摘要:

Gas Kinetics Group and Division de Chimie-Physique de la Societe Francaise de Chimie 9th International Symposium on Gas Kinetics To be held in Bordeaux, France on 20-25 July 1986 Further information from Dr R. Lasclaux, Lab. Photophys. Photochim. MolBculaire, Universite de Bordeaux I, 33405 Talence Cedex, France Poiymer Physics Group Biologically Engineered Polymers To be held at Churchill College, Cambridge on 21-23 July 1986 Further information from Dr M. J. Miles, AFRC,Food Research Institute, Colney Lane, Norwich NR4 7UA Polymer Physics Group with the British Rheological Society Deformation in Solid Polymers To be held at the University of Leeds on 9-1 1 September 1986 Further information from Dr J. V. Champion, Department of Physics, City of London Polytechnic, 31 Jewry Street, London EC3N 2EY ~~_____________ ~~~~ Carbon Group Carbon Fibres- P ro pe rt i es and A p p I i cat i o ns To be held at the University of Salford on 1 5 1 7 September 1986 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1 X 8QX ~ ~~~~~~~~ ~ Division with the Surface Reactivity and Catalysis Group-Autumn Meeting Promotion in Heterogeneous Catalysis To be held at the University of Bath on 23-25 September 1986 Further information from: Professor F.S. Stone, School of Chemistry, University of Bath, Bath BA2 7AY (viii)Gas Kinetics Group and Division de Chimie-Physique de la Societe Francaise de Chimie 9th International Symposium on Gas Kinetics To be held in Bordeaux, France on 20-25 July 1986 Further information from Dr R.Lasclaux, Lab. Photophys. Photochim. MolBculaire, Universite de Bordeaux I, 33405 Talence Cedex, France Poiymer Physics Group Biologically Engineered Polymers To be held at Churchill College, Cambridge on 21-23 July 1986 Further information from Dr M. J. Miles, AFRC,Food Research Institute, Colney Lane, Norwich NR4 7UA Polymer Physics Group with the British Rheological Society Deformation in Solid Polymers To be held at the University of Leeds on 9-1 1 September 1986 Further information from Dr J. V. Champion, Department of Physics, City of London Polytechnic, 31 Jewry Street, London EC3N 2EY ~~_____________ ~~~~ Carbon Group Carbon Fibres- P ro pe rt i es and A p p I i cat i o ns To be held at the University of Salford on 1 5 1 7 September 1986 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1 X 8QX ~ ~~~~~~~~ ~ Division with the Surface Reactivity and Catalysis Group-Autumn Meeting Promotion in Heterogeneous Catalysis To be held at the University of Bath on 23-25 September 1986 Further information from: Professor F. S. Stone, School of Chemistry, University of Bath, Bath BA2 7AY (viii)

ISSN:0300-9599    

DOI:10.1039/F198581FX025

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

xxxij AUTHOR INDEX Singh, Km. S., 751 Sircar, S., 1527, 1541 Slade, R. C. T., 847 Smith, I. G., 1095 Snelling, C. M., 1761 Sobczyk, L., 311 Siiderberg, D., 17 15 Solar, S., 1101 Solar, W., 1101 Soma, M., 485 Somorjai, G. A., 1263 Somsen, G., 1015 Sorek, Y., 233 Souto, F. A., 2647 Spencer, S., 2357 Spichiger-Ulmann, M., 7 13 Spoto, G., 1283 Spotswood, T. M., 1623 Srivastava, R. D., 913 Stachurski, J., 1447, 2813 Staricco, E. H., 1303 Stock, T., 2257 Stockhausen, M., 397 Stokes, R. H., 1459 Stone, F. S., 1255 Strachan, A. N, 1761 Strohbusch, F., 2021 Stuckless, J. T., 597 Su, Z., 2293 Subrahmanyam, V. S., 1655 Sugimoto, N., 1441, 2959 Suminaka, M., 2287 Suprynowicz, Z., 553 Sutcliffe, L. H., 679, 1467, 1215 Suzanne, J., 2339 Suzuki, H., 3117 Swallow, A. J., 1225 Symons, M.C. R., 433, 565, 727, 2131, 2775, 1095, 1963, 242 1 Takagi, Y., 1901 Takahashi, Y., 3 117 Takeshita, H., 2805 Tamilarasan, R., 2763 Tamura, K., 2287 Tanaka, T., 1513 Taniewska-Osinska, S., 695, Tascon, J. M. D., 939, 2399 Taylor, M. J., 1863 Taylor, N., 2357 Tejuca, L. G., 939, 2399, 1203 Teller, R. G., 1693 Tempere, J-F., 1357 Teramoto, M., 2941 Theocharis, C . R., 857 Thomas, J. K., 735 Tielen, M., 2889, 3049 Tindwa, R. M., 545 Tissier, C., 3081 Toi, K., 2835 Tokuda, T., 2835 Torrez-Mujica, T., 343 Townsend, R. P., 1071, 173 1, Trasatti, S., 2995 Treiner, C., 2513 Trenwith, A. B., 745 Trifiro, F., 1003 Troncoso, G., 1631, 1637 Tseung, A. C. C., 1883 Tuck, J. J., 833 Turner, J. E., 1263 Uemoto, M., 2333 Uma, K., 2733 Valencia, E., 1631. 1637 Valigi, M., 813 Vallmark, T., 1389 Van Oort, M.J. M., 3059 Varma, M. K., 751 Vattis, D., 2043 Vecli, A., 433 Veseli, V., 2095 Vink, H., 1677, 1725 Vliers. D. P., 2009 Vukovid, Z., 1275 3081, 1913 3127 Waghorne, W. E., 2703 Ward, A. J., 2975 Watanabe, H., 1569 Waugh, K. C., 3073 Weckstrorn, K., 2947 Weinberg, N. N., 875 Weingartner, H., 1031 Wells, C. F.. 801, 1057, 1401, White, M. A., 3059 Williams, J. O., 271 1 Williams, P. A., 2635 Williams, P. B., 3067 Williams, R. T., 847 Wojcik, D., 1037 Wood, G. L., 265 Wood, R. M., 273 Woolf, L. A., 769, 2821 Wright, C. J., 2067 Wright, J. P., 1471 Wright, T. H., 1819 Wurie, A. T., 2605 Yadav, G. D., 161 Yadava, R. D., 751 Yamaguchi, M., 1513 Yamaguti, K., 1237 Yamasaki, S., 267 Yamashita, H., 2485 Yamatera, H., 127 Yelon, W., 1693 Yoshida, S., 1513, 2485 Yoshikawa, M., 2485 Zambonin, P.G.. 621 zdanov, S. P., 2541 Zecchina, A., 1283 Zelano, V., 2365 Zhan, R. Y., 2083 Zhao, Z., 185 Zhulin, V. M., 875 Zilnyk, A., 679, 1215 Zulauf, M., 2947 Zundel, G., 1425, 2375 1985. 2145, 2475, 3091xxxij AUTHOR INDEX Singh, Km. S., 751 Sircar, S., 1527, 1541 Slade, R. C. T., 847 Smith, I. G., 1095 Snelling, C. M., 1761 Sobczyk, L., 311 Siiderberg, D., 17 15 Solar, S., 1101 Solar, W., 1101 Soma, M., 485 Somorjai, G. A., 1263 Somsen, G., 1015 Sorek, Y., 233 Souto, F. A., 2647 Spencer, S., 2357 Spichiger-Ulmann, M., 7 13 Spoto, G., 1283 Spotswood, T. M., 1623 Srivastava, R. D., 913 Stachurski, J., 1447, 2813 Staricco, E. H., 1303 Stock, T., 2257 Stockhausen, M., 397 Stokes, R. H., 1459 Stone, F. S., 1255 Strachan, A.N, 1761 Strohbusch, F., 2021 Stuckless, J. T., 597 Su, Z., 2293 Subrahmanyam, V. S., 1655 Sugimoto, N., 1441, 2959 Suminaka, M., 2287 Suprynowicz, Z., 553 Sutcliffe, L. H., 679, 1467, 1215 Suzanne, J., 2339 Suzuki, H., 3117 Swallow, A. J., 1225 Symons, M. C. R., 433, 565, 727, 2131, 2775, 1095, 1963, 242 1 Takagi, Y., 1901 Takahashi, Y., 3 117 Takeshita, H., 2805 Tamilarasan, R., 2763 Tamura, K., 2287 Tanaka, T., 1513 Taniewska-Osinska, S., 695, Tascon, J. M. D., 939, 2399 Taylor, M. J., 1863 Taylor, N., 2357 Tejuca, L. G., 939, 2399, 1203 Teller, R. G., 1693 Tempere, J-F., 1357 Teramoto, M., 2941 Theocharis, C . R., 857 Thomas, J. K., 735 Tielen, M., 2889, 3049 Tindwa, R. M., 545 Tissier, C., 3081 Toi, K., 2835 Tokuda, T., 2835 Torrez-Mujica, T., 343 Townsend, R.P., 1071, 173 1, Trasatti, S., 2995 Treiner, C., 2513 Trenwith, A. B., 745 Trifiro, F., 1003 Troncoso, G., 1631, 1637 Tseung, A. C. C., 1883 Tuck, J. J., 833 Turner, J. E., 1263 Uemoto, M., 2333 Uma, K., 2733 Valencia, E., 1631. 1637 Valigi, M., 813 Vallmark, T., 1389 Van Oort, M. J. M., 3059 Varma, M. K., 751 Vattis, D., 2043 Vecli, A., 433 Veseli, V., 2095 Vink, H., 1677, 1725 Vliers. D. P., 2009 Vukovid, Z., 1275 3081, 1913 3127 Waghorne, W. E., 2703 Ward, A. J., 2975 Watanabe, H., 1569 Waugh, K. C., 3073 Weckstrorn, K., 2947 Weinberg, N. N., 875 Weingartner, H., 1031 Wells, C. F.. 801, 1057, 1401, White, M. A., 3059 Williams, J. O., 271 1 Williams, P. A., 2635 Williams, P. B., 3067 Williams, R. T., 847 Wojcik, D., 1037 Wood, G. L., 265 Wood, R. M., 273 Woolf, L. A., 769, 2821 Wright, C. J., 2067 Wright, J. P., 1471 Wright, T. H., 1819 Wurie, A. T., 2605 Yadav, G. D., 161 Yadava, R. D., 751 Yamaguchi, M., 1513 Yamaguti, K., 1237 Yamasaki, S., 267 Yamashita, H., 2485 Yamatera, H., 127 Yelon, W., 1693 Yoshida, S., 1513, 2485 Yoshikawa, M., 2485 Zambonin, P. G.. 621 zdanov, S. P., 2541 Zecchina, A., 1283 Zelano, V., 2365 Zhan, R. Y., 2083 Zhao, Z., 185 Zhulin, V. M., 875 Zilnyk, A., 679, 1215 Zulauf, M., 2947 Zundel, G., 1425, 2375 1985. 2145, 2475, 3091

ISSN:0300-9599    

DOI:10.1039/F198581BX027

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

JOURNAL OF T H E CHEMICAL SOCIETY F A R A D A Y TRANSACTIONS, PARTS I A N D I I The Journal of the Chemical Society is published in six sections, of which five are termed Transactions; these are distinguished by their subject matter, as follows: Dalton Transactions (Inorganic Chemistry). All aspects of the chemistry of inorganic and organometallic compounds; including bioinorganic chemistry and solid-state inorganic chemistry; of their structures, properties, and reactions, including kinetics and mechanisms; new or improved experimental techniques and syntheses. Faraday Transactions I (Physical Chemistry). Radiation chemistry, gas-phase kine tics, elect roc hemist ry (0 t her than preparative), surface and interfacial chemistry, heterogeneous catalysis, physical properties of polymers and their solutions, and kinetics of polymerization, etc.Faraday Transactions II (Chemical Physics). Theoretical chemistry, especially valence and quantum theory, statistical mechanics, intermolecular forces, relaxation phenomena, spectroscopic studies (including Lr., e.s.r., n.m.r., and kinetic spec- troscopy, etc.) leading to assignments of quantum states, and fundamental theory. Studies of impurities in solid systems. Perkin Transactions I (Organic Chemistry). All aspects of synthetic and natural product organic, organometallic and bio-organic chemistry, including aliphatic, alicyclic, and aromatic systems (carbocyclic and heterocyclic). Perkin Transactions I1 (Physical Organic Chemistry). Kinetic and mechanistic studies of organic, organometallic and bio-organic reactions.The description and application of physicochemical, spectroscopic, and theoretical procedures to organic chemistry, including structure-activity relationships. Physical aspects of bio-organic chemistry and of organic compounds, including polymers and biopolymers. Authors are requested to indicate, at the time they submit a typescript, the journal for which it is intended. Should this seem unsuitable, the Editor will inform the author. The sixth section of the Journal of the Chemical Society is Chemical Communications, which is intended as a forum for preliminary accounts of original and significant work, in any area of chemistry that is likely to prove of wide general appeal or exceptional specialist interest. Such preliminary reports should be followed up eventually by full papers in other journals (e.g.the five Transactions) providing detailed accounts of the work. NOTES 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 a&ounts 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 anarticle is a full paper or a Note, but since there is no sharp dividing line between the one and the other, either in terms of length or character of content, the right is retained to transfer overlong Notes to the full papers section. As a guide a Note should not exceed I500 words or word-equivalents. (i)NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet ‘Quantities, Units, and Symbols’ (1975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W 1 V OBN).These recommendations are applied by The Royal Society of Chemistry in all its publications. Their basis is the ‘Systeme International d’Unites’ (SI). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A , B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn).Nomenclature of Inorganic Chemistry (Butterworths, London, I97 1, now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society’s editorial staff.(ii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 20 Phase Transitions in Adsorbed Layers University of Oxford, 17-1 8 December 1985 Organising Committee: Professor J. S. Rowlinson (Chairman) Dr E. Dickinson Dr R. Evans Mrs Y. A. Fish Dr N. Parsonage Dr D. A. Young The aim of the meeting is to discuss phase transitions at gadliquid, liquidbquid and solid/fluid interfaces, and in other systems of constrained geometry or dimensionality less than three. Emphasis will be placed on molecularly simple systems, whereby liquid crystal interfaces and chemisorption phenomena are excluded. The preliminary programme may be obtained from : Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO.81 Lipid Vesicles and Membranes Loughborough University of Technology, 15-17 April 1986 Organising Committee : Professor D. A. Haydon (Chairman) Professor D. Chapman Mrs Y. A. Fish Dr M. J. Jaycock Dr I. G. Lyle Professor R. H. Ottewill Dr A. L. Smith Dr D. A. Young The aim of the meeting is to discuss the physical chemistry of lipid membranes anG their interactions, in particular theoretical and spectroscopic studies, polymerised membranes, thermodynamics of bilayers and liposomes, mechanical properties, encapsulation and interaction forces between bilayers leading to fusion but excluding preparation and characterisation methodology. The preliminary programme may be obtained from: Mrs Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN Full papers for publication in the Discussion Volume will be required by December 1985.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 82 Dynamics of Molecular Photof ragmentation University of Bristol, 15-1 7 September 1986 Organising Committee : Professor R. N. Dixon (Chairman) Dr G. G. Balint-Kurti Dr M. S. Child Professor R. Donovan Professor J. P. Simons The discussion will focus on the interaction of radiation with small molecules, molecular ions and complexes leading directly or indirectly to their dissociation. Emphasis will be given to contributions which trace the detailed dynamics of the photodissociation process. The aim will be to bring together theory and experiment and thereby stimulate important future work.Contributions for consideration by the Organising Committee are invited and abstracts of submitted as soon as possible, and abstracts of about 300 words by 30 September 1985, to: Professor R. N. Dixon, Department of Theoretical Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 21 Interaction-induced Spectra in Dense Fluids and Disordered Solids University of Cambridge, 10-1 1 December 1986 Organising Committee : Professor A. D. Buckingham (Chairman) Dr R. M. Lynden-Bell Dr P. A. Madden Professor E. W. J. Mitchell Dr J. Yarwood Dr D. A. Young Mrs Y. A. Fish Whilst interaction-induced spectra have been studied in the gas phase for many years, their importance in the spectroscopy of condensed matter has been appreciated only relatively recently. At present a considerable number of studies of induced spectra are taking place in what are (nominally) widely separated fields of study.It is highly desirable to bring these communities together so that common issues can be identified and the progress of one field appreciated in another. Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words shodd be sent by 25 October 1985 to: Professor A. D. Buckingham, University Chemical Laboratory, Lensfield Road, Cambridge CB2 1 EW30TH INTERNATIONAL CONGRESS OF PURE A N D APPLIED CHEMISTRY Advances in Physical and Theoretical Chemistry Manchester, 9-1 3 September 1985 The Faraday Division is mounting the following symposia as part of the 30th IUPAC Congress: A.B. C. D. Reaction Dynamics in the Gas Phase and in Solution This symposium will examine the ways in which modern techniques allow detailed study of the dynamical motion of molecules which are undergoing chemical reaction or energy exchange. Micellar Systems The symposium will discuss various aspects of micellization, including size and shape factors, micellization in biological systems, chemical reactions in micellar systems, micelle structure and solubilization. Emphasis will also be given to modern techniques of examining micellar systems, including small-angle neutron scattering, neutron spin echo, photocorrelation spectroscopy, NMR and use of fluorescent probes.Surface Science of Solids The symposium will centre on recent advances in the study of kinetics and dynamics at surfaces and of phase transitions in adsorbate layers on single crystal surfaces. Both experimental and theoretical aspects will be reviewed with an emphasis on metal single crystal surfaces. New Electrochemical Sensors (in collaboration with the Electroanalytical Group of the Analytical Division) The symposium will cover such topics as the fundamentals of the subject, new gas sensors based on membrane electrodes and on ceramic oxides, the development of new ion- (selective electrodes and the synthesis of new guest-host carriers, the development of CHEMFETS and other integrated devices together with the theory of the operation of such devices, and finally the development of biosensors including for instance enzyme electrodes, direct electron transfer to biological molecules and new potentiometric techniques for protein analysis.The full programme and application form may be obtained from: Dr J. F. Gibson, 30th IUPAC Congress, Royal Society of Chemistry, Burlington House, London W1 V OBNTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 83 Brownian Motion University of Cambridge, 7-9 April 1987 Organising Committee Dr M. La1 (Chairman) Dr R. Ball Dr E. Dickinson Dr J. S. Higgins Dr P. N. Pusey Dr 0. A. Young Mrs Y. A. Fish The aim of the meeting is to discuss new developments in the experimental and theoretical studies of Brownian motion of colloidal particles and macromolecules, with particular emphasis on the dynamics of aggregate formation and breakdown, computer simulation and many-body hydrodynamic interactions.Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 15 June 1986 to: Dr M. Lal, Unilever Research, Port Sunlight Laboratory, Bebington, Wirral L63 3JW Full papers for publication in the Discussion volume will be required by December 1986 JOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistry/chemical physics which have appeared recently in J.Chem.Research, The Royal Society of Chemistry's synopsis+microform journal, include the following: Quantum-mechanical Studies of Catalysis. Part 1. A Model for Nucleophilic Attack on Carbonyl, catalysed by Non-functional Cationic Surfactants Amiram Goldblum and Jehoshua Katzhendler (1 985, Issue 3) Cyclopropane Parameters for Molecular Mechanics Pekto M.lvanov (1 985, Issue 3) Stereochemical Applications of Potential Energy Calculations. Part 4. Revised Electron Spin Resonance Studies of the Ammonia-Boryl Radical (H,N -+ BH;); an Inorganic Analogue of the Ethyl Radical Jehan A. Baban, Vernon P. J. Marti, and Brian P. Roberts (1985, Issue 3) The Iron-Vanadium-Oxygen System at 11 23, 1273, and 1373 K. Part 1. Phase Equilibria Larbi Marhabi, Marie-Chantal Trinel-Dufour and Pierre Perrot (1 985, Issue 3) Solvent Effects on the Rotational Barriers of the N,N-Dimethylamides of 2- and 3-Furoic and 2- and 3-Thenoic Acids Gaetano Alberghina, Francesco Agatino Bottino, Salvatore Fisichella, and Caterina Arnone (1 985, Issue 4) A Partial Determination of the Stability Fields of Ferrierite and Zeolites ZSM-5, ZSM-48, and Nu-10 in the K20-A120,-SO2-NH, [CH2],NH2 System Abraham Araya and Barrie M .Lowe (1 985, Issue 6) The Level of Prochirality : the Analogy between Substitutional and Distortional Desymmetrization Amitai E. Halevi (1 985, Issue 6)FARADAY DIVISION INFORMAL AND GROUP MEETINGS Polymer Physics Group Biennial Conference : Physical Aspects of Polymer Science To be held at the University of Reading on 11-1 3 September 1985 Further information from Professor Bassett, J. J. Thompson Physical Chemistry Laboratory, University of Reading, Whiteknights, Reading RG6 2AF Statistical Mechanics and Thermodynamics Group Multicomponent Mixtures To be held at the University of East Anglia on 16-1 8 September 1985 Further information from: Dr M.J. Grimson, Food Research Institute, Colney Lane, Norwich NR4 7UA Carbon Group Strength and Structure in Carbons and Graphites To be held at the University of Liverpool on 16-1 8 September 1985 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1X 8QX Division with the Institute of Physics Seventh National Quantum Electronics Conference To be held at the Abbey Hotel, Great Malvern on 16-20 September 1985 Further information from: Dr E. Jakeman, Treasurer QE7, RSRE, St Andrews Road, Great Malvern WRl4 3PS Surface Reactivity and Catalysis Group with the Catalysis Section of the KNCV Mechanism and Structure in Heterogeneous Catalysis To be held at Noordwijkerhout, The Netherlands on 18-20 September 1985 Further information from: Dr R.Joyner, BP Research Centre, Chertsey Road, Sunbury on Thames TW16 7LN Industrial Physical Chemistry Group A Molecular Approach to Lubrication and Wear To be held at Girton College, Cambridge on 23-25 September 1985 Further information from Mr M. P. Dare-Edwards, Shell Research Ltd, Thornton Research Centre, Chester CH1 3SH Neutron Scattering Group jointly with the Materials Testing Group of the Institute of Physics Industrial Uses of Particle Beams To be held at the Institute of Physics, London on 26 September 1985 further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1 X 8QX Colloid and Interface Science Group with the Colloid and Surface Science Group of the SCI Interfacial Rheology To be held at Imperial College, London on 16 December 1985 Further information from Dr R.Aveyard, Department of Chemistry, The University, Hull HU6 7RX High Resolution Spectroscopy Group and Theoretical Chemistry Group Title to be Announced To be held at the University of York on 1 6 1 8 December 1985 Further information may be obtained from: Dr J. M. Hollas, Department of Chemistry, University of Reading, Whiteknights, Reading RG6 2AD Neutron Scattering Group Time-resolved Scattering and Transition Kinetics To be held at Imperial College, London on 17 December 1985 Further information may be obtained from: Dr J.S. Higgins, Department of Chemical Engineering, Imperial College, London SW7 2BY (vii)Molecular Beams Group with CCCPG Molecular Scattering-Theory and Experiment To be held at the University of Sussex on 19-21 March 1986 Further information from Dr A. Stace, School of Molecular Sciences, University of Sussex, Falmer, Brighton BN1 9QJ Division-Annual Congress Structure and Reactivity of Gas Phase Ions To be held at the University of Warwick on 8-1 1 April 1986 Further information from : Professor K. R. Jennings, Department of Molecular Sciences, University of Warwick, Coventry CV4 7AL Polymer Physics Group with the Statistical Mechanics and Thermodynamics Group Macromolecular Flexibility and Behaviour in Solution To be held at the University of Bristol on 16-1 8 April 1986 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1 X 8QX Industrial Physical Chemistry Group Physical Chemistry of Water Soluble Polymers To be held at Girton College, Cambridge on 1-3 July 1986 Further information from Dr I. D. Robb, Unilever Research Laboratory, Port Sunlight, Bebington, Wirral L63 3JW Polymer Physics Group Biologically Engineered Polymers To be held at Churchill College, Cambridge on 21-23 July 1986 Further information from Dr M. J. Miles, AFRC Food Research Institute, Colney Lane, Norwich NR4 7UA Carbon Group Carbon Fibres-Properties and Applications To be held at the University of Salford on 1 5 1 7 September 1986 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SWl X 8QX Division with the Surface Reactivity and Catalysis Group-Autumn Meeting Promotion in Heterogeneous Catalysis To be held at the University of Bath on 23-25 September 1986 Further information from: Professor F. S. Stone, School of Chemistry, University of Bath, Bath BA2 7AY (viii)

