Component-Integrated NOx-sensors with Doped Semiconducting Metal-Oxide Electrodes for Control and Monitoring of Exhaust Emission Reduction

Kurzfassung

Catalytic convertors of after-treatment systems take up a challenging task to reduce emission effectively and reliably in exhausts of gasoline and diesel engines. Exhaust emission fluctuates during the driving period and this may indicate engine malfunction. The concentration of carbon monoxide in combustion gases can give an indication about the general state of this process. Real-time signals from other gases (e.g. NOx) would also be needed in order to establish a basis for more sophisticated control procedures. Catalytic convertors can help to detect such failure by means of embedded and OBD-integrated sensors. In this respect, semiconducting materials can be employed. Among those, the metal oxides such as SnO2 and TiO2 can detect NO/NO2 if doped with trivalent elements (e.g. Cr3+, Al3+, etc.). It is reported that the selectivity of TiO2-sensors towards NO2 in oxygen containing environment increases by doping with aluminium at operation temperatures as high as 500°C (1). Moreover, the incorporation of Cr up to 10 at% into TiO2 yields p-type conductivity resulting in sensor stability and increased selectivity towards NO2 (2). Increased sensor selectivity is important for NOx-detection from exhaust gases, since most of which are oxidising and influence the conductivity of the sensor material. Basing on these previous investigations, we prepared Cr- and Al-doped TiO2 and Al-doped SnO2 sensor electrodes by reactive sputtering. Sensor characteristics towards NO and NO2 were tested at the temperature range of 300-700°C, depending on the application and material. Doping of semiconducting metal oxides resulted in the alteration of microstructure, phase sequence and consequently in the sensing properties of the electrode material. Moreover, the function of the sensor in context of the NOx-reducing catalyst material was also evaluated. Considering the complex chemical reactions and thermal variations occurring along a catalytic convertor, employment of different types of the sensing materials in form of arrays was required. Accordingly, sensing electrodes were tested either for detection of high NOx-concentrations at high temperatures (> 500°C) or alternatively ultra-low NOx-concentrations (<2 ppm) at relatively lower temperatures (300-400°C). Sensing properties of the semi-conducting oxide electrodes were adjusted by tailoring the doping to yield the specific sensing abilities. Moreover, the function of the sensor in context of the catalytic material was also evaluated.