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.     

Zum chemischen Transport von Wolfram mit HgBr2 – Experimente und Modellrechnungen

On the Chemical Transport of Tungsten using HgBr₂ – Experiments and Thermochemical Calculations Using HgBr₂ as transport agent tungsten migrates in a temperature gradient from the region of higher temperature to the lower temperature (e.g. 1 000 → 900°C). The transport rates were measured for various transport agent concentrations (0.64 ? C(HgBr₂) ? 11.74 mg/cm³; T? = 950°C) and for various mean transport temperatures (800 ? T? ? 1 040°C). Under these conditions tungsten crystals were observed in the sink region. To observe the influence of tungsten dioxide (contamination of the tungsten powder) on the transport behaviour of tungsten, experiments with W/WO₂ as starting materials were performed. According to model calculations the following endothermic reactions are important for the migration of tungsten: In the presence of H₂O or WO₂ other equilibria play a role, too. Using a special “transport balance” we observed a delay of deposition of tungsten (e.g. T? = 800°C; 15 h delay of deposition) with W and W/WO₂ as starting materials. The heterogeneous and homogeneous equilibria will be discussed and an explanation for the non equilibrium transport behaviour of tungsten will be given.     

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 ternärer Indiummolybdate

Chemical Vapor Transport of Ternary Indium Molybdates An isothermal section of the phase diagram of the system In/Mo/O at 1273 K was established by isothermal equilibration and XRD analyses of quenched samples. The chemical vapor transport of In₂Mo₃O₁₂ was investigated in dependence on mean transport temperature (823 K to 1123 K) and amount of transport agent (Cl₂ or Br₂). The observed transport behaviour is compared with results of thermodynamical calculations and the influence of mean temperature, transport agent and moisture contents is described in detail. Single crystals of the metal rich compound InMo₄O₆ were grown by chemical vapor transport in a temperature gradient 1273 K to 1173 K using H₂O as transport agent. The gaseous compound In₂MoO₄(g) accounts for the chemical vapor transport of molybdenium compounds in the metal rich part of the ternary phase diagram In/Mo/O.     

Beiträge zum thermischen Verhalten von Sulfaten. XVI. Zum chemischen Transport von Ga2(SO4)3 mit Cl2 und mit HCl. Experimente und Modellrechnungen

Contributions on the Thermal Behaviour of Sulphates. XVI. The Chemical Vapour Transport of Ga₂(SO₄)₃ with Cl₂ and HCl. Experimental Results and Calculations Crystals of anhydrous Ga₂(SO₄)₃ can be grown by means of CVT (e. g. 525°C → 475°C) in the less hot region of a closed silica ampoule. We investigated the dependance of the deposition rate on the concentration of the transport agent (Cl₂, HCl) and the transport temperature (475°C ≤ T ≤ 750°C; T₂ > T₁; ΔT = 50°C; T = 0.5(T₁ + T₂)). Experimental results and thermodynamic calculations on the basis of Δ_FH ${}_{\text{298}}^{\text{º}}$ (Ga₂(SO₄)₃) = ?686.5 kcal/mol show a good agreement.     

Chemischer Transport ternärer Cadmiummolybdate

Chemical Vapor Transport of Ternary Cadmium Molybdates The ternary phase diagram Cd/Mo/O at 923 K have been investigated. Single crystals of CdMoO₄ and Cd₂Mo₃O₈ have been obtained via chemical vapor transport using X₂ and NH₄X (X = Cl, Br, I) as transport agent. Deposition rates are very different: up to 10 mg/h for CdMoO₄, maximum 10^–3 mg/h for Cd₂Mo₃O₈. The observed transport behaviour is compared with results of thermodynamical model calculations. The influence of source composition, transport agent and temperature gradient is described in detail.     

