Showing papers on "Molybdenum published in 1994"

Fabrication and mechanical behaviour of Al2O3/Mo nanocomposites

Abstract: Two types of Al2O3/Mo composites were fabricated by hot-pressing a mixture of γ- or α-Al2O3 powder and a fine molybdenum powder. For Al2O3/5 vol% Mo composite using γ-Al2O3 as a starting powder, the elongated molybdenum layers were observed to surround a part of the Al2O3 grains, which resulted in an apparent high value of fracture toughness (7.1 Mpa m1/2). In the system using α-Al2O3 as a starting powder, nanometre sized molybdenum particles were dispersed within the Al2O3 grains and at the grain boundaries. Thus, it was confirmed that ceramic/metal nanocomposite was successfully fabricated in the Al2O3/Mo composite system. With increasing molybdenum content, the elongated molybdenum particles were formed at Al2O3 grain boundaries. Considerable improvements of mechanical properties were observed, such as hardness of 19.2 GPa, fracture strength of 884 MPa and toughness of 7.6 MPa m1/2 in the composites containing 5, 7.5, 20 vol% Mo, respectively; however, they were not enhanced simultaneously. The relationships between microstructure and mechanical properties are also discussed.

Isomerization of n-Heptane on an Oxygen-Modified Molybdenum Carbide Catalyst

Abstract: The isomerization of n-heptane in the presence of hydrogen has been carried out over a molybdenum carbide catalyst modified by an oxygen treatment. Heptane was isomerized selectively to isoheptanes, a reaction which is difficult over traditional bifunctional catalysts due to extensive cracking. The C[sub 7] products were mainly monomethylhexanes, 2-methylhexane and 3-methylhexane, in close to equilibrium ratios. A typical bifunctional catalyst (Pt supported on an acidic zeolite) gave similar isomerization products, but mostly propane and isobutane as the cracked products. The selectivity over the oxidized carbide was found to be a function of pressure but independent of the conversion; increased the hydrogen pressure led to a decrease in the C[sub 7] selectivity. This was found to be different from the Pt/zeolite catalyst, over which the selectivity was a function of the conversion; a high selectivity was only obtained at low conversions. The active carbide-based catalyst was probably an oxycarbide of molybdenum. The results obtained over the oxidized carbide catalyst are discussed in terms of a bond-shift mechanism via a metallocyclobutane intermediate.

Molybdenum : an outline of its chemistry and uses

Abstract: 1. Occurrence, extraction, production and uses of molybdenum (E.R. Braithwaite). 2. An introduction to the chemistry of molybdenum (M.L.H. Green). 3. The oxide chemistry of molybdenum (A.M. Chippindale, A.K. Cheetham). 4. Molybdenum sulphur chemistry (J.R. Dilworth). 5. The halides of molybdenum (R.L. Richards). 6. The organometallic and metal-organic chemistry of molybdenum (J.A. McCleverty). 7. The chemical nature of the molybdenum centres in enzymes (C.D. Gardner). 8. The biochemistry of molybdenum (R.-J.P. Williams). 9. Molybdenum requirements, toxicity and nutritional limits for man and animals (G.M. Ward). 10. Molybdenum compounds in heterogeneous catalysis (J. Haber). I 1. Homogeneous coordination catalysis with molybdenum complexes (A. Keller, J.M. Sobczak, J.J. Ziolkowski).

Multiple states of the molybdenum centre of dimethylsulphoxide reductase from Rhodobacter capsulatus revealed by EPR spectroscopy.

