Showing papers on "Nickel published in 2005"

Characterization of Ni, NiMo, NiW and NiFe electroactive coatings as electrocatalysts for hydrogen evolution in an acidic medium

Abstract: The paper reports a study on the influence of alloying nickel by left-hand side transition metals (Fe, Mo, W) on the electrocatalytic activity in hydrogen evolution in an acidic environment. A number of experimental techniques were used in research (dc and ac electrochemical techniques, XRD, SEM, EDS, and ICP). The results clearly demonstrate that alloying nickel with Fe, Mo or W results in an increased electrocatalytic activity in hydrogen evolution when compared to pure nickel. Two effects were found to be responsible for the observed behavior; an increase in surface roughness and intrinsic activity of the material. Ni7.3Mo was found to yield the highest overall electrocatalytic activity among the investigated materials mainly due the highest surface roughness, while Ni3.4W was found to yield the highest intrinsic electrocatalytic activity, which is explained on the basis of the modification of electron density in d-orbitals upon alloying nickel with tungsten. It was also noticed that an increase in crystallinity can contribute to an increased electrocatalytic activity in hydrogen evolution.

Removal of Heavy Metals by Waste Tea Leaves from Aqueous Solution

Abstract: In this paper, tea leaves were shown to be an effective, low-cost biosorbent. Removal of lead, iron, zinc and nickel from 20 mg/L metal solution by dried biomass of waste tea leaves amounted to 96, 91, 72 and 58 %, respectively, at equilibrium, which followed Langmuir and Freundlich adsorption isotherms. Adsorption of metal was in the order of Pb > Fe > Zn > Ni from 5-100 mg/L of metal solution. From a multi-metallic mixture, 92.5, 84 and 73.2 % of lead, iron and zinc, respectively, were removed. Fourier transform infrared (FTIR) studies indicated that the carboxyl group was involved in the binding of lead and iron, whereas the amine group was involved in the binding of nickel and zinc. A flow through sorption column packed with dried biomass demonstrated a sorption capacity of 73 mg Pb/g of biomass, indicating its potential in cleaning metal containing wastewater. The metal laden biomass obtained could be disposed off by incineration.

Grain boundary decohesion by impurity segregation in a nickel-sulfur system.

Abstract: The sulfur-induced embrittlement of nickel has long been wrapped in mystery as to why and how sulfur weakens the grain boundaries of nickel and why a critical intergranular sulfur concentration is required. From first-principles calculations, we found that a large grain-boundary expansion is caused by a short-range overlap repulsion among densely segregated and neighboring sulfur atoms. This expansion results in a drastic grain-boundary decohesion that reduces the grain-boundary tensile strength by one order of magnitude. This decohesion may directly cause the embrittlement, because the critical sulfur concentration of this decohesion agrees well with experimental data on the embrittlement.

Direct Conversion of Carbon Fuels in a Molten Carbonate Fuel Cell

Abstract: Anodes of elemental carbon may be discharged in a galvanic cell using a molten carbonate electrolyte, a nickel-foam anode-current collector, and a porous nickel air cathode to achieve power densities of 40-100 mW/cm{sup 2}. We report cell and anode polarization, surface area, primary particle size and a crystallization index for nine particulate carbon samples derived from fuel oil, methane, coal, charred biological material and petroleum coke. At 800 C, current densities of 50-125 mA/cm{sup 2} were measured at a representative cell voltage of 0.8 V. Power densities for cells with two carbon-anode materials were found to be nearly the same on scales of 2.8- and 60 cm{sup 2} active area. Constant current operation of a small cell was accompanied by constant voltage during multiple tests of 10-30 hour duration. Cell voltage fell off after the carbon inventory was consumed. Three different cathode structures are compared, indicating that an LLNL fabricated porous nickel electrode with <10 {micro}m pores provides improved rates compared with nickel foam with 100-300 {micro}m pores. Petroleum coke containing substantial sulfur and ash discharges at a slightly lower rate than purified petroleum coke. The sulfur leads to degradation of the anode current collector over time. A conceptual modelmore » for electrochemical reactivity of carbon is presented which indicates the importance of (1) bulk lattice disorder, which continually provides surface reactive sites during anodic dissolution and (2) electrical conductivity, which lowers the ohmic component of anode polarization.« less

Nickel and either tantalum or niobium-codoped TiO2 and SrTiO3 photocatalysts with visible-light response for H2 or O2 evolution from aqueous solutions.

