(426b) Math-Based Approach to Automotive Emission Control System Development: from Global Kinetics to Microkinetics

Catalytic converters, which have been in widespread use since the introduction in the U.S. in the fall of 1974, have proven to be effective at reducing automobile exhaust emissions of carbon monoxide, hydrocarbons and nitrogen oxides. In the face of the stricter future exhaust emission standards, it is of critical importance to further improve the performance and durability of a catalytic converter while reducing its noble metal usage requirements. Since the performance of a catalytic converter is a complex function of its design and operating parameters, mathematical modeling (rather than an empirical approach) promises to be helpful in accelerating the development of optimum emission control systems (i.e., those with high emission performance and low noble metal content) and their implementation on vehicles.

For late-model gasoline vehicles, the vast majority (typically > 80%) of tailpipe hydrocarbon emissions occurs during the cold-start period of the FTP (EPA-prescribed driving schedule used for vehicle emission testing). The 1-D “single-channel” monolith model with global rate expressions has been extensively used at GM to provide guidance in the design, optimization and implementation of emission control systems with improved cold-start emission control performance. Some examples of model applications include: (1) Quantify the impact of noble metal loading and catalyst volume on cold-start emission performance, (2) Quantify the effects of substrate properties, (3) Quantify the impact of changes in exhaust system architecture and engine management strategy, (4) Quantify the impact of poisoning and thermal aging, and (5) Optimize the design and operation of electrically heated converters. In this presentation, we will discuss the results of cold-start emission calculations to illustrate some of the model applications mentioned above.

Although the monolith model with global kinetics provides a useful design tool, we need converter models with more detailed kinetics based on elementary reaction steps in order to capture the reaction events and fast transients more accurately. Such an elementary reaction step-based model is better suited to handle a situation where rate-determining steps are likely to change within the converter, allows one to extend/adapt the model more easily to different catalyst formulations or more complex reacting mixtures, and can be extrapolated with more confidence to outside the ranges where experimental data exists. However, such a fundamental model requires quantitative understanding of the reaction mechanism, including the rate constants for all the elementary reaction steps involved. The limitations of global-kinetics as well as the benefits of the microkinetics-based modeling approach will be illustrated for simple reaction systems relevant to automobile exhaust catalysis, and technical issues and future research needs associated with its extension to the actual exhaust environment will be discussed.