Crystal phase-dependent generation of mobile OH radicals on TiO₂: Revisiting the photocatalytic oxidation mechanism of anatase and rutile

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]OH_f) 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]OH_f was investigated using tetramethylammonium (TMA) cation as a main probe compound for ^[rad]OH_f 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]OH_f). The surface trapped holes are not involved in ^[rad]OH_f formation, but lead to the generation of surface-bound OH radical (^[rad]OH_s) on both anatase and rutile. The generation of ^[rad]OH_f is favorable on anatase because more H₂O₂ are evolved (via dioxygen reduction) and adsorbed on the anatase surface. Rutile showed little sign of ^[rad]OH_f 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]OH_f generation mechanism on anatase involves the reductive pathway. Better understanding of ^[rad]OH_f production pathway in photocatalysis will provide a new insight leading to an engineering solution for how the production of ^[rad]OH_f can be maximized, which is critically important in achieving the efficient photocatalytic oxidation of various pollutants.