摘要:

ISSN 0300-9599 JCFTAR 82 ( I 2 ) 3525-371 9 (1 986) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases 3525 3535 3553 3561 3569 3587 360 1 361 1 3625 3635 3647 3657 3667 368 1 3697 3709 3717 CONTENTS Fourier Transform Infrared Studies of the Irreversible Oxidation of Cyanide at Platinum Electrodes A. S. Hinman, R. A. Kydd and R. P. Cooney The Dielectric Properties of Zeolites in Variable Temperature and Humidity A. R. Haidar and A. K. Jonscher The Time-domain Response of Humid Zeolites A. K. Jonscher and A. R. Haidar Molecular-orbital Studies of C-H Bond Scission induced by Ionizing Radia- tion T. Tada Zeolites treated with Silicon Tetrachloride Vapour. Part 2.-Sorption Studies M. W. Anderson and J. Klinowski Studies of Propene Oxidation over Mixed Uranium-Antimony Oxides F.J. Farrell, T. G. Nevell and D. J. Hucknall Adsorption and Desorption Kinetics of Oxygen on Tin-Antimony Oxide Cat a1 y st by Quasi -cons tan t Coverage Met hods M-C. Bacchus-Montabonel and CO Adsorption at 77 K on KCl Films. An Infrared Investigation D. Scarano and A. Zecchina The Utilization of Time-resolved Dielectric Loss to probe the Role of the Surface in Heterogeneous Photochemistry C. J. Dobbin, A. R. McIntosh, J. R. Bolton, Z. D. Popovic and J. R. Harbour Polysilicate Equilibria in Concentrated Sodium Silicate Solutions I. L. Svensson, S. Sjoberg and L-0. Ohman Potentials of Ion-exchanged Synthetic Zeolite-Polymer Membranes M. Demertzis and N. P. Evmiridis Adsorption and Reduction of Nitrogen Monoxide by Potassium-doped Carbon T.Okuhara and K. Tanaka Physicochemical Properties and Isomerization Activity of Chlorinated Pt/Al,O, Catalysts A. Melchor, E. Garbowski, M-V. Mathieu and M. Primet Excess Pressures for Aqueous Solutions M. J. Blandamer, J. Burgess and A. W. Hakin Effect of Temperature on the Point of Zero Charge and Surface Dissociation Constants of Aqueous Suspensions of y-Al,O, K. Ch. Akratopulu, L. Vordonis and A. Lycourghiotis Thermal Decomposition of Solid Sodium Bicarbonate M. C. Ball, C. M. Snelling, A. N. Strachan and R. M. Strachan Reviews of Books J-P. Joly I I7ISSN 0300-9599 JCFTAR 82 ( I 2 ) 3525-371 9 (1 986) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases 3525 3535 3553 3561 3569 3587 360 1 361 1 3625 3635 3647 3657 3667 368 1 3697 3709 3717 CONTENTS Fourier Transform Infrared Studies of the Irreversible Oxidation of Cyanide at Platinum Electrodes A.S. Hinman, R. A. Kydd and R. P. Cooney The Dielectric Properties of Zeolites in Variable Temperature and Humidity A. R. Haidar and A. K. Jonscher The Time-domain Response of Humid Zeolites A. K. Jonscher and A. R. Haidar Molecular-orbital Studies of C-H Bond Scission induced by Ionizing Radia- tion T. Tada Zeolites treated with Silicon Tetrachloride Vapour. Part 2.-Sorption Studies M. W. Anderson and J. Klinowski Studies of Propene Oxidation over Mixed Uranium-Antimony Oxides F. J. Farrell, T. G. Nevell and D. J. Hucknall Adsorption and Desorption Kinetics of Oxygen on Tin-Antimony Oxide Cat a1 y st by Quasi -cons tan t Coverage Met hods M-C.Bacchus-Montabonel and CO Adsorption at 77 K on KCl Films. An Infrared Investigation D. Scarano and A. Zecchina The Utilization of Time-resolved Dielectric Loss to probe the Role of the Surface in Heterogeneous Photochemistry C. J. Dobbin, A. R. McIntosh, J. R. Bolton, Z. D. Popovic and J. R. Harbour Polysilicate Equilibria in Concentrated Sodium Silicate Solutions I. L. Svensson, S. Sjoberg and L-0. Ohman Potentials of Ion-exchanged Synthetic Zeolite-Polymer Membranes M. Demertzis and N. P. Evmiridis Adsorption and Reduction of Nitrogen Monoxide by Potassium-doped Carbon T. Okuhara and K. Tanaka Physicochemical Properties and Isomerization Activity of Chlorinated Pt/Al,O, Catalysts A. Melchor, E. Garbowski, M-V. Mathieu and M. Primet Excess Pressures for Aqueous Solutions M. J. Blandamer, J. Burgess and A. W. Hakin Effect of Temperature on the Point of Zero Charge and Surface Dissociation Constants of Aqueous Suspensions of y-Al,O, K. Ch. Akratopulu, L. Vordonis and A. Lycourghiotis Thermal Decomposition of Solid Sodium Bicarbonate M. C. Ball, C. M. Snelling, A. N. Strachan and R. M. Strachan Reviews of Books J-P. Joly I I7

ISSN:0300-9599    

DOI:10.1039/F198682FX019

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

FA RA DA Y 1RA N SA CTlO N S AND SYMPOSIA From the Royal Society of Chemistry FARADAY TRANSACTIONS II Molecular and Chemical Physics SPECIAL ISSUE - AUGUST 1986 Professor Alan Carrlngton delbred the 1985 Faraday lecture at the Royal lnsmutlon on 10th December, 1985. As a compliment to Professor Carrington, a group of his colleagues and Mends submitted original papers on the general theme of Molecular Dynamics and Spectroscopy. These papers are collected in the present Issue. CONTENTs: The Faraday Lecture: Spectroscopy of Molecular Ions at thelr Dissociation Limits A. Carrington The Spectroscopy, Photophysics and Photochemistry of Clusters of Metrazlne D. H. Levy Spectroscopy of Transient Species produced by Photodissociation or Photoionizatlon in a Supersonlc Free-jet Expansion T.A. Mlller Molecukr-beam Infrared Spectroscopy of the Ar-N,O van der Waals Molecule J. Hodge, G. D. Hayman, T. R. Dykeand B. J. Howard The Estimation of Vlbrational Predissociation Ufetimes M. S. Chlld The Infrared Spectrum of H; and its Isotopomers. A Challenge to Theory and Experlment J. Tennyson and B. T. Sutcliffe The Augmented Secular Equation Method for calculating Spectra of van der Waals Complexes. Application to the Infrared Spechum of Ar-HCI J. M. Hutson Quantum-mechanical Wavepacket Dynamics of the CH Group in Symmetric- top X,CH Compounds using Effecttve Hamiltonians from Hlgh-resolution Spectroscopy R. Marguardt, M. Quack, J. Stohner and f. Sutcllffe Internal Dynamics of Subunits and Bondlng Force Constants In Weakly Bound Dlmers P. Cope, D.J. Mlllerand A. C. fegon Internal Dynamics and HF Bond Lengthenlng In the Hydrogen-bonded Heterodimer CH,CN . . . HF determined from Nuclear Hyper-fine Structure in its Rotational Spectrum P. Cope, D. J. Miller, L. C. Willoughbyand A. C. fegon Pumping and Rshing. Double-resonance Measurements on Molecular Jets U. Veeken, N. Damand J. Reuss nme-resow Fluorescence of Jet-cooled Carbazoles and thelr Weak Complexes A. R. Auty, A. C. Jones and D. Philllps Prediction of the Ct(*P, J/CI(P,J Branching Ratio in the Photodissociation of HCI S. C. GIvertzand 6. C. Ballnt-Kurt/ Hlgh-resolution Laser Photofragment Spectroscopy of CH' P. J. Sam, J. M. Walmsleyand C. J. Whitham Asymmetric Uneshapes associated with Predissociating levels M. N. R. Ashfold, R. N. Dixon, J. D. Prince, B.Tutcherand C. M. Westem A Threshold-photoelectron Ruorescence-Photon Coincidence Study of Radlatlonless TransMons in the B ,iI State of BCN+ €. Castellucci, G. Dulardin and S. leach Cornpetthe Channels in the Interaction of Xe('P,) with CI,, Br, and I,. Atom Transfer, Excitation Transfer, Energy Disposal and Product Alignment K. Johnson, R. Pease, J. P. Simons, P. A. Smith and A. Kvaran Mco Non-RSC Momkn P14.30 ($27.70) RSC Momkn M.00 Payment should accompany ordon for lhlr Ihm. RSC Members should send their orders to: Membershlp Manager, The Royal Society of Chemistry, 30 Russell Square, London WC1 B SDT. Non-RSC Members should send their orders to: The Royal Socleiy of Chemistry, Distribution Centre, Blackhorse Road. Letchwotth, Herts SG6 1 HN. Faraday Discussions No.80 Physical Interactions and Energy Exchange af the Gas-Solid Interface [his publication discusses aspects of current research on the gas-solid Interface: elastic, inelastic and dlssipattve scattering of atoms and molecules from cfystal surfaces; the structure and dynamics of physisorbed species, including overlayers. Emphasis Is placed on the themes of physical interactions and energy exchange rather than on molecular beam technology or the phenomenology of phase tmnsmons in overlayen. The interplay between theory and experlment is stressed as they relate to the nature of atom and molecule-surface interaction potentials including many body effects. Faraday Discussions No. 80 (1986) Softcevor Prico 531 .OO ($60.00) RSC Mombon J66.25 ROYAL SOCIETY OF CHEMISTRY Information Services (xiii)FA RA DA Y 1RA N SA CTlO N S AND SYMPOSIA From the Royal Society of Chemistry FARADAY TRANSACTIONS II Molecular and Chemical Physics SPECIAL ISSUE - AUGUST 1986 Professor Alan Carrlngton delbred the 1985 Faraday lecture at the Royal lnsmutlon on 10th December, 1985.As a compliment to Professor Carrington, a group of his colleagues and Mends submitted original papers on the general theme of Molecular Dynamics and Spectroscopy. These papers are collected in the present Issue. CONTENTs: The Faraday Lecture: Spectroscopy of Molecular Ions at thelr Dissociation Limits A. Carrington The Spectroscopy, Photophysics and Photochemistry of Clusters of Metrazlne D. H. Levy Spectroscopy of Transient Species produced by Photodissociation or Photoionizatlon in a Supersonlc Free-jet Expansion T.A. Mlller Molecukr-beam Infrared Spectroscopy of the Ar-N,O van der Waals Molecule J. Hodge, G. D. Hayman, T. R. Dykeand B. J. Howard The Estimation of Vlbrational Predissociation Ufetimes M. S. Chlld The Infrared Spectrum of H; and its Isotopomers. A Challenge to Theory and Experlment J. Tennyson and B. T. Sutcliffe The Augmented Secular Equation Method for calculating Spectra of van der Waals Complexes. Application to the Infrared Spechum of Ar-HCI J. M. Hutson Quantum-mechanical Wavepacket Dynamics of the CH Group in Symmetric- top X,CH Compounds using Effecttve Hamiltonians from Hlgh-resolution Spectroscopy R. Marguardt, M. Quack, J. Stohner and f. Sutcllffe Internal Dynamics of Subunits and Bondlng Force Constants In Weakly Bound Dlmers P.Cope, D. J. Mlllerand A. C. fegon Internal Dynamics and HF Bond Lengthenlng In the Hydrogen-bonded Heterodimer CH,CN . . . HF determined from Nuclear Hyper-fine Structure in its Rotational Spectrum P. Cope, D. J. Miller, L. C. Willoughbyand A. C. fegon Pumping and Rshing. Double-resonance Measurements on Molecular Jets U. Veeken, N. Damand J. Reuss nme-resow Fluorescence of Jet-cooled Carbazoles and thelr Weak Complexes A. R. Auty, A. C. Jones and D. Philllps Prediction of the Ct(*P, J/CI(P,J Branching Ratio in the Photodissociation of HCI S. C. GIvertzand 6. C. Ballnt-Kurt/ Hlgh-resolution Laser Photofragment Spectroscopy of CH' P. J. Sam, J. M. Walmsleyand C. J. Whitham Asymmetric Uneshapes associated with Predissociating levels M.N. R. Ashfold, R. N. Dixon, J. D. Prince, B. Tutcherand C. M. Westem A Threshold-photoelectron Ruorescence-Photon Coincidence Study of Radlatlonless TransMons in the B ,iI State of BCN+ €. Castellucci, G. Dulardin and S. leach Cornpetthe Channels in the Interaction of Xe('P,) with CI,, Br, and I,. Atom Transfer, Excitation Transfer, Energy Disposal and Product Alignment K. Johnson, R. Pease, J. P. Simons, P. A. Smith and A. Kvaran Mco Non-RSC Momkn P14.30 ($27.70) RSC Momkn M.00 Payment should accompany ordon for lhlr Ihm. RSC Members should send their orders to: Membershlp Manager, The Royal Society of Chemistry, 30 Russell Square, London WC1 B SDT. Non-RSC Members should send their orders to: The Royal Socleiy of Chemistry, Distribution Centre, Blackhorse Road. Letchwotth, Herts SG6 1 HN. Faraday Discussions No. 80 Physical Interactions and Energy Exchange af the Gas-Solid Interface [his publication discusses aspects of current research on the gas-solid Interface: elastic, inelastic and dlssipattve scattering of atoms and molecules from cfystal surfaces; the structure and dynamics of physisorbed species, including overlayers. Emphasis Is placed on the themes of physical interactions and energy exchange rather than on molecular beam technology or the phenomenology of phase tmnsmons in overlayen. The interplay between theory and experlment is stressed as they relate to the nature of atom and molecule-surface interaction potentials including many body effects. Faraday Discussions No. 80 (1986) Softcevor Prico 531 .OO ($60.00) RSC Mombon J66.25 ROYAL SOCIETY OF CHEMISTRY Information Services (xiii)

ISSN:0300-9599    

DOI:10.1039/F198682BX021

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

1673 1681 1687 1703 1713 1721 1733 1745 1755 1771 1781 1789 1795 1805 813 829 839 853 865 56 ISSN 0300-9599 JCFTAR 82 (6) 1 673-2009 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases CONTENTS Structural Relationships in the Reduction of the Vanadia-Molybdena Inter- mediate Compound M. Najbar Hydrogenation of Carbon Dioxide and Carbon Monoxide over Supported Platinum Catalysts T. Inoue and T. Iizuka Mechanism of n-Alkane Transformations over a Solid Superacid of Lewis Character, Al,O,/AlCl, M. Marczewski Ionization Equilibria of Cobalt(I1) Chloride in N,N-Dimethylformamide W. Grzybkowski and M. Pilarczyk An Electron Spin Resonance Study on the Re/ZrO, System T. Komatsu, M. Komiyama, Y. Ogino and M. Iwamoto Surface-charging Effects in the X-Ray Photoelectron Spectra of some Semi- conducting Oxides s.J. Cochran and F. P. Larkins Adsorption of Phosphate on Calcite T. Suzuki, S. Inomata and K. Sawada Stability of Monochloride Complexes of some Divalent Transition-metal Cations in N,N-Dimethylformamide W. Grzybkowski and M. Pilarczyk Interfacial Tension Minima in Oil-Water-Surfactant Systems. Effects of Alkane Chain Length and Presence of n-Alkanols in Systems containing Aerosol OT R. Aveyard, B. P. Binks and J. Mead Catalysis by Amorphous Metal Alloys. Part 4.-Structural Modification to- wards Metastable States and Catalytic Activity of Amorphous Ni,,B,, Ribbon Alloy H. Yamashita, M. Yoshikawa, T. Funabiki and S. Yoshida Electrode Kinetics of the Cd"/Cd-Hg System in Ethylene Glycol-Water Mixtures J.A. Garrido, R. M. Rodriguez, E. Brillas and J. Dome'nech Application of the Competitive Preferential Solvation Theory to Ion-Molecule Interactions B. Parbhoo and 0. B.Nagy Paramagnetic Metal and Oxygen Species observed with Rh/y-Al,O, and Rh/ZrO,. Dependence on the Decarbonylation Temperature of [Rh,(CO),,] on Alumina and Zirconia supports A. Gervasini, F. Morazzoni, D. Strumolo, F. F'inna, G. Strukul and L. Zanderighi Mixed Adsorption of a Non-ionic and an Anionic Surfactant at the Carbon-Aqueous Solution Interface M. J. Hey, J. W. Mactaggart and C. H. Rochester Determination of Micelle Size and Polydispersity by Fluorescence Quenching. Theory and Numerical Results Determination of Micelle Size and Polydispersity by Fluorescence Quenching. Experimental Results G. C.Warr, F. Grieser and D. F. Evans ' Thermodynamics of Three Toluene-containing Mixtures. Generalized van der Waals and Ising-like Models C. Saez, A. Compostizo, R. G. Rubio, A. C. Colin and M. D. Peiia Solution Properties of Water in Molten AgN0,-LiNO, Mixtures as derived from Vapour-pressure Measurements on AgN0,-LiN0,-H,O Melts Z. KodejB and G. A. Sacchetto Micellisation and Surface Properties of Oligo(oxyethy1ene) Methyl-n-alkyl and di-n-Alkyl Ethers S. G. Yeates, J. R. Craven, R. H. Mobbs and C. Booth G. C. Warr and F. Grieser FAR 1Con tents High-resolution Solid-state Magic Angle Spinning Nuclear Magnetic Resonance Investigations of Surface Hydroxy Groups on Modified Silica Gel T. Bernstein, P. Fink, V. M. Mastikhin and A. A. Shubin Protonation of the Dianion of Tetraphenylethylene by Alcohols and Water in Acetonitrile.Case of Mixed First-order and Second-order Kinetics in the Proton Donor G. Farnia, F. Maran and G. Sandona Chlorinated Alumina. Acidic Properties and Catalytic Activity towards n-Butane Isomerization A. Melchor, E. Garbowski, M-V. Mathieu and M. Primet Association Characteristics of Synthetic Non-ionic Surfactants in Aqueous Solution D. Attwood, P. H. Elworthy and M. J. Lawrence Infrared Studies of Probe Molecules adsorbed on Calcium Oxide J. A. Anderson and C. H. Rochester Contamination by Coherent Scattering of the Elastic Incoherent Structure Factor observed in Neutron Scattering Experiments B. Gabrys, J. S. Higgins and 0. Scharpf Short-range Order in Amorphous Poly(methy1 methacrylate) B.Gabrys, J. S. Higgins and 0. Scharpf A Comparative Study of the Protonation of myo-Inositol Hexakis(ph0sphate) H. Bieth and B. Spiess Re,O,/Al,O, *B,O, Metathesis Catalysts X. Xiaoding, C. Boelhouwer, J. I. Benecke, D. Vonk and J. C. Mol Molar Gibbs (Free) Energies of Transfer of Silver(I), Copper@) and Potass- ium(1) G. Gritzner Inhibition of the Thin-film Oxidation of n-Dodecane by p-Methoxyphenol A. D. Ekechukwu and R. F. Simmons Rates and Activation Parameters of Alkaline Hydrolysis of the 2- Carbomethoxypropionate Ion in Aqueous Mixtures of Dimethyl Sulphoxide P. K. Biswas and M. N. Das The Effect of Aqueous-phase Solubility on Free-radical Exit from Latex Particles M. Adams, D. H. Napper, R. G. Gilbert and D. F. Sangster Hydrogenolysis of Alkanes. Part 3.-Hydrogenolysis of n-Hexane and Methyl- cyclopentane over variously treated Ru/TiO, Catalysts R. Burch, G. C. Bond and R. R. Rajaram Corrigendum to: A Nuclear Magnetic Resonance Study of the Sodium Cryptate formed by 4,7,13,18-Tetraoxa-l,lO-diazabicyclo[8.5.5]eicosane (C211) in Various Solvents S. F. Lincoln, Y. M. Brereton and T. M. Spotswood Reviews of Books D. Langevin; G. H. Findenegg; K. E. Weale; R. G. Egdell; J. A. S. Smith; J. S. Higgins; J. J. Rooney; F. J. Humphreys; B. L. Booth; D. Eagland; A. J. Thomson 879 885 1893 1903 191 1 1923 1929 1935 1945 1955 1965 1973 1979 1985 1999 200 1

