title: Photochemical water oxidation by crystalline polymorphs of manganese oxides: structural requirements for catalysis.
abstract: 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 LiMn2O4 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(g)(1) 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.
authors: Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, United States.
authors: David M
authors: DM
authors: Robinson
authors: Yong Bok
authors: YB
authors: Go
authors: Michelle
authors: M
authors: Mui
authors: Graeme
authors: G
authors: Gardner
authors: Zhijuan
authors: Z
authors: Zhang
authors: Daniel
authors: D
authors: Mastrogiovanni
authors: Eric
authors: E
authors: Garfunkel
authors: Jing
authors: J
authors: Li
authors: Martha
authors: M
authors: Greenblatt
authors: G Charles
authors: GC
authors: Dismukes
mesh_terms: D002384:Catalysis; D003460:Crystallization; D017895:Manganese Compounds / Q000737:chemistry*; D008855:Microscopy, Electron, Scanning; D046529:Microscopy, Electron, Transmission; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry*; D055668:Photochemical Processes; D011108:Polymers / Q000737:chemistry*; D033362:Powder Diffraction; D014867:Water / Q000737:chemistry*
chemical_list: D017895:Manganese Compounds; D010087:Oxides; D011108:Polymers; D014867:Water; C027424:manganese oxide