Highly enhanced photocatalytic oxidation of CO on titania deposited with Pt nanoparticles: kinetics and mechanism

The effects of nanosized Pt deposits on TiO2 particles on the photocatalytic oxidation of gaseous CO were investigated. Platinum was photodeposited directly on a TiO2-coated glass plate with a typical Pt loading of ∼3 wt.%. Platinum deposition on TiO2 greatly enhances the photooxidation rate of CO in the concentration range 30–500 ppmv, whereas naked TiO2 samples exhibit much slower oxidation rates. The degree of Pt-induced enhancement strongly depends on the kind of TiO2 substrate. Three commercial TiO2 samples (Degussa P25, ISK STS-01, Hombikat UV-100) were platinized and compared for their photoactivity of CO oxidation. Platinized Hombikat TiO2 shows the best activity among them. The photoconversion of CO to CO2 is quantitative in the presence of O2. The oxidation states of photodeposited Pt include both Pt0 and PtIIO with the former species dominant. The photocatalytic CO oxidation rates and the amounts of Pt deposited on TiO2 surface show a close correlation. The CO photooxidation is strongly inhibited in the absence of O2 or in the presence of alternative oxidants such as N2O and H2O. The addition of water vapor little affects the CO photooxidation kinetics in the presence of O2. Hydroxyl radicals do not seem to play a significant role in CO photooxidation on Pt/TiO2. The main oxidants in CO photocatalytic oxidation seem to be produced from adsorbed O2. The active oxygen species generated on illuminated Pt/TiO2 surface are stable enough to survive for several minutes even in a dark period, and take part in a successive photooxidation reaction. The photocatalytic oxidation kinetics was investigated in detail as a function of Pt loading, coated TiO2 mass, CO concentration, O2 concentration, light intensity, additives, and humidity.