Electron spectroscopic studies of the reduction of WO3

Five different doublets corresponding to W 4f electrons were observed in the course of reduction of WO₃. On contact of WO₃ with hydrogen, W⁵⁺ ions are formed. Reduction results at first in formation of isolated W⁴⁺ ions. Clusters of edge-sharing octahedra on shear planes are then formed, in which pairing of W⁴⁺ ions occurs due to metal-metal bonding, corresponding to an apparent 2+ oxidation state.     

Thermodynamic study of nonstoichiometric tungsten trioxide WO3−x by EMF measurements at high temperature

The values of ΔG(O₂), ΔH(O₂), and ΔS(O₂) have been determined from electrochemical cell measurements, within the whole homogeneity range of WO_3?x, between 700 and 900°C. The samples have been previously prepared by equilibration of WO₃ pellets with COCO₂ mixtures and their composition has been determined by thermogravimetry. A single phase has been found between WO₃ and WO_2.9760. The results may be understood by considering a structure involving point defects, singly ionized oxygen vacancies V^·_O between WO₃ and WO_2.9880. For larger departure from stoichiometry, the variations of ΔH(O₂) and ΔS(O₂) suggest the formation of more complex defects. The enthalpy of formation of V^·_O has been calculated: 78 kcal · mole^?1.     

Crystal structure and charge carrier concentration of W18O49

The electrical resistivity of the tungsten oxide, W₁₈O₄₉, is 1.75 · 10^?3 Ω cm along the needle axis. The charge carrier density, as determined by reflectivity measurements, is 1.87 · 10²² cm^?3, thereby indicating that most of the charge carriers are delocalized. Hence the smaller conductivity along the needle axis than that expected for such charge carrier concentrations must be found in the structure, which has been refined using the data collected with an automatic diffractometer. The structure consists of WO₆ and WO₇ polyhedra which are linked along edges and/or corners. However, as the linkage parallel to b takes place only by sharing corners, an anisotropy in the electrical conductivity may be expected. Another explanation for the smaller conductivity may be found in the occurrence of defects such as tunnels in the structure, which may scatter the electrons. The refinement shows that the tungsten positions, determined by Magneli (Arkiv Kemi1, 223 (1950)), are essentially correct; but the positions of the oxygens, especially two of them, differ considerably. This results in one of the tungsten atoms getting an additional coordinating oxygen, the coordination number thereby becoming seven.     

Crystallographic shear in the Nb2O5WO3 and Ta2O5WO3 systems

Crystallographic shear (CS) phases occurring in the Nb₂O₅WO₃ and Ta₂O₅WO₃ systems near to WO₃ were characterized by X-ray diffraction and high-resolution transmission electron microscopy. The Nb₂O₅WO₃ samples were heated at 1600K. They contained ordered {104} and {001} CS planes and wavy CS which were composed of intergrowths of {104} and {001} CS segments. The composition range over which the {104} CS series extended was from (Nb,W)O_2.954 i.e., (Nb,W)₆₅O₁₉₂, to (Nb,W)O_2.942, i.e., (Nb,W)₅₂O₁₅₃. The composition range over which the {001} CS series extended was from (Nb,W)O_2.9375, i.e., (Nb,W)₁₆O₄₇ to (Nb,W)O_2.875, i.e., (Nb,W)₈O₂₃. The Ta₂O₅WO₃ samples were prepared at 1593, 1623, and 1672K. At lower temperatures ordered {103} CS phases were found, with a composition range extending between (Ta,W)O_2.960, i.e., (Ta,W)₅₀O₁₄₈, to (Ta,W)O_2.944, i.e., (Ta,W)₃₆O₁₀₆. At 1673K ordered {103} CS phases occurred, as did wavy CS composed of intergrowths of {103} and {104} CS segments.     

