Hydrophobic TaOx Species Overlayer Tuning Light-Driven Methane Chlorination with Inorganic Chlorine

Abstract

Halogenated methane serves as a universal platform molecule for building high-value chemicals. Utilizing sodium chloride solution for photocatalytic methane chlorination presents an environmentally friendly method for methane conversion. However, competing reactions in gas-solid-liquid systems leads to low efficiency and selectivity in photocatalytic methane chlorination. Here, an in situ method is employed to fabricate a hydrophobic layer of TaO_x species on the surface of NaTaO₃. Through in-situ XPS and XANES spectra analysis, it is determined that TaO_x is a coordination unsaturated species. The TaO_x species transforms the surface properties from the inherent hydrophilicity of NaTaO₃ to the hydrophobicity of TaO_x/NaTaO₃, which enhances the accessibility of CH₄ for adsorption and activation, and thus promotes the methane chlorination reaction within the gas-liquid-solid three-phase system. The optimized TaO_x/NaTaO₃ photocatalyst has a good durability for multiple cycles of methane chlorination reactions, yielding CH₃Cl at a rate of 233 µmol g^-1 h^-1 with a selectivity of 83%. In contrast, pure NaTaO₃ exhibits almost no activity toward CH₃Cl formation, instead catalyzing the over-oxidation of CH₄ into CO₂. Notably, the activity of the optimized TaO_x/NaTaO₃ photocatalyst surpasses that of reported noble metal photocatalysts. This research offers an effective strategy for enhancing the selectivity of photocatalytic methane chlorination using inorganic chlorine ions.

Keywords: TaOx; hydrophobicity; methane chlorination; photocatalysis; ultrathin layer.