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/F198278FX033

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/F198278BX035

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 II 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. iPART 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 (1) 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 1 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. Chern. SOC., Chern. Cornmun. J . Chern. SOC., Dalton Trans. J . Chern. SOC., Faraday Trans. 1 J . Chern. SOC., Faraday Trans. 2 J . Chern. SOC., Perkin Trans. I J. Chern. 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. 11THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY 1982 Bourke Lectures by Professor E. Clementi Professor E. Clementi (International Business Machines Corporation, U.S.A.) will give the 1982 Bourke Lectures at the following venues: Monday, 18 October at Birkbeck College, London Tuesday, 19 October at Cambridge University Wednesday, 20 October at Oxford University Thursday, 21 October at Sheffield University Friday, 22 October at St Andrews University Further information may be obtained from: Mrs Y.A. Fish, The Royal Society of Chemistry, Burlington House, London WIV OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 1 7 The Hydrophobic Interaction University of Reading, 15-1 6 December 1982 This term refers to interactions between chemically inert residues arising from perturbations in the unique spatial and orientational correlations in liquid water. These effects provide a major contribution to many of the non-covalently bonded structures that form the basis of life processes.Current advances in the statistical mechanics of polar fluids, intermolecular forces, computer simulation, and membrane physics are providing a new basis for the re-examination of various aspects of hydrophobic effects, their origin and their quantitative description. Such theoretical treatments will be confronted with recent experimental work on simple model systems which, it is hoped, will lead to a better understanding of hydrophobic interactions in more complex processes. The following have agreed to contribute to the symposium: A. Ben-Naim, H. J. C. Berendsen, D. L. Beveridge, S. D. Christian, L. Cordone, D. Eagland, D. Eisenberg, R. Lumry, P. J. Rossky, M. C. R. Symons, H. Weingartner, M. D. Zeidler The programme and application form may be obtained from: Mrs Y.A. Fish, The Royal Society of Chemistry Burlington House, London W1V OBN ... 111THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 75 Intramolecular Kinetics 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, 14-1 6 September 1983 The meeting will discuss the experimental investigation and the theoretical description of the properties of concentrated colloidal dispersions, i.e.those systems in which the particlc+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. Further information may be obtained from Professor R. H. OttewiII, 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 Basis 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 a 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, Bebington, 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 Dr 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 1TS VFARADAY DIVISION INFORMAL AND GROUP MEETINGS 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, University of Hull, Hull HU6 7RX Statistical Mechanics and Thermodynamics Group Liquids and Liquid Mixtures To be held at the University of Hull on 28-29 March 1983 Further information from Dr P. G. Francis, Department of Chemistry, University of Hull, 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 Colloid and Interface Science Group Proteins and Colloidal Systems To be held at the University of Leeds on 14-1 5 April 1983 Further information from Dr E. Dickinson, Procter Department of Food Science, University of Leeds, Leeds LS2 9JT Polymer Physics Group, Macrogroup UK and the Plastics and Rubber Institute Polyethylenes To be held in London on 8-10 June 1983 Further information from The Plastics and Rubber Institute, 11 Hobart Place, London SW1 W OHZ Industrial Physical Chemistry Group Crystallization Processes in Condensed Phases To be held at Girton College, Cambridge on 5-7 July 1983 Further information from Dr I. D. Robb, Port Sunlight Laboratory, Bebington, Wirral, Merseyside L63 3JW Molecular Beams Group with High Resolution Spectroscopy Group Molecular Beams and Excited States To be held at the University of Nottingham on 1S21 December 1983 Further information from Dr J.C. Whitehead, Department of Chemistry, University of Manchester, Manchester M13 9PL 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 f 17.50 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, lon- 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 ‘ I 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. 141. Also available as a soft-cover booklet from Pergahon Press, Oxford.) Surface Chemistry. ‘ Definitions, Terminology, and Symbols in Colloid and Surface Chemistry - I.’ (Purr and Appl. Chem., Vol. 31, No. 4, 1972, pp. 577-638.) ‘ Definitions, Terminology, and Symbols in Colloid and Surface Chemistry - 11. Heterogenous Catalysis. ’ (Pure and Appl. Chem., Vol. 46, No. 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 Chemistry in 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 relationships, reflectance, potentiometric ion activities, testing distillation columns); solution chemistry; spectrochemical analysis; surface chemistry; thermo- dynamics, and zeolites. Finally, the rules for the naming of organic and inorganic compounds are dealt with in the following publications from Pergamon Press: ‘Nomenclature of Organic Chemistry, Sections A, B, C , D, E, F, and H’, 1979. ‘Nomenclature of Inorganic Chemistry’, 1971. A complete listing of all IUPAC nomenclature publications appears in the 198 1 Index issues of J . Chem. SOC. pp. 71-90.) ... Vlll

ISSN:0300-9599    

DOI:10.1039/F198278FP065

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. 1, 1982,78, 2593-2598 Central-force Models for Proton-transfer Reactions BY RONALD P. BELL 5 Park Villa Court, Leeds LS8 1EB Received 6th October, 1980 A general central-force model is used to calculate free-energy changes and Bronsted exponents for proton-transfer reactions. The exponents assume different values for the separate variations of the acidic and basic reactants, the value being always greater for the former type of variation, and a symmetrical transition state does not correspond to an exponent equal to one-half. The results cast some doubt on the procedure commonly adopted for using the curvature of Bronsted plots to deduce the energy of solvent reorganisation in the transition state. A proton-transfer reaction can be formulated as AH++B+A++HB.(1) In this scheme both A and B can in principle bear any charge, positive or negative, but in the systems most frequently studied either both carry a single negative charge (proton transfer from an uncharged acid to an anion base) or both are uncharged (proton transfer from a cation acid, such as the hydronium ion, to an uncharged base). Both the velocity constant and the equilibrium constant will be changed by chemical substitution in A and B, and provided that these changes are not too drastic both experiment and theory suggest relations of the form where AG# is the free energy of activation, AG the standard free energy change for the overall reaction, the subscripts A and B refer to modifications in A and B, respectively, and is commonly termed a Bronsted coefficient.It has long been realized that P will not remain constant when AG and AG# vary over wide ranges, and much attention has been paid to the prediction or interpretation of the dependence of j3 on these free-energy changes or on the symmetry of the transition state. In the most commonly used model of proton-transfer reactions the transition state is assumed to be located at the intersection of the potential-energy curves for AH+ and +HB, and the effect of substitution is represented by the vertical or horizontal displacement of these curves, without any change in their shapes. Since both AG and AG# depend only on the relative position of the two curves this model implies that PA = PB, i.e. if we start with a given acid and a given base the effect of small modifications in either AH+ or B, or in both simultaneously, can be represented by a single Bronsted coefficient 8.However, there is no thermodynamic requirement for PA and PB to be equal, and there is little experimental information on this point, largely because in most of the systems studied (notably catalysis by acids or bases) either the acid or the base is too weak for reliable values of both 6, AG and BB AG to be obtained. In a few instances markedly different values of PA and PB have been although these differences have been commonly attributed to special features of the reactions concerned. The present paper reports model calculations which suggest that 25932594 P ROT ON-TR A N SFER RE A C TIONS different values of PA and /IB may be the rule rather than the exception, and which serve to explore the dependence of these coefficients on transition-state symmetry.It is assumed that the proton moves in a potential which is the sum of contributions from the two basic centres A and B. This central-force model seems a priori just as likely as the one based on the intersection of two non-interacting potential-energy curves, and although it is unlikely to provide a quantitative description of actual systems it furnishes a convenient means of exploring possible types of behaviour. A previous paper6 describes one model of this type, in which the proton moves between charges of - q and - (1 -q), the values of AG and AGZ being varied by varying q. However, this situation is rather artificial, since the simultaneous variation of the two charges automatically leads to a single value of /?, which is equal to one-half when AG = 0: this model does not permit the variation of one reactant while the other is held constant, as is normally the case in practice.The present treatment allows for the separate variation of A and B and for laws of force which are more general than a charge-charge interaction. A and B are represented as spheres of radius p, impenetrable to the proton. In the reaction complex their centres are held a fixed distance apart, conveniently taken as unity, so that p < f. Our first expression for the potential (model I) is eqn (3) (3) where x and I--x are the distances of the proton from the centres of A and B, respectively: x must lie between p and 1 -p.The function f (x) can be any monotonically decreasing function of x, and oA and oB are measures of the basic strength of A and B. For given values of oA and oB, V(x) will have a maximum at some value x = q, defining the position of the transition state. The condition 1 - p > q > p imposes some restrictions on the value of oA/oB, which is given by eqn (4) (4) - V(x) = b A f(X)+o, f(l -x) o A f ’ ( ~ ) + oBf’(1 - q ) = 0 where the prime indicates differentiation with respect to q. The expressions for AG and AG# for reaction within the complex corresponding to this model are given in eqn (5) and (6)* AG = (OA - [ f @) -f ( -p)l ( 5 ) (6) AGZ = o A [f(P) -f (r)] + OB If(’ - P ) -f (’ - q)]. Eqn (7) and (8) for PA and PB are then obtained by combining the last three equations : (8) f ( l -V)-f(l -PI \ An alternative way of representing the effect of substituents is to add terms to an originally symmetrical potential (model 11).Instead of eqn (3) we then have - V(x) = g ( x > + g ( l - - x ) + ~ U , f ( x ) + ~ B f ( l (3’) * Following general usage the potential energies of the model have been represented as free energies, although this identification may be open to criticism.R. P. BELL 2595 wheref(x) and g(x) are now different functions. Eqn (4), (5) and (6) are now replaced by (47, (5’) and (6’): g’(q)+g’(l -7) + P A f ( r > + PB f ( l -q) = (4’) AG = @A-PB)[f@)-f(l ( 5 ’ ) The resulting expressions for PA and aB are identical in form to eqn (7) and (8), although it is now the potential owing to the substituent rather than the total potential owing to A or B which is proportional to - f ( x ) .Eqn (7) and (8) thus apply both to model I and to model 11, and they lead to a number of general conclusions which are independent of the form off(x) provided only that it is convex to the axis of x, as will be the case for any realistic potential. These conclusions are as follows. (i) The coefficients QA and pB have different values, except in the limiting cases /3 = 0 or 1, corresponding to q = p or 1 -p. Otherwise we always have PA > BB, since f ( P ) +fQ -PI > fh) +fU - v). (ii) Neither PA nor aB is equal to one-half when q = 8 (AG = 0). At this point PA +/IB = 1 : elsewhere PA+aB >< 1 according as q 8 8 (AG 3 0). (iii) If PA and PB are regarded as functions of q, the equations show that BA(l -q) = 1 -BBq..The dependence of PA and BB on the extent of proton transfer in the transition state or on AG can be predicted if specific assumptions are made about the form off(x). The simplest assumption isf(x) = x - ~ , where the values n = 1, 2 and 4 correspond, respectively, to the motion of the proton between two negative charges, point dipoles (with their negative poles directed towards the proton), or polarisable centres.* It is also necessary to assume a value for p( < a), and the present calculations have been carried out with p = 0.3 and p = 0.4. Eqn (7) and (8) then give PA and PB as functions of q. Since q must lie between p and 1 -p, the extent of proton transfer y is best defined Y = (21 - PMl- 2P).(9) by eqn (9) Plots of PA and BB against y for n = 1, 2, 4 are shown in fig. 1 (a) (p = 0.3) and 1 (b) (p = 0.4). These conform to the general conclusions (i) and (ii) given above, and in virtue of (iii) the curves for /IB are obtained by reflecting those for PA through the point 0.) = i, B = t). The values of at y = a differ considerably from one-half: correspondingly, PA and PB are equal to one-half for different transition states which are far from symmetrical. These findings contrast with the predictions of a frequently used model7 in which the transition state is located at the intersection of two identical parabolae, and AG and AGf are varied by vertical displacement of the parabolae: according to this picture PA and & should be equal and should vary linearly with the coordinate of the transition state.The extent of proton transfer in the transition state is not an experimentally accessible quantity, and it has been usual to considera as a function of AG or AG/AG,Z, where AG,Z is the value of AG# when AG = 0. In the present treatment AG/AG,Z for model I depends only on the ratio oA/oB, which is determined by eqn (4), so that both AG/AG,Z and PA or PB can be expressed as functions of q or y . If we are * The case n = 3 corresponds to the motion of a point dipole between two dipoles, but does not represent any simple physical model for proton transfer, although it might be a useful approximation to some more complex situation.2596 1 0.75 P 0.50 0.25 0 4 , 0.7E P 0.50 0.25 0 PROTON-TR ANSFER RE ACTIONS Y 0.25 0.50 0.75 1 Y FIG.1.-Bronsted exponents as a function of transition state symmetry: (a) p = 0.3, (6) p = 0.4. (. . . .) n = 1, (-) n = 2, (---) n = 4.R. P. BELL 2597 1 0.75 - P 0.50 - - 0.25 - - ()--------------- I I ! I I I -6 -4 - 2 0 2 4 6 AG~AG; FIG. 2.-Bronsted exponents as a function of free energy change (model I): (a) p = 0.3, (6) p = 0.4. ( . . . . ) n = 1, (-) n = 2, (---) n = 4. considering variations in A and B in the neighbourhood of a particular pair of reactants defined by oA and oB, different values of AGg, say (AG$)A and (AGg)B, must be used in conjunction with QA and /IB, respectively. This is because QA refers to variations in which oB is kept constant, so that (AG0f)A is given by eqn (6) with both coefficients equal to oB, while for (AG,f)B both coefficients are equal to oA.Plots of PA and PB against AG/AG,Z for model I are shown in fig. 2(a) (p = 0.3) and 2(b) (p = 0.4). As before, the curves for QB can be obtained from those for #IA by reflexion through the point AG/AG$ = 0, /I = 4, since it follows from eqn (3)-(6) that AG/(AGoS;)A for a system with a transition state at x = 7 is equal but opposite in sign to AG/(AG$)B for a system with a transition state at x = 1 - q . The results2598 PR 0 TON-TR AN SFER R E A C TI ON S again differ widely from the predictions of a model of intersecting parabolae, which in its simple form7 gives a linear plot ofQ against AG/AG,Z of slope i. The central-force model could account for the differences between QA and QB which have been observed e~perimentally,~-~ and in particular for the facts that QA > QB and that for uphill reactions PA+& > 1.However, it is not consistent with the observation that PA is sometimes greater than Our model I1 [eqn (3')-(6')] gives the same expressions as model I for QA and &, so that fig. 1 (a) and (b) apply equally to both models. However, the values of (AG$)* and (AGE)* for model I1 depend on the relative contributions of g(x) andf(x) to the total potential, so that fig. 2(a) and (6) will be modified, although retaining the same general features. which requires some other explanation. All the above calculations refer to the process A-H+. . . B + A. . +H-B taking place within the collision complex at a fixed A. . . B separation. In dissociating solvents the observed values of AG and AG# will often refer to the interconversion of the separated reactants, as in eqn (l), and will therefore differ from the values in our treatment by the energies of formation of the collision complex from separate reactants or products, which may also be affected by substitution.The central-force treatment adopted here could in principle be extended to include the separate species, but this would hardly be realistic, since although a particular law of force may a good approximation for a limited range of configurations within the complex, it cannot be expected to remain valid outside this range. No allowance has of course been made for any solvent reorganisation accompanying the formation of the transition state : this factor is likely to be relatively insensitive to substitution, but it will affect AG,Z, and hence the form of the relation between Q and AG/AG$.Estimates of the part played by solvent reorganisation have in fact frequently been based on a comparison between the observed Q- AGIAG,' relation and that predicted by some simple model, commonly intersecting parabolae. The results of the present treatment show that such estimates must be accepted with caution, since different models predict a wide variety of relations even when no account is taken of solvent involvement. F. G. Bordwell, W. J. Boyle, J. A. Hautala and K. C. Yee, J . Am. Chem. SOC., 1969, 91, 4002. M. Fukuyama, P. W. K. Flanagan, F. T. Williams, L. Frainier, S. A. Miller and H. Schechter, J. Am. Chem. SOC., 1970, 92, 4689. F. G. Bordwell, W. J. Boyle and K. C. Yee, J. Am. Chem. SOC., 1970,92, 5926; 1972, 94, 3907. F. G. Bordwell, Faraday Symp. Chem. SOC., 1975, 10, 100. R. P. Bell and S. Grainger, Chem. SOC., Perkin Trans. 2, 1976, 1367. R. P. Bell, J. Chem. SOC., Faraday Trans. 2, 1976, 72, 2088. In the second expression of eqn (8) of this paper the numerator should be (1 - 2 ~ ) ~ , not (1 - 2 ~ ) ~ . ' R. A. Marcus, J. Chem. Phys., 1956, 24, 966; Discuss. Faraday SOC., 1960, 29, 21; J. Phys. Chem., 1963, 67, 853,2889; Annu. Rev. Phys. Chem., 1964, 15, 155; J. Chem. Phys., 1965,43, 679. (PAPER O / 1524)

