NH3 formation over a lean NOX trap (LNT) system: Effects of lean/rich cycle timing and temperature

•Below 300°C NOX release and reduction are slow and kinetically limiting.•Above 450°C NOX storage becomes the limiting factor.•Selectivities to NH3 and N2O are primarily functions of temperature.•Selectivities to NH3 and N2O decrease with increasing temperature.•Cycle timing affects both NOX conversion and NH3 yield. A commercial lean NOX trap (LNT) catalyst containing Pt, Pd, Rh, BaO, and CeO2 was evaluated in this investigation. The effects of lean/rich cycle timing on the NOX, CO and C3H6 conversions and on the NH3 and N2O selectivities were considered. Two distinct lean/rich cycling regimes were identified. At low temperatures, NOX release and reduction were relatively slow processes. As a result, a longer, lower-concentration rich dose favored increased cycle averaged NOX conversions. For example, extending the rich period from 5 to 15s at 250°C, while holding the overall reductant dose constant, resulted in an increase in cycle averaged NOX conversion from 59 to 87%. At high temperatures, the opposite was found to be true. Above 450°C, NOX release and reduction occurred very rapidly and shorter, higher concentration rich doses yielded significantly higher NOX conversions. For example, extending the rich period from 5 to 15s at 500°C, while holding the overall rich dose constant, resulted in a decrease in the cycle averaged NOX conversion from 76 to 54%. The selectivities to NH3 and N2O were found to be primarily a function of temperature, with both being higher at lower temperatures. The effect of cycle timing and reductant concentration were of secondary importance. In contrast, NH3 and N2O yields were significantly affected by the cycle timing since they depend on the NOX conversion. Therefore, any combination of changes in the lean/rich timing protocol or reductant concentrations that resulted in increased NOX conversion also resulted in increased NH3 and N2O yields for a given temperature. Thus, concerted control of NH3 generation by varying the lean/rich cycle timing was demonstrated for this LNT catalyst. At both lower and higher temperatures, variations in the rich cycle duration resulted in NH3/NOX ratios that could extend the region of operation for a close-coupled LNT-SCR system, with the greatest effect observed at temperatures between 250 and 450°C. However, N2O yield also increased with NH3 yield under the same conditions.