Thermal Chemistry of NH3 on Oxygen-Pretreated Ni(111) Studied by Metastable Induced Electron Spectroscopy and Ultraviolet Photoelectron Spectroscopy

Ammonia adsorption on Ni( 111) and on oxygen-pretreated Ni( 111) has been studied using metastable induced electron spectroscopy and ultraviolet photoelectron spectroscopy at room temperature. Both techniques allowed us to report on the electronic structure of the valence band. The nickel oxidation leads to the existence of an oxygen-adsorbed phase before NiO growth. So we compared the ammonia reaction toward three types of surfaces: Ni(111), NiO/Ni(111), and O-Ni(111). We observed different ammonia reactivities on these three types of surfaces at room temperature: ammonia molecules were molecularly adsorbed on Ni( 111), and were decomposed on oxygen-pretreated Ni(111). After surface heating, ammonia products were dissociated, and adsorbed hydroxyl groups were also seen on O-Ni(111). Above 450 K, no ammonia decomposition products were observed on NiO/Ni(111), whereas Ni(111) and the O-Ni(111) were covered by atomic nitrogen. Its coverage is higher on the O-Ni(111) surface than on the clean Ni(111) surface. This observation proves the high ammonia reactivity toward adsorbed oxygen. This study shows that the reactivity of oxygen atoms on Ni(111) toward ammonia depends on their characteristics (adsorbed oxygen or oxygen embedded into NiO).