Mixture Of Molybdenum Research Articles

SHS Metallurgy of Binary Silicides (MoW)Si2 for Sintering Composite Materials

The regularities of synthesis by the SHS metallurgy method of cast materials in the Mo-W-Si system are studied. The experiments were carried out in SHS reactors with a volume of 3, 20 and 30 L under the pressure of an inert gas (argon) P = 5 MPa. In the experiments, high-calorie mixtures of molybdenum (6) and tungsten (6) oxides with aluminum and silicon were used. Thermodynamic calculation of combustion parameters of the initial systems according to the Thermo program showed that they have high adiabatic combustion temperatures exceeding the melting points of the initial reagents and final products of synthesis. Studies have shown a strong effect of the ratio of the initial reagents on the regularities of synthesis. Cast (ingots) single-phase disilicides of molybdenum (MoSi2) and tungsten (WSi2), as well as their solid solutions MoSi2–WSi2, are obtained with any given ratio between them. Their microstructure and elemental and phase composition were investigated. Optimal modes of mechanical conversion of the obtained ingots of the target products into the powders of the required fractions are developed. The influence of the composition of composites on the strength and oxidation of sintered samples in air at different temperatures was studied. It was shown that sintered composites from MoSi2–WSi2 solutions have higher strength and are less susceptible to oxidation compared to ceramics obtained by solid-phase sintering from powders of individual MoSi2 and WSi2.

ChemInform Abstract: High‐Precision X‐Ray Diffraction Data, Experimental and Theoretical Study of 2H‐MoS2.

AbstractHigh‐quality single crystals of the title compound are prepared from a mixture of molybdenum and a twofold excess of sulfur (quartz tube, 1250 °C, 96 h) and used to determine the crystal structure of the compound by precise three‐dimensional single crystal XRD and quantum chemical DFT calculations.

Thermal decomposition of bulk K-CoMoSx mixed alcohol catalyst precursors and effects on catalyst morphology and performance

Cobalt molybdenum sulfide-type mixed alcohol catalysts were synthesized via calcination of precipitated bulk sulfides and studied with temperature programmed decomposition analysis. Precursors containing aqueous potassium were also considered. Precipitates thermally decomposed in unique events which released ammonia, carbon dioxide, and sulfur. Higher temperature treatments led to more crystalline and less active catalysts in general with ethanol productivity falling from 203 to 97g(kgcat)−1h−1 when the calcination temperature was increased from 375 to 500°C. The addition of potassium to the precursor led to materials with crystalline potassium sulfides and good catalytic performance. In general, less potassium was required to promote alcohol selectivity when added before calcination. At calcination temperatures above 350°C, segregated cobalt sulfides were observed, suggesting that thermally decomposed sulfide precursors may contain a mixture of molybdenum and cobalt sulfides instead of a dispersed CoMoS type of material. When dimethyl disulfide was fed to the precursor during calcination, crystalline cobalt sulfides were not detected, suggesting an important role of free sulfur during decomposition.

ChemInform Abstract: Ring-Closing Alkyne Metathesis with Simple Catalyst Systems: An Access to Molecular Triangles and Rhomboids.

Treatment of the siloxane monomer 2 with a mixture of molybdenum hexacarbonyl and 4-chlorophenol at 140 °C furnished the corresponding cyclotrimer 3 and the cyclotetramer 4.

Direct grafting of alkoxide promoters on vanadium hydrogen phosphate precursors: Improved catalysts for the selective oxidation of n-butane

New vanadyl phosphate (VPO) catalysts for the oxidation of n-butane to maleic anhydride were synthesized by grafting reactive metal alkoxides onto pre-formed vanadyl phosphorus oxide precursor, VO(HPO 4)1/2H 2O, followed by a calcination and activation protocol. A series of precursors grafted with these alkoxides was used to generate promoted catalysts containing molybdenum and mixtures of molybdenum with main group or transition metal promoters. For many catalysts, a large increase (greater than twofold) in oxidation activity for n-butane was observed and the high selectivity to maleic anhydride was not compromised. For these catalysts, the N 2 BET surface areas do not change significantly with respect to unpromoted catalysts. Large reactivity enhancements were also obtained by contacting the alkoxides with vanadyl phosphate catalyst precursors which had been spray dried with polysilicic acid to form attrition resistant microspheres. Promoted VPO catalysts containing molybdenum combined with other promoters such as Sn, Ti and, in catalysts without silica binder, Fe, Mn, and Zn show further performance improvements compared to the single promoter system containing molybdenum. Hence, the relative ordering of molybdenum co-promoters for VPO catalysts without silica binder is Sn > Fe > Zn > Mn ∼ Bi > Co ∼ Ni in order of decreasing activity. For catalysts spray dried with silica binder, the relative ordering of co-promoters is Ti > Sn > Cu > Al ∼ Fe ∼ Zr. The reactive grafting technique of promoters with VPO is well suited for rapid, high throughput screening methods for the preparation of materials in which a vanadyl phosphate precursor “scaffold” is used to generate the new materials. This technique may be extended to generate a variety of new catalysts.

