Rapid propylene pulsing for enhanced low temperature NOx conversion on combined LNT-SCR catalysts

Lean reduction of NOx (NO + NO2) was conducted over combined LNT-SCR dual-layer and zoned monolithic catalysts using rapid propylene periodic pulsing into a lean feed. We investigated the effects of cycling frequency, reaction exotherm, hydrocarbon (HC) intermediates, top-layer material, LNT ceria content and catalyst configuration on the performance of dual-layer catalysts under fast propylene pulsing. High frequency propylene injection expands the operating temperature window of a conventional NOx storage and reduction (NSR) system in both low and high-temperature regions. The combination of rapid propylene pulsing and the dual-layer catalyst architecture achieves a low-temperature NOx conversion of up to 80% at a feed temperature of ca. 200 degrees C and relevant space velocities (similar to 70 k h(-1)). The working mechanisms of rapid propylene pulsing on both LNT and LNT-SCR catalysts are discussed. Fast cycling facilitates the generation of partially oxidized hydrocarbon intermediates over the LNT that can then either directly react with NOx or act as oxygen scavenger to maintain Pt in a reduced state for direct NO decomposition. The SCR top-layer traps partially oxygenated species that desorb from the LNT layer, enabling further production of N-2 through a LNT-assisted HC-SCR pathway. Optimization of ceria content, top-layer material and catalyst configuration like SCR and PGM zoning can improve system performance at lower cost. (C) 2015 Elsevier B.V. All rights reserved.