Cobalt particle

We have shown in the preceding section that the IR spectra of well-defined metal carbonyls provide valuable information of the environment of the deposited metal atom. However, IR signals of CO molecules adsorbed on larger particles suffer from broad lines, which hamper a more detailed analysis of the data. In the forthcoming section we will present results on cobalt particles where carbonyl species are formed on larger particles containing hundreds of atoms. 

The presence of a site with a low metal-metal coordination is compatible with the non-crystalline nature of the cobalt deposits [64]. It is to be expected that these sites exhibit different chemical reactivity than the usual adsorption sites. This can be verified by subsequent deposition of a small amount (0.1 A) of Pd atoms, which are known to nucleate exclusively on the cobalt particles [64]. The corresponding IR spectrum is shown as the bottom trace in Fig. 6. It is seen that an additional peak appears at 2105 cm which is readily assigned to CO bound terminally to Pd. More importantly, the growth of this Pd feature is completely at the expense of the carbonyl species, indicating that Pd nucleates almost exclusively at these low coordinated sites and prevents the formation of the carbonyl species. 

The surface properties of these nano-objects match those of metal nano crystals prepared in ultrahigh vacuum, for example the C - O stretch of adsorbed carbon monoxide or the magnetic properties of cobalt particles embedded in PVP. This demonstrates the clean character of the surface of these particles and its availabihty for reactivity studies. 

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... 

Figure 11. The influence of cobalt particle size on the TOF (left) and on methane selectivity (right 220 °C, H2/CO = 2, 1 bar). (Reprinted from Reference [127], 2006, with permission from American Chemical Society).

Bezemer G.L., Bitter J.H., Kuipers H.P.C.E., Oosterbeek H., Holewijn J.E., Xu X., Kapteijn F., van Dillon A.J., and de Jong K.P. 2006. Cobalt particle size effects in the Fischer-Tropsch reaction studied with carbon nanofibre supported catalysts. J. Am. Chem. Soc. 128 3956-64. 

Zhang Y., Liu Y., Yang G., Sun S., and Tsubaki N. 2007. Effect of impregnation solvent on Co/Si02 catalyst with bimodal sized cobalt particles. Appl. Catal. A Gen. 321 79-85. 

Prior to functionalization the carbon nanomaterials were washed in concentrated nitric acid (65% Fisher Scientific) for 8 h using a Soxhlet device in order to remove catalyst residues of the nanomaterial synthesis as well as to create anchor sites (surface oxides) for the Co on the surface of the nanomaterials. After acid treatment the feedstock was treated overnight with a sodium hydrogen carbonate solution (Gruessing) for neutralization reasons. For the functionalization of the support media with cobalt particles, a wet impregnation technique was applied. For this purpose 10 g of the respective nanomaterial and 10 g of cobalt(II)-nitrate hexahydrate (Co(N03)2-6 H20, Fluka) were suspended in ethanol (11) and stirred for 24 h. Thereafter, the suspension was filtered via a water jet pump and finally entirely dried using a high-vacuum pump (5 mbar). 

As can be seen from Figure 2.1, cobalt was deposited on the carbon nanomaterials quite homogeneously. Hence, the cobalt particle sizes of the three catalyst types vary only little. The Co/nanofiber materials exhibit cobalt particle diameters of roughly 10 nm. In case of the nanotubes, particle sizes ranging from 5 to 7 nm were observed. 

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. 

Additives such as rare earth or noble metals are generally introduced into industrial cobalt FTS catalysts as structural or reduction promoters.92 The addition of various promoters to cobalt catalysts has also been shown to decrease the amount of carbon produced during the FTS.84 87 93 94 Also, the addition of promoter elements may decrease the temperature of regeneration, preventing the possible sintering of supported cobalt particles during such treatments.92... 

Most recently, we have attempted to use this procedure to alter the dispersion of cobalt particles over the more strongly interacting 25% Co/A1203 catalyst. However, as shown in Table 8.5, the cluster size was not found to change significantly, and the TPR profiles (not shown for the sake of brevity) were observed... 

In terms of the effect of water on the deactivation, several mechanisms have been identified, and they will influence the stability of the catalyst depending on the conditions and the support used. At high partial pressures of water oxidation is always a possibility, but the various reports are less clear to whether this is mainly surface oxidation of cobalt particles irrespective of particle size, or if small particles... 

Recently, the efficacy of LDHs as catalyst precursors for the synthesis of carbon nanotubes via catalytic chemical vapor deposition of acetylene has been reported by Duan et al. [72]. Nanometer-sized cobalt particles were prepared by calcination and subsequent reduction of a single LDH precursor containing cobalt(II) and aluminum ions homogeneously dispersed at the atomic level. Multi-walled carbon nanotubes with uniform diameters were obtained. 

Cobalt-based catalysts are effective in the ethanol reformation to hydrogen. Many oxides have been used to prepare supported cobalt catalysts of low cobalt content (circa 1 wt%) by impregnation from a solution of Co2(CO)8 catalysts were used in the ethanol reformation as prepared [156]. The performance of the catalysts in the steam reforming of ethanol was related with the presence, under reaction conditions, of metallic (ferromagnetic) cobalt particles and oxidized cobalt species. An easy exchange between small metallic cobalt particles and oxidized cobalt species was found. Comparison of Co/ZnO catalysts prepared from Co2(CO)8 or from nitrate precursor indicated that the catalyst prepared from the carbonyl precursor was highly stable and more selective for the production of CO-free hydrogen... 

Finely divided cobalt particles can be prepared by reduction of cobalt(ll) chloride by lithium naphthalenide in glyme. 

Generation of Co(0) can be effected either by reduction of Co(acac)2 by NaBH4 in CH2CI2 or by reduction of CoBt2 by Zn in toluene//-BuOH. With cobalt particles, the reaction requires rather forcing conditions, CO (30-40 atm), 100 °C, or high dose of cobalt (0.4equiv.) under the atmospheric pressure of CO. 

While there have been much activity in the literature addressing Fe, Ru and Ni F-T catalysts, the largest body of papers and patents in the last three decades have dealt with Co-based F-T catalysts in attempts to make more active catalysts with high wax selectivities. It is, however, remarkable to notice that modern Co F-T catalysts are still very similar to the ones prepared by Fischer and co-workers i.e., they consist of promoted cobalt particles supported on a metal oxide and most of, if not all, Co-based F-T catalyst compositions contain the following components ... 

Promoters leading to increased cobalt dispersion. The addition of promoter elements may also lead to increased cobalt dispersion after preparation. In the absence of the promoters, relatively large cobalt crystals are formed, whereas, by adding these additives, smaller supported cobalt particles can be made. Such promotion effect is illustrated in Figure 3C. 

Water-gas shift reaction. The water-gas shift (WGS) reaction (reaction (2)) made by particles composed of a promoter element close to a supported cobalt particle leads to a change in the local CO/H2 ratio, which may affect the surface coverage of cobalt. As a result, both the activity and the selectivity of the catalyst can be altered. Some transition metal oxides are known to act as WGS reagents. 

Coke burning during regeneration. Co F-T catalysts deactivate due to coke formation blocking the active sites. This coke can be burned off by an oxidative treatment. The addition of promoter elements may decrease the temperature of this oxidative treatment, preventing the possible clustering of supported cobalt particles. 

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