Formation Of VO Research Articles

Gas-Phase Chemistry of Bare V+ Cation with Oxygen and Water at Room Temperature: Formation and Hydration of Vanadium Oxide Cations

Mass spectrometric experiments at extremely low (<10-6 mbar) and moderate (0.5 mbar) pressures are used to examine the reactions of atomic vanadium cation with molecular oxygen and water. With O2, rapid O-atom abstraction gives rise to the formation of VO+ cation (k = 3 × 10-10 cm3 molecule-1 s-1). Interestingly, despite a similar thermochemistry, the O-atom transfer from water to bare V+ is less efficient by more than an order of magnitude (k = 8 × 10-12 cm3 molecule-1 s-1). Subsequent associations of VO+ with either O2 or H2O occur with very low efficiencies and involve termolecular stabilization mechanisms. The low probability of degenerate 16O/18O exchange between VO+ and water indicates the operation of a sizable kinetic barrier. Ab initio calculations using density functional theory lend further support to the interpretation of the experimental data and provide the first thermochemical information on VOn+ cations with n > 2, as well as some hydrated species. In general, the dipolar water ligand is found to be much more strongly bound to the cationic vanadium complexes than is dioxygen.

Vanadium (V) phosphate prepared using solvent-free method

A solvent-free method of preparation of a vanadium(V) phosphate is described and discussed. Reaction of V2O5 with H3PO4 in the absence of water at 150 °C leads to the formation of a new catalytic material that is designated as “anhydrous” VOPO4. The material readily hydrates to form VOPO4⋅2H2O and has been characterised using powder X-ray diffraction, in situ Raman spectroscopy and 31P MAS NMR spectrometry. On activation in dry N2 followed by reaction with butane/air another novel material is formed that has an intrinsic activity for maleic anhydride that is similar to catalysts derived from VOHPO4⋅0.5H2O under comparable conditions. Activation of VOPO4⋅2H2O under comparable conditions leads to the formation of αI-VOPO4 which exhibits no partial oxidation activity. Reaction of “anhydrous” VOPO4 with alcohols leads to the exclusive formation of VO(H2PO4)2 in further contrast to VOPO4⋅2H2O which under similar conditions leads to the synthesis of VOHPO4⋅0.5H2O.

Composition and structure of tin/vanadium oxide surfaces for chemical sensing applications

The aim of this study is to obtain data useful to elucidate the sensing mechanism of solid state microsensors for hydrocarbon detection based on mixed vanadium/tin oxides. The sensors studied here were prepared by deposition of an active layer of Sn and V oxides on pre-oxidized porous silicon. The surface composition of these sensors was studied by a combination of low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). Parallel studies were performed on a ‘model’ system prepared by depositing a thin film of vanadium on a single crystal SnO 2(1 1 0) surface and by successive thermal treatment. The results obtained show that vanadium is detectable by LEIS in the topmost atomic layer of both the polycrystalline and the s.c. sample in similar amounts. On the model system, the results of X-ray photoelectron diffraction (XPD) measurements show the formation of epitaxial VO 2. The main conclusions of the present study are that in these sensors vanadium and tin oxide form separate phases and that vanadium species are present in the outermost surface. Both phases may take an active part in the mechanism of hydrocarbon detection in these sensors.

The Unexpected Role of Aldehydes and Ketones in the Standard Preparation Method for Vanadium Phosphate Catalysts

VO(H2PO4)2 is formed as the exclusive product from the reaction of aldehydes or ketones with V2O5 and H3PO4 whether aqueous (85%) or crystalline (100%) orthophosphoric acid is used. This exclusive product formation has been observed with a broad range of aldehydes and ketones (C4–C10). This finding casts doubt on the current commercial preparation of vanadium phosphate catalysts used for the oxidation of n-butane to maleic anhydride. This catalyst is derived from a crystalline precursor VOHPO4·0.5H2O, formed from the reaction of V2O5 and H3PO4 with an alcohol. The alcohol acts as a reducing agent forming an aldehyde or ketone. These aldehydes and ketones, once formed, will lead to the formation of VO(H2PO4)2 as an impurity, at levels which will be difficult to detect but which are known to affect catalytic performance adversely. The use of isobutanol containing a small quantity of butanone for the reaction of V2O5 and H3PO4 (100%) confirmed that low levels of VO(H2PO4)2 can be formed together with VOHPO4·0.5H2O.

