摘要:

Ordinary Members PROFESSOR R. J. DONOVAN 1983 PROFESSOR M. C. R. SYMONS 1983 DR G. J. HILLS 1984 PROFESSOR J. M. THOMAS 1983 PROFESSOR A. J. LEADBETTER 1984 DR J. ULSTRUP 1985 DR I . W. M. SMITH 1985 PROFESSOR G. WILLIAMS 1985 PROFESSOR F. L. SWINTON 1983 DR D. A. YOUNG 1984 Honorarj, Secretarj-: DR G. J. HILLS Honorarj- Treasurer : PROFESSOR P. GRAY The President thanked the retiring members of Council, Vice-presidents Professor Sheppard and Professor Wagner, and Ordinary Members Professor King and Professor Purnell, for their services. 5. Reriew of Futurr Acfirifies A programme of future activities of the Division had been tabled and the President drew attention to the forthcoming General Discussions and Symposia. xiOrdinary Members PROFESSOR R. J. DONOVAN 1983 PROFESSOR M. C. R. SYMONS 1983 DR G. J. HILLS 1984 PROFESSOR J. M. THOMAS 1983 PROFESSOR A. J. LEADBETTER 1984 DR J. ULSTRUP 1985 DR I . W. M. SMITH 1985 PROFESSOR G. WILLIAMS 1985 PROFESSOR F. L. SWINTON 1983 DR D. A. YOUNG 1984 Honorarj, Secretarj-: DR G. J. HILLS Honorarj- Treasurer : PROFESSOR P. GRAY The President thanked the retiring members of Council, Vice-presidents Professor Sheppard and Professor Wagner, and Ordinary Members Professor King and Professor Purnell, for their services. 5. Reriew of Futurr Acfirifies A programme of future activities of the Division had been tabled and the President drew attention to the forthcoming General Discussions and Symposia. xi

ISSN:0300-9599    

DOI:10.1039/F198278FX013

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

3 708 REVIEW OF BOOKS is the absence of any reference to possible new and potentially significant applications for polymer latices. Novel applications may well be found in at least two directions, namely, those which exploit the large polymer-aqueous-phase specific surface area of latices, and those which exploit the electrical dissymmetry which is present at the interface between polymer and aqueous phase in the case of electrostatically stabilised latices. No reference is made in this book to the efforts which have so far been made to exploit for medical purposes the adsorptive and binding potentialities of the large area of polymer-aqueous-phase interface in latices. Nor is there any mention of possible catalytic applications of this large interfacial area. So far, catalytic applictions have been confined to those which rely essentially upon enhancement of the counter-ion concentration in regions of the electrical double layer which are near to the polymer surface.However, it is at least possible that the adsorptive capacity of the interface may also be useful in catalytic applications. Some discussion of possibilities such as these would have been welcome. D. C. BLACKLEY Received 14th April, 1982 Shock Waves in Chemistry. Ed. by ASSA LIFSHITZ. (Marcel Dekker, New York, 1981). Pp. ix + 390. Price SFr 182. After a somewhat hesitant start, the use of shock waves to study chemical and physical processes at high temperatures has become an accepted technique and reliable kinetic data can be obtained in this way. Several books have been written, notably by Bradley and by Gaydon and Hurle, which describe not only the underlying principles and the experimental procedures but also give some account of the early results obtained using shock waves to provide high temperatures for short, well defined times in the reactant gases.Inevitably, these books have become rather dated. This new book, edited by Lifshitz, is rather different. It is a collection of self-contained review articles on various aspects of shock waves. The first (by Khandelwal and Skinner) is concerned with hydrocarbon oxidation, and the next (by Tsang) describes the results obtained using the comparative rate technique which he has pioneered. Both these articles include extensive lists of references and represent useful summaries of the present situation.Boyd and Burns have contributed a chapter on dissociation-recombination reactions, while Kiefer describes the laser-schlieren method which he has done so much to develop. There is another chapter by an acknowledged expert, Just, on atomic resonance absorption spectrometry. Under shock-tube conditions it is very seldom that the concentrations of radicals and other species reach a steady state, and so the classical Bodenstein steady-state approximation cannot be used. Instead, it is necessary to integrate the differential equations describing the time-variation of species concentration, and Gardiner, Walker and Wakefield have provided a useful guide to the computational procedures available in this and other aspects of shock-tube work.In addition to these contributions there is another by Bar-Nun on Chemical Aspects of Shock Waves in Planetary Atmospheres which, although interesting in itself, fits rather uneasily with its companions. As is inevitable in a book of this type the standard and style of the chapters varies and there is some overlapping material; none of this, however. represents a serious drawback. What is more difficult to understand is the audience for whom the book is intended. Each chapter is a useful and interesting review which will appeal to a fairly restricted readership, but, in the opinion of this reviewer, the whole volume lacks coherence. The time-honoured phrase ‘should be on the shelves of every library’ probably applies, though the price, over &50 at the current exchange rate, must cause all university librarians to flinch in these days of U.G.C.cuts. There is still room for the definitive up-to-date book to be written on shock waves in chemistry. J. A. BARNARD Received 5th April, 19823 708 REVIEW OF BOOKS is the absence of any reference to possible new and potentially significant applications for polymer latices. Novel applications may well be found in at least two directions, namely, those which exploit the large polymer-aqueous-phase specific surface area of latices, and those which exploit the electrical dissymmetry which is present at the interface between polymer and aqueous phase in the case of electrostatically stabilised latices. No reference is made in this book to the efforts which have so far been made to exploit for medical purposes the adsorptive and binding potentialities of the large area of polymer-aqueous-phase interface in latices.Nor is there any mention of possible catalytic applications of this large interfacial area. So far, catalytic applictions have been confined to those which rely essentially upon enhancement of the counter-ion concentration in regions of the electrical double layer which are near to the polymer surface. However, it is at least possible that the adsorptive capacity of the interface may also be useful in catalytic applications. Some discussion of possibilities such as these would have been welcome. D. C. BLACKLEY Received 14th April, 1982 Shock Waves in Chemistry. Ed. by ASSA LIFSHITZ. (Marcel Dekker, New York, 1981). Pp. ix + 390.Price SFr 182. After a somewhat hesitant start, the use of shock waves to study chemical and physical processes at high temperatures has become an accepted technique and reliable kinetic data can be obtained in this way. Several books have been written, notably by Bradley and by Gaydon and Hurle, which describe not only the underlying principles and the experimental procedures but also give some account of the early results obtained using shock waves to provide high temperatures for short, well defined times in the reactant gases. Inevitably, these books have become rather dated. This new book, edited by Lifshitz, is rather different. It is a collection of self-contained review articles on various aspects of shock waves. The first (by Khandelwal and Skinner) is concerned with hydrocarbon oxidation, and the next (by Tsang) describes the results obtained using the comparative rate technique which he has pioneered.Both these articles include extensive lists of references and represent useful summaries of the present situation. Boyd and Burns have contributed a chapter on dissociation-recombination reactions, while Kiefer describes the laser-schlieren method which he has done so much to develop. There is another chapter by an acknowledged expert, Just, on atomic resonance absorption spectrometry. Under shock-tube conditions it is very seldom that the concentrations of radicals and other species reach a steady state, and so the classical Bodenstein steady-state approximation cannot be used. Instead, it is necessary to integrate the differential equations describing the time-variation of species concentration, and Gardiner, Walker and Wakefield have provided a useful guide to the computational procedures available in this and other aspects of shock-tube work.In addition to these contributions there is another by Bar-Nun on Chemical Aspects of Shock Waves in Planetary Atmospheres which, although interesting in itself, fits rather uneasily with its companions. As is inevitable in a book of this type the standard and style of the chapters varies and there is some overlapping material; none of this, however. represents a serious drawback. What is more difficult to understand is the audience for whom the book is intended. Each chapter is a useful and interesting review which will appeal to a fairly restricted readership, but, in the opinion of this reviewer, the whole volume lacks coherence. The time-honoured phrase ‘should be on the shelves of every library’ probably applies, though the price, over &50 at the current exchange rate, must cause all university librarians to flinch in these days of U.G.C. cuts. There is still room for the definitive up-to-date book to be written on shock waves in chemistry. J. A. BARNARD Received 5th April, 1982

ISSN:0300-9599    

DOI:10.1039/F198278BX015

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY FARADAY TRANSACTIONS, PARTS I A N D I 1 The Journal of The Chemical Society is issued in six sections: Journal of The Chemical Society, Chemical Communications Journal of The Chemical Society, Dalton Transactions Journal of The Chemical Society, Faraday Transactions, I Journal of The Chemical Society, Faraday Transactions, I I Journal of The Chemical Society, Perkin Transactions, I Journal of The Chemical Society, Perkin Transactions, I I Thus, five of the sections are directly associated with three of the Divisions of The Royal Society of Chemistry: the sixth is Chemical Communications. This continues to be the medium for the publication of urgent, novel results from all branches of chemistry. Communications shoul1 not normally exceed one printed page in length and authors are required to submit three copies of the typescript and two copies of a statement of the reasons and justification for seeking urgent publication of the work.This Section is intended to be essentially a journal for inorganic chemists containing papers on the structure and reactions of inorganic compounds and the application of physical chemistry techniques to, e.g. the study of inorganic and organometallic compounds and problems, including work on the kinetics and mechanisms of inorganic reactions and equilibria, and spectroscopic and crystallographic studies of inorganic com po und s. Journal of the Chemical Society, Faraday Transactions, I and I I These are, respectively, physical chemistry and chemical physics journals.P A R T I (physical chemistry) includes papers on such topics as radiation chemistry, gas-phase kinetics, electrochemistry (other than preparative), surface and interfacial chemistry, heterogeneous catalysis, physical properties of polymers and their solutions and kinetics of polymerization, etc. P A R T I I (chemical physics) contains theoretical papers, especially those on valence and quantum theory, statistical mechanics, intermolecular forces, relaxation phenom- ena, spectroscopic studies (including i.r., e.s.r., n.m.r., and kinetic spectroscopy, etc.) leading to assignments of quantum states, and fundamental theory, and also studies of impurities in solid systems, etc. Journal of The Chemical Society, Chemical Communications Journal of The Chemical Society, Dalton Transactions Journal of The Chemical Society, Perkin Transactions, I and II These are, respectively, the organic chemistry and the physical organic chemistry sections of the Journal.PART I (organic and bio-organic chemistry) is designed to contain papers on all aspects of synthetic, and natural product organic and bio-organic chemistry and to deal with aliphatic, alicyclic, aromatic, carboncyclic and heterocyclic compounds. Papers on organometallic topics are considered for either the Dalton or the Perkin Transactions .P A R T I I (physical organic chemistry) is for papers on reaction kinetics and mechanistic studies of organic systems and the use of physico-chemical, spectroscopic, and crystallographic techniques in the solution of organic problems.Notice to Authors ( I ) Although authors need not be members of the Royal Society of Chemistry it is hoped that they will be. (2) Authors must indicate the Part of the Journal they wish their paper to appear in. This preference will be respected unless it is obviously erroneous in terms of the scientific content of the paper. (3) Since all papers will be subjected to refereeing, in parallel, by two independent referees, the original typescript (quarto or A4 size) and two good-quality copies should be provided. (4) All papers should be sent to the Director of Publications, The Royal Society of Chemistry, Burlington House, Piccadilly, London W I V OBN. ( 5 ) For details of manuscript preparation, preferred usages, etc. the Instructions to Authors, previously available from the Faraday Society, and now obtainable from The Royal Society of Chemistry, should be consulted. ( 6 ) The Society will adopt the following abbreviations for the new journals in all its publications.J. Chem. SOC., Chem. Commun. J . Chem. SOC., Dalton Trans. J . Chem. SOC., Faraday Trans. I J . Chem. SOC., Faraday Trans. 2 J . Chem. SOC., Perkin Trans. I J . Chem. Soc., Perkin Trans. 2 * The author to whom correspondence should be addressed is indicated by an asterisk after his name in the heading of each paper. 11FARADAY D I V I S I O N O F THE ROYAL SOCIETY OF CHEMISTRY ASSOCIAZIONE I T A L I A N A D I C H I M I C A F l S l C A S O C l l f T i DE C H l M l E PHYSIQUE DEUTSCHE BUNSEN G E S E L L S C H A F T F U R P H Y S I K A L I S C H E C H E M I E FARADAY D I S C U S S I O N NO.7 4 Electron and Proton Transfer University of Southampton, 14-1 6 September 1982 This meeting will be concerned with fundamental aspects of the chemical kinetics of electron and proton transfer reactions in solution and with particular reference to well defined biological systems. Attention will be focused on (i) the theory of charge transfer, (ii) critical experiments designed to test those theories and (iii) their application to the understanding of charge transfer reactions in molecules of biological interest. The meeting will encompass well characterised reactions in solution, redox reactions and elementary biochemical reactions; particular attention will be paid to isotope effects, to electron and proton tunnelling, to intermolecular and intramolecular transfers and to related questions concerning the organisation of biological systems. Among those who have agreed to take part are R.A. Marcus, R. R. Dogonadze, H. Gerischer, J. Jortner, R. M. Kuznetsov, N. Sutin, R. J. P. Williams, H. L. Friedman, J. M. Saveant, J. F. Holzwarth, F. Willig, J. C. Mialocq, M. Kosower, L. I . Krishtalik, E. F. Caldin, H. H. Limbach, W. J. Albery, M. M. Kreevoy, J. J. Hopfield, P. Rich, H. A. 0. Hill, K. Heremans, C. Gavach and D. B. Kell. The preliminary programme may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry Burlington House, London W1V OBN ... IllFARADAY D I V I S I O N O F THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 1 7 The Hydrophobic Interaction University of Reading, 15-1 6 December 1982 This term refers to interactions between chemically inert residues arising from perturbations in the unique spatial and orientational correlations in liquid water.These effects provide a major contribution to many of the non-covalently bonded structures that form the basis of life processes. Current advances in the statistical mechanics of polar fluids, intermolecular forces, computer simulation, and membrane physics are providing a new basis for the re-examination of various aspects of hydrophobic effects, their origin and their quantitative description. Such theoretical treatments will be confronted with recent experimental work on simple model systems which, i t i s hoped, will lead to a better understanding of hydrophobic interactions in more complex processes.The following have provisionally agreed to contribute to the symposium : A. Ben-Naim, H. J. C. Berendsen, D. L. Beveridge, S. D. Christian, L. Cordone, D. Eagland, D. Eisenberg, R. Lumry, P. J. Rossky, M. C. R. Symons, H. Weingartner, M. D. Zeidler The preliminary programme may be obtained from : M r s Y. A. Fish, The Royal Society of Chemistry Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY O F CHEMISTRY GENERAL DISCUSSION NO. 75 I nt ra molecu I a r K i net ics University of Warwick, 18-20 A p r i l 1983 Organising Committee Professor J. P. Simons (Chairman) Dr M. S. Child Professor R. J. Donovan Dr G. Hancock Experimental and theoretical interest in the time-dependent behaviour of isolated molecules, radicals or ions is strong and increasing.The Discussion will be concerned with the kinetics of processes which occur in isolated species following their preparation in states with non-equilibrium energy distributions (e.g. by photon absorption or collisional activation). Topics covered will include: ( a ) theoretical and experimental studies of energy redistribution in isolated species; ( 6 ) observation and theoretical modelling of the competition between intramolecular energy redistribution and radiative decay or radiationless processes (e.g. internal conversion, fragmentation, isomerisation). Contributions for consideration by the Organising Committee are invited. Titles should be submitted as soon as possible and abstracts of 300 words by 31 May 1982. Full papers for publication in the Discussion Volume will be required by 15 December 1982.Titles and abstracts should be sent to: Professor J. P. Simons, Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD. Dr D. M. Hirst Professor K. R. Jennings Dr R. Walsh IVTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 76 Concentrated Colloidal Dispersions Loughborough University of Technology, 14-1 6 September 1983 The meeting will discuss the experimental investigation and the theoretical description of the properties of concentrated colloidal dispersions, i.e. those systems in which the particle-particle interactions are strong enough to cause significant deviations from ideal behaviour. Both the structural and dynamic features of concentrated systems as determined by scattering, rheological and other techniques will be considered.It is anticipated that a range of dispersion types will be discussed. These will include both 'model' systems and dispersions of importance to industry provided that the data from the measurements can be interpreted. Contributions for consideration by the organising committee are invited and abstracts of about 300 words should be sent by 31 st August 1982 to: Professor R. H. Ottewill. School o f Chemistry, University o f Bristol, Cantock's Close, Bristol BS8 1TS FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division: Half day symposium Laser Spectroscopy (including the Centenary Lecture by T. Oka) To be held at University College, London on 28 April 1982 Further information from Mr S.S. Langer, The Royal Society of Chemistry, Burlington House, London W1 V OBN Gas Kinetics Group Seventh International Symposium on Gas Kinetics To be held at the University of Gottingen, West Germany on 23-27 August 1982 Further information from Dr R. Walsh, Department of Chemistry, University of Reading, Whiteknights, Reading RG6 2AD Colloid and Interface Science Group with the Colloid and Surface Chemistry Group of the SCI Adsorption from Solution To be held at the University of Bristol on 8-1 0 September 1982 Further information from Dr W D Cooper, Department of Chemistry. University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ Industrial Physical Chemistry Group Supercritical Fluids: Their Chemistry and Application To be held at Girton College, Cambridge on 13-1 5 September 1982 Further information from Dr W.R. Ladner, National Coal Board, Coal Research Establishment, Stoke Orchard, Cheltenham GL52 4RZ Neutron Scattering Group and Polymer Physics Group with the Institute of Physics The Neutron and its Applications To be held in Cambridge on 13-1 7 September 1982 Further information from The Meetings Officer, Institute of Physics, 47 Belgrave Square, London SW1 X 8QX Molecular Beams Group Molecular Beams and Molecular Structure To be held at the University of Bristol on 16-17 September 1982 Further information from Dr J. C. Whitehead, Department of Chemistry, University of Manchester, Manchester M13 9PL VFARADAY DIVISION INFORMAL AND GROUP MEETINGS Division Autumn Meeting: Energy and Chemistry To be held at Heriot-Watt University, Edinburgh on 21-23 September 1982 Further information from Dr J.F. Gibson, The Royal Soclety of Chemistry, Burlington House, London W1 V OBN Statistical Mechanics and 7hermodynamic.s Group with the British Society of Rheofog y Microstructure and Rheology To be held at Trinity Hall, Cambridge on 21 -24 September 1982 Further information from Dr P. Richmond, Unilever Research, Port Sunlight, Wirral, Merseyside L62 3JW High Resolution Spectroscopy Group High Resolution Fourier Transform, Laser Infrared and Electronic Spectroscopy To be held at the University of Newcastle-upon-Tyne on 22-24 September 1982 Further information from Dr P. J. Sarre, Department of Chemistry, University of Nottingham, Nottingham NG7 2RD Polymer Physics Group Polymer Electronics To be held in London on 20 October 1982 Further information from the Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SWlX 8QX Division with Polymer Physics Group and Macrogroup UK Annual Chemical Congress: Copolymers To be held at the University of Lancaster on 11-1 3 April 1983 Further information from Dr J.F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN Polymer Physics Group, Macrogroup UK and the Plastics and Rubber Institute Polyethylenes To be held in London on 8-1 0 June 1983 Further information from The Plastics and Rubber Institute, 1 1 Hobart Place, London SW1 W OH2 viNOTES I t has always been the policy of the Faraday Transactions that brevity should not be a factor influencing acceptability for publication.In addition however to full papers both sections carry at the end of each issue a section headed "Notes", which are short self-contained accounts of experimental observations, results, or theory that will not require enlargement into "full" papers. The " Notes" section is not used for preliminary communications. The layout of a "Note" is the same as that of a paper. Short summaries are required. The procedure for submission, administration, refereeing, editing and publication of "Notes" is the same as for "full" papers. However, "Notes" are published more quickly than papers since their brevity facilitates processing at all stages. The Editors endeavour to meet authors' wishes as to whether an article is a full paper or a "Note", but since there is no sharp dividing line between the one and the other, either in terms of length or character of content, the right is retained to transfer overlong " Notes" to the " full papers" section.As a guide a " Note" should not exceed 1500 words or word-equivalents. NOMENCLATURE AND SYMBOLISM For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both rules themselves and guidance on their use are given.Physicochemical Quantities and Units. Manual of Symbols and Terminology for Physicochemical Quantities and Units. (Pure and Appl. Chem., Vol. 51, No. 1, 1979, pp. 1-41. Also available as a soft-cover booklet from Pergamon Press, Oxford.) Surface Chemistry. ' Definitions, Terminology, and Symbols in Colloid and Surface Chemistry - I.' (Pure and Appl. Chem., Vol. 31, No. 4, 1972, pp. 577-638.) ' Definitions, Terminology, and Symbols in Colloid and Surface Chemistry - 11. Heterogenous Catalysis. ' (Pure and Appl. Chem., Vol. 46, No. 1, 1976, In addition, the terminology and symbols for the following subject areas are available either in the form of soft-cover booklets from Pergamon Press (denoted by *) or have been the subject of articles in Pure and Applied Chemisrry in recent years: activities;* chromatography ; elect rochemistry ; electron spectroscopy ; equilibria, fluid flow ; ion exchange; liquid-liquid distribution; molecular force constants; Mossbauer spectra ; nuclear chemistry; pH ; polymers; quantum chemistry; radiation;* Raman spectra; reference materials (recommended reference materials for the realization of physico- chemical properties : general introduction, enthalpy, optical rotation, surface tension, opt ica 1 refract ion, molecular weight, absorbance and wavelength, pressure-volume- temperature relationships, reflectance, potentiometric ion activities, testing distillation columns); solution chemistry; spect rochemical analysis ; surface chemistry; t hermo- dynamics, and zeolites. Finally, the rules for the naming of organic and inorganic compounds are dealt with in the following publications from Pergamon Press: 'Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H', 1979.' Nomenclature of Inorganic Chemistry', 1971. A complete listing of all IUPAC nomenclature publications appears in the 198 1 Index issues of J. Chem. SOC. pp. 71-90.) viiNOTES I t has always been the policy of the Faraday Transactions that brevity should not be a factor influencing acceptability for publication. In addition however to full papers both sections carry at the end of each issue a section headed "Notes", which are short self-contained accounts of experimental observations, results, or theory that will not require enlargement into "full" papers. The " Notes" section is not used for preliminary communications.The layout of a "Note" is the same as that of a paper. Short summaries are required. The procedure for submission, administration, refereeing, editing and publication of "Notes" is the same as for "full" papers. However, "Notes" are published more quickly than papers since their brevity facilitates processing at all stages. The Editors endeavour to meet authors' wishes as to whether an article is a full paper or a "Note", but since there is no sharp dividing line between the one and the other, either in terms of length or character of content, the right is retained to transfer overlong " Notes" to the " full papers" section. As a guide a " Note" should not exceed 1500 words or word-equivalents. NOMENCLATURE AND SYMBOLISM For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers.In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both rules themselves and guidance on their use are given. Physicochemical Quantities and Units. Manual of Symbols and Terminology for Physicochemical Quantities and Units. (Pure and Appl. Chem., Vol. 51, No. 1, 1979, pp. 1-41. Also available as a soft-cover booklet from Pergamon Press, Oxford.) Surface Chemistry. ' Definitions, Terminology, and Symbols in Colloid and Surface Chemistry - I.' (Pure and Appl.Chem., Vol. 31, No. 4, 1972, pp. 577-638.) ' Definitions, Terminology, and Symbols in Colloid and Surface Chemistry - 11. Heterogenous Catalysis. ' (Pure and Appl. Chem., Vol. 46, No. 1, 1976, In addition, the terminology and symbols for the following subject areas are available either in the form of soft-cover booklets from Pergamon Press (denoted by *) or have been the subject of articles in Pure and Applied Chemisrry in recent years: activities;* chromatography ; elect rochemistry ; electron spectroscopy ; equilibria, fluid flow ; ion exchange; liquid-liquid distribution; molecular force constants; Mossbauer spectra ; nuclear chemistry; pH ; polymers; quantum chemistry; radiation;* Raman spectra; reference materials (recommended reference materials for the realization of physico- chemical properties : general introduction, enthalpy, optical rotation, surface tension, opt ica 1 refract ion, molecular weight, absorbance and wavelength, pressure-volume- temperature relationships, reflectance, potentiometric ion activities, testing distillation columns); solution chemistry; spect rochemical analysis ; surface chemistry; t hermo- dynamics, and zeolites. Finally, the rules for the naming of organic and inorganic compounds are dealt with in the following publications from Pergamon Press: 'Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H', 1979. ' Nomenclature of Inorganic Chemistry', 1971. A complete listing of all IUPAC nomenclature publications appears in the 198 1 Index issues of J. Chem. SOC. pp. 71-90.) vii

