Microcalorimetric study of the acid-base properties of bulk and silica or γ-alumina-supported vanadium oxides

The acid-base character of vanadium pentoxide, V₂O₅/SiO₂ and V₂O₅/γ-Al₂O₃ catalysts has been investigated by adsorption of ammonia and sulphur dioxide using microcalorimetry. By depositing vanadium oxide on silica; new surface sites are formed which present more acid strength than bulk vanadium pentoxide and pure silica. Alumina-supported vanadium catalysts can be regarded as acidic monolayers VO_x. Sulphur dioxide was found to be selective for uncovered alumina.     

Exafs and electron-microscopy studies on V/SiO2 catalysts

Extended x-ray-absorption fine structure and scanning electron microscopy have been applied to the structure of the vanadium oxide layers on impregnated and grafted vanadium aerosils. When aerosil is impregnated with NH₄VO₃ solution, V₂O₅ crystals are formed; when vanadium is grafted by reacting the oxychloride with carrier OH groups, there are no visible crystals. On the other hand, the EXAFS spectra for the grafted specimens show all the oscillations found for crystalline V₂O₅. It is concluded that the vanadium oxide layers in these grafted materials have a long-range order similar to that in V₂O₅ and contain microcrystals having sizes up to 5 nm.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 5, pp. 652–655, September–October, 1987.     

Role of vanadium species in the selective oxidation of ethanol on V2O5/TiO2 catalysts

The catalytic properties of a sample of 20% V₂O₅/TiO₂ and its derivative, 12% V₂O₅/TiO₂, which was obtained by the treatment of the catalyst with nitric acid and did not contain bulk V₂O₅ species, were compared. In spite of a significant difference in the vanadium contents, the activity of both of the samples in the process of the gas-phase aerobic oxidation of ethanol to acetaldehyde and acetic acid was found to be the same. It was hypothesized that a monolayer of vanadium oxide on the surface of TiO₂ made the main contribution to the catalytic activity.     

Structure Peculiarities of Reduced Vanadium–Titanium Oxide Catalysts

The properties of vanadium–titanium oxide catalysts, which contain coherent phase boundaries formed by V₂O₅ and TiO₂ crystallites during reduction by hydrogen at 150–500°C, are examined. The phase boundary is preserved over the entire examined temperature range regardless of the structure of vanadium oxide, which is formed. The state of vanadium ions at the phase boundary is determined. The presence of a phase boundary in the catalyst is responsible for the V₂O₅ V₂O₃ transition without the formation of intermediate structures.     

Tmperature programmed techniques study on the nature of the oxidant in/on V2O5catalyst ?

Summary The nature of the oxidising species in/on a vanadium pentoxide (V₂O₅) catalyst has been studied using a combination of transient techniques: (i) Temperature programmed desorption (TPD), (ii) Temperature programmed reduction (TPR), (iii) Temperature programmed oxidation (TPO) and (iv) Temperature programmed reaction (TPRn). Chemisorbed oxygen was found not to exist on a fully oxidised V₂O₅ catalyst by TPD. The TPR in CO over V₂O₅ catalyst gave three peak maxima at 930, 982 and 1043 K, indicating that three types of kinetically different oxygen states exist in/on the catalyst. Reoxidation of the CO reduced V₂O₅ catalyst by N₂O resulted in the quantitative replacement of the lattice oxygen. A further reduction of the N₂O reoxidised catalyst gave a significantly different TPR profile compared to the original material suggesting that a less crystalline material had formed. The presence of phosphorus in (VO)₂P₂O₇ was found to labilise the lattice oxygen.     

