1. Structure/Function Relations in Oxygen Evolution Catalysis by Manganese Oxides
See: Robinson, D, Go YB, Mui M, Gardner G, Zhang Z, Mastrogiovanni D, Garfunkel E, Li J, Greenblatt M, Dismukes GC. Journal of the American Chemical Society, 2013,135:3494-3501
Manganese oxides occur naturally as minerals in at least 30 different crystal structures, providing a rigorous test system to explore the significance of atomic positions on the catalytic efficiency of water oxidation. We chose to systematically compare eight synthetic oxide structures containing Mn(III) and Mn(IV) only, with particular emphasis on the five known structural polymorphs of MnO2. We have adapted literature synthesis methods to obtain pure polymorphs and validated their homogeneity and crystallinity by powder X-ray diffraction and both transmission and scanning electron microscopies. Measurement of water oxidation rate by oxygen evolution in aqueous solution was conducted with dispersed nanoparticulate manganese oxides and a standard ruthenium dye photo-oxidant system. No Ru was absorbed on the catalyst surface as observed by XPS and EDX. The post reaction atomic structure was completely preserved with no amorphization, as observed by HRTEM. Catalytic activities, normalized to surface area (BET), decrease in the series Mn2O3 > Mn3O4 λ-MnO2, where the latter is derived from spinel LiMn2O4 following partial Li+ removal. No catalytic activity is observed from LiMn2O4 and four of the MnO2 polymorphs, in contrast to some literature reports with polydispersed manganese oxides and electro-deposited films. Catalytic activity within the eight examined Mn oxides was found exclusively for (distorted) cubic phases, Mn2O3 (bixbyite), Mn3O4 (hausmannite), and λ-MnO2 (spinel), all containing Mn(III) possessing longer Mn–O bonds between edge-sharing MnO6 octahedra. Electronically degenerate Mn(III) has antibonding electronic configuration eg1 which imparts lattice distortions due to the Jahn–Teller effect that are hypothesized to contribute to structural flexibility important for catalytic turnover in water oxidation at the surface.
2. Structure/Function Relations in Oxygen Evolution Catalysis by Cobalt Oxides
See: Gardner, GP, Go YB, Robinson DM, Smith PF, Hadermann J, Abakumov A, Greenblatt M, Dismukes GC, Angew.Chem.Int.Ed. 2012, 51(7), 1616-1619.
Two polymorphs of nanocrystalline cubic spinel and rhombohedral layered lithium cobalt oxides have been prepared and their application in the photocatalytic oxidation of water examined. The main factor that determines the catalytic activity of the different phases is the presence of a Co4O4 cubic core, which is present in the cubic form of the catalyst, but not in the layered structure
3.Structure/Function Relations in Oxygen Evolution Catalysis by Mixed Oxides
Transition metal oxides containing cubic B4O4 subcores are noted for their catalytic activity in water oxidation (OER). We synthesized a series of ternary spinel oxides, AB2O4, derived from LiMn2O4 by either replacement at the tetrahedral A site or Co substitution at the octahedral B site and measured their electrocatalytic OER activity. Atomic emission and powder X-ray diffraction confirmed spinel structure type and purity. Weak activation of the OER occurs upon A-site substitution: Zn2+ > Mg2+ > A-vacancy > Li+ = 0. Zn and Mg substitution is accompanied by (1) B-site conversion of Mn(IV) to Mn(III), resulting in expansion and higher symmetry of the [Mn4O4]4+ core relative to LiMn2O4 (inducing greater flexibility of the core and lower reorganization barrier to distortions), and (2) the electrochemical oxidation potential for Mn(III)/IV) increases by 0.15–0.2 V, producing a stronger driving force for water oxidation. Progressive replacement of Mn(III/IV) by Co(III) at the B site (LiMn2–xCoxO4, 0 ≤ x ≤ 1.5) both symmetrizes the [Mn4–xCoxO4] core and increases the oxidation potential for Co(III/IV), resulting in the highest OER activity within the spinel structure type. These observations point to two predictors of OER catalysis: (1) Among AMn2O4 spinels, those starting with Mn(III) in the resting lattice (prior to oxidation) result in longer, weaker Mn–O bonds for this eg1 antibonding electronic configuration, yielding greater core flexibility and a higher oxidation potential to Mn(IV), and (2) a linear free energy relationship exists between the electrocatalytic rate and the binding affinity of the substrate oxygen (*OH and *OOH) to the B site.
4. Catalysis for Industrial Electrolysis Conditions
The two polymorphs of lithium cobalt oxide, LiCoO2, present an opportunity to contrast the structural requirements for reversible charge storage (battery function) vs. catalysis of water oxidation/oxygen evolution (OER; 2H2O → O2 + 4H+ + 4e−). Previously, we reported high OER electrocatalytic activity from nanocrystals of the cubic phase vs. poor activity from the layered phase – the archetypal lithium-ion battery cathode. Here we apply transmission electron microscopy, electron diffraction, voltammetry and elemental analysis under OER electrolysis conditions to show that labile Li+ ions partially deintercalate from layered LiCoO2, initiating structural reorganization to the cubic spinel LiCo2O4, in parallel with formation of a more active catalytic phase. Comparison of cubic LiCoO2 (50 nm) to iridium (5 nm) nanoparticles for OER catalysis (commercial benchmark for membrane-based systems) in basic and neutral electrolyte reveals excellent performance in terms of Tafel slope (48 mV dec−1), overpotential (η = ∼420 mV@10 mA cm−2 at pH = 14), faradaic yield (100%) and OER stability (no loss in 14 hours). The inherent OER activity of cubic LiCoO2 and spinel LiCo2O4 is attributed to the presence of [Co4O4]n+ cubane structural units, which provide lower oxidation potential to Co4+ and lower inter-cubane hole mobility. By contrast, the layered phase, which lacks cubane units, exhibits extensive intra-planar hole delocalization which entropically hinders the four electron/hole concerted OER reaction. An essential distinguishing trait of a truly relevant catalyst is efficient continuous operation in a real electrolyzer stack. Initial trials of cubic LiCoO2 in a solid electrolyte alkaline membrane electrolyzer indicate continuous operation for 1000 hours (without failure) at current densities up to 400 mA cm−2 and overpotential lower than proven PGM (platinum group metal) catalysts.
5. Oxidation state and coordination geometry of manganese oxide octahedra for water oxidation catalysis
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 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+O6sites, 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.