Robin Brimblecombe

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⁺.

Solar driven water oxidation by a bioinspired manganese molecular catalyst

A photoelectrochemical cell was designed that catalyzes the photooxidation of water using visible light as the sole energy source and a molecular catalyst, [Mn(4)O(4)L(6)](+) (1(+), L = bis(methoxyphenyl)phosphinate), synthesized from earth-abundant elements. The essential features include a photochemical charge separation system, [Ru(II)(bipy)(2)(bipy(COO)(2))], adhered to titania-coated FTO conductive glass, and 1(+) embedded within a proton-conducting membrane (Nafion). The complete photoanode represents a functional analogue of the water-oxidizing center of natural photosynthesis.

Sustained Water Oxidation Photocatalysis by a Bioinspired Manganese Cluster

The creation of efficient catalysts for splitting water into H2 and O2 is one of the greatest challenges for chemists working on the production of renewable fuel. The water oxidizing center (WOC) within photosynthetic organisms is the only natural system able to efficiently photooxidize water using visible light, and is thus a blueprint for catalyst design. One of the atomic structural models of the WOC derived from X-ray diffraction involves a “cubelike” core comprised of a {CaMn3O4} unit tethered to a fourth manganese atom through one or two bridging oxo units. A few nonbiological tetramanganese complex mimics of this site have been prepared that contain an incomplete or distorted cubic {Mn4Ox} core [4–7] or are part of a larger Mnx–oxo lattice. [4] However, none of these have shown activity towards water oxidation. We have previously synthesized a prototypical molecular manganese–oxo cube [Mn4O4] n+ in a family of “cubane” complexes [Mn4O4L6], where L is a diarylphosphinate ligand (p-R-C6H4)2PO2 (R=H, alkyl, OMe). The diphenylphosphinate complex (1, R=H, Figure 1) assembles spontaneously from manganese(II) and permanganate salts in high yield in non-aqueous solvents. The release of O2 by the {Mn4O4} 6+ core in 1 was shown to be possible on thermodynamic grounds, but cannot take place because of the rigidity of the core arising from the six diarylphosphinate ligands, which bridge pairs of manganese atoms on the six cube faces. The assembly of 1 is also driven by intramolecular van der Waals forces that attract three aryl rings from adjacent phosphinate ligands. The cubic core in 1 is a much stronger oxidant than any known dimanganese complex with {Mn2O2} 3+ cores. Cubane 1 abstracts hydrogen atoms from various organic substrates by breaking O H and N H bonds with dissociation energies greater than 390 kJmol . Titrations of 1 against compounds containing either amine or phenol groups reach an end point after the abstraction of four successive hydrogen atoms, yielding two water molecules (from corner oxo groups) plus [L6Mn4O2], the so-called “pinned butterfly” complex 2 (Scheme 1). {Mn4O4} cubane complexes are unique in releasing an O2 molecule upon photoexcitation of the Mn !O charge transfer band, which reaches a maximum at 350 nm. This process, which occurs with high quantum efficiency only in the gas phase, involves the core oxygen atoms and is triggered by ejection of one phosphinate ligand, thereby generating the [L5Mn4O2] + “butterfly” complex 3 (Scheme 1). In contrast, noncuboidal manganese molecular complexes possessing {Mn2O}, {Mn2O2}, and {Mn3O6} cores in the Mn or Mn oxidation states fail to release O2, but instead photodecompose into multiple fragments. Thus, O2 release is favored by complexes with a {Mn4O4} cubane core. The composition of the butterfly complexes 2 and 3 differs only by one phosphinate ligand (Scheme 1). This finding suggests the possibility of creating a catalytic cycle that could oxidize two water molecules bound to 2 along the reverse pathway in Scheme 1 (1-3H!1-2H!1-H!1), eventually forming 3 by photochemical release of O2 and a phosphinate ligand. Thus far it has proved impossible to realize a catalytic cycle, as in Scheme 1, because O2 is not photodissociated from 1 or 1 (the one-electron oxidized cubane) in condensed phases. This was attributed to a large activation barrier for O2 release when all the phosphinate ligands remain ligated or re-ligate by fast geminate recombination. Figure 1. X-ray crystal structure of 1.

