摘要:

physicochemical topics, thereby encouraging scientists of different disciplines to contribute their varied viewpoints to a coiiinion theme. A recent Discussion is :- The Royal Soci- of Chemistry- No.75 lntraamolecwlar Kinetics No. 75 in the series, this publication is the result of a general discussion held at the University of Warwick in April 1983. Contents: The Spiers Meniorlal Lecture; Vibrational Redistribution within Excited Electronic States of Polyatomic Molecules Inrraniolecular R e h u t i o n o f 1. vcited States lsomerization of Intcrnal~ncrgy-selected Ions Kinetics of Ion-Molecule Collision Coinple\es in the Gas Phase, E\periinent and Theory lntrainolccular Decay 01' Soinc Open-shell Pulya t o niic Ca lions On tlic Theory u i Iiitrdniolccul~r I n e r g y Transfer Pulsed Laser Preparation and Ouaniuin Superposition Statc Evolution in ReguLtr and Irregular Systems A Ouantuiii-iiicclianical Internal-collision Model for State-sclcctcd Uniinolccular Decoiiiposilio n The Correspondence Principle and Intramolecular Dynamics lntrainoleculdr Dcphasiiig.t'icusecond Evolution of Wavepacket States in a Molecule with Int erinediate-casc level Struct urc Energy Conversion in van der Waals C'u~~iplc\c\ ol s-Tetrarine and Argon Tim-dependent Processes in Polyatuinic Molecules During and After Intense Intrarcd Irradiation Energy Distributions in tlic (.N(X'L+) bragnient froiii tlie Infrared Multiplepholun Dissociation ol' CI. ICN. A Coinparison between 1:xperiiiiental Results and the Predictions ot Statistical Theories of ChFO + Product Energy Partitioning in the Decoiii- position of State-selectively Excited HOON and IIOOD Low-power Inl-rarcd Laser I'hoiolysis o f Tetramethy ldioxetan Uniinolecular Reactions lnduccd by Vibrational Overtone Excitation Uniiiiolecular Decomposition of t-Butylhydro- peroxide by Direct Excitation of the 6-0 0-11 Stretching Overtone I'icosecond-jet Spectroscopy and Photoclieinistry.Energy Redistribution and its Iiiipact'on Coherence, Isoincrization, Ihssociatiun and Solvalioii knergy Redistribution in Large Molecules. Duect St ud y o f In1 rainolucular Rehxa lion in the Gas Phase with Picosecond Gating Rotation-dependent Intrainolecuhr I'r~)cessc.sofSO:(A'A.) in a Superwnic Jct Role of Rotation-Vibration Interaction in Vibrational Keh\ation. Energy Kcdistribution in k,xcitcd Singlet I~'ornialdc1iyde Sub-lhppler.Spectroscopy of Benrcnc in the "('liaiinel-lliree" Region Intraiiiulccular 1:lectronic Kclau~tion and I'liotois~)iiieruati[)n Processes in tlie lsuhted Azabenrene Molecules Pyridinc, Pyrazinc and I'yriiiiidinc Softcover 434pp 0 85186 658 1 Price f25.00 ($48.00) Rest of the World f26.00 RSC Members f 16.25 Faraday Discussions of the Chemical Society 7< lnrruniolei u h r Kincrit I Faraday Symposia are usually held annually and are confined to more specialiscd topics than Discussions, with particular reference to recent rapidly developing lines of rescuch. A recent Symposium is :- No.l?The Hydrophobic Interadion No. 17 in the series, this publication is the result of a symposium on The Hydrophobic Interaction held at the Uiiiversity of Reading in December 1982.Contents: Hydrophobic Interdctionr a llistaric.11 Per spect ivr llydrupliobic Ilydration Geometric Kelaution in Water. Its Role in Precise Vapour-pressure Measureiiients of the SolubilkdtiorI of Benzene by Aqueous Sodiuiii Octylsulphate Solutions Nuclear Magnetic Resonance R e b u t i o n Investigation of Tetrahydrofuran and Methyl Iodide Clathrdtes Infrared and Nuckar Magnetic Kcwnance Studies Pertaining to the (age Model t o r Solutions oS Acetone in Water Irothernial Transport Properties in Solutions o f S y mmet r ica I Tet ra-alk y hmnioniuiii Bromides Thermodynamics of Cavity I'oriiiaiion in Water. A Molecular Dynamics Study Molecular Librations and Solvent Oricnt- ational Correlations in Hydrophobic Phenomena Monte Carlo Computer Siniulation Study of the Hydrophobic Effect.Potential ot Mean Force for ECfir)gaq at 25 and SOv C Hydroplicibic Moments and Protein Structure Application 01' the Kirkwood-Buff Theory to the t'roblcin 01 Hydrophobic Interactions Ihentangleinent of Ilydrophubic and IFlcctrosi~tic Contributions t o the I.ilni Pressures O i Ionic Surfactants llydrophobir. Intcracliuns in Dilute Su lut io ns u t 1'0 1 y (vin y I a Ico lio I) ('onioriii;tiionaI and 1:unc.i ional I'ropertics of tiaeiiwglobin in Water+Alcohol Mixtures. Dependence o f Bull. Electrostatic and tlydrupliohic I n t c r x t i o n s upon ptl and KCI concentrations Softcover 24Opp 0 85186 668 9 Price f36.50 ($70.00) Rest of the World f38.50 RSC Members f 23.75 ORDERING RSC Members should send their orders to: The Royal Society of Chemistry.The Membership Officer. 30 Russell Square, Non-RSC Members The Royal Society of Chemistry, Distribution Centre, Blackhorse Road, L London WC1 B 5DT. Letchworth, Herts SO6 IHN, England. Faradaj Symposia of the Chemical Society hGi 17 I hc HI drophohr' Inrcrm rron 1 9 X ? (viii)physicochemical topics, thereby encouraging scientists of different disciplines to contribute their varied viewpoints to a coiiinion theme. A recent Discussion is :- The Royal Soci- of Chemistry- No.75 lntraamolecwlar Kinetics No. 75 in the series, this publication is the result of a general discussion held at the University of Warwick in April 1983. Contents: The Spiers Meniorlal Lecture; Vibrational Redistribution within Excited Electronic States of Polyatomic Molecules Inrraniolecular R e h u t i o n o f 1.vcited States lsomerization of Intcrnal~ncrgy-selected Ions Kinetics of Ion-Molecule Collision Coinple\es in the Gas Phase, E\periinent and Theory lntrainolccular Decay 01' Soinc Open-shell Pulya t o niic Ca lions On tlic Theory u i Iiitrdniolccul~r I n e r g y Transfer Pulsed Laser Preparation and Ouaniuin Superposition Statc Evolution in ReguLtr and Irregular Systems A Ouantuiii-iiicclianical Internal-collision Model for State-sclcctcd Uniinolccular Decoiiiposilio n The Correspondence Principle and Intramolecular Dynamics lntrainoleculdr Dcphasiiig. t'icusecond Evolution of Wavepacket States in a Molecule with Int erinediate-casc level Struct urc Energy Conversion in van der Waals C'u~~iplc\c\ ol s-Tetrarine and Argon Tim-dependent Processes in Polyatuinic Molecules During and After Intense Intrarcd Irradiation Energy Distributions in tlic (.N(X'L+) bragnient froiii tlie Infrared Multiplepholun Dissociation ol' CI.ICN. A Coinparison between 1:xperiiiiental Results and the Predictions ot Statistical Theories of ChFO + Product Energy Partitioning in the Decoiii- position of State-selectively Excited HOON and IIOOD Low-power Inl-rarcd Laser I'hoiolysis o f Tetramethy ldioxetan Uniinolecular Reactions lnduccd by Vibrational Overtone Excitation Uniiiiolecular Decomposition of t-Butylhydro- peroxide by Direct Excitation of the 6-0 0-11 Stretching Overtone I'icosecond-jet Spectroscopy and Photoclieinistry. Energy Redistribution and its Iiiipact'on Coherence, Isoincrization, Ihssociatiun and Solvalioii knergy Redistribution in Large Molecules.Duect St ud y o f In1 rainolucular Rehxa lion in the Gas Phase with Picosecond Gating Rotation-dependent Intrainolecuhr I'r~)cessc.sofSO:(A'A.) in a Superwnic Jct Role of Rotation-Vibration Interaction in Vibrational Keh\ation. Energy Kcdistribution in k,xcitcd Singlet I~'ornialdc1iyde Sub-lhppler. Spectroscopy of Benrcnc in the "('liaiinel-lliree" Region Intraiiiulccular 1:lectronic Kclau~tion and I'liotois~)iiieruati[)n Processes in tlie lsuhted Azabenrene Molecules Pyridinc, Pyrazinc and I'yriiiiidinc Softcover 434pp 0 85186 658 1 Price f25.00 ($48.00) Rest of the World f26.00 RSC Members f 16.25 Faraday Discussions of the Chemical Society 7< lnrruniolei u h r Kincrit I Faraday Symposia are usually held annually and are confined to more specialiscd topics than Discussions, with particular reference to recent rapidly developing lines of rescuch.A recent Symposium is :- No.l?The Hydrophobic Interadion No. 17 in the series, this publication is the result of a symposium on The Hydrophobic Interaction held at the Uiiiversity of Reading in December 1982. Contents: Hydrophobic Interdctionr a llistaric.11 Per spect ivr llydrupliobic Ilydration Geometric Kelaution in Water. Its Role in Precise Vapour-pressure Measureiiients of the SolubilkdtiorI of Benzene by Aqueous Sodiuiii Octylsulphate Solutions Nuclear Magnetic Resonance R e b u t i o n Investigation of Tetrahydrofuran and Methyl Iodide Clathrdtes Infrared and Nuckar Magnetic Kcwnance Studies Pertaining to the (age Model t o r Solutions oS Acetone in Water Irothernial Transport Properties in Solutions o f S y mmet r ica I Tet ra-alk y hmnioniuiii Bromides Thermodynamics of Cavity I'oriiiaiion in Water.A Molecular Dynamics Study Molecular Librations and Solvent Oricnt- ational Correlations in Hydrophobic Phenomena Monte Carlo Computer Siniulation Study of the Hydrophobic Effect. Potential ot Mean Force for ECfir)gaq at 25 and SOv C Hydroplicibic Moments and Protein Structure Application 01' the Kirkwood-Buff Theory to the t'roblcin 01 Hydrophobic Interactions Ihentangleinent of Ilydrophubic and IFlcctrosi~tic Contributions t o the I.ilni Pressures O i Ionic Surfactants llydrophobir. Intcracliuns in Dilute Su lut io ns u t 1'0 1 y (vin y I a Ico lio I) ('onioriii;tiionaI and 1:unc.i ional I'ropertics of tiaeiiwglobin in Water+Alcohol Mixtures. Dependence o f Bull. Electrostatic and tlydrupliohic I n t c r x t i o n s upon ptl and KCI concentrations Softcover 24Opp 0 85186 668 9 Price f36.50 ($70.00) Rest of the World f38.50 RSC Members f 23.75 ORDERING RSC Members should send their orders to: The Royal Society of Chemistry. The Membership Officer. 30 Russell Square, Non-RSC Members The Royal Society of Chemistry, Distribution Centre, Blackhorse Road, L London WC1 B 5DT. Letchworth, Herts SO6 IHN, England. Faradaj Symposia of the Chemical Society hGi 17 I hc HI drophohr' Inrcrm rron 1 9 X ? (viii)

ISSN:0300-9599    

DOI:10.1039/F198480FX037

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

AUTHOR INDEX xxxv Sabbatini, L., 1029 Sacco, A., 2669 Sanders, J. V., 571 Sangster, D. F., 291 1 Sarka, K., 521 Sasahira, A., 473 Sasse, W. H. F., 571 Satchell, P. W., 2395 Sato, K., 841 Sato, Y., 341 Savino, V., 759 Sayers, C. M., 1217 Schiller, R. L., 1257 Schmidt, J., 1 Schmidt, P. P., 2017 Schneider, H., 3275, 3285 Schulz, R. A., 489, 1323 Scott, J. M. W., 739, 1651, 2287, Scott, S. K., 3409 Segall, R. L., 2609 Sehested, K., 2929, 2969 Seidl, V., 1367 Sem, P., 297 Serratosa, J. M., 2225 Seyama, H., 237 Seyedmonir, S. R., 87, 2269 Shanahan, M. E. R., 37 Sheppard, A., 2999 Sherwood, P. M. A., 135, 2099, Shindo, Y., 879, 2199 Shiotani, H., 2145 Shizuka, H., 383, 341 Siekhaus, W. J., 61 Sircar, S., 1101, 2489 Smart, R. St C., 2957, 2609 Smith, I. M., 3021 Smith, R., 3233 Snow, R.L., 3463 Solar, S., 2929 Solar, W., 2929 Solymosi, F., 1841 Soma, M., 237 Soupart, J-B., 3209 Sourisseau, C., 3257 Spink, J. A., 3469 Spoto, G., 1875, 1891 Spotswood, T. M., 3147 Staricco, E. H., 2631 Stassinopoulou, K., 3095 Stedman, D. H., 285 Stout, D. R., 3481 Strohbusch, F., 1757 Strumolo, D., 1479 Struve, P., 813, 2167 Styring, M. G., 3051 Subramanian, R., 2405 2881, 3359 2549, 2867 Sundar, H. G. K., 3491 Sutcliffe, L. H., 669, 3021 Sutton, H. C., 2301 Sutton, L. E., 635 Suzuki, H., 803 Suzuki, T., 1925, 3157 Symons, M. C. R., 423, 1005, Szamosi, J., 1645 Szczepaniak, W., 2935 Takagi, Y., 1925 Takahashi, K., 803 Takahashi, N., 629 Takanaka, J., 941 Takao, S., 993 Takasaki, S., 803 Takegami, H., 1221 Tam, S-C., 2255 Tamamushi, R., 2751 Tamaru, K., 29, 1567, 1595 Tamilarasan, R., 2405 Tanabe, S., 803 Tanaka, K., 2563,2981 Tanaka, T., 119 Taniewska-Osinska, S., 1409 Tascon, J.M. D., 1089 Teo, H. H., 981, 1787 Tetenyi, P., 3037 Thomas, J. K., 1163 Thompson, L., 1673 Thomson, M., 1867 Thomson, S. J., 1689 Tiddy, G. J. T., 789, 3339 Tittarelli, P., 2209 Tominaga, T., 941 Tomkinson, J., 225 Tonelli, C., 1605 Toprakcioglu, C., 13,413 Tran, T., 1867 Trasatti, S., 913 Tripathi, A. D., 1517 Tronc, E., 2619 Troncoso, G., 2127 Truscott, T. G., 2293 Tsurusaki, T., 879 Tuck, J. J., 309 Turner, P. S., 2609 Tusk, M., 1757 Tvarbikova, Z., 2639 Tyrrell, H. J. V., 1279 Ueki, Y.. 341 Ueno, A., 803 Unno, H., 1059 Valencia, E., 2127 van de Ven, T. G. M., 2677 van Ommen, J. G., 2479 van Truong, N., 3275, 3285 Vargas, I., 1947 2767, 2803, 21 1, 1999 Vedrine, J.C., 1017 Veith, J., 2313 Velasco, J. R., 3429 Vesala, A., 2439 Vickerman, J. C., 1903 Vincent, B., 2599 Vinek, H., 1239 Vink, H., 507, 1297 Waghorne, W. E., 1267 Wagley, D. P., 47 Walker, R. W., 435, 3187, 3195, Wallington, T. J., 2737 Wang, G-W., 1039 Watkins, P. E., 2323 Watkiss, P. J., 1279 Watt, R. A. C., 489 Webb, G., 1689 Webster, B. C., 255, 267 Weiner, E. R., 1491 Wells, C. F., 2155. 2445 Wells, J. D., 1233 Whang, B. C. Y., 291 1 Whittle, E., 2323 Wichterlova, B., 2639 Wiesner, S., 3021 Wilhelmy, D. M., 563 Williams E. H., 3147 Williams, P. A., 403 Williams, R. J. P., 2255 Wokaun, A., 1305 Wolff, T., 2969 Wood, S. W., 3419 Woolf, L. A., 549, 1287 Wright, C. J., 1217 Wu, D. C., 1795 Wiirflinger, A., 3221 Wyn-Jones, E., 1915 Yamabe, M., 1059 Yamamoto, S., 941 Yamashita, H., 1435 Yamauchi, H., 2033 Yamazaki, A., 3245 Yariv, S., 1705 Yasumori, I., 841 Yeates, S.G., 1787 Yide, X., 969, 3103 Ylikoski, J., 2439 Yokokawa, T., 473 Yoneda, N., 879 Yonezawa, T., 1435 Yoshida, S., 119, 1435 Zambonin, P. G., 1029 Zanderighi, L., 1605 Zecchina. A., 2209, 2723, 1875, Zipelli, C., 1777 Zundel, G., 553 348 1, 2827 1891AUTHOR INDEX xxxv Sabbatini, L., 1029 Sacco, A., 2669 Sanders, J. V., 571 Sangster, D. F., 291 1 Sarka, K., 521 Sasahira, A., 473 Sasse, W. H. F., 571 Satchell, P. W., 2395 Sato, K., 841 Sato, Y., 341 Savino, V., 759 Sayers, C. M., 1217 Schiller, R. L., 1257 Schmidt, J., 1 Schmidt, P. P., 2017 Schneider, H., 3275, 3285 Schulz, R. A., 489, 1323 Scott, J. M. W., 739, 1651, 2287, Scott, S.K., 3409 Segall, R. L., 2609 Sehested, K., 2929, 2969 Seidl, V., 1367 Sem, P., 297 Serratosa, J. M., 2225 Seyama, H., 237 Seyedmonir, S. R., 87, 2269 Shanahan, M. E. R., 37 Sheppard, A., 2999 Sherwood, P. M. A., 135, 2099, Shindo, Y., 879, 2199 Shiotani, H., 2145 Shizuka, H., 383, 341 Siekhaus, W. J., 61 Sircar, S., 1101, 2489 Smart, R. St C., 2957, 2609 Smith, I. M., 3021 Smith, R., 3233 Snow, R. L., 3463 Solar, S., 2929 Solar, W., 2929 Solymosi, F., 1841 Soma, M., 237 Soupart, J-B., 3209 Sourisseau, C., 3257 Spink, J. A., 3469 Spoto, G., 1875, 1891 Spotswood, T. M., 3147 Staricco, E. H., 2631 Stassinopoulou, K., 3095 Stedman, D. H., 285 Stout, D. R., 3481 Strohbusch, F., 1757 Strumolo, D., 1479 Struve, P., 813, 2167 Styring, M. G., 3051 Subramanian, R., 2405 2881, 3359 2549, 2867 Sundar, H.G. K., 3491 Sutcliffe, L. H., 669, 3021 Sutton, H. C., 2301 Sutton, L. E., 635 Suzuki, H., 803 Suzuki, T., 1925, 3157 Symons, M. C. R., 423, 1005, Szamosi, J., 1645 Szczepaniak, W., 2935 Takagi, Y., 1925 Takahashi, K., 803 Takahashi, N., 629 Takanaka, J., 941 Takao, S., 993 Takasaki, S., 803 Takegami, H., 1221 Tam, S-C., 2255 Tamamushi, R., 2751 Tamaru, K., 29, 1567, 1595 Tamilarasan, R., 2405 Tanabe, S., 803 Tanaka, K., 2563,2981 Tanaka, T., 119 Taniewska-Osinska, S., 1409 Tascon, J. M. D., 1089 Teo, H. H., 981, 1787 Tetenyi, P., 3037 Thomas, J. K., 1163 Thompson, L., 1673 Thomson, M., 1867 Thomson, S. J., 1689 Tiddy, G. J. T., 789, 3339 Tittarelli, P., 2209 Tominaga, T., 941 Tomkinson, J., 225 Tonelli, C., 1605 Toprakcioglu, C., 13,413 Tran, T., 1867 Trasatti, S., 913 Tripathi, A.D., 1517 Tronc, E., 2619 Troncoso, G., 2127 Truscott, T. G., 2293 Tsurusaki, T., 879 Tuck, J. J., 309 Turner, P. S., 2609 Tusk, M., 1757 Tvarbikova, Z., 2639 Tyrrell, H. J. V., 1279 Ueki, Y.. 341 Ueno, A., 803 Unno, H., 1059 Valencia, E., 2127 van de Ven, T. G. M., 2677 van Ommen, J. G., 2479 van Truong, N., 3275, 3285 Vargas, I., 1947 2767, 2803, 21 1, 1999 Vedrine, J. C., 1017 Veith, J., 2313 Velasco, J. R., 3429 Vesala, A., 2439 Vickerman, J. C., 1903 Vincent, B., 2599 Vinek, H., 1239 Vink, H., 507, 1297 Waghorne, W. E., 1267 Wagley, D. P., 47 Walker, R. W., 435, 3187, 3195, Wallington, T. J., 2737 Wang, G-W., 1039 Watkins, P. E., 2323 Watkiss, P. J., 1279 Watt, R. A. C., 489 Webb, G., 1689 Webster, B. C., 255, 267 Weiner, E. R., 1491 Wells, C. F., 2155. 2445 Wells, J. D., 1233 Whang, B. C. Y., 291 1 Whittle, E., 2323 Wichterlova, B., 2639 Wiesner, S., 3021 Wilhelmy, D. M., 563 Williams E. H., 3147 Williams, P. A., 403 Williams, R. J. P., 2255 Wokaun, A., 1305 Wolff, T., 2969 Wood, S. W., 3419 Woolf, L. A., 549, 1287 Wright, C. J., 1217 Wu, D. C., 1795 Wiirflinger, A., 3221 Wyn-Jones, E., 1915 Yamabe, M., 1059 Yamamoto, S., 941 Yamashita, H., 1435 Yamauchi, H., 2033 Yamazaki, A., 3245 Yariv, S., 1705 Yasumori, I., 841 Yeates, S. G., 1787 Yide, X., 969, 3103 Ylikoski, J., 2439 Yokokawa, T., 473 Yoneda, N., 879 Yonezawa, T., 1435 Yoshida, S., 119, 1435 Zambonin, P. G., 1029 Zanderighi, L., 1605 Zecchina. A., 2209, 2723, 1875, Zipelli, C., 1777 Zundel, G., 553 348 1, 2827 1891

