Microstructures occurring in reduced vanadium dioxide

Slightly oxygen-deficient VO₂ contains small clusters of (121) CS planes which ultimately order into the V_nO_2n?1 series of oxides with increasing degrees of reduction. The fact that CS is limited to (121) planes within this system may be due to a specific degree of ionic relaxation within the CS plane. Nominal compositions near to M_xVO₂ in the AlVO, GaVO, and FeVO systems contain microstructures which are essentially similar to those of the binary system and no new CS plane types are found at temperatures of the order of 1400°K.     

Novel vanadium phosphate phases as catalysts for selective oxidation

In our effort to induce novel modifications in the structure of some important vanadium phosphate phases used as selective oxidation catalysts, it has been observed that metal ions such as Zn²⁺, Ni²⁺, Pd²⁺can be incorporated into the vanadyl hydrogen phosphate VOHPO₄0.5H₂O phase in very different ways depending upon the medium of preparation. It has been found that the metal ions are either substituted into the lattice with retention of structure of the parent compound or intercalated between the layers of a new mixed-valent phase. These new metal-incorporated phases are catalytically active and the palladium incorporated compound in particular displays shape selective catalysis for different oxidation and reduction reactions. In another approach, the preparation of VOHPO₄0.5H₂O) has been modified to give a novel crystalline phase containing mixed-valentvanadium and having NH₃ species bound to the lattice. This phase could be a potential catalyst for ammoxidation reactions. In addition, novel mesostructured vanadium phosphate phases have been prepared using a long-chain amine as the templating agent involving a ligand templating mechanism of formation.     

Single-crystalline vanadium dioxide nanowires with rectangular cross sections

We report the synthesis of single-crystalline VO2 nanowires with rectangular cross sections using a vapor transport method. These nanowires have typical diameters of 60 (+/-30) nm and lengths up to >10 mum. Electron microscopy and diffraction measurements show that the VO2 nanowires are single crystalline and exhibit a monoclinic structure. Moreover, they preferentially grow along the [100] direction and are bounded by the (01) and (011) facets. These VO2 nanowires should provide promising materials for fundamental investigations of nanoscale metal-insulator transitions.     

New methodological approach for the vanadium K-edge X-ray absorption near-edge structure interpretation: application to the speciation of vanadium in oxide phases from steel slag

This paper presents a comparison between several methods dedicated to the interpretation of V K-edge X-ray absorption near-edge structure (XANES) features. V K-edge XANES spectra of several V-bearing standard compounds were measured in an effort to evaluate advantages and limits of each method. The standard compounds include natural minerals and synthetic compounds containing vanadium at various oxidation state (from +3 to +5) and in different symmetry (octahedral, tetrahedral, and square pyramidal). Correlations between normalized pre-edge peak area and its centroid position have been identified as the most reliable method for determining quantitative and accurate redox and symmetry information for vanadium. This methodology has been previously developed for the Fe K edge. It is also well adapted for the V K edge and is less influenced by the standard choice than other methods. This methodology was applied on an "environmental sample," i.e., a well-crystallized leached steel slag containing vanadium as traces. Micro-XANES measurements allowed elucidating the microdistribution of vanadium speciation in leached steel slag. The vanadium exhibits an important evolution from the unaltered to the altered phases. Its oxidation state increases from +3 to +5 together with the decrease of its symmetry (from octahedral to tetrahedral).     

Surface plasmon resonance of silver nanoparticles on vanadium dioxide

The localized surface plasmon resonance (SPR) spectrum of silver nanoparticles fabricated on a thermochromatic film, vanadium dioxide (VO2), is studied in this paper. Owing to the temperature-dependent dielectric function of VO2, the SPR band dramatically exhibits temperature dependence in the range of 30-80 degrees C. The peak extinction wavelength, lambda(SPR), blueshifts as temperature increases and reversibly redshifts as temperature decreases. The shift magnitude (DeltalambdaSPR) is strongly dependent on the silver mass thickness, dm; a value of 50 nm of DeltalambdaSPR is achieved for particles (mean diameter 51 nm) with dm=2 nm while a value of 250 nm is achieved for particles (mean diameter 133 nm) with dm=10 nm. Beyond the SPR band, it is interesting to find that the spectral line shape of silver particles is dominated by the imaginary part of the dielectric function of VO2. These results can be interpreted based on dynamical Maxwell-Garnett theory.     

Relaxation kinetic studies of sulfur dioxide oxidation over vanadium catalyst

Relaxation processes of approaching a steady state of the catalytic system in sulfurous anhydride oxidation over a vanadium catalyst have been studied by a transient response method. It is shown that the experimental results can be interpreted in terms of a two-route mechanism.

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Hysteresis phenomena in sulfur dioxide oxidation over supported vanadium catalysts

Catalyst deactivation and hysteresis behavior in industrial SO₂-oxidation catalysts have been studied in the temperature region 350–480°C by combined in situ EPR spectroscopy and catalytic activity measurements. The feed gas compositions simulated sulfuric acid synthesis gas and wet/dry deNO_x'ed flue gas. The vanadium (IV) compound K₄(VO)₃(SO₄)₅ precipitated during all the investigated conditions hence causing catalyst deactivation. Hysteresis behavior of both the catalytic activity and the V(IV) content was observed during reheating.     

