摘要:

Contents 3663 3669 3675 3683 3693 370 1 3709 3717 3725 3737 Normal and Abnormal Electron Spin Resonance Spectra of Low-spin Cobalt(r1) IN,]-Macrocyclic Complexes. A Means of Breaking the Co-C Bond in B12 Co-enzyme M. Green, J. Daniels and L. M. Engelhardt The Interaction between Superoxide Dismutase and Doxorubicin. An Electron Spin Resonance Approach V. Malatesta, F. Morazzoni, L. Pellicciari-Bollini and R. Scotti Biomolecular Dynamics and Electron Spin Resonance Spectra of Copper Complexes of Antitumour Agents in Solution. Part 2.-Rifamycins R. Basosi, R. Pogni, E. Tiezzi, W. E. Antholine and L. C. Moscinsky An Electron Spin Resonance Investigation of the Nature of the Complexes formed between Copper(I1) and Glycylhistidine D. B. McPhail and B. A. Goodman A Vibronic Coupling Approach for the Interpretation of the g-Value Temperature Dependence in Type-I Copper Proteins M.Bacci and S. Cannistr aro The Electron Spin Resonance Spectrum of Al[C,H,] in Hydrocarbon Matrices J. A. Howard, B. Mile, J. S. Tse and H. Morris N; and (CN); Spin-Lattice Relaxation in KCN Crystals H. J. Kalinowski and L. C. Scavarda do Carmo Single-crystal Proton ENDOR of the SO, Centre in y-Irradiated Sulphamic Acid N. M. Atherton, C. Oliva, E. J. Oliver and D. M. Wylie Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice 1,. Part 1.-The 0- Radicals Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice I,. Part 2.-The HO, Radicals J. Bednarek and A. Plonka J. Bednarek and A. PlonkaContents 3663 3669 3675 3683 3693 370 1 3709 3717 3725 3737 Normal and Abnormal Electron Spin Resonance Spectra of Low-spin Cobalt(r1) IN,]-Macrocyclic Complexes.A Means of Breaking the Co-C Bond in B12 Co-enzyme M. Green, J. Daniels and L. M. Engelhardt The Interaction between Superoxide Dismutase and Doxorubicin. An Electron Spin Resonance Approach V. Malatesta, F. Morazzoni, L. Pellicciari-Bollini and R. Scotti Biomolecular Dynamics and Electron Spin Resonance Spectra of Copper Complexes of Antitumour Agents in Solution. Part 2.-Rifamycins R. Basosi, R. Pogni, E. Tiezzi, W. E. Antholine and L. C. Moscinsky An Electron Spin Resonance Investigation of the Nature of the Complexes formed between Copper(I1) and Glycylhistidine D. B. McPhail and B. A. Goodman A Vibronic Coupling Approach for the Interpretation of the g-Value Temperature Dependence in Type-I Copper Proteins M. Bacci and S. Cannistr aro The Electron Spin Resonance Spectrum of Al[C,H,] in Hydrocarbon Matrices J. A. Howard, B. Mile, J. S. Tse and H. Morris N; and (CN); Spin-Lattice Relaxation in KCN Crystals H. J. Kalinowski and L. C. Scavarda do Carmo Single-crystal Proton ENDOR of the SO, Centre in y-Irradiated Sulphamic Acid N. M. Atherton, C. Oliva, E. J. Oliver and D. M. Wylie Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice 1,. Part 1.-The 0- Radicals Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice I,. Part 2.-The HO, Radicals J. Bednarek and A. Plonka J. Bednarek and A. Plonka

ISSN:0300-9599    

DOI:10.1039/F198783FX021

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

Electrochemistry Group Workshop on Electrochemical Techniques and Instruments To be held at the University of Warwick on 6-7 January 1988 Further information from Dr P. N. Bartlett, Department of Chemistry, University of Warwick, Coventry CV4 7AL Surface Reactivity and Catalysis Group with the Process Technology Group and the Institute of Chemical Engineers Opportunities for Innovation in the Application of Catalysis To be held at Queen Mary College, London on 6-7 January 1988 Further information from Professor J. Pritchard, Queen Mary College, London Division with the Institute of Mathematics and its Applications Mathematical Modelling of Semiconductor Devices and Processes To be held at the University of Loughborough on 7-8 January 1988 Further information from the Institute of Mathematics, Maitland House, Warrior Square, Southend-on-Sea SS1 2JY Division London Symposium: Modern Electrochemical Systems To be held at Imperial College, London on 12 January 1988 Further information from Mrs Y.A. Fish, Royal Society of Chemistry, Burlington House, London W1V OBN Polymer Physics Group with the 3Ps Group Plastics, Packaging and Printing To be held at the Institute of Physics, 47 Belgrave Square, London on 18 February 1988 Further information from Dr M. Richardson, National Physical Laboratory, Teddington, Middlesex l w 1 1 OLW Theoretical Chemistry Group Postgraduate Students’ Meeting To be held at University College, London on 2 March 1988 Further information from Dr G. Doggett, Department of Chemistry, University of York, York Colloid and Interface Science Group with The Society of Chemical Industry and British Radio frequency Spectroscopy Group Spectroscopy in Colloid Science To be held at the University of Bristol on 5-7 April 1988 Further information from Dr R. Buscall, ICI Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 40E Annual Congress: Division with Electrochemistry Group Solid State Materials in Electrochemistry To be held at the University of Kent, Canterbury on 12-15 April 1988 Further information from Dr J.F. Gibson, Royal Society of Chemistry, Burlington House, London W1V OBN Electrochemistry Group with The Society of Chemical Industry Electrolytic Bubbles To be held at Imperial College, London on 31 May 1988 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College of Science and Technology, South Kensington, London SW7 2AY Electrochemistry Group with The Society of Chemical Industry Chlorine Symposium To be held at the Tara Hotel, London on 1-3 June 1988 Further information from Dr S.P. Tyfield, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GLI 3 9BP Gas Kinetics Group Xth International Symposium on Gas Kinetics To be held at University College, Swansea on 24-29 July 1988 Further information from Dr G. Hancock, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 302 (xiii)Electrochemistry Group Workshop on Electrochemical Techniques and Instruments To be held at the University of Warwick on 6-7 January 1988 Further information from Dr P.N. Bartlett, Department of Chemistry, University of Warwick, Coventry CV4 7AL Surface Reactivity and Catalysis Group with the Process Technology Group and the Institute of Chemical Engineers Opportunities for Innovation in the Application of Catalysis To be held at Queen Mary College, London on 6-7 January 1988 Further information from Professor J. Pritchard, Queen Mary College, London Division with the Institute of Mathematics and its Applications Mathematical Modelling of Semiconductor Devices and Processes To be held at the University of Loughborough on 7-8 January 1988 Further information from the Institute of Mathematics, Maitland House, Warrior Square, Southend-on-Sea SS1 2JY Division London Symposium: Modern Electrochemical Systems To be held at Imperial College, London on 12 January 1988 Further information from Mrs Y.A. Fish, Royal Society of Chemistry, Burlington House, London W1V OBN Polymer Physics Group with the 3Ps Group Plastics, Packaging and Printing To be held at the Institute of Physics, 47 Belgrave Square, London on 18 February 1988 Further information from Dr M. Richardson, National Physical Laboratory, Teddington, Middlesex l w 1 1 OLW Theoretical Chemistry Group Postgraduate Students’ Meeting To be held at University College, London on 2 March 1988 Further information from Dr G. Doggett, Department of Chemistry, University of York, York Colloid and Interface Science Group with The Society of Chemical Industry and British Radio frequency Spectroscopy Group Spectroscopy in Colloid Science To be held at the University of Bristol on 5-7 April 1988 Further information from Dr R.Buscall, ICI Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 40E Annual Congress: Division with Electrochemistry Group Solid State Materials in Electrochemistry To be held at the University of Kent, Canterbury on 12-15 April 1988 Further information from Dr J. F. Gibson, Royal Society of Chemistry, Burlington House, London W1V OBN Electrochemistry Group with The Society of Chemical Industry Electrolytic Bubbles To be held at Imperial College, London on 31 May 1988 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College of Science and Technology, South Kensington, London SW7 2AY Electrochemistry Group with The Society of Chemical Industry Chlorine Symposium To be held at the Tara Hotel, London on 1-3 June 1988 Further information from Dr S. P. Tyfield, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GLI 3 9BP Gas Kinetics Group Xth International Symposium on Gas Kinetics To be held at University College, Swansea on 24-29 July 1988 Further information from Dr G. Hancock, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 302 (xiii)

ISSN:0300-9599    

DOI:10.1039/F198783BX023

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

ISSN 030C9599 JCFTAR 83(6) 1657-7892 (7987) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases CONTENTS 1651 1667 1685 1703 171 1 1719 1725 1731 1739 1751 1761 1771 1779 1783 1795 1805 1815 1823 1835 55 Isothermal Titration of Supported Platinum. Part 1 .-CO-O, Titration M. A. M. Luengo, P. A. Sermon and A. T. Wurie Isothermal Titration of Supported Platinum. Part 2.-Alkene Titration using Cyclohexene M. S. W. Vong and P. A. Sermon Metachromasy in Clay Minerals. Sorption of Pyronin Y by Montmorillonite and Laponite Z. Grauer, G. L. Grauer, D. Avnir and S. Yariv Forces between Mica Surfaces in Aqueous KNO, Solution in the range 1 0-4-10-1 mol dm-,, showing Long-range Attraction at High Electrolyte Con- centration Hydrogenation of CO, over Co/Cu/K Catalysts H.Baussart, R. Delobel, M. Le Bras and J-M. Leroy Determination of Stability Constants from Linear-scan or Cyclic-voltammetric Data using a Non-linear Least-squares Method H. Gampp The Molar Volume of a Large Polymeric Cation [A1130,,H4,]7+ J. W. Akitt, J. M. Elders and P. Letellier Selective lH--13C and lH-lH Nuclear Overhauser Enhancement Studies of Adenosine-Thymidine Interaction in Solution C. Rossi, N. Niccolai, A. Prugnola and F. Laschi Adsorption of Organic Molecules on Titanium Dioxide (Rutile) Surface Y. Suda and M. Nagao Origin of Boron Mobility over Boron-impregnated ZSM-5. A Combined High- resolution-Solid-state llB Nuclear Magnetic Resonance/Infrared Spectral Investigation M. B. Sayed Catalytic Activity and Structure of Mo Oxide Highly Dispersed on ZrO, for Oxidation Reactions T.Ono, H. Miyata and Y. Kubokawa Comments on the Mechanism of MTG/HZSM-5 Conversion M. B. Sayed Measurements of Tracer Diffusion Coefficients of Lithium Ions, Chloride Ions and Water in Aqueous Lithium Chloride Solutions K. Tanaka and M. Nomura Internal Pressures, Temperatures of Maximum Density and Related Properties of Water and Deuterium Oxide M. J. Blandamer, J. Burgess and A. W. Hakin Pulse Radiolysis Study of Salt Effects on Reactions of Aromatic Radical Cations with C1-. Rate Constants in the Absence and Presence of Quaternary Ammonium Salts Y. Yamamoto, S. Nishida and K. Hayashi A Kinetic Study of the Self-reaction of Prop-2-ylperoxyl Radicals in Solution using Ultraviolet Absorption Spectroscopy J.E. Bennett Electron Spin Resonance Characterization of Rotational Isomers of the n-Butane Radical Cation with Partially Deuterated Methyl Groups in Some Halogenated Matrices M. Lindgren and A. Lund Oxygen Reduction with Hydroxy- 1,4-naphthoquinones immobilized at Carbon Electrodes T. Nagaoka, T. Sakai, K. Ogura and T. Yoshino The Effect of Hydrogen Sulphide on the Adsorption and Thermal Desorption of Carbon Monoxide over Rhodium Catalysts S. D. Jackson, B. J. Brandreth and D. Winstanley C. Toprakcioglu, J. Klein and P. F. Luckham FAR 1Con tents 1 843 Nuclear Magnetic Resonance Self-diffusion Studies of Methanol-Water Mix- tures in Pentad-type Zeolites J. Caro, M. Bulow, J. Richter-Mendau, J. Karger, M. Hunger, D. Freude and L. V. C. Rees Synthesis of Montmorillonite-Viologen Intercalation Compounds and their Photochromic Behaviour H. Miyata, Y. Sugahara, K. Kuroda and C. Kato Reactive Intermediates for the Ethene Homologation Reaction on Molybdena- Silica Catalysts K. Tanaka and K. Tanaka An Electron Spin Resonance Study of Triplet Radical Pairs in Single Crystals of X-Irradiated L-Ascorbic Acid at 77 K J. T. Masiakowski and A. Lund Infrared Spectroscopic Studies of Hydrogen Bonding in Triethylammonium Salts. Part 4.-Rearrangement of Hydrogen-bonded Ion Pairs of Triethylam- monium Salts caused by Interaction with Tetrabutylammonium Salts in Sol- ution A. A. Mashkovsky, A. A. Nabiullin and s. E. Odinokov 1885 Electrostatic Interactions between Organic Ions. Part 2.-Phosphates with Amines H. R. Wilson and R. J. P. Williams I85 1 1859 1869 1879