ISSN:0300-9599    

DOI:10.1039/F198682FP067

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions II, Issue 6,1986 - Molecular and Chemical Physics 881 889 897 913 929 937 953 963 97 1 973 For the benefit of readers of Faraday Transactions I, the contents list of Faraday Transactions II, Issue 6, is reproduced below. Inhibitory Features of the Thermal Oxidation of Carbon Monoxide. A Kinetic Foundation to Dynamic Instabilities in Closed Vessels K. J. Chinnick and J. F. Griffiths Magnetic Susceptibility and Electron Spin Resonance in Quenched Metal Solu- tions S. C. Guy, P. P. Edwards and M. R. Harrison Absolute Third-order Rate Constants for the Recombination Reactions between Alkali-metal and Iodine Atoms and the Measurement for Rb + I + He J. M. C. Plane and D. Husain The Mechanics and Thermodynamics of Disc-shaped Micelles S.Ljunggren and J. C. Eriksson Reaction of Hydrogen Atoms with Cyclopropane in the Temperature Range 358-550 K R. M. Marshall, H. Purnell and A. Sheppard Kinetics of SiF(X217,) by Time-resolved Molecular Resonance Absorption Spec- troscopy following the reaction of Si(3 ,PJ, 3 lD,, 3 lS0), generated by Pulsed Irradiation, with Fluorinated Compounds D. R. Harding and D. Husain A General Inter-relationship between Transition-state Bond Extensions and the Energy Barrier to Reaction A. J. C. Varandas and S. J. Formosinho The Low-temperature Quenching of Singlet Molecular Oxygen [0, (a lAg)] A. P. Billington and P. Borrell Corrigendum to Bonding Modes in the Analogous and Isoelectronic Group VB/VA Compounds As(O)Cl, and Nb(O)Cl,. An Ultraviolet Photoelectron- spectroscopic and SSC-Xa Study s.Elbel, A. Blanck, H. Walther and M. Grodzicki Reviews of Books A. Hodgson; D. M. Adams; K. E. Weale; J. P. Simons The following papers were accepted for publication in J . Chem. SOC., Faraday Trans. I, during March 1986. 5/328 5/992 On the Limitations of Evaluation Methods for Inhibition Kinetics in Oxidative Systems K. Heberger, J. Lukacs and T. Vidoczy Electron Transfer Rates by Dielectric Relaxometry and the Direct-current Conductivities of Solid Homonuclear and Heteronuclear Mixed Valence Metal Cyanometallates of First-row Transition Metals D. R. Rosseinsky and J. S. Tong 5/1467 Effect of Support and Promoter on the Co-adsorption of Carbon Monoxide and Hydrogen on Fischer-Tropsch Cobalt Catalysts B. Viswanathan and R. Gopalakrishnan 5/1506 The Transfer of Atoms, Ions and Molecular Groups in Solution.Part 4.- Methods for Metropolis/Monte Carlo Simulations P. P. Schmidt51 1697 Adsorption of Organic Substances at the MercuryIEthylene Glycol Inter- face. Part 2.-Aromatic Compounds J. I. Japaridze, S. s. Japaridze, N. A. Abuladze, A. De Battisti and S. Trasatti 51 1714 The Partitioning of Solutes between Water-in-Oil Microemulsions and Conju- gate Aqueous Phases P. D. I. Fletcher 5 / 1840 Cooperative Effects in Heterogeneous Catalysis. Part 1 .-Phenomenology of the Dynamics of CO Oxidation on Pd embedded in a Zeolite Matrix N. I. Jaeger, 511912 511946 512047 512070 512161 512210 51221 1 5/22 12 51223 1 512232 512244 512277 512280 61009 61010 61053 61054 61093 61109 K. Moller and P.J. Plath Photocatalytic Dehydrogenation of Liquid Propan-2-01 by TiO,. Part 2.- Mechanism I. M. Fraser and J. R. MacAllum On the Accuracy of the Derjaguin Approximation for the Electrostatic Double- layer Interaction between Curved Surfaces bearing Constant Potentials E. Barouch, E. Matijevic and V. A. Parsegian Standard Thermodynamic Functions of the Gaseous Actinyl Ions Mop+ and of their Hydration Y. Marcus and A. Liewenschuss Inner-sphere Reorganisation Energy of Ions in Solution using the Ion-Dipole Orbiting Potential M. S. Tunuli and S. U. M. Khan Application of a Solvent-coupled Model to a Calculation of Partial Molar Isobaric Heat Capacities of Neutral and Ionic Solutes in Aqueous Solution; Estimates of Isobaric Heat Capacities of Activation M. J. Blandamer, J.Burgess, A. W. Hakin and J. M. W. Scott Ordered Solution Structure of Monodispersed Polystyrene Latex as studied by the Reflection Spector Method T. Okubo Transmitted Light Spector Measurements. A Novel and Convenient Technique for the Study of the Ordered Structure of a Monodispersed Polystyrene Latex in Solution and Film T. Okubo Ordered Solution Structure of a Monodispersed Polystyrene Latex as studied by the Transmitted Light Spector Method T. Okubo Optical Anisotropies of Alkyl Cyanobicyclohexyls and Related Compounds P. Navard and P. J. Flory Optical Anisotropies of Alkyl and Alkoxy Cyanobiphenyls and Related Com- pounds P. J. Flory and P. Navard Thermal Desorption and Infrared Studies of Butylamine adsorbed on SiO,, A1,0, and CaO R. Sokoll, H.Hobert and I. Schmuck Studies of Propene Oxidation over Mixed Uranium-Antimony Oxides F. J. Farrell, T. G. Nevell and D. J. Hucknall Normal Coordinate Analysis of Molecules absorbed on Zeolite Surfaces. Part 1 .-Cyclopropane adsorbed on Sodium Faujasites and Mordenites 0. Zkharieva-Pencheva, H. Forster and J. Seebode Dielectric Behaviour of Adsorbed Water. Part 1 .-Measurement at Room Temperature on TiO, T. Morimoto and T. Iwaki Dielectric Behaviour of Adsorbed Water. Part 2.-Measurement at Low Temperatures on TiO, T. Iwaki and T. Morimoto Irrelevance of Gas Evolution to Oscillations in the Belousov-Zhabotinsky System R. M. Noyes Reactions of Recoil 3sCl Atoms with Dichloroethanes K. Berei, L. Vasaros and I. Kiss A Nuclear Magnetic Resonance Study of the Reactions of the Tetrahydroxy- borate Ion with Polyhydroxy Compounds J.G. Dawber and S. I. E. Green Adsorption and Desorption Kinetics of Oxygen on Tin-Antimony Oxide Catalyst by Quasi-constant Coverage Methods M. C. Bacchus-Montabonel and J. P. Joly (ii)611 10 611 16 6/ 145 61168 61194 61204 61210 61217 612 18 61230 61272 61336 61448 The Identification of the Space Group and the Detection of Catonic Ordering in Iron Antimonate using Conventional and Covergent Beam Electron Diffrac- tion J. G. Holden, M. H. Loretto and F. Berry Decay of High-valent Manganese Porphyrins in Aqueous Solution and Catalysed Formation of Oxygen A. Harriman, P. A. Christensen, G. Porter, K. Morehouse, P. Neta and M. C. Richoux Coadsorption of Methanol and Carbon Dioxide on Alumina J. Lamotte, 0.Saur, J. C. Lavalley, G. Busca, P. F. Rossi and V. Lorenzelli The Relation between Immersion Calorimetry and the Parameters of the Water Adsorption Isotherm on Active Carbons F. Kraehenbuehl, C. Quellet, B. Schmitter and H. F. Stoeckli Neutron Scattering of Supercooled Water in Silica Gels C. Poinsignon and J. D. F. Ramsay A Non-equilibrium Theory of Polyelectrolyte Adsorption W. Barford, R. C. Ball and C. M. M. Nex A Single-turnover (STO) Study of the Effect of Heat on Catalyst Activity R. L. Augustine and K. P. Kelly Kinetics of Metal Oxide Dissolution. Oxidative Dissolution of Chromium(II1) Oxide by Potassium Permanganate M. G. Segal and W. J. Williams Phase Equilibria in Model Mixtures of Spherical Molecules of Different Sizes G. Jackson, J. S. Rowlinson and C.A. Leng The Utilization of Time-resolved Dielectric Loss to Probe the Role of the Surface in Heterogeneous Photochemistry C. J. Dobbin, A. R. McIntosh, J. R. Bolton, Z. D. Popovic and J. R. Harbour Infrared Study of the Adsorption of But-l-ene, Buta-1,3-diene, Furan and Maleic Anhydride on the Surface of Anhydrous Vanadyl Pyrophospate S. J. Puttock and C. H. Rochester Electrochemistry and Stability Studies of the Oxo-bridged Dinuclear Ruthen- ium(m) Complexes for Water Oxidation R. Ramaraj, A. Kira and M. Kaneko Thermodynamic Parameters of Electrolyte Solutions in Nitromethane A. F. Danil de Namor and L. Ghousseini (iii)Cumulative Author Index Abu-Gharib, E.-E. A., 1471 Adams, D. M., 1020 Adams, M., 1979 Aida, M., 1619 Al-Hakim, M., 1575 Albery, W.J., 1033 Allen, G. C., 1367 Alwis, U. de, 1265 Ammann, D., 1179 Anderson, J. A,, 191 1 Anderson, M. W., 569, 1449 Andersson, S. L. T., 1537 Andersson, T., 767 Antoniou, A. A., 483 Araya, P., 1351 Attwood, D., 1903 Avent, A. G., 1589 Aveyard, R., 125, 1031, 1755 Baldwin, R. R., 89 Balk, R. W., 933 Bartlett, J. R., 597 Bartlett, P. N., 1033 Baur, J., 1081 Belton, P. S., 451 Benecke, J. I., 1945 Berezin, I. V., 319 Bernstein, T., 1879 Berry, F. J., 1023 Bieth, H., 1935 Binks, B. P., 125, 1031, 1755 Biswas, P. K., 1973 Blake, P. G., 723 Blandamer, M. J., 1022, 1471 Bloemendal, M., 53 B.Nagy, O., 1789 Boelhouwer, C., 1945 Bond, G. C., 1985 Booth, B. L., 2007 Booth, C., 1865 Boucher, E. A., 1589 Brereton, I. M., 1999 Brett, C. M. A,, 1071 Brigandi, P. W., 1032 Brillas, E., 495, 1781 Bruckenstein, S., 1105 Buck, R.P., 1169 Bui, V. T., 899 Burch, R., 1985 Burgess, J., 1471 Cameron, P., 1389 Carley, A. F., 723 Carpenter, T. A,, 545 Casal, B., 1597 Cass, A. E. G., 1033 Castro, V. Di, 723 Cenens, J., 281 Cesteros, L. C., 1321 Champion, J. V., 439 Chang, C . D., 1032 Chiou, C. T., 243 Chitale, S . M., 663 Clark, B., 1471 Clark, S., 125 Cochran, S. J., 1721 Cohen de Lara, E., 365 Coller, B. A. W., 943 Compostizo, A,, 1839 Cooney, R. P., 597 Copperthwaite, R. G., 1007 Cortes, J., 1351 Corti, H. R., 921 Covington, A. K., 1209 Craston, D. H., 1033 Craven, J. R., 1865 Crespo Colin, A,, 1839 Crilly, J. F., 439 Danil de Namor, A. F., 349 Das, M. N., 1973 Dawber, J. G., 119 De Schrijver, F. C., 281 Dean, C. E., 89 Dearden, S. J., 1627 Delaval, Y., 365 Dharmalingam, P., 359 Diaz PEna, M., 1839 Domtnech, J., 1781 Duatti, A., 1429 Duce, P.P., 1471 Eagland, D., 2008 Ebeid, E-Z. M., 909 Egdell, R. G., 2003 Ekechukwu, A. D., 1965 El-Daly, S. A,, 909 Elbing, The Late E., 943 Elworthy, P. H., 1903 Espenscheid, M. W., 1051 Espinosa-JimCnez, M., 329 Evans, D. F., 1829 Ewen, R. J., 1127 Farnia, G., 1885 Feakins, D., 563 Fegan, S . G., 785, 801 Fernandez-Prini, R., 921 Findenegg, G. H., 2001 Fink, P., 1879 Fisher, D. T., 119 Foulds, N. C., 1259 Fraser, I. M., 607 Freiser, H., 1217 Fricke, R., 263, 273 Fukuda, H., 1561 Funabiki, T., 35, 707, 1771 1986 Fyles, T. M., 617 Gabrys, B., 1923, 1929 Garbowski, E., 1893 Garrido, J. A., 1781 Gellan, A., 953 Geoffroy, M., 521 Gervasini, A., 1795 Ghatak-Roy, A. R., 1051 Ghoneim, M.M., 909 Ghousseini, L., 349 Gilbert, R. G., 1979 Gilhooley, K., 431 Gonzalez-Caballero, F., 329 Gonzalez-Elipe, A. R., 739 Gonzalez-Fernindez, C. F., 329 Gormally, J., 157 Gorton, L., 1245 Gosal, N., 1471 Green, M. J., 1237 Grieser, F., 1813, 1829 Gritzner, G., 1955 Grzybkowski, W., 1381, 1703, Guardado, P., 1471 Haggett, B. G. D., 1033 Hakin, A. W., 1471 Halle, B., 401, 415 Hansen, O., 77 Heatley, F., 255 Hedges, W. M., 179 Hellring, S. D., 1032 Hemfrey, J. P., 1589 Hersey, A., 1271 Hewitt, E. A,, 869 Hey, M. J., 1805 Heyrovsky, M., 585 Higgins, J. S., 1923, 1929, 2004 Higson, S., 157 Hill, C. A. S., 1127 Hill, H. A. O., 1237 Hill, T., 349 Hitchman, M. L., 1223 Hobert, H., 1527 Hobson, D. B., 869 Homer, J., 533 Honeybourne, C. L., 1127 Honeyman, M.R., 89 Hooper, A., 11 17 Houghton, J. D., 1127 Hronec. M., 1405 Hsu, W. P., 851 Hubbard, C. D., 1471 Humphreys, F. J., 1020, 2006 Hunt, D. J., 189 Hutchings, G. J., 1007 Iizuka, T., 1681, 61 1745Ikeda, H., 61 Ikeda, O., 1561 Indelli, A., 1429 Inomata, S., 1733 Inoue, T., 1681 Issa, R. M., 909 Iwamoto, M., 1713 Jackson, S. D., 431, 189 Jaeger, N., 205 Jayasuriya, D. S., 457, 473 Jensen, M., 1351 Johnson, D. C., 1081 Johnson, J., 1081 Johnston, P., 1007 Jones, W., 545 Jonson, B., 767 Jose, C. I., 663, 681, 691 Kakuta, N., 1553 Kamat, P. V., 1031 Kaminade, T., 707 Katime, I., 1321, 1333 Kavetskaya, 0. I., 319 Kawaguchi, T., 1441 Kawai, S., 527 Kawai, T., 527 Kazusaka, A., 1553 Kelly, H. C., 1271 Kelly, R. G., 1195 Kevan, L., 213 Khoo, K. H., 1 Kleine, A., 205 Klinowski, J., 569, 1449 KodejS, Z., 1853 Komatsu, T., 1713 Komiyama, M., 1713 Koreeda, A., 527 Kuzuya, M., 1441 Lancz, M., 883 Lang, J., 109 Langevin, D., 2001 Larkins, F. P., 1721 Larsson, R., 767 Lawless, T.A., 1031 Lawrence, K. G., 563 Lawrence, M. J., 1903 Leaist, D. G., 247 Lkonard, J., 899 Lim, T.-K., 69 Lincoln, S. F., 1999 Llars, S., 767 Llinares, A., 521 Lockhart, J. C., 1161 Loewenschuss, A,, 993 Logan, S. R., 161 Lomen, C. E., 1265 Lowe, B. M., 785, 801 Lowe, C. R., 1259 Lundin, S. T., 767 MacCallum, J. R., 607 Mactaggart, J. W., 1805 Mahnke, R., 1413 Malliaris, A., 109 Manes, M., 243 Maran, F., 1885 Marcus, Y . , 233, 993 AUTHOR INDEX Marczewski, M., 1687 Martin, C. R., 1051 Maruthamuthu, P., 359 Mastikhin, V. M., 1879 Mathieu, M-V., 1893 Matsuda, T., 1357 McCarthy, S., 943 Mead, J., 125, 1031, 1755 Melchor, A., 1893 Miale, J.N., 1032 Miasik, J. J., 11 17 Midgley, D., 1187 Minami, Z., 1357 Mishima, S., 1307 Mishra, S. P., 521 Miura, H., 1357 Miyake, Y . , 1515 Miyamoto, A,, 13 Mobbs, R. H., 1865 Mol, J. C., 1945 Mollett, C. C., 1589 Molyneux, P., 291, 635 Moore 111, R. B., 1051 Morazzoni, F., 1795 Morgan, H., 143 Mori, K., 13 Moyes, R. B., 189 Mulla, S. T., 681, 691 Murakami, Y . , 13 Nagano, S., 1357 Najbar, M., 1673 Nakajima, T., 1307 Nakamatsu, H., 527 Nakanishi, M., 1441 Napper, D. H., 1979 Narayana, M., 213 Neto, M. M. P. M., 1071 Neuburger, G. G., 1081 Nikitas, P., 977 Nyasulu, F. W. M., 1223 Oesch, U., 1179 Ogino, Y . , 1713 Ohlmann, G., 263, 273 Okazaki, S., 61 Okuda, T., 1441 Oldham, K. B., 1099 Ooe, M., 35 Opallo, M., 339 Orchard, S.W., 1007 Oref, I., 1289 Ortiz, A,, 495 Owen, A. E., 1195 Parbhoo, B., 1789 Parsons, B. J., 1575 Pease, W. R., 747, 759 Peeters, G., 963 Peeters, S., 963 Penner, R. M., 1051 Perry, M. C., 533 Pethig, R., 143 Pham, H. V., 1179 Phillips, G. O., 1575 Piculell, L., 387, 401, 415 Piekarska, A,, 513 Piekarski, H., 513 (4 Pilarczyk, M., 1703, 1745 Pinna, F., 1795 Pletcher, D., 179 Polta, J. A., 1081 Polta, T. Z., 1081 Pouchly, J., 1605 Primet, M., 1893 Puchalska, D., 1381 Quintana, J. R., 1333 Radulovic, S., 1471 Rajaram, R. R., 1985 Ramdas, S., 545 Rebenstorf, B., 767 Richardson, P. J., 869 Rideout, J., 167 Rigby, S., 431 Rizkallah, P. J., 1589 Roberts, M. W., 723 Robinson, B. H., 1271 Robinson, P. J., 869 Rochester, C. H., 953, 1805, Rodriguez, R.M., 1781 Rooney, J. J., 2005 Rosenholm, J. B., 77 Rouw, A. C., 53 Rubio, R. G., 1839 Ruiz-Hitzky, E., 1597 Ryder, P. L., 205 Sacchetto, G. A., 1853 Saez, C., 1839 Salmon, G. A,, 161 Sanchez, F., 1471 Sandona, G., 1885 Sangster, D. F., 1979 Sirkany, A., 103 Sawada, K., 1733 Scharpf, O., 1923, 1929 Schlosserova, J., 1405 Schmelzer, J., 1413, 1421 Schmitt, K. D., 1032 Schoonheydt, R. A., 281 Scott, R. P., 1389 Segall, R. L., 747, 759 Seloudoux, R., 365 Shibata, Y . , 1357 Shigeto, M., 1515 Shindo, H., 45 Shubin, A. A., 1879 Siiman, O., 851 Simmons, R. F., 1965 Simon, W., 1179 Sircar, S., 831, 843 Smallridge, M. J., 1589 Smart, R. St C., 747,759 Smith, I., 869 Smith, J. A. S., 2004 Snowdon, S., 943 Sokoll, R., 1527 Solymosi, F., 883 Somsen, G., 53, 933 Soria, J., 739 Spiess, B., 1935 Spotswood, T.M., 1999 1911 Schulz-Ekloff, G., 205Strazielle, C., 1321 Strukul, G., 1795 Strumolo, D., 1795 Sugiyama, K., 1357 Suppan, P., 509 Sutherland, I. O., 1145 Suzuki, T., 1733 Swallow, A. J., 1575 Symanski, J. S., 1105 Symons, M. C. R., 167 Szentirmay, M. N., 1051 Tamura, H., 1561 Tamura, K., 1619 Tanaka, T., 35 Tang, A. P-C., 1081 Taniewska-Osinska, S., 1299 Taniewska-Osinska, S., 513 Tatam, R. P., 439 Tear, S. P., 1022 Tennakoon, D. T. B., 545 Teramoto, M., 1515 Thijs, A., 963 Thomas, J. D. R., 1135 Thomas, J. M., 545 AUTHOR INDEX Thomson, A. J., 2009 Tofield, B. C., 11 17 Townsend, R. P., 1019 Turner, P. S., 747, 759 Tyler, J. W., 1367 van de Ven, T. G. M., 457, 473 Vansant, E. F., 963 Vekavakayanondha, S., 291, 635 Venkatasubramanian, L., 359 Verhaert, I., 963 Veselp, V., 1405 Volkov, A.I., 815 Vonk, D., 1945 Waghorne, W. E., 563 Walker, R. W., 89 Wallwork, S. C., 1589 Walton, A. J., 1023 Wang, Z-C., 375 Warhurst, P. R., 119 Warr, G. G., 1813, 1829 Watts, P., 1389 Weale, K. E., 1020, 2002 Wells, P. B., 189 Whalley, P. D., 1209 Whyman. R., 189 Wiens, B., 247 Wilson, G. S., 1265 Wilson, I. R., 943 Wojcik, D., 1381 Woznicka, J., 1299 Wren, B. W., 167 Wright, K. M., 451 Wu, E. L., 1032 Wuthier, U., 1179 Wysocki, S., 715 Xiaoding, X., 1945 Yamashita, H., 707, 1771 Yamazaki, A., 1553 Yatsimirsky, A. K., 319 Yeates, S. G., 1865 Yeo, I-H., 1081 Yoshida, S., 35, 707, 1771 Yoshikawa, M., 707, 1771 Zana, R., 109 Zanderighi, L., 1795 Ziind, R., 1179NOMENCLATURE A N D S Y M B O L I S M Units and Symbols.The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet ’Quantities, Units, and Symbols‘ (1 975) (copies of this pamphlet and further details can be obtained from t6e Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Chemistry in all its publications. Their basis is the ‘Systbme International d’Unit6s’ (SI). A more detailed treatment of units and symbols with specific application to chemistry is given in the lUPAC Manual of Symbols and Terminology for Ph ysicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn).Nomenclature of Inorganic Chemistry (Butterworths, London, 1971, now publis- hed by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). A complete listing of all IUPAC nomenclature publications appears in the January issues of J.Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society‘s editorial staff. (vii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 82 Dynamics of Molecular Photof ragmentation University of Bristol, 15-1 7 September 1986 Organising Committee: Professor R. N. Dixon (Chairman) Dr G. G. Balint-Kurti Dr M. S. Child Professor R. Donovan Professor J. P. Simons The discussion will focus on the interaction of radiation with small molecules, molecular ions and complexes leading directly or indirectly to their dissociation. Emphasis will be given to contributions which trace the detailed dynamics of the photodissociation process.The aim will be to bring together theory and experiment and thereby stimulate important future work. The programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 21 Promotion in HeterogeneousCatalysis University of Bath, 23-26 September 1986 Organising Committee: Professor F. S. Stone (Chairman) Dr R. Burch Mrs Y. A. Fish Dr R. W. Joyner Professor J. Pritchard Dr D. A. Young (Editor) The symposium will form the Faraday Division Programme at the 1986 Autumn meeting of the Royal Society of Chemistry, however, it will be conducted as a discussion meeting, with pre-printed papers and subsequent publication, following the style of the traditional Faraday discussions and symposia.The role of promoters is of intrinsic interest as well as being important for many industrial processes. Promoters are used for three purposes, to improve catalyst activity, to increase selectivity for the desired reaction, and to prolong catalyst life at high activity and selectivity. There are current advances in both exprimental and theoretical aspects of promoter action, making this an opportune time for a Faraday symposium. Attention will be focussed on the role of promoters in enhancing activity and selectivity. Three areas will be highlighted - model studies using well-defined surfaces such as single crystals, characterization of promoter function in real catalysts, and theoretical aspects of promotion. The mechanisms of promoter action in metal, oxide and sulphide catalysts will be discussed.The final programme and application form may be obtained from: MrsY. A. Fish,TheRoyalSocietyof Chemistry, Burlington House, LondonWlVOBN. (viii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 22 Interaction-induced Spectra in Dense Fluids and Disordered Solids University of Cambridge, 1&11 December 1986 Organising Committee: Professor A. D. Buckingham (Chairman) Dr R. M. Lynden-Bell Dr P. A. Madden Professor E. W. J. Mitchell Whilst interaction-induced spectra have been studied in the gas phase for many years, their importance in the spectroscopy of condensed matter has been appreciated only relatively recently.At present a considerable number of studies of induced spectra are taking place in what are (nominally) widely separated fields of study. It is highly desirable to bring these communities together so that common issues can be identified and the progress of one field appreciated in another. Dr J. Yarwood Dr D. A. Young Mrs Y. A. Fish The preliminary programme may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 83 Brownian Motion University of Cambridge, 7-9 April 1987 Orgenising Committee Dr M. La1 (Chairman) Dr R. Ball Dr E. Dickinson Dr J. S. Higgins Dr P. N. Pusey Dr D.A. Young Mrs Y. A. Fish The aim of the meeting is to discuss new developments in t..3 experimental ant theoretica studies of Brownian motion of colloidal particles and macromolecules, with particular emphus on the dynamics of aggregate formation and breakdown, computer simulation and many-body hydrodynamic interactions. Further information may be obtained from: Dr M. Lal, Unilever Research, Port Sunlight Laboratory, Bebington, Wirral L63 3JWTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 84 Dynamics of Elementary Gas-phase Reactions University of Birmingham, 14-16 September 1987 Organising Committee: Professor R. Grice (Chairman) Or M. S. Child Dr J. N. L. Connor Or M. J. Pilling Professor I. W. M. Smith Professor J. P. Simons The Discussion will focus on the development of experimental and theoretical approaches to 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. 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 1986 to Professor R. Grice, Chemistry Department. University of Manchester, Manchester M13 9PL~ ~~ ~ JOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistrylchemical physics which have appeared recently in J.Chem.Research, The Royal Society of Chemistry's synopsis+microform journal, include the following: Closed-loop Odd-alternant Polycyclic Polyenes Christopher Glidewell and Douglas Lloyd (1 986, Issue 3) The Radical Cation of Dimethyl Sulphate: an Electron Spin Resonance Study Robert Jones and Martyn C.R. Symons (1 986, Issue 3) Analysis of the Kinetics of Two Consecutive First-order Reactions, and of Processes of Related Kinetic Form Roy B. Moodie (1986, Issue 4) Study by a Continuous Kinetic Method of the Rate of Progesterone Adsorption by Silica Gel Julio Casado, Santiago Rincon and Francisco Salvador (1986, Issue 4) Electron Spin Resonance Study of Anatase-supported Vanadia-Molybdena Catalysts Guido Busca and Leonard0 Marchetti (1 986, Issue 5 ) A Comparison of Some Linear Substituent-free-energy Relationships Martien C.Spanjer and C. Leo de Ligny (1986, Issue 5) Interception of the Electron-transport Chain in Bacteria with Hydrophilic Redox Mediators. Part 1. Selective Improvement of the Performance of Biofuel Cells with 2,6-Disulphonated Thionine as Mediator Anna M. Lithgow, Lorraine Romero, lvelisse C. Sanchez, Fernando A. Souto and Carmen A. Vega (1 986, Issue 5 ) Ionic Strength Dependence of Complex-formation Enthalpies: a Literature Data Analysis Alessandro de Robertis, Concetta de Stefano, Carmelo Rigano and Silvia Sammartano FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division with the Societe Franpaise de Chimie, Deutsche Bunsen Gesellschaft fur Ph ysikalische Chemie and Associazione Italiana di Chimica Fisica Dynamics of Molecular Crystals To be held at Grenoble, France on 30 June to 4 July 1986 Further information from Dr C.Troyailowsky, 10 rue Vauquelin, 75005 Paris, France Industrial Physical Chemistry Group Physical Chemistry of Water Soluble Polymers To be held at Girton College, Cambridge on 1-3 July 1986 Further information from Dr I. D. Robb, Unilever Research Laboratory, Port Sunlight, Bebington, Wirral, L63 3JW Division with the Institute of Physics, Institute of Mechanical Engineers, Plastic and Rubber Institute and Institute of Chemical Engineers Tribology in Powder Conveying and Processing: Wear Attrition in Powder Flows To be held at the University of Birmingham on 2 July 1986 Further information from Dr 6.Briscoe, Department of Chemical Engineering, Imperial College, London SW7 2BYGas Kinetics Group and Division de Chimie-Physique de la SocMte Franqaise de Chimie 31 lnternational Symposium on Gas Kinetics To be held in Bordeaux, France on 20-25 July 1986 Further information from Dr R. Lasclaux, Lab. Photophys. Photochim. MolBculaire, Universit6 de Bordeaux I, 33405 Talence Cedex, France Polymer Physics Group Biologically Engineered Polymers To be held at Churchill College, Cambridge on 21-23 July 1986 Further information from Dr M. J. Miles, AFRC Food Research Institute, Colney Lane, Norwich NR4 7UA Polymer Physics Group with the British Rheological Society Deformation in Solid Polymers To be held at the University of Leeds on 9-1 1 September 1986 Further information from Dr J.V. Champion, Department of Physics, City of London Polytechnic, 31 Jewry Street, London EC3N 2EY Colloid and Interface Science Group Surfactant Systems with Liquid-Liquid Interfaces To be held at the University of Hull on 9-10 September 1986 Further information from Dr R. Aveyard, Department of Chemistry, The University, Hull HU6 7RX Statistical Mechanics and Thermodynamics Group Fractals: a New Development in Physical Chemistry To be held at the University of Salford on 1 C-12 September 1986 Further information from Dr P. Francis, Department of Chemistry, The University, Hull HU6 7RX Carbon Group Carbon Fibres-Properties and Applications To be held at the University of Salford on 15-1 7 September 1986 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1 X 8QX Electrochemistry Group with the Electroanalytical Group New Electrode Materials for Electrochemistry and Electroanalytical Applications To be held at Imperial College, London on 15-1 7 September 1986 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College, London SW7 2AZ Division with the Surface Reactivity and Catalysis Group-Autumn Meeting Promotion in Heterogeneous Catalysis To be held at the University of Bath on 23-25 September 1986 Further information from Professor F. S. Stone, School of Chemistry, University of Bath, Bath BA2 7A Industrial Physical Chemistry Group Water Soluble Polymers and their Industrial Application To be held at Girton College, Cambridge on 24-26 September 1986 Further information from Dr I. D. Robb, Unilever Research Laboratory, Port Sunlight, Bebington, Wirral L63 3JW Colloid and Interface Science Group with Macrogroup UK Polymer-Polymer Interfaces To be held at the Scientific Societies Lecture Theatre, London on 15 December 1986 Further information from Dr R. Aveyard, Department of Chemistry, The University, Hull HU6 7RX Colloid and Interface Science Group with the Colloid and Surface Group of the SCI Nucleation and Growth in Colloidal Systems To be held at the Society of Chemical Industry, 14 Belgrave Square, London on 16 December 1986 Further information from Dr R. Aveyard, Department of Chemistry, The University, Hull HU6 7RX (xii)

