摘要:

ISSN 0300-9599 JCFTAR 82 ( I 2 ) 3525-371 9 (1 986) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases 3525 3535 3553 3561 3569 3587 360 1 361 1 3625 3635 3647 3657 3667 368 1 3697 3709 3717 CONTENTS Fourier Transform Infrared Studies of the Irreversible Oxidation of Cyanide at Platinum Electrodes A. S. Hinman, R. A. Kydd and R. P. Cooney The Dielectric Properties of Zeolites in Variable Temperature and Humidity A. R. Haidar and A. K. Jonscher The Time-domain Response of Humid Zeolites A. K. Jonscher and A. R. Haidar Molecular-orbital Studies of C-H Bond Scission induced by Ionizing Radia- tion T. Tada Zeolites treated with Silicon Tetrachloride Vapour. Part 2.-Sorption Studies M. W. Anderson and J. Klinowski Studies of Propene Oxidation over Mixed Uranium-Antimony Oxides F.J. Farrell, T. G. Nevell and D. J. Hucknall Adsorption and Desorption Kinetics of Oxygen on Tin-Antimony Oxide Cat a1 y st by Quasi -cons tan t Coverage Met hods M-C. Bacchus-Montabonel and CO Adsorption at 77 K on KCl Films. An Infrared Investigation D. Scarano and A. Zecchina The Utilization of Time-resolved Dielectric Loss to probe the Role of the Surface in Heterogeneous Photochemistry C. J. Dobbin, A. R. McIntosh, J. R. Bolton, Z. D. Popovic and J. R. Harbour Polysilicate Equilibria in Concentrated Sodium Silicate Solutions I. L. Svensson, S. Sjoberg and L-0. Ohman Potentials of Ion-exchanged Synthetic Zeolite-Polymer Membranes M. Demertzis and N. P. Evmiridis Adsorption and Reduction of Nitrogen Monoxide by Potassium-doped Carbon T.Okuhara and K. Tanaka Physicochemical Properties and Isomerization Activity of Chlorinated Pt/Al,O, Catalysts A. Melchor, E. Garbowski, M-V. Mathieu and M. Primet Excess Pressures for Aqueous Solutions M. J. Blandamer, J. Burgess and A. W. Hakin Effect of Temperature on the Point of Zero Charge and Surface Dissociation Constants of Aqueous Suspensions of y-Al,O, K. Ch. Akratopulu, L. Vordonis and A. Lycourghiotis Thermal Decomposition of Solid Sodium Bicarbonate M. C. Ball, C. M. Snelling, A. N. Strachan and R. M. Strachan Reviews of Books J-P. Joly I I7ISSN 0300-9599 JCFTAR 82 ( I 2 ) 3525-371 9 (1 986) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases 3525 3535 3553 3561 3569 3587 360 1 361 1 3625 3635 3647 3657 3667 368 1 3697 3709 3717 CONTENTS Fourier Transform Infrared Studies of the Irreversible Oxidation of Cyanide at Platinum Electrodes A.S. Hinman, R. A. Kydd and R. P. Cooney The Dielectric Properties of Zeolites in Variable Temperature and Humidity A. R. Haidar and A. K. Jonscher The Time-domain Response of Humid Zeolites A. K. Jonscher and A. R. Haidar Molecular-orbital Studies of C-H Bond Scission induced by Ionizing Radia- tion T. Tada Zeolites treated with Silicon Tetrachloride Vapour. Part 2.-Sorption Studies M. W. Anderson and J. Klinowski Studies of Propene Oxidation over Mixed Uranium-Antimony Oxides F. J. Farrell, T. G. Nevell and D. J. Hucknall Adsorption and Desorption Kinetics of Oxygen on Tin-Antimony Oxide Cat a1 y st by Quasi -cons tan t Coverage Met hods M-C.Bacchus-Montabonel and CO Adsorption at 77 K on KCl Films. An Infrared Investigation D. Scarano and A. Zecchina The Utilization of Time-resolved Dielectric Loss to probe the Role of the Surface in Heterogeneous Photochemistry C. J. Dobbin, A. R. McIntosh, J. R. Bolton, Z. D. Popovic and J. R. Harbour Polysilicate Equilibria in Concentrated Sodium Silicate Solutions I. L. Svensson, S. Sjoberg and L-0. Ohman Potentials of Ion-exchanged Synthetic Zeolite-Polymer Membranes M. Demertzis and N. P. Evmiridis Adsorption and Reduction of Nitrogen Monoxide by Potassium-doped Carbon T. Okuhara and K. Tanaka Physicochemical Properties and Isomerization Activity of Chlorinated Pt/Al,O, Catalysts A. Melchor, E. Garbowski, M-V. Mathieu and M. Primet Excess Pressures for Aqueous Solutions M. J. Blandamer, J. Burgess and A. W. Hakin Effect of Temperature on the Point of Zero Charge and Surface Dissociation Constants of Aqueous Suspensions of y-Al,O, K. Ch. Akratopulu, L. Vordonis and A. Lycourghiotis Thermal Decomposition of Solid Sodium Bicarbonate M. C. Ball, C. M. Snelling, A. N. Strachan and R. M. Strachan Reviews of Books J-P. Joly I I7

ISSN:0300-9599    

DOI:10.1039/F198682FX023

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

FA RA DA Y 1RA N SA CTlO N S AND SYMPOSIA From the Royal Society of Chemistry FARADAY TRANSACTIONS II Molecular and Chemical Physics SPECIAL ISSUE - AUGUST 1986 Professor Alan Carrlngton delbred the 1985 Faraday lecture at the Royal lnsmutlon on 10th December, 1985. As a compliment to Professor Carrington, a group of his colleagues and Mends submitted original papers on the general theme of Molecular Dynamics and Spectroscopy. These papers are collected in the present Issue. CONTENTs: The Faraday Lecture: Spectroscopy of Molecular Ions at thelr Dissociation Limits A. Carrington The Spectroscopy, Photophysics and Photochemistry of Clusters of Metrazlne D. H. Levy Spectroscopy of Transient Species produced by Photodissociation or Photoionizatlon in a Supersonlc Free-jet Expansion T.A. Mlller Molecukr-beam Infrared Spectroscopy of the Ar-N,O van der Waals Molecule J. Hodge, G. D. Hayman, T. R. Dykeand B. J. Howard The Estimation of Vlbrational Predissociation Ufetimes M. S. Chlld The Infrared Spectrum of H; and its Isotopomers. A Challenge to Theory and Experlment J. Tennyson and B. T. Sutcliffe The Augmented Secular Equation Method for calculating Spectra of van der Waals Complexes. Application to the Infrared Spechum of Ar-HCI J. M. Hutson Quantum-mechanical Wavepacket Dynamics of the CH Group in Symmetric- top X,CH Compounds using Effecttve Hamiltonians from Hlgh-resolution Spectroscopy R. Marguardt, M. Quack, J. Stohner and f. Sutcllffe Internal Dynamics of Subunits and Bondlng Force Constants In Weakly Bound Dlmers P. Cope, D.J. Mlllerand A. C. fegon Internal Dynamics and HF Bond Lengthenlng In the Hydrogen-bonded Heterodimer CH,CN . . . HF determined from Nuclear Hyper-fine Structure in its Rotational Spectrum P. Cope, D. J. Miller, L. C. Willoughbyand A. C. fegon Pumping and Rshing. Double-resonance Measurements on Molecular Jets U. Veeken, N. Damand J. Reuss nme-resow Fluorescence of Jet-cooled Carbazoles and thelr Weak Complexes A. R. Auty, A. C. Jones and D. Philllps Prediction of the Ct(*P, J/CI(P,J Branching Ratio in the Photodissociation of HCI S. C. GIvertzand 6. C. Ballnt-Kurt/ Hlgh-resolution Laser Photofragment Spectroscopy of CH' P. J. Sam, J. M. Walmsleyand C. J. Whitham Asymmetric Uneshapes associated with Predissociating levels M. N. R. Ashfold, R. N. Dixon, J. D. Prince, B.Tutcherand C. M. Westem A Threshold-photoelectron Ruorescence-Photon Coincidence Study of Radlatlonless TransMons in the B ,iI State of BCN+ €. Castellucci, G. Dulardin and S. leach Cornpetthe Channels in the Interaction of Xe('P,) with CI,, Br, and I,. Atom Transfer, Excitation Transfer, Energy Disposal and Product Alignment K. Johnson, R. Pease, J. P. Simons, P. A. Smith and A. Kvaran Mco Non-RSC Momkn P14.30 ($27.70) RSC Momkn M.00 Payment should accompany ordon for lhlr Ihm. RSC Members should send their orders to: Membershlp Manager, The Royal Society of Chemistry, 30 Russell Square, London WC1 B SDT. Non-RSC Members should send their orders to: The Royal Socleiy of Chemistry, Distribution Centre, Blackhorse Road. Letchwotth, Herts SG6 1 HN. Faraday Discussions No.80 Physical Interactions and Energy Exchange af the Gas-Solid Interface [his publication discusses aspects of current research on the gas-solid Interface: elastic, inelastic and dlssipattve scattering of atoms and molecules from cfystal surfaces; the structure and dynamics of physisorbed species, including overlayers. Emphasis Is placed on the themes of physical interactions and energy exchange rather than on molecular beam technology or the phenomenology of phase tmnsmons in overlayen. The interplay between theory and experlment is stressed as they relate to the nature of atom and molecule-surface interaction potentials including many body effects. Faraday Discussions No. 80 (1986) Softcevor Prico 531 .OO ($60.00) RSC Mombon J66.25 ROYAL SOCIETY OF CHEMISTRY Information Services (xiii)FA RA DA Y 1RA N SA CTlO N S AND SYMPOSIA From the Royal Society of Chemistry FARADAY TRANSACTIONS II Molecular and Chemical Physics SPECIAL ISSUE - AUGUST 1986 Professor Alan Carrlngton delbred the 1985 Faraday lecture at the Royal lnsmutlon on 10th December, 1985.As a compliment to Professor Carrington, a group of his colleagues and Mends submitted original papers on the general theme of Molecular Dynamics and Spectroscopy. These papers are collected in the present Issue. CONTENTs: The Faraday Lecture: Spectroscopy of Molecular Ions at thelr Dissociation Limits A. Carrington The Spectroscopy, Photophysics and Photochemistry of Clusters of Metrazlne D. H. Levy Spectroscopy of Transient Species produced by Photodissociation or Photoionizatlon in a Supersonlc Free-jet Expansion T.A. Mlller Molecukr-beam Infrared Spectroscopy of the Ar-N,O van der Waals Molecule J. Hodge, G. D. Hayman, T. R. Dykeand B. J. Howard The Estimation of Vlbrational Predissociation Ufetimes M. S. Chlld The Infrared Spectrum of H; and its Isotopomers. A Challenge to Theory and Experlment J. Tennyson and B. T. Sutcliffe The Augmented Secular Equation Method for calculating Spectra of van der Waals Complexes. Application to the Infrared Spechum of Ar-HCI J. M. Hutson Quantum-mechanical Wavepacket Dynamics of the CH Group in Symmetric- top X,CH Compounds using Effecttve Hamiltonians from Hlgh-resolution Spectroscopy R. Marguardt, M. Quack, J. Stohner and f. Sutcllffe Internal Dynamics of Subunits and Bondlng Force Constants In Weakly Bound Dlmers P.Cope, D. J. Mlllerand A. C. fegon Internal Dynamics and HF Bond Lengthenlng In the Hydrogen-bonded Heterodimer CH,CN . . . HF determined from Nuclear Hyper-fine Structure in its Rotational Spectrum P. Cope, D. J. Miller, L. C. Willoughbyand A. C. fegon Pumping and Rshing. Double-resonance Measurements on Molecular Jets U. Veeken, N. Damand J. Reuss nme-resow Fluorescence of Jet-cooled Carbazoles and thelr Weak Complexes A. R. Auty, A. C. Jones and D. Philllps Prediction of the Ct(*P, J/CI(P,J Branching Ratio in the Photodissociation of HCI S. C. GIvertzand 6. C. Ballnt-Kurt/ Hlgh-resolution Laser Photofragment Spectroscopy of CH' P. J. Sam, J. M. Walmsleyand C. J. Whitham Asymmetric Uneshapes associated with Predissociating levels M.N. R. Ashfold, R. N. Dixon, J. D. Prince, B. Tutcherand C. M. Westem A Threshold-photoelectron Ruorescence-Photon Coincidence Study of Radlatlonless TransMons in the B ,iI State of BCN+ €. Castellucci, G. Dulardin and S. leach Cornpetthe Channels in the Interaction of Xe('P,) with CI,, Br, and I,. Atom Transfer, Excitation Transfer, Energy Disposal and Product Alignment K. Johnson, R. Pease, J. P. Simons, P. A. Smith and A. Kvaran Mco Non-RSC Momkn P14.30 ($27.70) RSC Momkn M.00 Payment should accompany ordon for lhlr Ihm. RSC Members should send their orders to: Membershlp Manager, The Royal Society of Chemistry, 30 Russell Square, London WC1 B SDT. Non-RSC Members should send their orders to: The Royal Socleiy of Chemistry, Distribution Centre, Blackhorse Road. Letchwotth, Herts SG6 1 HN. Faraday Discussions No. 80 Physical Interactions and Energy Exchange af the Gas-Solid Interface [his publication discusses aspects of current research on the gas-solid Interface: elastic, inelastic and dlssipattve scattering of atoms and molecules from cfystal surfaces; the structure and dynamics of physisorbed species, including overlayers. Emphasis Is placed on the themes of physical interactions and energy exchange rather than on molecular beam technology or the phenomenology of phase tmnsmons in overlayen. The interplay between theory and experlment is stressed as they relate to the nature of atom and molecule-surface interaction potentials including many body effects. Faraday Discussions No. 80 (1986) Softcevor Prico 531 .OO ($60.00) RSC Mombon J66.25 ROYAL SOCIETY OF CHEMISTRY Information Services (xiii)

ISSN:0300-9599    

DOI:10.1039/F198682BX025

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ISSN 0300-9599 JCFTAR 82( 7) 201 1-2282 (1 986) 201 1 2025 2043 2057 2065 2079 2089 2103 2111 2123 2133 2141 2151 2155 2167 2175 2185 2195 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases CONTENTS Micellar Inhibition of the Aquation of Tris-(3,4,7,8-tetramethyl- 1,l O-phenan- throline)iron(rr) by Sodium Dodecyl Sulphate in Aqueous Acid Medium J. Ige and 0. Soriyan Catalytic Reactions on Metal-supported Semiconductors. Oxidation of CO over ZnO Films on Silver E. Weiss and M. Folman Spectroscopic Investigation of Sulphidation of Zinc and Lead Carbonates F. Garbassi and A. M. Marabini Mechanism of Permeation through Molecular-sieve Carbon Membrane. Part 1.-The Effect of Adsorption and the Dependence on Pressure J. E. Koresh and A.Soffer Concentration Polarization and Water Dissociation in Ion-exchange Mem- brane Electrodialysis. Mechanism of Water Dissociation Y. Tanaka and M. Sen6 lH and 19F Nuclear Magnetic Resonance Investigation of the Active Site of Catalase J. Oakes Thermodynamics of Formation of Inclusion Compounds in Water. a- Cyclodextrin-Alcohol Adducts at 298.15 K G. Barone, G. Castronuovo, P. Del Vecchio, V. Elia and M. Muscetta Reactions of Hydroxyalkyl Radicals with Uracil S. N. Bhattacharyya and P. C. Mandal Kinetic and Equilibrium Studies associated with the Formation of Inclusion Compounds involving n-Butanol and n-Pentanol in Aqueous Cyclodextrin Solutions D. Hall, D. Bloor, K. Tawarah and E. Wyn-Jones A Kinetic and Equilibrium Study of the Inclusion of Pyronine B by p- and y-Cyclodextrin R.L. Schiller, S. F. Lincoln and J. H. Coates The Reaction of Ferrimyoglobin with Methylhydroperoxide M. L. Kremer Characterization of Anion Solvation in N-Methylacetamide. Transfer Enthalpies of Anions and the Reaction Rates of Ethyl Iodide with Bromide Ion in N-Methylacetamide - Acetonitrile and N-Methylacetamide-NJV-Dimethylace- tamide Mixtures Y. Kondo, A. Nakano and S. Kusabayashi Superacid Sites in Zeolite H-Mordenite K. A. Becker and S. Kowalak Application of Infrared Spectroscopy to the Measurement of Surface and Bulk Oxidation/Reduction States of MOO, J. S. Chung and C. 0. Bennett Osmotic Coefficients of Aqueous NaCl and KC1 Solutions. Temperature- Concentration Behaviour of the Bahe Lattice Model J. L. G6mez-Estdvez Solution of Hydrogen in Thin Palladium Films S.Kishimoto, M. Inoue, N. Yoshida and T. B. Flanagan Deuterium Longitudinal Nuclear Magnetic Relaxation of Heavy Water in Alkylammonium Chloride Solutions. Effect of Deuterium Exchange between Water and Polar Heads M-P. Bozonnet-Frenot, J-P. Marchal and D. Canet The Viscosity and Structure of Solutions. Part 2.-Measurement of the B Coefficient of Viscosity for Alkali-metal Chlorides in Propanol-1-01-Water Mixtures at 25 and 35°C J. Crudden, G. M. Delaney, D. Feakins, P. J. O’Reilly, W. E. Waghorne and K. G. Lawrence 67 FAR 1Con tents 2207 The Viscosity and Structure of Solutions. Part 3.-Interpretation of the Thermodynamic Activation Parameters for Propan-1-01-Water-Electrolyte Systems J. Crudden, G. M. Delaney, D. Feakins, P. J. O’Reilly, W. E. Waghorne and K. G. Lawrence Adsorption, Desorption and Surface Decomposition of Formaldehyde and Acetaldehyde on Metal Films Nickel, Palladium and Aluminium J. M. Saleh and S. M. Hussain Thermal Conductivity of Argon, Nitrogen and Carbon Dioxide at Elevated Temperatures and Pressures A. I. Johns, S. Rashid, J. T. R. Watson and A. A. Clifford Entry Rate Coefficients in Emulsion Polymerization Systems I. A. Penboss, R. G. Gilbert and D. H. Napper Behaviour of Dinitrogen Species adsorbed on a Co/Al,Q, Catalyst with or without Coadsorbed Hydrogen studied by Infrared Spectroscopy N. Kinoshita, K. Kido, K. Domen, K. Aika and T. Onishi Catalysis by Colloidal Gold of the Reaction between Ferricyanide and Thio- sulphate Ions P. L. Freund and M. Spiro 222 1 2235 2247 2269 2277

