Ammonia on Ni(1 1 1) surface studied by first principles: Bonding, multilayers structure and comparison with experimental IR and XPS data

DFT calculations have been performed on the adsorption of NH₃ on Ni(1 1 1) to obtain information on the structure of the absorbed species, the nature of the chemical interactions between the adsorbate and the surface and the structure of multilayers formed at high coverage. A cluster model, using localized basis functions as well as an approach based on plane waves and periodic boundary conditions have been considered. The two approaches lead to similar results for the relative stabilities of investigated adsorption sites (atop > fcc > hcp) with an adsorption energy of about 15-24 kcal/mol (depending on the coverage). On the atop site, α-ammonia adsorbs molecularly with an equilibrium distance between the nitrogen atom and the surface of 2.03 ± 0.02 Å and a geometry close to the one of the molecule in the gas phase. The good agreement between the two DFT approaches clearly underlines the local nature of the adsorption reaction. The vibrational frequencies computed for NH₃ adsorbed in this site are in good agreement with experimental values. They show that the interaction with the surface leads to a weakening of the strength of the N-H bond while the angular stretching is stronger. Both orbital and topological analyses were used to investigate chemical interactions between the cluster and the molecule. The results strongly suggest an electrostatic (non-covalent) interaction between the substrate and the molecule. Calculations with NH₃ coverages above 0.25 confirm that saturation occurs at a coverage of 0.25. Above the saturation coverage, ammonia molecules in excess form multiple layers (β and γ ammonia) bonded to the first layer by intermolecular hydrogen bonds. N 1s core level shifts calculations performed for the several investigated coverages are also in good agreement with experimental XPS data. It is shown that the H-bond network more than the bond to the surface allows to understand the N 1s core level variations.