ISSN:0300-9599    

DOI:10.1039/F198783FP071

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

79 1 805 819 837 843 857 87 1 88 1 897 905 917 927 933 939 949 96 1 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions ll,lssue6,1987 Molecular and Chemical Physics ~~~~ For the benefit of readers of Faraday Transactions I, the contents list of Faraday Transactions II, Issue 6, is reproduced below. The Polanyi Lecture : Bond Dissociation Energies-A Continuing Story S. W. Benson An SCF Study of Spin Density in the CrF:- Ion G. S. Chandler and R. A. Phillips Conformational Properties of a Diblock Copolymer Molecule Y. S. Sdranis and M. K, Kosmas Transmission-line Modelling of the Liesegang Phenomenon D. de Cogan and M. Henini Transmission-line Matrix (TLM) : a Novel Technique for Modelling Reaction Kinetics D. de Cogan and M. Henini Dynamics of Bimolecular Association Reactions L.F. Phillips Multiplet Structure in Coupled Spin Systems containing High-spin Nuclei L. G. Werbelow, A. Allouche and G. Pouzard Monte Carlo Calculations of Thermodynamic Properties of the Restricted, Primitive Model of Electrolytes at Extreme Dilution using 32, 44, 64, 100, 216 and 512 Ions and ca. lo6 Configurations per Simulation P. Sloth, T. Smith Srrrensen and J. B. Jensen Nuclear Magnetic Relaxation for Coupled Spins of Oppositely Signed Magnetic Moments L. G. Werbelow Kinetics and Mechanism of Reactions of NF, with Olefins. Reinterpretation of Existing Data and Estimation of C-N Bond Energies of Alkyldifluoroamines W. A. Sanders and M. C. Lin Morphology of Poly(oxyethy1ene) Methyl-n-alkyl Ethers as determined by Small-angle X-Ray and Raman Scattering K. Viras, F.Viras, C. Campbell, T. A. King and C. Booth Application of the Perturbed-elastic-rod Model to the Longitudinal Acoustical Modes of Poly(oxythy1ene)s and their n-Hexadecyl Ethers F. Viras, K. Viras, C. Campbell, T. A. King and C. Booth Investigation of the Electric Quadrupole Reaction Field on Nuclear Magnetic Resonance Solvent Shifts Reaction Field of an Oscillating Electric Dipole and Solvent Chemical Shift D. J. Pennino and E. R. Malinowski Numerical Solution of a Poisson-Boltzmann Theory for a Primitive Model Electrolyte with Size and Charge Asymmetric Ions C. W. Outhwaite Standard Transition-structure Geometries D. J. Pennino and E. R. Malinowski C. A. Reynolds and C. Thomson97 1 985 99 1 1001 101 1 1023 1029 1041 1049 1059 6/115 6/997 6/1018 6/ 1057 6/1175 6 / 1 544 6/ 1666 Gas-phase Thermal Decomposition Reactions of 1,2-Dibromopropane K-H.Jung, S. J. Yun and D. S. Huh Fluorescence in the Dissociative Excitation of Sulphur Dioxide by Electron Impact C. A. F. Johnson, S. D. Kelly and J. E. Parker Rotational Spectrum and Properties of the Hydrogen-bonded Dimer CH,CN'-.HCN N. W. Howard and A. C. Legon Electron-transfer Mechanism between Excited Alkali Metals and Halogen Molecules A. M. Kosmas Topography of Potential-energy Surfaces. Spin-Orbit Interaction for H + F,, C1, N. C. Firth and R. Grice Topography of Potential-energy Surfaces. Spin-Orbit Interaction for F + HF, Cl+HCl Topography of Potential-energy Surfaces. Spin-Orbit Interaction for F + F,, Cl+Cl, N. C. Firth and R. Grice Low-energy-electron Collision Cross-sections in Silane K.J. Mathieson, P. G. Millican, I. C. Walker and M. G. Curtis The Photochemistry of Cyclobutyl Methyl Ketone. Part 1 .-Room-temperature Results and the General Mechanism P. J. Baldwin, C. E. Canosa-Mas, H. M. Frey and R. Walsh Reviews of Books G. Doggett; D. M. Hint; R. P. Wayne; K. R. Jennings; F. Wilkinson N. C. Firth and R. Grice The following papers were accepted for publication in Faraday Transactions I during March 1987. H,O/Triton X-100 Solvent Effect in the Micellar Catalysis of the Aquation of Tris-(3,4,7,8-tetramethyl- 1, 10-phenanthroline)iron( 11) J. Ige, J. N. Lambi and 0.0. Soriyan The Inclusion of Pyronine Y by Beta- and Gamma-Cyclodextrin. A Kinetic and Equilibrium Study R. L. Schiller, S. F. Lincoln and J.H. Coates The Inclusion of Diflunisal by a- and j?-Cyclodextrins. A "F Nuclear Magnetic Resonance and Visible Spectroscopic Study S. F. Lincoln, A. M. Hounslow, J. H. Coates and B. G. Doddridge The Dissolution of Magnetite by Nitrolotriacetatoferrate(11) M. del Valle Hidalgo, N. E. Katz, J. J. G. Maroto and M. A. Blesa Microalorimetric Measurement of the Enthalpies of Transfer of a Series of o-Alkoxyphenyls from Water to Octan- 1-01 and from Isotonic Solution to Escherichia coli Cells A. E. Beezer, M. C. P. Lima, G. G. Fox, W. H. Hunter and B. V. Smith Hydrazine Reduction of Transition-metal Oxides D. M. Littrell, D. H. Bowers and B. J. Tataruchuk Comparison between Heterogeneous and Homogeneous Electron Transfer in p-Phenylenediamine Systems A. Kapturkiewicz and W.Jaenicke (ii)6/ 814 Electron Spin Resonance Studies of Free and Supported 12-Heteropoly Acids. Part 6. The Investigation of Reduced H4(SiWl2O4,)- x H,O and Ag,(SiW,,O,,). xH,O and Oxygen Adsorption R. Fickel, H. G. Jerschkewitz and G. Ohlmann 6/ 933 Kinetics and Mechanism of Oxidative Dehydrogenation of Ethane with Nitrous Oxide over Doped Magnesium Oxide K. Aika, M. Isobe, K. Kido and T. Onishi 6/ 1934 Relative Reactivities of Small Alkanes with Adsorbed Oxygen over Cobalt- doped Magnesium Oxide K. Aika, T. Moriyama and T. Onishi 6/2012 Infrared Studies on Dinitrogen and Dihydrogen adsorbed over T,O, at Low Temperatures T. Sakata, N. Kinoshitsa, K. Domen and T. Onishi 6/2099 Temperature-programmed Desorption Study of the Interactions of H,, CO and CO, with LaMnO, LG.Tejuca and A. T. Bell 6/2 130 Medium Effect on the Electrochemical Behaviour of the CdII/Cd(Hg) System in Propane- 1 ,Zdiol-Water Mixtures R. M. Rodriguez, E. Brillas and J. A. Garrido 6/2 132 Determination of the Distribution of Aluminium in Zeolitic Frameworks T. Takaishi 6/2265 Catalysis by Amorphous Metal Alloys. Part 7.-Formation of Fine Fe Particles on the Surface of the Alloy in the Precrystallisation State prepared from Amorphous Fe,,Zr,,Powder Alloy H. Yamashita, K. Sakai, T. Funabiki, S. Yoshida and Y. Isozumi 6/2266 Catalysis of Amorphous Metal Alloys. Part 6.-Factors Controlling the Activity of Skeletal-nickel Catalysts prepared from Amorphous and Crystalline Ni-Zr Powder Alloys H. Yamashita, M. Yoshikawa, T. Funabiki and S. Yoshida 6/2274 Chemical Equilibria and Kinetics at Constant Pressure and at Constant Volume E.Whalley 6/23 15 An X.P.S. Study of the Influence of Hydrogen on the Oxygen-Silver Interaction L. Lefferts, J. G. Van Ommen and J. R. H. Ross 6/2339 One-dimensional Rare Gases in the Pore of Ferrierite and their Virial Coefficients 6/2341 Intermolecular Structure around the Lithium Monovalent Cation in Molten LiAlCl, Y. Kameda and K. Ichikawa 6/2383 The Thermodynamics of Solvation of Ions. Part 4.-Application of the TATB Extrathermodynamic Assumption to the Hydration of Ions and to Properties of Hydrated Ions Y. Marcus 6/2415 Binary Systems of 1,2-Dichloroethane with Benzene, Toluene, p-Xylene, Quinoline and Cyclohexane. Part 3 .-Dielectric Properties and Refractive Indices at 308.15 K J.Natha and G. Singh 6/2416 Radical Spectra and Product Distribution following Electrophilic Attack by the * OH Radical on 4-Hydroxybenzoic Acid and Subsequent Oxidation R. F. Anderson, K. B. Patel and M. R. L. Stratford 1 T. Takaishi, K. Nonaka and T. Okada (iii)6/2417 The Mechanism of Conductivity of Liquid Polymeric Electrolytes G. G. Cameron, M. D. Ingram and G. A. Sorrie 6/2447 Wetting of Graphite (0001) by Carbon Monoxide : a Stepwise Adsorption Isotherm Study Y. Larher, F. Angerand and Y. Maurice 6/2463 Pairwise Gibbs Function Cosphere-Cosphere Group Interaction Parameters for Alkylammonium Salts in Aqueous Solutions at 298 K. Solubilities of Hydrocarbons in Aqueous Salt Solutions M. J. Blandamer, J. Burgess, M. R. Cottrell and A. W. Hakin 6/2496 Adsorptive Properties of Semiconductive Thin NiO and TiO, Films combined with an Oppositely Polarized Ferroelectric Support Y.Inoue, K. Sat0 and 0. Hayashi 7/054 Neutron Spectroscopic Study of Polycrystalline Benzene and of Benzene adsorbed in Na-Y Zeolite H. Jobic, A. Renouprez, A. N. Fitch and H. J. Lauter Modes of Enhancement of Physical Adsorption of Nitrogen and Water Vapour on Metal Oxides Ion-pair Formation by Tetra-alkylammonium Ions in Methanol. An N.M.R. Study M. Krell, M. C. R. Symons and J. Barthel Dependence of the Inflection Time on the Initial Concentrations in Auto- catalytic Reactions. Maxima, Minima and Anomalous Changes G. Pota and G. Bazsa Kinetics and Mechanism of Autocatalytic Oxidation of Fe(phen)i+ and by Nitric Acid I. Lengyel, T.Barna and G. Bazsa Structure of Second Stage Graphite-Rubidium, C,,Rb G. R. S. Naylor and J. W. White N.M.R. Investigation of Internal Silanol Groups as Structural Defects in ZSM-5 Type Zeolites M. Hunger, J. Karger, H. Heifer, J. Caro, B. Zibrowius, M. Bulow and R. Mostowicz 7/083 7/ 130 7/243 P. A. Sermon and R. R. Rajaram 7/244 7/317 7/334Cumulative Author Index 1987 Agnel, J-P. L., 225 Akalay, I., 1137 Akitt, J. W., 1725 Alberti, A., 91 Allen, G. C., 925, 1355 Anderson, A. B., 463 Anderson, J. B. F., 913 Antholine,,W. E., 151 Ardizzone, S., 1159 Atherton, N. M., 37, 941 Avnir, D., 1685 Axelsen, V., 107 Baldini, G., 1609 B a h , M., 1029 Barratt, M. D., 135 Barrer, R. M., 779 Basosi, R., 151 Bastl, Z., 51 1 Bateman, J. B., 841 Battesti, C. M., 225 Baussart, H., 171 1 Becker, K.A., 535 Bennett, J.E., 1805 Berclaz, T., 401 Berleur, F., 177 Berroa de Ponce, H., 1569 Berry, F. J., 615 Bertagnolli, H., 687 Berthelot, J., 231 Beyer, H. K., 51 1 Bianconi, A., 289 Bjorklund, R. B., 1507 Blandamer, M. J., 559, 865, Blyth, G., 751 Boerio-Goates, J., 1553 Borbdy, G., 51 1 Boucher, E. A., 1269 Brandreth, B. J., 1835 Braquet, P., 177 Brazdil, J. F., 463 Briscoe, B. J., 938 Bruce, J. M., 85 Brustolon, M., 69 Budil, D. E., 13 Bulow, M., 1843 Burch, R., 9 13 Burgess, J., 559, 865, 1783 Burggraaf, A. J., 1485 Burke, L. D., 299 Busca, G., 1591, 853 Buscall, R., 873 Cairns, J. A., 913 Carley, A. F., 351 Caro, J., 1843 Cassidy, J. F., 231 Celalyan-Berthier, A., 401 Chalker, P. R., 351 1783 Chandra, H., 759 Chieux, P., 687 Chittofrati, A., 1159 Clark, B., 865 Clifford, A.A., 751 Colin, A. C., 819 Coller, B. A. W., 645, 657 Coluccia, S., 477 Compostizo, A., 819 Compton, R. G., 1261 Conway, B. E., 1063 Corvaja, C., 57 Couillard, C., 125 Courbon, H., 697 Craven, J. B., 779 Crossland, W. A., 37 D’Alba, F., 267 Danil de Namor, A. F., 1569 Dash, A. C., 1307 Daverio, D., 705 Davies, M. J., 1347 Davoli, I., 289 Dawber, J. G., 771 De Doncker, J., 125 De Laet, M., 125 De Ranter, C. J., 257 Declerck, P. J., 257 Delafosse, D., 1137 Delahanty, J. N., 135 Delobel, R., 1711 Di Lorenzo, S., 267 Diaz Peiia, M., 819 Dimitrijevik, N. M., 1193 Dodd, N. J. F., 85 Ducret, F., 141 Dudikova, L., 51 1 Dusaucy, A-C., 125 Eicke, H.-F., 1621 Elbing, E., 657, 645 Elders, J. M., 1725 Empis, J. M. A., 43 Endoh, A., 41 1 Engberts, J.B. F. N., 865 Evans, J. C., 43, 135 Fahim, R. B., 1601 Fan, G., 323 Fatome, M., 177 Fejes, P., 1109 Fletcher, P. D. I., 1493, 985 Flint, N. J., 167 Formaro, L., 11 59 Formosinho, S. J., 431 Forrester, A. R., 211 Forster, H., 1109 Fraissard, J., 451 Freude, D., 1843 Freund, E., 1417 Chu, D-Y., 635 Fricke, R., 1041 Fujii, K., 675 Fujitsu, H., 1427 Galli, P., 853 Gampp, H., 1719 Garbowski, E., 1469 Garrido, J., 1081 Garrone, E., 1237 Gellings, P. J., 1485 Geoffroy, M., 401 Gervasini, A., 705 Gilbert, B. C., 77 Gilbert, R. G., 1449 Goates, J. R., 1553 Goates, S. R., 1553 Goffredi, M., 1437 Golding, P. D., 1203 Gottschalk, F., 571 Gozzi, D., 289 Grampp, G., 16 1 Gratzel, M., 1101 Grauer, G. L., 1685 Grauer, Z., 1685 Gray, P., 751 Greci, L., 69 Grieser, F., 591 Grigorian, K.R., 1189 Grossi, L., 77 Groves, G. S., 1119, 1281 Grzybkowski, W., 281, 1253 Guardado, P., 559 Guilleux, M.-F., 1137 Hada, H., 1559 Hagele, G., 1055 Hakin, A. W., 559, 865, 1783 Halawani, K. H., 1281 Hall, D. G., 967 Hall, M. V. M., 571 Halpern, A., 219 Hamada, K., 527 Harland, R. G., 1261 Harrer, W., 161 Harris, R. K., 1055 Hartland, G. V., 591 Hasegawa, A., 759 Hatayama, F., 675 Hayashi, K., 1795 Heatley, F., 517 Hemminga, M. A., 203 Henriksson, U., 1515 Herold, B. J., 43 Hertz, H. G., 687 Hidalgo, J., 1029 Higgins, J. S., 939 Hilfiker, R., 1621 Holden, J. G., 615 Howe, A. M., 985, 1007 Howe, R. F., 813Hudson, A., 91 Hunger, M., 1843 Hunter, R., 571 Hussein, F. H., 1631 Hutchings, G. J., 571 Ikeyama, N., 1427 Imamura, H., 743 Imanaka, T., 665 Ismail, H.M., 1601 Ito, T., 451 Iwaki, T., 943, 957 Iwamoto, E., 1641 Jackson, S. D., 905, 1835 Jaenicke, W., 161 Janes, R., 383 Juszczyk, W., 1293 Kakuta, N., 1227 Kaneko, M., 1539 Kanno, T., 721 Karger, J., 1843 Kariv-Miller, E., 1169 Karpiliski, Z., 1293 Kato, C., 1851 Kawaguchi, T., 1579 Kazusaka, A., 1227 Kerr, C W., 85 Kira, A., 1539 Kiricsi, I., 1109 Kitaguchi, K., 1395 Kiwi, J., 1 101 Klein, J., 1703 Kobayashi, J., 1395 Kobayashi, M., 721 Koda, S., 527 Kondo, Y., 1089 Konishi, Y., 721 Koopmans, H. J. A., 1485 Kordulis, C., 627 Korf', S. J., 1485 Koutsoukos, P. G., 1477 Kowalak, S., 535 Kubelkova, L., 51 1 Kubokawa, Y., 675, 1761 Kumamaru, T., 1641 Kuroda, K., 1851 Kusabayashi, S., 1089 Kuzuya, M., 1579 La Ginestra, A., 853 tajtar, L., 1405 Lambelet, P., 141 Lamotte, J., 1417 Laschi, F., 1731 Lavagnino, S., 477 Lavalley, J-C., 1417 Lawin, P.B., 1169 Lawrence, S., 1347 Le Bras, M., 1711 Leaist, D. G., 829 Lecomte, C., 177 Lee, E. F. T., 1531 Leroy, J-M., 1711 Letellier, P., 1725 Lin, C. P., 13 Linares-Solano, A., 108 1 Korth, H-G., 95 AUTHOR INDEX Lindgren, M., 893, 1815 Lippens, B. C., Jr, 1485 Liu, T., 1063 Loliger, J., 141 Lorenzelli, V., 853, 1591 Loretto, M. H., 615 Luckham, P. F., 1703 Lund, A., 893, 1815, 1869 Lycourghiotis, A., 627, 1 179 Lynch, J., 1417 Lyons, C. J., 645 Lyons, M. E. G., 299 MacAleer, J. F., 1323 MacDonald, J. A., 1007 Machin, W. D., 1203 Maestre, A., 1029 Maezawa, A., 665 Makela, R., 51 Manfredi, M., 1609 Maniero, A. L., 57, 69 Marchese, L., 477 Marcus, Y., 339 Mari, C. M., 705 Markarian.S. A., 1189 Liu, R-L., 635 Nomura, H., 527 Nomura, M., 1779, 1227 Norris, J. 0. W., 1323 Norris, J. R., 13 Nukui, K., 743 Nuttall, S., 559 OBrien, A. B., 371 Odinokov, S. E., 1879 Ogura, K., 1823 Ohno, M., 1559 Ohno, T., 675 Ohshima, K., 789 Okabayashi, H., 789 Okamoto, Y., 665 Okuda, T., 1579 Okuhara, T., 1213 Ono, T., 675, 1761 Otsuka, K., 1315 Ott, J. B., 1553 Pallas, N. R., 585 Parry, D. J., 77 Patrono, P., 853 Pedersen, J. A., 107 Pedulli, G. F., 91 Penar, J., 1405 Martin Luengo, M. A., 1347,1651 PireZ-Tejeda, p., 1029 Martin-Maiinez, J. M., 1081 Mashkovsky, A. A., 1879 Masiakowski, J. T., 893, 1869 Masliyah, J. H., 547 Matralis, H., 1179 Matsuura, H., 789 Maxwell, I. A., 1449 McCarthy, S . J., 657 McLauchlan, K. A., 29 Mehandru, S. P., 463 Merwin, L.H., 1055 Micic, 0. I., 1127 Miyahara, K., 1227 Miyata, H., 675, 1761, 1851 Mochida, I., 1427 Molina-Sabio, M., 1081 Monk, C. B., 425 Montagne, X., 1417 Morazzoni, F., 705 Morimoto, T., 943,957 Moseley, P. T., 1323 Moyes, R. B., 905 Mozzanega, M-N., 697 Muiioz, M. A., 1029 Nabiullin, A. A., 1879 Nagao, M., 1739 Nagaoka, T., 1823 Nair, V., 487 Nakai, S., 1579 Nakajima, T., 1315 Napper, D. H., 1449 Narayanan, S., 733 Narducci, D., 705 Nayak, R. C., 1307 Nazer, A. F. M., 11 19 Nedeljkovic, J. M., 1127 Nenadovic, M. T., 1127 Niccolai, N., 1731 Nishida, S., 1795 Pethica, B. A., 585 Pethrick, R. A., 938 Pichat, P., 697 Pielaszek, J., 1293 Pilarczyk, M., 281 Pizzini, S., 705 Pogni, R., 151 Pomonis, P., 627 Pomonis, P. J., 1363 Primet, M., 1469 Priolisi O., 57 Prugnola, A., 1731 Puchalska, D., 1253 Purushotham, V., 21 1 Radulovic, S., 559 Raffi, J.J., 225 Ramaraj, R., 1539 Ramis, G., 1591 Rees, L. V. C., 1531, 1843 Reyes, P. N., 1347 Richter-Mendau, J., 1843 Ritschl, F., 1041 Riviere, J. C., 351 Roberts, M. W., 351 Robinson, B. H., 985, 1007 Rodriguez-Reinoso, F., 1081 Rollins, K., 1347 Roman, V., 177 Romlo, M. J., 43 Rosseinsky, D. R., 231, 245 Rossi, C., 1731 Rowlands, C. C., 43, 135 Rubio, R. G., 819 Rudham, R., 1631 Sakai, T., 743, 1823 Sanchez, M., 1029 Sangster, D. F., 657 Saraby-Reintjes, A., 271 Sato, T., 1559AUTHOR INDEX Saucy, F., 141 Savoy, M-C., 141 Sayed, M. B., 1149, 1751, 1771 Seebode, J., 1109 Segal, M. G., 371 Segre, U., 69 Sermon, P. A., 1347, 1369, 1651, Seyedmonir, S., 813 Sidahmed, I. M., 439 Simonian, L.K., 1189 Smith, D. H., 1381 Soderman, O., 15 15 Sokolowski, S., 1405 Steenken, S., 113 Stevens, D. G., 29 Stone, F. S., 1237 Suda, Y., 1739 Sugahara, Y., 1851 Suppan, P., 495 Sustmann, R., 95 Suzuki, T., 1213 SvetliEid, V., 1 169 Swartz, H. M., 191 Symons, M. C. R., 1, 383, 759 Szostak, R., 487 Tabner, B. J., 167 Taga, K., 789 Takaishi, T., 41 1 Tan, W. K., 645 Tanaka, H., 1395 1667 Tanaka, K., 1213, 1779, 1859 Tanaka, K-i., 1859 Tempkre, J.-F., 1137 Tempest, P. A., 925 Theocharis, C. R., 1601 Thidry, C. L., 225 Thomas, T. L., 487 Thurai, M., 841 Tilquin, B., 125 Tomellini, M., 289 Tonge, J. S., 231, 245 Toprakcioglu, C., 1703 Torregrosa, R., 1081 Toyoshima, I., 1213 Trabalzini, L., 151 Tsuchiya, S., 743 Tsuiki, H., 1395 Tsukamoto, K., 789 Turner, J.C. R., 937 Tyler, J. W., 925, 1355 Ueno, A., 1395 Ukisu, Y., 1227 Uma, K., 733 Unwin, P. R., 1261 Vachon, A., 177 van de Ven, T. G. M., 547 Varani, G., 1609 Vattis, D., 1179 Vincent, P. B., 225 Vink, H., 801,941 Vong, M. S. W., 1369, 1667 Vordonis, L., 627 Vuolle, M., 51 Waddicor, J. I., 751 Waller, A. M., 1261 Waters, D. N., 1601 Wells, C. F., 439, 939, 1119, Wells, P. B., 905 White, L. R., 591, 873 Whyman, R., 905 Williams, D. E., 1323 Williams, J. O., 323 Williams, R. J. P., 1885 Williams, W. J., 371 Wilson, H. R., 1885 Wilson, I. R., 645, 657 Winstanley, D., 1835 Wojcik, D., 1253 Wurie, A. T., 1651 Xyla, A. G., 1477 Yamada, K., 743 Yamamoto, Y., 1641, 1795 Yamasaki, S., 1641 Yanagihara, Y., 1579 Yanai, Y., 1641 Yariv, S., 1685 Yonezawa, Y., 1559 Yoshino, T., 1823 Zaki, M.I., 1601 Zhang, Q., 635 1281 (vii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEM4STRY GENERAL DISCUSSION N o . 8 4 Dynamics of Elementary Gas-phase Reactions University of Birmingham, 14-16 September 1987 Organ ising Committee : Professor R. Grice (Chairman) Dr M. S. Child DrJ. N. L. Connor Dr M. J. Pilling Professor I. W. M. Smith Professor J. P. Simons The Discussion will focus on the development of experimental and theoretical approaches t o the detailed description of elementary gas-phase reaction dynamics. Studies of reactions at high collision energy, state-to-state kinetics, non-adiabatic processes and thermal energy reactions will be included. Emphasis will be placed on systems exhibiting kinetic and dynamical behaviour which can be related to the structure of the reaction potential- energy surface or surfaces. The final programme and application form may be obtained from: Mrs.Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 23 Molecular Vibrations University of Reading, 15-16 December 1987 Organ ising Committee : Professor I. M. Mills (Chairman) Dr J. E. Baggott Professor A. D. Buckingham Dr M. S. Child Dr N. C. Handy Dr B. J. Howard The Symposium will focus on recent advances in our understanding of the vibrations of polyatomic molecules. The topics to be discussed will include force field determinations by both ab initiu and experimental methods, anharmonic effects in overtone spectroscopy, local modes and anharmonic resonances, intramolecular vibrational relaxation, and the frontier with molecular dynamics and reaction kinetics.The final programme and application form may be obtained from: Mrs. Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN (viii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 85 Solvation University of Durham, 28-30 March 1988 Organising Committee: Professor M. C. R. Symons (Chairman) Professor J. S. Rowlinson Professor A. K. Covington Dr I. R. McDonald The purpose of the Discussion is to compare solvation of ionic and non-ionic species in the gas phase and in matrices with corresponding solvation in the bulk liquid phase. The aim will be to confront theory with experiment and to considerthe application of these concepts to relaxation and solvolytic processes. Contributions for consideration by the organising Committee are invited in the following areas: (a) Gas phase non-ionic clusters (b) Liquid phase non-ionic clusters (c) Gas phase ionic clusters (d) Liquid phase ionic solutions (e) Dynamic processes including solvolysis Further information may be obtained from: Mr.Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBM. Dr J. Yarwood Dr A. D. Pethybridge Professor W. A. P. Luck Dr D. A. Young THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 86 Spectroscopy at Low Temperatures University of Exeter, 13-15 September 1988 Organising Committee: Professor A. C. Legon (Chairman) Dr P.6. Davies Dr 6. J. Howard Dr P. R. R. Langridge-Smith Dr R. N. Perutz Dr M. Poliakoff The Discussion will focus on recent developments in spectroscopy of transient species (ions, radicals, clusters and complexes) in matrices or free jet expansions. The aim of the meeting is to bring together scientists interested in similar problems but viewed from the perspective of different environments. Contributions for consideration by the Organising Committee are invited. Titles should be submitted as soon as possible and abstracts of about 300 words by 30 September 1987 to: Professor A. C. Legon, Department of Chemistry, University of Exeter, Exeter EX4 4QD. Full papers for publication in the Discussion volume will be required by May 1988.JOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistrykhemical physics which have appeared recently in J.Chem. Research, The Royal Society of Chemistry's synopsis + microform journal, include the following: Po I a ri za bi I i t y and Conform a ti o n of M o n 0- a n d 6 is- (t ri a I ky I si I y I ) -, - (t ri a I ky I g e r my I)-, a n d -(trialkylstannyl)-acetylenes Krystyna Kamienska-Trela, Hanna Ilcewicz, Maria Rospenk, Maria Pajdowska, and Lucjan Sobczyk (1 987, Issue 4) Tetraphenylcyclopentadienylidene Anion Radical: Factors influencing the Ease of Formation Donald Bethell and Vernon D. of Carbene Anion Radicals from the Diazo Compounds Parker (1 987, Issue 4) Photochemical Reduction of Chromium(V1) over Cadmium Sulphide, Zinc Sulphide, and Tungsten(V1) Oxide Javier Domenech and Javier Mufioz (1987, Issue 4) lnterstructural and Structure - Reactivity Correlations in Cycloalkane Derivatives and their Hans- Solvolysis Reactions: the Limitation of Geometry - Reactivity Relationships Jorg Schneider, Ulrich Buchheit, and Gunther Schmidt (1987, Issue 3) E.s.r.Studies on Carboxylic Esters, Part 8. E s r . Studies on Thioamides, Part 7. Radical Anions of 1,l-Dithio- and l,l,Z-Trithio-oxalic Esters and Amides Horst Gunther and Jurgen Voss (1 987, Issue 3) Is the PhCH2(N20)' Cation likely to be a Reaction Intermediate? A Theoretical Study Peter M. W. Gill, Howard Maskill, Dieter Poppinger, and Leo Radom (1 987, Issue 2) Conformational Structure of Bipyridine Radical Cations Hans-Jorg Hofmann, Renzo Cimiraglia, and Jacopi Tomasi (1987, Issue 2 ) Solid-Liquid Equilibria in the Quarternary System Monomethylhydrazine - Sodium Chloride - Sodium Hydroxide - Water a t 298.1 K Alexandrina Salas-Padron and Marie-Th6rese Saugier Cohen-Adad (1 987, Issue 2 ) Issue 5, 1987, contains seventeen papers presented at a meeting of the Societe Franqaise de Chimie on 'New Materials and Magnetic Effects' held in Paris in September 1986.~~ ~ FARADAY DIVISION INFORMAL AND GROUP MEETINGS Gas Kinetics Group Thermally and Photochemically Activated Reactions To be held at the University of Edinburgh on 9-10 July 1987 Further information from Professor R.Donovan, Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ Division Xlth International Symposium on Molecular Beams To be held at the University of Edinburgh on 13-17 July 1987 Further information from Dr J.F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN Industrial Physical Chemistry Group The Physical Chemistry of Small Carbohydrates (as part of the International Symposium on Solute-Solute-Solvent Interactions) To be held at the University of Regensburg, West Germany on 10-14August 1987 Further information from Dr F. Franks, Pafra Ltd, 150 Science Park, Milton Road, Cambridge CB4 4GG Industrial Physical Chemistry Group The Interaction of Biologically Active Molecules and Membranes To be held at Girton College, Cambridge on 8-10 September 1987 Further information from Dr T. G. Ryan, ICI New Science Group, PO Box 11, The Heath, Runcorn WA7 4QE ~ _ _ _ _ ~ _ _ _ _ Polymer Physics Group Biennial Meeting To be held at University of Reading on 9-1 1 September 1987 Further information from Dr D.Bassett, Department of Physics, University of Reading, Reading RG7 2AD Neutron Scattering Group Applications of Neutron and X-Ray Optics To be held at the University of Oxford on 14-1 5 September 1987 Further information from Dr R. K. Thomas, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ Surface Reactivity and Catalysis Group New Methods of Catalyst Preparation and Characterization To be held at Brunel University on 14-16 September 1987 Further information from: Dr M. Bowker, ICI New Science Group, PO Box 11, The Heath, Runcorn, Cheshire WA7 4QE Colloid and Interface Science Group Polydispersity in Colloid Science To be held at the University of Nottingham on 15-16 September 1987 Further information from Dr. R. Buscall, ICI plc, Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 4QE Polymer Physics Group New Materials To be held at the University of Warwick on 22-25 September 1987 Further information from Dr M. J. Richardson, Division of Materials Applications, National Physical Laboratory, Queens Road, Teddington, Middlesex TW11 OLW Division Autumn Meeting Spectroscopy of Gas-phase Molecular Ions and Clusters To be held at the University of Nottingham on 22-24 September 1987 Further information from Professor J. P. Simons, Department of Chemistry, University of Nottingham, Nottingham NG7 2RDPolymer Physics Group with the Institute of Marine Engineers Polymers in a Marine Environment To be held in London on 14-16 October 1987 Dr G. J. Lake, MRPRA, Brickendonbury, Herts SG13 8NL Electrochemistry Group with the SCI Electrosynthesis To be held at the University of York on 15-17 December 1987 Further information from Dr G. Kelsall, Department of Mineral Resources Engineering, imperial College, London SW7 2AZ Neutron Scattering Group Scattering from Disordered Systems To be held at the University of Bristol on 16-18 December 1987 Further information from: Dr R. J. Newport, Physics Laboratory, The University, Canterbury, Kent CT2 7NR (xii)

