Search

A hetero-dinuclear IrIII–CuII complex with two adjacent sites was employed as a catalyst for the aerobic oxidation of aromatic olefins driven by formate in water. An IrIII–H intermediate, generated through formate dehydrogenation, was revealed to activate terminal aromatic olefins to afford an Ir-alkyl species, and the process was promoted by a hydrophobic [IrIII–H]-[substrate aromatic ring] interaction in water. The Ir-alkyl species subsequently reacted with dioxygen to yield corresponding methyl ketones and was promoted by the presence of the CuII moiety under acidic conditions. The IrIII–CuII complex exhibited cooperative catalysis in the selective aerobic oxidation of olefins to corresponding methyl ketones, as evidenced by no reactivities observed from the corresponding mononuclear IrIII and CuII complexes, as the individual components of the IrIII–CuII complex. The reaction mechanism afforded by the IrIII–CuII complex in the aerobic oxidation was disclosed by a combination of spectroscopic detection of reaction intermediates, kinetic analysis, and theoretical calculations., A hetero-dinuclear IrIII–CuII complex with two adjacent sites was employed as a catalyst for the aerobic oxidation of aromatic olefins driven by formate and promoted by a hydrophobic interaction in water.

Novel heterodinuclear IrIII–MII complexes (M = Co, Ni, or Cu) with two adjacent reaction sites were synthesized by using 3,5-bis(2-pyridyl)-pyrazole (Hbpp) as a structure-directing ligand and emplo...

Chemiresistive sensors, which are based on semiconducting materials, offer real-time monitoring of environment. However, detection of nonpolar chemical substances is often challenging because of the weakness of the doping effect. Herein, we report a concept of combining a cascade reaction (CR) and a chemiresistive sensor array for sensitive and selective detection of a target analyte (herein, ethylene in air). Ethylene was converted to acetaldehyde through a Pd-catalyzed heterogeneous Wacker reaction at 40 °C, followed by condensation with hydroxylamine hydrochloride to emit HCl vapor. HCl works as a strong dopant for single-walled carbon nanotubes (SWCNTs), enabling the main sensor to detect ethylene with excellent sensitivity (10.9% ppm-1) and limit of detection (0.2 ppm) in 5 min. False responses that occur in the main sensor are easily discriminated by reference sensors that partially employ CR. Moreover, though the sensor monitors the variation of normalized electric resistance (ΔR/R0) in the SWCNT network, temporary deactivation of CR yields a sensor system that does not require analyte-free air for a baseline correction (i.e., estimation of R0) and recovery of response. The concept presented here is generally applicable and offers a solution for several issues that are inherently present in chemiresistive sensing systems.

A RuII complex, [RuII(tpphz)(bpy)2]2+ (1) (tpphz = tetrapyridophenazine, bpy = 2,2'-bipyridine), whose tpphz ligand has a pyrazine moiety, is converted efficiently to [RuII(tpphz-HH)(bpy)2]2+ (2) having a dihydropyrazine moiety upon photoirradiation of a water-methanol mixed solvent solution of 1 in the presence of an electron donor. In this reaction, the triplet metal-to-ligand charge-transfer excited state (3MLCT*) of 1 is firstly formed upon photoirradiation and the 3MLCT* state is reductively quenched with an electron donor to afford [RuII(tpphz˙-)(bpy)2]+, which is converted to 2 without the observation of detectable reduced intermediates by nano-second laser flash photolysis. The inverse kinetic isotope effect (KIE) was observed to be 0.63 in the N-H bond formation of 2 at the dihydropyrazine moiety. White-light (380-670 nm) irradiation of a solution of 1 in a protic solvent, in the presence of an electron donor under an inert atmosphere, led to photocatalytic H2 evolution and the hydrogenation of organic substrates. In the reactions, complex 2 is required to be excited to form its 3MLCT* state to react with a proton and aldehydes. In photocatalytic H2 evolution, the H-H bond formation between photoexcited 2 and a proton is involved in the rate-determining step with normal KIE being 5.2 on H2 evolving rates. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the reaction mechanism of H2 evolution from the ground and photo-excited states of 2 were performed to have a better understanding of the photocatalytic processes.

