Study of the electrical conductivities of the NO_x trap materials BaCO_3 and K_2C0_3/La-Al_20_3 during NO_x exposure as sensitive layers or for in-situ characterization of catalyst systems

摘要

Alkaline (earth-) carbonates are utilized in NO_x trap catalysts to reduce NO_x emissions in automotive exhausts given their ability to chemically accumulate and convert NO_2 gas to nitrates; reversible upon exposure to rich-burn gas atmospheres or at elevated temperatures. The dependency of their electrical properties on the NO_x loading state enables in-situ monitoring of catalyst systems and makes them suitable as NO_2 sensitive layers. The electrical and NO_2 sensing properties of pure BaCO_3 and K2C03 stabilized on La-Al_2O_3, both catalytically well-known NO_2 storage components, are compared. The resistance of both carbonates decreases in the presence of NO_2 while NO has no effect (missing oxidizing properties). Based on an analysis of time-dependent conductivity data, NO_2 uptake on finely dispersed BaCO_3 particles is found to be diffusion limited starting from about 170 s in NO_2. A similar diffusion limitation is observed for K_2CO_3 deposited on La-Al_2O_3 particles but without any measureable nucleation delay. While the conductivity of the BaCO_3 layer is orders of magnitude lower in both states than that of K_2CO_3/La-Al_2O_3, the maximal relative conductivity increase upon exposure to NO_2 and therefore the sensitivity was considerably higher for BaCO_3.