Nanoparticles cobalt

FIG. 10 Hysteresis magnetization loops obtained at T = 3 K. (A) Diluted liquid solution of cobalt nanoparticles in hexane. (B) Cobalt nanoparticles deposited onto freshly cleaved graphite (HOPG) and dried under argon to prevent oxidation. Substrate parallel (—) and perpendicular (—) to the field. 

For iron, cobalt, nickel, and their alloys, the most sensitive technique for characterizing the particle surface is the measurement of magnetic properties. Thus, we synthesized cobalt nanoparticles of 1.6 nm (ca. 150 atoms), 2 nm (ca. 300 atoms) and 4 nm (a few thousand atoms) mean size. The structure of the particles is hep in the latter case and polytetrahedral in the first two cases. The 4 nm particles display a saturation magnetization equal to that of bulk... 

Structure of the particles, which may be different from that thermodynamically stable in the reaction conditions, e.g., the polytetrahedral structure found for cobalt nanoparticles... 

In a different way, metallic-core nanoparticles [346-349] (prepared cf. Section 3.10) equipped with biocompatible coats such as L-cysteine or dextrane may be exploited for highly efficient and cell-specific cancer cell targeting, i.e., for improving diagnosis and therapy of human cancer. In a recent proof-of-principle experiment an unexpectedly low toxicity of the L-cysteine-covered cobalt nanoparticles was demonstrated [433] For diagnostic purposes, it is expected to use the advantageous magnetic properties of the metallic-core nanoparticles to obtain a contrast medium for MRI with considerably increased sensitivity, capable to detect micro-metastases in the environment of healthy tissues [434 37]. 

Later, Chung et al. successfully developed an intramolecular Pauson-Khand reaction in water without any cosolvent by using aqueous colloidal cobalt nanoparticles as catalysts. The catalyst was prepared by reducing an aqueous solution of cobalt acetate containing sodium dode-cyl sulfate (SDS) surfactant. The cobalt nanoparticle could be reused eight times without any loss of catalytic activity (Eq. 4.57).107... 

Delpeux S., Szostak K., Frackowiak E., Bonnamy S., Beguin F. High yield of pure multiwalled carbon nanotubes from the catalytic decomposition of acetylene on in-situ formed cobalt nanoparticles. J. Nanosc. Nanotech. 2002 2 481-4. 

Wang, M.-S. Bando, Y. Rodriguez-Manzo, J.A. Banhart, F. Golberg, D., Cobalt nanoparticle-assisted engineering of multi-wall carbon nanotubes. ACS Nano 2009,3 2632-2638. 

S. Son, S. Lee, Y. Chung, S. Kim, and T Hyeon, The first intramolecular Pauson—Khand reaction in water using aqueous colloidal cobalt nanoparticles as catalysts. Organ. Lett. 4,277—279 (2002). 

J. D. Carter, Y. R. P. Qu, L. Hoang, D. J. Masiel, and T. Guo, Silicon-based nanowires from silicon wafers catalyzed by cobalt nanoparticles in hydrogen environment, Chem. Commun. 211 A—2216 (2005). 

MetalHc cobalt nanoparticles were synthesized by the decomposition of THE solution of an organometallic precursor Colij -CgHia) )[197]... 

RuCNC heterobi metal lie ruthenium/cobalt nanoparticle immobilized on charcoal. 2-Pyridylmethyl formate is used. 

Substantial improvements are made by using the cobalt nanoparticles. Contrary to the previous cases, much lower CO pressure (5 atm) is enough for the completion of the reaction. However, high loading of catalysts (45 mol%) seems to be a drawback. Nevertheless, this system can be reused, and one of them is compatible with water solvent. ... 

Another difference between Co and Fe is their sensitivity towards impurities in the gas feed, such as H2S. In this respect, Fe-based catalysts have been shown to be more sulfur-resistance than their Co-based counterparts. This is also the reason why for Co F-T catalysts it is recommended to use a sulphur-free gas feed. For this purpose, a zinc oxide bed is included prior to the fixed bed reactor in the Shell plant in Malaysia to guarantee effective sulphur removal. Co and Fe F-T catalysts also differ in their stability. For instance, Co-based F-T systems are known to be more resistant towards oxidation and more stable against deactivation by water, an important by-product of the FTS reaction (reaction (1)). Nevertheless, the oxidation of cobalt with the product water has been postulated to be a major cause for deactivation of supported cobalt catalysts. Although, the oxidation of bulk metallic cobalt is (under realistic F-T conditions) not feasible, small cobalt nanoparticles could be prone to such reoxidation processes. 

Fischer-Tropsch synthesis making use of cobalt-based catalysts is a hotly persued scientific topic in the catalysis community since it offers an interesting and economically viable route for the conversion of e.g. natural gas to sulphur-free diesel fuels. As a result, major oil companies have recently announced to implement this technology and major investments are under way to build large Fischer-Tropsch plants based on cobalt-based catalysts in e.g. Qatar. Promoters have shown to be crucial to alter the catalytic properties of these catalyst systems in a positive way. For this reason, almost every chemical element of the periodic table has been evaluated in the open literature for its potential beneficial effects on the activity, selectivity and stability of supported cobalt nanoparticles. 

