Electron spin resonance studies of Cu(I)-NO complexes formed over copper-exchanged three- and unidimensional zeolites.

Cu(I)-NO complexes, one of the essential intermediates in the NO decomposition reaction, were formed over copper exchanged and autoreduced Cu-ZSM-5, Cu-MCM-58, Cu-ZSM-12, and Cu-L zeolites, and studied by electron spin resonance spectroscopy at X-, Q-, and W-band frequencies. The spin Hamiltonian parameters of the NO adsorption complexes formed over pretreated materials firmly confirm the formation of Cu(I)-NO moieties. Two different Cu(I)-NO species A and B that are formed on account of different numbers of framework oxygen atoms coordinating to the Cu(I) cation are observed for Cu-ZSM-5, Cu-MCM-58, and Cu-ZSM-12 zeolites, while formation of a single Cu(I)-NO species B is observed in Cu-L zeolite. On the basis of the isotropic copper hyperfine couplings and the different channel topologies of the studied zeolite frameworks, we assign species A and B to Cu(I)-NO complexes formed at M5(7)-type and I2-type Cu(I) cation sites with either three or two oxygen co-ligands, respectively, supporting the results of previous quantum chemical studies on the Cu-ZSM-5 reference system. Whereas accessible five- or six-membered rings are clearly a prerequisite for M5(7)-type Cu(I) adsorption sites, the formation of I2-type sites in the unidimensional Cu-MCM-48, Cu-ZSM-12, and Cu-L zeolites suggests that not just channel intersection but also other structural motifs with exposed AlO4 tetrahedra can constitute such I2-type sites provided that sufficiently large channels can freely accommodate the Cu(I)-NO species.