Cobalt cluster effects in zirconium promoted Co/SiO2 Fischer-Tropsch catalysts

The effect of zirconium addition to the catalyst formulation of Co/SiO2 Fis cher-Tropsch catalysts was investigated. With increasing zirconium content the strong interaction between silica and cobalt is reduced and a somewhat weaker cobalt-zirconium interaction is observed. Therefore the degree of re duction of catalysts, which were reduced at 400 degrees C for 16 h, increas es strongly. The cobalt crystallite size increases with increasing zirconiu m content, leading to smaller cobalt metal surface areas for the freshly re duced catalyst. Cobalt particles can be found in clusters on the silica sup port. The size of cobalt clusters decreases and thus the number of cobalt p articles within a cluster decreases with increasing zirconium content. At s teady-state conditions the GO-conversion of the promoted catalyst in the Fi scher-Tropsch synthesis increases with increasing zirconium content. The C5 +-selectivity and the secondary hydrogenation activity pass a maximum with increasing zirconium content. The observed changes in activity and selectiv ity are explained in terms of an increase in the amount of metallic cobalt available under reaction conditions, leading to an increased activity, and a decrease in the cobalt cluster size, which diminishes the probability for secondary reactions. Furthermore, it was concluded that secondary double b ond isomerization can be catalyzed to some extent by zirconia. (C) 1999 Aca demic Press.