Vanadium Oxidation Research Articles

Interface features between 30Li2O·47.5V2O5·22.5B2O3 glassy cathode and Li7La3Zr2O12 solid electrolyte

Glassy cathode deposition on solid electrolyte substrate applied to lithium middle-temperature all-solid-state batteries was investigated. The 30Li2O·47.5V2O5·22.5B2O3 (LBV) glassy cathode is crystallized on Li7La3Zr2O12 (LLZ) solid electrolyte substrate at various temperatures (650–850 °C) and holding times (0.5–5 min). The phase composition of the crystallized glass and vanadium oxidation state are determined by the XRD and XPS analysis. The different lithium vanadates form at all studied temperatures, and the boron-containing phases appear at temperatures greater than 750 °C. SEM investigation shows that the lithium borate-vanadate glass crystallized at 750 °C to create the optimal interface and close contact between the electrode and the electrolyte. Processing at lower temperatures does not allow for good contact, and processing at higher temperatures leads to the interaction between the cathode material and ceramic electrolyte. The optimal mode of 30Li2O·47.5V2O5·22.5B2O3 glassy cathode deposition on the substrate is found to be glass powder annealing for 0.5 min at 750 °C. This LBV|LLZ cell has the highest conductivity and the lowest polarization resistance on the cathode|electrolyte interface. The universality of this deposition mode is demonstrated using Li3.65Al0.05Ge0.95P0.2O4 ceramic substrate as an example. Thus, the LBV glass can be used for the formation of an optimal interface with other lithium-ion solid electrolytes.

Selective doping of Li-rich layered oxide cathode materials for high-stability rechargeable Li-ion batteries

We report the discovery of Li-rich Li1+x[(Ni0.225Co0.15Mn0.625)1−yVy]O2 as a cathode material for rechargeable lithium-ion batteries in which a small amount of tetravalent vanadium (V4+) is selectively and completely incorporated into the manganese sites in the lattice structure. The unwanted oxidation of vanadium to form a V2O5-like secondary phase during high-temperature crystallization is prevented by uniformly dispersing the vanadium ions in coprecipitated [(Ni0.225Co0.15Mn0.625)1−yVy](OH)2 particles. Upon doping with V4+ ions, the initial discharge capacity (>275mAhg−1), capacity retention, and voltage decay characteristics of the Li-rich layered oxides are improved significantly in comparison with those of the conventional undoped counterpart.

Development of Vanadium‐Coated Carbon Microspheres: Electrochemical Behavior as Electrodes for Supercapacitors

AbstractVanadium‐coated carbon‐xerogel microspheres are successfully prepared by a specific designed sol–gel method, and their supercapacitor behavior is tested in a two‐electrode system. Nitrogen adsorption shows that these composite materials present a well‐developed micro‐ and mesoporous texture, which depends on the vanadium content in the final composite. A high dispersion of vanadium oxide on the carbon microsphere surface is reached, being the vanadium particle size around 4.5 nm. Moreover, low vanadium oxidation states are stabilized by the carbon matrix in the composites. The complete electrochemical characterization of the composites is carried out using cyclic voltammetry, chronopotentiometry, cycling charge–discharge, and impedance spectroscopy. The results show that these composites present high capacitance as 224 F g−1, with a high capacitance retention which is explained on the basis of the presence of vanadium oxide, texture, and chemistry surface.

Properties, structure and stability of V(IV) hydrazide Schiff base ligand complex

The reaction of vanadyl sulfate, VOSO4·H2O, with Schiff base ligand based on 2-hydroxybenzhydrazide and 5-bromosalicylaldehyde, with addition of 1,10-phenantroline as co-ligand, in ethanolic solution results in isolation of oxovanadium(IV) complex [VO(phen)L]·H2O (1). The complex was characterized by elemental analysis, single crystal X-ray structure measurements, IR, UV–Vis, EPR spectroscopy and cyclic voltammetry. Magnetic susceptibility measurements indicated the +4 vanadium oxidation state. The molecular structure of the complex was confirmed by single-crystal X-ray diffraction, revealing that the central vanadium atom was located in a six-coordinate distorted octahedral geometry. The stability of the complex at native pH and pH = 2.00 was also measured and the results are promising for application the vanadium complex as insulin mimetic compound.