Sustained water oxidation by [Mn4O4]7+ core complexes inspired by oxygenic photosynthesis

The bioinspired Mn-oxo cubane complex, [Mn(4)O(4)L(6)](+) 1b(+) (L = (p-MeO-Ph)(2)PO(2)), is a model of the photosynthetic O(2)-evolving complex. It is able to electro-oxidize water at 1.00 V (vs Ag/AgCl) under illumination by UV-visible light when suspended in a proton-conducting membrane (Nafion) coated onto a conducting electrode. Electrochemical measurements, and UV-visible, NMR, and EPR spectroscopies are interpreted to indicate that 1b(+) is the dominant electro-active species in the Nafion, both before and after catalytic cycling, and thus correlates closely with activity. The observation of a possible intermediate and free phosphinate ligand within the Nafion suggests a catalytic mechanism involving photolytic disruption of a phosphinate ligand, followed by O(2) formation, and subsequent reassembly of the cubane structure. Several factors that influence catalytic turnover such as the applied potential, illumination wavelength, and energy have been examined in respect of attaining optimum catalytic activity. Catalytic turnover frequencies of 20-270 molecules O(2) h(-1) catalyst(-1) at an overpotential of 0.38 V plus light (275-750 nm) and turnovers numbers >1000 molecules O(2) catalyst(-1) are observed. The 1b(+)-Nafion system is among the most active and durable molecular water oxidation catalysts known.

Electrochemical investigation of Mn4O4-cubane water-oxidizing clusters

High valence states in manganese clusters are a key feature of the function of one of the most important catalysts found in nature, the water-oxidizing complex of photosystem II. We describe a detailed electrochemical investigation of two bio-inspired manganese-oxo complexes, [Mn(4)O(4)L(6)] (L = diphenylphosphinate (1) and bis(p-methoxyphenyl)phosphinate (2)), in solution, attached to an electrode surface and suspended within a Nafion film. These complexes contain a cubic [Mn(4)O(4)](6+) core stabilized by phosphinate ligands. They have previously been shown to be active and durable photocatalysts for the oxidation of water to dioxygen. A comparison of catalytic photocurrent generated by films deposited by two methods of electrode immobilization reveals that doping of the catalyst in Nafion results in higher photocurrent than was observed for a solid layer of cubane on an electrode surface. In dichloromethane solution, and under conditions of cyclic voltammetry, the one-electron oxidation processes 1/1(+) and 2/2(+) were found to be reversible and quasi-reversible, respectively. Some decomposition of 1(+) and 2(+) was detected on the longer timescale of bulk electrolysis. Both compounds also undergo a two-electron, chemically irreversible reduction in dichloromethane, with a mechanism that is dependent on scan rate and influenced by the presence of a proton donor. When immersed in aqueous electrolyte, the reduction process exhibits a limited level of chemical reversibility. These data provide insights into the catalytic operation of these molecules during photo-assisted electrolysis of water and highlight the importance of the strongly electron-donating ligand environment about the manganese ions in the ability of the cubanes to photocatalyze water oxidation at low overpotentials.

A Tandem Water‐Splitting Device Based on a Bio‐inspired Manganese Catalyst

As water is the most abundant molecule on the planet, and given the huge amounts of solar energy that strike the earth every day, the production of hydrogen by using sunlight to split water has the potential to provide large amounts of clean, renewable fuel. This can be achieved by coupling a water electrolyzer to photovoltaic cells, which has been previously demonstrated to yield solar-to-hydrogen conversion efficiencies of up to 7%. However, due to the large overpotentials required to oxidize water, typical electrolyzers operate at voltages of around 2 V. Thus, when using conventional silicon photovoltaic devices, four cells need to be assembled in series, making the process prohibitively expensive. A wide range of catalysts that lower the required overpotential have been developed in response to this challenge. An ideal water oxidation catalyst would remove the overpotential, so that only the thermodynamic energy would be required to drive the water-splitting reaction; equivalent to a voltage of 1.23 V (pH 0). A diverse range of metal oxides, including multimetal oxides containing various combinations of Ti, Nb, Ta, W, Ga, In, Ge, Sn, and Sb; narrow-band-gap semiconductors, such as CdS and CdSe; and other materials have been developed in an effort to achieve this goal. 5–13] This is a very active area of research because achieving the right balance between energy absorption, catalytic activity, and materials stability has proven difficult for a single material. Some of these limitations have been overcome by coupling appropriate combinations of materials. For example, the unsuitable band edge positions of WO3 and Fe2O3 can be surmounted by coupling these materials to a photovoltaic device in tandem. In this case, the extra potential required to oxidize water and reduce protons is provided by the photovoltaic device. Solar-to-hydrogen conversion efficiencies of 6% have been reported for these tandem systems. In other examples, the challenge has been addressed by integrating multiple materials into a single electrode, creating multi-junction devices in which the photoanode contains a layer of a water oxidation photocatalyst, such as GaInP2, and a layer of photovoltaic material, such as a GaAs p/n junction, which provides the extra potential required to complete the circuit. Other approaches include depositing doped thin-film oxides (NiFeO2 and Fe2O3) on multi-junction photocells. 7] These previous examples have focused on the use of solid films as the catalytic material. In addition to these systems, a wide range of molecular water oxidation catalysts have been developed. The majority of these catalysts are based on inorganic Ru, Ir, or Mn complexes. Of these catalysts only a few have been successfully attached to electrode surfaces, which is a prerequisite for their incorporation into photoelectrochemical devices. To the best of our knowledge there are no reports of the successful integration of these types of water oxidation catalysts with a solar cell into a tandem water-splitting device. We recently reported that a tetranuclear Mn-oxo cluster, [Mn4O4L6] + (1 ; L= (p-Me-C6H4)2PO2; Scheme 1A), [18,19] is able to catalyze the oxidation of water for extended periods when doped within the proton-conducting channels of a Nafion membrane, polarized at 1 V (vs Ag/AgCl) and illuminated with visible light. The development of this catalyst was inspired by the presence of a tetranuclear Mn cluster in the water oxida-

