Interactions of Ni Nanoparticles with Reducible CeO2(111) Thin Films

The interaction between Ni and ceria was investigated under ultrahigh vacuum conditions using model Ni/ceria systems consisting of Ni nanoparticles vapor-deposited on well-ordered CeOx(111) (1.5 < x < 2) thin films grown on Ru(0001). As indicated by X-ray photoelectron spectroscopy studies, metallic Ni is the only species present on the reduced ceria. Ni-0 is the predominate species observed upon deposition of a submonolayer of Ni on CeO2 at 300 K. However, a small amount of Ni is oxidized to Ni2+. A decreased ratio of Ni2+ to Ni-0 was observed with further increase of Ni coverage. Oxidation of Ni on CeO2 can be facilitated by annealing as well as by depositing Ni at 500 K. Scanning tunneling microscopy studies show that Ni forms two-dimensional particles on ceria at room temperature, which suggests a strong Ni-ceria interaction. Upon deposition at 500 K, metallic Ni particles as well as NiO particles can be formed on the reduced and oxidized ceria, respectively. NiO particles exhibit a flatter particle shape than that of Ni particles. Our results can be explained by thermodynamics as well as by previous computational studies.