EPR and TPR investigation of the redox properties of vanadia based ceria catalysts

Vanadium cerium oxides, with different V/Ce atomic ratios, were prepared using the impregnation method and calcined under air at 500°C. Physicochemical studies have shown that at low vanadium content, polymeric V-O-V chains are stabilized on the ceria surface. Increasing the vanadium content tends to favor the formation of the CeVO₄ and V₂O₅ phases. The redox properties of these oxides have been simultaneously investigated by TPR/TPO and EPR techniques. V-O-V chains and V₂O₅ species are more easily reducible than the CeVO₄ phase. The reduction of V₂O₅ to V₂O₃ proceeds in several steps, the intermediate species being V₆O₁₃, VO₂ and V₅O₉. The reduction of V₂O₅ species interacting with ceria support leads to VO oxide. EPR measurements performed at T = −269°C have permitted to observe progressively different signals of V⁴⁺ in addition to vanadium ions in V²⁺ (3d³) paramagnetic configuration. This attribution is based on an EPR signal at g = 3.956 with eight well resolved hyper fine lines (A = 96 Gauss), which may be attributed to the perpendicular components of one of the fine transitions corresponding to the V²⁺ spectrum. At high reduction temperature, CeVO₄ phase leads in one step to CeVO₃ and a continuous and partial reduction of CeO₂ into Ce₂O₃ is observed. Re-oxidation process shows that polymeric V-O-V chains, easily reducible, are hardly re-oxidized whereas V₂O₅ species, present in the high vanadium loading samples, are easily re-oxidized at low temperatures. However, redox processes seem to be reversible.     

EPR and magnetic susceptibility studies on V2 O5-P2O5-PbO glasses

EPR and magnetic susceptibility investigations on the glass system with 04+ ion content are explained using a simulation program on the assumption of the superposition of two signals, one with hyperfine structure (hfs) typical for isolated ions and another one consisting of a broad line without hfs characteristic for clustered ions. The hfs is not shown for x<3mol%V₂O₅. The EPR data show the presence of V₄₊ ions in a square-pyramidal co-ordination (C_4V) for 3x20mol%V₂O₅. The progressive disappearance of hfs for high V²O⁵ content (>20 mol %) suggests the increase of the associated ion number coupled by superexchange interactions. This result is consistent with magnetic susceptibility data for >3 mol % where the temperature dependence of magnetic susceptibility is described by the Curie–Weiss type law with a negative paramagnetic Curie temperature. The magnetic susceptibility results allowed the estimation of the V⁴⁺/V⁴⁺ + V⁵⁺ ratio in the sample studied.     

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.     

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.     

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.     

Preparation of V/ZrO2 catalysts by the sol-gel method: Physical and structural characterization

Some zirconia-supported vanadium oxides have been prepared by sol-gel method and characterized by different techniques. The vanadium content exerts an effect on the area and pore-size of precursors and samples calcined. The presence of vanadium also affects the decomposition of precursors into ZrO₂ and to its phase transformation, as well as the formation of fibres, as shown by SEM-EDX. XPS analyses show the presence of two species of vanadium at the surface, V^5– and V⁴⁺, the percentage of each one depending on the load of vanadium in the catalyst.     

Electron paramagnetic resonance spectra of CeO2 catalyst for CO oxidation

The EPR spectra of freshly prepared CeO₂ samples as well as of samples used as a catalyst in the oxidation of CO to CO₂ are reported. Attribution of these patterns to oxygen species O ₂ ^– , O₂, O^–, or to Ce³⁺ ions is discussed and dynamic phenomena involving these species are described. Bottlenecked systems between conduction electrons and Ce³⁺ ions are envoked to explain the observation of the Ce³⁺ EPR spectrum up to temperatures of about 370 K. Oxygen-deficient systems are obtained when the sample is used as a catalyst for CO oxidation in the presence of pyrex or quartz powder as a diluent, showing that the latter can play some role in stabilizing such species.     