ISSN:0300-9599    

DOI:10.1039/F198783BP073

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J . Chem. SOC., Faruduy Trans. 1, 1987,83, 1651-1665 Is0 thermal Tit ration of Supported Platinum Part 1 .-CO-0, Titration Marie A. Martin Luengo, Paul A. Sermon” and Alpha T. Wurie Department of Chemistry, Brunel University, Uxbridge UB8 3PH The rate and extent of titrations of preadsorbed oxygen by gaseous CO and of preadsorbed CO by gaseous 0, upon oxide-supported Pt vary with the titration temperature; results suggest that 423 K is close to the optimum and is to be preferred to previously used ambient conditions. The consumption of titrant CO and the total amount of heat liberated in the primary titration processes are directly proportional to the Pt surface area available to chemisorb CO and may thus be used to estimate Pt dispersions rapidly and reproducibly. However, they require careful calibration with traditional chemisorptive methods because of great temperature sensitivity if absolute metal surface areas are to be estimated.Importantly, the ratio of enthalpies of the two primary titration processes (QCOTo/QOTco) appears to be very sensitive to the average Pt particle size, possibly as a result of the presence of different types of Pt adsorption site thereon. The thermokinetic profiles dQ/dt us. t provide an insight into the kinetics of such titrations, which may be relevant to the catalysis of CO oxidation. The isothermal titrations of oxygen chemisorbed upon surface Pt atoms by gaseous hydrogenf (HT,), alkenes, such as cyclohexene (AT,) or carbon monoxide3 (COT,) can be represented by: 4n(Pt,,-0) + (2 + 4n) H, + 4(Pt,,-H) + 4nH20 2n’(Pt,,-O) + (1 + 2n’) CO + (Pt,,,-CO) + 2n’CO, 2n”(Pt,,-O) + + (PtncHfl-)x- c) + 2n”/nH(Pt,,-H)+ 2n”H,O + (2 - 2n” - n“/n,) H,.The reverse titrations of preadsorbed hydrogen by gaseous oxygenf (OT,) or alkenes, such as ethene, pent-1-ene or cyclohexene (AT,) and titration of preadsorbed CO by gaseous oxygen (OT,,) are then given by: 4(Pt,,-H) + (1 + 2n) 0, + 4n(Pt,,-O) + 2H,O (Pt,,,-CO) + (n’ +t) 0, + 2n’(Ptn,-O) + CO, 2n”’(Pt,,-H) + (1 + n”‘)O+ (Ptnc.,,-)n -0 + n’” 0 where nH2, no,, nco, nCH and nCH“ are the average numbers of surface Pt atoms adsorbing one molecule of H,, O,, CO, benzene and cyclohexane, irrespective of the real extent of adsorbate dissociation. n is nH2/n02, n’ is nco/nop, n” is nCHu (for benzene adsorption)/n,, and n”’ is nCHllt (for cyclohexene adsorption)/nHg.The number of surface Pt atoms adsorbing a hydrogen atom and an oxygen atom (z.e. nH and no) are taken to be a half of nH2 and no,. Each n will take a varying value, depending upon the adsorbate-titrant coverage, partial pressure and temperature. Thus nH2 and nco may be 2 o r 1. HT, and OT, titrations have been widely used to estimate dispersions of supported and unsupported Pt,l* but observed ratios of the extents of chemisorption to titration 16511652 CO-0, Titration on Pt (e.g. qHC : qoc : qHT,) vary between 2: 2 : 6 and 2: 1 : 4. nH2 and no, vary with Pt particle size and the concentration of Pt step and terrace sites. The extent of titration is also certainly affected by the presence of (i) surface inhomogeneities, (ii) the fraction of reversibly held hydrogen, (iii) the fraction of chemisorbed oxygen which is unreactive towards hydrogen and (iv) the presence of contaminants.However, the uncertainty concerning whether the product water is retained upon Pt (stabilised by fractional coverages of oxygen and interfering with titrations) or transferred to the support or desorbed may be equally important.l? Isothermal AT, and AT, titrations2 have been used far less extensively to estimate Pt dispersion and still involve uncertainty regarding the extent of hydrocarbon retention on the catalyst, since the hydrocarbon titrant can also be held on the surface as an ill-defined surface intermediate. Isothermal COT, and OT,, have been the least-used titrations for estimating metal surface areas, OT,, being very rarely used.Langmuir showed5 that both COT, and OT,, titrations proceed at and above 293 K on unsupported Pt in a reversible manner. How- ever, Akhtar and Tompkins6 carried out the CO titrations of 0 adatoms on Pt film at 195 K and found that only 20-50% of the chemisorbed oxygen was so titrated; it was therefore assumed that the titration was too slow at this temperature. Nevertheless, the COT, was apparently used successfully at 300 K to characterise unsupported, carbon- supported and alumina-supported Pt (with product CO, being evolved and removed using a side-arm at 77 K).3 It was also used to characterise the dispersion of carbon- supported Pt at 303 K7 Interestingly, the reverse OT,, titration was preferred for characterisation of alumina-supported Ir at 423 K (rather than 300 K and the number of CO, molecules produced was directly related to the Ir surface area).* Subsequently, there has been disagreement about the titration conditions ~elected.~ The COT, process differs significantly from HT, and AT, titrations in that the product CO, is only weakly held [i.e.the enthalpy of CO, adsorption on Group VIII transition metals is -= 40 kJ mol-1 and thus low compared with that of 0, and CO (which have enthalpies of up to 335 and 230 kJ mol-l on Pt, respectively)]. Therefore CO, desorbs even at 110-109 K and 1.3 mPal0 and is not measurably adsorbed under titration conditions, but rather is rapidly desorbed, thereby minimising uncertainties. This titration is also different in that it involves no greater titrant consumption than direct CO adsorption, unless CO, is intentionally removed from the gas phase. Therefore (without such action) even if COT, goes to completion with all CO, desorbed, the ratio of the extents of titration to chemisorption (i.e.qcoT,/qcoc) should be close to unity. The COT, titration could in principle proceed either via a Langmuir-Hinshelwood or an Eley-Rideal mechanism. However, the OT,, titration is assumed to require dissociative chemisorption of oxygen before reaction and would presumably involve a Langmuir-Hinshelwood mechanism; possibly with a titration induction period. This would be especially so if adsorbed CO blocks the surface and inhibits the rate of OT,,. It may not be surprising therefore that COT, has generally been preferred to OT,, for estimating the dispersion of * Bearing in mind the frequent use of supported metal catalysts, it is important to note that at very high temperatures some oxide supports may also reduce in the presence of the CO titrantll during COT,,, but this is probably no more serious than reduction by H, in HT,.The rates and extents of these COT, and OT,, titrations involving the reaction between CO and oxygen on the surface of oxide-supported Pt are now described. These have been studied because of their apparent simplicity in terms of the greater extent of product desorption. After optimisation, consideration has been given to the accuracy they provide in estimating Pt surface areas in comparison with existing alternative chemisorption methods.M .A . M . Luengo, P. A . Sermon and A . T. Wurie 1653 Table 1. Pt/SiO, catalysts prepared conditions/K (h) silica Pr support (% ) impregnation calcination reduction sample ~~ ~ Shell 2.5 3 Shell 1.5 3 Shell 2.5 3 Shell 1.5 3 Shell 2.5 3 Shell 1.5 3 Davison 923 6 Davison 923 1 Sorbsil 6.2 373 (5) 373 ( 5 ) 373 (2) 373 (2) 273 (5) 273 (5) 298 (4) 298 (i) - 523 (1) - 523 (1) - 523 (1) - 523 (1) 543 (3) 713 (1) 543 (3) 713 (1) - 673 (1) - 673 (1) 378 (24) 673 (4) 2.513731-1523 1.5/373/-1523 2.5 1273 1-1 523 1.5/273/-1523 2.5/373/543/713 1.5 1 373 1 543 17 1 3 Fa IU KU a Described in ref. (12), where the fractional dispersions of supported Pt were found to be 0.265, 0.755 and 1.038 for samples F, I and K using the average extents of CO, 0, and H, chemisorption at ambient temperature.Experiment a1 Catalysts Four silica-supported Pt catalysts used here (denoted F, I and K) have been described previously.12 In addition six Pt/SiO, samples containing 3% Pt were prepared and are indicated in table 1 ; the silicas of controlled porosity (Shell S980-1.5 and S980-2.5) were impregnated with hexachloroplatinic acid (Johnson Matthey; Specpure) aqueous solutions to the point of incipient wetness at 373 or 273 K. Samples containing the acid salt upon the surfaces of the silica were then calcined in air at 534 K (or not; these treatments were designed to produce different Pt dispersions) and reduced in H, at 523 or 713 K. Such samples are denoted in table 1 by the silica used, the temperature of drying, calcination and reduction. Sample K is EuroPt Pt/SiO,; a comparison of titration results suggests no significant effect of surface chlorine after reduction.One sample of 3% Pt/TiO, was also used and has been described previously.12 Calorimetric Methods of Titration A differential scanning calorimeter (Dupont 990 with a 910 analyser) was used under constant temperature conditions. The catalyst and its support were placed in separate A1 pans on a constantan disc using chromel-constantan thermocouples to measure the differential heat flow as a function of time. A sample (10-25 mg; previously held in air at ambient temperature and therefore exhibiting a chemisorbed monolayer of oxygen) and its support were introduced and their temperature raised to that of the titration in flowing He. Then 6% CO in N, (BOC; 99.995% purity which had been further purified by passage through a Pd/Al,O, catalyst and then a molecular sieve trap) was introduced at 45 cm3 min-l and dQ/dt followed during COT,.After flushing with He for 10 min, 12% 0, in N, was introduced (BOC; 99.995% purity; further purified by passage through a molecular sieve trap; 45 cm3 min-l) and dQ/dt again followed as a function of time during OT,,. To determine the repeatability and reproducibility of COTo-OT,, titrations, several cycles were performed at different temperatures. Areas under the primary thermokinetic peaks were integrated up to the point where d2Q/dt2 < 0.02 mJ s-, to give enthalpies of the primary titration processes. Areas of integration are indicated in fig. 1 by shading. For titrations where dQ/dt > 0 such1654 CO-0, Titration on Pt tlminM . A .M . Luengo, P . A . Sermon and A . T. Wurie 1655 enthalpies are not total titration enthalpies; for this reason the ratios of enthalpies are judged most useful, i.e. QcoTo/QoT,o. Volumetric Methods of Chemisorption The extent of chemisorption of O,, H, and CO (i.e. qoc, qHC and qcoc) on the above Pt catalysts has been measured in a conventional volumetric apparatus12 using the following procedure. A sample (20&300 mg) was evacuated to < 1 mPa and the temperature raised to 423 K over 15 min. 13 kPa H, was then introduced for 30 min, after which the sample was re-evacuated for 1 h while heating to 683-693 K, at which temperature it was held for a further 2 h with evacuation to 1 Pa.Samples were then cooled to ambient temperature for chemisorption measurement. The values of qi corresponding to monolayer capacity were then deduced by extrapolation of the isothermal data to the zero-pressure intercept and also from the gradient of the linear Langmuir isotherm. Pt surface areas were then calculated assuming that nHz, no2 and n,, were 2,2 and 1 , respectively, and that the supported Pt existed as homogeneous cubic particles with one face upon the support. Volumetric Methods of COT, Titration The extent of CO titration of pre-adsorbed oxygen (qCOTo) on the Pt catalysts was also measured in the same volumetric apparatus using the following procedure. A sample (200-300 mg; previously held in air at ambient temperature and hence with its surface covered by adsorbed oxygen) was outgassed to 1 Pa and the temperature raised to 423 K over 30 min.A typical COT, isotherm was then measured at 423 K up to 6.7 kPa CO. Equilibration times were 10-15 min. The value of qCOTo corresponding to monolayer consumption was deduced by extrapolation of isothermal titration data to zero pco or from the gradient of the linear Langmuir isotherm. These values were converted to Pt surface areas assuming no* and n,, were 2 and 1, respectively, and in addition making the other assumptions used in chemisorption. Results Optimisation of COT,-OT,, Titrations using Calorimetry Fig. 1 shows the thermokinetic profiles obtained by differential scanning calorimetry during repeated COT, and OT,, titrations over Pt/SiO, sample K at 373 K using 40 cm3 min-l of either 6 kPa CO in N, or 21 kPa 0, in N, and intermediate flushing.First, it is clear that the profiles of heat flux (dQ/dt us. t ) are repeatable; hence the total extent of heat generation must be reproduced in repeated titrations. Presumably it must follow that the extent of exothermic COT,-OT,, titrations is also repeatable and reversible. Secondly, the shape of these profiles differs in the two titration steps, with that for the COT, more symmetrical (and appearing after a shorter induction period and with less subsequent tailing) than that for OT,,. The ratio of the times to maximum dQ/dt (tOT,o/tCOTo = 6.4) might suggest that OT,, and COT, proceed via Langmuir- Hinshelwood and Eley-Rideal mechanisms, respectively. However, there is also further information from the OT,, thermokinetic profiles.Thus the main OT,, thermokinetic peak is preceded by a period when dQ/dt is positive but small; it is tempting to associate this period with the dissociative chemisorption of 0, on the CO-covered surface prior to titration. Maximum rates of titration have been observed13 in residual gas analysis, Fig. 1. Repeat COT,-OT,, titrations of Pt/SiO, (K) at 373 K measured as a rate of heat flux by differential scanning calorimetry as a function of time. A: (a) OTcol, (b) OTcoz, (c) OT,,,. B: (a) COT,,, (b)COT,,.1656 CO-0, Titration on Pt 6 P k M TIK Fig. 2. Extents of primary heat liberation in COT, (0) and OT,, (0) titrations of Pt/SiO, (F) at temperature between 323 and 523 K. Table 2. Induction periods before dQ/dt > 0 for OT,, titrations on Pt/SiO, ~~ titration induction titration titration induction temp./K period/s temp./K no.period/s 293 330 323 1 323 168 323 2 373 80 323 3 423 48 323 4 473 0 323 5 523 0 423 1 573 0 423 2 3 4 5 165 315 840 1340 1410 24.0 22.5 18.0 15.0 13.5 fast i.r. response, Auger and radiotracer studies which have suggested that on Pt both COT, and OT,, proceed by Langmuir-Hinshelwood mechanisms, even if the CO titrant in COT, is only a weakly held precursor. This being so, then the rate of both isothermal COT,, and OT,, titrations should be proportional to a function of the fractional coverages of oxygen and CO [i.e. f ( ~ , , O , ) ] and hence should be a maximum at intermediate 6,, and 6, at intermediate titration times as shown in the dQ/dt us.t profiles here. However, the profiles in fig. 1 suggest that the titrations may not be complete under the conditions used here; this is particularly true when tailing is significant. Langmuir showed5 that qCOT, and qOTCo titres increased during repeated titrations upon un- supported Pt foil as the temperature was raised from 293 to 473-603 K. Thus a complete and stoichiometric titration may be unlikely9 at the arbitrarily chosen partial pressures and temperatures selected in fig. 1. Certainly others have found6 that the titrations are incomplete at 195 K. Fig. 2 shows the integrated areas of the primary steps in COT, and OT,, titrations over sample F Pt/SiO, at different temperatures; such areas of thermokinetic profiles are measured by integration of dQ/dt us.t plots to the pointM . A . M . Luengo, P. A . Sermon and A . T. Wurie Table 3. Enthalpies (in J per g Pt) of COT, and OT,, titrations on Pt/SiO, at 373 K Pt/SiO, (I) Pt/SiO, (K) run Q C O T ~ Q o T ~ ~ Q C O T ~ Q o T ~ ~ 1 630 2240 2270 1260 2 660 2200 2740 1260 3 670 2170 2920 1240 4 670 2140 3030 1240 5 680 2100 3090 1230 +36 -2 overall + 8 -6 change % 1657 where d2Q/dt2-< 0.02 mJ s-, (see the shaded areas in fig. 1). Such areas correspond to the enthalpies of these primary titration processes, but in the presence of thermokinetic profile tailing these necessarily cannot be the total titration enthalpy. From fig. 2 it is clear that the total extent of primary heat generation (to the point where d2Q/dt2 < 0.02 mJ s - ~ ) and hence the extent of both primary COT, and OT,, titration must increase significantly with increasing temperature.The greatest extents were seen at ca. 473 K. As the titration temperature is increased (i) qcoc and Oco will decrease with CO desorption before titration, liberating free surface Pt atoms * available for 0, chemi~orptionl~ in OT,, and (ii) qoc will increase with the eventual bulk oxidation of Pt particles to the 2 + state,14 which can then only weakly adsorb C0.15 Thus a maximum in the extent of COT,-OT,, titrations is to be expected at temperatures intermediate between 423 K and ambient. The titration rate should naturally rise with temperature. It would also be expected that the induction period in OT,, should decrease as the titration temperature rises.Table 2 shows that this is indeed the case. It therefore appears that a titration temperature of ca. 423 K is optimum; a value well above the ambient conditions often chosen in the The effect of Pt dispersion and titration temperature on the repeatability of COT, -OT,, titration was determined with titration cycles on samples I and K Pt/SiO, at 373 K. Table 3 shows that the total extent of the primary titration process in OT,, changed only a little with repeated titrations, but that that for COT, does increase. Enhancements have been seen previously3 as they have also been observed in HT,-OT, ~yc1es.l~ It should also be noted that the ratio of primary titration enthalpies QCOTo/QoTco is very different for these two silica-supported platinum samples. Fig. 2 has also shown that this ratio is very temperature-dependent.In addition, table 2 shows that the induc- tion period for the OT,, titration is large and increases substantially at 323 K with repeated titrations, but that at 423 K it is very small and decreases; the former situation probably arises from a progressive CO blocking of the surface in the titration cycles, which is not as serious at the higher titration temperature where Oco is lower. Again this suggests 423 K as an optimum titration temperature, where extents and kinetics of titrations are reasonable. However, it is important to note that some adsorbate will desorb before t i t r a t i ~ n ; ~ indeed this has been observed16 with OT,,. This temperature was selected for further study; this was also the temperature used for titration on supported Ir.*1658 CO-0, Titration on Pt 3 ( n m N m \ 2 4 6 8 P/ 1 O3 Pa 0 .0 9 1 0.08 t m ru ? 0.07 I M 0 1 2 3 p i ' / 1 O4 Pa Fig. 3. (a) Isotherms of H,, 0, and CO chemisorption at 293 K and COT, titration at 423 K on Pt/SiO, 2.5/373/-/523 (0) and the SiO, support alone (0) measured volumetrically. (b) and (c) Linear Langmuir plots of data in (a).M. A . M. Luengo, P. A . Sermon and A . T. Wurie 1659 (h) o ~ o - - - o - - - - o - o - 2 0 2 4 6 8 Pco/ 1 O3 Pa Fig. 4. COT, titrations at 423 K on different Pt/SiO, samples measured volumetrically, where (a), (b), (c), (d), (e), (f), (g) and (h) denote 2.5/373/543/713, 2.5/373/-/523, 1.5/373/-/523, 2.5/373/-/523, 2.5/273/-/523, 1.5/373/543/713, 1.5/273/-/523/, and Silica 2.5, respectively.COT,, Titrations at 423 K COT, titrations were obtained at 423 K on all six samples of Pt/SiO, prepared on the Shell silica supports of controlled porosity. In addition, one titration was carried out on one sample at 298 K for purposes of comparison. The justification for the selection of 423 K is seen in the fact that the value of qCOTo at 298 K was 25% lower than that obtained at 423 K; presumably this is a reflection of the slower titration kinetics at the lower temperature. Fig. 3 shows the qi us. pi isotherms for the chemisorption of CO, H, and 0, on 2.5/373/---/523 Pt/SiO, at 298 K and also that for the COT, titration on the same catalyst at 423 K. Isotherms on the support alone are also indicated. After subtraction of consumption upon the support alone, data were extrapolated to zero pressure and plotted as linear Langmuir isotherms; in the latter plots good linearity was seen.Titration data were obtained for each Pt/SiO, and are shown in fig. 4. From table 4 it can be seen that there is good correlation between the CO monolayer adsorption and titration Capacities and also between capacities measured at the zero-pressure intercept and from the linear Langmuir gradient. Values of the ratio of titration to adsorption capacities (i.e. ~ ~ o T , / ~ C O c ) are also given in table 4. This ratio should be unity if the COT, goes to completion and all CO, is returned to the gas phase. However, the values of the ratio suggest only 62-100% completion of COT,. Therefore, although the COT,-OT,, titrations are repeatable in continuous titration cycles under isothermal conditions (see table 3 and fig. l), neither titration step appears to go to completion even under the optimised conditions selected here.Hence the results are self-consistent, but require calibration before absolute surface areas of Pt can be assessed by titration. Fig. 5 shows the heat liberated in the primary COT, process (QcoTo) at 498 K on Pt/SiO, samples prepared from Shell silicas of controlled porosity and that this varies linearly with the average Pt particle size (dpt)1660 CO-0, Titration on Pt Table 4. Extents of isothermal adsorbate or titrant consumption on Pt/SiO, samplesa qJpmol per g catalyst sample qHC 4oc 4coc qCOTo qCOTo/qOTco 2.5/373/-/523 16.57 (17.66) 9.81 (10.77) 24.1 1 (24.98) 24.46 (24.72)’ 1.02 0.78 0.62 1.98 9.36 7.29 0.78 0.83 8.84 0.88 1.5/373/-/523 14.31 7.06 27.00 21.04 2.5/273/-/523 10.08 6.44 19.76 12.21 1.5/273/---/523 4.80 2.5/373/543/713 19.31 10.30 29.94 24.93 1.5/373/543/713 5.81 2.09 10.04 ~~ Bracketed data are those derived from fig.3(b) linear-Langmuir isotherms at 290 or 423 K. qi data were obtained from H,, 0, and CO chemisorption at 298 K or COT, titrations at 423 K. * One COT, titration was also carried out on this sample at 298 K, producing a pressure intercept (18.21 pmol per g catalyst) significantly below the value observed in COT, at 423 K (24.46 pmol per g catalyst). Fig. 5. Total extent of heat liberated during COT, titration on different samples of Pt/SiO, at 498 K plotted against Pt particle size, estimated by volumetric measurement of extent of COT, titration at 423 K.M .A . M. Luengo, P. A . Sermon and A . T. Wurie 1661 estimated from volumetric measurement of qCOTo. Similar correlations have been seen7 between enthalpies of HT, titrations and the Pt dispersion in carbon-supported Pt. However, this correlation here suggests that the primary thermokinetic peaks in COT, and OT,, (measured here until d2Q/dt2 < 0.02 mJ s-,) do correspond to titration processes related to the Pt surface alone; it may be that the tailing is involved with a spillover process, in which case it is intriguing to wonder why it is more dominant in one titration step than another. Discussion The above results seem to indicate that after calibration the titration of an oxygen- covered silica-supported Pt catalyst by gaseous CO at 423 K can give reliable estimates of the Pt surface area from the volumetric extent of titration qCOTo or the enthalpy of the primary thermokinetic titration peak QCOTo.The Pt dispersions obtained are self- consistent, but lower than those measured by CO chemisorption at 298 K. This will be so if (i) less CO is chernisorbed at 423 K than 298 K, (ii) not all chemisorbed oxygen can be titrated by CO (either from a kinetic or a thermodynamic point of view) or (iii) product CO, is not totally desorbed, but remains on surface Pt sites (*) inhibiting further titration. Bearing in mind the ease of CO, desorption under titration conditions17 and the change in qcoc with temperature, (i) and (ii) seem most likely.However, the extent of CO chemisorption greatly exceeds the extent of 0, chemisorption (i.e. qcoc 9 2qOc in table 4) and it may be that qoc measured here is suppressed by a significant but variable fraction of unreactive oxygen. This is also reflected in the COT,-OT,, titrations. Nevertheless, under the titration conditions selected here both qCOTo and QCOTo are repeatable, consistent and proportional to the Pt surface area (see fig. 5 and table 4). However, OT,, may be more affected by the presence of unreactive oxygen. COT,-OT,, titrations are the precursors of the continuous catalysis of CO oxidation on Pt. Normally a measurable rate of steady-state CO oxidation is observed at a temperature above the ignition temperature (i.e. 383 K for Pt/Al,O,l*). The steady-state reaction is deemed19 to occur via a Langmuir-Hinshelwood mechanism over a wide range of coverage conditions on Pt surface sites * : co + * $ toads 0, + 2" 2oads Oads + coa& co2, g + 2*.Preliminary equilibria suggest some ti trant desorption before reaction ;9 this is seen as simultaneous CO desorption and liberation of CO, from Pt(l11) at 300-400 K.,* Certainly, CO adsorbs non-dissociatively on Pt at the present titration temperatures,21 while dissociative chemisorption of oxygen is thought,' a prerequisite for reaction. While Coa& inhibits co oxidation, Oads does n0t.l'~ 22 Therefore the incomplete removal of Oads seen here is not predicted. Naturally, surface imperfections affect the rate of CO oxidation on Pt23 (and also the extent of COads inhibition,,), but the induction periods in fig.1 are predicted by a Langmuir-Hinshelwood mechanism, although the temperature-dependence and early titration shoulder are not predicted. It may be that the titrations restructure the Pt surfaces to some extent.24 In future COT,-OT,, titrations could provide information on the oscillatory kinetics of CO oxidation on catalyst^,,^ since during the titrations all coverages of oxygen and CO are scanned and the rate will be given by d[CO,]/dt or dQ/dt, which will equal kf(Oco Oo), wherefis not a universal or simple function.26 It is also noteworthy that at 373 K the ratio of the primary titration enthalpies QcoTo/QOTco is initially 0.28 for Pt/SiO, (I) with a 75.5% dispersion of Pt and 1.80 for Pt/SiO, (K) containing Pt with a higher Pt dispersion.12 If not is 41662 CO-0, Titration on Pt k A t/min and nco is 1 then scheme (1) is valid, qCOT,/qoC is 1 and Q~~T,/QoT,, is (Qco, -2Qcoc - ~ Q O C ) / ( ~ Q C O , +SQOC - ~ Q c o c ) * Pt,O + 3CO + 2Pt-CO + CO, 2Pt-CO+ 1.50,+ Pt,O+ 2C0,.(1) However, if no, is 4 and n,, is 2 then scheme (2) is relevant, qCOT,/qoC is 1 and QcoT,/QoT,, is (Qco, + Qcoc -iQoc)/(Qco2 + tQoc - Qcoc). Pt,O + 2co + Pt,-CO + co, Pt,-CO + 0, + Pt,O + CO, (2)M . A . M . Luengo, P. A . Sermon and A . T. Wurie 1663 n t/min t/min Fig. 6. COT, and OT,, titration thermokinetic profiles dQ/dt us. t for Pt/TiO, (N) (A), (B) and Pt/SiO, (G) (C), (D) measured at 373 K. For Pt/TiO, the preceding 0, chemisorption in OT,, is far more significant than for Pt/SiO,. A: (a) OTcOl, (b) OT,,,, ( c ) OT,.,.B: (a) COTo1, (b) COT,,. C : (a) OTCol, (6) OT,,,. D: (a) COTo1, (6) COT,,. Here it is assumed that Qi is the enthalpy of chemisorption or the enthalpy of oxidation of CO to CO,, g. (i.e. QcOz is 283 kJ mol-1 of CO oxidised at 298 K). Clearly, the catalyst moves from regime (1) to (2) if it bridge-bonds CO rather than linearly adsorbing it. Qcoc and Qoc are 21-230 and 167-335 kJ mol-l, respectively, on different Pt planes.1° Taking the highest values of Qcoc and Qoc the ratio QcoTo/QOTco is 2.10 and 1.57 for linearly and bridge-bound CO. However, taking the lowest values of Qcoc and Qoc this ratio becomes 0.40 to 0.64. Returning to the data in table 3, there is a suggestion that catalyst I exhibits * surface sites on the larger Pt crystallites which chemisorb CO and 0, with low enthalpies, while sites on the surfaces of the smaller crystallites in sample K chemisorb CO and 0, with higher enthalpies.This could be related to the degree of site coordination, but unfortunately enthalpies of CO and 0, chemisorption are not very different on (1 1 l), (1 10) and (100) planes of Pt10 and differences in QCOTo/QOTco cannot be ascribed to different fractional contributions of these planes to the surfaces of Pt crystallites of different size. Further evidence is required, especially if total rather than primary titration enthalpies can be measured. Nevertheless, the results presented here1664 C0-0, Titration on Pt suggest that this ratio is very sensitive to: (i) Pt particle size and temperature and (ii) continued use and cycling of catalysts.In addition it may be that the support is also important. Compare the results shown in fig. 6 for Pt/TiO, (sample N12 with a Pt dispersion of 12.2%) with those of Pt/SiO, (sample G with a Pt dispersion of 3 1.5% 12). With the titania support the initial low dQ/dt shoulder in OT,, tentatively associated with oxygen adsorption onto the initially CO-covered surface is much more pronounced than when using the silica support. Conclusions The extents of COT, titrations on supported Pt measured as qCOTo or the primary enthalpy change QCOTo can be used to determine Pt surface areas after calibration, but titration at 423 K is preferred to earlier suggestions of ambient ternperat~re.~?~ The enthalpy ratio QCOTo/QoTco appears to probe Pt surface site coordination and particle size, the nature of the support and the extent of restructuring during titration in a manner not shown by the enthalpy ratio in HT,-OT, titrations,,' but is also temperature sensitive.The COT,-OT,, titrations are repeatable and partly reversible ; some surface Oads may remain unreated. Combined thermokinetic-catalytic data may provide more detailed kinetic information on catalysis of CO oxidation in future. Total titration enthalpies are not yet readily determined and absolute values so determined by differential scanning calorimetry28 may be less significant than their ratios and relative values used here. The provision of study leave for M.A.M.L. by the Consejo (Madrid) and of a studentship for A. T. W.by the Sierra Leone Government is gratefully acknowledged. References 1 J. E. Benson and M. Boudart, J. Catal., 1965, 4, 704. 2 P. A. Sermon and G. C. Bond, J. Chem. SOC., Faraday Trans. 1, 1976,72,745; G . Leclercq, J. Barbier, C. Betizeau, R. Maurel, H. Charcosset, R. Frety and L. Tournayan, J. Catal., 1977, 47, 389; G. C. Bond and P. A. Sermon, React. Kinet. Catal. Lett., 1974, 1, 3 ; R. L. Augustine, K. P. Kelly and R. W. Warner, J. Chem. SOC., Faraday, Trans. I , 1983,79, 2639. 3 P. Wentrcek, K. Kimoto and H. Wise, J. Catal., 1973, 33, 279. 4 M. A. Vannice, J. E. Benson and M. Boudart, J. Catal., 1970,16, 348; G. R. Wilson and W. K. Hall, J. Catal., 1970, 17, 190; J. Catal., 1971, 24, 306; D. E. Mears and R. C. Hansford, J. Catal., 1967, 9, 125; J. Freel, J.Catal., 1972, 25, 139, 149; E. Kikuchi, P. C. Flynn and S. E. Wanke, J. Catal., 1974, 34, 132. 5 I. Langmuir, J. Am. Chem. SOC., 1918,40, 1361. 6 M. Akhtar and F. C. Tompkins, Trans. Faraday SOC., 1971, 67, 2461. 7 A. Linares-Solano, F. Rodriguez-Reinoso and C. Salinas-Martinez de Lecea, Carbon, 1982, 20, 177. 8 J. L. Falconer, P. R. Wentrcek and H. Wise, J. Catal., 1976, 45, 248. 9 P. C. Flynn and S. E. Wanke, J. Catal., 1975, 36, 244; P. Wentrcek and H. Wise, J. Catal., 1975, 36, I0 G. A. Somorjai, Catal. Rev., 1978, 18, 173; P. R. Norton and P. J. Richards, Surf. Sci., 1975,49, 567. I I J. A. Pajares, J. E. Gonzalez de Prado, J. L. Garcia Fierro, L. G. Tejuca and S. W. Weller, J. Catal., 12 A. R. Berzins, M. S. W. Vong, P. A. Sermon and A. T. Wurle, Ads. Sci.Tech., 1984, 1, 51. 13 Non-Linear Phenomena in Chemical Dynamics, Springer Series in Synergetics 12, ed. C. Vidal and A. Pacault (Spnnger-Verlag, Berlin, 198 1). 14 E. Kikuchi, P. C. Flynn and S. E. Wanke, J. Catal., 1974, 34, 132; G. Blanchard, H. Charcosset, M. Guemin and L. Tournayan, Surf. Sci., 1981, 106, 509; P. C. Fiynn and S. E. Wanke, J. Catal., 1975, 36, 244; T. Matsushima, C. J. Mussett and J. M. White, J. Catal., 1976, 41, 397. 247. 1976, 44, 421. 15 H. Heyne and F. C. Tompkins, Trans. Faraday SOC., 1967,63, 1274. 16 H. P. Bonze1 and R. Ku, Surf. Sci., 1972,33,91. 17 G. Ertl and P. Rau, Surf. Sci., 1969, 15, 443. 18 V. Hlavacek and J. Votruba, Adv. Catal., 1978, 27, 59. 19 T. Engel and G. Ertl, Adv. Catal., 1979,28,1; T. Engel and G. Ertl, in Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, ed. D. A. King and D. P. Woodruff (Elsevier, Amsterdam, 1982), chap. 3; R. P. H. Gasser, Introduction to Chemisorpiion and Catalysis by Metals (Oxford University Press, Oxford, 1985); H. Okamoto, G. Kawaura and T. Kudo, J. Catal., 1984, 87, 1.M . A . M . Luengo, P. A . Sermon and A . T. Wurie 1665 20 J. L. Gland, M. R. McClellan and F. R. McFeely, J. Chem. Phys., 1983,79,6349; J. Vac. Sci. Technof., 21 T. Engel and G. Ertl, J. Chem. Phys., 1978,69, 1267; W. Andreoni and C. M. Varma, Phys. Rei!., 1981, 22 J. 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ISSN:0300-9599    

