摘要:

4369 4377 4387 4397 4407 4417 4427 4439 445 1 4457 447 1 4475 4487 4495 450 1 4509 Con tents A New Form of the High-temperature Isopiestic Technique and its Applica- tion to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury- Indium and Mercury-Tin Binary Amalgams Z-C. Wang, X-H. Zhang, Y-Z. He and Y-H. Bao The Derivation of Chemical-diffusion Coefficients of Oxygen in UO,,, over the range 180-300 "C. Spectroscopic Procedure and Preliminary Results T. R. Griffiths, H. V. St. Aubyn Hubbard, G. C. Allen and P. A. Tempest Pho tophysics at Solid Surfaces. Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on Silica-gel Surfaces T. Fujii, E. Shimizu and S. Suzuki Ordering in Monodispersed Polymer Latices induced by a Temperature Gradient K.Furusawa, N. Tobori and S. Hachisu X-Ray Diffraction Study of Molten Eutectic LiF-NaF-KF Mixture K. Igarashi, Y. Okamoto, J. Mochinaga and H. Ohno Viscosity Measurements of Some Tetra butylammonium, Copper( I), Silver( I) and Thallium( 1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C D. S. Gill and B. Singh The Ethane- 1,2-diol-Water Solvent System. The Dependence of the Dis- sociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Silver(1) Complexation with Tertiary Amines in Toluene M. Soledade Santos, E. F. G. Barbosa and M. Spiro Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film G. J. Yao, A. Kira and M. Kaneko Nature of Acid Sites in SAP05 Molecular Sieves.Part 1.-Effects of the Concentration of Incorporated Silicon C. Halik, J. A. Lercher and H. Mayer Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface T. Gu, Y. Gao and L. He Nuclear Magnetic Resonance Relaxation in Micelles. Deuterium Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography K. D. Bartle, A. A. Clifford, J. P. Kithinji and G. F. Shilstone Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne The Radical Cation of Formaldehyde in a Freon Matrix. An Electron Spin Resonance Study Phase Transition of the Water confined in Porous Glass studied by the Spin- probe Method H.Yoshioka G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi 0. Soderman, G. Carlstrom, U. Olsson and T. C. Wong C. J. Rhodes and M. C. R. Symons C. J. Rhodes and M. C. R. Symons4369 4377 4387 4397 4407 4417 4427 4439 445 1 4457 447 1 4475 4487 4495 450 1 4509 Con tents A New Form of the High-temperature Isopiestic Technique and its Applica- tion to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury- Indium and Mercury-Tin Binary Amalgams Z-C. Wang, X-H. Zhang, Y-Z. He and Y-H. Bao The Derivation of Chemical-diffusion Coefficients of Oxygen in UO,,, over the range 180-300 "C. Spectroscopic Procedure and Preliminary Results T. R. Griffiths, H. V. St. Aubyn Hubbard, G. C. Allen and P. A. Tempest Pho tophysics at Solid Surfaces.Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on Silica-gel Surfaces T. Fujii, E. Shimizu and S. Suzuki Ordering in Monodispersed Polymer Latices induced by a Temperature Gradient K. Furusawa, N. Tobori and S. Hachisu X-Ray Diffraction Study of Molten Eutectic LiF-NaF-KF Mixture K. Igarashi, Y. Okamoto, J. Mochinaga and H. Ohno Viscosity Measurements of Some Tetra butylammonium, Copper( I), Silver( I) and Thallium( 1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C D. S. Gill and B. Singh The Ethane- 1,2-diol-Water Solvent System. The Dependence of the Dis- sociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Silver(1) Complexation with Tertiary Amines in Toluene M.Soledade Santos, E. F. G. Barbosa and M. Spiro Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film G. J. Yao, A. Kira and M. Kaneko Nature of Acid Sites in SAP05 Molecular Sieves. Part 1.-Effects of the Concentration of Incorporated Silicon C. Halik, J. A. Lercher and H. Mayer Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface T. Gu, Y. Gao and L. He Nuclear Magnetic Resonance Relaxation in Micelles. Deuterium Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography K. D. Bartle, A. A. Clifford, J. P. Kithinji and G. F. Shilstone Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne The Radical Cation of Formaldehyde in a Freon Matrix. An Electron Spin Resonance Study Phase Transition of the Water confined in Porous Glass studied by the Spin- probe Method H. Yoshioka G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi 0. Soderman, G. Carlstrom, U. Olsson and T. C. Wong C. J. Rhodes and M. C. R. Symons C. J. Rhodes and M. C. R. Symons

ISSN:0300-9599    

DOI:10.1039/F198884FX025

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

NOMENCLATURE 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 Cemistry in all its publications. Their basis is the 'Systeme 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 Physicochemical Quantities and Units (Pergamon, Oxford, 1979). 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 published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987).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. (xiv)NOMENCLATURE 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 Cemistry in all its publications.Their basis is the 'Systeme 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 Physicochemical Quantities and Units (Pergamon, Oxford, 1979). 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 published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987). 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. (xiv)

ISSN:0300-9599    

DOI:10.1039/F198884BX027

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

ISSN 0300-9599 JCFTAR 84(7) 221 5-251 7 221 5 2227 224 1 2247 2259 2267 2279 2289 2297 2305 231 1 2317 2327 2335 2347 2357 2369 2377 2387 2397 2409 13 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions 1 Physical Chemistry in Condensed Phases CO NT E N TS Stability Criteria for Charged Interfaces and their Role in Double-layer Theory D. G. Hall A Thermodynamic Analysis of Common Intersection Points in Potentiometric Titration Studies of Solid Surfaces Localized Excess Electrons in Solubilized Water Clusters in Aerosol OT-n- Heptane Solutions Application of the Volterra Integral Equation to the Mathematical Modelling of Adsorption Kinetics under Constant-volume/Variable-concentration Con- ditions A New Conducting Polymer-coated Glucose Sensor P. C. Pandey Effect of Hydrogen in the Chemisorption of n-Hexane over Platinum Black A.Sarkany Volumetric Properties of Aqueous Solutions of Polyols between 0.5 and 25 "C S. Wurzburger, R. Sartorio, G. Guarino and M. Nisi Phase Transfer of the 12-Tungstosilicate Anion across the Water/Nitrobenzene Interface Volume and Compressibility Changes in Aqueous Mixed-salt Solutions at 25 "C K. Patil and G. Mehta Electron Spin Resonance of a y-Irradiated Single Crystal of Carbamylcholine Chloride Surface Reactions of Goethite with Phosphate R. G. Jonasson, R. R. Martin, M. E. Giuliacci and K. Tazaki Conductivity of Polypyrrole Films doped with Aromatic Sulphonate Deriva- tives S. Kuwabata, K. Okamoto and H. Yoneyama Infrared Spectra of CO adsorbed on Prismatic Faces of a-Fe,O, A. Zecchina, D. Scarano and A.Reller A Correlated X-Ray Photoelectron and Electron Spin Resonance Spectroscopic Study of Rhodium-exchanged X and Y Zeolites D. Goldfarb, S. Contarini and L. Kevan Deuterium Nuclear Magnetic Resonance Studies of the Molecular Dynamics of Benzene in Zeolites Photoelectrochemical Electron Spin Resonance. Part 2.-The Reduction of Crystal Violet in Acetonitrile R. G. Compton, B. A. Coles, G. M. Stearn and A. M. Waller The Selection of Experimental Conditions in Temperature-programmed Reduction Experiments P. Malet and A. Caballero Adsorption of Oxygen and Reactivity with HC1 on a Barium-doped Lead Surface Effect of Solvent on the Reactions of Coordination Complexes. Part 5.--Kinetics of Solvolysis of cis-(Bromo)-[(2-aminothiazole)-bis(ethylenedi- amine)cobalt(m) in Methanol-Water, Propan-2-01-Water and Ethylene Gly- col-Water A.C. Dash and J. Pradhan Ammoxidation of 2-Methylpyrazine. Characterisation of Catalyst L. Forni, C. Oliva and C. Rebuscini Calorimetric and Spectrophotometric Studies of Chloro Complexes of D. G. Hall M. H. Abdel-Kader and R. Krebs M. KoEiiik, G. Tschirch, P. Struve and M. Biilow E. Wang and Y. Liu F. Koksal and F. Celik B. Zibrowius, J. Car0 and H. Pfeifer M. Ayyoob and M. S. Hegde FAR IContents Manganese(I1) and Cobalt(I1) Ions in N,N-Dimethylformamide S. Ishiguro, K. Ozutsumi and H. Ohtaki Interaction of Water with the Surface of Magnesium Oxide Y. Kuroda, E. Yasugi, H. Aoi, K. Miura and T. Morimoto Theory of the Taylor Dispersion Technique for Three-component-system Diffusion Measurements W.E. Price Linear Correlation between Entropies of Complexation of Cryptand 222 with Metal Ions in Non-aqueous Solvents and Entropies of Solvation of these Ions in these Solvents The Structures and Synergistic Catalyses of FeRu/Al,O, Catalysts derived from Fe,Ri,-,(CO),, (x = 0, 1, 2, 3). Part 1.-The Structures of Al,O,- supported Fe,Ru,-,(CO),, Clusters Structures and Synergistic Catalyses of FeRu/ Al,O, Catalysts derived from Fe,Ri,-,(CO),, (x = 0, 1, 2, 3). Part 2.- Structures and Catalyses of FeRu Catalysts reduced with H, Fourier-transform Infrared Investigation of Intermediates in the Oxidation of Toluene on V,O,/TiO, H. Miyata, T.Mukai, T. Ono, T. Ohno and F. Hatayama Cr3+ Electron Spin Resonance Linewidth in ZnO-ZnCr,O,-(Pd) Solid Mixtures L. Forni and C. Oliva Influence of some Factors affecting the Stationary Value of the Electric Birefringence of Aqueous Solutions of Poly(styrene sulphonates) in the Presence of 0.01 mol dm-, NaCl Solvation of Thiols. An Infrared and Nuclear Magnetic Resonance Study of Ethanethiol Reactions of 1,1,3,3-Tetramethylcyclobutane on Evaporated Metal Films J. K. A. Clarke, B. F. Hegarty and J. J. Rooney A. F. Danil de Namor K. Asakura and Y. Iwasawa K. Asakura, Y. Iwasawa and M. Yamada S. S. Wijmenga and M. Mandel M. C. R. Symons and G. P. Archer 242 1 243 1 244 1 2445 2457 2465 2477 2483 2499 251 1

