The adsorption of NO onto copper-exchanged ZSM-5 zeolite has been studied by EPR spectroscopy. A distinct Cu+-NO species possessing an end-on è1 bent structure has been identified. Its EPR spectrum with a well-defined superhyperfine coupling with 63,65Cu nuclei and hyperfine coupling with 14N was interpreted in terms of completely anisotropic g and hyperfine tensors with noncoincident axes (monoclinic symmetry). The spin Hamiltonian parameters of this adduct have been analyzed in detail, leading to a semiquantitative molecular orbital correlation picture of the complex. The model developed shows that the unpaired electron resides mainly on the angularly coordinated NO and the copper superhyperfine structure arises from delocalization of the unpaired electron density onto Cu a¢ orbitals (3dz2, 3dxz, and 4s). The total spin density on copper is found to be equal to 0.2 and is shared among 3dz2 (0.079), 3dxz (0.021), and 4s (0.1) orbitals. The remaining part of the unpaired electron density is localized on nitrogen (0.55) and oxygen (0.25) atoms. The bonding interaction involves essentially an overlap of Cu 3dz2 and 3dxz with an antibonding 2p ð* and a lone pair n orbital of the NO ligand as well as a 3dyz overlap with a second orthogonal NO 2p ð* orbital. The appropriate molecular orbital diagram has been devised to account for the EPR data. The model is consistent with the observed magnetic properties of the investigated adducts and satisfactorily explains the previously unrecognized complex source of hyperfine couplings. The implications of this coordination mode on the possible molecular mechanism of the catalytic decomposition of NO over Cu/ZSM-5 catalysts are discussed.
EPR Investigation of the electronic structure of mononuclear Copper (I) nitric oxide adduct formed upon low pressure adsorption of NO onto Cu/ZSM5 zeolites
GIAMELLO, Elio
1997-01-01
Abstract
The adsorption of NO onto copper-exchanged ZSM-5 zeolite has been studied by EPR spectroscopy. A distinct Cu+-NO species possessing an end-on è1 bent structure has been identified. Its EPR spectrum with a well-defined superhyperfine coupling with 63,65Cu nuclei and hyperfine coupling with 14N was interpreted in terms of completely anisotropic g and hyperfine tensors with noncoincident axes (monoclinic symmetry). The spin Hamiltonian parameters of this adduct have been analyzed in detail, leading to a semiquantitative molecular orbital correlation picture of the complex. The model developed shows that the unpaired electron resides mainly on the angularly coordinated NO and the copper superhyperfine structure arises from delocalization of the unpaired electron density onto Cu a¢ orbitals (3dz2, 3dxz, and 4s). The total spin density on copper is found to be equal to 0.2 and is shared among 3dz2 (0.079), 3dxz (0.021), and 4s (0.1) orbitals. The remaining part of the unpaired electron density is localized on nitrogen (0.55) and oxygen (0.25) atoms. The bonding interaction involves essentially an overlap of Cu 3dz2 and 3dxz with an antibonding 2p ð* and a lone pair n orbital of the NO ligand as well as a 3dyz overlap with a second orthogonal NO 2p ð* orbital. The appropriate molecular orbital diagram has been devised to account for the EPR data. The model is consistent with the observed magnetic properties of the investigated adducts and satisfactorily explains the previously unrecognized complex source of hyperfine couplings. The implications of this coordination mode on the possible molecular mechanism of the catalytic decomposition of NO over Cu/ZSM-5 catalysts are discussed.
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