Preparation and magnetic properties of Ti1−xVxPO4

The solid solutions of Ti_1?xV_xPO₄ were prepared under the reductive circumstance, although there seems to be immiscibility near x=0.5. Their magnetic susceptibility was analysed by the temperature-independent and Curie-Weiss contribution. The temperature-independent contribution increased gradually with x until x=0.4 and then decreased to 0 at x=1.0. A bent was observed at x=0.5 in the curve for the Curie constant against x and at x>0.5 the Curie temperature became strongly negative with x.In the region of x≦0.4, the Curie constant corresponded to the electronic configuration of d¹ for a vanadium ion. On the other hand, the Curie constant reflected the d² configuration for a vanadium ion in the range of $\text{x}\text{≧0.6}$. This fact indicates that Ti³⁺/1bV³⁺ homopolar bonds are formed in accordance with the replacement of Ti³⁺ with V³⁺ at x≦0.4. Vanadium ions added further, which substitute for Ti³⁺ in Ti³⁺/1bV³⁺ pairs, do not form the homopolar bond with V³⁺ ions and both V³⁺ ions contribute magnetically as ions with d² configuration.     

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.     

Synthesis and properties of Pb1-xVxO2-x(OH)x, solid solutions

Pb_1-xV_xO_2-x(OH)_x (0 <x ≤0.30) solid solutions with the rutile structure were prepared by reacting humid β-PbO₂ + V₂O₅ mixtures at temperatures from 200 to 250°C. At higher vanadium contents, lead hexavanadate, PbV₆O₁₆, was formed, as inferred from IR spectroscopy data. The structural and electrical properties of the solid solutions were studied, and the valence state of the constituent ions was determined. All of the vanadium has a valence of 4+ forx ≤ 0.06 and 5+ forx > 0.15. The lead ions are in the oxidation states 4+ and 2+ over the entire range of solid solution     

Study of magnetic inhomogeneity in β-Cu3Fe4V6O24

The temperature dependence of dc magnetization and electron paramagnetic resonance (EPR) spectra of the ??-Cu₃Fe₄V₆O₂₄ multicomponent vanadate were investigated. Dc magnetic measurements showed the presence of strong antiferromagnetic interactions (Curie-Weiss temperature, ?? ?? 80 K) at high temperatures, while zero-field-cooled (ZFC) magnetization revealed a cusp-like maximum in low fields at T _f1 = 4.4 K, which coincides with the splitting of the ZFC and FC curves. Another maximum was registered at T _f2 = 3.0 K. These two temperatures (T _f1 and T _f2) could be regarded as freezing temperatures in the spin glass state of two magnetic sublattices of Fe1 and Fe2 ions. The EPR spectrum of ??-Cu₃Fe₄V₆O₂₄ is dominated by a nearly symmetrical, very intense and broad resonance line centered at g _eff ?? 2.0 that could be attributed to iron ions. Below 10 K, an additional EPR spectrum with g ₁ = 2.018(1) and g ₂ = 2.175(1) appears, as well as a very weak line at g_eff = 1.99(1). The former spectrum is probably is due to divalent copper ions, and the latter line due to vanadium V⁴⁺ complexes. The temperature dependence of EPR parameters (g-factor, linewidth, integrated intensity) was determined in the range of 3?C300 K. Two low-temperature maxima in the temperature dependence of the integrated intensity (at 40 and 6 K) were fitted with a function suitable for pairs of exchange-coupled Fe³⁺ ions. A comparison of dc magnetic susceptibility and EPR integrated intensity indicates the presence of spin clusters, which play an important role in determining the low-temperature magnetic response of _??-Cu₃Fe₄V₆O₂₄.     

EPR and magnetic investigations of MnO-B2O3-PbO glasses

Electron paramagnetic resonance (EPR) of Mn²⁺ ions revealed their distribution on different structural units of a 2B₂O₃PbO glass matrix doped with MnO. Octahedral symmetric sites, tetragonally distorted, were detected. The progressive clustering of manganese ions was evidenced when the MnO content increased. EPR and magnetic susceptibility data revealed that both Mn²⁺ and Mn³⁺ ions are simultaneously present in the glass matrix and the transition from isolated ions in samples with x1 mol % to those involved in dipole–dipole interactions for samples with x30 mol % and antiferromagnetically coupled ions when x>30 mol %.     

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.     

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.     

