Mechanistic and Electronic Insights into Efficient Carbon Dioxide Reduction Driven by Visible Light Using a Coordination Polymer

In this study, a comprehensive theoretical analysis was undertaken to elucidate the remarkably efficient conversion of CO_2 into HCOO^– employing a coordination polymer featuring Pb–S bonds, namely [Pb(tadt)]_n (where tadt stands for 1,3,4-thiadiazole-2,5-dithiolate), referred to as KGF-9. The catalytic activity of this visible-light responsive solid photocatalyst has been carefully compared with that of PbS, a typical compound that also contains the Pb–S bond. The former shows a very high catalytic activity, while the latter shows almost no activity. The photoreduction process of CO_2 on the KGF-9 surface was analyzed in detail using periodic density functional theory calculations. The reduced catalyst surface was modeled as a hydrogenated surface. The reaction at the active center of a formate dehydrogenase provides an interesting contrast, suggesting that the S–H group plays an important role in the conversion of CO_2 to HCOO^–. However, the S–H group on the reduced PbS surface does not facilitate the conversion to the same extent as KGF-9. This is because the electrons supplied to CO_2 on the PbS surface come from deep within the solid, whereas on KGF-9, they come from the top surface. This difference is due to differences in the electronic structure of the S–H bond, band gap, and valence band maximum position between the two surfaces, accounting for the marked difference in their catalytic activity. These insights are consistent with experimental and computational results on the thermodynamic and kinetic characteristics of the CO_2 reduction reaction of KGF-9 and PbS, and provide guidance for the design of CO_2 photoreduction catalysts.