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/F198581FX033

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/F198581BX035

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY F A R A D A Y TRANSACTIONS, PARTS I A N D 11 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 kinetics, electrochemistry (other 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 i.r., 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 II (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 s t ruc t ure-ac t ivi t y 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 accounts of experimental observations, results, or theory that will not require enlargement into ‘full’ papers. The Notes section is not used for preliminary comm mica t ions. 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 wishesas to whether an article is a full paper or a Note, but since there is no sharp dividing line between the one and the other, either in terms of length or character of content, the right is retained to transfer overlong Notes to the full papers section. As a guide a Note should not exceed I500 words or word-equivalents. (i)NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participatingmember, 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 symbcls 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 (Pergarnon, Oxford, 1979 edn).Nomenclature of Inorganic Chemistry (Butterworths, London, 197 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.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY Marlow Medal and Prize Applications are invited for the award of the Marlow Medal for 1986 and Prize of f 100. The award will be open to any member of the Faraday Division of the Royal Society of Chemistry who, by the age of 32, had made in the judgment of the Council of the Faraday Division, the most meritorious contribution to physical chemistry or chemical physics. The award will be made on the basis of publications (not necessarily in the Transactions) on any subject normally published in J. Chem. Soc., Faraday Transactions I and 11, that carry a date of receipt for publication not later than the candidate’s 32nd birthday. Candidates should be members and under 34 on 1 st January 1986, the closing date for applications, which may be made either by the candidate himself or on his behalf by another member of the Society.Copies of the rules of the award and application forms may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN (ii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 20 Phase Transitions in Adsorbed Layers University of Oxford, 17-18 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, liquid/liquid 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 final programme and application form 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 and their interactions, in particular theoretical and spectroscopic studies, polymerised membranes, thermodynamics of bilayers and Iiposomes, 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 (iii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 82 Dynamics of Molecular Photof rag mentation University of Bristol, 15-17 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 should be sent by 25 October 1985 to: Professor A.D. Buckingham, University Chemical Laboratory, Lensfield Road. Cambridge CB2 1 EWTHE 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 D. 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 THE FARADAY DIVISION O F THE ROYAL SOCIETY OF CHEMISTRY 1985 BOURKE LECTURES by Professor D. Chandler University of Pennsylvania, Philadelphia, U.S.A. Electrons in liquids, Geometrical Perspectives The Lecture will consider new ways to think about quantum-mechanical processes in liquids, in particular, the mediation of electronic states and chemical bonding by fluctuating liquid environments as they pertain to the behaviour of solvated electrons. Monday Physical Chemistry Laboratory, Oxford 22 October 1985 2.1 5 pm Wednesday 23 October 1985 2.30 pm Friday 25 October 1985 2.00 pm Chairman: Professor J.S. Rowlinson F.R.S. Department of Chemistry, Manchester University (Lecture Theatre G54) Chairman: Professor R. Grice Department of Chemistry, Leicester University Chairman: Professor M. C. R. Symons F.R.S. Admission to the Lectures is free and non-members will be welcome. Further information from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN (Tel: 01 437 8656)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) 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 K,O-AI,O,-Si0,-NH, [CH,J,NH, 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) Radical Cations of Di-, Tri-, and Tetra- bromoethane formed by Radiolysis: an Electron Spin Resonance Study Martyn C. R. Symons (1 985, Issue 8) Stereochemical Applications of Potential Energy Calculations. Part 4. Revised Electron Spin Resonance Studies of the Ammonia-Boryl Radical (H,N -+ BH,.); an 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 SWlX 8QX Division with the Institute of Physics Seventh National Quantum Electronics Conference To be held at the Abbey Hotel, Great Malvern on lfj-20 September 1985 Further information from: Dr E. Jakeman, Treasurer QE7, RSRE, St Andrews Road, Great Malvern WR14 3PSSurface 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 Division-Endowed Lecture Symposium Surface Science and Catalysis (including the Centenary Lecture by G.Ertl and the Tilden Lecture by J. Pritchard) To be held at the Scientific Societies Lecture Theatre, London on 4 November 1985 Further information from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN Division-Endo wed Lecture Symposium Molecular Spectroscopy and Dynamics (including the Faraday Lecture by A. Carrington) To be held at the Royal Institution, London on 10 December 1985 Further information from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN - 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 16-1 8 December 1985 Further information may be obtained from: Dr J. M. Hollas, Department of Chemistry, University of Reading, White knights, 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 ~~ ~~~~~~~ ~~ Molecular Beams Group with CCP6 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 Electrochemistry Group Novel Techniques for the Study of Electrodes and their Reactions To be held at St. Catherine’s College, Oxford on 7-9 April 1986 Further information from: Dr S. P. Tyefield, CEGB, Rs Dept, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB (vii)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 16-18 April 1986 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1 X 8QX Division with the Societe FranFaise de Chimie, Deutsche Bunsen Gesellschaft fur Ph ysikalische Chemie and Associazione ltaliana di Chimica Fisica Dynamics of Molecular Crystals To be held at Grenoble, France on 30 June to 4 July 1986 Further information from: Dr C. Troyanowsky, 10 rue Vauquelin, 75005 Paris, France ~~ ~ 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 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/F198581FP073

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1985, 81, 2009-2019 Synthesis and Luminescence of Ruthenium Tris(2,2’-bipyridine)-Zirconium Phosphates BY DOMINIQUE P. VLIERS AND ROBERT A. SCHOONHEYDT* Laboratory for Surface Chemistry, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, B-3030 Heverlee, Belgium AND FRANS C. DE SCHRIJVER Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3030 Heverlee, Belgium Received 22nd March, 1984 Three adducts of Ru(bpy)i+ with zirconium phosphates have been synthesized. The crystalline intercalate, Ru(bpy);+-HZrP, where a maximum of 25 % of the interlamellar protons of a-HZrP are replaced by Ru(bpy):+, has its emission maximum at ca. 61 5 nm. By ion exchange or impregnation of Ru(bpy):+ on zirconium phosphate the complex is adsorbed only on the external surface.A similar material is prepared by reflux in the presence of very small amounts of Ru(bpy);+. These materials have their emission maximum at 64&5 nm. When Ru(bpy),Cl,, ZrOC1, * 8H,O and H,PO, are refluxed a species is formed which emits at 590 nm when excited at 420 nm. Its diffuse reflectance spectrum has a weak band at 32000 cm-l due to protonated bipyridine and a weak, broad band at 10200 cm-l of unknown origin. The emission intensities vary with Ru(bpy)i+ loading in a characteristic way for the adducts synthesized. The adsorption of the photosensitizer tris(2,2’-bipyridine)Ru(11), Ru(bpy):+, is currently being investigated in an attempt to devise heterogeneous photocatalytic water-splitting systems. Insulating oxides (SiO, and A1202),17 semiconducting oxides (TiO,, SnO, and SrTi03),3-5 zeolites6- and clay minerals*-13 have been or are being explored.For all these systems the adsorption and emission spectra are similar to those in aqueous solution. Small shifts of the band positions can be rationalized in terms of solvent or environmental effects. This is also the case for Ru(bpy):+ intercalated into the interlamellar space of clay minerals. In addition the n-n* transition of the 2,2’-bipyridine ligands at 285 nm has a component at 273 nm. This could be an indication of severe distortion of the intercalated complex. An extreme case of intercalated Ru(bpy)i+ was reported by Yeates et aZ.14 when Ru(bpy)[+ was intercalated between the layers of a-Zr(HPO,), during its synthesis. Only one sample was reported with 50% of the protons of HPO, replaced by Ru(bpy):+. The spectroscopic study of the sample was limited to the 40&600 nm region.The objectives of this work were (i) a systematic investigation of the intercalation of Ru(bpy)i+ between the layers of a-Zr(HPO,), and (ii) spectroscopic characterisation of both the ground state and the excited state of the intercalated Ru(bpy):+ molecules. 20092010 SYNTHESIS AND LUMINESCENCE OF [R~(bpy),]~-Zr(H,P0,), EXPERIMENTAL Ru(bpy),2+ ON THE EXTERNAL SURFACE OF ZIRCONIUM PHOSPHATE EXCHANGE ON SEMICRYSTALLINE ZIRCONIUM PHOSPHATE Semicrystalline zirconium phosphate [HZrP(0.5/48), where P stands for (HPO,), H indicates that the phosphate is in the H form, the first number in parentheses is the molarity of the H3P0, solution used to reflux the ZrP gel and the second number is the refluxing time in hours] was prepared by the method of Clearfield et al.15 2 g of HZrP(0.5/48) in 100 cm3 water were mixed with 0.15-0.75 g Ru(bpy),Cl, - 6H,O.The suspension was shaken vigorously for 3 days at 295 K. The solid was then separated by centrifugation and dried in air. The Ru(bpy):+ loadings of these samples are given in table 1. In agreement with the findings of Yeates et al.14 all the Ru(bpy):+ is on the external surface. IMPREGNATION ON CRYSTALLINE ZIRCONIUM PHOSPHATES Crystalline a-HZrP was prepared by direct precipitation, as described by Alberti and Torracca.ls 2 g of a-HZrP in 10 cm3 H,O were mixed with 1.5 or 3 g Ru(bpy),Cl, .6H,O to obtain Ru(bpy):+ loadings of 1 and 2 mmol g-l, respectively.The paste was mixed in a mortar and dried in air. These samples are identified as RucrZrP (sample 3 and 4 in table 1). SYNTHESIS OF Ru(bpy)i+ ZIRCONIUM PHOSPHATE REFLUX METHOD Samples of different Ru(bpy)i+ content were prepared by the method of Yeates et al.14 To a solution of 0.8 g ZrOC1,-8H20 in 20 cm3 water, 2 cm3 concentrated HCl, 5 cm3 85% H3P0, and various amounts of Ru(bpy),C1;6H2O in the range 0.02-1.0 g w r e added. The mixture was then refluxed for 2 days or 1 week. The reaction product was washed with water and freeze dried. These samples are identified as RuZrP(2.7/48) and RuZrP(2.7/ 168) (samples 5-1 8 in table 1). In a second set of synthesis reactions Ru(bpy),2+-ZrP was prepared by adding to a solution of 2.5 g ZrOC1;8H20 in 20cm3 water, 20cm3 85% H3P0,, 2 cm3 40% HF and various amounts of Ru(bpy),C1,.6H20 in the range of 0.2-1.0 g.This solution was constantly stirred on a waterbath at 330-335 K for 3 days. (The first crystals were formed after ca. 36 h.) Again the reaction product was washed with water and freeze dried. These samples are identified as RuZrP(HF) (samples 19-24 in table 1). This procedure was analogous to that for obtaining highly crystalline a-HZrP.16 ANALYSIS The Ru(bpy),2+ content was determined by dissolving a known amount of the Ru(bpy);+-ZrP sample, suspended in water, with HF, diluting the solution with water and measuring the absorbance on a Shimadzu QV-50 spectrophotometer at 452 nm (e = 14600 dm3 mol-l cm-l). In some cases the Ru(bpy)i+ content was determined by measuring spectrophotometrically the Ru(bpy);+ content of the synthesis solutions before and after synthesis.The water content was calculated from the weight loss after drying at 385 K for 2 days. The results are given in table 1. PROCEDURES AND TECHNIQUES X-Ray diffraction (X.r.d.) patterns (Ni-filtered Cu K , radiation) were obtained with a Siemens diffractometer connected to an HPl 000 computer, equipped with a position-sensitive detector (M. Braun) and a multichannel analyser (Mikras, Frieseke and Hoepfner). The surface areas were measured with the nitrogen single-point B.E.T. method on a Quantachrome- Quantasorbinstrument. 1.r. spectra were recorded with a Perkin-Elmer 580B spectrophotometer at 295 K from 4000 to 200 cm-l on KBr pellets.Reflectance spectra of the samples under N, were taken at 295 K in a cell that also allowed the emission spectra to be recorded, using a Cary 17 spectrophotometer with a type I diffuse-reflectance attachment. The Eastman Kodak white reflectance standard was used as a reference. The procedures for processing and display of the spectra have been described e1~ewhere.l~ For the same samples in the same cell emissionD . P. VLIERS, R. A. SCHOONHEYDT AND F. C. DE SCHRIJVER 201 1 Table 1. Ru(bpy):+ content, water content, surface area and relative emission intensities of Ru( bpy):+-ZrP. sample Ru(bPY):+ water surface area /mmol g-' /mmolg-' /m2 g-' E/Er 1. RuZrP(O.5/48)-1 2. RuZrP(0.5/48)-2 3. RuorZrP-1 4. RuaZrP-2 5. RuZrP(2.7/48)-1 6. RuZrP(2.7/48)-2 7.RuZrP(2.7/48)-3 8. RuZrP(2.7/48)-4 9. RuZrP(2.7/48)-5 10. RuZrP(2.7/48)-6 1 1. RuZrP(2.7/48)-7 12. RuZrP(2.7/48)-8 13. RuZrP(2.7/48)-9 14. RuZrP(2.7/48)-10 15. RuZrP(2.7/168)-1 16. RuZrP(2.7/168)-2 17. RuZrP(2.7/168)-3 18. RuZrP(2.7/168)-4 19. RuZrP(HF)- 1 20. RuZrP(HF)-2 2 1. RuZrP(HF)-3 22. RuZrP(HF)-4 23. RuZrP(HF)-5 24. RuZrP(HF)-6 0.056 0.172 1 .o 2.0 0.02 1 0.082 0.140 0.147 0.151 0.193 0.232 0.265 0.427 0.955 0.0064 0.072 0.190 0.319 0.0165 0.147 0.471 0.541 0.647 0.788 n.d.a n.d. n.d. n.d. 2.93 3.83 1.27 1.49 2.26 n.d. 1.95 3.97 4.85 4.1 1 1.36 1.07 1.61 1.34 2.95 2.5 0.33 n.d. n.d. 0 n.d. n.d. n.d. n.d. 1.4 0.8 13.7 3.4 1.9 n.d. 5.5 5.5 0.4 0.4 22.9 23.7 17.3 33.4 0.7 8.25 8.05 n.d. n.d. 4.2 0.136 0.152 0.186 0.134 0.130 0.189 0.219 0.253 0.172 0.2 16 0.141 0.126 0.127 0.120 0.058 0.158 0.23 1 0.276 0.3 18 0.509 0.807 0.888 0.790 1 a n.d.= not determined. spectra (500-800 nm) were obtained using a Spex Fluorolog instrument. The samples were excited at 452,420 or 480 nm and measured in front face. The widths of the exit and entrance slits of the sample compartment were 2.5 mm. The excitation and emission band widths were set at 10 nm. The spectra were corrected for the sensitivity of the photomultiplier. The emission intensities, E, are the areas under the luminescence spectra when excited at 452 nm. They are calculated by integration of the digitized spectrum on a PDP-11 computer. The relative emission intensity of a sample is E/E,, where E and E, are the emission intensities of the sample and the reference, respectively.We used sample 24 (see table 1) as the reference. RESULTS SYNTHESIS AND PHYSICOCHEMICAL CHARACTERIZATION OF Ru(bpy)i+-ZrP The characteristics of the samples are given in table 1. The efficiency of intercalation of Ru(bpy)i+ by synthesis in the presence of HF (samples 19-24) is ca. 55% of the initially added amount of Ru(bpy)i+ at low loadings (0.147 mmol per g ZrP) and ca. 30% for a sample with 0.788 mmol Ru(bpy)i+ per g ZrP. For refluxed samples the efficiency of incorporation increases with increasing loading of Ru(bpy)i+. Samples with low loadings (up to 0.32 mmol g-l) adsorb 60-75% of the initial amount of20 1 2 SYNTHESIS AND LUMINESCENCE OF [ Ru( bpy),],-Zr( H,PO,), 3.0 6.0 9.0 12.0 15.0 18.0 21.0 24.0 27.0 30.0 33.0 36.0 39.0 281" Fig.1. X.r.d. patterns for (a) RuZrP(HF)-2, (b) RuZrP(HF)-3 and (c) RuZrP(HF)-6. 3.0 6.0 9.0 12.0 15.0 18.0 21.0 24.0 27.0 30.0 33.0 36.0 39.0 2e/" Fig. 2. X.r.d. patterns for (a) a-HZrP, (b) RuZrP(2.7/168)-1, (c) RuZrP(2.7/168)-4 and ( d ) RuZrP(2.7/48)-10. Ru(bpy)g+ and almost all the Ru(bpy)g+ complexes (98%) are adsorbed by samples with 0.427 and 0.955 mmol g-l (samples 5-18). The X.r.d. spectra of RuZrP(HF) samples synthesised in the presence of HF are shown in fig. 1. The sharp lines at a 28 value of 11.71" (0, 0,2) and the sharp lines at 28 values of 19.77" (T,1, I), 20.0" (2,0,0), 24.97' (T,l, 3), 25.25" (2,0,2) and 34.15' (3, 1,1) are due to the a-HZrP phase. The numbers in parentheses refer to the (h, k, I )D . P. VLIERS, R. A. SCHOONHEYDT AND F.C. DE SCHRIJVER 2013 values. The Ru(bpy)i+-ZrP phase has 2 characteristic peaks at 28 = 5.55" and 1 1.05". The first intense peak can be interpreted as the (002) reflection by analogy with the spectra of a-HZrP and the organic derivatives Zr(O,POR),.ls It is shown in fig. 1 that at small loadings [0.147 mmol Ru(bpy)i+ per g ZrP] two phases co-exist, in a ratio of ca. 9: 1. They are the a-HZrP and the Ru(bpy):+-ZrP phases, respectively. The ratio is estimated from the peak heights of the (002) reflections and is therefore not accurate. At 0.471 mmol and 0.788 mmol Ru(bpy)i+ per g ZrP only the Ru(bpy)i+-ZrP phase is visible in the spectra. The well resolved, narrow lines are indicative of the excellent crystallinity of the samples. Attempts to synthesize large single crystals of Ru(bpy)i+- ZrP suitable for monocrystal X-ray analysis, by synthesis under more dilute conditions, in water and in ethanol and at lower temperatures, failed.The X.r.d. spectra of RuZrP(2.7/48) and RuZrP(2.7/168) are shown in fig. 2. For the RuZrP(2.7/48) samples refluxed for two days only the a-HZrP diffraction pattern is visible at all loadings [fig. 2(b) and (d)]. The broadened lines indicate some deterioration of the crystallinity with respect to the reference compound a-HZrP. For the RuZrP(2.7/48) samples the Ru(bpy):+ intercalated phase becomes visible above 0.2 mmol Ru(bpy)i+ per g ZrP and coexists with the a-HZrP phase. An example is fig. 2(c). The reflections are broad and crystallinity is poor. The surface areas and H,O contents are shown in columns 3 and 4 of table 1 and do not show a clear dependence on the Ru(bpy):+ content or the method of preparation.The highest surface areas are observed for samples 15-18 prepared by refluxing for 168 h. However, the reproducibility is poor as illustrated by samples 7-9, which were prepared independently but with almost the same Ru content. For the samples prepared in the presence of H F (samples 19-24) the H 2 0 content tends to 0 as the amount of Ru(bpy)i+ increases. Such a trend is not observed for the refluxed samples. Representative i.r. spectra are shown in fig. 3. The characteristic bands of a-HZrP are the two OH stretching vibrations at 35 15 and 3600 cm-l. Note also the broad-band system of H-bonded OH groups in the range of 3500-3000cm-l. The effect of Ru(bpy)i+ on these bands depends on the method of synthesis.For the samples prep& by using HF, the two typical OH bands at 351 5 and 3600 cm-l are broadened and become poorly resolved. The broad 3-580-3000cm-1 band is intensified and shifted to lower wavenumbers. A new broad band is created at ca. 2400 cm-l, probably also due to a hydrogen-bond system. A bipyridine ring vibration is seen at 1605 cm-l. When the Ru(bpy)i+-ZrP system is synthesized by the method of Yeates et al., the two typical OH stretching bands at 3600 and 35 15 cm-l are only slightly affected even at a loading of 0.955 mmol Ru(bpy):+ per g ZrP. REFLECTANCE AND LUMINESCENCE SPECTROSCOPY The diffuse reflectance spectra of samples at low Ru(bpy)i+ loadings synthesized in the presence of HF are similar to the Ru(bpy)i+ solution spectrum.Above 0.4 mmol g-l the metal-ligand charge-transfer band (d-x* transition), originally at 22 120 cm-l, is shifted to 21 600 cm-l and the transitions in the U.V. region (35200, 40000 and 41 600 cm-l) are broadened and poorly resolved. Also the ratio of the intensity of the band at 35200 cm-l to that at 22 120 cm-l changes with loading. It is 1.8 for the RuZrP(HF)-1 sample and decreases to 0.86 for the RuZrP(HF)-6 sample. For the impregnated samples, RuaZrP, the diffuse reflectance spectra are similar to the diffuse reflectance spectra of Ru(bpy),Cl, - 8H,O crystals diluted with BaSO, white reflectance standard. Besides the Ru(bpy)i+ solution spectrum, a very pronounced shoulder at 18 700 cm-l is found for those spectra.The diffuse reflectance spectra of refluxed samples are displayed in fig. 4. Note the shoulder at 32 100 cm-l, which is2014 SYNTHESIS AND LUMINESCENCE OF [Ru(bpy),],-Zr(H yPO,), 4000 3000 2000 wavenumber/cm -' Fig. 3. Infrared spectra of (a) a-HZrP, (b) RuZrP(HF)-2, (c) RuZrP(HF)-3, ( d ) RuZrP(HF)-6, (e) RuZrP(2.7/48)-8 and cf) RuZrP(2.7/48)- 10. " I I . 30 40 - '--'a 1 K 6 8 10 12 14 20 wavenumber/ 1 O5 cm-' Fig. 4. Reflectance spectra of (a) RuZrP(2.7/48)-1 and (b) after outgassing at 385 K; (c) RuZrP(2.7/48)-10 and ( d ) after outgassing at 385 K. more pronounced than in solution spectra and which decreases in intensity after outgassing at 385 K. The ratio of the intensity of the band at 35200 cm-l to that at 22 120 cm-l decreases with loading as for the HF samples.It is 2.62 for RuZrP(2.7/48)- 1 and 0.80 for RuZrP(2.7/48)-10. The vibrational overtone and combination bands of water and bipyridine in the n.i.r. region are superimposed on a slope, increasing from 4500 to 8000 cm-l. This slope is nearly independent of the Ru(bpy)i+ content, as can be observed by comparing the spectra of RuZrP(2.7/48)-1 and RuZrP(2.7/4)-10 (fig. 4), and decreases afterD. P. VLIERS, R. A. SCHOONHEYDT AND F. C. DE SCHRIJVER I 5 9 4 0 3952 h m U .- c a -f’ 2964 m v U .- 5 1976 .- 988 0 500 600 700 wavelength/ nm 8 Fig. 5. Emission spectra of (a) RuZrP(0.5/48)-2 and (b) RuZrP(HF)-3. 2015 0 500 6 00 700 wavelength/ nm 800 Fig. 6. Emission spectra of RuZrP(2.7/48)-6 excited at (a) 420, (b) 452 and (c) 480 nm. The intensity of spectrum (b) is reduced by a factor of 2.outgassing at 385 K. A broad band with a maximum at ca. 10200 cm-l is very weak at low loadings [for RuZrP(2.7/48) F(R,) is 0.051 but becomes intense at higher Ru(bpy):+ loadings [F(R,) for RuZrP(2.7/48)-10 is 0.421. After outgassing at 385 K this band almost disappears (fig. 4):The band at 10200 cm-l is not observed for the impregnated samples nor after refluxing in the absence of either ZrOC1;8H20 or2016 SYNTHESIS AND LUMINESCENCE OF [ Ru(bpy),],-Zr(H,PO,), 1.05 0.90 0.75 - - 0 - 0 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 Ru(bpy):+/rnrnol g-' Fig. 7. Relative emission intensity expressed as a function of the Ru(bpy),2+ content of: 0, crystalline materials (HF synthesis); x , materials refluxed for 168 h; A, materials refluxed for 48 h; 0, ion-exchanged materials.H,PO,, indicating that it is not due to decomposition of Ru(bpy)i+ in solution followed by adsorption of the decomposition products. The luminescence maximum of Ru(bpy)i+ on the external surface (samples 1-4 in table 1) is at 645 nm. The spectrum of RuZrP(0.5/48)-2 is shown in fig. 5. The emission spectra of the RuZrP(HF) samples have their maximum intensity at 615 nm (fig. 5). They are independent of the excitation wavelength: excitation at 420,452 or 480 nm gives emission spectra which, after normalization, are totally superimposable. For the refluxed samples the situation is more complicated. At very low loadings (sample 15 and 5 ) the band maximum is at 645 nm. At high loadings the emission changes with the wavelength of excitation (fig.6). Excitation at 420 nm leads to an emission band maximum at 585-590 nm. The maximum moves to 610 nm when the samples are excitated at 452 or 480 nm, although a shoulder, which is very pronounced at intermediate loadings (sample 12), remains at 590 nm. The relative emission intensities are strongly dependent on the loading and the method of preparation of the samples. This is shown in fig. 7. For the samples prepared in the presence of HF and those which were refluxed for 168 h the relative emission intensity increases with loading, but for the same loading the emission intensity of the former exceeds that of the latter by a factor of 2-3. For the samples refluxed for 48 h the relative emission intensity follows the curve for the sample refluxed for 168 h at small Ru(bpy)i+ loading to drop to a constant emission intensity of 0.12-0.14 above 0.20 mmol Ru(bpy)i+ per g ZrP.DISCUSSION SYNTHESIS AND ORGANIZATION OF Ru(bpy);+ ON THE SURFACE The synthesis of Ru(bpy)i+-ZrP intercalates in the presence of HF yields highly crystalline products. The basal spacing of 1.59 nm is indicative of the formation of a monolayer intercalate. The thickness of a-HZrP is 0.756 nm, which leaves 0.834 nm for the Ru(bpy)i+ molecule. This is ca. 0.086 nm less than in the crystalD. P. VLIERS, R. A. SCHOONHEYDT AND F. C. DE SCHRIJVER 201 7 Ru(bpy)i+(PF,).lg Thus the complex is present in the interlamellar space in a strained configuration. The maximum amount of intercalated Ru(bpy)i+ can be calculated as follows.Ru(bpy)i+ has a cross-section area of 0.92 nm2.19 The area per site in the interlamellar soace of a-HZrP is 0.24 nm2.18 Thus each complex occupies 8 sites and there are 4 sites on each ZrP layer with which it is in contact. As the theoretical number of sites is 6.64 mequiv. g-l, the maximum amount of intercalated Ru(bpy)i+ is 0.83 mmol g-l. This value is in good agreement with the value of 0.788 mmol g-l found experimentally. There are two consequences of this intercalation process: (i) only 25% of the protons are replaced by Ru(bpy)i+ at maximum intercalation and (ii) the Ru(bpy):+ molecules replace H,O in the interlamellar space. At complete filling no water is left on the interlamellar surface. This is confirmed by the measurements of the water content (table 1).The broadening of the sharp H,O bands in the i.r. spectra is also indicative of intercalation and distortion of the residual H,O molecules by Ru(bpy):+. The situation is completely different for the refluxed samples. Intercalates are only formed after refluxing for 168 h and at a loading > 0.2 mmol Ru(bpy):+ per g ZrP. The quality of the X-ray pattern is inferior to that of the HF samples and the crystallinity of the product is lower. Thus it is possible that not all of the adsorbed Ru(bpy)i+ is intercalated but that part of it is agglomerated on the external surface. This conclusion is strengthened by the observation that the H,O content does not decrease with increasing Ru(bpy)i+ loading as in the case of the HF samples. After refluxing for 48 h no intercalates are formed, as shown by the absence of the typical intercalate X-ray pattern and by the fact that the 3600cm-l i.r.bands of intercalated H,O are not affected whatever the loading with Ru(bpy):+. The large amounts of Ru(bpy);+ absorbed can only be explained by the formation of agglomerates of Ru(bpy):+ molecules on the external surface. Clearly, the size of the RuZrP particles and agglomeration depend on the method of synthesis and no definitive statement can be made from the water-content and surface-area data of table 1. SPECTROSCOPIC PROPERTIES When crystalline a-Zr(HPO,), - H,O is impregnated with Ru(bpy),Cl,, particles of Ru(bpy),Cl, are formed on the external surface. Thus the reflectance spectra are identical to those of Ru(bpy),Cl, diluted with BaSO,. When these samples (samples 3 and 4 in table 1) are excited at 452 nm, the emission maximum obtained at 640-645 nm is characteristic of these Ru(bpy),Cl, particles on the external surface of a- Zr(HPO,), - H,O.The ion-exchanged samples (samples 1 and 2 in table 1) and refluxed samples with very small loadings (samples 5 and 15) also have an emission maximum at 640 nm. Thus it is indicative of the presence of Ru(bpy),Cl, agglomerates on the external surface. Crystalline samples, RuZrP(HF), give emission spectra with the maximum at ca. 61 5 nm. This is approximately the same position as for Ru(bpy):+ exchanged into clay minerals and in aqueous solution.20 It is therefore an emission spectrum characteristic of independent Ru(bpy)i+ molecules in solution or organised as a monolayer on an inorganic surface.The fact that the emission spectra are independent of the loading indicates that the situation of independent Ru(bpy):+ molecules persists from very small to very large loadings. For the sample RuZrP(HF)-6 all the available interlamellar space is filled and the Ru(bpy):+ molecules form a close-packed two-dimensional array, with almost no diffusional mobility. From the emission spectra of the refluxed samples (with loadings > 0.05 mmol g-l,2018 SYNTHESIS AND LUMINESCENCE OF [R~(bpy)~],-zr(H,PO,), samples 6-14 and 16-1 8 of table 1) it is evident that two Ru(bpy)i+ species are present on the surface. The species excited at 452 or 480 nm and emitting at 620 nm is the ‘classical ’ Ru(bpy)i+ as described above for the crystalline samples.The species excited at 420 nm and emitting at 590 nm is a ‘new’ Ru(bpy)i+ molecule. There are two possible explanations: (i) a very strongly bound Ru(bpy):+ molecule similar to the Ru(bpy),2+ buried near the surface of polymerised SiO, particles and emitting at 573 nml or, more likely, (ii) the absorption at 420 nm and the emission at 590 nm are indicative of a chemical change in the adsorbed Ru(bpy)$+, because of the additional features in the diffuse reflectance spectra of the refluxed samples at 32000 and 10200 cm-l. This is associated with the formation of RuZrP phases under refluxing conditions. The 32000 cm-l shoulder can be ascribed to protonated bipyridine. However, we have no explanation for the band at 10200 cm-l; it cannot be ascribed to an intervalence charge transfer (RuII to RulI1) in bridged dimer complexes of R u ~ ~ - ~ ~ for the following reasons: (i) the experimental bandwidth (2600-3160 cm-l) is far below that predicted by Hush’s theory (4850 ~ m - l ) , ~ ~ * 25 (ii) the band broadens beyond the detection limit upon dehydration and (iii) no bridged dimers have been found with 2,2’-bipyridine bridged ligands.21-23 The best explanation we can offer is partial protonation of bipyridine ligands to monoprotonated species caused by refluxing in a strongly acidic medium : The resulting five-coordinated species can dimerise. It is these dimers which give the characteristic spectral features found in this work. The state of Ru(bpy):+ is also reflected in the relative emission intensities (fig.7). Thus, for crystalline samples or at very small loadings the Ru(bpy):+ ions behave as isolated entities and the relative emission intensities increase with the loading. For impregnated and refluxed samples the Ru(bpy)i+ molecules are agglomerated or partially protonated and dimerised. The relative emission intensities are much lower and do not vary with the loading above 0.2 mmol g-l. The difference between samples refluxed for 48 h and for 168 h in fig. 7 can be ascribed to the difference in crystallinity, as the latter show the X-ray pattern of an intercalated RuZrP phase. We thank the National Fund of Scientific Research (Belgium) for supporting this work and Prof. Kinget for the use of the Quantasorb. J. Wheeler and J. K. Thomas, J .Phys. Chem., 1982,86,4540. T. Kajiwara, K. Hasimoto, T. Kawai and T. Sakata, J. Phys. Chem., 1982,86, 4516. I. Willner, J. M. Yang, C. Laane, J. W. Otvos and M. Calvin, J . Phys. Chem., 1981,85, 3227. R. Memming, Surf. Sci., 1980, 101, 551. J. W. Perry, A. J. McQuillon, F. C. Anson and A. H. Zewall, J. Phys. Chem., 1982,87, 1480. W. de Wilde, G. Peeters and J. H. Lunsford, J . Phys. Chem., 1980, 84, 2306. W. H. Quayle and J. H. Lunsford, Znorg. Chem., 1982, 21, 97. D. Krenske, S. Abdo, H. Van Damme, M. Cruz and J. J. Fripiat, J. Phys. Chem., 1980, 84, 2447. S. Abdo, P. Canesson, M. Cruz, J. J. Fripiat and H. Van Damme, J . Phys. Chem., 1981, 85, 797. lo H. Nijs, M. Cruz, J. J. Fripiat and H. Van Damme, J . Chem. Soc., Chem. Commun., 1981, 1026. l1 H. Nijs, J. J. Fripiat and H. Van Damme, J. Phys. Chem., 1983, 87, 1279. R. A. DellaGuardia and J. K. Thomas, J . Phys. Chem., 1983, 87, 990. l 3 R. A. Schoonheydt, J. Pelgrims, Y. Heroes and J. B. Uytterhoeven, Clay Miner., 1978, 13, 435. l4 R. C. Yeates, S. M. Kuznicki, L. B. Lloyd and E. M. Eyring, J . Znorg. Nucl. Chem., 1981, 43, 2355. l5 A. Clearfield, A. Oskarsson and C. Oskarsson, Zon Exch. Membr., 1972, 1, 91. G. Alberti and E. Torracca, J . Znorg. Nucl. Chem., 1968, 30, 317. R. A. Schoonheydt, Diflbse ReJectanceSpectroscopy in the CharacterizationofHeterogeneous Catalysts, ed. F. Delannay (Marcel Dekker, New York, 1984), pp. 125-160.D. P. VLIERS, R. A. SCHOONHEYDT AND F. C. DE SCHRIJVER 2019 M. B. Dines and P. M. DiGiacomo, Inorg. Chem., 1981, 20, 92. l* D. P. Rillema, D. S. Jones and H. A. Levy, J . Chem. SOC., Chem. Commun., 1979, 849. *O R. A. Schoonheydt, P. De Pauw, D. Vliers and F. C. De Schrijver, J . Phys. Chem., 1984,88, 51 13. *l C. Creutz and H. Taube, J. Am. Chem. SOC., 1969, 91, 3988. 2* 0;. M. Tom, C. Creutz and H. Taube, J. Am. Chem. SOC., 1974,%, 7827. 23 T. J. Meyer, Ace. Chem. Res., 1978, 11, 94. 24 G. C. Allen and N. S. Hush, Prog. Inorg. Chem., 1967, 8, 357. 2s N. S. Hush, Prog. Inorg. Chem., 1967, 8, 391. (PAPER 4/468)

