Showing papers on "Molybdenum published in 1993"
TL;DR: The formation of especially stable metal-carbon clusters, which have been designated metallo-carbohedrenes by Castleman and co-workers, has previously been reported for the early transition metals (Ti, Zr, Hf, V) Clusters having the formula M[sub 8]C[sub 12] were recognized as especially stable species which were formed in plasmas containing the metal and a carbon containing precursor.
Abstract: The formation of especially stable metal-carbon clusters, which have been designated metallo-carbohedrenes by Castleman and co-workers, has previously been reported for the early transition metals (Ti, Zr, Hf, V) Clusters having the formula M[sub 8]C[sub 12] were recognized as especially stable species which were formed in plasmas containing the metal and a carbon-containing precursor A similar laser induced plasma technique is employed to synthesize chromium, molybdenum, and iron analogues of these so-called [open quotes]met-cars[close quotes] clusters and to study their photodissociation behavior These observations suggest that met-cars cluster formation is a more general phenomenon than previously recognized While chromium and molybdenum form the [open quotes]super-magic[close quotes] 8/12 stochiometry efficiently, met-cars formation competes with other special stoichiometries in the iron clusters Implications for the bonding properties in met-cars systems are discussed 22 refs, 5 figs
168 citations
TL;DR: The unexpected discovery that several hyperthermophilic organisms possess pterin-containing tungsten enzymes promises to stimulate the development of oxo-thio-tungsten chemistry.
Abstract: Publisher Summary This chapter presents a discussion on the bioinorganic chemistry of pterin-containing molybdenum and tungsten enzymes. The chapter focuses on the current results and the interplay of model and enzyme chemistry. Attention is directed to sulfite oxidase and xanthine oxidase, the archetypal examples of molybdenum enzymes, containing, respectively, dioxo-Mo(VI) and oxo-thio-Mo(VI) oxidized centers. Biochemical and model studies of molybdopterin, Mo-co, and related species are described in this chapter. A brief survey on the physical and spectroscopic techniques employed in the study of the enzymes is presented and their impact on the current understanding of the coordination about the molybdenum atom in sulfite oxidase and xanthine oxidase is discussed. Structural and spectroscopic models are also presented in the chapter. The chapter also explains the xanthine oxidase cycle and facets of intramolecular electron transfer in molybdenum enzymes. The pterin-containing tungsten enzymes and the evolving model chemistry are discussed. Current descriptions of the coordination environment of the molybdenum centers of the enzymes rest primarily upon the comparisons of the spectra of the enzymes with the spectra of well-characterized molybdenum complexes. The unexpected discovery that several hyperthermophilic organisms possess pterin-containing tungsten enzymes promises to stimulate the development of oxo-thio-tungsten chemistry.
155 citations
TL;DR: In this article, a new oxo transfer reaction system of relevance to the molybdenum oxotransferase group of enzymes has been developed and a set of ligands and their sterically hindered Mo VI O 2 complexes have been prepared.
Abstract: A new oxo transfer reaction system of relevance to the molybdenum oxotransferase group of enzymes has been developcd. A set of ligands and their sterically hindered Mo VI O 2 complexes have been prepared.
142 citations
TL;DR: In this article, a series of coprecipitated TiO 2 -Al 2 O 3 supports containing between 0 and 100% TiO2 were prepared and characterized by various methods.
Abstract: A series of coprecipitated TiO 2 -Al 2 O 3 supports containing between 0 and 100% TiO 2 were prepared and characterized by various methods. These solids were impregnated with molybdenum and tested in their sulphide form as catalysts in the hydrodesulphurization (HDS) of thiophene. The results showed that in the prepared mixed oxides supports, an interaction between TiO 2 and Al 2 O 3 exists and that the coprecipitated solid is different, in its surface properties, from a mechanical mixture of the two oxides. Acidity measurements show that Lewis and not Bronsted sites exist in the mixed oxides. The number of acid sites increases sharply at high TiO 2 content. High-resolution electron microscopy shows that a better dispersion of MoS 2 on the supports is obtained when the TiO 2 content is increased. Activity measurements in the thiophene hydrodesulphurization reaction show that the intrinsic activity, expressed on a molybdenum atom basis, increases sharply at high TiO 2 loadings in the mixed oxides supports. Comparison of the activity and acidity measurements point to a correlation between HDS activity and the number of acid sites on the support that affects the dispersion during the impregnation step and the reactivity of the MoS 2 sites.
