Number Of Vanadium Atoms Research Articles

Adsorption and activation of CO2 molecule on subnanometer-sized anionic vanadium carbide clusters VnC4− (n = 1–6): A theoretical study

Adsorption and activation of CO2 onto small-sized transition metal clusters is of practical importance for the conversion of this molecule to value-added products. We study the efficacy of small anionic vanadium carbide clusters, VnC4− with n = 1–6 towards the aforementioned key step by employing density functional theory based method. One of such clusters has been shown to perform very well in activating N2 molecule as revealed in a recent experiment. The results of our calculations suggest that the binding of CO2 molecule with the anionic vanadium carbide clusters becomes stronger with increase in number of vanadium atoms in the cluster. This adsorption energy trend can be properly rationalized by using the d-band center model. The strong adsorption of CO2 molecule is also accompanied with its high degree of activation. The high activation of CO2 is characterized by a significant amount of charge transfer from the anionic clusters to the molecule. We further find that the activation barrier heights for the dissociation of CO2 to CO and O fragments are considerably small for V4C4− and V5C4− clusters. In particular, V5C4− cluster with an activation barrier of 0.83 eV may serve as a good candidate for activation and dissociation of a CO2 molecule.

Catalysts Based on High-Vanadium Solutions of Molybdovanadophosphoric Heteropolyacids: Achievements, Challenges, and Prospects

A retrospective review based on the authors’ work is devoted to homogeneous catalytic systems based on aqueous solutions of Mo–V–P heteropolyacids (HPA-x, x is the number of vanadium atoms). These catalytic systems were first proposed for oxidative reactions at Boreskov Institute of Catalysis. The properties of HPA-x solutions, methods for their synthesis, promising processes occurring in their presence, problems related to the use of these solutions, and ways of solving these problems are considered. Prospects for the creation of industrial processes based on modified high-vanadium HPA-x solutions of non-Keggin compositions are shown.

Diene Synthesis and One-Pot Processes in the Presence of Solutions of Mo–V–P Heteropoly Acids

This review—which is based on the authors’ original studies—is devoted to application of aqueous solutions of Mo–V–P heteropoly acids (HPA-x, where x is the number of vanadium atoms) as bifunctional (acidic and oxidative) catalysts. The focus is on the development of one-pot processes in which the acid-catalyzed reaction of diene synthesis is combined with the oxidation of the produced adducts. Prospects for the design of a number of industrial processes for the production of substituted 1,4-naphtho- and 9,10-anthraquinones in the presence of solutions of modified non-Keggin HPA-x are shown.

Hybrid catalysts of molybdovanadophosphoric acid and g-C3N4 with tunable bandgaps.

The integration of semiconductors and polyoxometalates provides promising benefits for the rational tuning of hybrid materials' electronic band structures; however, the intrinsic influence of certain hybridization approaches on the resulting bandgaps of their complexes has seldom been noted. Herein, graphitic carbon nitride and a series of phosphovanadomolybdates (H3+xPMo12-xVxO40, x = 0-3) have been complexed through electrostatic charge attraction, and their optical and electronic properties are fully explored to investigate the effect of minor variations of the polyoxometalate structures on the hybrid bandgaps and electronic structures. The conduction band edge of the hybrids increases along with the expansion of the number of vanadium centers in the phosphovanadomolybdate, providing potential guidance for the design of hybrid catalysts.

Novel catalyst based on mono- and di-vanadium substituted Keggin polyoxometalate incorporated in poly(acrylic acid-co-acrylamide) polymer for the oxidation of sulfides

Composite materials based on [PVxW12-xO40](3+x)− with x = 1 or 2 (PVW and PV2W respectively), included in poly(acrylic acid-co-acrylamide) gel (PAACA), were synthesized. The samples were characterized by different techniques such as FT-IR, 31P MAS-NMR, 51V-NMR, XRD, DTA-TGA, UV–vis DRS, and the acidic properties were estimated by means of potentiometric titration with n-butylamine. Samples containing 10, 20 and 30% (w/w) of polyoxotungstovanadate (POTV) were prepared by inclusion of the POTV in the polymer during its synthesis. According to Fourier transform infra-red and magic angle spinning-nuclear magnetic resonance studies, the predominat anion present in the samples is [PVxW12-xO40](3+x)−, and there is no evidence of its decomposition during the synthesis of hybrid materials and the drying step. According to XRD results, these anions are greatly dispersed in the PAACA or present as amorphous phases.UV–vis DRS data reveal that the samples synthesized using POTV with two vanadium atoms (PAACA-PV2W) exhibit lower values of absorption edge energy than those prepared using PVW (PAACA-PVW), which correlates with a higher oxidizing capacity.The potentiometric titration shows strong acid sites of the hybrid materials, and their number increases with the number of vanadium atoms and with the amount of POTV incorporated in the PAACA grid.The hybrid materials prepared by inclusion of POTV during the polymer synthesis exhibit appropriate physicochemical features to catalyze the oxidation of diphenyl sulfide (DPS) to its sulfone employing acetonitrile as solvent H2O2 as a green oxidant. The samples with 30% w/w of POVT, which show higher catalytic performance, are suitable for the DPS oxidation and can be reused without remarkable drop of their catalytic activity. Furthermore, they show high activity as a catalyst in the oxidation reaction of 4,4´-diamino diphenyl sulfide to the corresponding sulfone (dapsone) used for malaria treatment.

