Coordination Geometry and Oxidation State Requirements of Corner-Sharing MnO6 Octahedra for Water Oxidation Catalysis: An Investigation of Manganite (γ-MnOOH)

Surface-directed corner-sharing MnO6octahe-dra within numerous manganese oxide compounds containingMn3+or Mn4+oxidation states show strikingly differentcatalytic activities for water oxidation, paradoxically poorestfor Mn4+oxides, regardless of oxidation assay (photochemicaland electrochemical). This is demonstrated herein bycomparing crystalline oxides consisting of Mn3+(manganite,γ-MnOOH; bixbyite, Mn2O3), Mn4+(pyrolusite,β-MnO2) andmultiple monophasic mixed-valence manganese oxides. Like allMn4+oxides, pureβ-MnO2has no detectable catalytic activity,whileγ-MnOOH (tetragonally distorted Mn3+O6,D4hsymmetry) is significantly more active and Mn2O3(trigonal antiprismatic Mn3+O6,D3dsymmetry) is the most active.γ-MnOOH deactivates during catalytic turnover simultaneous with the disappearance of crystallographically defined corner-sharingMn3+O6and the appearance of Mn4+. In a comparison of 2D-layered crystalline birnessites (δ-MnO2), the monovalent Mn4+form is catalytically inert, while the hexagonal polymorph, containing few out-of-layer corner-sharing Mn3+O6, has∼10-foldhigher catalytic activity than the triclinic polymorph, containing in-plane edge-sharing Mn3+O6. These electronic and structuralcorrelations point toward the moreflexible (corner-shared) Mn3+O6sites, over more rigid (edge-shared) sites as substantiallymore active catalytic centers. Electrochemical measurements show and ligandfield theory predicts that, among corner-sharedMn3+O6sites, those possessingD3dligandfield symmetry have stronger covalent Mn−O bonding to the six equivalent oxygenligands, which we ascribe as responsible for more efficient and faster electrolytic water oxidation. In contrast,D4hMn3+O6siteshave weaker Mn−O bonding to the two axial oxygen ligands, have separated electrochemical oxidation waves for Mn and O, andare catalytically less efficient and exhibit slower catalytic turnover. By controlling the ligandfield geometry and strength to oxygenligands, we have identified the key variables for tuning water oxidation activity by manganese oxides. We apply thesefindings topropose a mechanism for water oxidation by the CaMn4O5catalytic site of natural photosynthesis.