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/F198278FX029

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/F198278BX031

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, II Journal of The Chemical Society, Perkin Transactions, I Journal of The Chemical Society, Perkin Transactions, II 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 should 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 compounds. 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.P A R T 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. iP 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. 11F A R A D A Y D I V I S I O N OF THE ROYAL SOCIETY OF CHEMISTRY A S S O C I A Z I O N E 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 I i T i DE C H l M l E PHYSIQUE DEUTSCHE BUNSEN GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE 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, t o 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 final programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry Burlington House, London W1V OBN FARADAY 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, 15-16 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 o 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, it is 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 ! non-covalently bonded structures that The preliminary programme may be obtained from : Mrs Y. A. Fish, The Royal Society of Chemistry Burlington House, London W1 V OBN . . . 111THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 75 I nt ra mo lecu I a r K i net i cs University of Warwick, 18-20 April 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; ( b ) observation and theoretical modelling of the competition between intramolecular energy redistribution and radiative decay or radiationless processes (e.g. internal conversion, fragmentation, isomerisation). The preliminary programme may be obtained from : Mrs Y. A. Fish, The Royal Society of Chemistry Burlington House, London W1V OBN Dr D. M. Hirst Professor K. R. Jennings Dr R. Walsh THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO.76 Concentrated Colloidal Dispersions Loughborough University of Technology, 141 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 31st August 1982 to: Professor R. H. Ottewill, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS ivTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 18 Molecular and Microstructural 5asis of Viscoelasticity and Related Phenomena Robinson College, Cambridge, 8-9 December 1983 Organising Committee Sir Geoffrey Allen (Chairman) Professor Sir Sam Edwards Dr M. La1 The past few years have witnessed the development of new concepts which provide deeper understanding of the relationship between molecular dynamic and microstructural features of systems and their viscoelastic behaviour.This Symposium is designed to bring together original contributions involving theoretical, computational and experimental studies which represent significant advances in this important field of current activity. It is hoped that such contributions, together with the discussion that they will generate, will lead to new insights into the molecular mechanisms underlying the viscoelastic/rheological behaviour of, for example, flexible and rigid rod-like polymer molecules, liquid crystals and composites. In addition to three oral sessions (at which the main papers will be presented and discussed), the Symposium may include a poster session. Such poster papers will not be published in the Symposium volume. Contributions for consideration by the organising committee are invited.Abstracts of ca. 300 words should be sent to: Dr M. Lal, Unilever Research, Port Sunlight Laboratory, Bebiragton, Wirral L63 3JW not later than 29 October 1982. Full papers for publication in the Symposium volume will be required by 19 August 1983. Dr R. A. Pethrick Dr P. Richmond Or D. A. Young (Editor) THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 77 Interfacial Kinetics in Solution University of Hull, 9-11 April 1984 This Discussion will focus attention on reactions involving liquid-gas, liquid-liquid and liquid-solid interfaces (but it will not include electrode kinetics as such). The subject encompasses processes of fundamental, industrial and environmental importance and includes such topics as the rate of dissolution of reactive gases, kinetics at liquid membranes, metal and solvent extraction, Marangoni effects, heterogeneous catalysis and photocatalysis in solution, and the kinetics of dissolution of minerals and drugs.The aim of the meeting is to bring together workers in these diverse fields to highlight the complementary nature of the problems encountered and of the results obtained, and to disseminate ideas concerning new and effective experimental techniques and novel theoretical approaches. Contributions for consideration by the organising committee are invited. Titles should be submitted as soon as possible, and abstracts of about 300 words by 15th April 1983, to: Professor D. H. Everett, Department of Physical Chemistry, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1 TS VFARADAY DIVISION INFORMAL AND GROUP MEETINGS Theoretical Chemistry Group Molecular Electron Structure Theory and Potential Energy Surface To be held at the University of Bristol on 15-1 6 September 1982 Further information from Dr G.G. Balint-Kurti, School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 ITS Molecular Beams Group Molecular Beams and Molecular Structure To be held at the University of Bristol on 16-1 7 September 1982 Further information from Dr J. C. Whitehead, Department of Chemistry, University of Manchester, Manchester M13 9PL Surface Reactivity and Catalysis Group The Characterisation of Surface Layers in Chemisorption and Catalysis To be held at the University of East Anglia on 20-21 September 1982 Further information from: Dr M.A. Chesters, School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ 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 Society of Chemistry, Burlington House, London W1 V OBN Statistical Mechanics and Thermodynamics Group with the British Society of Rheology 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 Electrochemistry Group Spectroscopic Studies of Electrode Surfaces To be held in Oxford on 13-1 4 December 1982 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College, London SW7 2AZ Colloid and Interface Science Group Physical and Biological Aspects of Insoluble Monolayers and Multilayers To be held at the Scientific Societies Lecture Theatre, London on 14 December 1982 Further information from: Dr R.Aveyard, Department of Chemistry, The University, Hull HU6 7RX 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 W1 V 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, 11 Hobart Place, London SW1 W OH2 viPublications from The Royal Society of Chemistry SPECIALIST PERIODICAL REPORTS Catalysis Vol. 4 Senior Reporters: C. Kemball and D. A. Dowden This volume reviews the recent literature published up to mid 1980.Brief Contents: The Design and Preparation of Supported Catalysts: Aspects of Characterization and Activity of Supported Metal and Bimetallic Catalysts; Metal Clusters and Cluster Catalysis; Olefin Metathesis; Superbasic Heterogeneous Catalysts; Hydration and Dehydration by Heterogeneous Catalysts; Sulphide Catalysts: Characterization and Reactions including Hydrodesulphurization; Carbon as a Catalyst and Reactions of Carbon. Hardcover 266pp 0 851 86 554 2. Price f29.00 ($62.00). RSC Members €1 7.50 Gas Kinetics and Energy Transfer Vol. 4 Senior Reporters: P. G. Ashmore and R. J. Donovan A review of the literature published up to early 1980. Brief Contents: Reactions Studied by Molecular Beam Techniques; Reorientation by Elastic and Rotationally Inelastic Transitions; Infrared Multiple Photon Excitation and Dissociation: Reaction Kinetics and Radical Formation; Ultraviolet Multiphoton Excitation: Formation and Kinetic Studies of Electronically Excited Atoms and Free Radicals; Gas Phase Reactions of Hydroxyl Radicals; Gas Phase Chemistry of the Minor Constituents of the Troposphere.Hardcover 252pp 0 85186 786 3. Price f45.00 ($96.00). RSC Members f25.00 Mass Spectrometry Vol. 6 Senior Reporters: R. A. W. Johnstone This volume reviews the literature published between July 1978 and June 1980. Brief Contents: Theory and Energetics of Mass Spectrometry; Structures and Reactions of Gas-phase Organic Ions; Gas-phase Ion Mobilities, Ion - Molecule Reactions, and Interaction Potentials; Interaction of Electromagnetic Radiation with Gas-phase Ions; Aspects of Secondary Ion Emission; Development and Trends in Instrumentation in Mass Spectrometry; Applications of Computers and Microprocessors in Mass Spectrometry; Gas Chromatography- Mass Spectrometry and High- performance Liquid Chromatography- Mass Spectrometry; Reactions of Negative Ions in the Gas Phase; Natural Products; The Use of Mass Spectrometry in Pharmacokinetic and Drug Metabolism Studies; Organometallic, Co-ordination, and Inorganic compounds Investigated by Mass Spectrometry.Hardcover 368pp 0 85186 308 6. Price f39.50 ($88.00). RSC Members f23.00 ORDERING RSC Members should send their orders to: The Royal Society of Chemistry, The Membership Officer, 30 Russell Square, London WC1 B 5DT.Non-RSC Members should send their orders to: The Royal Society of Chemistry, Distribution Centre, Blackhorse Road, Letchworth, Herts SG6 1 HN. The Royal Society of Chemistry Burlington House Piccadilly London W1V OBN 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 I500 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. Chern., 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. I , 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 iil recent years: activities;* chromatography ; electrochemistry; 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, optical refraction molecular weight, absorbance and wavelength, pressure-volume- temperature rela t i on ships, reflectance, potent iome t ric ion activities, testing distillation columns); solution chemistry; spectrochemical analysis; surface chemistry; thermo- dyfiamics, 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. pp. 71-90.) A complete listing of all IUPAC nomenclature publications appears in the 1981 Index issues of J. Chem. SOC. ... Vlll

ISSN:0300-9599    

DOI:10.1039/F198278FP057

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1982, 78, 2313-2320 Application of the Elovich Equation to the Kinetics of Occlusion Part 1 .-Homogeneous Microporosity BY CHAIM AHARONI* AND YAAKOV SUZIN Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa, Israel Received 6th April, 1981 Some properties of the equations for the kinetics of occlusion, obtained by integrating Fick’s equation, are examined. The plot of the reciprocal of the rate z = (dv,/dt)-’ against the time t is sigmoid and has an inflexion point at t = t,. When t does not differ greatly from t, the kinetics are approximated by an ut = A+(l/b)ln(t/t,+t,) Elovich equation where A, b and t , are constants independent of the diffusion coefficient and length of the diffusion path and determined by the geometry of the particles.The term occlusion is applicable to a sorption process involving a porous solid and a fluid when the following conditions are fulfilled: (a) a change of phase takes place in the fluid when it enters a pore; (b) the fluid in the pore is in a single phase, i.e. no distinction can be made in a pore between an adsorbed phase and a non-adsorbed one, These conditions apply to the uptake of gases by solids when the width of the pores is of the order of the size of the gas molecules; they also apply to various cases of sorption of ionic and non-ionic solutes from liquid solutions. If the molecules of the fluid do not meet any significant resistance on their way to the entrances of the micropores, the rate of the occlusion process is entirely determined by the motion of the occluded molecules in the micropores.In many cases one can assume that this motion is governed by activated surface diffusion and obeys Fick’s equation with the diffusion coefficient D independent of concentration and given by dc/dt = D(d2c/dX2) (1) (2) D = Doexp (- E/RT). In these equations cis the concentration of the sorbate in an element of the pore volume at a distance x from the entrance of the pore and at time t ; Eis the energy of activation for diffusion and Do is a constant. A rate equation for the occlusion process is obtained by integrating eqn (1) with appropriate boundary conditions. Integration is easily performed for systems with ‘homogeneous microporosity ’, i.e. when the coefficient of diffusion is the same in all micropores, the lengths of the diffusion paths travelled by the occluded molecules are equal and the entrances of the micropores are equally accessible from the fluid phase surrounding the solid.However, the expressions obtained even in the simpler cases are not always easy to apply to experimental data, and it is often convenient to take advantage of the fact that they are approximated by simpler equations under certain conditions. The most widely used approximation is the parabolic law valid when t is 231323 14 KINETICS OF OCCLUSION small [see eqn (18), (24) and (25)]. An exponential approximation valid at large t is also well known [see eqn (19), (26) and (27)] but less useful. In the present discussion it is shown that in many cases there is a range of t intermediate between the two above-mentioned ranges at which the kinetic expressions can be approximated by another simple expression, an Elovich equation.The Elovichian approximation can be conveniently used for testing assumed models and estimating parameters . Moreover, an important feature of the Elovichian approximation is the fact that it is also applicable to adsorbents with ‘ heterogeneous microporosity ’, possessing pores with different coefficients of diffusion and different lengths. In contrast to the small-t parabolic approximation and the large-t exponential approximation, the range of validity of the Elovichian approximation increases with the heterogeneity of the system (see Part 3).l The kinetics of occlusion by microporous solids and the kinetics of chemisorption on heterogeneous non-porous surfaces can be described by equations of the same form, and it is possible that the kinetics of chemisorption are also governed by some process of diffusion.THE z(t) PLOT It has been shown2 that a kinetic expression can be approximated by an Elovich equation at a limited range of time when the plot of the reciprocal of the rate against time is sigmoid and has an inflexion point. Referring to an occlusion process we (3) consider the expression where vt is the amount of fluid taken up at time t andflt) is a function o f t ; we assume that eqn (3) is such that a plot of the reciprocal of the rate, z = (du/dt)-l, against t is sigmoid U t = f ( 0 d2z/dt2 < 0 at t < t, (4) and d2z/dt2 > 0 at t > t, ( 5 ) d2z/dt2 = 0 at t = t,.(6) where t, is an inflexion point, and we also have When t does not differ considerably from t,, the plot of z against t can be approximated by the equation of the tangent at the inflexion point z = zP + b( t - t,) (7) where zp is the value of z at tp and b is the slope of the tangent. Reintroducing z = (du,/dt)-l and integrating gives the Elovichian expression ut = A’+(l/b)ln(t+t,) (8) where to = (z,/b - 1,) A ’ = up - (l/b) In (t, + to) and up is the amount taken up at t,. Eqn (8) can also be written as ut = A + (1 /b) In (t/tp + tr) (1 1) with t, = z,/bt,- 1 = t,/tp (12) and A = v,+(l/b)ln(bt,/z,) = A’+(l/b)lnt,. (1 3)C. AHARONI A N D Y. SUZIN 2315 In the following discussion it is shown that the kinetic expressions obtained on integrating eqn (1) with various boundary conditions often give z ( t ) plots in agreement with relations (4)-(6). It follows that they are approximated at certain ranges of t by equations of the form of eqn (11) with particular values of the parameters, A , b and t,. OCCLUSION I N A SINGLE MICROPORE It is useful to consider first a system comprising a single micropore and a sorbable gas.The pore of length 1 is opened to the gas phase at x = 0 and closed at x = 1. The gas is maintained at constant pressure throughout the process and the concentration co of the occluded sorbate at the entrance to the pore is also constant. Integrating eqn (1) with appropriate boundary conditions and averaging the concentration along x gives [see for instance ref. (3)] n--a, n -0 ut/u, = zt/co = 1 - (8/n2) C [ 1 /(2n + 1)12 exp [ - (2n + 1)2 ( t l z , ) ] (14) where ct is the mean concentration of the sorbate in the pore at time t, u, the amount of sorbate at t = co and n an integer varying between 0 and GO ; z, is a parameter related to the diffusion coefficient and to the length of the pore by zl = 412/n2D.(15) An expression for z is obtained by differentiating eqn (14) and inverting -1 z = (n2zz/8u,) i 1: C [exp( -2n+ l)'(t/q)]} . (16) The plot of u,z/z, against ( t l z , ) calculated according to eqn (16) is depicted in fig. 1, curve 1. It is convex towards the z axis at low t and concave at large t, and it has an inflexion point in agreement with relations (4)-(6). In order to determine the coordinates of the inflexion point it is convenient to (17) rewrite eqn (6) as The derivatives in eqn (17) are calculated for various values of t / z , using eqn (14), and the value of t / z , for which they satisfy eqn (1 7) is tp/zz, the inflexion point value.The parameters up and zp can be determined by inserting tp/zz in eqn (14) and (16). b is dz/dt at t = t,. It is mentioned above that the presence of an inflexion point in the plot of z against t implies validity of the Elovich equation, eqn (1 l), near that point. The parameters A and t, can be found by inserting t,, up, zp and b as calculated above in eqn (12) and (13) (see table 1, column 1). A plot of ut/u, against In ( t / t p + t,) is depicted in fig. 2, curve 1 . It is calculated according to eqn (14) using tp/z, and t, given in table 1, column 1 .The plot is practically linear over a considerable range around ut/uoo = u p / u , = 0.494 as predicted by eqn (1 1). The slope of the linear segment has the predictable value l/bu, = 0.438. Other simple expressions to which eqn (14) reduces at limited ranges of time are (1 8) the parabolic equation valid at small t, and the exponential equation 2 ( d ' ~ ~ / d t ~ ) ~ - (dut/dt) (d3ut/dt3) = 0. V t / U , = (4/$) (t/T,)a2316 KINETICS OF OCCLUSION 6 4 c t4 8 f3 --- / 3 / / ,” * I/’/ / 1 0.5 1.0 t f r FIG. 1.-Plots of v,z/7 against t / 7 : (1) single pore or slabs, (2) cylinders, (3) spheres. OCCLUSION BY SOLIDS WITH HOMOGENEOUS MICROPOROSITY The above treatment can be extended to systems comprising a fluid and a particulate solid, if the particles have the same regular shape and the same size and if the micropores form a homogeneous network with constant D.If the particles are slabs with parallel surfaces and negligibly narrow edges, eqn (14) itself is applicable and the equations and data derived from it in the preceding section are also applicable. For elongated cylinders with radius rc the equation corresponding (20) where ln are the roots of the equation Jo(x) = 0 where Jo is a Bessel function. z, is defined by For spheres with radius Y, the pertinent equation is to eqn (14) is 12-03 n=1 u t / ~ m = 1 - (4/<2,) ~ X P [ - ( t n / t i ) ’ t/zcl z, = rE/<fD. (21) n-a, n-o v t / v , = 1 - (6/7t2) ( l / r ~ ) ~ exp ( -n2t/zs) (22) where z, is defined by z, = r,2/n2D. (23) The plots of z against t corresponding to eqn (20) and (22) are sigmoid in a similar manner to that derived from eqn (14) (see fig.1, curves 2 and 3). However, the coordinates of the inflexion point and the slopes of the tangents at the inflexion point differ for each geometry (see table 1). Eqn (1 1) is obeyed around the inflexion point in all cases, but the values of A and t , differ (see again table 1). Plots of v t / u , againstC. AHARONI A N D Y. SUZIN 2317 FIG. 2.-Plots of ut/u, against ln(t/t,+t,): (1) single pore or slabs, (2) cylinders, (3) spheres. TABLE 1 .-COORDINATES OF THE INFLEXION POINT IN THE z(t) PLOT AND PARAMETERS OF THE ELOVICH EQUATION single pore or slabs cylinders spheres t p l z 0.474 Z p U m l ~ 1.938 u p l u , 0.494 bum 2.281 A l u m 0.238 tr 0.793 0.455 1.986 0.549 3.136 0.444 0.391 0.349 1.645 0.530 3.535 0.449 0.333 In ( t / t p + t,) calculated according to eqn (20) and (22) with appropriate parameters are depicted in fig.2, curves 2 and 3; they are linear at a considerable range around t , and have the predicted slopes. Similarly to eqn (14), eqn (20) and (22) reduce to parabolic expressions at small t and to exponential expressions at large t. At small t eqn (20) becomes (24) V t I V , = (4/L .4 (tltC!Y2318 KINETICS OF OCCLUSION 0 0.5 I .o ti71 0 FIG. 3.-Plots of ut/u, against t / T calculated according to non-approximated equations (solid lines) and according to various approximations : 0, small-t approximations; A, large-? approximations; 0, intermediate-? approximations. (a) Single pore or slabs, eqn (1 l), (14), (18) and (19), with data from table 1, column 1 ; (b) cylinders, eqn (1 l), (20), (24) and (26), with data from table 1, column 2; (c) spheres, eqn (1 l), (22), (25) and (27), with data from table 1, column 3.and eqn (22) becomes ut/u, = (6/7&) (t/t,)*. (25) U t / % = 1 - (4/t3 exp ( - t / q ) (26) t l t / u , = 1 - (6/n2) exp (- t / ~ , ) . (27) At large t eqn (20) becomes and eqn (22) becomesC. AHARONI A N D Y. S U Z I N 2319 RANGE OF APPLICABILITY OF THE APPROXIMATIONS It is useful to compare the ranges at which the approximations (parabolic, Elovichian and exponential) can be subsituted into the accurate expression for each geometry. Fig. 3(a) refers to slabs (or single pores) and shows plots of vJv, against t/z calculated according to eqn (1 l), (14), (18) and (19).Similar plots for cylinders are depicted in fig. 3 ( b ) ; they are calculated according to eqn (1 l), (20), (24) and (26). Plots for spheres are depicted in fig. 3 (c) and are calculated according to eqn (1 l), (22), (25) and (27). [Eqn (1 1) is used with the parameters appropriate to each case.] For slabs one can cover the entire range of vt/v, with reasonable accuracy by jointly using the parabolic and exponential approximations, the former for say u t / v , < 0.5 and the latter for vt/v, > 0.5. This is not possible in case of cylinders or spheres. With spheres, for instance, the parabolic approximation is accurate for low vt/v, only, say vt/v, < 0.25, and the exponential approximation is applicable to vt/v, > 0.6; at vt/v, = 0.51 the plots for both approximations give a value of t/z lower than the real one by 30%.The Elovichian approximation has a wide range of validity at intermediate v t / v , in all cases. The range of validity widens in the sequence slabs, cylinders, spheres, whereas the range of validity of the other approximations becomes narrower. For spheres, the Elovichian approximation is reasonably accurate between say ut/v, = 0.25 and vt/v, = 0.8. CRITERION FOR THE APPLICABILITY O F AN ELOVICHIAN APPROXIMATION TO A DIFFUSION EQUATION It is often possible to find out if the z(t) plot corresponding to a given diffusion equation is sigmoid, and if the Elovichian approximation is applicable, without necessarily calculating z. where k, and z are constants, z is given by For a parabolic equation vt/v, = k,(t/z)i (28) V, z = (2~/k,) (t/z)i.(29) ut/v, = 1 - k, exp (- t/z) (30) v, z = (z/k,) exp (tlz). (31) u,(d2z/dt2) = - (1/2k, Z) (t/z)-a < 0 (32) v,(d2z/dt2) = (1 /k, z) exp (t/z) > 0. (33) It follows that for any diffusion equation that reduces to a parabolic equation at small t and to an exponential equation at large t , the plot of z against t is sigmoid (convex at low t, concave at large t and with an inflexion point at an intermediate t ) and it also follows that there is a range o f t at which the diffusion equation approaches an Elovich equation. On the other hand, for an exponential equation where kf is a constant, z is given by The derivative d2z/dt2 corresponding to eqn (29) is negative and the derivative d2z/dt2 corresponding to eqn (31) is positive2320 KINETICS OF OCCLUSION This reasoning is applicable to the diffusion equations considered above [eqn (14), (20) and (22)], which became parabolic at low t [eqn (18), (24) and (25)] and exponential at large t [eqn (1 9), (26) and (27)], and it leads to results consistent with the results obtained by actually calculating z. The criterion can also be applied to other diffusion equations (other geometries, varying D, varying Co), if it is known that the equation is parabolic at low t and exponential at large t . C . Aharoni and Y. Suzin, J. Chem. SOC., Faraday Trans. I , 1982, 78, 2329. C. Aharoni and M. Ungarish, J. Chem. SOC., Faraday Trans. I , 1976, 72, 400. W. Jost, Diffusion in Solids, Liquids and Gases (Academic Press, New York, 3rd edn, 1960), chap. I, sect. VIII. (PAPER 1 /548)

