Thermal,IR, Raman characteristics,Raman gain coefficient and bandwidths in quaternary glasses

Tellurite glasses with composition 75TeO₂–5WO₃–15Nb₂O₅–5M_xO_y in mol%, where M_xO_y = (Na₂O, Ag₂O, ZnO, MgO, CuO, NiO, TiO₂, MnO₂) have been prepared by using the conventional melt-quenching method. Thermal characteristic of prepared glasses were investigated by using DTA techniques. It was found that the glass with the composition 751TeO₂–5WO₃–15Nb₂O₅–5TiO₂ had high thermal stability (ΔT = 122 °C at heating rate 15 K/min). Raman gain coefficients and bandwidths of prepared glasses for Raman gain media were evaluated. The glass with composition 75TeO₂–5WO₃–15Nb₂O₅–5Na₂O had the maximum value of Raman gain coefficient (g = 4.43 × 10⁻¹² m/W) and it was 24 times as large as silica glass. The highest value of full width half maximum (FWHM ≈ 185 cm⁻¹) was observed in glass system 75TeO₂–5WO₃–15Nb₂O₅–5NiO. Finally, the structure of the glasses was investigated through deconvolution Raman and IR spectra.     

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.     

Atomic layer deposition of crystalline molybdenum oxide thin films and phase control by post-deposition annealing

Molybdenum forms a range of oxides with different stoichiometries and crystal structures, which lead to different properties and performance in diverse applications. Herein, crystalline molybdenum oxide thin films with controlled phase composition are deposited by atomic layer deposition. The MoO₂(thd)₂ and O₃ as precursors enable well-controlled growth of uniform and conformal films at 200–275 °C. The as-deposited films are rough and, in most cases, consist of a mixture of α- and β-MoO₃ as well as an unidentified suboxide MoO_x (2.75 ≤ x ≤ 2.89) phase. The phase composition can be tuned by changing deposition conditions. The film stoichiometry is close to MoO₃ and the films are relatively pure, the main impurity being hydrogen (2–7 at-%), with ≤1 at-% of carbon and nitrogen. Post-deposition annealing is studied in situ by high-temperature X-ray diffraction in air, O₂, N₂, and forming gas (10% H₂/90% N₂) atmospheres. Phase-pure films of MoO₂ and α-MoO₃ are obtained by annealing at 450 °C in forming gas and O₂, respectively. The ability to tailor the phase composition of MoO_x films deposited by scalable atomic layer deposition method represents an important step towards various applications of molybdenum oxides.     

Electron-deficient adduct site in the ring opening of methylcyclopentane (MCP) on tungsten-oxide-supported Pt,Ir and Pt–Ir catalysts

The activities of Pt/WO₂, Ir/WO₂ and Pt–Ir/WO₂ toward the conversion of methylcyclopentane (MCP) were investigated. The catalysts were prepared using impregnation and co-impregnation methods and were characterized by SEM, XRD, N₂-sorption and TEM investigations. The most active catalyst toward the conversion of MCP, irrespective of the temperature, was Ir/WO₂. The order of the reactivity was Ir/WO₂ > Pt–Ir/WO₂ > Pt/WO₂. Strong metal–support interactions (SMSI) were observed for all the catalysts over the entire investigated temperature range. At 400 °C, the Pt and Pt–Ir showed 100% selectivity toward ring-enlargement reactions associated with the presence of electron-deficient adduct sites on the reducible acidic WO₂ support. Ring opening occurred over all the catalysts in three positions, resulting in the formation of 2-methylpentane (2-MP), 3-methylpentane (3-MP), and n-hexane (n-H). Difficulty in breaking the secondary – tertiary carbon bonds was observed predominantly on the Ir catalyst, which opens the MCP ring via a selective mechanism.     

