Photochemical water oxidation by crystalline polymorphs of manganese oxides: Structural requirements for catalysis

Manganese oxides occur naturally as minerals in at least 30 different crystal structures, providing a rigorous test system to explore the significance of atomic positions on the catalytic efficiency of water oxidation. In this study, we chose to systematically compare eight synthetic oxide structures containing Mn(III) and Mn(IV) only, with particular emphasis on the five known structural polymorphs of MnO₂. We have adapted literature synthesis methods to obtain pure polymorphs and validated their homogeneity and crystallinity by powder X-ray diffraction and both transmission and scanning electron microscopies. Measurement of water oxidation rate by oxygen evolution in aqueous solution was conducted with dispersed nanoparticulate manganese oxides and a standard ruthenium dye photo-oxidant system. No Ru was absorbed on the catalyst surface as observed by XPS and EDX. The post reaction atomic structure was completely preserved with no amorphization, as observed by HRTEM. Catalytic activities, normalized to surface area (BET), decrease in the series Mn₂O₃ > Mn₃O₄ ≪ λ-MnO₂, where the latter is derived from spinel LiMn ₂O₄ following partial Li⁺ removal. No catalytic activity is observed from LiMn₂O₄ and four of the MnO₂ polymorphs, in contrast to some literature reports with polydispersed manganese oxides and electro-deposited films. Catalytic activity within the eight examined Mn oxides was found exclusively for (distorted) cubic phases, Mn₂O₃ (bixbyite), Mn₃O₄ (hausmannite), and λ-MnO₂ (spinel), all containing Mn(III) possessing longer Mn-O bonds between edge-sharing MnO₆ octahedra. Electronically degenerate Mn(III) has antibonding electronic configuration e_g¹ which imparts lattice distortions due to the Jahn-Teller effect that are hypothesized to contribute to structural flexibility important for catalytic turnover in water oxidation at the surface.