Coordination Geometry and Oxidation State Requirements of Corner-Sharing MnO6 Octahedra for Water Oxidation Catalysis

An Investigation of Manganite (γ-MnOOH)

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

Research output: Contribution to journalArticle

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Abstract

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, 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+. 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 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. In contrast, D4h 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.

Original languageEnglish
Pages (from-to)2089-2099
Number of pages11
JournalACS Catalysis
Volume6
Issue number3
DOIs
Publication statusPublished - Mar 4 2016

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Catalysis
Ligands
Oxidation
Manganese oxide
Geometry
Water
Oxides
Catalyst activity
Oxygen
Polymorphism
Crystalline materials
Photosynthesis
Electrochemical oxidation
manganite
Assays
Tuning
manganese oxide

Keywords

  • birnessite equilibria
  • electrocatalysis
  • Jahn-Teller distortion
  • manganese oxide
  • photosystem II
  • water oxidation

ASJC Scopus subject areas

  • Catalysis

Cite this

Coordination Geometry and Oxidation State Requirements of Corner-Sharing MnO6 Octahedra for Water Oxidation Catalysis : An Investigation of Manganite (γ-MnOOH). / Smith, Paul F.; Deibert, Benjamin J.; Kaushik, Shivam; Gardner, Graeme; Hwang, Shinjae; Wang, Hao; Al-Sharab, Jafar F.; Garfunkel, Eric; Fabris, Laura; Li, Jing; Dismukes, G Charles.

In: ACS Catalysis, Vol. 6, No. 3, 04.03.2016, p. 2089-2099.

Research output: Contribution to journalArticle

Smith, Paul F. ; Deibert, Benjamin J. ; Kaushik, Shivam ; Gardner, Graeme ; Hwang, Shinjae ; Wang, Hao ; Al-Sharab, Jafar F. ; Garfunkel, Eric ; Fabris, Laura ; Li, Jing ; Dismukes, G Charles. / Coordination Geometry and Oxidation State Requirements of Corner-Sharing MnO6 Octahedra for Water Oxidation Catalysis : An Investigation of Manganite (γ-MnOOH). In: ACS Catalysis. 2016 ; Vol. 6, No. 3. pp. 2089-2099.
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abstract = "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, 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+. 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 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. In contrast, D4h 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.",
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author = "Smith, {Paul F.} and Deibert, {Benjamin J.} and Shivam Kaushik and Graeme Gardner and Shinjae Hwang and Hao Wang and Al-Sharab, {Jafar F.} and Eric Garfunkel and Laura Fabris and Jing Li and Dismukes, {G Charles}",
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T1 - Coordination Geometry and Oxidation State Requirements of Corner-Sharing MnO6 Octahedra for Water Oxidation Catalysis

T2 - An Investigation of Manganite (γ-MnOOH)

AU - Smith, Paul F.

AU - Deibert, Benjamin J.

AU - Kaushik, Shivam

AU - Gardner, Graeme

AU - Hwang, Shinjae

AU - Wang, Hao

AU - Al-Sharab, Jafar F.

AU - Garfunkel, Eric

AU - Fabris, Laura

AU - Li, Jing

AU - Dismukes, G Charles

PY - 2016/3/4

Y1 - 2016/3/4

N2 - 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, 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+. 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 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. In contrast, D4h 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.

AB - 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, 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+. 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 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. In contrast, D4h 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.

KW - birnessite equilibria

KW - electrocatalysis

KW - Jahn-Teller distortion

KW - manganese oxide

KW - photosystem II

KW - water oxidation

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