Mediator enhanced water oxidation using Rb4[Ru(II)(bpy)3]5[{Ru(III)4O4(OH)2(H2O)4}(γ-SiW10O36)2] film modified electrodes

Binuclear Ru(III)-Containing Polyoxometalate with Efficient Photocatalytic Activity for Oxidative Coupling of Amines to Imines

A novel binuclear ruthenium-based polyoxometalate, K₆H[{Ru₂Cl(H₂O)(CH₃COO)₂}{WO(H₂O)}₂(PW₉O₃₄)₂]·14H₂O (1), was successfully synthesized by the conventional hydrothermal method. Compound 1 was well-characterized by single-crystal X-ray diffraction, X-ray powder diffraction (PXRD), infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), electrospray ionization-mass spectrometry (ESI-MS), thermogravimetric analyses (TGA), and elemental analysis. The structural unit of compound 1 contains two [A-α-PW₉O₃₄]^9- building blocks at the upper and lower positions connected by two W atoms and two Ru atoms, where the W atoms and Ru atoms are arranged in a trapezoidal arrangement and the Ru atoms are bridged by acetic acid. Furthermore, compound 1 features characteristic absorption bands in the visible region, which allows the investigation of its photocatalytic properties in visible light. Under simulated sunlight radiation (λ > 400 nm), compound 1 exhibits high photocatalytic activity and good circularity toward the oxidative coupling of amines to imines at room temperature with O₂ as the sole oxidant.

WVI‒OH Functionality on polyoxometalates for water reduction to molecular hydrogen

ABSTRACT: Grafting a WVI‒(OH)2 functionality on polyoxometalates (POMs)’ surface makes the concerned POM compounds, Na6[{CoII(H2O)3}2{WVI(OH)2}2{BiIIIWVI9O33)2}]·8H2O (1) and Na4(Himi)2[{MnII(H2O)3}2{WVI(OH)2}2{BiIIIWVI9O33)2}]·28H2O (2) prominent heterogeneous electrocatalysts for water reduction to molecular hydrogen. We have...

Multinuclear transition metal-containing polyoxometalates constructed from Nb/W mixed-addendum precursors: synthesis, structures and catalytic performance

Four new transition metal-containing Nb/W mixed-addendum POM trimers with the formula H19[M4(H2O)x(P2W15Nb3O62)3]·m(HCOOH)·nH2O (M = Cu, x = 15, m = 0, and n = 21, Cu-POM; M = Co, x = 7, m = 0, and n = 15, Co-POM; M = Mn, x = 7, m = 6, and n = 18, Mn-POM; and M = Zn, x = 7, m = 0, and n = 23, Zn-POM) have been synthesized by a solvothermal method in a water-ethanol mixed solvent. All the four compounds were characterized by single-crystal X-ray diffraction, powder X-ray diffraction (XRD), IR spectroscopy, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). These compounds can serve as efficient heterogeneous catalysts for the cyanosilylation of different carbonyl compounds under ambient temperature and solvent-free conditions, and Cu-POM shows much better catalytic performance than the other three compounds. The cycle experiment showed that Cu-POM can be reused for at least five cycles without significant loss of catalytic activity. The IR spectroscopy and XRD analysis revealed that Cu-POM can retain its integrity after catalysis.

Discovery of a New Family of Polyoxometalate-Based Hybrids with Improved Catalytic Performances for Selective Sulfoxidation: The Synergy between Classic Heptamolybdate Anions and Complex Cations

A series of functional cation-regulated isopolymolybdate-based organic-inorganic hybrid compounds, Na₂H₂[Mo₄O₁₂(C₈H₁₇O₅N)₂]·10H₂O (1), Na₂[M(Bis-tris)(H₂O)]₂[Mo₇O₂₄]·10H₂O [M = Cu, 2; Ni, 3; Co, 4; Zn, 5; Bis-tris = 2,2-Bis(hydroxymethyl)-2,2',2″-nitrilotriethanol], and (NH₄)₂[M(Bis-tris)(H₂O)]₂[Mo₇O₂₄]·6H₂O (M = Zn, 6; Cu, 7), were synthesized and characterized toward advanced molecular catalyst design. Compound 1 is a covalently bonded adduct, and its self-assembly process can be probed by electrospray ionization mass spectrometry (ESI-MS). Compounds 2-7 are polyoxometalate (POM)-based hybrids containing classic heptamolybdate anions and complex cations with Bis-tris ligands. All of these compounds showed remarkable catalytic effects for selective sulfide oxidation. To the best of our knowledge, compound 5 presents the best catalytic activity so far among the reported hybrid materials with common easily synthesized small-molecule POM clusters and also exhibits outstanding reliability. The conclusion of the catalytic effect is drawn from the results that Zn-based compounds have better catalytic effects than other transition-metal-containing compounds and the compound constructed by Na⁺ has higher catalytic activity than that constructed by NH₄⁺. The mechanism studies show that the improvements of the catalytic performance are caused by the synergy between classic heptamolybdate anions and complex cations. ESI-MS data and UV-vis spectra revealed that the POM anions can form intermediate peroxomolybdenum units during catalytic reaction. Further, the combination of the substrate thioanisole with complex cations was characterized by NMR experiments and UV-vis spectra. Thus, a new synergistic mechanism of anions and cations is proposed in which the activated thioanisole is used as a nucleophile to attack the peroxomolybdenum bonds, and this provides a new strategy in the design of reliable POM-based catalysts.

