Microporous zeolite modi®ed yttria stabilized zirconia YSZ) sensors for nitric oxide NO) determination in harsh environments Nicholas F. Szabo, Hongbin Du, Sheikh A. Akbar, Ahmed Soliman 1 , Prabir K. Dutta * Center for Industrial Sensors and Measurements, The Ohio State University, 2041 College Road, Columbus, OH 43210, USA Received 30 July 2001; received in revised form 25 October 2001; accepted 31 October 2001 Abstract This study is focused on development of a nitrogen monoxide NO) sensor capable of operation in harsh environments, as exempli®ed by automotive exhaust streams. The basis of the sensor is a mixed potential response generated by exposure of gases to a platinum±yttria stabilized zirconia Pt±YSZ) interface. The asymmetry between the two Pt electrodes on YSZ is generated by covering one of the electrodes with a zeolite, which helps promote the NO/NO 2 equilibrium prior to the gases reaching the electrochemically active interface. The mixed potential generated is logarithmically related to NO concentration 0±1000 ppm) at temperatures between 500 and 700 8C, the optimum temperature being 500 8C. The microporosity of the zeolite makes it permeable to oxygen, thus, minimizing the interference to O 2 . The sensor shows interference from CO and NO 2 . Three sensor designs have been examined, including a planar design that can be packaged appropriately for surviving automotive exhaust streams. Automotive tests indicate that the sensor is capable of detecting NO x in engine exhausts. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Zeolite Y; Automotive sensor; Sensor design 1. Introduction Processes that involve combustion of fossil fuels produce various emission gases such as nitrogen oxides NO x ), carbon monoxide CO), carbon dioxide CO 2 ), hydrocar- bons C n H 2n2 , referred to as HC), and sulfur oxides SO x ) [1]. These combustion sources can include gas heating furnaces, power plants, and internal combustion engines. Other anthropogenic sources of these gases can arise from non-combustion sources, an example being formation of NO x in chemical plants during nitric acid production or chemical nitration with subsequent venting into the atmo- sphere via stacks [2]. NO x includes several nitrogen oxide species, N 2 O, NO, N 2 O 3 and NO 2 , with nitrogen in different oxidation states and their impact on the environment and health has been well documented [3,4]. Tighter federal regulations for automotive and power plant emissions have been instituted. To control these gas emissions, monitoring is essential. Various instrumental methods, such as infrared IR) spectroscopy, chemiluminescence detection, and gas chromatography have been used to measure NO x species, but often have the disadvantage of large size and cost [5±7]. The use of high-temperature solid state gas sensors to monitor emissions in combustion related processes show great pro- mise due to small size, lower cost, and real-time measure- ment capabilities [8]. Of particular interest to this study is the class of sensors based on mixed potential measurements [9,10]. If several electrochemical reactions occur simultaneously on an elec- trode, the rates of these individual reactions determines the overall electrode potential and is referred to as the mixed potential or corrosion potential in the corrosion literature). Often, a mixed potential sensor is composed of a solid electrolyte that is an oxygen ion conductor such as yttria stabilized zirconia YSZ), and reactions involve oxidation/ reduction of the sensing gas. For mixed potential systems, the voltage does not obey a Nernstian response with con- centration of O 2 , indicating a kinetic-based origin of the mixed potential. Bruser et al. presented one of the earliest papers on NO x determination with asymmetric electrodes Pt and perovskites) on YSZ [11]. Miura et al. have exam- ined various strategies for NO x sensing using zirconia based mixed potential sensors. Their focus was to evaluate a series of metal oxide electrodes and found that several spinel-type oxides, including CdCr 2 O 4 and CdMn 2 O 4 exhibited high NO x sensitivity [12±14]. Sensors and Actuators B 82 2002) 142±149 * Corresponding author. Present address: Department of Chemistry, The Ohio State University, 120 W 18th Avenue, Columbus, Ohio 43210, USA. E-mail address: dutta.1@osu.edu P.K. Dutta). 1 Center for Automotive Research, The Ohio State University, 120 W 18th Avenue, Columbus, Ohio 43210, USA. 0925-4005/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0925-400501)00999-6