Improved NO_x Storage/Release Properties of Ceria-Based Lean NO_x Trap Compositions with MnO_x Modification

Ceria/spinel-based lean NO_x trap compositions with and without barium were modified with MnO_x via incipient wetness impregnation. The effect of the MnO_x layer on the aged materials (850 °C) as to the NO_x storage and release properties was investigated via NO_x adsorption (500 ppm NO/5% O₂/balance N₂) carried out at 300 °C in a dual-bed with a 1% Pt/Al₂O₃ catalyst placed upstream of the samples to generate sufficient amounts of NO₂ required for efficient NO_x storage. Subsequent temperature programmed desorption (TPD) experiments were carried out under N₂ from 300 °C to 700 °C. The addition of MnO_x to the barium free composition led to a slightly reduced NO_x storage capacity but all of the ad-NO_x species were released from this material at significantly lower temperatures (ΔT ≈ 100 °C). The formation of a MnO_x layer between ceria/spinel and barium had a remarkable effect on ageing stability as the formation of BaAl₂O₄ was suppressed in favour of BaMnO₃. The presence of this phase resulted in an increased NO_x storage capacity and lower desorption temperatures. Furthermore, NO_x adsorption experiments carried out in absence of the Pt-catalyst also revealed an unexpected high NO_x storage ability at low NO₂/NO ratios, which could make this composition suitable for various lean NO_x trap catalysts (LNT) related applications.