Measures to reduce the N_2O formation at perovskite-based lean NO_x trap catalysts under lean conditions

The net oxidising atmosphere of lean burn engines requires a special after-treatment catalyst for NO_x removal from the exhaust gas. Lean NO_x traps (LNT) are such kind of catalysts. To increase the efficiency of LNTs at low temperatures platinised perovskite-based infiltration composites La_0.5Sr_0.5Fe_1-xM_xO_3-δ/Al₂O₃ with M = Nb, Ti, Zr have been developed. In general, platinum based LNT catalysts show an undesired, hazardous formation of N₂O in the lean operation mode due to a competing C₃H₆-selective catalytic reduction (SCR) at the platinum sites. To reduce N₂O emissions an additional Rh-coating, obtained by incipient wetness impregnation, besides the Pt coating and a two-layered oxidation catalyst (2 wt.% Pd/20 wt.% CeO₂/alumina)-LNT constitution, has been investigated. Though the combined Rh-Pt coating shows a slightly increased NO_x storage capacity (NSC) at temperatures above 300^◦C, it does not decrease N₂O formation. The layered oxidation catalyst-LNT system shows a decrease in N₂O formation of up to 60% at 200^◦C, increasing the maximum NSC up to 176 µmol/g. Furthermore, the NSC temperature range is broadened compared to that of the pure LNT catalyst, now covering a range of 250–300^◦C.