ISSN:0300-9599    

DOI:10.1039/F198278FP025

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982,78, 1001-1009 Hydration and Ion-exchange Processes in Carboxylic Membranes Part 2.-Infrared Spectroscopic Investigation of Alkali-metal Salt Membranes B Y LEON LEVY, MARC MUZZI AND HENRI D. HURWITZ* Laboratoire de Thermodynamique Electrochimique, Facultk des Sciences, C.P. 160, Universitk Libre de Bruxelles, 50, avenue F.D. Roosevelt, 1050 Brussels, Belgium Received 14th January, 198 1 An infrared investigation has been performed on totally neutralized membranes of poly(acry1ic acid) grafted on a Teflon FEP or PTFE matrix. The changes in the principal absorption bands characterizing the vibration modes of the carboxylate group and water in Li+, Na+ and K+ salt membranes are analysed as a function of the degree of humidity and of the exchange capacity.The strength of the hydrogen bonding between water and the carboxylate ion and the nature of the ionic,interactions in the membrane are discussed. Carboxy-containing polymeric ion exchangers have recently come under close scrutiny. The conformational characteristics of these systems are to a large extent determined by hydrogen bonds and strong ionic interactions. The assessment of these effects and the determination of their role will be pursued in this series of investigations using carboxy-containing polymeric grafted membranes and ionomers based on them. In a previous publication' we used infrared spectroscopy in order to investigate the acid form of thin membranes made of poly(acry1ic acid) grafted onto a perfluorated (FEP or PTFE) matrix.In the present publication, attention is focused on the totally neutralized form of these membranes. The neutralization of the carboxy-containing polymers by metal ions leads to the appearance of a series of specific properties. We have previously discussed the prominent role of the intensive network of interchain contacts via hydrogen bonding and of the dimerization of carboxy groups in the acid membrane.' This configuration confers a rigidity to the polyelectrolyte structure which explains the relatively small degree of swelling of the membrane and its stability towards the total ionic exchange of hydrogen ions by metal ions. Certain observations2~3 have shown that the neutralization process of grafted carboxylic acid membranes is limited when the membrane is equilibrated with alkali-metal or alkaline-earth halide solutions.The degree of dissociation of the carboxy groups has been estimated to be ca. 15% with the alkali-metal ions and to reach 30% in the presence of alkaline-earth ions. It is also well known that the ion-exchange process is rather slow and occurs essentially at high pH. Moreover, the presence of ionic contacts and ionic cross-linking between the ionomer macrochains leads to a change in the kinetics and thermodynamic parameters of the absorption of the ~olvent.~ The transition temperature in ionomers is also to a large extent determined by their content of acid groups and their degree of ne~tralization.~ Infrared spectroscopic data for ionomers suggest the formation of a whole set of ionic associated species of different structures.s~ The analysis of available data shows that the nature of the cation might affect the transition temperature.For I0011002 I.R. INVESTIGATION OF ALKALI-METAL SALT MEMBRANES the same degree of neutralization, the transition temperature in Na+ ionomers indicates stronger ionic cross-links than in Rb+ ionome~s.~ At large degrees of neutralization, ionic cluster frameworks and supermolecular configurations might significantly stabilize the ionomer structure.8, It is thus expected that the infrared spectroscopic investigation presented here will provide information on the properties of PAA grafted membranes caused by the interplay of hydrogen bonding and ionic interactions. This work deals with membranes totally neutralized by alkali-metal ions.Such neutralization is achieved only with Li+, Na+ and K+ halides. EXPERIMENTAL PREPARATION OF SAMPLES To convert the membranes into their salt form, they are placed in a 0.1 mol dm-3 alkali-metal carbonate and 1 mol dm-3 alkali-metal chloride solution for at least seven periods of 24 h. After this first operation, the same successive experimental steps are used as described for the membranes in their acid form.' The characteristics of the membranes (supplied by Progil, France) are as follows: membranes 1 1 A, 12A and 13A have a FEP matrix and a thickness of 17 pm; membrane 14 has a PTFE matrix and a thickness of 9 pm. The respective capacities expressed in meq g-l are 0.97 (llA), 1.25 (12A), 0.64 (13A) and 0.69 (14). RESULTS All i.r. spectroscopic measurements are carried out on samples of 32 mm radius with a Beckman I.R.9 double-beam spectrophotometer.Table 1 shows the main results obtained with totally neutralized carboxylic membranes. The spectra are displayed in Analysis of the doublet situated at 1420-1450 cm-l shows that the intensity of the peak at 1420 cm-l depends neither on the amount of water nor on the ionic species (the transmission is ca. 7%) while the intensity of the peak at 1450 cm-l is higher the larger the ratio of the ionic charge Ze to the ionic radius a, Ze/a. Furthermore, the intensity of this peak decreases during the drying process of the membrane, with the exception of Li+ salts where it remains constant. In the case of neutralized membranes, the absorption bands A and B at, respectively, 2650 and 1970 cm-l disappear. Note that these bands are associated with dimer formation through strong hydrogen bonding of the carboxylic groups.' The peak situated at 860 cm-l is not observed for a degree of neutralization below 70%.Some authorslo have assigned this band to the bending vibration of the carboxylate group COO-, although it is generally ascribed to a splitting of the rocking vibration of the CH, group found at 790 cm-l.ll Dehydration of the membrane does not change the intensity of this band. Another peak appears at ca. 1330 cm-l in the spectra of membranes in their salt form and is not observed in the acid membrane. It is assigned1' to the wagging vibration mode of the CH, group and is more intense as the electric field (thus the ratio Ze/a) at the surface of the ion is increased. This band is situated at 1330 cm-l for Li+, 1324 cm-l for Na+ and 1320 cm-l for K+.Its intensity decreases with drying of the membrane and it transforms into a faint peak for Li+ salts, a strong shoulder on the CF, band for Na+ and a weak shoulder for K+ . It is known that the bending vibration do, of water in the membrane yields a band near However, in the case of the systems considered here, this absorption peak cannot be isolated. Quite probably the very strong absorption band fig. 1-3.TABLE 1 .-PRINCIPAL SPECTRAL CHARACTERISTICS OF VARIOUS ALKALI-METAL SALT MEMBRANFS vs coo- vAS COO- dCHo + YOH (4 duration of drying /h 12A 11A 13A 14 12A 11A 13A 14 12A 1 l A 13A 14 Li+ 1 0 1424 1420 2 3 1424 1420 3 18 1424 1420 4 80 1424 1420 Na+ 1 0 1415 1410 2 3 1415 1410 3 20 1415 1410 405 1421 1577 1575 1553 1574 1450 1459 1435 1450 405 1421 1578 1578 1555 1576 1450 1459 1435 1451 405 1421 1577 1578 1555 1576 1451 1458 1435 1450 405 1421 1577 1578 1555 1575 1451 1457 1435 1450 399 1412 1575 1570 1551 1570 1461 1457 1445 1458 399 1412 1575 1573 1553 1571 1461 1457 1445 1458 399 1412 1575 1573 1554 1571 1461 1457 1446 1458 4 80 1415 1410 1399 1412 1575 1573 1553 1571 1461 !457 1446 1458 K+ 1 0 1410 1405 1392 1408 1579 1575 1558 1572 1460 1452 1440 1457 2 3 1410 1405 1392 1408 1584 1578 1558 1572 1460 1452 1440 1457 3 24 1410 1405 1392 1408 1586 1578 1558 1572 1460 1452 1440 1457 4 80 1410 1405 1392 1408 1586 1578 1558 1572 1460 1452 1440 1457 (b) vAscoo- of complex duration dCOO- Or PCHl or do, of water '0 H of drying /h 12A 11A 13A 14 12A 11A 13A 14 12A 1 l A 13A 14 ~~~~ ~ Li+ 1 2 3 4 Na+ 1 2 3 4 K+ 1 2 3 4 0 3 18 80 0 3 20 80 0 3 24 80 860 862 864 864 856 857 858 858 852 848 84 1 840 858 859 860 86 1 854 856 857 857 848 846 842 84 1 857 858 859 860 854 855 855 856 845 843 842 840 857 1660e 858 859 860 853 165Oe 855 166Oe 856 167Oe 857 1669e 849 1654e 847 1662 843 1668 842 1670 - - - 1660e 16% 1662e 1670e 167Oe 165Oe 1660e 1668 1669 1650e 1650e 3381 3368 3357 3380 I - 3365 3346 3340 3354 - - 3330 3327 3327 3333 - - 3313 3309 3311 3328 16% 1650e 3375 3362 3351 3370 - - 3341 3330 3330 3341 - - 3326 3300 3312 3328 - - 3306 3289 3293 3318 165Oe 165Oe 3316 3320 3315 3314 1655e - 3305 3307 3308 3305 - - 3299 3300 3300 3301 - - 3290 3295 3294 3296 CI 0 0 w1004 I.R.INVESTIGATION OF ALKALI-METAL SALT MEMBRANES I 4000 3600 3200 2800 2LOO 2000 1000 1600 l L O O 1200 1000 800 600 waven umber/ cm - I FIG. 1.-1.r. spectra of Li+ salt of a FEP PAA membrane (1 1A) after various times of drying. (-) Initial hydrated state; (---) 3 h ; ( . . . ) 80 h. LO00 3600 3200 2800 2LOO 2000 1800 1600 1400 1200 1000 800 600 wavenumberf cm-' FIG. 2.-1.r. spectra of Na+ salt of a FEP PAA membrane (1 1A) after various times of drying. (-) Initial hydrated state; (- . - .) 3 h ; (---) 20 h; (. . .) 80 h. 4009 3600 3200 2800 2600 2000 1800 1600 :LO0 1200 1000 800 600 wavenumber/cm-' FIG. 3.--I.r. spectra of K+ salt of a FEP PAA membrane (1 1A) after various times of drying. (--) Initial hydrated state; (- . - .) 3 h; (---) 24 h ; (.. . ) 80 h.L. LEVY, M. B. M U Z Z I AND H. D. HURWITZ 1005 of the COO- group at 1575 cm-l overlaps with the a,, absorption band in the region around 1640 cm-l. The contribution of do, produces a strong asymmetry in hydrated membranes on the higher-frequency side of the vAs band of COO-. With decreasing water uptake in the membrane a surprising result is observed. In the case of Li+, the asymmetry (or intensity of the shoulder) is reduced. In the case of Na+, the asymmetry also first decreases and then the band is changed into an absorption step in the dryest membrane. In the case of K+, the asymmetry is first reduced, then at lower water content enhanced to the extent of taking the shape of a peak. Note that the vibration frequencies of the methylene group at 2940-2860 cm-l (stretching), 1330 cm-l (wag- ging) and 790-860 cm-l (rocking) depend on the nature of the counter-ion and the water content in the membrane.DISCUSSION The neutralization procedure, as performed in this investigation, leads to complete dissociation of the carboxylic groups. The total replacement of acid groups by salt groups is confirmed by the disappearance of the vco absorption band at ca. 1700 cm-l,ll corresponding to the stretching vibration of the carbonyl group in the undissociated carboxylic acid. Furthermore, the absence of the A and B absorption bands near, respectively, 2650 and 1970 cm-l and of the peak at 940 cm-l is noteworthy. It indicates that dimers with carboxylic groups cannot be formed in the membrane.l The band at ca.3300 cm-l can be ascribed to the OH stretching vibration vOH of water molecules linked through hydrogen bonds to the carboxylate ion. In the alkali-metal salt membranes, the vOH band intensity and wavenumber depend on the degree of hydration of the membrane. This contrasts with the behaviour observed for the absorption band vOH at ca. 3150 cm-l in the acid membrane,l where vOH was attributed to the OH vibration pertaining to the carboxylic acid. In all systems investigated here, the wavenumber of the maximum of the band at ca. 3300 cm-l decreases with increasing drying. From this shift it is inferred that the hydrogen bonds between the water and the carboxylate anions are strengthened as a function of the decreasing amount of absorbed water. Note that this vOH vibration at ca.3300 cm-l is found at much lower wavenumbers than the equivalent vOH vibration observed at 3400-3500 cm-l in the sulphonic- salt-containing membranes.l2* l3 Obviously, the carboxylate groups are more strongly hydrogen bonded to the water than the sulphonic groups. This can be regarded as a consequence of the basic character of the COO- ion which strongly interacts with the hydrogen ion or with small cations with a rare-gas electronic configuration. Such behaviour is exemplified in homogeneous solutions of alkali-metal acetate as the activity coefficients increase from Li+ to Cs+. Accordingly, Diamond14 has classified the acetate ion as among the water-structure-promoting ions and similarly Gurney15 has assumed that the acetate ion might be linked to a small cation by means of strongly polarized water molecules.From the frequenciesrecorded in table 1 (b)it can be suggested that the hydrogen-bond strength increases following the sequence Li+ < Na+ 6 K+. No significant effect is observed as a function of the ion-exchange capacity during the evolution of the drying process. In the wettest membranes, however, the vOH frequencies are shifted towards smaller values with decreasing exchange capacities in the presence of Li+ and Na+. In the cases of these two salts, one also observes some influence due to the matrix. The presence of the hydrophobic -CF3 group in the FEP membrane enhances the strength of the hydrogen bonds as compared with the PTFE matrix.1006 I.R. INVESTIGATION OF ALKALI-METAL SALT MEMBRANES It is assumed that the observed asymmetric stretching frequency vAS of the carboxylate ion is closely dependent on the interaction between a cation and the COO- group.Indeed, the bonding between a metallic ion and a ligand like -COT or -SO, lowers the symmetry of the latter and therefore enhances the complexity of the vibrational spectra.l6 Since the symmetry of the free carboxylate group is already low (C2,) and all its vibration modes are active in the infrared, the bonding of this group with a particle might have a marked effect on the values of the absorption frequencies. However, the formation of the bond will keep the number of absorption bands constant, contrary to the behaviour observed in the case of the sulphonate groups of C,, symmetry. The formation of a metal-oxygen bond in a structure of type I M - 0 \ O//C-R affords a redistribution of the electron densities between the CO bonds in the ionized carboxy group With increasing strength of the M-0 bond, the asymmetric COO- stretching frequency vAS will rise towards the frequency characteristic of the double-bonded C=O group, whereas the symmetrical COO- stretching frequency vs will fall towards the value characteristic of the single-bonded C-0.As a consequence, the difference Av, = vAS - vs yields a measure of the interaction of the counter-ion with the fixed carboxylate sites." Sawyer and Paulsen18 have suggested in an i.r, investigation of EDTA complexes that, for a difference Avl larger than 225 cm-l, the metal-oxygen bond should be predominantly covalent and that for a smaller difference the bond should be of prevailing ionic character.On the other hand, contrary to the properties of a structure of type I, a characteristic feature of the formation of a metal-oxygen bond in a structure of type I1 and of type I11 0 0 M/ \ C-R \ .;;;i M - 0 M - 0 *\ gc-R (11) (111) consists of the occurrence, in both cases, of two equivalent C-0 bonds which are similarly affected by the metal ion. Therefore, in many complexes of types I1 and 111, the replacement of one metal ion by another will lead to the shift of vAs and vs in the same direction. * From the values recorded in table 1 it is observed that vs and vAS behave quite differently as functions of the nature of the ion. Furthermore, the values of Avl given in table 2 are lower than 225 cm-l but increase as a function of the ionic radius.Therefore, it can be inferred that structure I is formed, the prevailing ionic character of the metal-ion-carboxylate bond being larger the smaller the counter-ion. No significant effect on Avl is observed as a function of the ion-exchange capacity and the nature of the matrix. * It has been established,'" for example, that the nickel, zinc and copper acetate salts form, respectively, structures of types I, 11 and 111. The v, wavenumbers of the bidentate copper acetate complex and zinc acetate complex are, respectively, 1604 and 1550 cm-', whereas for the monodentate nickel acetate complex vAS = 1530 cm-l.L. LEVY, M. B. MUZZI AND H. D. HURWITZ 1007 The absorption band found at ca.1450 cm-l has been assignedlg to two overlapping peaks corresponding first to the scissoring vibration SCHp of the methylene groups and secondly to the coupling of the C-0 stretching vibration with the out-of-plane bending mode yOH of the OH groups belonging to water molecules linked to COO- sites via hydrogen bonds. This last contribution is responsible for the fact that the intensity of this band is stronger the larger the water uptake by the membrane. TABLE 2.-DIFFERENCE BETWEEN THE VIBRATION FREQUENCIES OF THE CARBOXYLATE GROUP FOR DIFFERENT ALKALI-METAL IONS A v ~ = v A S - V S Av2 = ( ~ c , , + Y O H ) -vs 12A 11A 13A 14 12A 11A 13A 14 Li+ 153 158 150 154 27 28 30 29 Na+ 160 163 155 159 46 47 47 46 K+ 174 173 166 164 50 47 48 49 The intensity of the S,,2+yo, band increases following the sequence K+< Na+ < Li+ at any degree of swelling of the membrane, which leads us to believe that, even in the dryest specimen, the ionic pair COO-Lit is created with the Li+ ion at least partially hydrated.This assumption is in accordance with the ionic interaction model suggested by Gurney15 and Harned and Robinson.20 As regards the difference Av, = (dCH2 + yOH) - vs shown in table 2, it appears that its value depends neither on the exchange capacity nor on the type of matrix, but that it is strongly influenced by the type of counter-cation. At the present stage of our research, it is still difficult to elaborate more on the meaning of Av,. The step and peak of absorption, respectively, which take the place of the So, absorption band contribution at 1669 cm-l for the Na+ and K+ salt membranes at their lowest degree of swelling, raise some problems of interpretation.Concerning this particular morphology of the spectra, two hypotheses can be proposed, each of which tries to explain the absorption band pattern at low water content. First, the idea can be put forward that some specific hydration structure arises yielding a new contribution to SOH. Secondly, the occurrence of a splitting in the vAS stretching vibration can be suggested which ensures the presence of a new peak. If we take the first assumption for granted, we might suggest that conformational changes in the grafted polyelectrolyte occur during the drying process of membranes neutralized by large cations. In this respect it is significant that the pattern of the different ,CH, vibrations (stretching, bending, wagging, rocking, etc.) are influenced by the degree of hydration and the type of cation.The wagging vibration band at ca. 1320 cm-l is strongly attenuated in the presence of K+ and affected to a lesser extent in the presence of Na+ salts of the membrane at their lowest degree of swelling. The absence of this band in the acid membrane is indicative of the high rigidity of the PAA chains.l In view of these conformational changes, the residual water molecules should be compelled to organize into highly rigidly associated structures. Strong hydrogen bonding of water molecules in the environment of hydrocarbon chains is well known.,' Under such conditions the do, bending frequency is shifted towards higher values.This might determine the appearance of a step or a peak at ca. 1669 cm-l. A similar result for the So, of water has been observed with Na+, K+ and Cs+ salts of PSSA mernbranes.l3 Alternatively, it might be suggested that a framework of ionic clusters \1008 I.R. INVESTIGATION OF ALKALI-METAL SALT MEMBRANES appears which significantly stabilizes the ionomeric structure at low water content of the membrane. The concept of aggregation of counter-ions and carboxylate ions (clusters and multiplets) in carboxylate-containing ionomers constitutes the basis of the interpretation of a large number of experimental data in recent years.** 9 9 l3 The building of these clusters is prevented by a large amount of water and by strong ionic pairing of the fixed anions with small cations like Li+.The ionic clustering is strongest with Cs+ salts and K+ salts of the polyelectrolytes. The model of clustering predicts further the loss of individual ionic hydration shells, the remaining water molecules being accomodated within the interstices of the ionic network in the vicinity of the charged sites. There the joint action of the hydrophobic matrix and of the polar groups ensures strong hydrogen bonding. We proceed now with the second assumption. In this case, some reasonable argument must be found in order to account for the splitting of the vAs vibration mode of the carboxylate ion notwithstanding the low symmetry of this group. The C,, symmetry prevents the spectral splitting of this band unless there exist simultaneously two or more molecular structures sufficiently different so as to be characterized by different wavenumbers.It might be suggested that the band at 1669 cm-l corresponds to a molecular structure existing to a lesser degree than the -COO--metallic-ion pair which is related to the absorption band at ca. 1570 cm-l. In this respect, it becomes extremely significant that the difference Avl corresponding to this new configuration will be 254 cm-l. This means that a small proportion of the carboxylate ions forms bonds of type I, as considered above. The number of these bonds increases along the sequence Li+ < Na+ < K+. Note that Chuveleva et aZe2, have detected in the i.r. spectroscopic investigation of uranyl ion complexes with carboxylic resins a band at ca.1640 cm-l in addition to that located at 1540 cm-I. The two bands have been related to two different types of bonds between the carboxylate group and the uranyl ion. It has also been suggested by these authors that the appearance of a band at 860 cm-l (after 65% conversion of the resin to uranyl form) might be interpreted as \ a splitting of the rocking mode of the ,CH, group (at 790 cm-l) caused by complex ion formation. However, it is worth stressing the fact that this band at 860 cm-l ought to be assigned to the bending motion of the carboxylate group. This attribution is all the more pertinent in view of the presence of the band at 860 cm-l, independent of the existence of the band at 1669 cm-l, and on account of the strong influence of the nature of the counter-cation on this band.CONCLUSIONS The neutralization of the carboxylic groups in the membrane determines the formation of a set of different ionic associated species. At this stage of our investigation it is difficult to assess whether ionic clustering occurs in our membranes. Some complementary experiments with D,O have to be performed in order to ascribe precisely the peak appearing at 1669 cm-l either to the bending vibration mode of water or to the vAS of the carboxylate ion linked covalently to the metal ion. However, note the important role played by the water with respect to the ionic configuration. The same is true for the exchange capacity which enhances considerably the ability of the ions to interact by means of strong bonds. As regards the hydrogen-bond strength between the water and the carboxylate ion, it is observed that it is significantly increased in the presence of the K+ ion.L.LEVY, M. B. MUZZI AND H. D. HURWITZ 1009 I L. Y. Levy, A. Jenard and H. D. Hurwitz, J. Chem. SOC., Faraday Trans. I , 1982, 78, 17. E. Selegny, communication presented at the Advanced Study Institute on Charged and Reactive Polymers, Forges-les-Eaux, 1973. L. Y. Levy, Ph.D. Thesis (Universite Libre de Bruxelles, 1979). L. M. Kalyuzhnaya, A. N. Krasovskii,Yu. N. Panov, A. G. Zam, I. S. Lishanksiiand S. Ya. Frenkel, Vysokomol. Soedin., Ser. A, 1975, 17, 993. S. R. Rafikov, Yu. B. Manakov, I. A. Ionova, G. P. Gladyshev, A. A. Andrusenko, 0. A. Ponomarev, A. I. Vorobeva, A. A. Berg, L. F. Antonova, E. I. Ablyakimov, M. F. Sisin and A. A. Smorodin, Vysokomol. Soedin., Ser. A, 1973, 15, 1974. Yu. N. Boyarchuk, E. D. Andreeva, L. V. Konovalov and V. N. Nikitin, Zh. Prikl. Spektrosk., 1975, 23, 101. E. D. Andreeva, V. N. Nikitin and Yu. M. Boyarchuk, Vysokomol. Soedin., Ser. B, 1975, 17, 773. A. Eisenberg and M. King, Ion-containing Polymers. Physical Properties and Structure, ed. R. S . Stein (Academic Press, New York, 1977), vol. 2. A. I. Grigorev, Zh. Neorg. Khim., 1963, 8, 802. Wilson (Elsevier, Amsterdam, 1976), vol 6. 13 M. G. Marina, Yu. B. Monakov and S. R. Rafikov, Usp. Khim., 1979,443,722. l1 K. Eross, Analytical Infrared Spectroscopy, in Comprehensive Analytical Chemistry, ed. Wilson and l2 G. Zundel, Hydration and Intermolecular Interaction (Academic Press, New York, 1969). l3 L. Y. Levy, A. Jenard and H. D. Hurwitz, J. Chem. SOC., Faraday Trans. I, 1980, 76, 2558. l4 R. M. Diamond, J. Am. Chem. SOC., 1958,80, 4805. l5 R. W. Gurney, Ion Process in Solutions (Dover, New York, 1953). l6 J. R. Ferraro and J. S. Ziomek, Introductory Group Theory (Plenum Press, New York, 1969). l7 K. Nakamoto, J. Fujita, S. Tanaka and M. Kobayashi, J. Am. Chem. SOC., 1957, 79, 4904. In D. T. Sawyer and P. J. Paulsen, J. Am. Chem. SOC., 1959, 81, 816. 2o R. H. Stokes and R. A. Robinson, J. Am. Chem. SOC., 1948, 70, 1870. 21 G. Nemethy and H. A. Sheraga, J. Chem. Phys., 1962, 36, 3382. 22 E. A. Chuveleva, N. K. Yufryakova, P. P. Nazarov and K. V. Chmutov, Russ. J. Phys. Chem., 1970, J. De Villepin and A. Novak, Spectrochim. Acta, Part A , 1971, 27, 1259. 44, 313. (PAPER 1 /058)

