Reduction Of V2O5 Research Articles

Synthesis and characterization of transparent poly(2-methyl-2-oxazoline) (POZO)–vanadium oxide (V2O5) hybrids with reversible formation

Optically transparent and homogeneous polymer hybrids of poly(2-methyl-2-oxazoline) (POZO) and vanadium oxide (V2O5) can be prepared by utilizing the hydrogen bonding interactions between the amide groups of POZO and V–OH groups formed in the hydrolysis of vanadium(V) oxytripropoxide (TPVO). POZO acts as a stabilizer in the sol–gel reaction of TPVO, since this sol–gel reaction is too fast to obtain a transparent film without POZO. The amorphous state of the POZO–V2O5 hybrid was confirmed by DSC, SEM, and XRD. The polymer hybrids obtained can be reversibly dissolved in the solvents and cast again because of the amorphous state of vanadium(V) oxide. Moreover, the solubility of the polymer hybrids was dramatically changed by the reduction of V2O5 from V(V) to V(IV).

Optical and electrical properties of vanadium pentoxide xerogel films: modification in electric field and the role of ion transport

Reversible modification of the optical and electrical properties of V2O5×nH2O gel films under the action of an electric field is studied. It is shown that under cathodic polarization (I = 10-6-10-5 A, t~10min, film thickness ~10 µm), the films turn from brownish-yellow to red. This internal electrochromic (EC)effect is caused by the redistribution of hydrogen ions inside the film, instead of insertion from the outside (from an electrolyte). A local increase in the hydrogen concentration occurs near the cathode, and this in turn results in modification of the optical properties. The rise of transmittance in the long-wavelength region of the spectrum, as well as the shift of the absorption edge near hν~2.5eV towards longer wavelengths, is observed. In addition, according to the infrared data, some increase in water content also contributes to the process of colouration. The change in the optical properties is accompanied by a change in the electrical properties, namely, an increase in ionic conductivity from ~4×10-5 to 10-4Ω-1 cm-1. At higher currents, electroforming resulting from the transport of oxygen ions occurs. This process leads to the formation of a channel consisting of vanadium dioxide due to reduction of V2O5 to VO2. Electrical switching with the S-type negative resistance, associated with an electrothermally driven metal-insulator transition in the channel, is observed in the sandwich M/V2O5×nH2O/M devices (unlike the planar devices described in the literature). Finally, applied potentialities of these phenomena for micro- and opto-electronics (EC devices, sensors) are discussed.

A novel procedure for the synthesis of NH3 incorporated VPO phases

It has been observed that as a function of a time delay introduced between the onset of the reduction of V2O5 with hydroxylamine hydrochloride and the addition of phosphoric acid, in the aqueous route preparation of the catalytically important VOHPO4·0.5H2O phase, a series of NH3 incorporated phases are obtained. Up to delay times of 3 min the crystalline VOHPO4·0.5H2O phase is obtained whereas a weakly crystalline hemihydrate is formed at a delay time of 4 min. Longer delay times result in X-ray amorphous phases until at delay time of 10 min and above a new crystalline phase containing mixed-valent vanadium emerges. All the phases obtained with delay times of 4 min and above have NH3 incorporated into them. The weakly crystalline VOHPO4·0.5H2O phase as well as the X-ray amorphous phases undergo transformation to a crystalline vanadyl pyrophosphate phase on calcination at 723 K in spite of having NH3 incorporated into them. The in-situ generation of NH3 during the preparation procedure is a hitherto unreported aspect of VPO chemistry and provides a convenient method for the preparation of novel NH3 incorporated VPO phases which could have strong potential as catalysts for alkane oxidation as well as for ammoxidation reactions.

Thermal decomposition of divanadium pentoxide V2O5: towards a nanocrystalline V2O3 phase

Thermal decomposition of V2O5 was studied by means of transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). Samples were heated in a specimen chamber of an electron microscope up to 600 °C in vacuum at 10-7 Torr. TEM and EELS reveal a sequence of transformations from V2O5 via VO2 to V2O3, which differs from the electron-beam-induced reduction of V2O5. The phase transformation does not proceed topotactically. Our observation reveals that the initial thermal decomposition of V2O5 to V2O3 is followed by a combination of diffusion, coalescence, and stabilization processes. Our experiments open a new way for the preparation of single-crystalline V2O3 nano-particles.