ISSN:0300-9599    

DOI:10.1039/F198480BX039

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY FARADAY TRANSACTIONS, PARTS I AND 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 11 (Chemical Physics). Theoretical chemistry, especially valence and quantum theory, statistical mechanics, intermolecular forces, relaxation phenomena, spectroscopic studies (including i.r., e.s.r., n.m.r., and kinetic spec- troscopy, etc.) leading to assignments of quantum states, and fundamental theory. Studies of impurities in solid systems. Perkin Transactions I (Organic Chemistry). All aspects of synthetic and natural product organic, organometallic and bio-organic chemistry, including aliphatic, alicyclic, and aromatic systems (carbocyclic and heterocyclic). Perkin Transactions II (Physical Organic Chemistry). Kinetic and mechanistic studies of organic, organometallic and bio-organic reactions.The description and application of physicochemical, spectroscopic, and theoretical procedures to organic chemistry, including 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 It 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. (i)NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet ‘Quantities, Units, and Symbols’ (1975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W 1 V OBN).These recommendations are applied by The Royal Society of Chemistry in all its publications. Their basis is the ‘ Systeme International d’Unites’ (SI). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A , B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn).Nomenclature of Znorganic 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.FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY Marlow Medal and Prize Applications are invited for the award of the Marlow Medal for 1985 and prize of f 100. The award will be open to any member of the Faraday Division of the Royal Society of Chemistry who, by the age of 32, had made in the judgement of the Council of the Faraday Division, the most meritorious contribution to physical chemistry or chemical physics. The award will be made on the basis of publications (not necessarily in the Transactions) on any subject normally published in J. Chem. SOC., Faraday Transactions I and 11, that carry a date of receipt for publication not later than the candidate’s 32nd birthday. Candidates should be members and under 34 on 1 st January 1985, the closing date for applications, which may be made either by the candidate himself or on his behalf by another member of the Society.Copies of the rules of the award and application forms may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN (ii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 19 University of Cambridge, 1-3 April 1985 The object of the meeting will be to discuss all aspects of the developing subject of polymeric liquid crystals. The hope is to bring together scientists from the fields of conventional polymer science and monomeric liquid crystals who are active in this field. The discussion is aimed at understanding the following facets: (a) The chemical characteristics that give rise to polymer liquid crystalline behaviour. (b) The nature of the high local anisotropy of these systems and their structural organisation at the molecular, micron and macroscopic levels.' (c) The physical properties and their industrial exploitation, with particular reference to the 1 influence of external force fields such as flow, electric and magnetic fields. 1 (d) The inter-relations of polymer liquid crystals with small-molecule mesophases, conventional flexible polymers and biopolymers which exhibit liquid-crystalline behaviour. The conference will include a Poster Session for which contributions are invited. Abstracts (300 words) should be sent as soon as possible (and in any event no later than 31 January 1985) to the Chairman of the Organising Committee: Professor B.R. Jennings, Efectro-Optics Group, Department of Physics, Brunel University, Uxbridge UB8 3PH The preliminary programme may be obtained from : Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN Molecular Electronic Structure Calculations-Methods and Applications University of Cambridge, 12-1 3 December 1984 Molecular electronic structure calculations have now developed into a powerful predictive tool and are necessary in several different fields to aid the understanding and interpretation of experimental observations. The meeting will review the current state of this rapidly developing discipline and will bring together experts on some of the most advanced methods and their applications. The meeting will provide an opportunity for discussion and comparison of the various techniques currently in use.I t will therefore not only be a valuable forum for discussion among research workers in the field, but should also show the non-specialist what theoretical calculations can be expected to achieve now and in the near future. The final programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry. Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRP GENERAL DISCUSSION NO. 79 (in conjunction with the Polymer Physics Group) Polymer Liquid Crystals~ THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY 1 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 preliminary programme 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 Phase Transitions in Adsorbed Layers University of Oxford, 17-18 December 1985 Organising Committee : Professor J. S. Rowlinson (Chairman) Dr E. Dickinson Dr R. Evans Mrs Y. A. Fish Dr N. Parsonage Dr D. A. Young The aim of the meeting is to discuss phase transitions at 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. Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 12 October 1984 to: Professor J. S. Rowlinson, Physical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 302. Full papers for publication in the symposium volume will be required by August 1985.~ THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY 1 GENERAL DISCUSSION NO. 81 Lipid Vesicles and Membranes ~ Loughborough University of Technology, 15-17 April 1986 ~ Organising Committee: Professor D. A. Haydon (Chairman) Professor D. Chapman Mrs Y.A. Fish ~ Dr M. J. Jaycock Dr I. G. Lyle Professor R. H. OttewiII 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, mechanical properties, encapsulation and interaction forces between bilayers leading to fusion but excluding preparation and characterisation methodology. Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 1 May 1985 to: 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.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY 1984 BOURKE LECTURES by Professor V. Ponec State University of Leiden, The Netherlands Monday 22 October 1984 6.00 pm Wednesday University College, Dublin 24 October 1984 4.15 pm reactions on alloys Thursday University of Bath 25 October 1984 4.15 pm oxygen-containing molecules Friday Queen Mary College, London 26 October 1984 2.0G-5.30 pm heterogeneous catalysis (half-day Symposium) Teesside Surface Science Club, Norton Hall, Stockton-on-Tees Catalysis of CO hydrogenation, and the synthesis of oxygen -containi ng molecules Ensemble size and ligand effects in the catalysis of hydrocarbon Catalysis of CO hydrogenation, and the synthesis of Particle size effects, promotion and metal-support interaction in Admission to the Lectures is free and non-members will be welcome.Further information from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBNFARADAY DIVISION INFORMAL AND GROUP MEETINGS Polymer Physics Group Fundamental Aspects of Polymer Flammability To be held in London on 29 October 1984 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1X 8QX Polymer Physics Group with the Institute of Marine Engineers Polymers in a Marine Environment To be held at the Institute of Marine Engineers, London on 31 October-2 November 1984 Further information from Dr G. J. Lake, MRPRA, Brickendonbury, Hertford SG13 8ML ________ ~~ High Resolution Spectroscopy Group CARS, Diode Laser and Microwave Spectroscopy To be held at the University of Reading on 16-1 8 December 1984 Further information from Dr J.M. Hollas, Department of Chemistry, University of Reading, Whiteknights, Reading RG6 2AD Colloid and Interface Science Group with the Colloid and Surface Chemistry Group of the SCI Colloidal Aspects of Cohesive Sediments To be held at the SCI Headquarters, London on 18 December 1984 Further information from Dr J. W. Goodwin, School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS Neutron Scattering Group Neutrons in Magnetism To be held at the University of Southampton on 20 December 1984 Further information from Dr R. 6. Rainford, Department of Chemistry, University of Southampton, Southampton SO9 5NH Polymer Physics Group New Year Soiree To be held in Cambridge on 2 January 1985 Further information from Dr A.Windle, Department of Metallurgy and Materials Science, Pembroke Street, Cambridge CB2 3QZ ~~ Division Annual Congress: Solid State Chemistry To be held at the University of St Andrews on 25-28 March 1985 Further information from Professor P. A. H. Wyatt, Department of Chemistry, University of St Andrews, The Purdie Building, St Andrews KY16 9ST ~~ Polymer Physics Group 6th Churchill Conference To be held at Churchill College, Cambridge on 1-4 April 1985 Further information from Professor I. M. Ward, Department of Physics, University of Leeds, Leeds LS2 9JT Statistical Mechanics and Thermodynamics Group Liquids-Dynamic and Static Properties To be held at the University of Bristol on 1 &11 April 1985 Further information from Dr E.Dickinson, Procter Department of Food Science, University of Leeds, Leeds LS2 9JT Neutron Scattering Group Small-angle Neutron Scattering from Organised Systems To be held at Imperial College, London on 17-18 April 1985 Further information from Dr R. W. Richards, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1 XLIndustrial Physical Chemistry Group with the Food Chemistry Group Water Activity: A Credible Measure of Technological Performance and Physiological Via bi I i ty 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 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 ~ ~~ 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 Dr J. G. Booth, Department of Chemistry, University of Salford, Salford M5 4WT ~~~~ ~ 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 30TH INTERNATIONAL CONGRESS OF PURE AND APPLIED CHEMISTRY Advances in Physical and Theoretical Chemistry Manchester, S13 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 of the dynamical motion of molecules which are undergoing chemical reaction or energy exchange.Micellar Systems The symposium will discuss various aspects of micellization, including size and shape factors, micellization in biological systems, chemical reactions in micellar systems, micelle structure and solubilization. Emphasis will also be given to modern techniques of examining micellar systems, including small-angle neutron scattering, neutron spin echo, photocorrelation spectroscopy, 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 second circular, giving details of all the symposia of the Congress and listing invited speakers may be obtained from: Dr J. F. Gibson, 30th IUPAC Congress, Royal Society of Chemistry, Burlington House, London W1 V OBN (vii)Michel Boudart and G. Djega- Mariadassou Kinetics of Heterogeneous Catalytic A critical account of the principles of the kinetics of heterogeneous catalytic reactions in the light of recent developments in surface Reactions science and catalysis science. The book will be of great value both to teachers and researchers interested in the synthesis and in the physicochemical study of the catalyst .T-- - - - *r-----, = 2t3 I 1 I I * , I I 1 a I I ’ a’ 4-- and to engineers who wish to understand and improve the rate equations they use in research, design, or plant operation. Physical Chemistry: Science and Engineering Edited by John M. Prausnitz and Leo Brewer P $13.50 (U.S.). C $45.50 (U.S.) ------ Order from your local bookseller or from U n i v e r s i t y Press 15A Epsom Road. Guildford. Scirrey GU1 3JT (viii)

ISSN:0300-9599    

DOI:10.1039/F198480FP077

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1984,80, 2609-2617 Surface Area and the Mechanism of Hydroxylation of Ionic Oxide Surfaces BY COLIN F. JONES, ROBYN A. REEVE, RUPERT RIGG, ROBERT L. SEGALL, ROGER ST. C. SMART* AND PETER S. TURNER School of Science, Griffith University, Nathan, Queensland 41 1 1, Australia Received 19th April, 1983 The nearly perfect {loo} surfaces of MgO smoke cubes formed in air do not show significant v(0H) absorption in infrared spectra from thin (10 mg cm-2), coherent films exposed to H 2 0 vapour for several hours. It is shown that perfect, five-fold-coordinated sites are not protonated and that the proportion of protonated, low-coordination (i.e. less than five-fold) sites is < 5%. These results are in accord with theoretical predictions for H, adsorption.In contrast, v(0H) [and v(CO,)] absorptions are observed in identical preparations subjected to prior abrasion. The increase in protonated sites is more than ten-fold. Electron microscopy shows that only minor initial alteration of surface structure at edges and corners is caused by abrasion but a major increase in the rate of surface roughening is observed when new, nucleating sites appear and develop in regions across the surface. This process results in a major, time-dependent increase in hydroxylation but it is substantially complete before infrared spectra can normally be obtained. B.E.T. studies, using N, adsorption, do not measure the change in surface area produced by t h s surface roughening. Multilayer water adsorption followed by desorption more than doubles the B.E.T.surface area because of the formation of platelets decomposed from the Mg(OH), brucite surface layer. The particle-size distribution is altered and the B.E.T. method correctly measures this change. The influence of surface structure, through defect (e.g. edge, corner or vacancy) sites, on the reactions that take place at the surface of ionic oxides is well established.l-ll For the adsorption of simple molecules, both experimental and theoretical evidence has illustrated the importance of low-coordination (i.e. less than five-fold) sites. For instance, Boudart et a1.I have shown that the chemisorption (and D, exchange) of H, by MgO depends on the presence of an Mg2+ ion vacancy at a cube corner leaving a micro { 11 l } face consisting of three 0- ions in a triangular array (i.e.a Vl centre). Kunz and Guse,2 using a self-consistent field unrestricted Hartree-Fock approximation, confirmed that the local modification of electronic structure at this Vl centre leads to adsorption of three H atoms. In contrast, a defect-free MgO {OOl) surface is unable to dissociate H, molecule^.^ Extension of the calculations by Mackrodt and coworker^^-^ to consider other defect sites revealed that binding of both CO and H atoms to all low-coordination sites is much higher than to five-fold-coordinated (001 ) surface sites. A recent review by Bermudez' summarises much of the theoretical and experimental evidence for these centres. We are interested in the mechanism of hydroxylation of ionic oxides through the adsorption of H 2 0 molecules from vapour or solution.The dramatic effects of reaction with H,O on surface structure and the consequences of these surface rearrangements for the initial dissolution kinetics in dilute acid solution have been Water, being a triatomic molecule, is a more difficult proposition in computer modelling of adsorption processes and site preferences, and has not yet been 26092610 HYDROXYLATION OF IONIC OXIDE SURFACES studied in this way. Several experimental approaches have, however, been important in relating H,O reactivity to low-coordination sites. For MgO, it is possible to carry out a comparison of material from two preparation methods which result in substantially different proportions of low-coordination ions. MgO smoke, formed by burning Mg in air, produces small (r.m.s.edge length 80 nm), dislocation-free cubes with nearly perfect (001 ] faces.* Thermal decomposition of magnesium hydroxide or carbonate gives high specific surface area crystallites in plate-like pseudo-structures of the starting materials and with highly defective surfaces which contain a number of high-index planes. Collucia et al. lo have compared photoluminescence (and reflectance) spectra of these different surfaces with the different morphology studied by electron microscopy to show that the optical spectra are associated with three- and four-fold coordinated sites in the surface of the oxide. The number of three-fold coordinated sites on MgO smoke is considerably increased by exposure to water vapour as a result of erosion of the edges and corners of the cubes.Further work by Collucia et all1 has shown that this etching of smoke cubes and increase of reactive sites of low coordination does not produce any parallel increase in B.E.T. surface area. This last result has important consequences for any statement of dissolution rate per unit surface area when comparing different surfaces (i.e. with different defect concentrations) resulting from different preparation procedures. In this paper we describe results from infrared, electron-microscope and B.E.T. studies that extend our understanding of the process of hydroxylation and provide a basis for an explanation of several of these effects. We are able, first, to conclude that perfect five-fold-coordinated sites in (100) surfaces do not appear to form OH groups after exposure to H,O vapour whereas defect (i.e.low-coordination) sites do protonate, a result which is the same as that predicted for H, ad~orption.~ In contrast abrasion results in a major increase in v(0H) [and v(C03)] i.r. absorption. Electron microscopy reveals that only a minor alteration of surface structure is caused initially at edges and corners of the cubes by abrasion. The increase in i.r. absorption appears to be out of proportion to (i.e. much greater than) the concentration of new low-coordination sites. However, with time electron microscopy shows that this is caused by an increase in the rate of hydroxylation in new, nucleating regions de- veloping across the surface after this treatment, resulting from surface-structural alteration roughening the surface at the unit-cell level.The B.E.T. surface-area technique does not measure these detailed changes in surface structure. Only when an excess of water vapour, resulting in multilayer formation, is desorbed at high temperature is the B.E.T. surface area doubled because of the formation of platelets decomposed from the Mg(OH), surface brucite formation. The B.E.T. technique in this instance correctly measures the change in size distribution as new surface area is created. EXPERIMENTAL OXIDE PREPARATION MgO smoke was prepared by burning Mg ribbon (B.D.H. laboratory grade reagent) in air. The oxide was variously collected on clean, dry KBr discs, glass plates or e.m. grids held 0.1 m above the ignition. The smoke forms a very coherent, continuous, evenly distributed film on these surfaces, which consist of the nearly perfect cubes studied previously8 by transmission and scanning electron microscopy.INFRARED STUDIES For infrared studies, the sample was transferred, either deposited on KBr discs or after scraping from the glass plate and pressing between two KBr discs, to a Specac variable- temperature vacuum cell. A second method of preparation involved slurrying unmodified orc. F. JONES et al. 261 1 pretreated MgO smoke with carefully dried propan-2-01, followed by drying at 80 "C onto AgCl discs, before transfer to the infrared cell. [There was no evidence of v(CH) absorption in any spectra after this treatment.] The second method allowed control of the mass per unit area in the beam.Considerable care was taken to ensure that all oxide films were continuous and evenly distributed. The infrared cell could be evacuated under conventional vacuum conditions to Pa and pure H,O vapour admitted to the cell. Infrared spectra were recorded on a Perkin-Elmer 62 1 spectrometer using reference-beam attenuation to compensate for the low transmission of the samples (usually < 7% at 2000 cm-l). Adequate spectrometer response generally gave a resolution of ca. 9 cm-l at 4000 cm-l with slow scan rate. Experiments using MgO smoke prepared under glove-box conditions will also be described. This system was flushed with a N,+O, mixture (20 min) and sealed with P,O, as a dessicant for 12 h. The H,O content in the box was estimated to be < 100 v.p.m., but some H,O vapour was clearly present since the formation of the cubic particle shape is dependent on its presence.The MgO samples, prepared by resistive heating to ignition in the box, were pressed between KBr discs and mounted in a vacuum cell similar to that of Cant and Little1, fitted with a windlass system for removing the sample from the i.r. beam, before removal from the box. ELECTRON-MICROSCOPE STUDIES Specimens for electron microscopy were obtained by dispersing the powder ultrasonically in acetone or petroleum ether and holding clean gold grids in the spray above the dispersion. Grids were inserted into the microscope vacuum as quickly as possible (usually < 1 min) and kept in the vacuum until all micrographs had been recorded. Samples prepared in the glove box were deposited directly on to e.m.grids, kept under vacuum and treated identically to the samples for i.r. examination before transfer to the microscope. A JEM-100C instrument was used operating at 100 kV; micrographs were recorded at magnifications of lo5 or higher, with slight underfocus to reveal surface details by phase contrast. SURFACE-AREA STUDIES B.E.T. surface areas were determined using the Perkin-Elmer model 212D sorptometer with N, adsorption (a = 0.162 nm2) at 77 K and have errors of < f 10%. The values quoted here for the same conditions are similar to those of Coluccia et al." determined with a Sartorious microbalance 4102 using N, adsorption at 77 K. RESULTS The fresh, unattacked MgO smoke particles have been carefully characterised. They are nearly perfect cubes with some small cubic projections a few unit cells in dimensions on the (100) faces.899v13 The degree of roughening from these few projections depends on the ambient water vapour pressure during ignition but they are generally ca.5 nm apart. A few crystals show anomalous forms, such as plates or twins,14 the concentrations again being dependent on water vapour pressure. Dislocations occur in < 1 in lo6 particles. The particle-size distrib~tionl~ is extremely asymmetric with most particles having edge lengths in the range 45-85 nm but ca. 1 in 1 O4 having edge lengths > 1.5 pm. INFRARED STUDIES When MgO is deposited directly onto a KBr plate as an even, continuous film to 10 mg cm-2 and transferred into a vacuum cell in < 10 min, we find no measurable absorptions in the infrared spectra between 4000 and 750 cm-l.v(0H) and v(C0,) only become measurable after the sample has been standing in air for periods > 12 h. If the thickness of the deposited film is increased until total absorption from MgO vibrations is found below 800 cm-l (i.e. ca. 50 mg cm-2), very weak absorptions (i.e. absorbance increase above background of c 0.05) are seen at 3500 and 1 600- 1 400 cm-l.2612 HYDROXYLATION OF IONIC OXIDE SURFACES 4 000 3500 31 1 I I I I \ 5 1600 1L 00 1000 6 wavenum ber/cm-' Fig. 1. Infrared spectrum of fresh MgO collected on a glass plate scraped onto a KBr plate, with a second KBr plate used to spread the MgO (10 mg cmP2) evenly by gentle rotation and the two plates clamped together. Discontinuities in the spectra are caused by changes in reference- beam attenuation.4 000 3500 3000 1600 1400 1200 1000 Fig. 2. Ratio-recorded infrared spectra of (i) sample beam containing lightly abraded (mortar and pestle) MgO smoke solvent-dried, as described in the text, onto a KBr plate and (ii) reference beam containing the same thickness of fresh, unabraded MgO smoke solven t-dried on to a KBr plate. wave number /cm 0 In contrast, when MgO smoke is deposited on to a glass plate, scraped off and then dispersed into an even, continuous film of 10 mg cmP2 between two KBr windows, a spectrum like that in fig. 1 is found. Two broad v(0H) absorptions16--'o are evident near 3650 and 3500 cm-' with broad v(C0,) absorptions16-20 at 1650,1520,1380,1250, 1000 and 850 cm-l.J .Chem. SOC., Faraday Trans, 1, Vol. 80, part 10 Plate 1 Plate 1. Phase-contrast electron micrograpns or ivigu srnoKe C U D ~ S LaKen ai uiiiereni Limes after lightly grinding in petroleum ether. (a) Initially, within minutes of preparation, all regions of the specimen examined show smooth (100) faces with a few broken cube corners (arrowed). (b) After 10 min in the microscope vacuum (< lop5 Pa H,O) all cubes exhibit the onset of surface roughening; this is most readily visible in phase contrast on steeply inclined faces (as at arrows). (c) After a further 15 min all cubes show a high degree of roughening. C. F. JONES et al. (Facing p . 26 12)J. Chem. Soc., Furaduy Trans. 1, Vol. 80, purt 10 Plate 2 Plate 2. MgO cubes exposed to H,O vapour for 20 h at 24 "C and outgassed at 1200 K.The original cubes (marked A) ae substantially eroded. The eroded material forms sheets of hydroxide which are reduced to finely divided MgO (arrowed) upon heating. The dark regions such as the one marked B arise from strong Bragg diffraction.c . F. JONES et al. 2613 The only processes likely to have caused this change were the scraping and pressing operations involving particle contact. To test this possibility, fresh smoke was very lightly ground in a mortar and pestle and examined by both i.r. and e.m. techniques immediately after drying off the alcohol. The effect of this treatment on surface hydroxylation can be seen in the infrared spectrum of fig. 2, where the lightly ground smoke (sample beam) is run against the unmodified smoke (reference beam) using the same number ofmg cm-, in each beam.The samples were dried down from propan-2-01 slurries as described in the Experimental section. There has been a substantial change in the background below 1400 cm-l giving rise to the steep slope evident in the spectrum but, in addition, clear differences in absorption are seen at 350&3400, 1600 and 550 cm-l with a shoulder at 1000 cm-l. If the smoke surfaces were prepared in the glove box and disordered by scraping and pressing between KBr plates, weak v(0H) absorption was still found in the i.r. spectra [although no v(CO,) was detected] despite the low H,O vapour content (< 100 v.p.m.) and short time of exposure to the glove-box atmosphere (i.e.< 5 min). This evidence slfggests that there has been a major increase in the concentration of sites at which protonation and CO, adsorption can take place as a result of the abrasion process. ELECTRON-MICROSCOPE STUDIES It is tempting to assume from these results that the cubes have been damaged by abrasion so as to expose extensive areas of low-coordination sites compared to the virgin smoke. The electron microscope reveals that the effects of abrasion are more complex. Although there is some damage to cube corners and edges, initially the cube surfaces appear almost unaffected [plate 1 (a)]. After relatively short times ( i e . < 10 min) in the microscope vacuum (< Pa H,O vapour), the cube surfaces begin to show changes in surface structure or roughening effects [plate l(6) and (c)].In this experiment it was important to examine different areas of the specimen at successive times and to ensure that these areas had not been exposed to the electron beam while the roughening process proceeded. This is because the electron beam can itself induce surface damage on water-exposed smoke.21 With this precaution, however, we consistently observed new regions of surface alteration developing across the cube surfaces with time. This structural alteration introduces a high concentration of small projections and intrusions, still based on {loo} facetting, of a few unit cells in any dimension. The result of this process is a substantial increase in the number of surface sites which can be hydroxylated (or adsorb CO,). In view of this evidence, it is not surprising that the increase in v(0H) and v(CO,) is much greater than that expected from the damage inflicted by the abrasion process alone.This initial damage appears to be catalytic in inducing much more rapid surface alteration. This alteration is qualitatively similar to that found in our work immediately after immersion in water and before any dissolution occurs, i.e. ' castellation '.8 SURFACE-AREA STUDIES A surface area of 10.5f 1 m2 g-l is consistently found for the virgin smoke. A reproducible reduction of 15 % (i.e. to 9.0 1 m2 g-l) is found for all samples of lightly ground smoke. There has been major alteration in surface structure but, of course, the atomic-scale rearrangements described in plate l(b) and (c) do not alter the particle-size distribution in any significant way.We believe that this result illustrates the inability of the B.E.T. adsorption process to detect the atomic detail of the disordering, but its ability to detect the comparatively unimportant change in the particle-size distribution caused by some pressure-induced wringing together of particle surfaces.2614 HYDROXYLATION OF IONIC OXIDE SURFACES 1 I 00 3500 3 000 25 11800 1600 1400 1200 1000 wavenumber/cm -' Fig, 3. Infrared spectrum of MgO smoke exposed to 3 dm3 of H,O vapour at 2 kPa for 25 h, heated to 900 "C for 4 h and solvent-dried on to a KBr plate. Further evidence for this contention is provided by two more experiments. We have repeated the experiment of Collucia et al." They exposed smoke to a fixed quantity of H,O vapour for 20 h at 300 K and outgassed at 1200 K for 1 h.This has been shown to produce substantial surface disordering.1° We confirm that this treatment does not change the B.E.T. surface area despite the considerable change in surface structure. This is true also for the smoke after exposure to a limited quantity of H,O and evacuation at 300 K but before 1200 K outgassing; again the B.E.T. surface area is unchanged. The i.r. spectrum shown in fig. 3 confirms the increase in adsorption sites for H,O and CO,, leading to the conclusion that low-coordination sites are involved. In our experiments we estimated that the total quantity of H,O vapour available to the surface to produce this change corresponds to less than two monolayers. The second experiment of interest involved exposure of the smoke particles to a continuous supply of H,O vapour from bulk liquid in a closed dessicator over 20 h at 24 "C.The particles were then outgassed at 1200 K. The new surface area was found to be 23 f 1 m2 g-l. It is interesting that the sample after this exposure to H 2 0 vapour followed by evacuation at 24 "C, i.e. before 1200 K outgassing, gave the same surface area of 23 1 m2 g-l. Electron microscopy after 1200 K outgassing reveals (plate 2) the presence of extensive sheet-like particles amongst the heavily eroded MgO particles. In this experiment the MgO platelets have formed from decomposition of Mg(OH), in the layered structure of the brucite lattice. It appears that extensive hydroxide layers have been formed on the surfaces as multilayers of water are adsorbed.The particle-size distribution is significantly shifted towards smaller particles with the reduction in size of the original cubes and the addition of the small, very high surface area platelets. [As expected, the infrared spectrum shows very intense v(0H) absorption on exposure of the specimens to air.] The increase in measured area is associated with the shift in the particle-size distribution towards smaller particles. In all of these experiments the evidence is that the changes in B.E.T. surface area reflect only the changing particle-size distribution rather than the detailed atomic-scale disordering.c . F. JONES et al. 2615 DISCUSSION DEFECT CONCENTRATIONS ON MgO SURFACES The lack of v(0H) absorption on unattacked smoke surfaces and the very slow rate of hydroxylation in air can be explained in terms of the concentration of low- coordination sites on these surfaces.An estimate of the concentration of three- and four-coordinated sites on virgin MgO smoke surfaces can be made with a few simplifying assumptions. If we assume a 65 nm edge-length cube, with cubic projections of two-unit-cell dimensions spaced 5 nm apart across the (100) faces, this, including edge and corner sites on these projections plus those on the cubes themselves, gives ca. 5% of surface sites with less-than-perfect coordination. If we use (@, which has a value of 120 nm from the size distribution, this estimate reduces to 4% of all surface sites. If these are the sites that become protonated, as suggested by the theoretical StudieF and the work of Collucia et al.,lo7 l1 then we can also estimate the likely v(0H) infrared absorption from these sites.Assuming a relatively large extinction coefficient for the hydrogen-bonded v(0H) absorption near 3400 cm-l of ca. 13 mol-1 m2 and an absorbance increase (above background) of ca. 0.01 as a measurable limit in a conventional infrared spectrometer, then we need ca. 8 x lo-* mol of OH groups rn-, of infrared beam. For MgO smoke with a lattice parameter 4.20 A and a surface area of 10.5 m2 g-l dispersed as 10 mg ern-,, if we assume that 5% of all surface sites are OH groups, then we arrive at an estimate of 9.5 x a01 m-, for the actual MgO films. From these estimates it is clear that we are very close to the limit of detectability of infrared absorption from OH groups associated with low-coordination sites but, more importantly, it is obvious that if all surface sites were protonated we would detect the infrared absorption at 10 mg cm-, dispersion without difficulty.The estimates are also consistent with the observation of weak absorption when the dispersion thickness is increased to ca. 50 mg crn-,. We have now seen that major surface alteration is catalysed by the abrasion process although this process produces relatively little initial damage. It is not clear from our results whether the surface roughening occurs first (followed by protonation of the new defect sites produced) or whether protonation itself is enhanced by the process resulting in surface reordering to accommodate the changes in site energies.The i.r. results appear to indicate that the number of hydroxyl groups on the surfaces has increased at least ten-fold because of the surface disordering since we observe an absorbance increase > 0.1 in v(0H) absorption. This increase is easily achieved by the creation of pits and protrusions of one- and two-unit-cell dimensions on the sur- face since each edge or corner oxygen site in such defects becomes a centre for ready prot~nation.~, We may speculate that the mechanism for production of surface sites may involve hydroxylation of a five-fold site by pressure. It is certain that physically adsorbed H,O molecules will be present over most surfaces of the smoke cubes shortly after production of the 2o We surmise that pressure-induced electrostatic forces on an H,O molecule are sufficient to hydroxylate a surface oxygen ion.Then even light grinding is sufficient to produce many active sites across a smooth surface, which in turn acts to nucleate the hydroxylation, and roughening, of the surface in times very short compared with the time it takes undamaged smoke exposed to H,O vapour to be totally roughened. and to explain their e.s.r. results of the number of 0; ions observed. For 170 nm perfect cubes, the size used in their estimates, 4.5 x 1014 g-' three-coordinated sites are expected from corner sites.ll This figure is at least one order of magnitude less than It is now possible to consider some estimates in the work of Coluccia et2616 HYDROXYLATION OF IONIC OXIDE SURFACES the amount of 0; actually found on unmodified smoke, although these ions are believed to form on only a fraction of the three-coordinated sites actually present.For a particle-size distribution such that (P): = 120 nm,ll the number of three-coordinated sites from the cube corners is only ca. 2 x 1 OI4. Now, ifwe include the same two-unit-cell projections spaced ca. 5 nm apart on these surfaces as discussed above, this estimate increases to > lo1’ g-l. This increase can explain the 0; concentration observed.6 Attack by water vapour is found in the work of Coluccia et al. to increase the population of these sites by at least a factor of ten, which is in accord with our infrared results. SURFACE STRUCTURE AND SURFACE AREA The results from these studies strongly suggest that conventional B.E.T. surface-area measurements cannot detect, the atomic-level rearrangements involved in the surface roughening associated with hydroxylation and abrasion. There are two possible reasons for this insensitivity.The first is that the N, molecule with an area of 16 A2 is sufficiently large that steps of one- and two-unit-cell dimensions make little difference to the total count of adsorbed N, molecules. It is conceivable that the physically adsorbed N, molecules, which will have some significant lateral quadrupolar inter- actions, are acting like a coherent skin over the surface rather than as site-specific adsorbates. The second interesting possibility arises from the recent work by Colbourn and Ma~krodt,~ where it has been shown that ions at the corners and edges of small cubes or projections relax inwards whilst ions at the base of steps and ledges relax outwards (by 0.1-0.4 lattice parameter).This relaxation has the effect of partly ‘smoothing’ the surface and may contribute to the insensitivity of the B.E.T. measurements to these changes. CONCLUSIONS The nearly perfect surfaces of MgO smoke cubes formed in air containing a very small concentration of water vapour do not show significant v(0H) absorption in i.r. spectra (at 10 mg ern-,) because the proportion of surface oxide ions that are pro- tonated (i.e. less than five-fold-coordinate sites) is < 5 % . The five-fold-coordinate sites on perfect (100) surfaces are not protonated, and thus there are too few OH groups in the infrared beam.In contrast, v(0H) [and v(CO,)] are observed on specimens identical to those above apart from prior abrasion through pressure or light grinding. A more than ten-fold increase in active, low-coordination sites is observed. Electron microscopy shows that this difference results from a major change in the rate of hydroxylation and extensive surface roughening because of the presence of new, pressure-induced active sites and it is not caused by any difference in surface structure arising directly from abrasion. B.E.T. studies, using conventional N, adsorption, do not measure the change in surface area produced by this major alteration in surface structure . An excess of water vapour, resulting in multilayer formation, followed by desorption gives more than double the surface area, because of the formation of platelets decomposed from the Mg(OH), brucite structure.The size distribution is altered and new surface area created. The B.E.T. method correctly measures this change in particle- size distribution. Support from the Australian Research Grants Scheme is gratefully acknowledged.c. F. JONES et al. 261 7 I M. Boudart, A. Delbouille, E. G. Derouane, V. Indovina and A. B. Walters, J. Am. Chem. SOC., 1972, 94, 6622. A. B. Kunz and M. P. Guse, Chem. Phys. Lett., 1977, 45, 18. E. A. Colbourn and W. C. Mackrodt, Surf. Sci., 1982, 117, 571. E. A. Colbourn and W. C. Mackrodt, Solid State Ionics, 1983, 8, 221. E. A. Colbourn, J. Kendrick and W. C. Mackrodt, Surf. Sci., 1983, 126, 550. J. Kendrick, E. A. Colbourn and W. C. Mackrodt, Radiat. Eff., in press. R. L. Segall, R. St. C. Smart and P. S. Turner, J. Chem. SOC., Faraday Trans. I , 1977, 73, C. F. Jones, R. L. Segall, R. St. C. Smart and P. S. Turner, Proc. R. SOC. London, Ser. A, 141. lo S. Coluccia, A. J. Tench and R. L. Segall, J. Chem. SOC., Faraday Trans. I , 1979, 75, 176' l 1 S. Coluccia, A. Barton and A. J. Tench, J. Chem. SOC., Faraday Trans. I , 1981,77, 2203. l 2 N. W. Cant and L. H. Little, Can. J. Chem., 1968, 46, 1373. l3 A. F. Moodie and C. E. Warble, J. Cryst. Growth, 1971, 10, 26. ' V. Bermudez, Prog. Surf. Sci., 1981, 11, 1. 1710. 98 1, 374, l4 R. R. Cowley, R. L. Segall, R. St. C. Smart and P. S. Turner, Philos. Mag., Ser. A, 1979, 39, 163. l5 C. F. Jones, R. L. Segall, R. St. C. Smart and P. S. Turner, Philos. Mag., Ser. A , 1980, 42, 267. lCi L. H. Little, Infrared Spectra of Adsorbed Species (Academic Press, London, 1966). l7 M. L. Hair, Infrared Spectroscopy in Surface Chemistry (Edward Arnold, London, 1967). l9 J. V. Evans and T. L. Whateley, Trans. Faraday SOC., 1967, 63, 2769. 2o S. J. Gregg and J. D. Ramsay, J. Chem. SOC., 1970, 2784. 21 C. F. Jones, R. L. Segall, R. St. C. Smart and P. S. Turner, Radiat. Eff., 1982, 60, 167. R. St. C. Smart, T. H. Slager, L. H. Little and R. G. Greenler, J . Phys. Chem., 1973, 77, 1019. (PAPER 3/627)