ISSN:0300-9599    

DOI:10.1039/F198884FP091

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY 79 I 813 84 1 86 I 885 893 899 913 93 1 949 Faraday Transactions II, lssue7,1988 Molecular and Chemical Physics For the benefit of readers of Faraday Transactions I, the contents list of Faraday Transactions II, Issue 7, is reproduced below. Dynamics of Intercalated Molecules. Part 1 .-Sorption Potentials and Structure of ‘C,,Cs’ (CH,) Dynamics of Intercalated Molecules. Part 2.-A Molecular-dynamics Simulation of Methane in ‘C,,Cs’ (CH,) Dynamics of Intercalated Molecules. Part 3.-Structure of C,,Cs(CD,) F. R. Trouw and J. W. White Dynamics of Intercalated Molecules. Part 4.-Neutron Inelastic Scattering from Methane in C,,Cs(CH,) F. R. Trouw and J. W. White Molecular Motions in 2-Iodo-2-methylpropane, (CH,)CI. Proton Nuclear Magnetic Resonance Measurements at Variable Temperatures T.Hasebe, J. H. Strange and J. M. Chezeau Theoretical Studies of Vibrational Frequency Shifts upon Hydrogen Bonding. The Carbonyl Stretching Mode in Complexes of Formaldehyde X. Q. Lewell, I. H. Hillier, M. J. Field, J. J. Morris and P. J. Taylor The Reactions of NO, with OH and H R.B.Boodaghians, C.E. Canosa- Mas, P. J. Carpenter and R. P. Wayne Kinetic Study of the Reactions CH,O,+CH,O, and CH,O+H,O using Molecular Modulation Spectroscopy M. E. Jenkin, R. A. Cox, G. D. Hayman and L. J. Whyte Formation of O,(alA,) and Vibrationally Excited OH in the Reaction between 0 Atoms and HO Species S. T. Lunt, G. Marston and R. P. Wayne A Classical Trajectory Study of the x-State Photodissociation of the Water Molecule F. R. Trouw and J.W. White F. R. Trouw and J. W. White J. Guo and J. N. Murrell The following papers were accepted for publication in Faraday Transactions I during April 1988. 7/1871 CO Absorption on MgO and CaO: Spectroscopic Investigations of Stages prior to Cyclic Anion Cluster Formation 7/2090 Diffusion of Hi and OHo in Porous Solids Lewnard, J. J., Petersen, E. E. and Radke, C. J. 7/2 120 Diffusion Phenomena and Metal Complex Formation Equilibria. Part 2.-Cd” Benzimidazole and Ni” - Oxalate Systems in Aqueous Solution Crow, D. R. A Chronocoulornetric Investigation of Benzophenone Electroreduction on Mercury from Acidic Aqueous Media Carrone, E. and Zecchina, A. 7/2 130 Ruiz, J. J. and Foresti, M. L.7/2 184 Electrochromism of a Conducting Polypyrrole - Phosphotungstate 7/2228 Surface Solubilization Zhu, B-Y., Zhao, X.and Gu, T. 7/2230 Spectrophotometric Study of the Pseudo-tetrahedral Bromo-complexes of Cobalt(r1) in Hexamethylphosphoramide Solution Pilarczyk, M., Grzybkouski, W. and Klinszporn, L. ESR Study of Charge Carrier Stabilization in ZnO and Soria, J. Hazoume, R. P. and Hounkomou, N. M. Composite Electrode Shimidzu, T., Ohtani, A., Aiba, M. and Honda, K. 7/2231 Energetic Effects of Solvation Pawel, G. 7/2246 Gonzalez-Elipe, A. R. 7/2280 A Theoretical Study of the Kinetics of Deoxyhaemoglobin Aggregation 8/00053K/ FIP 8/00054I/ FIP 8/00066B/ FIP 8/00123E/FIP 8/00212F/FIP 8/002 1 61/ FIP 8/00222C/FIP 8/00278I/FIP 8/00333E/FIP 8 /00503 F/ FIP Conductance Stopped-flow Study of the Association Reaction of Colloidal Spheres with Poly(vinylpyrro1idone) Conductance Stopped-flow Study of the Association Reactions of Anionic Colloidal Spheres with Cationic Spheres and with Bovine Serum Albumin Molecules Okubo, Tsuneo, Kitano, Hiromi and Iwai, Satoru Ionic Solvation. Part 4.-Copper(r) Solvation, Disproportionation and Halide Complex Formation in Propylene Carbonate Lewandowski, Andrzej The Kinetics of Cathodic Generation of R,N(HgS) Ryan, Christopher M., Svetlicic, Vesna and Kariv-Miller, Essie Local Reaction Environments and their Properties for Ethene Deuterogenation on the Surfaces of SMSI Catalysts Yoshitaka, Hideaki Asakura, Kiyotaka and Iwasawa, Yashiro Photoelectrochemical E.S.R.Part 3.-The Reduction of Fluorescein - A ‘PHOTO-DISP2’ Reaction Compton, Richard G., Coles, Barry A.and Pilkington, Matthew B. G. A New Form of the High- temperature Vapour-pressure Method and its Application to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury-Indium and Mercury-Tin Binary Amalgams Wang, Zhi-Chang, Zhang, Xin-Hua, He Yu-Zhi and Hao, Yu-Hong Infrared Study of Ammonia/Carbon Monoxide Reactions on Silica- supported Iron Catalysts Johnston, Colin, Jorgensen, Norman and Rochester, Colin H. Calorimetric Study of Nonionic Surfactants, Enthalpies and Heat Capacity Changes for Micelle Formation in Water of C8E4 and Triton X-100 and Micelle Size of C8E4 Andersson, Boel and Olofsson, Gerd Reduction-Agglomeration Model for the Metal Dispersion in Platinum-exchanged NaX Zeolite Exner, Dieter, Jaeger, Nils, Kleine, Andreas and Schulz-Ekloff, Gunter Okubo, Tsuneo (ii)8/00553B/FIP 8/00554K/ FIP 8/00696B/FIP 8/007 19E/FIP 8/0072 1 G/ FIP 8/0977 1 C/FIP 8/008 17E/FIP 8 /00925 B / FIP 8/01039K/FIP 8/01 105B/FIP 8/01 169I/FIP 17 1 K/FIP 442F/FIP 8/0 1474D/FIP 8/01497C/FIP 8/01521J/FIP 8/01522H/FIP Radiation Damage in Organic Phosphates, Crystal Structure of 3-0- Diphenoxyphosphoryl- 1,2-0-isophopylidene 5-0-trityl a-D-ribo- furanose and ESR Study of the X-Irradiated Single Crystal Berclaz, Theo, Bernardinelli, Gerald, Celalyan-Berthier, Alice and Geoffroy, Michel Infrared Study of the Adsorption of C12E5 on Silica immersed in Carbon Tetrachloride Quadrupole Nutation NMR in Solids Janssen, R.and Vlemans, W. S. The Excess Surface Tensions of Simple Binary Mixtures Rowlinson, John S. Acidic Properties of Molybdenum Oxide Highly Dispersed on Titania Miyata, Hisashi, Mukai, Iadashi, Uno, Takehiko and Kubokawa, Yutaka Kinetics of Beta-hydroxyl Elimination from [(H,O),Cu"- CH,C(CH,),)H] + in Aqueous Solutions.A Pulse Radiolysis Study Cohen, Haim and Meyerstein, Dan Kerr Constants and Dielectric Polarisations of Amides and Thio- amides in Water and Dioxan Solutions Aroney, Manuel J., Dowling, Karen M., Patsalides, Emilios, Pierens, Raymond K. and Filipczuk, Stephen W. Photocatalytic Oxidation of Iodide Ions by Titanium Dioxide Harvey, Paul R. and Rudham, Robert NMR Relaxation in Micelles. 2H Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Soderman, O., Carlstrom, G., Olsson, Ulf and Wong, T. C. The NMR of "'Xe trapped in Solids Ripmeester, J.A., Ratcliffe, C. I. and Tse, J. S. E.S.R. Studies of the Reactions of Aluminium Atoms with some Alkenes in a Rotating Cryostat Howard, James A., Joly, Helen A., Mile, Brynmur and Histed, Michael Generation and Reactions of the Chlorine Atom in Aqueous Solution Gilbert, Bruce C., Stell, Jonathan K., Peet, Wendy J. and Radford, Karen J. Room-temperature Powder ENDOR Spectra of /?-Protons in some Organic n-Radicals Atherton, Neil M. and Oliver, Claire F. Enhancement of the Effect of Small Anisotropies in Magic Angle Spinning NMR Raleigh, Daniel P., Kolbert, Andrew C., Das, Terrence G., Levitt, Malcolm H. and Griffin, Robert G. Phase Transition of the Water confined in Porous Glass studied by the Spin Probe Method Peptide Backbone Conformation by Solid-state NMR Spectroscopy Opella, S. J., Stewart, P.L. and Tycko, R. Magnetic Resonance Reinvestigation of Mn"-S'ATP Equilibria in Solution Rossi, Claudio, Laschi, Franco, Pogliani, Lionello, Pogni, Rebecca, Tiezzi, Enzo and Basosi, Riccardo Keith, Shona and Rochester, Colin H. Yoshioka, H. (iii)8/01 523F/FIP 8/01 524D/FIP 8/01 525B/FIP 8/0 1526KIFIP 8/01 527I/FIP 8/01 529E/FIP 8/01 530I/FIP 8/01 548A/FIP 8/01551A/FIP 8/01 556B/FIP 8/01 558I/FIP 8/01 562G/FIP 8/01 563E/FIP E.S.R. and ENDOR Reinvestigation of Iminoxyl Radicals from 1 -Halogenofluoreneone Oximes Kirste, Burkhard, Grothe, Klaus and Kurreck, Harry An E.S.R. Study of Azolakane and Imine Radical Cations Rhodes, Christopher J. E.S.R. Studies of Cycles and Bicycles MacCorquodale, Finlay and Walton, John C.An E.S.R. and ENDOR Investigation of the Ion Pairs of 4,4’-Dicyano- benzophenone Ketyl with Alkali Metal Cations Barzaghi, Mario, Gamba, Aldo, Oliva, Cesare, Branca, Mario and Saba, Antonio ENDOR Studies of Flavins and Flavoproteins Kurreck, Harry, Bretz, Norbert H., Helle, Norbert, Henzel, Norbert and Weilbacher, Ellen Crystalline Heterocyclic Radical Cation Salts as Stable Intermediates of New Redox-mediating Systems Schulz, Andreas, Kaim, Wolfgang and Hausen, Hans-Dieter Substituent Effects in Anthrasemiquinones Pedersen, Jens A. 2D ENDOR Imaging based on Difference in Oxygen Concentration Kotake, Yashige, Oehler, Uwe M. and Janzen, Edward G. Kinetic E.S.R. Spectroscopy in Complex Reaction Systems. Solvent Effects on the Self-termination of 2,6-Di-ti-butylphenoxyl Radicals Ruegge, D.and Fischer, H. Electron Spin Resonance Identification of Irradiated Strawberries Raffi, Jacques J., Agnel, Jean-Pierre I., Ruscarlet, Lousis A. and Martin, Christine C. E.S.R. and ENDOR Spectra of Radicals formed by the Addition of Superoxide Ions to Dimethylformamide Boon, Philip J., Olm, Myra T. and Symons, Martyn C. R. Is Gram-negative Shock a Free-radical-mediated Condition? Jackson, S. K., Stark, J. M., Rowlands, C. C. and Evans, J. C. Radicals Formed by U.V. Irradiation of Substituted 4- Chlorophenols Evans, J. C., Rowlands, C. C. and Turkson, L. A.Cumulative Author Index 1988 Abdel-Kader, M. H., 2241 Abe, H., 511 Abraham, M. H., 175, 865, 1985 Abraham, R. J., 1911 Adachi, H., 1091 Aicart, E., 1603 Allen, G.C., 165, 355 Amorelli, A,, 1723 Anazawa, I., 275 Anderson, S. L. T., 1897 Anpo, M., 751 Antonini, A. C. R., 1889 Aoi, H., 2421 Aoyama, T., 2209 Aracil, J., 539 Archer, G. P., 2499 Arora, K. S., 1729 Asakura, K., 1329, 2445, 2457 Aveyard, R., 675 Ayyoob, M., 2377 Baba, K., 459 Bagchi, S., 1501 Baglioni, P., 467 Baldini, G., 979 Barna, T., 229 Barone, G., 1919 Baulch, D. L., 1575 Bazsa, G., 215, 229 Benmouna, M., 1563 Benoit, H., 1563 Berei, K., 367 Berroa de Ponce, H., 255, 1671 Bertoldi, M., 1405 Beyer, H. K., 1447 Binks, B. P., 675 Blandamer, A. H., 1889 Blandamer, M. J., 1243, 1889 Blesa, M. A., 9 Blinov, N. N., 1075 Bloor, D. M., 2087 Bonnefoy, J., 941 Borbely, G., 1447 Borckmans, P., 1013 Borgarello, E., 261 Bordwko, M., 1961 Bourdillon, C., 941 Brandreth, B.J., 1741 Breen, J., 293 Briggs, B., 1243 drown, M. E., 57, 1349 Brydson, R., 617, 631 Biilow, M., 2247 Burgess, J., 1243, 1889 Burget, U., 885 Busca, G., 237, 1405, 1423 Buxton, G. V., 1101, 1113 Caballero, A., 2369 Caceres, M., 539 Caceres-Alonso, M., 1603 Carbone, A. I., 207 Caro, J., 2347 Carr, N. J., 1357 Castronuovo, G., 1919 Cavani, F., 237 Cavasino, F. P.. 207 Celik, F., 2305 Centi, G., 237 Chagas, A. P., 1065 Chandra, H., 609 Che, M., 751 Cheek, P. J., 1927 Cheng, V. K. W., 899 Chien, J. C. W., 1123 Chinchen, G. C., 2135 Chirico, G., 979 Chudek, J. A., 1145, 1737 Clarke, J. K. A., 251 1 Clarke, R. J., 365 Clint, J. H., 675 Coates, J. H., 365 Coles, B. A., 2357 Coller, B. A. W., 899 Coluccia, S., 751 Compton, R. G., 473, 483, 2013, 2057, 2155, 2357 Contarini, S., 2335 Cook, A., 1691 Costas, M., 1603 Covington, A.K., 1393 Crowther, N. J., 1211 Danil de Namor, A. F., 255, 1671, 2441 Das, S., 1057 Dash, A. C., 75, 2387 Dash, N., 75 Davydov, A., 37 Dawber, J. C., 41 Dawber, J. G., 41, 713 Day, M. J., 2013 de Bleijser, J., 293 Del Vecchio, P., 1919 Diaz Peiia, M., 539 Dickinson, E., 871 Disdier, J., 261 Domen, K., 511 Dougal, J. C . , 657 Duarte, M. Y., 97, 367 Duce, P. P., 865 Duckworth, R. M., 1223 Dyster, S., 11 13 Eagland, D., 1211 Eaton, G., 2181 Egawa, C., 321 Einfeldt, J., 931 Ekechukwu, A. D., 1871 Eley, D. D., 2069 Elia, V., 1919 Elliot, A. J., 1101 Engel, W., 617, 631 Eszterle, M., 575 Evans, J. C., 1723 Everett, D. H., 1455 Eyears, J. M., 1437 Fernandez, A., 1543 Fernandez-Pineda, C., 647 Flanagan, T.B., 459 Fletcher, P. D. I., 1131 Foresti, E., 237 Foresti, M. L., 97 Forni, L., 2397, 2477 Forster, H., 491 Foster, R., 1145, 1737 Fraenkel, D., 1817, 1835 Franklin, K. R., 687 Fubini, B., 1405 Fiiredi-Milhofer, H., 1301 Gal, D., 1075 Gabrail, S., 41 Galwey, A. K., 57, 729, 1349, Gans, P., 657 Gardner, P. J., 1879 Geblewicz, G., 561 Geertsen, S., 1101 Georges, V., 1531 Giamello, E., 1405 Gill, D. S., 1729 Gill, J. B., 657 Gilot, B., 801 Girault, H. H., 2147 Giuliacci, M. E., 231 1 Goldfarb, D., 2335 Gopalakrishnan, R., 365 Grampp, G., 366 Gratzel, M., 197, 1703 Gray, A. C., 1509 Gray, P., 993 Green, P., 2109 Green, S. I. E., 41 Green, W. A., 2109 Grepstad, J. K., 1863 Griffiths, J. F., 1575 Grigo, M., 931 Grimson, M. J., 1563 Gritzner, G., 1047 Grzybkowski, W., 1551 Guardado, P., 1243 Guarini, G .G. T., 331 1357Guarino, G., 2279 Guglielminotti, E., 2195 Guidelli, R., 97, 367 Gupta, D. Das, 1057 Hadjiivanov, K., 37 Hakin, A. W., 1889 Hall, D. G., 773, 2087, 2215, Hall, N. D., 1889 Halle, B., 1033 Hamada, K., 1267 Hanawa, T., 1587 Handreck, G. P., 1847 Hanson, G. R., 1475 Harrer, W., 366 Harriman, A., 2109 Hasebe, T., 187 Hashimoto, K., 87 Hatayama, F., 2465 Hayashi, K., 2209 Hazra, D. K., 1057 Heatley. F., 343 Hegarty, B. F., 251 1 Hegde, M. S., 2377 Herley, P. J., 729 Herrmann, J-M., 261 Hey, M. J., 2069 Heyward, M. P., 815 Hidalgo, M. del V., 9 Hill, A,, 255 Hubbard, C. D., 1243 Hudson, B. D., 1911 Huis, D., 293 Hunter, R., 131 1 Hutchings, G. J., 1311 Ige, J., 1 Ikeda, S., 151 Imai, H., 923 Jmamura, H., 765 Imanaka, T., 851, 2173 Inoue, A., 1195 Irinyi, G., 1075 Ishiguro, S., 2409 Ishikawa, T., 1941 Isobe, T., 1199 Tto, D., 1375 Ittah, B., 1835 Iwamoto, E., 1679 Iwasawa, Y., 321, 1329, 2445, Iyer, R.M., 2047 Jackson, S. D., 1741 Jaeger, N. I., 1751 Jaenicke, W., 366 Jeminet, G., 951 Jens, K-J., 1863 Johnson, G. R. A., 501 Johnson, I., 551 Johnston, C., 309, 2001 Jonasson, R. G., 2311 Jonson, B., 1897 Jorge, R. A., 1065 Jorgensen, N., 309, 2001 Joiwiak, M., 2077 Juillard, J., 951, 959, 969, 1713 2227 2457 AUTHOR INDEX Kaizu, Y., 1517 Kakei, K., 1795 Kane, H., 851 Kaneko, K., 1795 Kanno, T., 281, 2099 Kasahara, S., 765 Kato, S., 151 Katz, N. E., 9 Kawasaki, Y., 1083 Keeble, D. J., 609 Kemp, T. J., 2027 Kermode, M. W., 1911 Kevan, L., 467, 2335 Kimura, T., 2099 Kinnaird, S., 2135 Kirby, C., 355 Kiricsi, I., 491 Kiss, I., 367 Kiwi, J., 1703 Klinszporn, L., 1551 Klissurski, D., 37 Kobayashi, A., 1795 Kobayashi, H., 1517 Kobayashi, M., 281, 2099 Koeiiik, M., 2247 Koda, S., 1267 Koksal, F., 2305 Kondo, J., 511 Kondo, S., 1941 Kondo, Y., 11 1 Konishi, Y., 281 Kordulis, C., 1593 Kornhauser, I., 785, 801 Kosugi, N., 1795 Kowalak, S., 2035 Kraehenbuehl, F., 1973 Krausz, E., 827 Krebs, P., 2241 Kristyan, S., 917 Kubelkova, L., 1447 Kubokawa.Y., 751, 2129 Kumamaru, T., 1679 Kurimura, Y., 841, 1025 Kuroda, H., 1329, 1795 Kuroda, Y., 2421 Kusabayashi, S., 11 1 Kuwabata, S., 1587, 2317 Lahy, N., 1475 Laing, M. E., 2013 tajtar, L., 19 Lambi, J. N., 1 Larsson, R., 1897 Laubry, P., 969 Laval, J-M., 941 Lawrence, K. G., 175 Lea, J. S., I181 Leaist, D.G., 581 Lefever, R., 1013 Lefferts, L., 1491 Lengyel, I., 229 Levy, A., 1817 Levy, M., 1835 Lewis, T. J., 1531 Leyendekkers, J. V., 397, 1653 Leyte, J. C., 293 (vi) Lilley, T. H., 1927 Lincoln, S. F., 365 Lindner, Th., 631 Lips, A., 1223 Llewellyn, J. P., 153 1 Logan, S. R., 1259 Lycourghiotis, A., I593 MacKay, R. L., 1145, 1737 Maezawa, A., 851 Malanga, C., 97 Malet, P., 2369 Mandel, M., 2483 Marcus, Y., 175, 1465 Markovid, M., 1301 Maroto, A. J. G., 9 Martin, R. R., 231 1 Martins, L. J. A., 2027 Maruya, K., 511 Mason, D., 473, 483, 2057 Matsumoto, T., 1375 Matsumura, Y., 87 Matsuoka, K., 1277 Matteoli, E., 1985 Mayagoitia, V., 785, 801 McAleer, J. F., 441 McMurray, N., 379 Mead, J., 675 Medda, K., 1501 Mehta, G., 2297 Mensch, C. T. J., 65 Merkin, J.H., 993 Meunier, F., 1973 Mills, A., 379, 1691 Mines, J. R., 1911 Mintchev, L., 1423 Mirti, P., 29 Mitsushima, I., 851 Miura, K., 2421 Miyagawa, S., 2129 Miyakawa, K., 1517 Miyanaga, T., 2173 Miyata, H., 2129, 2465 Mohamed, M. A-A., 57, 729, Moiroux, J., 941 Moller, K., 1751 Morimoto, T., 2421 Morris, J. J., 865 Morterra, C., 1617 Morton, J. R., 413 Moseley, P. T., 441 Mousset, G., 969 Muhler, M., 631 Mukai, T., 2465 Murray, B. S., 871 Nagao, M., 1277 Nakagawa, Y., 2129 Nakamura, T., 1287 Nakamura, Y., 111 Nakao, N., 665 Nakayama, N., 665 Nandan, D., 2047 Narayanan, S., 521 Nazhat, N. B., 501 Neta, P., 2109 1349Newman, K. E., 1387, 1393 Nicolis, G., 1013 Nishihara, C., 433 Nishikawa, S., 665 Nishio, E., 1639 Nisi, M., 2279 Nomura, H., 151, 1267 Norris, J.0. W., 441 Northing, R. J., 2013 Noszticzius, Z., 575 Nucci, L., 97 Ohno, T., 2465 Ohshima, K., 1639 Ohtaki, H., 2409 Ohtani, S., 187 Okabayashi, H., 1639 Okamoto, K., 2317 Okamoto, Y., 851 Okubo, T., 703, 1163, 1171, Oliva, C., 2397, 2477 Oliver, S. W., 1475 Olofsson, G., 551 Ommen, J. G. van, 1491 Onishi, T., 51 1 Ono, T., 2465 Ono, Y., 1091 Oosawa, Y., 197 Ozeki, S., 1795 Ozutsumi, K., 2409 Page, F. M., 1145 Painter, D. M., 773, 2087 Pal, M., 1501 Pan, C.-f., 1341 Pandey, J. D., 1853 Pandey, P. C., 2259 Pang, P., 1879 Pappin, A. J., 1575 Parrott, D., 1131 Passelaigue, E., I7 13 Patil, K., 2297 Patterson, D., 1603 Pelizzetti, E., 261 Pena-Nuiiez, A. S., 2181 Penar, J., 739 Penman, J. I., 2013 Pezzatini, G., 367 Pfeifer, H., 2347 Piccini, S., 331 Pichat, P., 261 Pickl, W., 1311 Piekarski, H., 529, 591 Pilarczyk, M., 1551 Pilbrow, J.R., 1475 Pilkington, M. B. G., 2155 Plath, P. J., 1751 Pointud, Y., 959, 1713 Pota, G., 215 Pradhan, J., 2387 Preston, K. F., 413 Price, W. E., 2431 Prior, D. V., 865 Pushpa, K. K., 2047 Quist, P-O., 1033 Radulovic, S., 1243 I949 AUTHOR INDEX Rai, R. D., 1853 Rajam, S., 1349 Rajaram, R. R., 391 Rao, B. G., 1773, 1779 Rao, K. J., 1773, 1779 Rao, K. M., 2195 Rebenstorf, B., 1897 Rebuscini, C., 2397 Reller, A., 2327 Renuncio, J. A. R., 539 Rhodes, C. J., 1187 Richoux, M-C., 2109 Riis, T., 1863 Riva, A., 1423 Rochester, C. H., 309, 2001 Rojas, F., 785, 801, 1455 Rooney, J. J., 251 1 Ross, J. R. H., 1491 Rowlands, C. C., 1723 Rubio, R. G., 539 Saadalla-Nazhat, R. A., 501 Saito, M., 1025 Saito, Y., 275 Sakaiya, H., 1941 Sakamoto, Y., 459 Sakata, Y., 511 Salvagno, S., 1531 Sarkany, A., 2267 Sartorio, R., 2279 Sato, T., 275 Sauer, H., 617 Sawabe, K., 321 Sayari, A., 413 Sbriziolo, C., 207 Scarano, D., 2327 Schelly, Z .A., 575 Schiffrin, D. J., 561 Schiller, R. L., 365 Schlenoff, J. B., 1123 Schlogl, R., 631 Schmelzer, N., 931 Schulz, R. A., 865 Schwarz, W., 1703 Scott, S. K., 993 Seidl, V., 1447 Sellers, R. M., 355 Senna, M., 1199 Senoda, Y., 1091 Sermon, P. A., 391 Serpone, N., 261 Shindo, H., 433 Shukla, A. K., 1853 Shukla, R. K., 1853 Sidahmed, 1. M., 1153 Simmons, R. F., 1871 Sinclair, G. R.. 1475 Singh, B., 1729 Singh, P. P., 1807 s’Jacob, K. J., 1509 Smith, E. R., 899 Smith, T. D., 1475, 1847 Sokolowski, S., 19, 739 Somsen, G., 529 Soriyan, 0. O., 1 Speight, J.M., 2069 (vii) Spoto, G., 2195 Stainsby, G., 871 Stearn, G. M., 2155, 2357 Stevens, J. C. H., 165 Stirling, C. J. M., 1531 Stocker, M., 1863 Stoeckli, F., 1973 Stone, W. E. E., 117 Stramel, R. D., 1287 Struve, P., 2247 Subba Rao, M., 1703 Sun, L-M., 1973 Suzuki, T., 1795 Sykes, A. F., 1575 Symons, M. C. R., 609, 1 181, Szamosi, J., 917 Taga, K., 1639 iagawa, T., 923 Takada, T., 765 Takagi, Y., 1025 Takato, K., 841 Takisawa, N., 2087 Tamaki, J., 2173 Tanaka, F., 1083 Tanaka, K., 601 Tanaka, K-i., 601 Taniewska-Osinska, S., 2077 Tardajos, G., 1603 Taylor, D. M., 1531 Taylor, P. J., 865 Tazaki, K., 2311 Tewari, J., 1729 Thampi, K. R., 1703 Theocharis, C. R., 1509 Thomas, J. K., 1287 Thomas, J. M., 617, 631 Tissier, C., 951, 969 Tofield, B. C., 441 Torres-Sanchez, R-M., 117 Townsend, R.P., 687 Tra, H. V., 1603 Trifiro, F., 237, 1405, 1423 Tschirch, G., 2247 Tsuchiya, S., 765 Tsukamoto, K., 1639 Twiselton, D. R., 1145 Uematsu, R., I11 Uma, K., 521 Unwin, P. R., 473, 483, 2057 Vaccari, A., 1405, 1423 van Rensburg, L. J., 131 1 van Veen, J. A. R., 65 van Wingerden, R., 65 Varani, G., 979 Vasaros, L., 367 Vazquez-Gonzalez, M. I., 647 Vidoczy, T., 1075 Viguria, E. C., 255 Vink, H., 133 Viswanathan, B., 365 Vogel, V., 1531 Vordonis, L., 1593 Walker, R. A. C., 255 Waller, A. M., 2013, 2357 1187, 2181, 2499AUTHOR INDEX Wang, E., 2289 Ward, J., 713 Webb, G., 2135 Wells, C . F., 815, 1153 Welsh, M. R., 1259 Wijmenga, S. S., 2483 Williams, B. G., 617, 631 Williams, D. E., 441 Williams, R. A., 713 Winstanley, D., 1741 Wong, J., 1773, 1779 Wood, N.D., 11 13 Wormald, C . J., 1437 Wurzburger, S . , 2279 Wyn-Jones, E., 773, 2087 Yamada, M., 2457 Yamada, Y., 751 Yamamoto, Y., 2209 Yamane, T., 2173 Yamasaki, S., 1679 Yamashita, S., 1083 Yao, S . , 1375 Yasugi, E., 2421 Yoneyama, H., 1587, 2317 Yoshida, S., 87 Yuqing, L., 2289 Zecchina, A., 751, 2195, 2327 Zeitler, E., 617, 631 Zelano, V., 29 Zibrowius, B., 2347 Zielinski, R., 151 Zundei, G., 885 (viii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY WITH THE ASSOCIAZIONE ITALIANA DI CHIMICA FISICA, DIVISION DE CHlMlE PHYSIQUE OF THE SOCIETE FRANQAISE DE CHlMlE AND DEUTSCHE BUNSEN GESELLSCHEFT FUR PHYSIKALISCHE CHEMIE JOINT MEETING Structure and Reactivity of Surfaces Centro Congressi, Trieste, Italy, 13-16 September 1988 Organising Committee: M.Che V. Ponec F. S. Stone G. Ertl R. Rosei A. Zecchina The conference will cover surface reactivity and characterization by physical methods: (i) (ii) (iii) The meeting aims to stimulate the comparison between the surface properties of dispersed and supported solids and the properties of single crystals, as well as the comparison and the joint use of chemical and physical methods. Further information may be obtained from: Professor C. Morterra, lnstituto di Chimica Flsica, Corso Massimo D’Azeglio 48,10125 Torino, ltafy Metals (both in single crystal and dispersed form) Insulators and semiconductors (oxides, sulphides, halides, both in single crystal and dispersed forms) Mixed systems (with special emphasis on metal-support interaction) THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No.86 Spectroscopy at Low Tern peratu res University of Exeter, 13-1 5 September 1988 Organising Committee: Professor A. C. Legon (Chairman) Dr P. B. Davies Dr B. J. Howard Dr P. R. R. Langridge-Smith Dr R. N. Perutz Dr M. Poliakoff The Discussion will focus on recent developments in spectroscopy of transient species (ions, radi- cals, clusters and complexes) in matrices or free jet expansions. The aim of the meeting is to bring together scientists interested in similar problems but viewed from the perspective of different envi- ronments. The Introductory Lecture will be given by G. C. Pimentel and speakers include: L. Andrews, K. H. Bowen, B. J. Howard, L. B. Knight Jr, E. Knozinger, D. H.Levy, J. P. Maier, J. Michl, M. Moskovits, A. J. Stace, M. Takami, M. Poliakoff, A. J. Barnes, J. M. Hollas, M. C. R. Symons and P. Suppan. The final programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBNTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMIST.RY SYMPOSIUM Orientation and Polarization Effects in Reactive Collisions To be held at the Physikzentrum, Bad Honnef, West Germany, 12-14 December 1988 Organising Committee: Dr S. Stolte Professor R.A. Levine Dr K. Burnett Professor R.N. Dixon Professor J.P. Simons Dr H. Loesch The Symposium will focus on the study of vector properties in reaction dynamics and photodissoci- ation rather than the more traditional scalar quantities such as energy disposal, integral cross-sec- tions and branching ratios.Experimental and theoretical advances have now reached the stage where studies of Dynamical Stereochemistry can begin to map the anisotropy of chemical interac- tions. The Symposium will provide an impetus to the development of 3-D theories of reaction dyna- mics and assess the quality and scope of the experiments that are providing this impetus. Contribu tionsfor consideration by the Organising Committee are invited in the following areas: (A) Collisions of oriented or rotationally aligned molecular reagents (B) Collisions of orbitally aligned atomic reagents (C) Photoinitiated ’collisions’ in van der Waals complexes (D) Polarisation of the products of full- and half-collisional complexes 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. 87 Catalysis by Well Characterised Materials University of Liverpool, 11-13 April 1989 Organising Committee: Professor R. W. Joyner (Chairman) Professor A. K. Cheetham Professor F. S. Stone The understanding of heterogeneous catalysis is an important academic activity, which compliments industry’s continuing search for novel and more efficient catalytic processes. The emergence of relevant, in particular in situ techniques and new developments of well established experimental approaches to catalyst characterisation are making a very significant impact on our knowledge of catalyst composition, structure, morphology and their inter-relationships.Well characterised catalysts, which will be the subject of the Faraday Discussion, include single-crystal surfaces, whether of metals, oxides or sulphides; crystalline microporous solids, such as zeolites and clays, and appropriate industrial catalysts. The elucidation of structure/function relationships and catalytic mechanism will be important aspects of the scientific programme. Contributions describing novel methods for synthesising well characterised catalysts and also reporting important advances in characterisation techniques will also be welcome. Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 31 May 1988 to: Professor R.W. Joyner, Leverhulme Centre for Innovative Catalysis, Department of Inorganic, Physical and Industrial Chemistry, University of Liverpool, Grove Street, P. 0. Box 147, Liverpool L69 3BX. Full papers for publication in the Discussion volume will be required by December 1988. Dr. K. C. Waugh Professor P. B. WellsFARADAY DIVISION INFORMAL AND GROUP MEETINGS Electrochemistry Group with the Electroanalytical Group and The Society of Chemical lndustry Electrochemical Dynamics To be held at the University of Strathclyde on 5-10 September 1988 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB Statistical Mechanics and Thermodynamics Group Dense Fluids To be held at the University of Cambridge on 14-16 September 1988 Further information from Dr P.Francis, Department of Chemistry, University of Hull, Hull HU6 7RX Carbon Group with the Carbon and Graphite Group of The Society of Chemical Industry Carbon 88 To be held at the University of Newcastle upon Tyne on 18-23 September 1988 Further information from The Conference Secretariat, Carbon 88, Society of Chemical Industry, 14/15 Belgrave Square, London SWlX 8PS Surface Reactivity and Catalysis Group Interfaces and Catalysis To be held at the University of Glasgow on 19-21 September 1988 Further information from Dr G. Webb, SCRG Meeting, Department of Chemistry, University of Glasgow, Glasgow G12 8QQ Division Autumn Meeting: Polymerisation and Polymer Behaviour To be held at the University of Birmingham on 20-22 September 1988 Further information from Professor I.W. M. Smith, Department of Chemistry, University of Birmingham, PO Box 363, Birmingham B15 2TT Colloid and Interface Science Group Structure in Colloidal Systems and its Characterisation To be held at the University of Bath on 21-23 September 1988 Further information from Dr R. Buscall, ICI plc, Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn, Cheshire WA7 4QE ~~ ~~ Polymer Physics Group jointly with Physical Crystallography Group Diffraction from Polymers To be held at the Geological Society, London on 30 November 1988 Further information from Dr M. Richardson, National Physical Laboratory, Teddington, Middlesex TW11 OLW ~~~ ~~~ ~~ Polar Solids Group with the Applied Solid State chemistry Group Computer Modelling of Inorganic Solid Structures To be held at the Scientific Societies’ Lecture Theatre, London on 2 December 1988 Further information from Dr A.E. Comyns, R & D Department, Laporte lndusties Ltd., Moorfield Road, Widnes WA8 OQJ Electrochemistry Group New Ideas in Electrochemistry To be held at the University of Cambridge on 15-16 December 1988 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB Colloid and Interface Science Group Aggregation in Colloidal Systems To be held at the Scientific Societies’ Lecture Theatre, London on 16 December 1988 Further information from Dr R. Buscall, ICI plc, Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn, Cheshire WA7 4QE Neutron Scattering Group Muon Spectroscopy To be held at the University of Nottingham on 20-22 December 1988 Further information from Dr S. Cox, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX1 1 OQXJOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistry or chemical physics which appear currently in J. Chem. Research, The Royal Society of Chemistry’s synopsis + microform journal, include the following: The Effect of Nitric Oxide on the Kinetics of Decomposition of Thionitriles Michael S. Garley and Geoffrey Stedman (1 988, Issue 2) Kinetics of the Solvolysis of Chloropenta-amminecobalt(iii) Ions in Water and in Water-Propan-2-01 Evaluation of the Broyden-Fletcher-Goldfarb-Shanno (BFGS) Variable Metric Method in Geometry Mixtures Kamal H. Halawani and Cecil F. Wells (1988, Issue 2) Optimisation using Semi-empirical SCF-MO Procedures Rzepa (1 988, Issue 3) 1. Brookes, Charles Kemball and H. Frank Leach (1 988, Issue 4) Lars Carlsen and Helge Egsgaard (1 988, Issue 4) Podmore and Martyn C. R. Symons (1 988, Issue 4) Dimitris K. Agrafiotis and Henry S. The Measurement of Exchangeable Hydrogen associated with Titanium Dioxide (Rutile) Beverley The Reaction between lmidogen and Elemental Carbon. An Alternative Route to Interstellar HCN ? Radical Cations of N,N-Dimethyluracil and N,N-Dimethylthymine Christopher J. Rhodes, Ian D. Inhibition or Acceleration of the lsomerisation of But-1 -ene on Titanium Dioxide (Rutile) by Adsorbed Molecules Beverley I. Brookes, Charles Kemball and H. Frank Leach (1 988, Issue 5) Methods Henry S. Rzepa (1988, Issue 7) Cheletropic Elimination of CO and N2. A Comparison of the MNDO, AM1 and ab initlb SCF-MO The Cyclopropenyl Anion: an ab inirb Molecular Orbital Study Wai-Kee Li (1 988, Issue 7) (xii)