ISSN:0300-9599    

DOI:10.1039/F198581FP057

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1985, 81, 1513-1525 Ab Initio Molecular-orbital Study on the Adsorption of Ethylene and Oxygen Molecules over Vanadium Oxide Clusters BY HISAYOSHI KOBAYASHI* AND MASARU YAMAGUCHI Faculty of Living Science, Kyoto Prefectual University, Shimogamo, Kyoto 606, Japan AND TSUNEHIRO TANAKA AND SATOHIRO YOSHIDA Department of Hydrocarbon Chemistry and Division of Molecular Engineering, Kyoto University, Kyoto, Japan Received 14th May, 1984 The ab initio Hartree-Fock molecular-orbital method has been used to investigate the electronic structures of a VO,H, cluster as a model for silica-supported vanadium oxide catalysts and the interactions between the cluster and ethylene and/or oxygen molecules. Wavefunctions have been obtained for the singlet ground state and the lowest triplet state formed by irradiation of ultraviolet light.The V=O bond in the triplet state is longer by 0.3 A than that in the ground state. The ethylene molecule is adsorbed in a stable form on the cluster only in the triplet state. The interaction between the cluster and oxygen is repulsive for both the singlet and triplet states. However, the coadsorption of both molecules is found to be stable for the triplet state, These results are consistent with recent experimental data, and the mechanism of electronic interactions is discussed in detail using the molecular-orbital expansion technique. Photoassisted reactions over transition-metal oxides have attracted much attention as a possible process for solar-to-chemical energy conversion, and many reports have been published.' However, most studies have been concerned with the yield or distribution of products.Only a few papers attempt to clarify the mechanism of photoassisted reactions at the molecular The elementary reaction steps and, especially, the structure of intermediates in the photo-oxidation of hydrocarbons have scarcely been discussed, although a general scheme from alkanes to aldehydes or ketones through alcohols and olefins has been pr~posed.~ In a previous experimental study2 we examined the nature of active oxygen species and the intermediates in the photo-oxidation of propene over vanadium oxide supported on silica, and proposed a mechanism initiated by photoadsorption of propene and oxygen molecules to form the first intermediate.A detailed discussion of the electronic state of the excited catalyst and of the electronic process of interaction between the catalyst and reactants, however, has remained for further study. The active state of vanadium oxide is generally recognized as an electronic triplet state and was first proposed by Kazansky et al.5 Although some evidence supporting the proposal has been reported,2v6 it remains as merely a concept explaining the observed experimental data. In the present paper we try to provide a quantum-chemical explanation of the first step in the photo-oxidation by performing an exact molecular-orbital (MO) calculation. The electronic state of the catalysts, the structure of the intermediates and electronic aspects of the catalyst-molecule interactions are discussed in detail.15131514 M.O. STUDY OF VANADIUM OXIDE CLUSTERS model A model B i model D: model E model C model G Fig. 1. Models used in the calculation. Model A is the VO,H, free cluster. The lengths of the V-0 and 0-H bonds and the V-0-H angle are taken to be 1.85 and 0.96 8, and 125", respectively. The VO, group is assumed to have tetrahedral symmetry (the 0-V-0 angle is 109.3"). The length of the V=O double bond is optimized. This optimized value is employed without further optimization in the following models for molecular adsorption. Models B-E are for ethylene adsorption. In models B and C the C=C axis of ethylene and the V=O axis of the cluster are perpendicular and T-shaped. Model C is obtained from model B by rotating the ethylene molecule by 90" around the V=O axis.Only the distance between the free cluster and the ethylene molecule is varied. The geometry of the free ethylene molecule is used throughout the calculation. In models D-G the V=O bond is inclined to the opposite side of the adsorbing molecules. In model D the V=O and C-C bonds form a trapezoid whose height is taken to be 1.8 A. In models E and G the angle c between the C,, axis of the VO, group and the line passing through the V atom and the middle of the C=C or 0=0 bond is fixed at 30". The distance between the C and V atoms is assumed to be 1.92 A in model E. Model F for oxygen adsorption is obtained from model E by replacing ethylene with oxygen. The standard values for 8 and c and the 0=0 and V-0, bond lengths are 45 and 30" and 1.44 and 1.87 8,, respectively, while other configurations are also examined.Model G is constructed from model F by adding an ethylene molecule. The angle q between the V=O axis and the line passing through the 0 atom and the middle of the C=C bond is varied. The VO-C,H, distance is fixed at the optimum value determined in models B and C.H. KOBAYASHI, M. YAMAGUCHI, T. TANAKA AND S. YOSHIDA 1515 MODELS A unit of vanadium oxide supported on silica is modeled by the VO,H, cluster, which hereafter is referred to as model A, as shown in fig. 1. Our e.s.r. and other data suggest that the active site in V205/Si02 is an isolated VO, unit rather than a V205 crystallite.' The VO, unit seems to be connected to the support, forming an 0-Si-0 linkage. Hydrogen atoms are used to saturate the valency of the oxygen atoms, which appears by a truncation of the cluster from the support. Although the cluster is improved by including silica units, the expansion of cluster size makes MO calculations on adsorption systems very difficult.The present truncation technique may not be the best, but we think it is better than a technique employing a highly charged cluster with an 0- anion instead of the OH group. Thus replacing the OSiO with the OH group is not an unreasonable approximation since we are mainly concerned with the reactivity of the V=O double bond. According to our recent experimental study, the propene molecule is thought to be adsorbed on the vanadium oxide through the C=C double bond.2 Since we are concerned with the initial modes of interaction between propene and the oxide, ethylene is used in place of propene in the present theoretical study to facilitate numerical calculations.Cluster-thylene interactions have been investigated by employing four adsorption models (models B-E), with model B the most extensively employed. In models B and C the C=C and V=O axes are perpendicular to each other, and ethylene forms an epoxide-type complex. In model D the C=C and V=O bonds constitute a trapezoid, i.e. a four-membered metallocycle. In model E ethylene is coordinated to the vanadium atom. In the latter two models, the V=O bond is inclined to the opposite side of the ethylene molecule in order to decrease steric repulsion. The oxygen molecule is assumed to be adsorbed on the vanadium atom in the side-on configuration shown in fig.1 (model F). This model is obtained from model E by replacing ethylene by an oxygen molecule. Model G shows the coadsorption state of ethylene and oxygen. METHOD OF CALCULATION The MO calculations were carried out at the Hartree-Fock (HF) level using the GAUSSIAN-80 program.8 For the triplet state the restricted HF (RHF) and unrestricted HF (UHF) methods were employed. Both methods give similar features for the free cluster, the ethylene-adsorbed cluster and the ethylene- and oxygen-adsorbed clusters with respect to the structure of stable configurations and the electron- and spin-density distributions. For oxygen adsorption a very large spin polarization occurs in the V=O group for the UHF calculation, but this is absent for the RHF calculation.Therefore we will discuss the electronic interactions mainly with reference to the RHF results, although the UHF results are also shown. The ( 1 2 ~ 6 ~ 4 4 Gaussian basis set for the vanadium atom reported by Roos et al. was contracted to [5~2pld],~ and a d orbital with the exponent 0.0885 was added to the set according to Hay.l0 In order to represent the 4p atomic orbitals (AO) a p orbital with the exponent 0.22 was added. Thus the basis set for V adopted in the present study is expressed as (12~7p54/[5~3p2d]. For the carbon and oxygen atoms and the hydrogen atom the 3-21G and 21G basis sets, respectively, were used?1516 0 - M.O. STUDY OF VANADIUM OXIDE CLUSTERS (C) .-? I 1.44 1.54 1.64 1.74 1.44 1.54 1.64 1.74 1.64 RESULTS AND DISCUSSION FREE CLUSTER The stable structures of the free cluster were calculated in both the ground (singlet) and the lowest triplet states by optimizing the V=O bond length, Both the RHF and UHF methods give a lower total energy for the triplet state when the V=O bond is elongated.An absolute comparison in energy between states with different spin multiplicities requires correlated wavefunctions beyond the HF level. In this work we examine the stable structure of clusters for the ground and triplet states individually without comparing the total energy between the two states. Fig. 2 shows the relative energy of the cluster as a function of the bond length. A value of 1.54 A was obtained for the singlet states. This agrees with the V=O bond length in the V,O, crystal.12 For the triplet state the value was calculated to be 1.84 A by means of both the RHF and UHF methods.Thus the bond length is elongated by 0.3 A. These optimal V=O bond lengths are used in later calculations for the chemisorbed clusters without further optimization. In the VOgH3 cluster the lowest twenty-nine orbitals are occupied in the singlet state as shown in fig. 3. The lowest thirteen MO (41-413) are composed of the core A 0 (the 1s to 3p A 0 of V and the 1s A 0 of 0). 419-417 are ascribed to the four 2s A 0 of 0 and 41a-420 the three 0-H bonds. 421 constitutes the V=O a-like bond,? and t The VO,H, cluster has C, symmetry. However, it is convenient to consider the VO bond as being composed of one a and two n bonds. So we refer these bonds as the ‘a-like’ and ‘n-like’ bonds.H.KOBAYASHI, M. YAMAGUCHI, T. TANAKA AND S. YOSHIDA 1517 -12 -13 b ' -14 - 15 - 1 6 -17 * 31 V = O ( f i c ] 30 LUMO O ( n ) 29 O ( n ) 28 27 26 v-0 - v- 0 25 24 23 v=o ( N v=o (fi) 21 v=o (6) - 22 Fig. 3. The energy and nature of the higher-lying MO for the VO,H, cluster. v=o (W v-0 m") SOMO 0 0 1.84 I I 1,54 I V V ground state triplet state Fig. 4. Orbital patterns for the n- and n*-like MO in the VO,H, cluster.1518 a M.O. STUDY OF VANADIUM OXIDE CLUSTERS 100 r 80 60 LO 20 ' ,y-7 , 0, '. 20 0 1.5 2.0 2.5 3.0 d&A d&A Fig. 5. Adsorption energy of ethylene on the VO,H, cluster. (a) Ground state, d = 1.54 A; (b) triplet state (RHF), d = 1.84 A; 0, model B; 0, model C; (c) triplet state (UHF). 422 and 423 the V=O n-like bonds.The higher-lying six MO (424-429) constitute the three V-0 single bonds and the three OZp lone-pair orbitals. Thus the n-like MO localizing in the V=O region are not the highest occupied MO (HOMO), while the lowest unoccupied MO (LUMO) is the corresponding n*-like MO. The orbital patterns for these two MO are shown in fig. 4(a). The triplet-state wavefunction was constructed by one-electron excitation from the bonding n-like MO to the antibonding n*-like MO, which spreads in the same plane as the n-like MO. The two SOMO are also shown in fig. 4(b). The V=O bond population decreases from 0.39 for the singlet state to 0.26 in the RHF result for the triplet state, which clearly shows a weakening of the bond. ADSORPTION OF ETHYLENE The interactions between ethylene and the free cluster were examined in both the singlet and triplet states.The changes in the total energy are shown in fig. 5 for models B and C. The distance between the oxygen atom in V=O and the carbon atoms in ethylene, do+, was varied. In the singlet state the energy increases monotonically as the molecule approaches the cluster. However, a stable intermediate is expected around 1.6 A of do.+ in the triplet state, although a potential barrier of ca. 40 kcal mol-l should be overcome.? Models B and C are compared with each other at the optimal do-c distance. The difference in energy is very small, as expected from the quasi-C,, symmetry of the free cluster. (The V04 species is of exactly C3, symmetry.) In order to understand the difference in reactivity for ethylene between the singlet and triplet states, the mode of electronic interactions has been analysed using the MO t 1 cal = 4.184 J.H.KOBAYASHI, M. YAMAGUCHI, T. TANAKA AND S. YOSHIDA 1519 C- C + n f 4 7 Fig. 6. Antibonding-orbital mixing between the n MO of ethylene and the MO localized in the V=O bond for the singlet state. expansion technique. The MO of the chemisorbed system (here V04H,-C,H4) are represented by a linear combination of the MO of the component subsystems (here VO,H, an+ C2H4). In the singlet state the n MO of ethylene mixes with the 2s A 0 of the V=O oxygen ( 4 1 7 ) and the V=O a-like MO (421). These three MO are all occupied in the respective component systems. In the chemisorption system all three orbitals constructed by mixing the bonding and antibonding modes are occupied.In this case the overall interactions bring about destabilization because of exchange repulsion among the occupied MO. The antibonding combination constructed from the 4 1 7 , 421 and n MO is shown in fig. 6 . This destabilization causes a monotonic increase in total energy. The orbital interactions for the triplet state are complicated, as shown in fig. 7. The upper part of fig. 7 shows the two SOMO, the V=O a-like MO for the V04H, cluster and the n MO of ethylene. The bonding combination of the V=O a-like bond and the ethylene IC MO is stabilized by the cluster-ethylene interaction. The corresponding antibonding combination, however, is destabilized and raised to the energy nearest to those of the SOMO even at a long cluster-ethylene distance (do-c = 2.6 A).Simultaneously, the two SOMO of the cluster are rehybridized to form two non-bonding orbitals localized on the 0 2p and V 3d AO, respectively. As a result of these changes in orbital energy the 0 2p orbital is doubly occupied in the VO,H,-C2H4 system and further stabilized by the back-donated charge transfer to the n* MO of ethylene. The new antibonding SOMO, composed of the V=O CT and ethylene n MO, is further destabilized by the approach of ethylene, and changes smoothly to an orbital localized within the V=O region. These replacements of the MO are characteristic of ethylene adsorption in the triplet state, and not only avoid electronic repulsion but also weaken both the V=O and C=C bonds. In fact the V=O bond population decreases from 0.26 for the free cluster to 0.10 for the chemisorbed state, and the C=C bond population decreases from 0.54 for a free molecule to 0.20 for the chemisorbed state.Thus the rearrangement of MO occurs in the triplet state which can not completely be described by the HF wavefunctions. However, this rearrangement also occurs in1520 M.O. STUDY OF VANADIUM OXIDE CLUSTERS SOMO SOM 0 0 SOMO Y V S O M O Fig. 7. Modes of orbital mixing between ethylene and the cluster in the triplet state. long-range interactions, and the potential barrier shown in fig. 5 is not an artifact caused by the HF wavefunetions. In order to examine other possibilities in the coordination of ethylene to the cluster, calculations have been carried out for models D and E.The results show that the interactions are repulsive even in the triplet state. The adsorption energies are calculated to be - 122(RHF) and - 106(UHF) kcal mol-l for model D, and - 118(RHF) and -95(UHF) kcal mol-1 for model E. These models are examined by single-point calculations where the distance between ethylene and the cluster is notH. KOBAYASHI, M. YAMAGUCHI, T. TANAKA AND S. YOSHIDA Table 1. Adsorption energy (AE) of the oxygen molecule on a VO,H, cluster 0 55 1.21 2.0 - 74.6 15 30 1 .44a 1 .87a - 70.9 45 30 1.44 1.87 - 35.4 45 30 1.22 1.83 - 50.3 45 30 1.33 1.85 - 36.5 45 30 1.60 1.90 -61.6 1521 a These values for the 0-0 and V-0 bond lengths are taken from ref. (15). In units of kcal mo1-I. Fig. 8. Two SOMO for the VO,H,-0, system. varied.However, we may conclude that the configurations represented by models D and E are not realistic, judging from the large negative values of the adsorption energy. According to the GVB calculations for chromyl chloride by Rappe and Goddard,13 the metallocycle is stable if the metal atom possesses more than two 0x0 bonds. Since the present VO,H, cluster possesses only one 0x0 (V=O) bond, the unfavourable interaction for model D is also consistent with their conclusion. ADSORPTION OF OXYGEN MOLECULE Adsorption of oxygen molecule is investigated using model F. The two angles (6' and r) and the 0-0 bond length (do-o) are varied. Table 1 shows the results of RHF calculations for the interactions between the cluster and the oxygen molecule. The interactions are found to be repulsive for all configurations.This repulsion is decreased by the inclination of the V=O bond to the opposite side of the oxygen molecule and the stretching of the 0-0 bond. The least unstable configuration is calculated to be 0 = 45", 5 = 30" and do-o = 1.44 A. These structural parameters are used in model G for the coadsorption of ethylene and oxygen. The two SOMO for the VO,H,-O, system are the 0 2p non-bonding orbital of the V=O bond and the n* MO of the oxygen molecule, as shown in fig. 8. As in the case of ethlene adsorption, exchange among the doubly occupied, singly occupied and vacant MO occurs as the oxygen molecule approaches. Another n* MO of the oxygen molecule is doubly occupied by donative charge transfer mainly from the V 3d,, AO, as shown in fig.9. Thus the adsorbed oxygen is formally written as O,, although the charge on the oxygen molecule is considerably less than - 1 due to a relaxation of electron density. In the UHF calculation very large spin polarization occurs for the V=O bond region, and a large negative (p) spin density appears on the V atom with a still larger1522 M.O. STUDY OF VANADIUM OXIDE CLUSTERS Fig. 9. Donative charge transfer from the V 3d A 0 to the x* MO of oxygen. Table 2. Adsorption energy (AE) of ethylene and oxygen on a VO,H, cluster 45 30 180 20.2 17.5 45 30 150 25.0 21.5 45 30 120 -28.4 - 30.4 a In units of kcal mol-l. positive value on the 0 atom. The eigenvalue of s2 is ca. 3 (for RHF results it is 2). Therefore we do not refer to the UHF results for this system. ADSORPTION OF ETHYLENE AND OXYGEN MOLECULES The calculations for the VO,H,-C,H,-O, system are carried out using model G, where the angle 7 (see fig.1) is varied to find a stable configuration. Table 2 shows that stabilization is obtained in the range 150 < q / O < 180. A slightly larger adsorp- tion energy is obtained for q = 150" than for q = 180". This result suggests that the interactions between the ethylene and oxygen molecules are attractive although their magnitude is small. The SOMO are the V 3d non-bonding orbital and the n* MO of the oxygen molecule. The orbital interactions between ethylene and the cluster and between oxygen and the cluster are qualitatively the same as those for the case of single-molecule adsorption. However, the individual interactions are strengthened by coadsorption ; the mechanism is discussed in the next section.GENERAL DISCUSSION We have examined the interactions for the individual chemisorption systems. The systems are compared with one another in order to elucidate the difference between single-molecule adsorption and coadsorption. Table 3 shows the charges estimated from Mulliken population analysis1, within the atom(s) and molecule before and after adsorption. Comparing the charge on the V and 0 atoms in the VO,H, cluster for the singlet and triplet states, we cannot confirm that the triplet state is the one-electron-transferred state from the 0 atom to the V atom. The excitation of one electron from the n-like MO to the n*-like MO certainly contributes to electron transfer to the V atom.However, reverse electron transfer occurs in the V=O a-like MO and almost completely compensates for the electron deficiency in the 0 atom. On adsorption, ethylene becomes positively charged. The electron density withdrawnH. KOBAYASHI, M. YAMAGUCHI, T. TANAKA AND S. YOSHIDA 1523 Table 3. Charges on the atom(s) and molecule before and after adsorption V 0 C2H4 0, 3(0-H) - 0.9 1 V0,H3 (singlet) +1.29 -0.38 - - 0.97 VO4H3 +1.34 -0.37 - V04H,-C2H4 +1.19 -0.66 +0.82 - - 1.35 V04H3-C2H4a + 1.20 -0.62 +0.77 - - 1.35 V04H3-02a + 1.45 -0.33 - -0.31 -0.81 V04H3-C2H4-02a +1.43 -0.66 +0.79 -0.48 -1.08 - - a V=O bond is inclined. Table 4. Changes in the MO population for the n MO of ethylene and the n* MO of ethylene and oxygen C2H4 0 2 AP(n) AP(n*) AP(n* + z * ) ~ V04H3-C,H4 (singlet) - 0.760 0.195 - -1.120 0.339 - 0.600 V04H3-C2H442 - 1 .1 4 4 0.359 0.737 - - V04H3-C2H4 V04H342 a Since the two n* MO are singly occupied before adsorption and singlet oxygen is taken as the reference state after adsorption, the tabulated value is the sum of the two n* MO populations subtracted by 2. from ethylene accumulates not only at the V=O bond region but also on the oxygen atoms represented by the OH group. Thus even local electron transfer influences the electron distribution of the whole cluster, otherwise the electron density would be accumulated within narrow regions. A comparison between the fourth and fifth rows of table 3 shows that electron transfer from the C,H, group is weakened by the inclination of the V=O axis.For the adsorption of molecular oxygen the third and sixth rows of table 3 should be compared with each other. The electron density transferred to the oxygen molecule comes from the V atom and also from the OH oxygen atoms. For the coadsorption of oxygen and ethylene the negative charge on the oxygen molecule is larger than that for the adsorption of oxygen only, i.e. charge transfer to the oxygen molecule is enhanced. The charges in the V and 0 atoms of the V=O bond are similar to those for the case of single-molecule adsorption by oxygen and ethylene, respectively. However, the charge on the 0-H group lies between the values for the two single-molecule adsorptions. These results suggest that the OH oxygen atoms work as an electron reservoir although they are not reactive to the adsorbing molecules.In order to obtain information on different aspects of the cluster-adsorbate interactions, the MO populations for important orbitals of the adsorbates are calculated. Table 4 shows the changes in population for the n and n* MO of the ethylene and oxygen molecules in the chemisorbed states. In the VO,H,-C,H, system1524 M.O. STUDY OF VANADIUM OXIDE CLUSTERS both the decrease and increase in the populations for the n and n* MO are enhanced in the triplet state, which suggests that the C=C bond of ethylene is more weakened in the triplet state. The decrease in the n MO population is larger than the increase in the II* MO population. In the case of oxygen adsorption the increase in the n* MO population of oxygen is ascribed to donative charge transfer mainly from the V 3 4 , AO.The accumulated electron density in the n* MO certainly works to weaken the 0-0 bond. On the coadsorption of both molecules the enhanced decrease in the n MO population in ethylene and enhanced increase in the n* MO population in ethylene and oxygen again suggest that coadsorption mutually strengthens the individual molecule-cluster interactions, which is favourable for the decomposition of adsorbed molecules and the reaction between them. CONCLUSION The present paper reports the electronic structures of vanadium oxide catalyst modelled by the VO,H, cluster and the interactions between the cluster and adsorbed molecules, i.e. ethylene and oxygen, from the viewpoint of quantum chemistry. The following results have been obtained by a series of ab initio MO calculations.(1) The optimal V=O bond length for VO,H, in the triplet state is longer by 0.3 A than that in the ground state. A population analysis also shows that the bond is weakened in the triplet state. (2) The triplet wavefunction is constructed by one-electron excitation from the V=O n-like MO to the n*-like MO. The V=O bond in the triplet state is not recognized as the one-electron-transferred state from 0 to V owing to a redistribution of electron density over the whole cluster. (3) Ethylene is adsorbed on the cluster in the triplet state but not in the ground state. Electronic repulsion between the II MO of ethylene and the occupied MO localized in the V=O bond region prevents stable adsorption in the ground state.In the triplet state the repulsion is considerably diminished by a rearrangement of the MO. (4) The interaction between the oxygen molecule and the cluster is repulsive even for the triplet state. The two 7t* MO of oxygen are singly and doubly occupied. Thus the adsorbed oxygen possesses anionic character but the charge on the oxygen is much less than - 1 because of relaxation of the electron density. (5) The adsorption of both ethylene and oxygen leads to a return to attractive interactions. The electron density is transferred from ethylene to oxygen through the cluster, which enhances the individual ethylene-luster and oxygensluster interactions and is considered to make the whole system stable. Information drawn from the present theoretical study may shed light on fundamental problems in heterogeneous catalysis using metal oxides, and particularly in pho t ocatal y sis.We thank the Data Processing Centre of Kyoto University for generous use of the FACOM M-200/382 computer and the Computer Centre of the Institute for Molecular Science for permission to use the HITAC M-200H computer. Part of this work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education of Japan.H. KOBAYASHI, M. YAMAGUCHI, T. TANAKA AND S. YOSHIDA 1525 R. I. Bickley, in Catalysis (Specialist Periodical Report, The Chemical Society, London, 1982), vol. 5, p. 308. S. Yoshida, T. Tanaka, M. Okada and T. Funabiki, J. Chem. SOC., Faraday Trans. I , 1984,80, 119. M. Anpo and Y. Kubokawa, J. Catal., 1982,75,204; M. Anpo, I. Tanahashi and Y. Kubokawa, J. Chem. SOC., Faraday Trans. I , 1982, 78, 2121 ; J. Cunningham and B. K. Hodnett, J. Chem. SOC., Faraday Trans. I , 1981,77,2777; J. Cunningham, M. Ilyas and E. M. Leahy, J. Chem. SOC., Faraday Trans. I , 1982, 78, 3297; J. M. Herrman, J. Disdier, M-N. Mozzanga and P. Pichat, J. Catal., 1979, 60, 369. * M. Formenti and S. J. Teichner, in Catalysis (Specialist Periodical Report, The Chemical Society, London, 1978), vol. 2, p. 87. A. M. Gritscov, V. A. Shvets and V. B. Kazansky, Chem. Phys. Lett., 1982,35, 51 1. M. Anpo, I. Tanahashi and Y. Kubokawa, J. Phys. Chem., 1980,84, 3440; 1982,86, 1. ' S. Yoshida, T. Iguchi, S. Ishida and K. Tarama, Bull. Chem. SOC. Jpn, 1972,45, 376. J. S. Binkley, R. A. Whiteside, R. Krishnan, R. Seeger, D. J. DeFrees, H. B. Schlegel, S. Topiol, L. R. Kahn and J. A. Pople, Quantum Chemistry Program Exchange, 1981, 13,406. B. Roos, A. Veillard and G. Vinot, Theor. Chim. Acta, 1971, 20, 1. lo P. J. Hay, J. Chem. Phys., 1977, 66, 4377. l1 J. S. Binkley, J. A. Pople and W. J. Hehre, J. Am. Chem. SOC., 1980, 102, 939. l2 A. F. Wells, in Structural Inorganic Chemistry (Clarendon Press, Oxford, 1975), p. 470. l 3 A. K. rap^ and W. A. Goddard 111, J. Am. Chem. SOC., 1980, 102, 5114; 1982,104,448; 3287. l4 R. S. Mulliken, J. Chem. Phys., 1955, 23, 1841. H. Mimoun, L. Saussine, E. Daire, M. Postel, J. Fischer and R. Weiss, J. Am. Chem. SOC., 1983, 105, 3101. (PAPER 4/790)

