Mechanistic aspects of facet-dependent CH_(4)/C_(2+)selectivity over aχ-Fe_(5)C_(2)Fischer-Tropsch catalyst

摘要

Structure-performance relationship is a complex issue in iron-catalyzed Fischer-Tropsch synthesis,and it is not easy to elucidate it by experimental investigations.First-principle calculation is a powerful method for explaining experimental results and guiding catalyst design.In this study,we investigated the reaction mechanisms of CH_(4)formation and C-C coupling on fourχ-Fe_(5)C_(2)surfaces and established the kinetic equations to compare the rates of CH_(4)formation and C_(1)+C_(1)coupling reactions and determine the CH_(4)/C_(2+)selectivity.The results show that the geometry of theχ-Fe_(5)C_(2)surfaces has little effect on the formation rate of CH_(4);however,the C_(1)+C_(1)coupling reactions are significantly affected by the surface geometry.The C_(1)+C_(1)coupling reaction rates on the terraced-like(510)and(021)surfaces are much higher than those on the stepped-like(001)and(100)surfaces.Based on these results,we established a Brùnsted-Evans-Polanyi(BEP)relationship between the effective barrier difference for CH_(4)formation and C_(1)+C_(1)coupling(ΔE_(eff))and the adsorption energy of C+4H(ΔE_(C+4H))onχ-Fe_(5)C_(2)surfaces.ΔE_(C+4H)can be used as a descriptor for CH_(4)/C_(2+)selectivity on different surfaces ofχ-Fe_(5)C_(2).