Abstract: The dimethylsulphoxide reductase of Rhodobacter capsulatus contains a pterin molybdenum cofactor molecule as its only prosthetic group. Kinetic studies were consistent with re-oxidation of the enzyme being rate limiting in the turnover of dimethylsulphoxide in the presence of the benzyl viologen radical. EPR spectra of molybdenum(V) were generated by reducing the highly purified enzyme under a variety of conditions, and with careful control it was possible to generate at least five clearly distinct EPR signals. These could be simulated, indicating that each corresponds to a single chemical species. Structures of the signal-giving species are discussed in light of the EPR parameters and of information from the literature. Three of the signals show coupling of molybdenum to an exchangeable proton and, in the corresponding species, the metal is presumed to bear a hydroxyl ligand. One signal with gav 1.96 shows a very strong similarity to a signal for the desulpho form of xanthine oxidase, while two others with gav values of 1.98 show a distinct similarity to signals from nitrate reductase of Escherichia coli. These data indicate an unusual flexibility in the active site of dimethylsulphoxide reductase, as well as emphasising structural similarities between molybdenum enzymes bearing different forms of the pterin cofactor. Interchange among the different species must involve either a change of coordination geometry, a ligand exchange, or both. The latter may involve replacement of an amino acid residue co-ordinating molybdenum via O or N, for a cysteine co-ordinating via S. Since the two signals with gav 1.96 were obtained only under specific conditions of reduction of the enzyme by dithionite, it is postulated that their generation may be triggered by reduction of the pteridine of the molybdenum cofactor from a dihydro state to the tetrahydro state.

Molybdenum Adsorption by Aluminum and Iron Oxides and Humic Acid

Abstract: It is hypothesized that soil adsorption properties can be estimated from contents and adsorption properties of individual soil components (additivity principle), if the data are consistent. Therefore, adsorption of molybdenum(VI) by synthetic aluminum oxide, ferrihydrite, goethite, extracted humic acid and a sandy soil sample was determined as a function of Mo concentration (0-1.2 mmol/L) and pH (3.5-8). For each pH and adsorbent, the Langmuir adsorption maximum (adsorption capacity) was calculated. For the oxides, plots of these capacities against pH resulted in adsorption envelopes exhibiting maxima at pH 4-5, i.e., near the pK a1 and pK a2 for molybdenum acid

Influence of molybdenum and tungsten additives on the properties of nickel steam reforming catalysts

Abstract: An introduction of small amounts of molybdenum and tungsten compounds into the nickel catalyst of the steam reforming of methane considerably reduces the detrimental effect of carbon deposit formation, while entailing no change in the catalyst activity.

New Developments in Deep Hydroconversion of Heavy Oil Residues with Dispersed Catalysts. 2. Kinetic Aspects of Reaction

Abstract: This paper discusses the results of hydrocracking experiments performed on a vacuum residue of Belaym crude in the presence of molybdenum naphthenate (MoN) as hydrogenation catalyst precursor and conducted in batch mode within a wide range of operating conditions and catalyst loading. The influence of severity on product distribution and quality is described as well as a kinetic analysis of the data. Results of this work demonstrate that the in-situ decomposition of MoN generates micrometer-sized particles of very active hydrogenation catalysts molybdenum disulfide utilizing the sulfur present in the feedstock.

Tungstate can substitute for molybdate in sustaining growth of Methanobacterium thermoautotrophicum. Identification and characterization of a tungsten isoenzyme of formylmethanofuran dehydrogenase.

Abstract: Methanobacterium thermoautotrophicum (strain Marburg) was found to grow on media supplemented with tungstate rather than with molybdate. The Archaeon then synthesized a tungsten iron-sulfur isoenzyme of formylmethanofuran dehydrogenase. The isoenzyme was purified to apparent homogeneity and shown to be composed of four different subunits of apparent molecular masses 65 kDa, 53 kDa, 31 kDa, and 15 kDa and to contain per mol 0.4 mol tungsten, <0.05 mol molybdenum, 8 mol non-heme iron, 8 mol acid-labile sulfur and molybdopterin guanine dinucleotide. Its molecular and catalytic properties were significantly different from those of the molybdenum isoenzyme characterized previously. The two isoenzymes also differed in their metal specificity: the active molybdenum isoenzyme was only synthesized when molybdenum was available during growth whereas the active tungsten isoenzyme was also generated during growth of the cells on molybdate medium. Under the latter conditions the tungsten isoenzyme was synthesized containing molybdenum rather than tungsten.

