Two kinds of pammagnetic 05 ions have been identified by contacting molecular oxygen wrth Ce-X zeolite heat treated at 200°C in vacua. The first 05 species corresponds to the usual 03 ions adsorbed on cerium oxide with the foIlowing parameters: gr =ga = 2.035 f 0.002, ga =ga = gl = 2.0095 -C 0.0005 and a hyperfine pattern of six and eleven lines centred about ga 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 unusual ESR parameters and hyperfme structure. One has: gr = 2.0242,g~. = 2.0208 andga = 2.0112 + 0.0003;cr = 24 f 3, c2 = 66 f 1 and cg = 12 t 3 Oe. The six and eleven hyperfine line pattern which has been observed shows that the unpaired spin is equally distributed between two oxygen nuclei. The g tensor values are interpreted in terms of almost complete averaging of gr and ga components due to rotational motion at -196'C about axis 3 (nx). The species is attributed to 05 ion formed according to the equtliirium: -) Ce(OH) '+ + 02 P s Ce(OH) 3+gx .
The Ce ions are located in cationic sites SII withii the zeolite supercages and are only we&ly bonded to the oxygen Lading to rotational motion and to ease of desorption, the species reversibly disappearing by outgassing at room temperature.