The addition of promoter elements to cobalt-based Fischer-Tropsch catalysts can affect (1) directly the formation and stability of the active cobalt phase structural promotion) by altering the cobalt-support interfacial chemistry, (2) directly affect the elementary steps involved in the turnover of the cobalt active site by altering the electronic properties of the cobalt nanoparticles electronic promotion) and (3) indirectly the behaviour of the active cobalt phase, by changing the local reaction environment of the active site as a result of chemical reactions performed by the promoter element itself synergistic promotion). 

It is far from easy to distinguish structural, electronic and synergistic promotion effects. Structural promotion is, in this respect, the most easily to observe. Most synergistic elfects are also widely discussed in the literature in enhancing the catalytic performance of supported cobalt nanoparticles. Instead, promotion as a result of electronic effects are much more difficult to detect. The main reason is that one has to discriminate between the number of surface cobalt sites and the intrinsic activity of a surface cobalt site (turnover frequency). This is especially difficult in view of the complexity of the catalyst material. It also requires spectroscopic tools, which are able to detect changes in the electronic structure of the supported cobalt nanoparticles. 

Kato, Y., Sugimoto, S., Shinohara, K., Tezuka, N., Kagotani, T. et al, Magnetic properties and microwave absorption properties of polymer-protected cobalt nanoparticles, Mater. Trans., JIM, 2002, 43, 406. 

Figure 4. TEM images of (A) a 2D assembly of the 11 nm Co nanoparticles, and (B) a 3D assembly of the 8 nm cobalt nanoparticles on an amorphous carbon surface.

Figure 5. TEM image of 11 nm polyhedral shaped cobalt nanoparticle assembly with the co-existence of stacking faults (S), twins (T) and particle dislocations (PD) [45].

Figure 9. TEM images of 8 nm cobalt nanoparticle assemblies of (A) before and (B) after...

Several non-carbonyl cobalt sources used recently show high efficiency in the catalysis of the PKR. Chung has reported different reusable catalysts like cobalt supported on mesoporous silica or on charcoal that work under high CO pressures [ 127]. Most recently they have described milder conditions with the use of colloidal cobalt nanoparticles, which react at lower CO pressures and can be used in aqueous media [128]. 

More recently this group has prepared a combination of palladium and cobalt nanoparticles immobilized on silica (PCNS) to form bicyclic enones after domino allylic alkylation-PKR [130]. 

A spectacular application allowed the synthesis of fenestranes by a three-step sequential action of cobalt nanoparticles and a palladium catalyst [131]. The cascade reaction started with a PKR of enyne 105, accomplished by the cobalt catalyst giving 106, followed by the formation of allyl-7r3 palladium complex 107 which reacted with a nucleophile derived from diethyl malonate, to give enyne 108. The final step was a second PKR that gave 109 in good yield. They used cobalt nanoparticles as with Co/charcoal the third step did not take place, apparently due to damage in this catalyst after the allylation step (Scheme 31). 

Keese envisioned the use of a tandem PKR for the synthesis of fenestranes. The second cycloaddition was in principle problematic as it involved an al-kene conjugated with a ketone. They were surprised when they observed the direct formation of the tetracyclic unit 136 from the endiyne 135 although with low yield [ 148]. Further studies from this group led to a mechanistic proposal that explained this result. It was clear from the fact that compound 140 failed to react, that the second PKR had to start from an intermediate metal-lacycle rather than from the uncomplexed final cyclopentenone. Thus, cobalt complex 137 would lead to 138 were both metal clusters would interact giving intermediate 139 which would evolve in the usual way to the final product (Scheme 42) [149]. These systems have been obtained later by Chung s group using cobalt nanoparticles as commented above (Sect. 2.4) [131]. 

Figure 4.16 Micrographs of filamentous (nanotubular) carbon synthesized through disproportionation of CO over small (less than 25 nm in diameter) cobalt nanoparticles. The external diameter of the filaments equals the diameter of the initial Co° nanoparticle, the wall thickness of the graphite nanotube being 3-5 nm [6],...

Figure 4.17 Regular layers Inside a cobalt nanoparticle larger than 20 nm in diameter, which are observed after the particle has been exposed to CO at the pressure of 1 bar and temperature 700 K. The light regions are the fine (approximately five atoms in thickness) hexagonal cobalt layers, dark region are the cubic cobalt layers [6].

Figure 5.4 Micrographs of carbon structures generated over large (A) and small (B) cobalt nanoparticles. The large nanoparticles (more than 25 nm in diameter) give rise to the graphite shell over their surface, while the small particles (less than 25 nm in diameter approximately 10 nm in the sample under study) to fine carbon nanotubes with the external diameter equal approximately to the diameter of the Co° particle and wall thickness of approximately 3-5 nm [5].

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