Photocatalytic Oxidative Coupling of Methane to Ethane Using CO₂ as a Soft Oxidant over the Au/TiO₂−V_o Nanosheets

Photocatalytic oxidative coupling of methane (OCM) offers an appealing route for converting greenhouse gas into valuable C₂ hydrocarbons. However, O_2, as the most commonly used oxidant, tends to result in inevitable overoxidation and waste of methane feedstock. Herein, we first report a photocatalytic OCM using CO₂ as a soft oxidant for C₂H₆ production under mild conditions, where an efficient photocatalyst with unique interface sites is designed and constructed to facilitate CO₂ adsorption and activation, while concurrently boosting CH₄ dissociation. As a prototype, the Au quantum dots anchored on oxygen-deficient TiO₂ nanosheets are fabricated, where the Au-V_o-Ti interface sites for CO₂ adsorption and activation are collectively disclosed by in situ Kelvin probe force microscopy, quasi in situ X-ray photoelectron spectroscopy and theoretical calculations. Compared with single metal site, the Au-V_o-Ti interface sites exhibit the lower CO₂ adsorption energy and decrease the energy barrier of the *CO₂ hydrogenation step from 1.05 to 0.77 eV via Au-C and Ti-O dual-site bonding. The adsorbed CO₂ on the photocatalyst reduces the energy barrier of *CH₄ dissociation to *CH₃ from 2.13 to 1.59 eV, contributing to CH₄ oxidation. Additionally, in situ Fourier-transform infrared spectroscopy unveils the Au site facilitates ethane production by engaging in *CH₃-Au interaction and accelerating CH₃-CH₃ coupling. Thus, the photocatalyst demonstrates a high C₂H₆ evolution rate of 2.60 mmol g^-1 h^-1 for OCM using CO₂ as the soft oxidant, surpassing most of previously reported photocatalysts regardless of OCM and nonoxidative coupling of methane. This work highlights the importance of soft oxidants for improving oxidation reaction efficiency and provides atomic scale insight into the design of photocatalysts for CH₄ conversion.