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 Co2C and Co/Co2C catalyst crystal facets on the selectivity of the C, oxygenate and hydrocarbon formation in Fischer Tropsch synthesis. Three representative low-index Co2C (101), (110), and (111) surfaces, varying in surface energy from low and medium to high, are model examples of different Co,C exposed crystal facets. Since CH, (x = 1-3), CO, and H species are the key intermediates critical to the C, oxygenate selectivity, all Fischer Tropsch reactions related to CH, (x = 1-3) species, including CO insertion into CH,. (x = 1-3) and CFI,. + CH,, (x, y = 1-3) coupling to form C, species (C,H,. and C,F1x(0)), as well as the hydrogenation and dissociation of CHx (x = 1-3) to form C-1 species (CH, and C), are used as examples examined at a typical FTS temperature of 493 K On Co2C (101) and (110) surfaces, CH and CH, species are dominant form of the CH species, CH self -coupling to C2H, and CH coupling with CH2 to CH2CH is dominant C-2 species. However, on a Co2C (III) surface, only CH monomer is a dominant CHx (x = 1-3) species, and CO insertion into CH to form CHCO is a dominant C, species. CH, and C production on these three surfaces is impossible. These results show that C, species formation, rather than C, species, is a preferable pathway on different Co,C crystal facets in FTS reactions. Moreover, the C, selectivity, quantitatively estimated from the effective barrier difference, is found to be sensitive to the Co,C crystal facet. The Co/Co,C (111) interface catalyst is more favorable for C, oxygenate formation in comparison to the pure Co,C (111) catalyst, whereas the Co/Co2C (110) and Co/Co2C (101) interface catalysts are unfavorable for C-2 oxygenate formation in comparison to the pure Co,C (110) and (101) catalysts. Therefore, for the FTS over Co2C and Co/Co2C catalysts, the Co,C (111) crystal facet is found to have an unexpectedly high selectivity for C, oxygenates, whereas the Co2C (101) and (110) crystal facets are found to have a high selectivity toward C, hydrocarbons. The results mean that controlling the crystal facets of Co,C catalysts using well-defined preparation methods can be an effective tool to tune the FTS selectivity toward the most desirable products.