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

Abstract

Surface-directed corner-sharing MnO 6 octahedra within numerous manganese oxide compounds containing Mn 3+ or Mn 4+ oxidation states show strikingly different catalytic activities for water oxidation, paradoxically poorest for Mn 4+ oxides, regardless of oxidation assay (photochemical and electrochemical). This is demonstrated herein by comparing crystalline oxides consisting of Mn 3+ (manganite, γ-MnOOH; bixbyite, Mn 2 O 3 ), Mn 4+ (pyrolusite, β-MnO 2 ) and multiple monophasic mixed-valence manganese oxides. Like all Mn 4+ oxides, pure β-MnO 2 has no detectable catalytic activity, while γ-MnOOH (tetragonally distorted Mn 3+ O 6 , D 4h symmetry) is significantly more active and Mn 2 O 3 (trigonal antiprismatic Mn 3+ O 6 , D 3d symmetry) is the most active. γ-MnOOH deactivates during catalytic turnover simultaneous with the disappearance of crystallographically defined corner-sharing Mn 3+ O 6 and the appearance of Mn 4+ . In a comparison of 2D-layered crystalline birnessites (δ-MnO 2 ), the monovalent Mn 4+ form is catalytically inert, while the hexagonal polymorph, containing few out-of-layer corner-sharing Mn 3+ O 6 , has ∼10-fold higher catalytic activity than the triclinic polymorph, containing in-plane edge-sharing Mn 3+ O 6 . These electronic and structural correlations point toward the more flexible (corner-shared) Mn 3+ O 6 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 Mn 3+ O 6 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 Mn 3+ O 6 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 CaMn 4 O 5 catalytic site of natural photosynthesis.

Nickel‐Doping Effect on Mn3O4 Nanoparticles for Electrochemical Water Oxidation under Neutral Condition

“…The catalytic cycle was facilitated by the defect sites in layers of edge‐sharing MnO 6 including corner‐sharing MnO 6 species . Also, the high OER efficiency of γ‐MnOOH (compared to various manganese oxides) shows that tetragonally distorted Mn(III)O 6 octahedra with corner‐sharing is regarded as more crucial structural motif for catalysis …”

“…To enhance the efficiency of synthetic Mn‐based electrocatalysts, structural parameters for activity enhancement and rate‐determining process have been copiously investigated . The superior OER activity for mixed‐valent MnO x to MnO nanocrystal indicated that the edge‐sharing MnO 6 octahedra with high valency served as a key structural motif for catalysis .…”

Computational Screening of Doped α‐MnO2 Catalysts for the Oxygen Evolution Reaction

“…[17] At potentials above 1.2 Vv ersus RHE, MnO 2 starts to dissolve. [34,35] On the basis of the currentu nderstanding, Mn 3 + is critical for enhancing the OER [36][37][38][39] and ORR [29] activities. with similar heats of formation that crystallize in different crystal structures.…”

Removing ammonium from underground water by strengthening Mn(III) generation through electrochemical reduction

“…Smith et al. () found that the electrocatalytic activity of Mn(IV) oxide for water oxidation was very poor, while Mn(III) oxide (MnOOH or Mn 2 O 3 ) showed significantly more active for water oxidation (Gorlin & Jaramillo, ). These results suggest that it is Mn(III) that perhaps plays a key role in the removal of ammonium by MnO x .…”