Showing papers on "Molybdenum published in 1998"

Reactions of transition metal complexes with fullerenes (c60, c70, etc.) and related materials

Abstract: A. Lanthanum, Yttrium, and Scandium 2126 B. Hafnium, Zirconium, and Titanium 2126 C. Tantalum, Niobium, and Vanadium 2127 D. Tungsten, Molybdenum, and Chromium 2127 E. Rhenium, Technetium, and Manganese 2128 F. Osmium, Ruthenium, and Iron 2128 1. Osmylation 2128 2. Reactions with Zerovalent Compounds 2131 3. Other Addition Reactions 2133 4. Redox Reactions 2134 5. Cocrystallizations 2134 G. Iridium, Rhodium, and Cobalt 2135 1. Adduct Formation with Vaska-Type Complexes, Ir(CO)Cl(PR3)2 2135

Semiconducting Properties of Passive Films Formed on Stainless Steels Influence of the Alloying Elements

Abstract: Passive films formed on stainless steels in a borate buffer solution (pH 9.2) have been investigated by capacitance measurements and photoelectrochemistry. The study was carried out on films formed on AISI type 304 and 316 stainless steels and high purity alloys with differing chromium, nickel, and molybdenum contents. Complementary research by Auger analysis shows that the passive films are composed essentially of an inner chromium region in contact with the metallic substrate and an outer iron oxide region developed at the film/electrolyte interface. The semiconducting properties of the passive films are determined by those of the constituent chromium and iron oxides which are of p-type and n-type, respectively. Thus the influence of the alloying elements on the semiconducting properties of the passive films is explained by changes in the electronic structure of each of these two oxide regions.

Formation of a Ring‐Shaped Reduced “Metal Oxide” with the Simple Composition [(MoO3)176(H2O)80H32]

Abstract: The largest inorganic molecular system so far, [(MoO3 )176 (H2 O)80 H32 ] (1; the picture on the right shows the polyhedral representation), which has been characterized by X-ray structure analysis, possesses a cavity of diameter 2.3 nm and remarkably shows the stoichiometry of a (reduced) protonated and hydrated "molecular molybdenum trioxide". It is formed by reduction of an aqueous solution of lithium molybdate with tin(II) chloride at very high H+ concentrations.

Simultaneous hydrodesulfurization, hydrodeoxygenation, and hydrogenation with molybdenum carbide

Abstract: Simultaneous hydrodesulfurization (HDS), hydrodeoxygenation (HDO), and hydrogenation (HYD) of model compounds were studied under high pressure conditions with a β-Mo2C catalyst. The β-Mo2C (hexagonal close packed) was synthesized by the temperature programmed reaction and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), CO chemisorption, and BET surface area measurements. Simultaneous HDS, HDO, and HYD were observed with minimal deactivation of Mo2C for up to 30 ppm of sulfur, 2000 ppm of oxygen, and 5 wt % cumene for more than 80 h. A noble metal catalyst (Pt/γ-Al2O3) deactivated immediately upon addition of sulfur. The reactivity results on β-Mo2C indicate there are two types of sites on the surface when simultaneous HDS, HYD and HDO are occurring. The first type of site is a molybdenum carbide site which is active for HYD and HDO. The second site is a molybdenum carbosulfide site which is active for HYD, HDS, and HDO. The results of this study may be applied not only to the refining of petroleum feedstocks, but also to the upgrading of synthetic fuels such as coal-derived liquids.

Molybdenum Phosphide: A Novel Catalyst for Hydrodenitrogenation

Abstract: A single phase molybdenum phosphide, MoP, with a moderate surface area was prepared using temperature programmed reduction of an amorphous phosphate precursor and the so-prepared sample was found to be a good catalyst for hydrodenitrogenation.

Human sulfite oxidase R160Q: identification of the mutation in a sulfite oxidase-deficient patient and expression and characterization of the mutant enzyme.

Abstract: Sulfite oxidase catalyzes the terminal reaction in the degradation of sulfur amino acids Genetic deficiency of sulfite oxidase results in neurological abnormalities and often leads to death at an early age The mutation in the sulfite oxidase gene responsible for sulfite oxidase deficiency in a 5-year-old girl was identified by sequence analysis of cDNA obtained from fibroblast mRNA to be a guanine to adenine transition at nucleotide 479 resulting in the amino acid substitution of Arg-160 to Gln Recombinant protein containing the R160Q mutation was expressed in Escherichia coli, purified, and characterized The mutant protein contained its full complement of molybdenum and heme, but exhibited 2% of native activity under standard assay conditions Absorption spectroscopy of the isolated molybdenum domains of native sulfite oxidase and of the R160Q mutant showed significant differences in the 480- and 350-nm absorption bands, suggestive of altered geometry at the molybdenum center Kinetic analysis of the R160Q protein showed an increase in Km for sulfite combined with a decrease in kcat resulting in a decrease of nearly 1,000-fold in the apparent second-order rate constant kcat/Km Kinetic parameters for the in vitro generated R160K mutant were found to be intermediate in value between those of the native protein and the R160Q mutant Native sulfite oxidase was rapidly inactivated by phenylglyoxal, yielding a modified protein with kinetic parameters mimicking those of the R160Q mutant It is proposed that Arg-160 attracts the anionic substrate sulfite to the binding site near the molybdenum

