Efficient Photooxidation of Methane to Liquid Oxygenates over ZnO Nanosheets at Atmospheric Pressure and Near Room Temperature
Shan Zhu, Xiaodong Li, Zhikang Pan, Xingchen Jiao, Kai Zheng, Li Li, Weiwei Shao, Xiaolong Zu, Jun Hu, Junfa Zhu, Yongfu Sun, Yi Xie
Abstract
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.