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

The effect of regeneration conditions on NH₃ formation in a fully formulated Pt-Rh/BaO/Al₂O₃ lean NO_x trap catalyst was investigated. Experiments were performed on a bench flow reactor under simulated diesel exhaust conditions, employing NO_x storage/reduction cycles. Using CO/H₂ as the reductant, the selectivity of NO_x reduction to NH₃ increased with increasing regeneration time, reductant concentration and space velocity, and decreased with increasing amount of stored NO_x and increasing temperature. At a given temperature the effect of these parameters on NH ₃ selectivity can be interpreted in terms of the local H ₂:NO_x 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 NH ₃ 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 NH ₃ ; this is attributed in part to lengthening of the NO_x storage-reduction zone, as demonstrated by SpaciMS data, and decreased OSC, resulting in decreased NH₃ consumption by NO_x and O ₂ downstream of the reductant front.