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 bimessite 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 mu-oxo and an OH- occurs after the neighboring Mn (III) is oxidized to Mn (IV). Compared to the CaMn4O5 in PSII, the mu-oxo on the surface of Ca-bimessite 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 mu-oxo and appropriate cations are predicted to be important factors governing the catalytic activity of manganese oxide catalysts.