ISSN:0300-9599    

DOI:10.1039/F19858102009

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1985,81, 2021-2025 Dissociation of Phenols and Phenolate Salts and Homocomplexation in the corresponding Phenol- Phenolate Systems in Benzonitrile BY ZENON PAWLAK AND J ~ Z E F MAGONSKI Institute of Chemistry, University of Gdansk, 80-952 Gdansk, Poland AND FRANK STROHBUSCH* Institute of Physical Chemistry, University of Freiburg, 78 Freiburg, Federal Republic of Germany Received 5th July, 1984 The acidities of 12 substituted phenols (pK,BN relative to the pKEN of HClO,) and the formation constants of the corresponding homocomplexes, KAHA, in benzonitrile have been determined from e.m.f. measurements. Comparison with previous results reveals that the stability of homocomplexes increases in aprotic solvents in the order acetonitrile = propylene carbonate < benzonitrile c acetone.Conductivity measurements have been carried out on the 12 tetrabutylammonium phenolates in benzonitrile. The data have been analysed using the Pitts equation and the limiting conductivities and dissociation constants of the salts have been calculated. Phenols are known to form hydrogen-bonded complexes with their parent bases in aprotic solvents. This association depends on the strength of solute-solvent interactions because of the competition between the phenolate ions and the solvent for hydrogen bonding with the phenol. Studies of some electrolytes1? and the kinetics of proton transfer3 in benzonitrile have been reported. However, too little is known about the acid-base and association properties of weak acids in benzonitrile. The purpose of this study was to collect suitable data on phenol-phenolate systems in benzonitrile and to make a comparison with the results obtained in a~etonitrile,~-~ propylene carbonate* and a~etone.~ Benzonitrile (BN) is a protophobiclO dipolar solvent with a dielectric permittivity E = 25.2, dipole moment 4 D, viscosity 0.0122 P and density 1.0008 g cmd3 at 298 K.ll EXPERIMENTAL Benzonitrile (Riedel de Haen) was dried with CaSO, and distilled in nitrogen under reduced pressure.Redistillation was carried out after 1 h refluxing with P,O, and the middle fraction of ca. 70% of the distillate was used. The specific conductivity of BN purified in this way was K = 2.9 x l2-l cm-l. Tetrabutylammonium perchlorate prepared from perchloric acid and tetrabutylammonium hydroxide in water was crystallized from the reaction mixture and then from ethyl acetate and dried.Elementary analysis was 56.00% C (calc. 56.21), 10.82% H (calc. 10.61), 4.21% N (calc. 4.10). All other salts were obtained and analysed as described previously.12 Anhydrous HClO, solution in BN (containing acetic acid) was prepared as described by Kolthoff and C0et~ee.l~ Potentiometric measurements were carried out at 25k0.05 "C using a N-517 Mera Elwro pH-meter and are accurate to within k0.5 mV. When not in use, the Radelkis glass electrode was stored under water. The reference electrode was a modified calomel electrode in which the 202 12022 DISSOCIATION OF PHENOLS AND PHENOLATES aqueous KCl solution was replaced by 0.1 mol dm-3 tetrabutylammonium chloride solution in BN.The glass sensor electrode was calibrated in buffer solutions containing tetrabutyl- ammonium perchlorate and HClO,. mol dm-3 salt solution was placed in the cell and appropriate volumes of a solution containing mol dm-3 acid were added. Thus the ionic strength was kept practically constant. After each addition of the titrant, the potential reached an equilibrium value within 2-8 min. The system was carefully dried. Two independent titrations were made with each system. The conductances of 8-10 samples of each quaternary salt in BN in the range (0.3-5) x lop3 mol dmP3 were measured using a bridge of type K-58-23/18 at 1 kHz (accuracy, 0 . 2 4 5 % ; cell constants, 0.0189 and 0.2950 cm-l). For pH measurements 20 cm3 of a mol dm-3 salt and 6 x RESULTS AND DISCUSSION POTENTIOMETRIC MEASUREMENTS In fig.1 the dependence of the e.m.f. on the ratio cHA/cA is shown for four systems characterized by different strengths of homoconjugation. The paH values were calculated from the Nernst equation E = Eo-SpaH, (1) where E, = 1280 mV [based on pK,BN(HCIO,) = 2.795'1 and S = 61 mV (obtained in the calibration of the glass electrode). The pK,BN values were determined from Eas (e.m.f. for cHA = c,) values: and are listed in table 1. The pK,BN of picric acid found in this way (11.1) is close to that (1 1 .O) determined by Kolthoff and Chantooni in acetonitrile.sa The pKa values in acetonitrile of 3-nitrophenol, pK,AN = 23.8, and of 4-nitrophenol, pK,AN = 20.9,6b are also very close to the pK,BN values reported here.However, the absolute pKa values in both solvents are uncertain since they are based on the pKa values of HClO, determined in the mixed solvent systems acetonitrile + acetic acid and benzonitrile+acetic acid. It has been shown that in the system HClO,+acetic acid+acetonitrile the dissociation of HClO, is due mainly to the protonation of the acetic acid.', The similarity of the measured pK, values of the phenols in both solvents indicates that the same holds for the system HClO, + acetic acid + benzonitrile. Even if the reference points of the pKa scales in both solvents are uncertain, the slopes of the plots of pK,BN and pK,AN against pKrater are still the correct measure of the difference between the solvent-solute interactions in the respective solvents.There is a linear correlation between the pK,BN and pKrater values of the phenols with a slope of 1.7, which denotes that BN is (like acetonitrile) a solvent which is better able to differentiate between the acidity of the phenols than water. Homoconjugation constants KAHA were calculated from the titration curves using the relation5* '7 KAHA = [AHAI/(CAH-[AHAI) (CS-[AHAI) (3) where = (Ka CA€€--aH+fA- cS)/(Ka-aH+fA-) and CAH and Cs denote the total concentrations of acid and salt, respectively. In eqn (3) the very small H+ activity -aH+ is neglected. One might expect that the formation of homocomplexes will increase, in general, with decreasing acidity of the parent phenols, because the more basic anions compete more favourably with the solvent for hydrogen-bond formation.This is confirmed byz. PAWLAK, J. MAGONSKI AND 0 a 0 o m 0 . 0 . F. STROHBUSCH . e 0 . 0 2023 I . 1 , 1 . 1 , 1 -0.5 -0.4-0.3-0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 log (C*H/C,) Fig. 1. Plot of e.m.f. values E against log cAH/cA for phenol-phenolate systems in benzonitrile. Open and closed points are from different titrations. Numbering and the log K A H A values (in brackets) are the same as those in table 1 : (a) 1 (2.0), (b) 4 (2.9,), ( c ) 10 (4.4) and ( d ) 12 (ca. 5.3). the data in table 1 when systems with similar steric requirements in proximity to the hydrogen bond, e.g. those with one ortho-substituent (no. 2,5 and 9) or those without ortho-substituents (no. 3, 10 and 12), are compared. The 2,6-disubstitued compounds (no. 1, 4, 6, 7, 8 and 11) show a much weaker interaction, as expected from steric considerations.The same result was also obtained for analogous systems in propylene carbonate8 and acetonitrile.7 In fig. 2 10gKAHA for phenol-phenolate systems in BN, propylene carbonate and acetone is plotted as a function of logK,,, in acetonitrile. Slightly stronger homocomplexation in BN arises from the lower permittivity of BN and its smaller hydrogen-bond acceptingpowercompared with acetonitrile and propy1ene~arbonate.l~ In acetone the effect of lower permittivity is of greater importance than the increase in basicity. The KAHA value of 4-nitrophenol has also been determined by visible spectroscopy at an ionic strength of 0.1 mol dmP3, giving log KAHA = 4.40, in full agreement with the value given in table 1 .16 The dependence of KAHA on the ionic strength was shown to be very small, as expected.162024 DISSOCIATION OF PHENOLS AND PHENOLATES Table 1.Dissociation constants of the phenols (K,) and homoconjugation constants (KAHA4) of the corresponding phenol-phenolate systems and dissociation constants ( K d ) and limiting conductances (Ao) of the tetrabutylammonium phenolates in benzonitrile at 298 K phenol, phenolate salt or K,B"/ 10-2 I I ~ / S Z - ~ no. phenol-phenolate system pK,SX log K z : , ?ia mol dm+ cm2 mol-l 1 2 3 4 5 6 7 8 9 10 11 12 2,6-dibromo-4-nitro- 2,4-dinitro- 3,4-dinitro- pentachloro- 2,5-dini tro- 2,4,6-tri-iodo- 2,4,6-tribromo- 2,4,6-trichloro- 2,3,5-trichloro- 4-nitro- 2,6-dichloro- 3-nitro- 15.6 16.9 18.1 18.25 19.15 19.9, 20.6 21.0 21.1 21.55 22.4 24.I 2.0 2.4 4.1 2.95 3.30 2.7 2.9 3.1 5.0 4.4 2.9 5.3 0.12 0.13 0.08 0.05 0.06 0.16 0.09 0.03 0.25 0.04 0.10 0.42 1.68 2.68 1.83 1.80 1.56 0.75 0.97 0.82 0.9 1 1.55 1.08 1.25 39.9 40.0 41 .O 41.3 39.1 41.1 37.4 38.8 40.8 37.7 42.0 38.7 a Average of the standard deviations of two measurements using 14 points of each titration curve. 0 0 0 O 0 0 0 b 0% 0 0 0 M - !! 2 0 2 3 & 5 log ) Fig. 2. LogK,,, values in benzonitrile (O), propylene carbonate (0) and acetone (0) as a function of log KAHA in acetonitrile for phenol-phenolate systems. Data for benzonitrile from this work, the rest from ref. (7)-(9). CONDUCTIVITY MEASUREMENTS The concentration dependence of the conductivity of tetrabutylammonium phenol- ates was evaluated using the Pitts equations for an incompletely dissociated electrolyte with a distance parameter R = 10 The equations were solved by an iterative procedure.18 The molar conductances A fell below the limiting Onsager slope, indicatingz.PAWLAK, J . MAGONSKI AND F. STROHBUSCH 2025 incomplete dissociation. Values of the dissociation constants Kd are ca. mol dm-3, in close agreement with published values for other tetra- alkylammonium salts in BN." There is no systematic substituent dependence of &. The limiting conductances A. fall within a relatively narrow range (table 1). With the value do = 15.7 R-l cm2 mol-1 for the limiting conductance of the tetrabutylammon- ium ion in BN, the ionic conductances Lo for the substituted phenolate ions range from 21.7 to 26.3 R-l cm2 mol-l.There is no direct relation between 1, and the sizes of the ions. Instead the degree of charge delocalization and polar interactions with the solvent seem to play some role. F. S . thanks the Deutsche Forschungsgemeinschaft and the Fonds of Deutsche Chemische Industrie for financial support. J. F. Coetzee and D. K. McGuire, J. Phys. Chem., 1963, 67, 1810. G. J. Janz, I. Anmad and H. V. Vankatasetty, J. Phys. Chem., 1964,68, 889. R. Suttinger and F. Strohbusch, Inorg. Chim. Acta, 1980, 40, 64. J. F. Coetzee and G. R. Padmanabhan, J. Phys. Chem., 1965, 69, 3193. I. M. Kolthoff, M. K. Chantooni Jr and S. Bhowmik, J. Am. Chem. SOC., 1966,88, 5430. I. M. Kolthoff and M. K. Chantooni Jr, J. Am. Chem. SOC. (a) 1965,87, 4428; (b) 1969,91,4621. J. Magonski, Dissertation (University of Gdarisk, 1983). Z. Pawlak and J. Magonski, J. Chem. Soc., Faraday Trans. I , 1982, 78, 2807. Z. Pawlak, B. Nowak and M. F. Fox, J. Chem. Soc., Faraday Trans. I , 1982, 78, 2157. l o I. M. Kolthoff, Anal. Chem., 1974, 46, 1992. l 1 Nonaqueous Electrolytes Handbook, ed. G. J. Janz and R. P. T. Tomkins (Academic Press, New York, l 2 J. Magonski and Z. Pawlak, J. Mol. Struct., 1982, 80, 243. l3 I. M. Kolthoff and J. F. Coetzee, J. Am. Chem. SOC., 1957, 79, 870; 1852; 6110. l4 M. Kinugasa, K. Kishi and S. Ikeda, J. Phys. Chem., 1972, 77, 1914. 1972), vol. I and 11. M. K. Chantooni Jr and I. M. Kolthoff, J. Phys. Chem., 1973, 77, 527. R. Suttinger and F. Strohbusch, Eer. Eunsenges. Phys. Chem., 1984, 88, 774. E. Pitts, B. E. Tabor and J. Dally, Trans. Faraday Soc., 1970, 66, 693. Z. Pawlak, R. A. Robinson and R. G. Bates, J. Solurion Chem., 1978, 7, 631. (PAPER 4/ 1 164)

