D4.2 - Intergrating NOx Gas Sensor: Concept, Sensitivity to NO/NO2 and Benefits of the Integrating Sensing Principle

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
D4 - Gas Sensors II
A. Geupel, R. Moos - Universität Bayreuth (Germany), D. Kubinski, J. Visser - Ford Research and Advanced Engineering, Dearborn, Michigan (USA)
578 - 582


On-Board-Diagnostics and emission regulations demand highly sensitive sensors for the detection of very low levels of NOx. For this purpose, the idea of an integrating NOx-sensor was developed. Via a lean NOx trap (LNT) NOx-molecules are accumulated in the sensitive layer and the total amount of NOx instead of the actual NOxconcentration, cNOx, is measured, promising an improved accuracy in the measurement of the accumulated NOx levels which is especially helpful for low ppm level emissions. In this presentation, the integrating sensing principle is explained and measurement data demonstrating the outstanding performance of this novel sensing concept are shown.

Concept: LNT materials contain precious metals like Pt to oxidize NO to NO2. The latter is successively chemically stored at the storage sites (e.g. BaCO3) in the form of nitrates. The material transformation is accompanied by changing electrical properties. Thus, recording the electrical properties of the sensitive layer enables the detection of the timely-integrated amount of NOx, ANOx. The relative resistance change |DeltaR/R0| is increasing in the presence of NOx but remains constant at 0 ppm NOx. Ideally, the characteristic line gives a linear correlation between ANOx and |DeltaR/R0|. Since the storage capacity is limited, regeneration (e.g. in the rich atmosphere) will be required once saturation effects lower the sensitivity.

Measurement results: The integrating properties of the LNT-coated samples were verified in various tests. |DeltaR/R0| increases during NOx exposure but remains constant at 0 ppm NOx (no desorption). Double cNOx results in double |DeltaR/R0| on the time-scale. The characteristic line gives a linear correlation between |DeltaR/R0| and ANOx independent on cNOx till saturation occurs. Regeneration with H2 is very effective to recover the integrating properties. Since the nitrate formation is controlled by the kinetics and thermodynamics, the sensitivity is also influenced by the temperature. At about 375°C, the presented integrating NO x sensor shows high sensitivity and sufficient nitrate stability. Due to the good catalytic properties of the LNT, NO is locally in equilibrium with NO2, and the sensitivity to NO emissions is the same as to NO2, which can be stored directly. The integrating sensing principle using a NOx storage material as sensitive layer shows great potential for the highly sensitive and accurate detection of very low levels of total NOx at 350°C in lean gas atmospheres.