Showing papers on "Cobalt published in 1997"

Design, synthesis, and use of cobalt-based Fischer-Tropsch synthesis catalysts

Abstract: Catalyst productivity and selectivity to C5+ hydrocarbons are critical design criteria in the choice of Fischer-Tropsch synthesis (FTS) catalysts and reactors. Cobalt-based catalysts appear to provide the best compromise between performance and cost for the synthesis of hydrocarbons from CO/H2 mixtures. Optimum catalysts with high cobalt concentration and site density can be prepared by controlled reduction of nitrate precursors introduced via melt or aqueous impregnation methods. FTS turnover rates are independent of Co dispersion and support identity over the accessible dispersion range (0.01–0.12) at typical FTS conditions. At low reactant pressures or conversions, water increases FTS reaction rates and the selectivity to olefins and to C5+ hydrocarbons. These water effects depend on the identity of the support and lead to support effects on turnover rates at low CO conversions. Turnover rates increase when small amounts of Ru (Ru/Co<0.008 at.) are added to Co catalysts. C5+ selectivity increases with increasing Co site density because diffusion-enhanced readsorption of α-olefins reverses, β-hydrogen abstraction steps and inhibits chain termination. Severe diffusional restrictions, however, can also deplete CO within catalyst pellets and decrease chain growth probabilities. Therefore, optimum C5+ selectivities are obtained on catalysts with moderate diffusional restrictions. Diffusional constraints depend on pellet size and porosity and on the density and radial location of Co sites within catalyst pellets. Slurry bubble column reactors and the use of eggshell catalyst pellets in packed-bed reactors introduce design flexibility by decoupling the characteristic diffusion distance in catalyst pellets from pressure drop and other reactor constraints.

Handbook of Nutritionally Essential Mineral Elements

Abstract: Calcium phosphorus sodium and chloride in nutrition magnesium potassium zinc copper iron magnesium cobalt chronium molybdenum nickel boron lithium lead selenium iodine fluorine silicon vanadium arsenic mineral-iron interaction as assessed by bioavailability and ion channel function radiotracers for nutritionally essential mineral elements.

An FT-IR study of ammonia adsorption and oxidation over anatase-supported metal oxides

Abstract: The adsorption and the oxidation of ammonia over sub-monolayer TiO2-anatase supported chromium, manganese, iron, cobalt, nickel and copper oxides, has been investigated using FT-IR spectroscopy. These materials are models of catalysts active in the Selective Catalytic Reduction of NOx by ammonia (SCR process) and in the Selective Catalytic Oxidation of ammonia to dinitrogen (SCO process). For comparison, the adsorption of ammonia and hydrazine over the TiO2-anatase support has also been studied. CrOx TiO2 adsorbs ammonia both in a co-ordinated form over Lewis acid sites and in a protonated form over Bronsted acid sites, involving high-valence chromium (chromyl species). However, simple outgassing at r.t. causes the desorption of ammonia from Bronsted acid sites showing that they are very weak. All other catalysts do not present any Bronsted acidity. Co-ordinated ammonia gives rise to several oxidation products over Fe2O3 TiO2, CrOx TiO2, CoOx TiO2 and CuO TiO2, among which hydrazine is likely present. Other species have been tentatively identified as imido species, NH, nitroxyl species, HNO, and nitrogen anions,N−2. NiOx TiO2 and MnOx TiO2 appear to be even more active in ammonia oxidation, because the adsorbed species disappeared completely at lower temperature (473 K) than in the other cases. However, possibly just due to their excessive activity, no adsorbed species different from co-ordinated ammonia can be found in significant amounts over these surfaces. Based on these data, the mechanism of the SCR and SCO processes over these catalytic materials is discussed. In particular, it is concluded that Bronsted acidity is not a requirement for SCR and SCO activity.