Speciation and NMR relaxation studies of VO(IV) complexes with several O-donor containing ligands: oxalate, malonate, maltolate and kojate

Potentiometric, spectral and 1H NMR relaxation studies are reported on the complex formation of VO(IV) with the bidentate chelating ligands oxalic and malonic acids, of a dicarboxylic nature, and maltol and kojic acid, of the hydroxy-4-pyrone type. Complexes with stochiometries VOA and VOA 2 were characterised in aqueous solution. Two main features were established for the complex systems. It was found that five-membered ring chelation favours the cis arrangement in the bis complexes of maltol and kojic acid to a higher extent than in those of oxalic and malonic acids. Moreover, previous investigations of VO(IV) binding to maltol or its analogues did not consider the formation of hydroxo species. Formation of dihydroxo bridged dimeric complexes of stoichiometry (VOAH −1) 2 and monomeric hydroxo species VOA 2H −1 or VOA 2(OH), are now taken into account in order to explain the equilibrium and spectroscopic results. 1H NMR relaxation measurements strongly suggest an equilibrium between the two isomers of the bis complexes.

Formation and phase transition of VO2 precipitates embedded in sapphire

Crystallographically coherent precipitates of vanadium dioxide (VO2) have been formed in the near-surface region of single crystals of sapphire (Al2O3) using a combination of ion implantation and thermal treatments. As in the case of either bulk VO2 single crystals or thin films of VO2, the thermally induced semiconductor-to-metal phase transition of the embedded VO2 precipitates is accompanied by a large hysteretic change in the infrared optical transmission. The VO2 precipitate transition temperature (Tc = 72 to 85 °C) is higher than that of bulk VO2 (Tc = 68 °C) and is sensitive to the implantation conditions. The present results show that the damage resulting from the coimplantation of vanadium and oxygen into an Al2O3 host lattice dictates the final microstructure of the VO2 precipitates and, consequently, affects the transition temperature, as well as the optical quality of the VO2/Al2O3 surface-nanocomposite precipitate system.

Chemical and Sonochemical Approaches to the Formation of VO2 Films and VO2-Impregnated Materials

ABSTRACTA new chemical and chemical/ultrasonic approach to the preparation of VO2 films and VO2-impregnated bulk materials has been developed. In this approach, a V2O5 sol prepared by quenching is used to coat SiO2 substrates. The resulting gel-film is heat treated in a reducing atmosphere to form a film identified as VO2 from the results of X-ray diffraction and both optical and resistivity measurements, which reveal the phase transition characteristic of vanadium dioxide. The advantage of this approach to the formation of VO2 is that the V2O5 sol can be used to impregnate porous materials, which are then heat treated to form an optically active composite material. The switching properties of the VO2 films are investigated using optical and resistivity measurements, and the results are compared to those obtained for VO2-films prepared by more-conventional methods.

Interaction of V 2O 5-NaY zeolite with H 2S and SO 2

V 2O 5-NaY zeolite catalyst was prepared by vacuum impregnation of ammonium metavanadate solution in water on NaY-zeolite (zeolite). The slurry was filtered, washed, dried, and calcined at 600°C. On heating the catalyst at 450°C in vacuum, VO 2+ species were detected by e.s.r. It was observed that the electron-accepting and -donating sites increased when V 2O 5 or vanadium species were loaded on it. Vanadium species poisoned the active sites of the zeolite that were responsible for the formation of SO 2 − ions. Pretreated V 2O 5-NaY zeolite with SO 2 plays an important role in the formation of VO 2+ species when it is treated with H 2S at room temperature. V 2O 5-NaY zeolite can be used for the reduction of SO 2 by H 2S.

Chapter 5.2 EPR characterization of V 2O 5/TiO 2 eurocat catalysts

The Eurocat catalysts TiO 2 (EL 10) and V 2O 5/TiO 2 with vanadium contents 1 and 8% (EL10V1 and EL10V8) have been studied by Electron Paramagnetic Resonance (EPR). No dispersed V( IV) ions were detected on the received samples degassed at room temperature. It has been shown that with V the transformation of the EL 10 anatase phase into rutile occurs at a relatively high temperature (1373 K.) compared to that reported in the literature for pure TiO 2. The inhibition in this transformation could be due to the presence in the solids of certain specific impurities including the vanadium added. On EL10V1 and EL10V8, a few V(IV) ions are present as V 4+ clusters in form of polycrystalline vanadia. These are easily removed either by acid attack or by calcination at high temperature, resulting in the formation of a few VO 2+ ions presumably as vanadyl sulfate, or as a solid solution of V 4+ in the TiO 2 matrix.

Pure and mixed titanium, niobium and vanadium oxides as sputtered thick and thin films: Crystallographic properties and phase transitions between 300 and 1800 K I: X-ray diffraction investigations of thick films

Crystallographic properties of transitien metal oxides (MO y and (Ti, M)O y , with y≈2 and M = Ti, Nb and V) as thick films (thickness e≈1microm) deposited by r.f. sputtering, were studied before and after annealing (up to 1100 K) by X-ray diffraction. TiO 2, NbO 2 and (Ti,Nb)O 2 compounds, initially amorphous, crystallized between 500 and 700 K according to the rutile structure. Sometimes the TiO 2 anatase phase was observed. After annealing above 900 K, α- and γ-Nb 2O 5 compounds formed for NbO 2-type samples. For (Ti,Nb)O 2 samples Ti x Nb y O 2 compounds formed. As-deposited VO y and (Ti,V)O y thick films were partially crystallized and textures, whatever the experimental conditions. Resistivity measurements and pattern indexation indicated the formation of rutile VO 2.

