ZIRCONIA-BASED GAS SENSORS USING OXIDE SENSING ELECTRODE FOR MONITORING NOx IN CAR EXHAUST

Solid-state electrochemical sensors using yttria-stabilized zirconia (YSZ) and oxide sensing electrode (SE) were fabricated and examined for NOx detection at high temperatures. Among various single-oxide SEs examined, NiQSE for the mixed-potential-type NOx sensor was found to exhibit rather high sensitivity to N@ even in the high temperature range of 800900°C. This sensor showed quicker response and recovery transients in the presence of water vapor, compared with that in the dry sample gas. The sensing mechanism of this type of sensor was discussed on the basis of the catalytic activities to the electrochemical and nonelectrochemical reactions. It was also shown that the new-type complex-impedance-based (impedancemetric) NOx sensor attached with ZnCrzOdjE exhibit good sensing characteristics to NOx at 70OOC. Furthermore. the sensitivity to NO was almost equal to that to NO2 in the concentration range from 0 to ca. 200 ppm at 700°C. A linear dependence was observed between the sensitivity of the impedancemetric sensor and the concentration of NOx even in the presence of 8 vol. % H20 and 15 vol.% C02. The planar laminated-type structure for the impedancemeteric NOx sensor was proposed for protecting NOx sensitivity from the influences of the co-existing combustible gases as well as the change in oxygen concentration in exhaust gas. lNTRODUCTION Recently, the demand for reliable, compact, low-cost solid-state sensors, which are capable of detecting nitrogen oxides (NOx) in different application, has been enhanced substantially. This demand has been driven by strong recent legislation in EU, USA and Japan. For example, in UK under Air Quality Regulations (1997) for NOx, standards of 150 ppb (hourly maximum) and 21 ppb (annual average) must be achieved by the end of 2005.' On the other hand, according to a new report by Fredonia Group, US demand for sensor products (including sensors, txansducers and associated housing) is projected to increase 7.8% annually to $13.8 billion in 2008: For monitoring in the automotive exhausts, the sensor should be. able to detect NO, concentration from 10 ppm up to 2000 ppm in very harsh environment where the temperature constantly fluctuates from 600°C up to 900°C. since the temperature of engine may occasionally reach up to 900°C. during vehicle acceleration. It is therefore vital to investigate thoroughly the SE materials of the NOx sensor in order to provide high NOx sensitivity and selectivity. long-term stability at high temperatures as well as fast response and recovery for a practical sensor. To the extent authorhj under the laws of the United States of America. nll copyright intcresta in thir publication arc (hc property of The A m e r i c ~ Cenmic Society. Any duplication. reproduction. M republication of this publication or MY parl thereof. without the express written consent of nK Amaic~ Ceramic Society or fee paid to the copy^@^ ClunnoC h e r . is prohibited. Advances in Electronic Ceramic Materials Editors-Sheng Yao ,Bruce Tuttle ,Clive Randall ,Dwight Viehland Co~vriahto2005. The American Ceramic Societv Last decade. an ultra leiin-bum (or direct-injection type) engine system has been developed to improve fuel efficiency as well as to reduce C02 and NOx emissions from engine. In this engine system, newlydeveloped NOx-storage catalyst should be used for compensating the low NOx-removal ability of the conventional three-way catalyst under the lean-bum (air rich) condition, as shown in Fig. I . I t is important, therefore, to hdve hiph-performance NOx sensors installed ~i,,. 1 catalytic system equipped with NOx sensors for the exhaust gas emitted from a new-type car engine. 3 at the ,in; after (or both before and after) the NOx-storage catalyst for such a system. The mixed-potential-type NOx sensors based on YSZ and oxide SE have been receiving considerable attention among the others YSZ-based NOx sensors.4 as potential candidates for practical sensor measuring car emissions. For example, last few years the most of the research groups have focused on the development of oxide SEs which are capable of working at high temperatures in car exhaust.'" The use of oxide SE in this type of NOx sensor was found to be very effective for sensitive and selective NOx measurement at hi h temperature." However, there are only a few designs of the NOx sensors reported to date"-"that can monitor total NOx (NO + N02) at high temperatures regardless of NOflO ratio in real exhausts. One of these sensors is the complex-impedance-based NOx sensor originally designed and developed by our group."*16 In addition, the most of results for oxide SEs published so far have revealed that the NOx sensors using the oxide SEs show relatively good sensing characteristics only in the temperature range of 450-700°C. The majority of these sensors have difficulty to operate at temperatures higher than 7 W C . Such a higher limitation of operating temperature is caused by the substantial decrease in the NOx sensitivity with increasing temperature. Based on the above-mentioned facts and keeping it in mind that there are no commercial high-temperature NOx sensors available on the market at the moment, further search for oxide SE has been done. As a result, it was found quite recently that NiO is a very effective SE for measuring NOx concentrations at temperatures higher than 800°C."~ In There were only limited number of publications reporting the properties and sensing characteristics of oxide SE that can measure NOx concentration at temperatures over 800°C.178 In Furthermore, we have also discovered that the new complex-impedance-type NOx sensor using ZnCrz04-SE shows rather good sensing characteristics for measurement of total NOx concentration at temperature as high as 700OC. Thus, we report here the sensing properties and sensing mechanism of the mixed-potential-type YSZ-based NOx sensor attached with NiO-SE at temperatures of 800-900°C as well as the main sensing performances of the new complex-impedance-type NOx sensor using ZnCr204SE. EXPERIMENTAL A tubular NOx sensor was fabricated by using a commercial one-closed-end YSZ tube (X wt.% Y203-doped, NKT). The tube is 300 mm in length and 5 and 8 mm in inner and outer diameters, respectively. NiO-powder paste was applied on the outer surface of the YSZ tube and sintered at 1400°C to form a sensing electrode (SE). A Pt lead wire was wound around the oxide layer to make a good electrical contact. Pt paste was painted onto the inside of the YSZ