Showing papers on "Molybdenum published in 2012"

Molybdenum Boride and Carbide Catalyze Hydrogen Evolution in both Acidic and Basic Solutions

Abstract: Molybdenum boride (MoB) and carbide (Mo2C) are excellent catalysts for electrochemical hydrogen evolution at both pH 0 and pH 14.

Hydrogen-evolution catalysts based on non-noble metal nickel-molybdenum nitride nanosheets.

Abstract: Hydrogen production through splitting of water has attracted great scientific interest because of its relevance to renewable energy storage and its potential for providing energy without the emission of carbon dioxide. Electrocatalytic systems for H2 generation typically incorporate noble metals such as Pt in the catalysts because of their low overpotential and fast kinetics for driving the hydrogen evolution reaction (HER). However, the high costs and limited world-wide supply of these noble metals make their application in viable commercial processes unattractive. Several non-noble metal materials, such as transition-metal chalcogenides, carbides, and complexes as well as metal alloys have been widely investigated recently, and characterized as catalysts and supports for application in the evolution of hydrogen. Nitrides of early transition-metals have been shown to have excellent catalytic activities in a variety of reactions. One of the primary interests in the applications of nitrides in these reactions was to use them in conjunction with low-cost alternative metals to replace group VIII noble metals. For example, the function of molybdenum nitride as a catalyst for hydrocarbon hydrogenolysis resembles that of platinum. The catalytic and electronic properties of transition-metal nitrides are governed by their bulk and surface structure and stoichiometry. While there is some information concerning the effect of the bulk composition on the catalytic properties of this material, there is currently little known about the effects of the surface nanostructure. Nickel and nickel–molybdenum are known electrocatalysts for hydrogen production in alkaline electrolytes, and in the bulk form they exhibited exchange current densities between 10 6 and 10 4 Acm , compared to 10 3 Acm 2 for Pt. Jaksic et al. postulated a hypo-hyper-d-electronic interactive effect between Ni and Mo that yields the synergism for the HER. Owing to their poor corrosion stability, few studies in acidic media have been reported.With the objective of exploiting the decrease in the overpotential by carrying out the HER in acidic media, we have developed a low-cost, stable, and active molybdenum-nitride-based electrocatalyst for the HER. Guided by the “volcano plot” in which the activity for the evolution of hydrogen as a function of the M H bond strength exhibits an ascending branch followed by a descending branch, peaking at Pt, we designed a material on the molecular scale combining nickel, which binds H weakly, with molybdenum, which binds H strongly. Here we report the first synthesis of NiMo nitride nanosheets on a carbon support (NiMoNx/C), and demonstrate the high HER electrocatalytic activity of the resulting NiMoNx/C catalyst with low overpotential and small Tafel slope. The NiMoNx/C catalyst was synthesized by reduction of a carbon-supported ammonium molybdate [(NH4)6Mo7O24·4H2O] and nickel nitrate (Ni(NO3)2·4H2O) mixture in a tubular oven in H2 at 400 8C, and subsequent reaction with NH3 at 700 8C. During this process, the (NH4)6Mo7O24 and Ni(NO3)2 precursors were reduced to NiMo metal particles by H2, and then they were mildly transformed to NiMoNx nanosheets by reaction with ammonia. The atomic ratio of Ni/Mo was 1/4.7 determined by energy dispersive X-ray spectroscopy (EDX) on the NiMoNx/ C sample. The transmission electron microscopy (TEM) images, as shown in Figure 1a, display NiMo particles that are mainly spherical. The high-resolution TEM image, as shown in the inset of Figure 1a, corroborated the presence of an amorphous 3 to 5 nm Ni/Mo oxide layer (see Figure S4 in the Supporting Information for resolved image), whereas NiMoNx is characterized by thin, flat, and flaky stacks composed of nanosheets with high radial-axial ratios (Figure 1b and Figure S5 in the Supporting Information for a magnified image). Figure 1c shows that some of the nanosheets lay flat on the graphite carbon (as indicated by the black arrows), and some have folded edges that show different layers of NiMoNx sheets (white arrows). The thickness of the sheets ranged from 4 to 15 nm. The average stacking number of sheets measured from Figure 1b is about [*] Dr. W.-F. Chen, Dr. K. Sasaki, Dr. J. T. Muckerman, Dr. R. R. Adzic Chemistry Department, Brookhaven National Laboratory Upton, NY 11973 (USA) E-mail: ksasaki@bnl.gov

The influence of hydrogenation and oxygen vacancies on molybdenum oxides work function and gap states for application in organic optoelectronics.

