(351d) Fast Cycling for Enhanced Low-Temperature NOx Reduction on Dual-Layer LNT-SCR Catalysts

Authors: 
Zheng, Y. - Presenter, University of Houston
Harold, M. - Presenter, University of Houston
Luss, D. - Presenter, University of Houston

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 (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.

 

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)