On the influence of the NOx equilibrium reaction on mixed potential sensor signals: A comparison between FE modelling and experimental data

The detection of NOx in automotive exhaust gas is a challenge for electrochemical gas sensors. In most cases, measuring systems are unsuitable to distinguish between NO and NO2. Mixed potential sensors can theoretically determine with sufficient selectivity NO or NO2. One issue to be considered is that heterogeneous catalysis brings the NOx mixtures partly into the thermodynamic equilibrium. Therefore, the influence of this NOx conversion is investigated in this paper. For this purpose, an FE model for mixed potential sensors is used to verify whether the NOx equilibrium must be taken into account in order to simulate the sensor behavior in the measurements. It is explained how the necessary parameters for the model are quantified. It is shown that the sensor behavior can be reproduced without considering the equilibrium if NO or NO2 is dosed alone. In the NO/NO2 mixture, on the other hand, it is no longer possible for a model without NOx equilibrium to agree with the experimental data. Only when considering the heterogeneously catalyzed NOx conversion, it is possible to explain numerically the measured data with astonishing agreement. Furthermore, the resulting data set can be used for other electrode geometries without having to characterize them electrochemically again.