Role Of Molybdenum Research Articles

The role of molybdenum in Fe–Mo–Al 2O 3 catalyst for synthesis of multiwalled carbon nanotubes from butadiene-1,3

MoO 3–Fe 2O 3–Al 2O 3 catalysts were prepared by co-precipitation of mixed hydroxides from soluble salts. It was found that the 55%Fe 2O 3–Al 2O 3 aluminum–iron catalyst calcined in air at 500–700 °C consisted of a solid solution based on hematite where a part of iron atoms was substituted for aluminum. The modification of the aluminum–iron catalyst with molybdenum results in the formation of a solid solution based on hematite where a part of iron atoms is substituted for aluminum and molybdenum ions. At 700 °C the MoO 3–Fe 2O 3–Al 2O 3 catalysts are reduced under the action of the feed. The dependence of the multiwalled carbon nanotube (MWNT) yield over MoO 3–Fe 2O 3–Al 2O 3 catalysts on the molybdenum concentration has a maximum. Small concentrations of MoO 3 (up to 6.5 wt.%) added to the aluminum–iron catalyst increase the dispersity of the active metal particles and alter their properties due to the formation of a Fe–Mo alloy. Its formation leads to the decrease of the nanotube growth rate and makes the catalyst more stable. The overall yield of the carbon nanotubes increases. Further increase of the molybdenum concentration leads to the decrease of the nanotube yield because the catalytically active Fe–Mo particles are enriched too much with molybdenum. In addition, excess molybdenum forms molybdenum carbide α-Mo 2C, in which part of the molybdenum atoms are substituted for iron atoms, and which is inactive in the MWNT growth.

Molybdenum limitation of asymbiotic nitrogen fixation in tropical forest soils

Biological nitrogen fixation limits plant growth and carbon exchange at local to global scales. Long-term nutrient manipulation experiments in forests and short-term manipulation experiments in microcosms suggest that the micronutrient molybdenum, a component of the nitrogen-fixing enzyme nitrogenase, limits nitrogen fixation by asymbiotic bacteria in tropical soils in Panama. Nitrogen fixation, the biological conversion of di-nitrogen to plant-available ammonium, is the primary natural input of nitrogen to ecosystems1, and influences plant growth and carbon exchange at local to global scales2,3,4,5,6. The role of this process in tropical forests is of particular concern, as these ecosystems harbour abundant nitrogen-fixing organisms1,4 and represent one third of terrestrial primary production4,7,8. Here we show that the micronutrient molybdenum, a cofactor in the nitrogen-fixing enzyme nitrogenase, limits nitrogen fixation by free-living heterotrophic bacteria in soils of lowland Panamanian forests. We measured the fixation response to long-term nutrient manipulations in intact forests, and to short-term manipulations in soil microcosms. Nitrogen fixation increased sharply in treatments of molybdenum alone, in micronutrient treatments that included molybdenum by design and in treatments with commercial phosphorus fertilizer, in which molybdenum was a ‘hidden’ contaminant. Fixation did not respond to additions of phosphorus that were not contaminated by molybdenum. Our findings show that molybdenum alone can limit asymbiotic nitrogen fixation in tropical forests and raise new questions about the role of molybdenum and phosphorus in the tropical nitrogen cycle. We suggest that molybdenum limitation may be common in highly weathered acidic soils, and may constrain the ability of some forests to acquire new nitrogen in response to CO2 fertilization9.

The role of molybdenum in agricultural plant production.

The importance of molybdenum for plant growth is disproportionate with respect to the absolute amounts required by most plants. Apart from Cu, Mo is the least abundant essential micronutrient found in most plant tissues and is often set as the base from which all other nutrients are compared and measured. Molybdenum is utilized by selected enzymes to carry out redox reactions. Enzymes that require molybdenum for activity include nitrate reductase, xanthine dehydrogenase, aldehyde oxidase and sulfite oxidase. Loss of Mo-dependent enzyme activity (directly or indirectly through low internal molybdenum levels) impacts upon plant development, in particular, those processes involving nitrogen metabolism and the synthesis of the phytohormones abscisic acid and indole-3 butyric acid. Currently, there is little information on how plants access molybdate from the soil solution and redistribute it within the plant. In this review, the role of molybdenum in plants is discussed, focusing on its current constraints in some agricultural situations and where increased molybdenum nutrition may aid in agricultural plant development and yields. Molybdenum deficiencies are considered rare in most agricultural cropping areas; however, the phenotype is often misdiagnosed and attributed to other downstream effects associated with its role in various enzymatic redox reactions. Molybdenum fertilization through foliar sprays can effectively supplement internal molybdenum deficiencies and rescue the activity of molybdoenzymes. The current understanding on how plants access molybdate from the soil solution or later redistribute it once in the plant is still unclear; however, plants have similar physiological molybdenum transport phenotypes to those found in prokaryotic systems. Thus, careful analysis of existing prokaryotic molybdate transport mechanisms, as well as a re-examination of know anion transport mechanisms present in plants, will help to resolve how this important trace element is accumulated.

