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

Abstract

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]⊂{[Mo₆O₁₂(OH)₃(HPO₄)₃(PO₄)]₄[Mn₆(H₂O)₄]}·16H₂O (NENU-605) and H{[Na₆CoMn₃(PO₄)(H₂O)₄]⊂{[Mo₆O₁₂(OH)₃(HPO₄)₃(PO₄)]₄[Co_1.5Mn_4.5]}·21H₂O (NENU-606). Both compounds have similar host inorganic polyoxometalate (POM) structures constructed with strong reductive {P₄Mo₆^V} units, homo/hetero transition metal ions (Mn^II/Co^IIMn^II) and alkali metal ions (K⁺ and/or Na⁺). It is noted that the {P₄Mo₆^V} 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%).