Showing papers on "Molybdenum published in 2004"

Development of High Strength Hot-rolled Sheet Steel Consisting of Ferrite and Nanometer-sized Carbides

Abstract: A ferritic steel precipitation-strengthened by manometer-sized carbides was developed to obtain a high strength hot-rolled sheet steel having tensile strength of 780 MPa grade with excellent stretch flange formability. Manganese in a content of 1.5% and molybdenum in a content of 0.2 % were added to 0.04 % carbon Ti-bearing steel in order to lower austenite-ferrite transformation temperature for fine carbides and to retard generating of pearlite and large cementites, respectively. Tensile strength of hot-rolled sheet steel increased with titanium content and it was achieved to 800 MPa in a 0.09 % Ti steel. Microstructure of the 0.09 %Ti steel was ferrite without pearlite and large cementites. Fine carbides of 3 nm in diameter were observed in rows in the ferrite matrix of the 0.09 % Ti steel with transmission electron microscope. The characteristic arrangement of the nanometer-sized carbides indicates that the carbides were formed at austenite-ferrite interfaces during transformation. By energy dispersive X-ray spectroscopy, the carbides were found to contain molybdenum in the same atomic concentration as titanium. Crystal structure of the nanometer-sized carbides was determined to be NaCI-type by X-ray diffractometry. The calculated amount of precipitation-strengthening by the carbides was approximately 300 MPa. This is two or three times higher than that of conventional Ti-bearing high strength hot-rolled sheet steels. Based on the results obtained in the laboratory investigation, mill trial was carried out. The developed hot-rolled high strength sheet steel exhibited excellent stretch flange formability.

Studies of the Hydrogen Evolution Reaction on Raney Nickel—Molybdenum Electrodes

Abstract: The hydrogen evolution reaction was studied in 1 M KOH at 25 °C on two types of electrodes:(i) pressed powders of Ni or NiMo with Al, heated at 700 °C; (ii) Ni—Al—Mo powders deposited by vacuum plasma spraying. These materials were treated with alkaline solution to leach out aluminum. Very active and stable Raney nickel—molybdenum electrodes were obtained from Al rich alloys. Adding molybdenum significantly improved catalytic activity. Electrochemical impedance spectroscopy was used to determine the apparent and intrinsic activities. Different ac models were tested and the appropriate one was selected. Intrinsic activities of these electrodes are smaller or equal to that of polycrystalline Ni and the origin of high apparent activities is related to the increase in real surface area.

The crystal structure of xanthine oxidoreductase during catalysis: Implications for reaction mechanism and enzyme inhibition

Abstract: Molybdenum is widely distributed in biology and is usually found as a mononuclear metal center in the active sites of many enzymes catalyzing oxygen atom transfer. The molybdenum hydroxylases are distinct from other biological systems catalyzing hydroxylation reactions in that the oxygen atom incorporated into the product is derived from water rather than molecular oxygen. Here, we present the crystal structure of the key intermediate in the hydroxylation reaction of xanthine oxidoreductase with a slow substrate, in which the carbon–oxygen bond of the product is formed, yet the product remains complexed to the molybdenum. This intermediate displays a stable broad charge–transfer band at ≈640 nm. The crystal structure of the complex indicates that the catalytically labile Mo—OH oxygen has formed a bond with a carbon atom of the substrate. In addition, the Mo⋕S group of the oxidized enzyme has become protonated to afford Mo—SH on reduction of the molybdenum center. In contrast to previous assignments, we find this last ligand at an equatorial position in the square-pyramidal metal coordination sphere, not the apical position. A water molecule usually seen in the active site of the enzyme is absent in the present structure, which probably accounts for the stability of this intermediate toward ligand displacement by hydroxide.

Structure of the molybdopterin-bound Cnx1G domain links molybdenum and copper metabolism.

Abstract: The molybdenum cofactor is part of the active site of all molybdenum-dependent enzymes1, except nitrogenase. The molybdenum cofactor consists of molybdopterin, a phosphorylated pyranopterin2, with an ene-dithiolate coordinating molybdenum. The same pyranopterin-based cofactor is involved in metal coordination of the homologous tungsten-containing enzymes found in archea3. The molybdenum cofactor is synthesized by a highly conserved biosynthetic pathway4. In plants, the multidomain protein Cnx1 catalyses the insertion of molybdenum into molybdopterin. The Cnx1 G domain (Cnx1G), whose crystal structure has been determined in its apo form, binds molybdopterin with high affinity and participates in the catalysis of molybdenum insertion. Here we present two high-resolution crystal structures of Cnx1G in complex with molybdopterin and with adenylated molybdopterin (molybdopterin–AMP), a mechanistically important intermediate. Molybdopterin–AMP is the reaction product of Cnx1G and is subsequently processed in a magnesium-dependent reaction by the amino-terminal E domain of Cnx1 to yield active molybdenum cofactor. The unexpected identification of copper bound to the molybdopterin dithiolate sulphurs in both structures, coupled with the observed copper inhibition of Cnx1G activity, provides a molecular link between molybdenum and copper metabolism.