Effect of the mechanochemical treatment of a V<Subscript>2</Subscript>O<Subscript>5</Subscript>/MoO<Subscript>3</Subscript> oxide mixture on its properties

The mechanochemical treatment of a V₂O₅/MoO₃ oxide mixture (V/Mo = 70/30 at %) was performed in planetary and vibratory mills under varying treatment times and media. The resulting samples were characterized using XRD analysis, micro-Raman spectroscopy, and XPS; their specific surface areas and catalytic activities in n-butane and benzene oxidation reactions were determined. It was found that the treatment of the oxide mixture in water resulted in chaotic degradation of the parent oxides, a decrease in crystallite sizes, and an increase in the specific surface area at a sufficiently uniform oxide distribution over the sample. The treatment in ethanol was accompanied by an anisotropic deformation of the V₂O₅ crystal by layer sliding in parallel to the vanadyl plane (010) and a chaotic degradation of MoO₃ crystals. This process was accompanied by the partial nonuniform supporting of vanadium oxide crystals onto the surface of molybdenum oxide to increase the V/Mo ratio on the sample surface. In this case, the particle size of oxides decreased and the specific surface areas of samples increased. It was found that the treatment of the oxide mixture in air (dry treatment) resulted in the most significant decrease in the sizes of V₂O₅ and MoO₃ crystals and a growth in the specific surface area. The amorphization of the parent oxides and the formation of MoV₂O₈ were observed as the treatment time was increased; in this case, an excess of amorphous vanadium oxide was supported onto the surface of this compound. It was found that, in all types of mechanochemical treatment, the binding energies of the core electrons of vanadium and molybdenum remained almost unchanged to indicate the constancy of the oxidation states of these elements. Mechanochemical treatment resulted in an increase in the activity of the samples in n-butane and benzene oxidation reactions and in an increase in the selectivity of maleic anhydride formation. In this case, an increase in the specific catalytic activity of the samples correlated with a decrease in the crystallite size of vanadium oxide, whereas selectivity correlated with an increase in the relative concentration of the V₂O₅ plane (010). In these reactions, samples after dry treatment exhibited a maximum activity, which can be related to the formation of MoV₂O₈.     

Cascading V2O3/N-doped carbon hybrid nanosheets as high-performance cathode materials for aqueous zinc-ion batteries

In recent years, especially when there is increasing concern about the safety issue of lithium-ion batteries（LIBs）, aqueous Zn-ion batteries（ZIBs） have been getting a lot of attention because of their costeffectiveness, materials abundance, high safety, and ecological friendliness. Their working voltage and specific capacity are mainly determined by their cathode materials. Vanadium oxides are promising cathode materials for aqueous ZIBs owing to their low cost, abundant resources, and multivale...     

Active component of supported vanadium catalysts in the selective oxidation of methanol

The structure of catalysts based on vanadium oxide supported on different oxides (SiO₂, γ-Al₂O₃, ZrO₂, and TiO₂) was investigated. Their catalytic properties in the selective oxidation of methanol in a temperature range of 100–250°C were studied. It was shown that the nature of the support determines the structure of the oxide forms of vanadium. The supporting of vanadium on SiO₂ and γ-Al₂O₃ leads to the preferred formation of crystalline V₂O₅; the surface monomeric and polymeric forms of VO_x are additionally formed on ZrO₂ and TiO₂. It was established that the crystalline V₂O₅ oxide is least active in the selective oxidation of methanol; the polymeric forms are more active than monomeric ones. The mechanism of the selective oxidation of methanol to dimethoxymethane and methyl formate on the vanadium oxide catalysts is considered.     

n-Butane Oxidation over γ-Al2O3 Supported Vanadium Phosphate Catalysts

Four vanadium phosphate catalysts supported onγ-Al_2O_3(20 wt%)were synthesized via wetness impregnation of VOHPO_4·0.5H_2O precursor and calcined for different durations(6,10,30 and 75 h)at 673 K in a reaction flow of n-butane/air mixture.The samples calcined for 6 and 10 h produced only a single phase of(VO)_2P_2O_7.However,the VOPO_4 phase(β-VOPO_4)was detected and became more prominent with only a minor pyrophosphate peaks were found after 30 h of calcination.All these pyrophosphate peaks disappeared after 75 h of calcination.The formation of V~(5 )phase was also observed in the SEM micrographs.The redox properties and the nature of oxidants of the catalysts employed in this study were investigated by H_2-TPR analysis.Selective oxidation of n-butane to maleic anhydride(MA) over these catalysts shows that the percentage of n-butane conversion decreases with the transformation of the catalysts from V~(4 )to V~(5 )phases.An appropriate ratio of V~(5 )/V~(4 )can enhance the performance of the VPO catalyst.However,a higher amount of V~(5 )and its associated oxygen species are responsible to promote the MA selectivity.     

