Cu-dinitrosyl species in zeolites: A density functional molecular cluster study

A small cluster model proposed earlier to examine bound Cu ions and their interaction with CO and NO in zeolites [Schneider, W. F.; Hass, K. C.; Ramprasad, R.; Adams, J. B. J. Phys. Chem. 1996, 100, 6032] is used to study Cu-bound dinitrosyl complexes. The possibility of a single-step, symmetric, concerted reaction occurring between the two nitrosyl ligands to form either a N-N bond or free N₂ and O₂ is addressed. Density functional theory is used to predict molecular and electronic structures and binding energies. N-down dinitrosyl binding to Cu⁰, Cu⁺, and Cu²⁺ can be represented as [Cu(I)-(NO)₂^-], [Cu(I)-(NO)₂], and [Cu(I)-(NO)₂⁺], respectively, with the dinitrosyl moiety closely resembling the free NO dimer, and having a long N-N bond (≈2.8 Å). Dinitrosyl species bound to Cu through the O display two distinct binding modes, one resembling the N-down dinitrosyl binding, again with a long N-N bond (≈2.0 Å), and the other similar to hyponitrite binding to a metal atom, displaying a short N-N bond (≈1.2 Å). The single-step, symmetric, concerted decomposition reaction of NO in the vicinity of Cu ion sites in zeolites is forbidden by orbital symmetry and is anticipated to have a comparable or higher activation barrier than the same reaction in the gas phase. Metastable hyponitrite complexes, on the other hand, display N-N coupling and may be precursors for a multistep decomposition of NO in the presence of Cu-exchanged zeolites.