Molecular oxygen enhances H ₂O ₂ utilization for the photocatalytic conversion of methane to liquid-phase oxygenates

Abstract

H ₂O ₂ 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 O ₂ is an efficient molecular additive to enhance the utilization efficiency of H ₂O ₂ by suppressing H ₂O ₂ adsorption on oxides and consequent photogenerated holes-mediated H ₂O ₂ dissociation into O ₂. In photocatalytic methane conversion over an anatase TiO ₂ nanocrystals predominantly enclosed by the {001} facets (denoted as TiO ₂{001})-C ₃N ₄ composite photocatalyst at room temperature and ambient pressure, O ₂ additive significantly enhances the utilization efficiency of H ₂O ₂ 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 TiO ₂{001}-C ₃N ₄ heterojunctions, photogenerated holes-mediated activation of CH ₄ into ·CH ₃ radicals on TiO ₂{001} and photogenerated electrons-mediated activation of H ₂O ₂ into ·OOH radicals on C ₃N ₄, and preferential dissociative adsorption of methanol on TiO ₂{001} are responsible for the active and selective photocatalytic conversion of methane to formic acid over TiO ₂{001}-C ₃N ₄ composite photocatalyst.

Abstract

The oxidation of methane to formic acid or related oxygenates relies on efficient reaction with H ₂O ₂. Here, the authors report a TiO ₂-based catalyst to selectively form formic acid by using molecular O ₂ additives to avoid unwanted side reactions.