The influence of metal crystallite size of the cobalt Fischer-Tropsch catalyst on the re-oxidation of cobalt with water
Catalyst productivity and selectivity to C5+ hydrocarbons are an important criteria in choice of Fischer-Tropsch catalyst. Cobalt-based catalysts appear to provide the best compromise between performance and cost for the synthesis of hydrocarbons [Iglesia, 1997]. The cobalt-based catalysts can be best applied in slurry reactor technology because of the high activity and the good heat transfer of the slurry reactor. Due to the nature of the slurry reactor, high partial pressures of water are generated when operated at high conversions. In addition, the back-mixing in the slurry reactors gives a homogenous concentration profile throughout the reactor with high water and low reactants concentrations. The cobalt catalyst is more expensive than the iron catalyst and to make the cobalt catalyst economically viable, the lifetime of the catalyst has to be stretched to 2-3 years. Possible deactivation by water thus becomes of major concern. Although the deactivation of bulk cobalt with water is not thermodynamically feasible, the re-oxidation of the metal surface might be possible. The surface atoms of a solid are exposed to a different environment than the atoms in the bulk of the solid. They have fewer neighbours than the bulk atoms do. The neighbours of each surface atom may be distributed anisotropically. Therefore the thermodynamic properties associated with the surface region are defined separately from the bulk thermodynamic properties. Studies of small particles have shown that these exhibit properties different from those of the bulk phase especially when dispersion approaches unity [Somorjai, 1994]. Significant surface oxidation is found when the Co/Al2O3 is treated with low H2O/H2 ratio [Schanke et. al. 1996]. In the present study the role of the cobalt crystallite size on the re-oxidation of a 5%(wt) Co/SiO2 catalyst has been investigated with TPR, TPO, H2 chemisorption, TEM and Fischer-Tropsch synthesis. The supported catalysts used in this study were prepared by impregnating a silica support with an aqueous solution of cobalt nitrate, using the incipient wetness technique. Impregnation was followed by drying and activation of the cobalt precursor.
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