Characterization of surface defects in CeO2 modified by incorporation of precious metals from chloride salts precursors:: an EPR study using oxygen as probe molecule

The influence of the incorporation of chloride, either by direct impregnation with HCl solution or during deposition of metals like Au and Rh in catalyst preparations using chloride salts as precursors, on the surface properties of CeO2 is studied with EPR, by following the effects of outgassing treatments at temperatures T-v = 373-773 K and the paramagnetic species generated by subsequent low temperature O-2 adsorption. Results on HCl-impregnated CeO2 show the generation of different Ce4+-O-2(-) species at chloride-free and chloride-influenced sites of the ceria surface, which are discerned by analysis of their g values, particularly g(x). The increasing relative contribution of the Cl--influenced sites with increasing T-v, as indicated by the increasing relative contribution to the adsorbed oxygen spectra of the Ce4+-O-2(-) signal with the g(x) component shifted to the highest value, reveals a trend of progressive dominance of chloride compounds on the sample surface with raising T-v. Analysis of the stability of the superoxide radicals reveals that chloride-induced modification of the adsorption centers confers a higher stability to a reduced state of these sites, but hinders an extensive reduction of the modified sample surface when treated at T-v > 573 K, a deeper reduction being produced in chloride-free ceria. In the case of metal-containing (gold or rhodium) samples, prepared using chloride salts as precursors, formation upon O-2 adsorption on samples outgassed at relatively low T-v of only the O-2(-) signal (as confirmed by adsorption of O-17-enriched O-2) with the highest g(x) value (g(z) = 2.026, g(x) = 2.023 and g(y) = 2,013), reveals that the whole sample surface has become affected by the presence of chloride, suggesting that these precious metals promote the formation of cerium oxychloride-like microphases, the process being more effective in the case of the rhodium-containing system. (C) 1999 Elsevier Science B.V. All rights reserved.