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/F198581FX021

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/F198581BX023

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY FARADAY TRANSACTIONS, PARTS I A N D I 1 The Journal of the Chemical Society is 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 I1 (Physical Organic Chemistry). Kinetic and mechanistic studies of organic, organometallic and bio-organic reactions.The description and application of physicochemical, spectroscopic, and theoretical procedures to organic chemistry, including structure-activity relationships. Physical aspects of bio-organic chemistry and of organic compounds, including polymers and biopolymers. Authors are requested to indicate, at the time they submit a typescript, the journal for which it is intended. Should this seem unsuitable, the Editor will inform the author. The sixth section of the Journal of the Chemical Society is Chemical Communications, which is intended as a forum for preliminary accounts of original and significant work, in any area of chemistry that is likely to prove of wide general appeal or exceptional specialist interest. Such preliminary reports should be followed up eventually by full papers in other journals (e.g.the five Transactions) providing detailed accounts of the work. NOTES I t has always been the policy of the Faraday Transactions that brevity should not be a factor influencing acceptability for publication. In addition however to full papers both sections carry at the end of each issue a section headed ‘Notes’, which are short self-contained accounts of experimental observations, results, or theory that will not require enlargement into ‘full’ papers. The Notes section is not used for preliminary communications. The layout of a Note is the same as that of a paper. Short summaries are required. The procedure for submission, administration, refereeing, editing and publication of Notes is the same as for full papers.However, Notes are published more quickly than papers since their brevity facilitates processing at all stages. The Editors endeavour to meet authors’ 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 1500 words or word-equivalents. (9NQMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet ‘Quantities, Units, and Symbols’ (1975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W 1 V OBN).These recommendations are applied by The Royal Society of Chemistry in all its publications. Their basis is the ‘ Systtme International d’Unites’ (SI). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D , E, F, and H (Pergamon, Oxford, 1979 edn).Nomenclature of Inorganic Chemistry (Butterworths, London, 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.(ii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 80 Physical Interactions and Energy Exchange at the Gas-Solid Interface McMaster University, Hamilton, Ontario, Canada, 23-25 July 1985 Organising Committee : Professor J. A. Morrison (Chairman) Dr M. L. Klein Professor G. Scoles Professor W. A. Steele Professor F. S. Stone Dr R. K. Thomas The discussion will be concerned with certain aspects of current research on the gas-solid interface: elastic, inelastic and dissipative scattering of atoms and molecules from crystal surfaces, and the structure and dynamics of physisorbed species, including overlayers. Emphasis will be placed on the themes of physical interactions and energy exchange rather than on molecular-beam technology or the phenomenology of phase transitions on overlayers.The interplay between theory and experiment will be stressed as they relate to the nature of atom and molecule surface interaction potentials, including many- body effects. The programme and application form may be obtained from: Professor J. A. Morrison, Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada L8S 4M1 or: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN, U.K. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 20 I. Phase Transitions in Adsorbed Layers University of Oxford, 17-1 8 December 1985 Organising Committee : Professor J. S. Rowlinson (Chairman) Dr E. Dickinson Dr R. Evans The aim of the meeting is to discuss phase transitions at gas/liquid, 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 preliminary programme may be obtained from : Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN Mrs Y. A. Fish Dr N. Parsonage Dr D. A. Young (iii;THE FARADAY D I V I S I O N OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 81 The discussion will focus on the interaction of radiation with small molecules, molecular ions and complexes leading directly or indirectly t o their dissociation. Emphasis will be given t o , 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. Titles should be 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 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 liposomes, mechaqical properties, encapsulation and interaction forces between bilayers leading to fusion but excluding preparation and characterisation methodology.Further information may be obtained from: Professor D. A. Haydon, Physiological Laboratory, Downing Street, Cambridge CB2 3EG Full papers for publication in the Discussion Volume will be required by December 1985. T H E FARADAY D I V I S I O N OF THE ROYAL SOCIETY O F CHEMISTRY GENERAL DISCUSSION NO. 82 Dynamics of Molecular Photof ragmentation University of Bristol, 1 5 1 7 September 1986 Organising Committee: Professor R.N. Dixon (Chairman) Dr G. G. Balint-Kurti Dr M. S. Child Professor R. Donovan Professor J. P. Simons30TH INTERNATIONAL CONGRESS OF PURE AND APPLIED CHEMISTRY Advances in Physical and Theoretical Chemistry Manchester, S 1 3 September 1985 The Faraday Division is mounting the following symposia as part of the 30th IUPAC Congress: Reaction Dynamics in the Gas Phase and in Solution This symposium will examine the ways in which modern techniques allow detailed study 01 the dynamical motion of molecules which are undergoing chemical reaction or energy exchange. Micellar Systems The symposium will discuss various aspects of micellization, including size and shape factors, micellization in biological systems, chemical reactions in micellar systems, micelle structure and solubilization.Emphasis will also be given to modern techniques of examining micellar systems, including small-angle neutron scattering, neutron spin echo, photocorrelation spectroscopy, NM R and use of fluorescent probes. Surface Science of Solids The symposium will centre on recent advances in the study of kinetics and dynamics at surfaces and of phase transitions in adsorbate layers on single crystal surfaces. Both experimental and theoretical aspects will be reviewed with an emphasis on metal single crystal surfaces. New Electrochemical Sensors (in collaboration with the Electroanalytical Group of the Analytical Division) The symposium will cover such topics as the fundamentals of the subject, new gas sensors based on membrane electrodes and on ceramic oxides, the development of new ion- [selective electrodes and the synthesis of new guest- host carriers, the development of CHEMFETS and other integrated devices together with the theory of the operation of such devices, and finally the development of biosensors including for instance enzyme electrodes, direct electron transfer to biological molecules and new potentiometric techniques for protein analysis.The full programme and application form may be obtained from: Dr J. F. Gibson, 30th IUPAC Congress, Royal Society of Chemistry, Burlington House, London W1V OBNTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 21 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 1 another. Interaction-induced Spectra in Dense Fluids and Disordered Solids Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 25 October 1985 to: I University of Cambridge, 10-1 1 December 1986 ~ 1 Cambridge CB2 1 EW Professor A. D. Buckingham, University Chemical Laboratory, Lensfield Road, 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 THE 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 1986FARADAY DIVISION INFORMAL AND GROUP MEETINGS Gas Kinetics Group with SERC Summer School in Gas Kinetics To be held at the University of Cambridge on 26 June to 3 July 1985 Further information from Dr I. W. M. Smith, Department of Chemistry, University Chemical Laboratory, Lensfield Road, Cambridge CB2 1 EP Industrial Physical Chemistry Group with the Food Chemistry Group Water Activity: A Credible Measure of Technological Performance and Physiological Viability To be held at Girton College, Cambridge on 1-3 July 1985 Further information from Professor F.Franks, Department of Botany, Downing Street, Cambridge CB2 3EA 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 Surface Reactivity and Catalysis Group with the Catalysis Section of the KNCV Mechanism and Structure in Heterogeneous Catalysis To be held at Noordwijkerhout, The Netherlands on 18-20 September 1985 Further information from: Dr R.Joyner, BP Research Centre, Chertsey Road, Sunbury on Thames TW16 7LN Industrial Physical Chemistry Group A Molecular Approach to Lubrication and Wear To be held at Girton College, Cambridge on 23-25 September 1985 Further information from Mr M. P. Dare- Edwards, Shell Research Ltd, Thornton Research Centre, Chester CH1 3SH -. Neutron Scattering Group jointly with the Materials Testing Group of the Institute of Physics Industrial Uses of Particle Beams To be held at the Institute of Physics, London on 26 September 1985 further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SWlX 8QX 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, Whitenkights Reading RG6 2AD (vii)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 7 ~~~~ 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 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 : Binary lonogenic Equilibria between some Phenols and Bases Geoffrey E. Holdcroft and Peter H. Plesch (1 985, Issue 2) The Radical Cation of Trimethyl Phosphate: E.s.r. Evidence for Bonding to CFCI, Glen D. G. McConnachie and Martyn C. R. Symons (1985, Issue 2) 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) Stereochemical Applications of Potential Energy Calculations. Part 4. Revised Cyclopropane Parameters for Molecular Mechanics Pekto M. lvanov (1 985, Issue 3) Electron Spin Resonance Studies of the Ammonia-Boryl Radical ( H,N -+ BH,.) ; an Inorganic Analogue of the Ethyl Radical Jehan A. Baban, Vernon P. J. Marti, and Brian P. Roberts (1 985, Issue 3) 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) The Iron-Vanadium-Oxygen System at 11 23, 1273, and 1373 K. Part 1. (viii)

ISSN:0300-9599    

DOI:10.1039/F198581FP049

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1985, 81, 1297-1302 Crystallization Field of Zeolite T at 100 "C for a SiO,/A1,0, Ratio of 28 and Crystallization Sequences in the Na,O-K,O-SO,-Al,O,-H,O System BY ANDRZEJ CICHOCKI Institute of Chemistry, Jagiellonian University, Karasia 3, 30-060 Krakow, Poland Received 16th August, 1983 A crystallization field of zeolite T has been found at n = 28 which differs in area and shape from those with lower values of n. Crystallization has been found to be very sensitive to changes in the ratio of OH to SiO, in the reaction mixture. Crystallization sequences of zeolite phases, depending on the composition of the reaction mixture, have been plotted and the disproportionation phenomenon has been observed. Crystallization fields of zeolite T (the erionite-offretite type) at 100 "C in the system Na,0-K20-Si02-A1203-H20 at n = Si0,/A1203 = 20.2 and 26.5 have been reported previously.' However, zeolite T can be crystallized ' relatively free from zeolites of similar crystalline structure' in the initial n range from ca.20 to ca. 2K2 Several successful attempts to synthesize zeolite T at n = 27.8 and 28.0 have been made.3 The purpose of this work was to define the limits of the zeolite T crystallization field at n = 28 and compare them with the limits at lower values of n ; other synthesis conditions were unchanged. Another purpose was to study the crystallization sequences of zeolite phases obtained from reaction mixtures (r.m.) of different compositions. EXPERIMENTAL Starting materials were, as before,l? 3, silica sol, sodium aluminate, KOH, NaOH and water, which were manufactured in Poland or at this laboratory.Syntheses were by the standard methods described previously;3* 100 g of silica sol was used in all cases. Hydrogels were always aged for 24 h at room temperature before crystallization, which was carried out in identical, sealed glass ampoules (Termisil, the Polish analogue of Pyrex) in an air oven at 100 "C for 168 h. The solid products were filtrated and washed with water to pH 9-10. In all experiments n had the value 28 and the content of water was 92 mol % . Two series of experiments were performed. In the first series (A-1 to A-9) the percentages of K,O and of the hypothetical compound A1,03. 28Si0, were changed while the Na,O content in the anhydrous raction mixture was kept constant.In the second series (A-10 to A-22) the percentages of Na,O and K,O were changed while A1,O3-28Si0, was kept constant. In experiment A-23 the percentages of Na,O and A1,0, - 28Si0, were changed. The samples were examined as before,l i.e. by X-ray powder diffraction, B.E.T. sorption of nitrogen and optical and scanning electron microscopy (s.e.m.). The ratios of the sums of the intensities of the chosen diffraction lines of the sample and of the standard were used to measure the zeolite contents in the sample (wt %). 12971298 CRYSTALLIZATION OF ZEOLITE T Table 1. Changes in chemical composition of the reaction mixture, 'concentration' of components in both series (mol per 1000 mol H20) and changes of the mole ratios OH/Si02 and Na/(Na + K) order of changes : series 1 series 2 experiments : A-1 A-9 A-10 A-22 A-23 wt% in anhydrous r.m.Na,O 20.0 20.0 25.0 10.0 18.0 K2O 14.0 6.00 5.50 20.5 14.0 A1203. 28 SiO, 66.0 74.00 69.5 69.5 68.0 'concentration ' change Na,O 18.1 17.7 22.0 9.30 16.4 K2O 8.40 3.50 3.20 12.5 8.37 2.10 2.30 2.10 2.20 2.14 SiO, 58.3 63.7 59.6 62.8 60.1 OH/SiO, Na/(Na + K) 0.91 0.67 0.85 0.70 0.82 0.68 0.83 0.87 0.43 0.66 outer scale: OH-/Si02 inner scale: Na+/(Na++ K+) 0.65 0.75 0.85 0.95 I I I I 0.87 1.00 "* 16 0 0.57 0.67 0.77 0.87 0.97 OI.89 0.'87 0.?7 0.bl 0.'65 I inner scale: OH-/Si02 outer scale: Na+/(Na++ K+) Fig. 1. Nitrogen sorption capacity (77 K, p / p o = 0.2) in cm3 liquid per 100 g against OH/Si02 and Na/(Na + K) in the r.m. : (a) series 1 and (b) series 2.A.CICHOCKI 1299 RESULTS AND DISCUSSION Table 1 shows changes in the composition of the r.m. of both series. Despite the constant water content in the system, the concentrations of all the components varied (mol per 1000 mol H,O), though to a different degree. A review of literature data regarding various zeolites showed that the result of a synthesis depends on both the absolute values of component concentration (i.e. the r.m. dilution) and the ratios of their concentration^.^-^ In bi-alkaline systems, besides the absolute OH- ion concentration (equal to approximately twice the concentration of Na,O + K,O) and the relative OH/SiO, concentration, an important part is played by the molar ratio Na+/(Na+ + K+). At constant n the OH/SiO, ratio characterizes the relative changes in the concentrations of skeleton-modifying (Na,O and K,O) and skeleton-forming (SiO, and Al,O,) components in the r.m.On the other hand, the Na/(Na + K) ratio characterizes the specific templating action of cations during the formation of structure of a given type. Therefore both ratios were chosen to characterize the composition changes of the r.m. (fig. 1-3). In both series changes in the r.m. composition caused strong changes in the sorption capacities (fig. 1) and phase compositions of the products (fig. 2 and 3). The sorption capacities of the materials for nitrogen correlate well with the phase composition after consideration of the decrease of the sorption capacities of zeolites with increasing occupation of sites by K+ ions (as Eberly has shown for erionite*).Generally, at n = 28 no completely pure zeolite T was obtained, as impurities such as zeolite L and chabazite (Ch) tended to crystallize. This can be linked to the change in n and the different absolute concentrations of Al,O, in the r.m.; at lower values of n a series of practically pure samples of zeolite T were obtained, but for n = 26.5 zeolite L impurities tended to form. Note that, almost independent of n, the highest amounts of zeolite T were obtained in the range OH/SiO, = 0.80-0.82.l Therefore this parameter should be included among the critical parameters for the synthesis of zeolite T. The plots in fig. 2 and 3 can be regarded as the so-called crystallization sequences, i.e. they illustrate the order of crystallization of the individual zeolite phases, depending on kinetic parameters of the synthesis.They are analogous to the plots given by Sandg and Dwyer et for ZSM-5 and ZSM-4 zeolites, with the exception that the time of crystallization was marked on the abscissa at constant temperature and composition of the r.m. Here the role of the kinetic parameter is played by the chosen composition parameters. The curves in fig. 2 and 3 show the existence and characteristic order of crystallization for zeolites T, L and Ch, which are superimposed partially or completely, depending on the composition of the r.m. However, obtaining pure zeolite T or another zeolite phase may only be a question of a careful choice of other factors which affect the kinetics of the crystallization. This can be shown specifically by the lack of zeolite Ph (Phillipsite type) as an impurity in the small-scale tests, carried out in glass ampoules, when the heat transport took place by air and the ampoule glass.On the other hand, zeolite T impurities existed at the same r.m. composition, time and crystallization temperature when the heat transport was more effective, i.e. in steel autoclavesll or glass vessels heated in oil or glycerol baths.12 The crystallization sequences agree with the order of stability of the zeolite phases, as devised ear1ier:ll L < T < Ch < Ph. In series 1 the phenomenon of disproportionation is observed. Instead of zeolite T the more stable zeolite Ch and less stable zeolite L are formed. Series 2, however,1300 CRYSTALLIZATION OF ZEOLITE T 40 c a 20 0 0.57 0.67 0.77 0.87 0.97 I I I I 1 J 0.89 0.83 0.77 0.71 0.65 inner scale: OH-/SiO, outer scale: Na+/(Na'+ K') Fig. 2.Crystallization sequences for series 1 (wt% zeolite) with simultaneous increase of OH/SiO, and decrease of Na/(Na+ K) in the r.m. : 0, zeolite T; x , zeolite L; 0, zeolite Ch. 100. 80 T c 60 .d 0 $ 8 24 40 c a 20 0 0.55 0.65 0.75 0.85 0.95 t 1 L I 1 I 0.06 0.33 0.60 0.87 1.00 inner scale: OH-/SiO, outer scale: Na"/(Na++ K') Fig. 3. Crystallization sequences for series 2 (wt% zeolite) with simultaneous increases of OH/SiO, and Na/(Na+K) in the r.m.: 0, zeolite T; x , zeolite L; 0, zeolite Ch.A. CICHOCKI 1301 Fig. 4. Crystallization fields of zeolite T plotted against n in the r.m. (compositions in wt%): (-)n = 28,(---*-.- ) n = 26.5 and (----) n = 20.2; 0, positive result; a, ‘limit’ result (on the limit of the crystallization field); x , negative result; transition (‘limit’) area is shaded.conforms with the Ostwald rule: as the speed of crystallization increases, more stable phases appear. The effect of glass corrosion on zeolite T synthesis has been discussed elsewhere.12 It was not investigated in this work and is treated only as a constant. This is corroborated by the correlation between crystallization sequences and stability series based on the results of crystallization in steel autoclaves.ll Results of X-ray, sorption and microscopic investigations were used to plot the crystallization-field borders of zeolite T on a triangular diagram of the r.m. compo- sitions (fig. 4).The criteria used to judge the results were as before.’ For comparison, the previously found crystallization-field borders for lower values of n were also plotted (other parameters remaining the same). For n = 28 the field is larger, has an elongated shape and diffusion of the field border into the border area occurs. Similar broadening of the crystallization field was observed by Robson et aZ.13 However, the elongation and its direction point to less sensitivity of the system to changes in Na/(Na+K) than in OH/SiO,. Knowledge of the crystallization fields, crystallization sequences and stability of zeolites, as well as the kinetic factors and their effect on the crystallization of zeolite, are important for the production of pure zeolites and for understanding the processes which take place. ’ A. Cichocki, Zesz. Nauk. Uniw. Jagiellon., Pr. Chem., 1977, 22, 259. D. W. Breck and N. A. Acara, U.S. Patent, 2950952, 1960. A. Cichocki, Zesz. Nauk. Uniw. Jagiellon., Pr. Chem., 1976, 21, 377. A. Cichocki, Zesz. Nauk. Uniw. Jagiellon., Pr. Chem., 1975, 20, 215. S. P. Zhdanov and E. N. Egorova, Chemistry of Zeolites (Nauka, Leningrad, 1968). D. W. Breck, Zeolite Molecular Sieves: Structure, Chemistry and Use (Wiley, New York, 1974). ’ R. M. Barrer, Hydrothermal Chemistry of Zeolites (Academic Press, London, 1982). P. E. Eberly, Am. Mineral., 1964, 49, 30.1302 CRYSTALLIZATION OF ZEOLITE T A. Erdem and L. B. Sand, J. Catal., 1979,60, 241. lo F. G. Dwyer and P. Chu, J. Catal., 1979, 50, 263. A. Cichocki, J. Grochowski and t. Lebioda, Krist. Tech., 1979, 14/1, 9. l2 A. Cichocki, Zeolites, in press. l3 H. E. Robson, G. P. Hamner and W. F. Arey Jr, Adv. Chem. Ser., 1971, 102, 417. (PAPER 3/1452)

