Surface modification of Fe₅C₂ by binding silica-based ligand: A theoretical explanation of enhanced C₂ oxygenate selectivity

Elucidating the role(s) of support-like ligands remains a challenge in catalytic reaction like the Fischer-Tropsch Synthesis (FTS) catalyzed by the iron catalyst supported on silica. We herein theoretically investigated surface modification of Fe₅C₂ by silica-based ligand and its influence on C₂ oxygenate selectivity in FTS, by carrying out DFT calculations on dissociation of CO and formations of CH₄ and C₂ on ha-SiO₂/Fe₅C₂(510). To mimic the structure of surface modification, the ha-SiO₂/Fe₅C₂(510) model was built up by binding the silica cluster (the ha-SiO₂ ligand) to Fe₅C₂(510). DFT calculations elucidated that the C + CH coupling with the lowest activation barrier among all the possible routes of C₂ formation on Fe₅C₂(510) is suppressed after modification with the ha-SiO₂ ligand because the binding ha-SiO₂ ligand limits the geometry relaxation caused by the C+CH coupling. However, CO molecule is anchored by the ha-SiO₂ ligand via hydrogen bond, suppressing the C–O cleavage because the d-valence band center of Fe₅C₂(510) lowers in energy by surface modification with the ha-SiO₂ ligand, but facilitating the C + CO coupling with the lowest activation barrier among all the possible routes of C₂ formation. Also, CH₄ formation in the ha-SiO₂/Fe₅C₂(510) case is not so easy as that on Fe₅C₂(510). Therefore, C₂ oxygenate is formed more easily in the ha-SiO₂/Fe₅C₂(510) case than in the Fe₅C₂(510) case. The result agrees with the experimental observation that C₂ oxygenate selectivity became high for iron-based FTS catalyst after surface modification of by silica-based ligand.