High-Selectivity Tandem Photocatalytic Methanation of CO₂ by Lacunary Polyoxometalates-Stabilized *CO Intermediate

Stabilizing specific intermediates to produce CH₄ remains a main challenge in solar-driven CO₂ reduction. Herein, g-C₃N₄ is modified with saturated and lacunary phosphotungstates (PW_x, x=12, 11, 9) to tailor the CO₂ reduction pathway to yield CH₄ in high selectivity. Increased lacuna of phosphotungstates leads to higher CH₄ yield and selectivity, with a superior CH₄ selectivity of 80 % and 40.8 μmol ⋅ g^-1 ⋅ h^-1 evolution rate for PW₉/g-C₃N₄. Conversely, g-C₃N₄ and PW_x alone show negligible CH₄ production. The conversion of CO₂ to CH₄ follows a tandem catalytic process. CO₂ is initially activated on g-C₃N₄ to form *CO intermediates, meanwhile photogenerated electrons derived from g-C₃N₄ transfer to PW_x. Then the reduced PW_x captures *CO, which is subsquently hydrogenated to CH₄. With the injection of two photogenerated electrons, PW₉ is capable of adsorbing and activating *CO. However, the reduced PW₁₂ and PW₁₁ are incapable of adsorbing *CO due to the small energy of occupied molecular orbitals, which is the reason for the poorer activity of PW_x/g-C₃N₄ (x=12, 11) compared with that of PW₉/g-C₃N₄. This work provides new insights to regulate highly selective CO₂ photoreduction to CH₄ by utilizing lacuna of polyoxometalates to enhance the interaction of metals in polyoxometalates with key intermediates.