Water Oxidation Mechanism on Alkaline-Earth-Cation Containing Birnessite-Like Manganese Oxides

The CaMn₄O₅ complex in Photosystem II (PSII) is known for its high efficiency in catalyzing the water oxidation reaction. A layered birnessite-like manganese oxide with intercalated metal cations has been prepared and reported to mimic the complex. However, the water oxidation reaction on the Ca²⁺-containing birnessite is far less active than the oxygen evolution complex (OEC). To determine the reason, the water oxidation mechanism has been studied on Ca-birnessite by the density functional theory. It is found that the OO bond formation between a μ-oxo and an OH^– occurs after the neighboring Mn (III) is oxidized to Mn (IV). Compared to the CaMn₄O₅ in PSII, the μ-oxo on the surface of Ca-birnessite is less flexible and therefore less active. Additionally, the overpotential for the water oxidation reaction depends on the cation and increases in the order of Ca²⁺ < Sr²⁺< Mg²⁺. Therefore, flexible μ-oxo and appropriate cations are predicted to be important factors governing the catalytic activity of manganese oxide catalysts