Application of spaciMS to the study of ammonia formation in lean NO _x trap catalysts

SpaciMS was employed to understand the factors influencing the selectivity of NO _x reduction in two fully formulated LNT catalysts, both degreened and thermally aged. Both catalysts contained Pt, Rh, BaO and Al ₂O ₃, while one of them also contained La-stabilized CeO ₂. The amount of reductant required to fully regenerate each catalyst was first determined experimentally based on the OSC of the catalyst and the NO _x storage capacity (NSC). In this way a correction was made for the change in catalyst OSC and NSC after aging, thereby eliminating these as factors which could affect catalyst selectivity to NH ₃. For both catalysts, aging resulted in an elongation of the NO _x storage-reduction (NSR) zone due to a decrease in the concentration of NO _x storage sites per unit catalyst length. In addition to decreased lean phase NO _x storage efficiency, stretching of the NSR zone affected catalyst regeneration. Three main effects were identified, the first being an increase of the NO _x "puff" that appeared during the onset of the rich front as it traversed the catalyst. Spatially, NO _x release tracked the NSR zone, with the result that the NO _x concentration peaked closer to the rear of the aged catalysts. Hence the probability that NO _x could re-adsorb downstream of the reduction front and subsequently undergo reduction by NH ₃ (formed in the reduction front) was diminished, resulting in higher rich phase NO _x slip. Second, the stretching of the NSR zone resulted in increased selectivity to NH ₃ due to the fact that less catalyst (corresponding to the OSC-only zone downstream of the NSR zone) was available to consume NH ₃ by either the NH ₃-NO _x SCR reaction or the NH ₃-O ₂ reaction. Third, the loss of OSC and NO _x storage sites, along with the decreased rate of NO _x diffusion to Pt/Rh sites (as a result of Pt/Rh-Ba phase segregation), led to an increase in the rate of propagation of the reductant front after aging. This in turn resulted in increased H ₂:NO _x ratios at the Pt/Rh sites and consequently increased selectivity to NH ₃.