A study was performed on a lean NOx 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 NOx trap generally increased with increasing amount of mixed oxide. The purge NOx release also increased with increasing mixed oxide level but was greatly diminished after thermal aging. The sulfur tolerance of the NOx 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.
After thermal aging, the NOx 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 NOx storage capability at low temperatures.
For both stabilized and thermally aged samples of the NOx traps, the sample with 0% mixed oxide had poor NOx 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 NOx trap in this study was maximized with 37% mixed oxide.