C₂ Oxygenate Synthesis via Fischer–Tropsch Synthesis on Co₂C and Co/Co₂C Interface Catalysts: How To Control the Catalyst Crystal Facet for Optimal Selectivity

Density functional theory (DFT) analysis is used to shed light on the intricate effects of the Co 2 C and Co/Co 2 C catalyst crystal facets on the selectivity of the C 2 oxygenate and hydrocarbon formation in Fischer–Tropsch synthesis. Three representative low-index Co 2 C (101), (110), and (111) surfaces, varying in surface energy from low and medium to high, are model examples of different Co 2 C exposed crystal facets. Since CH x ( x = 1–3), CO, and H species are the key intermediates critical to the C 2 oxygenate selectivity, all Fischer–Tropsch reactions related to CH x ( x = 1–3) species, including CO insertion into CH x ( x = 1–3) and CH x + CH y ( x, y = 1–3) coupling to form C 2 species (C 2 H x and C 2 H x O), as well as the hydrogenation and dissociation of CH x ( x = 1–3) to form C 1 species (CH 4 and C), are used as examples examined at a typical FTS temperature of 493 K. On Co 2 C (101) and (110) surfaces, CH and CH 2 species are dominant form of the CH x species, CH self-coupling to C 2 H 2 and CH coupling with CH 2 to CH 2 CH is dominant C 2 species. However, on a Co 2 C (111) surface, only CH monomer is a dominant CH x ( x = 1–3) species, and CO insertion into CH to form CHCO is a dominant C 2 species. CH 4 and C production on these three surfaces is impossible. These results show that C 2 species formation, rather than C 1 species, is a preferable pathway on different Co 2 C crystal facets in FTS reactions. Moreover, the C 2 selectivity, quantitatively estimated from the effective barrier difference, is found to be sensitive to the Co 2 C crystal facet. The Co/Co 2 C (111) interface catalyst is more favorable for C 2 oxygenate formation in comparison to the pure Co 2 C (111) catalyst, whereas the Co/Co 2 C (110) and Co/Co 2 C (101) interface catalysts are unfavorable for C 2 oxygenate formation in comparison to the pure Co 2 C (110) and (101) catalysts. Therefore, for the FTS over Co 2 C and Co/Co 2 C catalysts, the Co 2 C (111) crystal facet is found to have an unexpectedly high selectivity for C 2 oxygenates, whereas the Co 2 C (101) and (110) crystal facets are found to have a high selectivity toward C 2 hydrocarbons. The results mean that controlling the crystal facets of Co 2 C catalysts using well-defined preparation methods can be an effective tool to tune the FTS selectivity toward the most desirable products.