ISSN:0300-9599    

DOI:10.1039/F19858101297

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1985,81, 1303-1310 Non-Arrhenius Behaviour in the Reaction of CF, Radicals with CH,CN and CD,CN BY L. PASTERIS AND E. H. STARICCO* Instituto de Investigaciones en Fisicoquimica de Cordoba (INFIQC), Departamento de Fisico Quimica, Facultad de Ciencias Quimicas, Universidad Nacional de Cordoba, Sucursal 16, Casilla de Correo 61, 5016 Cordoba, Argentina Received 17th May, 1984 Evidence is presented that rate measurements over a wide temperature range, 298-648 K, for the reaction of CF, radicals with methyl cyanide show non-Arrhenius behaviour when the photolysis of trifluoromethyl iodide is used as a source of radicals. The results presented here support the fact that the curved Arrhenius plot may be explained in terms of two reaction channels giving the same product, CF,H.One is the abstraction reaction by the CF, radical and the other involves the iodine atom produced in the photolysis of CF,I. In order to obtain further information on the reaction mechanism the kinetic isotope effect has also been studied. Most of the Arrhenius parameters for hydrogen-abstraction reactions by alkyl and perfluoroalkyl radicals have been obtained over a short temperature range, mainly owing to problems with regard to the radical sources. In a previous study1 we reported the Arrhenius parameters for the abstraction of hydrogen and deuterium by the CF, radical from methyl cyanide over the temperature range 298-648 K using the photolysis of perfluoroacetic anhydride to generate the radicals and we compared the results with those obtained from CF,I between 498 and 648 K because the former source of radicals has not been used before at temperatures above 473 K.With the aim of obtaining further information on the behaviour of nitriles towards perfluoro- alkyl radicals, the measurements with CF,I were extended to lower temperatures and a pronounced curved Arrhenius plot was found. Thus this work is concerned with non-Arrhenius behaviour. We present results for the reaction of CF, produced in the photolysis of CF,I with CH,CN, the pressure dependence of the rate constant and the kinetic isotope effect between 473 and 298 K. EXPERIMENTAL The experiments were performed in a cylindrical quartz vessel 10.0 cm long and of 5.0 cm diameter. The photolysis of CF,I was carried out at wavelengths > 290 nm by filtering light from an Osram HBO 500 W high-pressure mercury lamp with a Corning glass 7740 plate.The main products of the reaction, CF,X (where X = H or D) and C,F,, were analysed by gas chromatography . Trifluoromethyl iodide (PCR, Research Chemicals), acetonitrile (Carlo Erba) and deuterated acetonitrile (KOR, Isotopes) were purified as described previously.' Argon (La Oxigena) was bubbled through pyragallol and sulphuric acid and passed through traps at 1 13 K. Helium (Air Products, 99.99% ) was used without further purification. Perfluorocyclobutane (ICN, K & K) was purified by trap-to-trap distillation and the middle portion was retained. Iodine was purified by successive sublimations. Distilled water was thoroughly degassed at 273 K by repetitive freeezing and thawing.13031304 REACTION OF CF, RADICALS Table 1. Hydrogen abstraction from c-C,H,, by CF, radicals radical source log [(A,/A~)/cm~ mol-i s”] (E,-&)/kJ mol-l ref. CF,COCF, CF,COCF, (CF,CO),O (CF,CO),O CF,I CF,I CF,I CF,I 4.76 5.82 5.18 5.00 5.20 5.48 5.35 5.44 20.92 25.94 23.26 22.40 23.30 26.73 25.35 26.19 This work. Recommended value in ref. (3). RESULTS In order to check our experimental and analytical techniques we studied the reaction of cyclohexane with CF, radicals generated from the photolysis of (CF,CO),O and CF,I in the temperature range 298-473 K. The Arrhenius parameters for the hydrogen abstraction CF, + C-CeHl2 -+ CF,H + c-C,H,l (1) relative to the recombination reaction k c CF, + CF, -+ C,F, are in agreement with those obtained by other authors in almost the same temperature region (table I).When CF,I was used as the radical source over the same temperature range in the presence of CH,CN, the values of RcF3H/&F6[cH3CN] were considerably higher than those for kH/kk obtained from hydrogen abstraction kH CF, + CH,CN -+ CF,H + CH,CN (3) using (CF,CO),O to produce the radica1s.l This suggests that reaction (3) is not the only source of CF,H; thus an additional reaction leading to the formation of CF,H in the CF,I + CH,CN system should be considered. Table 2 summarizes some experiments performed at various temperatures, indicating that RcF,H/&.FB[CH,CN] remains constant either with a five-fold variation of CF,I pressure or with an increase in CH,CN pressure up to 60 Torr* (approximately the vapour-pressure limit).RCF,H/&~,F,[CH~CN] was also independent of photolysis time up to 7 h at 473 K and 15 h at 373 K. The conversion percentage was always < 1 % at any temperature (calculated from the CF,H yield), although the mean amount of CH,CN was almost the same as that present initially. Table 3 shows the results obtained from CD,CN in the same range (473-295 K). * 1 Torr = 101 325/760 PaL. PASTERIS AND E. H. STARICCO 1305 Table 2. Results of the reaction between CF31 and CH3CNa T/K t / s CH3CN 295 97200 295 87540 353 14700 373 14400 373 11 040 373 10800 373 10 800 373 9240 373 54000 398 5 820 398 7 140 398 6420 423 7 800 423 10 800 423 11 580 423 7200 423 10 960 448 7 200 448 7 200 448 7080 473 5 640 473 5400 473 5 700 473 5 520 473 7200 473 10 800 473 10 920 473 25 800 473 5 520 473 1680 58.2 48.0 35.7 19.8 26.8 35.5 35.6 37.8 60.0 17.9 30.3 50.6 5.8 23.7 32.0 34.4 40.6 10.1 21.7 43.0 8.1 38.0 51.5 22.5 21.0 22.3 23.1 21.3 23.0 22.3 88.2 96.9 98.1 78.1 98.0 97.6 52.5 208.2 100.0 98.7 98.6 99.8 51.5 51.8 49.9 52.2 50.5 51.7 52.1 50.5 48.7 50.1 50.9 51.0 50.0 51.0 50.7 49.0 102.8 248.0 0.089 0.134 0.668 0.793 0.716 0.840 0.534 1.363 0.326 1.243 1.179 1.345 0.903 0.780 0.658 0.634 0.653 1.069 0.823 0.808 1.279 0.719 0.771 1.066 0.837 0.804 1.075 0.239 1.832 6.270 0.0 16 0.015 0.222 0.256 0.383 0.404 0.432 0.605 0.454 0.525 1.020 1.555 0.307 0.960 1.577 1.532 1.710 1.063 2.243 4.546 1.632 5.423 7.728 4.348 3.574 3.123 4.383 1.736 5.628 10.22 0.0054 0.0049 0.053 0.110 0.120 0.091 0.120 0.101 0.097 0.206 0.243 0.208 0.46 0.38 0.5 1 0.47d 0.43 0.90 1.01 1.04 1.66 1.57 1.59 1.74 1.73 1.45 1.71 1.55 1.69 1.71 a Volume of reaction vessel is 168 cm3.Rates of product formation, R, in units of Photolysis without 1013 mol cm-3 s-l. Corning glass filter 7740 quartz window. RcF3H/R~zFe[CH,CN] in units of cm: mol-* s-i. The data presented so far are consistent with the expression (where X = H or D) at any temperature. Averages of kobs at each temperature are plotted following the Arrhenius equation in fig. 1. We have included the results of our previous paper for comparison. The pronounced curvatures at temperatures < 498 K in the CF31 + CX3CN systems are clearly outside experimental error. I? order to obtain more information about the increase in kobs over the value of k,/kL, taken from runs with (CF,CO),O or CF31 at high temperatures' we performed some runs with the addition of different gases such as He, Ar, c-C,F,, H,O and I,.High pressures1306 REACTION OF CF, RADICALS Table 3. Results of the reaction between CF,I and CD,CNa reactant pressure/Torr RCFID/ T/K t / s CD,CN CF,I RCzFeb &c,F,[CD,CN]c 295 86400 45.9 99.0 353 21 600 25.5 99.8 373 14 520 25.1 97.8 373 14610 35.8 94.3 388 50400 33.0 42.6 398 12 600 17.0 95.1 398 10 800 44.1 93.2 423 10 800 7.2 100.0 423 9000 17.0 100.6 423 9 000 41.9 96.9 436 7 200 38.0 100.1 448 7 800 12.4 51.0 448 8400 38.2 51.1 473 7200 8.4 50.0 473 7 500 42.7 48.4 0.227 0.443 0.653 0.504 0.020 0.738 0.732 1.047 1.085 0.973 1.083 0.950 0.7 18 1.197 0.967 0.017 0.026 0.039 0.063 0.062 0.09 1 0.21 1 0.096 0.202 0.469 0.589 0.269 0.706 0.339 1.373 4.8 x 10-4 0.010 0.015 0.018 0.032 0.049 0.044 0.109 0.095 0.095 0.128 0.197 0.193 0.344 0.305 a Volume of reaction vessel is 168 cm3.R in units of lo1, mol cm-, s-l. In units of cm: mol-2 s-i. Fig. 1. Arrhenius plot: 0, CH,CN + (CF,CO),O from ref. (1); ., CD,CN + (CF,CO),O from ref. (1); a, CH,CN+CF,I; 0, CD,CN+CF,I. Points with CF,I at temperatures higher than 473 K correspond to ref. (1).L. PASTERIS AND E. H. STARICCO 1307 Table 4. Effect of pressure of added gas on kobs reactant pressure/Torr kobsl added gas CH,CN CF,I added gas cmt mol-: s-: a He 39.4 101.0 101.1 41.9 99.8 302.5 36.2 106.7 374.2 43.0 97.6 567.6 Ar 52.0 88.1 95.2 53.0 101.7 395.6 c-C,F, 36.7 96.2 25.8 36.8 102.1 46.3 39.9 99.5 75.5 H2O 33.9 101.5 5.5 36.2 100.1 12.0 32.9 93.4 16.5 12 39.5 99.1 < 1 37.4 102.4 < 1 0.098 0.07 1 0.052 0.053 0.10 0.050 0.104 0.081 0.057 0.098 0.062 0.059 0.140 0.10 a T = 373 K, photolysis time 12000 s; kobs (CFJ + CH,CN) = 0.1 1 f 0.02 without inert gas; VG-2 Schott and k H / e (373 K) = 0.055 5 0.005 from Arrhenius parameters given in ref.(1). Gen blue filter; photolysis time 42 h. of He and Ar in CF,I + CX&N mixtures had no effect on the rate constant at high temperatures, but a marked diminution of kobs was observed when inert gases were added in the temperature range considered in this work. The data in table 4, which correspond to 373 K, show that the pressure effect reduced the value of eqn (I) from that obtained in the CF31 + CH,CN system without adding any foreign gas to the value of the (CF,CO),O+CH,CN system.In other words, kobs is reduced to the hydrogen-abstraction value, k,/j$. The most efficient gas in table 4 seems to be water. Water may be considered as an inert gas at 373 K because the amount of CF,H produced by the reaction CF, + H,O -+ CF3H +OH (4) (studied previ~usly)~ is negligible. Nevertheless we carried out a blank experiment with 16.0 Torr of H,O and 104.3 Torr CF,I during the same photolysis time as that mentioned in table 4 and no CF3H peak was observed by gas chromatography. Conversely, the addition of iodine to a mixture of CF31 and CH,CN at a pressure < 1 Torr under typical conditions produced an increase of ca. 40% in kobs. Because of this result, iodine was added to mixtures of (CF,CO),O and CH,CN, and this enhancement was also observed though the values of RCF,H/RL,F,[CHsCr\rl being higher than the experimental scattering of the results.Although when adding iodine the reaction became important, it was still possible to measure the C,F, concentration. CF, + I, -+ CF31 + I ( 5 ) The last run in table 4 was performed with light that had passed through a VG-21308 REACTION OF CF, RADICALS (Schott and Gen) blue filter which removed wavelengths in the range ca. 450-650 nm. Even though this run might be considered semiquantitative because of the long reaction time and the small yield of CF,H, the result showed a decrease in kobs. DISCUSSION Recent experiments covering wide temperature ranges have shown that the rate constants of elementary bimolecular reactions cannot always be represented by a single Arrhenius expression.We attempted to represent our curved ‘ Arrhenius’ lines by the rate-constant expression k = A P exp (- E/RT), but the value of the parameter n which satisfactorily fits all the available data is outside reasonable limits. We also failed using an exponential temperature dependence of the form k = exp (a + bT). Measurements with CD,CN confirmed that the curvature could not be attributed to a tunnelling effect. Furthermore, the results obtained with cyclohexane show that the curvature is not related to experimental conditions. Indeed, our experimental results should be interpreted in terms of two reactions with different activation energies leading to the same product.In other words, the abstraction by CF, radicals is not the only way to produce CF,H when CF,I is used to generate the radicals. The pressure effect may be interpreted in terms of the participation of an excited species. If the excess of energy of that required to break the C-I bond in CF,I appeared as kinetic energy, hot CF, radicals would abstract hydrogen from CH,CN. If this were the other source of CF,H, there would be a wavelength dependence of the CF,H excess since at 300 nm, for example, ca. 55 kJ mol-l would appear as translational energy in the CF, radicals, whereas at 250 nm ca. 107 kJ m o t 1 would reside in the radical. Runs performed without filtering the light with the Corning plate yield the same value of kobs, so no wavelength effect is apparent (table 2).Furthermore, at high temperatures runs in which CF, radicals were generated by thermal decom- position of CF,I gave the same rate constants as those in which CF, was produced by U.V. irradiation.’ On the other hand, if translationally excited CF, radicals were involved we would observe ‘extra’ CF,H when CF:J was photolysed in the presence of c-C,H,, in the same temperature range. Since CF, radicals produced from (CF,CO),O photolysis do not carry significant amounts of excess kinetic energy, the same behaviour would be expected for both substrates, c-C,H,, and CH,CN. As mentioned above, the Arrhenius plots for hydrogen abstraction from c-CJI,~ by CF, produced from the photolysis of (CF,CO),O and CFJ are straight lines wirh almost identical Arrhenius parameters that satisfactorily agree with values found in the literature.Hence it is possible to rule out abstraction by hot CF, radicals in the present system. The photolysis of CF,T yields electronically excited iodine atoms with a broad-band quantum yield of almost one. Since I(2Pl/2) atoms possess ca. 90 kJ mol-I of energy above I(zP,,2) atoms and the rates of the chemical reaction are enhanced, in certain cases being comparable with the competing spin-orbit relaxation process U2Pl/2) + M --+ V2P3,2) + M (6) we should consider another possible mechanism involving iodine atoms. Therefore the other source of CF,H might be a hydrogen abstraction from CH,CN by 1(2P1,2). Considering that the C-H bond dissociation energy ranges from 300.9 to 388.7 kJ mol-l, the reaction will be exothermic or at least thennoneutral: 1(,&/2) + CHSCN -+ IH + CH,CN (7) followed by IH + CF, -+ CF,H + I .(8)L. PASTERIS AND E. H. STARICCO 1309 The steady-state relationship will be where k,/ki = k,/@c. No conversion effect was observed in runs with varying photolysis times. This might be due to the fact that reaction (8) has almost no activation energy6 so that IH seems to reach a stationary concentration. On the other hand, even though the quantum yield of the production of l(,P1/,) changes with wavelength, being 0.75,7 0.91* and 0.85’ at 248, 266 and 308 nm, respectively, we were not able to detect a wavelength dependence of the extra CF,H by removing the glass filter and letting light of all wavelengths irradiate the reaction vessel, since broadband measurements of the quantum yield gave a value > 0.9.9 The efficiency of the added gases in reducing RCF,H/~c,F,[CH3CNl to the value of k,/kk shown in table 4 is fully consistent with the presence of reaction (7).No effect in reducing kobs would be expected on increasing the pressure of CH,CN because reactive collisions rather than deactivating collisions occur. Also, a large pressure of CF,I has no apparent quenching effect, probably because during the time the lamp is switched on, excited iodine atoms are being produced in proportionally larger quantities. It is known that I, is a very good quencher for the relaxation of excited atoms, and the enhancement observed in kobs might be explained by a increase in the production of I(,&/,) through the reaction I, + h -P I(,Pl/,) + I(,A,) (9) which could be rejected by removing visible light. Based on the results of Callear et ~ 1 .~ ~ 7 l1 for hydrogen-atom abstraction from a series of alkanes by I(2&/2) produced from the photolysis of I, with visible light, IH should be observed between I, and CH,CN. Some experiments were performed with the object of detecting IH even though the yield might be very small. Runs with mixtures of I, (< 1 Torr) and CH,CN (ca. 40 Torr) were photolysed at 373 K and at room temperature with the same experimental set-up used in the CF,I + CH,CN system. Mass spectra of the reaction products showed the presence of a peak at m/e 128 which was not found when spectra were taken from the reactants alone.If reactions (7) and (9) took place in the I, + CH,CN system we would expect a larger yield of IH when CF31 was photolysed in presence of CH,CN. We confirmed that IH is a product of the latter system when the whole of the reaction vessel was transferred into the inlet of the mass spectrometer. Finally, since atomic iodine is involved in the production of the ‘extra’ RH (R = CF, or C,F,) in reactions of HCNl2,l3 and CH,CN with peduoroalkyl iodide, it is interesting to note the main differences in the behaviour of the nitriles. In the reaction with HCN a pronounced increase in RRH/fiR,[HCN] with reaction time was observed, whereas this effect was negligible in the case of CH,CN. Therefore, some species that accumulate as the reaction proceeds could be responsible for the ‘extra’ KH in the HCN system, and because of its high endothermicity hydrogen abstraction by 1(2P1/2) from HCN can be ruled out.In conclusion, although this last mechanism seems to conform to most of the observed experimental results, we do not have definite evidence to assess that hydrogen abstraction by excited iodine atoms is responsible for the additional source of CF3H. Hence, iodine is involved but its participation is not fully understood.1310 REACTION OF CF, RADICALS We thank CONICET (Argentina) for partial financial support through the Instituto de Investigaciones en Fisicoquimica de Cordoba (INFIQC). We also thank the referees for valuable comments. L. Pasteris, E. V. Oexler and E. H. Staricco, Ber. Bunsenges. Phys. Chem., 1984, 88, 568. S. W. Charles and E. Whittle, Trans. Faraday Soc., 1968, 64, 414. S. H. Jones and E. Whittle, Int. J. Chem. Kinet., 1970, 2, 479. A. Chamberlain and E. Whittle, J . Chem. Soc., Faraday Trans. I , 1972, 68, 88. L. Pasteris, E. V. Oexler and E. H. Staricco, Znt. J . Chem. Kinet., 1983, 15, 835. J. C. Amphlett and E. Whittle, Trans. Faraday SOC., 1967, 63, 2695. L. S. Ershov, V. Yu. Zalesskii and V. N. Sokolov, Sou. J. Quantum Electron., 1978, 8, 494. T. Donohue and J. R. Wiesenfeld, J . Chem. Phys., 1975, 63, 3130; V. S. Ivanov and V. A. Elokhin, Sov. J. Quantum Electron., 1980, 10, 566. ' E. Gerck, J. Chem. Phys., 1983, 79, 31 1. lo A. B. Callear and J. F. Wilson, Trans. Faraday Soc., 1967, 63, 1358. l1 T. W. Broadbent and A. B. Callear, Trans. Faraday SOC., 1971,67, 3030. l 2 L. Pasteris, E. V. Oexler and E. H. Staricco, Int. J. Chem. Kinet., 1983, 15, 187. l3 S. Lane, E. V. Oexler and E. H. Staricco, Ber. Bunsenges. Phys. Chem., 1983, 87, 158; S. Lane, E. V. Oexler and E. H. Staricco, Int. J. Chem. Kinet., in press. (PAPER 4/8 15)

