Preparation and characterization of Fischer–Tropsch active Co/SiO2 catalysts Original Research Article

Silica supported cobalt catalysts have been prepared by a new method combining the precipitation of the metal precursor (cobalt nitrate) by oxalic acid with the hydrolysis and condensation of the silicium precursor (tetraethoxysilane). Depending on the pH during preparation, the textural properties (BET specific surface area, porosity) of the Co/SiO2 catalysts can be modified. In an acid medium (pathway A), the resulting silica is constituted by a polymeric net with few branchings, the catalysts are microporous. In basic medium (pathway B) silica is composed of more branched polymers leading to mesoporous catalytic systems. After calcination, the only crystallized phase detected by XRD is the Co3O4 spinel. At 773 K, the surface degrees of reduction of the catalysts at cobalt isocontent (25 wt.% Co) as determined by XPS is of 81% and 69% for systems prepared by pathways A and B, respectively. The presence of small unreduced CoII suggests the existence of non-crystallized cobalt silicate formed during the reduction by reaction of CoO with silica. The activity for the CO + H2 reaction for the 25 wt.% catalysts prepared by pathway A increased with the specific surface area which can be controlled by the preparation parameters. The reduction degree has a direct influence on the selectivity for the Co/SiO2 catalysts. The presence of a part of unreduced cobalt (in interaction with the support) results in a better selectivity to the C5–C13 fraction (gasoline), whereas a higher reduction degree of cobalt favors the production of higher molecular weight hydrocarbons (waxes) (C22+ selectivity > 40%).