Abstract: Molybdenum oxide is used as a low-resistance anode interfacial layer in applications such as organic light emitting diodes and organic photovoltaics. However, little is known about the correlation between its stoichiometry and electronic properties, such as work function and occupied gap states. In addition, despite the fact that the knowledge of the exact oxide stoichiometry is of paramount importance, few studies have appeared in the literature discussing how this stoichiometry can be controlled to permit the desirable modification of the oxide’s electronic structure. This work aims to investigate the beneficial role of hydrogenation (the incorporation of hydrogen within the oxide lattice) versus oxygen vacancy formation in tuning the electronic structure of molybdenum oxides while maintaining their high work function. A large improvement in the operational characteristics of both polymer light emitting devices and bulk heterojunction solar cells incorporating hydrogenated Mo oxides as hole injection/ex...

Atomically thin layers of MoS2 via a two step thermal evaporation-exfoliation method.

Abstract: Two dimensional molybdenum disulfide (MoS2) has recently become of interest to semiconductor and optic industries. However, the current methods for its synthesis require harsh environments that are not compatible with standard fabrication processes. We report on a facile synthesis method of layered MoS2 using a thermal evaporation technique, which requires modest conditions. In this process, a mixture of MoS2 and molybdenum dioxide (MoO2) is produced by evaporating sulfur powder and molybdenum trioxide (MoO3) nano-particles simultaneously. Further annealing in a sulfur-rich environment transforms majority of the excess MoO2 into layered MoS2. The deposited MoS2 is then mechanically exfoliated into minimum resolvable atomically thin layers, which are characterized using micro-Raman spectroscopy and atomic force microscopy. Furthermore Raman spectroscopy is employed to determine the effect of electrochemical lithium ion exposure on atomically thin layers of MoS2.

A Review of Molybdenum Catalysts for Synthesis Gas Conversion to Alcohols: Catalysts, Mechanisms and Kinetics

Abstract: Recent literature on synthesis gas conversion to higher alcohols over Mo-based catalysts is reviewed. Density functional theory calculations show that Mo-CO adsorption is weakened by C, P, or S lig...

Large single crystals of graphene on melted copper using chemical vapour deposition

Abstract: A simple one-step method is presented for synthesizing large single crystal graphene domains on melted copper using atmospheric pressure chemical vapour deposition (CVD). This is achieved by performing the reaction above the melting point of copper (1090 °C) and using a molybdenum support to prevent balling of the copper from dewetting. By controlling the amount of hydrogen during growth, individual single crystal domains of monolayer graphene greater than 200 µm are produced, determined by electron diffraction mapping. Angular resolved photoemission spectroscopy is used to show the graphene grown on copper exhibits a linear dispersion relationship and has no sign of doping.

Molybdenum, Molybdenum Oxides, and their Electrochemistry

Abstract: The electrochemical behaviors of molybdenum and its oxides, both in bulk and thin film dimensions, are critical because of their widespread applications in steels, electrocatalysts, electrochromic materials, batteries, sensors, and solar cells. An important area of current interest is electrodeposited CIGS-based solar cells where a molybdenum/glass electrode forms the back contact. Surprisingly, the basic electrochemistry of molybdenum and its oxides has not been reviewed with due attention. In this Review, we assess the scattered information. The potential and pH dependent active, passive, and transpassive behaviors of molybdenum in aqueous media are explained. The major surface oxide species observed, reversible redox transitions of the surface oxides, pseudocapacitance and catalytic reduction are discussed along with carefully conducted experimental results on a typical molybdenum glass back contact employed in CIGS-based solar cells. The applications of molybdenum oxides and the electrodeposition of molybdenum are briefly reviewed.

Mechanistic Study of Codoped Titania with Nonmetal and Metal Ions: A Case of C + Mo Codoped TiO2

Abstract: To study the mechanism of metal- and nonmetal-ion-doped TiO2, TiO2 codoped with carbon and molybdenum prepared by a hydrothermal method following calcination post-treatment is chosen as the study object. The prepared samples are characterized by X-ray diffractmeter, Raman spectroscopy, X-ray photoelectron spectroscopy, and Brunauer–Emmett–Teller measurement. It is found that the doped carbon exists in the form of deposited carbonaceous species on the surface of TiO2, and molybdenum substitutes for titanium in the lattice and exists as the Mo6+ state. All the prepared samples have comparable large surface areas. The photocatalytic activities are tested by degradation of rhodamine-B and acetone under visible light irradiation. The results show that the codoped sample has the best performance in the degradation of both RhB and acetone. Briefly, the enhanced photocatalytic activity of codoped TiO2 is the synergistic effect of C and Mo. Mo substitutes in the Ti site in the lattice for the formation of the dopi...