Die Komplexe Pyren · 2 SbCl3 und Phenanthren - 2 SbBr3 Phasenverhalten,Präparation und Kristallstrukturen

Complexes Pyrene · 2 SbCl₃ and Phenanthrene · 2 SbBr₃: Phase Behaviour, Preparation, and Crystal Structures The melting diagrams of the systems pyrene-SbCl₃ and phenanthrene-SbBr₃ were studied by DTA. They are quasibinary and display one intermediate phase each of molar ratio 1:2 and melting congruently at 143 and 114°C, respectively. Their structures were determined by X-ray diffraction using single crystals obtained by sublimation. In pyrene. 2 SbCl₃ the two halogenide molecules are arranged on different sides, in phenanthrene - 2 SbBr₃ on the same side of the plane of the aromatic molecule. Sb ?π interactions lead to distances of the Sb atoms from this plane of 315 to 323 pm.     

Wanderung von SiAs im Temperaturgradienten ohne Zusatz eines Transportmittels – Experimente und Modellrechnungen

On the Migration of SiAs without using a Transport Agent – Experiments and Thermochemical Calculations SiAs migrates in a temperature gradient (T = 0.5 · (T₁ + T₂) = 850 to 1000°C) without adding a transport agent, into the cooler part of the silica ampoule. The migration rate depends on the temperature and the partial pressure of elemental arsenic in the silica tube. The migration rates were measured for various arsenic concentrations (0 ≤ n(As) ≤ 4 mmol/20 cm³) and for various mean transport temperatures (850 ≤ T le; 1000°C). In case of increasing the temperature the migration rate rises (e.g. T = 850°C, ?(exp.) = 0.006 mg/h; T = 1000°C, ?(exp.) = 0.044 mg/h). Adding arsenic (e.g. n(As) = 0.11 mmol, ?(exp.) = 0.067 mg/h; n(As) = 4.0 mmol, ?(exp.) = 0.82 mg/h), gives also the result of an increasing migration rate. Augmenting the pressure by adding argon as inert gas has only a small effect to the migration rate of SiAs. To explain the mechanism of the migration by using model calculations, the thermochemical data of the gaseous species SiAs_g and SiAs_{3, g} have to be estimated. According to model calculations an endothermic reaction like the following one is responsible for the migration of SiAs the region of the lower temperature: SiAs_s + 2 As_{n, g} = SiAs_{3, g} (1 ≤ n ≥ 4).     

Die relative Unterkühlung an Quarzglasoberflächen und anderen Substraten beim chemischen Transport fester Stoffe über die Gasphase

The Relative Undercooling on Silica Glass Surfaces and other Substrates during the Chemical Transport of Solids via the Vapor-phase . The relative undercooling on etched surfaces of silica glass at the chemical transport of ZnS in a stream of gaseous iodine at 652–854°C is found to be ΔT = 37–43°. Scrapers on silica glass lead to a substantial smaller undercooling. During the deposition of ZnS (made from “pure” components) in a temperature gradient one finds a remarkable fractionation. In closed (etched) silica glass ampoules the relative undercooling is determined for the systems ZnS/I₂, ZnSe/I₂, ZnS/HCl; ZnS/H₂, CdS/I₂, CdSe/I₂, Al₂S₃/I₂ and Nb₂O₅/NbCl₅ using a special furnace, The region free of nuclei (or crystals) for instance at T₂ ≈ 800°C depending on the system (and T₂) is ΔT ≈ 13–45°. The variation of the substrates showed: for fire polished silica surfaces for ZnS/I₂ is ΔT ≈ 56°; for the different quartz-faces and ZnS/I₂ is ΔT ≈ 13–35°. Generally, on different substrates (broken pieces, fragments) one finds for ZnS/I₂ ΔT ≈ 20–28°. Using another way for the systems MoO₃/Cl₂, MoO₃/Cl₂, Ar, MoO₃/Cl₂, O₂ and MoO₃/HgCl₂ with T₂ a strongly decreasing value of ΔT is found.     

