NO oxidation catalysts are used in conjunction with adsorption sites that bind NO₂ strongly as it forms.  In such systems, catalysis occurs at very low local concentrations of NO₂ (<1 ppm), a molecule that strongly inhibits NO oxidation reactions.  Chemisorbed oxygen (O*), at near-saturation coverages determined by prevalent NO₂/NO ratios, accounts for these inhibition effects.  Oxygen coverages are rigorously described in terms of a virtual O₂ pressure, determined by [NO₂]² [NO]^-2 K_R^-1,where K_R is the equilibrium constant for NO oxidation reaction.  This approach controls for the oxygen coverage in a comparison between isotopic O₂ exchange and the reverse NO oxidation rates two reactions with measurable O₂ formation rates. ¹⁸O₂-¹⁶O₂ exchange rates at a given dioxygen pressure were similar to the reverse NO oxidation rates at the same (virtual) O₂ pressure.  Both reactions have kinetically-relevant O₂ activation steps, which depends on the availability of isolated vacancies on nearly saturated Pt cluster surfaces.  Turnover rates for NO oxidation and O₂ exchange decreased markedly with increasing Pt dispersion, as a result of a concomitant increase in oxygen binding energy and a decrease in the concentration of vacancies as the coordination of Pt surface atoms decreases with decreasing Pt cluster size.  Similar size effects were observed for combustion of dimethyl ether and methane, both of which involve kinetically-relevant steps requiring isolated vacancies on surfaces nearly saturated with chemisorbed oxygen.  In contrast, CO oxidation rates, also limited by O₂ activation steps but on CO-saturated surfaces, did not depend on Pt cluster size, apparently because the availability of vacancies on chemisorbed CO monolayers is independent of Pt dispersion and controlled by intermolecular interactions that dampen the effects of Pt coordination at cluster surfaces.

NO oxidation rates increased markedly when Pt/Al₂O₃ was physically mixed with BaCO₃/Al₂O₃, which binds NO₂ as it forms via NO oxidation.  NO consumption rates decreased with time, as NO₂ binding sites and local NO₂ concentrations increased and inhibited NO oxidation catalysis.  Pt/Al₂O₃ pellets (250-425 micron) mixed with adsorbent gave similar initial rates as intimately ground mixtures at similar conditions.  These data indicate that there are no local NO₂ concentration gradients in the scale of these aggregates.  At all relevant conditions and prevalent low NO₂ concentrations, NO oxidation rates increase with further decreases in NO₂ concentrations, consistent with O*-covered surfaces even at undetectable levels of NO₂ during the operation of lean NO_x traps.