A high temperature amperometric NO sensor based on stabilized zirconia and CdCr2O4 electrode

Abstract

The emission of nitrogen oxides (NO and NO2 abbreviated as NOx) from automobiles and boilers is one of the main sources of atmospheric pollution. Generally speaking, combustion exhausts, as emitted, usually contain ten times as much NO as NO2, so that, for emission control, the sensitive and selective detection of NO in high-temperature combustion exhausts is more important than that of NO2. Several solid-state sensors to detect NO2 and/or NO have been reported [1±8], but these have problems in applications to emission control. For example, potentiometric or amperometric sensors using NASICON (Na-superionic conductor) and nitrite auxiliary phase [2±5] cannot be operated at temperatures above 250 °C because of the low melting point of the nitrites used (e.g., NaNO2: 271 °C). We recently proposed a new type of potentiometric sensor using a stabilized zirconia and oxide electrode (typically CdCr2O4). The devices are based on a sensing mechanism involving mixed potential at the oxide sensing electrode, and detect NO or NO2 in oxygen containing atmospheres at higher temperature, for example, 500±600 °C [6±8]. Unfortunately, however, one problem is that the sensitivity to NO is far smaller than that to NO2, a problem often encountered with potentiometric devices. A solution was suggested by recent reports that zirconia oxygen pumps may be used for the amperometric detection of NO in oxygen containing atmospheres at high temperature [9±12]. The sensor developed consisted of two serial oxygen pumps, which pumped out gaseous oxygen (®rst) and measured the limiting current due to the electrochemical decomposition of NO (second), respectively. This stimulated us to test the possibility of extending our device containing stabilized zirconia and the CdCr2O4 electrode to an amperometric NO sensor. As a result, this extension has been found to be possible by polarizing the oxide electrode appropriately. The amperometric sensor obtained can detect NO selectively in the presence of oxygen at 500 °C. For this sensor, it is not necessary to pump out the coexisting oxygen beforehand, as described below. 2. Experimental details