Synergetic Effect of Ligand Modification of a Ru(II) Complex Catalyst and Ag Loading for Constructing a Highly Active Hybrid Photocatalyst Using C₃N₄ for CO₂ Reduction

Hybrid photocatalysts comprising a semiconductor and a metal complex catalyst are promising for the efficient, visible-light-driven chemical conversion of CO2 to useful fuels. As a typical example, a hybrid photocatalyst comprising Ag-loaded carbon nitride (C3N4) and trans-(Cl)-[Ru{4,4 '-(PO3H2)(2)-2,2 '-bipyridine}(CO)(2)Cl-2] (RuP), RuP/Ag/C3N4, has been developed. However, the effects of the reduction potential of RuP on the photocatalytic CO2 reduction activity of hybrid systems have not yet been investigated. This study utilized a ligand-modified Ru catalyst (RuCP) to construct a new hybrid photocatalyst (RuCP/Ag/C3N4) in which a methylene spacer was inserted into RuCP between the bipyridine ligand and the phosphonic acid anchor groups on RuP to obtain a more negative reduction potential for the Ru catalyst. RuCP/Ag/C3N4 yielded formic acid with 2.5-fold turnover frequency and 6.5-fold turnover number (> 12,000, used RuCP base) in comparison to RuP/Ag/C3N4 under visible light irradiation. The superior photocatalytic activity of RuCP/Ag/C3N4 was attributed to the faster CO2 reduction reaction on RuCP owing to its more negative reduction potential. The faster CO2 reduction on RuCP was facilitated by Ag loading on C3N4, which accumulated photoexcited electrons on Ag-loaded C3N4 and improved electron supply to the RuCP. This finding sheds light on a new approach for the development of hybrid photocatalysts for CO2 reduction.