The nature of the oxygen species adsorbed on slightly reduced Ce02/Si02 supported catalysts has been investigated using oxygen enriched with I7O2. 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 thecentral component was defined as gxx. With this assumption, the e.s.r. spectra are consistent with the adsorption of oxygen as 0; at 77 K with gyy = 2.0109,gxx = 2.0158 and gzz = 2.028 with a hyperfinesplitting(A,,) of 75 G about gxx. The 0; 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 0; ion.Recently, catalysts based on pure or supported CeO, have been used in oxidation reaction^."^ In order to understand the reaction mechanisms involved it is useful to identify the nature of the oxidizing species present since this may greatly influence the activity of the ~a t a l y s t . ~A number of different oxygen species such as 0-, 0, and 0 ; have been observed recently on MgO 5-7 and their reactivity ~haracterized.~. The most reactive species appears to be 0and similar conclusions were reached by Kazansky for 0formed on V 2 0 5 supported by silica.The various possible species of oxygen can be distinguished unambiguously by the use of isotopically labelled oxygen in e.s.r. studies.'O As a continuation of the studies of adsorbed oxygen species,' we used the hyperfine structure resulting from the nuclear spin of 5/2 for 1 7 0 to identify the form of adsorbed oxygen on reduced CeO, /Si02 catalysts. EXPERIMENTAL Supported Ce02 was prepared by impregnation of a Si02 carrier (specific surface area 640m2/g) with a Ce(N03)3 aqueous solution of known concentration. The catalyst was further washed with doubly distilled water, dried at 380 K and calcined in an oxygen atmosphere at 800 K ; the catalyst was found to contain 5.4 % by weight of cerium. Before oxygen adsorption the samples were outgassed in vaczio at 800 K for 12 h in silica tubes and oxygen was adsorbed at pressure of 10-2-10-1 Torr at 77 K using 1 6 0 2 and isotopically enriched O2 (58 atom % of 1 7 0 ) on various samples. E.s.r. measurements were carried out on a Varian V 4502 spectrometer at 9.2 GHz. Diphenylpicrylhydrazyl was used as a standard for g value calibrations. The values of the g tensor components were measured at the turning points as described in ref. (6) and the absolute accuracy is believed to be within JrO.0010. The values are quoted to 4 decimal t On leave of absence from Institut de Recherches sur la Catalyse.

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Methyl alcohol absorbs on MgO to give a coverage of one molecule for each " ion pair " on the surface. Infra-red studies of successive additions of the alcohol indicate the build-up of various alcohol species on the surface which can be characterized mainly by their CO stretching bands and also by the presence of OH and CH bands. Infra-red and gas chromatographic desorption studies in vucuo at room temperature and at elevated temperatures are used to investigate the stability of the adsorbed species. The four major adsorbed species are characterized by CO stretching frequencies at 1030,1060,1114 and 1080 cm-'. This latter species is stable to above 600 K and its disappearance is accompanied by the evolution of alcohol decomposition products. Comparative studies with deuterated alcohols confirm the assignments and indicate that some exchange reactions involving hydrogens of the methyl group occur at elevated temperatures.

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 vucuo is characterised by the appearance of a band at 430nm in the diffuse reflectance spectrum and an e x . signal with g-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 X i ions was obtained. These results can be understood in terms of the initial formation of charge deficient groups of oxygen ions, such as 0;-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 0, from the surface indicating that this ion is only weakly bonded to the surface.

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