Interactions of ammonia with a nickel oxide (100) surface studied by high-resolution electron energy loss spectroscopy and temperature programmed desorption spectroscopy

Interactions of ammonia with well-defined NiO(100) films prepared on Mo(100) have been investigated using combined high-resolution electron energy loss spectroscopy (HREELS)/temperature-programmed desorption (TPD). The results show that ammonia is bonded to the surface cations of NiO(100) via the nitrogen lone pair of the molecules, with an accompanying transfer of electrons to the surface cations. At low coverages (theta < 0.2 monolayer), there are strong repulsive interactions between adsorbatess, resulting in a marked decrease in the apparent activation energy of desorption. Three distinct desorption states are observed below monolayer coverages. The presence of bilayers of ammonia following NH3 adsorption at 90K is also indicated. The HREELS data show that the umbrella mode of ammonia exhibits great charge sensitivity and that the frequency of this mode remains unchanged as a function of NH3 exposure. This result has an important implication in that the frequency of the umbrella mode can be utilized as a fingerprint of the nature of the cations of oxide materials, independent of the ammonia adsorption geometry and coverage.