ISSN:0300-9599    

DOI:10.1039/F198884BP093

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1988, 84(7), 2215-2225 Stability Criteria for Charged Interfaces and their Role in Double-layer Theory Denver G. Hall Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, Merseyside L63 3JW and Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT The thermodynamic criteria for interfacial stability with respect to separation into two coexistent surface phases are reviewed. Attention is focussed on systems containing electrolytes. The necessary conditions stated by Gibbs apply strictly only to components confined to the interfacial region. However, if specifically adsorbed ions or molecules and their bulk counterparts are considered as distinct species the stability criteria can be applied to systems in which there is specific adsorption from dilute solution. This is done by regarding equilibrium states as a subset of partial equilibrium states for which the electrochemical potentials of specifically adsorbed and bulk species differ, and by arguing that the stability criteria appropriate for partial equilibrium states apply.According to this viewpoint specifically adsorbed ions or molecules are by definition species confined to the interface. The application considered in this paper is to the electrical double layer, which is assumed to consist of inner and diffuse regions. It is shown that the repulsion between identical double layers may be lower than that given by the constant potential boundary condition without violation of the necessary conditions referred to above, and that systems of this kind are prone to separation into two coexistent surface phases at high electrolyte concentrations. When there is at least one ionic species in the system (usually an indifferent coion) which is not specifically adsorbed it is shown that if the solution composition is varied in an appropriate way, as described in the text, the outer Stern potential, tyd, for an isolated surface will show super- Nernstein behaviour for ' lower than constant potential ' interactions and vice uersa.Such behaviour can be investigated experimentally when wd can be identified with the zeta potential. The only assumption made concerning the nature of the double layer is that the split into inner and diffuse regions is legitimate.The additional assumption that ion distributions in the diffuse region conform to the Poisson-Boltzmann equation is not required. In a recent communication' it was shown that interactions at constant surface potential need not constitute a lower bound for the interaction between identical charged plates in solution. The aims of this paper are to provide the detailed reasoning underlying this conclusion and to extend the discussion to the case where several species may be specifically adsorbed. The treatment is based on the criteria for interfacial stability with respect to phase separation into two coexistent surface phases. Although these criteria we:re first derived by Gibbs over 100 years ago,2 it appears that some of their implications are not well recognised, particularly for ionic systems.Accordingly, in the first section of this paper a derivation of the appropriate conditions is given with ionic systems particularly in mind. The remainder of the paper is concerned with the implications of these conditions for double-layer interactions and with a more general discussion than 2215 13-22216 Double- layer Interactions that given previously of the effect of solution composition on the outer Stern potential for an isolated plate. In this discussion the specific adsorption of several ionic species is considered and solutions non-ideality is included formally. Conditions for Stability with respect to Phase Separation into Two Coexistent Surface Phases We consider two semi-infinite coexistent bulk phases, one or both of which are fluid, in contact through a planar interface and suppose that the system is homogeneous in the direction parallel to the interface.We choose as the system of interest a portion of volume V containing an area A of this interfacial region. We choose V so that the system includes some of both bulk phases. Let components present in either bulk phase be denoted by i, j , k etc., and let components present only in the interface be denoted by a, /I, y etc. We suppose for the moment that the components are all uncharged and are independent in the sense that none can be formed from any combination of the others. At equilibrium the fundamental equation for the system may be written as d E = TdS-pdV+C pidNi+C pudNu+adA (1) z U where E denotes internal energy, T temperature, S entropy, p pressure, p chemical potential, N amount of substance and o denotes interfacial tension or some other quantity which can be handled in the same way.It is apparent from eqn (1) that equilibrium states of the system are completely determined by the variables S, V, Ni, Nu and A . Since the system is homogeneous in two dimensions it follows that Let eqn (2) apply to one particular equilibrium state and let superscript ’ refer to a second equilibrium state which differs very slightly from the first. For both of these states to be stable with respect to continuous changes in phase the arguments applied by Gibbs to homogeneous bulk phases2 lead us to write E - E > T(S‘-S)-p(V- V)+C pi(N;--Ni)+c pa(Ni-Na)+a(A’-A) (3a) z U and E - E > T‘(S-S’)-p’(V- V)+C pi(Ni-Ni)+C pi(N,-N:)+a’(A-A’). (3b) 2 a Together the two equations give 0 < (T-T)(S’-S)-(p’-p)(V- V ) + c (’&-P~)(N~-N~) a +c (~~--~~)(Nj-N,)+(a’-a)(A’-A).(4) a The status of this expression is as follows. It is a necessary condition for the type of stability we are concerned with insofar as the system is unstable if the expression breaks down for any variation. However, it may not be sufficient for stability. It follows from eqn (4) that at constant intensive properties of the two bulk phases and at constant A a necessary condition for stability is that 0 G c ( P : - P a w : - r u ) ( 5 ) U where ra = NJA. Eqn (5) is the expression on which we base our discussion. If both bulk phases are stable but eqn ( 5 ) does not hold the interface will not be stable with respectD. G.Hall 2217 to continuous variations in composition but will break up into two or more coexistent surface phases. The above procedure follows that used by Gibbs for homogeneous bulk phases. There is no reason for supposing that it is inapplicable in the present case. When ions are present in a system eqn (1) and (2) still apply, but in this case pi and p, are electrochemical potentials which we denote by $. However, since our system is homogeneous it must also be electrically neutral in the absence of any externally applied electric field. Hence not all Ni and N, are independent in this case but are related by C v i N i + C v,N, = 0 a 0. where v denotes ionic valency, including the sign. Let species c be some ionic species present in one of the bulk phases.We may define the quantities Oi and 8, by and replace pi and pa in eqn ( 5 ) by Oi and 8,. Eqn ( 5 ) thus becomes o < c (o;-o,) (ri-r,). a For a system to be stable, eqn (3)-(5) and eqn (8) must apply however small the difference between the two states is. Hence the quantities (ri - r,) and (0; - 0,) may refer to infinitesimal variations. However, these infinitesimal variations must be construed strictly with all higher-order terms included and eqn (3)-(5) and eqn (8) must be interpreted as if they were expressions involving finite differences. Partial Equilibrium States The summation in eqn (8) is confined to species present only in the interfacial region. To include in this expression species which are also present in the bulk phase we invoke the notion of partial equilibrium ~ t a t e s .~ This notion is useful when it makes sense to talk about an adsorbed state. By this we mean that molecules or ions of the same species can be regarded as either adsorbed or non-adsorbed without ambiguity. When such a distinction is legitimate, adsorbed and non-adsorbed ions or molecules of a given type may be regarded as distinct species with their own chemical potentials, which happen to be equal at equilibrium. The adsorbed material is then, by definition, confined to the interfacial region. Partial equilibrium states are those whose free energy has its minimum value for given non-equilibrium amounts adsorbed. Such states are invoked in a variety of' situations, for example, in double-layer theory when we consider integrals such as where CT and b, respectively, performed at constant bulk refer to adsorbed and bulk material and the integration is intensive properties.Application to Double Layers The only assumption we make concerning the nature of the double layer is that it can be divided into inner and diffuse regions. We suppose, as previously, that the state of the diffuse region is determined entirely by the intensive properties of the bulk solution and the total charge on the other side of the boundary separating it from the inner regions. We take as our starting point eqn (8), where the summation now includes all specifically2218 Double- layer Interactions adsorbed species. For an isolated plate we let ly, denote the electrostatic potential relative to the bulk solution at the boundary referred to above. We define pa by where p: is bulk chemical potential of species c.We also define q by q = C r, v, e. 5 Substituting into eqn (8) we obtain 0 c (ri-ra)@i-p,)+(q’-q)(ly~-~~) a which applies to variations in amounts specifically adsorbed at constant intensive properties of the bulk solution. Consider now two overlapping double layers which are brought together at electrochemical equilibrium and let A refer to any changes which take place. During this process the 6, remain constant but the ra vary. Consequently 0 = c AraAOa because all AOm are zero. a However, under these conditions At?, = Ap, -k v, eAly (13) where y denotes the potential at the boundary referred to above when double layers overlap and where pa is defined by eqn (9).It follows that eqn (12) may be written as 0 = C Arm Apa+AqAly. a Since eqn (11) applies to all changes in ra at constant bulk solution composition, it applies when (r;-ra) = AT,. Moreover, when the diffuse region can be described by the Poisson-Boltzmann equation it follows, as has been shown previ~usly,~ that if (ri-r,) = Ara for all a then @:-pa) in eqn (1 1) is equal to Ap, in eqn (14). A similar result can also be expected to apply for more general treatments of the diffuse region based on local thermodynamics5 provided that the split into inner and diffuse regions can be made in such a way that the variables which completely determine the state of the former are those cited above. Under these circumstances, since we may now equate the first term on the right-hand side of eqn (1 1) with that of eqn (14), we find that for corresponding changes in the Fa Aly Aq (wli - lyd) (4’ - 4).(15) The left-hand side of this equation refers to the relationship between ly and q on one surface as the separation between two charged surfaces is varied at equilibrium. The right-hand side refers to the relationship between tpd and q for an isolated surface in the same bulk medium. This latter relationship is given by Gouy-Chapman theory when the Poisson-Boltzmann equation applies, and it is obvious under these circumstances that the right-hand side of eqn (1 5 ) is always positive. Eqn (1 5 ) may be stated in the following form According to eqn (1 5 ) and (1 6), Atp Aq arising from a double-layer interaction may be positive without violating our stability criteria so long as it is less than the corresponding quantity for an isolated plate.Since for identical plates q must decrease with decreasingD. G. Hall 2219 separation it follows that v/ may also decrease so that the work done is less than that which would be required if v/ remained constant. Behaviour of the Stern Potential We use the term Stern potential to refer to the potential at the boundary between inner and diffuse regions of the double layer relative to the bulk solution. Eqn (9) shows that during a double-layer interaction the Ap, are given by h + A v = 0. (17) v.2 It follows from our previous work4 that the changes which occur in the inner regions of the double layer during an interaction with another charged surface can be reproduced on an isolated plate by bringing about the same changes in all the p,.This will not be possible when all species in solution are specifically adsorbed because the pa cannot then be varied independently for the isolated surface without violating the electrical neutrality condition for the bulk solution. However, it may still be feasible when one or more species is not specifically adsorbed. The most likely candidates for such species are indifferent coions, but there may also be counterions whose specific adsorption is negligible when the concentration of supporting electrolyte is low and the surface charge is small. Let subscripts a, p etc. denote potential-determining species and species which are specifically adsorbed.Let subscripts i, j , k etc. denote species which are not specifically adsorbed. It is convenient to choose species c as one of these. Let x, y , z etc. denote ionic species in general. For an isolated plate We now consider species be species 1 u v,e v,e the behaviour of tyd when all Ap, satisfy eqn (17). Let the indifferent c and let (19) - A z . -- v,e From eqn (17)-(19) we have Av+AZ-Avd = 0 and it follows that the dependence of t,vd on Z at equilibrium is given by where (t?v//aq) refers to the dependence of ly on y which occurs when the plate separation is varied in the solution of interest and the subscript e denotes any variations at equilibrium for an isolated plate which satisfy eqn (19). As an example of such variations, consider the titration of an AgI suspension with KI solution in the presence of KNO,.If K+ ions but not NO, ions are specifically adsorbed then the variations which satisfy eqn (19) are the constant ion product of K+ and I-. To achieve this state of affairs and maintain bulk solution electrical neutrality it will be necessary to vary the concentration of NO, ions. If neither K+ ions nor NO, ions are specifically adsorbed it will not be necessary to vary the NO, concentration for eqn (19) to be satisfied. For an isolated plate tyd is determined by q and the bulk solution composition. In the2220 Double-layer Interactions case of interest the bulk composition depends only 2 and the Oi. Hence we may write at constant T and n Substituting for (aq/aZ),, as given by eqn (21), into eqn (23) and rearranging we obtain In general is positive for ideal and non-ideal systems alike.Thus when (aq/aly) is negative has the same sign as the right-hand side of eqn (24). When (aq/ay) is positive we have is a necessary condition for stability. In this case therefore is opposite in sign to the right-hand side of eqn (24). To evaluate this expression we turn our attention to the behaviour of vd at constant 2. We argue that at constant q, ly, depends on the 8, in the same way as the potential drop across a perfectly polarisable electrode at which there are no specific adsorption effects whatsoever.6 This enables us to make use of previous work concerning single ionic activities and the thermodynamics of charged interfaces.6* ' Accordingly we write at constant q where and r: is the contribution to ry from the diffuse double layer.? ny v,(vi - v:) fv = c Y1.z v, v l 5 where n denotes number density and the vf, are quantities defined in ref.(6) and (7). Evidently Cf, = 1 X t To be more precise, r; is the total amount of i per unit area on the solution side of the outer Stern plane minus the amount of i in that amount of bulk solution which contains the same amount of solvent.also, since the it follows that D. G. Hall electroneutrality condition for the interface ensures that q+C vxeTd, = 0 X c g x = 1. X When eqn (1 9) is valid the d8, are given by where, as is shown in ref. (6) and (7) We now substitute for the d8, in eqn (32) as given by eqn (31). This provides us with an expression for dpt in terms of the dei and dZ, which we may back-substitute into eqn (31) to express doa in terms of dZ and the dei.When this procedure is followed eqn (25) a a Hearing in mind that C & + C f a = I = C g i + C g a z a i a eqn (33) rearranges to give (34) where the summations over i in eqn (34) include species c. In the case that species c is the only non-specifically adsorbed species in the system, the final term of eqn (34) disappears and the leading term simplifies to (1 -g,/fc)edZ. We are now in a position to discuss further the right-hand side of eqn (24). At constant Oi eqn (34) gives Since the g, and thef, can be expected to be positive in most situations of practical interest it follows that at constant 8, the right-hand side of eqn (24) can be expected to be negative.Thus when (arylaq) is negative, corresponding to interactions between constant charge and constant potential, the dependence of ry, on Z at constant Oi is sub-Nernstian. When (ary/aq) = 0, corresponding to interactions at constant potential, the dependence of ry, on Z is Nernstian and this appears to be the case for all equilibrium variations. When (arylaq) is positive, corresponding to lower than constant potential interactions, the dependence of t,vd on Z at constant 8, is super-Nernstian. The situation where the variations of Z are not made at constant Oi is more complex and in this case the sign of ( ~ I , Y ~ / ~ Z ) ~ - 1 depends on the particular variations concerned. In practice, for the variations commonly made in which potential-determining ions are added at a constant2222 Double-layer Interactions concentration of supporting electrolyte, the changes in 0, will be quite small when the amount of potential-determining species concerned is small.If we identify ty, with the zeta potentia1819 then it may be possible to estimate the dependence of tyd on Z experimentally. Hence it may be possible to identify surfaces whose interactions are below constant potential from appropriate electrokinetic studies. Moreover, when the Helmholtz-Smoluchowski equation can be used, the assumption that the Poisson-Boltzmann equation applies is not necessary. When the Poisson-Boltzmann equation does not apply we may obtain explicit expressions for the dependence of Vd on q, Z and the Oi. In this case the various f are given by nY v: f, = _____ c n, v:.X Also ty, q and the n, are related through the expression which gives on taking differentials To express the dn, in terms of the doi and d Z we note that edZ dn, = n, v, - k T do. nivi a 'v,kT n,vc dn, = n.v. >+- dn, v,dn, = - c vidni-C v,dn, i # c a. (39 4 (39 b) (39 c) where the latter equation is merely the bulk electroneutrality condition. Using eqn (39) to substitute for the dn, in eqn (38) we find that the right-hand side of this expression may be rewritten as +C nivi [ exp ( -____ ':F")- 11%. (40) a nc vc From eqn (39) we may express dn,/n, v, in terms of the d6, and d Z as follows: a a We now substitute for dn,/n,v, in eqn (40), and after some rearrangement we find thatD. G. Hall 2223 where X (44) ]Eqn (42)--(44) enable us to calculate the required derivatives of v/d in eqn (24) explicitly.That they are entirely consistent with the more general expressions derived above for non-ideal systems may be verified as follows. When the Poisson-Boltzmann equation applies the r': are given by where x denotes distance and the lower limit of integration corresponds to the outer Stern plane. By changing the variable of integration from x to v/ eqn (45) may be rewritten as This gives Now from eqn (38) we find that whereas from Gauss's theorem we find that When eqn (50) is substituted into eqn (42) and (43) we obtain which are, of course, identical with the corresponding expressions as given by eqn (34).2224 Double-layer Interactions Phase Separation Consider eqn (1 1). For an isolated surface (av/aq), is positive but for a given value of q can be expected to decrease with increasing ionic strength.Also interfaces for which this equation is not satisfied are unstable with respect to phase separation into coexistent surface phases. Suppose now that we have a system for which the right-hand side of eqn (1 1) is positive even though the summation over a may be negative. There is no reason to suppose that such an interface is necessarily unstable. Now if we can increase the ionic strength of this system keeping all I‘‘ constant then (q’--q)(v’-v) will decrease in magnitude, whereas the summation over a will remain unaltered. Eventually the right- hand side of eqn (1 1) will change sign and phase separation will occur. Hence surfaces which interact at less than constant potential are prone to phase separation as the electrolyte concentration is increased, whereas surfaces which interact at greater than constant potential are not.However, for the electrolyte concentration to be increased with all r’ held constant we must have at least one counterion species and one coion species which are not specifically adsorbed over the range of conditions concerned. When q is fairly large this is a somewhat stringent requirement. Also at largish electrolyte concentrations it may no longer be legitimate to separate the double layer into inner and diffuse regions, It is therefore not certain but merely likely that surfaces which interact at less than constant potential will phase-separate on addition of electrolyte. Concluding Remarks The theory outlined in this work amplifies and generalises the work reported in ref.(1). The amplification consists of a more thorough and detailed development of the arguments concerning interfacial stability. The generalisations are, first, the extension to systems in which several ionic species are specifically adsorbed and, secondly, allowing for solution non-ideality in the handling of the diffuse double layer, thereby avoiding some of the assumptions inherent in the Poisson-Boltzmann equation. The main remaining assumption is that it is legitimate to regard the double layer as consisting of inner and diffuse regions. In order to apply the theory it is also necessary that there be at least one ionic species in the system, usually an indifferent coion whose specific adsorption is small enough to ignore.However, this may not be a particularly stringent requirement. Consequently the theory is much more likely to describe the behaviour of real systems than the simpler version outlined previously. In particular, when it is legitimate to identify the zeta potential with the potential at the boundary between inner and diffuse regions, it should be possible from electrokinetic studies to identify systems which are likely to interact at less than constant potential and to separate into coexistent surface phases on addition of electrolyte. The adsorption behaviour of such systems is also likely to show some interesting effects and to provide valuable information concerning the applicability of the above theory to real systems.Appendix A Sufficient Condition for Interfacial Stability? For homogeneous bulk phases the stability conditions first derived by Gibbs2 are both sufficient and necessary. However, the situation for interfaces is not so clear cut because eqn (4), which is a necessary condition for stability, may not be sufficient. Suppose for instance that the two bulk phases x and y are both stable. The inequalities analogous to eqn (4) for these bulk phases are2 0 < v- n ( ~ ; - x z ) - ( P ’ - P ) ( ~ ; - C)+C ( P ; - P i ) ( G - ~ i J (A 1) aD. G. Hall 2225 together with the corresponding expression for phase y , where S,, < and Nix denote the extensive properties of phase x. We may now choose amounts of phases x and y such that and define the quantities AS” and ATi by AS“ = s-s,-s, ‘I ATi = Ni - Nix - Ni,J where S, is the entropy of volume < of phase x and Nix is the amount of i in this volume.Evidently S“ and ri are excess quantities per unit area as normally understood. In a similar way we may also define quantities S’“ and ri corresponding to the neighbouring state . If we now put A’ = A , subtract eqn (A 1) for phase x and the corresponding expression for phase y from eqn (4), in both cases taking the amounts specified by eqn (A 2) for the original state and the corresponding amounts for the perturbed state ’, we obtain the quantity A(T - T ) (P- so) + A c @;-pa) (r;- r,) + A c @; - p i ) (r; - Q. i Evidently this quantity may be less than zero without violating eqn (4) because it may be outweighed by the contribution from the bulk phases as given by eqn (A 1). Under these circumstances the interface may be unstable. Hence compliance with eqn (4) for all variations may not be sufficient for stability. On the other hand, if the above quantity is greater than zero for all possible variations this may well be sufficient for stability. That it is not also a necessary condition is apparent from the fact that there are stable interfaces for which some are negative. As far as the author is aware a single set of conditions which are both sufficient and necessary for interfacial stability in the sense referred to above has yet to be derived. References 1 2 3 4 5 6 7 8 9 10 D. G. Hall, J. Colloid Interface Sci., 1985, 108, 411. J. W. Gibbs, in Scientific Papers (Dover, New York, 1961), vol. 1. D. G. Hall, J. Chem. Soc., Faraday Trans. 1, 1972,68 2169; J . Chem. Soc., Faraday Trans. I , 1980,76, 386. D. G. Hall, J. Chem. Soc., Faraday Trans. 2, 1977, 73, 101 ; 1978, 74, 1757. A. Sanfeld, in Introduction to the Thermodynamics of Charged and Polarised h y e r s , Monographs on Statistical Physics and Thermodynamics (Wiley, New York, 1968), vol. 10; D. G. Hall, in preparation. D. G. Hall, J. Chem. SOC., Faraday Trans. 2, 1978, 74, 405. D. G. Hall, J. Chem. Soc., Faraday Trans. 2, 1973, 69, 1391. A. L. Smith, J . Colloid Interface Sci., 1976, 55, 525. D. G. Hall, J. Chem. Soc., Faraday Trans. 2, 1980, 76, 1254. J. Th. G. Overbeek, Prog. Biophys. Biophys. Chem., 1956, 6, 57. Paper 7/716; Received 21st April, 1987

