Dinitrogen Irreversible Adsorption on Over-Exchanged Cu-ZSM-5

Dinitrogen (N2) adsorption on overexchanged Cu-ZSM-5 (Si/Al ) 25, 104% ion exchange from copper acetate)was studied at 302 K by diffuse reflectance infrared spectroscopy (DRIFTS), N2-adsorption isotherms, and temperature-programmed N2 desorption (TPD). We found that Cu-ZSM-5 adsorbs N2 at least in two different modes. The weaker mode, evidenced by the appearance of an IR band at 2295 cm-1, was already reported in the literature and assigned to N2 coordinated on-top on Cu(I) sites. This kind of adsorption occurs only under N2 atmosphere and is completely reversible at 302 K. The stronger mode, revealed by TPD and by the difference between total and reversible N2-adsorption isotherms, is completely irreversible at 302 K. The band at 2130 cm-1, previously assigned by us to this kind of strong N2 adsorption, is not observed if ultrapure gases are utilized. The introduction of controlled amounts of impurities, together with the utilization of 15N2 and 13CO shows that this band is instead related to hydrated Cu(I)-CO species formed by accumulation on the sample of traces of H2O in the N2 flow and from residual CO produced during activation. The coordination of N2 onto Cu(I)...Cu(I) dimers is proposed as an adequate model to describe this IR-silent irreversible N2 adsorption.