Showing papers on "Molybdenum published in 2007"

13 – Superlubricity of Molybdenum Disulfide

Abstract: Publisher Summary This chapter investigates the chemical composition of molybdenum disulfide (MoS2) coatings. High resolution TEM (HR-TEM) and extended X-ray absorption fine structure (EXAFS) are used to investigate the short-range order in the MoS2 crystallites of the coating. HR-TEM studies show that most of MoS2 crystallites exhibit many imperfections including faults, kinks, and curvature. Most of the vacuum-deposited MoS2 coatings contain significant amounts of oxygen in their structure, mainly because small amounts of water are present during the sputtering process. The oxygen incorporation affects the crystalline structure, the orientation, and the film morphology. A typical shift of the (100) peak is observed in X-ray diffraction and is strongly correlated with the oxygen content in the film. This shift has been attributed to the (100) lattice contraction. It is also correlated with a lattice expansion of the (002) basal planes in the c direction. For higher values of the oxygen content, molybdenum oxides (MoOy) are formed.

Applications of Single-site Photocatalysts Implanted within the Silica Matrixes of Zeolite and Mesoporous Silica

Abstract: The tetrahedrally coordinated metal oxide (titanium, vanadium, chromium, and molybdenum oxides) moieties can be implanted and isolated in the silica matrixes of microporous zeolite and mesoporous s...

Explosion temperatures and pressures of metals and other elemental dust clouds

Abstract: The Pittsburgh Research Laboratory of the National Institute for Occupational Safety and Health (NIOSH) conducted a study of the explosibility of various metals and other elemental dusts, with a focus on the experimental explosion temperatures. The data are useful for understanding the basics of dust cloud combustion, as well as for evaluating explosion hazards in the minerals and metals processing industries. The dusts studied included boron, carbon, magnesium, aluminum, silicon, sulfur, titanium, chromium, iron, nickel, copper, zinc, niobium, molybdenum, tin, hafnium, tantalum, tungsten, and lead. The dusts were chosen to cover a wide range of physical properties—from the more volatile materials such as magnesium, aluminum, sulfur, and zinc to the highly “refractory” elements such as carbon, niobium, molybdenum, tantalum, and tungsten. These flammability studies were conducted in a 20-L chamber, using strong pyrotechnic ignitors. A unique multiwavelength infrared pyrometer was used to measure the temperatures. For the elemental dusts studied, all ignited and burned as air-dispersed dust clouds except for nickel, copper, molybdenum, and lead. The measured maximum explosion temperatures ranged from ∼1550 K for tin and tungsten powders to ∼2800 K for aluminum, magnesium, and titanium powders. The measured temperatures are compared to the calculated, adiabatic flame temperatures.

Biology of the molybdenum cofactor

Abstract: The transition element molybdenum (Mo) is an essential micronutrient for plants where it is needed as a catalytically active metal during enzyme catalysis. Four plant enzymes depend on molybdenum: nitrate reductase, sulphite oxidase, xanthine dehydrogenase, and aldehyde oxidase. However, in order to gain biological activity and fulfil its function in enzymes, molybdenum has to be complexed by a pterin compound thus forming the molybdenum cofactor. In this article, the path of molybdenum from its uptake into the cell, via formation of the molybdenum cofactor and its storage, to the final modification of the molybdenum cofactor and its insertion into apo-metalloenzymes will be reviewed.

In vitro synthesis of the iron–molybdenum cofactor of nitrogenase from iron, sulfur, molybdenum, and homocitrate using purified proteins

Abstract: Biological nitrogen fixation, the conversion of atmospheric N2 to NH3, is an essential process in the global biogeochemical cycle of nitrogen that supports life on Earth. Most of the biological nitrogen fixation is catalyzed by the molybdenum nitrogenase, which contains at its active site one of the most complex metal cofactors known to date, the iron–molybdenum cofactor (FeMo-co). FeMo-co is composed of 7Fe, 9S, Mo, R-homocitrate, and one unidentified light atom. Here we demonstrate the complete in vitro synthesis of FeMo-co from Fe2+, S2−, MoO42−, and R-homocitrate using only purified Nif proteins. This synthesis provides direct biochemical support to the current model of FeMo-co biosynthesis. A minimal in vitro system, containing NifB, NifEN, and NifH proteins, together with Fe2+, S2−, MoO42−, R-homocitrate, S-adenosyl methionine, and Mg-ATP, is sufficient for the synthesis of FeMo-co and the activation of apo-dinitrogenase under anaerobic-reducing conditions. This in vitro system also provides a biochemical approach to further study the function of accessory proteins involved in nitrogenase maturation (as shown here for NifX and NafY). The significance of these findings in the understanding of the complete FeMo-co biosynthetic pathway and in the study of other complex Fe-S cluster biosyntheses is discussed.

