The effect of SO2 on the sensitive layer of a NOx dosimeter

The accumulating-type or integrating-type NOx sensor works like a dosimeter being well suited for long-term total amount detection of low levels of NO and NO2 while information about the instantaneous NOx concentration is obtained from the signal derivative. A potassium-based lean NOx trap material, known from automotive catalysts, is applied as sensitive material. It accumulates NOx in the form of nitrates and thereby changes its conductivity with the NOx loading state. Since SO2 is known to poison the catalytic properties of LNTs, the effect of SO2 on the sensing characteristics is addressed now. The competition between SO2 and NO2 for the accessible storage sites was found to affect the sensor signal in two ways: due to sulfate formation, the resistance of the LNT material decreases in the presence of SO2 and the accumulating NOx sensing properties are strongly reduced. However, a high reversibility of sulfur poisoning was obtained by desulfurization of the sensitive device at 650 °C in H2 containing gas. The results reveal that with the presented setup, SO2 detection is possible with a linear correlation between the conductivity and the exposed SO2 amount. Since at 380 °C, the sensitivity to SO2 is about half compared to NOx, cross sensitivities need to be concerned. At higher temperatures nitrates are decomposed and the sensor is selective to SO2.