Acceptor and donor functions of chemisorbed ammonia

Tight-binding-type calculations have been performed for N${\mathrm{H}}_3$ chemisorbed to the $C_{3v}$ site on fcc (111) transition-metal surfaces. Energetic considerations favor placing the N end of the N${\mathrm{H}}_3$ gas-phase geometry closest to the metal surface with H atoms staggered with respect to the metal atoms underneath. The acceptor function of N${\mathrm{H}}_3$ arises from partial population of ${\sigma }_{\mathrm{N}-\mathrm{H}}^{*}$ 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 N${\mathrm{H}}_3$ 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.