Cyclohexane photooxidation under visible light irradiation by WO3–TiO2 mixed catalysts

Cyclohexane photocatalytic oxidation was performed over WO3 photocatalysts under visible light irradiation with molecular oxygen. Cyclohexanone and cyclohexanol were obtained with high selectivity. The yield of cyclohexanol and cyclohexanone was increased by using a Pt-loaded WO3 catalyst (Pt/WO3). It was demonstrated by isotope-labeled experiments that cyclohexanone formed through the reaction with lattice oxygen in WO3. Electron spin resonance measurements showed that cyclohexyl radicals form as a reaction intermediate before reacting with the WO3 lattice oxygen. The binary catalyst obtained by physical mixing of TiO2 with Pt/WO3 exhibited a higher photocatalytic activity than that of the Pt/WO3 catalyst. The photocatalytic activity further increased with increasing BET surface area of TiO2 in the binary catalyst. The formation of cyclohexyl radicals was observed to be accelerated over the binary catalyst with respect to Pt/WO3. It is therefore speculated that the surface of TiO2 contributed to the formation of cyclohexyl radicals and/or their stabilization.