NH3 storage isotherms: a path toward better models of NH3 storage on zeolite SCR catalysts

Josh  Pihl, Oak Ridge National Laboratory

Achieving high NOx conversion while minimizing NH3 slip and efficiently utilizing reductant requires careful management of the NH3 inventory on the surface of SCR catalysts.  Development of control strategies that take into account NH3 inventory relies on SCR device models that accurately represent the NH3 storage process.  These models must account for the impacts of a range of key operating parameters, including temperature, gas composition, and catalyst age.  We have found that the widely used strategies for measuring NH3 storage generate data sets that convolute the effects of adsorption energetics, reaction rates, and mass transport, and thereby complicate the model parameter estimation process.  Furthermore, uncertainties regarding the appropriate model structure combined with the convoluted data sets generate models that do not work over a wide range of operating conditions.  To circumvent these challenges, we have measured NH3 desorption isotherms that isolate the energetics of the adsorption/desorption process.  We have also used a standard thermodynamic relation to estimate adsorption enthalpy as a function of NH3 coverage from the isotherm data.  In addition to providing direct estimates of key model parameters, the trends in how adsorption enthalpy changes with coverage provide insights into model structures for NH3 storage, including the number of storage sites and the appropriate model isotherm that should be used to capture the energetics of adsorption/desorption at each site.

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