ISSN:0300-9599    

DOI:10.1039/F19858102021

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1985, 81, 2027-2041 Comparison of the Surface Reactivity and Spectroscopy of Alkaline-earth-metal Oxides Part 2.-Dependences upon Temperature of Pre-activation for SrO BY JOHN NUNAN, JOHN A. CRONIN AND JOSEPH CUNNINGHAM* Chemistry Department, University College, Cork, Ireland Received 12th September, 1984 High-surface-area samples of strontium oxide, prepared by thermal decomposition of high-purity SrCO, in vacuo, have been examined for their reactivity and/or catalytic activity towards molecular gas probes and their luminescence spectra have been recorded. Temperature profiles are reported, as a function of prior outgassing temperature, for the development of room-temperature excitation and emission spectra. These are compared with temperature profiles for the development of room-temperature catalytic activity for &-type oxygen isotope exchange, 1 6 0 2 + leOp e 21s0180, and for the development of activity for nitrous oxide decomposition at various temperatures. Consideration is given to interpretations of the results in terms of ions at coordinatively unsaturated surface locations, to the extent to which these are exposed after thermal activation at various temperatures and to the involvement of point defects in the surface locations responsible for the various surface processes.Absorption features lying to the long-wavelength side of their absorption edges, and attributable to the photogeneration of correlated electron-hole pairs within the lattice (i.e. bulk excitons), have been reported for single crystals of high-purity alkali-metal halides1* and for some alkaline-earth-metal 0xides.~9 In terms of the tight binding approximation (generally considered applicable to these groups of highly ionic solids) such bulk-exciton transitions may be thought of as promoting an electron from an anion site enjoying full octahedral coordination onto a similarly coordinated adjacent cation site.Thus within the bulk of a perfect single crystal composed of Mi,+ and 0:; ions, the upward transition may be approximated by hv [M6c(nS0)2+, 0 6 C ( 2 p 6 ) 2 - ] [M6c(ns1)f, 06C(2p5)-l*' (1 a) Excitons thereby photogenerated, whilst highly mobile throughout perfect lattices at very low temperatures, are subject to radiative and non-radiative decay in real systems at normal temperatures, usually with a nett overall effect equivalent to the reverse of reaction (1 a).Radiative decay can occur via the intermediacy of self-trapped,2b defect- trapped5a or imp~rity-trapped~~ excitons and studies of such photoluminescence (emission) can yield information on the nature of the luminescence Spectroscopic observations reported in the present series of papers for the isostruc- tural alkaline-earth-metal oxides, MgO, CaO, SrO and BaO relate, however, to other electronic transitions, which are displaced to longer wavelength by 2-4 eV relative to bulk-exciton transitions. These are most readily observable in samples of high surface area, such as powdered oxide samples prepared by thermal decomposition in vacuo of the corresponding carbonates or hydroxides at high temperatures.Careful reflectance measurements, mainly by Stone et al., have revealed three partially resolved absorption 20272028 SURFACE REACTIVITY AND LUMINESCENCE OF SrO features, which they denote by I, I1 and 111 and which they interpret in terms of surface excitons, i.e. transitions analogous to reaction (1 a), except that the anion and/or cation sites involved are at surface locations and do not enjoy full octahedral c~ordination.~? * According to the interpretation espoused by Stone et al., features I1 and 111 in the reflectance spectra are to be associated with photogeneration of surface excitons involving 02- having only four- or three-fold coordination, e.g. hVl I [M~,(W~+, 0 , , ( 2 ~ 6 ) 2 - 1 ~ [M,,(w+, 0,,(2~5)-1* (1 b) where the subscript lc denotes low but unspecified coordination.Variation of these transition energies for the different alkaline-earth-metal oxides in accordance with the Mollwo-Ivey relation for localised excitons8 lent support to the interpretation of features I1 and I11 in terms of localised surface excitons. However, the transition energies of feature I were observed to vary in a similar fashion to freely diffusing excitons of the bulk, and this has been assigned to photogeneration of surface excitons at surface locations involving oxide ions having five-fold coordination upon flat { loo} terraces of the metal oxide: hVI [M~c(ns~)~+, 05c(2P6)2-I --+ [Mlc(nsl)+, 05c(2p5)-]* (14 Interpretations favouring steps, edges or corners as the surface topographical features mainly responsible for the existence of anions (and to lesser degree cations) with four- or three-fold coordination have been espoused not only by Stone and coworkers on the basis of reflectance measurements but also by Tench and coworkersg on the basis of luminescence spectroscopy .Recently, Duley has advanced an interesting variation upon the idea that coordin- atively unsaturated oxide ions are important in interpreting luminescence and absorption features of high-surface-area alkaline-earth-metal oxide Accor- ding to Duley's model, luminescence emission from MgO and CaO involves a charge-transfer-type transition from an electronically excited state of 0:; to the ground state of an adjacent F,+ centre: *Of;* * F , + d OFc * * F,* + hv. (2) An essential difference between this representation and those in reactions (1 a-d) is the recognition given to an important role of point defects at the surface [the defects of importance in reaction (2) being F,+ centres, consisting of vacant anion sites at the surface upon each of which a single electron has been localized].In another recent paper Duley has considered the involvement of coordinatively unsaturated hydroxide ions in luminescence from MgO and Mg(OH), powders at various stages of hydration deh~dration.~~ A preliminary account has recently been givenlo of the marked effects of Ba2+ dopant upon surface processes on MgO and of a probable influence of surface reconstruction and/or relaxation in modifying the degree of coordination of surface ions. A brief account was also given of problems attached to an interpretation of surface processes mainly in terms of ions at unrelaxed step, edge or corner locations.In view of these additional influences upon surface luminescence from alkaline-earth-metal oxide powders, and particularly in view of the very differing emphases possible for surface defects and surface impurities, we have initiated a programme to examine the extentJ. NUNAN, J . A. CRONIN AND J. CUNNINGHAM 2029 of correlation between surface luminescence and other surface properties likely to be dependent upon surface concentrations of point defects or impurities.'O Surface reactivity and catalytic activity for gaseous reactions mediated by charge transfer were selected as appropriate defect-sensitive processes.Furthermore, in view of the reported dependences of the surface reactivity and catalytic activity of alkaline- earth-metal oxide powders11-15 upon the temperature of prior activation, an important element of the strategy adopted in this second paper of the series is a careful comparison, made throughout with the same SrO material, between the dependences exhibited by the surface luminescence and the surface catalytic activity upon the temperature of prior activation. EXPERIMENTAL VACUUM PROCEDURES Both the thermal decomposition of strontium carbonate to SrO and its subsequent activity for N,O decomposition, or for the homomolecular oxygen isotope exchange (i.e. Oi6 + Ois + 2016018) were studied in a static quartz reactor using conventional vacuum procedures (base pressure 5 x Torrf) with mass-spectrometric analysis (base pressure Torr).In order to follow the progress of the thermal decomposition of high-purity SrCO, (Spex spectroscopically pure grade) in the quartz reactor, a powdered sample was slowly heated to 1273 K over a period of 10 h, during which the total pressure was monitored and the composition of gaseous decomposition products determined by leaking samples to a VG Micro Mass 6 mass spectrometer via a leak valve. The progress of thermal decomposition during activation in uacuo at temperatures up to 1273 K was also monitored by taking i.r. spectra at 300 K, after periods of activation at high temperature, of thin self-supporting discs, initially fabricated from SrCO, but converted progressively to SrO. Nitrous oxide was supplied by B.D.H.Chemicals and had a purity of > 99%. Before contacting PNzO z 3.5 Torr with the catalyst, the gas was further purified by a series of freeze-pumpthaw cycles, after which the purity was further checked by leaking the gas into the spectrometer through a by-pass valve. With the SrO catalyst at temperatures in the range 673-873 K, the reaction was initiated by admitting purified N,O and periodically obtaining the mass spectrum of gas samples leaked into the spectrometer. Reaction temperature and catalyst mass were chosen such that the reaction time for 50% conversion was > 10 min. Under these conditions the reaction rate was found not to be limited by diffusional effects. For oxygen-isotopic-exchange studies, an isotopically non-equilibrated mixture consisting of 50% Oi6+50% Oi8 was employed (Norsk Hydro). This gas was contacted with the catalyst at room temperature.The mass-spectrometric procedure was the same as for N,O decom- position, but oxygen pressures in the range 0.1-0.01 Torr were used. In general SrO was activated prior to reaction by heating under vacuum to the required activation temperature and maintaining this temperature for 1 h before cooling to room temperature in uucuo. The base pressure in the vacuum system employed was 5 x Torr. GAS-CHROMATOGRAPHIC PROCEDURE Nitrous oxide decomposition was also studied using g.1.c. and microcatalytic, flow-reactor systems operating at atmospheric pressure. A feature of these studies was the use of both the continuous-flow and pulsed-reactant procedures and full details have been given elsewhere of both the methods of operation.16 Strontium carbonate samples (200 mg) were decomposed in situ in the quartz microcatalytic reactor by heating at different temperatures, up to a maximum of 1273 K, for 1 h in a flow of dry helium at a flow rate of 40 cm3 min-l.Prior to the admission of nitrous oxide as pulses or as a continuous flow, the catalyst temperature was lowered to the reaction temperature in a flow of pure dry helium. The reactor was then isolated whle N,O + He mixtures were prepared such that the pressure of N,O was in the range 5&250 Torr. The total flow rate was then restored to 40 cm3 min-' t 1 Torr z 133.3 Pa2030 SURFACE REACTIVITY AND LUMINESCENCE OF SrO LUMINESCENCE PROCEDURES Instrumentation used in obtaining the excitation and emission spectra has previously been described in detail.Q Excitation was provided by a 250 W xenon lamp, from which appropriate wavelengths were sequentially selected using two Spex f4 monochromators whilst recording the excitation spectra.Another Spex monochromator, placed between the sample and the detector, was held at a fixed wavelength and augmented by appropriate cut-off filters when recording emission spectra. Spectra were automatically corrected for variations in excitation intensity with time, or changing wavelength, via instrumental comparisons (with an Ortec photon-counting system) of the pulse rates from two photomultipliers : one monitoring luminescence intensity and the other the intensity of the source at the same wavelength.Excitation spectra at acceptable signal-to-noise ratios were only obtained at wavelengths > 230 nm because of low source intensity at shorter wavelengths. When recording emission spectra under excitation at fixed wavelength, sharp cut-off filters (Corning 3-74 and 0-52) were placed between the sample and the emission monochromator to minimise scattered wavelengths from the xenon source reaching the photomultiplier. A band-pass of 5 nm was used for both the excitation and emission results. RESULTS SAMPLE ACTIVATION Decomposition profiles of SrCO, as a function of the outgassing temperature are shown in fig. I. Use of total pressure of gaseous products as the monitor indicated that the carbonate started to decompose at 873 K, reached a maximum at 1023 K and was almost complete at 1073 K.Mass-spectrometric analysis shows that the major decomposition products were CO, and CO, but some water vapour was also present. The ratio of CO to CO, changed markedly as the activation temperature was increased above 1073 K, with CO becoming the dominant decomposition product at the higher temperatures, presumably because of the onset of a dissociation of residual surface CO, upon sites exposed at the higher temperatures. Progress of the thermal decomposition of SrCO, and its conversion to SrO is also shown in fig. 2, which presents a summary of i.r. spectra obtained at 300 K from a self-supporting disc, following activation in vacuo at the indicated temperatures. Clearly evident is the progressive diminution of i.r.bands (at 2400, 1780 and 1090 cm-l) characteristic of bulk Cog- and the appearance of bands (at 1000-800 cm-l) characteristic of bulk Particular interest attaches, however, to the temperature needed for total removal of bands at 1090 and 1460 cm-l, which previous workers have identified with Cog- in the aragonite Our observation that these were not totally removed until vacuum outgassing reached 1173 K make it necessary to take into account the persistence of some SrC0,-like regions to considerably higher temperatures than might be suggested (ca. 1073 K) by the mass-spectrometric data in fig. 1. Another significant feature of the i.r. studies, which is not depicted in fig. 2, was the lack of evidence to indicate extensive hydroxylation at any of the stages represented in fig.2. Effects of prior outgassing at different temperatures on the emission and excitation spectra of SrO at room temperature are shown in fig. 3. The excitation and emission spectra appeared only on outgassing above 1073 K, which fig. 1 showed to correspond to decomposition of the bulk carbonate, Further increases in outgassing temperature brought about a progressive increase in the emission-peak intensity and an accom- panying increase in the emission-peak intensity, which fig. 2 would suggest to coincide with removal of the last traces of Cog-. Increasing the outgassing temperature also brought about significant changes in the peak shape and position. Thus in the excitation spectra, both a peak at 280 nm and an accompanying shoulder at 3 15 nm were observed to grow and experience changes in relative intensity on going from 10731 0 0 n E 80 8 + 60 LO 20 J.NUNAN, J. A. CRONIN AND J. CUNNINGHAM 3.0 I- 203 1 Fig. 1. Mass-spectrometric observations on the SrCO, -+ SrO conversion in uucuo as a function of outgassing temperature: A, total pressure of gaseous decomposition products; , combined peak heights of (CO+CO,); 0, ratio of CO to CO, peak heights. I I I I I I I I I I :OOO 2500 2000 1800 1600 1400 1200 1000 800 600 v/cm-' Fig. 2. Infrared spectra (at 300 K) of SrCO, + SrO coqversion after vacuum outgassing for 1 h at (a) 293, (b) 973, (c) 1073 ahd (4 1173 K.2032 SURFACE REACTIVITY AND LUMINESCENCE OF SrO 1 1 I 2 5 0 3 0 0 350 X/nm Fig. 3. (a) Excitation spectra and (b) emission spectra of high-surface-area strontium oxide obtained by outgassing SrCO, at various temperatures from 1073 to 1293 K: (i) 1 h at 1073 K, (ii) 1 h at 1123 K, (iii) 1 h at 1123 K, (iv) 1 h at 1173/1273 K and (v) 4 h at 1293 K.to 1273 K [cf. fig. 3(a), plots (i)-(iv)]. Upon outgassing for 4 h at 1293 K the relative intensity in the latter shoulder was observed to decrease again [cf. fig. 3(a), plot (v)]. Parallel observations upon the emission spectra of the same sample showed that outgassing at 1 123 K led to two broad peaks centred at 400 and 455 nm [cf. fig. 3 (b), plot ($1. Outgassing at the higher temperatures of 1173, 1223 and 1273 K resulted in a single peak at 465 nm, as shown in fig. 3 (b), plots (iii) and (iv). However, extensive outgassing at 1293 K for 4 h led to a shift in the emission maximum back to 450 nm and a reduction in its intensity of ca.40% [fig. 3(b), plot (v)]. Fig. 4 illustrates the effects of exciting the emission at different excitation wavelengths for a SrO saqple first outgassed at 1123 K and then at 1293 K. It is clear that for the sample pre-treated in this manner the emission-peak shape and position remained independent of the exciting wavelength. This was also found to be the case for samples outgassed at 1173, 1223 and 1273 K. These excitation and emission results agree well with those reported by Coluccia et aZ.9b In excitation the band having a maximum at 280 nm, similar to that previously attributed by Coluccia et aZ. to five-coordinate surface positions, appeared first. The other weaker band with a maximum at 3 15 nm, similar to that attributed to Coluccia et al.to four-coordinate surfaces locations, only developed as the outgassing temperature increased towards 1273 K. In emission, the bands shown in fig. 3(b) as the first to develop in our spectra, with maxima at 400 and 450 nm, are very similar to those assigned by Coluccia et al. to emission from surface locations involving five- and four-fold coordination, respectively. The development of the emission with a maximum at 465 nm only after outgassing at the higher temperatures, as illustrated by fig. 3(b), plot (iii), had previously been interpreted in terms of the involvement ofJ. NUNAN, J. A. CRONIN AND J. CUNNINGHAM 2033 ( a ) (iii) f : I . , . ... 1 1 1 1 1 350 400 450 500 550 Xlnm ( b ) (iv) ,.#-*\ i '\.i '. ; \ I 1 1 1 1 I 350 400 450 500 50 600 X/nm Fig. 4. Emission spectra of SrCO, after outgassing at (a) 1073 and (b) 1293 K, as excited by different wavelengths of exciting light: (i) R = 280 nm, (ii) R = 299 nm, (iii) ;Z = 315 nm and (iv) ;Z = 325 nm. surface ions with three-fold coordination. Furthermore, our observation that, regardless af whether excitation of SrO outgassed at the higher temperatures was made at 280 or 315 nm, emission was dominated by the band at 450 nm reproduces observations made earlier by Coluccia et al. and attributed by them to energy transfer from sites of five-fold coordination (excited by photons having R = 280 nm but emitting at 400 nm) to sites of four-fold coordination (excited by photons having R = 315 nm but emitting at 450 nm).It was encouraging that our spectral parameters for luminescence are qualitatively so similar to those of Colluccia et al., despite differences in starting materials and steps for its conversion to SrO. However, details of the dependence of luminescence upon temperature of sample outgassing, which are also contained within fig. 2-4, will, when compared with the temperature dependence of the surface reactivity/catalytic activity (see below), serve to call into question the adequacy of relying upon varying relative exposures of O;;, 0:; and 0:; as the sole arbiter of surface properties. SURFACE REACTIVITY The stoichiometry of the nitrous oxide decomposition at reaction temperatures of 673-873 K over SrO samples pretreated as outlined in the experimental section agreed within experimental error with the reaction N2O(g) + Ndg) + P2(g)- (3) The reaction exhibited first-order kinetics, whether studied by the mass-spectrometric (m.s.) procedure or by the gas-chromatographic (g.c.) procedure.In fig. 5 (a) are shown the first-order plots of N20 decomposition at 627,656 and 683 K obtained using mass spectrometry. Linear regression analysis gave correlation coefficients ranging from 0.992 to 0.997 for the three plots. In the gas-chromatographic continuous-flow studies, 67 FAR 12034 SURFACE REACTIVITY AND LUMINESCENCE OF SrO 0.74 I I I I 20 40 60 80 tlmin -13.8 - 1 3 . 6 - 1 3 . 4 Q) +d $ - 13.2 - -13.0 - 1 2 . 8 - 12.6 '\ 4.3 L.5 4 . 7 4 . 9 5.1 5.3 5 . 5 5.7 In (PN201TOrr) Fig. 5. Evidence from kinetic analysis for first-order character of N,O decomposition at 700 +_ 50 K following preactivation of the SrO material at higher temperatures.