Showing papers on "Cobalt published in 1987"

Electrochemical polymerization of amino-, pyrrole-, and hydroxy-substituted tetraphenylporphyrins

Abstract: are described. Oxidations of these monomers lead to thin polymeric coatings on electrodes by coupling reactions analogous to those operative in polyaniline, polypyrrole, and polyphenol electropolymerization. The films contain from two to several hundred layers of porphyrin sites and display reductive electrochemical and electronic spectroscopic properties similar to those of monomer solutions in acetonitrile or methylene chloride. The pyrtolyl- and amino-based polymers are polycationic and thus anion exchangers, whereas the hydroxy-based polymers (at pH >8) are polyanionic. Films prepared from electropolymerization of cobalt tetrakis(o-aminopheny1)porphyrin display a split Co(III/II) wave in aqueous acid and base and are good oxygen reduction catalysts, whereas films prepared from all other cobalt porphyrin monomers display only one Co(III/II) wave and are poor catalysts.

Catalyst and binder effects in the use of filamentous graphite for AMS

Abstract: Filamentous graphite produced by the reduction of carbon dioxide over a transition metal catalyst has been used as a sample material in the AMS counting of 14C for several years. While iron has been the most common catalyst used, cobalt and nickel have also been investigated. We have compared the reaction's total isotopic fractionation using these three metals in various powder forms. The graphites produced over these catalysts have been compared with respect to ion beam intensity and measured isotope ratio in our AMS system. The use of the reduction catalysts as graphite binders improved the very poor thermal conductivity of pure filamentous graphite. The relative advantages of the elements for modern contamination of the sample material were also determined.

Mechanistic studies on the cobalt(II) Schiff base catalyzed oxidation of olefins by O2

Abstract: The cobalt complex (bis(salicylidene-..gamma..-iminopropyl)methylamine)cobalt(II), CoSMDPT, has been shown to catalytically oxidize olefins in the presence of dioxygen or hydrogen peroxide. When terminal olefins are oxidized, the methyl ketone and corresponding secondary alcohol are produced selectively. Internal as well as terminal olefins are oxidized. The most common pathway for the oxidation of olefins catalyzed by first-row transition metals-autoxidation-has been ruled out in this system. A Wacker-type mechanism, oxidation by peracids, and mechanisms involving the formation of peroxymetallocycles have also been ruled out. A new mechanism for O/sub 2/ oxidations is proposed which involves oxidation of the primary alcohol solvent by CoSMDPT to produce the corresponding aldehyde and hydrogen peroxide. Reaction of hydrogen peroxide with CoSMDPT occurs to form a cobalt hydroperoxide, which can be viewed as a stabilized hydroperoxy radical which has spin paired with the d/sub z/sup 2// electron of CoSMDPT. The cobalt hydroperoxide then adds to the olefin double bond, leading to formation of an alkyl hydroperoxide. Haber-Weiss decomposition of this alkyl hydroperoxide by CoSMDPT produces the observed ketone and alcohol products. Deactivation of the catalyst is due to oxidation of the ligand system of the cobalt complex as well as formation of a ..mu..-peroxo-dicobalt complex.

Electrochemistry and spectroelectrochemistry of mononuclear and binuclear cobalt phthalocyanines

Abstract: : The electrochemistry of 9, 16, 23-TETRA(NEOPENTOXY)PHTHALOCYANATO cobalt, and some binuclear analogs, has been studied in dichlorobenzene and in dimethylformamide. The redox mechanisms and species on the electrode are discussed. Using an optically thin electrode, the electronic spectra of seven different oxidation states of the mononuclear derivative are reported. Data for a selection of oxidation states of several binuclear species are also presented.

Magnesium mechanical alloys for hydrogen storage

Abstract: Mechanical alloying is a promising new way to fabricate hydrogen storage materials consisting of unusual pairs of metals. Mechanical alloying can be used more easily for fabricating Mg2Ni, other MgNi and intermetallic systems than melting and sintering techniques. Systems such as MgFe, MgCo, MgNb and MgTi, that cannot be prepared by conventional techniques, can be fabricated by mechanical alloying, in addition to any other pairs or more complex mixtures of metals and non-metals. We have pointed out that intimate contact of metals can be achieved in high energetic ball mills. This contact plays an important role in hydriding and dehydriding processes. In this study we have used two groups of catalytic additives. The first group are “hydrogen pumps” such as CeHx. In the second group the added metal does not form any hydride in the conditions under study, but is known as an H2 → HH reaction catalyst, e.g., cobalt, nickel and probably iron.

Surface characterization and catalytic properties of La1−x -Sr x CoO3

Abstract: The surface chemical states of the perovskite-type compounds, strontium doped lanthanum cobalt oxides (La1−xSrxCoO3), have been investigated using X-ray photoelectron spectroscopy (XPS). Catalytic oxidations of both methane and CO have also been investigated using flow methods. The chemical composition of the surface of La1−xSrxCoO3 was very different from that in the bulk, which was measured by X-ray fluorescence spectroscopy (XRFS). The catalytic activity of La1−xSrxCoO3 increased with an increase in the quantity of cobalt atoms on the surface.

