(600c) Reduction of Precious Metal Loading in LNT-SCR Dual-Layer Monolithic Catalyst: Using H2 and CO As Reductants

Efficient and economical reduction of NO_x emissions from lean burn gasoline and diesel engines is still a major technological challenge. A recently developed catalyst for this application is the dual-layer catalyst in which a layer of Selective Catalytic Reduction (SCR) catalyst (Cu/ZSM-5) is deposited on top of a layer of Lean NO_x Trap (LNT) catalyst (Pt/Rh/BaO/CeO₂/Al₂O₃). The catalyst is operated with periodic switching between a fuel-lean to a fuel-rich feed. NO_x is stored on the LNT during the fuel-lean feed and generates NH₃ during the fuel-rich feed. The generated NH₃ is captured by the top SCR layer during the fuel-rich feed and reduces the fed NO_x during the fuel-lean mode. The objective of this study is to design the LNT/SCR to achieve improved performance (NO_x conversion and N₂selectivity) over a wide range of conditions and achieve this with a moderate loading of precious metal.

The NH₃ produced in the LNT layer is the most important link between the LNT and SCR catalyst layers. In this study we are investigating the impact of a reductant (either H₂ or a mixture of CO/H₂) on the NO_x conversion and NH₃ production in the LNT. The affinity of CO for the Pt leads to self-poisoning of the LNT below 250 ºC, lowering NO_x conversion and NH₃ production. The addition of ceria to the LNT layer mitigates the CO inhibition. However, excessive ceria load in the dual-layer catalyst intensifies the NH₃ oxidation above 250 ºC, lowering the NO_x conversion. To address this trade-off, zoned, dual-layer catalysts in which the first part contains no ceria while the second half contains ceria give improved performance for NO_x removal over a wide range of temperatures using feeds containing either H₂ or a mixture of CO/H_2. To examine if the PGM loading can be reduced while maintaining high NO_x conversion the SCR loading is manipulated. We show that the low-temperature NO_x conversion from an aged dual-layer catalyst can be increased with a higher SCR catalyst loading. Above 250 °C, the NO_x reduction from dual-layer catalyst was controlled by the NO_x storage and the sequentially NH₃ formation form the bottom LNT layer, while the SCR loading in the top layer has a little impact on the high-temperature NO_x conversion.