ISSN:0300-9599    

DOI:10.1039/F19827802313

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1982, 78, 2321-2327 Application of the Elovich Equation to the Kinetics of Occlusion Part 2.-Analysis of Experimental Data from the Literature BY CHAIM AHARONI* AND YAAKOV SUZIN Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa, Israel Received 7th August, 198 1 The Elovichian approximation and the parabolic approximation (see Part 1) have been applied to data on the kinetics of occlusion found in the literature. The data refer to occlusion of a gas, a liquid and solutes by zeolites, active carbon and a cation-exchange resin. The adsorbents were either spherical particles or powders. The equation for occlusion by ‘slabs’, i.e. parallel diffusion paths, is found to be applicable in all cases; the equation for occlusion by ‘spheres’, i.e.convergent diffusion paths, is not applicable. Rate equations for the kinetics of occlusion on adsorbents with homogeneous microporosity have been examined in Part 1 . l Diffusion equations can be approximated at small t by the parabolic expression vt/v, = k,(t/z)i (1) (2) and at intermediate t by the Elovichian expression Vt/V, = A/v, + l/bu,(t/t,+ tr). vt and v, are the amounts of adsorbate taken up at time t and at equilibrium, z is a residence time related to the length of the diffusion path, I or r, and to the diffusion coefficient D, while t , is the time at which the plot of z = (dv,/dt)-l against t has an inflexion point, and is proportional to z. The parameters k,, Alv,, bv, and t , are independent of z and consequently of the nature of the adsorbent-adsorbate pair, and they depend only on the geometry of the diffusion path.Table 1 gives the parameters for two simple geometries: (a) parallel diffusion paths, e.g. occlusion by thin slabs of TABLE 1 .-PARAMETERS OF EQN (1) AND (2) FOR TWO SIMPLE GEOMETRIES converging paths perpendicular to a parallel paths spherical surface (spheres of radius r ) (slabs of thickness 21) z = (4/n2) ( P / D ) z = ( l p ) (r2/D) t , = 0.3492 k, = 6/nf A / # , = 0.449 bv, = 3.535 t , = 0.333 t , = 0.4742 k, = 4/d A / # , = 0.238 bu, = 2.281 t , = 0.793 232 12322 KINETICS OF OCCLUSION equal thickness 21, and (b) convergent diffusion paths perpendicular to the surface of a sphere, e.g. occlusion by spherical particles of equal radius r. Eqn (1) and (2) are now applied to data from the literature.If microporosity is homogeneous, and if any of the above-mentioned idealisations for the geometry of the diffusion path are applicable, eqn (1) should be valid at low t , eqn (2) at intermediate t , and the values of z deduced from both equations should be equal. DETERMINATION OF t , t, is the only parameter in eqn (2) that depends on the nature of the adsorbent- adsorbate pair, and it must be known in advance in order to plot experimental data according to this equation. As t , is by definition the time at which the plot of z against t has an inflexion point, it can be determined by plotting the experimental data in that way. There are, however, methods for determining t , that are more convenient. The simplest method is based on the fact that up, the amount taken up at t,, can be predicted theoretically for simple geometries, e.g.up/u, = 0.494 for slabs and u,/v, = 0.530 for spheres [see ref. (111. Therefore t, can be taken as the time at which the experimental value ut/u, becomes equal to the predicted vp/v,. A more accurate procedure not based on a single point is as follows. Eqn (2) is solved for t,, and the validity of the resulting equation is extended to the entire range of u,/u, by replacing the constant t , by a variable t , t , = t (exp [bum(ut/u, --A/u,)] - tr}--l. (3) A plot of t , against t is drawn according to eqn (3) using the experimental values for the variables ut/u, and t and the theoretical values for the parameters Alum, bu, and t,. In the region in which eqn (1) is valid the plot becomes horizontal and t , becomes identical to t , (see fig.1). The plot according to eqn (3) can be used not only for the determination of t , but also as a direct test for the applicability of eqn (2). Experimental data can be 0.3 1 I I I 1.5 '*O t / r 0 0.5 FIG. 1.-Plots of t z / 7 against t / 7 : (1) slabs, (2) cylinders, (3) spheres.C . AHARONI AND Y. SUZIN 2323 considered to be in agreement with eqn (2) if: (a) the plot of t, against t has a flat minimum and (b) the point t = t, on the t axis, corresponding to t, = t,, is on the flat part of the curve. ANALYSIS OF EXPERIMENTAL DATA Experimental data for various systems were examined and the results are reported for some examples in fig. 2-5: (a) adsorption of n-hexane vapour by Ca-exchanged zeolite 4A in powdered form, data of Barrer and Clarke2 [the data in ref.(2), fig. 1 (b), curve 1 are replotted in fig. 2 of the present work]; (b) adsorption of mesitylene (liquid) 1 I 1 1 - 1.0 0.5 0 0.5 1.0 In (t/tp + t3 d t FIG. 2.-Adsorption of n-hexane vapour on Ca-exchanged zeolite 4A (data of Barrer and Clarke2): (a) plots oft, against t ; (6) plots of v t / v , against ln(t/t,+t,); (c) plots of v t / u , against z / t ( t in min).2324 KINETICS OF OCCLUSION 4 -0 1 I I I 1 ( a ) - - - I I 1 I 4 2 3 4 5 6 t d t FIG. 3.-Adsorption of mesitylene (liquid) by a Na Y zeolite (data of Satterfield and Cheng3): (a) plots of t , against t (0, spheres; x , slabs); (b) plots of ut/u, against dt ( t in min). by NaY zeolite with particles approximately spherical, data of Satterfield and Cheng3 [the data in ref.(3), fig. 4, 0 *C are replotted in fig. 31; (c) adsorption of isopropyl- N(3-chloropheny1)carbamate from aqueous solution by activated carbon in powdered form, data of Schwartz4 [the data in ref. (4), fig. 2, pH 6.9 are replotted in fig. 4.1; ( d ) uptake of potassium ions by a polystyrene sulphonic acid cation-exchange resin in the form of ‘perfect spheres’, data of Tetenbaum and Gregor5 [the data in ref. (9, table 1 are replotted in fig. 51. The kinetic data are plotted as t , against t using both the ‘slab’ and ‘sphere’ models. Both models were tried for each adsorbent-adsorbate pair, whether the adsorbent particles had a well-defined shape or comprised particles of various shapes [see fig.2(a), 3(a), 4(a) and 5(a)]. In one example, [fig. 4(a)] the ‘cylinder’ model is also examined; appropriate parameters may be found in ref. (1). The plots show that the Elovichian approximation is practically applicable to all the adsorbent-adsorbate pairs examined, with both the slab and sphere models. However, the condition concerning the position of the point t = t , is not fully satisfied in all the plots. In one case [fig. 4(a)] the slab model is definitely more satisfactory than the sphere model. The minima of the plots are distinct in all cases and values of tp for both models could be consistently estimated. From these parameters, values of z for both models were calculated using the relations in table 1. The values of t, and z are given in table 2.The kinetic data were also replotted as ut/u, against ln(t/t,+t,) using values of t , obtained from the plots oft, against t . The plots for one adsorbent-adsorbate pair are depicted in fig. 2(b). The plots are linear around t = t , as required by eqn (2), and again the slab and the sphere models seem to be equally applicable.C . AHARONI AND Y. SUZIN 2325 1 0 FIG. 4.-Adsorption of isopropyl-N(chlorophenyl)carbamate from aqueous solution by activated carbon (data of SchwartzQ): (a) plots oft, against t (0, spheres; 0, slabs; x , cylinders); (b) plots of ut/u, against l / t ( t in h). The applicability of the parabolic approximation and the consistency between the values of z predicted by the parabolic approximation and by the Elovichian approximation are tested in fig.2(c), 3(b), 4(b) and 5(b). The circles in these figures refer to the experimental data. The lines are calculated plots, obtained by introducing k, values taken from table 1 and z values taken from table 2 into eqn (1); one of the lines refers to the slab model and the other to the sphere model. In all the figures, the plot calculated according to the slab model is in good agreement with the experimental points, whereas the plot referring to the sphere model deviates consist- ently. This means that when the parabolic approximation is applied at low t and the Elovichian approximation is applied at intermediate t , the values of z obtained are consistent if the slab model is assumed.2326 5c KINETICS OF OCCLUSION 4c t x 3c 2c kn SPHERES \ ' t = t, ' t = t, 1 I I 10 20 30 t 0 2 I I 1 I I ( b ) .5 - FIG.5.-Uptake of potassium ions by a cation exchange resin (data of Tetenbaum and Gregor5): (a) plots of t, against t ; (b) plots of ut/u, against Z/t ( t in s). DISCUSSION The Elovichian approximation provides a simple method for testing the applicability of diffusion equations at a range oft beyond that in which the parabolic approximation is valid. Furthermore, it is possible to obtain additional information on the occlusion process by joint use of the parabolic and Elovichian approximations. In the examples examined in this paper, the microporosity behaves as though it were homogeneous, and diffusion takes place along parallel non-convergent paths, as in slabs; these results are not predictable from the nature of the adsorbent and the shape of the particles.Diffusion along parallel paths (the slab model) is applicable to particles of any shape if the material possesses a network of wide macropores in contact with the surrounding fluid atmosphere and a large number of narrow micropores of equal length and equalC . AHARONI AND Y. SUZIN 2327 TABLE 2.-tp AND 7 VALUES FOR DATA FROM THE LITERATURE^ t P 7 eqn (2) eqn (2) eqn (1) (slab) (sphere) (slab) (sphere) (slab) (a) zeolite + n-hexane 56 64 118 183 118 vapour (fig. 2)/min liquid (fig. 3)/min isopropyl-N(chloropheny1)- carbamate (fig. 4)/h resin + potassium ions (fig. 5)/s (b) zeolite + mesitylene 0.95 1.2 2.0 3.44 2.0 (c) active carbon+ 0.62 0.53 1.31 1.52 1.31 (d) cation-exchange 19.5 24.6 41.1 70.5 41.1 a t, and z are in the time units in which the data were published (1 h = 3.6 x lo3 s, 1 min = 60 s). D opening into the macropores (assuming that there is no adsorption and no resistance to the transport of the fluid in the macropores). In this case the kinetics of occlusion are entirely determined by the diffusion of the occluded sorbate in the micropores, and the process takes place at the same time and in parallel in all the micropores. Another case in which particles of any shape behave as slabs is when diffusion is hindered at a short distance from the surface of the particle so that the effect of curvature is not felt. In both cases the length of the diffusion path is likely to be much shorter than the length deduced from the size of the particle, and as z cc Z2/D the diffusion coefficient is likely to be considerably smaller than expected. C. Aharoni and Y. Suzin, J. Chem. SOC., Faraday Trans. I , 1982, 78, 2313. R. M. Barrer and D. J. Clarke, J. Chem. SOC., Faraday Trans. I , 1974, 70, 535. C. N. Satterfield and C. S. Cheng, AIChE Symp. Ser., 1971, 67, 43. H. G. Schwartz Jr, Environ. Sci. Technol., 1967, 1, 332. M. Tetenbaum and H. P. Gregor, J. Phys. Chem., 1954, 58, 1156. (PAPER 1/872)

