Generation of WO3-ZrO2 catalysts from solid solutions of tungsten in zirconia

WO₃-ZrO₂ samples were obtained by precipitating zirconium oxynitrate in presence of WO₄ species in solution from ammonium metatungstate at pH=10.0. Samples were characterized by atomic absorption spectroscopy, thermal analysis, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and energy filtered-TEM. The ammonia retained in the dried sample produced a reductive atmosphere to generate W⁵⁺ ions coexisting with W⁶⁺ ions to produce a solid solution of tungsten in the zirconia lattice to stabilize the zirconia tetragonal phase when the sample was annealed at 560 °C. When the sample was annealed at 800 °C, the W atoms near crystallite surface were oxidized to W⁶⁺, producing patches of WO₃ on the zirconia crystallite. The HR-TEM analysis confirmed the existence of the solid solution when the sample was annealed at 560 °C, and two types of crystalline regions were identified: One with nearly spherical morphology, an average diameter of 8 nm and the atomic distribution of tetragonal zirconia. The second one had a non-spherical morphology with well-faceted faces and dimensions larger than 30 nm, and the atom distribution of tetragonal zirconia. When samples were annealed at 800 °C two different zirconia crystallites were formed: Those where only part of the dissolved tungsten atoms segregated to crystallite surface producing patches of nanocrystalline WO₃ on the crystallite surface of tetragonal zirconia stabilized with tungsten. The second type corresponded to monoclinic zirconia crystallites with patches of nanocrystalline WO₃ on their surface. The tungsten segregation gave rise to the WO₃-ZrO₂ catalysts.     

TeO2-WO3 Glasses: Infrared, XPS and XANES Structural Characterizations

Transparent (1−x)TeO₂-xWO₃ glasses with 0≤x≤0.325 mol were synthesized by the fast quenching technique. Several complementary techniques as infrared, X-ray photoelectron and X-ray absorption spectroscopies were used to approach the structure of these tungsten oxide-tellurite glasses. Special attention was paid to the oxidation state and the coordination state of tungsten atoms. The structural results show that (1−x)TeO₂-xWO₃ glasses present characteristic tellurium environments which vary with their chemical composition while tungsten ions always adopt an octahedral configuration.     

Crystal structure and vibrational properies of Rb2MgWO2(PO4)2—A new framework phosphate

The new compound Rb₂MgWO₂(PO₄)₂ has been synthesized and characterized by a single-crystal X-structure determination, and IR and Raman spectroscopic studies. The crystal structure is orthorhombic, space group Pbca, with the unit cell dimensions a=9.891(2), b=12.641(2), , Z=8. Compared to the K₂M^IIWO₂(PO₄)₂ series, where M^II=Mg, Mn, Fe, Co, Ni, and Cd, the volume of the unit cell in the present compound is nearly doubled. The MgO₆ and WO₆ octahedra are arranged into polyhedral groups consisting of two edge sharing MgO₆ joined by corners with two WO₆ octahedra. These groups are interconnected through the PO₄ tetrahedra into layers in a×b plane. The Rb⁺ ions perform thermally activated displacements within the cavities formed between the polyhedral layers. The origin of various Raman and IR modes is discussed. These results indicate that a clear energy gap exists between the stretching and remaining modes. The most intense modes are shown to be due to vibrations of the W-O bonds.     

Bond lengths and valences in tungsten oxides

Equations relating bond strength (valence) to bond length have been developed for tungsten-oxygen and phosphorus-oxygen bonds. Bond-valence sums have been carried out for the different tungsten atoms in phosphate tungsten bronzes and other mixed-valence tungsten oxides and for the Mo atoms in TeMo₅O₁₆. Valences intermediate between 5 and 6 are generally found, in agreement with physical measurements that indicate delocalization of d electrons in these materials. Evidence is presented that shows that the degree of distortion of WO₆ octahedra increases with apparent oxidation state of tungsten.     