ISSN:0300-9599    

DOI:10.1039/F19858101513

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1985,81, 1527-1540 Excess Properties and Thermodynamics of Multicomponent Gas Adsorption BY SHIVAJI SIRCAR Air Products and Chemicals Inc., Box 538, Allentown, Pennsylvania 18105, U.S.A. Received 23rd May, 1984 The thermodynamics of multicomponent gas adsorption is developed by using surface excess variables, which constitute the true experimental properties for measurement of adsorption. It is shown that the thermodynamic properties of practical use such as the surface potential, the isosteric heats of adsorption and the heat capacity of the adsorption system can be expressed and evaluated using experimental surface excess data. New methods for estimating isosteric heats of adsorption from multicomponent gases and several new thermodynamic consistency tests are described.The true experimental variable in measuring the extent of adsorption from a single gas or a multicomponent gas mixture of i components is the surface excess, which was originally conceptualized by Gibbs.' The surface excess is the difference between the actual amount of component i adsorbed and the amount of component i that would be present in the adsorbed phase if the composition and the density of that phase were the same as those of the equilibrium gas phase. Fig. 1 describes a multicomponent adsorption system according to the Gibbs model. It consists of an adsorbed phase of volume V', an equilibrium gas phase of volume V and a unit amount of the adsorbent. It is assumed that the adsorbent is inert and non-volatile and that there is no absorption of the gases.The gas phase is at pressure P and temperature T and its composition (mole fraction of component i) is yi. The molar densities of the gas and the adsorbed phases are, respectively, p and p'. The amount of component i present in the adsorbed phase is ni( = V'p'x;) where xi( = n;/Xni) is its mole fraction in that phase. n'(= En;) is the total amount of all adsorbates in the adsorbed phase. ni and no are, respectively, the total amounts of component i and the total amount of all adsorbates present in the system. V" is the volume of the system accessible by the adsorbates. A volume and a material balance for the system gives V" = v+ V' (1) n: = Vpyi+ V'p'x; no = C ny = Vp+ V'p'. (3) P, T, p, yi, ni, no and Yo are the only experimental variables for the system.P , T and y i can be directly measured by properly instrumenting the gas phase. p can then be calculated using an equation of state for the gas phase. p is equal to P/RTfor an ideal gas. n: and no can be obtained by measuring how much of each adsorbate is contacted with the clean adsorbent prior to the adsorption process or by completely desorbing the system (cleaning the adsorbent) and measuring the total amount of the desorbed gas (no) and its composition (y:). ny is then equal to nor;. V" can be measured by introducing a known amount of a non-adsorbing gas into the completely evacuated system and measuring its pressure change. In principle there is no such thing as a 15271528 MULTICOMPONENT GAS ADSORPTION gas phase pressure = P volume= I/ temperature = T density = p mole fraction of component i = yi Gibbs interface adsorbed phase volume = V‘ temperature = T density = p ’ amount adsorbed of component i = nf adsorbent unit amount Fig.1. Schematic diagram of an equilibrium multicomponent gas adsorption system according to the Gibbs model. non-adsorbing gas, but helium at low P and high T is usually considered to have negligible adsorption compared with most adsorbates of practical interest. Everett has discussed in detail the negligible but real uncertainties caused by this assumption.2 It is assumed that the helium volume ( V ” ) is also accessible by the other adsorbates. The variables V, V’, p’ and ni, on the other hand, depend on the size of the adsorbed phase and its structure, which cannot be measured. V’ in fig.1 is arbitrarily defined by the dashed line known as the Gibb’s interface. It can, however, be shown from eqn (l)-(3) that n:- V”py, = V’(p’x;-pyi) (4) no - Vop = V’@’ -p). ( 5 ) The quantities on the left-hand sides of eqn (4) and ( 5 ) are experimental variables. Thus the quantities on the right-hand sides of these equations can also be estimated, although some of their terms depend on the choice of the location of the Gibb’s interface. The tight-hand-side quantities of eqn (4) and (5) are, respectively, called the surface excess of component i ( n y ) and the total surface excess (nm = C n p ) : n? = V‘(p‘x; - p y i ) (4’ ) nm = V’(p’ - p ) . (5’) There is only one surface excess variable [nm* = V’(p’-p)] for a pure-gas (x’ = y = 1) adsorption.n r and nm are the actually measured variables to represent the extents of adsorption from a multicomponent gas mixture. An alternative way to define the surface excess ( n f ) for component i of a multicomponent gas mixture is by n: = n’(xi-yi). ( 6 )S. SIRCAR 1529 It can be shown from eqn (1)-(3) that nf = n: -nay,. The right-hand-side quantities of eqn ( 6 ) are experimental variables. Thus nf can also be measured. This definition of surface excess does not require V” for its estimation. By this definition Clnf = 0, for a mixture (7) ne* = 0, for a pure gas. (8) Thus the second definition is only meaningful for representing adsorption from a gas mixture. The first definition is applicable to both the pure gas and the mixture adsorption.It can be shown that Eqn (9) relates the two types of surface excess variables for the ith component of a gas mixture. SURFACE EXCESS AND ACTUAL AMOUNT ADSORBED It follows from eqn ( 5 ’ ) that for a pure-gas adsorption n’ 1 --- nm* - ( I - C ) where C( = p/p’) is the ratio of the molar densities of the gas and the adsorbed phases. The difference between n’ and nm* depends on the value of C, which cannot be estimated without assuming the density of the adsorbed phase. For adsorption much below the critical temperature of the gas (c), p’ is often assumed to be of the same order of magnitude as pL, the liquid density of the ad~orbate.~ Thus for low pressures, p 4 pL (C 4 1) and n’ z nmL. This approximation need not be correct when T > T,. Furthermore, at high pressure, p can be comparable with p’ at all T and n’ may be significantly different from nm*.This is demonstrated by the maxima and the minima in the measured adsorption isotherms of pure gases (n: plotted against P) in the high P ~egion.~ n’, on the other hand, is a monotonically increasing function of P. For a binary gas mixture an experimental selectivity parameter (S) may be defined as Component 1 is more selectively adsorbed if S > 1. Eqn (4), (9, and (1 1) may be combined to obtain nk 1 + Cy,(S- 1) nF 1-c -- - Eqn (12) and (13) show that the differences between ni and n$ for a binary gas adsorption depend on S and yi as well as on C. They also indicate that ni z np when the gas pressure is low (C 4 1) but n;l # np, even for low P, when S is large.An order1530 MULTICOMPONENT GAS ADSORPTION Table 1. Differences between np and n; for ethane (l)+methane (2) on BPL activated carbon at 301.4 K Platm y1 nylmol kg-l a nplmol kg-l a S n;/ny n;l/np - 1.006 1.29 1.000 - - 1.29 0.501 2.41 0.18 13.1 1.005 1.29 0.255 1.66 0.33 14.6 1.002 - 1.030 6.80 1.000 - 6.80 0.501 3.73 0.57 6.5 1.017 6.80 0.255 2.87 0.95 8.9 1.010 - 1.062 - 13.60 1.000 5.10 - - .039 .025 - .111 .090 - 13.60 0.501 4.30 1.13 3.8 1.039 1.149 13.60 0.255 3.29 1.39 6.9 1.022 1.155 a Experimental. Calculated by eqn (12) and (13) assuming p’ = 9.5 mmol (liquid density). of magnitude for each of these differences was estimated for the adsorption of ethane (l)+methane (2) mixtures on BPL activated carbon using the data of Reich5 at 301.4 K. Ethane is selectively adsorbed from methane.p’ was assumed to be equal to the liquid density (9.5 mol ~ m - ~ ) of ethane at 301.4 K. Table 1 shows the results. n ; / n y x 1 in the pressure range of the data, but the difference between n;l and np can be significant even at moderate pressures and at moderate values of S. It may therefore be concluded that the surface excesses, rather than the actual amounts adsorbed, should generally be treated as the appropriate variables for representing the extent of adsorption. Significant error can be caused by approximating n f as n; for pure gases at high pressures or for the less strongly adsorbed species of a mixture even at moderate pressures when T < T,. The approximation is very speculative at all pressures when T > T,.The purpose of this work is then to develop the thermodynamics of multicomponent gas adsorption using the surface excess as the variable and to demonstrate its practicality. Most of the earlier developments of adsorption thermodynamics were based on the actual amounts adsorbed as the prime Everett and Hill have used surface excess variables to develop the thermodynamics of pure-gas THERMODYNAMICS OF MULTICOMPONENT ADSORPTION We define the excess thermodynamic properties (free energy, internal energy, enthalpy, entropy and volume) in accordance with the Gibbs model of fig. 1 as zm = V’(p’z’-pz) (14) follows : where z can be the molar internal energy (u), the molar free energy (g), the molar enthalpy (h), the molar entropy (s) or the molar volume (u). zm is the excess property per unit mass of the adsorbent for a given P, T and yi or for a given T and n p .z’ and z are the molar thermodynamic properties of the adsorbed and the bulk phases, respectively. Alternatively, the second version of the mass surface excess defined by eqn (6) may be used to express the excess thermodynamic properties ( z e ) as follows: (1 5 ) ze = n’(z’ - z).S . SIRCAR 1531 It can be shown by combining eqn (1)-(5), (14) and (15) that ze = zm - nmz. Eqn (16) relates the excess properties defined by the two versions of the Gibbs model. The total (gas + adsorbed phases) thermodynamic property (zo) of the adsorption system of fig. 1 may be obtained as (16) zo = zm + Vopz = ze + noz. (17) It follows from the definitions of the excess properties that they are related to each other like the ordinary thermodynamic properties.Thus g" = h"- Tsm; ge = he- Tse (18) hm =um+Pvm; he = ue+Pve. (19) Similar relationships also apply for the total thermodynamic properties (2') and the gas-phase properties (z). It also follows from the definitions that Urn = 0 (20) ve = --n"/p. (21) We use the thermodynamic framework developed by Bering et al. to describe the adsorbed phase, which gives8 u' = Ts' - Pv' + C pi xi + @In' (22) 0 = s'dT-u'dP+C xidpi+d@/n' (23) where @ is the surface potential and pi is the chemical potential of the ith component in the adsorbed phase. The term @ in eqn (22) replaces the term containing the product of the specific surface area and the surface pressure commonly used in thermodynamic developments.We prefer the use of @ because it applies to both microporous adsorbents and solids with defined surface areas. The bulk-phase thermodynamic properties are related by lo u = Ts-Pu+C piyi (24) 0 = s dT- u dP+C yi dpi. (25) pi = pf( T) + RT lnfi (26) where pt(T) is the chemical potential of the pure gas i at system temperature T and atmospheric pressure. p: is a function of T only. fi is the fugacity of component i in the gas phase.fi = Py, for an ideal gas. ,ai is the chemical potential of the ith component in the bulk phase defined bylo The criteria for equilibrium between two phases islo pi = pi. (27) gm = @ + Z p i n f ; ge = @+Zp,n? (28) dg" = -smdT+Epidnr; dge = -sedT+uedP+Cpidnf (29) dh" = Tds"+C pidn?; dhe = Tdse+uedP+Zpidnf (30) d@ = -s"dT-C nfdp, = -sedT+vedP-C nfdpi. (31) Eqn (4)-(6) and (14)-(26) may be combined to get1532 MULTICOMPONENT GAS ADSORPTION Eqn (18) and (19) and (28H31) form the key set of thermodynamic equations to describe the multicomponent adsorption system in terms of the surface excess variables.It is apparent from eqn (28H31) that the absolute values of zm or ze for a multicomponent system cannot be estimated from the experimental surface excess data; however, changes of these properties due to changes in P, T, yi, ny or n: can be calculated. The surface potential, on the other hand, can be calculated from the experimental equilibrium data by integration of eqn (3 1) under several specific conditions to be described later. For a multicomponent gas mixture the adsorption equilibrium data (nf) can be measured in three ways: (a) as a function of P at constant T and yi, (6) as a function of Tat constant P and yi and (c) as a function of yi at constant P and T. Experimental data from the last category of measurement can be conveniently used only for a binary mixture where a single adsorbate mole fraction defines the system.These measurements can be carried out by different methods. The simplest method is to saturate the adsorbent of known weight, contained in a chamber of known void volume ( Y O ) , with the gas mixture of composition yi at a given P and T. The total amount of adsorbate is then desorbed from the chamber by evacuation and heating and the total gas evolved (no) and its composition ( y i ) are measured. n f and nm can then be calculated using eqn (4) and (5). The experiment can then be repeated by varying one of the variables among P, T and yi while keeping the other two constant.For a pure-gas adsorption where there are only two variables (P, T) the conventional volumetric or gravimetric measurement technique can be used to obtain nmf as a function of P at constant T (isotherm) or as a function of T at a constant P (isobar). CALCULATION OF @ The surface potential can be calculated from the following experimental data. PURE GAS: CONSTANT T Eqn (26) and (31) may be combined for constant T to get d@* = -RTnm* d lnf where Qi* is the surface potential for a pure gas atfand to obtain (32) T. Eqn (32) can be integrated (33) IDEAL BINARY GAS: CONSTANT T, P Eqn (31) can be combined with eqn (7) and (26) for an ideal binary gas mixture at constant Tand P to obtain ne YlY2 d@ = - R T L dvl.Eqn (34) can be integrated to obtain Yi n: 0 YlY2 (a$-@) = R T J -dy, (34) (35) where Qi is the surface potential for the gas mixture at P, Tand y , and Qiz is the surface potential for the pure component i at P and T.S. SIRCAR 1533 IDEAL MULTICOMPONENT GAS: CONSTANT T, yi For constant T and y i eqn (26) and (31) can be combined for an ideal gas mixture to get d@ = -RTnm d In P. Eqn (36) can be integrated to obtain @(P, T , y i ) = -RT nm -. JOP (37) The last method of calculating @ from the experimental equilibrium data is most general. @ at any P , T and yi can be calculated by this technique if the necessary equilibrium data (at constant T and y i ) to carry out the integration of eqn (37) are available. Numerous experimental measurements may, however, be needed.Conse- quently, the temperature coefficient of @ at constant P and y i can also be estimated. TEMPERATURE COEFFICIENTS OF @ AND THE EXCESS PROPERTIES The temperature coefficients of gm and ge may be obtained from eqn (1 8) and (29) as (38) a - (gm/T),f = -hm/T2 8T Eqn (38) and (39) are analogous to the pure-gas propertylo where hi* is the enthalpy of pure gas i at T and atmospheric pressure. hi* is a function of T only. The molar enthalpy (h) of an ideal gas mixture at P, T and yi is given by lo h = Zyih,*. (41) Eqn (26) can be differentiated with respect to T and combined with eqn (40) to get for an ideal gas mixture where p i (= Pyi) is the partial pressure of component i in the gas mixture.Further- more, it can be shown from eqn (18), (26), (38), (41)-(44) and the temperature coefficient of eqn (31) that gm--Cnpp; =@+RT-Cnrlnpi (45)1534 MULTICOMPONENT GAS ADSORPTION The right-hand-side quantities of eqn (49447) can be estimated from experimental equilibriumdata. Consequently, theseequations show that the quantities (zm - C nf 2:) and (ze-C n& 2:) can also be experimentally evaluated for a multicomponent gas mixture at a given P, T and y , or T and np. Furthermore, eqn (46) and (47) show the ge*( = @*) and he*[ = @* - T(a@*/aT),, Yr] are also experimental variables for a pure-gas adsorption while gm* and hm* are not. The temperature coefficients of @ at constant n r and (P,nf) are related to that at constant ( P , y i ) by eqn (48) and (49), respectively.PHYSICAL SIGNIFICANCE OF EXCESS FUNCTIONS AND @ Consider the formation of the multicomponent adsorption system at P, T and y , described in fig. 1 from the pure ideal gases at temperature T and pressure P,*. This can be conceptually achieved by contacting n: moles of component i at P: and T with a unit amount of the clean adsorbent contained in an evacuated and thermostatted chamber fitted with a frictionless piston, as shown in fig. 2. The piston position can then be adjusted so that the final equilibrium gas phase is at P, T and y,. The initial free energy and enthalpy of the entire system may be obtained from eqn (24) and (26) as ginitial = C n:[pt + RT In Pf ] hinitial = C n: h,*. The final values of these properties may be obtained from eqn (17), (24), (26) and (41) as gfinal =ge +no C y i ( p f + RT In Pyi) hfinal = he + no C y , h,* .(52) (53) The integral changes in free energy (AG) and enthalpy (AH) for the formation of the equilibrium system of fig. (1) are then calculated by using eqn (6), (28) and (50)-(53): (54) ( 5 5 ) AG=@+XnyRTlnL PY . P: A H = he-C n& h:. A H is the change in enthalpy during the formation of the adsorption system which is removed as heat from the system to maintain isothermality. Eqn (46) shows that A H can be experimentally evaluated. For the special case of a pure-gas adsorption, A H is equal to he*. Thus the excess enthalpy for a pure gas is equal to the isothermal integral enthalpy of formation of the adsorption system. Eqn (54), on the other hand, shows that AG = @ when Pi* = Pyi.Thus @ is the isothermal integral free energy ofS . SIRCAR < pure gas i 1 I I constant-temperature bath at T' of 1535 Fig. 2. Schematic diagram of the process of formation of a multicomponent gas adsorption system from the pure-gas components. formation of the multicomponent adsorption system from pure gases at pressures equal to the respective partial pressures of each gas in the final equilibrium system. For a pure gas, AG = gel. Thus the excess free energy for a pure gas is the isothermal and isobaric integral free energy of formation of the adsorption system. DIFFERENTIAL HEAT OF ADSORPTION The total enthalpy (h") of the adsorption system of fig. 1 may be written for an ideal gas mixture by combining eqn (4), (1 7) and (41) as h" = hm+C (ny-n?) h,*.(56) We define a differential heat of adsorption, qi, for component i of the mixture by Thus qidnT is the amount of heat to be supplied to (desorption) or removed from (adsorption) the closed adsorption system (constant, I/", n:) in order to change the surface excess of component i by dn? at constant T and nEi. It follows from eqn (56), (57) and (30) that Eqn (29) shows that1536 MULTICOMPONENT GAS ADSORPTION Eqn (59) can be differentiated with respect to n f at constant T, n%i and equated with the temperature coefficient of eqn (60) at constant np to get Eqn (58), (61) and (43) may be combined to show that 8 In pi qi is a positive quantity so that pi increases with increasing T at constant n f . For a pure gas Eqn (63) shows that q: is the conventional isosteric heat of ad~orption.~ q: can be estimated as a function of nm* from pure-gas isotherms at different temperatures using eqn (63).Alternately 4: may be obtained calorimetrically.2 Analogously, qi may be called the isosteric heat of adsorption of component i of a multicomponent gas mixture. The isosteric heat of adsorption of component i of an ideal binary gas mixture (q’), defined by eqn (62), can be obtained from experimental equilibrium data using the Jacobian transformations1’ as follows. a In P 4: = RT2 (F)nm*. (63) Eqn (62) may be written for component 1 of the binary mixture as The ratio of the determinants of eqn (65) can be simplified by noting that p 1 = Py, to getS. SIRCAR Similarly, it can be shown that 1537 a, Eqn (66) and (67) show that q1 and q2 can be estimated for a given T and n p or P, T and yi from the experimental equilibrium data measured at constant (P, yl), (P, T ) and (T, y , ) as discussed earlier.This, however, could be an experimental nightmare. On the other hand, calorimetric measurements of qi for a binary system may also be very difficult. The author is unaware of any measurement of isosteric heats of adsorption for mixtures. HEAT CAPACITY OF THE ADSORPTION SYSTEM For non-isothermal heat transfer into the adsorption system of fig. 1, one needs to account for the heat capacity of the adsorbent. Thus eqn (56) is modified to h" = hm+C (n:-nT) h,*+C,(T-T,) (68) where C, is the specific heat capacity of the clean adsorbent and T, is a reference temperature. An overall specific heat capacity of the adsorption system, CA, is defined as cA = (z)np.Thus, CAdT is the differential change in the total enthalpy of the system for a differential temperature change (d T ) in the closed system (constant V", n:) at constant rzy. h" is a function of T and n?: h" = h"(T, nT). (70) It can be shown from eqn (70) by using the chain rule of calculus and eqn (57) and (69) that ah" d h " = C ( x ) dnT+(;;r> nP d T any T , ny+i = -c qidnT+CAdT. It follows from eqn (71) that A - (ri) +C qi (g) (72) - - P , y z P , Yi The differential form of eqn (68) is dh" = d(hm -C nT h,*)+C n: dh,* + C,dT. (73) Eqn (73) can be differentiated with respect to T at constant P,y, using eqn (46) to get where Czi is the heat capacity of pure gas i.l0 51 FAR 11538 MULTICOMPONENT GAS ADSORPTION Eqn (72) and (74) may be combined to get Eqn (75) shows that C , for a multicomponent adsorption system can be estimated from the experimental equilibrium data for a given T and nT or P, T and yi.For a pure ideal gas, nm* can be written as nm* = nm*(P, T ) . (76) It can be shown from eqn (76) using the chain rule of calculus and eqn (63) that Eqn (33) can be differentiated with respect to Tat constant P and combined with eqn (77) to show that . I d In P. P = -JOnrn* q* dnm* Eqn (78) may be differentiated with respect to T at constant P and combined with where C i is the overall specific heat capacity for a single-adsorbate system. adsorbents), eqn (79) reduces to For the special case where q* is not a function of nm* (as in most homogeneous (80) Thus the overall specific heat capacity of a single-adsorbate adsorption system with a unit amount of a homogeneous adsorbent is equal to the gas-phase heat capacity of the total number of moles of the adsorbate in the system plus the specific heat capacity of the adsorbent.C i = no Cz + C,. THERMODYNAMIC CONSISTENCY TESTS Several differential and integral consistency tests can be derived by using the above described thermodynamics of excess properties for the adsorption of ideal binary gas mixtures. CONSTANT P, T DATA INTEGRAL TEST Eqn (35) can be integrated between the limits of y , = 0 to y1 = 1 to get nf The right-hand side of eqn (81) can be evaluated by using binary n: against y , data at constant P and T.The left-hand side of eqn (81) can be independently evaluatedS. SIRCAR 1539 from the pure-gas isotherm data at T using eqn (33) by carrying out the integration up to pressure P. Thus an integral consistency check between the binary and the pure-gas data can be made. A similar test was earlier suggested by Broughton12 who used actual amounts adsorbed as variables. The original Broughton equations are different from eqn (81) but they reduce to this equation when the same nomenclature are used. DIFFERENTIAL TEST Eqn (35) can be differentiated with respect to P at constant Tand y , and combined with eqn (32) and (37) to get np* (P, T ) = nm(P, T, y , ) -P (I” dy,) . aP o yiy2 T,y1 The right-hand side of eqn (82) can be obtained from binary adsorption data while the left-hand side of eqn (82) can be obtained from pure-component adsorption data alone and thus it provides a consistency check between the two sets of data.CONSTANT T, y1 DATA Eqn (37) can be differentiated with respect to y , at constant T and P and equated with ( 8 @ / 8 ~ , ) ~ , ~ obtained from eqn (31) and (26) to get (83) P , T The integral in eqn (83) can be obtained from constant T,y, data at various P. Thus eqn (83) provides a consistency check of the binary data by comparing the variation of the integral of eqn (82) with y1 at constant P and T with the surface excesses of the components 1 and 2. A very useful practical application of eqn (83) is that it allows the calculation of ny from total surface excess (nm) measurement only. nm can be directly measured in a gravimetric apparatus at constant T but varying P and y,.This was originally proposed by Vanness’ using the actual amounts adsorbed as variables. CONSTANT P, T AND T, y1 DATA Eqn (31) can be combined with eqn (26) and differentiated with respect to P and y , , respectively, at constant T , y , and P, T to get Eqn (84) and (85) can then be differentiated with respect to, respectively, T (constant P , y l ) and P (constant T , y l ) and equated to get Eqn (86) provides a differential consistency test between constant P, T and constant T, y1 equilibrium data.1540 MULTICOMPONENT GAS ADSORPTION CONSTANT (P, T ) , (P,Yl) AND (T,yl) DATA It can also be shown from eqn (31) that T , Y1 ] +($) }. (87) Y2 aT P,yi T , y l T , Y1 Eqn (87) provides a differential consistency test between constant (P, T ) , ( T , y l ) and ( P , y l ) equilibrium data.CONCLUSIONS The surface excesses of the adsorbates, which are the true experimental variables for adsorption of gases, may be conveniently used to describe the thermodynamics of multicomponent adsorption from gases. All thermodynamic properties of practical usefullness, such as the surface potential, the isosteric and the integral heats of adsorption, the heat capacity of the adsorption system and the thermodynamic consistency tests, may be expressed in terms of the excess properties. Subsequently, these properties can be evaluated from the experimental data. Thus there is no need to define the actual amounts adsorbed for the adsorbates which require assumptions about the size and the structure of the adsorbed phase. These assumptions are speculative, at best, under most practical conditions of interest. It will be shown in a later article that the surface excess variables can also be used conveniently to describe the kinetics of adsorption and the dynamics of adsorption in packed adsorbent columns, which are needed for the practical application of adsorption for the separation and purification of gas mixtures. J. W. Gibbs, The Collected Works of J . W. Gibbs (Longmans and Green, New York, 1928), vol. 1. D. H. Everett, Colloq. Int. C.N.R.S., 1971, 201, 45. D. M. Young and A. D. Crowell, Physical Adsorption of Gases (Butterworths, Washington D.C., 1972), p. 139. A. Michels, P. G. Menon and C. A. ten Seldam, Red. Trav. Chim. Pays-Bas, 1961, 80, 483. R. Reich, Ph.D. Dissertation (Georgia Institute of Technology, 1974). T. L. Hill, J. Chem. Phys., 1950, 18, 246. B. P. Bering, A. L. Myers and V. V. Serpiniskii, Dokl. Akad. Nauk SSSR, 1970, 193, 119. ’ H. C. Vanness, Ind. Eng. Chem. Fundam., 1969,8,464. ’ T. L. Hill, Advances in Catalysis (Academic Press, New York, 1952), vol. 4, p. 21 1. lo J. M. Prausnitz, Molecular Thermodynamics of Fluid Phase Equilibria (Prentice Hall, Englewood Cliffs, l1 H. B. Callen, Thermodynamics (John Wiley, New York, 1962). l 2 D. B. Broughton, Ind. Eng. Chem., 1948,40, 1506. N.J., 1969). (PAPER 4/843)

