Cobalt Nanoparticles under Reaction Conditions

Cobalt is known for its catalytic activity for many reactions, and mostly for Fischer-Tropsch synthesis (FTS) to produce liquid fuels from syngas with high carbon efficiency [21-23]. Previous studies have shown that the use of cobalt NPs results in improved catalytic performance [24]. The chemical states of the Co NPs as well as the adsorbed species during the FT reaction are not attainable with the conventional ex situ techniques, preventing a complete understanding of the catalytic process. As such, in situ SXAS under reaction conditions have been carried out [25]. 

FIGURE 5.9 Schematic drawing of a flow reaction cell with the heating capability designed 

FIGURE 5.10 In situ X-ray absorption spectra of the Co L-edge of (a) 3nm and (b) lOnm Co nanocrystals supported on gold foils in the presence of 1 har (40m]/inin) and after heating to different temperatures. Reprinted with permission from Ref. [25]. American 

Following the reduction, the atmosphere was switched to pure CO or syngas (CO+mixtures) at room and elevated temperatures to mimic the FTS conditions, and SXAS was used to monitor the electronic structure of the Co NPs. Under aU conditions, no changes were observed in the Co L-edge spectra (Fig. 5.11) for the Co NPs with all sizes and also the Co foil, suggesting that the active phase of Co catalysts in FTS is the metallic state which remains unchanged during the reaction. 

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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. 

The Pauson-Khand reaction has also been carried out under different conditions, such as the use of chiral ligands, including PuPHOS, CamPHOS, and camphorsultam the use of aldehydes as the carbon monoxide source > the use of solid-supported cobalt catalyst to enhance purification, such as the dry-state adsorption the use of colloidal cobalt nanoparticle and the use of metallic cobalt supported on mesoporous silica prepared by decomposing Co2(CO)8 on mesoporous silica supports (SBA-15 and MCM-41) in the refluxing toluene solution.Other modifications include different promoting methods, such as photo-irradiation, microwave irradiation, molecular sieves, TEMPO,A-oxides, and supercritical fluids. Furthermore, the cycloaddition between allene and carbon monoxide under similar conditions is known as the allenic Pauson-Khand reaction,27,41 jjjg reaction among alkyne, carbodiimide, and CO is referred to as the aza-Pauson-Khand reaction. ... 

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