Abstract: TiO2 (band gap = 3.0 eV) and SrTiO3 (band gap = 3.2 eV) codoped with nickel and either tantalum or niobium ions showed photocatalytic activities for O2 evolution from an aqueous silver nitrate solution and H2 evolution from an aqueous methanol solution under visible light irradiation (λ > 420 nm). The visible-light responses were due to the charge-transfer transition from the electron donor levels formed by the 3d orbitals of doped Ni2+ to the conduction bands of the host materials. Codoping of a tantalum or niobium ion made the absorption intensity in the visible light region strong for TiO2 doped with nickel ions, resulting in the enhancement of the photocatalytic activity for O2 evolution under visible light irradiation. SrTiO3 codoped with nickel and tantalum showed higher activity for H2 evolution than SrTiO3 doped with only nickel. This is due to the fact that codoped Ta5+ ions played a role in charge compensation, resulting in the suppression of the formation of Ni3+ ions, which are expected to trap photogenerated electrons.

Corrosion behavior of carbon nanotubes–Ni composite coating

Abstract: Ni–carbon nanotube (CNTs) composite coatings were deposited on a carbon steel by electrodeposition The effect of incorporation of CNTs into the nickel coating on the morphology of the coating surface and corrosion properties was investigated Scanning electron microscopy (SEM) showed that the CNTs appear well dispersed in the nickel layer Corrosion tests were performed in aqueous NaCl (35 wt%) using a weight loss method and electrochemical measurements for bare, pure nickel coated and CNTs–nickel coated samples The results showed that addition of CNTs in the deposition process of nickel significantly increased the resistance to corrosion The anti-corrosion mechanism of the composite coating is also discussed

Reduction and Sintering of a Nickel–Dispersed‐Alumina Composite and Its Properties

Abstract: High-density nickel–dispersed-alumina (Al2O3/nickel) composites with superior mechanical properties were obtained by the hydrogen reduction and the hot pressing of alumina–nickel oxide (Al2O3/NiO) mixed powders. The mixtures were prepared by using NiO or nickel nitrate (Ni(NO3)2·nH2O) as a dispersion source of nickel metal. Microstructural investigations of the composite fabricated using nitrate powder revealed that fine nickel particles, } 100 nm in diameter, dispersed homogeneously at the matrix grain boundaries, forming the intergranular nanocomposite. High strength (.1 GPa) and high-temperature hardness were registered for the composite that contained a small amount of nickel dispersion. The ferromagnetic properties of nickel, such as high coercive force, were observed, because of the fine magnetic dispersions, which indicates a functional value of structural composites.

Carbon Formation on Nickel and Iron Oxide-Containing Oxygen Carriers for Chemical-Looping Combustion

Abstract: For combustion with CO2 capture, chemical-looping combustion with inherent separation of CO2 is a promising technology. Two interconnected fluidized beds are used as reactors. In the fuel reactor, a gaseous fuel is oxidized by an oxygen carrier, e.g., metal oxide particles, producing carbon dioxide and water. The reduced oxygen carrier is then transported to the air reactor, where it is oxidized with air back to its original form before it is returned to the fuel reactor. Carbon deposition on oxygen-carrier particles was investigated to assess whether it could have adverse effects on the process. The oxygen-carrier particles used were based on oxides of nickel and iron and produced by freeze granulation. They were sintered at 1300 degreesC for 4 h and sieved to a size range of 125-180 mum. The study of carbon deposition was performed in a laboratory fluidized-bed reactor, simulating a chemical-looping combustion system by exposing the sample to alternating reducing and oxidizing conditions. The particles with nickel oxide were tested at 750, 850, and 950 degreesC, and the particles with iron oxide at 950 degreesC. On the oxygen carrier with nickel oxide, only minor amounts of carbon formed during most of the reduction. However, when more than 80% of the oxygen available was consumed, significant carbon formation started. The formation of carbon was also clearly correlated to low conversion of the fuel. No carbon was formed on the oxygen carrier based on iron oxide. The interpretation for the actual application of this process is that carbon formation should not be a problem, because the process should be run under conditions of high conversions of the fuel.

Methanation of CO over Nickel: Mechanism and Kinetics at High H2/CO Ratios†

Abstract: The CO methanation reaction over nickel was studied at low CO concentrations and at hydrogen pressures slightly above ambient pressure. The kinetics of this reaction is well described by a first-order expression with CO dissociation at the nickel surface as the rate-determining step. At very low CO concentrations, adsorption of CO molecules and H atoms compete for the sites at the surface, whereas the coverage of CO is close to unity at higher CO pressures. The ratio of the equilibrium constants for CO and H atom adsorption, K(CO)/K(H), was obtained from the rate of CO methanation at various CO concentrations. K(H) was determined independently from temperature programmed adsorption/desorption of hydrogen to be K(H) = 7.7 x 10(-4) (bar(-0.5)) exp[43 (kJ/mol)/RT] and hence the equilibrium constants for adsorption of CO molecules may be calculated to be K(CO) = 3 x 10(-7) (bar(-1)) exp[122 (kJ/mol)/RT]. Furthermore, the rate of dissociation of CO at the catalyst surface was determined to be 5 x 10(9) (s(-1)) exp[-96.7 (kJ/mol)/RT] assuming that 5% of the surface nickel atoms are active for CO dissociation. The results are compared to equilibrium and rate constants reported in the literature.

