Crystalline V2O5 Research Articles

Preparation, characterisation and catalytic evaluation of MgF2 supported V2O5 catalysts for ammoxidation of 3-picoline

Non-conventional MgF2 supported V2O5 catalysts with different vanadia contents (2–15 wt.%) were prepared by impregnation (using NH4VO3), characterised and catalytically evaluated for selective ammoxidation of 3-picoline to nicotinonitrile. Oxygen and ammonia chemisorption uptakes increased continuously from 60 to over 600 μmol g−1 and 275 to >750 μmol g−1, respectively, with rise in V2O5 proportion indicating that the redox as well as acidic sites are increasing with increase in V2O5 content. Thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) analysis revealed endothermic as well as exothermic thermal effects mainly due to liberation of water and ammonia, and also due to structural changes. XRD patterns showed the formation of crystalline V2O5 in the fresh solids having 8 wt.% V2O5 and above and NH4VO3 phase in the spent samples. The conversion of 3-picoline is observed to increase continuously with increase in V2O5 loading. However, the selectivity of nicotinonitrile is found to be independent on conversion of 3-picoline. The catalyst with the highest V2O5 loading (15.7 wt.%) displayed the best activity (X > 90%) and selectivity (S > 95%) compared to all other catalysts of this series. The 3-picoline conversion of 10% at 548 K is increased to almost 100% with rise in temperature to 663 K. Increase in 3-picoline feed rate and NH3: 3-picoline ratio exhibited an inhibiting effect on the conversion, while an increase in air: 3-picoline ratio has no significant influence on the performance.

Hydrogen peroxide oxidation of toluene over V/Al systems

A series of γ-alumina supported vanadia catalysts have been prepared by adopting the wet impregnation method. Structural characterization of the catalysts was done by energy dispersive X-ray (EDX) analysis, X-ray diffraction (XRD) studies, BET surface area analysis, Fourier Transform-infrared spectroscopy (FT-IR) and UV-Vis Diffuse Reflectance spectroscopy (UVDRS). Liquid phase oxidation of toluene has been examined to investigate the catalytic activity. Spectroscopic investigation revealed the presence of highly dispersed tetrahedral vanadyl species at lower loadings and crystalline V2O5 at higher loadings. Incorporation of vanadia leads to greater activity and partial oxidation activity towards the liquid phase oxidation of toluene.

Ultrasound‐Assisted Synthesis of Hybrid Vanadium Oxide/Polyaniline Nanowires

AbstractA single‐step sonochemical procedure to synthesize hybrid vanadium oxide/polyaniline nanowires starting from crystalline V2O5 and aniline in aqueous medium is presented. The synthesis explores the effect of high power ultrasounds on heterogeneous solid–aqueous phases, which leads to 30 nm width wires of 5 to 10 µm in length. Monomer intercalation and oxidative polymerization within the inorganic matrix proceed simultaneously with morphological changes. The electronic conductivity of hybrid nanowires reaches 0.8 S · cm−1 at room temperature.magnified image

IL-assisted synthesis of V2O5 nanocomposites and VO2 nanosheets

Lamellar structures of alternating sheets of crystalline V2O5 (shcherbinaite) and cations from certain imidazolium-based ionic liquids (IL) (e.g. 1-ethyl-3-methyl-imidazolium tetrafluoroborate) were obtained starting from VOCl3. Surprisingly, growth along the [100] direction of shcherbinaite is suppressed due to the intercalation of the IL cations, which can be attributed to the higher charge density perpendicular to this plane compared to other crystallographic planes. In the lamellar nanostructure, V2O5 consists of sheets with just one single unit cell in the [100] direction (i.e. perpendicular to the sheets). Owing to the strong interaction with the imidazolium rings, crystallization of vanadium oxide occurs at relatively moderate temperature of ca. 100 °C. Depending on the heat treatment, diverse vanadium oxide nanostructures can be obtained from the lamellar IL-V2O5 nanocomposite, namely V2O5, V2O3 and VO2. The structure of the materials was characterized by X-ray scattering, scanning (SEM) and transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS).

