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/F198480FX009

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/F198480BX011

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

JOURNAL OF T H E CHEMICAL SOCIETY F A R A D A Y T R A N S A C T I O N S , P A R T S 1 A N D 11 The Journal of the Chemical Society is published in six sections, of which five are termed Transactions; these are distinguished by their subject matter, as follows: Dalton Transactions (Inorganic Chemistry). All aspects of the chemistry of inorganic and organometallic compounds ; including bioinorganic chemistry and solid-state inorganic chemistry ; of their structures, properties, and reactions, including kinetics and mechanisms; new or improved experimental techniques and syntheses. Faraday Transactions I (Physical Chemistry). Radiation chemistry, gas-phase kine tics, elect roc hemi st ry (0 t her than preparative), surface and inter facial chemistry, heterogeneous catalysis, physical properties of polymers and their solutions, and kinetics of polymerization, etc.Faraday Transactions I1 (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 Ncjtes section is not used for preliminary communications. The layout of a Note is the same as that of a paper. Short summaries are required. The procedure for submission, administration, refereeing, editing and publication of Notes is the same as for full papers.However, Notes are published more quickly than papers since their brevity facilitates processing at all stages. The Editors endeavour to meet authors’ wishes as to whether an article is a full paper or a Note, but since there is no sharp dividing line between the one and the other, either in terms of length or character of content, the right is retained to transfer overlong Notes to the full papers section. As a guide a Note should not exceed 1500 words or word-equivalents.NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participatingmember, has produced a set of recommendations in a pamphlet ‘Quantities, Units, and Symbols’ (1975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W 1V OBN).These recommendations are applied by The Royal Society of Chemistry in all its publications. Their basis is the ‘ Systeme International d’Unites’ (SI). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers.In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A , B, C , D, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 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 Index issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society’s editorial staff.(ii)1 THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 77 Interfacial Kinetics in Solution University of Hull, S l l April 1984 This Discussion will focus attention on reactions involving Iiquid-gas, Iiquid-liquid and Iiquid-solid interfaces (but it will not include electrode kinetics as such) The subject encompasses processes of fundamental, industrial and environmental importance and includes such topics as the rate of dissolution of reactive gases, kinetics at liquid membranes, metal and solvent extraction, Marangoni effects, heterogeneous catalysis and photocatalysis in solution, and the kinetics of dissolution of minerals and drugs The aim of the meeting is to bring together workers in these diverse fields to highlight the complementary nature of the problems encountered and of the results obtained, and to disseminate ideas concerning new and effective experimental techniques and novel theoretical approaches The programme and application form may be obtained from: Mrs Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 78 Radicals in Condensed Phases University of Leicester, 4-6 September 1984 Organising Committee Professor M C . R. Symons (Chairman) Dr K. A. McLauchlan Dr G. B. Buxton Dr T. A. Claxton Professor Lord Tedder Dr R. L. Willson The discussion will be primarily concerned with the structure and reactions of radicals in liquids and solids.It is designed to bring together theoretical work on structure, environmental effects and reactivity with spectroscopic and mechanistic studies directly concerned with radicals. Fundamental aspects will be stressed, and particular attention will be given to new developments including measurement at short time intervals, special solvent effects, and the effects of external fields. A special area for inclusion will be electron gain and loss processes including trapped and solvated electrons, electrochemical reactions, and specific electron capture and electron loss in low-temperature systems. Photochemical charge-transfer processes will also be included. The preliminary programme may be obtained from : Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBNTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO.1 9 Molecular Electronic Structure Calculations- Methods and Applications University of Cambridge, 12-1 3 December 1984 N.B. Please note change of date 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. It 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 preliminary programme may be obtained from : Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 7 9 Polymer Liquid Crystals 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 influence of external force fields such as flow, electric and magnetic fields. (d) The inter-relations of polymer liquid crystals with small-molecule mesophases, conventional flexible polymers and biopolymers which exhibit liquid-crystalline behaviour. Contributions are invited for consideration by the Organising Committee. A title and 300- word abstract should be submitted as soon as convenient and not later than 31 May 1984 Professor B.R. Jennings, Electro-optics Group, Department of Physics, Brunel University, Uxbridge UB8 3PH.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 80 Physical Interactions and Energy Exchange a t 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. Contributions for consideration by the Organising Committee are invited; titles should be submitted as soon as possible, and abstracts of about 200 words by 31 May 1984, to: Professor J. A. Morrison, Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada L8S 4M1~ ~ ~~ ~~~ ~~ FARADAY DIVISION INFORMAL AND GROUP MEETINGS Neutron Scattering Group Quantum Molecular Motion in Crystals and Intercalates and on Surfaces To be held at the University of Nottingham on 26-28 March 1984 Further information from Professor S.Clough, Department of Physics, University of Nottingham, Nottingham NG7 2RD Carbon Group (i) Sciences Related to the Usage of Pitch and Coke (ii) Pore Structure and Gas Flow in Graphites To be held at the Society of Chemical Industry, London on 29-30 March 1984 Further information from Dr H. Marsh, School of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU Statistical Mechanics and Thermodynamics Group Thermodynamics of Mixed Polymer Systems To be held at the University of Sheffield on 4-5 April 1984 Further information from Dr G. C. Maitland, Department of Chemical Engineering, Imperial College, London SW7 2BY Surface Reactivity and Catalysis Group Studies of Model Catalysts -Their Impact on Applied Catalysis To be held at UMIST on 8-1 0 April 1984 Further information from Dr J.C. Vickerman, Department of Chemistry, UMIST, PO Box 88, Manchester M60 1 OD Polar Solids Group with SERC Daresbury Laboratory Theory of Localized States in Condensed Matter To be held at Daresbury Laboratory, Warrington on 10-1 2 April 1984 Further information from Dr C. R. A. Catlow, Department of Chemistry, University College, London WC1 H OAJ Division with the Microprocessor Group Annual Congress: Electronic Processes in Thin Films and Novel Conductors To be held at the University of Exeter on 16-1 9 April 1984 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1 V OBN Electrochemistry Group with the SCI Electrochemical Technology Group Engineering Aspects of Electrochemistry To be held at Loughborough University of Technology on 17 April 1984 Further information from Professor N.A. Hampson, Department of Chemistry, Loughborough University of Technology, Loughborough LE11 3TU Polymer Physics Group Models of Polymer Deformation To be held in London at Easter 1984 Further information from Dr J. V. Champion, Department of Physics, City of London Polytechnic, 31 Jewry Street, London EC3 2EY Industrial Physical Chemistry Group The Metal-Polymer interface To be held at Girton College, Cambridge on 10-12 July 1984 Further information from Dr T. G. Ryan, ICI New Science Group, PO Box 11, The Heath, Runcorn, Cheshire WA7 4QEGas Kinetics Group 8th International Symposium on Gas Kinetics To be held at the University of Nottingham on 16-20 July 1984 Further information from Professor J.P. Simons, Department of Chemistry, University of Nottingham, Nottingham NG7 2RD Industrial Physical Chemistry Group Bacterial Adhesion To be held at Girton College, Cambridge on 11-1 3 September 1984 Further information from Dr I. D. Robb, Unilever Research, Port Sunlight, Wirral, Merseyside L63 3JW Electrochemistry Group with the SCI Electrochemical Technology Group E I ect rol yt i c Bubbles To be held at Imperial College, London on 13-1 4 September 1984 Further information from Dr G. H. Kelsall, Imperial College, London SW7 Polar Solids Group with British Ceramic Society, Institute of Ph ysics and Mineralogical Society of Great Britain Kinetics and Mass Transport of Silicate and Oxide Systems To be held at The Geological Society, London on 13-1 4 September 1984 Further information from Dr R.Freer, Department of Electrical and Electronic Engineering, North Staffordshire Polytechnic, Beaconside, Stafford STl8 OAD - Division with the Gas Kinetics Group Autumn Meeting: Combustion Chemistry in the Gas Phase To be held at the University of Hull on 18-20 September 1984 Further information from Dr R. W. Walker, Department of Chemistry, The University, Hull HU6 7RX Colloid and Interface Science Group with the SCI Foaming, Aeration and Dynamic Surface Tension To be held at Imperial College, London on 24-25 September 1984 Further information from Professor A.Bailey, Imperial College, London SW7 Division with the Deutsche Bunsen Gesellschaft fur Ph ysikalische Chemie, Societe de Chimie Ph ysique and Associazione ltaliana di Chimica Fisica Laser Studies in Reaction Kinetics To be held at the Evangelische Akademie, Tutzing, West Germany on 24-27 September 1984 Further information from Professor J. Troe, lnstitut fur Physikalische Chemie, Tammannstr 6, Gottingen, West Germany Electrochemistry Group with Statistical Mechanics and Thermodynamics Group The Electrical Double Layer To be held at the University of Southampton on 25-26 September 1984 Further information from Dr E. Dickinson, Procter Department of Food Science, University of Leeds, Leeds LS2 9JT Carbon Group Refactory Applications of Carbon To be held at the University of Sheffield on 2 5 2 7 September 1984 Further information from Dr B. Rand, Department of Ceramics, Glasses and Polymers, University of Sheffield, Sheffield S10 2TZ Neutron Scattering Group Neutron Scattering from Aqueous Solutions To be held at the University of Bristol on 27-28 September 1984 Further information from Dr G. W. Neilson, H. H. Wills, Physics Laboratory, University of Bristol, ,Tyndall Avenue, Bristol BS8 1TL (vii)~~ 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 30 October and 1 November 1984 Further information from Dr G. J. Lake, MRPRA, Brickendonbury, Hertford SG13 8ML Neutron Scattering Group Neutrons in Magnetism To be held at the University of Southampton in December 1984 Further information from Dr R. 6. Rainford, Department of Chemistry, University of Southampton, Southampton SO9 5NH Division Annual Congress: Solid-state Chemistry To be held at the University of St Andrews on 26-28 March 1985 Further information from Professor P. A. H. Wyatt, Department of Chemistry, University of St Andrews, The Purdie Building, St Andrews KY16 9ST (viii)

ISSN:0300-9599    

DOI:10.1039/F198480FP021

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I, 1984, 80, 521-529 Composition of Aqueous Solutions of Ammonium Sulphate and Sulphuric Acid BY JAN BALM*t AND FRANTISEK HANOUSEK Institute of Inorganic Chemistry, Czechoslovak Academy of Sciences, 16000 Prague, Czechoslovakia AND MILOS P I s ~ C I K AND KAMIL SARKA Institute of Inorganic Chemistry, Slovak Academy of Sciences, 80934 Bratislava, Czechoslovakia Received 1 1th October, 1982 Concentrations of SO:- and HSO; anions in aqueous solutions of ammonium sulphate and sulphuric acid have been determined by means of Raman spectroscopy. The molar ratio Po = mH2S04/m(NH4)nS04 and the total molality mT were in the range 1.0-2.5 and 3.38-18.21 mol kg-l, respectively. The results obtained have been used to establish the dependences of the stoichiometric dissociation quotient Q, = mH+mgo;/m,,,; and of the fraction a of total sulphate on the molar ratio Po and the total molality mT of the solution at 20 OC.Mixed ammonium sulphate + sulphuric acid aqueous solutions of various molar ratios are of importance in a number of industrial processes; for example, they constitute part of the scrubbing solutions for ammonia and sulphur dioxide absorption processes and find use in the production of artificial fertilisers and ammonium peroxodisulphate. The true ionic composition of these solutions is not, however, identical with the total analytical composition expressed in terms of the total content of sulphuric acid and ammonium sulphate. Since the second stage of sulphuric acid dissociation is incomplete, the solution contains H+, NH;, SO:- and HSO, ions whose concentrations depend on the equilibrium The true equilibrium constant of this reaction at 25 O C has most frequently been quoted to be in the range (1.02-1.04) x 10-2.1-s Covington et al. while the most recent critical study by Pitzer and Silvesterlo gives 1.05 x lop2. The temperature dependence of the dissociation constant3* 59 6* lo indicates that the degree of hydrogen sulphate dissociation decreases as the temperature is increased.To determine the equilibrium composition of real solutions, especially in the range of high concentrations, by means of the true equilibrium constant K2, one needs to know activity coefficients of the individual components in solutions of given total report a value of 1.06 x t Present address: Berchtoldstrak 17, D-8540 Schwabach, West Germany.52 1522 COMPOSITION OF (NH,),SO, + H,SO, MIXTURES compositions. However, these values are not always available and therefore the so-called stoichiornetric dissociation quotient Q , is frequently introduced : Q , = msoi- ~ H + / ~ H S O - (3) where mi denotes the true molality of ion i in the solution. Accordingly, K2 may be written as K2 = Qm YH+ YSO:-/YHSO; = Qm QY (4) where the activity quotient Q, = yH+~so;-/~Hso, expresses the deviations of the stoichiometric dissociation quotient Q , from the equilibrium constant K2 due to the deviations of the activity coefficients of individual ions yi from unity, especially in more concentrated solutions. There are a number of papers reporting values of the stoichiometric dissociation quotient Q , for when reaction (1) occurs in solutions of sulphuric acid alone and its mixtures with alkali-metal and ammonium sulphates3~ '7 11-15 and also with additions of other salts.ls Published results indicate that Q , increases with increasing total ionic strength up to a maximum and then decreases as the ionic strength is further increased.The results also show that the extent of dissociation of HSO, ions is largest in solutions of sulphuric acid alone, decreasing for other sulphates in the order H+ > NH+ > Na+ > K+.14 The extent of ionisation is reduced on the addition of other salts.lg The stoichiometric dissociation quotient Q, also decreases when the molar ratio Po = m2/ml decreases [where m, and m, denote the total analytical molality of (NH,),SO, and H2S04, respectively] ;3 this observation is consistent with the magnitude of Q , in solutions of sulphuric acid and hydrogen sulphates.l4+l5 Studies dealing with the system H2S04 + (NH,),SO, + H,0l2~ 14-16 present, however, data for solutions of molar ratio Po = 1.0 only, i.e.for NH,HSO, solutions. As part of extensive research into the theoretical foundations of the electrochemical production of peroxodi~ulphates,~~-~~ for which the knowledge of the true concen- trations of SO:- and HSO, ions in electrolysed solutions is of crucial importance for the evaluation of the kinetic equations for the individual electrode reactions, we carried out some measurements of the true compositions of mixed solutions of (NH,),SO,+H,SO, for various concentrations and molar ratios Po.The aim of this work was to derive, on the basis of the experimental data, an equation for calculating the true concentrations of SO:- and HSO; ions in such solutions from their molar ratio Po and the total molality mT. EXPERIMENTAL The true concentrations of SO:- and HSO; ions in solutions of known overall composition were investigated by recording the Raman spectra, which exhibit characteristic and well resolved peaks for both ions. The sulphate and hydrogen sulphate ion concentrations were determined from the integrated intensities of Raman lines at 981 and 1050 cm-l, respectively. The integrated intensities of the Raman lines were obtained by computer anaiysis of recorded spectra following Irish and Chen.14 First, the calibration graph was obtained for the relationship between the measured integrated intensity of the band at 981 cm-l for SO:- ions and the molality in pure (NH,),SO, solutions and good agreement was found.Similar results were obtained for pure solutions of H,SO,, which was used in the construction of a calibration graph for HSO; ion concentration. On the basis of earlier findings1, that aqueous solutions of pure ammonium sulphate are completely dissociated and that pure sulphuric acid solutions up to a concentration of 10 mol dm-3 (i.e. ca. 18 mol kg-l) are also completely dissociated into sulphate and hydrogen sulphate ions, leaving no undissociated H,SO, molecules, it was assumed that all solutions in our experiments with total molalities up to 18.2 are also completelyJ.BALEJ, F. HANOUSEK, M. PISARCIK AND K. SARKA 523 dissociated into SO:- and HSO, ions. Hence, the sum of the true molalities of SO:- and HSO; ions was assumed to be equal to the analytical molality of the solution under consideration "IS0:- + mHS0; = m, + 1112 = mT* (5) Because of the short time available, each sample of the mixed solutions of both components with various mT and p0 was measured only once. In some cases, however, the sum of the molalities of the SO:- and HSO, ions determined from the integrated intensities of the Raman lines using the calibration graphs was lower than the total analytical molality mT of the solution. As deviations were also observed for the most dilute solutions, where the absence of undissociated H2S0, molecules is beyond dispute, the discrepancy was therefore attributed to deviations in the measured integrated intensities arising from different positioning of the cell after changing the sample or from varying output of the light source.However, we assume that the deviations did not affect the magnitude of the fraction a of total sulphate species (6) because the factor by which the peak areas and hence the corresponding true concentrations of SO:- and HSO; ions are changed is the same for both ions during the single measurement of the given sample. Accordingly, values of the fraction a, along with the known total molality mT determined by chemical analysis of the measured solution, permit the calculation of true molalities of the two anions with sufficient accuracy a = mSO:-/(mSOq-+mHSO~) = mSO:-/mT and The spectra were recorded on a JEOL Raman spectrometer equipped with an argon laser (Ar+.488 nm, output on the sample ca.300 mW, sensitivity 8 x lo5 pulses per second, chart speed 100 cm min-l). A cell containing the solution was placed in a brass block maintained at 20k0.1 "C. The solutions were made up from ca. 96% sulphuric acid of analytical grade and ammonium sulphate of the same grade which had been additionally recrystallised and distilled water. The measurements were carried out on nearly saturated solutions with molar ratio Po = 1 .O, 1.5, 2.0 and 2.5, and on dilute solutions with the same molar ratios, the solutions being prepared by adding known amounts of water to the stock solutions. Each stock solution was analysed for the total molality mT.The composition of a dilute solution was calculated from the mass of the original stock solution and the amount of water added. The density of each solution was determined pyknometrically. RESULTS AND DISCUSSION The compositions of the solutions are given in table 1, along with experimental and calculated values of a. On the basis of the values of aeXptl, the true molalities of SO:- and HSO; ions and the corresponding values of the stoichiometric dissociation quotient Q,, exptl, expressed on the molality scale, were calculated using eqn (3), (7) and (8). For smoothing our experimental data for Q,, exptl the following correlation equation was assumed to be valid (9) loge,-log K, =-log Q, = Am&-5+Bm,+Cm$5+.... This equation was used in a recent correlation of the activity and osmotic coefficients of aqueous sulphuric acid.21 Fig.1 shows the dependence of the experimental values of the quantity ry = -log Qy/m0,.5 on m&-5. The same dependence for the more numerous data for solutions of ammonium hydrogen sulphate (Po = 1.0) and of sulphuric acid (6 = co) at 25 OC are also given, taken from measurements by Irish and Chen14. l5 after recalculating theirul h, P 8 v/ Qm f2 8 Table 1. Composition of the solutions and experimentally determined and recalculated values of a, v/ and Qm overall composition/mol kg-' a B density H2SO'l (NH4)2SO, mT Po /kg dm-3 exptl calc. exptl calc. exptl calc. n 6.9040 6.55 15 13.4555 1.0538 3.5080 3.3289 6.8389 1.0538 1.7354 1.6468 3.3822 1.0538 10.971 6 7.2341 18.2057 1.5167 5.8438 3.8532 9.9670 1.5167 2.6089 1.7203 4.3292 1.5167 10.9453 5.41 83 16.3636 2.0200 %i .3975 0.198 0.215 0.485 0.506 0.745 0.889 - .2730 0.307 0.302 0.731 0.725 1.009 0.974 2 .1633 0.375 0.351 0.989 0.950 0.814 0.689 e 0.151 0.145 0.462 0.455 1.154 1.076 Ls .4696 .3444 0.233 0.222 0.643 0.631 1.327 1.211 Ip .1987 0.288 0.307 0.887 0.864 0.864 0.980 4- .4594 0.171 0.153 0.530 0.512 1.717 1.454 Nx Ts 5.3285 2.6378 7.9663 2.0200 1.31 11 0.231 0.247 0.723 0.742 1.361 1.534 8 2.4441 1.2101 3.6542 2.0200 1.1775 0.296 0.310 0.992 1.012 0.974 1.061 - 1 1.4574 4.5315 15.9889 2.5282 1.4617 0.167 0.168 0.548 0.549 1.924 1.945 ' 6.62 16 2.6 190 9.2406 2.5282 1.3410 0.253 0.240 0.737 0.724 2.147 1.962 9 4.1087 1.625 1 5.7338 2.5282 1.251 1 0.297 0.294 0.900 0.897 1.769 1.739 EJ.BALEJ, F. HANOUSEK, M. PISARCIK AND K. SARKA 525 1.5 1.0 J, 0 . 5 0 1 1 0 2 4 6 8 Fig. 1. Plots of y~ against mg5 for various molar ratios 4: 1.0538 (a), 1.5167 (a), 2.02 (0) and 2.5282 (0) at 20 OC, and 1.0 (a) and m (0) at 25 'T, according to Irish and Chen.14*15 Lines: 1-4, as calculated from eqn (12) for the given values of Po at 20 "C; lines 5 and 6, as calculated from eqn (10) and (1 I), respectively. rn;' results in terms of molalities. For 20 "C, the value log K2 = - 1.907 98 and for 25 "C the value log K2 = - 1.978 8 1 were used as calculated from the general equation given by Pitzer et a1.lo As may be seen from fig. 1, there exists quite good agreement for Po = 1.0 between the more numerous data of Irish and Chen14 for 25 OC and our results for 20 OC.This may be taken as support for the reliability of our experimental data, although there exists greater scattering, and also for our approach to the evaluation of the fraction a as the primary experimental result. It may further be seen from fig. 1 that the dependence of v/ on m+5 follows a similar pattern for various Po, with the quantity ly decreasing as the total molality of the solution is increased, and shifting to higher values as Po is increased, in accordance with the curves for pure NH,HSO, and H,SO, solutions at 25 OC obtained by Irish and Chen.l4?l5 The experimental data of Irish and Chenl4? l5 have been fitted to eqn (9) with five coefficients, so that for NH,HSO, solutions the following expression v/ = 1.655 85 - 0.41 793 m$5 + 0.016388 mT + 0.002043 mk5 +0.00041156 m$ (10) and for H,SO, solution v/ = 1.75432-0.27002m!+5- 0.051 815mT+0.017934m+5-0.001 335m$ (1 1) have been obtained.* Our own experimental data for 20 OC, graphically smoothed with * For fitting the experimental data of osmotic and activity coefficients of H,SO, solutions at 25 O C the same eqn (9) with nine coefficients has been evaluatedg1 for the molality range 0.001-28 mol kg-'.526 COMPOSITION OF (NH,),SO, + H2S04 MIXTURES 't 0 - 1 4 3 - 2 1 I I I I 1 ~ 0 2 L 6 8 0.5 Fig.2. Plots of log Q, against n2g5 for various molar ratios Po: 1.0538 (a), 1.5167 (c)), 2.02 (0) and 2.5282 (0) at 20 O C , and 1 .O (0) and 00 (0) at 25 OC, according to Irish and Chen.l4. l6 Lines 1 4 , as calculated from eqn (15) for the given values of P, at 20 "C; lines 5 and 6, as calculated from eqn (14) and (1 3), respectively. respect to the course of similar curves for NH,HSO, and H,SO, solutions at 25 OC,14* l5 were fitted to eqn (9) with only three coefficients, because of the low number of measurements for any given value of Po.Expressing further individual coefficients as functions of the molar ratio Po, the following equation has been obtained for the dependence of the quantity on Po and mT at 20 OC v/ = -log Q,,/mp" = 1.5239+0.1616 Po-(0.4327+0.05091 P,)~Tz$~ +(0.03743+0.006515 Po)mT (12) valid in the range Po = 1.0-2.5 and mT = 3-18. This dependence for experimental values of Po is depicted in fig. 1 by the full lines, and similar dependences for NH,HSO, and H,SO, solutions at 25 O C according to eqn (10) and (1 1) are depicted by the dotted lines.As may be seen, the agreement between the experimental data and the correlation curves is quite good. From eqn (10)-(12), the expressions for the calculation of the stoichiometric dissociation quotient Q, from the given values of mT and Po may be obtained. Thus, for H,SO, solutions at 25 O C it follows that log Q, =-1.97881+ 1.75432m!$5-0.27002mT - 0.051 8 15 m+5 + 0.01 7934 m$ -0.001 335 mk5 (13) and for NH,HSO, solutions (8 = 1.0) at 25 OC log Q , = - 1.97881 + 1.65585.m$'-0.41793mT +0.016388 rnk5 +0.002043 m$+O.OOO411 56mg5 (14)J. BALEJ, F. HANOUSEK, M. PISARCIK AND K. SARKA 527 whereas for mixed solutions of (NH,),SO,+H,SO, at 20 "C log Q , = - 1.90798 +( 1 S239 + O . 1616 Po) mk5 -(0.4327+0.05091 P,)mT+(0.03743+0.006515 P,)mk5.(15) These are valid in the same ranges as eqn (12). The calculated plots of log Q , against m+5 for various Po are compared with the experimental data in fig. 2, where the calculated curves for mixed solutions at 20 "C are depicted by full lines and those for NH,HSO, and H2S04 solutions at 25°C by dotted lines. As can be seen, the correlation curves fit the experimental data quite well. As it is now possible to calculate the value of the stoichiometric dissociation quotient Qm knowing mT and Po, it is also possible to calculate the fraction a, thus allowing the computation of true molalities of both anions, mso;- and mHSOI, in mixed solutions (NH,),SO,+H,SO,. It is useful to start with eqn (3), into which the mo- lalities mso;- and mHSo4 given by eqn (7) and (8) and the molality mH+ given by mH+ = 2mTPo/(Po+ 1)-mT(l -a) = mT[P0(1 +a)-(l-a)]/(&+ 1) (16) are introduced.After rearranging we obtain For Po = 1, i.e. for NH,HSO, solutions, eqn (16) simplifies to Qm = mT a,/( 1 -a) which is generally valid for solutions of hydrogen sulphates of monovalent cations. From eqn (17) one obtains the final expression for calculating the fraction a for given values of mT and Po: (19) into which the appropriate value of Q, from eqn (1 5) is to be inserted. Comparison of the experimental values of a and those obtained from eqn (19) in table 1 shows reasonable agreement between two sets of data, as for the values of Qm. Some deviations are caused by errors in our method of measuring and evaluating the experimental data and also by the broader range of experimental errors in this method, as documented by Irish and Chen.14 Fig.3 shows plots of the calculated fraction a against the square root of the total molality at various molar ratios. Fig. 3 also shows the same dependence for pure solutions of sulphuric acid at 25 "C as to the data of Chen and Irish,15 in compari- son with its calculated curve (the dotted curve). In this case it is necessary to start with from which one obtains a = { - Qm -mT + [(em + m~), 4 m ~ Qm]"."}/2m~. (21) As can be seen, in all cases the fraction a goes through a minimum and a maximum with increasing total molality at a given Po, and then decreases above mT > 6. The distance between the extrema is greatest in the case of pure sulphuric acid solutions.With decreasing molar ratio Po the extrema become flatter, as has been observed by Wirth3 for mixed solutions of sulphuric acid and sodium sulphate (for mT d 4).528 COMPOSITION OF (NH4)$0, + H,SO, MIXTURES 0.8 0.6 (Y 0 . 4 0.2 0 i 0 2 1 6 8 m0.5 T Fig. 3. Dependence of the fraction a on rnk5 for various molar ratios P, at 20 OC, as calculated from eqn (15) and (19). Po = 1.0 (l), 1.5 (2), 2.0 (3) and 2.5 (4). Dotted line: H,SO, solutions at 25 "C, as calculated from eqn (13) and (21), in comparison with experimental data of Chen and Irish15 (0). Finally, we concluded that in spite of the scarcity of experimental data, the results enable us to gain reliable information about the real composition of mixed solutions of ammonium sulphate and sulphuric acid at higher concentrations and especially for molar ratios p0 > 1.0, for which it was not previously available.Simultaneously, the derived expressions enable us to calculate with sufficient reliability the true molalities of individual anions when we know the total molality mT and the molar ratio p0 of the solution under consideration. D. D. Pemn, Dissociation Constants of Inorganic Acids and Bases in Aqueous Solution (Butterworths, London, 1966), p, 200. H. E. Wirth, Electrochim. Acta, 1971, 16, 1345. J. M. Readnour and J. W. Cobble, Inorg. Chem., 1969,8, 2174. W. L. Marshall and E. V. Jones, J. Phys. Chem., 1966, 70, 4028. L. A. Pavljuk, B. S. Smoljakov and P. A. Kjukov, Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk, 1972, 3, 3. * M. H. Lietzke, R. W. Stoughton and T. F. Young, J. Phys. Chem., 1961,65, 2247. ' R. E. Lindstrom and H. E. Wirth, J. Phys. Chem., 1969,73, 218. * R. A. Robinson and R. H. Stokes, Electrolyte Solutions (Butterworths, London, 2nd edn, 1959), p. 385. A. K. Covington, J. V. Dobson and Lord Wynne-Jones, Trans. Faraday Soc., 1965,61, 2057. lo. K. S. Pitzer, R. N. Roy and L. F. Silvester, J. Am. Chem. Soc., 1977, 99, 4930. l1 C. F. Baes Jr, J. Am. Chem. Soc., 1959, 79, 5611. l 2 T. F. Young, L. F. Maranville and H. M. Smith, in The Structure of Electrolytic Solutions, ed. W. J. Hamer (Wiley, New York, 1959), p. 35.J. BALEJ, F. HANOUSEK, M. PISARCIK AND K. SARKA l 3 A. N. Fletcher, J. Inorg. Nucl. Chem., 1964, 26, 955. l4 D. E. Irish and H. Chen, J. Phys. Chem., 1970,74, 3796. l5 H. Chen and D. E. Irish, J. Phys. Chem., 1971,75,2672. 18 H. Chen and D. E. Irish, J. Phys. Chem., 1971,75,2681. J. Balej, M. Thumovd and H. Spalkova, Coll. Czech. Chem. Commun., 1981, 46, 2795. J. Balej, Coll. Czech. Chem. Commun., 1965, 30, 2663. lo J. Balej, and M. Kadeiavek, Coll. Czech. Chem. Commun., 1979,44, 1510. 2o J. Balej, CON. Czech. Chem. Commun., 1982, 47, 1539. B. R. Staples, J. Phys. Chem. Ref. Data, 1981, 10, 779. 529 (PAPER 2/ 1754)