The role of molybdenum in the hard-phase grains of (Ti, Mo)(C, N)–Co cermets

The evolution of the core–rim interface in Ti(C, N)-based cermets containing Mo is studied by scanning electron microscopy and conventional and high-resolution transmission electron microscopy as a function of the annealing temperature. A heavily disordered zone on the nanometric scale is observed close to the interface with the core of Ti(C, N) grains in as-sintered samples. It reorders after annealing at a high temperature. This disorder is correlated with the presence of Mo in the rim during sintering, which enhances the strength of the core–rim interface as a barrier for the diffusion of atoms from the core. It avoids the complete dissolution of small Ti(C, N) grains from the original powder and limits the grain growth. It also acts as a barrier against dislocation movements during plastic deformation. This effect, as well as the small final grain size determined by the presence of Mo, contributes to the good mechanical properties of Mo-containing cermets at intermediate temperatures.

Analysis of different acicular ferrite microstructures generated in a medium-carbon molybdenum steel

Different acicular ferrite morphologies have been produced in a medium-carbon steel by isothermal treatments carried out at 400, 450, 500 and 550°C. These treatments were interrupted at different times to analyse the evolution of the microstructure at each temperature. It has been found that at low transformation temperatures sheaves of parallel ferrite plates, similar to bainite but intragranularly nucleated, are formed, at intermediate temperatures a more interlocked microstructure is produced and at high temperatures, individual plates are generated following an incomplete reaction phenomenon. Taking as a reference a previous work, in the present one the role of molybdenum on the transition between these acicular ferrite morphologies has been investigated. The results show that molybdenum raises the temperature for which the transition between an acicular sheaf morphology and a more interlocked microstructure is produced, probably as a consequence of the reduction in the carbon mobility in ferrite and in austenite.

Role of molybdenum against ceria sulphur poisoning in the combustion of soot particles and the oxidation of propene

The influence of molybdenum on ceria activity, CO2 selectivity and resistance to sulphur poisoning was investigated. Different molybdenum-cerium catalysts, with various atomic ratios (Mo/Ce=0.001, 0.01, 0.1, 0.2 and 0.3), have been prepared and tested with respect to carbon black (CB) combustion and the oxidation of propene before and after treatment under SO2. Based on their catalytic behaviour and according to molybdenum loadings, the catalysts have been classified into two groups: (i) ceria and low molybdenum loading catalysts (1Mo1000Ce, 1Mo100Ce) which present a good activity, an excellent CO2 selectivity and a bad resistance to sulphur poisoning; (ii) catalysts containing higher molybdenum loadings (1Mo10Ce, 2Mo10Ce, 3Mo10Ce) show lower activity and CO2 selectivity but a better resistance to sulphur poisoning. The role of molybdenum against ceria sulphur poisoning has been evidenced.

Synthesis of Molybdovanadates Coordinated by Oxalato Ligands. The Crystal Structure of K6[Mo6V2O24(C2O4)2]·6H2O

The oxalato complex of a polyoxomolybdovanadate, K6[Mo6V2O24(C2O4)2]·6H2O has been obtained by reaction of potassium molybdate, ammonium vanadate and tartaric or ascorbic acid. Such conversion of dicarboxylate into oxalate ions indicates the catalytic role of molybdenum. Complexes of analogous composition also were obtained in the reactions of MoO3, V2O5 and potassium oxalate, or M 2CO3 (M = Rb, Cs) and oxalic acid. The centrosymmetrical molybdovanadate anion [Mo6V2O24(C2O4)2]6- consists of six MoO6 and two VO6 edge-sharing octahedra to give the n -[Mo6O26]4- structure. All complexes were characterized by powder and single crystal X-ray analyses, ESR and IR spectra and TG and DSC measurements.

