Modeling sulfur poisoning of NH3-SCR over Cu-SSZ-13

William  Epling, University of Virginia

Cu-SSZ-13 is currently being used commercially for NH₃-SCR in engine exhaust aftertreatment systems. Previous work has shown that sulfur poisoning inhibits low temperature (< 350 °C) SCR activity over Cu-SSZ-13. And despite such previous studies and Cu-SSZ-13’s apparent success, there are remaining questions around S poisoning. Using experimental data, we have developed a kinetic model that can describe sulfur poisoning, using a mechanism that includes copper sulfate and ammonium sulfate formation. Modeling using one cupper site type failed, and thus a two-site model was used, i.e. Z₂Cu and ZCuOH. This model is capable of predicting the performance of the fresh catalyst, one that has been exposed to sulfur and post-desulfation. Data from two model samples with different Si/Al ratios, and from a commercial samples were used to build, tune and validate the model. H₂ temperature programmed reduction (H₂-TPR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature programmed desorption (TPD) techniques were used to obtain the needed data.

GT-ISE v2017 software was used to define the reaction scheme and solve the governing equations for gas-phase and surface concentrations and temperatures. The mechanism input included NH₃ adsorption and desorption, SCR, ammonium sulfate formation and decomposition, and Cu bisulfite formation and decomposition. The developed kinetic model accurately predicts the experimental behavior of the catalyst observed in our micro-reactor setup.