Spectroscopy and Electrochemistry of Cobalt(III) Schiff Base Complexes

Abstract: The structural, spectroscopic, and electrochemical properties of cobalt(III) derivatives of acacen (H2acacen = bis(acetylacetone) ethylenediimine) and related ligands have been investigated. Electronic structure calculations indicate that the absorption between 340 and 378 nm in CoIII(acacen) spectra is attributable to the lowest π−π* intraligand charge-transfer transition. Equatorial ligand substitutions affect reduction potentials less than axial ligand changes, consistent with an electronic structural model in which dz2 is populated in forming cobalt(II). The crystal structure of [Co(3-Cl-acacen)(NH3)2]BPh4 has been determined: The compound crystallizes in the monoclinic space group P21/m (No. 11) with a = 9.720(2) A, b = 18.142(4) A, c = 10.046(2) A, β = 100.11(3)°, Dc = 1.339 g cm-3, and Z = 2; the complex cation, [Co(3-Cl-acacen)(NH3)2]+, exhibits a slightly distorted octahedral coordination geometry. The distances between the cobalt atom and the two axial nitrogen donor atoms differ only slightly ...

Magnetoresistance in the oxygen deficient LnBaCo2O5.4 (Ln=Eu, Gd) phases

Abstract: New “112” phases, LnBaCo2O5.4, with an ordered oxygen deficient perovskite structure, derived from the YBaFeCuO5-type were studied for Ln=Eu, Gd. The appearance of giant negative magnetoresistance in this structural type is demonstrated. Resistance ratio R0/R7 T reaches at least 10 at 10 K, i.e., is significantly larger than those observed in the other cobalt perovskites, such as La1−xSrxCoO3. These properties are linked to an original magnetic behavior of these materials that exhibit two types of transition—antiferromagnetic to ferromagnetic, and ferromagnetic to paramagnetic—as T increases. This magnetic behavior may be related to a possible I.S. and L.S. spin ordering of trivalent cobalt in pyramidal and octahedral coordinations, respectively.

Cobalt(III) Polyamine Complexes as Catalysts for the Hydrolysis of Phosphate Esters and of DNA. A Measurable 10 Million-Fold Rate Increase1

Abstract: Complexes between cobalt(III) and eight different 1,4,7,10-tetraazacyclododecane (cyclen) as well as two tris(3-aminopropyl)amine (trpn) derivatives are reported with varying numbers and structures of peralkylammonium groups in side chains of the ligands. The presence of additional positive charges has small effects on hydrolysis rates of nitrophenyl- and bis(nitrophenyl)phosphate esters but leads to substantially enhanced cleavage of plasmid DNA. Increasing the number of the charged side groups and/or their distance to the metal ion center provides for better binding to the DNA groove, as shown also by affinity measurements with calf-thymus DNA. In line with this, saturation kinetics of plasmid DNA cleavage yield a corresponding increase of efficiency in Michaelis−Menten-type KM values, with rather constant kcat parameters. A binuclear cobalt complex with two cyclen centers separated by a −(CH2)6-N+(CH3)2-(CH2)6-N+(CH3)2-(CH2)6− spacer shows, with only 5 × 10-5 M catalyst concentration, the largest known...

Optical recognition of CO and H2 by use of gas-sensitiveAu–Co3O4 composite films

Abstract: Gold–cobalt oxide (Au–Co 3 O 4 ) composite films have been prepared by the sputter-deposition of gold onto a glass plate substrate followed by pyrolysis of spin-coated cobalt 2-ethylhexanoate. The films comprise small Au particles and Co 3 O 4 nanocrystals, and exhibit different and independent optical responses to CO and H 2 in air. Carbon monoxide caused only a decrease in absorbance, while H 2 caused both a decrease and an increase in absorbance at different wavelengths. Among solid-state gas sensor materials, this Au–Co 3 O 4 composite film is probably the first example of an inorganic material which can be used for the recognition of CO and H 2 molecules through optical absorbance changes.

