NO Oxidation Catalysis on Pt Clusters: Elementary Steps, Structural Requirements, and Synergistic Effects of NO2 Adsorption Sites

Kinetic and isotopic methods show that NO oxidation on supported Pt clusters involves kinetically relevant reaction of O2 with vacancy sites on surfaces nearly saturated with oxygen adatoms (O*). The oxygen chemical potential at Pt surfaces that determines the O* coverage is rigorously described by an O2 virtual pressure and determined by the thermodynamics of NO2-NO interconversion reactions. NO oxidation and oxygen isotopic exchange processes are described by the same rate constant, consistent with similar kinetically relevant O2 dissociation steps for both reactions. NO oxidation, NO2 decomposition, and O2-O2 exchange rates increased markedly with increasing Pt cluster size (1-8 nm); these clusters remain metallic at all O2 virtual pressures prevalent during NO oxidation. These effects of cluster size reflect the higher vacancy concentrations and more facile oxygen desorption on larger Pt clusters, which bind oxygen adatoms weaker than more coordinatively unsaturated surface Pt atoms on smaller clusters. These trends are similar to those found for methane and dimethyl ether combustion on Pt and Pd catalysts, which also require vacancy sites on O*saturated cluster surfaces in their respective kinetically relevant steps. Inhibition of NO oxidation by NO2 persists to undetectable NO2 concentrations; thus, NO oxidation turnover rates increase significantly when NO2 adsorption sites present on BaCO3/Al2O3 are placed within diffusion distances of Pt clusters. NO oxidation rates on intrapellet catalyst-adsorbent mixtures are described accurately by a simple reaction-adsorption model in which NO2 adsorbs via displacement of CO2 on BaCO3 surfaces.