Molybdenum Salts Research Articles

Large-Diameter Single-Wall Carbon Nanotubes Formed Alongside Small-Diameter Double-Walled Carbon Nanotubes

Samples containing a majority of either single-wall carbon nanotubes (SWCNTs) or double-walled carbon nanotubes (DWCNTs) are prepared in the same catalytic chemical vapor deposition conditions but using slightly different catalytic materials, based on alumina impregnated with iron and molybdenum salts. There is a sharp SWCNTs-to-DWCNTs transition. By contrast to the usual findings, the selectivity is not correlated to the size of the iron-based catalyst nanoparticles, nor does the transition occur upon a decreasing carbon/catalyst ratio. The result is attributed to the increasing MoO3 concentration inducing modifications of the gas atmosphere, such as the formation of more reactive C2 species through C2H4 dissociation, which thus favors the nucleation and growth of a DWCNT. In the DWCNT sample, the average diameter of the SWCNTs is higher than the average outer diameter of the DWCNTs, which is uncommon, as many authors stress that SWCNTs show a lower diameter than DWCNTs. The study could provide guidelines for the synthesis of very small diameter DWCNTs.

Crystalline and solution chemistry of tetrameric and dimeric molybdenum(VI) citrato complexes

Reaction of citrate with molybdenum(VI) salts under controlled conditions of reactant ratios and pH values in aqueous solutions affords a family of molybdenum citrates. It includes tetranuclear complexes K2(NH4)2[(MoO2)4O3(Hcit)2]·5H2O (1), (NH4)5[(MoO2)4O3(Hcit)(cit)]·3H2O (2) and K2(NH4)4[(MoO2)4O3(cit)2]·7H2O (3), dinuclear complexes K2[(MoO2)2O(H2cit)2]·4H2O (5), (NH4)3[(MoO2)2O(H2cit)(Hcit)] (6), (NH4)14{[(MoO2)2O(Hcit)2][(MoO2)2O(Hcit)(cit)]2}·14H2O (8) and (NH4)6[(MoO2)2O(cit)2]·3.5H2O (9), as well as a mononuclear complex (NH4)4[MoO3(cit)]·2H2O (10) (H4cit=citric acid), where 2, 5, 6, and 8 are new type of citrato complexes. X-ray single-crystal structural analyses reveal that the molybdenum atom in these complexes is hexa-coordinated in an approximately octahedral geometry. Each citrate ligand uses its α-alkoxy, α-carboxy and one β-carboxy groups to act as a tridentate ligand while the other β-carboxy group is left free. Also, structural analyses show that some of the complexes possess strong hydrogen bonds that lead to one and two dimensional polymeric associations. All of these complexes have been investigated by 13C and 1H NMR spectroscopy. The NMR spectra demonstrate that these complexes except 1 and 8 obviously undergo decomposition in solution. Furthermore, four series of solutions with the molar ratio Mo:cit=2:1, 1:1, 1:2, and 1:4 in the pH range 0.1–9.0 were investigated by 13C NMR spectroscopy. The solution NMR studies reveal that the state of equilibriums among different species [4,2]n−, [2,2]n−, [2,1]n−, [1,2]n− and/or [1,1]n− being established under each set of specified conditions.

Electrodeposition and Properties of Nanocrystalline Ni-Based Alloys with Refractory Metal from Citrate Baths / Elektroosadzanie I Własciwosci Nanokrystalicznych Stopów Na Osnowie Niklu Z Trudnotopliwym Metalem Z Kapieli Cytrynianowych

The main aim of the present work was to determine the optimal conditions for electrodeposition of metallic Ni-Mo coatings of enhanced micromechanical properties. These alloys were electrodeposited on the ferritic steel substrate, under galvanostatic regime in a system with a rotating disk electrode (RDE), from an aqueous citrate complex solution containing nickel and molybdenum salts. The effect of the electrolyte solution pH (adjusted by sulphuric acid or ammonia) on the molybdenum content and on deposit quality as well as on the current efficiency of the electrodeposition process, has been studied. It was established that increase of bath pH is correlated with gradual increase of molybdenum content in deposits up to pH 7, where the maximum concentration of Mo(VI) electroactive citrate complex ions [MoO4(Cit)H]4- (Cit= C6H5O7-3 ) in plating bath was observed. In the selected bath of the optimum pH value, the effect of cathodic current density, as a crucial operating parameter which strongly controls the chemical composition and microstructure parameters (e.g. phase compositions, crystallite size), on the mechanical and tribological properties of the resulting coatings has been determined. It has been shown that - under all investigated current density range - crack-free, well adherent Ni-Mo coatings, characterized by microhardness of 6.5-7.8 GPa, were obtained. Alloys deposited at higher tested current densities (above 3.5 A/dm2) were characterized by compact and uniform microstructure, and thus had the highest wear and friction resistance.

