Room-Temperature Photooxidation of CH4 to CH3OH with Nearly 100% Selectivity over Hetero-ZnO/Fe2O3 Porous Nanosheets

Abstract

The huge challenge for CH 4 photooxidation into CH 3 OH lies in the activation of the inert C–H bond and the inhibition of CH 3 OH overoxidation. Herein, we design two-dimensional in-plane Z -scheme heterostructures composed of two different metal oxides, with efforts to polarize the symmetrical CH 4 molecules and strengthen the O–H bond in CH 3 OH. As a prototype, we first fabricate ZnO/Fe 2 O 3 porous nanosheets, where high-resolution transmission electron microscopy and in situ X-ray photoelectron spectroscopy affirm their in-plane Z -scheme heterostructure. In situ Fourier transform infrared spectra and in situ electron paramagnetic resonance spectra demonstrate their higher amount of ·CH 3 radicals relative to the pristine ZnO porous nanosheets, in which density functional theory calculations validate that the high local charge accumulation on Fe sites lowers the CH 4 adsorption energy from 0.14 to 0.06 eV. Moreover, the charge-accumulated Fe sites strengthen the polarity of the O–H bond in CH 3 OH through transferring electrons to the O atoms, confirmed by the increased barrier from 0.30 to 2.63 eV for *CH 3 O formation, which inhibits the homolytic O–H bond cleavage and thus suppresses CH 3 OH overoxidation. Accordingly, the CH 3 OH selectivity over ZnO/Fe 2 O 3 porous nanosheets reaches up to nearly 100% with an activity of 178.3 μmol –1 g cat –1, outperforming previously reported photocatalysts without adding any oxidants under room temperature and ambient pressure.