Crystal Facet Dependence for the Selectivity of C2 Species over Co2C Catalysts in the Fischer-Tropsch Synthesis

The effect of Co2C crystal facets on the selectivity of C-2 species (C-2 oxygenates and hydrocarbons) in Fischer-Tropsch synthesis (FTS) reaction was investigated using density functional theory calculations, and the selectivity comparisons among five exposed Co-termination (101), (011), (010), (110), and (111) crystal facets are examined to shed light on the essential relationship between FTS selectivity and the structure of Co2C crystal facets. The results show that the C-C bond of C-2 species prefers to be formed instead of C-1 species CH4 over Co2C catalysts in the FTS reaction, and the selectivity of C-2 species and the dominant existence form of key CH intermediates are sensitive to the crystal facet of Co2C catalysts, which are closely associated with Co2C crystal facet's electronic and structural properties. The electronic and structural properties of different Co2C crystal facets show that the high selectivity of C-2 oxygenates over the (011) and (111) facets are attributed to the upshift of their surface d-band centers, as well as the presence of the step B-5-type active unit with five Co atoms consisted of much denser surface active sites. However, both (101) and (010) facets exhibit high selectivity toward C-2 hydrocarbons, and the (110) facet presents high selectivity toward the formation of CH4. Thus, regulating the exposed crystal facets of Co2C catalyst can control the selectivity of desirable C-2 species. This work provides evidence at a molecular level to support that the sensitivity of Co2C crystal facet is a cause to affect the selectivity of desired products in the FTS reaction.