Aerobic oxidation of methane to formaldehyde mediated by crystal-O over gold modified tungsten trioxide via photocatalysis

General indirect transformation of methane into formaldehyde is based on methanol as intermediate yet leading to low selectivity. Herein, we report a direct photocatalytic conversion of methane to formaldehyde over gold modified cuboid-shaped tungsten trioxide with exposed (002), (020) and (200) facets containing crystal-O at room temperature (25 °C). Under light irradiation, high spin state crystal-O− ion as a hole can active CH bond in methane and subsequently be consumed to form formaldehyde, then renovated by the addition of oxygen. Isotopic 13CH4 and 18O2 tests reveal that formaldehyde indeed stems from the combination of methane and crystal-O on the exposed (002), (020) and (200) facets. Besides, metallic gold deposited on the surface of cuboid-shaped tungsten trioxide can extract the photoexcited electrons and further enhance the formation of crystal-O− ions. The productivity of formaldehyde over optimized catalyst can reach 7202 μmol g-1 with approximate 100 % selectivity within 24 h reaction time.