Oxidation of benzene to phenol by dioxygen over vanadium oxide nano-plate

Direct oxidation of benzene to phenol with dioxygen is an attractive and challenging subject. V⁴⁺ vanadium species play an important role in the reaction. Nano vanadium oxide which consists mainly of the quadrivalence vanadium species was synthesized via hydrothermal method and used to catalyze the reaction without addition of reducing reagent. Investigations about performances of catalysts show that 3.7% conversion of benzene and nearly 100% selectivity of phenol are given over the fresh catalyst VO-N-A. Morphology observations display that the VO-N-A is made of nano-plate. Thermogravimetric curve illuminates that the catalyst is stable under the reaction temperature.     

The First Neutral Dinuclear Vanadium Complex Comprising VO and VO2 Cores: Synthesis,Structure, Electrochemical Properties,and Catalytic Activity

A neutral dinuclear vanadium complex containing both oxido and dioxidovanadium cores with hydrazone based ligand, [VO(OCH₃)(CH₃OH)(HL)VO₂] ( 1 ) {H₄L = bis[(E)‐N′‐(5‐bromo‐2‐hydroxybenzylidene)]‐carbohydrazide}, was synthesized and fully characterized by X‐ray crystallography and spectroscopic methods (IR, UV/Vis, NMR). The ligand acts as a trinegative hexadentate N₃O₃ donor ligand to form a dinuclear complex and during the reaction V⁴⁺ is oxidized to V⁵⁺. The coordination polyhedra are a VO₅N distorted octahedron for the mono‐oxidovanadium core and a VO₃N₂ trigonal bipyramid for the dioxidovanadium core. The results of catalytic reactions indicate that 1 is a highly active catalyst in the clean epoxidation reaction of cis‐cyclooctene using aqueous hydrogen peroxide in acetonitrile. Cyclic voltammetric experiments of 1 in DMSO reveal two quasi‐reversible peaks due to the VO³⁺–VO²⁺ and VO₂⁺–VO₂ couples.     

Effect of direct ultrasound synthesis via a sesquihydrate route on bismuth-promoted vanadyl pyrophosphate catalysts

A series of 1, 3, and 5% Bi-doped vanadium phosphate catalyst catalysts were prepared via sesquihydrate route using direct ultrasound method and were denoted as VPSB1, VPSB3, and VPSB5, respectively. These catalysts were synthesized solely using a direct ultrasound technique and calcined in a n-butane/air mixture. This study showed that catalyst synthesis time can be drastically reduced to only 2 hr compared to conventional 32–48 hr. All Bi-doped catalysts exhibited a well-crystallized (VO)₂P₂O₇ phase. In addition, two V⁵⁺ phases, that is, β-VOPO₄ and α_II-VOPO₄, were observed leading to an increase in the average oxidation state of vanadium. All catalysts showed V2p_3/2 at approx. 517 eV, giving the vanadium oxidation state at approx. 4.3–4.6. Field-emission scanning electron microscopy micrographs showed the secondary structure consisting of thin and small plate-like crystal clusters due to the cavitation effect of ultrasound waves. VPSB5 showed the highest amount of oxygen species removed associated with the V⁵⁺ and V⁴⁺ species in temperature-programmed reduction in H₂ analyses. TheX-ray absorption near edge structure (XANES) measurement showed the occurrence of vanadium oxide reductions in hydrogen gas flow, indicating the presence of V⁴⁺ and V⁵⁺ species. Higher average valence states of V⁵⁺, indicating more V⁵⁺ phases, were present. The addition of bismuth has increased the activity and selectivity to maleic anhydride.     

A Novel Expansion Mode of Polyoxovanadate Clusters: Synthesis,Crystal Structure and Properties of {[Cu(H2O)(C5H14N2)2]2[V16O38(Cl)]}·4(C5H16N2)