ISSN:0300-9599    

DOI:10.1039/F19858101303

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1985, 81, 1311-1327 Tin Oxide Surfaces Part 14.-Infrared Study of the Adsorption of Ethane and Ethene on Tin(rv) Oxide, Tin(rv) Oxide-Silica and Tin(rv) Oxide-Palladium Oxide BY PHILIP G. HARRISON* AND BARRY MAUNDERS Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD Received 4th June, 1984 The adsorption of ethane and ethene onto tin(1v) oxide, tin(1v) oxide-silica, and tin@) oxide-palladium oxide at various pretreatment temperatures in the range 320-740 K has been studied by transmission infrared spectroscopy. In every case the ultimate surface product was a surface acetate, except in the case of ethane and tin(1v) oxide, where no evidence for adsorption could be obtained under any conditions. The proposed mechanism for the formation of acetate involves the oxidation of an initially formed surface ethoxide species, although this intermediate is thought to arise via dissociative chemisorption, with C-H bond fission in the case of ethane and an electrophilic addition of surface hydroxyl groups across the C=C double bond in the case of ethene.Decomposition of the surface acetate to a surface carbonate occurs at temperatures > 580 K. The partial oxidation of hydrocarbons to oxygen-containing organic products is a process of great industrial importance. Ethane in particular is an abundant raw material, yet the literature concerning ethane reactions over heterogeneous oxidation catalysts is rather sparse. Only physisorption appears to occur on nickel oxidel and zinc oxide2 at 293 K, and very little reaction occurs over nickel oxide at higher temperatures (573-6’73 K).3 However, ethane is oxidised at elevated temperatures in the absence of air over the oxides of vanadium (623-673 K), molybdenum (773-823 K) and tungsten (823-923 K) to give as oxidation products formaldehyde, acetic acid, carbon monoxide, carbon dioxide and, under certain conditions, ethene.* Conversion of ethane into ethene also occurs over mixed-oxide systems of molybdenum and vanadium combined with an oxide of Ti, Cr, Mn, Fe, Co, Ni, Nb, Ta or Ce at 473 K in the presence of ~ x y g e n .~ Ethene can undergo several different types of reaction over metal oxides. Polym- erisation occurs on oxides of nickel, copper and palladium supported on porous silica glass to give surface polymerised species whose nature depends upon the particular transition metal.In addition, the vapour phase over the nickel sample has been observed to contain trans-b~t-2-ene.~ Saturated surface species of the types -CH2-CH, and -CH2-CH2- have been demonstrated on alumina at low temperatures, whilst surface acetate is formed at temperatures > ca. 523-573 K.’-lo Ethene is both physisorbed and chemisorbed on zinc oxide pretreated at 773 K, the chemisorbed product being olefinic in character and bound to the surface by interaction of the TC e l e c t r o n ~ . ~ l - ~ ~ At temperatures > 573 K, ethene is oxidised by oxygen over zinc oxide to carbon dioxide and water, although again a surface n-olefin complex was p0stu1ated.l~ Oxidation to carbon dioxide, water and acetic acid occurs 131 11312 TIN OXIDE SURFACES over 10% Pd on titania.15 On titania itself, ethene is adsorbed as an alkoxide with cleavage of the C=C bond.16 No chemisorption of ethene occurs on ~i1ica.l~ There is, apparently, no description in the literature of reactions of ethene over tin(1v) oxide or mixed oxides containing tin(1v) oxide, and very few for ethene.Complete oxidation occurs over the neat oxide at temperatures > 623 K and the reactivity of the catalyst is enhanced by the incorporation of O.l-l% chromium in the surface.17 Oxidation to carbon dioxide occurs over tin@) oxide-molybdenum(v1) oxide, although acetaldehyde could also be detected at reaction temperatures -= 413 K.l* In this paper we report a detailed infrared study of the adsorption behaviour of ethane and ethene on tin(1v) oxide, tin@) oxide-silica and tin@) oxide-palladium oxide.EXPERIMENTAL The preparation of tin(rv) oxide, tin@) oxide-silica and tin(1v) oxide-palladium, the manufacture of infrared-transmitting discs therefrom and the general techniques employed have been described previo~sly.~~ Infrared spectra were recorded using a Perkin-Elmer 577 spectrometer. Spectral data are summarised in tables 1 and 2. RESULTS ETHANE ADSORPTION TIN@) OXIDE-SILICA A tin(1v) oxide-silica disc that had been evacuated at 320 K, exposed to ethane (0.33 kN m-2), re-evacuated and then subsequently heated to 488 K gave rise to new absorption bands at 1725, 1525 and 1431 cm-l, with weaker bands at 1380 and 1355 cm-l and a shoulder on the high-wavenumber side of the 1525 cm-l band (fig.1). Similar bands were observed when ethane (0.33 kN m-2) was adsorbed at 488 K on a tin(rv) oxide-silica disc pretreated by evacuation and oxygen treatment at 740 K (fig. 2). Additional weak bands were observed at 1630, 1330 and 1305 cm-l. The hydroxy stretching absorption bands of tin(rv) oxide-silica pretreated at 740 K exhibit a marked increase in intensity with the absorption of ethane, even before the appearance of the acetate structure (fig. 3). TIN(VI) OXIDE-PALLADIUM OXIDE A tin(1v) oxide-palladium oxide disc that had been evacuated at 320 K, exposed to ethane (0.21 kN mP2) and subsequently re-evacuated at 320 K gave rise to new absorption bands at 1720,1550-1 5 10 and 1422 cm-l, with weaker bands at 1370,1340 and 1290 cm-l (fig.4). Raising the evacuation temperature to 438 K left bands only at 1715, 1515 and 1422 cm-l, with a weak shoulder at 1380 cm-l. No new absorption bands were observed in the infrared spectrum of a tin(1v) oxide-palladium oxide disc pretreated by evacuation and oxygen treatment at 665 K then exposed to ethane vapour (0.27 kN m-2). Heating a similarly treated disc at 623 K resulted in the appearance of a band at 1590 cm-l, which like the tin(1v) oxide-silica system is most probably due to a surface carbonate decomposition product. In addition, the broad 1425-1 385 cm-l band shifts to 1390 cm-l and becomes much sharper. No surface adsorbed species were observed with pure tin(1v) oxide under any pretreatment conditions.Table 1.Infrared data for the adsorption on tin(1v) oxide-silica and tin(rv) oxide-palladium oxide 'd 9 pretreatment evacuation temperature temperature band position/cm-l /K /K 3: oxide Sn0,-SiO, 320/740 K 488 1725 1630 - 1525 1431 - 1380 1355 1330 1305 580 1725 - 1590 1525 1430 - - 1355 - 1305 8 Z 750 1720 - - 1510-1550 1422 - 1370 1340 - 1290 ' ? 320 K Z I M - - - 638 1705-1715 - 1590 1525 - 1390 - % - - - - - - - - 1590 - - - 438 1715 - - 1515 1422 - 1380 - assignments - 5 8 - W H J - - - - vaas(COO) V,(COO) - surface acetate - surface enolate - - v(C=C) - - - d(COH) - v(C-0) - - - - - - - - I1 I1 - I 320 K Sn0,-PdO I, physisorbed cyclic dimer of acetic acid; 11, carbonate formed by decomposition of surface acetate.e w P c Table 2. Infrared data for ethene adsorption on tin@) oxide, tin@) oxide-palladium oxide and tin(1v) oxide-silica pretreatment evacuation temperature temperature band position/cm-l oxide /K /K - 1535 1430 SnO, 320 320 - 1535 1425 445 1522 1420 475 - - - 1520 1422 Sn0,-PdO 320 320 513 - - 1520 1422 - 1520 1422 Sn0,-PdO 320/H, 320 Sn0,-SiO, 320 320 - - 1535 1435 1530 1430 566 632 1530 1428 - - 1530 1430 320 560 1720+1710 - - 1530 1430 660 - 1530 1430 320b - 1625 - 1550 1440 - 1530 1430 320 - - - - - - - - - - - - - - 1585 1530 - - - - - - Sn0,-SiO, 660 50P - - 1585-1590 1530 - - - - Sn0,-SiO, 660 506' - - assignments V,s(COO) Vs(CO0) - - - __ - surface acetate - - I11 I1 I 1340 - 1345 - 1350 - 1345 - - 1345 - 1345 - 1345 - 1350 - - - - - - - - - - 1395 1352 - 1352 - 1352 - 1395 - - 1352 - - 1352 - - - - - 1388 1305 - I, carbonate formed by decomposition of surface acetate; 11, hydrogen-bonded acetic acid; 111, physisorbed cyclic dimer of acetic acid.a In the presence of ethene vapour; in the presence of water vapour.P. G. HARRISON AND B. MAUNDERS 1315 1800 1600 ]LOO 1200 wavenumber/cm-' Fig. 1. Infrared spectra of tin(1v) oxide-silica: (1) evacuated 320 K, 18 h, 1.33 x lop4 N mp2, (2) exposed to ethane, 320 K, 2.5 h, 0.33 kN mp2; subsequent evacuation (3) 320 K, 20 h, 1.33 x N m+, (4) 440 K, 3 h, 1.33 x N mP2, (6) 580 K, 6 h, 1.33 x lo-* N m-2 and (7) 638 K, 22 h, 1.33 x lo-* N m-2. N m-2, (5) 488 K, 15 h, 1.33 x ETHENE ADSORPTION TIN(IV) OXIDE A tin(1v) oxide disc that had been evacuated, exposed to ethene vapour (0.13 kN m-2) and then re-evacuated, all at 320 K, for 23 h ( < 1.33 x lo-* N m-2) exhibited broad weak bands in the infrared spectrum centred at ca.1535 and 1430 cm-l and a weak shoulder at 1340 cm-l (fig. 5). Raising the evacuation temperature to 445 K increased the intensity of all three bands, with their maxima occurring at 1535, 1425 and 1345 cm-l. A further increase in the temperature to 475 K led to further increases in intensity with the maxima shifting to 1522, 1420 and 1350 cm-l. Evacuation at 538 K resulted in the removal of these bands. A tin@) oxide disc that had been evacuated and treated with oxygen at 563 K and subsequently exposed to ethene vapour did not exhibit any new absorption bands in the 1800-1 100 cm-l region. However, in the 4000-2000 cm-l region the initial hydroxy stretching envelope was seen to decrease in intensity during exposure to the ethene, but it was restored to its initial intensity upon subsequent evacuation (fig.6).1316 TIN OXIDE SURFACES r 1900 1700 1500 1300 1100 wavenurn ber/cm -' Fig. 2. Infrared spectra of tin(rv) oxide-silica: (1) evacuated 740 K, 1 1 h, 6.65 x lop5 N m-2, oxygen treated 740 K, re-evacuated; in the presence of ethane (0.33 kN mP2) (2) 320 K, 18 h, (3) 390 K, 5 h, (4) 488 K, 48 h; subsequent evacuation (5) 320 K, 16 h, 6.65 x N m-2, (6) 390 K, 3 h, 6.65 x N m-2, (7) 490 K, 6 h, 6.65 x N m-2, (8) 570 K, 2.75 h, 6.65 x N m-2, (9) 670 K, 4 h, 6.65 x N m-2 and (10) 750 K, 12 h, 6.65 x N m-2. TIN(IV) OXIDE-PALLADIUM OXIDE A tin(1v) oxide-palladium oxide disc that had been evacuated and then exposed to ethene vapour (0.23 kN m-2) for 1 h at 320 K led to the formation of new absorption bands at 1520, 1422 and 1345 cm-l, which grew in intensity after 3 h in the ethene vapour (fig.7). Subsequent evacuation at 320 K led to a further increase in the intensities, while raising the evacuation temperature to 513 K produced even more intense bands. Evacuation at 573 K produced a decrease in intensity, while the bands were virtually removed after evacuation at 645 K. A tin@) oxide-palladium oxide disc that had been evacuated and treated with oxygen at 673 K did not exhibit new absorption bands in the 1800-1 100 cm-l region upon exposure to ethene vapour (0.31 kN m-2) at 320 K. It did, however, exhibit a decrease in intensity of the band100 80 h s 2 60 e f 3 Y i .- E $ 4 0 L.Y 20 P. G . HARRISON AND B. MAUNDERS 1317 t t 01 I I I 4000 3500 3000 2 500 2000 waveiiumberlcm-' Fig. 3. Infrared spectra of tin(1v) oxide-silica: (1) evacuated 740 K, 11 h, 6.65 x N m-2, oxygen treated 740 K, re-evacuated; in the presence of ethane (0.33 kN m-2) (2) 320 K, 18 h, (3) 488 K, 48 h and (4) subsequent evacuation, 320 K, 16 h, 6.65 x low5 N m-2. in the hydroxy stretching region. When the disc was subsequently treated with hydrogen, evacuated and re-exposed to ethene vapour (0.23 kN mP2) at 320 K and then heated to 410 K, weak bands were observed in similar positions to the bands formed on the 320 K pretreated disc. A tin(1v) oxide-palladium oxide disc that had been evacuated, treated with hydrogen and then exposed to ethene vapour (0.37 kN m-2), all at 320 K, gave essentially the same spectra as observed in fig. 7 but with fewer bands of lower intensity.TIN(IV) OXIDE-SILICA A tin(rv) oxide-silica disc that had been evacuated, then exposed to ethene vapow (0.26 kN mP2) for 2.75 h and subsequently re-evacuated for 20 h, all at 320 K, gave rise to weak absorption bands at ca. 1535, 1435 and 1345 cm-l (fig. 8). The exact position of the 1535 cm-I band is obscured by the presence of the water bending mode of molecularly coordinated water, itself shifted slightly (ca. 10 cm-l) to 1600 cm-l. Increasing the evacuation temperature increased the intensity of these bands, with maximum intensity being observed after evacuation at 566 K. During this treatment, the 1535 and 1345 cm-l bands shifted slightly to 1530 and 1350 cm-l, respectively.The 1435 cm-l band, however, remained broad throughout and centred at ca. 1430 cm-l. The spectrum obtained after evacuation at 632 K indicated decomposition of the surface species, with the 1530 cm-l band greatly reduced and the 1430 cm-l band shifted to 1395 cm-l. A band at 1585 cm-l was also present. It is difficult to say at what temperature this band first appeared, as its position, coupled with its broad nature, is very close to that of the water binding mode. As such, it is hard to tell when the first was removed and the second formed. A tin(1v) oxide-silica disc that had been evacuated, treated with oxygen at 660K and then exposed to ethene vapour (0.32 kN mP2) at 320 K for 2.75 h exhibited no new absorption in the 1800-1 100 cm-ITIN OXIDE SURFACES 1318 1800 1600 1400 1200 wavenumber/crn-' Fig.4. 4 7 4 7 - 4 9 % 54 u m Y 'S 57 v) 3 6 0 - 10 O/O - I I I I I I 1000 1600 14 00 1200 wavenumber/cm-' Fig. 5. Fig. 4. Infrared spectra of tin(1v) oxide-palladium oxide: (1) evacuated 320 K, 19 h, 1.33 x N m-2, (2) exposed to ethane 320 K, 4 h, 0.21 kN m-2; subsequent evacuation (3) Fig. 5. Infrared spectra of tin(1v) oxide: (1) evacuated 320 K, 22 h, < 1.33 x N m-2, (2) exposed to ethene, 320 K, 25 h, 0.13 kN m-2; subsequent evacuation (3) 320 K, 3 h, < 1.33 x N m-2, (4) 320 K, 23 h < 1.33 x N m-2, (5) 445 K, 19.5 h, < 1.33 x 320 K, 17 h, 1.33 x N m-2 and (4) 438 K, 65 h, 1.33 x N mP2. N m-2 and (6) 475 K, 18.5 h, < 1.33 x N mF2. region of the infrared spectrum.However, after pumping off the ethene vapour, re-evacuating at 653 K with oxygen treatment and then exposing to ethene vapour (0.23 kN mb2) at 505 K, new bands were observed at 1530, 1428 and 1352 cm-l (fig. 9). Pumping off the ethene vapour had little effect on the spectra other than a small shift in the 1428 cm-l band to 1430 cm-l. Increasing the evacuation temperature to 560 K considerably increased the intensity of the three absorption bands, and a weak broad band, or doublet of bands, at 1720 and 1710 cm-l was also observed. The adsorbed species was observed to begin to decompose at an evacuation temperature of 615 K. In a similar manner to the disc treated at 320 K, the 1430 cm-l band broadened, then at 660 K a sharp band at 1395 cm-l was observed along with the 1585-1590 cm-l band.In the 4000-2000 cm-l region of the spectrum the effect ofP. G . HARRISON AND B. MAUNDERS 1319 100 8 0 h 5 8 60 5 € 2 2 40 Y c .- Y zoa 0 4000 3500 3000 2500 2000 wavenum ber1cm-I Fig. 6. Infrared spectra of tin(1v) oxide: (1) evacuated and oxygen treated at 563 K, re-evacuated; subsequent exposure to ethene, 320 K, 0.29 kN m-2 (2) 1.25 h, (3) 3 h and (4) re-evacuation, 320 K, < 1.33 x N m-2. ethene vapour on the hydroxy stretching bands can be observed. The most obvious effect of ethene adsorption is the specific reduction in intensity of the 3720 cm-l band, with a slight shift to 3700 cm-l, and the increase in the intensity of the broad band on the low-wavenumber side of the 3720 cm-l band. Pumping off the ethene vapour did not restore the original band.An evacuated tin@) oxide-silica disc (660 K with oxygen treatment) that had been treated with ethene to give the bands at 1530, 1430 and 1352 cm-l and then subsequently exposed to water vapour (0.13 kN m-2) exhibited absorption bands at 1625, 1550 (shoulder), 1440, 1388 (very weak) and 1305 ~ m - ~ (fig. 10). Subsequent evacuation restored the original bands. The shift in the acetate bands to 1550, 1440 and 1388 cm-l in the presence of water vapour in all three cases shows how the acetate structure is weakened by the water; indeed, some of it is protonated to acetic acid, which is bound to the surface by hydrogen bonding, some of the 1625cm-l band and the 1305cm-l band being attributable to the C=O stretching and 0-H deformation modes, respectively, of the hydrogen-bonded acetic acid.The remainder of the 1625 cm-l band can be assigned to the water bending mode. Pumping off the water vapour restores the positions of the acetate bands and removes most of the hydrogen-bonded acetic acid. DISCUSSION In all cases the observed absorption bands at ca. 1515-1535, 1420-1435 and 1345-1 355 cm-l can be assigned to the antisymmetric and symmetric v(C00) modes and the symmetric S(C-H) deformation mode, respectively, of a surface acetate species. We have previously reported the spectrum of surface acetate on tin(1v) oxide, whilst the spectra of surface acetate on tin(1v) oxide-silica on adsorption of acetic acid1320 TIN OXIDE SURFACES 1800 1600 1400 1200 wave nu rn ber/ c m - * Fig. 7.- I 1 I I I I I I 1800 1600 1400 1200 wavenumberlcm-' Fig. 8. Fig. 7. Infrared spectra of tin(rv) oxide-palladium oxide: (1) evacuated 320 K, 40 h, 1.33 x N m-2; subsequent exposure to ethene, 320 K, 0.23 kN mP2 (2) 1 h, (3) 3 h, re-evacuated, < 1.33 x 10-4 N m-2, (4) 320 K, 1 h, ( 5 ) 320 K, 21 h, (6) 513 K, 21 h and (7) 645 K, 17 h. Fig. 8. Infrared spectra of tin(1v) oxide-silica: (1) evacuated 320 K, 18 h, < 1.33 x N m-2, (2) exposed to ethene, 320 K, 2.75 h, 0.27 kN mP2; subsequent evacuation (3) 320 K, 20 h, < 1.33 x N m-2, (4) 445 K, 19 h, < 1.33 x loP4 N m-2, ( 5 ) 490 K, 3 h, < 1 . 3 3 ~ 10-4Nm-2.(6)535K,7h, < 1 . 3 3 ~ 1 0 - ~ N m - ~ , ( 7 ) 5 6 6 K , 2 0 h , < 1 . 