Theoretical investigations of the structures and properties of molybdenum-based sulfide catalysts

Abstract: The development of new hydrotreating catalysts to meet the need for producing cleaner transportation fuels requires a better understanding of the structures and properties of active sites of industrially relevant catalysts. Molecular modeling and simulation has made important contributions towards achieving this goal. This review summarizes theoretical results and conclusions regarding the structures and properties of molybdenum-based hydrotreating catalysts through a logical discussion of surface structures, hydrogen activation, and adsorption and reaction of organosulfur molecules. Ab initio calculations of molybdenum-based sulfide catalysts have been used to predict the equilibrium sulfur coverage on the edge planes of promoted and unpromoted MoS2 catalysts, identify the energetically favorable locations of promoter (Ni, Co) atoms, consider the mechanism of hydrogen dissociation and adsorption, and characterize the bonding of organosulfur molecules by evaluating the adsorption energies as a function of catalyst composition. We summarize published computational results from the previous decade, and in the process highlight the need to consider the appropriate model when performing calculations. In this review, we conclude the single slab MoS2 model containing two rows of molybdenum atoms is adequate for discussing general energetic trends when sulfur coverage changes on the edge surface, however more substantive models are required to obtain accurate energetic and structural data. The stable structures at reaction conditions also have to be considered when discussing the locations of promoter atoms, the dissociation of hydrogen, and the adsorption of molecules on the edge surface. Additional challenges and future applications to increase the fundamental understanding of molybdenum-based sulfide catalysts are discussed.

High-mobility, sputtered films of indium oxide doped with molybdenum

Abstract: Thin films of molybdenum-doped indium oxide, an n-type transparent conducting oxide, were deposited on glass substrates by a large-area deposition technique, radio-frequency magnetron sputtering, and their electrical properties were examined. Molybdenum content was varied from 1 to 4 wt%, and the highest mobility achieved was 83 cm2 V−1 s−1 at a carrier concentration of 3.0×1020 cm−3 without any postdeposition treatment for one of the films made from the target with 2 wt% Mo. Temperature-dependent Hall analysis indicated that this high mobility is limited by phonon scattering, whereas the method of four coefficients analysis showed that the conduction band is parabolic.

The DMSO Reductase Family of Microbial Molybdenum Enzymes

Abstract: Vol 35 No 3 December 2004 AUSTRALIAN BIOCHEMIST Page 17 Molybdenum is the only element in the second row of transition metals which has a defined role in biology. It exhibits redox states of (VI), (V) and (IV) within a biologically-relevant range of redox potentials and is capable of catalysing both oxygen atom transfer and proton/electron transfer. Apart from nitrogenase, all enzymes containing molybdenum have an active site composed of a molybdenum ion coordinated by one or two ene-dithiolate (dithiolene) groups that arise from an unusual organic moiety known as the pterin molybdenum cofactor or pyranopterin (1,2). The mononuclear molybdenum enzymes exhibit remarkable diversity of function and this is in part due to variations at the Mo active site that are additional to the common core structure. Prior to the appearance of X-ray crystal structures of molybdenum enzymes, EPR spectroscopy, X-ray absorption fine structure spectroscopy (EXAFS) and biochemical analysis had identified differences between the molybdenum hydroxylases, exemplified by xanthine dehydrogenase, and those enzymes that mostly functioned as 'oxotransferases' (3). The former possess a cyanolysable sulfido group at the Mo active site, while the latter are insensitive to cyanide and may possess oxo ligands (Fig. 1). Since 1995, X-ray crystal s t r u c t u r e s h a v e r a i s e d t h e u n d e r s t a n d i n g o f molybdenum enzymes to a much higher level and have led to a division of the oxotransferases into the sulfite dehydrogenase and the dimethylsulfoxide (DMSO) reductase families (Fig. 1) (4). The Mo hydroxylases and oxotransferases can act either as dehydrogenases or reductases in catalysis. This reaction can be summarised by the general scheme: X + H2O D X=O + 2H+ + 2eDuring this process the Mo ion cycles between the (IV) and (VI) oxidation states with electrons being transferred to or from an electron transfer partner or substrate. Experiments with xanthine dehydrogenase using 18O-labelled water have confirmed that the oxygen is incorporated into the product during substrate oxidation and this distinguishes the mononuclear molybdoenzymes from monoxygenases where molecular oxygen rather than water acts as an oxygen atom donor (5). The last decade has seen a resurgence of interest in Mo enzymes as a consequence of the new structural information and also because the remarkable b i o e n e r g e t i c d i v e r s i t y o f m i c r o o r g a n i s m s i s underpinned to a large degree by Mo enzymes (6). Molybdenum hydroxylases and examples of the sulfite dehydrogenase family can be found in all three domains of life (4). In contrast, the DMSO reductase family appears to be restricted to bacteria and archaea (5).