ISSN:0300-9599    

DOI:10.1039/F198682FP081

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY 977 99 1 1003 1021 1033 1043 1057 1067 1077 Faraday Transactions II, Issue 7, I986 Molecular and Chemical Physics For the benefit of readers of Faraday Transactions I, the contents list of Faraday Transactions 11, Issue 7 , is reproduced below. Molecular Orbital Calculation of Octahedral (M6) Clusters of Group VIII Metals N. C. Datta and B. Sen Interactions of Excited-state Porphyrin-Quinone in Reversed Micelles Studied by Time-resolved Fluorescence Spectroscopy S. M. B. Costa and R. L. Brookfield Photolysis of 3-Methylcyclobutanone in the Gas Phase R. Becerra and H. M. Frey Monte Carlo Simulation of a Cyclen Molecule in Water S. V. Hannongbua and B. M. Rode Microcanonical Variational Transition State Theory : Quasi-collinear Configura- tions.Application to the C1+ CH,I --+ ClI + CH, Excitation Function L. Baiiares, J. Alonso, M. Menzinger and A. Gonzalez Ureiia Effects of Isotopic Substitution and Vibrational Excitation on Reaction Rates. Kinetics of OH(u = 0, 1) and OD(u = 0, 1) with HCl and DC1 I. W. M. Smith and M. D. Williams Absorption Cross-sections and Mutual Reaction Rate Constants for C,H, and t-C,H, Radicals N. L. Arthur Valence Electronic Structures of Tetrakis(alky1thio)tetrathiafulvalenes K. Seki, T. B. Tang, T. Mori, W. P. Ji, G. Saito and H. Inokuchi Quasiclassical Trajectory Studies of the Dynamics of the F+I, --+ IF+I Reaction M. I. Urrecha, F. Castaiio and J. Iturbe The following papers were accepted for publication in J . Chem. Soc., Faraday Trans. I during April 1986. 511441 511442 51 1945 51 1982 512082 512120 The Dielectric Properties of Zeolites in Variable Temperature and Humidity A.R. Haidar and A. K. Jonscher The Time-domain Response of Humid Zeolites A. K. Jonscher and A. R. Haidar Thermal Properties, Thermochemistry and Kinetics of the Thermal Dissociation of Hydrochlorides of some Mono-nitrogen Aromatic Bases J. Lubkowski and J. Blazejowski The Structure and Properties of Pillared Clays D. T. B. Tennakoon, W. Jones and J. M. Thomas Effects of Solvent and Ionic Medium on the Kinetics of Axial Ligand Substi- tution in Vitamin BIZ. Part 5.-Solvent Effects on the Thermodynamics of Aquocobalamin Chloride and Model Compounds in Dioxane-Water Mix- tures S. Balt and A. M. van Herk Reaction of Hydrocarbons on Alumina-supported Pt-Ir Bimetallic Catalysts.Part 1 .-Exchange of Methane and Cyclopentane with Deuterium and Hydro- genolysis of Butane, Pentane and Cyclopentane D. Garden, C. Kemball and I). A. Whan (05/2121 61146 61 157 61 190 6/23 1 61253 6/29 1 61292 61309 61335 61348 61365 61370 61433 61465 61483 61642 61669 Reactions of Hydrocarbons on Alumina-supported Pt-Ir Bimetallic Catalysts. Part 2.-Exchange of Benzene with Deuterium, and Exchange and Hydrogen- olysis of 2,2-Dimethylpropane A. C. Far0 and C. Kemball Diffusion Phenomena and Metal Complex Formation Equilibria. Part 1 D. R. Crow Kinetics of the P-Hydroxyl Elimination Reactions from the (Protoporphyrin) Iron(rI1)-CHRCH,OH Complexes in Aqueous Solutions. A Pulse-radiolytic Study Y. Sorek and D. Meyerstein CO Adsorption at 77 K on KCl films: an Infrared Investigation D.Scarano and A. Zecchina Polysilicate Equilibria in Concentrated Sodium Silicate Solutions I. L. Svensson, S. Sjoberg and L-0. Ohman Catalytic Ignition on PdAu Wires Electron Spin Resonance Studies of Free and Supported 12-H.eteropoly Acids. Part 3.-The Stability of Unsupported H,+,(VP,Mo,,-,O,,) -xH,O in Air and in Vacuum Electron Spin Resonance Studies of Free and Supported 12-Heteropoly Acids. Part 4.-The Reduction of H,+,(PV,Mo,,-,O,,) -xH,O and the Influence of Supports on their Properties Physico-chemical Properties and Isomerization Activity of Chlorinated Pt/Al,O, Catalysts A. Melchor, E. Garbowski, M. V. Mathieu and M. Primet Excess Pressures for Aqueous Solutions M. J. Blandamer, J. Burgess and A. W. Hakin The Thermodynamics of Solvation of Ions.Part 3.-The Enthalpy of Hydration at 298.15 K Y. Marcus The Thermal Decomposition of Solid Sodium Bicarbonate M. C. Ball, C. M. Snelling, A. N. Strachan and R. M. Strachan The Identification and Characterisation of Mixed Oxidation States at Oxidised Titanium Surfaces A. F. Carley, P. R. Chalker, J. C. Riviere and M. Wyn Roberts Site Transfer Conductivity in Solid Iron Hexacyanoferrates by Dielectric Relaxometry, Voltammetry and Spectroscopy : Prussian Blue, Congeners and Mixtures D. R. Rosseinsky, J. S. Tonge, J. Berthelot and J. F. Cassidy Ionic Solvation in Water-Cosolvent Mixtures. Part 1 3.-Free Energies of Transfer of Single Ions from Water into Water-Tetrahydrofuran Mixtures I. M. Sidahmed and C. F. Wells Incorporation and Stability of Iron in Molecular-sieve Structures : Ferrisilicate Analogues of Zeolite ZSM-5 Setschenow Coefficients for Caffeine, Theophylline and Theobromomine in Aqueous Electrolyte Solutions P.Perez-Tejeda, A. Maestre, M. Balon, J. Hidalgo, M. A. Munoz and M. Sanchez Optical and Spectromagnetical Properties of Phosphate Glasses containing Ruthenium and Titanium Ions S. Pizzini, D. Narducci, D. Daverio, C. Mari, F. Morazzoni and A. Gervasini D. E. Eley and A. H. Klepping R. Fricke, H. G. Jerschkewitz and G. Ohlmann R. Szostak, V. Nair and T. L. ThomasCumulative Author Index 1986 Abu-Gharib, E.-E. A., 1471 Adams, D. M., 1020 Adams, M., 1979 Aida, M., 1619 Aika, K-i., 2269 Al-Hakim, M., 1575 Albery, W. J., 1033 Allen, G. C., 1367 Alwis, U. de, 1265 Ammann, D., 1179 Anderson, J.A., 1911 Anderson, M. W., 569, 1449 Anderson, S. L. T., 1537 Anderson, T., 767 Antoniou, A. A., 483 Araya, P., 1351 Attwood, D., 1903 Avent, A. G., 1589 Aveyard, R. 125, 1031, 1755 Baldwin, R. R., 89 Balk, R. W., 933 Barone, G., 2089 Bartlett, J. R., 597 Bartlett, P. N., 1033 Baur, J., 1081 Becker, K. A., 2151 Belton, P. S., 451 Benecke, J. I., 1945 Bennett, C. O., 2155 Berezin, I. V., 319 Bernstein, T., 1879 Berry, F. J., 1023 Bhattacharyya, S. N., 2103 Bieth, H., 1935 Binks, B. P., 125, 1031, 1755 Biswas, P. K., 1973 Blake, P. G., 723 Blandamer, M. J., 1022, 1471 Bloemendal, M., 53 Bloor, D., 21 11 Boelhouwer, C., 1945 Bond, G. C., 1985 Booth, B. L., 2007 Booth, C., 1865 Boucher, E. A., 1589 Bozonnet-Frenot, M-P., 21 85 Brereton, 1.M., 1999 Brett, C. M. A., 1071 Brigandi, P. W., 1032 Brillas, E., 495, 1781 Bruckenstein, S., 1105 Buck, R. P., 1169 Bui, V. T., 899 Burch, R., 1985 Burgess, J., 1471 Cameron, P., 1389 Canet, D., 2185 Carley, A. F., 723 Carpenter, T. A., 545 Casal, B., 1597 Cass, A. E. G., 1033 Castro, V. Di, 723 Castronuovo, G., 2089 Cenens, J., 28 1 Cesteros, L. C., 1321 Champion, J. V., 439 Chang, C. D., 1032 Chiou, C. T., 243 Chitale, S. M., 663 Chung, J. S., 2155 Clark, B., 1471 Clark, S., 125 Clifford, A. A., 2235 Coates, J. H., 2123 Cochran, S. J., 1721 Cohen de Lara, E., 365 Colin, A. C., 1839 Coller, B. A. W., 943 Compostizo, A., 1839 Cooney, R. P., 597 Copperthwaite, R. G., 1007 Cortes, J., 1351 Corti, H. R., 921 Covington, A. K., 1209 Craston, D. H., 1033 Craven, J.R., 1865 Crilly, J. F., 439 Crudden, J., 2195,2207 Danil de Namor, A. F., 349 Das, M. N., 1973 Dawber, J. G., 119 De Schrijver, F. C., 281 Dean, C. E., 89 Dearden, S. J., 1627 Del Vecchio, P., 2089 Delaney, G. M., 2195,2207 Delaval, Y., 365 Dharmalingam, P., 359 Domen, K., 2269 Domenech, J., 1781 Duatti, A., 1429 Duce, P. P., 1471 Eagland, D., 2008 Ebeid, E-Z. M., 909 Egdell, R. G., 2003 Ekechukwu, A. D., 1965 El-Daly, S. A., 909 Elbing, The Late E., 943 Elia, V., 2089 Elworthy, P. H., 1903 Espenscheid, M. W., 1051 Espinosa-Jimenez, M., 329 Evans, D. F., 1829 Ewen, R. J., 1127 Farnia, G., 1885 Feakins, D., 2195, 2207, 563 Fegan, S. G., 785, 801 Fernandez-Prini, R., 921 Findenegg, G. H., 2001 Fink, P., 1879 Fisher, D. T., 119 Flanagan, T. B., 2175 Folman, M., 2025 Foulds, N.C., 1259 Fraser, I. M., 607 Freiser, H., 1217 Freund, P. L., 2277 Fricke, R., 263, 273 Fukuda, H., 1561 Funabiki, T., 35, 707, 1771 Fyles, T. M., 617 Gabrys, B., 1923, 1929 Garbassi, F., 2043 Garbowski, E., 1893 Garrido, J. A., 1781 Gellan, A., 953 Geoffroy, M., 521 Gervasini, A., 1795 Ghatak-Roy, A. R., 1051 Ghoneim, M. M., 909 Ghousseini, L., 349 Gilbert, R. G., 1979, 2247 Gilhooley, K., 431 Gomez-Estevez, J. L., 2167 Gonzalez-Caballero, F., 329 Gonzhlez-Elipe, A. R., 739 Gonzilez-Fernandez, C. F., Gormally, J., 157 Gorton, L., 1245 Gosal, N., 1471 Green, M. J., 1237 Grieser, F., 1813, 1829 Gritzner, G., 1955 Grzybkowski, W., 1381, 1703, Guardado, P., 1471 Haggett, B. G. D., 1033 Hakin, A. W., 1471 Hall, D., 21 11 Halle, B., 401, 415 Hans&, O., 77 Heatley, F., 255 Hedges, W.M., 179 Hellring, S. D., 1032 Hemfrey, J. P., 1589 Hersey, A., 1271 Hewitt, E. A., 869 329 1745 (iii)AUTHOR INDEX Hey, M. J., 1805 Heyrovsk9, M., 585 Higgins, J. S., 2004, 1923, 1929 Higson, S., 157 Hill, C. A. S., 1127 Hill, H. A. O., 1237 Hill, T., 349 Hitchman, M. L., 1223 Hobert, H., 1527 Hobson, D. B., 869 Homer, J., 533 Honeybourne, C. L., 1127 Honeyman, M. R., 89 Hooper, A., 11 17 Houghton, J. D., 1127 Hronec, M., 1405 Hsu, W. P., 851 Hubbard, C. D., 1471 Humphreys, F. J., 1020, 2006 Hunt, D. J., 189 Hussian, S. M., 2221 Hutchings, G. J., 1007 Ige, J., 2011 Iizuka, T., 1681, 61 Ikeda, H., 61 Ikeda, O., 1561 Indelli, A., 1429 Inomata, S., 1733 Inoue, M., 2175 Inoue, T., 1681 Issa, R. M., 909 Iwamoto, M., 1713 Jackson, S.D., 431, 189 Jaeger, N., 205 Jayasuriya, D. S., 457, 473 Jensen, M., 1351 Johns, A. I., 2235 Johnson, D. C., 1081 Johnson, J., 1081 Johnston, P., 1007 Jones, W., 545 Jonson, B., 767 Jose, C. I., 663, 681, 691 Kakuta, N., 1553 Kamat, P. V., 1031 Kaminade, T., 707 Katime, I., 1321, 1333 Kavetskaya, 0. I., 319 Kawaguchi, T., 1441 Kawai, S., 527 Kawai, T., 527 Kazusaka, A., 1553 Kelly, H. C., 1271 Kelly, R. G., 1195 Kevan, L., 213 Khoo, K. H., 1 Kido, K., 2269 Kinoshita, N., 2269 Kishimoto, S., 2175 Kleine, A., 205 Klinowski, J., 569, 1449 KodejS, Z . , 1853 Komatsu, T., 1713 Komiyama, M., 1713 Kondo, Y., 2141 Koreeda, A., 527 Koresh, J. E., 2057 Kowalak, S., 2151 Kremer, M. L., 2133 Kusabayashi, S., 2141 Kuzuya, M., 1441 Lancz, M., 883 Lang, J., 109 Langevin, D., 2001 Larkins, F.P., 1721 Larsson, R., 767 Lawless, T. A., 1031 Lawrence, K. G., 563, 2195, Lawrence, M. J., 1903 Leaist, D. G., 247 LConard, J., 899 Lim, T.-K., 69 Lincoln, S. F., 2123, 1999 Llars, S., 767 Llinares, A., 521 Lockhart, J. C., 1161 Loewenschuss, A., 993 Logan, S. R., 161 Lomen, C. E., 1265 Lowe, B. M., 785,801 Lowe, C. R., 1259 Lundin, S. T., 767 MacCallum, J. R., 607 Mactaggart, J. W., 1805 Mahnke, R., 1413 Malliaris, A., 109 Mandal, P. C., 2103 Manes, M., 243 Marabini, A. M., 2043 Maran, F., 1885 Marchal, J-P., 2185 Marcus, Y., 233, 993 Marczewski, M., 1687 Martin, C. R., 1051 Maruthamuthu, P., 359 Mastikhin, V. M., 1879 Mathieu, M-V., 1893 Matsuda, T., 1357 McCarthy, S., 943 Mead, J., 125, 1031, 1755 Melchor, A., 1893 Miale, J. N., 1032 Miasik, J.J., 1117 Midgley, D., 1187 Minami, Z., 1357 Mishima, S., 1307 Mishra, S. P., 521 Miura, H., 1357 Miyake, Y., 1515 Miyamoto, A., 13 Mobbs, R. H., 1865 Mol, J. C., 1945 Mollett, C. C., 1589 Molyneux, P., 291, 635 Moore 111, R. B., 1051 Morazzoni, F., 1795 Morgan, H., 143 Mori, K., 13 Moyes, R. B., 189 2207 (iv) Mulla, S. T., 681, 691 Murakami, Y., 13 Muscetta, M., 2089 Nagano, S., 1357 Nagy, 0. B., 1789 Najbar, M., 1673 Nakajima, T., 1307 Nakamatsu, H., 527 Nakanishi, M., 1441 Nakano, A., 2141 Napper, D. H., 1979, 2247 Narayana, M., 213 Neto, M. M. P. M., 1071 Neuburger, G. G., 1081 Nikitas, P., 977 Nyasulu, F. W. M., 1223 Oakes, J., 2079 Oesch, U., 1179 Ogino, Y., 1713 ohlmann, G., 263, 273 Okazaki, S., 61 Okuda, T., 1441 Oldham, K. B., 1099 Onishi, T., 2269 Ooe, M., 35 Opallo, M., 339 Orchard, S.W., 1007 Oref, I., 1289 OReilly, P. J., 2195, 2207 Ortiz, A., 495 Owen, A. E., 1195 Parbhoo, B., 1789 Parsons, B. J., 1575 Pease, W. R., 747, 759 Peeters, G., 963 Peeters, S., 963 Peiia, M. D., 1839 Penboss, I. A., 2247 Penner, R. M., 1051 Perry, M. C . , 533 Pethig, R., 143 Pham, H. V., 1179 Phillips, G. O., 1575 Piculell, L., 387, 401, 415 Piekarska, A., 513 Piekarski, H., 513 Pilarczyk, M., 1703, 1745 Pinna, F., 1795 Pletcher, D., 179 Polta, J. A., 1081 Polta, T. Z., 1081 Pouchl9, J., 1605 Primet, M., 1893 Puchalska, D., 1381 Quintana, J. R., 1333 Radulovic, S., 1471 Rajaram, R. R., 1985 Ramdas, S., 545 Rashid, S., 2235 Rebenstorf, B., 767 Richardson, P. J., 869 Rideout, J., 167 Rigby, S., 431 Rizkallah, P. J., 1589 Roberts, M.W., 723Robinson, B. H., 1271 Robinson, P. J., 869 Rochester, C. H., 953, 1805, Rodriguez, R. M., 1781 Rooney, J. J., 2005 Rosenholm, J. B., 77 Rouw, A. C., 53 Rubio, R. G., 1839 Ruiz-Hitzky, E., 1597 Ryder, P. L., 205 Sacchetto, G. A., 1853 Saez, C., 1839 Saleh, J. M., 2221 Salmon, G. A., 161 Sanchez, F., 1471 Sandona, G., 1885 Sangster, D. F., 1979 Sirkany, A., 103 Sawada, K., 1733 Scharpf, O., 1923, 1929 Schiller, R. L., 2123 Schlosserova, J., 1405 Schmelzer, J., 1413, 1421 Schmitt, K. D., 1032 Schoonheydt, R. A., 281 Scott, R. P., 1389 Segall, R. L., 747, 759 Seloudoux, R., 365 Sen6, M., 2065 Shibata, Y., 1357 Shigeto, M., 1515 Shindo, H., 45 Shubin, A. A., 1879 Siiman, O., 851 Simmons, R. F., 1965 Simon, W., 1 179 Sircar, S., 831, 843 Smallridge, M.J., 1589 Smart, R. St C., 747, 759 Smith, I., 869 Smith, J. A. S., 2004 191 1 Schulz-Ekloff, G., 205 AUTHOR INDEX Snowdon, S., 943 Soffer, A., 2057 Sokoll, R., 1527 Solymosi, F., 883 Somsen, G., 53, 933 Soria, J., 739 Soriyan, O., 2011 Spiess, B., 1935 Spiro, M., 2277 Spotswood, T. M., 1999 Strazielle, C., 1321 Strukul, G., 1795 Strumolo, D., 1795 Sugiyama, K., 1357 Suppan, P., 509 Sutherland, I. O., 1145 Suzuki, T., 1733 Swallow, A. J., 1575 Symanski, J. S., 1105 Symons, M. C . R., 167 Szentirmay, M. N., 1051 Tamura, H., 1561 Tamura, K., 1619 Tanaka, T., 35 Tanaka, Y., 2065 Tang, A. P-C., 1081 Taniewska-Osiriska, S., 513, 1299 Tatam, R. P., 439 Tawarah, K., 21 11 Tear, S. P., 1022 Tennakoon, D. T. B., 545 Teramoto, M., 1515 Thijs, A., 963 Thomas, J. D. R., 1135 Thomas, J.M., 545 Thomson, A. J., 2009 Tofield, B. C., 1117 Townsend, R. P., 1019 Turner, P. S., 747, 759 Tyler, J. W., 1367 van de Ven, T. G. M., 457,473 Vansant, E. F., 963 Vekavakayanondha, S., 291,635 Venkatasubramanian, L., 359 Verhaert, I., 963 Veself, V., 1405 Volkov, A. I., 815 Vonk, D., 1945 Waghorne, W. E., 563, 2195, Walker, R. W., 89 Wallwork, S. C., 1589 Walton, A. J., 1023 Wang, Z-C., 375 Warhurst, P. R., 119 Warr, G. G., 1813, 1829 Watson, J. T. R., 2235 Watts, P., 1389 Weale, K. E., 1020, 2002 Weiss, E., 2025 Wells, P. B., 189 Whalley, P. D., 1209 Whyman, R., 189 Wiens, B., 247 Wilson, G. S., 1265 Wilson, I. R., 943 Wbjcik, D., 1381 Woinicka, J., 1299 Wren, B. W., 167 Wright, K. M., 451 Wu, E. L., 1032 Wuthier, U., 1179 Wyn-Jones, E., 21 11 Wysocki, S., 715 Xiaoding, X., 1945 Yamashita, H., 1771, 707 Yamazaki, A., 1553 Yatsimirsky, A.K., 319 Yeates, S. G., 1865 Yeo, I-H., 1081 Yoshida, N., 2175 Yoshida, S., 35, 707, 1771 Yoshikawa, M., 707, 1771 Zana, R., 109 Zanderighi, L., 1795 Zund, R., 1179 2207NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet ‘Quantities, Units, and Symbols‘ (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Chemistry in all its publications. Their basis is the Systbme International d’Unit6s‘ ( 9 ) . A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Ph ysicochemical Quantities and Units (Pergamon, Oxford, 1 979).Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn).Nomenclature of Inorganic Chemistry (Butterworths, London, 1971 , now publis- hed by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society’s editorial staff.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 82 Dynamics of Molecular Photof rag mentation University of Bristol, 1 S17 September 1986 Organising Committee: Professor R.N. Dixon (Chairman) Dr G. G. Balint-Kurti Dr M. S. Child Professor R. Donovan Professor J. P. Simons The discussion will focus on the interaction of radiation with small molecules, molecular ions and complexes leading directly or indirectly to their dissociation. Emphasis will be given to contributions which trace the detailed dynamics of the photodissociation process. The aim will be to bring together theory and experiment and thereby stimulate important future work. The programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 21 Promotion in HeterogeneousCatalysis University of Bath, 23-26 September 1986 Organising Committee: Professor F.S. Stone (Chairman) Dr R. Burch Mrs Y. A. Fish The symposium will form the Faraday Division Programme at the 1986 Autumn meeting of the Royal Society of Chemistry, however, it will be conducted as a discussion meeting, with pre-printed papers and subsequent publication, following the style of the traditional Faraday discussions and symposia. The role of promoters is of intrinsic interest as well as being important for many industrial processes. Promoters are used for three purposes, to improve catalyst activity, to increase selectivity for the desired reaction, and to prolong catalyst life at high activity and selectivity. There are current advances in both exprimental and theoretical aspects of promoter action, making this an opportune time for a Faraday symposium.Attention will be focussed on the role of promoters in enhancing activity and selectivity. Three areas will be highlighted - model studies using well-defined surfaces such as single crystals, characterization of promoter function in real catalysts, and theoretical aspects of promotion. The mechanisms of promoter action in metal, oxide and sulphide catalysts will be discussed. Dr R. W. Joyner Professor J. Pritchard Dr D. A. Young (Editor) The preliminary programme and application form may be obtained from: MrsY. A. Fish, The Royal Societyof Chemistry, Burlington House, London WlVOBN. (Vii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 22 Interaction-induced Spectra in Dense Fluids and Disordered Solids University of Cambridge, 10-11 December 1986 Organising Committee: Professor A.D. Buckingham (Chairman) Dr R. M. Lynden-Bell Dr P. A. Madden Professor E. W. J. Mitchell Dr J. Yarwood Dr D. A. Young Mrs Y. A. Fish Whilst interaction-induced spectra have been studied in the gas phase for many years, their importance in the spectroscopy of condensed matter has been appreciated only relatively recently. At present a considerable number of studies of induced spectra are taking place in what are (nominally) widely separated fields of study. It is highly desirable to bring these communities together so that common issues can be identified and the progress of one field appreciated in another. The preliminary programme may be obtained from : Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO.83 Brownian Motion University of Cambridge, 7-9 April 1987 Organising Committee Dr M. La1 (Chairman) Dr R. Ball Dr E. Dickinson Dr J. S. Higgins Dr P. N. Pusey Dr D. A. Young Mrs Y. A. Fish The aim of the meeting is to ~ k c u s s new developments in the experimental and theoretical studies of Brownian motion of colloidal particles and macromolecules, with particular emphasis on the dynamics of aggregate formation and breakdown, computer simulation and many- body hydrodynamic interactions. Further information may be obtained from: Dr M. Lal, Unilever Research, Port Sunlight Laboratory, Bebington, Wirral L63 3JW (viii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO.84 Dynamics of Elementary Gas-phase Reactions University of Birmingham, 14-1 6 September 1987 Organising Committee: Professor R. Grice (Chairman) Dr M. S. Child Dr J. N. L. Connor Dr M. J. Pilling Professor I. W. M. Smith Professor J. P. Simons The Discussion will focus on the development of experimental and theoretical approaches to the detailed description of elementary gas-phase reaction dynamics. Studies of reactions at high collision energy, state-to-state kinetics, non-adiabatic processes and thermal energy reactions will be included. Emphasis will be placed on systems exhibiting kinetic and dynamical behaviour which can be related to the structure of the reaction potential-energy surface or surfaces.Contributions for consideration by the Organising Committee are invited. Titles should be submitted as soon as possible and abstracts of about 300 words by 30 September 1986 to Professor R. Grice, Chemistry Department, University of Manchester, Manchester M13 9PLJOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistry/chemical physics which have appeared recently in J.Chem.Research, The Royal Society of Chemistry's synopsis+microform journal, include the following : Closed-loop Odd-alternant Polycyclic Polyenes Christopher Glidewell and Douglas The Radical Cation of Dimethyl Sulphate: an Electron Spin Resonance Study Robert Jones Lloyd (1 986, Issue 3) and Martyn C. R. Symons (1986, Issue 3) Analysis of the Kinetics of Two Consecutive First-order Reactions, and of Processes of Related Kinetic Form Roy B.Moodie (1 986, Issue 4) Study by a Continuous Kinetic Method of the Rate of Progesterone Adsorption by Silica Gel Julio Casado, Santiago Rincon and Francisco Salvador (1 986, Issue 4) Electron Spin Resonance Study of Anatase-supported Vanadia-Molybdena Catalysts Guido Busca and Leonard0 Marchetti (1986, Issue 5) A Comparison of Some Linear Substituent-free-energy Relationships Martien C. Spanjer and C. Leo de Ligny (1 986, Issue 5) Interception of the Electron-transport Chain in Bacteria with Hydrophilic Redox Mediators. Part 1. Selective Improvement of the Performance of Biofuel Cells with 2,6-Disulphonate Thionine as Mediator Anna M .Lithgow, Lorraine Romero, lvelisse C. Sanchez, Fernando A. Suoto and Carmen A.Vega (1 986, Issue 5) Ionic Strength Dependence of Complex-formation Enthalpies: a Literature Data Analysis Alessandro de Robertis, Concetta de Stefano, Carmelo Rigano and Silvio Sammartano (1 986, Issue 5) FARADAY DIVISION INFORMAL AND GROUP MEETINGS Polymer Physics Group with the British Rheological Society Deformation in Solid Polymers To be held at the University of Leeds on 9-1 1 September 1986 Further information from Dr J. V. Champion, Department of Physics, City of London Polytechnic, 31 Jewry Street, London ECBN 2EY Colloid and Interface Science Group Surfactant Systems with Liquid-Liquid Interfaces To be held at the University of Hull on 9-1 0 September 1986 Further information from Dr R. Aveyard, Department of Chemistry, The University, Hull HU6 7RX Statistical Mechanics and Thermodynamics Group Fractals: a New Development in Physical Chemistry To be held at the University of Salford on 1 &12 September 1986 Further information from Dr P. Francis, Department of Chemistry, The University, Hull HU6 7RXCarbon Group Carbon Fibres-Properties and Applications To be held at the University of Salford on 1 5 1 7 September 1986 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1X 8oX Electrochemistry Group with the Electroanal y tical Group New Electrode Materials for Electrochemistry and Electroanalytical Applications To be held at Imperial College, London on 15-17 September 1986 Further information from Professor W.J. Albery, Department of Chemistry, Imperial College, London SW7 2/42 Division with the Surface Reactivity and Catalysis Group-Autumn Meeting Promotion in Heterogeneous Catalysis To be held at the University of Bath on 23-25 September 1986 Further information from Professor F.S. Stone, School of Chemistry, University of Bath, Bath BA2 7AY Industrial Physical Chemistry Group Water Soluble Polymers and their Industrial Application To be held at Girton College, Cambridge on 24-26 September 1986 Further information from Dr I. 0. Robb, Unilever Research Laboratory, Port Sunlight, Bebington, Wirral L63 3JW Colloid and Interface Science Group with Macrogroup UK Polymer-Polymer interfaces To be held at the Scientific Societies Lecture Theatre, London on 15 December 1986 Further information from Or R.Aveyard, Department of Chemistry, The University, Hull HU6 7RX Colloid and Interface Science Group with the Colloid and Surface Group of the SCI Nucleation and Growth in Colloidal Systems To be held at the Society of Chemical Industry, 14 Belgrave Square, London on 16 December 1986 Further information from Or R. Aveyard, Department of Chemistry, The University, Hull HU6 7RXFARADAY TMNSACTIONS, DISCUSSIONS & SYMPOSIA From the Royal Society of Chemistry FARADAY fRANSACTlONS I SPECIAL ISSUE - APRIL 1986 This special issue of fafaday Transactlons l contains papers on electrochemical sensors, the result of a symposium held at the 30th International Congress of Pure and Applied Chemisty, Manchester, September 1985. The symposium covered five main areas - fundamentals, gas sensors, ion-selective electrodes, integrated devices and biosensors.Available as an individual publication, price €19.40 (S37.50), this special issue of fafaday Transactlons I wiii be of interest to those involved in electrical engineering, materials science, biochemistry, biotechnology, medical engineering as weii as analytical, physical and organic chemists. CONTENTS Keynote Lecture: Electrochemical Sensors - Theory and Experiment b y W. J. Albefy, P. N. hrtlett, A. E. G. Coss. D. H. Craston and B. G. D. Haggett. Sensors from Polymer Modified Electrodes b y M. W. Espenscheid, A. R. Ghotak-Roy. R. B. Moore, R. M. Penner, M. N. Szentlrmay and C. R. Martln. Trace-Metal Analysis in Hydroponic Solutions b y C. M. A. Brett and M. M. P. M. Neto. Anodic Detection in Flow-through Cells b y D.C. Johnson. Convolution of Voitammograms as a Method of Chemical Analysis b y K. 8. O#ham. Analytical Applications of Gas Membrane Electrodes b y S. Bruckensteln and J. S. Symnskl. Conducting Polymer G a s Sensors b y J. J. Mhxklk, A. Hooper and B. C. Tofleld. Semlconductlng Tetrapyrrole Gas Pigment Sensors b y C. L. Honeybourne, J. D. Houghton, R. J. €wen und C. A. S. HI//. Aspects of the Optimisation of PVC Matrix Membrane ion-selective Electrodes b y J. D. R. Thomas. Ion Recognition by Mactocyclic Hosts b y 1. 0. Sutherland. New Host - Guest Carriers b y J. C. Lockhart. Biosensors based on Reversible Reactions at Blocked and Unblocked Elecrodes b y R. P. Buck. Design of Anion-selective Membranes for Clinically Relevant Sensors b y U. Oesch, D. Ammnn, H. V. Pham, U. Wutheir, R. Zund and W. Simon. Ion-selective Electrodes based on Siderophones b y J. D. Mldgley. Solid-state Ion Sensors: Theoretical and Practical Issues b y R. G. Kelly and A. €. Aven. Recent Advances in Microelectronic Ion-sensitive Devices b y A. K. Covlngton and P. D. Whalley. Coated-wire Ion-selective Electrodes b y H. frelser. Potentiometric Monitoring of Proteins b y M. L. Hitchman and f. W. M. Atyasulu. Amperometric Enzyme Electrodes b y M. J. Green. Chemically Modified Electrodes for the Electrocatalytic Oxidation of Nicotinamide Coenzymes b y L. Gorton, Enzyme Entrapment in Eiecticaily Conducting Polymers b y N. C. Foulds and C. R. Lowe. Electrochemical Detection of Immunological Reactions b y G. S. Wlson. Price Non-RSC members E19.40 R3c momber E4.00. Payment should accompany orders tor thlr Item. ($37.50). RSC members should send their orders to: The Assistant Membership Officer, The Royal Society of Chemistry, 30 Russell Square, London WCIB 5DT. Non-RSC members should send their orders to: The Royal Society of Chemistry, Distribution Centre, Blackhorse Road, Leichworth, Herb SG6 IHN. Faraday Discussions No. 79 Polymer Liquid Crystals This publication discusses all aspects of the developlng subject of polymeric liquid crystals. Fafaday Dkcussion No. 79 (19sS) Softcover 296pp ISEN 0 85186 598 4 Price S28.W ($55.00) RSC members S6.50. Faraday Symposia No. 19 Molecular Electronic Structure Calculations - Methods and Applications This publicotlon reviews the current state of this rapidly developing dlsclpline provldlng valuable informotlon on recent methods and their applications. Forodcry Symposium No. 19 (1985) Softcover 208pp ISBN 0 85186 608 5 RSC Members SIO.OO. Prim E41.00 ($79.00) ROYAL SOCIETYOF CHEMISTRY lnformat ion ervices (xii)

