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-synthesizedframework of layered Mn-K oxide. These Mn oxides were heatedat 60-600 degrees C, and their electrochemistry and water oxidationreaction (WOR) were investigated at pH = 1. Using this strategy, theframework of the structure of catalysts is the same, and all are synthesizedin a layered Mn oxide containing K-(I) framework. Thus, comparing differentions in the same framework is easier. After calcination at 300-600 degrees C, X-ray absorption spectroscopy shows a layered structure withno change in bulk, but XRD shows that a few conversions regardinglayered Mn oxide -> alpha-MnO2 -> alpha-Mn2O3 occur. Indeed, after calcination at 300-600 degrees C, the formed layered Mn oxide converts into a better WOR catalyst.After calcination at >= 300 degrees C, small amounts of alpha-MnO2/alpha-Mn2O3 are formed in the structure,which could be important for WOR. At 600 degrees C, trace amounts ofcrystalline alpha-Mn2O3 are usually formedin the presence of redox-inert ions, which is not an efficient catalystfor WOR. In the presence of redox-active ions, the formation of metaloxides such as Fe, Co, Ni, and Ni oxides is possible at 600 degrees C.These metal oxides are usually catalysts for WOR. An open structureof layered Mn oxide could increase the WOR activity of these redox-activeions. Thus, Mn ions may not be active sites for WOR and could be asubstrate for these redox-active ions. Among the redox-inert ionsbetween the layers of Mn oxides, the presence of Ca-(II) at 500 degrees Cresults in a significant increase in WOR. Interestingly, a Ca/Mn clusteris present in the biological WOR catalyst.