Showing papers on "Cobalt published in 2007"

Electrocatalytic hydrogen evolution at low overpotentials by cobalt macrocyclic glyoxime and tetraimine complexes.

Abstract: Cobalt complexes supported by diglyoxime ligands of the type Co(dmgBF2)2(CH3CN)2 and Co(dpgBF2)2(CH3CN)2 (where dmgBF2 is difluoroboryl-dimethylglyoxime and dpgBF2 is difluoroboryl-diphenylglyoxime), as well as cobalt complexes with [14]-tetraene-N4 (Tim) ligands of the type [Co(TimR)X2]n+ (R = methyl or phenyl, X = Br or CH3CN; n = 1 with X = Br and n = 3 with X = CH3CN), have been observed to evolve H2 electrocatalytically at potentials between −0.55 V and −0.20 V vs SCE in CH3CN. The complexes with more positive Co(II/I) redox potentials exhibited lower activity for H2 production. For the complexes Co(dmgBF2)2(CH3CN)2, Co(dpgBF2)2(CH3CN)2, [Co(TimMe)Br2]Br, and [Co(TimMe)(CH3CN)2](BPh4)3, bulk electrolysis confirmed the catalytic nature of the process, with turnover numbers in excess of 5 and essentially quantitative faradaic yields for H2 production. In contrast, the complexes [Co(TimPh/Me)Br2]Br and [Co(TimPh/Me)(CH3CN)2](BPh4)3 were less stable, and bulk electrolysis only produced faradaic yields fo...

Catalytic decomposition of N2O over CeO2 promoted CO3O4 spinel catalyst

Abstract: A series of CeO2 promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N2O). Addition of CeO2 to Co3O4 led to an improvement in the catalytic activity for N2O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N2O conversion could be attained over the CoCeO.05 catalyst below 400 degrees C even in the presence of O-2, H2O or NO. Methods of XRD, FE-SEM, BET, XPS, H-2-TPR and O-2-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO2 could increase the surface area Of Co3O4, and then improve the reduction of Co3+ to Co2+ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N2O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO2, are responsible for the enhancement of catalytic activity Of Co3O4. (c) 2007 Elsevier B.V. All rights reserved.

Cobaloximes as functional models for hydrogenases. 2. Proton electroreduction catalyzed by difluoroborylbis(dimethylglyoximato)cobalt(II) complexes in organic media.

Abstract: Cobaloximes are effective electrocatalysts for hydrogen evolution and thus functional models for hydrogenases Among them, difluoroboryl-bridged complexes appear both to mediate proton electroreduction with low overpotentials and to be quite stable in acidic conditions We report here a mechanistic study of [Co(dmgBF2)2L] (dmg2- = dimethylglyoximato dianion; L = CH3CN or N,N-dimethylformamide) catalyzed proton electroreduction in organic solvents Depending on the applied potential and the strength of the acid used, three different pathways for hydrogen production were identified and a unified mechanistic scheme involving cobalt(II) or cobalt(III) hydride species is proposed As far as working potential and turnover frequency are concerned, [Co(dmgBF2)2(CH3CN)2], in the presence of p-cyanoanilinium cation in acetonitrile, is one of the best synthetic catalysts of the first-row transition-metal series for hydrogen evolution

Covalently functionalized cobalt nanoparticles as a platform for magnetic separations in organic synthesis.

Abstract: Synthesis in a day! Carbon-coated metal nanoparticles can be covalently functionalized by diazonium chemistry These colloidal reagents can now serve as a basis to magnetically functionalize molecules during synthesis, enabling their recovery within seconds

Fischer-Tropsch synthesis : Temperature programmed EXAFS/XANES investigation of the influence of support type, cobalt loading, and noble metal promoter addition to the reduction behavior of cobalt oxide particles

Abstract: TPR-XANES/EXAFS carried out using a novel multi-sample holder provided key information for verifying the nature of the chemical transformations occurring during cobalt Fischer–Tropsch synthesis catalyst activation in hydrogen. In the past, assumptions had to be made regarding the nature of the cobalt species present along the trajectory of a standard TPR experiment. The new technique directly provided insight into (a) the nature of the reduction process of cobalt oxide species and (b) the resulting cobalt crystallite size, as a function of the strength of the catalyst support interaction with the cobalt oxide species. A two-step reduction process involving Co 3 O 4 to CoO and CoO to Co 0 transformations over standard calcined catalysts was observed and quantified over all catalysts exhibiting both weak interactions (e.g., Co/SiO 2 ) and strong interactions (e.g., Co/Al 2 O 3 ) with the support. Noble metal promoter (e.g., Pt) addition strongly improved the reducibility of cobalt oxide species, most likely via a H 2 dissociation and spillover mechanism. Increasing cobalt loading, on the other hand, led to a measurable, but lesser, improvement on reducibility, due to the larger resulting particle size that resulted in less surface contact with the support. Higher reduction temperatures were needed to effectively reduce cobalt oxide particles deposited on strongly interacting surfaces in comparison with unsupported Co 3 O 4 or only weakly interacting supported cobalt catalyst. Nevertheless, despite lower extents of reduction, the smaller resulting Co particles on the more strongly interacting catalysts generally led to higher Co 0 active site densities. The addition of the noble metal promoter to strongly interacting supported catalyst significantly decreased the temperature required to reduce the cobalt oxides to Co 0 particles; this allows one to take advantage of the higher Co 0 surface areas arising from the combination of a smaller average Co 0 particle size and a higher extent of reduction.