ISSN:0300-9599    

DOI:10.1039/F19888402215

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

.J. Chem. SOC., Faraday Trans. I, 1988, 84(7), 2227-2240 A Thermodynamic Analysis of Common Intersection Points in Potentiometric Titration Studies of Solid Surfaces Denver G. Hall Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, Merseyside L63 3JW and Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT Potentiometric titration curves for solid/solution interfaces often exhibit common intersection points (CIP) at different levels of supporting electrolyte and, for a fixed electrolyte level, at different concentrations of cosolvents. Both situations are discussed. For the first, explicit expressions are derived which summarise the conditions for a CIP to occur in the presence of specific adsorption and which allow formally for solution non-ideality.Attention is focussed primarily on plots of amount adsorbed vs. activity of the potential determining species rather than concentration. However, the latter case is also discussed briefly. The treatment is extended to cases where several ionic species are strongly specifically adsorbed and leads to a useful test for the occurrence of specific adsorption which is applicable in non-ideal systems. The situation where the bulk concentrations of such species are not negligible is also considered. For mixed solvents a simple expression is derived which describes the effect of solvent composition on the con- centration of potential determining ions at the zero point of charge (z.P.c.). In this case CIP may arise from competition between medium effects associated with the adsorption of potential determining ions and electric fields in the diffuse double layer.The treatment is applicable to ionic crystals and to surfaces with fixed dissociable groups. Potentiometric titration is an important and widely used method for studying the ionisation behaviour of solid/solution interfaces and the effects of adsorption thereon. The information is usually presented as graphs of r, us. log C, or log a,, where r, is the surface excess of the species monitored in the titration experiment C, is its concentration and a, its activity with reference to an infinite dilution standard state. It is often found that a series of such curves at different concentrations of supporting electrolyte cross at a common intersection point (CIP).1.2 A CIP is also observed sometimes when the solvent composition is varied at constant electrolyte level^.^ Both types of CIP are discussed in some detail below.In the next section explicit expressions are derived for the conditions under which CIP can occur both when specific adsorption of supporting electrolyte ions takes place and when such adsorption is absent. The arguments are then extended in the following section to the situation where there are several strongly adsorbed species. Non-ideality of the bulk solution is allowed for in both cases. The discussions based in part on previous work concerning single ionic activities and the thermodynamics of charged interfaces.*! The section on mixed solvents highlights the interplay between specific adsorption and electric field effects in determining cosolvent adsorption.Due attention is given to the role of medium effects as discussed previously.6 Although some of the issues mentioned above have been considered by others, notably Lyklema and this earlier work tends to be less general and more qualitative than that given below. 22272228 Potentiometric Titration of Solids Electrolyte Effects in a Single-component Solvent (a) General Thermodynamics Consider a solid surface in contact with an aqueous solution which contains c independent ionic species, i = 1 + c. At constant T and p and at equilibrium the Gibbs adsorption equation may be written as where 0 denotes the quantity which plays the same role as does the surface tension for fluids and the are surface excesses defined according to the convention that To, where 0 denotes solvent, is zero.The Oi are given by where p may denote chemical or electrochemical potential and v denotes ionic valency including the sign. It is apparent from their definition that the Oi refer to electrically neutral combinations of It is useful to regard them as the chemical potentials of the electrically neutral ionic components of the system. At constant T and p they are independently variable at equilibrium. For the case under consideration, the 8, (together with T and p ) completely determine the state of the bulk solution. We denote by subscript 1 the species monitored in the titration experiments, otherwise known as the potential-determining species. Often, rl takes positive and negative values. Also, at constant T and p , pl, the single-ion chemical potential of 1 in bulk solution is completely determined by TI and the other Oi (i = 2 -+ c - 1). For simplicity we will suppose that the bulk concentration of species 1 is sufficiently small that the changes therein we shall be concerned with do not significantly affect the other Oi.According to ref. (4) and ( 5 ) we have: where and k C C f , = l . i - 2 (4) The summations in eqn (3) and (5) do not include species 1 because its concentration is very small. Eqn (1)-(3) lead straightforwardly to the expression : d(o+r1pl) = -C &+uf,) dBi+pu,drl i vi which gives on cross-differentiationD. G . Hall from eqn (6) and (7) it follows that at constant T and p RTdlna, = dp, = -C i *i 2229 were a, is the ionic activity of species 1.r, we must have It is apparent from eqn (8) that for a, to be independent of the Oi at constant T, p and for all i. Eqn (1)-(8) are completely general and are valid for all r, irrespective of whether or not there is any significant specific adsorption of the species i. When eqn (9) is valid for a given rl over a range of Oi, plots of r, us. lna, for different Oi exhibit a common intersection point at that r,. TEis equation too is a general thermodynamic result. (h) Explicit Treatment of Specific Adsorption So far, the occurrence of specific adsorption has been allowed for implicitly. Consider now the q. As previously, we suppose that we may write’.’ q = r;+rp (10) where r; and rp, respectively, are the contributions to q from specific adsorption and from the underlying diffuse layer.We now define the quantity q’ by and regard q’ thus defined as the total surface charge density due to the presence of the potential-determining species and specifically adsorbed ions. In terms of the Stern- Grahame model, q’ may be identified with the total charge per unit area in the inner regions of the double layerlo and for ideal solutions the may be calculated from Gouy-Chapman theory given q’ and bulk concentrations. Inherent in this definition of q’ is the idea that r, = 0 when the primary charge of the surface is zero. For ionic crystals this is the case when rl is defined as usual in terms of Gibbsian dividing surfaces. Surfaces with dissociable groups pose a slight problem in that this procedure is no longer feasible.In such cases r, must be defined relative to some reference state. However, there is no reason why this reference should not be chosen to give l?, = 0 at zero primary charge. A further issue concerns the choice of dividing surface to which the rp refer. In practice it makes sense to choose this surface as the boundary between inner and diffuse regions. Although this surface differs from that to which the & refer, the consequence is a slight modification of the r: which is usually trivial. Let By virtue of electrical neutrality it follows that Zigi = 1. Whereas the f , defined by eqn (7) depend only on the composition of the bulk solution, the gi depend in addition on 4’. We suppose, however, that the gi are completely determined by these variables irrespective of the nature of the surface concerned and point out that this notion is inherent in the idea of a diffuse double l a ~ e r .~ ’ ~ ’ When q’ = 0 we have gi = A (13)2230 Potentiornetric Titration of Solids because all specific effects are included in the ri. When # / v i e is positive we will usually have gi < f , and when q’/vi e is negative we will usually have gi > fi. These results always hold for ideal solutions and can also be expected to remain valid for most simple electrolytes when non-ideality is allowed for. For 2: 2 electrolytes they express the fact that the excess of counterions in the diffuse region exceeds the deficit of coions. They may break down in special cases such as solutions in which some species have negative transport numbers.In such cases some vC/v, may be negative. From eqn (10) and (12) it is apparent that Together with eqn (7), eqn (1 1) and (14) give as the condition for a CIP: for all i where dki denotes the Kronecker 6, (6 = 1 when i = k, 6 = 0 when i # k). Eqn (1 5 ) applies whenever it is legitimate to split the into contributions from specific adsorption and a diffuse double layer. For eqn (15) to be satisfied when q’ = 0 it is apparent from eqn (13) that the right- hand side must be zero for all i. A sufficient condition for this is that all (ar;/iX1) are zero. Under these circumstances the l-‘; themselves will usually be zero. This situation corresponds to the traditional interpretation of a CIP, namely the absence of specific adsorption when rl = O(l).However, it is not necessary that all (i3r;/Xl)oi = 0 for the right-hand side of eqn (15) to be zero because the equations concerned are linearly dependent. For example, in the case of a single supporting electrolyte the right-hand side of eqn (15) is zero if when q’ = 0. This state of affairs is not mentioned by Lyklerna.l3 Although improbable, it is not physically unreasonable on electrostatic grounds. For example if vi/vl is positive we expect (dr;/Xl)oi to be negative and (i3r;/i3rl)oi to be positive. Thus if eqn (16) is valid when q’ = 0 and r1 = ry then we expect 14’1 < Ivl e(Tl -r:)l in the neighbourhood Eqn (15) may also be satisfied when q’ # 0. The simplest case to consider in this category is that we have but a single supporting electrolyte and only one species is specifically adsorbed.For this case eqn (15) takes the form of r!. When v J v , is positive we expect (c?rq/L7r‘l),i to be negative, in which case eqn (17) can only be satisfied when gi < -& and q’/vi e is positive. As we increase r1 we expect @/vl e to increase so that ( g i - f , ) becomes more negative. At the same time we expect l(i3r;/Xl)eil to decrease. Hence it seems possible that eqn (1 7) will be met sometime on any titration curve having a point where q’ = 0 provided that l(iX’;/Xl)oil becomes small when q’ and rl both increase. Similarly when V J V , is negative we expect (i3r;/Xl)ei to be positive. Again (g,-f,) must be negative to satisfy eqn (17) and again q’/vie is positive. Clearly this is the situation we have just discussed.The case where vl/vi is positive and g, > f, requires a positive (ar;/c7rl),i for eqn (1 7) to hold. Although this may be possible in principle, it seems unlikely in practice.D. G . Hall 223 1 If both ionic species are specifically adsorbed eqn (15) is satisfied when Since (X’;/Xl)oi and (i3r;/c?rl)oi can be expected to have opposite signs the two terms on the right-hand side compete. If the first term dominates, q’/vi e will be positive when the condition is met. Evidently this situation is rather similar to that where only one species is specifically adsorbed and it appears that adsorption of k reduces the value of q’/v,e at which the CIP occurs. Eqn (15) is a necessary condition for a CIP and is likely to be met at some point on any titration curve.However, when specific adsorption occurs it seems unlikely that the value of rl, will be fixed. Consequently any CIP is likely to be smeared out to an extent depending on particular features of the systems concerned. To which of the above situations a given CIP corresponds can presumably be established from electrokinetic measurements. If q’ = 0 the mobility and zeta potential should both be zero at the a, concerned irrespective of the electrolyte level. Hence plots of zeta potential or electrophoretic mobility us. lna, should also give a common intersection point at the same a, as the titration curves. Moreover, if all r: and their derivatives are zero then we should have q’ = vleT, close to the ZPC and the tests proposed previously’’? l2 for the absence of specific adsorption should apply.These tests should fail, however, if the situation is that described by eqn (16). In this case the relationship between q’ and lna, in the neighbourhood of the ZPC as obtained from electrokinetic measurements should be the same as if there is no specific adsorption and the plane of shear is located within the diffuse region rather than at the outer Stern plane. If q’ is non-zero it is unlikely that the electrokinetic data will give a CIP. The case discussed above where only one ionic species is specifically adsorbed corresponds with that identified by Lyklema.2 (c) Replacement of a, by Other Quantities According to the Kirkwood-Buff theory of so1utions7~13 we may write for the case of interest, at constant T and p where c- 1 RTdlnn, = do,+ C Nlidei i f 2 The ni are number densities and the N: are defined by NZ = nj ( g i j - 1)dV (21) I where g,, denotes the pair distribution function for i and j .Ni*, is approximately equal to vi/uo, where the v are appropriate ionic or molecular volumes, respectively. Consequently Nli x Vi. The latter quantity is the contribution of i to the ion2232 Pot en t iomet r ic Tit ra t ion of Solids atmosphere of species 1. Since the charge of the atmosphere exactly balances the charge of the ion it is clear that we must have C I v,+ C Nfivi = 0 i f 2 C v l + C NTi vi = 0 i f 2 From eqn (1) it is readily shown that Hence, regarding 8, as a function of the other 8, and r, we may write We may now substitute this expression in eqn (19) to obtain RTdlnn, = 2 "Ti-(%) ] dOi+rA) dr,.i f 2 ar1 ei ari ei From eqn (25) it is clear that plots of r, us. In n, at different concentrations of supporting electrolyte will exhibit a CIP when for all i at the same r, over the range of Bi concerned. Clearly eqn (26) is analogous to eqn (9) and can be handled in the same way. Indeed we may proceed exactly as in the preceding section merely by replacing (v,/v,)f, with - NTi. In the ideal dilute limit N;, = xi where ni vi v, C n,v," lqi = -- a and it is apparent that in this situation, where we also have v' = vi, xi = -(v,/v,>fi. For non-ideal solutions the N;ti are usually greater than the right-hand side of eqn (27) both when vi/vl is positive or negative and the difference usually increases with increasing concentration up to levels of over 0.5 mol dm-3.The reasons for this difference are similar to those underlying the difference between g, andf, mentioned above. Suppose now that we have a 1 : 1 electrolyte for whichf, =f3 and that neither ion is specifically adsorbed. It follows that at moderate concentrations of electrolyte where non-ideality is significant eqn (26) will not be satisfied at the ZPC but will be satisfied at some greater value of r, where g, = - (vi/vl) N;i. If under these circumstances a CIP is observed it will not correspond strictly to conditions of zero electrophoretic mobility . The analysis in terms of n, does have the advantage that it avoids uncertainties due to liquid junction effects and other difficulties associated with the measurement of single ionic activities.However, the effects of solution non-ideality on the value of at which we can expect eqn (26) to hold in the absence of specific adsorption is not trivial as the calculation in Appendix 1 shows.D. G. Hall 2233 This drawback of working with n, may be avoided as follows. Let species 2 be such that v2/v1 = - 1. For this species we have in analogy with eqn (19) C RT d In n, = do2 + C Nii doi i-2 where the summation includes species 2. In practice when n, is very small be very small. Adding eqn (28) to eqn (19) we obtain c-1 RT d In n, + RT d In n, = do, + do, + C (NTi + Nli) doi. d0,+d02 = dpl+dp2 = RTd1nn,n2+2RTdlnyl2 i - 2 However, where y12 is the mean ionic activity coefficient of the neutral salt consisting of species 1 and 2.Together with eqn (30), eqn (29) gives 1 c-1 2 i - 2 0 = RTdlny,,+- C (Nli-Nii)dOi which in turn with eqn (19) gives 1 c-1 2 i-2 RTdlnnly12 = do,+- C (N:i-Nii)dSi. Substituting for do, as given by eqn (24) we now obtain Eqn (33) shows that plots of rl us. 1nn1y12 will give a common intersection point when NTi - Nii (g)oi = 2 (34) for all i, at the same l-, over the range of Oi concerned. Clearly eqn (34) is analogous to eqn (9) and (26) and can be handled in the same way. Because n1yl2 is measurable using thermodynamic methods eqn (34) like eqn (29) avoids the uncertainties associated with the measurement of single ionic activities. Eqn (34) is also free from the asymmetry inherent in eqn (27). It is, however, dependent on the choice of species 2 and reflects any specific effects associated with both species 1 arid 2 in the bulk solution.Specific Adsorption of Several Species Let subscripts x, y etc. denote species which are known to be strongly specifically adsorbed. Examples are potential-determining species and surfactant ions. For simplicity we will assume at this stage that the bulk concentrations of these species are sufficiently small for thef, to be negligible. Eqn (1) takes the form 2 52234 Potentiometric Titration of Solids Consider now variations for which de,/v, is the same for all x. For such variations eqn (35) leads to the expression where q = C rx v,e and d3, = dp5/v5 e. X (37 a, b) Evidently q is the surface charge density arising from the species x, and R has the dimensions of an electrical potential.The condition that dp5/v5 = dp,/v, = ed3, can be established experimentally by ensuring that the e.m.f. of a cell with electrodes reversible only to species x and y remains constant during the variations of interest. We may now proceed exactly as in parts (a) and (b) of the preceding section. In particular the condition that plots of q ZLS. 3, at different ei exhibit a common intersection point for a given q is that (?JOi+& = 0 (38) and like eqn (9) is a general result when allf, are negligible. Specific adsorption of the species i can also be dealt with in the same way as in part (b) above. The replacement of 3, by other quantities along the lines of part (c) above is less straightforward. In particular when dp5/vx = dp,/v, we do not in general have dlnn5/vx = dlnn,/v,.It is possible to express the do, in terms of the doi and the d In n, and to substitute for the do, so given in eqn (35). We may then consider variations for which dlnn,/v, = dlnn,/v,. For ideal systems this is of course equivalent to the procedure outlined above. For non-ideal systems the thermodynamic significance is somewhat obscure and the resultant expressions are a bit messy. Similar considerations apply to the quantities (1 /v5) d In n, yxz where x and 2 have opposite signs. When x and 2 have the same sign the definition of a suitable y l z poses additional problems. The situation in which the f, differ significantly from zero can be dealt with as follows. In this case we have d ~ = - ~ - d O i - ~ - - d 8 , . f, f5 VC i V i 5 vx For the variations of interest d e X dP - = e d A - 2 .V X VC From eqn (39) and (40) we find that 5 5 Substituting for dpc/vc in eqn (36) as given by eqn (41), we now obtain where z The condition that 3, is independent of Oi at a given q* is given by (39) (42) (43)D. G. Hall 2235 which is analogous to eqn (9) and (38). The final term arises because the bulk composition and hence thef, no longer remain constant when q* is varied at constant Oi. However, when q and q* are zero the term itself is zero. It is by no means obvious that when q = q* = 0 eqn (44) corresponds to the absence of specific adsorption of the species i. To reach a more explicit conclusion we consider partial equilibrium statesg' l4 in which specifically adsorbed and bulk material of a given type are regarded as separate species.Let superscript s denote specifically adsorbed material and let b denote the corresponding material in the bulk. As before we define q' r We consider variations in which all (d8",/v,) + (dpc/vJ = e dll, and (dO:/v,) + (dpc/vJ = t' dLb for all x. We note that dil, and dAb thus defined are not necessarily equal and that at constant T and p , As may be regarded as a function of the Oi, q' and Ab. Consider now the quantity (i3A/i30t)q*,oj at equilibrium. When q* = q = q' = 0 we may use partial equilibrium concepts to express this quantity as follows : If x includes all potential-determining and specifically adsorbed species then by definition there is no specific adsorption of species present in the bulk solution. Now by virtue of the arguments developed in the preceding section, L, does not depend on the bulk solution composition when q' = q* = q = 0.Consequently and it follows that (") =o. aoi q*,ei Hence if the species x include all specifically adsorbed species then plots of 3, vs. q for different values at Oi should exhibit a common intersection point when q = 0, even when thef, are non-zero. It is also apparent from eqn (35) that at such a CIP o should be independent of Oi. To observe a CIP of this kind in practice it is necessary that there be at least two ionic species which are not substantially adsorbed close to the isoelectric point (i.e.p.). If there is only one such species we cannot vary 1, at constant Oi. It is also apparent that when a CIP of this kind is observed plots of zeta potential or mobility us.3, at different Bi should give a zero zeta potential or mobility at the same A. The entire approach of this section is closely related to previous work concerning diffuse double layers which conform to the Poisson-Boltzmann equation.''? ''9 l4 However, the present work is applicable to non-ideal solutions. By varying the species whose Oi is constrained to comply with eqn (40) it is possible to establish which species are specifically adsorbed on a given surface close to its i.e.p. and which species are not. The treatment therefore may be useful in practical studies of adsorption onto solids. Mixed Solvents Common intersection points are sometimes observed in mixed solvents when the solvent composition is varied at a constant concentration of supporting ele~trolyte.~ When discussing this situation we will suppose for simplicity that the supporting electrolyte concentration is small enough for the assumption of ideality to be reasonable so that for species i we may write pi = @+RTInn,2236 Pot en t iome t r ic Titra t ion of Solids where py depends on T, p and solvent composition and n, denotes the number density of i.We will also suppose that eqn (48) applies to species 1 and that neither ion of the supporting electrolyte is specifically adsorbed. At constant T and p , we write for an arbitrary dividing surface where pLsolid is fixed at constant T and p and the Ts are surface excesses with respect to the dividing surface concerned. As before 0 denotes the principal solvent component.a denotes the cosolvent. We now argue, as above, that we may regard the double layer as consisting of an inner region and a diffuse region. We will suppose, for simplicity, that any contribution to r1 from the diffuse region is negligible and that any contribution to c from the inner region is negligible. If we now take as our dividing surface the boundary between the inner and diffuse regions we may if we so wish define the quantities l?: and Ti as the total amounts of these species found in the solid side of this dividing surface. We now write where T; and I': are the contributions to ro and ra from the solution side of the dividing surface. Again we suppose that the state of the solution on this side of the dividing surface is completely determined by the bulk solution properties and the total charge per unit area on the solid side.Consider now I'; and Tb. We recognise the following two contributions to these quantities: (1) if the supporting electrolyte ions are preferentially solvated by a then there will be a contribution to r': arising from the a associated with the non-uniform distribution of ions in the diffuse region; (2) the existence of an electric field leads to solvent sorting15-17 in that for a given T, po and pa the concentrations no and na depend on the field strength, E. To a first approximation this effect is proportional to E2, which in turn can be expected to vary as r:. Bearing these two effects in mind we 1 subdivide I?: and T: by writing r; = r;+r; r: = r:+r;J where the first terms on the right-hand sides refer to the ion distribution effect and the second terms refer to the field effect.At constant T i t is shown in ref. (6) that d$ = - Nif, dpo - Nif, dpa (52) where NL may be regarded as the average amount of a associated with an i ion and can take either positive or negative values. Eqn (52) also applies to species 1. It is apparent from the definitions of T; and Tz that we should have l-': = C riN2.j Consequently i and we may rewrite eqn (49) as do = - RT 2 ri d In nf - (I-: + r;) dpo - (rz + r:) dpa - Tl dpl. i (53) (54) (55)D. G. Hall 2237 However, since eqn (52) holds for species 1 , it follows that dp, = dpF+RTdlnn, = RTdlnn,-N~dpo-N~L,dpL,. (56) Also since n, and the n, are all taken to be very small, then at constant Tand p , according to the Gibbs-Duhem equation, we may write approximately where nJn0 is the number of moles of a per mole of solvent.Eqn (56) and (57) may be substituted into eqn (55) to give at constant T and p where and rgs may be regarded as the relative specific adsorption of a, and a similar interpretation may be given to and N1$3P. Remembering that we are concerned with only a single supporting electrolyte so that dlnn is the same for both species thereof, it is now straightforward to deduce from eqn (58) that RTdlnn,=-C - RTdlnn i ( l : ) n i , p z where n without subscript denotes the concentration of supporting electrolyte. Eqn (60) is the expression on which the subsequent discussion is based. Consider first the situation when r1 = 0. In the absence of specific adsorption of the supporting electrolyte ions the coefficient of RTdlnn = 0 for the reasons given above.Hence at constant pa we expect titration curves at different ni to have a common intersection point at the Z.P.C. The value of n, at this point depends on p,. This dependence is given by because, under these circumstances, r: can be expected to increase in proportion to r:, I5-l7 SO that (X:e/Xl)n7,iia is zero when rl = 0. The right-hand side of eqn (61) may be regarded as the relative adsorption of a by a po tential-determining ion in solution minus the corresponding differential quantity for an adsorbed ion. Indeed the integral with respect to pa of the right-hand side of eqn (61) is essentially the difference in primary medium effect between adsorbed and bulk ions of species I .Also the term (i3+:s/3rJn,, ~,~ is reminiscent of similar terms in the expression for the change in Galvani potential drop across a metal/dielectric interface as the dielectric composition is varied.152238 Potentiometric Titration qf' Solids intersection point is that The condition that titration curves with the same n but different pa have a common and does not depend on pa at the rl of interest. If the dielectric constant of a is less than that of water we expect (8r:e/Xl)nt,pz to be negative for positive rl and positive for negative r,. On the other hand, N:: does not depend on r, at all and we might expect the dependence of (X:'/8rl), on rl to be fairly small. Consequently we expect eqn (62) to be satisfied at some value of I-, for any pa.It is not clear, however, that this value of rl should not depend on pa and it is noteworthy that instances have been cited where 'common intersection points' of this type are not at all harp.^ In general the value of r, for which eqn (62) holds at a given pa will depend on n because altering n alters both the electric field and its dependence on position in the electrical double layer. To obtain an expression for this dependence we denote the left- hand side of eqn (62) by Xa. At constant Tand p , X , depends on n, the pz and r, so that we may write dX, = X,, dp, + Xai d In n + XZ1 dr, where Xu, Xai and XZ1 are the appropriate derivatives. For a CIP to be observed under conditions of varying pa the left-hand side of eqn (63) and the first term on the right- hand side must both be zero at the CIP itself.Hence the dependence of r, on n at the CIP is described by Evidently a2 Inn, xal = (-1 To obtain the dependence of n, on n which corresponds to that of r1 in eqn (64) we put the coefficient of dp, in eqn (60) equal to zero and write where (dT1/dlnn),,, is given by eqn (64). Experimental data on AgI appear to be in good qualitative agreement with the above ideas. For this system, increasing the cosolvent concentration usually decreases the iodide ion concentration at the Z.P.C. Thus suggests that [NT: - (?I'!e/?rl)nL,,,z] is negative. Since (i3r~e/i3rl)nt,l~z is also expected to be negative it is apparent that eqn (62) should be satisfied at some positive rl. This is indeed the case for most examples cited. To what extent the treatment can be used for quantitative predictions remains to be established.Concluding Remarks An important feature of this paper is the use that has been made of the work described in ref. (4)-(6). That concerning single ionic activities and the thermodynamics of charged interfaces4y5 provides a general background for the analysis of CIP as the electrolyte concentration is varied. The analysis allows for solution non-ideality and is applicable to any number of ionic components. When ideality is assumed by putting v: = v i and equating activities with concentrations the results given previously', are recovered forD . G. Hall 2239 electrolytes of all valence types. A further advantage of the present approach is that the conditions of interest are expressed concisely in the form of general equations.Although working with n, instead of a, circumvents the difficulties associated with measuring single ionic activities, it does pose other problems. In particular non-ideality of the solution leads to an undesirable asymmetry in the dependence off, on n, in the vicinity of the Z.P.C. This asymmetry can be avoided by working instead with n,y,,. When more than one ionic species is strongly specifically adsorbed working with quantities such as n, or n,y,, is less viable and the advantages of using single ionic activities are particularly apparent. This work leads to useful tests for the occurrence of specific adsorption based on titration data or electrokinetic measurements which can be expected to apply in non-ideal systems.The treatment of mixed solvents illustrates well the importance of previous work on rnedium effects6 in this context. This treatment applies to oxides and other surfaces with fixed dissociable groups as well as to ionic crystals such as AgI. It is also applicable with slight modification to mixed-solvent effects at a mercury electrode as also is the work involving several strongly adsorbed species. Appendix Calculation of Conditions for eqn (26) to be Satisfied Consider a solution in which all ions are univalent and have equal activity coefficients. The aim of this appendix is to estimate NTi for a typical 1 : 1 electrolyte at a concentration of 0.1 mol dm-3 and to calculate the corresponding value of rl required to satisfy eqn (26) in the absence of specific adsorption.For this latter calculation we will suppose that ion distributions in the double layer conform to the Poisson-Boltzmann equation. Although this procedure is strictly inapplicable to a non-ideal bulk solution it may well give a useful indication of the magnitude of rl involved. Calculation of NTi We suppose that the ionic activity coefficients are given by A d l+d lny = -~ where C denotes the electrolyte molarity and A = 1.172. Let species i be oppositely charged to species 1. If there are only three ionic species 1, i and c in the solution then I and c have the same charge. Eqn (19) now gives However, d6 do. dlnC, = L + N T i L RT RT' 3 = dlnC,-dlnC, RT doi - = dlnCi+dlnC,+2dlny RT where Ci and Cc are effectively equal because C, is very small. From eqn (A 2) and (A 3) we find after some manipulation that for a 0.1 mol dmP3 solution eqn (A 4) gives NTi z 0.56.2240 Potentiometric Titration of Solids Calculation of (a&/arl) By the methods outlined in ref. (18) it is readily shown that a & , - ex-1 XI ex---” where x = leYo/kTI and Yo is the electrostatic potential at the solid surface. The value of x which satisfies eqn (26) when Nil = 0.56 is the solution of (ez- l)/(e”-e-x) = 0.56 and is equal to 0.241. The surface charge which leads to this value of x is given by putting E = 81, E, = 1.11 x loplo C m-2 we find that q = 4.6 x C m-2. This value is quite significant in comparison with values of the order of 4 x lop2 C mp2 which are commonly observed well away from the zero point of charge. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 J. Lyklema, J. Electroanal. Chem., 1972, 37, 53. J. Lyklema, J. Colloid Interface Sci., 1984, 99, 109. B. H. Bijsterbosch and J. Lyklema, Adv. Colloid Sci., 1978, 9, 147. D. G. Hall, J. Chem. SOC., Faraday Trans. 2, 1973, 69, 1391. D. G. Hall, J. Chem. Soc., Faraday Trans. 2, 1978, 74, 405. D. G. Hall, J. Chem. SOC., Faraday Trans. 2, 1972, 68, 25. D. G. Hall, Trans. Faraday SOC., 1971, 67, 2516. D. G. Hall, B. A. Pethica and K. Shinoda, Bull. Chem. SOC. Jpn, 1975, 48, 324. D. G. Hall, J. Chem. SOC., Faraday Trans. 1, 1980, 76, 386. D. G. Hall, H. M. Rendall and A. L. Smith, Croat. Chem. Acta, 1980, 53, 147. D. G. Hall, in preparation. D. G. Hall and H. M. Rendall, J. Chem. SOC., Faraday Trans. 1, 1980, 76, 2575. J. G. Kirkwood and F. P. Buff, J. Chem. Phys., 1951, 19, 774. D. G. Hall, J. Chem. SOC., Faraday Trans. 2, 1978, 74, 1957. D. G. Hall and B. A. Pethica, Proc. R. SOC. London, Ser. A, 1978, 364,457. F. 0. Koenig, J. Phys. Chem., 1937, 41, 597. H. S. Frank, J. Chem. Phys., 1955, 23, 2023. J. Th. G. Overbeek, Prog. Biophys., Biophys. Chem., 1956, 6, 57. Paper 71717; Received 21st April, 1987

