Photo-active cobalt cubane model of an oxygen-evolving catalyst

Do multinuclear 3d metal catalysts achieve O-O bond formation via radical coupling or via water nucleophilic attack? WNA leads the way in [Co4O4]n

Catalytic water oxidation is a required process for clean energy production based on the concept of artificial photosynthesis. Here, we provide in situ spectroscopic and computational analysis for the closest known photosystem II analog, [Co₄O₄]ⁿ⁺ ([Co₄O₄Py₄Ac₄]⁰, Py = pyridine and Ac = CH₃COO^-), which catalyzes electrochemical water oxidation. In situ extended X-ray absorption fine structure detects an ultrashort, Co^IV=O (~1.67 Å) moiety, a crucial intermediate for O-O bond formation. Density function theory analyses show that the intermediate has two Co^IV centers and a Co^IV=O unit of strong radicaloid character sufficient to support a Co^IV=O + H₂O = Co-OOH + H⁺ transition, where the carboxyl ligand accepts the proton and the bridging oxygen stabilizes the peroxide via hydrogen bonding. The proposed water nucleophilic attack mechanism accounts for all prior spectroscopic evidence on the Co₄O₄⁴⁺ core. Our results are important for the design and development of efficient water oxidation catalysts, which contribute to the ultimate goal of clean energy from artificial photosynthesis.

Artificial photosynthesis: opportunities and challenges of molecular catalysts

Molecular catalysis plays an essential role in both natural and artificial photosynthesis (AP). However, the field of molecular catalysis for AP has gradually declined in recent years because of doubt about the long-term stability of molecular-catalyst-based devices. This review summarizes the development history of molecular-catalyst-based AP, including the fundamentals of AP, molecular catalysts for water oxidation, proton reduction and CO2 reduction, and molecular-catalyst-based AP devices, and it provides an analysis of the advantages, challenges, and stability of molecular catalysts. With this review, we aim to highlight the following points: (i) an investigation on molecular catalysis is one of the most promising ways to obtain atom-efficient catalysts with outstanding intrinsic activities; (ii) effective heterogenization of molecular catalysts is currently the primary challenge for the application of molecular catalysis in AP devices; (iii) development of molecular catalysts is a promising way to solve the problems of catalysis involved in practical solar fuel production. In molecular-catalysis-based AP, much has been attained, but more challenges remain with regard to long-term stability and heterogenization techniques.

Direct Synthesis of Two Inorganic Catalysts on Carbon Fibres for the Electrocatalytic Oxidation of Water

Two electrodes for anodic water oxidation made by direct synthesis of inorganic catalysts onto conductive carbon fibre sheets are evaluated. As catalysts two Co- and Sb-containing phases were tested, that is, Co₃ Sb₄ O₆ F₆ and the new compound CoSbO₄ . The compounds express large differences in their morphology: CoSbO₄ grows as thin needles whereas Co₃ Sb₄ O₆ F₆ grows as larger facetted crystals. Despite the smaller surface area the latter compound shows a better catalytic performance. When the compound Co₃ Sb₄ O₆ F₆ was used it gave a low increase of +0.028 mV h^-1 at an overpotential of η=472 mV after 10 h and a stability of +0.48 mV h^-1 at an overpotential of η=488 mV after 60 h. The leakages of Co and Sb were negligible and only <0.001 at % Co and approximately 0.02 at % Sb were detected in the electrolyte.

