Synergic effect of Pd/γ-alumina and Cu/ZSM-5 on the performance of NOx storage reduction catalyst

NOx reduction with a combination of catalysts, Pd catalyst, NOx storage reduction (NSR) catalyst and Cu/ZSM-5 in turn, was investigated to elucidate for the high NOx reduction activity of this catalyst combination under oxidative atmosphere with periodic deep rich operation. The catalytic activity was evaluated using the simulated exhaust gases with periodically fluctuation between oxidative and reductive atmospheres, and it was found that the NOx reduction activity with this catalyst combination was apparently higher than that of the solely accumulation of these individual activities, which was caused by the additional synergic effect by this combination. The Pd catalyst upstream of the NSR catalyst improved NOx storage ability by NO2 formation under oxidative atmosphere. The stored NOx was reduced to NH3 on the NSR catalyst, and the generated NH3 was adsorbed on Cu/ZSM-5 downstream of the NSR catalyst under the reductive atmosphere, and subsequently reacted with NOx on the Cu/ZSM-5 under the oxidative atmosphere.