Cobalt crystallites

The oxidation of cobalt metal to inactive cobalt oxide by product water has long been postulated to be a major cause of deactivation of supported cobalt FTS catalysts.6 10 Recent work has shown that the oxidation of cobalt metal to the inactive cobalt oxide phase can be prevented by the correct tailoring of the ratio Ph2cJPh2 and the cobalt crystallite size.11 Using a combination of model systems, industrial catalyst, and thermodynamic calculations, it was concluded that Co crystallites > 6 nm will not undergo any oxidation during realistic FTS, i.e., Pi[,()/I)i,2 = 1-1.5.11-14 Deactivation may also result from the formation of inactive cobalt support compounds (e.g., aluminate). Cobalt aluminate formation, which likely proceeds via the reaction of CoO with the support, is thermodynamically favorable but kinetically restricted under typical FTS conditions.6... 

Saib, A. M., Borgna, A., van de Loosdrecht, J., van Berge, P. J., and Niemantsverdriet, J. W. 2006. XANES study of the susceptibility of nano-sized cobalt crystallites to oxidation during realistic Fischer-Tropsch synthesis. Appl. Catal. A 312 12-19. 

Recently, it has been reported that a novel calcination procedure relying on nitric oxide gas in lieu of air also results in smaller cobalt crystallites over silica supports.15 17 The idea is to use a less oxidative gas to prevent rapid decomposition of the nitrate precursor during thermal nitrate decomposition, which has been observed when 02 is present.17 As a result, the mobility of the precursor on the oxide carrier surface is hindered, resulting in a smaller average Co oxide cluster... 

The results confirm that the novel metal nitrate conversion method using nitric oxide in place of air advocated by Sietsma et al. in patent applications WO 2008029177 and WO 2007071899 leads to, after activation in H2, catalysts with smaller cobalt crystallites, as measured by EXAFS and hydrogen chemisorption/ pulse reoxidation. In spite of the lower extent of cobalt reduction for H2-activated nitric oxide calcined catalysts, which was recorded by TPR, XANES, EXAFS,... 

Interestingly, Co(ll) exchanged NaY zeolite is also operative under the F-T conditions (26) However, large amounts of methane and higher hydrocarbons are produced. This behaviour is reminiscent of "classical" F-T catalysts ( ,25) and suggests the occurence of cobalt crystallites outside the Y zeolite cavities. 

A. M. Saib, A. Borgna, J. van de Loosdrecht, P. J. van Berge, J. W. Geus and J. W. Niemantsverdriet, Preparation and characterisation of spherical Co/Si02 model catalysts with well-defined nano-sized cobalt crystallites and a comparison of their stability against oxidation with water, J. Catal., 2006, 239, 326-339. 

Figure 5.4.6 Silica supported cobalt catalyst with a 10 nm cobalt crystallite (adapted from Saib et al. [34]).

The cobalt metal area of the reduced perovskites was determined by hydrogen chemisorption experiments. The results are shown in Table 1. The chemisorption measurements revealed that the cobalt metallic surface area was similar for all the perovskites. This is supported by the Co/Ln surface ratio (Table I) obtained by XPS which also suggests similar metallic dispersion. The XPS analyses of the reduced perovskites showed the presence of Co" (778.6 eV) but also a doublet at approximately 780.5 and 796.2 eV which correspond to Co 2p, and Co 2p , peaks respectively, for the Co ion. Shake-up satellite lines with 4.7 eV over the Co lines were also detected indicating the presence of Co [12]. These oxidised species of cobalt are probably formed by air oxidation during the transference of the reduced sample from the reactor to the XPS spectrometer. Also, Marcos el al. [15] have shown that the reduction of the perovskite LaCoO, produced a La,0, oxide covered by hydroxyl groups which upon heating and evacuation in the XPS pretreatment chamber partly reoxidises the cobalt crystallites. 

The selectivity of a series of catalysts, prepared by using a given salt and support, increased with increasing size of cobalt crystallites even in the hydrogenation of acrolein. 

The catalyst surface exposed to the reaction mixture at various temperatures was studied by SEM (Fig. 6). At 250°C, only the traces of carbon were found in the catalyst pores by the X-ray probe (picture A). The deposits grown at higher temperatures were similar in texture and all contained metallic cobalt crystallites on the surface. As an example, the deposit grown at 450°C is envisaged in picture B and C, respectively cobalt ciystallites can be spotted as white dots in picture B. The results of X-ray analyses taken at 100 points of the sample revealed that the higher amount of cobalt on the deposit produced at 450 C. 

Pangarov NA, Vitkova SD (1966) Preferred orientation of electrodeposited cobalt crystallites. Electrochim Acta 11 1733-1745... 

Finally, sintering of cobalt crystallites, the loss of catalytic surface area due to ripening or migration and coalescence of the cobalt phase, is mainly responsible for the loss of activity. 

Nevertheless, it is generally accepted that the oxidation behavior of cobalt catalysts is related to the FTS reaction conditions, especially, to water and cobalt crystallite size. Gong et studied the water for-... 

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