Reduced Vanadium Oxide Research Articles

Highly Exothermic and Fast Mechanochemical Redox and Intercalation Reactions of V2O5 with Sodium Hydride.

Vanadium oxides exhibit promising characteristics for electrochemical energy storage, owing to their capability to switch between different oxidation states, in combination with the incorporation of alkali metals. Here, we report on a systematic investigation of the mechanochemical reduction of V2O5 with NaH. In contrast to conventional high-temperature synthesis methods, the mechanochemical reaction occurs already after a few minutes. We observed a mixture of different (sodium) vanadium oxides with vanadium oxidation states ranging from +III to +V. Remarkably, these highly exothermic self-propagating reactions occur even within a rudimentary pistil-mortar setup. Hereby, the hydride concentration has a greater effect on the final sample composition than the milling time. In general, higher percentages of sodium vanadates are formed instead of vanadium oxides, and the lower oxidation states of vanadium are accessible with increasing amounts of NaH. Theoretical calculations confirm these experimental observations and emphasize the central role of sodium vanadates, especially with vanadium in the +V oxidation state, in carrying out the observed exothermic reactions. This comprehensive study sheds light on the mechanochemical reduction of vanadium oxides and underlines their potential for further development of electrochemical energy storage systems.

Catalytic Properties of Ethylene Reactions Using Silver and Reduced Vanadium Oxide Catalysts on MEMS Chips with Micro Hot Plate

Catalytic Properties of Ethylene Reactions Using Silver and Reduced Vanadium Oxide Catalysts on MEMS Chips with Micro Hot Plate

Preparation of SiC from carbonaceous vanadium-bearing stone coal via carbothermal reduction method

In the present study, silicon carbide (SiC) was prepared via carbothermal reduction method using carbonaceous vanadium-bearing stone coal at 1400 °C. Compared to other reported raw materials, SiC was prepared at a relatively low the reaction temperature. The effects of temperature, duration and vanadium catalysis on the process of SiC preparation were investigated, and the kinetics of SiC nucleation and growth were analyzed. The results indicate that the SiC conversion of samples with 0 wt% and 2 wt% V2O3 additions reached to 68.9 wt% and 80.7 wt% at 1450 °C for 5 h. It was observed that the reduction of vanadium oxides resulted in the formation of vanadium carbide (VC), which played an important role in catalyzing the nucleation of SiC and accelerating its growth. The mechanism of SiC nucleation and growth is consistent with the Johnson-Mehl-Avrami-Erofeev-Kolmogorov (JMAEK) model, and the apparent activation energy values of the carbothermal reduction reaction are 213.17 kJ/mol and 109.66 kJ/mol for samples with 0 wt% and 2 wt% V2O3 additions. Therefore, this method not only expanded the source of raw material but also reduced energy consumption and economic costs.

Ammonium Ion Enhanced V2O5-WO3/TiO2 Catalysts for Selective Catalytic Reduction with Ammonia.

Selective catalytic reduction (SCR) is the most efficient NOX removal technology, and the vanadium-based catalyst is mainly used in SCR technology. The vanadium-based catalyst showed higher NOX removal performance in the high-temperature range but catalytic efficiency decreased at lower temperatures, following exposure to SOX because of the generation of ammonium sulfate on the catalyst surface. To overcome these limitations, we coated an NH4+ layer on a vanadium-based catalyst. After silane coating the V2O5-WO3/TiO2 catalyst by vapor evaporation, the silanized catalyst was heat treated under NH3 gas. By decomposing the silane on the surface, an NH4+ layer was formed on the catalyst surface through a substitution reaction. We observed high NOX removal efficiency over a wide temperature range by coating an NH4+ layer on a vanadium-based catalyst. This layer shows high proton conductivity, which leads to the reduction of vanadium oxides and tungsten oxide; additionally, the NOX removal performance was improved over a wide temperature range. These findings provide a new mothed to develop SCR catalyst with high efficiency at a wide temperature range.

