Zum chemischen Transport von Molybdän mit HgBr2 — Experimente und Modellrechnungen

On the Chemical Transport of Molybdenum using HgBr₂ ? Experiments and Thermochemical Calculations . Mo migrates under the influence of HgBr₂ in a temperature gradient (e.g. 1 000→900°C). Besides elementary Mo we observed in some experiments the occurence of MoBr₂ and MoO₂ (from oxygen containing impurities) respectively. The transport behaviour (deposition sequence; deposition rates of various phases) has been enlightened by continous measurement of the mass change during the transport experiments using a special “transport balance”. Thus obtained deposition rates m(Mo) for molybdenum reached in the temperature region 800 ≤ T ≤ 1 040°C a maximum at T = 980°C independend from the starting material (Mo or Mo/MoO₂ mixtures). For variable densities D of transport agent at a constant temperature (T = 950°C) increasing values for m(Mo) were observed (m(Mo) = 23 mg/h, D_max = 8.61 mg HgBr₂/cm³). Thermochemical calculation give strong evidence for the migration of Mo via the endothermal reaction . The experimental deposition rates are about half as large than the calculated values. Good agreement between calculations and experiments were obtained only assuming the presense of oxygen in the starting materials.     

Untersuchungen an Metallkatalysatoren. XVII. Phasenaufbau,Dispersität und Dehydrieraktivität Molybdän- und wolframmodifizierter Palladiumkatalysatoren

Investigations on Metal Catalysts. XVII. Phase Structure, Dispersity, and Dehydrogenation Activity of Palladium Catalysts Modifided by Molybdenum and Tungsten Molybdenum and tungsten containing palladium catalysts were prepared by reduction of mixtures from Pd(NO₃)₂ with MoO₃ and WO₃, respectively, with hydrogen at 600°C and 800°C. The powders were characterized by means of several methods: Determination of the oxidation state for molybdenum and tungsten, X-ray measurements, N₂ adsorption, CO chemisorption, H₂ sorption, dehydrogenation of cyclohexane. The properties of the samples (heated at 600°C) are determined to a high degree by the co-existence of the palladium phase as well as the molybdenum and tungsten oxide, respectively, in the mean oxidation state +4. The after-reduction at 800°C leads to a great portion of metallic molybdenum and tungsten in the concerned catalysts. There are references that the treatment at 800°C in the presence of hydrogen causes for the Pd? Mo catalysts an increase of the palladium content in the surface of the crystallites.     

Experimente und Modellrechnungen zum chemischen Transport von WO2 mit HgCl2 oder HgBr2

Experiments and Calculations on the Chemical Transport of WO₂ with HgCl₂ or HgBr₂ Transport experiments with WO₂ or WO₂ + W₁₈O₄₉ or W + WO₂ as starting phases show that HgCl₂ or HgBr₂ are suitable transport agents. When using HgBr₂ we observed (in customary silica ampoules) unusual high transport rates n′ > 1000 mg/h. Experimental and calculated results agree to a large extent if the presence of small amounts of H₂O from the quartz glass wall and the resulting gaseous particles (for example HCl or HBr) formed under equilibrium conditions as well as an influence of convection are taken into consideration.     

Chemischer Transport intermetallischer Phasen. 3 [1]. Der chemische Transport von Mischkristallen im System Mo/W

Chemical Vapor Transport of Intermetallic Systems. 3. Chemical Transport of Mo/W-mixed Crystals Mo/W-mixed crystals can be prepared by means of chemical vapor transport with HgBr₂ (1000°C→900°C). It is known [2] that the transport reaction of tungsten begins hours or even days after starting the experiment. This is the reason for the unusual composition of deposited crystals: EDX-analysis show them to have a Mo-rich nucleus and a W-rich shell.     

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.     

Products of carbothermal reduction of tungsten oxides in argon flow

The investigation into carbothermal reduction of tungsten oxides has shown that this process involves several steps: WO₃ → WO_2.72 → WO₂ → W. The resulting oxide is rapidly reduced to tungsten metal at 950°C. The carbidization process has a diffusion mechanism. The staged character of carbothermal reduction of tungsten oxides and subsequent carbidization of tungsten are confirmed by scanning electron microscopy, which made it possible to determine the phase composition and size of the resulting particles.     

