Distinguishing Homogeneous from Heterogeneous Water Oxidation Catalysis when Beginning with Polyoxometalates

Kagóme Cobalt(II)-Organic Layers as Robust Scaffolds for Highly Efficient Photocatalytic Oxygen Evolution

Two Kagóme cobalt(II)-organic layers of [Co3 (μ3 -OH)2 (bdc)2 ]n (1) and [Co3 (μ3 -OH)2 (chdc)2 ]n (2) (bdc=o-benzenedicarboxylate and chdc=1,2-cyclohexanedicarboxylate) that bear bridging OH(-) ligands were explored as water oxidation catalysts (WOCs) for photocatalytic O2 production. The activities of 1 and 2 towards H2 O oxidation were assessed by monitoring the in situ O2 concentration versus time in the reaction medium by utilizing a Clark-type oxygen electrode under photochemical conditions. The oxygen evolution rate (RO2 ) was 24.3 μmol s(-1)  g(-1) for 1 and 48.8 μmol s(-1)  g(-1) for 2 at pH 8.0. Photocatalytic reaction studies show that 1 and 2 exhibit enhanced activities toward the oxidation of water compared to commercial nanosized Co3 O4 . In scaled-up photoreactions, the pH value of the reaction medium decreased from 8.0 to around 7.0 after 20 min and the O2 production ceased. Based on the amounts of the sacrificial oxidant (K2 S2 O8 ) used, the yield of O2 produced is 49.6 % for 2 and 29.8 % for 1. However, the catalyst can be recycled without a significant loss of catalytic activity. Spectroscopic studies suggest that the structure and composition of recycled 1 and 2 are maintained. In isotope-labeling H2 (18) O (97 % enriched) experiments, the distribution of (16) O(16) O/(16) O(18) O/(18) O(18) O detected was 0:7.55:92.45, which is comparable to the theoretical values of 0.09:5.82:94.09. This work not only provides new catalysts that resemble ligand-protected cobalt oxide materials but also establishes the significance of the existence of OH(-) (or H2 O) binding sites at the metal center in WOCs.

Visible-Light-Driven Water Oxidation by a Molecular Manganese Vanadium Oxide Cluster

Photosynthetic water oxidation in plants occurs at an inorganic calcium manganese oxo cluster, which is known as the oxygen evolving complex (OEC), in photosystem II. Herein, we report a synthetic OEC model based on a molecular manganese vanadium oxide cluster, [Mn4 V4 O17 (OAc)3 ](3-) . The compound is based on a [Mn4 O4 ](6+) cubane core, which catalyzes the homogeneous, visible-light-driven oxidation of water to molecular oxygen and is stabilized by a tripodal [V4 O13 ](6-) polyoxovanadate and three acetate ligands. When combined with the photosensitizer [Ru(bpy)3 ](2+) and the oxidant persulfate, visible-light-driven water oxidation with turnover numbers of approximately 1150 and turnover frequencies of about 1.75 s(-1) is observed. Electrochemical, mass-spectrometric, and spectroscopic studies provide insight into the cluster stability and reactivity. This compound could serve as a model for the molecular structure and reactivity of the OEC and for heterogeneous metal oxide water-oxidation catalysts.

Photocatalytic production of hydrogen peroxide from water and dioxygen using cyano-bridged polynuclear transition metal complexes as water oxidation catalysts

H₂O₂ was produced from H₂O and O₂ using cyano-bridged polynuclear transition metal complexes as water oxidation catalysts with a Ru photocatalyst in water under visible light irradiation.

Water oxidation by a soluble iron(iii)–cyclen complex: new findings

Herein, the role of iron oxide in the electrochemical water oxidation of an iron cyclen (cyclen = 1,4,7,10-tetraazacyclododecane) is considered.

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.

Nanoscale cobalt metal–organic framework as a catalyst for visible light-driven and electrocatalytic water oxidation

Co-ZIF-67 is proposed as an efficient water oxidation catalyst under visible light.

A carbon-free polyoxometalate molecular catalyst with a cobalt–arsenic core for visible light-driven water oxidation

Na₁₂[{Co^II₇As^III₆O₉(OH)₆}(A-α-SiW₉O₃₄)₂]·8H₂O with the first reported cobalt–arsenic core was synthesized, thoroughly characterized and employed to catalyze water oxidation under visible-light-driven conditions.

A newly designed softoxometalate [BMIm]2[DMIm][α-PW12O40]@hydrocalumite that controls the chain length of polyacrylic acid in the presence of light

We have controlled the radical as well as cationic polymerization of acrylic acid using [BMIm]₂[DMIm][α-PW₁₂O₄₀]@hydrocalumite (SOM 2) as a photocatalyst.

New findings and the current controversies for water oxidation by a copper(ii)-azo complex: homogeneous or heterogeneous?

In this paper, new findings for the water-oxidizing activity of [(L)Cu(II)(NO3)], (L = (E)-3-(pyridin-2-yldiazenyl)naphthalen-2-ol (HL)) under both electro-water oxidation conditions and in the presence of cerium(iv) ammonium nitrate are reported.

Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation

Artificial photosynthesis (AP) promises to replace society's dependence on fossil energy resources via conversion of sunlight into sustainable, carbon-neutral fuels. However, large-scale AP implementation remains impeded by a dearth of cheap, efficient catalysts for the oxygen evolution reaction (OER). Cobalt oxide materials can catalyze the OER and are potentially scalable due to the abundance of cobalt in the Earth's crust; unfortunately, the activity of these materials is insufficient for practical AP implementation. Attempts to improve cobalt oxide's activity have been stymied by limited mechanistic understanding that stems from the inherent difficulty of characterizing structure and reactivity at surfaces of heterogeneous materials. While previous studies on cobalt oxide revealed the intermediacy of the unusual Co(IV) oxidation state, much remains unknown, including whether bridging or terminal oxo ligands form O2 and what the relevant oxidation states are. We have addressed these issues by employing a homogeneous model for cobalt oxide, the [Co(III)4] cubane (Co4O4(OAc)4py4, py = pyridine, OAc = acetate), that can be oxidized to the [Co(IV)Co(III)3] state. Upon addition of 1 equiv of sodium hydroxide, the [Co(III)4] cubane is regenerated with stoichiometric formation of O2. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during this stoichiometric OER, implying that terminal oxo ligands are responsible for forming O2. The OER is also examined with stopped-flow UV-visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O2 formation requires disproportionation of the [Co(IV)Co(III)3] state to generate an even higher oxidation state, formally [Co(V)Co(III)3] or [Co(IV)2Co(III)2]. The mechanistic understanding provided by these results should accelerate the development of OER catalysts leading to increasingly efficient AP systems.

A Bioinspired Molecular Polyoxometalate Catalyst with Two Cobalt(II) Oxide Cores for Photocatalytic Water Oxidation

To overcome the bottleneck of water splitting, the exploration of efficient, selective, and stable water oxidation catalysts (WOCs) is crucial. We report an all-inorganic, oxidatively and hydrolytically stable WOC based on a polyoxometalate [(A-α-SiW9 O34)2Co8(OH)6(H2O)2(CO3)3](16-) (Co8 POM). As a cobalt(II)-based cubane water oxidation catalyst, Co8POM embeds double Co(II)4O3 cores. The self-assembled catalyst is similar to the oxygen evolving complex (OEC) of photosystem II (PS II). Using [Ru(bpy)3](2+) as a photosensitizer and persulfate as a sacrificial electron acceptor, Co8POM exhibits excellent water oxidation activity with a turnover number (TON) of 1436, currently the highest among bioinspired catalysts with a cubical core, and a high initial turnover frequency (TOF). Investigation by several spectroscopy, spectrometry, and other techniques confirm that Co8POM is a stable and efficient catalyst for visible light-driven water oxidation. The results offer a useful insight into the design of water oxidation catalysts.

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.

Oxidative decomposition of Au25(SR)18 clusters in a catalytic context

Gold nanoparticle catalysis of chemical transformations has emerged as a subject of intense interest over the past decade. In particular, Au25(SR)18 has emerged as a model catalyst. In an effort to investigate their potential as intact, homogeneous, unsupported catalysts, we have discovered that Au25(SR)18 clusters are not stable in oxidizing conditions reported for catalytic styrene oxidation. Further investigation suggests that the active catalytic species is an Au(I) species resulting from oxidative decomposition of the starting gold cluster. This conclusion appears independent of R-group on thiolate-ligated Au25(SR)18 clusters.

High catalytic activity of heteropolynuclear cyanide complexes containing cobalt and platinum ions: visible-light driven water oxidation

A near-stoichiometric amount of O2 was evolved as observed in the visible-light irradiation of an aqueous buffer (pH 8) containing [Ru(II) (2,2'-bipyridine)3 ] as a photosensitizer, Na2 S2 O8 as a sacrificial electron acceptor, and a heteropolynuclear cyanide complex as a water-oxidation catalyst. The heteropolynuclear cyanide complexes exhibited higher catalytic activity than a polynuclear cyanide complex containing only Co(III) or Pt(IV) ions as C-bound metal ions. The origin of the synergistic effect between Co and Pt ions is discussed in relation to electronic and local atomic structures of the complexes.

In situ Raman and surface-enhanced Raman spectroscopy on working electrodes: spectroelectrochemical characterization of water oxidation electrocatalysts

This perspective article gives an overview of the current developments regarding the use of in situ Raman and surface-enhanced Raman spectroscopy on working electrodes for probing the fate of molecular and heterogeneous catalysts during electrochemical water oxidation.

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.

Earth-abundant metal complexes as catalysts for water oxidation; is it homogeneous or heterogeneous?

This minireview focuses on the aspects that determine whether particular catalysts for the oxidation of water are homogeneous or heterogeneous.

Electron transfer and catalysis with high-valent metal-oxo complexes

High-valent metal-oxo complexes are produced by thermal and photoinduced electron-transfer reactions, acting as catalysts for oxygenation of substrates using water or dioxygen as an oxygen source.

Efficient light-driven water oxidation catalyzed by a mononuclear cobalt(iii) complex

A mononuclear Co complex, [Co^III(DPK·OH)₂]Cl, is reported as a robust and stable catalyst, and has the largest TON (1610) among metal–organic complexes for visible light-driven water oxidation.

Green and facile synthesis of an Au nanoparticles@polyoxometalate/ordered mesoporous carbon tri-component nanocomposite and its electrochemical applications

The one-pot synthesis of a well-defined Au nanoparticles@polyoxometalates/ordered mesoporous carbon (Au@POMs/OMC) tri-component nanocomposite is reported, which is facile, green and rapid. The polyoxometalates were used as both reductant and bridging molecules. The formation of these composite materials was verified by a comprehensive characterization using X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectra, scanning electron microscopy, and transmission electron microscopy. The novel nanohybrids of Au@POMs/OMC can provide new features of electrocatalytic activities, because of the synergetic effects of Au nanoparticles and OMC materials. Most importantly, the amperometric measurements show that the Au@POMs/OMC nanohybrids have a high catalytic activity with a good sensitivity, long-term stability, wide linear range, low detection limit, and fast response towards acetaminophenol, H2O2, and NADH detection for application as an enzyme-free biosensor.