Influence of ceria on the NO_x storage/reduction behavior of lean NO_x trap catalysts

The effect of La₂O₃-stabilized ceria incorporation on the functioning of fully formulated lean NO_x trap catalysts was investigated. Monolithic catalysts were prepared, corresponding to loadings of 0, 50 and 100 g CeO₂/L, together with a catalyst containing 100 g/L of ceria-zirconia (Ce_0.7Zr_0.3O₂). Loadings of the other main components (Pt, Rh and BaO) were held constant. Catalyst evaluation was performed on a bench flow reactor under simulated diesel exhaust conditions, employing NO_x storage/reduction cycles. NO_x storage efficiency in the temperature range 150-350 °C was observed to increase with ceria loading, resulting in higher NO_x conversion levels. At 150 °C, high rich phase NO_x slip was observed for all of the catalysts, resulting from an imbalance in the rates of nitrate decomposition and NO_x reduction. Optimal NO_x conversion was obtained in the range 250-350 °C for all the catalysts, while at 450 °C high rich phase NO_x slip from the most highly loaded ceria-containing catalyst resulted in lower NO_x conversion than for the ceria-free formulation. N₂O was the major NO_x reduction product at 150 °C over all of the catalysts, although low NO_x conversion levels limited the N₂O yield. At higher temperatures N₂ was the main product of NO_x reduction, although NH₃ formation was also observed. Selectivity to NH₃ decreased with increasing ceria loading, indicating that NH₃ is consumed by reaction with stored oxygen in the rear of the catalyst.