Supported titanium oxide species as photocatalysts in 2-propanol oxidation: Linking selectivity to structural and electronic properties

The photocatalytic performance of titanium oxide species supported on mesoporous silica (SBA-15) in the selective oxidation of 2-propanol is studied systematically as function of the titania surface loading. Monomers and oligomers of the supported titanium oxide species are formed on the silica surface in differing amounts with increasing loading, and the photocatalytic activity shows a strong dependence on the titanium content. A general advantage of supported samples in terms of net productivity could not be observed, but the photocatalysts’ longevity and selectivity improve with lower degrees of oligomerization. In-situ infrared spectroscopy in diffuse reflectance under irradiation revealed a likely explanation for this observation: The relative probability of the overoxidation of 2-propanol to acetate and possibly even total oxidation to CO₂ increases with loading and is most severe for crystalline bulk titania. Stably adsorbed acetate appears to block the active sites on crystalline TiO₂, but it is tolerable to a certain extent for the silica-supported samples. This work provides fundamental knowledge for the development of more stable and selective oxidation photocatalysts.