NO2/NO response of Cr2O3- and SnO2-based potentiometric sensors and temperature-programmed reaction evaluation of the sensor elements

The NO2 and NO sensing behavior of a MOx/YSZ/Pt (MOx = Cr2O3 and SnO2) potentiometric sensor in 1–21% O2 at 450–700 °C are described and their sensing mechanism is discussed based on temperature-programmed reaction (TPR) results. TPR results show a lower NO2 decomposition activity of MOx compared to Pt, resulting in a higher local NO2 concentration at the MOx/YSZ/gas triple phase boundary (TPB) than at the Pt/YSZ/gas TPB at the sensor operating temperature. The higher NO2 concentration at the MOx/YSZ/gas TPB provides for an increased cathodic reaction at this electrode as compared to the Pt electrode, resulting in a positive sensor emf. Selective NO2 detection is achieved with both MOx electrodes against CO and CO2 and is consistent with a CO- and CO2-independent NO2 profile in the TPR results. In contrast, the Cr2O3 sensor is not NO selective against CO. The NO + CO + O2 TPR profile showing no gas concentration change to 800 °C indicates that the CO-induced change in the NO sensitivity is non-catalytic in nature.