Photocatalytic reactivity of surface platinized TiO2:: Substrate specificity and the effect of Pt oxidation state

Surface platinized TiO2 (Pt/TiO2) has been frequently studied, but its photocatalytic reactivities reported in the literature are not consistent in some cases. To understand the discrepancies, the effects of Pt speciation on TiO2 on the photocatalytic degradation (PCD) of a few chlorinated organic compounds (trichloroethylene (TCE), perchloroethylene (PCE), dichloroacetate, etc.) were investigated with several Pt/TiO2 samples that were prepared differently. The oxidation state of Pt deposits was analyzed by X-ray photoelectron spectroscopy and was found to be the most important factor in determining the initial PCD rates of chlorinated organic compounds. TiO2 with oxidized Pt species (Pt-ox/TiO2) was less reactive than TiO2 with metallic Pt (Pt-0/TiO2) for all substrates tested. In particular, Pt-ox/TiO2 strongly inhibited the PCD of TCE and PCE whereas it was more reactive than pure TiO2 for the PCD of other compounds. The photocurrents obtained with the Pt-ox/TiO2 electrode were lower than those with the Pt-0/TiO2 electrode, which was ascribed to the role of Pt-ox species as a recombination center. It is proposed that TCE adsorbed on Pt-ox chemically mediates the charge recombination through the redox cycle of TCE. The Pt effects in photocatalysis are highly substrate-specific and depend on the Pt-substrate interaction as well as the properties of Pt deposits.