Co/A1203 catalyst

In the case of Co/A1203 catalysts, regression results showed a negative water effect on the catalyst, in good agreement with the results from the water co-feeding studies. 

Figure 4.1 summarizes the different routes that can potentially lead to carbon deposition during FTS (a) CO dissociation occurs on cobalt to form an adsorbed atomic carbon, which is also referred to as surface carbide, which can further react to produce the FT intermediates and products. The adsorbed atomic carbon may also form bulk carbide or a polymeric type of carbon. Carbon deposition may also result (b) from the Boudouard reaction and (c) due to further reaction and dehydrogenation of the FTS product (what is commonly called coke), a reaction that should be limited at typical FT reaction conditions. Carbon formed on the surface of cobalt can also spill over or migrate to the support. This is reported to readily occur on Co/A1203 catalysts.43 The chemical nature of the carbonaceous deposits during FTS will depend on the conditions of temperature and pressure, the age of the catalyst, the chemical nature of the feed, and the products formed. 

Agrawal et al.33 performed studies of Co/A1203 catalysts using sulfur-free feed synthesis gas and reported a slow continual deactivation of Co/A1203 methanation catalysts at 300°C due to carbon deposition. They postulate that the deactivation could occur by carburization of bulk cobalt and formation of graphite deposits on the Co surface, which they observed by Auger spectroscopy. 

FTS activity (215-232°C, 19-28 bar, H2/CO = 1.98-2.28) in a two-stage slurry bubble column using a Co/A1203 catalyst, (b) Correlation between amounts of residual carbon after 02 treatment and H2 chemisorption capacity. (Drawn from data provided in Gruver et al.34)... 

FIGURE 4.8 The correlation between polymeric carbon amount, as determined by TPO, with TOS for 20 wt% Co/A1203 catalysts taken from the slurry bubble column operated at realistic FTS conditions.73... 

Co/A1203 catalysts that contain higher amounts of less reactive polymeric carbon not only exhibited enhanced deactivation when tested in FTS when compared to the fresh catalyst, but also showed an increase in selectivity to olefinic products.31 The authors postulated that this was probably due to the reduction in hydrogenation ability of the carbon deposited catalyst to convert primarily formed olefins into the corresponding paraffins. 

FIGURE 8.1 Temperature-programmed reduction profiles of supported cobalt catalysts, including (top) Co/A1203 catalysts, (middle) Co/Ti02 catalysts, and (bottom) Co/ Si02 catalysts. 

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... 

FIGURE 14.2 TPR profiles of the Co/A1203 catalysts prepared by the slurry phase impregnation method. 

Xiong, H., Zhang, Y., Wang, S., and Li, J. 2005. Fischer-Tropsch synthesis The effect of A1203 porosity on the performance of Co/A1203 catalyst. Catal. Commun. 6 512-16. 

Jacobs, G., Patterson, P.M., Das, T.K., Luo, M., and Davis, B.H. 2004. Fischer-Tropsch synthesis Effect of water on Co/A1203 catalysts and XAFS characterization of reoxidation phenomena. Appl. Catal. A Gen. 270 65-76. 

Visconti, C.G., Tronconi, E., Lietti, L., Zennaro, R., and Forzatti, P. 2007. Development of a complete kinetic model for the Fischer-Tropsch synthesis over Co/A1203 catalysts. Chem. Eng. Sci. 62 5338 -3. 

Carbon deposition is much greater on Co/A1203 catalysts than on Ni/Al203 (240). The presence of MgO markedly decreased the carbon deposition on the surface of the cobalt catalyst (241). The role of MgO may be attributed to the formation of strongly adsorbed C02 species, which can easily react with the deposited carbon, thus preventing catalyst deactivation (241). 

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