Showing papers on "Cobalt published in 1995"

Kinetics of cadmium and cobalt desorption from iron and manganese oxides

Abstract: Oxides of Fe and Mn in soils are capable of sorbing large amounts of trace metal ions and can therefore be important in controlling trace metal concentrations in soil solution, and hence trace metal bioavailability in soils. There is, however, relatively little information on the rates of desorption of trace metals from oxide materials or on the factors affecting desorption rates. The objective of this study was to examine the kinetics of desorption of Cd and Co from two Fe oxides, goethite and ferrihydrite, and from two Mn oxides, hausmannite and cryptomelane. The concentrations of Cd and Co specifically sorbed by the oxides at pH 6.0 were greater for the Mn oxides than for the Fe oxides. The metals were also much less readily desorbed from the Mn than the Fe oxides and, in general, Cd was more readily desorbed than Co from all four oxides. Increasing the initial sorption period from 1 to 15 wk substantially decreased the proportion of sorbed Cd or Co subsequently desorbed from goethite, with a similar but much smaller effect also observed with the Mn oxides. Desorption kinetics for both Cd and Co were found to be described well by assuming either the occurrence of two simultaneous first-order desorption reactions, or by a continuous distribution of reaction sites, distributed lognormally with respect to desorption first-order rate constant. With increasing initial sorption period, the parameters obtained from fitting either type of kinetic equation to the experimental data could be interpreted as indicating a movement of metal ions to sites with slower desorption reactions.

Study of the deactivation mechanism of Al2O3-supported cobalt Fischer-Tropsch catalysts

Abstract: The influence of water on alumina-supported cobalt catalysts has been studied. The deactivation of supported Co catalysts was studied in a fixed-bed reactor using synthesis gas feeds containing different concentrations of water vapour. Supporting model studies were carried out using H2O/H2 feeds in conjunction with XPS and gravimetry. Rapid deactivation occurs on Re-promoted CO/Al2O3 catalysts when H2/CO/H2O feeds are used, whereas unpromoted CO/Al2O3 shows more stable activity. The results from the gravimetric studies suggest that only a small fraction of the bulk cobalt metal initially present reoxidizes to cobalt oxide during reaction. However, the XPS results indicate significant reoxidation of surface cobalt atoms or highly dispersed cobalt phases, which is likely to be the cause of the observed deactivation. Rhenium is shown to have a marked effect on the extent of reoxidation of alumina-supported cobalt catalysts.

Characterization of cobalt Fischer-Tropsch catalysts I. Unpromoted cobalt-silica gel catalysts

Abstract: The effect of preparation conditions on cobalt-silica gel catalysts has been investigated by temperature-programmed reduction, differential thermal analysis, X-ray photoelectron spectroscopy, and magnetic susceptibility measurements. The results have been correlated with the performance of the catalysts for the conversion of syngas. It has been found that active Fischer-Tropsch (F-T) cobalt catalysts can be partially or totally reduced to the metal phase during activation. However, reducibility of the catalysts is a necessary but not sufficient characteristic in ensuring good activity. The pH of the impregnating solution influences surface charge and solubility of silica gel. Consequently, the physical and chemical properties of the cobalt species on the surface of silica gel are strongly affected by this parameter. At pH

Cobalt-Schiff base complex catalyzed oxidation of para-substituted phenolics. Preparation of benzoquinones

Abstract: Para-substituted phenolics, serving as models for lignin (a renewable source of carbon), are oxidized to the corresponding benzoquinone with oxygen in the presence of catalytic amounts of Co-Schiff base complexes. The reaction products observed depend on the structure of the catalyst. The 5-coordinate catalysts (pyridine)[bis(salicylidene)ethylenediamine]cobalt[(pyr)Co(salen)]and[bis(salicylideneamino)ethylamine]cobalt [Co(n-Me salpr)] convert syringyl alcohol (3,5-dimethoxy-4-hydroxybenzyl alcohol) to 2,6-dimethoxybenzoquinone in high yield. In contrast, syringaldehyde (3,5-dimethoxy-4-hydroxybenzaldehyde) is unreactive toward these catalysts. However, the 4-coordinate Co(salen) converts syringaldehyde to 2,6-dimethoxybenzoiquinone in 72% isolated yield. Phenols bearing a single methoxy group on the ring are unreactive toward any catalyst in MeOH. However, vanillyl alcohol (3-methoxy-4-hydroxybenzyl alcohol) is converted to 2-methoxybenzo-quinone with Co(N-Me salpr) and oxygen in 43% yield in CH{sub 2}Cl{sub 2} and 58% yield in CH{sub 2}Cl{sub 2} in the presence of 1% CuCl{sub 2}. The success of the oxidations appears to be related to the ease of removal of the phenolic hydrogen by the Co/O{sub 2} complex. Competitive deactivation of the catalyst occurs with substrates of lower reactivity. 84 tabs.

