Carbon Permeation: The Prerequisite Elementary Step in Iron-Catalyzed Fischer–Tropsch Synthesis

Abstract

Carbon permeation into iron, a very important initial stage in iron-catalyzed heterogeneous reactions such as Fischer–Tropsch synthesis (FTS), is explored theoretically, to extend our thermodynamic and kinetic understanding of the process. The interaction of C atoms with five model surfaces (Fe (100), (110), (111), (211), (310)) was studied in six distinct ways. In the first, the random deposition of C atoms on the Fe surfaces was simulated by molecular dynamics, with C atoms released gradually. It shows that the early stages of carburization is a C permeation process, without much disturbance to the Fe surfaces. In the second approach, C atoms were approached to the surfaces sequentially. They bind readily (by 7–9 eV per C) to the surfaces, but to a different extent—strongest on Fe (100), and weakest on Fe (111). Addition of further C atoms proceeds with a slightly decreasing magnitude of the chemisorption energy, because of the increasing positive charges on the Fe atoms. At a certain coverage, different on each surface, C atoms prefer in calculation to go subsurface. C₂ units formed on some of the surfaces. In a third approach, detailed transition paths of C permeation subsurface were calculated, with associated barriers in the order Fe (100) > (111) > (310) > (211) > (110). Differences in stacking geometries of the Fe layers in these surfaces appear to be the main cause of the variation. Comparing C permeation with surface migration on clean surfaces, the barrier of the former is smaller than that of the latter for most of the surfaces, except Fe (111). At intermediate C coverage, the (100) surface also prefers migration to permeation. In a fourth approach, we calculate that with increasing carbon chemical potential, the surface energies of iron (110), (111), and (211) surfaces decrease, while those of (100) and (310) first decrease, then increase. Based on these surface energies, a Wulff construction of nanoparticle facets is made. In a fifth approach, the position in energy of the d-band centers of the Fe surfaces upon C permeation was studied. For all the surfaces, the d-band centers move away from the Fermi level with increasing C coverage, and start to resemble those of the bulk carbide phases at high C coverage. In the last approach, we show that C permeation not only lowers the barriers of model reactions for CH₄ formation and C–C chain propagation, two competing processes in FTS, but also changes the selectivity of the two competing processes. At high C coverage, chain propagation becomes preferred. A general picture emerges of C permeation on Fe surfaces as a stepwise process with opposite thermodynamic and kinetic preferences.

Graphical Abstract