Ein neues Syntheseverfahren für die Wolframbronze Cs0,29WO3

A Novel Synthetic Access to the Tungsten Bronze Cs_0.29WO₃ and its Crystal Structure The hexagonal tungsten bronze Cs_0.29WO₃ was obtained in form of black, prismatic crystal by the reduction of WO₃ with molten cesium iodide at 700°C. Its crystal structure was determined by X-ray diffraction (399 unique observed reflexions, R = 0.058). Crystal data: a = 741.2(3), c = 760.0(5) pm, space group P6₃22, Z = 6. It corresponds to the known structure of hexagonal tungsten bronzes, having tungsten atoms displaced from the octahedra centres by 11.9 pm and with three different W? O bond lengths (198, 191, 187 pm). The WO₆ octahedra are slightly titled mutually.     

<Emphasis Type="Italic">In silico</Emphasis> study of the atomic and electronic structure of quantum dots of the CdTe family doped with atoms of rare earth elements

An in silico study of semiconductor quantum dots of the CdTe family doped with atoms of rare earth elements is performed based of density functional theory. An ab initio computer design of quantum dots based on CdTe nanoparticles doped with Eu и Gd atoms is carried out. Partial densities of states of CdTe:Eu and CdTe:Gd quantum dots are calculated and analyzed. X-ray absorption near edge (XANES) spectra near the Eu K-, L₁-, and L₃- and Gd K-, L₁-, and L₃-edges of CdTe:Eu and CdTe:Gd quantum dots are calculated. The sensitivity of XANES spectroscopy for the verification of parameters of a nanosized atomic structure of quantum dots based on CdTe particles doped with atoms of rare earth elements and the determination of the local atomic structure around the atoms of rare earth elements in quantum dots is demonstrated.     

Highly efficient mesoporous WO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/KIT-6 catalysts for oxidative desulfurization of dibenzothiophene with hydrogen peroxide

Oxidative desulfurization (ODS) of organic compounds containing sulfur element from a model oil was performed using tungsten oxide catalysts supported on mesoporous silica with cubic Ia3d mesostructure, well-defined mesopores (7.2 nm), high surface area (719 m²/g), and three-dimensional pore network (WO _x /KIT-6). The prepared WO _x /KIT-6 catalysts (5–20 wt% WO _x ) were characterized by X-ray diffraction analysis, N₂ sorption measurements, electron microscopy, H₂-temperature programmed reduction, Raman spectroscopy, and thermogravimetric analysis. Among the mesoporous catalysts, 10 wt% WO _x /KIT-6 exhibited the best catalytic performance. Sulfur-containing organic compounds, such as dibenzothiophene, 4,6-dimethyldibenzothiophene, and benzothiophene, were completely (100 %) removed from the model oil over 10 wt% WO _x /KIT-6 catalyst in 2 h. In addition, the catalyst could be reused several times with only slight decrease in catalytic activity.     

IR-spectral study of the effect of WO3 on the structure of tellurite glasses

The comparison of IR-spectra of tellurite glasses and their crystal products containing from 5 up to 45 mol% WO₃ indicates that the modifier does not change the coordination of tellurium. The IR-spectra of glasses containing small WO₃ amounts show a band at 925 cm^?1, which shifts up to 950 cm^?1 with the increase in the tungsten concentration. The effect is specific of the vitreous state and may be explained by the change in the coordination of tungsten.     

Reactions of TiCl4 and ZrCl4 with IOCN

On Hexagonal Perovskites with Cationic Vacancies. V. Structure Determination on H? Ba₂Lu_2/3 □ _1/3WO₆ — a Novel Rhombohedral Stacking Polytype with 18 Layers Compounds of type Ba₂B□_1/3W^VIO₆ with B^III ? Gd—Lu, Y are polymorphic They crystallize in a cubic 1:1 ordered perovskite structure and in a new rhombohedral perovskite stacking polytype of 18 L respectively. By intensity calculations out of the three possible stacking sequences (4)(2), (5)(1) and (3)1(1)1 (all space group R3 m) the sequence (5)(1) can be selected. For H? Ba₂Lu_2/3□_1/3WO₆ the refined R′ factor is 14.1%. The structure contains groups of three octahedra connected with another by common faces which are linked with each other by three corner sharing octahedra. In the block of three face sharing octahedra the central octahedral lattice site is vacant, the two outer positions are occupied by tungsten atoms. According to this distribution a direct contact of occupied face sharing octahedra is absent.     