Efficient detoxification of harmful benzene rings into useful carboxylic acids in water is indispensable for achieving a clean water environment. We report herein that oxidative aromatic cracking (OAC) reactions in water were achieved using a catalytic system with a cobalt-substituted Keggin-type polyoxometalate (Co-POM) as a catalyst, an Oxone® monopersulfate compound as a sacrificial oxidant and sodium bicarbonate as an additive under mild conditions. Sodium bicarbonate plays a crucial role in the selective OAC reactions by Co-POM using ethylbenzenesulfonate as a model substrate. The reactive species was characterized to be a cobalt(III)-oxyl species based on 31P NMR, UV-vis spectroscopic, kinetic, and theoretical analyses. The electrophilicity of the cobalt(III)-oxyl species was demonstrated by a linear relationship with a negative slope in the Hammett plots of initial rates obtained from the OAC reactions of m-xylenesulfonate derivatives. Besides, we have verified the degradation pathway of the OAC reactions using benzene as a model substrate in the catalytic system. The degradation was initiated by an electrophilic attack of the cobalt(III)-oxyl species on benzene to yield phenol followed by producing catechol, muconic acid, maleic/fumaric acid, tartaric acid derivatives and formic acid on the basis of 1H NMR spectroscopic analysis.

We have synthesized a new Ni(II) complex having an S2N2-tetradentate ligand with two noncoordinating pyridine pendants as binding sites of Lewis-acidic metal ions in the vicinity of the Ni center, ...

Ag@TiO2 nanoparticles (NPs) with Ag metal cores and TiO2 semiconductor shells were prepared with a hydrothermal method and their structure was characterized by scanning electron microscopy, transmi...

We have designed and synthesized a hetero-dinuclear RuII–CoII complex with a dinucleating ligand inspired by hetero-dinuclear active sites of metalloenzymes. A synergistic effect between the adjace...

Stoichiometric electron-transfer (ET) oxidation of two diastereomeric μ-peroxo-μ-hydroxo dinuclear Co(III) complexes with tris(2-pyridylmethyl)amine (TPA) was examined to scrutinize the reaction mechanism of O2 evolution from the peroxo complexes, as seen in the final step in water oxidation by a Co(III)-TPA complex. The two isomeric Co(III)-peroxo complexes were synthesized and selectively isolated by recrystallization under different conditions. Although cyclic voltammograms of the two isomers in aqueous solutions showed one reversible wave at 1.1 V vs NHE at pH 2.0, two oxidation waves were observed at 1.0 and 1.4 V at pH 7.0 in the aqueous solutions, the latter of which is responsible for the O2-releasing process. At pH 7, one diastereomer showed higher reactivity than the other in O2 evolution, indicating the importance of structures of the μ-peroxo complexes in the reaction. In order to clarify the O2-evolving mechanism, we performed electron paramagnetic resonance (EPR) and resonance Raman (RR) measurements for characterizing one-electron oxidized species: The observed EPR and RR signals supported the formation of μ-superoxo-μ-hydroxo dinuclear Co(III) complexes; however, no characteristic difference was observed between two isomers in the EPR parameters including g values and superhyperfine coupling constants. ET-oxidation rate constants of the isomers were determined to be much faster than the O2-evolving rate constants, indicating that the O2-releasing step is the rate-determining step in the O2 evolution through the stoichiometric ET oxidation of the dinuclear Co(III)-μ-peroxo complexes. Therefore, the difference of reactivity in the O2 evolution for the two isomers should be derived from the thermodynamic stability of two-electron oxidized species of the dinuclear Co(III)-μ-peroxo complexes, μ-dioxygen-μ-hydroxo dinuclear Co(III) intermediates.

Hydrogen peroxide was directly produced from oxygen and formic acid, catalysed by a hetero-dinuclear Ir–Ni complex with two adjacent sites, at ambient temperature. Synergistic catalysis derived from the hetero-dinuclear Ir and Ni centres was demonstrated by comparing its activity to those of the component mononuclear Ir and Ni complexes. A reaction intermediate of Ir-hydrido was detected by UV-vis, ESI-TOF-MS, and 1H NMR spectroscopies. It was revealed that the Ir moiety serves as an active species of Ir-hydrido, reacting with oxygen to afford an Ir-hydroperoxide species through O2 insertion, which is the rate-determining step for H2O2 production. Meanwhile, the Ni moiety promotes H2O2 formation by activating solvents as proton sources. We also found that H2O2 production is strongly affected by the solvent dielectric constants (DE); the highest H2O2 concentration was obtained in ethylene glycol with a moderate DE. The catalytic mechanism of H2O2 production by the Ir–Ni complex was discussed, based on kinetic analysis, isotope labelling experiments, and theoretical DFT calculations.

