Influence of potassium adatoms on the dissociative adsorption of hydrogen on Pd(100)

Experiments by Solymosi and Kovács [Surf. Sci. 260 (1992) 139] have shown that potassium adsorbates on Pd(100) poison the dissociation of hydrogen molecules but tend to promote adsorption of the two separated H atoms to the surface. Using density functional theory and the full-potential linear augmented plane wave (FP-LAPW) method, we calculated the adsorption energies and geometries of hydrogen adatoms on potassium-covered Pd(100) and probed the potassium-induced modification of the potential energy surface (PES) for H₂ dissociation along two representative pathways. We found that potassium induces energy barriers for dissociation in the entrance channel of the PES, hindering the approach of the molecules to the surface. When the H₂ molecule is dissociated, potassium tends to stabilize the H-substrate bond. We confront these results with predictions of existing models of the influence of the adlayer on the PES of H₂ dissociation.