Density functional theory study of hydrogen adsorption on Fe5C2(001), Fe5C2(110), and Fe5C2(100).

Density functional theory calculations have been carried out for hydrogen adsorption on the (001), (110), and (100) surfaces of Fe5C2. At 1/3 and 2/3 monolyer (ML) on (001), the most stable hydrocarbon species is CsH, while CsH and CsH3 can coexist at 1 ML. On (110), only dissociated hydrogen is found at 2/5 ML, while CsH is the most stable hydrogen carbon species at 4/5 ML, and CsH and CH3 coexist at 6/5 ML. On (001) and (110) surfaces, CsH2 is less stable and can dissociate into CsH or convert into CsH3, respectively. These results are in agreement with the experimental observations. On the metallic Fe5C2(100) surface which lacks surface carbon atoms on the surface monolayer, dissociated hydrogen is found at 1/2 ML, while both dissociated hydrogen and activated H2 are found at 1 ML.