Titanium dioxide has been the most popular environmental photocatalyst of which role critically depends on the generation of OH radicals. In particular, the mobile free OH racial ([rad]OHf) generation and the subsequent diffusion from the surface are critical in achieving the mineralization of non-adsorbing substrates by extending the reaction zone from the surface to the solution bulk. Here the origin of the crystalline phase-dependent generation of [rad]OHf was investigated using tetramethylammonium (TMA) cation as a main probe compound for [rad]OHf in a UV/TiO2 photocatalytic system. We found a clear evidence that the mobile free OH radical is generated through a reductive conversion of dissolved O2 on anatase only (O2 → H2O2 → [rad]OHf). The surface trapped holes are not involved in [rad]OHf formation, but lead to the generation of surface-bound OH radical ([rad]OHs) on both anatase and rutile. The generation of [rad]OHf is favorable on anatase because more H2O2 are evolved (via dioxygen reduction) and adsorbed on the anatase surface. Rutile showed little sign of [rad]OHf formation. The generation of 18O-labelled p-hydroxybenzoic acid on anatase only (not rutile) from benzoic acid oxidation under 18O2-saturated condition provides a solid evidence that the [rad]OHf generation mechanism on anatase involves the reductive pathway. Better understanding of [rad]OHf production pathway in photocatalysis will provide a new insight leading to an engineering solution for how the production of [rad]OHf can be maximized, which is critically important in achieving the efficient photocatalytic oxidation of various pollutants.
- Hydroxyl radicals
- Photocatalytic oxidation mechanism