Effect of Zeolite Topology on Cu Active Site Formation for NO Direct Decomposition

The formation of Cu active sites for nitric oxide (NO) direct decomposition on ZSM-5, mordenite, and low Si/Al beta zeolites under similar Cu ion-exchange conditions was investigated. On all the Cu-zeolites applied in this work, it was observed that with an increase in the Cu ion-exchange level, catalytic activity for NO direct decomposition increased with an S-shaped curve. However, it was demonstrated that the efficiency of Cu active site formation for NO direct decomposition on the zeolites is affected by the zeolite topology. For the zeolites applied in this work, the efficiency of Cu active site formation follows the order: ZSM-5 ∼ mordenite > beta. It was revealed by NO probe IR measurements that Cu ion on different zeolites possesses different oxidation state distribution (Cu+/Cu2+ ratio) and coordination environment, suggesting that they are the origin of the different efficiencies for Cu active site formation for NO direct decomposition. The interpretation of our results combined with previous discussion on the structure of Cu active sites on the ZSM-5 zeolite suggests that the distribution of ion-exchange sites in the zeolite frameworks can affect the oxidation state distribution and coordination environment of Cu sites on the zeolites.