Understanding the Key Step of Co2C Catalyzed Fischer-Tropsch Synthesis

This research is designed to fundamentally understand CO activation acting as a crucial step in the initiation of the Fischer-Tropsch (FT) process over Co2C catalyst, the intrinsic activity, and structural sensitivity of the catalyst, which is vital to its FT performance, but remains unclear. Accordingly, CO activation over the commonly exposed Co2C-(101), (011), (010), (110), and (111) facets is investigated using density functional theory (DFT) calculations and microkinetic modeling. Results show that the CH monomer is the most abundant surface CHx species for CO activation over five Co2C facets, and the mechanism of CO activation and subsequent CH formation strongly depend on the Co2C facet. The formation rate of the CH monomer follows the order of (111) < (010) < (110) < (101) < (011). The (011) facet accounting for 35.21% of the surface is the most active for CO direct dissociation into C, followed by C hydrogenation to CH, which presents a comparable activity with the hexagonal close-packed (HCP) Co due to the presence of denser B-5-type active sites. Further, Co2C is a multifunctional catalyst for the FT process because its facets exhibit a different catalytic activity toward CO dissociation to form the key CH monomer, and as a result, C2+ hydrocarbons and oxygenates can be formed depending on the relative reaction rate between CO insertion into CHx reaction and CHx-CHx coupling. The finding of this research may lead a new avenue for rational design of Co2C-based catalysts with desirable activities and product selectivity.