ChemInform Abstract: Oxidative Transformation of Aryls Using Molybdenum Pentachloride

AbstractReview: 163 refs.

Microstructure and mechanical properties of nanocrystalline Ni-Mo protective coatings

Nickel-molybdenum alloys are of interest due to their functional properties, such as high hardness, corrosion resistance, as well as low wear and friction coefficients. Thus, they provide an excellent alternative to hard chromium coatings obtained from toxic electrolytes. Characterized coatings were electrodeposited on the steel substrates, from an aqueous citrate complex solution, containing nickel and molybdenum salts, in a system with a rotating disk electrode (RDE). The effect of cathodic current density on microstructure, chemical and phase composition, and crystallite size of the coatings was determined by the scanning and transmission electron microscopy, energy-dispersive X-ray spectrometry, and X-ray diffraction. The influence of microstructure as well as chemical composition on functional properties of coatings were also determined.

SYNTHESIS OF MO-CO ALLOY NANOPOWDER BY CO-PRECIPITATION METHOD FOLLOWED BY H2 REDUCTION

Cobalt-Molybdenum alloy nanopowder has magnetic properties and is used as catalyst for production of carbon nanotubes. Properties of this nanopowder depend on, for example, its size, morphology and internal residual stress. Synthesis of nanopowder of Molybdenum-Cobalt alloy by co-precipitation in an aqueous media using NH 4 OH as precipitating agent followed by calcination and reduction was investigated. The synthesis was started by dissolving salts of Cobalt and Molybdenum in water. A suspension of alumina or silica powders was used as a bed for precipitation. The effect of bed materials on size and morphology of the precipitate was investigated. The particles observed with scanning electron microscope possess a spherical shape and a needle shape for the samples participated on alumina and silica beds, respectively. XRD analysis of the calcined precipitate showed the formation of mixed oxide of CoMoO 4 as well as single oxides of Co 3 O 4 and MoO 3. Particle size of the precipitate observed with transmission electron microscope was about 100 nm. Finally, the powders were reduced by hydrogen gas in a tubular furnace to prepare metallic nanopowder with composition of Co 3 Mo .

Oxidative transformation of aryls using molybdenum pentachloride

Molybdenum pentachloride combines a strong Lewis acid character with an unusually high oxidation potential creating a powerful reagent for oxidative transformations. Since the oxidative coupling reaction of aryls is induced at an extraordinarily high reaction rate, a variety of labile groups, e.g. iodo, tert-alkyl, etc., are tolerated on the aromatic core. Furthermore, the co-formed molybdenum salts can either be exploited for template effects to obtain uncommon geometries in a preferred manner, or redox-play starts after aqueous workup. Therefore MoCl(5) represents a unique and easily available reagent.

Efficient recyclable catalytic system for deoxygenation of sulfoxides: catalysis of ionic liquid-molybdenum complexes in ionic liquid solution

Three newly synthesized room temperature ionic liquids containing molybdate anion, [DMIm][MoO2(NCS)4], [RPy][MoO2(NCS)4] (R = butyl or decyl), and previously reported [BMIm][MoO2(NCS)4] were used as catalysts for the reduction of sulfoxides. For facilitating the recyclability of the catalysts, the ionic liquids [RMIm][PF6] and [RPy][PF6] (RMIm = 1-n-alkyl-3-methylimidazolium, RPy = 1-n-alkylpyridinium) were applied as ionic solvents and toluene is used as solvent for the organic phase. Molybdenum salts immobilized in RTILs constituted a reusable and efficient heterogeneous catalyst system. A maximum conversion (>99%) was achieved for reduction of diphenyl sulfoxide to diphenyl sulfide as a model substrate by using triphenylphosphine as reducing agent at 50 °C after 20 h. This system gave the best results in terms of catalyst/solvent recycling and no loss of catalytic activity was observed after 8 cycles.