ISSN:0300-9599    

DOI:10.1039/F19827802593

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. 1, 1982, 78, 2599-2608 Electrochemical Decomposition of Li,SiO, and Li,TiO, in Solid-state Thermal Cells BY JUAN M. ACEVEST AND ANTHONY R. WEST* Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen AB9 2UE Received 29th January, 198 1 Cells of the type AuJLi,SiO,IAu and AulLi,TiO,lAu behave as secondary cells at high temperatures, 400 OC. The cell reactants are created in situ by charging the cells in air at e.g. 1.5 V. Electrochemical decomposition of the solid electrolytes occurs giving, as solid products, Li,CO, at the negative electrode and Li,SiO, and TiO,, respectively, at the positive electrode. Under different charging conditions other products may be obtained with the Li,TiO,-containing cell. The products of charging form as a layer on the surfaces of the pellet and the gold electrodes appear to take no part in the reactions.The charged cells have open-circuit voltages in the range 0.4-0.5 V at ca. 500 O C and give discharge currents of e.g. 10-100 pA through a lo4 R load resistance for several days. Gold is commonly used as the electrode material for a.c. conductivity measurements on solid electrolytes and is assumed to be blocking to the discharge of cations. Thus, at room temperature, the gold//3-alumina interface has been shown1 to behave as a double-layer capacitance of size 3 x lo-' F cm-2. At higher temperatures, however, completely different behaviour is observed. For instance, the cell AulLi,SiO,lAu may be charged and discharged above ca. 400 OC such that the quantity of charge passed in both directions is from 6 to 8 orders of magnitude larger than expected for double-layer This quantity of charge is sufficiently large that the cells may have applications in novel types of thermal battery.It was found that similar charging-discharging phenomena occur with a variety of solid electrolyte materials, including single-crystal and polycrystalline /&alumina. It was suggested that the phenomena were caused by unidentified chemical reactions occurring at the electrode /electrolyte interface. In the present work, experiments have been carried out to identify the chemical reactions responsible for the charging and discharging and a more detailed study of the behaviour of two cells has been made. Cells of the type AglRbAg,I,Igraphite may be charged and discharged5 in a manner that, superficially, is similar to the thermal cells Aulsolid electrolyte1Au.However, the former cells are, in fact, very large capacitors (e.g. of 50 F size) and can be charged and discharged at room temperature. The large capacitance values are achieved by greatly increasing the contact area between the electrode and the solid electrolyte. To do this, a powdered mixture of electrode and electrolyte is used to make contact between the electrode and the electrolyte pellet or membrane. The origin of the charge storage therefore appears to be capacitive, unlike the present thermal cells such as AulLi,SiO,(Au, in which electrochemical reactions occur. EXPERIMENTAL Li,SiO, and Li,TiO, were synthesised and pressed into pellets, as described previ~usly.~ Three methods of electrode preparation were used.(1) For most experiments, gold-foil electrodes were t Present address : Instituto de Investigaciones Electricas, Cuernavaca, Morelos, Mexico. 25992600 ELECTROCHEMICAL DECOMPOSITION OF Li,Sio, AND Li2Ti0, attached to the pellet faces using gold paste and the arrangement was fired at ca. 500 OC for 30 min to decompose the paste and harden the gold residue. (2) Gold foil was contacted directly with the pellet surface using a small vice; insulating glass discs were used to separate the cell from the metal vice. (3) Thin-film electrodes of Ag or Au were evaporated onto the pellet faces which was then placed in the above vice. The cells, attached to Pt electrode wires via the gold foil, were placed inside a vertical tube furnace whose temperature was controlled and measured to & 3 OC.The cells were charged at constant voltage, as described previ~usly,~ and the charging current recorded on a chart recorder. The cells were discharged through loads of either 10 or 10010 rZ and the currents again recorded. In other experiments, the cell voltage on open circuit was measured with a Keithley multimeter which had an internal impedance of ca. lo8 R. Two X-ray powder diffraction techniques were used to analyse pellet surfaces : a Philips, Hagg Guinier focusing camera and a Philips diffractometer. For the camera, small amounts of material (2-5 mg) were scraped from the pellet surface, crushed finely and placed in the sample holder. For the diffractometer, the whole of the pellet was placed in the instrument with the surface of interest angled towards the incident X-ray beam.The resulting films and traces were analysed by comparison with standard powder diffraction data reported in the powder diffraction file, Joint Committee for Powder Diffraction Standards, Swarthmore, U.S.A. RESULTS Previous experiment^^,^ had shown that cells such as Au(Li,TiO,lAu, with an electrolyte pellet of mass ca. 0.25 g, were capable of storing from 1 to 100 C of charge after charging at 2 400 OC with an applied voltage of e.g. 1.5 V. Experiments have now been carried out in order to analyse and identify the products formed on charging the cells. ANALYSIS OF CELL REACTIONS In the first experiment, the surfaces of a charged pellet which had been in contact with the electrodes were removed, dissolved in dilute acid and analysed by atomic absorption spectroscopy.This preliminary experiment showed that an excess concen- tration of lithium was present at the negative electrode after the cell had been charged. Details are as follows. A cell of the type AulLi,TiO,)Au was constructed, using gold paste for the electrodes, and charged; integration of the current-time curve showed that 3.5 C of electricity passed on charging. The gold electrodes were then removed from the pellets; they did not come off cleanly but instead a layer of white electrolyte material adhered to the gold. The electrodes were immersed in 0.5 mol dm-, HCI to dissolve any soluble Li+ compounds. The gold did not dissolve and a subsidiary experiment showed that Li,TiO, is not soluble in 0.5 moldm-3 HCl.Hence any Li2Ti0, adhering to the electrode surface did not interfere with the subsequent analyses. The resulting solutions were analysed by atomic absorption methods using standard procedures. The results showed a significant concentration of Li+ ions in the cathode (negative electrode) solution, but no detectable Li+ ion concentration in the anode (positive electrode) solution. Because only a portion of each pellet surface was effectively sampled, it was not possible to make an accurate estimate of the excess Li+ ion concentration at the negative electrode. However, the measured concentration was of the same order of magnitude as that calculated from the quantity of electricity passed on charging.This confirmed that Li+ ions were, in some way, deposited at the cathode during charging. The next stage was to identify the nature of the product(s) at the electrode/electrolyte interface(s). In order to obtain sufficient amounts of products to be detected by X-ray powder diffraction, the cells were charged for 2-3 days. The surfaces of unchargedJ. M. ACEVES AND A. R. WEST 260 1 TABLE 1 .-RESULTS OF X-RAY DIFFRACTION ANALYSES cell negative electrode positive electrode Li,SiO,, charged Li,SiO, + Li,CO, Li,SiO, + Li,SiO, Li,SiO,, uncharged Li,SiO, Li,SiO, Li,TiO,, charged Li,TiO, + Li,CO, Li,TiO, + TiO, (anatase) at 500 O C , 1.5 V Li,TiO,, charged Li,TiO, + unidentified Li,TiO, + unidentified at 700 OC, 0.5 V Li,TiO,, charged Li,TiO, + Li,TiO, Li,TiO, + TiO, (rutile) at 700 "C, 1.5 V Li,TiO,, charged Li,TiO, + Li,TiO, Li,TiO, + TiO, (rutile) at 900 OC, 1.5 V pellets were also analysed to serve as a reference.The results for Li,SiO, and Li,TiO, are summarised in table 1. For the cell containing Li,SiO,, the overall charging reaction may be written as Li,SiO, + CO, -, Li,CO, + Li,SiO, and the individual electrode reactions are probably as shown schematically in fig. 1. Under the action of the applied field, Li+ ions migrate preferentially towards the negative electrode where they react with CO, and 0, from the air to form Li,CO,. At the positive electrode, a Li+ depleted layer forms which achieves electroneutrality by condensation of the isolated, orthosilicate tetrahedra to form metasilicate chains, together with liberation of oxygen gas.This scheme is consistent with the phase diagram of the system Li,0-Si0,,6 although it does seem, in principle, that further decomposition of the Li,SiO, product at the anode could occur to yield, ultimately, SiO,. Li2C03 Li 2Si O3 negative e l e c t r o d e : 2 ~ i + + 2 e +1/ 2 o 2 + c o 2 + L i c o 3 2 p o s i t i v e e l e c t r o d e : S i 0 4 L- -+sio3 2- + 2e +V2o2 FIG. 1.-Schematic mechanism for charging the cell AulLi,SiO,lAu in air at e.g. 500 OC.2602 ELECTROCHEMICAL DECOMPOSITION OF Li4sio4 AND Li,TiO, For the cell containing Li,TiO,, several reactions were identified and all appear to fit the same pattern: lithium ions migrate towards the cathode on charging where they form a lithium-rich phase, e.g.at ca. 500 O C 2Li+ + 2e +40, + CO, -+ Li,CO, at 2 700 "C 2Li+ + 2e + 40, + Li,TiO, + Li,TiO,. The latter reaction occurs because Li,CO, is too reactive to form as a stable product above ca. 700 OC. A lithium-poor phase, often one of the polymorphs of TiO,, forms at the anode together with liberation of oxygen gas Ti0;- -+ TiO, + &02 + 2e. Thus the overall reactions may be written Li,TiO, + CO, + Li,CO, + TiO, or 2Li,TiO, -+ Li,TiO, + TiO,. At low charging voltage and high temperature (0.5 V, 700 "C) the products of charging could not be identified: the phase diagram of the system Li,O-TiO, is known7 and the products of charging did not correspond to any of the equilibrium phases on the diagram, i.e. Li,TiO,, Li4Ti50,, or Li,Ti,O,. Instead, the products appear to be new, probably metastable, lithium titanate phases.In order to confirm that cell reactions were indeed responsible for the charging- discharging processes, primary cells were constructed and their discharge monitored. These primary cells were : Au,Li,CO,~Li,SiO,~Li,SiO,,Au and Au,Li,CO,~Li,TiO,~TiO,,Au. The cells were constructed and studied in the same way as the cells Aulsolid electrolytelAu with one exception : the electrode reagents, e.g. Li,CO,, were introduced into the cell by mixing them with gold paste and the mixtures smeared onto the pellet faces in the same way that gold-paste electrodes are normally prepared. The discharge current obtained from one Li,TiO,-containing primary cell is shown in fig. 2; after 18 h discharge, 0.21 C of electricity had passed.Qualitatively, this is similar to the behaviour of the secondary cell AulLi,TiO,/Au shown in fig. 6 and 7 of ref. (3), although the actual magnitude of Q for the primary cell is several times lower than the values given in ref. (3) for the secondary cell. A more quantitative comparison of the discharge characteristics of the two types of cell has not been made because the two cells were prepared, in the charged condition, by entirely different routes. The results given in fig. 2 show clearly that chemical reactions are responsible for the operation of the cells. The cells are therefore quite distinct from the large capacitors which have been prepared5 by using Ag+-containing solid electrolytes and a very large area of electrode/electrolyte contact.For the latter, it cannot be possible to construct capacitors in the charged condition such that they give a discharge current without having first been charged, whereas this has been demonstrated with our thermal cells. Once the processes involved in the operation of the cells had been determined, it seemed likely that the presence of gold may be incidental to the cell reactions: the gold may function merely as an inert electrode. Two series of experiments were then conducted that showed this to be true. In the first series, a variety of electrode materials was used to construct cells, allJ. M. ACEVES AND A. R. WEST 0 2 4 6 8- 10 12 14 16 18 t l h FIG. 2.-Discharge of the primary cell Au,Li,CO,~Li,TiO,~TiO,,Au at 467 O C . TABLE 2.-cHARGE-DISCHARGE OF Li,TiO,-CONTAINING CELLS WITH DIFFERENT ELECTRODES time of time of electrodes T/OC Q (charging)/C charging/h Q (discharging)/C discharging/h Au paste 555 5.43 25 1.08 14 Pt paste 554 2.52 22 0.03 0.5 Au foil 487 0.78 26 0.09 4.5 Au evaporateda 463 0.53 44 0.27 44 Ag evaporated 463 0.26 22 0.09 1 1 a Cell operated in vacuum; all other cells operated in air.of which contained a similar Li,TiO, pellet as the solid electrolyte. Results of charging/discharging experiments on these are shown in table 2. With electrodes made from Pt paste, only a small discharge was obtained, but this was because the Li+ ions, which accumulated at the cathode on charging, reacted irreversibly with the Pt electrode 2Li+ + Pt + #02 + 2e -+ Li,PtO,. The Li,PtO,, together with some Li,PtO,, was identified by X-ray powder diffraction and formed a greenish-brown deposit on the surface of the Pt electrode. Once formed, this deposit did not disappear on subsequently allowing the cell to discharge.With evaporated-metal electrodes or with gold-foil electrodes in pressure contact with the pellet, significant charging and discharging currents were also obtained. The currents were smaller than in the first cell but this may have been because these cells were operated at lower temperatures (see later). It was not possible to confirm this since the evaporated-film electrodes deteriorated at higher temperatures, e.g. at 550 O C . In addition, the cell containing evaporated-gold electrodes did not function satisfactorily in air: the gold film separated into isolated clusters which turned a pink colour and the cell could not be charged and discharged satisfactorily.A similar cell did operate satisfactorily in vacuum, however.2604 ELECTROCHEMICAL DECOMPOSITION OF Li,sio, AND Li,TiO, TABLE 3.-EFFECT OF REPLACING AN ELECTRODE BY A FRESH ONE AFTER CHARGING time of replaceable charging charging and electrode T/OC voltage/V Q charging/C Q discharging/C discharging/h gold-foil 455 1.5 3.6 1.46 72 cathode gold-foil 463 1.5 0.25 0.04 50 anode In the second series of experiments, gold-foil electrodes were used which were replaced by fresh electrodes after charging. Details are as follows. Two identical Li,TiO,-containing cells were constructed. In each, one electrode was made from gold paste and the other was a piece of gold foil in pressure contact with a pellet surface.The cells were placed in a vice and charged: in one cell the gold-foil electrode was the cathode (negative electrode) and in the other it was the anode. After charging, the gold-foil electrodes were replaced by fresh pieces of gold foil and the cells allowed to discharge. Results are given in table 3. In the cell with the gold-foil cathode, the cells operated well, giving large values of Q, and the effect of replacing the gold-foil cathode after charging was minimal. In the second cell the effect of replacing the gold-foil anode after charging also did not appear to affect the subsequent discharge of the cell. However, the overall values of Q of this cell were an order of magnitude less than for the first cell.The results of both series of experiments taken together indicate that the gold electrodes take no part in the chemical reactions of charging and discharging. The possibility exists, however, that the gold may catalyse the reactions, and the nature of the anode, in particular, may be important. This is shown by the results for the second cell in table 3: use of gold foil as the anode instead of gold paste appears to give much reduced cell performance. TEMPERATURE DEPENDENCE OF CELL PERFORMANCE Most of the preliminary studies on the cells Aulsolid electrolytelAu were made in the temperature range ca. 450-600 oC,2y since at these temperatures most of the cells gave large charging and discharging currents. No appreciable discharge has been obtained for any cell below ca.350 'C. The behaviour of Li,TiO,-containing cells has now been studied over a wider range of temperatures, 350-900 O C . Separate cells were prepared for each temperature. All the cells were prepared in a similar way and had electrodes made from gold paste. All were charged and discharged for the same length of time, 24 h. Since the cells were not fully discharged after 24 h, this means that the values of Q obtained on discharge were underestimates of the actual amount of charge that was originally stored in the cells. Results are given in table 4. Over the range 350-810 O C , the values of Q on both charging and discharging increase by approximately four orders of magnitude. The amount of recoverable charge that is stored appears to pass through a maximum around 810 "C: the ratio Q (discharge): Q (charge) has values in the range 0.17-0.52 for temperatures in the range 350-810 OC, but at 900 O C the value drops to 0.007.This is probably because most of the excess lithium which appears at the cathode on charging is lost to the system at 900 O C either by volatilisation or by reaction with the Pt electrode wires,J. M. ACEVES A N D A. R. WEST 2605 TABLE 4.-EFFECT OF TEMPERATURE ON CHARGE-DISCHARGE OF AulLi,TiO,/Au CELLS Q (discharging) T/OC Q (charging)/C Q (discharging) Q (charging) 350 414 465 530 587 640 728 810 900 0.01 0.40 1.94 2.16 6.26 16.24 48 .OO 82.00 450.00 ~ 0.002 0.07 0.36 0.65 1.51 6.69 25.00 41.00 3.00 ~ 0.20 0.17 0.19 0.30 0.24 0.41 0.52 0.50 0.007 The results given in table 4 are values for the cells under operating conditions; they give no information as to the origin of the large temperature dependence that is observed.A large part of the temperature dependence is probably due to kinetic effects, i.e. the cells were charged for a constant time and not until they had reached the fully charged state (if, indeed, such a state can be defined). With increasing temperature the various cell reactions occur more quickly and hence more charge can be stored and subsequently released on discharge. ATMOSPHERE DEPENDENCE OF CELL PERFORMANCE Since the reactions that occur in the operation of the cells involve the liberation and absorption of gas at the electrodes, it was anticipated that the cell performance would be atmosphere dependent. Experiments were carried out which qualitatively showed this to be so.Four Li,TiO,-containing cells were prepared under similar conditions, using gold-paste electrodes. They were charged and discharged at 463 "C for the same length of time but in four different atmospheres. Results are given in table 5. The largest values of Q were obtained in oxygen and the smallest in nitrogen and in vacuo. The values of Q therefore decrease with decreasing partiaI pressure of oxygen. This is consistent with the reaction schemes proposed earlier since, on charging, oxygen gas must be present at the cathode in order for Li,CO, to be formed. The small values of Q obtained in nitrogen and in vacuo may have been due to a small amount of residual oxygen present in the system. TABLE 5.-EFFECT OF ATMOSPHERE ON CHARGE-DISCHARGE OF AulLi,TiO,lAu CELLS Q (charging) Q (discharging) atmosphere /c /c oxygen 2.74 0.90 air 0.80 0.25 nitrogen 0.22 0.04 vacuuma 0.13 0.03 a Estimated pressure 0.1 mmHg ( x 13.33 Pa).2606 ELECTROCHEMICAL DECOMPOSITION OF Li,SiO, AND Li,TiO, TABLE 6.-EFFECT OF CHARGING VOLTAGE ON THE CHARGE-DISCHARGE OF Aul Li,TiO,lAu CELLS charging T/OC voltage/V Q (charging)/C Q (discharging)/C 467 0.02 0.005 467 0.05 0.03 468 0.20 0.33 467 0.50 0.60 469 1 S O 0.80 0.001 0.012 0.10 0.25 0.26 EFFECT OF CHARGING VOLTAGE ON CELL PERFORMANCE Most of the previous experiments were carried out at a standard applied voltage of 1.5 V.Since chemical reactions are responsible for the charging of the cells it seemed reasonable that a minimum cut-off voltage (but < 1.5 V) would exist below which no charging occurred.However, experiments conducted at different charging voltages showed that this was not strictly true. A series of Li,TiO,-containing cells was prepared in the usual way and charged at voltages in the range 0.02-1.50 V. Results are given in table 6. The values of Q are very voltage dependent over the range 0.02-0.20 V but are approximately constant at higher voltages. It appears, therefore, that there is a minimum charging voltage for the cell to function normally which is in the range 0.2-0.5 V. At lower voltages, however, even as low as 0.02 V, the cell still functions but at a reduced level. OPEN-CIRCUIT VOLTAGE MEASUREMENTS ON PARTIALLY CHARGED CELLS It was shown earlier3 that in its charged state the cell AuJLi,TiO,JAu had an open- circuit voltage (o.c.v.) which was time dependent, but which after leaving for 24 h at 563OC had reduced to an approximately time independent value of 0.47V.This experiment has now been extended to include O.C.V. measurements at various levels of discharge of the cell. A Au(Li,TiO,lAu cell was prepared, charged in the normal way and then allowed to equilibrate on open circuit at 521 OC for 24 h. Results are shown in fig. 3. The initial O.C.V. value was ca. 1.5 V; with time, the O.C.V. decreased in an approximately exponential fashion to achieve a value of 0.425 after 6 h, after which the O.C.V. remained constant. After 24 h on open circuit, the cell was allowed to discharge through a load resistance of lo4 R for 3 h and for which Q = 0.12 C .The cell was then again switched to open circuit and its O.C.V. monitored. After ca. 2 h on open circuit the cell had regained its original O.C.V. value of 0.425 V, after which it stayed constant for a further 19 h. This procedure of partially discharging the cell and then returning it to open circuit was repeated three more times. For the first two of these, an O.C.V. of 0.425 V was regained but after the final cycle the O.C.V. value was only 0.400 V. The cell was therefore probably close to complete discharge. The sum of the four discharging steps gave Q = 0.50 C , which was ca. 10% of the value on charging the cell initially. These results, in which a partially discharged cell recovered its original O.C.V. value prior to discharge, are further proof that the origin of the behaviour of the cells is elect roc hemical rat her than capacitive.J.M. ACEVES AND A. R. WEST 2607 tlh FIG. 3.-Open-circuit voltage measurements on a cell at various levels of discharge. DISCUSSION The results presented above have various implications and applications. It has been shown clearly that the cells are not very large capacitors but are true electrochemical cells. There is, therefore, little similarity between them and capacitors such as Two cells have been studied in detail, those containing Li,SiO, and Li,TiO,. These were chosen for study because they were the cells that gave the highest values of Q on charging and discharging. However, similar effects, but of reduced magnitude, occur with a variety of electrolyte materials, including P-alumina.2* It is interesting to speculate as to the likely products of charging the cell Au(P-alumina(Au in air at e.g.500 OC. Probably, they are NaA10, or Na,CO, at the cathode and one of the polymorphs of A1,0, at the anode, but experiments are needed to confirm this. The mechanisms of operation of the cells are complex and several processes are likely to occur simultaneously, as shown, for instance, by the temperature dependence of the values of Q and the O.C.V. results. The results for Li,TiO, indicate that different processes occur at different temperatures and/or at different applied voltages. The results presented here are therefore rather crude and describe an overall effect, not lending themselves to quantitative interpretation and analysis.Free-energy data are not available for many of the phases involved and hence calculations of cell voltages cannot be made. The mechanism of operation of the cells has what is believed to be a novel characteristic in that the charging process, to give the electrode reagents, involves decomposition of the electrolyte. In normal cells and batteries charging occurs by transfer of material from one electrode compartment to the other and the electrolyte stays intact. For the present cells there is no distinction, in the discharged condition, between electrode and solid electrolyte. For example, Li,TiO, is both the electrolyte and the product of discharge at the anode. The electrode reagents, in this case Li,CO, (or Li,TiO,) and TiO,, are therefore generated in situ on charging the cell.As has been pointed out earlier,, the cells have possible applications in a novel type of thermal cell or battery. In the charged condition the cells may be stored indefinitely at low temperatures since the discharge reactions cannot occur to any extent below AglRbAg,I,lC.2608 ELECTROCHEMICAL DECOMPOSITION OF Li,Sio, A N D Li,TiO, ca. 300 OC. The O.C.V. measurements also indicate that charged cells have a considerable life at high temperatures, e.g. 520 OC, since a cell maintained its original o.c.v., with intermittent spells of discharge, over a period of several days: on continuous open circuit the cell life is probably much longer. The gold electrodes, which were used in the majority of the cells studied, appear to take no part in the cell reactions, apart from possibly acting as catalyst or active surface.It should be possible, therefore, to charge cells using other electrodes, such as silver. The cells may also have possible applications as novel temperature or gas sensors: preliminary results show that the O.C.V. of a charged cell is dependent on both temperature and atmosphere. It is reported that the cell A@-alumina] Au behaves, at 25 OC, as an ideal double-layer capacitor,l provided care is taken over the preparation of the electrode/electrolyte contact. The present results are not in conflict with this room-temperature behaviour. The gold is still behaving as an essentially inert electrode at high temperatures but now electrochemical reactions occur at the electrolyte surface even at applied voltages as low as 20 mV. The results obtained here confirm the belief that materials such as Li,SiO, and Li,TiO, are predominantly Li+ ion conductors. In order to explain the results that on charging (a) lithium-rich compounds form at the cathode, (b) lithium-poor compounds form at the anode and (c) the discharge efficiencies are relatively high, Li+ ions must be the predominant current carrier through the pellet. A.R.W. thanks the S.R.C. for financial aid. J.M.A. received a Scholarship from CONACYT (Mexico). The rutile (TiO,) was kindly donated by Tioxide International. We thank J. Kuwano for helpful discussions. R. D. Armstrong, T. Dickinson and P. M. Willis, J. Electroanal. Chem., 1976, 67, 121. J. M. Aceves, B. G. Cooksley and A. R. West, J. Electroanal. Chem., 1978, 90, 295. J. M. Aceves and A. R. West, J . Appl. Electrochem., 1980, 10, 379. J. M. Aceves, Ph.D. Thesis (University of Aberdeen, 1980). B. B. Owens, J. E. Oxley and A. F. Sammells, in Solid Electrolytes, ed. S. Geller (Springer-Verlag, Berlin, 1977), p. 67. F. C. Kracek, J. Phys. Chem., 1930, 34, 2645. G. Izquierdo and A. R. West, Mater. Res. Bull., 1980, 15, 1655. (PAPER 1 / 140)