DOI:10.1039/F19878301651

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1987, 83, 1667-1684 Isothermal Titration of Supported Platinum Part 2.-Alkene Titration using Cyclohexene Mariana S. W. Vong and Paul A. Sermon* Department of Chemistry, Brunel University, Uxbridge UB8 3PH Isothermal alkene titrations (AT) of Pt/SiO, have been studied on a continuous basis with simultaneous measurement of hydrogen, cyclohexene, cyclohexane and benzene concentrations. Cyclohexene either donates H atoms to the catalyst at 423 K or extracts them from the catalyst at 293-353 K, the temperature being determined by the kinetics and thermo- dynamics of the titration. After optimisation of titration parameters, standard temperatures for the dehydrogenation and hydrogenation steps in the titration were chosen to be 423 K and 310 K.At these temperatures rates of donation of H atoms (dehydrogenation) increased with increasing cyclohexene partial pressure, but subsequent rates of extraction of H atoms passed through a maximum at 500-630 Pa; the optimum partial pressure of cyclohexene selected for standard titrations was, therefore, ca. 460 Pa. Titration mechanisms are considered in detail and suggest that Pt surfaces areas can be estimated from the first phase of H-atom donation by cyclohexene; indeed these (51.8 m2 per g Pt) were in close agreement with those estimated by hydrogen chemisorption (53 m2 per g Pt). However, alkene adsorption can affect results from numerous repeated titrations. The alkene titration when operated in a dehydrogenation mode (AT,) with hydrogen donation to an oxygen-covered catalyst appeared far more satisfactory than previously used titrations involving alkene hydrogenation (AT,) by preabsorbed hydrogen.Since a complete differentiation of Pt-held from support-held hydrogen could not be achieved under isothermal conditions a temperature-programmed titration is proposed. Hydrogen can spill over or migrate from a supported metal to the adjacent support phase.l It may subsequently become directly involved in hydrogenation reaction,2 may be desorbed thermall~,~ reduce the support4 or migrate back to the surface of the supported metal for r e a c t i ~ n . ~ ? ~ The second and fourth processes are the most likely courses of action for hydrogen spilt over in Pt/Si02. Alkene titrations of hydrogen preadsorbed (AT,) on heterogeneous catalysts were first7 carried out isothermally at an arbitrary temperature and titrant partial pressure using pent-1-ene (and later ethene) with the aim of estimating the extent of metal-held hydrogen (and hence the metal dispersion) and also the extent to which spillover to the support was reversible.The first isothermal separations were based upon the kinetics with which these different surface hydrogen species were likely to be available for titration by an alkene and was not complete and hence not entirely successful. In an attempt to improve separation and resolution, other recent works has concentrated on titration of preadsorbed (and spilt over) hydrogen on catalysts with pulses of alkenes. The total quantities of hydrogen so extracted have been 5 to 25 times greater than the quantities thought to be held on the supported Pt alone in monolayer capacity, and presumably involved incomplete separation of metal-held hydrogen and support-held hydrogen.A suggestion8 for but-1-ene titration that the yield of butane was proportional to the number of corner Pt sites absorbing two H atoms, while the yield of but-2-enes was proportional t o the number of metal sites absorbing only one H atom was interesting 16671668 Cyclohexene Titration of Pt and potentially useful if alkene titration techniques can be optimised. One problem with pulse methods is that preadsorbed H, can be desorbed between pulses and re- equilibrationg between the support and Pt sites may disturb the initial hydrogen population of these surface sites and hence the ultimate separation.In addition, relatively high alkene concentrations constitute the pulses, and the adsorption characteristics for the alkene under these conditions is not well known. There is increasing need for such techniques of in situ catalyst characterisation. In the work presented here the new alkene titrant of cyclohexene was chosen to titrate silica-supported Pt catalysts since it readily donates and accepts hydrogen to and from the catalyst surface during dehydrogenation (AT,) and hydrogenation (AT,) phases of the titration. This is a radical but improved departure from past practice. To allow a good understanding of the kinetics of H-atom donation and extraction, this titrant was to be added continuously at constant concentration and temperature.The equilibrium position of the above reactions depends on the temperature and pressure, but temper- atures of 300 and 600 K favour hydrogenation and dehydrogenation, respectively ; relevant equilibrium constants for hydrogenation and dehydrogenation are : InK, = 30.31 at 298 K 18.08 at 400 K 5.83 at 600 K In Kd = -9.20 at 298 K 0.01 at 400 K 9.34 at 600 K. For this reason cyclohexene is an intriguing titrant for probing supported catalyst surfaces, but beside hydrogenation-dehydrogenation, cyclohexene can simultaneously undergo disproportionation to cyclohexane and benzene. However, this is not the dominant reaction under conditions of titration selected here. The extent of adsorption of cyclohexene and the extent of H, desorption has been measured on catalysts used here under conditions relevant to the titrationlo The continuous titration mode with simultaneous measurement of concentrations of hydro- carbons and H, was selected because of the ease with which kineticscould be interpreted and compared with these known absorptive and desorptive properties of the catalyst under the same reactant partial pressures and temperatures.Thus the continuous titration using cyclohexene has been optimised and the results subsequently obtained are now reported. Titration Model Fig. 1 represents possible mechanistic routes by which cyclohexene might interact with a Pt surface. Here * denotes a vacant site on a ‘clean’ metal surface, O,, an oxygen atom adsorbed in a bridged site and H, an absorbed hydrogen atom. In the new dehydrogenative alkene titration mode (with cyclohexene donating H atoms to the catalyst) the initial surface of the Pt may be ‘clean’ with a high concentration of vacant Pt surface sites and a high value of coverage 6, (in AT, titration) or may be partially covered by preadsorbed oxygen and have a moderate value of coverage (in AT, titration).In the older hydrogenative mode of alkene titration using cyclohexene the Pt surface on which the titration is initiated is one on which hydrogen has been preadsorbed and where 6,. % 8, (i.e. in AT, titration). Initially, in the dehydrogenative mode either k’ > k” or k’ < k” (depending upon the initial oxygen coverage of the supported Pt) and in the hydrogenative mode k > k’. It is important to note that surface coverages will change as the titration proceeds and that these inequalities will not be maintained.Under conditions of continuous hydrogenation on Pt, it has been suggestedll that half-hydrogenated C6H11* is the dominant hydrocarbon surface species. It is important to note here the effects upon the rate of titration of initial values of 8,, Bo,p or OH,.M . S . W. Vong and P . A . Sermon 1669 Fig. 1. Mechanisms of cyclohexene titration of vacant (*), oxygen-covered (Oe2) and hydrogen- covered (H,) sites on oxide-supported platinum where relevant primary forward adsorptive rate constants are k’, k” and k, respectively, and the first surface rate constants are k’, k . k ” is one of the rate constants defining conversion of 2H, into H,(g) and 2,. Two OH, may convert to H20(g) and O*2.Surface hydrocarbons may be mono-n, di-n, tri-n, mono-a, di-a or tri-a, adsorbed to * sites. In the hydrogenation mode Boudart et a1.12 have suggested that the half-hydrogenated species CGHll, dominates the surface hydrocarbon species. Experimental Materials Silica-supported Pt (E) prepared and characterised as reported previously12 was studied under isothermal titration conditions. Its main characteristics are given in table 1. Liquid cyclohexene (BDH research grade, purity > 99.8%) was purified by shaking with activated alumina before use to remove traces of peroxide which might be present. The purified cyclohexene contained no impurity detectable by gas chromatography. The hydrogen (BOC, 99.9% purity) and nitrogen (BOC, white spot, 99.9% purity) were used and were further purified by passage through beds of pre-reduced 1 % Pd-H, WO, (0.4 g) and MnO,-celite (7.0 g) powders, respectively, and cold traps at 195 K for the removal of traces of oxygen (< 1 ppb oxygen) and the vapour pressure of water to 0.9 ppm.1670 Cyclohexene Titration of Pt Table 1.Characteristics" of Pt/SiO, sample E 5 wt% Pt (3.9 wt% Pt determined by a.a.) Davison 70 support 3.35 x 1019 H atoms chemisorbed per g catalyst at monolayer capacity (qHC) 2.951 x 1019 ethene molecules per g catalyst at monolayer capacity (qCH) 16.83 x 1019 cyclohexene per g catalyst at monolayer capacity (qCH,,) 2.80 x 1019 H atoms desorbed per g catalyst in a, peak (380 K) 2.30 x 1019 H atoms desorbed per g catalyst in /?, peak (598 K) 7.68 x 1019 H atoms desorbed per g catalyst in y, peak (707 K) 12.78 x 1019 H atoms desorbed per g catalyst in all peaks of t.p.d.52.60 m2 per g Pt average surface area indicated by all chemisorptions 53.60 m2 per g Pt average surface area indicated by H, chemisorption 81.60 m2 per g Pt surface area indicated by a+/? t.p.d. peaks 5.33 nm average Pt particle size indicated by all chemisorptions 5.03-9.45 nm average Pt particle size indicated by TEM 5.30-5.90 nm average Pt particle size indicated by XRD ~ a Methods and results of chemisorption, X-ray diffraction, transmission electron microscopy and temperature-programmed desorption of H, have been described l2 Titration Apparatus A conventional flow system was used for cyclohexene titration on catalysts at atmos- pheric pressure.The cyclohexene saturator consisted of a 250 cm3 round-bottom flask (containing 25 cm3 of purified cyclohexene) and two empty bubblers, one which allowed the purified N, to be cooled to 243 K before entering the saturator flask in order to avoid any temperature gradient, which would affect the saturation vapour pressure of cyclohexene, and the second was to ensure gerfect cyclohexene saturation in N, before entering the catalyst reactor. Both bubblers and the flask were immersed in a thermostat bath (Grant LB8) filled with paraffin at 294 to 238 K with a sensitivity of kO.1 K and uniformity of f 0.3 K (measured using a precision mercury thermometer). Gas reactants entered a Pyrex microcatalytic reactor, passed through the sinter and reacted with the catalyst thereon.Samples of the resulting gas mixture were then taken by a gas-tight syringe for g.c. analysis. A mercury thermometer measured the temperature of the catalyst bed to f0.05 K. The temperature of the reactor was controlled by a stanton Redcroft linear temperature programmer (mark I11 LVP/CA4/R) and a Stanton Redcroft low-mass vertical furnace (LMVS 100). The flow rates of the gas were measured by a soap-film flowmeter (4 0.05 cm3 min-l). Gaseous hydrocarbons were analysed by gas chromatography (Perkin-Elmer gas chromatograph model F11 with a flame ionisation detector and a steel column packed with 5% polyethene glycol 400M coated on 100-200 mesh celite) and the peak areas were integrated (Minigrator, Spectra-Physics 2/947). Concentrations of H, in the gaseous product were analysed by a Perkin-Elmer gas chromatograph model F17) with a hot-wire conductivity detector and a glass column filled with molecular-sieve column packing material.Procedure Isothermal cyclohexene titrations were studied. Samples (0.4 g) of the catalyst with 0-covered surfaces in the reactor were purged with purified N, at room temperature. The sample was then heated to the titration temperature in N, and the dehydrogenation titration (AT,) commenced. Purified N, saturated with cyclohexene [p,(alkene partialM . S. W. Vong and P . A . Sermon 1671 pressure) 460 Pa] left the saturator and entered the reactor at a flow rate of 7 cm3 min-l. The concentrations of cyclohexane, cyclohexene, benzene and hydrogen in the reactor were monitored as a function of time by gas chromatography for 1 h.Then the cyclohexene-nitrogen reactant gas flow was stopped and the sample was cooled rapidly to room temperature before being purged with nitrogen (100 cm3 min-l). Titration AT, of the chemisorbed hydrogen at a low temperature was then recommenced by passing cyclohexene-nitrogen over the catalyst (7 cm3 min-l, p A = 460 Pa) and the reaction was followed until equilibrium was reached. Calculation Within certain temperature ranges, cyclohexene underwent slight disproportionation simultaneously with either hydrogenation or dehydrogenation over the catalyst. How- ever, the percentage dehydrogenation is given by (([C,H,] - [C,Hl,]/2) x 100/[inlet C,H,,,]} and the percentage hydrogenation is given by (([C,H,,] - 2[C,H,]) x lOO/[inlet C6Hlo]).Hence, the percentage conversion of the titrations in both hydrogenative and dehydrogenative directions was determined. From this the net rate of H-atom donation or retrieval from the catalyst could be determined by monitoring the gaseous H, concentration. Subsequently, these were converted to Pt surface areas assuming the number of Pt atoms preadsorbing an oxygen molecule in AT,(n,,) or a hydrogen molecule in AT,(H,,) was 4 and 2, respectively, irrespective of any adsorbate dis- sociation. Under conditions of dehydrogenative titration at 423 K, water produced was assumed to be released to the gas phase and not to interfere with the titration. Results for Optimisation of Isothermal Titration Thermal Reversibility First an attempt was made to study the thermal reversibility of the interaction of cyclohexene with a sample of Pt/SiO, (E), after this had been reduced in H, at 375 K for 1 h and stored in air at 295 K.Cyclohexene (460 Pa) in N, (101 kPa, predried at 195 K) was passed continuously over the catalyst at a rate of 7 cm3 min-l. As the temperature of the reaction was raised in a stepped manner from 273 to 495 K catalytic measurements at each step after 5 min equilibration revealed benzene produced by dehydrogenation at a rate which increased with temperature (see fig. 2). As the temperature was then reduced in steps after reaching 495 K, the rate of dehydrogenation decreased (fig. 2), reaching almost zero at 295 K, but the rate of dehydrogenation curve did not follow the same path when the temperature increased and decreased and in this context was hysteretic.It is reasonable to suppose that the Pt surface of the sample was initially covered by oxygen (as in an H,-0 titration) with the surface coverage of bridged oxygen on * sites, O0*,, being high; upon this the cyclohexene absorbs possible heterolytically and dehydrogenates (see fig. 1). Subse- quently surface OH groups release water and O0,, decreases. Thus B0*, decreases as temperature increases in fig. 2(a). In addition, as oxygen is removed, vacant platinum surface sites * are then occupied by further chemisorbed hydrogen (H*), also produced from cyclohexene dehydrogenatim (and some adsorbed cyclohexene, benzene or cyclohexane). As the reaction proceeds, the surface coverage of hydrogen 0,.(and hydrocarbon) increases until it reached an equilibrium value at the given temperature. Thus, when the experiment was continued with decreasing temperature on the same metal surface onto which hydrogen has been produced in the first run with increasing temperature, dehydrogenation of cyclohexene was slightly less favourable. Fig. 2 (b) shows the rate of desorption of hydrogen produced from the cyclohexene dehydrogena- tion in fig. 2(a), in addition to which some must be held by the Pt surface. Plots of1672 Cycluhexene Titratiun of Pt 37.5 - ( a ) c( E 3 c, CI k 27.5 - +.’ 0 M a % v) d 8 17.5- c1 z 0 - rl 2 7.5 - I 2 73 433 513 T/K T/K Fig. 2. Effect of increasing (0) and decreasing (a) temperature on the rate of cyclohexene dehydrogenation (a) and on the rate of H, desorption during cyclohexene dehydrogenation (b) on Pt/SiO, (E) with preadsorbed oxygen.p A = 460 Pa in N,; total flow rate 7 cm3 min-l. hydrogen desorption during the same experiment also show a hysteresis loop in which the rate of hydrogen released by cyclohexene and desorbed increased as the temperature increased, and decreased less slowly when the experiment was repeated with decreasing temperature on the same metal surface, even though dehydrogenation was less favourable. Thus the surface ultimately became saturated with hydrogen (or cyclohexene) during the dehydrogenation of cyclohexene, and subsequent hydrogen uptake by the catalyst is relatively low. Comparison of fig. 2(a) and (b) shows that the net rate of hydrogen retention by the catalyst during titration is low, but significant; at 433 K ca.21 x lo1’M . S. W. Vong and P . A . Sermon 1673 t/min Fig. 3. Rate of production of H atoms as a function of time during cyclohexene dehydrogenation (0, 0) and rate of their release from preoxidised Pt/SiO, (E) surface (@, a) at 474 K (0, @) and 423 K (0, m). Conditions as in fig. 2. Shaded areas relate to hydrogen retained by the catalyst. cyclohexene molecules are dehydrogenated per g catalyst min-l. This must produce 84 x 1017 H atoms per g catalyst min-l, but only 53 x 1017 H atoms per g catalyst were released to the gas phase during heating and 75 x lo1' H atoms during cooling [see fig. 2(b)] at this temperature. Thus a small but significant fraction of hydrogen is retained by the Pt/SiO, catalyst and this varies with the dominant surface coverages on Pt.The initial extents of hydrogen retention by the catalyst during dehydrogenation titration of cyclohexene might, therefore, be used to deduce the Pt surface area if adsorption stoichio- metries are known. However, one complication is the fact that hydrogen retention continued at a very low rate (see fig. 3), even at long titration times at 423 K and more so at 474 K, despite the titration reaching a state of dynamic equilibrium. This suggests that a slow continuing uptake of hydrogen by the catalyst is involved, which could be explained by hydrogen migration from the Pt to the silica support. This continued to occur at all titration times. More evidence will be discussed on these points wher, consideration is given to the two phases of titration.Surprisingly, cyclohexene hydro- genation, which was expected to be favourable at low temperature and high OH., was not observed. Fig. 4 shows t.he effect of temperature on the rate of cyclohexene disproportionation under the conditions in fig. 2; this passed through a maximum at ca. 373 K before falling to zero at ca. 503 K. At low temperature hydrogenation prevails, as does dehydrogen- ation at high temperature. Hence 310 K was chosen for the hydrogenative titration phases and 423 or 473 K for the dehydrogenative titration phases (see table 2). Under these titration conditions disproportionation was not dominant and correction could be made for the extent of its occurrence. Temperature of Titration The effect of the temperature of dehydrogenation titration was studied on fresh samples of Pt/SiO, (E) at 423 and 473 K.Results obtained are recorded in table 2 and fig. 3. The1674 Cycluhexene Titration of Pt 40 0 E 20- v) I; 2 10- 0 I 513 Fig. 4. Rate of disproportionation of cyclohexene over Pt/SiO, (E). Conditions as in fig. 2. Table 2. Effect of temperature of dehydrogenation titration on repeated hydrogenation titration cycles dehydrogenation titration hydrogenation ti tra tion no. H atoms donated* no. H atoms extracted run T/K ( x 1019 per g catalyst) T / K ( x 1019 per g catalyst) 423 5 6 473 6.85 8.10 3 10 310 310 310 3 10 3 10 310 310 3 10 3 10 5.39 2.64 2.37 2.02 I .36 0.76 5.25. 