ISSN:0300-9599    

DOI:10.1039/F19848002609

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1984, 80, 2619-2629 Ion Adsorption and Electron Transfer in Spinel-like Iron Oxide Colloids BY ELISABETH TRONC* Laboratoire de Chimie de la Matiere Condensee (LA 302), Ecole Nationale SupCrieure de Chimie de Paris, 11 rue Pierre et Marie Curie, 75231 Paris, France AND JEAN-PIERRE JOLIVET AND JEAN LEFEBVRE Laboratoire de Chimie des Polymeres Inorganiques, Universite Pierre et Marie Curie, 4 Place Jussieu, 75230 Paris, France AND RENE MASSART Laboratoire de Physico-Chimie Inorganique (ERA 608), Universite Pierre et Marie Curie, 4 Place Jussieu, 75230 Paris, France Received 25th July, 1983 The behaviour of aqueous spinel-like iron oxide colloids in the presence of metal ions, and especially Fe2+, has been studied protometrically and structurally. Fe2+ adsorption is shown to proceed reversibly with the release of two protons per adsorbed ion; subsequent implication of an Fe,O, outer shell is demonstrated. All the experimental features support electron transfer involving an octahedral site sub-lattice of the whole colloid.Phenomena occurring at the interface between metal oxides and aqueous electrolyte solutions are of interest in many areas such as corrosion and catalysis. Adsorption is generally treated using a surface complexation m ~ d e l l - ~ where protons or hydrolysed metal ions coordinate with surface oxygen species. In some cases, however, various processes may occur in the solid and the phenomena cannot be restricted to the interface. This is illustrated by the adsorption of protons or iron ions on iron oxides.Thus, in a-Fe203 adsorbed protons diffuse through the surface layer, suggesting a hydrated goethite-like interpha~e.~ Magnetite in a weakly acidic medium adsorbs protons but releases Fe2+ ions.6 Fe2+ adsorption on y-FeOOH may transform it to Fe304, probably via the formation of a surface intermediate.’ Paralleling the chemical insertion of lithium in Fe,04,8 diffusion of protons and Fe2+ ions through the spinel framework could also occur. Such phenomena are enhanced as the surface area of the solid increases, and in the colloid size range some remarkable features may result. We report here a study on the interactions in solution between spinel iron oxide colloids and metal ions, especially Fe2+, from both chemical and structural viewpoints.These colloidsg~ lo are the main constituents of aqueous magnetic sols and peptization of the solid results from the occurrence of surface electrical charges. Sol formation and charge evolutionll are understood classically in terms of acid-base reactions at amphoteric surface hydroxy group^.^^-^^ From their Fe2+ content, these materials stand between Fe,O, and y-Fe,O,. Magnetite has an inverse spinel structure and y-Fe20, differs from this in that it contains Fe3+ ions only and thus vacancies in the 26192620 ION ADSORPTION AND ELECTRON TRANSFER IN IRON OXIDE COLLOIDS octahedral metal sub-lattice. Stabilization of y-Fe,O, has been understoodl5-l7 in terms of protons occupying some or all of the vacancies up to the limiting composition HFe,O,; it has also been suggested that hydroxy groups take part in this stabilization.18 The structure of the (probably hydrogenated) colloids investigated, can be describedlO by an average pseudo-spinel unit cell with metal sub-lattices notably defective.EXPERIMENTAL TECHNIQUES AND ANALYTICAL METHODS The total Fe content of the sols was determined by atomic absorption spectrophotometry (Perkin-Elmer type 373) and the Fez+ content by potentiometric titration with K,Cr,O, after dissolution of the sample in concentrated HCI. Ultrafiltration of the sols was performed by using Millipore Hi-flux apparatus equipped with semipermeable membranes PSED with a retention limit of ca. 104-105 molecular weight. Protometric titrations and kinetic experiments were performed as follows : 20 cm3 of distilled water, 0.5 cm3 of the sol with an iron concentration of ca.1 rnol dmP3 and variable amounts (0.5-3 cm3) of FeC1, (0.1 mol dm-,) were introduced into a 50 cm3 cell. One glass electrode was connected to a potentiograph Metrohm E436 (forward titration) and another one to an automatic titrimeter Metrohm Combi-titreur 3D (back titration). The reference was a calomel electrode with a N(CH,),NC!, bridge. N, was continuously bubbled through the electrolyte solution to prevent oxidation. Forward titrations were performed by slowly adding N(CH,),OH (2 cm3 in ca. 15 min) and titration curves were recorded by the potentiograph; HCIO, was used for the back titrations. For the kinetic studies acid addition was adjusted automatically so as to keep the pH constant at 3.1.X-ray diffraction patterns were obtained in the transmission mode to check qualitatively the similarity between isolated colloids and those in the sol. Structural analysis was undertaken by powder X-ray diffraction methods as previously reported.1° Samples corresponded to colloids which were either ultrafiltered (sample H) or centrifuged (sample E), and kept under nitrogen in a dessicator at room temperature until the diffraction data were recorded. Integrated intensities were collected in air using a Philips PW 1050 diffractometer equipped with a graphite monochromator, using Co Ku radiation up to 26' = 120" at a 20 scanning rate of 0.25" min-l. The stability of the system was controlled by re-measuring the intensity of the two first lines at the end of each run.Data were corrected for background, Lorentz and polarization effects calculated at peak maximum positions. Scattering curves for neutral atomic species and the real component of the anomalous dispersion corrections were taken from ref. (19). No weighting coefficients were used and structural refinements were performed through a normal least-squares procedure. PREPARATION OF THE SOLS Tetramethylammonium hydroxide N(CH,),OH ( 10 % aqueous solution) was from Merck. All other reagents were RP Normapur Prolabo chemicals. The preparation of the sols was carried out by the procedure already d e s ~ r i b e d . ~ . ~ ~ An aqueous mixture of FeCI, (40 cm3, 1 rnol dm-,) and FeC1, (10 cm3, 2 mol dm-3, HC1 2 rnol dm-,) was added with vigorous stirring to a NH, solution (500 cm3, 0.7 mol drn-,) at room temperature. The black precipitate formed was separated from the supernatant and treated in one of two ways: (1) with concentrated N(CH,),OH ( 1 rnol dm-,), which led to peptization and anionic sol formation, excess hydroxide being removed by ultrafiltration and the sol being transferred to distilled water (pH ll), or (2) with concentrated HCIO, (3 mol dmW3), which gave a suspension, the flocculate being separated, resuspended in acid and centrifuged, after which it was easily peptized in distilled water, giving a cationic sol of The sols were made up of roughly spherical colloids of ca.100 A size. For cationic colloids the counterions were ClO,. The colloid average charge, defined as the molar ratio of protons titratable by a base to total iron, was ca.0.02-0.05. The overall composition of the sols corresponded to an Fe3+:Fe ratio of ca. 0.80-0.90. pH 2-3.E. TRONC, J-P. JOLIVET, J. LEFEBVRE AND R. MASSART 262 1 RESULTS PROTOMETRIC STUDY Unlike the anionic sol, which was stable in an air-free alkaline medium, the cationic sol underwent notable changes. As soon as the acidic flocculate was transferred to distilled water, it released Fe2+ ions. Similar behaviour has already been reported for magnetite in a weakly acidic medium.6 Fez+ migration into solution can be seen (fig. 1) by comparison of the N(CH,),OH titration curves of the sol before ultra- 10 PH .8 6 4 2 1 2 3 N( CH3)4 OH/cm3 Fig. 1. Protometric titration by N(CH,),OH (0.11 mol dm-3) of: (a) non-ultrafiltered cationic sol (20 cm3, [Fe] = 0.1 mol dm-3); (b) ultrafiltered cationic sol (20 cm3, [Fe] = 0.1 mol dmP3); (c) ultrafiltrate (20 cm3, [Fe] = 4.52 x lop3 mol dm-3).filtration [curve (a)], after ultrafiltration and transfer to water [curve (b)] and of the filtrate [curve (c)]. The latter is identical to the titration curve of ‘free’ Fe2+ ions (FeCl, solution) and yields an iron concentration in perfect agreement with the atomic absorption analysis of the filtrate. Cationic sol evolution resulted in the release of nearly all the Fez+ ions from the colloids. After ageing for a week at room temperature, the relative amount of Fe2+ in the colloids was lowered typically to ca. 0.02-0.05. All further experiments were performed on sols which were cleared of free Fez+ ions. Comparison of curves (a) and (c) of fig.1 shows that colloids alter the Fez+ neutralization process, preventing ferrous hydroxide precipitation. This is further illustrated in fig. 2, which shows titration curves for different Fe2+ concentrations. The2622 ION ADSORPTION AND ELECTRON TRANSFER IN IRON OXIDE COLLOIDS 1c e PH 6 4 2 Fig. 2. Protometric titration by N(CH,),OH of cationic colloid in the presence of Fez+ (FeCl,). [Fe2+],,/[Fe]initial = 0 (curve 1 ) ; 0.09(2); 0.19(3); 0.29(4); 0.31(5); 0.35(6); 0.36(7); 0.39(8); 0.48(9). ([Fe2+]/[Fe]),,,,,, = 0.04. Dotted line, titration of Fez+ without colloid. amount of FeC1, solution added to a fixed amount of colloid prior to titration will be called [Fe2+Iad. Two types of curve are apparent, one with a step at ca. pH 7 [type (a)], where flocculation occurs, and the other with a second step at pH 8.5-9 [type (b)].The latter corresponds to excess Fe2+ ions which no longer interact with the colloids and precipitate as Fe(OH),, it occurred only for values of [Fe2+Iad yielding a total concentration ratio [Fe2+] : [Fe] > ca. 0.33 ; i.e. cationic colloids can adsorb Fe2+ species up to a final concentration ratio [Fe2+]:[Fe3+] of ca. 1:2. Remarkably, this should characterize ideal magnetite, Fe,O,. Quantitative conservation of the Fe2+ and Fe3+ species involved was checked by dissolving the materials obtained at the end of the reaction (ca. pH 10) in 12 mol dm-3 HCl and titrating both iron ion species. Moreover, whatever the value of [Fe2+Iad, double this amount of base, [HO-I,,,, was needed up to the zero-charge point (near pH 10).This is shown in fig. 3, where the plot of [HO-I,,, against [Fe2+Iad is a straight line of slope 2. These results show that in the presence of the colloid, Fe2+ ions do not contribute to Fe(OH), precipitation. Though the straight line in fig. 3 does not define the end of iron fixation on the colloid, it indicates the implication of two HO- (or H30+) ions per adsorbed Fe2+ ion. The results can be understood in terms of an exchange reaction between Fe2+ species in the solution and protons in the colloid, or the reverse, particularly for the spontaneous changes of cationic sols in a weakly acidic medium. The expression ‘exchange’ does not necessarily imply that the reaction occurs between protons in the colloid surface layer and free Fe2+ ions.More complex situations, involving the adsorption of hydroxylated metal species or the hydrolysis of adsorbedE. TRONC, J-P. JOLIVET, J. LEFEBVRE AND R. MASSART 2623 Fig. 3. Variation of the amount of base added up to the end of the reaction (point of zero charge in protometric curves) with the amount of Fe2+ initially added. 10 pH 8 0 1 2 3 [HO-1 or [H,O+]/[ Fen] Fig. 4. Cationic colloid titration in the presence of Fez+: (a) forward titration by N(CH,),OH; (b) back titration by HClO, (same reactant addition rate).2624 ION ADSORPTION AND ELECTRON TRANSFER IN IRON OXIDE COLLOIDS 0 -4 m E Q - ; E B 0.35 B --- - + 3 Y 0.3 I I -1-- 5 10 15 tlh Fig. 5. Desorption kinetics of Fe2+ at pH 3.1 (HClO, addition) after sol neutralization in the presence of FeCl, ([Fe2+],,/[FeIinitial = 0.32).should also be considered. The pH associated with iron uptake covers the range over which Fe2+ irons are in solution; nevertheless, adsorption may proceed from species hydrolysed at the interface.20 Our results do not enable us to decide between such models. For the sake of simplicity, the cationic colloid in its initial form and the colloid brought to the alkaline medium in the presence of Fe2+ ions will be called pro tonated and exchanged, respectively . Back titrations confirmed the exclusive release of ferrous ions and proved the reversibility of the adsorption. Forward and back titration curves obtained under similar conditions (especially the rate of addition of titrant), are given in fig. 4. Similar features are observed in spite of a noticeable shift.This hysteresis effect is related to an important variation in the desorption kinetics and concerns ca. 30% of the ad- sorbed Fe2+ ions. The kinetics were further studied by the automatic measurement of acid consumption as a function of time at pH 3.1. Two distinct stages are seen (fig. 5) corresponding to high and low rates for the consumption of acid. The first step, up to 0.36 [H,O+] added, includes the titration of excess base (introduced during the forward titration), the positive charge on the colloid under the experimental conditions (evaluated by blank experiment on similarly treated colloid, but without additional Fe2+ ions) and partial re-exchange (ca. 70%) of Fe2+ ions. The second stage corresponds to desorption of the remaining Fe2+ ions which was very slow.After 17 h at pH 3.1, ca. lO-15% of the Fe2+ ions had still to be desorbed. Complete consistency between iron desorption and proton consumption, in the ratio I : 2, was also checked by colloid ultrafiltration and forward titration of the Fe2+ in the filtrate. The ability to adsorb other ions was also investigated. Addition of Co2+ ions prior to titration led to identical features (fig. 6). Hydroxide precipitation, however, occurred more rapidly with a Co2+ uptake which was ca. 3 times less than that of Fe2+.E. TRONC, J-P. JOLIVET, J. LEFEBVRE AND R. MASSART 2625 [ HO-I / [ Fe Iinitial Fig. 6. Protometric titration by N(CH,),OH of cationic colloid in the presence of Co2+ (CoC1,). ([Fe2+]/[Fe])initial = 0.04; [Co2+Iad/[Felinitial = 0 (curve 1); 0.04(2); 0.06(3); 0.09(4); 0.13(5); 0.20(6) ; 0.26(7).Dotted line, titration of Co2+ without colloid. The plot of [HO-I,,, against [Co2+Iad is also a straight line of slope 2; hence the occurrence of 2H+/Co2+ exchange is similarly deduced. Only one step with a high proton consumption rate could be detected in the desorption kinetics. This proves that all the Co2+ ions desorb readily. With Ni2+ any exchange reaction was hardly detectable; hydroxide precipitation occurred from an [Ni2+Iad: [Fe] ratio of 1 or 2: 100. Fe3+ and A13+ ions had different behaviour, Fe(OH), and Al(OH), precipitation was observed in acidic medium followed, as the pH increased, by neutralization of the colloid. The adsorption capacity of magnetite for metal ions is known to be high20-22 but it cannot compare with the Fe2+ uptake involved here.The ratio [Fe2+]:[Fe3+] in exchanged colloids and the drastic decrease in the affinity in the order Fe2+ > Co2+ > Ni2+ > Fe3+ emphasize the specificity of Fe2+ adsorption. Because of the magnitude of the phenomenon, structural information was thought to be within the scope of an X-ray diffraction investigation. STRUCTURAL STUDY X-ray powder diffraction data were obtained from protonated and exchanged colloids. Protonated colloids (H) correspond to a concentration ratio [Fe2+],: [Fe], = 0.07: 1 and exchanged colloids (E) to an uptake [Fe2+Iad : [Fe], = 0.35: 1, giving [Fe2+] : [Fe3+] ratios equal to 0.075 : 1 and 0.45 : 1, respectively.2626 ION ADSORPTION AND ELECTRON TRANSFER IN IRON OXIDE COLLOIDS Table 1.Observed and calculated diffraction line intensities for protonated (H) and exchanged (E) samples sample H sample E hkl IUbS Icalc! Iohs Icalc 111 220 222 400 33 1 422 333 53 1 620 622 444 71 1 642 73 1 800 R 90 1278 6721 2 525 0 1070 4 709 10858 545 1997 70 1 0 647 3 159 1743 - 114 1195 6 741 2475 68 1139 4685 10865 616 1996 820 91 720 3217 1749 0.02 1 119 1588 7 594 2 847 0 1445 5517 11 474 796 2 474 827 0 929 3 857 1966 - 119 1500 7 632 2 780 59 1471 5483 11 484 817 2457 986 87 99 1 3 794 1960 0.018 Table 2. Structural parametersa sample H sample E site 32e x x x x 0.2523(7) 0.2530(8) 16d 1/2 1/2 1/2 ab 12.84(35) 13.69(39) 8a 1/8 1/8 1/8 6.65(10) 7.22(12) 48s x 1/8 1/8 (X = 3/8) 1.66(15) 0.88(16) thermal factor/A2 1.71(7) 1.45(7) distance/A 0-Fe( 1 6 4 0-Fe(8a) 0-Fe(48f) Fe(48f)-Fe( 166) Fe(48f)-Fe( 8a) 2.07(1) 2.06( 1) 1.84( 1) 1.85( 1) 1.82( 1) 1.82( 1) 1.81 1.81 2.09 2.09 a Standard deviations are given in brackets.a refers to site occupancy.E. TRONC, J-P. JOLIVET, J. LEFEBVRE AND R. MASSART 2627 On1 spinel-type lines were detected and no variation in the unit-cell parameter observed broadening was dominated by the size effect. Colloid mean sizes were evaluated from the full-width at half-maximum, assuming Gaussian profiles for size and instrumental broadenings, and were found to be 104 A (H) and 116 A (E). Recalling our previous study,1° defective spinel structure models (space group Fd3m), based on a perfect oxygen atom framework (equipoint position 32e), were used and refinements focused on the statistical population of the various metal sub-lattices.First, unconstrained 16d octahedral and 8a tetrahedral site occupancies (five par- ameters: scale factor, overall isotropic thermal factor, oxygen coordinate x and two site occupancies) led to values of the reliability index, R = XI I, - I, I /E I,, of 0.090 (H) and 0.040 (E). The population of tetrahedral 48fsites (x 1/8 1/8, x = 3/8) by iron atoms significantly improved the fits (6 parameters), lowering R to 0.021 (H) and 0.018 (E). No significant residual electron density could then be detected in either site still available (8b, 16c). Observed and calculated diffraction data are listed in table 1 and structural parameters are given in table 2.Complete consistency with the previous investigationlo is obtained. Because of the order enhancement effect associated with X-ray diffraction it seems reasonable to consider that these data refer only to the crystalline bulk, ignoring eventual out-of-site atoms in the outer shell. The overall average cell contents deduced are Fe21.2(6)032 (H) and Fe21.8(,1032 (E). Such a small variation rules out any homogeneous distribution of the extra ions in the bulk [which would give Fe28.6032 (E)]. Even with a concentration gradient assumption, structural and chemical data cannot agree unless extension of the ordered oxygen array is taken into account. We therefore suggest that iron diffusion into the core is not prominent but that a growth occurs. The most simple description of colloid E is a coherent X-ray scattering two-phase system: a core made up of colloid H surrounded by an ordered outer shell.Mass balance, based on X-ray and chemical data and referred to unit-cell oxygen content, leads to partitioning: (8.35 K ) was observed. The angular dependence of the line-width confirmed that the which assesses the new layer composition at the magnetite one. Following this, site occupancy evolution is attributed to the outer layer, the values deduced from table 2 [8a+2.1(1), 16d+4.0(5), 48f-0.4(2)] show that iron species selectively locate in 8a tetrahedral and 16d octahedral sites in the ratio 1 : 2. This supports the above assumption and suggests that the outer shell has both the Fe and 0 concentration of Fe30, and its structure. As before,lO interpretation of 48f site occupancy, interstitials in the bulk or a surface effect, remains open.Neglecting any composition dependence of the unit-cell dimensions, a volume ratio, outer layer to core, of 0.31 : 1 is obtained. For a core diameter of 104 A (H) this yields a 5 A thick outer shell. This compares well with the size evolution deduced from the line-widths (6 A) and indicates that there are ca. 2 or 3 more ordered oxygen layers. X-ray diffraction data do not allow any differentiation between Fe3+ and Fe2+ species, but in conjunction with chemical data their respective amounts per unit cell may be evaluated. Corresponding Fe2+ contents are given in table 3. Comparison with values in table 2 shows that in cell E, overall 16d octahedral site population is twice the Fe2+ content.This suggests that, as in the inverse spinel structure, all Fe2+ ions locate in 16d sites with an equal number of Fe3+ ions in the outer shell and in the core.2628 ION ADSORPTION AND ELECTRON TRANSFER IN IRON OXIDE COLLOIDS Table 3. Iron content analysis from (1) chemical data (2) structural data and (3) both sample H sample E unit-cell H unit-cell E (2) Fe21.2032 Fe21.8032 Feg (3) Feii (7-4) 5.6 6.8 Fe",' - DISCUSSION The above structural model is schematic. Besides distribution in colloid mor- phology and surface irregularities, induced ordering of iron atoms in the hydroxylated interphase is ignored and the Fe,O, shell is assumed to be built up from adsorbed species only. Uncertainty remains as to the origin of the oxygen supply, which is from interphase ordering or from the solution or both.However, the perfect consistency obtained above and the complete coherence in the interpretation of the protometric features are highly corroborative and enable us to propose the following process. Adsorption of hydrolysed Fe2+ species by the colloid causes its growth by the development of a magnetite outer shell. Electrons produced by partial oxidation of adsorbed Fe2+ ions are pumped into the colloid core, causing the reduction of Fe3+ ions in 16d sites, up to the final Fe2+-Fe3+ equipopulation of these sites. This equilibrium state, given the initial amount of Fe2+ ions presumably in 16d sites, determines the magnitude of the electron transfer and thereby the Fe2+ uptake capacity. These itinerant 3delectrons may show a fast hopping, related to the pairwise electron hopping which exists in magnetite, thus stabilizing Fe2+-Fe3+ pairs in the defective material (interstitial atoms at 48f sites acting as impurities might help this electron transfer) unless they progressively fill the conduction band of octahedron chains and become delocalized.Information about this electron transfer may be provided by a Mossbauer study, even though a quantitative interpretation will be complicated by the superparamagnetic behaviour of the Within an ionic description, charge imbalance in the bulk could be compensated via hydrogen-species migration at the unit-cell level, or by inwards diffusion of surface protons. The small uptake capacity for Co2+ ions and Ni2+ ions corroborates that Fe2+-Fe3+ electron transfer is the driving force.Both Co2+ and Ni2+ ions have a marked tendency to occupy octahedral sites and form inverse-spinel ferrites. The slight variation in ionic radii from Fe2+ to Ni2+ 24 could not account, by itself, via a diffusion process, for the fundamentally different behaviour observed. Their oxidizability, on the other hand, parallels their decrease in uptake behaviour. Partial oxidation of Co2+ ions is possible under our experimental conditions and we suggest that the observed phenomena are governed by a redox reaction of the Co2+/Fe3+ couple. The exclusive location of cobalt at octahedral sites and the limited electron supply are probably responsible for the lower uptake capacity. Further information is obviously needed but note that such charge transfer is implicated in slightly non-stoichiometric cobalt f e r r i t e ~ .~ ~ Back exchange features may be understood by reversing the process. Iron desorbsE. TRONC, J-P. JOLIVET, J. LEFEBVRE AND R. MASSART 2629 exclusively in the ferrous state, in agreement with other observations.26* 27 We think this may be due to a charge effect, weakening Coulomb interactions and thus facilitating approach of the proton. This may even be reinforced by a distance effect since preferential occupation of octahedral sites leads to longer metal-oxygen bonds (table 2). Thus it is logically deduced that the fast desorption stage, concerning approximately two-thirds of the iron uptake, corresponds to leaching of Fe2+ ions from the Fe,O, layer, preferentially from octahedral sites, reduction to the ferrous state being assured by core-to-surface electron back transfer at the rate of one electron per two iron ions.The second stage implies slower processes. It may be connected, judging by the relative magnitudes, with iron in tetrahedral sites stabilized by its less easy reduction. Deeper octahedra may also be involved, hence implicating sluggish diffusion through an iron-deficient interphase constituted mainly of tetrahedrally coordinated ferric ions (possibly at 48fpositions). Whatever the process, the remaining mobile electrons are pumped back. Preferential exchange from octahedral sites is also corroborated by the fast desorption of all Co2+ ions. CONCLUSIONS The protometric results reported here demonstrate a reversible Fe2+ adsorption reaction at the colloid/solution interface in aqueous magnetic cationic sols.Structural investigations indicate that the uptake of ferrous ions proceeds with an extension of the spinel framework and formation of a Fe,O, outer shell. A consistent interpretation of all the experimental features leads us to conclude that it is governed by electron transfer between Fe2+ and Fe3+ ions involving the whole colloid, which behaves as an electron reservoir. The findings emphasize the specific properties of iron oxide colloids with a defective spinel structure when compared either with bulk materials or with Fe,O, colloids and may shed light on interesting catalytic properties. R. 0. James, P. J. Stiglich and T. W. Healy, Faraday Discuss.Chem. Soc., 1975, 59, 142. J. A. Davis, R. 0. James and J. 0. Leckie, J. Colloid Interface Sci., 1978, 63, 480. J. A. Davis and J. 0. Leckie, J. Colloid Interface Sci., 1978, 67, 90. M. M. Benjamin and J. 0. Leckie, J. Colloid Interface Sci., 1981, 79, 209. G. Y. Onoda and P. L. de Bruyn, Surf. Sci., 1966, 4, 48. Y. Tamaura, K. Ito and T. Katsura, J. Chem. SOC., Dalton Trans., 1983, 189. M . M. Thackeray, W. I. F. David and J. B. Goodenough, Mater. Res. Bull., 1982, 17, 785. R. Massart, C . R. Acad. Sci., Ser. C, 1980, 291, 1. ti D. P. Benton and G. A. Horsfall, J. Chem. SOC., 1962, 3899. lo E. Tronc, J. P. Jolivet and R. Massart, Mater. Res. Bull., 1982, 17, 1365. I 1 J. P. Jolivet, R. Massart and J. M. Fruchart, Nouv. J. Chim., 1983, 7, 325. l 2 G. A. Parks and P. L. de Bruyn, J. Phys. Chem., 1962, 66, 967. l 3 G. A. Parks, Chem. Rev., 1965, 65, 177. l4 R. T. Atkinson, A. M. Poster and J. P. Quirk, J. Phys. Chem., 1967, 71, 550. l 5 P. B. Braun, Nature (London), 1952, 170, 1123. l 7 T. W. Swaddle and P. Oltmann, Can. J. Chem., 1980,58, 1763. A. Aharoni, E. M. Frei and M. Schieber, J. Phys. Chem. Solids, 1962, 23, 549. K. P. Sinha and A. P. B. Sinha, 2. Anorg. Allg. Chem., 1957, 293, 228. D. T. Cromer and J. J. Waber, International Tables for X-ray Chrystallography (The Kynoch Press, Birmingham, 1974), vol. IV. B. Venkataramani, K. S. Venkateswarlu and J. Shankar, J. Colloid Interface Sci., 1978, 67, 187. 'O P. H. Tewari. A. B. Campbell and W. Lee, Can. J. Chem., 1972,50, 1642. p 2 H. Tamura, L. Meites and E. Matijevic, J. Colloid Interface Sci., 1983, 92, 303. 23 E. Tronc, D. Bonnin, J. P. Jolivet and R. Massart, to be published. 24 R. D. Shannon, Acta Crystallogr., Sect. A , 1976, 32, 751. 25 G. Benedek, F. Garbasse, G. Petrini and G. Parravano, J. Phys. C'hem. Solids. 1978,39, 645. 26 N. Valverde and C. Wagner, Ber. Bunsenges. Phys. Chem., 1976, 80, 330. 2i H. C . Chang and E. Matijevic, Finn. Chem. Lert., 1982, 90. (PAPER 3 / 1273)