3 3 ~ 1 O - ~ N m - ~ and (8) 632 K, 19 h, < 1.33 x N m-2. is shown in fig.1 1. No information could be gained from the 4000-2000 cm-l region of the spectra. However, spectra from the high-temperature-pretreated samples afforded indications as to the mechanism of the initial chemisorption reaction for both ethane and ethene. The increase in intensity of the hydroxy stretching band of tin(rv) oxide-silica that had been pretreated at 740 K indicates that new hydroxy groups are formed, either directly or from adsorption of water possibly produced in the reaction, as a result of carbon-hydrogen bond fission. The other species formed in this process would be expected to be an ethoxide group via dissociative chemisorption at two adjacent surface oxide sites (scheme 1, where M is tin or silicon). The mechanism of scheme IP. G. HARRISON AND B.MAUNDERS 1321 I 60 - 9 1 - 55- 6 5 - 6 9 - h aJ 0 m 4 4 Y .- : 2 Y 8 1- 8 5- - 10 "/o - I I I I I I 1800 1600 1400 1200 Fig. 9. wave number/ c m -' I I I I I I 1800 1600 1400 1200 wavenumber/cm -' Fig. 10. Fig. 9. Infrared spectra of tin(1v) oxide-silica: (1) evacuated and treated with oxygen at 660 K, re-evacuated < 1.33 x 10-4 N m-2, (2) exposed to ethene, 320 K, 2.75 h, 0.22 kN mP2, (3) evacuated and treated with oxygen at 653 K, exposed to ethene, 505 K, 1 h, 0.23 kN mP2; subsequent evacuation (4) 320 K, 16.5 h, < 1.33 x N m-2, (5) 560 K, 2 h, .< 1.33 x N m-2, (6) 615 K, 20 h, < 1.33 x N mP2 and (7) 660 K, 14 h, < 1.33 x lop4 N m-2. Fig. 10. Infrared spectra of tin@) oxide-silica: (1) evacuated and treated with oxygen at 653 K, exposed to ethene, 505 K, 1 h, 0.23 kN m-2, subsequent evacuation, 320 K, 16.5 h < 1.33 x N m-2, (2) exposed to water vapour, 320 K, 0.5 h, 0.13 kN mP2 and (3) re- evacuated, 320 K, 1.33 x N m-2.CH3 I I I I CHI i 7 i 0 0 0 6 6 0 0 0 0 CH3-CHz----H CH3-CH2-H I I I - I I I - I l l O/M\O/M\O/M\O O/M\O/M\O/M\O OOM\O/M\O/M\O 44 Scheme 1. FAR 11322 53 h 75 9 86 8 4 TIN OXIDE SURFACES 1900 1700 1500 1300 1100 wavenumber/cm-' Fig. 11. Infrared spectra of tin(rv) oxide-silica: (1) evacuated 750 K, 12 h, 6.65 x N m-2, oxygen treated 750 K, re-evacuated, (2) acetic acid vapour, 320 K, 1 h, 0.17 kN m-2; subse- quent evacuation (3) 320 K, 18 h, 6.65 x N m-2; (5) 560 K, 60 h, 6.65 x N m-2, (6) 658 K, 3.25 h, 6.65 x low5 N m-2 and (7) 750 K, 16 h, N m-2, (4) 470 K, 5 h, 6.65 x 6.65 x lod5 N m-2.is consistent with the nature of the oxide surface, but does not preclude the possible participation of surface 0,- and 0'- oxygen species, which have been proposed for other 21 Such species have been characterised by e.s.r. on tin@) oxide itself under similar pretreatment 23 Extensive interaction occurs between ethene vapour and the remaining surface hydroxy groups on high-temperature-pretreated samples of all three oxides. For both tin(1v) oxide and tin(1v) oxide-palladium oxide the hydroxy stretching bands initially shift to lower wavenumber and become broader, indicating a hydrogen-bonding interaction, then on prolonged exposure to ethene these bands decrease in intensity. This decrease can be rationalised in terms of addition of a surface hydroxy group across the carbon-carbon double bond of ethene.That the addition is reversible, under these conditions, is seen by the reappearance of the hydroxy band when the ethene vapour was pumped off in the case of tin(1v) oxide. With tin@) oxide-palladium oxide the hydroxy band did not reappear in its original form.P. G. HARRISON AND B. MAUNDERS 1323 However, as no other absorption bands were observed in the spectrum this could be caused by loss in transmission properties, the disc being orange-brown in colour before ethene absorption but much darker in colour afterwards, most probably because of oxygen depletion of the oxide. With the high-temperature-pretreated tin(1v) oxide-silica sample, the 3720 cm-l band was observed to decrease in intensity in the presence of ethene vapour and shifted to 3700-3710 cm-l with a slight increase in the broad OH band centred at 3360cm-l, indicating hydrogen bonding with surface Si-OH groups, presumably because of their higher Bronsted acidity.When the bands due to the surface acetate structure formed and ethene vapour was pumped off, the band due to isolated Si-OH groups was observed to increase again but did not return to its original value. Hence, although the initial surface adsorbate is formed reversibly, its transformation to surface acetate is irreversible. The initial interaction with ethene adsorption appears to be a reversible hydrogen- bonding interaction between the ethene and surface hydroxy groups, with subsequent formal addition of the 0-H band across the C=C double bond to generate a surface ethoxide group (scheme 2).In a previous study of the tin(v) oxide+ethene system, I CH3 I Scheme 2. Solymosi and B0Zs017 concluded that surface 0;- ions are the primary adsorption sites and that in the absence of gaseous oxygen surface lattice oxygen oxidises the hydrocarbon. However, no surface species were characterised and the only reaction products identified were carbon dioxide and water. Finklo has suggested that on alumina ethene possibly interacts with surface hydroxy groups. The greater reactivity of tin(v) oxide-silica towards ethene may be rationalised in terms of this mechanism. The rate-determining step in classical electrophilic addition to C=C double bonds is the addition of the positively charged part of the electrophile, i.e.: + I I > C = C < + f / W a >C-C-Y/w Hence, reaction with the more Bronsted-acidic surface silanol groups will be favoured compared with reaction with surface Sn-OH or Pd-OH groups, although these too undergo reaction. 44-21324 TIN OXIDE SURFACES Unfortunately, no direct evidence for a surface ethoxide intermediate could be detected from the spectra obtained in the present study. Bands that could be attributable to v(CH,) or v(CH,) stretches were obscured by the intensity and broadness of the hydroxy stretching modes, no deformation modes were observed and the region where a C-0 stretching band might have been observed was obscured by the strong absorptions of the oxide itself. Nevertheless, the occurrence of an intermediate surface ethoxide is highly probable and we have demonstrated previously the facile conversion of surface methoxy groups (generated by the chemisorption of either methanol or dimethyl carbonate) on tin(v) oxide into surface formate groups at temperatures as low as 320 K.24 Similar acetate formation from ethanol via an ethoxide intermediate has been reported by both G~eenler,~ and Kage126 on alumina and by Davydov et al.27 on chromia, whilst Fink2s has suggested that acetate formation from ethanol and alumina proceeds via a coordinated acetaldehyde species.Surface-bound acetaldehyde on tin(v) oxide also undergoes rapid oxidation at 320 K to produce surface acetate.24 Therefore the most probable mechanism for the formation of surface acetate from ethane and ethene on these oxides is by the further reaction of the ethoxide intermediate via the abstraction of a hydrogen atom from a surface ethoxide by a neighbouring oxide species giving an acetaldehyde molecule formally coordinated to a surface tin atom.This species then undergoes reaction with a surface hydroxy group to produce surface acetate and hydrogen (scheme 3). CH3 I \ 1: '\ I l l O/M\O/M\O/M\O CH3 H HC- H H HC,----? I \ I I l l O/M,O/M\o/y\o 0- d o d b 0 0 H CH, Scheme 3. No mass-spectroscopic analysis of the vapour phase was carried out in the present study. However, Kage126 has demonstrated that hydrogen is produced in the conversion of surface ethoxide to surface acetate on alumina. Note that in our previous study of the absorption of acetaldehyde onto tin(v) oxide we observed weak bands at 1620, 1380 and 1317 cm-l, which were assigned to the v(C=C), d(C0H) and v(C=O) modes, respectively, of a surface enolate, CH,=CH(OH),d,, species.24 Similar bands are also observed with ethane adsorption, but not with acetic acid adsorption, on tin(v) oxide-silica at 1630,1380 and 1330 cm-l, lending some support for the formation aldehyde-type intermediates.Ethane adsorption on the tin(v) oxide-silica disc that had been pretreated at 320 K was carried out at 320 K, after which it was pumped off, yet it was not until the discP. G. HARRISON AND B. MAUNDERS 1325 was heated at 488 K, still under vacuum, that the absorption band due to acetate appeared, because of further reaction of the initially surface-adsorbed intermediates. The two other weak bands, at ca.1720 and 1305 cm-l, are undoubtedly associated with the acetate structure, or acetic acid, for both these are observed with acetic acid adsorption as well as ethane adsorption. The 1720 cm-l band often appears split into two bands at ca. 1725 and 1710 cm-l; a similar band, and splitting, has been observed by Lorenzelli et ~ 1 . ~ ~ for acetic acid adsorbed on haematite, and it can be assigned to physisorbed cyclic dimers of acetic acid. The 1305 cm-l band may be associated with a C-H deformation mode or possibly the v(C0H) stretching mode of dimeric species. The nature of the acetate structure cannot be unambiguously determined from the infrared spectra. The separation of the v(C00) asymmetric and symmetric stretching modes for unidentate carboxylates [structure (I)] are greater than for the corresponding chelating bidentate carboxylates [structure (II)].For unidentate cyclopentadienyl titanium carboxylates [structure (111)] separations of up to ca. 300 cm-l have been observed, while for the bidentate chelate of structure (11) of carboxylate groups in Cp,Ti(CH,COO), and Cp,Ti(C,H,COO), separations of SO-60 cm-l were An alternative bidentate of structure (111), with the carboxylate groups bridging two titanium ions, in the dimers of CpTi(CH,COO), and CpTi(C,H,COO), exhibited a separation of ca. 170 cm-l. Lorenzelli et ~ 1 . ~ ~ observed a separation of 100 cm-l for an acetate structure on the surface of a-Fe,O,, which was assigned to the chelating bidentate configuration. In these studies the separation was observed to be 96 cm-l and consequently the chelating bidentate of structure (11) is proposed as the most likely structure, although the bidentate of structure (111) cannot be entirely ruled out.Decomposition of the surface acetate begins at 580 K, as observed by a reduction in the intensity of the acetate bands and the appearance of a band at ca. 1590 cm-l. A second band appears at 1390 cm-l, with the disappearance of the 143 1 cm-l band, on evacuation at 639 K. After evacuation at 750 K only the band at 1590 cm-l remains. The position of this band is characteristic of the v1(A1) mode of bidentate carbonate. The corresponding v,(B,) mode is expected to occur at ca. 1220 cm-l, but in the present case this region is marked by absorption of the bulk oxide material.,* Anshits et al.,l have reported that surface carboxylates on copper(1) oxide decompose to the formate at elevated temperatures over long periods. However, adsorption of formic acid on tin(1v) oxide-silica shows that this is not the case with this system.,, More likely is decomposition to a surface carbonate species.This type of decomposition occurs with acetate on magnesium oxide at 573-723 K, in which methane was also formed,,, and would be readily accounted for by reaction with neighbouring surface hydroxy groups (scheme 4). The results obtained in these systems demonstrate that tin(rv) oxide systems are very strongly oxidising towards hydrocarbons. In view of the poor activity of ethane over most metal oxides, the reactivity observed under the relatively mild conditions1326 TIN OXIDE SURFACES Scheme 4.in this study is quite surprising. A possible explanation for the reactivity of the tin(1v) oxide-silica is the net negative charge obtained on each condensed Sn-0-Si linkage. This would make these surface oxides more nucleophilic than on the pure tin@) oxide and thus more able to abstract a hydrogen from ethane. The activity of the tin@) oxide-palladium oxide is not likely to be caused by the same reason as the concentration ofpalladium oxide is much less than that of silicon oxide ; however, palladium-hydrogen bonds are well known in reactions involved with alkenes and alkynes, so it is possible that under those conditions the formation of a palladium-hydrogen bond may be the driving force.The reactions with ethene occur at much lower temperatures (320-445 K) than needed for surface acetate formation on alumina (523 K).l0 Again, the highly oxidising nature of the tin(1v) oxide surface, under such mild conditions, appears to be fairly unique amongst pure oxides, since only polymerisation of ethene was observed on the oxides of nickel, copper and palladium,6 while on zinc oxide ethene was observed to chemisorb by interaction of the n e l e c t r o n ~ . ~ ~ - ~ ~ Under more strongly oxidising conditions, in the presence of air or oxygen and at higher temperatures, ethene has been oxidised over zinc oxide14 and titania.l69 34 However, these studies did not observe the surface species formed, if any, but analysed the product gases to show exclusively the presence of carbon dioxide. The vapour phases of the present studies were not analysed, though it is possible that carbon dioxide was produced and that the surface acetate may either be an intermediate to CO, production or may form by a competing mechanism.Over mixed l8 quantities of acetic acid and acetaldehyde have been observed under certain conditions, as well as carbon dioxide, although when the conditions become harsher only carbon dioxide was observed, This may be because of oxidation of produced acetic acid or acetaldehyde, or it may just reflect that the harsh conditions favour carbon dioxide formation over acetic acid or acetaldehyde formation. The oxidising nature of tin(1v) oxide may well be caused by its ability to become non-stoichiometric by loss of oxygen, although, for formation of the surface acetate structure, the role of the surface hydroxy groups is also important.In reactions with ethene the tin@) oxide was always observed to discolour, which indicates the formation of non-stoichiometric tin oxide. Both tin(rv) oxide-palladium oxide and tin(1v) oxide-silica are strongly oxidising towards ethene under relatively mild conditions. The behaviour of tin(1v) oxide- palladium oxide is similar to that exhibited by palladium on titania, although in this case a temperature of 423 K was required for ethene conversion.15 Unlike the case for tin(rv) oxide itself and tin(xv) oxide-palladium oxide, a surface acetate is formed upon reaction of ethene with tin(1v) oxide-silica pretreated at a high temperature (660 K).This may be because of the retention of Bronsted-acidic silanol groups necessary for reaction under these conditions and/or the structural incompatibility between SnO, and SiO,, which prevents their heterolytic sintering and hence the propagation of oxygen deficiencies. Thus, the tin(1v) oxide retains its stoichiometry, as suggested by the lack of discoloration of the disc in this case, and still furnishes oxide ions for the oxidation of ethylene.P. G. HARRISON AND B. MAUNDERS 1327 We thank the S.E.R.C. and the International Tin Research Institute for support in the form of a CASE award (to B.M.). K. Kuchynku, Collect. Czech. Commun., 1968, 33, 3049. A. L. Dent and R. J. Kokes, J . Phys. Chem., 1970, 74, 3653. Y. F. Y. Yao and J. T. Kummer, J . Catal., 1973, 28, 124.H. Raudsepp and M. Mikkal, Tr. Tallin. Politekh. Inst., Ser. A, 1962, 198, 109; Chem Abs., 60, 150%. E. M. Thorsteinson, T. P. Wilson, F. G. Young and P. H. Kasai, J. Catal., 1978, 52, 116. L. H. Little, J . Phys. Chem., 1959, 63, 1616. P. J. Lucchesi, J. L. Carter and D. J. C. Yates, J. Phys. Chem., 1962, 66, 1451. F. Bozon-Veruraz and G. Pannetier, Bull. SOC. Chim. Fr., 1970, 3856. K. P. Zhdanova and N. I. Popova, Kinet. Catal. (Engl. Transl.), 1968, 9, 326. lo P. Fink, Rev. Roum. Chim., 1969, 14, 8 1 1 . l 1 A. L. Dent and R. L. Kokes, J . Phys. Chem., 1970, 24, 3653. l2 R. J. Kokes and A. L. Dent, Adv. Catal., 1972, 22, 1. l 3 A. G. Whitney and I. D. Gray, J. Catal., 1972, 25, 176. l4 Y. Kubekawa, T. Ono and N. Yano, J. Catal., 1973, 28,471. l5 A. Omar, G. Djega-Mariadassou, F. Bozon-Verduraz and G. Pannetier, Bull. SOC. Chim. Fr., 1974, l6 Y. M. Shchekochikhin, V. N. Filimonor, N. P. Keier and A. N. Terenin, Kinet. Catal. (Engl. Transl.), l7 F. Solymosi and F. BOZSO, Ber. Bunsenges. Phys. Chem., 1977, 81, 529. la S. Tan, Y. Moro-Oka and A. Ozaki, J. Cural., 1970, 17, 132. l9 P. G. Harrison and B. Maunders, J. Chem. Soc., Faraday Trans. I , 1975, 71, 461. 2o M. Iwamoto and J. H. Lunsford, J. Phys. Chem., 1980, 84, 3079. 21 S. L. Kaliguine, G. N. Shelimov and V. B. Kazansky, J. Catal., 1978, 55, 384. 22 J. C. H. Van Hooff, J. Catal., 1968, 11, 277. 23 M. Che, C. Naccache and B. Imelik, Bull. SOC. Chim. Fr., 1968, 12, 4791. 24 E. W. Thornton and P. G. Harrison, J . Chem. SOC., Faraday Trans. 1, 1975, 71, 2468. 25 R. G. Greenler, J . Chem. Phys., 1962, 37, 2094. 26 R. 0. Kagel, J. Phys. Chem., 1967, 71, 844. 27 A. A. Davydov, V. M. Shchekochikhin, P. M. Zaitsev, Yu. M. Shchekochikhin and N. P. Keir, Kinet. 28 P. Fink, Rev. Roum. Chim., 1969, 14, 81 1 . 29 V. Lorenzelli, G. Busca and N. Sheppard, J . Catal., 1980, 66, 28. 30 R. S. P. Coutts, R. L. Martin and P. C. Wailes, Aust. J . Chem., 1973, 26, 941. 31 A. G. Anshits, V. D. Sokolovskii, G. K. Boreskov, A. A. Davydov, A. A. Budneva, V. I. Ardeer and 32 P. G. Harrison and B. M. Maunders, unpublished data. 33 V. I. Yakerson and A. M. Rubinstein, Bull. Acad. Sci. USSR, Div. Chem. Sci., 1966, 1278. 34 B. Dmuchorsky, M. C. Freerks and F. B. Zienty, J . Catal., 1965, 4, 577. 12, 2740. 1964, 5, 113. Catal., 1971, 12, 611. I. I. Zakharov, Kinet. Catal., 1975, 16, 95. (PAPER 4/906)