Zum Chemischer Transport ternärer Übergangsmetall‐ und Erdalkaliwolframate MWO4 mit Chlor

On the Chemical Vapor Transport of Ternary Transition Metal‐ and Earth Alkaline Tungstates MWO₄ with Chlorine The chemical vapor transport of transition metal tungstates MWO₄ (M=Mn, Co, Ni, Cu, Zn, Cd) was investigated in dependence on mean transport temperature (923 K to 1223 K) and amount of transport agent Cl₂. All tungstates migrate in a temperature gradient ΔT = 100 K from the region of higher temperature to the lower temperature with migration rates of 0.5 to 8 mg/h depending on experimental conditions. The transport behaviour was determined by continuous measurement of mass change during the transport experiments. The results were compared to thermo chemical calculations and the influence of moisture content discussed in detail. MgWO₄ migrates under the influence of Cl₂ in a temperature gradient 1273 K to 1173 K (migration rate 0.7 mg/h), CaWO₄ and SrWO₄ in a temperature gradient 1423 K to 1323 K (migration rate <0.1 mg/h).     

Beiträge zur Chemie der Oxidhalogenide von Molybdän und Wolfram. IV. Der Transport von MoO2 mit J2 und die Existenz von MoO2J2

The possibility to transport MoO₂ with J₂ in a temperature gradient T₂/T₁ suggests the existence of MoO₂J₂. Starting from the reaction MoO₂ + J₂ ? MoO₂J₂ in the consideration of the function of temperature for the rates of chemical transport, the values ΔH^O_R ? 28.8 (±2) kcal/mole and ΔS^O_R ? 9.0 (±2) cl are deduced. From this the values ΔH^O(MoO₂J₂, g, 298) ? ?99.5 (±3.5) kcal/mole and S^O(MoO₂J₂, g, 298) ? 86 (±3) cl are derived. The comparison of the thermodynamic data for MoO₂X₂ and WO₂X₂ (X = Cl, Br, J) leads to the conclusion, that the existence of MoO₂J₂ in the vapour phase is very probable indeed.     

Untersuchungen zur Kristallisation von Rhodium(III)-Oxoverbindungen unter Beteiligung der Gasphase – Der chemische Transport von Rh2O3 mit Chlor

Investigations on the Crystallization of Rhodium(III) Oxo Compounds – Chemical Vapour Transport of Rh₂O₃ using Chlorine Rh₂O_3,s migrates in chemical transport experiments with chlorine as transport agent from the higher (T₂) to the lower (T₁) temperature of a gradient (Δ_T = 100°) due to endothermal reactions (900°C < T ≤ 1050°C; T = 0,5 · (T₂ + T₁)). Under the conditions of transport experiments RhCl_3,s is observed in most experiments as equilibrium solid besides the sesquioxide. The transport rates for Rh₂O_3,s and the sublimation rates for RhCl_3,s grow with increasing temperature T . The composition of the equilibrium solids, the rates of migration and the sequence of deposition (1. RhCl_3,s, 2. Rh₂O_3,s) is well reproduced by thermodynamic model calculations. As a result of this calculations the transport behaviour of the system Rh₂O_3,s/Cl₂ is determined by the two equilibria The influence of RhCl_2,g and RhCl_4,g on the transport behaviour of Rh₂O_3,s as well as the possible occurence of RhOCl_2,g in the equilibrium gas phase will be discussed. Predictions of the transport behaviour of ternary rhodium(III) oxo compounds will be made.     