Sensing of sulfur in molten metal using strontium β-alumina

A sulfur sensor based upon strontium β-alumina solid electrolyte was developed and tested in molten iron. The strontium β-alumina electrolyte was fabricated using an injection molding process and optimized for thermal shock resistance and toughness by incorporating up to 20 wt pct partially stabilized zirconia. A mixture of molybdenum and molybdenum sulfide was used as the reference electrode, while SrS, allowed to form in situ during the sensing of sulfur, formed the auxiliary electrode. The sensor behavior in terms of response time, lifetime to failure, and possible reaction mechanisms is discussed in this article. The results were in very good agreement with combustion analysis by conventional LECO.

Is molybdenum necessary for the growth of single-wall carbon nanotubes from CO?

Catalytic mixtures of molybdenum and cobalt have been considered essential for the growth of single-wall carbon nanotubes (SWCNT) from carbon monoxide by the chemical vapor deposition method. Here we demonstrated extensive growth of SWCNT with only cobalt as catalyst. The typically subsequent stage of annealing the catalyst in the presence of hydrogen was also eliminated. This process resulted in a near-100% yield of essentially defect-free chiral tubes. Use of electrochemical deposition of the catalyst seed particles may open the door for the production of massive electronic interconnect assemblies of these nanotubes.

In situ Raman spectroscopy for the characterization of MoVW mixed oxide catalysts

AbstractIn situ Raman spectroscopy was used to characterize the synthesis and activation steps of mixed metal oxide catalysts starting from the mixed solutions, through subsequent drying and activation procedures, to the catalytic propene partial oxidation reaction. Comparison with Raman spectra recorded for defined, well‐crystallized reference oxides allowed the assignment of the spectra obtained during the catalyst preparation to certain oxides, such as Mo5O14. Especially the latter oxide phase is relevant for selective partial oxidation catalysis. The selectivity for partial oxidation products could be improved between two‐ and three‐fold when the amount of Mo5O14‐type oxide increased. This phase forms from mixtures of molybdenum and tungsten and from mixtures of molybdenum, tungsten and vanadium, but not from binary mixtures of molybdenum and vanadium with high vanadium concentrations. Tungsten and vanadium play an important role as structural promoters in the formation and stabilization of this oxide and for high catalytic activity. A resonance Raman effect was proved for reduced molybdenum oxides due to resonant coupling of the exciting laser to the IVCT transitions between fivefold‐coordinated Mo5+ and sixfold‐coordinated Mo6+. This resonance enhancement renders possible the in situ characterization of operating, reduced molybdenum oxide catalysts. In contrast to the Mo5O14‐type phase, MoO3−x exhibits a high selectivity to total oxidation, and it even becomes re‐oxidized during propene partial oxidation. These different catalytic properties of MoO3−x and the Mo5O14‐type oxide led to the development of a structure–activity relationship which explains the behavior of industrial catalysts. A model is proposed on the basis of a bond order–Raman wavenumber relationship, which explains the different selectivities of these two oxides in terms of metal–oxygen bond strengths, i.e. oxygen basicity and oxygen lability, respectively. Copyright © 2002 John Wiley & Sons, Ltd.

The Role of Active Oxygen Transfer through the Gas Phase in Propylene Oxidation over a Bismuth–Molybdenum Catalyst

The transfer of atomic oxygen from the surface of bismuth oxide onto molybdenum oxide through the gas phase, including the case when ozone was fed to the catalyst bed, is experimentally studied. It is found that the transfer of atomic oxygen through the gas phase only leads to the formation of the products of complete oxidation of propylene under conditions of heterogeneous propylene oxidation on the mixture of molybdenum and bismuth oxides. No new sites are formed on the surface of molybdenum oxide.

