Coordination Geometry and Oxidation State Requirements of Corner Sharing MnO6 Octahedra for Water Oxidation Catalysis: An Investigation of Manganite (g MnOOH)
Surface directed corner sharing MnO6 octahedra within numerous manganese oxide compounds containing Mn3+ or Mn4+ oxidation states show strikingly different catalytic activities for water oxidation, paradoxically poorest for Mn4+ oxides, regardless of oxidation assay (photochemical and electrochemical) This is demonstrated herein by comparing crystalline oxides consisting of Mn3+ (manganite, g MnOOH; bixbyite, Mn2O3) Mn4+ (pyrolusite, b MnO2) and multiple monophasic mixed valence manganese oxides. Like all Mn4+ oxides, pure b MnO2 has no detectable catalytic activity, while g MnOOH (tetragonally distorted Mn3+O6, D4h symmetry) is significantly more active and Mn2O3 (trigonal antiprismatic Mn3+O6, D3d symmetry) is the most active. g MnOOH deactivates during catalytic turnover simultaneous with the disappearance of crystallographically defined corner sharing Mn3+O6 and the appearance of Mn4+ In a comparison of 2D layered crystalline birnessites (d MnO2) the monovalent Mn4+ form is catalytically inert, wh.