129 citations
TL;DR: In this paper, the surface characterization of supported molybdenum oxide thin films is undertaken following sulfidation treatments with H[sub 2]S. Surface characterization is carried out by in situ and ex situ laser Raman spectroscopy (LRS) and X-ray photoelectron spectrographs (XPS).
Abstract: Surface characterization of supported molybdenum oxide thin films is undertaken following sulfidation treatments with H[sub 2]S. Surface characterization is carried out by in situ and ex situ laser Raman spectroscopy (LRS) and X-ray photoelectron spectroscopy (XPS). Sulfidation of thin-film molybdenum oxides supported on planar alumina and graphite produces two sulfides species: MoS[sub 2] and a reduced form with a binding energy lower than MoS[sub 2]. The S(2p) spectra also show polysulfide species at higher binding energies to MoS[sub 2]. Sulfidation of thicker, octahedral MoO[sub 3] thin films results only in MoS[sub 2]. Extensive prereduction of octahedral MoO[sub 2] prior to sulfidation is shown not to be important in the conversion of oxide to sulfide phases. 52 refs., 6 figs., 1 tab.
126 citations
122 citations
TL;DR: In this article, an alumina-supported molybdenum nitrided catalyst was used for the selective C-S bond breakage of dibenzothiophene to produce biphenyl.
Abstract: An alumina-supported molybdenum nitrided catalyst was prepared and tested to determine its activity and selectivity during the hydrodesulfurization of dibenzothiophene. The nitrided catalyst was extremely active for the selective C-S bond breakage of dibenzothiophene to produce biphenyl.
121 citations
TL;DR: In this article, the authors used X-ray photoelectron spectroscopy to investigate the behavior of DeNOx catalysts and their individual components (V6Mo4O25, MoO3, V2O5, TiO2).
112 citations
TL;DR: In this paper, the authors used molybdenum-vanadium-niobium oxide catalysts in the temperature range of 350-450 °C for the oxidative dehydrogenation of ethane.
Abstract: The oxidative dehydrogenation of ethane has been studied at atmospheric pressure using molybdenum-vanadium-niobium oxide catalysts in the temperature range of 350–450 °C. The presence of all three oxides together is necessary in order to have active and selective catalysts. The best results have been obtained using a mixture having a Mo ∶ V ∶ Nb ratio of 19 ∶ 5 ∶ 1. Our studies of the variation of oxide composition suggest that the active phase is based on molybdenum and vanadium. Niobium enhances the intrinsic activity of the molybdenum-vanadium combination and improves the selectivity by inhibiting the total oxidation of ethane to carbon dioxide. The apparent activation energies for the conversion of ethane to ethylene, carbon monoxide and carbon dioxide were 18, 27 and 17 kcal/mol, respectively. The addition of water vapor to the gas stream does not affect the product distribution on this catalyst.
95 citations
TL;DR: In this paper, the anodic behavior of the major pure metals of which stainless steels are composed has been studied in chloride-containing aqueous solutions at neutral and acidic pH values, both before and after a nitriding treatment.
94 citations
TL;DR: In this article, the selective oxidation of methane to formaldehyde at atmospheric pressure was studied over a series of silica supported molybdena catalysts, and the performance of the two catalysts was compared for a wide range of experimental conditions, including the influence of the oxidant in the feed (O[sub 2] or N[ sub 2]O).
TL;DR: The local site symmetry of nanodispersed molybdenum oxide phases have been determined for the first time using X-ray absorption near edge spectroscopy (XANES) at the Mo L[sub 2,3]-edges as mentioned in this paper.
Abstract: The local site symmetry of nanodispersed molybdenum oxide phases have been determined for the first time using X-ray absorption near edge spectroscopy (XANES) at the Mo L[sub 2,3]-edges. Local site symmetries varying between pure tetrahedral coordination through mixed symmetries to pure octahedral coordination were determined for species present at concentrations above and below a submonolayer. X-ray absorption near-edge spectra at the Mo L[sub 2,3]-edges were obtained for a series of supported molybdate catalysts and Mo[sup 6+] reference materials with a range of oxygen coordination types. Basic ligand field concepts can be used to interpret the spectra. Tetrahedral molybdates display d-orbital splitting in the 1.8-2.0-eV range, whereas octahedrally coordinated molybdates display split peaks in the 3.0-4.5-eV range. The direct comparison between a complete set of molybdenum reference compounds and dispersed magnesium oxide-supported molybdate catalysts has been used to determine the detailed symmetries of the dispersed molybdenum phases that are present as amorphous, crystalline, and mixed phases. 26 refs., 5 figs., 1 tab.
