High Surface Area Molybdenum Research Articles

Synthesis of High Surface Area Molybdenum Nitride in Mixtures of Nitrogen and Hydrogen

The production of high surface area (150 m 2/g, passivated) unsupported γ-Mo 2N by temperature programmed reaction of MoO 3 powder with mixtures of H 2 and N 2 is reported. The addition of 1290 ppm H 20 or more to the synthesis gases leads to reduced product surface areas by either hydrothermal sintering or lattice fluidization mechanisms. Reduced surface areas in syntheses with lower N 2/H 2 space velocities and higher temperature ramping rates are attributed to increased concentrations of H 2O evolved by reaction. Elevated H 2O concentrations increase the temperature required for solid reduction. Observed reaction intermediates include MoO 2, Mo, and an unidentified molybdenum oxide, hydroxide, or hydrate. Intermediates in topotactic syntheses exhibited intermediate surface areas (up to 60 m 2/g). A thermodynamic analysis indicates that, in most cases, the intermediate solids are not in equilibrium with the gas phase and that solids may be reduced completely to Mo before nitridation. It is concluded that the rate of the gas/solid reaction rate is determined primarily by the rate of oxygen and nitrogen diffusion in the solid lattice but that competitive adsorption of H 2O and H 2 also influences the rate of the gas/solid reaction.

Gas Phase Synthesis of Molybdenum and/or Iron Nitrides, Carbides and Sulfides

ABSTRACTThe thermolytic decomposition of Mo(CO)6 with hydrogen sulfide or ammonia vapor (in a He carrier stream) at temperatures ranging from 300 to 1100 °C produces high surface area molybdenum sulfides (MoS2 or Mo2S3) or molybdenum carbides (hexagonal Mo2C) and carbonitrides, (hexagonal MoN(C) or cubic Mo2N(C)), respectively. The MoS2 surface areas range from 16.7 to 82.0 m2/g, while the surface areas of molybdenum carbides and carbonitrides vary from 14.9 to 21.1 m2/g. The maximum surface area for MoS2 is achieved at 500 °C and decreases with increasing or decreasing temperature. The surface area of the carbonitrides formed from 300 to 800 °C increases with increasing temperature up to 950 °C, where lower surface area Mo2C is formed. Crystallographically pure hexagonal MoN is prepared by decomposing Mo(CO) 6 in pure ammonia. Fe(CO) 5 decompositions in ammonia produce FexZ (where 5.8≥x≥1.6 and Z=C and N), and in some cases elemental Fe. Hexagonal Fe3 N(C) forms when Fe(CO) 5 is thermolyzed in ammonia from 300 to 600 °C, with surface areas ranging from 9.5 to 13.7 m2/g, whereas orthorhombic Fe3C and cubic Fe are produced at 700, 800, 900 and 1000 °C with surface areas of 6.7, 7.6, 2.2 and 2.0 m2/g, respectively. Within the same phase, the surface areas of the carbonitrides increase with increasing reaction temperature. These iron and molybdenum carbonitrides catalyze the conversion of CO/H2 to alkanes and methanol. Based on preliminary catalytic studies, the highest rate of methane (2850 g/kg/hr at 374 °C) and methanol (440 g/kg/hr at 284 °C) formation was accomplished with an FeMo carbonitride prepared by decomposing Mo(CO)6 and Fe(CO)5 in ammonia at 800 °C.

Compared activities of platinum and high specific surface area Mo 2C and WC catalysts for reforming reactions : I. Catalyst activation and stabilization: Reaction of n-hexane

The catalytic properties of high specific surface area (>150 m 2/g) molybdenum and tungsten carbides are studied in hydrocarbon-reforming reactions and compared to conventional platinum supported on alumina. The isomerization of n-hexane is used as a test reaction. TPR and XPS analyses show that the raw materials are contaminated by oxygen and need an activation process to become reactive. These analyses also show that Mo 2C is decomposed by pure hydrogen at 800°C to form methane and metallic molybdenum. Different methods of reductive activation are tested: high-temperature reduction (800°C) by hydrogen leads to metallic Mo and W on the surface (catalytically unreactive for reforming); coreduction by a mixture of pentane and hydrogen (700°C) gives active catalysts but less so than conventional platinum, probably because of the presence of carboneous residues formed by decomposition of the n-pentane. Trace amounts of different group VIII transition metals (≤500 ppm) can catalyze the activation process, probably by preventing the formation of the carboneous residues. Mo 2C activated by 500 ppm of Pt, Ir, or Ru can reach total specific activities 6 to 7 times higher than the conventional Pt catalyst. However in terms of yield, the best carbide, activated by Ir, only doubles the performance of conventional platinum, with a high amount of cracked molecules formed in parallel. Clean surfaces of Mo 2C or WC can be much more reactive than conventional Pt catalysts in terms of specific activity, isomerization, plus cracking; however, the best selectivity in isomers never exceeds 30% while selectivity on Pt is usually in the range 75 to 85%.

Catalytic hydrodesulfurization by molybdenum nitride

High surface area molybdenum nitride (up to 108 m 2/g) was synthesized, characterized, and tested for thiophene desulfurization activity. The surface area was found to depend on synthesis temperature profile, mass transfer, and passivation procedure. Passivated and sulfided catalysts retained the bulk structure of face-centered-cubic Mo 2N. X-ray diffraction and Raman spectroscopy showed no evidence for MoO 3 or MoS 2 formation in fresh catalysts or catalysts sulfided at 673 K. Thiophene desulfurization activity was measured over a broad range of Mo 2N surface areas and reactor conditions. Small amounts of tetrahydrothiophene were formed during desulfurization and low-conversion data at 673 K indicate that butane is one of the initial products of the thiophene desulfurization reaction, in addition to butadiene and the butenes.

ChemInform Abstract: PREPARATION OF HIGH SURFACE AREA REDUCED MOLYBDENUM OXIDE CATALYSTS

AbstractDie Darstellung der Mo‐oxid‐Katalysatoren erfolgt durch thermische Zers. und Reduktion von Mo(VI)‐oxalat.

High Surface Area Molybdenum Catalysts: Preparation, Characterization, and Activity

Methods for preparing high surface area Mo/MoO/sub 2/, MoO/sub 2/, and Co--Mo and Ni--Mo bimetallic compositions are described. Methods for preparing metal molybdates, such as Fe/sub 2/(MoO/sub 4/)/sub 3/, CoMoO/sub 4/, NiMoO/sub 4/, and MnMoO/sub 4/ of moderately high surface area are also described. All of the materials prepared were characterized by chemical analysis, x-ray, and surface area. Passivation and regeneration procedures were also devised for the pyrophoric materials. The Mo/MoO/sub 2/ composition was found to be active in the isomerization of n-hexane.

Preparation of high surface area reduced molybdenum oxide catalysts

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.