ISSN:0300-9599    

DOI:10.1039/F198682BP069

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1986, 82, 1673-1680 Structural Relationships in the Reduction of the Vanadia-Molybdena Intermediate Compound Mieczydawa Najbar Institute of Chemistry, Jagiellonian University, 30-060 Krakbw, Karasia 3, Poland The reduction of the powdered V,O,-MOO, intermediate phase (IP) has been investigated by X-ray powder diffraction. The samples were obtained by the fusion of oxides. The reduction was performed at 573 and 723 K in vacuo. The formation of MOO, epitaxial layers and the reorientation in the adjacent IP layers were found to be the main results of low-temperature reduction of IP crystals. The reduction of MOO, epitaxial layers was regarded as an additional result of the vacuum treatment at 723 K. Vanadia-molybdena catalysts, such as those used for selective benzene oxidation, represent labile physico-chemical systems in the conditions of a catalytic reaction.They can easily be reduced or oxidized, depending on the redox potential of the gaseous phase. It has been found that in Mo03-V205 solid solutions1 (SS) as well as in the molybdena-vanadia intermediate phase2 (both being the main components of freshly prepared catalysts), segregation of vanadium and molybdenum occurs in the course of catalyst reduction and oxidation. During the reduction of SS crystals as well as of crystals of the intermediate compound, both kinds of cations diffuse towards the bulk. However, the diffusion of vanadium is faster than that of molybdenum. As a consequence, an intermediate phase with the structure described by Kihlborg4 (see fig.2 and 3 in the Discussion) is formed near the (001) surface of SS crystals during their (V20, indices refer to the setting given in the Powder Diffraction File.)s As shown in ref. (7), vacuum reduction at room temperature of a powdered sample containing 10 mol % MOO, results in the formation of IP epitaxial layers with an orientation of (001) IP 11 (001) V205. The IP epitaxial layers with this orientation do not prevent further redox processes7 as there are some easy diffusion paths in their structures which run in the [OOl] direction and fit those in the V20, structure. On the other hand, vacuum reduction of SS crystals of the sample containing 30 mol % MOO, and composed of SS and IP leads to the formation of IP epitaxial layers with an orientation (100) IP 11 (001) V205 (perpendicular) as well as with an orientation of (001) IP 11 (001) V205 (~arallel).~ The reduced sample containing 30 mol % MOO, showed no oxidation, and this has been ascribed in ref.(7) to the formation of IP surface layers with a perpendicular orientation, thus blocking easy diffusion paths in both SS and IP crystals. On the other hand, prolonged use of the catalyst in a selective benzene oxidation process leads to a considerable loss of molybden~rn,~ suggesting that, besides the IP layers blocking diffusion in the [00 11 direction, layers of volatile molybdenum oxide appeared under the reducing conditions of the reaction. The present study was undertaken to investigate phase changes occurring in the IP crystals during their slow vacuum reduction at a particular temperature below the Tamman temperature as well as during their faster vacuum reduction at temperatures 1673 56-21674 M.Najbar 2810 Fig. 1. X-ray diffraction patterns of: (a) freshly prepared IP; (b) the same sample annealed at 573 K in air for 30 h; (c) sample (b) annealed at 573 K in vacuum for 20 h; ( d ) sample (a) annealed at 723 K in vacuum for 37.5 h (Mo,O,, reflections marked by e). Experiment a1 The catalyst, containing mainly the intermediate molybdena-vanadia phase, was prepared by fusion of analytically pure MOO, and V205. The ratio of the masses of V,05 and MOO, was the same as in the chemical compound V,Mo,O,,, which, in agreement with the Jarman investigations,1° has the structure described by Kihlborg., X-ray analysis of the pure oxides revealed only the presence of V20, and MOO,, both being orthorhombic phases. The mixture was heated at 973 K for 2 h and was subsequently slowly cooled (60" h-l).The X-ray diffraction patterns were obtained using a DRON-3 diffractometer with nickel-filtered Cu-K, radiation (A = 1 S418 A). Powder X-ray diffraction measure- ments were performed for: (a) a freshly prepared sample; (b) the same sample annealed at 573 K in air for 30 h; (c) sample (b) annealed at 573 K in a vacuum for 20 h; ( d ) (a) sample annealed at 723 K in a vacuum for 37.5 h. All the samples were prepared in the same way; they were passed through a 60 pm sieve before each measurement and then shaped by gentle pressing of the powder with a glass plate.Reduction of Vanadia-Molybdena 1675 Table 1.Intensities of the reflections in X-ray diffraction patterns presented in fig. 1 dhkl = 10.48 8, dhkl = 7.26 A IP (200) MOO, (020) v 2 0 5 (200) IP (400) IP (001) v2°5 (Ool) dhkl = 3.94 8, dhkl = 3.63 A MOO, (1 10) IP (110) IP (600) IP (401) v2°5 (110) dhkl = 3.26 A dhkl = 2.910 A v2°5 (400) dhkl = 2.716 A dhkl = 2.667 A IP (111) IP (510) IP (601) dhkl = 2.540 8, VZO, (401) MOO, (060) - 0.5 1 .o 0.2 4.8 1.3 - 100 - 0.5 2.0 0.5 6.7 0.7 0.2 0.7 0.7 0.8 - - - - 0.3 0.9 - 0.4 0.6 0.2 3.4 1 .o - 100 0.5 1.2 0.4 4.8 0.6 0.2 0.6 0.4 0.4 - - - - 0.3 0.8 0.2 0.8 0.4 0.3 1.1 0.2 0.3 0.3 5.6 2.3 1.4 1.4 0.3 0.6 0.2 2.0 1 .o 0.6 0.5 0.3 8.6 3.5 1.2 0.5 0.3 0.2 0.2 0.2 0.7 1 .o 0.1 1.3 0.7 0.6 0.4 0.3 0.3 0.2 1.2 0.8 - - 100 100 - - - - - - - - Results As shown in ref.(3), the powder sample is composed of plate-like crystals with (001) as the most developed face. This indicates that the (001) plane in IP, like the (001) plane in V205,11 is a cleavage plane. The X-ray diffraction patterns for samples (a)-(d) are shown in fig. 1. Relative intensities of the reflections are given in table 1. It is seen that, besides IP reflections, some reflections of orthorhombic M00,,12 and of V,O, are present in all the diagrams. The relative intensities of hOO IP reflections as well as of OkO in MOO, and 001 in V,O, reflections decrease during the annealing of the freshly prepared sample at 573 K in an oxidizing atmosphere [cf. fig. 1 (a) and (b) as well as the intensities given in lines 1 and 2 of table 13.On the contrary, the relative intensities of hOO IP, OkO in MOO, and 001 in V,O, reflections increase during the annealing of the oxidized sample at the same temperature in vacuum [fig. 1 (b) and (c), lines 2 and 3 in table 11. As the relative intensities of hOO IP reflections change in a direction opposite to that expected as a result of thickening (oxidation) or thinning (reduction) of crystals [see ref. (7)], it is necessary to assume that reorientation occurs in some areas of IP crystals during sample annealing. The results suggest that annealing in a reducing atmosphere leads to the formation of reoriented areas with the (001) plane tilted to the main surface of the platelet crystals,1676 M . Najbar while annealing in an oxidizing atmosphere results in a disappearance of the areas with (001) planes tilted with respect to the best developed (001) planes of platelet IP crystals.The same direction of changes of the relative intensities of hOO IP, OkO in MOO, and 001 in V205 reflections shows that the formation of the reoriented areas is connected with the phase segregation. On the other hand, the unification in the orientation of the IP phase is connected with phase homogenization. As phase segregation was observed previously as a result of cation segregation accompanying redox processes,2T 5-7 it is thought that the reorientation in IP crystals [fig. l(c)] occurred also because of cation segregation resulting in the enrichment of surface layers in molybdenum. The ‘ hybrid ’ crystals seemed to be formed as a result of the phase segregation connected with the reorientation in some IP layers.The disappearance of the reoriented areas [fig. l(b)], accompanied by phase homogenization, most probably occurs as a result of homogen- ization in atom distribution in ‘ hybrid’ crystals formed during solidification of the molten oxides. The slower the redox processes, the more homogenization influences phase composition. Thus phase composition of the sample (b) seems to be mainly influenced by an equalization in Mo distribution and not by the oxidation leading to segregation. In the diffraction pattern of the sample reduced at 723 K for 37.5 h [fig. l(d)] an increase of the intensity of the 001 V205 reflection, a decrease of the intensity of the OkO MOO, and hOO IP reflections, as well as the appearance of new weak reflections, was observed.The new reflections of dhkz = 10.48, 7.26, 3.94, 3.63, 3.26, 2.9 1, 2.72, 2.67 and 2.54 A can be ascribed to with some amounts of Mo17047 phase (the reflection with dhkl = 10.48 A may be regarded as a result of overlapping of the reflections of dhkz = 10.3 A of Mo,014 and of dhkl = 10.8 A of Mo17047). Note that there is also a reflection of dh = 3.78 A, present in all the diffraction patterns, as well as a reflection of dhkl = 3.83 W overlapping with that of dhkl= 3.81 A of MOO,, belonging to the set of reflections of Mo,O,,. The presence of Mo,O,, and Mo1704, is not surprising, as these are the only molybdenum oxides which can be obtained by MOO,, reduction at temperatures below 833 K.l3, l4 To explain the observed changes in the intensities of hOO IP, OkO MOO, and 001 V205 reflections, phase segregation, phase homogenization as well as MOO, reduction and MOO, sublimation should be taken into account.Discussion As shown in ref. (7), the easy cation diffusion paths in IP crystals run in the [OOl] and [OlO] directions. Simultaneously, as has been shown in ref. (3), the (001) surface is the best developed face of the IP platelet crystals. This suggests that reduction accompanied by inward diffusion of metal atoms, as well as oxidation accompanied by outward diffusion of metal atoms, occurs mainly on the (001) faces. As shown in ref. (7), storage of the powdered catalysts (containing mainly IP grains) in an oxygen-free atmosphere leads to the formation of layers enriched in molybdenum on the exposed (001) surface of these grains.Such layers block the paths of easy diffusion. A comparison of fig. 1 (b) and (c) suggests that vacuum reduction of the IP sample at 573 K results in some reorientation inside the crystals and in the formation of V20, and MOO, areas. A significant increase of the relative intensities of the 400 and 600 IP reflections, suggests that the orientation of the reoriented areas (r) with respect to the original crystals (0) may be described by the relation (loo),. (1 (OOl),. A significant increase of the intensity of the 020 and 060 reflections of MOO, and the 001 reflection of V205 suggests that the orientation of the MOO, and V205 areas formed with respect to the (001) surface of the original crystals can be described by the relation (010) MOO, 11 (OOl), 1) (001) V205.On the other hand, the reduction of IP crystals results in Mo-enrichment of the surface layers., Therefore MOO, layers are expected to be formed as surface layers of the ‘hybrid’ crystals and V205 as their bulk layers. From ref. (7), during the room temperature reduction of SS the ‘hybrid’ crystals, composed of IP and V203, are formed as a resultReduction of Vanadia-Molybdena 1677 C b MOO, I P t c U 1 a - Ib . MOO, a C Ib Moo3 P a C (b) b Fig. 2. Examples of crystallographic fit between: (a) MOO, (010) and IP (001); (b) MOO, (010) and IP (100). U t L C a T A C Fig. 3. Example of crystallographic fit between IP (100) and IP (001). of a good crystallographic fit between the (001) planes of both phases.Distinct crystallographic relationships between the phases formed during IP reduction showed a possibility of the formation of many-layer ' hybrid' crystals with MOO, in the surface layers. In order to appreciate the possibility of the formation of such crystals and to determine the sequence of the phases in them, the crystallographic fit between the proper planes of these phases was considered, and the misfit was estimated in the way proposed by Vejux and Courtine.ll Fig. 2 and 3 show examples of the crystallographic fit between : (0 10) MOO, and (1 00) IP, (0 10) MOO, and (00 1) IP (fig. 2), (1 00) IP and (00 1)1678 M. Najbar Table 2. Estimation of lattice misfit between MOO, (010) and IP (001) as well as MOO, (010) and IP (100) - - MOO, (010) a, = 3.96 C, = 3.70 IP (001) a, = 3.88 C, = c IP = 3.61 a, = -2.02 - IP (100) a, = c IP = 4.12 c, = b IP = 3.61 ax = 4.04 - a, = -2.43 nool = -4.37 - - a, = -2.43 n,,, = 1.50 Table 3.Estimation of lattice misfit between IP (100) and IP (001) ~ ~~~ (001) a, = 3.88 b, = 3.61 ax = 6.19 - (100) a, = c, = 4.12 b, = b, = 3.61 ag = 0 n,,,,,,, = 6.19 Table 4. Estimation of lattice misfit between MOO, (010) and V,05 (001) - MOO, (010) a, = 3.96 c, = 3.70 a, = 3.0 ‘Z05 (Ool) a, = 3.84 c, = b (V,05) = 3.56 a, = 3.78 no,, = - 6.69 IP (fig. 3). Examples between (001) V205 and (001) IP as well as (100) IP and (001) V205 were given in ref. (7). The estimation of the line (a) and plane (n) misfit between (010) MOO, and (001) IP as well as (010) MOO, and (100) IP is given in table 2, and the misfit between (100) IP and (001) IP is in table 3.In table 4 the misfit between (010) MOO, and (001) V205 is given in order to appreciate the probability of the formation of V,O, in the layer next to MOO,. The relatively low values of misfit given in tables 2 and 3 show that the formation of ‘hybrid’ crystals is possible, according to Ubbelohde’s theory.15 A comparison of the values of misfit between the (010) plane of MOO, and: (100) IP (n = 1.50)’ (001) IP (n = -4.37) and (001) V205 (n = -6.69) suggests that the IP, layer should be next to the MOO, layer. Comparison of the values of misfit between (100) IP and: (001) IP (n = 6.19) and (001) V,05 (n = S.86)7 suggests that the IP layer should be adjacent to IP,. As, additionally, the value of misfit between (001) IP and (001) V205 is very low (n = 2.64),’ the following phase sequence: (010) MOO, 11 (100) IP, 1) (001) IP 11 (001) V205 11 (001) IP may be expected in ‘hybrid’ crystals.MOO, layers should also be formed during solid solution reduction (if it is strong enough) as a result of further segregation occurring in the parallel IP layers. The intensities of the OM) MOO, reflections in the diffraction patterns of the reduced samples containing 30 mol % MOO, [ref. (7)] are too low to suggest MOO, formation in the surface layers of the ‘hybrid’ crystals. Therefore IP, layers are considered as responsible for the high resistance to oxidation of the samples. As the paths of easy-diffusion in MOO, epitaxial layer are parallel to the surface of plate-like ‘hybrid’ crystals, this layer can also play a role as a protective layer of the crystal.Reduction of Vanadia-Molybdena 1679 As shown in ref.(7), the phase segregation accompanying redox processes occurs through cation diffusion along the channels. Simultaneously, phase homogenization in the ‘hybrid’ crystals, possessing layers with a perpendicular orientation, can only occur via ‘ non-channel’ cation diffusion. Therefore it seems that ‘channel ’ diffusion plays a predominant role in the evolution of the IP sample treated at 573 K in a vacuum and ‘non-channel’ in the evolution of the sample treated at this temperature in air. To discuss the decrease of the intensities of OkO MOO, and hOO IP reflections, as well as the increase of the intensity of the 001 V205 reflection and the appearance of Mo,O,4 reflections during the vacuum treatment of the sample at 723 K for 37.5 h, the change in intensity of the 001 V,05 reflection was taken as a measure of the phase segregation. The increase in the intensity of 001 V20, [cf.fig. 1 (a) and (41 suggests that an additional phase segregation take place in the course of the vacuum treatment of the sample at 723 K. The extent of this segregation is, however, much smaller than of that occurring in the oxidized sample during its vacuum treatment at a lower temperature (573 K) for a shorter period of time (20 h). This suggests that the rate of homogenization increases with temperature more rapidly than the rate of segregation, thus the participation of the ‘ non-channel ’ diffusion in the evolution of the sample increases with temperature. A similar suggestion can be made on the basis of the results presented in ref. (7) showing that, at room temperature, ‘channel’ cation diffusion is the only possible diffusion in the V205-Mo0, system.The decrease of OkO MOO, intensities with simultaneous increase . of 001 V205 intensity may be explained by the reduction of the MOO, epitaxial layer (formed in the earlier stages of the vacuum treatment) to M05014. The participation of the sublimation of MOO, can be excluded because the Mo-content in the sample does not decrease during the experiment (we have shown this by atomic absorption spectrophotometry). A decrease in the intensity of the hOO IP reflections suggests that reduction of the MOO, epitaxial layer results in the return of the IP, layer to the initial orientation. The reduction of the MOO, layer may be considered as occurring either by ‘channel’ cation diffusion in the directions parallel to the surface of plate-like ‘hybrid’ crystals or by ‘non-channel’ diffusion in the [OOl], direction. On the basis of the above discussion, the considerable participation of ‘ non-channel ’ diffusion is postulated.Any suggestion concerning Mo5014 preferential orientation cannot be given from the intensities of the reflections. Summary Changes in powdered samples of the vanadia-molybdena intermediate phase (being one of the main components of V,O,-MoO, catalyst for selective benzene oxidation) during their reduction have been described. The powdered samples of the intermediate phase are composed of plate-like crystals with a well developed (001) face3 and with easy diffusion paths perpendicular to this face.’ Thus, the reduction processes tend to occur on this face.From X-ray diffraction measurements we have suggested that two types of cation diffusion play an important role in the evolution of crystals of the intermediate phase during their vacuum treatment: interstitial diffusion along the channels in the [OOl] and [OlO] directions and ‘ non-channel’ diffusion. At temperatures lower than the Tamman temperature, ‘channel ’ diffusion, resulting in phase segregation and ‘hybrid ’ crystal formation, is predominant. At temperatures higher than the Tamman temperature, both types of diffusion are appreciable. The segregation, suggested to occur by cation diffusion along the channels, is probably a basic result of the initial vacuum treatment, but after the formation of a protective layer of MOO, in the surface layer of the ‘hybrid’ crystals, phase homogenization, caused by ‘ non-channel ’ diffusion, should prevail.I thank the Polish Academy of Science for support.1680 M. Najbar References 1 A. Bielanski, J. Camra and M. Najbar, J. Catal., 1979, 57, 326. 2 M. Najbar, E. Bielanska, J. Camra and S. Niziol, Proc. VZ Znt. Symp. Heterogen. Catal., 1979, 1, 445. 3 M. Najbar, Proc. 8th Int. Congr. Catal., 1984, 5, 323. 4 H. A. Eick and L. Kihlborg, Acta Chem. Scand., 1966,20, 1658. 5 M. Najbar and S. Niziol, J. Solid State Chem., 1978, 26, 339. 6 M. Najbar and E. Bielanska, Proc. IX Int. Symp. React. Solids, 1980, 486. 7 M. Najbar and K. Stadnicka, J. Chem. SOC., Faraday Trans. 1, 1983,79, 27. 8 Powder Difraction File (Joint Committee on Powder Diffraction Standards, Philadelphia, Pennsylvania) 9 A. Bielanski, M. Najbar, J. Chrzqszcz and W. Wal, Studies in Surface Science and Catalysis, ed. 19 103, 9387. B. Delmon and G. F. Fromet (Elsevier, Amsterdam, 1980), vol. 6, p. 127. 10 R. H. Jarman, P. G. Dickens and J. Jacobson, Muter. Res. Bull., 1982, 17, 325. 11 A. Vejux and P. Courtine, J. Solid State Chem., 1978, 23, 93. 12 L. Kilhborg, Ark. Kemi, 1963, 21, 357. 13 L. Kilhborg, Ark. Kemi, 1963, 21, 427. 14 L. Kihlborg, Acta Chem. Scand., 1960, 14, 1612; 1963, 17, 1485. 15 A. R. Ubbelohde, Trans. Faraday SOC., 1937,33, 1198. Paper 51244; Received 11th February, 1985