ISSN:0300-9599    

DOI:10.1039/F19827802321

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1982, 18, 2329-2336 Application of the Elovich Equation to the Kinetics of Occlusion Part 3 .-Heterogeneous Microporosity BY CHAIM AHARONI* AND YAAKOV SUZIN Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa, Israel Received 29th May, 1981 A model is considered in which occlusion takes place in parallel in an array of micropores with different coefficients of diffusion. The rate equation for occlusion in a pore is approximated by a parabolic or by an exponential equation, and the rate for the overall process is obtained by summing the rates in the pores. The plot of &/ V , against In t, where & and V, are the amounts sorbed at times t and infinity, respectively, is sigmoid and has an intermediate part with greatest slope.The slope of this intermediate part is related to the heterogeneity of the system. If uE(E) is constant, where E is the energy of activation for diffusion and uE the pore volume for an energy between E and E+dE, the reciprocal of the slope is equal to the difference between the highest and lowest E divided by RT. If E is constant the slope is close to 0.2. The kinetics of occlusion by solids with a homogeneous network of micropores with the same diffusion coefficient and particles of the same shape and size have been discussed in Part 1.' It was shown that the plot of the reciprocal of the rate z = (dK/dt)-l against the time, t , is S-shaped and has an inflexion point at t = t,. Around t = t,, the rate equation for &(t) can be approximated by an Elovich equation (1) & = A + (1 / b ) In (IllI, + I,) where the constants A, b and t, are determined by the shape of the particles.In the present paper it is shown that Elovichian kinetics are also applicable to occlusion by sorbents possessing micropores with different lengths and different coefficients of diffusion. Occlusion is defined in this case also as a sorption process in which the fluid undergoes a change of phase when entering a micropore, and all the sorbate in a micropore is in the same phase. MODEL FOR THE OCCLUSION PROCESS We consider specifically the following model. The micropores open into a network of wide macropores that communicate with the surrounding atmosphere. There is no resistance to the flow of fluid and no adsorption in the macropores, and the fluid reaches the entrances of all the micropores rapidly and at the same time.The penetration of the fluid into the micropores and the accompanying change of phase are rapid. The movement of the condensed sorbate in the micropore is by activated surface diffusion, and this determines the rate of the overall occlusion process. The volume v; sorbed at time t in a micropore of length I with a coefficient of diffusion D is given by n--00 ui/ul, = 1 - (8/7r2) X [ 1 /(2n+ 1)2] exp [ - (2n + l)"t/z)] (2) n-0 23292330 KINETICS OF OCCLUSION where u; is the volume sorbed at t = o(> and is considered as the ‘available pore volume’ for the pressure and temperature at which occlusion is performed. z is a parameter related to I and D by z = 412/n2D.(3) If 1 and D are the same in all micropores, the volumes adsorbed in all micropores at t = t and t = co, & and Vm, are related to the volumes adsorbed in a single micropore, ( 4 ) ul and v;, by </Vm = u;/v; and eqn ( 2 ) applies to the overall system. The treatment for ‘slabs’ in ref. (1) is valid, and in the region t z t, = 0.4742 one can apply eqn (1) with A/Vw = 0.238, bVm = 2.281 and t, = 0.793. If the coefficients of diffusion or the lengths of the micropores vary, it is necessary to use a distribution function in order to characterize the system. It is convenient to use the function v,(z), where v,dz is the available pore volume for the micropores with z ranging from z to z+dz. The available pore volume is as defined above, the maximum volume of occluded adsorbate at the pressure and temperature considered.It is implicitly assumed that z can vary from micropore to micropore, but it does not vary in the same micropore. The total occlusion in all the micropores is given by where vTt is the volume of adsorbate in the micropores characterized by z at time t, and zi and z, are, respectively, the lowest and highest values of z in the system. In order to solve eqn (9, one has to introduce the appropriate distribution function v,(z) and replace vrt/v, = v;/ul, by its value according to eqn (2). The calculations are greatly simplified if one uses a suitable approximation instead of eqn (2). At small t eqn ( 2 ) reduces to the parabolic expression (6) u;/u:, = V,t/V, = (4/n9(t/z)4 and at large t it reduces to the exponential expression v;/v; = uTt/uT = 1 - [(8/n2) exp (- t / z ) ] .( 7 ) It has been shown1 that the plot of vl/v& against t calculated according to eqn (6) is practically congruent to the plot calculated according to eqn ( 2 ) for u ; / u k < 0.5, and similarly the plot according to eqn (7) is practically congruent to the one according to eqn ( 2 ) for u;/& > 0.5. A more strict test is to compare differential plots. Plots of z’vl,/z = v’,/z(du;/dt) against t / z calculated according to eqn (2), (6) and ( 7 ) are depicted in fig. 1. The figure shows that eqn (6) is valid with reasonable accuracy when 0 < t < t, and eqn (7) is valid in the range t, < t c co. In the system under consideration occlusion takes place in parallel in pores with different z, and there are at any moment t some pores in which t , is just being reached; these are the pores for which z = zt = t/0.474.In the pores with z < z~, z is beyond the inflexion point and the kinetics obey eqn (7). In the pores with z > zt the inflexion point is not reached and eqn (6) is still valid. Eqn (5) can thus be rewritten asC. AHARONI AND Y. SUZIN 233 1 fh FIG. 1.-Plots of Z’V&/T against t / t . The plots are calculated according to eqn (2) (solid line), eqn (6) (dots) and eqn (7) (squares). MICROPORES WITH vE CONSTANT Eqn (8) can be solved analytically for some simple distributions v,(z). An interesting case is the one in which the available pore volumes, correspondingto equal increments of the activation energy for diffusion, are equal.Denoting by vE(E)dE the available pore volume corresponding to an activation energy for diffusion between E and E+dE, we have u,(E) constant. As D = D,exp(-E/RT) (9) z is related to E by z = z, exp (E/RT) (10) where T o = 4l2/n2DO. (1 1) (12) Therefore v, = c / z (13) If Do is the same for all pores and the lengths I are equal, the distribution v,(z) is related to the distribution v,(E) by u, = U,RT/z. where C is a constant. The total available pore volume V, is given by V, = jrrm v,dz = Cln (zm/zi). (14) The volume adsorbed at t is obtained by combining eqn (13) with eqn (8) and normalising by using eqn (14)2332 KINETICS OF OCCLUSION Differentiating according to t and multiplying both sides by t gives [In (zrn/7i)] [(d &/ V,)/d In t] = 1 -(4/d) (t/tm)'-(8/n2) exp (- t/zi)* (16) The plot of VJV, against In t corresponding to eqn (16) has the following characteristics (see fig.2). 4 3 I 2 Int FIG. 2.-Plots of VJVm against In t : (1) Ei = 400, Em = 10 000; (2) Ei = 5400, Em = 15000; (3) Ei = 400, Em = 6000; (4) Ei = 400, Em = 2000 ( E in arbitrary units, RT = 600 arbitrary units, z, = 1). The points corresponding to t = ti and t = tm and the tangents at these points are shown for curve 1. The dotted line cutting curve 4 is a line with slope l/ln (zm/ri) = 0.375. (1) If zi is sufficiently small and z, sufficiently large, i.e. if the range of activation energies is sufficiently wide, there is a range of t for which the two last terms in eqn (16) are negligible, and eqn (16) then becomes (17) The plot of &/ V, against In t is linear in that range, and its slope is l/ln(zm/zi).This means that when heterogeneity increases (a) the range at which the curve is linear increases and (b) the slope of the linear part decreases. d (&/ V,)/d In t = 1 /In (zm/zi). (2) At small t eqn (16) is approximated by d( &/ V,)/d In t = [ 1 /In (zm/zi)] [ 1 - (8/n2) exp ( - t/zi)]. (18) The plot &/ V, against In t is concave towards the Q/ Vm axis in that range since d2 (&/ V,)/d (In t)2 = [ 1 /In (zm/zi)] (8t/n2zi) exp ( - t/zi) > 0. (19) When t = zi eqn (1 8) becomes d (&/ VJ/d In t = 0.702/ln (zrn/zi) i.e. the point at which t = zi is the point in the concave region for which the slopeC . AHARONI AND Y. SUZIN 2333 is 0.702 times the slope of the linear part.When t = 0 eqn (18) becomes d ( G / Vm)/d In t = 0.189/1n (zm/zi) (21) i.e. the minimum possible value of the slope at the beginning of a run is 0.189 its value in the linear region. (3) At large t eqn (16) is approximated by d ( &/ V,)/d In t = [ 1 /in (z,/zi)] [ 1 - (4/$) (t/zm)']. dz (&/ V,)/d (In t)2 = [ - 2/& In (z,/zi)] [t/zm]' < 0. d (Q'V,)/d In t = 0.282/1n (z,/zi) (22) (23) (24) The plot of &/ V, against In t is convex towards the VJ Vm axis since When t = z, eqn (22) becomes i.e. the point at which t = r , is the point in the convex region for which the slope is 0.282 times the slope of the linear region. Plots corresponding to eqn (1 6) were computed numerically and are depicted in fig. 2. Introducing uE(E) instead of u,(z) in eqn (8), replacing the integration by a summation and normalising by introducing one obtains wheref,(z) is uTt/u, as given by eqn (6),f2(z) is vJu, as given by eqn (7), Ei and Em are, respectively, the lowest and highest values of E in the system and Et = RTln ( z t / z o ) = RTln (t/0.474 zo).For the specific distribution uE constant eqn (25) becomes Curve 1 in fig. 2 refers to an array of 100 pores with vE constant and E varying progressively and by equal increments between Ei = 400 and Em = 10000; RT = 600 and z, = 1. (Ei, Em and RT are in the same arbitrary units.) The curve is calculated by introducing in eqn (26) the appropriate values of Ei, Em and AE. Curves 2, 3 and 4 refer to similar arrays of pores with other ranges of E and they are computed in a similar way: E ranges between 5400 and 15000 units for curve 2, between 400 and 6000 units for curve 3 and between 400 and 2000 units for curve 4.Curve 1 is practically linear over a wide range of &/Vm, between 0.15 and 0.85. The slope of this linear part is l/ln (z,/zi) = RT/(E,/Ei) = 0.0625 in agreement with eqn (17). The point corresponding to t = zi = 1 . 9 5 ~ ~ is on the concave part of the plot and the slope of the tangent at that point is 0.7/ln(z,/zi) = 0.044, as required by eqn (20). The point corresponding to t = z, = 1.73 x lo7 q, is on the convex part of the plot and the slope of the tangent at that point is 0.28/ln(z,/zi) = 0.018, as required by eqn (24). In curve 2 the ratio2334 KINETICS OF OCCLUSION z,/zi is the same as in curve 1, and the linear parts of these two curves are parallel.In curve 3 the ratio zm/ti is smaller, the linear part is shorter and steeper and its slope is l/ln(zm/zi) = 0.107. In curve 4 z, and zi are too close and it is not possible to discern a linear part with a slope of l/ln(zm/zi) = 0.375; if the region around the inflexion point is approximated by a straight line, the slope of this line is ca. 0.22. PLOT OF VJV, AGAINST lnt FOR A N ARRAY OF PORES WITH THE SAME z It is useful to compare the kinetics implied by eqn (16) with the kinetics for a homogeneous system in which the micropores have the same value of z. The kinetic equation for the homogeneous system, eqn (2), approaches eqn (1) at t = t, and eqn (7) at large t. The following expressions for d(&/V&J/dlnt are obtained by differentiating these equations with respect to In t and taking into consideration eqn (27) (4) : valid at intermediate t, and (28) valid at large t .Eqn (27) and (28) show that d(VJV,)/dlnt increases with t at intermediate t and decreases at large t . It follows that the plot of &/ V, against In t is sigmoid and has an inflexion point with maximum slope at a point t = tinf in the transition region between the regions described by eqn (27) and (28). This is illustrated by curve 1 in fig. 3 which depicts a plot of K/ V, against In t calculated according to eqn (2) for E = 5200, RT = 600 arid z, = 1. ( E and RTin the same arbitrary units.) d (K/ G ) / d In t = (l/b Gl)/(l+ tJ,/t) d (K/ Vm)/d In t = (8/n2z) t exp (- t / z ) In t In t In t In t In t FIG. 3.-Plots of &/ Vagainst In t for various distributions: curve (1) Econstant, (2) Gaussian distribution [eqn (30)], (3) triangular-shaped distribution [eqn (29)], (4) Rayleigh distribution [eqn (3 l)], (5) uE constant.The dotted curves refer to distributions with uE constant adjusted to fit the curves referring to the other distributions. The slope of the plot of against In t is known at the point t = tp. Introducing in eqn (27) t , / t = 1, bV, = 2.281 and tr = 0.793 [see ref. (l)], one obtains d(l$/V,)/dlnt = 0.244 for that point. At the point t = tinf the slope is > 0.244. If the plot &/ V, against In t is approximated by a straight line in the region aroundC. AHARONI A N D Y. SUZIN 2335 tinf this line will have a slope > 0.24.This property gives the possibility to distinguish between a strongly heterogeneous system with a small slope given by 1 /In (zm/zi) and a homogeneous or weakly heterogeneous system with a slope of the order of 0.24. The slopes for curves 1 , 2 and 3 in fig. 2 are 0.06, 0.06 and 0. 1, respectively, whereas the slopes for curve 4 in fig. 2 and curve 1 in fig. 3 are of the order of 0.24. OTHER DISTRIBUTIONS Eqn (16) is a solution of eqn (8) for the particular distribution u,(E) constant. For other distributions it is difficult or impossible to solve eqn (8) analytically, however plots analogous to those in fig. 2 can be computed by introducing in eqn (25) the appropriate function v,(@ and the assumed Ei, Em and AE. Three different distributions were examined, plots of v,/V, against E for these distributions are depicted in fig.4 , and the corresponding plots of &/Vm against In t are given in fig. 3. In all the cases a system comprising 100 pores is assumed; E is assumed to increase progressively and by equal increments between Ei = 400 and Em = 10000 arbitrary units, and therefore AE = (Em - Ei)/ 100 = 96 units; RT is 600 units and z, is 1. The distributions chosen are such that uE is maximum at the energy E = (Em - Ei)/2 = 5200 units. 2 3 4 5 2 5 5 7 5 'U' 2 5 5 7 5 E/ 103 E/ 103 ~1103 ~1103 EI 103 FIG. 4.-Plots of uE against E for the distributions given in fig. 3 (E in arbitrary units). Curves 2-4 have a maximum at E = 5200 units. Curve 3 in fig. 3 and 4 represents a triangular-shaped distribution vE(E) = 4(E- Ei)/(Em - Ei)2 for E < E and u E ( E ) = 4(Em-E)/(Em - EJ2 for E > E.The summation Curve 2 represents a Gaussian distribution uE(E) = (kG/n)4 exp [ - kG(E- IT)'] (30) k , = 3 x lo-' (energy units)-' was chosen; with this value vE at Ei and at Em are small as compared with uE at E Em Ei uE(Ei)/uE(E) = ~ E ( E ~ ) / V E ( E ) = uE(E)AE2336 KINETICS OF OCCLUSION can be taken as 1. Curve 4 represents a Rayleigh distribution, and distribution on which the Dubinin-Radushkevich equation is based2 (31) u,(E) = - 2k,(E- Ei) exp [ - k,(E- This distribution is unsymmetrical: uE is maximum at which differs from the average energy defined by Eav. = L?%,/Cu,. k , = (1 /2) (E- Ei)2 = 2.17 x (energy units)-2 was chosen giving U, maximum at Ev(max) = = 5200 units, and the resulting value for Eav.is 5500 units. v,(Ei) is zero, uE(Em)#O, EizEmVE(E)L\E = 0.86. Plots representing the systems E constant and vE constant are also depicted in fig. 3 and 4. With all the distributions examined the plot of K/ V against In t is sigmoid. If the intermediate section with greatest slope is approximated by a straight line, the slope of this straight line appears to depend on the distribution function u,(E) : it is greatest for E constant and decreases along the sequence v,(E) given by eqn (31), u,(E) given by eqn (29), u,(E> given by eqn (30) and finally uE(E) constant. This means that the distributions in which the pores with extreme values of E have a greater weight give plots of &/V, against In t with a lesser slope, a result consistent with the behaviour noted above: d (K/ Vm)/d In t decreasing with increasing heterogeneity.As all the plots in fig. 3 seem to be of the same form except for the variation in slope, it would be of interest to find out if they are significantly affected by the form of the distribution uE(E) or only by the implied heterogeneity. The dotted curves in fig. 3 represent plots for V , constant adjusted in order to give the best fit with the solid curves representing the. other distributions. The plots for u, constant were calculated by using a value of Eequal to that of the original distribution and a value of (Em - Ei)/RT equal to the reciprocal of the slope in the linear part for the original distribution [see eqn (17)]. These uE constant distributions are also depicted in fig. 4. In the three cases the plots of Q/Vm against In t for uE constant are congruent with the plots for the other distributions during most of the run. However, K/ Vm is slightly smaller at small t and slightly larger at large t, and the intermediate part of the curve is therefore closer to a straight line. It is unlikely that one could be able to assess the actual distribution by examining kinetic data obtained experimentally, although it should be possible in many cases to distinguish between more heterogeneous systems and less heterogeneous ones. CONCLUSION In conclusion, a simple test is suggested in order to assess the heterogeneity of a system of pores. K/ V, is plotted against In t, and if at the intermediate region with maximum slope the slope is ca. 0.24, the system is homogeneous or slightly heterogeneous, if it is + 0.24, the system is significantly heterogeneous. The reciprocal of the slope can conveniently be used as a parameter for expressing the degree of heterogeneity. This parameter increases as the heterogeneity increases and is indepen- dent of the form of the distribution uE(E). It is exactly equal to (Em-Ei)/RT for the distribution uE constant. C. Aharoni and Y . Suzin, J. Chem. Soc., Faraday Trans. I , 1982, 78, 2313. H. Marsh and B. Rand, J. Colloid Interface Sci., 1970, 33, 101. (PAPER 1 /872)

