Rediscovering Tuning Product Selectivity by an Energy Descriptor: CH4 Formation and C1-C1 Coupling on the FCC Co Surface

Despite extensive research work, how to control the chain growth by C-1-C-1 coupling on the catalysts of Fischer-Tropsch synthesis (FTS) remains an unsolved problem. The activity, chain growth, and selectivity of FTS had been investigated on Co(111), (100), (311), and (110) surfaces with spin-polarized density functional theory (DFT) calculations. It is clearly shown that the CO activity decreases in the order of Co(110) > Co(311) > Co(100) > Co(111). Surface carbon is successively hydrogenated to CH4 or undergoes chain growth to form heavier hydrocarbons. The effective barrier difference as a descriptor was introduced to evaluate the selectivity of CH4 formation and C-1-C-1 coupling. According to the effective barriers difference, the Co(100) surface has high selectivity toward C-1-C-1 coupling, which is attributed to the active site containing two 4-fold hollow sites. The Co(110) surface has the highest selectivity of CH4 formation. Moreover, it is revealed that the exposed specific cobalt crystal plane could tune the FTS selectivity to higher CO activity and lower CH4 selectivity. This work highlights the effects of surfaces and active sites on catalytic selectivity and promotes the design of Co-based catalysts of FTS with high selectivity for long-chain hydrocarbons.