Effect of direct ultrasound synthesis via a sesquihydrate route on bismuth-promoted vanadyl pyrophosphate catalysts

A series of 1, 3, and 5% Bi-doped vanadium phosphate catalyst catalysts were prepared via sesquihydrate route using direct ultrasound method and were denoted as VPSB1, VPSB3, and VPSB5, respectively. These catalysts were synthesized solely using a direct ultrasound technique and calcined in a n-butane/air mixture. This study showed that catalyst synthesis time can be drastically reduced to only 2 hr compared to conventional 32–48 hr. All Bi-doped catalysts exhibited a well-crystallized (VO)₂P₂O₇ phase. In addition, two V⁵⁺ phases, that is, β-VOPO₄ and α_II-VOPO₄, were observed leading to an increase in the average oxidation state of vanadium. All catalysts showed V2p_3/2 at approx. 517 eV, giving the vanadium oxidation state at approx. 4.3–4.6. Field-emission scanning electron microscopy micrographs showed the secondary structure consisting of thin and small plate-like crystal clusters due to the cavitation effect of ultrasound waves. VPSB5 showed the highest amount of oxygen species removed associated with the V⁵⁺ and V⁴⁺ species in temperature-programmed reduction in H₂ analyses. TheX-ray absorption near edge structure (XANES) measurement showed the occurrence of vanadium oxide reductions in hydrogen gas flow, indicating the presence of V⁴⁺ and V⁵⁺ species. Higher average valence states of V⁵⁺, indicating more V⁵⁺ phases, were present. The addition of bismuth has increased the activity and selectivity to maleic anhydride.     

Emptying and filling a tunnel bronze

The classical orthorhombic layered phase of V₂O₅ has long been regarded as the thermodynamic sink for binary vanadium oxides and has found great practical utility as a result of its open framework and easily accessible redox states. Herein, we exploit a cation-exchange mechanism to synthesize a new stable tunnel-structured polymorph of V₂O₅ (ζ-V₂O₅) and demonstrate the subsequent ability of this framework to accommodate Li and Mg ions. The facile extraction and insertion of cations and stabilization of the novel tunnel framework is facilitated by the nanometer-sized dimensions of the materials, which leads to accommodation of strain without amorphization. The topotactic approach demonstrated here indicates not just novel intercalation chemistry accessible at nanoscale dimensions but also suggests a facile synthetic route to ternary vanadium oxide bronzes (M_xV₂O₅) exhibiting intriguing physical properties that range from electronic phase transitions to charge ordering and superconductivity.     

Synthesis and Examination of Electrolytic Sodium–Vanadium Oxide Compounds Intended for Cathodes of Lithium Batteries: The Mechanism of Formation of Electrolytic Bronze β-NaxV2O5

To determine the mechanism responsible for the formation of electrolytic sodium–vanadium oxide bronze e-Na _x V₂O₅, synthesized earlier from acid vanadyl sulfate electrolyte, -bronze i-Na _x V₂O₅is synthesized by exposing electrolytic oxide e-V₂O₅in the same sodium-containing electrolyte under open-circuit conditions, with a subsequent annealing of the sample. It is established that the two modifications of -bronze (e-Na _x V₂O₅and i-Na _x V₂O₅) are identical and that electrolytic precursors of -bronze Na _x V₂O₅form via an ion-exchange mechanism.     

