Acceptor and donor functions of chemisorbed ammonia

Tight-binding-type calculations have been performed for NH₃ chemisorbed to the C₃ᵥ site on fcc (111) transition-metal surfaces. Energetic considerations favor placing the N end of the NH₃ gas-phase geometry closest to the metal surface with H atoms staggered with respect to the metal atoms underneath. The acceptor function of NH₃ arises from partial population of σ_N-H* molecular orbitals and can dominate the donor contribution to the heat of chemisorption on metals with nearly full d bands. An electron population analysis indicates that metal atoms attached to NH₃ are rehybridized to achieve a smaller d occupation and larger $s,p$ occupation compared to the bare surface. These findings show that ammonia can function principally as an acceptor on some metal surfaces.