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

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Monolithic catalysts consisting of a layer of SCR catalyst deposited on top of a LNT catalyst were optimized to provide high NOx conversion at both low and high temperatures with minimal precious group metal (PGM) loading for effective diesel NOx emission control. In this study we demonstrate the application of LNT & SCR zoning in dual-layer catalyst to improve NOx 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 deNOx performance under simulated diesel exhaust conditions. We investigated the NOx 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 NOx conversion over SCR layer at low temperatures (<= 225 °C). The roles of NH₃ and C₃H₆ as reductants for NOx 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 NOx 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.