Development of coupled models for NH3 capacity and SCR on Cu-SSZ-13 at various aging conditions and temperatures

Austin  Ladshaw, Oak Ridge National Laboratory

Under the anticipated regulatory changes to both NOx emissions limits and required emissions control system full useful life for heavy duty vehicles, urea selective catalytic reduction (SCR) systems will be required to achieve even higher performance over a much longer operating lifetime than what commercial systems currently deliver. Effective urea dosing strategies will be needed to meet the increasing performance requirements. Since urea dosing strategies are often developed through system simulations, the underlying SCR catalyst models must be able to accurately capture changes in ammonia storage capacity as a function of operating conditions and catalyst age, while also accurately predicting the SCR conversion efficiency over those same range of conditions. Thus, it is paramount that SCR models be closely coupled with NH3 storage capacity and track how various active sites change over the lifetime of the catalyst material.

In this work, experimental SCR protocols for NH3 oxidation, NO-SCR, and Fast SCR are analyzed using a modeling tool developed in python. The modeling tool utilizes a 1D-0D monolith catalyst model to simulate those protocols and approximate reaction rates for the various reactions involved with SCR at all aging conditions of interest. Storage of NH3 on the catalyst is assumed to occur at 3 unique sites: (i) Z1CuOH sites, (ii) Z2Cu sites, and (iii) Brønsted acid sites. The Brønsted sites in this model act primarily as a reservoir for NH3, while the Cu sites are active for SCR related reactions. In addition, we include the formation of NH4NO3 at surface sites as an intermediate species during Fast SCR. Preliminary results show that this model can adequately capture the changes in NOx conversion efficiency and rates across the various aging conditions with a common set of rate parameters, and that the effects of aging can be captured solely through changes in active site densities as the material ages.

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