Factors affecting molybdenum adsorption by soils and minerals

Abstract: Molybdenum adsorption behavior was investigated on a variety of crystalline and X-ray amorphous aluminum and iron oxide minerals, clay minerals, and arid-zone soils as a function of solution pH, molybdenum concentration, ionic strength, particle concentration, competing anion concentrations, and

Tough-coated hard powders and sintered articles thereof

Abstract: A sintered material and the tough-coated hard powder (TCHP) to make such a material is comprised of core particles that consist essentially of a first metal compound having the formula MaXb. M is a metal selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, aluminum, boron and silicon while X represents one ormore elements selected from the group consisting of nitrogen, carbon, boron and oxygen. The letters a and b represent numbers greater than zero up to and including four. The core particles are surrounded by an intermediate layer consisting essentially of a second metal compound, different in composition from the first metal compound thereby forming coated particles. The material of the intermediate layer has a higher relative fracture toughness than the material comprising the core particles and is capable of bonding with the metal compound(s) forming the core particles and also being capable of bonding with iron, cobalt or nickel. The coated particles are surrounded by an outer layer of iron, cobalt, nickel, their alloys, their mixtures and their intermetallic compounds. The intimate liaison of multiproperty alloys within the TCHP grains allows the combination of normally conflicting sintered article performance characteristics (e.g., strength and hardness) at levels heretofore unseen in the powder metallurgical art.

Partitioning of chromium and molybdenum in super duplex stainless steels with respect to nitrogen and nickel content

Abstract: Experimental data of elemental partitioning of Cr, Mo, and Ni in a 5% Mn containing SAF 2507 super duplex stainless steel (SDSS) with varying nitrogen and nickel mass fractions are presented. Experimental results on phase equilibria and alloying element partitioning are compared to values calculated using Thermo-Calc software revealing good agreement. A trilinear model is proposed to describe the partitioning ratio of Cr, Mo and Ni with nitrogen and nickel mass percent in the austenite and the annealing temperature as variables. While nitrogen is found to reduce the partitioning of chromium and molybdenum, nickel enhances the partitioning ratio of these to alloying elements. Based on this semi-empirical model, a guideline for the future development of improved SDSS is formulated. With respect to corrosion resistance, a higher nitrogen level is not beneficial by itself but needs to be paralleled by increasing molybdenum and decreasing chromium contents.

Hydrotreating catalyst composition and processes therefor and therewith

Abstract: A nitrided calcined modified zeolite composition having incorporated therein a molybdenum promoter and a co-promoter. A carburized nitrided calcined modified zeolite composition having incorporated therein a molybdenum promoter and a co-promoter. Producing the composition by incorporating a molybdenum promoter compound and a co-promoter compound into the zeolite followed by thermal treatment of the resulting zeolite with a nitriding agent and, optionally, further thermally treating the resulting zeolite with a hydrocarbon, preferably in the presence of hydrogen. A hydrotreating process that contacts a hydrocarbon-containing fluid with the nitrided, optionally carburized, calcined modified zeolite composition under a condition sufficient to effect the reduction of sulfur content in the hydrocarbon-containing fluid.

Examination of the role of molybdenum in passivation of stainless steels using AC impedance spectroscopy

Abstract: The role of Mo in the passivation of stainless steels was investigated using ac impedance spectroscopy. Six steels were examined: two ferritic (Fe-20Cr) alloys containing 0 and 1% Mo and four austenitic (Fe-20Cr-25Ni) alloys containing 0, 1, 2, and 4.5% Mo. The experiments were performed in 1 M HCl and, in some cases, 0.5 M H{sub 2}SO{sub 4}, 1 M HCl + 3 M NaCl, and 4 M HCl. Quasi steady-state polarization curves were obtained and the impedance was measured at potentials in the active region and through the active-to-passive transition. The impedance spectra exhibited up to four arcs with well-resolved time constants. Equivalent circuit parameters were evaluated in each case to examine the effect of Mo on the dissolution of the alloys. Mo appears to influence virtually all of the principal reactions involved in the dissolution, intermediate formation, and passivation of stainless steels. Its effect is complex and extends beyond a single reaction step in the mechanism for the dissolution and passivation of these alloys.

Infrared laser photodetachment of transition metal negative ions: studies on , , and

Abstract: Photodetachment threshold spectroscopy on the negative ions of chromium, molybdenum, copper and silver has yielded values for the electron affinities of 5451.0(10), 6027(2), 9967.2(3) and , respectively. The results agree well with previous measurements, with an improvement in the accuracy of up to a factor of 300.

