Enabling Specific Photocatalytic Methane Oxidation by Controlling Free Radical Type

Abstract

Selective CH 4 oxidation to CH 3 OH or HCHO with O 2 in H 2 O under mild conditions provides a desired sustainable pathway for synthesis of commodity chemicals. However, manipulating reaction selectivity while maintaining high productivity remains a huge challenge due to the difficulty in the kinetic control of the formation of a desired oxygenate against its overoxidation. Here, we propose a highly efficient strategy, based on the precise control of the type of as-formed radicals by rational design on photocatalysts, to achieve both high selectivity and high productivity of CH 3 OH and HCHO in CH 4 photooxidation for the first time. Through tuning the band structure and the size of active sites ( i.e., single atoms or nanoparticles) in our Au/In 2 O 3 catalyst, we show alternative formation of two important radicals, • OOH and • OH, which leads to distinctly different reaction paths to the formation of CH 3 OH and HCHO, respectively. This approach gives rise to a remarkable HCHO selectivity and yield of 97.62% and 6.09 mmol g –1 on In 2 O 3 -supported Au single atoms (Au 1 /In 2 O 3 ) and an exceptional CH 3 OH selectivity and yield of 89.42% and 5.95 mmol g –1 on In 2 O 3 -supported Au nanoparticles (Au NPs /In 2 O 3 ), respectively, upon photocatalytic CH 4 oxidation for 3 h at room temperature. This work opens a new avenue toward efficient and selective CH 4 oxidation by delicate design of composite photocatalysts.