Synthese und Kristallstruktur eines Natrium-tetraoxonitridowolframats(VI), Na5WO4N

Synthesis and Crystal Structure of Sodium Tetraoxo Nitrido Tungstate(VI), Na₅WO₄N Colourless crystals of Na₅WO₄N are obtained besides Na₄WO₂N₂ [1] by the reaction of WO₃ with NaNH₂ (15:1) at 350°C ≥ T ≥ 750°C in autoclaves to prevent early decomposition of sodium amide. X-ray single crystal investigations are characterized by the following data:

• Na₅WO₄N: Cmc2₁ (No. 36), Z = 4
• a = 9.873(2) Å, b = 5.769(1) Å, c = 10.648(2) Å
• Z(F)≥ 3σ(F) = 2182, Z(Var.) = 55, R/R_w = 0.029/0.039

The structure contains the tetragonal pyramidal ion WO₄N^5? with nitrogen at the apex connected via Na⁺ ions irregularly coordinated by one nitrogen and four oxygen atoms of different anions.     

Phases containing tungsten in Cu(1) positions

A tetragonal (123) phase of composition CdBa₂Cu₂WO₈ with the complete and selective substitution of tungsten atoms for copper in the Cu(1) positions is synthesized from CdWO₄ and BaCuO₂ at 800°C in flowing oxygen. The percentage of the (123) phase in the sample is at least 90%; the unit cell parameters are a = b = 0.4151(3) nm, c = 1.2537(8) nm. The X-ray diffraction pattern shows a superstructure with all three of the unit cell parameters being doubled.     

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.     

Darstellung,Struktur und thermisches Verhalten einer neuen hexagonalen Ge-W-Bronze

Preparation, Structure, and Thermal Behaviour of a New Ge? W Bronce A hexagonal Ge-tungsten bronce with the composition Ge_0.24 WO₃ is obtained by chemical vapour deposition of a mixture of GeO₂ and WO₂(temperature gradient: 930 to 830°C). By means of an X-ray crystal structure analysis the space group was determined to be P622 and the lattice constants a = 744.0 pm, c = 381.7 pm. In absence of oxygen Ge_0.24 WO₃ is stable up to 850°C; in contact with air it is oxydized at temperatures T > 670°C. An anomaly in the thermal lattice expansion at temperatures T > 260°C is discussed.     

Zum chemischen Transport von SiAs mit Iod — Experimente und Modellrechnungen

On the Chemical Transport of SiAs using Iodine — Experiments and Thermochemical Calculations Using iodine as transport agent siliconarsenide migrates in a temperature gradient. The direction of the migration depends on the chosen temperature and the concentration of the transport agent. The transport rates were measured for various transport agent concentrations (0.0002 ? C(I₂) ≥ 0,02 mmol/cm³) and for various mean transport temperatures (650 ? T? ? 1 000°C). For low temperatures (e.g. T₁ = 750°C→T₂ = 850°C), low iodine concentrations (e.g. C(I₂) = 0.001 mmol/cm³) and in the presence of H₂O (from wall of silica ampoule) the following exothermic reaction is responsible for the deposition of SiAs-crystals in the sink region:

• SiAs_s + 4HI_g = SiI_4,g + 2H_2,g + 1/4As_4,g

In case of higher temperatures (e.g. T₂ = 1 050°C→T₁ = 950°C) and higher iodine concentrations (e.g. C(I₂) = 0.02 mmol/cm³) SiI_4,g is the transport agent. According to model calculations the following endothermic reaction is responsible for the migration of SiAs to the region of the lower temperature:

• SiAs_s + SiI_4,g = 2SiI_2,g + 1/4As_4,g

The heterogeneous and homogenous equilibria will be discussed and an explanation of the non equilibrium transport behaviour of SiAs is given. Thermochemical data of SiAs are characterized by the quartzmembrane zero manometer technique and further verified by model calculations.     

