of solution composition on the rate at constant T , p , E and 8, are described by the expression RT(T) 2 In rate =(s) O j , E Bi, E which follows straightforwardly from eqn (26). In some cases it may be possible to estimate the right-hand side of eqn (56) using double-layer theory. As an illustrative example we consider the case where none of the species i = I -+ c or A are specifically adsorbed and their Ti are given by Gouy-Chapman theory. In this case, for a given bulk composition, the Ti are determined by the primary surface charge q* given by 4* - F = v*r* -re. (57) The activated complexes are included in this equation because they are confined to the interface and count as a specifically adsorbed species.Their contribution to q* is, of course, very small indeed but it is necessary to include them if we wish to form derivatives with respect to r*. If follows that However from eqn (57) it is immediately apparent that hence By calculating (i3ri/c3q*)oi we may obtain an estimate of the left-hand side of eqn (60) which in turn enables us to estimate the effect of changing Oi on the rate. Alternatively, if it is assumed that (c3r,/i3q*),i in the reacting system is the same as (X&) in a solution without reactants, and the latter quantity can be estimated from studies of the double layer, then the left-hand side of eqn (56) may be estimated empirically. However, if i is specifically adsorbed it may interact specifically with the activated complexes in which case the above identification is likely to be invalid.The effects of solution composition on p can also be determined from eqn (26). This expression gives (g) - _ - - ' [ a - ( a ' i ) - 1 a8i ojv 0,. E F aE ar' o,,e,,E B,.e, We note that the derivatives in eqn (61) at const T, p and Bi refer to the situation where activated complexes are ' in equilibrium with reactants '. Under these circumstances any change in r* due to the change in E are entirely neglibible and the quantities can be found simply from the effect of surface charge on the right-hand side of eqn (60). In a recent paper various model-based predictions of (ap/aT) have been discussed and1892 Transition-state Theory at Interfaces compared with experiment.15 An expression for this quantity can be obtained via the approach developed above.The expression concerned relates (t$/aT) to @AS* / d E ) where AS* is the entropy of activation. By so doing it provides a means of estimating this latter derivative but is somewhat sterile as a basis for predicting (8p/i3T). Consequently we do not pursue this issue further. Non-uniform Chemical Potentials So far we have discussed the case of reactions which are sufficiently slow that the reactant chemical potential may be regarded as uniform throughout the system. However there are many instances of reactions at interfaces where transport of one or both reactants to the surface is rate-determining. Conceptually this situation can be dealt with by regarding the overall rate as the volume integral of the local rate and arguing that the equilibrium hypothesis applies locally.This viewpoint is satisfactory provided that the surface may be regarded as in equilibrium with the solution immediately adjacent to it even though there may be diffusive transport of reactants from the bulk. For systems in which there is an energy barrier to adsorption the above situation may not apply. To deal with this case we proceed by regarding adsorbed molecules as separate species from their bulk counterparts which just happen to have equal chemical potentials at equilibrium. Similarly, activated complexes formed from adsorbed reactants may be regarded as separate species from bulk complexes. Thus if the reactive events can take place only between reactants both of which are adsorbed or both of which are in the bulk the overall rate is the sum of two terms involving the two types of complex.For this type of situation, in analogy with eqn (2) and (14), we write the equilibrium hypothesis as ~JA+G = ezu (62 a) 0; + 0; = 6Zb (62 b) where superscripts cz and b refer to adsorbed and bulk material, respectively. The reason for the dual subscripts 00 and bb will emerge as the discussion proceeds. If eqn (22) applies to activated complexes formed from adsorbed reactants and also to activated complexes formed from bulk reactants it follows that we may write where r'20 is the amount per unit area of complexes formed from adsorbed reactants and In general we expect @: to be dependent on T,p, the composition of the bulk solution immediately adjacent to the interface, on f$ and on the chemcial potentials (or surface excesses) of any other adsorbed species which are not in equilibrium with the adjacent bulk solution.Similarly 6; in eqn (63b) can be expected to depend on < and etc. as well as on the intensive properties of the solution concerned. Suppose now that we may also have reaction between adsorbed A and bulk B and between adsorbed B and bulk A. These reaction pathways are not allowed for in the above discussion. To take them into account we write, in anology with eqn (62) and (63), is the surface excess of complexes formed from bulk reactants. where routes concerned. and r'Cu are amounts per unit area of activated complexes formed via theD. G.Hail 1893 Now in analogy with eqn (20) we write for the overall excess surface rate per unit area (65) surface rate = k"[Tzo + I-& + rzb + If we so wish a similar subdivision of complexes may be made for a reaction which proceeds at equilibrium. For this case we may write 0: = e* +RTlnr: (66) where e* depends on T,p, O,, 0, and the other Oi and where r: is given by with the subscript e denoting that surface and bulk are in complete equilibrium. Now in this case we also have (68) 0: == g* = g* - g* = g* It follows therefore that uo bb -- uu bu' which shows how the various t P f are related when surface and bulk reactant chemical potentials are equal. The assumptions underlying this treatment are firstly that it makes sense to talk about an adsorbed state as distinct from a bulk state, and secondly that the chemical potential gradients in the solution immediately adjacent to the interface are such that changes in chemical potential with distance are negligible compared with the range of molecular interactions. However eqn (69a)-(69d) do not apply when surface and bulk reactant chemical potentials are unequal and the various standard chemical potentials depend on 6, and O R ' Application to Solutions of Macromolecules For solutions of non-interacting macromolecules which may bind or otherwise interact with reactants, a combination of multiple equilibrium theory and the Kirkwood-Buff theory of solutions leads at constant T and p to the expression d(0,- RTln C,) = - C ( N i + ri) dgi - N* dO* 1 where Ni denotes the average amount of i specifically bound per macromolecule and Ti denotes the average relative 'adsorbtion ' of i per macromolecule from bulk solution defined according to the convention that the adsorption of solvent is zero.Eqn (70) is in fact almost identical to eqn (34) of ref. (9) which in turn is based on the thermodynamic theory developed in ref. (7) and (8). Apart from the explicit inclusion of the term for the activated complexes, the only other difference arises from the factthat each aggregate contains one, and only one, macromolcule so that C, = C, and Pp =1894 Transition-state Theory at Interfaces NP = 1. Evidently eqn (70) is isomorphic with eqn (21) and can be handled in the same way. Solutions of interacting macromolecules pose additional problems.In principle these can be handled by making use of the general thermodynamic formalism developed in ref. (16) and (8), with the latter adapted as suggested therein for the case under consideration here. Alternatively, fairly dilute solutions of highly charged macro-ions of moderate ionic strengths can be dealt with using the thermodynamic treatment developed in ref. (17) together with the corrections for non-ideality developed in ref. (8). Although as it stands this treatment is only strictly applicable to solutions in which there is only a single counter-ion species, it can be extended to deal with counter-ion mixtures in the same way as the corresponding treatment for micelles has been extended. To deal with particular systems further specific details can be introduced as required.The above discussion provides a tractable framework on which such detail can be built. For example, eqn (1 9) may be applicable to enzyme-catalysed reactions but is unlikely to be useful unless explicit information is available concerning the environment of the active sites of the enzyme molecules. Conclusion The above treatment is neither exact nor in itself predictive. However, it is rigorous within the confines of transition-state theory provided that the effects of environment on the transition coefficient K make only a small contribution to changes in rates. Consequently, any model based on transition-state theory should be consistent with the above treatment unless large changes in IC are involved. The theory thus provides a framework in which models can be developed.It also acts as a brake on plausible but ill founded speculation. The approach can also be used to help interpret data. A good example is (53) which leads to the conclusion that for the reaction considered a positively charged transition state in effect repels electrons. Such a conclusion requires some explanation. However, in the present context the correctness or otherwise of the explanation proposed above is less important than the conclusion itself and the way in which it was obtained. This conclusion can be avoided only by arguing either that transition-state theory does not apply to the type of redox reaction considered or that K is very sensitive to changes in the electrode potential. For reactions in solution the approach developed above is entirely equivalent to the alternative formulation of the same issues based on the Kirkwood-Buff theory of solutions.' The latter approach can also be extended to reactions at interfaces and again the results should be entirely equivalent to those given in this paper. However, the Kirkwood-Buff theory as applied to interfaces, is couched in terms of local number densities which must be integrated across the interface to obtain the overall interfacial rate.The approach based on the Gibbs adsorption equation given above has the advantage that it avoids this complication. Consequently it is much more concise. It is also better suited to deal with discontinuities in chemical potentials arising from activation energies of adsorption. Finally we note that any thermodynamic theory which enables the effects of composition on p** to be calculated can be used in conjunction with the approach developed above to predict the effects of composition on reaction rates. References 1 D. G. Hall, J . Chem. SOC., Faraday Trans. 2, 1986, 82, 1297. 2 J. G. Kirkwood and F. P. Buff, J . Chern. Phys., 1951, 19, 774. 3 D. G. Hall, Trans. Faraday SOC., 1971, 67, 2516.D. G. Hall 1895 4 T. L. Hill, Thermodynamics of Small Sj-stems (Benjamin, New York 1963, 1964), vol. 1 and 2. 5 D. G. Hall and B. A. Pethica, Nonionic Surfhctants, ed. M Schick (Marcel Dekker, New York, 1967), 6 D. G. Hall, Trans. Faraday Soc., 1970, 66, 1351 ; 1359. 7 D. G. Hall, J . Chem. Soc., Faruday Trans. 2, 1972, 68, 1439; 1977, 63, 897; 1981, 77, 1121. 8 D. G. Hall, Aggregation Processes in Solution, ed. E. Wyn-Jones and J. Gormally (Elsevier, Amsterdam, 1983), chap. 2. 9 D. G. Hall, J . Phys. Chem., 1987, 91, 4287. chap. 16. 10 J. N. Brcansted, Z . Phys. Chem., 1922, 102, 109; 1925, 115,537; N. Bjerrum, 2. Phys. Chem., 1924,82, I 1 S . Glasstone, K. J., Laidler and H. Eyring, The Theory of Rate Processes (McGraw-Hill, New York, 12 R. Parsons, Can. J . Chem., 1959, 37, 308. 13 R. S. Hansen, J . Phys. Chem., 1962, 66, 410. 14 J. O’M. Bockris and A. K. N. Reddy, Modem Electrochemistry (McDonald, London, 1970), vol. 2. 15 J. O’M. Bockris and A. Gochev, J. Phys. Chem., 1986, 90, 9232. 16 D. G . Hall, J . Chem. SOC., Faraday Trans. 2, 1974, 70, 1526. 17 D. G. Hall, J . Chem. Soc., Faraday Trans. I, 1985, 81, 885. 108. 1941). Paper 7/00136C; Received 18th September, 1987

ISSN:0300-9599    

DOI:10.1039/F19898501881

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1989, 85(8), 1897-1906 Tin Oxide Surfaces Part 18.-Infrared Study of the Adsorption of very low Levels (2&50 ppm) of Carbon Monoxide in Air on to Tin(rv) Oxide Gel Philip G. Harrison* and Alan Guest Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD Three major types of surface species, unidentate carbonate, bidentate carbonate and carboxylate, are formed when tin(1v) oxide is exposed to dry air containing very low levels of CO (20-50 ppm). All three types appear to be formed immediately and simultaneously upon exposure to the CO-air mixtures, and the abundance of each continues to increase during the period of exposure. Little adsorption occurs at low oxide calcination temperatures when the surface retains a relatively high degree of hydroxylation, but abundances of the three surface species increase markedly a t temperatures > 570 K, and appear to reach a maximum at calcination temperatures of ca.590-610 K , declining thereafter. Although quite stable at lower tempera- tures, the surface bidentate carbonate and carboxylate slowly transform into surface unidentate carbonate at temperatures 2 ca. 400 K under an atmosphere of dry air. The chemisorption and electrical properties of tin(1v) oxide render it an excellent material for the detection of very low levels of carbon monoxide, and several solid-state sensors based on sintered pellets of SnO, are commercially To date, investigations of the mode of operation of such sensors have generally been restricted to studies of the electrical behaviour of the ~ x i d e .~ Little attention has been paid to the subtle relationship which must exist between the surface adsorption phenomena from atmospheres containing CO and the electrical changes thereby induced, afthough this relationship is absolutely critical to the successful operation of the sensor. It has previously4 been assumed that the chemical reactions occurring at the oxide surface involve CO adsorption, desorption of CO, (which produces an increase in conductance), and replenishment of the resulting surface oxygen vacancy by adsorption of molecular oxygen. Our own earlier infrared studies of the chemisorption from CO-0, mixtures onto tin(rv) oxide gel, whilst showing that surface carbonate species are indeed formed, employed gas mixtures far too rich in CO to be applicable to the concentration regime expected in the operation of a sensor (< 100ppm).5 In this paper, therefore, we report infrared studies of the chemisorption from very low levels (20-50 ppm) of CO in air.Experimental Tin(rv) oxide gel was prepared as described previously.6 CO-air mixtures containing nominally 20, 30 and 50 ppm (accurate to within 1 ppm) CO were obtained from Rank-Hilger and purified by passing through a ' U '-tube packed successively with KOH pellets and a glass wool-P,O, mixture. In the text, the term 'dry air' is used to refer to a dried synthetic 0,(20 %)-N2(80 YO) gas mixture. The infrared cell used for these measurements comprised two gas cells of nominally identical pathlength, one positioned in the reference beam and the other in the sample 18971898 Tin Oxide Surfaces beam of the spectrometer, connected by glass tubing and equipped with a matched set of four sodium chloride windows, thus allowing absorptions due to gas-phase species to be compensated for by the spectrometer.To the sample cell was attached a vertical silica tube surrounded by a cylindrical furnace controlled by a ' Digi ' temperature controller (AEI). A winch arrangement permitted the tin(1v) oxide disc sample under examination, which was supported in a gold wire 'W'-cradle, to be raised and lowered between the furnace section and infrared cell as required. In turn, the furnace section was connected to a conventional vacuum system operating at < T0rr.l. In order to eliminate interference from water vapour, the spectrometer was purged for 35 min prior to use and throughout experimental runs by high-purity nitrogen, which was dried by passing through a silica gel column.Thermal pretreatment and the subsequent recording of spectra were usually carried out to a strict protocol so as to permit as great a comparability as possible between runs on different tin(rv) oxide disc samples. The general procedure was as follows: (i) the pressed disc of tin(1v) oxide gel (60 mg) was heated in the furnace section for 16-20 h under a dynamic vacuum of < lo-* Torr at the desired temperature, followed if necessary by a further 2 h at the same temperature in an atmosphere of 2 ppm of pure oxygen; (ii) the disc was lowered into the sample compartment of the infrared cell which was evacuated, and a background spectrum of an average of nine or more scans recorded; (iii) the appropriate gas or gas mixture was then admitted to the system (time zero), and a spectrum of the region 11 50-1750 cm-' was recorded at the ambient beam temperature (ca.329 K) (nine scans taking 13 min); (iv) further spectra (9 scans) were recorded at 20 min intervals for ca. 180 min. Times quoted in the Results section refer to the mid-point of the 13 min scan period. An applied gas pressure of 1 atml was employed for all the experiments with CO-air mixtures; pressure of carbon dioxide are as indicated in the Results section. Infrared spectra were recorded using a Perkin-Elmer 683 spectrophotometer equipped with a 3500 data station.Results Absorption onto Tin(rv) Oxide from Atmospheres containing CO and CO, The same general spectral features were observed for the surface adsorbates formed by adsorption from atmospheres containing either CO or CO, onto tin(rv) oxide under a wide range of conditions. The principal features of the spectra (after subtraction of the oxide background following calcination and, where appropriate, oxygen treatment) comprise broad bands at ca. 1600 and 1440 cm-' together with shoulders on the low- and high-wavenumber sides, respectively, a sharp band at 1223 cm-', and weak broad features at ca. 1380 and 1300 cm-', but their relative intensities varied with the heat pretreatment temperature of the oxide, the application or otherwise of oxygen treatment following heat treatment in vacuo, the composition of the gas phase, and the time of exposure of the oxide to the gas phase. As we have noted previously, exposure of tin(1v) oxide to an atmosphere of pure CO initially gives a similar spectrum rapidly followed by a loss of transmission due to bulk reduction of the oxide.6 The Effect of Increasing Temperature on the Surface-absorbed Species A tin(1v) oxide disc was heated for 17 h at 553 K under a dynamic .vacuum of ca.Torr. The deep-rust-coloured disc was then oxidised by heating in oxygen (20 Torr), again at 553 K, for 2.5 h, after which the colour was canary yellow. The disc was then exposed to an atmosphere of 30 ppm CO in air for 1 h, and the resultant spectrum t 1 Torr z 133.322Pa. 1 1 atm = 101 325 Pa.P.G. Harrison and A . Guest 1899 ' O f 0 'Ai 0 0 \ temperature/K Fig. 1. Plots of absorbance peak heights uerms temperature for the (a) 0 , 1450 cm-', (b) V, 1370 cm-', (c) 0, 1585 cm-', (d) A, 1223 cm-' and (e) A 1300 cm-' bands after adsorption of 30 ppm CO in dry air. Point A on the abscissa scale gives the peak heights of the bands in the original spectrum. The remaining points show the variation of peak height with calcination temperature under an atmosphere of dry air. comprises bands at ca. 1600 cm-', with weaker shoulders on the low-wavenumber side, ca. 1450, 1380, 1300 and 1223 cm-l. Replacement of the CO-containing gas phase by rapid evacuation to a pressure of Torr and admitting 760 Torr of dry air had little effect on the spectrum. However, heating the disc under an atmosphere of dry air resulted in a decrease in the intensities of the 1600 cm-' band and shoulders and the bands at 1300 and 1223 cm-l.Concomitantly, the bands at 1450 and 1380 cm-' increased in intensity. The variation with temperature of the peak height absorbance values for the 1585, 1450, 1370, 1300 and 1223 cm-' bands is illustrated in fig. 1. Absorption from CO-Air Gas Mixtures containing 20, 30 and 50 ppm CO Time-resolved difference spectra over a period of 180 min of the surface species formed on tin(1v) oxide heated at temperatures of 533-613 K from CO-air gas mixtures containing 20, 30 and 50 ppm CO were all qualitatively very similar, and exhibited the same general bands in the 1700-1 150 cm-l region as observed in the previous experiments.More detailed information concerning the growth and relative abundances of the respective surface species was obtained by measuring the absorbance peak heights for the 1585, 1540, 1450, 1300 and 1223 cm-' maxima. Spectra increased in intensity with increasing pretreatment temperature, and increasing time of exposure to the gas mixture, although the greatest changes occurred in the first 60 min of exposure. For all bands the peak absorbance heights were larger for mixtures containing 50 ppm CO. Rates of growth decrease markedly under both gas mixtures after ca. 1 h, and some reduction in1900 Tin Oxide Surfaces intensity of the 1300 and 1540 cm-' bands is observed after relatively long times. Substantially more surface-adsorbed species are formed on tin(1v) oxide which has been heated at temperatures 2 593 K.Pretreatment at lower temperatures gives rise to bands of significantly lower intensity, and in particular the 1223 cm-' is apparent on oxide discs heated and oxygen-treated at 533 K only after quite long exposure times. Discussion The Nature of the Absorbate Species Formed on Tin(rv) Oxide from Atmospheres containing CO and CO, The similarity of the spectra obtained indicate that the same surface species are formed from CO/O, mixtures and CO,, although their relative abundances may vary with the particular conditions of each experiment. The positions of the bands indicate that the surface species are carbonato in nature.' Possible sites for adsorption of CO and CO, on tin(1v) oxide are surface hydroxyl groups, single and bridging surface oxide atoms, and bare Lewis-acidic surface tin atoms? In addition, surface 0; species may be pre~ent.