ISSN:0300-9599    

DOI:10.1039/F19888402227

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(7), 2241-2245 Localized Excess Electrons in Solubilized Water Clusters in Aerosol OT-n-Heptane Solutions Mahmoud H. Abdel-Kader*t Faculty of Applied Science, Arabian Gulf University, P.O. Box 26671, Manama, Bahrain Peter Krebs Institut f u r Physikalische Chemie und Elektrochemie der Universitat Karlsruhe, Federal Republic of Germany Excess electrons have been produced by flash photoionization of OH- in the water pool of reversed micelles. The optical absorption spectra of these electrons have been investigated as a function of the water cluster size. The results indicate that the shape of the absorption spectra as well as lifetimes of the excess electrons are in partial contradiction of the results reported in the literature. In some recent inve~tigationsl-~ excess electrons were used as a probe to obtain structural information on surfactant [sodium bis(2-ethylhexylsulphosuccinate), aerosol OT, AOTI-solubilized water pools in non-polar solvents (so-called reversed micelles).The number of water molecules in such a pool depends mainly on the molar ratio w = [H,O]/[AOT], and can easily be varied in a wide range. This will result in reversed rriicelles of different sizes having an almost monodispersive size di~tribution.~. Localized electrons2 in the water pools of the reversed micelles can be produced radiolytically or photolytically. The properties of these excess electrons depend strongly on the water content of the reversed micelles. The most salient results of these studies, some of which are controversial, are listed below.(i) With decreasing water content the yield of pulse radiolytically produced excess electrons is reduced." ' 9 ' Below a critical size of the water core (w < 5) localized electrons cannot be observed.'? (ii) For w > 15 the spectrum of the electrons in the water pool was found to be identical with that of hydrated electrons in the bulk water phase.* By lowering w, the maximum of the optical absorption spectrum of the excess electrons in the micellar sollutions becomes hypsochromically shifted compared with the spectrum of hydrated electrons in bulk water.l, ' v 4 Fendler and co-worker~,~ however, did not find any shift, even for very small water pools (w z 5). (iii) The bandwidth of the broad absorption spectrum increases with decreasing size of the water core,4 whereas the results of Wong et a1.l and Fendler and co-workers3 show a dramatic decrease of the bandwidth.The latter result was taken to indicate an ice-like structure of the water molecules in the (iv) The polarity of relatively large water clusters of radius R = 73 A (corresponding to w z 49) is still lower than that of bulk water.l It is not expected that this fact should be without any influence on the spectroscopic properties of the excess electrons, even in somewhat smaller water pools (w 2 15).4 7 Permanent address : Department of Chemistry, Faculty of Science, Tanta University, Tanta, Egypt. $ We use the term localized electrons instead of hydrated electrons since, especially in the small water pool of reversed micelles, where the number of Na+ ions is comparable to the number of H,O molecules in the pool, the electron is not hydrated in the literal sense.224 12242 Soluhilized Water Clusters (v) The lifetime of the electrons produced by pulse radiolysis was observed to be of the order of 100-200 ns1T4 and was strongly reduced by decreasing the water content of the reversed mi~elles.~ In contrast, the lifetime of photolytically produced electrons was found to be of the order of microseconds, increasing with decreasing water pool size.:' In both cases interpretation of the contradictory experimental observation was given. Here we report the results on the absorption spectra of excess electrons in the water pool of reversed micelles produced by laser flash photoionization of tetrabutyl- ammonium hydroxide (TBAOH).This compound is most probably solubilized at the micelle interface, thus leading to the formation of OH- in the water pool. The advantage of using OH- as electron donor in the photoionization processes is that it does not produce any additional transients in the wavelength region where the localized electron absorbs light. Experiment a1 Sodium bis(2-ethylhexylsulphosuccinate) (Fluka 98 %) was purified by column chromatography on activated charcoal as described by Calvo-perez et al.,3 using the mixing volume ratio (1 : 5 ) V/V cycholhexane/methanol. AOT was dried under vacuum at 320 K, n-heptane (Merck, for spectroscopy), tetrabutylammonium hydroxide (Fluka, 98 % ; 40 % in H,O) and KI (Merck, extra pure) were used as received.mol dm-3] in inverse micellar solutions of 0.1 mol dm-3 AOT in n-heptane containing different amounts of triply distilled water (the last distillation step was performed in a quartz apparatus) were prepared under vacuum in a special storage vessel with a 1 em spectrosil quartz cell (Hellma GmbH). These solutions were carefully degassed by several freeze-pumpthaw cycles. The excess electrons in the water pool were produced by photoionization of OH-, using the light pulse of a frequency quadrupled Nd-YAG laser (J. K. Lasers, System 2000, II = 265 nm, pluse duration 15 ns, pulse energy 10-25 mJ) according to (OH-)+hv -+ (HO')+e;& The absorption of e;",, was monitored by a pulsed high-pressure xenon arc lamp, detected by a fast Si-photodiode (modified UDT 600, United Technology), and displayed on a Tektronix 7633 storage oscilloscope.The experimental set-up is described el~ewhere.~ All experiments were carried out at room temperature. KI (4 x mol dm-3) in water and TBAOH [(2-8) x Results and Discussion In order to confirm the reproducibility of our experimental set-up we have measured the spectra of hydrated electrons produced by laser flash photolysis of I- and OH- in pure water. The results reveal that A,,, and the width of the spectrum of e& in bulk water was in agreement with the most reliable spectrum of Michael et al.,u where the electrons were generated by pulse radiolysis. Therefore possible variations of the shape of the electron spectra in inverse micellar solutions can be studied with this experimental set-up.The normalized spectra of localized excess of electrons in AOT-H,O-n-heptane solutions produced by photoionization of TBAOH are shown in fig. 1 for different H,O-AOT concentration ratios w in comparison with the results of other groups. Solutions containing 0.1 rnol dm-3 AOT, 2.22 mol H,O (w = 22.2) and 5 x lop3 mol dmP3 TBAOH are compared with the pulse-radiolytic result of Wong et a1.l for w = 49. Both spectra show approximately the same position of the maxi- mum as the hydrated electron in pure water (hv,,, = 1.72 eV, &,, = 720 nm), but their widths are strongly reduced. This finding is, however, in contradiction to the observations of Pileni et al.,4 who did not observe any decisive deviation from the shape of the hydrated electron spectrum in pure water.M .H. Abdel-Kader and P . Krebs 2243 1.0 - - - W G - - P, 2 - 2 0.5- 2 - - - D .* Y - m - - - - - ' # ' - I I I I I 400 500 600 700 800 900 1000 wavelength/nm Fig. 1. Normalized absorption spectra of excess electrons in large water pools of inverse micellar solutions in comparison with that of the hydrated electron in the bulk water phase (-.-). (-O-) pulse radiolysis: 6% v/v H,O in 3% w/v AOT-heptane solutions (MI = 49).l (-O-) This work: photoionization of 5 x mol dm-3 TBAOH in a solution of 0.1 mol dm-3 AOT and 2.22 mol dm-3 H,O in n-heptane ( w = 22.2). The different points at each wavelength demonstrate the reproducibility of these measurements. The insert shows the decay of the optical absorption with time at R = 700 nm (the small peak at t = 0 is due to scattered laser light).0.0 I ' 1 I I I I 1 1.0 0.0 t t t *$ + * t- * -I 1 1 V I I 1 i I 400 500 600 700 800 900 1000 wavelength/nm Fig. 2. Normalized spectra of electrons in small water cores of reversed micelles. (-F) Pulse radiolysis: 1 Yo v/v H,O in 3 YO w/v AOT-heptane solutions ( w = S).' (. + - ) photoionization of 1 x rnol dm-3 phenothiazine in a solution of 0.25 mol dm-3 H,O and 0.05 mol dm-3 AOT in heptane ( w = 5).3 (-.-) Pulse radiolysis: AOT-H,O-iso-octane solution with w = 5.4 (-O-) This work : photoionization of 5 x lop3 rnol dm-3 TBAOH in a solution of 0.55 mol dm-3 H,O and 0.1 mol dmP3 AOT in n-heptane ( w = 5.5). Fig. 2 compares our results for w = 5 with the results for w = 5-8 from several other groups (some of which used iso-octane instead of n-heptane).The following observations can be made. (1) All spectra shown, except these of Fendler and co-worker~,~ indicate a shift of hmaX to higher energies. One has to take into account that the spectrum published by Fendler and co-workers was observed by laser flash photolysis of phenothiazine in2244 Solubilized Water Clusters inverse micellar solutions. Here the wavelength region of the ' hydrated ' electron spectrum overlaps with the absorption of the phenothiazine cation radical. Subsequently, it was not easy to determine the electron spectrum exactly from these measurements. (ii) Wong et al. determined a shifted spectrum, whose shape has completely lost the typical characteristics of the excess electrons spectrum. (iii) The position of the maximum of the electron spectrum determined by Pileni et al.is almost the same as that of Wong et al. and ours. However, the bandwidth, which is in qualitative agreement with our experimental result, is twice as high as that Wong et al. (iv) It was asserted by different authors1Y4 that the absorption of excess electrons could not be observed in a solution with w < 5. In this case it was assumed that all water molecules participate as part of the solvation shell of the Na+ counterions of the surfactant molecules. In the pulse-radiolytic experiments the disappearance of any absorption was explained by the fact that there are no further water molecules in the pool to trap the electrons originally produced in the non-polar phase of the solution. However, our experiments with w z 1 yielded a broad transient absorption in the wavelength region 500 d L/nm < 700 (lifetime = 2ps).This may be ascribed to the absorption of excess electrons in an "a+-H,O matrix'. This interpretation is also supported by the experimental fact that the observed transient absorption disappears in the presence of typical electron scavengers such as N,O and 0,. In contrast to the results of Pileni et u Z . , ~ our measurements demonstrate that even for w = 22.2 the absorption spectrum of excess electrons in the water pool of reversed micelles is different from that in the bulk water (fig. I). This spectroscopic result is also supported by several independent experiments. Wong et al.' found from their n.m.r. studies that the micellar water is highly structured at least for w 5 8.In this case all water molecules are strongly bonded to the Na+ counterions of AOT. However Bakale e? al." showed (in an AOT-H,O-iso-octane solution) that the attachment of electrons (originally produced by pulse radiolysis in iso-octane) to the water pools does not become diffusion-controlled until w > 30. This implies that the properties of the excess electrons in the water pool should be different from those in the bulk water phase for w < 30. For w = 20 the mean Na+ concentration in the water pool is 2.24 mol dm-3. According to Kreitus et a1.l' only a small shift of the electron absorption maximum to higher energies is to be expected (e.g. for electrons in an aqueous 2 mol dmP3 LiCl solution Ahvmsx 5 0.1 eV with respect to hv,,, = 1.72 eV for the hydrated electrons).Indeed, apart from a decrease in the bandwidth, we have observed no noticeable shift of the spectrum. Therefore, it is concluded that the distribution of the Na+ ions is inhomogeneous and that the hydrated Na+ ions are located near the micelle interface. The rest of the water pool, however, seems to be different from bulk water, thus influencing the width of the broad electron absorption band. As shown in fig. 1, the observed narrowing of the absorption spectra of e;",, can be ascribed to the changes in the properties and organization of the water molecules in the pool which possesses a more ordered environment compared to the bulk water. Such narrowing is also rationalized in terms of increasing the viscosity with decreasing w value^.^ However, with a small water pool (w < 5 ) the bandwidth is broader than :hat observed for w = 22.2, in accordance with the experimental findings of Pileni et ~ 1 .~ This broadening may be attributed to the absorption of excess electrons in an Na+-H,O matrix' exhibiting a broad spectrum, but still narrower than that measured in bulk water. For w > 15 Pileni et al. determined a w-independent lifetime of ca. 230 ns for the localized electrons (they disappear according to a pseudo-first-order reaction). Hence the half-life is at least two orders of magnitude lower than in bulk water.4 Pileni et al. assumed that the electrons were reacting with AOT, because by diminishing w (for w < 15), i.e. by decreasing the distance between the electron and the surfactantM.H. Abdel- Kader and P. Krebs 2245 molecules, the electron lifetime is strongly reduced. However, Fendler and co- workers3 reported that the lifetime increases with decreasing size of the water pool of the inverse micelle. Moreover, the decay time of the excess electrons was found by them to be of the order of microseconds. The latter result is also supported by our own measurements (fig. 1). Even at very low ratios ( w = 1) the lifetime (measured at ;1 = 700 nm) is of the order of 2,us or greater. This would tend to contradict the interpretation of the fast decay of the electrons found by Pileni et aL4 The observed difference in the elo,, lifetime may be attributable to impurities which are reactive with electrons.Recently Pileni and co-workers12 reported the lifetime of e& for w 3 15 as 500 ns. Thus it can be concluded that the elo,, lifetime is very sensitive to the solution conditions and also to the method of production. It is worthwhile mentioning that our results, obtained by photoionization of OH- in aqueous solutions are similar to those reported using the pulse-radiolysis technique.* However, the results in microheterogeneous media seem to be very sensitive to the elo,, production method as well as the solution conditions in which the pH value is notoriously difficult to control. We thank Dr A. M. Braun, EPFL, Lausanne, for pointing out to us that TBAOH is a suitable ‘electron donor’ in the photoionization experiment, and Dr V. Giraud for assistance with the experimental work. Financial support by the Internationales Seminar an der Universitat Karlsruhe and by the Deutsche Forschungsgemeinschaft is acknowledged. References 1 M. Wong, M. Gratzel and J. K. Thomas, Chem. Phys. Lett., 1975, 30, 329. 2 J. K. Thomas, F. Grieser and M. Wong, Ber. Bunsenges. Phys. Chem., 1978, 82, 937. 3 V. Calvo-Perez, G. S. Beddard and J. H. Fendler, J. Phys. Chem., 1981, 85, 2316. 4 M. P. Pileni, B. Hickel, C. Ferradini and J. Puncheault, Chem. fhys. Lett., 1982, 92, 308. 5 M. Zulauf and H. F. Eicke, J. fhys. Chem., 1979, 83, 450. 6 P. D. I. Fletcher and B. H. Robinson, Ber. Bunsenges. fhys. Chem., 1981, 85, 863. 7 St. Jaenicke and P. Krebs, J. Phys. Chem., 1980, 84, 1119. 8 B. D. Michael, E. J. Hart and K. H. Schmidt, J . Phys. Chem., 1971,75, 2798. 9 M. Wong, J. K. Thomas and T. Nowak, J. Am. Chem. SOC., 1977, 99, 4730. 10 G . Bakale, G. Beck and J. K. Thomas, J. Phys. Chem., 1981, 85, 1062. 11 I. V. Kreitus, V. A. Benderskii, A. G . Krevenko and Yu. E. Tiliks, J. Electroanal. Chem., 1982, 133, 345. 12 C. Petit, P. Brochette and M. P. Pileni, J. Phys. Chem., 1986, 90, 6517. Paper 71741 ; Received 23rd April, 1987 74 FAR 1

