Microwave-Based State Diagnosis for Three-Way Catalysts – A Promising Technology for Future Gasoline Exhaust Gas Aftertreatment (2021)

Introduction One of the most important key elements for the exhaust gas aftertreatment of modern gasoline vehicles is the three-way catalytic converter (TWC). Its combination of oxygen storage (high-surface ceria) and precious metal loading (Pt, Pd, Rh) ensures the effective reduction of nitrogen oxides (NO x ) as well as the oxidation of unburnt hydrocarbons (HC) and carbon monoxide (CO) during stoichiometric operation. For optimal performance of the TWC, the determination of the current oxygen storage level is crucial. Today’s systems using oxygen sensors can only estimate the catalyst state by means of balancing approaches together with numerical models. Some time ago, however, a radio frequency-based measurement system was introduced that allows to determine directly the oxygen storage level [1]. In non-contact measurement, the catalyst housing serves as a cavity resonator in which standing electromagnetic waves propagate (GHz range). This article sheds light on the basic behavior of the radio frequency-based state diagnosis of three-way catalytic converters, deals with cross-sensitivities of the measuring principle, and provides an orientation for vehicle application. The advantages of different resonance parameters are considered. Methods and Setup In order to determine the oxygen storage level using RF technology, the metallic catalyst housing forms a cavity resonator. Its cylindrical geometry is limited by two inserted perforated plates, as shown in Fig. 1 [2]. In this case, the three-way catalyst is located in the center of the arrangement. Thermocouples and ceramic oxygen sensors (binary and wideband) are placed upstream and downstream of the resonator to analyze exhaust gas temperature and stoichiometry. For a transmission measurement, two coupling elements, also called antennas, are inserted upstream and downstream of the catalyst. At discrete frequencies, so-called resonance modes propagate within the canning, which are characterized by their resonance frequency and quality factor. The exact resonance properties are also a function of the dielectric properties of the catalyst and can be determined with the microwave cavity perturbation theory [3]. For the TWC, the dielectric properties (polarization and dielectric losses) change with the oxygen stoichiometry within the oxygen storage component. Therefore, reduction and re-oxidation of the oxygen storage material can be detected with the resonance frequency as well as with the quality factor [4]. To evaluate the method for the state diagnosis of three-way catalysts, both full-catalysts (Ø4.66'') at lower (gas hourly) space velocities ( GHSV = 1000 h -1 ) and smaller cores (Ø1.66'') at application-typical space velocities ( GHSV = 32.000 h -1 ) were investigated. The TE 111 mode was used for the radio frequency method as the electric field can be assumed to be almost constant over the entire length of the catalyst (Figure 1) and therefore a steady RF sensitivity can be assumed. Results In principle, both, resonance frequency and quality factor, are suitable to determine the current oxygen storage level, however, both signals also feature individual characteristics. As an example, Fig. 2 shows the relative signal amplitude of the resonance frequency as a function of the oxygen storage level of a TWC [2]. The signal amplitude is very low at temperatures just above the light-off (≈ 300°C) and increases noticeably with higher temperatures. Although the determination of the actual oxidation level is theoretically possible at low temperatures, the resonance frequency is more suitable for higher exhaust temperatures. An opposite behavior was found for the quality factor. Here, the relative signal amplitudes are high especially at low temperatures, but decrease at higher signal amplitudes at higher catalyst temperatures due to increasing resonator losses. Therefore, the quality factor is particularly suitable for the state diagnosis at low temperatures. It was shown that these resonance parameters are strongly dependent on the catalyst temperature, which can be attributed on the one hand to the thermal expansion of the catalyst housing and on the other hand to a temperature-related change in the dielectric TWC properties. A reliable diagnosis of the catalyst state therefore requires knowledge of the TWC temperature. In contrast, the effect of the water concentration can be neglected. The RF method also provides information on the aging condition of the catalyst. An increase of the temperature necessary for the activation of the oxygen storage component with proceeding catalyst ageing was clearly observed with the quality factor, due to its high sensitivity at temperatures close to the catalyst light-off. The quality factor therefore provides a new approach to determine TWC aging. Considering the different characteristics of resonance frequency and quality factor, a combined analysis of both resonance parameters does provide detailed information about the catalyst state and opens promising possibilities for technical application. Literature Reiß, S.; Spörl, M.; Hagen, G.; Fischerauer, G.; Moos, R.: Combination of Wirebound and Microwave Measurements for In Situ Characterization of Automotive Three-Way Catalysts. IEEE Sens. J. 2011, 11 , 434–438, doi: 1109/JSEN.2010.2058798 Steiner, C.; Malashchuk, V.; Kubinski, D.; Hagen, G.; Moos, R.: Catalyst State Diagnosis of Three-Way Catalytic Converters Using Different Resonant Parameters — A Microwave Cavity Perturbation Study. Sensors 2019, 19, 3559, doi: 3390/s19163559 Chen, L.; Ong, C.K.; Neo, C.P.; Varadan, V.V.; Varadan, V.K.: Microwave Electronics: Measurement and Materials Characterization. John Wiley &amp; Sons, Ltd, Chichester, 2006, doi: 10.1002/0470020466 Steiner, C.; Gänzler, A.M.; Zehentbauer, M.; Hagen, G.; Casapu, M.; Müller, S.; Grunwaldt, J.-D.; Moos, R.: Oxidation State and Dielectric Properties of Ceria-Based Catalysts by Complementary Microwave Cavity Perturbation and X-Ray Absorption Spectroscopy Measurements. Top. Catal. 2019, 62, 227–236, doi: 1007/s11244-018-1110-3 Figure 1