ISSN:0300-9599    

DOI:10.1039/F19827802599

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1982, 78, 2609-2618 Sorption of Mixed Methanol +Water Vapours by Ion-exchange Resins BY ROSA CROVETTO? AND E. 0. TIMMERMANN*$ Departamento de Quimica Inorganica, Analitica y Quimica Fisica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina Received 6th May, 1981 The sorption of mixed water+methanol vapours by ion-exchange resins in the sodium form has been studied gravimetrically (McBain quartz spring balance) at 25 O C , using the isopiestic method (equilibrating mixture : water + methanol +glycerol) for controlling the total vapour pressure and the vapour composition. The two resins selected for study, the gel-type Amberlite IR-120 and the macroporous Amberlite 200, give normal water sorption isotherms, but the gel resin shows irreproducible methanol vapour sorption.Systematic mixed sorption measurements were made on the macroporous resin, which presents a slight hysteresis loop for pure methanol vapour so that only rising experiments were performed. The results are analysed at constant total vapour pressure and at constant vapour composition. The mixed isotherms are, like the pure ones, of type I1 (B.E.T.) At saturation, capillary sorption between beds distorts the experimental results and a smooth extrapolation procedure is applied, which allows reliable data to be obtained for these states. In equilibrium, through the vapour phase to the liquid phase the resin shows a change of selectivity, preferring the minor component of the mixture. On the other hand, methanol is selected at constant vapour composition and decreasing total vapour pressure when using methanol-rich vapours.The behaviour of ion-exchange resins in mixed solvents is of interest in many different areas. The use of mixed solvents is very common in (a) batch operations of ion exchange, where the resins are in direct contact with the mixed liquid phase, and (b) dehydration processes that may be catalysed by ion-exchange resins, usually in the H-form, the resins in this case being in contact with a mixed vapour phase. These two cases correspond to the limits of a sorption isotherm; in case (a) the resins are always at saturation, as determined by the vapour-liquid equilibrium of the pure mixed solvents; in case (b) on the other hand, only relatively low vapour pressures are used.Since there are only limited experimental data available on mixed vapour sorption,' the Myers-Praunitz2 theory of the sorption of mixed gases is usually applied. For batch operations, there have been several investigation^,^-' but in all of them it was the equilibrium between the liquid phase and the resin that was taken into account without consideration of the vapour phase. In both cases the results indicate a selective uptake of one solvent by the resin, depending on the characteristics of the resin and its ionic form. We have not been able to find in the literature studies of sorption of mixed solvents from the vapour phase over the complete range of the isotherm, although the recent paper of Hall and Muller8 on mixed gas adsorption on glass, silica and graphite presents some experimental treatments which are similar to the present work.Besides temperature and total pressure measurements, in mixed sorption studies the equilibrium compositions of the vapour phase, the total mass sorbed by the resin and its f Present address : Cornision Nacional de Energia Atomica, Buenos Aires, Argentina. $ Present address : Instituto de Investigaciones Fisicoquimicas Teoricas y Aplicadas (INIFTA), Sucursal 4 - Casilla de Correo 16, (1900) La Plata, Argentina. 26092610 SORPTION OF METHANOL+WATER VAPOURS composition have to be determined. This makes the experimental aspects more complicated than in the case of single vapour sorption. Two of these quantities may be easily kept constant in the experimental procedure: (a) the temperature and composition of the vapour phase, and (b) the temperature and total equilibrium pressure.In case (a) a typical sorption isotherm will be obtained, which gradually changes from the isotherm of one pure sorbate to that of the other sorbate. Accordingly, the total saturation pressure can show a marked variation and thus a large range of pressure may have to be considered. In case (b) the effect of changing the composition of the sorbate is investigated, giving a characteristic picture of the mixed sorption. In the present work the isopiestic method was used to allow controlled variation of both the composition and the total pressure of the vapour phase. Two types of ion-exchange resins were used, the gel-type Amberlite IR- 120 resin and the macroporous Amberlite 200 resin, both in the sodium form.The two resins give reasonably good water isotherms, but with the gel resin no reproducible methanol sorption could be observed and therefore only systematic mixed sorption studies were made on the macroporous resin. EXPERIMENTAL MATERIALS The resin Amberlite IR-120 was provided by B.D.H. and by Mallinckrod and the resin Amberlite 200 by Rohm and Haas, Argentina. The beds were conditioned following HelfferichQ and a selection of beds with particle diameter 0.76-0.38 mm (20-40 mesh) was made. The capacities of the resins measured by batch potentiometric titration were as follows : Amberlite IR-120 3.46 mmol g-l (Mallinckrodt), 3.87 mmol g-' (B.D.H.) and Amberlite 200 2.87 mmol g-l, (per gram of dried resin in the sodium form).The Amberlite 200 had a nominal crosslinking of 20% DVB, a quoted specific area of 54.8 m2 g-l, a most probable pore radius of 80 8, and a pore distribution range of 60-300 A.l0 Doubly distilled water and methanol p.a. (Dorwill, Argentina) were used, the glycerol p.a. (Carlo Erba, Baker) was 9698% w/w and the remaining component was considered to be exclusively water. EQUIPMENT The experimental set-up consisted of a glass vacuum line, which could be pumped down to ca. 5 x Pa with a mercury diffusion pump, a sorption cell with a helical fused quartz spring [unloaded extension 10 cm, diameter 0.8 cm and fibre diameter 0.05 cm; sensitivity 31.37 (f0.07) cm g-l; usual load 0.1-0.5 g] for the gravimetric determinations, an auxiliary cell and a desorption unit, both for composition determination of the sorbed mass.The whole equipment was housed in an air-thermostatted chamber (2.0 m x 1.6 m x 2.1 m) based on a literature design,ll where the temperature could be kept constant within 0.1 OC. All the measurements were carried out at 25 OC. The main parts of the apparatus for mixed sorption are shown in fig. 1 (not to scale). The lower part of the sorption cell (1) is connected to a 30 cm3 flask (2) containing the liquid mixture used for controlling the vapour phase by the isopiestic method. The isopiestic mixture is the ternary system water + methanol + glycerol. The non-volatile glycerol was used to lower the total vapour pressure. The corresponding vapour-liquid equilibria have been studied previously12 at different constant glycerol contents.The composition and vapour pressure of the vapour phase may be conveniently fixed by selecting the appropriate proportions of water and methanol and the content of glycerol. Samples of the resin were placed in small bags made of a synthetic material with a mesh (300) suitable for containing the beds of the resin and for allowing equilibration with the vapour phase. A blank test showed that the synthetic material of the bags does not absorb or dissolve in water or methanol and their mixture, and that no capillary condensation takes place on the bags. Two bags were used in each experiment, one (3) for the gravimetric determination wasR. CROVETTO AND E. 0. TIMMERMANN 261 1 FIG. 1 .-Schematic diagram (not to scale) of the cell for mixed vapour sorption studies: 1, McBain spring balance; 2, equilibrating mixture with magnetic stirring; 3 and 3’, sample bags; 4, auxiliary cell; 5, cold ‘finger’; 6, condensation U-tube.hung on the quartz spring and the other (3’) was placed in the auxiliary cell (4) directly in the stopper. The second bag always contained twice as much as the first one. The quartz spring was kept in a cylindrical container of the following dimensions: height 60 cm and diameter 4.0 cm. The auxiliary cell was made as small as possible (5.4 cm3) to minimize the contribution of the vapour phase to the total mass absorbed in it, which is determined by desorption. The desorption unit consists of a condensation U-tube (6) (total length 18 cm, diameter 0.8 cm) kept in liquid air and a cold finger ( 5 ) for sampling the sorbed liquid. PROCEDURE At 25 *C the spring (1) was loaded with a sample bag (3) containing ca.0.2 g of resin while another sample bag (3’) with 0.3-0.4 g of resin was placed in the auxiliary cell. The equilibration mixture was charged into the flask (2) and degassed by a conventional method. The sorption line was pumped out until constant elongation of the loaded spring (ca. 12-16 h) was reached. During this time the flask (2) was disconnected from the line. The sorption process was started by isolating the sorption system (1,4) from the vacuum line and opening the stopcock between the equilibrating mixture and the sorption chamber. During the process the mixture was stirred magnetically and the vacuum line was connected for a few seconds at convenient intervals to ensure the homogeneity of the vapour phase.Normally the equilibrium was obtained after 24-48 h. The auxiliary cell was then disconnected from the sorption balance and the mass absorbed by the sample bag (3’) was desorbed into the U-tube cooled with liquid air. This process was completed within 20 to 24 h, as previous tests made by us and other investigators’ have shown. All the condensate was distilled into the cold finger (at liquid-nitrogen temperature), 1 to 1.5 cm3 of dioxan were then added2612 SORPTION OF METHANOLfWATER VAPOURS to allow a chromatographic composition analysis in a F & M 810 chromatograph with a Porapak Q column. Further details may be seen elsewhere.ll RESULTS AND DISCUSSION The pure vapour sorption isotherms for water are well defined and without hysteresis effects for both resins.The isotherms are of type I1 (B.E.T. classification) and the B.E.T. plot is linear over the relative pressure range 0.025 < p / p ' < 0.4-0.5. On the other hand, in the case of methanol different behaviour is observed. The macroporous resin Amberlite 200 gives a well defined and reproducible hysteresis effect and the gel resin IR-120 (Na-form) shows a strong kinetic effect (very slow sorption), which depends on the previous treatment of the resin (number and extension of dehydration cycles, exposure time to high vapour pressure and/or to liquid phase, etc.) and hinders the collection of reproducible equilibrium data. The same resin IR-120 in H-form gives a normal sorption isotherm.All the methanol isotherms are also of type 11. For both water and methanol the only point of the isotherm which remains uncertain is that of the saturation state. A combination of two processes, as seen kinetically, appears at this state and probably corresponds to the normal sorption plus a capillary sorption between the beds. The pure vapour sorption isotherms are obtained directly by the gravimetric method and are shown in fig. 2 and 3. Since the main part of this work is not concerned with pure vapour sorption, details of the experimental data are omitted but can be consulted e1sewhere.l' Fig. 6 shows the smoothed curves, which represent the limiting cases of the mixed isotherms. The sorption of the vapour mixtures by the resin Amberlite 200 in the sodium form is studied only for rising sorption processes.When the equilibration process is carried out with pure liquid mixtures of water and methanol (without glycerol) saturation always occurs and the capillary sorption process is observed. In the case of liquid mixtures with glycerol, however, it is possible to obtain sorption curves without anomalies, where the equilibrium is well defined. As will be shown below the 15- 10- p/kPa FIG. 2.-Pure water vapour sorption isotherm of the macroporous Amberlite 200 resin in the sodium form at 25 OC, q; : moles of water (1) per equivalent of resin.R. CROVETTO A N D E. 0. TIMMERMANN 2613 6 4 ; 4 2 0 FIG. 3.-Pure methanol vapour sorption isotherm of the macroporous Amberlite 200 resin in the sodium form at 25 OC (rising branch).q; : moles of methanol (2) per equivalent of resin. TABLE 1 .-EXPERIMENTAL DATA FOR THE SORPTION OF MIXED WATER (1) +METHANOL (2) VAPOUR BY THE RESIN AMBERLITE 200 IN THE SODIUM FORM AT 25 OC P P a Y1 9 21 41 92 XR 4.66 0.3 15 3.52 0.519 1.83 1.69 0.221 6.93 0.305 4.45 0.623 2.76 1.69 0.183 6.93 0.175 2.82 0.350 0.99 1.83 0.262 9.66 0.185 4.74 0.473 2.23 2.5 1 0.174 9.92 0.085 2.72 0.256 0.70 2.02 0.269 12.52 0.085 3.91 0.269 1.05 2.86 0.204 saturation data can be obtained by extrapolation either at constant total vapour pressure or at constant vapour composition. The results for the mixed sorption are summarised in table 1, where p is the total vapour pressure, y, is the molar fraction ofwater in the vapour (glycerol is non-volatile), q is the total number of moles sorbed by an equivalent of resin, z, is the mole fraction of water in the sorbed mass, determined by chromatographic analysis, q1 (= z, q) is the number of moles of water sorbed by an equivalent of resin, q2[ = (1 -z,)q] is the corresponding number of moles for methanol, and, finally, x,[ = l/(q+ l)] is the real mole fraction of the resin.As may be seen from the data in table 1 there are pairs of points which have either equal vapour pressure or equal vapour composition. This allows a smoothed extrapolation to the saturation states. The coordinates of the saturation points for the sorption mixtures are obtained by interpolation in the vapour-liquid equilibrium diagram of water + methanol studied previously12 and are given in table 2, where is the mole fraction of water in the liquid water +methanol mixture in equilibrium at saturation.Fig. 4 is a plot of total sorption q against the mole fraction of the vapour phase at constant total vapour pressure and it includes data corresponding to the sorption of pure methanol (y, = 0) and of pure water ( y , = 1) vapours. Taking the data of table 2 into account a smooth extrapolation is made up to the composition of the vapour at saturation, drawing a family of similar curves at constantp. The extrapolated values of qsat (filled symbols) are given in section A of table 3. As fig. 4 shows, qsat goes through a minimum as the water content of the vapour is increased. The sorption of each component is studied in fig. 5(a) (water) and fig.5(b) (methanol). The two components show completely different behaviour, conditioned by the fact that in the case of water the total vapour pressure is always higher than2614 SORPTION OF METHANOL+WATER VAPOURS TABLE 2.-vAPOUR-LIQUID EQUILIBRIUM DATA OF THE WATER (1) -k METHANOL (2) SYSTEM AT 25 O C 1 2 4.66 0.635 0.935 6.93 0.385 0.815 9.79 0.225 0.620 12.52 0.125 0.385 13.72 0.085 0.275 10.95 0.180 0.525 8.06 0.310 0.740 0 0 0.5 1 (CH30H) (H-20) Y l FIG. 4.-Mixed water (1) +methanol (2) vapour sorption of the Amberlite 200 resin at 25 O C . Total sorption (moles per equivalent) q( = q, + q2) against composition of the vapour phase ( y , : mole fraction of water) at constant total vapour pressurep. Open symbols: experimental points (data of table 1); constant pressure (kPa): (a) 3.20, (b) 4.66, (c) 6.93, ( d ) 9.79, (e) 12.52.Filled symbols: extrapolation to vapour composition at saturation (see table 2). + : extrapolation data of fig. 6. p i , the vapour pressure of pure liquid water at 25 O C , while in the case of methanol p is always lower than p i , the vapour pressure of pure liquid methanol at 25 O C . As can be seen, the extrapolation procedure is easier for water than for methanol. Care was taken that the relation qsat = q:at + qiat was always fulfilled, where qsat and qiat (filled symbols in fig. 5 ) are the numbers of moles of sorbed water and of methanol, respectively, at saturation. They are also given in section A of table 3. Both partial sorption isotherms at saturation (dotted line) resemble isotherms of type I1 of the B.E.T.classification, where the knee is better defined for water than for methanol. Plots of the mixed sorption at constant vapour composition are shown in fig. 6 as a function of the total vapour presure. The mixed sorption isotherms are characterized only by two experimental points, but taking into account (a) the experimentalR. CROVETTO AND E. 0. TIMMERMANN 2615 12 10 8 41 6 4 2 12 10 8 42 0 0 0.5 1 0 0.5 1 (CH,OH) (H20) (CH3OH 1 ( H2O 1 Y l Y l FIG. 5.-Partial sorption of (a) water and of (b) methanol by the Amberlite 200 resin at 25 OC. Symbols and parameters as in fig. 4. plkPa FIG. 6.-Mixed water (1) +methanol (2) vapour sorption isotherm of the macroporous Amberlite 200 resin in the sodium form at 25 OC. q against total vapour pressure p at constant vapour composition y1 : (a) 1 .O, (b) 0.31, (c) 0.18, ( d ) 0.085, (e) 0.0.Open symbols: experimental points as in fig. 4 (see table 1). +: extrapolation to total vapour pressure at saturation (see table 2). Filled symbols: extrapolation data of fig. 4.2616 SORPTION OF METHANOL -l- WATER VAPOURS TABLE 3.-EXTRAPOLATED SATURATION DATA FOR THE SORPTION OF MIXED WATER (1) -I- METHANOL 200 IN THE SODIUM FORM AT 25 OC (2) VAPOUR (AT EQUILIBRIUM WITH THE CORRESPONDING LIQUID PHASE) BY THE RESIN AMBERLITE (A) Data obtained by extrapolation at constant total vapour pressure. PlkPa 3.20 4.66 6.93 9.79 12.52 16.78 13.5 8.3 6.0 5.75 5.9 6.35 13.5 7.3 4.4 3.4 2.5 - 1 .o 1.6 2.35 3.4 6.35 9;; a 1.25 1.45 1.75 2.05 2.35 6.35 - Pt qsat 9Yt Y1 0.00 0.085 0.18 0.31 I .oo qsat 9Yt (B) Data obtained by extrapolation at constant vapour composition.6.35 6.0 5.8 5.8 13.5 - 2.0 3.0 3.85 13.5 - 9Yt 6.35 4.0 2.8 1.95 a Sorption of pure methanol vapour. isotherms of the pure components, (b) the vapour pressure at saturation for each vapour composition (table 2) and (c) the saturation points obtained from fig. 4, continuous curves can be drawn for each of them and the extrapolation to saturation is then fairly straightforward (filled symbols). The results of this extrapolation process are given in section B of table 3. In the analysis of all the sorption data care was taken that internal consistency was always maintained. In fig. 6 the minimum at saturation is not so well defined as in fig. 4 because the abscissa range is much more extended.The shape of the mixed isotherms is, as it should be, also of type I1 of the B.E.T. classification. Although based on only a few experimental points, the present case is the first one in this formulation, as far as the authors know, in which mixed sortion isotherms are given at high vapour pressure up to saturation. To show the effect of the composition of the sorbed mass the most convenient representation is the classic ternary diagram given by fig. 7, taking mole fraction as the composition variable. Using the saturation data of table 3, the saturation line has been drawn, the corresponding equilibrium pure water + methanol liquid phases (table 2) are indicated and also the tie lines. The area between the saturation line and the bottom of the triangle diagram defines the region of heterogeneous equilibrium between two condensed phases (the resin and the liquid mixture) and the mixed vapour phase.The diagram has several points of interest. The evolution of the direction of the tie lines with increasing water content in the liquid phase shows a change of selectivity of the resin at saturation. The resin always prefers the minor component of the liquid phase. With methanol-rich mixtures water is selected and with water-rich mixtures methanol is preferentially taken up. The representation of the difference (zi-z:) of the mole fractions of one component of the two condensed phases in equilibrium, a kind of selectivity variable, against the composition of the liquid phase would give an S-shaped curve.Such curves were obtained by Ra0,13 who studied the sorption of water + alcohol and water + acetone mixtures on silica gel. Curves of this nature were explained, following Williams,14 as being due to the simultaneous sorption of both solvents. This is always the case in our system. The selectivity behaviour of an ion-exchange resin in mixed solvents depends on the characteristics of the resin, the nature of the counterion and the pair of solvents. If we compare our results with those corresponding to gel resins in the sodium form (Dowex 50)’ in water + methanol mixtures we must conclude that our results are dueR. CROVETTO A N D E. 0. TIMMERMANN R NJ3) 2617 CH,OH(2) 0.5 1 ) FIG. 7.-Ternary diagram of the mixed water (1) +methanol (2) vapour sorption of the Amberlite 200 resin (3) at 25 O C ; composition variable: mole fraction.Symbols and parameters as in the previous figures. Additional total vapour pressure data (kPa) of the tie lines: (g) 8.06, (h) 10.95, (i) 13.72, cf> 16.78. to the large sorption area of the macroporous Amberlite 200 resin. With the gel resin a change of selectivity (from water to methanol) is not observed, but it may occur at very low methanol content. This effe.ct may be enhanced by a larger sorption area, although it is not observed with the macroporous Katex PX8 resin in water + ethano1,I5 perhaps because ethanol is not so competitive with water as methanol. Considering now the two-phase (resin-vapour) region above the saturation line, fig. 7 shows that the resin selects methanol in the presence of methanol-rich vapours at constant vapour composition if the total vapour pressure is decreased.On the other hand, isotherms at constant total vapour pressure are practically linear in the ternary diagram. An attempt to correlate the behaviour at constant vapour composition with the ideal theory of Myers-Prausnitz2 at very low total vapour pressures failed, as this theory predicts a selectivity from the vapour phase which is the opposite of that observed in fig. 7. In conclusion, some general trends of mixed vapour sorption by ion-exchange resins are shown. Although the discussion is based only on a few experimental points, which may not completely justify our diagrams, it is felt that the results may be valuable for other groups involved in research on this topic, since we are no longer able to continue this work. l J. Herlihy, Ph.D. Thesis (University of Washington, Seattle, 1968). * A. Myers and J. Prausnitz, AICHE J., 1965, 11, 121. * H. P. Gregor, D. Nobel and M. H. Gottlieb, J. Phys. Chem., 1955, 59, 10. C. W. Davies and J. J. Thomas, J . Chem. SOC., 1952, 1607. R. W. Gable and H. A. Strobel, J. Phys. Chem., 1956, 60, 513. G. L. Starobinets, L. V. Novitskaya and L. I. Sevostyanova, Russ. J . Phys. Chem., 1968, 42, 575. P. G. Hall and S. A. Miiller, J. Chem. SOC., Faraday Trans. I , 1978, 74, 948. 'I D. Nandam, A. R. Gupta and J. Shandar, Indian J . Chem., 1972, 10, 83. 85 FAR 12618 SORPTION OF METHANOL+WATER VAPOURS * F. Helfferich, Ion Exchange (McGraw-Hill, New York, 1962). lo K. Kunin and R. Kun, J. Polym. Sci., Part C, 1967, 16, 1457; K. Kunin, Amber-hi-lites No. 127 and l1 R. Crovetto, Ph.D, Thesis (Universidad de Buenos Aires, Argentina, 1974). l2 R. Crovetto and E. 0. Timmermann, An. Asoc. Quim. Argent., 1979,67, 51. l3 B. S. Rao, J. Phys. Chem., 1932, 36, 616. l4 A. M. Williams, Trans. Furaday SOC., 1914, 10, 167. l5 M. Simek, Collect. Czech. Chem. Commun., 1970, 35, 2275. 128, 1972 (Rohm and Haas Co.). (PAPER 1/724)

