Hetero-metallic active sites coupled with strongly reductive polyoxometalate for selective photocatalytic CO2-to-CH4 conversion in water

The photocatalytic reduction of CO2 to value-added methane (CH4) has been a promising strategy for sustainable energy development, but it is challenging to trigger this reaction because of its necessary eight-electron transfer process. In this work, an efficient photocatalytic CO2-to-CH4 reduction reaction was achieved for the first time in aqueous solution by using two crystalline heterogeneous catalysts, H{[Na2K4Mn4(PO4) (H2O)(4)]< subset of>{[Mo6O12(OH)(3)(HPO4)(3)(PO4)](4)[Mn-6(H2O)(4)]}16H(2)O (NENU-605) and H{[Na6CoMn3(PO4)(H2O)(4)]< subset of>{[Mo6O12(OH)(3)(HPO4)(3)(PO4)](4)[Co1.5Mn4.5]}21H(2)O (NENU-606). Both compounds have similar host inorganic polyoxometalate (POM) structures constructed with strong reductive {P4Mo6V} units, homo/hetero transition metal ions (Mn-II/(CoMnII)-Mn-II) and alkali metal ions (K+ and/or Na+). It is noted that the {P4Mo6V} cluster including the six Mo-V atoms served as a multi-electron donor in the case of a photocatalytic reaction, while the transition metal ions functioned as catalytically active sites for adsorbing and activating CO2 molecules. Additionally, the presence of alkali metal ions was believed to assist in the capture of more CO2 for the photocatalytic reaction. The synergistic combination of the above-mentioned components in NENU-605 and NENU-606 effectively facilitates the accomplishment of the required eight-electron transfer process for CH4 evolution. Furthermore, NENU-606 containing hetero-metallic active sites finally exhibited higher CH4 generation selectivity (85.5%) than NENU-605 (76.6%).