Structural Characterization and Oxidehydrogenation Activity of CeO2/Al2O3and V2O5/CeO2/Al2O3Catalysts

Structural characterization and oxidative dehydrogenation activity of CeO2/Al2O3 and V2O5/CeO2/Al2O3 catalysts for ethylbenzene (EB) to styrene were investigated systematically. The CeO2/Al2O3 catalyst was prepared by a deposition precipitation method, and a theoretical monolayer equivalent of 10 wt % V2O5 was dispersed over its surface by a wet impregnation method to obtain the V2O5/CeO2/Al2O3 catalyst. To understand thermal and textural stability, the synthesized catalysts were subjected to calcination at various temperatures (773−1073 K). Physicochemical characterization was performed using X-ray diffraction (XRD), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis, and BET surface area techniques. The XRD and RS results suggested that the CeO2/Al2O3 sample is thermally quite stable up to 1073 K and that the ceria exists in the form of an over layer on the surface of the alumina support. In the case of the V2O5/CeO2/Al2O3 sample, no crystalline V2O5 was observed from XRD results, indicating a highly dispersed form of vanadium oxide on the CeO2/Al2O3 carrier when calcined at 773 K. The XPS peak shapes and the corresponding electron binding energies indicate that the dispersed vanadium oxide selectively interacts with the ceria portion of the CeO2/Al2O3 support and forms a CeVO4 compound at higher calcination temperatures. Formation of CeVO4 is also established from XRD and RS measurements. Both CeO2/Al2O3 and V2O5/CeO2/Al2O3 samples were tested for oxidative dehydrogenation of EB to styrene by using dry air as an oxidant. The V2O5/CeO2/Al2O3 catalyst exhibits more conversion and selectivity than CeO2/Al2O3. The time-on-stream experiments further reveal that the V2O5/CeO2/Al2O3 catalyst exhibits stable activity and selectivity without fast deactivation.

Selective Oxidation of Toluene with Hydrogen Peroxide Catalyzed by V-Mo-based Catalyst

Abstract The selective catalytic oxidation of toluene with hydrogen peroxide over V-Mo-based catalysts under mild conditions was studied. The promotion effect of Mo on the catalysts was studied with V/Al2O3 and Mo/Al2O3 as reference samples. The catalysts were characterized by XRD, TPR, and XPS techniques. The results show that the addition of Mo to V/Al2O3 may change the distribution of V species on Al2O3 surface. Over V-Mo/Al2O3 catalyst, highly dispersed amorphous V species facilitates benzaldehyde formation, and crystalline V2O5 species increases the conversion of toluene but decreases the selectivity to benzaldehyde, while AlVMoO7 species favors both the conversion of toluene and the formation of cresols. The yield of benzaldehyde depends remarkably on the surface O/Al and Mo/V atomic ratios, and gets to a maximum value of 13.2% with a selectivity of 79.5% at an O/Al atomic ratio of 3.0 and Mo/V atomic ratio of 0.7.

Direct Observation of the Structural Component of the Metal−Insulator Phase Transition and Growth Habits of Epitaxially Grown VO2 Nanowires

We have grown epitaxially orientation-controlled monoclinic VO2 nanowires without employing catalysts by a vapor-phase transport process. Electron microscopy results reveal that single crystalline VO2 nanowires having a [100] growth direction grow laterally on the basal c plane and out of the basal r and a planes of sapphire, exhibiting triangular and rectangular cross sections, respectively. In addition, we have directly observed the structural phase transition of single crystalline VO2 nanowires between the monoclinic and tetragonal phases which exhibit insulating and metallic properties, respectively, and clearly analyzed their corresponding relationships using in situ transmission electron microscopy.

Optical and other physical properties of semiconducting cadmium vanadate glasses

The optical and other physical properties of semiconducting xV2O5−(1−x)CdO glasses have been reported in this article. The changes in both density and molar volume indicate structural modification of crystalline V2O5 due to addition of CdO. The glass transition temperatures are observed to decrease with an increase in V2O5 content in the compositions. It has been observed from the analysis of Fourier-transform infrared spectra that the network structure strongly depends upon the V2O5 content in the glass compositions. Introduction of CdO into the V2O5 matrix changes the two-dimensional layer structure of the crystalline V2O5 into a more complicated three-dimensional structure. The fundamental absorption edge for all the glasses is analyzed in terms of the theory proposed by Davis and Mott [E. A. Davis and N. F. Mott, Philos. Mag. 22, 903 (1970)]. The position of absorption edge and, hence, the value of the optical band gap are found to depend on the V2O5 content in the glass compositions.

