Cu-ZSM-5 (Si/Al=66), Cu-Fe-S-1 (Si/Fe=66) and Cu-S-1 catalysts for NO decomposition: preparation, analytical characterization and catalytic activity

We prepared H-ZSM-5 (Si/Al = 66), Fe-S-1 (Si/Fe = 66), and Silicalite-1 (S-1) according to the methods described by Bellussi and coworkers, and compared their ion-exchange capacity for copper ions. The Cu-ZSM-5, Cu-Fe-S and Cu-S-1 samples thus obtained have been investigated as catalysts for the NO decomposition reaction at 773 K. We found that using copper acetate solutions with concentrations in the range 0.004 M less than or equal to[Cu2+]less than or equal to 0.1 M, at room temperature and at 323 K, it is very easy to prepare overexchanged Cu-ZSM-5, Cu-Fe-S-1, and Cu-S-1 catalysts. XRD and Vis-UV DRS techniques show that after thermal treatments of the fresh samples in air at 823 K for 4 h no segregation of bulk CuO phase occurs, suggesting the presence of low nuclearity [CunOx(OH)(y)](q+) species [q=2(n-x)-y greater than or equal to 0]. These results were confirmed by TPR studies. At 773 K, only Cu-ZSM-5 and Cu-Fe-S-1 catalysts showed NO decomposition activity (NO 1% in He, W/F = 0.2 g s cm(-3)). Instead, the activities of Cu-S-1 and of the copper-free matrices (S-1, H-ZSM-5 and Fe-S-1) were not measurable under our experimental conditions. The negative results obtained with Cu-S-1 catalysts clearly demonstrate that the active sites for NO decomposition in Cu-MFI catalysts consist of polynuclear ionic copper species strongly anchored to framework [MO4](-) species (M=Al, Fe).