The adsorption of O2 (using either normal O or 17O-enriched mixtures), on samples which had been out-gassed at various temperatures, was studied by using electron paramagnetic resonance and FTIR. Differing signals, which were attributed to O2- species bonded to surface Ce ions, were observed; depending upon the vacuum treatment temperature. The latter parameter determined the type of defect which was generated at the oxide surface. Depending upon their line-widths and lowest g-values, the O2- signals which were observed in the electron paramagnetic resonance spectra could be grouped into 2 types which were related to species absorbed at isolated and aggregated O vacancies, respectively. The electron paramagnetic resonance parameters indicated that the bonds of these species to the surface had various degrees of covalency; which might also affect their infra-red absorption coefficient. Whereas the two O atoms in the O2- species which formed on isolated surface O vacancies were electron paramagnetic resonance-equivalent, they were not equivalent in those which formed on the aggregated vacancies that were produced at higher temperatures. The differing electron paramagnetic resonance parameters, and the variations in their intensities (for various treatment temperatures), indicated that the electron paramagnetic resonance of adsorbed O2- and related species could be used as a probe for the study of the generation and properties of defects on CeO2 surfaces.
J.Soria, A.MartÃnez-Arias, J.C.Conesa: Journal of the Chemical Society - Faraday Transactions, 1995, 91[11], 1669-78