Showing papers on "Molybdenum published in 2009"

The Hotter the Engine, the Better

Abstract: Jet turbine engines have benefited from decades of development of nickel-based superalloys, which have allowed a steady increase in engine operating temperatures and led to improved performance and efficiency. However, operating temperatures are now reaching limits posed by the melting temperatures ( T m) of these materials. New materials, including alloys based on metals with higher melting points, such as molybdenum (Mo) and niobium (Nb) alloyed with silicon (Si), are now being seriously examined as alternatives by academic and industrial groups.

Molybdenum cofactors, enzymes and pathways

Abstract: The trace element molybdenum is essential for nearly all organisms and forms the catalytic centre of a large variety of enzymes such as nitrogenase, nitrate reductases, sulphite oxidase and xanthine oxidoreductases. Nature has developed two scaffolds holding molybdenum in place, the iron-molybdenum cofactor and pterin-based molybdenum cofactors. Despite the different structures and functions of molybdenum-dependent enzymes, there are important similarities, which we highlight here. The biosynthetic pathways leading to both types of cofactor have common mechanistic aspects relating to scaffold formation, metal activation and cofactor insertion into apoenzymes, and have served as an evolutionary 'toolbox' to mediate additional cellular functions in eukaryotic metabolism.

Molybdenum limitation of asymbiotic nitrogen fixation in tropical forest soils

Abstract: Biological nitrogen fixation limits plant growth and carbon exchange at local to global scales. Long-term nutrient manipulation experiments in forests and short-term manipulation experiments in microcosms suggest that the micronutrient molybdenum, a component of the nitrogen-fixing enzyme nitrogenase, limits nitrogen fixation by asymbiotic bacteria in tropical soils in Panama. Nitrogen fixation, the biological conversion of di-nitrogen to plant-available ammonium, is the primary natural input of nitrogen to ecosystems1, and influences plant growth and carbon exchange at local to global scales2,3,4,5,6. The role of this process in tropical forests is of particular concern, as these ecosystems harbour abundant nitrogen-fixing organisms1,4 and represent one third of terrestrial primary production4,7,8. Here we show that the micronutrient molybdenum, a cofactor in the nitrogen-fixing enzyme nitrogenase, limits nitrogen fixation by free-living heterotrophic bacteria in soils of lowland Panamanian forests. We measured the fixation response to long-term nutrient manipulations in intact forests, and to short-term manipulations in soil microcosms. Nitrogen fixation increased sharply in treatments of molybdenum alone, in micronutrient treatments that included molybdenum by design and in treatments with commercial phosphorus fertilizer, in which molybdenum was a ‘hidden’ contaminant. Fixation did not respond to additions of phosphorus that were not contaminated by molybdenum. Our findings show that molybdenum alone can limit asymbiotic nitrogen fixation in tropical forests and raise new questions about the role of molybdenum and phosphorus in the tropical nitrogen cycle. We suggest that molybdenum limitation may be common in highly weathered acidic soils, and may constrain the ability of some forests to acquire new nitrogen in response to CO2 fertilization9.

Storage and bioavailability of molybdenum in soils increased by organic matter complexation

Abstract: The micronutrient molybdenum is necessary for nitrogen fixation, but is very rare and highly soluble in soils. X-ray spectroscopy analysis of forest soil samples indicates that most of the molybdenum in the litter layer binds to organic matter. The micronutrient molybdenum is a necessary component of the nitrogen-fixing enzyme nitrogenase1,2. Molybdenum is very rare in soils, and is usually present in a highly soluble form, making it susceptible to leaching3,4. However, it is generally thought that molybdenum attaches to mineral surfaces in acidic soils; this would prevent its escape into the groundwater, but would also impede uptake by microbes3. Here we use X-ray spectroscopy to examine the chemical speciation of molybdenum in soil samples from forests in Arizona and New Jersey. We show that in the leaf litter layer, most of the molybdenum forms strong complexes with plant-derived tannins and tannin-like compounds; molybdenum binds to these organic ligands across a wide pH range. In deeper soils, molybdenum binds to both iron oxides and natural organic matter. We suggest that the molybdenum bound to organic matter can be captured by small complexing agents that are released by nitrogen-fixing bacteria; the molybdenum can then be incorporated into nitrogenase. We conclude that the binding of molybdenum to natural organic matter helps prevent leaching of molybdenum, and is thus a critical step in securing new nitrogen in terrestrial ecosystems.