ISSN:0300-9599    

DOI:10.1039/F19858101527

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1985,81, 1541-1545 Excess Properties and Column Dynamics of Multicomponen t Gas Adsorption BY SHIVAJI SIRCAR Air Products and Chemicals Inc., Box 538, Allentown, Pennsylvania 18 105, U.S.A. Received 23rd May, 1984 The Gibbsian surface excesses are the true experimental variables in measuring the extent of adsorption from gases. The surface excess variables can be conveniently used to describe the dynamic adsorption of gases in adsorbent columns without defining adsorbed phase or the adsorbent structure. The resulting mass and enthalpy balance equations are as practical to solve as those conventionally written by using the actual amounts adsorbed as the variables to define adsorption. The separation and purification of gas mixtures by selective adsorption are usually carried out in packed adsorbent columns.Numerous theoretical studies of adsorption in packed columns have been published. It is generally assumed that the adsorbed phase in the column is well defined and the actual amounts of the gases adsorbed can be used as the primary variables to describe the extent of adsorption. However, it has been shown' that the actual amounts adsorbed may not be well defined (a) for the more selective component of the gas mixture when the gas pressure is moderate to high and its temperature is near or above the critical temperature of the adsorbate and (b) for the less selective component of the gas mixture even when the pressure is moderate to low. The Gibbsian surface excesses constitute the true experimental variables to describe the extents of adsorption under all conditions of opertion.' The purpose of this work is to demonstrate that the surface excess variables can be conveniently used to describe adsorption column dynamics without loss of practicality.SURFACE EXCESS CONCEPT Consider a closed adsorption system as described by fig. 1, where a multicomponent gas mixure is contacted with a unit amount of an inert and non-volatile adsorbent. It is assumed that the gases are not absorbed into the lattice structure of the adsorbent. At any time t the system consists of a gas and an adsorbed phase of volumes V and V', respectively, and their molar densities are p and p'. The gas-phase pressure is P, its temperature is T and its composition is y i (mole fraction of component i) at time t.The actual amount of component i adsorbed at time t is n; (= V'p'x;) where xi (= n;/X n;) is the mole fraction of that component in the adsorbed phase. V" is the total volume (gas+adsorbed phases) in the system accessible by the adsorbates. A mass and a volume balance at time t gives v o = v+V' (1) n: = V p ' x i + Vpyi no = C n: = Vp'+ Vp 15411542 MULTICOMPONENT GAS ADSORPTION gas phase pressure = P volume = V temperature = T density = p mole fraction of component i = yi Gibbs interface 4 ------------ ----. adsorbed phase volume = V' temperature = T density = p' amount adsorbed of component i = ni adsorbent unit amount Fig. 1. Schematic diagram of a non-equilibrium multicomponent gas adsorption system at time t. where n: and no are, respectively, the total amount of component i and the total amount of all adsorbates within the closed system.It is apparent from fig. 1 that the quantitites P , T, y i , p , V", n: and no are the only experimental variab1es.l P, T and y i can be measured as a function of time by appropriately instrumenting the gas phase. p can then be calculated using an equation of state for the gas. n: and no can be estimated by measuring how much of each component was introduced into the system prior to the adsorption process or by completely desorbing the system at any time so that the adsorbent is clean and measuring the total amount of the desorbed gas (no) and its composition 0.);). n: is then given by n"y:. V" is the helium void of the system.' It is assumed that the volume V" is accessible by all adsorbates.The parameters V, V', p' and n;, on the other hand, depend on the size and structure of the adsorbed phase, which cannot be measured. The adsorbed phase is arbitrarily defined by the dashed line (the Gibbs interface) in fig. 1 . The location of the Gibbs interface may change with time as P, T and y i change. It may be shown from eqn (1)-(3) that The quantities n f and nm are called the surface excess of component i and the total surface excess, respectively.' Eqn (4) and (5) show that they can be experimentally measured because the quantities on the left-hand sides of these equations are experimental variables. Thus nF and nm are independent of the choice of the location of the Gibbs interface while V', p' and xi in eqn (4) and ( 5 ) are not.S.SIRCAR 1543 The rates of change of ny or nm can be measured by monitoring the rate of change (6) dnf d of P, Tandy,: - = - V" z@y,) dt dnm d - = - V" -(p). dt dt (7) In fact these are the rates which are actually measured in any batch gas-phase adsorption experiment. For a pure gas there is only one surface excess variable, nm* [= V'(p'-p)]. The equilibrium surface excess quantities iif and tirn can also be measured using eqn (4) and (5) by measuring the equilibrium values of P, T, p and yi. The equilibrium surface excesses are functionally described by iif = iif (P, T, yi). (8) ti? and fim are the actually measured equilibrium properties. nm* x n' for a pure gas at low pressure when T + T, because p + p' under these conditions. n f (for a mixture) x n; when pyi < p'x;. This usually happens when T + T, and component i is very selectivity adsorbed. Otherwise a significant difference between n f and n; may exist.l ENTHALPY OF ADSORPTION The thermodynamics of multicomponent adsorption using the surface excesses as the variables have been developed.' The total enthalpy (h") of the adsorption system described by fig.1 is defined by a given n f and T (or P, T and y,), and the differential change in h" due to a change in n$ and T may be written as dh" = -2 qi dny+CA d T (9) where qi is the isosteric heat of adsorption of component i and C, is the overall specific heat capacity of the adsorption system. Both qi and CA are functions of P, T and yi (or T and n f ) and they can be experimentally estimated.l It is assumed in writing eqn (9) that the gas phase is ideal and that the gas and the adsorbed phases have the same T.Eqn (4)-(9) can be used to describe the column dynamics in terms of the excess properties as follows. COLUMN DYNAMICS Fig. 2 shows a schematic diagram of a packed adsorbent column of empty cross-sectional area A . Let V" be the total void (helium) volume in the column per unit mass of the adsorbent. A multicomponent gas mixture is passed through the column. Let Q be the molar gas flow rate through the column per unit cross-sectional area ( A ) at a distance x in the column at time t. Let P, T, p and yi be, respectively, the pressure, temperature, molar density and the mole fraction of component i in the gas phase of the columa at x and t.Let n y be the surface excess of component i per unit mass of the adsorbent in the column at x and t. Let ny and h" be, respectively, the total amount of component i and total enthalpy per unit mass of the adsorbent in the column at x and t. Then, using the conventional differential mass and enthalpy1544 MULTICOMPONENT GAS ADSORPTION molar flux into section at time t component i by convection = (QyJ, enthalpy by convection = Q c g (T- To)lz section of column of length Ax at a distance x molar flux out of section at time t pressure = P temperature at time t = T gas-phase mole fraction of density of gas at time t = p total moles of component i total enthalpy at overall specific heat component i by convection enthalpy by convection = ( Q ~ i ) l s + ~ z = Qcg (T- TO)~S+AZ component i at time t = yi at time t = A Axpb n: time t = A A x p , h" capacity of column = C, Fig.2. Schematic diagram of differential mass and enthalpy balances in a packed adsorbent column. balances across a differential section in the column (fig. 2) one can deduce the following conservation equations for an isobaric and adiabatic column : mass : The key assumptions in writing eqn (10) and (1 1) are that there is no surface flow of mass or heat in the adsorbed phase and there is no axial dispersion of mass and heat in the gas phase. Pb is the bulk density of the adsorbent. Cg in eqn (1 1) is the molar gas-phase heat capacity at P, T and yi and To is a reference temperature. Eqn (10) and (1 1) can be combined with eqn (4), (5) and (9) to obtain mass : Eqn (12) and (1 3) represent the mass- and enthalpy-balance equations for a column in terms of the surface excess variables.All physical quantities used in these equations, viz. Pb, cA, qi, np, p, yi, Q, c,, T, t and x, can be experimentally measured unambiguously without defining the size or the structure of the adsorbed phase. The rate of change of nT at any x depends on the local values of Tand np or P, T and yi at time t as well as the gas flow rate. This can be written in a functional notation as rs) =AQ, np, 7) =g(P,T,y, Q) ( i = 1, 2 . . .). (14) X The functionalityfor g can be independently measured in a closed-loop batch sorption system described earlier where the gas is circulated over the adsorbent with a flow rate of Q. Eqn (6) and (7) can then be used to obtain the rates.Various mechanistic models for kinetics of adsorption may be developed to define explicitly the functionsS. SIRCAR 1545 f o r g . A simple mechanistic model based on the phenomenological argument is the linear driving force model which assumes an? at - = Ki (ny-n?) ( i = 1 , 2 . . .) where ny is the local equilibrium surface excess of component i at P, T and yi. Ki is the mass-transfer coefficient for component i, which can generally be a function of Q, Tand n?. The variablesp,, p and C, can be obtained from the bulk properties of the adsorbent and the gaseous adsorbate. qi and C , may be measured from the equilibrium isotherms of the adsorbate-adsorbent system.' Thus there are (2k+ 2) unknowns ( n r , yi, Q and 7') in eqn (12)-(14) for a k-component (i = 1 , 2 .. . k ) gas mixture. Eqn (12)-( 14) and the constraint C yi = 1 provide (2k+2) independent equations relating them. Thus n?, yi, Q and Tcan be uniquely solved as functions of x and t for a given initial column and feed-gas conditions. In fact eqn (12)-( 14) are mathematically very similar to the conventional mass- and heat-balance equations written for column adsorption using n; in place of ny [ref. (2)] except that a heat capacity of the adsorbed phase (C,) is used in place of the overall system heat capacity (C,) and various void volumes for the column and the adsorbent such as the external, the macropore and the micropore volume fractions are needed to define the adsorbed phase instead of a single experimental total void fraction (pb Vo) for the packed column. The definitions of n;, C, and the different void fraction in the column can be ambiguous because the adsorbed-phase volume and structure are generally unknown. The present study shows that there is no need to define the adsorbed-phase or the adsorbent structure in order to describe the dynamics of adsorption in the columns. S. Sircar, J . Chem. Soc.. Faraday Trans. I , 1985, 81, 1527. N . H. Sweed, in Percolation Processes: Theory and Applications, ed. A. E. Rodrigues and D. Tondeur (Sijthoff and Noordhoff, 1981). p. 329. (PAPER 4/844)