Electrochemical and structural investigation of Mg-doped Li3V(2-2x/3)Mgx(PO4)3

X-ray absorption spectroscopy was used to characterize the local structural environment of the vanadium ions in Mg doped Li3V(2-2x/3)Mgx(PO4)3; x = 0.15, 0.30, 0.45, (LVP) in a functioning battery. For all three compositions the x-ray absorption near-edge structure spectra show evidence for only two spectroscopically distinguishable species when the potential is cycled between 3 and 4.5 V. These are attributed to octahedral V3+ and V4+ in LVP. If the potential is increased above 4.5 V a third species that is consistent with V5+ in a distorted, possibly tetrahedral environment is formed. For x = 0.30 and x = 0.45 this species disappears when the potential is lowered. In contrast, for x = 0.15 a fourth species is formed on reduction. This new species is consistent with V4+ in a distorted tetrahedral environment and may explain the loss of capacity in the 15% sample during cycling. Although the potential shows discrete plateaus, consistent with formation of mixed-valence species, these are not seen in the x-ray absorption. Rather, we see apparent deviations from Faraday's law that are consistent with a lag in oxidation of the bulk vanadium.

Post-deposition annealing of thin RF magnetron sputter-deposited VO2 films above the melting point

Vanadium dioxide (VO2) is a promising material for electronics and optics due to abrupt change in conductivity of VO2 that occurs at ∼68 °C, which is the metal-insulator transition temperature. The abruptness and magnitude of this transition is governed by the crystal structure of VO2. The growth of thin VO2 films therefore requires substrate heating during deposition and, as well as the occasional need for additional post-deposition annealing. The processes that occur during annealing depend upon the deposition technique. However, the published reports related to these studies are rarely systematic and lack consistency. The annealing of thin nanocrystalline VO2+x films at temperatures above the melting point is reported here. Thin films were deposited onto a polycrystalline corundum substrate by means of radio-frequency reactive magnetron sputtering, with the thin film composition fine-tuned by applying negative substrate bias during deposition and proceeding with ex-situ control of the vanadium oxidation state. Post-deposition annealing causes recrystallization, which is in accordance with the VO phase diagram. No epitaxial or other substrate-induced processes were detected. However, the sharpest transition was attributed to the film possessing the most uniform grain size, as opposed to the film with the largest VO2 crystallites. The temperature dependency of the microwave radiation transmittance was investigated over the 8–18 GHz range, with the samples measured in free-space conditions. This was intended to reveal any frequency-dependent features, which can possibly appear as benefits or obstacles during the development of designs in the microwave range that are based on the VO2 films being studied.

Synchrotron-based X-ray absorption spectroscopy study of vanadium redox speciation during petroleum coke combustion and gasification

Vanadium in petroleum coke can have environmental and technological impacts in gasification and combustion processes. The multitude of vanadium oxidation and coordination states has made it difficult to characterize vanadium-rich materials, particularly for materials where the vanadium is associated with an amorphous state and cannot be characterized by X-ray diffraction. This study presents two methods whereby vanadium is characterized by synchrotron-based X-ray absorption spectroscopy, which applies to both crystalline and amorphous phases. The first method, comparison of main edge energies, is relatively straightforward but presents multiple limitations. The other method, linear combination fitting, overcomes many of these limitations but relies on the availability of suitable reference materials. These methods, applied to pilot plant gasification and combustion samples, indicate various trends in vanadium speciation, such as higher oxidation states for combustion samples relative to gasification samples. Analysis of synthetic pseudo-binary materials, which is important for the development of thermodynamic equilibrium models, indicated that V5+ is stable with high oxygen partial pressures encountered during combustion. With a lower oxygen partial pressure, representative of gasification conditions, V5+ is stable if there is enough calcium present; otherwise, V3+ is stable.