ISSN:0300-9599    

DOI:10.1039/F19848000521

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1984, 80, 531-541 Infrared Study of the Interactions between NO and CO on Rh/Al,O, Catalysts BY EDWARD A. HYDE AND ROBERT RUDHAM Department of Chemistry, The University, Nottingham AND COLIN H. ROCHESTER* Department of Chemistry, The University, Dundee DD1 4HN Received 6th April, 1983 Infrared spectra of NO and NO+CO mixtures on Rh/Al,O, are reported. At ambient temperatures NO alone was adsorbed non-dissociatively and gave no detectable reaction products. At T 2 413 K NO underwent partial decomposition to N,O(g) at surface sites which were rapidly poisoned as the reaction proceeded. The addition of NO to preadsorbed CO eliminated spectral bands characteristic of linear and bridged carbonyl species, but maxima due to gem-dicarbonyl species remained.The main products of the CO + NO reaction detectable by infrared spectroscopy were N,O(g), adsorbed isocyanate and CO,, either gaseous or chemisorbed. The extent of isocyanate formation depended on the relative amounts of NO and CO present, the gas-phase pressures of NO and CO and the order of addition of NO and CO to the Rh/Al,O, catalyst. The order of addition did not affect the generation of N,O. The results are discussed in terms of reaction mechanism and types of catalytically active adsorption site. Dissociative adsorption of CO, on Rh dispersed on alumina led to an adsorbed CO species giving an infrared band at 2015 cm-'. The reaction on supported rhodium catalysts between carbon monoxide and nitric oxide to give nitrogen and carbon dioxide is incompletely understood, although of great importance with respect to vehicle emission-control catalysis.The most detailed relevant information concerns the non-dissociative adsorption of carbon monoxide which has been well characterized by infrared spectr~scopy.~-~ The formation of bridged species Rh,CO, linear species RhCO and gem-dicarbonyl species Rh(CO), depends on the preparation of the samples which in turn influences the dispersion of the metal. Significant factors are the size of the rhodium particles or two-dimensional arrays of rhodium atoms, interactions between metal atoms and the oxide support and the relative areas of different exposed crystal faces. Dissociative adsorption of carbon monoxide may also Nitric oxide is both dissociatively and non-dissociatively adsorbed on rhodium, and nitrogen, nitrous oxide and oxygen have been detected as products of subsequent desorption at elevated temperature^.^, 11-13 Infrared studies have been less detailed than for the adsorption of carbon monoxide but three bands in the 1700-2000 cm-l spectral region have been reported and are generally assigned to NO stretching vibrations of RhNO+, RhNO and RhNO- surface species.l*q l5 However, results from a study of interactions between nitric oxide and a RhY zeolite suggest that two of the bands could be attributed to the symmetric and asymmetric stretching vibrations of a gem-dinitrosyl complex, Rh(NO),, of rhodium.16 Evidence exists for the simultaneous adsorption of a nitric oxide and a carbon * probably at step or defect surface sitesg* lo 53 1532 REACTION OF co +NO ON RHODIUM monoxide molecule at the same rhodium atom to give a surface Rh(NO)(CO) species.14* l5 Mixtures of nitric oxide and carbon monoxide adsorbed on supported rhodium also lead to the appearance of infrared bands due to isocyanate species,14-17 probably adsorbed on the surface of the oxide 18* l9 A band at 2195 cm-l has been ascribed to the transitory appearance of isocyanate species liganded to rhodium.15 Excess of nitric oxide apparently inhibits the formation of isocyanate,15 possibly because dissociative adsorption of nitric oxide is increasingly unfavourable with increasing nitric oxide pres~ure.~~ l2 A previous paper7 reports a study of the adsorption of hydrogen and carbon monoxide on Rh/Al,O, catalysts prepared from dispersions of rhodium nitrate, rhodium sulphite and chloropentamminerhodium(II1) chloride on alumina.This study of the adsorption of carbon monoxide plus nitric oxide mixtures on Rh/Al,O, samples prepared by reduction of dispersions of the same three rhodium salts was aimed at probing the involvement of distinguishable forms of adsorbed carbon monoxide and nitric oxide in the formation of Rh(CO)(NO) and isocyanate species, and in assessing to what extent the various surface species were intermediates in the overall catalytic reaction to nitrogen and carbon dioxide. EXPERIMENTAL Rhodium (1.2 wt %) supported on y-alumina (Condea Chemie, surface area 132 m2 g-l) was prepared as before’ by reduction of dispersions of rhodium nitrate, rhodium sulphite and chloropentamminerhodium(m) chloride on alumina in a flow of hydrogen (1 atm,? 773 K, 1 h).Self-supporting discs compacted at 138 MN m-2 were at ca. 303 K during spectroscopic examination with a Perkin-Elmer 125 spectrophotometer. Nitric oxide was purified by passage over potassium hydroxide and repeated fractional distillation20 until the only impurity detectable by infrared spectroscopy was nitrous oxide at < 0.01%. Carbon monoxide, nitrous oxide and carbon dioxide (all research grade X) and nitrogen (spectroscopic purity) were used as received in 1 dm3 bulbs. RESULTS The spectroscopic results for the adsorption of NO and NO+CO mixtures on Rh/Al,O, prepared by reduction of Rh(NO,),/Al,O,, Rh,(SO,),/Al,O, and [Rh(NH3),C1]C1,/A1,0, were not significantly affected by the nature of the salt precursor to rhodium metal.Even for Rh,(SO,),, which underwent incomplete red~ction,~ the same infrared bands were observed as for samples prepared from the other two salts and no new bands, absent for the other salts, appeared in the spectra. In general, the infrared bands were more intense for Rh/Al,O, derived from Rh(NO,),/Al,O, and therefore it is primarily for these samples that spectra are shown here. Isotherms for the adsorption of carbon monoxide and hydrogen have established that, of the three salts, Rh(NO,), gives the most highly dispersed rhodium with the greatest number of exposed adsorption sites.7 Spectra of NO adsorbed on y-Al,O, which had been preheated in hydrogen at 773 K exhibited weak bands at 1590 and 1230 cm-l.The latter compares with a band at 1220-1 240 cm-l previously reported for NO on alumina which had been pre-evacuated at > 673 K 2 1 Three strong broad bands at 1910, 1830 and 1740 cm-l for NO on Rh/Al,O, (fig. 1) are assigned to NO stretching vibrations of NO molecules bonded to surface rhodium atoms.14-16 The exact band positions were a function of surface coverage. At low coverages a single maximum [fig. 1 (b)] appeared at ca. 1865 cm-l t 1 atm = 101 325 Pa.E. A. HYDE, R. RUDHAM AND C. H. ROCHESTER 533 shifting towards 1910 cm-l [fig. 1 (c) - (e)] with increasing pressures of NO. At high pressures the maxima at 1910 and 1830 cm-l contained contributions from the band envelopes of the R and P branches due to the fundamental stretching vibration of NO gas, which also gave a weak maximum at 1876 cm-l [fig.1 ( f ) ] . 2 2 Adsorbed NO was largely desorbed by evacuation at ca. 303 K although a weak band remained at 00 1800 2000 1800 wavenum berlcm -' Fig. 1. Infrared spectra of NO on Rh/A1,0, at equilibrium pressures/kN rn-, of (a) 0 ; (6) 0.051, (c) 0.205, ( d ) 1.31, (e) 8.81 and cf) 34.3, followed by subsequent desorption to (g) 8.2 N m-, and (h) evacuation (5 min). Prolonged contact between NO gas and Rh/Al,O, for several days at room temperature failed to give any reaction products in the gas phase with detectable infrared spectra. However, heat treatment of Rh/A120, at 413 K in NO (13 kN m-,) for 30 min generated small amounts ( < 0.15 %) of N,O gas. The intensities of maxima at 2218 and 2240 cm-l due to the P and R branch band envelopes of one of the fundamental vibrations of N20 decreased in intensity after subsequent heat treatment in NO at 633 K (30 min) and became very weak after heating the disc at 823 K (30 min) in NO.The removal of N,O was accompanied by the appearance of a band at ca. 1617 cm-l characteristic of NO,, also at very low partial pressure compared with the added pressure of NO. Infrared bands due to the fundamental vibration of NO gas were unchanged in apparent intensity during the sequence of thermal treatments, confirming that an undetectably small fraction of the NO had undergone reaction. However, in accordance with previous results,14$ l5 heat treatment of Rh/Al,O, in NO at increasing temperatures progressively caused the desorption of adsorbed species responsible for the infrared bands at 1830 and 1740 cm-l and the species were not reformed when the disc was cooled in NO from the treatment temperature to ca.303 K for spectroscopic examination. In contrast, the band at 1910 cm-l became more prominent and shifted to 1920 cm-l. Fig. 2 shows spectra of Rh/Al,O, after the addition of CO to adsorbed NO. After NO adsorption the gas pressure was reduced to a level for which the bands due to534 REACTION OF co +NO ON RHODIUM gas phase NO made negligible contributions to the spectrum but the bands at 1910, 1830 and 1740 cm-l due to adsorbed NO remained prominent [fig. 2(a)]. These three bands were considerably reduced in intensity following the addition of CO. The appearance of maxima at 2095 and 2025 cm-l due to vibrations of gem-dicarbonyl contrasted with tlie corresponding result in the absence of preadsorbed NO 1900 1700 wavenum ber/cm -* Fig.2. Infrared spectra of Rh/Al,O, after (a) exposure to NO (2.03 kN m-2) and evacuation to 16 N md2, followed by the addition of CO to total gas-phase pressures (in N m-2) of (b) 33, ( c ) 430 and ( d ) 980 N m-2. where a maximum at ca. 2050cm-l due to linearly adsorbed CO dominated the spectrum at low surface coverages7 The formation of an appreciable surface population of the linear complex RhCO was apparently unfavourable in the presence of NO. With increasing pressure of CO the maxima at 2095 and 2025 cm-l grew in intensity [fig. 2(c)] and showed evidence for splitting into four bands at 2100, 2085 (sh), 2040and 2010 cm-l [fig.2(d)]. Bands whichappeared at 2260,2235 and 2185 cm-l [fig. 2(c) and (d)] may be ascribed to products of a reaction between NO and CO catalysed by rhodium. The reaction was not catalysed by alumina alone in the absence of rhodium. The band at 2235 cm-l contained a contribution from the spectrum of a low partial pressure of nitrous oxide which was formed as a reaction product in the gas phase. Similarly the spectrum of CO gas made a small contribution to the band at 2 185 cm-'. However, the three bands were primarily due to vibrations of adsorbed products of reaction. Similar bands have previously been ascribed to isocyanate species.14-17 Increasing the CO pressure to 5.5 kN m-2 had little effect beyond the result shown in fig. 2(d), and subsequent evacuation reduced the intensities of the bands due to adsorbed isocyanate to approximately one-half their initial values before evacuation.Residual gem-dicarbonyl species were also retained on the surface after evacuation and a broad weak region of absorption remained in the spectra at 1 940- 1 700 cm-l. The maximum at 2260 cm-l was absent from spectra recorded after exposure of Rh/Al,O, to NO at high pressure [fig. 3(a)] followed by the admission of CO with the NO still present in the gas phase [fig. 3(b)]. Furthermore, the band at 2185 cm-' was not discernible and the maximum at 2235 cm-l could be largely ascribed to N20E. A. HYDE, R. RUDHAM AND C. H. ROCHESTER 535 in the gas phase [fig. 3(e); the spectrum of N,O gas overlaps the spectrum of CO in the 2000-2300 cm-l spectral region].Solymosi and Sarkany have reported a similar result for NO+CO on 5% Rh/A1,0, prepared from rhodium(II1) ch10ride.l~ Non- dissociatively adsorbed NO was gradually displaced from the rhodium surface [fig. 3(b) and (c)] and reductions in the intensities of the bands due to gem-dicarbonyl - 60- E z E + 40- s 0 .- m v) .3 8 0 / I b J - L 1 I I I I 2 0 k 2100 2000 2100 2000 1900 1700 wavenum ber/cm-' Fig. 3. Infrared spectra of Rh/A1,0, (a) in equilibrium with NO [8.12 kN m-2; spectrum ( d ) is of gas phase alone], (b) 30 min and (c) 19 h after the subsequent addition of CO to give a total pressure of NO+CO of 19.0 kN md2. (e) Spectrum of gas phase after an experiment similar to that which led to (c). species also occurred. The maximum at 2235 cm-l grew in intensity because of the continued generation of both N,O gas and some surface isocyanate contributing to the absorption intensity at that position. A new maximum also developed at 1590 cm-l When Rh/Al,O, discs were exposed to mixtures of NO and CO several usually weak bands appeared in the spectral region 1700-1200cm-1.The exact positions and relative intensities of the bands were a function of the particular experiment being carried out. Typical band positions were 1645, 1580, 1480, 1305 and 1230 cm-l, which compare with bands at 1638, 1477 and 1238 cm-I for CO, adsorption and at 1655, 1580, 1290 and 1230 cm-l for NO, adsorption on alumina which had been preheated in hydrogen at 773 K. Parkyns,' has reported that NO, on alumina gave two prominent bands at ca. 1600 and 1240 cm-l which were assigned to vibrations of nitrate ions.Exposure of Rh/Al,O, to N,O gave no infrared bands which could be ascribed to products of chemisorption on either rhodium or alumina although a band at 2242 cm-l due to weakly adsorbed N,O on alumina,, was observed. The present results suggest that trace amounts of surface carbonate and nitrate were formed on the alumina support during prolonged contact between NO plus CO and Rh/Al,O, at room temperature. Slight increases in absorption intensity at ca. 2350 cm-l, which [fig- 3 (c)l *536 REACTION OF co 4- NO ON RHODIUM were reversed by removal of the gaseous mixtures from contact with Rh/Al,O,, also provided evidence for the formation of trace amounts of CO, in the gas phase.The addition of NO to Rh/Al,O, which was already in contact with CO [fig. 4(a)] resulted in the disappearance of the infrared band at ca. 2050 cm-l due to linear RhCO species (fig. 4(b)]. Arai and Tominaga14 reported the same effect and also observed 2200 . 2000 1900 1700 wavenum berlcm-' Fig. 4. Infrared spectra of Rh/Al,O, (a) in equilibrium with CO (23 N m-2) and (b) 20 min and (c) 77 min after the subsequent addition of NO to give a total pressure of NO+CO of 158 N m-,. that heat treatment of Rh/A1,0, after the addition of CO followed by NO led to the appearance of a new infrared band at 2101 cm-l. The generation here, at ambient temperature, of the species responsible for this band was suggested by the marked increase in intensity at ca. 2100cm-l which coincided with the already existing maximum due to gem-dicarbonyl species7 A strong maximum at ca.1830 cm-l (fig. 4) due to NO stretching vibrations of adsorbed NO simultaneously appeared and dominated the 1950-1700cm-1 spectral region for low pressures of NO. At high pressures bands due to adsorbed NO also appeared at ca. 1905 and 1730 cm-l. Evidence for the displacement of bridged species Rh,CO [broad shoulder centred at 1895 cm-l in fig. 4(a)] was not always easy to identify because of overlap between broad bands due to adsorbed CO and NO. However, a maximum at 1890 cm-l due to Rh,CO species was particularly prominent in spectra of samples prepared by reduction of Rh,(SO,),/Al,O, and then exposed to C0.7 The maximum was increasingly reduced in intensity with increasing pressures of added NO, confirming14 that bridged CO was removed from the rhodium surface after exposure to NO.Gem-dicarbonyl species were more resistant than bridge-bonded or linearly adsorbed CO to removal by NO. However, reductions in the intensities of the bands at 2100 and 2035 cm-l occurred with time [fig. 4(c)] and were enhanced when the NO was in excess over the CO present. The destruction of gem-dicarbonyl species by NO was best exemplified in one experiment involving Rh/A1,0, derived from [Rh(NH,),Cl]Cl,/Al,O,. The disc was exposed to CO (29.3 kN m-,) and then evacuated at room temperature to remove bridged CO and a high proportion of linearly adsorbed CO. Bands remaining in the spectra at 2100 and 2035 cm-l [fig. 5 (a)] could be assigned primarily to Rh(CO), species although the latter also contained aE.A. HYDE, R. RUDHAM AND C. H. ROCHESTER 537 contribution due to RhC0.' The bands showed a progressive decrease in intensity with increasing added pressures of NO [fig. 5(b)-(e)] until the carbonyl species had been completely destroyed [fig. 5 ( f ) ] . At the same time maxima at 19 10,1830 and 1740 cm-l due to adsorbed NO increased in intensity, the band at 1830 cm-l being the most prominent at low pressures of NO but undergoing less growth relative to the other two bands as CO was increasingly displaced from the surface by NO. r-- ,Ot - 2300 2100 I ' / I I 1900- 1700 2200 2000 wavenum berlcm-' Fig. 5. Infrared spectra of Rh/Al,O, derived from [Rh(NH,),Cl]Cl,/Al,O, after (a) exposure to CO (29.3 kN m-,) and evacuation (16 h, 293 K), (b)-Cf) subsequent contact with NO at pressures/kN m-, of (b) 0.24 (8 min), (c) 0.24 (24 min), ( d ) 1.31 (7 min), (e) 1.31 (21 min) and cf) 2.57 (24 min), followed by evacuation and readmission of CO at pressures/kN rn-, of (g) 0.33 (10 min) and (h) 2.21 (36 min).The formation of surface isocyanate species by interactions between NO and preadsorbed CO apparently depended on the existence of linearly bonded and/or bridge-bonded CO on the rhodium surface. If the CO pressure was sufficient to ensure the presence of these species [fig. 4(a)] then the subsequent addition of NO led to the appearance of bands at 2260 and 2235 cm-' [fig. 4(b)] due to isocyanate. In contrast only a very weak broad increase in absorption intensity at 2230-2260 cm-l occurred [fig.5 ( b ) - ( f ) ] for Rh/Al,O, which had been exposed to CO (29.3 kN m-,) and subjected to evacuation before increasing pressures of NO were admitted to the cell. The displacement of gem-dicarbonyl species by the adsorption of NO was not accompanied by the formation of adsorbed isocyanate groups. However, partial removal of NO gas by brief evacuation followed by the readmission of CO led to the appearance in the spectra [fig. 5 (f)] of bands at 2260,2235 and 2 185 cm-l characteristic of isocyanate groups. An analogous result was obtained after the exposure of Rh/Al,O, to high pressures of NO followed by addition of CO, brief evacuation to remove both gases but not adsorbed species, and readmission of CO alone. Bands at 2260 and 2235 cm-l were present in the spectra after the second contact with CO whereas the band at 2260 cm-l was absent (fig.3) when NO was also present and had been admitted first.538 REACTION OF CO+NO ON RHODIUM Equilibration of Rh/Al,O, with NO at high pressure before mixing with CO inhibited the formation of isocyanate species responsible for the band at 2260 cm-l (fig. 3). However, this species was formed if the order of addition of the gases was reversed. Fig. 6 compares the spectra resulting from two experiments in which the final mixtures of CO and NO were identical. When NO was added first the 2260 cm-l 22002000 2 m 2 0 0 0 1900 1 700 wavenumberlcm-’ Fig. 6. Infrared spectra of Rh/Al,O, after (a) addition of CO (9.3 kN m-2) followed by NO (9.3 kN m-2) and (b) subsequent evacuation; (c) addition of NO (9.3 kN m-2) followed by CO (9.3 kN m-2) and ( d ) subsequent evacuation.(a’Hd’) Corresponding spectra of gas-phase alone in contact with discs when spectra (a) - ( d ) were recorded. band was non-existent or very weak but the clear maximum at 2235 cm-l was observed [fig. 6(c)] and could partly be ascribed to N,O in the gas phase [fig. 6(c’)]. Removal of the gas phase established the existence of a narrow band at 2235cm-l due to adsorbed isocyanate [fig. 6(d)]. In contrast, the addition of CO first followed by NO led to the appearance of considerably stronger bands at both 2260 and 2235 cm-l [fig. 6(a)] despite the fact that the amounts of N,O formed in the gas phase were similar in the two experiments [fig.6(a’) and (c’)]. The isocyanate species were again resistant to desorption by evacuation at ambient temperature [fig. 6(b)]. Because of the time dependence of the formation of N,O and surface isocyanate (fig. 3 ) the growth in intensities of the bands at 2235 cm-l were determined as a function of time until the rates of growth were negligible (ca. 1 h) when the spectra in fig. 6(a) and (c) were recorded. The half-life for the attainment of the maximum intensity at 2235 cm-l was ca. 8 min (at ca. 303 K) in both experiments. Exact comparisons of the relative intensities of the three bands ascribed to adsorbed NO were difficult because differing scattering effects from disc to disc significantly varied the slopes of the baseline spectra. However, in experiments involving highE. A.HYDE, R. RUDHAM AND C. H. ROCHESTER 539 pressures of CO and NO a maximum at ca. 1720-1 730 cm-l was more intense, relative to the maxima at 1910 and 1830 cm-l, in spectra of Rh/Al,O, exposed to NO and CO [fig. 3 (b)] than to NO alone [fig. 3 (a)]. In general, the spectra of non-dissociatively adsorbed CO (2100 and 2035 cm-l) andNO (1910,1830 and 1740 cm-l) on Rh/Al,O, in the presence of both gases were similar whether CO or NO was added to the catalyst first (fig. 6). The apparent enhancement of the intensity of the maximum at 1740 cm-l was more pronounced when CO was in excess over NO with total pressures of ca. 16-19 kN rn-, and pressure ratios in the range 1.0 < (pco/pNo) < 2.1. P I I I I 2200 2000 1900 1700 wavenum ber/cm -' Fig.7. Infrared spectra of Rh/Al,O, (a) in equilibrium with CO (1 1.2 kN rn-,), (6) after 64 h contact with CO (1 1.2 kN m-,) plus NO (5.25 kN m-,) and (c) - (s) a consecutive series of heat treatments in vacuum at temperatures/K of (c) 303 (30 min), ( d ) 380 (30 min), (e) 448 (30 min), cf) 303 (43 h) and (g) 469 (30 min). Desorption of isocyanate species by evacuation at elevated temperatures (fig. 7) generated no products, either in the gas phase or adsorbed state, other than those formed in the reactions at ambient temperatures. The two types of isocyanate responsible for the bands at 2260 and 2235 cm-l were similarly affected by thermal activation. The form of non-dissociatively adsorbed NO giving a band at ca. 1900 cm-l was most resistant to desorption. The enhancement in band intensity at 1900 cm-l following heat treatment at 380 K [fig.7(d)] was consistent with the present and previously reported1* effects of heating Rh/Al,O, in NO alone. Residual bands at 2090 and 201 5 cm-l after exposure of Rh/Al,O, to NO + CO and evacuation at high temperature [fig. 7cf> and (g)] emphasized an important general aspect of all the present results. The maxima assigned to vibrations of gem-dicarbonyl species Rh(CO), were at 2100 and 2035 cm-l in the absence of NO.' However, in the presence of NO the maxima were broadened towards lower wavenumbers with either resulting shifts in the positions of the maxima in the overall540 REACTION OF co + NO ON RHODIUM band envelopes or the appearance of distinct shoulders (fig. 2) due to contributions to the spectra from two new bands.The desorption spectra (fig. 7) confirmed the existence of these bands at 2090 and 2015 cm-l. The former was consistent with the growth in absorption intensity at ca. 2100 cm-l and the broadening of the band at 2100 cm-l towards lower wavenumbers when NO was added to Rh/Al,O, already exposed to CO (fig. 4). Spectra of CO, (13-17 kN m-,) adsorbed on Rh/Al,O, exhibited a band at 201 5 cm-l confirming,24* 25 despite a report to the contrary,17 that dissociative adsorption of CO, which occurs on rhodium single-crystal surfaces9* l1 also takes place on supported rhodium. Iizuka and Tanaka24 attributed a similar band at 2020 cm-l in spectra of CO, on Rh/A1,0, to RhCO species resulting from dissociative adsorption of CO,.The intensities of the present band for a given CO, pressure were greatest for Rh/Al,O, prepared by reduction of Rh(NO,),/Al,O,, less for reduced [Rh(NH3),C1]C1,/A1,0, and least for reduced Rh,(SO,),/Al,O,. This sequence is consistent with the relative surface areas of the dispersed rhodium prepared from the three precursor salts.' No bands due to CO on Rh other than that at 2015 cm-l were observed following the chemisorption of CO, at ca. 303 K. DISCUSSION The existence of infrared bands at 1910, 1830 and 1740 cm-l in spectra of NO adsorbed on Rh/A1,0, was consistent with the results of previous studies of Rh/A1,0,l47l5 and a RhY zeolite.ls For NO on Rh/A1,0, similar bands have been ascribed to surface RhNO+, RhNO and RhNO- species, re~pective1y.l~~ l5 However, isotopic labelling experiments for NO on a RhY zeolite strongly suggested that maxima at 1860 and 1780 cm-l could be assigned to vibrations of a gem-dinitrosyl complex Rh(NO), in which two NO molecules were simultaneously liganded to a single Rh' ion in the zeolite.ls Evidence for the assignment of the present bands at 1830 and 1740 cm-l to Rh(NO), species is provided by a comparison of results for the adsorption of CO and NO on Rh/Al,O,. Uptakes of CO by Rh/Al,O, prepared by the reduction of Rh(NO,),/Al,O, showed that a high proportion of the exposed Rh atoms were each able to act as a site for the adsorption of a pair of CO molecules to give a gem-dicarbonyl complex Rh(CO),.7 Bands at 2100 and 2035 cm-l in the infrared spectrum of adsorbed CO confirmed7 the presence of Rh(CO), after CO adsorption.s Yao et a1.26 have reported (NO/Rh) and (CO/Rh) values for NO and CO adsorption on alumina-supported Rh.The data showed that for samples similar to those studied here [(CO/Rh) z 1.6817 the values of (NO/Rh) exceeded those of (CO/Rh), providing strong evidence that NO also undergoes multiple adsorption on highly dispersed Rh. The assignment of bands at 1830 and 1740cm-l to vibrations of a rhodium dinitrosyl complex would be consistent with infrared spectra of dinitrosyl complexes of transition metals. Typically two bands are observed in the approximate ranges 1862-1 742 and 1764-1 699 cm-l and with a separation of ca. 41-107 cm-1.27-29 By analogy with proposals for gem-dicarbonyl species on supported Rh, the gem- dinitrosyl complex probably exists at individual Rh atom sites5 or edge sitess with cationic character generated through interactions between metal atoms and the alumina s ~ p p o r t .