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 150350 degrees C was observed to increase with ceria loading, resulting in higher NO(x) conversion levels. At 150 degrees 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, reduction. Optimal NO(x) conversion was obtained in the range 250-350 degrees C for all the catalysts, while at 450 degrees 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 degrees 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. (C) 2007 Elsevier B.V. All rights reserved.