CLUSTER-MODELS OF CU BINDING AND CO AND NO ADSORPTION IN CU-EXCHANGEDZEOLITES

A small cluster model is proposed and used to examine the properties o f 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 th e framework oxygens of the zeolite lattice that form the local coordin ation environment of the Cu ion. Variations in the oxidation state of the metal center are simulated by adjusting the net charge on the clus ters. Density functional theory is used to predict the molecular and e lectronic structures and binding energies of these model clusters, inc luding Cu(H2Oxn+), Cu(H2O)(x)COn+, and Cu(H2O)(x)NOn+ (x = 1-4, n = 0- 2). While quite simplistic, this model provides considerable insight i nto the behavior and interactions of zeolite-bound Cu ions. Both Cu+ a nd Cu2+ ions are found to bind strongly to H2O (or bridge oxygen) liga nds, with Cu2+ preferring higher and Cu+ preferring lower coordination numbers. CO and NO also bind strongly to both Cu ions. Cu2+ preferent ially binds the three ligands in the order Cu2+-NO > Cu2+-OH2 > Cu2+-C O while Cu+ exhibits an almost equal affinity for the three. Bare Cu-0 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 re duction processes. The Cu-OH2n+ and Cu-COn+ interactions are primarily electrostatic, but the Cu-NOn+ interactions have a large covalent com ponent that complicates their electronic structures and makes assignme nt of Cu oxidation states difficult. Three modes of NO binding on Cu a re predicted, represented approximately as [Cu(I)-(N=O)(+)], [Cu(I)-(N =O-.)], and [Cu(I)-(N=O)(-)]. The implications of these results for un derstanding Cu-exchanged zeolites is discussed, as are the limitations and possible extensions of the H2O ligand model.