Modeling and Optimization of Multi-functional Ammonia Slip Catalysts for Diesel Exhaust Aftertreatment

摘要

Ammonia slip catalysts (ASC) are used downstream of the NH_3-selective catalytic NO_x reduction (SCR) system, to minimize ammonia slip from transport sector SCR units. The ASC may consist of an SCR active layer for NO_x control, a layer with oxidation functionality for ammonia conversion, or a combination thereof, either as individual layers or as a single mixed layer. Mathematical models of such systems involve flow and mass transfer in the monolithic channels as well as coupled reaction/ diffusion processes in the catalytic layers. In this work, we develop a single-channel 1D-1D (pseudo 2D) steady-state tank-in-series mathematical model for monolithic ammonia slip catalysts with up to two layers. Kinetics of relevant reactions over the Cu-Beta zeolite and the Pt/TiO_2 layer, including SCR, ammonia oxidation, and the formation of N_2O, were derived based on an extensive flow reactor study. The model was validated based on experiments using four different small-scale ASC monoliths, showing good agreement between the model predictions and the experimental results. Comparison of ten ASC configurations for the oxidation of 200 ppm NH_3 at 300 °C shows that configurations with a pure layer of NH_3 oxidation catalyst generate undesirable high N_2O and NO_x levels. The best ASC configurations feature fast diffusion in the catalytic layers (a high effective diffusion coefficient), a thin top layer loading with some ammonia oxidation catalyst, and a mixed layer of ammonia oxidation catalyst and the SCR catalyst to limit the formation of N_2O and NO_x.