ISSN:0300-9599    

DOI:10.1039/F19888402241

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(7), 2247-2257 Application of the Volterra Integral Equation to the Mathematical Modelling of Adsorption Kinetics under Constant-volumelVariable-concentration Conditions? Milan KoEifiI J.- Heyrovsky-Institute of Physical Cheyistry and Electrochemistry, Czechos1oz;ak Academy of Sciences, Machoua 7, CSSR-12138 Prague 2, Czechoslovakia Gerhard Tschirch Institute of Informatics and Computing Techniques, Academy of Sciences oj‘ the G.D.R., Rudower Chaussee 5, DDR-1199, Berlin, German Democratic Republic Peter Struve and Martin Bulow* Central Institute qf Phjsical Chemistrjt, Academj) of Sciences of the G.D.R., Ruclower Chaussee 5 , DDR-I 199, Berlin, German Democratic Republic Mathematical solutions of adsorption kinetics on porous solids are usually developed for constant or variable concentration conditions. From both a theoretical and practical point of view the comparison of the kinetic parameters ( e g .intracrystalline diffusion coefficients of microporous adsorption systems) obtained from the various methods is valuable. This paper presents a solution which provides a numerical calculation of the kinetic curves at variable boundary conditions, e.g. constant-volume/ variable-concentration, based on the appropriate analytical solution of Fick’s diffusion law at constant boundary conditions, e.g. constant concentration. The investigation of adsorption kinetics on porous solids is of both theoretical and practical interest. Particularly in the case of ZSM-5-type zeolites the interplay between diffusion and reaction calls for new theoretical and experimental methods of analysis, especially for the measurement of real-time constants of adsorption kinetics and for the analysis of various rate mechanisms.Considerable effort has been made on this subject in several laboratories.’ New methods and experimental techniques8, ’ together with the classical batch-type procedure has led to progress in the investigation of intracrystalline mobility of sorbing species on It now becomes necessary to consider some additional properties of porous crystals, e.g. the anisotropy of microporous systems, and their influence on the molecular transport. Different experimental treatments of zeolite crystals, e.g. hydrothermal and coking conditions, may cause structural inhomogeneities both at the crystal interface and in the intracrystalline volume, which may be detected from the transport behaviour of adsorbed species.Many useful mathematical models to treat such problems are described in literature, but they usually consider only constant boundary conditions. Modelling of these problems for variable boundary conditions (e.g. batch arrangements) demands an additional parameter, which gives rise to much greater computational effort. This paper deals with a universal method of obtaining theoretical adsorption uptake curves under variable boundary conditions where the analytical solution of the Comparison of the modelling in the present paper with experimental data will be made in a paper yet to be submitted to Faraday Transactions.2247 74-22248 The Volterra Integral Equation corresponding problem at constant boundary condition has already been given. The procedure will cover isothermal adsorption kinetics in zeolite crystals in this instance. This work has been stimulated by the work of March and Weaver,13 who have given the analytical solution of one particular diffusion problem under variable boundary conditions by means of an appropriate integral equation. Although our approach is based on the same superposition principle, it differs from the above in several important ways. After formulating the general integral equation, application to the adsorption kinetics under consideration proceeds straightforwardly, following the theoretical and computational treatments1*-17 developed at the Institute of Informatics and Computing Techniques of the Academy of Sciences of the G.D.R., Berlin. Algorithms and programs developed by Micke and Tschirch are adapted to the relevant types of (weakly singular) Volterra equations.Development of the Model The assumptions in the model statement are as follows. (i) The mass balance for the batch arrangement to measure adsorption kinetics is given by Vp dp d a RT, dt dt -- + K - = O where p is the pressure of sorbing species in the gas phase of the volume %, T/s is the volume of zeolite crystals, T, is the temperature of the sample, R is the gas constant and a is the sorbate concentration averaged over the crystal, with the radius vector r, given a(r) dr by a= I,, (2) K (ii) Sorption equilibrium between the sorbing species in the gas phase and on the surface of the crystals is established instantaneously in accordance with the adsorption isotherm via the equation as = f ( P ) * (3) (iii) The initial conditions of the sorption experiment in a batch arrangement are as follows : for t + 0- I P = P(O-) a, = a,(O-) = a(0) for t --f 0,.I P = P(0,) a, = a,(O+) a(0,) = a(O-) = a(0) (4) The indices (-) and (+) at t = 0 denote the time regimes prior to and after the beginning of an experiment, respectively, i.e. prior to and after the gas expansion from a doser volume into a sorption vessel. (iv) The adsorption system, i.e. zeolite crystals, behaves linearly with respect to the disturbances of the concentration as and, therefore, in accordance with the superposition principle,l8> l9 one can write : where H(t) is the change of the molecular uptake y,(t) for the unit step change of a,, a( t ) - $0) a( co) - a(0) r d t ) = - (7)M.Koc'irik et al. 2249 and * indicates the operation of convolution of two time-dependent functions via the general relation : g(t) * h(t) = g(t - t ) h(z) dt; t 3 0. (8) (9) s: Integrating eqn (1) in the limits from 0, to t one obtains [ P W -P(O+)I + a,[a(t> - a,(O-)l = 0 with After substituting eqn (6) into eqn (9) and after some rearrangements one can write: with and The meaning of the parameter Sp is elucidated in fig. 1. The function @(t) is given by the equation For any given type of adsorption isotherm the function @(t) can be expressed on the basis of eqn (12) and (14) as a composite function of t depending on the values of two parameters ~ ( 0 , ) and A or p ( .o ) , respectively: Very often the function f ( p ) can be expressed by a Langmuir-type adsorption isotherm BP a = A - l + B p ' with constants A and B : Then after some rearrangements, eqn (1 1) can be written as: with and LJsing the nomenclature and denoting the integrand in eqn (17) as2250 The Volterra Integral Equation p(0-1 p(-) P (O+) p(t1 Fig. 1. Slope of the isotherm secant. one can recognize in eqn (1 7) a non-linear Volterra integral equation of the second kind, which is usually written as : I s , l7 with If the adsorption kinetics are measured stepwise the above description of the adsorption equilibrium can be made in a quite formal way for each pressure step with suitable constants A and B of the Langmuir isotherm equation.In this paper we restrict our considerations to the case that for the given pressure step so < s < s,. it holds that Thus w + 1 and eqn (1 7) reduces to [&@+)I 1 and il will be (- ABa,) [see eqn (IS)]. A similar equation to eqn (25) can be derived for yq(t): These are the linear ‘Volterra’ integral equations of the second kind for the functions y,(t) and y,(t), respectively. The usual form for this kind of equation is: The expression J s,, ?H(S- T ) c?T K(S, T ) = - represents the kernel of the integral equation, having the following properties for all problems of isothermal intracrystalline kinetics on zeolites : c3Ha(S- T) i3T - 3 0 for ( S - T ) > 0 ?H,(S- T ) (7T - = 0 for ( S - T ) < 0.M.Koc'iri'k et al. 225 1 Eqn (29) and (30) represent a mathematical formulation of the principle of causality, which claims that an impulse at an instant Tmay cause an effect in the future only. The kernels with the properties expressed by means of eqn (29) and (30) are called the Volterra kernels.20 The quantity A expresses the parameter of the integral equation. From eqn (13) it follows that for all adsorption kinetic problems A < 0. It is worth noting that both the right-hand side of the integral equation (27) in the form of eqn (23) and the kernel are expressed in terms of the same function H(S). According to the behaviour of the kernel at the diagonal of the region S 3 0, T 3 0, the adsorption kinetic problems can be classified into two groups as follows: (i) those with bound kernels, i.e.K(S, T ) < M for S+ T (referring to all mechanisms represented by a finite number of first-order kinetic steps); (ii) those with unbound kernels, e.g. all problems of diffusion into finite bodies with a single diffusional step. The function H ( S ) can often be expressed as: where A k , l , l ? > 0 and ak,I,m > 0 are constants with respect to S, depending on the summation indices k, I and m. For case (ii), the kernel K(S, T ) can be given as 5 3 2 % K(s, T , = c 2 2 a k , I , n~ 'k, 1 , m exp [-ak. 1 , m(s- (32) k=O 1=0 m=O which is evidently unbound at S+T. However, it can be shown that for all such diffusion problems there exists a number M , where and the quality is valid for S+ T. This behaviour is the consequence of the so-called 'square-root time law' of M is given by the expression where Sa and denote, respectively, the permeable part of the surface and the volume of the adsorbent body, while R, represents the characteristic dimension of the latter which enters the dimensionless time parameter with the diffusion coefficient D.Kernels for which the condition K(S, T ) = ~ * i < l (S- T)i' is fulfilled are called weakly singular. Computational Details To solve the Volterra equation [eqn (27)] numerically, the block-by-block method was used.17 Thus, the solution y ( S ) is computed in steps according to the following scheme : y ( S ) = yi(S), SE[S,,S,,J; i = l(1)N (3 7?2252 The Volterra Integral Equation where Si with i = 1 (1) N are the grid points in the interval [0, K ] and y,(S) and g,(S) are given as follows: (38) In this paper the algorithm developed by Micke14 is applied, where in every step on the subinterval [S,, Si+J the Volterra equation is treated as a Fredholm one.The algor- ithm realizes an adapted quadrature rule to guarantee an order of convergence O( 1 /nf), where n, is a number of knots per subinterval. Moreover, the algorithm contains a multi-grid method with error estimation and an automatic adjustment of the number of knots. It has been shown in the present paper that by using the so-called subtraction method, weakly singular kernel functions can be treated : where I i s a unit operator. To compute the right-hand side of eqn (40), g,(S), special procedures have been developed to guarantee the required accuracy. The algorithm includes an automatic step-size control for the interval length, which is described in detail in ref.(15)-(17).y Several problems solved by the methods mentioned above have been published by Tschirch. l7 Results The possibility of applying the Volterra equation to the analysis of adsorption kinetic curves under constant-volume/variable-concentration conditions has been proved for the following typical cases: (i) without singularity and with a known analytical solution, (ii) with a weak singularity and with a known analytical solution; (iii) with a weak singularity and without a known analytical solution. These cases will be considered separately . (i) Molecular Transport across a Crystal Interface with Resistance analogous to that of Surface Evaporation2s26 The function H ( S ) is given by H ( S ) = 1-exp(-S) (41) where S is given by and the kernel is K(S, T ) = exp [ - ( S - T ) ] 2 0 for S > T.(43) Eqn (42) is valid for an assembly of spherical crystals with a uniform radius R and a surface resistance parameter k,. Evidently, for S --f T it follows that K(S, T ) -+ 1 . This kernel is shown in fig. 2. Note that in this case (as with any kernel of the convolution type) the function K(S, T) requires a constant value along any line parallel to the diagonal of the region S 3 0, T b 0. The numerical solution of this integral equation for t The computational software cited [ref. (15)-(17)] can be obtained from the Institute of Informatics and Computing Techniques, Academy of Sciences of the G.D.R., Berlin-Adlershof, Rudower Chaussee 5 , DDR- 1 199.M.KotifiR et al. 2253 (0.0) Fig. 2. Kernel, K(S, T ) = exp [ - ( S - T)], of a Volterra equation. different values of A (full lines) together with the corresponding analytical solution (the points) is presented in fig. 3. The analytical solution of the problem is given by y, = 1 -exp[(l- 1) S ] . (44) (ii) Diffusion into a Crystal with a Plane Geometry In this case the function H ( S ) is given by (46) tD S = - L2 where and 2 L is the thickness of the plate (cf. ref. (23), p. 45). The form of the kernels which refer to all problems treated in this paper is similar to that shown in fig. 2. The only essential difference is their asymptotic behaviour in the vicinity of the diagonal. In fig. 4 the numerical solution of the integral equation is presented for several values o f 1 (full lines).The points in this figure were taken from the analytical solution of this problem given by Crank [cf. ref. (23), p. 551. The values of the parameter Iz are chosen in such a way that they correspond to the percentage x of the total amount taken up by the sheet (given in table 1). The parameter Iz is defined by eqn (13). For calculating A in comparison with the uptake curves given in ref. (23), we applied the equation l = x/(x- 100) (47) where X 1 100 - 1 +a2254 The Volterra Integral Equation 1 .o 2.0 3.0 4.0 T Fig. 3. Numerical solution of the integral equation for the case of transport across the crystal interface for various values of A corresponding to different amounts of solute taken up finally (lines); the points represent the analytical solution (1, -0.01, 0.99%; 2, -0.5, 33.33 YO; 3, - 1.5, 60%; 4, - 10.0, 90.91 YO of total solute finally taken up).1 .o 2 .o 3.0 4 .O 5 .O T Fig. 4. Numerical solution of the integral equation for the case of transport into a plane sheet (isotropic intracrystalline diffusion) for different amounts taken up finally (full lines) ; the points in the curves 2-5 represent the analytical solution, given in ref. (23) (p. 55, ~6 table 1). and a is a partition parameter, defined in ref. (23). In the case of a strictly linear isotherm one can conclude from eqn (13) that 2 is given by (iii) Anisotropic Diffusion into an Orthorhornbic Crystal approximated by an Infinite Parallelepiped An orthorhombic crystal is shown schematically in fig.5. If L, > &,La and D, < Do, D,, the approximation of the crystal by an infinite anisotropic parallelepiped seems to be reasonable.M. KoEifik et al. 2255 Table 1. 2 for the uptake by a plane sheet from a stirred solution of limited volume [the total amounts finally taken up by the sheet are given as percentages in ref. (23)] number of total amounts curve in 2 taken up, x fig. 4 -0.1 9.09 1" - 0.42857 30.00 2 - 1.0 50.00 3 - 2.3333 70.00 4 - 9.0 90.00 5 " Curve 1 has not been given in ref. (23). Fig. 5. Scheme of an orthorhombic crystal with the principal axes a, 6, c. If the principal axes of the tensor D of the diffusion coefficients are parallel to the axes a, b, c and if the concentration at the crystal interface is constant, the solution of the problem of anisotropic diffusion is given by the following equation :27 where and DL,m = kE2Otu+kk k , = (71/2)(2/+ 1) k , = ( ~ / 2 ) ( 2 r n + 1) $ = - A bu with D, and D, as the components of the tensor D.S is given by the expression (54) where t is the real-time coordinate. Fig. 6 shows the numerical solution (full lines) of this problem for several values of 3, and the special case characterized by O,, = 1, which likewise represents the isotropic diffusion into an infinite cylinder with circular cross-section. The dotted lines represent the values of the corresponding analytical solution for an infinite cylinder given in ref. (23) (c$ pp. 70-72) for 70, 50 and 30 % of total solute finally taken up by the cylinder2256 The Volterra Integral Equation T Fig.6. Numerical solution of the integral equation for the case of anisotropic diffusion into an infinite parallelepiped for different amounts taken up finally (full lines); special case: O,, = 1, isotropic diffusion. The dotted lines with curves 2 4 represent the analytical solution of the problem for the parallelepiped approximated by an infinite cylinder of radius equal to L, [given in ref. (23), p. 721. from a stirred solution with the initially uniform concentration C,. The values of i, have been calculated in the same way as in the case (ii): 1, -0.1 ; 2, -0.428 57; 3, - 1 .O; 4, Conclusions - 2.333 33. With the computational software developed at the Institute of Informatics and Computing Techniques of the Academy of Sciences of the G.D.R., it becomes possible to model various problems of adsorption kinetics at constant-volume/variable- concentration.The necessary conditions of the application of this procedure are: (i) the system is isothermal, (ii) it behaves linearly, (iii) the corresponding solution for the constant concentration conditions is known. The extension of this treatment to various cases of real adsorbents with a known distribution with respect to the size and habit of adsorbent particles is straightforward. Furthermore, it should be possible to extend the method to non-isothermal problems and to the problems with non-linear boundary conditions. The authors thank Dr A. Micke (Berlin) and Dr S. Pick (Prague) for support and valuable discussions. References 1 D.M. Ruthven, Principles of Adsorption and Adsorption Processes (Wiley, New York, 1984), 2 J. Karger, H. Pfeifer and W. Heink, in Proc. 6th Int. Conf. Zeolites, Reno 1983, ed. D. Olson and A. Bisio (Butterworths, Guildford, 1984), p. 184, 3 M. M. Dubinin, V. A. Gorlov and A. M. VoloStuk, in Proc. 5th Int. Con$ Zeolites, Naples 1980, ed. L. V. C. Rees (Heyden, London, 1980), p. 554. 4 R. M. Barrer, in Zeolites, Science and Technology, NATO AS1 Series, Ser. E., Appl. Sci., No. 80, ed. F. R. Ribeiro, A. E. Rodrigues, C. D. Rollmann and C. Naccache (Martinus Nijhoff, The Hague, 1984), p. 261. 5 L. V. C. Rees, in Structure and Reactivity of ModiJied Zeolites, ed. P. A. Jacobs, N. I. Jaeger, P. JirG, V. B. Kazansky and G. Schulz-Ekloff (Elsevier, Amsterdam, 1984), p. 1. pp.166-205.M. Koc'iri?c et al. 2257 6 M. Bulow, W. Mietk, P. Struve, M. KoCiiik and J. Karger, in ref. (2), p. 242. 7 Y. Yasuda, in Proc. 7th Int. Con$ Zeolites, Tokyo 1986, ed. Y. Murakami, A. Iijima and J. W. Ward 8 P. Struve, M. KoEiiik, M. Bulow, A. Zikanovri and A. G. Bezus, Z. Phys. Chem. (Leipzig), 1983, 264, 9 M. Bulow, W. Mietk, P. Struve and P. Lorenz, J . Chem. Soc.. Furaday Trans. I, 1983, 79, 2457. 10 M. Bulow, P. Struve, W. Mietk and M. KoCiiik, J . Chem. Soc., Faraduy Trans. I , 1984, 80, 813. 11 M. Bulow, P. Struve and W. Mietk, Z . Phys. Chem. (Leipzig), 1986, 267, 613. 12 A. Zikanova, P. Struve, M. Bulow, A. G. Bezus, M. KoEiEik and S. P. Zdanov, in Proc. 7th CHISA- 13 H. March and W. Weaver, Phys. Rec., 1928, 31, 1081. 14 A. Micke, Z.Ang. Math. Mech., 1987, 67, 37. 15 G. Lippold and T. Reiher, Report: ZfR-Information 1981, ZfR 80.09, Academy of Sciences of the G.D.R., Berlin, Programmpaket zur Losung von Anfangswertaufgaben fur Systeme expliziter gewohnlicher Differentialgleichungen. 16 M. Grabow and A. Micke, Report: ZfR-Information 1982, ZfR 82.09, Academy of Sciences of the G.D.R., Berlin, Programme zur Losung von Systemen hearer Fredholmscher Integralgleichungen zweiter Art. 17 G. Tschirch, Scientific Report 1985, Institute of Informatics and Computing Techniques, Academy of Sciences of the G.D.R., Berlin, Losung hearer Volterra-Integralgleichungen zweiter Art durch eine adaptive Blockmethode mit automatischer Schrittweitensteuerung. (Elsevier-Kodansha, Amsterdam-Tokyo, 1986), p. 587. 49. Conf., Prague 198 1, Progress in Measurement of Intracrystalline Sorption Kinetics on Zeolites. 18 J. Thewlis, Encyclopuedic Dictionary qf Physics (Pergamon Press, Oxford, 1962), vol. 7, p. 116. 19 R. Courant and D. Hilbert, Methoden der Muthematischen Physik, Teil IZ (Springer-Verlag, Berlin, 20 C. T. H. Baker, The Numerical Treatment of Integral Equations (Clarendon Press, Oxford, 1977). 21 R. M. Barrer and D. A. Ibbitson, Trans. Furaday Soc., 1944, 40, 206. 22 R. M. Barrer and M. Riley, J . Chem. Soc., 1948, 133. 23 J. Crank, The Mathematics of DgfSusion (Clarendon Press, Oxford, 1964), pp. 56, 73, 91. 24 N. N. Tunicki, V. A. Kaminski and S. F. TimaSev, Metody Fiziko-chimiteskoj Kinetiki (Chimija, 25 J. Karger and P. Herrmann, Ann. Phys., 1974, 31, 277. 26 M. Bulow, Z . Chem., 1985, 25, 81. 27 M. KoCiiik, G. Tschirch, J. Caro, P. Struve and M. Bulow, in preparation. 1937), p. 180. Moskva, 1972), p. 70. Paper 71908; Received 20th Muy, 1987