ISSN:0300-9599    

DOI:10.1039/F198682BP083

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1986,82, 2011-2023 Micellar Inhibition of the Aquation of Tris-( 3,4,7,8-tetramethyl- 1 , 10-phenanthroline)iron(II) by Sodium Dodecyl Sulphate in Aqueous Acid Medium Jide Ige" and 0. Soriyan Department of Chemistry, University of Ife, IEe-rfe, Nigeria The inhibition of the aquation of Fe(Me,phen):+ by sodium dodecyl sulphate (SIX) in aqueous acid media has been investigated and a mechanism which explains the pronounced inhibition and pre-micellar activity at low [SDS], has been proposed. Inhibition is due to favourable thermodynamic- /hydrophobic/electrostatic binding between the Fell complex and SDS monomer aggregates. The bound FeII complex is stabilised with respect to dissociation and binding takes place between the ridges of the Stern layer.The partitioning of the substrate between the bulk-water phase and the micellar phase is in favour of the latter at low [H+], and low [SDS],. From the rate law obtained and the observed kinetic data, the micellesomplex binding constant (KJ and micelle-acid binding constant (K3) were calculated to be (2.8 1 f 0.08) x 1 O5 and (1 3.80 f 0.16) dm3 mol-l, respectively, in acid media. Using the Scatchard method, Kl values of (3.95 k0.08) x lo5 and (3.04 f 0.16) x lo5 dm3 mol-1 were calculated for the binding in neutral medium (distilled water) and 2 x 10-5 mol dm-3 H+, respectively. The decrease in Kl in acid media is attributed to competition between H+ and the complex ion for the binding sites on the micelle. The k,-[SDS], profiles are structured owing to the evolution (size, geometry, aggregation number, etc.) of the micelle.The inhibition of the aquation rate by HSOh and SO:- ions which form a negative field around the Fe" complex is only significant at high acid concentrations. The micelle-bound complex and micelle-bound protons have opposing effects on the aquation rate. The degree of inhibition is therefore sensitive to the ratio of the concentration of these bound species. The parallel between the catalytic behaviour of macromolecles and enzymes has led to a renewed interest of physical chemists and biochemists in chemical reactions which take place on charged surfaces in solution. A large volume of work has been done on micellar catalysis of organic reactions1*2 such as solvolyses, rearrangements and decarboxylations, but there has also been an increasing level of activity in the study of aqueous and reverse micelles on metal-ligand complex f~rmation,~ metal redox reactions4 and metal complex diss~ciation.~-~ One important advantage of some of the metal dissociation studies performed in this laboratory is the relative simplicity of the system investigated coupled with the fact that the reactions are sufficiently slow to be monitored by conventional techniques.These features allow for a fair qualitative insight into the extent of electrostatic and hydrophobic interactions, substrate partitioning and substrate solubilisation effects. These effects were investigated in the present study of the aquation of tris-(3,4,7,8-tetramethyl- 1,lO- phenanthroline) iron@) sulphate in aqueous SDS. A previous report from this laboratory8 showed the aquation of Fe(phen)2+ to be catalysed by dodecyl pyrazinium chloride (DPC) (a cationic micelle-forming surfactant) in aqueous perchloric acid.The k,-[DPC] profiles were structured and this was attributed to the evolution of the micelle. For the present study, the effect of the increased hydrophobic character of the ligand (Me,phen) and 201 1 67-22012 Micellar Inhibition of Aquation the change to an anionic surfactant (SDS) on the aquation rate is of interest. Whereas in the Fe(phen)i+-DPC system no mechanism was proposed, the present work looks at the mechanistic aspect of the aquation process in more detai1.T Experiment a1 Materials Fe(Me,phen),SO, (G. F. Smith chemical company) was used as supplied.It was characterised by its visible spectrum which gave E = 13700 dm3 mot1 cm-l, in excellent agreement with the literatureg value of 13800 dm3 mol-l (A, = 500 nm). Specially purified SDS (B.D.H.) was used as supplied and its purity was ascertained by determination of the critical micelle concentration (c.m.c.) in aqueous solution at 25 "C, which correlated with the literaturelo value of 8.1 x mol dm-3. All other reagents were AnalaR grade. Kinetics Kinetic measurements involved monitoring the change in absorbance of Fe(Me,phen)i+ at Amax = 500 nm as a function of time using a Pye-Unicam SP500 series 2 spectro- photometer fitted with an automatic cell changer and four thermostable cell compartments. The reaction components were mixed in 1 cm quartz cells in the sequence water, acid, surfactant and complex to ensure minimum pre-mixing aquation.The concentration of the complex was maintained at 1.459 x mol dm-3 (except where stated otherwise) to avoid complications that may arise from the dependence of the aquation rate on the concentration of the complex in surfactant solution. Such a dependence was established for the Fe(phen);+-DPC system reportedg earlier. Owing to its inertness, the complex does not aquate appreciably in the absence of acid, hence all runs were carried out in acidified solution. Sulphuric acid was used because Fe(Me,phen);+ is precipitated from the solution of other mineral acids. The acid concentration range was 0.005-1 .OO mol dmW3 H,SO,. The acid was treated as essentially monobasic, but corrections were made for the slight dissociation of HSO, within the conditions of the experiment.A dissociation constant of 1.20 x lop2 was used for HSO,. A constant temperature was maintained using the combination of cryocool CC-60T compressor, a Gallenkamp thermostatting unit and a fast Austen pump which supplied the cell compartments. All the stock solutions except that of the complex were placed in a thermostatted water bath. All runs were performed assuming pseudo-first-order kinetics. The aquation rate constant k , was obtained from the slope of plots of In (A, -A,) vs. time. The plots were always linear to more than three half-lives. The investigation of binding of the FeII complex with SDS in a neutral medium and in 1-00 x lod5 mol dm-3 H+ at 25 "C used an SP6-400 spectophotometer and analysis by the Scatchard method.ll Results Dependence of the Observed Rate Constant (k,) on Complex Concentration At 25 "C the observed rate constant (k,) increases with increase in [Fe(Me,phen)i+] at fixed surfactant and acid concentrations. [Complex], was in the range (0.3623-3.623) x mol dm-3 and [€I+], was fixed at 0.017 mol dm-3 in all the runs.The choice of I A more logical extension of the Fe(phen)i+-DPC system is an investigation of the behaviour of Fe(phen)g+-SDS. Initial data show that SDS inhibits the aquation of Fe(phen)g+. More data are necessary for the comparison of mechanisms. We hope to report our detailed findings in the future.J . Ige and 0. Soriyan 2013 1.6- ( b ) 1.5 - 1.4 - - 1.3- m p, 1 . 2 - .-( \ 3 1.1 1.0 0.9 - * - - 0 0.5 1 .o 1.5 2 .o 0 1.0 2.0 3.0 L.0 [complex J T/ 1 O-' mol d ~ n - ~ Fig.1. Variation of k, with [Fe(Me,phen)i+], at fixed SDS concentrations [1.0 x lov4 mol dmP3 (a); 2.0 x lod4 mol dmd3 (0); 3.0 x mol dm-3 (a)] in the presence of Hy0.017 mol dm-3) at 25 "C. surfactant concentrations was based on the structure in the k,-[SDS], profile. Fig. 1 is a plot of k, us. [complex], at (0.1, 0.2 and 3.0) x mol dm-3 SDS. The first two concentrations represent the region of the minimum in the k,-[SDS], profile (fig. 2). Fig. 1 is representative for any surfactant concentration and shows that k, varies linearly with [complex],. The slope of the k , vs. [complex], plots increases with increase in [SDS],. Fig. 1 shows that [complex], must be kept constant in any study of the dependence of k , on [SDS], to allow for meaningful interpretation of the experimental data.Effect of [SDSIT on k , at Constant w + ] T and [c~mplex]~ Overall data at constant [H+],, [complex], and varying [SDS], show that aquation of the complex is inhibited by the addition of SDS. Fig. 2 shows a plot of k , us. [SDS], at 25 "C with [H+], = 0.017 and 1.1 16 mol dm-3. There is a minimum in k , at 2.0 x lo-* mol dm-3 SDS. At [SDS], < 3.0 x lo-* mol dm-3, k, values at 1.1 16 mol dm-3 H+ are higher than the corresponding k , values at 0.017 mol dm-3 H+. At surfactant concentrations > 3.00 x rnol dm-3, k , values obtained at 1.116 rnol dm-3 [H'], are lower. These results indicate acid catalysis at fixed low surfactant concentrations and acid inhibition at fixed higher surfactant concentrations. At both acid concentrations the absolute initial inhibition of the aquation rate by the surfactant is markedly sharp prior to the minimum point.At each fixed acid concentration the k,-[SDS], profile shows saturation at [SDSIT > 1.75 x mol dm-3 (data at higher [SDS], are summarised in table 1). The observed inversion in the comparative trend of the relative magnitudes of k , at high and low surfactant concentrations for the two fixed H+ concentrations necessitates a study of the acid dependence of the aquation rate at fixed surfactant concentrations. The acid dependence was studied at two SDS concentrations chosen from the regions of acid catalysis and acid inhibition suggested by the k,-[SDS], profile.The results are stated below.2014 Micellar Inhibition of Aquation 1 v) I 0 I,,,,,, 0 1.0 2.0 3.0 4.0 5.0 6.0 [ SDS],/ 1 0-3 rnol dm-3 Fig. 2. Variation of k , with [SDS], at 25 "C. [H+IT = 0.017 mol dm-3 (A); [HfIT = 1.1 16 mol dmP3 (0); [Fe(Me,phen):+] = 1.459 x mol dm-3. Table 1. Observed aquation rate constants (k,) at 25 "Ca 0 8 .o 0.993 0.607 10.0 0.994 0.606 12.0 0.998 0.584 14.0 - 0.583 16.0 - 0.591 20.0 - 0.575 25.0 - 0.556 30.0 - 0.561 a [FE(Me,phen):+], = 1.459 x 1 0-5 rnol dmP3. [H'], = 0.017 mOl dm-3. [H+IT = 1.1 16 mol dm-3. Effect of Acid on k , at Constant [SDSIT and [c~mplex]~ The aquation rate was determined at 25 OC, [SDS], = 1.0 x rnol dm-3 and [H+], = 0.007-0.129 rnol dm-3. The results (fig. 3) show that the aquation is catalysed by acid at this surfactant concentration.We could not determine k , at acid concentrations < 0.007 mol d ~ n - ~ because of the establishment of a rapid equilibrium. The initial kinetic data also show mixed first- and second-order kinetics. The observed complexities here are left for extended future investigation of this work at [HS], < 0.007 mol dmP3. At acid concentration 2 0.007 mol dm-3, the reaction shows first-order kinetics for more than three half-lives. mol dmP3, k , decreases with increase in [HfIT, contrary to the observed catalysis at low surfactant concentration discussed above, but consistent When [SDS], = 3.0 x4.0 3.9 - I ," 3 . 8 - 2 *3 3 . 7 - I . 3.6 3.5 2015 #. " - n A - - " I I I I I I I 0 0.02 0.04 0.06 0.08 0.10 0.12 [ H+IT/mol dm-3 [Fe(Me,phen)l+], = 1.459 x Fig.4. Variation of k , with [H+], at 25 "C. [SDS], = 3.0 x mol dm-3, with the observed trend in k,-[SDS], profile (fig. 2). This is a significant result. Fig. 4 shows the plot of k , us. [€I+], at 3.0 x mol dm-3 SDS. It exhibits an exponential decrease in the value of k, as the acid concentration is increased (kv approaching saturation at [H+], > 0.057 mol dm-3). The exponential decay of k , is observed to be valid for high [SDSIT (1.50 x < [SDS], < 3 x c.m.c.)t Binding of Fe(Me,phen)g+ by SDS The binding constant Kl was determined both in neutral medium and in 1.0 x mol dm-3 H+ at 25 "C. For each set of runs the absorbances of solutions containing f The c.m.c. decreases from 8.3 x to 7.04 x mol dmP3 for [H+IT in the range (0.0-1.0) x loP3 rnol dm-3.2016 Micellar Inhibition of Aquation 0 3.0 6.0 9.0 12.0 15.0 18.0 21.0 24.0 [ SDS],/ lo-' rnol dm-3 Fig.5. Typical plot of fractional saturation 6 = ( A -A,,)/(& -Ao) against [SDS],. 14.0 12.0 10.0 5 8.0 6.0 2.0 I I I I I I I I I I J 0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 l/[Fe(Me4Phen)s2'lf/ lo4 mol dm-3 Fig. 6. Reciprocal of average number of molecules of bound Fel* complex (1 /v) us. reciprocal of free [Fe(Me,phen)g+] in acid-free medium (0) and in [H+IT = 1.0 x mol dm-3 (A) at 25 "C. 1.459 x mol dm-3 Fe(Me,phen)g+ were measured at varying concentrations of SDS (< 2.00 x mol dm-3). Fig. 5 shows a typical plot of fraction of Fe(Me,phen):+ bound (6) us. [SDS],. 6 is given by the ratio (A-Ao)/(Aa-Ao).Using the Scatchard methodll [SDS]/ [Fe(Me,phen)f+]b,,,d, i.e. 1 /v was plotted against 1 /[Fe(Me,phen);+],,,, (fig. 6). Kl and the number of binding sites per molecule of SDS (n,) were calculated from the slope and intercept, respectively. K , values of (3.95k0.05) x lo5 and (3.04f0.16) x lo5 mol-1 dm-3 were obtained in neutral medium and 2.0 x mol-1 dm-3 H+, respect- ively; the corresponding values of n, were 0.25 and 0.20. Kl could not be determined at higher acid concentrations by this method owing to significant initial reaction which prevents accurate determination of the initial absorbance of the reaction mixture. Mechanism Some of the above results can be explained by the following mechanism. We suggest an equilibrium step involving the binding of the substrate [S2+ = Fe(Me,phen):+] with the micelle or monomer aggregates (M) of the surfactant, SDS K , (1) Mn- + S2+ MS2-nJ.Ige and 0. Soriyan 2017 where Kl is the binding equilibrium or association constant and n is the magnitude of the average charge on the micelle. The bound complex (MS2-") then reacts with H+ K2 present in solution MS2-" + H+ + MH1-n + S2+ where MH1-n is the protonated micelle. H+ competes with Fe(Me,phen)i+ for the binding sites on the micelle. Unbound micelle in solution can be protonated directly through the equilibrium pathway K3 Mn- + H+ e MHl-" where K3 is the acid-micelle binding constant. The dissociation of the substrate, S2+ to products occurs in two phases; in the bulk-water region where the complex molecules are unbound or free and in the micellar phase where the complex is bound to the SDS aggregates: (2) (3) kw S2+ -products MS2-" -products slow km slow where k, is the first-order dissociation constant of the substrate or complex in bulk water and k, is the first-order dissociation constant of the substrate or complex in the micellar phase.There is thermodynamic stabilisation of the complex with respect to dissociation via equilibrium 1. Hence k, would be less than k,, i.e. there will be inhibition of the aquation process due to the relative thermodynamic stability of the MS2-" micelle- complex ion, with respect to its dissociation. This is a reasonable explanation for the observed inhibitions in the present work. The micelle+omplex ion is either solubilised in the micellar core (as in the suggested SDS solubilisation of haemin)12 or the complex may be bound in the Stern layer of the micellar region.We define the following equilibrium concentrations : [S2+] = a [equilibrium concentration of Fe(Me,phen)I+] [MS2-"] = /3 (equilibrium concentration of bound complex) [MH1-n] = A (equilibrium concentration of micelle-bound proton) [H+] = y (equilibrium concentration of free hydrogen ions) [Mn-] = 8 (equilibrium concentration of free micelle). It follows from eqn (1)-(5) that [s2+]T = a+B K1K2 = K3 (1 1) where subscript T denotes total or initial concentrations. The rate of disappearance or dissociation of the complex is given by d -- + = kw[S2+] + k,[MS2-"] dt2018 Micellar Inhibition of Aquation and the observed rate constant k , is given by rate [S2+] [MS2-"] k , = - = k w T + k m p [s2+lT [s IT LS2+t]T i.e.k , = kw 4 r e e +km Ftmund (13) where is the fraction of free complex, lj',ound is the fraction of bound complex and but therefore From eqn (7) and (10) K2 B([H'lT - A) A = a Since A 6 [H+IT, or assuming K2P 4 a K2 PIHSIT A = a Substituting for a from eqn (6), eqn (19) yields From eqn (6) and (9) From eqn (8), (20) and (21) which can be rearranged to give At low surfactant concentrations the term p2 is negligible. Eqn (22) can then be solved for p to give From eqn (17) and (23) Eqn (24) can be rearranged to yieldJ . Ige and 0. Soriyan 2019 Table 2. Observed aquation rate constants (k,) at 25 OCa LSDS1T k; / lop3 mol dm-3 110-4 s-1 /10-4 s-1 0.00 1.733 (k,) 2.014 (kw) 0.02 1.114 - 0.04 0.835 0.896 0.06 0.748 0.687 0.08 0.639 0.544 0.10 0.385 0.423 a [Fe(Me,phen):+], = 1.459 x mol dm-3.mol dm-3. [H+IT = 0.017 mol dm-3. [H'], = 0.129 In 5 8.0 - n I I I I I I I I I I k - 0 2.0 4 .O 6.0 8 .O 10.0 [ M - " ] ~ / l o - ~ mOl dm-3 I I I I I I I Fig. 7. [Mn-IT (km-kw)/(kv-kw) against [Mn-IT at 25 "C. [H+] = 0.017 mol dm-3 (0) and 0.129 mol dm-3 (A). A plot Of [Mn-]~ (k, - k,)/(k, - k,) against [Mn-]~t at constant [S2+]~ and [H+]T should be linear with unit slope. To check the validity of this prediction, more data (table 2) were taken at surfactant concentrations < 1.0 x lop4 mol dmP3. Fig. 7 shows the required plot, at two acid concentrations (0.017 and 0.129 mol dmP3). The two plots are linear with slopes of 0.9 0.2 and 0.8 f 0.2, respectively, in excellent agreement with the prediction of eqn (25) within the limits of experimental error.From eqn (25), the intercept is given by 1 intercept = [s2+lT + K2[H+], +:. Kl [S2+], was fixed at 1.459 x mol dmP3 and [H+IT was constant for each set of runs. Substitution of the above acid concentrations and the extrapolated intercepts in fig. 7 into eqn (26) yields two simultaneous equations from which the binding constant Kl [(2.81+0.08) x lo5 dm3 mol-l], the equilibrium constant K2 [(4.91 k0.08) x dm3 mol-l] and hence K3 (= Kl K2) (13.80k0.16 dm3 mol-l) were obtained. It is important to note that the values of the above equilibrium constants are sensitive to variations in k,. E.g. a 10 % variation in k,, at 0.0 17 mol dm-3 H+, results in a 3.5-fold variation in K,, 6-fold variation in K, and 21-fold variation in K3.k , is the limiting value of k , at high surfactant concentrations. For the present calculation k , values were obtained by measuring k , at a surfactant concentration of 20 x c.m.c. t For [Mn-IT we have assumed [SDSIT. Because only a few monomer aggregates are involved at the low surfactant concentrations, we expect the error resulting from this assumption to be within acceptable limits.2020 Micellar Inhibition of Aquation concentration (fig. 1). Rearrangement of this equation yields Eqn (24) can be used to explain the observed dependence of k , on complex which can be reduced to the form 41 + kW[S2+1T 4 2 + [S2+IT k, = which then reduces to assuming [S2+] 6 42 where k , = 41/42 + kw[S2+1,/42 41 = K2 kw[H+lT+ km[M"-lT+ kw/K, 4 2 = [M"-]T + K~[H']T + 1 /K1.and #1 and d2 are constants at fixed surfactant and acid concentrations. From eqn (29) a plot of k , against [S2+] should be linear, in agreement with our observed experimental data (fig. 1). Kl, K2 and K3 can also be calculated from the intercept and slope, if these are known, at two or more surfactant and/or acid concentrations. Using eqn (27) it can also be shown that where 6 3 = kw[S2'~T+km[Mn-l~+kw/Ki (33) and 4 4 = [ s 2 + ] ~ + [Mn-]~ + 1 /Ki (34) within the range of acid concentration 0.007 0.129 mol dm-3 in this experiment and [SDS] < 2.0 x lo-* mol dm-3, d4 % K2[H+IT and 43 4 kWK2[H+lT. Under these conditions, eqn (32) reduces to (35) k , zz -[H+], kw K2 4 4 which predicts catalysis by acid at fixed low surfactant concentrations, in agreement with the observed data (fig.3). Discussion Binding Constant The binding constant K,, (2.81 +0.08) x lo5 dm3 mol-1 in acid medium (from fig. 7) is lower than the value of (3.95 Ifi 0.05) x lo5 dm3 mol-1 in acid-free medium (from fig. 6). From experimental observation, a significant increase in initial absorbance is obtained when SDS is added to a neutral aqueous solution of complex, while the change in initial absorbance is much smaller in the presence of acid. This experimental observation is in agreement with the higher binding constant obtained in an acid-free medium. Within the limits of experimental error, the agreement between Kl values determined in an acid medium using a Scatchard plot (3.04 0.16) x lo5 dm3 mob1 and the value obtained from kinetic data in an acid medium (2.81 -+0.08) x lo5 dm3 mol-l is reassuring.The number of binding sites per SDS molecule (0.25 and 0.20 in neutral and acid medium, respectively) suggest there are four SDS molecules in the aggregate in neutral medium and five SDS molecules in acid medium. This is a confirmation of premicellar activity and the higher value of monomer aggregates in acid medium is in agreement with the postulated binding of H+ to some sites originally occupied by Fe(Me4phen)i+. In addition, the plots of fractional bound complex against [SDS], (e.g. fig. 6) showJ. Ige and 0. Soriyan 202 1 no breaks, confirming that there is only one type of binding site for the complex at the surfactant concentration range covered; ([SDS], $ c.m.c.).With only one type of binding site and the given competition between H+ and complex molecules for this site, k , should increase with increase in [€€+IT at constant [SDS],. The reasoning is that as [H+], increases, more H+ is bound to the micelle via equilibrium ( 3 ) and the net surface negative charge of the micelle is reduced, i.e. there is a change in surface potential. This decrease in the net negative charge-density on the micellar surface reduces the strength of the electrostatic binding between the monomer aggregates and the FeII complex. Since fewer molecules of the complex are bound to the monomer aggregates, more of the complex will be found in the bulk-water phase where the aquation reaction is faster. If the postulation of electrostatic binding effect is correct, the bound complex molecules must be located on the micellar surface between the rough ridges of the Stern layer.Solubilisation of the complex in the micellar core, as postulated SDS for the solubilisation of haernin,l2 is ruled out. If there is solubilisation of the complex in the micellar core, inhibition of aquation should be more marked due to the low activity of water in this region and k , should be significantly insensitive to changes in [Hf],. This does not rule out hydrophobic interaction between the micelle and complex a few atoms below the Stern layer, with part of the complex still in the Stern layer. We propose, however, that electrostatic binding in the Stern layer has a predominant effect. There is evidence in the literature to support the type of binding proposed above.E.g. in the photolytic and radiolytic studies of Ru(bpy):+ in SDS micelle13 the kinetic results indicate that essentially all the Ru(bpy)g+ is bound to the micelle. In addition, the SDS catalysis of Ni(PADA)2+ formation14 has been postulated to be due to stabilisation of the Ni(PADA)i+ by binding on the micellar surface. In either case binding through both electrostatic and hydrophobic interactions were postulated. Complex Dependence The increase of k, with [complex], (fig. 1) at fixed [El+], and [SDS],, is due to changes in the distribution of the complex between the micellar phase and the bulk-water phase: [CompleXlrnicellar phase * [CompleXlbulk-water phase. (36) For the runs in fig. 1, [H+], was in at least 170-fold excess of [SDS],.Under this condition, most of the binding sites are already taken up by H+, leaving most of the complex molecules in the bulk-water solution. At fixed [SDS], and excess [H+IT, equilibrium (36) is therefore shifted to the right with increase in [complex], i.e. the distribution of the complex between the micellar phase and bulk-water phase is in favour of the later, where the aquation rate is faster. Rate - [Surfactant] Profiles At fixed [H+], and [Fe(Me,phen)t+],, we cannot readily explain the marked structure in k,-[SDS], profiles. However, we suggest that the evolution (size, geometry, etc.) of the micelle is an important factor. All the profiles show strong premicellar activity. This is attributed to the presence of premicellar aggregates consisting of few surfactant monomers which are more effective in the solubilisation/binding of the complex molecules. The increase in k, following the initial strong inhibition is the result of the expulsion of some of the solubilised complex molecules into the bulk water phase as the size of the aggregates increases.The equilibrium distribution of the complex molecules is therefore shifted progressively to the bulk-water region [equilibrium (36)]. This effect becomes less important once micellisation becomes significant. With [SDS], > 2.25 x lop3 mol dm-3 we observed that there is no significant change in k,,, with increase in [SDS],,2022 Micellar Inhibition of Aquation i.e. k, tends to its limiting value, k,. More data up to 3 x c.m.c. show little or no variation i n k .TKe observed overall inhibition suggests that solubilisation of the dissociated tetra- methylphenanthroline ligand is not a predominant factor as it was the case in the PDC-Fe(phen)i+ system,* where overall rate enhancement was observed and the unsubstituted phenanthroline ligand was used. Comparatively, the bulky tetramethyl- phenanthroline ligand may not be so readily solubilised in the present work. However, we have no supporting evidence in the literature for this type of steric effect. Acid Dependence Keeping [SDS], and [Fe(Me,phen)i+] constant, fig. 3 shows that the aquation is catalysed by acid at very low surfactant concentration (< 1 .OO x mol dm-3), while at higher surfactant concentration (3.0 x lop3 mol dm-3) k, decreases with increase in [H+] mol dm-3 SDS, Hf is in 70-fold excess at the lowest acid concentration.As [H+IT is increased, the ratio of the concentration of micelle-bound complex M W 2 to micelle-bound proton MHlPn decreases; i.e. less complex molecules are bound by the micelle. There is a shift in the partitioning of the complex between the micellar phase and the bulk-water phase, in favour of the later. As explained earlier, aquation is faster in the bulk-water phase. The presence of HSO; and SO:- ions produced from the addition of H,SO, will also inhibit the aquation rate. These ions form a negative field around the complex molecules and hence retard its attack by water molecules. This effect is, however, only significant at the higher acid concentrations. mol dm-3 SDS, [H+IT is in excess by only a factor of 2.3 to 13.3 (in the acid range from 7.0 x to 4.0 x lo-, mol dm-3).[H+], [SDS], is suf- ficiently low that [MS2-n]/[MH1-n] is significantly higher than at (1.0 and 2.0) x lo-* mol dm+ SDS. It is obvious that the rate of aquation will depend on the ratio [MS2-n]/[MH1-n] because of the opposing effect of the presence of and MH1-n on the aquation rate. Ordinarily, in acidified micelle-free media the aquation process is described by the following equilibria : (fig. 4). For fig. 3 at 1.0 x For fig. 1 at 3.0 x Fe(Me,phen)i+ + aq t- Fe(Me,phen)if,,, + Me,phen (37) Fe(Me,phen)i+ + aq Fe(Me,phen):; + Me,phen (38) Fe(Me,~hen)~+ + aq Fe&, + Me,phen (39) where the first dissociation is the rate-determining step. This dissociative mechanism is well established for iron(I1) phenanthroline complexes8 and is catalysed by acid through Me,phen + H+ Me,phen * H+ (40) Me,phen + 2H+ Me,phen - 2H2+. (41) A priori one would expect that in acidified SDS an additional catalytic pathway is the solubilisation of the hydrophobic dissociated ligand by SDS micelles. The absence of this effect and the observed overall inhibition are strong indications that Fe(Me,phen)$+ is stabilised with respect to dissociation in acidified SDS by favourable thermodynamic/ electrostatic/hydrophobic interactions between Fe(Me,phen)g+ molecules and SDS aggregates.J. Ige and 0. Soriyan 2023 References 1 E. D. Cordes and R. B. Dunlap, Acc. Chem. Res., 1969, 2, 329. 2 E. J. Fendler and J. H. Fendler, Adt.. Phys. Org. Chem., 1970, 3, 271. 3 S. Diekmann and J. Franhm, J. Chem. SOC., Faraday Trans. I , 1979,75, 2199. 4 E. Pelizetti and E. Pramauro, Inorg. Chem., 1979, 18, 1882. 5 C. J. O’Connor, E. J. Fendler and J. H. Fendler, J. Am. Chem. SOC., 1973, 95, 600. 6 C. J. O’Connor, E. J. Fendler and J. H. Fendler, J. Chem. SOC., Dalton Trans., 1974, 625. 7 J. Ige, J. N. Lambi and J. Jeje, to be published. 8 H. D. Burrow, J. Ige and S. A. Umoh, J . Chem. SOC., Faraday Trans. I , 1982,78, 947. 9 W. W. Brandt and G. F. Smith, Anal. Chem., 1949,21, 1313. 10 R. J. Williams, J. N. Phillips and K. J. Mysels, Trans. Faraday SOC., 1955, 51, 728. 11 G. Scatchard, J. S. Coleman and A. Shen, J. Am. Chem. SOC., 1957, 79, 12. 12 J. Simplicio, Biochemistry, 1972, 11, 2525. 13 P. Meisel, M. S. Matheso and J. Rabani, J. Am. Chem. SOC., 1978, 100, 117. 14 A. D. James and B. H. Robinson, J. Chem. SOC., Faraday Trans. I , 1978, 174, 10. Paper 4/2 I9 1 ; Receioed 3 1 st December, 1984