Effects of small MoO3 additions on the properties of nickel catalysts for the steam reforming of hydrocarbons. III. Reduction of Ni-Mo/Al2O3 catalysts

Abstract: Ni/Al2O3 catalyst modified by small amounts of Mo show unusual properties in the steam reforming of hydrocarbons. There are no data about the effect of small amounts of molybdenum on reduction of the Ni-Mo supported catalysts. The properties of these very complex systems depend on the conditions of successive preparation stages (calcination, reduction) or the process conditions. A series of Ni/Al2O3 catalysts modified by Mo were prepared in order to investigate the influence of promoter amounts and preparation sequence on their properties. Temperature programmed reduction (TPR) has been employed to study the reducibility of Ni-Mo/Al2O3 catalysts. Catalysts were further characterized by BET area, H2 chemisorption and X-ray diffraction measurements. The TPR curves of Ni-Mo/Al2O3 catalysts are very complex. Mo addition leads to the decrease of catalysts reducibility. However, complete reduction of NiO and MoO3 can be achieved at 800 °C. The reduction course depends on the sequence of nickel and molybdenum addition into the support. Precise measurements of Ni peaks positions in the XRD pattern of Ni/Al2O3 and Ni-Mo/Al2O3 samples show the possibility of Ni-Mo solid solution formation.

A series of NiMo/Al2O3 catalysts containing boron and phosphorus: Part I. Synthesis and characterization

Abstract: In this work, a series of nickel–molybdenum, nickel–molybdenum–boron and one nickel–molybdenum–phosphorus catalysts were prepared by incipient wetness impregnation method. Calcination temperature, nickel, molybdenum and boron concentrations in γ-Al2O3 were varied from 450 to 600 °C, 1.8 to 3.1 wt.%, 10.6 to 13.2 wt.% and 0.5 to 1.7 wt.%, respectively. Nickel–molybdenum–phosphorus catalyst was prepared using 2.7 wt.% phosphorus. All catalysts were thoroughly characterized. The catalyst containing 10.6 wt.% Mo and 2.4 wt.% Ni in γ-Al2O3 gave maximum BET area of 211 m2/g. In these catalysts, the molybdenum oxides were present predominantly as polymolybdate and tetrahedral form. Addition of boron to NiMo/Al2O3 caused an increase in weak acid centers, whereas phosphorus caused the formation of acid centers with intermediate strength. Also, addition of boron caused the formation of crystalline B2O3 and MoO3 on the catalyst surface, evident from both XRD and SEM analyses. Whereas addition of phosphorus caused the formation of irregular particle size and agglomeration on the catalyst surface, evident from SEM-EDS analysis. New Lewis and Bronsted acid sites on the catalyst surface were observed from FTIR analysis because of the addition of boron and phosphorus to NiMo/Al2O3.

CCVD synthesis of carbon nanotubes from (Mg,Co,Mo)O catalysts: influence of the proportions of cobalt and molybdenum

Abstract: Carbon nanotubes have been synthesised by catalytic chemical vapour deposition of a H2–CH4 mixture (18 mol% CH4) over (Mg,Co,Mo)O catalysts. The total amount of cobalt and molybdenum has been kept constant at 1 cat% and the proportion of molybdenum with respect to cobalt has been varied from x(Mo) = 0.25–1.0. This variation has important effects on both the yield and the nature (number of walls, straight walls or bamboo-like structures) of the carbon nanotubes. It also has an influence on the purity of the samples (amount of encapsulated metal particles, presence or not of amorphous carbon deposits). For x = 0.25, the nanotubes were mainly double- and triple-walled (inner diameter less than 3 nm); samples prepared from catalysts with higher molybdenum ratios contained larger multi-walled carbon nanotubes (inner diameter up to 9 nm), having up to 13 concentric walls. It is proposed that different growth mechanisms may occur depending on the initial composition of the catalyst.

