Coherent interfaces: Efficient boundary conditions in solid state reactivity. Study in V2O5AlNbO4, V2O5GaNbO4, and V2O5TiNb2O7 systems

The thermodynamically unexpected reduction of V₂O₅ in the presence of the mixed oxides AlNbO₄, GaNbO₄, and TiNb₂O₇ under nitrogen at 630°C is reported and gives a supplementary example of the kind of interfacial reaction observed in the V₂O₅TiO₂ system. It is shown that this phenomenon comes from the establishment of coherent interfaces between the cleavage planes of two crystals belonging to the same crystallo-chemical family. The reduction enables the system to diminish the elastic stress created by the slight interfacial misfit. A thermodynamic and kinetic explanation, based on structural factors, is given.     

Acidic properties of V2O5-WO3/ZrO2 catalysts

The adsorption of pyridine on V₂O₅−WO₃/ZrO₂ has been studied by FTIR. In V₂O₅/ZrO₂ (2 wt.%), the number of both Br?nsted and Lewis acidic sites increased with the addition of WO₃, while in V₂O₅/ZrO₂ (5 wt.%), Br?nsted sites increased and Lewis sites did not change.     

V2O5-TiO2复合半导体光催化材料结构及光响应性能研究

采用溶胶凝胶法制备了V₂O₅-TiO₂复合半导体材料，通过Raman、XRD及UV-Vis DRS等实验方法研究了V₂O₅与TiO₂复合对材料表面组成、晶体结构以及光响应性能的影响。结果表明：钒加入后优先与TiO₂作用形成较为稳定的金红石型TiVO₄晶相，其中V⁴⁺是促进TiO₂发生相变的关键；随着钒加入量的增加，V₂O₅由表面高分散状态逐渐聚集形成晶相，并释放部分Ti⁴⁺使之形成锐钛矿型TiO₂晶相，使得体相中金红石型TiO₂的含量有所下降；复合后形成的TiVO₄晶相显著提高了材料对可见光的吸收率，并使其吸光域红移至460 nm左右。     

Intergrowth between V2O5 and TiO2 (anatase): High resolution electron microscopy evidence

V₂O₅/TiO₂ (anatase) with 5 and 20 wt.% loadings of V₂O₅ have been studied using high resolution electron microscopy. This has provided the first direct evidence for the formation of a coherent interfacial boundary between two crystalline TiO₂ and V₂O₅ phases. The type of the interfacial stacking was found to be similar for all the samples, whereas the length of the boundary formed depended on the V₂O₅ loading. A model for the atomic arrangement of this boundary based on a comparison of structural positions of oxygen and metal atoms in the crystal lattices of both phases is proposed.     

Relationship between sulfur dioxide oxidation and selective catalytic NO reduction by ammonia on V2O5?TiO2 catalysts doped with WO3 and Nb2O5

The performance of V₂O₅–TiO₂ catalysts doped by WO₃ and Nb₂O₅ in sulfur dioxide oxidation, and in selective catalytic reduction (SCR) of NO by ammonia has been studied. Addition of tungsten and niobium oxides was found to suppresses sulfur dioxide oxidation thus increasing the catalysts resistance to SO₂ poisoning and their activity in SCR.     

Preparation and properties of homogeneous V2O5-SiO2 xerogel composite based on interpenetrating polymer network

In this paper, we report the conducting and electrochemical properties of a homogeneous V₂O₅-SiO₂ xerogel composite obtained from the simultaneous polymerization in both inorganic components (Si-O and V-O based polymers) forming an interpenetrating polymer network, where a mutual “solubility” due to cross-links and entanglements was observed. The presence of V₂O₅ inside the silica matrix has a strong effect on electrical conductivity; measurements showed room temperature conductivity almost 1000 times higher than what is found in the literature. In addition, the electrochemical behavior is quite similar to that found in V₂O₅ xerogel. Moreover, the effects of thermal treatment on the conducting and electrochemical properties were investigated. It was found that both properties were improved with no significant effect on V₂O₅ xerogel layered structure.     

Influence of the grain morphology of V2O5 on its reduction-reoxidation behaviour

The influence of the grain morphology of V₂O₅ on its reduction-reoxidation behaviour has been investigated by means of thermoanalytical methods. X-ray analysis and electron microscopy. Well-developed V₂O₅ platelets exposing predominantly the (010) face exhibited a significantly different reduction profile than poorly defined agglomerates of microcrystalline V₂O₅. Intermediate phases detected during reduction were V₆O₁₃ and VO₂ (rutile). The corresponding reoxidation profiles were found to be only weakly dependent on the grain morphology of V₂O₃. Electron microscopy showed that the original grain morphology of the V₂O₅ samples was not influenced markedly by the reduction-reoxidation cycle.     

