Fast Cycling for Enhanced Low-Temperature NOx Reduction on Dual-Layer LNT-SCR Catalysts

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: mharold@uh.edu

Abstract

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 [1]. 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 [2]. 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 [3].

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 (C_xH_yO_z and C_xH_yO_zN_t) 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.

 

References

1.       Bisaiji, Y., Yoshida, K., Inoue, M., et al., SAE Int. J. Fuels Lubr. 5(1):380-388 (2012).

2.       Perng, C.C.Y., Easterling, V.G., Harold, M.P., Catal. Today 231:125-134 (2014).

3.       Zheng, Y., Li, M., Harold, M., Luss, D., SAE Int. J. Engines 8(3) (2015)