~ Coordination of CO to bare surface tin atoms may be excluded since no bands are observed in the region 2250-1 850 cm-', where matrix-isolated tin carbonyl molecules absorb." Carbon monoxide does adsorb at Sn2+ sites supported on S O , at 128 K giving a band at 2170 cm"," but no significant adsorption occurred at 233 K.Adsorption of CO at bare Zn2+ sites on the surface of zinc oxide has also been observed, which single- crystal studies at 80 K has indicated as through the carbonyl atom.', Interaction with surface hydroxyl groups to form a surface bicarbonato species may also be ruled out since neither the v(0H) nor 6(OH) vibrations are perturbed, and the 1223 cm-' assignment as the 6(COH) of a surface bicarbonate is precluded since it remains unshifted on deuteration.l3 Candidate surface species in the present case are unidentate carbonate, bidentate chelating or bridging carbonate, and unidentate or chelating carboxylate anions. The free C0:- ion D,, symmetry exhibits one Raman-active band at 1063 cm-l [v,(A,), symmetric CO stretching], one infrared-active vibration at 879 cm-' [v,(AL), out-of-plane deformation], and two of E symmetry at 141 5 cm-' (v3, antisymmetric CO stretching) and at 680cm-' (v4), in plane deformation, respectively, which are both Raman- and infrared-active.'* Coordination of the free ion lowers the point symmetry and causes the v3 mode to be split into two bands.The magnitude of the separation of these bands, Au, [not strictly v, but the separation of the higher and lower energy v(C=O) vibrations] may be used to distinguish different modes of coordination of carbonato groups. Values of ca. 100 cm-' are characteristic of unidentate carbonate, values of ca. 300 cm-l to bidentate (chelating) carbonate, whereas values b 400 cm-' are assigned to carbonate bridging two metal centres. The splitting Av3 has also been related to the polarising power of the cation in bidentate c~mplexes.'~ Band growth and desorption characteristics in the present study indicate that the pair of bands ca. 1600 and 1223 cm-' belong to the same surface species, as do the pairs of bands at ca. 1540 and 1300 cm-', and the bands at 1450 and 1380 cm-', i.e.only three general types of surface species are formed. Indeed, an F.t.i.r. spectrum recorded after only 30 s exposure to a 50 ppm CO-air mixture exhibits only six relatively sharp bands (fig. 2). Hence the broad nature of the bands, except for the 1223 cm-' band, and the presence of fine structure in many cases would suggest that several slightly different, but essentially similar, adsorption sites are available. This would be in accord with the rather poorly defined (in crystallographic terms) nature of the oxide surface in the present study . The bands at ca. 1600 and 1223 cm-', which we have also observed previously8,'3 from the adsorption of CO, onto tin@) oxide, we assign to the v,(A ,)[v(C-O,,) + v(C-O,)] and v,(B,)[v(C-O,) + d(0, - CO,,)] modes, respectively, of a surface bidentate carbonateP.G. Harrison and A . Guesl 1901 0.051 carboxylate bidentate carbonate 1600 1400 12oc 0 w avenumtxr/cm-' Fig. 2. F.t.i.r. spectrum recorded 30 s after exposure of tin(1v) oxide to 50 ppm CO in air. (I). The separation of these bands (377 cm-l) is intermediate between the values expected for chelating and bridging carbonato species and substantially higher than examples of purely chelating carbonate {e.g. the complexes [Co(NH,),CO,]X (X = C1, Br, I, NO,, ClO, and SO,), for which the separation is in the range 308-330 cm-' and [Co(en),CO,]Br, for which the separation is 297 cm-1}.16-18 The formation of a sur- face peroxycarbonate (11) similar to the [O,CO] ligand in the rhodium complex Rh(4-MeC6H,)[OOC(0)](PhP(CH,CH,CH,PPh,),], which could be formed by reaction of CO with one surface 0; and a surface-adsorbed oxide, is excluded since C-0 stretching bands characteristic of this species would be expected at ca.1660 and 1255 cm-l.19 The band envelopes at 1500-1 422 and 140 1-1 320 cm-l (separation ca. 100 cm-l) are readily assigned to the antisymmetric and symmetric CO stretching modes of a surface unidentate carbonate (111). Bands due to unidentate carbonate have been observed in several other systems. For example, on La,O, exposed to CO, where the corresponding bands occur at the bands at 1500 and 1390 cm-1,20 whilst the unidentate carbonate ligand in the pentammino cobalt complexes, [Co(NH,),CO,]X (X = C1, Br, I, NO,, and CIO,), which exhibit bands in the ranges 1449-1488 and 1351-1370cm-' with separations of 79-131 cm-1.16-1s1902 Tin Oxide Surfaces The 1560-1 520 cm-' group of bands, which appear as a series of shoulders on the low- wavenumber side of the more intense 158&1600 cm-l bands are associated with the envelope of bands at ca. 1300 cm-l.Bands observed at 1560 and 1330 cm-' on alumina2' and at 1575 and 1342 ern-',,, and 1570 and 1380 ~ m - ' , , ~ on zinc oxide have been assigned to surface carboxylate species. Two types of molecular alkali-metal carboxylates, [M+CO;-], with significantly different spectra, have been discerned in cocondensates of alkali metal with CO, in inert matrices. For example, the lithium complexes exhibit bands at 1755.7, 1750.9, 1221.4 and 1208.7 cm-' for the unidentate (C,) isomer and at 1568.6, 1574.9, 1329.9 and 1304.9 cm-' for bidentate (C2\,) isomer.24 In the present case, therefore, the species giving rise to the observed bands at 1540 and 1300 cm-' is assigned as a surface bidentate carboxylate (IV).0 II The band at 1182 cm-' is always present in the spectra, regardless of whether CO or CO, is the adsorbing gas and of the pretreatment temperature of the oxide. The band was always of low intensity and therefore more prominent for oxide discs pretreated at relatively low temperatures, but was not removed by D,O exchange. A weak band has been observed at 1180 cm-' in the spectra of matrix-isolated molecular alkali-metal carbonates but was not assigned.24 However, bands at 1850 and 1 180 cm-' in spectra of CO, adsorbed on alumina have been assigned to the CO stretching modes of a bridging surface carbonate.Assignment to a similar structure (V) would not be unreasonable in the present case, with the corresponding high-wavenumber mode being the weak band which occurs at ca. 1700 cm-'. Formation of Surface Carbonato Species Any satisfactory model of the adsorption has to account for the following observations : (i) The formation of surface unidentate carbonate, bidentate carbonate, and carboxylate occurs simultaneously, the relative abundances depending upon, particularly, the thermal pretreatment history of the oxide. (ii) All three surface species are formed at oxide pretreatment temperatures of 2 593 K, but very little formation occurs at pretreatment temperatures < 573 K. (iii) The most abundant species formed from CO-0, is surface bidentate carbonate, whilst the most abundant species formed from C0,-0, is unidentate carbonate.? (iv) Surface bidentate carbonate and carboxylate convert into surface unidentate carbonate under an atmosphere of dry air at temperatures > ca.400 K. In order to understand the chemisorption processes, it is necessary to examine the nature of the tin(rv) oxide surface present under the experimental conditions employed. We have previously described reconstructions of the probable available surfaces derived from the [loo], [I 101 and [loll planes of rutile-structure tin(xv) oxide,' and it is likely that these planes also occur in the microcrystalline particles of the tin(xv) oxide gel sample in t Assuming that the extinction coefficients for the major bands for both surface unidentate and bidentate carbonate species are similar.P.G . Harrison and A . Guest 1903 Fig. 3. (a) Model reconstruction of the [loo] surface after total dehydroxylation but no surface oxygen loss. Possible origins of 0, desorption are also indicated (A, loss from geminal bridging-non- bridging ; B, loss from geminal bridging-bridging ; C, loss from adjacent bridging-non- bridging). (6) Model reconstruction of the [ 1001 surface after partial surface oxygen loss. this study. Physisorbed water is lost from the oxide gel by 423 K, and further heating in uamo removes the surface hydroxyl groups in stages, with few hydroxyl groups remaining at temperatures in excess of 673 K8 When heated at moderately high temperatures (> ca.573 K) in uacuo, the oxide also loses oxygen, the oxygen deficiency being at least partially restored by treatment with molecular oxygen. The final result of the dehydroxylation process is shown in along with a reconstruction of the surfaces following subsequent thermal oxygen desorption. Surface deoxygenation must necess- arily occur via associative desorption of adjacent pairs of surface oxygen atoms, which can either be attached to the same surface tin atom or on adjacent tin atoms in the surface (fig. 3-5). Examples of all possible types are shown although not all will occur with equal probability, since desorption energies for different pairs will be different. The resulting oxygen-depleted surfaces will exhibit a greater number of bare surface tin sites.At the ambient temperatures employed in the CO-adsorption studies, adsorption of molecular oxygen at these sites will also produce O;,,, and O& as reactive surface oxygen specie^.^ Thus, the overall picture of the surface of the oxide under the conditions employed in this study is quite complex. However, this picture of the surface assists greatly in rationalising the present observations. The nature of the CO-chemisorption products and the lack of any surface carbonyl species [cf. transition-metal-exchanged tin(1v) oxide gelsz5] indicate the surface oxygen species are the sites for CO adsorption. At low pretreatment temperatures (< 573 K) the surface is still highly hydroxylated, and hence relatively few surface oxide sites are available resulting in little CO-adsorption.At higher temperatures, the abundance of1904 Tin Oxide Surfaces Fig. 4. (a) Model reconstruction of the [loll surface after total dehydroxylation but no surface oxygen loss. Possible origins of 0, desorption are also indicated (A, loss from geminal bridging- non-bridging ; B, loss from geminal bridging-bridging ; C, loss from adjacent bridging-non- bridging). (b) Model reconstruction of the [loll surface after partial surface oxygen loss. Fig. 5. (a) Model reconstruction of the [110] surface after total dehydroxylation but no surface oxygen loss. Possible origins of 0, desorption are also indicated (A, loss from adjacent bridging-non- bridging ; B, loss from geminal bridging-bridging). (b) Model reconstruction of the [110] surface after partial surface oxygen loss.adjacent pairs lating-bridging P.G. Harrison and A . Guest 1905 of surface oxygen atoms increases as does the formation of che- bidentate carbonate : At high evacuation temperatures, the occurrence of isolated surface oxygen species also increases, resulting in the formation of surface carboxylate : That both surface bidentate carbonate and carboxylate transform into unidentate carbonate at moderately high temperatures under an atmosphere of dry air would indicate that unidentate carbonate is the more thermodynamically stable surface species under these conditions. The conversion of surface carboxylate is via an oxidation process at carbon most probably involving a reactive adsorbed O;;a,, species : whilst transformation of bidentate carbonate into unidentate carbonate must just involve a change in the mode of coordination to the metal, perhaps in order to relieve strain in the bridging chelation : A similar process is probably responsible for the production of unidentate carbonate as a primary product from CO adsorption by interaction of a CO molecule with more strained pairs of surface oxygen atoms as, for example, found on the [loll plane.*1906 Tin Oxide Surfaces In contrast, unidentate carbonate is readily formed as a primary product from CO, via adsorption at surface oxygen.o=c=o 0 0 C \.+/ We thank the S.E.R.C. for the award of a research grant. References 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17 18 19 20 21 22 23 24 25 J. Watson and R. A. Yates, Electronic Engineering, 1985, 47.S . Karpel, Tin and its Uses, 1986, 149, 1. J. F. McAleer, P. T. Moseley, J. 0. W. Norris and D. E. Williams, J. Chem. SOC., Faraday Trans. 1, 1987, 83, 1323. H. Windischmann and P. Mark, J. Electrochem. Soc., 1979, 126, 627. P. G. Harrison and E. W. Thornton, J. Chem. SOC., Faraday Trans. 1 , 1978, 74, 2597. P. G. Harrison and E. W. Thornton, J. Chem. SOC., Faraday Trans. 1, 1975, 75, 461. M. Aresta, C. F. Nobile, V. G. Albano, E. Forni and M. Manassero, J. Chem. SOC., Chem. Commun., 1975, 636. P. G. Harrison and A. Guest, J. Chem. SOC., Faraday Trans. I , 1987, 83, 3383. J. P. Joly, L. Gonzalez-Cruz and Y. Arnaud, Bull. SOC. Chim. Fr., 1986, 11. A. Bos, J. Chem. SOC., Chem. Commun., 1971, 26. B. Rebenstorf and R. Larsson, Acta Chem. Scand., Sect. A , 1980, 34, 239. K. L. D’Amico, M. Trenary, N. D. Shinn, E. I. Solomon and F. R. McFeely, J. Am. Chem. Soc., 1982, 104, 5102. P. G. Harrison and B. Maunders, J . Chem. SOC., Faraday Trans. I , 1984, 80, 1357. G. Hertzberg, Infrared and Raman Spectra of Polyatomic Molecules (Van Nostrand, New York, 1945), p. 178. J. P. Jolivet, Y. Thomas and B. Taravel, J. Mol. Struct., 1982, 79, 403. K. Nakamoto, J. Fujita, S. Tanaka and M. Kobayashi, J. Am. Chem. SOC., 1957, 79, 4904. J. Fujita, E. Martel and K. Nakamoto, J. Chem. Phys., 1962, 36, 339. J. A. Goldsmith and S. D. Ross, Spectrochim. Acta, Part A, 1968, 24, 993. L. Dahlenburg and C. Prengel, Organometallics. 1984, 3, 934. M. P. Rosynek and D. T. Magnuson, J. Catal., 1977, 48, 417. L. H. Little and C. H. Amberg, Can. J. Chem., 1962, 40, 1997. J. H. Taylor and C. H. Amberg, Can. J. Chem., 1961, 39, 535. S. Matsuchita and T. Nakata, J . Chem. Phys., 1962, 36, 665. Z. H. Kafafi, R. H. Hauge, W. E. Billups and J. L. Margrave, J. Am. Chem. SOC., 1983, 105, 3886. P. G. Harrison and E. W. Thornton, J. Chem. SOC., Faraday Trans. I , 1979, 75, 1487. Paper 8/01 11 1G; Received 18th March, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501897

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I , 1989, 85(8), 1907-1919 Tin Oxide Surfaces Part 19.-Electron Microscopy, X-Ray Diffraction, Auger Electron and Electrical Conductance Studies of Tin(rv) Oxide Gel Philip G. Harrison* Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD Martin J. Willett British Coal, HQ Technical Department, Ashby Road, Stafordshire DE15 OQD Physical and electrical properties of both unsintered and sintered tin(rv) oxide gel of high specific surface area and porosity are reported. Particle sizes range from 3 to ca. 5000 nm, the larger bodies apparently consisting of pressed agglomerations of smaller particles. The 3 : 1 0: Sn atomic ratio determined at the surface of the discs was consistent with a fully hydrox- ylated oxide surface.A slight oxygen deficiency from the ideal SnO, com- position is observed at all depths. Sintering for extended periods at 1273 K increased the minimum particle size from 3 to ca. 40 nm without causing any appreciable change in morphology on the micron scale. The electrical conductance of the unsintered oxide in air as a function of temperature was found to be extremely complex and exhibits hysteresis dependent upon the history of the sample. Unsintered discs initially displayed a sigmoid relationship between the conductance G and tem- perature T, with G typically reaching a maximum at 420 & 20 K, followed by a minimum at 520 +_ 50 K, thereafter increasing monotonically. Subsequent G-T cycles produced a wide range of behaviour. Although in general, the G-T characteristic exhibited a more linear form, the rate of change with temperature varied greatly.The initial sigmoid curve was largely unaffected by strict control of the water content of the test atmosphere, and hysteresis effects generally persisted, despite the adoption of more rigorous experi- mental methods. The air conductance behaviour of presintered samples was typically less complex, with an increasing tendency towards a monotonically rising G-T characteristic with extended duration of the sintering treatment. Significant differences were observed between the electrical behaviour of the unsintered tin(rv) oxide gel samples and that generally exhibited by ' commercial ' semiconductor gas sensors. However, these differences could be minimised by presintering of the oxide gel.The development of cheap solid-state electronic devices for the detection of toxic or flammable gases is a subject of prime importance in many areas. The electrical properties of tin(rv) oxide render it a n excellent material for the detection of very low levels of carbon monoxide, and several devices based on sintered pellets of SnO, are commercially available. '* * To date, investigations of operation of such sensors have generally been restricted t o studies of the electrical behaviour of the o ~ i d e . ~ - ~ Little attention has been paid t o the subtle relationship which must exist between the surface adsorption phenomena and the electrical changes thereby induced, although this intrinsic link is absolutely critical t o the successful operation of the sensor.Previous studies have demonstrated that a model in which the sensor conductance is effectively controlled by the population of negatively charged oxygen adsorbates is consistent with the observed 19071908 Tin Oxide Sutfaces beha~iour.~ The chemical reactions involved in the conductance mechanism, although unsubstantiated, have been assumed to involve CO adsorption, desorption of CO, (which produces an increase in conductance), and replenishment of the resulting surface oxygen vacancy by adsorption of molecular oxygen.’ As part of a programme designed to investigate more fully the mechanism of operation of tin(rv) oxide-based gas sensors, and particularly to elucidate the relationship between the nature of the adsorbate species and bulk oxide electrical conductance, we have already6 carried out infrared studies which have characterised the species formed on the surface of tin(rv) oxide from very low levels of carbon monoxide in air.However, the oxide employed in that study, tin(rv) oxide gel of high surface area, is substantially different in physical nature from that usually employed solely for electrical studies3-’ or used in commercial gas-sensing devices.’q2 We therefore have found it necessary to characterise the physical and electrical properties of the type of oxide used in our studies, and the results are described in this paper. Experimental Preparation of Tin(rv) Oxide Gel Tin(1v) oxide gel of high specific surface area was produced by the hydrolysis of redistilled tin(1v) chloride as described previ~usly.