ISSN:0300-9599    

DOI:10.1039/F19858101541

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1985,81, 1547-1554 Solvent Relaxation and Proton Transfer in the (H502 )+( H,O), Species BY M. ANGELES Mufirz Departamento de Quimica Fisica, Universidad de Sevilla, Sevilla, Spain AND JUAN BERTRP;N,* JOSE L. AND-, MIQUEL DURAN AND AGUST~ LLED~S Departamento de Quimica Fisica, Universidad Autonoma de Barcelona, Bellaterra (Barcelona), Spain Receiued 5th June, 1984 The proton-transfer process in the (H502)+ and (H502)+(H20), species has been studied by means of the semi-empirical CND0/2 and MIND0/3 methods and ab initio calculations using the 3-21G basis set. It has been found that the interoxygen distance does not remain constant in these processes. When four outer molecules are arranged such that they are solvating the four outer hydrogens of the (H,02)+ cation, the minimum in which the proton is located midway between the two oxygen atoms is broken down into two minima in which the hydrogen bonds are asymmetric.Solvent relaxation plays an important role in proton transfer, the proton adjusting its position upon changes in solvation. Proton-transfer reactions play an important role in many chemical and biochemical processes, and so it is not surprising that many theoretical works have been devoted to this subject and especially to proton transfer in (H,O,)+ species. Using ab initio methods the minimum-energy geometry of the (H,O,)+ species has been found to have a linear symmetric hydrogen bond in which the proton is located midway between the two oxygen atoms. The structure is of D,, symmetry with the planes of the two water molecules staggered by 90" with respect to one The potential-energy surface is extremely flat around the minimum, and thus a large proton shift of ca.0.3 A can occur with little change in Potential-energy curves for proton transfer between the two water molecules were obtained by calculating the energy of the system as a function of the distance between the central hydrogen and one of the oxygen atoms. All nuclei, with the exception of the central proton, were held stationary during the transfer. For small values of the interoxygen distance the potential-energy curve contains a single minimum, but for large values the potential-energy curve acquires double-well characteristics.2-10 The two equivalent energy minima correspond to a configuration in which the central proton is associated with one or other of the water molecules. This proton is midway between the two oxygens in the transition state of proton transfer, the transition state being defined as the top of the energy barrier separating the two minima.A rapid increase in the height of the energy barrier occurs as the two water molecules are gradually separated. To our knowledge, only two authorss* l1 have studied the effect of additional water molecules on the potential-energy curves for proton transfer. In Scheiner's work,6 one additional water molecule was added to each end of the H,O-H+-OH, species in order to enlarge the system. The transferring proton was moved along the central 15411548 SOLVENT RELAXATION AND PROTON TRANSFER interoxygen axis while the remainder of the system was held fixed.The tetramer barriers are only slightly different from the dimer barriers. Schusterll has studied the effect of hydration on proton transfer using the CND0/2 method. In the (H,O,)+ species four water molecules were arranged in equivalent positions around the cation, forming linear hydrogen bonds with the outer hydrogens of the (H,O,)+ cation. The water molecules led only to additional stabilization of the asymmetrical structures, thereby raising the height of the energy barriers between the two minima. Simultan- eously, the OH bond length of the H30+ unit was reduced at the energy minimum. The constant interoxygen separation may serve as a useful model for sudden proton transfer in which the heavier nuclei are not able to adjust their positions when the lightest proton is being transferred, as happens in quantum-mechanical tunnelling.This rapid-transfer model is also applicable to chemical systems where constant intermolecular separation is a result of various structural constraints.10-12 However, an adiabatic transfer model, in which the proton moves so slowly that the remaining nuclei are free to optimize their relative coordinates at each stage of the transfer, must be adopted and care must be taken when interpreting conclusions based on studies that have used fixed heteroatom distances.13 and Marcus15 have applied their theories for outer-sphere electron transfer to proton transfers. According to these theories, solvent relaxation must play an important role in the process.Hence, an essential aspect of the effect of solvent on proton-transfer processes was neglected when maintaining constant interoxygen distances. There has been considerable debate on two related questions about the details of proton transfers: is the proton motion coupled with that of the solvent and how much does the proton tunnel?16 De la Vega and c ~ w o r k e r s ~ ~ - ~ ~ have recently emphasized the role of symmetry in the tunnelling of the proton in double-minimum potentials. The energy profile for proton exchange between water molecules in the tetramer is symmetric when the arrangement of the molecules is that of the ice, but small changes in the solvent may destroy this symmetry; e.g. rotation of one of the external water molecules destroys the symmetry of the energy profile, quenching the tunnel1ing.l77 l8 In the present work our first purpose was to study proton transfer in the (H,O,)+ species and the second was to study the same process when four water molecules are arranged so that they are solvating the four outer hydrogens of the (H502)+ cation.The interoxygen distances are not kept constant. We shall analyse the approach or the removal of water molecules between which proton transfer occurs, together with the effect of solvent relaxation on proton transfer. Levich and METHOD OF CALCULATION Given the impossibility of calculating the potential hypersurface for the majority of reactions of chemical interest, there are two basic approaches that help in overcoming this difficulty.The first consists of reducing the dimensions of the surface and the second of locating the most interesting points on it. Both approaches are used in this paper. In order to reduce the dimensions of the potential surface, one or two geometric parameters are taken as independent variables and all the remaining geometric parameters are optimized. Because of the number of parameters to be optimized the semi-empirical MIND0/3,O and CND0/2,l methods were chosen in order to maintain computation time within reasonable limits. The GEOMO program,22 in which the geometric optimization is carried out by means of the Rinaldi alg~rithrn,,~ wasM. A. MURIZ, J. BERTRAN, J. L. ANDRES, M. DURAN AND A. LLEDOS 1549 3 = ? 9 2 - 1 ’ I I 1 2 3 dlA Fig. 1. Reduced potential-energy surface for the (H,O, )+ system, taking the distances between the proton and the oxygen atoms as independent variables.used extensively. Direct location of the stationary points was obtained with the SIGMA program, based on McIver’s and Komornicki’s direct location method.24 The nature of such points was determined using the FORCE program, by calculating the eigenvalues and eigenvectors of the force-constant matrix. In order to confirm the obtained results, some ab initio calculations using the 3-2 1 G 25 basis set were performed for the (H502)+ and (H5O2)+(H20)4 species. The GAUSSIAN 80 series of programs26 was used to carry out these calculations. The supermolecule model was used to represent the solvent. RESULTS AND DISCUSSION We will first present the results obtained for proton transfer in the (H502)+ species and then the results obtained for the same process when it is solvated by four water molecules.Fig. 1 shows the reduced potential-energy surface of the (H,02)+ species, obtained using the MIND0/3 method. The distances between the proton and the oxygen atoms have been taken as independent variables. Two of the three energy minima correspond to the (H,O)+ species solvated with one water molecule, whereas the third corresponds to a hydrogen bond with the proton located midway between the two oxygen atoms. This is shown in fig. 2. Note that the hydrogen bond is not linear at large distances. However, when imposing linearity on the hydrogen bond, a structure is obtained that is only 0.18 kcal mol-l less stable than the bent structure, and the linear structures are the most stable at small distances.From an energetic point of view, the use of linear hydrogen bonds is acceptable. Note also that the interoxygen distance does not remain constant during the proton-transfer process. From 3.67 A for the unsymmetric1550 SOLVENT RELAXATION AND PROTON TRANSFER H 1.01 1.66 eoi 'o.3eH ( c ) Fig. 2. Structures of the minima [(a) and (b)] and of the transition state (c) for the (H,O,)+ species. Components of the transition vector are shown in (c). hydrogen bond it becomes 2.36 A for the symmetric hydrogen bond. In the transition state, located by the SIGMA program, the value of this distance is 2.65 A. Fig. 2(c) shows the components of its transition vector, corresponding to the unique negative eigenvalue of the force-constant matrix.The reaction coordinate is thus defined by the approach of the (H,O)+ and H 2 0 species. Therefore, keeping interoxygen distance constant is incorrect. Taking the energeticminimum corresponding to the same structure with unsymmetric hydrogen bonds as the energy origin, the energy of the minimum corresponding to the structure with symmetric hydrogen bond is 1 kcal mol-' and that of the transition state is 2.38 kcal mol-l. These results, obtained using the MIND0/3 method, contrast with those obtained using the CND0/2 method, with which a unique energetic minimum for a linear structure is obtained, with the proton located midway between the two oxygen atoms, the OH distance being 1.17 A. If we compare the CND0/2 results with existing ab initio calculations' we can also note the tendency of the CND0/2 method to emphasize the stabilization of a structure with a symmetric hydrogen bond, With the CND0/2 method a symmetric double-minimum potential is found at a distance > 2.6 A,11 whereas with the 4-3 1G basis-set transformation from single- to double-well character occurs at an interoxygen distance of ca.2.4 A. The experimental enthalpy of formation of the (H,02)+ species from (H,O)+ and H 2 0 is 31.6 kcal m ~ l - ' , ~ ' whereas the energy of formation calculated using the CND0/2 method is 61.6 kcal mol-' and that calculated using the MIND0/3 method with a symmetric hydrogen bond is 9.2 kcal mol-l. An energy of 51.3 kcal mo1-128 is obtained with the 3-21G basis set, clearly greater than the experimental value.These results show that both the semi-empirical methods MIND0/3 and CND0/2 are deficient in their description of hydrogen bonds. The MIND0/3 method underesti- mates the energy of 32 30 while the CND0/2 method overestimatesM. A. MUGIZ, J. BERTR~N, J. L. AND&, M. DURAN AND A. LLED~S 1551 H, % 0.95 H U ', 2.61 ' 166' 0.96 . C C 0 H o'd H 0.95.F 0 0 3.30 ,/' / / Fig. 3. Structures of the minima for the (H,O,)+(H,O), species. We will now analyse the proton-transfer process in the (H502)S(H20)4 species. The reduced potential-energy surface, where the distances between proton and oxygen atoms are again taken as independent variables, differs little from that shown in fig. 1 for the (H502)+ species. The most important modification is that the central well is deeper and has two minima, corresponding to two structures with unsymmetric hydrogen bonds.These energetic minima are shown in fig. 3 and are only 0.07 kcal mol-1 above initial minima [fig. 3(a)]. The most remarkable feature is that the solvation parameters differ depending on which of the proton-donor or -acceptor water molecules is concerned. Thus the proton-transfer process is effected by solvent relaxation. The hydration shell is expanded around the proton-releasing water molecule and is contracted around the proton-accepting water molecule. If solvation parameters are kept constant at the initial minimum values [fig. 3(a)], solvent relaxation not being allowed, the reduced potential-energy surface presented in fig.4 is obtained. It can be proved that surface symmetry has disappeared, the final potential minimum being 2.47 kcal mol-1 above the initial minimum. The most important finding presented so far is that whereas (H502)+ has a symmetric 0-H-0 bond, when the outer hydrogens are solvated the bond becomes unsymmetric. To obtain greater insight into this, we have made the central 0-H-0 bond symmetric and taken a unique value for the four solvation parameters. Thus we have obtained a fully symmetric structure which lies 0.1 kcal mol-l above the minimum of fig. 3(6) and corresponds to the transition state in the proton-transfer process. To confirm this surprising MIND0/3 result, this calculation was repeated using the semi-empirical CND0/2 method and the 3-21G basis set.For all calculation methods the central well becomes two minima in the (H502)+(H20), species and the1552 I SOLVENT RELAXATION AND PROTON TRANSFER I 3 ? 9 2 1 r 0 3 - 0.2 I I I 1 2 3 dlA Fig. 4. Reduced potential-energy surface for the (H,O,)+(H,O), system, taking the distances between the proton and the oxygen atoms as independent variables and maintaining the solvation parameters at the values of fig. 3(a). Table 1. Lengths (in A) of the central and solvation hydrogen bonds for the proton-releasing water molecule (a) and proton-accepting water molecule (b) and relative energies (in kcal mol-') of the transition state for the proton-transfer processes MIND0/3 CND0/2 3-2 1 G asymmetric transition asymmetric transition asymmetric transition minimum state minimum state minimum state ~~ 40--HI (4 1.15 1.17 1.1 1 1.20 1.18 1.19 40-H) (b) 1.23 1.17 1.28 1.20 1.19 1.19 solvation 3.09 3.19 1.33 1.37 1.60 1.61 solvation 3.30 3.19 1.83 1.37 1.61 1.61 parameter (a) parameter (6) energy 0 0.1 0 1.5 0 0.00 1 fully symmetric structure becomes the transition state of the proton-transfer process.One can see from table 1 that the values obtained for the potential barriers, the asymmetry of the hydrogen bond and the solvation parameters depend on the method used. For the 3-21G0method, a transition state with a low energy barrier has been found, corresponding to a very flat region on the potential-energy surface. This contrasts with the sharp energy well found for the (H,O,)+ system. However, more efficient solvation is the reason for this new totally unsymmetric structure to appear.M.A. M U ~ ~ I Z , J. BERTRAN, J. L. ANDRES, M. DURAN AND A. LLEDOS 1553 Thus in all three cases can one consider proton transfer which requires relaxation of solvent parameters. On the other hand, if the relaxation parameter is inverted, then transfer occurs spontaneously, proving that the proton adjusts its position to the movement of solvent molecules. Three main conclusions follow from our results. First, the energetic minimum of the (H,02)+ cation, in which the proton is located midway between the two oxygen atoms, is broken down by solvation into two minima corresponding to structures with unsymmetric hydrogen bonds. This result is obtained using the MIND0/3 method, which underestimates solvation, and using the CND0/2 method, which overestimates it.The same result is obtained when the 3-21G basis set is used. Thus the physical significance of the unsymmetrical minima, i.e. they are caused by solvation effect, is without doubt. Secondly, when considering an adiabatic transfer model, solvation parameters take part in the reaction coordinate of the proton-transfer process. The inversion of solvation distances produces spontaneous proton transfer, the proton adjusting its position to the changes in solvation. Thirdly, given that there is an unsymmetrical contracted hydratation shell around the (H,O)+ unit and an expanded one around the H 2 0 part of the cation, the symmetry of the energetic profile is destroyed if solvent relaxation is not allowed. According to De la Vega and c ~ w o r k e r s , ~ ~ - ~ ~ this prevents proton tunnelling from taking place.In order for this to occur, the symmetry of the energy profile has to be re-established by means of solvent movement to make the hydratation shells around the two water molecules, between which the proton is transferred, identical. The rate-limiting step in such a transfer would then be the attainment of a W-shaped well. This solvent relaxation, similar to that required by Marcus33 and L e v i ~ h ~ ~ for the outer-sphere electron- transfer process, has not been properly considered for the protonic processes which take place. M. D. Newton and S. Ehrenson, J. Am. Chem. Soc., 1971,93,4971. P. A. Kollman and C. C. Allen, J. Am. Chem. SOC., 1970,92, 6101. W. P. Kraemer and G.H. F. Diercksen, Chem. Phys. Lett., 1970, 5, 463. R. Janoschek, E. G. Weidemann, H. Pfeiffer and G. Zundel, J. Am. Chem. SOC., 1972,94, 2387. W. Meyer, W. Jakubetz and P. Schuster, Chem. Phys. Lett., 1973, 21, 97. S. Scheiner, J. Am. Chem. SOC., 1981, 103, 315. S. Scheiner, J. Phys. Chem., 1982, 86, 376. S. Scheiner, J. Chem. Phys., 1982, 77, 4039. * S. Scheiner and L. B. Harding, J. Am. Chem. SOC., 1981, 103, 2169. l o S. Scheiner, M. M. SzczeSniak and L. D. Bigham, Int. J. Quantum Chem., 1983, 23, 739. l 1 P. Schuster, W. Jakubezt, G. Beier, W. Meyer and B. Rode, in Chemical and Biochemical Reacticity, l 2 S. Scheiner and L. B. Harding, J. Phys. Chem., 1983,87, 1145. l 3 J. Noel1 and K. Morokuma, J. Phys. Chem., 1976,80, 2675. l4 R. Dogonadze, A. M. Kuznetsov and V.G. Levich, SOC. Electrochem. (Engl. Transl.), 1967, 3, 648. l 5 R. A. Marcus, J. Phys. Chem., 1968,72,891; A. D. Cohen and R. A. Marcus, J. Phys. Chem., 1968, l6 W. J. Albery, Annu. Rer. Phys. Chem., 1980, 31, 227. ed. E. D. Bergmann and B. Pullman (Reidel, Jerusalem, 1974), p. 257. 72, 4249. J. H. Busch and J. de la Vega, J. Am. Chem. Sor., 1977, 99, 2397. J. de la Vega, Ace. Chem. Res., 1982, 15, 185. J. de la Vega, J. H. Busch, J. H. Schauble, K. L. Kunze and B. E. Haggert, J. Am. Chem. SOC., 1982, 104, 3295. 2o R. C. Bingham, M. J. S. Dewar and D. M. Lo, J. Am. Chem. SOC., 1975,97, 1285. 21 J. A. Pople and D. L. Beveridge, Approximate Molecular Orbital Theory (McGraw-Hill, New York, 22 D. Rinaldi, Quantum Chemistry Program Exchange, 1976, 290. 23 D. Rinaldi, Comput. Chem., 1976, 1, 109. 23 J. W. Mclver and A. Komornicki, J. Am. Chem. SOC., 1972,94, 2625. 1970).1554 SOLVENT RELAXATION AND PROTON TRANSFER 25 J. Stephen, J. S. Binkley, J. A. Pople and W. J. Hehre, J. Am. Chem. SOC., 1980, 102, 939. 26 J. S. Binkley, R. A. Whiteside, R. Krishnan, R. Seeger, D. J. DeFrees, H. B. Schlegel, S. Topiol, 27 A. J. Cunningham, J. D. Payzant and P. Kebarle, J. Am. Chem. SOC., 1972, 94, 7627; Y. K. Lan, 28 S. Yamabe, T. Minato and K. Hirao, J. Chem. Phys., 1984, 80, 1576. 29 P. Andreozzi, G. Klopman, A. J. Hopfinger, C. Kikuchi and M. J. S. Dewar, J. Am. Chem. SOC., 1978, 30 T. Zielinski, D. L. Breen and R. Rein, J. Am. Chem. SOC., 1978, 100, 6266. 31 P. Schuster, Int. J. Quantum Chem., 1969, 3, 851. 32 A. S. N. Murthy, K. G. Rao and C. N. R. Rao, J. Am. Chem. SOC., 1970,92, 3544. 33 R. A. Marcus, J. Chem. Phys., 1956, 24,966. 34 V. G. Levich, in Physical Chemistry, ed. H. Eynng, 0. Henderson and W. Jost (Academic Press, New L. R. Kahn and J. A. Pople, Quantum Chemistry Program Exchange, 1980, 406, 13. S. Ikuta and P. Kebarle, J. Am. Chem. SOC., 1982, 104, 1462. 100,6267. York, 1970), vol. 9B, p. 985. (PAPER 4/928)

