Performance of Cu-ZSM-5 in a Coupled Monolith NSR-SCR System for NO_x Removal in Lean-Burn Engine Exhaust

The viability of hybrid systems conformed by a combination of Pt–Ba/Al₂O₃ NSR and Cu-ZSM-5 SCR catalysts is studied with both catalysts over monolithic structures in order to maximize the removal of NO_x from lean-burn engines exhaust gases. To optimize performance of these coupled NSR-SCR systems, the NSR catalyst has to work under operational conditions completely different from those optimal when the NSR-only catalyst works, as in the coupled system the intermediate production of ammonia needs to be apparently increased, and then adsorbed in the SCR catalyst (Cu-ZSM-5) which is placed downstream in order to complete elimination of remaining NO_x (NO+NO₂) selectively to N₂ with minimal amounts of other nitrogen derivates, i.e. N₂O and NH₃. The suitable control of operational conditions, specifically H₂ concentration and duration of the rich period, allows establishing the optimal working conditions of the NSR-SCR double system in the practical application, which resulted in temperature around 200 °C and H₂ concentration in excess (3–4 %).