Selective oxidation of CH4 to valuable HCHO over a defective rTiO2/GO metal-free photocatalyst

Abstract

Direct conversion of CH₄ to oxygenates is regarded as a “holy grail” in the chemistry community. Herein, a specially designed metal-free rTiO₂/GO catalyst was reported for photocatalytic oxidation of CH₄. It was found that the presence of graphene oxide (GO) retarded the recombination of photo-generated electrons and holes (h⁺), whereas defective oxygen vacancies can be introduced upon reduction of TiO₂ (rTiO₂). The C–H bond of the CH₄ molecule can be readily activated with a considerably low energy barrier of 1.67 eV, and the generated CH₃OOH* intermediate can be stabilized by the oxygen vacancies. As such, CH₄ can be directly oxidized to liquid oxygenates with extraordinary efficiency (7.65 mmol g⁻¹ h⁻¹ yield and 90% selectivity). More interestingly, HCHO was found to be the major product rather than CH₃OH; such an extension was rarely reported, it provides not only a more valuable route for CH₄ upgrading, but also a new basis of natural gas-based chemistry upon tandem conversion of HCHO.