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 NO_x 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 NO_x performance, attributable to an increase in the oxidation of NO to NO₂. Conversely, lower PGM loadings were beneficial for high temperature NO_x 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 NO_x 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 NO_x performance up to 450°C.

The samples with low PGM loadings that were aged at 650°C had similar NO_x 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 NO_x trap aged behind an exhaust gas heat exchanger had much better NO_x 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 NO_x performance after thermal aging.