Hydrocarbon sensors

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

The success of the O2 sensor has made the auto manufacturers, regulators, and environmentalists anxious to extend chemical sensing to a variety of tailpipe gases, notably CO, NO, and short-chain hydrocarbons. Considerable research and development is needed for these molecules to be monitored in the hostile exhaust system environment (36). 

Hydrocarbon sensors (qv) placed directiy below the tank bottoms can be effective. However, old contamination or contamination from other tanks or piping can yield misleading results. In addition, the low permeabUity of some areas in the soil can prevent the migration of vapors to the sensing ports under the tank bottom. 

Aromatic solvents or polycyclic aromatic hydrocarbons (PAFI) in water, e.g. can be detected by QCM coated with bulk-imprinted polymer layers. Flere, the interaction sites are not confined to the surface of the sensitive material but are distributed within the entire bulk leading to very appreciable sensor responses. Additionally, these materials show high selectivity aromatic solvents e.g. can be distinguished both by the number of methyl groups on the ring (toluene vs. xylene, etc.) and by their respective position. Selectivity factors in this case reach values of up to 100. 

O2, and hydrocarbons, leading to a NH3 selective sensor. Arguments in favor of using a zeolite-based sensor in NH3 detection for automotive apphcations are low cost, high temperature stability, and suitability for use in thick-film technology, of common use in the automotive industry [72]. The sensors were tested on an engine test bank and the authors claim that the sensor itself meets aU the technological and economical demands of the automotive industry [73]. 

In a similar manner, Sahner et al. [76, 77] utilized a Pt-ZSM-5 layer to reduce the cross-sensitivity of a hydrocarbon (propane) sensor toward CO, propene, H2, and NO at 673 K. The zeolite layer was put on the sensor as a paste. The improved cross-sensitivity is attributed to selective oxidation of aU considered components except propane. Trimboli et al. [78] demonstrated the same concept by using a Pt-Y zeohte for the CO oxidation, maintaining the sensitivity for propane. 

Additionally, NO is reduced by H2 and by hydrocarbons. To enable the three reactions to proceed simultaneously - notice that the two first are oxidation reactions while the last is a reduction - the composition of the exhaust gas needs to be properly adjusted to an air-to-fuel ratio of 14.7 (Fig. 10.1). At higher oxygen content, the CO oxidation reaction consumes too much CO and hence NO conversion fails. If however, the oxygen content is too low, all of the NO is converted, but hydrocarbons and CO are not completely oxidized. An oxygen sensor (l-probe) is mounted in front of the catalyst to ensure the proper balance of fuel and air via a microprocessor-controlled injection system. 

Figure 10.1. Emissions of CO, NO and hydrocarbons along with the signal from the oxygen sensor as a function of the air/fuel composition X = 1 corresponds to the...

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Krska R., Kellner R., Schiessl U., Tacke M. and Katzir, Fiber optic sensor for chlorinated hydrocarbons in water based on infrared fibers and tunable diode lasers, Appl. Phys. Lett., 1993 63 (14), 1868-1871 A. 

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