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

Abstract Titanium(IV) oxide samples modified with platinum or rhodium chloride (H 2 PtCl 6 /TiO 2 or RhCl 3 /TiO 2 ) 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 H 2 PtCl 6 /TiO 2 was about two-times larger than that of RhCl 3 /TiO 2 . Only a small amount of released NO 2 was observed in the RhCl 3 /TiO 2 catalyst, whereas in the H 2 PtCl 6 /TiO 2 catalyst, the amount of NO 2 released to gas phase increased with an increase in oxidation products. The small amount of released NO 2 indicates that most of the NOx adsorbed on RhCl 3 /TiO 2 as an adsorption form of nonvolatile NO 3 − , whereas the amount of adsorbed NO 2 on H 2 PtCl 6 /TiO 2 was about four-times larger than that on RhCl 3 /TiO 2 . The results indicate that the oxidation rate of NO 2 to NO 3 − over RhCl 3 /TiO 2 was faster than that over H 2 PtCl 6 /TiO 2 . These results strongly suggest that the Cl radical induced by visible light was not directly related to the photo-oxidation of NO to NO 2 and NO 3 − and that the complex species of RhCl 3 and H 2 PtCl 6 contributed to the photo-oxidation.