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-functiona l calculations both with local density approximation (LDA) and general ized gradient approximation (GGA) exchange-correlation functionals. Th e surface is described by a two-dimensional slab with a frozen or rela xed geometry and a periodic adsorption of H atoms is considered. Among the surface sites, the fee hollow one is found to be the most stable, in agreement with other experimental and theoretical data. The GGA ad sorption 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 +0.8 5 Angstrom relative to the surface Pd layer, very close to the low-ene rgy electron diffraction determination. The hcp hollow site is signifi cantly less stable (+0.15 eV) than the fee site and its binding energy is similar to that of the bridge site. The octahedral subsurface site is stable with respect to H-2, except for the frozen surface with a c overage 1. Indeed, if the surface is relaxed, the subsurface site is o nly 0.1 eV less stable than the fee surface site. For the surface holl ow site, the first to second layer Pd spacing expands when H is chemis orbed, but only by 2.7%. A larger expansion is found for the subsurfac e site. In the eigenvalue spectrum, a new peak is clearly visible belo w the Pd band when H is adsorbed and the position of that peak correla tes with the H coordination. This surface state is mostly localized on the H and first layer Pd. The crystal orbital overlap population curv es show that the predominant Pd-H bonding character is contained in th e 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 relaxati on is explained on the basis of the overlap population analysis.