Combustion-Wave Propagation and Phase-Formation Mechanism in the “Molybdenum–Carbon-Active Additive” System

The combustion of a mixture of molybdenum and carbon activated by a high-energy magnesium–fluoroplastic mixture was studied. The adiabatic combustion temperature and the equilibrium compositions of the combustion products were calculated and, experimental dependences of the combustion temperature and rate on the main parameters of the process are obtained. It is shown that the activation of the formation of molybdenum carbides is of a thermal nature. The two observed mechanisms of formation of molybdenum carbides are found to differ in combustion temperature: at T 2470 K it proceeds by crystallization from a melt. The possibility of synthesis of single-phase Mo2C in the combustion regime is shown.

Ring-closing alkyne metathesis with simple catalyst systems: an access to molecular triangles and rhomboids

Treatment of the siloxane monomer 2 with a mixture of molybdenum hexacarbonyl and 4-chlorophenol at 140 °C furnished the corresponding cyclotrimer 3 and the cyclotetramer 4.

A characterization of permolybdate and its effect on cellular tyrosine phosphorylation, gap junctional intercellular communication and phosphorylation status of the gap junction protein, connexin43

Biological and analytical characterizations of permolybdate (a mixture of H 2O 2 and molybdate) were done. Molybdate (10 mM) and molybdenum(V) chloride (3 mM) did not affect gap junctional intercellular communication (GJIC), phosphorylation status of connexin43 (Cx43) or cellular tyrosine phosphorylation in early passage hamster embryonic cells (mainly fibroblast-like). High concentrations of H 2O 2 (3–10 mM) affected some of the parameters. Acidified permolybdate was clearly more stable than the unadjusted permolybdate. The maximum biological potency of acidified permolybdate was found at a molar ratio of 2:1 (H 2O 2:molybdate). The mixtures of molybdenum(V) chloride and H 2O 2 gave a maximum effect at 4:1 molar ratio (H 2O 2:molybdenum(V)). This can be explained by decomposition of H 2O 2 and by the generation of less biologically active compounds. Spectrophotometric analyses of the mixtures corroborated the biological results. The Mo(V) electron spin resonance spectrum disappeared upon addition of H 2O 2 to Mo(V) solutions, and no spectrum appeared when H 2O 2 was mixed with Mo(VI). Thus, permolybdate is probably diperoxomolybdate, a Mo(VI) compound. Regardless of the parent metal salt, the H 2O 2/metal salt mixtures showed concentration-dependent biphasic responses with an initial decrease in GJIC followed by an increase. A dissociation between alteration in Cx43 phosphorylation status and GJIC was obtained under certain conditions. The biological activities of permolybdate were only partially mimicked by phenylarsine oxide, an alternative protein tyrosine phosphatase inhibitor.

Reactive sintering of molybdenum alumino-silicide and its oxidation resistance.

An intermetallic compound, molybdenum alumino-silicide, MoAlxSi2−x, is produced from a mixture of molybdenum, aluminum and silicon powders by using a pseudo-hot-isostatic-pressing (PHIP) process and a reactive sintering method. The mechanical properties at room temperature and oxidation behavior at 773 K and 1500 K are investigated. The increase in the PHIP pressure leads to a remarkable increase in density and hardness of the alumino-silicide. The increase in aluminum content of the alumino-silicide leads to a slight increase in density, but it does not affect the hardness. The increase in aluminum content of the alumino-silicide dramatically improves its oxidation resistance.

Modeling and the adaptive solution of reactive vapor infiltration problems

We develop a mathematical model of a reactive vapor infiltration (RVI) process for manufacturing the matrices of fiber-reinforced ceramic composites. The model considers the diffusion of silicon at elevated temperature into a compressed powder pellet of either molybdenum or a mixture of molybdenum and molybdenum disilicide and its reaction to form the desired molybdenum disilicide matrix. Volume expansion and material distortion that may accompany the siliciding reactions are modeled by considering the materials capable of viscous deformation, for simplicity, and coupling this mechanical model with the reaction-diffusion system. The partial differential system comprising the model is solved by adaptive finite-element software having capabilities for automatic quadtree-structured mesh generation, mesh refinement/coarsening, method order variation, and mesh motion. Temporal integration is controlled within a method-of-lines framework by backward difference software. Meshes can be moved to track material distortion or to reduce discretization errors. Computational solutions of one- and two-dimensional problems indicate that the adaptive software is a robust and effective tool for addressing composite-processing problems.

