Optimization of LNT-SCR Dual-Layer Catalysts for Diesel NO _x Emission Control

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 C₃H₆/CO/H₂ as reductants. The non-NH₃ 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 °C). The roles of NH₃ and C₃H₆ as reductants for NO_x conversion over the SCR catalyst increase with temperature (> 225 °C). 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 °C) conversion loss caused by the SCR layer diffusion resistance and undesired NH₃ 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.