CLEERS Teleconference: William Epling

Title: Passive NOx Adsorbers Reaction Chemistry

William Epling, Professor, University of Virginia

Abstract:

Passive NOx adsorbers (PNAs) are being designed and developed to help mitigate cold start NOx emissions. Over the last few years, a variety of Pt- and Pd-containing formulations has been studied. Most of these materials adsorb NOx relatively well at low temperature, with different degrees of success in desorbing the NOx at what might be considered appropriate temperatures. This “appropriate” temperature is high enough such that downstream NOx reduction catalysts are active, but low enough to avoid having to go to higher than normal exhaust gas temperature ranges to induce desorption for a future cold-start need.

In this presentation we will discuss data obtained using a model Pd/BEA PNA formulation. There are two parts to the presentation. As highlights of the first, with the addition of CO, the temperature at which NO desorbs shifts and this can be to a more “appropriate” temperature. Without some CO, the desorption temperature was too low for a downstream NOx reduction catalyst to be efficient. The inclusion of NO2, a strong oxidant, led to lower temperature desorption. Thus, there appears to be some dependency on Pd oxidation state. With desorption temperature so critical, other parameters, including heat/ramp rate, other reductants, and extent of saturation, were investigated and each were found to affect the desorption TPD profile.

In the second part, observations regarding hydrothermal ageing treatments and exposure to gas conditions leading to both reversible and irreversible changes in NO storage capacity will be discussed. Here, highlights include reductant causing some portion of the adsorption sites to agglomerate and become lost, while other sites can apparently be reactivated via an oxidative treatment. We speculate that there is a distribution of particle sizes that forms upon reductant exposure – i.e. the reductant drives the Pd from the ion exchange site to particles. The smaller particles can be driven back to the ion exchange sites for NOx adsorption. The larger particles cannot and thus these represent the irreversible loss.