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

Model calculations on ${\mathrm{Al}}_{10}$ ${\mathrm{NH}}_3$ clusters are used to study the chemisorptive interaction of ${\mathrm{NH}}_3$ 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 ${\mathrm{NH}}_3$-Al interaction are described in detail. The interaction between the frozen charge distributions of the ${\mathrm{NH}}_3$ and ${\mathrm{Al}}_{10}$ 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 ${\mathrm{NH}}_3$ and ${\mathrm{Al}}_{10}$ 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 ${\mathrm{NH}}_3$-Al interaction leads to a 0.8 eV binding in the on-top site whereas the threefold hollow site is repulsive. In both sites, ${\mathrm{NH}}_3$ to metal $a_1$ donation is of minor importance. This is in contrast to the bonding scheme suggested from photoemission on ${\mathrm{NH}}_3$/metal systems. A large differential shift of the 3$a_1$ ionization potential (IP) for adsorbed ${\mathrm{NH}}_3$ has been interpreted as major involvement of the ${\mathrm{NH}}_3$ 3${\mathrm{a}}_1$ orbital in the ${\mathrm{NH}}_3$–metal bond. We show that the differential 3$a_1$ IP shift is not related to its involvement in the bonding.