New Developments in Deep Hydroconversion of Heavy Oil Residues with Dispersed Catalysts. 1. Effect of Metals and Experimental Conditions

Abstract: A comparative study of the effect of experimental conditions on the deep hydroconversion (up to 90 %) of an heavy oil residue (Safaniya) in the presence of dispersed catalysts is reported. Severe thermal conditions are required to obtain a high conversion level. With molybdenum naphthenate as catalyst precursor, a low metal concentration (0.02 wt %) and a moderate hydrogen pressure level (7.5 MPa at cold) are efficient to maintain a low coke yield (2 wt %). In contrast to molybdenum naphthenate, a higher amount of phosphomolybdic acid is needed to achieve the same result. The reason for its weaker catalytic activity was attributed to a lower sulfidation state of the Mo catalyst, as outlined by physical characterization of the catalyst (XRD, HREM, STEM-EDX).

Method for preparation of organic molybdenum compounds

Abstract: A novel process for the preparation of 2,4-heteroatom substituted-molybdena-3,3-dioxacycloalkane compounds by reacting diol-, diamino-, thiol-alcohol and amino-alcohol compounds with a molybdenum source and in the presence of a phase transfer agent of the imidazoline type.

Coloration Dynamics of Spin‐Coated MoO3 · n H 2 O Electrochromic Films Fabricated from Peroxo‐polymolybdate Solution

Abstract: A very smooth amorphous molybdenum oxide thin film can be fabricated easily by a spin‐coating technique using a solution of peroxo‐polymolybdic acid, which is formed by the direct reaction of metallic molybdenum with hydrogen peroxide. Amorphous resulting from heating such a film at 80–150°C exhibited reversible electrochromism in an organic electrolyte solution. However, its electrochromic characteristics depended on the heating temperature. Coloration dynamics of films treated at various temperatures were investigated by a potential step method. When the film was treated at temperatures as high as 150°C, its coloration process was diffusion controlled, while the transient current observed with films treated at lower temperature did not obey the Cottrell‐type relationship. Such behavior is discussed in relation to the structural features of the amorphous films.

Process to prepare amino carboxylic acid salts

Abstract: An improved process is disclosed to prepare an amino carboxylic acid salt. According to the process, an aqueous solution of an amino alcohol is contacted with an alkali metal hydroxide in the presence of an effective amount of a copper catalyst that has from about 10 parts per million to about 50,000 parts per million of an element selected from the group consisting of chromium, titanium, niobium, tantalum, zirconium, vanadium, molybdenum, manganese, tungsten, cobalt, nickel, bismuth, tin, antimony, lead, germanium, and mixtures thereof.

Autocorrelation function analysis of phase formation in the initial stage of interfacial reactions of molybdenum thin films on (111)Si

Abstract: Autocorrelation function (ACF) analysis has been applied to the high resolution transmission electron microscope images of amorphous interlayers formed in the interfacial reactions of ultrahigh vacuum deposited molybdenum thin films on (111)Si. Mo3Si was identified to be the first nucleated crystalline phase which is correlated to the stable structure of the amorphous Mo‐Si alloy. Both Mo3Si and Mo5Si3 were found to form simultaneously under certain annealing conditions. The results demonstrate the usefulness of the ACF analysis of atomic images in amorphous interlayers formed in the initial stage of reactions in metal thin films on silicon.

XPS analysis of lithium intercalation in thin films of molybdenum oxysulphides

Abstract: Thin films of molybdenum oxysulphides which could be used as positive electrode materials in microbatteries were analysed by XPS. The results have shown the existence of a new environment for molybdenum with S 2 2- pairs, S 2- and O 2- ions. The evolution of these thin films MoOySz at different stages of their cycle in experimental generators has also been studied; the main elements involved in the redox processes have been characterized: molybdenum atoms with+6 formal oxidation number and disulphide pairs S 2 2-

Sodium diffusion, selenization, and microstructural effects associated with various molybdenum back contact layers for CIS-based solar cells

Abstract: By varying the argon pressure during deposition, the authors have prepared a set of sputtered molybdenum films on soda-lime glass substrates with a range of mechanical and electrical properties. These films were subsequently exposed to several of the processing steps used in the fabrication of copper-indium-diselenide (CIS) solar sells. Processing steps of interest include heating in a vacuum, exposure to selenium vapor at elevated temperatures, and deposition of CIS and CIGS layers over the Mo. Resulting Mo films and structures were subsequently characterized using XPS, SEM, Auger, and SIMS. Here, they describe the results of these experiments and their implications for CIS cell fabrication.

