Nitrogen oxide gas sensor

Although the ISEs based on cobyrinates have good selectivity for nitrite over several anions, they also respond to salicylate and thiocyanate. To eliminate this interference, the nitrite-selective electrode based on ionophore 2 was placed behind a microporous gas-permeable membrane (GPM) in a nitrogen oxide gas-sensor mode (75). NOx was generated from nitrite in the sample at pH 1.7 and, after crossing the GPM, was trapped as nitrite by an internal solution that was buffered at pH 5.5 (0.100 M MES-NaOH, pH 5.5, containing 0.100 M NaCl). The internal solution was "sandwiched" between the nitrite-selective electrode and the GPM. 

Imanaka, N., Oda, A., Tamura, S. and Adachi, G.-Y. (2004) Total nitrogen oxides gas sensor based on solid electrolytes with refractory oxide-based auxiliary electrode. J. Electrochem. Soc., 151 (5), H113-16. 

Tamura, S. and Imanaka, N. (2007) Nitrogen oxide gas sensor based on multivalent ion-conducting solids. Sens. Mater., 19 (6), 347-63. 

Figure 8 Nitrogen oxide eiectrochemicai sensor response and sensitivity (a) the relationship of sensing current and the concentration of NO2 and (b) the effect of gas flow rate on the sensitivity of the sensor for various potentials of PANI/Au/Nafion working electrode. Reproduced with permission from Do, J. S. Chang, W. B. Sens. Actuators, B2001, 72(2), 101-107. ...

Amperometric gas sensors are - electrochemical cells that produce a - current signal directly related to the concentration of the - analyte by - Faraday s law and the laws of - mass transport. The schematic structure of an amperometric gas sensor is shown in Fig. 1. The earliest example of this kind of sensor is the - Clark sensor for oxygen. Since that time, many different geometries, membranes, and electrodes have been proposed for the quantification of a broad range of analytes, such as CO, nitrogen oxides, H2S, O2, hydrazine, and other vapors. 

Zirconia sensors have been used primarily in the exhaust system of automobiles to control the air-to-fuel ratio for meeting the federal requirements on such noxious gases as carbon monoxide, methane and nitrogen oxides. The applicability of zirconia sensors for this particular application is based on the assumption that, under thermodynamic equilibrium, the partial pressure of oxygen in the exhaust gas depends primarily on the air-to-fuel ratio. To compensate for the fact that in reality equilibrium is not reached, catalytic platinum electrics are incorporated in the zirconia sensor design [Stevens, 1986]. In the zirconia sensor, the outside of the zirconia tube is exposed to the exhaust gas while the inside is exposed to the ambient air as a reference atmosphere. 

The first concept is the closed-loop-controlled three-way catalyst. In this, one type of catalyst, which is placed in the exhaust gas stream, is able to promote all the main reactions that lead to the simultaneous removal of carbon monoxide, hydrocarbons and nitrogen oxides. To balance the extent of the oxidation and the reduction reactions, the composition of the engine-out exhaust gas is maintained at or around stoichiometry. This is achieved by a closed-loop engine operation control, in which the oxygen content of the engine-out exhaust gas is measured up-stream of the catalyst with an electrochemical oxygen sensor, also called lambda sensor. 

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