Cobalt Spheres Research Articles

Impact of noble-metals on the catalytic stability of cobalt nanoparticles for the acceptorless dehydrogenation of alcohols

Three different types of unsupported Co catalysts are prepared by the polyol process and investigated for the oxidation of alcohols through an acceptorless dehydrogenation pathway: cobalt spheres with a predominantly face-centered cubic structure, pure cobalt nanorods and Ru-decorated cobalt nanorods, both with a hexagonal compact structure. All the catalysts display a high chemoselectivity for the oxidation of long-chain model alcohols since only secondary alcohols can be transformed into the corresponding ketones. Besides the ketone generated, the highly valuable hydrogen molecule is produced as the only by-product. Irrespective of the shape, similar high conversions and high selectivities are measured for the first catalytic tests. Nevertheless, when the three types of particles are recycled, significant differences in terms of catalytic stability and morphological evolution are observed. All the Co crystal habits are substantially modified during catalysis although well-isolated particles are recovered after three consecutive runs. The highest catalytic stability is found for Co nanorods decorated by Ru nanoparticles.

Cobalt(ii) liquid metal salts for high current density electrodeposition of cobalt.

Cobalt(ii)-containing ionic liquids were synthesized using N,N-dimethylformamide, N-methylpyrrolidone (NMP), dimethylsulfoxide (DMSO), N,N-dimethylacetamide (DMAc), pyridine-N-oxide (py-O), 1,10-phenanthroline (phen), ethylenediamine (en) and dimethylimidazolidinone (DMI) as ligands. The weakly coordinating bis(trifluoromethylsulfonyl)imide (Tf2N-) was used as a counter anion. Several compounds had a melting temperature below 100 °C, and the compound [Co(DMAc)6][Tf2N]2 was liquid at room temperature, with a viscosity of only 18 mPa s at 80 °C. Several compounds were recrystallized to give high quality single crystals and their crystal structures were determined. EXAFS measurements were performed on [Co(DMAc)6][Tf2N]2 at different temperatures and it was observed that the [Co(DMAc)6]2+ ion partially dissociated at higher temperatures, which explains the temperature-dependent color change (thermochromism). The electrochemical properties of the compounds with the lowest melting points were also investigated. Adherent, crack-free metallic cobalt layers could be electrodeposited from [Co(DMAc)6][Tf2N]2, [Co(DMI)6][Tf2N]2 and [Co(NMP)6][Tf2N]2. From the first two, black deposits consisting of micrometer-sized needles were obtained, whereas the latter resulted in a dull grey cobalt layer consisting of micrometer-sized cobalt spheres. The Co(iii)/Co(ii) redox couple was not found to occur in any compound with an O-donor ligand, but the Co(iii)/Co(ii) redox couple was found to be quasi-reversible for [Co(phen)3][Tf2N]2 dissolved in [BMP][Tf2N].

Co nanocrystals engineering**Invited talk at 8th Int. Workshop on Advanced Materials Science and Nanotechnology (Ha Long City, Vietnam, 8–12 November 2016).

Organometallic chemistry plays an increasing role in the synthesis of nanoparticles, as it provides a reliable access to metal nanoparticles with efficient control over their morphology, organization and surface chemistry. In case of magnetic nanoparticles, the synthetic tools provided by organometallic chemistry allow access to nanomaterials of high magnetization, meaning that no dead surface magnetic layer is observed. These objects are thus good candidates to be used as building blocks in composite materials of high added value. This paper reports on the organometallic synthesis of composites made of cobalt nanoparticles and carbon nanotubes. TEM investigations show that attachment of cobalt spheres and rods along the carbon nanotubes is achieved, the rods and tube long axis being aligned parallel to one another.

Palladium nanoparticles supported on hollow mesoporous Tungsten carbide microsphere as electrocatalyst for formic acid oxidation

Hollow tungsten carbide and cobalt spheres (HTCCS) with cobalt concentration of 6% were prepared by spray conversion and reduction carbonization methods. The HTCCS with diameters of about 8–18 μm were covered with 0.3–1.0 μm microporous. The Pd/WC catalyst was prepared through the replacement reaction between Co nanoparticles on the surface of HTCCS and PdCl4. The electro-catalytic performances for formic acid electro-oxidation were investigated by cyclic voltammetry and chronoamperometry. It exhibited the low onset potential, excellent catalytic activity and stability compared with Pd/C due to the larger electrochemical surface area (ECSA) and the synergistic effect between Pd and WC.

