Energetic basis of catalytic activity of layered nanophase calcium manganese oxides for water oxidation

Abstract

Previous measurements show that calcium manganese oxide nanoparticles are better water oxidation catalysts than binary manganese oxides (Mn₃O₄, Mn₂O₃, and MnO₂). The probable reasons for such enhancement involve a combination of factors: The calcium manganese oxide materials have a layered structure with considerable thermodynamic stability and a high surface area, their low surface energy suggests relatively loose binding of H₂O on the internal and external surfaces, and they possess mixed-valent manganese with internal oxidation enthalpy independent of the Mn³⁺/Mn⁴⁺ ratio and much smaller in magnitude than the Mn₂O₃-MnO₂ couple. These factors enhance catalytic ability by providing easy access for solutes and water to active sites and facile electron transfer between manganese in different oxidation states.