First principles calculations of the adsorption and diffusion of hydrogen on Fe(1 0 0) surface and in the bulk

First-principles plane wave calculations based on spin-polarized density-functional theory (DFT) and the generalized gradient approximation (GGA) have been used to study the adsorption of hydrogen on Fe(1 0 0) surface and in the bulk. It was found that H2 adsorption takes place dissociatively with a classical activation energy of about 3.5 kcal/mol. In the low coverage regime at θ = 0.25, H atom adsorbs at both two-folded and four-folded sites with a slight preference for the four-folded site. In the full coverage regime, there is a clear distinction between two-folded and four-folded adsorption sites with a net preference for adsorption at four-folded site. The dependence of H binding energy on coverage in the range 1.0 ≤ θ ≤ 3.0 was also determined and the corresponding sequence of sites filling has been analyzed. After filling all four-folded sites, it was found that occupation of two-folded followed by one-folded sites is possible while adsorption at nearby mixed two-folded and one-folded sites leads to H–H recombination. The minimum energy pathways for surface diffusion of atomic H between selected pairs of local minima indicate the existence of small classical barriers with values of about 1.9 kcal/mol. These barriers increase slightly with the increase of coverage. When H diffuses from surface to subsurface sites, the corresponding barriers are larger than on the surface with values in the range 7.5–9.5 kcal/mol. At these subsurface sites, the absorption energy is still exothermic relative to gas phase H2 and increases with coverage. Once H penetrates the first two surface layers, the corresponding diffusion barriers decrease to values close to those obtained in bulk Fe. Absorption of H in bulk bcc Fe is endothermic relative to isolated gas phase H2 and takes place at tetrahedral sites. The most favorable diffusion pathway among tetrahedral sites was found to pass through a trigonal site and has a low barrier of about 1.1 kcal/mol.