Facile synthesis of hollow porous cobalt spheres and their enhanced electromagnetic properties

We report a template-free approach to prepare hollow porous cobalt spheres by combining a facile solvent-thermal route with a chemical reduction process. The synthesized cobalt microspheres with diameter of approximately 2–4 μm have a shell thickness of about 150 nm. The hollow Co microsphere is built from particle-like ligaments with diameter of ca. 80–150 nm and there are a lot of pores with size of several tens of nm on the shell. The hollow porous cobalt spheres exhibited saturation magnetization (Ms) of 163.0 emu g−1 and large coercivity (Hcj) of 307.9 Oe. The epoxy resin composites with 30 wt% and 60 wt% hollow porous cobalt sample showed efficient EM wave absorption characteristics (RL < −20 dB) in the ranges of 11.3–18.0 GHz and 4.0–12.0 GHz over absorber thicknesses of 1.4–2.0 mm and 1.4–4.0 mm, respectively. Our study demonstrates the potential application of hollow porous ferromagnetic materials as a more efficient electromagnetic wave absorber.

Hydrothermal synthesis and electrochemical properties of cobalt–carbon nanotubes nanocomposite

Cobalt–carbon nanotubes composite (Co–CNTs) is synthesized through a facile hydrothermal route. SEM and TEM characterizations reveal that the Co–CNTs composite contains abundance of carbon nanotubes connected by cobalt spheres and some of the CNTs are filled with metallic nanoparticles or nanorods. A series of electrochemical measurements show that the adding CNTs can remarkably enhance the electrochemical activity of the Co, leading to a notable improvement of the discharge capacity and the cycle performance. The practical maximum discharge capacity of the active Co is 495 mAh g −1 after deducting the weight contribution of CNTs, which is about 280 mAh g −1 higher than that of pure Co. The electrochemical reaction mechanism can be attributed to the dissolution–precipitation mechanism of Co in alkaline solution. And the functions of the CNTs are to improve dispersion of Co particles, increase contact area between Co and alkaline solution and promote the charge-transfer reaction.

Control of Magnetism in Cobalt Nanoparticles by Oxygen Passivation

We report on the preparation of ferromagnetic cobalt nanospheres with antiferromagnetic oxide capping layer and its implication for the variation in magnetic property. The hcp cobalt nanospheres were prepared by thermal decomposition of cobalt carbonyl in the presence of organic surfactants. The spherical nanoparticles thus prepared were oxidized to grow antiferromagnetic layers of varying composition and thickness on top of cobalt spheres. High-resolution transmission electron microscopy confirmed growth of Co3O4 in one case and CoO in another case. Strong exchange anisotropy and enhanced coercive field was observed due to the core−shell structure in the Co−CoO system. On the other hand only a marginal improvement was seen in the Co−Co3O4 system. A low-temperature paramagnetic behavior was also observed that is interpreted in the framework of crystal defects in the oxide shell.

New magnetorheological fluids based on magnetic fibers

In this work, we report the synthesis under the presence of external magnetic fields of two types of magnetic microfibers: (i) cobalt wires; and (ii) rigid filaments of chemically linked iron microspheres. Optical microscopy and scanning electron microscopy (SEM) were used for characterizing their shape and size. The effect of magnetic field intensity, mixing frequency and reagent concentration on the final size of the fibers was investigated. The particles were also characterized by means of energy dispersive X-ray spectroscopy (EDX). The magnetization curves of both the cobalt wires and the iron filaments were obtained and compared with those of cobalt spheres and iron spheres respectively. Finally, these magnetic fibers were used for the preparation of new smart composites by dispersing them in a liquid carrier. Interestingly, the suspensions of cobalt wires show a remarkable increase in the yield stress when compared to conventional (based on spherical particles) magnetorheological fluids.

One-step solution fabrication of magnetic chains consisting of jingle-bell-shaped cobalt mesospheres

Using a one-step solution phase approach, the authors have synthesized uniform jingle bell-shaped cobalt mesopheres (550–750nm) and assembled the mesospheres into long magnetic chains (20–30μm). All of the cobalt spheres are hollow with ∼40nm thick shells but each contains an ∼200nm diameter solid ball. The nano- to mesoscale structures were realized via reaction of CoCl2∙6H2O and N2H4∙H2O in the presence of polyvinylpyrrolidone (PVP) in an ethylene glycol solution. Magnetic measurements show a coercivity of about 75Oe with a remnance of 9.6emu∕g at 300K. We propose a possible mechanism for the formation of the nanoto mesoscale structures.

