Photocatalytic coupling of methane and CO₂ into C2-hydrocarbons over Zn doped g-C₃N₄ catalysts

Methane conversion into ethanol and associated derivates under mild reaction conditions is of great significance, yet remains a challenge to date. Herein, we report efficient photocatalytic methane conversion into C2-hydrocarbons by using CO₂ as a soft oxidant over a series of Zn-doped g-C₃N₄ composites. The results indicate that Zn atoms enter the lattice of g-C₃N₄ and are chemically coordinated therein in the form of Zn[sbnd]N bonds. The integration of Zn, on one hand, could increase the specific surface area thus surface alkaline sites, and broaden the light response ability as well, promoting the dehydrogenation of methane into methyl. One the other hand, it is found that the Zn[sbnd]N bonds could serve as electron channels, accelerating the charge separation inside g-C₃N₄ and enabling rapid electron transfer from g-C₃N₄ to surface photodeposited Ru cocatalyst. This unique behavior changes the photocatalytic pathway and promotes the formation of CH₃CHO and CH₃CH₂OH, as demonstrated by our in-situ infrared spectroscopy. Our photocatalytic tests indicate that the 0.5% Ru/Zn-g-C₃N₄-1/20 composite possesses the best activity, with the yields of CO, CH₃CHO, and CH₃CH₂OH up to 865.25, 330.38, and 1442.88 μmol/g in 3 h, respectively. A proposed reaction mechanism based on the experimental results and above characterizations was finally deliberated.