(669d) When Free Radicals Meet Solids, Ions, and Gasses in Light-Driven Selective Methane Oxidation

The activation and selective oxidation of methane (CH₄) under mild conditions constitutes outstanding challenges which are of fundamental and technological importance, spanning from biology to industry and environment. Methanol is a key feedstock for chemical industries. Currently, the methane to methanol conversion relies on the energy intensive process involving syngas intermediates. In recent decades, intensive efforts have been focused on developing the direct methane to methanol conversion as an alternative and more sustainable pathway. Free radicals are one of key mediators underpinning many active sites in thermal catalysis and these enhancement effects induced by the light and electric fields, including methane to methanol oxidation. Furthermore, earth relies on free radicals from photochemical reactions to remove methane and other pollutants from the atmosphere. However, it remains unclear how free radicals as active species mediate the C-H bond activation and dictate the products distribution.

In this talk, we combine rational design of TiO₂-noble metal composite photocatalysts, experimental design of methane oxidation experiments with in-situ free radical detection. Our results unravel interactions between these free radicals and the solid co-catalysts, ions, and gases molecules in water, as well as their impact on methane removal and its selective oxidation. Our studies highlight the neglected while important interplay between photogenerated radicals and co-catalysts, ion impurities, and gas molecules in promoting methane removal and controlling the products distribution. Furthermore, we report an inherent trade-off in photochemical oxidation of methane to methanol. A new design principle is developed to circumvent this trade-off. Furthermore, our findings and strategies are generally applicable to thermochemical methane oxidation using H₂O₂.

Our molecular-level insights into free-radical-mediated photochemical and thermochemical methane oxidation are expected to promote the methane removal and its selective functionalization and offer informative design principles for chemical bond activations driven by light, heat, and electric fields