Influence of vanadium content on structure and activity of V2O5–ZrO2–SiO2 catalyst for propane dehydrogenation

In order to create alternative vanadium oxide-supported catalysts for the process of non-oxidative propane dehydrogenation to propylene, we studied the effect of the increased content of vanadium oxide in the V2O5–ZrO2–SiO2 composition on its structure and catalytic properties. Zirconium silicate hydrogel in the form of finished spherical granules with the SiO2 content of more than 50% was prepared by direct sol-gel synthesis from zirconium oxychloride and sodium metasilicate using the droplet coagulation technology. Catalysts were fabricated by impregnation of hydrogel with an aqueous solution of vanadyl sulfate salt, hydrothermal treatment and calcination in air. By using scanning electron microscopy, X-ray diffraction analysis and low-temperature nitrogen adsorption/desorption method, we showed that amorphous samples with a developed mesoporous structure (with the pore diameter of ~6 nm and the specific surface area of ~300 m2 g–1) were formed when the content of the supporting V2O5 on zirconium silicate was 10, 20, 25, and 30 wt.%. In the course of temperature increase in the propane dehydrogenation reaction, the catalyst samples crystallized in the reaction mixture propane–inert gas with the formation of tetragonal zirconia. When the content of V2O5 was 25% or 30%, additional phases of reduced vanadium oxides and traces of the zirconium vanadate phase were formed. After the reaction, the specific surface area of the catalysts decreased significantly and the average pore size of the samples with 25% and 30% V2O5 increased to ~30 nm. The propylene yield reproducibly observed on the samples with 25% and 30% V2O5 was lower than that on the samples with 10% V2O5; however, it remained quite high, which was probably due to the expanded diameter of the pores and the appearance of additional ZrV2O7 sites that are active with respect to the dehydrogenation of light alkane.

Activation characterization of the Ti-Zr-V getter films deposited by magnetron sputtering

Non-evaporable getter (NEG) thin films provide distributed pumping properties, allowing to achieve ultrahigh vacuum in narrow and conductance limited chambers. It is an ideal solution to deal with the small aperture of vacuum chambers. In this paper, ternary Ti-Zr-V getter film with approximate equal atomic ratio was deposited on oxygen free copper (OFC) substrate by using a developed DC magnetron sputtering system with an alloy target. Microstructural characterization by scanning electron microscopy, atomic force microscopy and energy dispersive spectroscopy (EDX) indicates that the porous getter film exhibit a columnar growth pattern and the grain sizes are very distributed ranging from several up to hundreds of nanometres. The compositions and the corresponding chemical bonding states are analyzed by using in-situ synchrotron radiation X-ray photoelectron spectroscopy after activation for 1 h at different temperatures in ultra-high vacuum. The results indicate that Ti, Zr and V are initially in their high oxidized states, which are gradually reduced in steps to oxides of lower valence states and finally to their metallic states. The reduction of vanadium oxides is more preferable than that of titanium- and zirconium-oxides. Such activation commences at a temperature as low as 150 °C, and can be enhanced by increasing temperature.

Chemisorption of NO2 on V-Based SCR Catalysts: A Fundamental Study toward the Mechanism of “Fast SCR” Reaction

The “fast selective catalytic reduction (SCR)” process, with its superior efficiency, is promising for removing NOx from flue gas with low temperature and high gas velocity. Chemisorption of NO2 is a crucial step in this heterogeneous catalysis process. In this study, density functional theory calculations and experimental methods were implemented to study the chemisorption behaviors of NO2 on the industrial V-based SCR catalyst. Both the (001) and (101) surfaces of the TiO2 support were applied in the modeling and calculation process. Theoretical results show varied profiles of NO2 adsorption on the fresh and reduced V2O5/TiO2 catalysts. For the fresh catalyst, NO2 tends to adsorb on the TiO2 support rather than on the vanadium oxide. On the contrary, NO2 exhibits a strong interaction with the −OH site of the reduced vanadium oxide. NO2 temperature-programmed desorption, X-ray photoelectron spectroscopy, and a designed redox experiment were implemented to verify the calculation results. On the basis of t...

