Mechanism Of Fischer-Tropsch Synthesis On χ-Fe₅C₂ Catalyst:a Density Functional Theory Study

Fe-based catalyst is one of the most widely used Fischer-Tropsch synthesis (FTS) industrial catalysts. Understanding the mechanism of FTS on Fe-based catalyst is of paramount importance for the rational design of the catalyst with controlled product selectivity. Generally, x-Fe5C2 was regarded as the main active phase for Fe catalyzed FTS.In this thesis, theoretical calculations based on density functional theory were employed to systematically investigate the mechanism of FTS on different x-Fe5C2 surfaces.(1) The surface energies of different x-Fe5C2 facets were calculated, and then used to obtain the equilibrium shape and the composition of exposed surfaces of x-Fe5C2 crystallite by using Wulff construction. The results show that the thermodynamically most stable, high Miller-index (510) terraced-like surface is the largest exposed facet.(2) The mechanism of FTS on the x-Fe5C2 (510) surface was investigated. It is found that direct CO dissociation has lower barrier and thus is suggested as the preferred activation pathway. C-C coupling mechanism mainly follows the carbide mechanism, and the C+CR and CH+CR (R=H or alkyl) are the most likely C-C coupling pathways. The chain terminations of C1, C2 and C3 by hydrogenation have higher effective barriers than the C-C coupling reactions, suggesting the favorable formation of long-chain hydrocarbons. Based on the above results, plausible catalytic cycle for x-Fe5C2(510) surface and FTS was proposed.(3) The effects of surface structure of x-Fe5C2 catalyst on the FTS mechanism, activtity and selectivity were comparatively studied. The results indicate that the cleavage of C-0 bond and the formation of C-C bond are sensitive to the catalyst surface structure, while the formation of C-H bond is insensitive. The terraced-like (510) and (021) surfaces have higher activity toward CO activation and C-C coupling as well as lower selectivity toward CH4 than the stepped-like (001) and (100) surfaces, indicating that the (510) and (021) surfaces are active for FTS.