Structure and Site Evolution of Iron Oxide Catalyst Precursors during the Fischer−Tropsch Synthesis

Abstract

The contacting of Fe oxide precursors with synthesis gas (H2/CO mixtures) leads to structural and chemical changes and to the formation of the active sites required for the Fischer−Tropsch synthesis (FTS). The local structure and oxidation state of the starting Fe2O3 precursors promoted by Cu and/or K were probed using in situ X-ray absorption spectroscopy during these processes. The activation of these precursors occurs via reduction to Fe3O4 followed by carburization to form FeCx. FTS reaction rates increased markedly during the initial stages of carburization, suggesting that the conversion of near-surface layers of Fe3O4 to FeCx is sufficient for the formation of the required active sites. Thus, bulk structural probes, and any ex situ techniques without concurrent measurements of the products evolved during activation and FTS, can lead to misleading structure−function relations. The initial rate of carburization and the extent of carburization and the FTS rate at steady-state were higher for Fe2O3 precursors containing either Cu or K. These effects were stronger when both promoters were present. K and Cu provide CO and H2 activation sites, which lead to the nucleation of multiple carbide regions on Fe oxide surfaces. The larger number of nucleation sites leads to higher initial carburization rates and to smaller FeCx crystallites. These smaller crystallites, in turn, provide higher active surface areas, shorter bulk diffusion distances, more complete carburization of Fe2O3 precursors, and higher steady-state FTS rates. These effects of K and Cu on the number of available FeCx sites were confirmed by titrating surface sites using CO after FTS reactions. It appears that K and Cu act predominantly as structural promoters, which increase the surface area of the active FeCx phases. Chemical promotion of catalytic rates by these additives appears not to be required to explain the observed FTS rate enhancements in the presence of K or Cu. These structural promotion effects of Cu and K account for the apparent but non-causal correlation between bulk FeCx contents and FTS rates at steady-state, even though the carburization of only near-surface regions in bulk Fe3O4 is sufficient to achieve steady-state FTS turnover rates.