(a) First-order plots of data obtained by mass-spectrometric monitoring of N,O decomposition in a static reactor over SrO preactivated at 1133 K; PNZ0 (initial) = 2.5 Torr, reaction temperatures: 0, 627; A, 656 and U, 673 K. (b) Plot of In rate against In PNzO for steady-state decomposition in a continuous-flow reactor operated in the differential mode at 758 K over SrO preactivated at 1273 K for 6 hr in an argon flow. Inset shows plot of steady-state conversion against reciprocal space velocity at 740 K for the same sample.J. NUNAN, J. A. CRONIN AND J. CUNNINGHAM 2035 kinetic data were obtained under conditions where the reactor was operating in the differential mode.This was achieved by selecting the reaction temperature, catalyst mass and flow conditions such that the conversions were < 4%. Plots of the reciprocal space velocity against conversion within this range were found to be linear, as shown by the insert on fig. 5(b). Consequently the percentage conversion gives a direct measure of the rate of reaction. Applying the method of plotting In rate against In PNPO, the reaction was shown to be first order with respect to N,O pressure in the pressure range 50-250 Torr, as illustrated in fig. 5(b). In this manner first-order kinetics in N,O decomposition were shown to be obeyed over SrO (ex SrCO,) preactivated at various temperatures in the temperatures range 773-1 273 K, indepen- dent of whether the reaction was studied using m.s.or g.c. procedures. The effect of outgassing temperature on the reaction rate is shown in fig. 6, where the first-order rate constants obtained using g.c. are plotted against outgassing temperature. It is evident from fig. 6 that, while some steady-state activity for N,O decomposition became apparent after outgassing at 900 K, the rate only began to increase rapidly after outgassing at temperatures > 1073 K, i.e. in the range where fig. 2 indicated removal of residual traces of COi-. The use of the gas-chromatographic procedure in its pulsed-reactant mode allowed investigatior s to be made of any progressive changes brought about in sample activity by contacting a preactivated SrO sample with a succession ofindividual pulses of nitrous oxide.In this way low-exposure activity profiles (1.e.a.p.) of surface activity could be developed. Fig. 7 shows how such 1.e.a.p. plots varied with increasing pulse number (each pulse ca. 4 x mol of N,O) for different temperatures. Comparison of the three 1.e.a.p. profiles, all measured at an identical reaction temperature, demonstrate two important points: firstly, the activity maximum attained in a profile was lowest after preactivation in vacuo at 993 K and was increased by increased preactivation temperature, and secondly, although the drop-off in activity after the maximum was quite abrupt for samples preactivated at 993 or 1083 K (so that activity declined rapidly towards zero after a small number of pulses), no such abrupt drop-off in activity was found for the sample preactivated at 1273 K, but rather the activity for N,O decomposition continued at a high level up to large pulse numbers.The first of these points agrees well with existing hypotheses that the level of surface activity depends upon the extent to which more highly coordinatively unsaturated surface ions are exposed and/or developed at progressively higher preactivation temperatures. An adequate explanation of the second point requires some extension and modification of existing hypotheses, which is attempted in the Discussion. Such extension and modifications will also be relevant to observations made upon the ratio of N, to 0, products from N,O pulses delivered in the experiments depicted in fig.7. These showed that, whilst at any of the selected temperatures the N, to 0, product ratio did not vary significantly with pulse number, the ratio took the very different values of 2.0, 3.0 and 3.8 during sequences of pulses introduced for samples previously outgassed at 993, 1083 and 1273 K, respectively. Oxygen isotope exchange was studied using mass spectrometry.la After preactivation of the SrO (ex SrCO,) samples at adequate temperatures the reaction occurred readily at room temperature. This is illustrated in fig. 8, where the mole fractions of 1602, 160180 and 180, are plotted as a function of time. During the course of the reaction the atom fractions of l60 and l80 remained constant in the gas phase, thus indicating that an &-type oxygen-isotope-exchange process was occurring, i.e.l6O,(g) + l80,(g) e 21601SO(g) (4) The effect of temperature of prior activation upon the level of activity for &-type without significant accompaniment of an R,- or R,-type exchange with lattice l60. 67-22036 8 7 6 n 5 E S f A 4.4 .& 3 2 1 2, i ./. .-.--.--.- 723 823 923 1023 1123 1223 1323 Fig. 6. Influence of temperature of prior activation of SrO samples on the steady-state rate of N,O decomposition at 758 K in the continuous-flow g.c. procedure with PNzO = 380 Torr. 40 t 4 2 0 - \ \ 7 ‘.L1 10 - 0 1 0 2 4 6 8 10 N20/10-6 mol Fig. 7. Plots of low-exposure activity profiles (1.e.a.p.) of SrO samples, obtained by measurement in the pulsed-reactant g.c. procedure. These show the extent of non-steady state N,O decompo- sition at 758 K for each of a succession of individual pulses; PNs0 = 40 Torr.The three profiles were obtained for SrO samples preactivated at 0, 993; ., 1083 and A, 1273 K.J. NUNAN, J. A. CRONIN AND J. CUNNINGHAM - - - - - 2037 0.5 h E 0 0.L 2 It: 0.3 2 0 2 0 v 0.2 5 0.1 h c 0 . 0 I 1 1 1 1 1 1 1 t l s 0 10 20 30 10 50 60 Fig. 8. Progress of the &-type oxygen-isotope-exchange equilibration process at room temperature on admitting an isotopically non-equilibrated equimolar mixture of 1 6 0 2 + I S 0 2 into contact with an SrO sample preactivated in vacuo at 1223 K. Plots show the mole fraction of 1 6 0 2 and 1802 decreasing with time (a) and the mole fraction of 1601S0 increasing with time (m), in agreement with reaction (4). oxygen-isotope exchange subsequently measured at room temperature is clearly shown in fig.9. There the reciprocal of z, the time required for surface-assisted movement by 50% towards total equilibration, is used as a measure of rate of the isotopic equilibration process and is plotted as a function of prior outgassing temperature. For a constant preactivation period of 1 h at each temperature, fig. 9 indicates a rather sharp onset of activity after outgassing at 1023 K, followed by an essentially linear increase for higher outgassing temperatures. Fig. 9 represents for purposes of comparison the dependence upon temperature of preactivation observed in a related set of experiments where the magnitude of the first-order rate constant for N,O decomposition at 758 K was determined after 1 h preactivation of the SrO at various temperatures.Although the same static-reactor and mass-spectrometric method of analysis were employed for both sets of experiments, it is clear that the onset of activity is more sharply defined for the oxygen-isotope-exchange plot and that its initial slope is much steeper than for N,O decomposition. DISCUSSION The above results show the considerable success that there has been in the search for similarities between dependences upon temperature of prior activation in the development of surface photoluminescence and of surface reactivity. Such a correlation is most direct in the case of the &-type oxygen-isotope-exchange reaction, since surface luminescence and surface activity were both measured under similar conditions, i.e. at room temperature after cooling down from the higher preactivation temperatures. Furthermore, it is well established from previous r e p o r t ~ ~ - ~ of quenching of the surface luminescence by molecular oxygen that 0, species interact reversibly at 300 K with2038 SURFACE REACTIVITY AND LUMINESCENCE OF SrO 35 30 25 - I 0 v) 20 - 1 .y 15 10 5 800 1000 1200 TIK Fig.9. Influence of temperature of prior activation of SrO samples upon the rates of N,O decomposition (m) and of &-type oxygen isotope exchange (0) as monitored by mass spectrometry. The kinetic parameters utilised as measures of the rates of the processes were, respectively, the slopes of plots similar to those in fig. 5(a) and the time to 50% equilibration in plots similar to those in fig.8. the centres responsible for luminescence, even at the low pressures ( 10-1-10-2 Torr) employed in obtaining the surface reactivity data shown in fig. 8. The fact that the temperature profile for development of oxygen-isotope-exchange (cf. fig. 9) rises rapidly across the same temperature range (1073-1273 K) as was required for rapid enhancement of surface luminescence provides good support for the idea that a relationship exists between sites involved in surface photoluminescence and those conferring catalytic activity for oxygen-isotope exchange in accordance with reaction (4). It has been deduced from previous studies1*919 of the latter process upon ZnO surfaces that (a) the catalytically active sites involve surface cations at locations of high coordinative unsaturation and (b) the ready availability of at least one electron from a shallow energy level or trap in the immediate vicinity of the M& locations is also necessary to trigger oxygen isotope exchange (via an electron-transfer-catalysed chain reaction requiring negligible energy of activation).Both of these requirements seem likely to be satisfied by the Of;. . . F,+ surface locations proposed by D ~ l e y ~ ~ as being responsible for surface luminescence from high-surface-area MgO and CaO, since (a) the Sr2+ cations involved in the F,+ point defect necessarily have at least two degrees of coordination unsaturation and (b) relatively low energies can be expected for transfer of an electron from a ground-state 0;; or the adjacent F:. According to D ~ l e y , ~ ~ photoinduced transfer of an electron between these is responsible for a broad absorption centred upon 2 eV and detectable in MgO previously exposed to high-energy radiations in vacuo.Still lower energy could be expected for thermal,J. NUNAN, J. A. CRONIN AND J. CUNNINGHAM 2039 rather than optical, promotion of an electron from Of; to an adjacent F: for MgO. Further reductions in energy could be anticipated for SrO relative to MgO, and for Ot; . . . F,+, , locations relative to Of;. . . F,+ locations. * Thus it is reasonable to envisage oxygen-isotope-exchange upon SrO proceeding, in analogous fashion to that previously detected for ZnO surfaces, via activation of oxygen under the combined influence of a readily available electron and coordinatively unsaturated cation(s).Viewed in this context, the experimentally observed dependence of the extent of room- temperature oxygen-isotope-exchange activity of the SrO (ex SrCO,) surfaces upon temperature of prior activation (cf. fig. 9 ) should reflect the increase in number of 0:;. . . F,+, , surface locations with preactivation temperature (n = 1 or 2). The exact implications of the good agreement with first-order kinetics observed here [cf. fig. 5(a) and (b)] for decomposition of N,O at 758 K clearly depend upon the mechanism of the metal-oxide-assisted decomposition, and in particular upon which step is the rate-determining process. Electron transfer from active sites on the metal oxide surface to N,O adsorbate has been proposed20* 21 by many workers as the initial step.However, the rate-determining role has often been assigned to a step by which the necessary reverse electron transfer is accomplished.22 We may denote these electron-transfer processes by N,O(ads) + e-D, -+ N,O-(ads) -+ N2(g) + 0-(ads) D, ( 5 a) followed by O-D, + N,O(g) -+ N,(g) + 0, + e-D,. Note the adoption at this stage of the non-commital notation D, for surface locations with which an electron or an oxygen fragment can be loosely associated (as in e-D, and 0-D, respectively) and which are envisaged to play important roles in N,O decomposition. However, the ability of such a two-step mechanism to account for the pseudo-steady-state rate of N,O decomposition in the continuous-flow mode is largely independent of the nature of D,. Thus the operation of reaction (5b) as the slow rate-determining step in conditions such that available D, sites were effectively saturated by 0- would account for first-order dependence upon PNZO.Furthermore, the experimentally observed dependence of the pseudo-steady-state rate of decom- position on the temperature of preactivation (cf. fig. 6) may arise because the available surface concentration of the entity 0-D, in reaction (5b) was predetermined by the surface concentration, ODs, of active sites developed by prior activation. In the context of this two-step mechanism and of the similarities between the experimentally observed profiles for development of activity for N,O dissociation and oxygen- isotope-exchange (cf. fig. 9) it is of interest to consider whether the e-D, sites envisaged in reaction (5 a) may be similar to the 0;;.. . FZ, , locations favoured earlier as the active sites for oxygen-isotope-exchange on SrO. At first glance the fact that the profiles in fig. 9 do not reveal three distinguishable segments, corresponding to temperature regimes dominated by the growth of Ot;, 0;; and 0;; as envisaged by others in temperature profiles for development of photoluminescence features, might be construed as an argument against equating e-D, with the 0:;. . .F,+, 0;;. . . F,+ and 0:;. . . F: locations envisaged by Duley. However, an alternative interpretation which cannot yet be overruled and which would permit equivalence between e-D, and 0:;. . . F,+, sites, is that the electron transfer required to initiate N20 dissociation [cf.reaction (5 a)] or oxygen-isotope-exchange [cf. ref. (18)-(20)] can occur to adsorbed reactant with comparable facility from 0:;. . . Fi, s , 0;;. . . FA., , and 0;;. . . F:, , locations, thereby rendering molecular probes incapable of distinguishing between * Note added inproof: OkadaZ3 has recently associated the absorption at 2.17 eV in MgO with Ff centres.2040 SURFACE REACTIVITY AND LUMINESCENCE OF SrO such locations. In such,a situation, some differences between the degree of detail revealed by temperature profiles for development of surface activity and of surface photoluminescence could be expected. The value observed experimentally for the ratio of N, to 0, (between gaseous N, and 0, products detected as well formed peaks in the g.c. technique) represents a further criterion for assessing the mechanism involving reactions (5a) and ( 5 b).Thus the value of 2.0 observed for this ratio in the continuous-flow conditions under PNPO from 50 to 380 Torr agreed well with the mechanism. Note, however, the very significant upward deviation of this ratio observed in the pulsed-reactant mode whenever the temperature of preactivation of SrO was progressively increased (values of 2.0, 3.0 and 3.8 after preactivation at 993, 1083 and 1273 K, respectively). Such deviation would be consistent with enhanced competition, under the conditions prevailing in the pulsed-reactant mode, between the above two-step mechanism with reaction (5b) as the slow oxygen-producing step and another process which instead incorporated fragments from N,O decomposition into the oxide surface.Likely processes of the latter type are N,O(g) + mSrfz -+ N,(g) + OrnSrt: (6 b) which involve incorporation of an oxygen fragment from N,O into an anion vacancy doubly occupied by electrons, 2e-0,, or by surface groupings of m low-coordinate cations. Some approach towards equilibrium concentrations of point defects at the surface is thermodynamically expected during the preactivation treatment, greater con'centrations being expected during preactivation at high temperatures. A ' freezing- in' of some fraction of such defect concentrations during the rapid cooling to 758 K would result in the probability of reactions (6a) and/or (6b) occurring on contact with the initial pulses of N,O admitted in the pulse-reactant mode.The extent to which this could drive the N, to 0, ratio of the 1.e.a.p. plots away from the value of 2:l expected if only reactions (5a) and (5b) were significant would, as observed in the pulsed-reactant experiments, be greater after higher temperature of preactivation because of freezing-in of a greater number of defects. For the continuous-flow conditions only a low survival probability would be expected for such defects after the much higher integrated exposures (typically 2 x lo6 Torr s) to N,O which preceded measurement of steady-state activity for N,O decomposition. This would account for the negligible deviation of the N, to 0, ratio from 2.0 in the continuous-flow mode regardless of the sample temperature during preactivation. Deviation of the ratio from 2.0 during the determination of the 1.e.a.p.plots, with the extent of deviation observed to increase with the temperature of prior activation, contrasts with the steady state results and implies that surface defects at the SrO surface have a major role at low exposures. This work was supported by the U.S. Air Force Office of Scientific Research under grants AFOSR 82-0023 and 83-0074. We also thank the Department of Education of the Irish Government and University College, Cork for support (J.A.C.). (a) J. E. Ely, K. J. Tergarden and D. B. Dutton, Phys. Rev., 1959,116, 1099; (b) W. H. Hamill, Phys. Rev., 1969, 185, 1182. * (a) D. L. Dexter and R. S. Knox, Excitons (Wiiey Interscience, New York, 1965); (b) T. Higashimura, Y. Nukaoka and T. Iida, J .Phys. C, 1984, 17, 4127. B. Henderson and J. E. Wertz, Ado. Phys., 1978, 17, 749. V. M. Bermudez, Prog. SurJ Sci., 1981, 11, 1 .J. NUNAN, J. A. CRONIN AND J. CUNNINGHAM 204 1 (a) W. W. Duley, Philos. Mag. B, 1984,49,159; (b) K. Teegarden, in Luminescence of Inorganic Solids, ed. P. Goldberg, (Academic Press, New York, 1966). R. C. White1 and W. C. Walker, Phys. Reu., 1969, 188, 1380. A. Zecchina, M. G. Lofthouse and F. S. Stone, J. Chem. SOC., Faraday Trans. I , 1975,71, 1476. E. Garrone, A. Zecchina and F. S. Stone, Philos. Mag. B, 1980, 42, 683. (a) A. J . Tench and G. T. Pott, Chem. Phys. Lett., 1974, 26, 59; (b) S. Collucia, A. M. Deane and A. J. Tench, J. Chem. Soc., Faraday Trans, 1978,74,2913; (c) S. Collucia and A. J. Tench, J. Chem. SOC., Faraday Trans 1, 1983, 79, 1881. lo J. Nunan, J. Cunningham, A. M. Deane, E. A. Colbourn and W. C. Mackrodt, in Adsorption and Catalysis on Oxide Surfaces, ed. M. Che and G. Bond (Elsevier, Amsterdam, 1984). l 1 (a) D. Cordischi, V . Indovina and M. Occhiuzzi, J . Chem. SOC., Faraday Trans. I , 1978, 74, 456; (b) V . Indovina and D. Cordischi, J. Chem. SOC., Faraday Trans. I, 1982,78, 1705. l2 (a) M. J. Baird and J. H. Lunsford, J. Catal., 1972,26,440; (6) Y. Tanaka, H. Hattori and K. Tanaka, Chem. Lett., 1976, 37. l3 (a) H. Hattori and A. Satoh, J. Catal., 1976, 45, 32; (b) M. Mohri, K. Tanabe and H. Hattori, J. Catal., 1974, 32, 144. l4 (a) H. Hattori, K . Marayama and K. Tanabe, J. Catal., 1976,44, 50; (b) M. Mohri, H. Hattori and K. Tanabe, J. Catal., 1974,32, 144. l5 (a) L. Parrot, J. W. Rogers Jr and J. M. White, Appl. Surf. Sci., 1978, 1, 443; (b) H. Noeller and K. Thomke, J. Mol. Catal., 1979, 6, 375. l6 J. Cunningham, B. K. Hodnett, M. Ilyas, J. P. Tobin and E. L. Leahy, Faraday Discuss. Chem. SOC., 1981, 72, 283. l7 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, (Wiley Interscience, New York, 3rd edn, 1978); (b) S. Pinchas and I. Ilaalicht, Infrared Spectra of Labelled Compounds (Academic Press, New York, 1971). (a) J. Cunningham and E. L. Goold, J. Chem. SOC., Faraday Trans. I , 1981, 77, 837; (b) J. Cunningham, E. L. Goold and J. L. G. Fierro, J. Chem. SOC., Faraday Trans. I , 1982, 78, 785. lD J. Cunningham, in Comprehensive Chemical Kinetics, ed. C. F. Tipper and C. H. Bamford (Elsevier, Amsterdam, 1984), vol. 19, p. 362. 2o J. Cunningham, J. J. Kelly and A. L. Penny, J. Chem. SOC., Faraday Trans. I , (a) 1970, 74, 1992, (b) 1971, 75, 617. 21 N. B. Wong, Ben Taarit and J. H. Lunsford, J. Chem. Phys., 1974, 60, 2149. 22 (a) F. S . Sione, J. Solid State Chem., 1975,12,271; (6) G. Blyholder, J. Phys. Chem., 1971,75, 1037. 23 M. Okada, Cryst. Lattice Defects and Amorphous Materials, 1985, 11, 73. (PAPER 4/ 1579)

