Partial Oxidation of Alcohols on Visible-Light-Responsive WO₃ Photocatalysts Loaded with Palladium Oxide Cocatalyst

Particles of tungsten oxide loaded with a palladium oxide cocatalyst (PdO_x/WO₃) exhibit higher selectivity in comparison to other photocatalysts, such as Pt/WO₃, Pd/TiO₂, and Pt/TiO₂, in the partial oxidation of alcohols such as 2-propanol to the corresponding aldehydes or ketones (e.g., 80% selectivity for acetone production with ca. 96% conversion of 2-propanol) in water containing molecular O₂. A detailed investigation of 2-propanol oxidation as a model reaction revealed significant differences between the reactivities of the WO₃ and TiO₂ systems. On TiO₂ photocatalysts, complete decomposition to CO₂ proceeded readily, due to the occurrence of direct oxidation of 2-propanol and acetone adsorbed by holes, resulting in significantly low selectivity for partial oxidation. On the other hand, the rates of acetone peroxidation on WO₃ photocatalysts were much lower than those on TiO₂ due to the low affinity of the WO₃ surface to the substrates, particularly acetone. The low affinity of the WO₃ surface also enables preferential generation of hydroxyl radicals (^•OH) from water, which react with 2-propanol much more efficiently than with acetone, further increasing selectivity for acetone in the WO₃ system. Most importantly, the loading of a palladium oxide cocatalyst on WO₃ drastically improved the selectivity for acetone by almost completely suppressing the peroxidation of acetone during photoirradiation. A considerable amount of hydrogen peroxide (H₂O₂) was confirmed to accumulate during photoirradiation on PdO_x/WO₃ due to the high selectivity of the PdO_x cocatalyst for the two-electron reduction of O₂ molecules, while such accumulation was not observed for Pt/WO₃. The H₂O₂ in the PdO_x/WO₃ system preferentially reacted with photogenerated holes when the concentrations of acetone and H₂O₂ increased, suppressing the peroxidation of acetone by the holes. The PdO_x/WO₃ photocatalyst was more selective for partial oxidation of other alcohols than other photocatalysts, while the selectivity depended on the alcohols used, suggesting the availability of the unique reactivity of the PdO_x/WO₃ photocatalyst for partial oxidation of various organic compounds.