Theoretical Investigation of Water Oxidation Mechanism on Pure Manganese and Ca-Doped Bimetal Oxide Complexes

Understanding the role of Ca2+ ion in the oxygen-evolving complex of photosystem II is essential to design commercially viable and efficient water oxidation catalysts. To this end, small pure manganese oxide and calcium-doped manganese oxide model complexes saturated with water-derived ligands are investigated in this work. Density functional theory calculations are performed to investigate the water oxidation process on Mn-2(mu-OH)(mu-O)(H2O)(3)(OH)(5) (Mn2O4 center dot 6H(2)O) and CaMnO(mu-OH)(2)(H2O)(5)(OH)(2)(CaMnO3 center dot 7H(2)O) complexes. Many reaction pathways are considered, and the three lowest energy water oxidation mechanisms on CaMnO3 center dot 7H(2)O have highest reaction energy steps of 1.37, 1.67, and 1.81 eV compared to the highest reaction energy step of 2.25 eV for the lowest energy mechanism of the pure Mn2O center dot 6H(2)O complex. Doping of the manganese dimer complex with calcium decreases the highest reaction energy of the water oxidation process. Consequently, the inclusion of calcium appears to improve the catalyst's efficiency for water splitting.