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

Abstract

We prepared H-ZSM-5 (Si/Al=80), amorphous mesoporous silica–alumina (MSA, Si/Al=90), and Silicalite-1 (S-1) according to the methods described by Bellussi and co-workers, and compared their ion-exchange capacity for copper ions. The Cu-ZSM-5, Cu–MSA 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 0.008 M≤[Cu 2+]≤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 that 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 [Cu n O x (OH) y ] q+ species ( q=2( n− x)− y≥0). These results were confirmed by TPR studies. At 773 K only over-exchanged Cu-ZSM-5 catalysts showed NO decomposition activity (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 under our experimental conditions. The present results confirm that the framework topology and the presence of framework AlO − 4 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 AlO − 4 species. The most active sites, as they occur only on ZSM-5 support with the lower Si/Al atomic ratios, might consist of dimeric Cu species (Cu +⋯Cu 2+⋯O −) strongly anchored to next-nearest-neighbour framework AlO 4 − species.