Electron beam induced reduction of V2O5 studied by analytical electron microscopy

The reduction of V2O5 under electron irradiation was studied by means of electron energy-loss spectroscopy, electron diffraction, and high-resolution imaging. The decrease of spectral intensity of O 1s excitations indicates a preferential removal of oxygen. The observed chemical shifts of the V 2p3/2 and V 2p1/2 peaks reveal that V5+ is reduced to V2+. Electron diffraction and high-resolution imaging show a structural change from the orthorhombic V2O5 to cubic VO. The beam induced reduction is compared with thermal decomposition of V2O5.

Electrorefining of carbonitrothermic vanadium in a fused salt electrolytic bath

Electrorefining of impure vanadium was carried out in a fused LiCl–KCl–VCl2 bath. The impure vanadium, a solid solution of vanadium, nitrogen, carbon and oxygen (V,N,C,O), was prepared by carbonitrothermic reduction of V2O5. The major impurities present in (V,N,C,O) were: (wt.%) carbon 1.6, oxygen 1.2 and nitrogen 2.5. The maximum purity, current efficiency and metal yield, after refining, were 99.85, 90 and 86 wt.% respectively. The evaluation and characterization of the electrodeposited vanadium were carried out by chemical analysis, scanning electron microscopy, optical microscopy and hardness measurements.

Laser-induced colouration of V2O5

Vanadium oxide is one of the most important optoelectronic materialsthat are used in optical switching devices, gas and humidity sensors, andsecondary Li batteries. It has been recognized that V2O5 can becoloured through electro-, photo-, and thermochromism processes. We reporthere that the surface of V2O5 (annealed at 580 °C for4 h) pellets turned to a deep blue colour upon just one pulse ablation of theXeCl excimer laser (λ = 308 nm, pulse duration = 36 ns, output energy = 100 mJ/pulse). Also we found that when a yellow pentoxide vanadiumpellet was exposed to proper excimer laser irradiation, its surface electricalconductivity changed remarkably, that is from an insulator to a semiconductor.The pentoxide vanadium sample, before and after excimer laser irradiation, wasanalysed with scanning electron microscopy (SEM), UV-visible diffusereflectance spectral (UV-VIS), x-ray diffraction (XRD), Raman spectrum (RS)and x-ray photoelectronic spectrum (XPS). The presence of a peak at about 830 cm-1 in the Raman spectrum indicates that the sample after laserirradiation is sub-stoichiometric with an oxygen deficiency. XPS showed thepartial reduction of V2O5 and the appearance of conduction bandelectrons. SEM showed that the surface of V2O5 became smooth and lessporous after excimer laser irradiation. From our experimental results, it wasconcluded that the laser-induced colouration is attributed to thermochromism.

Vanadium Nitride Catalysts: Synthesis and Evaluation forn-Butane Dehydrogenation

A series of mesoporous vanadium nitrides with surface areas up to 60 m2/g was prepared via the temperature-programmed reaction of V2O5with NH3, and evaluated for the dehydrogenation ofn-butane. Thermogravimetric analysis coupled with X-ray diffraction and scanning electron microscopy indicated that the solid-state reaction proceeded by the sequential reduction of V2O5(V2O5→V4O9→VO2→V2O3→VO0.9) and concluded with the topotactic substitution of nitrogen for oxygen in VO0.9. An experimental design was performed to determine effects of the heating rates and space velocities on the VN microstructures. The heating rates had minor effects on the surface areas and pore size distributions; however, increasing the space velocity significantly increased the surface area and decreased the average pore size. We believe the effect of the space velocity was due to inhibition of the gas–solid reactions by H2O and/or N2. Temperature-programmed reduction and pretreatment studies indicated that the passivated VN could be activated by reduction in H2at 500°C for 3 h. Lower reduction temperatures or times resulted in suppressed surface areas and O2chemisorptive uptakes. Oxygen chemisorption on the fully reduced vanadium nitrides scaled linearly with the surface area. The corresponding O/Vsurfaceratio was 0.56 suggesting that each oxygen atom chemisorbed to approximately two surface vanadium atoms. The vanadium nitrides were highly active butane dehydrogenation catalysts with selectivities in excess of 98%. Volumetric reaction rates for the highest surface area VN were superior to those for a Pt–Sn/Al2O3catalyst. The average turnover frequency for the VN catalysts was 4.3×10−4s−1at 450°C assuming that oxygen chemisorbed atomically onto the active sites. The corresponding turnover frequency for the Pt–Sn/Al2O3catalyst was 6.3×10−2s−1suggesting that while selectivities for the VN and Pt–based catalysts were similar, VN had a lower intrinsic activity.

