Electron Microscopy Study of γ-Al2O3 Supported Cobalt Fischer-Tropsch Synthesis Catalysts

Three supported catalysts containing 20 wt% cobalt and 0.5 wt% rhenium were subjected to electron microscopy studies in their calcined state. The catalysts were prepared by incipient wetness impregnation of gamma-Al(2)O(3) supports of different pore characteristics with aqueous solutions of cobalt nitrate hexahydrate and perrhenic acid. The influence of the support on the Co(3)O(4) crystallite size and distribution was studied by X-ray diffraction and electron microscopy. There was a positive correlation between the pore diameter of the support and the post calcination Co(3)O(4) crystallite size. On all three gamma-Al(2)O(3) supports, Co(3)O(4) was present as aggregates of many crystallites (20-270 nm in size). Cobalt oxide did not crystallise as independent crystallites, but as an interconnected network, with a roughly common crystallographic orientation, within the matrix pore structure. The internal variations in crystallite size between the catalysts were maintained after reduction. Fischer-Tropsch synthesis was carried out in a fixed-bed reactor at industrial conditions (T = 483 K, P = 20 bar, H(2)/CO = 2.1). Although the cobalt-time yields varied significantly (4.6-6.7 x 10-3 mol CO/mol Co s), the site-time yields were constant (63-68 x 10-3 s-1) for the three samples. The C(5+) selectivity could not be correlated to the cobalt oxide aggregate size and is more likely related to the cobalt particle size and chemical properties of the gamma-Al(2)O(3) support.