Structures,thermal expansion properties and phase transitions of ErxFe2−x(MoO4)3 (0.0 ≤ x ≤ 2.0)

Structures, thermal expansion properties and phase transitions of Er_xFe_2−x(MoO₄)₃ (0.0 ≤ x ≤ 2.0) have been investigated by X-ray diffraction and differential thermal analysis. The partial substitution of Er³⁺ for Fe³⁺ induces pronounced decreases in the phase transition temperature from monoclinic to orthorhombic structure. Rietveld analysis of the XRD data shows that both the monoclinic and orthorhombic Fe₂(MoO₄)₃, as well as the orthorhombic Er_xFe_2−x(MoO₄)₃ (x ≤ 0.8) have positive thermal expansion coefficients. However, the linear thermal expansion coefficients of Er_xFe_2−x(MoO₄)₃ (x = 0.6–2.0) decrease with increasing content of Er³⁺ and for x ≥ 1.0, compounds Er_xFe_2−x(MoO₄)₃ show negative thermal expansion properties. Attempts for making zero thermal expansion coefficient materials result in that very low negative thermal expansion coefficient of −0.60 × 10⁻⁶/°C in Er_1.0Fe_1.0(MoO₄)₃ is observed in the temperature range of 180–400 °C, and zero thermal expansion is observed in Er_0.8Fe_1.2(MoO₄)₃ in the temperature range of 350–450 °C. In addition, anisotropic thermal expansions are found for all the orthorhombic Er_xFe_2−x(MoO₄)₃ compounds, with negative thermal expansion coefficients along the a axes.     

Phase relations and crystal structures in the systems (Bi,Ln)2WO6 and (Bi,Ln)2MoO6 (Ln=lanthanide)

Several outstanding aspects of phase behaviour in the systems (Bi,Ln)₂WO₆ and (Bi,Ln)₂MoO₆ (Ln=lanthanide) have been clarified. Detailed crystal structures, from Rietveld refinement of powder neutron diffraction data, are provided for Bi_1.8La_0.2WO₆ (L-Bi₂WO₆ type) and BiLaWO₆, BiNdWO₆, Bi_0.7Yb_1.3WO₆ and Bi_0.7Yb_1.3WO₆ (all H-Bi₂WO₆ type). Phase evolution within the solid solution Bi_2−xLa_xMoO₆ has been re-examined, and a crossover from γ(H)-Bi₂MoO₆ type to γ-R₂MoO₆ type is observed at x∼1.2. A preliminary X-ray Rietveld refinement of the line phase BiNdMoO₆ has confirmed the α-R₂MoO₆ type structure, with a possible partial ordering of Bi/Nd over the three crystallographically distinct R sites.     

Highly Efficient Mesoporous V<Subscript>2</Subscript>O<Subscript>5</Subscript>/WO<Subscript>3</Subscript>–TiO<Subscript>2</Subscript> Catalyst for Selective Catalytic Reduction of NO<Subscript>x</Subscript>: Effect of the Valence of V on the Catalytic Performance

Mesoporous WO₃–TiO₂ support was synthesized by hydrothermal method, mesoporous V₂O₅/WO₃–TiO₂ catalyst was synthesized by impregnation method and used for selective catalytic reduction (SCR) of NO_x with a excellent NO_x conversion at a wider operating temperature ranging from 200 to 460?°C. In the range of 260–440?°C, NO_x conversion reached to 98.6%, and nearly a complete conversion. Even with the existence of 300 ppm SO₂, NO_x conversion was only a little decline. The catalyst was characterized by a series of techniques, such as XRD, BET, XPS, TEM, Raman and H₂-TPR. It was concluded that V₂O₅/WO₃–TiO₂ catalyst was ascribe to antase TiO₂, and also the high crystallinity of anatase TiO₂ could improve the SCR performance. More interested, V₂O₅/WO₃–TiO₂ catalyst exhibited the typical mesoporous structure according to the BET results. In addition, the TEM results indicated that the active components of V and W were well-dispersed on the surface of TiO₂, while the enhancement of dispersion could improve the activity of catalysts. More importantly, the concentration ratio of V⁴⁺/(V⁵⁺?+?V⁴⁺?+?V³⁺) performed the key role in improving the activity of V₂O₅/WO₃–TiO₂ catalyst.     