~ Great care was taken to ensure the purity of all materials and apparatus used in the preparation, and the quality of the final product was checked by X-ray fluorescence analysis using a Philips XRS PWI400 spectrometer.No significant impurities were detected. Small quantities (ca. 0.5 g) were ground for 6 min in a ‘Grindex’ agate ball mill grinder (Research and Industrial Instrument Co.) immediately prior to pressing into self-supporting discs (60 mg, 25 mm diameter) using a pressure of 34.5 MN m-,. Presintering of discs was performed under ambient atmospheres at a temperature of I273 & 20 K in a silica boat. After sintering, the samples were virtually indistinguishable in colour from untreated discs, there being a creamy hue in some cases, but were quite brittle. The surface of the oxide appeared shiny in comparison with the relatively matt as-pressed samples.There was also evidence of some shrinkage (ca. 10 YO) in the physical dimensions of the samples as a result of the treatment. Physical Characterisation Electron-microscopic studies were carried out using a Pye Stereoscan 600 scanning electron microscope, which has a quoted resolution of 10 nm. However, the insulating nature of the oxide prevented meaningful observations below ca. 50 nm. Auger electron spectroscopy was performed by Loughborough Consultants Ltd. Depth profiling was performed by using argon-ion bombardment. Owing to the particulate nature of the discs, the absolute depth values are only approximate, although intercomparison between the samples is valid.The quoted uncertainties in the 0:Sn ratios represent & 5 %, which was the ‘worst-case’ estimate of the error. Electrical Resistance Measurements Two methods of sample resistance measurement were employed depending upon whether the measurements were obtained under a flowing ambient atmosphere or under a rigorously controlled atmosphere. For both methods, the pressed disc oxide samples (ca. 2-3 mm across) were mounted on self-heated Platfilms (Rosemount Ltd.), which were spot-welded on to nickel-plated TO5 headers (Wesley Coe Ltd). Temperature calibrations of the Platfilm as a function of heater current were accomplished by attaching a Pt/Pt-lO% Rh thermocouple fabricated from 0.05 mm diameter wire toP. G . Harrison and M .J. Willet 1909 Table 1. Atomic 0 : S n ratios from AES analysis calcination pretreatment/h at 1273 K _________________~___ depth/nm 0 72 168 256 0 2.86k0.14 2.86f0.14 3.03k0.15 3.06k0.15 4 1.95+0.10 2.02k0.10 2.02+0.I0 2.05+0.10 13 1.92k0.10 1.96k0.10 1.98k0.10 2.01 kO.10 25 1.88k0.09 1.98k0.10 1.97+_0.10 2.00+_0.10 100 1.88 rfI0.09 1.89 k0.09 1.95 rfIO.10 1.98 & 0.10 200 1.86+0.09 1.84kO.09 1.92k0.10 1.97kO.10 400 1.87k0.09 1.98k0.09 1.90k0.10 1.95k0.10 the centre of the upper tile surface using gold paste ('Hanovia' A1644 liquid gold, Engelhard Ltd). Gold paste was also employed in the fabrication of all electrical contacts to the pressed discs. Contact resistances were at least two orders of magnitude below the minimum resistance values expected for the oxides, and were therefore neglected.Electrical resistance measurements under ambient atmospheres (flow rate ca. 50 cm3 min-') were made, employing a simple series load resistor circuit. The voltage drop across the load was measured using a Keighley Model 616 electrometer. In order to minimise problems due to polarisation of the samples, particularly those exhibiting high resistances, the supply voltage was connected to the oxide sample under study for only a few seconds to allow the measurements to be completed, and the polarity of the supply voltage was reversed at random. No significant errors due to polarisation of the samples were noted. This method also minimised any self-heating effects due to the measurement current. Measurements under rigorously controlled atmospheres were achieved mounting samples in a vessel connected to a conventional vacuum frame and using dry bottled 0,(21 %)-N,(79 O/O) mixtures (purified by P20, and KOH filters prior to entry to the test apparatus).The sample temperature in these tests was controlled by an automatic heater circuit which produced a current cycle with a triangular profile and a period of 48 h, providing a mean heating/cooling rate of ca. 27 K h-'. The oxide sample resistance was obtained using a circuit based on a logarithmic amplifier (Intesil 8048). By comparing the internal current flowing through the unknown resistance with an externally generated reference current, the logarithmic amplifier produced a signal directly proportional to the resistance of the oxide.The system provided an output of 1 V per decade of resistance, and was capable of handling over six decades of current input. A 1 kR resistor in series with the oxide sample limited the current flowing through it to < 0.5 mA, ensuring minimal self-heating. Calibration curves were obtained by using a range of known resistors in place of the oxide. Errors caused by variations in the reference current and the stabilised voltage supply were insignificant compared with the measured range of the device, providing automatic monitoring of properties over a wide range. Using this arrangement, three samples could be examined concurrently under reproducible atmospheric and thermal conditions. Results Physical Characterisation Electron Microscope Studies The unsintered ground powder contained a very wide range of particle sizes, between < 1 pm and cu.200 pm. The large particles appeared to be pressed agglomerates of 64 F A R I1910 Tin Oxide Surfaces smaller particles, and were covered in networks of fine cracks. Comparison with similar photographs illustrating material prepared 7 years earlier indicated no significant differences in the appearance of the powder on the micron scale due to aging. The surface of an unsintered 60 mg pressed disc consisted of relatively smooth areas of irregular shape ca. 1-2pm across, separated by more broken regions where the particle size appeared to be @ 1 pm. The overall impression was of a relatively flat but discontinuous surface above a porous structure. Viewed in profile, however, the surface is quite uniform, although the porous body of the disc comprised a range of particle sizes.The larger (ca. 30 pm) particles appeared to have fractured by cleaving, indicating a brittle nature, but there was no evidence of these protruding up through the surface, since the thickness of any given sample was found to be quite uniform (typically 43+ 1 pm for an unsintered 60 mg disc). There was, however, a large variation (ca & 5 pm) between the thicknesses of nominally identical discs fabricated from the same powder, demonstrating the irreproducibilities inherent in the pressing process. Examination of discs presintered for various periods indicated that there was little change in the appearance of the oxide or the cross-section due to the extended period at high temperatures.There was not an unambiguous correlation between the oxide disc thickness and the duration of the presintering treatment, although, in general, sintered discs tended to be thinner than unsintered ones. Discs produced from 120 mg rather than 60 mg of oxide powder did not have double the thickness after an identical pressing treatment. The increase was typically only ca. 50 YO, presumably indicating an increased packing density or reduced porosity within the disc body. In all other respects, the appearance of the two types was found to be quite similar. X- Ray Difr ac t ion Studies' Minimum crystallite sizes were estimated from measurements of the full width at half- maximum height parameter of the pressed discs using the [I 101 peak at 28 = 26.7'.The minimum crystallite size in the unsintered material is ca. 3 nm, but increases to ca. 36 nm following sintering for 72 h at 1273 K in air. Sintering for 168 h at 1273 K resulted in a smaller increase to ca. 45 nm. These values are similar to the data of Kittaka,g whilst Fuller" quotes crystallite sizes of 4-5 nm for tin(1v) oxide gel prepared by an analogous route. Auger Electron Spectroscopy The AES data confirmed the purity of the as-pressed oxide, whilst the sintering process resulted in some surface contamination (1-2 atom YO of silicon and potassium was typically detected perhaps by contact with the silica boat used to support the samples in the furnace). However, there was no evidence that these impurities had penetrated into the porous body of the disc.Atomic 0 : Sn ratios as a function of depth and the length of presintering are presented in table 1. The depth profiles of the 0: Sn ratio indicated that the value at any given depth was only weakly affected by increased length of sintering. There was a slight tendency for the 0: Sn ratio to rise with the period of presintering at 1273 K, although values covering pretreatments of between 0 and 256 h all fall within the same range of experimental uncertainty. In each case, the results at a depth of 400 nm indicated a slight oxygen deficiency as compared with the idealised SnO, composition, but there was no evidence for high concentrations of other oxides, e.g. SnO. In contrast to these relatively minor effects, the change in the 0: Sn ratio with depth in a given sample was highly significant. The ratio typically fell from ca.3 at the surface to ca. 2 at a depth of ca. 4 nm, with only very small changes occurring on further etching to 400 nm. For the sintered samples, some of the relative increase in the oxygen signalP. G. Harrison and M. J . Willet 191 1 0.0 h 2 u, -5.( on - - 10.1 . - . . * 9 0 .0 I I oxide temperature/K Fig. 1. oxide temperature /K Fig. 2. Fig. 1. Ambient air conductance-temperature characteristics of (a) a pressed disc of tin(1v) oxide and (b) a SMRE-type" sensor (temperature varied randomly). Fig. 2. Effect of heating rate upon ambient air conductance-temperature characteristics of tin(1v) oxide pressed discs (steadily rising temperature). ( a ) 24 h per temperature; (b) 1 h per temperature.at the surface could be attributed to the presence of oxygen-containing impurity compounds (related to the silicon and potassium signals already mentioned), but the comparable rise in the 0: Sn ratio of the unsintered (and apparently uncontaminated) sample indicated a second effect. Given the relatively constant value of the 0:Sn ratio at all other depths, oxygen-tin segregation at the unetched surface was not thought to be likely. The effect has therefore been attributed to the presence of adsorbed H,O- or 0,-derived groups within the analysis region, which are almost completely desorbed due to thermal effects during the first etching period. It is interesting to note that a 3 : 1 0: Sn atomic ratio is the value which would be expected from a fully hydroxylated tin(rv) oxide surface.* Electrical Characterisation Results of Ambient Atmosphere Studies Preliminary studies of the electrical conductance of SnO, pressed discs were performed by leaving the sample for approximately 1 week at each temperature and making electrical measurements at regular intervals during this period. On the assumption that 64-21912 Tin Oxide Surfaces I . n . . ... ;. ... . ' * a .. . * .,: - : . .. 2 u - -5.0- M 0 -10.0 I I 250 650 65 0 1050 I I oxide temperature/K oxide temperature /K Fig. 3. Comparison of ambient air conductance-temperature characteristics of various Sn0,- based sensors (steadily rising temperature) (all axes are identical). (a) SMRE-type; (b) Taguchi gas sensor type 71 1 ; (c) Taguchi gas sensor type 812; ( d ) Taguchi gas sensor type 813.the disc properties would have stabilised within this time, the sample temperature was varied at random within the range 290-890 K to avoid the introduction of effects due to the extended exposure to progressively greater temperatures. As the examples of mean sample conductance in fig. 1 show, two main conclusions can be drawn : no clear pattern in the conductance-temperature distribution emerges for the pressed-disc samples, and the behaviour exhibited by an SnO, SMRE-type sensor'' tested under the same conditions was significantly different from that of the pressed disc type. Fig. 2 illustrates the mean conductances of two sintered pressed-disc samples heated from room temperature to ca. 950 K at different rates: (i) taking 10 days to traverse the range, i.e.ca. 24 h at each temperature setting, and (ii) taking 3 days to traverse th'e range, i.e. taking ca. 1 h at each setting. In both cases, the sample was left overnight at elevated temperatures (ca. 16 h). As the trace for protocol (ii) shows, this resulted in significant drifting of the measured conductance value, particularly at intermediate temperatures. However, both methods illustrated a clear pattern in the conductance-temperature distribution which was not observed under random thermal cycling. A conductance maximum at ca. 420 K was followed by a minimum (slightly below the room temperature value) around 550 K. Thereafter, the conductance was found to rise monotonically up to the highest temperature studied, typically reaching a value between these extremes.The overall shape of the distribution could therefore be described as sigmoid, and although significant variations in the absolute conductance levels were exhibited by nominally identical samples, the same general curve was observed without exception. The two different thermal protocols were subsequently employed in various types of experiment : (ii) was particularly suitable for studying the effect of other parametersP. G. Harrison and M. J. Willet O.O h r2 9 - 5 . 0 - eo - 1913 (b) .. . . ..* . = . * 0 . * 0 0 0 0 0 0 0 0 0 0 . 0 0 O -7.0 - I o - * o o , , -10.0 250 650 1050 oxide temperature/# Fig. 4. -2.01 -12.01 I I8 oxide temperature/K oxide temperature/K Fig. 5. Fig. 4. Extreme examples of ambient air conductance hysteresis of (nominally identical) unsintered 60 mg pressed disc samples., rising temperature ; 0, falling temperature. Fig. 5. Typical examples of ambient air conductance hysteresis of SnO, pressed discs. (-) Rising temperature ; ( - . - a -) falling temperature) (all axes are identical). Presintering at 1273 K for (a) 0, (b) 4, ( c ) 14, ( d ) 24, (e) 48 and (f) 72 h. upon the conductance-temperature relationship, whilst (i) was used in examining the changes caused by exposure to gas mixtures. Attempts were also made to traverse the range much more rapidly, completing a full 290-+950-+290 K loop in a period of ca. 8 h and then leaving the sample at room temperature overnight. This means a thermal cycling rate of ca. 200 K h-l produced the same general behaviour as illustrated in fig.2, but the period spent at each temperature could not be reduced below 1 h without causing unacceptable inaccuracies in the measured conductance values, due to relatively slow equilibration of the samples. A brief comparison of the conductance-temperature distributions of various sensor types (including commercial Taguchi Gas Sensors) under protocol (ii) confirmed that widely different behaviour was exhibited by the various samples. In some cases, there was also significant drifting during the overnight (ca. 16 h) period. A detailed analysis of these variations (see fig. 3) was not attempted, since investigation of the pressed-disc samples was of greater significance to the present study, but reference will be made to the results later,Tin Oxide Surfaces -2.0' h Ez 2 -7.0 M - ' i * i ' : ..I I I ! : . ! I , : * . . 0.0 h Ez $ -5.0 M - - 1o.c -10.0 0 40 80 250 650 1050 oxide temperature/K - 12.0 1273 K presintering period/h Fig. 6. Fig. 7. Fig. 6. Effect of sintering period upon ambient air conductance of 60 mg pressed discs measured at (a) 293 and (6) 935 K. Fig. 7. Effect of repeated cycling under dry 0,-N, upon conductance hysteresis curves for a 60 mg pressed disc : unsintered sample (-) Rising temperature ; (- .-. -) falling temperature. (a) 1st cycle; (b) 4th cycle. When the same protocol was used for longer experiments in which the sample temperature was raised from room temperature to ca. 950 K and then reduced, significant hysteresis in the conductance distributions was observed, as the examples shown in fig.4 demonstrate. During tests on more than 30 samples, the behaviour in the falling-temperature arm of the loop was found to be very unpredictable, as the two extremes in this figure show. No change in the test conditions which would account for these fluctuations could be identified. The similarities in the rising-temperature arms of the loop for nominally identical samples implied that the hysteresis was attributable to the effect of high temperatures. Fig. 5 shows typical examples of the hysteresis loops obtained in experiments on samples presintered in air at 1273 K between 0 and 72 h. For clarity in this and subsequent figures, the loops are represented by line plots linking the points (ca. 30) in each arm. Broken horizontal portions of the curves at low temperatures indicate regions where the sample conductance was below the working range of the electrical measurement system.As already emphasised, there was great variation in the behaviour within a batch of nominally identical sensors, particularly those subjected to little or noP. G. Harrison and M . J. Willet 0.0 1915 - - 1 o.o/ h rz - M -10.0 :-5.0-{-N -.-.--./ I - - - I I - 10.01 I J 0.0 sintering. The examples shown have been selected from the results of more than 100 tests, in order to convey a general impression of the trends observed. With increased sintering time, there was a tendency for the absolute conductance at room temperature to fall, whilst the pronounced sigmoid shape of the rising-temperature curve was gradually flattened.However, the conductance at higher temperature was affected only very slightly by the presintering, The degree of hysteresis was greatly reduced in samples which had been presintered for more than ca. 24 h, and for the 72 h examples the uncertainty in the measurements on the increasing temperature arm was of the same order as the differences between the two halves of the loop (i.e. no significant hysteresis was observed). Fig. 6 illustrates the effect of increased sintering periods upon sample conductance at 293 K and ca. 935 K (data for increasing temperatures). Even allowing for the wide scatter in the data (note the logarithmic ordinate scale in this figure), there is a clear difference between the two sets of data, confirming the previous comments.In an attempt to reduce the scatter observed in the results (from unsintered samples in particular), tests were performed on discs prepared using 120 mg of tin(rv) oxide gel, i.e. of double weight. It was thought that if microcracking of the discs were responsible1916 0.0 h 2 $ -5.0 M - - - --- 0.0 Tin Oxide Surfaces (4 r -10.01 I I -10.01 I I 250 650 1050 oxide temperature/K Fig. 10. Effect of repeated cycling under dry 0,-N, upon conductance hysteresis curves for a 60 mg pressed disc: 72 h presintering at 1273 K. (-) Rising temperature; (.-.-) falling temperature; (--) limit of measurements. (a) 1st cycle, (b) 4th cycle. for the unpredictable behaviour, increasing the sample thickness might improve the reproducibility. However, no noticeable improvement was found, and the effects of sintering upon the behaviour of 120 mg discs were similar to those already illustrated in fig. 5 and 6 for 60 mg samples.Results of Controlled Atmosphere Studies Fig. 7-1 0 inclusive show the first and fourth complete rising/falling temperature traverses of sensors constructed using disc material subjected to 0, 24, 48 and 72 h of presintering at 1273 K in air. Again, data have been selected from tests on many sensors to convey a general impression of the trends observed. The aim of cycling through the temperature range more than once was to provide data on the degree of hysteresis in the conductance behaviour during repeated testing. Unless stated, all of the cycles were performed under the same sealed atmosphere. The data for unsintered samples were again found to exhibit significant scatter, although the general impression was of slightly increased reproducibility over the ambient atmosphere tests.There were fewer cases in which the falling temperature arm of the first cycle resembled the conductance-temperature behaviour usually associated with a presintered sample (i.e. a very large degree of hysteresis). This observation may be partly attributed to the reduced time spent at temperatures towards the top of the range under automatic, as opposed to manual, heater control. The results showed thatP. G. Harrison and M . J. Willet 0.0 h rz 0, -5.0 w - 1917 - - -10.0 I -10.0 250 650 1050 oxide temperature/K Fig. 11. Effect of evacuation-re-dosing on conductance-temperature characteristic of a 60 mg pressed disc [dry 0,(21 %)-N,(79 %) atmosphere switched between first and second traverses].(a) 1st traverse, low -+ high temperature; (b) (-) 2nd traverse, low + high temperature; (-.-.) 3rd traverse, high -+ low temperature. hysteresis in the conductance loops persisted throughout multiple thermal cycling. The falling temperature arms exhibited little or no evidence of a peak at around 420 K and a trough near 650 K. This behaviour contrasted with the strongly sigmoid curve observed for the unsintered material as the temperature was raised, as illustrated in For presintered samples tested under controlled atmospheres, the room-temperature conductance fell sharply with increased sintering time, whilst towards the upper end of the measuring temperature range there was relatively little variation.There was no significant effect on the degree of conductance hysteresis due to repeated thermal cycling. Increasing the degree of presintering generalIy reduced the extent of the hysteresis, as was found for tests under ambient atmospheres. To further investigate the sigmoid air conductance curve, a test was performed in which an unsintered sample was run up through the temperature range to ca. 900 K and then subjected to a brief evacuation (ca. 0.2 Pa) whilst cooling to room temperature. The sample cell was then re-dosed with dry 0,(21 %)-N,(79 O h ) and another full thermal cycle performed. The results of this experiment are shown in fig. 1 1, and indicate that, whilst there are differences between all three traverses of the temperature range, no significant effect may be attributed to the change in gas atmosphere.Again, the large degree of hysteresis in the electrical properties prevents unambiguous interpretation of the data. fig. 7.1918 Tin Oxide Surfaces Discussion Physical Nature of the SnO, Discs The as-pressed discs consisted of pure tin@) oxide gel with a high specific surface area and a porosity of ca. 60% (assuming a thickness of 4.5 x m and a diameter of 25 mm for a 60 mg sample). The chemical composition appeared homogeneous, with a slight oxygen deficiency at all depths. On heating, there was a rapid and significant loss of both mass and surface area, the former being assumed to be due to the loss of adsorbed water and surface hydroxyl groups in a variety of configurations.