Promoter role of V2O5 on vanadium supported Al0.5Ga0.5PO4 catalysts during propane ammoxidation

Vanadium (5 wt%) was incorporated on the Al0.5Ga0.5PO4 support by impregnation and the resulting catalyst, 5V/Al0.5Ga0.5PO4 was characterized by BET surface area, pores distribution, X-ray diffraction, RAMAN spectroscopy and temperature-programmed reduction. Catalytic performances were measured on the propane ammoxidation. Crystalline V2O5 was observed on 5V/Al0.5Ga0.5PO4 catalyst; in addition, the originally amorphous A10.5Gao.5PO(4) Support Crystallises in presence of vanadium. The presence Of V2O5 on Al0.5Ga0.5PO4 enhances the propane conversion and the selectivity to acrylonitrile. (c) 2007 Elsevier B.V. All rights reserved.

Vanadium oxides supported on TiO2-Sepiolite and Sepiolite: Preparation, structural and acid characterization and catalytic behaviour in selective oxidation of toluene

The synthesis of vanadium oxide systems (5–20 wt% of V2O5) supported on TiO2-Sepiolite (with titania loading around the theoretical monolayer, 12 wt%) and on Sepiolite was carried out by the wet impregnation method. Materials were characterized by EDX, XRD, DR UV–vis, Raman, 29Si and 1H MAS NMR, TEM and nitrogen adsorption. Surface acid properties were evaluated through pyridine chemisorption. Their catalytic behaviour in the selective oxidation of toluene was studied. All solids exhibited the type IV isotherm, which corresponds to mesoporous solids. The decomposition of the Sepiolite into magnesium silicate and silica due to the effects of the vanadium incorporation process and calcination was not observed. The nature of the vanadium species depended on the support type and vanadium loading. Thus, in general, at the lowest vanadia loadings, isolated tetrahedral and polymetavanadate type species predominated. At higher loadings (15 wt% or above), vanadium oxides were also presented as crystalline V2O5 nanoparticles and, especially in %V/Sepc systems, as pyro- and meta-magnesium vanadates. On the other hand, in relation to the supports, the vanadium systems exhibited new acid sites whose density and strength both depended on the vanadia species structures. The catalytic performance of the vanadium oxide systems in the selective oxidation of toluene was also influenced by the structure and nature of vanadia species. Thus, the species formed on TiO2-coated Sepiolite support exhibited more activity in the formation of benzaldehyde (and benzoic acid) than the ones on Sepiolite. This formation is favoured by the polymeric vanadia species.

Cyclic voltammetry, electrochemical impedance and ex situ X-ray diffraction studies of electrochemical insertion and deinsertion of lithium ion into nanostructured organic–inorganic poly(3,4-ethylenedioxythiophene) based hybrids

Abstract In this work, we discuss the electrochemical insertion and deinsertion of lithium into poly(3,4-ethylenedioxythiophene) PEDOT–V2O5 nanohybrids by using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and powder ex situ X-ray diffraction (XRD) studies with special emphasis on their application potential as cathode materials for rechargeable Lithium batteries. The interlayer spacing of crystalline V2O5 expands from 0.43 to 1.41 nm by the intercalation of PEDOT nanoribbons prepared by a microwave irradiation method. Cyclic voltammetry studies of PEDOT–V2O5 hybrid at a slow scan rate of 0.058 mVs−1 between 2.0–4.3 V vs. Li/Li+, demonstrates that structural transitions of V2O5 are suppressed and more facile intercalation features appear in terms of well-defined reversible cathodic and anodic peaks when compared to that at higher scan rate. Electrochemical Impedance studies of PEDOT–V2O5 hybrid as cathode at ambient temperature with lithium metal foil as both counter and reference electrodes in 1 M LiPF6 in a mixture of ethylene and diethylcarbonate (50:50 by volume) between the range 2.0–4.3 V reveals the usefulness of these materials. The observed capacity fading during cycling in terms of surface passivation and structural transformations is also investigated by CV, EIS and XRD analysis.

