First-principles plane wave calculations based on spin-polarized density-functional theory and the generalized gradient approximation were used to study the adsorption of H on Fe(100) surface and in the bulk. It was found that H2 adsorption takes place dissociatively with a classical activation energy of about 3.5kcal/mol. In the low coverage regime at θ = 0.25, H atom adsorbs at both 2-folded and 4-folded sites with a slight preference for the 4-folded site. In the full coverage regime, there was a clear distinction between 2-folded and 4-folded adsorption sites with a net preference for adsorption at 4-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 was analyzed. After filling all four-folded sites, it was found that occupation of two-folded followed by one-folded sites was 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 1.9kcal/mol. These barriers increase slightly with the increase of coverage. When H diffuses from surface to subsurface sites, the corresponding barriers were larger than on the surface with values in the range 7.5–9.5kcal/mol. At these subsurface sites, the absorption energy was still exothermic relative to gas phase H2 and increased with coverage. Once H penetrates the first two surface layers, the corresponding diffusion barriers decreased to values close to those obtained in bulk Fe. Absorption of H in bulk body-centered cubic Fe was 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 1.1kcal/mol.

First Principles Calculations of the Adsorption and Diffusion of Hydrogen on Fe(100) Surface and in the Bulk. D.C.Sorescu: Catalysis Today, 2005, 105[1], 44-65