* Sintering in ambient air for extended periods at a temperature representing a significant fraction of the Tamman temperature increased the minimum particle size by at least one order of magnitude without causing any appreciable change in morphology of the disc on the pm scale.There was also evidence for a slight reduction in the oxygen deficiency within the lattice as a result of sintering, in agreement with the findings of Peterson,” who studied the effect of treating SnO, at 1323 K under oxygen atmospheres. Thermodynamic calculations performed by Peterson indicated that the equilibrium SnO(s) + +O,(g) -+ SnO,(s) (or the oxygen deficiency of the SnO,-, lattice) under various conditions could be used to rationalise the decrease in room temperature conductance of sintered as opposed to unsintered samples.Electrical Behaviour The electrical conductance of the as-pressed oxide in air as a function of measured temperature was found to be extremely complex and prone to significant hysteresis, dependent upon the history of the sample. On the first traverse of the range, unsintered discs displayed a sigmoid relationship between the conductance G and temperature T, with G typically reaching a maximum at 420+20 K, followed by a minimum at 520+ 50 K, increasing monotonically thereafter. Subsequent decrease of the temperature produced a wide range of behaviour. Although in general, the G-T characteristics exhibited a more linear form, the rate of change with temperature varied greatly. The initial sigmoid curve was largely unaffected by strict control of the water content of the test atmosphere, and hysteresis effects generally persisted, despite the adoption of more rigorous experimental methods.The air conductance behaviour of presintered samples was typically less complex, with an increasing tendency towards a monotonically rising G-T characteristic with extended duration of the sintering treatment. It is clear that there were significant differences between the electrical behaviour of the unsintered samples examined here, and that generally exhibited by ‘commercial ’ semiconductor gas sensors. However, these differences could be minimised by presintering of the oxide gel, which is perhaps not surprising since a sintering stage at temperatures well above the normal operating point is invariably a feature of the manufacturing of such devices.Gregg13 has discussed in detail the role of sintering in activated solids in some detail, noting that at relatively small fractions (ca. 30%) of the Tamman temperature ( qam), a surface diffusion is likely to reduce the number of defects in the near-surface crystal structure of the oxide without significantly altering the total pore volume. Goodman and Gregg14 have noted that for tin(rv) oxide, although the specific surface area is reduced by ca. one order of magnitude by sintering at 773 K due to surface diffusion (or, as suggested previously by us,’ by surface hydroxyl condensation), the total pore volume is virtually unaffected until qam is reached, when it collapses suddenly.These observations are consistent with the present findings ; treatment well below qam caused extensive losses in specific surface area without a largeP. G. Harrison and M . J. Willet 1919 fall in the porosity of the discs (as estimated from thickness measurements). No difference in the macroscopic morphology of the samples as a function of sintering time was detected despite the increase in minimum particle size. This is presumed to be because the plastic or viscous flow of particles during sintering occurs on a dimensional scale at least one order of magnitude below that resolved with the S.E.M. employed here. Therefore, the sintered discs suffered a large reduction in microscopic surface area, whilst retaining an open, porous macroscopic structure. These electrical data are discussed more fully in the following paper. M. J. W. is grateful for the support provided by British Coal during the course of this project, and for permission to publish this paper. The views expressed are those of the authors and not necessarily those of the British Coal Corporation. We thank Mr R. Hayward of British Coal for the electron microscopy studies and Mr B. Bellamy of the Materials and Surface Science Group, AERE, Harwell, for the XRD measurements. We thank S.E.R.C. for the award of a Research Grant. References 1 J. Watson and R. A. Yates, Electronic Engineering, 1985, 47. 2 S . Karpel, Tin and its Uses, 1986, 149, 1. 3 J. F. McAleer, P. T. Moseley, J. 0. W. Norris and D. E. Williams, J. Chem. SOC., Faraday Trans. 1, 4 J. F. McAleer, P. T. Moseley, B. C. Tofield and D. E.Williams, Proc. Br. Ceram. Soc., 1985, 36, 89. 5 H. Windischmann and P. Mark, J . Electrochem. Soc., 1979, 126, 627. 6 P. G. Harrison and A. Guest, J . Chem. SOC., Faraday Trans. I , 1989, 85, 1897. 7 E. W. Thornton and P. G . Harrison, J . Chem. SOC., Faraday Trans. 1, 1975, 71, 461. 8 See also P. G. Harrison and A. Guest, J . Chem. SOC., Faraday Trans. I , 1987, 83, 3383. 9 S. Kittaka, K. Morishige and T. Fujimoto, J. Colloid Interface Sci., 1979, 72, 191. 10 M. J. Miller, M. E. Warwick and A. Walton, J. Appl. Chem. Biotechnol., 1978, 28, 396. 11 U.K. Patent 1374375, 1974. 12 A. F. Peterson, Ph.D. Thesis (Rutgers State University, 1968). 13 S . J. Gregg, The Surface Chemistry of Solids (Chapman and Hall, London, 1961). 14 J. F. Goodman and S. J. Gregg, J. Chem. Soc., 1960, 1162. 1987, 83, 1323. Paper 8/01 165F; Received 21st March, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501907

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. 1, 1989, 85(8), 1921-1932 Tin Oxide Surfaces Part 20.-Electrical Properties of Tin(rv) Oxide Gel : Nature of the Surface Species Controlling the Electrical Conductance in Air as a Function of Temperature ’ Philip G. Harrison* Department of Chemistry, University of Nottingharn, University Park, Nottingham NG7 2RD Martin J. Willett British Coal, HQ Technical Department, Ashby Road, Stagordshire DE15 OQD The mechanisms controlling the electrical conductance of tin(1v) oxide gel have been investigated by examining the conductance-temperature charac- teristics of unsintered samples as well as samples sintered at 1273 K for various periods under atmospheres of both ambient (i.e. wet) air and dry 0, (2 1 %)-N, (79 %) mixture. The air conductance-temperature characteristics of unsintered samples are sigmoid in form, the conductance rising between ca.290 and 420 K and between 670 and 900 K. The majority of samples studied showed Arrhenius-type behaviour in both regions for both ambient air and dry 0,-N, as the test atmospheres. The mean values of the activation energy for unsintered materials are determined to be 0.41 f 0.14 eV (< 450 K) and 0.78 kO.11 eV (> 650 K) under ambient air, and 0.14_+0.08 eV (< 450 K) and 0.89+0.15eV ( > 650 K) under dry 0,-N,. Repeated thermal cycling under a sealed atmosphere of dry 0,-N, generally produced no change in activation energy in either temperature region, although there were variations in turning-point positions and absolute conductance levels. Experimental curves for sintered materials become more linear as the length of presintering is increased, although in most cases there is still a region of significantly lower activation energy at a temperature (ca.500-630 K) which correlates well with the minimum of the sigmoid curves found for unsintered material. The linear regions above and below this temperature have similar gradients, and there is a trend towards higher activation energies (ca. 1.7 eV) with increased length of sample presintering. The conductance mechanism is probably of the Schottky barrier type, where the magnitude of the barrier is modulated by the nature of the chemisorbed surface species, especially in samples with little or no presintering. With increasing degrees of sintering, however, the intergrain- neck model may become more important.In unsintered material in the temperature range ambient ca. 420 K, surface hydroxyl groups are the major conductance-modulating species. Above 670 K in the unsintered material, and in the sintered material, the conductance is primarily modulated by surface 0,- anions. Jarzebski and Marton’ have observed that the electrical conductance of SnO, single crystals may be satisfactorily interpreted via classical theories pertinent to the bulk behaviour of wide band n-type semiconductors. When in the form of a compacted particulate solid, however, the conductance-temperature characteristics of tin(rv) oxide are substantially different, as McAleer et al. have recently shown for a variety of different commercial sarnple~.~, ‘ Our investigations into the relationship between the surface adsorption phenomena and bulk electrical properties have employed a further type of 19211922 Tin Oxide Surfaces tin(1v) oxide, a high-purity, high-surface-area gel, which, as the previous paper shows, comprises very small particles (ca.3 nm) which have a very extensive internal pore structure.* In that paper we also described some of the general electrical conductance behaviour of this material. Here we analyse the conductance-temperature characteristics under atmospheres of both ambient air and rigorously dried 02(21 %)-N2(79 %) mixtures in terms of the conductance-controlling mechanisms. Results Arrhenius Analysis of the Conductance Data The Arrhenius equation is commonly used to analyse thermally activated processes, such as those considered here.It is of the form where G is the electrical conductance (s), A is a constant, EAct is the empirical Arrhenius activation energy (eV), k , is Boltzmann constant, T is the absolute temperature (K). In the general case,5 A and E A c t must be considered to be functions of T, but for the present discussion both are assumed to vary sufficiently slowly with T to allow the effect to be neglected. The air conductance-temperature characteristics of unsintered tin(1v) oxide gel samples on their first (upward) traverse of the range are sigmoid in form (fig. I), the conductance rising between ca. 290 and 420 K and between 670 and 900 K. The majority of samples studied showed Arrhenius-type behaviour in both these regions, although the high-temperature data were generally found to be more accurately represented by (1) than were those at the lower end of the range.Owing to variations in the temperatures at which the maximum and minimum conductances of the sigmoid G-T characteristics occurred for different samples, it was not possible to quantify the Arrhenius gradients within the same temperature limits in all cases. Therefore, the values of EAct were calculated by using straight lines manually fitted to the experimental data. The errors inherent in such a process are difficult to quantify, but the activation energies cannot be considered accurate to better than + 10 Oh. The same approach was retained in the analysis of data from presintered oxide, even though the trend towards more linear G-T characteristics was generally accompanied by a reduction in the variations between nominally identical samples.Tables 1 and 2 contain the results of this analysis of a total of more than 40 samples. For both ambient (i.e. wet) air and dry 02(21 %)-N2(79%) as the test atmosphere, the data generally indicate a trend towards higher activation energies with increased length of sample presintering at 1273 K. Allowing for the significant amount of scatter in each batch, the behaviour observed is qualitatively similar under the two different at- mospheres. The mean values of EAct for unsintered materials are summarised in table 3. These values may be compared with those obtained by McAleer et al.,293 who observed a sigmoid air conductance-temperature relationship very similar to that shown by most of the unsintered sensors examined here, and determined activation energies of 0.28 and 1.1 eV in the low- and high-temperature regions, respectively.The experimental curves for sintered samples become more linear as the length of presintering is increased, although in most cases there is still a region of significantly lower activation energy at a temperature (ca. 500-630 K) which correlates well with the minimum of the sigmoid curves found for unsintered samples. However, the linear regions above and below this temperature have similar gradients, as the example in fig. 1 shows. McAleer et al. also pointed out that a ratio of 1 : 4 between the activation energies at low and high measurement temperatures is consistent with a change in the unit charge of a conductance-controlling surface group from - 1 to -2.In such a model, EActP . G. Harrison and M. J . Willett - 7 . 0 ~ 1/1 2 -17.0- e - -27.0, -5.0 t 0 0 0 0 0 0 0 OOO 0 0 0 0 0 0 - - - - - o - I m 2 -10.0- c. - ?s * * * * * * *. -15.01 I 1 0.0 2.0 4.0 1923 (h) lo3 KIT lo3 KIT Fig. 1. Example Arrhenius plots of the ambient air conductance behaviour exhibited by ( a ) unsintered and (6) sintered (48 h in air at 1273 K) tin(1v) oxide samples. ( - - -) limit of reliable measurement. represents the desorption energy of the species concerned. The values of this ratio (R) in the present study exhibit significant variations between nominally identical samples and are highly dependent upon the presintering of the oxide disc (tables 1 and 2).Treatment at 1273 K for ca. 24 h is generally required to produce samples with R = 4 from the starting material employed here, although there does appear to be a general fall in R with further presintering, tending towards unity in the most highly sintered cases. The pellets employed by McAleer et a1.2q3 were sintered for 16 h at 1273 K prior to use, which correlates reasonably well with this observation, despite differences in other aspects of the oxide preparation. They also commented on the effect that microstructure can have on the measured values of the activation energy, and referred to a 'low porosity' SnO, sample which was found to exhibit a value of EAct = 0.72 eV across the full temperature range. No details of the preparation of this sample were given, but the results reported here indicate that such an effect might be achieved by increased severity of presintering.The data of Paria and Maiti,' who found values for EAct of 1.73 and 0.99 eV above and below 770 K, respectively, in SnO, samples presintered at 1823 K for 3 h, also serve to emphasise the importance of microstructural control via thermal pretreatment. '-I1 Such a treatment could influence the observed activation energy values in a variety of ways including (i) by modifying the bulk properties of oxide grains, (ii) by modifying the relative contributions of the bulk/surface electrical properties of the oxide (i.e. by changing the intergrain contact area), or (iii) by altering the nature of the surface by changing the relative concentrations of the various adsorbed species, as found by Joly et al.12 Consequently, it is difficult to propose a simple explanation of the observed variations in the value of EAct.The effect of repeated traversal of the temperature range upon the sample properties was used by McAleer et ~ 1 . ~ 9 ~ to investigate the influence of atmospheric water vapour upon the sample electrical behaviour. Such methods were more difficult to employ with unsintered oxides of the type used here, since the experimental temperatures were capable of producing morphological changes in the samples, resulting in hy~teresis.~ Therefore, the following discussion of the variations in EAct observed during cyclic experiments under controlled atmospheres is limited to results from samples where there was little evidence of significant thermal effects of this type.Presintering greatly affects the microstructure of tin(1v) oxide gel1924 Tin Oxide Surfaces Table 1. Arrhenius activation energy data under ambient air degree of low- temperature high-temperature ratio of activation presintering activation activation energies /h at 1273 K energy /eV energy/eV (high temp./low temp.) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4 4 8 8 14 14 20 20 24 24 24 24 48 48 72 72 0.30 0.16 0.45 0.43 0.60 0.53 0.46 0.47 0.44 0.56 0.49 0.38 0.10 0.25 0.12 0.58 0.45 0.17 0.15 0.16 0.18 1 S O 0.45 0.3 1 0.29 1.53 1.38 1.53 1.38 0.64 0.79 0.70 0.73 0.58 0.9 1 0.84 0.93 0.77 0.84 0.88 0.88 0.84 0.63 0.74 0.79 1.19 1.23 0.86 0.86 0.77 0.82 1.44 1.10 1.1 1 1.08 1.53 1.53 1.77 1.57 2.13 4.94 1.56 1.35 1.52 1.58 2.02 1.64 1.91 1.57 1.71 2.26 6.30 2.96 6.58 2.05 2.73 5.06 5.73 4.8 1 4.56 0.96 2.44 3.58 3.72 1 .oo 1.1 1 1.16 1.14 For unsintered samples, it was found that repeated thermal cycling under a sealed atmosphere of dry 0,-N, generally produced no change in EAct in either temperature region, although there were variations in turning point positions and absolute conductance levels.That these effects were largely independent of moisture introduced into the test vessel by desorption from the oxide and/or substrate was demonstrated by removing the test atmosphere by evacuation and replacing it with dry 0,(21 %)-N,(79%) at the end of the first traverse of the range (as the sample cooled from 900 K to ambient temperatures). Subsequent cycling produced broadly similar results.Presintered samples showed little change in the measured values of E,,, as a result of thermal cycling. McAleer et d 2 v 3 clearly demonstrated the effect of water during tests under flowing gas streams, reporting significant conductance hysteresis in the low temperature region (< 700 K). However, the values for EACt were found to be very similar in both wet and dry air, despite the observed hysteresis. This distinguishes such effects from the thermally induced (morphological) changes which occur as a result of presintering at 1273 K. The results of Arrhenius analysis of the conductance data obtained from nominallyP . G. Harrison and M . J . Willett 1925 Table 2. Arrhenius activation energy data under dry 0, (21 % t N , (79 YO) degree of low-temperature high- temperature ratio of activation presintering activation activation energies /h at 1273 K energy/eV energy/eV (high temp./low temp.) _ _ _ _ _ _ _ _ _ _ _ _ ~ _ _ 0 0 0 0 0 0 0 0 0 0 24 48 72 72 0.27 0.20 0.2 1 0.0 1 0.10 0.18 0.0 1 0.17 0.12 0.16 0.27 1.92 1.38 0.75 - 0.99 0.87 1.08 0.82 0.96 1.01 0.92 0.52 0.86 0.88 1.21 I .64 I .30 - 4.95 4.14 8.2 5.33 5.41 4.33 5.38 3.26 0.63 1.12 1.73 - - Table 3.Summary of Arrhenius activation energy data measurement temperature region/K in ambient air/eV in dry 0,-N,/eV 2 450 0.41 k0.14 0.14 k0.08 5 650 0.78k0.11 0.89 k0. 