A high-temperature structure for Ta2O5 with modulations by TiO2 substitution

Solid solutions in the series (1−x)Ta₂O₅−xTiO₂ with x=0.0-0.1 were prepared by high-temperature ceramic processing methods, and the crystal structure was determined at room temperature by transmission electron microscopy, electron diffraction and high-resolution lattice imaging. A structural model is proposed for the oxygen-deficient tantalum oxide (Ta₂O₅) phase with high TiO₂ doping level (x=0.08). The model is based on edge sharing of an oxygen octahedron-hexagonal bi-pyramid-octahedron molecular building block unit that repeats four times per unit cell. Electron diffraction reveals a monoclinic distortion from a pseudo-tetragonal model structure that is modulated primarily along 〈110〉. The modulation length varies with increasing TiO₂ content. Furthermore, by quantitative HREM analysis and matching of lattice images by simulation, it is shown that the modulation is associated with small ionic displacements in specific lattice planes that coincide with Ta ions in the model structure coordinated by oxygen hexagonal bi-pyramids. Based on this evidence, it is suggested that the modulation comes from a replacement of Ta with Ti ions, and the loss of inversion symmetry in the modulated structure is related to the dielectric properties of the material.     

The effect of reduction and temperature on the electronic core levels of tungsten and molybdenum in WO3 and WxMo1−xO3—A photoelectron spectroscopic study

X-Ray photoelectron spectroscopy (XPS) has been applied to electrochromic, reduced WO₃ and W_xMo_1?xO₃ crystals. In metal-reduced phases containing crystallographic shear planes the formation of Mo⁵⁺ (preferentially) and W⁵⁺ is observed in addition to that of the six-valent states. W⁵⁺ and W⁶⁺ are also dominant in H⁺-bombarded WO₃ indicating the formation of bronzes H_xWO₃. Significant differences are observed between single-crystal and “amorphous” oxides. The five-valent state is interpreted as being due to electron trapping and polaron formation. Under Ar⁺ bombardment the crystallinity of the surface is destroyed and a continuous distribution of W⁰, W⁴⁺, W⁵⁺, and W⁶⁺ is found similar to that observed for amorphous thin films. At low temperatures the (?-δ) metal-insulator (M-I) transformation of H⁺:WO₃ is accompanied by a spontaneous change in the linewidth of W⁵⁺ core levels but not of W⁶⁺ states. This is in accordance with recent theoretical approaches to M-I transformations.     

Enthalpy of formation of aqueous tungstate ion and of crystalline tungstic acid (H2WO4)

Calorimetric measurements of the enthalpy of reaction of WO₃(c) with excess OH^?(aq) have been made at 85°C. Similar measurements have been made with MoO₃(c) at both 85 and 25°C, to permit estimation of ΔH°=?13.4 kcal mol^?1 for the reaction WO₃(c)+2OH^?(aq)=WO^2?₄(aq)+H₂O(liq) at 25°C. Combination of this ΔH° with ΔH°_f for WO₃(c) leads to ΔH°_f=?256.5 kcal mol^?1 for WO^2?₄(aq). We also obtain ΔH°_f=?269.5 kcal mol^?1 for H₂WO₄(c). Both of these values are discussed in relation to several earlier investigations.     

Structural chemistry of Sr3Cr2WO9, Ca3Cr2WO9, and Ba3Cr2WO9

We have studied the preparation and crystallographic structure of three perovskite-type compounds: Sr₃Cr₂WO₉, cubic, the lattice parameter of which is a = 7.812Å; Ca₃Cr₂WO₉, tetragonal, the lattice parameters of which are a = 5.408 Å and c = 7.635Å; and Ba₃Cr₂WO₉, hexagonal, the lattice parameters of which are a = 5.691 Å and c = 13.957Å. We have compared these three structures and shown the relationship between the dimensions of the alkaline-earth metal and the existence of the different structures.     