Chemischer Transport und Sublimationsverhalten von CrBr3 — Experimente und Modellrechnungen

On the Chemical Transport and Sublimation of CrBr₃ — Experiments and Model Calculations The migration of CrBr₃ in the presence of high concentrations of bromine (for example D(Br₂) = 0,05 mmol/ml; closed silica ampoules) in the investigated temperature range (T? = 625°C to 875°C; T? = 50°C) is a result from the endothermic reaction The chemical transport of CrBr₃ is superimposed with the sublimation. With low concentrations of D(Br₂) and high temperatures T? is the sublimation decisive participated. This is a result of the homogenous equilibrium between CrBr_3,g and CrBr_4,g (2a) The reaction (2a) in comparison with the chemical transport of CrCl₃ with Cl₂ (Gl. (2b)) is more shifted to CrBr_3,g.     

Beiträge zum chemischen Transport oxidischer Metallverbindungen. III. Der Transport von NiO und MgO über ihre Metallchloride

Transport effect of HCl on NiO and MgO according to between T₂ = 1000°C and T₁ = 800°C was calculated by the model of diffusion in dependence of total pressure; for comparison, the classical transport of α-Fe₂O₃ was analogously treated. By experimental determination of the transport rates at total pressures from 0.009 to 6 atm hitherto not considered influences of the amount and surface of the starting material, and of the transport time were found. These effects are explained by a (not in detail defined) term of ?sorption”? of the transport gas onto the powder of the starting material. For an explanation of the transport rates estimations of the diffusion coefficients of the gas pairs FeCl₃–HCl and NiCl₂–HCl were performed and the vapour pressure diagrams of NiCl₂ and MgCl₂ evaluated.     

Über nichtstöchiometrische Wolfram-Verbindungen Synthese und Gitterkonstanten von Verbindungen der Mischkristallreihe WO3?NaWO3

On Nonstoichiometric Tungsten Compounds. Synthesis and Lattice Constants of WO₃? NaWO₃ Mixed Crystal Compounds In the range of temperature from 850 to 1300°C sodium tungsten bronzes with the formula Na_xWO₃ were prepared from a stoichiometric mixture of W, WO₃ and Na₂WO₄. The variation of lattice constant with nominal bronze composition was determined.     

Zum chemischen Transport der Wolframoxide WO2 und W18O49 mit HgI2. Experimente und Modellrechnungen

On the Chemical Transport of Tungsten Oxides WO₂ and W₁₈O₄₉ with Hgl₂. Experiments and Calculations Transport experiments with WO₂ or W + WO₂ or WO₂ + W₁₈O₄₉ show that HgI₂ is a transport agent as suitable as I₂. We observed transport rates up to 47 mg/h. We investigated the dependence of the transport rate on the concentration of the transport agent n°(HgI₂) as well as on the temperature. We also investigated the time dependence of the transport rates during transport experiments on a “transport balance”. Starting with WO₂ + W₁₈O₄₉, WO₂ is transported before W₁₈O₄₉. Thermodynamic calculations show that transport of W₁₈O₄₉ is understandable if the presence of small amounts of H₂O from the quartz glass wall are taken into consideration, while transport of WO₂ is possible with HgI₂ in the presence of H₂O as well as in absence of H₂O. is the most important reaction for the transport of WO₂.     

Synergistic effect of additional TiO2 support on metathesis activity of ethylene and 2-butene over supported tungsten-based catalysts for propylene production

Tungsten-based catalysts of different preparations mixed with TiO₂ support were investigated in the metathesis of ethylene and trans-2-butene to propylene. The catalytic activity of silica-supported tungsten oxide catalyst (WO₃/SiO₂) mixed with TiO₂ additional support had higher efficiency than that of mixed SiO₂-TiO₂ supported tungsten oxide (WO₃/SiO₂-TiO₂). The clean area of the TiO₂ additional support, which provides more space for tungsten migration, is an important key to explain the improved catalytic activity, due to the higher fraction of the isolated surface tetrahedral tungsten oxide species and better dispersion of the tungsten oxide species observed by FT Raman spectroscopy. In addition to the synergistic effect of the additional TiO₂ support on the metathesis activity, the similar synergy was also observed for the one–third diluted catalysts with additional SiO₂. It has been found that the synergistic effect exerted by the presence of additional SiO₂ support predominates over the one-third dilution effect of catalyst concentration. Thus, adding an additional support is another simple way to improve the catalytic activity of the catalysts and makes great benefit for being used in real chemical industry.     

