Kinetic study on photocatalytic hydrogenation of acetophenone derivatives on titanium dioxide

Acetophenone (AP) derivatives were photocatalytically hydrogenated to afford the corresponding secondary alcohols with excellent chemical efficiencies on titanium dioxide (Degussa P25, TiO2) under UV light irradiation. Maximum reaction rates (k(max)) and apparent adsorption constants (K-LH) under irradiation were obtained from the Langmuir-Hinshelwood kinetic analysis. The kmax values showed a tendency to decrease with the decreasing reduction potentials (E-red) of the AP derivatives, while the KLH values were distributed in the range of 280-780 L mol(-1). Among these, simple AP exhibited the greatest adsorptivity upon the UV irradiated TiO2 surface. Additionally, it was demonstrated that the electrons trapped at surface defect Ti (Ti-st) sites on the TiO2 actually hydrogenated the AP derivatives. The amount of reacted electrons also showed a tendency to decrease with decreasing E-red values, in accord with the dependence on k(max). These results indicate that the electrons accumulated at shallow Ti-st states easily participate in the hydrogenation of AP derivatives, whereas those trapped at deeper states hardly react with the substrates. The results strongly support the electron transfer reaction model via the Ti-st sites in the photocatalytic hydrogenation on TiO2.