In an effort to develop a semiconductor type NH
3 sensor, the NH
3 sensing properties of WO
3 semiconductors loaded with colloidal noble metals were investigated. Among the various noble metals tested, Au was proven to be the most excellent promoter for the WO
3-based NH
3 sensor, whereas Pt gave a very high NH
3 sensitivity at 200°C, but the response was too slow. Thus Au-WO
3 element could respond to NH
3 in air over a wide NH
3 concentration range of 5 ppb-50 ppm at 450°C, showing excellent characteristics in both NH
3 sensitivity and response rate. An optimum Au loading amount was shown to be 0.8 wt % with respect to the NH
3 sensitivity. The electrical resistance of Au-WO
3 element in air increased with an increase in Au loading. From the measure-ments of Seebeck coefficients it was revealed that the Au particles loaded interact with WO
3 and take the conduction electrons away when exposed to air, while those electrons are given back to WO
3 on exposure to NH
3-containing atmospheres. It is concluded that such an electronic interac-tion between Au and WO
3, susceptible to a change in atmosphere, is responsible for the prominent promoter action of Au.
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