2.40 0.77 0.27 a Up to 60 min titration. amount of hydrogen abstracted by the first hydrogenative titration at 310 K with an H-covered catalyst in both runs 1 and 7 was in reasonable agreement.However, the amount of hydrogen retained by the catalyst in dehydrogenation up to 60 min titration was slightly higher at 473 K (as revealed by the shaded areas in fig. 3); possibly as a result of an increase in the rate of hydrogen spillover from the platinum to its silica support.M . S . W. Vong and P. A . Sermon 1675 '0 tlmin Fig. 5. Rate of production of H atoms as a function of time during cyclohexene dehydrogenation (0) at 423 K (and other conditions as in fig. 2) and rate of H-atom desorption from the Pt/SiO, surface (a). Data as in fig. 3. Hatching indicates H held on Pt and the unshaded area between the profiles suggests H, retained by the catalyst, but spilt over onto the silica support.However, it must be noted that donation of H to the catalyst has not finished within this experimental time and total uptakes would have been much higher. Repeated HydrogenatiowDehydrogenation Titration Cycles Repeated dehydrogenation-hydrogenation cycles were carried out as described above. Results of the repeated titration cycles on catalyst E at two dehydrogenation tempera- tures and constant hydrogenation temperatures are recorded in table 2. This shows that the amount of hydrogen titrated and removed by cyclohexene hydrogenation at 310 K decreased with the number of titration cycles and the rate of decrease was more rapid at the higher temperature of intermediate dehydrogenation. During 0,-H, titration cycles on Pt catalyst surfaces, enhancement of hydrogen adsorption and retardation of oxygen adsorption has been reported;13 this effect was partly attributed to surface restructuring on exposure to oxygen.From the dehydrogenation results in runs 1 and 7 in table 2 there was no significant enhancement on repetition. However, in hydrogenation the catalyst has become deactivated by the accumulation of long-lived adsorbed cyclohexene or a carbonaceous overlayer of Pt particularly at higher temperatures.14 Hence, repeated characterisation of a sample by cyclohexene titration would induce deactivation and cannot be advisable. Effect of Cyclohexene Partial Pressure (pA) The effect of cyclohexene partial pressure on the hydrogenative titration at 310 K was studied over Pt/SiO, (E) after catalyst reduction by cyclohexene dehydrogenative titration at 423 K for 1 h.It was possible to estimate the extent of hydrogen donation to and held on the catalyst during dehydrogenative titration (see fig. 5 ) from the shaded areas and unshaded areas (relating to Pt and support sites, respectively) between rates1676 Cyclohexene Titration of Pt X I., I 1 I lo 0.4 0.8 1.2 1.6 P A IkPa Fig. 6. Effect of cyclohexene partial pressure on the rate of cyclohexene dehydrogenation at 423 K. Conditions as in fig. 2. Table 3. Effect of cyclohexene partial pressure on repeated hydrogenation titration cycles dehydrogenation titration hydrogenation titration cyclo hexene partial rH pressure rate H atoms donated no. H atoms extracted /Pa T/K ( x lOlg per g catalyst min-l) T/K ( x lOlS per g catalyst) 289 423 340 423 459 423 629 423 723 423 913 423 1080 423 1372 423 0.602 0.764 0.927 1.050 1.380 1.522 1.490 2.046 310 310 310 310 310 3 10 3 10 3 10 2.22 2.79 3.66 6.76 2.86 4.0 1 3.10 2.60 of hydrogen liberation and desorption as a function of time at 423 K (fig.5 ) or as a function of different reactant cyclohexene partial pressures (see fig. 6). The rate of cyclohexene dehydrogenation was found to increase with the partial pressure of cyclohexenep, (fig. 6), resulting in an increasing rate of hydrogen liberated by the reaction (table 3). However, the partial pressure of hydrogen in the system also increased as more product hydrogen was desorbed. Since the extent of chemisorption of hydrogen on Pt and the rate of hydrogen spillover from Pt to SiO, are directly proportional to the partial pressure of hydrogen,15 the total amount and rate of hydrogen retained by the catalyst was found to increase with pA (see fig.7 and table 3). However, with the hydrogenative titration it became more difficult to differentiate at higher alkene partial pressures the metal surface area owing to the influence of reverse hydrogen spillover. In hydrogenative titration, the total amount of hydrogen extracted from the catalyst first increased rapidly with p,, passed through a maximum at ca. 533 Pa and then fell slowly at higher p A (see table 3 and fig. 8). As discussed above, the surface coverage ofM . S. W. Vong and P. A . Sermon 1677 I I I 20 40 t/mh Fig. 7. Rate of H-atom liberation during cyclohexene dehydrogenation at 423 K at different partial pressures of cyclohexene: 0, 1370; A, 913; V, 629; 0, 459 Pa.I I 1 0.5 1 .o 1 .! PAlkPa 120 30 k 3 6 M a PI \ 40 0 Fig. 8. Quantities of H-atom consumption during cyclohexene hydrogenation at 3 10 K at different partial pressures of cyclohexene. The surface areas for Pt were deduced assuming Pt, : H = 1 : 1 (or nH* = 2). Conditions as in fig. 2.1678 Cyclohexene Titration of Pt hydrogen OH, resulting from cyclohexene dehydrogenation (and cyclohexene coverage) increased with pA. Hence in subsequent hydrogenation more hydrogen would be abstracted. However, at even higher p A , the concentration of surface carbonaceous species 8, would be substantial, modifying the behaviour of the catalyst for the hydrogenation reaction from that of the clean surface.16 Consequently, a decrease in the amount of recoverable hydrogen from the titration surface was observed at high alkene pressure.Thus it is possible to see the separate effects of increasing hydrogen and hydrocarbon coverages. Results of Isothermal Cyclohexene Titration under Optimum Conditions Dehydrogenative Titration Benzene and hydrogen were produced in the cyclohexene dehydrogenative titration as a result of dehydrogenation. Fig. 5 has already shown the rate of hydrogen liberated at 423 K as estimated from dehydrogenation and the actual amount of hydrogen desorbed into the gas phase. The area between two curves is the amount of hydrogen consumed by and remaining on the catalyst surface either on Pt or spillover onto the silica support.Fig. 9 shows in greater detail the rate of hydrogen donation to the catalyst during titration at 423 K. Initially the rate of hydrogen donated (i.e. reacting with the siirface oxygen and the subsequently chemisorbed on the Pt sites *) increased rapidly, passed through a maximum and then declined to a low value. All rates produced compositions consistent with thermodynamics equilibrium (i.e. favouring dehydrogenation at 423 K). It is proposed that in the first phase the area below the curve is a measure of hydrogen consumed in titration and chemisorbed on the Pt, while the area in the second phase measures the amount of hydrogen spillover from the Pt to the support (provided the rate of titration is allowed to reach zero).The average surface areas of Pt so estimated by cyclohexene titration from the shaded area in fig. 9 and from equivalent plots on this catalyst are given in table 4 together with assumptions made (i.e. no* = 4 on the initial surface in AT, and nH2 = 2 after AT,). Areas (51.8 10 m2 g-l) so calculated are comparable with those from hydrogen chemisorption at room temperature (53 m2 per g Pt). The subsequent continuing retention of hydrogen may be related to spillover and the displacement at say 40 min (where the rate of H-atom donation is changing only slowly) titration time in fig. 9 is presumably a measure of the spillover rate. Therefore both rates of spillover and metal surface areas in Pt/SiO, samples may be measured using cyclohexene dehydrogenation titration at 423 K and 460 Pa cyclohexene.Hydrogenative Titration A series of experiments was carried out with samples (0.2 g) Pt/Si02 (E) which were reduced using cyclohexene dehydrogenation at 423 K for 1 h before cooling to 295 K and then purged with N,. Cyclohexene titrations were then carried out at various temperatures and the results obtained are recorded in table 4 and fig. 10. The rate of cyclohexene hydrogenation also increased with time, passed through a maximum and then fell to zero and a small amount of dehydrogenation occurred. No hydrogen was detected in the gas phase during hydrogenation. Fig. 11 shows the variations of the total amount of hydrogen abstracted by cyclohexene at various temperatures in terms of metal-held hydrogen in the primary titration peak; this was a maximum at ca.313 K. Hydrogenation was not observed below 293 K (where the cyclohexene hydrogenation is probably kinetically controlled and fails to reach completion and therefore results in underestimation in surface-area measurement) or above 353 K (where although reaction is more kinetically favourable and it is also accelerated by reverse spillover of hydrogenM . S. W. Vong and P. A . Sermon I I I I I679 I Fig. 9. Rate of H-atom liberation and donation to catalyst E in cyclohexene dehydrogenation at 423 K. The shaded area is assumed to relate to Pt-held hydrogen. Conditions are as given in fig. 2. from the SiO, causing an overestimation of metal surface areas; dehydrogenation is also favoured by a lower coverage of hydrogen on Pt and higher Kd than Kh).The total amount of hydrogen extracted from the catalyst determined from the amount of cyclohexane produced from plots such as that in fig. 10 (corrected for cyclohexene disproportionation) is recorded in table 4. Integration of the area under the main peak gave the total amount of hydrogen titrated. This was found to be much in excess of the amount measured by hydrogen chemisorption on Pt alone and was also somewhat variable (see table 4). The results can in part be explained if adsorbed hydrogen atoms are extracted not only from the Pt but also as a consequence of reverse spillover hydrogen from the SiO, support. It is unlikely that an alkene can be hydrogenated directly by hydrogen spilt over on SiO,, even though this occurs on A1,0,., Therefore, it is believed that during titration cyclohexene reacts first with chemisorbed hydrogen held on the Pt until OH* becomes very low, then hydrogen spilt over on to the silica migrates back to the platinum to sustain the titration further.Summary The dehydrogenation mode of the alkene titration seems to give better estimates of supported Pt areas. This is a significant advance on earlier titrations which have used a hydrogenative mode, It is interesting to note that in titrations at 328 K a small second peak was observed1680 Cyclohexene Titration of Pt Table 4. Titration of catalyst E with cyclohexenea dehydrogenation titration hydrogenation titration no. H atoms donated no. H atoms extracted run T/K ( x 10lQ per g catalyst) T/K ( x 1019 per g catalyst) 1 423 2 423 3 423 4 423 5 423 6 423 7 423 8 423 9 423 6.06 309 5.64 273 5.995 295 7.87 306.5 6.34 328 6.93 345 10.06 373 3 10 - - 299.5 6.8 1 0 1.33 3.83 4.57 1.31 0 7.06 2.66 a The stoichiometries assumed for determining the Pt surface area from cyclohexene dehydro- genative titration of a surface with preadsorbed oxygen was defined by nO2 being 4 before the titration and nH2 = 2 after titration (i.e.Pt,: 0: H = 2: 1 : 2) and product water was assumed to be desorbed at the titration temperature (423 K). This titration was denoted AT,. The stoichiometry assumed for determining the Pt surface area from the cyclohexene hydrogenative titration of a surface with preadsorbed hydrogen (denoted AT,) was given by nHz = 2 (i.e. Pt,: H = 1 : 1). Other adsorption stoichiometries might need to be assumed at very high Pt dispersions.The average Pt surface area so determined by the dehydrogenation titration AT, in its first six runs was 51.8 m2 per g Pt; precise areas increased only a little 48.5 to 55.4 m2 per g Pt, but the 7th value was higher. Surface areas deduced from the hydrogenation titration AT, were much more variable. 40i tlmin Fig. 10, Rate of H-atom consumption-xtraction in cyclohexene hydrogenation at 309 K. Conditions as in fig. 2.M . S . W. Vong and P. A . Sermon 1681 I I 1 I I I 1 293 313 333 3 53 T/K Fig. 11. Total quantities of H atoms extracted during cyclohexene hydrogenation (conditions as in fig. 2) over Pt/SiO, (E). '2 30c I I I 1 0 20 40 60 80 t/min Fig. 12. Rate of extraction of H atoms during cyclohexene hydrogenation over Pt/SiO, (E) at 328 K.Conditions as in fig. 2. 56 FAR 11682 Cyclohexene Titration of Pt at the tail of the major titration peak (e.g. fig. 12). The appearance of this second peak at high temperature may support the assumption of the existence of spilt over hydrogen. Unfortunately, at high titration temperature, for reasons mentioned above, the total amount of hydrogen abstracted by cyclohexene was relatively small. Discussion The temperatures used here for the dehydrogenative and hydrogenative modes of the alkene titration involving cyclohexene are consistent with those used previously for continuous catalysis of cyclohexene dehydrogenation and hydrogenation (423 and 310-295 K). Thus cyclohexene has been observed12 to dehydrogenate over Pt(100) at 373 K and to hydrogenate over silica-supported Pt at 273-3 13 K or over stepped Pt(223) surfaces down to 298 K or over Pt black at 295 K.Such thermal conditions and the cyclohexene partial pressures used here have previously suggested structure-insensitivi ty. Earlier hydrogenative alkene titrations7 with pent- 1 -ene and ethene found that the rate of H-atom extraction from silica-supported Pt samples at 373 or 473 K was a maximum at times close to zero. Surprisingly, therefore, with cyclohexene titrant here maximum rates of H-atom extraction are only seen after 10-20 min. The causes of the induction period are worthy of further consideration. First, the extent of cyclohexene adsorption qCHfr measuredlo on Pt/SiO, (E) at 295 K was greater than that of ethene (see table 1) and so the extent of alkene adsorption could not be said to cause the different rates of attainment of maximum titration rates.However, qCH” did appear to be dependent on the average Pt crystallite size dpt, possibly as a result of differences in their abilities to strongly chemisorb hydrocarbons in multiply-bonded states.17 Cyclohexene is adsorbed more weakly on Pt than benzene.ls Nevertheless, on Pt(ll1) at 293 K it has been suggested that z-bound cyclohexene may be a long-lived surface species. l4 Certainly, dehydrogenation activity on Pt( 1 1 1) and higher-index planes has been inhibited by strong hydrogen and benzene retention.lg Bearing in mind the small effect of Oo2, and 0, on the kinetics of the dehydrogenative titration mode (see fig.2), it is interesting that preadsorbed 0 on Pt surfaces has been noted to increase dehydrogenation and hydrogenation activity of Pt at 423 K.20 Hence, it is unlikely that preadsorbed oxygen on the surface of supported Pt used here could reasonably be expected to have inhibited the dehydrogenative titration here. The hydrogenative titration model (see fig. 1) is based upon that of Horiuti and Polanyi.21 In both hydrogenative and dehydrogenative modes let =II be the surface hydrocarbon species whose hydrogenation (or dehydrogenation) is rate determining. =I+ will therefore differ from mode to mode and with pretreatment conditions. It is important to note that Oo2* and 0,. have little effect upon the maximum rate and induction period of cyclohexene titrations (see fig.2) where rates on a fully oxidised surface are only slightly different from those when OH* is high). The induction period before reaching the maximum rate of dehydrogenation titration (8 min) is less than that (16 min) for the hydrogenative titration, but then the latter was at a significantly lower temperature. The rate of hydrogenative titration [k,f(O,* 0,) where the orders with respect to OH, may be very low in order to explain its small effect on the rate in fig. 21 will be a maximum at intermediate times since at low t , 0, is zero and 0,. is a maximum, while at long t, 0, is high and 0,. near zero. Equally, the rate of dehydrogenative titration [k,f(Oo2* 0,) in AT, or kdflO, 0,) in AT,, where again orders with respect to 0, and OO2* may be very small] will also be a maximum at intermediate titration times for the same reason.The observation of maxima in titrations using cyclohexene (but not ethene or pen-1-ene) may at first sight be considered a reflection on its slower adsorption either as a result of diffusion-limitation into the porous silica or an inherently slower adsorption step. However, fig. 7 shows no effect upon the induction period of increasing the cyclohexene partial pressure and this mi tigates against diffusional control or adsorptive control.M. S . W. Vong and P. A . Sermon 1683 Therefore the stoichiometry of alkene titrations using cyclohexene may be consistent with the assumptions in table 4: 3(Pt,-O) + 2 0 --+ 2(Pt2-),- 0 + 3H,O+ 2(Pt-H) where the average numbers of surface Pt atoms * which adsorb one oxygen (n0J and one hydrogen molecule (n,,), irrespective of whether dissociative chemisorption is involved, are 4 and 2, respectively, and the average numbers of surface Pt atoms n-bonding product benzene (nCHf) and reactant cyclohexene (ncHrt) may also be 2.This recognises the importance of n-bound surface hydrocarbons,22 but in practice ncH8 # nCH” and both are likely to be larger than in the above equations. However, at present it is not possible to define nCH, or nCH” or the nature of this carbonaceous adsorbate very precisely. Nevertheless, this does not detract from the value of AT, and AT, titrations since these merely count the numbers of H atoms added to or extracted from the catalysts using measurements of the rates of formation of gaseous cyclohexane and benzene, without the need to know the precise number or mode of cyclohexene adsorption (important though this is in catalysis and studies of the involvement of carbonaceous residues therein), provided self-hydrogenation is not involved.It has been noted that cyclohexene adsorption on hydrogen precovered Ni/SiO, is more extensive at 307 K than either benzene or cy~lohexane.~~ This means that product adsorption is unlikely to be more important (and interfere with these titrations) than titrant adsorption and that the titration assumptions used here are valid. Conclusions The optimum conditions for the preferred cyclohexene dehydrogenation titration on Pt catalysts studied were 423 K, p A = 0.4 kPa and 7 cm3 min-l cyclohexene-nitrogen.The surface areas of Pt determined under the above conditions were comparable to the surface areas measured by hydrogen chemisorption at room temperature. At high cyclohexene pressure and high titration temperature, metal surface-area measurements would be complicated by the extent of hydrogen spillover from the Pt to the SiO, support and also hydrocarbon retention on the metal and its support. The techniques will be a factor or two more sensitive on an oxidised surface (AT,) than on a clean surface (AT*). Cyclohexene-hydrogenative titration (AT,) at 3 10 K will involve cyclohexene molec- ules reacting with preadsorbed hydrogen on the metal producing cyclohexane. When 8,. on Pt becomes very low, reverse spillover of hydrogen atoms retained on the silica support will extend the titration in this mode.However, the isothermal cyclohexene- hydrogenative titration technique fails to differentiate metal-held hydrogen from support- held hydrogen and is very temperature-sensitive. It seems that this might be turned to advantage if it could be undertaken in a temperature-programmed mode. The use of this approach is being investigated. Therefore, at present the isothermal cyclohexene titration is promising for in situ catalyst characterisation; for Pt/SiO, there are merits of the cyclohexene dehydrogen- ative titration over previously used hydrogenation modes of titration. It is interesting that it can be used to characterise metal catalysts without the requirement of a ‘clean’ surface ; pretreatments to achieve this may elsewhere have involved severe catalyst surface restructuring.Further studies of the importance of (i) surface hydrogen migration, (ii) sites of alkene titration and (iii) carbanaceous species in isothermal alkene titrations will be de~cribed.,~ The provision of a research studentship to M. S. W.V. by the S.E.R.C. is gratefully acknowledged. 56-21684 Cyclohexene Titration of Pt References 1 P. A. Sermon and G. C. Bond, Catal. Retl., 1973, 8, 21 1 . 2 D. Bianchi, G. E. E. Gardes, G. M. Pajonk and S. J. Teichner, J . Catal., 1975, 38, 135; D. Bianchi, M. Lacroix, G. Pajonk and S. J. Teichner, J . Catal., 1979,59,467; G. E. E. Gardes, G. M. Pajonk and S. J. Teichner, J. Catal., 1974, 33, 145; S. J. Teichner, A. R. Mazabrard, G. Pajonk, G. E. E. Gardes and C. Hoang-Van, J .Colloid. Interface Sci., 1977, 58, 88; P. A. Compagnon, C. Hoang-Van and S. J. Teichner, Proc. 6th Znt. Congr. Catal. (Chem. SOC., London, 1979), vol. 1, p. 117; J. C. Schlatter and M. Boudart, J . Catal., 1972, 24, 482; J. L. Carter, P. J. Lucchesi, J. H. Sinfelt and D. J. C. Yates, Proc. 3rdZnt. Congr. Catal. (North Holland, Amsterdam, 1965), vol. 1, p. 644; A. J. Moffat, J. Catal., 1972, 27, 456. 3 R. Kramer and M. Andre, J. Catal., 1979,58,287; J. R. A. Anderson, K. Foger and R. J. Breakspere, J . Catal., 1979,57,458; J. P. Candy, P. Fouilloux and A. J. Renouprez, J . Chem. Soc., Faraday Trans. I , 1980, 76, 616; Y. Amenomiya, J . Catal., 1971, 22, 109. 4 S. Khoobiar, J . Phys. Chem., 1964, 68, 41 1 ; P. A. Sermon and G. C. Bond, J. Chem. SOC., Faraday Trans.I , 1976, 72, 730. 5 P. A. Sermon and G. C. Bond, J. Chem. SOC., Faraday Trans. 1, 1980, 76, 889. 6 P. B. Wells and G. R. Wilson, J. Catal., 1967, 9, 70. 7 P. A. Sermon and G. C. Bond, J. Chem. SOC., Faraday Trans. 1, 1976,72,745; G. Leclercq, J. Barbier, C. Betizeau, R. Maurel, H. Charcosset. R. Frety and L. Tournayan, J . Catal., 1977, 47. 389; G. C. Bond and P. A. Sermon, React. Kinet. Catal. Lett., 1974, 1, 3. 8 R. L. Augustine, K. P. Kelly and R. W. Warner, J. Chem. Soc., Faraday Trans. I , 1983, 79, 2639. 9 N. Hoyle, P. Newbatt, K. Rollins, P. A. Sermon and A. T. Wurie, J . Chem. Soc., Faraday Trans. I , 1985,81,2605. 10 M. S. W. Vong and P. A. Sermon, to be published; M. S. W. Vong and P. A. Sermon, J. Chem. Soc., Faraday Trans. I , 1987, 83, 1651. 1 1 J. W. A. Sachtler, M. A. van Hove, J. P. Biberian and G. A. Somorjai, Phys. Rev. Lett., 1980,45,1601; G. Leclercq and M. Boudart, J . Catal., 1981,71, 127; E. Segal, R. J. Madon and M. Boudart, J . Caral., 1978, 52, 45; S. M. Davis and G. A. Somorjai, J . Catal., 1980,65, 78; D. J. O’Rea, D. G. Loffler and M. Boudart, J. Catal., 1985, 94, 225. 12 A. R. Berzins, M. S. W. Vong, P. A. Sermon and A. T. Wurie, Ads. Sci. Tech., 1984, 1, 51. 13 E. Kikuchi, P. C. Flynn and S. E. Wanke, J. Catal., 1974, 34, 132. 14 J. L. Gland, K. Baron and G. A. Somorjai, f. Catal., 1975,36,305; D. W. Blakely and G. A. Somorjai, 15 P. A. Sermon and G. C. Bond, J . Chem. SOC., Faraday Trans. I , 1976, 72, 745. 16 S. M. Davis and G. A. Somorjai, J . Catal., 1980, 65, 78. 17 V. Ponec, Adv. Catal., 1983,32, 149; B. Van Keulen, W. R. Wichers and V. Ponec, React. Kinet. Catal. 18 Z. Paal and P. Tetenyi, in Surface and Defect Properties of Solids, ed. G. C. Bond and G. Webb 19 R. K. Herz, W. D. Gillespie, E. E. Petersen and G. A. Somorjai, J . Catal., 1981, 67, 371. 20 S. M. Davis and G. A. Somorjai, Surf. Sci., 1980, 91, 73. 21 J. Horiuti and M. Polanyi, Trans. Faraday SOC., 1934, 30, 1164. 22 J. J. Rooney and G. Webb, J . Catal., 1964. 3, 488. 23 R. 2. C. van Meerten, A. C. M. Verhaak and J. W. E. Coenen, J. Catal., 1976, 44, 217. 24 M. S. W. Vong, P. A. Sermon, G. Georgiades and R. L. Augustine, J . Chem. Soc., Faraday Trans. I , J. Catal., 1976, 42, 181. Lett., 1979, 12, 125. (Specialist Periodical Reports, The Chemical Society, London, 1976), vol. 5, p. 8 1 . submitted for publication. Paper 512127; Received 4th December, 1985

