Enhanced catalytic activity of cobalt catalysts for Fischer-Tropsch synthesis via carburization and hydrogenation and its application to regeneration

In Fischer-Tropsch synthesis (FTS), cobalt carbide (Co2C) is not a catalytically active material, but rather an undesired cobalt phase associated with low catalytic performance. It is known that Co2C can be easily transformed back to metal cobalt in a H-2 environment at 220 degrees C. The transformed metal cobalt (hcp phase) even shows higher catalytic activity in low-temperature FTS, compared with the reduced cobalt metal from the cobalt oxide species. In this study, to obtain Co2C with high catalytic activity in FTS, we determined the optimum conditions for effective metal cobalt carburization and Co2C hydrogenation by monitoring the phase transformation of cobalt using X-ray absorption spectroscopy (XAS) and temperature-programmed hydrogenation (TPH). We also verified that the transitions effectively occur under the same conditions as those for FTS (2.0 MPa, 220 degrees C). Based on the conditions determined for the transitions, the deactivated cobalt catalyst can be completely regenerated in the FT reactor by simply altering the injected gases from syngas to CO and then H-2. Moreover, the regenerated catalyst shows enhanced catalytic performance compared with the fresh catalyst. The selective formation of hcp cobalt metal via carburization and hydrogenation of the spent catalyst was found to be the key for both the improved catalytic activity and the effective regeneration in situ. As a result, the formation of Co2C, which is mainly considered a nuisance, could provide valuable applications in investigations into catalyst activation and regeneration in FTS.