~ Cationic dinitrosyl complexes give i j (NO) bands at higher spectral positions than neutral complexes29 and this possibly explains why the bands for NO on Rh/Al,O, are at lower positions than for NO liganded to Rhl in a RhY zeolite.ls The edge sites for Rh dispersed on alumina have less cationic character than would be consistent with a formal + 1 oxidation state. However, this conclusion must beE. A. HYDE, R. RUDHAM AND C. H. ROCHESTER 54 1 tentative because the structure and total coordination at the adsorption site would also influence the infrared band positions.28 In contrast to the results for CO adsorption' no bands were observed which could be ascribed to bridging nitrosyl groups.3o Sites for the adsorption of bridged CO were perhaps active for the dissociative adsorption of The infrared band at 1910 cm-l is ascribed to RhNO+ species in accordance with previous assignmentsl47 l5 and with spectra of transition-metal complexes containing NO+.,' The formation of this species might be associated with the occurrence of both non-dissocative adsorption and dissociative adsorption of NO on the same surface.32 Loss of electronic charge from a chemisorbed NO molecule would be expected to be induced by an oxygen adatom liganded to the same or an adjacent Rh atom.Dissociative adsorption of NO on Rh occurs at temperatures at least as low as 330 K.12 The enhancement of the band at 1910 cm-l and the shift to 1920 cm-l with increasing temperature of contact between NO and Rh/A120, could be due to an increased amount of dissociative adsorption and hence a higher surface population of adjacent NO molecules and 0 atoms.The concomitant loss of the bands at 1830 and 1740 cm-l shows that reactions at edge sites were at least partly responsible for the results. A plausible scbheme would be that the reactions l1, l3 NO Rh + NO g Rh/ NO Rh/No + NO Rh/ (1 830 and 1740 cm-l) \NO occurred at ambient temperatures and led to the gem-dinitrosyl complex at edges sites. At higher temperatures pairs of NO molecules reacted at the metal surface in accordance with Rh + 2N0 + Rh/O + N,O (3) probably via dissociative adsorption of the first molecule and subsequent reaction between the resulting N adatom and the second NO molecule.Further addition of NO to vacant ligand positions gave Rh(O)(NO) species 0 0 Rh/ +NO + Rh( (1920 cm-l) (4) \NO . particularly when the sample was cooled for spectroscopic examination. The oxygen adatoms blocked the formation of the dinitrosyl complex and therefore the absorption bands at 1830 and 1740 cm-l did not reappear on cooling. Eventually the formation of the Rh(O)(NO) complex would also be hindered by total oxidation of the rhodium surface resulting from NO dissociation at high temperatures. This effect would be compatible with the growth of a band at 1920 cm-l in the spectra of Solymosi and Sarkany15 after heat treatment of Rh/A120, in NO at increasing temperatures up to 473 K but a decrease in intensity after subsequent heating at 673 K.Bands at 1838 and 1740 cm-l due to adsorbed NO were absent from spectra after heat treatment at 523 K.15 Reaction (3) accounts for the formation of N20. The small extent of conversion of NO to N20 arose because the reactive sites were simultaneously poisoned by an oxygen adatom which prevented further reaction. Oxygen is not readily desorbed from542 REACTION OF co -t- NO ON RHODIUM Rh below the maximum temperature (823 K) studied he~e.~q l2, l3 Nitrogen was probably also a product of reaction between N ad atom^.^, l2, l3 For Rh/Al,O, it is proposed that reactions (1)--(4) occur on Rh atoms at apex, edge or step sites where each atom has at least two vacant coordination positions.Stepped Rh (755) and (33 1) surfacesg* l3 and polycrystalline Rh wire1, adsorb NO dissociatively with the formation of N,, N,O and 0 adatoms. The results for NO adsorption and reaction over Rh/Al,O, were unhelpful in establishing whether dissociative adsorption of NO occurred on exposed single-crystal planes at the surface of dispersed particles. A LEED study of NO adsorption on Rh (1 1 1) and (100) surfaces showed that adsorption took place but did not establish whether dissociation Occurred.ll However, there is evidence for the dissociative adsorption of NO on Rh( 110) planes3, which suggests that some of the N,O formed could have resulted from reaction between N adatoms and NO molecules on single-crystal planes or two-dimensional arrays of Rh atoms.Sites available for the formation of dinitrosyl and dicarbonyl complexes should also be capable of adsorbing one CO plus one NO molecule to form a surface Rh(CO)(NO) complexls Rh/'O ( 5 ) Rh/No (+CO, -NO) , Rh/No (+CO,-NO) \NO ' (+NO,-CO) \ c O . (+NO,-CO) ' 'co . Bands at 2101 or 2105 and 1755 or 1760 cm-l have previously been ascribed to the Vc0 and vN0 vibrations, respectively, of the Rh(CO)(NO) species.l49 l5 The similarity between the spectra after addition of CO followed by NO or of NO followed by CO and the growth of the band at 2090 cm-l after addition of NO to a CO-covered surface were compatible with the establishment of equilibria (5) between Rh(NO),, Rh(NO)(CO) and Rh(CO), at edge or step sites. A contribution to the overall absorption intensity of the band with a maximum at ca.1740 cm-l could have resulted from the vN0 vibration of Rh(NO)(CO). However, the predominant effect which accompanied the growth of the band at 2090 cm-l involved a strong maximum at ca. 183Ocm-l [fig. 4(b) and 5(c)] which may have contained a contribution due to Rh(CO)(NO) as well as contributions due to the rhodium dinitrosyl complex and possibly carbonate species formed as reaction products on alumina by the NO+CO reaction on rhodium. Spectra of CO, adsorbed on Rh/A1,0, discs exhibited a broad maximum centred at 1815 cm-l with shoulders at 1850 and 1775 cm-l. The concomitant appearance of the present bands at 2090 and 1830 cm-l [fig. 4(b)] was analogous to the existence of bands at 2090 and 1810 cm-l in spectra of a RhY zeolite exposed to mixtures of CO and NO.ls The latter bands, together with a maximum at 2165 cm-l were assigned to vibrations of a [Rhl(CO),(NO)]+ complex.Here the 2165 cm-l band was absent. However, typical spectra of transition-metal complexes containing one NO and two CO molecules suggest that 2165 cm-l is a rather high spectral position for a CO vibration of such a complex.34 The present spectra for Rh/Al,O, do not provide unambiguous evidence for the existence of detectable equilibrium concentrations of [Rh(CO),(NO)] although it would not be unreas~nable,~ to assign bands at 2090, 2015 and 1830 cm-l to such a complex involving rhodium atoms of cationic character.' The bands at 2090 and 2015 cm-l generally appeared together in support of this assignment. However, the coincidence between the latter band and a maximum at 2015 cm-l in spectra of CO, adsorbed on Rh/Al,O, suggested the alternative possibility that the co-adsorption of NO and CO on Rh gave an adsorbed CO species similar to that resulting from the dissociative chemisorption of CO,.The band at 2185 cm-l (fig. 2) occurs in the same spectral region as bands due toE. A. HYDE, R. RUDHAM AND C. H. ROCHESTER 543 dinitrogen liganded to Rh atoms.35 Since nitrogen is a product of CO plus NO reactions over Rh/Al,O, the attribution of this band to RhNN species is conceivable. Spectra of nitrogen adsorbed on Rh/A1,0, exhibited a maximum at 2205 cm-I which may be ascribed to the NN stretching vibrations of surface RhNN species. The nitrogen was desorbed by evacuation at ambient temperatures.A similar band at 2236 cm-l has been reported for N, on Rh/Si0,.3sv 37. The present bands at 2185,2235 and 2260 cm-l cannot be ascribed to adsorbed nitrogen unless the band positions and/or strengths of adsorption for the adsorbed nitrogen were appreciably affected by the co-adsorption of CO, NO and other possible products of the CO+NO reactions. A more likely attribution of these bands is to surface i~ocyanate.'~-~~ Similar maxima in spectra of CO plus NO adsorbed on supported chromia were due to species containing both carbon and nitrogen atoms,18 which further precludes the asignment of the bands to dinitrogen species. Evidence, particularly from results for platin~m,'~ suggest that the band at 2185 cm-l should be ascribed to RhNCO speciesI5 but that the bands at 2235 and 2260 cm-l resulted from the migration of isocyanate groups from rhodium to the surface of the alumina upp port.^^^ l8 Variations in the relative intensities of the bands after different treatments may be compared with variations in the positions of band maxima reported elsewhere.In two studiesI59 l7 the predomi- nant band due to isocyanate was at 2269 or 2272cm-l whereas a third report14 described a single maximum at 2235 cm-l. These differences reflect the effects of the sequence of addition of NO and CO and the relative amounts of NO and CO present on the formation of surface isocyanate groups. The band positions reported here were independent of whether Rh(NO,),, [Rh(NH,),Cl]Cl, or Rh,(SO,), was the precursor salt in the catalyst preparation.The relative band intensities were also approximately independent of salt under similar experimental conditions, showing that variations in Rh dispersion' had little effect on the character of the isocyanate groups generated by the NO+CO reactions. The reactions N Rh/No + Rh/ +CO, (6) 'co NCO R h I N +CO + Rh/ (7) represent one proposed mechanism for the formation of isocyanate from NO + CO over Rh/Al,O, ~atalysts.'~q l5 Several factors suggest that this mechanism was not significant in the present reactions at ambient temperature. The intermediate species Rh(NO)(CO) would be expected to be formed at edge-site Rh atoms which give gem-dicarbonyl species in the presence of CO alone.s* The results in fig. 6 exemplify the mutual displacement of non-dissociatively adsorbed CO and NO at edge sites and the establishment of the equilibria represented by reaction (5).Accordingly the generation of Rh(NO)(CO) should have been independent of whether NO or CO was added to the system first. Hence, in contradiction to the experimental result, the extent of formation of isocyanate should also have been independent of the sequence of addition of the two gases. Furthermore, if the isocyanate group formed in reaction (7) migrated to the alumina support then the Rh atom would again be available for further reaction. The outcome would be build-up of the same amount of isocyanate whether CO or NO was added first. Finally, the absence of isocyanate after the gradual displacement of adsorbed gem-dicarbonyl species by excess NO (fig. 5) suggests that Rh atoms responsible for the formation of the dicarbonyl did not constitute active sites for the generation of adsorbed isocyanate.544 REACTION OF co +NO ON RHODIUM Bands at ca.2050 and ca. 1890 cm-l in spectra of CO adsorbed on Rh are ascribed to linear (RhCO) and bridge-bonded (Rh,CO) carbonyl species, respectively, on exposed crystal planes or two-dimensional arrays of rhodium atoms.'-' The absence of these bands after exposure of Rh/A1,0, to CO followed by NO, or to NO then CO, confirmed that NO adsorption took place on the same exposed planes. Arai and TominagaL4 have reported a similar result. The non-dissociative and dissociative adsorption of NO9? 1 1 - 1 3 9 33 may be represented by (8) 2Rh+NO -+ Rh-N+Rh-0 (9) (10) Rh + NO -+ Rh-NO reaction (9) producing N adatoms which could undergo the reaction Rh-N + CO -+ Rh-NCO to give adsorbed isocyanate groups.The present results do not preclude the occurrence of these reactions on exposed planar Rh surfaces. After the adsorption of NO at low pressures, or at high pressures followed by partial desorption, the plane surfaces probably contained adsorbed NO molecules, N and 0 adatoms and some vacant sites. The infrared band at 1910 cm-l may at least in part, particularly after adsorption at ambient temperatures [cf. reaction (4)], be due to adsorbed NO+ species liganded to Rh atoms influenced by the electron-withdrawing effects of adjacent 0 adatoms. Bands at 1860-1880 cm-l for NO on ruthenium have been interpreted in the same way.,, Subsequent addition of CO led to isocyanate formation [reaction (1 O)] possibly via a Langmuir-Hinshelwood mechanism involving adsorption of CO on vacant sites and reaction between N adatoms and adsorbed CO molecules.12 The pronounced appearance of the band at 2185 cm-l under these conditions [fig.2(c) and 5(h)] may be attributable to the existence of a high population of adjacent N atoms and vacant sites on the surface before the addition of CO. The inhibiting effect on isocyanate formation of NO preadsorbed at high pressure would then be ascribed to the absence of vacant sites for CO adsorption and subsequent reaction. This explanation would require that non-dissociatively adsorbed NO was not readily displaced by CO from the surface and that N adatoms did not undergo direct reaction with gaseous CO molecules to give isocyanate via a Rideal-Eley mechanism.The present proposals are supported by a suggestion that surface isocyanate possibly results from reaction between N adatoms and adsorbed CO molecules on a polycrystalline Rh wire.12 A further factor contributing to the inhibiting effect of NO on isocyanate formation might be that the extent of dissociative adsorption of NO is decreased by non-dissociative adsorption in dominant competition for sites at high NO press~res.~~ l2 The considerable enhancement in isocyanate formation for Rh/Al,O, exposed to high pressures of CO followed by NO (fig. 6) is explicable in terms of adsorption on planar surfaces if, in accordance with the removal of infrared bands at ca. 2050 and ca.1890 cm-l, linear and bridged CO were displaced from adsorption sites by the adsorption of NO. A plausible reaction scheme (RhCO sites) would be as follows: CO CO CO(+N:co) CO NO CO Rh Rh Rh Rh Rh Rh Rh, Rh Rh NCO I l l - 1 1 I - + I + co, . (1 1) An important factor could be that a non-dissociatively adsorbed NO molecule is initially surrounded by several adsorbed CO molecules, at least one of which promotes the dissociation of adsorbed NO and one of which becomes incorporated into theE. A. HYDE, R. RUDHAM AND C. H. ROCHESTER 545 isocyanate group. A possibility is that a concerted reaction led to the simultaneous formation of CO,, or a dissociatively adsorbed CO, species (band at 201 5 cm-l), and isocyanate. This would be consistent with a proposed mechanism for the generation of CO, and adsorbed isocyanate by the reaction of CO and NO on ruthenium catalysts.38 The present proposals support a previous conclusion15 that interactions between NO and CO molecules adsorbed on Rh at low temperatures enhance the dissociation of NO.The correct stoichiometry of one NO molecule and two CO molecules required for the formation of CO, and an isocyanate group would be contained in a [Rh(CO),NO] complex1* which might be formed on edge or apex Rh atoms with a high degree of coordinative unsaturation. The involvement of this complex as a precursor of isocyanate was unlikely to be the main reason for the appearance of isocyanate in the present systems although reaction at an edge site might be advantageous for the subsequent migration of isocyanate to the surface of the alumina support. The formation of N,O from NO at ambient temperature cannot be simply attributed to the occurrence of reaction (9) followed by Rh-N + NO 4 Rh + N,O (12) because no N20 appeared in the absence of CO.The generation of N,O must have required CO to be involved in one or both of two ways. First, the dissociation of NO was enhanced by the presence of CO. Secondly, the formation of N,O(g) via reaction ( 12) was promoted possibly by the displacement of reaction product from the surface by the adsorption of CO. The generation of carbon dioxide in the step involving NO dissociation would be consistent with the appearance of a weak band due to CO,(g) and of bands due to adsorbed carbonate species on the alumina support.However, N,O(g) formation became negligibly slow after ca. 1 h (fig. 6) even though the added pressures of CO and NO had not been appreciably depleted. Sites responsible for catalysing the N,O reaction were progressively poisoned either by isocyanate or by the retention of 0 adatoms formed by the dissociation of NO. Poisoning by isocyanate alone was inconsistent with the contrast between the similar amounts of N,O formed and the dissimilar amounts of isocyanate formed in the experiments for which results are shown in fig. 6. The retention of 0 adatoms was consistent with the relatively small amounts of CO, apparently formed by the reaction and with the presence of the maximum at ca. 201 5 cm-l similar to a corresponding maximum in spectra of CO, dissociatively adsorbed on Rh/A1203.The generation of CO,(g) by the reaction of 0 adatoms and adsorbed CO was apparently unfavourable at ambient temperatures. The infrared spectra do not enable an unambiguous decision to be made concerning which Rh atom sites were active for the formation of N,O. The production of isocyanate and N,O on the same sites in an exposed planar surface could only be rationalized if reaction (1 1) giving isocyanate did not lead to poisoning of the active sites but the reaction giving N,O simultaneously led to poisoning. The suggestion that isocyanate groups migrate to the alumina is in accordance with this possibility since the Rh atoms would then be available for further reaction to isocyanate or to N,O. The formation of N,O would continue until all the sites were poisoned and therefore the final amount of N,O would be, in accordance with the experimental result (fig.6), independent of the amount of isocyanate formed at the same time. However, this proposal seems unlikely because the secondary products of the two reactions are identical [CO,, CO(ads)+O(ads) or O(ads)] and therefore the poisoning effects of the products should be the same. With a limited total number of sites an increase in the extent of formation of isocyanate would then be expected 19 FAR 1546 REACTION OF CO+NO ON RHODIUM to cause a corresponding decrease in the amount of N,O product. This is not compatible with the present results (fig. 6) and therefore it appears that N,O and isocyanate formation must have involved different types of surface site.Iizuka and Lunsfordls reported that N,O was readily formed from NO+CO over a RhY zeolite and proposed that a complex containing one CO and two NO molecules liganded to a single Rh atom was an intermediate in the reaction mechanism. Arai and Tominaga14 suggested that a Rh(NO)(CO) complex [reaction (S)] was a precursor of N20 over Rh/Al,O, catalysts. Edge, apex or step sites may have been active for the formation of N,O. The reactive sites must have been progressively poisoned presumably by the retention of 0 adatoms which were resistant to desorption as oxygen or to reaction with CO and desorption as CO,. It is relevant to note that a dinitrosyl complex of iridium has been shown to undergo a ligand exchange reaction with CO with the resulting elimination of N,O and the concomitant formation of C0,.40 It is here proposed that a similar reaction occurs for Rh/Al,O, at Rh atoms which offer at least two vacant ligand positions for the adsorption of CO or NO.However, not all such sites were active since the formation of N20 was severely retarded whilst spectra still contained strong bands due to surface dicarbonyl and dinitrosyl species. We thank Drs B. Harrison and D. T. Thompson for helpful discussions, the S.E.R.C. for a studentship (E. A. H.) and Johnson Matthey for financial support. A. C. Yang and C. W. Garland, J. Phys. Chem., 1957,61, 1504. M. Primet, J. Chem. SOC., Faraday Trans. I , 1978,74, 2570. H. C. Yao and W. G. Rothschild, J. Chem. Phys., 1978,68, 4774. N. Sheppard and T. T. Nguyen, Adu. Infrared Raman Spectrosc., 1978, 5, 67.J. T. Yates, T. M. Duncan, S. D. Worley and R. W. Vaughan, J. Chem. Phys., 1979, 70, 1219. D. J. C. Yates, L. L. Murrell and E. B. Prestridge, J. Catal., 1979, 57, 41. E. A. Hyde, R. Rudham and C. H. Rochester, J. Chem. Soc., Faraday Trans. I , 1983,79, 2405. F. Solymosi and A. Erdohelyi, Surf. Sci., 1981, 110, 630. D. G. Castner and G. A. Somejai, Surf. Sci., 1979,83, 60. lo D. G. Castner, L. H. Dubois, B. A. Sexton and G. A. Somerjai, Surf. Sci., 1981, 103, 134. l1 D. G. Castner, B. A. Sexton and G. A. Somejai, Surf. Sci., 1978,71, 519. l2 C. T. Campbell and J. M. White, Appl. Surf. Sci., 1978, 1, 347. l3 L. H. Dubois, P. K. Hansma and G. A. Somerjai, J. Catal., 1980, 65, 318. l4 H. Arai and H. Tominaga, J. Catal., 1976, 43, 131. l5 F. Solymosi and J. Sarkany, Appl. Surf. Sci., 1979, 3, 68. l6 T. Iizuka and J. H. Lunsford, J. Mol. Catal., 1980, 8, 391. l7 M. L. Unland, Science, 1973, 179, 567; J. Catal., 1973, 31, 459. 19 J. Rasko and F. Solymosi, J. Catal., 1981, 71, 219. 2o R. E. Nightingale, A. R. Downie, D. L. Rotenberg, B. Crawford and R. A. Ogg, J. Phys. Chem., 21 N. Parkyns, Proc. 5th Znt. Congr. Catalysis, 1972, p. 255. 22 A. L. Smith, W. E. Keller and H. I. Johnston, J. Chem. Phys., 1951, 19, 189. 23 C. Mortena, F. Boccuzzi, S. Coluccia and G. Ghiotti, J. Catal., 1980, 65, 231. 24 T. Iizuka and Y. Tanaka, J. Catal., 1981, 70, 449. 25 F. Solymosi and A. Erdohelyi, J. Catal., 1981, 70, 451. 26 H. C. Yao, S. Japan and M. Shelef, J. Catal., 1977,50,407; H. C. Yao and M. Shelef, 7th Int. Congr. 27 W. Hieber and H. Fiihrling, 2. Anorg. Allg. Chem., 1971, 381, 235. 28 J. H. Enemark and R. D. Feltham, Coord. Chem. Rev., 1974, 13, 339. 29 F. J. Regina and A. J. Wojicicki, Znorg. Chem., 1980, 19, 3803. 30 H. Brunner and S . Loskot, 2. Nuturforsch., Teil B, 1971, 26, 757. 31 F. Bottomley, W. V. F. Brooks, S. G. Clarkson and S-B. Tong, J. Chem. SOC., Chem. Commun., 1973, 32 A. T. Davydov and A. T. Bell, J. Catal., 1977, 49, 332. J. Rasko and F. Solymosi, J. Chem. SOC., Faraday Trans. I , 1980,76, 2383. 1954,58, 1047. Catalysis, Tokio, 1980, paper A2 1. 919.E. A. HYDE, R. RUDHAM AND C. H. ROCHESTER 33 R. J. Baird, R. C. Ku and P. Wynblatt, Surf. Sci., 1980, 97, 346. 34 P. S. Braterman, Metal Curbonyl Spectra (Academic Press, London, 1975), chap. 7. 35 G. A. Ozin and A. Vander Voet, Can. J . Chem., 1973,51,3332. 36 Y. G. Borod’ko and V. S. Lyutov, Kinet. Catul., 1971, 12, 202. 37 V. S. Lyutov, V. A. Redosimov and Y. G. Borod’ko, Russ. J. Phys. Chem., 1972,46, 973. 38 M. F. Brown and R. D. Gonzalez, J. Cutal., 1976, 44, 477. 39 C. T. Campbell, S-K. Shi and J. M. White, Appl. Surf. Sci., 1979, 2, 382. 40 B. F. G. Johnson and S. Bhaduri, J. Chem. SOC., Chem. Commun., 1973,650. 547 (PAPER 3/524) 19-2