ISSN:0300-9599    

DOI:10.1039/F19827802329

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I , 1982, 78, 2337-2347 Pyrolysis of But-l-yne and the Resonance Energy of the Propargyl and 3 -Me thylpropargyl Radicals BY ANTONY B. TRENWITH* AND STEPHEN P. WRIGLEY Department of Inorganic Chemistry, School of Chemistry, The University, Newcastle upon Tyne NE1 7RU Received 6th July, 198 1 The pyrolysis of but-l-yne has been studied at eight temperatures over the range 652-731 K and pressures between 50 and 1200 Torr.? Measurements of the rate of formation of methane indicate that at the lower end of the temperature range studied, this product is formed mainly as a result of a bimolecular reaction. We propose the hydrogen transfer reaction 2CH3CH,C-CH -+ CH,CH,e:=CH, + CH,kHC-CH CH,CH,C:=CH, -+ .CH, + CH,=C=CH, (1) (1') as the rate determining bimolecular step, this being followed by the radical decomposition reaction and hydrogen abstraction by the methyl radical so formed.At higher temperatures the unimolecular dissociation reaction CH,CH,CrCH + *CH, + * CH,C=CH (2) contributes significantly to methane formation. The rate coefficient of this reaction is, under the experimental conditions employed, pressure-dependent. logl,(k,/dm3 mol-I s-l) = 1 1.3 0.2 - (47 800 600)/8 The expressions and loglo(kz/s-'), = 17.2&0.7-(74800f 2100)/8 have been obtained for the rate constants of the bimolecular and unimolecular reactions, respectively, where 0 = 2.303 RT/cal mol-'.$ These parameters yield values of 9.2k2.2 and 8.8k2.2 kcal mol-l for the resonance energies of the propargyl and 3-methylpropargyl radicals, respectively.Earlier results obtained from the pyrolysis of but- 1-ene and its derivatives have been re-examined in the light of the present findings. Only in the case of 2-methylbut-l-ene is there any indication of the occurrence of a bimolecular hydrogen-transfer reaction leading to the formation of methane. Measurement of the methane formed in the pyrolysis of but- 1 -ene and a number of its derivatives1t2 has provided a means of determining the resonance energy of the ally1 radical and various substituted allyls. The present work is an extension of these investigations to the pyrolysis of but-l-yne, the aim being to obtain by the same procedure a value for the resonance energy of the propargyl radical. The decomposition of but-l-yne has been found to resemble that of but-l-ene in that the methane formed results, in each case, from a unimolecular dissociation reaction in its pressure-dependent region.With but-l-yne there is a complication in that methane is also formed by a molecular process. From rate parameters obtained for both the unimolecular and bimolecular steps, values have been derived for the resonance energy of the propargyl and of the 3-methylpropargyl radical. 1 Torr = 101 325/760 Pa. 1 1 cal = 4.18 J. 76 2337 FAR 12338 PYROLYSIS OF BUT-I-YNE Earlier values obtained for the propargyl resonance energy varied markedly from 15.8 kcal mol-1 obtained by electron-impact studies3 to 4.1 kcal mol-1 from the iodine-catalysed isomerisation of methyl a~etylene.~ It is noteworthy that a number of estimates of resonance energies obtained by the latter procedure appear to be on the low side, and this may, in general, be due to under-estimates in the values taken for the activation energies of reactions involving hydrogen abstraction from HI by resonance-s tablised radicals.9 Comparative studies of the decomposition of various olefinic and acetylenic esters of p-cymene' indicate that the resonance energy of the ally1 radical exceeds that of the propargyl radical by 4 kcal mol-l. Taking a value of 12.6 kcal mol-1 for the former8 leads to 8.6 kcal for the latter, a figure which is in excellent agreement with the average of a number of estimates obtained by measuring bond dissociation energies of alkynes using the single-pulse shock-tube methodg as well as with those obtained by the very-low-pressure pyrolysis (v.1.p.p.) of but-l-ynelO and of 4-phenylbut- 1 -ynel1 and from more recent electron-impact studies.12 Data on resonance energies of derivatives of the propargyl radical are limited to values of 8.5 kcal mol-l for the 1-methylpropargyl radical [ * CH2C=CCH3] and 11 .O kcal mol-1 for the 3,3-dimethylpropargyl radical [(CH,),&CH] the former being obtained from shock-tube studiesg and the latter by the v.1.p.p.method.13 EXPERIMENTAL MATERIALS But-1-yne (Air Products) of 95% purity was outgassed and further purified by pumping at 175 K until impurities could no longer be detected by gas chromatography. APPARATUS AND PROCEDURE These were essentially the same as described previously,2 except that a precision pressure gauge (Texas Instruments) was connected to the reaction vessel to allow pressure changes to be monitored during pyrolysis.RESULTS Preliminary studies showed the major products of pyrolysis of but-1-yne at 719 K to be methane, ethylene, ethane and hydrogen. Minor products included allene, propene, propyne and but-I-ene together with trace amounts of acetylene and propane. The product-time curves shown in fig. 1 indicate that, whilst methane and allene are formed by both primary and secondary reactions having rates of formation which are significant initially and increase with time, other products are essentially secondary in character having negligible initial rates of formation. A complex mixture of higher-molecular-weight products is also formed, and a steady fall in pressure during reaction indicated the simultaneous occurrence of polymerisation processes.As the aim of this work did not necessitate a detailed analysis of all reaction products these high-molecular-weight products were not examined further. It was, however, noted from a carbon/hydrogen balance that as the reaction proceeds the high- molecular-weight products become progressively less saturated compared with the reactant but-1-yne. This suggests that association of but-1-yne molecules is followed by decomposition of the polymer by the splitting-off of alkyl radicals. Decomposition and hydrogen abstraction by these alkyl radicals will account for the formation of secondary methane, ethylene, ethane etc.A. B. TRENWITH AND S. P. WRIGLEY 2339 1.5 - 1 b Q a 1 .o- 5 1 4- 2 I 0 50 100 time/min FIG. 1 .-Products of the pyrolysis of 50 Torr but-1-yne at 719 K.a, propyne, propene and but-1-ene; A, allene; A, hydrogen; x , ethane; a, ethylene; 0, methane. time/min FIG. 2.-Plots of methane against time for the decomposition of but-1-yne at 686 K. x ,202 Torr in packed reaction vessel; 0, unpacked reaction vessel: (a) 870, (b) 580, ( c ) 387, (d) 290, (e) 242, (f) 202, (g) 169, (h) 140, (i) 118 Torr. Subsequently a series of runs was performed at eight temperatures over the range 652-731 K. At each temperature methane yields were obtained for a series of initial pressures of but-1-yne between 50 and 1200 Torr at various short reaction times so arranged that the decomposition of but-1-yne never amounted to more than 3%.76-22340 PYROLYSIS OF BUT-1-YNE A typical set of methane against time curves is shown in fig. 2. The initial slopes of these curves were obtained by computing best-fitting third-degree polynomial equations from the experimental data, the coefficients of the term of power unity then corresponded to the initial slopes. Experiments carried out using a packed reaction vessel with a surface/volume ratio six times greater than the unpacked vessel, showed that this change led to an increase in the rate of formation of secondary methane whilst the rate of formation of primary methane was unchanged. Double logarithmic plots of initial rate of methane formation against pressure were apparently linear and gave orders of methane formation which decreased steadily with increase in temperature as shown in table 1.From these orders of methane formation and their temperature dependence it is clear that reactions leading to the formation ofmethane from but-1-yne TABLE 1 .-ORDER OF REACTION LEADING TO METHANE FORMATION FROM BUT- 1-YNE AT VARIOUS TEMPERATURES reaction T/K order 652 2.00 660 2.00 674.5 1.92 686 1.84 697 1.80 708.5 1.70 719 1.69 73 1 1.58 must differ significantly from those occurring with but-1-ene and its derivatives, For the latter, orders of methane formation were 1.16 for but-1-ene itself, 1.05 for 3-methylbut-1-ene and 1.09 for 3,3-dimethylbut-l-ene, and these figures showed no clear variation with temperature being, within the experimental error limits, constant over the ranges studied. Since it is inconceivable that, under the experimental conditions employed in this work, the unimolecular dissociation of but- 1 -yne into methyl and propargyl radicals could be in its second-order region, it was concluded that at the lowest temperatures studied methane is formed predominantly as a result of a bimolecular reaction of but-1-yne.The decrease in order of methane formation with increase in temperature may be ascribed to the increasing participation of a higher-activation-energy uni- molecular process also leading to methane formation. A possible bimolecular reaction leading to the formation of methane is 2CH,CH,CrCH + CH,CH,&CH, + CH,eHCrCH (1) involving the transfer of a weakly bonded propargylic hydrogen atom to produce the methyl derivative of the propargyl radical and an unstable olefinic radical.Rapid decomposition of the latter by the reaction CH,CH,&CH, + *CH, + CH,=C=CH, (1’) is then the source of methyl radicals, and hence of methane, as well as the primary product allene. The most reasonable unimolecular process leading to the formation of methane is the dissociation reaction CH,CH,C=CH + .CH, + CHC-CH (2)A. B. TRENWITH AND S. P. WRIGLEY both (1’) and (2) being followed by the abstraction reaction 234 1 CH,CH2C=CH + CH, + CH, + CH,eHC-CH. (3) Justification for the proposed bimolecular step (1) arises from the cognate reactions which are believed to occur with propenel, and also in the reaction between ethylene and cyc10pentene.l~ Thus the rate of formation of methane, R(CH,), should be given by the equation and provided reaction (2) is in its first-order region, graphs of R(CH,)/[C4H6] against [C,H6] should be linear with slopes and intercepts corresponding to k , and k,, respectively.Examples of such graphs are shown in fig. 3. At the lowest temperature FIG. 3.-Plots of (d[CH,]/dt)/[C,H,] against [C,H,] for various temperatures. (a) 652, (b) 660, (c) 674.5, ( d ) 686, ( e ) 697, (f) 708.5, (g) 718, (h) 731 K. studied the experimental points lie close to a straight line passing through the origin. For each of the next four higher temperatures, data for pressures above 200 Torr again yield linear plots with small measurable intercepts with the ordinate, but for pressures below 200 Torr,the points deviate progressively from the straight line and the higher the temperature the more marked the deviation becomes.Since this deviation is a low-pressure phenomenon it may reasonably be ascribed to the fall-off behaviour of the rate coefficient for reaction (2). This is supported by the observation that at 731 K addition of a 10-fold excess of CO, leads to an increase in R(CH,) amounting to 22% for 50 Torr and 17% for 100 Torr but-l-yne. At the five lowest temperatures studied between 652 and 697 K, data in the pressure range where there is linear behaviour were sufficient to allow reasonable estimates of the slopes of the plots by the least-squares method. Values for k , obtained from these slopes had standard2342 PYROLYSIS OF BUT-I-YNE FIG. 4.-Arrhenius plots for the reactions CH,CH,CrCH -+ .CH,+ -CH,CrCH (0) and 2CH3CH,C-CH -+ CH,CHCrCH + CH,CH,&CH, (0).deviations of between 0.4 and 2.7% and yielded the equation logl,(kl/dm3 mol-1 s-l) = 1 1.3 k 0.2 - (47 800 f 600)/8 where 8 = 2.303 RT/cal mol-I. The Arrhenius plot is shown in fig. 4. Since at temperatures above 697 K data in the linear region of the graph were limited, and reliable estimates of the intercepts could not be obtained by extrapolation, the limiting high-pressure values foi. k , were obtained by the following procedure. For each of the five highest temperatures studied, values for k , were evaluated from the expression given above, and k,[C,H,] was then subtracted from R(CH,)/[C,H,], yielding values for k , over the pressure range studied. From these figures, using an extrapolation procedure described earlier,16 estimates of (k2)m were obtained for each of the five temperatures over the range 686-73 1 K.The Arrhenius plot for (k2)m shown in fig. 4 gives the equation iOg,,,(~,)~/s-~ = 17.240.7-(74800 k2100ye. To confirm that the deviations from linearity of the plots of R(CH,)/[C,H,] against [C,H6] which occur at low pressures are due to the unimolecular fall-off of k,, values obtained for k2/(k,)w at 73 1 K were compared with theoretical estimates derived using the Kassel equati0n.l’ In the latter, s was taken to be 13 as deduced from C,,,/R, and a collision diameter of 5.2 %i was used. This comparison is shown graphically in fig. 5 and indicates that, whilst there is some scatter in the experimental points, there is sufficient agreement between theoretical and experimental fall-off data to warrant the conclusion that values of k, should be pressure-dependent under the conditions of temperature and pressure involved in this investigation.This pressure dependence is, however, too small to produce an obvious curvature in the double logarithmic plots of rate of methane formation against pressure, although it was generally found that a least-squares analysis of the data for the lower end of the pressure range yielded a slightly higher order than that using the high-pressure values. According to the proposed mechanism allene is a product of reaction (1’) only,A. B. TRENWITH AND S. P. WRTGLEY n .y N --.. .$ -0.2 ' v 2343 - 0 . 4 t FIG. 5.-Plot of log,,(k,/k,,) against log,,([C,H,]/Torr). The curve is the Kassel curve derived as described in the text; x , experimental points for 731 K.whereas initially methane is obtained from the methyl radicals produced in reactions (1') and (2). If this is the case, the measured ratio of initial rates of formation of these products should correspond to that calculated from the expression The measured ratio was computed by fitting yields of allene and of methane for various reaction times from 50 Torr but-1-yne at 719 K to third-degree polynomial equations and this yielded (d[C,H,]/d[CH,]), = 0.14, in excellent agreement with the calculated ratio of 0.13. At the lowest temperature studied (652 K), where methane formation by the unimolecular dissociation of but-1-yne appears to be neglibible, initial rates of formation of allene and methane should be equal.At this temperature the rate of formation of allene from 100 Torr but-1-yne was found to fall off rapidly with time so that only a rough estimate of its initial rate of formation could be derived. This, however, gave a value of 0.93 for the ratio (d[C,H,]/d[CH,]),, indicating that at this temperature the initial rates of formation of these two products are indeed similar. DISCUSSION The main objective of this investigation was to determine the energy required for dissociation of but- 1 -yne into methyl and propargyl radicals, and consequently the information derived on the overall mechanism of decomposition of this molecule is rather limited. It is, however, clear that the reactions occurring differ significantly from those which take place in the pyrolysis of but-1-ene.Apart from the fact that methane appears to be formed as a result of both unimolecular and bimolecular primary processes, polymerisation reactions are more extensive with the alkyne, as are secondary reactions leading to the formation of low-molecular-weight products, which involve both homogeneous and heterogeneous steps. In the previous work on but-1-ene and its derivatives1v2y8 it was assumed that roughly 50% of the methyl radicals produced in the initial dissociation step reacted by addition to the parent olefin and that this process was not reversible. It has been suggested1* that this assumption is not realistic, and on further consideration this appears to be the case. Addition of methyl radicals to but- 1 -ene occurs predominantly at the terminal unsaturated carbon atom,19 and the secondary *C,H,, radical so formed can react either by hydrogen abstraction or by redissociation into methyl radical and but- 1 -ene.Rough calculations using estimated rate parameters for the2344 PYROLYSIS OF BUT-I-YNE dissociation of the free radical,20 together with data for hydrogen abstraction by radicals from but-1 -ene,21* 22 indicate that at the highest pressure studied (200 Torr) the adduct redissociates at a rate which is an order of magnitude greater than that of the abstraction reaction. Since at the experimental temperatures employed rates for addition and abstraction reactions between methyl radicals and but-1 -ene are similar,23 the above calculation shows that, at most, only 5% of the methyl radicals formed in the initial dissociation reaction are not converted into methane.This small difference is comparable to the reproducibility of the methane analyses and can therefore be neglected. The net result of the reassessment is that the A factors reported previously for the primary dissociation reactions should be reduced by 0.3 logarithmic units. This small modification leads in general to improved agreement between calculated and experi- mental A factors, since the latter tended to be slightly higher than those deduced from radical-recombination data and overall entropy changes. Methyl-radical addition to but-1-yne has also been disregarded in the present work and, in the absence of the necessary rate data, it has been assumed that the ratio of the rates of decomposition and abstraction reactions of the *C,H, radical is similar to that for the radical *C,Hll.Because of the simultaneous occurrence of the two reactions of but-I -yne, leading to the formation of methane, the temperature range over which data for each reaction could be obtained was limited. Despite the unavoidably high values for the estimated standard deviations, the Arrhenius parameters for the dissociation reaction merit consideration, being the first reported for this reaction by direct determination. The measured A factor of 1017-2 s-l may be compared with that obtained from calculated entropy changes for this reaction. At 700 K, ASP for reaction (2) was found to be 30.5 cal K-l mol-1 for a standard state of 1 mol dm-3 [entropy and specific-heat data for the propargyl radical were evaluated by the procedure described in ref.(24); for but-1-yne and methyl, data given in ref. (25) and (24), respectively, were used]. Assuming that propargyl radicals recombine at the same rate as do ally1 radicals,26 i.e. krecomb = lo9.’ dm3 mol-l s-l, and taking for methyl radicals27 krecomb = 1010*4 dm3 m0l-l S- l, the entropy of activation for the reaction (- 2) is given by AG = - 16.4 cal K-l mol-l, the activation energy for the radical-recombination reaction being taken to be zero. This leads to AS$ = 14.1 at 700 K for reaction (2) and A = 1016.7 s-l. Although slightly lower than the experimental value, this calculated result is sufficiently close to give reasonable confidence in the measured Arrhenius parameters.By contrast, the A factor of 1015.5 s-l assumed by Kinglo for reaction (2) in applying the R.R.K.M. theory to his v.1.p.p. data appears to be unreasonably low. With this value the rate constant for recombination of propargyl radicals becomes krecomb = dm3 mol-l s-l, a figure which, from available data, would appear to be at least two orders of magnitude too low. From the measured activation energy for reaction (2) we obtain for this step AHg8 = 75.4f 1.7 kcal mol-l, it again being assumed that the reverse reaction has a zero activation energy. Taking the value used previously1* of 34.1 kcal mol-1 for AH? of *CH3 with AHP = 39.7 kcal mol-l for b ~ t - l - y n e ~ ~ yields, for the propargyl radical, AHP = 81.0+2 kcal mol-l, all values being for 298 K. This figure is in good agreement with the most recently reported values9-12 and corresponds to a resonance energy of 9.2k2.2 kcal mol-l for the propargyl radical, assuming the primary C-H bond dissociation energy in propane2* to be 98 & 1 kcal mol-l.Although more recently reported figures29 for the primary C-H bond dissociation energy are slightly higher than the long accepted value used here, we have used the latter to avoid confusion when making comparisons with earlier work. The resonance energy for the propargyl radical so obtained confirms the lower value for this radical as compared with theA. B. TRENWITH AND S. P. WRIGLEY 2345 allyl radical, which has been ascribed to the differing energies of the two canonical forms of the former. An estimate of the A factor for reaction (1) may be obtained from the overall entropy change of the reaction and an assumed value for the rate constant of the radical disproportionation reaction (- 1). Use of data from the same sources as given above leads to A S 8 = 5.0 cal K-l mol-l for reaction (1).Assuming a recombination rate constant of dm3 mol-l s-1 for each of the radicals formed in reaction (1) we obtain, by the keometric mean rule, for the cross-combination reaction k , = lo9., dm3 mol-1 s-l; from the relationship between k,/k, and the entropies of combination and disproportionation k , for reaction (- 1) was found to be dm3 mol-l s-l. From this rate constant and the estimated overall entropy change, the calculated A factor for reaction (1) is lolo.' dm3 mol-1 s-l which, in view of the numerous approximations involved, is in satisfactory agreement with the measured value of The measured activation energy, in conjunction with specific-heat data for reactantsz5 and of reaction (I), yields for the enthalpy change of this reaction AH%, = 47.4k0.6 kcal mol-l.To obtain from this figure the heat of formation of the 3-methylpropargyl radical we need to know the heat of formation of the butenyl radical formed by the splitting of secondary C-H bond in an olefin, and this bond dissociation energy has not been reported. The C-H bond dissociation energy in ethylene is generally assumed28 to be 108 + 2 kcal mol-l and, if by analogy with primary and secondary C-H bond dissociation energies in saturated hydro- carbons we assume for the dissociation energy of the bond in question a value which is 3 kcal mol-l less than that of an ethylene C-H bond, we obtain AHfe = 52.9+2 kcal mol-l for the butenyl radical formed in reaction (1); hence for the 3- methylpropargyl radical AHfe = 73.7 & 2 kcal mol-l, which corresponds to a reson- ance energy of 8.8f2.2 kcal mol-l, assumingz8 D,,, (C-H)sec = 95+ 1 kcal mol-l.The similarity between the measured resonance energies of the propargyl and 3-methylpropargyl radicals is in line with the agreement already found1> between the resonance energies of the allyl radical and some of its derivatives and supports the conclusion that, in general, methyl substitution brings about no measurable change in the resonance stabilisation of free radicals. This may reasonably be linked to the orthogonality of the n and 0 bond systems involved, so that by changing the latter we cannot alter the energy of the former.In view of the evidence obtained in this work for the occurrence of a bimolecular hydrogen exchange reaction with but-1 -yne coupled with the similar processes believed to occur with other unsaturated molecules14* l5 it was considered worthwhile to re-examine the data obtained previously for but-1-ene and its derivatives.'* For but-1 -ene itself the overall order of methane formation was found to be 1.16 over the pressure range 1-200 Torr. Close agreement was observed between theoretical and experimental plots of log,, k / k m for the dissociation reaction against loglop, the former being obtained from the Kassel equation using the reported Arrhenius parameters for the reaction and taking s = 16 (Cvib/R = 16.5) and 0 = 5 A.On this evidence the departure from first-order behaviour of the reactions leading to the formation of methane results essentially from the fall-off with pressure of the rate coefficient for the unimolecular dissociation reaction, and if any methane is formed as a result of a bimolecular process this must be insignificant as compared with that from the accepted initial step. A similar conclusion was reached for 3 methylbut-1 -ene, where the order of methane formation was found to be 1.05, there being little variation in the rate coefficient for methane formation with pressure. With 3,3-dimethylbut- 1 -ene the absence of an allylic hydrogen precludes the occurrence of a bimolecular hydrogen dm3 mol-l s-l.2346 PYROLYSIS OF BUT-~-YNE exchange reaction, but with 2-methylbut- 1 -ene the presence of five allylic hydrogen atoms makes this a system where the exchange reaction could reasonably be expected to occur.The order of the reaction leading to the formation of methane from 2-methylbut-1-ene was found to be 1.4 and, since over the pressure range studied (10-500 Torr) the dissociation reaction of a molecule of this complexity must be very close to first order, some of the methane formed must result from a bimolecular exchange reaction followed by decomposition of the secondary radical so formed. Values for the rate constants of the unimolecular and bimolecular processes were obtained as described above from the intercepts and slopes of the linear portions of graphs of R(CH,)/[C,Hlo] against [C5Hlo].31 These yielded for the unimolecular dissociation of 2-methylbut- 1 -ene loglo(k/S-l), = 15.7 k 0.7 - 71 400 & 2000/8 and for the bimolecular reaction loglok/dm3 mol-1 s-l = 10.0 _+ 0.6 - 45000 k 1800/8.Whilst the activation energy for the unimolecular process is virtually the same as that reported previously, the A factor is 0.9 logarithmic units lower, but still in reasonable agreement with the calculated value [logl0(A/s-l) = 16.11. Assuming that the bimolecular process involves only the transfer of a secondary allylic hydrogen atom from one molecule to the terminal carbon atom of the other, the measured activation energy leads to a value of 10.0 kcal mol-l for, the resonance energy of the 2- methylbutenyl radical.This is roughly 2-3 kcal mob1 lower than would be expected, a variation which, in this instance, can be explained by the composite character of the bimolecular process. This must involve the transfer of both primary and secondary allylic hydrogen atoms. One significant difference between the data obtained from the decomposition of 2-methylbut-1-ene and those from but-1-yne is that with the former the plots of R(CH,)/[C5Hlo] against [C5Hlo] show, at the lower end of the temperature range studied (670-650 K), a progressive deviation from linearity at pressures above 200-300 Torr, indicating some loss of methane under these conditions. This may reasonably be ascribed to the occurrence at this comparatively low temperature of an hydrogen abstraction reaction by the sec-C5Hll radicals formed in the initial hydrogen-transfer reaction.At a sufficiently high pressure the rate of such a reaction must become comparable with that of the radical-dissociation reaction. The fact that such deviations were not observed with but-1-yne suggests that the alkenyl radical formed in reaction (1) is much less stable than a comparable alkyl radical, so that even at 660 K and a pressure of 1000 Torr the rate of decomposition of the alkenyl radical is much greater than that of the alternative hydrogen-abstraction reaction. One of the main points of interest arising from the work described herein is the apparent marked difference between the modes of decomposition of but-1-yne and but-1-ene under similar conditions, and in particular the fact that with but-1-ene the bimolecular hydrogen exchange process does not appear to occur at a measurable rate.The A factor for such a reaction may be estimated as already described, and this yields A = 10IO.O dm3 mol-1 s-l. Taking AH? = 30.6 kcal mol-1 for the 3-methylallyl radical,8 and assuming2s D(C-H),,, = 95 kcal mol-l, we obtain an activation energy of 44.1 kcal mol-l for the bimolecular step at 700 K. Since the Arrhenius parameters for the unimolecular dissociation of but-1-ene have been measured,l we can use these values to compare the rates of formation of methane by the two processes in the decomposition of but-1-ene. It emerges that at 700 K and a pressure of 150 Torr methane should be so formed from the bimolecular step at half the rate of that byA.B. TRENWITH AND S. P. WRIGLEY 2347 the unimolecular step. This is not significantly different from the observed behaviour of but-1 -yne, which under the same conditions yields methane from the two processes at similar rates. By contrast, using the recently proposed higher value31 for D(C-H),,, of 97.6 kcal mol-1 we obtain R(CH,),,i/R(CH,)bi = 12 for but-l-ene, other con- ditions being the same as before. This is more consistent with the observed behaviour and goes some way to explaining why, at the lower end of the temperature range studied, there was no evidence for the occurrence of the bimolecular step. This also provides support for the proposed higher secondary (C-H) bond dissociation energy. S.P. W. thanks the S.R.C.for the award of a Research Studentship. A. B. Trenwith, Trans. Faraday SOC., 1970, 66, 2805. A. B. Trenwith and S. P. Wrigley, J. Chem SOC., Faraday Trans. 1 , 1977, 73, 817. J. Collin and F. P. Lossing, J. Am. Chem. SOC., 1957, 79, 5848. R. Walsh, Trans. Faraday SOC., 1971, 62, 2085. H. M. Frey and A. Krantz, J. Chem. SOC. A, 1969, 1159. K. W. Egger and M. Jola, Int. J . Chem. Kinet., 1970, 2, 265. M. M. Martin and E. B. Sanders, J . Am. Chem. SOC., 1967, 89, 3777. A. B. Trenwith, J . Chem. SOC. Faraday Trans. 1, 1973, 69, 1737. W. Tsang, Int. J. Chem. Kinet., 1978, 10, 687. lo K. D. King, Int. J . Chem. Kinet., 1978, 10, 545. l1 K. D. King and Tam T. Nguyen, J . Phys. Chem., 1979, 83, 1940. l 2 D. K. Sen Sharma and J. L. Franklin, J . Am. Chem. SOC., 1973, 95, 6562. l3 K. D. King, Int. J. Chem. Kinet., 1977, 9, 907. l4 M. Simon and M. H. Back, Can. J . Chem., 1973, 51, 2934. l5 S . W. Benson, Int. J. Chem. Kinet., 1980, 12, 755. l6 A. B. Trenwith, J. Chem. SOC., Faraday Trans. 1, 1979, 75, 614. l7 P. J. Robinson and K. A. Holbrook, Unimolecular Reactions (Wiley, London, 1972). l a S. W. Benson, personal communication. lS J. A. Kerr and M. J. Parsonage, Evaluated Kinetic Data on Gas Phase Addition Reactions (Butterworths, 2o J. A. Kerr and A. C. Lloyd, Quart. Rev., 1968, 22, 549. 21 J. A. Kerr and M. J. Parsonage, Evaluated Kinetic Data on Gas Phase Hydrogen Transfer Reactions of Methyl Radicals (Butterworths, London, 1976). 22 P. Gray and A. A. Herod, Chem. Rev., 1971, 71, 247. 23 R. J. Cvetanovid and R. S . Irwin, J. Chem. Phys., 1967, 46, 1694. 24 H. E. O’Neal and S . W. Benson, Int. J. Chem. Kinet., 1969, 1, 221. 25 S. W. Benson, F. R. Cruickshank, D. M. Golden, G. R. Haugen, H. E. O’Neal, A. S. Rogers, R. Shaw 26 H. E. van den Berg and A. B. Callear, Trans. Faraday SOC., 1970, 66, 2681. 27 H. E. van den Berg, A. B. Cailear and R. J. Nordstrom, Chem. Phys. Lett., 1968,4, 101. 28 D. M. Golden and S . W. Benson, Chem. Rev., 1969, 69, 125. 20 R. R. Baldwin, R. W. Walker and R. W. Walker, J. Chem. SOC., Faraday Trans. 1, 1980, 76, 825. 30 R. A. Holroyd and G. W. Klein, J . Phys. Chem., 1963, 67, 2273. 31 S. P. Wngley, Ph.D. Thesis (Newcastle upon Tyne, 1978). 32 G. A. Evans and R. W. Walker, J . Chem. SOC., Faraday Trans. 1, 1979, 75, 1458. London, 1972). and R. Walsh, Chem. Rev., 1969, 69, 279. (PAPER 1 / 1068)

