(438c) Lean NOx Reduction Over LNT-SCR Dual-Layer Catalyst: Impact of SCR Layer

Authors: 
Luss, D. - Presenter, University of Houston
Zheng, Y., University of Houston
Wilhite, B. A., Texas A & M University



Lean NOx Reduction over
LNT-SCR Dual-Layer Catalyst: Impact of SCR Layer

Yang Zheng, Michael P. Harold*, Dan
Luss*

Department
of Chemical & Biomolecular Engineering

University
of Houston

Houston,
TX 77204

Abstract

Diesel
NOx emission control systems combining Lean NOx Trap
(LNT) catalysts with Selective Catalytic Reduction (SCR) catalysts offer
potential performance advantages for lean NOx reduction. The
combined LNT+SCR system development goal is to meet the more stringent upcoming
EPA Tier 3 emission regulation and/or California's Low Emission Vehicle (LEV III) standards. A promising
configuration for the combined LNT+SCR system is a dual-layer system, which
generates and utilizes NH3 during lean-rich switching. NH3 is
produced in the underlying LNT layer during the rich phase is captured in the
top SCR layer. The stored NH3 enables additional NOx
reduction during the ensuing lean phase. The main objective of this study is to
research and develop LNT-SCR dual-layer catalysts that will achieve high NOx
conversion (> 80%), especially at low temperatures (< 250 oC),
while minimizing the precious group metal (PGM) loading, at application
relevant space velocities (> 50K h-1). In order to meet this
objective we conduct lean-rich cycling for a family of dual layer catalysts
with the aim to elucidate the effect of catalyst composition on performance for
a range of conditions.

We
studied the impact of ceria loading to LNT catalysts on the low-temperature NH3
yield and NOx conversion. The addition of ceria to the LNT catalyst
used in dual-layer application enables a higher NH3 yield and NOx
conversion below 250 oC than does the ceria-free LNT [1]. The SCR
top layer increases the washcoat diffusion resistance. Meanwhile, NH3
trapped in the SCR layer is prone to be oxidized to NOx in the
adjacent LNT layer. The rate of this undesirable reaction increases with
temperature. Both adverse effects decrease the high temperature performance of
the dual-layer catalyst. In order to overcome the architectural drawbacks of
the dual-layer catalyst, we studied which type of SCR zoning can increase the
low-temperature NOx conversion while minimizing the decrease in the
high-temperature performance. To determine which SCR catalyst is best suited
for dual-layer applications, we compared the NH3 storage capacity
and standard SCR activity of Cu-ZSM-5 (10 membered ring) with that of Cu-SSZ-13
(8 membered ring). The use of Cu-SSZ-13 in the dual-layer catalyst improved the
low-temperature performance. Finally, an optimized LNT-SCR dual-layer system
capable of maximizing the low-temperature enhancement and minimizing
high-temperature loss in the cycle-averaged conversion

Fig.1 Comparison
of NOx conversion from LNT3, CuCHA-LNT3 (SCR 0.8 g/in3, 2cm)
and LNT3+CuCHA/LNT3 (SCR0.8g/in3,1cm) using 2.5% CO/H2
reductant mixtures.

Reference

[1]Y. Liu, Y. Zheng, M.P. Harold, and D.
Luss, "Lean NOx Reduction with H2 and CO in Dual-layer
LNT-SCR Monolithic Catalysts: Impact of Ceria Loading", Top. Catal., 56,
104¨C108 (2013).

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