ISSN:0300-9599    

DOI:10.1039/F19858102027

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J . Chern. SOC., Faraday Trans. I, 1985, 81, 2043-2051 Decomposition of N20 on Fe203/A1,03 Catalysts Relationships between Physicochemical and Catalytic Properties BY PHILIPPOS POMONIS Chemistry Department, University of Ioannina, Ioannina, Greece AND DIMITRIS VATTIS AND ALEXIS LYCOURGHIOTIS* Chemistry Department, University of Patras, Patras, Greece AND CHRISTOS KORDULIS Universite Catholique de Louvain, Groupe de Physicochimie Minerale et de Catalyse, 1348 Louvain La Neuve, Belgium Received 20th September, 1984 The influence of the iron(II1) content and the calcination temperature on the dispersion of supported Fe3+ species on alumina, on the semiconducting properties and on the catalytic activity of Fe,O,/Al,O, catalysts has been studied, with the decomposition of N,O into N, and 0, being used as a probe reaction.Above 250 "C iron(1II) is located on the surface of the carrier as a-Fe,O, and 'strongly associated iron(II1)' (denoted by Fe3+-S). The ratio of the amounts of these species remains constant from X = 0.172 to X = 0.615 mmol Fe3+ per g of alumina and then changes favouring Fe3+-S. The transformation of a-Fe,O, into Fe3+-S is not accompanied by any detectable change in the Fe3+ dispersion but causes an increase in the activation energy of conduction. A rise in the calcination temperature and iron(II1) content brings about an increase in the size of supported a-Fe,O, crystals shown by a decrease in the dispersion of Fe;+. it has been demonstrated that a-Fe,O, and Fe3+-S exhibit similar activity. This increases iinearly with the dispersion of the Fe3+ ions.Moreover, no relationship was found to exist between the semiconducting properties of the catalysts estimated by the activation energy of conduction and the catalytic activity. X-ray photoelectron spectroscopy was used to estimate the dispersion of the supported iron(III), conductivity experiments were performed to determine the activation energy of conduction and catalytic tests were carried out to determine catalytic activity. The catalytic properties of supported metal oxide catalysts are generally determined from the following physicochemical characteristics : (a) the kind of the species which the active ions form on the surface of the carrier, (b) the semiconducting properties of the solids estimated from the activation energy of conduction and (c) the dispersion of the active phase on the carrier surface.On which of rhe above the catalytic activity depends is one of the most important problems in catdysis. A research program was recently initiated in our laboratory to solve this problem in the case of the iron(Ir1)-supported alumina catalysts. Although this system had been extensively studied by various methods, especially Mossbauer spectroscopy, a close relationship between the catalytic properties and the above physicochemical characteristics has not been established. Thus we have studied'. by various techniques the influence of the calcination temperature and of the iron(m) content on the textural and structural features of both the active phase and the support. For the carrier it was found that a progressive increase in the calcination temperature brings about the removal of the physically adsorbed H,O followed by 20432044 DECOMPOSITION OF N20 ON Fe,O,/Al,O, an irreversible transformation of the a-Al,[OOH], to alumina.2 For the supported iron(1n) phase it was found that a rise in the calcination temperature from 90 to 250 "C causes a decomposition of the Fe(NO,), 9H,O followed by progressive dehydration of the supported a-Fe,O, - xH20 resulting from that decomposition.An additional rise in the calcination temperature from 250 to 320 "C causes a decrease in the amount of the supported a-Fe,O,; this takes place irrespective of the iron(m) content. Further increases in the calcination temperature do not change the concentration of the supported a-Fe,O,. Two alternative explanations have been proposed for this effect :2 (a) dilution of the Fe3+ into the bulk of the support or (b) formation of a strongly associated, and thus difficult to reduce, iron(Ir1) species on the surface of the carrier.The size of a-Fe,O, crystals formed at temperatures > 320 "C varies with the iron(II1) content. The samples calcined at the maximum calcination temperature, i.e. 665 "C, give the following results: (a) a-Fe,O, crystals were not detected for the specimens with iron content < 0.388 mmol Fe3+ per g of the support, (b) the formation of very small crystalls of a-Fe20, not detectable by X-ray spectroscopy was inferred from diffuse reflectance spectra and magnetic measurements on the samples with iron content in the range 0.388-0.974 mmol Fe3+ per g of support and (c) a-Fe,O, crystals detectable by X-ray spectroscopy were identified in the sample containing the maximum iron(rI1) content, i.e.0.974 mmol Fe3+ per g of support. In the present study we have attempted (a) to study the influence of the iron(m) content and calcination temperature on the state of dispersion of Fe3+ ions on the carrier surface as well as the semiconducting properties of the solid catalysts, (b) to elucidate the solid-state process responsible for the decrease in the amount of the supported a-Fe,O, when calcination occurs at 250-320 "C and (c) to relate the physicochemical characteristics mentioned above of samples calcined at temperatures > 320 "C with their catalytic properties.The series containing samples calcined at 665 "C was selected as typical and the decomposition of N,O was used as a model X-ray photoelectron spectroscopy (X.P.S.), conductivity experiments and catalytic tests under atmospheric pressure were the methods of investigation used. EXPERIMENTAL PREPARATION OF THE SPECIMENS Details of the method of preparation of the various specimens have been reported elsewhere.2 The specimens studied here are referred to as Fe-X-Y, where X denotes the iron(Ir1) content as mmol Fe3+ per g of alumina and Y denotes the calcination temperature ("C). CATALYTIC TESTS The catalytic tests were carried out in a flow system under atmospheric pressure. A catalyst sample of surface area 42.65 m2 was placed on a perforated-glass bed of volume of ca.0.35 cm3 and depth ca. 0.2 cm. An He+N20 (2: 1) gas mixture was passed through the bed at a rate of 180 & 5 cm3 min-'. The contact time was thus 0.12 s. A Varian 3700 gas chromatograph equipped with a thermal-conductivity detector was used to analyse both reactants and products. Two gas valves with 1 cm3 loops were used for sampling. The column (0.5 m long by 8 in? internal diameter) was of stainless steel and filled with 5A molecular sieve. Preliminary tests showed that no decomposition took place up to 700 "C in the absence of catalyst. The temperature range examined was 500-650 "C. In all the experiments the sequence of temperatures examined was 600 -+ 550 -+ 500 -+ 525 -+ 575 + 625 -+ 650 "C. t 1 in = 2 . 5 4 ~ 10+ m.P. POMONIS, D. VATTIS, A.LYCOURGHIOTIS AND CH. KORDULIS 2045 X-RAY PHOTOELECTRON SPECTROSCOPY X-ray photoelectron spectra were obtained using a Vacuum Generators ESCA 3 spectrometer equipped with an aluminium anode (A1 Ka = 1486.6 eV) which operates at 20 mA and 1.4 kV. The residual pressure inside the spectrometer was ca. Torr.? A signal averager (Tracor Northern 1710) was used to improve the signal-to-noise ratio. Binding energies were referenced to the C 1s line at 285 eV. The photoelectrons counted per unit time, i.e. the X.P.S. signal intensities, are represented by the areas under the corresponding peaks. The X.P.S. relative intensity measurements concern the ratios of the intensities of two peaks, I , and I,, associated with the supported Fe3+ and the carrier, respectively. Several models have been proposed to relate the X.P.S.intensity ratios to the dispersion of the deposited phase.12-ls The following equations have been derived,” based on these models, provided that the active phase is present as cubic crystallites on the surface of the carriers: (1 a-c) where Im/Is = IFezp/IA12p, n,/nm is the atomic ratio Al/Fe in the specimen, D,, is the dispersion of the iron(m) and C, C, and C, are proportionality constants. Eqn (1 a-c) were derived assuming that the length of the edge of the cubic crystallites of the deposited phase is much greater than, almost equal to and much smaller than the inelastic mean free path of the photoelectrons, respectively. From eqn (1) one can see that the quantity Im/Is [or (Im/&) (ns/n,) when n,/nm is not constant] is an increasing function of DF,, and therefore it can be used for an estimation of the dispersion of iron(m) on the surface of the carrier. Note that quantitative analysis based on X.P.S.intensity measurements relative to a series of specimens on a porous support is valid only when no variation in the repartition of the deposited phase takes place. By reparation we mean the relative amount of species deposited in the inner parts of the elementary catalyst particle and those located at the mouth of the pores or on the external surface of the particle.16 CONDUCTIVITY MEASUREMENTS The conductivity experiments were carried out in a system similar to that described in ref. (1 1). A pellet of the catalyst was prepared by compressing 400 mg of the sample under a pressure of 10 toncm-2$ for good electrical contact between the catalyst particles.The pellet was clamped between two platinum electrodes, which were connected to a 1.5 V cell. The current passing through the pellet was measured with a Hewlett-Packard volt-ammeter having a range from 10-l2 to A. The electrical measurements were performed under atmospheric pressure since the catalysts were examined for the N,O decomposition under similar conditions. The current i passing through the sample at different temperatures is given by i = VSa/l, where V is the voltage applied, S is the area of the electrode (ca. 0.5 cm2), 1 is the thickness of the pellet (ca. 0.3 cm) and CJ is the specific conductance related to the temperature by the relation applied to semiconductors, CJ = oo exp ( - E,/RT).Plots of log i against 1 /T were used to calculate the activation energy of conduction Eg. RESULTS CATALYTIC TESTS The degree of conversion at each temperature is given in table 1. These data have been normalised to a catalyst containing 1 mmol of supported Fe3+. The variation of activity with the amount of the supported iron(n1) is shown in fig. 1 for three different temperatures. Inspection of table 1 and fig. 1 shows a continuous drop in the activity with increasing iron (111) content. t 1 Torr z 133.3 Pa. 1 I ton cm-2 = 98.07 x lo6 Pa.2046 DECOMPOSITION OF N20 ON Fe,O,/Al,O, 8 - A Y ." .- t; 6 - 0 x - m ." c.l - *-' 4 - 2 - Table 1. Degree of conversiona for the decomposition of N,O temperature of reaction/OC FeX- Y 500 525 550 575 600 625 650 Fe-0.00&665 - (0.050) (0.080) (0.120) (0.220) (0.360) (0.530) Fe-0.172-665 0.878 1.316 2.487 3.949 6.377 8.864 11.116 Fe-0.245-665 0.756 1.418 2.317 3.735 4.964 6.902 8.037 Fe-0.388-665 0.419 0.805 1.449 2.319 2.834 4.348 5.314 Fe-0.615-665 0.408 0.730 1.332 1.933 2.578 3.437 - Fe-0.974-665 0.266 0.503 0.961 1.448 1.833 2.498 - a Nomalised to a catalyst containing 1 mmol of supported Fe3+, except values in parentheses.O L 1 I 1 I 1 I L 1 0.1 0.3 0.5 0.7 0.9 iron(rr1) content, X Fig. 1. Variation of catalytic activity of Fe-X-665 with iron(m) content for the decomposition of N,O at three temperatures: A, 525; e, 575 and 0, 625 "C. X-RAY PHOTOELECTRON SPECTROSCOPY The values of the binding energies, determined for the Fe 2p1,2, Fe 2p,,, and Fe 1s electrons, remain practically constant irrespective of the iron(m) content or calcination temperature.Fig. 2(a) and 3(a) illustrate the variation in the dispersion of the supported iron(rI1) with its concentration in the sample and calcination temperature, respectively. Note that the dispersion decreases as the iron(Ir1) content and calcination temperature increase. The decrease of (IFe 2p/IA1 2 p ) (nAl/nFe) with X is almost exponential, while the rate of the decrease of the dispersion with calcination temperature remains practically constant in the range 200-500 "C and then increases.P. POMONIS, D. VATTIS, A. LYCOURGHIOTIS AND CH. KORDULIS 2047 I 1 1 I I I , 3.0 n E + ._ ;iij 2.0 g W % G 9. a i - 4 1.0 0 .o 0.2 0.4 0.6 0.8 1 .o iron(II1) content, X Fig.2. Variation of (a) the dispersion of Fe3+, (b) the amount of the supported a-Fe,O, and ( c ) the activation energy of conduction with the iron(r1r) content for Fe-X-665. 4 3 n P N +: --. 3 0 2 1 Fig. 3. Variation of (a) the dispersion of Fe3+ and (b) the amount of the supported a-Fe,O, with the calcination temperature for Fe-0.974- Y.2048 DECOMPOSITION OF N,O ON Fe,O,/Al,O, 100 200 300 A00 'Fe 2 p nAl IA12p Fe Fig. 4. Variation of the catalytic activity of Fe-X-665 with the dispersion of Fe3+ for the decomposition of N,O at three temperatures: A, 525; 0, 575 and 0, 625 "C. The variations in the temperature-programmed-reduction signals, which are pro- portional to the amount of supported a-Fe,O,, with the nominal iron(II1) content and the calcination temperature are shown in fig. 2(b) and 3(b), respectively.Note that the values of the t.p.r. signal taken from the ref. (2) correspond to an amount of sample containing 2.2 mg of iron(II1). The variation of the activity with the dispersion of Fe3+ is shown in fig. 4 for three different temperatures. CONDUCTIVITY MEASUREMENTS The variation of the activation energy of conduction, Eg, determined for the Fe-X-665 catalysts with iron(II1) content is illustrated in fig. 2 (c). DISCUSSION INFLUENCE OF CALCINATION TEMPERATURE ON THE DISPERSION OF IRON(III) AND THE CONCENTRATION OF SUPPORTED Fe3+ SPECIES As stated above, the decrease in the amount of the supported a-Fe,O, observed in the range 250-320 "C is attributed to one of the following processes: (a) the high dilution of Fe3+ into the bulk of the support or (b) the formation of a strongly associated, and thus difficult to reduce, Fe3+ species on the carrier surface.Evidence as to which process actually occurs can be obtained by comparison of curves (a) and (b) of fig. 3. Such a comparison clearly demonstrates that the change in the concentration of a-Fe203 is not correlated with the change in the dispersion of Fe3+.P. POMONIS, D. VATTIS, A. LYCOURGHIOTIS AND CH. KORDULIS OL - Fe,O, Fe3'-S 2049 Fig. 5. Iron(rn) species present on the surface of the support for calcination temperatures > 320 "C. The absence of such a correlation in the range 250-320 "C suggests that process (a) does not occur. In fact, high dilution of Fe3+ in the range 250-320 "C should be shown by an abrupt decrease in the dispersion of Fe3+ in this temperature range, in disagreement with our experimental results.Thus it seems more probable that an increase in the calcination temperature from 250 to 320 "C promotes the formation of a strongly associated Fe3+ species on the surface of the support at the expense of supported a-Fe,O,. Brown et aZ.,18 working with iron catalysts, have proposed the formation of such a species to explain the appearance of a temperature-programmed- reduction peak at 950 "C. We denote this species as Fe3+-S (see fig. 5). Inspection of fig, 3(b) shows that a further increase in the calcination temperature does not cause a change in the concentration of supported a-Fe,O,. Additional but weak evidence for the occurrence of process (a) can be drawn from the constancy of the values of the binding energies.It thus seems reasonable to assume that a high dilution of Fe3+ into the alumina lattice would alter the Eb values, contradicting our experimental results. In view of the above considerations the decrease in the dispersion of iron(II1) on increasing the calcination temperature may, in principle, be attributed to one of the following processes : (a) solid-state transformation of a-Fe,O, into Fe3+-S, (b) surface diffusion of the loosely bound Fe3+ resulting in augmentation of the a-Fe,03 crystals or (c) an increase in the size of the Fe3+-S aggregates via surface diffusion of the strongly associated Fe3+. Careful examination of the results demonstrates that process (a) does not contribute to the decrease in the dispersion of Fe3+.In fact, this decrease takes place at a constant rate in the region 200-480 "C whereas the transformation of a-Fe,O, into Fe3+-S is complete at 320 "C. Moreover, since the rate of surface diffusion of the loosely bound Fe3+ must be higher than the rate of surface diffusion of the strongly associated Fe3+, the contribution of process (c) to the decrease in the dispersion is negligible. Based on the above considerations it seems reasonable to assume that process (b) is responsible for the decrease in the dispersion. Fig. 3(a) shows that the rate of this process increases at temperatures > 480 "C. The above does not exclude low dilution of the iron(n1) into the alumina lattice. INFLUENCE OF IRON(III) CONTENT ON THE PHYSICOCHEMICAL CHARACTERISTICS OF THE Fig.2(b) shows that the amount of supported a-Fe,O, remains almost constant in the region 0.173 < X < 0.615 and then decreases. This suggests that the formation of Fe3+-S at expense of a-Fe,O, is accelerated for X > 0.615. The absence of a discontinuity point on the smooth curve illustrating the drop in the dispersion with iron(m) content [fig. 2 (a)] implies that Fe3+ has a similar dispersion in both the a-Fe,O, and Fe3+-S phases. This is in excellent agreement with the conclusion stated above that the solid-state transformation of a-Fe,O, into Fe3+-S is not accompanied by a decrease in the dispersion of Fe3+. CATALYSTS2050 DECOMPOSITION OF N 2 0 ON Fe203/A1203 In line with the considerations mentioned above the decrease in the dispersion of Fe3+ is attributed to the increase in the size of the a-Fe203 crystals with increasing iron(II1) content.This is corroborated by the variation in the size of the a-Fe203 crystals with iron(rI1) content deduced from diffuse reflectance spectroscopy, magnetic measurements and X-ray spectroscopy. The activation energy of conduction [fig. 2(c)] shows a continuous decrease from pure alumina to X = 0.615, stabilising at ca. 1 eV. Bearing in mind the variation of the active-phase properties with iron(Ir1) content discussed above, the dependence of the Eg values on Fe3+ content can be interpreted in a simple way. The almost linear decrease of Eg up to X = 0.615 is justified because in this region the ratio of the Fe3+ ions distributed in the two supported iron(II1) species, i.e.a-Fe203 and Fe3+-S, is constant. Moreover, since the decrease in the dispersion of iron(II1) has no effect on the activation energy of conduction, an almost constant Eg value per mmol of Fe3+ is expected. The deviation from linearity is expected for X > 0.615 because in this range the above-mentioned ratio of Fe3+ ions is altered in favour of Fe3+-S, which is related to higher Eg values than a-Fe203. In conclusion, our electrical-conductivity results offer additional support for our view of the nature of the active phase of samples calcined above 320 "C. RELATIONSHIP BETWEEN PHYSICOCHEMICAL PROPERTIES AND CATALYTIC ACTIVITY Inspection of table 1 shows that the supported iron(rI1) ions are more active than the pure carrier.Moreover, from the smooth curve illustrating the drop in activity on increasing iron(rI1) content (fig. I), we obtain the following conclusions: (a) the semiconducting properties of supported iron(rr1) catalysts estimated by the activation energy of conduction do not govern the catalytic activity and (b) supported a-Fe203 and Fe3+-S have similar activity. If one of the above conclusions is not valid, then a discontinuity would appear in the curve at X = 0.61 5. The linear increase in activity with increasing dispersion of Fe3+ (fig. 4) suggests that the dispersion of the active phases is a key physicochemical parameter determining the catalytic activity. CONCLUSIONS In conclusion, we point out that the present study is a part of an attempt to obtain a better understanding of Fe203/A1203 catalytic ~ystems.l~-~~ The main results of this paper are as follows: (a) two species of iron(m), i.e.a-Fe203 and Fe3+-S, are formed on the carrier surface above 280 "C, and the ratio of the iron (111) species is constant from X = 0.172 to X = 0.61 5 mmol of Fe3+ per g of alumina, (b) Fe3+-S has a higher activation energy of conduction compared with a-Fe203, (c) an increase in the calcination temperature and iron(rI1) content causes a decrease in the dispersion of Fe3+ attributable to surface diffusion of the loosely bound Fe3+, resulting in an increase in the size of cc-Fe203 crystals, and (iv) the catalytic activity is determined by the dispersion of Fe3+ and not by the nature of the supported iron(II1) species nor by the semiconducting properties of the specimens.We thank the Services de Programmation de la Politique Scientific, Belgium for support (Ch. K.). A. Lycourghiotis and D. Vattis, React. Kinet. Catal. Lett., 1981, 18, 377. A. Lycourghiotis, D. Vattis, G. Karaiskakis and N. A. Katsanos, Rev. Chim. Miner., 1982, 19, 139. A. G. Keeman and R. D. Iyengar, J . Catal., 1966,5, 301. A. Cimino, R. Bosco, V. Indovina and M. Schiavello, J . Catal., 1966, 5, 271.P. POMONIS, D. VATTIS, A. LYCOURGHIOTIS AND CH. KORDULIS 205 1 M. L. Volpe and J. F. Ready, J. Catal., 1967, 7, 76. A. Cimino, V. Indovina, F. Pepe and M. Schiavello, Proc. 4th Int. Congr. Catal., Moscow, 1968, paper 12, p. 187. ' A. Cimino and V. Indovina, J. Catal., 1970, 17, 54. A. Cimino and F. Pepe, J. Catal., 1972, 25, 362. T. A. Egerton, F. S. Stone and J. C. Vickerman, J. Catal., 1974, 33, 299. lo T. A. Egerton, F. S. Stone and J. C. Vickerman, J. Catal., 1974, 33, 307. l 1 P. Pomonis and J. C. Vickerman, J. Catal., 1978, 55, 88. l 2 C. Defossk, P. Ganesson, P. G. Rouxhet and B. Delmon, J. Catal., 1978, 51, 296. l3 P. J. Angevine, J. C. Vartuli and W. I. Delgass, in Proc. 6th Int. Congr. Catal., ed. P . B. Wells and F. C. Tompkins (The Chemical Society, London, 1976), pp. 61 1-618. l4 S. C. Fung, J. Catal., 1979, 58, 454. F. P. J. M. Kerkhof and J. A. Moulijn, J. Phys. Chem., 1979,83, 1612. l6 M. Houalla, ScientiJic Basesfor the Preparation of Heterogeneous Catalysts, ed. G. Poncelet, P. Grange and P. A. Jacobs (Elsevier, Amsterdam, 1983), p. 273. l7 Ch. Kordulis, S. Voliotis, A. Lycourghiotis, D. Vattis and B. Delmon, Appl. Catal., 1984, 11, 179. R. Brown, M. E. Cooper and D. A. Whan, Appl. Catal., 1982,3, 177. l9 A. Andreev, E. Proinov, N. Neshev and D. Shopov, J. Catal., 1982, 74, 1. 2o F. F. Volkenshtein, Kinet. Katal., 1981, 21, 776. 21 M. Niwa, K. Yagi and Y. Murakami, Bull. Chem. Soc. Jpn., 1981,54,975. 22 B. Lover, J. Juhasz, K. Mihalyi and Z. G. Srabo, J. Res. Inst. Catal. Hokkaido Univ., 1980, 28, 223. 23 V. Perrichon, P. Turlier, J. Barrault, G. Forguy and J. C. Menezo, Appl. Catal., 1981, 1, 169. 24 D. Ying-Ru, Y. Qi-zie, H. Yuan-Fu, J. Yong-Shu and Q. Jin-Heng, Adv. Chem. Ser., 1981, 194, 609. 25 T. Tomov, D. Klissurski and I. Mitov, Phys. Status Solidi A, 1982, 73, 249. (PAPER 4/ 1630)