ISSN:0300-9599    

DOI:10.1039/F19848000531

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1984, 80, 549-551 Self-diffusion and Volumetric Measurements for Oc tame thylcyclo te trasiloxane under Pressure at 3 23 K BY ALLAN J. EASTEAL AND LAWRENCE A. WOOLF* Research School of Physical Sciences, Australian National University, Canberra, A.C.T. 2600. Australia Received 8th April, 1983 Self-diffusion coefficients have been measured for octamethylcyclotetrasiloxane (OMCTS) up to 58.4 MPa at 323 K and volume ratios have been determined at the same temperature to 81 MPa and fitted to a modified Tait equation. From the density of liquid OMCTS at the freezing pressure at 323 K, an equivalent hard-sphere diameter of 0.777 nm has been estimated and used in a rough hard-sphere interpretation of the diffusion data. Octamethylcyclotetrasiloxane (OMCTS) is of considerable interest because it is a large globular molecule. Self-diffusion measurements on OMCTS have been made by Levien and Mills’ at atmospheric pressure for the temperature range 288-318 K, and tracer1 and mutual2 diffusion coefficients for mixtures of OMCTS with benzene and carbon tetrachloride have been measured for the same temperature range.Substantial measurements of thermodynamic properties at atmospheric pressure have been made by Marsh and c o ~ o r k e r s . ~ Shear viscosity measurements were made by Dickinson4 at 30 and 52 MPa, at 323 K, but because of the lack of p V data he was forced to estimate densities at those pressures in order to interpret the viscosity data. As part of the present work on the self-diffusion of OMCTS under pressure, the density of the liquid has been measured at 323 K at pressures up to ca.20 MPa beyond the freezing pressure. EXPERIMENTAL The OMCTS used was Fluka ‘purum’ grade (> 98% purity) material which was stored over a molecular sieve. The p V data were obtained from volume-ratio measurements made at ca. 2.5 MPa intervals up to 30 MPa and at CQ. 10 MPa increments from 30 to 81 MPa, using a calibrated bellows vol~morneter.~ The self-diffusion measurements were made using the n.m.r. spin+ho technique. The volume-ratio data are estimated to have a precision of +0.02% and the diffusion data a precision of f 1 %. Pressures were measured using Heise-Bourdon gauges; for the diffusion measurements the pressures should be accurate to f0.3 MPa and for the p-V data to kO.025 MPa up to 25 MPa and f0.4 MPa above 25 MPa.Temperatures were maintained at 323.15 & 0.01 K and measured by platinum resistance thermometry. RESULTS AND DISCUSSION The p-V data were represented by p / ( l - k ) = a,+a,p+a3p2 0) < 81 MPa) (1) where k is the volume ratio [(volume at pressure p)/(volume at 0.1 MPa)], a, = 629.96326 MPa, a, = 5.30341503 and a3 = -0.010613265 MPa-l, with a root- 549550 SELF-DIFFUSION UNDER PRESSURE Table 1. p V data for octamethylcyclotetrasiloxane at 323.2 Ka 0.1 5 10 15 20 30 40 50 60 70 80 0.9998 0.9992 0.9853 0.9788 0.9727 0.961 5 0.95 15 0.9424 0.9341 0.9262 0.9 189 921.gb 928.8 935.4 941.7 947.6 958.6 968.7 978.0 986.8 995.1 1003.1 1.58 I .47 1.38 1.29 1.22 1.09 0.999 0.923 0.864 0.8 17 0.718 a k is the volume ratio and K is the isothermal compressibility; density at 0.1 MPa from K.N. Marsh, Trans. Faraday SOC., 1968, 64, 883. Table 2. Self-diffusion coefficients of OMCTS at 323 Ka D / 10-9 p/MPa m2s-' na3 A D 0.1 0.60 1 0.8794 0.204 5.02 0.547 0.8862 0.196 13.6 0.46 1 0.8970 0.180 22.6 0.403 0.9069 0.171 34.3 0.342 0.9188 0.162 48.5 0.281 0.93 18 0.152 58.4 0.246 0.9403 0.146 a Number density no3 is based on a molecular weight of 0.2966 kg mol-l and a diameter, 0, of 0.777 nm. mean-square deviation in p / ( 1 - k) of 0.09%. Volume ratios, densities and com- pressibilities, IC = - (a In klap),, obtained by analytic differentiation of eqn (1) are given at rounded pressures in table 1 and show that although OMCTS has a limited liquid range, it is a very compressible liquid at 323 K.The p-V measurements were made starting from a maximum pressure of 81 MPa after holding the liquid at that pressure for several hours. It was subsequently found that in the presence of sintered stainless steel OMCTS froze consistently and quickly at 60.6 f 0.2 MPa, implying that the liquid can exist for long periods in the metastable region. The self-diffusion results are given in table 2 and were fitted to where bo = -0.505025, b, = -0.021 7412, b,.= 2.041 259 x and b3 = - I .633 207 x with a root-mean-square deviation in 109D of 0.0045.A. J. EASTEAL AND L. A. WOOLF 55 1 It is usual to analyse such self-diffusion data by using the rough hard-sphere model developed by Chandler.' This relates the measured diffusion coefficient D to that of a smooth hard-sphere fluid DsHs by = AD DSHS (3) where A , represents the coupling of translational and rotational motion, DSHS = CDE, with C a number-density-dependent factor which corrects the Enskog dense-fluid- diffusion coefficient D , to correspond to molecular-dynamics simulation results.Application of eqn (3) relies conventionally on estimation of an equivalent hard-sphere diameter a which provides the best constancy of A , up to the maximum number density (no3 = 0.943) allowed by hard-sphere theory. For OMCTS this procedure is of little value because constancy of A , is not attained by varying a before the density limit is exceeded for all except D at 0.1 MPa. An alternative approach utilises the knowledge that a hard-sphere fluid has a liquid/solid phase transition at the reduced density no3 = 0.943.This density can be used with the molar volume Vm of the liquid at that freezing density to provide the hard-sphere diameter from a = (0.943 Vm/L)i. (4) This procedure has been used by Harris and Trappeniers* to obtain a a for methane. Recently Easteal et aL9 have shown that those a provide a prediction of reduced diffusion coefficients for methane from a smooth hard-sphere simulation method which are in excellent agreement with those obtaified from experimental data. For OMCTS the molar volume of the liquid at the freezing pressure of 60.6 MPa is 300.4 cm3 mol-l, which provides a/nm = 0.777. Use of this fixed value of a with the rough hard-sphere model then leads to a set of A , values which are not constant but decrease monotonically with increasing number density (pressure).Note that such variation in A , is not in accordance with the requirement of constancy stated by Chandler.' It does, however, fit in with the intuitive expectation that coupling of translational and rotational motion should increase with the closer packing of the particles consequent on increasing the pressure. A preliminary investigation of self-diffusion data for other globular liquids suggests that a similar dependence of A , on density exists when the a value is determined from freezing-pressure densities. J. Levien and R. Mills, Aust. J. Chem., 1980, 33, 1977. K. N. Marsh, Trans. Faraday SOC., 1968, 64, 894. K. N. Marsh, Trans. Faraday SOC., 1967, 64, 883; K. N. Marsh and R. P. Tomlins, Trans. Faraday SOC., 1970,66,783; B. J. Levien and K. N. Marsh, J. Chem. Thermodyn., 1970,2,227; K. N. Marsh, J. Chem. Thermodyn., 1971, 3, 355. E. Dickinson, J . Phys. Chem., 1977, 81, 2108. P. J. Back, A. J. Easteal, R. L. Hurle and L. A. Woolf, J . Phys. E, 1982, 15, 360. K. R. Harris, R. Mills, P. J. Back and D. S. Webster, J. Magn. Reson., 1978, 29, 473. K. R. Harris and N. J. Trappeniers, Physica, 1980, 104A, 262. A. J. Easteal, L. A. Woolf and D. L. Jolly, Physica, 1983, in press. ' D. Chandler, J. Chem. Phys., 1975, 62, 1358. (PAPER 3/554)

