ABSTRACT
Two unpromoted iron carbides, Hägg (χ-Fe5C2) and cementite (θ-Fe3C), were investigated for Fischer-Tropsch synthesis (FTS). Both iron carbides oxidized to Fe3O4 during FTS. Hägg carbide initially exhibited both higher FTS and water-gas shift (WGS) activity compared to cementite under similar reaction conditions. However, Hägg carbide deactivated faster than cementite, although the FTS activities of both catalysts leveled off at about the same productivity after a period of time on stream. Comparing at similar CO conversion levels, Hägg carbide displayed slightly higher methane selectivity compared to cementite. The fraction of FTS to WGS activity increased with increasing time on stream for the Hägg carbide, but the fraction remained nearly constant in the case of cementite. The observed FTS and WGS activity correlated to the percentage of carbide content in the FT catalysts, indicating that carbide is the active form of Fe for both FTS and WGS reactions.