High-Capacity Crystalline V2O5 Thick Films Prepared by RF Sputtering as Positive Electrodes for Rechargeable Lithium Microbatteries

We demonstrate that crystalline thick films up to thick can be synthesized by radio frequency (rf) sputtering using a vanadium target and without any postannealing treatment. Crystalline deposits in the range thick exhibit a rough surface morphology and a high porosity, allowing the electrolyte to penetrate the active material. The discharge–charge properties of the films are investigated in the two voltage windows and as a function of the thickness. We clearly show that such films allow one to get high specific capacities between 20 and in the range, depending on the thickness, even at a high rate of and between 40 and in the range. The influence of the current density indicates a little change in the capacity and the polarization, showing the homogeneous and efficient behavior of these sputtered films, in spite of their thickness, which can reach . The capacity is found to be a linear function of the thickness with a similar slope from the C/15 rate to the rate. The cycling behavior of these cathode film materials is attractive with, for instance, a high and stable capacity of after 90 cycles at at high voltage for the thick film and a capacity of found to be stable from the 50th cycle in the range at . The capacity fading observed after all of the first cycles in the potential limits and that found when the three insertion steps are involved is discussed in terms of structural data and impedance behavior.

Hydrothermal synthesis of single-crystal VO 2(B) nanobelts

Single crystalline VO 2(B) nanobelts with a metastable structure were obtained through a simple hydrothermal synthetic method. The VO 2(B) nanobelts were characterized by means of X-ray diffraction, transmission electron microscopy, selected area electronic diffraction, field-emission scanning electron microscopy and X-ray energy-dispersive spectroscopy techniques. The as-obtained VO 2(B) nanobelts are 400–600 nm long, typically 100–150 nm wide and 20–30 nm thick. The belt-like VO 2(B) with a high surface area may be beneficial to lithium insertion between the VO 6 layers for application in batteries.

Naphthalene oxidation over vanadium-modified Pt catalysts supported on γ-Al2O3

Alumina-supported platinum catalysts modified by vanadium were synthesised and tested for the complete oxidation of naphthalene. The catalysts were characterised by BET, pulsed CO chemisorption, powder X-ray diffraction, laser Raman spectroscopy and temperature-programmed reduction. Whilst BET and CO chemisorption results showed that the addition of vanadium modified both the textural properties of the support and the distribution of Pt, XRD and TPR data suggested the presence of V2O5 on catalysts with higher V loadings. TPR data showed that the concentration of V2O5 and possibly some other vanadium species increased as the V loading increased. Only 0.5%V was found to promote the activity of the 0.5%Pt/γ-Al2O3 catalyst. The activity enhancement has been related to the presence of a more easily reducible vanadium species coupled with the enhanced number of surface Pt sites. On the other hand, the reduced activity demonstrated by catalysts with higher vanadium content (1 – 12%) has been attributed to the presence of crystalline V2O5.

Synthesis, Structure, and Catalytic Reactivity of Isolated V5+-Oxo Species Prepared by Sublimation of VOCl3 onto H-ZSM5

