CH₃^•-Generating Capability as a Reactivity Descriptor for Metal Oxides in Oxidative Coupling of Methane

Abstract

Methyl radicals (CH3•) are the key intermediates in the heterogeneous–homogeneous reaction processes of catalytic oxidative coupling of methane (OCM). Here, by applying in situ synchrotron-based vacuum ultraviolet photoionization mass spectrometry, we quantitatively detected CH3• being desorbed from various metal oxides and validated the CH3•-generating capability as an effective descriptor for the catalytic performance of the metal oxides in OCM. It is found that the C2 yield is linearly correlated to the amount and the desorption temperature of CH3•, with the better OCM catalyst showing stronger CH3• intensity and lower CH3• desorption temperature. Furthermore, experimental characterizations together with density functional theory calculations suggest that the intrinsic electronic properties of metals and the subsequent generated electrophilic oxygen species are the decisive factors for CH3• generation. Then, the CH3•-generating capability can bridge the gap between the OCM performance and the structure of the catalyst and help us better understand the intrinsic structure–performance relationship in OCM over metal oxide catalysts.