Investigation of the End-Point Temperature Control Based on the Critical Temperature of Vanadium Oxidation During the Vanadium Extraction Process in BOF

To solve the problem related to the end-point temperature control during the vanadium extraction, industrial experiments and thermodynamic analyses were implemented to find out and verify the foundation for the new standard. The transition temperatures of carbon-vanadium oxidation (T tr ) and the molten bath temperature measured on the industrial experiments showed a negative temperature difference after smelting for 3.5 min. However, for about 30 wt% vanadium was removed when the molten bath temperature exceeded T tr . T tr was only suitable for achieving the goal of “keeping carbon while extracting vanadium” in the early and medium period. The critical temperature of vanadium oxidation (T cr ) stood for the thermodynamic equilibrium between the oxidation and reduction of vanadium in the later period. It served as the basis to control the end-point temperature to satisfy the demand of “deep vanadium extraction”. As the molten bath temperature increased above T cr , the residual vanadium neither decreased further nor increased, but showed an equilibrium state.

Coverage-dependent oxidation and reduction of vanadium supported on anatase TiO2(1 0 1)

Using a multi-technical approach, we studied the oxidation of anatase TiO2(1 0 1)-supported vanadium (V) clusters at room temperature. We found by ex situ XPS that the highest oxidation state is +4 at sub-monolayer coverage regardless of the O2 pressure, and STM studies revealed that the initial oxidation proceeds through oxygen-induced disintegration of V clusters into monomeric VO2 species. By contrast, for ∼2 monolayer V coverage, a partial oxidation to V5+ is achieved. By in situ APXPS measurements, we found that V can be maintained in the V5+ oxidation state irrespective of the coverage; however, in the sub-monolayer range, an O2 pressure of at least ∼1 × 10−5 mbar is needed. Our results suggest an enhanced reducibility of V in direct contact with the TiO2 support compared to V in the 2nd layer, which is in line with the observed optimum V2O5 loading in catalytic applications just slightly below a full monolayer.

Influence of vanadate structure and support identity on catalytic activity in the oxidative cleavage of methyl ketones

Oxidative ketone scissions are an interesting class of reactions; they yield aldehyde and carboxylic acids as products, and they proceed over reducible oxides in the presence of molecular oxygen. This chemistry can be leveraged in the production of bio-based maleic anhydride (MA) from levulinic acid (LA) over supported vanadium oxides. Here, we probe the role of active site structure and support identity in dictating the rate of ketone oxidation. Specifically, we have prepared supported vanadium oxides in a range of loadings on SiO2, γ-Al2O3, TiO2 and CeO2, and we have quantified their intrinsic activity in the oxidative cleavage of 2-pentanone. FT-Raman spectroscopy and temperature programmed surface oxidation (TPSO) of adsorbed methanol were employed to probe vanadium speciation and oxidation site densities. Our analysis suggests that the intrinsic activity of supported vanadates is sensitive to both vanadium oxide structure and support identity.

Oxidation state control of solution-processed vanadium oxide thin-films and resistive switching of VO2 thin-film in a metastable state

The solution processing of crystalline VO2 thin films has been intensively investigated for both fundamental studies and various electronic applications. However, previous studies have reported that the inevitable oxidation of vanadium cations in conventional precursor solutions hinders facile and reliable fabrication of VO2 thin films. Here we develop a kinetically stabilized precursor solution for solution-based VO2 thin-film fabrication. The stabilized solution precursor showed increased resistance to uncontrolled vanadium oxidation compared to conventional precursors. The synthetic conditions necessary for reliable VO2 thin-film fabrication were investigated at various oxygen concentrations and various annealing atmospheres. The oxygen level of an ambient atmosphere controls which of the three vanadium oxidation states arise, including V3+, V4+, and V5+ at a given temperature. The unusual occurrence of V3+ can be obtained without any reducing reagents. Nano-crystalline VO2 thin films subjected to rapid thermal shock exhibit metal-to-insulator transition at 58.9°C and resistive switching in a metastable state at 53.5°C, which could be further applied to future electronic devices such as memristors, Mott transistors, and Mott memories.