Solid state reactions in the system Nb2O5·WO3: An electron microscopic study

Lattice imaging electron microscopy has been used to study the mechanism of solid state reactions of the type: A_s → B_s + C_s, in which the product B is able to intergrow coherently with the starting material A, but the product C cannot do so. C must be formed by a fully reconstructive, heterogeneous process; formation of B is only partially reconstructive, and essentially homogeneous. Reactions were the reversible phase reactions in the system Nb₂O₅WO₃: disproportionation of the (5 × 4)₁ block structures of 8Nb₂O_5·5WO₃, to form (4 × 4)₁ blocks of 7Nb₂O_5·3WO₃ as coherent product, and that of 9Nb₂O_5·8WO₃ (with (5 × 5)₁ blocks), forming (5 × 4)₁ blocks of 8Nb₂O_5·5WO₃ as coherent product. The coherent product structure is formed in isolated rows of blocks, or small packets of such rows, running across each crystal. The reaction does not work in progressively from some surface initiating step, with an interface between unchanged and converted material, but represents a block-by-block conversion, linearly propagated. Nb₂O₅ and WO₃ must be abstracted, in appropriate stoichiometric ratio, from each block but must ultimately reach and react at the surface, to form the incoherent product (a pentagonal tunnel network structure, in both cases). Some homogeneous transport process involving lattice diffusion must be invoked. Domains of highly anomalous structure, regarded as relicts of transient conditions, are occasionally observed. From reactions at relatively low temperatures, these have structures that can be regarded as partially ordered nonstoichiometric solid solutions; after prolonged heating, and at higher temperatures they form well ordered strips of metastable block structures. Both types represent strong, spontaneous fluctuations of composition, which impose a corresponding structure locally. These fluctuations may be associated with the transport of WO₃ and Nb₂O₅ away from the locus of reaction. Evidence about the mechanism of the reactions, the role of dislocations and the nature of cooperative processes is considered.     

Structure and “oxidation behavior” of W24O70, a new member of the {103} CS series of tungsten oxides

Reduced tungsten trioxide crystals WO_3?x, formed by vapor transport from a preparation with bulk composition WO_?2.90, have been studied by X-ray diffraction and electron microscopy. A single-crystal X-ray investigation showed the existence of the ordered {103} CS-structure W₂₄O₇₀, a new member of the homologous series W_nO_3n?2. Electron diffraction patterns of crystal fragments, with a few exceptions, showed the presence of the W₂₄O₇₀ phase (composition WO_2.917). Lattice images, however, indicated a fairly ordered {103} CS-phase, W₂₄O₇₀, intergrown with slabs of WO₃ giving gross compositions of the examined crystals in the range WO_2.93WO_2.96. The wide WO₃ slabs were probably formed by an oxidation process during the preparation.     

REDUCTION OF NO BY CO OVER NiWO4, NiO, AND WO3 CATALYSTS

We present a comparative study of NiWO₄, NiO, and WO₃ catalysts for simultaneous conversion of NO and CO. Samples were synthesized by reacting ammonium metatungstate and/or nickel nitrate at high temperature (773 K to 903 K) under an oxygen stream. Catalysts were characterized by X-ray diffraction, surface area measurements, energy dispersive spectroscopy and scanning electron microscopy. The catalytic reduction of NO by CO took place in the temperature range (523 to 973) K under highly reductive conditions (NO:CO= 1:5) over NiWO₄NiO, and WO₃, respectively. The 100 % NO conversion at GHSV of 11460 h^-1 was achieved at 773 K over NiWO₄ and at 848 K over NiO. The WO₃ was deactivated at 898 K. However, in the range (523 to 723) K NiO was more active than NiWO₄ and WO₃ catalysts.     

An outline of the stucture of 7Bi2O3 · 2WO3 and its solid solutions

This paper gives an outline of the structure of a solid solution based on 7Bi₂O₃ · 2WO₃. The experimental results using X-ray diffraction methods (precession and powder) showed that 7Bi₂O₃ · 2WO₃ crystallizes in the space group $\text{I4}{}_1\text{a}$ with a = 12.5143(5)Å and c = 11.2248(6) Å. The number of formula weights per unit cell is 40, when the formula is considered to be of the oxygen-deficient fluorite-type Bi_0.875W_0.125O_1.6875. The compound has a substructure based on a defect fluorite-type pseudocubic subcell with $\text{a′ ? 5.6}$ Å. The axial relations between the supercell and subcell are $\text{a ? √a′}$ and $\text{c ? 2a′}$. The solid solution was formed over a limited range of WO₃ content between 21.3 mole% and 26.3 mole% at 700°C. The ordering of metal atoms is discussed and an ideal crystal structure is proposed.     

