Photo-oxidation of nitrogen oxide over titanium(IV) oxide modified with platinum or rhodium chlorides under irradiation of visible light or UV light

Titanium(IV) oxide samples modified with platinum or rhodium chloride (H2PtCl6/TiO2 or RhCl3/TiO2) were prepared by an impregnation method and post-calcination at various temperatures and were used for photo-oxidation of nitrogen oxide under irradiation of visible light or UV light. Turnover numbers of both the catalysts were maintained at temperatures up to 350 °C under 24-h irradiation of visible light, although the specific surface area of the catalysts decreased greatly with increase in post-calcination temperature. The turnover number of H2PtCl6/TiO2 was about two-times larger than that of RhCl3/TiO2. Only a small amount of released NO2 was observed in the RhCl3/TiO2 catalyst, whereas in the H2PtCl6/TiO2 catalyst, the amount of NO2 released to gas phase increased with an increase in oxidation products. The small amount of released NO2 indicates that most of the NOx adsorbed on RhCl3/TiO2 as an adsorption form of nonvolatile NO3−, whereas the amount of adsorbed NO2 on H2PtCl6/TiO2 was about four-times larger than that on RhCl3/TiO2. The results indicate that the oxidation rate of NO2 to NO3− over RhCl3/TiO2 was faster than that over H2PtCl6/TiO2. These results strongly suggest that the Cl radical induced by visible light was not directly related to the photo-oxidation of NO to NO2 and NO3− and that the complex species of RhCl3 and H2PtCl6 contributed to the photo-oxidation.