Redox-active ligand controlled selectivity of vanadium oxidation on Au(100).

Metal-organic coordination networks at surfaces, formed by on-surface redox assembly, are of interest for designing specific and selective chemical function at surfaces for heterogeneous catalysts and other applications. The chemical reactivity of single-site transition metals in on-surface coordination networks, which is essential to these applications, has not previously been fully characterized. Here, we demonstrate with a surface-supported, single-site V system that not only are these sites active toward dioxygen activation, but the products of that reaction show much higher selectivity than traditional vanadium nanoparticles, leading to only one V-oxo product. We have studied the chemical reactivity of one-dimensional metal-organic vanadium - 3,6-di(2-pyridyl)-1,2,4,5-tetrazine (DPTZ) chains with O2. The electron-rich chains self-assemble through an on-surface redox process on the Au(100) surface and are characterized by X-ray photoelectron spectroscopy, scanning tunneling microscopy, high-resolution electron energy loss spectroscopy, and density functional theory. Reaction of V-DPTZ chains with O2 causes an increase in V oxidation state from VII to VIV, resulting in a single strongly bonded (DPTZ2-)VIVO product and spillover of O to the Au surface. DFT calculations confirm these products and also suggest new candidate intermediate states, providing mechanistic insight into this on-surface reaction. In contrast, the oxidation of ligand-free V is less complete and results in multiple oxygen-bound products. This demonstrates the high chemical selectivity of single-site metal centers in metal-ligand complexes at surfaces compared to metal nanoislands.

XANES study of the dynamic states of V-based oxide catalysts under partial oxidation reaction conditions

Abstract A XANES study under reaction conditions has been performed with two different V-based catalytic systems, Mo-V-Nb-Te-O and V-Sb-O. For this study, an alumina-supported nanoscaled bulk catalyst has been used. In all cases XANES determined the average vanadium oxidation state during reaction. XANES also demonstrated that the nanosized phases are more dynamic, and able to participate in the redox catalytic cycle without significant changes either in their structure or in the overall vanadium oxidation state. Such a stability is also apparent under oxidizing conditions.

Influence of Steam on a Vanadyl Pyrophosphate Catalyst During Propane Oxidation.

We have investigated electronic and catalytic modifications of the p-type semiconducting selective oxidation catalyst vanadyl pyrophosphate (VPP) for propane oxidation in the presence and absence of steam. Steam changes propane conversion only slightly, but increases the selectivity toward oxygenates (acrylic acid, acetic acid) and the olefin propylene resulting in reduced selectivity of the undesired total oxidation products CO and CO2. Contact-free in operando microwave conductivity measurements at 0.1 MPa revealed that the modified catalytic performance is accompanied by a reduced electrical conductivity. Surface sensitive near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and X-ray absorption spectroscopy (XAS) measurements at 25 Pa showed that steam depletes the topmost surface of VPP in phosphorus and enhances the average vanadium oxidation state slightly. These findings are accompanied by a decreased work function, but no detectable shift of the valence band edge is observed. Thus, the chemical surface modification changes the surface dipole but leaves the barrier height of the surface induced space charge layer basically unaffected. Hence, we conclude that steam does not affect the electron hole concentration (majority charge carriers) and hence the oxygen vacancy concentration. Therefore, the reduced conductivity can be understood in terms of charge carrier mobility changes, which may affect the selectivity of VPP toward oxygenates with steam. In addition, the modification of local properties, such as the concentration of acid sites as well as the nature and number of adsorption sites may have an impact on the catalytic properties.