ISSN:0300-9599    

DOI:10.1039/F19858101311

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. Soc., Faruduy Trans. I, 1985, 81, 1329-1343 Tin Oxide Surfaces Part 15.-Infrared Study of the Adsorption of Propene on Tin(1v) Oxide, Tin(1v) Oxide-Silica and Tin(rv) Oxide-Palladium Oxide BY PHILIP G . HARRISON* AND BARRY MAUNDERS Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD Received 4th June, 1984 The adsorption of propene on tin(1v) oxide, tin(rv) oxide-silica and tin(1v) oxide-palladium oxide has been studied by infrared spectroscopy. Tin(1v) oxide-silica were found to chemisorb propene as a surface acetate species when the oxides were outgassed at ambient temperatures, while only tin(1v) oxide-silica reacted with propene after outgassing at 716 K, again to form a surface acetate species. Tin(1v) oxide-palladium oxide outgassed at ambient temperatures was found to chemisorb propene both as a surface acetate and acrylate, while after outgassing at 570 K reaction with propene leads only to a surface acetate species.The most probable mechanism for the formation of the surface acetate involves initial electrophilic addition of acidic hydroxy groups to the C=C double bond, giving a surface isopropoxide which under- goes oxidation to the acetate via an intermediate, coordinated acetone. Consistent with this hypothesis, only tin(rv) oxide-silica is active at higher temperatures of pretreatment, where Bronsted acidity for this sample remains high but is very low for the other two oxides. The surface acrylate is formed by a similar process, although the initial process appears to involve a palladium-induced C-H bond fission of the methyl group of propene, generating a surface allyloxide species which undergoes oxidation as before.The reaction of propene over metal oxides has been the subject of several studies and a variety of behaviour has been observed. Only physisorption occurs on silica gel,l but where chemisorption does take place both dissociative and associative mechanisms can occur, leading to the formation of surface n-allylic and isopropoxide species, respectively. The majority of studies are concerned solely with the composition of the product vapour phase, with relatively few diagnosing the nature of the surface-adsorbed species and intermediates. Infrared data for adsorption on y-alumina have been interpreted as coordination of propene via the C=C double bond to a surface A13+ ion and a methyl-group hydrogen to a surface oxide.This initial species was then reported to convert into a surface n-allylic species.2 Reversible dissociative chemisorption to surface n-allylic species occurs on zinc oxide, which has been shown to undergo oxidation by an 0; ion to ac~olein,~-~ although in another study oxidation to acetate plus fonnate was reported.6 In a preceding paper of this series we have studied the reaction of ethane and ethene with tin(1v) oxide, tin(1v) oxide-silica and tin(1v) oxide-palladium oxide, and although surface acetate was the ultimate product in each case, different chemisorption mechanisms operated.’ Here we report the results of a similar study of the adsorption of propene on the same oxides. 13291330 TIN OXIDE SURFACES 1 I I I I I I I wavenumber/cm-' Fig.1. Infrared spectra of tin(1v) oxide: (1) evacuated 320 K, 18 h, < 1.33 x N m-2, (2) exposed to propene, 320 K, 18.75 h, 0.27 k n mP2, (3) evacuation, 320 K, 2 h, < 1.33 x N mW2, (4) exposed to propene, 473 K, 2.5 h, 0.37 kN mP2; subsequent evacuation (5) 320 K, 2.5 h, < 1.33 x N m-2 and (7) 563 K, N m-2, (6) 523 K, 2.75 h, < 1.33 x 15.5 h, < 1.33 x N m-2. EXPERIMENTAL The preparation of tin(1v) oxide, tin(rv) oxide-silica and tin(rv) oxide-palladium, the manufacture of infrared-transmitting discs therefrom and the general techniques employed have been described previously.' Infrared spectra were recorded using a Perkin-Elmer 577 spectrometer. RESULTS TIN(IV) OXIDE N m-2) (fig.1) was exposed to propene vapour (320 K, 0.27 kN m-2) for 18.75 h. Only weak bands due to vapour-phase propene were observed on top of the background spectrum of the disc, and these were removed by evacuation (320 K, < 1.33 x lo-* n m-2). The disc was subsequently heated at increasing temperatures in the presence of propene vapour but no new bands were seen in the infrared spectrum until the sample was heated at 473 K. An evacuated tin(1v) oxide disc (320 K, < 1.33 x42 55 % 68 86 h riJ * c .- E - g 10 Y o h Y - 61 57 P. G . HARRISON AND B. MAUNDERS 1331 ~~~ 1800 1600 1400 1200 wavenumber/cm -' Fig. 2. Infrared spectra of tin(1v) oxide-palladium oxide: ( 1 ) evacuated 320 K, 2.5 h, <-1.33 x N m-2; exposed to propene, 320 K, 15.5 h, 0.27 kN m-2 and subsequently evacuated (2) 320 K, 2.5 h, < 1.33 x lo-* N m-2, (3) 445 K, 15 h, < 1.33 x lop4 N m-2, (4) 485 K, 19.5 h, < 1.33 x lop4 N m-2, ( 5 ) 564 K, 3.75 h, < 1.33 x N m-2 and (6) 61 1 K, 3.5 h, < 1.33 x N m-a.Under these conditions a broad band was observed centred at 1530 cm-l. Evacuating the cell (320 K, < 1.33 x N mA2) left stable absorption bands at 1515, 1425 and 1345 cm-l, the latter being very weak. The 1515 and 1425 cm-l bands increased in intensity upon heating under vacuum at 523 K. A tin(rv) oxide disc that had been pretreated by evacuation and then oxygen treatment at 605 K did not exhibit new absorption bands with propene at any reaction temperature, nor did a tin(1v) oxide disc that had been evacuated and oxygen treated at 738 K and then exposed to pro- pene + water-vapour mixtures; no rehydroxylation was observed in the latter case.TIN(1V) OXIDE-PALLADIUM OXIDE A tin(rv) oxide-palladium oxide disc that had been evacuated (320 K, < 1.33 x lob4 N mP2) (fig. 2), exposed to propene (320 K, 0.27 kN m-2) for 15.5 h and subsequently re-evacuated (320 K, < 1.33 x N m-2) exhibited new adsorption bands at 1630, 1515, 1428, 1365, 1270 and 980cm-l. The 1515cm-l band was unsymmetrical with a shoulder on the high-wavenumber side. In addition, an appreciable decrease in the intensity of the strong, broad band between 3600 and 2000 cm-l was observed. Increasing the evacuation temperature to 445 K increased1332 TIN OXIDE SURFACES I I I I I I I 1800 1600 1400 1200 wavenumber/cm-' Fig.3. Infrared spectra of tin(1v) oxide-palladium oxide: (1) evacuated 477 K, 15 h, < 1.33 x N m-2, (2) exposed to propene, 320 K, 0.75 h, 0.19 kN m-2, then evacuated 484 K, 3 h, < 1.33 x N m-2; exposed to propene 487 K, 2.5 h, 0.27 kN m-2 and then subsequently evacuated (3) 320 K, 43 h, < 1.33 x N m-2, (4) 579 K, 15.25 h, c 1.33 x N m-2 and (5) 666 K, 15 h, < 1.33 x lo-* N m-2. the intensity of the 15 15 and 1428 cm-l bands with respect to the other bands and exposed the presence of two other weak bands at 1065 and 820 cm-l, while at higher evacuation temperatures (485,521 and 564 K) the 1630,1365,1270,1065 and 820 cm-l bands decreased in intensity and were effectively removed after evacuation at 61 1 K. At this latter temperature a very weak band at 1350 cm-l replaced the 1365 cm-l band with the 1428 cm-l band shifted to 1420 cm-l.Evacuation at 659 K brought about almost complete removal of the 151 5 cm-l band, removal of the 1420 cm-l band and appearance of two new bands at 1585 and 1395 cm-l. A tin(rv) oxide-palladium oxide disc that had been evacuated, treated with oxygen at 570 K and re-evacuated at 477 K (1 5 h, < 1.33 x N m-2) was exposed to propene (320 K, 0.19 kN mP2) for 2 h, evacuated and heated to 484 K (fig. 3 and 4). No new bands appeared in the spectrum, although the broad band centred at 31 50 cm-l was removed. The disc was subsequently heated in propene (487 K, 0.27 kN rn-2), cooled and evacuated. New absorption bands were observed at 1630-1 540 (broad and weak), 1505 and 1420 cm-l, the latter band having a shoulder on the low-wavenumber side.In addition, a broad band centred at 3200 cm-l appeared. Evacuation at increasing temperatures had little effect until 579 K was reached. At this temperature the 1420 cm-l band was shifted to 1390 cm-l. Evacuation at 666 KP. G . HARRISON AND B. MAUNDERS 1333 wavenumber/cm-' Fig. 4. Infrared spectra of tin(1v) oxide-palladium oxide: (1) evacuated 477 K, 15 h, < 1.33 x 10-4 N m-2, (2) exposed to propene, 320 K, 0.75 h, 0.19 kN m-2 and then evacuated 484 K, 3 h, < 1.33 x N mP2, (3) exposed to propene, 2.5 h, 0.27 kN m-2 and then evac- uated 320 K, 43 h, < 1.33 x lo-* N m-2. greatly reduced the intensity of the 1505 cm-l band. Two bands at 1585 and 1390 cm-l were now present and were very similar in nature to those observed on the low-temperatdre pretreated tin(1v) oxide-palladium oxide after evacuation at 659 K.A tin(1v) oxide-palladium oxide disc that had been pretreated by calcination and oxygen treatment at 704 K exhibited no new absorption bands with propene under the conditions employed. TIN(IV) OXIDE-SILICA A tin(1v) oxide-silica disc that had been evacuated at 320 K was exposed to propene vapour (320 K, 0.43 kN mP2) for 1.5 h, and subsequent evacuation left a broad band between 1670 and 1550 cm-l (fig. 5). The disc was re-exposed to propene at 390 K and after pumping off the vapour a slightly broader band remained at 1670-1470 cm-l; upon subsequent evacuation at 443 K absorption bands were observed at 1520, 1425 and 1355 cm-l. The infrared spectrum of a tin(1v) oxide-silica disc that had been evacuated and treated with oxygen at 716 K and then exposed to propene vapour (0.16 kN m-2) at 320 K exhibited weak absorption bands attributable to propene (fig.6 and 7). In addition, the sharp band at 3720 cm-l was removed and a broad band centred at 3350 cm-l appeared. Pumping off the vapour left a weak, broad band centred at1334 64. 65 8 68. 69 69 U .- 2 77 - 1 0 - TIN OXIDE SURFACES 1800 1600 1400 1200 wavenurnber/cm -' Fig. 5. Infrared spectra of tin(rv) oxide-silica: (1) evacuated 320 K, 17 h, < 1.33 x lop4 N mP2, (2) exposed to propene, 320 K, 2 h, 0.43 kNm-2, (3) evacuated 320 K, 0.5 h, < 1.33 x N m--2 , (4) exposed to propene, 390 K, 1.5 h, 0.33 kN m-2 and subsequent evacuation (5) 320 K, 0.5 h, < 0.5 h, < 1.33 x N m-2 and (6) 443 K, 18.5 h, < 1.33 x lop4 N m-2.1580 cm-l. The 3720 cm-l band was partially restored but shifted to 3700 cm-l and the broad band at 3350cm-l slightly reduced in intensity. Raising the evacuation temperature to 470 K produced broad absorption bands with maxima at 1530 and 1420 cm-l. Raising the evacuation temperature to 565 K significantly increased the intensity of the bands, the 1530 cm-l band maximum shifting to 1520 cm-1 with a shoulder on the high-wavenumber side at 1580 cm-l. On heating at 661 K, the 1520 cm-l band was greatly reduced in intensity, the 1420 cm-l band was removed, and two new bands were seen at 1585 and 1395 cm-l. The infrared spectrum of a tin(1v) oxide-silica disc that had been evacuated, treated with oxygen at 708 K and then treated with D,O vapour (633 K, 60 h) exhibited a sharp absor tion band at 2740 cm-l and a weak broad band centred at 1345 cm-l.Exposure to propene (0.13 kN mP2, 320 K, 4.5 h) reduced the intensity of the 2740 cm-1 band with slight broadening on the low-wavenumber side while not greatly altering the intensity of the 2530 cm-l band. In addition, a weak, broad band with its maximum at 3400 cm-l appeared. No new bands, or increase in intensity of 2530 cm-l (fig. 8). Wea ., bands were also present at 1585, 1510, 1425, 1390 andP. G . HARRISON AND B. MAUNDERS 1335 I I I I I I I 1800 1600 1400 1200 wavenum ber/cm-' Fig. 6. Infrared spectra of tin(1v) oxide-silica: (1) evacuated and treated with oxygen, 716 K, 3 h, < 1.33 x N m-2, (2) exposed to propene, 320 K, 0.5 h, 0.16 kN m-2, then evacuated 320 K, 0.25 h, < 1.33 x N mP2, N m-2 and subsequent evacuation (3) 470 K, 3 h, < 1.33 x (4) 565 K, 2.75 h, < 1.33 x N m-2 and (5) 661 K, 3.5 h, .c 1.33 x N m-2.the existing bands, in the 1600-1300 cm-l region were observed. Pumping off the propene and evacuating for 5 min at 320 K did not fully restore the 2740 cm-l band but did largely remove the 3400 cm-l band, although a shallow band remained. Evacuation at 470 K (9.5 h, < 1.33 x lop4 N mp2) caused a decrease in intensity of the 2530 and 2740 cm-l bands with the latter becoming broader on the low-wavenumber side. At the same time the broad band at 3400 cm-l increased in intensity, with a weak shoulder being present at 3695 cm-l.Treatment with propene at this temperature removed the 2740 and 2530 cm-l bands almost entirely, while the 3400 cm-l band increased further in intensity with a definite shoulder at 3695 cm-l. No new bands were seen in the 1600-1300 cm-l region of the spectrum. Evacuating the cell had relatively little effect on the spectrum, causing only a slight restoration of the 2740 cm-l band. Finally, heating at 470 K in oxygen and then evacuating the cell resulted in the appearance of strong absorption bands at 1520 and 1428 cm-l, along with a weak band at 1345 cm-l. At the same time the 3400 cm-l band became a little more intense but narrower, with a definite band at 3710 cm-l (relatively intense). The 2740 and 2530 cm-l bands were partially restored, but not to their original intensity. In a separate experiment using a Perkin-Elmer 598 spectrometer with data-handling facilities, a tin(1v) oxide-silica disc was evacuated and treated with oxygen at 753 K1336 TIN OXIDE SURFACES I ~ ~ l o l l l l l r l 4 000 3500 3000 wave nu m ber/cm -' Fig.7. Infrared spectra of tin@) oxide-silica: (1) evacuated and oxygen-treated, 716 K, 3 h, < 1.33 x N m-2, (2) exposed to propene, 320 K, 0.5 h, 0.16 kN m-2 and subsequent evacuation, (3) 320 K, 0.25 h, < 1.33 x N m-2. N rnp2 and (4) 470 K, 3 h, < 1.33 x and a background spectrum recorded. Propene was admitted to the cell at 320 K for 0.5 h and then pumped off before a second spectrum was recorded. After normalising and smoothing the spectra, the latter was subtracted from the former to reveal an absorption band centred at 1180 cm-l.Infrared bands together with assignments are summarised in table 1. DISCUSSION The infrared absorption bands observed on low-temperature pretreated tin(1v) oxide (at 1515, 1425 and 1345 cm-l) and tin@) oxide-silica (at 1520, 1425 and 1355 cm-l) can in both cases be assigned to the antisymmetric and symmetric v(C00) stretching modes and the symmetric 6,(CH,) deformation mode, respectively, of surface-adsorbed acetate species. The absorption bands observed on the 716 K pretreated tin(rv) oxide-silica and the 570 pretreated tin(rv) oxide-palladium oxide discs can also be ascribed to the acetate structure. Tin(rv) oxide-palladium oxide, pretreated at low temperature, exhibited a different spectrum, with the absorption bands observed being attributable to two surface species : an acetate, antisymmetric v ( C 0 0 ) stretch, 15 15 cm-l, symmetric v(C00) stretch, 141 8 crn-l, with the symmetric d(C-H,) deformation mode combined with the band at 1365 cm-l, and a surface acrylate, v(C=C) stretch, 1630 cm-l, 6(C-H) deformation, 1270 cm-l, CH, rock, 1065 cm-l, CH, twist, 980 cm-l, and CH bend, 820 cm-l.The unsymmetrical nature72 68 78 h 5 78 2 80 c! 4- 79 .- E 82 - 10% - 3, P. G. HARRISON AND B. MAUNDERS 1337 83 f / 4 2 , I I I I I I I 10 3600 3300 - 2800 2500 Fig. 8. Infrared spectra of tin(1v) oxide-silica: ( I ) evacuated and treated with oxygen, 798 K, 3 h, < 1.33 x N m-2, exposed to D20, 633 K, 60 h, then evacuated and cooled to 320 K, 3 h, < 1.33 x N m-2, (2) exposed to propene, 320 K, 4.5 h, 0.13 kN m-2; subsequent evacuation (3) 320 K, 0.25 h, < 1.33 x N m-2, (5) exposed to propene, 470 K, 1.5 h, 0.13 kN m-2, (6) evacuated 320 K, 1.5 h, < 1.33 x N m-2 and (7) oxygen-treated 470 K, 14 h, then evacuated 320 K, 2 h, < 1.33 x N mP2.N mP2, (4) 470 K, 9.5 h, c 1.33 x of the 1515 cm-l band is due to its being composed of the antisymmetric v(C00) stretch of the acrylate as well as the acetate, the symmetric v(C00) stretch being obscured by the 1418 cm-l band. The lack of absorption bands assignable to surface-adsorbed oxidation products on the high-temperature-pretreated tin(1v) oxide and tin(rv) oxide-palladium oxide suggests that surface hydroxy groups play an important role in the oxidation process.Reactions of propene with many metal oxides have been reported in the literature. Water vapour is usually present under the reaction conditions studied. In general, the various authors report one of two initial reaction intermediates, a n-allylic-type species reported for chromia,s zinc 4 7 cuprous ~ x i d e , ~ bismuth molybdatelO and mixed tin-antimony oxide,ll or an isopropoxy species reported for mixed tin oxide- molybdenum oxide and cobalt oxide-molybdenum oxide,l27 l3 chromium oxide,l39 l4 and for mixtures of nickel oxide,14 titania13 and ferric oxidel33 l4 with molybdenum oxide. With the n-allylic intermediate the reaction products tend to be acrolein, although Kubokawa et aL6 have reported surface-absorbed acetate plus formate groups on zinc oxide, while the isopropoxy intermediate tends to lead to acetone or acetic acid as the main product.In the present case, the interactions of propene with tin(1v) oxide and tin(1v)c-' w w 00 Table 1. Infrared absorption bands observed for the adsorption of propene on tin(1v) oxide, tin(1v) oxide-silica and tin(1v) oxide-palladium oxide pretreatment evacuation temperature temperature band position/cm-l oxide /K /K SnO, 320 SnO, . PdO 320 SnO, . PdO 570 SbO, . SiO, 3 20 SnO, . SiO, 716 surface acetate - surface acrylate - surface carbonate - 473 320 445 61 1 487 1630-1 540(br)e 579 320 1670-1 550(br)e 443 - 320 1580(br)" 470 565 - - - - - - - - - 1515 1425 1630 - 1515 1428 1630 - 1515 1428 - 1515 1420 - 1505 1420 - - - 1580(sh) 1505 1 390d - 1520 1425 - - - 1530(br) 1420 - 1580(sh) 1520 1420 assignments v(C=C) - vas(COO) v,(COO)S(CH) - "as(CO0) V,(COO) - - Vas(C0) - - a Also due to dS(CH,) of acetate.Also observed: band at 980 cm-l (CH, twist). Also observed: bands at 1240 cm-, (epoxide-ring vibration of Also due to carbonate decomposition product. v(C=O) glycidaldehyde), 1065 cm-l (CH, rock), 980 cm-l (CH, twist), and 820 cm-l (CH bend). of coordinated ketone.P. G. HARRISON AND B. MAUNDERS 1339 oxide-silica to give a surface isopropoxide intermediate leading to surface-bound acetone, which is known to oxidise rapidly on tin@) oxide to form a surface-adsorbed acetate,15 would explain the observed results. However, if this type of mechanism is operating, an initial reduction of the hydroxy stretching bands in the 4000-2000 cm-l region of the spectrum would be expected.With the tin(1v) oxide disc such a reduction in intensity could not be observed because of the very intense nature of the hydroxy stretching band. On the 716 K pretreated tin(1v) oxide-silica disc, however, the sharp band at 3720 cm-l, assigned to the isolated SiOH groups, was removed in the presence of propene vapour while a broad band appeared at 3350 cm-l. These phenomena can be interpreted in terms of hydroxy-group addition across the carbon-carbon double bond together with a certain amount of hydrogen-bonding interaction. Some reversibility of this hydroxy addition was observed on evacuating the cell with the partial restoration of the 3720 cm-l band, though its shifted position (to 3700 cm-l) and the remaining broad band at 3350 cm-i are indicative of a significant amount of hydrogen bonding still being present.The fact that this occurred under conditions where the acetate structure was not observed, together with the appearance of the acetate structure on raising the evacuation temperature, is corroboration of there being an adsorbed intermediate. In an attempt to clarify the situation, the hydrogens of the hydroxy groups of a high-temperature-pretreated tin(1v) oxide-silica disc were exchanged by treatment with D,O. The absorption band of the isolated SiOD groups, at 2740 cm-l, was again observed to reduce in intensity in the presence of propene, and the new weak, broad band at 3400 cm-l could be due to hydroxy groups formed by hydrogen4euterium exchange between the surface and the propene (fig.8). H-D exchange has been shown by Buiten16 to occur readily for five of the propene hydrogens, but not for the hydrogen on carbon-2, consistent with the formation of an isopropoxide hydroxy addition to propene. Evacuation almost entirely removed the 3400 cm-l band, but only partially restored the 2740 cm-l band, suggesting the presence of an adsorbed species which arises from the 0-D interaction with propene. The decreased intensity of the 0-D bands and increased intensity of the 3400 cm-l band on evacuating at 470 K is further evidence for the H-D exchange mechanism. The eventual appearance of the acetate bands after further treatment with propene at 470 K followed by treatment with oxygen at this temperature again showed that the propene must be fairly strongly held to the surface as an intermediate.Infrared characterisation of the surface isopropoxide species formed by surface hydroxy addition to propene is difficult to obtain since the most intense absorption of this species, the v(C-0) stretching mode, is expected to fall at < ca. 1200 cm-l in a region where bulk oxide absorptions also occur. Nevertheless, a difference spectrum obtained using a data station of propene adsorbed at 320 K onto 753 K treated tin oxide-silica exhibited a band at 1 180 cm-l, in the region (1 185-1 130 cm-l) characteristic of molecular metal isopropoxides. No corresponding C-H deforma- tion modes were apparent, most probably because they were too weak to be observed (cf. the weak nature of the C-H deformation modes of the surface acetate species).As a surface alkoxide species, the isopropoxide is expected15 to undergo facile oxidation via coordinated acetate to the observed acetate species. The overall mechanism for the adsorption is shown in scheme 1, where M is Si or Sn. The oxidative clearage of ketones to give surface acetate and hydrocarbon has been previously well documented by us.17 In the present study, bands in the region 1670-1 550 cm-l may well correspond to the surface-coordinated acetone species [cf. the v(C=O) stretching mode of surface coordinated ketones which fall in the range 1680-1 585 ~ m - l ] . ~ ~1340 TIN OXIDE SURFACES D/HCH*-CH -CH3 H I '0 0 0- I l l - - o,Sn, ,M, /Sn 0 0 "0 Scheme 1. The hydroxy addition to the C=C double bond may be considered a classical electrophilic addition by the proton.Hence tin(rv) oxide-silica, which has a greater Bronsted acidity than tin(1v) oxide a10ne,18 exhibits a higher reactivity towards propene. The weak Bronsted-acidic sites on tin(1v) oxide are readily removed on heating, consistent with the lack of reactivity of the high-temperature-pretreated tin(rv) oxide. Scheme 2. Tin(1v) oxide-silica therefore resembles tin@) oxide-molybdenum(1v) oxide in its behaviour towards propene. In the latter case, acetone and acetic acid are formed, presumably via a similar reaction 2o The small amounts of acetaldehyde which are also formed may readily be incorporated into the same reaction scheme by elimination of methane, rather than hydrogen, from the surface isopropoxide (scheme 2).The formation of absorption bands due to both acetate and acrylate on the low-P. G . HARRISON AND B. MAUNDERS 1341 temperature-pretreated tin(1v) oxide-palladium oxide sample implies that at least two competing mechanisms are occurring. The identity of the second species as an acrylate was confirmed by comparison with samples exposed acrylic acid vapour.21 The acetate formation can be explained in an analogous manner to that put forward for the reaction of propene with tin(rv) oxide and tin(1v) oxide-silica. Since ethane is observed to react with tin(1v) oxide-palladium oxide under very mild conditions to afford surface acetate,' the most likely mechanism for the formation of surface acrylate is the formation of surface 1-0-CH,-CH-CH, species, followed by conversion into surface-coordinated acrolein and surface acetate in a manner similar to the formation of surface acetate (scheme 3).Indeed, separate studies have confirmed the formation of surface acrylate by adsorption of acrolein, identical to that obtained by adsorption of acrylic acid.21 The CH,=CH, /H c Scheme 3. iterature is unanimous that the formation of acrolein from propene over meta oxide catalysts involves a surface n-allylic species.4' 5 9 11* l9 In the present case, the palladium component of the oxide certainly plays an essential role in the initial chemisorption process, which leads to the formation of surface allyloxide (or other coordinated acrolein precursor). The driving force for the initial interaction may be the formation of surface Pd-H bonds (scheme 4), since palladium-hydrogen bonds Scheme 4.are known to be very stable in molecular complexes. Alternatively, a surface palladium--n-allylic species may be formed, which may subsequently be transferred 1.0 a neighbouring tin oxide surface site (scheme 5). The proposed mechanism for the oxidation of propene over zinc oxide is actually rather complicated and involves the interaction of a surface ally1 species with a surface1342 TIN OXIDE SURFACES H, H2C=CH-C<, O I I 0 I I I *,S*', 2% ,Sn, 0 0 Scheme 5. 0; species to give an intermediate allylperoxy species which is subsequently transformed to surface glycidol and gly~idaldehyde.~. The latter species was characterised by an infrared absorption at 1237 cm-l, ascribed to the epoxide ring vibration.In this study, a small band is observed at ca. 1240 cm-l, which disappears upon evacuation at higher temperatures before the disappearance of the acrylate absorption bands. However, the participation of a glycidaldehyde in the present study remains equivocal. The decomposition of a n-allylic intermediate to surface acetate plus formate as have been reported in one study of propene over zinc oxide6 can be definitely excluded by comparison of authentic infrared spectra obtained from formic acid adsorption.21 Tin@) oxide-palladium oxide, pretreated at 570 K, was found to react only weakly with propene to give a species attributable to a surface acetate only. With pretreatment at even higher temperatures, no surface species were observed, which is understandable in view of the loss of the weak Bronsted-acidic sites at these temperatures.With all three oxides when the evacuation temperature was raised high enough, decomposition of the adsorbed species occurred in the same manner as with the ethene and ethane reactions with these oxides. In summary, the reaction of propene over all three oxides evacuated at ambient temperatures gives surface acetate species. The most likely mechanism for the formation involves initial electrophilic addition of acidic hydroxy groups to their carbon-carbon double bond. At higher pretreatment temperatures tin(1v) oxide- silica is active in the formation of a surface acetate, reflecting the Bronsted-acidic nature of its surface, whilst underlying the very weak nature of the acid hydroxy groups of the other two oxides. Tin(rv) oxide-palladium oxide evacuated at ambient temperature, in addition to exhibiting absorption bands attributable to a surface acetate, also displays bands assignable to a surface acrylate. We thank the S.E.R.C. and the International Tin Research Institute for support in the form of a CASE award to (B.M.).P. G. HARRISON AND B. MAUNDERS 1343 L. Kubelkova and F. Trifiro, J. Catal., 1972, 26, 242. T. A. Gordymora and A. A. Davydov, Kinet. Catal. (Engl. Transl.), 1979, 20, 604. A. L. Dent and R. J. Kokes. J. Am. Chem. SOC., 1970,92, 1092; 6709; 6718. B. L. Kugler and R. J. Kokes, J. Catal., 1974, 32, 170. B. L. Kugler and J. W. Gryder, J. Catal., 1976, 44, 126. Y. Kubokawa, H. Miyata, T. Uno and S. Kawasaki, J . Chem. SOC., Chem. Commun., 1974,655. R. L. Burwell, G. L. Haller, K. C. Taylor and J. F. Read, Ado. Catal., 1969, 20, 1 . H. H. Voge, C. D. Wagner and D. P. Stevenson, J. Catal., 1963, 2, 58. 398. ' P. G. Harrison and B. M. Maunders, J. Chem. SOC., Faraday Trans. I , 1985,81, 1309. lo J. M. Peacock, A. J. Parker, P. G. Ashmore and J. A. Hockey, J. Catal., 1969,15, 15; 373; 379; 387; l 1 J. R. Christie, D. Taylor and C. C. McCain, J. Chem. SOC., Faraday Trans. 1, 1976,72, 334. l2 S. Tan, Y. Moro-Oka and A. Ozaki, J. Catal., 1970, 17, 132. l3 Y. Moro-Oka, Y. Takita and A. Ozaki, J. Catal., 1971, 23, 183. l4 Y. Moro-Oka, Y. Takita, S. Tan and A, Ozaki, Bull. Chem. SOC. Jpn, 1968, 41, 2820. l5 E. W. Thorton and P. G. Harrison, J. Chem. SOC., Faraday Trans. I , 1975, 71, 2468. l6 J. Buiten, J . Catal., 1969, 13, 373. P. G. Harrison and B. Maunders, J. Chem. SOC., Faraday Trans. I , 1984, 80, 1329. l 8 P. G. Harrison and B. Maunders, J. Chem. SOC., Faraday Trans. I , 1984, 80, 1341. Y. Moro-Oka, Y. Takita and A. Ozaki, J. Catal., 1972, 27, 177. *O J. Buiten, J. Catal., 1968, 10, 188. 21 P. G. Harrison and B. Maunders, unpublished data. (PAPER 4/908)

