A comparison between Cu-ZSM-5, Cu-S-1 and Cu-mesoporous-silica-alumina as catalysts for NO decomposition

We prepared H-ZSM-5 (Si/Al=80), amorphous mesoporous silica-alumina (MSA, S i/Al=90), and Silicalite-1 (S-1) according to the methods described by Bell ussi and co-workers, and compared their ion-exchange capacity for copper io ns. The Cu-ZSM-5, Cu-MSA and Cu-S-1 samples thus obtained have been investi gated as catalysts for the NO decomposition reaction at 773 K. We found that using copper acetate solutions with concentrations in the ran ge 0.008 M less than or equal to[Cu2+]less than or equal to 0.1 M, both at room temperature and at 323 K, it is very easy to prepare over-exchanged Cu -ZSM-5, Cu-S-1 and Cu-MSA catalysts. XRD and Vis-UV DRS techniques show tha t after thermal treatments of the fresh samples in air at 823 K for 4 h no segregation of 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 r esults were confirmed by TPR studies. At 773 K only over-exchanged Cu-ZSM-5 catalysts showed NO decomposition act ivity (NO 1% in He, W/F=0.1 gs/cm(3)) with a turnover frequency essentially determined by the Si/Al ratio, in agreement with previous literature data. Instead, the activity of Cu-S-1 and Cu-MSA catalysts was not measurable un der our experimental conditions. The present results confirm that the framework topology and the presence of framework AlO4-, species are fundamental to develop active copper species for NO decomposition. It is demonstrated that the active sites in Cu-ZSM-5 consist of copper species strongly anchored to framework AlO4- species. The most active sites, as they occur only on ZSM-5 support with the lower Si/A l atomic ratios, might consist of dimeric Cu species (Cu+... Cu2+... O-) st rongly anchored to next-nearest-neighbour framework AlO4- species. (C) 1999 Elsevier Science B.V. All rights reserved.