Valence State Of Vanadium Research Articles

XAFS study of BIMEVOX ionic conductors for ME = Mg, Si, Zr, Zn

BIMEVOX ceramics with different molar fraction of ME = Mg, Si, Zr, Zn have been investigated. X-ray absorption spectra have been measured for K edges of vanadium and zinc. The XANES at the vanadium K edge undergoes a conspicuous change for the metal dopant concentrations exceeding dopant solubility limits. Position of the pre-edge peak indicates the valence state of vanadium to be 5+ in all studied BIMEVOX'es. Vanadium coordination, as inferred from the EXAFS data, shows similarity for all dopants used, with two characteristic distances to oxygen atoms. An estimate of oxygen occupation numbers is given. Local environment of zinc atoms has an average coordination number ca. 4, and exhibits shorter interatomic distances to oxygen atoms than vanadium.

Evidence of lower valence state of vanadium on the dielectric relaxation of ferroelectric SrBi2 (V0.1Nb0.9)2O9

Dielectric properties of SrBi2(V0.1Nb0.9)2O9 (SBVN) were investigated in a broad range of temperatures (400–1000 K) and frequencies (1 Hz to 10 MHz). Strong dielectric relaxation at the ferroelectric transition temperature was observed. The effect of post-sinter annealing on the frequency dispersion of SBVN and the possible mechanisms for the observed dielectric relaxation are presented. Electron paramagnetic resonance and x-ray photoelectron spectroscopy investigations provide direct evidence for the existence of lower valence state of vanadium (V4+) in SBVN ceramics.

Crystal growth of vanadium doped YAlO3, LaGaO3, and CaYAlO4 crystals and spectroscopic studies of vanadium valence states

V 4+ doped YAlO3, LaGaO3, and CaYAlO4 crystals (3d1 ion in octahedral coordination) have been grown and characterized as potential broadband laser materials using optical and electron paramagnetic resonance techniques. The effect of charge compensation on the vanadium valence state and spectroscopic properties of the materials are discussed.

Charge-ordering phase transition and order-disorder effects in the Raman spectra ofNaV2O5

In the ac polarized Raman spectra of ${\mathrm{NaV}}_{2}{\mathrm{O}}_{5}$ we have found anomalous phonon broadening, and an energy shift of the low-frequency mode as a function of the temperature. These effects are related to the breaking of translational symmetry, caused by electrical disorder that originates from the fluctuating nature of the ${\mathrm{V}}^{4.5+}$ valence state of vanadium. The structural correlation length, obtained from comparisons between the measured and calculated Raman scattering spectra, diverges at $T<5 \mathrm{K},$ indicating the existence of the long-range charge order at very low temperatures, probably at $T=0 \mathrm{K}.$

A New Vanadium Oxide Bronze: [NH3(CH2)6NH3]10[V15O37(Cl)]2[V15O36(Cl)](OH)3(H2O)3 with Clusters [V15O36(Cl)]5− and [V15O37(Cl)]6− Textured by Diaminohexane

A novel mesostructured mixed-valence polyoxovanadate with formula [NH3(CH2)6NH3]10[V15O37(Cl)]2[V15O36(Cl)](OH)3(H2O)3 was synthesized hydrothermally under autogeneous pressure at 443 K for 5 days from an aqueous mixture of NH2(CH2)6NH2 and V2O5 in acidic HCl (1 M) medium. Its structure was solved from single-crystal X-ray diffraction data at room temperature. It crystallizes in the orthorhombic symmetry (space group Pnma) with a=23.616(5), b=44.785(9), c=18.568(4) Å, V=19638.3(5) Å3, and Z=4. Two types of polyoxovanadate clusters, [V15O37(Cl)]6− and [V15O36(Cl)]5−, have been characterized, around which the stacking of hexanediamine dications is different. XPS measurements are in reasonable agreement with the calculated average vanadium valence state of 4.60±0.1 in this hybrid compound. All things being similar, hexanediamine appears to be slightly less reducing than octyldiamine and therefore has been placed below octyldiamine in a phenomenological diagram implying the “effective” redox potential of diamines which have to be compared with that of VV/VIV couple in the solid.