ISSN:0300-9599    

DOI:10.1039/F19858102043

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1985, 81, 2053-2065 Structural Study of Microemulsions of Glycerol Stabilised by Cetyltrimethylammonium Bromide Dispersed in Heptane + Chloroform Mixtures BY PAUL D. I. FLETCHER,? MOHAMED F. GALAL AND BRIAN H. ROBINSON* Chemical Laboratory, University of Kent, Canterbury, Kent CT2 7NH Received 24th September, 1984 Dynamic light scattering, viscometry and phase-study results are presented for cetyltrimethyl- ammonium bromide- (CTAB)-stabilised dispersions of glycerol in 50/50 v/v mixtures of n-heptane and chlorofom (glycerol-in-oil microemulsions). Up to ten moles of glycerol per mole of CTAB may be solubilised in 50/50 v/v heptane + chloroform mixtures at 35 "C. The resulting solutions consist of thermodynamically stable, discrete droplets of glycerol stabilised by the surfactant.The droplet size depends primarily on the mole ratio (R) of glycerol to CTAB according to the following equation: hydrodynamic radius/nm = 1.8( & 0.4) + 1.2( f 0.2)R. The microemulsion phase separates into two phases if the temperature is decreased below a lower transition temperature or raised above an upper transition temperature. Attractive interactions between the droplets increase as the upper transition temperature is approached, but the droplet size remains constant. As the lower transition temperature is approached the droplets increase in size. This is attributed to loss of CTAB from the glycerol/oil continuous- solvent interface. Droplet swelling is also observed at very low surfactant concentrations. Viscosity data tentatively suggest that the extent of solvation of microemulsion particles increases with decreasing temperature or that the particles may be slightly non-spherical at low temperatures.The structural data are discussed with reference to the mechanisms leading to phase separation of the microemulsion system. In a recent paper1 we have reported results of a structural study of solutions of glycerol dispersed in n-heptane and stabilised by the anionic, branched-alkyl-tail surfactant Aerosol-OT (AOT). In the present study the surfactant used was the cationic, straight-chained cetyltrimethylammonium bromide (CTAB) in a mixed- solvent system of equal volumes of heytane and chloroform. This was necessary in order to produce microemulsions in a temperature region close to ambient.The previous studies1 revealed the presence of attractive interparticle interactions in the region of the upper transition temperature for that system. Unfortunately the study was limited to that region of the microemulsion stability map which is close to the upper-temperature transition. [The lower-temperature transition observed in the corresponding water system (i.e. AOT + H,O + n-heptane) was assumed to be present but at temperatures too low for study with our equipment.] In the CTAB system both the upper- and lower-temperature transitions are easily accessible experimentally. The aim of the present study was to investigate the structure of the microemulsion 7 Present address: Department of Chemistry, The University, Hull HU67 RX. 20532054 STRUCTURAL STUDY OF CTAB-STABILISED MICROEMULSIONS 10 R 5 icr oe mulsion single phase 0 20 L O 60 TI" C Fig.1. Microemulsion phase-stability map of the CTAB + glycerol + heptane + chloroform system. [CTAB] = 0.1 mol dm-3. solutions and also to measure the changes in particle structure and interparticle interactions that occur prior to phase separation at both the lower- and upper- temperature transitions. EXPERIMENTAL Cetyltrimethylammonium bromide was obtained from Sigma. n-Heptane (B.D.H.) was distilled over sodium, stored over type-4A molecular sieve and filtered prior to use. Chloroform (May and Baker, pronalys grade) was used as received, containing 1.5% by volume of ethanol as stabiliser. Glycerol was obtained from Fisons and used as received.Microemulsions of a particular mole ratio R (= [glycerol]/[CTAB]) were prepared by weighing the required quantity of glycerol into a volumetric flask. A known concentration of CTAB in the solvent n-heptane+chloroform (50/50 v/v) was added, making up to the mark with the solvent mixture. On shaking the mixture vigorously by hand either a clear or cloudy solution was obtained at room temperature according to the mole ratio (R) of glycerol to CTAB (fig. 1). Sonication accelerated microemulsion formation but did not affect the final equilibrium size distribution of the droplets. Precautions were taken to minimise changes in solution composition (due to solvent evaporation etc.) during the preparation procedure. Measurements were also made using chloroform containing smaller and larger amounts of ethanol. Qualitatively similar results were observed for solutions in which the chloroform contained 0-3% v/v ethanol.The effect of ethanol is to cause small shifts (0-10 "C) in the observed microemulsion phase stability region. At 6 % ethanol the microemulsion particles showed an increase in size, which was attributed to partitioning of the ethanol to the cores of the microemulsion particles. For measurements in the absence of ethanol, the ethanol was removed from the chloroform by using the procedure of ref. (2). Since the behaviour of the microemulsion systems was unchanged for different concentrations of ethanol (after allowance for the temperature shifts) the bulk of the results were obtained using untreated chloroform. The dynamic light-scattering apparatus and data-analysis procedures have been described previously.1 In such measurements it is necessary to minimise scattering perturbations due to the presence of dust in the samples.Microemulsions which were stable at room temperature were filtered through Millipore 0.22pm filters on injection into the Hellma measuring cell.P. D. I. FLETCHER, M. F. GALAL AND B. H. ROBINSON Table 1. Temperature of maximum dispersed-phase solubilisation system T/"C CTAB + H,Oa CTAB + Glycerola AOT + H,Ob AOT + glycerolb 15 35 9 < O 2055 a Solvent = 50/50 v/v heptane + chloroform. b Solvent = n-heptane. Microemulsions which were not stable at room temperature were not filtered in this way, but were left in the scattering cell for a few hours at the required temperature prior to making measurements. For some of the systems under study, the microemulsions were not stable at room temperature and attempts to pre-filter the (heated) solutions led to composition changes.For this reason solutions were not filtered before measurements were made, but care was taken to ensure that any anomalous signals arising from the presence of dust were rejected by computer control of the instrument. Phase stability was determined by visual inspection of the solutions and was taken as the temperature at which a sharp turbidity change occurred on slowly raising or lowering the temperature. Stability of the microemulsion solutions was further checked by the time- independence of the dynamic light-scattering behaviour or the turbidity determined using a Unicam SP8-200 spectrophotometer. Viscosities of the heptane +chloroform mixtures at different temperatures, obtained in connection with the analysis of light-scattering data, were determined using a calibrated Ubbelohde viscometer.The same viscometer was used for the determination of intrinsic viscosities. Refractive indices were determined using a thermostatted Abbe refractometer. Densities were measured using a standard pyknometer. RESULTS AND DISCUSSION MICROEMULSION STABILITY Fig. 1 shows the amount of glycerol that may be solubilised as a single-phase oil-continuous-microemulsion system in CTAB solutions of n-heptane + chloroform (50/50 v/v) as a function of temperature. The results are expressed as a plot of the molar ratio of glycerol to CTAB (R) against temperature.The area under the curve is the thermodynamically stable, clear-single-phase microemulsion region. A maximum R value of ca. 10 is observed at ca 35 "C. The figure shows the results for 0.1 mol dm-3 CTAB, but the stability region as defined by plots of R against T alters only slightly over the CTAB concentration range 0.02-0.2 mol dm-3. The lower-temperature phase transition shifts a few degrees to higher temperatures as the concentration of CTAB is decreased. No systematic variation of the upper- temperature phase transition is seen. It is useful to compare the effect of substituting glycerol for water in the various surfactant systems. They may be compared by noting the temperature at which a maximum mole ratio R of dispersed phase (water or glycerol) to surfactant (AOT or CTAB) is observed. Table 1 shows the results for the systems studied.The differences noted for the two different surfactants are presumably related to the energetics of solvation of the surfactant headgroups by water and glycerol, respectively. The separate phases produced at temperatures below the lower transition temperature and above the upper transition temperature were investigated. Solutions corresponding2056 STRUCTURAL STUDY OF CTAB-STABILISED MICROEMULSIONS in overall composition to 0.1 mol dm-3 CTAB (R = 7) were allowed to phase-separate at 15 and 55 "C, respectively. In each case two phases were produced, the compositions of which were determined using dynamic light scattering, refractive-index and density methods and by weighing residues left after solvent evaporation. To determine the composition of the separate phases the following procedure was adopted.Dynamic light-scattering measurements were made, on systems diluted as appropriate, to determine the droplet sizes (correlation lengths). Substitution into eqn (6), vide infra, gave an approximate value for R. After evaporation of solvent from a 1 cm3 sample of each phase, the weight of residual material (i.e. weight of CTAB+weight of glycerol) was obtained. The weight of CTAB in 1 cm3 was determined using the equation wt CTAB - MCTAB wt residual material - RM,,, + M,,,, where M is the molecular weight. From this value the concentration of CTAB was obtained. New solutions of the same compositions were then prepared and the refractive index and density were measured.These values were found to be consistent with those of the original samples. After one day at 55 "C the system separated into two clear phases. The upper phase (ca. 2/3 of the total volume) was shown to be ca. 0.012 mol dmP3 CTABIR = 7. The lower phase (ca. 1/3 of the total volume) was found to be ca. 0.3 mol dm-3 CTAB (R = 7). Thus the upper-temperature transition causes the microemulsion to separate into an upper dilute and a lower concentrated phase of glycerol droplets. The structures present in the separated phases are similar to those present at room temperature. After one day at 15 "C the solution separated into a clear upper phase (ca. 95% of the total volume) and a milky, viscous lower phase (ca. 5%).The upper phase was found to be 0.1 mol dm-3 CTAB (R = 3.5). The lower phase was therefore virtually pure glycerol. The lower-temperature transition is very different from the upper and involves the separation and sedimentation of some of the glycerol, leaving an upper phase containing essentially reversed micelles with a reduced amount of glycerol. As will be shown, the nature of the separated phases (A-D in fig. 1) is consistent with the interpretation of the temperature dependence of the structural data measured from solutions within the single-phase microemulsion region. DYNAMIC LIGHT-SCATTERING RESULTS From measurements of the intensity autocorrelation function (g(2)) as a function of delay time (t), values for the measured collective diffusion coefficient (D) and the corresponding correlation length ( I ) were obtained by means of eqn (2) and (3) D = z / 2 P (2) I = kT/6nqD (3) in which the scattering wavevector K = (4nn/;l) sin (8/2), where n is the refractive index of the medium, ;l is the wavelength of the incident light and 8 is the scattering angle.In eqn (2) and (3) q is the solvent viscosity and z is the limiting decay rate given by -d In (g(2))/dt as t -, 0. The measured correlation length, in general, contains a contribution from inter- particle interactions and can be equated with the particle hydrodynamic radius for the case of a spherical particle only in the limit of infinite dilution. Fig. 2 shows the measured correlation lengths for 0.1 mol dm-3 CTAB solutionsP. D. I . FLETCHER, M.F. GALAL AND B. H. ROBINSON 2057 20 40 60 T/"C Fig. 2. Correlation length plotted as a function of temperature. [CTAB] = 0.1 mol dmV3. R values: (a) 1.0, (b) 3.0, (c) 5.0, ( d ) 7.0, (e) 9.0. 20 Ec --. 5 M a, E: - .- Y - a, 10 I 1 I 0 0.1 0.2 Fig. 3. Correlation length plotted as a function of CTAB concentration. In ascending order the lines refer to R values of 1, 3, 5 and 7. T = 45.0 "C. [ CTAB]/mol dm -32058 STRUCTURAL STUDY OF CTAB-STABILISED MICROEMULSIONS 0 0.1 0.2 [CTAB]/mol dm-3 Fig. 4. Correlation length plotted as a function of [CTAB]: 0, 45 and 0, 30 "C. R = 7. of different R values as a function of temperature. The observed correlation lengths increase with R and with increasing temperature. At low CTAB concentrations (< 0.1 mol dm-3) an increase in correlation length is also observed as the temperature is decreased close to the low-temperature phase boundary.In order to separate the effects of particle size and interparticle interactions it is necessary to make measurements over a dilution series of the particles to obtain the infinite dilution limiting value. The data may then be analysed according to the following equation (for 4 < 1) D = Do(l +a4) (4) where Do is the infinite-dilution value of the diffusion coefficient, qi is the volume fraction of dispersed material (CTAB and glycerol) and a is a parameter reflecting any interparticle interactions. Fig. 3 shows the variations of correlation length with CTAB concentration for various R values. The figure refers to 45.0 "C. The variation is more pronounced at 55 "C but no significant variation is observed at 30 "C.These data were used to obtain values of the infinite-dilution limiting value of the correlations length (equal to the hydrodynamic radius for spherical particles) and the interaction coefficient a. An assumption made in this analysis is that the particle composition is independent of the dilution procedure (i.e. in this case depends only on R and not on the CTAB concentration). For any colloidal system where the dispersed particles are fluid, this assumption requires justification. In the present case the glycerol may partition between the particle central cores and a state of solution in the continuous-oil solvent. The solubility of glycerol in the solvent 50/50 v/v heptane +chloroform in the absence of CTAB was measured as < 0.65 mmol dm-3 over the temperature range 0-55 "C.The procedure was as follows: known weights (5, 10, 15, 20 and 25 mg) of glycerol were introduced into 100 cm3 volumetric flasks and filled with the solvent. The flasks were then tightly stoppered to prevent solvent evaporation. The samples were immersed in a constant-temperature bath (at different temperatures) and shaken regularly. The samples were checked for homogeneity by visual inspection and in this way an estimate of the solubility of glycerol in the solvent could be obtained. Since the lowest concentration of glycerol used in the present study was 20 mmol dm-3, the glycerol may be assumed not to partition from the particle to the solvent to any significant extent.Secondly, the possibility exists that the CTAB may partitionP. D. I. FLETCHER, M. F. GALAL AND B. H. ROBINSON 2059 0 0 0 0 0 'O. 0 0 I I I 20 30 40 50 T f "C *lL 0 20 30 Tf'C 40 0 Fig. 5. (a) Correlation length plotted as a function of temperature for 0.05 mol dm-3 CTAB / n -I\ m.11 3 * 1 . > . 1 I . 1 * * .* 1 C l t I * * = 1 ) : riiiea cirices represent aara coiiecrea on increasing me remperarure, unniiea circles apply to decreasing temperature. (b) Normalised variance for the same solution. between the glycerol/oil interface and the oil solvent. This would be expected to be more significant at very low concentrations of CTAB, i.e. in the concentration range of the solubility of monomeric CTAB in the oil solvent (analogous to a critical micelle concentration in aqueous solution).Any such partitioning of the surfactant to the oil solvent would lead to a decrease of the glycerol/oil interfacial area, i.e. an increase in size of the droplets. Such behaviour is indeed observed at very low CTAB concentrations and at temperatures close to the lower transition temperature. This is shown in fig. 4 and 5. Fig. 4 shows the variation of measured correlation length with CTAB concentration for R = 7 at 30 and 45 "C. At 30 "C the droplets swell at low concentration, but over a wide concentration range (at higher concentrations) the correlation length remains approximately constant. At 45 "C no significant swelling is observed but the correlation length increases at higher concentrations, showing the presence of interparticle attractions.Because of the two competing affects, the accuracy of the determinations of Do and a are limited. Data of the type shown in fig. 4 and 5 may be analysed using the equation to yield the apparent concentration of CTAB which is not interfacially bound at the glycerol/oil interface. [CTAB],,,, is the concentration of CTAB that is not inter- facially bound to the glycerol particles and [CTAB], is the weighed-in concentration of CTAB. Im and Z, are the measured correlation length and the particle hydrodynamic radius, respectively, when all the surfactant is bound and t is the thickness of the interfacial surfactant layer. Values of the free CTAB concentration estimated using eqn ( 5 ) range from 18 mmol dm-3 at 22 "C decreasing to 2 mmol dm-3 at 30 "C for [CTAB] = 50 mmol dm-3 (R = 7).The temperature dependence, expressed as R( d ln([CTAB],,,,)/d( 1 / T)), is shown in fig. 6. A linear plot is obtained from which2060 STRUCTURAL STUDY OF CTAB-STABILISED MICROEMULSIONS a value of ca. 190 kJ mol-1 may be obtained for the heat of adsorption. Values for other R systems lie in the range 150-200 kJ mol-l. These values imply that the adsorption of the surfactant from the ‘free’ state (the nature of which is unknown but which probably involves reversed micelles) to the interfacially bound state is a strongly endothermic process. To summarise : the increased correlation lengths observed at low CTAB concentrations and at temperatures close to the lower phase transition are attributed to an exothermic desorption of surfactant from the glycerol/oil interface, probably to form low-radius aggregates containing little glycerol.It should be emphasised that the postulate concerning the presence of non-interfacial surfactant is a rationalisation of the data only. There remains as yet no direct experimental evidence for the existence of such species. Indeed, it may be that the process is better described as an increase in polydispersity as the lower temperature boundary is approached; the apparent increase in average size in the light-scattering experiment being the result of the extreme weighting towards the largest particles. There is, however, no qualitative difference between the two descriptions, the first simply being an extreme case of the second. Some indirect evidence for non-interfacial surfactant at high R values has been observed in the AOT + water + heptane system.From observation of the hydrolysis rates of the surfactant it appears that some fraction of the AOT is unavailable for attack by aqueous base. This may be attributed to non-interfacially bound ~urfactant.~ Fig. 7 shows the value of the limiting correlation length as a function of R and as a function of temperature. The data are described by the equation limiting correlation length = hydrodynamic radius/nm = 1.8 k0.4 + 1.2 0.2(R). (6) If it is assumed that particles are spherical and monodisperse and also that all of the CTAB is located at the interface, then from considerations of the dispersed material volume and interfacial area we obtain r = 3 Vglycerol R / ~ C T A B where r is the radius of the glycerol core of the particle, Qlycero, is the volume of a glycerol molecule (0.121 nm3) and aCTAB is the apparent interfacial area per CTAB molecule.The slope of fig. 7 may be equated with (3V,1,,,,,,/aC,A,) to give an apparent value of aCTAB. A value of 0.3 1 f 0.04 nm2 is obtained by this method and may be compared with a value of 0.41 kO.07 nm2 obtained for the double-tailed surfactant AOT in ‘ glycerol-in-oil’ microemu1sions.l The intercept value of the plot shown in fig. 7 is the Stokes radius of a reversed micelle of CTAB containing no glycerol. The value obtained, 1.8 k0.4 nm, is a reasonable value for CTAB. (The all-trans configuration length is calculated to be ca. 2.8 nm.) Also the measured interfacial area multiplied by the measured length of the molecule yields a value of the molecular volume of 0.56 0.15 nm3. This is in reasonable agreement with the value of 0.605 nm3 calculated from the density.It therefore appears that the sizes of the microemulsion particles are temperature independent away from the low- temperature transition and are well described by the simple model of eqn (6). The interfacial area of the single-chained surfactant CTAB is considerably less than that of the double-chained molecule AOT. Fig. 8 shows the variation of the interaction coefficient a [from eqn (4)] as a function of R for three different temperatures. a becomes more negative as R or the temperature increases. For incompressible hard spheres a is thought to be 1.5, whilst it is predicted to beP.D. I. FLETCHER, M. F. GALAL AND B. H. ROBINSON 206 1 4.2 3 . 4 1 I I 3.31 3.35 3.39 103 KIT Fig. 6. Van’t Hoff plot for the low-temperature phase transition. 0.05 ml dm-3 CTAB. (R = 7). I’ 0 0 1 3 5 7 9 R Fig. 7. Infinite-dilution limiting values of the correlation length plotted as a function of R. Symbols refer to the following temperatures: 0, 30.0; +, 45 and 0, 55 “C. negative in the case of attractive interactions.* Attractive interactions may be produced by van der Waals forces, by an induced dipole over the complete colloidal particle caused by ion movement within the particle cores or by an intermingling of the surfactant interfacial layers on particle contact. There is also the possibility of ‘fusion’ of droplets in a small fraction of collisions between particles.The ‘dimer’ thus formed would be expected to be relatively long-lived (as compared with the water dimer).2062 STRUCTURAL STUDY OF CTAB-STABILISED -81- MICROEMULSIONS R O r 4 Fig. 8. Plot of a against R for various temperatures: 0, 30; +, 45 and a, 55 "C. The picture of the microemulsion solutions that emerges from the light-scattering study is the following. On increasing the temperature, the particles remain the same size as the upper-temperature limit is approached. Increasing the temperature causes an increased inter-particle attractive force. At the upper transition temperature the solution phase separates into high and low concentration phases, each containing particles of approximately the original size. Decreasing the temperature to approach the lower phase transition produces a swelling of the particles which is caused by desorption of the surfactant.This is also observed at very low concentrations of dispersed material, and this phenomenon accounts qualitatively for the CTAB concentration dependence of the microemulsion phase stability map. The type of phase separation observed at the lower-temperature transition is consistent with this interpretation of the data. It appears that the stability of the microemulsion phase may be rationalised in terms of the balance of affinities of the CTAB for the oil solvent and the dispersed phase. At low temperatures the surfactant has a high affinity for the oil solvent and a low affinity for the dispersed glycerol phase. The surfactant is then unable to stabilise the glycerol/oil interface and the microemulsion is consequently unstable.At temperatures higher than the upper transition the surfactant has a high affinity for the glycerol but low affinity for the oil solvent. The consequence of this is that the microemulsion particles are poorly solvated by the oil and so tend to cluster (i.e. to exhibit attractive interactions) rather than to remain isolated, oil-solvated particles. The microemulsion is stable only at temperatures intermediate between these two extremes where the affinity of the surfactant for the high and low polarity regions is the correct balance. The observed behaviour of the system is thus rationalised in terms of a consistently decreasing oil affinity (or increasing glycerol affinity) of the surfactant with increasing temperature.Information is contained within the dynamic light-scattering data concerning the polydispersity of the scattering particles. For a low concentration of monodisperse spherical particles the autocorrelation function of the scattered light intensity should decay exponentially. For a polydisperse system the decay deviates from exponentiality. For the case of homogenous particles (i.e. constant refractive index) our analysis ofP. D. I. FLETCHER, M. F. GALAL AND B. H. ROBINSON 2063 the data yields a size that may be equated with the z-averaged size.5 Microemulsion particles consist of a core of one substance and an annular shell of surfactant, and hence are not optically homogenous, so a more complex average is obtained.Note6 that the refractive-index increment of microemulsion particles containing aqueous cores may vary from positive to negative as a function of R, and therefore different microemulsion systems are expected to yield widely varying types of average of the size distribution as measured by this technique. For the case of the systems under study it is estimated that something between a weight- and z-average size is measured. The normalised variance of the computer-fitted autocorrelation decay curves normally was found to be in the range l-lO%. The variance can arise from causes other than polydispersity, e.g. poor choice of baseline or scattering from dust in the solution. Hence use of the variance as a measure of polydispersity is not very reliable. The magnitudes of the observed values are similar to those observed in measurements (using the same experimental apparatus) of the system AOT + water + heptane.For that system, the polydispersity measured by small-angle neutron scattering is found to be o,/K M 0.3 (0, is the root mean-square deviation from the mean size R). Hence it is concluded that polydispersity in the present system is of the same order. In most cases the variance does not show any systematic trends within the estimated errors. However, the inset of fig. 5 shows the measured values for 0.05 mol dm-3 (R = 7) as a function of temperature. The variance becomes significantly larger as the lower-temperature phase boundary is approached. This is in agreement with the size-distribution interpretation discussed earlier.VISCOSITY RESULTS The viscosity of dilute colloidal solutions is sensitive to dispersed particle shape and interparticle interactions but not particularly sensitive to particle size. It therefore provides useful complementary information to dynamic light-scattering studies. We have measured viscosities of microemulsions as a function of CTAB concentration at various R values and temperature. The data were analysed according to the equation where C is the concentration (in g cmP3) of dispersed phase, qsp is the specific viscosity { = [q(solution)-q(solvent)]/q(solvent)}, k , is the Huggins coefficient and [q] is the intrinsic viscosity. Fig. 8 shows plots of qsp/C against C for R = 3 microemulsions at 30 "C and at 45 "C. Values of [q] are obtained from the intercepts and values of k , are obtained from a combination of the initial slopes and the intercepts.The accuracy of these determinations is limited, probably owing to evaporation of the solvent during the course of the experiment, especially at 45 "C. Table 2 gives an indication of the parameters determined for the various R values and temperatures. Although the uncertainties are high, consideration of all the data suggests that the intrinsic viscosities decrease with increasing temperature and with increasing R value. The values of Huggins coefficient are again determined with low accuracy but they have a tendency to increase with increasing R and temperature. The intrinsic viscosity is dependent on both particle shape and solvation as shown (9) by the equation where B is the partial specific volume of the subscripted species, 6 is the weight of the solvent associated with 1 g of particles and v is a shape parameter, which is equal to 2.5 for sphere~.~ [tit] = 40particle + dosolvent)2064 STRUCTURAL STUDY OF CTAB-STABILISED MICROEMULSIONS 0 0.05 0.1 0 .15 Clg ~ r n - ~ Fig.9. Reduced-viscosity plot for R = 3: 0, 45 and 0, 30 "C. Table 2. Values of [q] and k, are derived from viscosity data 1 30 6.3 f 1 .O 0.2 f0.6 3 30 5.5 f 0.6 0.6 f 0.6 5 30 4.5 f 0.8 1 f 1 1 45 5.0 f 3.0 I f 1 3 45 3.5 f 3.0 4f4 5 45 3.0f 1.5 5 + 5 Partial specific volumes were calculated from our measured density data. (8-lCTAB = 0.998 f 0.03 g ~ m - ~ ; rlglycerol = 1.2 & 0.1 g ~ m - ~ and rlsolvent = [( 1.102 & 0.005) - (1.19 f 0.03) TI where Tis the temperature in "C.If it is assumed that the particles are unsolvated (i.e. 6 = 0) then the shape parameter v ranges from 3 to 7, which corresponds to axial ratios from near unity to 5 ; the particles appear close to spherical at the higher R values and temperatures. Alternatively if v is assumed to equal 2.5 (i.e. the particles are spherical) then 6 ranges from 0.3 to 1.7, the highest degree of solvation being found at lowest temperatures and R values. The viscosity data are therefore not inconsistent with the hypothesis that the microemulsion particles become less solvated as the temperature is increased, as was postulated in the discussion of the dynamic light-scattering results. The tendency of the CTAB microemulsion particles to show a high degree of solvation or alternatively to adopt non-spherical shapes contrasts with the AOT + glycerol and AOT + water systems. AOT is a double-tailed surfactant and is 'wedge- shaped'; hence it is expected to pack well in a spherical annular shell. This is not true for CTAB which, being a single-tailed amphiphile, might be expected to have voids in the interfacial region allowing the entrapment of larger amounts of solvent or alternatively the adoption of a non-spherical shape.P. D. I. FLETCHER, M. F. GALAL AND B. H. ROBINSON 2065 Theoretical estimates for Huggins coefficient in the case of hard spheres range from 0.7 to 1.0. Interactions, either attractive or repulsive, lead to an increased value.* Again, taking all the data into consideration, the values of k , suggest increased interactions as the upper-temperature transition is approached, in accordance with the behaviour of the a coefficient. We thank the S.E.R.C. and Tate and Lyle Research for financial support of this project. P. D. I. Fletcher, M. F. Gala1 and B. H. Robinson, J. Chem. SOC., Faraday Trans. I , 1984,80, 3307. A. J. Gordon and R. A. Ford, The Chemist’s Companion (Wiley Interscience, Chichester, 1972), p. 432. P. D. I. Fletcher, A. M. Howe, N. M. Perrins, B. H. Robinson, C. Toprakcioglu and J. C. Dore, Surfactants in Solution, ed. K. L. Mittal and B. Lindman (Plenum, New York, 1984), vol. 3, p. 1745. A. M. Cazabat and D. Langevin, J. Chem. Phys., 1981, 74, 3148. D. E. Koppel, J. Chem. Phys., 1972, 57, 4814. M. Zulauf and H-F. Eicke, J . Phys. Chem., 1979,83,480. W. J. Russell, J. Chem. SOC., Faraday Trans. 2, 1984, 80, 31. ’ C. Tanford, Physical Chemistry of Macromolecules (Wiley, New York, 1961). 68 (PAPER 4/ 1645) FAR 81

