Combined Dielectric Spectroscopy and Operando DRIFTS Analysis of Ba-Based NOx Storage Materials for Radio-Frequency-Based NOx Dosimeters

This study investigates the dielectric behavior and NOx storage properties of Pt/Ba–Al2O3 NOx storage materials using microwave cavity perturbation, operando DRIFTS, and impedance spectroscopy with respect to their applicability in a radio-frequency-based NOx dosimeter-type sensor. Dielectric losses (ε″) are identified as the most sensitive indicator of NOx storage, exhibiting a clear linear correlation with both the accumulated NOx 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 NOx 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 NOx storage at elevated temperatures. Capacitance measurements show a slight increase during NOx storage, indicating a moderate increase in permittivity. Overall, Pt/Ba–Al2O3 NOx storage materials exhibit a robust, quantitatively interpretable dielectric response that is well suited for radio-frequency-based, dosimeter-type NOx sensing.