The new polyoxovanadate (POV) compound {[Cu(H₂O)(C₅H₁₄N₂)₂]₂[V₁₆O₃₈(Cl)]} · 4(C₅H₁₆N₂) was synthesized under solvothermal conditions and crystallizes in the tetragonal space group I4₁/amd with a = 13.8679(6), c = 45.558(2) Å, V = 8761.7(7) ų. The central structural motif is a {V₁₆O₃₈(Cl)} cluster constructed by condensation of 16 square‐pyramidal VO₅ polyhedra. The cluster hosts a central Cl^– anion. According to valence bond sum calculations, chemical analysis and magnetic properties the cluster anion may be formulated as [V₁₅^IVV^VO₃₈(Cl)]^12–, i.e., only one vanadium atom is not reduced. To the best of our knowledge this is the first reported {V₁₆O₃₈(X)} cluster in this V^IV:V^V ratio. The presence of the two different vanadium oxidation states is clearly seen in the IR spectrum. An unusual and hitherto never observed structural feature is the binding mode between the [Cu(H₂O)(C₅H₁₄N₂)₂]²⁺ complexes and the [V₁₅^IVV^VO₃₈(Cl)]^12– anion. The Cu²⁺ ion binds to a μ₂‐O atom of the cluster anion whereas in all other transition metal complex‐augmented POVs bonding between the transition metal cation and the anion occurs through terminal oxygen atoms of the POV. The magnetic properties are dominated by strong antiferromagnetic exchange interactions between the V⁴⁺ d¹ centers, whereas the Cu²⁺ d⁹ cations are magnetically decoupled from the cluster anion. Upon heating, the title compound decomposes in a complex fashion.     

Understanding the nature of vanadium species supported on activated carbon and its catalytic properties in the aerobic oxidation of aromatic alcohols

Vanadium oxide catalysts supported on activated carbon (V/AC) with V loadings ranging from 1 to 20 wt.% were prepared by a wet-impregnation method. Various physicochemical characterization techniques, including nitrogen physisorption, X-ray diffraction (XRD), Raman spectroscopy, X-ray absorption (XANES and EXAFS), X-ray photoelectron spectroscopy (XPS), and electron spin resonance (ESR), were employed to understand the nature of vanadium species on activated carbon. The results revealed that vanadium oxide mainly existed in a highly dispersed state for 10 wt.% or less vanadium loadings; a large amount of vanadium resulted in aggregated microcrystalline phase. Vanadium species on activated carbon surface showed a similar local coordination structure to that of NH₄VO₃ with a distorted tetrahedral symmetry at low vanadium loadings, whereas octahedral coordination was dominant at high vanadium loadings (>10 wt.%). All V/AC samples showed V⁵⁺ as the major oxidation state. Nevertheless, V⁴⁺ centered in a distorted tetrahedral symmetry could be detected at a vanadium loading greater than 4 wt.%. The catalytic activity for the benzyl alcohol oxidation largely depended on the dispersion, oxidation state, and local coordination of vanadium oxides on activated carbon. Highly dispersed vanadium (5+) species with a distorted tetrahedral coordination were postulated to account for the excellent catalytic performances of V/AC catalysts (TOF = 39.1 h^?1).     

The Influence of Sulfate‐Doping on the Nature of V Sites in VOx/TiO2 Catalysts

EPR, UV/Vis and FTIR spectroscopy as well as thermal analysis (TA/MS) were applied to study the influence of sulfate species present in the anatase support on the specific nature of VO_x species in supported VO_x/TiO₂ catalysts. Those sulfate species modify the local structure of the supported vanadyl species and lead to the formation of two types of VO²⁺ sites instead of only one type being formed on sulfate‐free anatase. EPR and FTIR spectroscopic measurements revealed that a part of the VO²⁺ species are directly bound to the surface sulfate species. By TA/MS it was found that SO₂ is released at lower temperature from VO_x/TiO₂ in comparison to the vanadium‐free support. The direct bonding between sulfate and VO_x species stabilizes the latter on the surface of VO_x/TiO₂ resulting in three effects: 1) a higher V site dispersion in comparison to sulfate‐free TiO₂, 2) a better resistance of surface vanadyls against diffusion into the bulk of the support and 3) a much faster reoxidation of reduced V sites than observed on sulfate‐free TiO₂.     

Supported MgO–V<Subscript>2</Subscript>O<Subscript>5</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts for oxidative propane dehydration: Effect of the molar Mg : V ratio on the phase composition and catalytic properties of samples

The physicochemical properties of V₂O₅/Al₂O₃ and MgO–V₂O₅/Al₂O₃ supported catalysts (Mg : V = 1 : 1, 2 : 1, and 3 : 2) obtained by consecutive impregnation of the support with solutions of vanadium and magnesium precursors are studied using a complex of mutually complementary methods (XRD, Raman spectroscopy, UV–Vis spectrometry, and TPR-H₂). The effect of the formation of surface magnesium vanadates of various composition and structure on the catalytic properties of the supported vanadium oxide catalysts in the oxidative dehydrogenation of propane is studied. The introduction of magnesium in the samples and an increase in its content, accompanied by a change in the structure of the surface vanadium oxide phases from polymeric VO₆/VO₅ species to surface metavanadate species, magnesium metavanadate, and further to magnesium divanadate, significantly affects their catalytic properties in the reaction of the oxidative dehydrogenation of propane to propylene.     