Structural investigation of (CuI)0.45–(Ag2WO4)0.55 solid electrolyte using X-ray photoelectron and laser Raman spectroscopies

X-ray photoelectron spectroscopy (XPS) and laser Raman scattering (LRS) techniques have been employed to investigate the structure of amorphous (CuI)_0.45–(Ag₂WO₄)_0.55 solid electrolyte sample. XPS results reveal the presence of both Cu⁺ and Cu²⁺ ions whereas tungsten is found to exist only in the oxidation state of +6. The deconvolution of the O 1s spectrum into non-bridging and bridging oxygen atoms in conjunction with the laser Raman analysis tend to show that the amorphous (CuI)_0.45–(Ag₂WO₄)_0.55 solid electrolyte sample is composed mostly of octahedral WO₆ units that probably form [W₄O₁₆]⁸⁻ tetramer clusters, the existence of which is unique in the case of oxyhalide glasses.     

Local environment in Ba2In2−xWxO5+3x/2 oxide ion conductors

The actual oxygen environment of the tungsten dopant in the Ba₂In_2−xW_xO_5+3x/2 solid solution was revealed by combining X-ray absorption spectroscopy at the tungsten L_I and L_III edges and at the indium L_I edge. Whatever the substitution ratio, the tungsten atoms exhibit a regular octahedral environment. When the substitution ratio increases, the oxygen vacancies are progressively filled until their total occupancy for x=2/3. For x?0.3, the perovskite structure is stabilised; the tungsten atoms are randomly distributed in the structure. Although X-ray diffraction revealed a cubic symmetry for these compositions, a local distortion of the indium environment is observed when a tungsten atom is in its surrounding.     

Vibrational spectra and structure of mixed alkali borotungstate glasses: Evidence of mixed alkali effect

Mixed alkali borotungstate glasses with xLi₂O–(30 − x)Na₂O–10WO₃–60B₂O₃ (0 ≤ x ≤ 30) composition were prepared by melt quench technique. FT-IR and Raman spectroscopic studies were employed to investigate the structure of all the prepared glasses. Acting as complementary techniques, both IR and Raman measurements revealed that the network structure of the present glasses mainly based on BO₃ and BO₄ units placed in different structural groups. Raman spectra confirm the IR results regarding the presence of tungsten ions mainly as WO₆ groups. In the present work, the mixed alkali effect (MAE) has been investigated in the above glass system using FTIR and Raman studies.     

Crystal and molecular structure of guanidinium hexatungsto-bis(p-aminophenylarsonate) tetrahydrate,[C(NH2)3]4[p-NH3C6H4As)2W6O25]·4H2O

The crystal structure of a new type arylarsonic polytungstate [C(NH₂)₃]₄[p-NH₃C₆H₄As)₂W₆O₂₅]·4H₂O was determined by single-crystal X-ray diffraction analysis. It belongs to triclinic, space group \[ P\bar 1 \], with cell dimensions a = 12.863(3), b = 18.912(3), c = 21.383(4) Å α = 91.14(2)°, β = 93.65(3)°, γ = 92.25(3)°, V = 5185.9 ų, Z = 4, D_c = 2.753 g/cm³. The intensity data were collected on an Enraf-Nonius CAD4 diffractometer with Mo K_α radiation. The positions of all tungsten and arsenic atoms were determined by direct method. The other non-hydrogen atoms were revealed by difference Fourier synthesis. The structure was refined by fullmatrix least-squares procedure to a final R value of 0.070. The crystal structure contains two similar but nonidentical molecules. Two similar anions consist of a ring of six WO₆ octahedra, which are connected with one face-sharing, two corner-sharings and three edge-sharings, and two p-aminophenylarsonic tetrahedra capped above and below the ring. In each WO₆ ring, four tungsten atoms, which are joined with edge-sharing oxygen atoms, are almost coplanar, while the two others, which are joined with face-sharing oxygen atoms, protrude out of the ring towards the same side. The two arsenic atoms in each anion are not equivalent in their bonding manner. In each anion, all non-hydrogen atoms of each organic group are in the same plane. Each molecule contains one anion, four C(NH₂)₃+ cations and four water molecules. There are many hydrogen bonds between cations and anions throughout the whole crystal. The amino groups can accept protons, so that the charge of the resulting anion decreases and [(RAs)₂W₆O₂₅]^4- type complexes are formed.     