Thermal activation of charge carriers in ionic and electronic semiconductor β-AgIVVO3 and β-AgIVVO3@VV 1.6VIV 0.4O4.8 composite xerogels.

Silver vanadium oxide (SVO) and Silver Vanadium Oxide/Vanadium Oxide (SVO@VO) composite hydrogels are formed from the self-entanglement of β-AgVO3 nanoribbons and slightly reduced vanadium oxide (VO) (VV1.6VIV0.4O4.8) nanoribbons; respectively. Starting from randomly distributed nanoribbons within hydrogels, and after a controlled drying process, a homogeneous xerogel system containing tuneable SVO : VO ratios from 1 : 0 to 1 : 1 can be obtained. The precise nanoribbons compositional control of these composite system can serve as a tool to tune the electrical properties of the xerogels, as it has been demonstrated in this work by impedance spectroscopy (IS) experiments. Indeed, depending on the composition and temperature conditions, composite xerogels can behave as electronic, protonic or high temperature ionic conductors. In addition, the electric and protonic conductivity of the composite xerogels can be enhanced (until a critical irreversible point), through the temperature triggered charge carrier creation. As concluded from thermogravimetry, IR, UV-Vis and EPR spectroscopy studies, besides the SVO : VO ratio, the thermal induced oxidation/reduction of V5+ to V4+, and thermally triggered release of strongly bonded water molecules at the nanoribbon surface are the two key variables that control the electric and ionic conduction processes within the SVO and composite SVO/VO xerogels.

SYNTHESIS OF FINELY DISPERSED VANADIUM CARBIDE (VC<sub>0.88</sub>) USING NANOFIBROUS CARBON

The paper presents the experimental data on the synthesis of finely dispersed powder of vanadium carbide (VC 0.88 ). Vanadium carbide was prepared by the reduction of vanadium oxide (III) with nanofibrous carbon (NFC) in the induction furnace under an argon atmosphere. NFC is a product of catalytic decomposition of light hydrocarbons. The main characteristic of a NFC is a high specific surface area (~150 000 m 2 /kg), which is significantly higher than that of soot (~50 000 m 2 /kg). The content of impurities in the NFC is at the level of 1 % wt. Based on the analysis of the state diagram of the V – C system, the composition of the charge and the upper temperature limit of the carbide formation reaction for obtaining vanadium carbide in the powder state are determined. Based on the thermodynamic analysis, the temperature of the onset of the carbothermic reduction reaction of vanadium oxide (III) at various CO pressures was determined. The characteristics of vanadium carbide were studied using X-ray and elemental analyzes, pycnometric analysis, scanning electron microscopy using local energy dispersive X-ray microanalysis (EDX), low-temperature adsorption of nitrogen, followed by determination of the BET specific surface area, sedimentation analysis, synchronous thermogravimetry and differential scanning calorimetry (TG/DSC). The material obtained at optimal parameters is represented by a single phase – vanadium carbide VC 0.88 . The powder particles were predominantly aggregated. The average size of the particles and the aggregates equaled 9.2 – 9.4 μm within a wide range of size distribution. The specific surface value of the obtained samples was 1800 – 2400 m 2 /g. Oxidation of vanadium carbide began from the temperature of ~430 °C and practically ends at ~830 °C. Optimum parameters of synthesis are the ratio of reagents according to stoichiometry to obtain carbide of composition VC 0.88 at a temperature of 1500 – 1600 °С and a holding time of 20 minutes. It is shown that for this process nanofibrous carbon is an effective reducing agent and that vanadium oxide (III) is almost completely reduced to carbide VC 0.88

Understanding ferrovanadium smelting through computational thermodynamics modelling

Ferrovanadium is essential for the production of many alloy steels. It is made by the aluminothermic reduction of vanadium oxides, together with scrap steel and burnt lime as a flux at very high temperatures. In this work, the theory of aluminothermic reduction is discussed, and then a computational thermodynamics model is described and its input parameters are discussed and justified. The model predicted very satisfactorily the composition of both the ferrovanadium and waste slag from a heat at the Windimurra smelter in Western Australia. Moreover, the modelling revealed that published data on the activity coefficient of VO1.5 in slag were likely to be seriously in error. It was shown that increasing the recovery of vanadium to the ferroalloy was accompanied by a rapid increase in its aluminium content, which is limited by commercial specifications. The limit on the silicon content of ferrovanadium can only be met by limiting the input of SiO2 to the furnace. The model provides a useful basis for assessing process improvements.