ISSN:0300-9599    

DOI:10.1039/F19827802337

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. I , 1982, 78, 2349-2359 Kinetics of the Decomposition of Hydrogen Peroxide in Alkaline Solutions BY OTOMAR SPALEK,* JAN BALEJ AND Ivo PASEKA Institute of Inorganic Chemistry, Czechoslovak Academy of Sciences, 160 00 Prague 6, Majakovskiho 24, Czechoslovakia Received 14th July, 1981 Hydrogen peroxide decomposition in 1-5 mol dm-3 KOH and 1-2 mol dm-3 NaOH solutions is a first-order reaction with respect to undissociated hydrogen peroxide. The decomposition is catalysed by compounds of heavy metals (Fe, Cu) present as trace impurities in these solutions and is first order with respect to them. The hydroxyl ion concentration exerts a significant effect on the decomposition rate, which has been explained by its influence on the activity of catalysing species having colloidal character, which are probably the active sites for the decomposition.The kinetics of hydrogen peroxide decomposition in dilute solutions of alkali hydroxides have been studied by many authors, especially with regard to using these solutions for bleaching cellulose and textile materials. However, the results of these studies are difficult to compare, as the spontaneous decomposition of peroxide is apparently a catalysed reaction whose rate is also affected by the nature and concentration of trace impurities present. It has long been known that bases accelerate the decomposition of peroxide and that the content of trace heavy-metal impurities has the principal influence on the reaction rate.l According to Biirki and Schaaf,2 peroxide decomposition in dilute alkali hydroxide solutions is a first-order reaction with respect to the peroxide, the decomposition rate increasing with increasing hydroxide concentration (from 5 x lo-* to 0.2 mol dm-3 NaOH).It was later found that peroxide decomposition is fastest at a ratio of peroxide concentration to that of hydroxide of t ~ 0 ~ - ~ or three.6 From the former value, at which about one half of the peroxide is present as perhydroxyl ions, it was derived by AbeP that peroxide decomposition proceeds through the reaction of perhydroxyl ions with undissociated (A) peroxide molecules : HOT + H202 -+ H20 + 0, + OH-. The decomposition of peroxide in pure hydroxide solutions was considered to be an uncatalysed reaction, resulting from the oxidising properties of the peroxide molecules and the reducing properties of the perhydroxyl iom3 Similar dependences of the decomposition rate on the total alkalinity were found in pure solutions of potassium and sodium hydroxide in the presence and in the absence of colloidal catalysts (Pt, Au, Pd).3 In agreement with the results of Abe13, Duke and Haas' found that the experimental dependence of the peroxide decomposition rate on the total alkalinity at a constant total peroxide content is analogous to the dependence of the product c(H202) c(H0;) on the total alkalinity. They concluded that peroxide decomposition obeys the equation where a is the total peroxide content in the solution and c(H20,) and c(H0,) are the concentrations of undissociated peroxide and perhydroxyl ions, respectively.Peroxide da/dt = kc(H202) c(H0,) (1) 23492350 ALKALINE DECOMPOSITION OF H202 decomposition in these solutions was also considered as an uncatalysed reaction, as further purification of the solution (by complexation or coprecipitation of heavy metals) had no effect on the decomposition rate.' According to Erdey and InczCdy5 the formal reaction order with respect to total peroxide depends on the pH, attaining a maximum value of ca. 2 at pH 12, when approximately one half of the peroxide is dissociated. In contrast to Abel's hypothesis of a redox mechanism of peroxide decomposition, Erdey and Inczedy5 state that, in the reactions of peroxide with oxidants containing oxygen, the peroxide bond in the peroxide molecule is never dissociated, as proved by isotopic measurements. These authors proposed a mechanism involving interaction between H202 and HO; according to the scheme H20, + 0 2 H - - H \ / H 0 0 - II 0 2 + H20 + OH-. 0 H,O-l The conclusion that the 0-0 bond splits in the perhydroxyl ion is supported by the Raman spectra of these sol~tions.~ In other papers8-10 the decomposition of hydrogen peroxide in alkaline solution is suggested to proceed via two mechanisms, the radical-chain mechanism initiated by some catalytic species (e.g.Cu2+, Fe3+, Mn3+), and the non-radical one, via some unspecified intermediates. E.p.r. measurements have shown that in the system H202-NaOH-H20 the radical-chain mechanism takes place.'* In the system H20,- Na,SiO,-H,O no e.p.r. signal is observed, which has been explained by an interruption of the reaction chain and the capture of radicals by silicate ions.lo For the preparation of hydrogen peroxide by the reduction of oxygen,llg l2 rather concentrated potassium hydroxide solutions (1 -5 mol dm-3) or more dilute solutions of sodium hydroxide (1-2 mol dm-3) are used, to attain sufficient conductance.The degree of conversion of hydroxyl to perhydroxyl ions attains a value of 0.6-0.7 during the electrolysis, so that the leaving electrolyte contains up to several moles of peroxide per dm3. The current yield of peroxide is considerably reduced by spontaneous decomposition of peroxide. With the aim of finding conditions where the loss of product is minimal, we undertook a study of the kinetics of hydrogen peroxide decomposition in concentrated alkaline solutions.EXPERIMENTAL APPARATUS The peroxide decomposition was followed in a cylindrical thermostatted vessel made of Pyrex glass and closed by a Bunsen vent to permit the oxygen liberated to escape and protect the vessel contents against contamination. The solution temperature in the vessel was usually maintained at 20+_O.l0C and in a few measurements at 30f0.1 "C. In the measurements with high initial concentration of peroxide, the temperature sometimes increased by 1-3 O C as a result of the rapid exothermic reaction, but stabilised within the above range after a maximum of 15 min. The solution was stirred with a magnetic stirrer at 300 r.p.m., using a glass tube with a sealed iron core. To determine the specific effect of glass, the decomposition was carried out in a glass vessel in the presence of 10 g of ground glass (of the same kind as the vessel) with a surface area of 500 cm2.In a further experiment, the decomposition was carried out in a polyethylene vessel and the solution was stirred with a magnetic stirrer in a polyethylene tube.0. SPALEK, J. BALEJ AND I. PASEKA 2351 PURITY OF THE CHEMICALS EMPLOYED The hydroxide solutions were mostly prepared from chemicals of analytical grade (a.g.) purity. Measurements to determine the effect of the total alkalinity of the solution on the rate of decomposition of peroxide at constant impurity concentration were carried out using solutions of KOH purified by zone melting (z.m.).13 The heavy-metal contents in the concentrated alkali hydroxides were determined by atomic absorption and were recalculated to give the concentration in 1 mol dm-3 hydroxide (see table 1).TABLE 1.-cONCENTRATION OF HEAVY METALS IN 1 Ill01 dm-3 HYDROXIDE SOLUTIONS (mg dm-3) metal KOH (a.g.) NaOH (a.g.) KOH (z.m.) Fe 0.25 0.38 0.02 Ni 0.075 0.055 0.0008 c u 0.03 0.022 0.0008 The effect of the ionic strength on the decomposition rate was found using potassium chloride (Suprapur, Merck) with a declared content of < lop6% iron, copper and cobalt, corresponding to 7.5 x mg dm-3 metal in 1 mol dm-3 KC1. Hydrogen peroxide (analytical grade, Sokolov Chemical Works) was added to the solution as a ca. 85% unstabilised solution. The iron content determined by spectral colorimetry in this chemical recalculated to 1 rnol dm-3 H,O, was 0.02 mg dmp3.Redistilled water was used for preparation of the solutions. PROCEDURE The decomposition was followed by chemical analysis of samples taken at various time intervals. The total peroxide content was determined manganometrically and the total alkalinity was found by titration with 0.05 mol dmP3 sulphuric acid using methyl red indicator. RESULTS The decomposition of peroxide was measured in solutions containing 1-6.6 mol dm-3 KOH + 1-7 mol dme3 H,O, and several measurements were carried out in solutions of 1-3 mol dmp3 NaOH + 1 mol dm-3 H,O,. In contradiction of the findings of Biirki and Schaaf,, we found that the decomposition of peroxide in these solutions does not obey the integrated first-order rate equation with respect to the total peroxide content.In a further evaluation of the kinetics it was considered that hydrogen peroxide ionises in an alkaline medium to produce perhydroxyl ions H,O, + OH- e HO, + H,O (C) and that the decomposition rate can depend on the concentrations of the dissociated and undissociated forms of hydrogen peroxide. At the given temperature, the ratio of the two forms is primarily dependent on the molar ratio P of the total peroxide content a to the total alkalinity of the solution b (P = a/b). In solutions with P < 1 the concentration of molecular peroxide is more than an order of magnitude lower than the concentration of perhydroxyl ions.14 Only in solutions with P > 1 does the concentration of undissociated peroxide attain values comparable with the concen- tration of perhydroxyl ions.2352 ALKALINE DECOMPOSITION OF H,O, Previously derived relationship~l~ were used to calculate the instantaneous concen- trations of molecular hydrogen peroxide, perhydroxyl ions and hydroxyl ions for each experimental point.The instantaneous decomposition rates were found from the experimental dependences of the total peroxide content on time. On the basis of these data, dependences of the decomposition rate on the concentrations of the individual components of the solution and on the concentration product c(H20,) c(H0;) were sought. Treatment of the measured data indicated that the rate of the decomposition cannot be correlated to the concentration of perhydroxyl ions. For example, in a 3.1 rnol dm-3 KOH solution, with a decrease in total peroxide concentration from 4.8 to 1 rnol dm-3 the decomposition rate decreased roughly 200-fold, while the concentration of perhydroxyl ions decreased only three-fold. In certain concentration regions the decomposition rate was directly proportional to the product of the concentrations of perhydroxyl ions and molecular hydrogen peroxide [cf.ref. (3), (5) and (7)]. Further tests showed, however, that the validity of eqn (1) is limited to cases where the concentration of perhydroxyl ions does not change to an appreciable extent. On the other hand, if was found that the decomposition kinetics can be described over a much wider concentration range by an equation which gives the decomposition rate as a linear function of the concentration of the undissociated form of the hydrogen (2) peroxide - da/dt = k, c(H2O2) + k,.n 0 2 4 6 8 10 0.08 I 1 I I 1 1 I I I I I I 1 1 0 3 ~ ( ~ , 0 , ) 0 1 2 3 4 5 FIG. 1.-Dependence of the rate of decomposition of peroxide (mol dm-3 s-l) on the concentration of molecular hydrogen peroxide (mol dm-3, curve I), and on the product ll = c(H,O,) c(H0;) (rnol+ dm-6, curve 2), in 5.11 mol dm-3 KOH. The concentration range over which this relationship is valid extends from the lowest peroxide concentrations up to several mol dm-3. For comparison, fig. 1 depicts the dependences of the peroxide decomposition rates on the concentration of molecular peroxide (curve 1) and on the product c(H202)c(HO;) (curve 2) measured in 5.1 1 rnol dm-3 KOH at total peroxide concentrations of 2.7-0.2 rnol dm-3. The validity of eqn (2) was confirmed by the results measured for the decomposition in0.SPALEK, J. BALEJ AND I. PASEKA 2353 several dozen solutions containing potassium and sodium hydroxides in concentrations of 1-5.24 mol dm-3. The limiting total peroxide concentration for which eqn (2) is valid is higher, the higher the total alkalinity of the solution. Failure to obey eqn (2) at the highest peroxide concentrations (above 1-3.5 mol dm-3 H202) can be attributed to changes in the activity coefficients of H202 and HOT, giving rise to errors in the calculated solution compositions. 2 .o, I I I I I 1 . 5 r 1.0 0.5 0 1 2 3 4 5 6 b FIG. 2.-Dependence of the rate of peroxide decomposition (mol dmP3 s-l, curve I), concentration of molecular hydrogen peroxide (mol dm-3, curve 2), concentration of perhydroxyl ions (curve 3) and of product l7 = c(H,O,)c(HO;) (moP dm-6, curve 4) on the total alkalinity, b, of the solution (mol dm-3) at a total peroxide content a = 4 mol dm-3.Fig. 2 shows the dependences of the decomposition rate, the concentrations of perhydroxyl ions and molecular hydrogen peroxide, and the product c(H202) c(H0;) on the total alkalinity of the solution at a constant total peroxide content. While the value of the product c(H202)c(H0,) attains a maximum at a total alkalinity of roughly half the total peroxide concentration, the decomposition rate and the concentration of molecular hydrogen peroxide decrease with increasing total alkalinity. In contrast, the concentration of perhydroxyl ions increases with the total alkalinity.These dependences also indicate that the rate of peroxide decomposition in the solutions studied is roughly proportional to the concentration of its undissociated form. From measurements carried out under various conditions in a number of solutions with various initial compositions, rate constants k , and k, were evaluated by the least-squares method. The effects of reaction conditions on the magnitudes of the rate constants are discussed below. EFFECT OF STIRRING The decomposition was carried out in solutions with an initial composition of 4.6 mol dm-3 KOH+ 1 mol dm-3 H202, both with and without stirring with a magnetic stirrer at 100,200, 300 and 500 r.p.m. It was found that stirring has no effect on the rate-constant values.2354 ALKALINE DECOMPOSITION OF H202 EFFECT OF THE REACTION VESSEL WALLS The rate constants k, and k, found in 1 mol dm-3 KOH using a polyethylene vessel and stirrer were 14% and ca.60% lower, respectively, than the equivalent values obtained for the decomposition in a glass vessel with a glass stirrer. In experiments in which ground glass with a surface area of 500 cm2 was added to a glass reaction vessel (with a wetted wall surface area of ca. 100 cm2), k , and k, were 2.4 and 5 times larger, respectively, than the values found for a glass vessel without ground glass. kl 10 5 0 b FIG. 3.-Dependence of the rate constant k, (s-l) on the total alkalinity b (mol dm-3) in solutions of KOH (a.g.) (0) and NaOH (a.g.) (a). The curve includes points calculated from eqn (3). EFFECT OF ALKALI HYDROXIDE CATIONS No significant difference was found between the rate constant values found for measurements in sodium and potassium hydroxide solutions of the same concentration (see fig.3), in agreement with the findings of Duke and Haas.' This result is interesting considering the different contents of catalysing metals in the hydroxides used (see table 1). EFFECTS OF TEMPERATURE On increasing the decomposition temperature from 20 to 30 O C k , increased on average by 2.2 times and k, by roughly three times at the same potassium hydroxide concentration. EFFECT OF THE TOTAL SOLUTION ALKALINITY It was found from evaluation of measurements carried out on a number of KOH and NaOH solutions that both rate constants increase approximately with the square of the total alkalinity of the solution (see fig.3 and 4). It was found by a least-squares method that the rate constants obey the formal relationships k, = 0.43 b2 (3) k , = 2.3 x b2. (4)0. SPALEK, J . BALEJ AND I. PASEKA 2355 These relationships do not imply that the rate constants change only as a result of changes in the concentration of hydroxyl ions, as the concentrations of trace catalysing metals contained in the hydroxides also change. To differentiate the effect of total alkalinity from the effect of impurities, further measurements were carried out in solutions with various total alkalinities containing approximately the same amount of catalysing metals. 6 0 4 0 m 4 2 0 1 2 3 4 5 h FIG. 4.-Dependence of the rate constant k, (mol dm-3 s-l) on the total alkalinity h (mol dm-3).Points as in fig. 3. The curve includes points calculated from eqn (4). EFFECT OF THE TOTAL ALKALINITY AT CONSTANT CONCENTRATIONS OF HE A VY-MET AL CATALYSTS This was studied in mixed solutions containing 1 mol dm-3 KOH (a.g.) and various concentrations of KOH (z.m.), containing 1-2 orders of magnitude less heavy metals than the chemicals of a.g. purity. In 5 mol dm-3 KOH (z.m.) alone the rate constants were roughly one order lower than in solutions of the same concentration prepared from KOH (a.g.), in agreement with the ratio of iron concentrations, the predominant catalysing metal in both solutions. It follows from the rate-constant values obtained that at a constant concentration of catalysing metals rate constants k , and k , are a linear function of the total alkalinity (see fig.5 and 6). To differentiate the specific effect of hydroxyl ions from the effect of the ionic strength of the solutions, the decomposition of peroxide was also measured in mixed solutions 1 mol dm-3 KOH (a.g.) + x mol dmp3 KCl, where the chloride used contained ca. 100 times less iron than KOH (a.g.) (see Experimental). The effect of the ionic strength of the solution on k , in this series was far less marked than in solutions containing KOH (a.g.) and KOH (z.m.) (see fig. 5). In this series k , was equal to zero. The effect of the iron content in alkaline solutions on the rate of the decompo- sition of peroxide was studied in a further series of measurements in which various amounts of ferric chloride were added to 2.2 mol dmP3 KOH (a.g.) solution.The dependences of both rate constants on the overall concentration of iron (i.e. iron present as an impurity in the hydroxide and peroxide and that added as the chloride) are given in fig. 7.2356 ALKALINE DECOMPOSITION OF H202 6 4 k1 2 2 0 * 1 0 0 / / / / / //. / / /. P / ./ 0 1 2 3 4 5 6 0 1 2 3 4 5 b b FIG. 5 FIG. 6 FIG. 5.-Dependence of the rate constant k, (s-l) on the total alkalinity b (moldm-3) in solutions of 1 mol dm-3 KOH (a.g.)+x rnol dm-3 KOH (z.m.) (0) and in solutions of 1 rnol dm-3 KOH (a.g.)+x mol dm-3 KCI (Suprapur, Merck) (0). FIG. 6.-Dependence of the rate constant k, (mol dm-3 s-l) on the total alkalinity b (mol dmV3). Points as in fig. 5. 8 6 2 3 1 0 0 0.5 1.0 1.5 CF e FIG. 7.-Dependence of the rate constant k, (s-l) (0, curve 1) and k, (mol dm-3 s-l) (0, curve 2) on the total content of Fe (mg dm-3) in 2.2 mol dm-3 KOH (a.g.).The results of both series indicate that the rate constant k, is directly proportional to the content of heavy-metal catalysts, cM, and to the total alkalinity of the solution kI = kcMb. ( 5 ) k, is a linear function both of the concentration of catalysing metal and of the total alkalinity .0. SPALEK, J. BALEJ AND I. PASEKA 2357 In a further series of measurements the decomposition of peroxide was studied in more dilute solutions of potassium hydroxide (0.026-0.73 mol dm-3). The decom- position rate in these solutions, however, does not obey eqn (1) or (2). Fig. 8 shows the dependences of the measured decomposition rate, the decomposition rate calculated from eqn (2)-(4) and the dependence of the product c(H,O,)c(HO;) on the total alkalinity.The decomposition rate rapidly increases as the solution alkalinity falls below 1.4 mol dm-3. 0 -1 r 0 - M i - 0 - 2 - - 2 - 3 0 1 2 3 4 5 b FIG. 8.-Dependence of the measured rate of peroxide decomposition (mol dm-3 s-l) (0, curve l), rate of the decomposition calculated from eqn (2)-(4) (curve 2), and of product ll = c(H,O,)c(HO;) (moI2 dm-s, curve 3) on the total alkalinity of solution, b (mol dm-3) at a total peroxide content a = 1 rnol dm-3. Curve 2 of fig. 8, calculated from empirical equations, is in qualitative agreement with the measured dependence; however, the decomposition rates are lower and the maximum is shifted to higher hydroxide concentrations.The qualitative agreement between the dependences of the decomposition rate at constant peroxide content (curve 1) and of the product c(H,O,)c(HO;) (curve 3) on the total alkalinity found by Abe13 and by Erdey and Inczedy5 is maintained at low total alkalinities (0- 1 mol m-3) but breaks down in more alkaline solutions. DISCUSSION Our experimental data obtained at total alkalinity b > 1 can best be expressed by the rate equation (2). It follows therefore that hydrogen peroxide in alkaline solutions decomposes through two simultaneous pathways. The rate of the first pathway is directly proportional to the concentration of undissociated hydrogen peroxide molecules; the rate of the second does not depend on the concentrations of the individual reaction components (H,O,, HO;, OH-) during the decomposition.Note that in the alkaline solutions studied it is the hydrogen peroxide in an undissociated form that undergoes decomposition, in spite of its concentration being 1-3 orders of magnitude lower than that of the perhydroxyl ions.2358 A L K A L I N E DECOMPOSITION OF H,O, The direct proportionality between k , and the total alkalinity of the solution (at a constant concentration of catalysing ions) cannot be explained by participation of hydroxyl ions in the reaction because here, as a result of dissociation equilibrium (C), the decomposition rate would have to be directly proportional to the concentration of perhydroxyl ions, which is not in agreement with the dependence of the decom- position rate on the perhydroxyl ion concentration measured in more than fifty solutions.A different explanation of this dependence, based on the concept that the rate-constant values are affected primarily by the ionic strength of the solution,15 was refuted by the results of measurements of the decomposition in solutions containing potassium hydroxide and chloride, which indicated that the effect of chloride ions on k , is several times less than that of hydroxyl ions. It is apparent from the dependence of the rate constants on the total alkalinity in the mixed solutions studied that the decomposition of peroxide is catalysed by metals present as impurities (Fe, Cu, Mn). Simultaneously, the decomposition by the first reaction path is first-order with respect to the concentrations of these metals.Because these metals are present in alkaline solutions primarily as finely dispersed colloidal species of hydroxides or oxyhydroxides, the decomposition of peroxide by the first reaction path apparently proceeds through interaction of hydrogen peroxide molecules with the surface of these particles. In agreement with this concept, the specific effects of hydroxyl ions can be explained by the effect of pH on the activity of the catalysing species. It follows from the results of the decomposition carried out in the presence of ground glass and in a polyethylene vessel that in the usual experimental arrangement (glass vessel and stirrer) decomposition according to the first pathway proceeds to one fifth on the surface of glass. The second decomposition path is less clear; its rate is independent of the concentrations of molecular hydrogen peroxide, perhydroxyl ions and hydroxyl ions.In contrast, its rate increases both with increasing concentration of catalysing metals and with increasing total alkalinity of the solution, i.e. with the sum of concentrations of OH- and HOT ions. Peroxide decomposition by this reaction path in the given experimental arrangement proceeds on the glass surface. These conclusions on the mechanism of the decomposition of hydrogen peroxide in alkaline solutions are quite different from earlier according to which the decomposition of peroxide proceeds through interaction of its undissociated molecules with perhydroxyl ions. The opinion that hydrogen peroxide decomposes according to eqn (1) was derived3* from the analogous behaviour of the dependences of the decomposition rate and of the product c(H,O,)c(HO;) on the total alkalinity. This is valid, however, only in lower concentration regions (see fig. 8). We thank Mrs Bohdanecka and Dr Bludska for their assistance in the decomposition measurements. G. Tammann, Z . Phys. Chem., 1889, 4, 441. F. Biirki and F. Schaaf, Helv. Chim. Acta, 1921, 4, 418. E. Abel, Monatsh. Chem., 1952, 83, 422. L. Erdey, Acta Chim. Acad. Sci. Hung., 1953, 3, 95. L. Erdey and I. Inczedy, Acta Chim. Acad. Sci. Hung., 1955, 7 , 93. Ch. Dorfelt, Monatsh. Textil Znd., 1935, 50, 37, 67, 97, 117. F. R. Duke and T. W. Haas, J . Phys. Chem., 1961, 65, 304. I. Kh. Raskina, F. I. Sadov and G. A. Bogdanov, Zh. Prikl. Khim., 1966, 39, 35, 327. M. Ya. Kanter, I. Kh. Raskina, G. A. Bogdanov and F. I. Sadov, Zh. Prikl. Khim., 1970, 43, 447, 1449.0. SPALEK, J. BALEJ A N D I. PASEKA 2359 lo M. Ya. Kanter, I. Kh. Raskina, G. A, Bogdanov and Yu. N. Kozlov, Zh. Prikl. Khim., 1977, 50, l 1 J. Balej, K. Balogh and 0. Spalek, Chem. Zuesti, 1976, 30, 384, 61 1 . l2 0. Spalek, J. Balej and K. Balogh, Collect. Czech. Chem. Commun., 1977, 42, 952. l 3 J. Vepiek-SiSka, K. Eckschlager and V. Ettel, Chem. Prum., 1970, 20, 312. l4 J. Balej, 0. Spalek, Collect. Czech. Chem. Commun., 1979, 44, 488. l5 C . H. Rochester, in Prog. React. Kinet. (Pergamon Press, Oxford, 1971), vol. 6. 724. (PAPER 1/1114)

