Rosalie K. Hocking

Water-oxidation catalysis by manganese in a geochemical-like cycle

Water oxidation in all oxygenic photosynthetic organisms is catalysed by the Mn₄CaO₄ cluster of Photosystem II. This cluster has inspired the development of synthetic manganese catalysts for solar energy production. A photoelectrochemical device, made by impregnating a synthetic tetranuclear-manganese cluster into a Nafion matrix, has been shown to achieve efficient water oxidation catalysis. We report here in situ X-ray absorption spectroscopy and transmission electron microscopy studies that demonstrate that this cluster dissociates into Mn(II) compounds in the Nafion, which are then reoxidized to form dispersed nanoparticles of a disordered Mn(III/IV)-oxide phase. Cycling between the photoreduced product and this mineral-like solid is responsible for the observed photochemical water-oxidation catalysis. The original manganese cluster serves only as a precursor to the catalytically active material. The behaviour of Mn in Nafion therefore parallels its broader biogeochemistry, which is also dominated by cycles of oxidation into solid Mn(III/IV) oxides followed by photoreduction to Mn²⁺.

Highly active nickel oxide water oxidation catalysts deposited from molecular complexes

Nickel oxide (NiOx) water oxidation catalysts with high catalytic activity have been electrodeposited from [Ni(en)3]Cl2 (en = 1,2-diaminoethane, NiOx-en) in a 0.10 M borate buffer (NaBi) solution (pH = 9.2). Electrolysis experiments at a fixed applied potential of 1.1 V (vs. Ag/AgCl) established that the NiOx-en films sustain a stable current of 1.8 mA cm−2 for extended periods, compared with 1.2 mA cm−2 for films derived from [Ni(OH2)6](NO3)2 and [Ni(NH3)6]Cl2 when tested in a 0.60 M NaBi buffer. XAS studies indicate that the γ-NiOOH phase is formed in each case whereas SEM studies revealed significant differences in film morphology. The NiOx-en films were found to be more homogenous and to have a higher electroactive surface area, as determined from capacitance measurements. The results highlight the influence that the choice of molecular precursor can have on the activity and robustness of electrodeposited NiOx water oxidation catalysts.

Sorption of arsenic(V) and arsenic(III) to schwertmannite

This study describes the sorption of As(V) and As(III) to schwertmannite as a function of pH and arsenic loading. In general, sorption of As(V) was greatest at low pH, whereas high pH favored the sorption of As(III). The actual pH of equivalent As(V) and As(III) sorption was strongly loading dependent, decreasing from pH approximately 8.0 at loadings <120 mmol(As) mol(Fe)(-1) to pH approximately 4.6 at a loading of 380 mmol(As) mol(Fe)(-1). Sorption isotherms for As(V) were characterized by strong partitioning to the schwertmannite solid-phase at low loadings and sorption capacities of 225-330 mmol(As(V)) mol(Fe)(-1) at high loadings. In contrast, the As(III) isotherms revealed a weak affinity for sorption of As(III) versus As(V) at low loadings yet a greater affinity for As(III) sorption compared with As(V) at high loadings (when pH > 4.6). Sorption of As(V) and As(III) caused significant release of SO(4)(2-) from within the schwertmannite solid-phase, without major degradation of the schwertmannite structure (as evident by X-ray diffraction and Raman spectroscopy). This can be interpreted as arsenic sorption via incorporation into the schwertmannite structure, rather than merely surface complexation at the mineral-water interface. The results of this study have important implications for arsenic mobility in the presence of schwertmannite, such as in areas affected by acid-mine drainage and acid-sulfate soils. In particular, arsenic speciation, arsenic loading, and pH should be considered when predicting and managing arsenic mobility in schwertmannite-rich systems.

Highly active screen-printed electrocatalysts for water oxidation based on β-manganese oxide

