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. Copyright © 2005 John Wiley & Sons, Ltd.