ISSN:0300-9599    

DOI:10.1039/F19868202011

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1986,82, 2025-2041 Catalytic Reactions on Metal-supported Semiconductors Oxidation of CO over ZnO Films on Silver Eliezer Weisst and Mordechai Fohnan* Department of Chemistry and Solid State Institute, Technion I.I. T., Haifa 32000, Israel The catalytic activity of silver-supported zinc oxide films in CO oxidation has been studied. Attention has been directed to the influence of the semiconductor-metal contact on the catalytic behaviour of the semicon- ductor. Auger electron spectroscopy and electron microscopy have shown the presence of zinc oxide films uniform both in thickness and composition, which were free of contaminants. The kinetics of the catalytic reaction have been studied in a gas-flow system. The results fit two rate equations leading to two possible mechanisms: one with a rate-determining step which is a reaction between adsorbed CO and adsorbed oxygen, without mutual displacement; the other which corresponds to the Eley-Rideal mechanism.Both mechanisms assume a dissociative, asymmetric, acceptor-type adsorp- tion of the oxygen molecule, and a donor-type adsorption of the CO molecule. It has been found that the elementary constants in the rate equations for CO oxidation depend on the semiconductor film thckness. Good agreement between the experimental and the theoretical results has been found. The Schottky model, together with the rigid band model, have been used in the theoretical considerations. The Schottky barrier height for the ZnO-Ag system has been calculated using the kinetic results, and is in very good agreement with the measored values reported in the literature.According to the so-called ‘electron theory of catalysis’1-3 or the ‘ rigid band the concentration and distribution of electrons in the catalyst play a predominant role in catalytic processes and their interpretation. To determine the elementary steps of a catalytic reaction one has to change the distribution of the electrons over the quantum states of the bands at the surface of the solid, i.e. to change the Fermi level position in the energy gap at the surface. In a semiconductor (s.c.) catalyst supported by a metal the large number of electrons in the support can modify the spatial distribution and the average free energy of the relatively few electrons in the S.C.This takes place only near the interface. In fig. 1 (a) the phases of the metal and S.C. are separated. We choose the case where the metal has a larger work function (&J than the n-type S.C. (&). In fig. 1 (b) the two phases are in electric contact. As was suggested by Schottky,g electrons diffuse from the S.C. into the metal until their Fermi levels are equalized. This results in build-up of a space charge in the boundary layer, extending to a depth 1 in the s.c.; E, and E, have been bent upwards, and the distance of the Fermi level from Ec has increased. Within the boundary layer, this effect favours donor-type catalytic reactions, i.e. reactions where the reactant donates electrons to the catalyst to form the activated state. It is clear from this explanation that electronic interaction effects can change the catalytic activity only if the surface of the S.C.catalyst lies within the width of the boundary layer ( I ) from the metal support [fig. 1 (c)]. One can determine the dependence of the catalytic energy of activation (EaJ on the thickness ( d ) of the S.C. catalyst layer7 using the Poisson equation and the Schottky t Based in part on a D.Sc. dissertation presented to the Senate of the I.I.T. by E. Weiss.2026 CO Oxidation over ZnO Films on Silver C F F %a= &I-x C C I z I I 1 O d 1 Fig. 1. Energy bands in an n-type semiconductor in contact with a metal (&, > # s c ) : (a) before contact, (b) on contact, with S.C. layer thicker than the screening length ( I ) , (c) on contact, with S.C. layer thickness ( d ) smaller than the screening length.F-F: Fermi level. V-V: Top of valence band. C-C: Bottom of conduction band. barrier (+SB). For a donor-type reaction on a rectifying contact and an n-type S.C. (q4m > q4sc) one obtains Eat = aD-N +SB--d2 (1) ( E 2ne2N ) where a, is a constant, N is Avogadro’s number, N is the number of charge carriers within the s.c., e is the elementary charge and E is the S.C. dielectric constant. Metal-supported semiconductor catalysts were studied by Schwab et and Steinbach.15 As catalysts they used S.C. films on metal foils, S.C. powders distributed on metal foils or mixtures of powders of the two components. According to these authors, in some cases considerable differences in the apparent energy of activation were obtained.In one case only7y (p-type S.C. with q4sc > &J the results were compared with the theory and some correlation with an equation similar to eqn (1) was found. These former studies lack chemical and morphological characterization of the S.C. surfaces and the bulk, as well as the s.c.-metal interfaces. In those instances in which the S.C. layer was prepared by oxidizing a metal film supported by metal foil8? l3 there was no indication of the termination of the oxidation in the film-foil interface. There was no direct determination of the S.C. layer thickness, though this is the independent variable in eqn (1). In the studies where one or both phases were powders9? lo, l2 there was the possibility of bifunctional catalytic activity of the mixtures. The influence of the s.c.-metal interface on the S.C.catalytic activity was examined via the change in the apparent energy of activation. In those studies no detailed analysis of the energies of activation of the elementary steps was undertaken. The main objective of the present work was the study of the influence of the metal-s.c. interface on the rates of elementary reactions in a redox catalytic system. To achieve that, the catalyst was prepared under well controlled conditions and was characterized carefully to verify the presence of a continuous S.C. film of uniform thickness and composition. Special stress was put on the absence of contaminants, on the surface and in the bulk of the catalyst as well as at the metal-s.c. interface. The experiments were designed to enable the finding of the detailed rate equation and the influence of the metal-s.c.contact on the elementary rate constants.E. Weiss and M. Folman 2027 Experiment a1 Catalyst Preparation Thin zinc oxide films deposited on silver foils (Fine, Holland-Israel Ltd) served as catalysts. The films were produced by r.f. sputtering from a 99.99 wt % ZnO target, 6 in? in diameter, using a Perkin-Elmer model 2400 sputtering system. The distance between the target and the substrate was fixed at 1.5 in. After pumping the bell-jar down to < Torrt pure argon was admitted and sputtering was performed at 0.022 Torr Ar. Prior to deposition, the substrate was sputter-etched at 250 W for 5.5 min. Thin films were produced at a deposition rate of ca. 40 A min-l at 100 W and 500 V bias (for deposition times exceeding 30 min the deposition rate increases up to 80 A min-l at 120 min sputter-deposition). Catalyst Characterization Catalyst Composition Catalyst surfaces and chemical composition beneath the surface of the as-deposited films following the catalytic reaction were characterized by Auger electron spectroscopy (AES) in a PHI 590A system using a primary electron beam of 3 keV and 1 PA.Depth profiles were obtained by monitoring the peak-to-peak signal heights of the Auger transitions 0 KLL (503-510 eV), Zn LMM(994 eV) and Ag MNN (351 eV) during 4 keV Art ion-sputter etching. Catalyst Morphology The morphology of the catalysts was established by scanning electron microscopy (SEM) using JEOL JSM C-35 and JEOL JEM T-200 instruments.The electron beam energy was 25 keV. Electrical Conductivity The d.c. conductivity of ZnO films deposited on a microscope slide glass (6 x 6 mm) was measured by the van der Pauw method in a specially designed cell which was pumped down and then filled with gas up to 1 atm.$ As contacts served films of gold 3000 A thick deposited by electron gun evapQration. The sample was heated in the selected atmosphere for a pre-set period of time and was allowed to cool to room temperature for conductivity measurements. Film Thickness Step-height measurements were performed by means of a stylus, using a Sloan Dektak profilometer, and interference (wavelength 2950 A), using Reicher interferometer. For interference measurements the samples were coated with 1000 A gold layers.t 1 in = 2.54cm. $ 1 Torr M 133.3 Pa. 0 1 atm = 101 325 Pa.2028 CO Oxidation over ZnO Films on Silver +15 Fig. 2. Diagram of apparatus for kinetic experiments. 1, 0,-CO-He mixture cylinder; 2, He cylinder; 3, pressure control unit; 4, four-port isolation valve; 5, needle valve; 6, microcombustion furnace; 7, Pyrex reactor; 8, micro filter; 9, metering valve; 10, gas sampling valve; 11, gas chromatograph; 12, 13, external separation columns; 14, cooling bath; 15, line to flow meters and vent. Catalytic Experiments Apparatus The kinetics of the catalytic reaction were studied in a flow system operating at atmospheric pressure and described in fig. 2. The reactor was a Pyrex tube with Kovar ends 240 mm long (1 80 mm in the oven) and 14 mm i.d. The reactor ends were covered by asbestos paper to prevent light penetration into the reactor.The temperature of the reactor was controlled within f 1 K. Gas-flow velocity was varied from 5 to 30 cm3 min-l. No detectable reaction was found on the walls even at the lowest flow rate at temperatures below 400 "C. Detection of Reactants and Products The compositions of the gas mixtures were established by gas chromatography (g.c.) analysis. Separation was achieved using four columns, 1 m long and 1/4 in in diameter, filled with Porapak N, Porapak T (Waters Assoc.), Chromosorb 102 (Johns Manville) and Molecular Sieve 5A (B.D.H.). The grain size was 8&100 mesh. The first two columns were kept at 150 "C; the two others were at 14.5 "C. Helium served as the carrier gas. Detection was by means of the thermal conductivity method.Fig. 3 shows a representative chromatogram. The chromatograms were standardized by admitting pure gases or mixtures of known composition to the gas-sampling valve at various pressures directly from the vacuum system. A linear relationship between the chromatograms' heights and the gas pressures was found. Catalytic Procedures Reaction mixtures were prepared in an evacuated gas cylinder in the following manner : the cylinder was filled with CO (oxygen) up to a partial pressure of 670 Torr, oxygen (CO) was then admitted up to the preselected pressure (1-4 atm). Finally helium was added to the mixture up to a pressure of 4 atm.E. Weiss and M. Folman 2029 co c x cos AL I co il A c x z + c I1 i \ h I I I 0 1 2 3 4 5 6 7 8 9 1011 121314151617181920 tlmin Fig.3. Gas chromatograph separation for CO oxidation. (a) Superimposed chromatograms of pure gases simulating product mixture. (b) Typical chromatogram of product mixture. Flow through external columns (see fig. 2): I both, 11 12, 111 13. The catalyst was activated (in the reactor) for at least 12 h at 390 "C by passing the reaction mixture over it. The influence of activation on the catalytic activity is shown in fig. 4. Catalyst activity was determined by the composition of the product mixture as a function of flow rate, feed mixture composition and reactor temperature. Steady-state composition was established after attaining reproducibility of 1-274 . The system reached the steady-state composition in under 15 min.The steady-state stability was checked for hours and sometimes even for days.2030 CO Oxidation over ZnO Films on Silver 1.9 1 . 8 " I ; l a 7 E E ;=; 1.6 0" g1.5 1.4 '4 --. 1 . 3 1 0 5 10 15 20 25 tlh Fig. 4. Change of catalytic activity with time of activation (Po,/Pco = 6). Rate Calculations The rate calculations were based on the rate equation of a mixed-flow reactor.16 Since the feed mixture does not contain the product (CO,) the steady-state equation is given by where Pco, is the CO, partial pressure within the reactor and at its outlet (Torr), U is the flow velocity of the reaction products (cm3 min-l), V is the reactor volume (cm3), P is the reaction rate (Torr min-l) and t, is the residence time (t, = V/U min). Results and Discussion Catalyst Characterization Auger Electron Spectroscopy AES surveys of two 'as prepared' ZnO films on silver foils are shown in fig.5. It may be seen from the spectrum in fig. 5(a) that no detectable contaminants are present on the film surface. The thin film has the same composition as the pure zinc oxide powder which served as standard [fig. 5 ( c ) and (d)]. The ZnO-Ag interface is also free of contaminants and there is no additional layer of different composition at the interface. This can be seen from the AES surveys taken at the interface itself [fig. 5(6) and (e)]. In films of thickness < 100 A, holes (ca. 1 pm in diameter) in the ZnO layer were found, as judged from the AES line scans (fig. 6). The amount of Ag detected in those cases was ca. 5% . In films > 200 A the amount of silver detected was < 1 % (which is the limit of detection of the Auger system).The thickest film produced (ca. 1 pm thick) showed cracks after its use in the reactor. Catalysts, which were employed in the reactor for a number of days up to few weeks and maintained at 300-390 "C and 1 atm of 0,-CO mixture, were analysed by AES. In films of thicknesses > 100 A and < 10000 A, no changes in surface composition or depthE. Weiss and M . Folman 203 1 2 Ag Zn I , I l l , l I l 110 330 550 770 990 kinetic energyleV z6 .3 s 5 7 r 1 6 1 0 5 c I I l J l \ I I I 1 0 2 4 6 8 10 12 14 16 sputter time/min 110 330 550 770 990 kinetic energy/eV Fig. 5. AES of ZnO films on silver, prepared by r.f. sputtering. (a)-(c) Sample 315 (thickness: ca. 1200 A); (a) spectrum of the surface; (b) spectrum of the surface obtained after 8 min sputtering; (c) depth profile; sputtering rate of Ta,O, 200 A min-l; (d)-(e) sample 316 (thickness: ca.200 A); (d) depth profile; (e) spectrum of the surface obtained after 4 min sputtering; sputtering rate of Ta,O, 50 8, min-l. Zn 0 1 2 3 4 5 6 7 8 Erm Fig. 6. Auger line scan of zinc oxide film (100 8, thick) on silver. profiles were observed and no interdiffusion of the components of the two phases was noticed. Also, no traces of Ag were found on the films' surfaces. To avoid any possibility of catalytic activity of the Ag substrate, only films in the range 200-1 200 A were employed.Plate 1. SEM analysis of thin zinc oxide films on silver. Film thickness: (a) 1 pm; (b) 600 A.E. Weiss and M. Folman (Facing p . 2032)2032 CO Oxidation over ZnO Films on Silver Scanning Electron Micrc zopy SEM studies of the morphc ogy of the catalysts showed a granular structure. The grain size could not be determin i. The step structure [plate l(a)] indicates that the growth of the film shows no prefei zd crystallographic direction. Results for the thinner fi IC are LQC A.*A--- ,- - - instance, for a 600 A film, although a step was prepuLwu, IL Luuiu nave been detected only after the film w a s purposely damaged [plate 1 (b)]. Film Thickness Results of film-thickness variation within the deposition system are described in fig. 7. The longest dimension of the catalyst (its diagonal) is equal to the target radius, therefore the film-thickness variation along the catalyst is < 5 % .Very high reproducibility in the film thickness was obtained. Electrical Conductivity The room-temperature specific conductivity of a 600 A thick zinc oxide film as a function of oxidation time at 400 "C is shown in fig. 8. The high conductivities of the untreated film [point (a)] and after 15 min heating (at 400 "C) in helium lpoint (b)] are due to interstitial zinc, resulting from film deposition in an oxygen-free atm0~phere.l~ Oxidation of the film caused a monotonic decrease in conductivity, owing to oxidation of the interstitial zinc. This was noticed for the first 7.5 h, after which the conductivity reached a constant value of ca. s2-l cm-l. CO Oxidation over ZnO Films The understanding of the influence of the s.c.-metal interface on the catalytic behaviour of the S.C.requires a detailed understanding of the kinetic mechanism of the reaction. For that reason CO oxidation over the zinc oxide films had to be characterized kinetically. Inhibition by CO, It was found previously18 that the CO, produced may inhibit the CO oxidation. Therefore, a study of the kinetics of this reaction should start by determining the extent of the inhibition by CO,. The influence of flow velocity on the product pressure is described in fig. 9. The linear relationship means that in our system inhibition by CO, can be neglected. CO Oxidation Rate Equation To investigate the influence of the thickness of the S.C. catalyst on the reaction rate a suitable rate equation had to be found. Fig. 10 and 11 show some representative results in which the reciprocals of the reaction rates show a linear dependence on the reciprocals of the oxygen and CO pressures. These results obey the following rate equations :19 (3) r = kPo,Pco/[(1 +KO,PO,)(l + ~ c o ~ c o ) 1 which corresponds to a reaction between two adsorbed molecules with no mutual displacement; and: (4) = kP0, PCO/(l +KO, Po, + Kc0 PCO).E.Weiss and M . Folman 2033 10 000 8 000 2 3 v) 2 6000 % 4 000 2000 F t -- ---I I I ! I I I I 3 2 1 0 1 2 3 distance/in Fig. 7. Film thickness variation as a function of distance from the centre of the cathode in the r.f. sputtering system. 1 o3 lo2 1 0' - '5 loo 2 lo-' - I 10-2 1 d3 loq4 I I I I I I 0 2.5 5 7.5 10 12-5 oxidation timelh Fig. 8. tl s Fig. 9. Fig. 8. Room-temperature specific conductivity of a ZnO film (thickness ca.600 A) as a function of oxidation time at 400 "C. (a) Sample as deposited; (b) after 15 min heating (at 400 "C) in helium. Fig. 9. The influence of flow velocity on the product pressure.2034 CO Oxidation over ZnO Films on Silver 3 I 0.6 kz E E 2 0 . 4 4 -. 6 9 0.2 0 100 200 300 400 500 600 700 1 . 2 1 . o - I v) g O . 8 E .-O. 6 --... 4 n c 0.4 0.2 t / 0 100 200 300 400 500 600 700 Po, /mmHg Fig. 10. CO oxidation rates as a function of reactant pressure for several catalysts at two temperatures (in "C). Points are experimental values. The lines are calculated curves according to either eqn (3) or eqn (4). Eqn (4) corresponds to the Eley-Rideal reaction mechanism between two adsorbed molecules, of which one is weakly adsorbed.Although the latter is adsorbed on the adsorption sites of the second reactant, it does not react from this state. The two possible mechanisms differ only in the adsorption strength of the two reactants. The linear dependence in fig. 1 1 indicates that the adsorptions are first order. In accordance with the results presented above, inhibition by the product has not been included in eqn (3) and (4). (Additional mechanisms, such as Langmuir-Hinshelwood or two-step 'redox mechanism'20 were tested, but were not found to be applicable to our system). In order to find the elementary constants in the rate equations we shall designate by i, the intercept of the straight line obtained from plotting t - l us. P,: for a constant pressure of CO (Pc0,,), and by S, its slope.Likewise, the intercept and slope of r-l us. P& for a constant pressure of oxygen (Poz,y) are iy and S,, respectively. The rate constants in eqn (3) were calculated using the following expressions:E. Weiss and M. Folman 2035 7 I 25 E E 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 lo3 mmHg/Po2 1 O3 mmHg/Pc. Fig. 11. Inverse CO oxidation rates as a function of inverse reactant pressures for several catalysts at two temperatures (in "C). (a) Constant CO pressure. (b) Constant 0, pressure. The terms for k calculated by the last equation using the experimental results are practically equal. The rate constants in eqn (4) were calculated by k = Po,, yes, - cr p,O,,~l-l = [P,O,z(~& Po2, y)I-l KO, = i, kP,,,, = S, k- P& K,, = i, kPo2,, = Sx k- Pcb,,.(6) The two right-hand terms in each of eqn (6) are equal within experimental error. In a few cases differences between the two terns for KO, were found. The lines in fig. 10 are calculated plots of eqn (3) and (4) using the parameters obtained from either eqn ( 5 ) or (6), respectively. Both equations fit the experimental results well, therefore it is difficult to decide which of the two proposed rate equations describes better the oxidation of CO over ZnO. Mechanism of CO Oxidation over ZnO Films The detailed mechanism which gives the above rate equations will be outlined below. The mechanism will be based on the assumptionla* 21-23 that the rate-determining step is the reaction between adsorbed oxygen and adsorbed CO. that in the temperature range 373-453 K oxygen molecules adsorb on ZnO as 0; ions.On the other hand, at temperatures above 503 K the adsorption is dissociative and 0- is formed. Our experiments showed that the oxidation rate is linearly proportional to Po, (and not to Pg,"). Owing to this we have to assume that on adsorption a dissociation of the oxygen molecule into two different species takes place. Of the two species only one is charged. Therefore the adsorption of oxygen proceeds according to : Kl It has been 0 2 , g =O,,a, (7)2036 CO Oxidation over ZnO Films on Silver where ad stands for adsorbed, s for a surface atom, e- is a free electron and V& stands for a neutral oxygen vacancy at the surface. Of the two oxygen atoms obtained as a result of the reaction between 02,ad, a free electron and an oxygen vacancy, one is being adsorbed as 0-, while the other is incorporated into the surface.As was proposed by Doerfler and Hauffer21 we assume a donor-type adsorption of CO, and like Goepel and we designate it by K3 CO + 0, + C02 -V& +e-. (9) Here V$,s stands for an ionized oxygen vacancy at the surface. The main difference between the two mechanisms lies in the energy of the adsorption in the stage described by eqn (9). For the Eley-Rideal mechanism this adsorption is weak and the adsorption of CO on oxygen sites has to be taken into account: K 4 CO, + toad (on Old adsorption sites). co,d does not participate in the oxidation mechanism. The rate-determining step is (1 1) k co, ’ v:,~ + o k d -$ C02,ad. Since no inhibition by CO, was observed, the rate-determining step is followed by fast desorption of the product: C02,ad c02,g (fast)m (12) The surface-state energy of adsorbed oxygen atom lies very low with regard to the Fermi level of Zn0,23 therefore this adsorbed oxygen atom is completely ionized. Since the equilibria (7) and (8) are fast, the coverage is given (according to the Langmuir isotherm) by and for the Eley-Rideal mechanism by The surface-state energy of the donor V& (ED,s) is very close to the Fermi-level energy,23 therefore its degree of ionization is given by the Fermi-Dirac statistics.l. 2* 23 The surface coverage of the active species is given by and for weak adsorption KO2 - v;,s1 = F(E) K3 PCO.F(E), the Fermi-Dirac distribution function, is given byE. Weiss and M.Folman 2037 TI0C 390 370 350 330 I I I I I I I lo3 KIT Fig. 12. Arrhenius plots for the elementary constant k for various catalysts: (a) according to eqn (3), (b) according to eqn (4). Equating eqn (16a) and (16b) with eqn (3) and (4) one obtains k = k, Kl K, K3 F(E) KO, = 4 K,, = K3 or Kc, = K4. The Influence of the ZnO Film Thickness on its Catalytic Activity The influence of the ZnO film thickness on its catalytic activity can be studied in view of the rate equations for the oxidation of CO over ZnO films proposed in the former sections. This was done by studying carefully the elementary constants .of eqn (3) and (4) * The energies of activation (EaJ and the frequency factors ( A ) of the elementary constants k were determined from Arhenius plots for films of different thicknesses.Representative Arhenius plots are shown in fig. 12. Fig. 13 shows the change of E,, and A with the thickness ( d ) for the two rate equations. Inspection of fig. 13 reveals a pronounced influence of the thickness of the oxide layer on both the energy of activation for the rate constant k and its frequency factor. However there is no such influence for films >600 A thick. The results for K,, and KO, of eqn (3) and (4) are given in table 1. Inspection of the results for Kco according to eqn (4) shows that for all films (except for the thinnest2038 CO Oxidation over ZnO Films on Silver d l a 0200400 600 ,, 1200 I I I I / I .-. - I 0 E 3 30 3 \ w" 2 5 2 0 14 12 10 8 T c 3 6 4 2 0 1 2 3 " 9 10 li / 1 O2 minZ Fig. 13. Activation energy and frequency factor as a function of the semiconductor film thickness (d), according to eqn (3) (0) and eqn (4) (A).Table 1. Values of K,, and K02 in the rate equations of CO oxidation ... 4 n . 1 r, I , V T 1-1 m ,*a I, I , -1 \--1 aeposi tion iu- ~co/(mmng) A iu- n02/(mmng) A no. /min /102A T/"C: 340 355 370 385 340 355 370 385 catalyst time, t, d 316 31 1 304 315 nrrnrdino tn Pnn (A1 \ '/ ---"'-"'a c v .,-A* 5.0 2.0 1.8 1.6 2.4 2.1 11.2 4.5 0.25 0.66 0.91 0.98 15.0 6.0 1.8 2.7 3.2 3.7 30.5 12.0 3.5 3.8 4.7 5.6 316 311 304 315 according to eqn (3) 5.0 2.0 4.7 4.2 5.4 4.2 11.2 4.5 0.51 2.4 2.8 3.3 15.0 6.0 2.0 3.0 4.0 4.0 30.0 12.0 4.4 5.0 6.0 7.0 7.0 7.0 12.0 9.2 0.14 0.56 1.1 1.2 _- 12.0 11.0 13.0 13.0 0.20 - 2.3 - 5.2 9.0 0.22 1.1 5.0 11.0 0.24 1.1 9.2 12.0 0.34 2.2 8.0 16.0 0.36 2.1 catalyst where the results are scattered) E,, = 1Of 1 kcal mol-l and 1nA = 1.5f0.2.When Kco is calculated by means of eqn ( 3 ) one has to use values for k calculated by eqn (17). This results in a large error. Therefore we were not able to characterize the temperature dependence of the Kco constant. However, inspection of the results in table 1 reveals no dependence of Kco on the ZnO film thickness. KO, evaluated for both mechanisms does not show any clear dependence on temperature for all thicknesses. Its value differs from one catalyst to another with no correlation with its thickness.E. Weiss and M. Folman 2039 In light of the results presented above we conclude that both K,, and KO, do not depend on the thickness of the oxide film, while k does depend on it.Reduction of the film thickness causes a decrease in the activation energy as well as in the frequency factor. 26 Our catalyst was an n-type S.C. with dSc < dm. According to eqn (1) the energy of activation should decrease on reduction of the S.C. thickness. Inspection of eqn (16a), (16b) and (17) reveals that k depends on the Fermi-level position, while K,, and KO, do not, which is in accordance with our results. The dependence is via F(E). According to Goepel and ED,,-Ec = 0.2 eV (where, as above, ED,, is the energy of the donor surface state CO.V,,,); therefore, ED,s- E, > 0.2 eV. In the temperature range of our experiments (610-670 K) the Boltzmann approximation can be applied to eqn (17) (with an error < 5%). CO oxidation over ZnO is a donor-type k = k , Kl K2 K3 F(E) Only the last exponent in eqn (18) depends on the ZnO film thickness, and the changes in its value are the changes observed in Eac, shown in fig.13. As is expected from eqn (l), the energy of activation is linearly dependent on the square of the S.C. layer thickness up to a certain value above which the energy of activation remains constant. Schottky Barrier Height Calculation The foregoing results will be taken into account in calculating the Schottky barrier height in our catalytic system. The difference between the activation energy at thickness approaching zero and the maximum activation energy (of the free oxide) is ca. 12 kcal mol-1 (fig. 13). This is the extent to which the ZnO bands are bent due to the Ag support: Yo x 12 kcal mol-1 x 0.5 eV.Since the thickness of the space charge layer ( I ) is ca. 600 A (fig. 13) and assuming for the ZnO film a dielectric constant between 10 and 20,27 a charge carrier concentration of 1017cm-3 is obtained. Therefore the difference between the Fermi level and the conduction band edge (ECF) at 650 K (taking the electron effective mass for ZnO as 0.27 of the free electron mass)28 is:29 ECF x 0.25 eV. The Schottky barrier height should, therefore, be: $SB = Yo + E,, M 0.5 eV + 0.25 eV = 0.75 eV. This value agrees well with Mead’s results for the Schottky barrier height for ZnO-Ag (0.68 eV).30 Change of the Frequency Factor ( A ) Fig. 13 reveals also that the frequency factor ( A ) depends on the thickness of the S.C. layer.Although the catalysts had similar geometric areas their specific areas may depend on the film thickness. Generally for sputtered films the specific area, as well as the number of active sites, increase with their thickness. Therefore A also increases with the film thickness. The number of active sites is the dominant factor acting when activating a fresh catalyst. As shown in fig. 4 there was a continuous decrease in catalytic activity during the activation, until a constant value was reached. Heating the catalyst in an oxygen-rich2040 14 CO Oxidation over ZnO Films on Silver ' 12 10 T c 4 8 - 6 - - - -10 - 8, - - 6 - 1 1' 10 20 30 €,,/kcal mol-' Fig. 14. Compensation plots for the elementary constant k in the rate equation of CO oxidation over ZnO films of different thickness, according to eqn (3) (0) and eqn (4) (e).atmosphere caused an oxidation of the donor dopants, which caused an increase in ECF. Therefore, as explained above, this would have caused a decrease in Eat, i.e. an acceleration of the reaction. On the other hand the results in fig. 4 reveal that during the activation the decrease of the number of active sites plays the dominant role. As already suggested [eqn (9) and (1 l)] the active site is an oxygen vacancy at the surface of the ZnO. Under the conditions of the activation process the probability for the formation of this active site decreases drastically. The change of A in fig. 1.3 may also be a manifestation of the compensation effect,31 as can be seen in fig. 14. If this is the case, the compensation here is non-linear. Though this fact is not clear from fig.14, it is revealed by the absence of an isocatalytic temperature (a temperature where all Arrhenius plots for the different catalysts cross each other). This can be seen in fig. 12. Conclusions It has been shown that a ZnO thin film prepared by r.f.-sputtering yields an oxide, free of contaminants on its surface as well as in its bulk and at the s.c.-metal interface. This indicates that the supported ZnO films are suitable for the study of the influence of the s.c.-metal contact on the catalytic activity of the S.C. It was found that the rate-determining step in the oxidation of CO over ZnO is either a surface reaction between two adsorbed reactants without mutual displacement or a reaction between the strongly adsorbed oxygen with weakly adsorbed CO (Eley-Rideal mechanism).The proposed mechanism includes dissociative, asymmetric, acceptor-type adsorption of an oxygen molecule. While the adsorbed oxygen is completely ionized, in the donor-type adsorption of the CO molecule the degree of ionization is given by the Fermi-Dirac distribution, It was found that the activation energies of the elementary constants in the rate equations increase with the square of the thickness of the S.C. film. This agrees well with the theoretical results derived from coupling of the Schottky model, describing the s.c.-metal contact, with the rigid band model, describing catalysis on semiconductors. These models do not take into account surface parameters such as the energy spectrumE.Weiss and M . Folman 204 1 of donor and acceptor surface states and their influence on the position of the Fermi level at the surface of the s.c.; nevertheless one can apply the two models in the case of ZnO. The authors thank I. Lior, Dr R. Brener, C. Cytermann, A. Shai and A. Friedman for assistance with various aspects of the study. E. W. gratefully acknowledges the Wolf Fund for their generous support. References 1 Th. Wolkenstein, Adv. Catal., 1960, 12, 189. 2 F. F. Volkenstein, The Electron Theory of Catalysis on Semiconductors (Macmillan, New York, 1963). 3 B. Claudel, Chem. Phys. Aspects Catal. Oxid. (Proc. Spring Sch. CNRS Catal. Oxid., 1978) (CNRS, 4 S. R. Morrison, CHEMTECH, 1977,7, 570. 5 S. R. Morrison, The Chemical Physics of Surfaces (Plenum Press, New York, 1977).6 W. Schottky, Naturwissenschaften, 1938, 26, 843. 7 V. L. Vinetskii, I. V. Kel’man and D. V. Sokolskii, Russ. J. Phys. Chem., 1977, 51, 834. 8 G. M. Schwab and R. Sieged, 2. Phys. Chem. N.F., 1966,50, 191. 9 G. M. Schwab and H. Derleth, 2. Phys. Chem. N.F., 1967, 53, 1. 10 G. M. Schwab and K. Koller, J. Am. Chem. SOC., 1968,90, 3078. 11 G. M. Schwab and A. Kritikos, Helv. Phys. Acta, 1968,41, 1166. 12 G. M. Schwab and H. Zettler, Chimia, 1969,23,489. 13 G. M. Schwab and B. Matthes, 2. Phys. Chem. N.F., 1975, 94, 243. 14 G. M. Schwab, Adv. Catal., 1978, 27, 1. 15 F. Steinbach, Angew. Chem., Int. Ed. Engl., 1967, 6, 999; Nature (London), 1967, 215, 152; 2. Phys. 16 W. J. Moore, Physical Chemistry (Longmann, London, 5th Edn, 1975), pp. 350-352. 17 Handbook of Thin Film Technology, ed. L. I. Maissel and R. Glang (McGraw-Hill, New York, 1970), 18 W. R. Murphy, T. F. Veerkamp and T. W. Leland, J. Catal., 1976, 43, 304. 19 K. J. Laidler, Catalysis, ed. P. H. Emmett (Reinhold, New York, 1954), vol. 1, chap. 4. 20 P. Jim, B. Wichterlova and J. Tichy, Proc. 3rd Int. Congr. Catal., Amsterdam, 1964 (North 21 W. Doerfler and K. Hauffer, J. Catal., 1964, 3, 171. 22 I. Kobal, M. Senengancik and H. Kobal, J. Chem. Phys., 1983,78, 1815. 23 P. Esser, R. Feierabend and W. Goepel, Ber. Bunsenges. Phys. Chem., 1981, 85, 447, and references 24 P. Bonasewicz, R. Littbarski and M. Grunze, Curr. Top. Muter. Sci., 1981, 7, 371. 25 G. M. Schwab and J. Block, 2. Phys. Chem. N.F., 1954, 1, 42. 26 J. M. Thomas and W. J. Thomas, Introduction to the Principles of Heterogenous Catalysis (Academic 27 G. Heiland, E. Mollow and F. Soeckmann, Ado. Solid State Phys., 1959, 8, 191. 28 S. M. Sze, Physics of Semiconductor Devices (John Wiley, New York, 2nd edn, 1981). 29 W. Shockley, Electrons and Holes in Semiconductors (Van Nostrand, New York, 1953), sec. 16.3. 30 C. A. Mead, Solid State Elec., 1966, 9, 1023. 31 A. K. Galwey, Adv. Catal., 1977, 26, 247. Pans, 1980), p. 397. Chem. N.F., 1968,60, 126. chap. 3, p. 30. Holland, Amsterdam, 1965), vol. 1, p. 199. therein. Press, London, 1967). Paper 51493; Received 25th March, 1985 68 FAR 1

