The high temperature behaviour of the molybdenum (VI) -sulphur (-II) -water system in the presence of an ammoniacal buffer was studied. At 150°C. all species of the form MoO(4-x)S(2¯/x) were shown to exist and the stability constants of mono-, di-, tri-, and tetra-thiomolybdate were 2.3 x 10(2) M.(-1), 3.5 x 10(5) M.(-2), 2.7 x 10(8) M.(-3), 7.0 x 10(10) M.(-4), respectively. There were strong indications of the formation of protonated species, Mo(SH)(6), in solutions containing low concentration of free ammonia. Application of reducing gases to this system produced a mixture of a sulphide and an oxide of molybdenum, whose composition depended on the initial composition of the solution. Under hydrogen, the reduction reaction was autocatalytic, rate being first order in product amount and hydrogen pressure. A mechanism was proposed in which the rate determining step was heterogeneous activation of hydrogen on the surfaces of precipitates followed by two paths, one to produce the sulphide and the other to produce the oxide. The proportion of the sulphide to the oxide was dependent on the solution composition, the higher the fractional distribution of tetra-thiomolybdate and the higher the concentration of hydrogen ion, the more the sulphide being produced. Under carbon monoxide the reduction reaction was found to have an induction period. The molybdenum in solution then followed a linear decrease in concentration with time. The slope of this plot showed Langmuir type of dependence on both molybdenum concentration and pressure. A mechanism was proposed in which the rate determining step was a slow decomposition of some complex between thiomolybdates and carbon monoxide adsorbed strongly on catalytic precipitate which was produced during the induction period. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

A study has been made of the oxidation of molybdenite in solutions of hypochlorite. The reaction was found to be first order in reagent concentration with a reaction rate constant of 1.90 x 10⁻² min⁻¹ cm⁻² at pH 9 and 45°C. The oxidation rate increases with increasing pH and later decreases if the pH is raised above 9.5. An activation energy of 6.3 ± 0.8 kcal/mole was observed for this reaction which was controlled by either chemical or mixed diffusion and chemical reactions. Other factors studied were the agitation, surface area, surface characteristics and effect of sulfates, chlorides, chlorites and chlorates. It was observed that hypochlorite is capable of selectively leaching molybdenite from copper sulfides and positive results were obtained when applying this property to the extraction of molybdenite from copper rougher concentrates. The formation of oxides during drying and/or leaching of copper sulfides was found to be detrimental to the process because they act as catalysts in the decomposition reaction of hypochlorite. Rates of normal decomposition of the reagent were studied over the range 35 to 60°C and pH 7 to 10 and found to be negligible as compared to the rates of oxidation of molybdenite. The leaching of rougher concentrates with hypochlorite was found not to affect considerably the subsequent flotation operation. Data of on-site hypochlorite generator plants and factors that affect production and decomposition, have been presented with the purpose of proposing a process using the information obtained in this work for the extraction of molybdenite in the Copper Concentration Plants. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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Thesis--University of Wisconsin--Madison. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 188-196).

Thesis (M.S.)--West Virginia University, 2005. / Title from document title page. Document formatted into pages; contains vii, 42 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 40-42).

Based on rst principles calculations the structural, mechanical stability and electronic properties of molybdenum chalcogenides 2H-MoX2 (X = S, Se, Te) have been studied using density functional theory (DFT). The generalized gradient approximation (GGA) proposed by Perdew, Burke and Ernzerhof (PBE) was employed together with the projector augmented plane wave (PAW) method. The van der Waals interactions in the Grimme (DFT-D2) and Lundqvist and Langreth (vdW-DF) approximations have been added on top of PBE or revised PBE in order to take into account the weak interactions between layers of 2H-MoX2 (X = S, Se, Te). The structural properties include the equilibrium lattice parameters, the cohesive energy and the formation energy. Besides we have studied the mechanical stability of these compounds by examining the elastic constants using the PBE, vdW-DF, and DFT-D2 approximations. Other quantities related to the mechanical properties such as the Young's modulus, the Poisson's ratio and the bulk modulus were also calculated. Electronic properties of bulk 2H-MoX2 (X = S, Se, Te) have been investigated using density functional theory (via band structure and projected density of state), and a partially self-consistent GW (GW0) approximation. We have investigated electronic properties ( band structure) for monolayers 1H-MoX2 (X = S, Se, Te) using a single shot GW (G0W0) approximation. Optical properties for monolayer 1H-MoX2 (X = S, Se, Te) were studied using the Bethe-Salpeter equation (BSE).