Induced Codeposition III. Molybdenum Alloys with Nickel, Cobalt, and Iron

Abstract: A comparison was made between the codeposition behavior of NiMo, CoMo, and FeMo alloys on rotating cylinder electrodes when the molybdate concentration in the electrolyte was much lower than that of the iron-group species. More molybdate was codeposited with Co than with either Fe or Ni from an ammonia-citrate electrolyte at pH 7.4. During the NiMo codeposition, the molybdate species is mass transport controlled. Substitution of the nickel by cobalt in the plating bath does not influence the molybdenum deposition rate. However, a higher concentration of molybdate is found in the deposits because the rate of cobalt deposition is lower than that of nickel. On the other hand, substitution of the nickel by iron results in a dramatic lowering of the molybdenum deposition rate. These observations were described by a mathematical model which assumes that iron-group species can adsorb on the electrode surface, competing with the molybdenum intermediate for free surface sites. Thus the diminished partial current density of the molybdenum in iron containing electrolytes can be explained by a blocking mechanism due to the adsorbed iron intermediate.

Syntheses and Characterizations of Chiral Tetrahedral Cobalt Phosphates with Zeolite ABW and Related Frameworks

Abstract: The hydrothermal syntheses, X-ray crystal structures, and magnetic properties of a family of 3-D chiral framework cobalt phosphates are presented. Two different types of framework structures are described: one has the same topology as the well-known zeolite ABW framework and represents the only pure cobalt phosphate framework with a known zeolite topology; the other exhibits a framework connectivity which can be considered as a hybrid of tridymite and ABW frameworks. These new chiral materials were obtained by systematic chemical variation, accurate control of solution pH values, and use of non-aqueous solvents. Synthesis conditions were found which favored the crystallization of Co2+ oxygen tetrahedra in strict alternation with P5+ oxygen tetrahedra from cobalt phosphate solutions in the presence of Na+, K+, NH4+, or Rb+ ions. The transition from one framework type to the other is affected by the size of these extra-framework cations. The two ABW structures (NH4CoPO4-ABW, RbCoPO4) have eight-ring channe...

Nitrogen-containing carbon nanotubes

Abstract: Carbon nanotubes containing small amounts of nitrogen are produced by the pyrolysis of aza-aromatics such as pyridine, methylpyrimidine and triazine over cobalt nanoparticles in an Ar atmosphere; good yields of such nanotubes are obtained by carrying out the pyrolysis of a mixture of pyridine and Fe(CO)(5) in flowing Ar + H-2.

Polymerization of ethylene with specific iron or cobalt complexes, novel pyridinebis(imines) and novel complexes of pyridinebis(imines) with iron and cobalt

Abstract: Ethylene may be polymerized by contacting it with certain iron or cobalt complexes of selected 2,6-pyridinecarboxaldehydebis(imines) and 2,6-diacylpyridinebis(imines). The polymers produced are useful as molding resins. Novel 2,6-pyridinecarboxaldehydebis(imines) and 2,6-diacylpyridinebis(imines), and novel complexes of 2,6-pyridinecarboxaldehydebis(imines) and 2,6-diacylpyridinebis(imines) with iron and cobalt are also disclosed.

Synthesis and Characterization of Cobalt-Complex Functionalized MCM-41

Abstract: Silanation of the internal pore surface of the mesoporous molecular sieve MCM-41 (40 A) has yielded ethylenediamine (ED), diethylenetriamine (DET), and ethylenediaminetriacetic acid salt (EDT) functionalized sites. These grafted ligands were used for the preparation of cobalt(II) complexes covalently bound to the MCM-41 support. These materials were characterized by SEM, UV−vis, FTIR, ESR, and cyclic voltammetry as well as elemental analysis. Preliminary studies suggest the formation of a reversible cobalt−oxygen adduct with MCM-41 grafted ED and DET. The nature and reactivity of the MCM-41 bound complexes were found to differ significantly from functionalized amorphous silica.

Etch process selective to cobalt silicide for formation of integrated circuit structures

Abstract: An etch process selective to cobalt silicide is described for the selective removal of titanium and/or titanium nitride, unreacted cobalt, and cobalt reaction products other than cobalt silicide, remaining after the formation of cobalt silicide on an integrated circuit structure on a semiconductor substrate in preference to the removal of cobalt silicide. The first step comprises contacting the substrate with an aqueous mixture of ammonium hydroxide (NH 4 OH) and hydrogen peroxide (H 2 O 2 ) to selectively remove any titanium and/or titanium nitride in preference to the removal of cobalt silicide. The second step comprises contacting the substrate with an aqueous mixture of phosphoric acid (H 3 PO 4 ), acetic acid (CH 3 COOH), and nitric acid (HNO 3 ) to selectively remove cobalt and cobalt reaction products (other than cobalt silicide) in preference to the removal of cobalt silicide. In a preferred embodiment, the substrate is contacted by the respective etchant systems by spraying the respective etchant mixtures onto the substrate rather than immersing the substrate into baths containing the respective etchant mixtures.