Water Oxidation by Size-Selected Co27 Clusters Supported on Fe2 O3

The complexity of the water oxidation reaction makes understanding the role of individual catalytic sites critical to improving the process. Here, size-selected 27-atom cobalt clusters (Co₂₇ ) deposited on hematite (Fe₂ O₃ ) anodes were tested for water oxidation activity. The uniformity of these anodes allows measurement of the activity of catalytic sites of well-defined nuclearity and known density. Grazing incidence X-ray absorption near-edge spectroscopy (GIXANES) characterization of the anodes before and after electrochemical cycling demonstrates that these Co₂₇ clusters are stable to dissolution even in the harsh water oxidation electrochemical environment. They are also stable under illumination at the equivalent of 0.4 suns irradiation. The clusters show turnover rates for water oxidation that are comparable or higher than those reported for Pd- and Co-based materials or for hematite. The support for the Co₂₇ clusters is Fe₂ O₃ grown by atomic layer deposition on a Si chip. We have chosen to deposit a Fe₂ O₃ layer that is only a few unit cells thick (2 nm), to remove complications related to exciton diffusion. We find that the electrocatalytic and the photoelectrocatalytic activity of the Co₂₇ /Fe₂ O₃ material is significantly improved when the samples are annealed (with the clusters already deposited). Given that the support is thin and that the cluster deposition density is equivalent to approximately 5 % of an atomic monolayer, we suggest that annealing may significantly improve the exciton diffusion from the support to the catalytic moiety.

X-ray Spectroscopic Characterization of Co(IV) and Metal-Metal Interactions in Co4O4: Electronic Structure Contributions to the Formation of High-Valent States Relevant to the Oxygen Evolution Reaction

The formation of high-valent states is a key factor in making highly active transition-metal-based catalysts of the oxygen evolution reaction (OER). These high oxidation states will be strongly influenced by the local geometric and electronic structures of the metal ion, which are difficult to study due to spectroscopically active and complex backgrounds, short lifetimes, and limited concentrations. Here, we use a wide range of complementary X-ray spectroscopies coupled to DFT calculations to study Co(III)4O4 cubanes and their first oxidized derivatives, which provide insight into the high-valent Co(IV) centers responsible for the activity of molecular and heterogeneous OER catalysts. The combination of X-ray absorption and 1s3p resonant inelastic X-ray scattering (Kβ RIXS) allows Co(IV) to be isolated and studied against a spectroscopically active Co(III) background. Co K- and L-edge X-ray absorption data allow for a detailed characterization of the 3d-manifold of effectively localized Co(IV) centers and provide a direct handle on the t2g-based redox-active molecular orbital. Kβ RIXS is also shown to provide a powerful probe of Co(IV), and specific spectral features are sensitive to the degree of oxo-mediated metal-metal coupling across Co4O4. Guided by the data, calculations show that electron-hole delocalization can actually oppose Co(IV) formation. Computational extension of Co4O4 to CoM3O4 structures (M = redox-inactive metal) defines electronic structure contributions to Co(IV) formation. Redox activity is shown to be linearly related to covalency, and M(III) oxo inductive effects on Co(IV) oxo bonding can tune the covalency of high-valent sites over a large range and thereby tune E(0) over hundreds of millivolts. Additionally, redox-inactive metal substitution can also switch the ground state and modify metal-metal and antibonding interactions across the cluster.

Generation of Hydrogen by Visible Light-Induced Water Splitting with the Use of Semiconductors and Dyes

Photosynthesis that occurs in plants involves both the oxidation of water and the reduction of carbon dioxide. Plants carry out these reactions with ease, by involving electron-transport chains. In this article, hydrogen generation by the reduction of water in the laboratory by using semiconductor nanostructures through artificial photosynthesis is examined. Dye-sensitized photochemical generation of hydrogen from water is also discussed. Hydrogen generation by these means has great technological relevance, since it is an environmentally friendly fuel. The way in which oxygen can be generated by the oxidation of water using metal oxide catalysts is also shown.