ISSN:0300-9599    

DOI:10.1039/F19827801001

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1982, 78, 1011-1019 Influence of Some Solvents and Solutes on Illuminated Red Mercury(@ Sulphide Electrodes BY R. STEPHEN DAVIDSON,* CHARLES J. WILLSHER AND (IN PART) COLIN L. MORRISON Department of Chemistry, The City University, Northampton Square, London EClV OHB Receiued 20th February, 198 1 A study has been made of the irradiation of red mercury(@ sulphide electrodes in water, acetonitrile, pyridine and methanol containing dissolved tetra-alkylammonium salts. Photocurrents were found to be due to solvent oxidation, iodide oxidation and lattice decomposition, but which of these processes actually operates is determined by the individual solvent, solvation of the iodide ion and adsorption of the tetra-alkylammonium cation on the semiconductor surface.The length of the alkyl chain of the substituted ammonium ion was critical. Instability of the sulphide resulted from adsorption of tetra-alkylammonium cations in methanol and acetonitrile. In the case of pyridine, cation adsorption was not significant, but the presence of iodide induced a negative shift of the sulphide band edges. Exploration of semiconductor-electrolyte systems to harness solar energy is well d0cumented.l Much attention has been paid to the performance of electrodes in aqueous electrolyte solutions since water is a cheap, abundant solvent. A further attractive feature of water is that, if photoelectrolysis occurs, hydrogen (an ideal fuel) is produced, and this is of great economic interest.2 If progress is to be made in developing a successful cell then an understanding of semiconductor-electrolyte interactions is necessary.The choice of electrolyte can be important in determining the stability of a system, e.g. cadmium sulphide undergoes photocorrosion in aqueous solutions of many electrolytes but is stable in sulphide + polysulphide solution^.^ We have previously shown that the performance of mercury@) sulphide electrodes is highly dependent upon the type of electr~lyte.~ Use of solvents other than water has also been investigated. Thus both n-type molybdenum selenide5 and sulphide6 are photostable in non-aqueous systems such as acetonitrile and ethanol. Cadmium sulphide is also photostable in acetonitrile solutions containing sodium iodide' and will also reduce heptyl viologen in acetonitrile.* Valuable information concerning semiconductor-band edges, flatband potentials, surface states, stability and redox reactions of solution species has been accumulated in many of the reported studies.Semiconductor-solution ' energetics' (and some kinetics) have been evaluated, and the solvent is considered an inert medium (with the exception of alcohols, where solvent oxidation can occurQ~ lo) for dispersing interesting electroactive species, and a supporting electrolyte is included to assist conductivity. This electrolyte is deemed inert and is usually either a lithium or tetra-n-alkylammonium perchlorate, nitrate or other non-oxidisable anion. This paper describes an investigation of the effect of solvent and the structure of tetra-n-alkylammonium ions as supporting electrolyte upon the performance of mercury(r1) sulphide electrodes.In earlier work we found1' that irradiation of mercury(I1) sulphide electrodes in aqueous potassium nitrate solutions containing methyl viologen led to reduction of methyl viologen at the counter-electrode. The 101 11012 I L L u MI N A TED RED ME R c u R Y(II) s u L PHI D E E L E c T R o D E s characteristic blue colour of the viologen was clearly observed. However, the efficiency of the reaction (as measured by the photocurrent and the extent of reduced material) decreased as the irradiation was continued. When the mercury(I1) sulphide electrode was removed from solution, washed and then re-irradiated in a cell containing aqueous potassium nitrate (but no viologen), very little photoreactivity was observed.There were similar results when non-aqueous solvents, e.g. acetonitrile, were employed. We concluded that the viologen had poisoned the surface of the sulphide. Since the viologen is an ammonium cation it seemed worthwhile examining whether other systems containing ammonium cations would poison the material. EXPERIMENTAL ELECTRODE PREPARATION A platinum-mesh electrodeL0 was coated with mercury(1r) sulphide in the following way. Water (25 cm3, de-ionised) was de-oxygenated by purging with nitrogen for 30 min. The sulphide (2 g) was added and the mixture boiled and then sonicated (Dawe Instruments ultrasonic probe type 7530A). The electrode was dipped into the suspension produced by this procedure, taken out and then dried in a stream of hot air.The ‘dipping and drying’ procedure was repeated three to four times to obtain an even red coating (average weight 0.025 g) on the mesh. The electrode was stored in a solution of the electrolyte for 12 h to effect equilibration. IRRADIATION PROCEDURE The lamp source was a 1.6 kW xenon lamp, the output of which was focused onto the mesh by means of a concave mirror and appropriate lenses. The light was filtered through an aqueous copper(n) chloride (0.3 mol dm-3) solution which has a maximal transparency at 515 nm with a half-bandwidth of 90nm. The light intensity at the electrode surface was measured as 2.5+ W cm-2 (Macom light meter). MEASUREMENTS The cell was of conventional design accommodating the mercury sulphide electrode, a platinum counter-electrode and a reference electrode (sealed saturated calomel electrode, Electronic Instruments Ltd).The cell held 100 cm3 of electrolyte. Potentials were measured with a Phillips high-impedance voltmeter P.M. 2431 1 (internal resistance = lo6 - lo7 a). Potentials were applied to electrodes and currents measured using a Wenking LB 75L potentiostat. MATERIALS Water was distilled prior to use. Methanol, acetonitrile and pyridine were purchased with the lowest water assay (0.05 %) and used as received. All salts were of the highest possible purity, recrystallised from the appropriate solvent if necessary and stored under vacuum. All potentials were measured against a saturated calomel electrode (SCE) in both aqueous and non-aqueous solvents, and a platinum mesh, identical to the sulphide support mesh, formed the counter- electrode.Purging with dry nitrogen was employed to free the electrolyte of oxygen and was bubbled, very slowly, through the electrolytes during the experiments. Electrolyte agitation was effected by increasing the bubbling rate. Routine atomic absorption techniques were used to analyse the electrolytes for solubilised mercury. RESULTS AND DISCUSSION We have previously reported that irradiation of mercury(I1) sulphide in aqueous iodide solutions leads to dissolution and a colour change from red to l2 The darkened material has been identified as meta-cinnabar.13 We have now examined samples, darkened under various conditions by EDAX spectroscopy* and found that * We are grateful to Dr M. Phillips of the Physics Department, The City University, for carrying out the measurements.R.S. DAVIDSON, C . J. WILLSHER A N D C. L. MORRISON 1013 the colour change is not attended by a change in stoichiometry. Furthermore, as the amount of meta-cinnabar in the cinnabar increases so the conductance increases until that of meta-cinnabar is attained. This explains why the photocurrents of mercury(1r) sulphide electrodes rise on blackening.12 Since meta-cinnabar gives a very small photoeffect, there is an optimum level for the meta-cinnabar which produces the best electrodes. The extent of mercury solubilisation and colour change of the electrode is determined by the degree of interaction of the iodide with the sulphide.* The effect of added alkylammonium iodides upon the performance of mercury(I1) sulphide electrodes was therefore studied since the solubilisation and colour-change processes should give an indication of the effectiveness of the interaction of anions with the sulphide surface under these conditions.TABLE 1 .-PHOTOEFFECTS GENERATED BY RED MERCURY(II) SULPHIDE IN SOLUTIONS OF TETRA-ALKYLAMMONIUM SALTS IN WATER relative relative photocurrent/pA at indicated photo- applied potential/V us. SCEb dissolved potential/ salta mV - 0.6 -0.4 -0.2 0.0 +0.2 remarks ~~ ~~ ~~ NH,I - 30 -3.0 -2.3 +3.2 +6.8 +31.0 “C,Hd,I - 60 d C d - 100 -5.0 -3.0 + 11.0 +20.0 e C +3.0 +17.0 “CH3141 -0.1 +O.l +0.2 +2.0 N(C,H,),I - 75 - 1.1 “C4H,),I - 75 0.0 +0.1 f0.2 +0.5 +0.4 “C6 H 13141 - 50 -0.1 0.0 0.0 +0.1 + 0.4 N(C,Hd,NO, - 90 0.0 +0.4 +0.7 +2.4 +4.O C e f g f B a Salt concentration is 0.05 mol dm-3 and the substituent alkyl chains are primary.Potentiostatically controlled. Large (> 50pA) dark current. HgS blackened and HgS blackened, no solubilisation. f HgS remained red, underwent slight solubilisation. no solubilisation. HgS blackened very slightly, no solubilisation. The photoelectrochemical behaviour of mercury(I1) sulphide electrodes in water containing various tetra-n-alkylammonium salts is shown in table 1 . The results clearly show that, as the length of the alkyl chain on the ammonium cation is increased, the photocurrents decrease. Furthermore, the extent of photochemical and/or electro- chemical reaction decreases since mercury solubilisation decreases with increase in chain length and, in particular, the efficiency of blackening.We attribute the dependence of the efficiency of the photoinduced processes upon chain length as being due to the cations being adsorbed on the surface of the sulphide. As the chain length of the cation is increased, the more the approach of the counterion (e.g. -1 or OH) will be hindered, with a consequent decrease in photoreactivity. The rather low photopotentials observed in these systems (a value of ca. - 200 mV is observed in aqueous potassium nitrate) indicate that the surface of the sulphide (and probably the Helmholtz layer) * A referee has suggested that the formation of meta-cinnabar could be accounted for by the iodide solubilising the mercury as HgIz-, so allowing the concentration of S2- ions to build up in the semiconductor.Reaction of the S2- with solubilised mercury would result in the precipitation of the black form of mercury(r1) sulphide.1014 I L L UM I N A TE D R ED M E R c u R Y(II) s u L PHI D E E L E c T R ODE s TABLE 2.-hOTOEFFECTS GENERATED BY RED MERCURY(I1) SULPHIDE IN SOLUTIONS OF TETRA-ALKYLAMMONIUM SALTS IN ACETONITRILE relative relative photocurrent/pA at indicated photo- applied potential/V us. SCEb dissolved potential/ salta mV -0.6 -0.4 - 0 . 2 0.0 + 0 . 2 remarks NH,I -140 -7.0 - 3 . 2 -2.9 +3.0 -210 -2.0 0.0 + 3 . 2 +6.0 -140 -3.6 -0.4 +2.4 +8.0 +l.O +1.2 +7.5 - 170 +4.0 +9.5 - 220 0.0 + 1.6 +6.2 C d c e C f C 9 C C g N(CH,),I N(C,H,),I N(C3H441 H(C,H,),I C c C N(C6H13)41 - 145 C 9 N(C,H,),NO, -100 -0.3 0.0 +0.1 +0.4 +0.5 g a-c As table 1. HgS remained red, extensive mercury solubilisation.HgS blackened and underwent extensive solubilisation. f HgS blackened slightly and underwent extensive solubilisation. g No blackening took place, but extensive mercury solubilisation was noted. have been modified by the cations so as to either shift the sulphide band edges to more positive potentials or alternatively decrease the extent of band-bending. The results for acetonitrile as solvent are shown in table 2. With this solvent mercury solubilisation is extensive and increases as the chain length increases (fig. 1). Furthermore, the shapes of the photocurrent against time plots vary with the length of the alkyl chain (fig. 2). Very small photocurrents were observed with tetra- n-butylammonium nitrate.This probably reflects the fact that there is not a suitable redox couple present in solution. As a consequence of this the sulphide undergoes decomposition by reaction of the positive holes with the sulphide-producing Hg2+ ions. Decomposition via this route is thermodynamically feasible.4* l4 The decomposition that is observed with the long-chain alkylammonium cations is interpreted as being caused by the adsorption of the cations on the surface hindering reaction with the iodide ions, thereby favouring the decomposition reactions. The shorter-chain compounds are less well adsorbed, thereby allowing the interaction of the iodide ions with the sulphide. This interaction will account for some of the solubilisation, and apparently reprecipitation of the mercury as meta-cinnabar occurs since blackening is observed.The results for pyridine as solvent are shown in table 3. All the iodides behave in a similar manner. The small photocurrents for the nitrate are probably due to the lack of a suitable redox couple. Even under these conditions mercury solubilisation does not occur, whereas in acetonitrite extensive mercury solubilisation was observed. The stabilising influence of pyridine has been previously noted.15 The finding that most of the iodides behave in a similar way suggests that cation adsorption on the surface is not particularly important. This is probably due to the fact that pyridine is strongly adsorbed on the surface. This apparently still allows the iodide ions to react with the sulphide, giving rise to photocurrents as a result of iodide ion oxidation and sulphide solubilisation.The results for methanol are shown in table 4. The surprising feature of using this solvent is that positive photopotentials are observed. With the tetra-n-butylammonium nitrate a positive photopotential is recorded. It appears that the methanol isR. S. DAVIDSON, C. J . WILLSHER A N D C. L. MORRISON light off / t X 0 0 X X X 0 1015 number of carbon atoms in alkyl chain FIG. 1 .-Solubilised mercury as a function of chain length in tetra-alkylammonium iodides dissolved in acetonitrile ( x ) and methanol (0). w 5 min ---- 2 2 p A L -- -9.OpA I -- -30.OpA (i) (ii) (iii) FIG. 2.--Current against time plots for mercury(I1) sulphide held at -0.4 V us. SCE in acetonitrile containing (i) ammonium iodide, (ii) tetrapropylammonium iodide and (iii) tetra-n-hexylammonium iodide.1016 ILLUMINATED RED MERCURY(@ SULPHIDE ELECTRODES TABLE 3 .-PHOTOEFFECTS GENERATED BY RED MERCURY (11) SULPHIDE IN SOLUTIONS OF TETRA-ALKYLAMMONIUM SALTS IN PYRIDINE relative photo- applied potential/V us.SCEb relative p ho tocurren t /PA at indicated dissolved potential/ sal ta mV -0.6 - 0 . 4 -0.2 0.0 +0.2 remarks - 1.6 +3.8 +4.2 NHJ - 250 ? C d N(CH3)J N(C2H5)J e e C d C d - - - - - - - - - - - - N(C,H,)*I -275 +0.8 +1.9 +3.5 +7.0 N(C4He)J -230 + 1.3 +2.2 +3.6 +6.0 N(C6H 1314’ - 265 c d N(C,H,),NO3 -155 -0.8 -0.2 +0.2 +1.4 +1.9 0.0 +0.1 +0.6 +6.0 f a-c As table 1. Solubilisation occurred. Salts were sparingly soluble. f No blackening or solubilisation occurred. TABLE 4.-PHOTOEFFECTS GENERATED BY RED MERCURY(I1) SULPHIDE IN SOLUTIONS OF TETRA-ALKYLAMMONIUM SALTS IN METHANOL relative relative photocurrent/pA at indicated photo- applied potential/V us.SCEb dissolved potential/ salta mV -0.6 - 0 . 4 -0.2 0.0 +0.2 remarks C C NH,I + 180 -38.0 -5.0 -1.0 C d N(CH3)J + 60 -54.0 - 1.4 +3.0 C d N(C2H5)J +60 -27.0 -6.5 -14.0 -11.5 C d N(C,H,)J +90 -22.0 -5.5 -5.5 -5.0 C d + 70 -5.0 -1.5 0.0 + 4 . 0 C e + 30 -8.0 -1.6 +2.0 0.0 C f f N(C4He)J N(CI3H 13141 N(CIH9)4NO, +40 -5.0 -4.5 -2.6 +4.0 +3.5 a-c As table 1. Extensive blackening and mercury solubilisation. Partial blackening and considerable solubilisation of mercury. f Very slight blackening and considerable solubilisation of mercury. reacting with the sulphide to produce a species which is part of a redox couple that can undergo electrochemical reaction at the uncovered platinum surface of the electrode.This type of behaviour has been observed for electrodes fabricated from platinum mesh and powdered titanium dioxide, in the presence of reducing ions.16 That this typk of process is occurring is shown by the photoeffect against time plots depicted in fig. 3. Part (i) shows the photovoltage plot. At point A, the sulphide is fresh and red, and initial illumination commences. The shape of the response A-B is typical of the n-type behaviour of the sulphide,12 i.e. the voltage shifts negatively on illumination. Switching off the light at point B (at this stage the electrode is now black in colour) does not cause the e.m.f. to return to the initial dark reading, but it adopts a more negative value and point C is reached.Re-illumination produces a positive photovoltage shift; to attain the voltage observed at B, point D is reachedR. S. DAVIDSON, C. J. WILLSHER AND C. L. MORRISON 1017 (i) - - -220mV - light on light off light off - - -340 mV -- - 400mV E - 20 min Smin -- 16.0 PA --• 9.5pA --• 7 . O V A t++---+ Smin 10min FIG. 3.-Plots in methanol+ tetraethylammonium iodide for (i) voltage against time and (ii) current at 0.0 V us. SCE against time. on the voltage against time plot. The decline D-E on ending irradiation is analogous to B-C. This type of behaviour is typical of a blackened sulphide electrode in the presence of a reducing ion.* Part (ii) of fig. 3 shows a typical photocurrent against time plot.The most outstanding feature is the large anodic overshoot seen on ending the irradiation. The time taken for the decline of the overshoot is quite long, but it can be accelerated by vigorous stirring of the electrolyte. Since a similar effect is observed with tetra-n-butylammonium nitrate as electrolyte it is likely that the anodic currents and positive photo-e.m.f. are due to material produced by oxidation of the solvent. When iodides are used, iodine may also contribute to these effects. However, the reaction with methanol appears to dominate the reactions in all the alkylammonium iodides, with perhaps the tetra-n-hexylammonium salt being an exception. The solubilisation of the mercury may be due to the inherent instability of the1018 I L L U MI N A TE D R E D ME R C U R Y(I1) S U L P H I D E EL E C T R 0 D ES TABLE SUMMARY OF PROPOSED ELECTROLYTE EFFECTS AND REACTIONS ~ ~ ~~ reactions probably occurring at the illuminated sulphide solvent electrodea effects of the solute and solvent water 2H,O + 4h+ -+ 0, + 4H+b 21- + 2h+ + IZc HgS + 41- + 2h+ -+ HgI,,- + S HgS + 21-+ I, -+ HgI,,-+ S (i) Cation adsorption on the (ii) Water/iodide repelled by cations (iii) Water prevents anodic (iv) Solvation of iodide by water sulp hide .8 with long alkyl chains.8 decomposition." necessary for b1ackening.f acetonitrile 21- + 2h+ + (i) Cation adsorption on the sulphide! (ii) Acetonitrile/iodide hindered by cations with long alkyl chains.g (iii) Acetonitrile fails to prevent anodic decomposition.e (iv) Acetonitrile solvation necessary for b1ackeni'ng.f (v) Mercury solubilisation enhanced with longer alkyl chains.8 (i) Cation adsorption is unimportant.(ii) Pyridine solvation of iodide (iii) Pyridine prevents anodic decomposition.e (iv) Interaction of pyridine-solvated iodide with the sulphide causes a negative shift in the band edges. of the sulphide.8 HgS + 41- + 2h+ + HgI,,- + S 1 HgS + 21-+ I, -+ HgI,,-+ S J HgS + 2h+ + Hg2+ + Se pyridine 21- + 2h+ -+ I,c HgS + 41- + 2h+ -+ HgI,,- + S HgS + 21- + I, -+ HgI,,- + S 1' causes no blackening! methanol 21-+ 2h+ + I, (i) Cation adsorption on the surface (ii) The solvent is a blackening agent.f (iii) Methanol solvation of iodide necessary for b1ackening.f (iv) Mercury solubilisation enhanced with longer alkyl chains. (v) Methanol/iodide hindered by cations with long alkyl chains.g (vi) Other, undetermined effects.HgS + 41- + 2h+ -, HgI,,- + S HgS + 21- + I, + HgI,,- + S J other unknown decomposition reactions including HgS + 2hf + Hg2+ + S" a h+ is a light-generated positive hole in the semiconductor; water oxidation; iodide oxidation; iodide-induced solubilisation; anodic decomposition (in and the fate of elemental sulphur is unknown: it may be oxidised in water and solubilised in the organic solvents); f the blackening is discussed elsewhere;l5 ~7 the longer the alkyl chain, the greater the effect.R. S. D A V I D S O N , C. J. WILLSHER A N D C. L. MORRISON 1019 sulphide in this solvent or due to its reaction with the solvent. At present we cannot ascertain the extent to which each participates.However, the extensive solubilisation observed with the tetra-n-hexylammonium compound suggests that mercury sulphide is inherently unstable in this solvent. The various chemical reactions that can occur in the different solvents are summarised in table 5. Presumably the extents of mercury solubilisation and blackening are intimately connected since the reprecipitation of the mercury has to compete with the solubilised mercury escaping from matrix of the sulphide, and the efficiency of this process will depend upon the nature of the sulphide surface and the solvent. In our earlier work4 we have attempted to determine the flatband potential of the sulphide in aqueous solution. Inspection of tables 1-3 shows that the flatband potential apparently varies with solvent and with the alkylammonium cation.This affects the photostability of the electrode, e.g. no mercury solubilisation in water and extensive solubilisation in acetonitrile. Examples are known where a change of solvent has enabled the construction of stable photo electrode^.^? However, the results described show the importance of the supporting electrolyte. In the case of iodide ions in pyridine it is shifted to negative values. Furthermore, the chemical modification of semiconductors, e.g. by attaching silanes covalently,17 may lead to impairment of the reaction due to hindering the combination of photogenerated holes with reducing ions. We thank the N.R.D.C. for a fellowship to C . J. W. and the S.R.C. for a research assistantship to C. L. M. A. J. Nozic, Annu.Rev. Phys. Chem., 1978, 29, 189; K. Rajeshwar, P. Singh and J. Du Bow, Electrochim. Acta, 1978,23,1117; M. Tomkiewicz and H. Fay, Appl. Phys., 1979,18,1; M. A. Butler and D. S. Ginley, J. Muter. Sci., 1980, 15, 1 ; R. Memming, Electrochim. Acta, 1980,25,77; A. Heller and B. Miller, Electrochim. Acta, 1980, 25, 29. H. P. Maruska and A. K. Ghosh, Solar Energy, 1978, 20, 443. A. B. Ellis, S. W. Kaiser and M. S. Wrighton, J. Am. Chem. Soc., 1976, 98, 1635. R. S. Davidson and C. J. Willsher, J. Chem. Soc., Faraday Trans. I , 1980, 76, 2587. L. F. Scheenmeyer and M. S. Wrighton, J. Am. Chem. Soc., 1980, 102, 6964. L. F. Scheenmeyer and M. S. Wrighton, J. Am. Chem. Soc., 1980, 101, 6496. K. Nakatani, S. Matsudaira and H. Tsubomura, J. Electrochem. Soc., 1978, 125, 406. F. D. Saeva, G. R. Olin and J. R. Harbour, J. Chem. Soc., Chem. Commun., 1980,401. M. Miyake, H. Yoneyama and H. Tamura, Electrochim. Acta, 1976, 21, 1065. M. Miyake, H. Yoneyama and H. Tamura, Chem Lett., 1976,633. lo R. S. Davidson, R. R. Meek and R. M. Slater, J. Chem. Soc., Faraday Trans. I , 1979, 75, 2526. l1 R. S. Davidson and C. J. Willsher, unpublished results. l2 R. S. Davidson and C. J. Willsher, Nature (London), 1979, 278, 238. l3 R. S. Davidson and C. J. Willsher, J. Chem. Soc., Dalton Trans., 1981, 833. We apologise for the fact that we failed to reference the important paper by H.A. Kagi, Y. Fujita and E. Takabatabe (Photochem. Photobiol., 1977, 26, 373) in which some photochemistry of mercury(r1) sulphide is described. R. S. Davidson and C. J. Willsher, Faraday Discuss. Chem. Soc., 1980, 70, 177. 1979, 75, 2517. M. Tomkiewicz, J. Electrochem. Soc., 1980, 127, 1518. l4 C. J. Willsher, Ph.D. Thesis (University of Leicester, 1980). l6 H. H. Chambers, R. S. Davidson, R. R. Meek and R. M. Slater, J. Chem. SOC., Faraday Trans. I , (PAPER 1/296)