ISSN:0300-9599    

DOI:10.1039/F19868201673

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I, 1986, 82, 1681-1686 HydrogenatIan of Carbon Dioxide and Carbon Monoxide over Supported Platinum Catalysts Takashi Inoue and Tokio Iizuka" Department of Chemistry, Faculty of Science, Hokkaido Uniuersity, Sapporo 060, Japan The production of methanol from reactions between CO, + H, and CO + H, has been studied over platinum catalysts supported on Nb,O,, ZrO,, MgO, SiO, and TiO,. Zirconia- and niobia-supported Pt catalysts showed the highest activity for reactions of both CO, and CO. The magnesia-supported Pt catalyst was less active compared with Pt/ZrO, and Pt/Nb,O,, but exhibited the highest selectivity towards MeOH formation. The activation energy of the CO, + H, reaction was always lower than that of the CO reac- tion. Thus, the reaction of CO, + H, occurs easily at lower temperature com- pared with the CO + H, reaction, but the rate of CO hydrogenation exceeds that of CO, hydrogenation at higher temperatures.The selectivity towards methanol was higher for CO, hydrogenation than that for the CO+H, reaction over all the catalysts used. The hydrogenation reactions of CO over transition-metal catalysts have been extensively studied.l Most chemical studies have concentrated on maximizing the yields and optimizing the selectivity for the following main reactions : methanation,, methanol synthesis3 and the synthesis of higher molecular-weight hydrocarbon^.^ On the other hand, studies in the hydrogenation of C0,5-10 have been much less extensive than those of CO hydrogenation. In particular, for the synthesis of methanol from CO,, only a few papers have been published so far.l-14 This paper reports a study of the reaction of CO, + H, to form methanol and hydrocarbons in comparison to the hydrogenation of CO, using supported platinum as a catalyst.Experiment a1 Supported platinum catalysts were prepared by impregnating Nb205, ZrO,, TiO, or SiO, with an aqueous solution of hexachloroplatinic acid. For the preparation of Pt/MgO, a methanol solution of hexachloroplatinic acid was used to avoid the partial dissolution of MgO in water. The Pt content was 2.0 wt% on all the catalysts. Zirconium oxide was prepared by the hydrolysis of ZrOC1, with aqueous ammonia, followed by calcination at 500 "C for 2 h. Silica (a reference catalyst of the Catalysis Society of Japan, JRC-SiO- 1) and TiO,(JCR-TiD-1) were used after the calcination at 500 "C for 2 h.Nb,O, was used after washing Nb,05 .nH,O (CBMM-AD-108) with distilled water several times and drying at 100 "C. Magnesium oxide was prepared by the hydrolysis of Mg(NO,), with an aqueous ammonia solution followed by calcinating at 500 "C for 2 h. The catalysts were reduced at 400 "C for 2 h in a stream of H, before use. The reactions were carried out by using a flow reactor under 10 atm pressure. A premixed gas comprising CO, + H, (1 / l or 1/3) or CO + H, (1 / 1) (Nippon Sanso Co.) was used without purification. The flow rate of CO, (or CO) was 50 mmol h-l as a standard experimental condition. The products were analysed by f.i.d. and t.c.d gas. chromatographs. Adsorption experiments of CO to obtain the metal surface area were carried out by a conventional B.E.T.apparatus at room temperature. The adsorption uptake was determined as a function 16811682 7 1.0- Y 4- M I G 0 E '-I - Hydrogenation of CO, and CO over Pt n process time/min Fig. 1. Change in rate of CO, hydrogenation with time over Pt/Nb,O, ar 240 "C: 0, MeOH; 0, CH,. of pressure and the linear portion of the isotherm was extrapolated to zero pressure to obtain the amount chemisorbed. Results and Discussion The rate change for hydrocarbon and alcohol production in the course of CO, hydrogenation over Pt/Nb,O, is shown in fig. 1. In the initial stage of reaction, methane was formed predominantly. However, the formation of methane decreased and the selectivity to methanol formation increased gradually. Along with the formation of methanol, dimethyl ether formation was also observed.Since the Nb,O, support has acidic properties,15 the formation of dimethyl ether can be ascribed to the dehydration of methanol molecules produced on the oxide support. The activity and selectivity are summarized along with the metal dispersion in table 1. Pt/ZrO, and Pt/SiO, showed higher metal dispersions than the others. Although Pt/ZrO, exhibited the highest activity for CO and CO, reactions, there is nothing to choose between the activities of Pt/ZrO, and Pt/Nb,O, on the basis of metal dispersion because of the low dispersion on Nb,O,. The selectivity of Pt/ZrO, to methanol was 70% at 200-240 "C, but it decreased considerably at higher temperatures.In fig. 2 the effect of space velocity on the selectivity is shown. At lower space velocities, methanol selectivity decreased slightly. However, the selectivity change with space velocity was far less compared with the case of the CO, + H, reaction over Re/Zr0,.14 Thus, over Pt catalysts, although the possibility that some methane is formed via methanol cannot be ruled out, methane and methanol will mainly form via independent routes. Pt/Nb,O, and Pt/TiO, showed almost the same selectivity to methanol plus dimethyl ether (> 60%) at 240 "C. Pt/MgO had the highest selectivity to methanol, although the activity was least. Pt/SiO, showed a high selectivity to methanol at 260 "C, but the selectivity decreased drastically at higher temperatures and methane and higher hydrocarbons were formed.The results of CO hydrogenation are summarized in table 2. In CO hydrogenation over Pt catalysts, methanol formation with a high selectivity is known to occur.16*17 Pt/ZrO, and Pt/Nb,O, also showed the highest activity in this reaction, but the selectivity to methanol was lower than in CO, hydrogenation. In this reaction the selectivity to higher hydrocarbons increased in comparison with the case of CO,Table 1. CO, hydrogenation over supported Pt catalystsa rate of hydrocarbon reaction and alcohol catalysts temperature formation (dispersion)b /"C /mmol h-l g-cat-] turnover frequency 10-4 s-1 selectivity (molx ) CH, MeOH C, G+ Me20 Pt/Nb,O, (0.24) 240 235 260 Pt/MgO (0.16) 260 300 Pt/SiO, (0.54) 260 300 240 260 Pt/ZrO, (0.55) 200 Pt/TiO, (0.46) 200 0.06 0.05 0.14 0.15 0.03 0.04 0.01 0.12 0.04 0.06 0.08 6.7 2.5 6.9 7.5 5.0 6.7 0.6 6.1 2.5 3.6 4.7 37.1 24.3 48.8 50.5 21.5 32.7 44.8 81.8 26.8 33.7 43.5 56.2 71.7 48.8 47.3 78.5 66.9 52.7 4.5 72.9 65.8 55.6 2.9 0.4 3.4 0.8 0.2 0.9 0.1 0.8 0.1 0.4 0.1 1.3 0.6 8.5 5.0 0.3 trace trace 0.2 trace 0.3 0.3 0.1 0.3 - - - trace trace - - - - rate of CO formation /mmol h-l g-cat-l .y 3 2.34 0 0.64 8 3.25 2 2.72 Q 2.39 3 3 hl 5.36 3.41 10.10 E 2.52 2.62 3.28 ~ a CO,/H, = 1/1.Calculated from the amount of CO adsorption.1684 Hydrogenation of CO, and CO over Pt loot d,,, 3600 1200 2400 space velocity/cm3 h-l g-' Fig. 2. Effect of space velocity on selectivity over Pt/ZrO, at 240 "C: 0, MeOH; 0, CH,. I I 1 I I I I I I I 1 I I 1 1.8 1.9 2.0 2.1 1.8 1.9 2.0 2.1 lo3 KIT Fig.3. Arrhenius plots of (a) CO, and (b) CO hydrogenations. Activation energies are given in parentheses (kJ mol-I). (a) A. Pt/ZrO, (40.1); 0, Pt/TiO, (22.6) and ., Pt/MgO (41.6); (b) A, Pt/ZrO, (107.8); 0, Pt/TiO, (87.4) and ., Pt/MgO (95.6). hydrogenation. In particular, Pt/Nb,O, tends to stimulate the propagation of C-C bonds towards higher hydrocarbons. This higher propagation ability in the CO reaction is a marked difference between the CO and CO, hydrogenation reactions over all the catalysts. This will be due to the higher concentration of adsorbed CO species on the surface in the CO reaction. In the case of CO, hydrogenation over Pt/SiO, at 300 "C, the propagation towards higher hydrocarbons was faster than on the other catalysts.This is also due to the higher production rate of CO from the CO, + H, reaction over Pt/SiO, at 300 "C. The formation of ethanol was observed in CO hydrogenation over all the catalysts, in contrast to the case of CO, hydrogenation. Arrhenius plots of the reactions are shown in fig. 3. The activation energy of CO, hydrogenation was always lower than that of the CO reaction. Thus the reaction of CO, + H, towards methanol and hydrocarbons occurs easily at lower temperatures compared with the cases of CO reaction, but at a higher reaction temperature the rates of CO hydrogenation exceed those of the reactions of CO,. In previous studies the higherTable 2. CO hydrogenation over supported Pt catalystsa rate of hydrocarbon reaction and alcohol turnover selectivity (molx) rate of CO, catalysts temperature formation frequency formation 3 /mmol h-l g-cat-l /lO-4 s-l CH4 MeOH C, C, C,, Me20 EtOH /mmol h-' g cat-l 3 0 (dispersion)b /"C Pt/Nb20, (0.24) 240 240 260 Pt/MgO (0.16) 260 280 300 240 260 Pt/Zr02 (0.55) 200 Pt/Ti02 (0.46) 200 0.13 0.03 0.30 0.63 0.06 0.20 0.32 0.04 0.15 0.33 14.7 14.7 31.1 10.0 33.9 54.2 1.39 2.22 8.89 19.4 64.1 12.2 10.1 33.0 62.8 2.2 42.8 48.0 4.5 54.0 32.0 7.1 16.7 78.6 1.1 30.1 57.7 3.5 47.9 33.4 7.6 25.8 72.2 0.6 51.0 44.8 2.5 56.3 3.9 2.6 5.3 3.2 3.6 - - - 2.1 0.5 - 3.1 1.6 - 1.3 0.8 - 3.5 2.9 - 5.3 3.7 - 0.2 0.1 0.2 trace 1.2 0.3 trace 1.2 - 1.3 1.4 1.1 1.1 2.4 2.1 1.1 0.9 0.6 - E 0.04 0.01 3 0.1 1 0.47 3 0.14 3 2 0.29 ?? 0.47 $a 0.04 0.12 0.30 $a a a CO/H2 = 1 / 1.Calculated from the amount of CO adsorption.1686 Hydrogenation of CO, and CO over Pt reactivity of CO, towards H, compared with that of CO was emphasized and the reason for the higher reactivity of CO, was ascribed to the low CO coverage on the surface in CO, + H, reaction.g* lo In the CO + H, reaction, CO strongly absorbs on the metal surface and even acts as a poison for hydrogenation.This situation will occur even on a Pt catalyst surface; thus CO, reacts easily with H, at lower temperatures. In the reaction of CO, + H,, the product mainly comprises C, compounds, in contrast to the formation of higher hydrocarbons and alcohols in the CO+H, reaction. This is also due to the low probability of C-C propagation in the lower surface coverage of CO in the CO, reaction, because CO formed from CO, might be surrounded by hydrogen atoms on the surface and be easily hydrogenated before propagation.We thank Professor Tanabe for helpful discussions. References 1 C. K. Rofer-Depoorter, Chem. Rev., 1981, 81, 447. 2 G. A. Mills and F. W. Steffgen, Catal. Rev., 1973, 8, 159. 3 G. Batta, in Catalysis, ed. P. H. Emmett (Reinhold, New York, 1955), vol. 3. 4 R. B. Anderson, Catalysis, ed. P. H. Emmett (Reinhold, New York, 1956), vol. 4. 5 V. M. Vlasenco and G. E. Yusefovich, Russ. Chem. Rev., 1969, 38, 728. 6 B. A. Sexton and G. A. Somorjai, J. Catal., 1977, 46, 167. 7 F. Solymosi and A. Erdohelyi, J. Mol. Catal., 1980, 8, 471. 8 F. Solymosi, A. Erdohelyi and T. Bansagi, J. Catal., 1981, 68, 371. 9 T. Iizuka, Y. Tanaka and K. Tanabe, J. Mol. Catal., 1982, 17, 381. 10 T. Iizuka, Y. Tanaka and K. Tanabe, J. Catal., 1982,76, 1 . 1 1 R. Bardet, J. Cazat and Y. Trambouze, J. Chim. Phys., 1981,78, 135; Y. Ogino and M. Tani, Nippon 12 E. Ramaroson, R. Kieffer and A. Kienneman, J. Chem. SOC., Chem. Commun., 1982, 645. 13 B. Denise, R. P. A. Sneeden and C. Hamon, J. Mol. Catal., 1982, 17, 359. 14 T. Iizuka, Proc. 8th Int. Congr. Catal. (Verlag-Chemie, Weinheim, 1984), vol. 11, p. 221. 15 T. Iizuka, K. Osasawara and K. Tanabe, Bull. Chem. SOC. Jpn, 1983,56,2927. 16 M. L. Poutsma, L. F. Elek, P. A. Ibarbia, A. P. Risch and J. A. Rabo, J. Catal., 1978,52, 157. 17 M. Ichikawa and K. Shikakura, Proc. 7th Znt. Congr. Catal. (Kodansha-Elsevier, Tokyo, 1981), p. 925. Kagaku Kaishi, 1975, 1878. Paper 51528; Received 28th March, 1986

