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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry