Showing papers on "Molybdenum published in 2008"

Synthesis of Mo and W carbide and nitride nanoparticles via a simple "urea glass" route.

Abstract: A simple, inexpensive, and versatile route for the synthesis of metal nitrides and carbides (such as Mo2N, Mo2C, W2N and WC) nanoparticles was set up. For the first time, metal carbides were obtained using urea as carbon-source. MoCl5 and WCl4 are in a first step contacted with alcohols and an appropriate amount of urea to form a polymer-like, glassy phase, which acts as the starting product for further conversions. Just by heating this phase it was possible to prepare either molybdenum and tungsten nitrides or carbides simply by changing the metal precursor/urea molar ratio. In this procedure, urea plays a double role as a nitrogen/carbon source and stabilizing agent (necessary for the nanoparticle dispersion). Molybdenum and tungsten nitride and carbides synthesized are almost pure and highly crystalline. Sizes estimated by WAXS range around 20 and 4 nm in diameter for Mo and W nitrides or carbides, respectively. The specific surface area was found between 10 and 80 m2/g, depending on the metal and the initial ratio of metal precursor to urea.

Effects of molybdenum substitution on the photocatalytic behavior of BiVO4

Abstract: Transition metal doping has always suffered from the disadvantage of reduction of photocatalytic activity because the dopant ions can form a discrete level in the forbidden band of the photocatalyst resulting in low mobility of electrons and holes in the dopant level and thus lowered activity. However, in this study the photocatalytic activity of an efficient visible-light sensitive photocatalyst, BiVO4, for water oxidation or organic compound degradation was found to be remarkably enhanced by molybdenum doping. The role of molybdenum doping in enhancing the photocatalytic activity of BiVO4 was investigated and discussed based on the changes of the surface acidity of the photocatalyst.

Periplasmic Nitrate Reductase Revisited: A Sulfur Atom Completes the Sixth Coordination of the Catalytic Molybdenum.

Abstract: Nitrate reductase from Desulfovibrio desulfuricans ATCC 27774 (DdNapA) is a monomeric protein of 80 kDa harboring a bis(molybdopterin guanine dinucleotide) active site and a [4Fe–4S] cluster. Previous electron paramagnetic resonance (EPR) studies in both catalytic and inhibiting conditions showed that the molybdenum center has high coordination flexibility when reacted with reducing agents, substrates or inhibitors. As-prepared DdNapA samples, as well as those reacted with substrates and inhibitors, were crystallized and the corresponding structures were solved at resolutions ranging from 1.99 to 2.45 A. The good quality of the diffraction data allowed us to perform a detailed structural study of the active site and, on that basis, the sixth molybdenum ligand, originally proposed to be an OH/OH2 ligand, was assigned as a sulfur atom after refinement and analysis of the B factors of all the structures. This unexpected result was confirmed by a single-wavelength anomalous diffraction experiment below the iron edge (λ = 1.77 A) of the as-purified enzyme. Furthermore, for six of the seven datasets, the S–S distance between the sulfur ligand and the Sγ atom of the molybdenum ligand CysA140 was substantially shorter than the van der Waals contact distance and varies between 2.2 and 2.85 A, indicating a partial disulfide bond. Preliminary EPR studies under catalytic conditions showed an EPR signal designated as a turnover signal (g values 1.999, 1.990, 1.982) showing hyperfine structure originating from a nucleus of unknown nature. Spectropotentiometric studies show that reduced methyl viologen, the electron donor used in the catalytic reaction, does not interact directly with the redox cofactors. The turnover signal can be obtained only in the presence of the reaction substrates. With use of the optimized conditions determined by spectropotentiometric titration, the turnover signal was developed with 15N-labeled nitrate and in D2O-exchanged DdNapA samples. These studies indicate that this signal is not associated with a Mo(V)–nitrate adduct and that the hyperfine structure originates from two equivalent solvent-exchangeable protons. The new coordination sphere of molybdenum proposed on the basis of our studies led us to revise the currently accepted reaction mechanism for periplasmic nitrate reductases. Proposals for a new mechanism are discussed taking into account a molybdenum and ligand-based redox chemistry, rather than the currently accepted redox chemistry based solely on the molybdenum atom.

Synthesis and Characterization of Nanostructured Pd-Mo Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells

Abstract: Carbon-supported Pd100-xMox (0 ≤ x ≤ 40) nanoparticles have been synthesized by a simultaneous thermal decomposition of palladium acetylacetonate and molybdenum carbonyl in an organic solvent (o-xylene) in the presence of Vulcan XC-72R carbon, followed by heat treatment up to 900 °C in H2 atmosphere and characterized chemically and structurally. X-ray diffraction data reveal the formation of single-phase face-centered cubic solid solutions for 0 ≤ x ≤ 30 after heat treating at 900 °C and the occurrence of a Mo2C impurity phase for x = 40. The particle size of the Pd100-xMox samples increase with increasing heat treatment temperature as revealed by transmission electron microscopy. Cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements reveal that the alloying of Pd with Mo enhances the catalytic activity for the oxygen reduction reaction (ORR) as well as the stability (durability) of the electrocatalyst. However, the activity reaches a maximum at Pd90Mo10 and then decreases with increasing...

Redox behavior of molybdenum and tungsten in phosphate glasses.

Abstract: In this work, vitreous samples were prepared in the binary system (100 - x)NaPO3-xMO3 with M = Mo and W and x varying from 10 to 60. The transmittance properties in the UV, visible, and near-infrared were monitored as a function of MO3 concentration. In both cases, an increase in the amount of transition metal results in an intense and broad absorption band in the visible and near-infrared attributed to metal reduction under synthesis conditions. It was shown that this large absorption can be partially or totally removed using specific oxidizing agents or by improving synthesis parameters such as melting temperature or cooling rate of the melt. In addition, structural investigations by Raman and X-ray absorption spectroscopy suggest that reduction only occurs when the metal cation is in octahedral geometry and that the transmittance improvement is not related with any structural changes. These results were explained in terms of thermodynamic equilibrium of redox species in the melt and allowed to obtain for the first time transparent and chemically stable glasses containing high concentrations of MO3 with transition metals in octahedral geometry inside the glass network.

Hydrogen activation by high-valent oxo-molybdenum(VI) and -rhenium(VII) and -(V) compounds

Abstract: The high-valent oxo-molybdenum(VI) and -rhenium(VII) and -(V) derivatives MoO2Cl2, ReCH3O3 (MTO) and ReIO2(PPh3)2 catalyze the selective hydrogenation of alkynes to alkenes at 80 °C under 40 atm of pressure. The reduction of sulfoxides to sulfides has also been performed by oxo-rhenium and -molybdenum complexes using hydrogen as a reducing agent. Activation of hydrogen by MoO2Cl2 and MoO2(S2CNEt2)2 was shown by means of DFT calculations to proceed by H–H addition to the MoO bond, followed by hydride migration to yield a water complex.

Effect of Omega Phase on Mechanical Properties of Ti-Mo Alloys for Biomedical Applications

Abstract: The present work is a study of mechanical properties of cast binary Ti-Mo alloys (containing 7.5-20 wt% Mo) and the ω phase transformation of β-phase Ti-Mo alloys. The results indicated that when the molybdenum content increased to 9 wt% and higher, a significant amount of β phase was retained and a meta-stable phase called the athermal ω phase was formed. The appearance of the ω reflections changed from sharp to diffuse with increasing molybdenum content. In addition, in the high molybdenum content alloys, the diffuse reflections moved away from the positions an lines joining bcc reflections. X-ray diffraction (XRD) results indicated clearly that ω phase intensities of Ti-10Mo and Ti-12.5Mo alloys were higher than those of the other β-phase Ti-Mo alloys, so the microhardness values of the Ti-10Mo and Ti-12.5Mo alloys were higher than those of the other β-dominated Ti-Mo alloys, When the molybdenum content increased to 15 wt% higher, we almost could not read the characteristic diffraction peak of the ω phase in the XRD patterns. The Ti-15 Mo allay (β+ ω phase) had a higher modulus than that of Ti-7.5Mo (a”-dominated phase); this may be caused by the formation of the ω phase during quenching.

Hexavalent Molybdenum Reduction to Molybdenum Blue by S. Marcescens Strain Dr. Y6

Abstract: A molybdate-reducing bacterium has been locally isolated. The bacterium reduces molybdate or Mo6+ to molybdenum blue (molybdate oxidation states of between 5+ and 6+). Different carbon sources such as acetate, formate, glycerol, citric acid, lactose, fructose, glucose, mannitol, tartarate, maltose, sucrose, and starch were used at an initial concentration of 0.2% (w/v) in low phosphate media to study their effect on the molybdate reduction efficiency of bacterium. All of the carbon sources supported cellular growth, but only sucrose, maltose, glucose, and glycerol (in decreasing order) supported molybdate reduction after 24 h of incubation. Optimum concentration of sucrose for molybdate reduction is 1.0% (w/v) after 24 h of static incubation. Ammonium sulfate, ammonium chloride, valine, OH-proline, glutamic acid, and alanine (in the order of decreasing efficiency) supported molybdate reduction with ammonium sulfate giving the highest amount of molybdenum blue after 24 h of incubation at 0.3% (w/v). The optimum molybdate concentration that supports molybdate reduction is between 15 and 25 mM. Molybdate reduction is optimum at 35 °C. Phosphate at concentrations higher than 5 mM strongly inhibits molybdate reduction. The molybdenum blue produced from cellular reduction exhibits a unique absorption spectrum with a maximum peak at 865 nm and a shoulder at 700 nm. The isolate was tentatively identified as Serratia marcescens Strain Dr.Y6 based on carbon utilization profiles using Biolog GN plates and partial 16s rDNA molecular phylogeny.

