Density-functional periodic study of the adsorption of hydrogen on a palladium (111) surface

The adsorption of H on Pd(111) has been studied with density-functional calculations both with local density approximation (LDA) and generalized gradient approximation (GGA) exchange-correlation functionals. The surface is described by a two-dimensional slab with a frozen or relaxed geometry and a periodic adsorption of H atoms is considered. Among the surface sites, the fcc hollow one is found to be the most stable, in agreement with other experimental and theoretical data. The GGA adsorption energy ranges from -0.27 to -0.53 eV (the experimental value is -0.45 eV) while the LDA result for the adsorption energy is 0.6–0.7 eV larger in absolute value. The optimal height of the H atom is +0.85 Å relative to the surface Pd layer, very close to the low-energy electron diffraction determination. The hcp hollow site is significantly less stable (+0.15 eV) than the fcc site and its binding energy is similar to that of the bridge site. The octahedral subsurface site is stable with respect to ${\mathrm{H}}_2$, except for the frozen surface with a coverage 1. Indeed, if the surface is relaxed, the subsurface site is only 0.1 eV less stable than the fcc surface site. For the surface hollow site, the first to second layer Pd spacing expands when H is chemisorbed, but only by 2.7%. A larger expansion is found for the subsurface site. In the eigenvalue spectrum, a new peak is clearly visible below the Pd band when H is adsorbed and the position of that peak correlates with the H coordination. This surface state is mostly localized on the H and first layer Pd. The crystal orbital overlap population curves show that the predominant Pd-H bonding character is contained in the split-off band and indicate that the sp and d orbitals of Pd have a rather equal contribution to the Pd-H bond. The small surface relaxation is explained on the basis of the overlap population analysis. © 1996 The American Physical Society.