(608g) 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