Modeling and analysis of transport and reaction in washcoated monoliths: Cu-SSZ-13 SCR and dual-layer Cu-SSZ-13 + Pt/Al2O3 ASC

Abstract

Modeling and analysis of washcoated single- and dual-layer monolith catalysts is presented for selective catalytic reduction (SCR) on Cu-SSZ-13 and ammonia oxidation on Cu-SSZ-13 + Pt/Al₂O₃ ammonia slip catalyst (ASC). This work is a follow-up of the recent experimental study of enhanced transport in porosity-modified washcoated monoliths (Dhillon et al.). A 1 + 1 dimensional reactor model containing tuned multi-step kinetic formulations is utilized to simulate SCR and ASC performance for understanding and optimization. The SCR kinetic model combines multi-step Eley–Rideal kinetics with a dual-site description and the model parameters are systematically tuned to data sets for NH₃ uptake and TPD, steady state NH₃ + O₂ oxidation, and standard SCR (NH₃ + NO + O₂), the latter including differential kinetics. The SCR model is validated for a range of operating conditions and catalyst compositions. Among other findings, the dual-site SCR model predicts the two peaks during NH₃ desorption and captures the “sea gull” shaped conversion versus temperature dependence, underscoring the predominance of the two active sites. The model is used to simulate the performance of the modified porosity Cu-SSZ-13 washcoat for SCR on the single-layer catalyst and for SCR + ammonia oxidation on the dual-layer ASC. The model predicts that a ∼50% increase in the effective diffusivity serves to enhance the conversion in the intermediate temperature regime in which washcoat diffusion is controlling. The SCR and ASC findings provide direction for further catalyst and reactor optimization.