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The selective detection of CO gas by stannic oxide incorporated with ThO2(l − 10 wt%), in the presence of H2 and petroleum gases such as C3H8 and iso-C4H2 has been studied. Materials mixed with 5 wt% ThO2 showed high selectivity to CO gas at a sample temperature of 200−250‡C. The effects of hydrophobic or hydrophilic silica present in the samples on the detection sensitivity to CO gas have been investigated. From the results it is apparent that the removal of the hy-droxyl radical from the surface of SnO2 enhances the sensitivity to CO gas.

The temperature dependence of resistivities of gas sensors made of SnO2, Pd-doped SnO2, and ThO2-doped SnO2 with Pd has been investigated in air containing reducing gases such as CO, H2, and C3H8. The curves for ThO2-doped sensors were significantly influenced by the reducing gases as compared to the sensors without ThO2. From these results, it is found that in Pd-doped SnO2 sensors the dopant plays an important role in oxidizing the surface of SnO2 above 170°C, and that the addition of ThO2 to Pd-doped SnO2 enhances the effects of Pd by removing the adsorbed hydroxyl on SnO2. It is also apparent that the interactions between reducing gases in air and SnO2-based sensors depend upon the oxidizing rates of the surface of SnO2, as well as the amounts of the adsorbed hydroxyl on SnO2.