Photo-electro catalytic oxidation of aromatic alcohols on visible light-absorbing nitrogen-doped TiO2

Visible light illumination of nitrogen-doped TiO₂ brings about the selective oxidation of benzyl and cinnamyl alcohol to the corresponding aldehydes. The photocatalyst was prepared by a sol-gel method and characterised mainly by XRD, UV-vis diffuse reflectance and Raman spectroscopy. The conditions limiting the observation of visible light photoactivity are the use of dry nitriles as nonaqueous solvents and aromatic alcohols as the substrates. No visible light oxidation takes place in an aqueous medium. The efficiency of benzyl and cinnamyl alcohol photo-oxidation in nitrile solvents follows the order: CH₃CN > CH₃CH₂CN > CH₃(CH₂)₂CN. Conversely, since alcohol photo-oxidation occurs with 100% selectivity on electrodes in O₂ saturated solutions at potentials close to E_fb or under open-circuit conditions, suspensions of the photocatalyst can be advantageously employed. The process involves a relatively weak adsorption of the alcohol substrates which, however, do not readily capture the photogenerated holes. On the basis of the electrochemical and photoluminescence data, it appears that the solvent (e.g. acetonitrile), in addition to O₂ has an active role in the reaction mechanism.