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

Abstract

Particles of tungsten oxide loaded with a palladium oxide cocatalyst (PdO x /WO 3 ) exhibit higher selectivity in comparison to other photocatalysts, such as Pt/WO 3, Pd/TiO 2, and Pt/TiO 2, 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 O 2 . A detailed investigation of 2-propanol oxidation as a model reaction revealed significant differences between the reactivities of the WO 3 and TiO 2 systems. On TiO 2 photocatalysts, complete decomposition to CO 2 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 WO 3 photocatalysts were much lower than those on TiO 2 due to the low affinity of the WO 3 surface to the substrates, particularly acetone. The low affinity of the WO 3 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 WO 3 system. Most importantly, the loading of a palladium oxide cocatalyst on WO 3 drastically improved the selectivity for acetone by almost completely suppressing the peroxidation of acetone during photoirradiation. A considerable amount of hydrogen peroxide (H 2 O 2 ) was confirmed to accumulate during photoirradiation on PdO x /WO 3 due to the high selectivity of the PdO x cocatalyst for the two-electron reduction of O 2 molecules, while such accumulation was not observed for Pt/WO 3 . The H 2 O 2 in the PdO x /WO 3 system preferentially reacted with photogenerated holes when the concentrations of acetone and H 2 O 2 increased, suppressing the peroxidation of acetone by the holes. The PdO x /WO 3 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 /WO 3 photocatalyst for partial oxidation of various organic compounds.