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

Abstract

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 CeOx(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 CeO1.75(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 CeO1.75. Bridging and atop IR bands associated with CO adsorption over metallic Ni were observed on the Ni-CeO1.75 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.