Physico-chemical study of impregnated Cu and V species on CeO<Subscript>2</Subscript> support by thermal analysis,XRD, EPR, <Superscript>51</Superscript>V-MAS-NMR and XPS

CuVCe oxides were prepared by impregnation of copper and/or vanadium precursors on ceria support. The solids freshly prepared were calcined under air between 400 and 700 °C. Physico-chemical properties of these oxides were then studied using different techniques: Thermal Analysis (DSC/TG), X Ray Diffraction (XRD), Electron Paramagnetic Resonance (EPR), ⁵¹V Magic Angle Spinning Nuclear Magnetic Resonance (⁵¹V-MAS-NMR) and X-Ray Photoelectron Spectroscopy (XPS). X-ray diffraction and thermal analysis revealed cerium orthovanadate phase formation during the calcination of the 1Cu1V10Ce solid. This CeVO₄ phase is not observed for the 10Cu1V10Ce sample. The EPR study revealed two well-resolved copper signals: the first corresponds to isolated Cu²⁺ species and the second to Cu²⁺ dimers. The ⁵¹V-MAS-NMR confirmed the presence of CeVO₄ phase for 1Cu1V10Ce sample and revealed polymeric V–O–V chains in interaction with a copper ceria matrix for 10Cu1V10Ce sample. Finally, the XPS study indicated high vanadium content on the solid surface. This phenomenon is enhanced by the copper content in the solid and could explain the absence of the CeVO₄ phase in 10Cu1V10Ce sample. Thus the ceria orthovanadate phase formation is inhibited by the presence of a high copper content in the solid.     

EPR study of vanadium-loaded yttrium-cerium-zirconium solid solutions

5YO_1.5-10CeO₂-85ZrO₂ solid solution doped with vanadium was studied by means of electron paramagnetic resonance (EPR). The nature of the vanadium species formed by treatment of samples with O₂, CO, and H₂ was investigated. Three types of paramagnetic V⁴⁺ ions found in samples evacuated at 723 K have been attributed to tetragonally compressed octahedral complexes of vanadyl type with different interionic V–O distances. Two of them are supposed to be located on the surface of the oxide support, whereas the third type is in the bulk of the solid. EPR spectroscopy of all CO reduced samples showed the presence of isolated mononuclear V⁴⁺ species. Reducing treatment of the samples with H₂ also provoked the formation of aggregated V⁴⁺ species. The later species formed either by the agglomeration of surface isolated V⁴⁺ ions during high-temperature treatment or the reduction of polymerized V⁵⁺ species present in as-prepared samples. Obtained results can be used for the better understanding of vanadium effect on catalysts properties.     

Structural investigations of V<Subscript>2</Subscript>O<Subscript>5</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript>–CaO glass system by FT-IR and EPR spectroscopies

xV₂O₅·(100 − x)[0.7P₂O₅·0.3CaO] glass system was obtained for 0 ≤ x ≤ 35 mol% V₂O₅. In order to obtain information regarding their structure, several techniques such as X-Ray diffraction, FT-IR, and EPR spectroscopies were used. X-Ray diffraction patterns of investigated samples are characteristic of vitreous solids. FT-IR spectra of 0.7P₂O₅·0.3CaO glass matrix and its deconvolution show the presence in the glass structure of all structural units characteristic to P₂O₅. Their number are increasing for x ≤ 3 mol% V₂O₅ then, for higher content of vanadium ions, the number of phosphate structural units are decreasing leading to a depolymerization of the structure. The structural units characteristic to V₂O₅ were not evidenced but their contribution to the glass structure can be clearly observed. EPR revealed a well resolved hyperfine structure (hfs) typical for vanadyl ions in a C_4v symmetry for x ≤ 3 mol% V₂O₅. For 5 < x < 20 mol% V₂O₅ the spectra show a superposition of two EPR signals one due to a hfs structure and another consisting of a broad line typical for associated V⁴⁺–V⁴⁺ ions. For x ≥ 20 mol% V₂O₅ only the broad line can be observed. The composition dependence of the line-width suggests the presence of dipole–dipole interaction between vanadium ions up to x ≤ 5 mol% V₂O₅ and superexchange interactions between vanadium ions for x > 5 mol% V₂O₅.     