ISSN:0300-9599    

DOI:10.1039/F19888402247

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(7), 2259-2265 A New Conducting Polymer-coated Glucose Sensor Prem C. Pandey Department of Chemistry, Banaras Hindu University, Varanasi-221 005, India A sensor for glucose formed in a one-step process is described based on a new doped polyindole film coated onto a platinum surface; a polymer- entrapped glucose oxidase electrode can be operated as an amperometric glucose sensor. The new glucose sensor has fast response time (25-40 s) with high storage and operational stability (> 35 days). Electrochemical rate constants for the new enzyme electrode have been calculated using the expressions derived by Albery et al. (J. Chern. Soc., Faraday Trans. I , 1986, 82, 1033). It has been found that unsaturated enzyme kinetics are rate- limiting for conducting polymer-coated enzyme electrodes formed in a one- step process.The design of reliable, low-cost chemical sensors with high storage and operational stability is difficult. Enzyme electrodes with high sensitivity are very useful as analytical tools, since they provide simple and rapid assays or traces of substrates. An enzyme electrode is usually an amperometric sensor with which the rate of an enzyme-catalysed reaction is measured electrochemically. The first generation of enzyme electrodes (glucose sensors) was developed by coating an immobilized enzyme in a membrane matrix onto the electrode. A disadvantage in this case was the 0, dependence of the enzymic reaction. The second generation of enzyme electrodes was developed1 using electron-transfer mediators, while the third generation was developed2 using organic salts (NMP+ TCNQ- etc.) as electrode materials.In this case there was direct transfer of electrons to and from the enzyme. In the most recent type of enzyme electrode3 the enzyme is directly incorporated into the conducting electrode materials and is thus capable of producing an enzyme electrode in a one-step process. My own work has been concerned with the development of enzyme electrodes in a one-step process using polyindole as a new conducting electrode material. Recently a~etylene,~ pyrrole,'. ' thi~phene,~ phenylene' and phenylene sulphide' have been shown to be conductive electrode materials. The electrode response depends on the doping and conductance properties of the organic redox polymer film deposited onto the platinum surface.In the present investigation doped polyindole was prepared as a new conducting electrode material : it has a rapid sensitivity towards ammonia and other toxic gases. However, in this paper we only describe the application of polyindole as a conducting electrode material for glucose sensors. The sensor was developed by incorporating glucose oxidase in the polyindole film directly onto the platinum surface and thus producing the enzyme electrode in a one-step process. Experiment a1 Preparation of Enzyme Electrode Platinum foil ( 5 mm x 4 mm) was fixed onto a Perplex glass surface. The electrochemical preparation of the polyindole film on the platinum foil was carried out by the oxidation of 20 mmol dm-3 indole solution at 1.4 V (us. Ag/AgCl).The indole monomer was dissolved in an acetonitrile solution containing 100 mmol dmF3 tetraethylammonium 22592260 Polymer-coated Glucose Sensor percholorate. The average time for the synthesis was 1 h. Enzyme immobilization was performed by adding 0.15 pmol dm-3 type VII Aspergillus niger (Sigma Chemical Co.) to the indole solution prior to polymerization. All solutions were bubbled with nitrogen for 30 min prior to use and were maintained at 25 "C with a water thermostat (type U10 MLW Mechanick prufgerate, Medingen Sitz Freital DDR). The enzyme electrode prepared in this manner was thoroughly washed with phosphate buffer pH 7.0 and stored in 100 mmol dmP3 phosphate buffer at 4 "C when not in use. Operation of Enzyme Electrode as Amperometric Glucose Sensors Steady-state current measurements were made using enzyme electrode as working electrode, Ag/AgCl reference electrode and platinum counter-electrode. The cell system (working volume 10 cm3) was kept in an Ultra thermostat maintained at 25 "C and constant potential 1.6 V (us.Ag/AgCl) to the cell was supplied by POS73 Wenking potentiostat and current was measured as a function of time by Houston 2000 xy recorder. The potential supplied to the system had been predetermined by linear-sweep voltammetry as being in the diffusion-controlled plateau region for hydrogen peroxide oxidation on the polyindole working electrode at a scan rate of 200 mV s-l. All the solutions were bubbled with nitrogen for 30 min prior to use. After application of an appropriate potential to the working electrode, the background current was allowed to decay to a steady state.Aliquots of stock glucose were added, and after brief stirring (2 s) by means of a magnetic stirrer, the current in quiescent solution was recorded. The time required for the current to achieve 95 YO of the steady-state value, as measured from the steady-state background value, was donated as response time. Results and Discussion Fig. 1 shows the response curve for glucose using a polyindole/glucose oxidase electrode. The steady-state responses at 1.6 V (us. Ag/AgCl) in the polyindole- immobilized enzyme electrode were employed to construct a response curve for glucose. A fast response time (25-40 s), high storage and operational stability (> 35 days) have been recorded for the new glucose sensor.The typical response time increases from 25 to 40 s for the glucose concentration range 1-100 mmol dm-3 in 100 mmol dmP3 phosphate buffer at pH 7.0. The response time in 10 mmol dm-3 phosphate buffer at pH 6.5 was slightly longer ( 3 W 5 s). Since the electrode responses were kinetic, the apparent Michaelis-Menten constant ( k k ) has been calculated for the immobilized enzyme by an amperometric method as suggested by Shu and W i l ~ o n . ~ Fig. 2 shows a curve for the determination of the Michaelis constant. The maximum response for this electrode was 9.7 pA, with kk = 27.3 mmol dm-3. The latter value is of the same order of magnitude as that for soluble glucose oxidase from Aspergillus niger.l0 We have also investigated the storage and operational stability of the electrode under defined storage conditions in a phosphate buffer at pH 7.0 at 4 "C.The data plotted in fig. 3 show that the useful life of the electrode is > 35 days. The results in fig. 1 are analysed by the determination of the electrochemical kinetic terms, kdE, KME and kiatE. kdE represents the effective electrochemical rate constant for unsaturated enzyme kinetics, KME is the rate constant derived from three rate constants and kiatE describes flux per unit area. The expressions for the determination of these kinetic terms were derived by Albery et al." for organic salts, i.e. NMP+TCNQ-, as conducting electrode materials. I have calculated the values of these kinetic terms for two conducting polymer electrode materials, i.e.polypyrrole and polyindole. The experimental data for the calculation of these terms in the case of a polypyrrole/GOD (glucose oxidase) electrode were taken from a previous p~blication.~ In order to compensate for the different areas ( A ) of the electrodes, we have plotted the steady-stateP. C. Pandey 3 .O 2.5 4 2 2.0 Q M 9 5 2 1.5 2 2 \ - W 1.a 0.5 L -0.1 226 1 1 1 1 1 I 0 0.2 0.4 0.6 0.8 1.0 0 20 40 60 80 100 [glucose]/mmol dm-3 Fig. 1. Response curve for glucose using a polyindole/GOD electrode. Steady-state currents measured at 1.6 V (us. Ag/AgCl) in 100 mmol dm-3 phosphate buffer, pH 7.0.2262 Polymer-coated Glucose Sensor g*O* 8.0 4.0 I I I 1 I 1 1 0 5 10 15 20 25 30 35 tiday Fig. 3. Stability of the polyindole/GOD electrode on storage in 100 mmol dmV3 phosphate buffer, pH 7.0 at 4 "C.50t y 40 4 E 20 10 c 0 20 40 60 80 100 [glucose]/mmol dm-3 Fig. 4. Variation of current densities with a concentration of glucose electrodes made of two different conducting polymers for (0) polypyrrole and (0) polyindole. current change against substrate concentration in terms of current densities against substrate concentration for polypyrrole/GOD and polyindole/GOD electrodes. These plots are shown in fig. 4. The first stage of the analysis of the data given in fig. 4 is to find VME by making Hanes plot of [glucose]/j= 2AF [glucose]/i against [glucose] as described by Albery et a1.l' These plots are shown in fig. 5. The intercept I, of the Hanes plot at zero concentration determines kkE, where l/kaE = K,/(e, Lk,,,) + 1 /k's = I = (sn;/j),, the expressions for k,,, and Ksf/kc,at have been given by Albery and Knowles.12 k' s is the mass-transfer rate constant for the substrate, e, is the total concentration of the enzyme, L is the thickness of the polymer film and s, is the substrate concentration outside theP.C. Pandey 70 60- 2263 - 9 / / / / / / / Table 1. Results for polymer-entrapped enzyme electrodes electrode material k’,,/10-8 m s-’ Kh,,/mmol dm-3 k:,,,,/cm s-’ pol ypyrrole polyindole 6.7 9.1 32 29 1.6 1.5 polymer film. Values of ka for polypyrrole/GOD and polyindole/GOD electrodes are recorded in table 1. It may be concluded from the expressions of Albery et a1.l1 that for coricentrations below K,, the system becomes unsaturated, and above KME the system becomes increasingly saturated until at [substrate] 9 KbfE saturation is complete.A further analysis of the results in fig. 4 yields values of KME. This can be done provided values of the parameters p and y are known. p may be estimated using the following relation : p = Ij/[glucose] and p - l - 1 - 1 Pk;, E ’ = [glucose] - .___ K~~ (1 -i). Plots of y against p are shown in fig. 6 for polypyrrole and polyindole/GOD electrodes. In each case a horizontal straight line is obtained. This means that k’s % kaE, and thus the unsaturated enzyme kinetics are rate-limiting. From the plots the values of KME can be determined from the intercepts at p = 0. These values are recorded in table 1. The above conclusion can again be confirmed from the plots given in fig.7, since ka E/k’s < 1. Lastly, we have calculated the values of k:.,,E from the expression given by Albery et a1.l’ These values are given in table 1. The two conducting materials in table 1 are2264 Polymer-coated Glucose Sensor 40 0 0.2 0.4 0.6 0.8 1.0 P Fig. 6. Plots of the parameters y against p for the measurement of glucose using conducting polymers, (a) polypyrrole and (0) polyindole. 1 2 3 4 s, IK*E Fig. 7. Plots of the parameter @') against s,/K,, for two conducting polymers, (a) polypyrrole and (0) polyindole. indeed good electrocatalysts for the direct oxidation of glucose oxidase, since these values are greater than unity, thereby showing excellent transport of the flux per unit area. I thank a referee for constructive criticisms.I am also grateful to the Indian Council of Scientific and Industrial Research for financial assistance, and to Prof. R. P. Rastogi for helpful discussions. References 1 A. E. G. Gass, G. Davies, G. D. Francis, H. A. 0. Hill, W. J. Aston, I . J. Higgins, E. V. Plotkin, L. D. L. Scott and A. P. F. Turner, Anal. Chem., 1984, 56, 667.P. C. Pandey 2265 2 W. J. Albery and P. N. Bartlett, J. Electroanal. Chem., 1985, 194, 21 1. 3 Nicola, C. Foulds and Christopher R. Lowe, J . Chem. SOC., Furaday Trans. I , 1986, 82, 1259. 4 C. K. Chang, M. A. Druy, S. C. Gau, A. J. Heeger, E. J. Louis, A. G . MacDiarmid, Y. W. Park and 5 K. K. Kanazawa, A. F. Diaz, R. H. Geiss, W. D. Gill, J. F. Kwok, J. A. Logan, J. F. Rabot and 6 P. C. Pandey and A. P. Mishra, Analyst (London), 1988, 113, 329. 7 T. Yamatoto, K. Sanechica and A. Yamatoto, J. Polym. Sci., Polym. Lett. Ed., 1980, 18, 9. 8 Jan J. Miasik, Alan Hooper and Bruce C. Tofield, J. Chem. SOC., Faraduy Trans. I , 1986, 82, 1 1 17. 9 F. R. Shu and G. S. Wilson, Anal. Chem., 1976, 48, 1679. H. J. Shiraka, J. Am. Chem. SOC., 1978, 100, 1013. G. B. Street, J . Chem. Soc., Chem. Commun., 1974, 854. 10 B. E. P. Swoboda and V. Massey, J. Bid. Chem., 1965, 240, 2209. 11 W. J. Albery, P. N. Bartlett, A. E. G. Gass, D. H. Craston and B. G. D. Haggett, J. Chem. Soc., 12 W. J. Albery and J. R. Knowles, Biochemistry, 1976, 15, 5631. Faraday Trans. I , 1986, 82, 1033. Paper 7/990; Received 5th June, 1987