Novel heterodinuclear Ir

We report homogeneous electrocatalytic and photocatalytic H2 evolution using two Ni(II) complexes with S2N2-type tetradentate ligands bearing two different sizes of chelate rings as catalysts. A Ni(II) complex with a five-membered SC2S–Ni chelate ring (1) exhibited higher activity than that with a six-membered SC3S–Ni chelate ring (2) in both electrocatalytic and photocatalytic H2 evolution despite both complexes showing the same reduction potentials. A stepwise reduction of the Ni center from Ni(II) to Ni(0) was observed in the electrochemical measurements; the first reduction is a pure electron transfer reaction to form a Ni(I) complex as confirmed by electron spin resonance measurements, and the second is a 1e–/1H+ proton-coupled electron transfer reaction to afford a putative Ni(II)-hydrido (NiII–H) species. We also clarified that Ni(II) complexes can act as homogeneous catalysts in the electrocatalytic H2 evolution, in which complex 1 exhibited higher reactivity than that of 2. In the photocatalytic ...

A Ni(II) complex bearing an S2N2-type tetradentate ligand inspired by the active site of carbon monoxide dehydrogenase was found to selectively catalyze CO2 reduction to produce CO in a photocatalytic system using [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) as a photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as an electron donor. The Ni(II) complex shows a high turnover number over 700 with high CO selectivity of >99% and quantum yield of 1.42% in the photocatalytic system.

We have synthesized a new Ni(II) complex having an S

We investigated the selective oxidation of styrenes to benzaldehydes by using a non-irradiated TiO2–H2O2 catalytic system. The oxidation promotes multi-step reactions from styrenes, including the cleavage of a C=C double bond and the addition of an oxygen atom selectively and stepwise to provide the corresponding benzaldehydes in good yields (up to 72%). These reaction processes were spectroscopically shown by fluorescent measurements under the presence of competitive scavengers. The absence of the signal from OH radicals indicates the participation of other oxidants such as hydroperoxy radicals (•OOH) and superoxide radicals (•O2−) into the selective oxidation from styrene to benzaldehyde.

In this study, we report that selective aerobic oxidation of allyl phenyl ethers is attained by a Pd catalyst/polyoxometalate hybrid system to yield corresponding methyl ketones in water-enriched acetonitrile. The Pd(OAc)2/H5PV2Mo10O40 system exhibits higher conversions and yields of corresponding methyl ketone by Wacker-type oxidation of allyl phenyl ether as compared with the conventional PdCl2/CuCl2 system. The higher yields are attributed to the efficient re-oxidation of Pd0 to Pd2+ by H5PV2Mo10O40 using O2 as an oxidant as evidenced by electrochemical measurements. A reduced species of H5PV2Mo10O40 by Pd0 during the catalytic oxidation is revealed by UV–vis spectral measurements. The use of PdCl2 in place of Pd(OAc)2 in combination with [PV2Mo10O40]5− bearing tetraalkylammonium counter cations has also exhibited comparable conversions and product yields in the Wacker-type oxidation of allyl phenyl ethers. Para-substituted allyl phenyl ether derivatives are successfully oxidized in the Pd catalyst/polyoxometalate system to yield corresponding methyl ketones. The initial rate of products of para-substituted methyl ketones depended on the electronic effect of the substituents in which allyl phenyl ethers with electron-donating groups have accelerated the initial rate in the Pd catalyst/polyoxometalate system.

We report homogeneous electrocatalytic and photocatalytic H

Angesichts der Unvereinbarkeit eines zunehmenden Verbrauchs von Kraftstoffen und chemischen Produkten und einer nur endlichen Menge an Ressourcen versuchen wir Moglichkeiten zu finden, wie wir unsere Gesellschaft auf Dauer nachhaltig gestalten konnen. Die kunstliche Photosynthese nutzt das Sonnenlicht, um reichlich vorhandene Ressourcen in hochwertige Chemikalien umzuwandeln. Deshalb gilt sie als die aussichtsreichste Methode. Hier werden Entwicklungen und neueste Fortschritte sowie noch bestehende Herausforderungen fur ihre Schlusselprozesse, die photoelektrochemische Wasserspaltung und die elektrochemische CO2-Reduktion, beschrieben. Ein Uberblick uber die Fortschritte in der Katalyse, um den erneuerbaren Wasserstoff als Grundstoff fur die Produktion von Chemikalien einzusetzen, soll verdeutlichen, welche Rolle die kunstliche Photosynthese in einer nachhaltigen Chemie spielen wird.

