Size selectivity for CO-oxidation of Ag nanoparticles on highly ordered pyrolytic graphite (HOPG)

The Ag nanoparticles on HOPG were oxidized using atomic oxygen environments under ultrahigh-vacuum conditions, and the nature of the oxidized Ag particles, and their interactions with CO were studied using X-ray photoelectron spectroscopy (XPS). The oxygen uptake of a smaller Ag nanoparticle is significantly higher than that of a larger particle and bulk-like Ag. For the Ag nanoparticles larger than 3 nanometers (nm), two different oxygen species were identified, one of which readily reacts with CO to form CO2 at room temperature. Based on our XPS studies, the catalytically active oxygen species is assigned to be in Ag2O and/or AgO. For the smaller Ag nanoparticles, only a single oxygen species can form upon reacting with oxygen atoms in the gas phase, which is inert towards CO-oxidation. The nature of the catalytically inert oxygen species is discussed. The formation of different oxide species as a function of particle size can be responsible for the size selectivity in chemical reactions catalyzed by Ag nanoparticles.