15 identical tin(1v) oxide discs during 49 combined infrared conductance experimentsI3 are plotted in fig. 2, and demonstrate that the data for pretreatment temperatures below ca.570 K may be reasonably represented by an Arrhenius-type process with an activation energy of ca. 1.7 eV. This temperature correlates well with that at which the intensity of the surface G(Sn0H) falls to a negligible value. This experimental value of E,,, has tentatively been assigned to the effective activation energy for the removal of a surface hydroxyl group from the hydrated oxide during the calcination process. Discussion The assumption of an electrical conductance mechanism involving the surface chemisorbed species on tin(1v) oxide can be made without differentiating between models such as the Schottky barrier or intergrain necking.14 In both cases, the current flow through a porous body is controlled by regions of relatively high resistance, which are sensitive to the surface coverage of chemisorbed species.McAleer et al.2*3 have considered the relative merits of the Schottky barrier conductance mechanism as compared with the intergrain neck model for tin(1v) oxide, and have found that the former provided a more satisfactory explanation of their experimental data. However, it was noted that more highly sintered materials might well exhibit behaviour better described by the necking model. Comparisons with results obtained in the present study indicate that the behaviour of the oxide used by McAleer was equivalent to oxide gel used in this study after1926 Tin Oxide Surfaces -5.0 5 u -12.: S \ \ \ \ \ 1.4 1.9 lo3 KIT 2.4 Fig. 2. Arrhenius plot of pretreated tin@) oxide disc conductances from combined infrared- conductance experiments as a function of pretreatment temperature.EAct = 1.67 eV. presintering for ca. 24 h at 1273 K in air. This tends to suggest that in samples subjected to < 24 h presintering, Schottky barriers predominately control the conductance of the oxide body. In the absence of activation energy measurements at different oxygen pressures, which have been used2v3 to distinguish between the two models, it is not possible to distinguish which of the two is more active in the most highly sintered materials used in this work. The Dominant Processes in Unsintered Material It is generally recognised that in SnO, samples of the type discussed here, the air conductance behaviour mainly represents the influence of surface reactions involving electron exchange with the oxide, most notably those involving atmospheric oxygen and water.In order to interpret the conductance-temperature characteristics and activation energies observed in the present work, some consideration of previous desorption studies of 0,- and H,O-related species on tin(rv) oxide is required. A strict comparison of data from different laboratories can only be justified, however, as long as the oxide samples are sufficiently similar. Tables 4 and 5 summarise literature data in which particular emphasis was placed on the assignment of the observed effects to specific species and the determination (whenever possible) of their desorption energies. The most obvious conclusion from these data is that the situation under atmospheres containing both 0, and H,O is extremely complex, with no simple interpretation being apparent.P.G. Harrison and M . J . Willett 1927 Table 4. Desorption studies of 0,-derived species on tin(rv) oxide approximate surface method of desorp tion desorption species investigation temp/K energy/eV ref. 0; or 0-' TPD" 570 b C (0 0, TPD" 350 b d (ii) 0;' 420 (iii) 0-' or 02- 790 (iv) lattice oxygen 870 (i) 0;. (ii) 0-' or 0,- TPD" TPD" 320 0.28 670 0.48 (but possibly higher) e f (i) 0, 36W35 1.12 (ii) 0;' 1.29 (iii) 0-' 670-760 1.72 (iv) 0,- 820-940 2.45 (v) lattice > 1070 3.23 0;. e.s.r. > 430 b g (i) 0;. (ii) 0-' (iii) 0,- e.s.r. < 420 0.1 5-0.28h i > 420 b " Temperature-programmed desorption. ' Not determined. ' M. Egashira, M. Nakashima and S . Kawasumi, J.Chem. SOC., Chem. Commun., 1981, 1047. dN. Yamazoe, T. Fuchigami, M. Kishikawa and T. Seiyama, Surf. Sci. 1979, 86, 335. B. Gillot, C . Fey and D. Delafosse, J . Chem. Phys., Phys. Chim. Biol., 1976, 73, 19. fRef. 12. gA. M. Volodin and A. E. Cherkashin, React. Kinet. Cutal. Lett., 1981, 17, 329. ' Dependent on pressure. Ref. 17. Ambient Air Conductance Data The mechanism proposed to explain the sigmoid air conductance-temperature plot observed in unsintered oxide samples in this study, is that the initial conductance peak represents gradual loss of physisorbed H,O and hydroxyl groups in various sites on the polycrystalline oxide surface. The dehydroxylation process allows an increase in the surface coverage of charged oxygen adsorbates, corresponding to the region (ca.420-670 K) where the conductance falls. Finally, the desorption of these oxygen species again produces a positive temperature coefficient of conductance at the highest temperatures studied. The position of the conductance maximum in the sigmoid characteristics is subject to considerable variation between samples (ca. 320-400 K), but generally correlates well with typical values quoted for the loss of physisorbed water from the oxide (table 4).159 l6 It is generally accepted that molecular physisorbed water must be removed from the oxide surface before significant condensation of surface hydroxyl groups may commence. Thus, the temperature of the conductance maximum may be considered consistent with the region of Arrhenius behaviour below 420 K, where hydroxyl groups are the dominant surface species.This would imply that hydroxyl groups primarily govern the height of the potential barriers at the grain boundaries within the oxide at low temperatures.1928 Tin Oxide Surfaces Table 5. Desorption studies of H,O-derived species on tin(1v) oxide surface species appropriate method of desorption desorption investigation temp. / K energy/eV ref. (i) physisorbed water TPD" 330 0.46 b, c (ii) hydrogen-bonded water 420 d (lii) 1 various surface hydroxyl (lV) groups 520 1.04 (?) 69&7 1 5 0.94 760-770 1.35 (v) J 6) (ii) (ii) 0) (ii) (iii) ) simultaneous hydroxyl groups/surface O2 assorted water loss regions physisorbed water surface hydroxyl groups n.m.r. TPD" 880-890 270-280 600-63 5 770 > 870 380 1 670 hydroxyl groups hydroxyl groups adsorbed on [loo] face TPD" 513 adsorbed on [loll face 573 350-380 physisorbed water microbalance hydrogen- bonded water (vii) I 400-4 10 460480 540 1 590 630 1.72 e d d 1.47 g 1.68 or 2.29 h d a Temperature-programmeddesorption. Ref.16. M. Egashira, M. Nakashimaand S. Kawasumi, Nippon Kagaku Kaishi, 1982, 556. Not determined. E. W. Giesekke, H. S. Gutowski, P. Kirkov and H. A. Laitinen, Inorg. Chem., 1967,6, 1294. f N . Yarnazoe, T. Fuchigami, M. Kishikawa and T. Seiyama, SurJ Sci. 1979, 86, 335. Ref. 22. Ref. 15. The 420-670 K region of falling conductance corresponds to the various temperatures quoted for loss of hydroxyl groups from the oxide surface (table 5). In such circumstances, the sites vacated by the hydroxyl groups are likely to be reoccupied by oxygen as OBads or, with increasing probability at higher temperatures, O;', 0-' and conductance of the n-type solid, a mechanism thereby exists for the sample conductance to either increase, remain unaltered, or decrease, as a direct result of dehydroxylation.The behaviour observed is critically dependent upon the nature of the readsorbed species or, more accurately, the relative abundance of various types. Fig. 3 summarises the interactions which might be considered responsible for such changes, although it is not intended to imply that only one species of oxygen- or water-derived adsorbate species will be present at a given temperature. Apart from the range of desorption temperatures observed for each species, the situation in polycrystalline samples is complicated by the existence of different adsorption sites for the same group on the different crystal faces, which may exhibit varying desorption characteristics, and the conversion of one form of adsorbed oxygen species to another cannot be discounted.The work of Chang17 clearly 0 2 - . 1 7 Assuming the desorption of surface hydroxyl groups as H,O increases theP. G. Harrison and M . J. Willett 1929 0 2 (gas) + (site) 0 2 (ads) -e-bulk 1 i + e - b u l k +e-bulk O2 (gas) + (site) 02'(ads) -e-bulk -e-bulk 1 l + e - b u l k + 2e-bulk 9 (gas) + 2 (site) -= 207(ads) + 4e- b uI k I t O2 (gas) + 2 (site) 202-(ads) - 4e- b uI k Fig. 3. Possible routes for the formation of adsorbed oxygen species on tin(1v) oxide surfaces. indicates that the transformation O,g,, + 20,-ds and 20,-ds + 20;;; would be favoured at higher temperatures.Overall, it might be expected that dehydroxylation would initially result in a conductance rise, due to the low charge state of incoming chemisorbed oxygen groups. At higher temperatures, however, a reversal in this trend might be expected if the dominant surface species were considered to be doubly ionised (i.e. 02-). Under conditions where this group predominantly controls the electrical conductance processes within the solid, Arrhenius behaviour would again be expected, although at a different activation energy compared to that in the low-temperature region. The experimental observations reported in this study for unsintered oxides appear to be consistent with such a model. Similar conclusions were also reached by McAleer et al.2*3 The good correlation between the behaviour of the absolute conductance and the intensity of the infrared surface hydroxyl deformation G(Sn0H) band in the same samples13 lends further corroboration in support of an increase in electrical conductance as a direct result of dehydroxylation.Conductance Behaviour under Dry 0,(2 1 %)-N, (79 %) The assignment of the low-temperature increase in conductance of unsintered samples must also be viewed in relation to the experiments performed under dry 0, (21 %)-N, (79%) atmospheres.* In the first traverse of the range, hydroxyl groups are present, regardless of the test atmosphere, and so the sigmoid characteristics appear under both wet and dry air. On cooling under conditions where water is available for readsorption, the same type of processes might be expected to operate in reverse sequence, again producing a sigmoid characteristic (subject to any effects attributed to morphological changes in the sample).In cases where a sigmoid characteristic is still subsequently observed when dry gas is introduced for the second traverse (following a brief evacuation at ca. 900 K), it might appear that the model breaks down. However, this is not necessarily so, since the reappearance of a conductance maximum implies only that significant quantities of water-surface hydroxyl groups are still available. Indeed, we have previously shown that hydroxylation persists even on samples evacuated for 18 h at ca. 670 K, with hydroxyl groups trapped in deep pores inside the oxide parti~1es.l~ It is, therefore, quite feasible that under the experimental conditions employed in the present study, the porous oxide particles (identical to those used in our previous studies) still retain sufficient hydroxylation to produce effects associated with surface1930 Tin Oxide Surfaces hydroxylation, even under nominally dry atmospheres.In contrast, the samples employed by McAleer et al.,'l3 whilst being subjected to comparable periods at elevated temperatures to those under examination here, did exhibit an obvious change in behaviour when the test atmosphere was changed from wet to dry air. This effect may be attributed to the presintering treatment used in these studies, which would be sufficient to destroy the smaller pores within the oxide.Any surface hydroxyl groups would therefore be relatively easily lost from the sample after desorption. The Role of Surface Oxygen Species The assignment of the low-temperature conductance peak to the effects of dehydroxy- lation inherently implies relatively little contribution due to the desorption of surface oxygen species in this region. However, Takata and Yanagida14 have reported very similar behaviour on ZnO which was attributed to a switch between two different oxygen species as the conductance-controlling mechanism. It must be pointed out that the data reported here do not rule out contributions from such processes, but the dehydroxylation process is favoured as the predominant mechanism. The conclusion that the conductance increase in unsintered materials at temperatures above 700 K is attributable to the controlling influence of 0,- anions correlates satisfactorily with the majority of the desorption data quoted in table 4.The similarities in data obtained under wet and dry conditions appears to confirm the minor influence of hydroxyl groups upon the conductance at higher temperatures. Joly et a1.12 have previously observed significant changes in the relative abundances of chemisorbed oxygen species on SnO, according to the history of the sample, although the desorption activation energy for a given group was not found to be dramatically altered by such changes. However, it is clear that in a situation where several species contribute to the air conductance mechanism, and coexist on the oxide surface at a given temperature (as has already been implied in the present study), the observed value of EAct may be affected by abundance variations.In such circumstances, a single straight line would not adequately describe the Arrhenius data over a significant range of temperature if the species involved have different desorption energies. The uncertainties in the results of this study, together with the relatively sparse data in some cases, make it difficult to quantify the accuracy of the description provided by eqn (l), although the high- temperature data were more easily fitted to a straight line than those in the lower region. This reinforces the previous qualification regarding a simplified interpretation of low- temperature behaviour in terms of dehydration, but does not contradict the general hypothesis.The Dominant Processes in Sintered Material The electrical conductance data from sintered materials indicate similar activation energies at low and at high temperatures, usually with a region of lower activation energy between ca. 580 and 700 K. The data at lower temperatures exhibited greater scatter, but were still well described by a linear Arrhenius plot. The very low values of absolute conductance encountered near ambient temperatures prevented the collection of data below ca. 400 K, and so a further change of EAct in this region cannot be ruled out. The inflexion in what would otherwise be a linear plot corresponds to the temperature range in which relatively firmly bound hydroxyl groups are lost from the oxide surface.15 The presintered surface would be expected to contain far fewer surface hydroxyl groups than unsintered material, with a greatly reduced variety of environments (due to the annealing of surface defects at 1273 K,19-" which may explain the relatively well defined desorption temperature). Insufficient data were collected to allow EAct in this region to be quantified. The prime mechanism controlling the electrical behaviourP .G. Harrison and M . J . Willett 1931 above 700 K is assumed to be the desorption of chemisorbed 02- anions, by analogy with the case of the unsintered material. Although table 4 offers little evidence of 0,- desorption below about 650 K, the near equality of E,,, at low and high temperatures may imply that a similar surface process is responsible in both cases.The discussion of the conductance mechanisms in unsintered materials has referred to the action of hydroxyl groups in preventing the population of the oxide surface by chemisorbed oxygen species. Howqver, sintered materials have a greatly reduced hydroxyl coverage, and are therefore more susceptible to oxygenation and resultant lowering of the electrical conductance at temperatures below 650 K. It might be expected that singly charged 0;' or 0-' species would predominate under these conditions, but the experimental data clearly favour the interpretation of an O'--controlled conductance mechanism at all temperatures above 400 K except for the limited region where the remaining hydroxyl groups are desorbed.The findings of Chang17 are not inconsistent with such a model. His e.s.r. measurement on SnO, thin films show that the transition from 0;' to 0-- and 02--controlled processes occurred at around 420 K, i.e. close to the limit of reliable conductance measurements in sintered samples here. The apparent absence of effects due to singly charged oxygen adsorbates is most probably because of the relatively low population of such species compared with the relatively high abundance of 02- anions indicated by the studies of Joly et a1.12 at temperatures below 550 K. The air conductance characteristics recently reported by McAleer et al.3 resemble those of the highly sintered samples employed here. However, the various regions of behaviour were not conclusively assigned to the influence of specific surface species, although it was tentatively suggested that changes in the relative abundances of various forms of charged oxygen species alone could be responsible.However, the smooth transition of behaviour observed in the present study between the characteristics of unsintered and sintered materials tends to favour a mechanism in which there is a decreasing, but nevertheless significant, influence due to surface hydroxyl groups as the severity of the thermal conditioning is increased. It is also worth reiterating the observation that the non-stoichiometry of SnO,-, is postulated to decrease as a result of extended sintering in air.21 The steady reduction in the conductance of the oxide measured at 293 K as the length of the sintering treatment is increased would be consistent with such an effect superimposed upon the surface processes discussed previously.Conclusion In conclusion, the electrical resistance in tin(rv) oxide gel is probably of the Schottky barrier type, especially in samples with little or no presintering. The magnitude of the barrier is modulated by the coverage and charge of surface chemisorbed species. With increasing degrees of sintering, however, the intergrain neck model may become more important. In unsintered material in the temperature range ambient to ca. 420 K, the conductance increases with an EAct < 0.6 eV and surface hydroxyl groups are the major conductance-modulating species. Between ca. 420 and 670 K, the conductance falls as surface hydroxyl group desorption occurs, and surface sites are increasingly occupied by chemisorbed oxygen, with the relative abundance of the surface oxygen species increasing in the order Oi' < 0-' < 0,- as the temperature rises.In this temperature region, therefore, the nature of the primary conductance-controlling species is changing, with more than one surface species contributing to the conductance mechanism. Above 670 K the conductance again increases linearly with an activation energy in the range ca. 0 . 6 1 .O eV, and the conductance is primarily modulated by surface 02- anions. In sintered oxide, the conductance at temperatures both below ca. 550 K and above ca. 700 K is also primarily controlled by surface 02- anions with an activation energy in the range ca. 1-2 eV. In the intermediate temperature range the conductance increasesI932 Tin Oxide Surfaces with an activation energy 4 I eV due to the desorption of a few remaining tightly bound surface hydroxyl groups. It is pertinent to note that Egashiral' and Morishige22 have found EAct for hydroxyl desorption of between 1 and 2eV, with good evidence to suggest that the actual magnitude is highly dependent upon the precise surface environment of the desorbing species.In the present work, however, the electrical conductance of samples in air was found to exhibit a significantly lower value (E,,, < 0.6eV) in regions where the controlling mechanism has been assigned to dehydroxylation. For the cases where 02- has been cited as the major surface species, 2.4eV appears to be the most likely value for EACt,l2 whereas the present studies have generally produced values in the region 0.8-1.8 eV, depending upon the pretreatment of the sample.A major reason for these differences may be that desorption studies are generally performed in vacuo or under an inert gas stream. This procedure is generally adopted in order to simplify the assignment of effects observed upon desorption, although it has already been noted that highly complex situations may arise despite such precautions. The processes studied here for samples in air are subtly different from those reported in table 3, and the quantitative agreement of EAct is generally poor, although a value closer to those in the literature (1.7 eV) was obtained for samples dehydroxylated in vacuo (see fig.2). Furthermore, the comments made in previous sections regarding the influence of the precise nature of the surface12 upon the observed values of EAct in complex (multispecies) processes must again be stressed, and it should also be noted that the values of EAct quoted by McAleer et aL2v3 are similar to those reported here (0.28 and 1.1 eV). Hence, although the literature values for the activation energies of desorption of OH- and 02- groups do not correlate precisely with the results obtained in this study, the observed discrepancies may be rationalised by other, unquantified, effects. M. J. W. is grateful for the support provided by British Coal during the course of this project, and for permission to publish this paper. The views expressed are those of the authors and not necessarily those of the British Coal Corporation. We thank the S.E.R.C. for the award of a Research Grant. References 1 Z. M. Jarzebski and J. P. Martin, J. Electrochem. SOC., 1976, 123, 199C. 2 J. F. McAleer, P. T. Moseley, B. C. Tofield and D. E. Williams, Proc. Br. Ceram. Soc., 1985, 36, 89. 3 J. F. McAleer, P. T. Moseley, J. 0. W. Norris and D. E. Williams, J. Chem. SOC., Faraday Trans. I , 4 P. G. Harrison and M. J. Willett, J. Chem. Soc., Faraday Trans. I, 1989, 85, 1907. 5 M. Meunier, J. F. Currie, M. R. Wertheimer and A. Yelon, J. Non-Cryst. Solids, 1981, 46, 433. 6 M. K. Paria and H. S. Maiti, J. Mater. Sci., 1983, 18, 2101. 7 M. J. Fuller, M. E. Warwick and A. Walton, J. Appl. Chem. Biotechnol., 1978, 28, 396. 8 L. M. Sharygin and V. F. Gonchar, Kinet. Catal., 1974, 15, 123. 9 V. F. Gonchar and L. M. Sharygin, Kinet. Catal., 1974, 15, 404. 1987, 83, 1323. 10 L. M. Sharygin, V. F. Gonchar and V. M. Galkin, Kinet. Catal., 1974, 15, 1125. 11 L. M. Sharygin, V. F. Gonchar and A. P. Shtin, Kinet. Catal., 1975, 16, 178. 12 J. P. Joly, L. Gonzalez-Cruz and Y. Arnauld, Bull. SOC. Chim. Fr., 1986, 11. 13 P. G. Harrison and M. J. Willett, to be published. 14 S. R. Morrison, The Chemical Physics of Surfaces (Plenum Press, New York, 1977). 15 P. G. Harrison and A. Guest, J. Chem. SOC., Faraday Trans. 1, 1987, 83, 3383. 16 M. Egashira, M. Nakashima, S. Kawasumi and T. Seiyama, J. Phys. Chem., 1981, 85, 4125. 17 S. C. Chang, J. Vac. Sci. Technol., 1980, 17, 366. 18 M. Takata and H. Yanagida, J. Ceram. SOC. Jpn, 1979, 87, 13. 19 A. Jones, T. A. Jones, B. Mann and J. G. Firth, Sensors and Actuators, 1984, 5, 75. 20 S. J. Gregg, The Surface Chemistry of Solids, (Chapman and Hall, London, 1961). 21 A. F. Peterson, PhD. Thesis (Rutgers State University, 1968). 22 K. Morishige, S. Kittaka and T. Morimoto, Bull. Chem. SOC. Jpn, 1981, 53, 2128. Paper 8/01 166D; Receiued 18th April, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501921

