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Showing papers on "Molybdenum published in 1991"




Journal ArticleDOI
TL;DR: Alumina-supported molybdenum carbide catalysts have been prepared from MoO{sub 3}/Al{sub 2}O{Sub 3} by reduction with H{sub 4} followed by carburization, direct carburisation with CH{sub 6}/H{sub 7] followed by nitriding with NH{sub 5} as discussed by the authors.

160 citations


Journal ArticleDOI
TL;DR: In this article, unsupported molybdenum carbides, β-Mo 2 C and α-MoC 1-x, promoted by K 2 CO 3 were studied as catalysts for carbon monoxide-hydrogen reactions at 573 K and 8.0 MPa.

156 citations




Journal ArticleDOI
TL;DR: In this paper, a new interpretation of the Mo(3d) XPS spectra of reduced supported Mo catalysts is proposed, based on the assumption of a linear relation between the binding energies and the oxidation numbers of Mo states, with a slope of 0.8 eV per oxidation state.
Abstract: A new interpretation of the Mo(3d) XPS spectra of reduced supported Mo catalysts is proposed. It is based on the assumption of a linear relation between the binding energies and the oxidation numbers of Mo states, with a slope of {approx equal}0.8 eV per oxidation state. The proposal is substantiated by the results of an XPS investigation of the thermal decomposition of MoO{sub 3} and by a survey of evidence published in the literature. It implies that two different Mo(IV) states have to be considered, one with a binding energy obeying the linear relation with the oxidation state (isolated Mo(IV)) and a second one deviating for structural reasons (paired double-bonded Mo(IV), e.g., in MoO{sub 2}). The application of this approach to the fitting of Mo(3d) signal shapes in the spectra of reduced supported Mo catalysts yields estimates for the amounts of Mo(VI), Mo(O), and a state that may be Mo(II) or paired double-bonded Mo(IV) ions. For Mo(V) and isolated Mo(IV), ranges for possible amounts may be given.

138 citations


Journal ArticleDOI
TL;DR: The influence of impregnation pH and molybdenum content on the surface structure of catalysts has been studied by temperature-programmed reduction, X-ray diffraction and zeta potential measurements as discussed by the authors.

129 citations



Journal ArticleDOI
TL;DR: In this paper, the exothermicity of the reaction between molybdenum and silicon raised the temperature of the product to 1886 K, which is close to the theoretical adiabatic combustion temperature, 1900 K.
Abstract: Molybdenum disilicide was synthesized from elemental reactants in argon and vacuum atmospheres by utilizing the exothermicity of the reaction using self-propagating high temperature synthesis. Experiments were carried out using powdered reactants and compacts with varying densities. The reaction front propagated at a finite velocity depending upon the atmosphere, the diameter of the pellet and the particle sizes of the reactants. The exothermicity of the reaction between molybdenum and silicon raised the temperature of the product to 1886 K, which is close to the theoretical adiabatic combustion temperature, 1900 K. X-ray diffraction analysis of the product confirmed the product to be a single phase MoSi2 crystallizing in a tetragonal structure. Microstructural examination revealed melting of Si and its capillary flow, and chemical analysis indicated that the product is much purer than the reactants.

121 citations


Journal ArticleDOI
TL;DR: In this paper, a combined Raman spectroscopy-X-ray absorption study showed a significantly altered structure of the molybdenum oxide phase after dehydration, the total Mo-Mo coordination number drops from 3.27 to 0.20 after anin situ thermal treatment at 673 K.
Abstract: In contrast to the frequently reported lack of interaction between hexavalent molybdenum and SiO2 and the tendency of silica-supported MoO3 to coalescence, it has been found that on dehydration small molybdenum oxide clusters spread over a silica support. A combined Raman spectroscopy-X-ray absorption study shows a significantly altered structure of the molybdenum oxide phase after dehydration. In EXAFS the total Mo-Mo coordination number drops from 3.27 to 0.20 after anin situ thermal treatment at 673 K. The increase of the peak in the XANES region (Is -→ 4d) indicates that the coordination sphere of the molybdenum atoms strongly alters after dehydration. The Raman spectra reflect the change of the structure through a shift of the position of the terminal Mo=O bond from 944 to 986 cm−1 and the disappearance of the bridged Mo-O-Mo vibration at 880 cm−1. It is concluded that dehydration produces almost isolated molybdenum sites in this highly dispersed sample. Water ligands stabilize the oligomeric clusters under ambient conditions; the removal of water causes spreading of these clusters.