TL;DR: In this paper, results obtained by treating nickel and molybdenum oxides at a temperature of 523 K, at 10 -4 Pa (in vacuo) and at 10 5 PA (in hydrogen), are reported.
TL;DR: In this paper, the authors measured the spall strength of commercial grade molybdenum and polycrystalline moly bdenum single crystals in a wide range of load durations and intensities (∼5 − 100 GPa).
Abstract: Spall strength measurements for commerical grade molybdenum and molybdenum single crystals were made in a wide range of load durations (∼10−9 s – 10−6 s) and intensities (∼5 – 100 GPa). Resistance to fracture of pure single crystals was found to exceed two times the spall strength of polycrystalline molybdenum and to increase with shorter load duration. The value of the shock wave amplitude does not influence the spall strength of single crystals. The largest spall strength obtained under nanosecond load duration amounts to 30% of the ultimate theoretical strength.
Patent•
26 Jul 1993TL;DR: An activated carbon adsorbent for removing nosioux gases and vapors from a contaminated air stream, and method of making same is described in this paper, where activated carbon has impregnated therein at least one compound selected from each of three groups of compounds.
Abstract: An activated carbon adsorbent for removing nosioux gases and vapors from a contaminated air stream, and method of making same. The activated carbon has impregnated therein at least one compound selected from each of three groups of compounds. The three groups of compounds comprise sulfuric acid and sulfuric acid salts; molybdenum compounds, and copper compounds and zinc compounds. The amount of at least one sulfuric acid or salt thereof compound present is up to 10% by sulfate, by weight, the one molybdenum compounds is present in amounts up to 10% by weight, as molybdenum, and the total amount of copper or zinc compounds present is a total amount not greater than 20% by weight as total copper and zinc. The disclose adsorbents are desirably used in universal filters.
Patent•
22 Nov 1993TL;DR: In this paper, the second light transmit portion of a phase shift mask is formed of a molybdenum silicide nitride oxide (SILSO), or a moxybdenium silicide oxide (MSO), or chromium oxide, or a chromium carbide oxide (CNO) film.
Abstract: A second light transmit portion of a phase shift mask is formed of a molybdenum silicide nitride oxide, or a molybdenum silicide oxide, or a chromium nitride oxide, or a chromium oxide, or a chromium carbide nitride oxide film converting a phase of transmitted exposure light by 180° and having the transmittance of at least 2% and less than 5%. In the manufacturing method of the second light transmit portion, a molybdenum silicide nitride oxide film, or a molybdenum silicide oxide film, or a chromium nitride oxide film, or a chromium oxide film, or a carbide nitride oxide film is formed by a sputtering method. Consequently, with a conventional sputtering apparatus, the second light transmit portion can be formed, and additionally, etching process of the phase shifter portion is required only once, so that probabilities of defects and errors in the manufacturing process can be decreased.
TL;DR: Patients with isolated deficiencies of SO or molybdenum cofactor exhibit mental retardation, neurologic problems, and ocular lens dislocation, which seem to be caused by the toxicity of sulfite and/or inadequate amounts of inorganic sulfate available for the formation of sulfated compounds present in the brain.
Abstract: Molybdenum is found in most foods, with legumes, dairy products, and meats being the richest sources. This metal is considered essential because it is part of a complex called molybdenum cofactor that is required for the three mammalian enzymes xanthine oxidase (XO), aldehyde oxidase (AO), and sulfite oxidase (SO). XO participates in the metabolism of purines, AO catalyzes the conversion of aldehydes to acids, and SO is involved in the metabolism of sulfur-containing amino acids. Molybdenum deficiency is not found in free-living humans, but deficiency is reported in a patient receiving prolonged total parenteral nutrition with clinical signs characterized by tachycardia, headache, mental disturbances, and coma. The biochemical abnormalities in this acquired molybdenum deficiency include very low levels of uric acid in serum and urine (low XO activity) and low inorganic sulfate levels in urine (low SO activity). Inborn errors of isolated deficiencies of XO, SO, and molybdenum cofactor are described. Although XO deficiency is relatively benign, patients with isolated deficiencies of SO or molybdenum cofactor exhibit mental retardation, neurologic problems, and ocular lens dislocation. These abnormalities seem to be caused by the toxicity of sulfite and/or inadequate amounts of inorganic sulfate available for the formation of sulfated compounds present in the brain. XO and AO may also participate in the inactivation of some toxic substances, inasmuch as studies suggest that molybdenum deficiency is a factor in the higher incidence of esophageal cancer in populations consuming food grown in molybdenum-poor soil.
