Showing papers on "Molybdenum published in 2011"

Recent developments of molybdenum and tungsten sulfides as hydrogen evolution catalysts

Abstract: Recent work shows that nanoparticulate and amorphous molybdenum and tungsten sulfide materials are active catalysts for hydrogen evolution in aqueous solution. These materials hold promise for applications in clean hydrogen production technologies. In this perspective, the syntheses, structures and catalytic activities of nanoparticulate MoS2 and WS2, incomplete cubane-type [Mo3S4]4+ and amorphous MoSx films are summarized, compared, and discussed.

Low-Cost Molybdenum Carbide and Tungsten Carbide Counter Electrodes for Dye-Sensitized Solar Cells

Abstract: Carbide-based catalysts, MoC and WC embedded in ordered nanomesoporous carbon were developed for the redn. of triiodide in DSSCs. CV, EIS, Tafel polarization, and photocurrent/voltage tests confirm the excellent catalytic activity of the synthesized carbide-based composites - comparable to that of expensive Pt catalyst prepd. through pyrolysis. Com. Mo2C and WC particles also effectively catalyze the redn. of triiodide to iodide despite their large particle size. The results show that the addn. of P25 and CD (carbon dye) improves the adhesion, the catalytic activity and the cond. of Mo2C and WC electrodes. The optimum amts. of added P25 and CD added were also detd. Results demonstrate that molybdenum and tungsten carbides are potential alternatives to the expensive and scarce Pt in low-cost DSSCs.

Extending the Carbon Chain: Hydrocarbon Formation Catalyzed by Vanadium/Molybdenum Nitrogenases

Abstract: In a small-scale reaction, vanadium-dependent nitrogenase has previously been shown to catalyze reductive catenation of carbon monoxide (CO) to ethylene, ethane, propylene, and propane. Here, we report the identification of additional hydrocarbon products [α-butylene, n-butane, and methane (CH(4))] in a scaled-up reaction featuring 20 milligrams of vanadium-iron protein, the catalytic component of vanadium nitrogenase. Additionally, we show that the more common molybdenum-dependent nitrogenase can generate the same hydrocarbons from CO, although CH(4) was not detected. The identification of CO as a substrate for both molybdenum- and vanadium-nitrogenases strengthens the hypothesis that CO reduction is an evolutionary relic of the function of the nitrogenase family. Moreover, the comparison between the CO-reducing capacities of the two nitrogenases suggests that the identity of heterometal at the active cofactor site affects the efficiency and product distribution of this reaction.

Structural and Electronic Study of an Amorphous MoS3 Hydrogen‐Generation Catalyst on a Quantum‐Controlled Photosensitizer

