Showing papers on "Carbide published in 2014"

Hollow Spheres of Iron Carbide Nanoparticles Encased in Graphitic Layers as Oxygen Reduction Catalysts

Abstract: Nonprecious metal catalysts for the oxygen reduction reaction are the ultimate materials and the foremost subject for low-temperature fuel cells. A novel type of catalysts prepared by high-pressure pyrolysis is reported. The catalyst is featured by hollow spherical morphologies consisting of uniform iron carbide (Fe3 C) nanoparticles encased by graphitic layers, with little surface nitrogen or metallic functionalities. In acidic media the outer graphitic layers stabilize the carbide nanoparticles without depriving them of their catalytic activity towards the oxygen reduction reaction (ORR). As a result the catalyst is highly active and stable in both acid and alkaline electrolytes. The synthetic approach, the carbide-based catalyst, the structure of the catalysts, and the proposed mechanism open new avenues for the development of ORR catalysts.

Multiple Phases of Molybdenum Carbide as Electrocatalysts for the Hydrogen Evolution Reaction

Abstract: Molybdenum carbide has been proposed as a possible alternative to platinum for catalyzing the hydrogen evolution reaction (HER). Previous studies were limited to only one phase, β-Mo2C with an Fe2N structure. Here, four phases of Mo-C were synthesized and investigated for their electrocatalytic activity and stability for HER in acidic solution. All four phases were synthesized from a unique amine-metal oxide composite material including γ-MoC with a WC type structure which was stabilized for the first time as a phase pure nanomaterial. X-ray photoelectron spectroscopy (XPS) and valence band studies were also used for the first time on γ-MoC. γ-MoC exhibits the second highest HER activity among all four phases of molybdenum carbide, and is exceedingly stable in acidic solution.

Trends in the Hydrogen Evolution Activity of Metal Carbide Catalysts

Abstract: Metal carbide catalysts are alternative nonprecious electrode materials for electrochemical energy conversion devices, such as for H2 fuel cells or electrolyzers. In this article, we report the experimental exchange current densities for the hydrogen evolution reaction (HER) on eight mono- and bimetallic carbide electrocatalysts and correlate the current densities to hydrogen binding energies that we have calculated via electronic structure computations. We find these materials to have activities higher than those of their parent metals and intermediate between the catalytic activities of the Pt group and early transition-metal surfaces. Increased HER activities on metal carbides relative to their parent metals can be understood with a 3-fold higher sensitivity of metal carbides to the coverage-induced weakening of hydrogen adsorption relative to metal surfaces. The trends presented here can be useful for the design of bimetallic carbide electrocatalysts.

Segregation stabilizes nanocrystalline bulk steel with near theoretical strength.

Abstract: Grain refinement through severe plastic deformation enables synthesis of ultrahigh-strength nanostructured materials. Two challenges exist in that context: First, deformation-driven grain refinement is limited by dynamic dislocation recovery and crystal coarsening due to capillary driving forces; second, grain boundary sliding and hence softening occur when the grain size approaches several nanometers. Here, both challenges have been overcome by severe drawing of a pearlitic steel wire (pearlite: lamellar structure of alternating iron and iron carbide layers). First, at large strains the carbide phase dissolves via mechanical alloying, rendering the initially two-phase pearlite structure into a carbon-supersaturated iron phase. This carbon-rich iron phase evolves into a columnar nanoscaled subgrain structure which topologically prevents grain boundary sliding. Second, Gibbs segregation of the supersaturated carbon to the iron subgrain boundaries reduces their interface energy, hence reducing the driving force for dynamic recovery and crystal coarsening. Thus, a stable cross-sectional subgrain size $l10\text{ }\text{ }\mathrm{nm}$ is achieved. These two effects lead to a stable columnar nanosized grain structure that impedes dislocation motion and enables an extreme tensile strength of 7 GPa, making this alloy the strongest ductile bulk material known.