Enhanced sulfur‐resistant methanation performance over MoO3–ZrO2 catalyst prepared by solution combustion method

Sulfur‐resistant methanation of syngas was studied over MoO₃–ZrO₂ catalysts at 400°C. The MoO₃–ZrO₂ solid‐solution catalysts were prepared using the solution combustion method by varying MoO₃ content and temperature. The 15MoO₃–ZrO₂ catalyst achieved the highest methanation performance with CO conversion up to 80% at 400°C. The structure of ZrO₂ and dispersed MoO₃ species was characterized using X‐ray diffraction and transmission electron microscopy. The energy‐dispersive spectrum of the 15MoO₃–ZrO₂ catalyst showed that the solution combustion method gave well‐dispersed MoO₃ particles on the surface of ZrO₂. The structure of the catalysts depends on the Mo surface density. It was observed that in the 15MoO₃–ZrO₂ catalyst the Mo surface density of 4.2 Mo atoms nm^?2 approaches the theoretical monolayer capacity of 5 Mo atoms nm^?2. The addition of a small amount of MoO₃ to ZrO₂ led to higher tetragonal content of ZrO₂ along with a reduction of particle size. This leads to an efficient catalyst for the low‐temperature CO methanation process.     

Synthese,Kristallstrukturen und 121Sb-Mößbauer-Spektren von [SbBr3(15-Krone-5)], [SbBr2Me(15-Krone-5)] und [SbBr2Ph(15-Krone-5)]

Synthesis, Crystal Structure, and ¹²¹Sb-Mössbauer Spectra of [SbBr₃(15-Crown-5)], [SbBr₂Me(15-Crown-5)], and [SbBr₂Ph(15-Crown-5)] The compounds [SbBr₃(15-crown-5)] ( 1 ), [SbBr₂Me(15-crown-5)] ( 2 ), [SbBr₂Ph(15-crown-5)] ( 3 ), and [SbCl₂Me(15-crown-5)] ( 4 ) are formed by the reaction of 15-crown-5 with SbBr₃, SbBr₂Me, SbBr₂Ph, and SbCl₂Me, respectively, in toluene solution at ?40°C. The complexes were characterized by IR spectroscopy, ¹²¹Sb-Mössbauer spectroscopy, 1–3 as well as by X-ray structure determinations.

• 1 : Space group P2₁2₁2₁, Z = 4, 1735 observed, independent reflections, R = 0.050, Lattice dimensions at ?65°C: a = 787.03(7); b = 1313.0(2); c = 1619.3(2) pm.
• 2 : Space group Pca2₁, Z = 8, 2730 observed, independent reflections, R = 0.050, Lattice dimensions at ?65°C: a = 1308.2(2); b = 1611.8(2); c = 1640.5(3) pm.
• 3 : Space group P2₁/n, Z = 4,2458 observed, independent reflections, R = 0.040, Lattice dimensions at ?60°C: a = 900.3(3); b = 1390.6(6); c = 1618.5(7) pm, β = 96.32(3)°.

The complexes 1–3 have molecular structures, in which the antimony atoms are surrounded by the five oxygen atoms of the crown ether molecule and by three ligands Br₃, Br₂CH₃, Br₂Ph, respectively.     

Chemischer Transport von Mischkristallen im System CuMoO4/ZnMoO4

Chemical Vapour Transport of Solid Solutions in the CuMoO₄/ZnMoO₄ System Two solid solutions exist in the system CuMoO₄/ZnMoO₄: Cu_1‐xZn_xMoO₄ with x=0 to x=0.15 and x=0.20 bis x=1, respectively. Single crystals of Cu_1‐xZn_xMoO₄ were obtained by chemical vapor transport in the temperature gradient 973K→873K using Cl₂, Br₂ or NH₄Cl as transport agents. No difference of the Cu/Zn ratio between source and sink was observed for the transport agents Cl₂ and NH₄Cl. A slight shift to higher Zn amounts was observed for single crystals of Cu_1‐xZn_xMoO₄ grown using Br₂ as transport agent. The experimental results were compared with results of model calculations.     

