Comparison of the nature of the hydrogen-metal bond on Pd(111) and Ni(111) by a periodic density functional method

The adsorption of H on Pd(111) and Ni(111) has been studied with density functional calculations, including GGA to compute the exchange and correlation energies. The surface is described by a two-dimensional slab with a periodic (1x1) adsorption of H atoms. Among the surface sites, the fee hollow site is found to be the most stable on Pd, while on nickel, the fee and hcp hollow sites give the same binding energies. The adsorption energies corresponding to these sites are -0.27 eV and -0.49 eV for Pd and Ni, respectively. The energy, as well as the geometry, are in good agreement with experimental data. This study focuses on the electronic properties of the M-H bond in order to explain the differences that appear between the two metals. The existence of a split-off state is proven in the case of Pd, and rejected for Ni. A detailed investigation of the DOS and COOP curves explains the nature of the most stable site. It pinpoints the respective roles played by the d and sp bands in the adsorption phenomenon.