ε -Iron carbide as a low-temperature Fischer–Tropsch synthesis catalyst

Abstract: e-Iron carbide is a promising catalyst for low-temperature Fischer–Tropsch synthesis but is difficult to synthesize. Here, the authors report a rapid-quenching process for the synthesis of nanocrystalline e-iron carbide, and evaluate the catalytic activity and selectivity of the material.

Engineering non-sintered, metal-terminated tungsten carbide nanoparticles for catalysis.

Abstract: Transition-metal carbides (TMCs) exhibit catalytic activities similar to platinum group metals (PGMs), yet TMCs are orders of magnitude more abundant and less expensive. However, current TMC synthesis methods lead to sintering, support degradation, and surface impurity deposition, ultimately precluding their wide-scale use as catalysts. A method is presented for the production of metal-terminated TMC nanoparticles in the 1–4 nm range with tunable size, composition, and crystal phase. Carbon-supported tungsten carbide (WC) and molybdenum tungsten carbide (MoxW1−xC) nanoparticles are highly active and stable electrocatalysts. Specifically, activities and capacitances about 100-fold higher than commercial WC and within an order of magnitude of platinum-based catalysts are achieved for the hydrogen evolution and methanol electrooxidation reactions. This method opens an attractive avenue to replace PGMs in high energy density applications such as fuel cells and electrolyzers.

Investigation of molybdenum carbide nano-rod as an efficient and durable electrocatalyst for hydrogen evolution in acidic and alkaline media

Abstract: A well-defined dimolybdenum carbide (Mo 2 C) nano-rod with a porous structure is successfully synthesized through facile carburization of anilinium molybdate in hydrogen. As a catalyst for hydrogen evolution reaction (HER), we investigated its electrical conductivity and catalytic activity. It is found that, as a standalone catalyst, Mo 2 C nano-rod shows a competitive performance for catalysing HER in acidic media. A composite catalyst based on Ni impregnated Mo 2 C nano-rod is further synthesized to investigate its catalytic behaviour and reaction mechanism in alkaline condition. Compared to commercial Mo 2 C, the enhanced performance of the Mo 2 C nano-rod could be attributed to its porous structure with small particles as well as its high electrical conductivity.

Nickel-tungsten carbide catalysts for the production of 2,5-dimethylfuran from biomass-derived molecules.

Abstract: The development of new catalytic systems for the conversion of biomass-derived molecules into liquid fuels has attracted much attention. We propose a non-noble bimetallic catalyst based on nickel-tungsten carbide for the conversion of the platform molecules 5-(hydroxymethyl)furfural into the liquid-fuel molecule 2,5-dimethylfuran (DMF). Different catalysts, metal ratios and reaction conditions have been tested and give rise to a 96% yield of DMF. The catalysts have been characterized and are discussed. The reaction mechanism is also explored through capture of reaction intermediates. The analysis of the reaction mixture over different catalysts is presented and helps to understand the role of nickel and tungsten carbide during the reaction.

Room‐Temperature Carbide‐Derived Carbon Synthesis by Electrochemical Etching of MAX Phases

Abstract: Porous carbons are widely used in energy storage and gas separation applications, but their synthesis always involves high temperatures. Herein we electrochemically selectively extract, at ambient ...

Core-shell bimetallic carbide nanoparticles confined in a three-dimensional N-doped carbon conductive network for efficient lithium storage.

Abstract: Carbides represent a class of functional materials with unique properties and increasing importance. However, the harsh conditions in conventional synthetic strategies impede subtle control over size and morphology of carbides, which is highly imperative for their practical applications. Herein, we report a facile, simple approach to prepare porous Co3ZnC/N-doped carbon hybrid nanospheres. In this structure, the Co3ZnC nanoparticles exhibit a core–shell structure and they are uniformly confined in N-doped carbon conductive networks forming rather uniform nanospheres. The hybrid nanospheres have a specific surface area as high as 170.5 m2 g–1. When evaluated as an anode material for lithium ion batteries, they show an excellent lithium storage performance, which can be attributed to the combined effect of the core–shell Co3ZnC nanoparticles, the pore structure and the highly conductive and elastic N-doped carbon networks. This work provides an efficient route for the facile production of nanoscale carbides...

