Recent research has shown a large impact of cobalt particle size on the activity and selectivity in the Fischer-Tropsch (FT) synthesis. The best performance was reported for particles of 6 nm. Preparation of such catalysts demands full control over the effects of the individual preparation steps on the dispersion and particle distribution. In this respect, we explored the use of ordered mesoporous supports as model systems. Because of the well-defined pore system detailed information can be derived from 3D-TEM and N₂-physisorption. With 3D-TEM the particle size and distribution over the support can be monitored, while with physisorption the degree of pore blocking by the active phase or its precursor can be quantified.

We studied the preparation of cobalt on silica catalysts for FT synthesis, prepared by impregnation and drying using cobalt nitrate as precursor salt. Generally, catalysts obtained via this method display a poor dispersion, but the reason for this is not yet clear. Using SBA-15 we found that cobalt nitrate was evenly distributed over the support after drying. However, during calcination in air severe precursor redistribution and particle growth took place. Because of the use of SBA-15 as model support we were able to identify that calcination in a diluted stream of nitric oxide prevented redistribution. This enabled us to obtain mono-disperse and homogeneously distributed cobalt oxide particles of 5 nm on both SBA-15 and silica gel. Catalytic tests (1 bar, 220 °C) after reduction showed that a catalyst was obtained that combined a high loading (18 wt%) with an excellent activity.

In conclusion, using SBA-15 as model support yielded detailed information on the preparation steps that enabled us to control the cobalt particle size resulting in a highly active FT catalyst prepared by simple impregnation and drying using cobalt nitrate.

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