Phase equilibria in the system V2O3Ti2O3TiO2 at 1473°K

The phase equilibria in the V₂O₃Ti₂O₃TiO₂ system have been determined at 1473°K by the quench method, using both sealed tubes and controlled gaseous buffers. For the latter, $\text{CO}{}_2\text{H}{}_2$ mixtures were used to vary the oxygen fugacity between 10^?10.50 and 10^?16.73 atm. Under these conditions the equilibrium phases are: a sesquioxide solid solution between V₂O₃ and Ti₂O₃ with complete solid solubility and an upper stoichiometry limit of (V, Ti)₂O_3.02; an M₃O₅ series which has the V₃O₅ type structure between V₂TiO₅ and V_0.69Ti_2.31O₅ and the monoclinic pseudobrookite structure between V_0.42Ti_2.58O₅ and Ti₃O₅; series of Magneli phases, V₂Ti_n?2O_2n?1Ti_nO_2n?1, n = 4–8; and reduced rutile phases (V, Ti)O_2?x, where the lower limit for x is a function of the $\text{V}\text{(V + Ti)}$ ratio. The extent of the different solid solution areas and the location of the oxygen isobars have been determined.     

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.     

V2.38Nb10.7O32.7: A V2O5-Nb2O5 mixed oxide tunnel structure related to the tetragonal tungsten bronzes

A crystal structural model for the orthorhombic compound V_2.38Nb_10.7O_32.7, which is known as “V₂Nb₉O_27.5”, was developed by means of selected area electron diffraction (SAED), Rietveld refinement and high resolution electron microscopy (HREM). The metastable compound is obtained by thermal decomposition of freeze-dried precursors as chain-like agglomerated nanoparticles or by reaction of V₂O₅ with fresh-precipitated Nb₂O₅ as more compact micro-scaled crystals. With the latter, it was possible to identify its structure for the first time (space group Cmmm). The tetragonal tungsten bronze (TTB)-type structure shows high potential for ionic intercalation, since easily reducible [V⁵⁺₂O²⁻] units are implemented in the tunnels of a rigid niobium oxide framework.     

Phase diagram and infrared-spectral investigation of the 2TeO2 · V2O5-Na2O · V2O5 · 2TeO2 system

The phase diagram of the 2TeO₂ · V₂O₅-Na₂O · V₂O₅ · 2TeO₂ system is studied by X-ray diffraction, ir spectroscopy, and DTA. A new compound with a composition of Na₂O · 3V₂O₅ · 6TeO₂ is established. The ir spectra of the alkaline trivanadates are interpreted. They are considered as structural analogs of the new phase. As a result of this comparison, the postulate is made that the main structural units in the Na₂O · 3V₂O₅ · 6TeO₂ compound are V₂O₈ groups, while tellurium is present both in the TeO₃ and TeO₄ groups. Contrary to the crystal phases, in glasses the transition from VO₅ toward VO₄ does not proceed through the formation of new structural units of vanadium; but rather a gradual transition of the structure is observed with a change in the composition from 2TeO₂ · V₂O₅ to Na₂O · V₂O₅ · 2TeO₂.     

Preparation and characterization of highly dispersed V2O5/SiO2prepared by a CVD method

Highly dispersed V₂O₅/SiO₂(CVD catalyst) was prepared by the reaction of vaporized VO(OPrⁱ)₃ with silica at 293 K, whose process was followed by an IR technique. The rate of propylene photooxidation increased with an increase in V₂O₅ loading for CVD catalysts, but leveled off for impregnated ones. The CVD catalysts were characterized by XAFS and photoluminescence spectroscopy.     

Mechanochemical reactions between Ag2O and V2O5 to form crystalline silver vanadates

Solid-state reactions between Ag₂O and V₂O₅ were studied under ball-milling conditions. Single-phase crystalline Ag₄V₂O₇ was formed from the mixture of Ag₂O and V₂O₅ of corresponding (2:1) composition. The main component in the product when the Ag₂O mole fraction is less than V₂O₅ is amorphous AgVO₃, which is crystallized into needle-like α-AgVO₃ in the presence of water. Excess V₂O₅ was hydrated into V₂O₅·nH₂O intercalated with Ag⁺ ions. The mixtures with more than two parts of Ag₂O relative to V₂O₅ are composite materials of Ag₄V₂O₇ and Ag₃VO₄, together with Ag₂O. The crystalline phases in these systems resist attack by water.     

Lithium-promoted V2O5?TiO2 catalysts for o-xylene oxidation to phthalic anhydride

A V₂O₅–Li₂O–TiO₂ (a) based catalyst for o-xylene oxidation to phthalic anhydride has been synthesized. The activity and selectivity of the specimen obtained are comparable with those of industrial catalysts.     