ISSN:0300-9599    

DOI:10.1039/F19827802609

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982,78, 2619-2630 Counter-ion Intradiffusion in Sulphonic Ion-exchange Systems BY ERNESTO 0. TIMMERMANN Instituto de Investigaciones Fisicoquimicas Te6ricas y Aplicadas (INIFTA), Sucursal 4, Casilla de Correo 16, (1 900) La Plata, Argentina Received 6th May, 1981 The intradiffusion data of the sodium counter-ion in different sulphonic ion-exchange systems in the. sodium form (from linear polystyrene to commercial membranes of graft copolymers of styrene) are analysed as a function of q, the number of water molecules per ionic group. These data represent experimental results from a considerable number of workers during the last three decades. A general comprehensive picture is obtained, which shows how individual characteristics (water content, exchange capacity, crosslinking and inert material added) affect in a gradual and congruent way the value of the intradiffusion coefficient.The comparison with simple electrolyte systems such as aqueous sodium chloride and sodium p-ethylbenzene sulphonate shows not only a clear similarity, which shows that the transport mechanism is of the same type in all these systems, but also a clear differentiation owing to the polymeric and polyelectrolytic character of the ion-exchange systems. The complex nature of the obstruction to ion movement (tortuosity) is evidenced. The overall behaviour observed leads to the view that it is realistic to consider ion-exchange systems as homogeneous hydrated gels, since intermolecular interactions between charged sites, counter-ions and water are very important.Consequently, it is more convenient to study the transport properties as functions of the swelling variable q than in terms of the commonly used stoichiometric volume fraction of polymer. For several systems a linear relation between In I&,+ and 1 / q holds, within the range 2 < q < 70. The effect of cross-linking and of inert material on these relations is discussed. Furthermore, a similar analysis leads to the verification of a previously obtained linear equation between In @& and a,,, the thermodynamic activity of water. A large number of experimental studies1 of intradiffusion of small ions and other transport properties in different types of synthetic organic ion-exchange materials have been made since Boyd and Soldano2 published their now classical work on intradiffusion of counter-ions in sulphonated polystyrene cation-exchange polymers.In spite of the very important amount of empirical data accumulated since their study, several aspects of the transport phenomena in ion-exchange systems are still incompletely understood and open to discussion. One of the questions is: which is the appropriate composition variable or function that represents best the different factors affecting the transport quantities in these systems? These factors are: contents of water and simple electrolyte, ion valency, ionic form and exchange capacity, volume fraction of resin and of inert material, and degree of cross-linking. The purpose of this study is to introduce a new alternative for the composition variable.This new formulation is derived from a function recently proposed by Fernandez-Prini and phi lip^,^ based on volume fractions. Adequately simplified, this function turns out to be much more versatile, taking indirectly into account all the factors mentioned above. In the present paper we consider the intradiffusion coefficient Dk,+ of sodium counter-ions in several sulphonic ion-exchange systems in the sodium form, since for this case extensive experimental data are available. The result is a general comprehensive picture of the intradiffusion of sodium ions in these systems in the absence of sorbed simple electrolytes at different water contents, 2619 85-22620 SULPHONIC ION-EXCHANGE SYSTEMS exchange capacities, volume fractions of the macromolecular components and degrees of cross-linking.Moreover, it is shown that our earlier formulation4 based on the thermodynamic activity of water, although not so general, is still useful and gives important complementary information. THEORETICAL Fernandez-Prini and Philipp3 have found, based on some theoretical considerations, that in polystyrene sulphonate (PSS) ion-exchange systems the logarithm of the intradiffusion coefficient Dt of the counter-ion i depends linearly on the ratio bP/(l -4,) [=f(4,)], where 4, is the volume fraction of the polymer. It has been found earlier4 that In DZ is a linear function of the thermodynamic water activity a, (= p/po). Thus, it must first be decided whether both formulations are equivalent or if one is more fundamental and/or adequate.Independently of the interpretations given for each relationship, it may at once be concluded that a linear dependence between a, andf(4,) must hold. The existence of this correlation, a type of water sorption isotherm, is readily confirmed in the case of aqueous linear NaPSS in the range of interest (a, > 0.45), since data for partial molar volumes for this system are known.5 Other features of this isotherm will be discussed in another context. In general, however, volumetric data are not available in ion-exchange systems, except in some cases in the dry state; hence, in order to calculate bP in these systems some assumptions have to be made,3 and sof(&) will not be known with the desired accuracy. To overcome this difficulty an alternative function tof(#p) may be defined which is directly accessible (and usually determined) in experimental work. The thermodynamic expression for 4, is by definition where & and & are, respectively, the partial molar volumes of the equivalent unit of the sulphonated polymer and of water, np and no the corresponding mole numbers and q( = no/np) the water content or swelling ~ariable,~ i.e.the stoichiometric number of water molecules per equivalent of ion-exchanger. From eqn (1) it is immediately (2) derived that which shows that the function used by Fernandez-Prini and Philipp is directly proportional to 1 / q with a proportionality factor of V,/ &. For aqueous NaPSS5 and for swollen ion-exchangers61 & and 5 are almost constant for q > 2 and appreciably different from the corresponding values in the dry state (q = 0).So, instead off(d,), the inverse of q may be used as the composition variable for analysing transport phenomena in ion-exchangers. The inverse of q is equal to the molality mp of the polymer multiplied by the molar mass M, (kg mol-l) of the solvent. As only concentrated systems are of interest here the water content q gives a better physical description of the composition of the systems than m,. Thus the former will be used throughout. f(4p) = 4 p N -4,) = (&/G)(l/d RESULTS AND DISCUSSION EFFECT OF WATER CONTENT AND OF CROSS-LINKING In fig. 1 the experimental results available for I&,+ in synthetic ion-exchange systems are represented in a semilogarithmic form as a function of q.They correspond to sulphonic systems (the two lower curves) for which the water content is known orE. 0. TIMMERMANN 3 262 1 * o o q 0.4 I I I I I 1 0 10 20 30 40 50 60 4 FIG. 1 .-Intradiffusion coefficient DLa of the counter-ion in sodium sulphonic ion-exchange systems at 25OC. (a) Curve 3, NaPSS: 0, aqueous linear sodium polystyrene sulphonate solution^.^ (b) Curve 4, NaR, heterogeneous resins and membranes : (i) sulphonated polyethylene-styrene graft copolymers (sPES): V, C-103, C-1O4,l2 C-1O3C,l3 CK-l,14 C-60N, C-60E;16 (ii) formaldehyde-phenolsulphonic acid resins (FPS): x , A, ., C, both ref. (16), +, ref. (12), 'I, ref. (17); (iii) sulphonated perlluoroethylene membrane: *, Nafion 1 2O.ls (c) DVB-cross-linked polystyrene sulphonate resins (PSS): a, 2.3 % DVB,3.20 0, 2.39%,18 0, 1.7, 4 and 8%,s A, 4(?), 8.6, 16(?), 16, 24% and D-50(12%),12 8%.11 Intradiffusion coefficient DLa of the sodium ion in sodium comparison systems at 25 OC.(a) Curve 1, NaCl: A, aqueous sodium chloride solutions.8 (b) Curve 2, NapEBS: 0, aqueous sodium p-ethylbenzene sulphonate solutions.s (---) OK. could be estimated and to systems usually taken as a comparison (the two upper curves). The characteristics of these systems are given at the end of the paper with the corresponding references. Curve 1 represents Ilk,+ in aqueous sodium chloride solutions8 and curve 2 in aqueous sodium p-ethylbenzene sulphonate (N~PEBS),~ the model compound proposed by Boyd et aZ.l0 Curve 3 corresponds to Ilk,+ in aqueous sodium polystyrene sulphonate (NaPSS),4 while the discontinuous curve 4 gives D&,+ in various different heterogeneous sulphonic ion-exchange resins or membranes (NaR). Results for Dk,+ in several sulphonated polystyrene resins2* 9 9 l1 with different degrees of cross-linking with divinylbenzene (DVB) fall between these two curves, but always nearer the PSS curve.The data labelled with a number (nominal percentage of DVB of cross-linking) belong to PSS resins and those labelled with letters (characteristics of manufacture) to commercial resins or membrane^.^^-^^ These data correspond practically to the saturation state of each system in pure water, as the Donnan invasion of NaCl was always taken as negligible. On the other hand, unlabelled points belong to systems studied with isopiestic regulation and have water activities < 1.3*4-18 The ion-exchange systems included in fig.1 are all of the sulphonated polystyrene type, except those at higher water content (q > 30) of curve 4 which are formaldehyde-phenolsulphonic acid resins and one point which corresponds to a Nafion membrane (sulphonated perflu~roethylene).~~2622 S U L PHONIC I ON-E X C H A N G E SYSTEMS There is a clear parallelism between the four curves given in fig. 1. It may therefore be concluded, at a first instance, that the transport mechanism for intradiffusion of the sodium ion is the same in all the systems considered and furthermore that the mechanism in ion-exchange systems is definitely of the same type as that in aqueous solutions of simple electrolytes of moderate and high concentrations (mp > 0.5, q < 70).It may also be said that a different degree of ‘obstruction’ is present in the three lower curves compared to aqueous sodium chloride with the same water content: going from curve 1 to curve 4 the effect of ‘obstruction’ is progressively determined by the more voluminous, but of lower charge density, pEBS anion (curve 2), by the additional elongation of the anion by polymerization, which again increases the charge density per equivalent unit of the polyanion (electrostatic polyelectrolyte effect) (curve 3)’ and by the further additional obstruction of the inert material present in the heterogeneous systems (curve 4). It is by this last effect, other factors being constant, that in the PSS resins Ilk,+ gradually moves from curve 3 to approach curve 4 with increasing degree of cross-linking, as the fraction of the polymer directly affected by the cross-linking (usually not sulphonated) acts as inert material.These obstruction factors affecting the polymeric systems (curves 3 and 4) are thus the main causes of what is known as the tortuosity effect in membranes and resins2’ REPRESENTATION AS A FUNCTION OF THE NEW COMPOSITION VARIABLE, 1 / 4 In fig. 2(a) the data of fig. 1 are plotted against 1/q. As expected from the results obtained usingfl4,) and from eqn (2), the four curves of fig. 1 now become straight lines. However, fig. 2(a) indicates that there is a clear differentiation between the 200 70 I- k 10- \ t.2 7 - 9 L - 2 - 1 - 0.7 - 0 0.4 - I I 1 I 1 1 1 14 0.5 0.L 0.3 0.2 0.1 0 t I I I I I FIG.2.-Intradiffusion coefficient DLa of sodium ion in sodium sulphonic ion-exchange and in sodium comparison systems plotted against 1 / q at 25 OC. Symbols and nomenclature as in fig. 1 . (a) Curve 3‘, NaPSS resin, 2.3% DVB.3.20 Curve 5, NaPSS resins at different degrees at cross-linking and at saturation in water (constant activity of water a, x 0.99). (6) Curve 5, effect of desulphonation of PSS resins of 8.6% (i) and 16 % (ii) DVB.22E. 0. TIMMERMANN 2623 polymeric systems (curves 3 and 4) and the comparison systems (curves 1 and 2). Thus, although the NapEBS values of DL,+ approach those in ion-exchangers, the overall behaviour of the solutions of the model compound of Pikal and Boydg is still that of a simple electrolyte solution.The limiting values of the two upper lines at l/q = 0 are the same, and are practically equal to Dpa+, the trace diffusion coefficient of the sodium ion in pure water, while extrapolation of the two lower lines gives different values, which are much lower than Ora+. This difference is caused by the fact that the usual idea associated with the term ‘infinite dilution’ is not applicable to polyelectrolyte solutions, as the charged sites of the polymer chain may not be separated infinitely. In the case of resins and membranes (the DVB-cross-linked PSS resins as well as the heterogeneous systems of curve 4) the l/q = 0 condition is not accessible because cross-linking makes it impossible to separate the chains of the polymeric network beyond a certain limit; thus, typical polyelectrolyte behaviour4v l 8 9 23 is usually not observed in resins and membranes except at very high water contents, such as those exhibited by the FPS resins.ls In conclusion, the linear relations obtained in fig.2(a) should not be extended in the case of ion-exchange systems to over-high water contents, and the limit may be set at q w 70 (i.e. mp > 0.5-0.6). Besides this clear distinction between polymeric systems and systems of small particles, fig. 2(a) also illustrates some other important features of the diffusional behaviour of the sodium counter-ion in sulphonic ion-exchange systems. First, solutions of linear NaPSS (curve 3) are clearly the upper limiting states of these systems. All the results for DL,+ in DVB-cross-linked PSS resins fall below curve 3 (the exception of the 2.4% DVB resin18 will be considered later).Secondly, D&,+ in resins at constant degree of cross-linking and different water contents shows similar behaviour to that in linear NaPSS. The results for the 2.3% DVB resin3 fall slightly below curve 3 in fig. 1, and give an acceptable straight line (line 3’) in fig. 2(a) with a slightly smaller (negative) slope than line 3. This is the effect of the cross-linking, but the transport mechanism is of the same type in both systems. Finally, the results for the other PSS resins at different degrees of DVB-cross-linking must be considered. For these resins only one result for each degree of cross-linking is available, and it corresponds practically to the saturation state of the resins in pure water.The nominal degree of DVB-cross-linking in these gel-type resins varies from 1.7% to 24%, and the maximum water uptake decreases from q = 30.8 to q = 4.24, while the exchange capacities vary only slightly ( < 10 %). DL,+ in these resins decreases more drastically than in linear PSS solutions with decreasing water content because of the increasing degree of cross-linking. All the data, with the exception of two values (4X2 and 8%11 DVB), are well represented in fig. 2(a) by curve 5, which at low cross-linking (and therefore higher maximum water uptake) approaches asymptotically the straight line 3 for NaPSS (0% DVB) as l/qmax --* 0, an effect already observed by Pikal and Boydq9 Curve 5 is thus representative for the end points (maximum water content) of the straight lines of types 3 and 3’ corresponding to the variation of In Dka+ with l/q at different constant cross-linking.Therefore it may be concluded that the diffusion data for the ion-exchange resins correlate well with the diffusion results in linear PSS solutions and that curves 3 and 5 define the region within which the results for PSS resins at different percentages of DVB and different water contents should be found. Some of the exceptions mentioned above were considered by the original authors9 to be caused by an error in measuring the DVB content or to a mis-identi- fication of the resin lots by the manufacturers. The ‘16% DVB’ resin behaves as though it contained much more DVB, whereas in the case of the ‘4% DVB’ resin some other anomalies must also be present.2624 SU L PHONIC ION-EX CHANGE SYSTEMS Curve 4 and line 4 correspond to heterogeneous sulphonic resins and membranes of different chemical constitution.The transport properties of these systems have never been jointly considered in the literature, and fig. 1 and 2(a) show that, in the case of D&,+, these fit into the developed picture nicely. Besides the polyelectrolyte effect, typical of the PSS-systems so far considered, these materials include appreciable amounts of inert substances (allowing a higher mechanical, and chemical, resistance), which cause a marked decrease in Oha+ at constant water content q. Therefore it is surprising that results for sulphonic systems made by different manufacturers using quite diverse polymeric materials, such as grafted polyethylene-styrene, formalde- hydephenol and perfluoroethylene, respond to the same linear law when plotted against 1 / q (= l/qmax).In fact one might have expected these systems to be positioned in an irregular manner below curve 5. This is why curve 4 and line 4 have been drawn discontinuously, as they do not correspond to a single type of ion-exchange material. The most important conclusions to be gained from the observed behaviour are (1) that the principal factor determining the mobility of the counter-ion in these membranes is the hydration of the common -SO,Na group; (2) that these systems all present an equivalent obstruction effect owing to inert material; and (3) that the chemical nature of the polymeric backbone is of secondary importance.This is also true for the Nafion membranes, for which the PSS systems have been considered as inappropriate ana10gues.l~~ If a comparison is made with the CK-1 or C103 C membranes, which present similar degrees of hydration and inert content, the equivalence leaves no room for doubt. EFFECT OF EXCHANGE CAPACITY The exchange capacities of the PSS systems so far considered are high (within 90-100% of the theoretical values) and may be taken as roughly constant, so that no effect of the capacity on Dk,+ should be detected. Boyd et a1.22 investigated the effect of desulphonation of some resins on Oh,+. Their data are given in fig. 2(b), where for comparison the lines of fig. 2(a) are also represented. They correspond to an 8.5% DVB resin [curve (i)] and a 16% DVB resin [curve (ii)].These results fit into our picture well. At low degrees (< 30%) of desulphonation a decrease in the exchange capacity only has the effect of increasing the water uptake, which diminishes the obstruction effect of cross-linking and allows an increase in Oh,+ along curve 5 . At degrees of desulphonation > ca. 30% a relatively sharp decrease in I&,+ is observed, although the maximum water uptake is still increasing. This diminution of Ok,+ may be explained by the fact that at this stage a considerable part of the polymer has been converted by the desulphonation process into hydrophobic material, which exerts a more drastic obstruction to ionic diffusion than simple cross-linking between sul- phonated organic chains.In other words, polyelectrolytic character is beginning to be lost, and it seems correct to accept that some incipient ‘heterogeneity’ is beginning to appear, especially as Oka+ tends to present values which are characteristic of the heterogeneous ion-exchange membranes of line 4. DEPENDENCE ON WATER ACTIVITY, a, In order to complete this analysis some consideration about our earlier formulation4 will be made. It is evident that a representation of lnDk, (or any other quantity) as a function ofa,, the thermodynamic water activity, ismore limited than a representation as a function of 1 / q , as the former can only be applied if data for water contents below that corresponding to saturation (qmax, a, = 1) are available and also because the saturation data of different systems cannot be correlated in this way.Among more recent publications only Fernandez-Prini and Philipp, used the isopiestic method for the water-content regulation, and, as sorption-isotherm data become available,20 it is10 0 6 4 4 2 0 E. 0. TIMMERMANN 1 loo, 2625 0 FIG. 3.