Radio Frequency-Based Determination of the Oxygen and the NO_x Storage Level of NO_x Storage Catalysts

In recent years, research on radio frequency-based (also known as “microwave-based”) catalyst state diagnosis for diesel engines has focused mainly on SCR systems. To increase the efficiency of exhaust aftertreatment systems, SCR catalysts can be combined with NO_x storage catalysts (NSC). For optimum performance, the degree of loading of the storage components has to be monitored. Up to now, in commercial application this has been done indirectly by using mathematical models. In this paper, the opportunities of a simple radio frequency (RF) based method for a direct determination are highlighted. Therefore, a commercial NSC was investigated between 210 and 390 °C. A linear dependence between the RF signals and the stored amount of NO_x was found. Furthermore, the NO_x and oxygen gas concentrations do not directly affect the RF parameters, but they have an indirect influence, since the stored NO_x amount is NO_x concentration dependent. By evaluating multiple RF signals, which are based on different material effects, a separate determination of the oxygen and NO_x storage level could be realized.