The effect of vanadium oxides on the crystallisation of silicate glasses

The influence of vanadium oxides as catalysts for nucleation and crystal growth in CaO-MgO-Al₂O₃-SiO₂ glasses has been investigated. The effect of varying the total vanadium content and the ratio of oxidised to reduced vanadium ions has been observed. No internal nucleation was observed, but the rate of growth of crystals of anorthite and wollastonite from the surface was increased by additions of V₂O₅ and V₂O₃. The values of the growth rates and the analysed V⁵⁺ content in both oxidised and reduced glasses suggest that V⁵⁺ ions are the most active species. Increasing concentrations of V₂O₅ in the glass gave maxima in the growth rates between 2 and 4 wt % for crystallisation temperatures between 900 and 980° C.     

A XANES study of the valence state of vanadium in lithium vanadate phosphate glasses

Valence states of vanadium in Li₂O-V₂O₅-P₂O₅ electronic-ionic conducting glasses have been studied by X-ray absorption spectroscopy. The composition dependence of the XANES range of the spectra has been used to estimate relative abundance of the vanadium atoms in V⁴⁺ or V⁵⁺ charge states. This information is important for analysis of electronic transport in these glasses, which occurs via electronic hopping between aliovalent vanadium centers. It was found that in samples with the lowest V₂O₅ contents (10 mole% of V₂O₅) a vast majority of vanadium ions are in V⁴⁺ charge state. At higher V₂O₅ contents the proportion of V⁴⁺ ions decreases at the expense of V⁵⁺ ones, but remains substantial even in glasses with the highest contents of V₂O₅.     

Interaction of V2O5-NaY zeolite with H2S and SO2

V₂O₅-NaY zeolite catalyst was prepared by vacuum impregnation of ammonium metavanadate solution in water on NaY-zeolite (zeolite). The slurry was filtered, washed, dried, and calcined at 600°C. On heating the catalyst at 450°C in vacuum, VO²⁺ species were detected by e.s.r. It was observed that the electron-accepting and -donating sites increased when V₂O₅ or vanadium species were loaded on it. Vanadium species poisoned the active sites of the zeolite that were responsible for the formation of SO₂⁻ ions. Pretreated V₂O₅-NaY zeolite with SO₂ plays an important role in the formation of VO²⁺ species when it is treated with H₂S at room temperature. V₂O₅-NaY zeolite can be used for the reduction of SO₂ by H₂S.     

V4+ in quenched calcium silicates: An electron spin resonance spectroscopic investigation

Electron spin resonance (ESR) studies of vanadium(IV) have been carried out on CaO-SiO₂, CaO-MgO-SiO₂ and CaO-Al₂O₃-SiO₂ slags equilibrated with oxygen partial pressures over the range 10^–9–10^–2 atm and also CaO-SiO₂-V₂O₅-Fe₂O₃ slag melted in air. The slags were melted at 1873 K and their V₂O₅ level was varied between 1 and 10 mol%. Three different melt basicities and alumina contents were investigated. Magnesia content was varied between 3.5 and 4.9 wt%. An industrial slag sample was also examined. The spin Hamiltonian parameters were determined and it was concluded that the paramagnetic vanadium was in the form of a distorted octahedral vanadyl complex (VO²⁺). An overlapping signal from a yet unknown paramagnetic vanadium species was present in all slags. In general this signal diminished with reduction in the oxygen pressures below 10^–6 atm and V₂O₅ content. It also decreased with increase in basicity and magnesia content. The critical fraction of paramagnetic vanadium for induction of the second species was dependent on melt composition.     

Defect structure and ionic conductivity as a function of thermal history in BIMGVOX solid electrolytes

The defect structure of the oxide ion conducting solid electrolyte, Mg substituted Bi₄V₂O_11– (BIMGVOX), was examined by high-resolution powder neutron diffraction. A detailed explanation of interpretation of the defect structure is presented. The general formula for the BIMGVOX solid solutions Bi₂V_1–x Mg _x O_5.5–3x/2 assumes complete oxidation of vanadium to V^V. Analysis of the neutron diffraction data reveals the defect structure and indicates that there is, in fact, partial reduction of vanadium to V^IV. The extent of reduction is dependent on thermal history, with high temperature quenched samples showing a greater degree of reduction than exponentially slow cooled samples. This is correlated with differences in electrical behaviour at low and high temperatures. Differences in ionic conductivity and activation energies between samples with different thermal histories are explained in terms of the balance between charge carrier concentration and the extent of defect trapping effects.     