Hydrogen peroxide (H2O2) was produced from water and dioxygen using [RuII(Me2phen)3]2+ (Me2phen = 4,7-dimethyl-1,10-phenanthroline) as a photocatalyst and [Ir(Cp*)(H2O)3]2+ (Cp* = η5-pentamethylcyclopentadienyl) as a precursor of a water oxidation catalyst in the presence of Sc3+ in water under visible light irradiation. TEM and XPS measurements of residues in the resulting solution after the photocatalytic production of H2O2 indicated that [Ir(Cp*)(H2O)3]2+ was converted to Ir(OH)3 nanoparticles, which are actual catalytic species. The Ir(OH)3 nanoparticles produced in situ during the photocatalytic production of H2O2 were smaller in size than those prepared independently from hydrogen hexachloroiridiate (H2IrCl6), and exhibited higher catalytic reactivity for the photocatalytic production of H2O2. The photocatalytic production of H2O2 from water and dioxygen was also made possible when Ir(OH)3 nanoparticles were replaced by nickel ferrite (NiFe2O4) nanoparticles, which are composed of more earth abundant metals than iridium. The size of NiFe2O4 nanoparticles became smaller during the photocatalytic production of H2O2 to exhibit higher catalytic reactivity in the second run as compared with that in the first run. NiFe2O4 nanoparticles obtained by the treatment of NiFe2O4 in an aqueous solution of Sc3+ exhibited 33-times higher catalytic reactivity in H2O2-production rates than the as-prepared NiFe2O4. Thus, both the bottom-up method starting from a molecular complex [Ir(Cp*)(H2O)3]2+ and the top-down method starting from as-prepared NiFe2O4 to obtain nanoparticles with smaller size resulted in the improvement of the catalytic reactivity for the photocatalytic production of H2O2 from water and dioxygen.

A Ni(II) complex bearing an S

For exerting potential catalytic and photocatalytic activities of metal nanoparticles (MNPs), immobilization of MNPs on a support medium in highly dispersed state is desired. In this Research Article, we demonstrated that surfactant-free platinum nanoparticles (PtNPs) were efficiently immobilized on graphene oxide (GO) nanosheets in a highly dispersed state by utilizing oligopeptide β-sheets as a cross-linker. The fluorenyl-substituted peptides were designed to form β-sheets, where metal-binding thiol groups and protonated and positively charged amino groups are integrated on the opposite sides of the surface of a β-sheet, which efficiently bridge PtNPs and GO nanosheet. In comparison to PtNP/GO composite without the peptide linker, the PtNP/peptide/GO ternary complex exhibited excellent photocatalytic dye degradation activity via electron transfer from GO to PtNP and simultaneous hole transfer from oxidized GO to the dye. Furthermore, the ternary complex showed photoinduced hydrogen evolution upon visible light irradiation using a hole scavenger. This research provides a new methodology for the development of photocatalytic materials by a bottom-up strategy on the basis of self-assembling features of biomolecules.

Submicron-sized mesoporous nickel ferrite (NiFe2O4) spheres were prepared by an aerosol spray pyrolysis method using Pluronic F127 as a structure-directing agent, and their photocatalytic performance for hydrogen (H2) evolution was examined in an aqueous MeOH solution by visible light irradiation (λ > 420 nm). The structure of the spherical mesoporous nickel ferrites was studied by transmission electron microscopy, powder X-ray diffraction, and N2 adsorption–desorption isotherm measurements. Mesoporous NiFe2O4 spheres of high specific surface area (278 m2 g–1) with a highly crystalline framework were prepared by adjusting the amount of structure-directing agent and the calcining condition. High photocatalytic activity of mesoporous NiFe2O4 for H2 evolution from water with methanol was achieved due to the combination of high surface area and high crystallinity of the nickel ferrites.