ISSN:0300-9599    

DOI:10.1039/F19848000549

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I, 1984, 80, 553-561 Phenol-Amine Hydrogen Bonds with Large Proton Polarizabili ties Position of the OH. - . N * O - - * .H+N Equilibrium as a Function of the Donor and Acceptor BY GUNNAR ALBRECHT AND GEORG ZUNDEL* Institut fur Physikalische Chemie der Universitat Miinchen, D-8000 Miinchen 2, Federal Republic of Germany Received 9th May, 1983 1 : 1 mixtures of octylamine + chlorophenols and pentachlorophenol + aromatic amines have been studied in CC1, solution using infrared spectroscopy. It has been shown by conductivity measurements that no charged species are present in these systems. Therefore only the association constants K,, and the equilibrium constants, KpT, of proton transfer in the equilibrium OH * . - N 0- . . * H+N need be considered. Linear relations exist between log K, and the ApK, values for both families of systems.Log KPT increases with both systems in proportion to ApK,, i.e. the Huyskens equation is valid. This increase is caused by a decrease in the enthalpy term Ah$ due to increasing acidities of the donors or (in the other family of systems) increasing basicities of the acceptors. In addition to this direct effect, an increase in the size of the negative A 3 values due to the increasing interaction of these hydrogen bonds with their environment is responsible for this shift in the equilibrium. ApKS,O% amounts to 3.6 in the case of the family containing the aliphatic amine and 1.6 in the case of the family containing the aromatic amines, since with the latter the charge is spread over a more extended region if the proton approaches the N atom.The intensity of the continuum (and thus the proton polarizability) is largest if both proton-limiting structures OH . . . N and 0- * * H+N have almost the same probability. INTRODUCTION 0- - - H+N in solutions were first studied by Barrow and Yerger' using infrared spectroscopy. They studied such equilibria with carboxylic acid + amine systems in chlorinated hydrocarbons. Phenol + amine systems in CD,CN were investigated in ref. (2), and poly-L-lysine films with various phenols by Kristof and ZundeL3 It was shown in these studies2?, that if both proton-limiting structures have noticeable probability these hydrogen bonds show a large proton p~larizability~-~ due to proton motion, as indicated by the occurrence of continua in the infrared spectra.Furthermore, analogous intramolecular OH - - - N f 0- - - H+N bonds have been studied by various groups7-l0 with Mannich bases using U.V. and i.r. spectroscopy. Huyskens and Zeegers-Huyskensll have shown that the following equation is valid for the proton-transfer equilibrium constant KPT (1) where ApK, is the pK, of the protonated base minus the pK, of the acid and < and 6 are constants which are different for each family of systems. A family is a set of chemical compounds which possess the same donor and acceptor groups. The 553 Proton-transfer equilibria OH - - - N log Kp, = CApK, - 6554 PHENOL-AMINE HYDROGEN BONDS compounds have different ply, values because they possess different substituents ; however, these substituents show similar interaction properties with their environments.S is a function of the solvent in which the hydrogen bonds are present. It has been shown by various group^^^-^^ that the dipole moment of such OH * - N f 0- * * H+N bonds increases by 8-12 D* if the polar structure is realized. The influence of the environment on such hydrogen bonds is determined by the interaction of the hydrogen-bond dipole with the reaction field induced in the solvent, as demonstrated by Fritsch and ZundeP for phenol + amine systems in various solvents. In addition, specific interactions of these hydrogen bonds with their environments may influence the position of the proton-transfer equilibrium.l6* l7 Thus the thermodynamic properties determining the equilibrium are influenced by the properties of the donors and acceptors, and particularly by the interaction of these hydrogen bonds with their environments.Thus, as discussed in ref. (1 7), AH," + AH; AS; +AS; + R lnly,, = - RT whereby A= and A g are determined by the acidity and basicity of the donor and acceptor, and AG and A% by the interaction of the hydrogen bonds with their environments. In ref. (2) it was not taken into account that the proton-transfer equilibria AH - BAA-. 7 . . H+B are one stage in a sequence of various equilibria: Ka KPT Kd Kh AH+B$(AH * * * B A----H+B)SA-+H+B AH+B (AH.. . A - e - A . . . HA)+(B+H - - - B e B - - - H+B). (3) All these equilibria will be considered in this study. Furthermore, with two families of systems we will consider the influence of the acidity of the donor and the basicity of the acceptor on A%.Furthermore, we will compare a family of compounds having an aliphatic amine with one containing various aromatic amines. EXPERIMENTAL The substances were purchased from Fluka AG (Switzerland), EGA and Merck (Federal Republic of Germany). In every case, substances with the highest degree of purity available were used. CC1, (for spectroscop ) from Merck, Darmstadt (Federal Republic of Germany) dichloro-substituted phenols were dried over P,O,. All solutions were prepared in a water-free glove box. The concentrations of the donors and acceptors in the solution were 0.1 mol dm-3. For the i.r. investigations, cells with NaCl windows were used (layer thickness 0.2 or 0.5 mm). The solvent bands were compensated by a cell of variable layer thickness filled with pure solvent in the reference beam.The temperature of the samples was 298 K: The spectra were taken with a spectrophotometer (model 325, Bodenseewerk Perkin-Elmer, oberlingen, Federal Republic of Germany). It was flushed with dry and C0,-free air. The equilibria were determined by evaluating the bands corresponding to the donors or acceptors as described in the next section. The sigmoid curves shown later in fig. 3(b) and 4(b) are calculated from the curves in fig. 3(a) and 4(a) using the relation was used and was dried over 3 w molecular sieves, as were the pyridines. The mono- and 100 KPT percentage proton transfer = ~ KPT + 1 The pK, values were taken from ref. (1 8) and (19). * 1 D x 3.3356 x 10-lo C m.G.ALBRECHT AND G. ZUNDEL 555 0 4000 3200 2400 2000 1600 1200 800 wavenum ber/cm-' Fig. 1. (a) Infrared spectra of (-) 2,3,4-trichlorophenol+ octylamine in CCl,, concentration 0.246 mol dmV3, layer thickness 0.528 mm; (----) 2,3,4trichlorophenol in CCl,, concentra- tion 0.246 mol dm-3, layer thickness 0.528 mm. (b) Infrared spectra of (-) 3,5-lutidine +pentachlorophenol in CCl,, concentration 0.05 mol dm-3, layer thickness 0.5 mm; (- - - -) pentachlorophenol, concentration 0.05 mol/dm3, layer thickness 0.5 mm. The absorbance of the continuum was evaluated at ca. 1800 cm-l, i.e. in a region in which no bands are present. The values were referred to 1 mol dm-3 of absorbing hydrogen bonds in a reference layer of 1 cm thickness. RESULTS AND DISCUSSION Fig.1 shows i.r. spectra for examples of both families of systems, octylamine+ chlorophenols and pentachlorophenol+ aromatic amines. In the i.r. spectra of both examples intense continua are found, indicating that hydrogen bonds with large proton polarizabilities are formed between phenols and amines. As we shall see in the following, with these systems, both proton-limiting structures of the proton-transfer equilibria OH - 0 - N g 0 - * * * H+N have a significant probability. To clarify whether all species in the above-mentioned sequence of equilibria must be considered, we first performed conductivity measurements. For all systems studied, these measurements showed that the conductivity of the solutions is (within the limits of experimental error) not higher than the conductivity of the pure solvents.This result excludes the presence of all charged species. Thus we can limit our studies to the formation constant, K,, of OH * - N S O - - - - H+N complexes, and to the constant, KpT, describing the position of the proton-transfer equilibria within these hydrogen bonds. These data are summarized in table 1.556 PHENOL-AMINE HYDROGEN BONDS Table 1. Data for the association and the proton-transfer equilibria ~~ ~~ ~~ ~ ~ 1 2 3 4 5 6 7 8 donor acceptor K, ApK, KPT pGo% 6 4-chlorophenol 3-chlorophenol 3,4-dichlorophenol 3,5-dichlorophenol 2,4-dichlorophenol 2,3-dichlorophenol 2,3,4-trichlorop hen01 2,4,5-trichlorophenol 2,3,5-trichlorophenol 2,4,6-trichlorophenol pentachlorophenol 41 1.28 0.0 52 1.68 0.01 76 2.03 0.01 86 2.40 0.05 36 2.75 0.82 octylamine 39 2.94 0.63 58 3.68 1.30 85 3.93 2.51 62 4.22 2.51 86 4.66 5.90 - 133 5.91 3.6 0.92 3.29 pyridine 97 0.51 3-picoline 283 0.96 pentachlorophenol 2097 1.36 2,4-lutidine 185 1.96 1.73 2,4,6-collidine 227 2.76 6.8 1 1.6 0.75 1.22 ASSOCIATION EQUILIBRIA The association constants, K,, are determined from the integrated absorbance of the stretching vibration observed in the region 3610-3520 cm-l of non-hydrogen- bonded OH groups of phenols (see fig.1). This band is calibrated using the same band in the solution of the pure phenols. The K, values are given in table 1, column 3. They are shown in fig. 2 as a function of ApK, for both families of systems. With both families, two linear relations between log K, and ApK, are found, one in systems of ortho-substituted phenols and another for other phenols. When a donor-acceptor combination of the same ApK, is compared, log K, is smaller if a chlorine atom is present in the ortho-position of the phenol [fig. 2(a)], since with these phenols intermolecular association is reduced by the formation of intramolecular OH - * * C1 hydrogen bonds.Furthermore, with donor-acceptor combinations having the same ApK,, log K, is smaller if a CH, group is present in the ortho-position [points 4 and 5 in fig. 2(b)], since with these amines the intermolecular association may be slightly hindered sterically . PROTON-TRANSFER EQUILIBRIA The constants KPT for proton transfer in the OH - N e 0 - - - H+N equilibria are determined as follows: For octylamine + chlorophenol systems the concentration of the polar structure 0- - - - H+N was determined from the integrated absorbance of the antisymmetrical bending vibrations of the NHZ group at ca.1620 cm-l [see fig. 1 (a)]. This band was calibrated from the integrated absorbance of the corresponding band for the octylamine + pentachlorophenol complex. With this system, all protons are present on the amine molecule. This is demonstrated by the fact that the phenol band at 1185 cm-l is no longer observed in this system, whereas the phenolate band at 1210 em-' is very intense. The overall concentrations of the phenol + amine complexes were obtained from the respective K, values.G. ALBRECHT AND G. ZUNDEL 557 2 . 0 1 . 5 0 1 2 3 A PK, Fig. 2. Logarithm of the association constants K, plotted as function of ApK,.(a) Systems comprising octylamine + substituted phenols : (1) 4-chlorophenol, (2) 3-chlorophenol, (3) 3,4-dichlorophenol, (4) 3,5-dichlorophenol, (5) 2,4-dichlorophenol, (6) 2,3-dichlorophenol, (7) 2,3,4-trichlorophenol, (8) 2,4,5-trichlorophenol, (9) 2,3,5-trichlorophenol, (10) 2,4,6-trichloro- phenol, (1 1) pentachlorophenol. (b) Systems comprising pentachlorophenol + substituted pyridines : (1) pyridine, (2) 3-picoline, (3) 3,5-lutidine, (4) 2,4-lutidine, (5) 2,4,6-collidine. For the pentachlorophenol + aromatic amine systems, the concentration of the non-polar OH N structure was determined from the band at 1185 cm-l [see fig. l(b)]. This band, which shows large 6(OH) character, was calibrated with the pentachlorophenol + pyridine system. With this system, all protons are present on the chlorophenol molecule, as demonstrated by the fact that the phenolate band at 1215 cm-l is no longer observed. The overall concentrations of the phenol+amine complexes were obtained from the K, values.The KPT values of both families of systems are given in table 1, column 5. For the octylamine+chlorophenol systems, log KPT is shown in fig. 3(a) and the percentage proton transfer in fig. 3(b) as a function of ApK,. The corresponding plots for the chlorophenol+aromatic amine systems are shown in fig. 4(a) and (b). Fig. 3(a) and 4(a) show that the Huyskens relation eqn (1) is valid to good approximation for both families of systems. The intersections of the curves with the abscissa in the figures gives a value of Apeo% of 3.6 for the family of systems with the.aliphatic amine and 1.6 for the family with the aromatic amines.These values are given in table 1, column 6 and the respective < and 6 values, also obtained from these figures, in columns 7 and 8. The A% values are usually positive and relatively large, since in the gas phase the AH * - - BAA- T- - - - H+B equilibrium usually lies to the left. In liquids these equilibria become more or less strongly shifted to the right-hand side since 6% is overcompensated by A%, a term arising from the interaction of the hydrogen bonds with their environments. This term is always negative and relatively large, shifting the equilibria to the right-hand side. In addition, this term overcompensates the influence of the term ASO, which is negative with such equilibria'~ lo* 2ov 21 due to the increasing558 2 0 .1 1 I I I 1 - U L 5 0 , - 2 2 5 1 5 - 6,” pa (c) - - 2 - 2 a 3 4 s ‘, - b9 ‘\ 10 11 - s m 41 \ =--*- - 5 - 1 1 1 I 1 1 , PHENOL-AMINE HYDROGEN BONDS 2 0 . I I I 1 8 1 1 I I - 05,- 7 15 - yrl ( d , - 21 E * 1- g > g 10 : A 4 j 4 sip -; 2 \ ee,, 11 1 8-- - --@ - 5 - I l l l l l l l l l - 0 1 2 3 4 5 6 7 order in the solvent around the polar structure, which also shifts the equilibrium to the left-hand side. All these viewpoints are discussed in detail in ref. (1 7). In the octylamine + chlorophenol family, Kp,increases on going from 4-chlorophenol to 2,4,6-trichlorophenol (table 1, column 5). This increase in KPT is caused by a decrease in AI$ due to increasing acidity of the phenols.In addition, if the equilibrium becomes increasingly shifted to the right-hand side, the magnitude of the negative A% value also increases, since the interaction of the hydrogen bonds with their environments increases. By this effect, the equilibrium is also shifted to the right-hand side. Thus, increasing acidity of the acceptors favours a shift of the equilibria to the right-hand side, directly via 6% and indirectly via A%. With the pentachlorophenol + aromatic amine family, KPT increases on going from 3-picoline to 2,4,6-collidine (table 1, column 5). This increase in KPT is caused by a decrease in A G due to the increasing basicity of the hydrogen-bond acceptors. In addition, if the equilibrium becomes increasingly shifted to the right-hand side the magnitude of the negative A% value also increases.Hence, owing to this indirect effect, the equilibrium is also shifted to the right-hand side on increasing the basicity of the acceptors. ApK5,OX is the ApK, value at which both proton-limiting structures OH - - - N .C- 0- - - H+N have the same probability. For the family of systems containing an aliphatic amine, ApGo% is 3.6 and for systems containing the aromatic amines its value is 1.6.G. ALBRECHT AND G. ZUNDEL 559 1 0 - I - 2 0 1 2 3 APK, 100 80 2 6 0 E 40 h 5 m G 0 U g 20 0 - 1 0 1 2 3 4 APKa Fig. 4 (a) Log KPT of systems comprising pentac lorophenol+ substituted pyridines ploll:d as a function of ApK, : (1) pyridine, (2) 3-picoline, (3) 3,5-lutidine, (4) 2,4-lutidine, (5) 2,4,6-collidine.(b) Percentage proton transfer of the same systems plotted as a function of ApK,. (c) and (d) Absorbance of the continuum for the same systems plotted as function of ApK, and log KpT, respectively. This result demonstrates that the aromatic character of the amines favours proton transfer to the amines, i.e. A% decreases. This result is understandable since aromatic character leads to the positive charge being delocalized. In fig. 3(c) and ( d ) and 4(c) and ( d ) the absorbance of the continua is shown as a function of ApK, and log KPT. These figures show that the i.r. continua are most intense, and thus that the proton polarizability of these hydrogen bonds is largest in the region in which both proton-limiting structures have similar weight. This result is understandable, since in this case the fluctuation of the proton is largest.With the octylamine +chlorophenol family, however, a small shift of the intensity maximum of the continuum towards log KPT < 0 is observed. This result can be explained by the following considerations. The proton polarizability is determined by the proton potential and hence by the enthalpy term. The position of the equilibrium, however, is determined by the free energy, and thus by both enthalpy and entropy terms. With such proton-transfer equilibria, A 9 is negative8-l09 20, 21 owing to the large degree of order in the solvent around the polar proton-limiting structure. Thus this entropy term shifts the equilibrium to the left-hand side. Hence the proton potential is, on average, already symmetrical (AH0 = 0) before both limiting structures have the same probability (AGO = 0).This consideration may explain the slight shift in the intensity maximum of the continuum towards KPT < 1.560 PHENOL-AMINE HYDROGEN BONDS CONCLUSIONS With the two families of systems, octylamine + chlorophenols and pentachloro- phenol + aromatic amines, no charged species is found in CC1, solutions. The hydrogen bonds formed between the phenols and amines show large proton polarizabilities, as indicated by continua in the i.r. spectra. Linear relations exist between the logarithm of the complex formation constants, log K, and ApK, (pK, of the protonated base minus pK, of the acid). Log K, increases with increasing ApK,. These relations are different, however, when substituents are present in the ortho position.With the chlorophenols the formation of intramolecular OH - - - C1 bonds reduces complex formation. The same is true with methyl substituents in the ortho position of the N-bases, owing to steric hindrance. Log KpT, the logarithm of the OH - - * N +O- - - - H+N proton-transfer equilibrium constant, increases with both families of systems in proportion to the ApK, values, i.e. the Huyskens equation is valid. This increase in log KPT is caused by a decrease in the enthalpy term A%, resulting from the increasing acidity of the donors, or in the other family of systems, from the increasing basicity of the acceptors. This direct effect of the acidities and basicities is enhanced by an indirect effect, since the amounts of the negative A% values increase, too, due to the increasing interaction of the hydrogen bonds with their environment, if the probability of the polar structure increases.The ApK, value at which both proton-limiting structures have the same probability (ApK5,Ox) is 3.6 for the family containing the aliphatic amine and 1.6 for that containing the aromatic amines. This result is explained by the fact that the charge is delocalized for the aromatic amines if the proton becomes attached to the nitrogen atom. The intensity of the continuum, and thus the proton polarizability, is largest if both proton-limiting structures OH + . - N + 0- * - H+N have almost the same weight. We thank the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for providing the facilities for this work.G. M. Barrow and E. A. Yerger, J. Am. Chem. Soc., 1954, 76, 521 1; G. M. Barrow, J. Am. Chem. Soc., 1956, 78, 5802. G. Zundel and A. Nagyrevi, J. Phys. Chem., 1978,82, 685. W. Kristof and G. Zundel, Biophysical Structure and Mechanism, 1980, 6, 209. E. G. Weidemann and G. Zundel, 2. Naturforsch. Teil A , 1970, 25, 627. R. Janoschek, E. G. Weidemann, H. Pfeiffer and G. Zundel, J. Am. Chem. Soc., 1972,94, 2387. G. Zundel, in The Hydrogen Bond-Recent Developments in Theory and Experiments, ed. P. Schuster, G. Zundel and C. Sandorfy (North Holland, Amsterdam, 1976), vol. 11, chap. 15. ’ H. Schreiber, A. Koll and L. Sobczyk, Bull. Acad. Pol. Sci., Ser. Chim., 1978, 24, 651. * A. Koll, M. Rospenk and L. Sobczyk, J. Chem. Soc., Faraday Trans. I , 1981, 77, 2309. lo M. Rospenk, J. Fritsch and G. Zundel, J. Phys. Chem., in press. l1 P. Huyskens and Th. Zeegers-Huyskens, J. Chim. Phys. Phys. Chim. Biol., 1964, 61, 81; Th. Zeegers-Huyskens and P. Huyskens, in Molecular Interactions, ed. H. Ratajczak and W. J. Orville- Thomas (Wiley, New York, 1981), vol. 11. M. Rospenk, I. G. Ruminskaja and V. M. Schreiber, Zh. Priklad. Spektrosk. 1982, 36, 756. l2 H. Ratajczak and L. Sobczyk, J. Chem. Phys., 1969, 50, 556. l 3 R. Nouwen and P. Huyskens, J. Mol. Struct., 1973, 16, 459. l4 J. Jadzyn and J. Maiecki, Acta Phys. Pol., 1972 A41, 509. l5 L. Sobczyk, in The Hydrogen Bonh-Recent Developments in Theory and Experiments, ed. P. Schuster, l6 J. Fritsch and G. Zundel, J. Phys. Chem., 1981, 85, 556. G. Zundel and C. Sandorfy (North Holland, Amsterdam, 1976), vol. 111, chap. 20. G. Zundel and J. Fritsch, work in preparation.G. ALBRECHT AND G. ZUNDEL 56 1 l8 J. Drahonovsky and Z. Vacek, Coil. Czech. Chem. Commun., 1971, 36, 3431. D. D. Pemn, Dissociation Constants of Organic Bases in Aqueous Solutions (Butterworths, London, 1965), supplement (1972). 2o H. Baba, A. Matsujama and H. Kokobun, Spectrochim. Acta, Part A, 1969, 25, 1700. 21 G. S. Denisov and V. M. Schreiber, Vestn. Leningr. Univ., 1976, 4, 61. (PAPER 3/727)

ISSN:0300-9599    

DOI:10.1039/F19848000553

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1984,80, 563-570 Preparation and Properties of Monodispersed Spherical-colloidal Particles of Zinc Sulphide BY DEBORAH MURPHY WILHELMY AND EGON MATLTEVIC* Department of Chemistry and Institute of Colloid and Surface Science, Clarkson College, Potsdam, New York 13676, U.S.A. Received 23rd May, 1983 Spherical-colloidal zinc sulphide particles of narrow size distribution were prepared using a two-step process. First zinc sulphide ‘seeds’ were precipitated by aging a solution consisting of zinc nitrate, nitric acid and thioacetamide. The ‘seed’ crystals were then allowed to age further at an elevated temperature. X-ray analysis showed the particles to be crystalline with structure characteristic of sphalerite. Various properties such as electrokinetic and optical, the size distribution and the specific surface area of the solids were determined and the growth mechanism was analysed in terms of Nielsen’s chronomals.Although several studies have dealt with the preparation of uniform metal (hydrous) oxide particles,l only a few reports describe the precipitation of metal sulphide sols of narrow size di~tribution.~-~ The formation of colloidal dispersions of the latter depends on the same factors, as does the homogeneous precipitation of metal oxides (i.e. pH, temperature, nature of the anion and the reactant concentration). The adjustment of these parameters provides a valuable tool for controlling the generation of particle-forming solutes, as well as the nucleation and particle-growth processes which may lead to a monodispersed system.In an earlier report it has been shown that exceedingly uniform colloidal spherical, yet crystalline, particles of cadmium sulphide can be produced by homogeneous precipitation.2 Similar principles have been employed to obtain monodispersed zinc sulphide dispersions. Although the particles are perfectly spherical, the X-ray analysis showed them to have a crystalline structure consistent with sphalerite. The particle- growth mechanism was estimated from the chronomal analysise and the properties of the dispersions elucidated by light scattering and electrophoresis. Zinc sulphide is of interest in many applications, including pigments because of its opacity’, water purification because of its sorptive capacityg-B and electroluminescence.1° The monodispersed crystalline spheres described in this work can serve as excellent models for studies of interest in mineral technology.EXPERIMENTAL MATERIALS Thioacetamide (TA, Alfa Division, Ventron Corp.) was recrystallized in spectroscopic-grade benzene. The purified material was dried at 72OC in a vacuum oven and kept refrigerated under nitrogen. The concentration of the TA stock solution, which was freshly prepared before each experiment, was determined from absorbance measurements at 261 nm. All other chemicals were of highest-purity grade used without further purification. PREPARATION OF ZINC SULPHIDE SOLS The procedure consists essentially of homogeneous precipitation of zinc sulphide in zinc salt solutions using TA as the source of sulphide ions.The latter are slowly released in acidic media 563564 COLLOIDAL PARTICLES OF ZINC SULPHIDE at room temperature but the decomposition of TA can be accelerated by moderate heating. The most successful procedure involved first producing ‘seeds’ (ca. 0.08 pm in radius) which were then used to grow larger colloidal spheres of ZnS of a very narrow size distribution. The ‘seed’ crystals were obtained by keeping a solution of 0.024 mol dm-, Zn(NO,),, 0.062 mol dm-, HNO, and 0.1 1 mol dm-3 TA in a thermostatted water bath at 26 “C for 5 h. This dispersion was then placed in a second constant-temperature bath at 60 OC and the aging was continued for up to 350 min. The size of particles and their uniformity depended on the length of the reaction time at 60 OC.On prolonged heating a second population of colloidal zinc sulphide particles was formed and continued to grow while the first population, having reached a maximum size, settled on the bottom of the reaction vessel. It is essential to stress the purity of TA; unless a carefully recrystallized reagent was used polydispersed particles were obtained. The precipitation was carried out either in 50 cm3 Teflon-lined screw-capped Pyrex culture tubes or in 250 cm3 glass-stoppered Pyrex bottles. Zinc sulphide particles were separated from their mother liquor by spinning the suspension at ca. 3600 r.p.m. in a laboratory centrifuge. An ultrasonic bath was used to resuspend the particles in water. To follow the particle growth as a function of time, an aliquot of the dispersion was removed, quickly filtered through Nuclepore membrane, washed and resuspended for further study.The effect of hydrazine on the precipitation of ZnS was studied by preparing the ‘seeds’ in the usual manner, and after 5 h this reducing agent was added to yield a final concentration ranging between 1 and 4%. The system was then aged at 60°C for up to ca. 100 min. An attempt was made to prepare zinc sulphide solids in ammoniacal solutions. The initial solutions had the same composition of the zinc salt and TA as described above but a buffer containing 0.05 mol dm-3 ammonia, 0.05 mol dme3 ammonium nitrate and 0.4 mol dm-, sodium nitrate was used instead of nitric acid. Aging was carried out at 26°C. These systems resulted in immediate precipitation and could not be employed to generate monodispersed ZnS.ANALYSIS OF THE PARTICLES The size distribution of zinc sulphide particles was determined either by transmission and scanning electron microscopy or by light scattering. In the latter case the intensities of the horizontally and vertically polarized monochromatic light (436 or 546 nm wavelength) scattered from a zinc sulphide suspension were measured as a function of angle in a Brice-Phoenix series 2000 photometer fitted with Glan-Thompson prisms. Data were analysed by means of the polarization ratio method described elsewhere.l19 l2 The light scattering was also used to determine the relative refractive index, m, of the suspended colloidal zinc sulphide particles. The best fit for the size distribution between experimental and calculated values was obtained with m = 1.96 at 1 = 436 nm, which is close to the literature value of m = 1.8813 for the same wavelength.The crystal structure of zinc sulphide particles was established by X-ray diffraction and identified by comparison with data for standard samples.14 The specific surface area of the dried particles was obtained by the multipoint B.E.T. method15 in a Quantasorb apparatus equipped with a Quantasorb linear flow controller using helium as the carrier gas and nitrogen as the adsorbate. Electrophoretic mobilities of zinc sulphide particles suspended in aqueous solutions were measured in a Rank Brothers microelectrophoresis apparatus, using a cylindrical van Gils ce11.16 The particles were dispersed in lop2 mol dm-, KNO, solution and the pH was adjusted with HNO, or KOH.RESULTS FORMATION OF THE ZINC SULPHIDE SOLS The perfectly uniform spherical ZnS particles prepared by the described procedure are illustrated in plates l(a) and (b). Their size (and size distribution) was affected in a rather sensitive way’ by temperature. Plates 1(b) and ( c ) show two powders prepared under identical conditions except for temperature at which the ‘ seeds’ wereJ . Chem. SOC., Faraday Trans. I , Vol. 80, part 3 D. M. WILHELMY AND E. MATIJEVIC Plate 1 (a) and (b) a i M a (Facing p . 564)J. Chem. Soc., Faraday Trans. 1, Vol. 80, part 3 Plate 1 ( c ) and ( d ) 0 d .C( N D. M. WILHELMY AND E. MATIJEVIC301 D. M. WILHELMY AND E. MATIJEVIC 35-1 0.15 0.20 0.25 0.30 565 particle radius/pm Fig.1. Particle size distribution of a ZnS sol as determined from electron microscopy (histogram, solid lines) and by light scattering (dashed lines). The system was prepared as described in plate 1 (a). allowed to grow. These ZnS hydrosols showed higher-order Tyndall spectra (HOTS) confirming the uniformity of dispersed particles. A typical size distribution of such a zinc sulphide sol is illustrated in fig. 1. The histogram was obtained by counting ca. 800 particles on transmission electron micrographs and is compared with the corresponding data determined by light scattering. The good agreement indicates that the electron beam and mild drying have little effect on the precipitated solids. The change in the modal particle radius as a function of aging time of the ‘seeds’ at 60 O C is given in fig.2. Different symbols on the same curve represent duplicate runs carried out with solutions freshly prepared before each run. The particles reach a maximum size after longer reaction times (ca. 200 min). On further aging at elevated temperature a bimodal particle size distribution has developed [plate 1 (41. Using the appropriate value of the sedimentation coefficient (2 x cm s-l at 60 “C), it can be calculated that the first population of particles had already begun to settle at the onset of the second nucleation and/or precipitation process. The amounts of zinc ion and of TA left in solution as a function of time of growth of the ‘seed’ crystals is shown in fig. 3. A distinct discontinuity in the curve for zinc is observed between 200 and 225 min of aging.Polydispersed spherical particles precipitated when suspensions of ZnS ‘ seeds ’ were heated at 60 OC in the presence of hydrazine; their average size after a given growth time was larger than in the absence of hydrazine. Furthermore, a second population of particles did appear after 80 min at a final concentration of hydrazine of 1 %, which is a considerably shorter time than when the ZnS ‘seeds’ were grown without this additive.566 COLLOIDAL PARTICLES OF ZINC SULPHIDE time/min Fig. 2. Change in the radius of ZnS particles as a function time as determined by electron microscopy for systems grown at 26 OC from ‘seeds’ at 60 OC. The two symbols (0) and (0) represent data obtained in separate experiments using different stock solutions. The triangles indicate data derived from light scattering.PARTICLE CHARACTERIZATION AND PROPERTIES The chemical analysis (Schwarzkopff laboratory) showed that the ratio of Zn:S in dried particles was exactly 1 : 1. The X-ray diffraction data of the collected zinc sulphide solids displayed five major lines characteristic of sphalerite, confirming particle crystallinity. The B.E.T. analysis of samples with a modal diameter of 1.3 pm dried at 100 and 300 OC yielded the same specific surface area of 66 m2 gel, indicating no change in porosity on increasing the calcination temperature. Assuming a smooth spherical particle of the same diameter and a density of 4.1 g ~ m - ~ , the calculated specific surface area would be 1.1 m2 g-l; obviously, either porosity and/or surface roughness develops on drying.Fig. 4 illustrates the electrokinetic behaviour of the aqueous suspensions of zinc sulphide particles as a function of pH. The isoelectric point (i.e.p.) was found to be at the low pH value of 3.0. PARTICLE-GROWTH MECHANISM The particle-growth mechanism for the zinc sulphide sol was evaluated by the chronomal analysis,sv 1 7 9 which was developed for particles uniform in size and shape. In principle, these chronomals permit one to distinguish between diffusion-controlled, mononuclear and polynuclear layer crystal growth.567 2.4 I a E - E N 2 z \ 1 . 6 c( 0.8 time/min 1 I I 1 I 1 0.8 0 10 0 200 300 time/min Fig. 3. Concentration of zinc (upper) and of thiocetamide (lower) as a function of aging time of the ‘seed’ sol aged at 60 OC.Different symbols indicate duplicate runs. The experimental data were found to fit only the polynuclear chronomal, Ip, for p = 1 , in agreement with the earlier findings with the cadmium sulphide s01.~ The linear plot of Zp as a function of time for p = 1 is given in fig. 5. This result indicates that the zinc sulphide particles grow by surface nucleation on the initial ‘seed’ crystals. Using this plot the values of K p and of the rate constant were calculated to be 1.4 h and 6.3 x cm s-l, respectively. DISCUSSION The precipitation of metal sulphides by thioacetamide (TA) has found extensive use in analytical chemistry. Specifically, the formation of ZnS was studied as a function of different parameter~l~-~~ and found that in acidic solutions (pH -= 2.5) the process was controlled by the hydrolysis of TA and that at higher pH values the rate was determined by Zn-TA complexes.As a rule the aim was to obtain coarse, easily568 2 - I I I 1 - - - n 0 - 2 - 1 I I I COLLOIDAL PARTICLES OF ZINC SULPHIDE O l I I I I 1 1.0 I .5 2 .o 2.5 3 .O 3 . 5 time/h Fig. 5. Plot of the first-order polynuclear chronomal, I*, as a function of time for the growth data shown in fig. 2. filterable solids, and in no case was reported the preparation of well defined colloidal ZnS by this procedure. The slow hydrolysis of TA is the essential factor in the homogeneous precipitation of monodispersed ZnS. At lower temperature the sulphide ions are generated at a rate that allows for one burst of nuclei, which then uniformly grow to larger particles.As the temperature is increased the hydrolysis of TA is accelerated sufficiently forD. M. WILHELMY AND E. MATIJEVIC 569 sulphide ions to be generated faster than consumed in the particle growth. Thus a second critical supersaturation develops resulting in renewed nucleation and a bimodal size distribution [plate l(d)]. This mechanism is supported by the change in the content of zinc in solution during the precipitation process. After ca. 200 min the concentration of Zn2+ levels off to decrease again on continuous aging. At this time the secondary nucleation sets in and the new population of particles begins to grow causing further depletion of Zn2+ ions in solution. The precipitation at higher temperature (80 "C) resulted in polydispersed systems [plate 1 (c)].Apparently, continuous nucleation interferes with the growth of the ' seeds '. The effect of hydrazine on TA is to increase the rate of release of sulphide ions.22 The generation of the latter at 60 O C is too fast to be consumed in the particle-growth process, yielding a polydispersed system. These examples clearly show how critical is the kinetics of various reactions in homogeneous precipitation. The chronomal analysis, as applied only to the first population of particles, indicated a surface-controlled growth mechanism, which is to be expected since the process was initiated by the formation of 'seeds'. A good agreement in the particle size distribution as determined by light scattering and electron microscopy is the consequence of particle crystallinity and little hydration, which is supported by previous findings.24 The large discrepancy in the geometric specific surface area from that calculated using the B.E.T.method must be due to the development of micropores on drying the powder. The puzzling aspect of this system is the crystallinity of the perfectly spherical ZnS particles. The crystalline nature of colloidal spheres has been reported before, but either such particles were obtained by phase transformation of amorphous solids of the same morphology, as is the case with titania,25 or spherical particles were composites of much smaller subunits.1*26 In the case of ZnS reported here there is neither indication that recrystallization took place in the course of aging nor is there evidence for a substructure.Thus the question of the crystallization mechanism remains unresolved. This work was supported by the N.S.F. grant no. CHE 80 13684 and is part of D. M. W.'s Ph.D. Thesis. ' E. Matijevid, Ace. Chem. Res., 1981, 14, 22. E. Matijevid and D. M. Wilhelmy, J . Colloid Interface Sci., 1982,86, 476. G. Chin, J. Colloid Interface Sci., 1981, 83, 309. G. Chin and E. J. Meehan, J. Colloid Interface Sci., 1974, 49, 160. G. Chin, J . Colloid Interface Sci., 1977, 62, 193. A. E. Nielsen, Kinetics of Precipitation (Pergamon, Oxford, 1964). D. H. Parker, Principles of Surface Coating Technology (Interscience Publishers, New York, 1965), p. 79. M. S. Musalev, V. V., Popov and I. A. Dibrov, French Patent, 2,094,528. J. Shimoiizaka, M. Nanjo and S. Usui, Nippon Kogyo Kaishi, 1972, 88, 545. lo E. Schlam, Proc. I.E.E.E., 1973, 61, 894. l1 M. Kerker, The Scattering of Light and Other Electromagnetic Radiation (Academic Press, New l2 M. Kerker, E. Matijevid, W. F. Espenscheid, W. A. Farone and D. Kitani, J. Colloid Sci., 1964, 19, l 3 J. R. DeVore, J. Opt. Soc. Am., 1951, 41, 416. l4 Natl Bur. Stand. (US.) Circ. 539, 1953, 2, 16. l5 S. Brunauer, P. Emmett and E. Teller, J . Am. Chem. Soc., 1938, 60, 309. l6 A. M. James, Surf: Colloid Sci., 1979, 11, 121. l7 A. E. Nielsen, Acta Chem. Scand., 1959, 13, 784. I* A. Bell and E. Matijevid, J . Phys. Chem., 1974, 78, 2621. York, 1969). 213.570 COLLOIDAL PARTICLES OF ZINC SULPHIDE 19 D. F. Bowersox, D. M. Smith and E. H. Swift, Talanta, 1960, 3, 282. zo R. B. Hahn and F. M. Shellington, Anal. Chim. Acta, 1958, 19, 234. 21 D. H. Klein and E. H. Swift, Talanta, 1965, 12, 349. 22 D. M. King and F. C. Anson, Anal. Chem., 1961,33, 572. 23 R. B. Hahn and D. L. Pringle, Anal. Chim. Acta, 1964,31, 382. 24 L. Gordon, M. Salutsky and H. Willard, Precipitation from Homogeneous Solutions (Wiley, New 25 M. Visca and E. MatijeviC, J. Colloid Interface Sci., 1979, 68, 308. 26 T. Sugimoto and E. MatijeviC, J. Colloid Interface Sci., 1980, 74, 227. York, 1969), chap. 6. (PAPER 3/836)

