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 Hückel level on model systems. A hump in the energy vs Ni-NH3 separation curve was attributed to the initial repulsion of the NH3 lone-pair electrons by the electron-rich 3d orbitals of nickel. However, once the 3dz2 orbital was pushed above the Fermi level, the antibonding component of the Ni 3dz2-NH3 lone-pair interaction was depopulated and the Ni-NH3 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 NH3 prefers to attack directly at a surface Ni atom. From the principles derived in the NiO-NH3 study, qualitative predictions concerning the dynamics of NH3 adsorbing onto Cr2O3 and Cu2O were made. Only nickel-based substrates are expected to exhibit an activation energy toward adsorption of NH3. Further, predictions were made concerning the stability of the adsorbed NH3 toward dissociation into NH2 and smaller fragments. More tightly bound NH3 should be more susceptible to dissociation. The decomposition is expected to follow the trend on Cr2O3 > on NiO > on Cu2O. Since the activation energy for adsorption has been observed experimentally to be a function of coverage, calculations were performed assuming both half-coverage and full coverage of the available adsorption sites (surface Ni atoms). Our results indicated that most of the coverage effect could be attributed to NH3-NH3 steric repulsions. © 1990 American Chemical Society.