Photochemical Water Oxidation by Crystalline Polymorphs\nof Manganese Oxides: Structural Requirements for Catalysis

Abstract

Manganese oxides occur naturally\nas minerals in at least 30 different\ncrystal structures, providing a rigorous test system to explore the\nsignificance of atomic positions on the catalytic efficiency of water\noxidation. In this study, we chose to systematically compare eight\nsynthetic oxide structures containing Mn­(III) and Mn­(IV) only, with\nparticular emphasis on the five known structural polymorphs of MnO₂. We have adapted literature synthesis methods to obtain pure\npolymorphs and validated their homogeneity and crystallinity by powder\nX-ray diffraction and both transmission and scanning electron microscopies.\nMeasurement of water oxidation rate by oxygen evolution in aqueous\nsolution was conducted with dispersed nanoparticulate manganese oxides\nand a standard ruthenium dye photo-oxidant system. No Ru was absorbed\non the catalyst surface as observed by XPS and EDX. The post reaction\natomic structure was completely preserved with no amorphization, as\nobserved by HRTEM. Catalytic activities, normalized to surface area\n(BET), decrease in the series Mn₂O₃ > Mn₃O₄ ≫ λ-MnO₂, where the\nlatter is derived from spinel LiMn₂O₄ following\npartial Li⁺ removal. No catalytic activity is observed\nfrom LiMn₂O₄ and four of the MnO₂ polymorphs, in contrast to some literature reports with polydispersed\nmanganese oxides and electro-deposited films. Catalytic activity within\nthe eight examined Mn oxides was found exclusively for (distorted)\ncubic phases, Mn₂O₃ (bixbyite), Mn₃O₄ (hausmannite), and λ-MnO₂ (spinel),\nall containing Mn­(III) possessing longer Mn–O bonds between\nedge-sharing MnO₆ octahedra. Electronically degenerate\nMn­(III) has antibonding electronic configuration e_g¹ which imparts lattice distortions due to the Jahn–Teller\neffect that are hypothesized to contribute to structural flexibility\nimportant for catalytic turnover in water oxidation at the surface.