Syntheses and single-crystal structures of CsTh(MoO4)2Cl and Na4Th(WO4)4

Colorless crystals of CsTh(MoO₄)₂Cl and Na₄Th(WO₄)₄ have been synthesized at 993 K by the solid-state reactions of ThO₂, MoO₃, CsCl, and ThCl₄ with Na₂WO₄. Both compounds have been characterized by the single-crystal X-ray diffraction. The structure of CsTh(MoO₄)₂Cl is orthorhombic, consisting of two adjacent [Th(MoO₄)₂] layers separated by an ionic CsCl sublattice. It can be considered as an insertion compound of Th(MoO₄)₂ and reformulated as Th(MoO₄)₂·CsCl. The Th atom coordinates to seven monodentate MoO₄ tetrahedra and one Cl atom in a highly distorted square antiprism. Na₄Th(WO₄)₄ adopts a scheelite superlattice structure. The three-dimensional framework of Na₄Th(WO₄)₄ is constructed from corner-sharing ThO₈ square antiprisms and WO₄ tetrahedra. The space within the channels is filled by six-coordinate Na ions. Crystal data: CsTh(MoO₄)₂Cl, monoclinic, P2₁/c, Z=4, a=10.170(1) Å, b=10.030(1) Å, c=9.649(1) Å, β=95.671(2)°, V=979.5(2) Å³, R(F)=2.65% for I>2σ(I); Na₄Th(WO₄)₄, tetragonal, I4₁/a, Z=4, a=11.437(1) Å, c=11.833(2) Å, V=1547.7(4) Å³, R(F)=3.02% for I>2σ(I).     

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.     

Crystal structure and electrical conductivity of cubic fluorite-based (YO1.5) x (WO3)0.15(BiO1.5)0.85- x (0?≤ ?x ≤?0.4) solid solutions

(WO₃)_0.15(BiO_1.5)_0.85 exhibits a tetragonal structure derived from the fluorite subcell. The electrical conductivity of (WO₃)_0.15(BiO_1.5)_0.85 is lower than that of Y₂O₃-doped Bi₂O₃. The structure and electrical conductivity of samples formulated as (YO_1.5) _x (WO₃)_0.15(BiO_1.5)_{0.85- x} (x = 0.1, 0.2, 0.3, and 0.4) were investigated. The as-sintered (YO_1.5)_0.1(WO₃)_0.15(BiO_1.5)_0.75 exhibited a single cubic structure that is isostructural with δ-Bi₂O₃. For x = 0.2, 0.3, and 0.4, the as-sintered samples consisted of a cubic fluorite structure and rhombohedral Y₆WO₁₂. After heat treatment at 600 °C for 200 h, the cubic structures are stable for x = 0.1, 0.3, and 0.4. A transformation from cubic to rhombohedral phase after heat treatment at 600 °C for 200 h was observed in the sample originally formulated as (YO_1.5)_0.2(WO₃)_0.15(BiO_1.5)_0.65.     

Facile synthesis of WOx/ZrO2 catalysts using WO3·H2O precipitate as synthetic precursor of active tungsten species

Zirconia-supported tungsten oxide (WO_x/ZrO₂) catalysts were successfully synthesized using a suspension containing amorphous hydrous zirconia precipitates [ZrO_x(OH)_4-2x·yH₂O]_n and tungstate monohydrate (WO₃·H₂O) precipitates. The procedure involved the dissolution of the WO₃·H₂O precipitate during the aging process with the release of oxyanion [WO₄]^2- species, interaction of this species with the surface of the [ZrO_x(OH)_4-2x·yH₂O]_n precipitate and, formation of active WO_x species after thermal treatment. Non-bridging hydroxyl (OH^?) groups present in the [ZrO_x(OH)_4-2x·yH₂O]_n precipitate act as an active agent for the WO₃·H₂O dissolution. N₂ physisorption, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), temperature-programmed reduction using hydrogen (H₂-TPR), temperature-programmed desorption of ammonia (NH₃-TPD), Fourier-transform infrared (FTIR) spectroscopy of adsorbed pyridine, and Raman spectroscopy were used to elucidate the catalyst structure–performance relationship. The catalytic activity was evaluated for the oxidative desulfurization (ODS) of a model fuel containing dibenzothiophene (DBT). For a fixed WO₃·H₂O content, longer aging times improved the catalyst activity, reaching a maximum when WO₃·H₂O was completely dissolved. The increase in surface area and formation of more active Zr-WO_x clusters and polytungstates are observed for the highest active catalysts. A synergetic effect between local Lewis and Brønsted acid sites seems to have contributed to the observed superior activity. The proposed strategy provides an efficient approach to produce active WO_x/ZrO₂ catalysts and may be applicable for designing other heterogeneous catalytic systems.     

