X-ray ultrasoft spectra of vanadium in vanadium oxides

The Lα-emission spectra of vanadium in oxides V₂O₃, VO₂, V₂O₅, V₃O₅, V₄O₇, V₆O₁₁, V₆O₁₃, and V₃O₇ are investigated. It is discovered that the charge fluctuations in V₃O₅, V₄O₇, and V₆O₁₁ have a period greater than the $\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ hole life-time (～10^?15 sec), while in V₆O₁₃ and V₃O₇ this period is less than 10^?15 sec.     

Mechanism of electrochemical reduction of vanadium oxides

We specify the different electrochemical processes occurring when V₂O₅ is electrochemically reduced, yielding insertion products with lithium. Under low current density, V₂O₅ is of a ternary phase of approximate stoichiometry, V₂O₅Li_0.5. During the second step a further reversible insertion of Li⁺ occurs, yielding V₂O₅Li without any important modification of the crystalline structure, thus making the reduction reversible. During the two last steps, Li⁺ incorporation is much more difficult and rapidly causes an important and irreversible modification of the crystalline structure, thus making the reduction irreversible.V₂O₃, has nearly the same faradaic capacity as V₂O₅ but, unlike V₂O₅, it can be hardly be used in batteries since its discharge occurs in a wide potential range.     

Ligand field-actuated redox-activity of acetylacetonate

The quest for simple ligands that enable multi-electron metal–ligand redox chemistry is driven by a desire to replace noble metals in catalysis and to discover novel chemical reactivity. The vast majority of simple ligand systems display electrochemical potentials impractical for catalytic cycles, illustrating the importance of creating new strategies towards energetically aligned ligand frontier and transition metal d orbitals. We herein demonstrate the ability to chemically control the redox-activity of the ubiquitous acetylacetonate (acac) ligand. By employing the ligand field of high-spin Cr(ii) as a switch, we were able to chemically tailor the occurrence of metal–ligand redox events via simple coordination or decoordination of the labile auxiliary ligands. The mechanism of ligand field actuation can be viewed as a destabilization of the d_z² orbital relative to the π* LUMO of acac, which proffers a generalizable strategy to synthetically engineer redox-activity with seemingly redox-inactive ligands.

Tailoring the chemical surroundings of chromium(ii) allows reversible electron-transfer to the ubiquitous, purportedly redox-inactive acetylacetonate.     

Synthesis and characterization of vanadium oxides nanorods

Vanadium oxides nanorods with high crystallinity and high surface area were synthesized by hydrothermal method using laurylamine hydrochloride, metal alkoxide and acetylacetone. The samples characterized by XRD, nitrogen adsorption isotherm, SEM, TEM, and SAED. Uniformly sized B phase VO₂ nanorods had widths about 40-80 nm and lengths reaching up to 1 μm. V₂O₅ rodlike structured with the widths about 100-500 nm and the lengths of 1-10 μm were obtained by calcination at 400 °C for 4 h. This synthesis method provides a new simple route to fabricate one-dimensional nanostructured metal oxides under mild conditions.     

Reduction of nitrogen oxides with ammonia over complex vanadium and chromium oxides

Catalytic properties of -Al₂O₃ -supported complex vanadium and chromium oxides V_2–x Cr _x O_5– (0 < × s 1.3), amorphous to X-rays, in the reduction of nitrogen oxides with ammonia were studied. Vanadium exists in these catalysts mostly in a pentavalent state and chromium exists as Cr³⁺ and Cr⁶⁺. As the content of chromium in the catalysts increases, the optimal temperature of the process decreases, and the degree of conversion of nitrogen oxides increases.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 339–342, February, 1996.     

New investigations on acute toxicities of vanadium oxides

Summary The in-vivo-toxicity of the Vanadium-oxides V₂O₅ and V₂O₃ (administered orally, dermally and by inhalation) has been reinvestigated with particular emphasis on the safety and handleability of vanadium-oxides in the vanadium processing industry. Chemical-thermodynamic properties of vanadium-oxides make it likely that some earlier results on vanadium-toxicities have introduced artefacts as a consequence of the administration-techniques used. Special precautions have therefore been taken to avoid any chemical changes or artificial interactions during sample-preparation to ensure that the results significantly reflect the toxicities of the vanadium-compounds as exposure to them might occur. The LD₅₀(14d)-values indicate, that V₂O₅ should be classified as hamful (V₂O₅ techn. grade fused oral LD₅₀(14d): 716 mg/kg b.w. (rats m.) resp. 658 mg/kg b.w. (rats f.); inhal. LC₅₀ 16.2 mg/l (rats m.) resp. 4.0 mg/l (rats f.) for a 4-hour exposure), while V₂O₃ should be classified as relatively non toxic (V₂O₃ tech. grade powder oral: LD₅₀(14d): 5639 mg/kg b.w. (rats f.) resp. 8713 mg/kg b.w. (rats m.)) according to the EEC-commission directive of July 29, 1983 (83/467/EEC). Based on interaction-studies and considering new results reported in literature, a 3-level-model of the mechanism of vanadium-toxicity via oxygen-radicals is suggested.