A versatile screen-printing method is applied for the preparation of efficient water oxidation catalysts based on a nanostructured β-MnO2 material prepared by a redox-precipitation method, and commercial β-MnO2. The catalyst films were tested for activity in water oxidation over a range of neutral to alkaline pH. The onset of water oxidation in case of the nanostructured MnO2 films is found at an overpotential (η) of 300 mV at pH 13.6 (1.0 M NaOH), with current densities reaching 10 mA cm−2 at η = 500 mV. The screen-printed MnO2 (nano) is one of the most active manganese oxide-based catalysts reported to date, despite consisting mostly of the common pyrolusite (β-MnO2) phase, so far generally found inactive in water oxidation. The films prepared from commercial β-MnO2 were found to be moderately active, with an onset of water oxidation at η = 500 mV (pH 13.6), and currents up to 5 mA cm−2 at η = 800 mV. At pH 6, the two samples exhibit similar activity and also match or surpass the performance of recent benchmark manganese oxides. X-ray absorption spectroscopy (XAS) studies suggest that the crystal phase is unchanged after prolonged electrochemical cycling. Scanning electron microscopy (SEM) analysis indicates very little corrosion of the surface morphology after prolonged catalyst operation at alkaline pH. However, high-resolution transmission electron microscopy (HRTEM) analysis shows the formation of a small amount of an amorphous phase on the surface of the nanorods after oxygen evolution over 12 hours in alkaline media.

Improvement of Catalytic Water Oxidation on MnOx Films by Heat Treatment

Manganese oxides (MnOx ) are considered to be promising catalysts for water oxidation. Electrodeposited MnOx films from aqueous electrolytes have previously been shown to exhibit a lower catalytic action than films deposited from ionic liquids when tested in strongly alkaline conditions. In this study, we describe a thermal treatment that converts the MnOx films deposited from aqueous electrolytes to highly catalytic films with comparable activity to ionic-liquid-deposited films. The films deposited from aqueous electrolytes show a remarkable improvement in the catalysis of water oxidation after heat treatment at a low temperature (≤120 °C) for 30 min. The films were characterised by using XRD and SEM, and energy-dispersive X-ray (EDX), FTIR and Raman spectroscopy, which indicate that dehydration occurs during the heat treatment without significant change to the microstructure or bulk composition. The X-ray absorption spectroscopy (XAS) results show the growth of small amounts (ca. 3-10 %) of reduced Mn species (Mn(II) or Mn(III) ) after heat treatment. The dehydration process removes structural water and hydroxyl species to result in a conductivity improvement and a more active catalyst, thereby contributing to the enhancement in water oxidation performance.

Rates of water exchange for two cobalt(II) heteropolyoxotungstate compounds in aqueous solution.

Polyoxometalate ions are used as ligands in water-oxidation processes related to solar energy production. An important step in these reactions is the association and dissociation of water from the catalytic sites, the rates of which are unknown. Here we report the exchange rates of water ligated to Co(II) atoms in two polyoxotungstate sandwich molecules using the (17)O-NMR-based Swift-Connick method. The compounds were the [Co(4)(H(2)O)(2)(B-α-PW(9)O(34))(2)](10-) and the larger αββα-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-) ions, each with two water molecules bound trans to one another in a Co(II) sandwich between the tungstate ligands. The clusters, in both solid and solution state, were characterized by a range of methods, including NMR, EPR, FT-IR, UV-Vis, and EXAFS spectroscopy, ESI-MS, single-crystal X-ray crystallography, and potentiometry. For [Co(4)(H(2)O)(2)(B-α-PW(9)O(34))(2)](10-) at pH 5.4, we estimate: k(298)=1.5(5)±0.3×10(6) s(-1), ΔH(≠)=39.8±0.4 kJ mol(-1), ΔS(≠)=+7.1±1.2 J mol(-1) K(-1) and ΔV(≠)=5.6 ±1.6 cm(3) mol(-1). For the Wells-Dawson sandwich cluster (αββα-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-)) at pH 5.54, we find: k(298)=1.6(2)±0.3×10(6) s(-1), ΔH(≠)=27.6±0.4 kJ mol(-1) ΔS(≠)=-33±1.3 J mol(-1) K(-1) and ΔV(≠)=2.2±1.4 cm(3) mol(-1) at pH 5.2. The molecules are clearly stable and monospecific in slightly acidic solutions, but dissociate in strongly acidic solutions. This dissociation is detectable by EPR spectroscopy as S=3/2 Co(II) species (such as the [Co(H(2)O)(6)](2+) monomer ion) and by the significant reduction of the Co-Co vector in the XAS spectra.

