A novel molecular photocatalytic system with not only high reduction ability of CO but also high capture ability of CO has been developed using a Ru(II)-Re(I) dinuclear complex as a photocatalyst. By using this photocatalytic system, CO of 10% concentration could be selectively converted to CO with almost same photocatalysis to that under a pure CO atmosphere (TON > 1000, ΦCO > 0.4). Even 0.5% concentration of CO was reduced with 60% initial efficiency of CO formation by using the same system compared to that using pure CO (TON > 200). The Re(I) catalyst unit in the photocatalyst can efficiently capture CO, which proceeds CO insertion to the Re-O bond, and then reduce the captured CO by using an electron supplied from the photochemically reduced Ru photosensitizer unit.
A Ru(II)-Re(I) supramolecular photocatalyst and a Ru(II) redox photosensitizer were both deposited successfully on a NiO electrode by using methyl phosphonic acid anchoring groups and the electrochemical polymerization of the ligand vinyl groups of the complexes. This new molecular photocathode, poly-RuRe/NiO, adsorbed a larger amount of the metal complexes compared to one using only methyl phosphonic acid anchor groups, and the stability of the complexes on the NiO electrode were much improved. The poly-RuRe/NiO acted as a photocathode for the photocatalytic reduction of CO at E = -0.7 V vs Ag/AgCl under visible-light irradiation in an aqueous solution. The poly-RuRe/NiO produced approximately 2.5 times more CO, and its total Faradaic efficiency of the reduction products improved from 57 to 85%.
The novel supramolecular complexes, which are composed of an [Os(5dmb)2(BL)](2+)-type complex (5dmb = 5,5'-dimethyl-2,2'-bipyridine; BL = 1,2-bis(4'-methyl-[2,2'-bipyridin]-4-yl)ethane) as a photosensitizer and cis,trans-[Re(BL)(CO)2{P(p-X-C6H4)3}2](+)-type complexes (X = F, Cl) as a catalyst, have been synthesized. They photocatalyzed selective reduction of CO2 to CO under red-light irradiation (λ > 620 nm). The photocatalytic abilities were affected by the phosphine ligands on the Re unit, and the supramolecule with P(p-Cl-C6H4)3 ligands exhibited better photocatalysis (ΦCO = 0.12, TONCO = 1138, TOFCO = 3.3 min(-1)). The detailed studies clarified the electron balance and material balance; i.e., one molecule of the sacrificial electron donor (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH)) donated two electrons, one molecule of CO2 accepted the two electrons, and another CO2 molecule served as an "O(2-)" acceptor to give each molecule of the two-electron oxidized compound of BIH, CO, and HCO3(-).
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