Trends in Electrochemical Stability of Transition Metal Carbides and Their Potential Use As Supports for Low-Cost Electrocatalysts

Abstract: Early transition metal carbides (TMCs) can be used as supports for platinum-group metals as low-cost electrocatalysts. The determination of electrochemical stability of TMCs is important to identify their potential use in electrochemical and photoelectrochemical applications. Various TMC thin films were synthesized and characterized with X-ray diffraction and photoelectron spectroscopy. Chronopotentiometric-titration measurements were used to map the stability regions of the TMC thin films over a wide pH range. The stability of the TMC thin films was correlated to the oxygen binding energy of the parent metal. All of the TMCs studied are stable for hydrogen evolution/oxidation; most are stable for alcohol oxidation, and titanium, tantalum, and zirconium carbides are stable for oxygen evolution/reduction reactions.

Tensile Behavior of Ti,Mo-added Low Carbon Steels with Interphase Precipitation

Abstract: Tensile behavior and structure-property relationship of ferritic steels with nano-sized carbide dispersion were invesigated using Ti-added steel and Ti,Mo-added low carbon steels By austenitizing followed by isothermal heat treatment at 700°C, polygonal ferrites containing very fine carbides of TiC and (Ti,Mo)C were obtained in the Ti-added and the Ti,Mo-added steels, respectively The size of such carbides was finer in the Ti,Mo-added steel than in the Ti-added steel at the same isothermal holding The results of tensile tests for these samples showed that the strength is higher as the carbide size is smaller The structure-based strength calculation led to a good agreement with the experiments, when it was assumed that the Ashby-Orowan mechanism is dominant for precipitation strengthening of nano-sized alloy carbides It was also suggested that a relatively large tensile ductility is related to enhanced recovery during the tensile deformation, accompanied with promotion of secondary slips or cross slips in a finer scale due to the nano-sized particles

Oxidation behaviour of silicon carbide - a review

Abstract: Silicon Carbide as an inorganic material possesses properties like high thermo- chemical stability, high hardness and fracture toughness, low thermal expansion coefficient etc. It is therefore, widely used in the making of refractory, semiconductor devices, combustion en- gines, etc. Being a nonoxide, it has a tendency to get oxidized at elevated temperature under oxidizing atmosphere. Oxidation of silicon carbide can be either active or passive. Active oxidation reduces the strength of the samples whereas passive oxidation leads to the formation of coher- ent silica layer over silicon carbide surface, thereby improving its performances in several appli- cations. Being an interesting area of research, numerous works have been reported on the oxidation behaviour of silicon carbide. In this paper a comprehensive review has been made on different works related with the oxidation behaviour of silicon carbide.

Crystallization characteristics and chemical bonding properties of nickel carbide thin film nanocomposites

Abstract: The crystal structure and chemical bonding of magnetron-sputtering deposited nickel carbide Ni1−xCx (0.05≤x≤0.62) thin films have been investigated by high-resolution x-ray diffraction, transmissio ...

Effect of iron-carbide formation on the number of active sites in Fe–N–C catalysts for the oxygen reduction reaction in acidic media

