Applied and Fundamental Studies of LNT-SCR Dual-layer Monolithic Catalysts for Lean NOx Emission Control
The increasingly stringent both greenhouse gas (GHG) and tailpipe NOx emission standards have driven the continuous improvement of commercial deNOx technologies, NOx reduction & storage (NSR, also referred to as lean NOx trap (LNT)) and selective catalytic reduction (SCR) technologies. This dissertation conducts applied and fundamental studies of coupled LNT-SCR dual-layer catalysts with the aim of expanding the operating temperature window of a conventional NSR system at lower cost. This is accomplished by a systems approach to identify the influencing factors such as catalyst composition and architecture, types of reducing agents, operating and regeneration strategies, as well as synergistic interactions between the LNT and SCR. We start with performance evaluation of dual-layer catalysts under different regeneration conditions such as H2 alone, CO/H2 mixture and a simulated diesel exhaust containing the CO/H2/C3H6 mixture. Spatial analyses of NH3 yield and NOx conversion along the LNT monolith identify the upstream zone as major NH3 generator and NOx reducer, especially at temperatures exceeding 300 oC. Zoning of either or both the SCR and LNT having a dual-layer structure enables an increase in the low-temperature (200-250 oC) NOx conversion, and minimizes the high temperature (300-400 oC) conversion loss caused by the SCR diffusion resistance and undesired NH3 oxidation by the LNT. The hydrocarbon (HC) reductant leads to an alternative LNT-SCR synergy to classical NH3-pathway; a LNT-assisted HC-SCR pathway. The LNT promotes the formation of partially oxidized HC intermediates during the rich purge which are otherwise difficult to be generated by the Cu-zeolite layer at low temperatures. These activated intermediates can be captured and utilized by the SCR catalyst via HC-SCR during the ensuing lean phase. This pathway plays a major role at low temperatures (<= 225 oC) using the simulated diesel exhaust feed. We investigated the steady-state and transient effects of reductants (CO, H2 and C3H6) on Cu-SSZ-13 catalyzed NH3-SCR as the SCR component in the combined system is periodically exposed to a rich exhaust. The three reductants affect to different extent the NH3-SCR reactions. Propylene is most effective in promoting NO2 reduction to NO by formation of organic intermediates. CO effectively reduces nitrates to nitrites that react with NO2, releasing NO. H2 follows a similar pathway as CO but is less effective. Finally, the effects of the lean/rich cycling frequency on both LNT and combined catalysts are investigated. Rapid C3H6 pulsing into a lean exhaust steam expands the operating temperature window of a conventional NSR system in both low and high-temperature regions. The combination of rapid propylene pulsing and the dual-layer catalyst architecture achieves the highest low-temperature NOx conversion. The working mechanisms of rapid propylene pulsing on both LNT and LNT-SCR catalysts are elucidated. Optimization of top-layer material and catalyst configuration like SCR and PGM zoning can improve system performance at lower cost.
Lean NOx traps, Selective catalytic reduction (SCR), NOx reduction
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