Platinum/titanium dioxide catalysts. A photoconductivity study of electron transfer from the ultraviolet-illuminated support to the metal and of the influence of hydrogen

Abstract

Photoconductivity measurements have been carried out for a series of powder Pt/TiO₂ catalysts containing from 0.05 to 10 wt% Pt as well homodispersed particles (mean diameter ca. 2 nm). The presence of platinum, at least for contents ca. 1 wt%, caused a decrease in the anatase photoconductance σ_eq at equilibrium under vacuum and this decrease was more pronounced for the highest loadings (5 and 10 wt%). In addition, differences in the times required to reach the steady state were observed, and an attenuation of the u.v. light flux lowered σ_eq of the Pt-loaded samples, whereas it did not affect that of TiO₂ alone. These phenomena can be rationalized in terms of electron transfer from the titania to the platinum. Accordingly, the existence of such a withdrawal by small metal crystallites is substantiated. Conversely, in H₂ the Pt deposits decreased the resistance of TiO₂, illuminated or not, and this is tentatively attributed to a migration of adsorbed hydrogen atoms from Pt to TiO₂ where they form OH^– ions and release electrons.