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Since the ammonia (NH3) based selective catalytic reduction is today’s main DeNOx strategy to meet the emission limits for Diesel cars, research focuses on sensor and control approaches for higher conversion efficiencies and system robustness. One novel approach is the catalyst state monitoring based on radio frequencies (RF). It has proven its functionality to monitor the NH3 loading on SCR catalysts directly in previous studies. This study shows the first attempt to fully describe the whole RF SCR system by simulation. A divided model into the chemical reactions and the RF response is presented and parameterized on experimental test runs. The application of the adjusted model on a test run that was not included in parameter fitting correlates well with the simulated signals and the measured catalyst behavior as well as with the RF response.

The three-way catalytic converter (TWC) has become an integral part of the modern exhaust gas aftertreatment for gasoline engines. Some years ago, radio frequency technology has been successfully tested for monitoring and controlling ceria based TWC oxygen storage by contactless measuring the dielectric catalyst properties inside a cavity resonator. Applying the cavity perturbation method, we present results on CeO2, the oxygen storage component of three-way catalysts, and on differently prepared Pt–ceria model catalysts and combine it with in situ X-ray absorption spectroscopic (XAS) measurements conducted at the Ce L3-edge. The dielectric properties of the oxidized and reduced Pt–ceria and pure ceria powders were determined at 1.2 GHz between 250 and 550 °C. As the experiments show, the reduction of the material changes both, the polarization and the occurring dielectric losses. The complementary in situ XAS measurements, which allow monitoring variations in the redox state of ceria, gave a precise evaluation of the ceria reduction and oxidation. In addition, a substantial improvement of the low-temperature reducibility of ceria in the presence of Pt was demonstrated by both measurement techniques, in agreement with earlier studies. Furthermore, time resolved XAS data obtained during reducing/oxidizing cycles at 250 °C and 350 °C unraveled the faster and more pronounced reduction of ceria at higher temperature. Both methods proved to be valuable methods to provide insight into the oxygen storage and release process of ceria based materials, and capable to discriminate the impact of the noble metal or the ceria surface area in great detail, even under dynamic conditions, which is highly relevant for studying the function of state-of-the art three-way catalysis and giving input for OSC modeling studies.

In recent years, research on radio frequency-based (also known as “microwave-based”) catalyst state diagnosis for diesel engines has focused mainly on SCR systems. To increase the efficiency of exhaust aftertreatment systems, SCR catalysts can be combined with NOx storage catalysts (NSC). For optimum performance, the degree of loading of the storage components has to be monitored. Up to now, in commercial application this has been done indirectly by using mathematical models. In this paper, the opportunities of a simple radio frequency (RF) based method for a direct determination are highlighted. Therefore, a commercial NSC was investigated between 210 and 390 °C. A linear dependence between the RF signals and the stored amount of NOx was found. Furthermore, the NOx and oxygen gas concentrations do not directly affect the RF parameters, but they have an indirect influence, since the stored NOx amount is NOx concentration dependent. By evaluating multiple RF signals, which are based on different material effects, a separate determination of the oxygen and NOx storage level could be realized.

Diesel Particulate Filters (DPF) are an essential part of today’s diesel exhaust gas aftertreatment systems. For an effective filter regeneration strategy, the precise knowledge of the actual trapped soot mass is essential. Besides the state-of-the-art technology of determining the soot load via pressure drop and/or model-based, a microwave-based method enables direct and in situ soot load detection. Thereby, an electric field is impressed into the filter housing and the power transmission is measured, which correlates with the soot load. In this study, influencing parameters, especially temperature and humidity, are examined and compared to the sensitivity towards soot accumulation. Measurements were conducted in a laboratory test bench with a filter-core, which had been previously loaded with soot in an engine dynamometer. While humidity does not have a notable effect, the influence of temperature needs to be considered for real world application. Finally, a complete filter regeneration of the DPF-core in the laboratory test bench could be monitored with the microwave-based system. The carbon balance-derived burned soot mass coincides very well with the microwave derived transmission parameter.

For the development of catalyst materials a novel sensor tool is presented. It can determine catalyst light-off temperatures. The device measures temperature changes (exothermicities) due to catalytic reactions of the material which is coated on a very small catalyst coating area directly on the sensor device. By temperature modulation via an integrated heater, light-off temperature characteristics can be determined within a few minutes. In the present contribution, the sensor setup is introduced and several experimental results for an exemplarily investigated Pt-loaded Al2O3 catalyst are shown with typical exhaust gas components and with mixtures of their components in a lean base gas atmosphere. Several typical findings are light-off hysteresis, CO poisoning, alkane light-off-behavior or acetylene poisoning.

Three-way catalyst-based automotive exhaust gas aftertreatment is of high importance to meet today’s emission standards. To determine in situ the oxygen loading state of three-way catalysts, a microwave cavity perturbation method is used. In this study, it is investigated whether this measurement setup that had originally been described for full-sized catalysts can be transferred to a lab test bench using cores of 1″ diameter. The initial tests were successful and a high correlation between the oxygen loading degree dependent resonance frequency and the conversion was found. As an application example of the new in situ characterization technique, the steady state degree of oxidation of a three way catalyst was measured as a function of the exhaust stoichiometry. The experimental results are compared with the prediction of a recently published improved kinetic model that takes into account the oxidation of reduced ceria by H2O and CO2. It is shown that the experimental observations agree very well with this improved model. This result provides evidence that under typical operating conditions, the degree of oxidation of the three way catalyst is controlled by equilibrium effects.

Diesel particulate filters (DPF) are indispensable parts of modern automotive exhaust gas aftertreatment systems due to the stringent emissions legislation. For a fuel-efficient control strategy, it would be beneficial to determine directly and in-operation their actual trapped soot mass. Two novel approaches—based on the electrical conductivity of trapped soot particles—emerged recently. By measuring the electrical resistance between different single walls inside the filter, the soot load is determined with local resolution. The microwave-based technique is a contactless approach that gives an integral value depending on the soot mass in the DPF. We present investigations on loading and regeneration of DPFs in a dynamometer test bench applying both methods. The results are compared with each other and correlated with the differential pressure and the soot mass. Especially the microwave-based technique has a potential for serial application.

The oxygen loading degree in TWCs, the amount of stored ammonia in SCR catalysts, the NOx loading degree in LNTs, or the soot loading of DPFs play a key role in automotive exhaust gas aftertreatment. Today’s methods determine the catalyst state indirectly. They utilize gas sensors installed up- or downstream of the catalysts and the catalyst state is inferred from the sensor signals. This overview reports on the status of an alternative approach based on the interaction of electromagnetic microwaves with the catalyst material. Since the catalyst state is strongly correlated with the electrical properties of the catalyst material itself, this concept shows a great potential.