ISSN:0300-9599    

DOI:10.1039/F19868201681

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. I , 1986,82, 1687-1701 Mechanism of n-Alkane Transformations over a Solid Superacid of Lewis Character, A1,03/AlC13 Marek Marczewski Chemistry Department, Warsaw Technical University (Politechnika), Koszykowa 75, 00 662 Warsaw, Poland In the presence of Al,O,/AlCl, superacid catalysts light alkanes undergo low-temperature isomerization and decomposition. Isomerization is initiated by attack of a superacid Lewis centre (an AP+ cation with a pronounced deficit of electrons caused by the adsorption of AlCl, on an adjacent oxygen anion) on the electrons of an alkane C-H bond. The mechanism of n-alkane decomposition depends on the length of the carbon chain. n-Butane decomposition is catalysed by superacid Lewis centres and proceeds via dimer C,-transition-state cracking.In n-pentane and n-hexane decomposition dimers are not present in any reaction step and these reactions are catalysed by a different type of active centre. These sites (surface-bonded carbenium cations) are formed as a result of superacid Lewis centre attack on the C-C bonds of the alkane. Solid superacids are the most interesting and promising acid catalysts. Owing to their high acid strength, higher than 100% H2S04, they are able to catalyse the transformations of light alkanes at temperatures as low as 298 K.1-3 The mechanism of these reactions is not yet fully understood. In the case of protonic superacids such as Amberly~t/AlCl,,~, Al2O3/Et2C1Al - HC1,6 or graphite/SbF, - HF,' it is believed that reaction initiation step is the same as for superacid so1utions:l The carbonium cation produced in reaction (1) is unstable and decomposes to form hydrogen and the more stable carbenium cation R+. The latter can either isomerize or lose a proton8 to form an olefin.An alkene R, may be alkylated by a carbenium cation R+ to form a dimeric RR+ cation. The subsequent reactions of RR+ (i.e. /?-scission and hydrogen transfer) lead to the final reaction products8 In the case of aprotic Lewis superacids such as A1203/Si02/SbF5,2~ A120,/CCl,,10 or Al2O3/A1Cl3,l1 it is assumed that H- abstraction initiates the reaction R-H + L + S R++ L-H. (2) The carbenium cation R+ can undergo all the previously mentioned transformations. For the n-pentane reaction, the possibility of isobutane formation rather than dimer cracking has also been considered.Tanabe12 proposed that isopentane formed during the reaction decomposes with the evolution of methyl and methylene groups. These groups recombine to form isobutane. Marczewskil3 suggested that isobutane results the reaction of pentane with centres which are carbenium cations bonded to superacid Lewis sites : C5H12 + L-R+ -+ iC,H,, + L-RCH;. (3.) 16871688 Alkane Transformations over Superacids The growth of the surface carbenium cation chain resulting from reaction (3) is responsible for catalyst deactivation. Recently Commeyrasl4 proposed an entirely new approach to the mechanism of alkane reactions in the presence of superacids. He demonstrated that, depending on the acid strength of superacid solution, the reaction can have an ionic character [eqn (l)] or a radical one : HR+A -+ HR'++e-A (4) RFC where A is an electron acceptor HR'+ -, H+ + R'.-e- -H+ The radicals formed during reaction (5) can dimerize or disproportionate : 2R' e R= + RH. Either the dimer RR or the olefin R, can undergo the secondary reactions already described. All the above mechanisms of n-alkane transformations can be summarized in the following scheme : (6) * RR' R+ I R= I p-scission I R,+ RH I coke + RIH + R,H 1111 where: BSC is a Brarnsted superacid centre; LSC is a Lewis superacid centre; RFC is a radical forming centre; HT is a hydrogen transfer reaction. For n-butane RH = C4H10, R,H = C,H,, R2H = C5H12, iRH = iC4Hlo; for n-pentane RH = C5H12, R,H = iC4Hl,, R2H = iC6H14, iRH = iC,H12; and for n-hexane RH = C6H14, iRH = iC6H14, and R,H and R2H are products of C6H14 cracking.On the basis of the reaction scheme presented one can conclude that irrespective of the mechanism (i.e. ionic or radical) n-alkane transformations proceed via dimeric intermediates. Moreover the products of the reactions catalysed by Lewis, Brarnsted or radical-forming centres are identical. Hence, on the basis of knowledge of the reaction products, it is impossible to infer the mechanism of the alkane reaction initiation. In our previous papersll? l3 it was shown that the Al2O,/A1Cl, system obtained by AlCl, sublimation through alumina is a solid superacid of Lewis character. The aim of the present work was to study the mechanism of both initiation and transformation ofM . Marczewski 1689 light alkanes (c&) in the presence of this catalyst.To this end, the reactions of these alkanes with organic Lewis acids (carbenium cations)15 (model Lewis centres) as well as the reactions of longer hydrocarbons (possible dimer intermediates) were studied. Experiment a1 Alumina (grains of 0.5-1.0 mm, specific surface area 170 m2 g-l) was obtained by calcination of aluminium hydroxide in a flow of air at 823 K. The hydroxide was prepared by hydrolysis of aluminium isopropoxide (POCh, Gliwice). X-Ray examination showed that the Al,O, obtained was in the y form. Introduction of AlCl, onto the alumina was performed in the following manner. A known quantity of Al,O, (1 g) was first calcined at 823 K in vacuo (1.3 x lo-, N m-2) for 3 h. On cooling, the sample was transferred under dry, deoxidized nitrogen into a new container with aluminium trichloride.The container was heated to 573 K and evacuated to 1.3 N m-2. Under such conditions AlCl, sublimed through the A120, bed. In order to prevent the physical adsorption of dopant, heating and evacuating were continued for 1 h. Carbenium cations were generated in situ by introduction of alkyl chlorides into the reacting system. According to Olah16 Lewis type superacids transform alkyl chlorides into the corresponding carbenium cations. Reactions The catalyst, prepared as described above, was transferred into the reactor and used without additional pretreatment. Reactions were carried out using a 60 cm3 static reactor under the initial pressure of reagent vapours being 26.7 x lo3 N m-2 (200 Torr).In some experiments vapours of methyl, ethyl or propyl chlorides (8.0 x lo3 N m-2) were introduced with the alkanes into the reactor. The influence of pent- 1 -ene and ethene on n-pentane transformations was also studied. Reactions were carried out at 333 K using mixtures of pentane and pent-1-ene (molar ratio 10: 1) and of pentane and ethene (molar ratio 5 : 1) at an initial pressure equal to 26.7 x lo3 N m+ (200 Torr). The analyses of reaction products were made with a gas chromatograph fitted with a squalane capillary column (50 m). Results The results of n-butane reactions at temperatures of 333 and 473 K are presented in fig. 1. n-Butane isomerizes with 100% selectivity at 333 K. When the reaction takes place at a higher temperature (473 K) one can also observe small amounts of propane and pentane.The propane:pentane molar ratio observed during the reaction was close to unity for low conversion. The results of n-pentane reaction at 333 K are presented in fig. 2(a). The reaction products were isobutane, isopentane and isomeric hexanes. As it is seen from fig. 2, isobutane, the main product of the pentane transformation, forms after an induction period in parallel to the isomerization reaction. An increase of reaction temperature to 473 K causes the appearance of propane in the reactant mixture [fig. 2(b)]. The results of n-hexane reactions at 333 K are shown in fig. 3(a). In the presence of the Al,O,/AlCl, catalyst, n-hexane reacts with the formation of isobutane, isopentane and isohexanes.The isobutane, isopentane and isohexanes are formed in parallel reactions. Similarly, as in the case of n-pentane, the products of alkane decomposition (ie. isobutane and isopentane) appear in the reactant mixture after an induction period.1690 Alkane Transformations over Superacids - 40 E 20 G .C( c 00 20 40 60 80 total conversion (%) Fig. 1. Results of n-butane reaction over Al,O,/AlCl, catalyst at (A) 333 and (0) 473 K. g 40 g 20 E .- 8 20 40 60 8 0 total conversion (%) E .- g 20 8 20 40 60 8 0 total conversion (%) Fig. 2. Results of n-pentane reaction over A1,03/A1C1, catalyst at (a) 333 and (b) 473 K. 20 40 60 20 40 60 80 total conversion (%) total conversion (%) Fig. 3. Results of n-hexane reaction over Al,O,/AlCl, catalyst at (a) 333 and (b) 473 K.An increase of reaction temperature to 473 K causes the formation of new products propane, n-butane and n-pentane [fig. 3(b)]. The above results indicate that the change of reaction temperature from 333 to 473 K does not affect the reaction scheme. The main difference is that at higher temperature alkane cracking (propane formation from pentane and hexane) becomes visible. The influence of alkyl chlorides (CH,Cl, C,H,Cl and C,H,Cl) on different n-alkanes (from C, to Clo) was examined. A high reaction temperature 473 K was chosen to achieveM. Marczewski 1691 Table 1. The influence of alkyl chlorides on n-alkane transformationsa products (molx) reagent CH, C,H6 C,H8 nC,H,, iC,Hl0 iC5H,, nCsHlz iC6Hl, nC6H1, nC& nC,Hl8 - C4HlO C4H10 +I 0.9 C5HU 0.2 C5H12+1 0.7 C,H,,+II 0.5 C5H,, + I11 0.1 C6H14 0.1 C6H14 +I 0.5 C,H,,+II 0.2 C7H16 0.1 C7H16 +I 0.4 C8Hl8 0.1 C8Hl8 + 1 0.3 - 1.0 68.9 28.9 1.0 0.3 - - - 7.3 39.5 42.6 5.0 1.5 0.8 - - 6.7 0.3 6.9 10.1 80.2 1.4 - - 4.7 3.1 25.3 16.0 47.7 2.6 - - 3.5 1.8 20.9 11.9 58.5 1.4 - - 13.3 4.2 36.3 13.6 30.4 2.0 - - 4.1 1.0 21.0 9.8 0.9 13.9 49.2 - 5.1 1.4 25.4 10.7 1.2 9.6 46.1 - 13.8 2.6 34.3 14.2 1.5 9.1 23.4 - 15.2 1.7 43.1 10.1 0.7 1.7 0.1 - 17.3 2.3 47.4 11.2 0.9 1.1 0.2 - 4.8 3.2 64.4 13.5 1.1 1.2 0.1 - 4.6 3.3 66.6 14.9 1.1 1.5 0.1 27.3 - 17.3 - 4.5 5.9 - - a Reaction temperature 473 K, alkane initial pressure 26.7 x lo3 N rnb2 (200 Torr), alkyl chloride initial pressure 8.0 x lo3 N m-2 (60 Torr), 1 g of catalyst, reaction time 60 min.Abbreviations: CH,Cl = I, C2H,C1= 11, C3H,Cl= 111.n E 60 40 20 40 60 reaction time/min Fig. 4. Changes in the selectivity of isobutane formation: (a) from pentane and (b) from pentane-thene us. reaction time. conditions such that all substrates were in the gas phase. The results of both the n-alkane and the n-alkane/alkyl chloride reactions are gathered in table 1. The influence of olefins on the transformations of the alkanes was studied using pent-1-ene and ethene as a coreagents with n-pentane. Under the reaction conditions examined (333 K) the pent-1-ene was totally adsorbed and poisoned the superacid centres of the catalyst with the result that the n-pentane reaction was almost completely stopped (the total pentane conversion was less than 2 % ). The adsorbed pent-1 -ene did not react. On the other hand, ethene did not completely adsorb, it being observed in the gas phase even after 1500 min of reaction.The loss of catalyst activity under the influence of ethene was less pronounced than that caused by pent-1-ene adsorption. It was possible to observe changes in the selectivity of the n-pentane reaction. This is shown in fig. 4.1692 Alkane Transformations over Superacids Discussion Initiation of n-Alkane Reactions The catalyst obtained by AlCl,, adsorption on an alumina surface possesses superacid properties.ll7 l3 It was shown that AlCl, reacts with both surface hydroxyl groups and exposed oxygen anions of one-electron donor character. The i.r. study of adsorbed ammonia as well as e.s.r. examinations of adsorbed one-electron donor molecules (perylene) eliminated the possibility of the existence of Bransted acid centres and to link superacid properties with electron acceptor sites.These superacid centres are surface A13+ cations with a pronounced deficit of electrons caused by the adsorption of AlCl, on adjacent oxygen l3 AlC13 t 0 I -Al-O- Al- superacid centre (L+). The superacid centre attacks a molecule of adsorbed n-alkane, transferring it into corresponding carbenium cation. There is no detailed mechanism for the interaction of Lewis acid centres with alkanes. It seems sensible to use the mechanism of the reaction of alkanes with carbenium cations, the latter often being considered as organic analogues of Lewis acids.15 For example, methyl and ethyl cations react with butane in the following way? R+ + CH3CH2CH2CH3 t CH3-CH -CH2 CHJ I I R-H + CH3bHCH2CH3 I CH3CH(R)CH2CH3 + H+ (7) As a result of carbenium cation (Lewis acid) attack on the alkane C-H bond, a pentacoordinated transition complex is formed.It is not stable and decomposes, forming the product of a hydride anion to carbenium cation addition (CH,, C,H,), products of carbenium cation to alkane addition (2-methylbutane, 3-methylbutane) and product of H- abstraction of from alkane (butyl cation) which can react further. The Lewis acid attack can be directed not only on the C-H bonds but also on the C-C bonds of the reacting alkane.', In order to study the mechanism of the action of the Lewis centre on n-alkanes, the reactions of butane, pentane and hexane in the presence of alkyl chlorides were performed.When treated with solid Lewis superacids, alkyl chlorides react to form carbenium cations in situ. The transformations of butane, pentane and hexane caused by the presence of alkyl carbenium ions in the reacting system are shown in fig. 5. For any alkyl chloride usedM. Marczewski 1693 4 0 20 n 5 20 10 40 20 6o I 40 20 20 40 60 80 total conversion (%) Fig. 5. The influence of alkyl chlorides on the transformations of (a) n-butane, (b) n-pentane and (c) n-hexane: 0, CH3C1; 0, C,H,Cl; A, C3H,C1 and x , pure alkane. one can observe similar changes in the alkane reactions: an increase of overall reaction yield, accompanied by a decrease of isomerization selectivity and an increase of the selectivity towards products having a different number of carbon atoms from the substrate.This indicates that carbenium cations change not only the reaction rate, but also the direction of alkane transformations. The introduction of the CH: cation into the n-butane catalyst system caused an increase in conversion of the substrate to isobutane, propane and isopentane as well as the formation of methane-a new product (table 1). These results can be explained by the following reaction sequence:1694 Alkane Transformations over Superacids attack on C-H bond CH3,, >--CH2CH,CH,CH3 1 HO’ r - CH4 ++CH2CHZCH2CH3 7 CH3CH2CH2CH2CH3 + H+ l+ C H ; + C ~ H ~ ~ + L J + (8) ‘ CH3CH(CH3)CH2CH3 + H+ CH3 attack on C-C bond Methane and a butyl cation or a proton and the pentanes are thus the products of CHZ attack on the C-H bonds of n-butane.When a carbenium cation attacks the C-C bonds, ethane and a propyl cation or propane and an ethyl cation should be formed. All these hydrocarbons except ethane were found among the products of the butane-CH: reaction. The ethyl cation, the precursor to ethane, is very active and undergoes a secondary reaction, i.e. alkane alkylation : + CH2CH2CI-Id 1 + \ ’ H++ i-C6H14 . As a result of such a reaction, propane and isohexanes can be formed. Both products were observed in the butane-CH: reaction mixture. An additional proof of the possibility of the occurrence of the reaction is the result of the pentane-C,Ht transformation. As is seen from table 1, ethane was not formed, hence the ethyl cation had to react according to eqn (10).M. Marczewski 1695 In conclusion one may say that organic Lewis acid may attack the electrons of both C-H bonds (reaction products: CH,, C5H12, C,Ht, C,H,,) and C-C bonds (reaction products: C,H,).If n-pentane is used instead of n-butane then one can expect the formation of methane, hexanes and pentyl cation as a result of reaction (8) as well as that of ethane, propane and both propyl and butyl cations from transformation (9). The data gathered in table 1 indicate that reaction between pentane and the methyl cation leads to formation of methane, propane, isopentane and isobutane. This finding supports the idea that a Lewis acid can initiate n-pentane transformation through attack on both C-H and C-C alkane bonds. Assuming that there exists an analogy between the action of surface Lewis centres and organic Lewis acids, one can propose the following mechanism of the initiation of the n-alkane reaction.The superacid Lewis centre (L+) attacks electrons of the C-H bond of the alkane forming a carbonium cation, which is linked to the catalyst surface by a two-electron three-centre bond : r l+ A similar bond has been shown to exist in organoaluminium compounds:17 R I I I \ A1 A1 A 1 \ . The carbonium cation formed during reaction (1 1) decomposes with carbenium cation RCHZ formation, leaving a hydride anion on the surface. A different reaction of carbonium intermediates is also possible : a hydrocarbon chain RCH, could remain bonded with the surface, while a proton could be released. The superacid Lewis centre should be also able to attack electrons of C-C bonds in an alkane molecule.Such a reaction can be described by the following equation: A surface-bonded methyl group and a carbenium cation, RCH;, as well as a methyl cation and alkyl chain attached to the surface, will be products of decomposition of the carbonium cation intermediate. Summarizing, one can say that a Lewis centre attack on alkanes results in the formation of a carbenium cation and a surface hydrocarbon-like species. The presence of the former is indirectly confirmed by its chemical transformation, i.e. isomerization and alkylation, while the latter species were observed using i.r. spectroscopy. l31696 Alkane Transformations over Superacids Mechanism of n-Alkane Transformations n- Butane In the presence of the superacid catalyst Al,O,/AlCl,, n-butane undergoes a series of transformations.When the reaction temperature does not exceed 373 K, isobutane is the sole product. At higher temperatures equimolar quantities of pentanes and propane were observed in addition to isobutane. These findings support a hypothesis that butane reaction can proceed via a dimeric intermediate whose decomposition would give isobutane, propane and pentanes. The n-butane reaction through a dimeric state is energetically favoured compared with monomolecular butane transformation since the latter has to proceed via primary carbenium cation formation :18 CH3CH26HCH3 $ A C, dimer can be formed as a result of the reaction of a butyl cation with a previously formed olefin or it may result from butyl cation-butane alkylation [a reaction similar The octyl cation, the product of the reaction of a butyl cation and butene, can also be formed by the action of a Lewis acid on octane. The latter reaction was used to check the product distribution of octyl cation decomposition.The results are presented in table 1. Isobutane was the main product of the reaction. Propane and pentanes were also formed ; however, they were not in stoichiometric quantities. Octyl cation decomposition proceeds according to the mechanism of /?-scission : to (lo)]. Olefins (i.e. i-butene and propene) formed during reaction (14) undergo hydrogen transfer reactions and desorb as alkanes, the final reaction products. The other products of the reaction, i.e. carbenium cations, can abstract H- from neutral octane molecules or can lose H+, participating in coke formation processes.The latter reaction explains why propane and pentanes are not formed in a 1 : 1 ratio. Since it was observed that during the n-butane reaction propane and pentanes appeared in the reaction products in equimolar quantities, one can conclude that the C,Hf, cation is not an intermediate state in butane transformation. The C, dimer can be also formed in an alkylation reaction: 1' 1M . Marczewski 6o 1 1697 n E P s E: ._ e 40 60 8 0 100 total conversion (%) Fig. 6. Results of n-decane reaction over Al,O,/AlCl, catalyst at 473 K. i c 4 i Cg i Cg which gives pentanes and + C2HS-CHCH3 [ ii --CH2-C2H] i-CSHI2 + C3H$ (16) propane as final products. To confirm the possibility of butane reaction according to eqn (15) and (16), the alkylation of heptane with methyl chloride was performed.The intermediate product of reaction of heptane with methyl cation should have the same structure as that of the transition carbonium cation in reaction (1 5). Methyl cation with heptane interactions resulted in an increase of propane and pentane concentrations in reaction products (table 1). For low heptane conversion the C,/C, molar ratio was one. These findings strongly support the hypothesis that propane and pentane are formed from butane according to eqn (15) and (16). On the other hand, any product of butane alkylation with primary or secondary butyl cations does not lead to such a dimeric intermediate, which could form isobutane after decomposition. Hence, the main product of butane reaction (isobutane) cannot be formed as a result of c8 dimer dissociation.Thus, one can suppose that it is formed in intermolecular transformation of n-butane. n- Pentane In the presence of Al,O,/AlCl, catalyst, n-pentane reacts to form two main products, isopentane and isobutane, and small quantities of isomeric hexanes. In our previous work, we have shown that isopentane is formed during monomolecular isomerization of n-pentane catalysed by superacid Lewis centres.,, The mechanism of isobutane formation is more complicated. By analogy with butane transformations one can suppose that it is formed in C,, dimer decomposition. C, hydrocarbons should be the other product of this reaction. The results obtained (fig. 2) indicate that isomeric hexanes are formed, but in quantities much smaller than isobutane.To check if the cation C,,H& can be considered as an intermediate state of the pentane reaction, the possibility of its formation as well as its reactivity has been examined. According to Gates8 such a cation is formed in pentene with the pentyl cation reaction. To facilitate this reaction, pent-1-ene was added to n-pentane and then contacted with1698 Alkane Transformations over Superacids the superacid catalyst. The olefin poisoned the active sites of the catalyst instead of giving a rise in the formation of isobutane and isohexanes. On the other hand, the decane reaction gave isobutane and isopentane as main products (fig. 6). These two products formed together over the whole range of reaction time.In the case of pentane transformation, the decomposition reaction proceeds without any disturbance, while isomerization leading to isopentane stopped after a short reaction time (fig. 2). These findings exclude the C,,H& cation as a possible transition state of the n-pentane reaction. Another type of C,, dimer can be formed as a result of pentane alkylation by a pentyl cation : 1’ I C4H9-7H2 I 0 A [C2Hs- “CH2-C2HS CH3 I A IcH;’ ‘ ‘ CH2-C3 H7 C2H5-C-CH3 + C5Hl2 + C2HS-C/CH3/CH3 I I 1’ , C7H16+C3H: C8H18 + C2Hg - J 1’ l + J 1+ 1’ Hexanes, pentanes, octanes and nonanes, as well as ethyl, propyl and butyl cations, are the products of the decomposition of these dimers. In pentane reactions, however, neither the products heavier than hexanes nor propane, which should be formed as a result of theM. Marczewski 1699 reaction of C,H; with pentane (table l), were found.Hence, a C,,Hi3 dimer cannot be also considered as a source of isobutane and hexane during n-pentane conversion. In our previous work1, we have proposed that during the n-pentane reaction new acid centres are formed on the Al,O,/AlCl, catalyst surface. These sites, surface-bonded carbenium cations, are able to catalyse isobutane formation: + CH3\, /CH2-C3H, L-R++ C5H12 [ LL ] L-RCH3 ++CH2-C3H, I1 L-RCH; + CH3CH(CH3)CH3. (18) If pentane decomposition proceeds according to the proposed mechanism one can predict that products of pentane reaction with ethyl (L-R+ = C,Ht) or propyl (L-R+ = C,H:) carbenium ions should be propane and isobutane or n-butane and isobutane : 1 J (19) + (20) RH C4H; iC4H$ * iC4Hto + R+ where RH is pentane, propane or n-butane.The data presented in table 1 confirm the proposed mechanism of pentane decom- position. The addition of ethyl or propyl carbenium ions into the n-pentane/Al,O, - AlCl, system causes a substantial increase of pentane conversion to isobutane. The smaller amounts of propane and n-butane formed indicate that these compounds undergo secondary reactions such as hydrogen transfer [eqn (20)]. To confirm such a reaction pathway, an experiment has been performed with a mixture of ethene and pentane as substrate. Ethene should react with surface superacid centres of the Al,O,/AlCl, catalyst giving surface carbenium cations : L+ + C,H, L-CH,-CHi. The results obtained (fig. 4) support the possibility of the occurrence of reaction (18). In the presence of ethene, isobutane forms without any sign of the induction period observed in the C5H,,-A1,0,/A1C1, system.Artificial formation of surface carbenium cations allows the reaction to start from zero contact time.1700 Alkane Transformations over Superacids n- Hexane The main products of hexane transformations at 373 K are isobutane and isopentane (both forming with a certain induction period) and isohexanes [fig. 3(a)]. As was the case for n-pentane transformations, the decomposition reactions occur in parallel to the isomerization reaction. Taking an analogy with the pentane isomerization mechanism13 one can predict that isohexanes are formed by the action of superacid Lewis centres on n-hexane: Superacid Lewis centres can also attack the C-C bonds of hexane.This reaction leads to the formation of surface carbenium cation as well as to the disappearance of Lewis (L+) sites: L-CH, C,Hf, i-CSHfl * i-C5HI2 + L-CH; . The diminution of the concentration of Lewis centres influences the isomerization reaction [eqn (22)], which is practically stopped after 10 min of reaction. On the other hand, isobutane and isopentane formation remains unaffected. It seems possible that these compounds are formed, as in the case of pentane decomposition, under the influence of secondary active centres, surface carbenium cations : CH3\\ ,/ CH,-C4Hg Y LCH, I I + L-CH: + C6H14M . Marczewski 1701 The data presented in table 1 support the above mechanism of n-hexane decomposition.If ethyl carbenium ions (L-CH; = CH,CH$) are introduced into the n-hexane/ Al,O, - AlCl, reaction system one finds an increase of n-hexane conversion into propane, isobutane and isopentane. These compounds are formed in reactions where C,Ht attacks a or /3 carbon-carbon bonds in hexane molecules [see eqn (19) and (20)]. Increasing reaction temperature up to 473 K causes propane formation at the cost of isopentane and isobutane [fig. 3(b)]. This indicates that at higher temperatures, a classical /?-scission mechanism becomes more important in the hexane decomposition reaction. Conclusions Lewis superacid centres are able to initiate n-alkane transformations. When the Lewis acid centres attack the C-H bonds in the alkane, an isomerization reaction takes place.When the Lewis acid centres attack the C-C bonds in the alkane new active sites, surface carbenium cations, are formed. The n-butane decomposition reaction proceeds via a C , dimeric transition. n-Pentane and n-hexane decompose under the action of surface carbenium cations without dimer formation. References 1 G. A. Olah, G. Klopman and R. H. Schlosberg, J. Am. Chem. Soc., 1969,91, 3261. 2 H. Hattori, 0. Takahashi, M. Takagi and K. Tanabe, J. Catal., 1981,683, 132. 3 A. Krzywicki, M. Marczewski and S. Malinowski, React. Kinet. Catal. Lett., 1978, 8, 25. 4 V. L. Magnotta, B. C. Gates and G. C. A. Schuit, J. Chem. SOC., Chem. Commun., 1976, 342. 5 V. L. Magnotta and B. C. Gates, J. Catal., 1977, 46, 266. 6 J. P. Franck and J. F. le Page, Proc. 7th International Congress on Catalysis, Tokyo 1980 (Kodansha, 7 N. Yoneda, T. Fukuhara, T. Abe and A. Suzuki, Chem. Lett., 1981, 1485. 8 G. A. Fuentes and B. C. Gates, J. Catal., 1982, 76, 440. 9 K. Tanabe and H. Hattori, Chem. Lett., 1976, 1485. Tokyo, 1981), p. 1018. 10 G. A. Goble and P. A. Lawrence, Proc. 3rd International Congress on Catalysis (North Holland, 11 A. Krzywicki and M. Marczewski, J. Chem. Soc., Faraday Trans. I, 1980, 76, 1311. 12 0. Takahashi, T. Yamauchi, T. Sakuhara, H. Hattori and K. Tanabe, Bull. Chem. Soc. Jpn, 1980,57, 13 M. Marczewski, Catalysis by Acids and Bases, ed. B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit 14 A. Germain, P. Ortega and A. Commeyras, Nouv. J. Chim., 1979, 3, 415. 15 R. P. Bell, Acids and Bases (Butler and Tanner Ltd, London, 1969), p. 106. 16 G. A. Olah and J. Kaspi, J. Org. Chem., 1977, 42, 3046. 17 E. G. Hoffman, 2. Elektrochem., 1960, 64, 45. 18 D. M. Brouwer and H. Hogeveen, Prog. Phys. Org. Chem., 1972, 9, 179. Amsterdam, 1965), vol. 1, p. 320. 1807. and J. C. Vedrine (Elsevier, Amsterdam, 1985), p. 213. Paper 51630; Received 15th April, 1985