CRYSTAL STRUCTURE INVESTIGATION OF VANADYL COMPLEXES OF TRIDENTATE LIGAND (II)——SYNTHESE AND CRYSTAL STRUCTURE OF 1-(2-PYRIDYLAZO)-2-NAPHTHOLATO-DIOXOVANADIUM (V) DIMER [VO 2 (C 15 )H 10 N 3 O)] 2 (H 2 O 2 )(CHCl 3 ) 2 AND PYRIDINE-(1-(2-PYRIDYLAZO)

The title complexes crystallize in space groups C(?)-P(?) and C 2h 5 -P2 1 /c, and with unit cell parameters a =8.488. b =10.100, c =11.974. α =72.73°, β =78.56°, γ =73.55°, and a =11.496, b =9.883. c =16.360, β =100.18°, respectively. All the V coordinates are obtained from Patterson and direct methods, respectively, and then Fourier and difference Fourier methods are employed to deduce other non-hydrogen atoms. Structural parameters are refined with least-squares technic, yielding final discrepancy factors R=0.081 and 0.067, respectively. Structural analyses demonstrate that the dimer of VO 2 + complex is formed through the sixth-position bonding of a bridge oxygen atom of one VO 2 + group with another VO 2 + , and the formation of VO(O 2 )(C 15 H 10 N 3 O)(C 5 H 5 N) shows thatVO 2 + complex with strong chelating tridentate PAN-seems less difficult to transform into the pentagonal bipyramid VO 3+ complex. Since no peroxo species has been used in the synthesis, the fact that a peroxo group forms in the product is especially interesting.

Vanadium-51 NMR line-broadening in the mixture of dioxovanadium(V) and oxovanadium(IV) ions in perchloric acid Solution

Vanadium-51 NMR line-broadening in the mixture of dioxovanadium(V) VO 2 + and oxovanadium(IV) VO 2+ ions has been studied in perchloric acid media. The line-broadening is attributed to the exchange reaction between VO 2 + and a mixed-valence complex VO 2 +VO 2+. The rate law is expressed by rate = k 2 K[VO 2 +] o[VO 2 +] o 2/(1 + K[VO 2 +] o) where K and k 2 refer to the formation constant of VO 2 +VO 2+ and the rate constant for the exchange reaction between VO 2 +VO 2+ and VO 2 +, respectively, and [ ] o denotes the initial concentration of each ion. The result differs from that of the earlier study by McConnell et al. who predicted a mechanism involving the formation of VO 2 +VO 2 +.

An investigation of the kinetics and stability of VO 2

The kinetics of the formation of VO 2 and its stability is discussed. A Gulbransen-type vacuum microbalance was employed to monitor mass changes with time. The activation energy of the diffusion of vanadium is calculated and compared with that of parabolic oxidation. VO 2 was identified by means of X-ray diffraction techniques.

Electron paramagnetic resonance evidence for electrochemical formation of Mo vO 2, cis-Mo vO-(OH), Mo vO(S) and cis-Mo vO(SH) centers

Electron paramagnetic resonance evidence for electrochemical formation of Mo vO 2, cis-Mo vO-(OH), Mo vO(S) and cis-Mo vO(SH) centers

Crossed-beam chemiluminescent reactions of titanium and vanadium with O2

Titanium and vanadium have been reacted with O2 under crossed-beam conditions to form the TiO A 3Φ, B 3Π, C 3Δ, and E 3Π states and the VO B 4Π state. By heating and seeding the O2 nozzle beam the Ti reaction has been studied at relative collision energies of 4.0, 7.6, and 13.3 kcal/mol, and the V reaction at 3.9, 6.9, 7.6, and 13.1 kcal/mol. Computer simulations of the spectra yield relative rates for formation of TiO(A) and VO(B) vibrational states, which are slightly more excited than the ’’prior’’ model predictions. Increasing the relative collision energy does increase the production of all the electronically excited states, but the VO B state population does not increase as quickly as expected from the prior model. The dependence of the chemiluminescent signals on the metal source temperature suggests reaction of ground state Ti to form TiO(A), but metastable V to form VO (B).

