EPR study of the radicals formed upon UV irradiation of ceria-based photocatalysts

EPR measurements reveal remarkable differences on the type of radicals produced after UV illumination of TiO2, CeO2 and 0.8% CeO2/TiO2 photocatalysts. Photoactivation of the TiO2 sample in vacuum results in the formation of Ti4+-O- species and a small amount of Ti3+ centers. In the presence of adsorbed oxygen, irradiation of this material also generates Ti4+-O-3(-) radicals. In the case of the CeO2/TiO2 catalyst, the ceria component is present in a highly dispersed state, as indicated by XRD and UV-Vis diffuse reflectance spectra (DRS) results. Accordingly, the only type of Ce4+-O-2(-) adducts generated on the CeO2/TiO2 sample are indicative of the presence of two-dimensional patches of ceria on the anatase surface. On the other hand, photoactivation of the CeO2/TiO2 sample in the presence of oxygen also leads to the formation of some Ti4+-O- and Ti3+ centers. In the case of the CeO2 sample, superoxide radicals are observed upon irradiation in vacuum and subsequent oxygen adsorption. Further irradiation of this material in the presence of oxygen increases the amount of Ce4+-O- radicals and simultaneously generates new species, which are tentatively assigned to Ce4+-O2H radicals. Photocatalytic activity was tested for toluene oxidation, and the results obtained show that the photodegradation rate is slightly lower for CeO2/TiO2 than for the TiO2 sample. However, the selectivity towards benzaldehyde (6-13%) is comparable for both materials. In the case of CeO2 the photo-oxidation rate is an order of magnitude lower than for TiO2 although mineralization of toluene is almost complete. Photoactivity results are discussed in connection with the characteristics of the radicals observed. (C) 2002 Elsevier Science B.V. All rights reserved.