Elevated Serum Cobalt With Metal-on-metal Articulating Surfaces

Abstract: We determined serum cobalt levels in 55 patients by atomic absorption spectrophotometry before and after implantation of uncemented total hip arthroplasties. In a randomised, prospective trial 27 wrought Co-28Cr-6Mo-0.2C metal-on-metal articulations were compared with 28 ceramic-on-polyethylene hips which did not contain cobalt. Other sources of iatrogenic cobalt loading were excluded. The metal-on-metal group produced detectable serum cobalt levels (median 1.1 μg/l after one year) which were significantly different (p < 0.0001) from those of the ceramic-on-polyethylene control group (median below detection limit of 0.3 μg/l after one year). Our findings indicate that metal-on-metal bearings generate some systemic release of cobalt.

Low temperature catalytic activity of cobalt oxide and ceria promoted Pt and Pd: -influence of pretreatment and gas composition

Abstract: The influence of pretreatment, gas composition and metal (Ce or Co) oxide promotion on the low-temperature CO and C3H6 oxidation activity over alumina-supported Pt and Pd has been studied. The monolith catalysts have either been preoxidised in O2/N2 Or prereduced in H2/N2 prior to evaluation with respect to light-off performance, using either net oxidising or net reducing CO/C3H6/O2/N2 gas mixtures. Compared with unpromoted Pt, promotion with preoxidised ceria or cobalt oxide enhances the low-temperature activity significantly and lowers the light-off temperatures by about 60-70 degrees C for both CO and C3H6. Prereduction of a cobalt-oxide catalyst (without precious metals) gives a dramatically improved performance compared with a preoxidised catalyst in terms of light-off and overall conversion. Prereduction of metal oxide promoted Pt and Pd can shift the light-off temperatures for CO and C3H6 by up to 100 degrees C toward lower temperatures compared with preoxidised samples. When using gas mixtures containing both CO and C3H6, the conversion of CO always starts at lower temperatures than the conversion of C3H6 The catalysts have been characterised by temperature-programmed desorption (TPD) of carbon monoxide, X-ray photoelectron spectroscopy (XPS), and specific surface area measurements (BET). The reduced cobalt containing samples adsorb large amounts of CO. The high activity over the catalysts containing prereduced cobalt oxide is suggested to be due to the presence of reduced cobalt-oxide sites on the surface of those samples.

Electroreduction of O2 to H2O at Unusually Positive Potentials Catalyzed by the Simplest of the Cobalt Porphyrins

Abstract: Simple cobalt porphine, adsorbed on graphite electrodes, catalyzes the direct reduction of O2 to H2O, while most other monomeric cobalt porphyrins do not. The surprising behavior of cobalt porphine suggests possible new strategies for the design of electrocatalysts for the reduction of O2 to H2O using structurally similar but more oxidation-resistant analogs.

Monomeric and Polymeric Tetra-aminophthalocyanatocobalt(II) Modified Electrodes: Electrocatalytic Reduction of Oxygen

Abstract: The monomeric and polymeric tetra-aminophthalocyane to, cobalt(II) species adsorbed onto graphite electrodes are active in electrocatalytic oxygen reduction. While the monomeric species is unstable, the polymerized species is an effective and stable reduction catalyst over a wide pH range. Both the two-electron reduction of oxygen to hydrogen peroxide and the four-electron reduction of oxygen to water are characterized by cyclic voltammetry, rotating disc and rotating ring-disc studies with appropriate theoretical analysis. Some mechanistic information is obtained. This is the first cobalt phthalocyanine species to provide a four-electron reduction pathway which exists over a wide pH range and is stable. The stability is associated with the polymerization since the monomeric species is not stable.