Evidence for rapid chain growth in the Fischer-Tropsch synthesis over iron and cobalt catalysts

Abstract: The authors have examined hydrocarbon chain growth in the Fischer-Tropsch reaction by following both the rate and position of incorporation of /sup 13/C into the hydrocarbon products after an abrupt switch from /sup 12/CO to /sup 13/CO in the reactant gas. On two catalysts (cobalt supported on silica and precipitated iron combined with various promoters) they find that /sup 13/C appears after the isotope switch at the same rate in all positions of all products examined. Under the assumption that chain growth is unidirectional, this finding shows that chain growth on these catalysts is very rapid in comparison to the rate of isotope displacement in the precursors to chain growth. In addition, only a small portion of the active carbon on the surface can be in the form of hydrocarbon chains. Under these conditions no detailed information about the mechanism of chain growth is obtainable.

Dual-mode transport of molecular oxygen in a membrane containing a cobalt porphyrin complex as a fixed carrier

Abstract: Preparation de la membrane en dispersant dans du polymethacrylate de butyle le complexe [α, α', α″, α''' μeso-tetrakis (o-pivalamidophenyl) porphinato] cobalt II-methyl-1 imidazole, le transfert d'oxygene est ainsi ameliore

Synthesis, aggregation, electrocatalytic activity, and redox properties of a tetranuclear cobalt phthalocyanine

Abstract: Using pentaerythritol as a framework, it has been possible to synthesize a tetranuclear phthalocyanine with a spiro type linkage. Electrochemical and spectroelectrochemical data are presented for the cobalt complex in the Co/sup III/Pc(-2), Co/sup II/Pc(-2), Co/sup I/Pc(-2), Co/sup III/Pc(-1), Co/sup II/Pc(-1), and Co/sup I/Pc(-3) oxidation levels. The Co(II) species aggregates very strongly, dimerizing with an aggregation constant in o-dichlorobenzene of 2.4 x 10/sup 5/ M/sup -1/, 2 orders of magnitude greater than for the parent Co/sup II/TNPc. Monolayers of the cobalt(II) species laid upon an ordinary pyrolytic graphite electrode are shown to electrocatalytically reduce oxygen more efficiently than previously described analogous mononuclear and binuclear phthalocyanines. The Co(II) tetranuclear species disproportionates into a 1:1 mixture of Co(I) and Co(III) species upon reaction with hydroxide ion in a nondonating or weakly donating solvent. A film containing (Co/sup I/TrNPc(-2))/sub 4//sup 4 -/ reduces nitrite ion in NaOH slowly, with oxidation to the Co(II) tetranuclear species. 60 references, 8 figures, 5 tables.

The role of Co in sulfidized Co-Mo hydrodesulfurisation catalysts supported on carbon and alumina

Abstract: The thiophene hydrodesulphurisation activities of cobalt sulphide catalysts supported on activated carbon have been measured in a flow microreactor operating at atmospheric pressure. The cobalt content was varied between 1.3 and 13.3 wt% Co, and the oxidic precursor catalysts were dried by three different procedures. Structural characteristics and the degree of Co dispersion in the oxidic precursor and in the sulphidised state of the catalysts were obtained by X-ray photoelectron spectroscopy. The oxidic cobalt phase present in the precursor catalysts was found to be inhomogeneously dispersed over the carbon carrier surface. Dispersion decreased significantly during catalyst sulphidation. XPS results showed that the cobalt was fully sulphidised, although sulphur was found to be present in excess of the amount corresponding to stoichiometric Co9S8.The catalysts demonstrated very high hydrodesulphurisation activities, which were by far superior to those of corresponding carbon- or alumina-supported molybdenum catalysts. By extrapolation of the inverse activities to zero loading the activities of the optimally dispersed cobalt and molybdenum sulphide were determined. Comparison of the activity of carbon-supported Co–Mo with the activities of the optimally dispersed catalyst supported on carbon demonstrated that the so-called promoter effect in sulphidised Co–Mo catalysts can be explained completely by the exceptionally high catalytic activity of cobalt sites and showed that the role of MoS2 in these catalysts is mainly to function as a support for optimally dispersed cobalt ions.