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1989, 85(8), 1933-1943 Interaction between Amine Oxide Surfactant Layers Adsorbed on Mica Christina E. Herder," Per M. Claesson and Peter C. Herder Department of Physical Chemistry, The Royal Institute of Technology, S-100 44 Stockholm, Sweden Institute for Surface Chemistry, Box 5607, S-I14 86 Stockholm, Sweden A surface-force apparatus has been used to determine the interaction between two mica surfaces immersed in water solutions of a non-ionic surfactant dimethyldodecylamine oxide (DDAO). These interactions could be associated with three different adsorption states as the surfactant concentration was increased : hardly any adsorbed surfactant, a monolayer and a bilayer. The study has also been aimed at determining any temperature dependence in the interactions between bilayer-coated surfaces.In addition to the surface-force measurements, quantifications of the amount of adsorbed surfactant on mica sheets were made using X-ray photoelectron spectroscopy. The contact angles were also determined for the same mica samples. From these results we suggest an adsorption mechanism based on positively charged surfactant molecules adsorbing onto the negatively charged mica substrate. No temperature dependence is found of the forces between bilayer-coated surfaces and this is consistent with the absence of any temperature-induced phase separation of the DDAO-water system. Introduction The phase behaviour of surfactant-water systems is strongly dependent on the nature of the hydrophilic It is by now evident that while the occurrence of miscibility gaps and liquid-crystalline phases in surfactant systems, is to a large extent governed by the interactions between the solvated head groups, the nature of these interactions is not completely understood. We have therefore decided to measure directly the forces between layers formed by surfactants with identical hydrocarbon chains but different non-ionic hydrophilic head groups.These surfactants are dimethyldodecylamine oxide (DDAO), dimethyldodecylphosphine oxide (DDPO) and pentaoxyethylenedodecyl ether (C12E5). The results on the temperature-dependent interactions between Cl,E5 layers have been published previ~usly.~ Here we report on the adsorption of and the interactions between DDAO layers. DDPO is yet to be investigated in the same way.The phase behaviour of DDAO-water and DDPO-water systems is quite different in spite of the structural similarities of the two s u r f a c t a n t ~ . ~ ~ DDAGwater shows no miscibility gap and has quite large liquid-crystalline phases, whereas DDPGwater has a lower consolute point at 1 wt YO and 39 "C and smaller liquid-crystalline phases. It is important to note that in the hydrophilic head group there is a substantial charge separation between the negative oxygen atom and the positive nitrogen and phosphorous atoms, respectively. DDAO is a rather good proton acceptor having a pK, of 4.65, whereas the corresponding value for DDPO is - 1.5. In a neutral solution of DDAO ca. 1 % of the molecules are protonated and hence charged whereas in a neutral solution of DDPO there are almost no charged molecules.The surface-force apparatus of Israelachvili is well suited for studies of the interaction between adsorbed surfactant layers.'-'O The surfactant is either deposited onto a mica substrate surface by the Langmuir-Blodgett technique in advance or adsorbed from a 19331934 Amine Oxide Surfactant Layers surfactant solution during the surface force experiment. Interactions between surfaces coated with a monolayer of surfactants such as dioctadecyldimethylammonium bromide, hexadecyltrimethylammonium bromide and dihexadecyldimethylammonium bromide have demonstrated that in water a strong and long-range attractive force acts between hydrophobic surface^."-'^ This force, which is stronger than a van der Waals force, is often referred to as the hydrophobic force.By depositing or adsorbing a second layer of the same or another surfactant on top of the hydrophobic layer, it becomes possible to investigate interactions between the hydrophilic head groups. This has already been done for a few surfactants such as pentaoxyethylenedodecyl ether, dihexadecyl- dimethylammonium bromide, hexadecyltrimethylammonium bromide and some phos- pholipids. 3* 14-16 The interactions between non-ionic head groups of pentaoxyethyl- enedodecyl ether were found to be strongly temperature-dependent. The interaction is repulsive at low temperature but with increasing temperature the repulsion is weakened and turns gradually into an attraction. This could explain why ethylene oxide surfactant-water systems phase-separate at elevated temperatures.An overwhelming amount of evidence has already been gathered for the presence of short-range repulsive hydration forces between hydrophilic surfaces and more long- range attractive non-DLVO forces between hydrophobic surfaces. It is unknown to what extent these forces are present between weakly hydrophobic and weakly hydrophilic surfaces or what happens with these forces when the surfaces consist of a mixture of hydrophilic and hydrophobic groups. This investigation was partly aimed at clarifying these questions. Particular emphasis was given to the magnitude and temperature dependence of non-DLVO forces acting between DDAO bilayers. The interaction between hydrophobic DDAO monolayers was also investigated.All surface-force measurements were performed in water solutions of DDAO at different concentrations and at different temperatures. Adsorption isotherms of DDAO on mica were derived from X-ray photoelectron spectroscopy data using a quantitative evaluation method developed earlier by our group.17 In addition to this, contact angles were studied. The surface film was then characterized in terms of adsorption kinetics, orientation of molecules within the layer and number of layers formed on the surface. Chemicals The dimethyldodecylamine oxide (purum) used was obtained from Merck. It was recrystallized twice in acetone and stored over a drying agent, P205(s), in vacuum prior to use. The purity was checked by determining the c.m.c. from surface tension measurements, obtaining 1.9 x mol dm-3, which is in good agreement with values found in the literature.6 No minimum in the surface tension was detected above the c.m.c.Brown muscovite mica was obtained from Sciama (Paris) and green muscovite mica from Brown Mica Co. (Sydney). The kind of mica used did not affect the results. The water used in these experiments was purified by the following consecutive steps : decalcination and prefiltration followed by treatment with a reverse osmotic unit, two mixed-bed ion exchangers, activated charcoal, Organex (R) and a final filtration. All purification units were Millipore (R) products except the final filter which was a Zetapore (R) 0.2 pm. In addition to this, the water used in surface-force experiments was deaereated for at least 2 h.Method X-Ray Photoelectron Spectroscopy Mica sheets were cleaved along basal planes, so that both surfaces of the sample were freshly cleaved, and subsequently treated in two different ways. Some of the sheets,C. E. Herder, P. M . Claesson and P. C. Herder 1935 including the samples for contact-angle measurements, were immersed into a surfactant solution and allowed to equilibrate for 30 min before being pulled out slowly. The amount of surfactants adsorbed reflects the situation at a three-phase contact (mica/solution/ air). The other samples were prepared by cleaving the sheets again along a basal plane, but this time only half-way through the sheet. A small piece of Teflon was placed in between the two new surfaces to keep them apart during immersion in the surfactant solution.After equilibration the Teflon piece was removed and the mica surfaces were slowly forced together with a squeezing tool, applying a pressure of a few atm,? before they were reopened at the same cleavage plane. The amount of DDAO adsorbed represents the situation of two surfaces brought into molecular contact in surfactant solution. This mimics the contact situation in the surface force apparatus.'' The two types of samples were both blow-dried with nitrogen gas before being analysed in the X-ray photoelectron spectrometer, a Leybold-Heraeus LH2000 ESCA, with an X-ray source. In order to quantity the adsorption of dimethyldodecylamine oxide onto molecularly smooth mica basal planes, peak areas from the XPS were analysed as described in ref.(17). Muscovite mica is a layered aluminosilicate mineral, consisting of ca. 1 nm thick layers. There are covalent bonds within each layer that are much stronger than the ionic bonds between the layers, explaining why it is possible to cleave the crystal and obtain molecularly smooth surfaces. On average, one silicon atom out of four is substituted for an aluminium atom, which gives each layer a net negative charge. This charge is neutralized by potassium ions and to a smaller extent by sodium ions located between the layers in the mineral. When these ions are exposed on a surface they are easily exchanged for other cations, e.g. in water they are displaced by protons. This is an important factor in the adsorption mechanism of cationic surfactants, which will be discussed later.The known number of exchangeable cations on the molecularly smooth mica surface is used as an internal standard in the evaluation of the XPS data. In this work the N 1s and C 1s signals originating from the adsorbed surfactant and the K 2p signal from the mica substrate were analysed. Both the C 1 s and N 1 s signals were used to quantify the amount of DDAO in order to minimize errors due to carbon-containing contaminants which are a common problem with XPS. Contact-angle Measurements Contact angles were determined using a Rame-Hart goniometer. The samples were prepared as described in the previous section. A small droplet of the surfactant solution used for conditioning the surface was placed on the mica sheet and both the advancing and receding contact angles were measured.This was repeated for at least three different spots on each mica sheet in order to eliminate the influence of any local differences in surface properties. Surface-force Measurements Molecularly smooth mica sheets were glued with an epoxy resin (Epon 1004 from Shell Chemical Co.) onto two curved silica disks. The disks were mounted in a crossed cylindrical geometry and the interaction forces determined by measuring the deflection of a spring, supporting the lower surface. The separation between the surfaces is determined interferometrically by using multiple- beam interference fringes, fringes of equal chromatic order (FECO), which gives a distance resolution of ca. 0.1 nm.l0 In this experiment the main chamber was first filled with purified water and the surfactant was later added through a small inlet.The surface forces were measured at t I atm = 101 325 Pa.1936 Amine Oxide Surfactant Layers 6 - 5 - 'E - - 2 2 3 - 2 - 1 - 0 0 8 I t : 0 0 p. 0 0 0 o h 1 = & k 1 ' I ' ' I I O - ~ lo-& lo-* lo-' c/mol dm-3 Fig. 1. The number densities of adsorbed dimethyldodecylamine oxide as determined both from C 1s photoelectrons (0, 0 ) and from N 1s photoelectrons (0, m), as a function of surfactant concentration. The filled symbols represent the samples withdrawn from surfactant solutions whereas the unfilled symbols represent the 'pressed ' samples. Details about the sample preparation are found in the Method section. different surfactant concentrations and as a function of time and temperature. From the force between two curved cylindrical surfaces, Fc, one obtains the free energy of interaction between flat surfaces, GF, using the Derjaguin appro~imationl~ where R is the local geometric mean radius of the cylinders.The adhesion F(O), is calculated from the spring constant, k, and the distance of the outward jump, Dj, when the surfaces are separated from contact F(0) = kD,. The adhesion is usually normalized by the local radius, R, and given as F(O)/R. Results X-Ray Photoelectron Spectroscopy The number densities of adsorbed DDAO molecules were determined from the C 1 s and the N Is peak areas in the XP spectrum using the model of homogeneous adsorption which has been described e1~ewhere.l~ The resulting adsorption isotherms are shown in At DDAO concentrations below ca.lop3 mol dm-3 the adsorption density is low. For pressed mica surfaces no adsorption of DDAO is detected at concentrations below mol dmW3, which tells us that less than 0.5 x 1014 mol cm-2 have been adsorbed. On surfaces pulled out from dilute surfactant solutions (C 5 1 O-* mol dm+) some surfactant has been adsorbed. Quantification in this region is difficult to accomplish since the N 1s signal is weak and the C 1s signal may be affected by adsorption of airborne contaminants. At higher concentrations, the DDAO adsorptions on pressed and fig. 1.C. E. Herder, P . M. Claesson and P . C. Herder 90 1 I I I 1 I v I I 80 70 60 50 0 s 40 30 20 10 - - - - - - - - 0 0 000 0 e r 1937 Q 0 a f i b I L m P v lo-' c/mol dm-.' Fig.2. The advancing (0) and receding (0) contact angles as measured on mica immersed in dimethyldodecylamine oxide solutions, as a function of concentration. unpressed samples are quite different. For the mica sheets pulled out from the solution, the number density steadily increases to (6-7) x lOI4 molecules cm-2, whereas for pressed surfaces, the adsorbed amount is roughly constant around 2 x lo1: molecules cm-2. The latter corresponds to one surfactant molecule per negative site (48 A per site) on the mica surface, i.e. a monolayer. A larger surface density than this is interpreted as the adsorption of surfactant molecules in a second (or several) layer on top of the first layer on the surface. The number density determined from the N 1s signal has been calculated without taking any photoelectron adsorption in the layer into account, thereby giving a somewhat low value.The amount adsorbed may be underestimated by up to I5 %. On the other hand, the quantifications based on the C 1s signal may somewhat overestimate the amount adsorbed due to adsorption of carbon-containing airborne contaminants. Contact-angle Measurements The advancing and receding contact angles on mica sheets coated with DDAO are shown in fig. 2. The advancing contact angle has a value > 80" at 1 x and 3 x mol dm-3, indicating a hydrophobic, monolayer-covered mica surface. At higher concentrations both the advancing and the receding angles decrease, which could indicate a gradual formation of a bilayer. On these surfaces, which presumably expose dimethylamine oxide groups towards the solution, the advancing and receding contact angles are 50 and O", respectively.The variations of the advancing contact angles were < 2" at all concentrations. These results are consistent with the XPS results for surfaces pulled out from surfactant solutions. Surface-force Measurements In fig. 3 interaction curves in the low concentration region ( 5 10- rnol drnp3) are shown. For concentrations at and below l OP4 mol dm-3 dimethyldodecylamine oxide, the1938 Amine Oxide Surfactant Layers 1.2 1 .o 0.8 0.6 $ 0.4 0.2 'E ... I 0.0 -0.2 - 0.4 I 1 I 1 I I 1 I I 1 0 10 20 30 40 50 60 70 80 90 100 Dlnm Fig. 3. Measured forces, FIR, between mica surfaces in 1 x lo-' mol dm-3 (m) and 1 x mol dm-3 (0, 0 ) dimethyldodecylamine oxide solutions at 20 "C.The line drawn through the points for 1 x lo-* mol dm-3 DDAO is the force calculated from DLVO theory with constant charge boundary conditions and the following parameters: K - ~ = 36.4 nm, I&, = 60 mV and Aell = 2.2 x J. 4 represents the inwards jump to mica-mica contact. 0, 'Initial' interactions in 1 x rnol dmP3 obtained after 1-3 h of equilibration. The corresponding line is calculated from DLVO theory with constant charge boundary conditions and the following parameters : K - ~ = 21.5 nm, u/: = 35 mV and Aell = 2.2 x 10-20J. a marks the inwards jump, this time into a monolayer-monolayer contact. 0, Equilibrium interaction in 1 x rnol dm-3 obtained after more than 24 h equilibration. The line has no theoretical background and is only there to mark the small attractive jump and the very steep repulsive force.The theoretical lines plotted are based on all force measurements done at a particular concentration, whereas experimental points from only one force run have been included in the figure. interaction between the mica surfaces could be well described by the DLVO theory, when taking into account that ca. 10 % of the DDAO molecules in solution are charged through protonation. One force curve measured in a 1 x lo-* mol dm-3 DDAO solution is plotted in fig. 3, showing a long-range electrostatic force which is overcome by an attractive van der Waals force at small distances (3 nm). The contact position is unchanged from that in pure water, indicating that hardly any adsorption has occurred.mol dm-3 DDAO, considerable amounts of molecules are adsorbed on the surfaces and the interaction changes dramatically with time. In fig. 3 the interactions after two different equilibration times are shown. After 30 min a weak but long-range double-layer repulsion dominates at large distances. An attractive force, too strong to be a pure van der Waals force, sets in at a large separation. This attraction causes the surfaces to jump from 1 I to 15 nm into an adhesive minimum at a distance of I .6 nm out from mica-mica contact. The conclusion is that a monolayer has been adsorbed on each surface, giving rise to a hydrophobic attraction similar to that measured for other systems. 