| Fe based catalyst is one of the most widely used catalysts in Fischer-Tropsch synthesis.Understanding the reaction mechanism is of great significance to guide the design of selective Fe-based catalyst.Previous reasearch has proved that?-Fe3C plays an important role in enhancing the selectivity of C2-C4 olefins.In this paper,we studied FTS mechanism includingCO activation,CH4 formation,C1-C1 coupling on?-Fe3C by DFT calculation.The surface energies of different?-Fe3C surfaces were firstly calculated.On basis of Wulff construction theorem,the morphology of?-Fe3C crystallite was structured and terraced-like?031?was confirmed as the most exposed facet of?-Fe3C due to its thermodynamic stability.Then,the mechanism of CO activation on?-Fe3C?031?surface was investigated.It is supposed that H assisted CO dissociation?H+CO?HCO+H?CH2O?CH2+O?has lower reaction barrier?1.06eV?,which is suggested to be the preferred activation pathway.Investigation on CH4 formation and C1-C1 coupling reveals that CH4 formation exhibits a high effective barrier and C?d?H+C?d?H and C?d?H2+C?d?H2 are the dominant chain growth pathways duo to their relatively low effective barriers.The difference of effective barrier between CH4 formation and C1-C1 coupling(?Eeff)was employed to quantify the selectivity of CH4 to C2+.The high value of?Eeff indicates the preference of C2+formation to CH4.The influence of surface structure of?-Fe3C catalyst on CO activation was studied.Stepped-like?-Fe3C?010?surface was chosen.It is found that,similar with?-Fe3C?031?surface,H assisted CO dissociation?H+CO?HCO+H?CH2O?CH2+O?has lower reaction barrier?0.84eV?,which is suggested to be the preferred activation pathway.Comparing with terraced-like?-Fe3C?031?surface,the stepped-like?-Fe3C?010?surface has higher activity toward CO activation,indicating that stepped-like surfaces are more active for FTS. |
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