Optimization of LNT-SCR Dual-Layer Catalysts for Diesel NO_x Emission Control

摘要

Monolithic catalysts consisting of a layer of SCR catalyst deposited on top of a LNT catalyst were optimized to provide high NO_x conversion at both low and high temperatures with minimal precious group metal (PGM) loading for effective diesel NO_x emission control. In this study we demonstrate the application of LNT & SCR zoning in dual-layer catalyst to improve NO_x reduction efficiency and show the potential to reduce the expensive PGM loading by up to 40% from that of LNT only catalyst without degrading its deNO_x performance under simulated diesel exhaust conditions. We investigated the NO_x reduction pathway in the SCR layer of the dual-layer catalyst using simulated rich exhaust of C_3H_6/CO/H_2 as reductants. The non-NH_3 reduction pathway by N-containing organic intermediates via the synergy of LNT and SCR catalysts can play a major role in the incremental NO_x conversion over SCR layer at low temperatures (<= 225℃). The roles of NH_3 and C_3H_6 as reductants for NO_x conversion over the SCR catalyst increase with temperature (> 225℃). The impact of SCR and LNT zoning as well as SCR layer thickness, were studied. Zoning of either or both the SCR and LNT in the dual-layer catalysts improved the low-temperature NO_x conversion, and minimized the high-temperature (300-400℃) conversion loss caused by the SCR layer diffusion resistance and undesired NH_3 oxidation. The performance decline due to the Cu-zeolite layer diffusion resistance was confirmed by replacing it with an inert Na-zeolite layer with a high Si/Al ratio.