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

详细信息    查看全文

• 作者：Paul F. Smith ; Benjamin J. Deibert ; Shivam Kaushik ; Graeme Gardner ; Shinjae Hwang ; Hao Wang ; Jafar F. Al-Sharab ; Eric Garfunkel ; Laura Fabris ; Jing Li ; G. Charles Dismukes
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：March 4, 2016
• 年：2016
• 卷：6
• 期：3
• 页码：2089-2099
• 全文大小：719K
• ISSN：2155-5435

文摘

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, γ-MnOOH; 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, D_4h symmetry) is significantly more active and Mn₂O₃ (trigonal antiprismatic Mn³⁺O₆, D_3d 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⁺. 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-fold higher catalytic activity than the triclinic polymorph, containing in-plane edge-sharing Mn³⁺O₆. These electronic and structural correlations point toward 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 D_3d 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. In contrast, D_4h Mn³⁺O₆ sites have weaker Mn–O bonding to the two axial oxygen ligands, have 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.