-Intradiffusion coefficient Dka of the sodium counter-ion in NaPSS systems plotted against a,, the thermodynamic activity of water at 25 O C . (a) Water sorption isotherm of the NaPSS systems: 0, linear NaPSS, cap. 4.70 mmol g-1;4 a, 2.3% DVB resin, cap. 4.58 mmol g-1;3320 (>, 2.39% DVB resin, cap. 4.81 mmol g-'.l8 (b) InDL, plotted against a,; symbols and systems as in (a). possible to analyse their diffusion data on the a,-scale and compare them in this way with our earlier result^.^ In fig. 3(a) the sorption isotherms of the three systems studied by this method are given. While the 2.3% DVB resin3 sorbs more water than linear PSS,4 the 2.39% DVB resinla takes up markedly less, although they correspond to practically the same degrees of cross-linking.Because of this low water uptake the results4? la of DLa for this resin are the only ones which do not follow the general behaviour observed in fig. 2(a), and this is also the case for the two plots {as functions of log,,[( 1 - 4p)/( 1 + iP)l2 and (dp/ 1 - q&)} analysed by Fernandez-Prini and Philipp. [If, instead, the water contents of the 2.3% DVB resin at the corresponding a, are taken, these Ilka data will be displaced to the right in fig. 2(a) and will almost fall on line 3, joining the other results for PSS resins of similar degrees of cross-linking.] On the other hand, by interpolation on the sorption isotherm using the formula q = 8.43 (1 -4p/4p)20 the water activities corresponding to the diffusion data of the 2.3% DVB resin may be obtained.In fig. 3(b) In DLa is plotted against a, for linear PSS and the two resins. The data of the 2.3% DVB resin fall below those of linear PSS and are practically also linear in a, with the same slope (within the uncertaintly of the sorption isotherm and of the interpolation process) as for soluble PSS. Thus our earlier conclusions4 for CsPSS systems are now confirmed for NaPSS systems: the cross-linking only displaces the diffusion coefficient to lower values with respect to linear PSS, but does not affect the slope, i.e.the transport mechanism. The data for the 2.39% DVB resin follow the same law and are in this representation much more compatible with the other data than in the plot against l/q, although still a little too high.2626 S U L PHON I C I ON-E X C H A N G E SYSTEMS SOME NUMERICAL EVALUATIONS The linear relationships observed in fig. 2(a) and 3 (b) may be represented by the following equations : a, scale [fig. 3 (b)] Dt = Dl exp ba, l / g scale [fig. 2(a)] DT = DL exp - b’/q. (3) (3’) b and - 6’ are the slopes of the straight lines in fig. 3 (b) and 2(a), respectively, with DA and DL the corresponding pre-exponential factors. DJ corresponds to a hypothetical dry state of the exchanger having the same structure as in the swollen state and DL to a hypothetical infinitely swollen state of the resins of the same characteristics, as both equations are not valid at the limits a, = 0 and l/q = 0, respectively.The values of the four constants were obtained by a least-squares method and are given in table 1 for the sodium ion in several ion-exchange systems. In some cases the least-square adjustment has been made in two ways: (a) without restrictions and (6) with special consideration of some contour condition. TABLE PARAMETERS OF EQN (3) AND (3’) linear PSS PSS resin 2.3% DVB PSS resin 2.39% DVB PSS resin 24% DVB RNa 4.64 x 9.07 7.35 x 10-6 (1) 3.1 x 10-lo 9.4 (1’) 1.8 x 10-5 (2) 3.8, x 10-lo 9.1a (2’) 7.4x lO-6b - - (3’) 7.35 x 10-sa (1) 7.6 x 10-lo 8.3 (2) 4.4 x 10-10 9.1a - - - 7.35 x 10-6a - - 5.34 x 10-6 - 12.79 line 3 18.9 14.4c line 3’ 14.0d line 3’ 23.3 line 4 a Value fixed equal to the value for linear PSS; b y c calculated with eqn (5’) and (3, respectively (see text); calculated with eqn (6) (see text).The following facts may be stressed: (1) In the case of fig. 3(b) the lines corresponding to the resins have also been evaluated by fixing the slope equal to the value for linear PSS. The differences between both procedures are not significant, but the dependence of DJ on cross-linking is better established in the second case. (2) In fig. 2(a) line 3’ has been analysed freely and made compatible with curve 5, which represents the variation of Dk,+ at saturation with the maximum water uptake of PSS resins of different degrees of cross-linking. As the values of Dga, and of gmax for this resin are not known, they may be estimated by the following procedure.Curve 5 may be represented analytically by the expression DIat = DL exp - (b”/q,,, + c”/q&,,) (curve 5). (4)E. 0. TIMMERMANN 2627 By fixing the value of DL equal to the value obtained for linear PSS (DL = 7.35 x cm2 s-l) b” and C” were evaluated by a least-squares method and their values are: b“ = 13.2 C” = 21.26. (4’) As qmax is a complex (decreasing) function of the nominal percentage of DVB in the PSS resin, an interpolation of a graphical representation of qmax against %DVB, using data for the systems analysed in fig. 2(a), allows one to obtained qmax z 27 for the 2.3% DVB resin. The corresponding value of Dgat, given by eqn (4), is Diat = 4.38 x cm2 s-l (2.3% DVB resin, Na form, qmax = 27).(3) By introducing the saturation state eqn (3’) may be rewritten as The value of constant b’ may be determined from eqn ( 5 ) and the value of DL from the pre-exponential expression of eqn (5’). For the 2.3% DVB resin, as table 1 shows, this value of DL is practically the same as for linear PSS, while the value obtained without restrictions is higher than the value of the diffusion coefficient of the sodium ion in pure water (1.33 x lop5 cm2 s-l), which is physically unsound. b’ is also reduced and is in better agreement with the value for linear PSS. (4) The former conclusion is interesting, as it allows us to state (a) that the constant DL in eqn (3’) may be considered as independent of cross-linking in pure PSS resins and (b) that the value of slope -b’ may be obtained directly by the pre-exponential expression of eqn (5’) if Diat and qmax are known: 6’ = qmax In DL/Diat.This relation gives a b’-value of 14.0 for the 2.3 % DVB resin and of 18.2 for the highest cross-linked resin (24% DVB, DBat = 0.1 x cm2 s-l, qmax = 4.24).2 An increasing effect of cross-linking on b’ is observed, a fact which is consistent with the whole picture developed here. Finally, considering line 4, corresponding to heterogeneous systems, we see that the inert material has a strong influence on both constants, DL and b’ (table 1). Thus the independence of DL from cross-linking is restricted to pure PSS resins with high (near theoretical) exchange capacity, although further experimental results are needed to support this conclusion.FINAL CONSIDERATIONS The entire diffusional behaviour of the sodium counter-ion in hydrated sodium sulphonate ion-exchange systems, as shown in fig. 1 and 2, presents a smooth and gradual transition in passing from soluble PSS solutions to heterogeneous technical membranes. This behaviour can only be explained by accepting that the physico- chemical structures of the different systems involved are closely related and that these structures are of the hydrated-gel type. Transport in hydrophilic gels has recently been analysed by me are^^^ and we will use some of his arguments here. In ionizable polymers the first water molecules to enter hydrate the ionogenic groups,25 and consequently the exchangeable counter-ion loosens from the oppositely charged radicals of the polymer backbone and becomes able to migrate within the swollen polymer.At relatively high exchange capacity the ionizable groups are sufficiently close that water clusters around them may overlap at a certain relative humidity and form an aqueous phase interpenetrating the polymeric network. The2628 SU L PHON I C ION-EX C H ANGE SYSTEMS swollen polymer then constitutes a gel, and if there are no constraints due to cross-linking, as in the case of linear PSS, it may completely dissolve at high water contents. The enhancement of mobility of water molecules and of the loose counter-ions with increasing hydration is a cooperative phenomenon involving both constituents of the medium, water and polymer. In gels the polymer chains with ionizable groups must be regarded as osmotically active species dispersed at high concentration in the swelling solvent and are the principal constituents of this pseudophase within which transport phenomena take place.On the other hand, the cross-linking chains, usually not sulphonated, and the added inert polymers form the hydrophobic pseudophase of the gel. Both parts are intimately intermixed and linked by the polyelectrolytic fraction of the whole polymer. It becomes clear, therefore, that the stoichiometric volume fraction of the polymer, which includes the polyelectrolytic fraction, may not be considered as a proper variable for these systems and that q, the water molecules per ionizable group, is a more appropriate and representative quantity in this sense. In conclusion, the analysis on the l / q scale allows appropriate interrelation of the interdiffusion data for the sodium counter-ion in very different sulphonic ion-exchange systems in the sodium form.These data represent almost all experimental results from a considerable number of workers during the last three decades and which have remained rather unconnected in the literature. Thus, a general comprehensive picture is obtained which embraces all these data and shows how individual characteristics (water content, exchange capacity, cross-linking and inert material) affect in a gradual and congruent way the value of the intradiffusion coefficient of the counter-ions. On the other hand the conclusion that water activity is also an important variable in regulating the diffusional mobility of the counter-ions offers a congruent thermo- dynamic argument4 for regarding these systems as being made up of one unique homogeneous phase, in contrast to the usual interpretation of a two-phase structure, namely, the porous macromolecular network and the pore liquid.It also favours the cooperative phenomena between water and polymer already mentioned, For linear PSS eqn (3) is valid for 0.4-0.5 < a, < 0.9, the range within which the partial molar volumes are also ~ o n s t a n t . ~ Cross-linking extends the upper limit (by decreasing the maximum water uptake) towards a, = 1, a value which is reached at ca. 4-5% DVB. The lower limit corresponds to q z 2, i.e. after the first water molecules have entered.5* 25 Thus all these characteristics correspond to the region where the hydration shells are being completed, once the hydration nucleus has been formed, and the water clusters are beginning to overlap and stabilize the homogeneous gel structure.In conclusion, the a, representation, although more limited, is still useful and gives important complementary information. Theoretical interpretations of eqn (3) and (3’) may be made in terms of the absolute-rate theory26T or in terms of the free-volume 28+ but since both theories present quantities whose physical meanings are rather obscure and still open to discussion, such an analysis cannot be made in a few paragraphs and will be left to a future publication. In a later paper other transport quantities of these systems, such as electrical conductivity, electro-osmotic transport of water and permeability (interdiffusion), will be considered as well as those corresponding to these systems in the caesium form. Although in general the experimental data available are less numerous, similar behaviour to the case described here is found, indicating that these are general characteristics of sulp honic ion-exchange systems.E.0. TIMMERMANN 2629 The communication of additional data on the resin used by R. Fernandez Prini and M. P. Philipp is gratefully acknowledged. This work is part of the research program of the Electrochemistry Division of INIFTA, sponsored by the Universidad Nacional de La Plata (UNLP), the Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET) and the Cornision de Investigaciones Cientificas de la Provincia de Buenos Aires (CIC).( a ) F. Helfferich, Zon Exchange (McGraw-Hill, New York, 1962); (b) P. Meares, Transport in Ion-exchange Polymers, in Diflusion in Polymers, ed. J. Crank and G. S. Park (Academic Press, London, 1968), chap. 10; (c) N. Lakshminarayanaian, Transport Phenomena in Membranes (Academic Press, London, 1969). G. E. Boyd and B. A. Scoldano, J . Am. Chem. SOC., 1954, 75, 6091. PSS-resins: 4% DVB (cap.: 5.32 mmol g-I HR), 8.6% (5.25), Dowex 50x8 (5.20), 16% (5.10), 16% (5.16), 24% (4.36) (see table 8); q(NaR) is taken as 0.8 q (HR) (table l), relation deduced from data given in ref. (7). R. Fernandez-Prini and M. Philipp, J . Phys. Chem., 1976, 80, 2041. NaPSS resin, 2.3% DVB. E. 0.Timmermann, Z . Phys. Chem. (N.F.), 1970,70,195. Linear NaPSS, cap. : 4.70 mmol g-I NaPSS. E. 0. Timmermann, Z . Phys. Chern. (N.F.), 1970, 72, 140; as ref. (4). H. P. Gregor, B. R. Sundheim, K. M. Held and M. H. Waxman, J. Colloid Sci., 1952, 7, 51 1. ( a ) R. Mills, Rev. Pure Appl. Chem., 1961, 11, 78; (b) J. Anderson and R. Paterson, J . Chem. SOC., Furaday Trans. I , 1975, 71, 1335. H. J. Pika1 and G. E. Boyd, J . Phys. Chem., 1973,77,2918. Diffusion data for NapEBS and for NaPSS resins: 1.7% DVB (cap.:5.12 mmol g-I NaR), 4% (4.73) and 8% (4.82), q = l/NMo; external solution: 0.2 mol dm-, NaNO,, 0.02 mol dmP3 NaCl, 0.002 mol dm-, ZnC1,. G. E. Boyd, J. W. Chase and F. Vaslow, J . Phys. Chem., 1967, 71, 573; G. E. Boyd, F. Vaslow, A. Schwarz and J. W. Chase, J .Phys. Chem., 1967, 71, 3879. l 1 G. E. Boyd, J. Phys. Chem., 1974, 78, 735. Dowex 50.8% DVB, c(NaR) = 3.0 mmol cmP3 (wet), external solution: 0.1 rnol dm-, NaCl; q x 11.4 [ref. (7)]. l 2 N. Lakshminarayanaiah, J. Phys. Chem., 1970,74, 2385; N. Lakshminarayanaiah and F. A. Siddiqi Z . Phys. Chem. (N.F.), 1972, 78, 150. sPES-membranes: AMF C-103 [cap.:0.92 mmol cmP3 (wet)], q = cJcI = 10.27; AMF C-104 (l.lO), q = 7.37. FPS membranes: PSA (0.98), q = 39.0. External solution : 0.01 mol dmP3 NaCI. l 3 E. M. Scatergood and E. N. Lightfoot, Trans. Furaday Soc., 1968,64, 1135. sPES membranes: AMF C-103C [cap.: 1.03 mmol cm-3 (wet)], q = c,/c, = 13.20; external solution: 0.1 mol dm+ NaC1. l4 Y. Prigent, A. Poiteau and A. Chemla, J . Chim Phys., 1974,71, 75; 1975,72, 57.NaPSS resin: Asahi CK 1 (8% DVB), q = c,/cxa = 13.25; external solution: 0.01 rnol dm-3 NaCl. l5 H. Ferguson, C. R. Gardner and R. Paterson, J . Chem. Soc., Faruduy Trans. I, 1972,68,2021. sPES membranes: AMF C60N (cap.: 1.57 mmol ggl NaR), q = c3/c1 = 19.8, AMF C60E (1.70) q = 26.8; q values extrapolated to zero external concentration of NaC1. l6 A. 0. Jakubovic, G. J. Hills and E. A. Kitchener, J. Chim. Phys., 1958, 55, 263. FPS resins: A (cap. : 2.64 mmol g-' HR), C (1.42), q = l/m, M,. l7 W. J. McHardy, P. Meares and J. F. Thain, J. Electrochem. SOC., 1969, 116, 920. FPS membranes: Zero-Karb 3 15 {cap. : 1.45 mmol g-' NaR [ = 0.486/( l-O.665]}, q = 76 (= 0.665/ 18 x 0.486 x DLa value: extrapolated to zero concentration of NaBr (fig. 6). See also P. Meares J . Chim. Phys., 1958, 55, 273; Zeo-Karb 315 (cap.: 1.28 mmol g-' NaR), q = 92, DLa = 2.35 x cm2 s-l. A. E. Lagos and J. A. Kitchener, Trans. Faraday Soc., 1960, 56, 1245. NaPSS resin 2.39% DVB. cap. : 4.8 1 mmol g-' NaR. Is (a) H. L. Yeager and A. Steck, Anal. Chem., 1959,51,862. Sulphonated perfluoroethylene membrane: Nafion 120 (cap. : 0.83 mmol g-' HR) q = 11.9. (6) H. L. Yeager and B. Kipling, J. Phys. Chem., 1979, 83, 1836. Same membrane, diffusion data: DL, = 0.944 x 10+ cm2 s-l; external solution: 0.05 mol dmP3 NaCl. 2o R. Fernandez-Prini, personal communication. NaPSS resin of ref. (3); cap. : 4.58 mmol g-' NaR, p(NaR, q = 0) = 1.524 g cmP3, thus q = 8.43(1-&)/&; for sorption data see fig. 3(a). 21 (a) J. M. Mackie and P. Meares, Proc. R . SOC. London, Ser. A , 1955,232,498; (b) S . Prager, J . Chem. Phys., 1960, 73, 122. 22 G. E. Boyd, B. A. Soldano and 0. D. Bonner, J . Phys. Chem., 1954,58,456. Effect of desulphonation on PSS resins: 8.6% DVB (21.1 and 63.6% desulphonation), 16% (4.2, 9.0, 15.9, 37.4 and 50.6%), q(NaR) taken in the same way as in ref. (2). ' G. E. Boyd and B. A. Soldano, Z . Efektrochem., 1953, 57, 162. 23 R. Fernandez-Prini and A. E. Lagos, J . Polym Sci., Part A , 1964, 2, 2917. 24 P. Meares, Philos. Trans. R. SOC. London, Ser. B., 1977, 278, 113.2630 SULPHONIC ION-EXCHANGE SYSTEMS 25 E. Glueckauf and G. P. Kitt, Proc. R. SOC. London, Ser. A , 1955, 228, 322. 26 S. Glasstone, J. J. Laidler and H. Eyring, The Theory of Rate Processes (McGraw-Hill, New York, 27 M. H. Cohen and J. Turnbull, J. Chem. Phys., 1959, 31, 1164. ** (a) H. Yasuda, C. E. Lamaze and L. D. Ibenberry, Makromol. Chem., 1968, 118, 19; (b) H. Yasuda, A. Peterlin, C. K. Colton, K. A. Smith and E. W. Merril, Makromol. Chem., 1969, 126, 177; (c) H. Yasuda, C. E. Lamaze and A. Peterlin, J. Polym. Sci., Part A-2, 1971, 9, 1 11 7. 1941). (PAPER 1 /725)

