The Effect of Ceria Content on the Performance of a NOx Trap

Abstract

<div class="htmlview paragraph">A study was performed on a lean NO<sub>x</sub> trap in which the loading of a ceria-containing mixed oxide in the washcoat was varied. After a mild stabilization of the traps, the time required to purge the NO<sub>x</sub> trap generally increased with increasing amount of mixed oxide. The purge NO<sub>x</sub> release also increased with increasing mixed oxide level but was greatly diminished after thermal aging. The sulfur tolerance of the NO<sub>x</sub> trap improved as the mixed oxide content was increased from 0% to 37%. The sample with 0% mixed oxide was more difficult to desulfate than the other samples due to poor water-gas-shift capability.</div> <div class="htmlview paragraph">After thermal aging, the NO<sub>x</sub> reduction efficiency on a 60 second lean/5 second rich cycle was highest for the samples with 0% to 37% mixed oxide at evaluation temperatures of 400°C to 500°C. For the samples with higher amounts of mixed oxide, the higher oxygen storage capacities at these temperatures removed more of the reductants during the purges and led to incomplete purging of these samples. Conversely, the performance at 300°C was highest for the samples with 50 to 100% mixed oxide because the ceria provided NO<sub>x</sub> storage capability at low temperatures.</div> <div class="htmlview paragraph">For both stabilized and thermally aged samples of the NO<sub>x</sub> traps, the sample with 0% mixed oxide had poor NO<sub>x</sub> conversion at stoichiometry relative to the other samples, but the HC conversion at 0.98 lambda generally decreased with increasing content of mixed oxide. After thermal aging, the stoichiometric lightoff temperature for 50% conversion generally increased with increasing content of mixed oxide. Considering all of these factors, the performance of the NO<sub>x</sub> trap in this study was maximized with 37% mixed oxide.</div>