Correlation between electron state density change and the electrical resistivity and magnetic permeability changes in the nanostructured powder of the NiMo alloy

The nanostructured powders of the Ni 95.4Mo 4.6 and Ni 99Mo 1 alloys (average crystallite dimensions of 14 and 21 nm) were obtained by the electrochemical deposition from ammonium solutions of nickel and molybdenum salts. The method of differential scanning calorimetry (DSC) and measurement of temperature dependence of the powder's electrical resistivity, magnetic permeability and the thermoelectromotive force were employed to examine structural changes of the powders. The nanocrystalline alloys Ni 95.4Mo 4.6 and Ni 99Mo 1 were stable up to about 460 K. The thermal stabilization of the alloys takes place within the temperature interval of 460–570 K. As a result of this process, a decrease in the electrical resistivity and increases in magnetic permeability as well as electron state density in the proximity of the Fermi level are observed. The crystallization temperature depends upon the current density of powder formation. The nanocrystalline alloy Ni 95.4Mo 4.6 obtained at j=70 mA cm −2 becomes crystallized in the temperature range between 650 and 840 K, while the Ni 99Mo 1 alloy obtained at j=180 mA cm −2 crystallizes in the 580–950 K temperature interval. The electrical resistivity and magnetic permeability of the nanocrystalline alloy decreased while the alloy's electron state density near the Fermi level increased after the process of crystallization took place. The electrical resistivity decrease recorded during the structural changes was due to an increase in the electron state density in the proximity of the Fermi level, as well as to an increase in the mean free path of the conducting electrons.

Highly active NiMo/TiO2–Al2O3 catalysts: Influence of the preparation and the activation conditions on the catalytic activity

NiMo/TiO2–Al2O3 catalysts were prepared over a TiO2–Al2O3 mixed oxide (5% of Al2O3) by impregnation with nickel and molybdenum salts precursors at different pH solution and the catalysts have been activated by several calcination–sulfidation procedures. Evaluation of the catalysts in tetraline hydrogenation under pressure indicated that the sample obtained after impregnation under basic condition and sulfided without any previous calcination presents the highest activity. Characterizations of the samples by Raman and UV–vis diffuse reflectance spectroscopies, XPS and HRTEM allowed to follow the evolution of the Ni and Mo species along the different steps of the preparation and to understand how to get a high promoting effect of Ni over such a TiO2 rich support.

Silicate determination in sea water: Toward a reagentless electrochemical method

Silicate has been determined in sea water by four different electrochemical methods based on the detection of the silicomolybdic complex formed in acidic media by the reaction between silicate and molybdenum salts. The first two methods are based on the addition of molybdate and protons in a seawater sample in an electrochemical cell. Cyclic voltammetry presents two reduction and two oxidation peaks giving four values of the concentration and therefore increasing the precision. Then chronoamperometry is performed on an electrode held at a constant potential. A semi-autonomous method has been developed based on the electrochemical anodic oxidation of molybdenum, the complexation of the oxidation product with silicate and the detection of the complex by cyclic voltammetry. This method is tested and compared with the classical colorimetric one during ANT XXIII/3 cruise across Drake Passage (January–February 2006). The detection limit is 1 μM and the deviation between both methods is less than 3% for concentrations higher than 10 μM. Finally a complete reagentless method with a precision of 2.6% is described based on the simultaneous formation of the molybdenum salt and protons in a divided electrochemical cell. This latter method should be very useful for developing a reagentless sensor suitable for long term in situ deployments on oceanic biogeochemical observatories.

Environmentally Benign Catalytic Hydroformylation−Oxidation Route for Naproxen Synthesis

