Showing papers on "Cobalt published in 2010"

Synthesis and Characterization of Cobalt Hydroxide, Cobalt Oxyhydroxide, and Cobalt Oxide Nanodiscs

Abstract: Cobalt hydroxide, cobalt oxyhydroxide, and cobalt oxide nanomaterials were synthesized through simple soft chemistry. The cobalt hydroxide displays hexagonal morphology with clear edges 20 nm long. This morphology and nanosize is retained through to cobalt oxide Co3O4 through a topotactical relationship. Cobalt oxyhydroxide and cobalt oxide nanomaterials were synthesized through oxidation and low-temperature calcination from the as-prepared cobalt hydroxide. Characterization of these cobalt-based nanomaterials was fully developed, including X-ray diffraction, transmission electron microscopy combined with selected area electron diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and thermal gravimetric analysis. Bonding of the divalent cobalt hydroxide from the oxyhydroxide and oxides by studying their high-resolution XPS spectra for Co 2p3/2 and O 1s. Raman spectroscopy of the as-prepared Co(OH)2, CoO(OH), and Co3O4 nanomaterials characterized each material. T...

Design of Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency Dye-Sensitized Solar Cells

Abstract: Dye-sensitized solar cells (DSCs) with cobalt-based mediators with efficiencies surpassing the record for DSCs with iodide-free electrolytes were developed by selecting a suitable combination of a cobalt polypyridine complex and an organic sensitizer. The effect of the steric properties of two triphenylamine-based organic sensitizers and a series of cobalt polypyridine redox mediators on the overall device performance in DSCs as well as on transport and recombination processes in these devices was compared. The recombination and mass-transport limitations that, previously, have been found to limit the performance of these mediators were avoided by matching the properties of the dye and the cobalt redox mediator. Organic dyes with higher extinction coefficients than the standard ruthenium sensitizers were employed in DSCs in combination with outer-sphere redox mediators, enabling thinner TiO2 films to be used. Recombination was reduced further by introducing insulating butoxyl chains on the dye rather than...

Cobalt(II) single-molecule magnets.

Abstract: This short tutorial review covers recent progress in the field of polynuclear cobalt(II)-based complexes, which display slow magnetic relaxation at low temperature. Cobalt(II) single-molecule magnets (SMMs) can display much larger magnetic anisotropies and hence, potentially higher blocking temperatures than SMMs based on ions where the zero-field splitting originates from a second order spin–orbit coupling, such as manganese(III).

Cobalt-Catalyzed Cross-Coupling Reactions

Abstract: 3.1.2. From Vinylic Grignard Reagents 1437 3.1.3. From Aryl Halides and Alkenyl Acetates 1438 3.2. Aryl-Aryl Cross-coupling 1438 3.2.1. From Aromatic Organometallic Reagents 1438 3.2.2. From Two Aromatic Halides 1440 4. Csp2-Csp3 Cross-coupling Reactions 1440 4.1. Alkenylation 1440 4.1.1. From Aliphatic Organometallic Reagents 1440 4.1.2. From Aliphatic Halides 1442 4.2. Arylation 1442 4.2.1. From Aliphatic Halides 1442 4.2.2. From Aromatic Halides 1445 4.3. Allylation of Aromatic Organometallics 1446 5. Alkynylation 1446 5.1. Pioneering Works 1446 5.2. Benzylation of Acetylenic Grignard Reagents 1446 5.3. Alkylation of Acetylenic Grignard Reagents 1447 5.4. Alkenylation of Acetylenic Grignard Reagents 1447 6. Csp3-Csp3 Cross-coupling 1448 6.1. Allylation 1448 6.1.1. Allylation of Aliphatic Organozinc Compounds 1448

Nanostructured cobalt and manganese oxide clusters as efficient water oxidation catalysts

Abstract: Recent development of new methods of preparing cobalt oxide and manganese oxide clusters has led to oxygen evolving catalysts that operate under mild conditions and modest overpotentials at rates approaching practical utility. Synthesis of nanostructured Co3O4 and Mn oxide clusters in mesoporous silica scaffolds affords catalysts with very high densities of surface metal sites per projected area, with the silica environment providing stability in terms of dispersion of the clusters and prevention of restructuring of catalytic surface sites. Stacking of the nanoclusters of these earth abundant, durable oxide catalysts in the scaffold results in turnover frequencies per projected area that are sufficient for keeping up with the photon flux at high solar intensity. Opportunities for expanding the metal oxide/silica interface approach to heterogeneous water oxidation catalysis to a more general approach for multi-electron catalyst designs based on core/shell constructs are discussed. The results are reviewed in the context of all-inorganic materials for catalytic water oxidation reported recently from other laboratories, in particular electrodeposits generated from Co phosphate solutions, a molecular water oxidation catalyst based on a polyoxotungstate featuring a Co oxide core, and Mn oxide materials with incorporated Ca ions.

