Highly active water oxidation on nanostructured biomimetic calcium manganese oxide catalysts

Abstract

Water oxidation is a crucial reaction step in solar-to-chemical energy conversion processes such as photocatalytic water splitting and carbon dioxide reduction. In natural photosynthesis, the water oxidation reaction is catalyzed by μ-oxido-Mn₄Ca clusters in photosystem II (PSII). Herein, we report the fabrication of nanostructured biomimetic calcium manganese oxides (Ca_xMnO_y) via a simple process under mild conditions utilizing H₂O₂ as an oxidant and TMAOH (tetramethylammonium hydroxide) as an alkaline source. Ca_xMnO_y materials with x higher than 0.26 are composed of nanoparticles with particle sizes ranging from 15 to 30 nm according to the result of HRTEM. The results of X-ray absorption fine structure (XAFS) indicate that calcium manganese oxides have similar structural motifs to the catalytically active site for water oxidation in PSII. It was also found that the content of Ca and the concentration of H₂O₂ in the initial mixture could affect the crystallinity and the average Mn valence state of calcium manganese oxides. Water oxidation experiments for both chemical and photocatalytic systems suggest that the disordered structure of calcium manganese oxides and a modest valence state of Mn (+3.7 to +3.8) are necessary for achieving high activity. Our method provides a strategy for synthesis and modulation of nanostructured biomimetic water oxidation catalysts.