ISSN:0300-9599    

DOI:10.1039/F19858101547

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1985,81, 1555-1561 Thermodynamic Functions for the Transfer of 1 -Naphthoic Acid from Water to Mixed Aqueous Solvents at 298 K BY M. CARMEN PEREZ-CAMINO, ENRIQUE SANCHEZ, MANUEL BALON AND ALFREDO MAESTRE* Departamento de Fisicoquimica Aplicada, Facultad de Farmacia, Universidad de Sevilla, 41012 Sevilla, Spain Received 26th June, 1984 Values of free energies, AG?, enthalpies, AH?, and entropies, A F , of transfer of 1-naphthoic acid from water to water + ethanol, water + t-butyl alcohol and water + dimethyl sulphoxide (DMSO) mixtures at various mole fractions have been determined from solubility measurements at different temperatures. Variations of AG? with xcosolvent in the water-rich alcoholic mixtures are governed by the sign of the entropy term.For water + DMSO mixtures AGP is negative, but the variation of AH? with xDMSO shows different behaviour from that of water + alcohol mixtures. We have calculated molecular pair-interaction parameters (g,,, h,, and Ts,,) in water for the three systems studied; h,, > 0 for water+alcohol mixtures and h,, < 0 for water + DMSO mixtures. The results arediscussed in terms of solute-cosolvent and solute-water interactions. In the last few years, several studies, using different experimental techniques, have been made of mixed aqueous solvents in order to gain a better understanding of the structural properties of water. However, there is still disagreement as to the nature of the interactions responsible for certain thermodynamic functions, some of which have been studied using ele~trolytesl-~ and non-electrolytes6-10 as probes (transfer functions) and others using mixed aqueous solvents (excess functions).It has been observed that adding small quantities of an organic solvent to water (water-rich region) can reduce, increase or scarcely affect its three-dimensional structure, according to the properties of the cosolvent. It is known that alcohols enhance the structure of water," dimethylformamide (DMF) and dioxane scarcely affect it12 and sulpholane and acetonitrile reduce it.13 The concentration dependence of the transfer functions in mixed aqueous solvents is a measure of the perturbation of the water and cosolvent components in the vicinity of solute transferred as the concentration of the cosolvent is changed.Depending on the regions upon which the study is focused, different interactions will predominate.12 The purpose of this paper is to report thermodynamic transfer functions for un-ionized 1 -naphthoic acid by measuring the temperature dependence of its relative solubilities in water and three mixed solvents (water + ethanol, water + t-butyl alcohol and water + dimethyl sulphoxide) at various concentrations. Molecular pair- interaction parameters, solute-cosolvent, in water have been determined in order to obtain a more detailed description of the aqueous mixtures studied. 15551556 THERMODYNAMICS OF TRANSFER FROM WATER TO MIXED SOLVENTS EXPERIMENTAL The solvents ethanol (EtOH), t-butyl alcohol (ButOH), and dimethyl sulphoxide (DMSO) were all reagent-grade chemicals (Merck) and were used without purification.Water was distilled twice using an all-glass apparatus. 1-Naphthoic acid (Fluka, purum grade, > 97%) was recrystallized twice from water + EtOH, dried at 105 "C and stored in a vacuum desiccator over silica gel. The solvents were prepared by mixing weighted quantities of water and cosolvent. For the solubility measurements, an excess of 1-naphthoic acid was added to water or the mixed solvent (10-80 wt%) in order to obtain saturated solutions, which were then agitated by a magnetic stirrer for 24 h at 55-60 "C, transferred to a thermostatted bath at seven different temperatures (1040 "C) and left for at least 6 days to attain equilibrium. Two aliquots of the same sample and four samples for each experiment were removed and analysed by two different methods.(a) The total solubility of the 1-naphthoic acid was quantified by titrating aliquots with standard NaOH solution and phenolphthalein indicator for water and water + alcohol (when the cosolvent was DMSO we used either thymol blue or cresol red indicators because these exhibit a behaviour similar to that seen in pure water14). (b) A spectrophotometric technique using a Perkin-Elmer Lambda 5 ultraviolet-visible spectrophotometer was used to measure the total solubility of 1 -naphthoic acid (AH + A-) and that of the un-ionized form (AH). The two methods agree to within 3%. The precision in two runs was 3% at the lower concentration of cosolvent and pure water and 1 % at higher concentrations. It was considered that 1-naphthoic acid was un-ionized when the concentration of the cosolvent was 50 wt% EtOH, 50 wt% DMSO and 30 wt% ButOH.RESULTS The solubilities of the 1-naphthoic acid in water and water+cosolvent in terms of log S (table 1) were fitted by a non-linear regression analysis15 to the expression l o g s = A-B/T+ClnT (1) where S is the solubility in mol dm-3, T is the temperature in K and A , B and C are adjustable parameters. The derived values of AGP, A@ and AH? were calculated from the equations AGP = - 2.302 RT log (SJS,) (2) AGP = (AL-A&) T+(Bi-Bk)+(Ci-C&) Tln T (3) AH? = (BL-B&)+(Ck-CL) T (4) ASP = (Ak-AL)+(Ck-Ci)+(Ck-Ci) In T ( 5 ) where the subscripts w and s represent water and mixed solvents, respectively, A' = -2.303RA, B' = 2.303RB and C' = -2.303RC.The values of A , B and C in water and mixed solvent are given in table 2 for un-ionized 1-naphthoic acid along with AGP, A@ and AH? at 298.15 K. The maximum uncertainties in the transfer- function values were calculated from standard deviations associated with A , B and C and were f 5% in AGP and AH? and 10% in ASP. We have assumed that the ratio of activity coefficients of un-ionized 1 -naphthoic acid in the saturated solutions (water and mixed solvents) is unity. This may not be correct in the case of higher concentrations of DMSO (> 70 wt%), where the solubility of the acid is high.M. C. PEREZ-CAMINO, E. SANCHEZ, M. BALON AND A. MAESTRE 1557 Table 1. Values of -log (S/mol dm-3) for un-ionized 1 -naphthoic acid in various mixed aqueous solvents at different temperatures - mole T/"C fraction cosolvent (wt%) 10 15 20 25 30 35 40 - ethanol 10 20 30 50 70 20 30 50 70 dimethyl sulphoxide 10 20 30 50 65 70 75 80 t-butyl alcohol 10 water 3.62 3.47 3.34 3.19 3.04 2.93 2.82 3.19 3.08 2.91 2.81 2.62 2.47 2.32 2.53 2.40 2.27 2.09 2.01 1.87 1.75 1.34 1.21 1.15 1.05 0.95 0.86 0.78 0.80 0.70 0.64 0.57 0.50 0.42 0.33 3.62 3.47 3.31 3.14 3.08 2.92 2.73 2.87 2.65 2.41 2.26 2.15 1.92 1.79 1.73 1.60 1.52 1.41 1.28 1.17 1.08 1.06 0.97 0.85 0.81 0.73 0.66 0.57 0.75 0.66 0.55 0.51 0.42 0.35 0.29 3.59 3.46 3.31 3.20 3.12 2.99 2.87 3.19 3.08 2.97 2.86 2.67 2.56 2.49 2.83 2.74 2.62 2.49 2.34 2.23 2.11 1.77 1.68 1.59 1.48 1.35 1.21 1.11 0.82 0.71 0.59 0.50 0.40 0.25 0.14 0.40 0.29 0.17 0.11 -0.03 -0.05 -0.14 -0.05 -0.10 -0.13 -0.16 -0.22 -0.28 -0.33 -0.38 -0.40 -0.42 -0.43 -0.45 -0.46 -0.47 3.97 3.81 3.70 3.59 3.43 3.33 3.26 Table 2.Coefficients of eqn (1) and thermodynamic functions for transfer from water to water + cosolvent at 298 K mole fraction AG? As? AH? cosolvent (wt%) A BIK C /kJ mol-l /J K-l mol-l /kJ mol-l ethanol 10 20 30 50 70 t - but yl 10 alcohol 20 30 50 70 sulphoxide 20 30 50 65 70 75 80 dimethyl 10 water 63.741 13.596 -271.97 -11.661 - 80.630 - 89.134 201.74 - 77.556 41.563 86.230 33.266 95.503 - 79.875 - 280.72 - 177.32 178.50 - 145.05 34.597 131.1 1 5014.2 -8.7951 -9748.3 41.5070 2672.4 - 1.1856 918.9 2.4044 2422.1 12.626 - 1668.8 14.104 1 1 746.3 - 28.890 - 1742.0 12.341 3 075.4 - 5.6245 5005.0 - 12.275 3 395.9 - 1571.1 6 242.1 - 10728.0 -6 164.7 9288.4 1745.6 - 5 749.6 - 4.4029 12.598 42.697 27.406 22.104 - 13.520 - 25.876 - 4.9684 7 800.2 - 19.046 - 2.28 - 4.45 - 8.56 - 14.50 - 17.24 - 2.57 - 7.59 - 12.45 - 15.87 - 17.58 - 2.23 -4.17 - 6.28 - 12.05 - 17.64 - 19.87 -21.41 - 22.95 24 47 40 17 7 34 90 30 7 9 4 18 27 32 51 31 -11 - 43 5.03 9.66 3.43 - 9.43 - 15.07 7.55 19.16 - 3.62 - 13.94 - 14.92 - 0.95 1.22 1.74 - 2.53 - 2.34 - 10.52 -24.53 - 35.701558 THERMODYNAMICS OF TRANSFER FROM WATER TO MIXED SOLVENTS 4 52 Fig.1. Variation of AGf) and T A P for the transfer of 1-naphthoic acid from water to water + EtOH (O), water + BdOH (A) and water + DMSO (0) at 298 K. DISCUSSION TRANSFER FROM WATER TO WATER 4- ALCOHOL (EtOH OR BUtOH) The decrease in AG? with the mole fraction (fig. 1) indicates preferential solvation of the 1-naphthoic acid molecule by alcohol.In the water-rich region the rise in AH? is clearly compensated by a still larger contribution from - A p . Therefore, the entropy term governs the sign of AGP in this region. Both A@ and AH? reach maxima that have frequently been related to an enhancement of the lo, l6, l7 In general, the position, height and sharpness of these maxima depend on the nature and size of the solute transferred. For other non-electrolytesBv lo* the size seems to be the controlling factor for a given water+alcohol mixture; i.e. the larger the hydrophobic group of the non-electrolyte, the more the maximum is shifted towards lower mole fraction of alcohol. In addition, an increase in the hydrophobic nature of the alcohol (e.g. methyl to t-butyl) produces the same effect.M.C. PEREZ-CAMINO, E. SANCHEZ, M. BALON AND A. MAESTRE Table 3. Molecular pair-interaction parameters (in J kg m o P ) in water at 298 K 1559 ~ X Y ref. 1-naphthoic acid EtOH ButOH DMSO benzoic acid MeOH EtOH PriOH p-nitroaniline EtOH Pr’OH ButOH naphthalene EtOH - 478 -719 - 843 - 198 - 250 - 308 - 96 - 538 -116 - 409 1430 2776 - 755 419 733 1640 736 3060 553 1 1972 1908 3495 88 61 7 983 1948 832 3598 5617 2384 this work this work this work 9 9 9 10 10 10 26 s,,/J kg mol-I Fig. 2. Enthalpy+xtropy compensation plot for the transfer of naphthalene (A), 1 -naphthoic acid (O), benzoic acid (a) and p-nitroaniline (A), from water to water+EtOH mixtures. We have calculated the molecular pair-interaction parameters, 1 -naphthoic acid- cosolvent (J,,,f= g , h, Ts), in water (table 3) on the basis of the Gibbs free energies and enthalpies of transfer for 1-naphthoic acid from water to water +cosolvent mixtures.The Ts,, were derived from g,, and h,, using the expression Ts,, = h,,-g,,. For the sake of comparison, table 3 also shows f,, of other non-electrolytes. The compensating effect between AH? and - T A* is also shown by h,, and Ts,,. In water+alcohol mixtures h,, are positive and increase with the alkyl group of the alcohol. These positive values mean unfavourable interactions with the alcohol. The introduction of a 1-naphthoic acid molecule to a water-rich alcohol mixture can produce a breakdown of hydrogen bonds between the water and the1560 THERMODYNAMICS OF TRANSFER FROM WATER TO MIXED SOLVENTS alcohol (hZy increases) and dispersion interactions and dipole-dipole interactions should occur between the 1 -naphthoic acid molecule and the alcohol (hXy decreases).Evidently, the former effect is stronger than the latter, resulting in h,, > 0. The greater value of h,, for ButOH with respect to that for EtOH is a consequence of stronger hydrogen bonds. The same pattern is shown by other hydrophobic non-electrolytes with alcohols (table 3). Ts,, values are positive, which seems to be connected with the size of the alcohol and may be due to the competitive effect (alcohol-water against alcohol-solute interactions) previously mentioned for h,,. This type of enthalpy- entropy compensation behaviour is shown by plotting h,, against szy for various non-electrolytes in water + EtOH mixtures (fig.2). TRANSFER FROM WATER TO WATER + DMSO In the case of water + DMSO mixtures, AGP decreases as the proportion of DMSO in the solvent increases (fig. 1). However, whereas AS? passes through a maximum (xIIMSO = 0.30-0.35, fig. l), AH? behaves differently from water + alcohol mixtures. This sort of behaviour of AH? has been observed for the transfer of un-ionized o-methoxybenzoic acidlg and p-hydroxyaniline20 from water to water + DMSO. Several studies using different techniques on the effect of DMSO on the water structure have proved 22 maintains that DMSO is a typical non-aqueous non-electrolyte because its aqueous mixtures are in fact characterized by IAHEI > TIAS”\. However, it seems that in the water-rich region strong water-DMSO interactions occur,24 possibly of the dipole-dipole type.25 We have calculated the molecular pair-interaction parameters, f,,, in water (table 3) and they show a pattern that is different from that obtained for water+alcohol mixtures; i.e.although g,, is negative, h,, and Ts,, take negative and small positive values, respectively. A value of h,, < 0 for the 1-naphthoic acid-DMSO pair means that a favourable interaction occurs. If the interactions between water and DMSO are preferentially of the dipole-dipole type in the water-rich region, then adding 1-naphthoic acid to a water-rich DMSO mixture could conceivably produce a com- petitive effect on the DMSO. The h,, < 0 value may be explained by supposing that dipoledipole and dispersion interactions between DMSO and 1 -naphthoic acid are stronger than water-DMSO interactions.A small positive value of Ts,, would be in accordance with the above interpretation. CONCLUSIONS In the transfer of non-electrolytes from water to aqueous mixtures, AG? accounts for solute-medium interactions only. Therefore it is necessary to determine the entropy and enthalpy terms. This is supported by analysis of molecular pair-interaction parameters, since these parameters (hx, and Ts,,) represent not only solute-cosolvent interactions, but also cosolvent-water interactions. We thank Dr Sanchez-Burgos, University of Seville, for helpful discussions. K. Bose, K. Das, A. K. Das and K. K. Kundu, J . Chem. SOC., Faraday Trans. I, 1978,74, 1051 and references therein.0. Popovych and R. P. T. Tomkins, Nonaqueous Solution Chemistry (Wiley, New York, 1981), chap. 4. K. K. Kundu and A. J . Parker, J . Solution Chem., 198 1, 10, 847. M. H. Abraham, T. Hill, H. Chiong Ling, R. A. Schulz and R. A. C. Watt, J . Chem. SOC., Faraday Trans. I , 1984, 80, 489.M. C. PEREZ-CAMINO, E. SANCHEZ, M. BALON AND A. MAESTRE 1561 F. Rodante and M. G. Bonicelli, Thermochim. Acta, 1983, 66, 225. S. Murakami, R. Tamaka and R. Fujishiro, J. Solution Chem., 1974, 3, 71. M. Roseman and W. P. Jenks, J. Am. Chem. SOC., 1975,97, 631. H. Gillet, L. Avedikian and J. P. Morel, Can. J. Chem., 1975, 53, 455. K. Bose and K. Kundu, Can. J. Chem., 1977,553961. lo K. Das, A. K. Das, K. Bose and K. K. Kundu, J. Phys. Chem., 1978,82, 1242. l 1 F. Franks and D. J. G. Ives, Q. Rev. Chem. Soc., 1966, 20, 1. l2 C. Visser, G. Perron and J. E. Desnoyers, J. Am. Chem. SOC., 1977,99, 5894. l 3 G. Petrella and M. Petrella, Electrochim. Acta, 1982, 27, 1733. l4 M. Georgiera, P. Zokolov and 0. Budevsky, Anal. Chim. Acta, 1980, 115, 41 1. l5 K. J. Johnson, Numerical Methods in Chemistry (Marcel Dekker, New York, 1980), chap. 5. l6 N. Dollet and J. Juillard, J. Solution Chem., 1976, 77, 5 . l 7 M. C. R. Symons and M. J. Blandamer, in Hydrogen-bonded Solvent Systems, ed. A. K. Covington and P. Jones (Taylor & Francis, London, 1968), p. 21 1. ** N. R. Choudhury and J. C. Ahluwalia, J. Solution Chem., 1982, 11, 189. lQ F. Rodante, G. Ceccaron and F. Fantauzzi, Thermochim. Acta, 1983, 67, 45. 