Hydroformylation of 6-methoxy-2-vinylnaphthalene (MVN), using homogeneous Rh(CO) 2 (acac) as a catalyst and a chelating bidentate ligand (l,2-bis-(diphenylphosphino) ethane, dppe), followed by oxidation of the product (2-(6-methoxynaphthyl) propanal, 2-MNP) has been studied as an alternative route for the synthesis of D,L-naproxen. The feasibility of the MVN hydroformylation route has been demonstrated, and a detailed study has been reported on the key hydroformylation step. The roles of the catalyst, ligands, and solvents, as well as the effect of reaction conditions on the reaction rate and regioselectivity of the product 2-MNP, have been investigated. With Rh(CO) 2 (acac) as a catalyst and dppe as a ligand, >98% selectivity to 2-MNP (an important precursor to D,L-naproxen) has been achieved. A possible mechanism to explain the variation in regioselectivity with Rh(CO) 2 (acac) as a catalyst and dppe as a ligand has been discussed. The kinetics of the hydroformylation step has been investigated and a rate equation has been proposed. The second step in the proposed route for naproxen-the oxidation of 2-MNP to 2-(6-methoxynaphthyl) propanoic acid (2-MNPA, or naproxen)-has been studied using Na 2 WO 4 as a catalyst and tetrabutyl ammonium hydrogen sulfate (TBAHS) as the phase-transfer catalyst with H 2 O 2 as the oxidant for the first time. Screening of the catalysts that consisted of the early transition metals, such as salts of tungsten, vanadium, and molybdenum showed that Na 2 WO 4 gives the best performance for the oxidation step with > 80% selectivity to 2-(6-methoxynaphthyl)-propanoic acid (2-MNPA/naproxen). This study would be valuable in developing a new environmentally benign route for naproxen synthesis.

Influence of the preparation method on the activity of phosphate-containing CoMo/HMS catalysts in deep hydrodesulphurization

Abstract Phosphate-containing hexagonal mesoporous silica (P/HMS) materials were used as supports of hydrotreating CoMo catalysts. Two series of catalysts were prepared by sequential and simultaneous impregnation of P/HMS substrates with cobalt and molybdenum salts solutions. Both bulk and surface structures of calcined and sulphided samples were determined by several techniques (SBET, XRD, UV–vis, TPD-NH3, TPR, FTIR of adsorbed NO and pyridine, HRTEM and XPS). The activity of P-containing CoMo catalysts was examined in hydrodesulphurization (HDS) of dibenzothiophene (DBT) and compared to that of a conventional commercial CoMo/Al2O3 catalyst. It was found that the dispersion of oxide and sulphide Co and Mo species depends on the presence of phosphate and also on the sequence of Co and Mo incorporation being co-impregnation more favourable than sequential impregnation. HRTEM analysis of sulphided samples (673 K) showed that larger stacking degree of MoS2 phase and better dispersion of Co and Mo species are achieved by co-impregnation. Activity tests revealed that sequential incorporation of Mo and Co is less effective for S-removal from DBT than co-impregnation. Contrary to sequential impregnation, the presence of P2O5 (up to 1.5 wt.%) on support surface enhanced S removal from DBT on the catalysts prepared by co-impregnation but the selectivity in this reaction was not influenced by phosphate and catalyst's preparation method. A close parallelism between active phase surface exposure and catalytic response was found.

Structural and textural characterisation of NiMo catalysts supported on various zeolites

Structural and textural characterisation of NiMo catalysts supported on various zeolites Structural and textural properties of nickel-molybdenum catalysts supported on four various type of zeolites of different chemical composition and porous structure have been studied by the methods of IR, XRD and the low-temperature sorption of nitrogen. The catalysts have been obtained by the two-stage impregnation of the supports with the solutions of nickel and molybdenum salts by the method of incipient wetness. The catalysts have been shown to preserve the structure characteristic of the appropriate zeolite supports.

Influence of the Catalyst Type on the Growth of Carbon Nanotubes via Methane Chemical Vapor Deposition

The preparation of the catalyst is one of the key parameters which governs the quality of carbon nanotubes (CNTs) grown by catalyzed chemical vapor deposition (CVD). We investigated the influence of three different procedures of catalyst preparation on the type and diameter of CNTs formed under identical growth conditions via methane CVD. In the first one, chemically synthesized colloidal iron oxide or iron molybdenum alloy nanoparticles were used, which were homogeneously deposited on silicon substrates by spin coating to prevent them from coalescence under CVD growth conditions. The obtained multiwall CNTs (MWNTs) exhibited diameters corresponding to the catalyst particle size, whereas no formation of single-wall CNTs (SWNTs) was observed. In the second method, commercial porous alumina nanoparticles were used in association with iron and molybdenum salts and the Fe/Mo catalyst was formed in situ. We determined that the alumina concentration significantly influenced the morphology of the catalyst and that below a critical value of the range of 1 g/L no CNTs were formed. While yielding nearly defect-free SWNTs, their diameter could not be controlled using this procedure, resulting in a large distribution of tube sizes. In a third, new preparation method, associating alumina and iron-based nanoparticles, SWNTs of a different size and narrower diameter distribution as compared to the second method were obtained. Our results are evidence of the essential role of alumina particles in the formation of SWNTs, and the newly developed method opens up a way to the synthesis of diameter-controlled SWNTs via catalyzed CVD.

