Martin Votsmeier

Inverse Hysteresis Phenomena During CO and C3H6 Oxidation over a Pt/Al2O3 Catalyst

It is well known that conversion as a function of temperature hysteresis can occur during ignition and extinction exothermic reaction experiments, such as CO oxidation over Pt/Al2O3, with the activity during the ignition process not matching that during the extinction process. Conversions being higher during extinction than that during ignition are often observed. Several explanations have been proposed in which heat effects, different catalyst surface states, and different Pt oxidation states are the most common. In this work CO oxidation hysteresis behavior, when in a mixture with C3H6, was investigated. The results show that when C3H6 was absent, CO oxidation followed normal hysteresis behavior; however, when C3H6 was added to the mixture, the catalytic activity during the extinction phase decreased. As the C3H6 concentration in the mixture increased, the hysteresis loop became smaller and ultimately reverse hysteresis was observed. The decrease in catalytic activity during extinction was due to the formation of C3H6 oxidation intermediate species. These species competed with CO for active sites, thus inhibiting CO oxidation, and were not present during ignition as CO was the dominant adsorbed species when starting at low temperature.Graphical Abstract

Overview: Status of the Microwave-Based Automotive Catalyst State Diagnosis

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.

Microwave Cavity Perturbation as a Tool for Laboratory In Situ Measurement of the Oxidation State of Three Way Catalysts

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.

SI-Engine Control With Microwave-Assisted Direct Observation of Oxygen Storage Level in Three-Way Catalysts

To realize existing emission standards with a reduced amount of noble metals, advanced control strategies for spark ignition (SI) engines are of absolute priority. Such control strategies require precise and accurate information about the actual oxygen storage level of the three-way catalyst needed to reduce emissions. This precision is out of reach for state-of-the-art oxygen storage models on available on-board hardware. A novel approach toward the solution of this problem is the direct determination of the oxygen storage level. This contribution shows how this can be achieved with a model-based microwave measurement system. The method is validated by numerical experiments. It is demonstrated how this approach can be implemented into a new SI engine control algorithm.

Replacing the lambda probe by radio frequency-based in-operando three-way catalyst oxygen loading detection

A novel strategy for closed-loop controlling the stoichiometric combustion of gasoline engines is presented. In contrast to common strategies which use the signal two lambda probes up- and downstream of a three way catalyst to control the engine, here the oxygen loading degree of a threeway catalyst is directly and in-operando determined by a radio frequency-based method and used as the process variable. Initial tests at a high dynamic laboratory test bench show that such a controlling strategy may lead to much lower emissions. Hence, there is a chance to replace lambda probes.

Efficient interpolation of precomputed kinetic data employing reduced multivariate Hermite Splines

Abstract Detailed surface kinetics can be efficiently implemented into complex reactor simulations by the use of precomputed solutions of the chemical rate equations. In this work we propose reduced Hermite splines as an interpolation method for the precomputed rate data. The reduced Hermite splines require significantly less storage space and less execution time than the tensor product splines that have been previously used for this task. Using previously published test cases on NH 3 oxidation, we demonstrate that despite of the reduced storage requirements and faster interpolation times, reduced Hermite interpolation achieves the same interpolation accuracy as conventional full tensor product splines. In many cases, the derivatives of the interpolated function with respect to the input quantities can be cheaply obtained during the computation of the function values. If these derivatives are provided as additional information, the accuracy of the reduced Hermite method even outperforms conventional tensor product splines, in our demonstration example by a factor of three. The advantage of the reduced Hermite schemes with respect to storage requirements and evaluation time strongly increase with the dimensionality of the interpolation problem. It is therefore expected that the reduced Hermite splines will allow to significantly extend the application range of solution mapping methods to higher dimensional problems (i.e. problems with a larger number of relevant chemical species).

Modellbildung und Vergleich von linearen und nichtlinearen Regelungskonzepten für Drei-Wege-Abgaskatalysatoren

Zusammenfassung In diesem Beitrag wird eine neue Methodik zum Erstellen von Zustandsraummodellen aus physikochemischen Katalysatormodellen vorgestellt und verallgemeinert. Basierend auf dem resultierenden Zustandsraummodell des Katalysators wird eine systemtheoretische Analyse durchgeführt. Lineare und nichtlineare Regelungskonzepte zur Minimierung der Schadstoffemissionen werden entworfen und ihre spezifischen Vor- und Nachteile diskutiert.

Infrared Spectroscopic Study of the NH3-SCR on Fe-Zeolites under Transient Conditions

The standard NH3-SCR (selective catalytic reduction) of NO/O2-mixtures on Fe-zeolites shows significant maxima in the NO-conversion as transient response, when NH3 is turned on and off. Introducing in situ DRIFT-spectroscopy of the catalyst two remarkable effects were observed. On the one hand a kink in the kinetics of the bands assigned to the 2H- and 3H-structure of adsorbed NH3 was detected. Furthermore the kinetics of several bands show a maximum, which gives hint to the existence of an intermediate, in the following called SCR-SI (SCR-Surface Intermediate). Due to the observed correlation between both, the maximum in the NO-conversion and the maxima in the band kinetics on the catalyst, the SCR-SI plays an important role in the mechanism and seems to be responsible for the increased NO-conversion. In order to gain more information about the structure and role of SCR-SI the influence of temperature, iron content, O2 concentration and NO/NH3–ratio was studied.

Systematic Transformation of Chemical Catalyst Models for Control Design

Common system analysis and controller design methods are based on state-space models. However, most physical-chemical catalyst models are not represented in a state-space. In this paper, we present an approach to transform simplified physical–chemical models into the state-space, and hereby, we introduce the concept of weighting functions. The weighting functions represent the catalyst model in a compact form and are a convenient way for system analysis. The weighting functions will be analyzed from the system theoretical and chemical point of view. The generalization of this approach will be demonstrated. Exemplarily, three reaction mechanisms will be transformed into nonlinear state-space models. Based on the first model, the nonlinear state-space model will be linearized and a linear quadratic controller will be designed. Contrarily to the LQ controllers in the literature, the weighting matrix for the state vector is chosen in the model-based form. The only few design parameters are intuitive to choose.