ISSN:0300-9599    

DOI:10.1039/F19858101329

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. 1, 1985,81, 1345-1355 Tin Oxide Surfaces Part 16.-Infrared Study of the Adsorption of Formic Acid, Acrylic Acid and Acrolein on Tin(rv) Oxide, Tin(1v) Oxide-Silica and Tin@) Oxide-Palladium Oxide BY PHILIP G. HARRISON* AND BARRY MAUNDERS Department of Chemistry University of Nottingham, University Park, Nottingham NG7 2RD Received 1 lth June, 1984 Transmission infrared spectroscopy has been employed to study the surface species formed by adsorption of formic acid, acrolein and acrylic acid on tin(1v) oxide, tin@) oxide-silica and tin(1v) oxide-palladium oxide. The behaviour of the three adsorbents is similar on all three oxides. At low temperatures the two organic acids give surface carboxylate plus coordinated acid, which can be removed by evacuation at higher temperatures.Acrolein is adsorbed as a surface acrylate, although surface-coordinated acrolein is observed at lower temperatures. We have recently shown that ethane, ethene and propene are adsorbed on tin(rv) oxide, tin(1v) oxide-silica and tin(rv) oxide-palladium oxide to give surface acetate species as the ultimate pr0ducts.l In the case of propene on tin(rv) oxide-palladium oxide, a surface acrylate species was also formed. The most probable mechanism for the formation of surface acetate from ethane and ethene is via the oxidation of an initially formed surface-ethoxide species, which appears to be formed by hydrogen abstraction by surface oxide in the case of ethane and by the formal addition of surface hydroxy groups across the carbon+arbon double bond in the case of ethene.The surface ethoxide species was then postulated to undergo oxidation to afford the observed surface carboxylate, by analogy with our previous observation that surface alkoxide groups on tin(1v) oxide undergo facile oxidation to the corresponding carboxylate at temperatures as low as the ambient beam temperature of the spectrometer.2 Furthermore, the probable intermediacy of a surface-coordinated carbonyl species in these reactions gains considerable support from the known chemisorption behaviour of ketones of the type MeCOR (R = Me, Et, Pr", But or Ph) on tin(1v) oxide to give the surface carboxylate, 02CRads.29 Propene adsorption presents a more complicated case, since two initial surface alkoxide species, an isopropoxide (by hydroxide addition to the C=C bond) and an allyloxide (by hydrogen abstraction), may be generated, and also since dissociative chemisorption with C-C bond fission to give both surface acetate and formate, as has been observed on zinc oxide,* may also be possible.In order to resolve some of the possible ambiguities in the adsorption of propene on materials containing tin(1v) oxide (i.e. the possible occurrence of surface formate) and also to corroborate the formation of surface acrylate via the oxidation of coordinated acrolein, we here report the details of an infrared study of the adsorption of formic acid, acrolein and acrylic acid. 13451346 TIN OXIDE SURFACES Table 1. Infrared data for acrylic acid adsorption assignment position of absorption bands/cm-' coordinated SnO, - PdO SnO, SnO, * SiO, acrylate acrylic acid - 1630 1580 15 10-1 520 1410 1325 1270 1160 980 890 820 1425-1430 1355-1365 1245-1250 1050-1065 - 1 630-1 635 1500-1 530 1408-1 4 10 1365 1270 1170 1065 980 890 820 1585-1 590 1 430- 1 43 5 1328-1 330 1 250- 1 25 5 1655 1630 1580-1 590 1500-1 5 10 1430 1410 1325 1270-1273 1365-1368 1245-1250 - C-H head - v(C-C)? P W , ) S(C00) CH, twist CH bend v(C=C) or v(C-0) vc=o - - - CH, scissors SOH v(C-0) or S(C-H) - - v(C-C) - Table 2.Infrared data for acrolein adsorption assignment position of absorption bands/cm-l coordinated SnO, PdO SnO, SnO, . SiO, acrylate acrolein 1710-1 720 1655 1630 - 1 5 1 0- 1 520 1425- 1428 1365 1270 - 1150-1 170 1055-1060 980 820 1710 1630 1 5 10-1 520 1675-1680 - 1428- 1430 1405- 141 0 1365 1270 1250 1170 1060 980 - 820-825 1700- 17 10 1655 1630 1570-1590 1510-1515 1428-1432 1405- 14 10 1370 1310 1270-1 273 1250 - v(C=C) vas (C0O)U v, (COO) - S(C-H) d(C-H)" - - v(C-C)? PCH, CH, twist CH bend v(C=O) v(C=O) - CH, scissors - - CH bend v(C-C) Due to coordinated acrylic acid; see text for details.EXPERIMENTAL The preparation of tin(1v) oxide, tin(rv) oxide-silica and tin(1v) oxide-palladium, the manufacture of infrared-transmitted discs therefrom and the general techniques employed have been described previ~usly.~ Infrared spectra were recorded using a Perkin-Elmer 577 spectrometer. Spectral data are summarised in tables 1 and 2.P. G. HARRISON AND B. MAUNDERS 1347 I I I 1 1 I I 1800 1600 1400 1200 wavenumber/cm -' Fig. 1. Infrared spectra of tin(1v) oxide: (1) evacuated 320 K, 16 h, < 1.33 x N m-2, (2) exposed to formic acid vapour, 320 K, 5 min, 13.3 N m-2; subsequently evacuated (3) 320 K, 17 h, < 1.33 x N m-2, (4) 438 K, 3 h, < 1.33 x N rn+ and (5) 476 K, 18 h, < 1.33 x N m-2. RESULTS FORMIC ACID ADSORPTION Exposure of an evacuated tin(1v) oxide disc (320 K, < Torr) to formic acid vapour (1 3.3 N mP2, 320 K) gave very strong infrared absorption bands at 1630, 1600, 1550, 1380,1345 and 1260 cm-l (fig.1). Evacuation (320 K, 17 h) reduced the intensity of the 1630 and 1260 cm-1 bands with respect to the other bands; in addition, it could then be seen that the 1260 cm-1 band was composed of the original band plus a second band at 1285 cm-l and the 1345 cm-l band shifted to 1350 cm-l.Evacuation at 438 K (3 h, < z) reduced still further the 1630 and 1260 cm-l bands, whilst evacuation at 476 K reduced the intensity of all the bands leaving absorptions at 1548, 1382 and 1340 cm-l. Evacuation at 514 K caused the decomposition of the formate species. N mP2, 320 K) to formic acid vapour (13.3 N m-2, 320 K) led to the formation of very strong adsorption bands at 1600,1555,1370,1335 and 1280 cm-l (fig. 2). Evacuation (320 K, 1 h) removed the 1600 cm-l band, leaving a very weak band at 1290 cm-l and three strong bands at 1545, 1380 and 1340 cm-l. Exposure of an evacuated tin(rv) oxide-palladium oxide disc (1.33 x1348 TIN OXIDE SURFACES h E g 39 cd Y .- E 2 56 2 7 4 8 4 . Y - 10 O/O - 1 1 I I I I 1800 1600 1400 1200 wavenumber/cm-' Fig.2. Infrared spectra of tin(1v) oxide-palladium oxide: (1) evacuated 320 K, 96 h, < 1.33 x lop4 N m-2, (2) exposed to formic acid vapour 320 K, 5 min, 13.3 N rnP2 ; subsequently evacuated (3) 320 K, 1 h, < 1.33 x N m-2 and (4) 443 K, 3 h, < 1.33 x N m-2. Exposure of an evacuated tin(1v) oxide-silica disc (1.33 x lo-* N m-2, 320 K) to formic acid vapour (1 3.3 N m-2, 320 K) led to the formation of very strong absorption bands at 1640, 1600-1540, 1375, 1350 and 1260 cm-l (fig. 3). These bands can also be assigned to coordinated formic acid, 1640 and 1260 cm-l, and surface formate groups, 1600-1540, 1375 and 1350 cm-l. Evacuation (320 K, 16 h) left bands at 1580-1530, 1380, 1350 and 1280 cm-l, which agree well with similar bands seen on tin(1v) oxide.Evacuation at 443 K greatly reduced the width of the 1580-1530 cm-l band, exposing a weak broad band centred at 1700 cm-l, which can be assigned to the antisymmetric COO stretch of an unidentate formate, most probably on a silica site. A further weak band was observed at 1815 cm-l, which can be assigned to undissociated formic acid adsorped on a silicon site, since formic acid is known to adsorb undissociated on silica itself.6 This band was removed by evacuation at 470 K. Evacuation at 520 K again decomposed the formate structure. ACRYLIC ACID ADSORPTION Exposure of an evacuated tin(1v) oxide-palladium oxide disc (1.33 x lop4 N m-2, 320 K) to acrylic acid vapour (13.3 N mp2, 320 K) led to an infrared spectrum exhibiting adsorption bands at 1630, 1580, 1520, 1425, 1410, 1325, 1245, 1130, 1050, 980, 890 and 820 cm-l, along with shoulders at ca.1760, 1705 and 1355 cm-l (fig.P. G . HARRISON AND B. MAUNDERS 1349 1900 1700 1500 1300 wavenum berlcm-' Fig. 3. Infrared spectra of tin(rv) oxide-silica: (1) evacuated 320 K, 24 h, < 1.33 x N m-2, (2) exposed to formic acid vapour, 320 K, 5 min, 13.3 N m-2; subsequently evacuated (3) 320 K, 16 h, < 1 . 3 3 ~ 10-4Nm-2, (4) 443 K, 19 h, < 1 . 3 3 ~ 10-4Nm-2 and (5) 470K, 4h, < 1.33 x 1 O - O N m-2. 4). Subsequent evacuation (320 K, 1.5 h) removed the 1760 and 1705 cm-l shoulders, reduced in intensity the 1580, 1410 and 1325 cm-l bands, the 1410 cm-l band now being a shoulder on the 1430 cm-1 band shifted from 1425 cm-l, while the remaining bands became more intense with slight shifts in position (1 520 to 15 10, 1355 shoulder to 1365,1245 to 1250, 1130 to 1160 and 1050 to 1065 cm-l).A shoulder at 1270 cm-l on the 1250 cm-l band was also present. Evacuation up to 400 K had little effect on the spectrum, but evacuation at 463 K removed the 1580, 1410, 1325, 1250 and 1160 cm-l bands, with the appearance of a new band at ca. 1700 cm-l. The removal of the 1250 cm-l band exposed the shoulder at 1270 cm-l as a sharp intense band. At increasing evacuation temperatures up to 597 K, the absorption bands at 1630, 1365, 1270, 1065, 980, 890 and 820 cm-l were steadily reduced in intensity and effec- tively removed, while the two strong absorption bands, at 1510 and 1430 cm-l, were unaltered at 530 K. At 597 K they were slightly reduced in intensity with the 1430 cm-l band shifted to 1395 cm-l.Exposure of evacuated tin(1v) oxide and tin@) oxide-silica discs (1.33 x N m-2, 320 K) to acrylic acid vapour (13.3 N m-2, 320 K) gave infrared spectra which were very similar to those seen for tin@) oxide-palladium oxide (fig. 5 and 6, respectively). Absorption bands were observed on tin@) oxide at 1760 and1350 TIN OXIDE SURFACES 1800 1600 1400 1200 wavenum ber/cm-' Fig. 4. Infrared spectra of tin(rv) oxide-palladium oxide: (1) evacuated 320 K, 48 h, < 1.33 x N m-2, then exposed to acrylic acid vapour, 320 K, 5 min, 13.3 N m-2; subsequent evacuation (2) 320 K, 1.5 h, < 1.33 x N m-2, (4) N m+, (3) 463 K, 17 h, < 1.33 x 597 K, 5.5 h, < 1.33 x lop4 N m-2 and (5) 657 K, 3 h, < 1.33 x N m-2. 1700 cm-l due to weakly held acrylic acid, at 1585-1590, 1408-1410, 1328-1330, 1250-1255 and 1170 cm-l due to acrylic acid coordinatively bonded through the carbonyl oxygen to Lewis-acidic tin sites and at 1630-1635, 1500-1 530, 1430-1435, 1365, 1270, 1065,980,890 and 820 cm-I due to the surface acrylate species. On tin(1v) oxide-silica bands were seen at 1580-1590, 1410, 1325 and 1245-1250 cm-l due to coordinatively bound acrylic acid and at 1630, 1500-1 5 10, 1430, 1365-1 368 and 1270-1273 cm-l due to the surface acrylate species.No bands due to weakly held acid were seen in this case. In addition, a shoulder was observed at 1655 cm-I and it can be assigned to acrylic acid coordinatively bound to a surface site of different strength to the Lewis-acidic tin sites, responsible for the 1580 cm-l band, presumably an unsaturated surface silicon. No absorption bands were observed below 1200 cm-l because of the strong absorptions of the oxide itself.Both oxides showed the appearance of a 1700-1710 cm-l band after evacuation at 456 and 526 K for tin(rv) oxide and tin(1v) oxide-silica, respectively. The absorption bands and assignments for all three oxides are summarised in table 1 .P. G. HARRISON AND B. MAUNDERS 1351 8 6- 9 2. n E W C Y Y .- 5 5 5 65 - 10% - I I I I I I 1800 1600 1400 1200 wavenumber/cm -' Fig. 5. Infrared spectra of tin@) oxide: (1) evacuated 320 K, 3 h, < 1.33 x N m-2, then exposed to acrylic acid vapour, 320 K, 5 min, 13.3 N m-2; subsequent evacuation (2) 320 K, 18 h, < 1.33 x N mP2 and (3) 456 K, 3.5 h, < 1.33 x loP4 N m-2.ACROLEIN ADSORPTION The adsorption of acrolein on tin(1v) oxide-palladium oxide, tin(1v) oxide and tin(rv) oxide-silica gave infrared spectra attributable to surface acrylate species. In addition, weak absorption bands were observed at ca. 1720-1700, 1655-1675, 1405-1410, 1250 and 1150-1 170 cm-l. Detailed discussion and diagrams of spectra are not shown since the main features closely resemble those of acrylic acid absorption. The absorption bands are summarised in table 2. DISCUSSION FORMIC ACID ADSORPTION The adsorption bands observed upon exposure of tin(1v) oxide to formic acid vapour are readily assignable to surface formate and coordinated formic acid. The 1550, 1380 aiid 1345-1350 cm-l bands can be assigned to the v,,(COO), djeP.(CH) and y,(COO) modes, respectively, of a surface formate group, the bands being in good agreement with absorption bands reported for other formate species (see table 3). The 1630 and 1260 cm-1 bands can be assigned to the v(C=O) and v(C-0) bands of adsorbed formic acid, respectively. The large shift of the carboxy stretching band for the ad- sorbed formic acid is evidence for the acid being coordinatively bonded through the1352 TIN OXIDE SURFACES 1800 1600 1400 1200 wavenumber/cm - l Fig. 6. Infrared spectra of tin(1v) oxidesilica: (1) evacuated 320 K, 16 h, .c 1.33 x N m-,, then exposed to acrylic acid vapour, 320 K, 5 min, 13.3 N m-2; subsequent evacuation (2) 320 K, 0.5 h, < 1.33 x lop4 N m-,, (3) 448 K, 3.25 h, < 1.33 x N rn-, and (4) 526 K, 16 h, < 1.33 x lop4 N rn+.Table 3. Absorption band positions for formate species compound band position of formate ion/cm-l ref. ~ ~~~ Sn(OOCH), 1563 1385 1339 7 Sn(OOCH), 1608 1404 1368 7 (CW, Sn(OOCH), 1588 1373 1390 8 CH(CH,), Sn(O0CH) 1595 1325 1373 8 HCOONa 1567 1377 1366 9 assignment va(C00) &i.p.(CH) vs(C00) - - 1370 131 1 carboxy oxygen to Lewis-acid sites, as in structure (I). Hydrogen bonding between the acid and surface hydroxy groups may also occur. The nature of the bonding of the surface formate species may be distinguished by the separation of the symmetric and antisymmetric by surface oxide formate. The formation of both bridging [structure (11)] and chelating [structure (III)] surface formateP. G. HARRISON AND B. MAUNDERS 1353 groups may be readily rationalised by mechansims involving an initial coordination of the carbonyl oxygen atom to a Lewis-acidic tin site, the electron donation having the effect of increasing the negative charge density at a neighbouring surface oxide or hydroxy group, whilst also increasing the group or water molecule can then form (scheme 1).acidity of the acid proton. A hydroxy leaving the surface formate group .c\ 6+ H 0-H H O@ / / H I H c or 2"\ /sz Sn Sn i n i n / \ / \ / \ / \ 0 0 0 0 0 0 0 0 0 (11) H I Sn Sn Sn Sn ' ~ n Sn / \ / + \ / \ / \ / \ / \ 0 0 0 0 0 0 0 0 0 The bands observed for the adsorption of formic acid on tin(1v) oxide-silica and tin(1v) oxide-palladium oxide can be assigned in a similar fashion. All three oxides exhibited a decrease in the intensity of the hydroxy stretching bands upon evacuation of the formic acid vapour at 320K, suggesting that the former mechanism operates.The separation between the p(C=O) and p(C-0) stretching frequencies is 665 cm-l in the formic acid monomer and ca. 550cm-l in unidentate methyl and ethyl formates,1° while a separation of ca. 225 cm-l has been reported for a bidentate formate species adsorbed on ultrastable zeolite.ll In the present case, the difference in wavenumber between the antisymmetric and symmetric COO stretches of 205-210 cm-l suggests that the formate is adsorbed in a bidentate manner. However, a bidentate formate species may be either chelating or bridging, although no distinction between these two possibilities can be made on the infrared data alone.The formation of the formate species can be rationalised by a mechanism which involves the initial coordination of the carbonyl oxygen of the acid to a Lewis-acidic tin site. The electron donation has the effect of increasing the negative charge density 45 F A R 11354 TIN OXIDE SURFACES at a neighbouring oxide or hydroxy group whilst also increasing the acidity of the acid proton, which is abstracted by neighbouring surface oxide or hydroxy groups to give either bridging [structure (111)] (abstraction by surface hydroxyl) or chelating [structure (11)] (abstraction by surface oxide). However, a tin (IV) oxide-silica disc that has been evacuated at 740 K, exposed to formic acid vapour and then subsequently evacuated exhibited an increase in the hydroxy stretching band along with the appearance of the formate structure, which suggests that the second mechanism can occur also.Decomposition of the formate structure occurs above 500 K, which is close to the decomposition temperature of tin(I1) formate (47 1-473 K).12 ACRYLIC ACID ADSORPTION By comparison with the gas-phase spectrum of acrylic acid,13 the two shoulders at 1760 and 1705 cm-l can be assigned to the v(C=O) stretch of weakly held monomeric and dimeric acrylic acid; their easy removal at 320 K suggests only weak hydrogen- bonding interactions with the surface. The bands at 1580, 1410, 1325, 1245-1250 and 1160 cm-l can also be assigned to coordinated acrylic acid since their intensities alter in a similar manner and are removed on evacuation at 463 K.The large shift in the carbonyl stretching frequency, from 1725 to 1580 cm-l, suggests that the acid is coordinatively bonded through the oxygen to Lewis acid sites. The 1410 cm-l band can be ascribed to the CH, scissors mode of the methylene group, the 1325 and 1245-50 cm-’ bands are tentatively assigned to an OH deformation and either the v(C-0) or S(C-H) modes, respectively, while the 1160 cm-l band is best assigned to the y(C-C) stretching mode. The expected y(C=C) stretching mode will occur in approximately the same position as in the acrylate and is not, therefore, seen as a separate band. M M M / \ / \ / \ 0 0 0 0 The bands at 1630, 1510-1520, 1425-1430, 1355-1365, 1275, 1050-1065, 980, 890 and 820 cm-l are in similar positions to bands observed for sodium acrylate13 and can be attributed to the v(C=C), v,,(COO), vs(COO), S(C-H), (C-H) bending, p(CH,), CH, twisting, S(CO0) and (C-H) bending modes, respectively, of a surface-adsorbed acrylate.The 1 160-1 170 cm-l band assigned to the v(C-C) stretch- ing mode of coordinated acrylic acid may also be partly due to the same mode of the adsorbed acrylate. The same situation may exist for the assignment of the 14 10 cm-l band. Decomposition of the acrylate at 657 K caused absorption bands at 1590, 1510, 1440, 1390 and 11 80 cm-l. A possible explanation for these bands could be the formation of a polymeric acrylic acid ~ a 1 t . l ~P. G. HARRISON AND B. MAUNDERS 1355 The weak infrared bands observed at ca. 1700 cm-l on evacuation at 463 K occur with the disappearance of the coordinated acrylic acid and may be due to a carbonyl stretch of a decomposition product from acrylic acid at this temperature.The mechanism for acrylate formation is the same as for formate formation from formic acid. However, the position of the v(C=C) stretching band in all three cases is 5-10 cm-1 lower than in either the free-acid vapour or sodium acrylate,13 indicating that the carbon-carbon double bond is involved in interaction with surface Lewis-acidic sites [structures (IV) and (V)] or surface Bronsted-acid sites [structure (VI)]. ACROLEIN ADSORPTION Infrared spectra obtained from the adsorption of acrolein were very similar to those obtained from acrylic acid adsorption, indicating the very facile oxidation of acrolein by these tin(1v) oxide materials.The 1720-1700, 1405-1410, 1250 and 1150-1 170 cm-l bands can be attributed to the v(C=O) stretching, CH, scissors, CH bending, and p(C-C) stretching modes, respectively, of physisorbed acrolein, while the 1655-1675 cm-l band may be assigned to acrolein coordinated through the carbonyl oxygen to Lewis-acidic sites and be due to the v(C=O) stretching mode.15 The acrolein responsible for the 1720-1700 cm-l band may be physisorbed through the carbon- carbon double bond to Lewis-acidic sites or hydrogen-bonded to surface hydroxy groups. Note that two weak bands at 1570-90 and 1310 cm-l are observed after acrolein adsorption on tin(1v) oxide-silica; these agree well with the positions of the v(C=O) stretching and d(0H) deformation modes, respectively, of coordinated acrylic acid. This observation is not surprising since the tin(rv) oxide-silica is Bronsted acidic and may well protonate the acrylate (scheme 2). H, ,c= H Sn Sn Si / \ / \ / \ 0 0 0 0 Scheme 2. H H >c=c' 0-H H 'C' II 1 I Sn Sn Si / \ / \ / \ 0 0 0 0 0 oo We thank the S.E.R.C. and the International Tin Research Institute for support in the form of a CASE award (to B.M.). P. G. Harrison and B. Maunders, J. Chem. SOC., Faraday Trans. I , 1985,81, 1309; 1327. E. W. Thornton and P. G. Harrison, J. Chem. SOC., Faraday Trans. I , 1975, 71, 2468. P. G. Harrison and B. Maunders, J . Chem. SOC., Faraday Trans. I , 1984,80, 1329. Y. Kubokawa, H. Miyata, T. Ono and S. Kawasaki, J. Chem. SOC., Chem. Commun., 1974, 655. P. G. Harrison and B. Maunders, J. Chem. SOC., Faraday Trans. I , 1984,80, 1341. K. Hirota, K. Fueki, K. Shindo and Y. Nakai, Bull. Chem. SOC. Jpn, 1959, 32, 1261. J. D. Donaldson, J. F. Knifton and S. D. Ross, Spectrochim. Acta, 1964, 20, 847. R. Okawara, D. E. Webster and E. G. Rochow, J. Am. Chem. SOC., 1960,82, 3287. K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds (Wiley, London, 1970). lo A. R. Katitzky, J. M., Lagowski and J. A. T. Beard, Spectrochim. Acta, 1964, 20, 847. l 1 T. M. Duncan and R. W. Vaughan, J. Catal., 1981, 67,469. IL2 D. Hadzi and M. Pintar, Spectrochim. Acta, 1958, 12, 162. l3 W. R. Fairheller and J. E. Katan, Spectrochim. Acta, 1967, 23, 2225. l 5 For the infrared spectrum of acrolein see R. K. Harris, Spectrochim. Acta, 1964, 20, 1129. J. C. Leyle, L. H. Zuiderweg and H. J. Viedder, Spectrochim. Acta, 1967, 23, 1397. (PAPER 4/975) 45-2