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Neue Untersuchungen zur akuten Toxizität von Vanadiumoxiden

Zusammenfassung Die in-vivo-Toxizität der Vanadium-Oxide V₂O₅ und V₂O₃ bei oraler, dermaler und inhalativer Applikation wurde neu untersucht. Aufgrund einer Analyse der chemisch-thermodynamischen Eigenschaften dieser V-Oxide wird nahegelegt, daß die Resultate einiger früherer Toxizitätsuntersuchungen durch chemische Veränderungen der zu untersuchenden Stoffe bei der Probenvorbereitung verfälscht wurden. Nach den in-vivo-LD₅₀(14d)-Werten ist V₂O₅ als mindergiftig (V₂O₅ techn. fused oral LD₅₀(14d): 716 mg/kg b.w. (Ratten m.) bzw. 658 mg/kg b.w. (Ratten w.); inhalotiv LC₅₀ 16.2 mg/l (Ratten m.) bzw. 4.0 mg/l (Ratten w.) for a 4-hour exposure) bzw. V₂O₃ (techn. pulv. peroral LD₅₀(14d): 5639 mg/kg KG (Ratten w.) bzw. 8713 mg/kg KG (Ratten m.) als relativ nicht toxisch-nicht klassifiziert gemäß EEC-Commission-Directive vom 29. Juli 1983 (83/467/EEC) einzustufen. Basierend auf Studien der Interaktionswirkung bestimmter Substanzen und unter Eibeziehung der Resultate jüngst mitgeteilter Befunde zur Vanadium-Toxizität an Zellkulturen, wird ein Modell zum Vanadium-Toxizitäts-Wirkungsmechanismus vorgeschlagen, das 3 Hauptmechanismen — abhängig von der Konfrontations-Intensität (Konzentration und Expositionsdauer) — nahelegt.

     

Solid-state interactions of vanadium and phosphorus oxides in the closed systems

Physicochemical processes during thermal treatment of vanadium and phosphorus oxides mixture (1) as well as with diammonium hydrophosphate (2) in the closed system (autoclave) have been studied. In the first case, at 300 °C, the defective structure γ-VOPO₄ is formed and in the second case, there was established possibility of synthesis of vanadyl hydrophosphate—the precursor of vanadyl pyrophosphate (the catalyst of n-butane oxidation to maleic anhydride). At the same time, various phases of mixed ammonium and vanadium phosphates were obtained at lower and higher temperatures.     

Photocatalytic oxidation of propylene with molecular oxygen over highly dispersed titanium, vanadium, and chromium oxides on silica

Photocatalytic oxidation of propylene with molecular oxygen at room temperature was investigated over various silica-supported metal oxides with low loading. The photocatalytic active site is assumed to be the isolated tetrahedrally coordinated metal oxides in the ligand-to-metal charge-transferred state, such as (Mdelta- -OLdelta+). Photocatalytic epoxidation of propylene into propylene oxide was promoted over silica-supported V and Ti oxides at steady state. Over silica-supported Cr oxide, the propylene oxide formation rate was remarkably decreased with the time course in the reaction. The oxidation state and the coordination environment of the supported Ti, V, and Cr oxide species were determined by diffuse reflectance UV-vis spectroscopy (DRS) and electron spin resonance (ESR). During the photocatalytic oxidation, the oxidation state of the Ti4+ species was not varied. On the other hand, the V5+ species was partially reduced to V4+ and the Cr6+ species was successively reduced to Cr5+ and Cr3+. An isotopic tracer study of the C3H6-18O2 reaction suggests the difference of the active oxygen species between TiO2/SiO2 and V2O5/SiO2. The active oxygen species on TiO2/SiO2 is derived from molecular oxygen. On the other hand, the photogenerated products on V2O5/SiO2 incorporate the lattice oxygen of the surface metal oxide species. It is suggested that the kinds of terminal ligand (hydroxyl or oxo) of the tetrahedrally coordinated metal oxides on silica decide the active oxygen species in the photocatalytic oxidation. A photoinduced hole center on the monohydroxyl (SiO)3Ti-OH species activates molecular oxygen that reacts with propylene. In the case of the monooxo (SiO)3V=O and dioxo (SiO)2Cr=O2 species, the photoactivated lattice oxygen (OL-) directly reacts with propylene.     

