RESTRAINT OF NH3 DISSOCIATION ON OXYGEN-MODIFIED MO(112)

The adsorption and reaction of ammonia (NH3) on clean and oxygen-precovered Mo(112) surfaces has been studied by temperature-programmed desorption and low-energy electron diffraction. NH3 is dissociatively adsorbed on clean Mo(112) giving rise to the desorption of H-2 at 450 K and N-2 at 1050 K. With increasing oxygen precoverage (theta(0)), the saturation NH3 coverage at 330 K decreases, passes a minimum at theta(0) congruent to 0.8, and increases again at higher theta(0). The increase of the saturation NH3 coverage at high-theta(0) surfaces is due to the stabilization of non-dissociatively adsorbed NH2 (x approximate to 2) species on Mo(112)-(1 X 2)-O phase. The NHx(a) species stabilized on 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 ascribed to the successive surface reactions triggered by the N-H bond scission. The NHx(a) species, which persists on Mo(112)-(1 X 2)-O up to unusually high temperatures as compared to that on other transition metal surfaces, may provide a variety of catalytic reaction pathways with co-adsorbed species.