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

The influence of the incorporation of chloride, either by direct impregnati on with HCl solution or during deposition of metals like Au and Rh in catal yst preparations using chloride salts as precursors, on the surface propert ies of CeO2 is studied with EPR, by following the effects of outgassing tre atments at temperatures T-v = 373-773 K and the paramagnetic species genera ted by subsequent low temperature O-2 adsorption. Results on HCl-impregnate d CeO2 show the generation of different Ce4+-O-2(-) species at chloride-fre e and chloride-influenced sites of the ceria surface, which are discerned b y analysis of their g values, particularly g(x). The increasing relative co ntribution 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, r eveals a trend of progressive dominance of chloride compounds on the sample surface with raising T-v. Analysis of the stability of the superoxide radi cals reveals that chloride-induced modification of the adsorption centers c onfers 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 > 5 73 K, a deeper reduction being produced in chloride-free ceria. In the case of metal-containing (gold or rhodium) samples, prepared using chloride sal ts as precursors, formation upon O-2 adsorption on samples outgassed at rel atively 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 an d 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 mor e effective in the case of the rhodium-containing system. (C) 1999 Elsevier Science B.V. All rights reserved.