ISSN:0300-9599    

DOI:10.1039/F19848002619

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1984,80, 2631-2637 Reaction of i-Heptafluoropropyl Radicals with Cyanogen Chloride BY CECILIA M. DE VOHRINGER AND-EDUARDO H. STARICCO* Instituto de Investigaciones en Fisicoquimica de Cordoba (INFIQC), Departamento de Fisicoquimica, Facultad de Ciencias Quimicas, Universidad Nacional de Cbrdoba, Sucursal 16-C.C. 6 1, 50 16 Cordoba, Argentina Received 12th October, 1983 The gas-phase reaction of i-C,F, radicals with cyanogen chloride has been studied between The main product, 2-~hloroheptafluoropropane, is formed via chlorine-atom abstraction by (6) 70 and 330 OC, using heptafluoro-2-iodopropane as the free-radical source. i-C,F, : for which log,, [(k,/kk)/cm: mol-4 s-k] = 6.27-14.41/2.303 RT was obtained. At lower temperatures the Arrhenius plot shows curvature.This was interpreted as the occurrence of another reaction producing C,F,Cl, which could be i-C,F, + ClCN -+ C,F,Cl+ CN i-C,F, + ClCN s C,F,ClCN (9, -9) (10) C3F,C1CN -+ C,F,Cl + CN. The results are compared with those for the reaction of other perfluoroalkyl radicals with ClCN. It has been shown in previous that when perfluoroalkyl radicals, R,, react with ClCN, the following reactions may occur: R, + ClCN -+ R,Cl + CN R, + ClCN --r R,ClCN (1) (2, -2) (3) RfCICN + R,ClCN + CN kc R, + Rf -P @,I2 where Rf = CF3,C2F, or n-C3F,. Arrhenius plots for kobs = RRrCI/@Rf)2[ClCNl are strongly curved. The Arrhenius parameters for reactions (1) and (2) were obtained using an iterative method. The present paper deals with experiments on photolysis of i-C,F,I in the presence of cyanogen chloride.It was intended to corroborate the reaction pattern previously proposed and to compare Arrhenius parameters of different perfluoroalkyl radicals reacting with cyanogen chloride. The i-C,F, radicals were generated by photolysis of heptafluoro-2-iodopropane. 263 12632 REACTIONS OF 1-HEPTAFLUOROPROPYL RADICALS WITH ClCN EXPERIMENTAL Heptafluoro-2-iodopropane (ICN chemicals) was purified by gas chromatography on a Triacetin column followed by bulb-to-bulb distillation at low temperature. ClCN was prepared and purified as described in ref, (2). A conventional grease-free static vacuum system was used for all the experiments. Mixtures of i-C,F,I and ClCN were photolysed in the range 70-330 "C with ,I > 300 nm. Full details of the apparatus and procedure are given in ref.(2). The analysis was performed on a 3 m copper column packed with silica gel of 60/80 mesh. A good separation of the reacting mixture was achieved by operating the column at 120 "C. 2-Chloroheptafluoropropane was eluted first, followed by 2-trifluoromethyltetrafluoropropanenitrile, and then 2,3-bis(trifluoromethyl)- octafluorobutane. The products were identified by i.r. and mass spectroscopy. RECULTS AND DISCUSSION When illuminating 2-iodoheptafluoropropane in the presence of ClCN the products were 2-chlorohep tafluoropropane, 2,3 - bis( trifluorome thy1)octafluorobu tane, 24 tri- fluorome thy1)-2,3,3,3 te trafluoropropaneni trile, cyanogen iodide and iodine. At any given temperature and for each run was calculated.As shown in table 1, kobs was a true constant at each temperature. Neither four-fold variation in ClCN pressure nor three-fold variation in 1-C,F,I pressure had a significant effect on kobs. At higher temperatures the conversion percentage was also varied and no effect on the rate constant was found. The temperature dependence of the data in table 1 is shown in fig. 1. The following equation was obtained by the least-squares method in the range 160-330 "C: (14130+430) cal 2.303 RT log,, kobs = (6.25 + O . 19) - where the errors shown are two standard deviations. The photolysis of i-C,F,I involves the reactions i-C,F,I + hv + i-C,F, + I kc I-C,F, + I-C,F, + C,F,, I+I+M+I,+M. If ClCN were present, the production of 2-chloroheptafluoropropane could be explained in two ways: i-C,F, + ClCN + C,F,Cl + CN (6) or I-C,F, + ClCN + C,F,CN + C1 (7) followed by C1+ i-C,F, + C,F,Cl.(8 4 Although insufficient data for calculating enthalpy changes are available, reaction (7) is surely less endothermic than reaction (6). Taking D(C,F,-Cl) = 85 kcal mo1-I (as in chlor~trifluoromethane~) and 89 kcal as the lowest limit for D(C,F,-CN) both reactions have almost the same value of AH", but D(C,F,-CN) could be as high as 110 kcal mol-l (depending on the heat of formation of the cyanogen radical), making reaction (7) more favourable. However, it is well known that exothermicity is not the unique driving force which determines the occurrence of a reaction. A significantc . M. DE VOHRINGER AND E. H. STARICCO 2633 Table 1. Photolysis of i-C,F,I with CICNu reactant pressure/Torr photolysis T/"C time/s i-C,F,I ClCN R C ~ F ~ C I R c ~ F ~ ~ ~ F , C N b s b 70 70 80 80 100 120 160 190 240 240 300 300 300 330 79 200 79 200 79 200 79 200 28 800 54 000 14400 7 200 7 200 3 600 600 600 3 000 600 80.0 81.8 80.0 200.0 100.4 99.8 59.4 60.2 60.0 59.5 31.0 30.4 30.0 30.4 20.1 0.0242 79.7 0.0975 81.0 0.1039 81 .O 0.2132 79.5 0.360 40.0 0.280 39.7 0.769 39.8 3.283 20.0 11.789 20.6 15.215 6.7 30.243 40.0 160.93 40.0 151.03 6.5 88.10 0.9403 0.8961 0.6623 3.038 1.317 0.843 1.452 3.292 9.217 12.700 60.775 45.382 42.131 175.14 0.0184 0.075 1 0.085 0.175 0.300 0.278 0.743 3.20 11.85 15.03 30.95 153.03 150.45 89.30 0.0084 0.0087 0.01 10 0.0105 0.0290 0.0591 0.138 0.41 5 I .963 2.096 6.53 6.74 6.57 12.38 Volume of reaction vessel 100cm3.Rate of formation of products, R, in units of loi3 mol cm-, s-l. Units of cm: rno1-i s+. 1.5 1 .o - 1 . 5 -2.0 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2 . 0 3.0 3.2 103 KIT Fig. 1. Arrhenius plot for the reaction of i-C,F, radicals with ClCN. Solid line: least-squares plot obtained from data between 160 and 330 "C [eqn (5)]. 86 FAR 12634 REACTIONS OF I-HEPTAFLUOROPROPYL RADICALS WITH ClCN fraction of 7z electrons formally associated with the triple bond in ClCN is actually in the Cl-C bond.5 The charge distribution is highly concentrated at this end of the molecule so that the attack of an electrophilic perfluoroalkyl radical should be more favourable on the chlorine atom. Furthermore, reaction (7) followed by reaction (8a) would produce more C,F7CN than C,F,Cl, unless all chlorine atoms produced by reaction (7) recombined by reaction (8a), and yet the amounts of C,F,CN and C,F,Cl are equal.A good scavenger for chlorine atoms is ClCN :6 C1+ ClCN -+ products. (8 6) The relative rates of reactions ( 8 a ) and (8b) could be determined by:? Although ksb/ksa could be as low as [ClCN]/[i-C,F,] is certainly at least lo7, and therefore R8b would be 102R,,. All this would reduce C,F7C1 formation in comparison with that of C,F,CN, and RC3F,CN/RC3F7C1 would be higher than unity. From the data in table 1 this ratio is found to be lower rather than higher than unity and decreases with temperature. Therefore reactions (7) and (8) are rejected, and the Arrhenius parameters of eqn ( 5 ) are considered to be associated with genuine chlorine abstraction from cyanogen chloride by i-heptafluoropropyl radicals [reaction (6)].Fig. 1 shows that points at temperatures < 140 "C (lo3 K/T = 2.4) deviate manifestly from the straight line extrapolated from higher temperatures. According to previously published work1-, it was assumed that the enhancement of kobs at lower temperatures arises from the contribution of another reaction leading to C,F7C1 formation, i.e. i-C,F7 + ClCN e C,F,ClCN (9, -9) C,F,ClCN -+ C,F,Cl+ CN (10) Reactions similar to reactions (9) and (10) have been found in other systems such Instead of reaction (lo), the disproportionation reaction as OH + benzene7 and CF,(or C,F,) + benzene.s 1-C,F, + C,F,ClCN + C,F7Cl + C,F7CN (1 1) may be proposed. There are two reasons for disregarding reaction (1 1) : (a) kobs would depend on i-C,F,I pressure at low temperatures (not observed, see table 1) and (b) the ratio RC3F,CN/RC3F,CI would be unity at lower temperatures, when reaction (6) becomes negligible.Using the Arrhenius parameters of eqn ( 5 ) only 20% of kobs at 70°C can be attributed to chlorine abstraction. Hence 8072 of C,F7Cl arises from addition. If the reactions had been (9) and (1 1) instead of (9) and (lo), R C 3 F , C N / R C s F , c l would have been at least 0.80 at 70 "C. In fact, this ratio could have been > 0.80 because C,F,CN could also have been formed by i-C,F, + CN -+ CF,CF(CF,)CN (12) and should increase as the temperature decreases because the addition reaction (9) becomes more important. Because of the slowness of the overall reaction between i-C,F, and ClCN, data below 70 "C could not be obtained.However, for the three systems previously investigated1-, as well as for this one, R R e C N / R R e C I decreases with temperature, which 1- We thank one of the referees for valuable comments on these reactions.c . M. DE VOHRINGER AND E. H. STARICCO 1 . 5 1 I I I I I I I I 263 5 2 lo3 KIT Fig. 2. Arrhenius plot for kobs. Solid line: calculated from eqn (15) by an iterative method. (a) E-,-E,,, = 1 kcal, (b) E-,-E,, = 3 kcal. reinforces the occurrence of reactions (9) and (10) and further indicates the presence of other reactions [besides reaction (1 2)] consumming CN radicals. These reactions may be CN+12 +ICN+I (13) or CN + 1-C,F,I + I-C,F, + ICN (14) but not because no cyanogen was found among the products.Presumably reaction (14) would be responsible for the increase in RC6Fld with temperature (see table 1). If the reactions involved in C,F,Cl formation were (6), (9, - 9) and (lo), then CN + CN + (CN), The iterative method previously used1-, was inadequate to obtain Arrhenius parameters for the rate constants involved in eqn (1 5 ) because the temperature range investigated was more limited. The differences E-2-E3 were very similar for all three reactions (R, + ClCN) already reported., Therefore the curve for i-C,F,I + ClCN was fitted assuming that E-,-E,, = 2.0 kcal and A-,/A,, = 10. This, together with parameters from eqn ( 5 ) for A, and &9 provided estimates of A , and E, which were used in a new iteration. A good fit to the experimental points could be achieved with the following parameters, as shown in fig.2: 10g,,(A6/A$)/cm~ m0l-i S-i = (6.27k0.22) &-& = 14.41 k0.48 kcal m0l-l loglo(A~g/Alo) = (0.99 f 0.18) E-,-El, = 1.79 f 0.40 kcal mol-I 0.67 kcal mol-I. log,,(A,lA~)/cm~ mol-a s-4 = (3.39 f 0.28) E,+, = 8.33 86-22636 REACTIONS OF i-HEPTAFLUOROPROPYL RADICALS WITH ClCN Table 2. Absolute Arrhenius parameters for reactions (1 )-(3)" CF, C,F, n-C,F, 1-C,F, loglo(A1/cm3 mo1-I s-l) E,/kcal mol-1 loglo(kl/~m3 mol-l s-l) 1og10(A-2/A3) E-,-E3 loglo(A2/cm3 mol-1 s-l) E2/kcal mot1 loglo(k2/cm3 mol-l s-l) 13.11 12.99 11.62 12.20 8.68 8.34 0.95 0.98 2.0 2. I 10.43 10.40 6.35 7.27 5.70 4.98 12.95 12.95 12.62 14.41 8.14 7.46 0.91 0.99 1.9 1.8 10.40 10.07 7.68 8.33 4.67 3.86 (extrapolated) a Calculated with k, = 2.3 x lo1, cm3 mol-l s-l, log,,k, at 300 "C, loglok2 at 20 "C.6 .O 5.5 4 .O 3 . 5 dlA Fig. 3. Correlation of the addition constants at 20 "C for CF,, C2F,, CF,CF,CF, and (CF,),CF with diameters of the radicals. Fig. 2 also shows that the method for curve-fitting is highly sensitive to the selection In order to make a comparison with previously published work on the reactions of perfluoroalkyl radicals, it is necessary to assign a value for the self-coordination of 1-C,F, radicals. Although two independent determinations have been publi~hed,~~ lo it was assumed that k, was the same as k , for CF, radicals. For the sake of consistency with previous w ~ r k , l - ~ Asycough's valuell of k, = 2.3 x lo1, cm3 mol-1 s-l was used to obtain the Arrhenius parameters of table 2.If the lower value reported for k , for 1-C,F, recombination [1.2 x loll cm3 mol-l s-l, ref. (S)] were used to obtain absolute rate constants, a of E+,-Elo.c. M. DE VOHRINGER AND E. H. STARICCO 2637 subtraction of 1.15 would be necessary for the correction of log,,A, while the activation energy would be unaltered. However, it is interesting to note that with this correction the difference in reactivity observed will be enhanced. The A factors for the abstraction reactions (1) are the same within experimental error, and the difference in reactivity may be attributed to the difference in activation energy. The increase in E on going from CF, to 1-C,F, could partially be attributed to a reduced electron density on the carbon atom of CF, and hence the reduction in repulsion between CF, and ClCN in comparison with that between (CF,),CF and ClCN.Repulsion between the CF, groups of the heptafluoro-2-propyl radical and the non-bonding p electrons of chlorine may also contribute to the increased activation energy in the i-C,F, + ClCN reaction. Three of the addition reactions have almost identical A factors. The fourth, the addition of I-C,F, to ClCN, is certainly low. This suggests that steric effects are important. If steric hindrance is involved, a correlation between radical size and the rate of addition is expected. A simple correlation has been found between radical diameter', and the logarithm of the addition rate (fig. 3). The only absolute Arrhenius parameters of addition reactions reported are those of CF, with acetylene or eth~1ene.l~ This would be the first time that absolute values have been obtained for the addition reactions of perfluoroalkyl radicals more bulky than CF,. F. Cosa, E. V. Oexler and E. H. Staricco, J. Chem. Soc., Faraday Trans. I , 1981, 77, 253. C. de Vohringer and E. H. Staricco, J. Chem. Soc., Faraday Trans. I , 1982, 78, 3493. C. de Vohringer and E. H. Staricco, Anales Asoc. Quim. Argent., in press. J. W. Coomber and E. Whittle, Trans. Faraday Soc., 1967, 63, 2656. P. Politzer and S. D. Kasten, J. Phys. Chem , lC%, 80, 283. C. de Vohringer, Thesis (Universidad Nacional de Chrdoba, Argentina, 1981). K. Lorenz and R. Zellner, Ber Bunsenges. Phys. Chem., 1983,87, 629. G . A. Chamberlain and E. Whittle, Znt. J . Chem. Kiner., 1972, 4, 79. S. V. Kuznetsova, A. I. Maslov and V. N. Prished'ko, Krufk. Soobshch. Fiz., 1973, 10, 18. lo S. V. Kuznetsova and A. I. Maslov, Kuantovaya Electron. (hfoscow), 1978, 5, 1587. l 1 P. B. Ayscough, J . Chem. Phys., 1956, 24, 1944. l2 J. Tedder, J. Walton and L. Vertommen, J. Chem. Soc., Fnruday Trans. I , 1979, 75, 1040. l 3 A. El Soueni, J. Tedder and J. Walton, J. Chem. Soc., Faraday Trans. 1, 1981, 77, 89. (PAPER 3 / 1804)

