Assessment of zeolite-based Low temperature NOx adsorbers: Effect of reductants during multiple sequential cold starts

Abstract

Low temperature NOx adsorbers (LTNA) are designed to adsorb NOx from a diesel engine during a cold start and then release the stored NOx when the downstream SCR catalyst is above 200 °C and able to reduce the NOx with NH3. Monolithic catalysts with washcoats containing beta (BEA), chabazite (CHA), or ZSM5 zeolites with Pt and Pd (4.2 g/L at 1:4) were prepared, degreened, and evaluated for NOx storage performance with different reductants (C2H4, a CO/H2 mixture, both, or none). The reductants improved the NOx storage efficiency of all three zeolites. CHA provided the best NOx storage and release performance, as BEA released all of the stored NOx below 200 °C with or without reductants, and ZSM5 released most of the stored NOx below 200 °C with most feedgas compositions. After aging 80 h at high temperatures (650–800 °C) under lean conditions, BEA was the most thermally durable of the three zeolites, while ZSM5 was the least durable. The NOx storage performances of the three degreened zeolites were stable during multiple NOx storage tests without reductant, with C2H4 alone, and with H2 alone, but the performance degraded from test to test when the feedgas contained CO. This performance degradation was attributed to reduction of the Pd by the CO. Lean CH4 lightoff tests performed after the NOx storage tests confirmed that the Pd in all three zeolites was not reduced after multiple tests with C2H4, H2, or no reductant, but the Pd was significantly reduced after multiple tests with CO. The decrease in NOx storage capacity was less for ZSM5 than for BEA and CHA during multiple tests with 900 ppm CO/300 ppm H2, and the Pd in ZSM5 was also less reduced. However, tests with 3600 ppm CO/1200 ppm H2 reduced the Pd in ZSM5 much more rapidly and consequently caused a more rapid deterioration of the NOx storage performance.