11-13 The adhesion, F(O)/R, increased to 180 mN m-'. In fig. 4 this interaction is pictured on a logarithmic scale and compared with force curves calculated from DLVO theory.Clearly, the DLVO theory fails to explain the measured interaction in 1 x rnol dm-3 In 1 xC. E. Herder, P. M. Claesson and P. C. Herder 1939 1 XlOO 5 x lo-' 2 x lo-' 3 -; 1 x lo-' 5 x lo-2 2 x lo-2 1 x ld2 0 10 20 30 40 50 60 70 80 Dlnm Fig. 4. The non-equilibrium forces, FIR, as measured between mica surface in 1 x lop3 mol dm-3 after 1-3 h of equilibration (also plotted in fig. 3). Three different force runs are plotted. The solid lines represent theoretical forces calculated from DLVO theory with the following para- meters: K-' = 21.5 nm and t& = 35 mV. The two upper curves have a Hamaker constant, Aeff = 0.5 x lop2" J, representing hydrocarbon surfaces interacting across water calculated with constant charge and constant potential boundary conditions, respectively.The two lower curves have A,,, = 2.2 x J, thus representing bare mica surfaces, again plotted with both constant charge and constant potential boundary conditions. DDAO. However, at a DDAO concentration of 1 x mol dm-3 a second layer slowly builds up, and after leaving the system overnight a stable bilayer has formed. The long- range double-layer force has vanished and the attractive force has decreased. An inward jump occurs from a distance of 12 nm in to an attractive minimum at a separation of 6.4 nm. The adhesion between bilayers is a factor of of that between monolayers. When pushing the surfaces closer together a strong repulsive force is observed. It is possible to push out the second layer during its build-up (a few hours) using a strong force, but when the bilayer is fully developed it remains between the surfaces even under very high pressures.In one experiment the surface-force apparatus was drained at this stage and refilled with conductivity water. This results in desorption of most of the second surfactant layer leaving hydrophobic, monolayer-covered mica surfaces. A weak attraction was observed at distances below 15-20 nm and this attraction increased rapidly in magnitude below 7-9 nm. The normalized adhesion was in this case 350 mN m-l. The adhesion observed at this stage is thus larger than between the monolayer-coated surfaces obtained after a1940 Amine Oxide Surfactant Layers 0.2 0.1 - 'E 0.0 9 I z - 0.1 -0.2 - 0.3 -+-+I- d I A A A 0 5 10 15 20 Dlnm Fig.5. The measured equilibrium force, FIR, between mica surfaces in 1 x lop3 rnol dm-3 at three different temperatures: 0, 27; ., 18; and A, 14 "C. All the experimental points of the force curves at distances larger than 10 nm lie within the region below the dot-dashed line and above FIR = 0. The arrows indicate the limits of the standard deviation of the average inwards jump separations. The thin solid line is the van der Waals force calculated from Lifshitz theory as outlined in the Discussion section. The steep repulsive force at 6.2 nm is plotted in more detail in fig. 6 . few hours adsorption in mol dm-3 DDAO. This could indicate a difference in chain packing and degree of orientation. mol dmV3 dimethyldodecylamine oxide has also been investigated.The results are plotted in fig. 5. Three different temperatures were chosen, 14, 18 and 27 "C. The weak, long-range repulsive force was hard to determine accurately owing to the low surface charge density and the long Debye length, but all measured forces from many different force runs were found to be below the dot-dashed line. The standard deviation of the average inward jump distance is marked with arrows. The location and size of the attractive minimum did not change with temperature. The short-range repulsive force which is present at concentrations above 1 x lop3 mol dm-3 was also investigated at 4 x mol dm-3 and 17 "C. When a force of the order of 200 mN m-' is applied, the separation decreased from 6.4 to 4.1 nm.This behaviour is completely reversible and data both from pushing the surfaces together and from separating them have been included in fig. 6. This reversibility suggests that the molecules are not desorbed as the surfaces are pushed together. The temperature dependence of the interactions obtained after 24 h in 1 xC. E. Herder, P . M . Claesson and P. C. Herder 1941 103 lo2 - 10' loo 'E 2 I lo-' lo-* I I I I I I OO 00 0 0 0 0 0 0 .oo 8 @ 0 0 0 0 0 0 0 I I I I I I 0 1 2 3 4 5 6 7 Dlnm Fig. 6. The repulsive part of the force, FIR, us. distance in 4 x mol dmP3 DDAO at 17 "C. 0 is measured when pressing the surfaces together and when separating them. Discussion Adsorption Mechanism When discussing the adsorption mechanism of DDAO on mica, it is important to keep in mind the charge separation in the amine oxide head group.The negative oxygen atom makes it a good proton acceptor with a pK, of 4.65.2 In the concentration range studied the pH varied between 5.8 and 6.8, so between 1 and 10% of the molecules in the solution will be protonated and hence charged. Even though most of the molecules are uncharged, we would like to explain the monolayer adsorption with an ion-exchange mechanism. The protons adsorbed on the negatively charged mica surface in pure water are exchanged for protonated, cationic amine oxide molecules. This mechanism is strongly supported by the low surface potential obtained for the interaction of two monolayer-covered surfaces (fig. 3 4 ) . Contact-angle measurements on mica immersed in DDPO solutions indicate that the phosphine oxide surfactant is not adsorbed on mica, thus giving additional support to this mechanism.Phosphine oxide surfactants are far less protonated than amine oxide and hence they are not expected to be adsorbed on mica. 65 FAR 11942 Amine Oxide Surfactant Layers Table 1. A comparison of hydration forces between bare mica sheets in 5 x mol dm-3 NaCl and mica sheets coated with four different bilayers. D* /nma FR-l/mN m-l DDAOb Cl2ESr DLPC/DMPCd NaCP 0.1 2.1 - 2.1 > 3.4 1 1.7 2.0 2.0 2.8 10 1.1 1 .o 1.2 0.8 < 0.7 - 100 0.2 0 a Surface separation between bilayer-coated mica and bare mica is given for different values of FIR. In the case of bilayer-coated mica, D* is set to zero for a fully compressed bilayer. bThis work.Ref. (3). Ref. (16). Ref. (2). The monolayer formed in 1 x mol dm-3 solution is not thermodynamically stable and a second layer immediately begins to be adsorbed on top of the first. During this adsorption process, which requires several hours, the bilayer is not mechanically stable in the sense that the second layer could be squeezed out when applying high pressures. No double-layer forces are observed between DDAO-bilayers showing that mostly uncharged molecules build up the second layer. The formation of a bilayer is confirmed by the lowering of the advancing and receding contact angles and the increase in number density of DDAO molecules as determined with XPS. The values of the advancing contact angles on bilayer-covered mica are higher than expected for a completely hydrophilic surface.The reason for this might be the presence of two methyl groups in the dimethylamine oxide head group. Surface Interactions The interactions in 1 x mol dm-3 DDAO could not be explained by DLVO theory, as is shown in fig. 4. The attraction is more long-range than a van der Waals force but not as long-range as in the case of dioctadecyldimethylammonium-coated surfaces. This could be an indication of the lower hydrophobicity. There are too few experimental points in the attractive region to allow the force law to be determined in any detail. The small attractive minimum obtained between two bilayer-coated surfaces could be explained as a van der Waals attraction, using the Lifshitz' theory across a triple-layer film according to Ninham et al.,,' Our 'film' is comprised of a solution layer (3) of varying thickness D, two bilayers (2) of thickness T = 2.05 nm and mica (1) on the outside.The Hamaker constants used were J. The calculated force is included in fig. 5, showing good agreement with the experiment. No temperature dependence was found for this attractive minimum. This is consistent with the fact that no phase separation occurs in the DDAO-water system when changing the temperature. By contrast, the interactions between adsorbed layers of penta- oxyethylenedodecyl ether are strongly temperature-dependent at small separation^,^ At 15 "C the interaction between such layers was found to be purely repulsive, whereas at higher temperatures the force became partly attractive. This temperature-dependent = 0.5 x lo-,' J, A,,, = -0.5 x lo-,' J and A,,, = 0.5 xC .E. Herder, P . M . Claesson and P . C . Herder 1943 interaction between ethylene oxide groups was identified as the cause for phase separation occurring in the ethylene oxide surfactant-water system. This hypothesis is supported by the lack of temperature dependence in the interaction between DDAO bilayers. The repulsive hydration force measured between adsorbed bilayers of DDAO (fig. 6) is consistent with earlier measurements of hydration forces between bilayers of zwitterioniP and non-ionic surfactant l a y e ~ s . ~ Direct comparisons are made in table 1. Also included in the table are values of hydration forces between pure mica surfaces with 5 x moldrn-3 NaCl as measured by Pashley.21 These latter forces show an exponential decrease with increasing separation and are much more long-range than hydration forces between ' non-ionic ' bilayers.Conclusions A thermodynamically unstable monolayer is first formed on mica surfaces immersed in 1 x lop3 mol dm-3 surfactant solutions. The hydrophobic attraction between these surfaces is stronger than the expected van der Waals force but considerably weaker than the force between hydrophobic Langmuir-Blodgett-coated surfaces.ll The interaction between bilayers of DDAO is repulsive at short distances. The hydration force which has a range of ca. 2-3 nm shows no temperature dependence in the range 14-27 O C , whereas, in the case of pentaoxyethylenedodecyl ether bilayers, the repulsive force observed is strongly temperature-dependent. These dissimilarities are consistent with their different phase behaviour. References 1 R. G. Laughlin, Adu. Liquid Crystals, 1978, 3, 41. 2 R. G. Laugh, Adv. Liquid Crystals, 1978, 3, 99. 3 P. M. Ckaesson, R. Kjellander, P. Stenius and H. K. Christenson, J. Chem. SOC., Faraday Trans. I , 4 E. S . Lutton, J . Am. Oil Chemists SOC.. 1966, 43, 28. 5 K. W. Herrman, J. G. Brushmiller and W. L. Courchene, J. Phys. Chem., 1966, 70, 2909. 6 K. W. Herrman, J. Phys. Chem., 1962, 66, 295. 7 D. Tabor, F. R. S. Winterton and R. H. S. Winterton, Proc. R. SOC. London, Ser. A, 1969, 312, 435. 8 D. Tabor, J . Colloid Interface Sci., 1977, 58, 2. 9 J. N. Israelachvili and G . E. Adams, Nature (London), 1976, 262, 774. 1986, 82, 2735. 10 J. N. Israelachvili and G. E. Adams, J. Chem. SOC., Faraday Trans. I , 1978, 74, 975. 1 I P. M. Claesson, C. E. Blom, P. C. Herder and B. W. Ninham, J. Colloid Interface Sci., 1986, 114, 234. 12 J. N. Israelachvili and R. M. Pashley, J . Colloid Interface Sci., 1984, 98, 500. 13 R. M. Pashley, P. M. McGuiggan, B. W. Ninham and D. F. Evans, Science, 1985, 229, 1088. 14 R. M. Pashley, P. M. McGuiggan, B. W. Ninham, J. Brady and D. F. Evans, J. Phys. Chem., 1986,90, 15 R. M. Pashley, P. M. McGuiggan, R. G. Horn and B. W. Ninham, J. Colloid Interface Sci., submitted. 16 J. Marra and J. N. Israelachvili, Biochemistry, 1985, 24, 4608. 17 P. C. Herder, P. M. Claesson and C. E. Herder, J. Colloid Interface Sci., 1987, 119, 155. 18 P. M. Claesson, P. Herder, P. Stenius, J. C. Eriksson and R. M. Pashley, J. Colloid Interfuce Sci., 1986, 19 B. Derjaguin, Kolloid Z . , 1934, 69, 155. 20 B. W. Ninham and V. A. Parsegian, J. Chem. Phys., 1970, 52, 4578. 21 R. M. Pashley, J. Colloid Interface Sci., 1981, 80, 153. 1637. 109, 31. Paper 8/01331D; Received 5th April, 1988 65-2

ISSN:0300-9599    

DOI:10.1039/F19898501933

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J . Chem. SOC., Furaduy Trans. 1, 1989, 85(8), 1945-1962 Zeolites treated with Silicon Tetrachloride Vapour Part 5.-Catalytic Cracking of n-Hexane Michael W. Anderson, Jacek Klinowski and John M. Thomas? Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW Michael T. Barlow B. P . Research Centre, Chertsey Road, Sunbury-on- Thames, Middlesex T W16 7LN The catalytic activity of a series of samples of zeolite Y dealuminated with silicon tetrachloride vapour was tested by cracking of n-hexane at various temperatures. A reaction mechanism, involving high-molecular-weight intermediates, is proposed to account for the observed product distribution. Preliminary experiments with the cracking on dealuminated ferrierite and zeolite omega (synthetic mazzite) are also reported.Earlier papers' have described the preparation and characterization of a series of samples of zeolite Y dealuminated with silicon tetrachloride vapour at elevated temperatures. We now consider the catalytic performance of these zeolites with reference to their acidic properties and the catalytic behaviour of hydrothermally dealuminated samples. Several investigations have been carried out of the catalytic properties of aluminium-deficient zeolites. In a study of the rate of iso-octane cracking over ultrastabilised zeolite Y Barthomeuf and Beaumont2 found that all samples with more than 37 aluminium atoms per unit cell exhibited similar cracking activities. Below this aluminium content the activity dropped, suggesting that the strong acid sites, of which there are ca.40 per unit cell, were the seat of the cracking activity. The distribution of products did not change a great deal over a wide range of aluminium contents. Using aluminium-deficient zeolites Y prepared by various techniques Jacobs et aL3 found that the nature of the acid site involved varies from one catalytic reaction to another. For example, the site for toluene disproportionation appeared to be stronger than that required for cumene cracking. Abbas et aL4 investigated the isomerisation of cyclopropane over acid-leached and over steam-treated zeolite Y. They found a variation in activation energy with aluminium content, and related this effect to a change in acid strength of active sites upon dealumination. We have used the cracking of n- hexane as the test reaction for the measurement of catalytic activity.This approach was initiated by Miale et aL5 who found that cracking of n-hexane over a variety of zeolites and amorphous silica-aluminas revealed the existence of a wide range of catalytic activities. However, despite this variation, the apparent activation energy, E,, was in most cases close to 120 kJ mol-'. This fact was used to derive the activity parameter, a, defined as the activity of the zeolite relative to that of amorphous silica-alumina at an extrapolated temperature of 538 "C. a ranged from unity to over lo4, and the samples with the highest values are referred to as 'superactive'. t Present address : Davy-Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street. London WlX.19451946 Catalytic Cracking of n- Hexane Table 1. Heats of adsorption of n-hexane on zeolite Y no. of aluminium Si/AI atoms per unit cell AH/kJ mol-' 2.5 54.8 -42.7 6.2 26.7 -41.4 7.0 23.4 -48.1" 9.0 19.2 - 42.7 100 1.9 -48.3 "Sample which may have been partially dea- luminated (ultrastabilised) by deep-bed treat- men t. Experiment a1 The cracking of n-hexane was performed in a Pyrex microreactor holding I .5 cm3 of catalyst of 10-25 mesh crystal size. Hydrogen forms of the zeolites were prepared by calcining the ammonium-exchanged forms. Nitrogen was passed through the reactor at a rate of 10 cm3 min-l, which corresponds to a contact time of 9 s. The nitrogen could be passed either directly through the reactor or diverted via a bubbler containing 99% pure n-hexane maintained at 25 0.1 "C.Prior to testing, catalysts were activated by heating in air to 538 "C for 15 min. All glassware was packed with fused alumina in order to reduce dead space. Conversion was monitored after 5 min on stream by sampling with a syringe through a septum placed close to the catalyst. All products were analysed by gas chromatography and conversions were kept between 5 and 40% to avoid analytical inaccuracies and mass- and heat-transport effects. With some of the less siliceous material extensive coking of the catalyst meant that catalytic runs at different temperatures had to be performed on fresh samples. Initial heats of adsorption, H , of n-hexane over the dealuminated zeolites, were determined by the pulse-flow technique described later.The activation energy for n-hexane cracking, E, is related to the apparent activation energy E,, determined from kinetic studies, by E = E,+( - A H ) (1) where AH is the heat of adsorption. In order to determine AH a 6 cm chromatographic column was packed with zeolite of 60-80 mesh screen size. Hexane in a nitrogen carrier gas was passed over the catalyst at adsorption temperatures between 100 and 200 "C, and the retention times measured. The preparation of the SiC1,-treated catalysts was described in detail elsewhere. In Table 1 shows the Si/Al ratios of the zeolites tested. In the parent material 92% of the sodium cations where replaced by protons. In the dealuminated materials some of the cation-exchange capacity is satisfied by residual extra-framework cationic aluminium species.At Si/Al = 7.2 this accounts for half the exchange capacity. At higher Si/AI ratios of ca. 50 the extra-framework A1 content is very 10w.l~ Results and Discussion Heats of Adsorption of n-Hexane Determination of the heat of adsorption by gas-solid chromatography is described by Greene and Pust' and Hamerski' and the application of the method they used to the study of zeolite X was examined by Eberly et aZ.**' The relationship between the chromatographic data and the adsorption equilibrium constant, B', is Llt, ue = 1/B'M. W. Anderson, J . Klinowski, J . M. Thomas and M . T. Barlow 1947 2.4 4 a' I . 2.2 2.3 2.4 2.5 2.6 lo3 K/T Fig. 1. Logarithm of the corrected retention time us. 1/T for the absorption of n-hexane.Si/AI: 0 , 2.5; W, 6.2; 0, 7.0; A, 9.0; +, > 100. where L is the length of packed column, t, is the retention time of pulse maximum, Ue is the superficial linear gas velocity, i.e. velocity in a completely empty column and B is the adsorption equilibrium constant, defined as the ratio of the number of molecules adsorbed per unit volume of column at equilibrium to the number of molecules in the same volume of gas. Since B' is a thermodynamic equilibrium constant we may write B = Bexp(-AH/RT) (3) Int, = C - A H / R T (4) where B is a constant. Substituting eqn (3) in eqn (2) and rearranging we have where C = In (BL/ Uc). The observed retention time must be corrected to 25 "C and atmospheric pressure where V z is the limiting retention volume, pi is the column inlet pressure, p, is the column outlet pressure, is the column temperature, & is the temperature of flow measuring device and F is the flow rate of carrier gas.As the column is very short, the ratio pi/po is close to unity and therefore t,(corr) = t , </T. (6) In(?, TJT) = C-AH/RT. (7) Inserting t,(corr) into eqn (4) instead o f t , gives A plot of In(?, C / T ) t's. I/Tshould therefore be a straight line with slope - A H / R . The advantage of this method for determining AH over more conventional methods is that AH is calculated at high temperature and low sorbate loading under conditions which closely simulate those used for cracking tests. Plots of t,(corr) us. I / T for a series of dealuminated zeolites are given in fig. 1, and the values of AH are listed in table 1.AH is fairly constant, ca. 42 kJ mo1-', for samples with different Si/AI ratios. The sample with Si/AI = 7.0 which may have undergone deep-bed treatment simultaneously with treatment with SiCl, vapourfd has a slightly larger value of AH, as does the highly dealuminated sample (Si/AI = 100).1948 Catalytic Cracking of n- Hexane I I I I I I 1 1 1.9 1.8 1 7 1.6 1 5 1.4 1.3 1.2 103 K I T Fig. 2. Arrhenius plots for the cracking of n-hexane over zeolite Y. Cracking of n-Hexane The degree of conversion of n-hexane to different products over the various zeolite samples and different temperatures is listed in the Appendix, while the corresponding Arrhenius plots are given in fig. 2. The latter were calculated on the assumption that the reaction kinetics are first-order and obey the equation (8) k = A exp (- EJRT).The first-order rate constant is obtained from the degree of conversion, E , by 1 k = Aln [l/(l - E ) ] z (9) where t is contact time. This relationship was verified by varying t and plotting it us. In [1/(1 --&)I. As is shown in fig. 3 the plots are linear, confirming the assumption of first-order kinetics. Cracking on zeolites with high aluminium contents causes extensive coking of the sample and consequent deactivation. Upon dealumination the apparent activation energy for the cracking of n-hexane decreases, as does the activation energy. The profile shown in fig. 4 is very similar to that found by Abbas et al.4 for the isomerisation of cyclopropane over hydrothermally treated zeolite Y , although the absolute values are somewhat different.The activation energy, E, drops from 160 kJ mol-' for the parent material to ca. 112 kJ mol-' at an aluminium content of ca. 20 atoms per unit cell. Between 20 and 4 aluminium atoms E remains constant, but below 4 aluminium atoms it rises steeply. A plot of the framework A1 content, nA,, us. conversion, E , at 315 "C over the range of constant activation energy is linear (see fig. 5). For the parent material Ea is 117 kJ mol-l, which agrees with the values reported by Miale et al.5 for a variety of aluminosilicates. Abbas et aL4 suggested that the initial activation energy could be explained in terms of the Sanderson electronegativity model as described by Mortier. lo According to this, dealumination of the framework increases its electronegativity, which in turn increases the polarisation of the framework 0-H bonds, making acid sites stronger and reducing the activation energy for protonation of hydrocarbons.This implies further that the nature of the Brarnsted-acid sites plays a fundamental role in cracking.M . W. Anderson, J . Klinowski, J. M . Thomas and M . T. Barlow 1949 2 6 10 14 18 TI s Fig. 3. Contact times, t, us. In[l/(l -&)I. 0, Si/AI = 20.7; +, Si/AI > 100. 