Development of High Accuracy NOx sensor 2019-01-0749
For the purpose of coping with the strengthening of NOx exhaust gas control, vehicle manufacturers are developing high-performance SCR（Selective catalytic Reduction）system. For such systems to be realized, high accuracy NOx sensor is indispensable. In SAE 2018, it is reported that the technology (residual O2 and electronic conduction are cancelled by three-electrode structure) is established (SAE Technical Paper 2018-01-0334). To realize higher level accuracy urea injection control in high NOx concentration, in addition to this conventional technology, NOx detection electrode technology has been developed. Detection of NOx concentration by means of a NOx sensor is generally performed follows: O2 is discharged by means of an O2 detection electrode; remaining NOx is decomposed by a NOx detection electrode; NOx concentration is then detected as electric current through solid electrolyte. When NOx is decomposed at O2 detection electrode, NOx which NOx detection electrode can detect decreases, and accuracy becomes worse. So, the O2 detection electrode composition is generally Pt/Au alloy which is hard to react NOx, and NOx detection electrode composition is generally Pt/Rh alloy which can easily detect NOx. Besides, O2 and NOx detection electrodes are located in same space to realize smooth gas flow. However, when Au in O2 detection electrode is evaporated and reached to NOx detection electrode during sensor operation, the activation area to detect NOx decreases, so accuracy becomes worse (This is Au poisoning). It is clarified that the mechanism about Au poisoning and that Rh oxides covered partially on NOx detection electrode surface can prevent poisoning by Au from O2 detection electrode. Due to these insights, the suitable ratio of Pt/Rh to realize high accuracy and reliability is clarified, and a NOx sensor with high accuracy detection electrode in high NOx concentration after durability has been developed.
YUSUKE KAWAMOTO, Yusuke Todo, Hironobu Shimokawa, Keiichiro Aoki, Masashi Kawai, Koji Ide
DENSO Corporation, SOKEN Inc., Toyota Motor Corp