Therefore, the operating temperature was fixed at 350 8C in the following experiments. The sensitivity was lowered at high temperature above 350 C, which is consistent with Mcaleer and other’s reports that while the sensitivity increases as temperature increases since the activity of device surface increases, it begins to decrease above the threshold temperature because the desorption and re-absorption of oxygen absorbed species by the reducing gas occur very fast and all of the gas reacts on the surface to block the gas infiltrated into the device.