Potential energy hypersurface for ammonia adsorbing onto nickel oxide

The approach of an ammonia molecule to the surface of nickel oxide was probed theoretically by performing band calculations at the extended Hueckel level on model systems. A hump in the energy vs Ni-NH{sub 3} separation curve was attributed to the initial repulsion of the NH{sub 3} lone-pair electrons by the electron-rich 3d orbitals of nickel. However, once the 3d{sub z}2 orbital was pushed above the Fermi level, the antibonding component of the Ni 3d{sub z}2-NH{sub 3} lone-pair interaction was depopulated and the Ni-NH{sub 3} interaction became strongly attractive. This was demonstrated to be completely analogous to the intended crossing of occupied and unoccupied orbitals in rearrangements of small molecules. A limited study of the potential energy surface was performed, the results of which suggested that NH{sub 3} prefers to attack directly at a surface Ni atom. From the principles derived in the NiO-NH{sub 3} study, qualitative predictions concerning the dynamics of NH{sub 3} adsorbing onto Cr{sub 2}O{sub 3} and Cu{sub 2}O were made.