Microstructure of Supported Cobalt Fischer-Tropsch Catalysts

International audienceThe structures of metallic cobalt particles supported on silica (with and without Ru promoter) and on alumina have been investigated in-situ by conventional and anomalous X-ray diffraction. Comparison with simulated XRD patterns shows the structure to consist of a defect phase characterised by the presence of stacking faults, the density of which is directly related to the temperature at which the cobalt oxide is reduced. Controlled modification of the cobalt microstructure, while maintaining the same support and promoter has been achieved with limited variation of the particle size, notably via CO – H2 treatment leading to the formation and decomposition of cobalt carbide. Independently of the support or the presence of a promoter, low reduction temperatures increase the tendency to form hexagonal close packed (hcp) structure whilst at high reduction temperatures face centred cubic (fcc) stacking is preferentially formed. The catalytic activity for CO conversion in Fischer-Tropsch conditions is greatly increased for catalysts with a majority of hcp stacking compared to those containing predominantly fcc phase particles