(289e) Pt-Catalyzed NO Oxidation Pathways and the Effect of NO2 Adsorbents
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 KR-1,where KR 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 NOx traps.