ISSN:0300-9599    

DOI:10.1039/F19848000563

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faradoy Trans. I , 1984,80, 571-588 Colloidal Platinum Sols Preparation, Characterization and Stability towards Salt BY D. NEIL FURLONG,* ANTON LAUNIKONIS AND WOLFGANG H. F. SASE CSIRO, Division of Applied Organic Chemistry, G.P.O. Box 433 1, Melbourne, Victoria 3001, Australia AND JOHN V. SANDERS CSIRO, Division of Materials Science, University of Melbourne, Parkville, Victoria 3052, Australia Received 3rd June, 1983 Stable aqueous colloidal platinum sols have been prepared by the citrate reduction of chloroplatinic acid. Particle size at the completion of the reaction was found to increase with increased heat input during reduction. The u.v.-visible spectrum of the platinum sols depends upon particle size up to 4 nm. A stirring-centrifugation procedure was used to identify the extent of coagulation induced by added electrolytes. It was found that the smaller platinum particles required higher concentrations of added electrolyte before being destabilized.A survey was made of the coagulation of the platinum sols induced by mono-, di- and tri-valent cations. Hydrogen treatment of platinum sols leads to growth of particles up to a maximum size of ca. 5 nm and substantial linkage of primary particles to form extensive aggregates. Such growth and aggregation lead to significant changes in the u.v.-visible spectra and reduced resistance to electrolyte-induced coagulation. Aqueous dispersions of colloidal metal particles', have recently been studied as redox catalysts for the solar photolysis of water. They have been found useful in sacrificial photolysis systems both in dispersed form and when supported by polymers3 or inorganic oxide~.~ Bifunctional catalysts incorporating dispersed platinum have also been claimed5 to allow simultaneous reduction and oxidation of water, the so-called non-sacrificial photolysis process.By far the most used metal has been platinum because of its proven capabilities as a reduction catalyst in many areas of catalysis. Several methods of preparation of aqueous colloidal platinum sols have been proposed involving the reduction of aqueous platinum salt solutions either directly with hydrogen4v 6, or, for example, via ph~tochemical,~~ * pulse-radiolyti~~~ lo or thermal An obvious attraction of colloidal metal catalysts has been the promise of reproducible variation of particle size with the likely consequence of variations in catalytic activity.There has to date emerged an uncertain picture as to the dependence of the efficiency of solar reduction of water on the particle size of the colloidal platinum catalyst used.l29 l3 In addition to catalyst primary particle size the state of dispersion of catalyst particles in an aqueous photolysis reaction medium is likely to be of importance. Whilst this has been recognized, and in general polymer supports are used to prevent aggregation, little has been reported on aggregation behaviour of colloidal platinum metal particles dispersed in aqueous electrolyte solutions. The classic colloid literatureI4 does contain numerous studies on aqueous metal sols such as those of silver and gold with a view in general towards establishing light-scattering and aggregation behaviour.57 1572 COLLOIDAL PLATINUM SOLS However, such studies for platinum sols have been few; e.g. a brief report on preparation and spectral properties by Aika et all5 and some indirect references by Gratzel and coworkers on variation of particle size and aggregation behaviour.18 The aim of the present study was to investigate the citrate reduction method15 with the aim of enabling controlled preparation and physical characterization of various particle sizes of colloidal platinum. The aggregation behaviour of such platinum sols in aqueous electrolyte solutions has been investigated with the aim of assessing the likely behaviour of such sols in aqueous photolysis systems.EXPERIMENTAL REAGENTS Triply distilled water with a maximum conductivity of 0.8 p W 1 cm-’ was used to prepare all solutions and dispersions. Acetic acid/sodium acetate buffer solutions and KOH and HNO, solutions for pH control were prepared using analytical-reagent (A.R.) chemicals. Electrolyte solutions, including those of tris(2,2’-bipyridine)ruthenium(11) dichloride (RUB+) and methyl viologen dichloride (MV2+), were prepared using A.R. salts, except for aluminium chloride and lanthanum chloride solutions, for which Laboratory Reagent salts were used. Research-grade gases were used throughout the study. METHODS SOL PREPARATION AND ANALYSIS Platinum sols were prepared by the citrate reduction of chloroplatinic acid. The method used was similar to that described by Aika et all5 except that heating was achieved using an oil bath with temperature control of & 1 *C instead of a heating mantle.The details of the preparation procedure are as follows: 249cm3 of water were heated in a round-bottomed flask to the required reaction temperature, 30 cm3 of 1 wt % aqueous sodium citrate were then added and the temperature reestablished, 4 cm3 of aqueous chloroplatinic acid solution were then added and the reaction mixture maintained at temperature for 4 h. Vigorous stirring was maintained at all times. The reaction mixture was then cooled rapidly to room temperature by immersing the reaction flask in an ice-water mix. Any ongoing reduction was stopped by this lowering of temperature. Excess citrate, and in some cases unreduced chloroplatinic acid, were separated from the colloidal platinum by stirring with a mixed-bed ion-exchange (A.R.Amberlite MBl) resin until the conductivity was < 2 pi2-l cm-1; the conductivity of the sol before ion exchange was typically in the range 800-1200 pi2-l cm-’. The sol conductivity after ion exchange is estimated to correspond to a maximum citrate concentration of 8 x mol drn-,. Blank experiments on chloroplatinic acid solutions revealed that the quantities of resin used in the ion-exchange procedure would remove > 90% of the total starting amount of acid. It is assumed therefore that any acid remaining in solution after the citrate reduction procedure because of incomplete reduction to colloidal platinum was removed via the ion-exchange procedure used.The extent of reduction achieved during each citrate preparation was determined by dissolving the colloidal platinum (aqua regia) present after the ion-exchange treatment and analysing for aqueous platinum by atomic absorption It was observed that the ion-exchange procedure also resulted in the removal of a small amount of the colloidal platinum. The colloidal platinum sols exhibit a scattering-absorption spectrum in the u.v.-visible wavelength region15 which can be used to monitor changes in colloidal particle concentration. By measuring absorbance at any wavelength, in the present work 450 nm, removal of colloidal platinum during ion exchange could be monitored and hence atomic absorption analyses corrected to allow calculation of the real extent of reduction of chloroplatinic acid to colloidal platinum during the citrate reduction preparation.All u.v.-visible spectra were recorded using a Cary 219 spectrophotometer. At the concentrations used (< 3 x mol dm-3) the absorbance of H,PtCl, (or for that matter the PtC142- anion) at 450 nm was insignificant compared with that of the colloidal platinum sol; the latter typically were 0.154.65 (1 cm path-length cell). Electron micrographs were obtained using a Jeol 100 CX microscope. A few drops of theD. N. FURLONG, A. LAUNIKONIS, W. H. F. SASE AND J. V. SANDERS 573 sol of interest were put onto a carboncoated microscope grid. A number of different areas on each grid was examined and ca. 10 representative areas photographed for each sample.Particle sizes were measured on prints at 660000 magnification. The ‘average particle size’ in a sol (e.g. in table 1) is the arithmetic mean size estimated by measuring ca. 100 particles from representative micrographs. Dispersions are termed ‘monodisperse’ if the range of particle size was < 0.5 nm (e.g. in table 2 later). SOL STABILITY In the present study the relative particle concentration of a platinum sol was assessed by measurement of sol turbidity at 450 nm (Cary 2 19 spectrophotometer). Solcoagulation experiments were performed in 15 cm3 centrifuge tubes, and the rate of settling or more often the extent of coagulation was determined from turbidity measurement on the top 10 cm3 of sol. The concentration when expressed relative to the initial particle concentration gives the quantity ‘stability (%)’ in all figures.All platinum sols prepared in the present study (table 1) from which excess citrate had been removed were stable for at least several months if left under gravity alone. It has been reported18* le that gold particles prepared by the citrate reduction method undergo pairing of primary particles on standing for a few days after the removal of excess citrate. Electron micrographs of the platinum sols of the present study did not reveal any such systematic pairing. When electrolyte was added to the platinum sols of the present study the rate of settling of particles was found to depend upon the concentration of added electrolyte. Even sols that eventually completely settled out often required in excess of 24 h to do so.Often sols at electrolyte levels below that necessary to induce total coagulation were still slowly settling out a week or more after the addition of electrolyte. This ‘ standing/settling’ approach has been used by Brugger et a1.16 in a brief look at the effect of added salt and polymer on platinum sol stability. In the present study, however, it was felt that the long settling times required with this method led to unnecessary imprecision. The criterion for coagulation was chosen in the present study to be centrifugation for 15 min at 6000 g (corresponding to 6000 r.p.m. with the centrifuge used). Simple Stokes-law20 calculation indicates that platinum aggregates of diameter > ca. 50 nm would have been removed by this centrifugation from that part of the supernatant that was subsequently withdrawn and analysed for platinum particles.Given that the platinum particles in the present study are generally < 5-8 nm in diameter when prepared it can be said that coagulation has proceeded extensively to produce these aggregates of size 50 nm. In all coagulation experiments sols were stirred by magnetic stirrer at ca. 100 r.p.m. for 20 h subsequent to addition of salt and prior to centrifugation. The excellent reproducibility of sol preparation with respect to coagulation is shown in fig. 1 for sol preparation F (table 1) at pH 5. Electron micrographs of dispersions corresponding to points I to V down the ‘coagulation edge’ of fig. 1 confirmed the above calculation, showing that the coagulation criterion used corresponded to growth of aggregates to > ca.50 nm. In addition the data in fig. 1 also show that the coagulation edge was independent of platinum concentration at least over the range 10-100 mg dm-3 (ca. 101s-1017 particle dm-3). The inset in fig. 1 shows that, as expected, the assessed extent of coagulation under a given set of conditions depends upon the applied centrifugal force. The data of fig. 1 also show the stabilizing effect of citrate on the platinum particles, an effect found with a variety of anions.21 All coagulation experiments discussed in the following sections were performed using sols which had been treated with ion-exchange resin. In summary, then, it was found that the stirring-ntrifugation procedure adopted identifies the occurrence of gross particle aggregation and gives a reproducible assessment of sol stability.RESULTS AND DISCUSSION PLATINUM SOL PREPARATION Some physical properties of platinum sols prepared in the present study are given in table 1 . Sols denoted A, B, C and F were prepared using oil baths at various temperatures, whilst samples D and E were prepared by the methods of Aika et all5 (also citrate reduction of H,PtCl,, heating mantle) and Rampino and NordZ2Table 1. Physical properties of colloidal platinum sols conversion aver age initial [Pt] colloidal to colloidal particle preparation (PPW [Ptl (PPm) Pt (%I size/nm dispersi ty aggregation Sa A oil bath at 85OC ii 1 oil bath at 90°C ( C oil bathat 140°C D Aika et al.b E Rampino and NordC oil bath at 165 "C F2 112 56 112 56 56 100 56 112 45 35 67 46 not available 92 52 104 40 1.5-2.0 63 2.0 59 1.5 82 2.5 2.5 - 92 3 .O 93 ca.4 93 ca. 4 monodisperse monodisperse a few larger particles monodisperse particle sizes from 2.0 to 4.0 nm 2.0 to 3.0 nm 2.0 to 8.0 nm 1.5 to 6.0 nm none none a little formation none none of small aggregates none some linking into substantial linking chains into chains 3.6 0 2.8 3.2 5 U 2.3 $ 1.8 * 4 1.8 g 1.8 8 1.4 G a S = -d log A /d log A where A = sol absorbance at wavelength A nm. Sol prepared according to method of Rampino and Nord.22 Sol prepared according to method of Aika et al.I5 using a heating mantle.D. N . FURLONG, A. LAUNIKONIS, W. H. F. SASE AND J. V. SANDERS 575 100 90 80 70 n E x 6o + .- - 2 50 40 30 20 10 0 w repeat - 3 -2 -1 - 0 1 log [ KCI 1 Fig.1. Coagulation of platinum sol with KC1 at pH 5.0 (solid symbols, excess citrate not removed; open symbols, excess citrate removed): O,., sol preparation F1, 50 ppm colloidal platinum; 0, +, repeat preparation F1, 50 ppm; 0, ., sol preparation F1, 20 ppm; 0, sol preparation F2, 100 ppm; V, repeat preparation F2, 100 ppm; m, repeat preparation F2, 30 ppm; A, repeat preparation F2, 10 ppm. Colloidal platinum concentration refers to that used in coagulation experiments. Inset: Centrifugation of an F1 sol. (reduction of K,PtCl,, heating mantle), respectively. The latter two sols are included for comparison in table 1 and are situated in the table according to platinum particle size. During the preparation of sols C and F (and also D and E) a substantial temperature gradient existed across the boundary between aqueous-sol/glass- vessel/oil-bath; such a gradient was not present during preparations A and B.Some clear trends between oil bath temperature and sol characteristics are evident from table 1. (1) Particle size (column 5 ) increases substantially with temperature. Plates l(a) and (6) for 90 and 165 OC sols, respectively, depict this variation in primary particle size as well as the higher degree of monodispersity found for the lower-temperature sols. Aika et al.15 produced a sol of 3.4k0.9 nm diameter particles. Unfortunately these workers did not specify the exact heating conditions present when using a heating mantel; direct comparison thgm with the sol D (2.5 nm) prepared in the present study using a heating mantle is possibly not appropriate. For similar reasons, the claim23 by Kiwi and Gratzel that their variation of the method of Rampino and Nord22 produces 1 1 .O nm particles is difficult to assess with respect to the particle size of sol E (3.0 nm).Gratzel and coworkers16 claim that the citrate reduction method produced particles of different sizes over the temperature range 85-95 OC: from the results of the present study (sols A, B, C and F) no significant variations in size were found over this temperature range (table 1). Interestingly Wilenzick et aZ.24 prepared a 4.0 nm sol with 90 "C heating conditions; unfortunately they did not specify clearly their method of preparation. (2) The extent of reduction of H,PtCl, increased with temperature (column 4).In576 COLLOIDAL PLATINUM SOLS f a a, 0 - I I I I 1 300 400 500 600 h/nm Fig. 2. Normalised absorbance (A) of platinum sols: (-) A, (- -) B2, (- - - -) B2 (hydrogen treated), (---) C, (--------) F2. Inset: logA against IogE, plots for the same sols. contrast to the average particle size the degree of reduction increased significantly when the temperature was increased from 85 to 90 O C . The mechanism of synthesis of colloidal gold with citrate as reductant has been discussed in det-1: by Turkevich et af.,25 who carried out a comprehensive study of the nucleation and growth of colloidal particles and of the role of the citrate ion and its derivativesz6 as reductants and surface-active species. Whilst nucleation is very rapid, the growth of gold particles was found to proceed much more slowly at lower temperature^.^^ Wilenzick et ~ 1 .~ ~ were thus able to increase the size of gold (and platinum) particles by seeding with small particles and allowing reduction to proceed for prolonged periods. The effects of temperature on platinum particle size and degree of reduction seen in the present study are similar to those reported for gold sols. Turkevich and Kim27 have prepared palladium sols of various sizes and found that if the temperature was reduced to 50 "C no reduction by citrate occurred. An important observation of Turkevich et af.25 was that surface adsorption of both citrate ion derivatives and ionic metal complexes, in the case of platinum sols most probably PtClG2-, is involved in the nucleation and growth of the colloidal metal particles.It is also probable that these species play an important role in determining the physical stability of the platinum sol. ( 3 ) The nature of the u.v.-visible absorption-scattering spectrum exhibited by a platinum sol varied with the temperature of preparation. As reported by Aika et af.15 these sols give a smoothly increasing absorbance ( A ) as incident wavelength (A) is decreased; no spectral peaks are observed for 200 < A/nm < 800 (fig. 2). In principle the exact variation of A with A can be calculated from Mie theoryz8* 29 if the electronicD. N. FURLONG, A. LAUNIKONIS, W. H. F. SASSE AND J. V. SANDERS 577 properties of the colloidal metal are known. Such calculations have been performed, for example, on gold sols (> 20 nm particles) by Turkevich et aL30 These workers were able to fit theoretical with experimental spectra by using the refractive indices as adjustable parameters.In the case of gold sols, which are more strongly absorbing than those of platinum28 and exhibit an absorption band at ca. 500 nm, there may be some value in comparing theory with experiment, as have Turkevich et aL30 Henglein and on the other hand chose not to attempt calculation of spectra for his gold sols of 3-4 nm particles. The electronic properties of colloidal platinum are not known, a problem recognized by Aika et aZ.15 in their unsuccessful attempt to calculate spectra, and in the present study we have not attempted to pursue Mie theory calculations to fit our observed spectra. In general terms a most useful observation is that the variation of A with A for the platinum sols, as represented by S = - d log A/d log A, is dependent upon particle size; some data are presented in fig.2. For ease of comparison the spectra are normalized at 450 nm. The inset in fig. 2 shows the spectra on a log/log scale, from which it is evident that S is in most cases constant from ca. 250 to 600 nm. S values for sols A-F are given in column 8 of table 1 . In general terms S decreases with increasing particle size. Interestingly the S value calculated by us from spectral data of Aika et for a platinum sol also produced by citrate reduction corresponds very well with that of the present study. By contrast Delcourt et aZ.10 claim to produce platinum particles of average diameter < 1 nm radiolytically from H,PtCl, and report spectra for which S is ca.1.6. This value is much less than that of sols of corresponding particle size in the present study: Delcourt et al. also recognise that their sol displays different spectral characteristics. Whilst it is not clear that Delcourt et aZ. were accurate with their estimated particle size, their electron micrographs indicate in fact that their particles were larger than 1 nm, it is possible that S values of sols of different preparative procedures may not be directly comparable. S values are useful, however, in the present study in characterizing the size of platinum particles after preparation and changes induced by change of sol conditions. For example, when a sol of particles smaller than 4 nm was destabilized by the addition of electrolyte, S was observed to decrease rapidly to ca.1.3, that is to approximately the value found for sols F, during the first few minutes of coagulation and thereafter remain constant. The S value remained constant even though the sol became unstable to the centrifugation procedure described in the previous section, indicating that aggregate growth had continued to give aggregates larger than ca. 50 nm. It appears then that S is insensitive to changes in aggregate size, and probably also in primary particle size, at sizes greater than ca. 4 nm. It is possible that platinum particles prepared at different temperatures and thence exhibiting different ( < 100%) reduction of platinum also exhibit different electronic properties which result in the variations in S.However, the changes in S values described for the coagulating system would appear to indicate that particle-size effects are dominant. (4) Particle size, degree of reduction and S values did not change significantly when the initial H,PtCl, concentration was doubled (table 1, sols B and F). TREATMENT WITH HYDROGEN As the platinum sols were being prepared with a view to their use as reduction catalysts in hydrogen-producing photolysis systems, it was of interest to observe their behaviour in an atmosphere of hydrogen. Table 2 summarizes electron microscope and spectroscopic analyses of hydrogen-treated sols. Note that prior to all bubbling experiments sols had been treated with ion-exchange resin to remove unreduced H,PtCl,.Two types of change to the particles were induced by hydrogen treatment. 20 FAR 1Table 2. Hydrogen treatment of platinum sols initial state of sol sol Pt particle size/nm aggregation S F1 oil bath at 165OC C oil bath at 14OOC oil bath at 9OoC A oil bath at 85OC A 5 x mol dm-3 glu ta t hione added to 85 OC sol 2.0-8.0 some linking 1.8 into chains 2.5 average dispersed 2.3 1.5 average dispersed 3.2 2.0 average dispersed 2.8 2.0 average dispersed 3.6 2.0 average dispersed 3.6 final state of sol Pt particle shoulder at cI r 4.5-8.0 linked into chains 1.4 none 275 nm size/nm aggregation S 8 c 4.5-5.5 linked into chains 1.4 very slight cd 3.0-4.5 extensive linking 1.6 slight" r- $ 5.0 average extensive linking 1.7 noneb 5.0 average extensive linking 1.6 slight" 2.0 average dispersed 3.6 none z a Shoulder on spectrum 275 nm removed when sol bubbled with oxygen.Sol coagulated after hydrogen treatment without centrifugation.J . Chem. SOC., Faraday Trans. I , Vol. 80, part 3 Plate 1 Plate 1. Electron micrographs (magnification 660000); (a) sol preparation B1, (b) sol preparation F 1 . D. N. FURLONG et al. (Facing p . 578)J . Chem. SOC., Faraday Trans. I , Vol. 80, part 3 Plate 2 Plate 2. Electron micrographs (magnification 660000, sol preparation Bl): (a) bubbled with H, (sol requires centrifugation to destabilize), then (b) bubbled more extensively with H, (sol settles rapidly on standing). D. N. FURLONG et al.D. N. FURLONG, A. LAUNIKONIS, W. H. F. SASSE AND J. V. SANDERS 579 First, for all sols particle-particle linkage was induced.Enustun and Turkevich18 have commented that their gold particles underwent fusion if left standing for 3 months but did not offer any explanation for the effect. Secondly, for sols of particle size < ca. 5 nm hydrogen treatment also induced primary particle coalescence, i.e. growth of primary particle size to ca. 5 nm consequent to the linkage of particles. For example, for the polydisperse sol F1 (oil bath at 165 "C) hydrogen treatment resulted in growth of the smaller particles (table 2). In the case of the lower-temperature preparations all particles were substantially increased in size; plates l ( a ) and 2 illustrate the changes for a 90 OC sol. This sol after CQ. 15 h slow bubbling with hydrogen [plate 2(a)] clearly consisted of primary particles of ca.4-5 nm diameter. This particular sol did not at this stage exhibit substantial particle-particle linkage [plate 2(a)] and was stable unless centrifuged. Further extensive hydrogen treatment (several days) produced substantial particle linkage [plate 2(b)], and in fact these aggregates settled out rapidly without centrifugation. It was found for all sols that similar prolonged bubbling with oxygen or nitrogen produced no apparent changes in sol characteristics. As might be expected from the previous discussion, particle growth due to hydrogen treatment was accompanied by spectral change and a reduction in the S value (table 2). The spectra for a 90 OC sol before and after hydrogen treatment are shown in fig. 2.On some occasions prolonged hydrogen treatment of a small-particle sol produced a small shoulder on the spectrum at ca. 275 nm. This peak may be due to a small amount of absorbed PtCli-, which has a spectral peak at around this wavelength32 and is possibly generated from absorbed PtCli- by hydrogen treatment. The shoulder at 275nm was removed by oxygen treatment. Delcourt et aZ.l0 have observed a weak band at 265 nm on spectra of platinum sols produced radiolytically from H,PtCl,. They claim however that it cannot indicate PtCli- due to the absence of an accompanying strong band at 217 nm. The origin of the shoulder at 275 nm remains therefore uncertain. The mechanism of the hydrogen- induced coalescence of particles is not known. It is probable that reduction of surface adsorbed ions, such as PtC1,2- and PtC1,2-, would lead to a lowering of surface charge density and reduce particle-particle repulsion. The metal surfaces thus formed would be highly hydrophobic in that polar surface groups would be absent, and 'contact' of surfaces upon aggregation may not involve any intermediary solvent layer.It can only be speculated then that at this stage surface-energy and curvature effects for particles in the < 5 nm size range are such as to encourage the tightly bound aggregates of small particles to coalesce into larger uniform units. STABILITY OF SOLS TOWARDS SALT PARTICLE SIZE The destabilization of sols A, B, C and F by KCl is shown in fig. 3. The concentration ( C ) of added electrolyte that induces coagulation quite clearly increases with decreasing particle size.The coagulation experiments represented in fig. 3 were performed with each sol containing 10 ppm (lod2 g dm-3) of platinum. This means that the smaller particle-size sols, those more resistant to coagulation by KCl, were at higher particle concentrations; for example sol B2 would have contained ca. lo1' particle dm-3 compared with ca. loLg particles dm-3 for sols F. The experiments of fig. 1 for sols F indicated that C was independent of particle concentration at least over the range 101s-1017 particle dm-3. It seems then that the different coagulation behaviour of sols shown in fig. 3 cannot be ascribed to different particle concentrations. Particle-size effects on coagulation have been observed with other ~ o l l o i d s ~ ~ - ~ ~ and theoretical attempts made to explain such effects.Classical DLVO theory14 considers the total energy of interaction between particles (V,) to be the sum of electrical 20-2588 COLLOIDAL PLATINUM SOLS 100 90 80 70 - 60 3 A 0 m 3 50 .