Interpreting the structural and electrochemical complexity of 0.5Li2MnO3·0.5LiMO2 electrodes for lithium batteries (M = Mn0.5−xNi0.5−xCo2x, 0 ≤x≤ 0.5)

Abstract: The structural and electrochemical features of layered 0.5Li2MnO3·0.5LiMO2 electrodes, in which M = Mn0.5−xNi0.5−xCo2x (0 ≤ x ≤ 0.5), have been studied by powder X-ray diffraction, electrochemical differential-capacity measurements, 7Li magic-angle-spinning nuclear magnetic resonance, and X-ray absorption near-edge spectroscopy. Li2MnO3-like regions in the as-prepared samples were observed for all values of x, with transition-metal cation disorder between the LiMO2 and Li2MnO3 components increasing with cobalt content (i.e., the value of x). The structural disorder and complexity of the electrochemical redox reactions increase when the Li2MnO3-like regions within the electrode are activated to 4.6 V in lithium cells; interpretations of structural and electrochemical phenomena are provided.

Fischer−Tropsch Synthesis: Comparison of Performances of Iron and Cobalt Catalysts

Abstract: Biomass represents a source of syngas that can be further processed to hydrocarbon fuels. This paper examines the Fischer−Tropsch technology for the biomass-to-clean fuels scenario. A comparison of the activities, selectivities, and lifetimes of iron and cobalt catalysts for Fischer−Tropsch synthesis is made. For the more severe conditions, iron is the more active catalyst, whereas a cobalt catalyst may be more active at low-severity conditions. In spite of many reports, there are still considerable differences in defining catalyst activity. The selectivity for methane likewise shows a wide range of reported results. Under the proper conditions, both catalysts are capable of operating for 6 months or more.

Nanocrystalline cobalt oxide immobilized on titanium dioxide nanoparticles for the heterogeneous activation of peroxymonosulfate

Abstract: Recently, sulfate radical-based advanced oxidation technologies have shown significant implications for environmental remediation to decompose water pollutants. In this study, we evaluated the performance of heterogeneous activation of peroxymonosulfate (PMS) to generate sulfate radicals using cobalt catalyst immobilized on titanium dioxide nanoparticles (Co/TiO2). The Co/TiO2 catalyst was prepared via an incipient wetness impregnation method employing Degussa P-25 TiO2 and Co(NO3)·6H2O. The activity of Co/TiO2 system was compared with those of Co(NO3)2 solution for homogeneous PMS activation and neat Co3O4 for heterogeneous PMS activation. More emphasis was given to the effect of cobalt loading and heat treatment on the physicochemical properties of Co/TiO2 and cobalt leaching. The results showed that heat treatment of Co/TiO2 at 500 °C, where cobalt existed as Co3O4, induced negligible Co leaching and enhanced catalytic activity to decompose 2,4-dichlorophenol. The Co/TiO2 catalyst at Co/Ti molar ratio of 0.1 showed the highest activity via heterogeneous PMS activation. On the other hand, Co/TiO2 catalysts with Co/Ti molar ratio of above 0.2 exhibited rather much lower activity which was initiated predominantly via a homogeneous pathway from leached cobalt, although they contained considerable amounts of Co3O4. The formation of Co OH complexes at the surface of Co/TiO2 nanoparticles, due to the ability of TiO2 to dissociate H2O for the formation of surface hydroxyl groups, was proposed to facilitate the heterogeneous PMS activation. However, high cobalt loading covering the TiO2 surface diminished the beneficial role of TiO2 due to the reduction in the concentration of surface hydroxyl groups and thus decreased the heterogeneous PMS activation. The activity of Co3O4 in Co/TiO2 catalysts was much higher than that of neat Co3O4 due to the presence of surface hydroxyl groups and uniform distribution of well-defined 10−15 nm nanocrystalline Co3O4 particles at the surface of 30−40 nm TiO2 nanoparticles.

