Au accelerates the conversion of intermediates to achieve efficient photocatalytic methane conversion to chemicals

Methane, as the primary component of natural gas and shale gas, serves as a critical pillar of the fossil fuel system. This is due to its abundant global reserves and well-established energy applications. However, its combustion, which releases CO2, worsens climate concerns. Consequently, the selective catalytic oxidation of methane into high-value chemicals has emerged as a strategic approach, combining both economic and environmental benefits. Traditionally, methane is converted into high-value chemicals through an indirect two-step process. The methane reforms into syngas, which is followed by the Fischer-Tropsch synthesis under high temperature and pressure to produce methanol and other valuable chemicals. However, this method suffers from high energy consumption, limited yield, and significant CO2 emissions, necessitating breakthroughs in alternative low-carbon technologies. The key challenges with direct methane oxidation is the activation of its stable C–H bonds (439 kJ/mol), while controlling the product selectivity to avoid over-oxidation. Photocatalysis, which drives reactions through photogenerated charge carriers at ambient conditions, offers a promising solution. However, during the photocatalytic methane conversion, CH3OH is frequently formed as an intermediate. This intermediate readily reacts with reactive oxygen to produce undesired byproducts like CO2, leading to lower selectivity compared to traditional indirect methods. Metal oxides (e.g., TiO2, ZnO) are widely used as photocatalysts due to their excellent chemical stability, environmental friendliness, and abundant surface-active sites. However, pure metal oxide catalysts suffer from rapid charge recombination, low utilization efficiency of photogenerated carriers, and difficulty in intermediate transformation, which results in low catalytic activity. TiO2 photocatalysts with different Au loadings (0.25wt%–1.5wt%) have been synthesized by impregnation, and their methane conversion performance has been evaluated. The results showed that 1.0wt% Au/TiO2 exhibited the best photocatalytic performance, with a 2.1-fold increase in CH4 conversion compared to pure TiO2 under mild conditions (60°C, 0.1 MPa, no additional oxidant). The yield of oxygenated liquid products (CH3OH and HCHO) reached 1057.9 μmol/(g h), with a total selectivity of 84.2%. Characterization techniques, including XRD, XPS, and TEM, revealed well-dispersed Au nanoparticles on the TiO2 surfaces, confirming that Au was present in its metallic state without altering the crystalline structure or surface electronic states of TiO2. Also, high-resolution TEM clearly distinguished the lattice fringes of Au (111) and TiO2 (101), indicating an intimate contact between the two phases. The analysis with in situ DRIFTS elucidated the reaction mechanism. CH4 was initially adsorbed on the oxygen sites of the TiO2 surface, where the C–H bond was polarized and a cleavage generated methoxy intermediates. Also, the H2O activation on the Ti sites produced hydroxyl radicals that oxidized CH3O into CH3OH and subsequently HCHO. Notably, the Au loading did not alter the reaction pathway, as confirmed by the nearly identical DRIFTS spectra for both Au/TiO2 and pure TiO2. However, the photocurrent and PL measurements showed that the Au nanoparticles functioned as electron traps in reducing the charge recombination. More importantly, the in situ spectroscopic results indicated that Au facilitated the conversion of CH3O* into methyl radicals. This enhanced the methyl radical formation and enabled their rapid coupling with ·OH, thereby improving the selective production of oxygenated products while suppressing over-oxidation. These findings provided fundamental mechanistic insights into the role of noble metal nanoparticles in photocatalytic methane conversion, offering a promising route for efficient and selective methane valorization under mild conditions. This study has successfully enhanced the photocatalytic methane conversion performance of the catalyst through the loading and optimization of Au nanoparticles. The reasons for the improvement of performance were elucidated using relevant characterization methods, thereby providing a valuable basis for the future development of efficient photocatalysts that convert methane into high-value chemicals.