Bioanorganische Chemie des Vanadiums. Leben ohne Vanadium?

Vanadium is used by microorganisms as an electron acceptor in respiration, and as an essential transition metal in enzymatic reactions. An example for the employment in respiratory function is the soil bacterium Shewanella, which reduces vanadate(V) to oxidovanadium(IV). Examples for enzymatic reactions are the nitrogen fixation (by the proteobacterium Azotobacter and the cyanobacterium Anabaena), and the two‐electron oxidation of halide X^– to a species {X⁺} by marine macro‐algae, fungi and lichen. In vanadium nitrogenase, vanadium is constituent of a {Fe₇VS₉} cluster, in vanadate‐dependent haloperoxidases it is present in the form of H₂VO₄^– bound to a histidyl residue of the protein matrix. Mushrooms of the genus Amanita store vanadium in the form of amavadin, a “bare” (non‐oxo) vanadium(IV) complex. Several sea squirts and fan worms accumulate vanadium from sea water and store it as an aqua complex of vanadium(III). “Tailored” vanadium complexes with organic ligands have been shown to be active as insulin‐mimics in vivo and in vitro: They are able to stimulate the cellular uptake of glucose and to inhibit the degradation of lipids. These functions are related to the phosphate‐vanadate antagonism.     

CeVO4 Nanozymes Catalyze the Reduction of Dioxygen to Water without Releasing Partially Reduced Oxygen Species

In this study, we report a remarkably active CeVO₄ nanozyme that functionally mimics cytochrome c oxidase (CcO), the terminal enzyme in the respiratory electron transport chain, by catalyzing a four‐electron reduction of dioxygen to water. The nanozyme catalyzes the reaction by using cytochrome c (Cyt c), the biological electron donor for CcO, at physiologically relevant pH. The CcO activity of the CeVO₄ nanozymes depends on the relative ratio of surface Ce³⁺/Ce⁴⁺ ions, the presence of V⁵⁺ and the surface‐Cyt c interactions. The complete reduction of oxygen to water takes place without release of any partially reduced oxygen species (PROS) such as superoxide, peroxide and hydroxyl radicals.     

A non‐oxo methanolate‐bridged divanadium(IV) complex with tris(2‐sulfanidylphenyl)phosphane ligands: synthesis,structural characterization and magnetic investigation

Vanadium chemistry is of interest due its biological relevance and medical applications. In particular, the interactions of high‐valent vanadium ions with sulfur‐containing biologically important molecules, such as cysteine and glutathione, might be related to the redox conversion of vanadium in ascidians, the function of amavadin (a vanadium‐containing anion) and the antidiabetic behaviour of vanadium compounds. A mechanistic understanding of these aspects is important. In an effort to investigate high‐valent vanadium–sulfur chemistry, we have synthesized and characterized the non‐oxo divanadium(IV) complex salt tetraphenylphosphonium tri‐μ‐<!?tlsb=‐0.11pt>methanolato‐κ⁶O:O‐bis({tris[2‐sulfanidyl‐3‐(trimethylsilyl)phenyl]phosphane‐κ⁴P,S,S′,S′′}vanadium(IV)) methanol disolvate, (C₂₄H₂₀P)[V^IV₂(μ‐OCH₃)₃(C₂₇H₃₆PS₃)₂]·2CH₃OH. Two V^IV metal centres are bridged by three methanolate ligands, giving a C2‐symmetric V₂(μ‐OMe)₃ core structure. Each V^IV centre adopts a monocapped trigonal antiprismatic geometry, with the P atom situated in the capping position and the three S atoms and three O atoms forming two triangular faces of the trigonal antiprism. The magnetic data indicate a paramagnetic nature of the salt, with an S = 1 spin state.     

掺Ni钒磷氧化物催化剂的合成和表征

以VOPO4.2H2O为原料制备了钒磷氧化物催化剂,考察了镍掺杂(1%,2%和5%)对该催化剂的影响.低掺杂量的Ni明显提高了活性晶格氧物种O-的数量,降低了V5 和V4 相的还原峰温.粉末X射线衍射、程序升温还原和化学分析结果表明,高掺杂量的Ni促使V5 物相生成并抑制V4 物相出现.高含量与V5 相关的氧物种会降低正丁烷的转化率,但会提高马来酸酐的选择性.     