Amorphous molybdenum nitride thin films prepared by reactive sputter deposition

Abstract: Amorphous molybdenum nitride thin films were prepared by pulsed direct current (dc) reactive sputter deposition. The effect of the sputtering gas nitrogen content on the structure of as-deposited thin film was investigated. The nitrogen content in the sputtering gas affected the crystallinity of deposited thin films. When the sputtering gas contained from 10 to 30% nitrogen, crystalline γ-Mo 2 N was observed. A decrease in the γ-Mo 2 N peak intensity along with peak shifting and broadening was observed in X-ray diffraction (XRD) spectra as the deposition nitrogen content increased. An amorphous molybdenum nitride thin film was obtained as the nitrogen content in the sputtering gas increased to 50%. X-ray photoelectron spectroscopic analysis (XPS) of the as-deposited thin films showed that the binding energy of Mo 3d 3/2 , Mo 3d 5/2 and Mo 3p 3/2 peaks as well as the amount of nitrogen in thin film increased as the nitrogen content in sputtering gas increased. Thermal treatment of the amorphous thin film showed that the amorphous film had survived 700 °C thermal annealing with no evidence of crystallization and reaction. However, crystallization of amorphous film and reaction of the amorphous thin film with Si substrate were observed after thermal annealing at 800 °C.

Electrochemical studies of arsenite oxidase: an unusual example of a highly cooperative two-electron molybdenum center.

Abstract: Arsenite oxidase from Alcaligenes faecalis, an unusual molybdoenzyme that does not exhibit a Mo(V) EPR signal during oxidative-reductive titrations, has been investigated by protein film voltammetry. A film of the enzyme on a pyrolytic graphite edge electrode produces a sharp two-electron signal associated with reversible reduction of the oxidized Mo(VI) molybdenum center to Mo(IV). That reduction or oxidation of the active site occurs without accumulation of Mo(V) is consistent with the failure to observe a Mo(V) EPR signal for the enzyme under a variety of conditions and is indicative of an obligate two-electron center. The reduction potential for the molybdenum center, 292 mV (vs SHE) at pH 5.9 and 0 degrees C, exhibits a linear pH dependence for pH 5-10, consistent with a two-electron reduction strongly coupled to the uptake of two protons without a pK in this range. This suggests that the oxidized enzyme is best characterized as having an L(2)MoO(2) rather than L(2)MoO(OH) center in the oxidized state and that arsenite oxidase uses a "spectator oxo" effect to facilitate the oxo transfer reaction. The onset of the catalytic wave observed in the presence of substrate correlates well with the Mo(VI/IV) potential, consistent with catalytic electron transport that is limited only by turnover at the active site. The one-electron peaks for the iron-sulfur centers are difficult to observe by protein film voltammetry, but spectrophotometric titrations have been carried out to measure their reduction potentials: at pH 6.0 and 20 degrees C, that of the [3Fe-4S] center is approximately 260 mV and that of the Rieske center is approximately 130 mV.

Role of combined addition of niobium and boron and of molybdenum and boron on hardnenability in low carbon steels

Abstract: Effects of the combined addition of niobium (Nb) and boron (B) and of molybdenum (Mo) and B on hardenability were investigated using low carbon steels. Strength synergically increases due to the combined addition of Nb and B and that of Mo and B. It is thought that strength increases due to these combined additions because austenite (y) to ferrite (a) transformation is retarded and bainite transformation is promoted due to the increase in the segregated B along the y grain boundary before y to a transformation. The mechanism for the increase in the segregated boron along the y grain boundary by these combined additions is considered below. Fe 23 (C, B) 6 precipitates formed along the y grain boundary are suppressed by these combined additions because of the suppression of C diffusion towards the y grain boundary due to the precipitation of the fine dispersive niobium-titanium carbonnitride (Nb, Ti)(C, N) or titanium-molybdenum carbonnitride (Ti, Mo)(C, N) and the formation of C clusters of Nb and Mo during rolling or during cooling after rolling. Therefore, the segregated B along the y grain boundary increases and y to α transformation is retarded. The combined addition of Nb and B or that of Mo and B in low C bainitic steel is effective for increasing strength without deteriorating low temperature toughness. It is clarified that the increments of hardenability by the combined addition of Nb and B is different from that of Mo and B due to the difference of the amount of carbide precipitates.