Synthetic models of oxygen evolving complex (OEC) are used not only to gain better understanding of the mechanism and the roles of cofactors for water oxidation in photosynthesis, but also as water oxidation catalysts to realize artificial photosynthesis, which is anticipated as a promising solar fuel production system. However, although much attention has been paid to the composition and structure of active sites for development of heterogeneous OEC models, the cofactors, which are essential for water oxidation by the photosynthetic OEC, remain little studied. The high activity of CoO(OH) nanoparticles for electrocatalytic water oxidation is shown to be induced by a CO cofactor. The possibility of CO ions acting as proton acceptors for O-O bond formation based on the proton-concerted oxygen atom transfer mechanism is proposed. The O-O bond formation is supposed to be accelerated due to effective proton acceptance by adjacent CO ions coordinated on the Co center in the intermediate, which is consistent with Michaelis-Menten-type kinetics and the significant H/D isotope effect observed in electrocatalysis.

A heterogeneous catalyst incorporating an inorganic ion cofactor for electrochemical water oxidation was exploited using a CoO(OH) nanoparticle layer-deposited electrode. The significant catalytic current for water oxidation was generated in a NaBO solution at pH 9.4 when applying 0.94 V versus Ag/AgCl in contrast to no catalytic current generation in the KSO solution at the same pH. HBO and BO ions were indicated to act as key cofactors for the induced catalytic activity of the CoO(OH) layer. The NaBO concentration dependence of the catalytic current was analyzed based on a Michaelis-Menten-type kinetics to provide an affinity constant of cofactors to the active sites, K = 28 ± 3.6 mM, and the maximum catalytic current density, I = 2.3 ± 0.13 mA cm. The I value of HBO and BO ions was 1.4 times higher than that (1.3 mA cm) for the previously reported case of CO ions. This could be explained by the shorter-range proton transfer from the active site to the proton-accepting cofactor because of the larger size and more flexible conformation of HBO and BO ions compared with that of CO ions.

No abstract

No abstract