Study of Methane Dehydroaromatization on Impregnated Mo/ZSM-5 Catalysts and Characterization of Nanostructured Molybdenum Phases and Carbonaceous Deposits

Abstract: Methane dehydroaromatization was studied over a series of impregnated Mo/ZSM-5 catalysts with different molybdenum contents and Si/Al atomic ratios in the parent H-ZSM-5 zeolites. The maximum catal...

Efficient catalysts with controlled porous structure obtained by anodic oxidation under spark-discharge

Abstract: The anodic oxidation under spark discharge (ANOF technique) is presented as a promising alternative for the preparation of catalytic oxide layers on metallic supports. The influence of the substrate (Al, Ti, and Mg), electrolyte composition and the treatment time to obtain egg-shell catalysts with tailor-made properties for the effective oxydehydrogenation (ODH) of cyclohexane to cyclohexene is studied. Chromium, molybdenum and nickel were chosen as precursors for active species in order to obtain catalysts able to perform the substrate oxidation by electrophilic (chromium oxide), nucleophilic (molybdena) or both kind of oxygen species (nickel oxide). It was found that compared to nickel oxide, the chromium oxide containing egg-shell catalyst was less selective while the molybdenum oxide showed very high selectivity, but lower activity. The intrinsic activity of nickel containing egg-shell catalysts is an order of magnitude higher than that of a classic alumina supported nickel oxide catalyst obtained by impregnation. The selectivity to the intermediate cyclohexene is also favored on the egg-shell catalyst with a regular pore structure. Moreover, the selectivity performance of the catalysts can be improved by designing catalysts with shorter pores.

Corrosion resistance of thermally sprayed high-boron iron-based amorphous-metal coatings: Fe 49.7 Cr 17.7 Mn 1.9 Mo 7.4 W 1.6 B 15.2 C 3.8 Si 2.4

Abstract: An iron-based amorphous metal, Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4 (SAM2X5), with very good corrosion resistance has been developed. This material was prepared as a melt-spun ribbon, as well as gas atomized powder and a thermal-spray coating. During electrochemical testing in several environments, including seawater at 90 °C, the passive film stability was found to be comparable to that of high-performance nickel-based alloys and superior to that of stainless steels, based on electrochemical measurements of the passive film breakdown potential and general corrosion rates. This material also performed very well in standard salt fog tests. Chromium (Cr), molybdenum (Mo), and tungsten (W) provided corrosion resistance, and boron (B) enabled glass formation. The high boron content of this particular amorphous metal made it an effective neutron absorber and suitable for criticality control applications. This material and its parent alloy maintained corrosion resistance up to the glass transition temperature and remained in the amorphous state during exposure to relatively high neutron doses.

Synthesis of bulk and supported molybdenum carbide by a single-step thermal carburization method

Abstract: A facile and effective method for the preparation of bulk and alumina-supported molybdenum carbide is described. The synthesis of molybdenum carbide β-Mo2C has been conducted by a single-step heat treatment of a mixed salt precursor containing molybdenum HMT (HMT = hexamethylenetetramine) complex and HMT in argon atmosphere up to 700 °C. The formation process of the carbide has been investigated. Alumina-supported nanocrystalline β-Mo2C has also been successfully prepared with this method. The resultant catalyst exhibits good hydrodesulfurization (HDS) activity. It indicates that this simple method is an effective way to synthesize active carbide catalysts.