Improvement of a grass-clover silage-fed biogas process by the addition of cobalt

Abstract: Batch assays were performed with samples from a silage-fed mesophilic biogas process accumulating acetate to examine if the addition of single trace elements (iron, nickel, cobalt and molybdenum) or a mixture of trace elements could improve the process. The results from the batch assays led to the addition of cobalt to reach a concentration of 0.2 mg L −1 . This made it possible to increase the organic loading rate (OLR) above that in a parallel process not receiving any extra cobalt. Problems with low gas production and decreasing pH were overcome by the daily supply of this single element in small amounts. Acetate conversion to methane was improved, which was confirmed by increased specific methanogenic activity (SMA) with acetate as a substrate. As a consequence, an OLR of 7.0 g VS L −1 day −1 was achieved at a hydraulic retention time (HRT) of 20 days with an equivalent increase in methane production. Without cobalt addition, an OLR of 5.0 g VS L −1 day −1 with an HRT of 20 days was reached after 70 weeks operation.

Determination of cobalt in seawater by catalytic adsorptive cathodic stripping voltammetry.

Abstract: A new procedure for the direct determination of picomolar levels of cobalt in seawater is presented. Cathodic stripping voltammetry is preceded by adsorptive accumulation of the cobalt-nioxime (cyclohexane-1,2-dione dioxime) complex from seawater containing 6 μM nioxime and 80 mM ammonia at pH 9.1, onto a hanging mercury drop electrode, followed by reduction of the adsorbed species. The reduction current is catalytically enhanced by the presence of 0.5 M nitrite. Optimized conditions for cobalt include a 30 s adsorption period at -0.7 V and a voltammetric scan using differential pulse modulation. According to the proposed reaction mechanism, dissolved Co(II) is oxidized to Co(III) upon addition of nioxime and high concentrations of ammonia and nitrite; a mixed Co(III)-ammonia-nitrite complex is adsorbed on the electrode surface; the Co(III) is reduced to Co(II) (complexed by nioxime) during the voltammetric scan, followed by its chemical reoxidation by the nitrite, initiating a catalytically enhanced current. A detection limit of 3 pM cobalt (at an adsorption period of 60 s) enables the detection of this metal in uncontaminated seawater using a very short adsorption time. UV digestion of seawater is essential, as part of the cobalt may occur strongly complexed by organic matter and rendered nonlabile. The method was applied successfully to the determination of the distribution of cobalt in the water column of the Mediterranean.

Cation-driven electron transfer involving a spin transition at room temperature in a cobalt iron cyanide thin film

Abstract: The electronic structure of a cobalt iron cyanide thin film, Na0.4Co1.3[Fe(CN)6]5H2O, was switched from FeIII(low spin)−CN−CoII(high spin) to FeII(low spin)−CN−CoIII(low spin) by exchanging cations in interstitial sites from Na+ to K+. The reverse process was also induced by replacing Na+ with K+. That is, the alkali cations in the interstitial sites dominate the electronic and spin states of the host compound. This type of switching phenomenon can be achieved by choosing a compound with both electronic degeneracy and zeolitic properties.

Comparison of TiSi2 , CoSi2, and NiSi for Thin‐Film Silicon‐on‐Insulator Applications

Abstract: TiSi2, CoSi2, and NiSi self-aligned silicide processes have been studied, compared, and applied to thin-film silicon-on-insulator technology. Compared to TiSi2, CoSi2 and NiSi have the advantages of wider process temperature window, no significant doping retarded reaction, narrow runner degradation, and thin-film degradation. Therefore, they are more suitable for thin-film silicon-on-insulator technology. N-type field effect transistors have been fabricated in a complementary metal oxide-semiconductor compatible thin-film silicon-on-insulator technology with titanium, cobalt, and nickel self-aligned silicide processes for low-voltage, low-power microwave applications. The initial thicknesses of titanium, cobalt, and nickel are 30, 13, and 25 nm, respectively. The gate sheet resistances are 6.2, 4.4, and 2.9 Omega/square, respectively, and the total source/drain series resistances are 700, 290, and 550 Omega mu m, respectively. High-frequency measurement results are also presented.