Growth and characterization of molybdenum oxide nanorods by RF magnetron sputtering and subsequent annealing

Abstract: High-densely packed and uniformly distributed molybdenum oxide nanorods have been grown onto glass substrates by RF magnetron sputtering and subsequent annealing in an oxygen atmosphere. A two-step growth mechanism (sputtering redeposition and enhanced rearrangement during annealing) for the formation of MoO3 nanorods has been proposed. The morphological, structural, optical and electrical properties of the nanorods have been investigated systematically using atomic force microscopy, scanning electron microscopy, x-ray diffraction, micro-Raman, UV–visible, photoluminescence (PL) spectroscopy and dc resistivity studies. The nanorods in the as-sputtered film and the film annealed at 473 K are amorphous in nature. However, the nanorods in the films annealed at 573 and 673 K exhibit the presence of monoclinic Mo8O23 and orthorhombic MoO3, respectively. Vibrational analysis of the molybdenum and oxygen atoms in the nanorods is carried out by micro-Raman spectra. The nanorods show room temperature PL in the UV–visible region. The PL emission is found to be strongly enhanced by post-deposition annealing. The low temperature resistivity measurement is done on the as-deposited film; the activation energy and polaron hopping energy for electrical conduction are calculated. The MoO3 nanorods are expected to exhibit enhanced functionality, particularly in nanoscale, photochromic and gas sensing applications.

Atomic-scale analysis on the role of molybdenum in iron-catalyzed carbon nanotube growth.

Abstract: We have elucidated the synergetic role played by molybdenum in iron-catalyzed chemical vapor deposition growth of carbon nanotubes (CNTs) by in situ environmental transmission electron microscopy. Molybdenum can be well accommodated by Fe-based carbide nanoparticle catalysts of M23C6-type structure (M = Fe and Mo). We have also shown that molybdenum suppresses the nucleation of iron compounds that are known to exhibit no catalytic activity for the growth of CNTs.

Effect of alloying elements Ti, Zr on the property and microstructure of molybdenum

Abstract: Mo–Zr and Mo–Ti alloy were fabricated by adding alloying element Zr, Ti in molybdenum via powder metallurgy process. The effect of Zr, Ti alloying elements on the tensile strength and microstructure of molybdenum are investigated. Two types of alloying additives are used: one is elemental powder of Zr or Ti; the other is their respective hydride powders, TiH2 or ZrH2. The experimental results show that the formed Mo–Zr, Mo–Ti alloy possess much higher mechanical properties than pure sintered molybdenum. The tensile strength of Mo–Zr alloy is more affected by adding elemental Zr powder than ZrH2 powder, the optimal tensile strength is obtained with addition of 0.1 wt% Zr, most of Zr forms Zr-oxide existed both inside the grain and at the grain boundaries, while very little Zr diffuses into the Mo matrix. Meanwhile, for Mo–Ti alloy, the tensile strength is more influenced by addition of TiH2 powder than elemental Ti powder, the tensile strength is optimized at the content of 0.8 wt% TiH2, some Ti diffuses into molybdenum and forms Mo–Ti solid solution and some forms as MoxTiyOz oxide particles uniformly distributed at the grain boundaries.

Tribological and corrosive characteristics of electrochemical coatings based on cobalt and iron superalloys

Abstract: The tribological and corrosive characteristics of binary and ternary alloys electrodeposited from CoW, FeW, CoMoP, and CoWP citrate solutions are studied. The tungsten content of CoW alloys reaches 31 at.% and of FeW alloys 34 at.%. The introduction of phosphorus into the alloys reduces the content of tungsten from 26.7 to 19.7 at.% (at 4.5 at.% P). The molybdenum content of CoMoP alloys is 0.7–1.0 at.% at 5 to 8 at.% P. The electrolytic tungsten alloys are nanocrystalline and subgrains are 4 to 7 nm in size. The nanohardness of tungsten-rich alloys (∼13 GPa) is comparable with that of electrolytic chromium coatings. The wear resistance of the deposited alloys is quite high and is commensurable with that of hard coatings such as TiB2, TiN, and TiAlN. The tribooxidation of FeW alloys in dry friction is revealed. The corrosion resistance of the coatings is similar to that for electrolytic chromium.

Application of molybdenum(VI) dichloride dioxide (MoO2Cl2) in organic transformations

Abstract: The role of MoO2Cl2 and some complexes containing MoO2Cl2 as catalysts for various Lewis acid catalysed organic transformations, oxidation and reduction reactions is reviewed.

Cemented carbide-metallic alloy composites

Abstract: A macroscopic composite sintered powder metal article including a first region including cemented hard particles, for example, cemented carbide. The article includes a second region including one of a metal and a metallic alloy selected from the group consisting of a steel, nickel, a nickel alloy, titanium, a titanium alloy, molybdenum, a molybdenum alloy, cobalt, a cobalt alloy, tungsten, and a tungsten alloy. The first region is metallurgically bonded to the second region, and the second region has a thickness of greater than 100 microns. A method of making a macroscopic composite sintered powder metal article is also disclosed, herein. The method includes co-press and sintering a first metal powder including hard particles and a powder binder and a second metal powder including the metal or metal alloy.