First-principles calculation of the electronic structure and EELS spectra at the graphene/Ni(111) interface

Abstract: A spin-polarized first-principles calculation of the atomic and electronic structure of the graphene/Ni(111) interface is presented. Different structural models have been considered, which differ in the positions of the carbon atoms with respect to the nickel topmost layer. The most probable structure, which has the lowest energy, has been determined. The distance between the floating carbon layer and the nickel surface is found smaller than the distance between graphene sheets in bulk graphite, in accordance with experimental measurements. The electronic structure of the graphene layer is strongly modified by interaction with the substrate and the magnetic moment of the surface nickel atoms is lowered in the presence of the graphene layer. Several interface states have been identified in different parts of the interface two-dimensional Brillouin zone. Their influence on the electron energy loss spectra has been evaluated.

Thermal decomposition of nickel acetate tetrahydrate: an integrated study by TGA, QMS and XPS techniques

Abstract: The thermal decomposition of nickel carboxylates is a feasible route to synthesize metal nanoparticles suitable for catalytic purposes. The aim of this work is the characterization of the thermal decomposition course of nickel acetate tetrahydrate, Ni(CH 3COO)2·4H2O. A thermogravimetric (TGA) decomposition study carried out in three different atmospheres (air, helium and hydrogen) showed that the dehydration of the parent salt occurs between 118 and 137 ◦ C. However, irrespective of the chosen atmosphere, the sample weight loss registered in this stage remains invariable, suggesting the formation of a an intermediate basic acetate with the formula 0.86Ni(CH 3COO)2·0.14Ni(OH)2. The dehydration step was followed at ca. 350 ◦ C by the subsequent one-step major decomposition of the acetate group, producing NiO and Ni, in treatment atmospheres of air and hydrogen, respectively, but there was some indication of an additional step when the thermolysis was conducted in helium. The conclusions possible from thermal analysis were confirmed by monitoring evolved gases employing quadrupole mass spectrometry (QMS), and a set of reactions linked to the decomposition of the acetate group has been proposed to account for most of the gas products detected. X-ray photoelectron spectroscopy (XPS) was used to investigate the solid phases obtained during the thermal decomposition of the salt in He atmosphere. © 2004 Elsevier B.V. All rights reserved.

Nickel-catalyzed cross-coupling reaction of aryl fluorides and chlorides with grignard reagents under nickel/magnesium bimetallic cooperation.

Abstract: Nickel-catalyzed cross-coupling of Grignard reagents with aryl (poly)fluorides or (poly)chlorides can be achieved efficiently in the presence of a new triarylphosphine ligand bearing a nearby hydroxy group. The high reactivity and the unique chemoselectivity (ArF > ArOTf ≫ ArSR) of the catalysis have been attributed to synergy of nickel and magnesium atoms preorganized on the ligand, as has been surmised on the basis of theoretical modeling of the reaction mechanism.

Deposition precipitation for the preparation of carbon nanofiber supported nickel catalysts.

Abstract: Deposition precipitation of nickel hydroxide onto modified carbon nanofibers has been studied and compared to deposition onto silica. The carbon nanofiber support materials consisted of graphite-like material of the fishbone-type with a diameter of 20−50 nm and a specific surface area of 150 m2/g. Modification involved surface oxidation (CNF-O) optionally followed by partial reduction (CNF-OR) or thermal treatment (CNF-OT). Titration of the support materials showed the presence of 0.17 and 0.03 mmol/g carboxylic acid groups for CNF-O and CNF-OR, respectively. For the CNF-OT only basic groups were present. The deposition precipitation of 20 wt % nickel onto these supports has been studied by time dependent pH and nickel loading studies. With silica, nickel ion adsorption did not occur prior to nucleation of the nickel hydroxide phase at pH = 5.6. With CNF-O, nickel ion adsorption took place right from the start of the deposition process at pH = 3.5, and at pH = 5.6 already 4 wt % nickel was adsorbed. Nucle...

Carcinogenic effect of nickel compounds.

Abstract: Nickel is a widely distributed metal that is industrially applied in many forms. Accumulated epidemiological evidence confirms that exposures to nickel compounds are associated with increased nasal and lung cancer incidence, both in mostly occupational exposures. Although the molecular mechanisms by which nickel compounds cause cancer are still under intense investigation, the carcinogenic actions of nickel compounds are thought to involve oxidative stress, genomic DNA damage, epigenetic effects, and the regulation of gene expression by activation of certain transcription factors related to corresponding signal transduction pathways. The present review summarizes our current knowledge on the molecular mechanisms of nickel carcinogenesis, with special emphasis on the role of nickel induced reactive oxygen species (ROS) and signal transduction pathways.