ISSN:0300-9599    

DOI:10.1039/F19827801011

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I, 1982, 78, 1021-1028 Extended X-ray Absorption Fine Structure (EXAFS) Study of Cobalt-Porphyrin Catalysts Supported on Active Carbon BY RICHARD W. JOYNER*~ School of Chemistry, University of Bradford, Bradford BD7 1DP AND J. A. ROBERT VAN VEEN AND WOLFGANG M. H. SACHTLER Koninklijke/Shell-Laboratorium, Amsterdam, (Shell Research B.V.), The Netherlands Received 13th March, 1981 Two samples of an active-carbon-supported cobalt-porphyrin catalyst have been studied by EXAFS. The first was the ‘as prepared’ catalyst, the second had been heated in nitrogen at 973 K. For the ‘as prepared ’ catalyst the experimental result agreed well with that calculated with two coordination shells : (1) Co-N, R, = 1.95 0.03 A, CN = 4 and (2) Co-C, Rj = 3.08 & 0.03 .A, CN = 8.The structure is thus very similar to that in the unsupported porphyrin, indicating that little decomposition had occurred. The EXAFS result shows no evidence for a cobalt-support interatomic distance. For the treated sample the experimental spectrum agrees well with that calculated for only a single shell, Co-N, R, = 1.95 k0.03 A, CN = 4. It is suggested that the Co-C interatomic distances have been randomized by the heat treatment through elimination of the meso-carbon atoms of the porphyrins and subsequent reaction of the remaining fragments with the carbon surface. The CON, group is retained in the heat-treated catalyst. Heat treatment, e.g. for 1 h at 970 K in nitrogen, converts metalloporphyrins supported on an active carbon into reasonably active oxygen-reduction electrocatalysts.The chemical reactions during the heat treatment are insufficiently understood and form the subject of the present research. In a previous study2 it was found that (i) with MeTPP/Norit BRX [TPP = meso-tetraphenylporphyrin, fig. 1 (a)] benzene was released upon heating, indicating loss of the phenyl groups; (ii) heat-treated carbon-supported CoPc [Pc = phthalocyanine, fig. 1 (b)] and CoTBP [TBP = tetrabenzoporphyrin, fig. 1 (c)] catalysts have about equal activity, which suggests that the bridging nitrogen atoms are removed from the Pc ligand during heat treatment [loss of nitrogen was indeed found (with X.P.S.) for CoPc but not for CoTBP] and (iii) e.s.r. spectra of CoPc/Norit BRX and Mossbauer spectra of FeTPP/Norit BRX changed very little upon heating, pointing to the existence of the MeN, group even in the heat-treated catalysts.To check these inferences we judged it desirable to obtain information about Me--N nearest-neighbour distances and the coordination of the Me ion. The extended X-ray absorption fine structure (EXAFS) technique appears eminently suitable for this p~rpose.~ In this paper EXAFS results are reported and interpreted for two samples of CoTPP/Norit BRX. The ‘ as prepared ’ catalyst (adsorption from acetone solution, 0.20 mg CoTPP per mg carbon) is referred to as Co-UN; the second sample has been heat treated and is referred to as Co-TRT. t Present address: B.P. Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN. 10211022 EXAFS STUDY OF SUPPORTED CO-PORPHYRIN CATALYSTS Ph Ph FIG.1 .-Structures of (a) Co-meso-tetraphenylporphyrin (CoTPP), (b) Co-phthalocyanine (CoPc) and (c) Co-tetrabenzoporphyrin (CoTBP). EXPERIMENTAL The samples for measurement were prepared by adding 33% by volume polyethylene to the catalysts and pressing the resulting mixtures in the form of discs at 133 bar, 413 K for 1 h. The EXAFS measurements were performed with radiation from the DORIS storage ring, Hamburg, Germany. The circulating current WTLS ca. 20 mA at 5 GeV, yielding an X-ray flux of typically 1Olo photon s-l eV-l bandwidth. The EXAFS apparatus, constructed by European Molecular Biology Laboratory, (EMBL), Hamburg, was of standard con~truction.~ The monochromator consisted of two silicon (220) crystals with rocking curves slightly offset to eliminate higher harmonics and parasitic reflections from the transmitted X-ray beam.Sealed ionization chambers filled with appropriate Xe + He mixtures were used. The cobalt K absorption edge, hv x 7720 eV, was studied. Five spectral runs were averaged for each sample, each spectrum being acquired in ca. 1 h. Normalization experiments, in which the sample was absent, were also performed. RESULTS AND DATA ANALYSIS The absorption spectrum is a plot of In ( & / I ) against photon energy, where I,, is the incident photon flux and I is the transmitted flux at energy E. Normalised, averaged absorption spectra for the two samples are shown in fig. 2. The term ‘extended X-ray absorption fine structure’ refers only to oscillations in absorption which occur above the absorption edge.For a free atom no EXAFS is observed and the absorption curve above the edge is structureless. The magnitude of the EXAFS oscillation is defined byR. W. JOYNER, J. A. R. VAN VEEN A N D W. M. H. SACHTLER 1023 5 1.901 , , , , , 7700 7800 7900 8000 8100 photon energy/eV 7700 7800 7900 8000 8100 photon energy/eV FIG. 2.-Normalized cobalt K edge absorption spectra: (a) Co-UN, untreated cobalt porphyrin catalyst; (b) Co-TRT, heat-treated cobalt catalyst. Each spectrum is an average of five experimental runs. where ,u is the absorption in the sample of interest and ,uo is the absorption in the free atom, The magnitude of the absorption in the free atom is determined by fitting an appropriate polynomial both below and above the absorption edge.For fitting below the edge a second-order polynomial is usually adequate; higher orders up to N = 5 are appropriate above the absorption edge. During the ‘background removal process’ the energy zero is redefined to be at or near the absorption edge. In EXAFS the true energy zero is the photon energy sufficient to excite the K shell electron just to the so-called ‘muffin-tin zero’ of the solid. Since the true zero is not accurately known this is often treated as a disposable parameter in EXAFS ~tudies.~ In this study the energy zero has been located 0.5 hartrees (1 hartree = 27.2 eV) below the inflection point in the absorption edge.61024 EXAFS STUDY OF SUPPORTED CO-PORPHYRIN CATALYSTS 0.0 2 s 0.00 - 0.02 - 0.04 - 0.06 - photoelectron energy/eV X -0.08 - 0.10 t -0 .l o 1 FIG. 3.-Experimental EXAFS: (a) Co-UN; (b) Co-TRT. For clarity curves are drawn through the experimental points (circles). The EXAFS results for the two samples are shown in fig. 3. Distinct structure is observed up to ca. 200 eV above the absorption edge. DISCUSSION AND SPECTRAL CALCULATION The origin of the EXAFS fine structure is described by the fundamental equation (2) which has been deduced by several k is the wave vector, given in atomic units by Ikl = 2/(2E). Rj is the interatomic distance of thejth shell of Nj atoms, with X(k) = C A2 sin (2kRj + 26 + y j ) E(n)l exp (- U,Z k2) exp (- 2 5 Rj k ) IklRjR. W. JOYNER, J. A. R. V A N VEEN A N D W. M. H. SACHTLER 1025 Debye-Waller factor U f . V;(n)) is the magnitude of the backscattering amplitude of thejth shell, while wj is the backscattering phase shift.26 is the phase shift from the central, emitting atom. The term exp (- 2 & Rj k ) accounts for inelastic losses, < being the so-called imaginary part of the self-energy . For a single coordination shell the last three terms in eqn (2) vary slowly with energy and the EXAFS oscillations are described by X(k) = C(k) sin (2kRj + 26 + y j ) ( 3 ) where C(k) varies slowly with energy. Unknown values of Rj can be determined by fitting calculated to experimental spectra provided that the values of the phase shifts 26+ y j are known. If lfj(n)I and < are known then N j and Uj can also be determined. The following preliminary remarks may be made before the fitting of the experimental spectra is described.The spectra show similarities, but also a number of important differences. The untreated sample shows twin maxima at ca. 60 and 85 eV above the absorption edge (fig. 3 ) compared with a single maximum observed at ca. 78 eV for the heat-treated sample. As will be seen, the spectrum of the treated sample is characteristic of the case where only a single shell of nearest neighbours contributes to the EXAFS spectrum. The Co-UN spectrum suggests the contribution of at least two shells of nearest neighbours. The spectra also show differences in the shape of the absorption edge, the untreated sample displaying a small maximum at ca. 7725 eV photon energy [fig. 2(a)], which is not observed for Co-TRT. The similarities in the spectra are the broad minima at ca.145 eV, the maxima at ca. 165 eV and the lack of structure above ca. 250 eV. For light atoms the backscattering amplitude falls off rapidly with increasing energy, resulting in the rapid decay in the intensity of the EXAFS oscillation. Spectral oscillations may, however, be observed up to ca. 450 eV in crystalline samples with low-2 backscattering atoms6 and the cut-off observed here at ca. 250 eV suggests a relatively high Debye-Waller factor. We now describe the calculation of EXAFS spectra and their comparison with the experimental results. The calculations were performed on an IBM 370/ 165 computer, using programs written by Gurman and Pendry. For energies above 150 eV the outgoing photoelectron wave may be assumed to be planar, and the calculation uses eqn (2) to compute X(k).Below this energy the sphericity of the outgoing wave must be considered, and the more complicated ‘curved wave’ expression [ref. (8), eqn (2.16) and (2.19)] is used. Calculated EXAFS amplitudes are invariably greater than those observed exper- imentally, as a result of electron shake-up. In the present case calculated amplitudes have been multiplied by 0.5 to allow for this effect. The atomic phase shifts used to calculate V;(n)l and y j were calculated in the local density approximation with overlapping charges, using the Daresbury Laboratory program MUFPOT, written by Pendry. Hartree-Fock wave functions determined by Clementi and RoettilO were used and the Slater exchange parameter was set to 1.0. To calculate the central-atom phase shift 6 for cobalt (2 = 27), the wave functions for a singly charged nickel atom ( Z = 28) were used, together with wave functions for neutral ligand atoms.CALCULATION FOR CO-UN The structure of the unsupported porphyrin is shown in fig. 1 (a). The central cobalt atom has four nitrogen nearest-neighbour atoms at a distance of ca. 1.97 A.11 The experimental EXAFS spectrum suggests contributions from at least two shells. The next nearest shell of eight carbon atoms [C, in fig. 1 (a)], Co-C distance ca. 3.05 A, was also included in the calculation. The coordination number for each shell was taken 34 FAR 11026 EXAFS STUDY OF SUPPORTED CO-PORPHYRIN CATALYSTS o .oa 0.06 0 -04 0.02 0.00 -0.02 -0.04 h X -0.06 -0.08 -0.1 0 0.06 o*08t. b O0 O0 photoelectron energyJeV -0.10 I- FIG.4.-Experimental EXAFS (circles) and calculaied EXAFS (lines), using the parameters given in the text: (a) Co-UN; (b) Co-TRT. as fixed, and the interatomic distances and Debye-Waller factors were adjusted to give the best fit with experiment. The best fit, which was judged by eye, is shown in fig. 4(a),andwasachievedwithCo-N = 1.95 A,Co-C, = 3.08 Aand theDebye-Waller factor for each shell, U2 = 1.4 x A2. The calculation duplicates well all of the features of the experimental spectrum including the double maxima between 50 and 100 eV. The quality of the fit observed indicates that the spectrum shows no clear evidence of any cobalt-support interatomic distance. The errors in the reported interatomic distance arise from the calculated phase shifts and also from the fitting approach used.For a model compound the approach used here has yielded results within kO.02 A of those known from X-ray and neutronR. W. JOYNER, J. A. R. VAN VEEN A N D W. M. H. SACHTLER 1027 diffraction.6 Because of the noise level observed in this study and the relatively short data range the errors here are thought to be slightly larger, & 0.03 A. The interatomic distances determined by EXAFS are therefore CO-N = 1.95 k 0.03 A CO-CA = 3.08 k0.03 A. These are all within the ranges observed in the unsupported complex.11 The calculated and experimental amplitudes are also in reasonable agreement. The EXAFS results thus provide good evidence that the structure of the unsupported porphyrin is retained in the supported catalyst. No evidence being obtained for any specific cobalt-support atomic distance, the results do not indicate any ordering of the complex on the support.This is not surprising in view of the highly disordered nature of the carbon surface. The noise level and short data range mean that the error in Debye-Waller factor is large, U 2 = 1.4k0.7 x A2. Note that the Debye-Waller factor determined in EXAFS differs from that measured in an X-ray diffraction experiment. The latter gives the root mean-square amplitude of vibration of an atom about its rest position. In EXAFS the Debye-Waller factor measures the motion of the backscattering atom with respect to the central atom. For nearest neighbours motion is often strongly correlated, so that EXAFS Debye-Waller factors are expected to be smaller than those noted in X-ray l2 The EXAFS Debye-Waller factor U 2 is related to the root mean-square variation in bond length d by u2 = 2c2.For the samples under study d = 0.08 k0.02 A. (4) CALCULATION FOR CO-TRT For the treated cobalt sample the calculation was performed in the way already described and used the same sets of atomic phase shifts. The main difference in the spectrum for the treated sample is that the double maximum in the range 50- 100 eV is replaced by a sharper, single maximum, [cf. fig. 3 (a) and (b)]. The spectrum for the treated sample thus resembles the modulated sine function in d E , which is predicted by eqn (3) when a single shell of scattering atoms is present. We have therefore calculated the EXAFS spectrum for cobalt surrounded only by a shell of four nitrogen atoms.The best fit is shown in fig. 4(b), and the Co-N distance is estimated to be 1.95kO.03 A, identical to the Co-N distance of the untreated sample. The Debye-Waller factor, U2 = 1.4 k 0.7 x A2, is also the same as in the untreated sample. Again, reasonable agreement is obtainable between the calculated and the experimental spectrum. This result thus suggests that the shell of four nitrogen atoms, Co-N = 1.95 A, is retained after the heat treatment, while the next nearest shell of eight carbon atoms, at CO-C, = 3.08 A, is modified. The extent of the modification is not certain, since it can occur by complete removal of carbon atoms or by inducing variation in the CO-C A distances. We propose the following interpretation of this result. During heat treatment some bonds between CA atoms and the meso-carbon atoms [e.g.CB in fig. 1 (a)] are broken, destroying the rigidity of the ligand framework; this restores the freedom of rotation of the pyrrole fragments around the Co-N axis, and these fragments will take positions of optimum interaction with the highly irregular carbon surface. As a result, the carbon (C,) atoms in the pyrrole fragments are located in a variety of positions 34-21028 EXAFS STUDY OF SUPPORTED CO-PQRPHYRIN CATALYSTS with respect to the Co atom, and this large static disorder results in eliminating the effect of the Co-C, distance from the EXAFS spectrum. The EXAFS results are therefore in agreement with the model deduced on the basis of e.s.r., X.P.S.and other evidence as outlined above. The CON, group is retained in the active, heat-treated catalyst. CONCLUSIONS The spectrum of the untreated sample corresponds closely to that expected from the unsupported complex. Good agreement between experimental and calculated spectra is obtained with the following two shells: CO-N, Rj = 1.95fi0.03 A, N = 4, 8 = 0.08&0.02 W (1) CO-C, Rj = 3.08fi0.03 A, N = 8, 8 = 0.08+0.02 A. (2) No interatomic distance is observed corresponding to a cobalt-support interaction. For the heat-treated catalyst good agreement is obtained with the following single shell : CO-N, Rj = 1.95f0.03 A, N = 4, cr = 0.08+0.02 A. The results suggest that the carbon atoms originally present at R, = 3.08 A have been displaced, although the CON, group is retained in the heat-treated material. We are grateful to the S.R.C. and the European Molecular Laboratory, Hamburg Outstation, for facilities. Experimental assistance was received from Drs J. Bordas, J. C. Philips, A. D. Cox and G. N. Greaves. Assistance in calculating the phase shifts was received from Dr G. Aers and Prof. J. B. Pendry. J. A. R. van Veen, J. F. van Baar, C. J. Kroese, J. G. F. Coolegem, N. de Wit and H. A. Colijn, Ber. Bunsenges. Phys. Chem., 1981, 85, 693. J. A. R. van Veen, J. F. van Baar and C. J. Kroese, J. Chem. Soc., Faraday Trans. I , 1981,77, 2827. E. A. Stem, J. Yac. Sci. Technol., 1977, 14, 461; P. Eisenberger and B. M. Kincaid, Science, 1978, 200, 1 4 4 1 ; R. W. Joyner, in Characterization of Catalysts, ed. J. M. Thomas and R. M. Lambert (J. Wiley, Chichester, 1980), p. 237. B. M. Kincaid, Thesis (Stanford University, 1975). B. K. Teo and P. A. Lee, J. Am. Chem. SOC., 1979, 101, 2815. R. W. Joyner, Chem. Phys. Lett., 1980, 72, 162. P. A. Lee and J. B. Pendry, Phys. Rev. B, 1975, 11, 2795. C. A. Ashley and S. Doniach, Phys. Rev. B, 1975, 11, 1279. ’ D. E. Sayers, F. W. Lytle and E. A. Stem, Phys. Rev. Lett., 1971, 27, 1204. lo E. Clementi and C. Roetti, Atomic Data Nucl. Data Tables, 1974, 14, 177. l1 J. L. Hoard, in Porphyrins and Metalloporphyrins, ed. U. M. Smith (Elsevier, Amsterdam, 1975), l2 W. Bohmer and P. Rabe, J. Phys. C, 1979, 12, 2465. p. 317. (PAPER 1/421)

