In order to improve the sensitivity to NO in air, WO3 based sensor elements were loaded with a small amount (0.004 wt%) of noble metals. Among the various noble metals added, Au and Ru were found to be effective promoters. The addition of Au enhanced the sensitivity to 200 ppm NO in air by about one order of magnitude, making the Au-loaded element attached with Pt electrodes the most sensitive to NO in the temperature range (200∼500°C). Ru-WO3 element also exhibited high NO sensitivity at 250 and 300°C, but the promoting effect of Ru quickly decreased as temperature increased. Such promoting effects were hardly consistent with the catalytic activity of the noble metals for the oxidation of NO to NO2, which were in the order Pt>Ru>Au=0. TPD experiments provided evidence supporting that the enhancement of the oxidative adsorption of NO as NO2 would be a major reason for the promoting effects of Au and Ru.
