Journal Articles (before-1995)
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Item Reaction between ethylene and O– ions on the surface of magnesium oxide(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (07), 1977) Taarit, Younes Ben; Symons, Martyn C. R.; Tench, Anthony J.Interaction between ethylene and O– centres on magnesium oxide surfaces gives a radical having a nearly isotropic hyperfine coupling to two equivalent hydrogen atoms (58 G), a very weak coupling to a single proton, and strong coupling to 13C which indicates nearly unit spin-density in a 2p orbital on a single carbon atom. These results agree with expectation for H2C[double bond, length as m-dash]Ċ– anions, isoelectronic with the relatively stable H2C[double bond, length as m-dash]N radical in every respect. We suggest that after initial abstraction to give H2C[double bond, length as m-dash]ĊH radicals (not detected), ionization of the acidic acetylenic type proton occurs but this remains hydrogen-bonded to the radical anion: H2C[double bond, length as m-dash]Ċ–—HO–. In contrast, reaction of ethylene oxide with F+s centres on the surface gave H2Ċ—CH2O– radicals.Item Photoluminescent Spectra of Surface States in Alkaline Earth Oxides(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (09-12), 1978) Coluccia, Salvatore; Deane, A. Michael; Tench, Anthony J.Photoluminescence has been observed from high surface area alkaline earth oxides with exciting light of a much lower frequency than that expected from the band gaps of the bulk oxides. This luminescence can be quenched by oxygen and hydrogen and the quenching is reversible under some conditions. It seems probable that both the excitation and the luminescence spectra observed arise from excitons in the surface region. The different behaviour towards oxygen and hydrogen has been used to develop a model in which the excitation site, where absorption of light occurs, is thought to be associated with anions on the surface in states of unusually low coordination such as might be found at step, edge or corner sites on the surface. The luminescence is thought to be associated with a cation on the surface in a similar state of coordination.Item Luminescence and Other Spectroscopic Studies of the Reaction of Pyridine and Oxygen with Thermally Activated SrO(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (09-12), 1978) Coluccia, Salvatore; Hemidy, Jean F.; Tench, Anthony J.The reaction of pyridine and O2 on SrO powder has been studied using several different spectroscopic techniques to characterise the intermediate and the final products. Reflectance spectroscopy has been used to demonstrate the formation of the 4,4′-bipyridyl negative ion when pyridine is adsorbed on SrO. Subsequent adsorption of oxygen leads to the formation of the O–2 ion by a one-to-one electron transfer process from the organic ion and photoluminescence studies confirm that the 4,4′-bipyridyl molecule is also formed on the surface when the electron transfer occurs.Item Electron paramagnetic resonance study of molybdenum supported catalysts labelled with 95Mo. Evidence for molybdenyl ions(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (09-12), 1978) Che, Michel; McAteer, James C.; Tench, Anthony J.Reduction of molybdenum supported on silica leads to X-band e.p.r. spectra with g⊥= 1.940 and g‖= 1.882. When molybdenum enriched in 95Mo is used, the hyperfine tensor can be measured as A⊥= 44 G and A‖= 98 G. The simulation of the experimental spectra has been carried out and is found to be satisfactory with the above magnetic parameters. Q-band spectra show an increase in the linewidth which prevents observation of the h.f. lines. This is due to a distribution in the components of the g tensor reflecting the presence of a number of molybdenum sites. The relative magnitude of the g-tensor components is discussed by comparing by comparing e.p.r. spectra of (NH4)2 MoOCl5 in dilute form or supported on silica with other molybdenyl compounds. It is observed that the relative magnitude of the g-tensor components of various molybdenyl compounds is very sensitive to the value of the spin–orbit coupling constant λ of the ligands and it is concluded that the present e.p.r. spectra can be assigned to pentavalent molybdenum surrounded by oxide ions and retaining one short Mo[double bond, length as m-dash]O bond.Item Electron Spin Resonance Studies of Isotopically Labelled Oxygen Species Adsorbed on Supported Molybdenum(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (1-6), 1974) Che, Michel; Tench, Anthony J.The molecular ion (O2)5 has been observed on the surface of the moIybdenum/SiO2 and molyb- denum/Al2O3 systems by e.s.r. Using oxygen isotopically enriched in 17O, two different hyperfine splittings were observed in the Mo/Si system corresponding to A^ = 85 and 72 G respectively; the Ayy and Az: values of the tensor were too small to be resolved. The observed values for A^ are shown to arise from an unequal interaction of the unpaired electron with two oxygen nuclei in the same molecular ion. This result is contrary to the previous work for (Ops on other oxides where the oxygen nuclei have been found to be equivalent; the difference is attributed either to a more covalent bonding with the surface ions or to adsorption at an unusual site at the surface. This is thought to be a sensitive function of the surface environment since the oxygen nuclei appear to be nearly equivalent for the Mo/Al system although the (/-tensors for both systems are almost identical.Item Reaction of Halogens with Oxide Surfaces(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (1-6), 1974) Kibblewhite, John F. J.; Tench, Anthony J.Charge transfer reactions at oxide surfaces are discussed for the halogens, chlorine, bromine and iodine as strong electron acceptors. Adsorption of chlorine and bromine on MgO prepared in vacuo is characterised by the appearance of a band at 430 nm in the diffuse reflectance spectrum and an e.s.r. signal with ^-factors of 2.0099 and 2.0020. These features are destroyed on heating and oxygen is evolved. Chlorine and bromine react with the lattice oxygen ions to give halide ions corresponding to about 20 and 10 % respectively of the surface MgO ion pairs ; no evidence of the formation of molecular X2 ions was obtained. These results can be understood in terms of the initial formation of charge deficient groups of oxygen ions, such as O"~ and subsequent release of oxygen. The reaction of bromine with MgO would not be expected purely on a consideration of the bulk heats of formation, and indicates an appreciable concentration of surface oxygen ions with less than 5-fold co-ordination. Iodine, as expected from the low heat of formation of the iodide, does not displace oxygen ions on the surface but some iodide ions are formed. All the halogens are able to displace oxygen adsorbed as O2 from the surface indicating that this ion is only weakly bonded to the surface.