ISSN:0300-9599    

DOI:10.1039/F19848002631

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1984, 80, 2639-2645 Chromium Ions in Zeolites Part 4 . X - r a y Photoelectron Spectroscopic Study of Chromium Valence States in the Surface Layers of CrY Zeolites BY BLANKA WICHTERLOVA,* LYDIE KRAJC~KOVA, ZDENKA TVARIXKOVA AND STANISLAV BERAN The J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, 121 38 Prague 2, Czechoslovakia Receiaed 14th October, 1983 X-ray photoelectron spectroscopy has been used to elucidate the state of chromium in the sur- face layers of CrHY and stabilized CrSY zeolites. Values of both the binding energy (EB) and the spin-orbit splitting (AE) of the Cr 2p level for chromium in the zeolite differ from those of bulk chromium oxide compounds or those of chromium supported on silica.The counter-Cr"' ions in the surface layers of Y zeolites are reduced by zeolite heat treatment both in carbon monoxide and in U ~ C U O to Cr", unlike the Crl*I ions in the zeolite bulk, which are not self- reduced. The stable isolated CrV and CrV1 ions present in the oxidized zeolite bulk were not detected by X.P.S. in the surface layers because of their reduction in U ~ C U O and/or by X-ray irradiation in the ESCA system. In previous the occurrence of various valence states of chromium in the zeolite bulk and their relevance to the zeolite's catalytic activity in ethylene polymerization have been presented. No detailed study dealing with the Cr valence states in the zeolite surface layers, which are of great importance in catalytic processes, has been performed.As far as we know, information is available only from the X.P.S. studies of Minachev et aZ.,4-6 who found that the CrrI1 ions in zeolite Y are transformed into either a Cr,O, or a CrO, phase, depending on whether the zeolite heat treatment is carried out under reducing or oxidizing conditions. On the other hand, the surface layers of bulk chromium oxidic compounds as well as chromium supported on silica and alumina have been studied extensively by X.p.s.'-13 Finely dispersed supported Cr ions exhibit higher binding energies (EB) for the Cr 2p levels and a greater ability for the CrVr ions to be photoreduced in comparison with bulk oxidic compounds. This was considered to be a consequence of a strong interaction between the support and dispersed Cr cations.7- The present X.P.S.study deals with the state of Cr ions within the surface layers of CrHY and stabilized CrSY zeolites, identical to those studied previously.'. The Cr ions were introduced into the zeolites by CrIII ion exchange and their valence changes resulting from zeolite treatment in vacuo, in oxygen and in carbon monoxide have been followed and compared with the behaviour of chromium in the zeolite bulk. EXPERIMENTAL The zeolites used were prepared by Cr"' ion exchange with both NH,Y and stabilized SY zeolites. [For details of their preparation and composition see ref. (I).] They are denoted as CrHY-27 and CrHY-55 for Cr contents of 1-60 and 2.75 wt% , respectively; and CrSY-27 and 26392640 CHROMIUM IONS IN ZEOLITES Table 1. X.p.spectra of the Cr 2p level for the chromium compounds studied EB(Cr 2p3/2)a /eV AEIeV ref. CrO, 579.1 9.2 Cr203 576.2 9.8 CrF, 578.1 9.6 Na,CrO, 579.3 9.1 Na,CrO, 576.7 9.6 CrF, 579.4 9.9 Na3Cr0, 577.3 9.3 NaCrO, 576.0 9.9 this study this study 8 8 9 9 9 9 a The binding-energy values are related to the C 1s level at 285.0 eV. CrSY-55 for Cr contents of 1.61 and 3.01 wt%, respectively; the numbers 27 and 55 refer to the approximate extent of ion exchange. The Cr,O, and CrO, used were Merck, pure grade. X.p. spectra were obtained by using an ESCA 3 MK 11-VG spectrometer. The base pressure in both the preparation and analyser chambers was lops Pa. The A1 Ka,, line (1486.6 eV) was used as the excitation source with a power of 200 W. A correction for charging effects upon the photoemission of electrons was referred to the Si 2p level (102.8 eV).The binding energy of the C 1s level was then 285.0 eV and that of the Si 2s level was 154.0 eV. For chromium oxides the binding energies were referred to the contamination C 1s level (285.0 eV). The reproducibility of the binding energy was kO.2 eV and that of the spin-orbit splitting was f O . l eV. The zeolites were deposited on a steel holder in the form of a water suspension and were subject to various treatments in the preparation chamber of the ESCA system. The spectra were recorded on zeolites as received and after their treatment at elevated temperatures in uacuo ( lop6 Pa) as well as in oxygen (4 x 1 O2 Pa) and carbon monoxide (4 x 1 O2 Pa). RESULTS AND DISCUSSION X.P.S. measurements made on pure Cr,O, and CrO, oxides are presented in table 1.Their Cr 2p level values match satisfactorily other published data for these oxides [cf. ref. (8), (10) and (1 l)] and therefore verify our measuring procedure. X.P.S. data for other chromium compounds which will be used in further discussion are also included in table 1 . The X.P.S. data given in the literature for CrI" in zeolite Y4 and CrV1, CrV, CrIrl and CrI1 ions supported on silica and aluminas* l3 are listed in table 2. Table 3 lists the X.p. spectra of the Cr 2p level for Cr zeolites measured as received and after treatment in vacuo at elevated temperatures for 1 h. In another experiment the spectra were monitored on zeolites reduced in carbon monoxide at 770 K for 1 h. Spectra of oxidized zeolites (treated in oxygen at 770 K for 1 h) were recorded immediately after the sample had been cooled to room temperature and had reached the appropriate vacuum conditions and then after evacuation at elevated temperatures for 1 h.For illustration the X.p. spectra of the Cr 2p level of CrHY-55 zeolites treated in various ways are presented in fig. 1. In the low-binding-energy part of the Cr 2p level spectrum a broad low-intensity peak, more pronounced for the oxidized zeolite, is present. It was also detected on the parent HY and SY zeolites (see fig. 1) and is obviously due to electrons from the 0 1s level (532.2eV) which have undergone inelastic collisions. With regard to the presence of this background superimposed onB. WICHTERLOVP;, L. KRAJC~KOVA, z.TVAR~J~KOVP; AND s. BERAN 264 1 Table 2. X.P.S. data of the Cr 2p level for chromium in various matrices E,(Cr 2p,12)a/eV AE/eV ref. Cr-SiO, Cr" (3 wt %) Cr"I CrV1 Cr-SiO, Cr"I (8 wt 7 3 Cr"' Cr-A1203 Cr (1&20 wt %) CrV CrV1 CrNaY CrIII 576.6 577.6 58 1.6 577.1 580.4 577.7 577.8 579.7 577.3 9.6 8 9.5 9.0 10.0 13 8.2 9.8 11 9.0 9.0 5 a The binding-energy values are related to the C Is level 285.0 eV Table 3. X.p. spectra of the Cr 2p level for chromium in zeolites evacuated at reduced evacuated at Cr/Sia as in Co oxidized zeolite bulk surface received 620 K 770 K at 770 K at 770 K 620 K 770 K CrHY-27 CrSY-27 CrHY-55 CrSY-55 CrHY-27 CrSY-27 CrHY-55 CrSY -55 0.032 0.030 0.046 0.050 0.032 0.030 0.046 0.050 0.016 0.0 16 0.020 0.022 0.016 0.016 0.020 0.022 578.3 578.4 578.2 578.4 9.2 9.2 9.5 9.5 EB(cr 2P3/,>blev 577.5 577.2 577.5 577.2 577.7 576.9 577.1 577.1 AE/eV 9.1 9.2 9.1 9.2 9.6 9.5 9.4 9.4 577.1 577.1 577.1 577.0 9.3 9.2 9.6 9.5 578.5 578.4 578.3 579.2 9.0 9.1 9.2 9.2 578.6 578.1 578.1 578.3 9.0 9.0 9.3 9.3 577.7 577.5 577.1 577.5 9.2 9.2 9.4 9.5 a Relative molar concentration.The binding-energy values are related to the Si 2p level at 102.8 eV corresponding to the C Is level at 285.0 eV. the Cr 2p spectrum the concentrations of chromium in the surface layers of the zeolites could only be estimated; these values are presented in table 3. The Cr concentrations in the zeolite surface layers roughly correspond to half the concentration of Cr in the zeolite bulk. This implies that the Cr ions are not supported on the surface but are probably in a highly isolated state. The full width at half maximum of the Cr 2p,,, peak for Cr zeolites did not exhibit any meaningful changes with the zeolite treatments, taking a value of ca. 4-5 eV.However, one should note that the background spectrum can contribute to the uncertainty in the estimation of this value. Nevertheless, the values of the full width at half maximum for Cr zeolites are comparable to those published for Cr supported on silica, but much higher than those for bulk chromium oxides.* This finding probably reflects the presence of various ligand-field symmetries of the Cr ions in the zeolite surface layers.2642 CHROMIUM IONS IN ZEOLITES 59 0 580 570 E , lev Fig. 1. X.p. spectra of the Cr 2p level for CrHY-55 zeolite (corrected for charging effects): (1) as received, (2) evacuated at 770 K for 1 h, (3) reduced by CO at 770 K for 1 h, (4) oxidized in 0, at 770 K for 1 h, ( 5 ) parent HY zeolite, as received.ORIGINAL CrIII ZEOLITES The formal valance of chromium in the original ‘as received’ zeolites was CrIII, since these ions were incorporated into the zeolites by ion exchange from CrIII solutions [cf. ref. (l)]. The binding energy of the Cr 2p level for CrlI1 in all the zeolites studied (the Cr 2p3,2 peak was at 578.3 eV) are much higher (2.1 eV) than that for Cr,O, oxide (cf. tables 1-3). This is not surprising, since other metal cations located in zeolites (e.g. CoII or NiII) also exhibit higher binding energies (of ca. 2 eV) compared with their corresponding oxides [cf.ref. (4) and (6)]. Similarly, low concentrations of Cr ions (3 wt %) supported on silica give a higher binding energy, while for larger amounts of Cr (8 wt % , exceeding the silica saturation coverage) the binding energy has a similar value to that for the bulk Cr,O, oxide.1° Apparently, the EB values forB. WICHTERLOVA, L. KRAJC~KOVA, z. TVART~~KOVA AND s. BERAN 2643 the Crl*I ions are affected by the degree of dispersion of chromium as well as by the nature of their interaction with the matrix in which they are embedded. The higher binding energy for CrIII in the zeolites studied in comparison with CrIII on silica might reflect the occurrence of highly isolated Cr complexes. Moreover, the differences in binding energies between Cr,O,, CrlI1 supported on silica and CrIII in zeolites may be also caused by the different contributions of the Madelung potential for the individual systems.The higher binding energies of the Cr 2p,,, peak for our zeolites (ca. 1 eV) compared with that found by Minachev et aL4 are probably not the result of calibration, as in both cases the Si 2s peak was found at 154.0 eV and the C 1s peak at 285.0 eV. This fact probably illustrates the different state of the CrI" ions in the surface layers of the zeolites studied; this is also obvious from the different behaviour of these chromium zeolites during oxidation and reduction procedures [cf. ref. (4)]. Unfortu- nately, values of the spin-orbit splitting for Cr ions are not available in ref. (4)-(6). In addition to the differences in binding energies, values of the spin-orbit splitting (AE) for the above-mentioned systems also differ (see tables 1-3).AE decreases from a value of 9.8 eV for Cr,O, to 9.5 eV for 3 wt % CrlI1 on silica8 or in zeolites, eventually attaining a value of 9.2 eV for zeolites with 1.6 wt % CrIII. This indicates that the AE values for Cr complexes (even though they contain similar ligands, i.e. oxygen atoms) depend not only on the formal valence state of Cr but also on the degree of isolation of the complexes in the matrix. Therefore, values of the spin-orbit splitting reflect not only the number of 3delectrons interacting with the 2p electrons but also, for example, the delocalization of these valence electrons, ligand field symmetry and its strength etc. [cf. ref. (14)-( 16)].Similar variations in AE for a given valence state of a cation were also found for various types of CoII and CoIII For this reason, the values of both the binding energies and the spin-orbit splitting must be considered and related to the corresponding values for CrTII during the investigation of the redox behaviour of Cr ions in zeolites. REDUCTION OF CrlI1 ZEOLITES The interaction of CrlI1 zeolites with carbon monoxide at 770 K for 1 h resulted in a lowering of the binding energy of the Cr 2p level by ca. 1.2 eV, while the spin-orbit splitting remained almost unchanged (see table 3). This change in binding energy, with respect to C P , is comparable to the shift between CrF, and CrF, as well as CrlI1 and CrI1 supported on silica [see tables 1 and 3 of ref.(7)]. There is a disagreement only in the change of AE values, which differ for CrF, and CrF,, but are very close (within experimental error) for CrlI1 and CrlI supported on silica. According to these results, and bearing in mind that the reduction of CrIII ions in the zeolite bulk by hydrogen or carbon monoxide proceeds completely to CrI1 [ c - ref. (1) and (1 €91, one may state that the binding energies found for Cr at 577.1 eV, AE = 9.2-9.3 eV for samples CrHY-27 and CrSY-27 and AE = 9.5-9.6 eV for samples CrHY-55 and CrSY-55, show that CrI1 cations occur in the surface layers of the zeolites studied. Furthermore, these Cr 2p level parameters indicate (in comparison with those for CrlI1 ions) that the CrII ions in the surface layers of the reduced zeolites still act as counter-ions.The X.P.S. parameters obtained for the Cr 2p level of CrIII and CrII ions in zeolites permit a study of the influence of high-temperature zeolite treatment in vacuo (an operation frequently used in zeolite studies) on the state of the Cr ions. Evacuation of all zeolites at temperatures of first 620 and then 770 K led to a gradual decrease in binding energy with respect to the original values. Zeolites evacuated at 770 K (Cr2644 CHROMIUM IONS IN ZEOLITES 2p,,, at 577.2-577.1 eV) exhibited a binding energy identical to that of the reduced Cr" zeolites. The spin-orbit splitting was found to be very close for CrlI1 and Cr", similar to the case of Cr supported on silica (see above). These values clearly demonstrate the gradual reduction of surface CrlI1 to C P , which seems to be quantitative after zeolite evacuation at 770 K.The presence of a considerable amount of carbon with the C Is level at 285.0 eV in the surface layers indicates that the reduction of CrlI1 ions occurred owing to hydrocarbon impurities. The reduction of surface CrlI1 ions in vacuo is in sharp contrast to their behaviour in the zeolite bulk, where they remain trivalent [cf. ref,( l)]. However, note that zeolite evacuation in the ESCA pretreatment chamber occurred at a much lower pressure Pa) and probably in the presence of other contaminants at different concentrations than in the case of measuring procedures used during the investigation of the state of Cr ions within the zeolite bulk. OXIDATION OF CrI1 ZEOLITES CrV1 ions are known to be easily photoreduced or reduced in vacuo depending on the nature of their complexes.Since the X.P.S. technique requires the sample to be placed in a relatively high vacuum and under irradiation by X-rays, the determination of the true state of chromium in the surface layers is a rather complicated problem and one must be very careful in taking measurements as well as in interpreting the X.p. spectra. De Angelis' reported negligible reduction of pure CrO, following X-ray irradiation of up to 2 h. In contrast, most of the CrV1 ions in CrO, supported on silica were reduced to CrIII after 1 h X-ray irradiation. Similarly, evacuation of K,CrO, in an ESCA chamber for 30 rnin 570 K led to the reduction of part of the CrV1 to C F . In considering the nature of Cr complexes in the bulk of the oxidized zeolites (at 770 K for 1 h) it is necessary to review information obtained previously [cf.ref. (l)]. The oxidized zeolites contain rather unstable CrV1 and CrV complexes with oxygen ligands easily removable from their coordination sphere by heat treatment in vacuo. The uptake of oxygen from CrV1 complexes into the gas phase takes place at an evacuation temperature of 370 K and is connected with the reduction of CrV1 to CrV. The oxygen ligands of CrV complexes, which are bound more firmly, are removable only at much higher evacuation temperatures, > 770 K. Accordingly, the X.p. spectra of oxidized zeolites were measured as quickly as possible (scanning for < 15 min) to minimize both photoreduction and reduction due to the high-vacuum treatment.Components of the X.p. spectra of the Cr 2p level for the oxidized zeolites are listed in table 3. The binding energies of the Cr 2p peak for all the oxidized zeolites exhibited practically identical values to those of the original CrlI1 zeolites. A significantly higher value was found only for the oxidized CrSY-55 zeolite (see below). For oxidized samples CrHY-27 or CrSY-27 and CrHY-55 or CrSY-55, values of AE were 0.2 and 0.3 eV lower, respectively, compared with corresponding original CrlI1 zeolites (cf. table 3). It follows that the X.p. spectra indicate a change in the state of CrIII ion, but the binding-energy values do not reflect the presence of a substantial number of CrV1 and CrV ions, which should exhibit much higher binding energies compared with C F , in a similar manner to Cr on silica (table 2) or in chromate-like compounds (table 1).These findings are not surprising, because only some of the Cr ions in the zeolite bulk are oxidized and also because CrV1 and CrV complexes are very unstable. [cf. ref. (l)], probably already having being reduced in high vacuum at room temperature and/or by (even short) X-ray irradiation in the ESCA system. Therefore it is clear that, under the conditions in which X.P.S. measurements are made, the surface layers of oxidized zeolites containing unstable CrVT and CrV complexes exhibit only a very small amount, if any, of Cr icns in higher oxidation states than CrIII, which is responsible for the binding-energy value of 578.4 eV and values of AE of 9.0 andB.WICHTERLOVP;, L. KRAJC~KOVA, z. TVAR~J~KOVA AND s. BERAN 2645 9.2 eV. However, the exact identification of this valence state is questionable, and some changes in the state of chromium which involve the isolation of its complexes are also not fully excluded. The application of an evacuation procedure at elevated temperatures (at 620 and 770 K for 1 h) to the oxidized zeolites changed both the binding energy and AE values in the expected way (see table 2). EB gradually decreased and eventually reached a value (Cr 2p3,2 at 577.7 eV) slightly higher than that for CrL1 cation. AEchanged again to values of 9.2 and 9.5 eV, depending on the chromium content of the zeolite. It follows that the Cr ions are gradually reduced, most attaining divalency. These results imply that the higher oxidation states of highly isolated Cr ions in zeolites are less stable than those observed for Cr supported on silica. However, the formation of some more stable Cr species in higher oxidation states was indicated in the oxidized CrSY-55 zeolite (table 3, the Cr 2p3/2 peak at 579.2 eV) and in all the Cr, zeolites studied oxidized for a longer time (30 h); the binding energy of Cr 2p,,, was found to be ca.580.5 eV. Both these findings can be accounted for by some clustering of the Cr ions, which obviously increases with the concentration of Cr ions as well as with the length of the oxidation process. The Cr species formed in such a way are likely to be similar to those observed for Cr supported on silica. We thank Dr Bast1 for helpful discussions.B. Wichterlova, 2. TvarSikova and J. Novakova, J. Chem. SOC., Faraday Trans. 1, 1983, 79, 1573. S. Beran, P. JirS and B. Wichterlova, J. Chem. SOC., Faraday Trans. 1, 1983, 79, 1585. Z. TvarSikova and B. Wichterlova, J. Chem. SOC., Faraday Trans. I , 1983 79, 1591. Kh. M. Minachev, G. V. Antoshin and E. S. Shpiro, Izv. Akad. Nauk USSR, Ser. Chim., 1974, 1012. Kh. M. Minachev, G. V. Antoshin, E. S. Shpiro and Yu. A. Jusiphov, Proc. 1st All-union Conf. Zeolites in Catalysis (Academy of Sciences of the U.S.S.R., Novosibirsk, 1976). Kh. M. Minachev, G. V. Antoshin, E. S. Shpiro and Yu. A. Jusiphov, Proc. 6th Znt. Congr. Catal. (The Chemical Society, London, 1977), vol. 2, p. 621. B. A. DeAngelis, J. Electron Spectrosc. Relat. Phenom., 1976, 9, 81. R. N. Merryfield, M. McDaniel and G. Parks, J. Catal., 1982, 77, 348. L. Lavielle and H. Kessler, J. Electron Spectrosc. Relat. Phenom., 1976, 8, 95. lo A. Cimino, B. A. DeAngelis, A. Luchetti and G. Minelli, J. Catal., 1976, 45, 316. l1 Y. Okamoto, M. Fujii, T. Imanaka and S. Teranishi, Bull. Chem. Soc. Jpn, 1976, 49, 859. l2 G. C. Allen, M. T. Curtis, A. J. Hooper and P. H. Tucker, J. Chem. Soc., Dalton Trans., 1973, 1675. l3 S. A. Best, R. G. Squires and R. A. Walton, J. Catal., 1977, 47, 292. l4 J. C. Carver and G. K. Schweitzer, J. Chem. Phys., 1972,57, 973. l 5 D. C. Frost, C. A. McDowell and I. S. Woolsey, Chem. Phys. Lett., 1972, 17, 320. l6 V. I. Nefedov, Bull. Acad. Sci. USSR, Phys. Ser., 1964, 28, 774. l 7 Y. Okamoto, H. Nakano, T. Imanaka and S. Teranishi, Bull. Chem. SOC. Jpn, 1975, 48, 1163. l a C. Naccache and Y. Ben Taarit, J. Chem. SOC., Faraday Trans. 1, 1973, 69, 1475. (PAPER 3/ 1825)

