| 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 thatθ-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 onθ-Fe3C by DFT calculation.The surface energies of differentθ-Fe3C surfaces were firstly calculated.On basis of Wulff construction theorem,the morphology ofθ-Fe3C crystallite was structured and terraced-like（031）was confirmed as the most exposed facet ofθ-Fe3C due to its thermodynamic stability.Then,the mechanism of CO activation onθ-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 ofθ-Fe3C catalyst on CO activation was studied.Stepped-likeθ-Fe3C（010）surface was chosen.It is found that,similar withθ-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-likeθ-Fe3C（031）surface,the stepped-likeθ-Fe3C（010）surface has higher activity toward CO activation,indicating that stepped-like surfaces are more active for FTS. |
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