In Situ Temperature-Programmed Diffuse Reflectance Infrared Fourier Transform Spectroscopy (TPDRIFTS) of V2O5/TiO2 Catalysts

Temperature-programmed diffuse reflectance infrared Fourier transform spectroscopy (TPDRIFTS) is proposed to follow in situ temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD) experiments of catalytic systems, as a method for observing relationships between the surface structure and the reducibility of the catalyst under the actual reaction conditions. TPDRIFTS studies of desorption and reduction of a conventional V2O5/TiO2 catalyst have been carried out, showing the presence in both processes of four stages. Both desorption and reduction experiments result in similar DRIFTS spectra for temperatures below 400 °C, accounting for two processes: (1) water desorption and (2) condensation of highly dispersed vanadium species in between V2O5 islands on the anatase surface. V2O5 reduction by H2 takes place between 400 and 680 °C, being characterized by elimination of the first-overtone bands of V= O groups. DRIFTS is shown to be a very powerful tool to characterize both hydrogen-consuming and nonconsuming processes such as O= migration above 725 °C (stage IV). Relationships between the area of V = O overtones bands obtained by TPDRIFTS and the oxidation state of vanadium are proposed as a method to estimate this parameter under actual reaction conditions.

The Role of Fe and Co Dopants during the Activation of the VO(HPO4) · 0.5H2O Precursor of the Vanadium Phosphorus Catalyst as Studied by in Situ Laser Raman-Spectroscopy: I. Study of VO(HPO4) · 0.5H2O Precursors Prepared by Reduction of V2O5 by Isobutanol

The activation of two Fe- and Co-doped VO(HPO4).0.5H2O precursors under n-butane/air catalytic atmosphere (1.5%) has been compared to the corresponding undoped precursor prepared under the same conditions by the organic route of reduction of V2O5 by isobutanol. The modification of the structure of the VPO materials has been followed by in situ laser Raman spectroscopy during the activation on line. It is evidenced that doping promotes the first time the oxidation of the catalysts favoring the VOPO4/(VO)2P2O7 balance. This is confirmed by an analysis of the bulk and surface composition of the final catalysts by UV-visible, XPS spectroscopy and 31P NMR by spin-echo mapping. The role of the two dopants is shown to be different on the catalytic performances: both Co and Fe dopants improve strongly selectivity to maleic anhydride but n-butane conversion is decreased for Co and increased for Fe. This has been explained by a different dispersion of VOPO4, phases on the (VO)2P2O7 matrix during the activation process as compared to the same undoped VPO catalyst.

Temperature-programmed reduction of V2O5 and coprecipitated V2O5−TiO2 by hydrogen

Temperature-programmed reductions (TPR) with H2 of both pure V2O5 and coprecipitated V2O5−TiO2 systems with different titanium concentrations was performed. The original and the reduced samples following each TPR step were characterized by X-ray diffraction, Fourier transform infrared analysis and scanning electron microscopy. Within the temperature range in which TPR analysis was carried out (100–600°C) the V2O5 phase was reduced in two or three steps, while no variation in the TiO2 phase (anatase or rutile) was observed. In the first reduction step only superficial reduction of the oxides was detected. In the following steps, the H2 reacted with oxygen atoms of the V=O and V−O−V bonds. This led to important changes in the structure and morphology of the system. The experimental evidence allowed the conclusion that titanium stabilizes certain phases of vanadium oxides in which vanadium appears as V(+4) or as a mixture of V(+4) and V(+5). Moreover, when moderate and high titanium concentrations were used, the reduction temperature of the bulk V2O5 decreased markedly.