Electrocatalytic oxidation of water at a polyoxometalate nanoparticle modified gold electrode

Polyoxometalate nanoparticles, [H₃PMo₁₂O₄₀]NPs, modified gold electrode showed excellent electrocatalytic activity towards water oxidation reaction at an overpotential of 350 mV with a current density of 1.7 mA cm⁻² in neutral pH medium.

Bioinspired Electrocatalytic CO2 Reduction by Bovine Serum Albumin-Capped Silver Nanoclusters Mediated by [α-SiW12O40 ](4-)

Silver nanoclusters capped with bovine serum albumin (AgNC@BSA) were synthesized and applied for the electrocatalytic reduction of CO2 . Inspired by the fact that many enzymes function only in the presence of coenzymes/cofactors that transfer electrons/protons or other groups, an electron transfer mediator was introduced to facilitate the electrical communication between the electrode and the strongly protected catalyst. The AgNC@BSA catalyst mediated by [α-SiW12 O40](4-) anions showed high electrocatalytic activity for CO2 reduction, which was not observed with either component on its own, in dimethylformamide containing 1 % (v/v) water. CO was the major product with excellent faradaic efficiency (>75 %). The onset potential for this catalytic CO2 reduction process is about 400 mV more positive than that found at a bulk silver electrode. This mediator-enhanced catalysis concept provides a general approach to investigate the electrocatalytic activities of nanoclusters, which remain largely unexplored.

Water splitting with polyoxometalate-treated photoanodes: enhancing performance through sensitizer design

Visible light driven water oxidation has been demonstrated at near-neutral pH using photoanodes based on nanoporous films of TiO₂, polyoxometalate (POM) water oxidation catalyst [{Ru₄O₄(OH)₂(H₂O)₄}(γ-SiW₁₀O₃₆)₂]^10- (1), and both known photosensitizer [Ru(bpy)₂(H₄dpbpy)]²⁺ (P2) and the novel crown ether functionalized dye [Ru(5-crownphen)₂(H₂dpbpy)](H₂2). Both triads, containing catalyst 1, and catalyst-free dyads, produce O₂ with high faradaic efficiencies (80 to 94%), but presence of catalyst enhances quantum yield by up to 190% (maximum 0.39%). New sensitizer H₂2 absorbs light more strongly than P2, and increases O₂ quantum yields by up to 270%. TiO₂-2 based photoelectrodes are also more stable to desorption of active species than TiO₂-P2: losses of catalyst 1 are halved when pH > TiO₂ point-of-zero charge (pzc), and losses of sensitizer reduced below the pzc (no catalyst is lost when pH < pzc). For the triads, quantum yields of O₂ are higher at pH 5.8 than at pH 7.2, opposing the trend observed for 1 under homogeneous conditions. This is ascribed to lower stability of the dye oxidized states at higher pH, and less efficient electron transfer to TiO₂, and is also consistent with the 4^th1-to-dye electron transfer limiting performance rather than catalyst TOF_max. Transient absorption reveals that TiO₂-2-1 has similar 1^st electron transfer dynamics to TiO₂-P2-1, with rapid (ps timescale) formation of long-lived TiO₂(e^-)-2-1(h⁺) charge separated states, and demonstrates that metallation of the crown ether groups (Na⁺/Mg²⁺) has little or no effect on electron transfer from 1 to 2. The most widely relevant findings of this study are therefore: (i) increased dye extinction coefficients and binding stability significantly improve performance in dye-sensitized water splitting systems; (ii) binding of POMs to electrode surfaces can be stabilized through use of recognition groups; (iii) the optimal homogeneous and TiO₂-bound operating pHs of a catalyst may not be the same; and (iv) dye-sensitized TiO₂ can oxidize water without a catalyst.

Electrocatalytic oxidation of water by a self-assembled oxovanadium(iv) complex modified gold electrode

An oxovanadium(iv) complex modified gold electrode showed efficient electrocatalytic activity towards water oxidation at neutral pH.