ADSORPTION OF AMMONIA ON NI(111)

The adsorption of ammonia on the [111] surface of Ni is treated by using a many-electron embedding theory to describe the electronic bonding, modeling the lattice as a 28-atom, three-layer cluster. Ab initio valence orbital CI calculations carried out on a local surface region permit an accurate description of bonding at the surface. Molecular NH3 adsorbed on the Ni(111) surface at an atop site and a fee threefold site as well as a bridge site are of comparable stability, with adsorption energies of 19, 18, and 17 kcal/mol and with Ni-N equilibrium distances of 2.12, 2.78, and 2.74 Å, respectively; ammonia bonds to the substrate via the N atom with the H's pointing away from the surface. The equilibrium geometry of NH3 is calculated to have a pyramidal configuration with the molecular axis normal to the substrate, but tilting the molecular axis or rotating the molecule about the surface normal requires little energy. Calculated frequencies of the surface-NH3 group vibration, perpendicular to the Ni(111) surface, are between 460 and 500 cm-1, and the H-N stretching frequency in NH3 on the surface is 3340 cm-1. The work function decreases by ≈0.6 eV on NH3 adsorption. © 1990 American Chemical Society.