Redox Properties of VPO Ammoxidation Catalysts

The purpose of this study is to explore correlations between the molarity of V x IV/V O y proportions of (NH 4 ) 2 (V IV O) 3 (P 2 O 7 ) 2 catalysts, the vanadium valence states, and the reducibility of the vanadium on the one hand and the catalytic activity of the solids on the other. Pure (NH 4 ) 2 (V IV O) 3 (P 2 O 7 ) 2 and two ((NH 4 ) 2 (V IV O) 3 (P 2 O 7 ) 2 +V x IV/V O y ) solids (precursor V/P=1 and 1.5) are used as catalysts; the specimens are characterized by chemical analyses, different X-ray techniques (energy dispersive analysis of X-rays (EDX) and X-ray photoelectron spectroscopy (XPS)), temperature-programmed reduction (TPR), and catalytic measurements using the ammoxidation of toluene to benzonitrile as a model reaction.

Flow Injection Catalytic Determination of Vanadium using the Indicator Reaction between Victoria Blue B and Bromate and A Citric Acid Activator

ABSTRACT A flow injection catalytic method is proposed for determination of trace vanadium based on its catalytic action on a new indicator reaction between Victoria blue B (VBB) and potassium bromate in the presence of citric acid as an activator and in dilute sulfuric acid medium. The reaction is monitored spectrophotometrically by measuring the decrease in absorbance of VBB injected at the maximum absorption wavelength of 618 nm. The detection limit is 0.5 ng ml−1 and is independent of the initial valence state of vanadium. The relative standard deviation is 1.5% obtained from 11 standard solutions each containing 40 ng ml−1 of vanadium. The sampling rate is about 15 samples per hour. This method is very simple and has been applied to the determination of trace vanadium in some natural waters with recoveries of 93-103%.

Inversion Symmetry in the Spin-Peierls Compound α'-NaV2O5

At room-temperature, sodium pentaoxodivanadate, NaV2O5, was found to have the centrosymmetric space group Pmmn. This space group implies the presence of only one kind of V site, in contrast with previous reports [Carpy & Galy (1975). Acta Cryst. B31, 1481–1483] of the non-centrosymmetric P21mn counterpart. This indicates a non-integer valence state of vanadium. Furthermore, this symmetry has consequences for the interpretation of the transition at 34 K, which was ascribed to a spin-Peierls transition of one-dimensional chains of V4+.

Study by XPS and TPD of the interaction of n-pentane and n-butane with the surface of `non-equilibrated' and `equilibrated' V–P–O catalysts

Surface characterization of V–P–O catalysts used in the oxidation of n-butane and n-pentane to maleic and phthalic anhydrides is reported. Two different states for the same catalyst are selected: after a short time under reaction conditions (100 h), the `non-equilibrated' system, and after a much longer time-on-stream (1000 h), the `equilibrated' catalyst. Characterization has been carried out by following the desorption of molecules with a mass spectrometer after catalysts had been in contact with the hydrocarbons and/or O 2 mixtures at 613 K. Valence state of vanadium under environmental conditions similar to those used in the desorption studies was obtained by XPS. It was concluded that both V V and V IV are capable of interacting with the hydrocarbon and that not all the V IV present at the (VO) 2P 2O 7 surface can interact. On the other hand, desorption of molecules resulting from such interaction occurs at lower temperatures from V IV-rich surfaces (`equilibrated' catalyst), than from the V V richer surface (`non-equilibrated' catalyst).

TPD, XPS and ESR Studies of the Surface Processes Involved in the Oxidation ofn-Pentane on a (VO)2P2O7 System

Surface-sensitive techniques such as XPS, temperature-programmed desorption–mass spectrometry (TPD–MS) and electron spin resonance (ESR) have been used to study the effect of O2 treatment on the interaction ofn-pentane with the surface of an ‘equilibrated’ (VO)2P2O7 catalyst. The study was carried out by means of TPD studies after the catalyst had been in contact withn-pentane at 613 K in the presence and in the absence of O2 and by comparing the results, depending on whether the sample was subjected or not to O2 pretreatment at 613 K previously, ton-pentane adsorption. The valence state of vanadium and the changes in the short-range order around vanadium at the surface after former pretreatments were studied by XPS and ESR techniques, respectively. It was concluded that no oxygen radicals can be created on this catalyst, thus excluding the possibility that these kinds of radicals are involved in the oxidation process. On the other hand, despite the fact that the valence state of vanadium at the surface is an important variable, surface topology around vanadium centres at the surface of the ‘equilibrated’ (VO)2P2O7 sample clearly influences the interaction ofn-pentane with the surface. The short-range order around the vanadium sites does not seem to have much effect on the first steps of interaction ofn-pentane on the catalyst, but instead alters the further oxidation processes acting on the molecules resulting from the first steps.© 1997 by John Wiley & Sons, Ltd.

