Occurrence, Extraction, Production and uses of Molybdenum

Abstract: Ab initio density functional theory cluster studies on the MoO 3 (0 1 0) surface as well as ultraviolet photoemission (UPS) experiments on well-crystallized single phase Mo oxides are carried out to examine the electronic structure of the oxide systems. In addition, electronic details of different surface oxygen vacancies are studied by appropriate vacancy clusters. Calculations on embedded clusters as large as Mo 15 O 56 H 22 confirm the mixed covalent/ionic character of the oxide. The computed width of the O 2sp dominated valence band region of MoO 3 , about 7 eV, agrees well with the present photoemission data for MoO 3 (0 1 0) samples. The overall shape of the computed densities of states (DOS) in the O 2sp region of MoO 3 is rather similar to the measured UPS intensity curves indicating weak energy dependence of corresponding transition matrix elements. Calculated vacancy energies for the different surface oxygen sites at MoO 3 (0 1 0) yield rather large values, 6.8–7.6 eV, which shows that oxygen is bound quite strongly to the substrate. Vacancy formation leads to reduction of neighboring molybdenum centers which expresses itself by increased metal d electron occupation and corresponding DOS contributions above the O 2sp region. This is consistent with the experimental UPS data for MoO 3 (0 1 0) where oxygen vacancies have been introduced by mild ion bombardment. It is further supported by the present UPS data for well-crystallized intermediate molybdenum oxides, such as Mo 18 O 52 , Mo 8 O 23 , or Mo 4 O 11 . These oxides show, depending on the degree of reduction, one or two additional peaks above the valence band. Characteristic changes in the intensity ratios of the O 2sp peaks can be interpreted on the basis of the theoretical DOS results as a preferential loss of bridging oxygen from the MoO 3 lattice. Mild ion bombardment, a technique which is often used to clean surfaces in UHV experiments, results in the case of MoO 3 (0 1 0) in considerable surface reduction. The reduced Mo species is comparable to that in MoO 2 as indicated by the UP spectra. Therefore, mild ion bombardment cannot be considered a suitable tool for the preparation of molybdenum oxide surfaces.

Abstract: Structure and electronic properties of the α and β-MoO3, have been studied with periodic LAPW calculations. The structure and electronic properties of the α-MoO3 are in quite agreement with experimental and previous theoretical results. The oxide is partially ionic and the symmetrically bridging oxygens exhibit more ionic feature while the terminal oxygens are more covalent. The lattice scaling of the β-MoO3 give results in excellent agreement with the reported experimental pseudo-cubic results. It has been found from density of states (DOS) that β-MoO3 is not fully ionic system and some covalent contributions are still appreciable. These covalent contributions to the bonding appear derisory compared to the covalent contributions of the α phase. The β-MoO3 → α-MoO3 transformation is explained by metal off-center displacement toward O1 (and a little less toward O2) centers which is stabilized by an increase in covalency between the Mo and oxygen atoms.

Abstract: Cluster model studies by means of ab initio DFT method are performed to examine electronic properties of different surface O atoms in several VOX systems and to correlate them with catalytic behavior. Since chosen compounds vary in metal oxidation state and in X element, reactivity of differently coordinated surface O is studied as a function of different parameters such as coordination number, geometrical and chemical environment. In addition, the mobility of different surface oxygen species, formation of surface oxygen vacancies and the adsorption of O2 molecule at the different vacancy is discussed on the examples of the V2O5 and MoO3 systems.

Abstract: The atomic structure of a reconstructed Mo(1 1 2)–O(2 × 3) surface has been revisited using photoelectron spectroscopy with synchrotron radiation, scanning tunneling microscopy, infrared reflection absorption spectroscopy and density functional theory. In contrast to previous models, the results are rationalized in terms of the formation of one-dimensional, Mo O terminated molybdenum oxide involving corner sharing distorted [MoO 6 ] octahedra on the (1 × 3) reconstructed Mo(1 1 2) surface.

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.

Abstract: The problem of the nature of the interatomic farces in the elementary metals and in intermetallic compounds and other alloys continues to be puzzling, despite the clarification of some questions which has been provided by quantum mechanical considerations. It has been my opinion, contrary to that of other investigator that the metallic bond is very closely related to the covalent (shared-electron-pair) bond, and that each atom in a metal may be considered as forming covalent bonds with neighboring atoms, the covalent bonds resonating among the available interatomic positions. It was shown in the first paper of this series that the number of covalent bonds resonating among the available positions about an atom (the metallic valence of the atom) increases from one to nearly six (5.78) in the sequence K, Ca, Sc, Ti, V, Cr in the first long period of the periodic table, remains nearly constant from Cr to Ni, and begins to decrease with Cu. This concept, which is substantiated by the magnetic properties of the metals and their alloys, provides a qualitative explanation of many properties of the transition metals (including those of the palladium and platinum groups), such as characteristic temperature (heat capacity at low temperatures), hardness, compressibility, coefficient of thermal expansion, and the general trend of interatomic distances. It will be shown in the following pages that it also permits the formulation of a system of atomic radii which can be used for the calculation of interatomic distances in metals and intermetallic compounds and for the interpretation of observed interatomic distances in terms of the electronic structure of the crystals. These atomic radii (which may be called metallic radii) are found, as expected, to show an intimate relation to the covalent radii of the atoms-a relation which, in its general nature, permitted Goldschmidt over twenty years ago to use data taken from both metals and ordinary covalent crystals in formulating a table of atomic radii, and which has been recognized3 as providing very strong support for the concept that metallic bonds are essentially resonating covalent bonds.

Abstract: An equation is presented for the evaluation of electronegativity from the force of attraction between a nucleus and an electron from a bonded atom. From this simple electrostatic approach, the electronegativities of forty-four elements are calculated. The results are compared with certain previously proposed scales of electronegativity.

Abstract: Copper and molybdenum deposits, potassic, phyllic, argillic, and propylitic alteration, emphasis on San Manuel-Kalamazoo deposit of Arizona, tabulated data on characteristics of North and South American deposits