A transition metal oxofluoride offering advantages in electrocatalysis and potential use in applications

The recently described solid solution (Co,Ni,Mn)₃Sb₄O₆F₆has proved stable and efficient as a catalyst for electrocatalytic water oxidation. The end component Co₃Sb₄O₆F₆was found to be most efficient, maintaining a current density ofj= 10 mA cm⁻²at an overpotential of 443 mV with good capability. At this current density, O₂and H₂were produced in the ratio 1 : 2 without loss of faradaic current against a Pt-cathode. A morphological change in the crystallite surface was observed after 0.5 h, however, even after 64.5 h, the overall shape and size of the small crystallites were unaffected and the electrolyte contained only 0.02 at% Co. It was also possible to conclude fromin situEXAFS measurements that the coordination around Co did not change. The oxofluorides express both hydrophilic and hydrophobic surface sites, incorporate a flexible metalloid element and offer the possibility of a mechanism that differs from other inorganic catalytic pathways previously described.

Artificial photosynthesis: a molecular approach to photo-induced water oxidation

By the use of a molecular approach we performed photo-induced water oxidation by combining different photosensitizers and catalysts in order to obtain an efficient system that pave the way to the construction of an artificial photosynthetic system. Different types of molecular catalysts, such as ruthenium and vanadium polyoxometalates or cobalt core stabilized by different organic ligands were combined with ruthenium (II) polypyridine complexes of different nuclearity, mononuclear species like [Ru(bpy)3]2+ or a tetranuclear dendrimer.

Water oxidation catalysis upon evolution of molecular Co(iii) cubanes in aqueous media

The increasing global energy demand has stimulated great recent efforts in investigating new solutions for artificial photosynthesis, a potential source of clean and renewable solar fuel. In particular, according to the generally accepted modular approach aimed at optimising separately the different compartments of the entire process, many studies have focused on the development of catalytic systems for water oxidation to oxygen. While in recent years there have been many reports on new catalytic systems, the mechanism and the active intermediates operating the catalysis have been less investigated. Well-defined, molecular catalysts, constituted by transition metals stabilised by a suitable ligand pool, could help in solving this aspect. However, in some cases molecular species have been shown to evolve to active metal oxides that constitute the other side of this catalysis dichotomy. In this paper, we address the evolution of tetracobalt(iii) cubanes, stabilised by a pyridine/acetate ligand pool, to active species that perform water oxidation to oxygen. Primary evolution of the cubane in aqueous solution is likely initiated by removal of an acetate bridge, opening the coordination sphere of the cobalt centres. This cobalt derivative, where the pristine ligands still impact on the reactivity, shows enhanced electron transfer rates to Ru(bpy)₃³⁺(hole scavenging) within a photocatalytic cycle with Ru(bpy)₃²⁺as the photosensitiser and S₂O₈²⁻as the electron sink. A more accentuated evolution occurs under continuous irradiation, where Electron Paramagnetic Resonance (EPR) spectroscopy reveals the formation of Co(ii) intermediates, likely contributing to the catalytic process that evolves oxygen. All together, these results confirm the relevant effect of molecular species, in particular in fostering the rate of the electron transfer processes involved in light activated cycles, pivotal in the design of a photoactive device.

Ligand-Mediated Ring → Cube Transformation in a Catalytic Subnanocluster: Co4O4(MeCN)n with n = 1-6

We studied the Co4O4 subnanocluster and its MeCN-coated species using density functional theory, and we found that the Co4O4 core presents distinctive structures in bare and ligand-coated species. We propose a possible ligand-mediated ring → cube transformation mechanism during the ligand-coating process of the Co4O4 core due to the stronger binding energies of the MeCN ligands to the 3D distorted cube structure than to the 2D ring and ladder structures; theory indicates that three ligands are sufficient to stabilize the cube structure. Both ring and cube structures are ferromagnetic. Our finding is potentially useful for understanding the catalysis mechanism of Co4O4 species, which have important applications in solar energy conversion and water splitting; these catalysis reactions usually involve frequent addition and subtraction of various ligands and thus possibly involve core rearrangement processes similar to our findings.