The development of efficient water reduction and oxidation catalysts is the key issue for achieving solar energy conversion to sustainable energy sources to replace of fossil fuels. Platinum group metal (PGM) catalysts have been recognized as active catalysts for both water reduction and oxidation. However, it is highly desired to replace precious and scarce PGM catalysts by earth-abundant metal catalysts for water reduction and oxidation. In the past 5 years, there has been significant progress in the development of water reduction and oxidation catalysts based on earth-abundant metals such as iron, nickel, copper, and manganese, which have been combined with organic photocatalysts. This work describes the state of the art and future challenges in bioinspired photocatalytic water reduction and oxidation with earth-abundant metals.

Recent developments in thermal and photochemical water oxidation by using homogeneous and heterogeneous catalysts is described together with the conversion of the homogeneous catalysts into heterogeneous catalysts during the course of water oxidation. The use of homogeneous catalysts is advantageous in the elucidation of detailed catalytic mechanisms including the detection of active intermediates for water oxidation. In contrast, heterogeneous catalysts are advantageous for practical applications, because of their high catalytic activity and the ease with which they can be separated by filtration. However, it is quite difficult to identify the active intermediates on the surfaces of heterogeneous catalysts, and therefore, the heterogeneous catalytic mechanism of water oxidation has not been clarified. Although investigations on homogeneous and heterogeneous catalysts for water oxidation have been performed rather independently, the link between homogeneous and heterogeneous catalysts is becoming more important for the development of efficient WOCs. This microreview focuses on factors to determine if the actual catalysts for water oxidation are homogeneous or heterogeneous depending on the conditions under which the catalysts are used. Ligand oxidation of homogeneous catalysts sometimes results in dissociation of the ligands to form nanoparticles, which act as much more efficient catalysts for water oxidation.

© 2016 American Chemical Society. Conversion of the greenhouse gas carbon dioxide (CO2) to value-added products is an important challenge for sustainable energy research, and nanomaterials offer a broad class of heterogeneous catalysts for such transformations. Here we report a molecular surface functionalization approach to tuning gold nanoparticle (Au NP) electrocatalysts for reduction of CO2 to CO. The N-heterocyclic (NHC) carbene-functionalized Au NP catalyst exhibits improved faradaic efficiency (FE = 83%) for reduction of CO2 to CO in water at neutral pH at an overpotential of 0.46 V with a 7.6-fold increase in current density compared to that of the parent Au NP (FE = 53%). Tafel plots of the NHC carbene-functionalized Au NP (72 mV/decade) vs parent Au NP (138 mV/decade) systems further show that the molecular ligand influences mechanistic pathways for CO2 reduction. The results establish molecular surface functionalization as a complementary approach to size, shape, composition, and defect control for nanoparticle catalyst design.

Water oxidation by cerium(IV) ammonium nitrate, CAN, with [IrIII(Cp*)(4,4′-R2-2,2′-bipyridine)(H2O)]2+ (R = OH, OMe, Me or COOH) to evolve oxygen has been investigated together with the possible oxidation of the ligands by CAN. The apparent catalytic reactivity is highly dependent on the substituent R and the highest catalytic reactivity was obtained when R = OH. The apparent turnover frequency (TOF) of the catalyticwater oxidation by CAN with [IrIII(Cp*){4,4′-(OH)2-2,2′-bipyridine}(H2O)]2+, which acts as a precatalyst, gradually increased during the reaction to reach the highest value among the Ir complexes. In the second run, the apparent TOF value was the highest from the beginning of the reaction. 1H NMR and dynamic light scattering measurements for solutions after the first run indicated formation of insoluble nanoparticles, which exhibited a much higher catalytic reactivity as compared with iridium oxide prepared by a conventional method. The 4,4′-R2-2,2′-bipyridine ligand was also efficiently oxidized by CAN up to CO2 only when R = OH. TG/DTA and XPS measurements of nanoparticles produced after the water oxidation suggested that the nanoparticles were composed of iridium hydroxide with a small amount of carbonaceous residue. Thus, iridium hydroxide nanoparticles act as an excellent catalyst for the water oxidation by CAN.

The apparent incongruity between the increasing consumption of fuels and chemicals and the finite amount of resources has led us to seek means to maintain the sustainability of our society. Artificial photosynthesis, which utilizes sunlight to create high-value chemicals from abundant resources, is considered as the most promising and viable method. This Minireview describes the progress and challenges in the field of artificial photosynthesis in terms of its key components: developments in photoelectrochemical water splitting and recent progress in electrochemical CO2 reduction. Advances in catalysis, concerning the use of renewable hydrogen as a feedstock for major chemical production, are outlined to shed light on the ultimate role of artificial photosynthesis in achieving sustainable chemistry.

