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.