ISSN:0300-9599    

DOI:10.1039/F19868201687

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1986,82, 1703-1712 Ionization Equilibria of Cobalt(I1) Chloride in N , N-Dime t h ylformamide Waclaw Grzybkowski" and Michal Pilarczyk Department of Physical Chemistry of the Institute of Inorganic Chemistry and Technology, Technical University of Gdarisk, 80-952 Gdarisk, Poland Visible absorption spectra and the molar conductance curve for CoC1, in N,N-dimethylformamide (DMF) have been determined at 25 "C. The results indicate the formation of the Co(DMF)i+ - 2CoC1,DMF- complex electro- lyte which controls the electrolytic properties of the solution. The formation constants of the individual chloro-complexes of cobalt(I1) have been calculated. A knowledge of transition-metal complexes in non-aqueous media and an understanding of the influence of solvents upon their equilibria are of major interest.However, there is a lack of information concerning the simplest transition-metal salts dissolved in non-aqueous donor solvents. It has been shown by Libui et a1.l that the transition-metal cations in strongly polar donor solvents, such as acetonitrile,,. dimethyl sulphoxide,*~ dimethylacetamide6 and dimeth~lformamide,~ exist in the absence of coordinating anions as MLi+-type solvated complexes (M = Mn2+, Co2+, Ni2+, Cu2+ or Zn2+ and L denotes the solvent molecule). Unlike perchlorates and tetrafluoroborates, the divalent transition-metal halides dissolved in dimethyl sulphoxide8 or acetonitrileg* lo exhibit a variety of electrolytic behaviour, as is shown by their molar conductance curves and absorption spectra. The properties of the solutions can be described in terms of the coordinative disproportionation reactions producing complex electrolytes consisting of a hexacoordinate cation and a tetracoord- inate anion.The formation of tetrahedral anionic species in N,N-dimethylformamide solutions of the transition metal chlorides was shown by Katzinll and ascribed to the equilibrium (original notation used) : 2°CtMC1, e OctMC1+ + tetMC1;. The proposed scheme, however, does not describe a large range of electrolytic behaviour. The dissolution of CoCl, in acetonitrilelo is accompanied by the equilibrium 3CoC1,(CH3CN), + 2CH3CN Co(CH,CN)i+ + 2CoC1,CH3CN- while in dimethyl sulphoxide solutions of NiCl, the pseudotetrahedral complexes are practically absent and the NiCl(DMSO),+ * C1- complex electrolyte is responsible for the electrolytic behaviourq8 In a previous paper1, we have shown that the dissolution of NiCl, in DMF results in the formation of the NiCl(DMF)$ NiC1,DMF- complex electrolyte being the main form of the solute in DMF.The present work was undertaken in order to establish the ionization equilibria of CoCl, in DMF. Experiment a1 N,N-Dimethylformamide (analytical grade) was dried using 4A molecular sieves and distilled under reduced pressure at 45-50 "C. The specific conductance of the purified 57 1703 FAR I1704 Ionization Equilibria in DMF solvent was in the range (3.0-8.0) x S cm-l. The density at 25 "C was 0.94402 g ~ m - ~ DMF-solvated CoCl, and C0(C104), were prepared from the corresponding hydrates by dissolving them in DMF, followed by removing any excess of the solvent under reduced pressure at 60 "C.On cooling, crystalline solids were obtained and were recrystallized twice from anhydrous DMF. Analytical-grade tetraethylammonium chloride was recrystallized twice from anhy- drous acetonitrile and dried in vacuo at 65 "C. The stock solutions of the salt were analysed by standard EDTA titrations. Solutions for measurements were prepared by weighed dilutions. The concentrations were calculated using densities determined independently. Details of the procedures for spec t rop ho t ome tric and conduct ome tric measurements were identical to those described previo~sly.~. ' 9 l2 All the preparations and manipulations were performed in a dry box. (literature values are 0.94387 and 0.94407 g cm- 3 ).13, 1 4 Results and Discussion Fig. 1 shows the visible absorption spectra of a series of solutions of CoCl, in DMF within the concentration range 0.002-0.02 mol dm-3 at 25 "C. As is seen, the spectrum consists of a broad band with maxima located at 610 and 680 nm. The band position, contour, and high intensity are typical of cobalt(I1) in a pseudotetrahedral environment8* 9; l5 Further inspection of fig. 1 shows that the band position is independent of salt concentration, while a variation of the intensity with the increase of CoCl, concentration can be observed. The effect of concentration becomes more distinct when the molar absorption coefficients of cobalt(I1) at the maxima are plotted against the square root of the concentration of CoC1, (fig.2). The most characteristic feature of this dependence is the relatively sharp decrease in the intensity with the decrease of CoCl, concentration below 0.0004 mol dm-3. This fact may be interpreted in terms of replacing the tetrahedral I I I 5 7 300 - ,-I 'E 200 "E - 0 - E a IUJ 1 550 600 650 700 750 wavelength/ nm Fig. 1. Visible absorption spectra of CoCl, solutions in DMF at 25 "C. The molar concentrations of CoCl, are: (1) 0.00 1935; (2) 0.002429; (3) 0.004837; (4) 0.01083; and (5) 0.016 18.W. Grzybkowski and M . Pilarczyk - 7 --. I E 300 E 200- ; m -a IIU . 100 1705 - r-=yGT'' n n ,. V " , I , ] I I I I I , , , cobalt(I1) complex with an octahedral one. The hexacoordinated species do not exhibit absorption in this spectral range.l The independence of the band position and contour of concentration of CoCl, suggest that only one tetracoordinated complex of cobalt(r1) exists in solution.Moreover, an increasing concentration of CoC1, in the solution brings about an increase in the relative content of the complex. Fig. 3 shows the molar conductance curve of CoCl, in DMF at 25 "C. The experimental values are listed in table 1. In the same figure is presented the molar conductance curve of Co(ClO,), reported previous1y.l It can be seen that the molar conductance curve of CoCl, runs well below the curve of Co(ClO,),, known to exist in the form of a Co(DMF)i+ - 2C104-type complex electrolyte, being only slightly associated. As is seen the molar conductance curve of CoC1, exhibits a slight decrease in conductivity with increasing concentration at the lowest Concentration range, while the experimental points run almost horizontally at higher CoCl, concentrations.The relatively low value of the molar conductance suggests a high degree of complex formation. The limiting molar conductance calculated for the Co(DMF)i+ * 2C1- complex electrolyte from the known ionic conductancesly l6 amounts to 188.4 S cm2 mol-l, and the corresponding conductometric curve is expected to run close to the curve for Co(ClO,),. As can be seen, the variation of the molar conductances is roughly reflected in the concentration changes of the spectrum of cobalt(I1). Such behaviour can be due to the formation of ionic species in the solution rather than neutral ones. The effect of increasing the conceqtration of the ionic complexes may compensate the effect of decreasing ionic mobilities due to increasing ionic strength.Thus, we infer that the pseudotetrahedral complex of cobalt(I1) is the CoC1,DMF- anion. A qualitative confirmation of this conclusion is provided by the effect which addition of a non-coordinating diluent of low polarity exerts on the spectrum of CoCl, dissolved in DMF. The spectra of cobalt(I1) observed at high toluene contents are shown in fig. 4 along with the spectrum of CoCl, in DMF. Inspection of fig. 4 shows that addition of toluene results in drastic changes in the spectrum of cobalt(I1). The effect consists of the development of a new spectrum with absorption maxima at 582,640 and 670 nm. Similar spectral changes were induced by addition of chlorobenzene.Moreover, the effects are accompanied by the essential decrease in conductivity. The molar conductance of a 0.00289 mol dm-, solution of CoC1, in mixed solvent (87 mol % of chlorobenzene) amounts to 1.1 S cm2 mol-1 only, while the value for the corresponding DMF solution is 26.5 S cm2 mol-l. It may be expected that the decrease in dielectric constant of the medium favours formation of a neutral species. Thus, it is clear that the pseudotetrahedral complex of cobalt(@ existing in the presence of the inert diluents is CoCl,(DMF),. Fig. 5 shows the visible absorption spectra of a series of solutions containing Co(ClO,), 57-21706 200 150 r( - I 0 E *s 100 e LA \ 50 0 Ionization Equilibria in DMF \ \ \ \ \ 1 I I I I I I 1 I 0.02 0.04 0.06 0.08 (clmol dm-3)3 Fig.3. Plot of the molar conductivity against the square root of concentration for CoCl, in DMF solution at 25 "C (0). The broken line represents the conductometric curve obtained previously for Co(ClO,),. at an approximately constant concentration of 0.012 mol dm-3 and Et,NCl at a number of different concentrations not exceeding the 2:l mole ratio of Et,NCl to Co(ClO,),. As is seen, an addition of Et,NCl brings about a development of an intensive absorption band with maxima at 610 and 680 nm. The same maxima were observed for solutions of CoCl, in DMF (fig. 1). With increasing concentration of Et,NCl the band changes in intensity only, indicating the presence of one tetrahedral complex of cobalt(I1) in the solutions. Table 1.Molar conductivities of CoCl, in DMF at 25 "C c Am C Am / 1 O-* mol dm-3 / S cm2 mol-l / 1 0-4 mol dm-3 / S cm2 mol-1 1.9450 2.8773 4.0909 5.3970 8.4335 12.134 14.377 21.890 33.52 26.103 27.33 33.04 32.671 26.90 3 1.44 43.936 26.46 30.57 52.140 26.21 29.30 63.528 25.84 28.68 81.639 25.39 28.26 99.587 25.10 27.52 - -400 300 r( I E "E - 2 200 a IUJ 1 100 0 W. Grzybkowski and M. Pilarczyk I 1 I I 1707 550 600 650 700 7 50 wavelength/nm Fig. 4. Absorption spectra of CoCl, in DMF-toluene mixtures at 25 "C. The concentrations (mol dm-3) of CoCl, and the mole fractions of toluene in the mixed solvent are, respectively: (1) 0.003 025,O.OO; (2) 0.003 180,0.156; (3) 0.003077,0.428; (4) 0.003009,0.592; (5) 0.003 158,0.755; and (6) 0.001 675, 0.925. Further changes in the spectrum of cobalt@) induced by an increasing concentration of Et4NCl added to a 0.0012 mol dm-3 solution of C0(C104), in DMF are illustrated in fig.6. At Et4NCl to Co(ClO,), ratios exceeding 2.8 an increase of Et4NC1 concentration results in the gradual disappearance of the maximum at 610 nm and the simultaneous development of a new spectrum consisting of three bands with maxima at 637.5,673 and 700 nm. Subsequently, three well defined isosbestic points can be observed on the spectra at 622,648 and 661 nm. When the Et,NCl to Co(ClO,), ratio exceeds 50, further increase of chloride concentrations does not affect the spectrum. The limiting spectrum shows the characteristics of the tetrachloro-complexes. The facts indicate a two-species equilibrium : CoC1,DMF- + C1- + CoC1;- + DMF (1) established within the 30-50 range of the C1- to Co2+ ratio. The above results permit calculation of the equilibrium concentrations of CoC1,DMF- and CoC1;- complexes.Thus, ignoring absorption due to the octahedral complexes, the mole fraction of the tetrahedral complexes may be calculated as1708 300 2 00 r( I 8 - 0 E E m a Iu, 1 100 0 Ionization Equilibria in DMF I I I 550 600 650 700 750 w aveleng t h/nm Fig. 5. Absorption spectra of Co(ClO,),-Et,NCl solutions in DMF at 25 "C. The concentrations (mol dm-3) of Co(ClO,), and Et,NCl are, respectively: (1) 0.01 1896, 0.004528; (2) 0.01 1947, 0.006713; (3) 0.012187, 0.008912; (4) 0.012475, 0.01272; (5) 0.012 119, 0.01561; (6) 0.01253, 0.01887; and (7) 0.012 195, 0.021 97. where E denotes the measured mean molar absorption coefficient of cobalt(I1) at the wavelength of the isosbestic point where the molar absorption coefficient of the two complexes have the common values of e3,.For any solution in which the tetrachloro- complex is absent eqn (2) has the form: On the other hand, for the solutions in which the octahedral complexes are absent, as indicated by an existence of the abovementioned isosbestic points, we have c4 - &,--e c E 3 - E 4 -- - (4) where E relates to any selected wavelength at which the molar absorption coefficients, e3 and E,, of CoC1,DMF- and CoCli-, respectively, are markedly different. The necessary values of E, were taken from the limiting spectrum, and those of E , were calculated as E, = (G)/C, from the absorption curves of the Co(ClO,),-Et,NCl solutions in which the tetrachloro-complexes were absent.The concentrations of CoC1,DMF-, C,, were calculated using eqn (3), valid for the wavelength of the isosbestic points. The mole fraction of the CoC1,DMF- complex in the CoC1, solutions amounts to 60% for the more concentrated solutions. Thus, the octahedral species must be the Co(DMF),2- solvo-complex. The same is indicated by the results obtained for the Co(ClO,),-Et,NC1 solutions; the chloride ions for the most part are consumed for CoC1,DMF- complex formation. An illustrative example is the mole fraction of the CoC1,DMF- complex calculated for the solution at the mole ratio of Et4NC1 to C0(C104), equal to 2.0. In this solution 65% of cobalt(r1) exists as the trichloro-complex.800 70 0 600 500 - I E c( 400 m E a IUJ \ 300 200 I 100 W.Grzybkowski and M. Pilarczyk 1709 1o.11 I A I 550 6 00 6 50 700 750 wavelength/nm Fig. 6. Absorption spectra of Co(ClO,),-Et,NCl solutions in DMF at 25 "C. The concentrations (mol dm-3) of Co(ClO,), and Et,NCl are, respectively: (1) 0.001 244, 0.002642; (2) 0.001 248, 0.003043; (3) 0.001 254,0.003366; (4) 0.001 261, 0.003627; ( 5 ) 0.001 240; 0.003957; (6) 0.001 247, 0.005626; (7) 0.001 236, 0.01 103; (8) 0.001 225, 0.021 51 ; (9) 0.001 226, 0.03271 ; (10) 0.001 268, 0.08858; and (11) 0.001241, 0.1808.1710 Ionization Equilibria in DMF 100 h E E .- % 50 5 E P) 0 c-. 0 0.1 1 10 100 Et 4 Ncl~~Co(clo4 )2 Fig. 7. The ranges of existence of the chloro-complexes of cobalt@) in DMF solutions of Co(ClO,), in the presence of Et,NCl at 25 "C.The fact that its abundance is somewhat higher than the corresponding value for the CoCl, solution is due to a presence of Et,N+ and C10, ions enhancing the ionic strength of the solution. The results presented suggest that the solute undergoes a coordinative disproportion- ation producing a complex electrolyte of the Co(DMF)i+ - 2CoC1,DMF- type, being the main form of existence of CoCl, in DMF solution. However, formation of the corresponding mono- and di-chloro-complexes cannot be ignored. This is indicated by the 60% abundance of the trichloro-complex in the CoCl, solution and some non- linearity of the plot of the abundance vs. mole ratio for the Co(ClO,),-Et,NCl solution. However, the lack of spectral evidence suggests their octahedral structure.The above results provide the possibility of calculating stability constants of all the complexes assumed to be formed in the solutions under consideration, provided that proper allowance can be made for the variation in the activity coefficients. In the calculation actually performed we used the data obtained for the Co(ClO,),-Et,NCl-DMF system. The results obtained for the three-component solutions were much more useful than the data derived for the CoC1, solution. For the latter system the differences in the intensity of the spectral bands are rather small at the higher concentration range, while the data obtained for the very dilute solutions are less accurate. In the computer analysis performed we assumed formation of the CoCl+, CoC1, and CoCl; formal complexes, the corresponding formation constants being defined as KO, = cn Yn (5) Cn-1 LC1-1 where n = 1, 2 or 3 and Yn is quotient of the respective activity coefficient.Variations in the activity coefficients were assumed to follow the Debye-Huckel equation involving the ion-size parameter, BH, the latter being estimated from conductivity data7 as 3.29. Taking into account the equations arising from the material balance for the cation and anion, we attempted to find the best set of KO, values describing the spectral properties of the three-component solutions in which the CoCli- complexes were absent. The resulting values of the logarithms of the formation constants of CoCF, CoC1, and CoC1; were 3.5k0.5, -2.Ok0.5 and 1l.Ok 1. The high uncertainty of the derived values is related to the very high stability of the CoC1,DMF- complex.The data suggest also that the existence of the dichloro-complex can be ignored, while the monochlorideW. Grzybkowski and M. Pilarczyk 171 1 complex is present in the solution containing the relatively low amount of the chloride ion donor. In order to complete the characteristics of the chloro-complexes of cobalt(I1) in DMF solution we calculated the formation constant of the CoCli- complex. The calculation was based on the C,/C values determined for the Co(C104),-Et,NC1 solutions at the range of the coexistence of the two tetrahedral complexes only. The corresponding logarithm is equal to 1.9kO.l. The equilibrium concentrations of the single complexes found at the same time for the different Et,NCl to Co(ClO,), ratios have been used for preparing the distribution diagram shown in fig.7. Commenting on the results obtained, we note that the derived formation constant of the CoC1,DMF- complex is much higher than that previously found for the trichloro- complexes of cobalt(I1) in DMSOs and DMA17 solutions. This striking difference cannot be related to the donor properties of the solvents, since their donor numbers are rather similar and close to the donor number of C1- ion.ls Conclusions The observed behaviour of CoC1, in DMF solutions is a consequence of the high stability of the CoC1,DMF- complex resulting in the formation of the Co(DMF);+ - 2CoC1,DMF- complex electrolyte. We note that >60% of cobalt(I1) exists in the form of the pseudotetrahedral trichloro-complex.In the previous paper1, from this laboratory it has been shown that the dominating form of an existence of NiCl, in DMF solution is the NiCl(DMF)'; NiC1,DMF- complex electrolyte and almost 50% of nickel@) exists as the trichloro-complex. It is interesting to compare the abovementioned coordination forms of CoCl, and NiCl, with those being formed in DMSO solutions. It has been shown by Libui et aL8 that the corresponding coordination forms are the CoCl(DMS0)'; * CoC1,DMSO- and NiCl(DMS0): - Cl- complex electrolytes. As is seen, CoCl, exhibits a significant tendency towards tetrahedral complex formation, while NiCl, prefers the octahedral structure. Similar abilities were found for CoBr, and NiBr, dissolved in a~etonitrile.~? lo However, it should be noted that the abovementioned complex electrolytes are formed at a higher concentration range.In very dilute solutions the simple ionization and association equilibria are responsible for the properties of the systems. The conclusion follows that the main factor governing the coordination states of transition metal salts in solution is the relative stability of tetrahedral and octahedral complexes. References 1 W. Libus, J . Solution Chem., 1981, 10, 631. 2 W. LibuS and H. Strzelecki, Electrochim. Acta, 1970, 15, 703; 1971, 16, 1749. 3 W. LibuS, B. Chachulski and L. Frqczyk, J. Solution Chem., 1980, 11, 355. 4 W. LibuS and M. Pilarczyk, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1972, 20, 359; 1974, 22, 717. 5 W. LibuS, W. Grzybkowski and R. Pastewski, J. Chem. Soc., Faraday Trans. 1, 1981,77, 147. 6 E. Kamienska and I. Uruska, Electrochim. Acta, 1977, 22, 181. 7 W. Grzybkowski and M. Pilarczyk, J . Chem. Soc., Faraday Trans. 1, 1983, 79, 2319. 8 W. LibuS, Electrochim. Acta, 1975, 20, 831; 1982, 27, 573. 9 W. LibuS, W. Grzybkowski and M. Walczak, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1970, 18, 141. 10 W. LibuS and W. Grzybkowski, Electrochim. Acta, 1978, 23, 791. 11 L. Katzin, J. Chem. Phys., 1962, 36, 3034. 12 M. Pilarczyk and L. Klinszporn, Electrochim. Acta, in press. 13 M. R. J. Dack, K. J. Bird and A. J. Parker, Aust. J . Chem., 1975, 28, 955. 14 D. Hamilton and R. H. Stokes, J. Solution Chem., 1972, 1, 213. 15 L. Sestili, C. Furlani and G. Festuccia, Inorg. Chim. Acta, 1970, 4, 542.1712 Ionization Equilibria in DMF 16 A. C. Covington and T. Dickinson, Physical Chemistry of Organic Solvent Systems (Plenum Press, 17 E. Kamienska and I. Uruska, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1976, 24, 576. 18 V. Gutman and U. Mayer, Monatsh., 1968,99, 1383. London, 1973), p. 677. Paper 5/759; Received 2nd July, 1985

