Co particles

For the physico-chemical measurements and practical utilisation in some cases the purification of nanotubules is necessary. In our particular case, purification means the separation of filaments from the substrate-silica support and Co particles. 

Figure 14. Size-selective formation of air stable Co particles via thermolysis of Co2COg controlled by aluminium alkyl. (Reprinted from Ref [53], 2007, with permission from Wiley-VCH.)...

As was demonstrated in the preceding sections, structure-sensitivity phenomena are mostly confined to particle size regimes smaller than 3-4 nm. A process of industrial relevance was investigated by de Jong et al. [127] in their study on cobalt particle size effects in the Fischer-Tropsch reaction. Earlier works noted distinct drop in activity for Co particles smaller than lOnm and ascribed this phenomenon to either a partial oxide or carbide formation which should be enhanced for particles in this size regime [128-139]. In order to avoid similar effects, de Jong used... 

In agreement with the DEMS results the ECTDMS work of Wilhelm et at. (1987b) also showed that the nature of the predominant adsorbate is a function of both coverage and methanol concentration. The authors performed ECDTMS measurements at two different methanol concentrations. The areas under the H2 and CO desorption peaks were calibrated and allowed the number of desorbed H and CO particles, nH and nc0, to be calculated. nH represents the number of COH species present on the surface and nco the total number of CO and COH adsorbates. From these values the mole fraction of C=Oada and COHads can be calculated, as shown in Figure 3.40. 

To some extent the cobalt particles in Figure 2.1(c) seem to be distributed within the tubular structure of the multiwalled nanotubes. TEM analysis could not fully clarify if this is an artifact or if the particles are truly situated inside the hollow space of the tubes. However, Tavasoli et al.14 observed Co particles captured inside the tubes after incipient wetness impregnation. Thus, it can be assumed that this is the case here as well. 

There is mounting evidence to suggest that water effects during FTS may also be linked to Co particle size, as determined by the support interaction [18,19], even though the turnover frequencies of CO over the surface Co metal atoms on... 

Figure 2. Differential spectra of CO chemisorbed on alumina-supported Co particles both before and after heating in hydrogen to 415 K. The chemisorbed CO is seen to react and form hydrocarbons in the tunnel junction. This hydrocarbon species is distinct from that formed on Rh as seen by vibrational modes near 1600...

In the very active field of unmodified nanoparticles recent discoveries have been made on size-selective Fischer-Tropsch catalysts that convert selectively CO and H2 into hydrocarbons there is a strong dependence of activity, selectivity and Hfetime on Co particle size. This topic of unmodified, supported or unsupported, nanoparticles is outside the scope of this chapter [74, 75]. Nevertheless, we mention discoveries made by Degussa, who have patented a process for H2O2 synthesis from molecular oxygen and molecular hydrogen with nanosized Pd particles (6 A) [76]. 

In the limit co J. 0 the radius R(co) becomes infinite and both potentials differ by a constant A everywhere. We will now show that A is unequal to zero. If we now insert v dN to) = o (JV) + A in the Kohn-Sham equations for the N -1- co-particle system and use the fact that for small to the orbitals in the region... 

The aim of this review paper is to give an extensive overview of the different promoters used to develop new or improved Co-based F-T catalysts. Special attention is directed towards a more fundamental understanding of the effect of the different promoter elements on the catalytically active Co particles. Due to the extensive open and patent literature, we have mainly included research publications of the last two decades in our review paper.In addition, we will limit ourselves to catalyst formulations composed of oxide supports, excluding the use of other interesting and promising support materials, such as, e.g., carbon nanofibers studied by the group of de Jong. ... 

Co active phase due to the promoter element, which either avoids the formation of metal-support compounds, or prevents the agglomeration and sintering of the Co particles under F-T operation conditions. 

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