Density Functional Theory Study of CO Adsorption on Fe5C2(001), -(100), and -(110) Surfaces

Density functional theory calculations have been carried out to study CO adsorption on the (001), (100), and (110) surfaces of Fe5C2, which are considered as active catalysts in Fischer−Tropsch synthesis. It is found that CO prefers to adsorb at three 3-fold sites (three iron atoms) on the three surfaces at low coverage with maximum adsorption energies of −2.10, −2.21, and −2.34 eV, respectively. The adsorption energy of CO adsorbed around surface carbon atoms on (001) is higher than that on (110) by up to 0.94 eV. The difference is attributed to the C 2p−Fe 3d bands that mainly lie at the lower energy level on bare (110) than that on bare (001). This indicates that the surface Fe−C bonds on (110) are stronger than those on (001), which in turn hinders local CO adsorption on (110). In contrast to (001) and (110), a monolayer of the (100) surface only consists of iron atoms. The adsorption energy at low coverage on (100) is similar to those on (001) and (110), but the highest at high coverage among all surfaces. These results indicate the diversity of the atomic structure on the underlying Fe5C2 surface site. Apart from the (100) surface at high coverage, other surfaces may play dominant roles when practical working conditions permit.