Structural,optical and microwave dielectric properties of Ba1−xSrxWO4 ceramics prepared by solid state reaction route

In this paper, Barium Strontium Tungstate (Ba_1−xSr_x)WO₄ crystals with (x = 0; 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0) were prepared by standard wet milling ceramic preparation method. These crystals were structurally characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman) and Fourier transform infrared (FT-IR) spectroscopic techniques. The shape, growth and average crystal size distribution of these crystals were investigated by a scanning electron microscope (SEM). Their optical properties were investigated by ultraviolet visible (UV–vis) absorption and photoluminescence (PL) measurements. XRD patterns, Rietveld refinements data, FT-Raman and FT-IR spectroscopies indicate that all the crystals present a scheelite-type tetragonal structure without deleterious phases. FT-Raman spectra exhibited 6 Raman active modes in range from 100 to 1000 cm⁻¹, while the FT-IR spectra presented 2 infrared active modes in range from 500 to 1000 cm⁻¹. SEM micrographs showed well sintered BaWO₄ grains, while the substitution of Sr induced modifications in the shape and reduction in the grain size. UV–vis absorption measurements evidenced an increase in the values of the optical band gap (from 4.36 to 4.53 eV) with the increase of Sr into BaWO₄ lattice. Dielectric constant, temperature coefficient of resonant frequency (τ_f), quality factors were measured with Hakki–Coleman technique. The value of τ_f found −43.68 ppm/°C for BaWO₄ which increased to −21.40 ppm/°C for the SrWO₄.     

Effect of the rhenium additive to VOx/TiO2 and MoOx/TiO2 catalysts on their properties in oxidative dehydrogenation of propane

VO_x/TiO₂ and MoO_x/TiO₂ catalysts with the addition of Re (Re/V or Mo = 0.5) were synthetized and tested in oxidative dehydrogenation of propane and in reduction by propane. XPS measurements showed depletion of the surface in Re. The Re additive does not affect the total conversion of propane, but increases the selectivity to propene. The effect is more pronounced for the MoO_x/TiO₂ catalyst. The increase in the selectivity to propene is accompanied with the increase in the reducibility of the catalysts.     

NaBO<Subscript>2</Subscript>-Na<Subscript>2</Subscript>CO<Subscript>3</Subscript>-Na<Subscript>2</Subscript>MoO<Subscript>4</Subscript>-Na<Subscript>2</Subscript>WO<Subscript>4</Subscript> quaternary system

This is the first study of the NaBO₂-Na₂CO₃-Na₂MoO₄-Na₂WO₄ quaternary system by differential thermal analysis. Na₂[MoO_4(x)WO_{4(1 − x)}] solid solutions in the quaternary system are found to not decompose.     

Synthesis and thermal stability of rare-earths molybdates and tungstates with fluorite- and scheelite-type structure

Polycrystalline samples of rare-earths molybdates and tungstates, i.e., CdRE₄Mo₃O₁₆ (RE = Eu, Gd, Y, Ho) and Pb_1–3x□_xEu_2xWO₄ (0 < x ≤ 0.1296 and □ denotes cationic vacancies) have been successfully prepared by high-temperature annealing of adequate CdMoO₄/RE₂MoO₆ and PbWO₄/Eu₂(WO₄)₃ mixtures, respectively. According to the X-ray diffraction analysis, the CdRE₄Mo₃O₁₆ compounds crystallize in a cubic, fluorite-related-type structure with space group \(Pn\bar{3}n\). In turn, new Pb_1–3x□_xEu_2xWO₄ phases crystallize in the scheelite-type, tetragonal symmetry, space group I4₁/a. Cadmium and rare-earth molybdates decompose in the solid state and the solid products of their decomposition are two RE molybdates, i.e., RE₂MoO₆ and RE₂(MoO₄)₃. Thermal stability of CdRE₄Mo₃O₁₆ decreases with decreasing of RE³⁺ radius. The melting point of each sample of Pb_1–3x□_xEu_2xWO₄ solid solution is lower than melting point of pure matrix, i.e., PbWO₄ (1116 °C), and it decreases with increasing in Eu content. Both CdRE₄Mo₃O₁₆ as well as Pb_1–3x□_xEu_2xWO₄ samples are insulators, and their optical band gap (E _g) is bigger than 3 eV.     

