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The radio-frequency (RF) or microwave-based catalyst state determination offers the opportunity to operate an automotive catalyst at its optimal point. This has already been proven for the oxidation state of TWCs, the soot loading state on DPFs/GPFs, and the ammonia storage state of vanadium and zeolite based SCR catalysts. However, the latter has only been demonstrated in laboratory scale with synthetic exhaust using gaseous ammonia. This work presents first results on an engine test bench with a serial-type zeolite-based SCR catalyst, using urea solution and the RF tool to detect the current ammonia loading in real time and to control directly the urea dosing system without any additional sensors. The original catalyst volume was reduced by 50 % to operate deliberately the SCR system under high space velocities and to challenge its function. Stationary conditions and operation points with continuously changing NOx emissions and space velocities were observed.

Ammonia adsorption on the catalyst surface is a crucial step in the selective catalytic reduction of nitrogen oxides over zeolites with NH3 as the reducing agent. In this study, two small pore zeolites with chabazite frameworks, H-SSZ-13 and Cu exchanged SSZ-13, are examined. Adsorption of NH3 on the zeolite causes changing electrical properties of the material. They can be detected by a radio frequency based technique. We have found that with this method it is possible to determine the amount of adsorbed NH3 on these catalysts, examining both the influences of temperature and NH3/NO feed gas ratio. At constant temperature, a fairly linear correlation between the resonance frequency and the amount of adsorbed ammonia was observed. Furthermore, this method also allows differentiation between some of the NH3 adsorption sites.

The state of catalysts and filters plays a key role in automotive exhaust gas aftertreatment. The soot or ash loading of particulate filters, the oxygen loading degree of three-way catalysts, the amount of stored ammonia in SCR catalysts, or the NOx loading degree in NOx storage catalysts are important parameters. Today, they are determined indirectly and/or model-based, calibrated by gas sensors installed up- or downstream of the catalysts or by a differential pressure sensor. This contribution overviews a novel approach to determine directly the catalyst state by a microwave-based technique. For that purpose, the catalyst housing serves as a cavity resonator. As “sensing” element, one or two simple antennas are mounted in the catalyst canning. The electrical properties of the catalyst device, i.e., of the ceramic honeycomb incl. coating and storage material, can be measured.

In order to control NOx reduction with NOx storing lean NOx traps (LNT), a gas sensor downstream of the LNT is presently preferred. It is a disadvantage that no means are available to gauge directly the LNT NOx loading level and the catalyst quality. The presented novel sensor consists of interdigital electrodes that are deposited on a planar substrate. On its reverse side, a temperature sensor is applied. Both sides are covered with the original catalyst coating, allowing detecting directly electrical impedance and temperature of the coating. Such sensors were integrated in different positions of an LNT. It is shown in synthetic exhausts as well as in engine tests that in-situ measurements of the electrical impedance of the LNT coating are appropriate to determine directly the catalyst status. Hence, the local degree of NOx loading as well as the local regeneration status can be measured. Furthermore, sulfur poisoning, desulfurization, and thermal ageing can be directly seen.

To determine the oxygen storage capacity of three-way catalysts in gasoline-operated vehicles, lambda probes are best practice to measure the A/F ratio of the exhaust for monitoring indirectly the conversion efficiency. In this paper, initial tests of a novel approach are described to determine the oxygen loading of the TWC more directly by means of radio frequency without the use of lambda probes. Experiments in synthetic exhaust gas as well as dynamometer tests using ceria-containing commercial catalysts showed that characteristic parameters of the radio-frequency signal coincide with the oxidation/reduction state of the TWC.