Adsorption of ammonia on the Al(111) surface: Theoretical studies

Model calculations on Al₁₀{NH}₃$ clusters are used to study the chemisorptive interaction of ${NH}₃$ with the Al(111) surface. It is assumed that the ammonia molecule stabilizes on the surface with the N center pointing towards the metal and two different surface sites, on top and threefold hollow, are examined. The energetics of the NH₃-Al interaction are described in detail. The interaction between the frozen charge distributions of the NH₃ and Al₁₀ subunits is repulsive for both sites. However, the repulsion is much larger in the threefold hollow site than in the on-top site. The charge rearrangements that are important for the bonding are the intra-unit polarizations of both NH₃ and Al₁₀ in the presence of the other (frozen) subunit. These polarizations make attractive electrostatic contributions. They are larger for the on-top site. As a consequence, the NH₃-Al interaction leads to a 0.8 eV binding in the on-top site whereas the threefold hollow site is repulsive. In both sites, NH₃ to metal a₁ donation is of minor importance. This is in contrast to the bonding scheme suggested from photoemission on NH₃/metal systems. A large differential shift of the 3a₁ ionization potential (IP) for adsorbed NH₃ has been interpreted as major involvement of the NH₃ 3a₁ orbital in the NH₃--metal bond. We show that the differential 3a₁ IP shift is not related to its involvement in the bonding.