Hydrodeoxygenation of guaiacol over carbon-supported molybdenum nitride catalysts: Effects of nitriding methods and support properties

Abstract: Molybdenum nitride catalysts supported on activated carbon materials with different textural and chemical properties were synthesized by nitriding supported Mo oxide precursors with gaseous NH 3 or N 2 /H 2 mixtures using a temperature-programmed reaction. The supports and catalysts were characterized by N 2 physisorption, XRD, chemical analysis, TPD, FT-IR and XPS. Guaiacol (2-methoxyphenol) hydrodeoxygenation (HDO) activities at 5 MPa and 300 °C were evaluated in a batch autoclave reactor. Molybdenum nitrides prepared using a N 2 /H 2 mixture resulted in more highly dispersed catalysts, and consequently more active catalysts, relative to those prepared using ammonolysis. The HDO activity was also related to pore size distribution and the concentration of oxygen-containing surface groups of the different carbon supports. Increased mesoporosity is argued to have facilitated the access to active sites while increased surface acidity enhanced their catalytic activity through modification of their electronic properties. The highest activity was thus attributed to the highest dispersion of the unsaturated catalyst species and the highest support mesoporosity. Surprisingly, addition of Co did not improve the HDO activity.

Nanostructuring of molybdenum and tungsten surfaces by low-energy helium ions

Abstract: The formation of metallic nanostructures by exposure of molybdenum and tungsten surfaces to high fluxes of low energy helium ions is studied as a function of the ion energy, plasma exposure time, and surface temperature. Helium plasma exposure leads to the formation of nanoscopic filaments on the surface of both metals. The size of the helium-induced nanostructure increases with increasing surface temperature while the thickness of the modified layer increases with time. In addition, the growth rate of the nanostructured layer also depends on the surface temperature. The size of the nanostructure appears linked with the size of the near-surface voids induced by the low energy ions. The results presented here thus demonstrate that surface processing by low-energy helium ions provides an efficient route for the formation of porous metallic nanostructures.

Mo-Substituted Lanthanum Tungstate La28–yW4+yO54+δ: A Competitive Mixed Electron–Proton Conductor for Gas Separation Membrane Applications

Abstract: Molybdenum substituted lanthanum tungstate, La28–y(W1–xMox)4+yO54+δ (x = 0–1, y = 0.923), was investigated seeking for an enhancement of the n-type electronic conductivity for its use as a mixed electron–proton conductor in hydrogen gas separation membrane applications. The materials were synthesized by the freeze-drying precursor method, and they were single phase after firing between 1300 and 1500 °C for x ≤ 0.8. The crystal structure changed from cubic (x ≤ 0.4) to rhombohedral (x ≥ 0.6) with increasing the molybdenum content. Transmission electron microscopy (TEM) investigations revealed an ordering of the oxygen vacancies with increasing Mo-content, giving rise to superstructure domains. The dependency of the conductivity with the oxygen and water partial pressure showed that these materials are good mixed electron–proton conductors under wet reducing conditions for x ≤ 0.4. The conductivity of the materials with x ≥ 0.6 was dominated by electrons, and they are expected to be less chemically stable d...

Carbon-supported molybdenum carbide catalysts for the conversion of vegetable oils.

Abstract: Ordered mesoporous carbon (OMC)-supported molybdenum carbide catalysts were successfully prepared in one pot using a solvent-evaporation-induced self-assembly strategy accompanied by a carbothermal hydrogen reduction reaction Characterization with nitrogen sorption, small-angle XRD, and TEM confirmed that the obtained materials had high surface areas, large pore volumes, ordered mesoporous structures, narrow pore size distributions, and uniform dispersions of molybdenum carbide particles With nitrogen replaced by hydrogen in the carbothermal reduction reaction, the formation temperature of molybdenum carbide could be reduced by more than 100 °C By changing the amount of molybdenum precursor added from less than 2 % to more than 5 %, molybdenum carbide structures could be easily regulated from Mo(2) C to MoC The catalytic performance of OMC-supported molybdenum carbide catalysts was evaluated by hydrodeoxygenation of vegetable oils Compared with Mo(2)C, MoC exhibited high product selectivity and excellent resistance to leaching in the conversion of vegetable oils into diesel-like hydrocarbons

