How Pt nanoparticles affect TiO₂-induced gas-phase photocatalytic oxidation reactions

The effect of Pt nanoparticles on the gas-phase photocatalytic oxidation activity of TiO₂ is shown to be largely dependent on the molecular functionality of the substrate. We demonstrate that Pt nanoparticles decrease rates in photocatalytic oxidation of propane, whereas a strong beneficial effect of Pt was observed in oxidation of ethanol. On the basis of oxygen conversion, Pt nanoparticles result in an increase in rates of TiO₂ from 1.55 mmol O₂/g/h to 4.65 mmol O₂/g/h, at a light intensity of 8 mW/cm² at 375 nm. The latter value is comparable to obtained in propane oxidation in the absence of Pt and represents a photonic efficiency of approximately 2%. Besides an effect on oxygen conversion rate, we also observed significant effects of Pt nanoparticles on reaction selectivity. DRIFT analysis demonstrates that acetone is a rather abundant surface-bound intermediate when propane is oxidized in the presence of Pt nanoparticles, while this is barely observed in the absence of Pt nanoparticles. In ethanol oxidation, both surface-bound and gas-phase acetaldehydes are produced more significantly in the presence than in the absence of Pt. The activity data are discussed on the basis of adsorption affinity of the reactants toward TiO₂, much higher for ethanol as compared to propane. The changes in (surface) selectivity are discussed on the basis of Pt-induced alterations in the rate-determining steps.