(69b) NOx Storage and Reduction by Multilayer Monolithic Catalysts

It is essential to remove NO_x emissions from diesel engine effluents. This can be accomplished by a double-bed combination of a Lean NOx trap (LNT) with a Selective Catalytic Reduction (SCR) system. During the lean operation (i.e., oxygen in excess) the Pt in the LNT catalyzes the oxidation of NO to NO₂ which is captured as nitrates and nitrites by an alkaline-earth metal oxide component, typically comprised of Barium. The stored NOx is then reduced to a mixture of N₂, N₂O, and NH₃ during a short fuel rich period containing a mixture of H₂, CO, and hydrocarbons. The ammonia that is formed and released during the reduction step is adsorbed by the acidic zeolite component in the SCR bed and later reacts with NO_x to form N₂.To improve the operation of this system and to reduce the amount of required expensive Pt it has been suggested to use short in series sequences of LNT and SCR catalysts.

As an alternative, we synthesized and tested catalysts consisting of multilayers of LNT and SCR deposited on a multi-channel monolith to capture and reduce NO_x to N₂ under simulated exhaust lean-rich atmosphere. We investigated which arrangement of the layers yielded the best NO_x removal efficiency. To determine the best balance between the alkalii sites for NO_x storage, the acidic sites (ZSM-5) for NH₃ storage, and the optimal metal loadings on both LNT (Pt) and SCR (Fe or Cu) catalysts, we conducted experiments to determine the NO_x and NH₃ storage on LNT and SCR catalysts separately under different monolith temperatures. The multilayer catalysts led to higher N₂ selectivity and lower NH₃ selectivity during a larger temperature window than those in series LNT or SCR catalysts. The Cu zeolite exhibited a higher redox activity than Fe-zeolite at low temperature. The implications of the findings in terms of the optimal catalyst design will be discussed.