Abstract
Mixed potential sensors were fabriated using yttria-stabilized zirconia (YSZ) as a solid electrolyte and a mixture of Au and various metal oxides as a sensing electrode. The effects of calcination temperature ranging from 600 to 1,000 °C and acid-base properties of the metal oxides on the sensing properties were examined. The selective sensing of ammonia was achieved by modification of the sensing electrode using MoO3, Bi2O3 and V2O5, while the use of WO3, Nb2O5 and MgO was not effective. The melting points of the former group were below 820 °C, while those of the latter group were higher than 1,000 °C. Among the former group, the selective sensing of ammonia was strongly dependent on the calcination temperature, which was optimum around melting point of the corresponding metal oxides. The good spreading of the metal oxides on the electrode is suggested to be one of the important factors. In the former group, the relative response of ammonia to propene was in the order of MoO3 > Bi2O3 > V2O5, which agreed well with the acidity of the metal oxides. The importance of the acidic properties of metal oxides for ammonia sensing was clarified.
Highlights
The Uera-SCR (Selective Catalytic Reduction) technique is known to be an effective technology for the removal of nitrogen oxide (NOx) emissions from heavy-duty diesel engine cars [1,2,3,4]
As we have reported in a previous paper [17], the electronegativity can be used as an indicator of the acid-base properties of metal oxides: the higher electronegativity implies higher acidity of the oxides
The effect of calcination temperature and acid-base properties of metal oxides used as a sensing material for an ammonia sensor have been investigated
Summary
The Uera-SCR (Selective Catalytic Reduction) technique is known to be an effective technology for the removal of nitrogen oxide (NOx) emissions from heavy-duty diesel engine cars [1,2,3,4]. Semiconductors of n-type metal oxides such as WO3 [11], MoO3 [12,13,14,15], V2O5 [16,17], SnO2 [18,19], TiO2 [20], In2O3 [21,22,23] and Ru/ZnO [24] have high hydrothermal stability, and have been extensively investigated as sensing materials They usually act at lower temperatures (below 300 °C) than those needed in the automobile industry, but show low cross-sensitivity to NH3 in the presence of various interfering gases. It is highly desirable to develop thermally stable ammonia sensors which show high cross-sensitivity to NH3 at high temperatures
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