Mechanistic Insight into the C2 Hydrocarbons Formation from Syngas on fcc-Co(111) Surface: A DFT Study

A comprehensive density functional theory (DFT) calculation of C 2 hydrocarbons formation in Fischer–Tropsch synthesis (FTS) on the close-packed fcc-Co(111) surface has been carried out. The activation barriers and reaction energies for CO dissociation, CH x hydrogenation, CH x + CH y coupling and C(HO) insertion into CH x, CH x CH y –O bond scission, and successive hydrogenation reactions involved in C 2 hydrocarbons formation have been examined, and the following conclusions could be concluded: (i) CH is the dominant monomer, which is formed via CO + H → CHO → CH + O; (ii) CHO insertion is more plausible for C–C chain formation compared with CO insertion and CH x –CH y coupling. The rate-determining steps for C 2 hydrocarbons are CO + H → CHO and CHCH + H → CH 2 CH. Meanwhile, CH 3 hydrogenation to form CH 4 is more facile than C 2 hydrocarbons, which will lead to the low productivity and selectivity to C 2 hydrocarbons. (iii) Stepped-Co(111) surface has been modeled to clarify the role of defects during C 2 hydrocarbons formation, and the calculation results indicate that CHO and CH 2 CH formation could be facilitated and CH 4 formation could be suppressed, suggesting that the step sites could effectively promote the catalytic activity and selectivity for C 2 hydrocarbons formation.