Solid state synthesis and photocatalytic activity of bio-inspired calcium manganese oxide catalysts

Abstract

Calcium manganese oxides have structural similarities to the μ-oxo-Mn4Ca cluster in Photosystem-II protein that is responsible for water-splitting in natural photosynthesis. As such, calcium manganese oxides have attracted great interest in recent years as a new class of biomimetic redox catalysts in many energy-related applications. However, photo-induced catalytic activities of calcium manganese oxides have not been fully understood. This work reports the experimental investigation of the synthesis of different calcium manganese oxides nanoparticles (Ca2Mn3O8, CaMn2O4 and CaMnO3) by mechanochemical processing, a potentially green scalable method, and their photocatalytic activities on the decolourization of Rhodamine 6G aqueous solutions. Band gap energies of the three calcium manganese oxides showed their ability to absorb visible light. Valence bands of all the as-synthesized samples were more positive than the electrochemical potential required for the formation of hydroxyl free radicals from water. Without addition of any catalytic powder, 32% of the dye was decolorized within 3 ​h. The addition of Ca2Mn3O8 did not increase the dye decolourization upon 3 ​h of light irradiation. On the other hand, the presence of CaMn2O4 and CaMnO3 resulted in 61% and 62% degradation of the dye, respectively, within the same irradiation time. The photocatalytic activity of the prepared powders, per unit specific surface area, increased in the order Ca2Mn3O8 ​< ​CaMnO3 ​< ​CaMn2O4.