Mechanism and kinetics of early transition metal hydrides, oxides, and chlorides to enhance hydrogen release and uptake properties of MgH2

Selected hydrides (TiH2, ZrH2), chlorides (VCl3, ScCl3) or oxides (V2O5) utilized as additives facilitating hydrogen release and uptake for magnesium hydride were investigated using mechano-chemical treatment and in-situ synchrotron radiation powder X-ray diffraction studies. The fastest hydrogen desorption and absorption kinetics for MgH2 was observed for a sample with 5 mol% V2O5 at 320 °C. Additional activation of the system (2 cycles, vacuum/p(H2) ~150 bar, 450 °C) leads to significant improvement of the kinetics even at lower temperatures, 270 °C. The observed prolific effect is achieved through the full reduction of vanadium oxides and formation of an efficient vanadium catalyst as nanoparticles and possibly interfacial effects in the MgO/Mg/MgH2/V system introduced during cycling hydrogen release and uptake in hydrogen/dynamic vacuum at 450 °C. Nanostructuring as well as hydrogen permeability via vanadium nanoparticles may improve kinetics and reduce the apparent activation energy for hydrogen release. Thus, the enhancement of hydrogen release/uptake in the MgH2 owe to “in situ” formation of vanadium nanoparticles by reduction of V2O5.

Mesoporous carbon-vanadium oxide films by resol-assisted, triblock copolymer-templated cooperative self-assembly.

Unlike other crystalline metal oxides amenable to templating by the combined assemblies of soft and hard chemistries (CASH) method, vanadium oxide nanostructures templated by poly(ethylene oxide-b-1,4-butadiene-b-ethylene oxide) (OBO) triblock copolymers are not preserved upon high temperature calcination in argon. Triconstituent cooperative assembly of a phenolic resin oligomer (resol) and an OBO triblock in a VOCl3 precursor solution enhances the carbon yield and can prevent breakout crystallization of the vanadia during calcination. However, the calcination environment significantly influences the observed mesoporous morphology in these composite thin films. Use of an argon atmosphere in this processing protocol leads to nearly complete loss of carbon-vanadium oxide thin film mesostructure, due to carbothermal reduction of vanadium oxide. This reduction mechanism also explains why the CASH method is not more generally successful for the fabrication of ordered mesoporous vanadia. Carbonization under a nitrogen atmosphere at temperatures up to 800 °C instead enables formation of a block copolymer-templated mesoporous structure, which apparently stems from the formation of a minor fraction of a stabilizing vanadium oxynitride. Thus, judicious selection of the inert gas for template removal is critical for the synthesis of well-defined, mesoporous vanadia-carbon composite films. This resol-assisted assembly method may generally apply to the fabrication of other mesoporous materials, wherein inorganic framework crystallization is problematic due to kinetically competitive carbothermal reduction processes.

Elementary steps of the catalytic NOx reduction with NH3: Cluster studies on reaction paths and energetics at vanadium oxide substrate

We consider different reaction scenarios of the selective catalytic reduction (SCR) of NO in the presence of ammonia at perfect as well as reduced vanadium oxide surfaces modeled by V2O5(010) without and with oxygen vacancies. Geometric and energetic details as well as reaction paths are evaluated using extended cluster models together with density-functional theory. Based on earlier work of adsorption, diffusion, and reaction of the different surface species participating in the SCR we confirm that at Brønsted acid sites (i.e., OH groups) of the perfect oxide surface nitrosamide, NH2NO, forms a stable intermediate. Here adsorption of NH3 results in NH4 surface species which reacts with gas phase NO to produce the intermediate. Nitrosamide is also found as intermediate of the SCR near Lewis acid sites of the reduced oxide surface (i.e., near oxygen vacancies). However, here the adsorbed NH3 species is dehydrogenated to surface NH2 before it reacts with gas phase NO to produce the intermediate. The calculations suggest that reaction barriers for the SCR are overall higher near Brønsted acid sites of the perfect surface compared with Lewis acid sites of the reduced surface, examined for the first time in this work. The theoretical results are consistent with experimental findings and confirm the importance of surface reduction for the SCR process.