Subsolidus phase relations and crystal structures of the mixed-oxide phases in the In2O3-WO3 system

Subsolidus phase relationships in the In₂O₃-WO₃ system at 800-1400°C were investigated using X-ray diffraction. Two binary-oxide phases—In₆WO₁₂ and In₂(WO₄)₃—were found to be stable over the range 800-1200°C. Heating the binary-oxide phases above 1200°C resulted in the preferential volatilization of WO₃. Rietveld refinement was performed on three structures using X-ray diffraction data from nominally phase-pure In₆WO₁₂ at room temperature and from nominally phase-pure In₂(WO₄)₃ at 225°C and 310°C. The indium-rich phase, In₆WO₁₂, is rhombohedral, space group (rhombohedral), with Z=1, a=6.22390(4) Å, α=99.0338(2)° [hexagonal axes: a_H=9.48298(6) Å, c=8.94276(6) Å, a_H/c=0.9430(9)]. In₆WO₁₂ can be viewed as an anion-deficient fluorite structure in which 1/7 of the fluorite anion sites are vacant. Indium tungstate, In₂(WO₄)₃, undergoes a monoclinic-orthorhombic transition around 250°C. The high-temperature polymorph is orthorhombic, space group Pnca, with a=9.7126(5) Å, b=13.3824(7) Å, c=9.6141(5) Å, and Z=4. The low-temperature polymorph is monoclinic, space group P2₁/a, with a=16.406(2) Å, b=9.9663(1) Å, c=19.099(2) Å, β=125.411(2)°, and Z=8. The structures of the two In₂(WO₄)₃ polymorphs are similar, consisting of a network of corner sharing InO₆ octahedra and WO₄ tetrahedra.     

Synthesis and characterization of Zr-SBA-15 supported tungsten oxide as a new mesoporous solid acid

A series of materials WO₃/Zr-SBA-15 were synthesized by modifying zirconium-incorporated SBA-15 mesoporous molecular sieve with various loadings of tungsten oxide, followed by calcining at different temperatures. The structures and the surface states of these materials were determined by XRD, TEM, N₂ adsorption–desorption and Raman spectroscopy, while the surface acidities were characterized by FT-IR spectroscopy of pyridine adsorption, NH₃-TPD, and the Hammett indicator method. To evaluate the catalytic activities of the prepared materials, the benzoylation of anisole was chosen as the model reaction. All the results reveal that the synthesized samples are strong solid acids, even solid superacids under some conditions, with uniform mesoporous structure and high surface area. The dispersion state of the supported WO₃, which depends on the WO₃ loading and the calcination temperature, has a direct influence on the acidity and catalytic activity of the materials. Moreover, the high acid strength is attributed to the WO bond nature of the complex formed by the interaction between WO₃ and the surface of Zr-SBA-15.     

Electronic structure and one-electron properties of the MoO2Cl2 molecule. Electronic spectra of the MoO2Cl2, MoO2Br2 and WO2Br2 molecules

The SCF-X -SW method in an overlapping atomic spheres approximation has been used to calculate the electronic structure, ionization potentials, energies and oscillator strengths of the allowed optical transitions and also some of the one-electron properties of the MoO₂Cl₂ molecule. The electronic absorption spectra of vapours over molybdenum and tungsten dioxodibromides have been measured. Interpretation of the experimental electronic absorption spectra of the MoO₂Cl₂, MoO₂Br₂ and WO₂Br₂ molecules is discussed.     

Ba0.15WO3: A bronze with an original pentagonal tunnel structure

The structure of a new barium tungsten bronze, Ba_0.15WO₃, has been established by X-ray diffraction and high-resolution microscopy studies. This bronze is orthorhombic, space group Pbm2 or Pbmm, with a = 8.859(3) Å, b = 10.039(8) Å, and c = 3.808(2)Å. The “WO₃” framework is built up from corner-sharing WO₆ octahedra forming pentagonal tunnels where the barium ions are located. Structural relationships with hexagonal tungsten bronze and tetragonal tungsten bronze structures are discussed.     