Cobalt selenium oxohalides: catalysts for water oxidation

Two new oxohalides Co4Se3O9Cl2 and Co3Se4O10Cl2 have been synthesized by solid state reactions. They crystallize in the orthorhombic space group Pnma and the monoclinic space group C2/m respectively. The crystal structure of the two compounds are made up of similar building blocks; Co4Se3O9Cl2 is made up of [CoO4Cl2], [CoO5Cl] and [SeO3] polyhedra and Co3Se4O10Cl2 is made up of [CoO4Cl2] and [SeO3] polyhedra. As several Co-containing compounds have proved to be good catalysts for water oxidation, the activities of the two new compounds were compared with the previously found oxohalide Co5Se4O12Cl2 in reference to CoO and CoCl2. The one electron oxidant Ru(bpy)3(3+) was used as oxidizing species in a phosphate buffer and it was found that the activities of the oxohalide species were in between CoO and CoCl2. The roles of Cl(-) and PO4(3-) ions are discussed.

Dicobalt-μ-oxo polyoxometalate compound, [(α(2)-P2W17O61Co)2O](14-): a potent species for water oxidation, C-H bond activation, and oxygen transfer

High-valent oxo compounds of transition metals are often implicated as active species in oxygenation of hydrocarbons through carbon-hydrogen bond activation or oxygen transfer and also in water oxidation. Recently, several examples of cobalt-catalyzed water oxidation have been reported, and cobalt(IV) species have been suggested as active intermediates. A reactive species, formally a dicobalt(IV)-μ-oxo polyoxometalate compound [(α2-P2W17O61Co)2O](14-), [(POMCo)2O], has now been isolated and characterized by the oxidation of a monomeric [α2-P2W17O61Co(II)(H2O)](8-), [POMCo(II)H2O], with ozone in water. The crystal structure shows a nearly linear Co-O-Co moiety with a Co-O bond length of ∼1.77 Å. In aqueous solution [(POMCo)2O] was identified by (31)P NMR, Raman, and UV-vis spectroscopy. Reactivity studies showed that [(POMCo)2O]2O] is an active compound for the oxidation of H2O to O2, direct oxygen transfer to water-soluble sulfoxides and phosphines, indirect epoxidation of alkenes via a Mn porphyrin, and the selective oxidation of alcohols by carbon-hydrogen bond activation. The latter appears to occur via a hydrogen atom transfer mechanism. Density functional and CASSCF calculations strongly indicate that the electronic structure of [(POMCo)2O]2O] is best defined as a compound having two cobalt(III) atoms with two oxidized oxygen atoms.

A functionalized, ethynyl-decorated, tetracobalt(iii) cubane molecular catalyst for photoinduced water oxidation

A new tetracobalt(iii)-oxo cubane 1 was prepared. The ethynyl groups do not affect the photocatalytic properties of 1, which, in contrast, appear to be improved.

Importance of trivalency and the e(g)(1) configuration in the photocatalytic oxidation of water by Mn and Co oxides

Prompted by the early results on the catalytic activity of LiMn2O4 and related oxides in the photochemical oxidation of water, our detailed study of several manganese oxides has shown that trivalency of Mn is an important factor in determining the catalytic activity. Thus, Mn2O3, LaMnO3, and MgMn2O4 are found to be very good catalysts with turnover frequencies of 5 × 10(-4) s(-1), 4.8 × 10(-4) s(-1), and 0.8 × 10(-4) s(-1), respectively. Among the cobalt oxides, Li2Co2O4 and LaCoO3--especially the latter--exhibit excellent catalytic activity, with the turnover frequencies being 9 × 10(-4) s(-1) and 1.4 × 10(-3) s(-1), respectively. The common feature among the catalytic Mn and Co oxides is not only that Mn and Co are in the trivalent state, but Co(3+) in the Co oxides is in the intermediate t2g(5)e(g)(1) state whereas Mn(3+) is in the t2g(3e(g)(1) state. The presence of the e(g)(1) electron in these Mn and Co oxides is considered to play a crucial role in the photocatalytic properties of the oxides.