2o F. Rodante and M. G. Bonicelli, Thermochim. Acta, 1983, 66, 225. 2 1 G. Petrella, M. Petrella, M. Castagnolo, A. Dell’Atti and A. De Giglio, J. Solution Chem., 1981, 10, 22 G. Petrella and M. Petrella, Electrochim. Acta, 1982, 27, 1733. 23 F. Franks, in Hydrogen-bonded Solvent Systems, ed. A. K. Covington and P. Jones (Taylor & Francis, 24 M. F. Fox and K. P. Whittingan, J. Chem. Soc., Faraday Trans. 1, 1975,71, 1407. 25 R. K. Wolford, J. Phys. Chem., 1964,68, 3392. 26 D. Bennet and W. J. Canady, J. Am. Chem. SOC., 1984, 106,910. 129. London, 1968), p. 34. (PAPER 4/1098)

ISSN:0300-9599    

DOI:10.1039/F19858101555

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1985, 81, 1563-1567 Multistability and Sustained Oscillations in Isothermal, Open Systems Cubic Autocatalysis and the Influence of Competitive Reactions BY BRIAN F. GRAY AND STEPHEN K. SCOTT* School of Chemistry, Macquarie University, North Ryde, New South Wales 21 13, Australia Received 1 1 th July, 1984 The simple, isothermal model of cubic autocatalysis : A+2B+3B, rate = kab2 B+C, rate = k,b (0 (ii) exhibits ‘exotic’ patterns of behaviour, including mushroom and isola dependences of the stationary-state catalyst concentration on residence time and sustained oscillations in well stirred, open systems. In the light of recent criticisms of similar schemes, the effects on these features of various additional steps, reversibility and detailed balance are investigated comprehensively.The two B molecules on the left-hand side of reaction (i) may be regarded as acting as third bodies, providing additional energy. The reactant A may also play such a role. Lower-order catalytic steps and uncatalysed conversions of A and B are also considered, along with multiple conversions such as A+A+(A or B)-+B+B+(A or B). Thermodynamic requirements suggest that these additional steps and their reverse should have strictly non-zero (but otherwise quite arbitrary) rate constants. It is shown that all of the above patterns are still found when these extra steps are included, although the range of values of some of these rate constants over which they exist is quite narrow. Simple model kinetic schemes have been widely studied for a number of years.Prime motives for this have been the need to establish that various complex patterns of behaviour, in particular sustained or long-lived oscillations, do not require complex mechanisms and do not violate thermodynamic principles. Perhaps the simplest of all ~chemesl-~ is the cubic autocatalysis A+2B -+ 3B (1) coupled with a first-order decay B 3 C, rate = k, b in an isothermal, well stirred open system. The termolecular step, reaction (l), should not be regarded as an elementary process: rather the corresponding rate equation, rate = k , ab2, might represent an empirically determined expression. The interpretation of processes such as reaction (1) has, however, been subject of some recent debate?. Real chemical reactions are subject to certain restrictions imposed by thermodynamic requirements.In particular, these suggest that a number of additional reaction steps, 15631564 OSCILLATIONS IN AUTOCATALYTIC SYSTEMS proceeding in parallel with reaction (I), may not be completely ignored (i.e. may not be assigned strictly zero rates). It is the purpose of this paper to show that the cubic autocatalysis ‘core’ above is stable to the inclusion of such processes: both multiplicity (ranges of experimental conditions for which more than one stationary state may exist) and sustained oscillations survive although only for quite small ranges of the various rate constants of the additional steps. Whether rate constants falling in these narrow ranges are physically meaningful or not is a question which can only be answered by appeal to molecular theory.ADDITIONAL REACTIONS TO BE CONSIDERED IN PARALLEL WITH REACTION (1) Reaction (1) has the stoichiometry A + B and the two B molecules that appear on both sides of the equation may be regarded as playing the role of third bodies, providing additional energy to the system. If this is so, then molecules of A which are also present may act in the same way, although perhaps not with the same efficiency. Thus the additional reactions A+A+B~’.A+B+B (2) k3 A+A+A+A+A+B (3) can be assigned non-zero rate constants. Furthermore the lower-order or quadratic catalysis and the uncatalysed conversion of A into B A+Bk”. 2B (4) ( 5 ) k A+A”.A+B have the same overall stoichiometry and hence the same changes in the thermodynamic quantities AG* etc.They must also be assigned non-zero rate constants and may compete in parallel with reaction (1). Finally, there may be sufficient energy in the two- or three-body reactions (1)-(5) for more than one A molecule to be converted into product B. Thus there are three double conversion steps: A+A+Bk’.3B (7) (9) k A+A--&,2BB. F. GRAY AND S. K. SCOTT 1565 and a triple conversion: A+A+Ak'".3B. (10) All of these reactions are, in principle, reversible. However, the ten reverse rate constants k-l-k-lo cannot all be chosen independently. Because reactions (1)-(6) have the same overall stoichiometry they must have the same equilibrium constant, i.e. These equalities are independent of concentration and hence must apply at all times and not just at the state of equilibrium.The double-conversion reactions (7)-(9) have a stoichiometry which is twice that of the single-conversion steps and hence the change in the thermodynamic quantities AG* etc. is doubled. This, in turn, means that the equilibrium constant of these steps is the square of that for the single-conversion processes, i.e. Similarly, for the triple-conversion reaction (1 0) we have Thus, specification of the equilibrium constant and the forward rate constants fixes all the reverse rate constants. MULTISTABILITY, ISOLAS, MUSHROOMS AND SUSTAINED OSCILLATIONS The conditions under which multiple stationary states of reactions (1HlO) can exist for some residence times can, for the case of an infinitely stable catalyst (kd = 0), be determined in terms of the rate constants, inflow concentrations and equilibrium constant using the method presented by Gray et aZ.4 These also represent the conditions for multiplicity when the catalyst B undergoes subsequent decay, although there is then an additional requirement in terms of a limitation on the value of k,.In general, the ranges of multiple solutions and sustained oscillations are decreased by increases in the rate constants k,-k,, and k , with respect to the value of k , and by decreases in the equilibrium constant. For this general system, the dependence of the stationary-state concentrations of reactant A and autocatalyst B on the residence time may be evaluated numerically for any given combinations of the various rate constants and inlet concentrations.At the same time the local stabilities of the stationary states, which determine the response of the system to small perturbations, may be evaluated by the standard method^.^, Fig. l(a)-(d) show four of the patterns possible with non-zero values of k,-k,, and k, and a finite value of K,. For example, fig. l(a) is found for a system with k, = 0.05 k,, k3 = 0.01 k , and k, = 0.025 kla& The other rate constants are chosen to have much smaller (but still non-zero) magnitudes compared with k,. This response shows a range of multistability at short residence times and a range of conditions at longer residence times over which the reaction undergoes sustained oscillations. Decreasing k, and k3 leads to a pattern of the form shown in fig. l(b). This response is termed an isola and has three stationary-states over some range of residence time.The lowest branch may also support sustained oscillations.1566 OSCILLATIONS IN AUTOCATALYTIC SYSTEMS 1. 0 II a" h U" a" -% u" d III II " .:.-is I I 1 (-;>(b) - I I 1 1 0 -% d U" II 0 Fig. 1. Typical dependences of the stationary-state concentration of reactant on the residence time t,,, = k,ai c,,, for full scheme of reactions (1)-(10); letters on branches indicate local stability as follows : s, stable; u, unstable; 0, sustained oscillatory reaction. Including some B in the inflow opens out the isola first to a mushroom [fig. 1 (c)] and then to a curve showing no multistability [fig. l(d)]. The latter shows that multiple stationary states are not necessary for oscillatory behaviour.DISCUSSION The impetus for the present study has been to show that the 'exotic' patterns of behaviour exhibited by the simple cubic autocatalysis model, reaction (I), may be genuine chemical features and do not arise solely because certain thermodynamic requirements have not been met. Thus we have demonstrated that multistability and sustained oscillations in this system are stable to the consideration of the strictly non-zero, but otherwise arbitrary, values of rate constants for reactions (2)-( 10). This is by no means a trivial point. It has been shown elsewhere that the ' Brusselator' model of Prigogine and Lefevera ceases to display oscillations if the scheme is required to satisfy the principle of detailed balance. For the same model King5 has found that decomposition of the cubic reaction (1) into two steps involving a transition state (AB)S, e.g.A + B e (AB)~ (AB)S + B -+ 3B also removes the oscillatory behaviour. This is because the cubic terms in the governing rate equations are then replaced by a series of quadratic terms which are not, in this case at least, sufficiently non-linear to support oscillations. The required degree of non-linearity can, however, be retained by breaking reaction (1) down as follows: A + B + €3 e (A&)$ (1 1) (AB,)f -, 3B. (12)B. F. GRAY AND S. K. SCOTT 1567 It can be readily shown for a number of examples that provided the intermediate (AB2)t decomposes very quickly all the above patterns of multistability and sustained oscillation are permissible. In this case the two-variable system provides a good approximation to the three-variable scheme involving (AB,)S.The numerical values of the rate constants k,-k,, used in computing the examples shown in fig. 1 have, in all cases, been a number of orders of magnitude smaller than the rate constant for reaction (l), for example we have taken k, lo-’ k , at. If one examines the stability of the ‘Oregonator’ m ~ d e l , ~ which forms the basis of interpretations of the Belousov-Zhabotinskii reaction and which contains the step X + Y +2X, to the introduction of the uncatalysed step X + Y, one again finds1° that the catalysed step must have a rate constant approximately four orders of magnitude greater than the latter. The questions arising here, as to whether multistability or oscillatory behaviour are physically real, is now essentially molecular, i t .we need to say something about the likely values of the ratios k , / k , and kJk, before we can assert that this scheme shows multistability. As A and B are isomers, we might hazard a first guess that these ratios would be of the order of unity and the system would not show exotic behaviour. Clearly further statements can only be made with particular physical examples to provide guidance and we regard this circumstance as a welcome move away from pure formalism. S. K. S. thanks the Australian Research Grants Committee for an award under the Queen Elizabeth I1 Postdoctoral Fellowship Scheme. P. Gray and S. K. Scott, Chem. Eng. Sci., 1983, 38, 29. S. K. Scott, Chem. Eng. Sci., 1983,38, 1701. P. Gray and S. K. Scott, Chem. Eng. Sci., 1984, 39, 1087. B. F. Gray, S. K. Scott and P. Gray, J. Chem. SOC., Faraday Trans. 1, 1984,80, 3409. G. A. M. King, J. Chem. Soc., Faraday Trans. 1, 1983, 79, 75. B. F. Gray and T. Morley-Buchanan, J . Chem. SOC., Faraday Trans. 2, 1985,81, 77. I. Prigogine and R. Lefever, J . Chem. Phys., 1968,48, 1695. R. J. Field and R. M. Noyes, J . Chem. Phys., 1974, 60, 1877. ’ A. A. Andronov, A. A. Vitt and S. E. Khakin, Theory of Oscillators (Pergamon Press, Oxford, 1966). lo B. F. Gray and T. Morley-Buchanan, in preparation. (PAPER 4/ 1205)

