Insight into the Intrinsic Active Site for Selective Production of Light Olefins in Cobalt-Catalyzed Fischer–Tropsch Synthesis

Abstract

Selective production of light olefins remains a great challenge in cobalt-catalyzed Fischer–Tropsch synthesis (FTS). Understanding the nature of the active site and the structure–performance relationship can assist in the optimal design of more efficient catalysts to manipulate the product selectivity. In this work, on the basis of our previous experimental study, we performed a detailed computational study on the mechanisms of CO activation, methane formation, and C–C coupling on three cobalt phases, namely Co, Co2C, and Co3C, aiming at unraveling the intrinsic active site that determines the selectivity to light olefins in cobalt-catalyzed FTS reactions. The effective barrier difference was used as a descriptor to evaluate the catalytic selectivity. The structural and electronic properties were deeply analyzed to unveil the nature of the active site. Our results indicate that Co(111) favors the production of alkanes because both hydrogenation and CH2*–CH3* coupling have lower activation barriers. Co2C(111) has the highest selectivity toward CH4, which is attributed to its valley-type surface structure and strong binding energy for CH2* species, inhibiting the C−C coupling. Co3C(101) is beneficial for the production of light olefins by controlling C–C coupling, suppressing deep hydrogenation, and preventing methane formation, which originates from the synergistic effect between its ridge-type surface structure and the downward shift of the d band center. The reactions occurring at the Co/Co3C interface were also studied, and the results suggest that the electron accumulation at the interface prominently facilitates CO dissociation and benefits the formation and desorption of light olefins. Furthermore, we systematically compared the theoretical results in this work with our previous experimental results and discussed the effects of Mn promoter and CO coverage. This study provides theoretical guidance for the rational design of effective Co-based FTS catalysts to tune the FTS selectivity toward the desired light olefins.