Electronic structure of ZrCuSiAs and ZrCuSiP by X-ray photoelectron and absorption spectroscopy

The electronic structures of quaternary pnictides ZrCuSiPn (Pn=P, As) were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES). Shifts in the core-line XPS and the XANES spectra indicate that the Zr and Cu atoms are cationic, whereas the Si and Pn atoms are anionic, consistent with expectations from simple bonding models. The Cu 2p XPS and Cu L-edge XANES spectra support the presence of Cu¹⁺. The small magnitudes of the energy shifts in the XPS spectra suggest significant covalent character in the Zr-Si, Zr-Pn, and Cu-Pn bonds. On progressing from ZrCuSiP to ZrCuSiAs, the Si atoms remain largely unaffected, as indicated by the absence of shifts in the Si 2p_3/2 binding energy and the Si L-edge absorption energy, while the charge transfer from metal to Pn atoms becomes less pronounced, as indicated by shifts in the Cu K-edge and Zr K, L-edge absorption energies. The transition from two-dimensional character in LaNiAsO to three-dimensional character in ZrCuSiAs proceeds through the development of Si-Si bonds within the [ZrSi] layer and Zr-As bonds between the [ZrSi] and [CuAs] layers.     

Synchrotron X-ray absorption of LaCoO3 perovskite

LaCoO₃ perovskite was prepared at 700°C using citrate precursors. The product was then characterized with X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). The powder XRD pattern indicates rhombohedral or its monoclinic I2/a subgroup symmetry. The electronic configuration and the short-range atomic structure of the LaCoO₃ perovskite at room temperature were investigated using synchrotron near-edge X-ray absorption spectroscopy (XANES) and extended X-ray absorption spectroscopy (EXAFS). From the XANES region of the XAS we conclude that Co(III) is at least partly in its intermediate- or high-spin state, which is in accordance with most of the published literature on LaCoO₃ perovskite. The EXAFS region of the LaCoO₃ perovskite spectrum, which up to now was almost not investigated, was simulated satisfactorily for the first two radial structure peaks in terms of the dominant scattering contributions generated with the FEFF8 code and the structural information available from crystallographic data. The best simulation results were obtained with I2/a symmetry. The obtained amplitude reduction factor, zero-energy shift and Debye-Waller factors are useful reference values for data analyses of similar compounds like partly substituted LaCoO₃ perovskite, such as La_1−xCa_xCoO₃ or La_1−xSr_xCoO₃, which are materials of technical interest in catalyst and other applications.     

Crystal structure studies of tetragonal sodium tungsten bronzes,NaxWO3. I. Na0.33WO3 and Na0.48WO3

The crystal structures of two sodium tungsten bronzes, Na_0.33WO₃ and Na_0.48WO₃, have been determined by three-dimensional single-crystal X-ray analysis. They were found to crystallize in the tetragonal space groups $P\overline{4}{2}_{1}m(a=12.097,c=3.754\AA ,Z=10)$ and P4/mbm (a = 12.150,c = 3.769Å, Z = 10), respectively. The structures were solved by standard Patterson and Fourier techniques and refined by full-matrix least-squares to final conventional discrepancy indices of 8.9% for Na_0.33WO₃ and 8.4% for Na_0.48WO₃. In general, the oxygen atoms were found to be either twofold or fourfold disordered, suggesting that the WO₆ octahedra do not have axes exactly aligned parallel to the crystallographicc-axis. The structure found can be viewed as a composite of two kinds of domain structures. These domain structures would require a doubling of thec-axis along with selection of newa- andb-axes along the [1 1 0] and [$[\overline{1}10]$] directions. There exist pentagonal and tetragonal sites in both these sodium tungsten bronzes for sodium atoms occupancy. In Na _xWO₃, x = 0.48, all the pentagonal sites are filled and 40% of the smaller tetragonal sites are also occupied. As x decreases to 0.33 though, only the pentagonal sites are occupied. A relation between the x value and the Na _xWO₃ crystal structures is postulated, extrapolating from the results found in these structure determinations.