Gas Chromatography on Self-Assembled, Single-Walled Carbon Nanotubes

High-performance stationary phases, which provide high resolutions and are stable at high temperatures, are of significant importance in gas chromatographic analysis. Carbon nanotubes are nanosized carbon-based sorbents that have a high surface area and a large aspect ratio and are known to be stable at high temperatures. It is, therefore, conceivable that gas chromatography can benefit from their unique properties. This paper reports gas chromatography separations in an open tubular format on self-assembled, single-walled carbon nanotubes (SWNTs) inside silica-lined steel capillary tubing. A SWNT film with an average thickness of 300 nm was self-assembled by a unique, single-step, catalytic chemical vapor deposition (CVD) process consisting of dissolved cobalt and molybdenum salts in ethanol. A variety of organic compounds with varying polarity were separated at high resolution, and the column efficiency demonstrated approximately 1000 theoretical plates/m. Comparison of capacity factors' and isosteric heats of adsorption with a packed column containing a commercial sorbent (Carbopack C) showed comparable results. This demonstrated high capacity and strong sorbate-sorbent interactions on the SWNT phase. Evaluation of the McReynolds constants suggested that the SWNT was a nonpolar phase.

New Method To Minimize High-Temperature Corrosion Resulting from Alkali Sulfate and Chloride Deposition in Combustion Systems. II. Molybdenum Salts

The use of fuels other than natural gas in gas turbine generators still is fraught with blade corrosion problems that result from the formation of sodium sulfate or potassium sulfate, in the presence of chlorine. The present work illustrates that the addition of molybdenum salts to synthetic fuels (synfuels) modifies this deposition process and benign protective coatings of alkali polymolybdates are produced instead. This study is a follow-up to research published earlier in this journal that showed closely similar behavior with tungsten salt additives. In the case of molybdenum, a new preferential ranking of product formation on the surface is established that is closely related to the thermodynamic stabilities, namely, Na2Mo2O7 > Na2SO4 > Na2MoO4 > NaCl and K2Mo4O13 > K2Mo3O10 > K2Mo2O7 > K2SO4 > K2MoO4 > KCl. This chemistry exists under fuel-lean conditions and is otherwise not sensitive to the fuel, combustion conditions, surface temperature, or material. The required additive trace levels of molybdenum salt are on the order of twice that of the gaseous alkali, on an atomic basis. Consequently, when used as a final pre-gas turbine polishing technique and following other fuel precleaning methods, the approach offers a very practical and inexpensive solution.

The electrodeposition of cobalt–nickel–iron high aspect ratio thick film structures for magnetic MEMS applications

Thick (up to 100 μm) CoNiFe soft magnetic films were electrodeposited for micro-system technology application. The solid composition was characterized and compared to the bath composition and the deposition conditions. The samples obtained were either single or two-phased depending primarily on the Fe concentration; the highest attained BS was of about 20 kG. It was shown that film resistivity of 50 μΩ cm can be reached by adding molybdenum salts to the bath. The films were deposited either on blanket wafers or on pre-defined photoresist patterned wafers. The resultant island structure was used to define small magnetic components.

Simple Preparation of Nitroso Benzenes and Nitro Benzenes by Oxidation of Anilines with H2O2 Catalyzed with Molybdenum Salts.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Catalytic Activity of Metal Ions in Redox Processes in Polymer–Salt Systems during Synthesis of Mixed Oxides

Data are presented on the catalytic activity of metal ions in the oxidative destruction of poly(vinyl alcohol) in polymer–salt (nitrate) systems. The results indicate that the catalytic activity is due to the coexistence of oxidized and reduced forms of metals and that the oxidation potential for the conversion from one oxidation state to another must be high enough for catalysis. To assess catalytic activity, one can also compare the Gibbs energies of redox reactions involving oxides. Metal ions exhibit catalytic activity in solutions (of iron nitrate) and films (of molybdenum, tungsten, and silver salts) and also during the pyrolysis of polymer–salt systems in the synthesis of mixed oxides or in the presence of silver nitrate.