Mineralogical reconstruction of Titanium-Vanadium hematite and magnetic separation mechanism of titanium and iron minerals

In this paper, magnetization roasting - low-intensity magnetic separation was performed on a pre-concentration concentrate obtained from an Australian hematite containing titanium and vanadium to realize the separation of titanium and vanadium. The results showed that an iron concentrate with TFe (total iron) grade of 58.71%, iron recovery of 86.72% and V₂O₅ grade of 1.00%, V₂O₅ recovery of 93.97% could be obtained under the optimum roasting conditions (roasting temperature of 580 °C, total gas-flow rate of 1.44 m³/h, CO concentrate of 30%, roasting time of 20 min), meanwhile the iron tailings (i.e., TiO₂ concentrate) with TiO₂ grade of 43.76%, TiO₂ recovery of 63.42% was obtained. XRD analysis, phase iron chemical analysis, and magnetic analysis were conducted on the raw sample and the roasted products. The results showed that after magnetization roasting, the hematite and limonite in raw sample were transformed into magnetite with strong magnetism, and the vanadium hematite of hexagonal system was transformed into vanadium magnetite of equiaxed system. As the weakly magnetic ilmenite does not participate in the reaction during reduction roasting, the separation of ilmenite and vanadium can be easily achieved via magnetic separation.     

Nature of V ions in SnO2: EPR and photoluminescence studies

SnO₂ and 5 at.% V doped SnO₂ samples were prepared by citrate-gel method. From Raman study on vanadium doped SnO₂, the existence of phase separated V₂O₅ clusters has been established. EPR study on the V doped sample clearly revealed the existence of V⁴⁺ ions, which are incorporated in SnO₂ lattice and the existence of conduction electrons with g = 1.993. For vanadium doped SnO₂ sample, there is a decrease in luminescence at 400 nm and an increase in activation energy of electrical conduction compared to undoped SnO₂, and this has been attributed to the decrease in oxygen vacancies brought about by the incorporation of V⁵⁺ in the SnO₂ lattice.     

Structural and electrochemical behaviour of sputtered vanadium oxide films: oxygen non-stoichiometry and lithium ion sequestration

Structural and electrochemical aspects of vanadium oxide films recently reported from ICMCB/ ENSCPB have been examined using appropriate structural models. It is shown that amorphous films are nonstoichiometric as a result of pre-deposition decomposition of V₂O₅. It is proposed that the structure of amorphous films corresponds to a nanotextured mosaic of V₂O₅ and V₂O₄ regions. Lithium intercalation into these regions is considered to occur sequentially and determined by differences in group electronegativities. Open circuit voltages (OCV) have been calculated for various stoichiometric levels of lithiation using available thermodynamic data with approximate corrections. Sequestration of lithium observed in experiments is shown to be an interfacial phenomenon. X-ray photoelectron spectroscopic observation of the formation of V³⁺ even when V⁵⁺ has not been completely reduced to V⁴⁺ is shown to be entirely consistent with the proposed structural model and a consequence of initial oxygen nonstoichiometry. Based on the structural data available on V₂O₅ and its lithiated products, it is argued that the geometry of VO_n polyhedron changes from square pyramid to trigonal bipyramid to octahedron with increase of lithiation. A molecular orbital based energy band diagram is presented which suggests that lithiated vanadium oxides, Li_xV₂O₅, become metallic for high values ofx.     

Oxidation-reduction equilibria of vanadium in CaO-SiO2, CaO-Al2O3-SiO2 and CaO-MgO-SiO2 melts

A series of redox studies of vanadium have been carried out in CaO-SiO₂, CaO-MgO-SiO₂ and CaO-Al₂O₃-SiO₂ melts/slags equilibrated over oxygen partial pressures (pO₂) range 10^–2–10^–9 atm at 1600°C. V₂O₅ level was varied from 1–5 mol%. Three different melt basicities and alumina contents were investigated. Magnesia content was varied between 3.5 and 4.9 wt%. A newly developed analytical technique based upon electron paramagnetic resonance (EPR) spectroscopy was successfully applied to these melts. Two redox equilibria corresponding to V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ pairs followed the O-type redox reaction over the oxygen partial pressure range investigated. Higher oxidation states of vanadium were stabilized with increasing basicity of slags. Two redox pairs coexisted within oxygen pressure region 10^–4–10^–6 atm. However, redox ratios did not indicate clear trends with increasing V₂O₅ content in CaO-SiO₂-V₂O₅ system. In CaO-SiO₂-Al₂O₃-V₂O₅ slags, a slight increase in redox ratios (V³⁺/V⁴⁺) was obvious when alumina quantity was changed from 3.22 to 5.44% at a basicity ratio 1.3, indicating an increase in slag acidity. CaO-MgO-SiO₂-V₂O₅ slags showed a sharp decrease in redox ratios (V⁴⁺/V⁵⁺) between 10^–2–10^–6 atm with addition of 3.5 wt% MgO, due to increasing free oxygen ions in slags.     