ISSN:0300-9599    

DOI:10.1039/F19827802349

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1982, 78, 2361-2368 Adsorption from Solutions onto Solid Surfaces Effects of Topography of Heterogeneous Surfaces on Adsorption Isotherms and Heats of Adsorption BY WLADYSLAW RUDZINSKI* AND JOLANTA NARKIEWICZ-MICHALEK Department of Theoretical Chemistry, Institute of Chemistry, Maria Curie-Sklodowska University, 20-03 1 Lublin, Nowotki 12, Poland AND STANISLAW PARTYKA Centre de Recherches de Microcalorimetrie et de Thermochimie du C.N.R.S., 26 rue de 141e, R.I.A., F-13003 Marseille, France Received 7th August, 1981 A model investigation is presented, showing the effects of the topography of heterogeneous solid surfaces on adsorption from binary liquid mixtures onto solid surfaces. Equations are developed for excess adsorption isotherms and heats of immersion, for both patchwise and random surface topography, when the adsorbed solution is a quadratic mixture.Using the developed equations, numerical model calculations are carried out for some typical values of heterogeneity and interaction parameters. It has long been thought that the heterogeneity of a solid surface may play a dominant role in adsorption from liquid mixtures onto real surfaces.’ More than two dozen papers have been published in the last ten years providing either experimental evidence for this fact or a theoretical and numerical analysis of appropriate experimental data. We have referred to them in detail in a previous publication.2 We have shown that the contribution from the surface heterogeneity to the excess adsorption and the excess heat of immersion is comparable with the contribution due to the interactions between the adsorbed molecules.Only a few published papers correctly take into account both these physical factor^.^-^ The mutual interference of these factors raises new questions about the role of surface topography in adsorption from binary liquid mixtures onto heterogeneous surfaces of real solids. To our knowledge, no one has yet investigated this problem either experimentally or theoretically. It is the purpose of this paper to provide a model investigation showing the effect of surface topography on the adsorption isotherms and heats of immersion. In order to show this effect clearly we have confined ourselves to the simple model of adsorption of molecules of equal sizes forming a quadratic mixture in both the surface and the equilibrium bulk phase.THEORETICAL The starting point of our consideration will be the general equation describing adsorption from mixtures of molecules of equal sizes’ 236 12362 ADSORPTION FROM SOLUTION ONTO SOLID SURFACES where x denotes the mole fraction of components A and B in both the surface (s) and the equilibrium bulk phase (b) and y denotes appropriate activity coefficients. For the case of the regular bulk and surface solutions assumed here, the activity coefficients take the following form: = exp (2 [xg)]~) (2) where ap and a, are the mole fractions of the nearest-neighbour adsorption sites in directions parallel (p) and vertical (v) to the surface plane which fulfil the condition (ap + 2a,) = 1.The meaning of the parameter E, is known from the theory of regular bulk sol~tions.~ Further, it will be assumed in this paper that the structure of the solution adjacent to the surface is not perturbed by the presence of the solid. For the purpose of our illustrative numerical calculation, we will further assume that ap = f and a, = a. The constant K can be expressed in the following way? K = exp (s) (4) where Here and E~ are the energies of adsorption of molecules A and B from their gaseous state onto the solid surface, qAS and qss are the molecular partition furlctions of molecules A and B in the adsorbed state, and cAA and EBB are appropriate energies of the A-A and B-B pair interactions. In the case of adsorption from binary liquid mixtures onto heterogeneous solid surfaces, various adsorption sites will exhibit various adsorption energies, E,.It is usually assumed in theories of adsorption of single gases on heterogeneous solid surfaces that the adsorption energy eA or E~ may vary from some minimum (positive) value up to infinity.' If we accept these assumptions, then we have to assume that the adsorption energy E, may vary from - co to + co. Following the results at our previous publication, we assume that the differential distribution of adsorption sites among various values of the adsorption energy, E,, is described by the following quasi-gaussian function : where Y describes the width of the distribution function and E: is the most probable value of E, on the heterogeneous surface. When I + 0, the distribution function [eqn (S)] tends to the Dirac delta distribution B ( E , - E ~ ) , which means that the surface becomes homogeneous and is characterized by the adsorption energy E, = E:. Because of the dispersion of the adsorption energy, &,,the surface mole fraction of component A will be represented by the following average:w. R U D Z I N S K I , J.NARKIEWICZ-MICHALEK AND s. PARTYKA 2363 The form of the energy distribution function in eqn (8) makes an analytical integration in eqn (9) possible:s where xgb is the value of xg) ate, = E:, and Bm is Bernouli's number. We have shown2 that for typical adsorption systems a sufficient accuracy of integration is achieved when only the first term of the sum in eqn (10) is retained. Thus, truncating expansion (10) after the first term we arrive at the following equation for x f i : where D&') is the second derivative of x f ) taken at the point eS = ez.Now let us consider the effect of the topography of a heterogeneous surface. As in the theories of adsorption of single gases on heterogeneous solid surfaces, we shall consider two extreme models of the surface topography. The first is the 'patchwise' model, which assumes that adsorption sites having the same adsorption energy are grouped into patches on the heterogeneous surface. These patches are sufficiently large that statistical thermodynamics can be applied to any of them, but interactions between admolecules on different patches are neglected in calculating the state of the adsorption system. Thus the adsorption system can be considered as a collection of independent subsystems, being in only thermal and material contact.The potential of the average force acting on a molecule adsorbed on a certain patch will depend only on the surface concentration of this patch. Consequently, the surface activity coefficient, yfb, referred to this patch will be given by the expression [ I - x2)(Es)]2 + a, 2 (xg))~) (12) where the subscript p in ygk refers to the patchwise topographical model. The other extreme model of surface topography considered so far is the 'random' model of heterogeneous surfaces. It is assumed in this model that no spatial correlations exist between adsorption sites having the same adsorption energy. In other words adsorption sites having various adsorption energies are distributed on the heterogeneous surface completely at random.Thus any local concentration on such a heterogeneous surface will be the same as the average surface concentration. Consequently the surface activity coefficient 72; should be written in the following form : yk; = exp ( a 5 (xf3~)2 + a, 5 (xg)>2 (13) kT kT where the subscript r in yg! refers to the random surface topography. For this case, the first and second derivatives of xg) take the following explicit form:2364 ADSORPTION FROM SOLUTION ONTO SOLID SURFACES For the patchwise surface topography, we obtain Now let us consider the heat of wetting of a heterogeneous surface by a solution of varying composition. As usual we start by writing an appropriate expression for the heat of wetting of a hypothetical homogeneous surface, Qw, G?, = 4) Q ~ A +xP Q ~ B +P (18) where QwA and QwB are the heats of wetting of the homogeneous surface by the pure liquids A and B.With certain approximations they can be written in the following (19) form2 Q ~ B = M(EB-~~EBB) (20) where M is the total number of the adsorption sites on the homogeneous surface. Further /? is an interaction term, which for the Bragg-Williams approximation takes the following form (21) Q ~ A = M(%-~v&AA) /? = Mcin [a, xg) xfjs) + a, (xg)xf3b) + xfjs) xff) - xff) xf3b))l where &in = E m - (3). The interaction term, /?, describes the change in energy of the pair interactions A-A, B-B and A-B owing to the formation of the solid/solution interface, relative to the situation in an infinite bulk solution.The surface heterogeneity will affect this interaction term, but in different ways, depending on the kind of surface topography present. In the case of surfaces characterized by random surface topography, the interaction term p will have the same form as in eqn (21), except that appropriate local surface concentrations have now to be replaced by their averaged values (23) Pr = P o +Ex (24) /?r = M&in [a, xg! xgi + a, (xfl xf3b) + xgi xff) - xp) @))I. We will now write Pr in the following form: where Po is the value of /3 at E, = E:. Thus, Ex is the contribution to the interaction term owing to the dispersion of the adsorption energy E,. Or, in other words, this is an excess of caused by the surface heterogeneity. In the case of patchwise surface topography, the interaction term Pp will be represented by the following average : Using the expansion (10) and neglecting terms of order higher than Co(r/kT)2, we obtain Pp = Mcin [ a, (xgb x& + (W [ Dg) (xfjsb - x f i ) - 2( DP))’]) + a, (xpi xf3b) + xgi xp) - xp) xg))] .(26)w. RUDZINSKI, J. NARKIEWICZ-MICHALEK AND s. PARTYKA 2365 As in the case of random surface topography, we can write Pp in the following form: Pp = Po+8”,”. (27) Now, let us consider the contribution to the heat of wetting, Q,, due to solid-solution interactions. This contribution is represented by the first two terms on the right-hand side of eqn (18). We can rearrange them in the following form:2 ~ 2 ’ QwA + xg’ QwB = Q,, + M x ~ ) E, - M x ~ ) t?, (28) where A = kT1n (qAs/qBS).(29) Thus we can see that the total heat of immersion of a heterogeneous surface, Q,,, can be represented by the following expression: +a3 Q,, = QwB+Pt+MJ -a3 X & ~ ~ ~ E ~ - M X ~ ! A (30) where Pt is P,. or Pp depending on the topography of the heterogeneous surface. Using the expansion (lo), and neglecting the terms of order higher than O ( T - / ~ T ) ~ , we arrive at the following expression for Qwt: Finally, we rewrite eqn (31) in the following condensed form: where Q,, is the value of Q, at E , = E: Qwo = Qw~+Po+Mxfb($-A) and Q$$, is the excess in Qwt due to the heterogeneity of surface (33) NUMERICAL RESULTS AND DISCUSSION Simultaneous measurements of the excess adsorption isotherms and heats of immersion have rarely been reported in the literature. Thus in order to illustrate our theoretical consideration from the previous section we have performed appropriate model calculations.Their results are shown in fig. 1-4. Fig. 1 shows the excess isotherms, evaluated using various interaction and heterogeneity parameters, for both patchwise and random surface topographies. The values of the heterogeneity parameter, r, assumed here, are comparable to, or even smaller than, the value found by us for the system benzene-cyclohexane on silica geL2 Also the values of the interaction parameter, E,/kT, accepted here represent only moderate deviations from the model of an ideal solution. The general conclusion which can be drawn from fig. 1 is as follows. The increasing degree of surface heterogeneity causes the maximum in the excess isotherm of the preferentially adsorbed solvent to increase, and to be shifted to smaller solvent concentrations.In the case of negative deviations from Raoult’s law, this effect is stronger for random surface topography. On the other hand, this effect is stronger for patchwise surface topography in the case of positive deviations from Raoult’s law. In this case the surface heterogeneity causes azeotropes to appear.2366 ADSORPTION FROM SOLUTION ONTO SOLID SURFACES bulk mole fraction, x t ) FIG. 1.-Excess adsorption isotherms nzt = ( x ~ ~ - x ~ ) ) , evaluated for the models of random (-) and patchwise (---) surface topography. The assumed heterogeneity parameters are r/kT = 1 .O (circles), r/kT = 0.5 (crosses) and r/kT = 0.0 (free solid line).The most probable value of the adsorption energy cz assumed here was 2542 J mol-l. (A) shows the case of negative deviation from Raoult’s law, characterized by the parameter &,/kT = - 1.0. (B) shows the situation for a positive deviation from Raoult’s law, characterized by c,/kT = 1 .O. 0.3 1 -o,21 \ \ d / bulk mole fraction, xf“ FIG. 2.-Contribution to the excess adsorption isotherm due to surface heterogeneity : (-) surface excess for hypothetical homogeneous surface, characterized by the adsorption energy cz; (--), (---) adsorption excess nxt when the heterogeneity parameter r/kT = 1.0, for random and patchwise topography, respectively; (-@---@-), (-0- - - @-) contribution (nr)2 Db2)/6 due to surface heterogeneity, for random and patchwise topography, respectively.Other notation as in fig. 1.w . RUDZINSKI, J. NARKIEWICZ-MICHALEK AND s. PARTYKA 2367 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 bulk mole fraction, xf“ FIG. 3.-Behaviour of the heat of immersion, Qwt. The notation is the same as in fig. 1. Other parameters are: QwB = 0.0, T = 25 OC and 1 = 418.5 J mol-l. bulk mole fraction, x f ) FIG. 4.-Contribution to the heat of immersion due to surface heterogeneity. The notation is the same as in fig. 2, but (-@--.--) and (@---a) now denote @,“h for random and patchwise topography, respectively. Fig. 2 shows separately the contribution to the excess adsorption isotherm ( ~ r ) ~ Di2)’/6 caused by the surface heterogeneity. Fig. 3 shows the behaviour of the heats of immersion found for the same interaction and heterogeneity parameters.As in our previous paper,2 we have assumed that the term T(aem/i3T) is negligible when compared with E,. Further, we have accepted that 2 is equal to 418.5 J mol-l. This value is close to the A value found by us for the system benzene-cyclohexane on silica gel.9 Fig. 4 shows separately the contribution to the heat of immersion (3;; caused by surface heterogeneity. The general conclusion which can be drawn from fig. 3 and 4 is as follows: Surface2368 ADSORPTION FROM SOLUTION ONTO SOLID SURFACES heterogeneity causes large positive contributions to the heat of immersion, no matter what type of surface topography is present or in which direction deviation from Raoult’s law occurs. We hope that our present model calculations may be helpful in understanding the complicated interference of the basic physical factors governing the behaviour of solid-solution systems, i.e. the non-ideality of the bulk and adsorbed phase, the energetic heterogeneity of real solid surfaces, and the topography of these surfaces. We also believe that a simultaneous numerical analysis of the excess isotherms and the heats of immersion should provide much more reliable information on the nature of solid-solution systems. D. H. Everett, Trans. Faraday Soc., 1965, 61, 2478. A. Dqbrowski and M. Jaroniec, J. Colloid Interface Sci., 1980, 77, 571. E. A. Guggenheim, Mixtures (Oxford University Press, 1952). J. E. Lane, Aust. J. Chem., 1968, 21, 827. R. Sips, J . Chem. Phys., 1950, 18, 1024. J. Horiuti, J . Res. Znst. Catal. Hokkaido Univ., 1961, 9, appendix. C. Y . Lu and R. F. Lama, Trans. Faraday Soc., 1967, 63, 727. * W. Rudzinski and S. Partyka, J . Chem. SOC., Faraday Trans. 1, 1981, 77, 2577. .I M. Borbwko, M. Jaroniec and W. Rudzinski, Monatsh. Chem., 1981, 112, 59. (PAPER 1 / 1247)

