A capacitive type NO sensor which is operable at high temperature and under low oxygen partial pressure has been investigated in this study for monitoring NO in exhaust gases from gasoline engine vehicles. It was found that a mixed oxide of SrSnO3 and WO3 exhibited the high sensitivity at temperatures higher than 773 K. However, the SrSnO3-WO3 capacitor required a long period to response and recovery even at temperatures higher than 773 K. Improvement of the response characteristics was further studied and it became clear that addition of Pt was effective for improving the response and recovery behavior. In particular, loading Pt to WO3, followed by mixing with SrSnO3, was effective for improving the response characteristics without decreasing the sensitivity at high temperatures. On the mixed oxide of SrSnO3-(Pt/WO3), the 90% response and recovery to 100 ppm NO were attained within 6.5 and 1.0 min, respectively. Furthermore, the capacitance of SrSnO3-(Pt/WO3) increased with increasing NO concentrations in the range of 10 to a few 100 ppm. In addition, SrSnO3-(Pt/WO3) was less sensitive to H2O, CO, CO2 and NO2 than that to NO. Therefore, the mixed oxide capacitor of SrSnO3-(Pt/WO3) was usable for monitoring NO at high temperature and low oxygen partial pressure. Effects of Pt addition on the equivalent circuit of the sensor element was further investigated with the impedance measurement. Addition of Pt increased the interface resistance of the mixed oxide. However, the electrical equivalent circuit was not changed by the Pt doping. Therefore, improved response characteristics seem to be assigned to acceleration in the surface nitration reaction by gaseous NO with the Pt addition.
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