Journal ArticleDOI
TL;DR: In this paper, a range of mixed Mo/V/Nb oxide catalysts were investigated for the oxidative dehydrogenation of ethane, and a correlation was observed between the ease of reduction of the catalyst and the activity and selectivity, with the most easily reduced catalyst being the most active and selective for the formation of ethene.

Journal ArticleDOI
TL;DR: In this article, the role of platinum in aluminide coatings on selected nickel-based superalloys was investigated using various microscopy, spectroscopy and diffraction techniques.
Abstract: An investigation has been carried out to determine the role of platinum in aluminide coatings on selected nickel-based superalloys. Various microscopy, spectroscopy and diffraction techniques were used to characterize the microstructure. It is found that platinum improves the protective nature of the coating by (1) increasing its diffusional stability, (2) eliminating chromium-rich precipitates from the outer coating layer and (3) preventing refractory transition elements such as molybdenum, vanadium and tungsten from diffusing into the outer coating layer. However, the oxidation behaviour of the coating varies from one alloy substrate to another depending upon its chemical composition and, in particular, the rare earth elements.

Journal ArticleDOI
TL;DR: In this article, the synthesis of the silicides of molybdenum (Mo3Si, Mo5Si3, and MoSi2) by self-propagating combustion method is investigated.
Abstract: The synthesis of the silicides of molybdenum (Mo3Si, Mo5Si3, and MoSi2) by the self-propagating combustion method is investigated. Only the reactants corresponding to the last phase can be reacted in a self-sustaining mode without preheating. The product of such a reaction is single-phase MoSi2. Although reactant mixtures corresponding to the other two silicides can react in a self-sustaining mode with prior heating, the products of combustion were multi-phase. The dependence of the combustion temperature and velocity on the stoichiometry of the silicide was determined. The activation energy for the combustion synthesis of MoSi2 determined in this investigation, 139.4 kJ mol−1, is considerably lower than activation energies reported for the diffusion of silicon in molybdenum or in MoSi2.








Journal ArticleDOI
TL;DR: In this paper, metal and ceramic materials were sprayed on polished steel and iron substrates using atmospheric arc (nickel, molybdenum) and plasma (Al 2 O 3, ZrO 2 -7Y 2O 3 ) spraying.
Abstract: Metal and ceramic materials were sprayed on polished steel and iron substrates using atmospheric arc (nickel, molybdenum) and plasma (Al 2 O 3 , ZrO 2 -7Y 2 O 3 ) spraying. The bonding quality at the substrate-coating interface depends on the size and depth of the contact zones. The chemical-metallurgical interactions (diffusion, reactions) between the sprayed particles and the metal substrate as a function of the properties of the sprayed materials, contact temperature and solidification time were analysed. Higher contact temperature and longer interaction time for spraying of metals result in better bonding. Moreover, oxidation of the metal substrate to a limited degree by spraying of oxides allows one to improve the bonding quality. It is possible to correlate the results of microscopical investigations of the interface phenomena with the results of quantitative measurement of the bonding tensile strength.

Patent
09 Dec 1991
TL;DR: In this article, the authors describe an ethylene oxide process using a carrier consisting of at least 85 percent by weight of alpha alumina, from about 0.01 to about 6 percent of an added alkaline earth metal in the form of an oxide.
Abstract: The invention relates to ethylene oxide catalysts containing silver, alkali metal promoters, rhenium promoters and optionally rhenium co-promoters selected from sulfur, molybdenum, tungsten, chromium and mixtures thereof supported on a carrier consisting of at least 85 percent by weight of alpha alumina, from about 0.01 to about 6 percent by weight (measured as the oxide) of an added alkaline earth metal in the form of an oxide, from about 0.01 to about 5 percent by weight (measured as the dioxide) of added silicon in the form of an oxide and from zero to about 10 percent by weight (measured as the dioxide) of added zirconium in the form of an oxide. The invention also relates to an ethylene oxide process using the above catalyst.