TL;DR: In this article, a newly developed analogue reaction system of the molybdenum oxotransferases, MoO 2 (tBuL-NS) 2 (2)+X⇄ MoO(tBu L-NS 2 (3)+XO (TBTN), exhibits broad substrate reactivity.
Abstract: A newly developed analogue reaction system of the molybdenum oxotransferases, MoO 2 (tBuL-NS) 2 (2)+X⇄ MoO(tBuL-NS) 2 (3)+XO (tBuL-NS= bis(4-tert-butylphenyl)-2-pyridylmethanethiolate(1-)), exhibits broad substrate reactivity. The kinetics of the reactions with X=Et 3 P and XO= Me 2 SO, Ph 2 SO, (CH 2 ) 4 SO, Ph 3 AsO, Ph 2 SeO, and pyridine N-oxide and three substituted variants have been measured in DMF solutions at 298 K
TL;DR: Nitrogenase-3 of Azotobacter vinelandii is synthesized under conditions of molybdenum and vanadium deficiency and can incorporate the Mo-nitrogenase cofactor (FeMoco) to form a functional enzyme.
Abstract: Nitrogenase-3 of Azotobacter vinelandii is synthesized under conditions of molybdenum and vanadium deficiency. The minimal metal requirement for its synthesis, and its metal content, indicated that the only transition metal in nitrogenase-3 was iron [Chisnell, Premakumar and Bishop (1988) J. Bacteriol. 170, 27-33; Pau, Mitchenall and Robson (1989) J. Bacteriol. 171, 124-129]. A new species of nitrogenase-3 has been purified from a strain of A. vinelandii (RP306) lacking structural genes for the Mo- and V-nitrogenases and containing a mutation which enables nitrogenase-3 to be synthesized in the presence of molybdenum. SDS/PAGE showed that component 1 contained a 15 kDa polypeptide which N-terminal amino acid sequence determination showed to be encoded by anfG. This confirms that nitrogenase-3, like V-nitrogenase, comprises three subunits. Preparations of the nitrogenase-3 from strain RP306 contained 24 Fe atoms and 1 Mo atom per molecule. Characterization of the cofactor centre of the enzyme by e.p.r. spectroscopy and an enzymic cofactor assay, together with stimulation of the growth of strain RP306 by Mo, showed that nitrogenase-3 can incorporate the Mo-nitrogenase cofactor (FeMoco) to form a functional enzyme. The specific activities (nmol of product produced/min per mg of protein) determined from activity titration curves were: under N2, NH3 formation 110, with concomitant H2 evolution of 220; under argon, H2 evolution 350; under 10% acetylene (C2H2) in argon, ethylene (C2H4) 58, ethane (C2H6) 26, and concomitant H2 evolution 226. The rate of formation of C2H6 was non-linear, and the C2H6/C2H4 ratio strongly dependent on the ratio of nitrogenase components.
TL;DR: In this article, the metal and metal carbide colloids were derived from the colloids by centrifuging and washing with various solvents, and the results showed that the powders were comprised of agglomerates of the primary 1-5nm-sized crystallite.