Isomer distribution in α-Keggin structures [XW12−nVnO40]−(q+n) X = Si, P (0 ≤ n ≤ 4): A DFT study of free energy and vibrational spectra

Potassium salts of Keggin anions with tungsten and vanadium as addenda atoms ([α-PV n W (12− n ) O 40 ] (3+ n )− 0 ≤ n ≤ 4 and [α-SiV n W (X− n ) O 40 ] (4+ n )− 0 ≤ n ≤ 3) were prepared and analyzed by infrared and Raman spectroscopy. Geometry optimization at a Density Functional Theory (DFT) level was performed and vibrational spectra were obtained for all 54 possible isomers of these anions. The results have been improved by using Polarizable Continuum Model (PCM) to simulate solvent effects. Gibbs free energy values were calculated for all the members of the series. The Boltzmann distribution of the isomers with a given number of vanadium atoms was calculated. The results suggest that (i) the most stable state corresponds to a homogeneous distribution of vanadium between Keggin species and (ii) potential relations between synthetic route and isomer distribution in α-Keggin ions are unjustified on thermodynamic grounds. Calculated Raman spectra are in overall agreement with the experimental data. However, specific bands associated to vanadium substitution are predicted but are not experimentally observed. Les sels de potassium de différents ions de Keggin contenant du tungstène et du vanadium comme additifs ([α-PV n W (12− n ) O 40 ] (3+ n )− 0 ≤ n ≤ 4 et [α-SiV n W (X− n ) O 40 ] (4+ n )− 0 ≤ n ≤ 3) ont été synthétisés et analysés par spectroscopies infrarouge et Raman. La géométrie a été optimisée et les spectres vibrationnels ont été determinés par la méthode DFT pour les 54 isomères possibles de ces ions, l'utilisation de PCM pour simuler les effets de solvant ayant permis d'améliorer les résultats. Les valeurs de l'énergie libre de Gibbs ont été calculées pour chacun des isomères, ainsi que la distribution de Boltzmann pour les isomères associés à un nombre donné d'atomes de vanadium. Les résultats indiquent que l'état le plus stable correspond à la repartition homogène du vanadium entre les ions de Keggin. Une relation éventuelle entre la méthode de synthèse et la distribution des isomères α des ions de Keggin demeure injustifiée sur une base thermodynamique. Les spectres Raman calculés sont globalement en accord avec les données expérimentales. Cependant, certaines bandes sécifiquement dues à la substitution du tungstène par le vanadium sont prévues, mais ne sont pas observées.

Inhibitory effect of H 3+x PMo 12−x V x O 40 -T on the self-polymerization of methyl methacrylate

In this work, a series of molybdovanadophosphoric heteropoly acid quaternary ammonium salts (H3 + xPMo12 − xVxO40-T) were synthesized and employed as a reaction inhibitor in the self-polymerization of methyl methacrylate (MMA). The polymerization inhibition effect of H3 + xPMo12 − xVxO40-T with different number of vanadium atoms and reaction dosages was investigated using differential scanning calorimetry (DSC). It shows that the inhibitory effect was improved with the increasing dosages of H3 + xPMo12 − xVxO40-T, and the polymerization inhibition was also affected by the number of vanadium atoms in the H3 + xPMo12 − xVxO40-T. Furthermore, cyclic voltammograms (CV) was used to probe the mechanism of the inhibition reaction with H3 + xPMo12xVxO40-T. The result of CV indicates that the inhibition reaction is an oxidation–reduction reaction. H3 + xPMo12 − xVxO40-T can react directly with the MMA monomer radicals, which eliminated the MMA monomers, and therefore the self-polymerization of the MMA can be effectively inhibited by H3 + xPMo12 − xVxO40-T.

