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.

The initiation, growth and termination mechanism of the C-C chain from syngas on the Co(0001) surface have been investigated using DFT calculations. Our results show that CH (x = 1-3) formation is easier than CHOH, both CH and CH species are the dominant forms of CH (x = 1-3), both CH and CH species dominantly interact with CHO to form CHCHO and CHCHO, and realizes the initial C-C chain formation. Then, CHCHO and CHCHO prefer to be successively hydrogenated to CHCHO, followed by C-O bond cleavage to give CHCH; subsequently, CHO insertion into CHCH can realize the further chain growth to form CHCHCHO, followed by dissociation and hydrogenation to give CHCHCH and CHCHCHO, respectively; further, CHCHCH hydrogenation or CHCHCHO dissociation via the C-O bond cleavage can form the CHCH-like species CHCHCH intermediate. Thus, the mechanism of a C-C chain growth cycle has been proposed that starts from a CHCHCH intermediate, followed by repeating the above C-C chain growth cycle via CHCH intermediates, and the C-C chain growth to higher C hydrocarbons and oxygenates can be realized, in which RCHCH prefers to interact with CHO to form RCHCHCHO, followed by its C-O bond cleavage and its hydrogenation to form R'CHCH (R' = RCH) and R'CHCHO (R' = RCH), respectively, where R'CHCH hydrogenation and C-O bond cleavage of R'CHCHO will produce R'CHCH. Moreover, aldehyde intermediates R'CHCHO are expected to undergo C-O bond cleavage to five R'CHCH (R' = RCH) rather than its desorption and its hydrogenation to alcohol. The C-C chain termination occurs at three possible positions along the growth cycle: R'CHCHO desorption, R'CHCH with successive hydrogenation steps to alkanes or alkenes, and R'CHCH hydrogenation to alkanes, in which the relative importance of termination of R'CHCH and R'CHCH with hydrocarbons will depend strongly on the hydrogen coverage on the metal surface. The results of this work not only illustrate the initiation, growth and termination mechanism of the C-C chain involved in FTS on the Co(0001) surface, but also serve as a basis for the rational design of other Co surfaces toward desirable higher hydrocarbons or oxygenates.