Preparation and Properties of Stoichiometric Vanadium Oxides

Stoichiometric films of the V₂O₃–V₂O₅system, including the Magneli phases V _n O_{2n– 1}(3 n 9), were prepared by thermal oxidation of metallic vanadium between 720 and 950 K in quartz tubes evacuated to a residual pressure below 10^–3Pa. The oxidation was performed in the presence of powder mixtures of two vanadium oxides with adjacent stability fields so as to ensure an oxygen partial pressure corresponding to the stoichiometry of the required vanadium oxide. The oxide films thus prepared showed a sharp metal–insulator transition, with a conductivity change comparable to that in single crystals.     

Effect of thermal treatment on various characteristics of undoped and V2O5-doped Co3O4/TiO2 catalysts

The thermal treatment of undoped and V₂O₅-doped Co₃O₄/TiO₂ catalysts was studied in the temperature range, 330–600° C both in vacuum and in air. The wide difference in the catalytic behaviour of the two catalysts could be attributed to surface as well as bulk diffusion of the active cobalt oxide particles. Although in both cases the total Co³⁺ ions of various energy states were considered to be the active species for the given reaction, the distribution of various cobalt species, namely Co-t and Co-o, occupying tetrahedral and octahedral sites in the support-defective structure, seemed to be seriously affected by doping with V₂O₅. This dopant was supposed to have two-fold effect: part is incorporated into the surface Co₃O₄ crystallites leading to smaller more mobile particles, easily reducible and more dispersed, and another part diffuses a few atomic layers deeper in the support causing the redistribution of cobalt species. Upon heating, the increased mobility and the increased availability of the support tetrahedral sites may be responsible for the deactivation behaviour. The bulk diffusion enhanced by doping might cause some modification in the porosity characteristics of the titania support.     

Phase Separation Derived Core/Shell Structured Cu11V6O26/V2O5 Microspheres: First Synthesis and Excellent Lithium‐Ion Anode Performance with Outstanding Capacity Self‐Restoration

Novel amorphous vanadium oxide coated copper vanadium oxide (Cu₁₁V₆O₂₆/V₂O₅) microspheres with 3D hierarchical architecture have been successfully prepared via a microwave‐assisted solution method and subsequent annealing induced phase separation process. Pure Cu₁₁V₆O₂₆ microspheres without V₂O₅ coating are also obtained by an H₂O₂ solution dissolving treatment. When evaluated as an anode material for lithium‐ion batteries (LIBs), the as‐synthesized hybrid exhibits large reversible capacity, excellent rate capability, and outstanding capacity self‐recovery. Under the condition of high current density of 1 A g^?1, the 3D hierarchical Cu₁₁V₆O₂₆/V₂O₅ hybrid maintains a reversible capacity of ≈1110 mA h g^?1. Combined electrochemical analysis and high‐resolution transmission electron microscopy observation during cycling reveals that the amorphous V₂O₅ coating plays an important role on enhancing the electrochemical performances and capacity self‐recovery, which provides an active amorphous protective layer and abundant grain interfaces for efficient inserting and extracting of Li‐ion. As a result, this new copper vanadium oxide hybrid is proposed as a promising anode material for LIBs.     

X-ray Photoelectron Spectra and Electrical Properties of the K4.3V6O16.2 Polyvanadate

The valence state of vanadium atoms in polycrystalline K_4.3V₆O_16.2 is studied by x-ray photoelectron spectroscopy. By decomposing the V 2p _3/2 peak into Gaussian components, the relative amounts of V⁵⁺, V⁴⁺, and V³⁺ ions in the polyvanadate are evaluated before and after Ar⁺ ion milling. The top surface layer of vacuum-sintered K_4.3V₆O_16.2 is shown to be enriched in potassium and oxygen ions. The 288-K dielectric permittivity, carrier mobility, and carrier concentration are determined by complex-impedance measurements.     

Novel structural properties of the lead–vanadate–tellurate glass ceramics

In this paper, we have examined and analyzed the effects of systematic intercalation of the lead ions on vanadate–tellurate glass ceramics with interesting results. The structural properties of the lead–vanadate–tellurate glass ceramics of compositions xPbO·(100 − x)[6TeO₂·4V₂O₅], x = 0 − 100 mol%, are reported for the first time. It has been shown by X-ray diffraction that single-phase homogeneous glasses with a random network structure can be obtained in this system. Among these unconventional lead–vanadate–tellurate glass ceramics, we found that network formers are good host material for lead ions and are capable to intercalate a variety of species such as Te₂V₂ ⁵⁺O₉, Pb₃(V⁵⁺O₄)₂, Pb₂V₂ ⁵⁺O₇, and V₂O₅-rich amorphous phase. On the other hand, these glass ceramics contain V⁴⁺ and V⁵⁺ ions necessary for the electrical conduction. Based on these experimental results, we propose that the V⁴⁺=O bonds are created by two different mechanisms: the first of reduction of V⁵⁺ ions to V⁴⁺ ions and thus of creation of V⁴⁺=O bonds.     

