Abstract
A mordenite-type zeolite exchanged with Pd(NH 3) 4 2+ ions has been studied by EPR spectroscopy after heat treatment at 500°C in flowing oxygen and then in vacuo. The signal with g ∥ = 2.97 and g ⊥ = 2.146 was unambiguously assigned to Pd(I) ions bonded to four lattice oxygen ions in a pseudo-square-planar symmetry at the center of an eight- or a twelve-membered ring of the zeolite. Pd(I)CO or Pd(I)OH (or H 2O) adducts were formed and characterized by g ∥ = 2.41 and g ⊥ = 2.105 or g ∥ = 2.35 and g ⊥ = 2.105 , respectively The decrease in the g values with respect to initial Pd(I) ions was interpreted in terms of the ligand effect, with the CO or the OH adduct in an axial position of a pyramidal square-planar complex, and the equatorial positions occupied by the four lattice oxygen ions. The adsorption of molecular oxygen led to a new EPR signal with g ⊥ = 2.050 and g ∥ 1.99 and a hyperfine structure of a sextet and an eleven-line pattern of 77-gauss splitting due to singly or doubly 17O-labeled diatomic oxygen species. The spectrum was attributed either to an O 2 − ion freely rotating about the internuclear axis or to an O 2 3− ion of V-center type.
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