XPS Study on Al2O3- and AlPO4-Coated LiCoO2 Cathode Material for High-Capacity Li Ion Batteries

Abstract: The impact of aluminum oxide coatings on LiCoO2 materials for commercial lithium ion batteries has been investigated by X-ray photoelectron spectroscopy (XPS). A low binding energy component in the Al 2p core peak spectra was observed and attributed to the formation of a LiAlx Co(1-x) O2 solid solution interphase for both Al2 O3 - and AlPO4 -coated LiCoO2. The surface chemistry of pristine and Al2 O3 -coated LiCoO2 cathodes and graphite anodes have been investigated after cycling up to 4.2 V or 4.4 V cutoff voltage and after various levels of capacity fade. The Al2 O3 coating enhances the capacity retention at both 4.2 V and 4.4 V cutoff voltages. XPS analyses provided evidence of the inhibition of cobalt dissolution from the LiCoO2 positive electrode by the aluminum coatings. Moreover, the Al2 O3 coating lowers the kinetics of degradation of electrolyte species, especially the LiPF6 salt. © 2007 The Electrochemical Society.

Chromium and cobalt ion release following the Durom high carbon content, forged metal-on-metal surface replacement of the hip

Abstract: We evaluated the concentrations of chromium and cobalt ions in blood after metal-on-metal surface replacement arthroplasty using a wrought-forged, high carbon content chromium-cobalt alloy implant in 64 patients. At one year, mean whole blood ion levels were 1.61 microg/L (0.4 to 5.5) for chromium and 0.67 microg/L (0.23 to 2.09) for cobalt. The pre-operative ion levels, component size, female gender and the inclination of the acetabular component were inversely proportional to the values of chromium and/or cobalt ions at one year postoperatively. Other factors, such as age and level of activity, did not correlate with the levels of metal ions. We found that the levels of the ions in the serum were 1.39 and 1.37 times higher for chromium and cobalt respectively than those in the whole blood. The levels of metal ions obtained may be specific to the hip resurfacing implant and reflect its manufacturing process.

Transplacental transfer of cobalt and chromium in patients with metal-on-metal hip arthroplasty: a controlled study.

Abstract: Metal-on-metal bearings are being increasingly used in young patients. The potential adverse effects of systemic metal ion elevation are the subject of ongoing investigation. The purpose of this study was to investigate whether cobalt and chromium ions cross the placenta of pregnant women with a metal-on-metal hip resurfacing and reach the developing fetus. Whole blood levels were estimated using high-resolution inductively-coupled plasma mass spectrometry. Our findings showed that cobalt and chromium are able to cross the placenta in the study patients with metal-on-metal hip resurfacings and in control subjects without any metal implants. In the study group the mean concentrations of cobalt and chromium in the maternal blood were 1.39 microg/l (0.55 to 2.55) and 1.28 microg/l (0.52 to 2.39), respectively. The mean umbilical cord blood concentrations of cobalt and chromium were comparatively lower, at 0.839 microg/l (0.42 to 1.75) and 0.378 microg/l (0.14 to 1.03), respectively, and this difference was significant with respect to chromium (p 0.05). The mean cord blood level of cobalt in the study patients was significantly greater than that in the control group (p 0.05) The transplacental transfer rate was in excess of 95% in the controls for both metals, but only 29% for chromium and 60% for cobalt in study patients, suggesting that the placenta exerts a modulatory effect on the rate of metal ion transfer.

Uniform Magnetic Chains of Hollow Cobalt Mesospheres from One‐Pot Synthesis and Their Assembly in Solution

Abstract: Magnetic chains up to 10 μm in length formed of hollow cobalt mesospheres (480-850 nm) with a 60 nm thick shell are synthesized by a new soft-assembly protocol. The obtained chains show a saturation magnetization of 37.5 emug -1 , a remnant magnetization of 1.55 emu g -1 , and a coercivity of ca. 66 Oe at 300 K. A possible mechanism for the formation of the chainlike hollow structures is proposed.

Highly efficient decomposition of organic dyes by aqueous-fiber phase transfer and in situ catalytic oxidation using fiber-supported cobalt phthalocyanine.

Abstract: A novel metallophthalocyanine derivative, cobalt tetra (2,4-dichloro-1,3,5-triazine) aminophthalocyanine (Co-TDTAPc), was prepared and immobilized on cellulosic fiber by covalent bond to obtain a supported oxidation catalyst (Co-TDTAPc-F). Co-TDTAPc-F/H2O2 system based on phase-transfer catalytic oxidation for decomposing dyes, including acid, reactive, and direct dyes, has been investigated thoroughly. Compared to traditional adsorption technologies and advanced oxidation processes (AOPs) for dye treatment, Co-TDTAPc-F/H2O2 combines the advantages of both and is more efficient and more effective. Azo dyes such as C. I. Acid Red 1 (AR1) can be quickly adsorbed onto/into the fiber from aqueous solution and decomposed in situ simultaneously in the presence of Co-TDTAPc-F and H2O2. It has been found that the reaction process is not affected by the visible light. Furthermore, it turns the negative effect of NaCl normally observed in homogeneous catalysis into positive one. The catalytic reaction can proceed a...