Nitrogen Monoxide Interaction with Cu(I) Sites in Zeolites X and Y: Quantum Chemical Calculations and IR Studies

The NO molecules adsorbed on Cu(I) sites in FAU lattice show strong red shift of stretching bond frequencies in comparison to the gas-phase NO. Three IR mononitrosyl bands can be found in Cu(I)Y (1815, 1793, and 1780 cm⁻¹) and two in Cu(I)X (1772 and 1757 cm⁻¹), respectively. Theoretical models allow association of all these bands with the species adsorbed on site II. The highest band in Cu(I)Y (1815 cm⁻¹), not observed in this work, is assigned to mononitrosyl species inside 6T rings with 1Al and Cu(I) coordinated to a single tetrahedron; the middle one (1793 cm⁻¹) derives both from 6T/1Al rings with 2(2) coordination and from 6T/2Al rings with 2(1) coordination, while the lowest band (1780 cm⁻¹) is due to the 6T/2Al rings with copper bound with two framework tetrahedrons. As for Cu(I)X, the higher (1772 cm⁻¹) and the lower (1757 cm⁻¹) bands are assigned to the adsorption complexes inside 6T/3Al rings with 2(1) and 3(3) coordination, respectively. No experimental mononitrosyl band is assigned to the species in site III, which may be populated by Cu(I) ions in Cu(I)X. This results from easier formation of dinitrosyls in this site, due to its better accessibility for the adsorbing molecules and slightly higher heat of the second NO adsorption. Dinitrosyls bands, about 1720 and 1820 cm⁻¹ for Cu(I)Y and 1700 and 1820 cm⁻¹ for Cu(I)X, do not show clear site specificity. Cu(I) in site II binds NO molecule much weaker than it does in site III. It is due to very stable planar 3-fold coordination to the framework O atoms, which is achieved in site II. The heats of the second NO molecule adsorption are only slightly higher at site III than at site II. This is because upon first NO molecule adsorption on site II, Cu(I) ions are displaced to the position more resembling site III. Low heat of adsorption may be regarded as one of the reasons responsible for lower Cu(I)FAU activity in the direct decomposition of NO in comparison to Cu(I)MFI.