ISSN:0300-9599    

DOI:10.1039/F19868202025

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1986,82,2043-2055 Spectroscopic Investigation of Sulphidation of Zinc and Lead Carbonates Fabio Garbassi* Istituto Guido Donegani S.p.A., Centro Ricerche Novara, Via Fauser 4 , 28100 Novara, Italy Anna M. Marabini Istituto per il Truttarnento dei Minerali (C. N . R.), Via Bolognola 7, 00138 Roma, Italy The sulphidation of smithsonite (ZnCO,) and cerussite (PbCO,) mineral ores by Na,S solutions has been studied by X-ray photoelectron spectroscopy and infrared spectroscopy. Results have shown that the surface of the two minerals is sulphidized in a different manner under similar experimental conditions. HighNa,S concentration (0.208 mmol cmP3) andlow temperature (20 "C) have been demonstrated to be the most effective conditions for obtaining a good sulphidation.Results have been interpreted considering the formation of compact and very thin film of ZnS (with a depth of 1-2 atomic layers) in the case of smithsonite and of a non-continuous porous layer of PbS for cerussite. On the latter, a higher amount of hydrated species has been detected after treatment and some dissolution of the mineral has occurred. A process widely used on an industrial scale for the flotation of oxidized ores of zinc and lead consists of two stages: first the sulphidation of the ore by soaking in Na,S solution and secondly, treatment with suitable collector agents. The latter consist of primary amines in the case of smithsonite or sulphur compounds like xanthates in the case of cerussite.l The sulphidation of the oxidized ore is a particularly delicate stage in this process.In fact it occurs through a sequence of complex chemical reactions whose nature is only partially clear. Therefore an in-depth study of this process would seem to be necessary. The use of a technique of surface analysis such as X-ray photoelectron spectroscopy (XPS) seems particularly suitable in order to achieve this objective. In fact, XPS is able to give information on both the composition and the chemical state of detected elements. It has recently been used in studies related to the flotation of ores of Pb,2* Cu4 and Fe.5 In this paper is presented a study of the first stage of the process, i.e. the sulphidation of smithsonite (ZnCO,) and cerussite (PbCO,). The study has been carried out mainly by XPS and i.r.measurements in the frame of a general investigation including also the effect of collector agents.6 Experiment a1 Samples Pure smithsonite and cerussite have been obtained by repeated wet treatment of ores from the Buggerru-Coitas and San Giovanni mines (Sardinia, Italy), respectively. The smithsonite sample had 99.4% of zinc carbonate, the remainder being calcite. The cerussite sample had 98% PbCO, and 2% PbS. Samples were ground and sieved to ca. 10pm. The specific surface areas, measured by the B.E.T. method, were 6.12 m g-l for smithsonite and 1.60 m g-l for cerussite. 2043 68-22044 Sulphidation of Zinc and Lead Carbonates Table 1. Treatment conditions for smithsonite (Sx) or cerussite (Cx) samples sample Na2S concentration /mmol cmP3 T/"C s1 (Cl) s2 (C2) s3 (C3) s 4 (C4) s5 (C5) S6 (C6) s7 (C7) S8 (C8) s9 (C9) s10 (C10) 0.208 0.208 0.208 0.042 0.042 0.042 0.004 0.004 0.004 saturated H2S 20 40 60 20 40 60 20 40 60 20 Sulphidation Tests Sulphidation tests were performed at three different temperatures (20,40 and 60 "C) by stirring 1 g of powder sample for 15 min with 20 cm3 Na,S solutions having three different concentrations in the range 0.004-0.208 mmol ~ m - ~ .After treatment, the suspension was centrifuged and the solid was washed twice with 20 cm3 of distilled water. The powder was then dried at 60 "C and stored in a vacuum-sealed flask. Some experiments were also made using a saturated solution of H,S as sulphidizing agent. In table 1 all treatment conditions are summarized. Spectroscopic Characterization For the i.r.measurements, a Perkin-Elmer 580B spectrophotometer with a data station was used, on samples prepared as KBr pellets. The XPS measurements have been carried out using a Physical Electronics 548 XPS-AES system connected to a PDP 1 1 /50 computer for the collection of data. Powder samples, drawn from sealed flasks, were pressed onto a sheet of pure indium, introduced into the analysis chamber and examined using Mg Ka radiation. In order to minimize sample charging effects, a flood electron gun. was switched on during the collection of spectra. Quantitative analysis was carried out after processing raw intensity data by smoothing and background subtraction. Surface compositions were drawn from peak-area values measured by digital integration, corrected for the respective sensitivity factor.? Curve fitting of spectra was performed, adopting pure Gaussian profiles and without introducing constraints in the fitting procedure.s Results Untreated Samples The i.r.spectra of untreated smithsonite and cerussite are shown in fig. 1. On smithsonite the three characteristic frequencies of the calcite group are evident, as well as the four frequencies of orthorombic aragonite on ceru~site.~ X P S data obtained on the same samples are collected in table 2. While 0 Is, Zn 2p3,, and Pb 4f7,, peaks appeared single and symmetric, the experimental C 1s profile was evidently due to the contribution of at least two species (fig. 2). Fitting with Gaussian curves revealed that three species of carbon are present, shown in table 2 as CI, CII and CIII.These components have similar binding energies in the two samples. However, whileF. Garbassi and A . M . Marabini 2045 Fig. 1. Infrared spectra of untreated smithsonite (a) and cerussite (b) samples. Table 2. XPS data of untreated carbonate samples smithsoni te cerussite binding energya f.w.h.m. binding energy f.w.h.m. species at. % /ev /eV at. % /ev /ev 0 51.9 53 1.6 2.6 53.8 53 1.4 2.6 Zn 14.7 1021.8 2.6 - - Pb - 15.7 139.2 2.4 CI I 1.5 286.5 1.9 8.8 286.7 2.9 16.1 289.1 3.0 13.7 290.0 2.9 S - - - CI 15.8 285.0 2.9 5.9 284.9 2.0 CIII - - - - - 2.1 a A = 2009.7 eV. in smithsonite CII contributes < 5% of the total carbon peak intensity, in cerussite its relative intensity is much greater. Comparing the measured binding energies with those of reference compounds, CI has been assigned to carbon in a contaminant, C,, to C-OH bonds and C,,, to the carbonate group.'.lo The binding energy of zinc, 1021.8 eV, is in good agreement with the value found by Brion for ZnCO,.ll We also measured the Auger parameter ( A ) of zinc : A = EB+hv-EA2046 Sulphidation of Zinc and Lead Carbonates I I 1 I , I I * 293 291 289 287 285 283 binding energy/eV Fig. 2. C 1s photoelectron spectra of untreated smithsonite (a) and cerussite (b) samples. where EB is the binding energy of the Zn 2p,,, peak, hv is the energy of the incident radiation (1253.6 eV for Mg Ka) and EA is the position of the Auger peak Zn L, M,,, in the binding energy scale. The Auger parameter has been found in many cases to be more sensitive to chemical variations than EB.73 l1 In particular, binding energy values of zinc in ZnS and in ZnCO, are not appreciably different, while the corresponding A values are 201 1.7 and 2009.7 eV for sulphide and carbonate, respectively.The latter value is in good agreement with that of 2009.3 eV from ref. (1 1). A characteristic of the cerussite sample examined in this work is the presence in the photoemission spectrum of a small sulphur peak. In spite of the fact that some PbS was found in the sample by chemical analysis, the presence of a sulphide species on the surface can be excluded on the basis of its binding energy. Tentatively, this peak has been assigned to elemental sulphur, while the presence of S,O:- ions is less probable. In fact, in the latter case, the presence of a sulphur species in the spectrum region typical of S(V1) ought to be observed also., It is likely that the sulphur species observed on untreated cerussite is a product of the surface degradation of PbS present in the mineral ore.Considering the quantitative analysis of the surface and the contribution of the carbonate CIIl species only, the formula ZnOa,CO,., is derived for smithsonite, in rather good agreement with the expected stoichiometry. In the case of cerussite the agreement is poorer, probably owing to the presence of hydroxy species. Treatment with Na,S I.R. Spectra 1.r. spectra of samples treated with Na,S and H,S, together with those of some reference compounds, are shown in fig. 3. Comparing spectra of Na,S-treated smithsonite with that of the untreated sample, no significant variations are observed, as was expected since the absorption bands of ZnCO, and ZnS are quite similar.A slight absorption increase at 1625 cm-l was attributed to hydroxide and hydroxycarbonate forms, while theI;. Garbassi and A . M . Marabini 2047 I I I I I V I I I I I * 3000 2000 1500 1000 500 3000 2000 1500 1000 500 vlcm-' Fig. 3. Infrared spectra of (A) Zn-containing and (B) Pb-containing samples. (A): (a) smithsonite treated with 0.208 mmol cmb3 Na,S, (h) smithsonite treated with H,S, (c) hydrozincite, ( d ) sphalerite, ZnS. (B): (a) cerussite treated with 0.203 mmol cmP3 Na,S, (6) cerussite treated with H,S, (c) hydrocerussite, ( d ) galena, PbS. absorption bands of water at 2920-2850 cm-l showed a remarkable decrease.After treatment with H2S only, a marked decrease of the physically adsorbed water was observed. After treatment with Na,S, the v1-v4 peaks of cerussite weakened considerably, and the i.r. spectrum became quite similar to that of hydrocerussite Pb(OH), - 2Pb(CO,),, with the appearance of a peak at 1631 cm-1 and an increase of band intensity at 2923-2856 cm-l and 241 1-2364 crn-l, attributable to OH- ions. This was accompanied by the appearance of peaks characteristic of galena at 466 and 374 cm-l and a peak at 986 cm-l, attributable to sulphoxide forms like PbS,0,.12 Finally, the doublet band of water at 2789 and 2856 cm-l decreased. As in the case of smithsonite, H2S did not cause any apparent modification of the i.r. spectrum of cerussite. Particularly, neither the formation of PbS nor that of PbS,O, were observed.XPS Measurements The treatment of smithsonite with Na,S solutions causes several modifications to the sample surface, as detected by XPS. Surface atomic concentrations are reported in table 3. From the point of view of the surface composition, the major modifications observed are the occurrence of a sulphur peak, the decrease of the oxygen peak intensity and the different distribution of the relative abundance of carbon species. The relationship among the atomic ratios O/Zn and S/Zn with the Na,S concentration are reported in fig. 4. Both ratios tend asymptotically to unity. For a particular concentration of Na2S there is inverse proportionality between O/Zn and the solution temperature, while for S/Zn the relationship with this variable is less clear.In fact, the faster increase of S/Zn has been observed at the intermediate temperature of 40 "C. Apart from the carbon peak, which showed a compound profile, others peaks of species like 0, Zn and S did not vary their position and shape after treatments. The 0 1s peak maintained a binding energy of 53 1.6 eV and a f.w.h.m. of 2.75 0.15 eV. The S 2p2048 Sulphidation of Zinc and Lead Carbonates Table 3. Surface composition of smithsonite samples after treatment with sulphide solutions composition (at. %) sample 0 Zn CI CII G I 1 s s1 19.7 22.4 20.8 7.7 8.2 21.2 s2 21.7 22.5 19.4 9.5 4.7 21.7 s3 21.1 24.7 14.2 16.5 2.7 20.8 s4 33.1 21.0 20.3 10.1 3.6 11.9 s5 34.2 19.0 14.5 10.2 7.2 14.9 S6 32.9 19.3 16.0 4.2 12.0 5.6 s7 42.9 15.1 12.1 9.0 17.9 3.0 S8 51.3 18.1 8.5 9.6 11.4 1.1 s9 51.2 15.1 10.5 7.7 15.3 0.2 SlO 47.2 16.8 14.5 3.0 18.4 - 4 0.05 01 0 0.'15 0:20 [Na,S]/mmol cmW3 Fig.4, O/Zn and S/Zn surface atomic ratios as a function of Na,S concentration at various temperatures: 0, 20 "C; .,40 "C; x , 60 "C; (-) O/Zn; (----) S/Zn. peak had a binding energy of 162.0f0.3 eV and a f.w.h.m. of 2.85k0.20 eV. Finally, Zn 2p3,2 had a binding energy of 1021.9 0.2 eV. Its f.w.h.m., unlike other peaks, showed variations with treatments. Such behaviour is described in fig. 5, which shows that the f.w.h.m. remains substantially unchanged with respect to the untreated sample, when the sample is treated at the lowest Na,S concentration. As the concentration increases, the f.w.h.m.decreases to a slightly, but meaningfully lower value. The Auger parameter of zinc varies in a similar way (fig. 5). In fact, by increasing the Na,S concentration A passes from a ZnC0,-like situation to a ZnS-like situation. This variation is more evident at the lowest rather than at the highest temperature of treatment. The binding energy value of the sulphur peak is in good agreement with that measured by Brion in ZnS.llF. Garbassi and A . M. Marabini 2049 20141 % 2012 p: l i 2010 (b) 7 - O * - B e ~ Untreated sample level x-X The C 1s peaks were resolved into three Gaussian components, C,, C,, and CIII, at 284.8 & 0.3, 286.7 0.4 and 289.5 0.4 eV, respectively. Their binding energies, as in the case of the untreated sample, correspond to -C-H (hydrocarbon), =C-OH (hydroxy) and Cog2 (carbonate groups).The atomic ratio CII,/Zn is little affected by treatments with the most dilute solutions, while at higher sulphide concentrations it decreases strongly. C,,/Zn, on the contrary, increases with respect to the untreated smithsonite in all conditions, roughly in a similar amount for all concentrations, Both relationships are illustrated in fig. 6. Surface atomic compositions of cerussite samples after sulphidation treatments are shown in table 4. Variations of O/Pb and S/Pb atomic ratios as a function of Na,S concentration are shown in fig. 7. The role of temperature seems more complex than in the case of smithsonite, since results obtained at different values of this variable are sometimes superimposed or intersecting.Values of S/Pb near unity are reached only at the highest treatment temperature. O/Pb is always higher than 1.4. The binding energies of 0 1s and Pb 4f,,z peaks are reported in table 5. It appears that the binding energy of Pb decreases from 139.2 f 0.3 to 137.5 0.2 eV. The first value is rather higher than that of 138.2 eV measured by Pedersen on 2PbC0, * Pb(0H),.l3 We assume it is the characteristic value for cerussite. After the sulphidation treatment the binding energy approaches (depending on the Na,S concentration) a value typical of PbS.,> l3 Simultaneously, the oxygen binding energy decreases from a value around 53 1.5 to 530.5 eV. This shift is accompanied by a broadening of the peak, which tends to assume a rather large f.w.h.m. Similar behaviour has not been observed for the Pb peak, which on the contrary tends to become sharper.Treatment with H,S (sample 10) does not affect the 0 and Pb binding energies with respect to the untreated sample. In most cases, the S 2p peaks of treated samples have a compound shape and can be \ /2050 Sulphidation of Zinc and Lead Carbonates 1.2 I, I - I I I I - 0.05 0.10 0.15 0.20 [ Na2S]/mmol cm-3 Fig. 6. C,,,/Zn and C,,/Zn atomic ratios as a function of Na,S concentration (C,,,/Zn: open symbols and continuous lines; C,,/Zn: full symbols and dotted lines). 0,@, 20 "C; 0, ., 40 "C; A, A, 60 "C. Table 4. Surface composition of cerussite samples after treatment with sulphide solutions composition (at. %) Cl 37.5 21.4 15.2 6.6 2.6 4.2 12.5 - C2 35.9 22.1 9.9 9.6 4.1 4.5 13.9 - C3 32.9 23.2 13.8 7.4 2.1 4.6 16.0 - C4 42.5 23.7 9.0 7.7 3.2 7.7 6.2 - C5 37.4 22.5 8.5 6.2 12.8 2.4 10.2 - C6 46.7 21.5 7.1 6.6 12.4 - 3.9 1.8 C7 45.4 20.3 13.3 6.7 9.0 - - 5.3 C8 47.2 16.6 9.3 7.4 13.8 - 3.5 2.2 C9 53.1 18.5 8.3 2.9 14.6 - 0.6 2.0 C10 54.6 21.1 7.5 3.6 10.9 - - 2.3 resolved into two or sometimes three Gaussian curves.On samples treated at the lowest Na,S concentration, a species at 161.0k0.3 eV is detected near that already found on untreated cerussite at 162.7k0.3 eV. The above species can be attributed to PbS., It becomes the prevailing species with increasing Na,S concentration. At intermediate values, a new species appears at 159.0 f 0.3, that can be attributed to molecular sulphur., Finally, at the highest sulphide concentration, only the sulphide and the lowest binding energy sulphur species are detected.As in the case of smithsonite, the C Is experimental profiles were fitted by three Gaussian components, CI, C,, and C,,,, having binding energies of 284.8 & 0.3, 287.1 0.4 and 289.5 & 0.4 eV, respectively. While C , and CIII are easily attributed to hydrocarbon and carbonate species, respectively, C,, tends to assume a binding energy slightly higher than the corresponding peak in smithsonite, suggesting that a more oxygenated species like carbonyl or acetal can be present.lOF. Garbassi and A . M. Marabini 205 1 3- 0 .* + 2- g Y 1- . 0 X I I I I * 0.05 0.10 0.1 5 0.2 0 [Na,S]/mmol dm-3 Fig. 7. O/Pb and S/Pb surface atomic ratios as a function of Na,S concentration at various temperatures: 0, 20 "C; a, 40 "C; x 60 "C.(a) O/Pb, (b) S/Pb. 1 .o f Table 5. Binding energies of oxygen and lead in cerussite samples sample 0 ls/eV Pb 4f7,,/eV C l c 2 c3 c 4 c 5 C6 c7 C8 c 9 c10 530.5 530.5 530.5 530.5 530.5 53 1 .O 531.0 531.8 531.9 531.9 137.6 137.4 137.5 137.8 137.9 138.1 138.6 139.0 139.5 139.6 I I I I I 0.05 0.10 0.1 5 0.20 & [Na2S]/mmoI ~ r n - ~ Fig. 8. C,,,/Pb and C,,/Pb atomic ratios as a function of Na,S concentration (C,,,/Pb: open symbols and continuous lines; C,,/Pb: full symbols and dotted lines). 0, @, 20 "C; 0, ., 40 "C; A7 A7 60 "C.2052 Sulphidation of Zinc and Lead Carbonates C,,,/Pb shows a behaviour rather similar to C,,,/Zn (fig. 8), with a strong decrease brought about by increasing the Na,S concentration.On the contrary, C,,/Pb behaves somewhat differently than the corresponding atomic ratio in smithsonite. In fact, while it seems quite independent of the Na,S concentration, a decrease with respect to the untreated sample has consistently been observed. Discussion Smi t hsoni te In spite of the fact that i.r. observations did not show significant variations of the spectra of treated samples with respect to the untreated ones, apart from some slight increase of the hydroxide and hydroxycarbonate forms, XPS results showed important changes. In particular, all the parameters taken into account, e.g. the surface composition, the relative occurrence of the carbon species and the Auger parameter of zinc, show variations that are well interpreted in terms of the formation of zinc sulphide on the surface of smithsonite at the expense of carbonate species, i.e.ZnCO,(s) + SB-(aq) --+ ZnS(s) + COi-(aq). Results indicate that the reaction is favoured by the Na,S concentration and retarded by an acidic pH and high temperature. In order to gain a more quantitative description of the surface of the mineral after sulphidation, we can take into consideration the results reported in table 3, making some rough stoichiometric considerations. Let us consider the possible presence on the sample surface of ZnCO,(A), ZnS(B) and Zn(OH),(C) and the experimental atomic concentrations from XPS analysis, Zn(X), O(X), S(X) and C(X). The following relations can be written: Zn(X) = Zn(A) + Zn(B) + Zn(C) O(X) = O(A)+O(C) S(X) = S(B) C(X) = C(A) and Zn(A) = C(A) = 8 O(A) Zn(B) = S(B) Zn(C) = O(C).Normalized results are reported in table 6. Samples treated with more dilute Na,S solution or by H2S remain essentially zinc carbonate. Zinc sulphide becomes the more important surface compound on samples treated at intermediate concentrations. However, for at least two of samples, the presence of hydroxide species must be envisaged in order to justify the surface composition. Sample S4, which is supposed to possess the highest Zn(OH), amount, also presents the broadest oxygen peak (f.w.h.m. = 2.95 eV, compared to an average value of 2.73k0.11 eV), so confirming the reliability of the above considerations. Finally, the surface compositions of samples treated at the highest Na2S concentration are justified by considering the presence of both ZnS (prevailing) and ZnCO,.From the point of view of XPS only, this result could be due to two different limiting situations, i.e. the presence of unreacted carbonate regions near regions covered by a relatively thick layer of sulphide, or alternatively the presence of a thin and continuous layer of ZnS over all the zinc carbonate. In the latter situation, it is possible to evaluate the thickness d of such a layer, considering the relationship between the photoemission intensity and the thickness itself. In fact, when a substrate (s) containingF. Garbassi and A. M. Marabini 2053 Table 6. Normalized concentration (mol %) of zinc compounds at the surface of smithsonite sample ZnCO, ZnS Zn(OH), s1 S2" s 3 s4 S5" S6 s7 S8 s9 s10 33 20 20 25 32 60 100 85 100 100 67 65 80 50 25 55 30 10 5 10 - - - - - - a Some lack of Zn is observed.an element 2 is covered by a coating (c) containing an element X, the intensities of the photoemission peaks of the two elements depend on the relations: I,(Z) = I,(Z) exp ( - d/sin 8&) Z,(X) = I,(X) [ 1 - exp ( - d/sin 81x)] where & and Ax are the mean free paths (escape depths) of the photoelectrons of the elements Z and X, respectively, loo are the intensities from samples of the same chemical nature having infinite thickness and 8 is the take-off angle of the emitted electrons. Considering the intensity ratio : I J X ) - I , ( X ) [ 1 - exp ( - d/sin 8Ax)] I,(Z) - the left term can be drawn from experimental measurements, while the right term contains d(in A).The ratio Im(X)/I,(Z) can be evaluated from the density of compounds and ionization cross-sections of emitted e1ectr0ns.l~ In such a way, thickness values of the ZnS overlayer not exceeding 3.6 A have been calculated, corresponding to very few atomic layers. In a previous work,6 we measured the amount of sulphur and Na,S abstracted at equilibrium from the same samples varying the sulphide concentration. Results were represented by straight lines described by a Freundlich-like equation: r = jycl/n Z,(Z) exp (- d/sin 8Az) where n is very near unity. This result was interpreted considering that only one layer is involved, accounting for all the abstracted sulphur. Considering that XPS concentration results have only a semi-quantitative value, the conclusions drawn from XPS and from abstraction tests appear in good agreement, allowing the conclusion that sulphidation of smithsonite is limited to a very superficial range and, under sufficiently high Na,S concentrations, give rise to a very thin and compact zinc sulphide layer.Cerussite The interpretation of XPS results on cerussite is rather complicated by the fact that some sulphur was always detected on the surface of the sample. Furthermore, several sulphur species are present and finally, also the nature of oxygen seems to vary with the strengthening of the sulphidation treatment. Also i.r. spectra indicate rather complex2054 Sulphidation of Zinc and Lead Carbonates Table 7. Normalized concentration (molx ) of lead compounds and carbon groups at the surface of cerussite sample PbCO, PbS Pb(OH), PbS,O, =C(OH), c 1 c 2 c 3 c 4 C5" C6" c 7 C8 C9" C10" 10 11 7 11 62 63 37 70 73 50 44 37 52 36 38 20 23 18 2 21 11 16 30 10 29 10 40 - - " Some lack of oxygen is observed.modifications of the nature of samples after the treatment: absorption bands attributable to hydrocerussite appear in the spectrum, together with other bands characteristic of PbS and PbS,O,. The first occurrence is justified by the fact that in the presence of sulphide the treating solution is strongly alkaline, so contributing to the formation of the hydroxycarbonate species. According to the electrochemical diagram reported by Stumm and Morgan,15 such a species is prevalent at pH values between 7.3 and 12.5, while over the latter value the soluble hydroxide Pb(0H); begins to form.The possibility that the lead-containing system forms soluble species allows us to interpret the result of the abstraction tests, that can be approximately represented by a Freundlich-like equation with exponent n = 1.2.6 In fact it is reasonable to interpret this result as being due to dissolution of cerussite, diffusion of sulphide ions in the mineral and formation of a relatively thick PbS layer, that was evaluated to be formed by cu. ten atomic layers.6 In such a complicated situation, and considering the semi-quantitative character of XPS surface-composition data, stoichiometric considerations similar to those carried out in the case of smithsonite ought to be considered with care. Taking into account the binding energies, we considered the possible presence of PbS, PbCO,, Pb(OH),, PbS,O, and =C(OH), groups, following a hierarchy of presence as written (i.e.PbS at first, followed by others). Results are reported in table 7. Only for samples treated at the highest Na,S concentration the above considerations can justify rather well the experi- mental surface compositions, with some excess of oxygen in two cases. For other samples, the presence of C,, and sometimes of S,, was hard to prove. However, the above analysis allows, all the same, some interesting considerations: (a) PbCO, is always accompanied by Pb(OH),, so accounting for the presence of hydrocerussite as detected by i.r.; (b) the sulphidation of the cerussite surface is less effective than that of smithsonite, giving rise to a surface where both reacted and unreacted regions are present; (c) again the treatment with H,S does not give rise to an appreciable sulphidation.The presence of a supposedly incontinuous sulphide layer hinders considerations on the thickness of the cerussite layer involved in the sulphidation process. It seems, however, reasonable that the formation of sulphide domains involves ca. ten atomic layers of material: in such conditions a remarkable fraction of PbS is not detected by XPS owing to the escape value of electrons. The above conclusion accounts well for the dark colour of treated cerussite and the previous considerations of Rey,16 who emphasized the inevitable penetration of the sulphide ions into the crystal lattice of cerussite, owing to the porous structure of the latter and the strong alkalinity of the medium.F. Garbassi and A .M . Marabini 2055 The simultaneous detection on the treated samples of molecular sulphur and PbS,O, near the sulphide species is well accounted for by the mechanism proposed by Eadington and P r o ~ s e r ~ ~ for oxidation of galena in water, when elemental sulphur is present in an alkaline medium : 4 PbS+4 H20+2 O , f 4 Pb+,+8 OH-+4 So 41 2 PbS + PbO * PbS,O, * xH20 + (4 - X) HZO. References 1 M. Rey, Trans. Inst. Min. Metall., 1979, 88, C245. 2 A. S. Manocha and R. L. Park, Appl. Surface Sci., 1977, 1, 129. 3 K. C. Pillai, V. Y. Young and J. O’M. Bockris, Appl. Surface Sci., 1983, 16, 322. 4 J. Mielczarski and E. Suoninen, Surface Interface Anal., 1984, 6, 34. 5 K. C. Pillai, V. Y. Young and J. O’M. Bockris, J. Colloid Interface Sci., 1985, 103, 145. 6 A. M. Marabini, V. Alesse and F. Garbassi, Reagents i.2 the Minerals Industry, ed. M. J. Jones and 7 C. D. Wagner, W. M. Rigs, L. E. Davis, J. F. Moulder and G. E. Muilenberg, Handbuok of X-Ray 8 F. Garbassi, Surface Interface Anal., 1980, 2, 165. 9 E. E. Augmo, Am. Mineral., 1967, 52, 137. 10 T. Takahagi and A. Ishitani, Carbon, 1984, 22, 43. 11 B. Brion, Appl. Surface Sci., 1980, 5, 133. 12 J. Leja, L. H. Little and G. W. Foling, Trans. Inst. Min. Metall., 1963, 62, 407. 13 L. R. Pedersen, J. Electron Spectrosc. Relat. Phenom., 1982, 28, 203. 14 Y. M. Cross and J. Dewing, Surface Interface Anal., 1979, 1, 26. 15 W. Stumm and J. J. Morgan, Aquatic Chemistry (Wiley, New York, 1970), pp. 187 and 322. 16 M. Rey, Trans. Inst. Min. Metall., 1953154, 63, 541. 17 P. Eadington and A. P. Prosser, Trans. Inst. Min. Metall., 1969, 78, C74. R. Oblatt (The Institution of Mining and Metallurgy, London, 1984), p. 125. Photoelectron Spectroscopy (Perkin-Elmer C o p , Eden Prairie, 1979). Paper 5/942; Received 3rd June, 1985

