Effect of Different Metal Ions between Nanolayers of Manganese Oxide for Water Oxidation Reaction under Acidic Conditions

Abstract

A new approach is used to synthesize various metal ions [Mg(II), Ca(II), Ba(II), Al(III), Zn(II), Cd(II), Mn(II), Fe(III), Co(III), Ni(II), and Cu(II)] between layers of Mn oxide in a pre-synthesized framework of layered Mn–K oxide. These Mn oxides were heated at 60–600 °C, and their electrochemistry and water oxidation reaction (WOR) were investigated at pH = 1. Using this strategy, the framework of the structure of catalysts is the same, and all are synthesized in a layered Mn oxide containing K(I) framework. Thus, comparing different ions in the same framework is easier. After calcination at 300–600 °C, X-ray absorption spectroscopy shows a layered structure with no change in bulk, but XRD shows that a few conversions regarding layered Mn oxide → α-MnO2 → α-Mn2O3 occur. Indeed, after calcination at 300–600 °C, the formed layered Mn oxide converts into a better WOR catalyst. After calcination at ≥300 °C, small amounts of α-MnO2/α-Mn2O3 are formed in the structure, which could be important for WOR. At 600 °C, trace amounts of crystalline α-Mn2O3 are usually formed in the presence of redox-inert ions, which is not an efficient catalyst for WOR. In the presence of redox-active ions, the formation of metal oxides such as Fe, Co, Ni, and Ni oxides is possible at 600 °C. These metal oxides are usually catalysts for WOR. An open structure of layered Mn oxide could increase the WOR activity of these redox-active ions. Thus, Mn ions may not be active sites for WOR and could be a substrate for these redox-active ions. Among the redox-inert ions between the layers of Mn oxides, the presence of Ca(II) at 500 °C results in a significant increase in WOR. Interestingly, a Ca/Mn cluster is present in the biological WOR catalyst.