Role of molybdenum in acicular ferrite formation under continuous cooling in a medium carbon microalloyed forging steel

The role of molybdenum on acicular ferrite formation during both continuous cooling and isothermal transformation processes has been investigated. It has been found that molybdenum not only affects the transformation kinetics but also modifies the morphology of the acicular ferrite.

On limit of molybdenum content of pitting-corrosion-resistant austenitic steels

The role of molybdenum in the pitting resistance of iron–chromium–nickel–molybdenum experimental alloys and their welds is investigated. The accelerated tests are performed in a 5% solution of FeCl3. It is shown that alloying with molybdenum enhances the pitting resistance of an alloy only until of molybdenum the solubility limit of molybdenum in the matrix is reached; the limit in turn grows with the Ni content. Thus, to enhance the pitting resistance of an alloy corrosion, one should hold a balanced proportion between the contents of Mo and Ni in the alloy.

The role of molybdenum in Mo-doped V–Mg–O catalysts during the oxidative dehydrogenation of n-butane

A detailed study on the influence of the addition of molybdenum ions on the catalytic behaviour of a selective vanadium–magnesium mixed oxide catalyst in the oxidation of n-butane has been performed. The catalysts have been prepared by impregnation of a calcined V–Mg–O mixed oxides (23.8 wt% of V 2O 5) with an aqueous solution of ammonium heptamolybdate, and then calcined, and further characterised by several physico-chemical techniques, i.e. S BET, XRD, FTIR, FT-Raman, XPS, H 2-TPR. MgMoO 4, in addition to Mg 3V 2O 8 and MgO, have been detected in all the Mo-doped samples. The incorporation of molybdenum modifies not only the number of V 5+-species on the catalyst surface and the reducibility of selective sites but also the catalytic performance of V–Mg–O catalysts. The incorporation of MoO 3 favours a selectivity and a yield to oxydehydrogenation products (especially butadiene) higher than undoped sample. In this way, the best catalyst was obtained with a Mo-loading of 17.3 wt% of MoO 3 and a bulk Mo/V atomic ratio of 0.6. From the comparison between the catalytic properties and the catalyst characterisation of undoped and Mo-doped V–Mg–O catalysts, the nature of selective sites in the oxidative dehydrogenation of n-butane is also discussed.

Studies on the surface behavior of non-chromium type high temperature co shift catalysts

The activity of an iron-base catalyst modified by molybdenum and cerium for CO shift reaction was investigated in a flow microcatalytic reactor. The results show that the activity of the modified catalyst is obviously improved. The surface behavior of Fe−Mo−Ce catalyst and the role of molybdenum and cerium in the catalyst are discussed.

A role of molybdenum and shape selectivity of catalysts in simultaneous reactions of hydrocracking and hydrodesulfurization

The hydrocracking and hydrodesulfurization (HDS) of n-heptane containing 0.2 mole% dibenzothiophene (DBT) were performed simultaneously using NiPtMo catalysts supported on HZSM-5, LaY and γ-Al2O3 in a high pressure fixed bed reactor. Molybdenum played an important role in both hydrocracking and hydrodesulfurization (HDS). We found that the sulfur compound, dibenzothiophene (DBT). in the reactant was adsorbed on a molybdenum site and converted to hydrogen sulfide so that the active sites of the catalysts for hydrocracking were less poisoned by DBT and the conversion of n-heptane over molybdenum impregnated catalyst was higher than that over molybdenum-free catalyst. The crystal structures of the molybdenum supported on the zeolite and γ-Al2O3 were mainly MoO2.5 (OH)0.5[021] and MoO3[210] respectively as shown by XRD analysis. The structure of MoO2.5(OH)0.5 was easily reduced to MoS2[003] during the reaction. After the reaction of 100 hours over the catalyst supported on γ-Al2O3 the crystal structure of MoO3[210] partially changed to MoO3[300] and the structure of MoS2[003] was not observed. Because of the reactant shape selectivity of zeolite, the acid and the metal sites in the intracrystalline of the catalysts supported on zeolites were less poisoned by DBT. Therefore, both hydrocracking and HDS using n-heptane containing 0.2 mole% of DBT were successfully demonstrated over the prepared catalysts.

