Interactions between hydrogen and tungsten carbide: a first principles study
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Tungsten carbide has been proposed to be a potential inexpensive alternative to platinum for catalyzing hydrogenation reactions. Using density functional theory (DFT) calculations, we performed a systematic study on the geometries and electronic structures of small (WC)n (n = 1–10) clusters. A cubic-like growth of WC clusters was found to be the preferred pathway. The interactions between the WC clusters and hydrogen were compared to interactions of hydrogen with the WC(0001) surface. Both atomic clusters and the crystalline surface of WC can lead to facile dissociation of H2 while the activation energy on the former is even lower; the H diffusion barrier on WC clusters was found to be significantly higher than that on the WC(0001) surface, yet both of the calculated diffusion barriers are well above the values on the platinum counterpart. The lack of hydrogen mobility may exert a profound influence on the kinetics of hydrogen for participating in reactions on WC. The desorption strength of H on (WC)n (n = 2, 4, 6, 9) at various hydrogen coverages was explored and the hydrogen saturated (WC)9 cluster was used to rationalize the catalytic formation of hydrogen tungsten bronzes. The study provides insights which may be useful for efforts to replace Pt with WC in catalytic processes involving hydrogen.Keywords:
Tungsten Carbide
TRACER
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Organometallic Chemistry
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Positron lifetimes and intensities were compared, with and without external mixing, for simulated hydrogenation reaction mixtures consisting of 2/1, 1/1, 0.25/2, 0.5/2, 0.75/2, 1/2, and 1.5/2 ethanol/cis-2-pentene molar ratios to which 0.6 mg platinum powder or 5% platinum on charcoal were added. For platinum mixtures marked quenching of all o-Ps lifetimes and intensities occurred with external mixing. For 1/2 and 1.5/2 platinum-on-charcoal mixtures, the combined o-Ps intensity increased from 66% to 76% and from 30% to 76%, respectively, with external mixing.
Charcoal
Annihilation
Pentene
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Considerable attention has been paid to Mg-Ni based hydrogen storage alloys because of their high capacity and low cost.The Mg_(2)Ni alloys have been successfully synthesized under 900 ℃ by means of a metallurgical method in an induction furnace.The studies of X-ray diffraction and metallorgraphic structure reveal that there are pricipally Mg_(2)Ni phases in the alloys.The measurements of the hydrogen storage properties for the alloys have been carried out.The results show that the hydrogen storage capacities of 3.14 wt% and 3.49 wt% of the alloy powders are observed after the first and the second activation respectivelly.The latter is closed to the maximum of hydrogen storage capacity of Mg_(2)Ni.The hydrogen storage capacity of the bulk samples is much less than the alloy powders after the second activation and there is not obviously increase in hydrogen storage capacity of the samples after the third activation.
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Magnesium borohydride Mg(BH4)2,having gravimetric and volumetric hydrogen densities of 14.9 wt.% and 112g/L,respectively,is considered as one of the most promising materials for hydrogen storage.Extensive investigations have been paid on this complex hydride in the past few years.We summarized research progresses on the synthesis,crystal structure and hydrogen storage performance of Mg(BH4)2 in this paper.Pending issues,such as kinetic barrier and reversibility of hydrogen storage in Mg(BH4)2,were discussed,and further development of this storage material was suggested.
Borohydride
Gravimetric analysis
Magnesium hydride
Cryo-adsorption
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Surface oxides on smooth platinum, electrodeposited platinum black, and platinum black‐polytetrafluoroethylene molded electrodes have been characterized by anodic oxidation and cathodic stripping. The oxidation sequence for smooth platinum and platinum black electrodes is . At higher potentials smooth platinum forms and while platinum black forms . The initial oxidation of platinum to and subsequent reduction has been used as a basis for the determination of platinum electroactive surface areas. The formation of a "tight" structure on smooth platinum at higher potentials may account for some of the unusual properties of smooth platinum electrodes. The failure of platinum black to form tight may be due to irregular platinum atom arrangement and related to its catalytic properties.
Platinum black
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A micro-structuring of the tungsten plasma-facing surface can strongly reduce near surface thermal stresses induced by ELM heat fluxes. This approach has been confirmed by numerical simulations with the help of ANSYS software. For experimental tests, two 10 × 10 mm2 samples of micro-structured tungsten were manufactured. These consisted of 2000 and 5000 vertically packed tungsten fibres with dimensions of Ø240 µm × 2.4 mm and Ø150 µm × 2.4 mm, respectively. The 1.2 mm bottom parts of the fibres are embedded in a copper matrix. The top parts of the fibres have gaps about of 10 µm so they are not touching each others. The top of all tungsten fibres was electro-polished. A Nd:YAG laser with a pulse duration 1 ms and a pulse repetition frequency of 25 Hz was used to simulate up to 105 ELM-like heat pulses. No damage on either of the micro-structured tungsten samples were observed. Neon plasma erosion rate and fuel retention of the micro-structured tungsten samples were almost identical to bulk tungsten samples.
Pulse duration
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Tungsten Carbide
Carbon fibers
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Tungsten Carbide
Carbon fibers
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Feasibility and cost data are supplied for the production of tungsten-rhenium alloys from tungsten targets in the N-Reactor. The two types of target elements assumed were: (a) tungsten containing 90 a/o tungsten-186, 9 a/o tungsten-184 and 1 a/o tungsten-183 and 182, and (b) tungsten of natural isotopic composition (28.4 a/o tungsten-186, 30.6 a/o tungsten-184, 14.4 a/o tungsten 183, and 26.4 tungsten-182). It is assumed that the average thermal neutron capture cross section for the tungsten-186 is 32 barns.
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