Search

A new numerically efficient simulator for dual layer catalysts is implemented and its performance is demonstrated using the example of the dual layer ammonia oxidation catalyst. For the solution of the radial mass balances, each washcoat layer is represented by a single volume element and the diffusive mass fluxes into and within the washcoat are computed based on the concept of the internal mass transfer coefficients. The performance of the new simulator is compared against a reference simulator that fully resolves the concentration profiles in the washcoat. For a steady state test case, the error introduced by the new solution scheme is below 2 % for all relevant exhaust components. For a transient test case, the deviations become more significant. For the simulation of a WHTC, the maximum deviation in the NH3 outlet concentration is 23.3 %, whereas the cumulated NH3 emissions deviate by 10.6 %.With the new simulator, the 1800 s of the transient WHTC cycle can be simulated within 53 s on a standard laptop, 30 times faster than real time.

A new experimental setup is introduced, enabling a preheated exhaust gas mixture to abruptly heat up an initially cold monolith catalyst and, thus, reproduce transient catalyst light-off behavior. With this setup, cold start experiments have been carried out on an aged monolithic Pd three-way catalyst (TWC). This study aims at systematically investigating the effect of diverse hydrocarbons, including methane, acetylene, ethene, ethanol, acetaldehyde, propane, propene, toluene, and xylene in TWC exhaust. It was shown that the commonly used model components propane and propene do not represent the behavior of aromatics and other hydrocarbons present in a typical TWC exhaust very well, since they do not account for their variability in inhibition strength and reactivity. Among the studied hydrocarbons, acetylene proved to be the strongest inhibitor; even at low concentrations of 70 ppm (a typical value found in TWC exhaust), acetylene inhibition increases CO cold-start emissions by 40 %. Furthermore, aromatic and liquid hydrocarbon species (like ethanol) first condense on the initially cold catalyst surface and evaporate again as the catalyst heats up, resulting in a desorption peak at the catalyst outlet.

Knowledge of the actual catalyst state plays a key role in automotive exhaust gas aftertreatment. The oxygen loading degree of three-way catalysts (TWC), the amount of stored ammonia in selective reduction catalysts (SCR), or the NOx loading degree in NOx storage catalysts (NSC) are important parameters. Today, they are determined indirectly and/or model-based, applying models that are typically calibrated by gas sensors installed up- and/or downstream of the catalysts. A novel approach to determine directly the catalyst state by microwaves (radio frequencies, rf) emerged recently. For this method, the catalyst housing serves as an electrical cavity resonator. As “sensor”, one or two simple antennas are mounted in the canning. The electrical properties of the honeycomb incl. coating change with gas loading, affecting either the resonance frequencies or the power transmission. Such contactless-obtained information is strongly correlated with the catalyst state as will be discussed here for TWC and SCR systems. This contribution reviews the progress in the past 3 years that exceeds by far the status of initial studies.

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.

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.

A microkinetic model is developed for the NO-oxidation on platinum. The approach we followed employs semi-empirical methods, optimization of the parameters of elementary-like reactions, and thermodynamic constraints. The model is capable of capturing reasonably light-off experimental data.

The oxidation of ammonia on a Pt/Al2O3 coated monolith has been studied under automotive NH3-slip catalyst conditions. Ammonia conversion as well as the selectivities towards the products N2, N2O and NO are well described by a mechanistic model that is based on reaction mechanisms originally developed for NH3 oxidation in nitric acid production plants.

This paper describes the development and validation of a global kinetic model for a diesel oxidation catalyst. The model is based on an extension of the classical Voltz model, and incorporates CO, H2, NO and HC oxidation, and NO reduction by HC. The model is tested against experimental light-off curves for a variety of scenarios. A one dimensional single channel model for the reactor was used. Parameter optimization was performed using a general pattern search algorithm. The model is shown to be able to capture the ignition curves reasonable well.

The dynamic behaviour of the NO SCR-reaction on Fe-exchanged zeolites was investigated by the transient response method (TRM). The results were used to develop a mathematical model for catalysts containing Fe-exchanged zeolites capable of describing the dynamic phenomena observed.

Different global kinetics for the oxidation of NO on platinum are compared in this work. The general form of the equation for the reaction rate is $r = k \cdot c_{NO}^\alpha \cdot c_{O_2 }^\beta \cdot c_{NO_2 }^\gamma \cdot TD^\delta$ with $TD = \left( {1 - c_{NO_2 } \cdot c_{NO}^{ - 1} \cdot c_{O_2 }^{ - 0.50} \cdot K^{ - 1} } \right)$ . Furthermore steady-state and temperature-programmed experiments were performed. The best results of simulation coupled with parameter estimation were obtained using α = 0.28, β = 0.49, γ = 0 and δ = 1, along with an activation energy of 47.5 kJ mol−1.

The use of the Diesel engine is increasing, while at the same time emission limits are becoming more stringent. To reduce emissions, novel catalyst converter designs have been proposed, including the placement of a small converter before the turbocharger. The role of this catalyst is to provide some reduction of the pollutant level prior to the main catalyst. This catalyst is typified by high flow rates. In this paper the performance of small catalytic converters operated at high flowrate is examined using computer simulation. The oxidation of CO is used as a model reaction. It is shown that for a typical oxidation catalyst, conversions of the order of 30% can be achieved at typical temperatures and a GHSV of 5 million h−1.

This paper presents the results of a series of simulations of the light-off of carbon monoxide in excess oxygen in a single channel monolith reactor. The catalytic washcoat geometry is tested for its effect on the light-off position and shape. The simulations used a combination of two and three-dimensional modelling with non-linear oxidation kinetics. Different temperature ramp rates were employed. It is seen that the shape of the washcoat has an influence on the shape of the light-off curve, especially for high values of gas hourly space velocity and larger temperature ramp rates. The washcoat geometry that gives the fastest rise to complete conversion is the one that is the most non-uniform in shape; that is, the one that has thin sections.