Abstract: In this work Fe–N–C catalysts were prepared by the oxalate-supported pyrolysis of FeTMPPCl or H2TMPP either in the presence or absence of sulfur. The well-known enhancing effect of sulfur-addition on the oxygen reduction activity was confirmed for these porphyrin precursors. The pyrolysis process was monitored in situ by high-temperature X-ray diffraction under synchrotron radiation (HT-XRD) and thermogravimetry coupled with mass-spectroscopy (TG-MS). It was found that the beneficial effect of sulfur could be attributed to the prevention of iron-carbide formation during the heat-treatment process. In the case of pyrolysis of the sulfur-free precursors an excessive iron-carbide formation leads to disintegration of FeN4-centers, hence limiting the number of ORR active sites on the final catalyst. Physical characterization of the catalysts by bulk elemental analysis, X-ray diffraction (XRD), Raman and 57Fe Mosbauer spectroscopy confirmed the outcome from HT-XRD and TG-MS. It could be shown that the avoidance of carbide formation during pyrolysis represents a promising way to enhance the density of ORR active sites on those catalysts. This can be done either by sulfur-addition or the performance of an intermediate acid leaching. As iron carbide is often found as a by-product in the preparation of Fe–N–C catalysts this work gives some general strategies for enhancing the density of active sites enabling higher current densities.

Departures from the Adsorption Energy Scaling Relations for Metal Carbide Catalysts

Abstract: The activity of heterogeneous catalysts is often limited by a strong correlation between the chemisorption energies of reaction intermediates described by the “scaling relations” among the transition metals. We present electronic structure calculations that suggest that metal carbides do not in general follow the transition-metal scaling relations and tend to exhibit a carbophobic departure relative to the transition metals, meaning they tend to bind carbon-bound species weakly compared to oxygen-bound species. This contrasts with the oxophobic departure exhibited by Pt and Pd. Relative to the parent metals, carbides tend to bind carbon and oxygen more weakly and hydrogen more strongly. The departures are rationalized with the adsorbate–surface valence configuration and the energy of the metal sp-states. We employ these general trends to aid in the understanding of various catalytic properties such as the high activity of iron carbides for Fischer–Tropsch synthesis and Pt-group catalysts for partial oxida...

Co-optimization of wrought alumina-forming austenitic stainless steel composition ranges for high-temperature creep and oxidation/corrosion resistance

Abstract: A series of candidate alumina-forming austenitic (AFA) stainless steels designed to evaluate the effects of variation in Al, C, Cr, Mn, Nb, and Ni content on high-temperature tensile properties, creep, and oxidation/corrosion resistance were studied. The compositions assessed were based on medium Ni (20–25 wt%) and low Ni (12 wt%) AFA variations strengthened primarily by MC and/or M23C6 carbide precipitates, and a high Ni (32 wt%) AFA superalloy variation strengthened primarily by γ′-Ni3Al intermetallic precipitates. Tensile and creep properties were measured at 650 and 750/760 °C, oxidation resistance from 650 to 900 °C in air with water vapor and steam environments, and sulfidation–oxidation resistance in Ar–20%H2–20%H2O–5% H2S at 550 and 650 °C. Optimized composition ranges for different use temperatures ranges based on these evaluations are presented.

Tungsten Carbide–Nitride on Graphene Nanoplatelets as a Durable Hydrogen Evolution Electrocatalyst

Abstract: Alternatives to platinum-based catalysts are required to sustainably produce hydrogen from water at low overpotentials. Progress has been made in utilizing tungsten carbide-based catalysts, however, their performance is currently limited by the density and reactivity of active sites, and insufficient stability in acidic electrolytes. We report highly active graphene nanoplatelet-supported tungsten carbide-nitride nanocomposites prepared via an in situ solid-state approach. This nanocomposite catalyzes the hydrogen evolution reaction with very low overpotential and is stable operating for at least 300 h in harsh acidic conditions. The synthetic approach offers a great advantage in terms of structural control and kinetics improvement.