Study on the formation process of MoO3/Fe2(MoO4)3 by mechanochemical synthesis and their catalytic performance in methanol to formaldehyde

Mechanochemical method has applied to the green preparation of iron-molybdenum catalyst efficiently, and their catalytic performance was evaluated by the oxidation of methanol to formaldehyde. In order to investigate the formation process of iron-molybdenum catalyst based on mechanochemical method, various characterization techniques have been employed. Results indicate that iron-molybdenum catalyst could not be generated during ball milling process without calcining, and calcination is crucial step to regulate the ratio of MoO₃ and Fe₂(MoO₄)₃. For the formation of MoO₃ and Fe₂(MoO₄)₃ phase, 180 °C could be the key turning temperature point. Fe₂(MoO₄)₃ and MoO₃ phases are concurrently emerged when Mo/Fe atomic ratio exceeds 1.5. The aggregation of Fe₂(MoO₄)₃ is severe with the increasing calcination temperature. Fe₂(MoO₄)₃ is stable below 600 °C, while MoO₃ phase could be subliming with the increasing temperature. The catalytic performance of iron-molybdenum catalyst has closely correlation with the phase compositions, which can be controlled by synthesis temperature and Mo/Fe molar ratio. The iron-molybdenum catalyst with Mo/Fe atomic ratio of 2.6 calcined at 500 °C for 4 h showed the best methanol conversion (100%) and formaldehyde yield (92.27%).

     

Chemischer Transport ternärer Oxide in den Systemen Ca/Mo/O und Sr/Mo/O

Chemical Vapour Transport of Ternary Oxides in the Systems Ca/Mo/O and Sr/Mo/O The chemical vapour transport behaviour of ternary phases in the Ca/Mo/O and Sr/Mo/O systems has been investigated using Cl₂ as transport agent in a temperature gradient 1423 to 1323 K. MMoO₄ (M= Ca, Sr) migrate in the above‐mentioned temperature gradient with rates of 0.1 to 0.2 mg/h. Starting from three phase mixtures crystals of the compounds MMo₅O₈ have been grown (migration rates: M = Ca 0.1 mg/h, M = Sr 0.01 mg/h). The observed transport behaviour is compared with predictions given by thermo dynamical model calculations and the influences of source composition and the moisture contents are described in detail.     

NMR study of water reorientation in molybdic acids: MoO3 · 2H2O and yellow MoO3 · H2O

Proton NMR relaxation times (T₂, T₁, T_1?) are reported for powder samples of MoO₃ · 2H₂O and yellow MoO₃ · H₂O in the temperature range 150–325 K and at 20 and 60 MHz. No translation of hydrogen atoms is detected but the spin-lattice relaxation behavior indicates reorientation of H₂O molecules. The waters coordinated to Mo atoms undergo 180° flips (about their C₂ axes) with similar motional parameters in both compounds. The interlayer waters in MoO₃ · 2H₂O undergo 180° flips with different parameters. An assumed Arrhenius-type temperature dependence of correlation times leads to preexponential factors which are “anomalously” low. The possible involvement of temperature-dependent activation barriers is discussed.     

Kristallstruktur des Molybdändioxiddichlorid-Phosphoroxidtrichlorid-Addukts MoO2Cl2 · POCl3

Crystal Structure of the Molybdenum Dioxide Dichloride — Phosphorus Oxide Trichloride Adduct MoO₂Cl₂ · POCl₃ The crystal structure of MoO₂Cl₂ · POCl₃ was determined by X-ray methods (R = 0.046; 2497 independent reflexions). MoO₂Cl₂ · POCl₃ crystallizes monoclinic in the space group P2₁/c with Z = 8. It forms nearly linear chains in which the Mo atoms are linked together via weakly bent and asymmetric oxo bridges (Mo? O = 172 and 218 pm). The Mo atoms are surrounded in a distorted octahedral coordination by one O and two Cl atoms (Mo? Cl = 230–232 pm) as terminal ligands and by the POCl₃ molecule and the bridging O atoms as well. The POCl₃ molecule (Mo? O = 233 pm) is located in trans position to the terminal oxo ligand (Mo? O = 166 pm).