Corrosion protection of high copper aluminium alloys by surface modification

Abstract: In a project devoted to the elimination of toxic materials in methods of corrosion protection, a surface modification process has been developed to improve the pitting resistance of high copper aluminium alloys such as 2024 and 7075 without the use of chromates. This process involves immersion in boiling Ce(NO3)3, anodic polarisation in Na2MoO4, and immersion in boiling CeCl3. Very corrosion resistant surfaces were produced on 7075–T6 after applying a pretreatment step which removes copper from the outer surface layers. A chemical and an electrochemical copper removal process have been developed with the actual procedure depending on alloy composition. During exposure in 0·5M NaCl for 30 days no measurable corrosion was indicated by continuous monitoring with electrochemical impedance spectroscopy or by visual observation. For 2024, significant improvement in the resistance to localised corrosion was also obtained. Surface analysis showed increased levels of cerium and molybdenum at locations whic...

The Structure of Highly Dispersed SiO2-Supported Molybdenum Oxide Catalysts during Sulfidation

Abstract: The structure of sulfided Mo-catalysts and their oxidic precursors has been abundantly studied, but the genesis of the active phase has remained much less investigated. The sulfidation (in H2S/H2 atmosphere) of a series of MoO3/SiO2 catalysts has been examined by means of temperature-programmed sulfidation, X-ray absorption fine structure, and transmission electron microscopy.The oxidic, oligomeric clusters in a 5.6 wt 5% MoO3/Si02 catalyst are transformed into partly sulfided particles (MOOS,,) by 0-S exchange at room temperature. A molybdenum sulfide species the structure of which resembles the MoS, structure is formed during sulfidation at 423 K. The MoS2 phase is formed at temperatures between 523 and 573 K, depending on the dispersion of the initial Moo3 phase. The transition of MoS3 into MoS2 can be monitored by the evolution of H2S from the catalyst with a simultaneous consumption of H2. The two-dimensional size of the MoS2 slabs can be derived from the EXAFS Mo-Mo coordination number by means of a theoretical model. TEM is required to elucidate the stacking height of the slabs. The application of supported molybdena-based catalysts in industrial processes, such as partial oxidation’-5 and hydrotreating,+lB has rendered supported molybdenum oxide catalysts a main subject of investigations. Many studies are dedicated to the preparation and the structure of Moo3 catalysts on different supports. The sulfded counterpart of the oxidic precursors of Co(Ni)/Mo(W) catalysts is one of the most thoroughly investigated catalyst systems. Conversion of the oxidic catalyst into the active sulfide occurs under reaction conditions, due to presulfiding or the presence of sulfur-containing feedstocks. It is industrial practice to “spike” the feedstock with agents, such as CS2, dimethyl sulfide (DMS), or dimethyl disulfide (DMDS), to accelerate the sulfidation of the catalyst.I9-20 As an alternative, thecatalyst can besulfided exsitu by impregnation with a sulfurcontaining liquid. In situ heating of the catalyst subsequently leads to sulfidation.2’ In most of the laboratory tests the catalyst is sulfided by a mixture of H2S/H2 at high temperat~re~~v~~ or in a temperature program. A number of models have been proposed to explain the activity of the Co(Ni)/Mo(W) catalysts. Consensus exists on the structure of the molybdenum sulfide phase: under typical hydrotreating conditions (i.e., p = 10-200 bar, T = 573-723 K, excess H2, and a sulfur-rich feedstock”) MoS2 is present, in which molybdenum is trigonal prismatically coordinated by sulfur However, the position and the role of the cobalt atoms in the catalysts are still debated. The earlier proposed models are based on the assumptionof a monolayer,34~ontactsynergy,’~J~ inter~alation,3S33~ and remote control.37 The most popular model is based on publications by Topsoe et al.30 and by Wive1 et a1.38 The latter reported a linear relationship between the activity in thiophene HDS and the concentration of Co present in the “Co-Mo-S” species. This phase had been designated as ‘COMOS”~~ because its Mbssbauer spectrum did not resemble any of the known cobalt sulfide or cobalt molybdenum sulfide spectra. The CoMoS species is envisioned as a MoS2 layer (slab), with cobalt atoms decorating the edges.30J9 Since the introduction of this model, some