Activated carbon supported cobalt catalysts for advanced oxidation of organic contaminants in aqueous solution

Abstract: A heterogeneous cobalt catalyst was prepared by impregnation of cobalt ion on an activated carbon (AC) and used to activate peroxymonosulphate (PMS) for advanced oxidation of phenol in aqueous solution. The Co/AC catalyst was characterised by several techniques such as XRD, EDS, SEM, and TGA. It was found that Co2O3 was the major form of Co species and was homogeneously distributed on the activated carbon surface. Co/AC exhibited high activity in oxidation of phenol with sulphate radicals and 100% decomposition and 80% TOC removal could be achieved in 60 min at the conditions of 500 ml phenol solution of 25 ppm, 0.1 g catalyst and 1 g peroxymonosulphate. The catalyst also exhibited stable performance after several rounds of regeneration. Several operational parameters such as catalyst and oxidant amount, temperature on the rate of oxidation were found to influence the phenol oxidation.

Shape-controlled synthesis of porous Co3O4 nanostructures for application in supercapacitors

Abstract: In this work, we report a facile approach for the shape-controlled synthesis of cobalt carbonate/hydroxide intermediates. Three different structures, viz., one-dimensional (1D) needle-like nanorods, two-dimensional (2D) leaf-like nanosheets, and three-dimensional (3D) oval-shaped microparticles, have been synthesized through varying experimental parameters such as precursor (cobalt acetate) concentrations and volume ratio of polyethylene glycol to water. Phase-pure tricobalt tetroxide (Co3O4) has been obtained by annealing these as-prepared intermediates without significant alterations in morphology. With relatively high specific surface areas of 86.1–121.5 m2 g−1, these products with distinct nanostructures were tested for their potential application in supercapacitors. The results show that these porous Co3O4 structures exhibit promising capacitive properties with high capacitance and good retention. The needle-like nanorods show the highest capacitance of 111 F g−1, and 88.2% of which can still be maintained after 1000 charge–discharge cycles.

Nanophase Transition Metal Oxides Show Large Thermodynamically Driven Shifts in Oxidation-Reduction Equilibria

Abstract: Knowing the thermodynamic stability of transition metal oxide nanoparticles is important for understanding and controlling their role in a variety of industrial and environmental systems. Using calorimetric data on surface energies for cobalt, iron, manganese, and nickel oxide systems, we show that surface energy strongly influences their redox equilibria and phase stability. Spinels (M(3)O(4)) commonly have lower surface energies than metals (M), rocksalt oxides (MO), and trivalent oxides (M(2)O(3)) of the same metal; thus, the contraction of the stability field of the divalent oxide and expansion of the spinel field appear to be general phenomena. Using tabulated thermodynamic data for bulk phases to calculate redox phase equilibria at the nanoscale can lead to errors of several orders of magnitude in oxygen fugacity and of 100 to 200 kelvin in temperature.

'Carbene radicals' in Co-II(por)-catalyzed olefin cyclopropanation

Abstract: The mechanism of cobalt(II)−porphyrin-mediated cyclopropanation of olefins with diazoesters was studied. The first step—reaction of cobalt(II)−porphyrin with ethyl diazoacetate (EDA)—was examined using EPR and ESI-MS techniques. EDA reacts with cobalt(II)−porphyrin to form a 1:1 Co(por)(CHCOOEt) adduct that exists as two isomers: the ‘bridging carbene’ C′ in which the ‘carbene’ is bound to the metal and the pyrrolic nitrogen of the porphyrin that has a d7 configuration on the metal, and the ‘terminal carbene’ C in which the ‘carbene’ behaves as a redox noninnocent ligand having a d6 cobalt center and the unpaired electron residing on the ‘carbene’ carbon atom. The subsequent reactivities of the thus formed ‘cobalt carbene radical’ with propene, styrene, and methyl acrylate were studied using DFT calculations. The calculations suggest that the formation of the carbene is the rate-limiting step for the unfunctionalized CoII(por) and that the cyclopropane ring formation proceeds via a stepwise radical proces...