- w 40 30 20 10 -3 -2.5 -2 -1.5 - I -0.5 0 log [KCI] Fig. 3. Coagulation of platinum sols with KC1 at pH 5.0, [Pt] = 10 ppm: (>, A; 0, B2; a, B2 treated with hydrogen; 0, B1; e, B1 treated with hydrogen; 0, a, C; ., C treated with hydrogen; (= = =) F (range of preparations). -4rrows link sols before and after hydrogen treatment. double-layer repulsions and van der Waals-London attractions (at greater than molecular distance particle-particle separation) and results in an expression for a monodisperse system of equivalent particles of the form V = flu) d r y , , K , 4 Ho) where a is the particle radius, ry0 the particle surface potential, K the Debye-Huckel electrical double-layer parameter, A the particle Hamaker constant and H, the particle-particle separation.The variation of VT with particle-particle separation, say during a Brownian collision, reveals the kinetic nature of colloidal stability. The energetically favourable reduction of surface area resulting from coagulation may be hindered by the presence of an energy barrier at some particle separation. One approach to the coagulation of colloids is then to relate the size of the energy barrier (V,,,) to the rate of coagulation. The coagulating ability of electrolyte is seen uia the dependence of V,,, on ionic strength and adsorption of ions on particle surfaces.Clearly from the equation above if the V,,, value required for coagulation is set at zero then no particle-size dependence will result. If, more realistically, V,,, at coagulation is related to the average Brownian energy of particles then the above equation will predict that bigger particles are more stable, a trend opposite to that of the present study and also to that found with some polymer latex 34 The DLVO approach outlined above centres around the prevention of particle-particle contact due to a maximum in the interaction-energy-separation profile. The approach has been m ~ d i f i e d ~ ~ - ~ * to allow for the possibility that larger particles can mutuallyD. N. FURLONG, A. LAUNIKONIS, W. H. F.SASSE AND J. V. SANDERS 58 1 ‘adhere’ at relatively large separations because of the presence of the so called ‘ secondary minimum ’ in the energy-separation profile. Smaller particles when interacting may not have the option of secondary minimum coagulation and are therefore required to descend into the primary potential energy well, i.e. to pass over any energy barrier, in order to be stable at small, i.e. aggregated, separations. This barrier, as well as the depths of the primary and secondary minima, is affected by ionic strength, i.e. by added electrolyte. In general terms the above argument was reasonably successful in eFplaining previously o b ~ e r v e d ~ ~ - ~ ~ particle-size effects on coagulation by added electrolyte. In these previous studies only particles of diameter > 25 nm were investigated.In general it is believed that secondary minima are not important for very small particles. For the 1.5-4 nm sols of the present study, however, the range of the electrical double layer as characterized by the decay length K - ~ , ~ ~ even at (monovalent) electrolyte concentrations in excess of 10-l mol dm-3, is comparable to particle size. It is not clear that the sec~ndary-minirnum~~-~~ coagulation theory as proposed, and in particular some of the approximations used, is entirely appropriate for metal particles in such situations. Theoretical verification that secondary minima do exist for the platinum particles in the 1.5-4.0 nm size range requires knowledge of particle parameters (yo, A ) as well as supposition as to the nature of redistribution of ions at particle surfaces within the time scale of Brownian collision, The surface potential during coagulation of the platinum particles is unknown, and estimates for A values of small metal particles vary greatly.It does not therefore seem profitable to pursue extensive calculation at this stage. Calculations performed40 using a surface regulation model for the variation of surface charge and potential during particle- particle encounter do indicate that secondary minima are not likely to be significant for the platinum particles of the present study. Interestingly the theoretical treatments of coagulation described above all refer to coagulation under the influence of Brownian motion and in the absence of external hydrodynamic shear.The incor- poration of shear can41 in some cases lead to the prediction that smaller particles will be more resistant to coagulation by added electrolyte. The magnitude of the shear in the present study is, however, believed4’ to be too small for this effect to occur. The explanation for the observed size dependence would seem then to lie in a variation of surface charge density and/or the metallic nature of particles with particle size. It is frequently observed that particles coagulate when the interacting potential is less than some critical value: numbers like 22 mV (polymers)4o and 18-28 mV (minerals)38 are often quoted. For such relatively small potentials it has been that the relationship between charge (a) and potential (y) around a sphere can, with some approximation, be expressed as where % is a constant and the term in brackets is a correction for double-layer curva- ture (a correction particularly important for small particles in concentrated electrolyte solutions).K a values for sols F, C and B1 at the onset of coagulation (fig. 3) are 0.4, 1.0 and 1.3, respectively, revealing that at coagulation the particles in these sols are exhibiting very curved electrical double layers ( K - ~ - a); hence many of the established theories of colloid stability may not be valid. If it is proposed that the platinum particles of various sizes are coagulating at some common value of y then inclusion of the appropriate K a values in the above equation reveals that oBl > crc > oF. As already noted the degree of conversion from PtC1,2- to PtO increases from sol F to sol C to sol B 1.The particle-size effect on stability would therefore suggest that particle stability is enhanced by surface adsorption of anions, most probably chloroplatinum582 COLLOIDAL PLATINUM SOLS complex anions, and that this adsorption is more extensive for smaller particles. Work is in progress to investigate these properties further. TREATMENT WITH HYDROGEN Bubbling with hydrogen rendered all sols less resistant to subsequent coagulation by KCl (fig. 3). These changes are consistent with the hydrogen-induced growth of primary particles observed with the smaller particle sols (table 2) and with the induced linking of primary particles that occurred during hydrogen treatment of all sols.The extent of destabilization by hydrogen depended upon the duration of hydrogen treatment. The data in fig. 3 are representative: the duration of hydrogen treatment of each sol varied. The data indicate the common tendency of all sols towards less stability with hydrogen treatment. Treatment of any sol for longer periods with hydrogen resulted in progressively less resistance to added salt and eventually to rapid coagulation and settling out even in the absence of added salt. The change in stability of sol B2 depicted in fig. 3 is that corresponding to the extent of hydrogen treatment sufficient to increase primary particle size from ca. 1.5 to 4.5 nm [plate 2(a)]. Following such hydrogen treatment sol B2 showed remarkably similar stability with respect to added electrolyte as sols F, the latter also consisting of particles of average size ca.4 nm. This similarity indicates that the hydrogen-induced changes in sol stability are mainly associated with particle/aggregate size. SPECIFIC CATION EFFECTS The destabilization of sol F1 at pH 5 by a range of mono-, di- and tri-valent cation electrolytes is shown in fig. 4. Sol F1 was chosen to allow for the possibility of increased stability in the presence of other cations whilst avoiding the use of salt solutions > 1 mol dm-3. In all cases the electrolyte anion was chloride. The platinum sol at pH 5 behaves as a typical negative sol in that coagulation is induced in the main by the electrolyte cation and is very dependent, in particular, on the valence (2) of that cation. Variations in behaviour arising from the use of anions other than chloride are in general much less than those of various cations; specific anion effects are discussed elsewhere.21 The coagulation of sol F1 for any added electrolyte shown in fig. 4 proceeds to completion over an electrolyte concentration range of ca. 0.5-1 order of magnitude. It is not constructive then to discuss a critical coagulation concentration. For the present discussion the electrolyte concentration corresponding to 50 % coagulation, C5,, will be used to characterize the effect of various electrolytes. Log C,, against log Zplotscorresponding to the data offig. 4 are shown in fig. 5 , together with previously reported data14 for silver iodide, arsenic sulphide and gold sols. The spread of C,, values for ions of the same valence indicates that whilst valence effects dominate other specific ion effects contribute to sol stability.It is possible that under the pH/total concentration conditions of fig. 4 the aluminium and lanthanum ions exist as hydrolysed species. However, the coagulation induced by these species (fig. 5 ) seems consistent with their being trivalent ions and that in fact such hydrolysis is not significant at the effective equilibrium concentration of these ions in the platinum sols. A least-squares fit through the platinum data of fig. 4 has a slope of 8.4, a value significantly higher than that predicted by electrical double-layer theory embodied in the so-called Schultz-Hardy Z6 rule. The Schultz-Hardy rule does describe the reported behaviour of many 2o the data for arsenic sulphide given in fig.5 are an example. However it is also true that other sols, as was observed for those of platinum in the present study, do not follow the Schultz-Hardy rule: the silver iodide and gold sols represented in fig. 5 are also examples of a greater than sixth powerD. N. FURLONG, A. LAUNIKONIS, W. H. F. SASSE AND J. V. SANDERS 583 chlorides: +, A13+; 0, La3+. 0 -7 t 0 0.1 0.2 0.3 0.4 0.5 0.6 log z Fig. 5. Log C,, plotted against log (cation valence). Experimental points from fig. 4, literature data on AgI, As$, and Au sols from ref. (12). (a) AgI (6.8), (b) As$, (5.9), (c) Pt (8.4), ( d ) Au (7.4).584 COLLOIDAL PLATINUM SOLS dependence on counterion valence. It appears then that the Z6 dependence may not be observed for high-Hamaker-constant materials such as metal The variations of C50 within the groups of mono-, di- or tri-valent cations (fig.4) constitute the so-called lyotropic 43-45 often observed in coagulation studies. Theoretical attempt^*^-^^ to explain the basis of the lyotropic series have centred around the variation in size of the ions concerned. L ~ k l e m a ~ ~ claimed that specific adsorption of cations (for negatively charged sols) was necessary and sufficient for quantitative explanation. On the other hand Pieper et af.45 have criticised the assumptions of the Lyklema approach (both experimental and theoretical) and moveover show that inclusion of ion size in calculations of the interaction potential between particles can explain the lyotropic effect without the need to propose specific adsorption of counterions.These latter workers do, however, recognise that the requirement of their approach that coagulation occur with no change in the surface potentials of interacting particles may be unacceptable in rapidly coagulating systems. Lyklema and de Wit44 reappraised the earlier Lyklema calculations in a manner related to the approach of Pieper et al. but maintained the claim that specific adsorption of cations is necessary for the theory to match experimental trends. Whether or not specific adsorption is involved it seems certain that smaller ions will be better able to induce coagulation than larger ions of the same valence. When considering ion size it is uncertain whether for all types of colloid that the hydrated ion size is of relevance, as specific adsorption if it occurs may involve partial removal of hydration sheaths.The Cs0 data of fig. 4 for monovalent cations are plotted in fig. 6 against the reciprocal of the hydrated ion radius (rH) using rH values given by Nightingale.46 These rH values differ from those calculated by Pieper et Reported coagulation data14 for arsenic sulphide and gold sols are also included in fig. 6 for com- parison. The correlation between log Cj0 and rfil is good for the three sols: for reasons outlined above the quantitative relationship between log C,, and rfil is not clear. The least-squares fit through the platinum data of fig. 6 is included as it allows the coagulation induced by hydrogen ion to be assessed, with particular regard to the possible role of the hydrogen ion as a primary potential-determining ion.It has been reported that platinum sols prepared as in the present study exhibit a pH dependent electrokinetic potential with an isoelectric point at pH < 2.47 This behaviour suggests that the hydrogen ion functions as a primary potential-determining ion for these platinum particles. The implication is therefore that the surfaces of these particles contain chemical functionality, such as hydroxyl groups, with dissociation behaviour related to pH. It might be proposed that this surface functionality corresponds to a thin surface-oxide layer on particles dispersed in aqueous solution. The data of fig. 4, in the form presented in fig. 6, indicate that the hydrogen ion functions in a similar manner to the other monovalent cations studied, i.e.not as a primary potential- determining ion, at least up to concentrations of ca. mol dm-3 (pH > 3). The coagulation curve for HCl in fig. 4 in fact deviates slightly from the shape typical of the other monovalent cations at concentrations > ca. mol dm-3. The deviations if significant are such, however, as to indicate that the platinum sol is slightly more stable than expected. This would not be so if in fact the inherent surface charge was decreasing as the pH was lowered below 3. Of course electrokinetic measurements at pH values below 3 lack precision due to the increasing ionic strength and consequent decrease in measured electrokinetic potential (or mobility). It needs also to be recognised that the interpretation of electrokinetic data on sols consisting of electrically conducting particles is often complex;14 it is not clear that reported electrokinetic studies of platinum sols have been adequate in this respect.Fig. 7 shows the pH stability of sols A, B2, C and F2; the increased stability of the smaller particle sols,D. N. FURLONG, A. LAUNIKONIS, W. H. F. SASSE AND J. V. SANDERS 585 -1.5 -2.0 3 - 2 . 5 - ?? -3.0 -3.5 1 1 1 1 1 1 1 1 1 l I I I I ~ 3.0 3.5 4.0 r i l 1nrn-I Fig. 6. 100 90 80 70 h 60 < 50 40 A 4- .- - m 30 20 10 0 0 1 2 3 4 5 PH Fig. 7. Fig. 6. Coagulation of platinum sols with monovalent chlorides: dependence on hydrated radius of monovalent cation. Coagulation data from fig. 4. 0, Li+; 0, Na+; 0, K+; a, Cs+; Q, H+.(a) As$,, (b) Au, (c) Pt. Fig. 7. Stability of platinum sols with pH. [Pt] = 20 ppm: 0, A; 0, B2; (>, C; 0, F2. as found for destabilization by KCl (fig. 3) is evident. All the results of the present study indicate therefore that the aqueous platinum sols behave as negatively charged particulate sols and that this negative charge is essentially constant at pH > 3. CATION COMPETITION In any photolysis system involving colloidal platinum catalysts there are often several cationic reaction components. A typical photolysis mixture might contain 5 x mol dm-3 RU2+ as sensitizer, mol dmP3 MV2+ as electron relay, 1 O-' mol dm-3ethylene diamine tetra-acetic acid as sacrificial donor and lo-' mol dm-3 acetate buffer. Of course addition of colloidal platinum particles to such a reaction mixture renders them unstable towards coagulation and often polymer supports are used to retard such coagulation.' The destabilization of platinum particles in the photolysis reaction mixture is the result of the combined effects of all cations (and anions21) present.Fig. 8 illustrates one example of the complex coagulation behaviour of platinum sol F1 in the presence of divalent MV2+ and monovalent K+. K+ alone destabilizes the sol at ca. lop2 mol dmP3 whilst MV2+, in line with the higher valence, is effective at concentrations > ca. 3 x mol dm-3. Note that in sols represented586 COLLOIDAL PLATINUM SOLS 100 90 80 70 h 60 2 50 A & .- 4 +-I 40 30 20 10 0 \ - 3 -2.5 - 2 log [ KCl] -1.5 Fig. 8. Coagulation of platinum sols with mixed electrolyte.Pt sol preparation F1 at 20 ppm, pH 5.0. Added MV concentration: 0, 0; 0, 5 x 0, 0, 3 x and 0, 5 x mol dm-3. Numbers on curves refer to destabilization in the presence of the added MV only. by the data in fig. 8 the added concentration of K+ is > 10-fold greater than that of added MV2+. Marked on the various curves in fig. 8 are the corresponding extents of destabilization of the sol with the designated amount of MV2+ alone added. It is clear from the data that small amounts of MV2+, whilst themselves not causing significant sol instability, substantially reduce the subsequent tolerance of the sol to added K+. Given that the MV2+ is in fact at much lower concentrations than K+ its adsorption affinity for the platinum surfaces is then much greater than that of K+ and it is strongly preferentially adsorbed. Of course in a reaction mixture incorporating other polyvalent cations, particularly for example the divalent Ru complexes used as sensitizers, competition for available platinum surface would occur between all ions present.We will report competitive adsorption studies el~ewhere.~~ It can be seen from fig. 4 that in fact RU2+ is more efficient in coagulating the platinum sol than is MV2+. RU2+ is a bigger i ~ n ~ ~ ~ ~ ~ and so the relative effects of MV2+ and RU2+ follow the observed trend in dication size observed in fig. 4 for the series Mg2+, Ca2+, Sr2+, Ba2+. CONCLUSIONS (1) Both the primary platinum particle size and the extent of reduction to colloidal platinum increased with increased heat input during the reduction of chloroplatinic acid.(2) The platinum sols exhibit a u.v.-visible spectrum that shows a stronger dependence on wavelength as particle size, or aggregate size, is reduced to < CQ. 4 nm.D. N. FURLONG, A. LAUNIKONIS, W. H. F. SASE AND J. V. SANDERS 587 (3) Platinum sols with particles in the size range 1.5-4 nm exhibit greater resistance to coagulation by electrolyte as particle size decreases. (4) The ability of any cation to induce coagulation of the platinum sol increases dramatically with cation valence. This dependence is greater than the sixth power dependence found for many sols of non-metallic particles and predicted by classic electrical double-layer theory. Smaller cations were found to be more effective destabilizers for platinum sols than larger ions of the same valence.( 5 ) Hydrogen treatment of platinum sols lead to coalescence of particles and consequent primary particle growth up to a maximum size of ca. 5 nm. We thank Prof. T. W. Healy, Prof. R. Hogg, Dr L. R. White and Mr I. M. Metcalfe for stimulating discussion. 1 (a) Solar Power and Fuels, ed. J. R. Bolton (Academic Press, New York, 1977); (b) Photochemistry, ed. R. P. Wayne (Elsevier Sequoia, Amsterdam, 1979), vol. 10; (c) Photochemical Conversion and Storage of Solar Energy, ed. J. S . Connolly (Academic Press, New York, 1981); ( d ) Photochemical Conversion and Storage of Solar Energy, ed. J. Rabani (The Weizmann Science Press of Israel, 1982). 2 R. I. Bickley, in Catalysis (Specialist Periodical Report, The Royal Society of Chemistry, London, 1981), vol.5, p. 308. (a) A. Moradpour, E. Amouyal, P. Keller and H. Kagan, Nouu. J. Chim., 1978,2,547; (b) A. Harriman and G. Porter, J. Chem. SOC., Faraday Trans. 2, 1982,82, 1937. A. Mills and G. Porter, J. Chem. SOC., Faraday Trans. I , 1982,78, 3659. E. Borgarello, J. Kiwi, M. Gratzel, E. Pelizzetti and M. Visca, J. Am. Chem. SOC., 1982, 104, 2996. W. W. Dunn and A. J. Bard, Nouu. J. Chim., 1981, 5, 651. J-M. Lehn, J-P. Sauvage and R. Ziessel, Nouv. J. Chim., 1981, 5, 291. B. Kraeutler and A. J. Bard, J. Am. Chem. SOC., 1978, 100, 4317. @ A. Harriman, G. Porter and M-C. Richoux, J. Chem. Soc., Faraday Trans. 2, 1982, 78, 1955. lo M. 0. Delcourt, N. Keghouche and J. Belloni, Now. J. Chim., 1983,7, 131. l1 A.Henglein, J. Phys. Chem., 1982,86,2291. l2 P. Keller and A. Moradpour, J. Am. Chem. SOC., 1980, 102,7193. l3 M. Spiro and P. L. Freund, J. Chem. SOC., Faraday Trans. I , 1983, 79, 1649. I4 Colloid Science, ed. H. R. Kruyt (Elsevier, New York, 1952), vol. 1. l5 K. Aika, L. L. Ban, I. Okura, S. Namba and J. Turkevich, J. Res. Inst. Catal. Hokkaido Univ., 1976, 24, 55. l6 P. A. Brugger, P. Cuendet and M. Gratzel, J. Am. Chem. SOC., 1981, 103, 2923. A. E. Pitts, J. C. Van Loon and F. E. Beamish, Anal. Chim. Acta, 1970, 50, 181. la B. V. Enustiin and J. Turkevich, J. Am. Chem. SOC., 1963,85, 3317. A. K. Halliday, Trans. Faraday SOC., 1947,43, 661 ; 1950,46,440. 2O P. C. Hiemenz, Principles of Colloid and Surface Chemistry (Marcel Dekker, New York, 1977), p. 89. 21 D. N. Furlong and W. H. F. Sasse, Aust. J. Chem., in press. 22 L. D. Rampino and F. F. Nord, J. Am. Chem. SOC., 1941,63, 2745. 23 J. Kiwi and M. Gratzel, J. Am. Chem. SOC., 1979, 101, 7214. 24 R. M. Wilenzick, D. C. Russell, R. H. Morriss and S. W. Marshall, J. Chem. Phys., 1967, 47, 533. 25 J. Turkevich, P. C. Stevenson and J. Hillier, Discuss. Faraday SOC., 1951, 11, 55. 26 A. C. Kuyper, J. Am. Chem. SOC., 1933,55, 1722. 27 J. Turkevich and G. Kim, Science, 1970, 169, 873. 28 G. C. Papavassiliou, Prog. Solid State Chem., 1979, 12, 185. 2@ B. A. Maguire, AerosolSci., 1971, 2, 417. 30 J. Turkevich, G. Garton and P. C. Stevenson, J. Colloid Sci., Suppl., 1954, 1, 26. 31 J. Westerhausen, A. Henglein and J. Lilie, Ber. Bunsenges. Phys. Chem., 1981, 85, 182. 32 D. S. Martin Jr, Adv. Chem. Ser., 1971, 98, 74. 33 A. Watillon and A-M. Joseph-Petit, Discuss. Faraday SOC., 1966, 42, 143. 34 R. H. Ottewill and J. N. Shaw, Discuss. Faraday SOC., 1966, 42, 154. 35 G. D. Parfitt and N. H. Picton, Trans. Faraday SOC., 1968, 64, 1955. 3e G. R. Wiese and T. W. Healy, Trans. Faraday SOC., 1970,66, 490. 3' R. Hogg and K. C. Yang, J. Colloid Interface Sci., 1976, 56, 573. 38 K. C. Yang and R. Hogg, in Recent Developments in Separation Science, ed. N. N. Li (C.R.C. Press, 3@ R. J. Hunter, Zeta Potential In Colloid Science - Principles and Applications (Academic Press, New Florida, 1978), vol. IV, p. 71. York, 1981), p. 24.588 COLLOIDAL PLATINUM SOLS 40 I. M. Metcalfe, D. Y. C. Chan, T. W. Healy and L. R. White, unpublished results. 41 R. Hogg, personal communication. 42 H. Ohshima, D. Y. C. Chan, T. W. Healy and L. R. White, J. Colloid Interface Sci., 1983, 92, 232. 43 J. Lyklema, Croat. Chem. Acta, 1970, 42, 151. 44 J. Lyklema and J. N. de Wit, J. Electroanal. Chem., 1975, 65, 443. 45 J. H. A. Pieper, D. A. De Vooys and J. Th. G. Overbeek, J. Electroanal. Chem., 1975, 65,429. 46 E. R. Nightingale Jr, J. Phys. Chem., 1959, 63, 1381. 47 D. Duonghong, E. Borgarello and M. Gratzel, J. Am. Chem. SOC., 1981,103,4685. 48 D. N. Furlong and Y-M. Tricot, J. D. Swift and W. H. F. Sasse, Aust. J. Chem., in press. 49 D. N. Furlong, Aust. J. Chem., 1982, 35, 911. 50 D. N. Furlong and W. H. F. Sasse, Colloids and Surfaces, 1983, 7 , 29. (PAPER 3/905)