Homogeneous catalysts with a mechanical ("machine-like") action.

Chemical reactions may be controlled by either: 1) the minimum threshold energy that must be overcome during collisions between reactant molecules/atoms (the activation energy, E(a)), or: 2) the rate at which reactant collisions occur (the collision frequency, A)--for reactions with low E(a). Reactions of type 2 are governed by the physical, mechanical interaction of the reactants. Such mechanical processes are unusual, but not unknown in molecular catalysts. In this work we examine the machine-like nature of the action in various abiological mechanical catalysts and consider the implications for mimicry of biological catalysts.

Energy from photosystem II: Manganese water oxidation catalysts

Photosystem II is a blueprint for the design of water oxidation catalysts for incorporation into photoelectrochemical devices capable of efficient solar hydrogen production. In this chapter, we review ongoing efforts to develop manganese water oxidation catalysts. These catalytic systems embody one or more of the key features observed in the PSII water oxidizing complex – the concentration of high energy oxidation states of multiple manganese centres, the ability to facilitate di-oxygen bridge formation, a dynamic supporting environment that prevents dissociation of the complex, assists in electron and proton removal, and aids coupling to a photoactive charge separation centre – with the most successful examples incorporating most or all of these key features. Promising advances have been made towards achieving solar water oxidation, ranging from the direct coupling of Mn complexes to Ru dyes or TiO2 to demonstrate successful oxidation of Mn centers, to achieving direct light driven water oxidation by coupling a Nafion supported Mn catalysts to a Ru-dye sensitized TiO2 electrode, which should stimulate further interesting developments.

A bio--inspired molecular water oxidation catalyst for renewable hydrogen generation: An examination of salt effects

Most transport fuels are derived from fossil fuels, generate greenhouse gases, and consume significant amounts of water in the extraction, purification, and/or burning processes. The generation of hydrogen using solar energy to split water, ideally from abundant water sources such as sea water or other non-potable sources, could potentially provide an unlimited, clean fuel for the future. Solar, electrochemical water splitting typically combines a photoanode at which water oxidation occurs, with a cathode for proton reduction to hydrogen. In recent work, we have found that a bioinspired tetra-manganese cluster catalyzes water oxidation at relatively low overpotentials (0.38 V) when doped into a Nafion proton conduction membrane deposited on a suitable electrode surface, and illuminated with visible light. We report here that this assembly is active in aqueous and organic electrolyte solutions containing a range of different salts in varying concentrations. Similar photocurrents were obtained using electrolytes containing 0.0 - 0.5 M sodium sulfate, sodium perchlorate or sodium chloride. A slight decline in photocurrent was observed for sodium perchlorate but only at and above 5.0 M concentration. In acetonitrile and acetone solutions containing 10% water, increasing the electrolyte concentration was found to result in leaching of the catalytic species from the membrane and a decrease in photocurrent. Leaching was not observed when the system was tested in an ionic liquid containing water, however, a lower photocurrent was generated than observed in aqueous electrolyte. We conclude that immersion of the membrane in an aqueous solution containing an electrolyte concentration of 0.05 - 0.5M represent good conditions for operation for the cubium/Nafion catalytic system.