Hydrogen temperature programmed reduction based on water analysis

Profiles of temperature programmed reduction (TPR) of V2O5 under H2 have been studied through water analysis. This new approach enhances the sensitivity of the method. The influence of experimental parameters on the temperature corresponding to the maximum reduction rate,T m, has been studied. The hydrogen concentration and heating rate are the two major factors that influenceT m. In our method, the amount of solid and the flow rate are also important factors in monitoring of profile distortions attributed to water condensation at the outlet of the reactor. An approach of the mechanism of reduction of V2O5 has been carried out. The best kinetic model which describes the reduction rocess is the Avrami-Erofeev tridimensional nucleation model.

Electron Optical Studies of Fresh and Reduced Vanadium Pentoxide-Supported Rhodium Catalysts

One percent Rh/V2O5 catalysts prepared by wet impregnation with RhCl3 · xH2O and subsequent reduction in hydrogen were shown to be subject to the formation of lower oxides of vanadium via an Rh-catalysed reduction process. V4O9, VO2, and V2O3 compounds were observed at reduction temperatures of 250, 450, and 600°C, respectively, as characterised by IR, XRD, and electron optical methods. Maximum hydrogen uptake by the Rh-loaded V2O5 occurred at 155°C according to TPR data, and this contrasts with the onset of reduction of V2O5 alone at ∼450°C. Mechanistic and structural aspects of the Rh-catalysed reduction process are discussed.

ChemInform Abstract: Reduction of V2O5. Part 2. Reduction by Hydrogen.

AbstractIn the course of the exothermic reduction reaction of V2O5 by H2 to yield V2O3, various intermediates with the general composition VnO2n‐1 with 4 ≤ n ≤ 8 are observed.

ChemInform Abstract: Reduction of V2O5. Part 1. Reduction by Carbon Monoxide.

AbstractThe final product of the reduction of V2O5 by CO is V2O3, while V6O13 and VO2 are observed to be formed as intermediates.

Irradiation Effect by Ruby Laser on Vanadium-Pentoxide Thick Films

A process for reduction and simultaneous sintering in air was developed by a laser beam irradiation method, and VO2 films were prepared from V2O5 printed on alumina substreates. Thick V2O5 films 40μm thick screen-printed on alumina substrates were irradiated by a ruby laser beam for 2ms in air at 350°C. The energy density of the beam was varied from 0 to 26J·cm-2. X-ray analysis and measurement of the electrical resistance were carried out at various temperatures including the phase transformation temperature. The change of film thickness by the irradiation was measured also. Irradiation energy densities lower than 9J·cm-2 caused only the sintering of V2O5 films. However, the reduction of V2O5 into lower oxides took place at the energy densities higher than 9J·cm-2. By 9J·cm-2 irradiation, a mixture of V3O7 and VO2 was obtained. Between 10 and 26J·cm-2, only VO2 was obtained. In the latter case, concentration ratio of VO2 to residual V2O5 in a specimen was estimated to be about 1:2. VO2 crystals thus obtained had a tendency to orient with their [001]r axes parallel to the substrate surface, where the suffix “r” indicates the axes for the rutile-type lattice. The logarithmic electrical resistance of specimens irradiated with 26J·cm-2 beam decreased by a factor of 2.1 above the critical temperature. After 300 heat cycles between 40° and 100°C, the logarithmic resistance ratio decreased to 1.1. The lattice parameters and the unit volume were determined by X-ray analysis.

Microstructural changes in vanadium pentoxide in controlled environments

Abstract Direct observations have been made of the reduction of V2O5 between room temperature and ∼600°C in reducing gases such as H2, balanced by He, CO and C2H4, using a gas-reaction cell in a high-voltage electron microscope. The systematic study has revealed microstructural changes relevant to structural and catalytic properties of the system. The reduction of the surface begins at low temperatures of ∼80–100°C. The defect structures formed are analysed, and it is shown that these are important in understanding redox reactions. The results could also perhaps explain chemical studies of oxygen desorption from V2O5.

