No abstract

Reduction of 12wt.%Co/0.5wt.%Re/a-Al 2 O 3 Fischer-Tropsch catalyst has been studied in-situ in an environmental transmission electron microscope. Reduction of Co 3 O 4 to metallic cobalt was observed dynamically at 360°C under 3.4 mbar H 2 . Structural and morphological changes were observed by high resolution transmission electron microscopy and scanning transmission electron microscopy imaging. The cobalt particles were mainly face centred cubic while some hexagonal close packed particles were also found. Reoxidation of the sample upon cooling to room temperature, still under flowing H 2 , underlines the reactivity of the nanoparticles and the importance of controlling the gas composition and specimen temperature during this type of experiment. Similar behaviour was observed for a non-promoted catalyst. Imaging and analysis of the promoted sample before and after reduction indicated a uniform distribution of the promoter.

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 c-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 c-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 9 10 -3 mol CO/mol Co s), the site-time yields were constant (63-68 9 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 c-Al 2 O 3 support.