Impact of cyclic lean\u2013rich aging under DeSOx condition on the lean-gas light-off and hydrogen formation ability of a lean NOx trap (LNT)

Abstract

In this paper, the aging impact of desulphation (DeSOx) procedures on lean NOx traps (LNT) was investigated. With accelerated aging procedures on an engine test bench and on a synthetic-gas test bench, LNTs were stressed with lean rich cycling under realistic desulphation conditions. Exhaust gas chassis dynamometer tests showed the impact on emissions of the lean rich treatment. High carbon monoxide (CO) slips were detected in NEDC tests during NOx regeneration (DeNOx). With light-off tests, the pattern of damage was further investigated. A pronounced deactivation in CO-rich gas conversion was found to be the main reason for the carbon monoxide emissions in the chassis dynamometer tests. A correlation between DeSOx duration (cumulated duration of rich pulses) and the inhibited CO conversion was observed. Determinations of oxygen storage capacities of aged catalysts indicated that the lean–rich cycling mainly damaged the ceria oxide of the LNT. Variations of the rich gas components indicated that hydrogen in the feed gas as well as in situ generated hydrogen out of feed gas components (steam reforming, water gas shift) is accountable for the degradation in carbon monoxide conversion in rich purges. Investigations for lower desulphation temperatures showed that the effect is negligible for temperature <350 °C. Therefore, catalyst deactivation throughout NOx regeneration events with much lower temperatures than at DeSOx was not observed. As reference to other aging treatments used in the literature, samples aged hydrothermally at 750 °C, a phosphorus poisoned and hydrothermally aged LNT as well as a LNT sample from a vehicle endurance run were compared to the DeSOx aged catalysts. All LNTs had the same conventional LNT coating.