Application of spaciMS to the study of ammonia formation in lean NOx trap catalysts

SpaciMS was employed to understand the factors influencing the selectivity of NOx reduction in two fully formulated LNT catalysts, both degreened and thermally aged. Both catalysts contained Pt, Rh, BaO and Al2O3, while one of them also contained La-stabilized CeO2. The amount of reductant required to fully regenerate each catalyst was first determined experimentally based on the OSC of the catalyst and the NOx 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 NH3. For both catalysts, aging resulted in an elongation of the NOx storage–reduction (NSR) zone due to a decrease in the concentration of NOx storage sites per unit catalyst length. In addition to decreased lean phase NOx storage efficiency, stretching of the NSR zone affected catalyst regeneration. Three main effects were identified, the first being an increase of the NOx “puff” that appeared during the onset of the rich front as it traversed the catalyst. Spatially, NOx release tracked the NSR zone, with the result that the NOx concentration peaked closer to the rear of the aged catalysts. Hence the probability that NOx could re-adsorb downstream of the reduction front and subsequently undergo reduction by NH3 (formed in the reduction front) was diminished, resulting in higher rich phase NOx slip. Second, the stretching of the NSR zone resulted in increased selectivity to NH3 due to the fact that less catalyst (corresponding to the OSC-only zone downstream of the NSR zone) was available to consume NH3 by either the NH3-NOx SCR reaction or the NH3-O2 reaction. Third, the loss of OSC and NOx storage sites, along with the decreased rate of NOx 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 H2:NOx ratios at the Pt/Rh sites and consequently increased selectivity to NH3.

Figure optionsDownload as PowerPoint slideHighlights▸ Ammonia formation in LNT catalysts was studied using spaciMS. ▸ Catalyst aging resulted in elongation of the NOx storage–reduction zone. ▸ Ammonia emissions increased due to shortening of the OSC-only zone. ▸ After aging the rate of propagation of the reductant front increased. ▸ Increased H2:NOx ratios at the Pt/Rh sites resulted in increased selectivity to NH3.