EPR study of molybdenum-lead-phosphate glasses

Abstract: The local symmetry and interaction between paramagnetic ions in x MoO 3 (1 − x )[2 P 2 O 5 PbO] glasses with 0.5 ⩽ x ⩽ 50 mol% are investigated by EPR spectroscopy. For x ⩽ 10 mol% the isolated Mo 5+ ions surrounded by five oxygen ligands in a square-pyramidal form ( C 4v symmetry) prevail. The short range disorder in the environment of Mo 5+ ions is not significantly (Δ R / R ≈ 2%). At high molybdenum content ( x > 20 mol%) the dipole–dipole and superexchange coupled Mo 5+ ions appear and their number increases with the MoO 3 content. These two aspects are also correlated with the network modifier and former role of molybdenum oxide in function of its concentration. Thus a strong depolymerization of the phosphate structure and the formation of P–O–Mo or Mo–O–Mo bonds in studied glasses appear.

Synthesis and isolation of molybdenum atomic wires.

Abstract: One of the main challenges in nanoscience and nanotechnology consists in the production and isolation of metallic atomic-scale nanowires (Benzryadin, C. N.; Lau, A.; Tinkham, M. Q. Nature 2000, 404, 971-974; Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000, 290, 2120-2123; Nilius, N.; Wallis, T. M.; Ho, W. Science 2002, 297, 1853-1856.). Here we report a unique and controllable way of isolating individual atomic molybdenum (Mo) chains by their encapsulation inside double-walled carbon nanotubes, exhibiting inner diameters ranging from 0.6 to 0.8 nm. We have found that these individual atomic chains form spontaneously within the hollow core of tubes in the absence of any reducing agent. We believe that these atomic-scale nanowires could now be studied without suffering oxidation, so that their physical and chemical properties are elucidated.

The effect of deposition parameters on radiofrequency sputtered molybdenum thin films

Abstract: Employing r.f. (radiofrequency) magnetron sputtering, molybdenum thin films were fabricated on soda-lime glass substrates for use in Cu(In,Ga)Se2 based solar cells. The physical, electrical and structural properties of the films were studied as a function of two deposition parameters: argon pressure and r.f. power. The strain reversal from tensile to compressive occurs at high pressure and is found to decrease with increasing applied r.f. power. The grain sizes of films deduced from x-ray diffraction measurements (full width at half-maximum), and consistent with atomic force microscope images, increase with increasing argon pressure and power. The resistivity of the films was found to increase with increasing argon pressure and decrease with increasing r.f. power.

Effect of molybdenum on the microstructure and wear resistance of Fe-based hardfacing coatings

Abstract: Fe-based hardfacing alloys containing molybdenum compound have been deposited on AISI 1020 steel substrates by shield manual arc welding (SMAW) process. The effect of Mo on the microstructure and wear resistance of the Fe-based hardfacing alloys were investigated by means of X-ray diffraction, optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis, as well as wear test. The results indicated that cuboidal and rod-type complex carbides were synthesized in the lath martensite matrix. The fraction of carbides in hardfacing layer increased with an increasing of Mo content. The hardfacing layer with good cracking resistance and wear resistance could be obtained when the amounts of Fe–Mo was controlled within a range of 3–4 wt.%. The improvement of hardness and wear resistance of the hardfacing layers attributed to the formation of Mo 2 C carbide and the solution strengthening of Mo.

Π-Bonding in Complexes of Benzannulated Biscarbenes, -germylenes, and -stannylenes : An Experimental and Theoretical Study

Abstract: Benzannulated bisstannylenes, exhibiting a CH 2 C(CH 3 ) 2 CH 2 linking unit and CH 2 tBu (1) or CH 2 CH 2 CH 2 NMe 2 (2) N-substituents, and their molybdenum tetacarbonyl complexes 3 and 4 have been prepared. The complexes 3 and 4 exhibit remarkably short Mo-E bond lengths compared to the related biscarbene and bisgermylene complexes. The experimentally determined bonding parameters of the molybdenum bisstannylene complexes are discussed based on DFT calculations.