Thermal barrier coating system with intermediate phase bondcoat

Abstract: A thermal barrier protected nickel based or cobalt based superalloy component for use in a gas turbine engine includes a thermal barrier coating system having a multi-layered structure. The first bondcoat layer of the thermal barrier coating system comprises a chemical vapor deposited, platinum modified diffusion aluminide layer on the superalloy component (substrate). The diffusion aluminide layer includes an inner diffusion zone proximate the substrate and an outer layer region comprising a platinum modified (platinum-bearing) intermediate phase of aluminum and at least one of nickel and cobalt depending on the superalloy composition. The intermediate phase is a non-ordered solid solution having a range of compositions and is free of other phase constituents. The intermediate phase has an average aluminum concentration in the range of about 18 to about 26% by weight, an average platinum concentration in the range of about 8 to about 35% by weight, and an average nickel concentration in the range of about 50 to 60% by weight and is non-stoichiometric relative to intermetallic compounds of aluminum and nickel, aluminum and cobalt, and aluminum and platinum. An adherent alpha alumina layer is thermally grown on the diffusion aluminide layer and receives an outer ceramic thermal barrier layer deposited thereon.

Survey of lead, cadmium, fluoride, nickel, and cobalt in food composites and estimation of dietary intakes of these elements by Canadians in 1986-1988.

Abstract: During the period 1986-1988, foods were purchased at the retail level in 5 Canadian cities and, for each city, prepared for consumption and combined into 113 composites and 39 composite subsets. Lead and cadmium were determined in all the samples; fluoride, in samples from Winnipeg; and cobalt and nickel, in samples from Montreal. Means and ranges of concentrations (ng/g) found in individual samples were lead, 23.2 (< 0.4-523); cadmium, 9.96 (< 0.02-167); fluoride, 325 (11-4970); nickel, 196 (< 0.6-2521); and cobalt, 9.4 (< 0.3-75.7). Estimated dietary intakes (microgram/day) of the elements over all ages and sexes were lead, 24; cadmium, 13; fluoride, 1763; nickel, 286; and cobalt, 11. During the period 1985-1988, the average level of lead in canned foods decreased from 73.6 to 46 ng/g.

Electrochemical preparation of nanostructured bimetallic clusters

Abstract: Recently the authors reported an electrochemical method for the size-selective preparation of tetraalkylammonium salt-stabilized transition-metal clusters in the size range of 1-6 nm. In this procedure, a sacrificial anode such as a palladium or nickel sheet is electrochemically oxidized with the formation of the corresponding metal ions which are reduced back to the zero oxidation state thereby forming metal clusters which are stabilized by the ammonium salts (which also functions as the electrolyte). The advantages include high yield, easy isolation, and simple control of particle size by adjustment of the current density. A combined TEM/STM study showed that the clusters are surrounded by a monomolecular layer of tetraalkylammonium salt. Electrochemically prepared cobalt clusters of this type have been shown to be superparamagnetic. 8 refs., 3 figs., 1 tab.

Magnetic switching in cobalt films by adsorption of copper

Abstract: MAGNETIC storage of information requires the ability to manipulate the magnetization of thin films with high sensitivity and spatial resolution. Non-magnetic overlayers are known to affect the characteristics of magnetic films: for example, the direction of magnetization of cobalt and iron films can be altered by deposition of a monolayer of copper and gold, respectively1–3. The magnetic properties of cobalt films seem to be particularly sensitive to copper overlayers—deposition of only sub-monolayer amounts of copper will decrease the coercive field required to invert the magnetization direction4. Here we show that copper coverages as small as three-hundredths of a monolayer are sufficient to rotate by 90° the magnetization of Co films up to 20 atomic layers thick. This implies that the spins of about 500 cobalt atoms switch direction for each copper atom added. Adding more copper eventually switches the magnetization back to its original direction. This fine tuning of thin-film magnetism might be useful for developing sensitive magnetic-field sensors, as well as for magnetic recording.