ISSN:0300-9599    

DOI:10.1039/F19827801021

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J, Chem. Soc., Faraday Trans. I, 1982, 78, 1029-1038 Support Effects on the Catalytic Activity and Selectivity of Ruthenium in CO and N, Activation BY ALESSANDRO BOW, FABIO GARBASSI* AND GUIDO PETRINI Istituto Guido Donegani SPA, Centro Ricerche Novara, Via G. Fauser 4, 28 100 Novara, Italy AND LUCIANO ZANDERIGHI Istituto di Chimica Fisica, Universita di Milano, Via Saldini 50, 20133 Milano, Italy Received 17th March, 1981 CO and N, isotopic equilibration reactions have been studied on ruthenium catalysts supported on A1,0,, SiO, and MgO. The reactivity in CO equilibration was found to follow the sequence: A1,0, > MgO > SO,. However, the reactivity on Ru/MgO catalysts was found to be strongly dependent on the activation procedure: samples heated in oacuo at 673 K and then reduced at the same temperature showed the highest activity.The activation does not influence the reactivity of Ru/MgO catalysts towards N, isotopic equilibration, where samples directly reduced in H, were found to be slightly more active. The reactivity scale in NH, synthesis is as follows: MgO %- SiO, > A1,0,, with an apparent activation energy of ca. 100 kJ mol-l for all the supports. The Fischer-Tropsch reaction camed out on samples supported on A1,0, or MgO at ca. 50 atm and 530 K gave rise to a noticeable amount of oxygenated compounds such as alcohols. Results obtained on the various supports are discussed in terms of the presence of interactions between the metal species and the support itself, which were found to influence the activity and selectivity of the investigated reactions.Recent studies have shown that highly dispersed supported metals interact with the support losing some of their metallic characteristics. The Fourier transform of the EXAFS signal obtained on the Pt/Al,O, system1 shows a peak when the metal particles have a diameter of ca. 10 A; this peak was assigned to an interaction between the Pt atoms and the support oxygen atoms. Similar interactions have been found in the Ru/SiO, system., CO and hydrogen chemisorption studies on the Group VIII metals (Pt, Os, Ir) revealed a significant decrease in the gas uptake when the metals were highly dispersed on oxide support^.^ This decrease was explained, according to theoretical calculations on the Pt/TiO, system, as being caused by a direct Pt-Ti intera~tion.~ In the Ru-silica system, on the contrary, the metal-support interaction is likely to occur through the oxygen ions, in agreement with the interpretation of the EXAFS results., The aim of the present work is to verify the catalytic behaviour of some highly dispersed Ru catalysts when metal-support interactions are present.SiO,, AI,O, and MgO were chosen as supports, due to their different Lewis surface acidity (i.e. to a first approximation, their different abilities as electron acceptors). The isotopic equilibration of carbon monoxide or nitrogen was used as the test reaction. It is kn0wn~9~ that Ru can activate both molecules. However, while the activation of N, depends strongly on the electron density of R u , ~ different mechanisms are involved in the CO a~tivation.~-ll Activity tests using the Fischer-Tropsch reaction and ammonia synthesis were also carried out.10291030 EFFECT OF SUPPORT ON Ru CATALYSTS EXPERIMENTAL PREPARATION OF CATALYSTS The catalysts were prepared by the incipient wetness impregnation method, using silica (Akzo F5) or alumina (Akzo grade A) powders and proper solutions of ruthenium trichloride (RudiPont). Ru/Al,O, pellets were prepared by dipping the tablets in an excess solution of the Ru compound with the correct pH. Different Ru concentrations were occasionally used. Both Ru/SiO, and Ru/A1,0, catalysts were dried overnight at 383 K. Ru/MgO samples were prepared by making into a paste freshly prepared Mg(OH), and an aqueous solution of Ru nitrosonitrate (Pfaltz & Bauer).The paste was dried overnight at 343 K. All samples were activated by heating in an H, stream up to 673 K for 4 h and maintaining this temperature for 2 h. The samples were cooled to room temperature in the hydrogen stream. By chemical analysis, the silica supported catalysts were found to be nearly free from C1-, while the Ru/Al,O, samples contained relevant amounts of such anions, probably bonded to the A13+ ions.', Small amounts of nitrate ions were found in the Ru/MgO catalysts even after high-temperature red~cti0n.l~ CHARACTERIZATION Total specific surface areas were determined by N, adsorption at 77 K (B.E.T. method). Metal surface areas were measured by oxygen chemisorption at room temperature, using the static or pulse method. A good agreement was found between the values obtained by the two methods.Before chemisorption, the samples were treated for 4 h in an H, stream, then degassed in U ~ C U O and cooled to room temperature. In the measurements by the pulse method, hydrogen removal was carried out using a He stream. Surface composition analysis was carried out in a commercial PHI X.p.s.-A.e.s. spectrometer (Physical Electronics) using methods described elsewhere.12 Table 1 gives the characterization data of the basic samples. TABLE 1 .-FHYSICO-CHEMICAL CHARACTERIZATION DATA OF SUPPORTED Ru CATALYSTS chemical total specific 0, uptake shift /eVb bulk surface surface area /molecules sample compositiona composition" /m2 g-l (Ru atom)-' no air air Ru/Al,O, 0.72 0.5 170 0.53 +0.7 +0.7 Ru/SiO, 3.0 2.5 680 0.37 0 + 0.7 Ru/ MgO-B 5.0 5.6 25 0.23 n.d.+ 1.2 Ru/MgO-A 5.0 4.4 200 0.02 0 + 1.2 + 1.4 a lo4 Ru atoms (g catalyst)-'. With respect to Ru metal binding energy. KINETIC A N D CATALYTIC MEASUREMENTS Kinetic measurements of CO or N, isotopic equilibration were performed in a Temkin-type gradientless glass reactor. The isotopic composition analysis of the reaction mixtures was carried out using a quadrupole mass analyser (E.A.I. model 250B) connected to a PDP 11/50 computer (Digital Equipment Corp.). Further details on the apparatus and the measurement methods are reported elsewhere.' The Fischer-Tropsch synthesis was carried out in a Bennett-type stainless-steel reactor with internal circulation. A CO+H, mixture (10:90) was used in the range (1.5) x los Pa and 423-573 K.Condensable products were gathered in a liquid-nitrogen trap or in a CO, +alcoholA. BOSSI, F. GARBASSI, G . PETRINI A N D L. ZANDERIGHI 1031 ’0201J 10’8-j 1016 t I I 2 x i ~ 3 2A04 2x1103 2x1104 2X1103 zX104 zX103 zX\o4 RU /A12 0 3 Ru/MgO A RulMgO B RulSi02 PlPa FIG. 1 .-CO isotopic equilibration data for supported Ru catalysts activated at different temperatures, TA : 0, 673; 0, 723; A, 773 K. trap, identified by mass spectrometry and quantitatively analysed by a gas chromatograph. Gaseous products were analysed on line at the outlet of the trap. The NH, synthesis was carried out in a glass flow reactor at atmospheric pressure with a stoichiometric reaction mixture. Experiments were performed in the temperature range 573-773 K. The ammonia formed was bubbled into a titrated solution of H,SO, and determined by excess titration or colorimetric analysis.RESULTS co EQUILIBRATION Equilibration rates at 373 K on different samples are reported in fig. 1 as a function of the total pressure. The kinetic parameters of the pseudo-homogeneous rate model are reported in table 2. In most cases the reaction order decreases on increasing the activation temperature, indicating that a stronger thermal treatment in H, gives rise to a larger CO adsorption. The Ru/SiO, catalyst appeared to be the least active. The low reaction order indicates that CO is easily adsorbed, but its low activity with respect to the other samples (at the same reduction temperature) suggests that only a small fraction of adsorbed CO is active for the equilibration reaction.Since X.P.S. measurements on the sample activated at 673 K in hydrogen indicated the presence of metallic Ru only, it was decided not to carry out experiments on samples treated at higher temperatures. Ru/Al,O, is at least one order of magnitude more active than the sample supported on silica and shows a weak effect due to the reduction temperature. The sample reduced at 723 K, however, shows a higher activity than those treated at 673 or 773 K. The activity of Ru/MgO samples strongly depends on the activation procedure. A1032 EFFECT OF SUPPORT ON RU CATALYSTS TABLE 2.--KINETIC PARAMETERS FOR THE PSEUDO-HOMOGENEOUS EQUATION r = kpn FOR THE CO ISOTOPIC EQUILIBRATION (r IN molecules s-l m-2) reduction sample temperature/K k( x 10-ls) n Ru/Si02 673 Ru/AI,O, 673 723 773 Ru/MgO-A 673 723 773 Ru/MgO-B 673 723 773 0.2 6.1 16.0 10.5 1 .o 0.06 0.09 0.04 0.16 0.45 0.57 0.35 0.21 0.17 1.72 1.62 1.02 1.3 0.72 0.58 sample treated at 673 K under vacuum before reduction (hereafter called A) shows very high activity when reduced at the same temperature.This activity falls sharply when the reduction temperature is increased. All the samples exhibit a reaction order greater than 1, suggesting a low CO adsorption. Samples of Ru/MgO prepared by the standard reduction procedure (hereafter called B) always have catalytic activity lower than the catalyst supported on alumina. By increasing the reduction temperature from 673 to 723 K, both the activity and reaction order decrease suggesting that a stronger reduction causes an increase in the number of sites available for CO adsorption, accompanied by a decrease in the amount of activated CO.A further increase in the reduction temperature to 773 K gives rise to a more active catalyst, showing CO adsorption similar to that of the preceeding samples, but with a higher number of activated CO molecules. A comprehensive analysis of all the data indicates that the activity of supported Ru in the CO equilibration reaction is in the following sequence : A1,0, > MgO > SO,. However, the behaviour of the Ru/MgO-A sample, when reduced at 673 K, is anomalous with respect to the above sequence. N, EQUI L I B R A T I 0 N Equilibration rates of N, on Ru/MgO catalysts are shown in fig. 2 as a function of the total pressure. The kinetic parameters of the pseudo-homogeneous rate model are reported in table 3.Only the results relative to catalysts Ru/MgO-A and Ru/MgO-B are reported, as they showed behaviour remarkably different in the case of CO equilibration. Experiments were carried out at reaction temperatures of 673 or 723 K on samples previously reduced at 723 or 773 K. On both catalysts the reduction temperature does not influence the equilibration rate. Furthermore, their behaviour is similar, with a reaction order of 0.7-0.9. However, Ru/MgO-B appears to be slightly more active. The apparent activation energy in the range 673-723 K is 100 and 104 kJ mol-1 for A and B, respectively. It can be concluded that the two samples appear to be almost indistinguishable with respect to the nitrogen isotopic equilibration.A.BOSSI, F. GARBASSI, G. PETRINI A N D L. ZANDERIGHI 1033 FIG. 2.-N, isotopic equilibration data for Ru/MgO catalysts at different reaction temperatures T, : sample A : 0, 673; ., 723 K; sample B: 0, 673; 0, 723 K. TABLE 3.-KINETIC PARAMETERS FOR THE PSEUDO-HOMOGENEOUS EQUATION r = kpn FOR THE N, ISOTOPIC EQUILIBRATION (r IN molecules s-l mP2) reaction sample temperature/K k( x n Ru/MgO-A 673 0.05 0.96 723 0.17 0 . 8 1 Ru/M go-B 673 0.18 0.82 723 0.68 0.73 F I sc H ER-T R o P s c H s Y N THE s I s The F.T. synthesis was studied bearing in mind the results of the CO equilibration. In fact, only catalysts showing a noticeable activity in the CO equilibration reaction were taken into account, namely Ru/Al,O, and Ru/MgO-A. As an increase in the reduction temperature decreases the activity in the CO equilibration, catalysts not previously reduced were also tested.A possible influence of the metal content was studied by preparing and testing catalysts with different Ru contents. The results are reported in table 4. No correlation between the metal content and the reaction rate has been observed on alumina-supported ruthenium. Remarkable selectivity towards alcohols is an outstanding feature of the present results. Previous literature data showed that in the F.T. synthesis Ru catalysts give selectively hydrocarbons while alcohols were observed only in traces in the reaction products.14 We found that such oxygenated compounds are produced with higher selectivity on1034 EFFECT OF SUPPORT ON RU CATALYSTS TABLE 4.-ACTIVITY AND SELECTIVITY DATA FOR RU CATALYSTS IN FISCHER-TROPSCH SYNTHESIS activation conversion selectivity sample Rua (673 K in H,) T/K p/atm (%) (%> rateb Ru/AI,O, 0.05 Ru/Al,O, 0.33 Ru/AI,O, 0.73 Ru/Al,O, 0.73 Ru/A1,0, 0.77 MgO 0.0 Ru/MgO-A 5.0 Ru/MgO-B 5.0 Yes no no Yes no no no no 533 51 527 53 533 52 523 52 527 50 533 53 523 51 533 53 0.94 3.1 18.0 20.0 6.5 2.3 3.6 29.0 9.5 1.05 40.0 0.58 32.0 1.78 6.9 1.95 13.0 1.07 29.0 - 41 .O 0.11 27.0 0.43 a lo4 Ru atoms (g catalyst)-' N (cm3 CO) s-' (g Ru)-'.catalysts not previously reduced and with a lower Ru content. Since in these conditions oxidized Ru species are likely to be present, a mutual relationship involving the non-dissociative CO exchange mechanism and the formation of alcohols can be envisaged.Pure MgO is active in F.T. synthesis giving rise to a noticeable level of alcohol production. Ru/MgO-A shows similar behaviour, suggesting that the metal is almost unavailable for the reaction. On the other hand, Ru/MgO-B, pre-treated in the reactor with the reaction mixture, shows an activity higher by almost one order of magnitude than that of the pure support, maintaining a similar selectivity. Therefore we conclude that Ru probably acts as a promoter of MgO. NH, SYNTHESIS Experiments carried out using several Ru-containing catalysts showed that the reaction rate at low conversion depends strongly on the contact time and decreases on increasing the partial pressure of NH,, in agreement with the recent results published by Holtzmann et aI.l5 Since an exact determination of the kinetic equation was not the aim of the present work, we simply compare the various catalysts by determining the reaction rate in the same experimental conditions by a differential method.For this purpose the conversion was maintained below 10% of the equilibrium value. Moreover, since the catalyst activity changes with time, only steady-state data were considered. Such conditions were reached in a few hours for Ru/SiO, and Ru/Al,O,, while for Ru/MgO samples a longer time was necessary. In fig. 3 reaction rates are plotted against 1 f T. In the investigated temperature range, alumina- or silica-supported catalysts show a similar behaviour, even if in duplicated runs Ru/SiO, generally revealed a slightly higher activity.Both catalysts have an apparent activation energy of 83 kJ mol-l. A significantly higher activity was found for magnesia-supported catalysts. However, the behaviour of A and B samples is different. Catalyst B suffers from a sharp decrease in activity at the beginning of the reaction and reaches a steady state in a few hours. Treatment in hydrogen at high temperature (up to 873 K) of the partially deactivated catalyst has no effect or, in some cases, an adverse influence on the performance. On the contrary, catalyst A increases its activity during the experiments, up to a steady-state value similar to that of catalyst B. Thermal treatments in hydrogen cause an increase in activity, followed by a rapid decrease towards the steady state. ResultsA. BOSSI, F. GARBASSI, G.PETRINI A N D L. ZANDERIGHI 1035 I I 1 5 1 ‘6 17 103 KIT 3 FIG. 3.-NH, synthesis data for supported Ru catalysts. 0, Ru/A1,0,; @, Ru/SiO,; x , Ru/MgO. concerning Ru/MgO-A previously reduced at 673 K and after a run of 300 h are reported in fig. 3. The apparent activation energy is 113 kJ mol-l. DISCUSSION X.P.S. analysis of Ru/A120, samples reduced at different temperatures and handled in an inert atmosphere revealed the presence of Ru species not reducible with hydrogen at 873 K.12 The appearance of the X.P.S. spectra did not change on exposure to air. One may conclude that this surface species is stable in oxidizing or reducing conditions: such a stability may arise from a strong interaction between Ru and the surface oxygen atoms of alurnina.l2 The X.P.S.analysis of the reduced sample revealed only metallic Ru in the case of the Ru/Si02 system, while the presence of RuO, was found after exposure to air. The lack of non-reducible species on SiO, does not agree with the EXAFS results2 or with theoretical calculation^.^ Note, however, that the Ru 3d photoelectron peak was weak and disturbed by overlaps with the C 1s peak owing to contamination. In such a condition interaction of traces of Ru with the support may not be detected. Therefore, while the presence of Ru-SiO, interactions cannot be excluded, it is possible to state that the fraction of non-reducible Ru is greater on alumina than on silica. The analysis of the reduced Ru/MgO-A sample revealed the presence of two different Ru peaks. On the basis of their binding energy values they were attributed to metallic Ru and an oxidized species different from that found on alumina.After1036 EFFECT OF SUPPORT ON RU CATALYSTS exposure to air, the metallic Ru peak disappeared. It was not possible to obtain results on the reduced Ru/MgO-B sample owing to the experimental difficulties. On exposure to air the sample gave a spectrum similar to that of the corresponding A catalyst. The existence of not easily reducible oxidized Ru species on MgO has already been suggested by i.r. studies of CO chemisorption.l69 l7 Furthermore, thermal treatments in oxidizing conditions gave rise to the formation of perovskite-type compounds, e.g. MgRu0,.18 Our results show that the heating of the precursor at 673 K in vacuo favours the formation of a Ru species that is relatively stable in hydrogen.In order to verify the stability of such a species, some samples were reduced at 773 K in flowing H, and only metallic Ru was found. We may conclude that oxidized Ru species on MgO are less stable than those on alumina. When the precursor is directly reduced in hydrogen, only metallic Ru was observed. It is likely that the reduction rate is greater than the rate of formation of an interaction compound between Ru and magnesia. Oxygen chemisorption data support this view. The value of 0.02 oxygen molecules per Ru atom obtained on Ru MgO-A can be explained either by the growth of Ru particles having a size of ca. 400 d or by the presence of a Ru compound unable to adsorb oxygen. However, the first hypothesis was ruled out by the X.r.d.analysis, since Ru diffraction lines were not detected. Further studies are in progress in order to clarify the chemical nature and stability of such an intera~ti0n.l~ From the present results and literature data2?, we conclude that metallic Ru and species interacting with the support (Rus+) are present on A1,0, and SO,, but in different proportions in the two cases: Rus+ is the most abundant species on alumina while Ruo is prevalent on silica. The presence of oxidized Ru species on MgO depends on the thermal treatment conditions: in any case such species are different from those present on Al,O, and not stable under strong reducing conditions. These conclusions must be taken into account when interpreting kinetic and chemical activity results.The CO isotopic equilibration experiments clearly show that: (1) in the adopted experimental conditions neither detectable formation of CO, nor poisoning of the catalyst by carbon take place. This suggests that the equilibration reaction mechanism does not occur through dissociation of the CO molecule; (2) the amount of l80 in the gas phase does not change during the reaction. Therefore the support oxygen atoms do not play any part in the exchange reaction, e.g. in the formation of surface carbonate species at the boundary of the metal particles. If the intermediate step of the reaction is not the cleavage of a carbon-oxygen bond, it must be assumed that the isotopic equilibration proceeds through the simultaneous exchange of oxygen atoms between two adsorbed CO molecules in the following way: Ru-*C=O + Ru-CE*O + Ru-*CF*O + Ru-C=O.In previous work7 we concluded that the exchange reaction occurs on Ru/Al,O, when the CO molecules adsorb on an oxidized species (Ru&). This species gives a weak back-donation allowing the bending of the chemisorbed CO and the formation of a four-atom biradical intermediate involving two vicinal CO molecules. According to this view, the reaction rate is expected to increase with the number of oxidized Ru sites. Taking into account the CO isotopic equilibration results, the following reactivity sequence is proposed: Al,O, > MgO-A > MgO-B > SO,. This sequence also holds for the amount of oxidized Ru species on the different catalysts. It is likely that the residual chlorine ions present on Al,O,, increasing its surface acidity, stabilize the metal-support interaction. The Ru/MgO-A sample reduced at 673 K does not follow the above sequence. However, this catalyst has a high percentage of Ru interacted with the support andA.BOSSI, F. GARBASSI, G. PETRINI A N D L. ZANDERIGHI 1037 is very active in the CO equilibration reaction. In any case, both catalysts supported on MgO reach the same activity level after a reduction treatment at a sufficiently high temperature. The Ru/SiO, catalyst shows an activity so low that in previous work we were not able to measure it.7 N, isotopic equilibration was carried out only on samples supported on magnesia. On both catalysts the reaction rate is unaffected by the reduction temperature.The apparent activation energy, around 100 kJ mol-l for both samples, is in agreement with the value of 108 kJ mol-l found by Ozaki on bulk Ru metaL5 Assuming that the rate-determining step is the dissociative adsorption of nitrogen, an increase in the electron density on Ru will favour the dissociation. The slightly lower activity of catalyst A can therefore be attributed to the residual presence of oxidized Ru species, which are also active in the CO equilibration. The NH, synthesis results allow us to propose the following reactivity scale: MgO % SiO, > A1,0,. By analogy with the N, equilibration reaction and in agreement with the literat~re,~ the activity can be related to the electron density of the supported metal. The oxidized surface Ru species on MgO is not stable in the strongly reducing conditions under which ammonia synthesis is carried out and consequently both catalysts show quite similar activity.The lower activity of catalysts supported on alumina or silica can be attributed to the presence of Ru6+ species, stable under the reaction conditions, which inhibit nitrogen activation, owing to a decrease in the electron density of Ru atoms in metal crystallites. The data for the F.T. synthesis can also be interpreted on the basis of two ruthenium species being present on the surface of the catalysts. Indeed, the formation of significant amounts of alcohols can be explained by assuming a non-dissociative activation mechanism of CO under the reaction conditions. CONCLUSIONS The activity and selectivity of supported Ru catalysts for the reactions studied here depend on the ‘history’ of the catalyst itself.In fact, strong interactions between the support surface and the ruthenium particles can occur as a function of the nature of the support, the preparation procedure and the activation conditions. On Ru/MgO catalysts, a surface compound is formed which is unstable under high-temperature reduction conditions. On alumina or silica, interactions with Ru can also be established, probably through the oxygen anions. The amount of metal interacting appears to be larger on A1,0, than on SO,. The nitrogen isotopic equilibration and ammonia synthesis reactions preferentially take place on metallic Ru, while the CO equilibration, which occurs through a non-dissociative mechanism, is favoured by the presence of oxidized Ru sites. The selectivity towards oxygen-containing organic compounds in the Fischer-Tropsch reaction is attributed to the presence of such sites.J. Lornston and J. R. Katzer, 4th SSRL Users Group Meeting, Stanford, 1977. F. W. Lytle, G. H. Via and J. H. Sinfelt, .T. Chem. Phys., 1977, 67, 3831. S. J. Tauster, S. C. Fung and R. L. Garten, J . Am. Chem. Soc., 1978, 100, 170 and J . Catal., 1978, 55, 29. J. A. Horsley, J . Am. Chem. Soc., 1979, 101, 2870. K. Aika, H. Hori and A. Ozaki, J . Catal., 1972, 27, 424. H. H. Storch, M. Golumbic and R. B. Anderson, The Fischer-Tropsch and Related Syntheses (J. Wiley, New York, 1951). ’ A. Bossi, G. Carnisio, F. Garbassi, G. Giunchi, G. Petrini and L. Zanderighi, J . Catal., 1980,65, 16. J. C. Fuggle, T. E. Madey, M. Steinkilberg and D. Menzel, Surf. Sci., 1979, 52, 521.1038 EFFECT OF SUPPORT ON R U CATALYSTS J. W. A. Sachtler, J. M. Kool and V. Ponec, J. Catal., 1979, 56, 284. lo D. J. Elliott and J. H. Lunsdorf, J. Catal., 1979, 57, 1 1 . l1 J. G. McCarthy and H. Wise, Chem. Phys. Lett., 1979, 61, 323. A. Bossi, F. Garbassi, A. Orlandi, G. Petrini and L. Zanderighi, in Preparation of Catalysts IZ (Elsevier, Amsterdam, 1979), p. 405. l3 A. Bossi, A. Cattalani, F. Garbassi, G. Petnni and L. Zanderighl, in preparation. l4 R. C. Everson, E. T. Woodburn and A. R. M. Kirk, J. Catal., 1978, 53, 186. l5 P. R. Holzmann, W. K. Shiflett and J. A. Dumesic, J. Catal., 1980, 62, 167. Is A. A. Davydov and A. T. Bell, J. Catal., 1977, 49, 332 and 345. la S. J. Tauster, L. L. Murrell and J. P. De Luca, J. Catal., 1977, 48, 258. le K. Urabe, K. Aka and A. Ozaki, J. Catal., 1976, 42, 197. M. F. Brown and R. D. Gonzales, J. Catal., 1977, 48, 292 and J. Phys. Chem., 1976, 80, 1731. (PAPER 1/435)

