Photocatalytic reactivity and diffusing OH radicals in the reaction mediumcontaining TiO2 particles

The generation of OH radicals on UV-illuminated TiO2 surface is mainly resp onsible for the photocatalyric oxidation of pollutants in various contamina ted environmental media. Although the reactivity of OH radicals is largely limited within the surface region, the possibility of OH desorption and dif fusion into the reaction medium has been often raised. This study provides several examples for the presence of diffusing OH radicals in aqueous solut ion and polymer matrix containing TiO2 particles. The photocatalytic degrad ation rates of (CH3)(4)N+ in TiO2 suspension were comparable between acidic and alkaline conditions, which could not be explained by a simple electros tatic surface charge model. From the present mechanistic study, it is sugge sted that the photocatalytic oxidation of (CH3)(4)N+ at acidic pH mainly pr oceeds through free OH radicals in the solution bulk, not on the surface of TiO2. The diffusing OH radicals also played the role of main oxidants in t he solid phase. The photolysis of TiO2-embedded PVC composite films generat ed cavities around the imbedded TiO2 particles and the development of cavit y diameter continued even after the direct contact between the PVC and TiO2 was prohibited. This implied that active oxygen species that were photogen erated on TiO2 surface desorbed and diffused across a few micrometers to re act with the polymer matrix.