Experimental and modeling study of a dual-layer (SCR + PGM) NH3 slip monolith catalyst (ASC) for automotive SCR aftertreatment systems. Part 1. Kinetics for the PGM component and analysis of SCR/PGM interactions

摘要

We present herein the first of two parts in the development and validation of a chemically and physically consistent mathematical model of a commercial dual-layer (SCR+PGM) monolithic NH3 slip converter (ASC). The overall project followed a systematic approach of growing complexity, and its results emphasize the beneficial features of a dual-layer configuration with the SCR catalyst on top of the PGM component. Specifically, we report in this paper NH3/O2/NO—NO2 steady-state and transient kinetic runs performed over the PGM component of the dual-layer NH3 slip catalyst. The PGM component was tested in a representative temperature range (150-550 °C) in the form of precursor washcoat powders at high space velocities in order to gain kinetic information. From these data an original global PGM kinetic model was developed, which fully accounts for the effects of temperature and of NO2/NO_x feed ratio (0-1) on NH3 oxidation. The model considers NO2 inhibition on NO oxidation, as well as a novel NO2 inhibition effect on the NH3 oxidation reactions. Comparative NH3/O2/NO—NO2 steady-state runs were performed also over two combinations of SCR+PGM powders (sequential double-bed and mechanical mixture). The N2 selectivity was greater over the mechanical mixture, as in this configuration the unselective NH3 oxidation products (NO_x) formed over the PGM catalyst had a chance to further react selectively with NH3 over the SCR catalyst. Such a positive interaction between the PGM and the SCR catalytic chemistries was satisfactorily predicted by a model involving the simple superposition of the PGM and SCR kinetics. In the following part of the project the herein developed PGM kinetics, together with consistent SCR kinetics, will be incorporated in a novel dual-layer monolith catalyst model and validated against both lab-scale and engine test bench data collected over dual-layer ASC systems.