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 chi-Fe5C2(010), Fe7C3(001), theta-Fe3C(001), and eta-Fe2C(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 chi-Fe5C2(010), Fe7C3(001), and theta-Fe3C(001), whereas for eta-Fe2C(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 chi-Fe5C2(010) surface shows the strongest adsorption, with a di-sigma adsorption mode. For n-alkanes, the strongest physisorption was found for the smoothest surface, i.e., the theta-Fe3C(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.