Olefin isomerization and metathesis catalyst

Abstract: A process for the production of propylene, the process including: contacting ethylene and a hydrocarbon stream comprising 1-butene and 2-butene with a bifunctional isomerization-metathesis catalyst to concurrently isomerizes 1-butene to 2-butene and to form a metathesis product comprising propylene; wherein the bifunctional isomerization-metathesis catalyst comprises: a catalyst compound may include at least one element selected from tungsten, tantalum, niobium, molybdenum, nickel, palladium, osmium, iridium, rhodium, vanadium, ruthenium, and rhenium for providing metathesis activity on a support comprising at least one element from Group IA, IIA, IIB, and IIIA of the Periodic Table of the Elements; wherein an exposed surface area of the support provides both isomerization activity for the isomerization of 1-butene to 2-butene; and reactive sites for the adsorption of catalyst compound poisons. In other embodiments, the catalyst compound may include at least one element selected from aluminum, gallium, iridium, iron, molybdenum, nickel, niobium, osmium, palladium, phosphorus, rhenium, rhodium, ruthenium, tantalum, titanium, tungsten, and vanadium.

Thin film sodium species barrier method and structure for cigs based thin film photovoltaic cell

Abstract: A method for fabricating a thin film solar cell includes providing a soda lime glass substrate comprising a surface region and a concentration of sodium oxide of greater than about 10 wt % and treating the surface region with one or more cleaning process, using a deionized water rinse, to remove surface contaminants having a particles size of greater than three microns. The method also includes forming a barrier layer overlying the surface region, forming a first molybdenum layer in tensile configuration overlying the barrier layer, and forming a second molybdenum layer in compressive configuration using a second process overlying the first molybdenum layer. Additionally, the method includes patterning the first molybdenum layer and the second molybdenum layer to form a lower electrode layer and forming a layer of photovoltaic material overlying the lower electrode layer. Moreover, the method includes forming a first zinc oxide layer overlying the layer of photovoltaic materials.

Molybdenum Trafficking for Nitrogen Fixation

Abstract: The molybdenum nitrogenase is responsible for most biological nitrogen fixation, a prokaryotic metabolic process that determines the global biogeochemical cycles of nitrogen and carbon. Here we describe the trafficking of molybdenum for nitrogen fixation in the model diazotrophic bacterium Azotobacter vinelandii. The genes and proteins involved in molybdenum uptake, homeostasis, storage, regulation, and nitrogenase cofactor biosynthesis are reviewed. Molybdenum biochemistry in A. vinelandii reveals unexpected mechanisms and a new role for iron-sulfur clusters in the sequestration and delivery of molybdenum.

N-CN bond cleavage of cyanamides by a transition-metal complex.

Abstract: N−CN bond cleavage of cyanamides (R2N−CN) has been attained at room temperature in the reaction of R2N−CN with Cp(CO)2Fe(SiEt3). The mechanistic investigation revealed that silyl migration from Fe to CN nitrogen of cyanamide gave an N-silylated η2-amidino iron complex, which was isolated and characterized by X-ray analysis. Catalytic N−CN bond cleavage was also attained using a methyl molybdenum complex under thermal conditions.

Robustness of RF MEMS Capacitive Switches With Molybdenum Membranes

Abstract: This paper compares the characteristics of an RF microelectromechanical systems (MEMS) capacitive switch with a molybdenum membrane versus that of a switch with similar construction but with an aluminum membrane. In comparison, the molybdenum switch exhibits a significantly reduced sensitivity to ambient temperature change so that its pull-in voltage varies by less than 0.035 V/°C. In addition, large-signal RF performance of the switches was compared under both continuous wave and pulse conditions. The results show that under large RF signals, the self-biasing effect is exacerbated by the self-heating effect and the self-heating effect is in turn amplified by nonuniform current and temperature distributions on the membrane. Measurements of both molybdenum and aluminum switches demonstrate a hot-switched power-handling capacity of approximately 600 mW. Since aluminum has been used as a membrane material for over a decade while molybdenum is new, the above results indicate that molybdenum is a promising membrane material for RF MEMS capacitive switches.

Cell biology of molybdenum

Abstract: The transition element molybdenum (Mo) is an essential micronutrient that is needed as catalytically active metal during enzyme catalysis. In humans four enzymes depend on Mo: sulfite oxidase, xanthine oxidoreductase, aldehyde oxidase, and mitochondrial amidoxime reductase. In addition to these enzymes, plants harbor a fifth Mo-enzyme namely nitrate reductase. To gain biological activity and fulfill its function in enzymes, Mo has to be complexed by a pterin compound thus forming the molybdenum cofactor. This article will review the way that Mo takes from uptake into the cell, via formation of the molybdenum cofactor and its storage, up to the final insertion of the molybdenum cofactor into apometalloenzymes. © 2009 International Union of Biochemistry and Molecular Biology, Inc.