ISSN:0300-9599    

DOI:10.1039/F19848000571

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. Soc., Faraahy Trans. I, 1984,80, 589-597 An Absorption Edge and Extended X-Ray Absorption Fine Structure Study of a Dispersed Manganese Dioxide Oxidation Catalyst BY NORMAN M. D. BROWN* AND JAMES B. MCMONAGLE School of Physical Sciences, The New University of Ulster, Coleraine, N. Ireland BT52 ISA AND G. NEVILLE GREAVES S.E.R.C., Daresbury Laboratory, Warrington WA4 4AD Received 6th June, 1983 Variously treated samples of a low-cost Mn0,-SiO, catalyst designed in the first instance for the complete vapour-phase oxidation of hydrocarbon and other vapours in pollution-control applications have been examined. The EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near-edge structure) characteristics of samples deliberately activated, deactivated or thermally degraded, i.e.respectively oxidised, reduced or destroyed, are compared. The oxidation number or effective atomic charge of the manganese ion in these samples is identified, as are also changes in the Mn-0 bond distances. For comparison and reference, analogous data from bulk manganese oxides of known stoichiometry were also measured and are briefly considered. The work herein concerns one member of a series of low-cost catalysts based on transition-metal oxides of variable valency designed in the first instance1 to give complete oxidation of organic vapours at relatively low temperatures; e.g. 90% conversion of 1% n-heptane in air to carbon dioxide and water is readily achieved2 around 200 "C with the manganese 'dioxide' formulation, Mn0,-SiO,, described below.The enhanced activity of these catalysts is a function of their preparation;l? polysaccharides are used as dispersing agents and as such give a much more potent active phase than in, for example, normal 'activated' MnO,. For the MnO,-SiO, system discussed here permanganate, sodium silicate and starch (or another polysaccharide) in water serve as starting materials. In preparing the active catalyst the aqueous permanganate oxidises the carbohydrate and the manganese is reduced from MnV" to a nominal MnIV, gelation then occurs with coprecipitation of manganese 'dioxide'. After removal of any excess of polysaccharide the resultant gel is dried and promoted as necessary with cerous ions (< 1 % is usual) to give a catalyst with an oxidative performance substantially better than that of manganese dioxide prepared in more conventional fashion.The Mn0,-SiO, system of interest, as prepared, is in the form of black glassy fractured pieces (as examined by scanning electron microscopy) of reproducible surface area (100-150 m2 g-l; B.E.T., N,), porosity (mean pore radius F z 35 A, N,; r = 80 A, Hg) and Mn content [typically ca. 55 % (w/w) for a bulk analysis calculated as MnO,]. Note that a closely similar composition is found via an XPS-ESCA analysis of the catalyst in fine powder form, i.e. the bulk and surface compositions are the same. 589590 EXAFS OF A DISPERSED MnO, OXIDATION CATALYST X-ray powder photographs show no long-range order, with the manganese ions therefore dispersed through a glassy silica matrix.Under appropriate mild conditions this catalyst is resistant29 to poisoning effects : vapours of hydrocarbons, alcohols, amines, phenols, aldehydes, ketones, sulphides etc. and the common solvents can all be oxidised. However, the catalyst does show a progressive loss in oxidative power at operating temperatures > 350 "C; it is effectively destroyed if heated to 700 "C, but below 300 "C extended performance with high activity is usual. In addition, other recent work has shown4 the utility of this Mn0,-SiO, system for oxidations in aqueous media where, as well as complete or near complete oxidation, the use of a suitable regime leads, with a range of organic substrates, to controlled partial oxidations in good yield at ambient temperature. Prior to the present work flow-reactor kinetics, conventional adsorption-desorption methods, XPS-ESCA, electron spin resonance and infrared spectroscopies and in situ resistance measurements have all been used2 to study the catalyst.The oxidation process in the vapour phase involves a reversible Mn" MnIV redox couple with MnIZ1 playing an intermediary role; oxygen adsorption activation/dissociation occurs at the catalyst surface to give 0; and/or 0-. The details of this redox system follow from an analysis2 of the kinetic and related data, the ESCA results and changes in the electrical characteristics of the catalysts in reactant gas streams. In the present work we report a complementary X-ray spectroscopic study of the dispersed Mn0,-SiO, system described above. The work now elaborated substantiates our understanding of the catalyst by probing further the oxidation state and the local environment of the manganese ion in the catalyst.In particular the changes in the oxidation level of the manganese ion following deliberate activation (oxidation), reversible deactivation (reduction) and thermal treatment leading to irreversible deleterious structural changes are deduced from a consideration of the near-edge structure and extended-edge features of the Mn K-shell absorption edge. EXPERIMENTAL The Mn0,-SiO, catalyst samples examined were all derived from the same unpromoted batch (with preparation, properties and characteristics as described above). Prior to the X-ray absorption measurements 1-2 g aliquots of controlled fine particle size (< 1 pm) were prepared and treated as follows.Sample 1 was activated by exposing the finely ground catalyst in a heated quartz reactor to a metered supply of dry oxygen (360 cm3 min-l) for 3 h at 350 O C , conditions known2 to maximise the oxidative performance of the system. Sample 2 was treated similarly in a stream of dry hydrogen (360 cm3 min-l) for 5 h at the same temperature, an exposure leading2 to a reversibly reduced form of the catalyst (which is coincidentally a remarkably good oxygen scavenger). Sample 3 was destroyed by heating the catalyst at 700 "C in dry air for 3 h; as a result2 this has no remaining oxidative power compared with that of the active precursor. All three samples, treated as described off-site, were kept sealed under nitrogen till just prior to collection of the X-ray absorption data.In addition to these catalyst samples, X-ray absorption spectra were also obtained from similarly ground samples of the reference manganese oxides: MnO (99.5 %), Mn,03 (98 %) and MnO, (99.9%). These were obtained from the normal commercial sources and were pretreated in the following manner. MnO was heated at 300 "C for 3 h in a stream of dry hydrogen (360 cm3 min-'), Mn20, was heated at 150 "C in dry air under the same flow regime and the MnO, was heated at 80 "C in the same flow of dry air for the same time. These procedures were used to ensure as far as possible that these reference materials were of the correct stoichiometry and oxidation state. Manganese metal (a-Mn 98.7%, 10 pm foil encapsulated in polythene) and powdered KMnO, (A.R.grade) were also used as reference materials. In addition these were utilised as calibrantsN. M. D. BROWN, J. B. McMONAGLE AND G. N. GREAVES 59 I for the energy of the Mn Kabsorption edge. Here, as is customary, the position of the absorption edge of interest was taken as the first major peak in the corresponding derivative spectrum. For a-Mn this was taken as 6538.0 eV.5 Compared to this, KMnO, has a sharp pre-edge feature at 6538.0+4.3 = 6842.3 eV (see fig. 4 later). By measuring each specimen, be this treated catalyst sample or reference oxide, in combination with KMnO, and alone, it was possible to calibrate the observed absorption edges with precision. An Si 220 monochromator was used with entrance slits set to give an energy resolution of 0.4eV at the manganese K edge at 6538.0eV.This matches the step size with the reproducibility of the monochromator table setting, typically +_ 0.1 eV. The actual EXAFS measurements were made in transmission with a synchrotron radiation source (SRS) beam energy of 1.9 GeV and a circulating current of ca. 100 mA. The transmission of the reference ionization chamber was 90% and that of the signal chamber 30%. The comparatively low energy of the SRS and the ion-chamber settings throughout helped reduce the harmonic contamination of the transmission measurement. All samples were measured at room temperature and several spectra were repeated with the samples held at 77 K. Whilst some improvement in fine structure was apparent in these instances, this did not affect the precision of the absorption edge; neither did it affect the location of the first shell of oxygen atoms surrounding the manganese centres, the primary parameters of interest in the present work. RESULTS AND DISCUSSION The resultant raw data were processed using the Daresbury suite of EXAFS programs.The required Fourier transforms were obtained without the use of calculated phase shifts. In consequence the positions of the first coordination shells found were foreshortened as expected. To a first approximation the phase-shift reduction in shell radius was estimated by comparing the observed first-shell radii with the analogous crystallographic data6 of the reference oxides, a-Mn and perman- ganate. This is shown in fig. I, where the position R(EXAFS-FT) of the first peaks in the EXAFS Fourier transforms (calculated over the same energy range, with the same k weighting and window function) of the reference materials is plotted against the known crystallographic nearest-neighbour Mn-0 distances.The good linear correlation shown?. gives a mean distance correction of AR = 0.62 k0.07 A. This correction is then used to establish from the linear plot the shell radii of the variously treated catalyst samples. The actual EXAFS spectra and the corresponding Fourier transforms for the treated catalyst samples 1, 2 and 3, i.e. activated, reversibly reduced and destroyed, respectively, are given in fig. 2. The analogous XANES spectra and the resultant first derivatives are shown in fig. 3, while for comparison the XANES spectra of the reference oxides and pennanganate are shown in fig.4. Table 1 contains the collected data of interest. Therein are the observed edge positions and the edge shifts, AE, for the variously treated samples along with those of the reference materials used. Also listed are the corresponding formal oxidation numbers and effective of the manganese ions involved. The sample-related data were obtained by appropriate interpolation, as in fig. 6 described later. In addition table 1 gives the empirically corrected Mn-0 first shell distances taken from the Fourier-transformed EXAFS spectra, i.e. R (EXAFS-FT) + AR. The edge-position data agree closely with those derived8 from conventional X-ray absorption measurements; likewise the EXAFS spectra and near-edge structure match those obtained more recently by Belli et aL9 using synchrotron radiation.However, t The linearity of fig. 1 indicates that the phase shifts for this range of manganese oxides are transferable within the more than adequate error limits indicated. A least-squares regression-fit of the data gives a correlation coefficient of 0.98 1 3.592 EXAFS OF A DISPERSED MnO, OXIDATION CATALYST 3 .O 2.0 5 n - rn u v F % 1 . o 0 1 .o 2.0 3.0 R (EXAFS-FT)/A Fig. 1. Position of the first peak in the unmassaged Fourier transforms of the EXAFS spectra obtained from the reference materials plotted against the corresponding crystallographic nearest-neighbour Mn-0 distances. the MnO; calibration procedure described here ensured an absolute energy error of +0.4 eV or better in the present work.This is also the first time the edge positions a i d oxygen shell radii have been directly compared using X-ray absorption spectroscopy. From table 1 it is clear that there is a correlation between the edge positions and first-shell radii. The first-shell radius (Mn-0) bond length increases as the energy of the absorption edge decreases, as shown in fig. 5; starting with metallic manganese and going to permanganate ion the manganese-nearest-neighbour distance decreases from 2.83 to 1.70 whilst the K edge shifts from 6538.0 to 6557.1 eV, a change of > 19 eV. The ‘universal’ curve obtained provides a grid with which a comparison of the oxidation state of manganese in the differently treated catalyst samples can be made.It is clear from fig. 5 , in terms of the edge shift, that the activated catalyst has manganese in an oxidation state between that in MnO, and MnO;. Correspondingly, when reduced the manganese ion has an oxidation level falling between that of the MnO and Mn,O, systems. On the other hand the behaviour of the ‘destroyed’ sample in these terms is more akin to that of Mn,O,. The sequence between K-edge shifts and the Mn-0 bond lengths matches closely changes in the formal oxidation state of the manganese ion. The manganese oxidation states concerned, together with the effective atomic charge’ of the reference materials (given in table l), are plotted against edge shift in fig. 6 where, by interpolation, the corresponding valence and charge information of catalyst samples 1 , 2 and 3 is deduced, i.e.for the activated, reversibly reduced and destroyed samples, respectively. It is now clear why the oxidation catalyst system is more effective than the more usual MnO, formulations in that the activated form has the manganese ion in a significantly higher oxidation state than in MnO, itself. Conversely, in the reducedN. M. D. BROWN, J . B. McMONAGLE AND G. N. GREAVES 593 0 1 2 3 4 5 6 energy/ 1 O2 eV c 0 1 2 3 4 R / A Fig. 2. Normalised EXAFS spectra (plotted in absorption on arbitrary scales) of the variously treated catalyst samples [( 1 ) activated, (2) reduced and ( 3 ) destroyed] along with the correspond- ing Fourier transforms. form the catalyst system has the manganese in an oxidation state lower than that in Mn,O,.The efficacy of the catalytic system in normal usage must therefore involve the redox chemistry of these two limiting oxidation states with Mnrrr and the promoter CelI1 ions, when present, involved in electron transfer. In this regard the charge or oxidation state data thus found are in accord with the analogous XPS-ESCA, gas-kinetic and other arguments given elsewhere.2 A detailed consideration of the near-edge structure of the treated catalyst samples shows these to be similar to the analogous XANES features of the locally octahedral reference manganese oxides, and as such are distinct from the a-Mn and MnO; cases. Cycling of the catalyst from an activated to a reversibly reduced form is accomplished with retention of similar octahedral oxygen coordination round the manganese centre.Note, however, that the Fourier transforms of the activated and reduced samples differ beyond the first-shell radius. This difference is consistent with the view that charge594 EXAFS OF A DISPERSED MnO, OXIDATION CATALYST rl----- 0 20 40 energy/eV 0 20 LO energy/eV Fig. 3. XANES spectra and the analogous first derivatives of the catalyst samples: (1) activated, (2) reduced and (3) destroyed. Table 1. X-ray absorption-edge positions, edge shifts, first-shell radii (corrected) and related data for the catalyst samples and reference materials edge edge first-shell oxidation effective samplea position/evb shift, AE/eV radii/A number charge, qc 1 (activated) 2 (reduced) 3 (destroyed) a-Mn MnO MnO, KMnO, Mn203 6553.1 6546.6 6548.3 6538.0 6543.6 6548.5 6551.3 6557.1 15.1 8.6 10.3 0 5.6 10.5 13.3 19.1 1.89 1.95 1.91 2.83 2.22 1.92 1.94 1.70 5.0d 2.gd 3.4d 0 2 3 4 7 3.46d 1 .83d 2.36d 0 1.35 2.03 2.67 4.72 a Treated as described in text. Taken as the first major maximum in the derivative of the K-edge spectrum with A E measured with respect to the corresponding feature of a-Mn metal (6538.0 eV).5 Derived by interpolation in fig.6, as indicated. Ref. (7) and (9); q is in units of electrons per atom.N. M. D. BROWN, J. B. McMONAGLE AND G. N. GREAVES 595 I I I I I I ' h I I I I y I I MnOz # I 1 I 1 -10 0 20 LO energy /eV , pre-edge c I 1 1 1 1 1 1 energy/eV -10 0 20 LO Fig. 4. XANES spectra of the reference oxides and permanganate. 3*00 I AE Fig. 5. Observed edge shifts, AE, (see table 1) of the reference materials and the treated catalyst samples [( 1) activated, (2) reduced and (3) destroyed] plotted against the empirically corrected (see fig.1) first-shell radii obtained from the Fourier transforms of the corresponding EXAFS spectra.596 AE AE EXAFS OF A DISPERSED MnO, OXIDATION CATALYST 0 1 2 3 L 5 6 7 oxidation number Fig. 6. Oxidation number and effective atomic charge. q. plotted against obsened edge shift, AE, for the reference systems and treated catalyst: (1) activated. ( 2 ) reduced and (3) destroyed. is transferred from manganese to the oxide matrix in and beyond the first coordination shell during the MnlI1/I\’ -+ Mn\- activation process. Belli et al.9 concluded that electronic-structure screening effects make the interpre- tation of edge shifts with respect to oxidation state of the metal ion in\-ol\sd ambiguous.None the less. both Mn-0 distances and A€. the edge shifts. are found to correlate well with oxidation state. Starting with the metallic structure. screening effects must decrease monotonically with depletion of yalence electrons as the le\.el of oxidation of the manganese ion increases. This is accompanied by a co\dent strengthening of the Mn-0 bond. There are therefore tw.0 ma$or contributions to the edge shifts: an increasing gap between bonding and antibondins states together with a contribution in the same sense from decreased screening of the core potential. In the limit the local structure becomes molecular in t ? p s ( r k . MnOy). CONCLUSIONS From the above it can be concluded that the acti\.ated fomi of the hIn0,-SiO, catalyst (sample 1) has the manganese ion centres.;it the local le\el. in ri higher oxidation state than in the normal dioside. i.e. close to hln’ rather than \In1V. InN. M. D. BROWN, J . B. McMONAGLE AND G. N. GREAVES 597 turn the associated K-edge shift is some 2 eV greater than that found in manganese dioxide itself. Both the edge shift and oxidation state found are thus measures of the markedly enhanced oxidative power of the catalyst under study. In the reversibly reduced form (sample 2) the X-ray absorption data point to a local oxidation state for the manganese between that in MnII and MnlI1. Note2 that X-ray photoelectron and electron spin resonance spectroscopic data also give evidence for the presence of MnII-like ions following the reduction regime.The edge shift found for the destroyed system (sample 3) is consistent with an intermediate MnlI1/IV state, as is other evidence.2 In this regard it is known2 that a gross change in the macroscopic and microscopic structures of the thermally destroyed catalyst arises when the catalyst is treated as described. The surface area is reduced by ca. 70% and the porosity characteristics change accordingly, the mean pore radius, ?, increasing from ca. 35 to ca. 60 A, N,, and from ca. 80 to ca. 400 A, Hg. Moreover the XPS data available2 show clearly that the MnII character of the system is enhanced and a greater surface concentration of SiO, is present than is found prior to thermal degradation. We thank the S.E.R.C. for support. J. B. McM. is grateful to the Northern Ireland Department of Education for a research studentship. Brit. Patent 1 436 700, Lambeg Industrial Research Association, Lisburn, N. Ireland. J. B. McMonagle, D.Phi1. Thesis (New University of Ulster, 1982); N. M. D. Brown, D. J. Cowley and K. Kitson, unpublished results. E. Robinson, (Lambeg Industrial Research Association, Lisburn, N. Ireland), unpublished results. N. M. D. Brown and J. B. McMonagle, unpublished results; Brit. Patent 8222613. G. N. Greaves, P. J. Durham, G. DiakunandP. Quinn, Nuture(London), 1981,294,139; J. A. Bearden and A. F. Burr, Rev. Mod. Phys., 1967, 39, 125. R. W. G. Wycoff, Crystal Structures (Wiley-Interscience, New York, 2nd edn, 1963); A. F. Wells, Structural Inorganic Chemistry (Oxford University Press, London, 4th edn, 1975). P. R. Sarode, S. Ramaseshe, W. H. Madhusudan and C. N. R. Rao, J. Phys. C, 1979, 12, 3439; cJ J. P. Suchet, Phys. Status Solidi, 1962, 2, 167. * M. Y. Apteand C. Mande, J. Phys. C, 1982,15,607; J. Phys. Chem. Solids, 1980,41,307; S . I. Salem, C. N. Chang, P. L. Lee and V. Severson, J. Phys. C, 1978,11,4085; R. M. Nayak and B. D. Padalia, Phys. Status Solidi B, 1979, 96, 259. M. Belli, A. Scafati, A. Bianconi, A. Mobilo, L. Palladino, A. Reale and E. Burattini, Solid State Commun., 1980, 35, 355. (PAPER 3/9 1 8)

ISSN:0300-9599    

DOI:10.1039/F19848000589

出版商:RSC    

年代:1984

数据来源: RSC