Global Rate Expressions for Reactions In Automotive Catalytic Converters

In an effort to better understand the underlying chemistry in three-way catalytic converters (TWC), several detailed micro-kinetic models have been developed in the literature [1]. Microkinetic models can capture the observed features at a wide range of operating conditions. They are especially useful under conditions of intrinsic reaction control. However, incorporating detailed microkinetic models, consisting of hundreds of reactions, in computational fluid dynamics simulations for TWC can be prohibitively computationally expensive.

The overall reactions method [2] provides a convenient means of analysis of microkinetic reaction mechanisms. Its analytical framework lends itself extremely well to determination of rate limiting steps, and development of corresponding reduced reaction rate expressions. In this work the focus is on analyzing various reaction systems of importance in catalytic converters, using the overall reactions method.  

First, a reduced rate expression is to be developed and validated for the formation of ammonia under automotive aftertreatment conditions. Second, the methodology is to be extended to the reduction of NO by CO. The microkinetic mechanism for the reduction of NO by CO has been proposed recently [3]. While the ammonia synthesis reaction is a simple sequence of elementary reaction steps in series, the NO-CO reaction is a series-parallel scheme, with the simultaneous formation of N₂ as a main product, and N₂O, as a side product. The rate determining step, and consequently the overall reaction rate expression are expected to vary with the operating conditions, in this example. For both the reaction schemes, overall reaction rate expressions are to be developed and validated against literature experiments, in this work. These validated global rate expressions will provide appropriate kinetics for reactor-scale modeling of the TWC, and enable further analysis and optimization of the device, for future applications.

REFERENCES

1.     R. Burch, J.P. Breen, F.C. Meunier, "A review of the selective reduction of NOx with hydrocarbons under lean-burn conditions with non-zeolitic oxide and platinum group metal catalysts," Applied Catalysis B: Environmental 39 (2002) 283–303

2.     S. A. Vilekar, I. Fishtik, R. Datta. "The steady- state kinetics of a catalytic reaction sequence," Chemical Engineering Science 64 (2009) 1968-1979

3.     D. B. Mantri , P. Aghalayam ,"Detailed Surface Reaction Mechanism for Reduction of NO by CO," Catalysis Today 119 (2007) 88-93