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

Lean NO_x 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 NO_x emission control systems combining Lean NO_x Trap (LNT) catalysts with Selective Catalytic Reduction (SCR) catalysts offer potential performance advantages for lean NO_x 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 NO_x 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 NO_x 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 NO_x conversion. The addition of ceria to the LNT catalyst used in dual-layer application enables a higher NH₃ yield and NO_x 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 NO_x 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 NO_x 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 NO_x 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 NO_x Reduction with H₂ and CO in Dual-layer LNT-SCR Monolithic Catalysts: Impact of Ceria Loading", Top. Catal., 56, 104¨C108 (2013).