ISSN:0300-9599    

DOI:10.1039/F19827802619

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982,78, 2631-2642 Thermodynamic Analysis of the Thermal Decomposition of Dolomite BY GIORGIO SPINOLO* Centro di Studi per la Termodinamica ed Elettrochimica dei Sistemi Salini Fusi e Solidi del C.N.R., c/o Istituto di Chimica Fisica e di Elettrochimica dell’universita, Viale Taramelli 16, I 27 100 Pavia, Italy AND DARIO BERUTO Laboratorio di Chimica della Facolta di Ingegneria, Universita di Genova, Piazzale J. F. Kennedy, Pad. D, I 16129 Genova, Italy Received 3rd August, 1981 The role of CaC0,-MgCO, solid solutions in the decomposition of dolomite is discussed in terms of an asymmetrical function of their free energy of mixing. The results are in fairly close agreement with experimental phase diagrams. The thermodynamic analysis is used to describe the behaviour of the CaO-MgO-CO, ternary system and to gain a better understanding of the different reaction mechanisms proposed for the decomposition of dolomite.1. INTRODUCTION Among the thermal decompositions of two-component solid solutions, that of dolomite, CaMg(CO,),, has been widely investigated1-18 owing to its technological importance. In fact two different decomposition reactions occur : the ‘ full decompo- sition’ described by eqn (1) CaMg(CO,), (dolomite) -+ CaO(s) + MgO(s) + 2CO,(g) (1) and the ‘half decomposition’ CaMg(CO,), (dolomite) -+ [ 1 /( 1 - x)] Ca,-, Mg,CO, (calcite) which is usually written in a simplified manner by assuming x = 0. Several authors agree that below a critical value of the CO, partial pressure the full decomposition takes place, whereas under high pressures eqn (2) should be considered.However, it is not clear whether this threshold pressure is a thermodynamic or a kinetic parameter and whether it is temperature-dependent or not. Moreover, mechanisms suggested for both reactions are often conflicting and their thermodynamic bases are not always unquestionable. In a recent paper18 such mechanisms have been discussed in detail. Accordingly, CaC0,-MgCO, solid solutions seem to play a key role in decomposition of dolomite. Unfortunately, with the exception of the valuable contribution by Goldsmith and Newton,lo little information exists about the free energy of mixing function for these 263 12632 DOLOMITE DECOMPOSITION compounds and its temperature dependence. Consequently, any thermodynamic approach to the feasibility of the reaction steps involving these solid solutions is difficult.The first goal of this paper is to provide a thermodynamic model to calculate the free energy of mixing function of the CaC0,-MgCO, solid solutions in the temperature range 673-1 173 K. In previous attempts these compounds were assumed to behave as regular solutions.1° This approach, as shown in the following, predicts a composition- temperature phase diagram which is not in agreement with the experimental one, while the model discussed here gives a fairly close fit. Finally, on the basis of this model a thermodynamic analysis of the CaO-MgO-CO, ternary system is used to provide a better understanding of the different reaction mechanisms proposed for dolomite decomposition and to show the thermodynamic inconsistency of some of them.2. THE CALCITE-MAGNESITE SYSTEM AND ITS SIMPLIFIED MODEL Calcite and magnesite, i.e. pure CaCO, and pure MgCO,, have the same crystal structure, formed by alternate planes of alkaline-earth and carbonate ions perpendicular to a threefold axis according to a cubic stacking sequence. Random substitution of Ca2+ for Mg2+ (or of Mg2+ for Ca2+) is possible to some extent, and leads to solid solutions usually named ‘magnesian calcites ’ (or ‘calcian magnesites ’). In the following both will be called calcites regardless of whether a Ca-rich or a Mg-rich solution is concerned. The structure of dolomite is closely related to the calcite type, the main difference being that the cation planes are alternately occupied by Mg2+ and by Ca2+.The structure is less symmetric and slightly distorted. This ordered compound also shows a measurable homogeneity range around its stoichiometric composition. The isobaric or polybaric solubility-temperature relations in the CaC0,-MgCO, pseudobinary system have been experimentally investigated’~ 9 9 lo* l2 in the temperature range 623-1473 K and under CO, pressures up to 2.5 x lo9 Pa, i.e. always high enough to prevent decomposition. Three monophasic fields are present, at least to 1273 K, the calcite-structured phases being separated from the intermediate field of the ordered compound (dolomite) by two diphasic regions. At higher temperatures the binary system approaches complete solubility in the solid state, as dolomite shows an order-disorder transition to the random solution and the diphasic fields tend to disappear. The phase relations are best discussed with reference to a G against x plot at constant temperature and pressure, where G is the integral molar free energy and x is the MgCO, molar fraction.The topology of the CaC0,-MgCO, phase diagram discussed above suggests that the underlying free-energy function should show the following distinctive features (see fig. 1): (a) at low temperatures it should be curved upwards in three distinct ranges, near x = 0, x = 1 and x = 0.5 (the former branches are pertinent to quasi-random solutions, whereas the latter describes an almost completely ordered solid solution); (b) at higher temperatures the three branches tend to merge into one large curve (random solutions over the whole composition range).As our aim is to discuss the role of the calcite-structured solutions in the thermal decomposition of the ordered compound, we may take into account only the low-temperature part (T .c 1200 K) of the phase diagram, i.e. where no significant change in the degree of order occurs in dolomite. Under such a restrictive hypothesis a more empirical and approximate model is appropriate. In particular, the closeG. SPINOLO AND D. BERUTO I calcite 2633 calcite + dolomite Tb 1400 1300 1200 % 2 1100 1000 900 800 To 1 I I I 1 ~ 1 1 ~ 1 1 1 1 ~ 1 1 I I CaC03 0.2 0.4 0.6 0.8 MgC03 CaC03 MgC03 x(MgCO3) X (M&O3) FIG. 1 .-CaC0,-MgCO, pseudobinary system: schematic plots of the experimental phase diagram (left-hand side) and of the composition dependence of the integral molar free energy (G) at temperatures and & (right-hand side); x = mole fraction of MgCO,.Curve (c) and point (d) indicate the model proposed here for the low-temperature range. structural relationship between calcite (c) and dolomite (d) will be neglected and they will be taken as completely independent phases to be described by distinct free-energy functions, G,(x, T, P) and G,(x, T, P), which respectively correspond to the side- and middle-branches of the ‘exact’ model, i.e. to almost random and to almost ordered solutions. Moreover, it is clear that, although of fundamental importance in defining homogeneity range of dolomite, the precise shape of the G, curve plays a minor part in determining the stability limits (x’ and x”) of the calcite phase.It is therefore reasonable to assume that dolomite is a line compound, i.e. a phase of constant thermodynamic properties (at fixed temperature and pressure) and with a unit Ca/Mg ratio. Concerning the G, function, the experimental phase diagram again allows one to formulate a thermodynamic model for the free energy. Indeed, its strong asymmetrical shape suggests that the excess free-energy function, G,E, should also be asymmetrical. This simple statement has so far received little attention,* and will be developed in the following. * Following the submission of this paper an article appearedl9 in which this feature is discussed in connection with the equilibria between calcite, dolomite and aqueous solutions.The present approach is slightly different in considering the shape of the GF function and stresses the importance of the above conclusion in interpreting the mechanisms of decomposition.2634 DOLOMITE DECOMPOSITION Generally speaking, a GE function can be expressed as series of powers of mole fractions through the series expansion of the excess chemical potentials : n Epl = RTln fl = x akx;+, 0 n (3) where the a, and b, are coefficients (dependent upon temperature and pressure) to be determined; M 2 2 is an integral number. The latter statement is in agreement with experimental data and with theoretical models for liquid and solid solutions, except in cases where Debye-Huckel forces are Since the a, and b, are obtained through an analysis of the phase diagram, i.e.with two points for each temperature, it is reasonable to limit the series expansion to the first term and to take n = 0. Under these conditions, the excess integral molar free energy can be written as (4) where A = (a, + b0)/2 and B = (b, - a0)/2. The shape of this function is asymmetrical if B # 0, whereas the equality leads to the regular solution model applied by Goldsmith and Newton.lo If Gd is known as a function of temperature and pressure, the values of A and B at a particular temperature may be obtained from the experimental x’ and x” data at the same temperature. Since a considerable difference exists among (x‘ against T ; 21 G,E = ~ ( l -x)[A+(l -2x)BI TABLE 1 .-INTERACTION PARAMETERS FOR CALCITE SOLID SOLUTIONS T / K x ’ ~ X”U A/RTb - B/RTb X” X//d A/RTb -B/RTb 1173 1123 1073 1023 973 923 873 823 0.272 0.232 0.194 0.161 0.130 0.105 0.082 0.063, 0.98 1 0.985 0.988 0.990, 0.994, 0.996 0.997, 0.992, 1.55 1.70 1.87 2.05 2.25 2.44 2.66 2.87 1.83 0.245, 0.972, 1.62 1.46 1.81 0.203 0.979, 1.79 1.45 1.86 0.165 0.985 1.99 1.44 1.73 0.134 0.988, 2.18 1.43 1.68 0.108 0.992 2.38 1.45 1.65 0.088 0.994, 2.58 1.50 1.64 1.66 Smoothed values from polybaric data of ref.(12); referred to 1 g-ion of carbonate; smoothed values from smoothed values from isobaric (P = 1 atm) data [ref. (lo), table 41; polybaric data of ref. (7) and (8). x” against T ) data of different authors7-107 l2 it is difficult to average them. For each set of data we therefore drew a smoothing curve on which x’ and x” values have been taken at 50 K intervals in the 823-1 173 K range (see table 1).The A and B coefficients have been evaluated by assuming23 Gd/RT = -(739.74/T) + 0.2026 and by neglecting the pressure dependence of G, and G,. The results are given in table 1 and plotted in fig. 2. The coefficients may be empirically described by ( 5 ) A/RT = (5.96f0.02)- (3.72 k0.15) (T/K - 1000) B/RT = - (1.58 & 0.15).G. SPINOLO AND D. BERUTO 263 5 2.0 L m I 3 1.5 873 973 1073 1173 TIK FIG. 2.-Interaction parameters for calcite solid solutions. Empty circles and left-hand scale: AIRT; filled circles and right-hand scale: -B/RT. Values obtained from two sets of experimental data are shown together (see also table 1). FIG. 3.-Calcite-magnesite pseudobinary phase diagram. 0, ref.(12); 0, ref. (7) and (8); A, 1 atm data of ref. (10); A, 10 kbar data of ref. (10); *, ref. (9); (-)calculated from the present model; (---) regular solution model.lo2636 DOLOMITE DECOMPOSITION Fig. 3 shows the phase diagram of the CaC0,-MgCO, pseudobinary system as calculated through the above A , B and G, values. Note that the present approach correctly describes the experimental behaviour over almost the whole temperature range. Indeed, it represents a type of best fit of the experimental data and is well within their accuracy and precision. In contrast the regular solution model fails, because it assumes a symmetrical shape for G,. One point must be stressed in discussing the accuracy of the present model: it regards G, and Gd as being independent of pressure.This assumption is proved wrong by the experimental data,lo as can be seen from fig. 3 (triangles). However, the pressure dependence of the A and B coefficients can be roughly evaluated in the following way. The composition dependence of the molar volume of the calcite phase can be written in a Redlich-Kister form2, as uc = x uO(MgC0,) + (1 - x) uO(CaC0,) + x( 1 - x) [ A ’ + (1 - 2x) B’]. (6) A’ = (aA/aP),; B’ = (aB/aP),. (7) The experimental u, against x data at room temperature and pressure24 allow one to (8) evaluate Such small values, the scattering among the experimental (x’ and x’’ against T ) data and the small departure of the dolomite molar volume from u,(x = 0.5) justify the assumed pressure independence of G, and Gd. Since u, = (aG,/aP), one obtains A’ = -0.38 x 10-6m3m01-1; B’ = -0.25 x 10-6m3moi-1.3. THE CaO-MgO-CO, TERNARY SYSTEM AND THE P(C0,) AGAINST T DIAGRAM FOR DOLOMITE DECOMPOSITION To describe the ternary system let us define the independent composition variables as y = n(MgO)/n(tot) = ~ / 2 z = n(CO,)/n(tot); n(tot) = n(Ca0) + n(Mg0) + n(C0,) and let us indicate by 0, p and g the phases of the pure components CaO (lime), MgO (periclase) and CO, (gas). According to the assumptions of section 2, the calcite (c) phase exists only for z = 0.5 and 0 < y < 0.5, whereas the dolomite (d) phase occurs only at the composition point ( y = 0.25, z = 0.5). In order to compute the phase diagram, the dependence on pressure, temperature and composition must be stated for the following integral molar free energies : Go = Go(T); G , = G,(T); G, = G,(T,P); G , = G,(y, T ) ; Gd = Gd(T) where G, and Gd are now half of the corresponding values of the previous section, because of the different choice of composition variable.For convenience, the standard states can be defined according to G,(T,P = 1 atm,ideal) = 0; Gc(y = 0, T ) = 0 ; G,(y = 0.5, T ) = 0 where the compositions y = 0 and y = 0.5 stand for pure calcite and pure magnesite, respectively. It is easy to verify that, as a consequence of this choice of standard states, the Go value is equal to the standard free-energy change of the reaction CaCO,(c) --+ CaO(o) + CO,(g, P = 1 atm, ideal)G. SPINOLO AND D. BERUTO 2637 and that the equilibrium CO, pressure for the decomposition of pure CaCO,(c) is given by peQ(CO2) = exp( -G,/RT); p = P(CO,)/P"; Po = 1 atm = 101 325 Pa (9) if, as a first approximation, ideal behaviour is assumed for carbon dioxide: Gg = RT In p .In an analogous way, the G, value is the standard free-energy change for the decomposition reaction of pure magnesite, and the CO, equilibrium pressure is given by a law similar to eqn (9), with G, replacing Go. To describe the dolomite decomposition reactions it is best to consider again the free energy against composition diagrams at constant temperature and pressure for the CaC0,-MgCO, system (see fig. 4). In these diagrams, points A and B are, respectively, relevant to the heterogeneous mixtures CaO-CO, and MgO-CO, having a CO, mole fraction z = 0.5. Both of them depend on the pressure in the same way: and therefore the straight line connecting them should be shifted in parallel (AB -+ A'B' -+ .. .) when the carbon dioxide pressure changes. On such lines, the equimolecular points (D, D', . . . ) correspond to the free energy of the right-hand side I '. '. '. '. ' '. \. 4' \. '. '. '. '. '. 4" '. \ \ ' / \ \ '. '. '. '. '. '. B \ . '. 1 I I CaC03 x (MgcO,) MgC03 FIG. 4.-Molar free energy against composition diagram, at constant temperature, for the CaC0,-MgCO, section ( z = 0.5) through the ternary system CaO-MgO-CO,. Curve (c), calcite solid solutions; point F, dolomite; points A, A', . . . , heterogeneous CaO+CO, mixtures; points B, B', . . . , heterogeneous MgO + CO, mixtures.2638 DOLOMITE DECOMPOSITION of eqn (1). To investigate the feasibility of the full decomposition reaction at different temperatures and CO, pressures, these points must be directly compared with the point (F) indicating the free energy of the dolomite phase (Gd) at the considered temperature.The CO, equilibrium pressure for the full decomposition is graphically shown by the A”FB” line, because in such a condition the molar free energies of both sides of eqn (1) are equal. In the same diagram, the G,(y) function at the same temperature is also plotted. This function is pressure-independent and allows one to obtain data for reaction (2) (‘half decomposition’): the CFB’ line defines the calcite composition (x, = x’) and the CO, pressure at the equilibrium between magnesium oxide and dolomite. Note that the carbon dioxide pressure relevant to this equilibrium is always higher than the equilibrium value for the full decomposition, eqn (I), at the same temperature.Therefore this result, which is well known from a kinetic point of view, has a thermodynamic background. Moreover, the parameter x of eqn (2) depends only on temperature and is the same as x’ of the previous section, viz. the phase boundary of the Ca-rich calcite solid solution. The equilibrium described by the line CFB’, or by eqn (2), is invariant. If one removes carbon dioxide, its pressure and the calcite composition may change only after the whole dolomite phase has been consumed: a monovariant equilibrium is then established and a relation exists between calcite composition and CO, pressure. At a still lower CO, pressure a new invariant equilibrium occurs: Ca,-, Mg,CO,(c) -+ (1 - r ) CaO(o) + r MgO(p) + CO,(g, p = p”’) (10) where r is a pressure-independent but temperature-dependent parameter and indicates the lowest Mg content of a Ca-rich calcite in equilibrium with magnesium oxide and carbon dioxide.The CO, equilibrium pressure for eqn (10) (see line A”’C’B” in fig. 4) is definitely lower than the equilibrium pressure for the direct decomposition of dolomite into oxides at the same temperature. Also, its value is practically equal to the CO, equilibrium pressure for the thermal decomposition of pure calcite. It is now possible to calculate the r(C0,) against Tcurves for the equilibria discussed above.* The results are shown in fig. 5, where curve (a) illustrates the decomposition of dolomite into magnesian calcite, magnesium oxide and carbon dioxide [eqn (2)], curve (b) illustrates eqn (lo), whereas the lines marked by 20, 10, 5, .. . , 0.05 are the lines of constant MgCO, content (20, 10,5, . . . , 0.05 mol%, respectively) of the calcite phase in monovariant equilibrium with MgO and CO,. The direct decomposition of dolomite into oxides is described by the dashed curve (c). For the sake of comparison, the p(C0,) against T relation for pure magnesite decomposition is also plotted [curve (d)]. The analogous curve for pure calcite decomposition, as said, is practically coincident with curve (b). Fig. 6 reports the results on a smaller scale and shows fairly good agreement with the experimental data.7r10y12 In the same plot the calculated data for the regular solution model are also drawn.Although having been proved wrong in the above discussion, the regular solution model gives a reasonable fit of the data relevant to reaction (2). This is not surprising, since when one is dealing with equilibrium (2) only Ca-rich calcites need be considered. In this composition range the asymmetrical shape of the free energy of mixing curve does not become apparent, and therefore either model can be safely applied. However, for a complete description of dolomite * The phase diagram of fig. 5 and 6 has been calculated by assuming G,/RT = 20923/T- 18.169, GJRT = 13551.4/T- 19.9295 [mean values in the range of interest, taken from ref. (25)] and half of the above-stated values for Gd and for the coefficients A and B.G .SPINOLO AND D. BERUTO 2639 tP c , , 9?0 ,8?0 ,7?0 6?0 500 , , I I I I I I 8 10 12 14 104 K I T FIG. 5 . 2 0 , partial pressure against temperature phase diagram for dolomite, as computed from the proposed model; p = P(CO,)/Po, Po = 1 atm = 101 325 Pa; (a) invariant equilibrium of dolomite with magnesian calcite, magnesium oxide and carbon dioxide, eqn (2); (b) invariant equilibrium of magnesian calcite with CaO, MgO and COT, eqn (10); (c) metastable invariant equilibrium of dolomite with CaO, MgO and CO,, eqn (1); (d) invanant equilibrium of pure magnesite with MgO and CO,. The lines marked with 20, 10, . . ., 0.05 are lines of constant magnesium composition x = 20%, lo%, . . ., 0.05% in the magnesian calcites (x = mole fraction of MgCO,). decomposition mechanisms one must deal with the G, against x curve over the whole composition range.Accordingly, the asymmetrical curve of the present model should be preferred to the regular solution function. 4. DISCUSSION On the basis of the previous analysis, some thermodynamic predictions relevant to dolomite decomposition will now be described. From the data plotted in fig. 5, it is possible to state which of the overall reactions, (1) or (2), is thermodynamically feasible under fixed T and P(C0,) conditions. For instance, when T = T,, MgO and CaO are the final solid products of the decomposition only if the reaction is carried on under CO, pressures lower than pA. The widely2640 4 3 2 Q. 2 011 - 1 a DOLOMITE DECOMPOSITION t J T 900 800 700 600 500 I I 1 1 1 I 8 9 10 1 1 12 13 lo4 KIT FIG.6.-CO, equilibrium pressure for dolomite decomposition to magnesium oxide, magnesian calcite and carbon dioxide, eqn (2). Thin line: computed from the present model; dashed line: computed from the regular solution model; thick line: interpolation of the experimental data from ref. (12) (circles) and from ref. (7) (squares). The filled symbols indicate runs which yielded magnesian calcite and magnesium oxide, the empty symbols indicate runs which yielded dolomite. recognized fact that the thermal decomposition of dolomite proceeds, in vacuu, to MgO plus CaO follows straight from an observation of the diagram. In the same way, at the same temperature, point B indicates the P(C0,) value pertinent to the decomposition of dolomite into a calcite solid solution and MgO. If one wishes to obtain, at the same CO, pressure, MgO and CaO as final solid products, one must raise the temperature to reach again the range below line (b), i.e.T > T,. As far as the thermal decomposition of dolomite into calcite and MgO [eqn (2)] is concerned, essentially two kinds of mechanisms have been suggested.13 The first assumes direct calcite formation, the second a primary formation of the oxides followed by recarbonatization. The two sets of mechanisms can be summarized by the following equations: (a) DIRECT CALCITE FORMATION (1) dissociation into the pure carbonates and successive decomposition of MgCO,:G. SPINOLO A N D D. BERUTO (2) intermediate formation of solid solution: 264 1 (12) with n changing with time from 0 to 1 .(b) PRIMARY FORMATION OF THE OXIDES (1) primary formation of the oxides followed by successive recarbonatization by gaseous carbon dioxide : CaMg(CO,),(d) +CaO(o) + MgO(p) + 2CO,(g) CaO(o) + CO,(g) -+ CaCO,(c) ( 1 3 4 (13b) (2) primary formation of the oxides, reaction (13a), followed by solid-state recarbonatization : CaO(o) + CaMg(C03)2(d) -+ 2CaC03(c) + MgO(p) + CO,(g). (14) Arguments exist for choosing from among the above mechanisms. Recently Hashimoto et aZ.18 provided experimental evidence in favour of a direct calcite formation mechanism according to eqn (1 2). We think that the thermodynamic approach discussed here can be useful in making the choice. At fixed temperature and CO, pressure, the free-energy change of each step of the proposed mechanism should be negative.Under this condition, some of the conclusions reached by Hashimoto et aZ. seem to be thermodynamically sound. These authors were working in a P(C0,) against T range around line (6) of fig. 5: this is a peculiar range because the overall reaction can be either (1) or (2). As far as eqn (2) is concerned, the direct formation of calcite as in eqn (1 1) does not seem feasible because the free energy of the dolomite is always lower than that of the parent carbonates, and therefore step (1 1 a) would require a positive AG value. The second set of equations, eqn (12), proposed for direct formation assumes the possibility of forming a solid solution whose composition changes with time. However, if one is dealing with near-equilibrium reaction, this hypothesis is thermo- dynamically misleading, because the system, at fixed temperature, is invariant, i.e.only one solid solution (having a fixed composition) can be in equilibrium with dolomite, magnesium oxide and carbon dioxide. If the intermediate formation of a calcite solid solution is not an equilibrium step, its composition must still be within the intrinsic stability limits, i.e. outside the convex portion of the G, function evaluated at the same temperature. Table 2 shows the compositions to be discarded. In conclusion, the mechanism relevant to the formation of a calcite solid solution can be thermodynamically accepted if the solid solution itself is on the Ca-rich side. TABLE 2.-sPINODAL POINTS OF THE FREE-ENERGY CURVE FOR THE CALCITE SOLID SOLUTIONS 1173 0.544 0.896 1073 0.507 0.904 973 0.465 0.912 873 0.429 0.919 773 0.395 0.9242642 DOLOMITE DECOMPOSITION This could be a good thermodynamic basis for the conclusions drawn by the above authors. If one is dealing with the mechanism of primary formation of the oxides, followed by successive recarbonatization, one must conclude that the sequence of steps described by eqn (1 3) and (14) is thermodynamically meaningful.Indeed, according to fig. 4, in the P(C0,) and T range experimentally explored by Hashimoto et al. the first step is characterized by a negative value of the reaction free-energy change. In this respect a thermodynamic approach does not help in choosing between the direct formation of calcite, eqn (12), and the primary formation of oxides.However, note that within the above p T range the free-energy change for reaction (12) is more negative than that relevant to the first step of eqn (13). In this respect, again, for the half decomposition the direct formation of a calcite solid solution seems to be thermodynamically preferred. Kinetic reasons, such as the exact local CO, pressure within the sample or the amount of strain connected with structural rearrangement, actually determine which mechanisms is favoured, thus explaining the conflicting experimental results obtained by various authors. The only mechanism which has to be thermodynamically excluded is that leading to the dissociation into two carbonates. The same line of argument holds when the overall reaction is that described by eqn (1).The full decomposition should be obtained either in a single step or by the intermediate formation of calcite and MgO followed by decomposition of the calcite. Both mechanisms can be thermodynamically accepted, even if the direct formation of the oxides, according to fig. 4, is favoured in low CO, partial-pressure regimes. We thank Alan W. Searcy for insight into the subject of this paper. G.S. has been partially supported by a C.N.R. fellowship. R. A. Rowland and D. R. Lewis, Am. Mineral., 1951,36, 80. R. A. W. Haul, L. H. Stein and J. D. Louw, Nature (London), 1951, 167, 241 and 727. R. A. W. Haul and H. Wilsdorf, Acta Crystallogr., 1952, 5, 250. D. L. Graf, Am. Mineral., 1952, 37, 1. W. F. Bradley, J. F. Burst and D. L. Graf, Am. Mineral., 1953, 38, 207. ' D. L. Graf and J. R. Goldsmith, Geochim. Cosmochim. Acta, 1955, 7 , 109. D. L. Graf and J. R. Goldsmith, Geochim. Cosmochim. Acta, 1958, 13, 218. * J. R. Goldsmith and H. C. Heard, J. Geol., 1961, 69, 45. lo J. R. Goldsmith and R. C. Newton, Am. J. Sci., 1969, 267A, 160. l1 H. T. S. Britton, S. J. Gregg and G. W. Winsor, Trans. Faraday SOC., 1952, 48, 63. l2 R. I. Harker and 0. F. Tuttle, Am. J. Sci., 1955,253, 209; 274. l3 P. A. Lange and W. Roesky, Ber. Dtsch. Keram. Ges., 1964, 41, 497. l4 M. Vivaldi, F. Girela and J. Linares, Acta Crystallogr., Sect. A, 1969, 25, S231. l5 J. W. Smith, D. R. Johnson and M. Muller-Vonmoos, Thermochim. Acta, 1974, 8, 45. l6 W. R. Bandi and G. Krapf, Thermochim. Acta, 1976, 14, 221. l7 E. K. Powell and A. W. Searcy, J. Am. Ceram. Soc., 1978, 61, 216. la H. Hashimoto, E. Komaki, F. Hayashi and T. Uematsu, J. Solid State Chem., 1980, 33, 181. lS F. C. M. Driessen and R. M. H. Verbeeck, Ber. Bunsenges. Phys. Chem., 1981, 85, 713. 2o C. Sinistri, Z. Phys. Chem. (N.F.), 1961, 30, 349. 21 R. Haase, Z. Naturforsch., Teil A, 1953, 8, 380. 22 0. Redlich and A. T. Kister, Ind. Eng. Chem., 1948, 40, 341. 23 J. W. Stout and A. Robie, J. Phys. Chem., 1963,67, 2248. 24 J. R. Goldsmith, D. L. Graf and H. C. Heard, Am. Mineral., 1961, 46, 453. 2L K. H. Stem and E. L. Weise, Nut1 Bur. Stand. (US.) Publ. no. NSRDS-NBS-530 (1969). * R. A. W. Haul and H. Heystek, Am. Mineral., 1952, 37, 166. (PAPER 1/1217)