UHV AFM based colloidal probe studies of adhesive properties of VAlN hard coatings

Abstract The adhesion of polystyrene (PS) on V0.27Al0.29N0.44 and the related influence of the oxidation states of both surfaces was investigated using X-Ray Photoelectron Spectroscopy (XPS) and Colloidal Force Spectroscopy (CFS) in Ultra-High Vacuum (UHV). Complementary, the intimate relation between the adhesion force, the chemical structure and surface polarizability was investigated by XPS valence band spectroscopy and the calculation of non-retarded Hamaker coefficients using Lifshitz theory based on optical data as derived from Reflection Electron Energy Loss Spectroscopy (REELS) spectra. The combined electron and force spectroscopic analysis of the interaction forces disclosed quantitatively the separation of the adhesion force in van der Waals and Lewis acid-base contributions. Further, the surface polarizability of VAlN was shown to be unaffected by oxygen incorporation due to the formation of an only gradually oxidized surface comprising a range of vanadium oxidation states. In contrast, the adhesion force analysis revealed additional Lewis acid-base interactions between the oxidized and non-oxidized VAlN surfaces and carboxyl groups present in the surface of PS after an oxidative oxygen beam treatment.

Development of a Li-Ion Electrochemical Platform for in-Situ TEM of Solid State Electrode/Electrolyte Interfaces

The interface of electrode materials in contact with solid-state electrolytes is of current interest for the development of devices with high energy density and good safety. In-situ transmission electron microscopy (TEM) of energy storage materials allows for the detailed study of structure-property relationships near solid interfaces during electrochemical charge and discharge. Here, we will describe the design of an in-situ TEM platform to investigate the structure-property relationship of a model energy storage oxide, V2O5, in contact with a state-of-the-art solid-state electrolyte Li7La3Zr2O12 (LLZO). The cell was fabricated by the atomic layer deposition (ALD) of V2O5 (~ 50 nm thick) onto the electrolyte and subsequent focused ion beam (FIB) milling and lift-out of a micron-long electrode/electrolyte architecture. The lithium metal anode was electrochemically deposited directly onto the gold electrode of an in-situ TEM chip. Assembly of the electrochemical cell was performed in a scanning electron microscope using a micromanipulator. Bright-field TEM and electron energy loss spectroscopy (EELS) showed that the deposited V2O5 layer formed a well-defined interface with LLZO and this allowed for the determination of the vanadium oxidation state. This work demonstrates the fabrication of a platform for the in-situ TEM study of materials at electrode/solid-state electrolyte interfaces that can readily be adapted to other types of solid state battery chemistries.

Visualizing atomic-scale redox dynamics in vanadium oxide-based catalysts

Surface redox processes involving oxygen atom exchange are fundamental in catalytic reactions mediated by metal oxides. These processes are often difficult to uncover due to changes in the surface stoichiometry and atomic arrangement. Here we employ high-resolution transmission electron microscopy to study vanadium oxide supported on titanium dioxide, which is of relevance as a catalyst in, e.g., nitrogen oxide emission abatement for environmental protection. The observations reveal a reversible transformation of the vanadium oxide surface between an ordered and disordered state, concomitant with a reversible change in the vanadium oxidation state, when alternating between oxidizing and reducing conditions. The transformation depends on the anatase titanium dioxide surface termination and the vanadium oxide layer thickness, suggesting that the properties of vanadium oxide are sensitive to the supporting oxide. These atomic-resolution observations offer a basis for rationalizing previous reports on shape-sensitive catalytic properties.