Selective Reduction of Nitrogen Monoxide with Hydrogen on Pt/Al2O3 Containing a Metal Oxide

Abstract By using a three-way catalyst containing a metal oxide, NO can be selectively converted to N2 and N2O without the formation of NH3 even at the “rich” side of the air/fuel ratio. In order to elucidate the mechanism of this action of the added metal oxide, the NO–H2 reaction on Pt–V2O5/Al2O3 catalyst was investigated using the pulse reaction technique. In the NO–H2 reaction on Pt/Al2O3 catalyst, a significant amount of NH3 was formed in the “rich” region of the reactants (H2/NO>1), although NO was completely removed. In the NO–H2 reaction on the Pt–V2O5/Al2O3 catalyst, on the other hand, no NH3 was formed even in the “rich” region of the reactants; in addition, NO was completely removed. During the NO–H2 reaction on the Pt–V2O5/Al2O3 catalyst, the V2O5 in the catalyst was found to be reduced from V5+ to V4+. The reaction of NO with NH3 and the reaction of NH3 or H2 with Pt–V2O5 mixed catalysts for various states of mixing were also investigated. From these results, the NO–H2 reaction on the Pt–V2O5/Al2O3 catalyst has been shown to be composed of the following three main steps: Step I, the formation of NH3 by the reduction of NO with H2 on Pt; Step II, the NO–NH3 reaction on Pt to form N2 or N2O; and Step III, the reduction of V2O5 in the catalyst by H2 which is accelerated by the “hydrogen spil-lover.” In the NO–H2 reaction on Pt/Al2O3 catalyst, NO is first reduced to NH3 in Step I and the NO–NH3 reaction in Step II follows. In the “rich” region, the formation of NH3 takes place more readily than the subsequent NO–NH3 reaction. In the NO–H2 reaction on the Pt–V2O5/Al2O3 catalyst, the formation of excess NH3 is suppressed by the removal of hydrogen from Pt in Step III, and the NH3 thus produced reacts with NO to form N2 or N2O selectively in Step II. The rates of Steps I, II, and III were also discussed in terms of the structure of the catalysts and composition of the reactants.

ESR study of V2O5 reduction in butyllithium

physica status solidi (a)Volume 59, Issue 2 p. K167-K169 Short Note ESR study of V2O5 reduction in butyllithium A. Pasturel, A. Pasturel Spectrochimie du Solide L. A. 302, Paris Search for more papers by this authorC. Sanchez, C. Sanchez Spectrochimie du Solide L. A. 302, Paris Search for more papers by this authorJ. Livage, J. Livage Spectrochimie du Solide L. A. 302, Paris Search for more papers by this author A. Pasturel, A. Pasturel Spectrochimie du Solide L. A. 302, Paris Search for more papers by this authorC. Sanchez, C. Sanchez Spectrochimie du Solide L. A. 302, Paris Search for more papers by this authorJ. Livage, J. Livage Spectrochimie du Solide L. A. 302, Paris Search for more papers by this author First published: 16 June 1980 https://doi.org/10.1002/pssa.2210590266Citations: 2 4 Place Jussieu, 75230 Paris Cédex, 05, France. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume59, Issue216 June 1980Pages K167-K169 RelatedInformation

Consolidation and fabrication of vanadium and vanadium-base alloys

AbstractA review is presented on the current state of technology on the consolidation and fabrication of vanadium and vanadium alloys. Commercial quantities of pure vanadium metal (99.9+%) have been produced by the metallothermic reduction of V2O5 followed by electron-beam refining. Lack of demand stopped further development work on vanadium and vanadium alloys in the USA around 1969. Currently, there is not any active capacity in the USA for producing commercial quantities of high-purity vanadium. Vanadium and vanadium alloys can be processed to all the common mill shapes using procedures similar to those followed for processing niobium and tantalum alloys. To avoid contamination during processing, all elevated-temperature working operations must be accomplished with suitable protection for the workpiece, Le. encapsulation for hot extrusion, forging, and rolling and in process and final annealing a pressure of ≤1. 3 × 10-3 Pa. Gas-tungsten arc welds in unalloyed vanadium containing 1560 ppm C+O+N are rep...