The apparent incongruity between the increasing consumption of fuels and chemicals and the finite amount of resources has led us to seek means to maintain the sustainability of our society. Artificial photosynthesis, which utilizes sunlight to create high-value chemicals from abundant resources, is considered as the most promising and viable method. This Minireview describes the progress and challenges in the field of artificial photosynthesis in terms of its key components: developments in photoelectrochemical water splitting and recent progress in electrochemical CO2 reduction. Advances in catalysis, concerning the use of renewable hydrogen as a feedstock for major chemical production, are outlined to shed light on the ultimate role of artificial photosynthesis in achieving sustainable chemistry.

Thermal water oxidation by cerium(IV) ammonium nitrate (CAN) was catalyzed by nonheme iron complexes, such as Fe(BQEN)(OTf)2 (1) and Fe(BQCN)(OTf)2 (2) (BQEN = N,N'-dimethyl-N,N'-bis(8-quinolyl)ethane-1,2-diamine, BQCN = N,N'-dimethyl-N,N'-bis(8-quinolyl)cyclohexanediamine, OTf = CF3SO3(-)) in a nonbuffered aqueous solution; turnover numbers of 80 ± 10 and 20 ± 5 were obtained in the O2 evolution reaction by 1 and 2, respectively. The ligand dissociation of the iron complexes was observed under acidic conditions, and the dissociated ligands were oxidized by CAN to yield CO2. We also observed that 1 was converted to an iron(IV)-oxo complex during the water oxidation in competition with the ligand oxidation. In addition, oxygen exchange between the iron(IV)-oxo complex and H2(18)O was found to occur at a much faster rate than the oxygen evolution. These results indicate that the iron complexes act as the true homogeneous catalyst for water oxidation by CAN at low pHs. In contrast, light-driven water oxidation using [Ru(bpy)3](2+) (bpy = 2,2'-bipyridine) as a photosensitizer and S2O8(2-) as a sacrificial electron acceptor was catalyzed by iron hydroxide nanoparticles derived from the iron complexes under basic conditions as the result of the ligand dissociation. In a buffer solution (initial pH 9.0) formation of the iron hydroxide nanoparticles with a size of around 100 nm at the end of the reaction was monitored by dynamic light scattering (DLS) in situ and characterized by X-ray photoelectron spectra (XPS) and transmission electron microscope (TEM) measurements. We thus conclude that the water oxidation by CAN was catalyzed by short-lived homogeneous iron complexes under acidic conditions, whereas iron hydroxide nanoparticles derived from iron complexes act as a heterogeneous catalyst in the light-driven water oxidation reaction under basic conditions.

NiFe2O4 is an excellent catalyst for photocatalytic water oxidation with Na2S2O8 as a sacrificial oxidant and [Ru(bpy)3]2+ as a photosensitizer.

Single or mixed oxides of iron and nickel have been examined as catalysts in photocatalytic water oxidation using [Ru(bpy)(3)](2+) as a photosensitizer and S(2)O(8)(2-) as a sacrificial oxidant. The catalytic activity of nickel ferrite (NiFe(2)O(4)) is comparable to that of a catalyst containing Ir, Ru, or Co in terms of O(2) yield and O(2) evolution rate under ambient reaction conditions. NiFe(2)O(4) also possesses robustness and ferromagnetic properties, which are beneficial for easy recovery from the solution after reaction. Water oxidation catalysis achieved by a composite of earth-abundant elements will contribute to a new approach to the design of catalysts for artificial photosynthesis.

Cs5[Ru(H2O)SiW11O39] (I) and Cs5[Ru(H2O)GeW11O39] (II) are robust catalysts for water oxidation to generate oxygen with (NH4)2[Ce(NO3)6] (III) as a one-electron oxidant in water.

