Mn-based water oxidation catalyst under neutral pH - Proton transfer regulation toward high catalytic activity -
Akira Yamaguchi, Riko Inuzuka, Toshihiro Takashima, Toru Hayashi, Kazuhito Hashimoto and Ryuhei Nakamura
Professor Kazuhito Hashimoto and their colleagues at the University of Tokyo Graduate School of Engenireering, working with researchers at RIKEN (Team Leader Ryuhei Nakamura and Junior Research Associate Akira Yamaguchi), developed the artificial Manganese (Mn)-based catalyst which can extract electrons from neutral water by utilizing the mechanism which the natural photosynthetic system adapts.
Water molecule is one of the most abundant electron sources in nature and is important chemical source to create hydrogen and organic fuels. In nature, photosynthetic organisms such as plants utilize Mn-containing enzyme to obtain electrons from water, by which hydrocarbons are produced from carbon dioxides. Inspired by this enzyme, artificial structure-mimicking Mn-based catalysts have been developed to extract electrons from water. Although artificially developed Mn-based catalysts can obtain electrons from water efficiently under acidic or basic conditions, its activity drastically decreases under neutral pH. The reason why these catalysts don’t work under neutral pH and the origin of the activity difference between the natural and artificial Mn-based catalysts had been unknown.
The research group considered the activity difference in terms of the electron and proton transfer mechanism, and investigated the electron and proton transfer process of artificial Mn-based water oxidation catalysts. After the examination, the research group has found that the electrons and protons were transferred at the different timing during the water splitting process（2H2O → O2 + 4e- +4H+）of artificial catalysts, while they are transferred simultaneously during the process of the natural enzymes. Based on this result, the research group intentionally added base reagents which have large proton-accepting ability to a reaction system in order to adjust the timing of the electron and proton transfer. The addition of the base enhanced the water splitting activity approx. 15 times under neutral pH, and it reached 60 % of activity under alkaline condition.