Cluster models of Cu binding and CO and NO adsorption in cu-exchanged zeolites

A small cluster model is proposed and used to examine the properties of bound Cu ions and their interactions with CO and NO in Cu-exchanged zeolites, such as Cu-ZSM-5. The model uses H₂O ligands to represent the framework oxygens of the zeolite lattice that form the local coordination environment of the Cu ion. Variations in the oxidation state of the metal center are simulated by adjusting the net charge on the clusters. Density functional theory is used to predict the molecular and electronic structures and binding energies of these model clusters, including Cu(H₂O)_xⁿ⁺, Cu(H₂O)_xCOⁿ⁺, and Cu(H₂O)_xNOⁿ⁺ (x = 1-4, n = 0-2). While quite simplistic, this model provides considerable insight into the behavior and interactions of zeolite-bound Cu ions. Both Cu⁺ and Cu²⁺ ions are found to bind strongly to H₂O (or bridge oxygen) ligands, with Cu²⁺ preferring higher and Cu⁺ preferring lower coordination numbers. CO and NO also bind strongly to both Cu ions. Cu²⁺ preferentially binds the three ligands in the order Cu²⁺-NO > Cu²⁺-OH₂ > Cu²⁺-CO while Cu⁺ exhibits an almost equal affinity for the three. Bare Cu⁰ is weakly bound to H₂O and is unlikely to be stable within a zeolite, but both CuCO⁰ and CuNO⁰ may exhibit some stability as products of reduction processes. The Cu-OH₂ⁿ⁺ and Cu-COⁿ⁺ interactions are primarily electrostatic, but the Cu-NOⁿ⁺ interactions have a large covalent component that complicates their electronic structures and makes assignment of Cu oxidation states difficult. Three modes of NO binding on Cu are predicted, represented approximately as [Cu(I)-(NsO)⁺], [Cu(I)-(N=O*)L and tCu(I)-(N=O)^-]. The implications of these results for understanding Cu-exchanged zeolites is discussed, as are the limitations and possible extensions of the H₂O ligand model.