The reaction mechanism of the selective oxidation of butane on (VO) 2P 2O 7 catalysts: The influence of the valence state of vanadium

The selective oxidation of n-butane to maleic anhydride (MA) on (VO) 2P 2O 7-based catalysts having different vanadium oxidation states (+3.18 to +4.90) was studied by a vacuum transient technique applying the Temporal-Analysis-of-Products (TAP) reactor. Experiments in the absence of gas-phase oxygen (anaerobic conditions) revealed that all steps of the oxidation to MA may proceed on V 5+ as well as on V 4+ sites and that the step from furan to MA needs the catalyst state with the highest oxidation potential. In equilibrated (VO) 2P 2O 7 catalysts the valence state of the vanadium in the surface layers involved in the selective reaction amounted to +4 and +5. Under such conditions all reaction steps primarily occur on V 5+ centers but a contribution of the redox couple V 4+ V 3+ in these steps cannot be excluded.

Effect of the surface vanadium valence state on activity and selectivity properties of (VO)2P2O7 used as catalyst in the ammoxidation of toluene

The effect of the surface vanadium valence state of a (VO)2P2O7 catalyst, which was changed by reductive as well as oxidative pretreatment, on its catalytic properties in toluene ammoxidation to benzonitrile is reported.

Lithiation characteristics of α-CuV 2O 6 and other nCuO · V2O5 oxides

Lithiation characteristics of a complex oxide containing V(V) and Cu(II), α-CuV 2O 6, were investigated. Lithiation was carried out chemically using an n- BuLi/ hexane solution. The valence states of copper and vanadium and the structural changes upon lithiation were analyzed using ESR, XRD and OCV measurements. It was revealed that reduction processes occurred consecutively with an increasing extent of lithiation, x = Li (Cu + 2V) and Cu(II) component was reduced to Cu(I) preferentially over V(V). The single phases exist at 0 < x < 0.35 and 1.5 < x < 2.0, and at 0.35 < x < 1.5 and 2.0 < x < 3.0 lithiation occurs in multi-phases of the original and the second phases and the second and Cu(O) phases, respectively. Electrochemical lithiation behaviours of a series of the complex oxides nCuO · V 2 O 5 were compared by the galvanostatic intermittent titration technique.

Lithiation characteristics of Cu 5V 2O 10

Lithiation characteristics of a complex oxide containing V(V) and Cu(II), Cu 5V 2O 10, were investigated. Lithiation was carried out chemically using an n-BuLi/hexane solution. The valence states of copper and vanadium and the structural changes upon lithiation were analyzed using esr, XRD and ir measurements. It was revealed that reduction processes occurred consecutively with an increasing extent of lithiation, x = Li/(5 Cu + 2 V). Up to x = 3, preferential reduction of Cu(II) to Cu(I) occurred in the single phase. At 3 < x, Cu(II) was reduced to Cu(O) and Cu(II) (LiCuVO 4) in the multiphase system. Above about x = 6, LiCuVO 4 was further reduced to metallic copper. The open-circuit potentials of lithiated Cu 5V 2O 10 decreased with x gradually at 0 < x < 3, and has a plateau at x > 3.

The mechanism of n‐butane oxidation. Characterization of titania‐supported V‐P‐O catalysts by pulse‐flow experiments and in‐situ XANES spectroscopy

AbstractWell‐dispersed supported V‐P‐O (Vanadium Phosphorus Oxyde) catalysts were prepared and tested in the selective oxidation of n‐butane to maleic anhydride. Because of their high activity, it was possible to test titania‐supported catalysts in a two‐step oxidation reduction process at low temperatures. After n‐butane adsorption and activation in the absence of molecular oxygen, desorption took place after subsequent introduction of molecular oxygen into the gas stream, leading to the selective formation of maleic anhydride.In‐situ XANES spectroscopy was performed under reaction conditions in the two‐step oxidation reduction process with a special titania‐supported V‐P‐O catalyst with a high loading. The results of these experiments showed that, after equilibration, no more changes in the valence state of vanadium were observed after reduction or subsequent re‐oxidation. This indicates that the generally assumed Mars‐Van‐Krevelen mechanism is not operative for the titania‐supported V‐P‐O catalyst.

