The Origin of the Size Effect in the Oxidation of CO on Supported Palladium Nanoparticles

Abstract

Two Pd/TiO2 catalysts with mean particle sizes of 1 and 3 nm were prepared and tested in the low-temperature oxidation of CO. It was found that the first catalyst with higher dispersion is more active. Turnover frequencies varied for these catalysts by almost six times. In contrast, the apparent activation energy of the oxidation of CO on the catalyst with smaller Pd nanoparticles was estimated at 76 kJ/mol, and for the catalyst with larger Pd nanoparticles at 58 kJ/mol. According to in situ XANES studies, the particle size effect originates from the oxidation of small palladium nanoparticles under reaction conditions, whereas larger nanoparticles are stable and consist of palladium atoms mainly in the metallic state. Palladium oxide is more active in the low-temperature oxidation of CO than metallic palladium. This means that the origin of size-dependent activity of Pd nanoparticles in the low-temperature oxidation of CO is associated with the change in the chemical composition of nanoparticles that leads to a change in the reaction mechanism and, as a result, in their activity.