Synthesis, crystal structure and spin exchange coupling in polynuclear carboxylates with {Li2(VO)2} metal core

Crystals of binuclear complex [(VO)2(µ-OH)(µ-Piv)2(bpy)2](OTf) (1) have been obtained by the reaction of [VO(H2O)5](OTf)2 (OTf=CF3SO3−, Piv=(CH3)3CCO2−) with 2,2′-bipyridine (bpy) and lithium trimethylacetate (pivalate), Li(Piv), in a ratio of 1:1:2 in MeCN. Change of the reagent ratio to 1:1:3 resulted in the bimetallic compound [Li2(VO)2(µ-Piv)6(bpy)2] (2), which was isolated in the form of 2·MeCN and 2·CH2Cl2 solvates. Replacement of Li(Piv) with lithium trifluoroacetate Li(Tfac) (Tfac=CF3CO2−) results in the bimetallic compound [Li2(VO)2(µ-Tfac)6(bpy)2] (3). A similar reaction with lithium trichloroacetate, Li(Tcac) (Tcac=CCl3CO2−), in ethanol gives the mononuclear complex [VO(Tcac)(bpy)2](OTf) (4). The structures of 1–4 were determined by single crystal X-ray diffraction. The magnetic properties (2–300K) and EPR spectra of compounds 1–3 have been studied. Using the ESEEM (electron spin echo envelope modulation) method it has been shown that the mononuclear cation [VO(Tcac)(bpy)2]+ of complex 4 has partial spin density transfer from the VIV atom to donor ligands (in particular, the bpy nitrogen atoms). According to EPR spectroscopy data and quantum-chemical calculations, a similar spin density distribution is observed for 2 and 3, though spin exchange in these compounds is hindered by lithium ions, which contain no spin density and block electron density delocalization in the bridging system between the “magnetic” vanadium ions. Electrochemical properties of compounds 2 and 3 confirm the vanadium(IV) oxidation state.

Application of lime in two-stage purification of leaching solution of spent vanadium catalysts for sulfuric acid production

Two-stage hydrolytic purification of acidic solution of reductive leaching of spent vanadium catalysts for sulfuric acid production from arsenic and iron impurities has been investigated. After oxidation of Fe(II) and As(III) with hydrogen peroxide and neutralization of vanadium (IV) solution with lime to рН2.8 at 25°С, the concentration of iron and arsenic has been reduced from 4.5 to 1.6g·L−1 and from 1.0 to 0.6g·L−1, correspondingly. After neutralizing excessive acidity to рН8.5 at 60°С and oxidation of vanadium, the concentration of iron and arsenic in obtained solutions of sulfates and vanadates of alkali metals does not exceed 0.1mg·L−1. Lime's calcium ions are bound with excessive sulfate ions to form gypsum, which allows to preserve the composition of promoter salts and to return them into production of catalysts after vanadium separation. Washing of the second stage precipitation with initial leaching solutions allows to avoid vanadium losses, guarantees return of impurities to the first stage and their extraction in a convenient form for arsenic burial. The kinetic equation for an oxidation of vanadium (IV) solutions with atmospheric oxygen during their neutralization with lime in presence of ferric compounds has been acquired.

Optimization Mechanism of Additive of Composite Sodium Salts on Vanadium Oxidation of Siliceous Shale

The promotion of strengthening of vanadium oxidation by compound additive of sodium salts is described from the changes of the vanadium valence state and mineral phase. The results are as follows: during the roasting process, dehydroxy mica converts to the melt, which the composite system mainly contains Na, K, Al, Si, and O. Under the action of NaCl-Na2CO3, ion exchange between sodium and potassium promotes the crystallization of albite from the melt. Na2CO3 enhances the reactivity of quartz. The albite reacts with activated quartz, which promotes the migration of sodium ions and the generation of vanadate. Under the action of NaCl-Na2SO4, The crystallization of spinel in the melt is promoted and that of the albite is inhibited. Under the action of Na2SO4-Na2CO3, the permeability of ore is deteriorated. The ion exchange process and the vanadium oxidation is inhibited from the melt. The composite additive with three sodium salts shows a stronger promoting effect on vanadium oxidation. The contribution of NaCl and Na2CO3 to V(V) in roasted slag was the highest among three sodium salts. With a certain ratio of NaCl and Na2CO3, the low content of Na2SO4 did not significantly affect the V(V) content in roasted slag.