The effect of regeneration conditions on the selectivity of NOx reduction in a fully formulated lean NOx trap catalyst

The effect of regeneration conditions on NH3 formation in a fully formulated Pt–Rh/BaO/Al2O3 lean NOx trap catalyst was investigated. Experiments were performed on a bench flow reactor under simulated diesel exhaust conditions, employing NOx storage/reduction cycles. Using CO/H2 as the reductant, the selectivity of NOx reduction to NH3 increased with increasing regeneration time, reductant concentration and space velocity, and decreased with increasing amount of stored NOx and increasing temperature. At a given temperature the effect of these parameters on NH3 selectivity can be interpreted in terms of the local H2:NOx ratio at the precious metal sites and the extent to which NH3 is consumed in the reductant front as it propagates through the catalyst. However, selectivity to NH3 increased with increasing temperature (>300 °C) during rich purging using C3H6 as the reducing agent. It was shown that NH3 selectivity was governed by the steam reforming activity of the catalyst, selectivity to NH3 increasing with increasing H2 generation. Experiments using a second catalyst to which ceria had been added as an OSC material confirmed these trends, although the presence of the ceria resulted in lower selectivity to NH3 when using H2 and/or CO as the reductant. After aging, the catalysts displayed increased selectivity to NH3; this is attributed in part to lengthening of the NOx storage-reduction zone, as demonstrated by SpaciMS data, and decreased OSC, resulting in decreased NH3 consumption by NOx and O2 downstream of the reductant front.