Study on the Reduction of Molybdenum Dioxide by Hydrogen

Abstract: The reduction of MoO 2 powder by hydrogen is one of the most important steps for manufacturing ferromolybdenum alloy and molybdenum powder. The results of experiments on the kinetics of this reaction are presented in this paper. The experiments were carried out under nonisothermal condition in hydrogen atmosphere using TGA equipment. The nonisothermal experiments were carried out at various linear heating rates up to 1273 K. It was found that the reduction reaction is very fast under the whole heating rate until the reduction ratio of MoO 2 approaches to about 0.92. The reduction ratio of MoO 2 was about 0.98 after finishing the reduction reaction at a heating rate of 4 K/min. Kinetics of the reaction was analyzed from the dynamic TGA data by means of Coats and Redfern equation. The nucleation and growth model yielded a satisfactory fit to these experimental data.

Structure of Dimeric Molybdenum(VI) Oxide Species on γ-Alumina: A Periodic Density Functional Theory Study

Abstract: The structure and stability of dimeric Mo(VI) oxide species on γ-alumina are investigated with a periodic DFT approach. A large number of Mo dimers of various geometries, located on the (100) and (110) surfaces of γ-Al2O3, are modeled. The most stable dimeric species on the (100) plane are mixed dioxo−monooxo or double-monooxo species that are 3-, 4-, or 5-fold bonded to the surface. In most cases, the molybdenum atoms are pseudotetrahedrally coordinated. In the case of the (110) surface, the most stable dimeric Mo(VI) forms are double-monooxo species, which are 6- or 5-fold bonded to the support. The coordination of the molybdenum atoms is most frequently a distorted square pyramidal, but distorted octahedral or distorted tetrahedral geometry also happens. The energetic stability of the dimeric Mo(VI) species, relative to the corresponding monomeric Mo(VI) centers, is determined. On the minority (100) surface, the complex−surface interaction is moderate and the dimeric Mo(VI) species are more stable than...

Heavy metal ions inhibit molybdoenzyme activity by binding to the dithiolene moiety of molybdopterin in Escherichia coli.

Abstract: Molybdenum insertion into the dithiolene group on the 6-alkyl side-chain of molybdopterin is a highly specific process that is catalysed by the MoeA and MogA proteins in Escherichia coli. Ligation of molybdate to molybdopterin generates the molybdenum cofactor, which can be inserted directly into molybdoenzymes binding the molybdopterin form of the molybdenum cofactor, or is further modified in bacteria to form the dinucleotide form of the molybdenum cofactor. The ability of various metals to bind tightly to sulfur-rich sites raised the question of whether other metal ions could be inserted in place of molybdenum at the dithiolene moiety of molybdopterin in molybdoenzymes. We used the heterologous expression systems of human sulfite oxidase and Rhodobacter sphaeroides dimethylsulfoxide reductase in E. coli to study the incorporation of different metal ions into the molybdopterin site of these enzymes. From the added metal-containing compounds Na2MoO4, Na2WO4, NaVO3, Cu(NO3)2, CdSO4 and NaAsO2 during the growth of E. coli, only molybdate and tungstate were specifically inserted into sulfite oxidase and dimethylsulfoxide reductase. Other metals, such as copper, cadmium and arsenite, were nonspecifically inserted into sulfite oxidase, but not into dimethylsulfoxide reductase. We showed that metal insertion into molybdopterin occurs beyond the step of molybdopterin synthase and is independent of MoeA and MogA proteins. Our study shows that the activity of molybdoenzymes, such as sulfite oxidase, is inhibited by high concentrations of heavy metals in the cell, which will help to further the understanding of metal toxicity in E. coli.

Effect of molybdenum and chromium on hardenability of low-carbon boron-added steels

Abstract: The hardenability of low-carbon boron-added steels containing molybdenum or chromium was studied using dilatometry, thermodynamic calculations, and secondary ion mass spectroscopy (SIMS). The combined addition of boron and molybdenum was found to be more effective than that of boron and chromium in enhancing the hardenability of boron-added steels. In particular, the addition of 0.5 wt.% molybdenum to the boron-added steel almost completely suppressed the formation of polygonal ferrite even at a slow cooling rate of 0.5℃/s. The synergistic effect of the combined addition of molybdenum and boron is thought to be due to both the suppression of M23(C,B)6 precipitation resulting from the deterioration of phase stability and the reduction of carbon diffusivity by the molybdenum addition.