Use of gas-diffusion electrodes for high-rate electrochemical reduction of carbon dioxide. II. Reduction at metal phthalocyanine-impregnated electrodes

Abstract: The use of polytetrafluoroethylene-bonded, carbon gas-diffusion electrodes, prepared with carbon impregnated with metal phthalocyanines, for the electrochemical reduction of carbon dioxide in aqueous, acidic solution has been investigated High rates of reduction of carbon dioxide to carbon monoxide were demonstrated at electrodes impregnated with cobalt (II) phthalocyanine In contrast, formic acid, and not carbon monoxide, was produced at low rates at electrodes impregnated with either manganese, copper or zinc phthalocyanine This marked variation in reaction product on changing the central metal ion of the organometallic complex is rationalized in terms of a reaction mechanism involving, as the first step, the electrochemical reduction of cobalt (II) to cobalt (I)

Spin-crossover cobalt(III) complexes: steric and electronic control of spin state

Abstract: The series of octahedral Co(III) spin-crossover complexes (III) is synthesized from the corresponding Co(II) complexes (Ia) by oxidation with the radical cation of (II).

Catalyst composition and process for its preparation

Abstract: What is described is a process for preparing a more effective catalyst for the vapor-phase reaction of 1,1,1,3-tetrachloropropane with anhydrous hydrogen fluoride to produce 3,3,3-triflurorpropene-1. The process comprises (A) mechanically mixing, in the absence of water, of (i) aluminum fluoride which has been prepared at temperatures less than about 400° C. and (ii) a transition metal compound, the transition metal compound being selected from a group consisting of compounds of cobalt, chromium, iron, manganese, nickel, titanium, and vanadium; and (B) contacting the mechanical mixture with sufficient anhydrous HF to convert the transition metal compound to a transition metal fluoride. Also described is the catalyst. Additionally, a process for the preparation of 3,3,3-trifluoropropene-1 in which the catalyst is utilized is described.

Cobalt(II) tetrasulfophthalocyanine on titanium dioxide: a new efficient electron relay for the photocatalytic formation and depletion of hydrogen peroxide in aqueous suspensions

Abstract: A novel synthesis for the covalent linkage of cobalt(II) tetrasulfophthalocyanine (Co^(II)TSP) to the surface of titanium dioxide (TiO_2) particles (d ≤ 0.5 µm) is described. Upon irradiation with light that exceeds the bandgap energy (E_g) of TiO_2 (i.e., ⋋ 5 380 nm), Co^(II)TSP is reduced to Co^ITSP under anoxic conditions both as a dry powder and in aqueous suspension. The photochemical reduction is shown to be fully reversible in the presence of molecular oxygen (O_2). Hydrogen peroxide (H_2O_2) is produced upon irradiation of an aerated aqueous suspension of the 'hybrid" catalyst, TiO__2-Co^(II)TSP. Formation kinetics are followed in situ with a polarographic detector (detection limit ≃ 10^(7-) M H_2O_2); quantum yields, ϕ_(H_2O_2) between 0.16 and 0.49 have been determined. The reactive photocatalytic center appears to be generated by the attachment of molecular oxygen in the open apical coordination site of the hybrid Co^(II)TSP complex. Formation of Co^(II)TSP-O_2^(•-) is enhanced by the binding of TiO^- surface groups in the opposite apical position. Hydrogen peroxide is produced in a two-step electron transfer from the conduction band via the Co(III) center. The involvement of free radical intermediates in this mechanism appears to be highly unlikely. On the basis of its observed chemical and photochemical stability and the high quantum yields for O_2 reduction, the newly developed hybrid material is proposed to be applicable as a potent and stable oxidation catalyst.

Magnetic recording medium

Abstract: PURPOSE:To improve coercive force and to realize a high-density medium having excellent corrosion resistance and wear resistance by substituting iron at a specific ratio with cobalt (Co). CONSTITUTION:The coercive force is increased by substituting the iron of hexagonal iron nitride among iron nitride, i.e., epsilon-iron nitride with <=15atom% cobalt. However, the Curie point of the hexagonal iron nitride decreases with an increase in nitrogen content and the satd. magnetic flux density thereof is about 60emu/g with Fe2.5N although said density varies with a production process, raw material, etc. The satd. magnetic flux density is decreased by the substn. of the cobalt and therefore, the amt. of the substitution of the iron by the cobalt is practicably up to 15%. The coercive force is improved without decreasing the other magnetic characteristics such as satd. magnetic flux density to practicable values or below. Corrosion resistance and wear resistance are improved and the need for forming a protective layer on the surface is eliminated.

Mechanisms in the simultaneous erosion-oxidation attack of nickel and cobalt at high temperature

Abstract: An apparatus is described by which a specimen may be exposed to a well-defined erosive gas stream containing 20 μm alumina particles. Samples of nickel and cobalt were exposed to conditions of erosion and high temperature oxidation simultaneously, using 90 deg impact angle, at velocities up to 140 m/s-1 and at temperatures up to 800 °C. From the results of erosion kinetics, surface morphologies, and cross section metallographic observations it is concluded that several definite regimes of interaction between erosion and oxidation exist. These regimes are described in qualitative terms, and it is shown how they arise as a result of the balance between the relative intensities of the erosion and oxidation processes.