An Atom Probe Study of Kappa Carbide Precipitation and the Effect of Silicon Addition

Abstract: The influence of silicon on κ-carbide precipitation in lightweight austenitic Fe-30Mn-9Al-(0.59-1.56)Si-0.9C-0.5Mo cast steels was investigated utilizing transmission electron microscopy, 3D atom-probe tomography, X-ray diffraction, ab initio calculations, and thermodynamic modeling. Increasing the amount of silicon from 0.59 to 1.56 pct Si accelerated formation of the κ-carbide precipitates but did not increase the volume fraction. Silicon was shown to increase the activity of carbon in austenite and stabilize the κ-carbide at higher temperatures. Increasing the silicon from 0.59 to 1.56 pct increased the partitioning coefficient of carbon from 2.1 to 2.9 for steels aged 60 hours at 803 K (530 °C). The increase in strength during aging of Fe-Mn-Al-C steels was found to be a direct function of the increase in the concentration amplitude of carbon during spinodal decomposition. The predicted increase in the yield strength, as determined using a spinodal hardening mechanism, was calculated to be 120 MPa/wt pct Si for specimens aged at 803 K (530 °C) for 60 hours and this is in agreement with experimental results. Silicon was shown to partition to the austenite during aging and to slightly reduce the austenite lattice parameter. First-principles calculations show that the Si-C interaction is repulsive and this is the reason for enhanced carbon activity in austenite. The lattice parameter and thermodynamic stability of κ-carbide depend on the carbon stoichiometry and on which sublattice the silicon substitutes. Silicon was shown to favor vacancy ordering in κ-carbide due to a strong attractive Si-vacancy interaction. It was predicted that Si occupies the Fe sites in nonstoichiometric κ-carbide and the formation of Si-vacancy complexes increases the stability as well as the lattice parameter of κ-carbide. A comparison of how Si affects the enthalpy of formation for austenite and κ-carbide shows that the most energetically favorable position for silicon is in austenite, in agreement with the experimentally measured partitioning ratios.

Wear and corrosion resistance of niobium–chromium carbide coatings on AISI D2 produced through TRD

Abstract: Niobium carbide, chromium carbide and niobium–chromium carbide coatings were deposited using the thermo-reactive diffusion (TRD) deposition technique on AISI D2 steel substrates, and their wear and corrosion resistance was studied. The morphology of the coatings was characterized through optical and scanning electron microscopy (SEM), and the crystalline structure was studied through X-ray diffraction (XRD). Chemical composition was evaluated via energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The hardness of the coatings was measured through nanoindentation, and their wear was studied using the ball on disk test. The electrochemical behavior was assessed with potentiodynamic polarization and electrochemical impedance (EIS) tests. The XRD results show the formation of the NbC for the niobium carbide coating, Cr 23 C 6 and Cr 7 C 3 for the chromium carbide coating, and NbC, Cr 23 C 6 and Cr 7 C 3 for the niobium–chromium coating. Hardness value for the niobium–chromium carbide coating was 27.62 ± 2.56 GPa, which was higher in comparison to 21.66 ± 0.5 GPa for niobium carbide, 14.7 ± 1.1 GPa for chromium carbide and 6.70 ± 0.28 GPa for the uncoated steel. The wear resistance obtained was higher for the niobium–chromium carbide coating. However, its corrosion resistance was lower than the corrosion resistance for binary coatings.

Synthesis and characterization of sintered hybrid aluminium matrix composites reinforced with nanocopper oxide particles and microsilicon carbide particles

Abstract: The application of nano-sized particles is increasing because it strengthens the Metal Matrix composites (MMCs) and maintains the ductility of the matrix alloy. The present investigation deals with the synthesis and characterization of hybrid aluminium matrix reinforced with micro SiC particles, and nanocopper oxide (CuO) particles prepared by sintering process. First the powder mixtures containing fixed weight (wt)% of SiC and different wt% of nanocopper oxide as reinforcement constituents that are uniaxially cold pressed. Afterwards the green compacts are sintered in an electric muffle furnace. Microstructure and mechanical properties such as tensile strength, microhardness and density of the composites are examined. Microstructure of the samples has been investigated by using scanning electron microscope (SEM), X-ray diffraction (XRD) and Atomic Force Microscope (AFM). The results indicated that the increase in weight% of nano Cuo particles improves the mechanical properties.