Photocatalysis of CO2 Reduction by a Ru(II)–Ru(II) Supramolecular Catalyst Adsorbed on Al2O3

Various hybrid systems consisting of metal-complex photocatalysts and semiconductor materials have been developed for photocatalytic CO2 reduction via the Z-scheme mechanism. Such systems are often composed of a supramolecular photocatalyst with Ru(II) photosensitizer and Ru(II) catalyst (RuRu). In this study, RuRu was adsorbed on Al2O3 particles, and the photocatalytic activity of RuRu/Al2O3 in CO2 reduction was investigated in detail, especially by changing the adsorption density of RuRu. The production of HCOOH, which is the main product of CO2 reduction, decreased upon increasing the adsorption density of RuRu owing to the formation of Ru polymers through the reductive coupling of the Ru catalyst units of RuRu moieties at high adsorption densities. This behavior contrasts with the previously reported behavior of RuRe/Al2O3 with the same Ru(II) photosensitizer and a Re(I) catalyst unit. Moreover, the co-adsorption of mononuclear Ru(II)-complex photosensitizers (Ru) close to RuRu on Al2O3 substantially improved the photocatalytic activity of RuRu (turnover number of HCOOH formation increased more than 10 times) because Ru photochemically supplies electrons to the intermediate formed from the reduced Ru species to suppress the decomposition of RuRu.