Catalytic water oxidation to generate oxygen was achieved using all-inorganic mononuclear ruthenium complexes bearing Keggin-type lacunary heteropolytungstate, [Ru(III)(H(2)O)SiW(11)O(39)](5-) (1) and [Ru(III)(H(2)O)GeW(11)O(39)](5-) (2), as catalysts with (NH(4))(2)[Ce(IV)(NO(3))(6)] (CAN) as a one-electron oxidant in water. The oxygen atoms of evolved oxygen come from water as confirmed by isotope-labeled experiments. Cyclic voltammetric measurements of 1 and 2 at various pH's indicate that both complexes 1 and 2 exhibit three one-electron redox couples based on ruthenium center. The Pourbaix diagrams (plots of E(1/2) vs pH) support that the Ru(III) complexes are oxidized to the Ru(V)-oxo complexes with CAN. The Ru(V)-oxo complex derived from 1 was detected by UV-visible absorption, EPR, and resonance Raman measurements in situ as an active species during the water oxidation reaction. This indicates that the Ru(V)-oxo complex is involved in the rate-determining step of the catalytic cycle of water oxidation. The overall catalytic mechanism of water oxidation was revealed on the basis of the kinetic analysis and detection of the catalytic intermediates. Complex 2 exhibited a higher catalytic reactivity for the water oxidation with CAN than did complex 1.

NiMnO3 was found to be an efficient catalyst for light-driven water oxidation using [Ru(bpy)3](2+) and S2O8(2-) as a photosensitiser and a sacrificial oxidant, respectively. NiMnO3 exhibited remarkably high catalytic activity in comparison with manganese oxides and nickel oxide. For electrochemical water oxidation, the highest catalytic current was also obtained with NiMnO3 among the manganese oxides.

Cobalt-containing metal oxides [perovskites (LaCoO(3), NdCoO(3), YCoO(3), La(0.7)Sr(0.3)CoO(3)), spinel (Co(3)O(4)) and wolframite (CoWO(4))] have been examined as catalysts for photocatalytic water oxidation with Na(2)S(2)O(8) and [Ru(bpy)(3)](2+) as an electron acceptor and a photosensitizer, respectively. Catalysts with the perovskite structure exhibited higher catalytic activity as compared with the catalysts with the spinel and wolframite structures. LaCoO(3), which stabilizes Co(III) species in the perovskite structure, exhibited the highest catalytic activity in the photocatalytic water oxidation compared with CoWO(4), Co(3)O(4) and La(0.7)Sr(0.3)CoO(3) which contain Co(II) or Co(IV) species in the matrices. The high catalytic reactivity of LaCoO(3) possessing perovskite structure was maintained in NdCoO(3) and YCoO(3) which exclusively contain Co(III) species. Thus, the catalytic activity of Co ions can be controlled by the additional metal ions, which leads to development of highly reactive and robust catalysts for the photocatalytic water oxidation.

The photocatalytic water oxidation to evolve O2 was performed by photoirradiation (λ > 420 nm) of an aqueous solution containing [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine), Na2S2O8 and water-soluble cobalt complexes with various organic ligands as precatalysts in the pH range of 6.0–10. The turnover numbers (TONs) based on the amount of Co for the photocatalytic O2 evolution with [CoII(Me6tren)(OH2)]2+ (1) and [CoIII(Cp*)(bpy)(OH2)]2+ (2) [Me6tren = tris(N,N′-dimethylaminoethyl)amine, Cp* = η5-pentamethylcyclopentadienyl] at pH 9.0 reached 420 and 320, respectively. The evolved O2 yield increased in proportion to concentrations of precatalysts 1 and 2 up to 0.10 mM. However, the O2 yield dramatically decreased when the concentration of precatalysts 1 and 2 exceeded 0.10 mM. When the concentration of Na2S2O8 was increased from 10 mM to 50 mM, CO2 evolution was observed during the photocatalytic water oxidation. These results indicate that a part of the organic ligands of 1 and 2 were oxidized to evolve CO2 during the photocatalytic reaction. The degradation of complex 2 under photocatalytic conditions and the oxidation of Me6tren ligand of 1 by [Ru(bpy)3]3+ were confirmed by 1H NMR measurements. Dynamic light scattering (DLS) experiments indicate the formation of particles with diameters of around 20 ± 10 nm and 200 ± 100 nm during the photocatalytic water oxidation with 1 and 2, respectively. The particle sizes determined by DLS agreed with those of the secondary particles observed by TEM. The XPS measurements of the formed particles suggest that the surface of the particles is covered with cobalt hydroxides, which could be converted to active species containing high-valent cobalt ions during the photocatalytic water oxidation. The recovered nanoparticles produced from 1 act as a robust catalyst for the photocatalytic water oxidation.