2 00 160 120 80 I =' & I I I I I 1 10 20 30 40 50 60 no. of Al atoms per unit cell Fig. 4. E (+), E, (m) and - A H (0) us. aluminium content of the samples for the cracking of n-hexane over zeolite Y. An increase in the acid strength with decreasing aluminium content for the same samples reported in this work was observed previously.le This was based on a shift of the infrared band at 3640 cm-', corresponding to an 0-H stretch, to lower frequency.Interestingly, at ca. 15 aluminium atoms per unit cell the 3640 cm-' band no longer moves to lower frequency. This correlates closely with the point at which the activation1950 Catalytic Cracking of n- Hexane 1.0 0-8 0.6 & 0-4 0.2 0 0 4 8 12 16 20 24 nAl Fig. 5. Conversion of n-hexane, extrapolated to 315 "C us. the number of framework aluminium atoms per unit cell, nA,, for a series of SiC1,-treated samples of zeolite Y. energy for n-hexane cracking levels off. This indicates a direct correlation between acid strength and n-hexane cracking activity. It is also possible that activation energy is reduced because the reaction takes place in the newly formed Lewis acid sites thought to be associated with extra-framework aluminium.It is well known that in the presence of strong Brsnsted acids Lewis acids are ideal for forming a carbonium ion from an alkene. This may provide a mechanism for the isomerisation of unsaturated reaction products but is less likely to be the driving force behind the initial carbocation formation. Reaction Mechanisms It has been proposed1' that the formation of a carbonium ion is initiated by thermal production of an olefinic species: R,-CH,-CH,-R, --+ R,-CH=CH-R, + H,. This is a gas-phase reaction and its activation energy is likely to be high. Protonation of the olefinic species leads to the formation of a carbocation:12 R -CH = CH-R*+ HZ -+ R - C H ~ H - R +z- I HM .W. Anderson, J. Hinowski, J. M. Thomas and M. T. Barlow 1951 where HZ denotes the Brernsted-acid site. From studies of the rates of cracking of n-hexane13 it is known that the second reaction is over 200 times faster than the first. It would therefore appear that olefin formation is the rate-determining step. If this is the case, modification of the acid sites (for example by dealumination of the zeolitic framework) should have little effect on the activation energy, unless changed electrostatic potentials in the interstitial space alter the activation energy for the production of an olefinic species. However, as is evident from fig. 4, the activation energy changes considerably, and it is therefore necessary to postulate another mechanism for carbocation formation in which framework hydroxyls play a fundamental role in altering the rate-determining step.There are two possible routes to carbocation formation which would intimately involve the catalyst. First, the catalytic dehydrogenation of n-hexane may be initiated by extra-framework aluminium. This would then be followed by protonation by the zeolite as, indicated in the above reaction, to form a carbenium ion. The other possibility is the formation of an Olah-type non-classical carbonium ion1* by direct protonation of the n-hexane by the zeolite. This is shown below: This reaction would then be followed by a cleavage of the three-centre bond to yield a conventional carbenium ion : The above mechanism offers an explanation for the drop in activation energy in the cracking of n-hexane upon dealumination.Indeed C i species have only been shown to exist in superacidic environments (such as fluorosulphuric acid, H , = - 15.6) which are similar to the intracrystalline zeolitic environment. After the formation of the carbonium ion the reaction path becomes rather complicated. Cleavage of carbonxarbon bonds may take place via a a-scission one carbon away from the electric charge,". l5 producing an olefin and a new carbonium ion : H H H H H I I I l l + I I I I H 3 C : C : C : C : C : C : R H H H H H H H 1 1 1 7 7 : C : C : R I I H H This primary carbonium ion is very unstable and must isornerise to carbonium The olefin is then very likely to form a carbonium ion a secondary itself, which1952 Catalyric Cracking of n- Hexane would then undergo isomerisation.The possible reaction paths for this mechanism are given in scheme 1. nC4 +C2 + iC4 +C2 C3 +C2 +C, + iC4 +C2 iC5 +C1 P H -H2 + nC6 +/\/\I+ /\/\/+'ic6 ;c6 nC5 +C1+ iC5 +C1 L c3 +c2 +c* J L iC4 +C2 iC5 + C t Scheme 1. Assuming that secondary reactions are insignificant, the underlined species will be the major products. The absence of secondary reactions was verified by varying contact times which did not alter the distribution of reaction products to any great extent. According to the reaction scheme the product distribution should contain all species from C, to C,. Inspection of the product distributions given in the Appendix reveals that for all samples except those with very high Si/Al ratios there are virtually no C, or C, species.This discrepancy between the expected and observed product distributions is well do~umented.'~~ 16-19 The distribution found for the parent material closely resembles that reported by Tung and McIninch." The main product for cracking over the low Si/A1 ratio materials is propane followed by isobutane and isopentane. The straight- chain hydrocarbons are in lower abundance, while skeletal isomerisation amounts to ca. 5% of the product. If the reaction scheme shown above were to be operating then the ratio of the rates of formation of each kind of secondary carbonium ion would be approximately : + + with C , w C, and C, w C4, w products for cracking over a samples except that with the The distribution of reaction ,here the Cs are concentrations.variety of dealuminated samples is given in table 2. For all very high Si/Al ratio the amounts of C, and C, species are very low. The need to postulate a reaction mechanism other than a simple p-scission of the carbocation was first realised by Bolton and Lanewalal' when interpreting theM. W. Anderson, J. Klinowski, J. M . Thomas and M. T. Barlow Table 2. Distribution of products for the cracking of n-hexane on zeolites Y 1953 2.5 2.5 2.5 2.5 2.5 4.8 7 .O 7.0 7.6 7.6 20.7 33.7 33.7 48.8 48.8 75.0 75.0 75.0 > 100 > 100 > 100 ~~ 54 33 1 54 340 54 354 54 363 54 372 33 23 1 24 230 24 300 22 276 22 282 8.8 300 5.5 312 5.5 353 3.8 316 3.8 349 2.5 403 2.5 445 2.5 494 I .9 499 1.9 538 I .9 548 > 100 > 50 > 100 > 100 > 100 > 100 > 100 > 100 > 100 > 100 > 100 > 50 > 100 > 100 > 100 > 30 23 2.3 3.4 2.3 2.1 21 15 18 18 18 > 50 > 50 > 100 > 50 > 100 > 50 > 30 18 21 17 7 7 3 .O 1.9 1.8 I .7 3.6 3.1 3.1 3.1 3.1 7.0 4.9 5.3 7.5 8.1 5.8 4.9 4.6 4.2 4.2 2.9 2.3 2.2 0.8 1 .o 0.9 I 5.5 4.6 4.7 6.2 5.0 2.9 6.0 6.3 6.3 7.2 10.8 6.0 6.4 4.7 5.2 2.5 3.4 1.6 1 .o 0.9 1.1 ~ ~~ N , , denotes the number of aluminium atoms per unit cell.I11 Scheme 2. IV I1 c c c \ I \ / \ c c c I I c c c /c\ /c\ 7 0.40 0.57 0.42 0.45 0.46 0.12 0.10 0.13 0.16 0.12 0.22 0.24 0.33 0.23 0.35 0.47 0.74 1.07 I .25 1.46 1.15 distribution of products of isomerisation of hexane over Pt-loaded zeolite Y. They considered disproportionation with the production of four intermediates based on cyclohexane, as shown in scheme 2.Bolton and Lanewala were able to show that the main products of catalytic cracking of these species were 2- and 3-methylpentane, with some 2,3-dimethylbutane but no 2,2-dimethylbutane. Bolton and Bujalski suggested1954 Catalytic Cracking of n-Hexane rr 0 100 200 300 400 TI "C Fig. 6. The number of supercages per molecule of adsorbed n-hexane on dealuminated zeolite Y us. temperature. later18 that the formation of C,, C, and C, species could be understood in terms of the reactions : 2C6 + K 1 2 l + 3c4 + 4c, +c,+c4+c5 with no C, or C, species being formed. Closer examination of the products formed by scission of the reaction intermediates in scheme 2 suggests that the main product would always be C,.Haag et al.', demonstrated that the structure of zeolite ZSM-5 imposes shape selectivity on the activity of n-hexane and interpreted their findings in terms of a large reaction complex H c c which is anchored in the zeolite as a charge-balancing cation. This seems quite likely if we consider the reaction as follows. First a carbocation is formed, probably by direct protonation of hexane by a framework hydroxyl. The carbocation cleaves according to the rules of P-scission : + /\\I + c; The reactions (i) are less likely than reaction (ii) since they result in a primary carbocation which may only be stabilised by hydride transfer.12 Reaction (ii) will now be considered in some detail. The primary carbocation forn. -d by reaction (ii) will isomerise via a hydride shift to give the secondary isopropyl carbocation.Because of its positive charge the carbocation may become pinned to the site at which it was formed by the negative charge on theM . W. Anderson, J . Klinowski, J . M. Thomas and M. T. Barlow 1955 zeolitic framework. If it were not adsorbed in this manner it would be unlikely to come into contact with another molecule of n-hexane, as the concentration of the reactant in the zeolite at reaction temperatures is very low. Also, the kinetics of the reaction would probably be second-order. It can be seen from fig. 6, which shows the uptake of n-hexane on zeolite Y at elevated temperatures, that above 150 "C there is less than one molecule per supercage. At 300 O C , a typical reaction temperature, there is only one molecule of n-hexane per 10 supercages.The probability of two molecules coming together under these conditions is very low. By being in close contact with the zeolite framework, the secondary carbocation, which is inherently unstable, may stabilise by spreading its charge on to the framework. Such hyperconjugation could occur by overlap of the vacant p orbital of the carbocation with the lone pair of electrons in the sp*-hybridised orbitals of the framework oxygen. Reaction of the secondary carbocation with another n-hexane molecule to form a larger tertiary carbocation would further increase its stability according to scheme 3, which also gives the four possible tertiary carbocations and their products. C c c c C+-H+C C C / \ / \ I \ / C C Ci +iC4 Ct + iC5 C; +23DMB c; +2MP 23DMB = 2,3-dhethylbutane 2MP = 2-methylpentane Scheme 3.1956 Catalytic Cracking of n-Hexane Reactions which result in the primary CT and C l carbocations have been neglected for reasons given previously.In this way a product distribution consisting of mainly C,, C,, C, and iso-C, compounds will result as observed experimentally. We suggest a reaction mechanism involving the following: ( I ) the lifetime of carbocations C l and C;t is too short for them to react with n-hexane molecules to a significant extent. They are neutralised by hydride transfer. (2) Once formed, a carbocation is either neutralised by hydride transfer or becomes pinned to the framework to await an n-hexane molecule. (3) Carbocations rearrange by hydride shift to tertiary carbocations. (4) Large reaction intermediates are formed which crack via p-scission. ( 5 ) Reaction intermediates leading to products larger than C, are negligible. (6) Secondary reactions other than simple isomerisation to more stable species are negligible.For very low aluminium contents (Si/AI 2 75) much more C, and C, is produced, the ratio C,/2(C4 + C,) approaches unity, there is little isomerisation, not all the olefins become saturated, and the activation energy increases rapidly. All these factors are characteristic of thermal cracking, which is consistent with the fact that higher temperatures are required for cracking over low-aluminium-content samples and that they contain very few catalytically active centres. The role of Lewis acidity in all these reactions is not clear.We know that the Brarnsted sites are playing an important role in the catalytic cracking as the 3640 cm-l infrared band, corresponding to Brarnsted-acid sites in the supercage, is eliminated during the cracking process.lf However, the possibility that Lewis acidity also plays a role in carbocation formation by hydride extraction cannot be ruled out: where [L] denotes a Lewis-acid site. Isomerisation of olefins by Lewis acids is well known, but their interactions with the much more stable paraffins is less well understood. We have shownld that Lewis acidity is present not only in the dealuminated samples but in the parent zeolite H-Y as well. The formation of Lewis acidity upon dealumination cannot therefore be the only cause of the changes in activation energy and product distribution.a-values could not be calculated for these catalysts as the activation energy for cracking varies between different zeolite samples. Such a comparison, as reported by Miale et al.,, is only valid if all the catalysts exhibit the same activation energy. Cracking by Other Dealuminated Zeolites Cracking of n-hexane over zeolite omega (synthetic mazzite) and zeolite omega treated with silicon tetrachloride leads to a similar distribution of reaction products to that found for zeolite Y of similar Si/AI ratio (see Appendix). The fact that the parent zeolite omega shows a similar product distribution to dealuminated Y with about the same Si/AI ratio suggests that Lewis acidity, which is only present to any significant extent in the latter, does not play an important role in the process.The activation energy for zeolite omega is ca. 65 kJ mol-', rather lower than for zeolite Y. The correspond- ing Arrhenius plots are given in fig. 7. 1.r. spectra of zeolite omega in the hydroxyl stretch region indicate the presence of only one kind of acid site, and the frequency (ca. 3580 cm-l) suggests that it is stronger than in zeolite Y. This is likely to be respon- sible for the low activation energy. In the case of ferrierite the product distribution is rather different from that for zeolites Y and omega (see table 3). Over the parent sample the products are mostly saturated hydrocarbons, which is indicative of catalytic, rather than thermal, cracking. The C, species are the most abundant, and the ratios C,/C, and CJC, are much closer to unity than for zeolite Y.There is also little isomerisation. The most likely reason for thisM. W. Anderson, J . Klinowski, J . M . Thomas and M . T. Barlow 1957 0 9 - 2 - -Y s - 4 - 2.2 2.0 1.8 lo3 KIT 1.6 Fig. 7. Arrhenius plots for the cracking of n-hexane over parent zeolite omega (m, Si/AI = 4.24) and partially dealuminated omega (0, Si/Al = 6.00). Table 3. Distribution of products for the cracking of n-hexane on zeolites on zeolites omega and ferrierite Si/AI N* I T/"C CJC, C,/C, iC,/nC, iC5/nC5 C3/2(C, + C,) __ __ - - . __ _ ~~ __ 4.24 4.24 4.24 4.24 6.0 6.0 6.0 4.6 4.6 4.6 9.3 9.3 9.3 9.3 6.9 6.9 6.9 6.9 5.1 5.1 5.1 6.4 6.4 6.4 3.5 3.5 3.5 3.5 omega 200 > 100 > 100 232 > 100 > 100 252 > 100 > 50 268 > 100 > 100 209 > 100 > 100 235 > 100 > 100 252 > 100 > 50 ferrierite 280 9.4 2.6 300 8.I 2.8 320 5.4 2.5 376 1.9 0.9 389 2.2 1 .O 410 2.2 1.2 430 2.1 1.2 8.8 6.7 5.0 4.6 8.5 5.3 4.4 0.5 0.5 0.5 0.6 0.6 0.7 0.8 > 100 7.9 6.0 5 .O 6.4 5.1 > 100 0.4 0.6 0.3 0.0 0.0 0.0 0.0 0.05 0.08 0.10 0.12 0.07 0.09 0.12 0.57 0.56 0.58 1.07 0.97 0.87 0.84 product distribution is the steric restrictions imposed on the prqducts and reaction intermediates by the small pore size of ferrierite (4.3 x 5.5 A). Large reaction intermediates of the kind shown in scheme 3 can no longer form, and cracking must proceed along the more conventional routes shown in scheme 1. This gives further support to our postulate that large reaction intermediates are formed in wide-pore zeolites.Cracking over ferrierite treated with SiCl, produces olefinic C, and C , species, which suggests that insufficient hydrogen is available for the products to be saturated hydrocarbons. Thermal cracking is unlikely as the activation energy is low. Restricted1958 Catalytic Cracking of n-Hexane 0 - 2 * - 4 Ei - 6 - 8 / 0@ a /@ /@' 1.74 1.58 1.4 2 103 K/T Fig. 8. Arrhenius plots for the cracking of n-hexane over parent ferrierite (m, Si/AI = 4.6) and partially dealuminated ferrierite (@, Si/AI = 9.3). deposition of coke in the narrow channels of ferrierite might reduce the amount of available hydrogen. The activation energy is ca. 85 kJ mol-' and the corresponding Arrhenius plot is given in fig. 8.Conclusions It is clear that zeolite Y treated with silicon tetrachloride vapour exhibits interesting catalytic properties. The activation energy for the cracking of n-hexane is reduced by nearly 30% on dealumination. This is probably caused by an increase in acid strength resulting from an increased electronegativity of the silicon-rich zeolitic framework. The postulated reaction mechanism requires the formation of a carbonium ion via a non- classical CRS species followed by a sequence of reactions during which the carbonium ion is pinned to the zeolitic framework as a charge-balancing cation and large reaction intermediates are formed. In the medium-pore zeolite ferrierite no such large reaction intermediaries can be formed and this is reflected in the product distribution. We are grateful to B.P.Research Centre, Sunbury-on-Thames, for support and to Professor J. H. Lunsford, Texas A & M University, for his comments on the manuscript. References I ( a ) M. W. Anderson and J. Klinowski, J. Chem. SOC., Faraday Trans. 1, 1986, 82, 1449; (6) M. W. Anderson and J. Klinowski, J . Chem. SOC., Faraday Trans. 1, 1986, 82, 3569; (c) M. W. Anderson, J . Klinowski and J. M. Thomas, J . Chem. SOC., Faraday Trans. I , 1986,82,2851; ( d ) M. W. Anderson and J. Klinowski, Zeolites, 1986, 6, 150; (e) M. W . Anderson and J . Klinowski, Zeolites, 1986, 6, 455; (.f) M. W. Anderson, Ph.D. Thesis (Cambridge University, 1984). 2 D. Barthomeuf and R. Beaumont, J . Catal., 1973, 30, 288. 3 P. A. Jacobs, H. E. Leeman and J. B. Uytterhoeven, J .Catal., 1974, 33, 31. 4 S. H. Abbas, T. K. Al-Dawood, J. Dwyer, F. R. Fitch, A. Georgopoulos, F. J. Machado and S. M. Smyth, in Catalysis by Zeolites, ed. B. Imelik, C. Naccache, Y. Ben Taarit, J. C . Vedrine, G. Coudurier and H. Praliaud (Elsevier, Amsterdam, 1980), p. 127.M . W. Anderson, J . Klinowski, J . M. Thomas and M. T. Barlow 1959 5 J . N. Miale. N. Y. Chen and P. B. Weisz, J. Cural., 1966, 6, 278. 6 S. A. Greene and H. Pust, J. Phys. Chcm., 1958, 62, 55. 7 J. J. Hamerski, Ph.D. Thesis (University of the Pacific, 1963). 8 P. E. Eberly Jr, J . Phys. Chem., 1961, 65, 68. 9 P. E. Eberly Jr and E. H. Spencer, Trans. Furuday Soc., 1961, 57, 289. 10 W. J. Mortier, J. Cutul., 1978, 55, 138. 1 1 P. A. Jacobs, in Carhoniogenic Activity qf Zeolifes (Elsevier, Amsterdam, 1977).12 V. Haensel, Adz?. Cutul., 1951, 3, 179. 13 W. 0. Haag, R. M. Lago and P. B. Weisz, Furuduy Discuss. Chem. Soc., 1981, 72, 317. 14 G . A. Olah, G . K. Surya Prakash and J. Sommer, Science, 1979, 206, 13. 15 F. Whitmore, Chem. Eng. News, 1948, 26, 673. 16 S. E. Tung and E. McIninch, J. Catal., 1968, 10, 175. 17 A. P. Bolton and M. A. Lanewala, J. Cutuf.. 1970, 18, 1. 18 A. P. Bolton and R. L. Bujalski, J. Cutul.. 1971, 23, 331. 19 J. A. Rabo, Cutul. Rev. Sci. Eng., I98 I , 23, 293. Paper 8/01890A; Received 13th May, 1988 Appendix Distribution of products and the total conversion (YO) for the cracking of n-hexane by various zeolitic catalysts at temperatures indicated. (a) Catalyst: H-Y, Si/Al = 2.5 T/"C 33 1 340 354 363 372 0.0 1 0.02 0.02 0.03 0.12 0.22 0.32 0.47 C2H6 0.03 0.05 0.06 0.09 3.78 6.14 8.46 12.37 iC, 2.5 1 2.98 5.29 7.48 nC4 0.69 0.96 1.70 2.42 nc, 0.24 0.26 0.52 0.54 iC, 0.70 I .2 2.5 0.6 conversion 9.43 13.04 21.38 27.36 CH4 C2H4 c3 iC5 1.31 1.20 2.46 3.34 0.04 0.60 0.12 14.70 9.68 3.14 4.5 1 0.90 2.0 35.71 (6) Catalyst: Na-Y treated with SiCI,, Si/Al = 7.0 T/"C 240 260 0.0 I 0.0 1 0.09 0.07 0.04 0.04 c3 2.91 4.65 iC, 9.61 14.00 iC5 5.10 8.44 iC, 3.0 8.9 conversion 22.9 39.3 iC, are mostly 2- and 3-methylpentane with some 2.3-dimethylbutane and very little 2,24imethylbutane.CH4 CJ-4 C*H, nC4 1.32 1.75 nc, 0.59 0.77 .~1960 Catalyric Cracking of n- Hexane (c) Catalyst: Na-Y treated with SiCl,, Si/AI = 20.7 T/"C 276 284 306 CH4 0.0 1 C2H4 0.0 I C2H6 0.0 1 iC, 3.45 iCS 1.99 c3 I .68 nC4 0.5 I nC5 0.26 iC, 3.96 conversion 2.25 0.00 0.03 0.02 2.34 4.9 I 0.69 2.97 0.35 10.34 21.78 0.0 I 0.09 0.04 4.42 7.73 1.19 4.85 0.55 13.94 32.85 (d) CataIyst: Na-Y treated with SiCI,, Si/AI = 33 T/"C 292 312 3 34 353 CH4 0.06 C2H4 0.04 c3 0.56 iC, 0.62 iC, 0.36 nCS 0.16 conversion 3.47 'ZH6 0.02 nC4 0.20 iC, I .43 ~~ 0.0 1 0.0 1 0.02 0.04 0.08 0.19 0.02 0.03 0.07 1.37 2.09 4.91 1.59 2.15 3.77 0.32 0.44 0.82 0.76 1.43 2.48 0.20 0.26 0.39 3.80 5.25 6.59 8.11 11.74 19.24 (e) Catalyst: Na-Y treated with SiCI,, Si/AI = 49 T/"C 306 316 349 369 0.00 0.04 c3 0.75 iC, 0.99 iC, 0.67 nC5 0.13 iC, 3.29 conversion 6.12 CH4 C2-4 0.01 nC4 0.22 C2H4 0.0 1 0.01 0.02 0.05 0.17 0.29 0.02 0.05 0.08 1.14 4.31 7.16 1.12 3.05 4.81 0.29 0.73 1.12 0.80 1.98 3.01 0.17 0.38 0.56 3.7 1 4.57 6.1 7.38 15.3 23.24M .W. Anderson, J . Klinowski, J . M . Thomas and M . T. Barlow 1961 (f) Catalyst: Na-Y treated with SiCl,, Si/AI = 75 T/"C 303 403 445 494 524 0.00 0.19 c3 0.23 iC, 0.39 nC4 0.23 iC5 0.26 nC5 0.03 iC6 negligible conversion 1.43 CH4 '2*6 0.00 C2H4 0.04 0.3 1 0.05 3.57 1.88 0.65 0.89 0.35 7.39 0.05 0.28 0.09 5.67 1.87 0.80 0.88 0.26 9.61 0.54 1.60 0.52 16.41 4.37 2.02 0.77 0.47 27.76 0.66 2.05 0.80 20.93 4.45 2.63 1.45 0.56 33.57 (g) Catalyst: Na-Y treated with SiCI,, Si/Al > 100 T/"C 403 499 532 538 548 CH4 0.0 1 C2H4 0.02 'ZH6 0.0 1 c3 0.39 iC, 0.06 nC4 0.04 iC5 0.03 nC5 0.03 iC6 negligible conversion 0.6 0.10 0.22 0.29 3.33 0.45 0.54 0.17 0.17 0.25 0.62 0.68 7.25 1.10 1.28 0.34 0.3 I 0.26 0.85 0.60 9.32 1.29 1.30 0.29 0.32 5.3 11.87 14.26 0.44 1.1 I 1.08 10.80 1.76 2.0 1 0.49 0.43 18.17 (h) Catalyst : NHi-exchanged zeolite omega T/OC 200 232 252 268 0.00 0.00 0.04 0.00 0.00 0.02 'ZH6 0.00 0.02 0.06 c3 0.12 0.66 2.37 n c , 0.06 0.29 1.08 iC5 0.69 1.74 4.21 nC5 0.00 0.22 0.70 iC6 0.87 8.72 9.69 conversion 2.26 13.58 23.63 CH4 C2H4 iC, 0.53 1.94 5.45 0.03 0.02 0.06 3.92 7.84 1.72 6.00 1.19 12.74 33.501962 Catalytic Cracking of n-Hexane (i) Catalyst: NHf-exchanged dealuminated omega, Si/AI = 5.4 T/"C 209 0.00 0.00 c3 0.05 iC, 0.17 iC5 0.17 iC, 4.12 conversion 4.55 CH, C2H, C2H6 0.00 nC, 0.02 nc, 0.00 235 252 0.00 0.02 0.00 0.02 0.00 0.03 0.30 1.20 0.74 2.28 0.14 0.52 0.64 1.77 0.10 0.45 4.47 6.44 6.40 12.65 . .. ~ . __ - - ( j ) Catalyst: NHf-exchanged synthetic ferrierite, Si/AI = 4.6 T/ "C 280 300 320 0.12 0.09 C2H6 0.93 iC, 0.95 nC4 1.74 iC5 0.30 nC5 0.83 iC6 negligible conversion 9.3 I CH, c3 4.34 C2H4 0.25 0.15 1.69 8.13 1.66 3.51 0.73 1.29 17.42 0.5 I 0.27 2.93 12.44 2.68 5.25 0.71 2.05 26.85 (k) Catalyst : NHi-exchanged SiCl,-treated ferrierite, Si/AI = 9.3 ~ _ ~ ~ ~ ~ _ _ _ ~ _______. - ~ T/OC 280 376 389 410 430 0.00 0.19 0.25 0.03 0.46 0.66 C2H6 0.07 1.21 I .57 iC, 0.04 0.56 0.91 nC, 0.00 0.90 1.39 nC5 0.00 0.37 0.56 iC, negligible conversion 0.48 7.62 10.88 CH4 C2H4 c3 0.26 3.92 5.54 iC5 0.07 0.00 0.00 0.4 1 1.16 2.3 1 8.92 1.18 2.40 0.00 0.9 1 17.92 0.64 1.84 3.23 12.30 2.72 3.32 0.03 1.29 25.38

ISSN:0300-9599    

DOI:10.1039/F19898501945

出版商:RSC    

年代:1989

数据来源: RSC