Combined Dielectric Spectroscopy and Operando DRIFTS Analysis of Ba-Based NO_x Storage Materials for...

This study investigates the dielectric behavior and NO_x storage properties of Pt/Ba-Al₂O₃ NO_x storage materials using microwave cavity perturbation, operando DRIFTS, and impedance spectroscopy with respect to their applicability in a radio-frequency-based NO_x dosimeter-type sensor. Dielectric losses (ε″) are identified as the most sensitive indicator of NO_x storage, exhibiting a clear linear correlation with both the accumulated NO_x dose and the utilization of Ba storage sites. Approximately 35% of the available Ba sites participate in nitrite and nitrate formation, and the absolute dielectric loss response increases proportionally with the Ba content of the NOx storage catalyst. In contrast, the permittivity (ε') shows only minor changes, which are mainly influenced by temperature. Temperature-dependent experiments reveal stable NO_x storage with negligible desorption up to 350 °C, whereas pronounced desorption processes at 400 °C significantly limit the linear dosimeter behavior. Operando DRIFTS measurements on Pt/Ba-Al₂O₃ functional films confirm temperature-dependent formation of nitrites and nitrates, with nitrates dominating the NO_x storage at elevated temperatures. Capacitance measurements show a slight increase during NO_x storage, indicating a moderate increase in permittivity. Overall, Pt/Ba-Al₂O₃ NO_x storage materials exhibit a robust, quantitatively interpretable dielectric response that is well suited for radio-frequency-based, dosimeter-type NO_x sensing.