Abstract: The group 6 metal chlorides CrCl[sub 3], MoCl[sub 3], MoCl[sub 4], and WCl[sub 4] were reduced in toluene solution with NaBEt[sub 3]H at room temperature to form the corresponding metal colloids in high yield. When the same metal chlorides were reduced in THF solution with LiBEt[sub 3]H and NaBEt[sub 3]H metal carbides (M[sub 2]C) were formed in approximately 95% yield. The metal and metal carbide colloids were shown to be comprised of 1-5-nm-sized particles by TEM. The metal and metal carbide powders derived from the colloids by centrifuging and washing with various solvents were characterized with SEM, TEM, EDS, and ED which showed that the powders were comprised of agglomerates of the primary 1-5-nm-sized crystallite. X-ray powder diffraction studies of these black powders exhibited broad peaks for the s-prepared powders at room temperature where the crystallite size estimated from the broadening analysis generally corresponded to the primary particle size as determined by TEM. Variable-temperature X-ray powder diffraction allowed unambiguous identification of the crystalline metal and metal carbide phases present. Carbon and hydrogen combustion elemental analysis was also used to identify the metal and metal carbide materials. Thermogravimetric analysis in air provided information on the oxidation behavior of these materials andmore » also enabled distinction between the formation of metals versus metal carbides, M[sub 2]C. Experiments to determine the exact origin of the carbide carbon were inconclusive. Treatment of nanocrystalline W with THF and THF/LiBet[sub 3]H solutions did not result in further reaction to form the metal carbide. 38 refs., 10 figs.« less
Patent•
21 May 1993
TL;DR: In this paper, the authors define a multimetal oxide composition of the formula I [X.sup.sub.a X 2 b O.sub O y ] q (I) where X 1 is bismuth, tellurium, antimony, tin and/or copper, X 2 is molybdenum, thallium and/ or samarium, X 3 is an alkali metal, X 4 is an alkaline earth metal, nickel, cobalt, copper, manganese, zinc, tin, cadmium, and
Abstract: Multimetal oxide compositions of the formula I [X.sup.1.sub.a X.sup.2.sub.b O.sub.x ].sub.p [X.sup.3.sub.c X.sup.4.sub.d X 5 e X 6 f X 7 g X 2 h O y ] q (I) where X 1 is bismuth, tellurium, antimony, tin and/or copper, X 2 is molybdenum and/or tungsten, X 3 is an alkali metal, thallium and/or samarium, X 4 is an alkaline earth metal, nickel, cobalt, copper, manganese, zinc, tin, cadmium and/or mercury, X 5 is iron, chromium, cerium and/or vanadium, X 6 is phosphorus, arsenic, boron and/or antimony, X 7 is a rare-earth metal, titanium, zirconium, niobium, tantalum, rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium, indium, silicon, germanium, lead, thorium and/or uranium, a is from 0.01 to 8, b is from 0.1 to 30, c is from 0 to 4, d is from 0 to 20, e is from 0 to 20, f is from 0 to 6, g is from 0 to 15, h is from 8 to 16, x and y are numbers determined by the valency and frequency of the elements in I other than oxygen, and p and q are numbers whose ratio p/q is from 0.1 to 10, containing three-dimensional regions with a chemical composition X 1 a X 2 b O x which are delimited from their local environment due to their chemical composition which is different from their local environment, and whose maximum diameter is from 1 to 25 μm.
TL;DR: A cubane-type compound with a molybdenum-indium-sulfur core is described in this article, where it is regarded as an indium complex with the ligands H[sub 2]O, pts[sup [minus]], and A, where the cluster A functions as a ligand having three ligating sulfur atoms.
Abstract: A molybdenum-indium mixed-metal cluster, [Mo[sub 3]InS[sub 4](pts)[sub 2](H[sub 2]O)[sub 10]][sup 3+] (B), was synthesized from [Mo[sub 3]S[sub 4](H[sub 2]O)[sub 9]][sup 4+] (A) and indium metal. The cluster B may be regarded as an indium complex with the ligands H[sub 2]O, pts[sup [minus]], and A, where the cluster A functions as a ligand having three ligating sulfur atoms. The cluster B described here is the first cubane-type compound with a molybdenum-indium-sulfur core. 15 refs., 2 figs.
TL;DR: In this paper, single phase Molybdenum nitride thin films were deposited using reactive r.f.c. magnetron sputtering, and the activation energy was found to be temperature dependent, indicating that conduction in these films is dominated by carrier hopping through localized states in the intergranular region.
TL;DR: In this article, the effects of alloying elements such as niobium, copper, chromium, molybdenum, and titanium in high-strength, low-alloy steels on hydrogen induced cracking (HIC) were investigated.
Abstract: The effects of alloying elements such as niobium, copper, chromium, molybdenum, and titanium in high-strength, low-alloy (HSLA) steels on hydrogen induced cracking (HIC) were investigated. The HIC test was performed to NACE TM-02-84 in BP (saturated H2S synthetic sea water) and NACE solutions. The test data were analyzed with an optical microscope, scanning electron microscope, electron probe microanalyzer, Auger electron spectrometer, transmission electron microscope, and an image analyzer. Results showed the addition of Nb increased the aspect ratio of inclusions and decreased HIC resistance. In BP solution, Cu-bearing steel formed a protective film of γ-Fe2O3 enriched with Cu on the outside and FeS on the inside. Cr and Mo showed a synergistic effect on the formation of low-temperature transformation products and the increase of HIC sensitivity. The suitable parameter for these two elements on HIC sensitivity was , and the critical value of this parameter was 0.1%. Additionally, coarse TiN pre...
TL;DR: The resistances to charge transfer, measured using complex ac impedance, are affected dramatically by film composition as discussed by the authors, and diffusion coefficients of intercalating ions do not vary greatly with film composition.