A soft hydrogel reactor for cobalt nanoparticle preparation and use in the reduction of nitrophenols

Abstract: Bulk poly(2-acrylamido-2-methyl-1-propansulfonic acid) (p(AMPS)) hydrogels were prepared by irradiation of an aqueous solution of AMPS in the presence of crosslinker and photoinitiator. These p(AMPS) hydrogel networks were utilized for in situ cobalt nanoparticle synthesis by reduction of metal ions absorbed into the hydrogel network with a reducing agent, i.e., NaBH4. TEM images confirmed that Co particles are about 100 nm in size. The hydrogel network with embedded Co nanoparticles was utilized as a catalyst in the reduction of 4-nitrophenol (4-NP) and 2-nitrophenol (2-NP) in aqueous media in the presence of an excess amount of NaBH4. The kinetics of the reduction reaction under different reaction conditions was investigated to determine the activation parameters. Activation energies are 27.8 kJ mol−1 and 39.3 kJ mol−1 for 4-NP and 2-NP, respectively. It was found that hydrogel–Co composites were 99% active after 5 days storage.

Visible-light sensitive cobalt-doped BiVO4 (Co-BiVO4) photocatalytic composites for the degradation of methylene blue dye in dilute aqueous solutions

Abstract: A series of visible-light sensitive Co-BiVO4 photocatalysts were synthesized by heteronuclear complexing method using diethylenetriamine pentaacetic acid (DTPA) as the chelating agent. The photocatalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM). UV-vis diffuse reflectance spectroscopy (DRS), and Raman spectroscopy. Results indicated that all Co-BiVO4 photocatalysts had a crystal structure of monoclinic scheelite. Loading BiVO4 with cobalt did not alter the crystal structure of the composites. Cobalt was present as oxides and was deposited on the surface of larger BiVO4 particles. The oxidation state of cobalt varied with its content in Co-BiVO4. The photocatalytic activity of Co-BiVO4 was studied by the decolorization of methylene blue (MB). BiVO4 containing 5% (molar wt) of cobalt exhibited the greatest photocatalytic activity with a 85% of MB removal versus 65% by pure BiVO4 in 5 h. Factors such as pH, initial MB concentration, electrolytes, and irradiation conditions that may affect the photodegradation of methylene blue were studied. High pH and low initial MB concentration resulted in fast photocatalytic reaction. Electrolytes, especially those capable of scavenging hydroxyls, can inhibit MB degradation. The stability of the photocatalysts was confirmed using reclaimed Co-BiVO4 in three successive runs. There was no loss of photocatalytic ability in three successive runs each of which lasted for 6h with BM removal remained high at >90%. Results demonstrated clearly that Co-BiVO4 was stable and resistant to photocorrosion during the photocatalytic oxidation of organic compounds such as methylene blue. (C) 2010 Elsevier B.V. All rights reserved.

Cobalt-Catalyzed Hydroarylation of Alkynes through Chelation-Assisted C−H Bond Activation

Abstract: Ternary catalytic systems consisting of cobalt salts, phosphine ligands, and Grignard reagents promote addition of arylpyridines and imines to unactivated internal alkynes with high regio- and stereoselectivities. Deuterium-labeling experiments suggest that the reaction involves chelation-assisted oxidative addition of the aryl C−H bond to the cobalt center and insertion of the C−C triple bond into the Co−H bond, followed by reductive elimination of the resulting diorganocobalt species.

Anion-Exchange Membrane Fuel Cells: Dual-Site Mechanism of Oxygen Reduction Reaction in Alkaline Media on Cobalt−Polypyrrole Electrocatalysts

Abstract: The oxygen reduction reaction (ORR) processes in alkaline media that occur on a family of electrocatalyst materials derived from a Co containing precursor and a polypyrrole/C composite material (PPy/C) are investigated here. The effects of Co loading and heat treatment temperature on the CoPPy/C materials are revealed through structural evaluations and electrochemical studies. Principle component analysis (PCA), a mutivariant analysis (MVA) technique, is used to establish structure-to-property correlations for the CoPPy/C materials. In all cases, pyrolysis leads to formation of a composite catalyst material, featuring Co nanoparticles coated with Co oxides and Co2+ species associated with N−C moieties that originate from the polypyrrole structures. Based on these correlations, we are able to propose an ORR mechanism that occurs on this class of non-platinum based fuel cell cathode catalysts. The correlations suggest the presence of a dual site functionality where O2 is initially reduced at a Co2+ containi...