Термодинамическое описание процесса сорбции ванадия на углеродсодержащем сорбенте

The paper describes a method of vanadium adsorption from acidic solutions on a highly-developed surface of the activated charcoal, modified by cationic surfactants. It is estimated that in an acidic media vanadium polyanions are mainly sorbed. It was proved that ions of copper, nickel, iron, calcium, magnesium, sodium and potassium do not interfere the sorption extraction of vanadium from the aqueous solution. The best values of vanadium sorption on the modified charcoal sorbent are obtained in the pH range from 2 to 4, i.e. in the acidic medium. In the acidic medium the hydroxides of other metals do not form the precipitate, thus enabling to extract the pure product. The reduction of the vanadium sorption with the increase of pH is connected with the fact that vanadium polyanions to which polyacids Н 2 V 4 O 11 , Н 6 V 10 O 28 , H 4 V 6 O 17 correspond, are decomposed into anions to which acids Н 4 V 2 O 7 , HVO 3 , H 3 VO 4 correspond. Therefore, one sorption center captures an anion containing a fewer number of vanadium atoms. Filling the sorbent surface with vanadium compounds is similar to the Langmuir type sorption enabling to make thermodynamic description of this process and to determine main energy components. Thermodynamic studies showed that the extraction of vanadium corresponds to physical adsorption of polyanions on a positively charged surface of charcoal (the degree of vanadium extraction is about 84%). Physical adsorption is confirmed by the fact that the D G of the reaction decreases with the increasing temperature, and the heat effect of the reaction does not exceed 13 kJ/mol. Additionally saturated sorbents were studied using X-ray diffraction. Phases of vanadium oxide V 2 O 5 and a composite oxide of vanadium and manganese MnO∙V 2 O 5 . were detected on the X-ray photograph. Thus, the end product of calcination of saturated sorbent is vanadium pentoxide doped with manganese. Calcination of saturated sorbent together with a reducing agent enable to obtain metallic vanadium, the final product purity being about 99%.

Electronic structure and elasticity of Z-phases in the Cr–Nb–V–N system

Structural properties and energetics of Cr-based Z-phases (CrNbN, Cr(Nb,V)N and CrVN) were investigated using the Vienna ab initio simulation package (VASP) code employing the projector augmented wave (PAW) pseudopotentials by means of both local density approximation (LDA) and generalized gradient approximation (GGA) for the exchange and correlation term. The geometry of all studied phases including NbN, VN and elemental constituents (nonmagnetic bcc Nb and V and antiferromagnetic bcc Cr) was fully relaxed, providing the equilibrium structure parameters and total energies. The calculated lattice parameters of Z-phases correspond very well to the experimental data and decrease with increasing molar fraction of vanadium. Enthalpies of formation show that all three Z-phases are stable at T = 0 K. The electronic structures of Z-phases including densities of states and charge densities were analysed. The calculated bulk moduli and elastic constants were used to evaluate stability conditions and elastic anisotropy ratios. It was confirmed that Z-phases are mechanically stable. Additional information on ductility was obtained from Cauchy pressures, Pugh ratios, Young moduli, and Poisson ratios. The ductility evaluated using the Pugh ratio decreases with number of vanadium atoms.

Stereochemistry of vanadium in oxygen-containing compounds

Using Voronoi-Dirichlet (VD) polyhedra and the method of overlapping spheres, we analyze the coordination of 714 crystallographically different vanadium atoms in the structure of compounds containing VO n polyhedra. Vanadium atoms are found to be bonded to 4, 5, 6, or 7 oxygen atoms. The effect of the valence state and coordination number of vanadium atoms on the main parameters of their VD polyhedra is considered. A unified linear dependence between the solid angles of VD polyhedron faces corresponding to V-O bonds and the respective interatomic distances varying in a range of 1.55 A to 2.79 A is stated to exist. It is shown that the parameters of VD polyhedra can be used to determine the valence state of vanadium atoms.

Total oxidation of Cl-containing VOCs over mixed heteropoly compounds derived catalysts

Catalysts derived from mixed PMoV and PWV heteropoly compounds with Keggin structure supported on titania were prepared. The samples were characterized by the S BET method, Fourier transform infrared spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The effect of the temperature pretreatment of the samples, the number of vanadium atoms in the starting heteropolyanions and the nature of the peripheral atom on the surface properties and catalytic behavior of the samples in total oxidation of Cl–benzene was studied. The results provided clear evidence for the Keggin unit preservation in the heteropolyanions for pretreatments at temperature not exceeding 723 K. It was shown that the high-temperature (723 K) pretreated catalysts and/or replacement of W or Mo in the heterepoly anions with V atoms lead to an increase in the catalytic activity due to the redox capability of vanadium centers.