Sättigungsdrucke,Bildungsenthalpien von WOBr4 und WO2Br2, die Reaktionsgleichgewichte 2WO2Br2 = WO3 + WOBr4 und 2WOBr4 + SiO2 = 2WO2Br2 + SiBr4

The saturation vapour pressures of WOBr₄ and WO₂Br₂ and their reaction equilibria have been determined by means of a membrane zero manometer and ampoule quenching experiments, respectively. From the pressuretemperature dependence the following sublimation data were estimated: Δ H° (subl., WOBr₄, 298) = 29.4 (± 1.0) kcal/mole; Δ H° (subl., WO₂Br₂, 298) = 36.6 (±1.5) kcal/mole; Δ S° (subl., WOBr₄, 298) = 50.1 (± 1) cl; Δ S° (subl. WO₂Br₂, 298) = 53.0 (±1.5) cl. For the decomposition reaction of solid WO₂Br₂ were obtained: Δ H° (s, 690) 37.5 (± 0.7) kcal/mole, Δ S° (s, 690) = 49.0 (± 0.5) cl; and for the decomposition of gaseous WO₂Br₂: Δ H° (g, 690) = ?29.6 (± 2.0) kcal/mole, Δ S°. (g, 690) = ?44.5 (± 1.5) cl.     

Molecular structure of WO2Br2

Molecular structure of WO₂Br₂ has been studied by electron diffractometry. Structural parameters for the molecule with C_2v symmetry are: r_α(W=O)=1.710(6) Å, r_α(W?Br)=2.398(5) Å, r_α(O?O)=2.815(30) Å, r_α(Br?Br)=4.021(16) Å, r_α(O?Br)=3.347(10) Å. The OWO and BrWBr bond angles are close to tetrahedral:L _αOWO=110.8(2.0)°, L_αBrWBr=113.9(1.0)°. The W=O bond was found to be characteristic in the series of tungsten dioxyhalides.     

Electrical and electrochemical characteristics of La<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>CoO<Subscript>3-δ</Subscript> cathode materials synthesized by a modified citrate-EDTA sol-gel method assisted with activated carbon for proton-conducting solid oxide fuel cell application

The electrical conductivity and electrochemical performance of a La_0.6Sr_0.4CoO_3-δ (LSC) cathode produced by a modified citrate-EDTA sol-gel method assisted with activated carbon are characterized for a proton-conducting solid oxide fuel cell (H⁺??SOFC) application at intermediate temperature. Thermogravimetric analysis revealed that the decomposition of the unrequired intermediate compounds in the precalcined powder was completed at 800?°C. A single LSC perovskite phase was formed at a calcination temperature of 900?°C, as confirmed by X-ray diffraction analysis. The particle size, crystallite size, and BET-specific surface area of the powder are 219–221?nm, 18?nm, and 9.87?m²?g^?1, respectively. The high index value of the extent of agglomeration (5.53) showed that the powder was barely agglomerated. Bulk LSC sintered at 1200?°C for 2?h showed the highest direct-current electrical conductivity (σ_d.c) compared to that of bulk LSC sintered at 1000?°C and 1100?°C. The value of σ_d.c was affected by the density and porosity of the sintered samples. The area specific resistance (ASR) of screen-printed LSC working on a proton conductor of BaCe_0.54Zr_0.36Y_0.1O_2.95 (BCZY) decreased from 5.0?Ω?cm²–0.06?Ω?cm² as the temperature increased from 500?°C to 800?°C with an activation energy of 1.079?eV. Overall, in this work, the LSC material produced with the aid of activated carbon meet the requirements for the application as a cathode in an intermediate temperature H⁺-SOFC.

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