Review on Radio Frequency Based Monitoring of SCR and Three Way Catalysts

Knowledge of the actual catalyst state plays a key role in automotive exhaust gas aftertreatment. The oxygen loading degree of three-way catalysts (TWC), the amount of stored ammonia in selective reduction catalysts (SCR), or the NOx loading degree in NOx storage catalysts (NSC) are important parameters. Today, they are determined indirectly and/or model-based, applying models that are typically calibrated by gas sensors installed up- and/or downstream of the catalysts. A novel approach to determine directly the catalyst state by microwaves (radio frequencies, rf) emerged recently. For this method, the catalyst housing serves as an electrical cavity resonator. As “sensor”, one or two simple antennas are mounted in the canning. The electrical properties of the honeycomb incl. coating change with gas loading, affecting either the resonance frequencies or the power transmission. Such contactless-obtained information is strongly correlated with the catalyst state as will be discussed here for TWC and SCR systems. This contribution reviews the progress in the past 3 years that exceeds by far the status of initial studies.