ISSN:0300-9599    

DOI:10.1039/F19858102053

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans, 1, 1985, 81, 2067-2082 Molecular Diffusion in Monolayer Films of Water Adsorbed on a Silica Surface BY JONATHAN W. CLARK AND PETER G. HALL Department of Physical Chemistry, University of Exeter, Exeter EX4 4QD AND ALAN J. RDDUCK Materials Centre, Director of Quality Assurance, Royal Arsenal East, London SE18 6TD AND CHRISTOPHER J. WRIGHT*? Materials Physics & Metallurgy Division, AERE Harwell, Didcot, Oxfordshire OX1 1 ORA Received 1 1 th October, 1984 The dynamics of water adsorbed in near-monolayer films at the surface of silica has been measured and characterized by quasielastic neutron scattering. The analysis shows the coexistence of two phases of sorbed water molecules with different dynamics. One component is immobile on the experimental timescale, while the other has a two-dimensional diffusion coefficient of ca.6 x 10-lo m2 s-l and a residence time during reorientation of 4 x s. The precise details of the reorientation process in the mobile phase are still unclear. Modelling the surface structure of microcrystalline or amorphous solids and the structure and dynamics of their adsorbates is an important goal in surface science. One such material is amorphous silica, and the interactions of its surface with water have been investigated by numerous workers because of the widespread use of this material as a drying agent, catalyst support and filler. Much of the reported experimental evidence concerning the adsorption of water is consistent with the concept that at low partial pressures individual water molecules adsorb at pairs of surface hydroxyl groups.Differential heats of adsorption of water on various silicas are ca. 50 kJ mol-1 at ON* = 0.5, which can be shown to be generally consistent with the double hydrogen bonding of sorbed water molecules to pairs of surface OH groups of favourable geometry. Inelastic-neutron-scattering experiments2 have shown that the librations of adsorbed water can also be explained on this basis. The near-infrared spectra of water adsorbed on silicas at low coverages show only small departures from the spectrum of critical water at 5300 cm-l, the position of the unperturbed v,+ v2 combination band.3q4 As a consequence, therefore, water must adsorb so that its oxygen atoms form new hydrogen bonds with the surface.At higher coverages, but still sub-monolayer, the same evidence suggests that some water molecules bond through their hydrogen atoms as water clusters begin to form. Direct measurements of the dynamical behaviour of adsorbed water at a silica surface are, however, incomplete. Translational diffusion coefficients, DT, of adsorbed water at room temperature have been measured by spin-echo magnetic resonance techniques and at low coverages, 1.0 > ON* > 0.5, were found5 to be Wycombe, Buckinghamshire HP12 3QR. -f Present address: RHM Research Ltd, The Lord Rank Research Centre, Lincoln Road, High 2067 68-22068 DIFFUSION OF WATER ON SILICA (3-5) x loplo m2 s-l. In contrast unequivocal and quantitative information about the reorientation of such molecules has not been obtained although an attempt has been made to obtain their dipole correlation function by analysing the width of the v, + v2 band in the infrared spectrum of adsorbed water.In this paper we report results obtained with quasielastic-neutron-scattering techniques, since they provide insight into the translational and rotational diffusion behaviour of adsorbed molecules leading to important structural information relevant to the surface-bonding models discussed earlier. The experiments have been performed on a well characterized, high-surface-area silica called Spherisorb. This material and its interactions with water have been investigated by inelastic2 and quasielastic6 neutron scattering and by diffra~tion,~ in addition to measurements of its pore and surface properties.* It has a very narrow pore-size distribution created by the close packing of spherical particles.As a consequence, therefore, the micropore volume fraction is very small and all the water molecules in the first monolayer can be assumed to be adsorbed at a single surface where the adsorption potential is unperturbed by that of any neighbouring surface. EXPERIMENTAL MATERIALS The silica samples were of the Spherisorb type and their physical properties have been documented elsewhere.27 The preparations of the degassed samples and those equilibrated with water were also identical to those which have been described before., Details specific to the samples examined in this work are collated in table 1. The coverage, 8oH, is the ratio of adsorbed water molecules to surface OH groups given a surface OH group concentrations of 4.6 nm-2 and an N, B.E.T.surface area of 232 m2 g-1.2 8Hz0 is the coverage related to the water B.E.T. monolayer concentration of 0.021 & 0.001 g g-l obtained from water isotherm measurement at 302 Ks This B.E.T. plot was linear over the range of PIP, = 0.05 + 0.4 with a c constant of 12&4. ON* is the proportion of the N, B.E.T. surface area covered by water assuming each water molecule occupies 0.108 nm2. The adsorption experiments could not be performed in situ because of the time (ca. 24 h) required for equilibration to take place. Consequently the sample cans were selected to be sufficiently well matched that the sample volumes were identical to within 3%. In all cases the time between sample preparation and irradiation was sufficient for equilibrium to be attained.NEUTRON DIFFRACTION Diffraction experiments were performed on silica samples encapsulated in 12 mm diameter vanadium cans on the 'guide-tube' diffractometer at AERE Harwell using an incident wavelength of 4.7 A. QUASIELASTIC NEUTRON SCATTERING The quasielastic-scattering data were obtained at the Institut Laue-Langevin, Grenoble using the time-of-flight spectrometer INS. The instrument was used with an incident wavelength of 6.1 A and an elastically scattered energy resolution with f.w.h.m. of 80 peV. Each sample was positioned in transmission geometry at an angle of 35 f 1" to the incident beam in a helium-filled container at room temperature. The detectors were grouped in sixes such that the angular range covered by each group was 4" in 28.The mean 28 values of each group were 30, 44.3, 63.5, 83.5, 104 and 128". The raw IN5 data were initially treated by subtracting the container and degassed silica scattering (Is+c) from that of the sorbed water samples (Is+w+c) according to Iw = Is+,+, - 4 + c T,+w+,/T,+cTable 1. Details of the preparation of Spherisorb samples quantity outgassing of water coverage conditions scattering (%) adsorbed mean thickness equilibrium, sample /g g-l OoH &IzO ONz T/K t/h of sample/mm total fromH,O P/POC - 1 0 0 0 0 3 53 20 2.97 8.5" - - 2 0.020 0.6 1.0 0.3 353 20 2.96 12.0" 3.5" 5.2b 0.20 3 0.029 0.9 1.4 0.45 353 20 3.04 13.4" 5.1" 6.6b 0.38 4 0.040 1.3 1.9 0.6 353 20 3.00 15.2" 6.7" 8.0b ca.0.55 "Calculated ; bobserved ; ccalculated from isotherm. X r F s ?2070 DIFFUSION OF WATER ON SILICA where the variables T,,,,, and T,,, were the theoretical transmissions of the samples with and without sorbed water, respectively. This was performed on data which had been normalised to incident neutron flux and corrected for counter efficiency, using the ILL program CROSSX. A 2 mm thick slab of vanadium was used to record the experimental resolution function. THEORY For water, the incoherent scattering from lH so dominates the cross-section that only the incoherent contribution to the scattering intensity needs to be considered. Therefore we have where fiQ is the neutron momentum transfer, Q is defined as k,-k, k, and k are the incident- and scattered-neutron wavevectors, fiw is the neutron energy transfer, agC is the incoherent cross-section of the protons, S(Q, cu) is the symmetrised scattering law and is an experiment-independent quantity related to the self-correlation function for diffusive motions, GJr, t), by Fourier transformation in space and time:9 Sinc(Q, 0) = - JI G,(r, t) exp [i(Qr - cut)] dr dt.(3) 2z As is usual in quasielastic neutron scattering, the Q range and quality of data is insufficient to allow calculation of Gs(r, t) by numerical transformation. Hence quasielastic data are interpreted by comparing Sinc(Q, LO) derived from experimental data from isotropic powder systems using eqn (2) with theoretical forms derived from models for G,(r, t). Rotational and translational diffusive motions of lH nuclei will both contribute to the quasielastic broadening. For analysis to be tractable, it is assumed that both these two motions are uncorrelated. GJr, t ) is then factorisable, and the total quasielastic scattering law can be expressed as a convolution product: (4) where exp ( - Q2( u2)), the Debye-Waller factor, and B, the level of cu-independent background, represent the contribution from vibrational motions, and 3( u2) is the total mean-square vibrational amplitude of lH nuclei if the vibrations are isotropic.Sinc(Q7 0 ) = ~ X P (- Q2(u2>) [Xk?YQ7 0) x sfnotC(Q, W ) + BQ21 Models for Strans(Q, cu) and Fot(Q, cu) can now be considered. TRANSLATIONAL DIFFUSION The simplest model for translational diffusion is that of continuous isotropic diffusion which leads to a Lorentzian-shaped scattering law: 1 - Y(DQ2, 0).1 DQ2 ~ c u ~ + ( D e ~ ) ~ z qE:ns(Q, W) = - The translational diffusion coefficient D can be obtained by plotting the width of the quasielastic broadening against Q2 at small Q values (i.e. for QL < 1, where L is the diffusion step length). Another model which leads essentially to the same result for small Q is the isotropic jump-diffusion model,lo where the diffusion coefficient is related to the jump distance L by D = L2/6ti, where ti is the mean time between jumps. At higher Q (such that QL 2 1) the Lorentzian broadening reaches a limit at a f.w.h.m. of 2fi/t;. For two-dimensional jump diffusion the form of the scattering law will alsoJ. W. CLARK, P. G. HALL, A. J. PIDDUCK AND C. J.WRIGHT 207 1 approximate to a Lorentzian if its f.w.h.m. is small compared with the resolution width. In this case the f.w.h.m. is 2?i/ti [ l -Jo(QL)],ll where Jo is the cylindrical Bessel function of zeroth order. The true form of the scattering law for powder averaged continuous diffusion in two dimensions is $i:ns(Q, LO) = - i,” LY(D2D sin2 0, LO) sin 0 d 0 (6) 27r which is more peaked around LO = 0 than a Lorentzian. LOCALIZED ROTATIONAL MOTION The scattering law for a localized rotational process can be written generally as12 1 N-1 qgk(Q, 0) = Ao(Qa) &LO) + C An(Qa) (G1, 0) (7) n-1 where, for powder-averaged, uniaxial, jump rotation I N A,(@) = _f_ C jo[2Qa sin (xP/N)] cos (2nnP/N) N P-1 t0 sin2(7r/N) t , = 1 - [cos (27r/N)] sin2(nn/N) ‘ N is the number of residence sites of the proton on the circle of radius a describing the motion and to is the proton residence time.The delta function in eqn (7) arises from the bound nature of the motion. Its relative intensity, A,(Qa), the elastic incoherent structure factor (e.i.s.f.), has a form which depends on the geometry of the rotation. The series of Lorentzians in eqn (7) have widths which vary with n, reflecting the higher-order harmonics of the motion which have correlation times t,. A model of powder-averaged, uniaxial, jump rotation is a plausible model of the uncorrelated rotation of water protons about the axis of a fixed hydrogen bond, such as those in solid or sorbed water. N can lie between 2 (two-fold uniaxial rotation) and (30 (uniaxial rotational diffusion), but it has been pointed out previously12 that the scattering laws calculated from eqn (7)-(9) for N < 6 and at Qa < 3 are virtually indistinguishable.The computation with N = 6 thus gives a satisfactory model scattering law for uniaxial rotational diffusion with D,, the rotational diffusion coefficient, given by l/tl. RESULTS COHERENT SCATTERING Fig. 1 (A) shows the diffraction data recorded on the Harwell diffractometer for a degassed silica sample and two further samples containing 0.003 and 0.015 g g-l water. The instrumental background is also shown. Subtractions showing the contribution to the scattering from the adsorbed water are shown in fig. 1 (B). The principal features of the dry silica diffraction pattern are the intense small-angle scattering at Q 5 0.3 A-1 and the broad Bragg maximum near Q = 1.55 A-l, neither of which is affected by the presence of sorbed water.The scattering enhancement from the water is virtually isotropic for 1.7 > Q/A-l > 0.4, and as a consequence it was concluded that in this Q region elastic and quasielastic scattering from the water could be assumed to be completely incoherent for the purpose of subsequent data analysis.2072 8000 h m U c 8 2 f 2 '? c) .- e N & a m 4000 c 3 * 8 v A r: E Y m .- * .3 0 DIFFUSION OF WATER ON SILICA . . . . It.:. . . . . ' . .'. ,.. 0 . .. e m p t y can .-. bac k g r o u n d -.._ ., . . .".*a. ..-%--.a. " . - -~.~.-.~-.~..'...-,...~,.,~~ I I I I I I I I 1 L ) I 0 3 0 5 0 . 7 5 1 0 1 ' 5 1 7 Q1A-l I , I B I I I 0 . 5 1 . o 1 .5 Q1A-l Fig. 1. (A) Diffraction traces recorded for silica samples containing (a) 0.0 15 and (b) 0.003 g g-l of water and (c) degassed silica. (B) Subtracted spectra showing the contribution to the scattering from the adsorbed water. MOLECULAR VIBRATIONS Vibrational spectra, in the form of an amplitude-weighted density-of-states function G( Q, m), have been reported previouly.2 The vibrational frequencies were interpreted in terms of the development of a stable network of doubly hydrogen-bonded water molecules attached to surface hydroxyl groups. Eqn (4) indicates that mean vibrational amplitudes can be calculated from the attenuation of quasielastic peak area (q.e.p. area) with increasing Q. Plots of In (q.e.p. area) against Q2 are shown in fig. 2. The lowest Q point appears to be systematically 7 4 % higher than expected, and so this point was omitted from the least-squares measurement of the slopes.The slopes are, at x = 0.020 g g-l, -0.07(0) k2; at x = 0.029 g g-l, -0.06(1) k2; and at x = 0.040 g g-l, -0.09(4) k2. They corre- spond to r.m.s. thermal cloud radii, (3(u2)):, of 0.46, 0.43 and 0.53 A, respectively, which are comparable with previously reported values for ice.13 QUASIELASTIC SCATTERING Typical examples of quasielastic-scattering laws obtained at three different scattering angles are shown in fig. 3. The narrow component in each spectrum has a width equal to that of the resolution function. They correspond to the sample with a water coverage of 0.29 g g-l. In view of the fact that the surface hydroxyl groups cover < 50% of the available surface area of the silica it could have been expected that these groups might be undergoing rotational diffusion, which would cause quasielasticJ.W. CLARK, P. G. HALL, A. J. PIDDUCK AND C. J. WRIGHT 2073 - 0 . 7 I I I 0 -0.9 h $ - 1 ' 1 iJ + p! 3 - 1 . 3 v c I - l o I i j 3 . 0 L . 0 Q2/A-' Fig. 2. Plots of the area of the quasielastic peaks against momentum transfer squared, Q2: 0, silica plus 0.02 g g-' H,O; +, silica plus 0.03 g g-l H,O; 0, silica plus 0.04 g g-' H,O. Fig. 3. Typical examples of experimental scattering laws at a coverage of 0.29 g g-l. The continuous curves are the two components which result from fitting the data with eqn (1 1). The difference function is also shown on a x 10 scale. Values of S/" and Q/w-l: (a) 128.0,1.858; (b) 103.9, 1.628; (c) 83.5, 1.376; ( d ) 63.5, 1.008; (e) 44.3, 0.779; (f) 30, 0.535.scattering. Analysis of the scattering from the degassed silica, however, showed no detectable broadening, which implies that no further account need be taken of the hydroxyl group rotation in the subsequent analysis. ANALYSIS OF QUASIELASTIC SCATTERING BY CONVOLUTION TECHNIQUES Model scattering laws were convoluted with the instrumental resolution function and tested against the experimental data using an iterative procedure. To imitate the2074 DIFFUSION OF WATER ON SILICA experimental process, the convolution was performed on scattering laws expressed in time-of-flight using the relationship where zo and zi are the incident time-of-flight and the time-of-flight in channel i, respectively, Smodel(Q, z) is the model scattering law converted to time-of-flight units and Rj(r) is the experimental resolution in time-of-flight units.Rj(z) is defined over a range of 2K channels, which are sufficient to include its complete width and where channel K contains its centre of gravity. 24 channels were used, corresponding to 3 f.w.h.m. The least-squares minimisation program V A ~ S A D ~ ~ was used to obtain the values of variables which lead to the best agreement between a model and the experimental data. In addition to the variables required to calculate the model scattering law, a further two parameters, Am and B, were needed. Am was the fractional difference between the channels containing the centres of gravity of the model and experimental scattering laws, and BQ2 was the level of the one-phonon inelastic scattering which was added to the convoluted, model scattering law.Data were fitted over a range of energy transfer, from -0.75 to 1.0 meV, which was more than sufficient to include the observed broadenings. Account was taken of the small variation of Q in the energy transfer across the quasielastic peak. The delta function in the model was represented by allocating the intensity to the two channels closest to zero energy transfer, in the proportions of their mean energy transfers. This results in a negligible perturbation of the resolution function after c o n v ~ l u t i o n . ~ ~ A Simpson’s rule integration was used to calculate the Lorentzian broadenings when they approached the magnitude of a channel width.16 Finally, note that the neutron-counting statistics do not vary symmetrically across the quasielastic S(Q, m) peaks.Graphical outputs have been used to supplement values of 02 (averaged over the data points of the area-normalised scattering laws) for assessing the quality of a fit. QUASIELASTIC DATA ANALYSIS MODEL-INDEPENDENT ANALYSIS A useful model scattering law, for a preliminary analysis of data where the broadenings are small, is that composed of a Lorentzian of variable f.w.h.m. AE, and a delta function of variable relative intensity A :I7 Physically, the delta function represents the e.i.s.f. in a rotational model, but it also takes account of any molecules which are immobile on the timescale dictated by the resolution width.In the limit of poor resolution, the Lorentzian is appropriate for diffusion in 2 or 3 dimensions and for a rotation of undefined periodicity.l* Using this model four parameters ( A , A E , B and Am) were refined whilst the exponent (- Qz (1.2)) was obtained from the slopes of fig. 2. The values of A and AE obtained from the refinements are plotted in fig. 4 against momentum transfer and momentum transfer squared, respectively. The complete set of fits obtained at the coverage of 0.029 g g-l is shown in fig. 3. Since the quality of the fits is good, the model and the data points being nearly coincident, the model scattering laws are shown withJ. W. CLARK, P. G. HALL, A. J. PIDDUCK AND C. J. WRIGHT 2075 \ ( a ) \ 1.0 - 0.21 + + a" o v - i 0 1 Q21A-2 Fig.4. Values of (a), A , the elastic fraction, and (6) AE, obtained from fitting eqn (1 1) to the experimental data. The straight line in (b) is the relationship between AE and Q2 for pure water at 25 "C. 0, silica+0.02 g g-l H,O; A, silica+0.029 g g-l H,O; 0, silica+0.04 g g-l H,O; +, values of A , from eqn (12); (----) predicted e.i.s.f. for a = 0.90 A. the difference function (data -model) multiplied by a factor of 10. Typical values of 02 are given in table 2. Inspection reveals a systematic tendency for the model to overestimate the intensity in the wings of the quasielastic peaks between 0.1 and 0.2 meV. Differences in the elastic region are cu. 1 % of peak height, reflecting the counting statistics. The values of A in fig. 4 show a tendency to decrease with both increasing Q and coverage.The e.i.s.f. for uniaxial rotation with a radius of 0.90 A, corresponding to the radius of motion of one hydrogen atom in a water molecule rotating around a fixed hydrogen bond containing the other hydrogen atom of the same molecule, is shown as a dotted line. Whilst the Q dependence is indicative of rotation, the quantitative agreement with the e.i.s.f. is poor. The Lorentzian broadening increase2076 DIFFUSION OF WATER ON SILICA Table 2. Values of fit factor, 02, for best fits to data at an adsorbed water coverage of 0.03 g g-l model 0.53 0.148 0.146 0.136 0.066 0.068 0.78 0.179 0.172 0.155 0.088 0.086 1.09 0.105 0.092 0.079 0.042 0.035 1.38 0.178 0.156 0.142 0.088 0.087 1.63 0.132 0.1 13 0.105 0.082 0.078 1.86 0.179 0.148 0.159 0.150 0.132 a Eqn (1 1) (fits shown in fig.3). Eqn (1 1) with Lorentzian replaced by summation in eqn Eqn (1 1) with Lorentzian replaced by integral Eqn (1 3) (7) with N = 6 (uniaxial rotational diffusion). in eqn (6) (two-dimensional continuous diffusion). with an additional delta function to represent an immobile concentration of 0.01 g g-l. Eqn (1 3) (fits shown in fig. 5). with increasing Q and are virtually independent of coverage. They do not however extrapolate to AE = 0 at zero Q, as would be expected for translation. The relation- ship between AE and Q2 for bulk water is shown as a solid line. The results are therefore inconsistent with either simple translation or uniaxial rotation about a single axis. This conclusion was confirmed by further fits in which the Lorentzian was replaced both by the correct scattering law for continuous two-dimensional diffusion [eqn (6)] and by that for uniaxial rotational diffusion [eqn ( 7 ) with N = 61.In the first case the new refined values of A were lower at all angles and coverages by between 0.03 and 0.05 and the values of 2fiDZDQ2 were larger than AE at all angles and coverages by a factor of between 1.3 and 1.5. In the second case virtually identical values of A were obtained and the refined values of 2?i/t, were lower than AE in fig. 4 at the highest Q points by a maximum of 15 % . The modified scattering laws resulted, in most cases (see table 2) with a slight improvement in the quality of the fit, most notably at the two highest angles. ANALYSIS ASSUMING A BOUND LAYER The main differences between the refined parameters obtained so far for the different data sets is that the elastic fraction decreases with increasing coverage.Conversely, the values of AE and the form of the plots of A against Q are similar, suggesting that the magnitude and the mechanism of diffusion is independent of coverage. The increase in A with decreasing coverage can be accounted for if a constant number of immobile molecules are present, such as those which might be held at particularly active sites filled at the lower vapour pressures. The word immobile is used with respect to the timescale dictated by the resolution function [lOfi/AR(w) = s]. The variation of A with coverage is given by A , = A M ( l - x , / x ) + x , / x (12) where x, is the surface concentration of immobile molecules and AM is the elastic fraction due to the motion (i.e.the e.i.s.f.). A plot of A , against l / x will be of slopeJ. W. CLARK, P. G. HALL, A. J. PIDDUCK AND C. J. WRIGHT 2077 Fig. 5. Fits of eqn (13) to the experimental scattering laws. Details as for fig. 3. xB( 1 -A,) and intercept A,. Linear regression analysis of these plots at each Q gave values for xB which lay in the narrow range from 0.008 to 0.01 1 g g-l, although the individual correlation coefficients at each individual scattering angle were poor. The values of A , are shown in fig. 4. A , appears to be almost independent of Q, with the exception of the lowest Q point. ANALYSIS ASSUMING UNIAXIAL ROTATION A previous study of the libration frequencies of adsorbed water on Spherisorb silica concluded that the adsorbate is largely composed of doubly hydrogen-bonded molecules at low coverages.2 Basing further analysis of the present data on this model it is possible to imagine that diffusion of these molecules occurs as a result of the consecutive rupture of the two bonds.Rupture of one bond leaves the molecule free to rotate, with an unspecified periodicity, around the axis of the remaining bond, which may or may not be fixed in space. If sufficient thermal energy is then available to break the second bond, a translational jump can occur. A suitable model scattering law for such a process, assuming that the motions are uncorrelated and that uniaxial rotation around a hydrogen bond is the major rotational motion, is then given by where AER 9 AET.This model, which involves one additional variable over those specified above, was fitted to the data to give the results shown in fig. 5 and 6. A significant improvement in the quality of fit was obtained at all values of Q, as evidenced both by the plots shown in fig. 5 and the values of 02 given in table 2. Comparing fig. 5 with fig. 3 shows that much of the systematic deviation in the wings of the elastic peak has now disappeared. Just as significant are the virtually constant refined values of AE, (all values lay in the range 0.285k0.5 meV) and the more pronounced decays of A with increasing Q.2078 4 Q 0 . 1 1 , I1 DIFFUSION OF WATER ON SILICA \ 9‘ + A I I I ; .o 1 .5 2.0 0.21 0.5 Q1A-l $ 0.012 --- h € 4 0.008 3 i v t $ 0.004 .The values of A again show a smooth tendency to decrease with increasing coverage, and applying eqn (12) to these values shows that xB lies in the range 0.009f0.001 g g-l, which is close to the value obtained previously. The data were refined further with a model which included an additional delta function to represent this 0.010 g g-l of bound water. Fig. 7 shows the results of these fits, which showed expected behaviour in that the values of A were similar at all coverages and close to the values of A , in fig. 6. The values of AET were approximately twice those corresponding to the highest coverage in fig. 6 and fairly similar at the three coverages. Though the fitted values of AET do not extrapolate to zero at zero Q, they do pass through a maximum in the range 0.5 < Q/A-l < 2.5.Since J,(QL) exhibits a shallow maximum near QL = 4, this allows us to estimate the jump distance, L, to be ca. 3.0 A. This value is physically reasonable since it is close to the diameter of a water molecule. The maximum values of AET at each coverage lay between 0.024 and 0.040 meV, giving a value for th of (4.5-8) x s.J. W. CLARK, P. G. HALL, A, J. PIDDUCK AND C. J. WRIGHT 2079 1 I 0 0 . 5 1 .o 1 .s 2 .o 0 . 3 2 2 0 . 2 8 0 . 2 4 E --. e 5 0 . 2 0 3 5 0 1 6 Q 0 . 1 2 Q c- 0 . 0 8 0 . 0 4 0 1 2 3 Q21A-2 0 . 5 , I O .:o' 0 . 5 1 .o 1 . 5 2-0 QIA-' Fig. 7. Values of (a) A , the translationally broadened elastic fraction, (b) AET and (c) AER, obtained by fitting eqn (13), together with an additional delta function representing 0.01 g g-l of bound water, to the experimental data.Key as for fig. 4. The two lower hatched regions (model a) represent predicted values of A for a = 0.90 and 0.77 A in the interval N 2 2. The uppermost hatched region (model p) represents the predicted value for N 2 2 for a = 0.90 A and for 25% of the protons remaining stationary during the reorientation. DISCUSSION It has been found that a two-component model of the dynamics of the adsorbed water produces a good fit to the experimental scattering laws (see table 2 and fig. 5). One component, which has been shown to have a concentration of 0.01 g ggl at all coverages, is immobile on the experimental timescale of s, whilst the other component undergoes both translational and rotational diffusion. Surface hydroxyl groups have been shown to be immobile on this timescale so that the mobile component can be fully ascribed to adsorbed water molecules.Similar observations2080 DIFFUSION OF WATER ON SILICA of two-phase behaviour have been made for water intercalated between clay lamellae, although in the clays the phenomenon of ion hydration provides a complicating factor which is absent in the si1i~a.l~ The refined variables from the best fits to the experimental scattering laws are shown in fig. 7. These were obtained with a model in which the mobile fraction undergoes consecutive uniaxial rotation and two-dimensional jump translation with t , = 4.4 x s. Thus approximately ten reorientations take place between each translational jump. The translational jump distance has been shown above to be ca.3 x m, so that the two-dimensional diffusion coemcient, D,,,, for the mobile phase is ca. 6 x 10-lo m2 s-l, a factor of four less than the diffusion coefficient of bulk water at the same temperature. This value of D,, for the mobile film agrees well with the diffusion coefficients measured by spin-echo n.m.r. techniques for complete monolayers of water adsorbed upon silica^.^ s and t, z 6 x GEOMETRY OF REORIENTATION The values of A derived from the fits to the experimental scattering laws have been compared in fig. 4, 6 and 7 with predictions of this quantity based upon a model in which all the water protons execute N-fold uniaxial rotation about an axis which is 0.9 A removed from both hydrogen atoms. This distance corresponds to the separation between one hydrogen atom in a water molecule and the ‘pseudo C3v’ axis of the doubly hydrogen-bonded water molecule, which lies on one of these hydrogen bonds.The comparisons shown in fig. 7 are unsatisfactory and there is a tendency for the results to be systematically lower than the predictions for A , especially at lower angles. In the introduction we discussed the currently accepted model for adsorption at the silica surface in which most of the ‘first down’ oxygen atoms in the adsorbed water molecules act as double donors of electron density to form hydrogen bonds. As a consequence there are two alternative uniaxial rotation models that can be postulated ; the one that has been described above and one in which both hydrogen atoms rotate about the CZv axis of the water molecule.In this latter model the distance between the axis of rotation and the hydrogen atoms is 0.77 A, and A , for this model is shown in fig. 7. Motion about the CZv axis requires the rupture of both hydrogen bonds, whereas the rupture of a single hydrogen bond followed by either 360” rotation or the formation of a hydrogen bond to another -OH group might be expected to have a higher probability. At higher coverages of water there is an increasing probability that hydrogen-bond formation occurs through the hydrogen atoms of the water molecules and in the limit that all orientations of the adsorbed water have equal probability there will be a 25% chance that one proton will remain stationary during the reorientation.In this limit the e.i.s.f. will equal 0.75A0(Q) + 0.25, and this prediction is plotted for a 0.9 A radius of rotation, with N 2 2, as model B in fig. 7. It is compared with the standard model in which all hydrogen atoms diffuse during the reorientation, designated model a in fig. 7. Fig. 7 shows that the standard model, with a radius of rotation of 0.9 A, is in marginally better agreement with the data than the other models. At the same time it is clear that none of the fits to the e.i.s.f. is very good and that the experimental data do not provide strong support for any of the different geometric models that have beer, considered. STRUCTURE OF THE IMMOBILE PHASE It has been found that 0.01 g g-l of water is adsorbed into an immobile phase at the surface of the silica, which is a quantity equivalent to a,, = 0.3 and OHIO z 0.5.The specific mass of this immobile water remains constant at the three coverages investigated and so it is reasonable to assign this water to the molecules which wereJ. W. CLARK, P. G. HALL, A. J . PIDDUCK AND C. J. WRIGHT 208 1 the first to adsorb at each surface. Infrared evidence shows that for a silica surface prepared under similar conditions to our own, none of this water is involved in a rehydroxylation process and that the concentration of non-hydrogen-bonded, ' free', Si-0-H groups initially decreases uniformly with the concentration of adsorbed water. At the same time a,, = 0.3 is close to the value (0.26) previously determined8 for the concentration of non-hydrogen bonded hydroxyl groups at the surface of Spherisorb and we propose, therefore, that these immobile water molecules are those 'first down' water molecules which each form new hydrogen bonds to two surface hydroxyl groups, at least one of which was previously non-hydrogen bonded.STRUCTURE OF THE MOBILE PHASE As adsorption proceeds it is generally thought that clusters of water molecules can form where each newly arriving molecule has, on average, a smaller number of hydrogen bonds than the 'first down' molecules. This accounts for the commonly observed decrease in adsorption enthalpy, AHad, as adsorption proceeds. Initial values Of AHad are in the range 60-70 kJ mol-l, decreasing to near 50 kJ mol-l at ON* z 0.5'. It is probable that this reduction in adsorption enthalpy is associated with the enhanced mobility of the water molecules adsorbed at higher coverages, as has been observed in this paper.Linear and cyclic clusters are consistent both with the inelastic-neutron-scattering evidence, which shows these mobile molecules to be doubly hydrogen bonded, and with the observation that their dynamical behaviour is independent of coverage at concentrations up to 1 molecule per OH group. MACROSCOPIC DIFFUSION Diffusion within the mobile phase has been characterised by a two-dimensional diffusion coefficient D,,, = 6 x m2 s-l. In contrast a calculation of the Knudsen diffusion constant for the transport of water through the 90 A diameter pores of Spherisorb, assuming a zero residence time at the walls of the pores, is given by20 D = = 4.6 x lo-' m2 s-l where T is the absolute temperature, M is the molecular weight, V is the pore volume, S is the surface area, p is the bulk density and Fis the tortuosity factor.The residence time at the walls, however, will not, in practice, be equal to zero but will be given t" = t: exp QIRT where Q is the enthalpy of adsorption of a water molecule and t,N is the inverse frequency of the perpendicular vibration of a water molecule against the silica surface. In this case by 1 3t" +- - 1 - DKnudsen d2 where d is the pore diameter. For a vibration frequency of 100cm-l and Q = 50 kJ mol-1 D = 1.8 x m2 s-l. Since this value is substantially less than the surface diffusion coefficient, the bulk of the mass transport along the pores at partial pressures > 0.2 must be via surface diffusion.2082 DIFFUSION OF WATER ON SILICA CONCLUSIONS This work has shown the coexistence of two phases of sorbed water molecules with different dynamics in monolayer and near-monolayer films on the surface of Spherisorb silica.One component (0.01 g g-l, or 1 H,O per 3 OH groups) is immobile on the experimental timescale (< s). The other component, at concentrations between 0.01 and 0.03 g g-I, gives quasielastic broadening, which is fitted well by a model of consecutive uniaxial rotation (tR = 4 x lo-’, s) and two-dimensional jump translation (D2D x 6 x m2 s-l). The e.i.s.f. obtained from this fit is not satisfactorily accounted for by any of the models that have been considered and the geometry of the reorientation process in the mobile phase is still unclear.We propose that initial adsorption at the SiO, surface results in formation of the immobile phase. ‘First down’ water molecules interlink pairs of surface OH groups, one of which is non-hydrogen bonded and both of which are rotationally immobile on the experimental timescale. Subsequent adsorption on this network forms mobile, molecular clusters, which must be doubly hydrogen bonded to be consistent with librational frequencies reported previously2 and with near-infrared and adsorption measurements on related systems.’? 3 9 Comparison of the surface and Knudsen diffusion coefficients shows that most of the mass transport occurs via surface diffusion. Yu. Babkin and A. V. Kiselev, Russ. J. Phys. Chem., 1963, 37, 118. P. G. Hall, A. Pidduck and C. J. Wright, J. Colloid Interface Sci., 1981, 79, 339. K. Klier and A. C. Zettlemoyer, J. Colloid Interface Sci., 1977, 58, 216. V. Ya. Davydov, A. V. Kiselev, V. A. Lokutsievskii and V. I. Lygin, Rum. J. Phys. Chem., 1974, 48, 1342. R. Mills and V. V. Morariu, Z. Phys. Chem. (Frankfurt am Main), 1972, 79, 1. P. G. Hall, A. J. Leadbetter, A. Pidduck and C. J. Wright, Neutron Inelastic Scattering 1977 (Interna- tional Atomic Energy Authority, Vienna, 1978), vol. 11, p. 51 l . J. C. Dore, D. C. Steytler and C. J. Wright, Mol. Phys., 1983, 48, 1031. R. A. C. Gray, Ph.D. Thesis (UMIST, 1976); A. J. Pidduck, Ph.D. Thesis (University of Exeter, 1980). L. Van Hove, Phys. Rev., 1954, 95, 249. lo K. S. Singwi and A. Sjolander, Phys. Rev., 1960, 119, 863. l1 A. Renouprez, P. Fouilloux, R. Stockmeyer, H. M. Conrad and G. Goeltz, Ber. Bunsenges. Phys. l 2 A. J. Dianoux, H. Hervert and F. Volino, Mol. Phys., 1975, 30, 1181. l3 P. von Blanckenhagen, Ber. Bunsenges. Phys. Chem., 1972, 76, 891. l4 R. M. Richardson, Ph.D. Thesis (University of Exeter, 1976). l5 Numerical Algorithms Group (NAG) Library Manual, E04lSystem 4, Document no. 42F (1975). l6 R. L. Ponczek, N. V. de Eastro Faria and A. P. Guimares, Nucl. Instrum. Methods, 1975, 126, 125. l7 P. L. Hall, D. K. Ross and I. S. Anderson, Nucl. Instrum. Methods, 1979, 159, 347. l8 C. J. Wright and C. Riekel, Mol. Phys., 1978, 36, 695. Chem., 1977,81,429. J. J. Tuck, P. L. Hall, M. H. B. Hayes, D. K. Ross and C. Poinsignon, J. Chem. SOC., Faraday Trans. I , 1984,80, 309. 2o C. N. Satterfield and T. K. Sherwood, The Role of Diffusion in Catalysis (Addison-Wesley, New York, 1963). (PAPER 4/ 1745)

