Oxygen species adsorbed on CeX zeolites identified by ESR spectroscopy

Two kinds of paramagnetidc O^?₂ ions have been identified by contacting molecular oxygen with CeX zeolite heat treated at 200°C in vacuo. The first O^?₂ species corresponds to the usual O^?₂ ions adsorbed on cerium oxide with the following parameters: g₁ = g_? ≈ 2.035 ± 0.002, g₂ = g₃ = g_⊥ = 2.0095 ± 0.0005 and a hyperfine pattern of six and eleven lines centred about g₃ and split by about 78 Oe, the other two components being not resolved. The second paramagnetic species of much higher intensity (larger by 2 or 3 orders of magnitude) presents rather ESR parameters and hyperfine structure. One has: g₁ = 2.0242, g₂ = 2.0208 and g₃ = 2.0112 ± 0.0003; c₁ = 24 ± 3, c₂ = 66 ± and c₃ = 12 ± 3 Oe. The six and eleven hyperfine line pattern which has been observed shows that the unpaired spin is qually distributed between two oxygen nuclei. The g tensor values are interpreted in terms of almost complete averaging of g₁ and g₂ components due to rotational motion at ?196°C about axis (3 (χ_ξ). The species is attributed to O^?₂ ion formed according to the equilibrium:

. The Ce ions are located in cationic sites SII within the zeolite supercages and are only weakly bonded to the oxygen leading to rotational motion and to case of desorption, the species reversibly disappearing by outgassing at room temperature.