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Surface-directed corner-sharing MnO¬6 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, γ-MnOOH and bixbyite, Mn2O3), Mn4+ (pyrolusite, β-MnO2) and multiple monophasic mixed-valence manganese oxides. Like all Mn4+ oxides, pure β-MnO2 has no detectable catalytic activity, while γ-MnOOH (tetragonally distorted Mn3+O6, D4h symmetry) is significantly more active, and Mn2O3 (trigonal antiprismatic Mn3+O6, D3d symmetry) is the most active. γ-MnOOH deactivates during catalytic turnover simultaneous with the disappearance of crystallographically defined corner-sharing Mn3+O6 and the appearance of Mn4+. Comparing 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-fold higher catalytic activity than the triclinic polymorph, containing in-plane edge-sharing Mn3+O6. These electronic and structural correlations point towards the more flexible (corner shared) Mn3+O6 sites, over more rigid (edge shared) sites as substantially more active catalytic centers. Electrochemical measurements show and ligand field theory predicts that among corner shared Mn3+O6 sites, those possessing D3d ligand field symmetry have stronger covalent Mn-O bonding to the six equivalent oxygen ligands, which we ascribe as responsible for more efficient and faster electrolytic water oxidation. By contrast, D4h Mn3+O6 sites have weaker Mn-O bonding to the two axial oxygen ligands, separated electrochemical oxidation waves for Mn and O, and are catalytically less efficient and exhibit slower catalytic turnover. By controlling the ligand field geometry and strength to oxygen ligands we have identified the key variables for tuning water oxidation activity by manganese oxides. We apply these findings to propose a mechanism for water oxidation by the CaMn4O5 catalytic site of natural photosynthesis. |
