A computational study on N2 adsorption in Cu-ZSM-5

The present computational study investigates the adsorption of N2 by Cu-ZSM-5, with particular regard to the interaction with pairs of Cu+ ions, employing simple cluster models in the calculations. It shows that several interaction patterns between N2 and couples of Cu+ sites are possible within the Cu-ZSM-5 structure. In particular, when pairs of Cu+ ions are located at opposite sides of ten-membered rings, in the region where linear and sinusoidal channels intersect each other, a quasi-linear Cu–N–N–Cu adsorption occurs. Although lattice restraints cause small deviations from linearity, such interaction turned out to be more favourable than other adsorption patterns within the Cu-ZSM-5 structure. The linearity of the Cu–N–N–Cu fragment and the relatively low concentration of the related sites cause a low extinction coefficient for the N–N IR stretching mode, which is usually detected with very low intensity or not detected at all. The results of the present calculations may explain the experimental evidence for a nearly IR-silent fraction of nitrogen strongly adsorbed in the Cu-ZSM-5 catalyst which, as shown in a previous work, is linearly related to the number of active sites for NO decomposition.