Magnetic susceptibility of amorphous and heat-treated vanadate glasses

X-ray diffraction and magnetic susceptibility measurements were used to study the effect of addition of iron on the crystallization process and magnetic properties of the vanadium borophosphate system before and after heat treatment. The glass composition was 78 mol% V₂O₅-15 mol% P₂O₅-7 mol% B₂O₃, the Fe₂O₃ was added with concentration ranging from 0.05 up to 1 mol%. The X-ray diffraction showed that the V₂O₅ was the only phase separated during the heat treatment process. The intensity of the characteristic V₂O₅ peaks increased with increasing Fe₂O₃ content. The magnetic susceptibility was found to decrease for the sample containing 0.05 mol% Fe₂O₃. This could be explained by the presence of most of the iron ions in the ferrous state and the presence of a covalent bond between the ferrous ions and the oxygen ions. An increase in the magnetic susceptibility was found in samples containing an iron content of more than 0.05. This increase could be explained according to the change of Fe²⁺ ions to Fe³⁺ which has higher paramagnetic behaviour.     

Luminescent properties of vanadium-doped SnO2 nanoparticles

This paper reports the influence of doping degree and annealing temperature on XRD, Raman, EPR and PL spectra of Sn_1−xV_xO₂ nanoparticles with x = 0, 0.01 and 0.05 annealed at 600 and 800 °C. XRD studies reveal a tetragonal rutile crystalline phases of tin oxide, while the formation of V₂O₅ secondary phase was evidenced for all doped nanoparticles only by Raman scattering. In function of the doping degree and annealing temperature, from EPR spectroscopy was evidenced the presence of three different positions for V⁴⁺ ions in the samples: isolated ions disposed on the nanoparticles surface, ions which are coupled by dipolar or exchange interactions and cluster ions. The luminescence emissions associated with oxygen vacancies and structural defects are influenced by doping degree and annealing temperature and could be correlated with the crystallite size determined from XRD patterns.     

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.     

Optical and structural investigations on iron-containing phosphate glasses

The article reports the preparation and complex characterization of iron-containing phosphate glasses considered to be ecological materials, as they contain non-toxic compounds related to environment. The oxide system Li₂O?CMgO?C(CaO)?CAl₂O₃?CP₂O₅?C(FeO/Fe₂O₃) was investigated in respect to its structural changes caused by MgO replacement with CaO and by the iron addition. UV?Cvis?CNIR (ultraviolet?Cvisible?Cnear infrared) spectroscopy as well as thermo-gravimetric (TG) measurements, differential thermo-analysis (DTA), X-ray diffraction (XRD) analysis, electronic paramagnetic resonance (EPR), and Mossbauer (nuclear gamma resonance) spectroscopy have been used to investigate redox states and coordination symmetry of iron, together with vitreous network changes during the heat treatment up to 1000 °C. UV?Cvis?CNIR transmission spectroscopy revealed no structural modifications when MgO was substituted by CaO, but noteworthy absorption bands attributed to Fe²⁺/Fe³⁺ species. TG analysis made in the 20?C1000 °C range shows low weight loss accompanied by several thermal effects, as evidenced by DTA. XRD patterns for the glass samples heat treated at about 700 °C revealed the presence of different phosphate crystalline phases containing Mg, Al, and Fe ions. EPR spectroscopy revealed the presence of paramagnetic Fe³⁺ ions and the change of the first coordination symmetry, when the samples are heated below the vitreous transition temperature. Mossbauer spectroscopy has evidenced two paramagnetic species, Fe²⁺ and Fe³⁺, both in octahedral coordination symmetry and a clustering process supported by only Fe³⁺ ions.     

Coordination of V<Superscript>4+</Superscript> in Vanadium Titanium Oxide Xerogels

V_{2 ? y}Ti _y O_{5 ? δ} · nH₂O (0 < y < 1.33, layered structure) and Ti_{1 ? y}V _y O_{2 + δ} · nH₂O (0 < y < 0.25, anatase structure) solid solutions are synthesized by a sol-gel process. ESR spectra are used to assess the state of tetravalent vanadium in the solid solutions before and after calcination at 350°C and after intercalation with K⁺. Six types of ESR signals arising from paramagnetic ions in different environments are identified. Computer simulation is used to evaluate the ESR parameters of the materials studied.     

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.