ISSN:0300-9599    

DOI:10.1039/F19858102067

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1985, 81, 2083-2093 Electron Spin Resonance and Electron Spin-echo Spectroscopic Studies of Supported-molybdenum Catalysts Interaction between Molybdenum, Adsorbate and Oxygen Molecules BY RUIYUN Y. ZHAN,? MYSORE NARAYANA AND LARRY KEVAN* Department of Chemistry, University of Houston, Houston, Texas 77004, U.S.A. Received 15th October, 1984 The interaction of various molybdenum species, obtained by high-temperature reduction of the supported catalyst, Mo/SiO,, with various adsorbates and molecular oxygen has been examined by the techniques of electron spin resonance (e.s.r.) and electron spin-echo modulation (e.s.e.m.). Two species observable by e.s.r. after such reduction, Mo(A) with glI = 1.865 and Mo(B) with g,, = 1.895, seem to be insensitive to oxygen adsorption, indicating that the electron-donating centre causing the formation of 0; is not observable by e.s.r.at room temperature, 77 K or 4 K. If polar adsorbates such as H,O, CH,OH, NH, or C,H,N are adsorbed prior to exposing the reduced Mo/Si02 surface to oxygen, 0; is not formed. If a non-polar adsorbate such as C2H4 is adsorbed before or after 0, adsorption, 0; is readily formed. 0, can also be formed if the adsorbate is CH,CN. Mo(A) is sensitive to all adsorbates, leading to the formation of a new Mo5+ species, Mo(C), presumably through the completion of the coordination sphere. It is suggested that Mo5+ or Mo4+ in symmetric tetrahedral coordination is a likely centre for the formation of an 0,species. Both e.s.r. and e.s.e.m. indicate that when small olefins are adsorbed after the formation of 0; a reaction takes place, even at room temperature.The study of the valence state and coordination of molybdenum ions on a catalyst surface and of their interaction with various molecules is of considerable interest since this catalyst is widely used for oxidation of organic substances with molecular oxygen. In a number of studies1-15 concerning this subject it has been demonstrated with the aid of electron spin resonance (e.s.r.) that the pentavalent molybdenum ion on such a catalyst surface is an active centre for polar-molecule and oxygen-molecule adsorption. However, the Mo5+ ion on the surface can take various coordination symmetries upon reduction or other treatments, depending on the type of support and the method of preparation.Thus further study of the coordination symmetries of Mo5+ ions on an oxide surface is important in determining how organic molecules adsorb on the surface and affect catalytic selectivity. It is also important to study the nature of oxygen molecules adsorbed on such surfaces and their role in the subsequent oxidation of olefins. Recently we have used the e.s.r. and electron spin-echo modulation (e.s.e.m.) techniques to study16 the interaction of Mo5+ ions on silica, formed by reduction with hydrogen at 773 K, with polar molecules such as H,O, CH,OH and NH,. These studies indicated the presence of at least two types of Mo5+ with different coordination geometries, only one being the prime candidate for the adsorption of polar molecules.t Present address: Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Chang- chun, The People’s Republic of China. 20832084 E.S.R. STUDIES OF MO CATALYSTS Further studies on the interaction of similarly prepared Mo5+ with less polar molecules such as CH,CN, C,H,N, C,H, and C,H, have been carried out in the absence and presence bf oxygen to determine the reactivity of different types of Mo5+ on silica. The results indicate the presence of another Max+ (x = 4 or 5) species, most likely in tetrahedral coordination, as suggested by Hall and coworkers. These results also indicate the role of non-framework oxygen anions in oxidation reactions of olefins on Mo supported on ~i1ica.l~ EXPERIMENTAL Silica gel from Fisher Scientific (grade 950,60-200 mesh, 700 m2 g-l) was used after thorough washing with HC1 to remove impurity Fe3+ ions.The supported-molybdenum catalysts were prepared by impregnating the silica gel with aqueous solutions of ammonium paramolybdate, (NH,),Mo,O2;4H,O. The material was subsequently dried at 393 K for 4 h in air and then calcined by heating at 773 K in flowing air for 24 h. The molybdenum content in these samples was 2 wt % . No differences in the e.s.r. data of samples with 0.8 or 3.3 % was seen. The catalyst was placed in 3 mm 0.d. Suprasil quartz tubes for vacuum treatment at various temperatures and subsequent e.s.r. experiments. Most samples were treated at 773 K for 5 h in vacuum to a residual pressure of Torr.? The catalysts were reduced at the same temperature using 100 Torr of H, (Union Carbide, 99.99% pure) for 1 h followed by evacuation of the residual H, for 30 min. To study adsorbate interactions the samples were cooled to ambient temperature and exposed to various adsorbates.Liquid adsorbates were previously vacuum-distilled over activated MgSO, or 3A zeolite. Ammonia, ethylene and propylene were further purified by vacuum distillation using a freeze-pumpthaw method. High-purity oxygen (Union Carbide, 99.99 % pure) was used without further treatment. DC1 from Stohler Isotopes was used as received. E.s.r. spectra of reduced catalysts with and without various adsorbates were recorded with a modified Varian E-4 spectrometer at room temperature and 77 K. The g values were calculated by comparison with a dilute polycrystalline sample of diphenyl picrylhydrazine with a g factor of 2.0036.E.s.e.m. spectra were recorded on a home-built e.s.e.m. spectrometerl8* l9 at 4.2 K. RESULTS On thermal reduction of Mo/SiO, catalysts at a high temperature in H, several paramagnetic molybdenum species in different coordination states are formed. Fig. 1 shows an e.s.r. spectrum at 77 K of Mo/SiO, reduced at 773 K in H,. The two gll lines indicate two different Mo5+ species with similar g , values. These are denoted as Mo5+(A) with gll = 1.865 and Mo5+(B) with gI1 = 1.895. On the low-field side of the g , line two other peaks are detected which have been attributed19,v9 to hyperfine structure resulting from 9 5 M ~ and 9 7 M ~ isotopes (both with nuclear spin I = 5/2) with natural abundances of 15.78 and 9.60%, respectively.No changes were seen in the spectra at 293 K and 77 K, indicating the absence of motional averaging and a long spin-lattice relaxation time for these paramagnetic species. The e.s.r. signal shapes of Mo5+ ions on the silica surface were only slightly different, but the signal intensities changed significantly over a reduction temperature range of 773-973 K. No signal was observed when reduction was carried out below 473 K, and the maximum signal intensity appeared for the temperature range 773-873 K. For all the adsorption experiments with polar and non-polar molecules, samples reduced at 773 K were used. Adsorption of oxygen (1-2 Torr) leads to the formation of a new e.s.r. signal at lower field with orthorhombic symmetry (see fig.2). No temperature dependence was seen either in its lineshape or in its g value between 293 and 77 K. It has been suggested -f 1 Torr = 101 325/760 Pa.R. Y. ZHAN, M. NARAYANA AND L. KEVAN 208 5 200 G g II Fig. 1. E.s.r. spectrum at 77 K of Mo/SiO, after reduction at 773 K in H,. i 100 G . ! * H Fig. 2. E.s.r. spectra at 77 K of Mo/SiO, reduced at 773 K in H, and with adsorbed 0, (1-2 Torr). in earlier studies1~2~s~9~14 that this new species is 0;. We find that this 0; signal is formed by electron transfer from some surface species other than the Mo5+(A) and Mo5+(B) species observed by e.s.r. spectroscopy since these Mo5+ e.s.r. signals on reduced Mo/SiO, showed little or no variation in intensity after adsorption of 0,. The intensity of 0; depends sensitively on the sample reduction temperature.No 0; could be formed in samples reduced at 473 K even though a reasonable amount of Mo5+ could be seen by e.s.r. spectroscopy. On increasing the reduction temperature from 500 to 800 K the intensity of 0; rapidly increases; in fact it increases 100 times more than Mo5+ does.2086 E.S.R. STUDIES OF MO CATALYSTS I 11'1 - Ir X I 0 t Q 3 Fig. 3. E.s.r. spectra at 77 K of Mo/SiO, reduced at 773 K in H, (a) with adsorbed CH,CN for 24 h and (b) with adsorbed CH,CN and subsequently adsorbed 0,. To study the specific site dependence, if any, for the formation of O,, oxygen adsorption was carried out in two different types of experiments. In one type the reduced catalyst was exposed to various polar and non-polar molecules such as NH,, C,H,N, C,H,, etc.before oxygen adsorption. In the other type, the reduced catalyst was first exposed to 2 Torr of oxygen and then exposed to various polar and non-polar molecules. Fig. 3 shows the e.s.r. spectrum of Mo/SiO, with adsorbed CH,CN and 0,. The order in which these two molecules were adsorbed did not make any difference in the final e.s.r. spectrum. However, this was not the case when NH, and 0, were adsorbed separately on Mo/SiO,. No 0; was seen in the samples first adsorbed with NH,, and in the samples with the order of adsorption reversed 0; was seen initially but in a few hours at room temperature it disappeared. An intermediate situation is shown in fig. 4. For samples with adsorbed water, methanol or pyridine no 0; could be observed, irrespective of the order of adsorption (see fig.5 ) . These adsorbates cause conversion of Mo5+(A) into another species, Mo5+(C), which is characterized by a change in gl from 1.95 1 to 1.937. Interesting changes were observed in the e.s.r. spectrum of Mo/SiO, with adsorbed ethylene or propylene, as shown in fig. 6 and 7. Unlike the other adsorbates, adsorption of ethylene and propylene show broader peaks in the gl region which can be assigned to two components. Exposure of these samples with adsorbed small olefins to oxygen immediately results in the formation of 0; with very little change in the other features of the e.s.r. spectrum. On raising the temperature of the sample withR. Y. ZHAN, M. NARAYANA AND L. KEVAN 2087 Fig. x 10 7 H "i t 100 G 9 3 4. E.s.r.spectra at 77 K of Mo/SiO, reduced at 773 K in H, (a) with adsorbed NH, Torr) and (b) with adsorbed 0, (2 Torr) and subsequently adsorbed NH, (50 Torr). 200 G - H 9 3 ' I Fig. 5. E.s.r. spectrum at 77 K of Mo/SiO, reduced at 773 K in H, with adsorbed pyridine. 0; could not be formed by subsequent adsorption of 0,.2088 E.S.R. STUDIES OF MO CATALYSTS Fig. 6. E.s.r. spectra at 77 K of Mo/SiO, reduced at 773 K in H, (a) with adsorbed C2H4 and (b) with adsorbed C2H4 and subsequently adsorbed 0,. Fig. 7. E.s.r. spectra at 77 K of Mo/SiO, reduced at 773 K in H, (a) with adsorbed C,H, and (b) with adsorbed C,H, and subsequently adsorbed 0,. adsorbed olefin and oxygen to 393 K significant changes take place in the e.s.r. spectra, as can be seen in fig. 8.The 0; signal disappears and a new spectrum appears around the free-spin g-value. The g line of the Mo5+ becomes sharp and the gb2) line of Mo5+(A) disappears. In samples with the order of adsorption reversed, i.e. Mo/SiO, first exposed to oxygen and then to the olefin, these changes in the e.s.r. spectra take placeR. Y. ZHAN, M. NARAYANA AND L. KEVAN 2089 91 + 200 G g I/ Fig. 8. E.s.r. spectrum at 77 K of Mo/SiO, reduced at 773 K in H, with adsorbed 0, and subsequently adsorbed C,H, after heating to 393 K. at room temperature over a few hours, the end product being identical to that shown in fig. 8. The relevant e.s.r. parameters of Mo5+ and 0; are summarized in tables 1 and 2. Adsorption of DC1 on reduced Mo/SiO, led to spectral changes reported by other w ~ r k e r s . ~ ~ ~ ~ ~ In addition, we find that absorption of DCl followed by 0, does not result in the formation of 0;.Electron spin-echo spectra of such samples, however, failed to show the expected strong deuterium modulations. Fast Fourier transform of a two-pulse electron spin-echo spectrum is shown in fig. 9 (a). Three peaks are easily discernable in this spectrum, of which the first two at 2.06 and 4.22 MHz correspond to the free precession frequency of deuterons at H = 3350 G and its second harmonic, respectively. The origin of the third relatively broad apparent peak at 5.88 MHz is unclear at this point but the peak was reproducible. A poorly resolved shoulder on the main peak occurs at 2.82 MHz and is consistent with the second harmonic of the free precession frequency of the most abundant isotope of chlorine, 35Cl.Very strong electron spin-echoes were observed in the Mo/SiO, samples with adsorbed CD,CN, however, the amplitudes of the deuterium modulations were weak. Fig. 9(6) shows the fast Fourier transform of a two-pulse e.s.e.m. spectrum for the sample with adsorbed CD,CN. The peak at 2.16 MHz corresponds to the free-precession frequency of deuterium at H = 3370 G, and the much weaker apparent peak at 4.40 MHz is its second harmonic. The origin of the broad peak at 6.86 MHz is not known. A significant difference was seen in the phase memory times, T', of the echo decay envelopes for samples with adsorbed oxygen and then ethylene compared with those of samples with the order of adsorption reversed, with T, changing from 1.40 to 0.45 p s , respectively.In both cases the electron spin echoes were quite intense but the amplitude of the deuterium modulations was quite weak. In the samples where oxygen was the first adsorbate, a strong echo was seen at the free-spin g value, but in the samples with ethylene as the first adsorbate no such echo was seen at the free spin g value even after the sample was heated to 393 K prior to the 4 K measurement of the electron spin-echo.2090 E.S.R. STUDIES OF MO CATALYSTS Table 1. E.s.r. parameters of the Mo5+ ion in Mo/SiO, matrix method of preparation g," gla ref. Mo/SiO, + H,O + CH,OH + NH, + CH,CN + C,H,N + C2H4 + 02/C2H4 + C3H6 + DC1 reduction in H, at 773 K ads. at r.t.O for 2 h ads. at r.t. for 2 h ads. at r.t. for 2 h ads.at r.t. for 24 h ads. at r.t. for 24 h ads. at r.t. for 24 h reaction at 398 K ads. at r.t. for 24 h ads. at r.t. for 4 h 1.951 1.865 this work 1.951 1.895 this work 1.936 1.893 16 1.937 1.896 16 1.956c 1.887 16 1.928 1.947c 1.887 this work 1.929 1.946 1.896 this work 1.926 1.955 1.889 this work 1.944 1.858 1.943 1.896 this work 1.954 1.891 this work 1.942 1.858 1.945 1.968 this work a Estimated errors in g values are kO.003. Adsorbed at room temperature. Rhombic g with g, split into two values designated as g, and g,. Table 2. E.s.r. parameters of 0, species on various molybdenum catalysts species method of preparation g1 g2 g, ref. Mo/SiO," thermal reduction and 2.018 2.012 2.004 this work Mo/SiO, photoreduction at 77 K 2.016 2.009 2.003 14 Mo(CO),/SiO, partial reduction and 2.017 2.001 2.005 8 Mo/MgO partial reduction and 2.070 2.009 2.003 20 Mo/Al,O,b partial reduction and 2.070 2.009 2.009 20 adsorption of 0, 2.029 2.012 2.004 in H, and adsorption of 0, 2.021 2.009 2.004 adsorption of 0, adsorption of 0, and adsorption of 0, a Estimated errors in g values are fO.OO1.g, = gll, g, = g, = gl. DISCUSSION Before reduction at 400-800 K no e.s.r. was seen in the Mo/SiO, samples. We have shown16 that of the two species clearly seen in the e.s.r. of reduced Mo/SiO, samples, Mo5+(A) with gll = 1.865 reacts with several polar adsorbates and we have suggested it to have square-pyramidal coordination. On adsorption of water and methanol and perhaps other polar molecules, one additional molecule enters the first coordination sphere of Mo5+, causing the formation of Mo5+(C). On adsorption of the nitrogen-containing molecules CH,CN, NH, and C,H5N the e.s.r.spectra become quite anisotropic. For CH,CN some additional hyperfineR. Y. ZHAN, M. NARAYANA AND L. KEVAN 2 - 209 1 structure is observed on the low-field lines which could be due to superhyperfine interaction with the nitrogen (14N, I = 1). We have already shown16 that the electron spin-echo spectra of Mo/SiO, with adsorbed NH, or ND, exhibit strong coupling of the nitrogen to Mo5+ and to the deuterons. However, for samples with adsorbed pyridine we did not observe any nitrogen modulation. While the formation of the anion radical 0; is known to occur by electron transfer from the catalyst to the adsorbed oxygen molecule, there is considerable controversy as to the type of molybdenum species involved in such electron transfer.Che and coworkers', suggested that the Mo5+(A) with gll = 1.865 is reactive towards oxygen but our observation does not support this (see fig. 2). Furthermore, we have been able to form 0; on samples exposed to CH,CN vapour for 10 h, by which treatment Mo5+(A) disappears. This indicates that the adsorption sites of CH,CN and 0; are different. Experiments with C,H, and C,H, also lead to the same conclusion that 0; forms on sites other than Mo5+(A). Such a conclusion is substantiated by the experimental observation that Mo5+(A) does not change on the adsorption of 0, but disappears on subsequent adsorption of CH,CN, C,H, or C,H,. Since the other species observed by e.s.r.in the reduced Mo/SiO,, Mo5+(B) with gll = 1.895, also does not change on adsorption of O,, it follows that the molybdenum ion donating the electron to oxygen is not observable by e.s.r. spectroscopy.8 Spiridonov et aL20 studied the interaction of Mo/MgO and Mo/Al,O, with oxygen molecules. They suggested that a Mo4+ ion could be involved in the formation of O;, which is stabilized on the host Mg or A1 ion. However, the 0; observed in our samples has different g-values (see table 2). KasaiZ1 has shown by theoretical treatment of the g-factors of 0; that g, (or g,,) reflects the charge of the cation on which the oxygen is adsorbed, decreasing as the charge of the cation increases. Thus the relatively low value of gl observed for 0; on Mo/SiO, suggests molybdenum in the pentavalent or hexavalent state to be the adsorption centre.Pershin et aI.l4 produced Mo5+ by U.V. irradiation in the presence of H, at 77 K and concluded that this Mo5+ is in tetrahedral coordination with sufficient distortion of the tetrahedral symmetry to permit its observation by e.s.r. spectroscopy. They also showed that such an Mo5+ species is very reactive towards O,, leading to the formation of 0;.2092 E.S.R. STUDIES OF MO CATALYSTS It was mentioned earlier that on samples with adsorbed H,O or CH,OH, 0; could not be seen irrespective of the order of adsorption. This is probably due to the competition of the oxygen-containing molecules for the same site which is responsible for the formation of 0; . Such a competition between 0, and NH, could be observed by the gradual disappearance of 0; when NH, was adsorbed after the adsorption of 0,.Since 0; could be formed on the samples with adsorbed C2H4, C3H6 or CH,CN, it follows that these ligands do not favour the site responsible for 0; formation, possibly owing to steric reasons. If one were to postulate an Mo5+ species having tetrahedral coordination with very little distortion, represented as Mo5+(D), some of the above-mentioned observations may be understood. Because of very short relaxation times due to low-lying excited states, Mo5+ in highly symmetric tetrahedra is not observable by e.s.r. spectroscopy.lOT l1 Thus the lack of significant changes in the e.s.r. spectrum of Mo5+(A) or Mo5+(B) upon formation of 0; becomes under- standable. That C,H,, C,H, or CH,CN are bulky ligands is apparent from the experimental observation of more anisotropic g-factors when these molecules were adsorbed.Possibly owing to such steric reasons, these molecules may not coordinate with Mo5+(D), thus leaving it open for oxygen adsorption. It is not clear why 0; formation preferentially occurs on Mo5+(D) and not on Mo5+(A). It.is also not clear how pyridine is able to prevent the formation of 0;. From the adsorption results of NH, and 0, in either order it is clear that NH, binds better to Mo and hence displaces oxygen from Mo5+(D). The ability to form 0; after adsorption of ethylene or propylene in our samples contrasts with the observations made by Howe and Leith* on Mo/SiO, prepared from Mo(CO),. They suggested that propylene poisons the sites favourable for the formation of 0,.It is also interesting that when oxygen is adsorbed prior to the adsorption of ethylene, some reaction takes place even at room temperature, leading to the formation of an Mo5+ species similar to that observed for oxygen-containing adsorbates such as CH,OH or H,O (see fig. 8).16 The sharp reduction in the phase memory time of the electron spin-echo decay in such a sample is apparently due to spin-spin interaction between Mo5+ and the radical resulting from oxidation of the olefin. Consequently one may need to consider the role of 0; in addition to the role of lattice oxygens in the oxidation of unsaturated hydrocarbons on silica-supported Mo ~ata1ysts.l~ CONCLUSIONS The two species of Mo5+, Mo(A) and Mo(B), observable by e.s.r.after high- temperature reduction of Mo/SiO, do not change on adsorption of 1 to 2 Torr of 0,, but 0; is readily observed, indicating the presence of a third e.s.r.-inactive reduced Mo species. Mo(A), while being inactive towards 0,, readily interacts with polar as well as non-polar adsorbates, leading to the formation of a new species, Mo(C). If the reduced Mo/SiO, is exposed to H,O, CH,OH, NH, or C5H5N prior to exposure to oxygen, 0; cannot be formed. However, if the reduced Mo/SiO, is exposed to C,H,, C,H, or CH,CN, subsequent adsorption of 0, readily yields 0;. Furthermore, if the surface with 0; formed is exposed to H,O or NH,, 0; decays quickly but is stable to exposure to CH,CN. On adsorption of C,H, or C,H, after 0; is formed on reduced Mo/SiO,, both 0; and Mo(A) gradually disappear at room temperature, forming a free radical and an Mo5+ species similar to that observed following adsorption of H,O.This indicates that 0; might play a significant role in the oxidation of unsaturated hydrocarbons on supported-molybdenum catalysts.R. Y. ZHAN, M. NARAYANA AND L. KEVAN 2093 This research was supported by the National Science Foundation, the Robert A. Welch Foundation and the Energy Laboratory of the University of Houston. M. Dufaux, M. Che and C. Naccache, C.R. Acad. Sci., Ser. C, 1969, 268, 2255. V. M. Vorotyntsev, V. A. Shvets and V. B. Kazanskii, Kinet. Katal., 1971, 12, 1249. V. A. Shvets and V. B. Kazanskii, J. Catal., 1972, 25, 123. M. Akimoto and E. Echigoya, J. Catal., 1972, 29, 191 1 .C. Naccache, J. Bandiera and M. Dufaux, J. Catal., 1972, 25, 334. Y. Ben Taarit and J. H. Lunsford, J. Phys. Chem., 1973, 77, 1365. J. H. Lunsford, Catal. Rev., 1973, 8, 135. M. Che, A. J. Tench and C. Naccache, J. Chem. SOC., Faraday Trans. I , 1974, 70, 263. * R. F. Howe and I. R. Leith, J. Chem. SOC., Faraday Trans. I , 1973, 69, 1967. lo S. Abdo, M. Lo Jacono, R. B. Clarkson and W. K. Hall, J. Catal., 1975, 36, 330. l1 W. K. Hall and M. Lo Jacono, Proc. VZth Znt. Congr. Catal. (The Chemical Society, London, 1977), p. 246. S. Abdo, R. B. Clarkson and W. K. Hall, J. Phys. Chem., 1976, 80, 2431. l 3 (a) M. Che, F. Figueras, M. Forissier, J. C. McAteer, M. Perrin, J. I. Portefaix and H. Praliaud, Proc. VIth Znt. Congr. Catal. (The Chemical Society, London, 1977), p. 261 ; (b) M. Che, J. C. McAteer and A. J. Tench J. Chem. SOC., Faraday Trans. I , 1978,74,2378; (c) A. R. Elipe-Gonjalez, C. Louise and M. Che, J. Chem. SOC., Faraday Trans. I , 1982, 78, 1297. l4 A. N. Pershin, B. N. Shelimov and V. B. Kazanskii, Kinet. Katal., 1979, 20, 1298. l5 (a) R. Fricke, W. Hanke and G. Ohlman, J. Catal., 1983,79,1; (b) S. R. Seyedmonir and R. F. Howe, l6 M. Narayana, R. Y. Zhan and L. Kevan, J. Phys. Chem., 1985,89,636. lR T. Ichikawa, L. Kevan and P. A. Narayana, J, Phys. Chem., 1979,83, 3378. l 9 P. A. Narayana and L. Kevan, J. Photochem. Photobiol., 1983, 37, 105. *O K. N. Spiridonov, G. B. Pariiskii and D. V. Krylov, Kinet. Katal., 1971, 10, 2161. 21 P. H. Kasai J. Chem. Phys., 1965, 43, 3322. J . Chem. SOC. Faraday Trans. I , 1984, 80, 87. D. B. Dadyburjor, S. S. Jewur and E. Ruckenstein, Catal. Rev. Sci. Eng., 1979, 19, 293. (PAPER 4/ 1763)

ISSN:0300-9599    

DOI:10.1039/F19858102083

出版商:RSC    

年代:1985

数据来源: RSC