Vapour phase synthesis of isobutyraldehyde from methanol and ethanol over mixed oxide supported vanadium oxide catalysts

The vapour phase synthesis of isobutyraldehyde from methanol and ethanol in one step was investigated over titania-silica, titania-alumina, titania-zirconia, titania-silica-zirconia, and magnesia supported vanadium oxide catalysts at 623 K and under normal atmospheric pressure. Among various catalysts the titania-silica binary oxide supported vanadia provided higher yields than the other single or mixed oxide supported catalysts. The high conversion and product selectivity of V₂O₅/TiO₂-SiO₂ catalyst (20 wt% V₂O₅) was related to the better dispersion of vanadium oxide over titania-silica mixed oxide support in addition to other acid-base and redox characteristics. A reaction path for the formation of isobutyraldehyde from methanol and ethanol mixtures over these catalysts was described.     

Preparation and Electrochemical Performance of V2O3-C Dual-Layer Coated LiFePO4 by Carbothermic Reduction of V2O5

The V₂O₃-C dual-layer coated LiFePO₄ cathode materials with excellent rate capability and cycling stability were prepared by carbothermic reduction of V₂O₅. X-ray powder diffraction, elemental analyzer, high resolution transmission electron microscopy and Raman spectra revealed that the V₂O₃ phase co-existed with carbon in the coating layer of LiFePO₄ particles and the carbon content reduced without graphitization degree changing after the carbothermic reduction of V₂O₅. The electrochemical measurement results indicated that small amounts of V₂O₃ improved rate capability and cycling stability at elevated temperature of LiFePO₄/C cathode materials. The V₂O₃-C dual-layer coated LiFePO₄ composite with 1wt% vanadium oxide delivered an initial specific capacity of 167 mAh/g at 0.2 C and 129 mAh/g at 5 C as well as excellent cycling stability. Even at elevated temperature of 55 oC, the specific capacity of 151 mAh/g was achieved at 1 C without capacity fading after 100 cycles.     

Vapor-phase oxidation of β-picoline to nicotinic acid on V2O5 and modified vanadium oxide catalysts

Modification of V₂O₅ with Ti, Sn, Zr, Nb, and Al oxides improves the activity and selectivity of the vanadium oxide catalyst in vapor-phase oxidation of β-picoline to give nicotinic acid. It is shown that the conversion of β-picoline and the yield of nicotinic acid on two-component V₂O₅-TiO₂, V₂O₅-SnO₂, V₂O₅-ZtrO₂, V₂O₅-Nb₂O₅, and V₂O₅-Al₂O₃ catalysts may be several times those on the V₂O₅ catalyst. It was found that, on passing from V₂O₅ to double-component vanadium-containing catalysts, the proton affinity of active oxygen bonded to vanadium, calculated by the quantum-chemical method, grows simultaneously with the increase in the activity of the catalysts in the oxidation reaction.     

Hydrothermal synthesis and electrochemistry of a δ-type manganese vanadium oxide

A new manganese vanadium oxide containing double sheets of V₂O₅ layers has been synthesized hydrothermally in the presence of tetramethylammonium ions. It has the electrochemically active δ-V₂O₅ structure, variants of which are found in V₆O₁₃ and xerogel vanadium oxides. The manganese ions, together with the N(CH₃)₄ ions, reside in a disordered manner between the oxide sheets. The δ-type [N(CH₃)₄]_zMn_yV₂O₅·nH₂O has a monoclinic structure, a=11.66(2) Å, b=3.610(9) Å, c=13.91(4) Å, β=108.8(2)°. It reacts readily with lithium with a capacity exceeding 220 mAh g⁻¹; the organic ions do not impede reaction as in the single sheet V₂O₅ materials, such as N(CH₃)₄V₃O₇.     