Preparation of plastic ferromagnetic composite materials for magnetic encoders

Monodispersed spheres of ferromagnetic metals like iron, cobalt and nickel, can be synthesized on a micrometric and sub-micrometric scale by simply reducing their salts with boiling polyols. These magnetic particles embedded into a polymeric matrix can be exploited for a number of technological applications (e.g., barcodes for magnetic encoders, magnetic storage media, electromagnetic shielding, etc.). The spherical shape of particles allows to prepare composites with high-filling factors and very uniform filler distribution. In addition, since ferromagnetic particles show size-dependent magnetic properties (e.g., coercivity), the possibility to control the particle size is of primary importance. Here, we have found that monodispersed cobalt spheres of different sizes can be obtained by reduction of cobalt hydroxide with diethylene glycol in presence of oleic acid. This organic compound absorbs on the metal surface, increasing compatibility with polymer in the resulting composite. The average particle size was related to the precursor amount and it was widely varied by simply controlling this parameter. Disproportion in ethylene glycol medium of in situ generated iron hydroxide (at 140 °C) has been used to produce monodispersed iron nanoparticles.

Synthesis and Characterization of Large Colloidal Cobalt Particles

Large colloidal environmentally stable silica-coated cobalt particles were synthesized by combining the sodium borohydride reduction in aqueous solution and the Stöber method. Low size polydisperse cobalt spheres with an average size of 95 nm were synthesized by using a borohydride reduction method and were subsequently coated with a thin layer of silica by means of hydrolysis and condensation of tetraethylorothosilicate (TEOS) in an aqueous/ethanolic solution. The large uniform cobalt spheres consist of smaller metallic Co clusters, explaining the superparamagnetic behavior of the spheres. The particles were investigated by transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM).

Plastic Ferromagnetic Composite Materials for Magnetic Encoders

Abstract Monodispersed spheres of ferromagnetic metals like iron, cobalt, and nickel can be synthesized on a micrometric and sub-micrometric scale by simply reducing their salts with boiling polyols. These magnetic particles embedded into a polymeric matrix can be exploited for a number of technological applications (e.g., barcodes for magnetic encoders, magnetic storage media, electromagnetic shielding). The spherical shape of particles allows to prepare composites with high filling factors and very uniform filler distribution. In addition, because ferromagnetic particles show size-dependent magnetic properties (e.g., coercivity), the possibility to control the particle size is of primary importance. Here, we have found that monodispersed cobalt spheres of different sizes can be obtained by reduction of cobalt hydroxide with diethylene glycol. The average particle size was related to the precursor amount and it was widely varied by simply controlling this parameter. Disproportion in ethylene glycol mediu...

Cobalt–Carbon Spheres: Pyrolysis of Dicobalthexacarbonyl‐Functionalized Poly(p‐phenyleneethynylene)s

Micrometer-sized carbon–cobalt spheres are demonstrated to be formed by the pyrolysis at 650 °C of a dicobalthexacarbonyl-complexed poly(p-phenyleneethynylene) derivative (see Figure). The spheres are obtained in quantitative yield and are shown to be composed of cobalt, carbon, and potentially a small amount of oxygen, rather than being all-carbon, as might have been expected.

Structure of Cobalt Nanosphere Superlattice Films by Small Angle X-ray Scattering

AbstractCobalt nanocrystals, recently synthesized with narrow size distributions and controlled shapes, organize in a wide range of arrays as a function of shape, size and interparticle interactions. The nanocrystals (NCs) consist of a cobalt metal core with a nominal diameter of 11 nm, and an organic surfactant surface layer with a chain length of ∼1.7 nm. For the simplest case (ε-Co nanospheres, super-paramagnetic at room temperature) a hexagonal arrangement of NCs is observed in transmission electron microscope (TEM) images when precipitated from solution onto carbon films. For practical applications and for further understanding of the self-assembly process, long range order of the super lattice must be probed over regions that are greater in extent than may be examined by TEM. Hence, small angle x-ray scattering (SAXS) measurements were performed on cobalt nanospheres randomly dispersed in solution and assembled on glass substrates. Least squares fit to the intensity distribution as a function of the scattering vector q gave an average particle diameter of 11.0 ± 0.8 nm. Structure factor contribution to the intensity profile agrees well with a quasi-random model for scattering from a face centered cubic (FCC) superlattice composed of uniform radius cobalt spheres. The measured nearest neighbor interparticle spacing, 14.1 nm, agrees to within 2% of the predicted value of 14.4 nm based on a free energy model that governs the self-assembly of the nanoparticle system.