ISSN:0300-9599    

DOI:10.1039/F19827802361

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1982, 78, 2369-238s Structure and Dynamics of Graphite Intercalation Compounds Part 1.-Neutron Diffraction and the Structure of C&, C,KH$ and C,KD+j BY TIMOTHY TREWERN, ROBERT K. THOMAS, GEOFFREY NAYLOR AND JOHN W. WHITE* Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ Received 7th August, 198 1 The neutron diffraction patterns from C,K, C,KDt and C,KHg have been measured at 300 K using samples prepared from a large variety of different graphites. The effect of sample texture on the degree of order that can be produced in the intercalation compounds is reported. A difference synthesis is made for the diffraction patterns of C,KDI and C,KHI and locates the hydrogen species in the median plane between the graphite sheets and potassium layers, confirming recent models.A model structure is fitted to the powder diffraction data and leads to a three-dimensional arrangement for a hydride ion species in the intercalated region of C,KHt materials. The intercalation compounds of graphite have properties, intermediate between metals and insulators, which may be varied systematically by choice of the intercalating and stoichiometry. There is currently much interest in their band structure4 and associated physical proper tie^,^?^ and their widely variable electron affinity gives them some properties of the transition metals, which may be responsible for their observed catalytic a ~ t i v i t y . ~ ~ ' 9 This series of studies concerns the ternary compounds of graphite, alkali metals and hydrogen and aims to resolve some outstanding questions on structure, kinetics of formation and excitations by studies with neutron diffraction and neutron-scattering spectroscopy.A study is presented of the texture of powder and single-crystal samples prepared by different methods. In this first paper neutron diffraction from C&, C,KHj and C,KDj is reported. Entry of potassium between layers of graphite increases their spacing from 3.35 to 5.4 A, a process which involves a translation of the carbon network so that planes on either side of an intercalate layer are superimposable. If we denote reactant layers by I, then the process of the first-stage formation can be denoted by ABABAB+AIAIAIA.. .. Structural studiesg have shown that in the first-stage compound every fifth intercalate layer is superimposable, so that it can now be represented as: A a ADA y A G A a ...giving a c-axis lattice parameter of 4 x 5.4 A = 21.6 A. The packing of the metal species within the layers for the stage 1 compound is a centred hexagonal arrangement. X-ray studies of the second- and subsequent-stage compounds have shown that the intercalate layers are disordered in a liquid-like fashion but pass through a transition into an ordered state on l1 2369 77 FAR 12370 GRAPHITE INTERCALATION COMPOUNDS TERNARY COMPOUNDS Two types of behaviour are seen in the reaction between the graphite intercalates C,M (M = K, Rb, Cs) and hydrogen or deuterium. In the first type (resembling physisorption) it has been reported by Watanabe et aZ.12 that the second-stage intercalates C,,M (M = K, Rb, Cs) take up hydrogen, deuterium and other gases such as nitrogen and methane at temperatures below 196 K.Although C,,K absorbs hydrogen or deuterium up to a limiting composition of C24K(Hz)2.1, C,K was found to be non-sorptive in this temperature range. The enthalpies of sorption (for half coverage) for this type of behaviour are ca. 30 kJ mol-l. A quite different behaviour at high temperature has been noticed by Herold et al.13914 who reported the action of hydrogen and deuterium on the first- and second-stage intercalates (C,M, C24M) of both natural and artificial graphite. The limiting composition for C8K is the dark blue C,KH+, which is obtained at a pressure < 1 atm. Replacement of potassium by caesium decreases the degree of hydrogenation at a given pressure.For C24K the product at a hydrogen pressure of 1 atm* had an approximate composition of C,,KH,,,,. For C,KH+ Colin and Herold14 have proposed an orthorhomic unit cell with parameters a = 4.92 A, b = 8.52 A, c = 47.52 A. According to them, intercalation of hydrogen into C,K increases the carbon interlayer spacing from ca. 5.4 to 5.94 A. Lagrange et aZ.15 have suggested that this process involves a major structural transformation whereby the first-stage intercalate passes to a second-stage material with a potassium bilayer. As evidence of this they found the ternary intercalate C,KH+ to be susceptible to attack by further alkali metal, forming a new stage 1 compound in the following manner: 2C,K&+M + C,KH+.C,M (M = K, Rb, CS).Transition of the stage 1 C,K to the stage 2 C8KHt completely empties the space between pairs of carbon planes. An objective of the work reported here was to verify this conclusion by measuring neutron diffraction along the crystalline c axis of C,KHt and C,KD#, if possible at the same time learning something of the lateral organisation of the hydrogen species by measurements on well dispersed, powdered samples. EXPERIMENTAL PREPARATION AND CHARACTERISATION OF SAMPLES The preparation and characterisation by neutron diffraction of all samples used for diffraction, kinetic and neutron inelastic scattering studies to be reported now and in subsequent papers are summarised. For the present work highly ordered single-crystal texture samples of C,K, C,KDg and C,KH, were required as well as samples of powdered C,K and its hydrogen intercalates.The two-bulb techniquegi l6, l7 was used for all preparations with a vacuum above the sample of always better than 10-5mmHgt at the start of experiments. Our modification of the apparatus for single-crystal samples is shown in fig. 1. Potassium, free from organic impurities, was obtained by vacuum distillation of carefully cleaned metal. Five different types of graphite were used (a) pyrolytic graphite (Union Carbide), (b) turbostatic graphitised carbon Graphon (86 m2g-l), (c) highly ordered pyrolytic graphite (Union Carbide, h.o.p.g.), ( d ) monochromator- grade pyrolytic graphite (Union Carbide), (e) exfoliated graphite (H,SO,) from A. Wedgewood, A.E.R.E.Harwell. * 1 atm = 101 325 Pa. t 1 mmHg = 13.5951 x 980.665 Pa.T. TREWERN, R. K. THOMAS, G. NAYLOR AND J. W. WHITE 237 1 A silica support to vacuum line PTFE tap\ graded :seal \( potassium *\ silica FIG. 1 .-Apparatus for preparing oriented specimens of intercalation compounds. The powder samples were prepared from powdered pyrolytic graphite of particle size ca. 0.2 mm. Following outgassing of the graphite at 623-673 K and 5 x lop6 Torr,* ca. 20% excess potassium was distilled in and the sealed system was maintained at 573 K for 18 h. The temperature was lowered to 473 K and the excess potassium was allowed to distill off by opening the tap. An analogous procedure was followed for otRer alkali intercalates. Incorporation of a PTFE tap allowed subsequent transfer of the product under vacuum.With potassium the product was always a copper-coloured, free-flowing powder. Thomy and Duvall* have reported that intercalation into graphitised carbon black is not easy, since the exposed crystal faces are almost exclusively basal planes and not the edges into which the guest species diffuses. With this in mind the heating time for samples using Graphon was extended to 36 h to ensure reaction. The products appeared a very dark brown colour and remained free-flowing powders. The preparation of single-crystal samples presented a difficulty with regard to their support. Since the crystal undergoes a net expansion of some 60% during intercalation of potassium it was necessary that the crystal support should also be able to expand.The solution adopted was to support the crystal between silica hooks as shown in fig. 1. The tension in the hooks was previously adjusted so as to grip the graphite firmly and yet allow expansion during reaction without fracture. The intercalation was carried out in a similar manner to that used for the polycrystalline materials. The temperature gradient within the furnace was such that there was no condensation of alkali metal on the surface of the graphite. A summary of the samples prepared and neutron experiments done is shown in table 1. * 1 Torr = 101 325/760 Pa. 77-22372 GRAPHITE INTERCALATION COMPOUNDS TABLE 1 .-SAMPLE SUMMARY sample no. graphite substrate neutron experimentsa 1 2 3 4 9 13 14 15 powdered pyrolytic graphite (p.p.g.) (Union Carbide) P*P*g.P.P*g* Graphon (surface area P*P*g- P.P*g- P*P.g- highly ordered pyrolytic graphite P.P*g- monochromator grade 86 m2 g-l) (h.0.p.g.) pyrolytic graphite h.0.p.g. 16 exfoliated graphite Curran 13. = 1.37 8, as C,K/H2/D2 beryllium filter as C,K/H, beryllium filter as C,K/D2 Curran 13. = 1.37 8, as C,K 4H5 13. = 4.8 8, as C,K 6H 13. = 5.8 A as C,K/H2 INIB as C,K/H2 HRPD 13. = 1.522 8, as C, C,K DIB as C,K/H,/D, Curran 13. = 2.63 8, Mk VI 13. = 1.09 (6) 8, w scan as C 0 scan as C,K HRPD 13. = 1.17 8, as C,K/H2 Mk VI 13. = 1.09 A w scan as C w 20 scan as C,K w and 020 scans as C,K o 20 scan as C,K/H, Mk VI A = 1.09 A w 20 scan as C,K/H2 Mk VI 13. = 1.09 8, w scan as C HRPD 13. = 1.29 8, w 20 scan as C,K w and 20 scans as C,K/D2 w 20 scan as C,K/H2 Curran 13.= 1.37 Mk VI 13. = 1.09 A a Curran, beryllium filter, 4H5, 6H, high-resolution powder diffractometer (HRPD), mark VI (Mk VI) are or were instruments at A.E.R.E. Harwell on the DIDO and PLUTO reactors. DIB and INIB are powder diffractometer and beryllium filter instruments at Institut Laue- Langevin, Grenoble. METHODS OF CHARACTERISING THE SAMPLES The quantities available from analysis of neutron diffraction patterns are the lattice parameters before and after intercalation, the mosaic spread of the crystals before and after intercalation and, in the case of powder samples, the dimensions of the two-dimensional crystals characterised by the parameters of Warren.lB To determine the mosiac spread, the mark VI single-crystal diffractometer of A.E.R.E.T. TREWERN, R.K. THOMAS, G. NAYLOR AND J. W. WHITE 2373 Harwell was used. A reflexion from the sample was chosen as near as possible in 28 to the take-off angle of the monochromator for the wavelength used, and rocking of the sample about the o axis was performed; the full width at half maximum, f.w.h.m., B, of the rocking curve is related to the mosaic spread B by where 2 is an instrumental parameter determined by measuring several reflexions at 28 values away from the take-off angle. The effect of 2 was found to be never more than 0.5O. The data are summarised in table 2 . (In all cases intercalation increases the f.w.h.m. of the rocking curve by ca. 5O.) Note, however, that in every case the crystal did not remain single but was extensively cleaved, so that relative misorientations account for a certain amount of the final width.The above data are relevant to current interest in graphite intercalates as monochromator crystals for long-wavelength neutrons. The reflectivities are quite good, in some cases the second orders of diffraction are very weak, and the possibility of sandwiching crystals with slightly different lattice parameters opens up the possibilities of band-pass monochromators. /P = B2-Z TABLE 2.-EFTECT OF POTASSIUM INTERCALATION ON THE ROCKING CURVES OF SOME GRAPHITE SPECIMENS rocking curve f.w.h.m. ~ ~~~ specimen diffractometer before intercalation after intercalation UCAR highly oriented HRPD A = 1 . 5 2 A pyrolytic graphite 10' sollers /? = 0.8 & 0.2 c = 6.716 A Curran A = 1.37 8, pyrolytic graphite Curran A = 2.63 A Mk VI A = 1.09 A (monochromator grade) pyrolytic graphite Mk VI A = 1.09 A exfoliated graphite exfoliated graphite Mk VI A = 1.09 A Mk VI A, = 1.09 A (1) (11) 0.88 f 0.02" 28 = 85.80 7.0 f 0.5O 28 = 50.6 (sample 8) 5.0 & 0.3 28 = 14.7 (sample 15) 0.7 f 0.1' 28 = 630 6.3 f 0.2O 28 = 47.60 (sample 13) 50 5.0' 1 1 .O f 0.5' 1 1.4 f 0.5" 28 = 18.60 28 = 18.60 2e= 18.60 15.5+ 1.0" 28 = 5.30 (sample 16) " After exposure to deuterium.PRELIMINARY RESULTS ON ORIENTED CRYSTALS The first oriented intercalate prepared, C,K (sample 8, table l), used a relatively large piece of ordinary pyrolytic graphite (dimensions $" x x g). Although the intercalation proceeded smoothly only three (001) diffraction peaks were seen on the high-resolution powder diffrac- tometer [A = 1.52 (1) A] at d = 5.39, 2.61 and 1.78 A.This gave a carbon<arbon spacing of 5.32 A. The loss of high orders was attributed to uneven intercalation in this large sample, which2314 GRAPHITE INTERCALATION COMPOUNDS was not further used. It is possible that the ordinary quality of the pyrolytic graphite contributed to the effect also. By starting with monochromator-grade pyrolytic graphite (sample 13) a better sample of C,K was obtained showing seven diffraction orders of (001) and a carbon layer spacing of 5.35 A using the Mk VI diffractometer [A = 1.09 (6) A]. This measurement showed up an important source of error in measurements on large mosiac crystals owing to the effects of counter aperture. The initial set of integrated intensities from sample 13 when corrected for the Lorentz factor were compared with those calculated from the accepted structure of C,K: I 1 2 3 4 5 6 7 It heory 100 67 35 36 23 28 20 *expt 100 56 22 17 8 7 4 We believe that the large discrepancy is due mainly to two effects: first no thermal factor has been included but a more important effect in this case is due to beam divergence.Implicit in the use of the Lorentz factor, 1 /sin 28, is the assumption that the detector is capable of accepting the entire diffracted beam, a questionable assumption for large mosiac spread crystals. Following Saxena and Schoenborn20 a suitable correction factor, L’, was applied which, with a physically meaningful thermal factor B = 0.9 (cf. B, for graphite = 1.321), gave good agreement between experiment and theory as shown: I 1 2 3 4 5 6 7 Itheory 100 55 22 17 8 7 4 Iexpt 100 56 22 17 8 7 4 Careful checks were made on all data to ensure that this phenomenon was adequately controlled. HYDROGENATED A N D DEUTERATED SAMPLES Admission of hydrogen (deuterium) at atmospheric pressure to the C,K powder samples at 373 K led to a colour change brown -+ blue in a matter of minutes.Samples were allowed to equilibrate overnight under 1 atm of hydrogen. Formation of the hydrogenated sample from large pieces of potassium intercalated pyrolytic graphite was much more difficult. Exposure of the C,K crystal (sample 13) at ca. 373 K to hydrogen led only to surface discolouration; diffusion of hydrogen into the bulk of the crystal occurred exceedingly slowly so that the preparation of the ternary intercalate in this case was not feasible.In order to overcame this problem the size of the next specimen (sample 15) was reduced. The dimensions of the highly ordered pyrolytic graphite substrate were al/ x a” x &” thick. As already described, potassium intercalation proceeded smoothly. Examination of the samples on the Curran diffractometer (A = 1.37) showed that intercalation had increased the mosaic spread from < lo to ca. 5O and the C-C separation from 3.35 to 5.35 A [derived from five orders of (001) peaks]. In spite of the ease of potassium intercalation, conversion to the hydrogenated intercalate was again inconveniently slow and was, in fact, never complete, as shown by the persistence of peaks due to C,K in the diffraction pattern.The final attempt to prepare an oriented specimen of C,KH, was made using exfoliated graphite (sample 16). Diffraction patterns were recorded on the HRPD [A = 1.29 (6) A], both after intercalation of potassium and following exposure at ca. 150 O C to deuterium for 18 h. In this sample C,K diffraction peaks were not visible, and further patterns were recorded on the Mk VI diffractometer [A = 1.09 (6) A], both of C,KDz and of the hydrogenated specimen. The statistics of the weaker peaks were improved by dividhg the angular range of 28 into three sections and successively increasing the monitor setting. The patterns from this crystal are shown in fig. 2.3 Q 2 2 2 C 0 1 0 T. TREWERN, R. K. THOMAS, G. NAYLOR A N D J. W. WHITE 2375 20 16 m 2 12 8 \ Y C a 4 Mon.= 1.25~1 L I I I I I I I I I I I I Mon. =2.5x106 I Mon.=5.0x106 I I I I I I I I I I 40 60 80 100 I I I I I Mon. = 6. 5x105 I I Mon. = 1 3 ~ 10' I Mon.=26x105 I I I ! I I I 0 20 40 60 80 100 2 e p FIG. 2.-Diffraction patterns of oriented C,KDI (A) and C,KHI (B) along the (001) direction measured with 1.09 A neutrons at 300 K. RESULTS APPEARANCE OF THE DIFFRACTION PATTERNS Fig. 2 (A) and (B) show CI), 28 scans along the c axis of single-crystal texture samples (sample 16) of C,KDi and C,KHi, respectively. Two points for consideration arise from the general appearance of the diffraction patterns: (a) There was a small proportion of a contaminant present in the specimen, as shown by a second series of weak diffraction maxima. The d-spacing of the first peak in this contaminant series corresponds to that of the second-stage material, C,,K.