Effect of aging on the NO_x storage and regeneration characteristics of fully formulated lean NO_x trap catalysts

In order to elucidate the effect of washcoat composition on lean NO_x trap (LNT) aging characteristics, fully formulated monolithic LNT catalysts containing varying amounts of Pt, Rh and BaO were subjected to accelerated aging on a bench reactor. Subsequent catalyst evaluation revealed that in all cases aging resulted in deterioration of the NO_x conversion as a consequence of impaired NO_x storage and NO_x reduction functions, while increased selectivity to NH₃ was observed in the temperature range 250-450°C. Elemental analysis, H₂ chemisorption and TEM data revealed two main changes which account for the degradation in LNT performance. First, residual sulfur in the catalysts, associated with the Ba phase, decreased catalyst NO_x storage capacity. Second, sintering of the precious metals in the washcoat occurred, resulting in decreased contact between the Pt and Ba, and hence in less efficient NO_x spillover from Pt to Ba during NO_x adsorption, as well as decreased rates of reductant spillover from Pt to Ba and reverse NO_x spillover during catalyst regeneration. For the aged catalysts, halving the Pt loading from 100 to 50g/ft³ was found to result in a significant decrease in overall NO_x conversion, while for catalysts with the same 100g/ft³ Pt loading, increasing the relative amount of Pt on the NO_x storage components (BaO and La-stabilized CeO₂), as opposed to an Al₂O₃ support material (where it was co-located with Rh), was found to be beneficial. The effect of Rh loading on aged catalyst performance was found to be marginal within the range studied (10-20g/ft³), as was the effect of BaO loading in the range 30-45g/L.