Illustrating the Fate of Methyl Radical in Photocatalytic Methane Oxidation over Ag−ZnO by in situ Synchrotron Radiation Photoionization Mass Spectrometry

Abstract

Photocatalysis has emerged as an ideal method for the direct activation and conversion of methane under mild conditions. In this reaction, methyl radical (⋅CH₃ ) was deemed a key intermediate that affected the yields and selectivity of the products. However, direct observation of ⋅CH₃ and other intermediates is still challenging. Here, a rectangular photocatalytic reactor coupled with in situ synchrotron radiation photoionization mass spectrometry (SR-PIMS) was developed to detect reactive intermediates within several hundred microseconds during photocatalytic methane oxidation over Ag-ZnO. Gas phase ⋅CH₃ generated by photogenerated holes (O^- ) was directly observed, and its formation was demonstrated to be significantly enhanced by coadsorbed oxygen molecules. Methoxy radical (CH₃ O⋅) and formaldehyde (HCHO) were confirmed to be key C1 intermediates in photocatalytic methane overoxidation to CO₂ . The gas-phase self-coupling reaction of ⋅CH₃ contributes to the formation of ethane, which indicates the key role of ⋅CH₃ desorption in the highly selective synthesis of ethane. Based on the observed intermediates, the reaction network initiated from ⋅CH₃ of photocatalytic methane oxidation could be clearly illustrated, which is helpful for studying the photocatalytic methane conversion processes.