ISSN:0300-9599    

DOI:10.1039/F19827801029

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1982, 78, 1039-1042 Theoretical Evaluation of the Velocity of Sound in Liquid Mercury at Elevated Pressures BY BRIJ R. CHATURVEDI, RAMESHWAR P. PANDEY AND JATA D. PANDEY* Department of Chemistry, University of Allahabad, Allahabad-2 1 1002, India Received 19th March, 1981 An attempt has been made to evaluate the velocity of sound in liquid mercury over a wide range of temperatures and pressures in the light of Flory ’s statistical theory. The agreement between theoretical and experimental values is satisfactory. The theoretical estimation of sound velocities in molecular liquids and binary liquid mixtures from Flory ’s statistical in conjunction with Auerbach’s relati~n,~ has been discussed recently by Pandey5p and Mishra.’? Pandey6 and Mishra8 have also extended Flory’s theory to enable the velocity of sound to be evaluated in pure molecular liquids at elevated pressures.Recently Pandey et aL9 have also successfully calculated the surface tension of molten salts using the Flory theory. It appears from the literature that no attempt has so far been made to examine the applicability of Flory’s theory to liquid metals. In the present paper such an attempt is made. The theoretical evaluation of sound velocities in the case of liquid metals is rare in the literature. Although KonyuchenkolO calculated sound velocities for a variety of molten metals and compared the results with experiment, he did not utilize Flory’s theory for this purpose. We have computed here the velocity of sound in liquid mercury over a wide range of temperatures and pressures in the light of Flory ’s theory. THEORETICAL According to Auerbach4 the velocity of sound, U, is expressed by the relation where c7 and p are the surface tension and density, respectively. According to Flory’s statistical theory the surface tension is expressed as c7 = c7* 6(V) (4 where c7* and 6( V ) are the characteristic surface tension and reduced surface tension, respectively.Patterson and Rastogi’ll in their extension of the corresponding states’ theory to the case of surface tension,l29l3 obtained the following relations for characteristic and reduced surface tensions : o* = ki p*$T*f (3) 10391040 VELOCITY OF SOUND I N LIQUID MERCURY Here k is Boltzmann's constant and M is the fractional reduction in the number of neighbours of a cell owing to migration from the bulk phase to the surface phase.Different values have been given for M.l1* 14-16 Mishra', and Pandey5? have suggested some modification to its previous values11v14-16 in order to obtain better results at elevated pressures. Recently Pandey et al. used an alternative value in the case of molten salts. This value9 differs from those used for molecular liquids and liquid mixtures. In the present case we have used 0.69 for M. Other symbols in the eqn (3) and (4) have their usual meanings. These equations have been derived in the light of the reduced equation of as described by Flory using the following relati~nsl-~v 6 v where a and BT are the thermal expansion coefficient and isothermal compressibility, respectively .17501 1700 1400- 1700 - I I " I ' ' I I I ' FIG. 2 4 6 8 10 12 13 0 p/108 Pa p , for various temperatures: (a) 294.9, (b) 313.5, (c) 325.9 K. 1.-Theoretical (0) and experimental (0) values of the velocity of sound as a function of pressure,B. R. CHATURVEDI, R. P. PANDEY A N D J. D . PANDEY 1041 TABLE 1 .-REDUCED VOLUME, CHARACTERISTIC PRESSURE, AND CALCULATED AND EXPERIMENTAL SOUND VELOCITIES IN LIQUID MERCURY Ucak Uexpt T/K p/108 Pa v p*/107 Pa /m s-' /m s-' A (%) 294.9 1 x 10-3 1 2 3 4 5 6 7 8 9 10 1 1 12 13 1 2 3 4 5 6 7 8 9 10 1 1 12 13 1 2 3 4 5 6 7 8 9 10 1 1 12 13 313.5 1 x 10-3 325.9 1 x 10-3 average % error 1.0516 1.0503 1.0492 1.048 1 1.047 1 1.0462 1.0452 1.0444 1.0435 1.0429 1.042 1 1.0416 1.0407 1.0402 1.0546 1.0533 1.0520 1.0509 1.0498 1.0488 1.0478 1.0469 1.0462 1.0453 1.0441 1.0438 1.0429 1.0423 1.0566 1.0552 1.0539 1.0527 1.0516 1.0505 1.0495 1.0486 1.0476 1.0467 1.0461 1.0452 1.0446 1.0439 - 146.7 148.0 149.2 150.3 151.3 152.5 153.2 154.2 155.0 156.7 156.9 158.0 158.3 159.5 153.2 154.6 155.7 157.1 158.3 159.3 160.4 161.4 163.0 163.6 163.7 165.1 165.2 166.7 157.5 159.0 160.3 161.6 162.7 163.9 164.9 166.1 166.7 167.2 169.1 169.1 170.5 172.0 - 1465 1473 1489 1500 1510 1521 1530 1540 1548 1560 1565 1575 1581 1590 1491 1504 1516 1521 1540 1550 1560 1570 1581 1589 1595 1604 161 1 1622 1508 1521 1534 1547 1558 1569 1579 1589 1598 1606 1617 1623 1632 1642 - 1450 1472 1493 1512 1531 1550 1567 1585 1601 1618 1633 1649 1664 1679 1442 1464 1485 1505 1524 1543 1561 1578 1595 1612 1628 1643 1659 1674 1436 1454 1465 1500 1520 1538 1556 1574 1591 1608 1624 1639 1655 1670 - - 1.0 - 0.4 0.2 0.8 1.4 1.9 2.4 2.8 3.3 3.6 4.2 4.5 5.0 5.3 - 3.5 - 2.8 -2.1 - 1.0 -1.0 - 0.5 0.1 0.5 0.9 1.4 2.0 2.4 2.9 3.1 - 5.0 - 4.6 - 4.8 -3.1 - 2.5 - 2.0 - 1.5 - 1.0 - 0.4 0.1 0.4 1 .o 1.4 1.7 2.11042 VELOCITY OF SOUND I N LIQUID MERCURY RESULTS AND DISCUSSION The values of reduced volume, v, and characteristic pressure, p*, as obtained through eqn (5) and (6), respectively, in the present case of liquid mercury over a wide range of temperatures and pressures are enlisted in table 1. The essential data required for the calculations have been taken from the 1iterat~re.l~ The values of reduced surface tension and charscteristic surface tension have been obtained with the help of these paraFeters, viz.V , p* and T*, vide eqn (3) and (2), respectively. These values of a* and 6 ( V ) have been utilized to obtain the values of surface tension which are in turn employed to predict the sound velocity through eqn (1). Although eqn (1) is empirical in nature its validity is well justified,le as it gives a maximum error of only & 4 % for theoretical sound-velocity results if all the experimental data are precise. Since in the present case the experimental surface tension data for liquid mercury at elevated pressures are not available in the literature, only a comparison of the experimental sound velocity with those predicted theoretically is possible. The predicted values of the sound velocities so obtained, along with the experimental values,17 are recorded in table 1.Both the theoretical and experimental values of the sound velocity are also represented graphically in fig. 1 as a function of pressure at all three temperatures. An inspection of the last three columns of table 1 and of fig. 1 reveals that the percentage deviation between theoretical and experimental values lies between 0.4 and 5.3, with an average of 2.1. Thus the agreement between experimental and theoretical results appears to be satisfactory. The results of the present calculation show that the rate of change of sound velocity with temperature is positive, whereas experimentally it is negative. Our main aim is to show the general applicability of the theory in a qualitative way. It appears that theory cannot explain the reverse trend of sound velocity with temperature, although the pressure variation of the sound velocity predicted theoretically is similar to those observed experimentally.All the reduced and characteristic parameters have been computed from the reduced equation of state as given by Flory,2y which is assumed to hold universally but which avoids the unrealistic parameterization of the intermolecular energy as stipulated by the theorem of corresponding states. One may therefore conclude that Flory’s theory can also be applied in the case of liquid metals over a wide range of temperatures and pressures. B. R. C. and R. P. P. are grateful to the Indian Council of Scientific and Industrial Research, New Delhi, for financial assistance. P. J. Flory, R. A. Orwall and A.Vrij, J. Am. Chem. SOC., 1964, 86, 3507. P. J. Flory, J. Am. Chem. SOC., 1955, 87, 1833. A. Abe and P. J. Flory, J . Am. Chem. SOC., 1965, 87, 1838. N. Auerbach, Experientia, 1948, 4, 473. J. D. Pandey, J. Chem. SOC., Faraday Trans. I , 1980, 76, 1215. J. D. Pandey, J . Chem. SOC., Faraday Trans. I , 1979, 75, 2160. R. L. Mishra, Acoustics Lett., 1979, 3, 1. R. L. Mishra, J. Chem. Phys., 1980, 73, 5301. J. D. Pandey, B. R. Chaturvedi and R. P. Pandey, J. Phys. Chem., 1981,85, 1750. lo G. V. Konyuchenko, Zzv. Vyssh. Uchelon. Zaved., Fiz., 1972, 15, 145. l1 D. Patterson and A. K. Rastogi, J. Phys. Chem., 1970, 74, 1067. l2 I. Prigogine and L. Saraga, J. Chim. Phys., 1952,49, 399. l3 R. Defay, I. Prigogine, A. Bellemans and D. H. Everett, Surface Tension and Adsorption (Longmans, l4 M. D. Croucher and M. L. Hair, J. Phys. Chem., 1977, 81, 163. Is D. V. S. Jain, S. Singh and R. K. Wadi, Trans. Faraday SOC., 1974, 70, 961. l6 R. L. Mishra and J. D. Pandey, Chem. Scr., 1977, 11, 117. l7 L. A. Davis and R. B. Gordon, J. Chem. Phys., 1967,46, 2650. London, 1966), chap. XI. R. L. Mishra, D. Phil. Thesis (University of Allahabad, 1977). (PAPER 1/447)