The mineral-graphite electrode and its application to the analysis of vanadium oxides

Vanadium in different oxidation states in a sample of mixed oxides can be determined by a voltammetric method without the need for decomposition of the sample. The oxide is mixed into a paste with graphite and 1-bromonaphthalene, and packed into the electrode holder. Current maxima on the polarization curves give a measure of the amount of vanadium present in each oxidation state.     

On the nature of oxygen particles in the low-temperature desorption from vanadium oxides

The nature of oxygen particles produced in low-temperature desorption from V₂O₅ and V₂O₅ · MoO₃ was studied. It was found that there was no low-temperature desorption from samples in the presence of a quartz trap cooled to −195°C, which was evidence of the absence of possible desorption of O₃ and the presence of ¹Δ_g O₂. Under these conditions, the latter was frozen completely, and ozone was not.     

Quantitative determination of the speciation of surface vanadium oxides and their catalytic activity

A quantitative method based on UV-vis diffuse reflectance spectroscopy (DRS) was developed that allows determination of the fraction of monomeric and polymeric VO(x) species that are present in vanadate materials. This new quantitative method allows determination of the distribution of monomeric and polymeric surface VO(x) species present in dehydrated supported V(2)O(5)/SiO(2), V(2)O(5)/Al(2)O(3), and V(2)O(5)/ZrO(2) catalysts below monolayer surface coverage when V(2)O(5) nanoparticles are not present. Isolated surface VO(x) species are exclusively present at low surface vanadia coverage on all the dehydrated oxide supports. However, polymeric surface VO(x) species are also present on the dehydrated Al(2)O(3) and ZrO(2) supports at intermediate surface coverage and the polymeric chains are the dominant surface vanadia species at monolayer surface coverage. The propane oxidative dehydrogenation (ODH) turnover frequency (TOF) values are essentially indistinguishable for the isolated and polymeric surface VO(x) species on the same oxide support, and are also not affected by the Br?nsted acidity or reducibility of the surface VO(x) species. The propane ODH TOF, however, varies by more than an order of magnitude with the specific oxide support (ZrO(2) > Al(2)O(3) > SiO(2)) for both the isolated and polymeric surface VO(x) species. These new findings reveal that the support cation is a potent ligand that directly influences the reactivity of the bridging V-O-support bond, the catalytic active site, by controlling its basic character with the support electronegativity. These new fundamental insights about polymerization extent of surface vanadia species on SiO(2), Al(2)O(3), and ZrO(2) are also applicable to other supported vanadia catalysts (e.g., CeO(2), TiO(2), Nb(2)O(5)) as well as other supported metal oxide (e.g., CrO(3), MoO(3), WO(3)) catalyst systems.     

Determination of stoichiometry of vanadium oxides by 14-MeV neutron activation

A method was developed for the determination of oxygen/vanadium ratios in vanadium oxides by 14-MeV neutron activation analysis by means of the reactions ⁵¹V(n, p)⁵¹Ti and ¹⁶O(n, p)¹⁶N. A linear relationship between activity ratios and weight ratios of six mixtures composed of vanadium carbide and oxalic acid was obtained with a precision of 0.18% after suitable corrections for /gg- and β-ray absorption. Four commercial vanadium oxides and three specially prepared vanadium oxides were analyzed. The V/O ratio varied between 1.980 and 3.015. The percent standard deviation on these values was between 0.53% and 1.09%.     

Reactivity of some vanadium oxides: An EPR and XRD study

V₂O₅, VO₂ and V₂O₃ fresh samples and at different times after purchase or preparation (aged samples) were investigated by chemical analysis, redox treatments, XRD and EPR. The ageing process through a reaction with water and oxygen slowly oxidize crystalline VO₂ and V₂O₃, leading to a quasi-amorphous phase with bariandite structure (V₁₀O₂₄·12H₂O). The role of water is the progressive demolition of the compact structures and formation of hydrated phase. Kinetic study of VO₂ oxidation by O₂ and O₂+H₂O mixture indicates that increasing the temperature up to 723 K the effect of water becomes less important. The reaction leads to partially oxidized products with decreasing water content: bariandite at room temperature, V₃O₇·H₂O at 383 K and V₃O₇ at 723 K. Kinetic investigation of V₂O₅ reduction by CO at 633-723 K showed that the reduction process proceeds trough the formation of V⁴⁺ and of electrons delocalized in the conduction band.     