Synthesis and characterization of anisometric cobalt nanoclusters.

Abstract: Sonication of aqueous Co2+ and hydrazine resulted in the formation of anisometric (disk-shaped) cobalt nanoclusters that averaged about 100 nanometers in width and 15 nanometers in thickness. Electron diffraction from single particles revealed that they were oriented (001) crystals that conformed to a trigonal or hexagonal unit cell four times the size of the cell adopted by bulk α-cobalt. Lorentz microscopy indicated that these were single-magnetic domain particles, with the axis of magnetization located in the (101) plane, offset at some appreciable angle from the (001) axis.

Investigation of bifunctional zeolite supported cobalt Fischer-Tropsch catalysts

Abstract: ZSM-5, ZSM-11, ZSM-12 and ZSM-34 were examined as supports for cobalt bifunctional Fischer-Tropsch catalysts in order to determine the influence of zeolite pore structure on the synthesis. All supports produced reasonably active catalysts, with activity increasing with the channel size of the zeolite support (i.e. in the order of ZSM-34 ZSM-5 > ZSM-11 > ZSM-12. Contrary to initial expectations, the hydrocarbon products became lighter, and contained less n-alkanes as the acidity of the zeolite was reduced (which was in the same order as when the channel size of the zeolite was increased). This indicated that for this strongly acidic series of zeolites, accessibility to internal acid sites was more important in influencing the degree of secondary acid catalysed restructuring reactions of the primary Fischer-Tropsch products than was the strength or concentration of these acid sites.

The formation of cobalt silicates on Co/SiO2 under hydrothermal conditions

Abstract: A Co/SiO2 catalyst recovered after both gas and liquid phase Fischer-Tropsch synthesis exhibited significant decreases in the amount of reducible metal. Hydrothermal conditions similar to those occurring during Fischer-Tropsch synthesis were simulated in order to study compound formation in this Co/SiO2 catalyst. Hydrothermal treatment at 220°C led to a catalyst with lower reducibility, attributable to the formation of both reducible and nonreducible (<900°C) Co silicates. The formation of these compounds occurred only when metallic Co was present, was more pronounced in the presence of hydrogen, and was inhibited by air.

NO Reduction by Activated Carbon. 6. Catalysis by Transition Metals

Abstract: Some first series transition metals (Cr, Fe, Co, Ni, and Cu) have been investigated as catalysts of the NO reduction by carbon. An activated carbon was loaded by impregnation from an excess solution of metal nitrate. The NO-carbon reaction was studied in a fixed-bed flow reactor at atmospheric pressure, using two types of experiments : (i) a temperature-programmed reaction (TPR) in a NO/He mixture ; and (ii) an isothermal reaction at 300-600 °C. The products of the reactions were monitored allowing detailed oxygen and nitrogen balances. All the metals used catalyze the NO-carbon reaction, causing an important decrease in the activation energy and a substantial shift of the NO reduction curves toward lower temperatures. The TPR curves, the isothermal reactivity data, and the distribution of the products can be explained by means of an oxidation/reduction mechanism that implies a different oxidation states of the metal and the carbon matrix. The results show that, at low temperatures, iron, cobalt, and nickel are the most effective, as they are metals able to chemisorb NO dissociatively ; at high temperatures the activity is larger for cobalt and copper, metals whose oxides are reduced by carbon at a lower temperature. This order of activity seems to indicate that, at low temperatures (T 500 °C), the reduction of the intermediate oxidized metal species is the rate-limiting step.

Surface supported cobalt catalysts, process utilizing these catalysts for the preparation of hydrocarbons from synthesis gas and process for the preparation of said catalysts

Abstract: A supported particulate cobalt catalyst is formed by dispersing cobalt, alone or with a metal promoter, particularly rhenium, as a thin catalytically active film upon a particulate support, especially a silica or titania support. This catalyst can be used to convert an admixture of carbon monoxide and hydrogen to a distillate fuel constituted principally of an admixture of linear paraffins and olefins, particularly a C10+ distillate, at high productivity, with low methane selectivity. A process is also disclosed for the preparation of these catalysts.