MULTIFUNCTIONAL (NOx/CO/O2) SOLID-STATE SENSORS FOR COAL COMBUSTION CONTROL

Abstract

Sensing properties of a La 2 CuO 4 -and WO 3 -based potentiometric NO x sensor were investigated both in N 2 and in a simulated exhaust gas. We performed temperature programmed reaction (TPR) and desorption (TPD) experiments to determine the reaction and adsorption characteristics of O 2 , NO x , CO, CO 2 , and their mixtures on the electrodes, and related the results to sensor performance.The relative responses of the La 2 CuO 4 -based sensor under varied concentrations of NO, NO 2 , CO, CO 2 and O 2 were studied. The results showed a very high sensitivity to CO and NO 2 at 450 ºC in 3 % O 2 , whereas the response to O 2 and CO 2 gases was negligible. The NO response at 400 -500 ºC agreed with the NO adsorption behavior. The high NO 2 sensitivity at 450 ºC was probably related to heterogeneous catalytic activity of La 2 CuO 4 .The adsorption of NO was not affected by the change of O 2 concentration and thus the sensor showed selective detection of NO over O 2 . However, the NO sensitivity was strongly influenced by the existence of CO, H 2 O, NO 2 , and CO 2 , as the adsorption behavior of NO was influenced by these gases.The WO 3 -based sensor was able to selectively detect NO in the presence of CO 2 in 3 % O 2 and at 650 ºC. The NO sensitivity, however, was affected by the variation of the NO 2 , CO, and H 2 O concentration. No gas-solid reactions were observed using TPR in the NOcontaining gas mixture, indicating that the NO response was not obtained by the conventionally accepted mixed-potential mechanism. At the same condition the sensor had high sensitivity to ~ 10 ppm NO 2 and selectivity in the presence of CO, CO 2 , and H 2 O, showing it to be applicable to the monitoring of NO 2 . Significantly different sensing properties of NO in simulated exhaust gas suggested the occurrence of gas composition change by the gas-phase and gas-solid reactions, and strong adsorption of water on the electrodes. The NO 2 sensitivity in simulated exhaust gas was modified by O 2 and H 2 O, but not by CO and CO 2 .A positive voltage response was obtained for NO 2 but negative for NO at 650 ºC with the n-type semiconducting WO 3 -based sensor. In contrast the opposite response direction for NO x was observed at 450 ºC with the La 2 CuO 4 (p-type semiconductor).