Spectroscopic and transient kinetic studies of site requirements in iron-catalyzed Fischer-Tropsch synthesis

The structure, reduction/carburization, and catalytic performance of K- and Cu-promoted Fe2O3 during initial contact with synthesis gas were examined by combining kinetic analysis of the initial stages of Fischer-Tropsch synthesis (FTS) with X-ray absorption spectroscopy. Oxygen removal initially occurs without FTS reactions as Fe2O3 is reduced to inactive oxygen-deficient Fe2O3 species. Hydrocarbon synthesis reactions become detectable only as Fe3O4 forms and rapidly converts to FeCx. FTS reactions require only the incipient conversion of the surface layers to a dynamic and active surface phase, which consists of FeCx with steady-state surface coverages of vacancies, formed via oxygen and carbon removal during the formation of monomers, CO2, and H2O. Such surfaces tend to respond to changes in the contacting gas phase within turnover times by changing the relative surface concentration of carbon, oxygen, CO, and hydrogen. The catalytic behavior of these dynamic surfaces is largely independent of the carbide or oxide nature of the particle cores. These findings, combined with the rapid formation and interconversion of Fe3O4 and FeCx within characteristic FTS turnover times, make the definite assignment of FTS activity to either phase neither appropriate nor kinetically rigorous. The presence of K and Cu increases FTS rates, and the steady-state extent of carburization by providing nucleation sites for the formation of smaller FeCx crystallites. These smaller active domains lead, in turn, to shorter diffusion paths and to a larger number of sites for CO adsorption/dissociation and for FTS reactions. These structural promotion effects of K and Cu were consistent with reaction rates, X-ray absorption spectra, and site density measured on promoted and unpromoted catalysts as a function of time in contact with synthesis gas.