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

Commercial-intent lean NO_x 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 NO_x storage capacity, NO oxidation, NO_x reduction, and materials properties. Additionally, DRIFTS analysis was used to determine the effects of high temperature aging on surface chemistry and NO_x 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 NO_x that is successfully reduced also increases with aging, and it is suggested that this is accounted for by a slower release of more stable NO_x species resulting from thermal aging. NO_x storage and NO_x release experiments performed with DRIFTS at 200, 300, and 400°C indicate that a substantial amount of NO_x 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 NO_x 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.