The compound Cr2TiO5 in the system Cr2O3TiO2

The compound Cr₂TiO₅ could be synthesized as a stoichiometric single phase above 1660°C in air. Application of selected area electron diffraction, high resolution electron microscopy and powder X-ray diffraction studies showed that Cr₂TiO₅ is isomorphous with CrFeTiO₅, with V₃O₅ type structure. It is monoclinic, a = 7.020(1)Å, b = 5.025(1)Å, c = 9.945(2)Å and β = 111.43(2)°. It was found that Cr₂TiO₅ is unstable relative to a mixture of Cr₂O₃ (ss) and a so-called “E” phase, below 1660°C.     

Vanadium- and molybdenum-containing compositions prepared by mechanochemical and following thermal treatments of V2O5/(NH4)2Mo2O7 (V/Mo = 0.7/0.3)

X-ray powder diffraction, DTA, FT-IR spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), nitrogen adsorption, and mercury porometry were used to characterize samples prepared as a result of mechanochemical treatment (MCT) of a V₂O₅/(NH₄)₂Mo₂O₇ (V/Mo = 0.7/0.3) composition in water, ethanol, and air, as well as after calcining them at temperatures from the range 300–700°C. The MCT of nonporous powders in water yields porous materials with definite meso- and macropore sizes. Heat treatment in air at 300–450°C enhances the formation of a molybdenum substitutional solid solution in V₂O₅ and conserves rather high values of specific surface areas and pore volumes. An increase in heat treatment temperature is accompanied by the degradation of the solid solution and the formation of a V₂MoO₈ phase.     

Nonstoichiometry and defects in V2O3

The structural mechanism which accommodates nonstoichiometry in V₂O₃ was investigated by transmission electron microscopy. The existence of distinct diffuse scattering was observed as the boundary of the homogeneity range was approached. The analysis of the diffuse scattering indicates the formation of one-dimensional microdomains in the c direction having a structure similar to VO₂. Orientation relations between V₂O₃ and V₃O₅ (which is formed from V₂O₃ by heat treatment in an oxidizing atmosphere) show that V₃O₅ is formed by redistribution of vanadium ions among the octahedral interstitial sites of common close-packed sublattice consisting of oxygen ions. Possible relations between the present observations and physical properties in nonstoichiometric V₂O_3+x are discussed.     

Oxidative dehydrogenation of propane over chromium and nickel oxide modified V2O5/ZrO2 catalysts

The effect of addition of chromium and nickel oxides on the physicochemical properties and performance of V₂O₅/ZrO₂ catalysts was studied for the oxidative dehydrogenation of propane. Addition of chromium oxide increased, whereas addition of nickel oxide lowered the activity. Selectivity for propene was lower for the doped catalysts. The selectivity was lowered by higher total acidity as well as the higher concentration of stronger acid sites in doped catalysts.     

Synthesis and characterization of inorganic solid electrolytes of Li2O-SiO2-P2O5 and Li2O-TiO2-SiO2-P2O5 systems

The lithium-ion-conducting inorganic solid electrolytes in the oxide systems Li₂O-SiO₂-P₂O₅ and Li₂O-TiO₂-SiO₂-P₂O₅ were prepared by the solid-state reaction, and the electrolyte pellet made by cold-pressing method had diameter of 13 mm and was about 1 mm thick. Phase identification and surface morphology of the products were carried out by X-ray diffraction and scanning electron microscopy. Ionic conductivity of the pellets was investigated through ac impedance. The results show that the adding of other cations can improve the ionic conductivity of the solid electrolyte, and the sintering temperature and duration can influence the ionic conductivity. The maximum ionic conductivity in the samples is 9.9 × 10⁻⁴ S/cm in the Li₂O-TiO₂-SiO₂-P₂O₅ system. Original Russian Text ? W. Li, M. Wang, Z.H. Li, X.F. Shang, H. Wang, Y.W. Wang, Y.B. Xu, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 11, pp. 1341–1345.     

Kinetic Peculiarities in the Low-Temperature Oxidation of H2S on Vanadium Catalysts

H₂S oxidation by oxygen on catalysts V₂O₅/Al₂O₃, V₂O₅/TiO₂, V₂O₅/Al₂O₃/TiO₂ was studied at temperatures below the sulfur dew point. High activity and the oscillation character of the oxidation were demonstrated by catalysts with low contents of V₂O₅ (3–5 wt.%). The increase in the V₂O₅ concentration to 10–20 wt.% results in the reduction of the catalytic activity and oscillation ability. On a pure V₂O₅ catalyst, the oscillations were not detected. The difference between the catalysts with the high and low concentrations of V₂O₅ is explained in terms of the structures of the V⁵⁺ species formed in the catalysts. This revised version was published online in June 2006 with corrections to the Cover Date.