The electrical properties of NOx-storing carbonates during NOx exposure

In order to gain a deeper understanding of the different components in multi-component automotive catalyst systems and integrating type NOx-sensors, in-situ electrical methods are examined. The conductivity of BaCO3 and K2CO3, key components in automotive NOx storage catalysts, was examined as a function of gas composition and temperature. Impedance measurements performed on BaCO3, K2CO3, and equimolar mixtures of both, demonstrated that, in all cases, the resistance decreases in the presence of NO2 gas with progressive NOx sorption on the carbonate surface. Mixing the hygroscopic but low resistivity K2CO3 with the very highly resistive but stable BaCO3 resulted in greater stability than K2CO3 alone, as well as improved ability to measure resistivity changes. The resistance of the mixture recovered about 50% of its value after removing NO2 from the gas stream, believed to result from migration of sorbed NOx from K to Ba, instead of desorption alone. These results on NOx trap materials show that in-situ impedance spectroscopy measurements offer real-time characterization of NOx loading levels in lean NOx trap catalyst components.