ISSN:0300-9599    

DOI:10.1039/F19858101345

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. 1, 1985,81, 1357-1367 Influence of Support Acidity and Ce3+ Additives on the Reactivity of Nickel Particles Highly Dispersed on Various Oxide Supports BY GUY-NOEL SAUVION, JEAN-FRANCOIS TEMPERE,* MARIE-FRANCE GUILLEUX, GERALDJEGA-MARIADASSOU~ AND DENISE DELAFOSSE Laboratoire de Chimie des Solides, E.R. 133 ‘Reactivite de Surface et Structure’, Universite P. et M. Curie, 4 Place Jussieu, 75230 Paris Cedex 05, France Received 2nd July, 1984 Important modifications to the reactivity of metallic nickel highly dispersed on various oxide supports have been observed. These vary according to the acidity of the support and the presence of Ce3+ additives. The loss or attenuation of hydrogen chemisorption properties and catalytic activity in butane hydrogenolysis seems to be correlated to the acidity of the support, which leads to a total or partial coverage of the metallic surface during the reduction process by hydrogen, which remains strongly chemisorbed up to high temperatures. The presence of Ce3+ additives causes an increase in the catalytic activity in the CO/H, reaction and a significant shift of the selectivities in favour of the formation of heavier hydrocarbons from both the hydrogenation of CO and the hydrogenolysis of butane.These results are discussed in terms of strong metal-support interactions and concomitant modifications of the electronic environment of the metal. Numerous studies have recently been devoted to metal-support interactions, which can strongly modify the reactivity of Group VIII metals. In the case of reducible oxide supports like titania,l-* a strong metal-support interaction, occurring when the reduction of the catalyst is performed at a relatively high temperature, reduces the ability of the metal to adsorb hydrogen or CO and decreases or suppresses its catalytic activity in hydrogenolysis reactions.However, it leads to an increase in methanation activity and a shift in product distribution towards higher hydrocarbons. These modifications of the catalytic and chemisorption properties have been explained as the result of an electronic interaction between the support and the metal crystallites rather than as the effect of crystallite size or surface structure. In Nio/TiO, systems5 spectroscopic measurements have shown an electron enrichment of the metal particle when the cations of the support were reduced to lower valence (Ti3+).In acidic supports like zeolites, metal-support interactions may also occur for well dispersed metallic systems.6* Lewis-acid sites interact strongly with the metal crystallites causing them to become electron deficient.6 The increased activities of these systems in hydrogenation and hydrogenolysis reactions, when observed, cannot unambiguously be interpreted in terms of electron deficiency of the metal. Other factors can account for these modifications, such as the shape selectivity of zeolites, their electrostatic field and their acidity. More generally, the nature of the support, its reducibility but particularly its acidity 7 Present address: Laboratoire de Cinetique Chimique, 1 Rue Guy de la Brosse, 75230 Pans CCdex 05, France.13571358 REACTIVITY OF NICKEL PARTICLES or basicity and the presence of additives can induce some electron transfer from the support towards the metal or, conversely, from the metal towards the support, especially in well dispersed systems. These changes in electron density of the metal can significantly modify its chemisorptive and catalytic properties. In an earlier works we studied the reactivity of well dispersed metallic nickel (0.7-1.2 nm diameter) on Ce3+ zeolites differing mainly in their acidity, the presence of the Ce3+ cation leading to stabilization of nickel particles inside the zeolitic cavities. The preliminary experimental results have shown that nickel supported on the more acidic supports does not chemisorb hydrogen and has no activity in butane hydrogenolysis.These results have been explained by the presence on the sample surface of large quantities of hydrogen species remaining strongly chemisorbed until desorbed by heating to 923 K. However, after similar treatment the dispersed nickel is not catalytically active in butane hydrogenolysis. This behaviour of NiCeX zeolites reduced at a relatively low temperature (623 K) has to be compared with that of metals supported on reducible oxides reduced at high temperatures. In order to explain the origin of these profound changes in the catalytic and chemisorptive properties of nickel highly dispersed on non-reducible carriers, we have extended this work to the study of the various parameters capable of modifying the metallic reactivity, namely the nature of the support, its acidity and morphology and the effects of the presence of Ce3+ additive and metallic dispersion. The catalytic activity and chemisorption properties of nickel well dispersed on X and A zeolites and silica have been studied in butane hydrogenolysis, CO+H, reactions and hydrogen chemisorption.EXPERIMENTAL MATERIALS The zeolite samples were prepared by ion exchange, contacting dilute solutions of nickel and cerium(1n) nitrates with NaX zeolites (samples X,-X, ) or NaA molecular sieve (sample Al ). Silica sample S, was obtained by precipitation of nickel and cerium(1n) nitrates in ammonia medium. Sample S,, without cerium, was studied for comparison.1° Sample compositions and nomenclature are reported in table 1. Generally, zeolite samples were reduced at 623 K in a flow ( 5 dm3 h-l) of high-purity hydrogen for 16 h, while silica systems were pretreated under a helium flow up to 773 K before reduction at 923 K.METHODS The oxidation state of cerium before reduction was evaluated either by X-ray determinations or potentiometric titration of Ce3+ against Fe3+ ions. The metallic-particle-size distribution and the extent of reduction of the sample were determined by magnetic measurements using the Weiss extraction method.ll The experiments were generally carried out in the temperature range 77-300 K and magnetic fields up 18 kOe. In some cases a superconductive coil reaching 70 kOe (for measurements at 4.2 K) was used. A ferromagnetic resonance (f.m.r.) study was performed at 300 K using a Varian spectrometer (model C.S.E., 109-X band) in order to follow the modifications of the magnetic properties of the metallic particles during the various treatments to which the samples were subjected.Hydrogen chemisorption measurements on reduced samples degassed at the reduction temperature were carried out at room temperature and 100 Torrt gas pressure in a classical micromanometric apparatus equipped with a Texas Instruments pressure gauge. The temperature-programmed-desorption measurements (t.p.d.) were performed on reduced t 1 Torr = 101 325/760 Pa.SAUVION, TEMPERE, GUILLEUX, DJEGA-MARIADASSOU AND DELAFOSSE 1 3 59 Table 1. Sample compositions and characterizations chemical Ni ions Ce ions degree of nickel particle metallic sample composition (wt %) (wt %) reduction diameter/nm dispersion X, Ni,Ce,Na,,H,,X 2.53 4.09 0.28 X, Nil,Ce7Na,,H,,X 4.03 5.47 0.73 X, Ni,,Ce,,Na,H,X 3.52 13.3 0.54 X,a Ni,,Ce,~,Na,,H,X 6.83 4.25 0.95 Ni,.7Ce1.7Na1H0.5A 7.22 11.85 1 S, NiCe/SiO, 1.20 8.60 1 SZb Ni/SiO, 4.55 0 1 0.7 0.92 1 .o 0.85 1.2 0.78 87% 0'7 0.87 13% 3.0 2.5 0.49 2.5 0.49 2.5 0.49 a Sample reduced at 623 K after desorption at the same temperature. Sample prepared and characterized by Martin and Dalmon.lo catalysts cooled to room temperature still under hydrogen flow.The evolved gas was characterized by mass spectrometry to be a mixture of water and hydrogen. Quantitative measurements of these gases were made by calibration of the apparatus with known amounts of nickel sulphate hydrate and hydrogen gas, respectively.The activities of the supported nickel samples as catalysts of butane hydrogenolysis at atmospheric pressure were measured in a differential mode in a fixed-bed flow reactor. A ternary gas mixture (total flow 5 dm3 h-l) comprising pure, dry butane, hydrogen and helium, was contacted with 40 mg of catalyst at temperatures in the range 473-573 K. The reaction products were analysed using an Intersmat I.G.C. 112 F gas chromatograph fitted with an ionization detector and a 2 m Porapak N column. The areas of the product peaks were determined using an L.T.T. ICAP 5 integrator. Catalytic activities and selectivities were measured at 523 K (see table 4 later). The rates are expressed in moles of butane decomposed per second per gram of metallic nickel (rl) or per second per metal surface area ( r , ) .The metallic surface area was calculated as being 6 x 103/8.9 dnm, where d,, is the average particle diameter determined by magnetic measurements. The selectivity is defined by either S,,, = rclt/rt or SC1 = rcl/rt, where rt is the total butane hydrogenolysis rate, rclt the overall rate of production of methane formed during the reaction and rcl the rate of production of methane due to the complete cracking of the butane molecule (C,Hlo+3H, + 4CH,). For the CO + H, reaction, kinetic experiments were carried out at atmospheric pressure using a fixed-bed flow reactor in a differential mode. The total gas flow rate was equal to 3.61 dm3 h-' and the catalyst weight was 100 mg.The standard conditions were a CO:H, ratio of 1 :4. Gas analysis was performed by on-line gilts chromatography with catharometric and flame ionization detectors. Apart from methane, ethane was the only product detected. Catalytic activities and selectivities are given for the reference temperature of 573 K. The rates are expressed in moles of CO hydrogenated per second per gram of metal ( r , ) or per second per metal surface area (r,). The selectivity is relative to the C,H,/CH, + C,H, molar ratio. RESULTS SAMPLE CHARACTERIZATION As shown by X-ray determinations and potentiometric titration, the cerium ions in X-zeolite samples are all located in the crystallographic sites of the zeolite framework and are always in the trivalent state. In samples A, and S, cerium is present partly in the + 4 oxidation state.For sample A, a CeO, phase was detected by electron microdiffraction. An X-ray determination1360 REACTIVITY OF NICKEL PARTICLES of the location of the Ce3+ ions in cationic sites could not be carried out for this sample, owing to the partial damage of the zeolitic structure. For sample S,, no CeO, phase could be detected by microdiffraction, probably because of its high dispersion or low concentration. The reduced samples exhibit superparamagnetic or paramagnetic behaviour, as shown by the magnetic study. In such cases saturation magnetization, os, is difficult to attain. Thus after magnetic measurements had been made on the reduced samples, these samples were sintered under flowing helium at 1073 K. The large ferromagnetic crystallites thus formed are easily saturated and the os value was measured precisely.Nevertheless, some samples were oxidized during this treatment. In this case the reduction degree, which leads to the o, value, was obtained from volumetric measurements under static conditions of hydrogen consumption by unreduced Ni2+ on samples reduced under dynamic conditions. This measurement was performed at 100 Torr hydrogen pressure and 923 K, the temperature necessary to achieve a complete reduction of the samples. The difference between the total nickel content and the value obtained by this measurement gives the degree of reduction. Values of the extent of reduction, particle size distribution and metallic dispersion are listed in table 1. The metallic dispersion was estimated from values of the metallic particle diameter.HYDROGEN CHEMISORPTION The results, expressed by the ratio H/Ni$ where Ni; is the total number of metallic nickel atoms, are summarized in table 2, as are the values of the metallic dispersion. As shown previously,8 no hydrogen chemisorption occurs for samples X, and X,. These samples possess the greater metallic dispersion. Sample X,, in the same range of metallic dispersion but containing a small percentage of nickel particles of 3 nm diameter, shows a weak but detectable hydrogen chemisorption. It is tempting to attribute this chemisorption uniquely to the 3 nm particles. In all cases the H/Ni$ ratio of samples containing cerium is less than that corresponding to the metallic dispersion, especially for the X-zeolite samples.These results led us to characterize the surface state of the X samples after the reduction treatment by qualitative t.p.d. analysis. These samples retain large quantities of strongly chemisorbed hydrogen up to 923 K. The chemisorbed hydrogen was released during t.p.d. as water (in the temperature range 273-623 K) and as hydrogen (at temperatures above 700 K). The spectra are given in fig. 1. The present work affords further quantitative measurements of desorbed water and hydrogen below 923 K. TEMPERATURE-PROGRAMMED DESORPTION The results, expressed as the number of H,O and H, molecules desorbed per nickel atom, are summarized in table 3. For samples X, and X, the number of H atoms desorbing as H, molecules per nickel atom is greater than unity.Moreover, the amount of water desorbed is large, with the major fraction desorbing between 273 and 623 K. Blank t.p.d. experiments performed on non-reduced samples previously degassed under helium flow at the reduction temperature (623 K) showed that no water desorption occurs below 623 K. Water desorbed above this temperature arises from the dehydroxylation of OH groups chemisorbed on the zeolite. On the reduced samples, the amounts of water desorbing below 623 K cannot be attributed only to the contribution of OH groups generated during the reduction process, which would lead to an H,O/Ni$ ratio equal to or, more probably, lower than unity. The present results strongly suggest that, during the reduction process, hydrogen is dissociatively adsorbed on the nickel with part of it diffusing onto the zeolite.This adsorbate is desorbed during t.p.d. first as water1, and secondly as hydrogen. The loss of adsorbedSAUVION, TEMPERE, GUILLEUX, DJEGA-MARIADASSOU AND DELAFOSSE 1 36 1 Table 2. Hydrogen chemisorption hydrogen chemical metallic chemisorption,a sample composition dispersion H/Ni$ X, Ni,Ce5Na,,H,,X 0.92 0 X, Ni12Ce,Na,,H,,X 0.85 0 X, Ni,,Ce,,Na,H4X 0.78 0.15 x4 Ni21Ce6. 5Na20H4X 0.87 0.04 Ni2.7Ce1.7Na1H0.5A 0.49 0.24 Sl NiCe/SiO, 0.49 0.39 s2 Ni / Si 0, 0.49 a Hydrogen chemisorption expressed by the H/Ni& ratio, where Ni; is the total number of metallic nickel atoms. 400 500 600 700 800 900 TI K Fig. 1. Temperature-programmed-desorption spectra of samples X, (a) and X, (b) : (-) H,O and (---) H,.hydrogen as water is corroborated by i.r. and f.m.r. results. Indeed, the i.r. spectra of the reduced samples show a band at 1990cm-l which may be ascribed to a hydrogen-metal bond.13 This band disappears, during t.p.d., in the same temperature range over which water is desorbed. Furthermore, the f.m.r. spectra of all the samples, except sample XI, remain unchanged during this treatment, indicating no change in the magnetization of the nickel particles. This result suggests that the hydrogen species dissociating on the metal donates an electron and is removed as H+ without any change in the electronic state of the metal previously covered by the chemisorbed hydrogen during the reduction process. At higher desorption temperatures, when hydrogen molecules are evolved, the magnetization of the samples increases significantly as the hydrogen species leaves the metallic particle in the atomic form.In the case of sample X,, f.m.r. measurements recorded during the desorption treatment have shown that at high temperature this sample is progressively oxidized to total oxidation. The metallic nickel supported on the more acidic supports is stable only in presence of adsorbed hydrogen.l4? l51362 REACTIVITY OF NICKEL PARTICLES Table 3. Temperature-programmed-desorption measurements chemical nickel particle sample composition diameter/nm H,O/Ni$a H/Ni$* Xl Ni,Ce5Na,,H3,X 0.7 5.66 0.53 x2 Ni,,Ce,Na,,H,,X 1 .o 6.68 0.40 2.91 0.82 x4 Ni21Ce6.5Na20H4X 13% 3.0 x3 Nil ,Ce,,Na,H,X 1.2 3.35 0.21 Number of water and hydrogen molecules evolved up to 923 K per metallic nickel atom, Ni$ being the total number of metallic atoms.All the above results explain why, according to the hydrogen coverage of nickel crystallites, the hydrogen chemisorption capacities are small or suppressed. Similar observations have been reported by Menon and Froment16 with Pt/Al,O, systems. The attenuation of the hydrogen-adsorption properties of these solids, when reduced above 723 K, is ascribed to the presence of strongly chemisorbed hydrogen on the metal in this temperature range. BUTANE HYDROGENOLYSIS In table 4 are reported the catalytic activities of the various samples studied and their selectivities, defined as the degree of fragmentation of the butane molecules, towards me thane formation. From these results the following points may be deduced.(1) Samples X, and X, are totally inactive in the reaction. In no case was metal sintering observed during the course of the catalytic runs. Taking into account the assumption that the hydrogenolysis mechanism requires the existence of a sufficient number of contiguous free metallic sites accessible to the reactant molecules,17 it seems obvious that samples X, and X,, with high hydrogen coverage after reduction, cannot hydrogenolize butane. (2) Sample X,, with a bidispersed metallic distribution, shows weak but detectable activity. It seems likely that the activity of this sample can be ascribed, as suggested for hydrogen chemisorption, uniquely to the 3 nm nickel particles. (3) Higher catalytic activities can be observed with nickel particles dispersed on silica in the presence or absence of cerium additives.(4) For X, and A, samples, which display lower activities than nickel/silica systems, it may be noted that these activities, expressed per total metallic surface area, are obviously underevaluated for samples partly covered by hydrogen species. ( 5 ) The catalytic selectivities depend strongly on the presence of cerium additives. Thus the selectivity values are greatly lowered in the presence of cerium-containing nickel catalystscompared with thecerium-free nickel/silica reference sample. lo These results concerning butane hydrogenolysis show that, when the nickel catalysts present an accessible metallic surface to the reactants, the activities seem to be of the same order of magnitude for all samples whether or not they contain Ce3+ ions, whereas the selectivities are modified in the presence of cerium in the 3 + state.The partial or total loss of reactivity of the reduced catalysts, when observed, may be attributed to the coverage of the metallic surface by hydrogen entities. In the case of samples XI, X, and X,, the particularly large amount of hydrogen species retained after the reduction process seems to imply the existence of a hydrogen-spillover effect.SAUVION, TEMPERE, GUILLEUX, DJEGA-MARIADASSOU AND DELAFOSSE 1 363 Table 4. Activity and selectivity for butane hydrogenolysis at 523 K butane hydrogenolysis rate hydrogen selectivitya sample composition H/Ni% C4Hl0 s-' g& C4H10 s-' m& SClt SC, chemical chemisorption, rl/ mol r z / lop8 mol - - - X, Ni,Ce,Na,,H,,X 0 0 0 X, Ni,,Ce7Na2,Hl,X 0 0 0 - - X, NillCe17Na,H,X 0.15 62.3 11.0 1.43 0.09 x.4 Ni21Ce,.,Na20H4X 0.04 5.2 0.6 - 0.24 47.0 17.4 1.57 0.12 S, NiCe/SiO, 0.39 121.0 44.8 1.57 0.12 S, Ni/SiO, 102.7 38.0 2.28 0.35 - A 1 Ni2.7Ce1. 7Na1 HO. SA a For definition see text. It was of interest to study the reactivity of these samples towards a reaction which, unlike hydrogenolysis, would not be inhibited by the presence of hydrogen preadsorbed on the metallic sites and would consume these species. Two reactions seem to meet these conditions : the hydrogenation of ethylene and the hydrogen-deuterium exchange reaction. Both reactions might occur only by the introduction of ethylene or deuterium onto the reduced samples. REACTIVITY OF HYDROGEN SPECIES IN THE PRESENCE OF ETHYLENE OR DEUTERIUM Of the X-zeolite samples, X, was chosen for this study on account of its homogeneous particle-size distribution, its stability and its high coverage by adsorbed hydrogen.After reduction at 623 K and evacuation of the hydrogen gas phase at room temperature, the sample was submitted for 2 h to ethylene or deuterium at 433 or 273 K, respectively, in a static system under a pressure of 100 Torr of the considered gas. Afterwards, the gas phase of the reactor was analysed by on-line mass spectroscopy and a t.p.d. spectrum of the catalyst was obtained and compared with the reference t.p.d. spectrum of sample X, reduced at 623 K. Quantitative experimental results are reported in table 5. After introduction of ethylene into the reactor according to the preceding conditions the following results are obtained.(1) The gas phase evacuated at room temperature consists of 90% ethane. (2) During the course of the t.p.d. experiment heavier hydrocarbons are evolved in large quantities up to high temperatures. (3) The amounts of water and hydrogen molecules desorbed during the t.p.d. treatment are negligible. (4) In addition, after the ethylene reaction the i.r. band located at 1990cm-l, previously ascribed to an NiO-H bond, can no longer be observed, while the f.m.r. signal of the sample increases. All these results show that ethylene consumes the chemisorbed hydrogen, yielding ethane and higher hydrocarbons. With regard to deuterium reactivity at 273 K we can make the following observations.(1) The gas evolved at room temperature is composed of D, and HD molecules. (2) By temperature-programmed desorption and as compared with the reference sample (table 5 ) the solid yields an identical number of water molecules, while the amount of hydrogen molecules evolved is diminished significantly. (3) The f.m.r. signal of the sample is increased.1364 REACTIVITY OF NICKEL PARTICLES Table 5. Temperature-programmed-desorption measurements after reaction of sample X, with ethylene and deuterium treatment H,O/Ni$" H,/Nigb X, taken as reference X, after reaction with C2H4 at 433 K X, after reaction with D, at 273 K 6.7 0.4 0 0 6.8 0.2 Number of water and hydrogen molecules evolved up to 923 K per metallic nickel atom, Ni; being the total number of metallic atoms.Table 6. Activity and selectivity for CO hydrogenation at 573 K CO hydrogenation rate hydrogen chemical chemisorption rl/ lop6 mol,, r 2 / mol,, selectivity, sample composition H/Ni$ s-' g,;o s-1 S" X, Ni,Ce,Na,,H,,X 0 8.4 9 0.04 X, Ni,,Ce,Na,,H,,X 0 10.1 15 0.06 X, Ni, Ce, Na,H,X 0.15 203 361 0.04 Ni2. 7Na1H0.,A 0.24 157 585 0.04 Sl NiCe/SiO, 0.39 94 1 3.500 0.07 s2 Ni/SiO, 44.6 166 0.0 1 a S is the selectivity defined by the C,H,/CH, + C2H6 molar ratio at 573 K. Thus in contrast to ethylene the deuterium molecule only reacts with part of the hydrogen species which left the surface as molecular hydrogen during the thermo- desorption of the freshly reduced sample. It follows that sample X,, inactive for butane hydrogenolysis, presents a degree of catalytic activity in hydrogenation reactions and for H,-D, isotopic exchange.The hydrogen species present on the reduced sample surface are completely consumed in the presence of ethylene, but only partially in the presence of deuterium. CARBON MONOXIDE HYDROGENATION We have shown that highly dispersed nickel catalysts displayed little or no activity with regard to butane hydrogenolysis. This does not imply that these solids cannot present good activities for other reactions, and particularly for the hydrogenation of CO as mentioned above. The results of this study are summarized in table 6 , which lists the catalytic activities and selectivities of the various samples. The reactivity of these catalysts differs widely according to the sample considered.The activities of cerium-containing catalysts, expressed per metallic surface area, do not seem dependent on the particle size or cerium content, but rather on the hydrogen surface coverage of the reduced samples; this is shown for all the catalysts by the difference observed between the values of the metallic dispersion and the H/Ni& ratio. Thus samples X, and X, are by far the less active.SAUVION, TEMPERE, GUILLEUX, DJEGA-MARIADASSOU AND DELAFOSSE 1 365 In addition, sample S,, with the same metallic dispersion as sample S,, is twenty times more active than the latter. This greater activity must be attributed to the presence, on sample S,, of cerium additives. Furthermore, it appears that samples X, and A,, with a metallic surface which is mostly inaccessible to hydrogen at room temperature, possess higher activities than sample S,, with a fully accessible metallic surface.The presence in sample X, of cerium only in the trivalent state seems to show that the Ce3+ ion is responsible for the increase in activities observed. DISCUSSION The above results clearly show how the catalytic properties of nickel metal dispersed on various oxides can be modified as a function of the support acidity, the metallic dispersion and the presence of additives. For X-zeolite samples it has been shown that correlations exist between the support acidity, the average particle diameter, the metallic surface accessible to hydrogen and the catalytic activity for butane hydrogenolysis. It thus appears that the greater the acidity of the support, the smaller the nickel particles and the lower the hydrogen chemisorption and hydrogenolysis rates.Similar relations are observed with the other catalytic systems having the same metallic dispersion, but differing in the acidity of the support (A zeolite or SiO,). The loss or decrease in activity observed for the more acidic samples could be ascribed to the total or partial coverage of the metallic surface by strongly chemisorbed hydrogen species. The coverage of the metal by hydrogen could be correlated with the presence of Lewis-acid sites on the zeolitic supports. During the reduction process these electron-acceptor centres can induce some electron depletion of the small nickel particles in their formation, leading to the establishment of strong metal-support interactions and to the enhancement of metallic-surface affinity for the chemisorption of hydrogen molecules, which can donate their electrons to the nickel atoms.In butane hydrogenolysis the extensive coverage of the metallic surface of the more acidic samples by hydrogen species hinders the dissociative chemisorption of the hydrocarbons. After temperature-programmed desorption of the X-zeolite samples up to 923 K, these hydrogen species are removed from the metallic surface as water and as molecular hydrogen. The nickel particles supported on the more acidic sample are oxidized in the absence of hydrogen. In contrast, the less acidic samples retain their metallic stability without sintering. An attempt to observe activity after the removal of these hydrogen species has been carried out on sample X,, which is not oxidized during t.p.d.treatment and whose metallic-particle size remains unchanged during this treatment. Again, no hydro- genolysis activity can be observed. An additional experiment has been performed on this catalyst thermodesorbed at 923 K, by contacting it with a hydrogen flow at 523 K, the reference temperature for the hydrogenolysis reaction. It appears from the thermodesorption curve of this sample that during this treatment the hydrogen uptake is negligible. The inactivity observed for the bare metallic nickel surface in H, chemisorption and hydrogenolysis could originate from superficial metal oxidation during this treatment. However, taking into account that the chemisorbed hydrogen leaves the metallic surface mainly as H+, one could also propose that the metal remains electron-enriched and consequently does not chemisorb hydrogen or hydrogenolize butane, as do the Nio/TiO, systems when reduced at high temperatures. Further investigation by X-ray spectroscopy might afford valuable information on the electronic state of the supported nickel.1366 REACTIVITY OF NICKEL PARTICLES With regard to CO hydrogenation, the presence of chemisorbed hydrogen species on the metallic surface could prevent the adsorption of the reactant molecules as in butane hydrogenolysis.However, the temperature at which the CO + H, reaction activity is measured (573 K) is higher than that taken as reference in butane hydrogenolysis and refers to a more important desorption of the hydrogen entities, rendering the surface more accessible to the reactants and thus justifying the weak but non-negligible activities observed.For the active samples, which have part of the metallic surface accessible to the reactants, it appears that another factor can also strongly modify the catalytic properties of nickel metal, namely the presence of Ce3+ ion additives. For X-zeolite samples, cerium cations are always present in the trivalent state, whereas for samples A, and S, it has been shown that, prior to reduction, cerium is partly present as a well dispersed CeO, phase. After reduction CeO, could no longer be detected by electron microdiffraction, and it is likely that during the reduction process the CeO, phase is much dispersed or reduced.Consequently, on all cerium- containing samples cerium is present, for the most part, in the trivalent state. In butane hydrogenolysis the low values of hydrocarbon fragmentation obtained on these samples, which generally occurs when the dehydrogenated intermediates formed during the reaction are less strongly adsorbed on the metallic surface, may be correlated with the presence of trivalent cerium. The electron-donating power of Ce3+ might induce charge transfer towards nickel crystallites and, as a result, increase the electron density of the metal leading to weaker hydrocarbon-metal interactions. In addition, the substantial increase in methanation activity and the selectivity modification observed for cerium-containing catalysts seem to be correlated with the electron-donor properties of cerium in the + 3 state.An explanation of the role of Ce3+ can be proposed by again assuming that the trivalent cerium cations in the neighbourhood of the metallic nickel particles initiate electron transfer towards the metal, leading to an electron-density enrichment of the metallic surface. Using the (a, n) bond scheme for CO chemisorptionl* this electron transfer would decrease the carbon-oxygen bond strength and thus increase the C-0 bond-dissociation probability. In the same way Kao et aL5 have ascribed the increased methanation activities observed in Ni/TiO, systems, compared with Ni/SiO,, to an excess concentration of electrons on metallic nickel caused by electron transfer from the support to the metal. Concerning the selectivities of cerium-containing catalysts, in all cases these samples are four to seven times more selective towards ethane production than sample S,.This is again in good agreement with the preceding assumption, according to which Ce3+ cations could induce some electron transfer from the support towards the metal. The electron enrichment of the metal surface might increase the dissociative chemisorption of CO compared with that of H, and lead to the formation of hydrogen-poor hydrocarbons. The results of the present investigation emphasize the importance of two factors able to modify considerably the catalytic properties of nickel metal highly dispersed on non-reducible oxide supports. One is the acidity of the support, which can generate strong metal-support interactions leading to electron depletion of the metal and consequently to its coverage, during the reduction process, by strongly chemisorbed hydrogen.In this case, the loss of reactivity is to be correlated to the absence of free metallic surface sites. The second is the presence of Ce3+ additives, which could induce electron enrichment of the metallic surface, leading to enhancement of methanation activity and to a shift in product distribution, as observed for Ni/TiO, systems when reduced at high temperatures.SAUVION, TEMPERE, GUILLEUX, DJEGA-MARIADASSOU AND DELAFOSSE 1 367 We thank Dr G. A. Martin (Institut de Recherches sur la Catalyse, Villeurbanne) for the gift of the nickel-silica sample. We are also grateful to Mrs J. Jeanjean for assistance with the X-ray diffraction study and to Mr M. Lavergne, who recorded the electron microdiffraction spectra. S. J. Tauster, S. C. Fung, R. T. K. Baker and S. A. Horsley, Science, 1981, 211, 1121. C. H. Bartholomew, R. B. Pannell and J. L. Butler, J . Catal., 1980, 65, 335. M. A. Vannice and C. C. Twu, J. Catal., 1983, 82, 213. R. Burch and A. R. Flambard, J . Catal., 1982, 78, 389. Chia-Chieh Kao, Shou-Chin Tsai and Yip-Wam Chung, J. Catal., 1982, 73, 136. P. Gallezot, Catal. Rev. Sci. Eng., 1979, 20, 121. J. C. Vedrine, M. Dufaux, C. Naccache and B. Imelik, J. Chem. SOC., Faraday Trans. I, 1978,74,440. G. N. Sauvion, M. F. Guilleux, J. F. Tempkre and D. Delafosse, J . Chim. Phys., 1982, 79, 395. S. Djemel, M. F. Guilleux, J. Jeanjean, J. F. Tempere and D. Delafosse, J . Chem. SOC., Faraday Trans. I , 1982, 78, 835. P. Weiss and R. Forrer, Ann. Phys. (Paris), 1926, 5, 153. lo G. A. Martin and J. A. Dalmon, C.R. Acad. Sci., 1978, 286, 127. '* T. M. Apple, P. Gajardo and C. Dybowski, J . Catal., 1981, 68, 103. l3 T. Nakata, J . Chem. Phys., 1976, 65, 487. l4 P. Leroux, Th&e (Universitk de Lyon, 1977). Is G. Martino, Studies in Surface Science and Catalysis (Elsevier, Amsterdam, 1980), vol. 4, p. 399. l6 P. G. Menon and G. F. Froment, Appl. Catal., 1981, 1, 31. J. A. Dalmon and G. A. Martin, J . Catal., 1980, 66, 214. G. Blyholder, J. Phys. Chem., 1964, 68, 2722. (PAPER 41 1 1 33)