ISSN:0300-9599    

DOI:10.1039/F19878301667

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1987, 83, 1685-1701 Metachromasy in Clay Minerals Sorption of Pyronin Y by Montmorillonite and Laponite Zvi Grauer,? Goldye L. Grauer, David Avnir" and Shmuel Yariv*f Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel The adsorption of pyronin Y by montmorillonite and laponite has been studied by visible, infrared and X-ray diffraction spectroscopies. The saturation point is much higher in montmorilionite, being 100 and 41 mmol pyronin per 100 g montmorillonite and laponite, respectively. The adsorbed cationic dye is located in the interlayer space. In montmorillonite adsorption leads to metachromasy of the dye and the appearance of a new band at shorter wavelengths than the original band (480 and 545 nm, respectively) even at very small coverages.In laponite, on the other hand, no meta- chromasy is observed with small amounts of dye. It is observed only when the degree of saturation approaches the saturation point. In montmorillonite the organic cation is oriented with the plain of the rings parallel to the silicate layer. In this parallel orientation n interactions between the oxygen plane of the aiuminosilicate and the aromatic dye give rise to metachromasy of the dye. In laponite the plain of the aromatic ring is tilted relative to the silicate layer and n interactions between the oxygen plane and the aromatic dye do not occur. Metachromasy is observed when dimers or aggregates of dye cations are formed in the interlayer space or in the interparticle space of flocs of laponite.Adsorption of dyes by clay minerals often results in significant spectral changes, especially in the electronic spectrum. Little is known, however, about the adsorption interactions which cause these alterations. There is also a practical point to this problem: smectite group minerals are often used as fillers in various dyeing and painting processes and often the use of this additive leads to colour changes of the dye molecule. In previous publications from our laboratory the adsorption of three cationic dyes by montmorillonite was described :1-3 methylene blue, acridine orange and rhodamine 6G. The adsorption of these dyes takes place by the mechanism of cation exchange. The adsorption of the first two dyes is accompanied by metachromasy, i.e. the appearance of a hypsochromically shifted band.From X-ray measurements and i.r. spectroscopic studies it was concluded that metachromasy resulted from n interactions between the oxygen planes of montmorillonite and the aromatic rings of the organic dye, and not due to the aggregation of the organic cation in the interlayer space, as was previously suggested by Bergmann and O'Konski.* The idea of n interactions between the aluminosilicate layer and the aromatic entity was later supported by comparing the adsorption of dibenzotropone and dibenzosuberone by rnontmoril1onite.j Only in the first molecule is aromaticity induced, and its adsorption by montmorillonite results in n interactions between the organic molecule and the clay mineral. In rhodamine 6G the phenyl ring is sterically constrained to be roughly perpendicular to the planar xanthene group.6 Owing to this steric effect, n interactions between the organic dye cation and the aluminosilicate layer cannot occur.Thus, no metachromasy is observed when rhodamine 7 Present address: Department of Chemistry, Columbia University, New York, N.Y. 10027, U.S.A. J On sabbatical leave at the Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T'6G 2G2. 16851686 Metachromasy in Clay Minerals 6G penetrates into the interlayer space of montmorillonite. It may therefore be concluded that metachromasy of cationic dyes in montmorillonite suspensions is expected only if there is no steric hindrance and when 7t interactions may occur between the aromatic rings and the oxygen plane of the aluminosilicate layer.Laponite is a synthetic hectorite. This clay is of industrial importance as it often replaces montmorillonite as a filler in various painting and dyeing processes. In spite of the observed differences in the staining between these two minerals7* * very little work has been done on the interactions between cationic dyes and laponite. In our previous study on the adsorption of rhodamine 6G by both minerals we showed that their behaviour towards this dye is similar, namely both adsorb the dye by the mechanism of cation exchange and no 71 interactions occur between the clay oxygen plane and the aromatic rings. In the present study we show that in the absence of steric hindrance, 7t interactions between the dye cation and aluminosilicate layer do occur.Differences in spectroscopic behaviour between montmorilloni te and laponite dye suspensions were observed. The cationic dye which is examined in the present study is the xanthene dye, pyronin Y (PY). Unlike rhodamine 6G, this dye has no side rings which may prevent metachromasy due to steric hindrance, as is the case in adsorption of acridine orange.2' pyronin Y (PY) Experimental Pyronin Y was supplied by Aldrich and was recrystallized from ethanol. Laponite XLG (a synthetic Na-hectorite) was from Laporte Industries Inc. Wyoming bentonite (Na-montmorillonite) was supplied by Wards Natural Establishment Inc. and was ground to 80 mesh. Quartz was separated by sedimentation. The preparation of the clay suspensions, the recording of the spectra of the clay-dye suspensions, the preparation of samples for X-ray diffraction and the recording of the X-ray diffractograms have been described previously.2y The specimens which were examined by X-ray were also examined by i.r.spectroscopy. Infrared spectra were recorded on Perkin-Elmer 457 and 597 grating instruments. The specimens were examined in the form of KBr discs and as oriented deposits on polythene windows. Oriented deposits were tilted by 45" or 90" with respect to the direction of the i.r. beam. Clay-dye suspensions and supernatants were examined in tubes transparent to U.V. radiation. Results Electronic Spectra Absorption Spectrum of PY in Aqueous Solution The effect of concentration on the visible absorption spectrum of PY in aqueous solution is shown in fig.1 . The very dilute solution (2.1 x mol dmP3) gives a single absorption band with a maximum at 545 nm, assigned as band a. More concentrated dye solutions show metachromasy, i.e. the appearance of a new band with a maximum at 512 nm, assigned as band /I. Further increase in the dye concentration results in the increase in the intsnsity ratio p/a.2. Grauer et al. 1687 ‘.-.I 500 550 600 650 wavelength/nm Fig. 1. Absorption spectrum of aqueous solution of PY: (a) 4.2 x mol dm-3 and (b) 2.1 x mol dmP3. The Eflect of Montmorillonite on the Absorption Spectrum of PY A series of clay-dye suspensions (montmorillonite or laponite) with varying clay concentrations were prepared, keeping the concentration of the dye in each sample at a constant value of 1.57 x mol dm-3.Thus, the formal degree of saturation (expressed in mol dye per 100 g clay) increased with the decreasing clay concentration. When the solid phase was separated from the solution by centrifugation it showed an intense coloration. The supernatant was either colourless (in the presence of high clay concentrations) or showed weaker absorption intensity than the clay-free dye solution. This clearly indicates adsorption of PY by montmorillonite and by laponite. Some representative spectra of montmorillonite-PY suspensions at different clay concentra- tions are shown in fig. 2. Comparison of fig. 1 and 2 shows that the adsorption of PY by montmorillonite results in metachromasy, namely a pronounced B band appears. In the presence of small amounts of clay (or a high formal degree of saturation) soluble PY contributes to the adsorption spectrum [fig.2(d)] and the spectrum shows a high intensity of band a at 545 nm. With increasing clay concentrations (or decreasing formal degree of saturation) no PY remains in the aqueous phase and the /3 band increases [fig. 2(c)]. Adsorbed PY shows a bathochromic shift of a to 550 nm. The intensity of this shifted a band increases with increase of clay concentration. Fig. 3(a) shows the effect of clay concentration in the suspension on the locations of bands a and in the absorption spectrum of the dye. As a result of adsorption, bandI688 Metachromasy in Clay Minerals I I I I 1 500 550 600 650 wavelength/nm Fig. 2. Absorption spectrum of PY (1.57 x lop5 mol dm-3) in the presence of montmorillonite.(a) 27.5 x lop3 wt % montmorillonite (formal degree of saturation 3 mmol PY per 100 g clay), (b) 2.75 x wt % (60 mmol PY per 100 g clay) and ( d ) 0.625 x 1 0-3 wt % (1 30 mmol PY per 100 g clay. wt % (30 mmol PY per 100 g clay), (c) 1.375 x a shifted to a longer wavelength (from 545 nm in the aqueous solution up to 550 nm), whereas band b shifted to a shorter wavelength (from 512 nm in the aqueous solution up to 48 1 nm). A bathochromic shift of the a band due to adsorption by montmorillonite was observed previously with acridine orange and rhodamine 6G29 and was interpreted as reflecting a surface polarity which is not as high as that of water. Similarly, as long as the added PY is completely adsorbed by the clay, the band maximum appears at ca.550 nm. As soon as free PY is present in the suspension the maximum of the a band is shifted towards 545 nm. Fig. 3 shows that the PY adsorption capacity of montmorillonite (' saturation point ') was 100 & 5 mmol dye per 100 g clay. The spectroscopic behaviour of the /3 band is different. The location of its maximum depends on the formal degree of saturation. As this degree increases, the p band shifts2. Grauer et al. 560 t 1689 50 100 150 200 250 50 I00 I50 200 250 degree of saturationlmmo1 per 100 g clay Fig. 3. (a) The effect of the formal degree of saturation of montmorillonite by PY on the locations of bands a ( j and /? (A j and (b) the effect on the absorption intensities of these bands. All spectra were recorded at a fixed PY concentration of 1.57 x mol dmP3.The clay concentration was between 0.3 x and 27.5 x wt % . to shorter wavelengths, reaching a minimum at 481 with 45 mmol PY per 100 g montmorillonite. Further increase in the degree of saturation shifts band /l to longer wavelengths. X-Ray studies (see below) indicate that this minimum is associated with the occurrence of two types of PY-H,O-montmorillonite complexes. At 100 mmol PY per 100 g clay, the /3 maximum is located at 487 nm. With higher formal degrees of saturation this band shifts to longer wavelengths owing to the presence of unadsorbed PY molecules in the suspension. Similar gradual hypsochromic shifts of the /l band with increasing adsorption of the dye, followed by bathochromic shifts, were observed during the adsorption of methylene blue and acridine orange by montmorillonite.l* If we assume that metachromasy in montmorillonite is the consequence of n interactions between the oxygen plane and the aromatic dye, then the hypsochromic shift is an indication of stronger n interactions.Such a strengthening trend is expected in the system owing to the increase in hydrophobic character of the interlayer space. This occurs because of the exchange of the inorganic hydrophilic cation with the organic cationic dye.1° The bathochromic shift of the /l band, detected at the higher saturations, was previously attributed to a transition of the organo-clay complex from one type into a second type. This will be discussed again in connection with the X-ray measurements.1690 Metachromasy in Clay Minerals Since the dye concentration was constant in all clay suspensions of the present series, it was expected that the absorbance of band B would increase as a had decreased and vice versa.Fig. 3 shows the absorbance of both bands as a function of the formal degree of clay saturation and shows that the adsorption of the dye by the clay resulted in a considerable decrease in the intensities of both bands. It was previously shown3 that absorbance depends on the particle size of the solid fraction, decreasing with clay flocculation. The shapes of the absorbance intensity curves (fig. 3) can be interpreted similarly, as reflecting flocculation and peptization. Flocculation of the clay starts with a small degree of saturation; it becomes most effective at the stage in which the amount of adsorbed dye is slightly below the saturation point of montmorillonite.At higher formal degrees of saturation the clay was peptized. Two important conclusions may be deduced from the present series of spectroscopic measurements: (1) metachromasy in montmorillonite takes place even at very dilute solutions of the dye, which in the absence of the clay do not show this effect; (2) metachromasy in montmorillonite is already observed at a very low formal degree of saturation, namely its occurrence does not depend on the surface concentration of the dye. Eflect of Laponite on the Absorption Spectrum of PY Some representative spectra of laponite-PY suspensions with different clay concentra- tions are shown in fig. 4.Comparison of fig. 2 and 4 shows that the effect of laponite on the metachromasy of the dye is much smaller than that of montmorillonite. Band a is almost always the principal absorption band, whereas two metachromic bands (B and y at 510-515 and 480 nm, respectively) became significant at a formal degree of saturation of 33-50 mmol PY per 100 g laponite [fig. 4(b)-(d)]. In the presence of small amounts of clay (or a high formal degree of saturation), soluble PY is the principal contributor to the absorption spectrum and the spectra show a high intensity of band a [fig. 4(e) and (f)]. Owing to the fact that in most spectra bands p and y appear as weak shoulders, it was difficult to follow the effect of the degree of saturation on their locations and on their absorbance values, as was done for montmorillonite. Fig.5 shows the effect of the clay concentration in the suspension (expressed by the formal degree of saturation) on the location and absorbance of band a. As with montmorillonite, the adsorption of PY by laponite results in a bathochromic shift of band a to 550 nm. As soon as the clay is saturated and free PY is present in the suspension, the maximum of the a band is shifted towards 545 nm. Fig. 5 shows that the PY adsorption capacity of laponite (‘saturation point’) was 41 f 5 mmol dye per 100 g clay. X-Ray measurements (see below) show that adsorption of PY by laponite continues to some extent at a formal degree of saturation above the saturation point. At this stage only part of the dye is adsorbed by the clay.The absorbance intensity of the band a depends on the formal degree of saturation. Up to 15 mmol PY per 100 g clay the intensity changes only very slightly. Increasing the amount of dye per clay results in flocculation of the particles, accompanied by a decrease in intensity of absorption. A minimum is obtained at 41 mmol PY per 100 g clay. At higher degrees of saturation the clay is peptized and the intensity increases. From this series of laponite spectroscopic measurements it is concluded that meta- chromasy on this clay depends on the formal degree of saturation. No metachromasy occurs when the amount of adsorbed dye is either small or high (i.e. peptization below or above the ‘saturation point’). Metachromasy is significant when the amount of adsorbed dye is equal to the saturation point of laponite and the clay is flocculated.The significant differences between metachromasies in laponite and in montmorillonite lead us to conclude that the weak metachromasy in laponite does not result from n interactions between the aromatic rings and oxygen plane, but is due to n interactions1691 w avelength/nm Fig. 4. Absorption spectrum of PY (1.57 x (a) 25 x 100 gclay); (c) 2.0 x lop3 wt % (41 mmol PY per 100 gclay); ( d ) 1.65 x 100 g clay); (e) 1.4 x mol dm-3) in the presence of laponite: wt % laponite (3.3 mmol PY per 100 g clay); (b) 2.5 x lop3 wt % (33 mmol PY per wt % (49 mmol PY per wt% (66 mmol wt% (58 mmol PY per 100 g clay); (f) 1.25 x PY per 100 g clay).1692 0.2 0.1 Metachrornasy in Clay Minerals - - I I I I I 520 "'9 I I I I I I 0 50 I00 I50 200 2 50 Fig.5. The effect of the formal degree of saturation of laponite by PY on the location of band a (a) and on the absorption intensity (b). All spectra were recorded in a fixed PY concentration of 1.57 x mol dm-3. The clay concentration was between 0.3 x and 27.5 x wt % . between neighbouring dye cations either adsorbed on the clay surface or trapped in the intertactoid space of a floc. Eflect of Ultrasound on the Absorption Spectrum of PY A series of clay-dye suspensions (montmorillonite or laponite) were prepared in which the clay concentration was kept constant (0.036 wt % ) but the dye concentration varied between 1.4 x mol dm-3. For samples with a degree of saturation above 150 mmol PY per 100 g clay, the clay concentration was only 0.018 wt % and the recorded absorbance values were normalized to 0.036 wt % .Spectra of the suspensions were recorded immediately after their preparation and also after an ultrasonic treatment of 6min. The supernatants were examined for the presence of free PY. With small amounts of dye, below the saturation points of the clays, the dye was totally adsorbed by both clays. Further addition of PY resulted in coloration of the supernatants. Fig. 6 and 7 show the absorbance values of bands a and p in the spectra of montmorillonite-PY and laponite-PY suspensions, respectively, as a function of the formal degree of saturation, before and after the ultrasound treatment. Both systems showed significant deviation from Beer's law, before and after the ultrasound treatment.Each absorbance curve can be divided into three distinct regions. In the first region there is a linear increase in absorbance as a function of the dye concentration; namely, at a low formal degree of saturation the system obeyed Beer's law. In the second region and 41 .O x2. Grauer et al. 10 9 - 8 - 8 f - p 6 - 9 5 - 2 1693 - (a) - 3 - 2 - / I I I I 1 5 0 100 IS0 200 250 50 1 0 0 I50 200 2 so degree of saturation/mmol per 100 g clay Fig. 6. Absorption intensity of bands a and p [(a) and (b), respectively] as a function of the formal degree of saturation of montmorillonite. Spectra were recorded before (a, A) and after (0, A) an ultrasound treatment of 6min. The suspensions contained a fixed clay concentration of 0.036 wt % . The dye concentration was between 1.4 x and 41.0 x mol dm-3.absorbance values either remained constant, or decreased with increasing dye concen- tration. In the third region absorbance values increased with increasing dye concentra- tion. As one could observe with the naked eye, flocculation occurred in the samples in the second region, whereas samples in the first and third regions were well peptized. Flocculated systems should be affected by ultrasound treatment, resulting in a better dispersed system. Indeed, fig. 6 and 7 show that the ultrasound treatment had a tremendous effect on the absorbance of both bands in the spectra of samples from the first region. Comparison of fig. 6 and 7 reveals a significant difference in the adsorption mechanism between montmorillonite and laponite.At a low degree of saturation on montmorillonite, band p is more intense than band a, owing to 71 interactions between the dye cation and the clay layer. Band a becomes significant only above the saturation point when part of the dye molecules are not adsorbed. Band a, which in the third region of the absorbance curve represents soluble PY, is not affected at this stage by the ultrasound treatment, whereas band p, which represents mainly adsorbed PY, was affected by the same treatment. With laponite, band a was more intense than band /3 during most stages of this series of experiments owing to the absence of 71 interactions between the dye and the oxygen1694 Metachromasy in Clay Minerals 50 100 I50 degree of saturation/mmol per 100 g clay 200 Fig.7. Absorption intensities of bands a and D [(a) and (b), respectively] as a function of the formal degree of saturation of laponite. Spectra were recorded before (@, A) and after (0, A) an ultrasound treatment of 6 min. The suspensions contained a fixed clay concentration of 0.036 wt % . The dye concentration was between 1.4 x and 41 .O x mol dm-3. plane of the mineral. When the formal degree of saturation is above 140 mmol PY per 100 g clay, dimerization of dye cations in the soluble state occurs to a high extent and band becomes more intense than band a. At lower dye concentrations some dimerization and polymerization of dye cations occurs in the solid-liquid interphase, giving rise to the appearance of band P. With a low degree of saturation this kind of surface dimerization and polymerization is very low, but, as one would expect, it increases with dye concentration.There is a significant difference in ultrasound effect on band a between the two clays. The intensity of this band increases in laponite, mainly because of peptization of the flocs; in montmorillonite this increase is cancelled by transfer of adsorbed species responsible for a absorption into P-absorbing species. This supports the assumption that monomeric PY is the principal species adsorbed by laponite. Determination of Saturation Point from Fluorescence Intensity As already observed for rhodamine 6G, adsorption quenches the fluorescence markedly.3* l1 The same effect is observed with PY: adsorption on montmorillonite quenches the fluorescence completely; adsorption on laponite significantly reduces the fluorescence intensity.Consequently, even traces of unadsorbed PY cause the whole suspension to fluoresce intensively (when illuminated with an ordinary He lamp). This phenomenon was used for determination of saturation point.3 At this point the fluorescence of the suspension is quenched. Furthermore, because of peptization, a conventional centrifuge is sometimes not sufficient to separate between the solid clay and2. Grauer et al. I695 Table 1. Basal spacings (nm) of montmorillonite and laponite treated with various amounts of PY : (A) equilibrated for one week at room temperature under an atmosphere of 40% humidity; (B) heated for one week in U ~ C U O at 215 "C formal degree of montmorillonite laponite saturation/mmol PY per 100 g clay A B A B 0 12 24 36 48 60 72 96 120 144 1.28 s 1.40 s 1.39 s 1.47 m, 2.01 s 1.58 w, 2.01 s 2.01 s 2.01 s 1.96 s 2.01 s 2.01 s 0.98 1.34 s 1.23 1.30 s (1.92 wsh) 1.29 1.32 s (1.96 sh) 1.32 1.36 s, 1.96 w 1.32 1.36 m, 2.10 m 1.28 1.38 m, 2.10 s 1.30 1.43 w, 2.10 s 1.34 (1.47 sh), 2.10 s 1.35 (1.49 sh), 2.10 s 1.34 (1.47 sh), 2.10 s 1.08 1.18 1.23 1.28 1.28 1.31 1.36 b 1.39 b 1.40 b 1.38 b s, strong; m, medium; w, weak; sh, shoulder; wsh, weak shoulder; b, a very broad and asymmetric peak.the aqueous solution in clay-dye systems, and the supernatant contains peptized clay samples. The fluorescence measurement is therefore very useful to identify non-adsorbed dye in the presence of adsorbed dye. There was a very good correlation between this method and the visible spectroscopy method, in which the saturation point had been determined from the location of the band a (fig.3 and 5): the suspension started to fluoresce when the degree of saturation was 100 and 41 mmol PY per 100 g montmo- rillonite and laponite, respectively. It should be noted that for laponite the saturation point was determined from the fluorescence measurements of the supernatants (which contained small amounts of peptized clay), whereas for montmorillonite the suspensions as well as the supernatants could be used for this purpose. X-Ray Study Oriented specimens of the clay samples treated with various amounts of PY were examined by X-ray diffraction under ambient conditions after being equilibrated at 40% humidity and after drying at 215 "C in a vacuum oven for 7 days.The results are summarized in table 1. All samples gave non-integral higher orders of reflections, indicating random interstratification of layers with different c spacings. By comparing the c spacings obtained before and after the thermal treatment it is clear that the interlayer space of samples equilibrated at 40% humidity contained water. The thermal treatment of natural montmorillonite and laponite resulted in a dehydration process with c spacings of 0.98 and 1.08 nm, respectively. Larger c spacings were recorded after the thermal dehydration of PY-treated montmorillonite or laponite (table I), indicating that PY was located in the interlayer space. Table 1 shows that for both minerals two types of PY-H,O-clay associations can be identified.The first (with c spacings of 14.0-15.8 and 13.0-14.3 nm in montmorillonite and laponite, respectively) predominated with small amounts of PY, whereas the second type (with c spacings of 20.1 and 19.6-21.0 nm in montmorillonite and laponite, respectively) predominated with larger amounts of PY. These observations are in agreement with the visible spectra observations. With montmorillonite, two types of associations could be deduced from the location of the band /I [fig. 3 (a)]. The first type1696 Metachrornasy in Clay Minerals with the low c spacing was gradually obtained mainly when the degree of saturation was below 45 mmol PY per 100 g clay. The second type of association, with the higher c spacing, was mainly formed with higher degrees of saturation.With laponite, the first type of association corresponded to samples with amounts of PY below the adsorption capacity of the mineral [fig. 5 (a)], whereas the second type corresponded to samples with amounts of PY above the adsorption capacity. As long as the c spacing is not above 1.4 nm, there is no doubt that the adsorbed PY forms a monolayer in the interlayer space with the aromatic rings parallel, or almost parallel, to the silicate layer. With such a c spacing there is no possibility for any kind of aggregation of the dye cation to take place inside the interlayer space. Spacings of 1.47-1.58 nm may account for the presence of a bilayer of water and/or of PY. The tilting of the cationic dye relative to the silicate layer is also possible at this stage.Spacings of 2.01-2.10 nm (second-type associations) may populate four water layers and/or aggregates of the cationic dye, but X-ray measurements cannot serve as conclusive evidence for this. All thermally dehydrated montmorillonite and laponite samples gave c spacings of ca. 1.3 nm, indicating that a monolayer of PY was formed in the interlayer space with the aromatic rings parallel, or almost parallel, to the aluminosilicate layer. We showed previously that polylayers of organic molecules in the interlayer space of montmorillonite persisted during thermal treatment similar to that given in the present study.5 It is therefore supposed that the monolayer of PY in the interlayer space of both clay minerals already existed before the thermal treatment.The first-order reflections in the diffractograms of thermal dehydrated laponites were always much broader than those of dehydrated montmorillonites. This broadening was significant when the formal degree of saturation was equal to or above the adsorption capacity of laponite. Such a broadening is characteristic for an inhomogeneous material. It is possible that the c spacings of a small number of interlayers were high enough to populate dimers and higher aggregates of PY. These metachromic aggregates could be responsible for the appearance of the weak p and y bands in the visible spectra of laponite-PY suspensions. In the case of montmorillonite, the presence of a monolayer of PY in the interlayer space was unequivocally proven from X-ray measurements for samples with a formal degree of saturation less than 30 mmol PY per 100 g clay.It is therefore concluded that metachromasy which was observed by visible spectroscopy of montmorillonite PY suspensions with low degrees of saturation resulted from the 7c interaction of the organic cation with the clay surface. The presence of a monolayer of PY in the interlayer space of laponite with low degrees of saturation was also unequivocally proven from the X-ray data. Nevertheless, there was almost no metachromasy in suspensions of laponite-PY with a low degree of saturation. It is therefore concluded that n interactions do not occur between PY and the oxygen plane of laponite. Infrared Study Fig. 8 shows the infrared spectrum of crystalline PY. The spectra of three samples of each of the clays saturated with increasing amounts of PY are also shown. All spectra were recorded as KBr discs.The locations of the different bands between 1100 and 1700 cm-l are summarized in table 2, together with their assignments. It is obvious that PY in the crystalline state is aggregated, forming n interactions between neighbouring cations. On the other hand, according to the visible and X-ray spectroscopic observations which were described in the previous sections, PY in laponite is in the monomeric state with no 71 interactions, for at least as long as the degree of saturation is below 30 mmol PY per 100 g clay. By comparison between the spectrum of the KBr disc of PY and that of laponite with small amounts of PY, the differences2.Grauer et al. 1697 \ v c H wavelength/ nm Fig. 8. Infrared spectra of PY: (a) in KBr disc; (b)-(d) adsorbed on montmorillonite; (e)-(g) adsorbed on laponite: 20, 50 and 120 mmol PY per 100 g clay, respectively.1698 Metachromasy in Clay Minerals Table 2. Absorption maxima (cm-l) and assignments of bands recorded in the infrared spectra of PY in KBr discs and adsorbed by montmorillonite and laponite [formal degrees of saturation for a and b are 20 and 100 mmol PY per 100 g clay, respectively] montmorillonite laponite band assignmentb KBr disc a b a b A B C D E F G H I 1650 - ring (i) - ring (0) - ring (i) 1525 ring (i) 1497 CZH5 1430 1404 Ar-N 1356 R-N 1166 1 588-1 600 - 1660 1606 1593 1532 1502 1435 1410 1357 1168 1657 1596-1 606 1527 1502 1435 1406 1175 - - 1357-1368 1660 1610 1595 1530 1502 I435 1410 1358 1170 1658 1607 1595 1530 1502 1435 1410 1360 1170 a The assignment which is suggested here follows the treatment of Rao concerning group frequencies.13 i, in plane; 0, out of plane (see paragraph on i.r. spectra in polarized light).between the spectrum of aggregated PY and that of the monomeric species can be envisaged. The locations of all the bands slightly changed with the aggregation of the organic dye, always shifting to lower wavenumbers (table 2). Most of the bands were sharper in the monomeric variety compared to the aggregated one [compare curves (a) and (e) in fig. 81. Bands B and C were very sensitive to aggregation. Both bands were nicely seen in the spectrum of the monomer. They became broad in the aggregated variety.Owing to overlapping, they appeared in the recorded spectrum of the dye in KBr as a single broad band. Band H also became broad as a result of aggregation. The effect of the degree of saturation of PY on the infrared spectrum was very small in the case of laponite, but was very significant in the case of montmorillonite. Most of the bands remained sharp in the spectra of laponite even when the formal degrees of saturation were high. Only bands B, C and H became broader, indicating that some aggregation of the organic cation was taking place on the surface of laponite. This aggregation seems to be responsible for the small amount of metachromasy which was observed during the study of the electronic spectrum of the aqueous suspensions of laponite (fig.4). Metachromasy was observed in the electronic spectrum of montmorillonite-PY with very small degrees of saturation (fig. 2). Nevertheless, the infrared spectrum differed from that of PY in KBr discs (table 2). This is an indication that in the present system metachromasy does not result from aggregation of the cationic dye, but stems from a different process. Significant spectroscopic changes resulted from increasing the forrnal degree of saturation. Bands A, D and I shifted gradually from 1660, 1532 and 1168 cm-l to 1655, 1527 and 1 175 cm-l, respectively. Band B became relatively more intense than the rest of the bands. At 60 mmol per 100 g clay this band and band C became broad and overlapped, Band H was sharp at small dye concentrations but became broad at 60 mmol per 100 g clay, extending between 1357 and 1368 cm-l.These observations support the assumption that during the adsorption of PY different types of bonding occur in montmorillonite and laponite which lead to metachromasy. Infrared Spectra in Polarized Light Oriented specimens of layer silicates are commonly used to distinguish between possible adsorption orientations relative to the basal plane. It is expected that if the adsorbed2. Grauer et al. 1699 A 1 I I I 1600 1400 1200 wavelength/nin Fig. 9. Infrared spectra of PY: (a) and (b) adsorbed on montmorillonite; (c) and ( d ) adsorbed on laponite (20 mmol PY per 100 g clay), oriented deposits on polyethylene, (a) and (c) normal incidence; (b) and ( d ) 45" incidence (polyethylene reference band is between E and F).organic cationic dye is parallel to the clay surface, the infrared spectra in polarized light can distinguish between in-plane and out-of-plane ring vibrations. If such a distinction is not observed, one may conclude that the adsorbed cationic dye is not parallel to the clay surface. Oriented specimens of montmorillonite and laponite treated with various amounts of PY were obtained by sedimentation of the organo-clay complexes from dilute aqueous suspensions on polyethylene. Spectra were recorded at normal and 45" incidence without the separation of the film from the supporter (fig. 9). When PY-saturated montmorillonite1700 Metachromasy in Clay Minerals films were tilted by 45" with respect to the i.r. beam, changes occurred in the relative intensities of some of the skeletal vibrations. The intensity of band C increased considerably relative to that of the band at 1475 cm-l (polyethylene reference band in fig.9), whereas bands B, D and E became weak. Similar observations were obtained with various formal degrees of saturation. It may be concluded that most PY cations are oriented with their aromatic rings parallel to the clay surface.' Laponite treated with PY did not show this phenomenon. The infrared spectra of the oriented films showed the same intensities whether the film was normal or at an angle of 45" to the direction of the incident beam. It may be concluded that the cationic dye is tilted at a definite angle or randomly relative to the clay surface. These observations support our assumption that n interactions occur between the oxygen planes of the silicate in montmorillonite but not in laponite.Conclusions All spectroscopic studies show that the adsorption of PY by montmorillonite differ from adsorption by laponite. (1) The features of the visible spectra of montmorillonite-PY suspensions differed from those of laponite-PY suspensions. Metachromasy in montmorillonite takes place from very dilute dye solutions and does not depend on the surface concentration of the dye. In laponite it takes place only when the degree of saturation equals the saturation point (or slightly above and below this point) and the clay is highly flocculated. (2) Ultrasound treatment of the flocculated laponite-PY system has a strong effect on the intensity of band a.On the other hand, the same treatment has a strong effect on the intensity of band p in the spectrum of montmorillonite-PY. This observation indicates that a metachromic PY species governs the montmorillonite-PY system, whereas a monomeric, non-metachromic species occurs in the laponite-PY system. (3) The intensity of the fluorescence of PY decreases with the adsorption of PY by the clay mineral. The quenching in montmorillonite is much stronger than that in laponite. According to Schoonhedt et al.12 the quenching of the fluorescence upon adsorption by clays is due to iron impurities in the clays. Indeed for laponite, which contains less iron, the quenching was less severe, It seems to us that the metachromasy in montmorillonite should also contribute to the quenching of the fluorescence of PY in this mineral, (4) X-Ray measurements showed that a monolayer of PY was formed in the interlayer space of montmorillonite and of laponite.From these data, together with the information gained from visible spectroscopy, it is clear that interactions occur between the organic cation and the oxygen plane in montmorillonite, but not in laponite. (5) Infrared spectroscopy showed that the organic cation was parallel to the silicate layer in montmorillonite, but not in laponite. Such an orientation should facilitate x interactions between the organic dye and the oxygen plane of montmorillonite. At present there is no convincing explanation for the differences in behaviour between the two smectite minerals. The different behaviours should be associated with structural differences.Montmorillonite is dioctahedral, whereas laponite is trioctahedral. The negative charge of laponite results from octahedral substitution, whereas in Wyoming bentonite, the montmorillonite used in the present study, a very small amount of tetrahedral substitution occurs as well. Such a substitution leads to increasing basic strength of the oxygen plane in montmorillonite compared to laponite.1° It is possible that the tetrahedral substitution is responsible for the n interactions occurring in montmorillonite. Further investigation is needed with other expanding clay minerals. This work was completed while S. Y. was spending a sabbatical year at the Department of Chemistry, The University of Alberta, Edmonton, Canada. His stay in Canada wasZ . Grauer et al. 1701 possible owing to the support of the Natural Sciences and Research Council of Canada (N.S.E.R.C.), The Alberta Oil Sands Technology and Research Authority (AOSTRA) and the Hebrew University of Jerusalem. These financial supports are gratefully acknowledged. D.A. is a member of the F. Haber Research Center for Molecular Dynamics, Jerusalem. References 1 S. Yariv and D. Lurie, Isr. J. Chem., 1971, 9, 537; S. Yariv and D. Lurie, Zsr. J. Chem., 1971, 9, 553. 2 R. Cohen and S. Yariv, J. Chem. SOC., Faraday Trans. I , 1984,80, 1705. 3 2. Grauer, D. Avnir and S. Yariv, Can. J. Chem., 1974, 62, 1889. 4 K. Bergmann and C. T. OKonski, J. Phys. Chem., 1963, 67, 2169. 5 Z. Grauer, S. Yariv, L. Heller-Kallai and D. Avnir, J. Thermal Anal., 1983, 26, 49; Z. Grauer, 6 F. L. Arbeloa, I. L. Gonzales, P. R. Ojeda and I. L. Arbeloa, J. Chem. SOC., Faraday Trans. 2, 1982, 7 T. Furukawa and G. W. Brindley, Clays Clay Miner., 1973, 21, 279. 8 E. F. Vansant and S. Yariv, J. Chem. SOC., Faraday Trans. I , 1977, 73, 1815. 9 A. Yamagishi and V. Soma, J. Phjx Chem., 1981,85, 3090. H. Pelled, D. Avnir, S. Yariv and L. Heller-Kallai, J. Colloid Interface Sci., 1986, 111, 261. 78, 989. 10 S. Yariv and H. Cross, Geochemistry of Colloid Systems (Springer Verlag, Berlin, 1979). 1 I D. Avnir, Z. Grauer, D. Huppert, D. Rojanski and S. Yariv, Now. J . Chim., 1986, 10, 153. 12 R. A. Schoonheydt, P. de Pauw, D. Voiers and F. C. de Schrijver, J. Phys. Chem., 1984, 88, 51 13; 13 C. N. R. Rao, Chemical Applications of IR Spectroscopy (Academic Press, New York, 1963). J . Mol. Catal., 1984, 27, 1 1 1. Paper 6/695; Received 8th April, 1986

