Comparison of photooxidation and photoreduction reactions on TiO2 nanoparticles

Photooxidation and photoreduction reaction rates on nanoparticulate TiO2 have been studied using a series of silica coated samples prepared from the same parent material—a high area rutile. Oxidation of propan-2-ol was measured by GC analysis of acetone product. Reduction of DPPH (1,1-diphenyl-2-picrylhydrazyl radical) was followed by the decrease in absorbance at 520 nm and photogreying of TiO2 was measured as a change in L, the reflectance of the CIE Y illuminant centred at 550 nm, using a Minolta chroma meter.The activity for oxidation of propan-2-ol correlated well with two measures of photoreduction, the reduction of the stable DPPH and TiO2 photogreying in an oxygen free system.In all cases, low silica loadings resulted in a small increase in activity. For silica levels >1 wt.% the dependence of activity decreased with increasing levels of silica treatment. The pattern of activity versus silica-level was consistent with elimination of both photooxidation and photoreduction at 12–16% silica. A speculative interpretation of these changes was based on changes in the silica environment as determined by magic angle spinning NMR.