ISSN:0300-9599    

DOI:10.1039/F19868202043

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1986,82, 2057-2063 Mechanism of Permeation through Molecular-sieve Carbon Membrane Part 1.-The Effect of Adsorption and the Dependence on Pressure Jacob. E. Koresh* and Abraham Soffer Chemistry Division, The Nuclear Research Center, Negev, P.O. Box 9001, Beer Sheva, Israel The permeation in narrow-pore molecular-sieve carbon membranes (MSCM) is governed by adsorption and activated transport and proceeds exclusively through the ultramicropores. Contribution from the solution- desolution mechanism is unlikely. The dependence of permeabilities on pressure and extent of pore opening has been studied. On the basis of adsorption isotherms the relation between adsorption and permeation is established. It is maintained that the slope of the adsorption isotherm of the penetrant determines the concentration gradient responsible for the flow through the membrane.For weakly adsorbing gases, permeability is inde- pendent of pressure, as corresponding to a free molecule in the pore system or to a linear adsorption isotherm. For adsorbing gases permeability decreases with pressure, owing to the decrease in the slope of the type I adsorption isotherm. Gas separation processes are based mainly on fractional distillation, adsorption and selective chemical reactivity. These processes are essentially cyclic, and so relatively elaborate and energy demanding in most cases. During the last few decades the polymer membrane separation processes from liquid systems had been utilized in several industrial applications. Nowadays reverse osmosis, ultrafiltration and dialysis are well known membrane processes for liquid treatments.Although gas permeation through polymer membranes was also under current fundamental study,l it was only in 1977 that an industrial gas-separation process was introduced.2 Since then considerable efforts have been devoted to studying and utilizing polymer membranes for gas separation through this simple and energy-saving process. New polymers with better permeability-selectivity combinations are currently being developed. So far the relatively young science and technology of membrane gas separation has been combined primarily to synthetic polymer materials. During the recent years of our research on molecular-sieve carbon adsorbent~~-~ we have shown that the molecular- sieving effect of non-graphitizing carbonslO is extremely specific and adjustable by mild activation and sintering steps to the discrimination range 2.8-5.2 A.Recognizing that pyrolysis of polymers yields an exact mimic of the morphology of the parent material if it does not proceed through a melt, it took a short while to produce a carbon molecular-sieving membrane from a thermosetting polymer membrane. l1 The permeation characteristics of the MSCM can be readily varied by changing the high-temperature treatment parameters. This is in contrast to polymer membranes, where such variations frequently involve the synthesis of a new membrane material. As with carbon molecular-sieve adsorbents, the pore size will serve as a core parameter in studying the MSCM properties. Comparing carbon with polymer membranes, the former may be considered as a refractory porous solid into which the permeates are non-soluble and merely penetrate the pore system.In this sense the permeation mechanism through the MSCM is much 20572058 Molecular-sieve Carbon Membrane simpler than that of glassy polymer membranes, whereas the dissolution-diffusion- desolution mechanism may take place in addition to penetration through domains of low polymer density. Therefore the influence of adsorption of permeates on the pore walls of carbon has to be taken into consideration, especially in the case of penetrants with relatively high boiling points. As a membrane material, carbon was expected to exhibit different and unique properties as compared with synthetic polymers. These are in addition to its important advantage of high-temperature stability. In a preliminary study we demonstrated some of its unique properties, namely the molecular-sieving transport mechanism, its high- temperature stability up to 900 "C and its extraordinary high separation power PS (where P is the intrinsic permeability and S the selectivity), which is 1-2 orders of magnitudes greater than that of any known polymeric membrane.In the same paper it was shown that enlarging the pore dimensions by mild oxidation steps can be carried out with the carbon membrane in a similar manner as with the carbon molecular-sieve adsorbents. This is considered to be initial evidence for the molecular-sieving mechanism of permeation through the carbon membrane.In this paper further evidence of the molecular-sieving mechanism of permeation is given, and the dependence of permeabilities and selectivities on temperature, pressure and extent of pore opening is discussed, for both adsorbing and non-adsorbing permeates. Experiment a1 Adsorption measurements were carried on in a conventional volumetric high-vacuum system. The permeability system and the membrane cell were the same as described previously.ll Since the hollow fibres were end-sealed with epoxy resin which cannot withstand high temperatures, the heating zone skipped these ends. As a result, part of the hollow-fibre membrane was colder than the centre when it was heated to several hundred "C. Our high-temperature permeability data refer to the hottest zone.The gases studied were H,, He, N,, Ar, Xe, O,, CO,, N,O, SF, and CH,. All the permeability measurements were carried on with pure gases. The permeability unit used is the barrer, expressed as a flow of loplo cm3 (s.t.p.) s-l of a fluid passing through a membrane of 1 cm2 in surface area and 1 cm in thickness. The pressure difference across the membrane was 500 Torrf- or less. The average pressure was spanned up to 6000 Torr. Results and Discussion Permeability, Adsorbability and Pore Size We have shown in a previous study3 that degassing a molecular-sieve carbon at gradually higher temperatures enlarge the pores first, owing to abstraction of surface carbon atoms as carbon oxides. At higher temperatures, pore closure commences owing to progressive annealing.To demonstrate this trend, adsorption-rate experiments were employed. It would be of interest to find out whether permeation, like the adsorption rate, follows the same trend, A series of carbon membranes of different pore dimensions were produced by treating a carbon membrane at gradually higher temperatures. In parallel, the same polymer precursor (not necessarily in a membrane form) was treated similarly and prepared for adsorption experiments. The adsorbability data are summarised in table 1. Adsorbability is estimated using four qualitative degrees of adsorption kinetics: (i) n.a. - implies no detectable adsorption; (ii) v.s.a. - very slow adsorption, i.e. adsorption equilibrium is not attained within tens of hours; (iii) s.a. - slow adsorption; adsorption equilibrium would require few hours; (iv) a fast adsorption denoted by specifying the amount adsorbed at the plateau of the typical type I isotherm in mmol ggl.Using this qualitative scale is satisfactory, since the adsorption rate on carbon molecular sieves changes by orders of t 1 Torr = 101 325/760 Pa.J. E. Koresh and A . So@- 2059 Table 1. Adsorption data for different carbon membranes (in mmol g-') membrane CO, at H, at N, at Xe at SF, at materiala 80 "C 196 "C 196 "C 80 "C 80 "C M-300 M-400 M-500 M-600 M-700 M-SO0 M-900 M- 1 000 v.s.a. s.a. 3.6 3.4 3.4 3.4 3.0 2.6 n.a. n.a. v.s.a. n.a. s.a. 4.4 4.4 4.4 s.a. v.s.a - - - - - - n.a. n.a. n.a. > 106b 1 06b 21 n.a. ma. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. a The membrane designations indicate the high-temperature treatment temperature.In pmol min-1. magnitude upon slight variations in pore size.3 Therefore, precise adsorption-rate data are unnecessary for a coarse adsorbability scale. The membrane designation shows the highest temperature of heat treatment. In table 1 adsorbability data are given for membrane materials that had been heated for gradually higher temperatures. Recognizing that the adsorbability sequence of the molecules in table 1 is CO, > H, > N, > Xe > SF,, it is evident that, starting from low HTT, the M-300 membrane acquires the narrowest porosity. Pore dimensions gradually increase up to the M-600 or M-700 membrane, then pore closure commences. The permeabilities of hydrogen and methane through a similar series of carbon membranes is given in fig.1. As for adsorbability, the permeability through the membrane passes through a maximum at 600-700 "C, thus furnishing further evidence for the molecular-sieving mechanism of permeation through the carbon membrane and for the simplicity of modifying its properties. Dependence of Permeability on Pressure In fig. 2 and 3 are shown the permeabilities of various gases plotted against the average pressure applied across the membrane. It is evident that, within the accuracy of the experimental results, the permeabilities of the low-boiling-point gases hydrogen, helium, oxygen and argon are independent of pressure (fig. 2). The permeability of methane, on the other hand, drops upon increasing the pressure. This drop is even more significant for the case of carbon dioxide and nitrous oxide.This trend seems to follow the sequence of intermolecular interactions i.e. adsorption of CH,, CO, and N,O impedes the permeation of these gases. These results may be interpreted as follows. The local flux, J , of a permeate through a unit membrane surface area is given by J = - P(dp/dx) ( 1 ) where P is the permeability (by definition) and dp/dx is the local pressure drop across the membrane. The pressure p in these equations is a local pressure determined as the bulk pressure that would determine the amount adsorbed locally according to an isotherm. It is an intermediate between the two bulk pressures on both sides of the membrane. Fick's first law is given by J = - D(dC/dx) (2) where D is the diffusion coefficient and dC/dx is the gradient of the local concentration of the permeate within the membrane.Combining the above two equations leads to P = D(dC/dp). (3)2060 1 2 10 g P N 1 a , 4 0 0. 0. 0. 0. 2 0. b P N I ; 0. 0. 0. 0. 1 I I I I I - - 1 . 2 1 . 4 *d 0 * - - 1 . 0 3 5 6 - - 0 . 8 ,p 2 L g - I O - 7 m Q - 0 . 6 0 - 0.4 L 2 - I I I I I ' - 0 . 2 Molecular-sieve Carbon Membrane I T/OC Fig. 1. Permeability of CH, and H, in membranes heat-treated to various temperatures: 0, CH,; 0, H,.J. E. Koresh and A. Sofler 206 1 5 - I I I I I I 4 - 33: - 3- 2 0 1 I I I I I 1 . - l-4 E P N 9 - 2 ;2- 7 - I I I I I I 0 1 2 3 4 5 6 5 3 . O 2 . 5 LI E m P 2.0 7 2 ;2 1.5 1 .o p/mTorr Fig. 3. Permeabilities of CO,, N,O and CH, plotted against the average pressure applied across the membrane.0, CH,; 0, N,O and *, CO,. LI 2 m .Q N I 2 k Fig. 4. Permeabilities of CO, plotted against the average pressure applied across the membrane at various temperatures: 0, 25; *, 100 and 0, 200 "C. To a first approximation we may assume that, at the presently encountered very low relative pressures p/po (where po is the vapour pressure of the penetrant), adsorption is low and interaction between the penetrant molecules is negligible, so that the diffusion coefficient is independent of the local concentration. Therefore the dependence of permeability on pressure should be analysed only through the derivative dC/dp in eqn (3). The local concentration C of the permeate obeys the adsorption isotherm c = F(P) (4) whereas dC/dp is its slope at a pressure p .It is well known that adsorption isotherms in molecular-sieving adsorbents are of type I, namely the isotherm slope is constant and high at low pressures and decreases upon increasing pressure. With respect to eqn (3), this readily explains the decrease in permeability upon increasing the pressure. This behaviour could be formulated mathe-2062 I I I I t I , 3 .O 2.5 h W 5 P P) 2.0 2 2 1.5 Molecular-sieve Carbon Membrane I 1 1 1 I- \ * - 1 matically by employing a type I adsorption isotherm, Let us take the Langmuir isotherm as a representative : where C, is the adsorption saturation capacity and b is an interaction parameter. By differentiating this equation and substituting in eqn (3), we obtain which shows that the permeability is a decreasing function of pressure.This is a general result of the curvature of a type I isotherm: it is not specific to the Langmuir mode. At high temperatures and the same pressure range, the interaction parameter of the Langmuir type isotherm becomes exponentially smaller,12 so that the isotherm resides at its linear portion, i.e. dC/dp is independent of pressure; therefore the decrease in the dependence of permeability on pressure at higher temperature is as shown in fig. 4 and 5. It is of interest to refer at this point to our previous work on the mechanism of penetration of an adsorbate into a particle of molecular-sieving ad~orbent.~ In that case we made use of the experimental fact that the adsorption rate in microporous media decreases upon increasing the amount adsorbed, so that a thin adsorption saturation layer is created at the outermost face of the porous adsorbent particle at the moment of exposure to the gas.Through this layer the transport rate is slowest, so that it becomes the rate-determining step for adsorption into the depth of the particle. In this work we provide an interpretation of this phenomenon, namely that it originates from the decreasing slope of the type I isotherm. Adsorption in a porous particle is a rate process, just as is penetration through a membrane. Both should essentially obey the same mechanism. However, unlike the previous adsorption study, in the present membrane study the pressure-temperature combination is such that adsorption saturation is not achieved. Therefore, the previously anticipated amplification of selectivity is not likely to occur for the case of membranes at or above room temperature.Returning to the light gases Ar, O,, H, and He, their interaction parameters which give rise to adsorption are much smaller than that of CO, for the same temperature; their adsorption isotherms are thus linear and the corresponding permeabilities are independent of pressure at lower temperatures as is observed in fig. 2. A comprehensive discussion of the permeability dependence on temperature must take into account the variation with temperature of the diffusion constant D in eqn (3). Thus a prolonged residence time of the adsorbed molecule at the pore's wall corresponds to C = Co bp/( 1 + bp) ( 5 ) P = DC, b / ( l + bp)2 (6)J . E. Koresh and A .Sofer 2063 blocking the pores with the penetrant at low temperatures, while a virtually zero residence time will approach the limiting case of Knudsen diffusion at high temperatures. This more complex situation will be discussed later. This work was supported by the Israel National Council for Research and Development. We are indebted to S. Saggi and D. Rosen for skilled technical assistance. References 1 S. A. Stern, in Membrane Separation Processes, ed. P . Meares (Elsevier, Amsterdam, 1976), chap. 8. 2 R. J. Gardner, R. A. Crane and J. F. Hannan, Chem. Eng. Progr., (Oct 1977), p. 76; J. M. S. Henis and M. K. Tripodi, Sep. Sci. Technol., 1980,15, 1059; S . G. Kimura and G. A. Walmet, Sep. Sci. Technol., 1980, 15, 11 15. 3 J. Koresh and A. Soffer, J. Chem. SOC., Faraday Trans. 1, 1980, 76, 2457. 4 J. Koresh and A. Soffer, J . Chem. Soc., Faraday Trans. 1, 1980, 76, 2472. 5 J. Koresh and A. Soffer, J. Chem. Soc., Furaday Trans. 1, 1981, 77, 3005. 6 J. Koresh, J. Colloid Interface Sci., 1982, 88, 398. 7 J. Koresh and A. Soffer, J. Colloid Interface Sci., 1983, 92, 517. 8 S. S. Barton and J. E. Koresh, J. Chem. SOC., Faraday Trans. 1, 1983, 79, 1147. 9 S . S. Barton and J. E. Koresh, J. Chem. Soc., Furaday Trans. 1, 1983, 79, 1 1 57. 10 J. Koresh and A. Soffer, J. Chem. Soc., Faraday Trans. 1 , 1980, 76, 2507. 11 J. Koresh and A. Soffer, Sep. Sci., 1983, 18, 723. 12 A. W. Adamson, in Physical Chemistry of Surfaces (Interscience, New York, 1967), p. 572. Paper 51980; Received 10th June, 1985

ISSN:0300-9599    

DOI:10.1039/F19868202057