Laser surface alloying of nickel by molybdenum and aluminium— microstructural studies

It is now well established that a considerable improvement in the mechanical and chemical properties of the near surface regions of materials may be achieved by the use of high energy laser beams. By manipulating the laser power density and the time of interaction of the laser beam with an appropriately coated work piece, it is possible to achieve a surface chemistry that would have an improved resistance to wear, fatigue and corrosion failures. The change in chemistry at the surface is attained through the process of melting and mixing of the coating and a thin layer of the substrate. Solidification of this molten region at the surface results, due to an interplay of various forces, in the development of very complex microstructures. To analyse these in a piece of nickel that had been coated with a mixture of molybdenum and aluminium powders and then treated with a continuous wave CO 2 laser, extensive transmission electron microscopy was done on a thin foil obtained from near the bottom region of the recast pool. The foil was found to have a cellular microstructure comprising cells of the Ni 3Al (γ′) ordered phase. In the intercellular regions, two dispersed phases, namely, martensitic Ll 0 phase and Ni 2Al 3, and a contiguous phase, identified as a close derivative of the equilibrium δ-NiMo phase were found. A gradient in the degree of order could be observed within the γ′ cells. In the central portion of the cells, rapid solidification resulted, by the process of sequential ordering, in the development of a partially ordered alloy with very fine domains. In the peripheral regions of the cells, the alloy solidified by direct ordering into larger domains. This paper discusses some of these results.

Change of state for oxygen contaminant and reaction of components during vacuum annealing of a mixture of molybdenum and boron powders

Change of state for oxygen contaminant and reaction of components during vacuum annealing of a mixture of molybdenum and boron powders

Synthesis of molybdenum(IV) sulphide: I. Reaction of hydrogen sulphide with molybdenum(vi) oxide in the presence of ammonium chloride

AbstractMolybdenum(VI) oxide reacted with hydrogen sulphide to give, either a mixture of molybdenum(IV) sulphide and molybdenum(IV) oxide, which is very difficult to separate, or, at higher temperatures molybdenum(IV) oxide only. In the presence of ammonium chloride, the reaction gave pure lubricant grade molybdenum(IV) sulphide. The reaction was studied with different process variables. Under optimum can be conditions 81.5% pure MoS2 was obtained at 700°C. The overall reaction represented as: The densities of the products were measured. X‐ray diffraction patterns of pure samples were taken.

A Spectrophotometric Study of Phosphomolybdenum Blue Formed by the Reaction of Phosphate with a Mixture of Molybdenum(V) (Mo2O42+) and Molybdenum(VI) and Application to the Spectrophotometric Determination of Small Amounts of Phosphates

Abstract The spectrophotometric determination of trace amounts of phosphates, based on the color development of phosphomolybdenum blue formed by the reaction of phosphate with a mixture of molybdenum(V) (Mo2O42+) and molybdenum(VI), is described. The rates of formation of phosphomolybdenum blue were measured at 25°C in a perchloric acid medium. The maximum constant color development was obtained in the perchloricacid range from 0.20 to 0.41 M under given conditions by warming the solution for 10 min at 80°C. The method obeys Beer’s law in the phosphate-concentration range from 0.08 to 1.16 μg/ml. The molar absorptivity at 840 nm was calculated to be 2.4×104 1 mol−1 cm−1. The rate is inverse fifth-order with respect to the perchloric-acid concentration in the range from 0.20 to 0.41 M and is first-order with respect to the molybdenum(V) concentration. The formation rates increase with the phosphate and molybdenum(VI) concentrations, although the rate dependence on the concentrations of these reactants is complicated. The ratio of P: Mo(V) for phosphomolybdenum blue was determined to be 1 : 1 using the molar-ratio method.

Densification and alloy formation in the sintering of molybdenum-iron powders

1. The sintering of a mechanical mixture of molybdenum and iron powders (10 and 40% Fe) in the temperature range 900–1000° C is accompanied by growth of specimens induced by the formation of the intermetallic compound Mo2Fe3. A solid solution of Fe in Mo does not appear to form under these conditions. 2. Mo -10% Fe alloy powders produced by the coreduction of higher oxides are characterized by good sinterability thanks to their small particle size and specific phase composition, enabling relative densities of the order of 90–92% to be obtained in specimens sintered at 1200°C. With Mo-40% Fe alloy powders the factor of active shrinkage is counterbalanced by the factor of specimen growth brought about by intense formation of ɛ-phase. 3. In the sintering of molybdenum-iron alloy powders produced by the coreduction of oxides the concentration of Fe in the Mo grows with rise in sintering temperature.

A Very Rapid Two-Stage Solid–Solid Reaction and the Mosaic Structure of Potassium Permanganate

STUDIES of self-sustained reaction by the temperature profile method1 have recently been extended to compressed mixtures of molybdenum and potassium permanganate and have shown that reaction occurs in two stages.