Catalytic mechanism, detailed kinetics and converter model for NH3-SCR of NOx emissions from vehicle

Enrico  Tronconi, Politecnico di Milano

We present a systematic fundamental study of the catalytic and kinetic features of NO/NO2 – NH3 reactions over a V2O5-WO3/TiO2 commercial catalyst, and the related development of a dynamic simulation model of SCR monolithic converters for vehicle applications.
The SCR reactions were first investigated in a microreactor by unsteady experiments over a powdered catalyst, leading to the proposal of a global mechanistic scheme accounting for stoichiometry, selectivity and kinetics of NO/NO2 NH3-SCR. On this basis a Mars-Van Krevelen dynamic kinetic model was derived which unifies “Standard” and “Fast” SCR into a single Redox-type approach. Intrinsic rate parameters were accordingly estimated and incorporated into a transient heterogeneous 1D+1D mathematical model of SCR monolithic reactors, which fully accounts for gas/solid mass and heat transfer, as well as for intraporous diffusion/reaction of reactants and products within the active catalytic mass. On the basis of relevant geometrical and morphological characteristics of the honeycomb catalyst, the model was validated first against transient SCR runs performed over core monolith samples (up to 10 cm3) in a laboratory rig, and then against extensive test bench data collected in full scale SCR monolithic converters loaded with catalysts up to 43 litres in size, using real Diesel engine exhaust gases. The model is currently applied by DaimlerChrysler to model-based design and optimization of SCR converter parameters (e.g. urea dosing strategy, size, cell density and position of the monolith catalyst).
Extension of the study to zeolite-based SCR catalysts is in progress: similarities and differences with the catalytic behavior of vanadium-based systems will be highlighted.

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