NOx Gas Sensors

NOx (NO and NO2) gas is known to be very harmful to humans and is one of the main causes of acid rain. They are typical noxious gases released from combustion facilities and automobiles. The lower exposure limits (LEE) have been provided in the previous section. Solid-state NOx sensors are widely demanded for monitoring NOx ID the environmental atmosphere as well as in combustion exhausts. Particularly, NOx monitoring is indispensable for the feedback control of combustion systems or de-NOx systems. The NO and NOx have quite different properties from each other. 

Various types of solid-state NO2 sensors have been proposed based on semiconducting metal oxides (including heterocontact materials) [42-50,58,59,234-238], solid electrolytes [1,239,240], metal phthalocyanine [241], and SAW devices [242]. Among these NO2 sensors, the semiconducting metal oxides and solid electrolytes appear to be the best. Specifically, semiconducting metal oxide gas sensors are most attractive because they are compact, sensitive, of low cost, and have low-power consumption. Their basic mechanism is that the NO2 gas is adsorbed on the surface of the material this decreases the free electron density into the space-charge layer and results in a resistance increase [243]. 

FIGURE 1.15 Sensitivity of Ti02-W03 and WO3 gas sensors as a function of NOj gas concentration. (Reprinted from IEEE Sensors J., 1, Lee D.-D. and Lee D.-S., Environmental gas sensors, 214-224, 2001, with permission from Elsevier.) 

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Figure 6 shows the selectivity behavior of this NOx gas sensor. The sensor had a sub-Nernstian response toward nitrite, with slopes in the range of -45 to -50 mV/decade. Further, the response observed with salicylate and thiocyanate was diminished substantially, as compared to that obtained with the original nitrite-selective electrode (Figure 3). In addition, the gas sensor described here does not suffer interferences from nitrate, bicarbonate, acetate, benzoate, or chloride. These excellent selectivity properties of the sensor are a combination of the selectivity characteristics of the nitrite-selective electrode and the additional discrimination provided by the GPM.

Figure 6. Selectivity pattern of the NOx gas sensor. The sensor was exposed to 0.010 M H2SO4 containing the following anions nitrite (1), salicylate (2), thiocyanate (3), benzoate (4), nitrate (5), chloride (6), bicarbonate (7), acetate (8). (Adapted from ref. 15.)...

Yang J-C, Dutta PK (2007) Influence of solid-state reactions at the electrode-electrolyte intraface on high-temperature potentiometric NOx-gas sensors. J Phys Chem C 111(23) 8307-8313... 

A NOx gas sensor constructed from a molecularly-imprinted nitrate-selective electrode has also been developed in our laboratory (30). This gas sensor is produced by placing the nitrate-selective electrode behind a gas-permeable membrane. Also, there is a buffer compartment present between the gas-permeable membrane and the ISE. As before, NOx species that pass through the gas-permeable membrane are trapped in the buffer compartment as NO2 and NOs . In this case, the electrode responds proportionately to the amount of nitrate in the buffer, rather than nitrite as in the NOx sensor described above. The gas sensor based on the N03 -selective electrode offers advantages over the Severinghaus arrangement similarly to the other aforementioned gas sensors (SO2 and NOx). Detection limits for this gas sensor were on the order of 1x10 M, with response characteristics being retained for over 80 days. This lifetime is consistent with lifetimes of the molecularly-imprinted nitrate-selective electrodes. 

Tuyen LIT, Vinh DX, Khoi PH, Gerlach G (2002) Highly sensitive NOx gas sensor based on a Au/n-Si Schottky diode. Sens Actuators B 84 226-230... 

Fergus JW (2007b) Materials for high temperature electrochemical NOx gas sensors. Sens Actuators B 121 652-663 Fergus JW (2008) A review of electrolyte and electrode materials for high temperature electrochemical CO and SO ... 

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. 

Zosel, J. and Guth, U. (2004) Electrochemical solid electrolyte gas sensors-hydrocarbon and NOx analysis in exhaust gases. Ionics, 105 (5-6), 366—77. 

Further development of new impedance-based gas sensors is likely to allow their introduction to niche applications in the field of zirconia-based, solid-state gas sensors. Impedance spectroscopy has the potential to measure changes not detectable with simple current or voltage measurements. Although it is not practical to implement complete impedance spectroscopy in an operating sensor, an optimized frequency suitable for a specific gas sensor may be used. So far, these sensors are still more complex and expensive as compared to the potentiometric zirconia-based gas sensors. However, they have two important advantages (1) measurement of total NOx concentration, regardless of the NO/NO2 ratio in exhausts and (2) near equal sensitivity to NO and NO2 at 700°C. These are essential prerequisites to their practical implementation in vehicle exhausts. Therefore, further investigation is... 

Electrochemical type-III sensors have been constructed for measuring NOx, C02, S02, and many other environmentally harmful gases in very low concentration ranges [5]. In these multilayer type-III gas sensors, no direct relation between a solid electrolyte and the target gas exists without the mediation of an attached auxiliary phase. 

Electrodes and catalytic materials in front of the electrode (pre-catalyst) play an important role in the design of specific functions or characteristics of exhaust gas sensors. For excellent 02 detection in nonequilibrium raw emissions, platinum has become established as the dominant electrode material, because it efficiently supports the ionic-atomic transfer reaction of oxygen. Additionally, it ensures that the raw gas quickly achieves thermodynamic equilibrium, to enable an accurate lambda measurement. Sometimes it can be useful to add additional metals such as Rh, Pd, or Au to improve or prevent NOx reduction or to add ceramic additives such as zeolites or Ce02 to adsorb oxygen or other species [14—16]. 

In Type III, there is an auxiliary phase attached on the surface of the solid electrolyte so as to be sensitive to the gas, and it is produced by a compound that contains the same ionic species as derived from the gas. The auxiliary phase can act as a sort of poor ion-conducting solid electrolyte, which forms a half cell of Type I or II as shown in Figure 1.1 [1]. Type III sensors can be divided into three subgroups depending on the types of the half cells combined [6]. Since a NASICON solid electrolyte potentiometric gas sensor using alkali metal carbonate as an auxiliary phase solid electrolyte is known to be sensitive to CO2, Type III sensors have been of immense importance as sensors for oxygenic gases such as CO2, NOx, and SOx... 

Bur, C., Reimann, R, Schtitze, A., Andersson, M. and Lloyd Spetz, A. (2012), New method for selectivity enhancement of SiC field effect gas sensors for quantification of NOx, Microsystem Technologies/Smart Sensors, Actuators and MEMS, Springer-Verlag, Berlin, 18,7,1015-25. Doi 10.1007/s00542-012-1434-z... 

Schalwig, X, Ahlers, S., Kreisl, P, Bosch-v. Braunmtlhl, C. and Muller, G. (2004), A solid-state gas sensor array for monitoring NOx storage catalytic converters. Sensors and Actuators B, 101,63-71. 

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