Valence State Of Vanadium Research Articles

Catalytic and spectroscopic studies of Ag 1.2V 3Ce yO 8+ x catalysts for oxidation of toluene

The effects on structure and chemical composition of adding silver and cerium oxide to vanadium oxide catalysts with the general formulae Ag 12V 3Ce y O 8+ x , were investigated by measurements with BET, XRD, ESCA, SIMS, and titration of vanadium valence states. These properties were correlated with the catalytic performance in oxidation of toluene. The addition of silver results in multiphasic systems and in a considerably increased V 4+ concentration of both fresh and used catalysts. The catalysts consist of the β, ϵ, and σ silver-vanadium oxide bronzes. With reduction during use, x varies from 0 to -0.5 and transformations of these phases occur. The surface is considerably more reduced than the bulk. The degree of reduction of used catalysts increases with the addition of cerium oxide. No cerium phase is identified by XRD for used catalysts, but possibly CeVO 4 is present in fresh catalyst. The surface cerium concentration is considerably higher than the nominal bulk composition when Ce/V ≤ 0.02, but lower at higher Ce/V ratios. Activity for toluene oxidation increases with the addition of silver. This is also generally the case for cerium oxide additions. However, surface enrichment of cerium upon use at high temperatures gives lower activity. Selectivity for mainly benzaldehyde increases with the addition of silver, while oxidative coupling products are almost absent. Cerium addition further increases selectivity for benzaldehyde and benzoic acid, the latter almost linearly with cerium concentration.

Surface structure and reactivity of VTiO catalysts prepared by solid-state reaction 2. Nature of the active phase formed during o-xylene oxidation

The modifications occurring to V 2O 5 crystallites supported on TiO 2 (anatase or rutile) in contact with an o-xylene/air flow at about 600 K are characterized by determining the change in the valence state of vanadium through chemical analyses, the variation in the vanadium coordination environment through FT-IR and solid-state 51V NMR spectroscopies, and the modification in the nature and distribution of the vanadium-oxide phase by means of XRD and XPS analyses. Results are analyzed with reference to the time-on-stream modifications in the catalytic behavior in o-xylene oxidation to phthalic anhydride of these catalysts and to the catalytic behavior and physicochemical characteristics of V 3O 7 supported on TiO 2 and of an unsupported partially reduced vanadium-oxide phase. On anatase samples, the TiO 2 surface is covered homogeneously by a V IVV V partially hydrated mono- or bilayer. Overlaying this phase a hydrated V 3O 7-like phase (V V: V IV ratio of about 2 : 1) is also present as amorphous multilayer patches. In rutile samples, the former phase is no longer present, but instead islands of partially oxidized V 2O 4 are found together with the same partially reduced V-oxide phase as in the anatase samples. Catalytic results indicate that both phases have roughly similar catalytic behaviors and that the role of the TiO 2 support both in the anatase and rutile forms is to increase the number of surface sites, but not to modify their nature which depends only on the modifications occurring during the catalytic tests.

Catalytic oxidation of toluene over V2O5-WO3 catalysts

The catalytic performance of V2O5-WO3 catalysts in the oxidation of toluene was investigated and correlated with features shown by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and chemical analysis of vanadium valence states. At low vanadium concentrations high selectivity and activity was obtained and these catalysts consisted of highly dispersed V2O5 supported on WO3. V4+ formation upon use was highest in this region. Increased vanadium concentrations resulted in decreased dispersion with formation of less active and less reducible crystallites, and both activity and selectivity decreased towards values for V2O5. Dissolution of W6+, increasing with WO3 concentration, affects crystallization of V2O5. The (010) to (101) ratio, from XRD data, increases at the lowest addition levels, but decreases strongly at higher levels. Activity and selectivity are not simply correlated with this ratio, but overall are inversely correlated with it. Activity and selectivity correlate with the increase in the V4+-to-ΣV ratio with use in the reaction, indicating the importance of reduced phases and surface defects.

Laser-produced reduction of pentavalent vanadium in aqueous solutions and V2O5 powder

The reduction of pentavalent vanadium ions in aqueous solutions upon laser irradiation has been confirmed by using optical absorption and fluorescence techniques. X-ray photoelectron spectroscopy (XPS) study revealed that V2O5 was reduced to lower oxides upon laser treatment. Furthermore, the presence of reduced valence states of vanadium has been detected through XPS in vanadium-containing solids such as V2O5/P2O5 glass, thin films prepared separately from V2O5 powder and the glass.

