Palladium enhanced resistance to deactivation of titanium dioxide during the photocatalytic oxidation of toluene vapors

Different Pd/TiO2 samples were prepared with metal loading ranging from 0.1 to 1% (w/w) and using two different titanium dioxide powders samples as support. The XRD data reveal that the incorporation of palladium can increment slightly the rutile to anatase ratio, but it does not change the textural parameters of TiO2 extensively. The H-2 chemisorption measurements indicate that the dispersion of palladium decreases on increasing the metal loading. The Pd particles remain in a metallic state after reduction in hydrogen and the subsequent exposure to atmospheric air, as revealed by UV-Vis DRS. Most of the Pd/TiO2 catalysts show a considerable increment of the conversion of toluene vapor after prolonged UV-irradiation, meanwhile the benzaldehyde production decreases slightly with respect to bare TiO2. However, this enhancement of the photoactivity is not observed initially and it is independent of the palladium content and dispersion. Therefore, the main effect of palladium is to hinder the deactivation of the photocatalysts. On the other hand, the improvement of the photocatalytic performance also depends on the characteristics of TiO2, since for pure anatase of low surface area the addition of palladium is slightly detrimental for the photoactivity. The presence of water vapor in the feed stream is beneficial for the photoactivity for both Pd-loaded and neat TiO2. In contrast, a moderate increment in the operation temperature is not beneficial for the photoactivity, despite Pd/TiO2 samples present a significant thermal activity in the dark. FTIR analyses of the used Pd/TiO2 catalysts indicate that the surface of both the metallic particles and the support are partly covered with by-products of the photo-oxidation of toluene. A limited oxidation of the palladium particles also takes place during photoreaction. (C) 2003 Elsevier B.V. All rights reserved.