Andrea Groß

Dual mode NOx sensor: measuring both the accumulated amount and instantaneous level at low concentrations.

The accumulating-type (or integrating-type) NOx sensor principle offers two operation modes to measure low levels of NOx: The direct signal gives the total amount dosed over a time interval and its derivative the instantaneous concentration. With a linear sensor response, no baseline drift, and both response times and recovery times in the range of the gas exchange time of the test bench (5 to 7 s), the integrating sensor is well suited to reliably detect low levels of NOx. Experimental results are presented demonstrating the sensor’s integrating properties for the total amount detection and its sensitivity to both NO and to NO2. We also show the correlation between the derivative of the sensor signal and the known gas concentration. The long-term detection of NOx in the sub-ppm range (e.g., for air quality measurements) is discussed. Additionally, a self-adaption of the measurement range taking advantage of the temperature dependency of the sensitivity is addressed.

The Effect of the Thickness of the Sensitive Layer on the Performance of the Accumulating NOx Sensor

A novel and promising method to measure low levels of NOx utilizes the accumulating sensor principle. During an integration cycle, incoming NOx molecules are stored in a sensitive layer based on an automotive lean NOx trap (LNT) material that changes its electrical resistivity proportional to the amount of stored NOx, making the sensor suitable for long-term detection of low levels of NOx. In this study, the influence of the thickness of the sensitive layer, prepared by multiple screen-printing, is investigated. All samples show good accumulating sensing properties for both NO and NO2. In accordance to a simplified model, the base resistance of the sensitive layer and the sensitivity to NOx decrease with increasing thickness. Contrarily, the sensor response time increases. The linear measurement range of all samples ends at a sensor response of about 30% resulting in an increase of the linearly detectable amount with the thickness. Hence, the variation of the thickness of the sensitive layer is a powerful tool to adapt the linear measurement range (proportional to the thickness) as well as the sensitivity (proportional to the inverse thickness) to the application requirements. Calculations combining the sensor model with the measurement results indicate that for operation in the linear range, about 3% of the LNT material is converted to nitrate.

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

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.

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

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.

5.2.2 The influence of SO2 and the thickness of the sensitive layer on the performance of the Integrating NOx Sensor

Due to its chemical accumulation abilities, the integrating NOx sensor is potentially well suited for low ppm-level sensing of NO and NO2. The sensitive layer consists of a potassium-based automotive lean NOx trap (LNT) catalyst storing NOx chemically by forming nitrates. The accumulative sensing principle and the NOx concentration detection properties have been published recently. Now, two factors influencing the sensing performance are addressed: Sulfur poisoning and the thickness of the sensitive layer. The measurement results reveal that the competition between SO2 and NO2 for the available storage sites, known from LNT catalysts, affects the sensor signal in two ways: First, the resistance of the sensitive layer (even without NOx) decreases in the presence of SO2 (sulfate formation), allowing for integrative SO2 detection. Secondly, the linear NOx measurement range decreases due to a diminished NOx storage capacity upon SO2 blocking the storage sites. A high reversibility of sulfur poisoning was obtained by desulfation at 650 °C in H2 containing gas. The resistance in the unloaded state was found to correlate with the inverse thickness of the LNT layer. More relevant, the thickness was found to highly influence the sensitivity and the linear measurement range of the integrating NOx sensor. Therefore, variations in the thickness are an effective tool to adapt the sensor performance (sensitivity and linear measurement range) to the application requirements without loosing the benefits of the integrating sensing principle.