ISSN:0300-9599    

DOI:10.1039/F19827802631

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

3. Chem. SOC., Faraday Trans. I , 1982,78, 2643-2648 Capillary Phenomena Part 19.-Systems Subjected to Various Gravitational Field Strengths BY ERNEST A. BOUCHER* AND TIMOTHY G. J. JONES School of Molecular Sciences, University of Sussex, Brighton BNl 9QJ Received 2nd September, 198 1 A scheme is described which allows quantities relating to axially symmetric capillary systems to be computed for various values of the gravitational field strength, or of the fluid-phase density difference, or of the fluid/fluid interfacial tension. Examples are given using pendent and sessile drops. The treatment deals with zero-gravity conditions (or Ap = 0) under which these drops become spherical, but fluid bridges, for instance, possess a variety of shapes. Analytical formulae exist for many gravity-free axially symmetric cases, but they are not always convenient to use and may not be available for sufficient quantities.The approach used here involving an explicit capillary parameter is quite general. In dealing with the behaviour of axially symmetric fluid bodies, such as pendent and sessile drops and fluid bridges,l the interface shape is given by combining the equations which relate fluid-phase (a, 8) hydrostatic pressures pa and fl at height z to the fluid densities pa and and the mean surface curvature j, and involve the acceleration g due to gravity and the interfacial tension y@. It is customary to use reduced or dimensionless quantities, e.g. 2 = z / a where a = (2yap/Apg)i: whether or not the factor 2 is used is immaterial, and Ap is the fluid-phase density difference.Having obtained numerical results, e.g. the maximum stable volume of a pendent drop in reduced terms,2* the actual maximum volume for a drop at the end of a tip of radius r is simply obtained by evaluating a, or, if the volume is already known from experiment, a can be used to give (say) yaB. However, this common approach is not suitable for ' gravity-free ' cases, including the neutral buoyancy condition Ap = 0, because as a 4 m the reduced quantities degenerate to zero. What is required is a scheme whereby: (i) a pendent drop, for example, becomes a spherical segment of finite volume as g + 0; (ii) the solution by numerical integration of the appropriate Laplace differential form of eqn (1) is compatible with known analytical solutions, e.g.involving elliptic integrals, for gravity-free systems of simple symmetry; and (iii) systems can be described in terms of varying gravitational field strengths, including the zero-gravity limit (or near-zero for space experiments*), terrestrial gravity and greater or lesser field strengths. Analogous variations in Ap or in yap would be similarly accommodated, e.g. the slow decrease in yap leading eventually to pendent drop detachment as described experimentally by Tornberg and L ~ n g h . ~ The purpose of this paper is to show briefly, by example, in terms of varying gravitational field strength, how these objectives can be met. The method of calculation by numerical integration was first used to obtain the generating curves for the axially symmetric zero-gravity bodies the unduloid, the nodoid and the catenoid, given on a small scale in fig. 7 of ref.(1): it was considerably more convenient than 26432644 CAPILLARITY I N VARYING GRAVITY checking existing analytical meridian equations for misprints before evaluating the elliptic integrals. Interfacial areas, fluid volumes and centres of mass where appropriate can also be evaluated by the direct computational technique to be described [see ref. (1) for relevant formulae]. Axially symmetric systems are convenient for study experimentally and theoretically, but the principle of allowing for variation in the quantities in the conventional capillary constant a is quite general. It can be argued that computed meridian curves can be unreduced by using a sequence of a values corresponding to a range of gravitational field strengths.While this is true in principle, except for a = 0, one cannot expect tabulated values, e.g. of Hartland and Hartley,e to cover all the meridian shapes and fluid-body quantities that are required. Furthermore, an ad hoc approach hinders the interpretation of capillary phenomena. The variations using c are explicit and lead to the occurrence of [ in thermodynamic formulae for varying conditions including the zero-gravity case. COMPUTATIONAL TECHNIQUE The essence of the technique is to scale an actual linear dimension 1 by a*, giving L = l/a* as before,192 using the usual values for the density difference Ap*, the interfacial tension y* and the acceleration g* due to terrestrial gravity to define a*.More generally, however, there is a capillary variable a = (2y/Apg)a which can cover a range of conditions. Starting from a chosen vertical position (in reduced terms 2 = z/a*) 2 = 0, where the pressure difference APo is 2H, the shape factor H specifies the mean surface curvature of the fluid/fluid interface. The variation of the pressure difference A P with 2 depends on the value of the scaling or capillary parameter c, introduced to take account of the values of Ap, y and g defining a for the system A P = 2(H--Z); C2 = a*/a. (2) One now obtains by numerical computation' an interface shape or meridian curve for each [ and a given H instead of a single meridian for [ = 1. Finite ( X , 2) meridians are obtained where [ = 0, corresponding to zero-gravity conditions, or to Ap = 0 or y + CQ. Eqn (2) is reminiscent of the scheme introduced earlier1y2 to give pendent drops, emergent bubbles, sessile drops, captive bubbles and fluid bridges from a single set of equations: a quantity d (= _+ 1) was used.It is now seen how the general parameter c can give, e.g., pendent drops for [ > 0 and sessile drop for [ c 0, using these as examples in the following account. The equation to be solved numerically when the meridian arc length S is the independent variable is d@/dS = 2(H-[Z)-sin@/X (3) dX/dS = COSQ,, dZ/dS = sin@,. (4) with the usual relationships The meridian angle is defined as arctan(dZ/dX). Computation is started at X = 2 = S = 0 and Q, = Oo for chosen H and [. The computational procedure is similar to that already de~cribed.l-~ EXAMPLES AND DISCUSSION Fig.1 shows meridian curves for H = 2 and five values of c, including the gravity-free case of [ = 0. Without the procedure introduced here the meridians would have culminated in a point at the origin when [ = 0. A set of meridians meeting variousE. A. BOUCHER AND T. G. J. JONES 2645 2 z 1 0 0.5 1 1.5 X FIG. 1.-Set of pendent-drop meridian curves for shape factor H = 2 and the C values indicated: (a) 0, (6) 0.5, (4 1, (4 3, (4 5. 1 1 I I FIG. 2.-Set of pendent-drop meridian curves for R = 1 and Va = 2.09 beneath a solid tip when [ takes the values: (a) 0, (b) 1, (c) 1.4, ( d ) 1.473, (e) 1.3.2646 CAPILLARITY I N VARYING GRAVITY z FIG. 3.-Dependence of the meridian angle 4 at the tip on C for R = 1 and (a) Va = 2.09, (b) Va = 0.60 and (c) Va = 0.38.prescribed physical conditions can be obtained with varying c as shown in fig. 2 for fixed pendent drop volume [Va([ = 1) = 2.091 and solid tip radius R = 1 (c = 1). c reaches a definite maximum of 1.473 for the example, as is shown in fig. 3 in terms of the three-phase-confluence (tip) meridian angle #. The free energy F of the pendent drop can be written as an extension of earlier treatmentsly where A afi is the reduced area of the drop a/#l interface, 2. is the drop centre of mass and Z* is the drop height. An increase in c is regarded as an increase in the gravitational field strength or in Ap, or a decrease in y : the computations leading to the diagrams herein represent equilibrium configurations, but [ can be used in variational tests for thermodynamic stability.Fig. 4 shows that for the case Va = 2.09 there is a distinct cusp in F as a function of c at the limit of [ corresponding to the strain-controlled limit of stability, i.e. the drop will detach when 5 reaches 1.473. Taking the normal a/mm x 4 for the air/water interface, the tip radius r is 4 mm and the volume of the water drop is ua/mm3 = 130.6. The field strength can be steadily increased to give nearly 14 times terrestrial g before this drop must (partially) detach. Fig. 3 and 4 also show, respectively, plots of # and Fagainst c for the cases Va = 0.6 and 0.38. In neither of these cases is there a maximum in c or a cusp in F: c reaches an extreme value corresponding to # = Oo at the tip when detachment will occur.The inset in fig. 4 shows the case where the cusp occurs at [ = 1 for Va = 2.907, i.e. this is Vgax for terrestrial conditions. The case of Va = 0.6 and 4 x 0' at the limit in c corresponds very closely to the 'Lohnstein point', where the curves of maximum volume at constant contact angle and at constant tip radius coincide at 8 = # = Oo, for the limiting value, [ = 5. Finally, fig. 5 shows how a sessile drop of fixed volume (Va = 4.1) meeting a plane solid at contact angle 4 (= 1 50°) changes shape as [ is systematically varied. F = Aap+2cVa(Z*-Ze) ( 5 )E. A. BOUCHER AND T. G. J. JONES 2647 3 2 F 1 0 - 1 5 FIG. 4.-Dependence of pendent-drop free energy F on [ for (c) V" = 2.09, (b) V" = 0.60, (c) V" = 0.28 and (inset) ( d ) YOr = 2.91. 0 1 X 2 FIG. 5.-Sessile-drop meridians for Va = 4.1 and 6 = 150° and the lrl values: (a) 0, (b) 0.3, (c) 1, ( d ) 4.4, (e) 10.6.2648 CAPILLARITY I N VARYING GRAVITY T. G. J. Jones acknowledges a S.R.C. studentship, and discussions with Dr M. J. B. Evans are gratefully acknowledged. E. A. Boucher, Rep. Prog. Phys., 1980, 43, 497. E. A. Boucher and M. J. B. Evans, Proc. R . Soc. London, Ser. A , 1975,346,49. E. A. Boucher, M. J. B. Evans and H. J. Kent, Proc. R. Soc. London, Ser. A , 1976, 349, 81. European Space Agency, Special Publication no. 114 (1976). E. Tornberg and G. Lungh, J. Colloid. Interface Sci., 1981, 79, 76. S. Hartland and R. W. Hartley, Axisymmetric Fluid-Fluid Interfaces (Elsevier, Amsterdam, 1976). (PAPER 1 / 138 1)

