Quantitative Determination of C–C Coupling Mechanisms and Detailed Analyses on the Activity and Selectivity for Fischer–Tropsch Synthesis on Co(0001): Microkinetic Modeling with Coverage Effects

Abstract

The Fischer–Tropsch synthesis plays a significant role in re-forming natural resources to meet global demand for commodities, while there is ongoing oil depletion and population growth. Mechanisms have long been investigated, but they are still a heavily debated issue. In this work, all of the possible elementary reaction steps on a flat cobalt surface were calculated using density functional theory (DFT) with van der Waals interactions. Kinetic simulations using standard DFT data (free energies and barriers at low coverages), the so-called non-coverage-dependent kinetic model commonly used in the literature, are compared to those from a coverage-dependent kinetic model for the system. We show that the coverage-dependent kinetic model gives rise to a TOF which is approximately 6 orders of magnitude larger than the TOF calculated using the non-coverage-dependent kinetic model. Furthermore, it is found that Co(0001) is highly selective to olefin production, and it is very likely to produce long-chain hydrocarbons. Both models demonstrate that the CO insertion mechanism is the dominant mechanism on Co(0001). Our calculations also reveal that high coverage of CHx leads to the carbide mechanism being significant and low coverage of CHx results in the CO insertion mechanism being more favored. Direct CO dissociation is difficult on Co(0001), which leads to monomers CHx being unable to occupy a certain amount of surface coverage, causing the carbide mechanism to be inhibited. The reaction pathway through CO + H → CHO, CHO + H → CHOH, and CHOH → CH + OH is the main channel to form the monomer CH on the basis of the coverage-dependent kinetic model simulations. The temperature considerably affects the surface coverage and the total reaction rate, leading to the selectivity being highly temperature dependent. Our coverage-dependent kinetic model predicts that the selectivity of oxygenates is high in comparison to methane in the low-temperature region from 425 and 475 K. From 475 to 525 K, the selectivity toward CH4 increases. From 525 to 700 K, the selectivity of C2 decreases significantly and the selectivity of CH4 increases remarkably.