Influence of the oxide matrix on the electronic state of vanadium ions

ESCA spectra of V₂O₅, V₂O₅/TiO₂ and V₂O₅ dissolved in TiO₂ were recorded after oxidation and reduction. V⁴⁺ ions have the same effective charge in reduced V₂O₅ and in TiO₂, whereas the effective charge of V⁵⁺ ions in TiO₂ is higher than in V₂O₅, indicating enhanced electron acceptor properties.

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V₂O₅, V₂O₅ TiO₂ V₂O₅ TiO₂ . , V⁺⁴ V₂O₅ TiO₂ V⁺⁵ TiO₂ V₂O₅, .

     

Application of AXAFS spectroscopy to transition-metal oxides: influence of the nearest and next nearest neighbour shells in vanadium oxides

The influence of changes in coordination number, interatomic distances, and oxidation state on the intensity and centroid position of the Fourier transform (FT) of the atomic X-ray absorption fine structure (AXAFS) peak of vanadium oxide bulk model compounds and alumina-supported vanadium oxide clusters has been investigated. Using Na3VO4 and V2O5 as model compounds, it has been shown that the nearest neighbour shells have a pronounced influence on the AXAFS intensity; specifically, a 40 % decrease in intensity was observed between these two compounds. Secondly, the influence of partial reduction of the vanadium oxide species has been determined; this led to a 50 % decrease in the AXAFS intensity and to an increase in the centroid position. Furthermore, the influence of the vanadium oxide loading has been assessed. A non-linear relationship between the vanadium oxide loading and the AXAFS intensity has been found, indicating that the AXAFS intensity is sensitive to the formation of V-O-V bridging bonds between the vanadium VO4 clusters. The results show that AXAFS can be used to probe the relative energy level of the vanadium valence orbitals.     

Synthesis of vanadium oxides nanosheets as anode material for asymmetric supercapacitor

Vanadium oxides (V₂O₅) have been intensely investigated for advanced supercapacitors due to its extensive multifunctional properties of typical layered structure and multiple stable oxide states of vanadium in its oxides. In this study, V₂O₅ nanosheets are synthesized via V₂O₅ xerogel solvothermal reaction in ethanol solvent at 200 °C for 12 h. The V₂O₅ nanosheets facilitate the easy accessibility of ions and can provide more area available for electrochemical reactions. We have achieved the highest specific capacitance of 298 F/g and good rate discharge for V₂O₅ electrodes. Notably, the capacitance still retains a high retention rate of 85% after 10,000 cycles at 200 mV/s. Furthermore, asymmetric supercapacitors is assembled based on V₂O₅ nanosheets and active carbon electrode, and a specific capacitance of 13.2 F/g is obtained at 1 A/g, with a energy density of 4.7 Wh/kg at a power density of 0.798 kW/kg and remains 2.28 Wh/kg at 7.992 kW/kg. Based on these results, the asymmetric supercapacitor exhibits a good cycle life with 77.3% capacitance retention after 3000 cycles. It suggests that the V₂O₅ nanosheets are promising electrode material for electrochemical supercapacitors.     

The fluorination of uranium and vanadium oxides with some metal fluorides

UO₃ reacts with CrF₃ or CuF₂ forming UO₂F₂ and Cr₂O₃ or CuO respectively. Further fluorination occurs above 800°C to form UF₆ though the presence of excess CrF₃ gives mainly UF₄. The fluorination of U₃O₈ with CrF₃ gave UO₂F₂, UF₄ and Cr₂O₃ but with CuF₂ gave UO₂F₂, CuO and Cu₂O. VO₂ reacts with excess CrF₃ forming VF₃, VF₅ and Cr₂O₃. If there is a deficiency of CrF₃ the products are VOF₃, V₃O₅ and Cr₂O₃. CuF₂ and VO₂ form VOF₃, CuO and Cu₂O.     

Stability of molecular ions

Physical Institute, Leningrad State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 5, pp. 133–134, September–October, 1991.