Fast cycling for enhanced low-temperature NOx reduction on dual-layer LNT-SCR catalysts
Yang Zheng, Michael P. Harold*, Dan Luss
Department of Chemical & Biomolecular Engineering
University of Houston
Houston, TX 77204
*Corresponding author: firstname.lastname@example.org
The increasing stringent fuel economy standards have been driving exhaust conditions towards net-lean and low temperatures. This poses a great challenge to NOx emission-control systems to meet the ever-tightening tailpipe standards like Tier 3. Toyota recently developed a Di-Air (Diesel NOx aftertreatment by Adsorbed Intermediate Reductants) system, in which rapid short-cycle injection of hydrocarbon (HC) on a lean NOx trap (LNT) catalyst results in generation of short-lived HC intermediates that show high reactivity with NOx . The Di-Air operation significantly increased the LNT NOx conversion at temperatures exceeding 400 oC. A follow-up study from our group showed the rapid propene pulsing expands high NOx conversion temperature window of a LNT catalyst to not only high temperatures but also low temperatures . In this study, we have developed a new system with enhanced low-T deNOx efficiency which utilizes high-frequency HC pulsing over a dual-layer LNT-SCR monolithic catalyst under lean feed conditions. Our proposed system exploits the reported Di-Air mechanism, in which a fraction of partially oxidized HC intermediates generated in the LNT layer can be captured and utilized by a SCR-zeolite (Cu-SAPO-34) top layer via-HC-SCR pathway .
Our experiments consisted of rapid short cyclic injection of propene into a lean NOx stream fed to an aged LNT-SCR dual-layer catalyst. High frequency HC pulsing on the dual-layer reduced the light-off temperature by ca. 50 °C below that of an LNT catalyst subjected to conventional NOx storage and reduction (NSR) cycling. The enhanced low-T performance can be attributed to both chemical and thermal effects. Under high frequency operation the deposited SCR layer enables utilization of intermediate species (CxHyOz and CxHyOzNt) and exotherm from oxidation of the hydrocarbon by O2/NOx in the underlying LNT layer. Increased PGM loading of the dual-layer catalyst from 90 to 120 g/ft3 decreases the light-off temperature to a feed temperature of around 175 °C. We also discuss how variations of the catalyst configuration (layered or sequential), ceria loading, top-layer material (Cu or Ag) may affect the performance of this new deNOx system.
Fig.1 Cycle-averaged NOx conversion as a function of feed temperature for LNT alone and dual-layer catalysts with different cycle timing. Conditions: lean feed: 300 ppm NO + 10 % O2 in carrier gas of 3.5% H2O, 5% CO2 and balanced Ar; rich feed: 1.8% C3H6 + 300 ppm NO + 5% O2 in carrier gas. Flow rate: GHSV = 80,000 h-1.
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