Grain boundary segregation and intergranular fracture in Ferrite containing Mo, Si or Al

Roles of molybdenum, silicon or aluminum in ferrite on grain boundary segregation and hence on intergranular fracture have been investigated by using Auger electron spectroscopy (AES) and tensile test. Competitive segregation between sulfur and carbon or nitrogen, which caused the decrease below 700°C of sulfur content at the grain boundaries, was observed in the pure iron. The intergranular brittleness of the pure iron was caused by sulfur at the grain boundaries. When molybdenum was added to the pure iron, the sulfur contents at the grain boundaries were lowered in comparison to those in the pure iron. The molybdenum-bearing alloy showed higher fracture strength than that of the pure iron, and fractured mostly in the transgranular mode. This arises from the intrinsic effect of molybdenum on the grain boundaries as well as the decrease in sulfur content. Tn the 3.37 wt.%Si alloy, silicon and carbon or nitrogen competitively segregated to the grain boundaries, and such a competitive segregation was also observed between sulfur and carbon or nitrogen. The sulfur content at the grain boundaries decreased with increasing silicon content. The fracture modes in the 3.37- and 4.26 wt.%Si alloys were transgranular in the rolling direction, but were mostly intergranular in the transverse direction and in the as-rolled condition. The intergranular characteristic in the fracture behavior may be attributed to the detrimental effect of silicon as well as sulfur on the intergranular cohesion. Carbon and aluminum only were found at the grain boundaries of the aluminum-bearing alloy. This suggests that aluminum is a strong repulser of sulfur or nitrogen at the grain boundaries. Additionally, it was found that aluminum has a detrimental effect on grain boundary strength of ferrite.

ChemInform Abstract: Role of Molybdenum at the Iron Oxide Surface.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Role of molybdenum in chronic hemodialysis patients

Role of molybdenum in chronic hemodialysis patients

Role of Molybdenum at the Iron Oxide Surface

The structure of Mo at the surface of Fe2O3/SO4 and ferrihydrite catalysts was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. Molybdenum is found to be surrounded by 3-4 oxygens at 1.73-1.75 Å with no extended coordination shells beyond the nearest oxygen shell. X-ray diffraction results for the Mo/ferrihydrite catalysts after high temperature annealing demonstrate that the incorporation of Mo at the ferrihydrite surface effectively prohibits crystal growth and allows the catalyst to maintain its dispersion to higher temperatures.

ChemInform Abstract: The Role of Molybdenum in the Dissolution and the Passivation of Stainless Steels with Adsorbed Sulfur.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The role of molybdenum in the dissolution and the passivation of stainless steels with adsorbed sulphur

The effects of pre-adsorbed sulphur on the dissolution and the passivation of Fe17Cr12.6Ni (100) and Fe17Cr14.5Ni2.3Mo (100) alloys in sulphuric acid were studied. Sulphur was pre-adsorbed at high temperature in H 2H 2S gas. The surface concentrations of sulphur were measured with radioactive sulphur (the 35S radioisotope). The results demonstrate that adsorbed sulphur catalyses the dissolution of the alloys, but the effect is much less pronounced on the molybdenum-containing alloy. It is shown that the presence of molybdenum induces a desorption of sulphur from the surface at the corrosion potential. The kinetics of this molybdenum-induced desorption of sulphur was investigated. The mechanisms of this effect are discussed and compared with the mechanisms which have been reported previously for NiMo alloys.

The Roles of Molybdenum and Ion-Mixing in Producing Friction-Reducing Films

The roles of molybdenum and ion-mixing on the tribological characteristics of friction-reducing films have been investigated in a laboratory test. The effect of an electric field on the friction stability and the localization of phosphate formation were also studied. Electro-coated iron-molybdenum piston ring specimens demonstrated better friction stability and lower friction coefficients than specimens of uncoated iron-molybdenum. This is believed to be due to the formation of localized phosphate films in molybdenum-poor regions. In addition, based on results for samples with different degrees of ion-mixing, ion-mixed Fe/Mo ring specimens further reduced slidring friction and enhanced friction stability. Fewer stick-slip or wear-induced vibrations occurred for the sample with increased ion-mixing. In addition, for this same sample, no significant stick-slip or wear-induced vibrations were observed during a continuous lubricated sliding process of 400 hours. Surface analyses indicate that increased ion-mixing promotes a broad Mo/Fe interface and increases phosphate penetration. These results suggest that electro-coated and ion-mixed Fe/Mo films both have potential for use in reducing sliding friction.

Roles of molybdenum, FAD and iron-sulphur domains in molybdenum-containing hydroxylases: molecular genetic, kinetic and spectroscopic studies

Roles of molybdenum, FAD and iron-sulphur domains in molybdenum-containing hydroxylases: molecular genetic, kinetic and spectroscopic studies