Influence of Ceria on the NOx Storage/Reduction Behavior of Lean NOx Trap Catalysts

The effect of La 2 O 3 -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 CeO 2 /L, together with a catalyst containing 100 g/L of ceria-zirconia (Ce 0.7 Zr 0.3 O 2 ). 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. N 2 O was the major NO x reduction product at 150 °C over all of the catalysts, although low NO x conversion levels limited the N 2 O yield. At higher temperatures N 2 was the main product of NO x reduction, although NH 3 formation was also observed. Selectivity to NH 3 decreased with increasing ceria loading, indicating that NH 3 is consumed by reaction with stored oxygen in the rear of the catalyst.