ISSN:0300-9599    

DOI:10.1039/F19868201703

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC. Faraday Trans. I, 1986,82, 1713-1719 An Electron Spin Resonance Study on the Re/ZrO, System Takashi Komatsu, Masaharu Komiyama and Yoshisada Ogino" Department of Chemical Engineering, Faculty of Engineering, Tohoku University, Aramaki Aoba, Sendai 980, Japan Masakazu Iwamoto Department of Industrial Chemistry, Faculty of Engineering, Nagasaki University, Bunkyo-cho 1-14, Nagasaki 852, Japan The clear room-temperature e.s.r. signal observed for a Re/ZrO, catalyst at 9.5 GHz has been analysed to obtain the following values for the e.s.r. parameters g,, = 1.76, guy = 1.83, g,, = 1.96, lAlzx = 17.1 mT, lAlyy = 45.2 mT and ~ A ~ , , = 91.6 mT. The analysis has revealed that hexa- valent rhenium cations (58, I = g, S = 4) are responsible for the e.s.r. signal observed. A qualitative discussion both of the crystal field around the rhenium cation and of the 0; e.s.r.signal observed for the oxidized sample is presented. Supported rhenium catalysts are known to promote various chemical reactions such as naphtha reforming,l olefin metathesis,, carbon monoxide hydr~genation,~ e t ~ . , ~ ? hence physico-chemical characterizations of rhenium on the catalyst supports deserve intensive studies. Although several workers6-* have published interesting aspects of the supported rhenium catalyst, many problems remain unsolved. Thus the present authors have undertaken systematic studies on the rhenium catalysts using modern experimental techniques such as X.P.S.,~ A.e.s.1° and i.r. spectroscopy.21 The present work, which mainly employs e.s.r. techniques, also aims to clarify the chemical state of rhenium supported by a catalyst carrier, i.e.ZrO,. It has been found that the Re/ZrO, system exhibits a clear e.s.r. signal and enables us to make a theoretical analysis. The purpose of the present paper is to report the experimental results together with a discussion of the chemical state of rhenium on ZrO,. Experiment a1 Materials The Re/ZrO, catalyst which served as a sample for the e.s.r. measurement contained 5 wt% rhenium unless otherwise stated. Details of the catalyst preparation and the CO hydrogenating activity of this catalyst have been reported el~ewhere.~ Methods A Varian e.s.r. spectrometer E-4 (X-band; 9.5 GHz) was mainly used in recording the e.s.r. signal. Except for special cases, room-temperature e.s.r signals were recorded at a microwave power of 2 or 5 mW.Prior to the e.s.r. recording, the freshly prepared catalyst sample was evacuated at 400 "C for 1 h in an e.s.r. tube. In addition to this standard recording, special recordings of the e.s.r. signals were made at several stages in a reduction-oxidation cycle : reductions with hydrogen under 0.1 MPa pressure for 1 h at 150, 200, 300 and 400 "C; oxidations with air under 0.1 MPa pressure for 20 min at 100, 200, 300 and 400 "C. DPPH and Mn2+/Mg0 were used as standard substances in determining exact g-values and spin concentrations. 17131714 E.S.R. Study of the Re/ZrO, System I 119. - h h 111 100 mT H/mT I / Fig. 1. A typical e.s.r. signal of the Re/ZrO, catalyst. 150 200 300 400 r" - 200 300 H/mT 300 400 H/mT Fig.2. Changes in the e.s.r. signal of the Re/ZrO, catalyst during a reduction-oxidation cycle: the sample was (a) reduced sequentially at 150,200,300 and 400 "C with hydrogen under 0.1 MPa pressure for 1 h at each temperature, then (b) oxidized sequentially at 100, 200, 300 and 400 "C with air under 0.1 MPa pressure for 20 min at each temperature.T. Komatsu et al. 1715 I g 3 = 2.003 5 mT Fig. 3. A typical e.s.r. signal for 0, formed on the Re/ZrO, catalyst in an oxidation stage of the oxidation-reduction cycle. Results E.S.R. Behaviour of Re/ZrO, Fig. 1 shows a typical e.s.r. signal of the Re/ZrO, catalyst. This signal appears when the catalyst of 5 wt% rhenium content was evacuated at 400 "C for 1 h. Catalysts with lower rhenium contents, i.e.1,2 and 3 % gave e.s.r. signals similar to that shown in fig. 1. The decrease in the rhenium content was accompanied by a slight decrease in the signal intensity. On the other hand, the catalyst with 10 wt% rhenium content showed no measurable e.s.r. signal. A change of the rhenium source from ammonium per-rhenate to rhenium decacarbonyl brought about no significant changes in the e.s.r. signal. Similarly, a cooling of the sample temperature from a room temperature to 77 K did not alter the signal. In the reduction-oxidation cycle, the e.s.r. signal of the Re/ZrO, catalyst varied in a manner shown in fig. 2: signals around 300 mT are shown in order to reduce the complexity of the figure. Reduction with hydrogen at temperatures up to 300 "C did not bring about appreciable changes in the e.s.r.signal, but reduction at 400 "C resulted in a great depression of the signal intensity. Reoxidation of the reduced catalyst gave an e.s.r. signal similar to the original one together with a sharp peak at g x 2.0. Fig. 3 shows the details of this new sharp peak. Discussion E.S.R. Signal of Re/Zr02 It appears that the e.s.r. signal obtained for the Re/ZrO, catalyst deserves special discussion. Such a clear room-temperature signal as shown in fig. 1 has not been reported previously for any supported rhenium catalysts. For instance, the e.s.r. signals reported by Yao,12 Johnson13 and Echigoya14 for the Re/Al,O, catalyst are very broad. It is expected that information derived from the theoretical analysis of the e.s.r.signal of the Re/Zr02 system would contribute to advances in the chemistry of supported rhenium cat a1 y s t s. Hexavalent cations of rhenium (Re6+) are most likely to be the species responsible for the e.s.r. signal found for Re/ZrO, at a room temperature, though rhenium can take various oxidation states15 and the majority of these (Re2+, Re4+, Re5+ and Res+) is capaple of giving its own e.s.r. ~ i g n a l . l ~ - ~ ~ Divalent cations15 give e.s.r. signals which differ greatly1716 E.S.R. Study of the Re/Zr02 System from that shown in fig. 1. Although tetravalent cations16* 18- 2 0 ~ 22 give clear e.s.r. signals with well resolved hyperfine structure, the conditions are limited to temperatures below 15 K.23 Similarly, Re5+ cations give an e.s.r. signal below 20 K.l6, l7 On the other hand, Garif'yanov,l9 and later Borisova et aZ.,21 found that ReOCl, dissolved in concentrated sulphuric acid gave a clear e.s.r.signal of Re6+ and suggested that the signal would appear even above a room temperature. The theoretical justification of assigning the e.s.r. signal to Re6+(5dl, electron spin quantum number S = i, nuclear spin quantum number I = j) has been made on the basis of the following equations proposed by Wilson and K i ~ e l s o n ~ ~ for the glassy solid system containing cations with S = 4: W, = gPo H/A + AM+ (A:, + A;,) (A& + A2) [I(I+ 1) - M2]/8A2g/?o H/A ( 1 ) ( 2 ) ( 3 ) g = (gEz cos2 8 + g i x sin2 8 cos2 4 + giU sin2 o sin2 4)* gA = (Agz g& cos2 8 + A i x g i x sin2 8 cos2 4 + A;, g i y sin2 8 sin2 4); where w, is the angular frequency of microwave, H is the magnetic field, Po is the Bohr magneton, A is Planck's constant divided by 27r, M is the nuclear spin magnetic quantum number, g j j is the j-component of the spectral splitting factor, A, is the j-component of the hyperfine splitting constant, 0 is the angle between the magnetic field applied and the unique molecular axis, 4 is the azimuthal angle, the subscripts x, y and z are the molecule-fixed Cartesian coordinates.With an approximation proposed by Rollman and char^,^^ eqn (1)-(3) can be converted into a more convenient form [eqn (4)-(7)] under the following three extremes: (i) 8 = 0 (ii) 8 = n / 2 , 4 = 0- (iii) 8 = 4 2 , 4 = 7r/2 Hzz = R z - A z z M - 2 A Z z [ I ( I + 1)-M2] (4) Hxx = Pxx -Axx M - 2Axx [I(I+ 1) - M2] ( 5 ) Hyy = H i y - A,, M - 2A,,[I(I+ 1)- M2] (6) where the following abbreviations are made; (qj)j - x, y, z = ( ~ 0 fi/gjj P o ) j - x, y, z, Axx = fiAxxlPo gxx = AjtxIgxx A,, = fiAyy/Pogyy = Ak,/gyy9 Azz = fiAzz/Pogzz = A;z/gzz Axx = (A:x+AZy) (Az+AL%)/16A:xg:xH0,x A,, = (A;% +A&) (A;; + AZy)/ 1 g;, HE, Azz = (4: +Aj2,)/8g:z Rz.Evidently the forms of eqn (4)-(6) are identical with each other and hence they are expressed by the following equation : Hjj = H$'j - Ajj M - 2Ajj[I(I+ 1) - M2]. (7) Thus the discussion with eqn (7) holds for every case ofjy = xx, y y or zz. Since M for Re6+ ( I = 4) takes six different values (-8, - $, -4, 0, $,#, 4) eqn (7) predicts that, in every case of j j = xx, y y or zz, six e.s.r. peaks would appear at six different positions on the magnetic-field axis.Thus if an appropriate set of six e.s.r. peaks can be found on the observed e.s.r. spectra it is possible to obtain six different values for Hjj. Putting these Hjj values and the corresponding values for M into eqn (7) gives six simultaneous equations in which qj, A , and Ajj are the unknown quantities to be evaluated. It is an easy task to solve the equations and to find an appropriate value for each of qj, A , and Ajj.T. Komatsu et al. 1717 loot I I I l l I I - 5 4 - 3 4 - 1 4 112 3 4 512 M Fig. 4. Straight-line relationships between corrected e.s.r. signal positions (Hjj)corr and the nuclear spin quantum numbers M. Table 1. E.s.r. parameters for the Re/ZrO, catalyst JJ parameter xx YY zz Ajja/mT 3.15 0.98 1.6 gii 1.76 1.83 1.96 IA Ijj/mT 17.1 45.2 91.6 qj/mT 345 370 383 a Coefficients of the second-order perturbation terms.The calculations mentioned above are, of course, possible only after the classification of e.s.r. peaks into three different groups corresponding to]] = xx, y y and zz: in principle each group consists of six e.s.r. peaks. This classification needs some trials, but is facilitated by the recognition that the separation between the neighbouring two peaks must be identical for the six peaks belonging to the samejj-group when the second-order perturbation term [the third term on the right-hand side of eqn (7)] can be neglected. This situation is somewhat modified by the interference of the second-order perturbation, which cannot be neglected in the present case, but the recognition mentioned above is nevertheless useful for the classification of the e.s.r.peaks. The three e.s.r. signal groups shown in fig. 1 have been classified by the method mentioned above. The validity of the classification of the e.s.r. signals and that of the evaluation of the e.s.r. parameters can be seen by the linearity between M and the corrected magnetic field (Hjj)corr which is defined by (Hjj)corr = Hyj - A , M. Since the magnitude of the second- order perturbation coefficient Ajj is already known, it is easy to evaluate (Hjj)corr with eqn (7). Straight lines shown in fig. 4 demonstrate that the e.s.r. signals for the Re/ZrO, catalyst has been satisfactorily analysed by the method mentioned above.1718 E.S.R. Study of the Re/ZrO, System Table 1 summarises the e.s.r.parameters for the Re/ZrO, catalyst: gjj were obtained from the position of the centre of gravity of each (Hjj)corr group. The gjj which has a value closest to the free-electron value has been assigned to g,, and the other two components have been assigned to g,, and to g,, arbitrarily. The e.s.r. parameters listed in table 1 also support the consideration that Re6+ cations are responsible for the e.s.r. signal shown in fig. 1. Namely, the g-values listed are close to those (1.77, 1.90) reported by Garif yanov19 for Re6+. Furthermore, the isotropic hyperfine coupling constant d = (~)~(Az,+Ayy+A,,)~ has been calculated to be 9.8 mT by assuming that A,, has a sign opposite to those of A,, and of A,,. This d value is essentially equal to the value reported by Garif yanov19 (10.6 mT).Local Crystal Field around Re6+ The facts that g,, # g,, # g,, and A,, # A,, # A,, (table 1) infer that the local crystal field around Re6+ in the Re/ZrO, system is not axially symmetrical. According to the results of trial applications of several typical crystal fields, the most probable crystal field exerted upon the Re6+ cations is an octahedral field with a compressive tetragonal distortion. It is 27 that cations in the local crystal field mentioned above exhibit an e.s.r. signal with the following gjj parameters; g,, = g,( 1 - il/6,,-,,), g,, = g,( 1 - il/6,-,) and g,, = g,( 1 - 4il/6(,~-,2)-,,), where g , is the g-value for free electron, il is the spin-orbit coupling constant of Re6+, and 6,,-,,, BYz-,, and 6(,~-,2~-,, are the respective energy gaps between the ground state (dz,) and each of the d,,, d,, and d,2-,2 orbitals.With experimentally obtained values of gii. and with the value of il = 2500 cm-1 28 (free Re6+) x 0.5 (ionicity correcti~n),~~~ 30 it is easy to obtain 6,z-sy = 10400 cm:l, 6,z-T, = 14700 cm-l and 6(,2-y2)-sy = 25000 cm-l. With respect to the supported rhenium catalysts, electronic spectra are not available in the present stage, but Moffitt et aL31 have reported that the absorption peaks for ReF, appear at 35000 cm-l, 24000 cm-l and at 5200 cm-l. In addition, Borisova et aLZ1 have found spectral peaks at 24000 and 18 000 cm-l for ReOCl, dissolved in concentrated sulphuric acid. These literature data indirectly support the consideration that the 6 values shown above show reasonable order of magnitude.Note that the following crystal fields do not give results consistent with the experimental results, or otherwise give unrealistically large values of 6; a tetrahedral field and a cubic field which are accompanied by a tetragonal distortion coupled with a rhombic distortion; an octahedral field with a tetragonal elongation coupled with a rhombic distortion; an octahedral field with a C, distortion. Information from the 0; Signal A peculiar shape and the g-values of the e.s.r. signal shown in fig. 3 indicate that superoxide ions 0; are formed by the heat treatment of Re/ZrO, in air. Furthermore, the g, value of 2.033 infers that trivalent or tetravalent surface cations would be the sites stabilizing the superoxide ions.It is most likely that the Zr4+ cations of the catalyst support are the sites for the superoxide ions. The g, value of the e.s.r. signal of 0; anions captured by ZrO, has been reported to be 2.032.32 The superoxide ions formed on the rhenium sites probably migrate on the catalyst surface toward the Zr4+ sites. An explanation similar to this has been reported by Beringhelli et aZ.33 for the e.s.r. behaviour of 0; formed on the Rh/Al,O, catalyst. It is not likely that the hexavalent rhenium sites which have been discussed in the preceding section are acting as the adsorptive sites for 0; anions even if a smaller ionicity of the rhenium ion is taken into account. According to matt he is^,^ and Broclawik et aZ.,30 the ionicity of ReO, can be expressed by (Re3+) (01-)3 or (Re4+) (Oj-),.Therefore it is not improbable that the effective electrostatic fields exerted by the surface hexavakntT. Komatsu et al. 1719 rhenium cations are about the same order of magnitude as those exerted by purely trivalent or tetravalent cations. However, this view is excluded for the following reasons. First, the super-hyperfine structure, which is expected to appear [owing to interaction between the rhenium nuclear spin ( I = 8) and the electron spin of OF] has not been observed even at liquid nitrogen temperature. Secondly, as Che and T e n ~ h ~ ~ have pointed out, a strong line broadening, which probably smears out the e.s.r. signal, would result if paramagnetic ions such as Re6+ cations are the adsorptive sites for 0;.The authors thank Professors M. Matsuda and M. Iwaizumi of the Chemical Research Institute of Non-aqueous Solutions, Tohoku University for their permission to use the e.s.r. equipments in their laboratories. This work was partially supported by a Grant for Scientific Research no. 5843001 5 from the Japanese Ministry of Education, Science and Culture. References 1 F. G. Ciapetta, D. N. Wallace and W. R. Grace, Catal. Rev. Sci. Eng., 1972, 5, 67. 2 R. Nakamura, H. Iida and E. Echigoya, Chem. Lett., 1972, 272. 3 T. Tsunoda, H. Ogasawara, M. Komiyama, S. Ozawa and Y. Ogino, Chem. Lett., 1981, 819. 4 T. Iizuka, M. Kojima and K. Tanabe, J. Chem. SOC., Chem. Commun., 1983, 638. 5 R. L. Moss, Catalysis (The Chemical Society, London, 1977), vol. 1, p. 45. 6 R.A. Dalla Betta, A. G. Piken and M. Shelef, J. Catal., 1975, 40, 173. 7 G. V. Antoshin, E. S. Shpiro, 0. P. Tkachenko, S. B. Nikishenko, M. A. Ryashentseva, V. I. Avaev and Kh. M. Minachev, Proc. 7th Znt. Congr. Catal. (Kodansha Ltd., Tokyo; Elsevier, Amsterdam, 1981), part A, p. 302. 8 S. Palfi, A. Sarkany and P. Tetenyi, J. Chem. SOC., Faraday Trans. 1, 1981, 77, 177. 9 M. Komiyama, Y. Ogino, Y. Akai and M. Goto, J. Chem. SOC., Faraday Trans. 2, 1983, 79, 1719. 10 M. Komiyama, T. Tsunoda and Y. Ogino, Carbon, in press. 1 1 M. Komiyama, T. Okamoto, and Y. Ogino, J. Chem. SOC., Chem. Commun., 1984, 618. 12 H. C. Yao and M. Shelef, J. Catal., 1976,44, 392. 13 M. F. L. Johnson and V. M. LeRoy, J. Catal., 1974, 35, 434. 14 R. Nakamura, K. Ichikawa and E. Echigoya, Nippon Kagaku Kaishi, 1978, 36.15 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry (Interscience, New York, 2nd edn, 16 J. H. E. Griffiths, J. Owen, and I. M. Ward, Proc. R. SOC. London, Ser. A , 1953, 219, 526. 17 A. Carrington, D. J. E. Ingram, D. Schonland and M. C. R. Symons, J. Chem. SOC., 1956,4710. 18 W. Low and P. M. Llewellyn, Phys. Rev., 1958, 110, 842. 19 N. S. Garif’yanov, Sou. Phys. JETP, 1964,18, 1246. 20 R. 0. Rahn and P. B. Dorain, J. Chem. Phys., 1964,41, 3249. 21 L. V. Borisova, I. N. Marov, Yu N. Dubrov and A. N. Ermakov, Russ. J. Znorg. Chem., 1971,17,1607. 22 H. H. Pieper and K. Schwochau, J. Chem. Phys., 1975,63,4716. 23 M. Valigi, D. Cordischi, D. Gazzoli, C. P. Keijzers and A. A. K. Klaassen, J. Chem. SOC., Faraday 24 R. Wilson and D. Kivelson, J. Chem. Phys., 1966,44, 154. 25 L. D. Rollmann and S. I. Chan, J. Chem. Phys., 1969, 50, 3416. 26 H. G. Hecht and T. S. Johnston, J. Chem. Phys., 1967,46, 23. 27 E. Serwicka, J. Solid State Chem., 1984, 51, 300. 28 J. C. Eisenstein, J. Chem. Phys., 1960, 33, 1530. 29 L. F. Mattheiss, Phys. Rev., 1969, 8, 135. 30 E. Brocklawik, J. Haber and L. Ungier, J. Phys. Chem. Solids, 1981, 42, 203. 31 W. Moffitt, G. L. Goodman, M. Fred, and B. Weinstock, Mol. Phys., 1959, 2, 109. 32 J. H. Lunsford, Catal. Rev. Sci. Eng., 1973, 8, 135. 33 J. Beringhelli, A. Gervasinic, F. Morazzoni, D. Strumolo, S. Martinego and L. Zanderighi, J. Chem. 34 M. Che and A. J. Tench, Advances in Catalysis (Academic Press, New York, 1983), vol. 32, p. 1. 1966), p. 45. Trans. 1, 1981 77, 1871. SOC., Faraday Trans. 1, 1984, 80, 1479. Paper 51925; Received 31st May, 1985