Lithiation characteristics of Cu 11V 6O 26

Lithiation characteristics of a complex oxide containing V(V) and Cu(II), Cu 11V 6O 26, were investigated. Lithiation was carried out chemically using an n-BuLi/hexane solution. The valence states of copper and vanadium and the structural changes upon lithiation were analyzed using ESR, XRD and IR measurements. It was revealed that reduction processes occurred consecutively with an increasing extent of lithiation, x=Li/(11Cu+6V). Up to x=3, Cu(II) was reduced preferentially to Cu (I) in the single phase. At 3< x, V(V) was reduced to V(IV) parallel to the reduction of Cu(II) in the two phase system, and the second phase was amorphous in nature. Further Cu(O) was formed at 7< x, and at 14< x the mother oxide disappeared. The open-circuit potentials of lithiated Cu 11V 6O 26 were 3.4 to 3.0 V at 0< x<20.

Lithium insertion characteristics of ß-Cu 2V 2O 7

Lithium insertion characteristics of a complex oxide containing V(V), ß-Cu 2V 2O 7, were investigated. Lithium insertion was carried out chemically using an n-BuLi/hexane solution. The valence states of copper or vanadium and the structural changes after lithium insertion were analyzed by ESR and XRD measurements, respectively. It was revealed that reduction processes occured successively with an increasing extent of lithium insertion, x = Li/2(V+Cu). Up to x = 2, Cu(II) was reduced preferentially to Cu(I), and Cu(I) and a half of V(V) were reduced to Cu(0) and V(IV), respectively at 2< x. Preferentially is predominant for β-phase oxide than for α-phase oxide.

Valence and magnetic properties of YSr 2−xCa xV 3O 9−y

Compounds YSr 2−xCa xV 3O 9−y have an orthorhombic symmetry. XPS results show that the vanadium ions exist in the mixing valence in the system. Temperature dependence of magnetic susceptibility represents the Curie-Weiss law. The valence state of vanadium obviously affects the magnetic properties of YSr 2−xCa xV 3O 9−y. The system exhibits a paramagnetic behavior from 300K to 1073K.

Investigation of the electrical and magnetic properties of YSr 2− xCa xV 3O 9− y

All the members of the solid solution of YSr 2− x Ca x V 3O 9− y have the orthorhombic symmetry. Their electrical and magnetic properties have been studied. The magnetic susceptibility and electrical resistivity increase gradually with x. The system shows paramagnetic behavior both at 300 K and at 77 K. It is shown that a change of valence state of vanadium obviously affects the electrical and magnetic properties of the solid solution.

Vanadium Oxide Deposited on an Rh Foil: CO and CO2 Hydrogenation Reactivity

The effects of submonolayer deposits of vanadia on the rates of CO and CO2 hydrogenation of an Rh foil have been examined. Auger electron spectroscopy (AES), ion scattering spectroscopy (ISS), and temperature-programmed desorption (TPD) have been used to determine the oxide coverage, and X-ray photoelectron spectroscopy (XPS) has been used to determine the oxidation state for VOx deposits on a rhodium foil. After oxidation (3 × 10−6 O2 at 350°C), the vanadium valence state is 3+, which corresponds to an oxide stoichiometry of V2O3. CO chemisorption reduces 34% of the V3+ to V2+ at low oxide coverages. The absolute amount of V2+ in the oxide overlayer after CO titration is found to maximize at a coverage of 0.5 ML. The rates of CO and CO2 hydrogenation increase upon addition of VOx to an Rh foil. For CO hydrogenation, the rate maximizes at two times that of the rate on the clean surface at an oxide coverage of 0.4 ML. For CO2 hydrogenation, the rate maximizes at six times that of the clean surface rate at an oxide coverage of 0.6 ML. The oxide promoter also alters the activation energies, partial pressure dependences, and selectivities for the hydrogenation reactions. A comparison of the kinetic and spectroscopic data reveals a strong correlation between the degree of reducibility of the oxide overlayer and the amount of rate enhancement for CO and CO2 hydrogenation.