摘要 : Lean reduction of NO_x (NO and NO2) was studied using monolith-supported catalysts consisting of a layer of a metal-exchanged (Fe, Cu) zeolite (ZSM-5) selective catalytic reduction (SCR) catalyst deposited on top of a Pt/Rh/BaO/Ce... 展开 Lean reduction of NO_x (NO and NO2) was studied using monolith-supported catalysts consisting of a layer of a metal-exchanged (Fe, Cu) zeolite (ZSM-5) selective catalytic reduction (SCR) catalyst deposited on top of a Pt/Rh/BaO/CeO2 lean NO_x trap (LNT) catalyst. During periodic switching between lean and rich feeds, the LNT layer reduces NO_x to N2 and NH3. The SCR layer traps the latter, resulting in additional NO_x reduction. The dual-layer catalysts exhibited high N2 selectivity and low NH3 selectivity over the temperature range of 150-300 °C The NO_x conversion was incomplete due to undesired side reactions in the LNT layer, such as NH3 oxidation to N2O at low temperature and NO_x at high temperature. The NO_x conversion and N2 selectivity of the Cu-exchanged ZSM-5 was higher than that of the Fe-exchanged ZSM-5. This was due to a higher low temperature SCR activity and a higher NH3 storage capacity on the Cu-zeolite. The dual-layer catalyst had a higher NO_x conversion than the LNT catalyst below 300 °C and a higher N2 selectivity over the entire temperature range when H2O and CO2 were present in the feed. The NO_x storage capacity and NH3 generation increased upon addition of CeO2 to the LNT layer below 250 °C. It also led to increased NH3 oxidation at high temperatures. The addition of ceria mitigated the undesirable migration of Pt between the LNT and SCR layers. Hydrothermal aging had a smaller effect on dual-layer catalysts containing ceria. 收起