ISSN:0300-9599    

DOI:10.1039/F19827801039

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982, 78, 1043-1050 Use of Hydrogen Atoms for the Low-temperature Reduction of Oxides B Y MICHEL CHE,* BEGONA CANOSA? AND AGU s T IN R. G ON z A LE Z-E L I PET Laboratoire de Chimie des Solides, ER 133, CNRS, Universite P. et M. Curie, 4 Place Jussieu, 75230 Paris Cedex 05, France Received 6th April, 1981 The interaction of hydrogen atoms with the surface of supported (MOO, or V,O, on SiO,) or bulk oxides (TiO,) kept in liquid nitrogen has been studied by e.p.r. after the oxides were pretreated in oxygen at high temperatures. It is shown that reduction occurs as evidenced by the appearance of e.p.r. signals assigned to (MO=O),+ (gL = 1.952, gI1 = 1.890), (V=O)z+ (gL = 1.982, gll = 1.938, Al = 64 G, All = 190 G) and Ti3+ (sl = 1.976, gll = 1.950).These paramagnetic centres are found to be inert towards oxygen, showing that they are located below the surface, in contrast to the result; obtained after thermal reduction in uacuo or in hydrogen. The presence of F centres (sli = 2.0015, gl = 2.0000), observed in the supported oxides only, is related to the support, as shown by experiments performed with SiO, alone. The formation of reduced transition metal ions and of other centres (F centres, O;, OzHo) is discussed on the basis of the reaction of Ho atoms with 0,- oxide ions. The most widespread method of reduction of oxides is to heat them in uacuo or in a reducing atmosphere (hydrogen or carbon monoxide).l Some other methods have also been used, for instance low-temperature irradiation in vacuo or in a reducing atmosphere with U.V.light2 or y-rays.l? Recently, it has been shown by ferromagnetic resonance that hydrogen atoms can be used to produce nickel particles of small diameter by reduction of Ni2+ cations within the framework of exchanged Nix zeolites, due to the small size of the hydrogen atom.4 The present work reports the use of hydrogen atoms for the reduction at low temperatures of supported (MOO, or V,O, on SiO,) or unsupported (TiO,) transition metal oxides, as monitored by e.p.r. There are several reports on the interaction of atoms with oxide surfaces. One of the first examples is the production of a blue colour when MOO, was exposed to a stream of hydrogen atoms. This colour reaction has been used to obtain the rates of hydrogen atom reactions and a similar effect has been reported for WO,.,q6 Smith and Tench' were able to produce surface F centres on high surface area MgO bombarded with hydrogen atoms produced by a microwave discharge.They also studied the reaction of such hydrogen atoms with alcohols preadsorbed on Mg0.8 Transition metal oxides have also been considered. Thus, the interaction of hydrogen atoms with TiO, has been followed by conductivity meas~rements.~ Oxygen atoms have also been prepared and their interaction with various surfaces (zeolite 4A,1° MgO,ll Ti0212) investigated. In relation to the present work, Wintruff et al.13 described the reactivity toward oxygen of surface defects produced by the interaction of a hydrogen plasma with SiO,. The present report has a direct implication for hydrogen spillover studies.In t Present address: Institut fur Physikalische Chemie, Sophienstrasse 1 l,8000%lunchen, West Germany. 10431044 REDUCTION BY HYDROGEN ATOMS particular, if the active and migrating species involved in spillover is the neutral atom, it should behave like the hydrogen atom produced from the microwave discharge. We have thus restricted our choice to the oxides M00,,14 V,o5l5 and Ti0,,16 whose reduction by spilled-over hydrogen using Pd or Pt metal has been investigated in detail. EXPERIMENTAL The unsupported oxide was a TiO, prepared by the flame method (anatase, Degussa, P25). The supported oxides (MOO, and V,O, on SiO,) were prepared by the grafting method by reacting surface silanol groups with MoCl, dissolved in chloroform' or directly with vapour phase VCl,.17 Prior to reduction with hydrogen atoms, the samples were heated first in U ~ C U O for one hour and then in oxygen for one hour at 400 OC for TiO, and 500 "C for MOO, and V,O, on SiO,.The catalysts had a white colour after this oxidizing treatment, which was carried out in the same cell where the reduction was taking place. 'C FIG. 1 .-Experimental set-up for the reduction of oxides with hydrogen atoms. A, e.p.r. tube; B, microwave cavity; C, liquid-nitrogen dewar; D, microvalve; E, Pirani gauge; F, stopcocks; G, sintered disc. The experimental set-up in the present work is a modified version of the one described earlier17* l8 and is represented schematically in fig. 1. The hydrogen had a purity of 99.95 % and was used after passing through a trap at 77 K.Its flow could be controlled by measuring the pressure with the aid of a gauge placed in E. The cell could be separated and isolated from the rest of the system by means of the stopcocks F. The catalyst was maintained in the liquid- nitrogen bath C during the reduction on the sintered disc G. The origin of time for the various reduction periods was taken when the microwave discharge was switched on. It was produced with a Microtron 200 microwave power generator mark 111 (Electro Medical Supplies, Wantage, UK) equipped with a microwave discharge cavity B tuned at 2450MHz. The apparatus was made of Pyrex except for the silica tubing passing through the microwave cavity. Typical operating conditions were: pure H, flow (4 x lop2 dm3 min-l) under 1 Torr pressure with microwave discharge power of 150 W.After reduction by Ho atoms, the catalyst was transferred to an e.p.r. tube A which was finally sealed off for e.p.r. observation. The entire process was performed within a polystyrene vessel filled with liquid nitrogen. For most experiments, e.p.r. spectra were recorded at 77 K on a Varian spectrometer (model E3, X band) with 100 kHz modulation. The spectrometer was also equipped with a 77-513 K variable temperature accessory (E-257). The g values are measured relative to a DPPH sample (g = 2.0036). The magnetic field increases from left to right in the figures.M. CHE, B. CANOSA A N D A. R. GONZALEZ-ELIPE 1045 RESULTS After 3 min exposure of TiO, to hydrogen atoms, the solid had a brown colour associated with an e.p.r.spectrum at 77 K (fig. 2). Two kinds of species could be detected, one with g, = 1.976, gll = 1.950 and the other with g, = 2.000, g , = 2.008 and g, hardly resolved. The first signal is very similar to that observed after thermal reduction of TiO,, and thus assigned to Ti3+ ions. The second is similar to that detected after oxygen photoadsorption on hydroxylated TiO, surfaceslg and assigned to an O,Ho radical. These two species (Ti", O,Ho) were not stable at room temperature. After exposure of MoO,/SiO, (1 h) and V,O,/SiO, (10 min) to hydrogen atoms, the solids turned brown and gave e.p.r. spectra. ' I Y 2.000 FIG. 2.-E.p.r. spectrum (X band, 77 K) of TiO, (anatase) maintained in liquid nitrogen and reduced by hydrogen atoms for 3 min.The e.p.r. spectrum obtained at 77 K for the MoO,/SiO, sample is represented in fig. 3. It is composed of a signal with g, = 1.952, gll = 1.890, similar to that observed after thermal reduction of M00,/Si0,~* and assigned to (Mo=O),+. Another signal can also be identified with the following g tensor components: g, = 2.018, g , = 2.010 and g, = 2.0045, which correspond to the parameters reported for 0; absorbed onto Mo0,/Si0,.21 The last signal which can be identified is very narrow and almost symmetrical with gll = 2.001 5 and g, = 2.0000. It is saturated easily with microwave power above 4 mW either at 77 K or at room temperature. The g values of this species, close to the free electron g value, and its facility to become saturated with low microwave power make it likely to be assigned to an F centre,,, which is in line with previous work on MgO where F centres were produced at the surface of the solid by interaction with Ho atoms.' After heating the sample at room temperature, the 0; species was no longer observed at 77 K or room temperature, in contrast to the (Mo=O),+ species and the F centre which did not change in either its g values or its intensity.The e.p.r. spectrum for the V,O,/SiO, system is shown in fig. 4. A signal with parametersg, = 1.982,g11 = 1.938, A , = 64 G, A , , = 190 G, similar to those obtained after thermal reduction of V,O,/SiO,, can be identified and assigned to (V=O)2+.23 F centres similar to those observed for MoO,/SiO, could also be found in the spectrum. The spectrum recorded at 77 K did not change after warming the sample to room temperature.1046 REDUCTION BY HYDROGEN ATOMS FIG.9 F centre 3.-E.p.r. spectrum (X band, 77 K) of MoO,/SiO, maintained in liquid nitrogen and reduced by hydrogen atoms for 1 h. 2.0045’ 1 g,! = 1.938 IDPPH n Y F cent re FIG. 4.-E.p.r. spectrum (X band, 77 K) of V,O,/SiO, maintained in liquid nitrogen and reduced by hydrogen atoms for 10 min. Note that on exposure to oxygen at 77 or 300 K, the three oxides reduced by this method did not lead to the formation of superoxide 0; ions, as normally observed for samples reduced by the thermal method. This indicates that the reduced transition metal ions were located below the surface and were not accessible to oxygen at 77 or 300 K. It also demonstrates that the coordination sphere of the cation is complete and does not change on adsorption of oxygen.This situation is similar to that observed for (V=O)2+ cations.24 When the bombardment of the oxides by hydrogen atoms was performed at room temperature, the same results as previously were obtained except for the centres which were not stable at room temperature (Ti3+, 02H0, 0;).M. CHE, B. CANOSA A N D A. R. GONZALEZ-ELIPE 1047 Experiments performed with the silica support alone (Degussa, 300 m2 g-l) led to the formation of an e.p.r. spectrum at 77 K characterised by gl = 2.0000 and gll = 2.0015 with a peak-to-peak line width AHpp N 4 G. This e.p.r. signal, which did not disappear on warming to room temperature and was easy to saturate on increasing the microwave power, is assigned to F centres.Similar results have been obtained by Wintruff et aZ.13 in the case of SiO,. DISCUSSION The small concentration of Ho atoms detected by their e.p.r. spectra for the solids reduced and maintained throughout at 77 K shows that the Ho atoms produced by the microwave discharge have mainly reacted with surface 0,- ions followirig the reaction Ho + 02- -+ OH- + e-. (1) ( 2 ) The electrons are then trapped by transition metal ions according to the process e- + Mn+ -+ M(n-1)+ as shown by the e.p.r. spectra of Ti3+, (Mo=O)~+ and (VZO)~+ species. The inertia of these reduced cations is expected since the pretreatment 'in oxygen' is believed to fill any vacancy in the coordination sphere of surface transition metal ions with adsorbed oxygen. In this sense, the case of MoO,/SiO, is very helpful since it has been shown earlier that reduction of such catalysts in the proper conditions could produce two kinds of (Mo=O)~+ species characterised by gl = 1.958, gI1 = 1.856 and gl = 1.941, gll = 1.885' assigned to bulk octahedrally hexacoordinated and surface pentacoordinated (Mo=O)~+ species, respectively.20 The former was found to be inert on exposure to oxygen in contrast to the latter which led to the formation of 0; species. Note that Ho atoms have been shown to oxidize Ti3+ ions in acidic organic according to the reactions (3) H0-Ti1I1+H+ -+ H,+TiIV.(4) HO +Ti111 + HO-TiIII In the present work, it is shown that Ti3+ ions are octahedrally coordinated and not reactive eliminating reaction (3) as a possibility.Furthermore, process (4) requires the reaction between two ' mobile' species H-Ti111 and H+ which is not possible in the present work since one is dealing with adsorbed species at 77 K and since Ti3+ ions are finally detected. The presence of F centres observed only in the case of supported oxides is to be associated with the support, since experiments performed on SiO, alone indicate the presence of F centres. This is consistent with: (i) The absence of hyperfine structure in the F centre signal for V205/Si0,. This indicates that the trapped electron is not interacting with V nuclei ( I = S, 100% natural abundance) and thus not located in the supported V,05 oxide but rather in the support SiO, oxide. (ii) The fact that transition metal ions are better traps than anion vacancies, if any, in the transition metal oxides (MOO,, V,O,).In contrast, in the support oxide (SiO,) the anion vacancies are more efficient traps than Si4+ which are not known to be reduced in normal conditions. Accordingly, F centres were not observed for TiO,, where the only electron traps are the Ti4+ ions.1048 REDUCTION BY HYDROGEN ATOMS The presence in some cases of 0; or O,HO radicals can be explained following the ( 5 ) (6) reactions : 0,+e- -+ 0; 0, + Ho -+ O,HO. The two radicals 0; and O,HO are not independent as shown by the reaction O,HO -+ 0; + H+. (7) In the case of a hydroxylated TiO, surface, it has been shownlg that reaction (7) was displaced to the left. Thus, according to reactions (1) or (7), an increase in the degree of hydroxylation is likely even if, at the beginning of the reduction by Ho atoms, the oxide surfaces can be considered as almost free of OH-.The hydrogen atom method appears attractive since reduction can occur at low temperatures as indicated by the present results. From available thermodynamic data, it can be calculated that the two following reductions are exothermic: 2 TiO,(s) + 2 H ' (g) -+ Ti,O,(s) + H,0(1) AH2,, K = - 84.4 kcal m0l-l v2°5(s) + * (g) v2°4(s) + H20(1) AHZg8~ = - 143.2 kcal m01-l (9) where (s), (g) and (1) refer to solid, gas and liquid, respectively. The oxides Ti,O, and V,04 have been chosen to correspond to the reduction state deduced from the e.p.r. data. The previous enthalpy variations have been obtained using Hess's law from the enthalpy changes of the following reactions : 2 TiO,(s) -+ Ti,O,(s) +i O,(g) AH298 = + 88.1 1 kcal mo1-l 26 a v2°5(s) '2O4(') + i O2(g) = +29.3 kcal mol-l 26b H,(g) + f 02(g) -+ H20(1) AHsg8 K = - 68.3 1 kcal m0l-l 26c In reactions (8) and (9), the formation of liquid H,O has been considered.However, the addition of a term including the enthalpy of adsorption on the surface would not change the reasoning. As a matter of fact, for the reaction 02-(s) + H,O --+ 20H-(s) (14) the enthalpy value estimated from desorption studies is - 107 2 kJ mol-l for TiO,,' while a similar value of - 117 kJ mot1 can be obtained from immersion studies.28 For MOO,, although this calculation cannot be made because of the lack of data for Mo205, a reasonable value can be obtained by comparison with data concerningM.CHE, B. CANOSA AND A. R. GONZALEZ-ELIPE 1049 (16) leading to 2 W03(s) + 2 H’ (8) -+ W205(s) + H20(1) = - 109.7 kcal mol-l. Measurements of the temperature by means of a thermocouple located just below the sintered disc indicate that, during the reduction, the temperature increases to a few degrees above 77 K. The ‘local’ temperature where the reduction actually takes place is, however, difficult to measure, although attempts are being made. Note also the absence of reactivity of the transition metal ions. As outlined above, this is due to the pretreatment in oxygen at high temperature. It is likely that if the reduction is performed after a pretreatment in uacuo, the exposed transition metal ions should be obtained with higher reactivity, This has been observed in the case of MgO pretreated in uacuo.l1 Experiments are presently being performed along this line. Finally, our results indicate that hydrogen atoms produced by a microwave discharge are able to produce isolated reduced transition metal ions at low temperature. It is thus not surprising that the final state of reduction is different from that obtained at substantially higher temperatures with spilled-over hydrogen which was able to reduce MOO, to M0,0,,~9~ CrO, to Cr20530 and TiO, to Ti,O,.ls In the latter case, one would not expect, for complete reduction, to detect any e.p.r. spectra because of dipolar interaction between neighbouring Mo5+, Cr5+ or Ti3+ in the reduced oxides. The fact that we have also used supported oxides (MOO, and V205 on SO,) is also to be considered.It has been shown that supported MOO, is less reducible than bulk We are thus investigating, for the sake of comparison, the reduction of bulk MOO,, V205 and CrO, by hydrogen atoms produced by a microwave discharge. We thank B. Morin for recording some of the e.p.r. spectra. A. R. G-E. acknowledges a grant from the CSIC, Madrid, Spain. Partial support of this work from the ‘Ministkre des Universites’ (grant 78 C 527 E) is also acknowledged. M. Che, F. Figueras, M. Forissier, J. C. McAteer, M. Perrin, J. L. Portefaix and H. Praliaud, Proc. VZth Znt. Congr. Catal. (The Chemical Society, London, 1977), vol. I, p. 251. P. Mbriaudeau, M. Che, P. C. Gravelle and S. J. Teichner, Bull. SOC. Chim., 1971, 13. M. Che, M. Dufaux and C.Naccache, Tagung Hoch-frequenzspektroskopie (Karl-Marx Universitat, Leipzig, 1969), Erganzugband, p. 10. M. Che, M. Richard and D. Olivier, J. Chem. SOC., Faraday Trans. 1, 1980, 76, 1526. T. H. Johnson, J. Franklin Znst., 1929, 207, 629. H. W. Melville and J. C. Robb, Proc. R. SOC. London, Ser. A, 1949, 196, 445. D. R. Smith and A. J. Tench, Chem. Commun., 1968, 1 113. D. R. Smith and A. J. Tench, Can. J. Chem., 1969,47, 1381. P. Dumont and P. de Montgolfier, J. Chim. Phys., 1972, 69, 16. lo P. Svejda and D. Hermerschmidt, Ber. Bunsenges. Phys. Chem., 1976, 80, 491. l1 P. Svejda, R. Haul, D. Mihelcic and R. N. Schindler, Ber. Bunsenges. Phys. Chern., 1975, 79, 71. l2 P. Svejda, W. Hartmann and R. Haul, Ber. Bunsenges. Phys. Chem., 1976, 80, 1327. l3 W. Wintruff, R.Herrling and H. J. Tiller, Chem. Phys. Lett., 1976, 38, 524. l5 M. A. Ibanez and G. C. Bond, unpublished results. l6 R. T. K. Baker, E. B. Prestridge and R. L. Garten, J. Catal., 1979, 56, 390; 1979,59, 293. l7 B. N. Shelimov, C. Naccache and M. Che, J. Catal., 1975, 37, 279. l8 D. Olivier, M. Richard, L. Bonneviot and M. Che, Growth and Properties of Metal Clusters, ed. J. l9 A. R. Gonzalez-Elipe, G. Munuera and J. Soria. J. Chem. SOC., Faraday Trans. 1 , 1979, 75, 748. 2o M. Che, M. Fournier and J. P. Launay, J. Chem. Phys., 1979, 71, 1954. 21 M. Che, A. J. Tench and C. Naccache, J. Chem. SOC., Faraday Trans. I , 1974, 70, 263. 22 B. Henderson and A. K. Garrison, Adu. Phys., 1973, 22, 423. 23 L. L. Van Reijen and P. Cossee, Discuss. Faraday SOC., 1966, 41, 277. 24 V. B. Kazansky, V. A. Shvets, M. Ya. Kon, V. V. Nikisha and B. N. Shelimov, Catalysis, ed. J. ,G. C. Bond and J. B. P. Tripathi, J. Less-Common Met., 1974, 36, 31. Bourdon (Elsevier, Amsterdam, 1980), p. 193. Hightower (North Holland, Amsterdam, 1973), vol. 11, p. 1423.1050 REDUCTION BY HYDROGEN ATOMS 25 D. Behar and A. Samuni, Chem. Phys. Lett., 1973, 22, 105. 26 I. Barin and 0. Knacke, Thermochemical Properties of Inorganic Substances (Springer-Verlag, Berlin, 27 G. Munuera and F. S. Stone, Discuss. Faraday Soc., 1971, 52, 205. 2* C. M. Hollabaugh and J. J. Chessick, J. Phys. Chem., 1961, 65, 109. 29 Selected Values of Chemical Thermodynamic Properties (Department of Commerce, Washington 30 J. B. P. Tripathy and G. C. Bond, J . Indian Chem. Soc., 1978, 55, 950. 31 J. Masson and J. Nechtschein, Bull. SOC. Chim., 1968, 3933. 1973), (a) p. 784, (6) p. 830, (c) p. 323. D.C., 1961), p. 296. (PAPER 1/547)