Structural relationships in the series AxP4O8(WO3)2m (A = K,Rb): A high-resolution electron microscopy study

The first members of the series A_xP₄O₈(WO₃)_2m were studied by means of electron microscopy. These bronzes can be classified into two groups on the basis of ReO₃-type block composition: even- and odd-m members. High-resolution lattice images of tungstophosphate crystals (m ≤ 10) allow us to establish a correlation between the image contrast and the framework of the structure. The structural mechanism proposed for this series is discussed and compared to the possibility of intergrowth, and to the crystallographic shear phenomena observed in tungsten and molybden oxides.     

Determining the structure of tetragonal Y2WO6 and the site occupation of Eu dopant

The compound Y₂WO₆ is prepared by solid state reaction at 750 °C using sodium chloride as mineralizer. Its structure is solved by ab-initio methods from X-ray powder diffraction data. This low temperature phase of yttrium tungstate crystallizes in tetragonal space group P4/nmm (No. 129), Z=2, a=5.2596(2) Å, c=8.4158(4) Å. The tungsten atoms in the structure adopt an unusual [WO₆] distorted cubes coordination, connecting [YO₆] distorted cubes with oxygen vacancies at the O₂ layers while other yttrium ions Y₂ form [YO₈] cube coordination. Y³⁺ ions occupy two crystallographic sites of 2c (C_4v symmetry) and 2a (D_2d symmetry) in the Y₂WO₆ host lattice. With Eu³⁺ ions doped, the high resolution emission spectrum of Y₂WO₆:Eu³⁺ suggests that Eu³⁺ partly substituted for Y³⁺ in these two sites. The result of the Rietveld structure refinement shows that the Eu³⁺ dopants preferentially enter the 2a site. The uniform cube coordination environment of Eu³⁺ ions with the identical eight Eu-O bond lengths is proposed to be responsible for the intense excitation of long wavelength ultraviolet at 466-535 nm.     

Cr2(WO4)3和Cr2(MoO4)3的负热膨胀特性研究

用液相反应-前驱物烧结法制备了Cr₂(WO₄)₃和Cr₂(MoO₄)₃粉体。298～1 073 K的原位粉末X射线衍射数据表明Cr₂(WO₄)₃和Cr₂(MoO₄)₃的晶胞体积随温度的升高而增大, 本征线热膨胀系数分别为(1.274±0.003)×10^-6 K^-1和(1.612±0.003)×10^-6 K^-1。用热膨胀仪研究了Cr₂(WO₄)₃和Cr₂(MoO₄)₃在静态空气中298～1 073 K范围内热膨胀行为，即开始表现为正热膨胀，随后在相转变点达到最大值，最后表现为负热膨胀，其负热膨胀系数分别为(-7.033±0.014)×10^-6 K^-1和(-9.282±0.019)×10^-6 K^-1。     

Structure and electrical conductivity of La0.84Sr0.16MnO3

X-Ray diffraction, density, and electrical conductivity measurements were performed on the perovskite-like mixed oxide La_0.84Sr_0.16MnO₃. A rhombohedral crystalline structure with lattice parameters a = 3.893 Å and α = 90°29′16″ was assigned to the powder prepared by standard ceramic technique. Its theoretical density is therefore 6.576 g/cm³, while the experimental density was determined as 6.48 g/cm³. The conductivity measured at 1000°C is 133 Ω^?1 cm^?1. The temperature dependence of the conductivity indicates that the charge carriers are small polarons. The activation energy of the mobility is 9.6 kJ/mole.     

