Reduction Of Molybdenum Oxide Research Articles

Spatiotemporal Multitechnique Imaging of a Catalytic Solid in Action: Phase Variation and Volatilization During Molybdenum Oxide Reduction

Caught in the act: A novel combined experimental setup is demonstrated, which uses very high energy/flux synchrotron X-rays and allows the measurement of spatiotemporal data on larger reactors and the use of techniques such as fluorescence spectroscopy and Compton scattering. Wide-angle X-ray and Compton scattering reveal information on the causes of molybdenum volatilization in partial oxidation catalysts.

Effects of RE2O3 doping on the reduction behavior of molybdenum oxide and properties of molybdenum powder

The reduction of molybdenum oxide doped with rare earth oxides (RE2O3) and the properties of molybdenum powder with RE2O3 additions have been studied by TG-DTA, X-ray diffraction, temperature programmed reduction (TPR), and X-ray photoelectron spectrometry (XPS). The TPR results show that the reduction peak of doped MoO3 shifts to lower temperature as compared with that of pure MoO3. The interaction between rare earth and molybdenum ions leads to this decrease of the reduction temperature of RE2O3 doped molybdenum oxide. RE2O3 is decreasing the particle size of molybdenum by the pinning effect of RE2O3 at the molybdenum grain boundaries. Moreover, RE2O3 nanoparticles on the surface of molybdenum powder increase the escape depth of free electrons in molybdenum, decrease the height of the characteristic energy loss peak of molybdenum, and thus increase the height of the XPS-Mo3d peak.

X-ray photoelectron study of the interaction of H2 and H2+O2 mixtures on the Pt/MoO3 model catalyst

The effects of H2 and H2 + O2 gas mixtures of varying composition on the state of the surface of the Pt/MoO3 model catalyst prepared by vacuum deposition of platinum on oxidized molybdenum foil were investigated by X-ray photoelectron spectroscopy (XPS) at room temperature and a pressure of 5–150 Torr. For samples with a large Pt/Mo ratio, the XP spectrum of large platinum particles showed that the effect of hydrogen-containing mixtures on the catalyst was accompanied by the reduction of molybdenum oxide. This effect results from the activation of molecular hydrogen due to the dissociation on platinum particles and subsequent spill-over of hydrogen atoms on the support. The effect was not observed at low platinum contents in the model catalyst (i.e., for small Pt particles). It is assumed for the catalyst that the loss of its hydrogen-activating ability is a consequence of the formation of platinum hydride. Possible participation of platinum hydride as intermediate in hydrogen oxidation to H2O2 is discussed.

Magnesothermic synthesis of disperse molybdenum carbide in sodium carbonate melts

The physicochemical aspects of the synthesis of molybdenum carbide powders through the magnesothermic reduction of molybdenum oxide in sodium carbonate melts are considered. The reduction reactions are thermodynamically characterized. The composition of the ionic melt is shown to influence the phase composition of the products. In sodium carbonate melts Mo2C is formed, while sodium chloride melts under the same conditions yield molybdenum powders. The product yield reaches 97% with an about 30% excess of the reducing agent relative to the stoichiometry. Minor metal concentrations are within ∼2%. The particle sizes of the powders are determined. The specific surface area of molybdenum carbide powders is 5.53 × 105 m−1; that of molybdenum is 20.19 × 105 m−1.

Polymer‐Assisted Growth of Molybdenum Oxide Whiskers via a Sonochemical Process.

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Polymer-Assisted Growth of Molybdenum Oxide Whiskers via a Sonochemical Process

Whiskers of molybdenum oxides with high aspect ratios were synthesized from peroxomolybdate precursor solutions in the presence of small amounts of poly(ethylene glycol) (PEG) via a sonochemical process at temperatures of 25-70 degrees C. Irradiation with ultrasound reduces the time needed for the growth of micrometer-sized whiskers from weeks to a few hours. The simplicity of the sonochemical approach also compares favorably to a hydrothermal/solvothermal process. The morphology, crystal structure, and other characteristics of the whiskers were characterized by scanning electron microscopy, transmission electron microscopy, selective area electron diffraction, energy-dispersive X-ray spectroscopy, wide-angle X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and the Brunauer-Emmett-Teller method. The surface area of the calcified molybdenum oxide whiskers (55.4 m2/g) was found to be much higher than those of molybdenum oxide nanofibers (35 m2/g) or nanorods (13.4 m2/g) The growth rate of various crystal faces could be postulated to be controlled by the binding of peroxomolybdate ions to pseudo-crown ether cavities formed by PEG. The reduction of molybdenum oxide to produce mixed-valent oxides and their growth could also be controlled by the reducing ability of PEG. The aspect ratio of the molybdenum oxide whiskers increased with decreasing concentration in the initial peroxomolybdate precursor solution. Whether the precursor solution species was H2Mo2O3(O2)4(H2O)2, H2MoO2(O2)2, or MoO2(OH)(OOH), the peroxide group in all the species disproportionates to give the final product MoO3 by a catalytic process. On the basis of experimental evidence of the dual role of glycols, a mechanism for the growth of the molybdenum oxide whiskers is proposed.