Influence of Ni doping on vanadium oxide/hexadecylamine multiwall nanotubes

In this work, we present a characterization of a hybrid nanocomposite constituted by vanadium oxide (VO x )/ hexadecylamine (Hexa) and the effect of the addition of Ni ions on the magnetic properties. Micrographs of transmission electron microscopy (TEM) show that the nanostructures are multiwall nanotubes. The walls are constituted by the successive intercalation of VO x and Hexa layers. The number of vanadium atoms and the ratio Ni/V were measured by instrumental neutron activation analysis (INAA) and by energy dispersive spectroscopy (EDS), respectively. Measurements of electronic paramagnetic resonance (EPR) and DC magnetic susceptibility, as a function of temperature, reveal that the Ni doping reduces considerably the amount of V 4+ present in the system.

Infrared and Computational Investigation of Vanadium‐Substituted Keggin [PVnW12‐nO40](n+3)‐ Polyoxometallic Anions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Infrared and Computational Investigation of Vanadium-Substituted Keggin [PVnW12-nO40](n+3)- Polyoxometallic Anions

This study investigates complex salts of [PW(12)O(40)](3-), [PVW(11)O(40)](4-), [PV(2)W(10)O(40)](5-), and [PV(3)W(9)O(40)](6-) Keggin anions using Fourier transform infrared spectroscopy under ambient atmospheric and high-vacuum conditions and by computational methods. A detailed vibrational study suggests that the presence and amount of vanadium in these anions can be tested quickly and reliably with the help of infrared spectroscopy because the salts of these anions exhibit distinctly different infrared absorption signatures (particularly in the 1025-1250 cm(-1) region, where the P-O stretch is surrounded by a set of complex vibrations involving V-O and W-O bonds). Investigation of these salts with different countercations suggests that the overall appearance of the spectra, and thus the ability to use infrared as a diagnostic tool to identify the presence and number of vanadium atoms in Keggin structure anions, is not affected significantly. Last, it is shown that all the structures investigated are extremely stable across a wide range of temperatures.

Ferromagnetism in one-dimensional vanadium-benzene sandwich clusters.

A molecular beam of multilayer vanadium-benzene organometallic complexes Vn(C6H6)m was produced by a laser vaporization synthesis method. The magnetic moments of the complexes were measured by a molecular beam magnetic deflection technique, and were found to increase with the number of vanadium atoms in the cluster, showing that the unpaired electrons, which occupy the nonbonding dsigma orbitals localized on the metal atoms, couple ferromagnetically. These sandwich species represent a new class of one-dimensional molecular magnets in which the transition metal atoms are formally zerovalent.

Formation, structure and bond dissociation thresholds of gas-phase vanadium oxide cluster ions

The formation and structure of gas-phase vanadium oxide cluster anions are examined using a guided ion beam mass spectrometer coupled with a laser vaporization source. The dominant peaks in the anion total mass distribution correspond to clusters having stoichiometries of the form (VO2)n(VO3)m(O2)q−. Collision-induced dissociation studies of the vanadium oxide species V2O4–6−, V3O6–9−, V4O8–10−, V5O11–13−, V6O13–15−, and V7O16–18− indicate that VO2, VO3, and V2O5 units are the main building blocks of these clusters. There are many similarities between the anion mass distribution and that of the cation distribution studied previously. The principal difference is a shift to higher oxygen content by one additional oxygen atom for the stoichiometric anions (VxOy−) as compared to the cations with the same number of vanadium atoms, which is attributed to the extra pair of electrons of the anionic species. The oxygen-rich clusters, VxOy(O2)−, are shown to more tightly adsorb molecular oxygen than those of the corresponding cationic clusters. In addition, the bond dissociation thresholds for the vanadium oxide clusters ΔE(V+–O)=6.09±0.28 eV, ΔE(OV+–O)=3.51±0.36 eV, and ΔE(O2V−–O)=5.43±0.31 eV are determined from the energy-dependent collision-induced dissociation cross sections with Xe as the collision partner. To the best of our knowledge, this is the first bond dissociation energy reported for the breaking of the V–O bond of a vanadium oxide anion.