Health, Safety & Environment
Molybdenum is widely distributed in nature. It is found in the minerals molybdenite,MoS2 which is the major ore of molybdenum, wulfenite, ferrimolybdate, jordisite, and powellite. Most molybdenum compounds are derived from molybdenum trioxide which is prepared by roasting molybdenum disulfide ores in air. Commonly used molybdenum-oxygen compounds are molybdenum trioxide, MoO3, sodium molybdate, Na2MoO4.2H2O, ammonium dimolybdate, (NH4)2 Mo2O7, and ammonium heptamolybdate, (NH4)6Mo7O24.4H2O.
Molybdenum is used mainly as an alloying element in steel, cast iron, and superalloys and in the electronics industries.
Leichtfried, G., in Ullmann's Encyclopedia of Industrial Chemistry ,5th ed. 1990, A16 , 668.
Molybdenum trioxide and molybdenum-oxygen compounds are added to steel and corrosion-resistant alloys. They are used in industrial catalysts, corrosion inhibitors, pigments, glass, ceramics, and enamels, flame retardant for polyester and polyvinyl chloride resins, as crop nutrients in agriculture, and as reagents in chemical analyses.
Vukasovich, M. S., in Ullmann's Encyclopedia of Industrial Chemistry ,5th ed., 1990, A16 , 682.
Molybdenum-sulfur compounds are used in lubrication to reduce friction and wear: molybdenum disulfide as a dry or suspended lubricant and molybdenum-sulfur complexes as soluble oil additives.
Mitchell, P. C. H., Wear , 1984, 100 , 281.
Other molybdenum compounds which find some application, for example, in the electronics industries and in chemical vapour deposition, are molybdenum pentachloride and molybdenum hexacarbonyl.
Environmental release of molybdenum compounds from industrial activities can occur in air (stack emissions), water (liquid effluents), or solid wastes (sludge): see Industrial and Environmental Exposure of Humans to Molybdenum.
MOLYBDENUM METAL ALLOY BIOACCESSIBILITY
The European chemical framework REACH requires that hazards and risks posed by chemicals, including alloys and metals, that are manufactured, imported or used in different products (substances or articles) are identified and proven safe for humans and the environment. Metals and alloys need hence to be investigated on their extent of released metals (bioaccessibility) in biologically relevant environments. Read-across from available studies may be used for similar materials. This study investigates the release of molybdenum and iron from powder particles of molybdenum metal (Mo), a ferromolybdenum alloy (FeMo), an iron metal powder (Fe), MoO2, and MoO3 in different synthetic body fluids of pH ranging from 1.5 to 7.4 and of different composition. Spectroscopic tools and cyclic voltammetry have been employed to characterize surface oxides, microscopy, light scattering and nitrogen absorption for particle characterization, and atomic absorption spectroscopy to quantify released amounts of metals. The release of molybdenum from the Mo powder generally increased with pH and was influenced by the fluid composition. The mixed iron and molybdenum surface oxide of the FeMo powder acted as a barrier both at acidic and weakly alkaline conditions. These findings underline the importance of the surface oxide characteristics for the bioaccessibility of metal alloys.
Morsdorf, A.,Wallinder, I. O.,and Hedberg, Y.,Bioaccessibility of micron-sized powder particles of molybdenum metal, iron metal, molybdenum oxides and ferromolybdenum - Importance of surface oxides, Regul Toxicol Pharmacol, 2015.
Time-of-flight neutron powder diffraction data have been collected from Na2MoO4 and Na2WO4 to a resolution of sin (theta)/lambda = 1.25 A(-1), which is substanti-ally better than the previous analyses using Mo Kalpha X-rays, providing roughly triple the number of measured reflections with respect to the previous studies [Okada et al. (1974 ). Acta Cryst. B30, 1872-1873; Bramnik & Ehrenberg (2004 ). Z. Anorg. Allg. Chem. 630, 1336-1341]. The unit-cell parameters are in excellent agreement with literature data [Swanson et al. (1962 ). NBS Monograph No. 25, sect. 1, pp. 46-47] and the structural parameters for the molybdate agree very well with those of Bramnik & Ehrenberg (2004 ). However, the tungstate structure refinement of Okada et al. (1974 ) stands apart as being conspicuously inaccurate, giving significantly longer W-O distances, 1.819 (8) A, and shorter Na-O distances, 2.378 (8) A, than are reported here or in other simple tungstates. As such, this work represents an order-of-magnitude improvement in precision for sodium molybdate and an equally substantial improvement in both accuracy and precision for sodium tungstate. Both compounds adopt the spinel structure type. The Na(+) ions have site symmetry .-3m and are in octa-hedral coordination while the transition metal atoms have site symmetry -43m and are in tetra-hedral coordination.
Fortes, A. D.,Crystal structures of spinel-type Na2MoO4 and Na2WO4 revisited using neutron powder diffraction, Acta Crystallogr E Crystallogr Commun, 2015, 71, 592.
The crystallographic structure and morphology of redox active transition metal oxides have a pronounced effect on their electrochemical properties. In this work, h-MoO3 nanostructures with three distinct morphologies, i.e., pyramidal nanorod, prismatic nanorod and hexagonal nanoplate, were synthesized by a facile solvothermal method. The morphologies of h-MoO3 nanostructures were tailored by a controlled amount of hexamethylenetetramine. An enhanced specific capacitance about 230 F g(-1) at an applied current density of 0.25 A g(-1) was achieved in h-MoO3 pyramidal nanorods. Electrochemical studies confirmed that the h-MoO3 pyramidal nanorods exhibit superior charge-storage ability. This improved performance can be ascribed to the coexistence of its well exposed crystallographic planes with abundant active sites, i.e., hexagonal window (HW), trigonal cavity (TC) and four-coordinated square window (SW). The mechanism of charge-storage is likely facilitated by the vehicle mechanism of proton transportation due to the availability of the vehicles, i.e., NH4(+) and H2O. The promising, distinct and unexploited features of h-MoO3 nanostructures reveal a strong candidate for pseudocapacitive electrode materials.
Kumar, V., Wang, X., and Lee, P. S.,Formation of hexagonal-molybdenum trioxide (h-MoO3) nanostructures and their pseudocapacitive behavior, Nanoscale, 2015, 7, 11777.