Browsing by Author "Tench, A. J."

Now showing 1 - 3 of 3

Oxygen Species Adsorbed on CeO2/SiO2 Supported Catalysts
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (5), 1973) Che, M.; Kibblewiiite, J. F. J.; Tench, A. J.
The nature of the oxygen species adsorbed on slightly reduced CeO2/SiO2 supported catalysts has been investigated using oxygen enriched with 17O2. The observed hyperfine splitting was found to be associated with the central component of the g tensor; it indicated that the corresponding orbital contained the unpaired electron and therefore the central component was defined as gxx. With this assumption, the e.s.r. spectra are consistent with the adsorption of oxygen as O–2 at 77 K with gyy= 2.0109, gxx= 2.0158 and gzz= 2.028 with a hyperfine splitting (Axx) of 75 G about gxx. The O–2 is adsorbed with the internuclear axis along the surface of the catalyst at a site corresponding to a cerium ion. There is some indication that interaction with the metal ion orbitals is perturbing the energy levels of the adsorbed O–2 ion.
Oxygen Species Adsorbed on Oxides: Part 1.—Formation and Reactivity of (O_)s on MgO
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (7), 1972) Tench, A. J.; Lawson, T
Nitrons oxide reacts with F*(H) centres (surface oxygen ion vacancies with a single trapped electron) to give (O-)/' ions adsorbed at the anion vacancy. The reaction, FS+(H)+N2O ->(O_)t± + N2 is quantitative and the reflectance spectrum indicates a band centred at 2 eV which is ascribed to the (O~)+ centre. Comparison of the optical transition energy calculated from the e.s.r. data with the observed transition shows that the normally accepted value of 135 cm’1 for the spin-orbit coupling of the O_ ion is too low and that a value of >200 cm-1 is more appropriate for the solidstate. Nitrous oxide also reacts with other surface centres to give an adsorbed species which may be (O2_)s or (O2)s- The reaction of (O_)s+ with hydrogen reforms the FS+(H) centre which then reacts with more nitrous oxide; the react ion can be cycled several times. The reactions of (O-)+ with O2, CO2, alcohols and olefins are also investigated. To facilitate reference to various types of surface defects and adsorbed species a nomenclature is proposed which is compatible with a recently suggested notation for defects in bulk lattice.
Oxygen Species Adsorbed on Oxides: Part 2.—Formation of (O3X on MgO)
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (7), 1972) Tench, A. J.
MgO surface containing adsorbed (O~)s ions reacts with excess oxygen to form the molecular ion (Oajs- This ion has been characterized by e.s.r. using oxygen enriched with 17O and shows a complex hyperfine structure. Heating at 400 K converts the (Oj)s ions to (O2)s and an isotopic exchange occurs between the various oxygen nuclei. The optical data indicate that a band close to 420 nm is associated with the (O2)s species. Calculations using electron affinity and bond energy data indicate that the reaction O“+O2->O2 is exothermic and that (O2)s must be considered as a possible adsorbed oxygen species on oxides.