Infrared reflection absorption spectroscopy and temperature-programmed desorption studies of CO adsorption on Ni/CeO₂(111) thin films: The role of the ceria support

Ceria-supported Ni has shown unique catalytic activity due to unique properties of small Ni particles and strong metal-support interaction. Identification of adsorption sites and understanding the chemical interaction over Ni-ceria at the fundamental level provide crucial insights into the reaction pathways of complex catalytic processes. In this study, to probe the surface sites, the adsorption of CO was carried out with model Ni/ceria systems consisting of Ni nanoparticles vapor-deposited on well-ordered CeO_x(111) (1.5 < x < 2) thin films using infrared reflection absorption spectroscopy (IRRAS) and temperature-programmed desorption (TPD) under ultrahigh vacuum (UHV) conditions. Fully oxidized CeO₂(111) (Ce⁴⁺) and partially reduced CeO_1.75(111) (Ce⁴⁺/Ce³⁺) thin films were grown on Ru(0001) to examine the role of the ceria support. Ni with low coverages (e.g., 0.2 ML) grows forming small two-dimensional particles on ceria at 300 K, which develop into three-dimensional clusters after heating to 700 K. In the absence of Ni, CO adsorption at 1 mTorr at 100 K shows distinct IR bands at 2158 cm⁻¹ on CeO₂ and 2165 cm⁻¹ on CeO_1.75. Bridging and atop IR bands associated with CO adsorption over metallic Ni were observed on the Ni-CeO_1.75 surface at 300 K under UHV conditions. CO adsorption over Ni⁰ was also observed over as-deposited Ni on CeO₂. However, a new IR band at 2146 cm⁻¹ due to CO adsorption over Ni²⁺ species was detected at 100 K over the annealed Ni particles on CeO₂. CO IRRAS data suggest the oxidation of Ni to Ni²⁺ on CeO₂ and the formation of predominant Ni²⁺ species with heating, which is further confirmed with CO TPD data and previous x-ray photoelectron spectroscopy results.