Insight into the mechanism about the initiation, growth and termination of the C–C chain in syngas conversion on the Co(0001) surface: a theoretical study

Abstract

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 (x = 1–3) formation is easier than CH₃OH, both CH and CH₂ species are the dominant forms of CH_x (x = 1–3), both CH and CH₂ species dominantly interact with CHO to form CHCHO and CH₂CHO, and realizes the initial C–C chain formation. Then, CHCHO and CH₂CHO prefer to be successively hydrogenated to CH₃CHO, followed by C–O bond cleavage to give CH₃CH; subsequently, CHO insertion into CH₃CH can realize the further chain growth to form CH₃CHCHO, followed by dissociation and hydrogenation to give CH₃CHCH and CH₃CH₂CHO, respectively; further, CH₃CHCH hydrogenation or CH₃CH₂CHO dissociation via the C–O bond cleavage can form the CH₃CH-like species CH₃CH₂CH intermediate. Thus, the mechanism of a C–C chain growth cycle has been proposed that starts from a CH₃CH₂CH intermediate, followed by repeating the above C–C chain growth cycle via CH₃CH intermediates, and the C–C chain growth to higher C₂₊ hydrocarbons and oxygenates can be realized, in which RCH₂CH prefers to interact with CHO to form RCH₂CHCHO, followed by its C–O bond cleavage and its hydrogenation to form R′CHCH (R′ = RCH₂) and R′CH₂CHO (R′ = RCH₂), respectively, where R′CHCH hydrogenation and C–O bond cleavage of R′CH₂CHO will produce R′CH₂CH. Moreover, aldehyde intermediates R′CH₂CHO are expected to undergo C–O bond cleavage to five R′CH₂CH (R′ = RCH₂) rather than its desorption and its hydrogenation to alcohol. The C–C chain termination occurs at three possible positions along the growth cycle: R′CH₂CHO desorption, R′CHCH with successive hydrogenation steps to alkanes or alkenes, and R′CH₂CH hydrogenation to alkanes, in which the relative importance of termination of R′CHCH and R′CH₂CH 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.