Mechanisms of hydrogen reduction of molybdenum oxides : W.V. Schulmeyer, H.G. Ortner. (Darmstadt University of Technology, Darmstadt, Germany.) Int J. Refac. Metal/Hard Mater., vol 20, no 4, 2002, 261–269

Mechanisms of hydrogen reduction of molybdenum oxides : W.V. Schulmeyer, H.G. Ortner. (Darmstadt University of Technology, Darmstadt, Germany.) Int J. Refac. Metal/Hard Mater., vol 20, no 4, 2002, 261–269

Mechanisms of hydrogen reduction of molybdenum oxides

Mechanisms of hydrogen reduction of molybdenum oxides

Mechanisms of the hydrogen reduction of molybdenum oxides

The two stages of the hydrogen reduction of MoO 3 to Mo were investigated in a thermal balance under well defined reaction conditions. Starting with different grain and agglomerate sizes for both stages, the influence of a set of parameters (temperature, local partial pressure of H 2O, gas flow, etc.) on the reaction progress and the final result were studied in detail. Depending on the set of parameters used, different reaction mechanisms like pseudomorphic transformation or chemical vapour transport (CVT) were observed. Taking into account that grains and agglomerates deviate from a spherical shape and a definite grain size, the extent of reaction is well described by standard theoretical gas–solid reaction models such as the shrinking core model or the crackling core model (CCM). Thermo-gravimetric analysis, X-ray diffraction, scanning electron microscopy, surface area measurements (BET method) and laser diffraction were used for these studies. Under all conditions, the first stage shows a reaction path MoO 3→Mo 4O 11→MoO 2 via CVT. The reaction extent follows the CCM. Depending on the local partial pressure of H 2O during reduction, the formed Mo 4O 11 and MoO 2 exhibit different size distributions and shapes of the grains. The extent of reaction of the second stage develops according to the shrinking core model. Depending on the local dew point, two different reaction paths can occur: Pseudomorphic transformation at low dew points and transformation via CVT at high dew points. This paper is an extract from the Ph.D. thesis of W.V. Schulmeyer “Mechanismen der Wasserstoffreduktion von Molybdänoxiden”, 1998, Darmstadt University of Technology, Institute of Material Science, Department of Chemical Analytics, FRG. It focuses on a phenomenological description of the most important results.

Nitridation of vacuum evaporated molybdenum films in H2/N2 mixtures

The nitridation behavior of molybdenum (Mo) films has been studied in relation to the crystal structure of the films. Vacuum evaporated Mo films are nitrided to form γ-Mo2N by annealing in H2/N2 mixtures in the temperature region of 400–900 °C. The temperature region becomes narrow with increasing H2/N2 flow ratio, and the Mo films are not nitrided by annealing in only nitrogen. These results are interpreted as caused by the reaction process involving hydrogen reduction of molybdenum oxide followed by direct nitridation of Mo. During annealing, the Mo films show uniform nitrogen distribution throughout the film and the nitrogen content increases with increasing annealing time. This behavior is well explained by the grain-boundary nitridation model in which each columnar grain of a Mo film is nitrided from the periphery to the center of the grain.

Kinetics and mechanism of carbothermic reduction of MoO 3 to Mo 2C

The kinetics of carbothermic reduction of molybdenum oxide with homogeneously mixed carbon black was studied under non-isothermal heating conditions using a thermobalance. The evolved gases (CO and CO 2) were analysed by a gas chromatograph. A mechanism is proposed to explain the stepwise conversion of oxide to carbide. A model representing stepwise conversion of the above reduction has been developed and the probable rate controlling steps, for different reactions, have been identified.

Production and Technical Characteristics of a New Class of Amorphous Metals

AbstractA new technology for obtainment of amorphous single-component metals is presented.For the first time the reduction of molybdenum oxide with formation of its amorphous phase is realized in conditions of a given quantum-chemical technology by means of vibrationally excited to the third quantum level hydrogen molecules with 1.5 ± 0.2 eV energy. The evidences of formation of this nonequilibrium amorphous phase are presented along with certain physicochemical properties of the obtained amorphous molybdenum.A model is proposed for the origin of amorphous phase under the influence of nonequilibrium quantum-chemical technology.

