Facile Fischer–Tropsch Chain Growth from CH2 Monomers Enabled by the Dynamic CO Adlayer

Abstract

Numerous chain growth mechanisms, namely CO insertion and carbide, and active sites (flat and stepped surfaces) have been proposed to explain how hydrocarbons are formed from syngas during the Fischer–Tropsch reaction on Ru catalysts, particularly active and selective toward long-chain products. While these reaction pathways are supported by density functional theory (DFT) calculations, computational models often considered surfaces at rather low adsorbate coverage. A systematic comparison of chain growth mechanisms including the CO adlayer present on the catalyst’s surface under reaction conditions is therefore not available due to the challenging representation of co-adsorbate interactions in DFT models. Here, we show that the high coverage of chemisorbed CO on the metal surface favors the carbide mechanism on flat surfaces according to ab initio molecular dynamics simulations, which introduce the complex adlayer effects at the reaction temperature of 200 °C. At the considered CO and H coverages (0.50–0.72 and 0.24 monolayer, respectively) hydrocarbon formation involves CH2 monomers yielding ethylene and propylene as primary products, consistent with the selectivity observed in experiments. Such mechanism is favored by the presence of the CO adlayer. Indeed, in the absence of co-adsorbed CO, methane may be formed on the flat surface and the first C–C bond occurs preferentially on stepped surfaces via CH and CH2 monomers with a higher free-energy barrier (55 kJ mol–1) compared to the coupling of two CH2 species at high CO coverage on the flat surface (20 kJ mol–1). Therefore, the CO adlayer strongly modulates the nature of chain growth monomers and active sites and drives the formation of hydrocarbons during Ru-catalyzed Fischer–Tropsch. Overall, these results show how adsorbate–adsorbate interactions dictate reaction mechanisms operating in adlayers, ubiquitous in heterogeneous catalysis.