ISSN:0300-9599    

DOI:10.1039/F19858101563

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I , 1985,81, 1569-1576 Reduction of Carbon Monoxide on a Mediated and Partially Immersed Electrode BY KOTARO OGURA* AND HIROYASU WATANABE Department of Applied Chemistry, Yamaguchi University, Ube 755, Japan Received 1 1 th July, 1984 The reduction of carbon monoxide into methanol has been performed using Everitt's salt {K,Fe"[Fe"(CN),]} as a mediator in the presence of homogeneous catalysts at a three-phase (electrode/solution/gas) interface. These conditions enhance the reduction of CO owing to the rapid transport of CO to the active zone of the electrode. The logarithm of the reduction current is linearly related to log (time) with a slope of - 1/2 and reaches a constant value after an extensive period of polarization. A theoretical equation for the current against time curve is derived based on a surface-diffusion model in which CO from the gas phase is first adsorbed on the electrode surface and then diffuses along the surface toward the intersection of the three phases; the computed plot of current against time fits the experimental one well. The conversion of CO into an organic substance is an important process as it can be used as chemical feedstock.We have previously reported that CO may be converted into methanol using Everitt's salt (K2Fe11[Fe11(CN),], ES} using methanol and penta- cyanoferrate(r1) complex as homogeneous catalysts at room temperature under atmospheric pressure.1.2 In order to obtain a higher conversion efficiency the CO reduction is carried out here at a three-phase boundary at a partially immersed flat electrode in which CO gas is in contact with both solution and metal, as described later.Grove3 found that a marked increase in current results when part of a platinum electrode is exposed to hydrogen gas above the electrolyte. This finding led to the emergence of various gas-diffusion electrodes made up of a large number of cylindrical pores, with each pore consisting of partially immersed flat plates. These electrodes have been widely studied from both and e~perimental~-~ aspects. There are theories relating to the mechanism by which the reaction takes place at the three-phase boundary : (i) the surface-diffusion mechanism and (ii) the electrolyte-film mechanism. In the former chemisorption of the reaction species occurs on the electrode surface above the electrolyte meniscus.The reactive species then diffuse along the electrode surface to the area where the electrochemical reaction takes place, and surface diffusion of the reaction species is the rate-determining step. In the latter mechanism the electrode is considered to be covered with a thin film of electrolyte which separates the gas phase from the metal surface. The gaseous species first dissolves in the outermost layer of the film and then diffuses through the film. In this model it is assumed that the rate-determining step is the diffusion of the reaction species through the liquid meniscus and that the existence of the three-phase boundary is not necessarily related to the electric current. The present work was undertaken using a partially immersed electrode in order to enhance the conversion efficiency of CO into methanol. The reduction of CO in this system is caused by the oxidation of Everitt's salt, coated on a platinum plate, to 1569 F A R 1 521570 REDUCTION OF co INTO METHANOL Prussian blue (PB).However, this redox reaction can only be activated by the presence of methanol and metal complexes.'$ The amount of methanol produced was considerably enhanced under these three-phase conditions, and the experimental plot of current against time was in agreement with the theoretical curve obtained on the assumption of the surface-diffusion model. EXPERIMENTAL The test cell had two compartments separated by a fine-porosity glass frit. The main compartment, in which a platinum plate coated with ES was partially immersed in solution, was connected by a Luggin capillary to a saturated calomel reference electrode.This compartment was constructed so as to permit CO or nitrogen gas to be introduced above the electrolyte. The other compartment contained a platinum-plate counter-electrode. Everitt's salt {K2Fe11[Fe11(CN),]} coated on a platinum plate was prepared by the electro- chemical reduction of Prussian blue {(KFelll[Fell(CN),J}. The preparation of PB on a platinum plate was performed under similar conditions to those in ref. (10) and (1 1). A platinum plate of ca. 4.2 cm2 area was first submerged for 3 min in an equal-volume mixture of concentrated nitric and sulphuric acids and for 2 min in 1 mol dm-3 hydrochloric acid solution in a supersonic syringe.The electrode was then cathodized in 1 mol dm-3 HCl for 15 min at 0.1 mA cm-2 and thoroughly rinsed with distilled water. The platinum plate was cathodically polarized at a constant current density of 0.08 mA cm-2 for 5 min in a mixed solution (1 : 1) of 0.01 mol dm-3 K,Fe(CN), and 0.01 mol dm-3 FeC1;6H20. The electrode was again rinsed, allowed to stand for 1 h in distilled water and dried in air for 24 h. The amount of ES on the platinum plate was evaluated coulometrically in 0.1 mol dm-3 KCl, and an amount of ES corresponding to (1.0-2.0) x lo-' mol cm-2 was formed in each run. The electrolyte meniscus formed on the partially immersed platinum plate coated with ES was observed microscopically. The experi- ments were performed for various electrode positions.The electrolytes were 0.1 mol dm-3 KC1 solution (the pH was adjusted with HCl) containing a given amount of aquapentacyanoferrate(I1) (Na,[Fe(CN),(H,O)]) and methanol, which function as homogeneous catalysts. Aquapentacyanoferrate(I1) was synthesized from penta- cyanonitrosylferrate(Ir1) {Na,[Fe(CN),(NO)]} by the method of Hieber et af.12 The carbon monoxide used in this experiment was supplied by the Seitetsu Kagaku Co. and contained c 500 ppm nitrogen, c 100 ppm carbon dioxide, c 100 ppm oxygen, < 100 ppm hydrogen and < 15 ppm water. 70 cm3 of the prepared solution was transferred to the test cell, the platinum plate coated with ES was partially immersed in the solution and CO was introduced above the solution level. The electrochemical experiments were performed using a Nichia NP-G 1 OOOG potentiostat, a Nichia S-5A function generator, a Nichia N-CR564 coulometer and a Watanabe WX440 X- Y recorder.The reaction products, such as methanol, formic acid and formaldehyde, were analysed by the following methods. A JGC-110 gas chromatograph with a thermal-conductivity detector and a Poropak Q column was used to determine the amount of methanol. The sampling procedure for the gas chromatograph was performed in the following way. 2.5 cm3 of the sample solution was transferred to ace11 which wasconnected to vacuum through a stopcock. A side-port of the cell was fitted with a rubber septum in order to withdraw samples, and the solution was evaporated under 1 Torr pressure at 70 "C. After nitrogen gas was introduced into the cell, gas samples (2 cm3) were taken with a Pressure-Lock air-tight syringe.The calibration curve for this sampling procedure was linear for methanol up to at least 30 mmol dm-3. The amount of methanol was calculated from the difference in the methanol contents of the initial and final solutions. The experimenlal error involved in such a method was within f 2 % . Formic acid and formaldehyde were determined by colorimetric analysis using chromotropic acidI3 but they were present in negligible amounts.K. OGURA AND H. WATANABE 1571 8 0.5 I I I [ n l R I O3 I 2 3 4 S,/cm2 Fig. 1. Relationship between the amount of methanol produced and the electrode area exposed to the gas phase (S,) in 0.1 mol dm-3 KC1 solutions at pH 3.5 containing (A, 0) 5 mmol dm-3 Fe(CN)i-+20 mmol dm-3 CH30H or (a) 1 mmol dm-3 Fe(CN)X-+ 20 mmol dm-3 CH,OH.Electrolysis potential -0.9 V; time 3 h. 0 is the ratio of the area exposed to the gas phase to the total surface area. 0, 0, in an N,-saturated solution under an atmosphere of CO; A, in a CO-saturated solution under an atmosphere of N,. RESULTS AND DISCUSSION A marked enhancement of CO conversion was obtained when the platinum plate coated with ES was partially immersed in the electrolyte. The amount of methanol produced is shown in fig. 1 as a function of the electrode area exposed to the gas phase under an atmosphere of CO or nitrogen gas. In the solution saturated with CO the production of methanol decreases with increasing S, (the area exposed to the gas phase), which is reasonable since the reduction of CO is dependent only on the electrode surface submerged below the level of the electrolyte.In an atmosphere of CO, with the electrolyte saturated with nitrogen gas, the amount of methanol increases with increasing S,. In this case the conversion process is independent of the area below the electrolyte surface, but depends on the area exposed to the gas phase. At 6 = 0.5 the amount of methanol obtained under the three-phase condition is about four times as large as that in the corresponding two-phase state. Using the completely submerged electrodel the formation of methanol was enhanced on raising the concentration of added pentacyanoferrate(r1) complex. A similar result was observed here. In the present case two different concentrations of pentacyanoferrate(I1) were added, and 52-21572 REDUCTION OF co INTO METHANOL I 2 3 4 5 t / h Fig.2. Relationship between the amount of methanol produced and the polarization time at -0.9 V in 0.1 mmol dmP3 KCI solutions at pH 3.5 containing 5 mmol dmP3 Fe(CN)E- and 20 mmol CH30H. 0, Partially immersed electrode in an N,-saturated solution under an atmosphere of CO; a, completely submerged electrode in a CO-saturated solution. methanol formation was always higher in the presence of the higher concentration of pentacyanoferrate(n), as seen from fig. 1. The amount of methanol produced is plotted against the polarization time in fig. 2 for two cases in which the electrode is partially and completely submerged. The three-phase condition always gives more methanol than the two-phase condition.Except for the initial period, the amount of methanol formed increases linearly with time in the three-phase state for the timescale of this study, although the amount obtained in the two-phase state approached a final value over a long time of polarization. In the present system CO is not converted into methanol by direct electrochemical reduction but rather CO reduction is brought about by the oxidation of ES to PB. This redox reaction is activated by homogeneous catalysis in the presence of methanol and pentacyanoferrate(1r). Thecatalytic process has been discussed in detail previous1y.l The net reduction reaction of CO is represented as The external current is consumed in the recovery of the mediator, i.e. the reduction of PB to ES: PB+K++e-+ES.(2)K. OGURA AND H. WATANABE 1573 I I I I J 10 I o2 lo3 I o4 tl s Fig. 3. Plots of current against time at - 0.9 V in solutions containing 5 mmol dm-3 Fe(CN):- and either 20 mmol dmP3 CH,OH (0) or 10 mmol dmP3 CH30H (0). 8 = 0.766. CO must reach the active zone of the ES surface by transport through the surrounding gas and liquid. The above results show an enhancement of the formation of methanol in the three-phase condition, which indicates that the transport of CO to the active zone of the electrode is more favourable through the gas phase than solution. Plots of current against time are shown in fig. 3, where -0.9 V is applied at 8 = 0.766. Log (current) varies at first in a linear way with log (time) with a slope of - 1 /2, and reaches a constant value after a long period of polarization.The current is ascribed to the electrochemical reduction of PB to ES, with PB being provided by chemical reaction.' In such a coupled process the transport of CO to the active zone of the ES surface is the rate-determining step, as discussed below. We will try to understand the physical meaning of the - 1 /2 slope obtained in fig. 3 by a theoretical analysis of the reduction process in the three-phase condition. The surface-diffusion model seems to explain the present experimental results well. The three-phase interface is schematically represented in fig. 4: CO from the gas phase first adsorbs on to the electrode surface, and then CO diffuses along the surface from the point of adsorption towards the intersection of the three phases.In the present treatment the following situation is supposed : before electrolysis CO is adsorbed on the site of ES exposed to the gas phase, and the adsorbed CO is depleted as reaction (1) proceeds, but the concentration of CO at the top of the electrode is always constant; i.e. CO is only supplied from the top (x = 0) of the electrode surface during electrolysis, and the adsorption of CO from the gas phase at various values of x is neglected. Under such an assumption the amount of CO is given by the total number of adsorption sites on the electrode surface multiplied by the ratio (8) of S, to the total area S( = S, + S,): t = 0,o < x < 1: c = K * / S (3)1574 REDUCTION OF co INTO METHANOL x = o - x = l . co solution Fig.4. Schematic representation of the three-phase (metal/solution/gas) interface. S , is the area exposed to gas phase, S, is the area immersed in solution, x is the distance from the top of the electrode to the three-phase interface and rn is the width of the electrode. where C is the concentration (in mol cm-2) of CO at the electrode surface, C* is the amount (in mol) of adsorption sites and S is the electrode surface area (in cm2). We have the following boundary condition: t > O , x = l . . c=o. (4) After a long time of reduction the concentration of CO at the top of electrode (x = 0) reaches a constant value, C# : t + m , x = O : C = C # . ( 5 ) Under these boundary conditions the Fick equation for semi-infinite linear diffusion was solved, and the Laplace transform is given by c c"" C= s+s X (exp {-(s/D):[(2n+l)I+xl}-exp {-(s/o):[(2n+l)f-x]}) (6) n-o where C is the initial concentration of adsorbed CO and D is the surface-diffusion constant of CO.The inverse transformation of c i s given as (7) n-0 The concentration of CO at the three-phase interface can be converted into the diffusion current by the relation i = -zFDm - where z is the number of electrons involved in the CO reduction and F is Faraday's constant. Differentiating eqn (7) with respect to x and substituting the resulting equation in eqn (8) we have I = . zFmOC*Da n:t:s n-0 [exp ( - g ) + e x p (-%)I Dt ' (9)K. OGURA AND H. WATANABE 1575 I O - ~ I d4 d 2 10-5 lo-' 0.012 I I I I I 10 I 0' I o3 I o4 I 0' t l s Fig. 5. Computed current against time curves at various values of 8: C* = 1.036 x mol, S = 10 cm2, m = 1.532 cm, z = 4 and D,, = 1.0 x cm2 s-l.I o - ~ I o - ~ 4 2 lo-s I O-E I I I I I I 10 I o2 103 I o4 IC t/ 5 Fig. 6. Computed current against time curves at various values of Dco: (a) lop3, (b) lo-*, (c) and ( d ) C* = 1.036 x mol, S = 10 cm2, m = 1.532 cm, z = 4 and B = 0.806.1576 REDUCTION OF co INTO METHANOL This equation can be converted into a dimensionless one: where i* = zFmC*D/lS f = (Dt):/l. Fig. 5 shows plots of current against time computed from eqn (9). In this computation CO is assumed to be adsorbed on accessible cations in the ES film and C* to be equal to the amount of ES present on the electrode surface. The number of electrons involved in CO reduction is four per mol of methanol formed.Except for extremely exposed or submerged electrodes the log (current) changes in a linear manner with log (time) with a slope of - 1/2, reaching a constant vaIue after a long period of cathodic polarization, which agrees well with the experimental result shown in fig. 3. Fig. 5 shows that the ratio of the initial to final current is dependent on 8. This is because of the difference in the values of the initial concentration of CO; i.e. if 8 or C* is large, the initial current should be large, but with a small value of C* the initial current cannot be large. However, in both cases the final current should be small because of the depletion of CO. Such a correlation was observed experi- mentally. In fig. 6 the computed plots of current against time are exhibited at various values of D.The curve given for D = 1 .O x lop5 cm2 s-l fits the experimental curve well. Accordingly, in three-phase reduction, CO from the gas phase is first adsorbed on the electrode surface; this is followed by surface diffusion to the three-phase interface. This situation enhances the reduction of CO into methanol, which is based on the rapid transport of CO to the active zone of the electrode. K. Ogura and S. Yamasaki, J . Chem. Soc., Faraday Trans. I , 1985, 81, 267. K. Ogura and M. Kaneko, J . Mol. Catal., in press. W. R. Grove, Philos. Mag., 1893, 14, 127. E. Justi, M. Pilkuhn, W. Sceibe and A. Winsel, High-Drain Hydrogen-Diflusion Electrodes Operating at Ambient Temperature und Low Pressure, (Verl. Akad. Wiss. Lit., Wiesbaden, 1959). R. P. Iczkowski, J . Electrochem. Soc., 1964, 1078. S. Srinivasan and H. D. Hurwitz, Electrochim. Acta, 1967, 12, 495. ' J. O'M. Bockris and B. D. Cahan, J . Chem. Phys., 1969,50, 1307. H. C. Weber, H. P. Meissner and D. A. Sama, J . Electrochem. Soc., 1962, 109, 884. F. G. Will, J . Electrochem. SOC., 1963, 110, 145. l o D. Ellis, M. Eckhoff and V. D. Neff, J . Phys. Chem., 1981, 85, 1225. l 1 K. Itaya, H. Akahoshi and S. Toshima, J . Electrochem. Soc., 1982, 129, 1498. l 2 W. Hieber, R. Nast and C. Bartenstein, Z . Anorg. Chem., 1953, 272, 32. l 3 Colorimetric Analytical Method, ed. L. C. Thomas and G. J . Chamberlin (Tintometer Ltd, Salisbury, 9th edn, 1980). (PAPER 4/ 1206)

ISSN:0300-9599    

DOI:10.1039/F19858101569

出版商:RSC    

年代:1985

数据来源: RSC