Isolated and uniform V(5+)-oxo species were grafted onto H-ZSM5 at V/Al(f) ratios of 0.2-1 via sublimation of VOCl(3) precursors. These methods avoid the restricted diffusion of solvated oligomers in aqueous exchange, which leads to poorly dispersed V(2)O(5) at external zeolite surfaces. Sublimation methods led to stable and active V-ZSM5 catalysts for oxidative dehydrogenation (ODH) reactions; they led to an order of magnitude increase in primary C(2)H(6) ODH rates compared with impregnated ZSM5 catalysts at similar V/Al(f) ratios and showed similar activity to impregnated VO(x)/Al(2)O(3). The structure of grafted V(5+)-oxo species was probed using spectroscopic and titration methods. Infrared spectra in the OH region and isotopic exchange of D(2) with residual OH groups showed that exposure to VOCl(3(g)) at 473 K led to stoichiometric replacement of H(+) by each (VOCl(2))(+) species. Raman spectra supported by Density Functional Theory electronic structure and frequency calculations showed that, at V/Al(f) < 0.5, hydrolysis and subsequent dehydration led to the predominant formation of (VO(2))(+) species coordinated to one Al site with single-site catalytic behavior (0.7-0.9 x 10(-3) mol C(2)H(4) V(-1) s(-1), 673 K). At higher V/Al(f) ratios, simulation of extended X-ray absorption fine structure spectra indicated that V(2)O(4)(2+) dimers coexisted with VO(2)(+) monomers and led to an enhancement in ODH rates as a result of bridging V-O-V (1.3 x 10(-3) mol C(2)H(4) V(-1) s(-1)). These V(5+)-oxo species form via initial reactions between VOCl(3(g)) and OH groups to form HCl((g)), hydrolysis of grafted (VOCl(2))(+) to form HCl((g)) and (VO(OH)(2))(+), and intramolecular and intermolecular condensation to form monomers and dimers, respective with the concurrent evolution of H(2)O. Raman and X-ray spectroscopies did not detect crystalline V(2)O(5) at V/Al(f) ratios of 0.2-1, but V(2)O(5) crystals were apparent in samples prepared by impregnation or physical mixtures of V(2)O(5)/H-ZSM5. Framework Al atoms and zeolite crystal structures are maintained during VOCl(3) treatment and subsequent hydrolysis; (27)Al and (29)Si MAS NMR showed that these synthetic protocols removes <10% of the framework Al atoms (Al(f)).

Lithium Insertion into Vanadium Oxide Nanotubes: Electrochemical and Structural Aspects

The electrochemical and structural aspects of lithium insertion into vanadium oxide nanotubes have been studied. Structural changes, induced as lithium was inserted between the vanadium oxide layers, were followed by in situ synchrotron X-ray diffraction recorded during potential steps. Two separate processes were identified: a fast decrease of the interlayer distance followed by a slow two-dimensional relaxation of the intralayer vanadium oxide structure. The nanotubes were synthesized using crystalline V2O5 as a precursor, instead of the normally used vanadium alkoxide, which produced a more cost-effective material with similar performance. Electrochemical measurements showed that surface processes, i.e., charge transfer and/or ohmic drop, were rate-controlling, as can be anticipated for the thin layer electrochemical conditions used in this study.

Reactivity of some vanadium oxides: An EPR and XRD study

V 2O 5, VO 2 and V 2O 3 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 VO 2 and V 2O 3, leading to a quasi-amorphous phase with bariandite structure (V 10O 24·12H 2O). The role of water is the progressive demolition of the compact structures and formation of hydrated phase. Kinetic study of VO 2 oxidation by O 2 and O 2+H 2O 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, V 3O 7·H 2O at 383 K and V 3O 7 at 723 K. Kinetic investigation of V 2O 5 reduction by CO at 633–723 K showed that the reduction process proceeds trough the formation of V 4+ and of electrons delocalized in the conduction band.

On the Structure of Vanadium Oxide Supported on Aluminas: UV and Visible Raman Spectroscopy, UV−Visible Diffuse Reflectance Spectroscopy, and Temperature-Programmed Reduction Studies