Fe L- and K-edge XAS of Low-Spin Ferric Corrole: Bonding and Reactivity Relative to Low-Spin Ferric Porphyrin

Corrole is a tetrapyrrolic macrocycle that has one carbon atom less than a porphyrin. The ring contraction reduces the symmetry from D(4h) to C(2v), changes the electronic structure of the heterocycle, and leads to a smaller central cavity with three protons rather than the two of a porphyrin. The differences between ferric corroles and porphyrins lead to a number of differences in reactivity including increased axial ligand lability and a tendency to form 5-coordinate complexes. The electronic structure origin of these differences has been difficult to study experimentally as the dominant porphyrin/corrole pi --> pi* transitions obscure the electronic transitions of the metal. Recently, we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e., the differences in mixing of the metal d orbitals with the ligand valence orbitals) using a valence bond configuration interaction model. Herein, we apply this methodology, combined with a ligand field analysis of the Fe K pre-edge to a low-spin ferric corrole, and compare it to a low-spin ferric porphyrin. The experimental results combined with DFT calculations show that the contracted corrole is both a stronger sigma donor and a very anisotropic pi donor. These differences decrease the bonding interactions with axial ligands and contribute to the increased axial ligand lability and reactivity of ferric corroles relative to ferric porphyrins.

Fe L-edge X-ray absorption spectroscopy determination of differential orbital covalency of siderophore model compounds: electronic structure contributions to high stability constants.

Most bacteria and fungi produce low-molecular-weight iron chelators called siderophores. Although different siderophore structures have been characterized, the iron-binding moieties often contain catecholate or hydroxamate groups. Siderophores function because of their extraordinarily high stability constants (K(STAB) = 10(30)-10(49)) and selectivity for Fe(III), yet the origin of these high stability constants has been difficult to quantify experimentally. Herein, we utilize Fe L-edge X-ray absorption spectroscopy to determine the differential orbital covalency (i.e., the differences in the mixing of the metal d-orbitals with ligand valence orbitals) of a series of siderophore model compounds. The results enable evaluation of the electronic structure contributions to their high stability constants in terms of sigma- and pi-donor covalent bonding, ionic bonding, and solvent effects. The results indicate substantial differences in the covalent contributions to stability constants of hydroxamate and catecholate complexes and show that increased sigma as well as pi bonding contributes to the high stability constants of catecholate complexes.

Arsenic Mobilization in a Seawater Inundated Acid Sulfate Soil

Tidal seawater inundation of coastal acid sulfate soils can generate Fe- and S0(4)-reducing conditions in previously oxic-acidic sediments. This creates potential for mobilization of As during the redox transition. We explore the consequences for As by investigating the hydrology, porewater geochemistry, solid-phase speciation, and mineralogical partitioning of As across two tidal fringe toposequences. Seawater inundation induced a tidally controlled redox gradient Maximum porewater As (~400 μg/L) occurred in the shallow (<1 m), intertidal, redox transition zone between Fe-oxidizing and S0(4)-reducing conditions. Primary mechanisms of As mobilization include the reduction of solid-phase As(V) to As(lll), reductive dissolution of As(V)-bearing secondary Fe(lll) minerals and competitive anion desorption. Porewater As concentrations decreased in the zone of contemporary pyrite reformation. Oscillating hydraulic gradients caused by tidal pumping promote upward advection of As and Fe(2+)-enriched porewater in the intertidal zone, leading to accumulation of As(V)-enriched Fe(lll) (hydr)oxides at the oxic sediment-water interface. While this provides a natural reactive-Fe barrier, it does not completely retard the flux of porewater As to overtopping surface waters. Furthermore, the accumulated Fe minerals may be prone to future reductive dissolution. A conceptual model describing As hydro-geochemical coupling across an intertidal fringe is presented.

Anodic deposition of NiOx water oxidation catalysts from macrocyclic nickel(II) complexes

Molecular complexes have been found to be excellent precursors for the deposition of catalytically active metal oxide films. Here, three macrocyclic Ni(II) amine complexes have been used for the electrochemical deposition of NiOx films from either a borate buffer solution (pH 9.2) or more basic conditions (pH = 12.9). The cyclic voltammetry of the complexes in both electrolytes shows very similar features and indicates the deposition of a catalytically active nickel oxide film. The NiOx films have been characterised using infrared (IR) and Raman spectroscopy complemented by scanning electron microscopy. Testing of the water oxidation activity at pH 9.2 and 12.9 showed that the films deposited from macrocyclic Ni(II) complexes exhibit similar catalytic activity to those derived from Ni2+ salts. The macrocyclic complexes offer the advantage of greater solubility and solution stability over a wider range of deposition conditions. At pH 9.2, the catalytic activity of the NiOx films was significantly higher when using a borate buffer and, in addition, the films were more active at pH 12.9 than at pH 9.2. The NiOx films deposited from molecular complexes were also found to show electrochromic properties. The oxidation of water by these films was enhanced by visible light. Water oxidation currents were observed to increase by ∼20% under simulated solar radiation.