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 N-2 and O-2 is addressed. Density functional theory is used to predict molecular and electronic structures and binding energies. N-down dinitrosyl binding to Cu-0, 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 (approximate to 2.8 Angstrom). 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 (approximate to 2.0 Angstrom), and the other similar to hyponitrite binding to a metal atom, displaying a short N-N bond (approximate to 1.2 Angstrom). 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.