Vanadia species on aluminas (delta- and gamma-Al2O3) with surface VOx density in the range 0.01-14.2 V/nm2 have been characterized by UV and visible Raman spectroscopy, UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), and temperature-programmed reduction in hydrogen. It is shown that the alumina phase has little influence on the structure and reducibility of surface VOx species under either dehydrated or hydrated conditions. Three similar types of dispersed VOx species, i.e., monovanadates, polyvanadates, and V2O5, are identified on both aluminas under dehydrated conditions. Upon hydration, polymerized VOx species dominate on the surfaces of the two aluminas. The broad Raman band at around 910 cm(-1), observed on dehydrated V/delta-, gamma-Al2O3 at all V loadings (0.01-14.2 V/nm2), is assigned to the interface mode (V-O-Al) instead of the conventionally assigned V-O-V bond. The direct observation of the interface bond is of significance for the understanding of redox catalysis because this bond has been considered to be the key site in oxidation reactions catalyzed by supported vanadia. Two types of frequency shifts of the V=O stretching band (1013-1035 cm(-1)) have been observed in the Raman spectra of V/Al2O3: a shift as a function of surface VOx density and a shift as a function of excitation wavelength. The shift of the V=O band to higher wavenumbers with increasing surface VOx density is due to the change of VOx structure. The V=O stretching band in dispersed vanadia always appears at lower wavenumber in UV Raman spectra than in visible Raman spectra for the same V/Al2O3 sample. This shift is explained by selective resonance enhancement according to the UV-Vis DRS results. It implies that UV Raman has higher sensitivity to isolated and less polymerized VOx species while visible Raman is more sensitive to highly polymerized VOx species and crystalline V2O5. These results show that a multiwavelength excitation approach provides a more complete structural characterization of supported VOx catalysts.

Thin films of vanadium oxide grown on vanadium metal: oxidation conditions to produce V2O5 films for Li-intercalation applications and characterisation by XPS, AFM, RBS/NRA

Thin films of vanadium oxide were grown on vanadium metal surfaces (i) in air at ambient conditions, (ii) in 5 mM H2SO4 (aq), pH 3, (iii) by thermal oxidation at low oxygen pressure (10−5 mbar) at temperatures between 350 and 550 °C and (iv) at near-atmospheric oxygen pressure (750 mbar) at 500 °C. The oxide films were investigated by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-Ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA). The lithium intercalation properties were studied by cyclic voltammetry (CV). The results show that the oxide films formed in air at room temperature (RT), in acidic aqueous solution, and at low oxygen pressure at elevated temperatures are composed of V2O3. In air and in aqueous solution at RT, the oxide films are ultra-thin and hydroxylated. At 500 °C, nearly atmospheric oxygen pressure is required to form crystalline V2O5 films. The oxide films grown at pO2 = 750 mbar for 5 min are about 260-nm thick, and consist of a 115-nm outer layer of crystalline V2O5. The inner oxide is mainly composed of VO2. For all high temperature oxidations, the oxygen diffusion from the oxide film into the metal matrix was considerable. The oxygen saturation of the metal at 450 °C was found, by XPS, to be 27 at.% at the oxide/metal interface. The well-crystallized V2O5 film, formed by oxidation for 5 min at 500 °C and 750 mbar O2, was shown to have good lithium intercalation properties and is a promising candidate as electrode material in lithium batteries. Copyright © 2005 John Wiley & Sons, Ltd.

The Relationship Between Nanoscale Structure and Electrochemical Properties of Vanadium Oxide Nanorolls

AbstractIn this paper, we explore the relationship between the nanoscale structure and electrochemical performance of nanoscale scrolls of vanadium oxides (vanadium oxide nanorolls). The vanadium oxide nanorolls, which are synthesized through a ligand‐assisted templating method, exhibit different morphologies and properties depending upon the synthetic conditions. Under highly reducing conditions, nearly perfect scrolls can be produced which have essentially no cracks in the walls (well‐ordered nanorolls). If the materials are produced under less reducing conditions, nanorolls with many cracks in the oxide walls can be generated (defect‐rich nanorolls). Both types of samples were examined by X‐ray diffraction (XRD), transmission electron microscopy (TEM), and X‐ray photoemission spectroscopy (XPS) to characterize their local structure, local redox state, and nanoscale structure. After ion‐exchange to replace the templating ammonium ions with Na+, the ability of these materials to electrochemically intercalate lithium reversibly was investigated. In sweep voltammetry experiments, the well‐ordered nanorolls showed responses similar to those seen in crystalline orthorhombic V2O5. In contrast, the defect‐rich vanadium oxide nanorolls behaved electrochemically more like sol–gel‐prepared vanadium oxide materials. Moreover, the specific capacity of the well‐ordered nanorolls was about 240 mA h g–1 while that of the defect rich nanorolls was found to be as much as 340 mA h g–1 under these same conditions. Disorders on both the atomic and nanometer length scales are believed to contribute to this difference.