Abstract
New composite effects of Au and La 2O 3 and/or Y 2O 3 have been examined. (1) The La 2O 3 addition effect: characterization of the tin oxide surface involving a stabilization of the surface oxygen (e.g., O 2−) and a decrease in the number and/or the strength of Lewis acid sites, (e.g., Sn 4+), which are supposed to be active sites of adsorption or oxidation, resulting in a reduction of the sensitivity of CO, H 2, CH 4 and i-C 4H 10. (2) The Au addition effect: a selective sensitization (activation) to CO; other precious metals or metal oxides increase the sensitivity not only to CO, but also to H 2 and iso-C 4H 10. The special effect of Au is due to its selective adsorption for CO and the absence of active oxygens on the surface of the Au grains. The present surface chemical reactions causing the sensitivity to CO are supposed to occur in the grain boundary consisting of the three grains of tin oxides, lanthanum oxide and gold. The above composite effects are thus able to cause successfully the activation to CO inherent in gold at a comparatively high temperature (e.g., 360°C), and will make it possible to detect CO selectively and stably without periodic purging at high temperature.
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