ISSN:0300-9599    

DOI:10.1039/F19868201713

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1986,82, 1721-1731 Surface-charging Effects in the X-Ray Photoelectron Spectra of some Semiconducting Oxides Simon J. Cochran Department of Chemistry, Monash University, Clayton, Australia 3168 Frank P. Larkins* Department of Chemistry, The University of Tasmania, Hobart, Australia 7001 The irradiation of poor electrical conductors in X-ray photoelectron spectroscopy induces an equilibrium charge at the sample surface. The resulting shifts in the spectra have been measured for a number of semi- conducting metal oxides (NiO, Co,O, and ZnO) with a variety of pretreat- ment conditions. Heating the samples, or the adsorption of electron-donating or-withdrawing gases, causes potential shifts which could be related to the electrical properties of the substrate.These effects were investigated for nickel, cobalt and zinc oxides with the adsorbates NO, NO,, NH,, CO and 0,. The reactive system NO + 0, was also investigated with the nickel oxide catalyst. It is found that, for a given sample configuration, this effect is reproducible, and can provide important information on the nature of adsorbed species. Electricalconductivity measurement is a frequently used method for obtaining information on the electronic properties of a solid.l-, Absorption may cause changes in the bulk conductivity of the substrate. Measurement of such changes is, however, subject to a number of experimental difficulties. In particular, sample fragility and poor reproduci- bility have limited the accuracy which can be obtained from the conductivity.Conductivity changes have also recently been shown to be reflected in surface-charging effects in photoelectron This effect is evident in earlier work,’ although its significance was apparently not realised at the time. Most of the earlier studies using classical techniques have employed relatively poor vacua, and it has been demonstrated in a previous paper8 that surface cleanliness is important in the electronic behaviour of transition metal oxides. Therefore, a study of the effects of impurities under u.h.v. conditions may provide further insight into earlier work. Irradiation of insulators and semiconductors with X-rays results in the presence of an equilibrium surface charge as a consequence of the loss of photoelectrons during irradiation.This changes the effective surface potential, resulting in shifts in the kinetic energies of the photoelectron lines. The origins and problems associated with this effect have been discussed by Ebel and Ebel.9 The magnitude of the charge depends primarily upon (a) the rate of loss of electrons by photoionization, i,, (b) attraction of stray electrons to the positively charge sample surface, i,, and (c) conduction through the sample from the earthed holder, i,. This is illustrated diagrammatically in fig. 1. There have been several investigations into the surface charge on insulators and insulated con- ducting films9-12 where the contribution from (i,) has been shown to be very small. However, there have been few attempts to study the charging effects on polycrystalline semiconducting sample^.^ Ley et al.13 did however establish a broad relationship between sample charging and band gap for a series of freshly cleaved single crystals of semi- conductor.For a fixed X-ray flux and sample configuration and assuming the thermal electron 17211722 Surface-charging Efiects - I Fig. 1. Schematic diagram of electron flow under irradiation by X-rays, i, is the current due to ejected photoelectrons, i, represents stray thermal electron and i, conduction through the sample. current (iz) to be constant, the relative charge depends only on i,, thus giving an indirect measure of sample conductivity. It has recently been shown5 that X.P.S. can provide useful information on the donation or withdrawal of electrons by adsorbates.Thus, measurement of sample charge offers additional evidence for the assignment of the spectra of adsorbed species. Sample conductivity depends on the following factors : (a) temperature ; (b) adsorbate coverage ; (c) impurities, e.g. contamination, doping; (d) non-stoichiometry ; and (e) contact resistance between particles of the polycrystalline material and between the sample and the holder. The doping of semiconducting oxides with foreign ions has been the subject of many studies of conductivity.14~ l5 The doping of a p-type oxide such as nickel oxide with univalent ions such as lithium or potassium produces a corresponding increase in the number of NilIJ ions in the NiII lattice. This is equivalent to a downward bending of the conduction band and results in an increase in the conductivity of the material.Likewise, the doping with trivalent iron or chromium ions reduces the number of NiIII charge carriers and therefore the conductivity. It has also been shown that adsorption of a gas capable of accepting electrons from the lattice of a p-type semiconductor also increases the number of charge carriers, while for an electron donating species, the reverse is true.16 By contrast, in an n-type oxide such as zinc oxide, where electrical conduction is by an excess of electrons in the lattice, doping or adsorption with a similar species will have the reverse effect. For example, electron- donating gases such as CO decrease the bulk conductivity of NiO, whereas CO and H, increase the conductivity of Zn0.l' Gases which are only weakly adsorbed as a molecular species, such as CO,, do not change the substrate conductivity.Two potential applications of the variations in surface charge are (1) as an aid in the identification of adsorbed species and (2) in understanding the differences in catalytic properties of materials. The aim of this work is to analyse systematically the data obtained for selected transition-metal oxides, namely nickel oxide (NiO), cobalt oxide (Co,04 ) and zinc oxide (ZnO) with a range of adsorbates, NO, NO,, NH,, CO and 0,. Our goal is to examine the feasibility of quantifying the charging phenomenon and to examine the various factors which contribute to the effect.S. J. Cochran and F. P . Larkins 1723 Experiment a1 X.p. spectra were recorded on a modified AEI (Kratos) ESlOO equipped with a sample preparation chamber as described previously,s using A1 K, radiation operating at 15 kV and 15 mA.Base pressui ,s in the analyser and sample preparation chambers were better than 2 x 10-lo Torr (1 Torr z 133.3 Pa) and 1 x Torr, respectively. Samples were either pressed directly into an earthed copper holder which was attached to a variable- temperature probe or first pressed into a pellet using a 10 tonne press and then mounted on a stainless-steel holder. The samples were rectangular (5 mm x 16 mm) with a thickness of < 0.5 mm when pressed directly into the holder and a thickness of ca. 1 mm when prepared as pellets. The area of the sample irradiated by the X-ray beam was ca. 25% of the exposed surface. Spectra were calibrated using the Au(4f,,,) line binding energy = 83.98 eV).18 Pure oxide samples of nickel, cobalt and zinc were obtained from Johnson Matthey (Specpure grade).A high-surface-area nickel oxide (140 m2 g-l) was obtained from Univar. Samples were normally pretreated in situ in the sample preparation chamber by heating at 300 "C in 100 Torr oxygen for 1 h, followed by evacuating and heating at 200 "C in vacuo to remove excess oxygen. C.P. grade nitric oxide from Matheson was purified by multiple freeze-pumpthaw cycles on a separate vacuum line and passed through a dry-ice-acetone bath before admission to the spectrometer. Gas purity was verified by use of a mass spectrometer. Ammonia and NO, were similarly purified. C.P. grade CO from Matheson and high-purity 0, from CIG were passed through a dry-ice bath before use.The adsorption conditions involved exposure of 0.1 Torr of gas for 15 min at room temperature in the sample preparation chamber for nickel and cobalt oxides prior to evacuation and recording of spectra. For zinc oxide the adsorptions were at 100 K because at room temperature no marked charging effect was observed. Potential shifts were measured as an effective shift in the binding energy of the 0 (1s) photoelectron line of the lattice oxygen from that of the uncharged oxide. This is generally the sharpest line in the spectra of metal oxides and is not sensitive to minor changes in the chemical environment of the surface layer. The lattice 0 (1s) binding energies for the uncharged oxides were 529.6, 529.6 and 530.1 eV for nickel, cobalt and zinc oxides, respectively.The shifts of several other lines in the spectrum in addition to the lattice 0 (1s) line were also measured in a number of experiments, including Auger lines, and all shifts were identical within experimental error. This result was also obtained by Wagner. l1 Peak areas were determined by deconvolution of spectra using Gaussian lineshapes after suitable smoothing, background subtraction and satellite removal. Spectral inten- sities were corrected using theoretical cross-sectionslg and an experimental analyser sensi tivi ty function.20 Results Temperature-dependent Charging Effects The effect of temperature on the surface charge of the nickel, cobalt and zinc oxide samples pressed directly into an earthed copper holder is shown in fig.2. These samples were pretreated in oxygen as outlined in the previous section. After the pretreatment zinc oxide will be oxygen deficient (n-type semiconductor) and therefore conductive throughout the temperature range investigated. Nickel and cobalt oxides behave as p-type semiconductors and are almost stoichiometric after pretreatment. The p-type oxides have relatively low conductivity under high-vacuum conditions and the surface potential depends strongly on temperature. Semiconductor conductivity is known to obey the:1724 Surface-charging Eflects T/K Fig. 2. Effect of temperature on the surface potential of (a) high surface area NiO, (b) Specpure Co,O, and ( c ) Specpure ZnO. equation log (c) = - k / T , where c is the bulk conductivity and k is a constant.21 Because of the variation in conductivity values reported in the literature for these oxides, which demonstrates that the sample history is of great importance, it was not practical to use such values in analysing the present data.Smart22 found that a plot of log (Kh) US. T1 was linear over part of the temperature range investigated. The results of the present study, where the temperature range of interest is below room temperature, are not consistent with a linear plot over a significant temperature range. Charging was found to be completely reversible in all cases, and reproducible to within k0.2 eV between samples with the same configuration and pretreatment. The major uncertainty was in the temperature of the sample surface.This uncertainty poses a restriction on the quantitative information which may be obtained from these results, amounting to ca. & 5% in typical cases. Adsorbatedependent Charging Effects The effect of adsorption on the surface charge for a number of adsorbate-substrate systems is shown in table 1. The oxide samples for this work were pelletised before mounting on the metal holder. Adsorbate coverages, estimated either from signal attenuation or by the method of Carley and were in good mutual agreement and were generally in the range 0.1-0.5 mon01ayers.~~ The measured oxygen adsorption on freshly prepared nickel and cobalt oxides was, however, more than one monolayer. This result was due to oxygen incorporation into the lattice. Comparison of the results for NO and CO on NiO with those of Roberts and Smart5 shows that the present results are consistent in sign, but vary in magnitude.Roberts and Smart recorded shifts of - 2.2 and 0.7 eV for NO and CO adsorbed on a sample calcined at 1450 "C. However, the potential shift on the clean sample was only 3.6 eV, and values obtained on samplesS. J. Cochran and F. P. Larkins 1725 Table 1. Effect of adsorbate on surface charge at room temperaturea substrate NO -0.7 -0.3 1 .o NO2 -2.1 -1.2 2.9 co 0.6 0.2 -2.4 -6.7 -3.5 - NH3 -1.6 -0.7 1.1 0 2 NO+02 - 7.4 - - a Values in the table are potential shifts (eV) relative to the potential on clean surfaces of 8.6, 4.2 and 3.1 eV for NiO, Co304 and ZnO, respectively. ZnO recorded at 100 K, NiO and Co30, at 300 K. Gas exposures were 0.1 Torr for 15 min followed by evacuation at the temperatures indicated.calcined at lower temperatures were less. By comparison, shifts of up to 8.6 eV were observed in this study. Large shifts are by no means uncommon, as shifts of 10 eV or more have been reported by others.2s The differences are attributed to somewhat cleaner surfaces resulting from the pretreatment procedure, variations in contact resistance and a different spectrometer configuration used in the present study: Several of the spectra presented by Roberts and Smart5 also show evidence for metallic nickel due to surface reduction.8 The adsorption of ammonia on NiO provides an illustration of the use of charging effects in the assignment of absorbate spectra. It can be seen from table 1 that ammonia adsorbed at room temperature is electron-withdrawing overall, producing a decrease in surface potential of 1.6 V.However, exposure of NiO to 0.1 Torr of ammonia for 15 min at 100 K in a separate experiment produced an increase in the equilibrium surface potential of 0.4V. This is consistent with the proposition that ammonia absorbs molecularly at 100 K with electron donation into the substrate through the lone pair. A single N (1s) peak is observed at 399.7 eV. As the temperature is raised above 200 K a second N (1s) peak is observed at 398.0 eV and the surface potential decreases. This observation is assigned to the formation of the NH2-species2* at the higher temperature. The effect of raising the temperature on the surface charge of a nickel oxide sample pressed directly into the holder and exposed to 0.1 Torr of NO for 15 min at 100 K followed by evacuation is shown in fig.3(b). Fig. 3(a) shows the curve produced by a clean NiO surface [fig. 2(a)] which was included for comparison. The range over which the two curves diverge (150-200 K) may be associated with the formation of the electron-withdrawing species on the sample surface. At 100 K the dominant adsorbate species has an N (1s) peak at 402 eV, attributed to the molecular species, but on warming the sample a new N (1s) peak at 400.5 eV is observed, which increases in intensity with temperature relative to the 402 eV peak. From the binding energy and surface potential changes the adsorbate species is assigned as NO-.24 It is the relative, not the absolute, potential changes for a particular sample with various treatments which should be reproducible for different spectrometers and experimental conditions.The relative changes provide the information to interpret behaviour at the oxide catalyst surface.1726 Surface-charging Efects Effect of Contamination on Charging The change in the potential shift for a fresh sample pelletised and mounted on the stainless steel holder as a function of pretreatment temperature is shown in fig. 4. In these experiments, samples were heated in vacuo in the spectrometer for 1 h at the temperature indicated and then cooled to room temperature and the spectrum recorded. They were not pretreated with oxygen, as was the case for the results presented in fig. 2. In the case of nickel oxide [curve (a)], as the preheating temperature is increased the surface charge, as measured by the shift in the 0 (1s) line, increases, reaching a maximum at ca.200 "C. This corresponds to the loss of loosely bound water, some of the carbon contamination and other species. Further heating results in the reduction of the oxides, but does not alter significantly the surface potential.* Thus, following reduction, Ni atoms must form small isolated clusters on the sample surface. Only after very significant reduction of the highly active Univar oxide does the charge begin to reduce, indicating some metallic conduction. A similar result is seen for cobalt oxide [fig. 4(b)]. However, zinc oxide [fig. 4 (c)] displays a different characteristic. As the temperature is raised the potential goes through a maximum before reducing to zero.This may be attributed in the first instance to the loss of mobile hydroxyls on the surface, which decreases conductivity, followed by the loss of lattice oxygen to produce the oxygen-deficient non-stoichiometric oxide with increased conductivity. The surface oxygen can also be correlated with the change in surface potential of theS. J. Cochran and F, P. Larkins ' O n 1727 0 100 200 300 pretreatment temperature/"C Fig. 4. Effect of preheating temperature on the surface potential of (a) high surface area NiO, (b) Specpure Co,O, and (c) Specpure ZnO. high-surface-area nickel oxide pellet mounted on a metal holder. Fig. 5 shows the effect of the ratio of the surface oxygen peak at 53 1.4 eV to the lattice oxygen peak at 529.6 eV as measured by the relative spectral intensities as described earlier.The surface/lattice oxygen ratio was varied by preheating the sample for an appropriate time in order to remove surface oxygen species. There is clearly a considerable electron-withdrawing effect by such species. With a large amount of surface oxygen, the number of charge carriers is greater, and the potential shift due to chxging is lower. Effect of X-Ray Flux on Surface Charge Fig. 6 illustrates the variations of surface charge with X-ray power for high-surface-area nickel oxide and alumina samples pressed into a copper holder. The result for alumina, obtained at 298 K, is consistent with previous studies9* 22 It is obvious that the characteristics of the two materials are quite different.Whereas the charge on alumina saturates at a power well below that normally used in X.P.S. experiments, for high- surface-area nickel oxide studied at 100 K the charge does not saturate until a substantially higher flux is used. It is clear that in this case sample photoconductivity does reduce the surface charge on nickel oxide at low temperature. For samples held at low temperature, the thermal effect of the X-rays may also be important. However, on exposure of a sample with an adsorbed gas to the X-ray beam, only a small increase in system pressure was observed as a result of sample heating. Ebel and Ebelg have investigated the relationship between sample charge and X-ray photo flux for a number of insulators. They found that for their system charging was saturated at ca.4 kV and 160 W. The present work is primarily concerned with the effect on charge as a function of sample treatment on semiconductors. However, fig. 6 shows1728 Surface-charging Efects 10 8. > 2 6 .A Y 5 Y 0 a 0 g 4 v) 2 0 o( 1s)531.6/0( lS)529.6 Fig. 5. Variation of surface potential with non-stoichiometric oxygen measured by the spectral intensity ratio on high surface area NiO. 10- 210 1 1 1 0 5 10 15 X-ray genera tor volt age/ kV Fig. 6. Effect of X-ray flux on surface potential: (a) alumina at 298 K, (b) high surface area NiO at 100 K.S. J. Cochran and F. P. Larkins 1729 that under normal operating conditions in our spectrometer (15 kV, 15 mA) the semiconductors can be charged further by increasing the X-ray flux.Discussion Correct potentials are, in general, non-ohmic and will vary with crystallite size, the contact resistance between the particles of the polycrystalline material and the contact resistance between the sample and the holder. In this work, samples which were pressed into a pellet and then mounted on a stainless steel holder were in poor electrical contact with the spectrometer and exhibited larger charging shifts and line-broadening effects than those which were pressed directly into a copper holder. For example, the high-surface-area nickel oxide mounted as a pellet and used for the adsorbate study reported in table 1 had a potential shift of 8.6 eV at 300 K for the clean surface, while for a sample of the same oxide directly pressed into the holder and used for the temperature-dependence study presented in fig.2 and 3, the potential shift at 300 K was only 0.1 eV. An initial relatively high surface potential was an advantage for adsorbate studies because changes due to the presence of different adsorbate species were more easily observed at room temperature. The data in table 1 indicate, for example, that when NO is adsorbed on a high-surface-area nickel oxide pellet at 300 K a potential shift of - 0.7 eV was recorded relative to the potential on the clean surface; however, for a sample pressed directly into the holder and used to obtain the data presented in fig. 3 the relative potential shift was only - 0.1 eV at 300 K with the NO adsorbate present. Although the magnitudes of the energy shifts are different, they have the same sign in both cases and one can conclude that the adsorbate species is electron withdrawing from both experiments.Similar circumstances prevailed for the study of the effect of surface contamination and the charging study presented in fig. 4 and 5. For pressed samples relatively good electrical contact is established between the thin sample (< 0.5 mm thickness) and the metal support; consequently, the major conduction path for such polycrystalline oxides is in this direction and not across the irradiated surface. This does not imply, however, that bulk conductivity is higher than conductivity on the surface of the particles. This observation is the consequence of a geometric effect related to the shape and size of the pellets and the fact that polycrystalline materials are used.Measurement of surface charging can aid interpretation of adsorption results. The temperature dependence of the charging associated with ammonia adsorption on nickel oxide mentioned earlier and that for nitric oxide presented in fig. 3 are interesting examples. In the latter case the potential shift relative to the clean oxide surface is -4.8 eV at 150 K, compared with only -0.1 eV at 300 K. The change can be related to the concentration and nature of the adsorbed species. Another interesting finding is that the coadsorption of NO and 0, results presented in table 1 shows an additive effect on the surface charge. This finding is consistent with the independent adsorption of these gases. NO may adsorb onto surface nickel atoms, while 0, may dissociate into 0,- ions which are incorporated into the lattice.From the data in table 1 it is concluded that the adsorption of oxygen on NiO has a more dramatic effect on the surface potential (- 6.7 eV) than either NO (-0.7 eV) or CO, (0.6 eV), consistent with the incorporation as 02-. Previous did not observe such changes in the spectra on exposure to oxygen below 350 "C. The difference is attributed to the higher surface area and stoichiometry of the oxide used in the present study. The absence of significant spectral broadening effects in our work due to differential charging indicates that the part of the catalyst surface being analysed acquires an effectively uniform potential. For example, in the case of NiO, the lattice 0 (1s) line was broadened from 1.4 to 1.8 eV when the surface charge increased from 0.0 to 8.6 V at1730 Surface-charging Efects room temperature.However, more severe broadening effects of up to 2.8 eV, probably due to thermal effects, were observed on samples cooled to 100 K. Smart2, has investigated the effect of temperature on the surface charge on nickel oxide in the range 20-400 "C and has interpreted the change in terms of a band model and calculated an activation energy of 0.23 V. However, it was assumed in that paper that the bulk conductivity (a,) was independent of temperature. This is clearly not a good assumption, as demonstrated by the present work, and it may be that about half of this activation energy can be accounted for by variations in oo.For adsorbate studies clearly the relationship between surface charge in X.P.S. and bulk conductivity is not a simple one, and further work is necessary for quantitative interpretation. In the case of a p-type semiconductor the adsorption of an electron-withdrawing species effectively increases the number of charge-carrying positive holes and thus the conductivity. The reverse is the case for an n-type semiconductor. This may be explained in band theory by a bending of the conduction bands downwards, thus they will have a higher occupancy and the material will become more conductive. It has generally been assumed in previous r e ~ e a r c h ~ ? ~ that the current from stray electrons (i, in fig. I) is independent of the sample charge. This is not necessarily a valid assumption, particularly where large potential changes are observed.Unfortunately, this assumption is not easy to test experimentally. Smart2, has proposed a mechanism such as Zener breakdown to account for the surface charge reaching a maximum as conductivity decreases. However, an increasingly positive surface should become a more effective sink for stray electrons, and this dependence cannot be neglected. Conclusions Changes in the effective surface potential arising from temperature and adsorption effects can provide important information on the electronic properties of semiconducting surfaces. Electron donation and withdrawal by adsorbates can readily be detected, even when spectra of the adsorbates are very weakly bound. NO, NO, and NH, adsorb as electron withdrawing species, while CO is electron donating. Although a comprehensive quantitative theory has yet to be developed, semi-quantitative data can provide a basis for comparison between different adsorption systems.Surface charging is a complex phenomenon depending upon several factors, but with careful work the effects are reproducible. By suitable choice of sample geometry and temperature; the effects of adsorption can be studied for a range of systems more conveniently than by traditional techniques. Provided the X.P.S. technique can be adequately calibrated, it offers a rapid and simple means of obtaining further information about the adsorption process which will assist in the assignment of adsorbed species. Financial assistance from the Australian Research Grants Scheme for this research is gratefully acknowledged. References 1 S.P. Mitoff, J. Chem. Phys., 1961, 35, 882. 2 R. W. Wright and J. P. Andrews, Proc. Phys. Soc., 1949,62A, 446. 3 E. Heiland, E. Mollwo and F. Stockman, Solid State Phys., 1959,8, 191. 4 M. W. Roberts and R. St. C. Smart, Chem. Phys. Lett., 1980, 69,234. 5 M. W. Roberts and R. St. C. Smart, Surf. Sci., 1980, 100, 590. 6 M. W. Roberts and R. St. C. Smart, J. Chem. Soc., Faraday Trans. I , 1984,80,2957. 7 W. P. Dianis, Ph.D. Thesis (Northwestern University, 1974). 8 S. J. Cochran and F. P. Larkins, J. Chem. Soc., Faraday Trans. I , 1985,81,2179. 9 M. F. Ebel and H. Ebel, J. Electron Spectrosc. Relat. Phenom., 1974, 3, 169. 10 H. Ganska, H. J. Freund and G. Hohlneicher, J. Electron Spectrosc. Relat. Phenom., 1977, 12, 435.S. J. Cochran and F. P. Larkins 1731 1 1 C. D. Wagner, J. Electron Spectrosc. Relat. Phenom., 1980, 18, 345. 12 R. B. Bjorklund and J. Lundstrom, J. Catal., 1983, 79, 314. 13 L. Ley, R. A. Pollak, F. R. McFeely, S. P. Kowalczyk and D. A. Shirley, Phys. Rev. B, 1974, 9, 600. 14 A. Bielanski, J. Deren, J. Haber and J. Sloczynski, Trans. Faraduy Soc., 1962, 58, 166. 15 J. Notwotny and J. B. Wagner, Bull. Pol. Acad. Sci., Ser. Sci. Chem., 1973, 21, 931. 16 I. A. Myasnikov, Russ. J Phys. Chem. (Engl. Transl.), 1960, 34, 183. 17 Y. Kobokawa, Bull. Chem. SOC. Jpn, 1960,33,739. 18 R. J. Bird and P. Swift, J. Electron Spectrosc. Relat. Phenom., 1980, 21, 227. 19 J. H. Scofield, J. Electron Spectrosc. Relat. Phenom., 1976, 8, 129. 20 A. Lubenfeld, MSc. Thesis (Monash University, 1977). 21 J. B. Goodenough, Prog. Solid State Chem., 1971,5, 145. 22 R. St. C. Smart, Surf: Sci., 1982, 122, L643. 23 A. F. Carley and M. W. Roberts, Proc. R. SOC. London., Ser. A, 1978,363,403. 24 S. J. Cochran and F. P. Larkins, unpublished results. 25 M. W. Roberts and R. St. C. Smart, Surf. Sci., 1981, 108, 271. 26 K. S. Kim and N. Winograd, Surf. Sci., 1974,43,625. Paper 51945; Received 3rd June, 1985

ISSN:0300-9599    

DOI:10.1039/F19868201721

出版商:RSC    

年代:1986

数据来源: RSC