Dosimeter-Type NOx Sensing Properties of KMnO4 and Its Electrical Conductivity during Temperature Programmed Desorption

Abstract

An impedimetric NOx dosimeter based on the NOx sorption material KMnO4 is proposed. In addition to its application as a low level NOx dosimeter, KMnO4 shows potential as a precious metal free lean NOx trap material (LNT) for NOx storage catalysts (NSC) enabling electrical in-situ diagnostics. With this dosimeter, low levels of NO and NO2 exposure can be detected electrically as instantaneous values at 380 °C by progressive NOx accumulation in the KMnO4 based sensitive layer. The linear NOx sensing characteristics are recovered periodically by heating to 650 °C or switching to rich atmospheres. Further insight into the NOx sorption-dependent conductivity of the KMnO4-based material is obtained by the novel eTPD method that combines electrical characterization with classical temperature programmed desorption (TPD). The NOx loading amount increases proportionally to the NOx exposure time at sorption temperature. The cumulated NOx exposure, as well as the corresponding NOx loading state, can be detected linearly by electrical means in two modes: (1) time-continuously during the sorption interval including NOx concentration information from the signal derivative or (2) during the short-term thermal NOx release.

Overview on conductometric solid-state gas dosimeters

“…Dependent on the interaction between the sorbent and the analyte, different regeneration procedures are effective to recover the sorption capacity as well as the sensor signal. They include thermal decomposition at elevated temperatures (Katayama et al, 2004;Helwig et al, 2008;Kubinski and Visser, 2008;Brunet et al, 2008;Brandenburg et al, 2013;Groß et al, 2013a), chemical reactions, e.g., by a changed gas atmosphere (Helwig et al, 2007(Helwig et al, , 2008Groß et al, 2012a) or optical-induced desorption, e.g., by UV light (Li et al, 2003;Helwig et al, 2008).…”

“…the sensor is designed to provide an output that intrinsically depends on the history of the input quantities" (Angelini et al, 2009) and therefore the sensor response correlates with the cumulated exposure. Hence, the electrical detection of the analyte might occur either during the sorption or during the release period (Kubinski and Visser, 2008;Groß et al, 2013a). In the literature, this change in the accumulationlevel is detected mainly optically (Tanaka et al, 1999;Sasaki et al, 2001;Maruo, 2007) or gravimetrically (Matsuguchi et al, 2005).…”

“…Hence, the analysis of the sensor signal change during the regeneration interval is another method for operating gas dosimeters. It was demonstrated on a potassium and manganese containing NO x sensor that, after the sorption period under sorption conditions, the change of the electrical conductance during thermal decomposition of the formerly formed nitrates correlates with the preceding sorbed analyte amount (Groß et al, 2013a). A schematic drawing of this novel method is given in Fig.…”

“…In this operation mode, the gas dosimeter can be seen as passive sampler with an internal sensor function to determine the sampled analyte dose electrically instead of applying an external subsequent gas analysis in the laboratory. By combining these two operation modes (measurement during sorption and desorption), it is expected that the redundant information enables a plausibility consideration of the timeresolved sensor signal (Groß et al, 2013a).…”

“…2, the temperature strongly influences the gas sensing behavior (Reyes et al, 2006;Andringa et al, 2012;Shu et al, 2012;Brandenburg et al, 2013;Groß et al, 2013a). Since conventional gas sensors are mostly based on metal oxides as sensitive layer and these materials are known to be able to adsorb gases, e.g., SO 2 (Mathieu et al, 2013), they might be suitable for applications in a gas dosimeter under modified operation conditions.…”