Dehydrogenation of ethylbenzene with CO2 over V2O5/Al2O3–ZrO2 catalyst

Abstract V-containing catalysts supported on Al 2 O 3 , modified with varying amounts of ZrO 2 , were prepared by impregnation method. Dehydrogenation of ethylbenzene with CO 2 was run over these catalysts in a fixed-bed downflow stainless steel reactor. Compared with pure Al 2 O 3 support, a small amount of ZrO 2 in the support led to a significant increase in catalytic activity. Partial reduction of vanadium oxides and carbon deposition were the main reasons for the decreased catalytic activity.

Role of Lattice Oxygen of Metal Oxides in the Dehydrogenation of Ethylbenzene under a Carbon Dioxide Atmosphere

The mechanism for the dehydrogenation of ethylbenzene over V, Cr, and Fe oxides loaded on activated carbon, powdered diamond, Al(2)O(3), and MgO was studied in the presence of CO(2). Vanadium oxide-loaded catalysts provided higher styrene yields under CO(2) than Ar flow. The transient response method was carried out to understand the reaction behaviors of lattice oxygen of various metal oxides on the support. The results showed that lattice oxygen of vanadium oxide (V=O) was consumed in the dehydrogenation reaction and that reduced vanadium oxide was reoxidized with CO(2). A similar redox cycle was observed on iron oxide-loaded activated carbon catalyst. Spectroscopic characterization revealed that vanadium oxide and iron oxide on the support were reduced to a low valence state during the dehydrogenation reaction, and that CO(2) could oxidize the reduced metal oxides. In contrast, chromium(III) oxide was not reduced during dehydrogenation. From these findings, the redox cycle over vanadium oxide- and iron oxide-loaded catalysts was concluded to be an important factor in promoting the catalytic activity with CO(2).

Synthesis of vanadium-phosphorus oxide catalysts supported on pyrogenic silica and titanium dioxide

Various methods for the preparation of vanadium-phosphorus oxide (VPO) catalysts supported on aerosils A-300 and A-50 and TiO2 were studied: a traditional method (in an organic solvent under varying the support addition time, the nature of the reducing agent, and the degree of reduction of vanadium oxide) and barothermal and mechanochemical syntheses. With the use of XRD analysis, it was found that the composition of the resulting VPO phase depends on the time of support addition to the synthesis and the temperature of thermal treatment. Conditions for the formation of a supported phase of VOHPO4·0.5H2O, the precursor of the active component (VO)2P2O7, were determined. The presence of vanadium in an oxidation state of +4 was demonstrated using EPR and UV-VIS spectroscopy. The specific surface areas and pore structures of the synthesized catalysts were determined. The catalytic properties of samples in the reactions of n-butane oxidation in an excess of the hydrocarbon and oxidative ethane dehydrogenation were studied. It was found that, as compared with traditional bulk VPO catalysts, the use of the synthesized supported VPO catalysts made it possible to improve the process characteristics of n-butane oxidation and did not change these characteristics in the reaction of oxidative ethane dehydrogenation.

Direct reduction of vanadium oxide in molten CaCl2

A combined process of electrolysis of CaO in molten CaCl2 and calciothermic reduction, the so called OS process, was applied to prepare metallic vanadium directly from V2O3. Fine metallic powder containing 1860 wt ppm oxygen was obtained using a 0·5 mol.-%CaO–CaCl2 melt following processing for 10·8 ks (3 h) at 1173 K. The oxygen level did not decrease further even when the applied charge was doubled. Because metallic vanadium can be electrochemically deoxidised to 10 ppm oxygen from 1·48%, the high oxygen content in the reduced powder was attributed to the large amount of byproduct CaO due to reduction and the large surface area of the fine particles.

