Browsing by Author "Che, Michel"

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    Electron paramagnetic resonance study of electron transfer at the surface of alkaline earth oxides. Reversible pyridine-induced formation of superoxide ions labelled with 17O
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (05), 1978) Che, Michel
    In the absence of irradiation, it is necessary to use the method of electron transfer between adsorbed species (ETBAS) to stabilise the superoxide ions at the surface of alkaline earth oxides. Identification of the oxygen species as O–2 ions was carried out using 17O-enriched oxygen; results thus obtained were in line with those previously reported for other oxides: two equivalent oxygen nuclei, Axx= 76 G, Ayy and Azz too small to be resolved. The splitting of the 2pπ* orbitals by the surface crystal field, as measured from the gzz component of the g-tensor depends on the polarizing power q/r of the surface cation acting as the adsorption site. The electron donor sites producing the O–2 molecular ions are the surface, coordinatively unsaturated, O2– ions.
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    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.
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    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.