Abstract: The design and synthesis of catalysts, especially for the production of solar fuels, is a major challenge in developing sources of renewable energy. Catalyst development requires an understanding of the mechanism(s) involved and the nature of the active site. While platinum group metals have unrivalled activity for both hydrogen and oxygen evolution, they are scarce and expensive. Photocatalytic systems relying on earth-abundant materials are therefore desirable for large scale energy production. Herein, we examine the structure and electronic properties of an amorphous molybdenum sulfide species and its possible use for photocatalytic hydrogen evolution. The catalyst was grown on a seeded quantum-rod sensitizer, a model system for investigating the photophysics of solar fuel generation. This catalyst s activity is shown experimentally to be associated with under-coordinated molybdenum centers, and we document that a reduced form of MoS3 is an active species for hydrogen generation. Molybdenum sulfides are prevalent in both biological enzymes and industrial catalysts. Mo metalloenzymes are involved in carbon, nitrogen, and sulfur metabolism, while synthetic molybdenum sulfides serve as industrial hydrotreating catalysts and are proven electrocatalysts for the hydrogen evolution reaction (HER). MoS2, [13,14] incomplete cubane [Mo3S4] 4+ clusters, molecular molybdenum catalysts, and amorphous MoS2 made by a reduction of MoS3 [19] have been shown to be active HER catalysts. Highly active HER catalysts, including Pt, have a Gibbs free energy of H adsorption (DGH) close to zero. [14] Density functional theory calculations show that the equatorial sulfur atoms in Fe–Mo cofactors in nitrogenase enzymes as well as the bridging S atom on the edge sites of MoS2 bind H atoms with DGH 0. These calculations, coupled with scanning tunneling microscopy (STM) studies, have indicated that molybdenum sulfide based hydrodesulfurization and HER catalysts derive their activities from under-coordinated atoms. Recent investigations of MoS2 nanoparticles using STM combined with electrochemical measurements have revealed that HER activity scales with the number of edge sites, rather than nanoparticle area, adding substantial evidence that undercoordination is critical to activity. There is also substantial current interest in molecularly thick and structurally disordered metal oxide and sulfide layers supported on electrodes, surfaces, and nanoparticles as potential catalysts for the HER and oxygen evolution reaction (OER). Such ultrathin films can support a variety of unusual and possibly favorable bonding geometries and may retain flexibility in healing and recovering. Despite their potential, such systems remain very difficult to characterize, impeding reproducibility and the communication of results between groups. Mechanisms are difficult to pin down when structural and electronic characterization is lacking. In this work, we use X-ray absorption techniques to obtain structural information on a catalytically active disordered molybdenum chalcogenide species that was grown on a wellcontrolled seeded quantum rod photosensitizer system with very high surface area. The high surface area of the colloidal system enables us to employ a variety of X-ray characterization techniques. Yet the system is also well-defined: Amorphous layers of MoS3 are deposited on quantumcontrolled photosensitizers. We take advantage of recent work showing that cadmium chalcogenide nanocrystals can be engineered to systematically control the separation of photogenerated holes and electrons, thus allowing us to modulate the photochemical yield of hydrogen. Nanorods of CdS grown on CdSe seeds with varying diameters and pure CdS nanorods of differing length were synthesized by a seeded-growth method previously reported. These particles have been of interest as a model system for investigating photochemical HER because their [*] Dr. M. L. Tang, D. C. Grauer, Dr. L. Amirav, Prof. J. R. Long, Prof. A. P. Alivisatos Department of Chemistry, University of California, Berkeley Material Sciences, Lawrence Berkeley National Laboratory (LBNL) Berkeley, CA 94720 (USA) E-mail: alivis@berkeley.edu jyano@lbl.gov

Synthesis and characterization of single-crystalline α-MoO3 nanofibers for enhanced Li-ion intercalation applications

Abstract: High quality, single-crystalline alpha-MoO3 nanofibers are synthesized by rapid hydrothermal method using a polymeric nitrosyl-complex of molybdenum(II) as molybdenum source without employing catalysts, surfactants, or templates. The possible reaction pathway is decomposition and oxidation of the complex to the polymolybdate and then surface condensation on the energetically favorable 001] direction in the initially formed nuclei of solid alpha-MoO3 under hydrothermal conditions. Highly crystalline alpha-MoO3 nanofibers have grown along 001] with lengths up to several micrometres and widths ranging between 280 and 320 nm. The alpha-MoO3 nanofibers exhibit desirable electrochemical properties such as high capacity reversibility as a cathode material of a Li-ion battery.

Laser ablation of thin molybdenum films on transparent substrates at low fluences

Abstract: The selective structuring of thin molybdenum (Mo) films is a major challenge for the monolithic interconnection of CIS thin film solar cells during their production. Here we present the structuring of ca. 0.5 μm thin molybdenum films on glass substrates with picosecond laser pulses (pulse duration 10 ps, wavelength 1064 nm) without any visible thermal effect on both, the remaining film and the substrate material. When the molybdenum film is irradiated from the transparent substrate side with a fluence level below 1 J/cm2 a “lift-off” process is initiated, which seems to be induced by a direct effect in the removed molybdenum film. At that fluence level, the energy input per ablated volume of ca. 30 J/mm3 is much less than would be needed for a thermodynamic heating, melting and vaporization of the complete film with ca. 78 J/mm3. Therefore we conclude that the molybdenum is only evaporated partially. Parts of the ablated Mo-film can be found as structurally intact debris. We assume that partial melting and vaporization with high-pressure formation play an important role during that picosecond laser ablation without thermal side effects. Due to its remarkable physical nature we called that process “directly induced laser ablation”.

