Occurrence, Extraction, Production and uses of Molybdenum
About: This article is published in Studies in Inorganic Chemistry.The article was published on 1994-01-01. It has received 10 citations till now. The article focuses on the topics: Extraction (chemistry) & Molybdenum.
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TL;DR: In this article, a density functional theory (DFT) analysis of the O 2sp dominated valence band region of MoO 3 was carried out to examine the electronic structure of the oxide systems.
156 citations
TL;DR: In this paper, the structure and electronic properties of the α-MoO3 were studied with periodic LAPW calculations with the results in excellent agreement with the reported experimental pseudo-cubic results.
41 citations
TL;DR: In this paper, the authors performed cluster model studies by means of ab initio DFT method to examine electronic properties of different surface O atoms in several V2O5 and MoO3 systems and correlate them with catalytic behavior.
27 citations
TL;DR: In this paper, the electronic properties of MoO 2 bulk and (0, 1, 1) surface are discussed. But the surface retains the metallic character of the bulk, and it is found that Fermi level is located within the band dominated by d molybdenum orbitals.
Abstract: Electronic properties of MoO 2 bulk and (0 1 1) surface are discussed. It is found that Fermi level is located within the band dominated by d molybdenum orbitals, thereby reflecting the metallic character of the system. Results for (0 1 1)MoO 2 surface indicate that the surface retains the metallic character of the bulk. Depending on the thickness of the slab used to model the surface (1-layer or 2-layers) the electronic structure and properties change. In the 2-layer slab, bands close to the Fermi level originate both from regular six-fold coordinated Mo(6) centers as well as from five-fold coordinated Mo(5) centers occurring due to surface formation. In the 1-layer slab, peaks right below the Fermi level are dominated by the surface centers that are six-fold coordinated Mo(6) but also centers which are effectively four-fold coordinated Mo(4). This has a profound effect on the reactivity as was tested by a probe reaction of H 2 adsorption, which did not interact with the surface described by the 2-layer slab, but underwent dissociation on the 1-layer slab. The Mo–Mo pairs with bonds of approximately single character, characteristic for the bulk structure, are also present on the surface, both on 1-layer and 2-layer slabs. The local properties of (0 1 1)MoO 2 surface are very similar to other transition metal oxides. Metal–oxygen bonds are of a mixed ionic and covalent nature and the nucleophilicity of oxygen increases with the increase of coordination numbers of the corresponding oxygen atoms.
26 citations
TL;DR: In this article, the atomic structure of a reconstructed Mo(1/1/2)-O(2/2/3) surface has been revisited using photoelectron spectroscopy with synchrotron radiation, scanning tunneling microscopy, infrared reflection absorption spectrography and density functional theory.
22 citations
References
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TL;DR: In this paper, a series of MoSi2 compacts were fabricated at increasing hot-pressing temperatures to achieve different grain sizes, and the materials were evaluated by Vickers indentation fracture to determine room-temperature fracture toughness, hardness, and fracture mode.
Abstract: A series of MoSi2 compacts were fabricated at increasing hot-pressing temperatures to achieve different grain sizes. The materials were evaluated by Vickers indentation fracture to determine room-temperature fracture toughness, hardness, and fracture mode. From 1500° to 1800°C, MoSi2 had a constant 67% transgranular fracture and linearly increasing grain size from 14 to 21 μm. Above 1800°C, the fracture percentage increased rapidly to 97% transgranular at 1920°C (32-μm grain size). Fracture toughness and hardness decreased slightly with increasing temperature. MoSi2 processed at 1600°C had the highest fracture toughness and hardness values of 3.6 MPa.m1/2 and 9.9 GPa, respectively. The effects of SiO2 formation from oxygen impurities in the MoSi2 starting powders and MoSi2–Mo5Si3 eutectic liquid formation were studied.
37 citations
TL;DR: In this paper, it was found that the initial trioxide formed is highly oxygen deficient and that the composition depends on the method of preparation, varying from MoO2.98 to MoO 2.82 for an oxide prepared by decomposing the oxalate in nitrogen.
