Theoretical Insights into Morphologies of Alkali-Promoted Cobalt Carbide Catalysts for Fischer-Tropsch Synthesis

Co2C catalysts were found to be structure-sensitive during Fischer-Tropsch synthesis (FTS), and reaction-induced Co2C nanoprisms exposing the (101) and (020) facets exhibited superior olefin selectivity, low methane selectivity, and high activity under mild conditions. Alkali metal promoters benefit the formation and stabilization of Co2C nanoprisms, although mechanistic understanding and theoretical support are lacking due to the great complexity of this catalytic reaction and the difficulty in characterizing the detailed structural changes. Here, density functional theory (DFT) calculations and ab initio atomistic thermodynamics simulations are combined to elucidate the "promoter-structure" relationship of Co2C catalysts decorated with different alkali metal promoters. Co- and C-terminated and stoichiometric low-index surfaces including (020), (101), (111), (011), and (110) are considered. Evolution of the equilibrium morphology versus K2O coverage (theta(K2O)) and carbon chemical potential (mu(C)) as determined by the temperature, pressure, and the H-2/CO ratio are predicted. We find that increasing theta(K2O) facilitates the preferential exposure of the (020) and (101) facets with different terminations at diverse mu C, which are the active surfaces for Fischer-Tropsch to olefins (FTO). For theta(K2O) = 1/6 ML, these two surfaces are predicted to cover 100, 53.4, and 48.4% of the exposed surface area at mu C of -7.5, -8.5, and -9.5 eV, respectively, compared with 8.6, 18.3, 26.2% without this promoter. The dominant exposure of these two facets explains the experimentally observed structure of Co2C nanoprisms with a parallelepiped shape. Additionally, the promotional effect extends the preference of most Co- and C-terminated facets over stoichiometric facets to a larger range of mu C. Furthermore, Na, K, and Rb promoters are predicted to have stronger effects than Li in stabilizing these two facets, which is also consistent with available experimental observations. Insights from this theoretical work provide a rational understanding of the promotional effect of alkali metals on the morphology of the Co2C catalyst, which may facilitate the design of more efficient FTO catalysts with controlled surface structures.