New Co-based γ-γ′ high-temperature alloys

Abstract: New cobalt-based alloys containing ordered L12 precipitates have been investigated. With additions of Cr, Mo, Ni, Re, Ta, and V to the ternary Co-Al-W system, two phase γ-γ′ microstructures have been established. Solidus and liquidus temperatures are 100°C–150°C higher than advanced nickel-based single-crystal alloys strengthened with the L12 phase. An anomalous rise in flow stress with temperature is observed. Single crystals have been solidified and partitioning during solidification is limited in the ternary system, suggesting a high resistance to convective instabilities. Oxidation at 900°C results in the formation of cobalt oxide. Following oxidation, an inner layer of Al2O3 is observed in uncoated Cr-containing alloys and Cr2O3 is observed in alloys subjected to chromization.

Cobalt Complexes as Antiviral and Antibacterial Agents

Abstract: Metal ion complexes are playing an increasing role in the development of antimicrobials. We review here the antimicrobial properties of cobalt coordination complexes in oxidation state 3+. In addition to reviewing the cobalt complexes containing polydentate donor ligands, we also focus on the antimicrobial activity of the homoleptic [Co(NH3)6]3+ ion.

Oxygen reduction reactivity of cobalt(II) hangman porphyrins

Abstract: Cobalt(II) hangman porphyrins are delivered from easily available starting materials, in two steps, in good yields, and with abbreviated reaction times. Selected compounds from a library of Co(II) hangman porphyrins immobilized on multiwall carbon nanotubes establish that the four-electron four-proton catalytic reduction of oxygen to water in aqueous solution can be achieved at the single cobalt center of the hangman platform. Reaction trends within the library reveal that the selective reduction of O2 to H2O occurs at electron deficient hangman porphyrin platforms possessing a distal group that is capable of proton transfer.

Redox-Active Ligand-Mediated Oxidative Addition and Reductive Elimination at Square Planar Cobalt(III): Multielectron Reactions for Cross-Coupling

Abstract: Square planar cobalt(III) complexes with redox-active amidophenolate ligands are strong nucleophiles that react with alkyl halides, including CH2Cl2, under gentle conditions to generate stable square pyramidal alkylcobalt(III) complexes. The net electrophilic addition reactions formally require 2e− oxidation of the metal fragment, but there is no change in metal oxidation state because the reaction proceeds with 1e− oxidation of each amidophenolate ligand. Although the four-coordinate complexes are very strong nucleophiles, they are mild outer-sphere reductants. Accordingly, addition of alkyl- or phenylzinc halides to the five-coordinate organometallic complexes regenerates the square planar starting materials and extrudes C−C coupling products. The net 2e− reductive elimination reaction also occurs without a oxidation state change at the cobalt(III) center. Together these reactions comprise a complete, well-defined cycle for cobalt Negishi-like cross-coupling of alkyl halides with organozinc reagents.

Controlled Synthesis and Morphology-Dependent Electromagnetic Properties of Hierarchical Cobalt Assemblies

Abstract: Hierarchical cobalt assemblies such as spheres, flowers with dendritic petals, and flowers with sharp petals are successfully synthesized via a facile liquid-phase reduction method by simply adjusting the reaction conditions. The morphology evolution process and transformation mechanism from spheres to dendrites and finally to flowers have been systematically investigated. It is determined that coercivity Hc depends more on sample size than on shape anisotropy, while saturation magnetization Ms is greatly affected by pinned surface magnetic moment. Even at a thinner thickness, as-synthesized cobalt samples exhibit stronger microwave absorbing ability compared with reported cobalt in the same frequency band. Especially, the cobalt flowers with dendritic petals exhibit the strongest absorption in middle frequency because incident wave and reflected wave are totally canceled at matching thickness. The architectural design of material morphologies is critical for improving properties toward future application.