Synergistic Catalysis of SnO2/Reduced Graphene Oxide for VO2+/VO2+ and V2+/V3+ Redox Reactions

In spite of their low cost, high activity, and diversity, metal oxide catalysts have not been widely applied in vanadium redox reactions due to their poor conductivity and low surface area. Herein, SnO₂/reduced graphene oxide (SnO₂/rGO) composite was prepared by a sol–gel method followed by high-temperature carbonization. SnO₂/rGO shows better electrochemical catalysis for both redox reactions of VO²⁺/VO₂⁺ and V²⁺/V³⁺ couples as compared to SnO₂ and graphene oxide. This is attributed to the fact that reduced graphene oxide is employed as carbon support featuring excellent conductivity and a large surface area, which offers fast electron transfer and a large reaction place towards vanadium redox reaction. Moreover, SnO₂ has excellent electrochemical activity and wettability, which also boost the electrochemical kinetics of redox reaction. In brief, the electrochemical properties for vanadium redox reactions are boosted in terms of diffusion, charge transfer, and electron transport processes systematically. Next, SnO₂/rGO can increase the energy storage performance of cells, including higher discharge electrolyte utilization and lower electrochemical polarization. At 150 mA cm⁻², the energy efficiency of a modified cell is 69.8%, which is increased by 5.7% compared with a pristine one. This work provides a promising method to develop composite catalysts of carbon materials and metal oxide for vanadium redox reactions.     

(NH4)3[VO2(SO4)2(H2O)2]·1.5H2O

The title compound, tri­ammonium cis‐di­aqua‐cis‐dioxo‐trans‐disulfatovanadate 1.5‐hydrate, was obtained by oxidizing V^IV to V^V in a 2 M sulfuric acid solution of vanadyl­ sulfate and adding ammonium sulfate. Here, the V atom is sandwiched by two sulfate groups by corner‐sharing to form a discrete [VO₂(SO₄)₂(OH₂)₂]^3? anion. The water mol­ecules occupy cis positions in the equatorial plane of the vanadium octahedron.     

High‐Field Pulsed EPR Spectroscopy for the Speciation of the Reduced [PV2Mo10O40]6− Polyoxometalate Catalyst Used in Electron‐Transfer Oxidations

An in‐depth spectroscopic EPR investigation of a key intermediate, formally notated as [PV^IVV^VMo₁₀O₄₀]^6? and formed in known electron‐transfer and electron‐transfer/oxygen‐transfer reactions catalyzed by H₅PV₂Mo₁₀O₄₀, has been carried out. Pulsed EPR spectroscopy have been utilized: specifically, W‐band electron–electron double resonance (ELDOR)‐detected NMR and two‐dimensional (2D) hyperfine sub‐level correlation (HYSCORE) measurements, which resolved ⁹⁵Mo and ¹⁷O hyperfine interactions, and electron–nuclear double resonance (ENDOR), which gave the weak ⁵¹V and ³¹P interactions. In this way, two paramagnetic species related to [PV^IVV^VMo₁₀O₄₀]^6? were identified. The first species (30–35 %) has a vanadyl (VO²⁺)‐like EPR spectrum and is not situated within the polyoxometalate cluster. Here the VO²⁺ was suggested to be supported on the Keggin cluster and can be represented as an ion pair, [PV^VMo₁₀O₃₉]^8?[V^IVO²⁺]. This species originates from the parent H₅PV₂Mo₁₀O₄₀ in which the vanadium atoms are nearest neighbors and it is suggested that this isomer is more likely to be reactive in electron‐transfer/oxygen‐transfer reaction oxidation reactions. In the second (70–65 %) species, the V^IV remains embedded within the polyoxometalate framework and originates from reduction of distal H₅PV₂Mo₁₀O₄₀ isomers to yield an intact cluster, [PV^IVV^VMo₁₀O₄₀]^6?.     

Transport properties of vanadium ions in cation exchange membranes:: Determination of diffusion coefficients using a dialysis cell

Diffusion coefficients of vanadium ions in cation exchange membranes are of interest because they allow to calculate the ion exchange across the membrane in an all vanadium redox flow battery which leads to undesired cross contamination and energy losses in the battery system. Diffusion coefficients of V²⁺, V³⁺, VO²⁺ and VO⁺₂ ions in CMS, CMV and CMX cation exchange membranes have been determined by measuring the ion exchange fluxes of these ions with H₃O⁺ ions using a dialysis cell. The experimental data are evaluated on the basis of integrated flux equations which require also ion exchange sorption equilibria obtained already in previous work. The lowest diffusion coefficients are observed in the CMS membrane for all vanadium ions. This membrane turns out to be the most suitable one for being applied in a vanadium battery since it is expected to prevent most effectively cross contamination of vanadium ions.     

