Efficient Photooxidation of Methane to Liquid Oxygenates over ZnO Nanosheets at Atmospheric Pressure and Near Room Temperature.

Direct CH4 photoconversion into liquid oxygenates under mild conditions still represents a huge challenge. Herein, two-dimensional oxide semiconductors are designed to generate abundant active O- species for activating C-H bond of methane. Taking the synthetic ZnO nanosheets as an example, in situ electron paramagnetic resonance spectra verified their lattice oxygen atoms could capture photoexcited holes and generate active O- species, which could efficiently abstract H from CH4 to generate ·CH3 radicals. Gibbs free energy calculations and in situ Fourier-transform infrared spectroscopy corroborated the rate-limiting step was the first C-H bond activation process, whereas the exoergic oxidation of *CHO to HCOOH was easier than the endoergic overoxidation to CO, accounting for the selective production of liquid oxygenates. As a result, the formation rate of liquid oxygenates over ZnO nanosheets reached 2.21 mmol g-1 h-1 with a selectivity of 90.7% at atmospheric pressure and approximately 50 °C, outperforming previously reported photocatalysts under similar conditions.