The adsorption and reaction of ammonia (NH3) on clean and oxygen-preco
vered Mo(112) surfaces has been studied by temperature-programmed deso
rption and low-energy electron diffraction. NH3 is dissociatively adso
rbed on clean Mo(112) giving rise to the desorption of H-2 at 450 K an
d N-2 at 1050 K. With increasing oxygen precoverage (theta(0)), the sa
turation NH3 coverage at 330 K decreases, passes a minimum at theta(0)
congruent to 0.8, and increases again at higher theta(0). The increas
e of the saturation NH3 coverage at high-theta(0) surfaces is due to t
he stabilization of non-dissociatively adsorbed NH2 (x approximate to
2) species on Mo(112)-(1 X 2)-O phase. The NHx(a) species stabilized o
n Mo(112)-(1 X 2)-O gives rise to simultaneous desorption of H2O, H-2,
and NH3 at 650 K with N(a) remaining on the surface, which is ascribe
d to the successive surface reactions triggered by the N-H bond scissi
on. The NHx(a) species, which persists on Mo(112)-(1 X 2)-O up to unus
ually high temperatures as compared to that on other transition metal
surfaces, may provide a variety of catalytic reaction pathways with co
-adsorbed species.