ISSN:0300-9599    

DOI:10.1039/F19888402259

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

<J, Chem. SOC., Faraday Trans. I , 1988, 84(7), 2267-2277 Effect of Hydrogen in the Chemisorption of n-Hexane over Platinum Black Antal Sarkany Institute of Isotopes of the Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 77, Hungary The chemisorption and transformation of n-hexane have been investigated over Pt black in the presence of hydrogen between 450 and 600 K. Direct gravimetric measurements have confirmed that only 3-8% of the total hydrocarbon coverage can be removed by evacuation or by purging the system with an inert gas. Hydrogenation experiments have been used to separate reactive and irreversibly bound hydrocarbons ; their coverages have been measured as a function of temperature and partial pressure of hydrogen. The variation of the product selectivity with the partial pressure of hydrogen has been interpreted considering the effect of the irreversibly bound hydrocarbons on ensemble size and on availability of hydrogen.While in the transformation of alkanes the reactivity and selectivity of various types of Pt catalysts, including films,’ blacks,2 supported and single crystal~,~-~ have been systematically investigated, less attention has been directed towards the surface state of the working catalyst^.^. lo High-temperature (T > 623 K) reforming studies have unambiguously confirmed that the carbonaceous deposits form a component of the catalytic In the low-temperature reactions, however, direct data for the hydrocarbon coverage are scarce.8-10 Considering the negative reaction order with respect to hydrogen it has been generally assumed that the surface coverage of the hydrocarbon in a large excess of hydrogen should be negligible.l4,l5 The positive hydrogen order and the deceleration of the reaction rate at low hydrogen/hydrocarbon ratios point on the other hand to the formation of trapped hydrocarbons.l6-ls Recent studies by Davis et al.8.9 seem to prove that Pt single crystals even at carbon coverages C/Pts = 2-3, where Pts is the number of surface sites, still exhibit remarkable catalytic activity in the transformation of n-hexane.However, the results of Davis et aL8*’ are a.t variance with the suggestions that the chemisorption of alkanes requires a large number of contiguous Pt or Pt-H sites.14*15 The toxicity of carbon atoms1’ observed in hydrocarbon transformations over supported Pt catalysts also cannot be reconciled with the results measured on single crystals.8.The temperature-independent initial carbon coverage reported by Luck et a1.l’ does not agree with experimental results observed in other laborat~ries.~~ l3 The obvious discrepancies in the literature concerning activity and carbon coverage have directed our attention to a study of the nature of hydrocarbon transformations on working Pt catalysts. The work presented here is aimed at an investigation of the chemisorption of n-hexane in H, over Pt black using a microbalance, since in the case of gravimetric detection the analytical procedure does not influence the hydrocarbon coverage. Systematic studies have shown that the variation of the partial pressure of hydrogen significantly affects the product selectivity of the individual reaction routes.2 It was therefore decided to pay special attention to the effect of the partial pressure of hydrogen on the coverage of the chemisorbed species formed from n-hexanes.In particular, attempts have been made to answer the following questions. (i) What are the total and reversible hydrocarbon coverages in the presence of H, at low temperature 22672268 Chemisorption of n-Hexane on Pt Black (450 < T/K < 600) for the interaction of n-hexane with Pt? (ii) What is the coverage of the reversibly bound hydrocarbons which are removed by evacuation and therefore not detected in microreactor studies? Experimental In order to avoid support effects the experiments were carried out with Pt black.The preparation of the black has been described in earlier paper^.^'.'^ In the experiments 0.265 g Pt black was used. As the black reduced by formaldehyde contains some carbon, the sample was repeatedly oxidized at 623 K in 1.33 kPa 0, and reduced in H,. After a few 0,-H, runs the amount of carbon measured by temperature-programmed oxidation (t.p.0.) was < 0.005 cm3 (s.t.p.) CO, g;:. The number of surface sites, Pt’, inferred from 0,-H, titrations was 1.25 x lo1’ g;:. All the gases were carefully cleaned: H, and H,-He mixtures taken from cylinders were purified and dried over Pd/A1,03 and molecular-sieve contacts. He and Ar were passed through MnO,/Al,O, and molecular-sieve columns. n-Hexane was purified by preparative gas chromatography.No impurities could be detected in the n-hexane used in the kinetic studies. In the gravimetric measurements the purity of n-hexane was 99.96 %, the impurities were 3-methylpentane and traces of 2-methylpentane. The gravimetric measurements were performed by means of a Sartorius microbalance. The ‘ hangdown ’ tubes and the upper part of the balance were separated by orifices and the upper part was purged continuously with H, (0.5 cm3 min-l). The hydrogen and the H,-He-n-hexane mixture entered at the bottom of the hangdown tubes, moved upwards and left the tubes at side-ports. Prior to the measurement of n-hexane chemisorption the Pt black was oxidized (1.33 kPa 0,) and reduced in a stream of H, at 573 K. As soon as a stable baseline was reached in the H, or H,-He stream at the temperature of the experiment the carrier gas was passed through a saturator filled with n-hexane, and the n-hexane-H,-He stream (flow rate 6 1 5 cm3 min-l) was introduced into the micro- balance.The experimental set-up used for the gravimetric measurements is not suitable for determining the rate of n-hexane transformation because only part of the reaction mixture ‘meets ’ the catalyst surface. For this reason additional experiments were performed in a microreactor working under differential conditions. The ‘free’ Pt sites (surface sites which are capable c.f chemisorbing CO or H, and 0, after hydrocarbon reaction)lg were measured at 298 K. In the microreactor studies the amount of surface carbon was inferred from temperature-programmed oxidation.l3 Results and Discussion Typical gravimetric measurements are collected in fig. 1-3. Fig. 1 shows the temperature dependence, while in fig. 2 and 3 the effect of hydrogen partial pressure on the formation of the chemisorbed species are presented. Further data on the n-hexane/Pts ratio are summarized in table 1, while tables 2 and 3 relate to the results of the microreactor experiments. The gravimetric measurements in fig. 1-3 show that the concentration of the chemisorbed species reaches a steady state within a few minutes. Because of back-mixing the concentration profile of n-hexane in the H,-He stream deviates from a stepwise function. The shape of the adsorption curve, as confirmed by a mass-spectrometric analysis in the vicinity of the Pt black, follows the concentration profile of n-hexane (nH). This observation suggests that the build-up of hydrocarbon coverage is fast on Pt sites.From the n-hexane/Pt’ ratios observed it can be concluded that under the experimental conditions investigated n-hexane effectively competes with hydrogen forA . Sarkany 2269 I 1 Fig. 1. 0.3 0.2 0.1 0 10 20 30 tlmin Chemisorption of H,-n-hexane as a function of temperature. 4.92 kPa. (a) 593, (b) 563, (c) 510 and (d) 453 K. I 1 6 tz --- 2 pHz = 93.5 kPa, pnH = 0 10 20 30 tlmin Fig. 2. Chemisorption of H,-n-hexane as a function of the partial pressure of hydrogen 543 K. Chemisorbed hydrocarbon was hydrogenated with 98.4 kPa H, after He purging. pnH 4.92 kPa. (a) pHz = 0; (b) pHz = 4.67 kPa; (c) pH, = 18.7 kPa and (d) p H z = 93.5 kPa.adsorption sites. Already at -55 K with HJnH = 19 the nH/Pts ratio is 0.083, indicating that in the kinetic interpretations the carbon coverage of Pt sites should be considered. This suggestion is also supported by the fraction of ‘free’ sites, lOOf= 63, even though the chemisorption phase is obviously perturbed by the procedure used to measure the ‘free’ sites. In the course of the He treatment (or upon evacuation of the2270 Chemisorption of n-Hexane on Pt Black I 1 6 w 4 I 2 e . rn v) a c 1 I I 0 10 20 30 t/min Fig. 3. Chemisorption of H,-n-hexane as a function of the partial pressure of hydrogen at 543 K. pnH = 4.92 kPa. (a) 1, (b) 5, (c) 19 and ( d ) 100 YO H,/He. Table 1. Carbon coverage and number of free sites on a Pt black as measured in a gravimetric system experi- ment T/K H,/nH nHt/Pts nH/Pt" nHi/Pts lOOf,, PtS x (1 -fco)/nH - 1 453 19 0.082 0.076 0.02 - 2 455 19 0.080 0.075 - 65.8 4.55 3 515 19 0.148 0.135 55.3 3.3 1 4 516 19 0.151 0.143 0.023 89.9 4.36 5 563 19 0.231 0.22 0.055 81.3 3.4 6 563 19 0.236 0.22 - 38.2 2.8 1 - pnH = 4.92 kPa; nHt/Pts = hydrocarbon coverage in equilibrium with nH/H,; nH/Pt" = hydrocarbon coverage after He treatment ; nH'/Pts = hydrocarbon coverage after 10 min H,; f,, = free fraction measured by 0.65 kPa CO (adsorption 5 min, evacuation 5 min).In experi- ments (4) and (5) lOOf,, and Pt" x ( 1 -f,,)/nH refer to nHi/Pts, otherwise to nH/Pts. H,-nH mixture) the chemisorbed hydrocarbons transform into more firmly held forms, and thus the coverage of the free sites (this term corresponds to 1 - OnH) might be less than the amount in equilibrium with the H,-nH mixture.The total hydrocarbon coverage as shown in fig. 1 and tables 1 and 2 increases with temperature in accordance with the exothermicity of hydrogen adsorption. With a decrease in the partial pressure of hydrogen (fig. 2 and 3) the hydrogen coverage decreases, so that more sites become available for n-hexane adsorption; hence the carbon coverage increases. The coverage of 'free' sites changes in the opposite direction. Plugging the nH-H,-He inlet and purging the balance with He permitted measurements of the reversibly bound proportion of the adsorbed hydrocarbons, which is not detected in u.h.v. systems. The reversibly bound hydrocarbons representA . Sarkany 227 1 Table 2.Carbon coverage (nH/Pts) and number of free sites on a Pt black as measured in a microreactor experiment nH/Pt” 1 oop Pt“ x (1 -f>/nH 1 2 3 4 5 6 7 8 9 10 11 12 13 455 48 1 508 536 563 563 563 563 563 563 563 563 563 19 19 19 19 0 0.95 0.95’ 0.95‘ 6.46 6.46’ 6.46’ 19 19’ 0.078 0.1 1 0.13 0.21 0.39 0.3 1 0.24 0.21 0.25 0.101 0.055 0.24 0.066 66.0 67.1 53.4 46.1 22.1 33.6 35.6 21.5 61.4 82.8 34.3 - 4.35 2.99 3.58 2.56 2.5 1 2.76 3.06 3.14 3.82 3.12 2.73 P , , ~ = 98.42 kPa, pnH = 4.92 kPa; time on stream, 5 rnin after that 5 rnin in Ar. a Surface was saturated with 0.1 cm3 H, pulses at 273 K, and after 10 rnin Ar purging 0, titration followed. In experiments (b) and (c) the nH stream was closed and there was 3 and 15 rnin hydrogenation, respectively, with H,/He [experiments (7) and (8), 31 YO H,/He; experiments (10) and (1 I), 5 YO 4 /He].Table 3. Product selectivity (YO) in catalytic test (R) and in hydrogenation-desorption (HD) t / s <C, 2MP 3MP nH MCP B R”HU 448 R 448 HD 448 HD 463’ HD 526 R 526 HD 54 1 R 541 HD 54 I HD 563 R - 20-30 0.4 30-60 3.6 2MO 0.3 - 85.6 2 w o 3.9 - 80.9 15-30 2.0 3 w 5 3.9 - 82.1 reaction is not observed 1.5 0.2 61.1 36.6 21.5 1.3 53.9 19.4 0.7 0.1 89.3 9.6 1.3 9.9 3.2 - 10.5 2.3 63.2 20.1 12.4 4.7 - 2.0 4.9 1.1 20.2 70.7 4.2 0.5 65.4 25.7 1.2 12.6 4.1 - - - 0.00 0.00 0.00 - 0.02 0.00 1.63 x 0.8 0.1 0.00 6.33 x - 0.00 2.51 x 10-9 - - - pntr = 4.92 kPa, pH, = 93.5 kPa; R, time on stream 6 min; HD, catalyst Ar treated for 6 rnin before hydrogenation flow of H, = 17 cm3 min-l.a R,, = rate of n-hexane consumption in mol g;: s-l. ’ Sample cooled to 373 K in nH-H,, sample heated in Ar to 463 K for 1 min and H, addition. < C,, fragments; 2-MP, 2-methylpentane; 3-MP, 3-methylpentane ; nH, n-hexane ; MCP, methylcyclopentane ; B, benzene. chemisorbed species which are able to recombine with surface hydrogen. Because of the high temperature the presence of physisorbed species on the surface is not likely. With n-hexane over Pt black under the experimental conditions investigated the reversible fraction is 3-8% of the total carbon coverage. The close similarity of nH/Pt” values measured gravimetrically and in a microreactor by t.p.0. after Ar purging of the catalyst also confirms that the reversible fraction is only a small part of the total carbon coverage (tables 1 and 2).In the presence of gas-phase hydrogen a considerable fraction of surface species can be hydrogenated off the surface. Two types of experiment were performed. (i) After the2272 E 5 4 j: 3 3 5 2 1 0 Chemisorption of n-Hexane on Pt Black - 0.3 0.2 w --. a 2 0.1 0 I 0 50 H, in He (70) 1 - 100 Fig. 4. Ratio of the strongly bound hydrocarbons to the reactive ones, nHi/nHr, and the coverage of the reactive hydrocarbon, nHr/PtS, measured after 5 min hydrogenation as a function of percentage H, in He. pnH = 4.92 kPa,p,,, = 98.4 kPa. 0 , 6 0 3 K (B); 0 , 5 4 3 K (B); x ,563 K (R); 0, 523 K (B) (B = balance, R = microreactor test). formation of the steady-state carbon coverage in nH-H,-He, the n-hexane stream was closed and only the H,-He mixture was passed through the system (fig.3). In fig. 4 the surface coverage of the hydrocarbons hydrogenated from the surface within 5 min in the H,-He stream and the ratio of the irreversibly to reversibly bound hydrocarbons are summarized. (ii) After He or Ar treatment of chemisorbed hydrocarbons, hydrogenation was performed with 94.5 kPa hydrogen (fig. 2). Owing to the hydrogen loss in the trapped hydrocarbons, the coverage of the irreversibly bound forms is higher than in the direct hydrogenations with H,-He, with the exception of experiments in which the partial pressure of hydrogen in the nH-H,-He inlet is kept low (fig. 5). The coverage of the reversibly bound hydrocarbons, like that of the irreversibly bound ones, is a sensitive function of the temperature and the nH/H, ratio.The chemisorbed hydrocarbon which could be removed by H, might be regarded as a reactive surface intermediate which is responsible for n-hexane transformation on Pt sites. Of course a sharp dividing line cannot be drawn between reversibly and irreversibly bound forms.2o It is likely, however, that hydrocarbons residing on the surface for 5 min or longer might contribute only to a very limited extent to the formation of the products. The product composition measured after cooling and Ar purging of the microreactor (table 3) confirms that at low temperatures a large fraction of chemisorbed n-hexane maintains its molecular identity. This result is in accordance with the fact that below 523 K only a limited transformation of n-hexane could be detected.Previous deuterium- exchange studies with ethane,20*21 propane,17 n-pentane,, and cyclopentane22 have also shown that the rate of deuterium exchange (this reaction was regarded as a weakA . Sarkany 0'4 I 0.3 - LZ 0.2- --. % 0.1 - 0 - 2273 0 50 H, in He (%) 100 Fig. 5. Coverage of strongly bound hydrocarbons, nHi/Pts, measured after 5 min hydrogenation as a function of percentage H, in He. pnH = 4.92 kPa, ptot = 98.4 kPa. @, 603 K (B); x , 563 K (R); (), 543 K (B); 0, 523 K (B); A, 543 K (B), hydrogenation after He treatment with 98.4 kPa H,. intera~tion,~ of hydrocarbons with Pt sites) is at least two orders of magnitude faster than that of hydrogenolysis (strong interaction) at 1015 mol m-'s-l fission rate. The chemisorption results reported have over Pt black and the previous kinetic con- siderations2, seem to support the view that in alkane transformation over poorly dispersed Pt catalyst distinction can be made between chemisorption and reaction sites, and that the chemisorbed hydrocarbons are likely to reach the reaction sites by surface migration rather than direct adsorption on reaction sites.In a recent paper Davis et a1.' have argued that the low hydrogenolysis activity of Pt and Pd within the 5 d and 4d series, respectively, stems from the ease of formation of carbonaceous deposits on these metals. These chemisorption results with a Pt black do not support this conclusion. The h ydrogenolysis activity of Pt (and Pd) is usually measured under experimental conditions (high H,/hydrocarbon ratio, low temperature) when the surface coverage of the irreversibly bound hydrocarbons is relatively small (fig.3). The exchange activity of Pt and the ease of hydrogenation of the surface species in equilibrium with nH-H, point rather to the low concentration of surface sites being able to catalyse the hydrogenolysis of alkanes.21* 22 The conclusion of Davis et aZ.8y ' has been based upon ' total ' hydrocarbon coverages measured after evacuation of the H,-nH mixture from the reactor. The ' total ' hydrocarbon coverage, as is clearly shown by the results in this paper, also includes the coverage of the reactive forms. The carbon coverage and the fraction of free sites enable an average site requirement of the chemisorbed species, Pts x (1 -f)/nH, to be calculated.The site requirement has been observed to range from 2.5 to 4.5 under the experimental conditions reported. The kinetic site requirement determined from a comparison of the slope of the hydrogen isotherm and the reaction order with respect to hydrogenl47l5 has been proposed to be larger than eight. The obvious discrepancy between chemisorption and kinetic site requirement might be explained if it is assumed that the kinetic site requirement also involves the free and hydrogen adsorption sites which are required for a given reaction. One cannot exclude that the discrepancy stems from the fact that the procedure used to measure the free sites perturbs the surface state of the chemisorbed hydrocarbons. Taking for example the first row in table 2, T = 455 K, H,/nH = 19, nH/Pts = 0.0782274 Chemisorption of n-Hexane on Pt Black L 10 20 30 tlmin B 10 20 30 tlmin Fig.6. n-Hexane chemisorption and product selectivity observed at 603 K in a 20% H,/He stream, F = 12 cm3 min-l, ptOt = 98.4 kPa; inlet nH (a) 0.31, (b) 0.55, (c) 0.88, (d) 1.53, (e) 4.7, (f) 4.9 and ( g ) 4.8 kPa. Selectivity: 0, S<c,; V, S21\.II,+3MP; a, SMcp; A, S,. and 100 f = 66, it is observed for lOOf < 37.6 that one obtains a site requirement higher than eight. If the hydrocarbon-covered Pt sites are capable of chemisorbing a limited amount of H,, 0, or CO, this would increase the value of lOOJ In this particular experiment this additional adsorption would mean 3.6 0 atoms for each chemisorbed n- hexane molecule, which does not seem probable. In experiments with nH/Pt” > 0.125 the site requirement is less than eight, even if the fraction of the free sites is neglected.Clearly, a thorough investigation is required to clarify the problem. The ratio of the irreversibly to reversibly bound hydrocarbons shows a systematic variation as a function of the partial pressure of hydrogen. As is illustrated in fig. 4, at 563 K the hydrocarbon species adsorb only reversibly in the presence of hydrogen with H,/nH > 19. Since H,/nH is related to the appearance of the entirely reversible adsorption of n-hexane, it shifts to lower values with a decrease in the reaction temperature : obviously the n-hexane transformation at large H,/nH ratios is not affected by the presence of firmly held hydrocarbons. In other words, the reaction rate and product formation are governed only by the hydrogen coverage on the surface : the ensemble of surface sites can be regarded as an H-Pt system.At low H,/hydrocarbon ratios the transformation of n-hexane takes place in the presence of firmly held hydrocarbons : it might be suggested that the catalytically active surface consists of H-Pt-C ensembles. Systematic studies over Pt,2 Pd,24 Rh2, and Ni2, have shown that the variation of the partial pressure of hydrogen significantly influences the product selectivity. Over Pt in the transformation of alkanes, considering the rate maxima for various reactions, the hydrogen requirement of the individual reaction routes was proposed to increase in. the following order : 2 , 2 5 , 2 7 , 2 8 dehydrogenation and dehydroisomerization < C, dehydro- cyclization < hydrogenolysis < C, cyclic reactions < bond-shift skeletal isomerization.The hydrogen sensitivity of the product distribution has been interpreted by the inhibiting effect of hydrogen on the degree of dissociation of C-H bonds in the reacting surface intermediate2*25g 27 and by the role of hydrogen as an astoichiometric reactant. InA . Sarkany 2275 previous papers we have shown that a remarkable change can be observed in the product selectivity when the rate of consumption of the parent compound exhibits positive hydrogen 29 The likely connection between the product selectivity and the accumulation of the firmly held hydrocarbons on Pt sites also appears in the gravimetric measurements.A typical run is presented in fig. 6: the n-hexane partial pressure (the actual pressures denoted by arrows were calculated from the g.1.c. analysis) was continuously raised in 19 YO H,-He at 593 K. In a large excess of H,, when the reaction proceeds essentially on a 'clean' surface, the main reaction route is fragmentation. With a decrease in the H,/nH ratio, as soon as the surface of the Pt crystallites becomes covered with trapped hydrocarbons (the H-Pt system is transformed into an H-Pt-C one) non-destructive reactions such as isomerization (2MP + 3MP) and methylcyclo- pentane (MCP) and benzene (B) formation come into prominence. The decrease of the partial pressure of hydrogen allows the transformation of reversibly bound forms into more dehydrogenated surface species, which by blocking the hydrogenolysis sites increase the selectivity of the non-destructive reactions.The retardation of the hydrogenolysis activity has already been observed in the poisoning of Pt sites by carbonaceous deposits formed at elevated temperature~.~*'~~~ The alloying of Pt with a catalytically inactive metal to form P~-Au,~'* 31 Pt-Cu3, and Pt-Sn13> 3 3 3 34 catalysts corroborated that hydrogenolysis is a reaction route whose selectivity is drastically reduced by the increase in the surface concentration of the second metal. The discrepancy between product distribution observed in the activity tests and in hydrogenation-desorption of the trapped hydrocarbons at 526 and 541 K (table 3) might also be interpreted by the formation of firmly held hydrocarbons.With H,/nH = 19 at 526 and 541 K the principal reaction is fragmentation: the selectivity of this reaction route reaches 86 and 81 YO, respectively. Purging the H,-nH inlet with Ar and keeping the trapped hydrocarbons for 6 min in Ar (this time being much longer than the average residence time of n-hexane molecules in the reactor in the activity tests) allow the transformation of chemisorbed n-hexane. One of the remarkable features of the product distributions observed upon hydrogenating the trapped hydrocarbons is the very limited amount of fragments among the products. This observation emphasizes again that the surface sites, which in the presence of sufficient hydrogen produce fragments, are the most likely to be poisoned by the decrease of the partial pressure of hydrogen. In previous papers this route of poisoning, which is apparently connected with fragmentation, is referred to as the C, or hydrogenolysis route of site blo~king.l~*~* On the basis of the systematic variation of the carbon coverage on the catalysts it is proposed that the hydrogen affects the product selectivity through the alteration of geometric/steric and electronic properties of Pt sites caused by the transformation of a H--Pt surface to a H-Pt-C one.The partial pressure of hydrogen governs the coverage of the irreversibly held hydrocarbons; thus the lower the H,/hydrocarbon ratio, the less the size of the working Pt ensembles. Owing to the decrease of the ensemble size the dissociation of C-H bonds becomes restricted, even though the low HJhydrocarbon ratio should promote the rupture of C-H bonds.Moreover, the carbon atoms on the surface might result in the formation of localised hydrocarbon-metal bonds rather than delocalised ones.35 The presence of firmly held hydrocarbons causes crowding on Pt sites, which in turn might exert steric effects upon the configuration of the reacting species. The effect of hydrocarbon congestion has already been assumed in the reactions of a l k e n e ~ ~ ~ and in chain lengthenir~g.~'. 38 The high selectivity of a-alkene formation in an He stream might also be explained by geometric/steric effects of firmly held deposits rather than electronic ones.39 Electronic effects are likely to contribute to the ease of product de~orption.~' 13esides ensemble size, hydrogen availability on the ensembles formed might be a factor which influences the dissociation of C-H bonds and the desorption of products.However, the ensemble size and the hydrogen availability might not be independent of2276 Chemisorption of n-Hexane on Pt Black each other. It might be assumed that the blocking of surface sites by deposits decreases the hydrogen coverage on the surface for geometric/electronic reasons. With the accumulation of deposits a new source of hydrogen appears on the surface as the trapped hydrocarbons, whose hydrogen content expressed by the ratio H/C is 1.2-1.5,’ might serve as a hydrogen donor.’ At low H,/hydrocarbon ratios the main reaction routes are C, cyclisation, aromatization and alkene formation. All these reactions yield hydrogen atoms which might ensure the steady activity of Pt ensembles formed.Over highly dispersed Pt/A1,0334 and Pt/SiO,lg the steady-state formation of carbonaceous deposits from n-hexane at 673 K in absence of added H, provides evidence that the dissociated hydrogen atoms are able to maintain a constant activity of Pt sites in the formation of coke precursors and gas-phase products. Conclusions (1) Under the experimental conditions normally used to study the low-temperature activity of Pt catalysts n-hexane successfully competes with hydrogen for Pt sites on a Pt black sample. (2) Hydrogenation experiments confirm that the ratio of firmly held species to reactive ones increases with temperature and with a decrease in the partial pressure of hydrogen.The reversibly bound hydrocarbons which can be directly removed by He treatment or by evacuation represent only 3-8% of the total carbon coverage. (3) With a change in the partial pressure of hydrogen the composition of the surface sites shows a systematic variation: the H-Pt system transforms into an H-Pt-C one upon a decrease in the hydrogen pressure. The geometric/steric and electronic effects of firmly held hydrocarbons should be considered in the interpretation of the effect of hydrogen upon product selectivity. References 1 J. R. Anderson and Y. Shimoyama, Proc. Sth Int. Congr. Catal., Miami, 1972, p. 695. 2 Z. Paal, Advances in Catalysis (Academic Press, New York, 1980), vol. 29, p. 273. 3 F. M. Dautzenberg and J. C. Platteeuw, J. Catal., 1970, 19, 41.4 Z. Pail, H. Zimmer and P. Tetenyi, J. Mol. Catal., 1984, 25, 99. 5 F. M. Dautzenberg and J. C. Patteeuw, J. Catal., 1972, 24, 264. 6 P. P. Lankhorst, H. C. DeJongste and V. Ponec, in Catalyst Deactivation, ed. B. Delmon and 7 F. Garin, S. Aeiyach, P. Legare and G. Maire, J. Catal., 1982, 77, 323. 8 S. M. Davis, F. Zaera and G. A. Somorjai, J. Catal., 1984, 85, 206. 9 S. M. Davis, F. Zaera and G. A. Somorjai, J. Catal., 1982, 77, 439. C. Froment (Elsevier, Amsterdam, 1980), p. 43. 10 F. Luck, S. Aiyach and G. Maire, in Proc. 8th int. Congr. Catal., Berlin, 1984, vol. 2, p. 695. I 1 J. Barbier, P. Marecot, N. Martin, L. Elassal and R. Maurel, in Catalyst Deactivation, ed. B. Delmon 12 R. Bacaud, H. Charcosset, M. Guenin, R. Torrellas-Hidalgo and L. Tournayan, Appl.Catal., 1981, 1, 13 A. Sarkany, H. Lieske, T. Sziligyi and L. Toth, in Proc. 8th int. Congr. Catal., Berlin, 1984, vol. 2, 14 A. Frennet, G. Lienard, A. Crucq and L. Degols, J. Catal., 1978, 53, 150. 15 P..Parayre, V. Amir-Ebrahimi, F. G. Gault and A. Frennet, J. Chem. SOC., Faraday Trans. I , 1980,76, 16 R. S. Dowie, D. A. Whan and C. Kemball, J. Chem. SOC., Faraday, Trans. 1 , 1972, 68, 2150. 17 L. Guczi, A. Sarkany and P. Tetknyi, J. Chem. SOC., Faraday Trans. I , 1974, 70, 1941. 18 A. Sa’rkany, L. Guczi and P. TCtenyi, React. Kinet. Catal. Lett., 1974, 1, 169. 19 A. Sarkany, in Catalyst Deactivation, ed. B. Delmon and C. Froment (Elsevier, Amsterdam, 1987), 20 L. Babernics, L. Guczi, K. Matusek, A. Sarkany and P. Tetenyi, in Proc. 6th Znt. Congr. Catal., 21 A. Sarkany, K. Matusek and P. TCtCnyi, J. Chem. Soc., Faraday Trans. I , 1977, 73, 1699. 22 A. Sarkany, J. Gaal and L. T6th, in Proc. 7th Int. Congr. Catal., Tokyo, 1980, vol. 1, p. 291. and C. Froment (Elsevier, Amsterdam, 1980), p. 53. 81. p. 613. 1704. p. 125. London, 1976, vol. 2, p. 456.A . Sarkany 2277 23 L. Guczi and P. Tetknyi, Ann. N.Y. Acad. Sci., 1973, 213, 173. 24 Z. Paal and P. Titenyi, Appl. Catal., 1981, 1, 9. 25 Z. Paal and P. G. Menon, Catal. Rev. Sci. 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Clarke and J. J. Rooney, Adv. Catal.,*1976, 25, 125. 40 A. Palazov, Ch. Bonev, D. Shopov, G. Lietz, J. Volter and A. Sarkany, J. Catal., 1987, 103, 249. Paper 711029; Received I Ith June, 1987 FAR I

ISSN:0300-9599    

DOI:10.1039/F19888402267

出版商:RSC    

年代:1988

数据来源: RSC