Journal ArticleDOI
TL;DR: In this article, six samples of silica-supported molybdenum disulfide were prepared by decomposition of MOS3/SiO2 in argon and hydrogen at 623, 673, and 723 K. The hexagonal MOS2 structure was confirmed by XRD.

Journal ArticleDOI
01 Jan 1991-Zeolites
TL;DR: In this paper, the results of the water-gas shift reaction were reported for sulfided MoHNaY zeolites prepared from Mo(CO)6 and ammonium heptamolybdate.

Journal ArticleDOI
TL;DR: In this article, it was shown that the "glassy" phase observed in powder-processed MoSi 2 consists of amorphous SiO 2 and that formation of this phase is intrinsic to powder processing.
Abstract: MoSi 2 has been produced by hot pressing of commercial and in-house powders and also by arc melting of pure molybdenum and silicon. It is shown that the “glassy” phase observed in powder-processed MoSi 2 consists of amorphous SiO 2 and that formation of this phase is intrinsic to powder processing. In addition, the Mo 5 Si 3 phase, which is often present in MoSi 2 , is found to be a carbon-stabilized form of approximate composition CMo 5 Si 3 .


Journal ArticleDOI
TL;DR: In this article, the hydrogenation of carbon dioxide was carried out on iron catalysts, either unpromoted or promoted with chromium, manganese, or molybdenum.

Patent
03 Apr 1991
TL;DR: In this article, a fixed-bed multipipe reactor is used to produce unsaturated aldehyde or an unsaturated acid by catalytically oxidizing propylene or at least one compound selected from isobutylene, tert-butyl alcohol and methyl-tert.-butyl ether in a gaseous phase with molecular oxygen or a molecular oxygen-containing gas.
Abstract: In a process for producing an unsaturated aldehyde or an unsaturated acid by catalytically oxidizing propylene or at least one compound selected from isobutylene, tert.-butyl alcohol and methyl-tert.-butyl ether in a gaseous phase with molecular oxygen or a molecular oxygen-containing gas using a fixed bed multipipe reactor, characterized in that (a) a plurality of composite oxides different in occupied volume, represented by formula MoaWbBicFedAeBfCgDhOx ... (I) wherein Mo denotes molybdenum; W denotes tungsten; Bi denotes bismuth; Fe denotes iron;A denotes at least one element selected from cobalt and nickel; B denotes at least one element selected from an alkali metal, an alkaline earth metal and thallium; C denotes at least one element selected from phosphorus, tellurium, arsenic, boron, niobium, antimony, tin, lead, manganese, cerium and zinc; Ddenotes at least one element selected from silicon, alminum, titanium and zirconium; Odenotes oxygen; a, b, c, d, e, f, g, h and x denote numbers of atoms of Mo, W, Bi, Fe, A, B, C, D and O; when a=2 to 12, b=0 to 10 and a+b=12, c=0.1 to 10, d=0.1 to 10, e=2 to 20, f=0.005 to 3, g=0 to 4, h=0.5 to 30 and x=value determined by an oxidized state of each element, are used as catalysts, (b) a plurality of reaction zones are provided along an axial direction in each reaction pipe of the fixed bed multipipe reactor, and (c) the plurality of the catalysts different in occupied volume are filled in the plurality of the reaction zones such that the occupied volumes become lower from the starting gas inlet side to the outlet side.

Journal ArticleDOI
TL;DR: In this article, carbon-supported molybdenum HDS catalysts were investigated using X-ray photoelectron spectroscopy and temperature-programmed desorption, Xray diffraction, titrations and electrophoresis.