Mechanistic detailed kientic modeling of the NH3-NO/NO2 SCR reactions over a Cu-zeolite catalyst

Massimo  Colombo, Politecnico di Milano

ORAL PRESENTATION: Tightening NOx emission standards for heavy-duty and passenger car engines require the development of highly efficient exhaust aftertreatment systems. The selective catalytic reduction (SCR) of NOx with NH3/Urea is currently the technology of choice for heavy-duty applications to comply with the latest emissions standards. The new generation of SCR converters is based on zeolite catalysts promoted with transition metals (e.g. Cu, Fe) washcoated onto honeycomb monoliths. Such systems can better exploit the so called “Fast” SCR chemistry resulting from partial conversion of NO to NO2 over an upstream oxidation pre-catalyst; however little engineering work has been reported so far on the transient kinetics of this complex reacting system over zeolites. A growing interest in the NO2 related SCR reactions is emerging in the last years due to the increasing need of high NOx removal efficiencies at progressively lower temperatures. In this framework the elucidation of the NO2 related SCR chemistry and reactions mechanisms, and the development of mechanistic mathematical models is of key importance. Drawing from our previous experience on V2O5-WO3/TiO2 and Fe-zeolite SCR catalysts, we investigated by dynamic methods the intrinsic kinetics of the NH3-NO/NO2 reactions over a commercial copper exchanged zeolite powdered catalyst.
A dynamic micro-kinetic model was developed in close agreement with all the details of the NO2-related SCR catalytic chemistry over a Cu-zeolite catalyst. The rate parameters were estimated from transient kinetic runs (i.e. isothermal concentration step changes, temperature programmed desorption and temperature programmed surface reaction experiments), designed to address the individual steps in the surface mechanism. Such a mechanism includes adsorption of ammonia, adsorption of NO2 in the form of surface nitrites and nitrates, decomposition of nitrites to nitrogen via reaction with ammonia, reversible reduction of nitrates to nitrites by NO (Fast SCR route), and direct reduction of nitrates by ammonia (NO2-SCR route).
The fitted set of rate equations was able to predict the complex transient behaviours associated with the global Fast-SCR and NO2-SCR reactions observed in T-ramp validation experiments.

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