ISSN:0300-9599    

DOI:10.1039/F19858101357

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1985, 81, 1369-1373 Ion-pair Formdtion as a Determining Factor in the Effectiveness of the Interaction of Electrolytes with Amphiphilic Azo Dyes in Water BY MICHEL DE VIJLDER Laboratorium voor Fysische Scheikunde, Rijksuniversiteit Gent, Krijgslaan 28 1, B-9000 Gent, Belgium Received 5th July, 1984 The order of effectiveness of added simple electrolytes (BaCl,, CaCl,, SrCl,, LiC1, NaC1, KC1, RbCl and CsCl) in influencing the spectrum of four amphiphile azo-dye sulphonates (Methyl Orange and its homologues) has been determined spectrophotometrically. The order found for Benzyl Orange corresponds to that for Methyl Orange but differs from those for Ethyl Orange and Orange IV. As with Benzyl Orange, the order of effectiveness does not correspond to either a lyotropic series or a ‘salting-out power’ series but can be related 1.0 the previously reported association constants for azo-dye anions and added cations.This correlation confirms the importance of competitive hydration and ion-pair formation as intermediate steps in processes involving electrolytes and amphiphilic molecules in water. Interactions of foreign ions with the chromophoric groups of amphiphilic compounds are of interest in several areas of chemistry and biochemistry, where it has been established that neutral salts can influence the conformation of proteins and other macromolecules by affecting the prevalent hydrophobic or ionic interactions. Such processes can usually be explained by invoking the breaking up of the water structure around the amphiphilic compounds by the lyophilic electrolytes, the amphiphilic compounds becoming insufficiently solvated and inclined to aggregate or form micelles, possibly with the assistance of the reduction of the electrostatic repulsion forces by the counterions.An important class of amphiphiles are the ionic azo dyes. We recently reported’ the effect of added electrolytes on the spectrum of Methyl Orange; the effect consists of a marked decrease in the intensity of light absorption by the dye anion at Amax = 465 nm and of the appearance of a new absorption band at 358 nm. This new band cannot be attributed to some aggregated species because of the lack of evidence of a distinct dimer or polymer band in the absence of salt, even at dye concentrations near saturation. We believe that the altering of the electronic configuration of the chromophoric system from its more stable level because of the interactions between the counterions and the charge distributed over the chromophoric system is responsible for our observations, and we found that the order of effectiveness of the cations is Mg2+ > Ca2+ > Na+ > K+ > Li+.However, we overlooked the abnormal order of Na+ and K+ : the power of cations to break the ‘ iceberg’ structure surrounding hydrocarbon molecules2 follows the lyotropic series, while the ability of ions to bind water to themselves at the expense of hydrated colloids3? follows the order K+ > Na+ > Li+ or K+ < Na+ < Li+. Also, the effectiveness of added cations for promoting the aggregation of the higher 13691370 INTERACTION OF AZO DYES WITH ELECTROLYTES Table 1.Concentration of electrolyte (in mol dmP3) required to induce a decrease in the absorptivity of azo-dye anions in water (the accuracy is ca. 10%) [dye1 [ele~trolyte]~/mol dmP3 dye dm-3 /nm Ca2+ Ba2+ Sr2+ Cs+ Rb+ K+ Na+ Li+ mol A,,, Methyl Orange 5.065 465 0.75 0.55 0.65 1.5 1.5 2.25 1.5 3.25 Benzyl Orange 5.156 435 2.5 x 1.5 x 5 x lop4 0.25 0.25 0.25 0.05 0.70 Orange 1V 5.232 443 4 x 3.5 x lop5 9 x - - 0.25 0.40 0.90 Ethyl Orange 4.958 475 - 0.6 1.3 - - 2.1 2.75 3.10 (Tropaeolin 00) a As chloride. 1.5 X E x m 4 d T Ba" No' 0 0.5 1 1.5 0 1 x10' 2x10 0 [ electrolyte]/mol dm-3 Fig. 1. Decreasing absorbance ( A ) of Benzyl Orange in water (5.156 x lop5 mol dmP3, A,,, = 435 nm, pH 10, cell pathlength = 1 cm, water at the same pH as reference) in the presence of increasing concentrations of electrolytes.homologues of Methyl Orange (Pentyl Orange and Hexyl Orange) is in the order K+ > Na+ > Li+.5 We thus decided to examine more accurately the influence of these cations and others on the spectra of different axo dyes and to try to find an explanation for the irregularities found in their order of effectiveness. EXPERIMENTAL The spectrophotometric measurements were performed with a Varian Techtron 635 spectro- photometer. Methyl Orange and Ethyl Orange were purchased from B.D.H. Chemicals, Orange IV (Tropaeolin 00) from Fluka and Benzyl Orange from Merck. All dyes were recrystallized twice from water. The electrolytes were commercial products of analytical grade, used without further purification.Each sample, prepared by dilution from stock solutions, was allowed to stand overnight to reach equilibrium before measurements were undertaken.M. DE VIJLDER 1371 RESULTS The spectra of four amphiphilic azo dyes [Methyl Orange, Ethyl Orange and two higher homologues : Benzyl Orange and Orange IV, chosen to illustrate the effect of increasing aromatic bulk (a feature which has been studied by Takagishi5)] in aqueous solution at increasing electrolyte concentrations (BaCI,, CaCl,, SrCl,, LEI, NaC1, KC1, RbCl and CsCl) were recorded. We noted the amount required to induce a decrease in the dye absorptivity, indicating a deviation from Beer’s law. The results are summarized in table 1 . How the absorptivity of Benzyl Orange decreases as a function of added electrolyte concentration is illustrated in fig.1. Note the differences between univalent and bivalent cations. DISCUSSION The results are unexpected both qualitatively (the difference in order of effectiveness on the almost identical Methyl and Ethyl Orange anions) and quantitatively (a factor of > lo3 between the concentrations of univalent and bivalent cations inducing spectral effects for Benzyl Orange and Orange IV). The order of effectiveness of the electrolytes on the behaviour and the structure of amphiphiles in aqueous solution, especially in the colloidal state, is currently related to their places in lyotropic series. However, the proposed orders in such series (Dobry-D~claux:~ Sr2+ > Ca2+ > Ba2+ > K+ > Na+ > Li+; Hofmeister:* Ca2+ > Sr2+ B Ba2+ > Li+ > Na+ > K+ > Rb+ > Cs+)arenotalwaysvalidanddependonthe nature of the colloid being precipitated.A comparison with the colloidal state, characteristic of most other amphiphiles, is also not self-evident for the azo dyes considered here: indeed, Reeves6 has reported that the association of azo dyes occurs in a stepwise manner, without cooperativity and without evidence of a c.m.c., by stacking like a pile of coins. Since the hydrophobic part of such dyes consists of benzene rings connected by heteroatoms, which can also be hydrated along with the terminal polar groups, the growth of aggregation is not as limited by a balance between hydrophobic interactions and coulombic repulsions as in the case of micelles formed by other ionic surfactants.An alternative explanation of the effects of electrolytes follows from their unques- tionable inauence on the ‘hydration structure’ around the hydrocarbon part of the azo dyes. Evidently the mechanism of such hydration is qualitatively different from that for a simple polar species and is characterized by intensification of the hydrogen-bonded structure in water.” The energy of such a system is quite high and electrolytes will more or less easily break up this system. Unfortunately, there exist no rules which relate the operational criteria for modifications to the structure of water to any properties of the ions. In brief, we postulate that if salt effects generally can be accounted for in terms of their power of ‘ salting out’ the amphiphile molecule, the mechanism of this process remains obscure.Deviations from fixed criteria are often observed, because of the interactions of the various ions with water (charge size, polarizability) but also more specifically because of association of the ions with some part of the am~hiphile.~ This association, the result of competitive hydration between hydrated electrolyte cations (M+) and hydrated dye anions (D-), forming a solvated ion pair has already been considered by Reeves.lO The probability of ion pairs being formed between the anions of Methyl Orange1372 1.5 1 .o G Do d 0.5 0 INTERACTION OF AZO DYES WITH ELECTROLYTES r --- Ba2' - 0 25 0 0.25 0.50 - L - 3 - 2 -1 0 log c Fig. 2. Double logarithmic plot of the values of the association constants for the cations of added chlorides and the anions of the dyes calculated in ref.(1 1) against the critical concentration of the corresponding electrolytes required to induce deviations from adherence to Beer's law in (a) Methyl Orange and (b) Benzyl Orange solutions. The accuracy of the reported log K,,, values may be estimated to be ca. 0.05. and Benzyl Orange on the one hand and cations of simple electrolytes on the other has been assumed on the basis of the anomalous spectra recorded at pH = pKdye; association constants have been calculated.'l We find that our unexpected order of effectiveness of the cations corresponds to the order of the calculated association constants and that there exists a linear relationship between the logarithms of both (fig.2). Whether this relationship also applies to the two other dyes we examined (Ethyl Orange and Orange IV) is not yet known, as the values of the association constants are not available; however, we hope to obtain them in forthcoming work. The question as to why the order of effectiveness holds for Methyl and Benzyl Orange is not clear: the cations may partly retain their hydration layers in such processes, as is sometimes postulated for alkali-metal counterions in rnice'les,l2 and there may also be doubt about the site where association occurs. Recently reported Raman spectral3 confirmed that the structure of the anion of Methyl Orange may be represented as a quinoidal : and that protonation occurs at the p-azo nitrogen as well as at the dimethylamino nitrogen, the former being the dominant form: Under appropriate circumstances other cations may take the protons' places.However, the structural influence of the auxochromic groups, as experienced here (uia the different behaviour of Methyl and Ethyl Orange) can only be invoked if association occurs at the amino nitrogen.M. DE VIJLDER 1373 1 I I I t 1 I I 0.1 0.2 0.3 0.4 log ( [NaCl],d/mol dm-3) Fig. 3. Double logarithmic plot of the Methyl Orange anion concentration against the critical NaCl concentration. On considering the critical concentration values of an electrolyte, the quantitative differences between Methyl and Ethyl Orange as against Benzyl Orange and Orange IV can be accounted for by the size of the hydrophobic volumes: the larger the hydrophobic volume the greater will be the solvation needed to keep it protected and the lower will be the electrolyte concentration needed to interfere with it.This leads us to consider the effect of decreasing the concentration of dye. The results shown in fig. 3 confirm that more salt is needed when less dye (and thus more hydration water) is present. The double-logarithmic relationship obtained conforms to Schiilze-Hardy's law. M. De Vijlder, J. Chem. SOC., Faraday Trans. I , 1982, 78, 137. M. Kaminski, Discuss. Faraday Soc., 1957, 24, 171. A. Dobry-Duclaux, Chem. Ztg, 1952,76, 805. S. H. Maron and J. B. Lando, Fundamentals of Physical Chemistry (Macmillan, New York, 1974). T. Takagishi, S. Fujii and N. Kuroki, J. Colloid Interface Sci., 1983, 94, 114. R. L. Reeves and Sh. A. Harkaway, J. Colloid Interface Sci., 1978, 64, 342. ' M. Mastroianni, M. Pika1 and S. Lindenbaum, J. Phys. Chem., 1972, 76, 3050. * R. L. Reeves and R. S. Kaiser, J . Org. Chem., 1970, 35, 3670. A. Ray and G. Nemethy, J. Am. Chem. SOC., 1971, 93, 6787. lo R. L. Reeves and R. S. Kaiser, J . Phys. Chem., 1969, 73, 2279. l1 M. De Vijlder and W. Rigole, Anal. Chem., 1971,43, 1234; M. Meganck, Lic. Thesis (University of l2 B. Lindman and H. Wennerstrom, Amphiphile Aggregation in Aqueous Solution (Springer-Verlag, l 3 K. Machida, B-K. Kim, Y. Saito, K. Igarashi and T. Uno, Bull. Chem. SOC. Jpn, 1974, 47, 78; Ghent, 1974); H. Huysmans, Lic. Thesis (University of Ghent, 1975). Berlin, 1981), p. 55. J. C. Merlin, J. L. Loriaux, A. Dupaix and E. W. Thomas, J. Raman Spectrosc., 1981, 11, 131. (PAPER 41 1 166)

ISSN:0300-9599    

DOI:10.1039/F19858101369

出版商:RSC    

年代:1985

数据来源: RSC