Preparation of MoS2 and WS2 catalysts by in situ decomposition of ammonium thiosalts

Abstract: The in situ decomposition of ammonium thiometallates during the hydrodesulfurization (HDS) of dibenzothiophene (DBT), to obtain molybdenum disulfide and tungsten disulfide catalysts, was investigated. It was found that very efficient catalysts for the HDS of DBT were obtained by in situ decomposition. Mechanical uniaxial pressing of the precursors (ammonium thiometallates) affected both textural and catalytic properties of the catalysts. Surface areas of molybdenum and tungsten disulfides increased as a function of uniaxial pressing, while catalytic activities went through a maximum. For MoS2, the hydrogenation selectivity was much higher for in situ catalysts than for ex situ ones. For WS2 catalysts, the hydrogenation selectivity was less sensitive to the condition of decomposition (ex situ/in situ). The surface S/M (M = Mo, W) atomic ratio from the Auger signal decreased as a function of uniaxial pressing, while the C/M ratio remained almost constant at 1.6. The best catalyst showed an experimental S/Mo ratio that is slightly higher than the stoichiometric value. The effect of in situ decomposition and mechanical deformation of thiometallate precursors is discussed.

Syntheses, Structures, and Reactions of Binary and Tertiary Thiomolybdate Complexes Containing the (O)Mo(Sx) AND (S)Mo(Sx) Functional Groups (x = 1, 2, 4)

Abstract: Publisher Summary This chapter discusses syntheses, structures, and reactions of binary and tertiary thiomolybdate complexes containing the (O)Mo(Sx,) and (S)Mo(Sx,) functional Groups. Molybdenum is one of the most abundant elements on the planet (found as molybdenite—MoS 2 and wulfenite—PbMoO), and in the oceans molybdenum is the most abundant of the redox-active transition elements. The importance of molybdenum sulfur compounds in nonbiological catalysis is exemplified by the hydrodesulfurization (HDS) reaction. The chemistry of molybdenum ions with sulfur ligands is unique when compared to other transition metal ions. At times, the ground states of electron-redistribution isomers of isoelectronic complexes are close enough in energy so that minor perturbations (like crystal packing forces, solvent dielectrics, and ion pairing) are sufficient to stabilize preferentially one isomer over another. A multitude of heterometallic complexes are obtained whereby the thiomolybdate anions serve as ligands. The Mo atoms found at the edges of the MoS 2 crystallites may be bound by any of several oxo, thio, or sulfide ligands and these units may be directly or indirectly involved in the catalytic process.

Surface atomic structures, surface energies, and equilibrium crystal shape of molybdenum

Abstract: Using first-principles calculations, we calculated the atomic structures and surface energies of molybdenum surfaces in the (100), (110), (111), and (211) orientations. The equilibrium crystal shape of molybdenum is then found using the Wulff construction. We find that all four orientations appear on the Wulff plot and hence they are stable.

Zeolite-Supported Metal Oxide Sorbents for Hot-Gas Desulfurization

Abstract: This paper describes the results of the development of supported mixed-metal oxide sorbents for hot-gas desulfurization capable of withstanding multiple sulfidation/regeneration cycles at 871 °C (1600 °F). The sorbents consisted of various combinations of copper, molybdenum, and/or manganese oxides supported on a high silica-containing zeolite. These sorbents were tested in a fixed-bed reactor with simulated coal gas at 205 kPa (15 psig). The combination of all three metal oxides displayed synergism in enhancing efficiency for H2S removal and improved the crush strength on the pellets. Copper oxide was the most active component for reaction with H2S, while molybdenum and manganese oxides appeared to act as catalysts/promoters. During multicycle testing, this ternary metal oxide sorbent retained its reactivity and mechanical strength.

Derivatization of Dinitrogen by Molybdenum in Triamidoamine Complexes

Abstract: The reduction of [N3N]MoCl by an excess of Mg powder in THF under dinitrogen (1 atm) yields the diazenido species {[N3N]Mo−NN}2Mg(THF)2 (1), which reacts with MCl2(PPh3)2 (M = Ni or Pd) or ZnCl2 to...

X-ray Absorption Spectroscopy of the Molybdenum Site of Escherichia coli Formate Dehydrogenase

Abstract: X-ray absorption spectroscopy at the molybdenum and selenium K-edges has been used to probe the active site structure of Escherichia coli formate dehydrogenase H. The active sites of both oxidized and reduced wild-type protein, and of a variant containing cysteine instead of selenocysteine, were studied. The oxidized and reduced enzymes were found to be very similar, both containing a novel des-oxo molybdenum site, with four Mo−S ligands at 2.35 A, (probably) one Mo−O at 2.1 A, and one Mo−Se ligand at 2.62 A being indicated from the Mo K-edge data. The selenium K-edge EXAFS not only is in good agreement with the Mo K-edge data but also indicates the unexpected presence of Se−S ligation, with a bond length of 2.19 A. We suggest that the active site of Escherichia coli formate dehydrogenase H contains a novel seleno-sulfide ligand to molybdenum, where the selenium and sulfur originate from selenocysteine and one of the pterin-cofactor dithiolenes, respectively.