Abstract: High surface area molybdenum oxides have been prepared by the thermal decomposition and reduction of molybdenum (VI) oxalate. It has been found that the initial trioxide formed is highly oxygen deficient and that the composition depends on the method of preparation, varying from MoO2.98, for an oxide prepared by decomposing the oxalate in nitrogen, to MoO2.82 for an oxide prepared in vacuum. The rate of reduction was found to be dependent on: the method used to decompose the oxalate; the temperature of the decomposition; the partial pressure of hydrogen; and the partial pressure of water vapour. Vacuum-prepared trioxides are reduced in a single stage to MoO2–x. Nitrogen-prepared trioxides reduce first to Mo4O11 or Mo2O5 depending respectively on whether “wet” or “dry” conditions are used. In most cases the reduction-time curves are essentially sigmoidal in character. Possible reduction mechanisms and rate-determining steps are discussed.
37 citations
TL;DR: In this article, the thermal decomposition of ammonium di-, hepta-, tri-and octa-molybdate has been investigated by means of thermogravimetric analysis, ammonia determination and the use of an omegatron mass-spectrometer.
Abstract: The thermal decomposition of ammonium di-, hepta-, tri- and octa-molybdate has been investigated by means of thermogravimetric analysis, ammonia determination and the use of an omegatron mass-spectrometer. Both weight-decrease curves and ammonia evolution curves exhibit stepwise decomposition, indicating that there should be a difference in bonding strengths of water and of ammonia, presumably as ammonium ions in the crystal, with molybdate ions. Hepta-, di- and trimolybdate decompose in three stages, and octamolybdate in two stages. A certain crystal structure should correspond to each stage. Dehydration precedes deammoniation. Dehydration is complete at about 130, 150 and 280°C with hepta-, tri- and octa-molybdate respectively. Deammoniation is complete in all at the same temperature of 360∼375°C. The existence of nitrogen above 250°C in vacuo has been established. Nitrogen is yielded in the process of the thermal decomposition through oxidation as well as through the catalytic decomposition of ammonia....
37 citations
15 Mar 1937
TL;DR: In this paper, it was shown that at high temperatures, hydrogen dissolves to an appreciable extent in a number of metals (Sieverts and Gotta 1928), but at low temperatures the only examples of hydrogen dissolving appreciably without forming covalent links (as GeH4) or negative ions (as LiH) are the “metallic” hydrides of transitional elements such as palladium, tantalum, titanium, etc.
Abstract: An investigation of the properties of metallic hydrogen is of particular interest, on account of the simple structure of a metal composed of protons and electrons. A calculation of the energy change on forming a body centred lattice of metallic hydrogen from hydrogen atoms has recently been made by Wigner and Huntingdon (1935). They find that the energy of formation of metallic hydrogen from hydrogen atoms would be 10 kcal., and of metallic deuterium from deuterium atoms 11·6 kcal., but that metallic hydrogen (Graham’s hydrogenium) would only be stable relatively to covalent hydrogen at pressures not less than 2·5 × 105 atmospheres. Experiments on hydrogen at these pressures have not yet been realized. It is however possible to study the properties of metallic hydrogen in a number of alloys. At high temperatures, hydrogen dissolves to an appreciable extent in a number of metals (Sieverts and Gotta 1928), but at low temperatures the only examples where hydrogen dissolves appreciably without forming covalent links (as GeH4) or negative ions (as LiH) are the “metallic” hydrides of transitional elements such as palladium, tantalum, titanium, etc. The exceptional position of these low temperature alloys of metallic hydrogen is referred to again below. The evidence that hydrogen is in the metallic state has been reviewed (Ubbelohde 1931) and may be summarized by the statements that the hydrogen dissolves as atoms, and that at least a portion of these atoms is ionized to give electrons and protons.
34 citations
TL;DR: The structure of a discrete polyanion, Mo7O246−, in the crystal of ammonium heptamolybdate tetrahydrate, previously determined by Lindqvist, has been redetermined and refined by a two-dimensional, single crystal X-ray diffraction study.
Abstract: The structure of a discrete polyanion, Mo7O246−, in the crystal of ammonium heptamolybdate tetrahydrate, previously determined by Lindqvist, has been redetermined and refined by a two-dimensional, single crystal X-ray diffraction study. The positional parameters of the oxygen atoms, which were left undetermined in the previous investigation, have been determined. Lindqvist’s description of the structure still applies. However, the distortion of the polyanion has been found to be more pronounced than was previously assumed. Comparisons with molybdenum oxides and oxyacids are discussed with regard to the structure, the Mo–Mo distances and Mo–O distances.
34 citations