Phase equilibria and thermodynamics of the double oxide phase Cu3TiO4

Phase equilibria in the system CuCu₂OTiO₂ were investigated in the temperature range of 1160–1270 K by means of thermogravimetry and measurements of the oxygen partial pressure. The tie lines on the isothermal phase diagram run from the phase Cu₃TiO₄ to CuO, Cu₂O, and TiO₂. The existence of Cu₃TiO₅ and Cu₂TiO₃ could not be confirmed in this temperature range. The phase “Cu₃TiO₄” is only stable above about 1140 K and its composition fluctuates between about Cu₃TiO_4.3 and Cu₃TiO_3.9. The formation of Cu₃TiO_4.3 according to the reaction 1.6 CuO + 0.7 Cu₂O + TiO₂ = Cu₃TiO_4.3 is endothermic: (1160 < T < 1270 K) ΔH° = (7600 ± 450 J-mole^?1; ΔS° = (6.7 ± 0.4) J·K^?1·mole^?1. The standard Gibbs free energy, enthalpy, and entropy of formation of Cu₃TiO_4.3 at 1200 K are ΔG°_f = ?101.39 kJ, ΔH°_f = ?1115.84 kJ, and S°_f = 466.76 J·K^?1. Rather similar values were found for Cu₃TiO_3.9.     

Electrical conductivity and defect structure of Nd2O3+x

The electrical conductivity and departure from the stoichiometry of Nd₂O₃ have been measured over the temperature range of 900° to 1100°C and oxygen partial pressure of 1 to 10^?16 atm. The hole conductivity of Nd₂O₃ is found to be proportional to , where n are 4.6, 4.9, and 5.1 at 900°, 1000°, and 1100°C, respectively. From the oxygen partial pressure dependence of the hole conductivity, it is shown that the predominant point defects in nonstoichiometric NdO_1·+x are fully ionized and partially doubly ionized metal vacancies. From the thermogravimetric measurements, the departure from stoichiometry, x in NdO_1·5+x, is 2.0 × 10^?3 at 1000°C and 1 atm. By combining the electrical conductivity and weight change data, it is shown that the hole mobility is 6.3 × 10^?4 (cm²/V·sec) at 1000°C and 1 atm.     

Bi2WO6/TiO2纳米管异质结构复合材料的多模式下的光催化活性比较

以TiO₂纳米管为模板,采用多组分自组装结合水热法制备Bi₂WO₆/TiO₂纳米管异质结构复合材料。通过多种技术如X射线衍射（XRD）,X射线光电子能谱（XPS）,N₂吸附-脱附,扫描电镜（SEM）,高分辨透射电镜（HRTEM）和紫外可见漫反射吸收光谱（UV-Vis DRS）考察所制备样品的组成、结构、形貌、光吸收和电子性质。Bi₂WO₆纳米片或纳米粒子分布在TiO₂纳米管上,形成异质结构。随后,通过在紫外、可见和微波辅助光催化模式下降解染料罗丹明B（RhB）来评价复合催化剂的光催化活性。与TiO₂纳米管和Bi₂WO₆相比,Bi₂WO₆/TiO₂-35纳米管在多模式下表现出更优异的光催化活性。与紫外和可见降解模式相比,Bi₂WO₆/TiO₂-35纳米管在微波辅助光催化模式下对RhB的降解效率最高。这种增强的光催化活性源于适量Bi₂WO₆的引入、纳米管独特的形貌特征和降解模式所引起的增强的量子效率。降解过程中的活性物种被证明是h⁺,·OH和·O₂^-自由基。而且,在微波辅助光催化模式下,可产生更多的·OH和·O₂^-自由基。     

Formation of pentagonal tunnels bordered with P2O7 groups in diphosphate tungsten bronzes (P2O4)2(WO3)2m: A HREM investigation

During the investigation of the phosphate bronzes (PO₂)₄(WO₃)_2m [MPTB_P] and K_x(P₂O₄)₂WO₃)_2m [DPTB_H] crystals of a new type were observed. HREM images of these crystals showed twinned ReO₃-type slabs the junction of which was parallel to the (102)_ReO3 plane. The proposed model identified the twin boundary as built from P₂O₇ groups involving the formation of pentagonal tunnels. The structure of this new type of extended defects is quite original: it corresponds to a new structural type named “diphosphate tungsten bronzes with pentagonal tunnels” [DPTB_P], for which no regular member could be synthesized. Image calculations were performed to confirm the junction model. Apart from the disordered stacking of the ReO₃-type slabs, very few defects were observed and shear planes were only obtained in reduced samples. This new structural type takes its place in the large family of phosphate tungsten bronzes where all members (DPTB_H, MPTB_H, MPTB_P) are very closely related.