Co-precipitation synthesis,humidity sensing and photoluminescence properties of nanocrystalline Co2+ substituted zinc(II)molybdate (Zn1–xCoxMoO4; x = 0, 0.3, 0.5, 0.7, 1)

Zinc-cobalt molybdate composites (Zn_1–xCo_xMoO₄; x = 0, 0.3, 0.5, 0.7, 1) were synthesised by a simple co-precipitation method and characterised by thermogravimetric/differential thermal analysis (TG/DTA), Fourier transform-infrared (FT-IR), Fourier transform Raman (FT-Raman) spectroscopy, X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM/EDAX) and transmission electron microscopy (TEM). The surface area was calculated by BET analysis in the adsorption/desorption isotherm. The humidity sensing properties of zinc-cobalt molybdates were tested by dc electrical measurements at different relative humidity environments (RH = 5–98%). The electrical resistance of the composites linearly decreases and the maximum sensitivity of 3672 ± 110 was observed for the Zn_0.3Co_0.7MoO₄ (ZnCM-4) composite towards humidity, which is calculated by the relation S_f = R_5%/R_98%, where the response time is 200 s and the recovery time is 100 s. Photoluminescence (PL) measurement at the room temperature of ZnM-1 composite exhibited a blue emission peak at 475 nm (λ_em) when excited at a wavelength (λ_ex) of 430 nm. During Co²⁺ substitution in Zn²⁺ matrix, a green and red emission peak was observed when excited at a wavelength (λ_ex) of 520 nm.     

Functionalized SBA-15 and its Catalytic Applications in Selective Organic Transformations

Ordered, mesoporous SBA-15 functionalized with organic and inorganic moieties exhibits efficient catalytic activity in a variety of organic transformations. In this account, reviewing our own work, three-sets of surface-modified SBA-15 materials have been investigated. The first-set of materials consists of SBA-15 modified with organo-acidic (propyl thiol and propyl sulfonic acid) and basic (propyl amine and propyl adenine) moieties. The second-set of materials was prepared by grafting Mn complexes to the organo-functionalized SBA-15. The third-set composes of nanocrystalline metal oxides supported on SBA-15. All these catalysts have been characterized by structural and spectroscopic techniques. Catalytic activities of the first-set of solid materials have been investigated in acid/base-catalyzed reactions viz., ring-opening of epoxides with amines (producing β-amino alcohols), esterification, three-component-Mannich reactions and cycloaddition of CO₂ to epoxides. The Mn complexes grafted on organofunctionalized SBA-15 are efficient catalysts for the chemo-, regio- and stereoselective aerial oxidation of monoterpenes at ambient conditions. TiO_x, VO_x, MoO_x and WO_x supported on SBA-15 catalyzed biomimetic oxyhalogenation of aromatic compounds. In all these reactions, the functionalized SBA-15 showed high selectivity.     

Mechanism by which Tungsten Oxide Promotes the Activity of Supported V2O5/TiO2 Catalysts for NOX Abatement: Structural Effects Revealed by 51V MAS NMR Spectroscopy

The selective catalytic reduction (SCR) of NO_x with NH₃ to N₂ with supported V₂O₅(‐WO₃)/TiO₂ catalysts is an industrial technology used to mitigate toxic emissions. Long‐standing uncertainties in the molecular structures of surface vanadia are clarified, whereby progressive addition of vanadia to TiO₂ forms oligomeric vanadia structures and reveals a proportional relationship of SCR reaction rate to [surface VO_x concentration]², implying a 2‐site mechanism. Unreactive surface tungsta (WO₃) also promote the formation of oligomeric vanadia (V₂O₅) sites, showing that promoter incorporation enhances the SCR reaction by a structural effect generating adjacent surface sites and not from electronic effects as previously proposed. The findings outline a method to assess structural effects of promoter incorporation on catalysts and reveal both the dual‐site requirement for the SCR reaction and the important structural promotional effect that tungsten oxide offers for the SCR reaction by V₂O₅/TiO₂ catalysts.     

Kinetic study on photochromism of WO3 aqueous sol and its enhancement accompanying spectral changes by the addition of TiO2 aqueous sol

Photochromism of a WO₃ aqueous sol has been investigated in a nitrogen atmosphere under controlled temperature. Effects of ageing of the WO₃ sols, concentrations of WO₃ sols or Cl⁻ ion and temperature on the coloring rate were examined. The coloring rate was the first-order with respect of the WO₃ concentration. The coloring process was accelerated by an addition of TiO₂ aqueous sols. Spectral changes were measured using the mixing sol with various molar ratios (γ) of WO₃ and TiO₂. The absorption spectra changed from those having the single peak at 775 nm to those with two peaks at 640 and 980 nm. Such spectral transformation was ascribed to the structural change of the WO₃ nanoclusters, depending on the γ value and the concentration of Cl⁻ ion.     

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.