Molecular oxygen enhances H2O2 utilization for the photocatalytic conversion of methane to liquid-phase oxygenates

Abstract

H₂O₂ is widely used as an oxidant for photocatalytic methane conversion to value-added chemicals over oxide-based photocatalysts under mild conditions, but suffers from low utilization efficiencies. Herein, we report that O₂ is an efficient molecular additive to enhance the utilization efficiency of H₂O₂ by suppressing H₂O₂ adsorption on oxides and consequent photogenerated holes-mediated H₂O₂ dissociation into O₂. In photocatalytic methane conversion over an anatase TiO₂ nanocrystals predominantly enclosed by the {001} facets (denoted as TiO₂{001})-C₃N₄ composite photocatalyst at room temperature and ambient pressure, O₂ additive significantly enhances the utilization efficiency of H₂O₂ up to 93.3%, giving formic acid and liquid-phase oxygenates selectivities respectively of 69.8% and 97% and a formic acid yield of 486 μmol_HCOOH·g_catalyst⁻¹·h⁻¹. Efficient charge separation within TiO₂{001}-C₃N₄ heterojunctions, photogenerated holes-mediated activation of CH₄ into ·CH₃ radicals on TiO₂{001} and photogenerated electrons-mediated activation of H₂O₂ into ·OOH radicals on C₃N₄, and preferential dissociative adsorption of methanol on TiO₂{001} are responsible for the active and selective photocatalytic conversion of methane to formic acid over TiO₂{001}-C₃N₄ composite photocatalyst.

The oxidation of methane to formic acid or related oxygenates relies on efficient reaction with H₂O₂. Here, the authors report a TiO₂-based catalyst to selectively form formic acid by using molecular O₂ additives to avoid unwanted side reactions.