Adsorption of NH3 on oxygen pre-treated Ni(111)

The adsorption of NH3 on oxygen pre-treated Ni(111) surfaces has been studied at room temperature using X-ray photoelectron spectroscopy (XPS). Oxygen pre-treatments have been performed at 650 K. This protocol leads to a "two-phase domain" (O-adsorbed phase + NiO islands) over a large range of oxygen exposures. The investigation of the surface reactivity towards NH3 shows that ammonia is adsorbed provided that the O-adsorbed phase is present; the surface reactivity increases with the O-adsorbed phase coverage. Two N-adspecies have been detected from the N 1s core level peaks at 399.8+/-0.2 and 397.8+/-0.2 eV and assigned to molecular NH3 and dissociated NH2 species, respectively. The molecular adsorption results from direct impingement of the NH3 molecules, whereas the dissociated one results from the dissociation of a part of the preadsorbed molecular species. At saturation, the dissociated species is the more abundant one (about 4/5 of the total N 1s peak) whatever the initial coverage (>0) of the surface by the O-adsorbed phase. The XPS data indicate that this dissociation is correlated to the formation of OH from oxygen of the adsorbed phase and hydrogen abstraction from the molecular ammonia. When the O-adsorbed phase is absent on the surface, i.e. for clean Ni(111) or the complete NiO layer, none of these surface reactions with ammonia occurs, under the same adsorption conditions. (C) 2003 Elsevier Science B.V. All rights reserved.