ISSN:0300-9599    

DOI:10.1039/F19827801043

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982, 78, 10514063 Reversed-flow Gas Chromatography for Studying Heterogeneous Catalysis B Y NICHOLAS A. KATSANOS Physical Chemistry Laboratory, University of Patras, Patras, Greece Received 6th April, 1981 The rate constants of surface-catalysed reactions can be determined in a gas-chromatographic column, containing the catalyst, by introducing the reactant as a pulse at a middle position of the column, and then repeatedly reversing the direction of flow of the carrier gas. Following each reversal of the gas flow, an extra function is recorded by the detector, and then the chromatographic signal returns to the original elution curve. The analytical mathematical expressions, describing the elution curves when the gas flow is reversed, are derived for a general case and then applied to two specific reaction schemes, namely a simple first-order reaction and two consecutive reactions.By plotting the logarithm of the area under the ‘extra peaks’ as a function of time when the respective reversal of,the flow was made, the rate constants of the surface steps of the reaction can be extracted. Arrhenius plots constructed from these rate constants give the ‘true’ activation parameters, free from heats of adsorption. The main advantage of differential methods for studying chemical kinetics over integration methods is that they are based on determination of reaction rates rather than on concentrations as functions of time. These reaction rates have a closer and more direct relation to reaction orders and reaction mechanisms than integrated rate equations.Differential methods have been used in heterogeneous catalysis but, as far as we know, only for the determination of initial rates. However, initial rates can give little information concerning intermediates and other details of the reaction. One exception is the ‘ stopped-flow gas chromatography’ invented by Phillips et a1.l and developed mathematically by Katsanos.2 This method permits a direct determination of reaction rates, not only for small conversions to products or for reaction times around zero, but in the whole range of conversions covering an extended period of time. Probably the main drawback of the method is that it continuously switches the system under study from a flow dynamic one to a static system and vice versa, by repeatedly closing and opening the carrier-gas flow.Diffusion and other related phenomena, which are usually negligible during the gas flow, may become important when the flow is stopped. A new differential technique termed ‘ reversed-flow gas chromatography’ has been reported in a preliminary comm~nication.~ It serves to determine rates of surface- catalysed reactions over the whole range of conversions; from these rates true rate constants for catalytic reactions can be calculated. The method is based on rates of formation of product(@ rather than on rates of disappearance of the reactant, and it does not suffer from the disadvantage of the stopped-flow technique mentioned above. It is the object of the present paper to describe and analyse the reversed-flow technique in greater detail.10511052 REVE R SED-F L 0 W GAS CHROMATOGRAPHY EXPERIMENTAL APPARATUS The experimental set-up for the application of the reversed-flow method is very simple. A conventional gas chromatograph with a high-sensitivity detector, e.g. a flame ionization detector (fid.), is modified as shown diagrammatically in fig. 1. The catalyst is contained in two chromatographic column lengths 2' and I connected in series with an injector between them. The two columns have the same diameter and the same or different length. By means of a suitable valve S (four-port, or six-port with two alternate ports connected through a small piece I1 $tube needle valve Fi Y a i r H2 amp FIG. 1 .-Schematic arrangement for the reversed-flow technique.V, two stage reducing valve and pressure regulator; G, gas flow controller for minimizing variations in the gas flow-rate due to small changes in temperature and/or other factors; S, six-port gas-sampling valve with a short in. tube connecting two alternate ports; H, a restrictor or a 1 : 10 splitter; F1, bubble flowmeter; amp, signal to amplifier. of in. tube,* as in fig. l), the carrier gas can be made to flow in one of two opposite directions. Thus, with the valve S in one position (represented by the solid lines) the carrier gas enters at point D,, flows through Z' and I and meets the detector at point D,. This is termed direction F (Forward). When the valve is switched to the other position (dotted lines in fig. 1) the direction of the gas flow is reversed.It now enters at D, and the detector is placed at position D,. This is direction R (Reverse). A restrictor or a 1 : 10 splitter with its side-arm closed is placed at H to prevent the flame of the detector from being extinguished when the valve is turned from one position to the other. * 1 in. = 2.54 x m.N. A. KATSANOS 1053 PROCEDURE After conditioning the catalyst by heating it in situ at an appropriate temperature, under carrier gas flowing in either direction, some preliminary injections of the reactant in both directions are made to establish constant catalytic activity. Kinetic experiments are conducted after the last reactant introduced has been exhausted to a negligible amount. A few mm3 (0.5-10) of liquid reactant using a microsyringe, or a few cm3 of gas reactant at atmospheric pressure using a gas-tide syringe, are introduced onto the column length f through the injector placed between the two columns, and with the carrier gas flowing in direction F.If the reactant is more strongly adsorbed than the product(s), the latter is recorded by the detector as an asymmetric elution curve. At a time (measured from the moment of injection) greater than the retention time of the product(s) on the column length f, the direction of the carrier gas is reversed by means of the valve S. After a certain dead time, when no signal is 80 r 45 40 35 time/min 30 25 FIG. 2.-A reversed-flow chromatogram obtained with a 3.8 mm i.d. glass column of length I' = 7 and 1 = 100 cm, filled with 80-100 mesh 13X molecular sieve, activated at 665 K for 21 h.The carrier gas was nitrogen at a flow-rate 0.446 cm3 s-l, the column temperature 477.3 K and the injected reactant 0.5 mm3 propan-2-01 giving di-isopropyl ether as the main product. recorded by the detector, the chromatographic elution curve rises abruptly, then follows a function with a form depending on the mechanism of the surface reaction, and finally returns to the previous or a slightly different in height continuous curve (fig. 2, peak 1). After a time (from the moment of reversal) greater than the total retention time of product(s) on the total column length f+f', the carrier gas is again turned to the direction F. This is followed by a new extra signal (fig. 2, peak 2). The procedure is repeated several times, until two series of peaks are obtained, one with the carrier gas flowing in direction R (R-peaks) and another series in direction F (F-peaks). From these peaks rate constants for the surface reactions are calculated as described later.THEORETICAL ANALYSIS The analytical mathematical expressions which describe the two series of elution curves (R- and F- curves) as functions of time depend on the mechanistic reaction model and the chromatographic parameters of the reactant and of the product(s). The1054 RE VE R SED-F LO W GAS C HRO MA TOG R A P HY present paper analyses the case where the gaseous reactant A is rapidly and reversibly adsorbed on the active centres S of the catalytic surface, giving the species A-S. This transforms to the adsorbed final product D-S in equilibrium with the gaseous product D.Thus, equilibration of the reactant and of the product between the gas and the solid phases is instantaneous, i.e. chromatography is assumed to be ideal. Furthermore, only the case is treated where the reactant A is strongly adsorbed on the surface, i.e. KA % 1. injection point A 0 ; X - direction f i l’+l “ + I / d i r e c t i o n R - X ‘ -0 FIG. 3.-Representation of the chromatographic column in the reversed-flow method. The surface is assumed to contain only one kind of active sites S and in a large concentration compared with that of the adsorbed species in view of the very small amounts of the reactants being used. Additional assumptions are: (a) the adsorption isotherms are linear; (6) axial diffusion of the gases in the bed is negligible, which is justified for high enough flow-rates; (c) the reacting vapour is introduced at the point x = 2’ of a chromatographic column of total length l’+l (fig.3), and this is done as an instantaneous pulse, which can be described by a Dirac delta function, 6(x - 2‘). NOTATION volume of gas phase per unit length of column (cm2) volume of solid phase per unit length of column (cm2) concentration defined by the last term of eqn (14) (mol ~ m - ~ ) concentration of D in the gas phase with the carrier gas flowing in direction F or R, respectively (mol ~ m - ~ ) Laplace transform of cD or cb with respect to to Laplace transform of cD with respect to t’ double Laplace transform of cb with respect to to and t’ area under elution curve of D (mol) fraction of A adsorbed on reactive surface sites height above the baseline defined by eqn (18) and (19) (mol ~ m - ~ ) rate constants for surface reactions (s-l) partition ratio of D (dimensionless) partition coefficients of A and D (dimensionless) column lengths defined in fig.3 (cm) total mass of A injected (mol) constant defined by eqn (27) (mol cmd3) transform parameter with respect to to and t’, respectively concentrations of adsorbed species B-S and D-S, respectively, per unit volume of solid (mol ~ m - ~ ) rate of production of D from the chemical reaction (mol cm-2 s-l), and its t’ Laplace transform, respectively Laplace transform of r with respect to to double Laplace transform of r with respect to to and t’N. A. KATSANOS 1055 S f 0 ftot t , t‘ tR, t;t V v x, x’ 8, 9 2, 2’ response of the detecting system (cm s mol-l) time interval from the injection of A to the first reversal of gas flow (s) total time passed from the injection of A to the last reversal of gas flow (s) time measured from the last reversal of the gas, now flowing in direction F or R, respectively (s) retention times of D on the column lengths I or 1‘, respectively (s) linear velocity of carrier gas in interparticle space (cm s-l) volume flow-rate of carrier gas (cm3 s-l) distances from end D, or D, of column, respectively, as defined in fig.3 (cm) functions defined by eqn (4) and (8), respectively time from the last reversal of the flow diminished by the retention time of D in the flow direction, eqn (12) and (15) (s) The problem will be considered separately for various time intervals, in which the concentration of D (as a function of time and distance x or x’) is determined by certain differential equations with given initial conditions.I N I T I A L F-IN TERV A L This extends from the time of injection of the reactant A until the first flow reversal. The concentration c ~ ( x , to) is determined by the following mass balance equation for D and D-S: Since adsorption isotherms are assumed to be linear, qD = KDcD = (a/a’)kDcD and this can be substituted for qD in eqn (1). The rate of production of D from the chemical reaction on the surface, r(x, to), is generally a function of time and distance along the column. However, the reactant A is assumed strongly adsorbed on the surface, and the chromatographic process on A is repeatedly changing direction (forwards and backwards).Thus, a useful approximation which greatly simplifies the calculations is that the x distribution of r(x, to) can be described by the initial x distribution of A, namely by a delta function S(x - Y). Writing r(to) S(x - 1’) in place of r(x, to), where the rate r(to) is only a function of time and expressed in mol cm-, s-l, i.e. per unit a, eqn (1) reads (2) Taking the to Laplace transform of this equation, under the initial condition c ~ ( x , 0) = 0, an ordinary differential equation for CD as a function of x results. This is easily integrated to give (3) where 8 = (1 +kD)(x-I‘)/v (4) ac, acD a t 0 ax (1 +kD) - = -v-+r(t,) ~ ( x - I ’ ) . CD =- R’~) exp ( -Po e) u(x - 1’1 V and u(x - Y) is the Heaviside unit step function, which equals 0 for x < I’ and 1 for x > I’.At the detector, i.e. at x = I’+Z, u(x-1‘) becomes u(l) = 1 for I > 0, and 8 becomes (1 + k,)l/v = tR, i.e. the retention time of the product D on the column length 1. Then, taking thep, inverse transform of the resulting equation, using the well-known property ‘translation’ of Laplace transforms, one finds the break-through curve of D at position D, (fig. 3):1056 RE VE R SED-F LO W GAS CHROMATOGRAPHY FIRST R-INTERVAL At a time to > fR the direction of the carrier gas flow is reversed and the time measured from the moment of reversal is called t’. The distance co-ordinate x is now changed to x’ defined by the relation x’ = Z’+l-x (6) and the concentration ck(x’, t’) in this time interval is given by an equation analogous to eqn (2): (2’) acg acg ax (1 +kD) =-U,+r(?,, 2‘) d(X’-/).As with eqn (2), one proceeds by taking Laplace transforms of this equation with respect to time, but now the to transform is taken first, and then the t’ transform with initial condition Ck(X’,PO, 0) = - R’~, exp (pot) [I - u(x’- 111. (7) V This is obtained from eqn (3) by substituting 1 - u ( i - 0 for u(x-2‘) and replacing -9, as defined by eqn (4), by its equivalent: 8’ = (1 +k,)(x’-Z)/u = -8. (8) The above double Laplace transformation leads to an ordinary differential equation for cb as a function of x’, which is easily integrated by using x’ Laplace transforms, giving CD(x’,p0,p’): - -exp [-(I + k D ) @ o l + p ’ x ’ ) / u l ) + y R’~,p‘) exp (-p’ e ) u(x’- I ) .(9) At the detector, i.e. for x’ = I’ + 1, u(x’ - 1 ) becomes u(l‘) = 1 for Z’ > 0, and 6’ becomes (1 + k,) Z’/v = t;, i.e. the retention time of the product on the column Z’. Then eqn (9) becomes The break-through curve of D at the end D, of the column is now obtained by taking the inverse transforms, first with respect to p’ and then with respect to po. The result is r( to + z‘) CD = ~ [u(z’) - u(z’ - f R ) ] u(to - z’) + - V V where 7‘ = t‘-& (12) and r(to, t’- tk) is written r(to+z’), since the time t’ is a continuation of to. The behaviour of eqn (1 1) is of some interest. For z’ c 0, i.e. for t’ < tk, c6 = 0 and no signal is recorded by the detector until the retention time tk is reached. Then u(7’) = 1, and the chromatographic signal abruptly rises to r(to - z’)/v + r(to + z‘)/u.It falls again to r(t,+z’)/v when z’ > t,, i.e. when t’ 3 tk+tR, because the squareN. A, KATSANOS 1057 function in square brackets becomes zero. The u(to - z’) factor remains as unity in the above interval, since the flow was reversed at to > tR. Thus, the concentration of the. product r(to)/v, produced by the chemical reaction at time to, is shifted in time on reversing the flow direction. This time-shift takes place in two opposite directions. One, which occurs ‘forwards’ to r(to+z’)/v, starts at t’ = t k and continues uninterrupted. It is nothing else than the continuation of eqn (5) at the other end of the column. The other shift r(to-z’)/v occurs ‘backwards’ and is barred in the interval 0 < z’ < tR.Therefore, it starts with the concentration r(to)/v and ends with that of a preceding time, namely r(to - tR)/v. This extra signal (R-peak) adds to the forward shift, constituting a kind of ‘chromatographic sampling’ (cf. fig. 2). SECOND F-INTERV AL At a time t’ > t&+ tR the carrier-gas flow is again turned to the direction F, the time from this moment being denoted by t . The distance co-ordinate is changed from x’ back to x, according to eqn (6). The concentration c,(x, t) is again described by eqn (2) with t substituted for to. To solve this equation by the method of Laplace transformations we need the initial condition at t = 0. This, in the form of its t’ transform, is obtained from the last term of eqn (9), since all other terms of this equation disappear at z’ > tR.The remaining term, after taking the po inverse transform, changing 8’ to - B and u(x’ - I ) to 1 - u(x - I ) , gives the desired condition The rest of the solution in the t interval follows the same procedure as that described for eqn (2’), namely the t’ transform is taken first, followed by the t transform with initial condition eqn (1 3). This leads to a differential equation in x, which in turn gives the equivalent of eqn (9), and finally the counterpart of eqn (1 1) is obtained: V CD = r(ttot--z) [u(z>-u(z-t&)] u(t’-z)+ V where z = t-tR (15) ttot = to + t’. and This equation is the F-direction equivalent of eqn (1 1) and its behaviour is analogous to that. It predicts that cD = 0 for z < 0, and at z > 0 two functions are recorded as a sum.One is given again by r(to)/v with the total time ttot in place of to, and shifted forwards by z. This continues uninterrupted as the last term of eqn (14) shows. In the other function the ttot is shifted backwards by z and this function vanishes when z 2 t k . Thus, an extra signal (F-peak) is predicted in the interval 0 < z < t k , positioned on top of the otherwise continuing chromatographic curve. The u(t’ - z) function in eqn (14) is kept at unity in the above interval because of the condition that the new reversal was at t’ > tk+tR. Repeating the reversal of the carrier-gas flow in the direction R for a second time at t > tR + t k , then in the direction F for a third time at t’ > tR + t k , and so on, two series of peaks, R-peaks and F-peaks, are produced.The periods between reversals, t or t’, must be kept smaller than the retention time of A, so that the latter is never eluted from the column. The R-peaks are described by eqn (1 1) with ttot in place of to, while the F-peaks are given by eqn (14). Since the terms forward and reverse are arbitrary, and the two above equations have the same form, one of them suffices to describe both kinds of 35 FAR 11058 REVER SED-F LO W GAS CHROMA TOGR A PHY peaks. In the following we make use only of eqn (14), in which ttot is taken to mean the total time passed from the injection of A to the last reversal of the gas flow, z the time measured from the last reversal of the flow diminished by the retention time of the product D in the direction of the flow [cf.eqn (12) and (1 91, and tk the retention time in the opposite direction of the flow. The function u(t’-r) in eqn (14) is unity provided that the time elapsing between any two successive reversals is greater than the total retention time t , + tk, as mentioned before. CALCULATION O F RATE CONSTANTS FROM THE ELUTION CURVES Two relations are useful for calculating rate constants from F- or R-peaks. The first is that giving the area under the peaks, taking the last term of eqn (14) as baseline. For this purpose one can use eqn (10) and the well-known relation where in place of cD we use only the first term on the right-hand side of eqn (lo), the last term giving rise to the baseline. The result is The area under the peaks is found by simply taking the po inverse transform of this expression.The second useful relation is that giving the height of the F- and R-peaks above the continuous chromatographic signal (baseline) at either of the two discontinuities of the peak, namely at T = 0 or z = tk. Denoting by cb the last term of eqn (14) (baseline), one finds for these heights h,=o = (c, -Cb)r=O = r(ttot)/v h,=& = (CD-c&=& = r(ttot-t&)/v- (18) (19) Both f and h are functions of ttot, the analytic forms of which depend on the analytic expression for r(ttot), and this in turn is determined by the reaction mechanism. These analytic forms forfor h can be used (by linearizing them, etc.) to calculate the rate constant(s) of the reaction. REPRESENTATIVE RESULTS In this section two examples are given for the application of the theoretical equations of the previous section. The simplest possible case of mechanistic models is fast k fast A+S + A-S + D-S f D+S K A KD i.e.a simple first-order decomposition of the absorbed reactant A-S to give the adsorbed product D-S. In this case where g is the fraction of A adsorbed on reactive sites of the surface. The relevantN. A. KATSANOS 1059 equations for the R- and F-elution curves can be obtained from eqn (14) by substituting the r.h.s. of eqn (21) for r(ttot). The result is The area under the R-peaks is obtained by applying eqn (17): f = mg [exp ( k t R ) - 11 exp ( - kttot). (23) Thus a plot of lnfagainst ttot gives k from the slope of the straight line obtained. For the F-peaks the same equation applies with tk substituted for t R .The physical meaning of eqn (23) becomes obvious if it is written as f = mg exp [ - k(ttot - tR)] - mg exp ( - k ttot). It means that when the direction of the gas flow is reversed at time ttot, a sampling is made from the chromatographic column and the product formed between the times t t o t - t R and ttot is exhibited as an extra peak. It follows thatfis proportional to t R (or tk for the F-peaks) and thus a short column length I or I' produces small narrow peaks, such as 2 in fig. 2. FIG. 4.-Plots of eqn (23) for the dehydration of propan-2-01 (0.5 mm3) to di-isopropyl ether, at 452 K, over 13X molecular sieve (80-100 mesh, Applied Science Laboratories) activated at 673 K for 12 h. The lengths of the glass column were I = 7 and 1 = 100 an, and the carrier gas nitrogen (99.99% purity), at a volume flow-rate of 0.446 cm3 s-l.0, R-peaks; A, F-peaks. In fig. 4 an example of plotting Infagainst ttot, according to eqn (23), is given. It refers to the dehydration of propan-2-01 to di-isopropyl ether over 13X molecular sieve at 452 K. The rate constant k calculated from the R- and F-peaks is given in table 1. A ' t-test ' of significance, performed on the coefficients of regression of lnf on ttot, shows that these are significant at a level better than 1%, showing that the probability for the corresponding ' t '-value of being exceeded is < 1 %. This is a measure of the goodness of fit of the experimental data by the linear plots of fig. 4. In fig. 5 the Arrhenius plots are given constructed from rate constants of the above 35-21060 RE VERSED-FLO W GAS CHROMA TOG R A PHY reaction at various temperatures.The Arrhenius parameters calculated are listed in table 1. The literature4 value for Ea on a similar surface is 110 kJ mol-l. Note that these are ‘true’ activation parameters, not involving heats of adsorption, since they are derived from rate constants pertaining to the surface step of the reactions. As a second example, we quote the deamination of aminocyclohexane to cyclohexene on y-aluminium oxide at 525 K. Fig. 6 and 7 show the time-dependence of the height at z = 0 of R- and F-peaks, respectively. The appearance of these plots suggests consecutive first-order reactions: Past k, k* fast A+S .C- A-S + B-S + D-S + D+S. KA .- KD Here A is the gaseous aminocyclohexane, A-S its form adsorbed on reactive sites S, B-S a surface intermediate, D-S the adsorbed cyclohexene and D its vapour. (24) adsorbed species TABLE l.-vARIOUS KINETIC PARAMETERS FOUND BY THE REVERSED-FLOW METHOD parameter from R-peaks from F-peaks dehydration of propan-2-01 at 452 K over 13X k/ 10-4 S-1 2.48 f 0.14 2.56 f 0.12 In (A/s-l) 21.12 & 0.08 20.1 f 0.7 EJkJ mol-l 110.6f0.3 107 f 3 kl/lo-4 S-1 I .36 f 0.02 1.15 f 0.03 k2/10-4 s-1 6.5 & 0.2 7.3 f 0.4 102 g (% conversion) 11.6 12.9 deamination of aminocyclohexane at 525 K over y-A120, The rate of production of the final product D is now proportional to qB and this is given by the classical equation for an intermediate in consecutive reactions. Thus When this expression is used for r(ttot) in eqn (14), the analytic function describing the R- and F-elution curves is obtained: cD = M(exp [ - kl(ztot - 2)1 -exp [ - k2(ttot - z)]) [U(t) - U(z - tk)] + M W P - kl0tOt + 41 - exp - k2(ttot + z)l> u(z) (26) where To determine the rate constants k, and k2 in this reaction, we employed eqn (1 8) which, by using (25), gives (28) in agreement with the general appearance of the experimental curves of fig.6 and 7. From the slopes of the last linear part of the plots (after the induction period) the rate constant with the smaller value, say k,, is obtained. Then, the term Mexp ( - k2 ttot) is found from the difference h’ = Mexp (- k, ttot) - h, i.e. by extrapolating back the h = M b P (-kAot)-exP (-k2 tt0t)lN. A. KATSANOS 1061 last linear part and subtracting from it the experimental values of h.If now lnh’ is plotted against ttot, k , is found from the slope of this new linear plot. All these plots are shown in fig. 6 and 7. The various curve-fittings were obtained by standard least-square procedures. The rate constants found are given in table 1. The two sets of rate constants are not significantly different, bearing in mind the difficulty and the low precision with which rate constants of surface-catalysed reactions are generally determined. Needless to say that by the present method it is not possible to ascertain which rate constant belongs to the first and which to the second step of mechanism 1 1 I I 2 05 2 10 2 15 2.20 lo3 KIT FIG. 5-Arrhenius plots for the dehydration of propan-2-01 to di-isopropyl ether over 13X molecular sieve. The experimental conditions are those of fig.4. 0, Rate constants determined from R-peaks (lower abscissa); 0, rate constants from F-peaks (upper abscissa). Another important quantity of eqn (28) is the pre-exponential factor M, given by the intercepts of the plots of fig. 6 and 7. However, only relative values of M/cm are obtained from the plots. The absolute value of M/mol ~ m - ~ can be determined by the relation where S/cm s mol-1 is the response of the detecting system. This can be determined by injecting known amounts of the pure product D into an empty column and integrating the resulting elution curve. The experiment must be done for the same flow- rate ratio of hydrogen and carrier-gas as that used in the reaction, because the response changes with this ratio. Alternatively, the response can be found for various ratios M,,,/mol cm-3 = (M,,,/cm)/ VS (29)1062 REV E R SED-F LO W GAS CHROMATOGRAPHY 9.0 - 8.5 - n s \ 5 s 8.0 - ttot/lo3 s 0 1 2 3 4 I I I z5 - I I I I I 1 1 1 I I I 0 2 4 6 8 10 FIG.6.-Time-dependence of the In h values for R-peaks at ‘s = 0, for the deamination of aminocyclohexane (1 mm3) to cyclohexene at 525 K, over y-aluminium oxide (60-72 mesh, H,-415 Houdry-Huls) activated at 673 K for 2 h. Both lengths 2‘ and I of the glass column (i.d. 4 mm) were 40 an, and the carrier gas was nitrogen (99.99% purity) at a volume flow rate of 0.30 cm9 s-l. 0, (Lower abscissa), experimental points h; a, (upper abscissa), values of h’ obtained by subtracting the experimental points h from the corresponding values Mexp ( - k, I,,,) found by the extrapolation of the last linear part to the times of the ascending experimental curve.HJcarrier-gas and that corresponding to the reaction can be determined by inter- polation. In the present case it was found that S = 7.23 x 1013 cm s mol-l. By using Mabs in eqn (27) and the values of k, and k, determined from the slopes, g can be calculated. Its values are presented in table 1. Since the reactant aminocyclohexane is not eluted from the column over a period of several days, its partition coefficient must be very high and therefore its fraction on the surface must be close to unity. The considerable deviation of the g values from unity on the other hand probably shows that the active catalytic surface is only a relatively small fraction of the total surface.This possibility has been pointed out many times in the literature, and actual experimental confirmation can be found if it is noted that ‘% conversion’, so often quoted in heterogeneous catalysis, is here nothing other than 100 g. This can be seen by deriving the fractional conversion to cyclohexene from eqn (28): 1 * 1 conversion = jo hd V = - m hdt,,, = g. Experiments reported elsewhere5 showed that the conversion of aminocyclohexane to cyclohexene on aluminium oxide, determined directly from the the area under the product elution curve, was 18.9-32.3 %, depending on the flow rate. Of course these values differ considerably from those found here, but this could be due to the different origin and type of aluminium oxide used there. The important point is that these conversions, like the present ones, deviate considerably from 100%. It was also shown5N. A. KATSANOS ttot/103 s 1063 ttot/lO3 s FIG. 7.-Time-dependence of the In h values for F-peaks at r = 0, for the deamination of amiaocyclo- hexane to cyclohexene, over y-aluminium oxide. The experimental conditions are those of fig. 6. A, (Lower abscissa), experimental points h; A, (upper abscissa), values of h' = Mexp(-k, ttot)-h (see legend of fig. 6). that this phenomenon was not due to irreversibly adsorbed cyclohexene on the aluminium oxide surface, since this latter was found experimentally to be only 3-5%. The two representative cases of surface reactions, presented as examples in this section, show that the assumptions on which the theoretical analysis was based are justified by experimental evidence. Thus, eqn (22), (23), (26) and (28), derived from the general eqn (14), (1 7) and (1 8), describe the experimental findings fairly well and are consistent with them. The author gratefully acknowledges the help of Dr G. Karaiskakis and Mrs Margaret Barkoula. C. S. G. Phillips, A. J. Hart-Davis, R. G. L. Saul and J. Wormald, J. Gas Chromatogr., 1967,5,424. N. A. Katsanos and I. Georgiadou, J. Chem. SOC., Chem. Commun., 1980, 242; 640. S. J. Gentry and R. Rudham, J. Chem. SOC., Faraday Trans. 1, 1974, 70, 1685. D. Vattis, N. A. Katsanos, G. Karaiskakis, A. Lycourghiotis and M. Kotinopoulos, J. Chrornatogr., 1981, 214, 171. * N. A. Katsanos, J. Chromatogr., 1978, 152, 301. (PAPER 1/553)

ISSN:0300-9599    

DOI:10.1039/F19827801051

出版商:RSC    

年代:1982

数据来源: RSC