XPS, auger, electrical and optical studies of vanadium phosphate glasses doped with nickel oxide

X-ray photoelectron and X-ray excited Auger spectra, optical absorption and DC conductivity of vanadium phosphate glasses containing nickel oxide have been studied as a function of nickel content. The shift in the binding energy of the V 2 p 3 2 photoelectron peak with the change in nickel content provided evidence of a change in the valence states of vanadium. The Ni 2 p 3 2 core level in NiO powder is a doublet and is normally used as a fingerprint for NiO. However, a single peak, shifted to higher binding energy, was observed in the glasses. The Ni (LVV) Auger spectrum for NiO powder showed a doublet structure. In glasses, however, two well-separated peak were observed in this spectrum. The results suggest that Ni in the glass may exist in two different phases, viz. nickel oxide and nickel vanadate. Initial addition of nickel (2 wt% of NiO) increased the optical absorption and electric conductivity but large additions, (5 wt% of NiO) led to a small decrease. The conductivity had a minimum value for a concentration of about 10 wt% of NiO. An attempt has been made to account for the observed changes in optical and electrical properties in terms of the XPS and Auger results.

Electron Paramagnetic Resonance and Optical Absorption Spectra of VO2+ in CsCl Single Crystals

In vanadium-doped CsCl crystals grown from aqueous solutions anisotropic EPR spectra due to VO2+ are observed and analyzed at room temperature. Evidence is presented that isotropic spectra of this ion observed in this and other compounds are due to inclusions of growth solution and not to rapid rotation of the vanadyl ion in the solid as normally assumed. At 77 K a well resolved vibrational progression of about 820 cm −1 is observed in the first ligand field band of this ion. The optical absorption spectra indicate the presence of a second valence state of vanadium, most likely V3+, in varying proportions depending on the crystal growth temperature.

The role of spin-spin interaction in the electrical conductivity of vanadium phosphate glasses

Representative glass series of different vanadium content and valence state have been studied with special emphasis on V 4+−V 4+ spin-spin interaction. From temperature dependent EPR studies paramagnetic Curie temperatures and J values have been determined. We demonstrate that this method is informative in the whole glass formation range. It is shown that there is an antiferromagnetic → ferromagnetic transition at c = V 4+/(V 4+ + V 5+) ∼ 0.17 (2.3×10 21 spin cm −3). At this composition the glass is essentially homogeneous and displays maximal conductivity. In glasses of higher V 4+ content, in which the dominating interaction is ferromagnetic, the conductivity gradually decreases. Therefore, ferromagnetically coupled V 4+ − V 4+ pairs seem to contribute to conductivity less effectively.

Role of the zeolitic carrier in the change of the selectivity of the P-V-O active component for the catalytic oxidation of butadiene

Zeolite catalysts of the faujasite type were prepared with a supported active component of the type P-V-O (P/V = 1.0) as well as, for comparison, with supported compounds of either vanadium or phosphorus alone. The catalysts were characterized by IR spectra in the region of the skeletal vibrations of the zeolite and by the sorption capacities for argon. The catalytic behaviour of the zeolite catalysts, of the unsupported active component as well as of the pure H-faujasite in the selective oxidation of butadiene is discussed in detail. An important role in the supported catalysts is played by the active sites formed by the interaction of the acidobasic centres of the zeolite with vanadium, these new sites giving the possibility of the formation of furan. The interaction of vanadium with the zeolite weakens the stabilizing effect of phosphorus on the valence state of vanadium; this explains the very low selectivity of the supported catalysts for the formation of maleic anhydride.

Correlation between structure and properties in vanadium phosphate glasses and amorphous V 2O 5− x/ O< x>1/ films

There is an apparent contradiction between the predictions of Mott-theory and experimental conductivity data of vanadium phosphate glasses. In parallel investigations we measured the molar volume of V 2O 5-P 2O 5 glasses with 55, 60, 70, 75 and 85 mol % V 2O 5 systematically varying the vanadium valence state, and characterized their microstructure by SEM, TEM and EDAX. We demonstrate that phase separation is a characteristic feature of this glass system and there is a compositional gradient at phase boundaries. It is suggested that amorphous V 2O 5−x/O<x<1/ films prepared by chemical vapour deposition can be appropriate prototype materials for testing the predictions of polaron theory for conduction.

Polaronic conduction and spectroscopy of borate glasses containing vanadium

Barium borate glasses containing 0-35 mole% V2O5 were found to be homogeneous. Optical absorption spectra indicated that only V4+ of the lower valence states of vanadium was present. The molar volume data suggested that a change in the structure associated with vanadium ions occurred at around 22% V2O5. ESR spectra showed that a more continuous change occurred in the electron structure associated with the V4+ ions as the V2O5 content was varied. Resistivity and activation energy decreased rapidly with decreasing vanadium-vanadium separation, while, at constant % V2O5, they showed a minimum at V4+/(V4++V5+)=0.45. Thermoelectric power of glasses with 25-30% V2O5 showed very low activation energies, in agreement with a polaron hopping mechanism.