Electron-Beam Float Zone and Vacuum Purification of Vanadium

Electron-beam float zone melting vanadium three passes at 3.9 in./h in 4×10−10 Torr vacuum resulted in resistance ratios of about 700 and a total impurity content of 50 wt ppm. The zoning reduced both carbon and oxygen content. However, while carbon removal was always accompanied by a lowering of the oxygen content, the reverse was not true. The removal of oxygen was probably accomplished by the formation and volatilization of VO. The mechanism for carbon removal was not clearly resolved. Nitrogen was not appreciably affected by electron-beam float zone melting. All metallic impurities, with the exception of Ta and W and possibly Si, were effectively volatilized. No evidence was found for zone refining action on any impurity. Vacuum outgassing further reduced the carbon and oxygen level. However, the oxygen removal was far more effective than the carbon. The nitrogen content increased, particularly on the specimens that lost about 40% in weight due to vanadium evaporation. However, the effect of vacuum outgassing was less on the highest purity as-zoned specimens. The same purification mechanisms apparently operated.

Studies on the murexide complexes of vanadium, thorium and cerium and their colorimetric determination with murexide

Complex formation of VO 2+, Th 4+ and Ce 3+ with murexide has been studied both spectrophotometrically and by pH titration. The use of the 1:1 coloured complexes for the analytical determination of the respective ions, and the effects of interfering ions, are discussed.

Pilot plant manufacture of vanadium carbide

A manometric study was carried out of the reduction of vanadium oxide with carbon, and it was found that the pressure vs time curve exhibits three discontinuities, at temperatures of 1200°C (formation of the monoxide VO), 1400°C (formation of VCX-VO solid solutions), and 1700°C (formation of the carbide VCX).

Chemical transport reactions in the vanadium-silicon-oxygen system and the ternary phase diagram

Vanadium and silica glass react at 800 °–1200 °C to form V 3Si(s) and VO(s) if the reaction vessel contains gaseous vanadium chlorides or iodides causing a chemical transport of the metal to the glass. In the presence of chloride species, V 3Si and VO are chemically transported from the hot to the cooler portions of the reaction vessel. The chemical reactions accounting for the transport are proposed. The following equilibrium tie-lines in the ternary V-Si-O phase diagram were determined: SiO 2-VO, SiO 2-V 3Si, V 3Si-VO, V 3Si-V 2O, and V 3Si-V 4O; and the phase diagram is presented. Considerations of the phase data and the heats of formation of VO(s) and SiO 2(gl) showed that − ΔH o f (V 3Si,s) must be >9 kcal/mole. Relative to one another, the set of values for the heats of formation of V 3Si, V 5Si 3, and VSi 2 as given in the literature was found to be thermodynamically inconsistent with V-Si-O phase data.

Determination of Vanadium with Square Wave Polarograph

It has been a usual method in the polarographic determination of vanadium to utilize the anodic wave produced by the oxidation of vanadyl ion (IV) to vanadate ion (V) in a supproting electrolyte composed of 1 M sodium hydroxide and 0.08 M sodium sulphite.1) Recently, Musha and Takao studied on the simultaneous determination of titanium and vanadium using a base solution of 0.1 M EDTA and 0.02 M sodium citrate.2) However, these supporting electrolytes can not be applied to the high sensitive determination with square wave polaroglaph, since the reactions of vanadium ions in these supporting solutions appear to proceed irreversibly, or to be accompanied by a chemical reaction. Only an available reversible reaction for this purpose in the consecutive reduction processes of vanadium ions is a reaction between vanadic (III) and vanadous (II). As the half wave potential of this reaction in 1 N sulphuric acid is -0.508±0.002 V. res. S.C.E. and that of the reaction between vanadyl (IV) and vanadous (II) in 0.1 N sulphuric acid is -0.85 V. res. S.C.E., it is impossible to reduce vanadyl (IV) ion direct-ly to vanadic (III) ion. Thus, for the high sensitive determination of total vanadium with square wave polarograph it is necessary to convert aII vanadium ions into the state of either vanadic (III) or vanadous (II). It is accepted in general that when a oxidation-reduction reaction invoIVes a process of formation or breaking of covalent metal-oxygen bond, the reversibility of the redox reaction becomes very poor. The irreversibility of the reaction between vanadyl (IV) and vanadous (II) may be caused from the formation of a metal-oxygen bond VO++ which is invoIVed in the reaction process between vanadyl (IV) and vanadic (III). Therefore, it is expected that the reversibility of the reaction between vanadyl ion (IV) and vanadous ion (II) may be increased by preventing the formation of VO++ ion, using a much higher concentrated sulphuric acid as a supportinng electrolyte, in which VO++ ion is scarcely formed. The aim of the present study was to obtain informations for the determination of vanadium with square wave polarograph, utilising the reaction between vanadyl (IV) and vanadous (II) in a highly concentrated sulphuric acid on the basis of the abovementioned expectation.