Autocatalytic effect in the processes of metal oxide reduction. II. Kinetics of molybdenum oxide reduction

Reduction by hydrogen of molybdenum oxide unsupported or deposited on different carriers has been studied at different reductant pressures and temperatures. It has been found that the model of the consecutive autocatalytic reaction (CAR) accounts for changes in the reaction kinetics, induced by changes in the parameters of the process as well as by the presence of supports. The model describes quantitatively the reduction of the bulk molybdenum oxide. For MoO 3 on Aerosil, the CAR model was modified by a factor related to aggregation of the intermediate product, in order to account for the additional hindrance of the reduction.

Catalyst based on nickel and molybdenum of a high specific surface

Reduction of mixed oxides NiO-MoO3 of varying composition by hydrogen in the temperature range 653-773 K was studied as well as several physico-chemical parameters of the reduction products - methanation catalysts.Initial oxides contain - depending on their composition - a varying concentration of nickel molybdate as a free phase. The reduction of molybdenum oxide is catalytically influenced by nickel reduced and takes place, depending on its actual concentration, to the lower oxidation degrees eventually to the metallic molybdenum. On the other hand, the molybdenum present stabilizes nickel microcrystallites during the reduction by hydrogen. Resulting active catalytic systems exhibit a high specific surface. The rate, degree, and apparent activation energy of the reduction are markedly dependent on the initial sample composition. The reduction kinetics can be qualitatively described by an equation of the zone model of solid phase decomposition.

Disproportionation of Propylene over MoO3-γ-Al2O3

Disproportionation of propylene over MoO2-Al2O2- or CoOoO2-Al2O2- catalyst was studied using a flow system. The effect of the pretreatment temperature and the effects of the atomic ratio (Mo/A1) on the catalytic activity and various physical properties of the catalyst were investigated in order to elucidate the active center of the catalyst.No catalytic activity revealed by the pretreatment below 300°C. The activity which appeared upon the pretreatment above 300°C increased with increase in the pretreatment temperature. The disproportionation activity over M0O2-Al2O2- catalyst pretreated with nitrogen gas was correlated with the relative intensity of Mo5+ and also with the activity of double bond isomerization of olefin over r-alumina. On the contrary, the yield of isobutene decreased with increase in the pretreatment temperature. It was also found that the maximum activity of catalyst appeared at 5% Mo content while the activity virtually disappeared at 10% Mo content in the case of N2-treatment. It seemed likely that the catalytic activity was associated with the lattice defects of M0O2-Al2O2- catalyst. Reduction of all these catalysts with hydrogen, however, increased the catalytic activity markedly. The maximum activity appeared at 10% Mo content. On the basis of these results, it is suggested that the active center was produced at the initial stage of the reaction by the reduction of molybdenum oxide with olefin adsorbed on the acid site of alumina (or lattice defects).

Studies on the catalytic Synthesis of Cyano benzenes. I.

A vapor phase catalytic reaction between toluene and ammonia in the presence of molybdenum oxide and vanadium oxide, respectively, was investigated. The activityfor formation of benzonitrile was better in molybdenum oxide carrying catalystt but the selectivity for benzonitrile was superior in vanadium oxide carrying catalyst. The activity of vanadium oxide-alumina catalyst did not change during the experimental period of about 10 hours, but that of molybdenum oxide-alumina catalyst decreased with the time of reaction: however, it was regenerated by the heating in air. A preliminary treatment of vanadium oxide catalyst in an atmosphere of ammonia at 550°c showed practically no change in the activity of catalyst, but that of molybdenum oxide catalyst showed considerable decrease in its activity. It was considered to be mainly due to the formation of lower oxide by the reduction of molybdenum oxide in this reaction atmosphere. It was found that the acidity of catalyst carrier was injurious, as by the action of alumina gel or silicagel of acidic nature on molybdenum oxide, the formation of benzene was accele rated. The catalyst with sintered alumina carrier treated at 1200°c was effective por to inhbit the formation of benzene as a by-product and to improve the selectivity formation of benzonitrile. It was indicated that the rate determining step was before the formation of benzylamine, that the formation of benzonitrile from benzylamine with molybdenum oxide catalyst was proceeded mainly through dehydrogenation process, and that the benzonitrile formation with vanadium oxide catalyst was proceeded mainly through disproportionation.