Effect of valence state of vanadium on stereoblock polymerization of methyl methacrylate with VOCl3–AlEt2Br catalyst system

The polymerization of methyl methacrylate with the VOCL3–ALEt2Br catalyst system in n-hexane has been studied. The first-order dependence of rate of polymerization on catalyst and monomer concentrations, activation energy of 6.67 kcal/mole, and NMR spectra of polymer lend support to a coordinate anionic mechanism of polymerization. It has been shown that the vanadium in V+2 oxidation state is less active than V+3 oxidation state of active complex.

Separation and determination of the valence states of metal ions by ring colorimetry incorporating tropolone: Application to chromium and vanadium

Rapid separation and determination of chromium and vanadium valence states inμg quantities from their mixed solutions have been accomplished by the Weisz ring oven. Aqueous solution of tropolone is used as the developing reagent and evaluation of the separated ions is based on ring colorimetry.

Mössbauer effect on the spinel oxide FeCoVO4

The valence states in the spinel FeCoVO4 are studied. After recalling the crystalline structure and the magnetic properties, the different possibilities for valence states of iron and vanadium are discussed. Mossbauer measurements at 342 K give for iron an isomer shift relative to metallic iron IFe = (0.34 ± 0.02)mm/sec and a quadrupole splitting 14e2qQ = (0.17 ± 0.02)mm/sec. Mössbauer spectra at low temperatures are investigated. The formula (CoII) {FeIIIVIII}O4 is the most probable.

The phase diagram and phase transition of the V 2O 3-V 2O 5 system

The insulator-metal transition (IMT) in vanadium dioxide (VO2) which occurs above room temperature (67 °C) is highly sensitive to atomic defects caused by oxygen stoichiometry. The strained growth and the degree of oxygen deficiency in VO2 epitaxial films result in lowering of transition temperature below room temperature as well as the broadening of transition parameters such as transition width and hysteresis width, which limit its application potential. Here we demonstrate the growth of highly oriented strain-relaxed VO2 thin films on (001)-oriented TiO2 substrates at various oxygen partial pressures, exhibiting the narrow transition and hysteresis width. The cross-sectional transmission electron microscopy and x-ray diffraction analyses of the films reveal the highly oriented growth of insulating monoclinic VO2. The IMT parameters associated with temperature-dependent phase transition vary with the oxygen partial pressure used during the deposition. The presence of multiple and mixed valence states of vanadium in the films was confirmed by Raman and XPS analyses. We have achieved a narrow transition width (2.3 °C) and hysteresis width (1.2 °C) through controlling the oxygen stoichiometry during the growth of VO2/TiO2 films.

Infrared Spectra of GeO2‐P4O10‐V2O5 Glasses and Their Relation to Structure and Electronic Conduction

Infrared spectra of binary and ternary compositions in the glassy state and in the devitrified crystalline state in the system GeO2‐P4O10‐V2O5 were studied and compared with infrared spectra of crystalline spinels, which are known to contain V5+ ions in sixfold coordination. Results indicated that the fundamental vibration frequency of the V—O bond occurred at wave number 1015 cm−1. The spectra also provided evidence that the V5+ ion existed in sixfold coordination in the glassy state, as well as in the devitrified crystalline state, and that the VO6 octahedron retained its identity even at low concentrations and melting temperatures of 1450°C. Conductivity measurements indicated that, as the concentration of V2O5 decreased below the critical range of 5 to 10 mole %, there was an abrupt loss in the electronic conductivity of the glasses; the conductivity decreased with increasing concentration of the lower valence states of vanadium. A mechanism of conduction compatible with the structure of glasses in the system is suggested to explain these observations.

Optical Spectra of the Various Valence States of Vanadium in Na2 O.2SiO2 Glass

Optical absorption data are presented for Na2O2SiO2 glass containing vanadium equilibrated in various states of oxidation. Spectra of 5+, 4+, and 3+ vanadium were derived. The variation in amounts of the three species is in accord with mass action considerations. The spectra are related to crystal field assignments.

Structure of the soluble complex organometallic catalyst for the polymerization of acetylene

1. The system AlEt3 and VC10H14O5 is homogeneous in benzene medium. 2. The valence state of the vanadium in the catalytic complex was established. 3. A hypothesis was given for the structure of the catalytic complex.