Porous cobalt spheres for high temperature gradient magnetically assisted fluidized beds

Porous metallic cobalt spheres have been prepared as high temperature capable media for employment in gradient magnetically assisted fluidization and filtration technologies. Cobalt impregnated alginate beads are first formed by extrusion of an aqueous suspension of Co 3O 4 into a Co(II) chloride solution. The organic polymer is thermally decomposed yielding cobalt oxide spheres, followed by reduction to the metallic state, and densification. Cobalt beads have been produced with porosities ranging between 10 and 50%, depending upon sintering conditions. The product media have been characterized by scanning electron microscopy (SEM), nitrogen adsorption porosimetry, and vibrating sample magnetometry.

Preparation of metallic cobalt and cobalt-barium titanate spheres as high temperature media for magnetically stabilized fluidized bed reactors.

Fluidized bed reactors are extremely efficient when mass transfer limitations dominate global reaction rates, such as in combustion or incineration [1, 2]. Because fluidized beds operate using a balance between direct fluid dynamic forces and the gravitational restoring force, the maximum mass transfer rates for a given reactor configuration are limited by gravity. This limitation has been overcome using magnetic fields acting upon magnetically susceptible fluidization media to augment the force of gravity [3–7]. However, the use of magnetically stabilized fluidized beds in high temperature reactions has been limited by the lack of suitable media. Three magnetic properties are required to obtain suitable media: high magnetic susceptibility, high Curie temperature, and low coercivity. Cobalt, with a Curie temperature of 1121 ◦C, meets these requirements. We have investigated the preparation of two forms of cobalt containing spherical fluidization media: metallic cobalt, and cobalt impregnated barium titanate. Novel ferromagnetic media including cobalt impregnated barium titanate and metallic cobalt spheres have been prepared for high temperature application in magnetically stabilized fluidized beds. Spherical polymeric beads were initially formed by gelation of an alginateprecursor oxide suspension. Water was then eliminated from the hydrous gel at low temperature and the organic polymer was removed by oxidation in air. The temperature was then raised and the firing atmosphere changed to a reducing mixture containing hydrogen. Under these conditions, cobalt oxide was first reduced to the metallic form, and at a sufficiently high temperature, densification occurred via sintering. Cobalt impregnated barium titanate was prepared by intimately mixing BaCO3, TiO2, MgSO4, SiO2, and Co3O4 in a ball mill using zirconium oxide milling media, with molar ratios corresponding to one mole of Ba0.95Mg0.05TiO3, 0.064 moles of TiO2, 0.311 moles of Co, and 0.0207 moles of SiO2. The excess TiO2 and SiO2 form a low melting eutectic, predominately BaTiSi2O7, which melts at 1245 ◦C and acts as a sintering aide. The cobalt content corresponds to 10% (w/w) of the resulting BaTiO3. The oxides, carbonates, and sulfates were initially added to 1.4 times their weight

Preparation and Properties of Monodisperse Magnetic Cobalt Colloids Grafted with Polyisobutene

A single-step method to synthesize monodisperse metallic cobalt particles of diameter around 8 nm is described. The particles are sterically stabilized by modified polyisobutene and form stable ferrofluids in toluene. The core–shell cobalt spheres have a narrow size distribution and are nonaggregated. Various techniques, such as infrared spectroscopy, X-ray diffraction, and elemental analysis establishes the presence of grafted modified polyisobutene on the surface of cobalt particles. The hydrodynamic thickness of the grafted polyisobutene calculated from sedimentation and viscosity measurements and independently from the dynamic light scattering is ca. 10 nm. The dependence of magnetization has been measured for ferrofluid, showing superparamagnetic behavior above 247 K. Due to anisotropy, superposition of magnetic curves is not observed below this temperature. The concentration dependence of the sedimentation coefficient agrees qualitatively with the theory for dipolar spheres. SAXS data on concentrated ferrofluid dispersion indicate interpenetration of polyisobutene chains grafted to the cobalt particles.

Particle impact phenomena of silicon nitride ceramic

Abstract By using a new impact test apparatus, the particle impact behaviour of ceramic materials at room temperature and elevated temperatures up to 1400°C and under tensile loads were carried out. Samarium cobalt spheres of 1mm diameter and 4mg mass were used as impact particles. The particle velocities ranged from 200 to 1000ms−1. Silicon nitride, which is a candidate material for gas turbines under development, was chosen as the test material. At elevated temperatures there was a case where the impact fracture strength significantly decreased. At room temperature, the critical impact velocity for crack initiation in thin disc specimens seems to be proportional to (bending strength)5/6 × (thickness)5/3 in line with the results of Tsuruta, Masuda, Soma and Matsui. For particle impact events, electrical signals induced by the impact, photon emission and acoustic waves were observed simultaneously.