(b) The intensities of the2376 GRAPHITE INTERCALATION COMPOUNDS C,KHQ peaks fell rapidly with increasing scattering angle. It was again necessary to take into account the effect of beam divergence. The problem of contamination by C2,K was approached first. The observed integrated intensities are shown in table 3 to ether with the peak positions. The c-axis repeat spacing was determined as 11.88 x . For comparison the positions and TABLE 3.-TOTAL OBSERVED INTENSITIES FOR THE DIFFRACTION PATTERNS OF THE TERNARY INTERCALATES TOGETHER WITH DATA FOR THE SECOND-STAGE CONTAMINANT (Ao = 1.09 A) total integrated intensities order position C24K 1 2e CSKH, CSKD, 1 20 F (relative) 10 11 12 13 14 15 16 17 5.3 10.6 15.9 21.3 26.7 32.1 37.7 43.3 49.1 54.1 61.0 67.2 73.7 80.5 87.6 95.1 103.3 - 58.44 2.63 16.74 18.18 0.36 1.46 5.60 2.80 0.02 1.34 1.89 0.39 0.16 1.65 1.05 0.01 0.61 53.90 0.92 43.50 30.49 1.45 3.60 9.88 6.28 0.20 1.96 4.82 0.71 0.18 2.63 1.76 0.82 (0.00) 1 2 3 - - 5 6 7 8 9 10 11 12 - 7.2 14.4 21.7 - 36.5 44.1 52.0 60.1 68.6 77.6 87.1 97.5 - - - - 1 2a 56 92 - - - 97 42 34 100 4 85 64 8 - - - - a F2 values are those calculated; no account has been taken of the factors which will cause the intensities of these peaks to decrease rapidly with scattering angle, 28.calculated structure factors for C2,K are also shown. The only contaminant peak which was completely free from overlap was the fourth order which has a low structure factor and was experimentally unobservable.The method of separating the intensities eventually adopted involved the use of a Du Pont curve analyser. This allowed the observed profile to be constructed as the sum of two peaks of defined shape (based on the fifth order for C,KDj). This allowed the impurity peaks to be scaled relative to the C,KHj (Di) peaks and subtracted. The impurity was of the order of 8% by weight. Correction for the Lorentz factor (sin 28) and normalisation followed. The values of FF’ and FF’ are shown in table 4 and were used in several Fourier syntheses. All syntheses were simplified as shown by the following reduction 1 L l p(z) = - I= r;,, cos 2nlz n 1 - 1 = F,,,+2 C Fool cos 2nlzT. TREWERN, R. K. THOMAS, G . NAYLOR AND J. W. WHITE 2377 TABLE 4.-NORMALIZED STRUCTURE FACTORS CORRECTED FOR CONTAMINATION AND LORENTZ FACTOR 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 12.53 3.77 11.25 13.52 2.18 4.77 9.50 7.33 0.67 5.67 6.54 3.25 2.12 6.76 5.44 0.54 4.17 9.25 1.75 13.85 13.01 3.40 5.83 9.13 8.31 1.64 5.34 7.80 3.39 1.75 6.39 5.35 0.06 3.77 t ! 2.4SA A ! 3.3sA A FIG.3.-Model for the layer stacking in C,KHi. where the first n orders of (001) peaks are observed. In all cases the zero-order term was neglected. A Patterson difference synthesis was calculated first using coefficients I FF’ -FF’ 12. The intensity at z = 0 and 1 1.88 A suggested that the hydrogen/deuterium was located at a centre of symmetry. A more useful form of the Patterson difference in this case results from the use of the modified coefficients I FF’ l2 - I FF’ 12.Evaluation for the model (fig. 3) shows that the dominant peaks are those associated with interatomic vectors from hydrogen or deuterium [fig. 4(b)]. When the observed coefficients are used the pattern [fig. 4(a)] is seen to be dominated by the C-H (D) vector at ca. 4.3 A. This is fairly conclusive evidence for the central position of the hydrogen. Comparisons of Fourier maps from the observed intensities (a) with those predicted from the model (b) are shown in fig. 5 . On changing hydrogen for deuterium the scattering density markedly increases in the centre of the unit cell in both the calculated and the observed maps.2378 GRAPHITE INTERCALATION COMPOUNDS I I I I I ~ 0 4 8 1 dlA FIG. 4.4ne-dimensional Patterson difference functions from C,KH, and C,KD$ patterns (a) using coefficients 1 @’ - FF’ l2 and (b) using I e’ l2 FF’ 12.QUANTITATIVE TREATMENT In order to place the results on a more quantitative basis account was taken of the neutron beam divergence. Using parameters appropriate to the experimental conditions on the Mk VI instrument and following the treatment of Saxena and Schoenborn,28 the correction factor, L’, was calculated and applied to the observed intensities. The corrected structure factors FF and F , were then compared with those calculated from the model in a least-squares refinement. The temperature factors, B, were arbitrarily fixed at 1.0 for both carbon and potassium, 4.0 for hydrogen (by comparison with the values derived from the study of the motion of hydrogen in metals30) and 2.8 (4/ 4 2 ) for deuterium.Agreement between the calculated and observed intensities could be improved by slight adjustment of the contamination correction. A useful, but by no means infallible, measure of the fit between the observed and calculated structure factors is the residual index R. This is defined by The values of R calculated for the two sets of data are R, = 0.22 and R, = 0.15, and the agreement between the actual values is shown in table 5. The quality of the original data is probably insufficient to justify further reductions and interpretation. There is, however, some evidence that the carbon-carbon plane separation is lower than the usual 3.35 A. The lowest R factor (9%) was obtained with an interplane spacing of 3.31 A for C,KDi.This was approximately duplicated by the C,KHi data, although whether it is a genuine effect is not certain.T. TREWERN, R. K. THOMAS, G. NAYLOR A N D J. W. WHITE 2379 4 s I I I I 1 0 4 8 12 dlA s s 0 4 8 12 dlA FIG. 5.-One-dimensional Fourier syntheses for (A) C,KH3 and (B) C,KD3. TABLE 5 .-COMPARISON OF CALCULATED AND OBSERVED STRUCTURE FACTORS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 56 11 106 100 31 63 98 81 24 68 97 26 41 78 59 3 57 55 11 99 103 29 54 92 88 18 63 96 43 23 89 77 1 57 70 25 91 112 18 51 108 70 15 76 89 33 46 72 63 7 53 75 25 81 108 19 45 96 78 7 68 82 42 29 94 79 8 642380 GRAPHITE INTERCALATION COMPOUNDS 6000- v1 c1 c g 4000- SUMMARY Preparation of an oriented specimen of C,KHj and C,KDj has allowed the layer-stacking sequence to be derived.The hydrogen (deuterium) species is situated mid-way between the potassium planes. The X-ray work of Herold on the carbon and potassium plane positions has been confirmed. Structure factor calculations agree reasonably well with observed values, especially in view of the rather severe corrections imposed by a contaminant and the mosiac spread of the sample. ( b ) . POWDER DIFFRACTION C8K Fig. 6(a) shows the neutron diffraction pattern from C,K powder (sample 1) at 295 K taken with 1.37 neutrons on the Curran diffractometer at A.E.R.E. Harwell. The sample was prepared from powdered pyrolytic graphite. For comparison the 6000 m U E s E 8 4000 0 2 000 0 20 40 60 80 2810 2elo FIG. 6.--(a) Powder diffraction pattern from C,K (a) prepared from graphite and (6) prepared from turbostatic graphite (Graphon).T. TREWERN, R.K. THOMAS, G. NAYLOR A N D J. W . WHITE 238 1 pattern from C,K originating in Graphon (sample 4) is shown in fig. 6(6). The chief differences lie in the intensities and sharpness of peaks at 28 = 30, 39 and 6 9 O , the peaks in the Graphon sample being invariably weaker and wider with strong evidence of compound structure. The most general feature in common for the two samples is the differentiability of peaks into two categories, the (001) series of narrow, more or less gaussian peaks, and the sawtoothed, asymmetric peaks with a sharp onset at low 28, characteristic of the two-dimensional layer latticelg of limited size. To analyse the powder patterns, intensities were corrected for multiplicity, Lorentz factor and a consistent thermal factor chosen. In the first instance the theoretical diffraction pattern of perfect, polycrystalline C,K was considered.An examination of the symmetry of the material led to the conclusion that the space group was not C222 (0;) as proposed by Wolten22 but was of higher symmetry: Fdd2(C3, in agreement with Nixon and Parry.23 Results are given in TABLE 6.-PREDICTED DIFFRACTION DATA FOR C,K, 2 = 1.37 A index Bragg angle, structure factor multiplicity Lorentz (FpL) h k l 20/O F P factor L (relative) - 0 0 4 0 0 8 0 4 0 , 2 2 0 0 4 4 , 2 2 4 0012 - 0 4 8 , 2 2 8 0412, 2 2 12 2 6 0 , 4 0 0 2 6 4 , 4 0 4 0416, 2216 2 6 8 , 4 0 8 0020 - 0 8 0 , 4 4 0 0 8 4 , 4 4 4 2612, 4012 0 8 8 , 4 4 8 - 0016 - 14.7 29.7 37.2 40.2 45.2 48.3 59.9 61.6 67.1 69.1 74.2 75.0 79.6 79.3 81.2 84.6 86.8 39.60 45.52 18-01 23.93 39.60 18.01 23.93 45.52 45.52 39.60 18.01 54.52 39.60 18.93 24.85 39.60 18.93 - 18.47 24.39 18.47 24.93 45.52 39.60 18.47 54.52 18.01 23.93 39.60 18.01 - - - 2 2 2+4 4+8 2 4+8 4+8 2 4+2 8+4 4+8 8+4 2 2+4 4+8 8+4 4+8 62.4 16.1 10.4 9.0 7.48 6.59 4.68 4.51 3.93 3.77 3.48 3.41 3.22 3.19 3.1 1 3.00 2.92 100 34 11 32 12 14 17 10 25 36 7 43 5 3 11 29 6 table 6.Recent studies by Lagrange et al.24 suggest that the space group is Fddd, but the observed diffraction peaks for the pyrolytic graphite samples were consistent with the Fdd2 space group with unit-cell parameters a = 4.96 A, b = 8.59 A, c = 21.4 A, a = a = y = 90°, and an attempt to fit Fddd was less successful than the fit to Fdd2.The c parameter derived from the pattern was 21.4 (2) A. Comparison of the experimental and theoretical intensities confirmed that the material was indeed turbostatic, no general (hkl ) reflexions being present and the observed reflexions falling into two separate sets. The experimental intensities are in reasonable agreement with the predicted values. From the powder pattern it is possible to obtain values for the average extent of the crystallites both within the basal planes and perpendicular to them by deconvoluting instrumental broadening. This is relatively easy if the final profile is analytically simple and especially if it can be assumed Gaussian. This assumption can be made for the2382 GRAPHITE INTERCALATION COMPOUNDS (001) reflexions but the form of the (hkO) reflexions is not simple and deconvolution has not been attempted. Analyses of peak widths show that for C8K and using the expression of Warren19 L, [from the (008) peak] is ca.160 A (or 110 A if instrumental effects are neglected). The riean value of La, derived from the two-dimensional peaks (100) and (1 10) and neglecting instrumental broadening, is 90 A. It is probably safe to say that minimum crystalline dimensions are ca. 100 A. L, = 1.84 A/B cos 8, L, = 0.89 L / B cos e. C8K/D$/H$ The C,KD$ pattern [fig. 7(a)] has a similar appearance to that of C8K in that, once again, both symmetric and asymmetric peaks are present. The C,KHg-pattern [fig. 01 4- E g 2000 -0 e, 0 c E 8 1000 0 28 I" 0 20 40 60 80 261" FIG. 7.-Powder diffraction patterns of (a) C,KD and (b) C,KHt taken with 1.37 8, neutrons at 300 k.T.TREWERN, R. K. THOMAS, G. NAYLOR A N D J . W. WHITE 2383 7(b)] is also similar, the most noticeable difference being the higher background due to the high incoherent scattering from the hydrogen. The sharp symmetric peaks are the (001) series of the expanded lattice, indexing on the d spacing of the first reflexion (1 1.9 A). The asymmetric peaks appear in roughly the same positions for all samples, implying that the carbon skeleton remains, to a first approximation, unchanged in the ab plane in both C,K and the hydrogen intercalates. The results of indexing are summarised in table 7. TABLE 7.-sUMMARY OF DIFFRACTION RESULTS FOR THE POWDER SPECIMENS 8.6 & 0.2 5.35 k0.15 2.84 & 0.06 2.69 f 0.02 2.12 & 0.03 1.78k0.01 1.35 0.01 1.23 f 0.01 1.07+0.01 - vw - vs - - W m S W - - vw VS W - - 3.98 & 0.03 2.95 f 0.02 - - C24K 0 0 8 0 4 0, 2 2 0 0 0 12 2.11 k0.02 1.71 kO.01 1.48 f 0.01 1.23f0.01 1.06 f 0.01 - - 0 0 16 2 6 0, 4 0 0 0 8 0 , 4 4 0 - vs I 1.55 f 0.45 5.80 k 0.20 s 3.93f0.08 s 2.92f0.03 - - vs vw m S - s 2.10f0.01 m 1.69f0.02 w 1.47 k 0.02 s 1.22f0.01 w 1.06f0.01 - - - - S W vw S W It is impossible to say whether crystallite dimensions are significantly changed upon intercalation of the hydrogen, although measurement of peak breadths yielded La values which were generally slightly lower than those for the potassium intercalate itself.INTERPRETATION OF THE POWDER DIFFRACTION PATTERNS Using the one-dimensional hydrogen structure and known stoichiometry, three- dimensional structures can be guessed for the ordered state of C,KD; or C,KHj. We suggest as one plausible ordered structure that a hydride ion sits in the approximately tetrahedral sites of the sandwich formed by juxtaposing two, 2 x 2 potassium/graphite, C8K structures.As drawn this has the stoichiometry of C,KHi, i.e. that of the compound most readily formed by the reaction of C,K and hydrogen.l3~l4 This structure is sketched and shown in plan in fig. 8, and should be considered as a basis from which a three-dimensional structure could be built. The higher hydrogen density in C,KH$ can be modelled by filling more of the unoccupied sites with H-, or on chemical arguments it may be reasonable to suppose filling of the sites with H; ions. We regard these as extreme models of the structure which cannot be distinguished on present evidence.If one takes as ionic radii 1.33 A (K+) and 1.54 A (H- and H;) the model of fig. 8 is plausible, since the predicted distance between carbon sheets on each side of the2384 GRAPHITE INTERCALATION COMPOUNDS FIG. 8.-Possible three-dimensional structure for C,KH$ based upon the oriented and powder diffraction patterns. TABLE 8 .-EFFECTIVE IONIC RADII FOR POTASSIUM IONS IN RELATED GRAPHITE INTERCALATION COMPOUNDS c axis compound stage spacing/A radius of K+/A ref. KCl - - 1.33 - C,KH, I1 8 x 5.94 1.32 (14) CZ4K(DZ)ZU I1 8.96 1.13 (12) C*K I 4 x 5.4 1.025 (9) G*K I1 8.67 0.99 (12) a Measured at 77 K. Molecular diameter of D, is 2.4 A (D; = H; assumed = 3.08 A). C-C plane/plane distance is always taken as 3.35 A.potassium bilayer is 8.95 A (found 8.61 A). By looking at related compounds, tat 8, there are grounds for taking a smaller K+ ionic radius, which would bring the: figures into close accord. These numbers suggest that the potassium ions are 'packed into' the graphite sheet or alternatively that the van der Waals distance between bilayer graphite sheets is < 3.35 A. This last possibility echoes the result from the least-squares refinement of the one-dimensional diffraction pattern of C,KDi (above). The above model has been tested against the powder diffraction patterns of C,KDi and C,KHi to attempt a characterisation of any proton disorder to be expected by analogy with C,K1'* l1 but without a conclusive result.T.TREWERN, R. K. THOMAS, G. NAYLOR AND J. W. WHITE 2385 GENERAL CONCLUSIONS The preparation of CEK, CEKHt and CEKDf samples with a wide range of textures is reported. The most crystalline samples give eighteen orders of the (001) series in neutron diffraction, allowing the hydrogen pention to be definitely placed within a bilayer of potassium ions in a second-stage intercalation structure. One possible model for a basic unit of this structure is proposed. The powder diffraction measurements support the model used but were inadequate to allow a three-dimensional structure to be proposed. Yu. N. Novikov and M. E. Vol’pin, Russ. Chem. Rev., 1971, 40, 733. A. Herold, M. Colin, N. Daumas, R. Diebold and D. Saehr, Chem. Soc. Spec. Pub., 1966, 22, 309. M. A. M. Boersma, Catal. Rev., 1975, 10, 243. W. Eberhardt, I. T. McGovern, E. W. Plummer and J. E. Fischer, Phys. Rev. Lett., 1980, 44, 200. F. L. Vogel, G. M. T. Foley, C. Zeller, E. R. Farlardeau and J. Gan, Muter. Sci. Eng., 1977,31,261. J. Parrod and G. Bienert, J. Polym. Sci., 1961, 53, 99. W. Rudorff and E. Schulze, Z. Anorg. Allgem. Chem., 1954, 277, 156. .5 A. R. Ubbelohde, L. C. F. Blackman and J. F. Mathews, Nature(London), 1959, 183, 454. ’ H. B. Kagan, Chem. Technol. 1976, 6, 510. lo G. S. Parry and D. E. Nixon, Nature (London), 1967, 216, 909. l 1 G. S. Parry, D. E. Nixon, K. M. Lester and B. C. Levene, J. Phys. C, 1969, 2, 2156. l2 K. Watanabe, T. Kondow, M. Soma, T. Onishi and K. Tamaru, Proc. R. Soc. London, Ser. A, 1973, l3 D. Saehr and A. Herold, Bull. Soc. Chim. Fr., 1965, 3130. l4 M. Colin and A. Herold, Bull. Soc. Chim. Fr., 1971, 1982. lS P. Lagrange, A. Metrot and A. Herold, C.R. Acad. Sci., Ser. C, 1974, 278, 701. l8 A. Herold, C.R. Acad. Sci., Ser. C, 1951, 232, 838. l7 A. R. Ubbelohde and F. A. Lewis, Graphite and its Crystal Compounds (Oxford University Press, 333, 51. 1960). A. Thomy and X. Duval, J. Chim. Phys. Physicochim. Biol., 1969, 66, 1966. *’ B. E. Warren, Phys. Rev., 1941, 59, 693. 2o A. M. Saxena and B. P. Schoenborn, Acta. Crystallogr., Part A, 1977, 33, 813. 21 R. Chen, P. Trucano and R. F. Stewart, Acta Crystallogr., Part A , 1977, 33, 823. 22 G. M. Wolten, Atomic Energy Commission Report NAA, SR4545 (1960). 23 D. E. Nixon and G. S. Parry, J. Phys. D, 1968, 291. 24 P. Lagrange, D. Guerard and A. Herold, Ann. Chim. E, 1978, 3, 143. (PAPER 1 / 1248)

ISSN:0300-9599    

DOI:10.1039/F19827802369

出版商:RSC    

年代:1982

数据来源: RSC