Fischer - Tropsch synthesis catalysts based on Fe oxide precursors modified by Cu and K : structure and site requirements

The reduction, carburization, and catalytic properties of Fischer-Tropsch synthesis (FTS) catalysts based on Fe-Cu were examined using kinetic and spectroscopic methods at reaction conditions. Fe2O3 precursors reduce to Fe3O4 and then carburize to form a mixture of Fe2.5C and Fe3C in both CO and H2/CO mixtures at 540-720 K. Oxygen removal initially occurs without FTS reaction as Fe2O3 forms inactive O-deficient Fe2O3 species during initial contact with synthesis gas at 523 K. FTS reactions start to occur as Fe3O4 forms and then rapidly converts to FeCx. The onset of FTS activity requires only the conversion of surface layers to an active structure, which consists of FeCx with steady-state surface coverages of oxygen and carbon vacancies formed in CO dissociation and O-removal steps during FTS. The gradual conversion of bulk Fe3O4 to FeCx influences FTS rates and selectivity weakly, suggesting that the catalytic properties of these surface layers are largely independent of the presence of an oxide or carbide core. The presence of Cu and K increases the rate and the extent of Fe3O4 carburization during reaction and the Fischer-Tropsch synthesis rates, apparently by decreasing the size of the carbide crystallites formed during reaction.