Study of the electrical conductivities of the NOx trap materials BaCO3 and K2CO3/La-Al2O3 during NOx exposure as sensitive layers or for in-situ characterization of catalyst systems

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Groß, Andrea ; Weller, Tina ; Tuller, Harry L. ; Moos, Ralf:
Study of the electrical conductivities of the NOx trap materials BaCO3 and K2CO3/La-Al2O3 during NOx exposure as sensitive layers or for in-situ characterization of catalyst systems.
In: 14th International Meeting on Chemical Sensors : IMCS 2012. - Wunstorf : AMA Verband für Sensorik und Messtechnik e.V. , 2012 . - S. 1664-1667
ISBN 978-3-9813484-2-2
DOI: https://doi.org/10.5162/IMCS2012/P2.8.6

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Abstract

Alkaline (earth-) carbonates are utilized in NOx trap catalysts to reduce NOx emissions in automotive exhausts given their ability to chemically accumulate and convert NO2 gas to nitrates; reversible upon exposure to rich-burn gas atmospheres or at elevated temperatures. The dependency of their electrical properties on the NOx loading state enables in-situ monitoring of catalyst systems and makes them suitable as NO2 sensitive layers. The electrical and NO2 sensing properties of pure BaCO3 and K2CO3 stabilized on La-Al2O3, both catalytically well-known NO2 storage components, are compared. The resistance of both carbonates decreases in the presence of NO2 while NO has no effect (missing oxidizing properties). Based on an analysis of time-dependent conductivity data, NO2 uptake on finely dispersed BaCO3 particles is found to be diffusion limited starting from about 170 s in NO2. A similar diffusion limitation is observed for K2CO3 deposited on La-Al2O3 particles but without any measureable nucleation delay. While the conductivity of the BaCO3 layer is orders of magnitude lower in both states than that of K2CO3/La-Al2O3, the maximal relative conductivity increase upon exposure to NO2 and therefore the sensitivity was considerably higher for BaCO3.