Theoretical study of transition metal cation exchanged zeolites: Interaction with NO

Abstract

DFT studies are applied to Cu(I) exchanged Y zeolite with FAU topology and Cu(I), Cu(II) and Co(II) exchanged silico-alumino-phosphate SAPO-34 with CHA topology; the reactivity of the cations is probed by NO adsorption. Divalent cations are more stable at positions with higher coordination number; they occupy SI sites in the center of D6R (double six-rings) of SAPO-34; the relative stability of these sites depends on the local ordering of negative framework charges. Cation mobility was observed upon NO adsorption: an approaching NO molecule is able to shift a Co(II) cation from the D6R center of SAPO-34 to the more favorable position SII. Cu + cations interact with NO more strongly than Cu(II) and Co(II). In addition to the SII site, where they are most stable, they occupy SIII and SIII′ sites, located in the vicinity of the tilted six-rings of SAPO-34. Cations at these sites are coordinatively unsaturated; they are less stable, but of highest reactivity. Monovalent copper cations which are located at site SIII form a metastable adsorption complex; the Cu + cation shifts upon NO adsorption to the more stable SIII′ site of SAPO-34. The accessibility of cation sites inside the sodalite cages of Y zeolite (SI′ sites) for adsorbate molecules (NO) is assessed by the cluster method, using hybrid functional (B3LYP) and by the ONIOM method, where B3LYP is used for the model system and the semiempirical PM6 method is applied to the real system. The stability of the adsorption complex at different cation sites is examined also by periodic DFT. Cu + cations at SI′ sites form adsorption complexes of comparable stability to those at the SII sites in the supercage.