ISSN:0300-9599    

DOI:10.1039/F19878301685

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1987, 83, 1703-1709 Forces between Mica Surfaces in Aqueous KNO, Solution in the range 10-4-10-1 mol dm-3, showing Long-range Attraction at High Electrolyte Concentration Chris Toprakcioglu Cavendish Laboratory , Madingle y Road, Cambridge Jacob Klein*T Polymer Department, Weizmann Institute of Science, Rehovot 76100, Israel and Cavendish Laboratory, Madingley Road, Cambridge Paul F. Luckham Department of Chemical Engineering and Chemical Technology, Imperial College, Prince Consort Road, London S W7 2BT Interactions between mica surfaces immersed in 10-4-10-1 mol dm-, aque- ous KNO, have been measured: in contrast to earlier studies in this system, at the higher concentration range (> 4 x mol drn-,) the potential of the diffuse double-layer drops to values ry, < 30 mV.In 0.1 mol dmP3 KNO, only attraction is observed down to the range of ‘hydration forces’ (ca. 2 nm). The discrepancy with the earlier results is attributed to adsorption on the mica of surfactant originating in the Millipore filter used in our previous studies. The measurement of the forces acting between molecularly smooth mica surfaces immersed in aqueous electrolyte media has been the subject of active experimental interest. The pioneering investigation of Israelachvili and Adamsl and subsequent s t u d i e ~ ~ - ~ have revealed that for a variety of electrolytes and concentrations these forces are generally well described by DLVO with a double-layer repulsion predominating at long distances, and van der Waals attraction becoming appreciable closer in, while strong ‘hydration’ forces were found to operate at distances close to molecular contact.Similar experiments by Klein and Luckham*$ (although only with KNO, at two values of electrolyte concentration) performed as a preliminary to the determination of the forces between macromolecular layers adsorbed on mica, yielded substantially similar results. More recently, however, we performed a series of experi- ments which clearly demonstrate the presence of attractive forces (at high electrolyte concentrations) at surface separations where the earlier studies indicated a strong repulsion. We report here the results of this investigation of interactions between mica surfaces in aqueous KNO, as a function of electrolyte concentration.Experimental Materials The water used was first deionized and then doubly distilled from an all-quartz apparatus. It has a resistivity > 2 x lo6 i2 cm and a pH in the range 5.5-6 owing to dissolved atmospheric CO,. Potassium nitrate (analytical grade) was obtained from B.D.H. and t Permanent address and address for correspondence : Polymer Research, Weizmann Institute of Science, Rehovot 76100. Israel. 17031704 Forces between Mica Surfaces in Solution from Fisons, and electrolyte solutions were prepared by dissolving the salt in doubly distilled water. The mica used was best quality FS/GS, grade 2, Muscovite ruby mica, obtained from Mica & Micanite Ltd. Apparatus The apparatus used in these experiments was similar to that described earlier'? * and will not be described in detail here.The technique employs multiple-beam interferometry, from which the separation between two mica surfaces can be determined to a typical accuracy o f k 3 A and the force acting between the two surfaces to withink0.1 pN. Procedure All parts of the apparatus that came into contact with the electrolyte solution were thoroughly cleaned before each experiment. Metal and Delrin parts of the apparatus were left in toluene for 24-48 h, washed with absolute ethanol and ultrasonicated in 0.1 mol dm-, nitric acid. Glassware was immersed in chromic acid for 24 h at room temperature or in hot concentrated aqueous NaOH for a few minutes. All components were finally rinsed in doubly distilled water and further washed with filtered absolute ethanol (0.2 pm Gellman Teflon filters).They were then dried in a laminar flow cabinet. After assembly of the apparatus and measurement of the contact position of the mica surfaces in air, the surfaces were separated to ca. 3 mm and potassium nitrate solution, filtered through 0.22 pm Millipore Triton-free (TF) grade filters, was introduced into the cell until the two surfaces were immersed in the electrolyte. After allowing ca. 30 min for thermal equilibration, the force us. distance profile in the electrolyte was determined. The temperature of our experiments was 20f 1 "C. Results and Discussion Fig. 1 shows the force, F(D), vs. distance, D, profile between two mica surfaces (radius of curvature R % 7 mm) in aqueous solutions of KNO, at three different concentrations and with pH 5.7 k0.2.The force axis is normalized with respect to the radius of curvature of the mica surfaces. According to the Derjaguin approximationlo the quantity F/2R gives the interaction energy per unit area of two flat parallel surfaces a distance D apart obeying the same force law. At large surface separations the forces are repulsive owing to electrical double-layer interactions. At sufficiently short separation, however, the attractive van der Waals forces operating between the two surfaces exceed the double- layer repulsion (i.e. the potential curve has a 'primary' minimum) and the surfaces jump into contact. On separation of the surfaces, long outward jumps are observed (ca. 5 x lo4 A) which give a measure of the depth of this potential well, given that the spring constant of the leaf spring on which the lower mica surface is mounted is K = 110+10Nm-l.These results are consistent with DLVO theory,' and the observed forces are well described by the approximate DLVO expression for a 1- 1 electrolyte, incorporating a double layer on the mica surfaces: F 641 kT A --- tanh2 (zT) exp ( - K D ) - - - 2nR - K 12;nD2 where n is the number of ions per unit volume, k is Boltzmann's constant, T is the absolute temperature, e is the electronic charge, K is the Debye-Huckel parameter, Wd is the potential of the diffuse double layer and A is the Hamaker constant. Debye lengths I / K and potentials of the diffuse double layer t,u calculated from the experimental data of fig. 1 give the values 1/K=175*51 and Wd=105+10mV for 3x1OW4 mol dm-, KNO,, and l / ~ = 37+ 5 A and tyd = 1 lo+ 10 mV for mol dm-, KNO,.C .Toprakcioglu, J . Klein and P . F. Luckham 1705 Fig. 1. F(D)/R us. D profile between two curved mica surfaces (radius of curvature R z 7 mm) in aqueous KNO,: A, 3 x lo-* mol dm-, KNO, ; 0, mol dm-, KNO,. The Debye lengths for 3 x lop4 and mol dm-, were calculated from the slopes of the linear parts of the continuous lines for these concentrations, while the curved parts of the profiles are best-fit by eye. For 4 x mol dm-, the entire continuous line is a theoretical DLVO-type interaction with 1 / ~ = 20 A, ‘yd = 32 mV and A = 2.5 x J [eqn (l), see text]. The broken line corresponds to the force observed on further compression at this concentration (4 x rnol drn-,), attributed to ‘hydration’ effects.Inset: Variation of surface potential vd of mica surfaces with KNO, concentration c. The solid circles are experimental points from the same mica sheets. The vertical bar at 10-l mol dm-, KNO, indicates the range within which ly, must lie (see text). The broken line indicates the trend of the data. mol dm-, KNO,; 0, x , + , 0,4 x At these lower concentrations the results are in substantial agreement with those of previous investigations.’> *- At 4 x mol dm-, KNO, however, the force (and hence vd) is considerably lower. The figure shows results from consecutive approaches, while the continuous line is a theoretical DLVO interaction based on eqn (1). The parameters of this curve are the Hamaker constant of mica, A = 2.5 x J,’ vd = 32 mV and 1 / ~ = 20 A.On compression there is an indication of a small jump into a (primary) minimum and the emergence of an additional force which rises sharply on further compression (dashed line in fig. 1). On decompression there is a reproducible outward jump (ca. 2500 A). These observations appear to be consistent with the presence of ‘ hydration forces’ operating at short distances. These forces have been previously observed and attributed to solvent (water) structuring around cations adsorbed at the mica-water interface; they manifest themselves particularly at higher electrolyte concentrations and have been extensively investigated by Pashley and Israelachvili.2y Shown as an inset to fig. I are the calculated vd values as a function of the electrolyte concentration.At 10-1 mol dm-, KNO,, however, there is a complete absence of long-range repulsion (as shown in fig. 2). As the surfaces are brought closer, there is no indication of any interaction until a separation of 150-200 A is reached, when a small attractive force becomes evident. On further approach the magnitude of this attraction increases, until a jump into ‘contact’ (strictly s eaking, a new equilibrium position) is recorded from a distance in the range 5Ck100 On separation (without further compression) there is1706 0 I E % 5 5 -100 -200 Forces between Mica Surfaces in Solution I I I I / f w/ I I I Fig. 2. Force us. distance profile between mica surfaces at lo-' mol dm-, KNO,. Each set of symbols corresponds to a different experiment (different pairs of mica sheets).The continuous lines are theoretical DLVO-type interactions for different t,ud values [eqn (l), see text] (a) 25, (b) 30 and (c) 35 mV. The broken line represents pure van der Waals attraction. The solid stars (*) were obtained in an experiment where no filter was used. an outward jump of 300-1000 A which appears to be mica-dependent. Both inward and outward jumps are reproducible within the same experiment, and have been observed systematically in a large number of experiments at this concentration. The 'contact' position associated with the inward jump is typically within ca. 20 A out from air contact. Further compression is met with a stiff repulsion, and longer outward jumps are recorded on decompression; although we did not investigate the behaviour in this range in detail, both observations are consistent with the presence of hydration force^.^ Each set of symbols in fig.2 represents a different experiment with different mica sheets. Although our results are in good agreement with those of earlier studies1? for KNO, concentrations 3 x and lop2 mol dm-3 there is a discrepancy at higher concentra- tions, since earlier investigations reported repulsive forces only (with a weak secondary minimum in 10-1 mol dm-3 KNO,), and found t,vd to be essentially independent of electrolyte concentration.l* 8 $ In order to check the possibility of artefacts arising from surface impurities inherent in our cleaning procedure, we have also varied the latter using hot concentrated NaOH instead of chromic acid.No change in the results could be observed. Likewise, different batches of KNO, and mica produced similar results, while atamic absorption analysis of our electrolyte solutions showed the concentration of Fe and Cu (likely contaminant ions) to be below the detection limit of 0.1 ppm. The possibility of surface impurities is in any case improbable since the results are quite reversible and reproducible as a function of electrolyte concentration. Thus within the context of the same experiment (i.e. the same mica sheets), cycles of increasing and decreasing concentration could be performed with reproducible results at each concentration. The origin of the discrepancy appears rather to be contamination arising from the use of GS-grade Millipore filters in our earlier investigations.8* These contain up to 5 wt% water-extractable materia1,ll including Triton X- 100, a non-ionic polyethoxy-type surfactant (octylphenol with an average of 10 ethoxy units per molecule, but with a broadC .Toprakcioglu, J . Klein and P . F. Luckham 1707 I o4 lo3 E 1 ?i n a: 9 ,02 10 I \ j\ x \ \ \ \ \ b\ \ y.7 \ I I \ I 100 200 DlA 10 Fig. 3. Force us. distance profile for two mica surfaces in 10-' mol dm-3 KNO, filtered through a standard (GS grade) Millipore filter, 0.22 pm. (a) First approach after 1 h (0). (6) Subsequent compression-decompression cycles (v, A, m). The crosses ( x ) represent experimental points from fig. 1 of ref. (8). distribution) employed as a wetting agent12. This surfactant is water-soluble, and surface-tension measurements (kindly performed by Dr J.Mingins) on the first three 200 cm3 portions of filtered water yielded values of 65.0, 69.9 and 69.2fO.l mN m-l, respectively, at 293 K. These results indicate a low (< 5 ppm) level of Triton X-100 present in the water.13 A similar surfactant, Triton X-405, is known to adsorb on to mica;14 thus it is probable that the repulsion below 100 A observed in previous studies is due to adsorbed Triton X-100. In this context we note that the toxic effect, on cultured cells, of Triton X-100 eluted from unwashed Millipore filters has been earlier remarked on by Cahn,12 and more recently the presence of surfactant contamination arising from the use of Millipore filters has been reported by Israelachvili and Pa~h1ey.l~ We have therefore repeated experiments at 0.1 mol dm-3 KNO, using GS-grade Millipore filters instead of the TF (Triton-free)-type filters which were used in the measurements shown in fig.2. The first 150-200 cm3 of filtered water was discarded and the second 200 cm3 portion was used to prepare the electrolyte solution. The force-distance profile for this solution was then determined (fig. 3). On a first approach [curve (a)] repulsive interaction commences at a surface separation of 140 A and increases with decreasing D. All subsequent compression or decompression of the surfaces results in a reduced range of interaction [fig. 3(b)]. These profiles show an exponential decay of force with distance, with a decay length of ca. 12 A, deceptively close to the Debye length one might expect at lo-' mol dm-3 KNO,.No strong attraction was observed, although a weak secondary minimum attraction (not shown on the logarithmic plot of fig. 3) was noticed, in agreement with results reported in our earlier studies. The onset separation for the observed interaction in fig. 3(b) is reasonable, given the size and polydispersity of the polyethoxy chain of Triton X-100.131708 Forces between Mica Surfaces in Solution Although the interpretation of these experiments is not straightforward this should not obscure the fact that they indicate a repulsion below ca. 8-10 nm (typically with an exponential decay length of ca. 12 A) in contrast to the experiments performed using TF filters, which clearly show attraction in this range (fig. 2). As an additional check we also carried out experiments (in 0.1 mol dm-, KNO,) in which no filter was used: these are shown as solid stars in fig.2, and are essentially identical to the force profiles obtained using TF filters. These results lead us to suggest that the repulsive forces previously reported at 10-1 mol dm-, KNO, in studies using non-TF grade Millipore filters were due to the interaction of adsorbed layers of surfactant. For example, a plot of FIR us. D from an earlier study,8 also shown in fig. 3 (crosses), which was obtained after ca. 1 h following addition of the electrolyte filtered through the non-TF Millipore filter, is close to the results of the present study. Surfactant adsorption could also account, at least in part, for the hysteresis effects, as well as the anomalously high apparent surface potentials occasionally observed in previous investigation^.^ The original force-distance measurements of ref.(1) are also similar to the profiles of fig. 3, and to our own previous studies,8j9 and thus differ qualitatively from the results obtained in the present work using TF filters or no filter at all (fig. 2). While we cannot be certain why this is, this discrepancy indicates that contamination was present in some of the original experiments of Israelachvili and Adams,' as also suggested to us by Dr Israelachvili.16 Finally, it is appropriate in this context to note that, in separate experiments,I4 addition of a high-molecular-weight polymer (polyethylene oxide) to a solution of Triton X-405 in 0.1 mol dm-, KNO, in which mica sheets had been incubated results in the desorption of adsorbed Triton from the mica surfaces, and its replacment by the polymer; subsequent force measurements are then characteristic of interactions between the adsorbed polymer layers 1 4 9 l7 Conclusion The forces between mica surfaces in aqueous KNO, solutions are well described by DLVO theory in the range 3 x to 10-1 mol dm-, KNO,.The potential of the diffuse part of the electrical double layer vd decreases as a function of electrolyte concentration from ca. 100 mV at 3 x lo-* mol dm3 to ca. 30 mV at 4 x mol dm-, KNO,. In contrast to all earlier studies in this electrolyte system, we find that at higher electrolyte concentrations wd c 30 mV, with the result that attractive van der Waals forces are dominant at distances from 200 A down to the range of hydration forces (ca.20 A). We conclude that the repulsive forces at surface separations below 100 A observed at 10-l mol dm-, KNO, in our previous investigations were likely to have been caused by adsorbed surfactant originating from the filters employed in these studies. We are grateful to Dr J. Mingins of F.R.I. Norwich, for carrying out the surface-tension measurements noted in the text and for helpful discussions. We thank Prof. D. Tabor for interest and encouragement, Prof. A. Silberberg for pointing out to us ref. (12), and particularly Prof. J. N. Israelachvili for many suggestions and discussions and for useful correspondence. We also thank the S.E.R.C. for support to C.T. References 1 J. N. Israelachvili and G. E. Adams, J. Chem. SOC., Faraday Trans. 1, 1978, 74, 975. 2 R. M. Pashley, J . Colloid Interface Sci., 1981, 80, 153. 3 R. M. Pashley, J . Colloid Interface Sci., 1981, 83, 531. 4 R. M. Pashley, J . Colloid Interface Sci., 1984, 102, 23. 5 R. M. Pashley and J. N. Israelachvili, J . Colloid Interface Sci., 1984, 97, 446. 6 B. V. Derjaguin and L. Landau, Acta Phys. Chim. USSR, 1941, 14, 633. 7 E. J. W. Verwey and J. Th. G. Overbeek, ' Theory of the Stability of Lyophobic Colloids' (Elsevier, Amsterdam, 1948).C. Toprakcioglu, J . Klein and P . F. Luckham 8 J. Klein and P. F. Luckham, Nature (London), 1982, 300, 429. 9 P. F. Luckham and J. Klein, J. Chem. SOC., Faraduy Trans. 1, 1984,80, 865. 10 B. V. Derjaguin, KolloidZ., 1934, 69, 155. 11 Membrane Technology (Millipore Corp., Mass., 1979). 12 R. D. Cahn, Science, 1967,155, 195. 13 E. J. Colichman, J. Am. Chem. SOC., 1950, 72, 4037. 14 P. F. Luckham and J. Klein, J. Colloid Interface Sci., in press. 15 J. N. Israelachvili and R. M. Pashley, J. Colloid Interface Sci., 1984, 98, 500. 16 J. N. Israelachvili, personal communication. 17 P. F. Luckham and J. Klein, Macromolecules, 1985, 18, 721. 1709 Paper 6/ 1330; Received 24th June, 1986

