Effect of lean/rich high temperature aging on NO oxidation and NOx storage/release of a fully formulated lean NOx trap

Abstract

Commercial-intent lean NOx traps (LNTs) containing Pt, Pd, Rh, Ba, Ce, Zr, and other proprietary additives were thermally aged at 750, 880, 930, and 1070°C using lean/rich cycling and then investigated for effects of aging on NOx storage capacity, NO oxidation, NOx reduction, and materials properties. Additionally, DRIFTS analysis was used to determine the effects of high temperature aging on surface chemistry and NOx storage. As platinum group metal (PGM) dispersion decreases with aging, the NO turnover frequency (TOF) for NO oxidation at 200, 300, and 400°C is shown to increase. The fraction of stored NOx that is successfully reduced also increases with aging, and it is suggested that this is accounted for by a slower release of more stable NOx species resulting from thermal aging. NOx storage and NOx release experiments performed with DRIFTS at 200, 300, and 400°C indicate that a substantial amount of NOx is stored on Al2O3 as nitrates at 200 and 300°C before aging. However, almost no nitrates are seen on alumina after aging at 900 and 1000°C, resulting in a significant reduction in NOx storage capacity. This is most likely due to a 45% reduction in total surface area and a high temperature redispersion of Ba over remaining alumina sites. No evidence of BaAl2O4 was observed with XRD.