Oxidative dehydrogenation of propane: Differences between N 2O and O 2 in the reoxidation of reduced vanadia sites and consequences for selectivity

The role of vanadyl and peroxovanadate oxygen species in the oxidative dehydrogenation of propane (ODP) was analyzed by density functional theory (DFT). The dehydrogenation of C 3H 8 to C 3H 6 and the oxidation of C 3H 6 occurred over vanadyl oxygen of VO x species, yielding reduced VO x species. The vanadyl oxygen species were restored via reoxidation of reduced VO x species with O 2 and N 2O. The ODP reaction with N 2O occurred via a V V(d 0)/V III(d 2) redox couple, whereas both V V(d 0)/V IV(d 1) and V V(d 0)/V III(d 2) redox cycles were active with O 2. O 2 was a more active oxidizing agent than N 2O. Peroxovanadates as precursors of vanadyl species were formed on reoxidation of reduced vanadium oxide species with O 2, but not with N 2O. Peroxovanadates were highly reactive for propene oxidation. The absence of peroxovanadates may explain the superior performance of N 2O compared with O 2 in the selective ODP reaction over highly dispersed VO x species.

Direct Reduction of Vanadium Oxide in Molten CaCl2

A process consisted of electrolysis of CaO in the molten CaCl2 and of calciothermic reduction was applied to prepare metallic vanadium directly from V2O3. The fine metallic powder containing 1860 mass ppm oxygen was successfully obtained using 0.5mol%CaO-CaCl2 melt for 10.8 ks at 1173 K. The oxygen level from the oxide fitted with the demand for hydrogen storage application, but it remained nearly constant even when the twice electricity was supplied. The commercial grade powder contained 1.48% oxygen was, however, well deoxidized to 10 ppm. The large amount of by-product, CaO, due to reduction and the large surface area of the fine particles are the possible reasons of high oxygen content in the reduced powder.

Mixed-Valence and Neutral Framework Inorganic−Organic Hybrid Materials: Synthesis, Characterization, and Magnetic Properties of [{Co(N(CH)4N)}(V2O5)2] and [{Co(N(CH)4N)}V2O6

Two inorganic−organic hybrid materials [{Co(N(CH)4N)}(V2O5)2] (1) and [{Co(N(CH)4N)}V2O6] (2) have been synthesized and characterized by FT-IR spectroscopy, thermogravimetric analysis, elemental analysis, manganometric titration, temperature-dependent magnetic susceptibility measurement, bond valence sum calculations, and single-crystal X-ray diffraction analyses. The neutral framework materials were prepared by the amalgamation of vanadium oxide chains and layers with metal−organoamine coordination polymers. The three-dimensional framework structures of the crystals in 1 and 2 are composed of vanadium oxide layers (1) and chains (2) combined with {Co(N(CH)4N)}2+ motifs that cross-link the reduced vanadium oxide layers (containing mixed-valent vanadium centers) in 1 and oxovanadium chains in 2. Besides crosslinking the metal oxide chains and layers in these materials, cationic metal−pyrazine polymers provide charge balance leading to the neutral framework in these solids. The compounds show good thermal stability. Crystal data for 1 is as follows: C4H4N2O10CoV4, orthorhombic space group Cmcm, a = 14.3071(6), b = 6.9965(3), c = 11.4510(5) Å, V = 1146.24(9) Å3, Z = 4, Dcalcd = 2.913 Mg·m-3, R1 = 0.0385 (all data), wR2 = 0.1019. Crystal data for 2 are as follows: C4H4N2O6CoV2, orthorhombic space group Pnna, a = 10.2524(8), b = 11.5653(9), c = 7.4719(6) Å, V = 885.96(12) Å3, Z = 4, Dcalcd = 2.526 Mg·m-3, R1 = 0.0199 (all data), wR2 = 0.0547.