ISSN:0300-9599    

DOI:10.1039/F19878301703

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. Soc., Faraduy Trans. 1, 1987, 83, 1711-1718 Hydrogenation of CO, over Co/Cu/K Catalysts Herv6 Baussart, Rene' Delobel, Michel Le Bras and Jean-Marie Leroy* Laboratoire de Physicochimie des Solides, E.N.S.C.L., U.S.T.L., B.P. 108, 59652 Villeneuve D' Ascq Cedex, France The influence of the different elements Co, K, Cu as components of catalysts, has been investigated for the reaction of hydrogenation of CO,. Cobalt, without promoter, appears to be predominant in the methanation process. The presence of potassium in the matrix changes the nature of bonds between the other chemical species and favours the formation of CO. The addition of Cu reduces the specific role of potassium; it limits the quantity of products formed and decreases the selectivity in CO. In the absence of potassium, copper lowers the temperature of reduction of the catalysts, decreasing their activity, and affects the distribution of the active carboneous species formed during the transient regime.The development of techniques incorporating C, carbon sources to compete with or even supplant oil as a source of raw materials, explains the interest in the compounds CO and CO,. In this context, the catalytic hydrogenation of CO to CH, and hydrocarbons of high molecular weight has been particularly studied and de~eloped.l-~ On the other hand, despite the existence of many cheap sources, the development of the hydrogenation reactions of carbon dioxide is still limited. The catalysts used are generally the same as those used in the methanation of C0.49 In this paper, we have particularly studied the catalytic hydrogenation of carbon dioxide to methane with Co-Cu-K catalysts.Previously, only Russells has studied such a catalytic system on purpose to synthesize oils. The aim of the specific work is to bring about a contribution to the knowledge of the specific part played by each of the elements constituting the catalyst on the distribution of the reaction products. Experimental Materials Russell Catalysts ( R) Russell precursors were obtained according to the process described by the author :6 precipitation of basic carbonates of cobalt and copper by a solution of potassium carbonate. The precipitates obtained were filtered, washed and dried at 373 K, then pelletized in cylinders (diameter: 5 x m) under a pressure of 1.47 x los Pa.These pellets are treated as described in table 1. m, depth: 3 x C U , - ~ C O ~ + ~ O ~ Catalysts (C) All the Cu,-,Co,,,O, catalysts were obtained by the same procedure. The oxalates formed in the first step were decomposed into Co,O, and CuO. The mixture of the two oxides was then heated under oxygen at 450 "C using conditions described previ~usly.~ This temperature allows one to obtain defined compounds and consequently an optimal dispersion of the elements, as proved by electron probe microanalyses which showed the homogeneity of the catalysts. For Co/Cu > 3.92, X-ray analyses showed that these 171 11712 Co/ Cu/ K Hydrogenat ion Catalysts Table 1. Catalyst composition and conditions for the activation pretreatments catalyst precursors Co/Cu K/Co catalyst activation Co basic carbonate - - Co-K carbonate - :} Co-Cu-K carbonate 20 1.8 1 h under N, at 423 K, 24 h under CO,:H, = 1:4 at 723 K l a 8 1 R2 C, cobalt oxalate - - 24 - 12 - 6.9 - ' Co-Cu oxalates 5.5 - 3.9 - 2.3 - 2 - 360 h under 0, at 723 K, 1 h under N, at 423 K and 24 h under CO,: H, = 1 : 4 at different temperatures I compounds presented the spinel structure.For x < 3.92, a mixture of monoclinic CuO and a spinel phase was always observed. The pelletized oxides obtained were treated as described in table 1. Textural Characterization The B.E.T. surface areas were measured by a standard method using N, physisorption at 77 K (Carlo Erba Sorptomatic). They are in the ultimate range of the apparatus (< 1 m2 g-l). The low values obtained were corroborated by scanning electron micro- graphs using a Stereoscan mark 2A electron microscope. The observed images of the specimens show that the grains may be assimilated roughly with spheres without any apparent rugosity.As the surface areas could not be precisely determined, only the specific rates of the reactions or the fractions of CO, converted (x in %) were compared. Thermogravimetric Studies Thermogravimetric studies were carried out using a Setaram M.T.B. lo-* microbalance. This apparatus allowed the measurement of the temperature TR which corresponds to the beginning of the reductions of the samples. These reductions were carried out under continuous flow of the reactant gases (CO,:H, = 1 :4, flow rate: 1.2 dm3 h-l) and the relative losses of weight of the catalysts, A W% , were measured after working in the catalytic conditions.Catalytic Apparatus Measurements of activities and selectivities were performed in a continuous stirred reactor at ca. 101 325 Pa. This stirred gas-solid reactor (s.g.s.r.) has been described elsewhere8 and was placed in a furnace whose temperature could be controlled to within f 1 K. Products were analysed by gas chromatography (Intersmat I.G.C. 15 gas chromatograph). CH,, C2H4, C3H,, n-C,H,,, n-C5H12, n-C,H,,, CH30H and C2H50H were separated in a 2 m packed column containing Porapak Q ; an active carbon column was used for the separation of H,, CO, CH4 and C,H,. Determination of the Kinetic Range The CO,: H, (1 :4) mixture at ca. 101 325 Pa total pressure reacted in the absence of catalyst in the s.g.s.r.at temperatures up to 623 K. This homogeneous process limitsH . Baussart et al. 1713 contact time/g h dm-3 Fig. 1. Amount of CO, reduced in CH, us. contact time ( T = 433 K, CO: H, = 1 :4). the temperature range below 623 K. To ensure the mixing in the gas space is adequate, the evolution of the conversion rate was studied at a series of different agitation speeds in the presence of the catalysts. Under the conditions of reaction, satisfactory mixing tests were run in the range 500-5000 r.p.m., for which the external mass-transfer resistance was negligible. The curves in fig. 1 show the linear relation at 433 K between the percentage of CO, reduced in CH, and the contact time 6 = m / d (g h dm-3), where m is the catalyst mass and d the volumetric flow of reactant gases into the reactor.The relation, corroborated when d is high for a given m, proves the absence of diffusional effects. Such a relation was always observed in the temperature range used. In the kinetic regime, x remained below 2%, Throughout this study the constant experimental conditions were: catalytic charge, 4 g; total flow, 6 dm3 h-l; reactant mixture, CO,:H, = 1 :4; rotation speed, 3500 r.p.m.; pressure, ca. 101 325 Pa. Results Russell-type catalysts were taken as a reference; an activation process of these R catalysts was carried out using the conditions defined by the author: pretreatment under reactant mixture (flow rate, 6 dm3 h-l) in the temperature range 423-503 K. This procedure does not allow observation of any catalytic activity with R,.To obtain R catalysts with significant catalytic performances, it is necessary to activate the specimens at 723 K, as described in table 1. On the other hand, C-type catalysts do not require such a high temperature for activation. Table 2 shows the evolution of the catalytic performances of C, as a function of the temperature of the activation process. Note that the increase of this temperature involves a drop in the selectivity for methane as well as a decrease of the conversion of CO,. In this paper, the activities of the catalysts are compared using the temperature T, for which x is 1 % (specific activity: 1.34 x mol g-l h-l). The chosen convention allows the comparison of the selectivities at a defined level of x in the kinetic regime.This comparison between R, and C, activated at 433 K shows that C, is slightly more active and selective than R,. Owing to these performances, every C catalyst was activated at 433 K. Among the C catalysts, samples containing copper have activation profiles markedly 57 F A R 11714 C o / C u / K Hydrogenation Catalysts Table 2. The influence of the temperature of activation on the activities and the selectivities for C, selectivity (%) activation sample TIK T,/K CH.4 C,H, C,H* C*KO 433 424 97.7 1.6 0.6 0.04 Cl 533 428 96.6 1.9 1 . 1 0.09 633 434 92.5 4.7 2.5 0.3 723 447 90 5.9 3.5 0.5 RO 723 433 95.5 2.9 0.9 0.7 I I . c7 5 10 15 20 25 time/h Fig. 2. Activation of the C catalysts at 433 K. different from that of C, (fig. 2). For C , the activity increases regularly before reaching a.steady state. With the other C compounds, the initial sharp rise in activity is followed by a slow decline. The steady state is reached after ca. 30 h of activation. It is obvious that a comparison with the activation profiles of the R-type catalysts is not possible because the process is, in that case, carried out in the temperature range where the homogeneous reaction takes place. The CH, production is shown in fig. 3, in Arrhenius form. Apparent activation energies E,, calculated from the slopes of the lines, are summarized in tables 3 and 4. They are virtually the same for all the catalysts (83 < E,/kJ mol-1 < 102), except for the catalyst containing Co, Cu and K. In that case, the value of E, is comparatively low (64.6 kJ mol-l).A drop in total activity of R-type catalysts is observed when they contain K. The addition of copper slightly reduces this evolution. Furthermore, R, is selective for the formation of methane. The presence of potassium favours the synthesis of carbon monoxide ; primary alcohols, alkanes and ethylene are formed as by-products. Unlike Russell, the formation of oils has never been observed. The addition of copper decreases the selectivity for CO and limits the number of by-products. Table 4 shows that activities and selectivities for CH, of C-type catalysts decrease when the content of Cu increases, whereas their selectivities for ethane are in the opposite order. These catalysts are not active for the catalytic formation of CO.H. Baussart et al. 1715 - 8 - 9 - I s I ca 0 \ n d - c.E -10 a" F: - - 11 103 KIT Fig. 3. Arrhenius plot for CH4 conversion from CO, : H, = 1 : 4 on R- and C-type catalysts. Table 3. Catalytic performances of the R catalysts selectivity (% ) E a sample TJK /kJ mo1-I CH4 C2H6 c3H6 C4H10 RO 433 87.8 95.5 2.9 0.9 0.7 529 86.5 20.4 0.4 0.9 0.5 Rl R2 515 64.6 34.2 0.7 0.4 - selectivity (% ) sample C5Hl2 C13H14 C2H4 CH,OH C,H,OH co - - - - - - RO Rl 0.54 0.17 1.8 0.09 0.2 75 - - - R2 0.6 0.1 1 64 57-21716 C o / C u / K Hydrogenation Catalysts Table 4. Comparison of the catalytic performances of C-type catalysts selectivity (% ) Ea sample 7JK /kJ mol-l CH* C2H6 C3H8 C4HlO Cl 424 c3 44 1 c5 456 c2 437 c4 449 C6 477 c, 493 C8 500 85.5 86.9 83.6 88.6 84.0 95.3 83.6 102.6 97.7 97.2 97.9 96.7 96.8 96.8 96.7 95.2 1.8 2.0 1.7 2.5 2.5 2.5 2.6 4.1 0.4 0.7 0.3 0.7 0.6 0.6 0.6 0.6 0.04 0.09 0.06 0.06 0.07 0.08 0.07 0.06 Discussion The interpretation of the role in the reaction process of the different elements constituting the catalysts is particularly interesting.This task is not an easy one to accomplish, particularly in complex systems such as multicomponent catalysts. In spite of the existence of several techniques which have been developed to evaluate the active specie^,^ some investigators suggested that no standard method exists.1° This situation has led us to use surface reaction studies alone as probes of catalyst surfaces. The specific role of the element Co and the eventual synergisms which result from the association with potassium and/or copper have been investigated.A bibliographic study reveals that the presence of cobalt favours the formation of alkanes., Our work verifies this result: unsaturated species are never found in the reaction products when R, and C, are used. Two hypotheses can be proposed to explain the formation of the saturated compounds CH,, C,H,, C,H, and C4H1,. First, a classical mechanism of competition between chain-propagation and hydrogenating desorption : Secondly, it may be considered that methane is directly produced from CO,, whilst the other alkanes are formed by hydrogenation of C0.l1 It has been observed that this process occurs when CO is the major product, so this mechanism seems improbable. Our results show that the presence of potassium (R,, R,) favours the formation of CO.This compound is probably produced by the reverse water-gas shift reaction: CO, + H, CO + H,O. The formation of small quantities of ethylene is also observed. It is well known that the presence of potassium in the catalysts used for Fischer-Tropsch synthesis favours the formation of olefins. In our case, such a reaction cannot be rejected because CO and H, exist in the reaction medium; the presence of C2H4 can thus be explained. In a more general way, the reaction process may be explained taking into account the electron-donor character of potassium.12 This character favours the adsorp- tion of CO, increases the metal-C bond strength and, at the same time, weakens the C-0 bond. On the other hand, it does not favour the chemisorption of H, and decreases the hydrogenating properties of the catalyst, explaining the formation of unsaturated species.l3H. Baussart et al. 1717 Table 5. Comparison between temperature of beginning of reduction, TR, weight losses in the s.g.s.r. and temperatures T, catalyst c, 493 6.15 424 c5 443 8.36 456 c3 448 7.5 441 c, 43 8 12.16 493 The presence of copper in Russell catalysts increases the activity and the selectivity for CH, and reduces the number of products. On the other hand, in C catalysts copper leads to a loss of activity and minor changes in the distribution of the products. It may be proposed that copper, when introduced in a matrix containing potassium, presents a weakened electron-donating effect. Moreover, as suggested by Russell, if the alkali metal is a poison for the methanation sites, a competition between copper and potassium may be considered to be responsible for the increase of the carbon dioxide conversion to methane.If the matrix is free of potassium, the electron-donor character of copper may play its part: the temperature of reduction of the catalysts is lowered and, according to the literature,14 superficial carbides would be formed and would react as intermediates. When a comparison between activities and reducibilities of the samples is performed (table 5 ) an inverse relationship is observed between TR and the Cu content. The activation profiles which describe the evolution of the activity as a function of the time can explain this result. In the case of C,, the non-existence of an extremum during activation can be explained by considering that the catalyst is the least reducible, so the reduction process involves the lowest superficial change, thereby limiting the formation of active sites. For all the other catalysts which contain Cu we observe a maximum.This suggests a transient regime during which the passage through the maximum activity corresponds to the rate of production of active sites being equal to that of their spontaneous decay.15 More precisely, like Sachtler,14 we may propose the formation on these sites of active adsorbed carboneous species: Cads, CH,,,, CH,, ads. In this scheme, copper acts during the transient regime on the distribution of these species and determines their evolution either to products or to less-active intermediates which may go as far as graphitic carbon.The quasi-stability of the selectivity may be explained by the presence of active species defined above, but in a lower concentration when the amount of copper increases. This evolution may also be explained by a sintering phenomenon, the intensity of this process being a function of the copper content. It is then obvious that if TE decreases the sintering increases and the number of superficial active sites falls, these sites remaining unchanged. Finally, experimental results lead us to consider that the presence of Cu favours, in a first stage, the reduction of CO, to active carboneous species probably via formation of CO. In a second stage there is a competition between the synthesis of methane from active carboneous species and the reduction which leads to the formation of carboneous deposits on the surface of the catalyst.As pointed out by Somorjai,16 in the first stage (which corresponds in our pattern to the reduction of the oxide) we cannot exclude the formation of metallic clusters. The amount of these clusters and their coexistence with an oxidized form would condition the catalysis. Finally, it may be pointed out that, unlike Russell catalysts, the formation of oils on C-type catalysts has never been observed. This difference may be explained by differences in the hydrodynamic and thermal regimes of the reactor, the diffusional processes having a significant effect on the distribution of the products.1718 Co/Cu/K Hydrogenation Catalysts Conclusion The present work enables us to characterize the specific role played by Cu, K and Co in the hydrogenation of carbon dioxide at low pressure.The chemical nature of the elements plays a very important part in the orientation of the reaction. In a matrix containing cobalt, our results show the essential part played by potassium, which modifies the nature of the bonds between the other chemical species and favours the formation of Co. The presence of copper reduces the temperature of reduction of the catalyst, decreasing their activity, and affects the distribution of the active carboneous species, intermediates of the reaction, which are formed during the transient regime. References 1 G. A. Mills and F. W. Steffgen, Catal. Rev., 1975, 8, 159. 2 M. A. Yannice, Catal. Rev. Sci. Eng., 1976, 14, 153. 3 Y. Ponec, Catal. Rev. Sci. Eng., 1978, 18, 151. 4 W. W. Russell and G. H. Miller, J. Am. Chem. Soc., 1950, 72, 2446. 5 E. J. Gibson and C. C. Hall, J. Appl. Chem., 1954,4,464. 6 L. F. Cratty and W. W. Russell, J. Am. Chem. Soc., 1958, 80, 267. 7 H. Baussart, M. Lx Bras and J. M. Leroy. C . R. Acad, Sci. Paris, Ser. C, 1977, 281, 735. 8 M. L. Brissk, B. L. Day, M. Jones and J. B. Waarren, Trans. Znst. Chem. Eng., 1968, T3,46. 9 J. L. Lemaitre, P. Goving Menon and F. Delannay, in Chemical Industries, Characterization of Heterogeneous Catalysts, ed. F. Delannay (Marcel Dekker, New York, 1984), vol. 15, pp. 299-362. 10 E. G. Baglin, G. B. Atkinson and L. J. Nicks, Ing. Eng. Chem., Prod. Res. Den, 1981, 20, 87. 1 1 M. Pijolat and Y. Perrichon, C . R. Acad. Sci. Paris, Ser. IZ, 1982, 295, 343. 12 D. E. Pebbles, D. W. Goodman and J. M. White, J. Phys. Chem., 1983, 87, 4378; F. Solymosi, I. Tombacz and J. Koszta, J. Catal., 1985, 95, 578. 13 M. Papadopoulos, R. Kieffer and A. Deluzarche, B.S.C.F., 1982, no. 3-4,I-109. 14 P. Biloen and W. M. H. Sachthler, Adv. Catal., 1981, 30, 165. 15 A. Amariglio, M. Lakhdar and H. Amariglio, J. Catal., 1983, 81, 247. 16 D. J. Dwyer and G. A. Somorjai, J. Catal., 1978, 52, 291. Paper 6/ 1398 ; Received 14th July, 1986

ISSN:0300-9599    

DOI:10.1039/F19878301711

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1987,83, 1719-1723 Determination of Stability Constants from Linear-scan or Cyclic-voltammetric Data using a Non-linear Least-squares Method Harald Gampp Institut de Chirnie de r Universitk, Avenue de Bellevaux 51, CH-2000 Neuchcitel, Switzerland Stability constants of binary complexes can be determined using linear-scan or cyclic-voltammetric data. In contrast to a recently applied graphical method, numerical treatment based on a non-linear least-squares refinement allows one to make optimal use of the information contained in the data set. This is illustrated by re-evaluating recent literature data. In a recent paper by Killal it was shown that linear-scan or cyclic-voltammetric data can be used to determine stability constants. This was demonstrated for metal ions M forming a series of binary complexes ML, ( i = 1, .., p ) with a given ligand L. The stability constants were obtained using a procedure based on the classical approach by DeFord and H ~ m e : ~ ? ~ for each type of complex ML,, the measured data are transformed into a polynomial of degree p - i in ligand concentration cL and plotted against cL. The respective stability constant is obtained by linear extrapolation to zero concentration. Clearly, this method was, despite its inherent disadvantage^,^^^ optimal at a time where no computing facilities were available. These disadvantages are as follows. (i) The data are transformed non-linearly and thus obtain different weights, which is neglected in the graphical evaluation. (ii) For complexes ML, with i < p - 1, the plots are not linear and, accordingly, the extrapolation to zero concentration cannot be made unambiguously.(iii) No unbiased estimates of the confidence limits of the calculated parameters are obtained. (iv) Errors in the stability constants are accumulated since in the determination of K,, the previous constants K,, ..., are used. (v) The data evaluation is rather time-consuming. Some attempts have been made to overcome these drawbacks, either by using linear regression analysis4v5 or by solving sets of simultaneous equations.6 However, these methods are not generally applicable (vide infra), and accordingly have never become popular among chemist^.^ It is the purpose of this note to demonstrate that a non-linear least-squares procedure allows a straightforward data analysis that does not suffer from any of the abovemen- tioned drawbacks.The re-evaluation of some recent literature data1 will show that in contrast to the graphical method, the full information contained in the data set is obtained by the rigorous evaluation. Theoretical Background A complex ML, is generally reduced at a more negative potential than the solvated metal ion. If both the complexation equilibria and the electron transfer at the electrode are fast, a cyclic voltammogram of an equilibrium mixture containing species ML, ( i = 0, I , . . . , p ) shows a single wave with a shape typical for a reversible (or Nernstian) system.8 Hence, the reduction occurs at the level of the free metal ion (i.e. the species which is the easiest to reduce) and the only effect of complexation is a shift of the wave.17191720 Stability Constants from Voltammetry In the following we consider a reversible process at a mercury working electrode, M n + + n e e M o , where the reduced metal is not complexed by L. Assuming constant activity coefficients, the Nernst equation can be rearranged into eqn (l).398 Thus the measured shift of the half-wave potential, AE,, can be described as a function of the concentration of free ligand, [L], and of the stability constants& (& = [MLi]/[M][LIi); all charges omitted, i.e., M stands for Mn+): Eqn (1) can be used as a fitting function and the parameters pi can be determined by any non-linear least-squares method. In the present case a simple Newton-Gauss procedureg-10 was chosen.In the general case, where the total concentration of ligand (c,) is not in large excess over that of metal (cM), the program uses the Newton-Raphson algorithmll in order to calculate the concentration of free ligand ([L]). If cL 9 cM, [L] in eqn (1) can be replaced by cL and eqn (1) can be transformed into a polynomial form : exp AEL- -1 = &(c& ( 1;) Accordingly, the unknown parameters pi can be calculated non-iteratively by linear regression anal~sis.~-~ Obviously, the experimental data are non-linearly transformed and appropriate weighting factors have to be introduced in order to obtain meaningful results. Since the optimal selection of weights is not always easy to make and since eqn (2) is not generally applicable, the non-linear fit using eqn (1) is to be preferred.Results and Discussion Three of the data sets measured by Killal have been evaluated by using a non-linear least-squares pr0gram~3~~ based on the Newton-Gauss method. In each case two different models (i.e., with and without pl) were fitted. The results of the numerical analysis together with the respective literature values are compiled in table 1. For the Cd2+-oxalate system the excellent fit of the experimental data by the calculated curve can be seen in fig. 1. The standard error of fit increases from 1.5 to 2.6 mV when is omitted from the model. Subjecting the corresponding variances to the F-testg shows that including p1 leads to a statistically significant improvement of the fit at a 95 % level (at a 99% level /I1 has to be rejected, however). Quite in contrast, is not necessary in order to describe the experimental data obtained from the other two equilibrium systems.Although the stability constant calculated for the 1 : 1 complex between Cd2+ and 1,3-diaminopropane seems to agree with the reported values (table l), the F-test clearly shows that it is a coincidence, thus including p1 does not improve the quality of fit. The same is true for the Cu2+-oxalate system, where the experimental data are well explained by considering only p2, as illustrated in fig. 2. In this case not even statistical criteria are necessary since the program finds that the best fit is obtained with a physically meaningless, negative value of B1. The fact that in the last two systems D1 could not be determined is by no means due to a poor mathematical procedure.Using the reported stability constants,' one easily calculates that at the lowest ligand concentration (0.01 mol dm-3) in the Cu2+-oxalate system ML, is already formed to > 93%, ML to < 7% (its concentration decreases rapidly with increasing ligand concentration), and that uncomplexed metal is never present in significant amounts. Clearly, under these circumstances p1 cannot be determ- ined. The Cd2+-1 ,3-diaminopropane system has been studied under similar conditions,l where M is not present to > 0.02% and where ML is formed to > 20% only at the two lowest ligand concentrations. Obviously, in these equilibrium systems reliable values forH. Gampp 1721 Table 1.Stability constants and standard errors obtained by nonlinear least-squares fit to cyclic vol tamme tric da taa system no. of parameters SE/mVb log & log P 2 Cd2+-oxalate C d e Cd2+-1 ,3-di- C aminopropane c d f Cu2+-oxalate C d e 3 1.5 2.42 & 0.10 2 2.6 - 2.69 2.73 f 0.03 3 1 .o 4.95 f 0.48 2 1.1 - 5.477 4.5k0.2 2 1.2 -Q 1 1.6 - 6.00 5.53+ 1 3.86 k 0.14 4.24 k0.06 4.04 4.1 f O . 1 7.58 f 0.04 7.63 & 0.02 7.59 7.2f0.5 9.15 k0.02 9.1 1 k0.02 9.13 9.54f0.5 5.1 0 f 0.04 4.95 0.07 5.16 5.1 8.30 & 0.05 8.25 f 0.05 8.3 1 8.0 - - - - a Data from ref. (1) ( I = 1.0 mol dm-3, 25 "C). * Overall standard error of fit. This work (non-linear least-squares fit). From ref. (1) (graphical method). A. E. Martell and R. M. Smith, Critical Stability Constants (Plenum Press, New York, 1982), vol.5. f A. E. Martell and R. M. Smith, Critical Stability Constants (Plenum Press, New York, 1975), vol. 2; constants refer to 1 = 0.1 mol dm-3 and 25 "C. Best fit obtained with a negative value for PI. 1oc > E 2 6C 1 cl 20 0 0.1 0.2 0.3 0.4 [oxalate]/mol dm-3 Fig. 1. Dependence of the shift of the half-wave potential AEL on the concentration of ligand in the Cd2+-oxalate system. [Experimental values from ref. (1) are represented as open squares of height 3 times the standard error of fit; calculated values obtained from a non-linear least-squares fit of three parameters are represented as a solid line.]1722 200 > E U --- I;i' 150 100 Stability Constants from Voltammetry 0.0 0.1 0.2 [ oxalate]/mol dmd3 0.3 Fig. 2. Dependence of the shift of the half-wave potential AEL on the concentration of ligand in the Cu2+-oxalate system.(Symbols are as in fig. 1; only a single parameter, &, was fitted.) p1 can only be obtained under experimental conditions where the ligand is not in large excess over the metal. Since the concentration of ligand then no longer equals its total concentration but depends on the unknown stability constants [eqn (2)], and thus the graphical method cannot be used (uide supra). Nevertheless, one might argue that the good agreement between the parameters obtained by the graphical method and by the least-squares procedure (see table 1) at least in the present case does not really disfavour the former method. Therefore, the graphical evaluation was repeated for the two oxalate equilibriq.Inspection of the plots for the Cd2+ system clearly reveals that an unbiased extrapolation does not lead to log p1 > 2.4 or to log p2 > 3.5 [cf. fig. 1 in ref. (I)]. The respective plots are such that zero or even negative p values could equally well be deduced, hence neither B1 nor Bz can be determined by this method. Only for p3 is a reliable value obtained (5.2 < log p3 < 5.3). Similarly, in the Cu2+-oxalate system the graphical method predicts a physically meaningless, negative value for pl, whereas p2 can be determined (9.0 < log p2 < 9.2). Thus, in the abovementioned equilibrium systems the graphical evaluation is strictly limited to the determination of the maximum number of coordinated ligands, p , and the respective formation constant pp.By using a non-linear least-squares method the data evaluation is considerably improved and the full information contained in the experimental data is readily obtained. Especially helpful is the fact that unbiased statistical criteria can be applied in order to decide whether a certain complex and the respective stability constant are significant or not. This is of decisive importance where unknown equilibrium systems are to be studied. To conclude, cyclic voltammetry is indeed well suited for the determination of stability constants. A least-squares procedure which is easily implemented even on inexpensive microcomputers allows one to perform a safe, complete and straightforward data analysis in a short time. Clearly, other electrochemical techniques like linear-scan voltammetry or polarography which equally allow one to determine El values can be used as well.Accordingly, these electrochemical measurements should be considered as an alternative to the commonly used pH-potentiometric or spectrophotometricH. Gampp 1723 titrations since they can be used for studying equilibria in strongly acidic or basic solution (where pH-potentiometry cannot be applied12) or for systems which do not show well developed spectral characteristics (e.g. complexes of the d 1 O metals). This work was supported by the Swiss National Science Foundation. References 1 H. M. Killa, J. Chem. Soc., Faraday Trans. I , 1985, 81, 2659. 2 D. D. DeFord and D. N. Hume, J. Am. Chem. Soc., 1951,73, 5321. 3 H. L. Schlafer, Komplexbildung in Losung (Springer, Berlin, 1961), p. 205. 4 P. Kivalo and J. Rastas, Suomen Kemi., 1957, B 30, 128; J. Rastas and P. Kivalo, Suomen Kemi., 1957, 5 L. N. Klatt and R. L. Rouseff, Anal. Chem., 1970,42, 1234. 6 J. G. Frost, M. B. Lawson and W. G. McPherson, Inorg. Chem., 1976, 15, 940. 7 F. R. Hartley, C. Burgess and R. Alcock, Solution Equilibria (Ellis Horwood, Chichester, 1980), 8 A. J. Bard and L. R. Faulkner, Electrochemical Methods (Wiley, New York, 1980). 9 P. R. Bevington, Data Reduction and Error Analysis for the Physical Sciences (McGraw-Hill, New York, B30, 143. pp. 165. 1 969). 10 H. Gampp, M. Maeder and A. D. Zuberbuhler, Talanta, 1980, 27, 1037. 11 H. Margenau and G. M. Murphy, The Mathematics of Physics and Chemistry (Van Nostrand, 12 H. Gampp, D. Haspra, M. Maeder and A. D. Zuberbuhler, Znorg. Chem., 1984, 23, 3724. Princeton, 1968), pp. 492. Paper 6/1408; Received 15th July, 1986

ISSN:0300-9599    

DOI:10.1039/F19878301719

出版商:RSC    

年代:1987

数据来源: RSC