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 hydrogenation, CHx + CHy coupling and C(HO) insertion into CHx CHxCHy-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 CHx-CHy coupling. The rate-determining steps for C-2 hydrocarbons are CO + H -> CHO and CHCH + H -> CH2CH. Meanwhile, CH3 hydrogenation to form CH4 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 CH2CH formation could be facilitated and CH4 formation could be suppressed, suggesting that the step sites could effectively promote the catalytic activity and selectivity for C-2 hydrocarbons formation.