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1986,82,2065-2077 Concentration Polarization and Water Dissociation in Ion-exchange Membrane Electrodialysis Mechanism of Water Dissociation Yoshinobu Tanaka" and Manabu Sen6 Central Research Institute, Salt Division, Japan Tobacco Inc., 6-2, Umegaoka, Midori-ku, Yokohama 227, Japan A relationship between the current density i and the current efficiency of H+ and OH- ions generated by dissociation of water above the limiting current density in ion-exchange membrane electrodialysis has been pro- posed. Generation rates of H+ and OH- ions in the dissociation of water, H,O e H+ +OH- are given as the difference between the forward and the reverse reaction rate. The transport of H+ and OH- ions generated is described according to the Nernst-Planck equation.The relationship between i and q is derived by using the conservation law for the generation and the transport of H+ and OH- ions. Comparing the calculated values with experimental ones, it is clear that the water dissociation reaction rate constant increases with increasing i. Such an increase is usually larger for an anion-exchange membrane than for a cation-exchange membrane. An especially remarkable increase is seen for a cation-exchange membrane placed in aqueous MgCl,, NiCl, or CoC1, solution. This is due to violent water dissociation generation by cascade (snow slide) splitting of water molecules in the water dissociation layer. ki kz _ _ _ ~ ~ In the course of demineralization and concentration of ionic solutions by ion-exchange membrane electrodialysis, the concentration of ions in the boundary layer formed on the membrane surface rises at the concentrating side, while it falls at the desalting side owing to concentration polarization.If the product of the concentrations of scale-forming ions in the layer at the concentrating side exceeds the solubility product, the formation of scale becomes feasible. On the other hand, if a depleted layer is formed at the desalting side, not only does the cell voltage rise but also, in extreme cases, water splitting, i.e. water dissociation takes place and results in a decrease in current efficiency and scale formation. It is, therefore, necessary to operate the electrodialytic apparatus at or below a given level of current density to avoid these problems. Concentration polarization has been widely investigated,l-l* and is said to be due to the difference in transport numbers of the counter ions between the membrane and the solution. From a consideration of transport numbers, it is expected that concen- tration polarization would occur more easily on a cation-exchange membrane than on an anion-exchange membrane, and the resultant water dissociation is therefore expected to occur easily on the cation-exchange membrane.However, on the contrary, an inspection of the experimental results of Peers2 and Rosenberg and Ti~-rel,~ shows that water dissociation on a cation-exchange membrane tends to be suppressed. Similar phenomena have been reported in ref. (5) and (11)--(17) and mechanisms have been discussed. Even from these results, however, it cannot be said that the mechanism of water dissociation has been sufficiently clarified.In order to avoid the problems caused by the water dissociation it is necessary to understand the mechanism. This problem will be discussed in this report. 20652066 Ion-exchange Membrane Electrodialysis \ water d i s s o c i a t i o n layer Fig. 1. Water dissociation layer formed on a membrane surface. water dissociation l a y e r ' A I /O X boundary layer -+x a x i s I Fig. 2. Formation and transport of H+ and OH- ions in a water dissociation layer. Theory When an electric current passes across an anion-exchange membrane placed in an ionic solution, a boundary layer is formed on the desalting surface of the membrane (fig. 1). If an electric current larger than the limiting value passes, a salt-depleted layer (water dissociation layer) is formed at the membrane-solution interface as depicted in fig.2. In the water dissociation layer, H+ and OH- ions are generated (generation rates are oH and ooH, respectively), and are transported under the potential difference (transport rates are JH and JOH). Water dissociation is a reversible reaction : kl kz H,O + H+ +OH-. (1) The generation rates of H+ and OH- ions are given by the difference between the forwardY. Tanaka and M. Sen6 2067 where oH is the generation rate of H+ ions at &x; and oOH is the generation rate of OH- ions at x-l; C,, COH and CH20 are the concentrations of H+ ions, OH- ions and H,O respectively, at x; x is the axis drawn in the water dissociation layer; and 1 is the thickness of the water dissociation layer.H+ and OH- ions generated are transported by electromigration and diffusion. The transport rates (fluxes) are obtained from the Nernst-Planck equation as foilows : where JH and JoH are the fluxes of H+ and OH- ions at x, DH and DOH are the diffusion constants of H+ and OH- ions, F is the Faraday constant, zH and zOH are the charges of H+ and OH- ions, R is the gas constant, T is the absolute temperature and # is the electric potential. The following formulae hold between generation and transport from the mass conservation law : (4) f l I T = JH = (i/F)qH OOH = -JOH = (i/F)qOH Current efficiencies are defined as follows: where q,, qoH, qA and q y are the current efficiencies for H+ and OH- ions, cations A and anions Y, respectively. The following equation is obtained from eqn (2)-(4): 1 ( i / F ) q =k,CH201-k,J 0 CHCOHdx --DH---- dCH FDHCHdb dCOH FDOH COH 2 RT d x ' - dx RT dx+Do,-- dx The distributions of C , and CoH in the water dissociation layer in eqn (6) are given by (cf.Appendix): exp (KZ) - 1 exp (Kx) - 1 cH = J( D,, c"~ C G ~ exp ( ~ K z ) exp (- ~ K I ) + [I - exp (- ~ K Z ) ] exp (KZ) - 1 exp [K(l - x)] - 1 DH c,, = J ~ ~ C R C ; , exp(rK~) exp(-rK~)+[l -exp(-rK~)] CH and CgH in eqn (7) are the concentrations of H+ and OH- ions at both ends of the water dissociation layer. K is given by where p is the apparent specific resistance of the water dissociation layer, po is the specific resistance of the water dissociation layer and 4m is the membrane surface potential.2068 Ion-exchange Membrane Electrodialysis 10-‘6t- I I I I I I I 0 0.2 0.4 0.6 0.8 1.0 x / 1 0-4 cm Fig.3. Concentration distributions of H+ and OH- ions in a water dissociation layer. i=0.lAcm~2,l=lO~4cm,p=lO5Rcm.r:(a)0.5,(b)0.3,(c)0.l,(d)O. r i ( I - r ) I I Fig. 4. Concentration distributions of H+ and OH- ions in a water dissociation layer (model).Y. Tanaka and M . Sen6 2069 1 0.2 0.4 0.6 0.8 1.0 x/ 1 o-4 cm Fig. 5. Ionic product distributions of H+ and OH- ions in a water dissociation layer. i = 0.1 A cm-2, I = cm, p = lo5 Q cm. r : (a) 0, (b) 0.1, (c) 0.3, ( d ) 0.5. Taking r from eqn (7), C, and C,, are plotted against x in fig. 3 and illustrated in fig. 4 as a simplified model. Fig. 4 indicates the phenomenological meaning of r as follows : r = (thickness of the region in which the concentrations of H+ and OH- (10) Moreover, it is understandable that r becomes zero at or below the limiting current density and increases with increasing i, but never exceeds 0.5.The ionic product CH C,, is plotted against x in fig. 5 which shows a tendency to have reduced values in the water dissociation layer above the limiting current density. This behaviour is schematically shown in fig. 6. ions change)/(thickness of the water dissociation layer). From eqn (6) and (7), the relationship between i and 1 is obtained as follows: a = k, CHzO [exp (KZ) - 112 exp (rKZ) B = k, c"~ {[exp (KZ) - exp (rKZ)l2 + exp (KZ) [exp (rKZ) - 112 2 +a [exp (KZ) - I] [exp (rKZ) - 11 [exp (KZ) - exp (rKZ)]> y = &'(OH Do, CH COH) [exp (KZ) - 11 exp (rKZ) (exp [(2 - r)KZ/2] -exp (rKZ/2)).(1 5)2070 Ion-exchange Membrane Electrodialysis r l 1 / 2 ( I - r l l I Fig. 6. Ionic product distributions of Hf and OH- ions in a water dissociation layer (model). X Experiment a1 Fig. 7(a) and (b) illustrate the apparatus for measuring water dissociation on a cation-exchange membrane @ and an anion-exchange membrane @ , respectively. The effective areas of membranes @ and @ were reduced to 0.264 cm2 by a gasket in order to generate more easily water dissociation on these membranes rather than on the other membranes used in the apparatus. D and C are the desalting chamber and the concentrating chamber, respectively. Dimensions of D and C are shown in fig. 8. If an electric current passes across @ or @, the ionic concentration changes in D are small, but the ionic concentration in the boundary layer formed on @ or @ decreases owing to concentration polarization and water dissociation takes place at a value of i above the limiting current density.0.1 mol dm-3 NaCl aqueous solution or 0.05 mol dmA3 MgC1, aqueous solution were supplied to D by a pump at a rate of 0.1 cm3 s-l. 1 mol dm-3 aqueous solution of the electrolytes mentioned above were put in D’. 1 eq dm-3 NaCl aqueous solution were put in C and D”, respectively. 1 mol dm-3 NaCl aqueous solution was pumped to the electrode chambers. After these preparations, an electric current was passed for 10 min using AgIAgCl electrodes. When water dissociation takes place on the surface of @ or @ in D, H+ or OH- ions generated are transported into C and accumulated there.Then the electrodialyses were repeated by changing the current densities incrementally. Current efficiencies for H+ or OH- ions were calculated from measurements of pH changes of the solution in C and these were plotted against current densities. At the same time, voltage drops V across the membrane were measured and were plotted against current densities. Results and Discussion i vs. qH plots and I/ vs. i plots are shown in fig. 9 and 10. From the inflection of the curves, the limiting current density ilim is evaluated. i us. rH plots and i vs. qoH plots are shown in fig. 11.Y. Tanaka and M. Sen6 207 1 ( b ) Fig. 7. Electrodialytic apparatus to measure water dissociation : K, cation-exchange membrane; A, anion-exchange membrane; D, desalting chamber; C, concentrating chamber; G, gasket.Fig. 8. Dimensions (mm) of desalting chamber and concentrating chamber. Now, we will make clear theoretically the relationship between i and q by using eqn (1 1)-(15), and compare the result with the observed values. For this purpose, we must know the values of k , and k,. The ratio k,/k,, being the equilibrium constant, must remain constant at a given temperature, so that k, and k, could be expressed using a parameter 2 as follows; k, = 2 x 2 s-l k, = 1.5 x 1014 2 cm3 mot1 s-l. As will be explained, the calculated values of 7 are not consistent with the observed values unless k , and k, increase drastically with increase in i. So, we could tentatively assume that Z is unity below the limiting current density and k , and k, have the values observed2072 Ion-exchange Membrane Electrodialysis 6 5 4 2 1 Fig. 9.Current 0 0.05 0.10 0.15 0.20 ilA cm-2 25 "C. density us. qH plot. Cation membrane Selemion CMV, 0.20 0.1 5 N I 2 0.10 2 0.05 0 0.5 1.0 1.5 2.0 2.5 VlV 0.1 mol dm-3 NaCl, Fig. 10. Current density us. voltage drop plot. Cation membrane Selemion CMV, 0.1 mol dm-3 NaCl, 25 "C. by Eigen,18 and above the limiting current density the values of 2, and therefore, k, and k,, increase with increasing i. Taking 2 and p as variables, the calculated values of q were plotted against i at r = 0.3 and the result gave the curves in fig. 11. By comparing the observed values with the calculated ones, it is clear that 2 (i.e.k, and k2) greatly increases with increasing i. (p seems to increase simultaneously, but it is difficult to identify its changes.) It is recognizable, moreover, that in aqueous NaCl, the increase of Z is more intensely restricted on a cation-exchange membrane than on an anion-exchange membrane. In aqueous MgCl,, however, it was confirmed that Z increases remarkably and violent water dissociation proceeds on the cation-exchange membrane, as indicated by the large increase of qH. In previous reports,lg* 2o it was recognized that water dissociation is generally moreY. Tanaka and M. Sen6 2073 1 6 ~ to2 10’ 1 10 l o 2 i/A cm-2 Fig. 11. Relations between i and qH or qoH. Y = 0.3. solution membrane 0.1 mol dm-3 NaCl 0.05 mol dm-3 MgC1, cationa 0 anionb 0 a A 104 1 O6 1 O8 1 102 1 O6 1 1 o2 104 104 1 O8 1 O8 1 O8 1 O6 1 O6 1 O6 1 os 104 104 104 a ACIPLEX K-102.ACIPLEX A-102. restricted on cation-exchange membranes than on anion-exchange membranes. Such a restriction was proved to be brought about by the acceleration of ionic transport under the high electric potential developed on the desalting surface of cation-exchange membranes. On the other hand, it was recognized that extraordinarily violent water dissociation takes place on cation-exchange membranes placed in aqueous MgCl,, NiC1, or CoC1,. It also became clear that water dissociation is promoted when hydroxides such 69 FAR 12074 Ion-exchange Membrane Electrodialysis + + + + + + + + + cation membrane water dissociation layer Fig. 12. Cascade (snow slide) splitting model of water dissociation. as Mg(OH),, Ni(OH), or Co(OH), are placed under the conditions of low ionic concentration and high electric potential.These conditions were observed to be reproduced easily on the desalting surface of a cation-exchange membrane placed in aqueous MgCl,, NiCl, or CoCl,, so that such circumstances were estimated to cause the extraordinarily violent water dissociation. From the considerations described in this report, it is concluded the reaction rate constants k , and k, increase remarkably in the violent water dissociation. Furthermore, it is clear that the drastic increase in the reaction rate which was observed on the cation-exchange membrane placed in aqueous MgC1, is caused by the precipitation of Mg(OH), on the desalting surface of the membrane.It could be thought that this violent water dissociation is brought about by the accelerated cascade (snow slide) splitting of water molecules as shown schematically in fig. 12. In the cascade water splitting, H+ and OH- ions generated by water dissociation interact with adjacent water molecules on the precipitates of Mg(OH),, with the resultant promotion of water dissociation, accompanied by the generation of majorities of H+ and OH- ions. Appendix Concentration Distribution of H+ and OH - Ions in a Water Dissociation Layer The concentrations of H+ and OH- ions (C, and COH) multiplied by the diffusion constant (DH and DOH), respectively, in a water dissociation layer, and the distributions of these quantities (the converted ionic concentration XH and XOH), are expressed as polynomials of x as follows : n: X,=D,C,= C a n x n n=o cc XOH = DOH CO, = bn(l-x)". n=o From eqn (A 1) and eqn (6) we getY.Tanaka and M. Sen6 2075 Substitution of eqn (A 1) into (A 2) gives eqn (A 3): Eqn (6) indicates that ( i / F ) v should be independent of x. Therefore, with the exception of the first term, all other terms on the right-hand side of eqn (A 3 ) can be regarded as zero and then the relationship of eqn (A 4) is obtained. ... - a, x + b2(l - x) a, x + b,(l-x) = (n+ 1 ) a3 x2 + b,(l- x ) , a, x2 + b , ( l - ~ ) ~ K = 2 = 3 .... an+, xn +bn+l(l-x)n - - an xn + bn(l- x ) ~ Putting x = 1 in eqn (A 4), we get K = 2(a,/a,) = 3(a,/a,) = . . . = (n -/- 1) (an+,/an) = . . . . (A 5 ) From eqn (A 5), the coefficients in the polynomials are obtained as follows: a, = (K/2)a1 = (K/2!)a1 a, = (K/3)a2 = (K2/3!)a1 a, = (K/n) an-, = (Kn-l/n!) a, In the same way, putting x = 0 in eqn (A 4), we get K = 2(b2/b,) = 3(b,/b,) = .. . = (n + 1 ) (b,+,/b,) = . . . (A 7) (A 8) (A 9) 1 b, = (K/2)b, = (K/2!)b1 b, = (K/3) b, = (K2/3!) b, b, = (K/n) b,-, = (Kn-l/n!) b,. Using eqn (A 6) and (A 8), eqn (A 1) can be simplified as: XH = a,+(a,/K) (Kxll ! +K2x2/2! + . . . + Knxn/n! + . . .) = a, + W K ) [exp (Kx) - 11 = bo + (b,/K) (exp [K(l-x)] - 1). X O H = bo+(bl/K) [ K ( l - ~ ) / l ! +K2(1- ~ ) ~ / 2 ! + . . . + Kn(l-x)n/n! + . . .] Further, in order to extinguish the coefficients of the polynomials remaining in eqn (A 9), the distribution of XH and XOH in the water dissociation layer are depicted as the model in fig.13. Xf, and XkH are XH and XOH at x = 0; X,A and XbH are those at x = 1. S is the ionic product of H+ and OH- ions at the end of the water dissociation layer. The following relation exists between these values : x& XLH = XHTAH = s > XHXOH. (A 10) Whereupon, using eqn (A lo), the ratio of the concentrations of H+ and OH- ions at x = 0 and x = 1 is defined as: m = X&/Xf, = XLH/XGH. (A 1 1 ) 69-22076 Ion-exchange Membrane Electrodialysis '0 1 anion exchange water dissociation mern brane layer Fig. 13. Concentration distribution of H+ and OH- ions in the water dissociation layer. Because the quantities of Hf ions generated in the water dissociation layer are equal to those of OH- ions, the following equations are obtained from eqn (A 10) and (A 11).Using eqn in eqn (A the coefficients remaining in eqn (A 9) can be replaced by rn as In eqn (A 13), if i increases above the limiting current density, KZ and rn will also increase. We define the ratio of lnm to KZ by r = lnrn/(KI) (A 14) where r is a parameter concerned with the structure of the water dissociation layer and its meaning is given by eqn (10). Eliminating m by the substitution of eqn (A 14) into (A 13) and putting A', = D , C , and XOH = DOH COH, we get the distribution of C, and C,, in the water dissociation layer described by eqn (7). References 1 T. R. E. Kressman and F. L. Tye, Discuss. Faruduy SOC., 1956, 21, 185. 2 A. M. Peers, Discuss. Faraday SOC., 1956, 21, 124. 3 N. W. Rosenberg and C . E. Tirrel, Znd. h'ng. Chem., 1957, 49, 78. 4 D. A. Cowan and J. H. Brown, Ind. Eng. Chem., 1959,51, 1445. 5 B. A. Cooke, Electrochim. Actu, 1961, 3, 307; 1961, 4, 179. 6 Ch. Forgacs, DECHEMA Monog., 1962 47, 601. 7 Y. Onoue, J. Electrochem. SOC. Jpn, 1962, 30, 415. 8 K. S. Spiegler, Desalination, 1971, 9, 376. 9 N. Takemoto, J. Chem. SOC. Jpn, 1972, 2053; 1973,44. 10 A. J. Makai and J. C. Turner, J . Chem. Soc., Faraduy Trans. I , 1978,14, 2850. 1 I V. J. Frilette, J. Phys. Chem., 1957, 61, 168. 12 T. Uchino, S. Nakaoka, H. Hani and T. Yawataya, J. Electrochem. Soc. Jpn, 1957, 26, 366.Y. Tanaka and M. Sen6 13 M. Sen6, K. Yamagata and T. Yamabe, J. Electrochem. Soc. Jpn, 1966, 34, 770. 14 T. Yamabe and M. Sen& Desalination, 1972, 2, 148. 15 M. Block and J. A. Kitchener, J. Electrochem. SOC., 1966, 113, 947. 16 I. Rubinstein, J . Chem. Soc., Faraday Trans. 2, 1981, 77, 1595. 17 R. Simons, Electrochim. Acta, 1984, 29, 151. 18 M. Eigen, Discuss. Faraday SOC., 1966, 113, 947. 19 Y. Tanaka, S. Matsuda, Y. Sat6 and M. Sen& J, Electrochem. Soc. Jpn, 1982,50, 667. 20 Y. Tanaka and M. Sen6, J. Electrochem. SOC. Jpn, 1983,51, 267; 1983,51,465. 2077 Paper 511019; Received 17th June, 1985