ISSN:0300-9599    

DOI:10.1039/F19848002639

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1984, 80, 2647-2654 Estimation of Apparent Permeability in Heterogeneous Membranes Part 1 .-Model Calculations through Cubic Chequer Assembled Membranes B Y AKON HIGUCHI, JIRO KOMIYAMA AND TOSHIRO IUJ.MA* Department of Polymer Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Received 3rd November, 1983 The apparent permeability of a specific model of a heterogeneous polymer membrane has been estimated by applying a finite-difference method of integration to Fick’s first law and the continuity equation. The model was assumed to be a simple chequer one composed of alternate rectangular cells of two different kinds. The cells were defined so as to give repeating concentration profiles under periodic boundary conditions.The apparent permeability of a permeant was calculated from the concentration in sufficiently small lattices in the cell by the iteration method. The effect of membrane heterogeneity or cell dimensions on the permselectivity of permeants is discussed by taking an example, 0, + N,, with polystyrene (PSt)-poly(dimethy1 siloxane) (SR) as membrane substrates. Heterogeneous polymer membranes1’ composed of two components are interesting because of their promising permselective properties, good mechanical strength and high permeability. However, it has been difficult to predict gas or solute permeabilities in such membranes from knowledge of their components and structures, except for the cases of simple series and parallel composite the apparent permeabilities (F) are written as a P = x li P , f x li parallel composite membrane P = li/E (li/pi) series composite membrane i i i where P, is the permeability coefficient in phase i and li is the width of phase i.In recent years, three-dimensional arrangements of the components in heterogeneous polymer membranes have become meticulously de~igned.~ This prompted us to apply a numerical method to estimate apparent permeabilities in such membranes. There have been some theoretical and experimental approaches to the estimation of apparent permeability coefficients in heterogeneous media. One is the so-called ‘potential theory’ developed by Maxwell6 and Rayleigh.’ It treats the effective conductance in a uniform medium interrupted by spheres or spheroids. Kulkarni and Mashelkars recently developed a modified Maxwell model to study diffusivity-structure correlations in network polymers.Connelly and Turner9 measured permeabilities of He, Ar, 0, and p-xylene through blends of polyethylene and polypropylene and compared the results with Maxwell’s predictions. Their results, however, neither proved nor disproved the validity of the Maxwell theory, since the permeabilities in the polymer phases were insignificantly different. 2647PERMEABILITY IN HETEROGENEOUS MEMBRANES I flow direction Fig. 1. Heterogeneous membrane model. Barrer and PetropouloslO have considered the apparent permeability of a hetero- geneous membrane in which a regular array of rectangular parallelepipeds of phase A is embedded in a continuum of phase B. A phenomenological method was employed, since the potential theory could not be applied because of mathematical difficulties.They took into account the effects on the permeability of the surface resistance at the upstream and downstream faces of the parallelepipeds. However, they wrote down their equations with three unpredictable parameters to divide the total flux into those in component phases. Hence the permeant flow perpendicular to the permeation direction was taken into account only implicitly. Bell and Crankll have studied the effective diffusion coefficient for a similar but two-dimensional model. In their study, however, numerical solutions of the Laplace equation were obtained for an extreme case in which one phase is impermeable to the permeant. The purpose of this work is to give a method for estimating the apparent permeability in a heterogeneous membrane with microphase structure from a knowledge of the permeabilities of the component phases. A finite-difference method of integration*? l2 has been applied to Fick’s first law and to the continuity equation.Calculations for some model heterogeneous membranes were also performed. The effects of heterogeneity of the membrane of the permeability and the selectivity of two gases are discussed in some detail, using oxygen and nitrogen as examples. MODEL AND METHOD The constitution of the model membrane, a chequer of parallelepipeds of two phases (phases A and B), is shown in fig. 1 . Cartesian coordinates are determined as shown in fig. 1 , where the z axis is taken to be the direction of permeation.The unit cell is defined by the repetition of the concentration profiles under the periodic boundary condition.ll7 l3 W,, W,, W,, We and W, are the widths of the phases as defined in fig. 1. The unit cell is assumed to be composed of cubic lattices of width W, (W, < W,, W, or W, and y> < Wd). Therefore, the cell consists of lattices of L( W,+ W,) W,/ W,A. HIGUCHI, J. KOMIYAMA AND T. IIJIMA 2649 numbers, where L is the membrane thickness. Permeant concentrations at the lattices are determined using the following assumptions. (1) There is no boundary effect between any two adjacent phases, (2) the permeant concentration in each lattice is constant, (3) Henry’s law holds between the lattices at the membrane surface and the gas phase and (4) the periodic boundary condition holds.It follows from the boundary condition [assumption (4)] that the unit cell, rather than the membrane, can be used for estimation of the apparent permeability of the membrane. The concentrations in the lattices on the surface of the upstream or downstream side are C(x, y , 4 W,) = s i p , , C(x, y , L - ; q.) = sip2 (2) where Si is the solubility of phase i (i = A or B), p 1 is the upstream pressure, p 2 is the downstream pressure and C(x, y , z ) is the concentration at coordinate (x, y , z). The periodic boundary condition leads to the following boundary conditions : (3) i C(X, $ Wc, z ) = C( W a + Wb -x, - i We, z ) C(x, Wd-$Wc,z) = C(x, Wd+$Wc,z) C(W,+ W,-&W,,y,z) = C(-$W,,y,z) C($ wp, JJ, z ) = C( W a + Wb + i Kl, y , z).The concentrations in the lattices can be estimated by using the following iteration method. When lattice i is next to lattice j , the flux, Jij, between lattices i and j is obtained from Fick’s first law: Ji j = ( 1 / c. + 1 / 4) (Cj - C,)/2 W, (4) where pi and 4 and Ci and Cj are the permeabilities and the concentrations of lattices i andj, respectively. The total flux, Jtotal, for lattice i is the sum of the fluxes between lattice i and the adjacent 6 lattices. Ci is incremented by Ci (new) = Ci (old) + Jtotal A? ( 5 ) since Jtotal should be zero from Fick’s second law at steady state. A? is chosen to be short enough to fulfill the condition that Jtotal Wz, A1 is c 1 % of Ci (old) except for the first 20 iterations. With the initial concentrations of lattices assumed to be zero, .Itotal in every lattice was calculated and the concentrations were iterated according to eqn (5).This procedure was continued until a steady state was established. It is possible to calculate the apparent permeabilities between adjacent lattice layers. Since there exist L/W, lattice layers in the direction of the z coordinate, the apparent permeability of the adjacent layers are given as a function of the z coordinate where qx, y, is the permeability of the lattice at (x, y , z). P(z) should not depend on z and should be equal to the apparent permeability, P, at steady state. Therefore, the calculation was repeated until P(z) (0 < z < L - 2 W,) becomes independent of z to within 0.5%. The calculation was performed using a HITAC 200H (Hitachi Co.Ltd) computer, with the iteration being made ca. lo5 times for the three-dimensional model (W, > W,) and ca. lo4 times for the two-dimensional model (Wd = Wc).2650 PERMEABILITY IN HETEROGENEOUS MEMBRANES Table 1. Permeability, diffusion and solubility coefficients of homogeneous membrane9 polyethylene* 2.89 4.6 6.28 0.97 3.20 3.03 2.9, poly(viny1 0.04 0.044 10 0.0115 0.010 11.5 3.8, (PE) chloride) (PVC) (PW siloxane) (S R) polystyrene 2.01 22 0.0914 0.3 15 3.0 1.05 6.3, poly(dimethy1 3 52 189 18.6 181 123 14.7 1.9, a Units: P/ 10-lo cm2 s-l cmHg-l; D / 1 O-' cm2 s-l; S / 1 0-4 cmHg. Low-density polyethylene. I I r" E d I VI CI E 2 2 . 4, I I I 1 I 0.0 0.1 0.2 0.3 0.4 0.5 ' 1.0 lattice size/ I 0-3 cm Fig. 2. Dependence of permeability on lattice size for the three-dimensional model (m), the two-dimensional model (a), the two-dimensional model using the method of subdivision with W, = 0.25 x cm (0) and the two-dimensional model using the method of subdivision with cm (A).W, = 0.10 x RESULTS AND DISCUSSION Calculations were performed for model membranes composed of poly(viny1 chloride) (PVC)-polyethylene (PE) (PVC for phase A and PE for phase B) and polystyrene (PSt)-poly(dimethylsi1oxane) (SR) at 25 "C. These were selected to investigate the effect of the combination of polymers of different permeabilities and permselectivities. Values which characterize the component polymers are summarized in table l.14-17 LATTTCE-SIZE EFFECT The dependence of oxygen permeability on the lattice size is shown in fig.2 for the PVC-PE system. The conditions of the calculation are W, = W, = W, = 1 .O x lo+ cm, L = We = 2W, and wd = W, for the three-dimensional model or W d = W, for theA. HIGUCHI, J. KOMIYAMA AND T. IIJIMA 265 1 membrane f We I s line X- 1 I I Fig. 3. Subdivision of four lattices near line X , N = 4. two-dimensional model. For both the two- and three-dimensional models the permeability increases with decreasing W,. A membrane model with 6W, = W, and W d = W, is shown in fig. 3 to illustrate the reason for the above dependence. The gas permeates mainly through the PE phase, since this phase has a much higher per- meability than phase A (PVC). The upper PE phase is in contact with the lower one at the line of contact, which is denoted as line X .Therefore the gas in the upper PE phase must pass through the PVC phase to enter the lower PE phase and flow through the bilayer. Thus a decrease in Wc leads to a reduction in the detour length in the gas flow in the PVC phase, and so the apparent permeability increases. The value of W, must be small enough to reproduce the apparent permeability with high precision, as discussed by Fidelle and Kirk1, for heat flux in composite solids. However, this requires a tremendous time for the calculation. A method of subdivision for the two-dimensional model was developed to reduce the lattice-size effect. With this method only four lattices near line X are subdivided, as shown in fig. 3. When the subdivision is repeated N times, the smallest lattice size is W,/N.The results obtained by this method are shown in fig. 2 for the smallest lattice size. The inset in fig. 2 shows the permeability as a function of 1/N. It is seen that the permeability becomes independent of N when N > 9 (0.1 1 1 > l/N). The following results were calculated with W, = 0.1 x lop3 cm, N = 15 and W d = W, (two-dimensional model). A typical model is shown in fig. 4. CONCENTRATION PROFILES AND PERMEABILITY Concentration profiles for 0, at given z values under the conditions W, = W, = @ = 1.0 x cm are shown in fig. 5. The pressure drop observed in the PVC phase is greater than that in the PE phase. Fig. 5 shows that in the upper phase the gas permeates from the PE phase to the PVC phase in the x direction. In the lower phase, however, the gas permeates from the PVC phase to the PE phase.This arises from the higher permeability of PE than that of PVC. The apparent permeability for this model was estimated to be 0.317 x cm2 s-' cmHg-l. The permeabilities for a bicomponent series of cm and L = 2We = 2.0 x2652 PERMEABILITY IN HETEROGENEOUS MEMBRANES I -2(wa*wb)+ Fig. 4. Model of a two-dimensional heterogeneous membrane. 1.0 - 2 0.5 Lc 0 z= 0 z= L / 5 Z=3L/5 2=4L/5 2 = 2 L/5 Z=L 0 1.0 Fig. 5. Concentration profiles at z = L / 5 (a), 2L/5 (W), 3L/5 (0) and 4L/5 (0). The conditions cm, L = 2We = 2.0 x are W, = W, = = 1.0 x cm and W, = 0.1 x cm. composite membrane and a bicomponen t parallel composite membrane were calculated to be 0.867 x lo-', and 0.147 x cm2 s-l cmHg-l, respectively. PERMEATION AND PERMSELECTIVITY The dependence of permselectivity on the widths of the model shows some interesting features.Table 2 shows the permeabilities of Oz(Fo2) and N,(FNP) and the permselectivity (&lx/FN2). All the calculated permeabilities of 0, are > 10 times greater than that for a PSt homogeneous membrane, while the permselectivities are found to be ca. 5 , which is close to the value, 6.38, for a PSt homogeneous membrane. A plot of l&/PN2 against Fo2 is shown in fig. 6.A. HIGUCHI, J. KOMIYAMA AND T. IIJIMA 2653 Table 2. Permeability of 0, and N, in various two-dimensional models, PSt-SR system, W, = 0.1 x lop3 cm, We = 2.0 x cm. W,/10-3 cm W,/ 1 Op3 cm &/ cm Po2 'N 2 Po, / P N 2 1 .oo 1 .oo 1-00 0.198 0.0379 5.23 0.20 1.80 1 .oo 0.176 0.0355 4.96 0.20 1.80 1.70 0.274 0.0504 5.44 1.80 0.20 1.70 0.151 0.0324 4.65 0.50 0.50 1 .oo 0.337 0.0668 5.05 0.20 0.20 1 .oo 0.628 0.137 4.60 a Units: P/lOP cm2 s-l cmHg-l 6.0 5.0 4.0 3.0 2 .o 1.0 1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Fig.6. Plot of the permeability ratio of 0, to N, against the permeability of 0, by heterogeneous membranes (0) and a homogeneous PSt membrane (0). Po2/10-8 mi2 s-' crnHg-' CONCLUDING REMARKS For a simple model of polymeric heterogeneous membranes, the present method enables us to predict the apparent permeabilities of gases. Although the calculation is performed here only for the chequer membrane, the method is easily applicable to other models, such as a regular array of rectangular parallelepipeds embedded in a continuum or dispersed spheres in a continuum.Thus, the method and results may also be applied to other transport problems in heterogeneous media, such as thermal or electrical conductivity. The iteration method used here can be substituted by the numerical solution of a linear equation system. The latter method needs the second power of the memory numbers used here, although the calculation time is reduced. Note that real heterogeneous membrane processes are more complicated than assumed here. In polymeric heterogeneous membranes certain boundary effects between the adjacent microphases occur.lS The concentration dependence of the permeant diffusion coefficient is another example of possible complications. These factors may seriously affect the membrane permeability. For this rational evaluation, the present method may afford a tool for estimating the purely geometrical factors.2654 PERMEABILITY IN HETEROGENEOUS MEMBRANES We would like to thank a referee for his careful reading of the manuscript and his constructive comments.J. A. Barrie and K . Munday, J . Membr. Sci., 1983, 13, 175. J. A. Barrie and J. B. Ismail, J . Membr. Sci., 1983, 13, 197. Dzflusion in Polymers, ed. J . Crank and G. S. Park (Academic Press, London, 1968), chap. 6. J. Crank, The Mathematics of DzHwion (Oxford University Press, Oxford, 1975), chap. 12. Y. Mohajer, G. L. Wilkes, I. C. Wang and J . E. McGrath, Polymer, 1982, 23, 1523. C . Maxwell, Treatise on Electricity and Magnetism (Oxford University Press, Oxford, 1873), vol. 1 , p. 365. Lord Rayleigh, Philos. Mag., 1892, 34, 481. M. G. Kulkarni and R. A. Mashelkar, Polymer, 1981, 22, 1658. T. M. Connelly and J. C. R. Turner, Chem. Eng. Sci., 1979, 34, 319. lo R. M. Barrer and J. H. Petropoulos, Br. J. Appl. Phys., 1961, 12, 691. G. E. Bell and J . Crank, J . Chem. SOC., Faraday Trans. 2, 1974, 70, 1259. ** T. P. Fidelle and R. S. Kirk, AIChE J., 1971, 17, 1427. l3 G. R. Gavalas and S. Kim, Chem. Eng. Sci., 1981,36, I I 1 1 . l5 C. E . Rogers, J . Pol-vm Sci., Polym. Phys. Ed., 1965, 10, 93. T. Nakagawa, H. B. Hopfenberg and V. Stannet, J. Appl. Polym. Sci., 1971, 15, 231. A. S. Michaels and H. J . Bixler, J . Polym Sci., 1961, 50, 413. S. Yamada and T . Nakagawa, 26th IUPAC Conference, Tokyo, 1977. H . Odani, M. Uchikura, K. Taira and M. Kurata, J . Macromol. Sci., 1980, B17, 337. (PAPER 3/ 1960)