Structural, mechanical and tribological properties of Mo–S–C solid lubricant coating

Abstract: Mo–S–C self-lubricating coatings were deposited by d.c. magnetron sputtering from carbon and molybdenum disulphide targets. The power ratio of the targets was varied in order to prepare films with carbon content in the range 0–55 at.%. Whatever the carbon content, the S/Mo ratio was higher than 1.25. The hardness of the films increased almost linearly with the carbon content. X-ray photoelectron spectroscopy showed evidence of Mo–C bonds; nevertheless, the size of molybdenum carbide grains was expected to be very small, since X-ray diffraction did not reveal any peaks related to any Mo–C phase. The coatings tested by pin-on-disc exhibited low friction, decreasing with increasing carbon content, when humid air was present. In nitrogen, the friction of all films was lower than 0.02 except for the reference MoS2 (0.04). Mo–S–C outperformed the wear resistance of MoS2; on the other hand, the results were in some cases hindered by the low adhesion of the coatings. The films were very sensitive to air exposure leading to surface oxidation.

Molybdenum Carbide-Catalyzed Conversion of Renewable Oils into Diesel-like Hydrocarbons

Abstract: In the paper, we report for the first time that the conversion of renewable oils into diesel-like hydrocarbon mixtures can be realized on molybdenum carbides with high activity and selectivity. The molybdenum carbide catalyst exhibited much better resistance to leaching than noble metals and could be reused consecutively for sixteen times without deactivation. Mechanism investigations indicated that molybdenum carbide and palladium showed different reaction selectivities and it was speculated that the level of difficulty in acyl-to-alkyl rearrangement of surface acyl intermediates on molybdenum carbide and palladium resulted in the different product selectivity.

Synthesis and Characterization of Nanostructured Niobium and Molybdenum Nitrides by a Two-Step Transition Metal Halide Approach

Abstract: Nanosized NbNx (1.64>x>1.33) and MoNx (1.32>x>0.77) crystallites were synthesized at a relatively low temperature (≥600°C) through ammonolysis and nitridation of the corresponding metal chlorides. The ammonolysis reaction in anhydrous chloroform at room temperature and the nitridation mechanism during subsequent heat treatment was investigated using Fourier transform infrared spectroscopy, X-ray diffraction (XRD), thermogravimetric (TG), and mass spectroscopy analysis. The effects of the synthesis parameters on the stoichiometry, structure, specific surface area, density, and electronic conductivity of the final nitrides were studied using TGA, XRD, N2 adsorption, high-resolution transmission microscopy, helium pycnometry, and four-point probe technique. Preliminary electrochemical responses of the obtained nitrides were studied using cyclic voltammetry to explore their possible use as electrochemical capacitors.

Nanoelectronics: Flat transistors get off the ground.

Abstract: The presence of a large bandgap means that a single layer of molybdenum disulphide can be used to make field-effect transistors with high on/off ratios and reasonably high mobilities.

Effect of neodymium oxide on the solubility of MoO3 in an aluminoborosilicate glass

Abstract: High molybdenum and rare earth concentrations in soda–lime aluminoborosilicate glasses may lead to the crystallization of molybdenum-rich phases such as alkali and alkaline-earth molybdates (Na2MoO4, CaMoO4) and also rare earth (RE)-rich phases such as apatite (Ca2RE8(SiO4)6O2) during melt cooling that must be controlled particularly during the preparation of highly radioactive nuclear glassy wasteforms. To understand the effect of neodymium addition (from 0 to 16 wt.% Nd2O3) on the phase separation and the crystallization tendency of molydbate phases and on the structure of a Mo-bearing nuclear glass belonging to the SiO2–B2O3–Al2O3–Na2O–CaO–MoO3 system, crystallization and structural studies have been performed by X-ray diffraction, scanning electron microscopy, electron microprobe analysis, Raman and optical absorption spectroscopies. The results obtained show that the addition of an increasing amount of Nd2O3 induces a significant increase of the solubility of molybdenum in the glass, characterized by a decrease of the phase separation and of the crystallization tendency of molybdate phases. The increase of chemical disorder in the structure of Mo-bearing glasses when Nd2O3 is added – and more precisely in the depolymerized regions where Nd3+ cations and [MoO4]2− entities are located – could be at the origin of the evolution of the molybdenum solubility in the glass.