Catalytic properties of platinum-promoted acid cesium salts of molybdophosphoric and molybdovanadophosphoric heteropoly acids in the gas-phase oxidation of benzene to phenol with an O<Subscript>2</Subscript> + H<Subscript>2</Subscript> mixture

Acid salts Cs _x H_{3 + n ? x} PMo_{12 ? n} V _n O₄₀ (n = 0, 1, 2, or 3; x = 2.5 or 3.5) with coprecipitated or supported platinum were studied using thermogravimetry, IR spectroscopy, and temperature-programmed reduction. The thermal region of the full stability of these salts is limited by the decomposition temperature of the corresponding acid H₃PMo₁₂O₄₀ (～400°C) or H_{3 + n} PMo_{12 ? n} V _n O₄₀ (～300–350°C). The degree of reduction of heteropoly anions with hydrogen is regulated by temperature. Deeply reduced heteropoly anions (at 300°C) are slowly oxidized with oxygen with structure and composition regeneration. The states of molybdenum and vanadium on the surface of samples with coprecipitated platinum Pt_0.1-Cs_2.5H_0.5PMo₁₂O₄₀ (1) and Pt_0.1-Cs_2.5H_2.5PMo₁₀V₂O₄₀ (2), which were studied using XPS, correspond to reduced or reoxidized heteropoly anions in the bulk. Platinum metal particles of ～5 nm in size were observed in high-resolution TEM images obtained after the reduction and storage of sample 1 in air. A heteropoly compound forms two texture levels: spherical nanoparticles of 10–20 nm in size are collected in closely packed globules of 100–300 nm in size. Detailed texture studies, which were performed using nitrogen adsorption isotherms, demonstrated texture mobility under the ambient conditions. The cesium salts of the heteropoly acids were tested in the gas-phase oxidation of benzene to phenol with an O₂ + H₂ mixture at 180°. The effect of platinum concentration on the specific catalytic activity in the presence of deeply reduced heteropoly anions was monitored. The samples containing the salt Cs_2.5H_0.5PMo₁₂O₄₀ exhibited the highest activity in the formation of phenol. The introduction of vanadium into the heteropoly anion impaired the catalytic performance of both deeply and slightly reduced samples.     

V2O5 xerogel-poly(ethylene oxide) hybrid material: Synthesis, characterization, and electrochemical properties

In this work, we report the synthesis, characterization, and electrochemical properties of vanadium pentoxide xerogel-poly(ethylene oxide) (PEO) hybrid materials obtained by varying the average molecular weight of the organic component as well as the components’ ratios. The materials were characterized by X-ray diffraction, ultraviolet/visible and infrared spectroscopies, thermogravimetric analysis, scanning electron microscopy, electron paramagnetic resonance, and cyclic voltammetry. Despite the presence of broad and low intensity peaks, the X-ray diffractograms indicate that the lamellar structure of the vanadium pentoxide xerogel is preserved, with increase in the interplanar spacing, giving evidence of a low-crystalline structure. We found that the electrochemical behaviour of the hybrid materials is quite similar to that found for the V₂O₅ xerogel alone, and we verified that PEO leads to stabilization and reproducibility of the Li⁺ electrochemical insertion/de-insertion into the V₂O₅ xerogel structure, which makes these materials potential components of lithium ion batteries.     

Electrolytic Sodium-Vanadium Oxide Compounds in Intercalation Cathodes of Lithium Battery

Electrolytic mixtures of vanadium(V) oxide and -Na_xV₂O₅ bronze were synthesized and studied by X-ray diffraction and thermal analyses, step galvanostatic titration, IR absorption spectroscopy, mass spectroscopy, and electrochemical methods.     

Oxidation of 3- and 4-methylpyridines on modified vanadium oxide catalysts

The partial oxidation of 3- and 4-methylpyridines on V₂O₅ and vanadium oxide catalysts doped with TiO₂, Al₂O₃, and ZrO₂ was studied. The catalytic activities of the studied catalysts were correlated with the calculated proton affinities of the vanadyl oxygen. A possible mechanism of the surface stages of the partial oxidation of 3- and 4-methylpyridines on the vanadium oxide catalysts was discussed.