The Effects of Aging Temperature and PGM Loading on the NOx Storage Capacity of a Lean NOx Trap 2005-01-1117
A laboratory aging study was performed on samples of a lean NOx trap with platinum group metal (PGM) loadings of 0.53, 1.06, 2.12, and 3.18 g/liter. The LNT samples were aged at inlet temperatures of 650°C, 750°C, 800°C, and 850°C behind samples of a three-way catalyst that were aged on a pulse-flame combustion reactor with a Ford-proprietary durability schedule representing 80,000 km of customer use. For all aging temperatures, higher PGM loadings were beneficial for low temperature NOx performance, attributable to an increase in the oxidation of NO to NO2. Conversely, lower PGM loadings were beneficial for high temperature NOx performance after aging at 650°C and 750°C, as higher loadings promoted the decomposition of the nitrates during lean operation and thereby decreased the NOx storage capability at high temperatures. Also, higher PGM loadings increased the OSC of the trap and thereby increased the purge requirements. After aging at 800°C and 850°C, higher PGM loadings were beneficial for NOx performance up to 450°C.
The samples with low PGM loadings that were aged at 650°C had similar NOx and HC performance as the samples with high PGM loadings that were aged at 850°C. The laboratory results were confirmed with full-size converters aged on an engine-dynamometer with the Ford-proprietary durability schedule, as a NOx trap aged behind an exhaust gas heat exchanger had much better NOx performance than a trap aged without the heat exchanger. These results suggest that an exhaust gas heat exchanger could allow reductions in the PGM loading of the LNT while maintaining good NOx performance after thermal aging.