Molybdenum isotope variations in molybdenite: Vapor transport and Rayleigh fractionation of Mo

Abstract: Molybdenum isotopes in 20 molybdenite samples, dated by the Re-Os method and representing a range of geologic settings, show mass-dependent fractionation spanning 0.63‰ per atomic mass unit (amu). Previous Mo isotope data for molybdenite reveal variations in fractionation of <0.5‰/amu. Interpretation of these data is hampered, however, by limited sample numbers in each study, lack of a common standard for interlaboratory comparison, and limited range of geologic settings. Here we show that Mo isotope compositions of molybdenites do not correlate with crystallization temperature, age, geographic distribution, or geologic conditions. Rather, Rayleigh distillation may explain variations of as much as 0.34‰/amu in a single molybdenite occurrence, exceeding the proposed variability in average continental crust. Vapor transport and rapid precipitation of Mo in propagating fractures may account for isotope fractionation of Mo (and perhaps other metals) at very small scales. If so, the average isotopic composition of Mo at each molybdenite occurrence may be representative of bulk crust. Our results suggest that the isotopic composition of Mo delivered to the oceans is uniform geographically and through geologic time.

Space environmental effects on MoS2 and diamond-like carbon lubricating films: Atomic oxygen-induced erosion and its effect on tribological properties

Abstract: Effect of 5 eV atomic oxygen beam exposure on the surface properties of sputter-deposited molybdenum disulfide (MoS2) and diamond-like carbon lubrication films is experimentally evaluated with relevance to space environmental effect in low Earth orbit. X-ray photoelectron spectra indicate that the loss of sulfur and oxidation of molybdenum at the atomic oxygen-exposed MoS2 surface are significant. Depth profiles of sulfur and molybdenum indicate that the oxidation is restricted within 3 nm from the surface. This is due to the fact that Mo oxide plays as a protective layer against further atomic oxygen attack. The surface oxidation affects the friction coefficient, however it is recovered with sliding. Due to the delamination of oxide layer, wear-life of the film is reduced dramatically by certain exposure conditions. On the other hand, no severe oxidation states of carbon atoms are detected at DLC surface from the synchrotron radiation photoemission spectra. In contrast, loss of the DLC film itself is measured by Rutherford backscattering spectroscopy. These analytical results can be explained by the oxidative gasification of carbon atoms at DLC surface.

Synthesis and characterization of high surface area molybdenum phosphide

Abstract: A high surface area molybdenum phosphide (MoP) was successfully synthesized by combining citric acid (CA) and temperature-programmed reduction (TPR) (CA-TPR) method. Reduction of the precursor which was modified by citric acid produced Mol? with a high surface area of 122.0 m(2) g(-1) under optimum conditions. Fourier transform infrared (FTIR) results showed that the chelating interaction between the moderate amount of citric acid and the molybdenum ion was effective in suppressing the aggregation of Mo during drying through the formation of a molybdenum citrate, which was decomposed in calcination. Reduction of the precursor from the CA-TPR method showed an obvious decrease in degree of the aggregation of the MOP particles when compared to that from the conventional TPR method. The Mol? prepared by the CA-TPR method was characterized by X-ray diffraction (XRD), N-2 adsorption-desorption, TPR, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and CO adsorption microcalorimetry. An increase in reduction temperature led to the formation of MoP crystalline and a change of morphology. The increase of surface area in the reduction process was a result of the formation of pores. The porous MoP from the CA-TPR method had a higher CO chemisorption uptake than from the conventional TPR method, indicating that the high surface area MoP possessed more active sites. The preliminary testing showed that the high surface area MoP exhibited a superior activity for hydrazine decomposition with a conversion of 85%, which was much higher than the conventional Mol? of 55%. (c) 2006 Elsevier B.V. All rights reserved.

The structural investigations of PbO-P2O5-Sb2O3 glasses with MoO3 as additive by means of dielectric, spectroscopic and magnetic studies

Abstract: 40PbO–40P 2 O 5 −(16+ x )Sb 2 O 3 :(4− x )MoO 3 glasses with nine values of x ranging from 4 to 0 were prepared. A number of studies viz., differential thermal analysis, infrared, optical absorption, Raman and ESR spectra, magnetic susceptibility and dielectric properties (constant e ′, loss tan δ , AC conductivity σ AC over a range of frequency and temperature) of these glasses have been carried out. The results have been analyzed in the light of different oxidation states of molybdenum ions. The analysis indicates that when the concentration of Sb 2 O 3 is less than 19.0 mol% in the glass composition, molybdenum ions are observed to exist mostly in Mo 5+ state, occupy network-modifying positions and decrease the rigidity of the glass network.