Dual-Layer Ammonia Slip Catalysts for Automotive SCR Exhaust Gas Aftertreatment: an Experimental and Modeling Study

A chemically and physically consistent mathematical model of a commercial dual-layer (SCR + PGM) monolith catalyst for control of the NH3 slip (ASC, Ammonia Slip Catalyst) from NH3/urea-SCR converters is herein presented. In a first stage NH3/O2/NO–NO2–N2O steady-state and transient kinetic runs were performed at high space velocities in a representative temperature range (150–550 °C) over each one of the two ASC component catalysts (Fe-zeolite SCR and Pt/Al2O3 PGM) in the form of powders to provide intrinsic kinetic information on the prevailing reaction pathways and to enable the development of original kinetic models for each catalytic phase. In a subsequent stage the two powdered catalysts were also tested jointly, both in a sequential dual-bed configuration and in the form of a mechanical mixture, thus acquiring information on the interactions between the SCR and the PGM catalytic chemistries. Two global kinetic models were fitted to the SCR and to the PGM catalyst data, respectively, and then suitably combined to generate predictive simulations of the runs over the double bed and over the mechanical mixture. Lab-scale NH3/O2/NO–NO2 catalytic activity experiments were performed over washcoated core monolith single-layered SCR and PGM catalysts and over a full dual-layer ASC honeycomb catalyst in order to evaluate the importance of transport phenomena and to scale-up the kinetics. Finally, an original heterogeneous 1D + 1D dynamic mathematical model of a single monolith channel with dual-layer washcoat was developed for the simulation of ASC catalysts. The rate parameters derived from the kinetic analysis of the powdered SCR and PGM catalyst data were incorporated, together with the relevant geometrical and morphological catalyst properties, into such a model, which was eventually validated by simulating data from engine test bench runs, collected over a full scale dual-layer ASC monolith catalyst using real exhausts from a Diesel engine.