V2O5/TiO2 Catalytic Xerogels Raman and EPR Studies

Raman spectroscopy and Electron Paramagnetic Resonance (EPR) studies were performed on a series of V₂O₅/TiO₂ catalysts prepared by a modified sol-gel method in order to identify the vanadium species. Two species of surface vanadium were identified by Raman measurements, monomeric vanadyls and polymeric vanadates. Monomeric vanadyls are characterized by a narrow Raman band at 1030 cm^–1 and polymeric vanadates by two broad bands in the region from 900 to 960 cm^–1 and 770 to 850 cm^–1. The Raman spectra do not exhibit characteristic peaks of crystalline V₂O₅. These results are in agreement with those of X-ray Diffractometry (XRD) and Fourier Transform Infrared (FT-IR) previously reported (C.B. Rodella et al., J. Sol-Gel Sci. Techn., submitted). At least three families of V⁴⁺ ions were identified by EPR investigations. The analysis of the EPR spectra suggests that isolated V⁴⁺ ions are located in sites with octahedral symmetry substituting for Ti⁴⁺ ions in the rutile structure. Magnetically interacting V⁴⁺ ions are also present as pairs or clusters giving rise to a broad and structureless EPR line. At higher concentration of V₂O₅, a partial oxidation of V⁴⁺ to V⁵⁺ is apparent from the EPR results.     

CO Oxidation Promoted by the Gold Dimer in Au2VO3− and Au2VO4− Clusters

Investigations on the reactivity of atomic clusters have led to the identification of the elementary steps involved in catalytic CO oxidation, a prototypical reaction in heterogeneous catalysis. The atomic oxygen species O^.? and O^2? bonded to early‐transition‐metal oxide clusters have been shown to oxidize CO. This study reports that when an Au₂ dimer is incorporated within the cluster, the molecular oxygen species O₂^2? bonded to vanadium can be activated to oxidize CO under thermal collision conditions. The gold dimer was doped into Au₂VO₄^? cluster ions which then reacted with CO in an ion‐trap reactor to produce Au₂VO₃^? and then Au₂VO₂^?. The dynamic nature of gold in terms of electron storage and release promotes CO oxidation and O? O bond reduction. The oxidation of CO by atomic clusters in this study parallels similar behavior reported for the oxidation of CO by supported gold catalysts.     

Physico-chemicals and catalytic properties of manganese-promoted vanadium phosphate (VPO) catalyst

The addition of 1% Mn promoter to vanadium phosphate catalyst led to doubling of the specific surface area from 20.3 (unpromoted) to 39.4 m² g⁻¹. The XRD pattern of the Mn-promoted catalyst gave only the characteristics of the (VO)₂P₂O₇ phase, indicating that the Mn was incorporated into the crystal lattice of the catalyst. The Mn-promoted catalyst was also twice as active in removing the total amount of oxygen. However, since the only oxygen species related to V⁴⁺ being removed and no oxygen species associated with V⁵⁺ was observed, the n-butane conversion was not much improved as compared to the unpromoted counterpart. A necessary amount and distribution of the V⁵⁺ phase in a well crystalline V⁴⁺ phase is essential in order to enhance the catalytic performance in the mild oxidation of n-butane to maleic anhydride.     

Substrate temperature effects on the structural,compositional, and electrical properties of VO2 thin films deposited by pulsed laser deposition

The vanadium dioxide (VO₂) thin films were deposited on silicon (100) substrate using the pulsed laser deposition technique. The thin films were deposited at different substrate temperatures (500°C, 600°C, 700°C, and 800°C) while keeping all the other parameters constant. X‐ray diffraction confirmed the crystalline VO₂ (B) and VO₂ (M) phase formation at different substrate temperatures. X‐ray photoelectron spectroscopy analysis showed the presence of V⁴⁺ and V⁵⁺ charge states in all the deposited thin films which confirms that the deposited films mainly consist of VO₂ and V₂O₅. An increase in the VO₂/V₂O₅ ratio has been observed in the films deposited at higher substrate temperatures (700°C and 800°C). Scanning electron microscope micrographs revealed different surface morphologies of the thin films deposited at different substrate temperatures. The electrical properties showed the sharp semiconductor to metal transition behavior with approximately 2 orders of magnitude for the VO₂ thin film deposited at 800°C. The transition temperature for heating and cooling cycles as low as 46.2°C and 42°C, respectively, has been observed which is related to the smaller difference in the interplanar spacing between the as‐deposited thin film and the standard rutile VO₂ as well as to the lattice strain of approximately −1.2%.