Highly selective CO sensor using stabilized zirconia and a couple of oxide electrodes

2006

Solid-state sensors were developed for coal combustion control and the understanding of sensing mechanisms was advanced. Several semiconducting metal oxides (p-type and n-type) were used to fabricate sensor electrodes. The adsorption/desorption characteristics and catalytic activities of these materials were measured with Temperature Programmed Desorption (TPD) and Temperature Programmed Reaction (TPR) experiments. The sensitivity, selectivity, and response time of these sensors were measured for steps of NO, NO 2 , CO, CO 2 , O 2 , and H 2 O vapor in simple N 2 -balanced and multi-component, simulated combustion-exhaust streams.The role of electrode microstructure and fabrication parameters on sensing performance was investigated. Proof for the proposed sensing mechanism, Differential Electrode Equilibria, was demonstrated by relating the sensing behavior (sensitivities and cross-sensitivities) of the various electrode materials to their gas adsorption/desorption behaviors and catalytic activities.A multifunctional sensor array consisting of three sensing electrodes and an integrated heater and temperature sensors was fabricated with tape-casting and screen-printing and its NO x sensing performance was measured. The multifunctional sensor demonstrated it was possible to measure NO 2 independent of NO by locally heating one of the sensing electrodes.The sensor technology was licensed to Fuel FX International, Inc. Fuel FX has obtained investor funding and is developing prototype sensors as a first step in their commercialization strategy for this technology.iv

Section: Differential Electrode Equilibriamentioning

confidence: 99%

Multifunctional (NOx/CO/O2) Solid-State Sensors For Coal Combustion Control

2006

“…The differential electrode equilibria, created between the gas molecules and the dissimilar sensor electrode surfaces, give rise to a potential gradient between the electrodes resulting in a voltage response [8]. [11,12]. Among the above oxides, WO 3 is of particular interest because it has been known to be sensitive to NO x without significant sensor signal drift.…”

Section: Differential Electrode Equilibriamentioning

confidence: 99%

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

2005

We have made great progress in both developing solid state sensors for coal combustion control and understanding the mechanism by which they operate. We have fabricated and tested numerous sensors and identified the role electrode microstructure plays in sensor response. We have developed both p-type (La 2 CuO 4 ) and n-type (WO 3 ) semiconducting NO x sensing electrodes. We have demonstrated their respective sensing behavior (sensitivities and cross-sensitivities), related this behavior to their gas adsorption/desorption behavior and catalytic activity, and in so doing verified that our proposed Differential Electrode Equilibria is a more comprehensive sensing mechanism.These investigations and their results are summarized below.The composition and microstructure of the sensing electrode is the key parameters that influence the sensing performance. We investigated the effect of electrode microstructure on the NO x sensitivity and response time using a La 2 CuO 4 -based potentiometric sensor.Temperature dependence, cross-sensitivity and selectivities 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.In order to optimize the sensor electrode microstructure, powders were prepared using four different powder synthesis routes, resulting in different particle size distributions and BET surface areas. Different sintering conditions were also applied. The microstructure of electrodes, synthesized with the same composition, has a dramatic effect on both sensitivity and response time of potentiometric NO sensors, showing that large surface areas generate a porous morphology with smaller grain size, and that smaller grain size results in a sharper response and faster response time.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 NO, 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 iv 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 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 mixtur...

“…We also used the TPD technique to determine the adsorption characteristics of O 2 FeO 3 , and ZnO, where both electrodes were on the same face, showed fast and stable response to NO 2 and CO [9,10]. Gas sensing properties of potentiometric tubular sensors based on semiconducting metal oxides such as WO 3 , SnO 2 , ZnO, CdO, MoO 3 , In 2 O 3 , etc., were also investigated [11,12]. Among the above oxides, WO 3 is of particular interest because it has been known to be sensitive to NO x without significant sensor signal drift.…”

Section: Differential Electrode Equilibriamentioning

confidence: 99%

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

2005

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).