Guaiacol transformation over unsupported molybdenum-based nitride catalysts

Abstract: Unsupported Mo 2 N catalysts were prepared by thermal decomposition of ammonium heptamolybdate to form MoO 3 followed by nitridation in either flowing ammonia or a N 2 /H 2 mixture. The nitridation was achieved at two different space velocities for each reaction mixture. This resulted in Mo 2 N catalysts with different surface areas as well as different phases present as were determined by X-ray diffraction (XRD). The activity and selectivity of the catalysts were evaluated for hydrodeoxygenation (HDO) of guaiacol in a batch autoclave at 300 °C and 5 MPa hydrogen pressure. The catalyst prepared with flowing ammonia and a space velocity of 29 h −1 displayed the highest activity (guaiacol conversion) and highest phenol/catechol ratio within the products indicative of HDO activity. This highest activity was attributed to the catalyst containing only γ-Mo 2 N phase (as measured by XRD) and having the highest N/Mo atomic ratio. An initial attempt to promote this catalyst with Co, similar to that done for hydrodesulfurization and hydrodenitrogenation catalysts was unsuccessful in forming a single phase cobalt molybdenum nitride, however did show modest improvements in the production of deoxygenated compounds, and suggests further studies into this material for HDO catalysis.

Method for manufacturing molybdenum oxide-containing thin film, starting material for forming molybdenum oxide-containing thin film, and molybdenum amide compound

Abstract: PROBLEM TO BE SOLVED: To provide a method for manufacturing a molybdenum oxide-containing thin film by a CVD process, in which excellent transportation performance for a precursor is achieved, the supply amount of the precursor to a substrate is easily controlled, the precursor can be stably supplied, and a high-quality molybdenum oxide-containing thin film can be manufactured with good mass-productivity.SOLUTION: The method for manufacturing a molybdenum oxide-containing thin film comprises: introducing vapor containing a molybdenum amide compound obtained by vaporizing a starting material for forming a thin film containing a compound expressed by general formula (I), onto a substrate; and further introducing oxidative gas to induce decomposition and/or a chemical reaction to form a thin film on the substrate. In the formula, Rand Rare each 1-4C straight-chain or branched alkyl; Ris t-butyl or t-amyl; y is 0 or 2; x is 4 when y is 0, and x is 2 when y is 2; and a plurality of Rand Rmay be respectively the same or different.

A molybdenum MEMS microhotplate for high-temperature operation

Abstract: This paper presents a fabrication process for high-temperature MEMS microhotplates that uses sputtered molybdenum as a conductive material. Molybdenum has a high melting point (2693 °C, bulk) and is simpler to deposit and pattern in larger series than platinum. Molybdenum is sensitive to oxidation above 300 °C, so during fabrication it is protected by PECVD silicon oxide. The electrical resistivity is linear with the temperature up to 700 °C at least. Molybdenum microhotplate has a higher maximum operating temperature than platinum which is demonstrated by the observation of the boiling of barium carbonate (BaCO3) microcrystals at 1360 °C. Annealing at 1100 °C is effective in extending the operating range. The molybdenum microhotplate performs far better than platinum also in terms of long-term resistance drift. The material properties of the sputtered molybdenum are studied and the overall performances of the microhotplate are tested, with regard to power consumption, temperature uniformity and dynamic behavior.

Synthesis and applications of molybdenum (IV) oxide

Abstract: Molybdenum dioxide (MoO2) is a transition metal oxide with unusual metal-like electrical conductivity and high catalytic activity toward reforming hydrocarbons This review covers the synthesis techniques used to fabricate MoO2 in a variety of morphologies and particle sizes Processing from molybdenum ore and reduction from MoO3 are also covered, with emphasis on reduction mechanisms and kinetic considerations Discussions of various solution-based and gas phase synthesis techniques shed light on strategies to achieve various unique morphologies, which leads into a brief discussion of nanoscale MoO2 Nanoscale MoO2 is of interest for important technological applications including catalysts for partial oxidation of hydrocarbons, solid oxide fuel cell anodes, and high-capacity reversible lithium ion battery anodes

Facile synthesis of graphene–molybdenum dioxide and its lithium storage properties

Abstract: Graphene–molybdenum dioxide composites in several ratios have been prepared through a facile synthesis method. Depending on the ratio, the as synthesized composites have either 2-dimensional graphene sheets with MoO2 particles anchored to them or a clustered agglomerate morphology. The composites have been characterised using Raman spectroscopy, X-ray diffraction, and electron diffraction to confirm the monoclinic MoO2 phase that is present. Lithium storage properties of the as-synthesised samples were tested in a coin-type half cell assembly to determine the relationship between the ratio and the electrochemical performance. The sample with highest amount of MoO2 (78 wt%) displayed the most promising lithium storage properties, with stable cycling performance at 0.2 A g−1 that shows negligible capacity loss over 50 cycles, retaining a capacity of 640 mA h g−1. The rate capabilities were also tested, and show a capacity of 380 mA h g−1 at 2.0 A g−1, which is comparable to the theoretical capacity of graphite and previously reported work on similar materials.