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

摘要

The photocatalytic reduction of CO _(2) to value-added methane (CH _(4) ) 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 CO _(2) -to-CH _(4) reduction reaction was achieved for the first time in aqueous solution by using two crystalline heterogeneous catalysts, H{[Na _(2) K _(4) Mn _(4) (PO _(4) ) (H _(2) O) _(4) ]?{[Mo _(6) O _(12) (OH) _(3) (HPO _(4) ) _(3) (PO _(4) )] _(4) [Mn _(6) (H _(2) O) _(4) ]}·16H _(2) O ( NENU-605 ) and H{[Na _(6) CoMn _(3) (PO _(4) )(H _(2) O) _(4) ]?{[Mo _(6) O _(12) (OH) _(3) (HPO _(4) ) _(3) (PO _(4) )] _(4) [Co _(1.5) Mn _(4.5) ]}·21H _(2) O ( NENU-606 ). Both compounds have similar host inorganic polyoxometalate (POM) structures constructed with strong reductive {P _(4) Mo _(6) ~(V) } units, homo/hetero transition metal ions (Mn ~(II) /Co ~(II) Mn ~(II) ) and alkali metal ions (K ~(+) and/or Na ~(+) ). It is noted that the {P _(4) Mo _(6) ~(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 CO _(2) molecules. Additionally, the presence of alkali metal ions was believed to assist in the capture of more CO _(2) 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 CH _(4) evolution. Furthermore, NENU-606 containing hetero-metallic active sites finally exhibited higher CH _(4) generation selectivity (85.5%) than NENU-605 (76.6%).