Nonsteady-state kinetic model of sulfur dioxide oxidation on vanadium catalysts

Rate constants and activation energies of individual steps for the previously suggested mechanism of SO₂ oxidation on vanadium catalysts have been determined from experimental data on nonsteady-state kinetics.

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New zirconium kappa phases

The compounds Zr₉W₄S, Zr₉Mo₄S, and Zr₉W₄S_1?xNi_xO₃ have been prepared by high temperature techniques. They belong to the general group of K phases. Single crystal x-ray diffraction analysis of the mixed nickel/sulfide (Zr_9.2W_3.8S_0.7Ni_0.3O_3.0, R = 2.6%) reveals nickel substitution for sulfur, and confirms oxygen occupancy of interstitial sites.     

A Raman spectral study of hydrated vanadium sulfate (VSO5) phases

Raman spectra were measured for various hydrated VSO₅ materials. Spectral differences were noted and interpreted for α-VSO₅, VSO₅ · H₂O possessing vanadium atoms bonded to H₂O molecules in the interlayers, and other hydrated phases which involve broken VOS linkages.     

Molten salt synthesis and localized surface plasmon resonance study of vanadium dioxide nanopowders

Rutile-type vanadium dioxide nanopowders with four different sizes were successfully synthesized by carbothermal reducing V₂O₅ in KCl-LiCl molten salt. XRD and TEM characterizations suggested that vanadium dioxide particles formed by a broken and reunited process of vanadium oxide. Molten salt and organic carbon sources are crucial to the size of final particles. In the presence of the molten salt, the organic carbon with a shorter chain length would induce smaller particles. The UV-VIS-IR spectral measurements for as-prepared vanadium dioxide announced an obvious localized surface plasmon resonance band in the near infrared region at 90 °C.     

Specifics of adsorption and chemical processes on the surface of gamma-irradiated vanadium dioxide

The effect of γ-irradiation on the electrophysical properties and processes of thermal desorption of water from the surface of vanadium oxides V₂O₃-VO_2?δ-VO_2+δ-V₂O₅ were studied by the thermal analysis and electric conductivity techniques. It was shown that the amount of adsorbed water on the surface and the phase composition of the surface changed under exposure to low doses. The observed effects for vanadium oxides as semiconductors were correlated with analogous processes for alumina as a dielectric. It was found that the surface conductivity in irradiated VO_2?δ samples during the chemical reaction of ethanol with benzoic acid adsorbed after irradiation was substantially higher than that in their unirradiated counterparts. It was assumed that irradiation enhances the metal-semiconductor phase transition on the VO_2?δ surface during the chemical reaction.     

Studies of vanadium catalysts for sulfur dioxide oxidation by51V-NMR

Studies of the⁵¹V-NMR spectra of a series of industrial catalysts and systems modelling the active catalyst component indicate that the catalyst composition depends on the preparation method and during thermal treatment the support interacts with the active component. The local environment of vanadium is being formed in the process of catalytic reactions.

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⁵¹V , -I-4, , : KVO₃, KVO₃–SiO₂, KVO₃–K₂SO₄ , , . .

     

New liquid-crystalline polymers with chiral phases

Liquid-crystalline polyesters with cholesteric and probably chiral smectic C* phases were prepared using combined liquid-crystalline polymers (that is polymers with the mesogenic groups in the main chain as well as in the side groups). Copolyesters of these polymers and polymers with olefinic double bonds could be cross-linked retaining the liquid-crystalline phases. This resulted in cross-linked polymers with elastic properties.     

X-ray and neutron diffraction study of intermediate phases in nonstoichiometric cerium dioxide

Powder samples of reduced ceria, CeO_2?x, of known compositions in the range 0 < x < 0.3 have been examined by X-ray and neutron diffraction techniques in order to determine which intermediate phases belonging to the homologous series Ce_nO_2n?2 (with n = integer) truly exist. Through the appearance of superlattice lines in the neutron diffraction patterns, the existence of four distinct phases, corresponding to n = 7, 9, 10 and 11 was established. Aside from the phase Ce₇O₁₂, the structures of these phases cannot be accounted for with rhombohedral cells based on 〈111〉 vacancy strings, but indicate lower (monoclinic or triclinic) symmetry. The structure of Ce₉O₁₆ and Ce₁₀O₁₈ do not agree with structures proposed for the analogous Pr_nO_2n?2 compounds.     

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.

     

Activation diffusion of oxygen under conditions of the metal-semiconductor phase transition in vanadium dioxide

Density functional theory is used to calculate the energies of formation of oxygen vacancies and migration of oxygen in the monoclinic and rutile phases of vanadium dioxide. The results are compared to estimates of the parameters of activation diffusion of oxygen using data from the electron-beam modification of thin film structures of vanadium dioxide and their subsequent reduction in the temperature range of 20–100°C. It is shown that diffusion in both phases of vanadium dioxide has a preferential direction of oxygen migration along axis а in the monoclinic phase and axis с in the rutile phase. The difference between the rate of oxygen vacancy generation upon electron-beam exposure above and below the temperature of metal–semiconductor phase transition is explained by the jump (~150%) in the activation energy of oxygen diffusion upon the structural transition of rutile–monoclinic phase. The mobility of oxygen (oxygen vacancies) correspondingly changes by more than an order of magnitude.