ISSN:0300-9599    

DOI:10.1039/F19868202065

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1986,82,2079-2087 lH and 19F Nuclear Magnetic Resonance Investigation of the Active Site of Catalase John Oakes Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, Merseyside L63 3J W In order to pinpoint the location of a water molecule in the vicinity of the active site of catalase water-proton relaxation times have been determined for aqueous solutions of beef-liver catalase in the presence and absence of fluoride and formate. Additionally, fluorine relaxation times have been measured for solutions containing sodium fluoride. Surprisingly, it is shown that water does not coordinate directly to the haem active centre. Instead, there is evidence for the presence of a water molecule strategically situated some 3.6-3.8 8, away from the haem iron and hydrogen-bonded to neigh- bouring amino-acid residues, histidine and asparagine.This may have important consequences mechanistically, since its replacement by peroxide allows direct coordination of peroxide to the haem group and simultaneous interactions with neighbouring amino-acid residues may facilitate efficient peroxide bond cleavage. The iron protoporphyrin IX group CH3t__lCH =CH2 c72 CH=CH2 CH2COOH CHzCOOH is present in the active site of a variety of enzymes having functions as widely different as oxygen transport, electron transfer and catalytic utilisation of hydrogen per0xide.l Evidently, the fundamental differences in properties of these haem enzymes result from subtle interactions between the haem group and the surrounding protein matrix of amino acids and from changes in axial ligation.2 Of particular interest is the participation of water as an axial ligand, since reactions take place in aqueous media and because it is implicated in reaction mechanisms.2 Coordination of water to haem iron can be determined in the crystal by X-ray crystallography33 and in solution by 1 7 0 or lH n.m.r.relaxation studies5 of water molecules. The existence of a water molecule bound to the haem iron in metmyoglobin and met- haemoglobin has been established by X-ray studies.3* Similarly, n.m.r. studies have elucidated the dynamics of water-proton exchange and, in addition, located the position 20792080 N.M.R. Study of Cutaluse of a histidine proton in the fluorinated met derivatives.61 However, the situation for peroxidases and catalases is far from clear.For example, whilst it has been proposed that horse-radish peroxidase (HRP) does not contain a water molecule8~ in its active site, similar studies have suggested that liver catalase does.1°-12 On the other hand, X-ray studies have been unable to detect coordinated water in liver catalase,l39 l4 but clearly indicate its presence in cytochrome c peroxidase.lj More recent studies have queried the presence of water in liver and erythrocyte catalases16 and other studies have queried the absence of water in HRP 1 7 9 la and lactoperoxidase. In the author's opinion, the main reasons for the uncertainty in interpretation of n.m.r. measurements are: (i) there has been a tendency to assume that the contribution to relaxation arising from the paramagnetic haem group is constant from enzyme to enzyme, e.g.simple comparisons of molar relaxivities have been used8 to give an indication of the presence or absence of bound water in peroxidases; (ii) uncertainties in the extent to which haem exists in the high-spin state and in the value of z, the electronic spin relaxation time, from frequency dependence studies; (iii) the failure to appreciate the significance of the finding that exchange of water protons is slow6y7 on the n.m.r. timescale in cases where a directly coordinated water molecule has been established; and (iv) uncertainties in estimated values of the diamagnetic contribution arising from the protein moiety to the overall relaxation. Whilst its estimation from the cyanide derivative of certain haem proteins, e.g.metmyoglobin, is beyond doubt, its extension to the cyanide derivative of peroxidases and catalases is questionable. In the work reported here, an n.m.r. relaxation study of water protons in solutions of beef-liver catalase has been carried out. Impetus for this study has come from (i) the recent crystal structure determination of beef-liver catalasel3? l4 and (ii) the successful determination of hydration numbers of small complexes using n.m.r. technique~.l~-~~ Experimental Beef-liver catalase was obtained as a suspension (1 g per 50 ~ m - - ~ ) from Boehringer Mannheim GmbH. A 3% solution was prepared by centrifugation of 1.5 cm3 of suspension, followed by dissolution in 1 cm3 of I-lO% NaCl solution.This produced typical haem concentrations of cu. 3 x lop4 mol dm-3 required for n.m.r. investigations. Haem concentrations were determined using a Cary 14 spectrometer by measuring Soret band intensities at 403 nm of a 0.09% solution, using a molar absorptivity of 1.485 x lo5 dm3 mol-1 cm-l per haem. Assuming a molecular weight of catalase of 250000 it can be estimated that there are at least 2.7 intact haem groups per protein molecule. Spin-lattice, q, relaxation time measurements were made using a Bruker BKR-322s pulse n.m.r. spectrometer operating at 60 MHz. q was determined from a 180"-~-90" pulse programme in which the n.m.r. free-induction signal following the 90" pulse was obtained as a function of the pulse separation, z. In the majority of cases, relaxation decay and recovery curves were single exponentials and q was calculated from semi-log plots of signal intensity us. time.As in earlier investigations, the intensity of the protein signal was found to be too low to contribute to overall proton relaxation times. Results and Discussion General The addition of a haem protein to an aqueous solution results in an increase in the water-proton relaxation rates through two contributions: (i) the effect of the protein (diamagnetic contribution) and (ii) the paramagnetic effect of the haem group, which dominates relaxation T-l = T-l (diamagnetic) + T-l (paramagnetic).J . Oakes 208 1 Table 1. 19F and lH relaxation times in catalase solution at 294 K spin-lattice relaxation time/ms sample water proton fluoride __ ____ ~ _ _ _ _ _ catalase (3%)"/10% NaCllpH7 380 - catalase (3 % )/ 10 % NaCl/ 1 mol dm-3 NaF/pH 7 365 390 495 - bovine serum albumin (1 2 % ) 850 - catalase (3 % )/ 1 0-1 mol dmP3 formate/ 10 % NaCl a [Haem] = 3.06 x lo-* mol dmP3.It is customaryg~ lo to estimate the diamagnetic contribution to relaxation by recording data in solutions of the low-spin cyanide derivative under the same conditions. Whilst this is sound practice for metmyoglobin and methaem~globin,~~ it is less satisfactory for peroxidases and catalases. For example, whilst there is good agreement between data for the cyanide and the carbon monoxyferrous derivatives of metmyoglobin,23 this is apparently not the case for liver catalase.lO7 l5 Similarly, replacement of bound water in metmyoglobin by cyanide results in a marked reduction in relaxation, but addition of cyanide to liver catalase has little effect.12 (Reasons for this will become apparent later.) In this work, the diamagnetic contribution has been taken from our earlier work for a corresponding concentration of the non-paramagnetic protein, bovine serum albumin.24.25 Magnitude of Relaxation Enhancement The values of the spin-lattice relaxation times (table 1) determined for 3% catalase solutions, with and without addition of formate mol dm-3), are similar to those previously reported.l09 l1 The similarity of relaxation rates for liverlO9 l1 and erythrocyte16 catalases suggest little contribution from high-spin FeIII-biliverdin groups in liver catalase. The diamagnetic contribution to relaxation was estimated as 1.2 s-l, cf.1.5 s-l obtained from the cyanide derivative.l0> l1 This yields a value of 4.6 x lo3 for the relaxation per haem group (table 2). Inspection of table 2 shows that this is much higher for catalases and lactoperoxidase than for HRP and other enzymes and is the basis10-12t l8 for assigning a water molecule as being directly coordinated to the haem group in catalase and lactoperoxidase. Addition of F- to haem enzymes produces marked increases in molar haem relaxation rates (table 2) of methaemoglobin, metmyoglobin and HRP, but only a slight change in catalase. The fluoride ion is well known to be coordinated directly to the haem iron in these enzymes and the observation of hyperfine coupling from fluorine in the e.s.r. spectrum of erythrocyte catalase16 confirms this is the case for catalase.The increase in molar relaxitivity for methaemoglobin and metmyoglobin has been explained6$ in terms of (i) increase in magnetic moment of the haem group (by increasing the proportion of the high spin state); (ii) a corresponding increase in z, the electronic relaxation time; and (iii) an efficiently relaxed outer-sphere histidine proton in rapid exchange with bulk water, instead of the slowly exchanging water proton in water molecules directly coordinated to the native enzymes. The first two arguments have also been used to explain the more efficient relaxation observedg for protons outside the first coordination sphere of HRP when F- becomes bound. On the other hand, catalase exists entirely in the high-spin state in its native form26 so that no changes in magnetic moment or z, are anticipated on binding of F-.If a water molecule directly coordinated to haem were2082 N.M.R. Study of Catalase responsible for in relaxation, * Table 2. Comparison of relaxation rates due to the haem group (R,) in different enzymes R1/s-l (fluoride enzyme RJs-l derivatives) methaemoglobina ca. 5 x 102 1.2 x 103 metmyoglobinb (2-5) x lo2 1.4 x 103 HRPC ca. 5 x 102 2.5 x 103 lactoperoxidased 3.5 x 103 - catalase ( 3 4 ) x 103 e , f 4.5 x 103f 5 x 1039 4.6 x 1036 a Ref. (7). Ref. (6). Ref. (9). Ref. (16). Ref. (10). f Ref. (15). g This work. relaxation then displacement by F- should produce a marked decrease Conversely, the marginal increase observed in relaxation is the first indication of the absence of a haem-coordinated water molecule in catalase.Dynamics of Water-proton and Fluoride Exchange The contribution to overall relaxation from the haem group is given by (2) nN 55.6 Tcl - Tcl (diamagnetic) = - ( qM + zM)-l when n is the number of water molecules bound to the haem group, N is the concentration of the haem group, qM is the relaxation time for protons of a water molecule bound to haem and rM is the residence time for a bound water proton. The paramagnetic contribution to relaxation does not increase with temperature,l09 l6 from which it is concluded that T~ 4 qM. Assuming one water molecule is coordinated to haem, then qM = 4 x s. Consequently, rkl, the frequency of exchange of (water) protons between haem group and bulk water > 2.5 x lo5 s-l.This value is similar to that determined for exchange of water molecules from the coordination sphere of many high-spin ferric ion systems,10 suggesting water-molecule rather than proton exchange. The proton spin-lattice relaxation rate for water protons coordinated to the haem where yI is the nuclear gyromagnetic ratio, g is the spectroscopic splitting factor, p is the Bohr magneton of the electron, S is the total electron spin, r is the haem- iron-water-proton distance and wT and ws are the nuclear and electron Larmor precession frequencies, respectively. r;l is given as follows : z,1= r,'+z;'+zG1 where z, is the rotation correlation time and z, is the electron spin-relaxation time of the haem iron. * This decrease could conceivably be offset by hydrogen bonding between bound fluoride and neighbouring t The merits and limitations of the Soloman-Bloembergen equations when applied to catalase have been protons on the distal side of the haem.discussed previously.l0, l6J . Oakes 2083 Eqn (2) reduces to 7'c1 (paramagnetic) = 1.48 x for conditions of fast exchange. For fluorine nuclei, eqn (4) becomes Tcl (paramagnetic) = 1.8 x (4) wherefis the fraction of fluoride ions bound to the haem. For 1 mol dm-3 fluoride solution f = 3 x and taking pz = 35, r = 2.1 Tcl (paramagnetic) = 6.67 x 10l2 z,. For z, = z,, varying between 2 x s and 1 x s, Tcl (paramagnetic) is calculated to range from 7.50 to 1.48 ms. Since the observed value is 390 ms, it is con- cluded that zM % GM, hence eqn (2) becomes Tcl (paramagnetic) = f z ~ l and for fluoride nuclei 2 ~ 1 = 8.3 x 103 s-1.Consequently, exchange of F- from the first coordination sphere of catalase haem with F- in bulk water occurs at a rate of 8.3 x lo3 s-l, i.e. slow exchange of fluoride ions prevails. In metmyoglobin6 and rnethaemogl~bin,~ both fluoride and protons in water molecules directly coordinated to the haem exhibit slow exchange. The fact that fast exchange is observed for (water) protons in catalase indicates that n.m.r. ' sees' outer-sphere (water) protons rather than protons in water molecules directly coordinated to the haem group. This is the second indication* for the absence of a haem-coordinated water molecule, consistent with the failure to detect bound water by X-ray studies. * Strictly speaking, n.m.r.investigations alone cannot unambiguously rule out the existence of a directly bound water molecule. However, there is no reason to suppose it would exhibit anything other than slow-exchange characteristics.2084 N.M.R. Study of Catalase Location of Outer-sphere Water Molecule Calculations were carried out using eqn (4) to estimate the proximity of exchangeable protons to the haem iron. The distance of up to four exchangeable proteins from the haem iron is plotted in fig. 1 vs. the correlation time, z,, of the haem group. Assuming values of 2, (which is dominated by z,) of (2-7) x 10-lo s, it is estimated that there are at least two protons at a distance of 3-3.6 away from the iron. Since protons in amino acids on the distal side of the haem are much more distant,13* l4 relaxation is attributed to an outer-sphere water molecule.Indeed, it has been suggested that the small peak of electron density detected between the distal histidine and asparagine amino-acid residues may be due to such a water molecule hydrogen-bonded between these residues and situated ca. 3.8 A from the iron. The location of this outer-sphere water molecule and its interaction with neighbouring amino-acid residues is schematically illustrated below. The presence of an outer-sphere water molecule explains: (i) why exchange of water protons/molecules is fast on the n.m.r. timescale rather than slow (expected for directly coordinated water); (ii) the similarity of relaxation rates in solution of catalase and catalase fluoride;* and (iii) the small effect of cyanide on relaxation in solutions of catalase compared with metrnyoglobin.l2 (In native metmyoglobin, the directly bound water dominates relaxation and this is displaced by CN-.) Conversely, the sizeable outer-sphere relaxation observed12 in the cyanide derivative of catalase shows that there must be reservations in using low-spin derivatives for estimating diamagnetic contributions to relaxation.Whilst there is little doubt that relaxation in the fluoride derivative of metmyoglobin results from the exchangeable proton of a distal histidine, caution must be exercised in assigning relaxation to a distal histidine in HRP and its derivatives. The present investigations indicate that relaxation could be due to an outer-sphere water molecule, particularly for lactoperoxidase which has a similar relaxivity to catalase (table 2).However, for very much the same reasons as given here it does seem likely that a water molecule is not directly coordinated at the sixth site in HRP, particularly supporting evidence being the absence of 170 hyperfine structure in e.s.r. spectrag and the invariance of haem molar relaxivity when the haem spin-state is changed.8 The increase in water-proton relaxation time upon addition of formate ( > mol dm-3) to catalase solutions (table 1) almost certainly arises from displacement of the outer-sphere water molecule. (The similar increaseg when benzohydroxamic acid is added to HRP may also be related to displacement of an outer-sphere water molecule; * It seems unlikely that a water oxygen atom can get closer than 3 .6 8 k to iron in catalase fluoride.J. Oakes 2085 c a t a l a t i c action P P P 0 compound I ( b l u e / g r e e n 3 unpaired electrons peroxidatic action ( red I brow n) 0 2 compound III inactive Fig. 2. Mechanisms of catalase action. however, the observation of 1 7 0 hyperfine coupling in the e.s.r. spectra and the observation of slow exchange on the n.m.r. timescaleZ7 suggests this is accompanied by introduction of a water molecule bound to haem.) Mechanism of Catalytic Action X-Ray inve~tigationsl~? l4 have shown that the haem active site in catalase is buried within the protein structure some 20 A below the molecular surface and at a distance of only 23 A from the molecular centre. It is accessible through a channel 30 A long, formed by interlinking protein subunits, and has a maximum width of 15 A at its mouth.The channel is lined with hydrophobic amino acids and the entrance contains a high proportion of hydrophilic amino-acid residues. The fifth ligand is a tyrosine group. Histidine, asparagine and phenyl alanine amino-acid residues surround the distal side of the haem active site and their disposition is such that they are believed to interact with reactive intermediates.14 Although there is still some uncertainty about how catalase works, it is generally believed that the mechanism follows the scheme shown in fig. 2. The structure of intermediates I, I1 and 111 may differ slightly from one peroxidase to another, but the structures given are now fairly well established for at least one peroxidase and recent evidence has narrowed the range of possibilities for the others.Compound I is a powerful oxidising agent formed by double oxidation of catalase. It can react either catalatically (the liberated oxygen coming from the second molecule of H,O,) or peroxidatically to form a red/brown complex (compound 11), possibly as a result of one-electron reduction of the porphyrin ring. The peroxidatic reaction is completed by reduction of compound 11, again by organic molecules (S), to regenerate catalase, although in the presence of excess H,O, there is a competing reaction in which compound I1 is converted to a catalytically inactive FeIIO, complex, compound 111. This can degrade still further in the presence of H,O, to produce bile pigments.What is the significance of there being a water molecule with protons situated some 3-3.6 A away from the haem iron? The most straightforward explanation is that the water molecule is so strategically located to be displaced by the second oxygen atom (0,) of an incoming peroxide group, whilst allowing coordination of the first oxygen atom (0,) to the haem iron (fig. 3). Scission of the 0-0 bond will be controlled by the same forces, i.e. interaction with neighbouring protein amino-acid residues and haem iron, responsible2086 N.M.R. Study of Catalase Fig. 3. Schematic illustration of compound I formation. for dictating slow exchange on atoms situated in the 0, position and fast exchange in position 0,. These will lower the activation energy for bond breakage, as will stabilisation of intermediates by amino-acid residues.The mechanism given in fig. 3 is similar in many respects to that previously proposed for cytochrome c peroxidase.28 No attempt is given to explain differences in selectivity between catalase and peroxidases, but undoubtedly the disposition of amino-acid residues and the elaborately constructed channel sur- rounding the distal haem site are responsible for promoting the interaction of compound I, with a further molecule of peroxide and inhibiting its peroxidase reaction with larger organic substrates. The function of the fifth ligand, tyrosine, may well be to stabilise the FeIII state of iron and higher oxidation state intermediates. I am indebted to Olive Laurie for preparation of samples, Jeff Rockliffe for measurement of relaxation times and Ed Smith for helpful discussions. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 M.W. Makinen, in Techniques and Topics in Bio Inorganic Chemistry, ed. C. A. McAuliffe (McMillan, London, 1985). H. Theorell, Ark. Kemi, 1942, MA, 1. J. C. Kendrew, R. E. Dickerson, B. E. Strandberg, R. G. Hart, D. R. Davies, D. C. Phillips and V. C. Shore, Nature (London), 1960, 185, 422. M. F. Perutz, Nature (London), 1970, 228, 734. R. A. Dwek, Adv. Mol. Relaxation Processes, 1972, 4, 1. M. E. Fabry and M. Eisenstadt, J. Biol. Chem., 1974,249, 2915. R. K. Gupta and A. S. Mildvan, J. B i d . Chem., 1975, 250, 246. A. Lanir and A. Schejter, Biochem. Biophys. Res. Commun., 1975, 62, 199. R. K. Gupta, A. S. Mildvan and G. R.Schonbaum, Arch. Biochem. Biophys., 1980,202, 1. A. Lanir and A. Schejter, Biochemistry, 1976, 15, 2590. A. Lanir and A. Schejter, FEBS Lett., 1975, 55, 254. R. D. 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Soc., Dalton Trans., 1984, 1133. 23 T. L. Fabry, J. Kim, S. Koenig and W. E Shillinger in Probes of Structure and Function of Macro- molecules and Membranes, ed. B. Chance, T. Yonetani and A. S. Mildvan (Academic Press, New York, 1971), vol. 2. 24 J. Oakes, J. Chem. Soc., Faraday Trans. I , 1976, 72, 216. 25 J. Oakes, J. Chem. Soc., Faraday Trans. I , 1976, 72, 228. 26 D. E. Eglinton, P. M. A. Gadsby, G. Severs, J. Peterson and A. J. Thomson, Biochem. Biophys. Acta, 27 R. K. Gupta, A. S. Mildvan and G. R. Schoubaum in Biochemical and Clinical Aspects of Oxygen 28 T. L. Poulos and J. K. Kraut, J. Biol. Chem., 1980, 225, 8199. 1983, 724, 648. (Academic Press, New York, 1979). Paper 5/1183; Received 12th July, 1985

ISSN:0300-9599    

DOI:10.1039/F19868202079

出版商:RSC    

年代:1986

数据来源: RSC