Electrical conductivity study of NOx trap materials BaCO3 and K2CO3/La-Al2O3 during NOx exposure

Alkaline (earth-) carbonates are applied in NOx storage and reduction catalysts, given their ability to accumulate NO2 in excess oxygen environments, and releasing it under rich-burn conditions. The dependency of their conductivity on the NOx loading state makes them suitable as NO2 sensitive layers and enables the in situ characterization of the status of such catalyst systems. The conductivity of the NOx trap components BaCO3 and K2CO3, with different morphologies, is analyzed in various NOx containing atmospheres by impedance spectroscopy. While the stepwise uptake of NO2 can be detected electrically on both carbonates, only a weak corresponding influence of NO is detected. The increase in the conductivity of the carbonates during NO2 accumulation can be described as a diffusion controlled process with a characteristic nucleation delay in the case of BaCO3. Arrhenius-like plots comparing the conductivities of the carbonates in the unloaded and NO2 saturated states from 250 to 600 °C reveal that the conductivity of BaCO3 is more sensitive to NO2 than those of K2CO3/La-Al2O3, but the latter exhibiting about five orders of magnitude higher conductivity.