Roles of the active species involved in the photocatalytic oxidation of benzyl alcohol into benzaldehyde on TiO₂ under UV light : Experimental and DFT studies

Active species such as holes, electrons, hydroxyl radicals (•OH), and superoxide radical anions (O₂ ^•-) involved in the photocatalytic oxidation of benzyl alcohol (BA) into benzaldehyde (BAD) on TiO₂ under ultraviolet (UV) light were investigated by adding corresponding scavengers. The experimental results show that the oxidation of BA into BAD is driven mainly by the holes and •OH radical. The role of dissolved O₂ was studied by bubbling N₂ through the reaction. The results show that the dissolved O₂ is, to some extent, the source of •OH. Additional density functional theory (DFT) calculations identified two H-abstraction pathways (R_absOH and R_absα). The structure of the R_absOH product is less stable and tends to transfer resulting in the R_abs α product. The holes are the main driving force for the abstracting of H from BA. The •OH radical combines with the H-abstract product to form a precursor of product BAD, detected by LC-MS technique (m/z 123), which then undergoes an intramolecular dehydration reaction to form BAD. The results will help to select the optimal active species for the photocatalytic oxidation of BA into BAD on TiO₂ under UV light.