First Principle Study on the Adsorption of Hydrocarbon Chains Involved in Fischer–Tropsch Synthesis over Iron Carbides

The adsorption of n -alkyls, 1-alkenes, and n -alkanes has been studied on the χ-Fe 5 C 2 (010), Fe 7 C 3 (001), θ-Fe 3 C(001), and η-Fe 2 C(011) surfaces which have been reported as active phases for Fe-based Fischer–Tropsch catalysts. Spin-polarized density functional theory calculations with the vdW-DF2 functional were performed. The most stable adsorption configuration for n -alkyls is a bridge position for χ-Fe 5 C 2 (010), Fe 7 C 3 (001), and θ-Fe 3 C(001), whereas for η-Fe 2 C(011) the most stable configuration is on-top. The surfaces with the larger surface carbon content present a higher affinity for the adsorption of n -alkyls. For adsorption of 1-alkenes, the χ-Fe 5 C 2 (010) surface shows the strongest adsorption, with a di-σ adsorption mode. For n -alkanes, the strongest physisorption was found for the smoothest surface, i.e., the θ-Fe 3 C(001). The adsorption energies are found to be independent of the length of the hydrocarbon chain from a chain length of three carbon atoms for n -alkyl species, five carbon atoms for 1-alkenes, and four for n -alkanes.