ISSN:0300-9599    

DOI:10.1039/F19848002647

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1984,80, 2655-2668 Activity and Selectivity in the Oxidation of Benzene on Supported Vanadium Oxide Catalysts BY KENJI MORI AND MAKOTO INOMATA Kinuura Research Department, JGC Co., Sinosaki-cho, Handa, Aichi 475, Japan AND AKIRA MIYAMOTO* AND YUICHI MURAKAMI Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan Received 7th November. 1983 The dependences of activity and selectivity in the oxidation of benzene on unsupported and supported V205 catalysts on the catalyst structure have been investigated. It was found that the reaction rate at various concentrations of 0, was proportional to the amount of V5+=0 species in the catalyst, indicating that surface V=O is an active oxygen species for the reaction.The specific activity of surface V=O species on unsupported V,05 was the same as that of the fused catalyst, while the surface structures were markedly different from each other. This means that the oxidation of benzene on V,05 is a structure-insensitive reaction. The activity of the V205/Ti02 and V205/Al,0, catalysts was mainly determined by the number of surface V=O species. The specific activity of surface V=O species of V,O,/TiO, was greater than that of unsupported V205 or V,O,/AI,O,, indicating the promoting effect of the TiO, support on the activity of surface V=O species. The selectivity to maleic anhydride was found to be determined only by the number of V205 layers on the support for both V205/Ti02 and V,05/Al,0, catalysts.When the number of V205 layers was 1 or 2, the selectivity to maleic anhydride was very low, while it increased markedly on increasing the number of V,O, layers to 5 and attained a constant value above 5 layers. The change in the oxidation state of the catalyst did not affect the selectivity. Attention has been given to the oxidation of benzene on vanadium oxide catalysts because of its significance for the commercial production of maleic anhydride. Interesting results have been obtained in studies of the kinetics and mechanism of the reaction.l-18 However, the activity and selectivity of the reaction have not been clarified in terms of the structure of the catalyst. This is because of the lack of a well established method for characterizing the surface of vanadium oxide catalysts. Recently we proposed a method using the rectangular-pulse technique for a deter- mination of the number of surface V=O species (A) and the number of V,O, layers (v) on the s ~ p p o r t .~ ~ ~ ~ * This technique is based on the following reactions: V=O+NO+NH,- V-OH +N, + H,O (1) bulk V=O V-OH-V=O + fH20 together with the introduction of the NO+NH, mixture in a rectangular pulse to the preoxidized catalyst and the subsequent detection of the concentration profile of N, produced by reaction (1). The separation of the N, formed on the initial surface V=O species from that formed on the reproduced V=O species leads to a determination 26552656 OXIDATION OF BENZENE ON SUPPORTED v,o, of A, while detailed analysis of the concentration profile of the N, formed on the reproduced V=O leads to a determination of v.Furthermore, the structures of the V,O,/TiO, and V,O,/Al,O, catalysts have been determined by various physico- chemical techniques as well as the rectangular-pulse method.21* 22 The purpose of this study was thus to study the activity and selectivity in the oxidation of benzene on unsupported and supported vanadium oxide catalysts in terms of the structure of V,O, on the support. EXPERIMENTAL CATALYSTS A V,O,-U catalyst was prepared by thermal decomposition of NH,VO, in a stream of 0, at 773 K. A V,O,-F catalyst was prepared by fusing the V,O,-U catalyst at 1073 K for 18 h in air, followed by gradual cooling to room temperature. The number of surface V=O species (A) on the V,O,-U and V,O,-F catalysts was determined by the rectangular-pulse techniquelg' 2o and the results are shown in table 1 , together with the B.E.T.surface areas (SB. E.T.). The number of V,O, layers (v) was calculated from A using where M(V,O,) is the molecular weight of V,O,. From the results of X-ray diffraction studies, u.v.-visible spectroscopy, i.r. spectroscopy, X-ray photoelectron spectroscopy and scanning electron micrographs of the catalysts,2". 24 the electronic properties of the catalysts are the same while the surface of V,O,-U is rougher than that of V,O,-F. Anatase TiO, was prepared by hydrolysis of Ti(SO,), followed by calcination in air at 873 K, while A1,0, was commercially available (Sumitomo, y-Al,O,). The B.E.T. surface areas of TiO, and A1,0, were 48.2 and 230.0 m2 g-l, respectively.Vanadium oxides supported on the carriers were prepared by impregnation of the carrier with an oxalic acid solution of NH,VO, followed by calcination at 773 K in a stream of 0, for 3 h. V,O,/TiO, and V,O,/Al,O, monolayer catalysts were prepared from V,O,/TiO, (10 mol x V,O,) and V,O,/A1,0, (25 mol % V,O,), respectively, in a manner similar to that described by Yoshida et al.,, The number of surface V=O species (A), the number of V,O, layers on the support (v) and the B.E.T. surface areas of the supported catalysts were determined and the results are shown in table 1. p is the amount (percent) of theoretical monolayer of V205,26 which is calculated from the V,O, content (x) and SB.bl.7r. using (ii) where N is Avogadro's number, a(V,O,) is the area occupied by a V,O, unit (20.6 A,) and M (support) is the molecular weight of the support (TiO, or A1,0,).Unless otherwise stated, the particle size of the catalyst was in the range 2848 mesh. CATALYTIC-ACTIVITY MEASUREMENTS Kinetic studies were carried out using the continuous-flow reaction technique under the followingconditions: total pressure 1 atm (1 atm = 101.3 kPa), temperature 61 3-723 K, partial pressure of 0, (Po) 04.20 atm, partial pressure of benzene (PB) 0.004-0.017 atm and W / F = 144-13400 g s mo1-l. Helium was used as a carrier gas and conversion of benzene was kept to < 20% by controlling the amount of catalyst. Maleic anhydride (MA), benzoquinone (BQ), CO and CO, were identified as reaction products. The rate of formation of MA was determined by titration with 0.01 mol dm-, Na,CO, following collection in water, while that of BQ was determined by titration with 0.01 mol dm-, Na,S,O, following collection in a methanol trap cooled by a dry-ice + methanol mixture.CO, CO, and benzene were analysed by gas chromatography. CHARACTERIZATION OF THE CATALYSTS Catalysts in the steady-state reaction were rapidly cooled to room temperature and the steady-state catalyst structure studied. 1.r. spectra of the catalyst were observed on a Jasco-IR-GTable 1. Physical characteristics of catalystsa and rates (R), turnover frequencies (TF) and selectivities (S) for the oxidation of benzeneb V205/Ti02 1 2 5 10 25 50 monolayer VZO,/A1203 2 5 10 25 35 50 monolayer v205-u V205-F 47.2 45.2 26.3 22.8 10.3 7.4 32.1 221.3 219.1 167.6 114.1 101.1 65.8 174.2 5.4 0.8 56 120 184 135 60 31 126 3 77 355 405 365 249 20 22 4 32 67 276 602 2847 6382 - 11 27 67 222 329 662 __ - - 1-2 1-2 2-3 5-8 3WO 50-60 1 1-2 1-2 1-3 2 4 3-7 5-1 5 1 504 2750 10.2 15.4 18.6 16.7 6.06 1.92 9.70 0.17 1.54 10.7 17.4 15.7 10.5 0.80 1.25 0.19 182 128 101 124 101 62 77 (58) 20 30 43 43 41 40 57 48 5 9 43 52 53 53 13 (1 1) 10 37 51 45 45 8 50 50 95 91 50 36 39 40 87 (69) 89 57 42 45 46 92 44 45 a SB.E,T., the B.E.T.(Brunauer-Emmett-Teller) surface area; A, the number of surface V=O species; p, value calculated by eqn (ii); u, number S(MA), selectivity to maleic anhydride. S(BQ), selectivity to benzoquinone. of V205 layers on the support. Reaction conditions: temperature 662 K, PB = 0.0143 atm and Po = 0.20 atm.S(C0 + CO,), selectivity to CO + CO,.2658 OXIDATION OF BENZENE ON SUPPORTED v,o, i 0.8 2000 4 000 6000 ( W / O / g s mol-' Fig. 1. Effect of W/F on R, R(MA), R(BQ) and R(CO+CO,) for V,O,-U at 662 K; PB = 0.0143 atm and Po = 0.20 atm. 0, R; 0, R(MA); A, R(BQ); 0, R(CO+CO,). spectrometer using KBr discs. The e.s.r. signal of the V4+ ions in the catalyst was measured at X-band frequencies with a Jeol ME 1X Spectrometer. Desorption of oxygen from the catalyst surface was investigated using temperature-programmed desorption (t.p.d.) apparatus similar to that used by Amenomiya et RESULTS EFFECTS OF CATALYST WEIGHT, PARTIAL PRESSURE OF BENZENE AND PARTICLE SIZE OF CATALYST ON THE REACTION RATES In the oxidation of benzene on vanadium oxide catalysts, the following reactions were found to take place: Taking into account the stoichiometries of reactions (3)-(5), the rate of formation of each product [R(MA), R(BQ) or R(CO+CO,)] is defined as the rate at which ben- zene is converted into the product.The total reaction rate (R) and selectivities to MA [S(MA)], BQ [S(BQ)] and CO+CO, [S(CO+C02)] are given by R = R(MA) + R(BQ) + R(C0 + CO,) (iii) S(MA) = R(MA)/R (iv) S(BQ) = R(BQ)/R (v) ( 4 S(C0 + CO,) = R(C0 + CO,)/R. Fig. 1 shows plots of the reaction rates against weights of V,O,-U catalyst. The rates are proportional to the catalyst weight. This indicates that MA, BQ and CO+CO,K. MORI, M. INOMATA, A. MIYAMOTO AND Y. MURAKAMI 2659 1.51 I I .5 0 P,/atm Fig. 2. 100 80 n E 5 60 .- > 0 .d c1 - x 40 20 0 A A A ~ A A A 0 b L 1 1 1 0.05 0.1 0.15 0.2 Polatm Fig.3. Fig. 2. Effect of Po on R, R(MA), R(BQ) and R(CO+CO,) for V,O,-U at 662 K; PB = 0.0143 atm and W/F = 3744 g h mol-l. Symbols as fig. 1. Fig. 3. Effect of Po on S(MA), S(BQ) and S(C0 +CO,) for V,O,-U at 662 K; PB = 0.0143 atm. 0, %MA); A, S(BQ) ; 0, S(C0 + CO,). were formed concurrently from benzene and consecutive oxidation of MA or BQ to CO + CO, was negligible under our experimental conditions. The rates of formation of the individual products were found to be independent of the dimensions of catalyst particles; the rates for V,O,-U of 28-48 mesh were the same as those for V,O,-U of 48-68 mesh. This indicates that mass transfer has a negligible effect on the reaction rate. It was also found that the reaction rates were approximately first order with respect to PB.EFFECT OF 0, PARTIAL PARTIAL PRESSURE ON REACTION RATES AND CATALYST STRUCTURES Fig. 2 shows the effects of Po on R, R(MA), R(BQ) and R(C0 + CO,) for the V,O,-U catalyst at 662 K. The rates increased gradually with increasing Po up to 0.12 atm and they were almost constant for Po > 0.12 atm. In spite of the change in the reaction rates with Po, selectivities to MA, BQ and CO+CO, were independent of Po, as shown in fig. 3. Fig. 4 shows the change in the reaction rates after the supply of 0, was ended. The rates decreased gradually with increasing time and were negligible 8 h after the reaction began. It was confirmed that both the reaction rate and the infrared spectrum of the catalyst attained steady states under these conditions.Fig. 5 shows i.r. spectra of the V,O,-U catalyst in the steady-state reaction at various partial pressures of 0,. Catalysts in the steady-state reaction at Po > 0.021 atm gave absorption bands at 1020 and 825 cm-l assigned to the stretching vibration of V=O species and the coupled vibration between V=O and V-0-V, respectively.28* 29 New absorption bands at 990 and 910 cm-l were observed for catalysts in the steady-state reaction at Po < 0.09 atm. These bands are assigned to lattice vibrations of V,0,.30 In order to quantify the change in the amount of V=O species with Po, absorbances at 1020 and 825 cm-l were calculated from the spectra in fig. 5 and the results are shown in2660 OXIDATION OF BENZENE ON SUPPORTED v,o, time on stream/min Fig.4. I 10 OIO I I I 1 1200 1000 800 600 waven um berlcm-' Fig. 5. Fig. 4. Change in R, R(MA), R(BQ) and R(CO+CO,) after the end of the supply of 0, for V,O,-U at 630 K ; PB = 0.0143 atm, Po = 0.20 atm at t 6 0, Po = 0 atm at t > 0 and W/F = 11 556 g h mol-'. Symbols as fig. 1. Fig. 5. Infrared spectra of vanadium oxides in the steady-state reaction for various values of Po at 662 K; PB = 0.0143 atm. Po/atm = (a) 0.203, (b) 0.143, (c) 0.10, (d) 0.091, (e) 0.06, (f) 0.021 and ( g ) 0. fig. 6. As shown, the amount of V=O species increased almost linearly with Po up to 0.14 atm and was constant above this value. E.s.r. signals assignable to the V4+ species ( g = 1.96) were observed for the V,O,-U catalyst in the steady-state reaction for Po < 0.14 atm.Fig. 6 also shows the relative intensity of the e.s.r. signal at various Po values. As shown, the amount of V4+ species decreases monotonically with in- creasing Po up to 0.14 atm, while it was negligibly small at Po > 0.14 atm. At any Po no e.s.r. signal assignable to O;, 0- or 0; species was observed.K. MORI, M. INOMATA, A. MIYAMOTO AND Y. MURAKAMI 266 1 1 .o v) 0, u 0, .- 2 0 II 0 . 5 L 0 E 0 * 5 6 0 0.05 0.1 0.15 0.2 1 .o ul u .- 2 0.5? + 0 E 0 CI 5 0 Po/atm Fig. 6. Absorbances in infrared spectra and relative intensity of e.s.r. signal for vanadium oxides in the steady-state reaction for various values Po at 662 K. 0, Absorbance at 1020 cm-l; A, absorbance at 825 cm-l; e, relative intensity of e.s.r. signal at g = 1.96. 20 100 > .- c1 - a + 9 - a r ' p - L o 2 x 0.05 0.1 0.15 0.2 Po/atm Fig.7. Effect of Po on R, S(MA), S(BQ) and S(CO+CO,) for V,O,/TiO, (10 mol % V,O,) at 662 K ; PB = 0.0143 atm and W/F= 360 g h mol-l. a, I?; 0, S(MA); A, S(BQ); 0, S(C0 + CO,). As shown in fig. 7-9, similar results to those for the V,O,-U catalyst were obtained for the V,O,/TiO, (10 mol% V,O,) catalyst. The rate increased monotonically with Po up to 0.12 atm and was constant above this value (fig. 7), while the selectivities to MA, BQ and CO+CO, were independent of Po (fig. 9). The amount of V=O species as calculated from the absorbance at 1020 cm-l in the i.r. spectra of V,O,/TiO, (1 0 mol % V,O,) at various Po (fig. 8) changed in a similar manner to the reaction rates with respect to the change in Po (fig. 9), while the amount of V4+ ions as determined by e.s.r.decreases monotonically with increasing Po up to 0.12 atm (fig. 9). No e.s.r. signal assignable to O,, 0-, or 0, was observed for any of the catalysts. This is reasonable since adsorbed species such as O;, 0- or 0; are usually detected2662 OXIDATION OF BENZENE ON SUPPORTED V,05 I 1 I 1200 1000 800 wavenum ber/cm -I Fig. 8. ( 0 0 0 0 A 1 .I 0.05 0.1 0 .I5 0.2 Po/atm Fig. 9. 1.0 w u .- 4 + 0 3.5 2 5 0 E 0 - 0 Fig. 8. Infrared spectra of V,O,/TiO, (10 mol % V,O,) In the steady-state reaction values of various Po at 662 K; PB = 0.0143 atm. Po/atm = (a) 0.20, (b) 0.11, (c) 0.09, (d) 0.05, (e) 0.026 and (f) 0. Fig. 9. Absorbances in infrared spectra and relative intensity of e.s.r. signal for V,O,/TiO, (10 mol % V,O,) in steady-state reaction for various values of Po at 662 K.0, Absorbance at 1020 cm-l; @, relative intensity of e.s.r. signal at g = 1.96. when the V,O, content in the supported catalyst is very low and when prereduced vanadium oxide is treated with 0, or N,O at a low temperature (e.g. room temperat~re).~l-~~ In this study, however, the content of V205 was not very low and the catalyst was treated with a mixture of benzene, 0, and He at a high temperature (e.g. 662 K). ACTIVITY AND SELECTIVITY IN THE OXIDATION OF BENZENE IN THE PRESENCE OF AN EXCESS OF OXYGEN Table 1 shows values of R, S(MA), S(BQ) and S(CO+CO,) for V,O,-U, V,05-F, V,O,/TiO, and V,O5/Al,O3 catalysts for excess-oxygen conditions where the reaction rate was zero order with respect to Po and where the catalyst was confirmed t o be in the highest oxidation state, i.e.V5+. In accordance with the decrease in SB.E.T., RK. MORI, M. INOMATA, A. MIYAMOTO AND Y. MURAKAMI 2663 for V,O,-F was much smaller than that for V,O,-U, while the selectivities did not change significantly. TiO, or Al,03 had a negligible activity for the oxidation of benzene. The reaction rate (R) for V,O,/TiO, increased markedly with increasing V,O, content from 0 to 5 mol% , passed a maximum at 5 mol% V,O, and then decreased to the value for V,O,-U for any further increase in V,O, content. The selectivity to MA [S(MA)] was very low for V,O,/TiO, containing 1 or 2 mol% V,O,, while it was as high as 50% for catalysts containing > 5 mol % V,O,. V,O,/Al,O3 with low V,O, content (1 or 2 mol%) had only negligible activity.R increased markedly with increasing V,O, content from 5 to 25 rnol % . R for V,O,/AI,O, (25 rnol % V,O,) was considerably higher than R for V,O,/TiO, (5 mol % V,O,). S(MA) for V20,/A1203 with low V,O, content (1 , 2 or 5 mol % ) was very low, while it increased markedly from 5 to 25 mol % and attained a constant value above 25 mol % . S(MA) for monolayer V,O,/TiO, and V,O,/Al,O3 was low and the total oxidation of benzene to CO and CO, takes place preferentially. Although table 1 shows results for the reaction at 662 K, similar relationships were found to hold at any temperature examined (613- 723 K). The apparent activation energy for R was 22 kcal mol-l for the unsupported V,O, catalysts, 20-23 kcal mol-l for V,O,/TiO, and 29-32 kcal mob1 for V,0,/A1203.The selectivity to MA decreased only slightly with increasing temperature. DISCUSSION ACTIVE OXYGEN SPECIES FOR THE OXIDATION OF BENZENE As shown in fig. 2 and 7, the rate of benzene oxidation increased with increasing Po up to 0.12 atm for V,O,-U and V,O,/TiO, and remained constant above this value. The steady-state amount of V5+=0 species in the catalyst changed with Po in a similar manner to the reaction rate for both unsupported and supported catalysts, while the amount of V4+ changed inversely with the reaction rate (fig. 6 and 9). This suggests that the reaction proceeds by a reduction-oxidation mechanism (or Mars-van Krevelen mechanism)6 and that the active oxygen species for the oxidation of benzene is surface V=O, a conclusion similar to that obtained by Tarama et aZ.34 and Hirota et aL3, for the oxidation of CO on V,05 catalyst.It can be shown that adsorbed oxygen species are not responsible for this reaction as follows. No adsorbed oxygen species, such as O;, 0- or OF, were detected on the catalysts by either e.s.r. or t.p.d. measurement. Moreover, even if the surface of the V,O,-U was initially covered completely with adsorbed oxygen species, these species should be consumed by reaction with benzene within 2 s of stopping the supply of 0, and the rate should decrease abruptly, contrary to the experimental result shown in fig. 4. Since the amount of bulk V=O in the V,O,-U is greater than the amount of surface V=O and the bulk V=O species can diffuse easily from the bulk to the surface, the result shown in fig.4 can satisfactorily be interpreted by assuming that the active oxygen species for the reaction is surface V=O. When supply of 0, is stopped, the surface V=O species is consumed by the reaction with benzene to form the reduced site (V4+ ion) and the reaction products. The reduced site is then reoxidized by bulk V=O species to form surface V=O species, and therefore the rate of benzene oxidation decreases gradually until all of the bulk V=O is consumed by the reaction. Some researchers have reported that reduced states of vanadium oxide are more active than V205 for benzene oxidation.4* l2 For example, using the pulse microcatalytic reactor Schaefer found that the specific activities for different oxides of vanadium increased in the sequence12 VO,., < VO,.,, < VO,.,, < VO,.,, -= VO,.,,.This order is different from the results shown in fig. 2, 6, 7 and 9. Taking into account the2664 OXIDATION OF BENZENE ON SUPPORTED V,O, difference in experimental conditions, however, the results obtained by Schaefer are consistent with the above-mentioned conclusion that surface V5+=0 is the active oxygen species for the oxidation of benzene. According to Anderss~n,~ and Miyamoto et aZ.,37 the number of surface V=O species on V,013 or VO, changes with the condition of the surface: when the oxidation state in the surface is equal to that in the bulk, the amount of surface V=O decreases with decreasing oxidation state of the vanadium oxide. When only the surface of V,O1, or VO, is oxidized to V5+, the number of surface V=O species on V,O,, or VO, is larger than that on V,O,.The catalytic activity in this study was measured after the steady-state reaction rate and steady-state catalyst structure had been attained. Since it took a long time to reach the steady state (e.g. 8 h), the oxidation state of the surface in the pulse experiments of Schaefer is considered to be significantly different from that of the bulk. In other words, the apparent difference in specific activities of various oxides of vanadium may be caused by differences in the experimental conditions. Further details concerning the catalytic activities of vanadium oxides in various oxidation states will be investigated by measuring the number of surface V=O species present under the reaction conditions.The observed relationship between reaction rate and Po (fig. 2 and 7) can be explained in terms of the reduction-oxidation mechanism as follows. In the absence of 0, (Po = 0 atm), the catalyst is in the reduced state (V4+) and no surface V=O species are present to oxidize benzene. As Po increases, reoxidation of the reduced catalyst by 0, increases the oxidation state of the catalyst to increase the number of surface V=O species. This means that the reaction rate increases with increasing Po. In the presence of an excess of oxygen (Po > 0.12 atm), the catalyst is in the highest oxidation state, i.e. V,O,. Therefore, the increase in Po does not lead to a further increase in the oxidation state or in the number of surface V=O species and so the reaction rate does not increase with Po when an excess of oxygen is present.STRUCTURE SENSITIVITY OF THE REACTION The surface of V,O,-F was found to be significantly different from that of V,O,-U. The surface of V,O,-F is much smoother than that of V2O5-U. This is reasonable since fusion of a solid would generally lead to a smooth surface. According to the results of preliminary experiments, the difference between the surface structures of V,O,-U and V,O,-F affects the catalytic activity, e.g. the specific rates of oxidation of CO, butane or buta-1,3-diene on V,O,-U was much higher than on V,0,-F.239 24? 38 Since the surface V=O species have been found to be the active sites for the oxidation of benzene, the turnover frequency (TF) for this reaction can be defined by TF = RIA.(vii) Values of T F at 662 K were calculated from the results of R and A for various catalysts and the values are shown in table 1 . In spite of the significant difference between the surface structures of V,O,-U and V,O,-F, the value of TF for V,O,-F is almost equal to that for V,O,-U, suggesting that the oxidation of benzene on vanadium oxide catalyst is a structure-insensitive reaction. ACTIVITY OF SUPPORTED CATALYSTS In general, the activity of supported catalysts is determined by two factors: (i) the number of active sites and (ii) the specific activity of the active site, i.e. the turnover frequency. The separation of these two factors is indispensable for a detailed under- standing of the role of the support in a given reaction. However, this has not been done for the supported metal oxide catalyst, because of the lack of a well established method for determining the number of active sites.Since the number of surface V=OK. MORI, M. INOMATA, A. MIYAMOTO AND Y. MURAKAMI 2665 1 2 10 100 500 2000 number of V,O, layers, v Fig. 10. Relationship between the turnover frequency (TF) for the oxidation of benzene and the number of V205 layers of the support (v). 0, V,O,/TiO,; A, V,O,/Al,O,; 0, V,O,-U; A, V,O,-F. species has been determined for the supported vanadium oxide catalyst^,^^-^^ the role of support is discussed below in terms of the number of surface V=O species (A) and the specific activity of the surface V=O, i.e. the turnover frequency (TF) as defined by eqn (vii). As shown in table 1, the rate (R) for V,O,/Al,O, is greater than that for V,O,-U and V,O,-F, indicating the promoting effect of the A1,0, support.The number of surface V-0 species on V,O,/Al,O, is much greater than for unsupported V,O,, while the turnover frequency for V,O,/Al,O, is less than that for the unsupported V,O,. This means that the promoting effect of A1,0, is to increase the number of surface V=O species without increasing the specific activity of these species. The rate for V,O,/TiO, is also much larger than that for unsupported V,O,. As can be seen from table 1, both A and TF are increased by supporting V,O, on TiO,. This means that the promoting effect of the TiO, support is caused by two factors: an increase in the number of surface V=O species and an increase in the specific activity of these species.Fig. 10 shows the relationship between the turnover frequency and the number of V,O, layers (v) for V,O,/TiO, and V,O,/AlO,. The turnover frequency for V,O,/TiO, increases monotonically with decreasing number of V,O,, layers on TiO,. This indicates that the promoting effect of TiO, on the specific activity of the surface V=O species increases with decreasing number of V,O, layers. It is known that oxygen evolution from V,O, in V,O,/TiO, (anatase) occurs at a temperature much lower than in unsupported V205.39-41 Thus it is believed that the V=O species on V,O,/TiO, are catalytically more active than those on unsupported V,O,. The relationship shown in fig. 10 provides experimental evidence for the validity of this inference. SELECTIVITY IN BENZENE OXIDATION As shown in fig.6 and 9, the oxidation state of the catalyst changes with Po. Under the condition of an excess of oxygen, the catalyst is kept in the highest oxidation state, i.e. V5+. As Po decreases, the amount of V4+ increases and the amount of V5+ decreases. As shown in fig. 3 and 9, the selectivities of MA, BQ and CO + CO, do not 81 F A R 12666 OXIDATION OF BENZENE ON SUPPORTED V,05 1 2 10 100 500 2000 number of V , 0 5 layers, v Fig. 11. Relationship between the selectivity to maleic anhydride [S(MA)] and the number of V,O, layers on the support (v). 0, V,O,/TiO,; A, V,O,,/Al,O,; 0, V,O,-U; A, V,O,-F. change with Po. This means that the selectivity in the oxidation of benzene on unsupported and supported V,O, catalysts is independent of the oxidation state of the catalyst at least when conversion of benzene is low, consecutive oxidation of MA to CO and CO, is negligible and the catalyst is in its steady state at a given level As shown in table 1, the selectivities to MA, BQ and CO+CO, under conditions of an excess of oxygen change greatly with the catalysts.Fig. 11 shows a plot of the selectivity to MA [S(MA)] against the number of V205 layers on the support (v). When v = 1 or 2, S(MA) has a very low value, but it increases as the value of v increases to 5 and reaches a constant value at v > 5. Note that the relationship between S(MA) and v is common to both V,O,/TiO, and V,O,/Al,O,, while the structures of the V,O,/TiO, catalysts differ significantly from those of the V20,/A1,0, catalysts.219 22 Although we have tried to correlate the change in S(MA) with various parameters, the relationship between S(MA) and v shown in fig.11 is the best result. This indicates that the number of V205 layers is an important factor for determining selectivities in the oxidation of benzene; V,O, layers are necessary for the selective oxidation of benzene to MA. Although further studies are necessary to clarify the molecular mechanism for the correlation between S(MA) and v, the following points are noted as a possible mechanism. As described above, the reaction proceeds by the reduction- oxidation mechanism and the surface V=O species are the active oxygen species. It has also been shown that there are Bronsted-acid sites adjacent to the surface V=O species on the supported vanadium oxide 2 2 q 28 This suggests that the ben- zene molecules are adsorbed and activated on the Bronsted-acid sites and that the re- action is initiated by the nucleophilic attack of oxygen atoms of surface V=O species on the adsorbed benzene molecule to form an intermediate species.From the stoichiometry of reaction (3), the subsequent introduction of oxygen atoms (6-8 atoms per benzene molecule) is necessary to complete the reaction. Since the reaction proceeds by the reduction-oxidation mechanism, these oxygen atoms are not directly supplied from gaseous 0, but supplied through the oxygen of the catalyst. It is known that the oxygen of V,O, can migrate from the bulk to the surface. When the V205 layers on the support are thick enough, the oxygens in the V205 layers can be used to oxidize the intermediate species.For catalysts with monolayer V,O, or with very of Po.K. MORI, M. INOMATA, A. MIYAMOTO AND Y. MURAKAMI 2667 thin V205 layers on the support, the oxygens cannot be supplied from the bulk but are supplied by migration of surface oxygens. This change in the mode of oxidation of the intermediate species with the number of V,05 layers may provide one of the reasons for the correlation between S(MA) and v . Further details of the reaction mechanism will be discussed in a subsequent paper, which will also include additional data from 180-tracer experiments, a detailed kinetic and dynamic study and identifi- cation of the intermediate species. Recently, Bond et al. have discovered that monolayer V205/Ti02 is effective for the selective oxidation of o-xylene to phthalic anhydride, i.e.the oxidation of a side- chain of a benzene ring.42* 43 Since the oxidation of benzene to maleic anhydride is the oxidation of the benzene ring, the relationship between the structure of the catalyst and its selectivity is considered to change with the type of reaction. It may therefore be interesting to investigate different reactions on well characterized vanadium oxide catalysts. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan (no. 57 470 055). C. F. Cullis and D. J. Hucknall, Catalysis (The Royal Society of Chemistry, London, 1982), vol. 5, p. 273 and references therein.M. S. Wainwright and N. R. Foster, Catal. Rev., 1979, 19, 21 1 and references therein. D. B. Dadyburjor, S. S. Jewur and E. Ruckenstein, Catal. Rev., 1979, 19, 293. A. Bielanski and J. Haber, Catal. Rev., 1979, 19, 1 and references therein. J. M. Weiss, C. R. Downs and R. M. Burns, Ind. Eng. Chem., 1923, 15,965. P. Mars and D. W. van Krevelen, Chem. Eng. Sci., 1951, 3, 41. I. I. Ioffe and A. G. Lyubarskii, Kinet. Katal., 1962, 3, 261. R. Hayashi, R. R. Hudgins and W. F. Graydon, Can. J. Chem. Eng., 1963,41, 220. ’ I. I. Ioffe, Z. I. Ezhkova and A. G. Lyubarskii, Kinet. Katal., 1962, 3, 194. lo J. D. Butler and B. G. Weston, J. Catal., 1963, 2, 8. l1 B. Dmuchovsky, M. C. Freerks, E. D. Pierron, R. H. Munch and F. B. Zienty, J . Catal., 1965,4,291. l2 H. Schaefer, Ber.Bunsenges. Phys. Chem., 1967, 71, 222. l3 I. S. Jaswel, R. F. Mann, J. A. Juusola and J. Downie, Can. J . Chem. Eng., 1969, 47, 284. l 4 J. E. Germain and J. C. Peuch, Bull. SOC. Chim. Fr., 1969,6, 1844. l5 A. Bielanski and A. Inglot, React. Kinet. Catal. Lett., 197.7, 6, 83. l6 A. Inglot, React. Kinet. Catal. Lett., 1979, 11, 167. l7 L. Lucas, D. Vandervell and K. C. Waugh, J. Chem. SOC., Faraday Trans. I , 1981, 77, 15, 31. R. W. Petts and K. C. Waugh, J. Chem. SOC., Faraday Trans. 1, 1982,78, 803. le A. Miyamoto, Y. Yamazaki, M. Inomata and Y. Murakami, J. Phys. Chem., 1981,245, 2366. 2o M. Inomata, A. Miyamoto and Y. Murakami, J. Phys. Chem., 1981, 85, 2372. 21 M. Inomata, K. Mori, A. Miyamoto, T. Ui and Y. Murakami, J. Phys. Chem., 1983,87, 754. 22 M. Inomata, K. Mori, A. Miyamoto and Y. Murakami, J. Phys. Chem., 1983, 87, 761. 23 K. Mori, A. Miyamoto, T. Ui and Y. Murakami, J. Chem. SOC., Chem. Commun., 1982, 260. 24 K. Mori, A. Miyamoto and Y . Murakami, J. Phys. Chem., in press. 25 S. Yoshida, T. Iguchi, S. Ishida and K. Tarama, Bull. Chem. SOC. Jpn, 1972, 45, 476. 26 F. Roozeboom, T. Fransen, P. Mars and P. J. Gellings, Z. Anorg. Allg. Chem., 1979, 25, 449. 27 R. J. Cvetanovic and Y. Amenomiya, Adu. Catal., 1967, 17, 103. 28 M. Inomata, A. Miyamoto and Y. Murakami, J. Catal., 1980, 62, 140. 29 K. Tarama, S. Yoshida, S. Ishida and H. Kakioka, Bull. Chem. SOC. Jpn, 1969, 41, 2840. 30 L. D. Frederickson and D. M. Hansen, Anal. Chem., 1963, 35, 818. 31 V. A. Shvets, M. E. Sarichev and V. B. Kazansky, J. Catal., 1968, 11, 378. 32 J. H. Lunsford, Adv. Catal., 1972, 22, 265. 33 M. Che and A. J. Tench, Adu. Catal., 1982, 31, 77. 34 K. Tarama, S. Teranishi, S. Yoshida and N. Tamura, in Proc. 3rd Int. Congr. Catal., ed. W. M. H. 35 K. Hirota, Y. Kera and S . Teratani, J. Phys. Chem., 1968, 72, 3133. 36 A. Anderson, J. Solid State Chem., 1982, 42, 263. 37 A. Miyamoto, A. Hattori and Y. Murakami, J. Solid State Chem., 1983, 47, 373. Sachtler, G. C. A. Schmit and P. Zwieterling (North-Holland, Amsterdam, 1965), p. 282. 87-22668 OXIDATION OF BENZENE ON SUPPORTED v,o, 38 K. Mori, A. Miyamoto and Y . Murakami, Appl. Catal., 1983, 6, 209. 39 D. J. Cole, C. F. Cullis and D. J. Hucknall, J . Chem. SOC., Faraday Trans. I , 1976, 72, 2185. 40 A. Vejux and P. Courtine, J . Solid State Chem., 1978, 23, 93. 41 G. C. Bond, A. J. Sarkany and G. D. Parfitt, J . Catal., 1979, 57, 476. 42 G. C. Bond and K. Bruckman, Faraday Discuss. Chem. SOC., 1981,72, 235. 43 G. C. Bond and P. Konig, J . Catal., 1982, 77, 309. (PAPER 3/ 1990)