Seebeck coefficient of V2O5-R2O3-TeO2 (R = Sb or Bi) glasses

The thermoelectric power of glasses in the systems V₂O₅-Sb₂O₃-TeO₂ and V₂O₅-Bi₂O₃-TeO₂ was measured at temperatures in the range 373–473 K. The glasses in both systems were found to be n-type semiconductors. The Seebeck coefficient, Q, at 473 K was determined as –192 to –151 VK^–1 for V₂O₅-Sb₂O₃-TeO₂ glasses, and –391 to –202 VK^–1 for V₂O₅-Bi₂O₃-TeO₂ glasses. For these glasses in both systems, Heikes' formula was satisfied adequately for the relationship between Q and In [C _v/(1-C_v)] (C _v = V⁴⁺/V_total, C _v is the ratio of the concentration of reduced vanadium ions), and discussions confirmed small polaron hopping conduction of the glasses in both systems. Mackenzie's formula relating to Q and V⁵⁺/V⁴⁺ was also applicable to the glasses in both systems, and it was concluded that the dominant factor determining Q was C _v.     

Reactivity of layered vanadium pentoxide hydrate with ultrafine metal oxide, TiO2 and ZrO2, particles in an aqueous system

The interactions between vanadium pentoxide hydrate (V₂O₅·nH₂O) sol and colloid solutions of ultra fine titanium dioxide TiO₂ and zirconium dioxide particles ZrO₂ were studied. When mixed with an intrinsic V₂O₅·nnH₂O sol, TiO₂ particles in the mixed sol are sandwiched by V₂O₅·nH₂O layer sheets to form intercalation compounds. An Interlayer distance of V₂O₅·nH₂O was increased by this treatment and the surface area was also increased from 7.9 m² g^–1 for the V₂O₅·nH₂O to ca. 50 m² g^–1. When the TiO₂ sol was contacted with K-type V₂O₅·nH₂O, microporous nature appeared in the sample and the surface area incrased up to ca. 100 m² g^–1. The porous structure was maintained up to 300°C, above which materials were separated into two phases, anhydrous V₂O₅ and anatase type TiO₂. Ultrafine ZrO₂ particles were intercalated stoichiometrically in both intrinsic and K-type V₂O₅·nH₂O giving ZrO₂-V₂O₅·nH₂O for all the mixing ratios from ZrO₂/V₂O₅ = 5 to 20. Physico-chemical properties were almost unvaried and the materials were nonporous. Their surface areas are around 50 m² g^–1 for the former and around 60 m² g^–1 for the latter. The layered structure was maintained up to 300°C above which the sample was crystallized into ZrV₂O₇. The reaction temperature is about 150°C lower than that the heated mixture of ZrO₂ and V₂O₅ powders. The electron microscope observations of the prepared materials showed that the number of the stacked layers was decreased from more than 10 sheets for the sample before intercalation to about 2–4 sheets by exfoliation. This indicates that V₂O₅·nH₂O is exfoliated by ion exchangeably reacting to ultrafine titanium oxide and zirconium oxide particles.     

Structural and electrochromic properties of molybdenum doped vanadium pentoxide thin films by sol-gel and hydrothermal synthesis

Molybdenum-doped vanadium pentoxide (Mo-doped V₂O₅) thin films with doping levels of 3-10 mol% were prepared by dip-coating technique from a stable Mo-doped V₂O₅ sol synthesized by sol-gel and hydrothermal reaction. The Mo-doped V₂O₅ films had a layered V₂O₅ matrix structure along c-axis orientation with Mo⁶⁺ as substitutes. Values of the inserted and extracted charge density of 21.4 and 21.3 mC·cm^− 2 and the transmittance variation (ΔT at 640 nm) between anodic (+ 1.0 V) and cathodic (− 1.0 V) colored states of 41% were observed for the films with 5 mol% Mo⁶⁺ doping. Above this dopant concentration, the charge capacity and ΔT decreased. The enhancement of the electrochemical and electrochromic properties of the films is related to changes in the electronic properties of V₂O₅ films due to the creation of energy levels in the band gap of V₂O₅ by the Mo doping, accompanied by the reduction of the forbidden-band width and the increase of the conductivity.