Influence of NOx Species and the Residual CO and C3H6 on NOx Storage Performance of NSR Catalysts at Low Temperatures

Abstract

As an important approach to improve the fuel economy of lean-burn engines, cutting down the thermal loss will lower the exhaust temperature. In response to this technical challenge, the enhancement of the low temperature performance turns out to be an imperative issue with the NOx storage and reduction (NSR) catalysts. Targeted at gaining technical information on upgrading practical NSR catalysts, NOx storage reactions over a model NSR catalyst under a series of lean-burn exhaust atmospheres and a corresponding thermodynamic analysis were conducted in this research. It was found that, below 400°C, the residual reducing agents of CO and C3H6 in the lean gas reduced NO2 to NO over the NSR catalyst; hence, the outlet NO2 concentration fell lower than the value derived merely from thermodynamic equilibrium with NO oxidation reaction. With CO and C3H6 existing in the reaction atmosphere, it did not matter if NO2 or NO was used as the inlet NOx species; the outlet NO2 concentration would turn out the same. Thus, the NOx storage amount with NO2 as an inlet NOx species is approximately the same as that with NO. If no CO and C3H6 are present in the reaction atmosphere, the outlet NO2 concentration equaled the thermodynamic equilibrium value with NO oxidation reaction, while an increase in the NOx storage amount with NO2 as an inlet NOx species had been observed. These results indicated that the enhancement of NO oxidation to NO2 and the removal of the residual reducing agents from the exhaust gas are both necessary for improving the NOx storage performance at low temperatures.