ISSN:0300-9599    

DOI:10.1039/F19848002655

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1984,80, 2669-2675 Enthalpies of Solution of Tetra-alkylammonium Salts in Hexamethylphosphotriamide at 298.15 K BY MAURIZIO CASTAGNOLO,* ANTONIO SACCO AND ANGELO DE GIGLIO Department of Chemistry, University of Bari, Via Amendola 173, 70126 Bari, Italy Received 1 1 th November, 1983 The enthalpies of solution of some tetra-alkylammonium salts and of tetraphenylphosphonium bromide in hexamethylphosphotriamide (HMPT) at 298.15 K have been obtained. Ionic enthalpies of transfer from water to HMPT and from HMPT to propylene carbonate (PC) have been calculated on the basis of the extrathermodynamic assumption AfC"(Ph,As+) = AKJBPh;). The results indicate that this assumption is valid for enthalpies of transfer from HMPT to PC, but not for transfer from water to HMPT.The enthalpies of transfer of tetra-alkylammonium ions from HMPT to PC are discussed in terms of the ability of HMPT to solvate alkyl chains. In a previous paper1 we reported the enthalpies of solution of some uni-univalent electrolytes in hexamethylphosphotriamide (HMPT) at 298.15 K. In order to obtain further information about ion-solvent interactions in such an interesting solvent we have determined the enthalpies of solution of some tetra-alkylammonium salts, choosing those whose rate of solution and solubility give reasonable results. EXPERIMENTAL MATERIALS The methods used for the purification of water,' hexamethylphosphotriamide' and dimethyl- sulphoxide (DMSO)2 have been given previously. Tetramethylammonium perchlorate (Me,NC10,) (K & K Laboratories), tetraethylammonium (Et,NClO,) and tetrahexylammonium (Hex,NClO,) (Fluka, purum) perchlorates were crystal- lized from acetone and dried in vacuo at 333 K for 24 h.Tetrapropylammonium iodide (Pr,NI) (Fluka, purum) was recrystallized from methanol +ether mixtures (1 : 1) and dried at 333 K in vacuo for 2 days. Tetrapropylammonium perchlorate (Pr,NC10,) (Eastman Kodak) was recrystallized from conductivity water and dried in vacua at 323 K for 24 h. Tetrabutylammonium chloride (Bu,NCl) (K & K Laboratories) was dissolved in a little acetone, reprecipitated from ether and dried in vacuo at 333 K for 48 h. All purifications and weighings of this salt were performed in a nitrogen-filled dry-box. Tetrabutylammonium bromide (Bu,NBr) (Fluka, puriss.) and tetra-amylammonium bromide (Am,NBr) (K & K Laboratories) were recrystallized from acetone +ether mixtures and dried in vacuo at 333 K for 48 h.Tetrabutylammonium iodide (Bu,NI) (Carlo Erba R.S.) was dissolved in acetone, reprecipitated from ether and dried in vacuo at 363 K for 48 h. Tetrabutylammonium perchlorate (Bu,NClO,) (Carlo Erba R.S.) was recrystallized from acetone +ether mixtures and dried in vacuo at room temperature for 48 h (melting point 486-487 K). Tetra-amylammonium iodide (Eastman Kodak) was dissolved in acetone, reprecipitated fromether and dried in vacuo at 323 K for 48 h. Tetraphenylphosphonium bromide (Ph,PBr) (Fluka, purum) was dissolved in anhydrous ethanol, reprecipitated from ether and dried in vucuo at 333 K for 48 h (melting point 570-571 K).Tetrahexylammonium bromide (Hex,NBr) (Fluka, purum) was washed with ether and dried in vucuo at room temperature for 3 days. Tetrabutylammonium tetraphenylboride (Bu,NBPh,) was prepared by precipitation 26692670 ENTHALPIES OF SOLUTION Table 1. Enthalpies of solution at 298.15 K (kJ mol-l) H2O HMPT PC Me,NC10, Et,NClO, Pr,NI Pr,NC10, Bu,NCl Bu,NBr Bu,NI Bu,NC10, Bu,NBPh, Am,NBr Am,NI Am,NC10, Hex,NBr Hex,NC10, Ph,PBr 49 .04a 30.75" 1 1.55b 27.24b - 29.87b - 8.49" 17.74a 10.75" 24.1 Od 3.77" 34.48a 42.97" 14.14a 50.63b 10.42e - 5.8 f 0.2 4.0 f 0.1 7.4 f 0.2 8.09 & 0.01 13.4 14.6 17.2 k 0.1 - 4.65 f 0.02 3.92 f 0.01 25.2 32.7 f 0.1 25.9 f 0.2 1.7k0.2 28.2 f 0.2 2.72 f 0.08 a Calculated from ref. (3); ref. (3); ref. (13); ref. (14); ref.(15). from Bu,NBr and sodium tetraphenylboride (NaBPh,) in water. The salt was dried in V ~ C U O at 353 K for 48 h. Tetra-amylammonium perchlorate (Am,NClO,) was prepared from Am,NBr and KClO, in water and dried in vucuo at 333 K for 3 days. The purities of Pr,NI, Bu,NCl, Bu,NBr, Am,NBr and Ph,PBr were checked by single measurements of the enthalpies of solution at 298.15 K in pure water. The purity of Me,NC10, and Et,NC10, was checked by single measurements of the enthalpies of solution in pure DMSO at 298.15 K. DMSO values were in agreement to within kO.2 kJ mol-l with the values reported in ref. (3). APPARATUS AND PROCEDURE The calorimetric apparatus and procedure have been described elsewhere.* All the salts studied dissolved rapidly, except Me,NClO, which dissolved within 15-20 min. The observed enthalpies of solution, AHs, were extrapolated to infinite dilution using the limiting Debye-Hiickel equation.The enthalpies of solution of Bu,NCl, Bu,NBr and Am,NBr were dependent on concentration. Conductometric measurements indicated that these salts are associated in HMPT, with association constants at 298.15 K of 157,5 55f3 and 59 & 2s mol-1 dm3, respectively. The enthalpies of solution at infinite dilution were obtained using an extrapolation method described by Wu and Friedman.' We attempted to measure the enthalpies of solution of tetramethylammonium chloride, bromide and iodide, tetraethylammonium bromide and iodide, tetrapropylammonium bromide and tetraphenylphosphonium perchlorate, but these salts were poorly soluble or dissolved too slowly in HMPT to allow their use with our calorimeter.All measurements were performed at 298.15 2 0.05 K. RESULTS Solution enthalpies, A e , of the tetra-alkylammonium salts and of Ph,PBr at in- finite dilution in HMPT at 298.15 K are reported in table 1. The mean deviation is given for each salt except Bu,NCl, Bu,NBr and Am,NBr, for which the extra- polation method described in the Experimental section makes the uncertainty in A X values hard to define. The solution enthalpies of the same electrolytes in water and in propylene carbonate (PC) at 298.15 K are also reported in table 1.M. CASTAGNOLO, A. SACCO AND. A. DE GIGLIO 267 1 Table 2. Enthalpies of transfer from H,O to HMPT at 298.15 K (kJ mol-l) - c1- Li+ Na+ K+ Rb+ cs+ Me,N+ Et,N+ Pr,N+ Bu,N+ Am,N+ Hex,N+ Ph,As+ Ph,P+ ionic values - 57.4 - 50.6 -46.6 -43.8 - 45.0 - 34.1 -6.1 1.6 5.3 3.6 - 1.8 -25.8 - 25.4 38.2 - 19.2 (- 19.2) - - - - - - - 43.3 (43.5) - - - - Br- I- ClO; BPh; 17.7 - 39.7 (- 39.7) - - - - - - - 23.1 (23.0) 21.4 (21.3) 15.8 (1 5.9) - 8.2 (-8.1) - 7.70 (- 7.7) - 5.8 - 62.8 (- 63.2) - 56.4 (- 56.4) - 52.3 (- 52.4) - - - - - 4.2 (-4.2) -0.5 (-0.5) - 1.8 (- 2.2) - -31.6 ( - 3 1.6) - - 20.7 -78.1 (- 78.1) - 70.9 (-71.3) - 67.7 (- 67.3) - 64.4 ( - 64.5) - 65.7 (- 65.7) - 54.8 (- 54.8) - 26.8 (- 26.8) - 19.15 (- 19.1) - 15.40 (- 15.4) - 17.1 (- 17.J) - 22.4 (- 22.5) - - -25.8 - - 76.4 (- 76.4) - 72.4 (- 72.4) - 69.7 (- 69.6) - 70.8 (- 70.8) - - - - 20.18 (- 20.5) - - - - The transfer enthalpies of all the salts studied in HMPT, presented according to the scheme recommended by Wu and Friedman* and Cox et aZ.,g are reported in tables 2 and 3.The ionic enthalpies of transfer from water to HMPT and from HMPT to PC at 298.15 K were calculated using the assumption Aer(BPhy) = AKr(Ph4As+) and are reported in tables 2 and 3. AGr values obtained by adding ionic AKr are given in parentheses. DISCUSSION The transfer enthalpies of the electrolytes studied in this paper are presented, together with those previously reported,' in tables 2 and 3. The ionic values for anions, alkali-metal cations and the tetraphenylarsonium ion are the same as those reported in ref. (1). As can be seen, the agreement between the experimental values and the calculated values is generally within 0.1 kJ mol-l, the exceptions being the electrolytes which presented experimental problems.Note that some disagreements are not present in both tables for the same salt so the differences probably reflect errors in A% values in the reference solvents rather than in HMPT. Ionic values for Me4N+, Et,N+ and Ph,P+ have been calculated from a single salt so their uncertainty is not easily defined.2672 ENTHALPIES OF SOLUTION Table 3. Enthalpies of transfer from HMPT to PC at 298.15 K (kJ mol-l) - Li+ Na+ K+ Rb+ c s + Me,N+ Et,N+ Pr,N+ Bu,N+ Am,N+ Hex,N+ Ph,As+ Ph,P+ ionic values 60.6 40.2 24.8 19.2 17.9 17.8 6.7 10.0 13.1 16.1 19.4 11.4 11.7 c1- Br- I- c10, BPh; - 12.0 - 3.9 48.6 56.3 (48.6) (56.7) 0.7 9.3 (1.1) (9.2) - 12.2 (12.2) 15.5 (1 5.5) 8.0 (7.5) 7.78 (7.8) - - - 2.5 63.1 (63.1) 42.8 (42.7) 27.2 (27.3) - - - - 12.4 (12.5) 15.6 (1 5.6) 18.6 (1 8.6) - 13.9 (13.9) - 4.3 64.9 (64.9) 44.2 (44.5) 29.4 (29.1) 23.4 (23.5) 22.5 (22.2) 22.1 (22.1) 11.0 (1 1 .O) 14.38 (14.3) 17.33 (1 7.4) 20.7 (20.4) 23.7 (23.7) - - 11.4 - 51.6 (51.6) 36.1 (36.2) 30.7 (30.6) 28.9 (29.3) - - - 23.95 (24.5) - - - - Let us consider the enthalpies of transfer from HMPT to PC.As can be seen in table 3, AGr(Ph,P+) is very similar to Aer(Ph4As+). This supports the equivalence of the extrathennodynamic assumption AGr(Ph,P+) = AKr(BPh;) and Aer(Ph4As+) = Aqr(BPh;). For the tetra-alkylammonium ions, AKr are positive and, with the exception of Aer(Me4N+), they increase regularly from Et,N+ to Hex4N+. For these values the equation Aer(R4,N+) = 0.788 4n + 0.46 0.1 kJ mol-1 (1) is valid, where n is the number of methylenic groups in each alkylic group of cations.The same behaviour had been observed by Friedman and coworkers for the enthalpies of transfer of tetra-alkylammonium halides and perchlorate from DMS03 and dimethylformamide (DMF)l0 to PC. With the exception of the tetramethylammonium salts, it was found that b e r of the tetra-alkylammonium salts wereelinear with respect to n. The intercepts of these straight lines, interpreted by Friedman and coworkers as the transfer enthalpies of Cl-, Br-, I- and C10, ions from DMSO and DMF to PC, are equal to the values obtained using the convention AKr(PH4As+) = AKr(BPh;). This is considered as an indication that, for the solvent pairs DMSO-PC and DMF-PC, the conventional ion-transfer enthalpies are closeM.CASTAGNOLO, A. SACCO AND, A. DE GIGLIO 2673 20 10 0 - I H 0 E - * -10 ?G U -20 - 30 0 1 2 3 4 5 6 n Fig. 1. Ionic enthalpies of transfer of tetra-alkylammonium ions from water to HMPT (O), from HMPT to PC (a), from DMSO to PC (A) and from DMF to PC (A) as a function of n, the number of carbon atoms in each alkyl chain. to the real ones. As a consequence of the agreement between the conventional ionic AKr values and those obtained by extrapolation to n = 0 of the AKr of the tetra-alkylammonium salts, the intercepts of Aer(R4N+) as a function of 4n for the systems DMSO-PC and DMF-PC are practically zero, as can be seen in fig. 1 . In our case the value of 0.46 kJ mol-1 for the constant term in eqn (1) shows that the convention adopted works for the obtainment of ionic enthalpies of transfer from HMPT to PC to within 0.5 kJ mol-l.Eqn (1) indicates that, with the exception of the tetramethylammonium ion, the enthalpies of transfer from HMPT to PC of the tetra-alkylammonium ions studied by us can be expressed as the sum of the contributions from each methylenic group, with A~r(-CH2-),MPT-PC = 0.79 kJ mol-l. This value can be compared with the values of -0.42, 0.22 and 0.0 obtained using the same method for AKr(-CH2-) to PC from DMS0,3 DMFlO and acetonitrile (ACN),ll respectively. Such acomparison indicates that, among the solvents considered, HMPT is the best solvent, in an enthalpic sense, for aliphatic chains. The anomalous behaviour of Me4N+, already observed in other organic solvents, indicates that the interactions of this ion with HMPT, a strongly basic solvent, are similar to those of an alkali-metal cation such as Rb+ or Cs+ with a high positive charge density.We now consider the results in table 2. Ionic enthalpies of transfer of tetra- alkylammonium ions from water to HMPT as a function of n are reported in fig. 1 .2674 ENTHALPIES OF SOLUTION Table 4. Ionic enthalpies of transfer from H,O to HMPT based on the convention Aqr(I-) = AGr(Cs+) at 298.15 K (kJ mol-l) Li+ - 37.8 Me,N+ Na+ -31.0 Et,N+ K+ - 27.0 Pr,N+ Rb+ - 24.2 Bu,N+ cs+ - 25.4 Am,N+ Hex,N+ Ph,As+ Ph,P+ 14.5 c1- 18.6 13.5 Br- - 1.9 21.2 I- - 25.4 24.9 ClO, -40.3 17.8 SCN- - 24.2 23.2 BPh, - 45.4 - 6.2 - 5.8 As can be seen, the overall shape is no longer a straight line as in the case of enthalpies of transfer from HMPT to PC.Such behaviour can be attributed to the water structure, where the contribution of hydrophobic interactions to the enthalpy of hydration is not a linear function of n. The non-linearity of the A G r values from water to HMPT as a function of n for tetra-alkylammonium salts and for tetra- alkylammonium ions indicates that it is impossible to obtain ionic A G r values by extrapolation, as in the case of the HMPT-PC transfers. However, following the previous procedure, we see that, even if we overlook A G r for the tetramethylam- monium ion, whose behaviour in water is anomalous with respect to that of other tetra-alkylammonium ions, the extrapolation of the water to HMPT AGr values gives a negative intercept (fig.1). This could be an indication that the solvation in water of tetraphenylarsonium ion is different from that of the tetraphenylboride ion, and so the extrathermodynamic assumption on which the ionic values in table 2 are based is not valid. It seems that, as in the studies carried out by Jolicoeur et a1.,12 the results for enthalpies of transfer from water to PC and from water to DMS03 are in agreement with this conclusion. If we assume, as suggested by Wu and Friedman,* that for transfer from water to non-aqueous solvents the assumption AKJI-) = AGr(Cs+) is valid, we can calculate the ionic A q r reported in table 4. As can be seen, AKr(BPh,) assumes a much more negative value than Aqr(Ph,As+), as already obtained for enthalpies of transfer from water to PC determined using a similar assumption in ref.(10). Note that preliminary measurements of enthalpies of solution of non-electrolytes in HMPT performed in our laboratory allowed us to calculate a value of -3.0 kJ mol-1 for the enthalpy of transfer of benzene from water to HMPT. The value of - 12 kJ mol-l for transfer from water to HMPT of 4 mol of benzene is in better agreement with the data for Ph,P+ and Ph,As+ in table 4 than with the data in table 3. However, the qualitative interpretation of enthalpies of transfer from water to HMPT reported in our previous paper' and that concerning tetra-alkylammonium ions still appear to be valid on the basis of new data in table 4. Inspection of these data shows that, among the tetra-alkylammonium ions, only the Me,N+ ion has an exothermic AK',, in agreement with its slightly structure-breaking character. M. Castagnolo, G. Petrella, A. Inglese, A. Sacco and M. Della Monica, J. Chem. Soc., Faraday Trans. I , 1983,79, 221 1 . G. Petrella, M. Petrella, M. Castagnolo, A. Dell'Atti and A. de Giglio, J. Solution Chem., 1981, 10, 129. C. V. Krishnan and H. L. Friedman, J . Phys. Chem., 1969,73, 3934. M. Castagnolo, G. Petrella, M. Della Monica and A. Sacco, J. Solution Chem., 1979, 8, 501. P. Miiller and P. Siegfiied, Helv. Chim. Acta, 1972, 55, 2965.M. CASTAGNOLO, A. SACCO AND. A. DE GIGLIO P. Bruno and M. Della Monica, Gazz. Chim. Ztal., 1974, 104, 757. Y-C. Wu and H. L. Friedman, J. Phys. Chem., 1966, 70, 2020. B. G . Cox, G. R. Hedwig, A. J. Parker and D. W. Watts, Aust. J. Chem., 1974, 27, 477. ' Y-C. Wu and H. L. Friedman, J. Phys. Chem., 1966,70, 501. lo C. V. Krishnan and H. L. Friedman, J. Phys. Chem., 1971,75, 3606. l1 M. H. Abraham, J. Chem. SOC., Faraday Trans. I , 1973, 69, 1375. l2 C. Jolicoeur, N. D. The and A. Cabana, Can. J. Chem., 1971, 49, 2008. l3 M. Castagnolo, A. Sacco and G. Petrella, J. Chem. SOC., Faraduy Trans. 1, 1981, 77, 9. l4 L. L. Brigth and J. R. Jezorek, J . Phys. Chem., 1975,79, 800. l5 S. Sunder, B. Chawla and J. C. Ahluwalia, J. Phys. Chem., 1974, 78, 738. 2675 (PAPER 3/2018)

ISSN:0300-9599    

DOI:10.1039/F19848002669

出版商:RSC    

年代:1984

数据来源: RSC