We have conducted an experimental investigation of the solubility and speciation of Mo in HCl-, NaCl-, and NaOH-bearing water vapour, and of the partitioning of Mo between coexisting aqueous liquid and vapour at 300 to 370°C and up to saturated pressure. Our results indicate that Mo concentration is enhanced in HCl-bearing water vapour at fHCl > 0.1 bar, and in NaOH-saturated water vapour, but is unaffected by the presence of NaCl. This suggests that Mo speciates as MoO3·nH2O in water vapour at equilibrium with NaCl or fHCl < 0.1 bar. The dependence of SigmafMo on fHCl at higher acidity points to the formation of Mo oxychloride (MoO2Cl 2). For the system MoO3-NaOH-H2O, log Sigma fMo increases with increasing fH2O, and with log SigmafNa in a ratio of 0.28+/-0.4, but Sigma fNa does not change with increasing fH2O. This suggests the formation of a small proportion of sodium molybdate (Na 2MoO4) in addition to MoO3·nH2O. Our partitioning experiments show that at lower temperature and fluid density, Mo partitions more strongly into the liquid than the vapour, but the Mo concentration in the vapour increases as the temperature-pressure conditions approach those of the critical point of water (374°C and 221 bar), surpassing that in the liquid at ∼360°C. The results of our experiments indicate that both the liquid and vapour phases may be important for the transport of Mo in porphyry ore-forming systems, and that vapour-phase solubility is enhanced in high fHCl magmatic gases.

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This research has been directed at the study of both the enzyme sulfite oxidase and molybdenum model chemistry. A modification of a previously published procedure has been used to purify sulfite oxidase in high yield which is well-suited for experiments requiring prosthetically intact enzyme and which is not contaminated with extraneous heme or with other redox active proteins. Laser flash photolysis was used to study the reaction of photoproduced 5-deazariboflavin, lumiflavin, and riboflavin semiquinone radicals with the redox centers of purified sulfite oxidase. Two distinctly different intramolecular electron transfer processes were observed between the molybdenum and heme sites of the enzyme, and these assignments were supported by flash photolysis studies of the cyanide-inactivated enzyme and the sulfite oxidase heme peptide. Microcoulometric experiments on sulfite oxidase have shown that the enzyme requires the addition of three electrons for complete subunit reduction. Midpoint potentials for the Mo(VI)/Mo(V), Mo(V)/Mo(IV), and Fe(III)/Fe(II) couples have been obtained under varied buffer conditions. The midpoint potentials obtained under High-pH and Low-pH conditions provided a means for reductively titrating the enzyme to the Mo(V) oxidation state for EXAFS studies. EXAFS of sulfite oxidase under High-pH and Low-pH conditions have provided the first example of a structural study of the three accessible oxidation states (Mo(VI), Mo(V), and Mo(IV)). A biologically relevant synthetic model for the formation of the Mo(V) Low-pH form of sulfite oxidase has been developed. The Mo(V) model compound closely resembles the minimum coordination environment for the Mo(V) Low-pH form of sulfite oxidase as determined by EXAFS. Using synchrotron radiation, molybdenum L-edge x-ray absorption spectra have been obtained for a variety of oxomolybdenum(V) compounds which serve as models for sulfite oxidase. An attempt has been made to correlate unique features of the molecules to the observed 2P → 4d electronic transitions.

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