ISSN:0300-9599    

DOI:10.1039/F19827802643

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. 1, 1982, 78, 2649-2659 Spectroscopic Study of Anatase Properties Part 5.-Surface Modifications Caused by K 2 0 Addition BY CLAUDIO MORTERRA,* ANNA CHIORINO AND GIOVANNA GHIOTTI Istituto di Chimica Fisica dell’universita di Torino, Corso Massimo d’Azeglio 48, 10125 Torino, Italy AND EMILIA FISICARO Centro Ricerche Sibit, Gruppo Montedison, 15047 Spinetta Marengo (Al), Italy Received 10th September, 1981 Infrared spectroscopy has been used to investigate the surface properties of anatase gels containing up to ca. 1 % K,O, one of the additives most commonly employed to give TiO, pigmentary characteristics. It was found that K,O mostly collects at the surface of the material, as revealed by the spectral behaviour of surface hydroxyls and of surface sulphate contaminants.During the thermal treatment leading to the pigmentary material, K,O progressively modifies TiO, surface acidity, so that on the low-surface-area final product no Ti ions are revealed by suitable admolecules (pyridine, CO), but only coordinatively unsaturated K ions, acting as weak Lewis centres. The surface modifications caused by K,O are slowly reversible on contact with water vapour. Our previous studies (Parts 1-41-4) have been devoted to the characterization of the surface properties of pure anatase and to the identification of the nature and activity of various anionic impurities that arise from the manner of preparation. We considered these studies as being preliminary to an investigation of the role played by the various additives that are needed to give TiO, its well-known pigmentary characteristics.This work deals with the addition of K20, whose concentration in TiO, pigments ranges between 0.05 and 0.4 wt % and whose presence is thought to be essential for adequate particle growth during the thermal treatment leading to the production of white pigment. In this work i.r. spectroscopy has been used to elucidate the changes produced by the presence of K20 and revealed by modifications to both the background spectrum of the solid and the spectral features of the interaction of the solid with suitable test molecules. EXPERIMENTAL The anatase used here as a starting material is a ‘ via-sulphate preparation’ (hereafter referred to as TS) coming from the H,SO, hydrolysis of pure titanyl sulphate, whose characterization has been reported elsewhere.2* As the addition of K,O to TiO, pigment preparations occurs in the early stages of anatase-gel production, gel samples containing K,O (hereafter referred to as TSK) were studied after various thermal treatments at temperatures ranging from 298 K (i.e.after ambient-temperature drying) to 1073 K (i.e. just before the anatase-rutile phase transformation). Two K,O concentrations were investigated, namely 0.2 and 1.0 wt %, but all data reported refer to the latter concentration unless otherwise stated : this concentration highlights the 86 2649 FAR 12650 I.R. STUDY OF THE TiO,/K,O SYSTEM differences between TS and TSK, and no qualitative differences were observed between the two preparations of different K 2 0 content. 1.r.spectra were run on a Perkin-Elmer 580/B double-beam spectrophotometer. All details concerning experimental procedures and instrumentation have been reported previously. 1v All gases and chemicals were of the highest-purity grade available and were further purified in situ using standard techniques RESULTS AND DISCUSSION No appreciable differences between TS and TSK were observed in the spectral features either of the surface hydroxyls during dehydration-rehydration experiments or after contact with CO, of specimens dehydrated in various ways. In the former case the lack of resolution at i j 2 3000 cm-l, caused by severe scattering,, can also be invoked as a factor limiting the spectroscopic selectivity, whereas in the latter case, based on spectra run in a well-resolved range, the addition of up to ca.1 % K,O does not affect the scarce surface basicity of the anatase revealed by CO, chemi~orption.~ Significant differences between TS and TSK were observed in the low-frequency background spectrum, where sulphate impurities absorb, and in the activity towards acid-revealing test molecules, such as pyridine (Py) and CO. The relevant results are described separately. BACKGROUND SPECTRUM I N THE 1000-1800 Cm-' RANGE Fig. 1 compares this spectral range of a TS and TSK starting samples outgassed at up to 673 K, and then rehydrated and outgassed again at ambient temperature. This is the spectral range of coordinated molecular water (having a band at ca. 1630 crn-l)l* and of sulphate impurities., Three facts can be observed.(i) On passing from TS to TSK there are no changes of shape, spectral position, thermal stability, and reversibility of the band owing to coordinated water. This indicates that, at least in the early stages of thermal treatment (Le. in the temperature interval needed to remove coordinated molecular water), no surface acidic sites of those species responsible for water coordination are either ascribable to or appreciably affected by the presence of potassium. (ii) Although the level of sulphur contamination remains unchanged on passing from TS to TSK gel, on TSK samples there is a definite increase in the amount of surface sulphates that are typical of TS., This is particularly evident for samples treated at T > 473 K, characterized by a surface sulphate of highly covalent character absorbing at ca.1375 and ca. 1130 cm-l. Moreover, this sulphate is definitely more loosely held on TSK than on TS [cf., e.g., the band at 1375 cm-l in spectra 4 and 5 of fig. 1 (a) and (b)]. (iii) On TSK samples there is a new sulphate species whose low-8 SO, stretching mode presumably lies in the unresolved band at ca. 1130 cm-l but whose high-v mode is at ca. 1275 cm-l on samples activated at low temperatures and moves to 1300 cm-l following activation at T 2 473 K. The reversibility of this spectral behaviour on rehydration (spectrum 6 of fig. 1) indicates that this sulphate species, obviously ascribable to the presence of potassium, is also at the surface, at least in the early stages of the thermal treatment considered here.The assignment of sulphates which are characteristic of TSK samples, both at high and low dehydration stages, is not straightforward, and requires a close comparison with the behaviour of pure TS samples. The high-ij SO, stretching mode observed on TSK, at both dehydration stages, isC. MORTERRA, A. CHIORINO, G. GHIOTTI AND E. FISICARO 265 1 wavenumber/cm-' FIG. 1.-The 1050-1725 cm-' spectral range of a previously untreated sample of TS (a) and TSK (b), respectively. S1-5: the sample was treated in vacua at 298, 373, 473, 573 and 673 K, respectively. S6: the sample of spectrum 5 was rehydrated and further dehydrated at 298 K. compatible with that of sulphato complexes of Czv symmetry, such as chelating and bidentate sulphato c~mplexes.~ On TS only, the sulphates typical of low dehydration stages had frequencies compatible with that modeL2 It is also recalled that, on TS, dehydration causes the reversible transformation of chelating or bidentate sulphato complexes into covalent sulphates,6 characterized by a higher splitting of the two SO, stretching modes.The similar spectral behaviour of TSK sulphates on dehydration- rehydration suggests that the sulphate species typical of K-containing preparations undergoes the structural changes previously suggested for TS, following the overall polarity of the surface layer. Thus on highly dehydrated materials, characterized by a low polarity of the surface layer, there can be two types of covalent sulphate,6 of which the one containing potassium exhibits a smaller splitting between the two SO, stretching modes, owing to the more polar character induced by the alkali-metal ion(s).2652 I.R.S T U D Y OF THE TiO,/K,O SYSTEM ADSORPTION OF PYRIDINE The use of pyridine as a selective probe molecule to characterize both qualitative and quantitative aspects of surface acidity is widespread [see, for instance, ref. (7) and (8) and references therein], and the suitability of 8 a-8 b and 19a-19 b ring-stretching modes as analytical tools has been previously suggested.** The reasonable number of data available in the literature, concerning both homogeneous and heterogeneous Py compounds and complexes, allowed us to outline the 1400- 1700 cm-l spectral range as reported in fig. 2: in it the width of the schematic bands only concerns the range of occurrence, rather than the actual bandwidth, whereas the height only indicates the difference in intensity between fundamental and combination modes.wavenum ber/cm-' FIG. 2.-Schematic representation of the spectral range of some Py modes in various Py-containing homogeneous and heterogeneous compounds. (No reference is made in the scheme to any actual intensity ratio, but rather to the difference between fundamental and combination modes.) PYRIDINE INTERACTION W I T H HIGHLY HYDRATED MATERIALS Fig. 3(a) shows the interaction of Py with a TSK starting sample activated at ambient temperature (i.e. still possessing most of the surface sulphate impurities and highly hydrated), whereas fig. 3 (b) shows the interaction of Py with a sample that was first freed from all sulphate contaminants* and then rehydrated and outgassed at ambient temperature.Most of the spectral features previously reported for TS evacuated at the same temperature3 are found for TSK as well, including the formation of PyH+ species when in the presence of sulphate contaminants, shown by the band * It was previously shown2 that the complete elimination of sulphate contaminants can be achieved upon a prolonged outgassing at temperatures as high as 873 K. In the present work another technique has been adopted, leading to the elimination of sulphate at lower temperatures and in shorter times, and thus with a less severe surface-area loss. It consists of the vacuum decomposition at ca. 773 K of previously chemisorbed Py : the reducing atmosphere owing to the pyrolysis of the organic admolecules causes an easier elimination of sulphates as well as partial reduction of TiO, 1,2 that is completely reversible upon 0, treatment at T 2 673 K.2C.MORTERRA, A. CHIORINO, G. GHIOTTI A N D E. FISICARO 2653 I 1650 1600 1550 1500 wavenumber/cm-' FIG. 3.-Interaction of Py with a TSK sample dehydrated at 298 K. (a) Virgin sample, (b) the same sample after the elimination of sulphate contaminants and rehydration. S1: background; S2: after contact with Py (266 N mP2); S3: after Py evacuation at 298 K. at ca. 1545 cm-l and by the intensity of the band at 1640 cm-l (see scheme of fig. 2). Also the almost complete ligand displacement of undissociated molecular water, which has been shown3 to lead to the formation of a Py species strongly Lewis-coordinated to coordinatively unsaturated Ti ions (Ticus ions, 8a mode at 1608-1614 cm-l), is observed on TSK, in both the presence and absence of residual sulphates.The following facts can also be observed. (i) There is a band at ca. 1600cm-l, ascribable to the 8a mode of Py hydrogen-bonded to surface hydroxyls, whose poor2654 I.R. STUDY OF THE TiO,/K,O SYSTEM intensity is nearly equal to that of the same species on TS outgassed at 298 K and that is nearly unchanged on passing from the sample of fig. 3(a) to that of fig. 3(b). (ii) Py species that are easily removed by evacuation at ambient temperature (namely, the liquid-like species with an unresolved 8a-8b mode at 1582 cm-l and the hydrogen-bonded species with an 8a mode at 1600 cm-l) possess a well-resolved 19a component at ca.1484 cm-l, whereas the sum of the 19a modes owing to both Lewis and Brarnsted species lies at ca. 1494 cm-l. (iii) There is a weak band at 1588- 1590 cm-l, not observable on TS,3 that is probably caused by the 8a mode of a new Py species, whose 8b mode is believed to lie in the unresolved 8b envelope at 1578 cm-l. The scheme of fig. 2 suggests the assignment to Py coordinated to weak Lewis-acidic sites, and the absence of such a species on TS allows us to ascribe it to Py interacting with coordinatively unsaturated K ions (K,,, ions). The proposed assignment is consistent with the i.r. and Raman frequencies of 8 a-8 b modes of Py Lewis-coordinated to K ions in zeolites.lO*ll Also, the small splitting between the two v8 ring modes is consistent with the weak electrostatic field produced by ions characterized by a large radius and low charge.1° (iv) On passing from fig.3(a) to fig. 3(b) there is no appreciable change in intensity of the band owing to Kc,,-coordinated Py species. It is thus argued that surface sulphates, which the spectral features of fig. 1 clearly indicate to be affected by the presence on TSK of K ions, when removed do not leave many K,,, ions capable of interacting with Py. PYRIDINE INTERACTION WITH DEHYDRATED MATERIALS Fig. 4 shows the 1400-1700 cm-l spectral range of Py adsorbed at ambient temperature and desorbed at various temperatures on a TSK sample dehydrated at 673 K, i.e. almost complete dehydration,2 and from which all sulphate impurities had been removed.The following observations can be made. (i) The very few residual hydroxyls2 still involve some Py by hydrogen bonding, yielding an 8a mode band at ca. 1600 cm-l of much the same intensity as that produced on the highly hydrated TSK sample of fig. 3, whereas on TS after outgassing at temperatures as low as 498 K hydrogen-bonded Py could no longer be observed in the spectrum. This suggests that potassium does not induce in anatase hydroxyls a higher capacity to undergo An-- 1650 1603 1550 1500 14 wavenumber/cm-' FIG. ,4.-Interaction of Py with a sulphate-free TSK sample dehydrated at 673 K. S1: background; S2: after contact with Py (266 N m-2); S3 and S4: after Py evacuation at 298 and 373 K, respectively.C. MORTERRA, A. CHIORINO, G.GHIOTTI A N D E. FISICARO 2655 hydrogen bonding with Py, but, unlike TS, such hydroxyls are desorbed last during thermal activation. (ii) The 19a mode of the easily removable Py species (1484 cm-l) is far more intense than on the sample activated at 298 K: as this increased intensity cannot be ascribed to hydrogen-bonded Py, whose intensity is at most unchanged, it must be caused by the liquid-like Py species (8a-8b modes at 1582 cm-l), thus confirming that this species cannot be thought of as being physically adsorbed, but that a weak specific interaction of the acid-base type must also be involved in this case, as previously s~ggested.~ (iii) The 8a mode owing to Py Lewis-coordinated to K,,, ions is much stronger than on the 298 K sample, and is completely desorbed within 373 K (spectrum 4 of fig.4). It was previously shown that Py strongly Lewis-coordinated to Ti,,, ions (8a mode at 1608-1614 cm-l) adsorbs at almost its maximum concentration even on samples activated at ambient temperature, through a ligand-displacement mechanism at the sites that coordinate water in the undissociated form. This is also true for TSK samples [see fig. 3(a) and (b)], so that the almost complete coordinative saturation of K ions on a 298 K sample, both in the presence and in the absence of residual sulphates, cannot be primarily ascribed to undissociated coordinated water. It is thus deduced that, as far as the production of activity towards Py is concerned, the thermal treatment of TSK samples in the 298-673 K range enhances the concentration of sites responsible for the adsorption of liquid-like Py species (as does the thermal treatment of TS samples) and creates Lewis-acid centres coming from the production of K,,, ions and that this is somehow connected with the surface dehydroxylation process rather than with the desorption of coordinated water or of surface sulp hate contaminants.PY R I D I N E INTER ACT I O N FOLLOW I N G HI GH-TE M PER A T U R E TREAT MEN T When the thermal treatment of pure TS anatase is carried out at temperatures higher than 673 K, i.e. at temperatures above complete dehydration, there is a rapid surface-area loss; this can be revealed by B.E.T. measurements2 and, upon Py chemisorption, by a severe weakening of the spectrum, although the overall features t c ... m '2 ;=" 2 i 6'XI 1600 1550 l650 l6bo 1550 1650 6;oo 1550 wavenumberlcm-' FIG.5.--Interaction of Py with a TS sample outgassed at 923 K (a), with a TSK sample outgassed at 923 K (b), and with a TSK sample outgassed at 1073 K (c). S1: background; S2: after contact with Py (266 N m-"; S3: after Py evacuation at 298 K.2656 I.R. STUDY OF THE TiO,/K,O SYSTEM remain almost unchanged. [See fig. 5(a), relative to a sample treated at 923 K: only the liquid-like Py species exhibits a decreased relative intensity.] If the same thermal treatment is carried out on TSK and Py is allowed to contact the sample, fig. 5(b) shows that the overall shape of the spectrum changes drastically. (i) The band owing to the 8a mode of K-coordinated Py possesses a much higher relative intensity with respect to all other components, both in the presence of Py and after Py evacuation at ambient temperature.As for the absolute intensity of the band at 1590 cm-l, it is higher on a sample treated at 923 K [fig. 5(b)] than on a sample treated at 673 K (fig. 4) or at 773 K (not shown in the figures), despite the surface-area loss. This fact indicates that the complete elimination of surface hydroxyls (very few OH groups remain on the 673 K sample and none on the 773 K one, on the basis of the i.r. data) is not the sole mechanism through which K,,, ions, acting as Lewis-acidic centres, are produced in this temperature range. (ii) If the intensity of the band envelope at 1582 cm-l(8a-8b mode of liquid-like Py) and at 1578 cm-l(8b mode of all other species) is assumed as a rough estimate of the total amount of chemisorbed P Y , ~ ~ * fig.5(a) and (b) tell us that on the relevant samples the total amounts of Py are comparable, but after Py evacuation at ambient temperature much less Py remains adsorbed on TSK. In particular, the number of Py species Lewis-coordinated to Ti,,, ions is halved by such a treatment, whereas it is slightly decreased on TS. The quicker reversibility of this species on TSK is made particularly evident in fig. 5(c), showing that the small amount of Py still Lewis-coordinated to Ti,,, ions on a TSK sample activated at 1073 K is fully reversible at ambient temperature. Also, the fraction of Py coordinated to K,,, ions reversible at ambient temperature is higher the higher the sample activation temperature.The above experiments suggest that, even at temperatures above 1073 K (the temperatures employed to prepare TiO, pigments), all Lewis acidity owing to Ti,,, ions and characteristic of the early stages of thermal treatment would be destroyed by the presence of potassium. REVERSIBILITY OF SURFACE ACIDITY MODIFICATIONS The reversibility of TSK surface acidity, modified by high-temperature treatment, has been checked as follows. A sample treated as in fig. 5(c) and freed from all chemisorbed Py was kept in long contact with water vapour, and further dehydrated at various temperatures. After each treatment the acidity was checked by Py ad- sorption at ambient temperature, and the relevant results are summarized in fig. 6. Even after dehydration at ambient temperature, the ratio of Py Lewis-coordinated at K,,, sites to Py Lewis-coordinated at Ti,,, sites is higher than on the starting material.The incomplete reversibility on rehydration of the acidity changes is made even more evident by parts (b) and (c) of fig. 6: on the sample treated at 673 K the 8a band at 1590 cm-l is already stronger than the 8a band at 1610 cm-l, and after thermal treatment at 823 K we obtain the same profile for the Py bands and rapid reversibility which are typical of previously untreated samples treated at higher temperatures [see fig. 5(b) and (c)]. Note also that the Py 8 b mode at 1578 cm-l and 19a mode at ca. 1490 cm-l (not reported in fig. 6) are similar in the three sections, showing that no further surface-area changes have occurred, and that the overall amount of chemisorbed Py does not vary appreciably with activation temperature.It is thus confirmed that, both before and after complete dehydration, the Lewis acidity of the K,,, ions is produced at the expense of the Lewis acidity of the Ti,,, ions. Prolonged contact with water at ambient temperature of TSK samples freed fromC. MORTERRA, A. CHIORINO, G. GHIOTTI AND E. FISICARO 2657 1650 1600 1650 1600 1550 wavenurnber/cm-' FIG. 6.-Interaction of Py with a TSK sample outgassed at 973 K, rehydrated at 298 K (14 h) and further outgassed at: (a) 298, (b) 673 and (c) 823 K. S1: after contact with Py (266 N m-z); S2: after Py evacuation at 298 K. all sulphate contaminants (see previous footnote) previously outgassed at high temperatures restores the ratio between stronger and weaker Lewis-acid sites only partly.In fact, on passing from a sample rehydrated at ambient temperature after outgassing at 773 K to a sample rehydrated after outgassing at 973 K, there is a strong decrease in the ratio of the total amount of coordinated water to the amount that remains coordinated after evacuation at ambient temperature : the ratio between the absorbances at the H,O bending maximum passes from ca. 0.40 to ca. 0.16. ADSORPTION OF CARBON MONOXIDE In a previous paper3 is was shown that TS samples still carrying residual sulphate impurities behave differently towards CO chemisorption from anatase samples having undergone different preparation paths. After only a thermal elimination of sulphates2 very little can be said about the activity towards CO, as the high temperature required brings about such a severe surface-area loss that all the i.r.activity of CO adsorption is lost. In contrast, the thermal-chemical procedure adopted in the present work to eliminate sulphate impurities (see previous footnote) allows TS samples to maintain a sufficient surface area to yield well-detectable i.r. bands on CO chemisorption. Fig. 7(a) shows that sulphate-free rehydrated TS samples behave like all other anatase sample^,^' l2 giving rise to one CO band (2184-2187 cm-l) after activation at T -= 473 K and two CO bands (2188 and 2208 cm-l) after activation at T 2 473 K. These bands reach maximum intensity following complete surface dehydration, and for still higher temperatures slowly decline in a parallel way owing to sintering of the material.Fig. 7(b) shows that a sulphate-free rehydrated TSK sample behaves quite differently: after dehydration at 473 K there is only the 2188 cm-l CO component, as on TS, and with an intensity comparable to that of TS. However, this CO band remains the sole component even on activation at higher temperatures; its intensity starts to decline long before complete surface dehydration, and reduces to zero for CO adsorption after thermal treatment at T 2 923 K. Volumetric measurements indicate that the decrease in surface area with activation temperature is much the same2658 t % e E 0 ." ." E u I.R. STUDY OF THE TiO,/K,O SYSTEM 2250 2150 2250 2150 wavenumber/cm-' FIG. 7.-Interaction of CO (5.3 x lo3 N m-*) with a sulphate-free TS sample (a) and a sulphate-free TSK sample (b) after outgassing at: S1, 473; S2, 673; S3, 773 and S4, 973 K.on TS and TSK, so that the above data suggest the following. (i) No CO adsorption occurs on the K,,, Lewis-acidic sites, whose production occurs at temperatures > 298 K and continues above the temperature of complete dehydration. This is not unexpected, in view of the weak polarizing field produced by alkali-metal ions. (ii) Consistent with what has been suggested by Py adsorption, the production of K,,, Lewis-acidic sites progressively destroys the strong Ti,,, Lewis-acidic sites ; this process starts at medium activation temperatures for the fraction of Ti,,, sites responsible for the 21 88 cm-1 CO band, but is complete at any activation temperature for the Ti,,, sites responsible for the 2208 cm-l CO component, i.e.for the sites that are formed last upon TS dehydration and for which a more polarizing field causes a higher upward shift of the CO stretching frequency. CONCLUSIONS The present study indicates that K,O, added in small amounts to anatase gels, mostly collects at the surface at any stage of thermal treatment. In fact the spectrum of surface sulphate impurities is modified to an extent that cannot be justified by a 1 wt % concentration of K,O, if homogeneously distributed. Although surface hydroxyls on TSK do not reveal appreciable spectral differences from those on TS, owing to poor resolution in the relevant spectral region, they show in different ways their interaction with added K,O, thus confirming the surface location of potassium ions.In particular, unlike TS, all surface OH groups must be eliminated for hydrogen bonding with adsorbed Py to disappear, and reasonable amounts of surface hydroxyls must be eliminated for potassium ions to reveal themselves through direct interaction with suitable test molecules.C. MORTERRA, A. CHIORINO, G. GHIOTTI AND E. FISICARO 2659 The surface location of K20 is confirmed by its easy extraction from anatase powders in water suspension. At the concentration levels employed here and following the thermal treatments which we adopted, no K20 segregates as a separate phase. If this were in fact the case, changes in the activity towards CO, adsorption would be expected, and these were never observed.The presence of K20 mostly affects, in a destructive way, strong surface Lewis acidity which was previously ascribed to Ti,,, ions. Even if K 2 0 is present at the surface at any stage of the preparation, high-temperature treatment is needed to affect appreciably strong Lewis acidity. In fact Ti,,, acidity, mostly produced by the cationic centres responsible for undissociated water coordination, manifests itself in the ‘normal’ way of a K-free sample after a thermal treatment at low temperatures, but at higher temperatures is progressively replaced by a weaker Lewis acidity, owing to K,,, ions, while the remaining part is rendered more and more vulnerable to evacuation. The interaction with CO, that can distinguish between the two Ti,,, families of sites whereas Py can not, and the interaction with water are vital in order to confirm that the elimination of Ti,,, strong Lewis sites occurs for the strongest ones first.An extrapolation of the present spectroscopic data seems to indicate that, on the final product, no Ti,,, ions are exposed, and that the entire surface layer should be thought of as a layer of potassium titanate, partly reversible to water vapour in the absence of other additives. A rough calculation can be attempted in order to follow this process. Complete dehydration of anatase was shown to occur with the elimination of ca. 8 water molecules per nm2,3 leaving as many coordinatively unsaturated cations at the surface. If we assume that the K 2 0 is all at the surface and that the Ti02-K titanate surface transformation occurs through the direct substitution of Ti cations for K cations, a K,O concentration of 1 % and a surface area of some 100 m2 g-l (at ca.773 K) would mean 16% K surface ions, and a surface area of some 15 m2 g-l (at ca. 1023 K) would mean 100% of surface K ions. The roughness of this calculation is evident, especially in comparison with the spectral data, which indicate a small residual Ti,,,/Py activity after treatment at temperatures as high as 1023 K, but the overall trend is confirmed. As the final pigmentary product, ready for the grain-coating process (to be dealt with elsewhere), has a surface area of ca. 2-4 m2 g-l, this calculation tells us that a K,O concentration of ca. 0.2% (the K 2 0 Concentration usually adopted) is sufficient for a complete surface-layer transformation. Finally, the present results indicate what is the most probable surface chemical composition of the material undergoing the next preparation step (i.e. the coating step), and also suggest that what actually controls the mechanism of crystal growth and the process of grain-shape modification during preparation is the nature and concentration of the surface acidic centres. C. Morterra, A. Chiorino, A. Zecchina and E. Fisicaro, Gazz. Chim. Ital., 1979, 109, 683. C. Morterra, A. Chiorino, A. Zecchina and E. Fisicaro, Gazz. Chim. Ital., 1979, 109, 691. C. Morterra, G. Ghiotti, E. Garrone and E. Fisicaro, J. Chem. Soc., Faraday Trans. I , 1980,76,2102. C. Morterra, A. Chiorino, F. Boccuzzi and E. Fisicaro, 2. Phys. Chem. (N.F.), 1981, 124, 21 1 . K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds (J. Wiley, New York, 1978), p. 241. L. J. Bellamy, Advances in Infrared Group Frequencies (Methuen, London, 1968), p. 224. H. Knozinger, Adu. Catal., 1976, 25, 222. C. Morterra, A. Chiorino, G. Ghiotti and E. Garrone, J. Chem. Soc., Faraday Trans. I , 1979,75,271. C. Morterra, S. Coluccia, A. Chiorino and F. Boccuzzi, J . Catal., 1978, 54, 348. lo J. W. Ward, J . Catal., 1968, 10, 34. l1 T. A. Egerton, A. H. Hardin and N. Sheppard, Can. J . Chem., 1976, 54, 586. M. Primet, J. Bandiera, C. Naccache and M. V. Mathieu, J. Chim. Phys., 1970, 67, 535. (PAPER 1 / 1425)

ISSN:0300-9599    

DOI:10.1039/F19827802649

出版商:RSC    

年代:1982

数据来源: RSC