Pyridine-induced formation of 17O –2 adsorbed on thermally activated CaO

Abstract

Whatever the pretreatment of CaO (thermal activation or γ-irradiation in the presence of H₂), the molecular ion O^–₂ could not be observed by direct oxygen adsorption. It was necessary to adsorb pyridine before oxygen to stabilize the molecular ion O^–₂ characterised by g′_zz= 2.169, g″_zz= 2.093 (major site), g_yy= 2.0059 and g_xx= 2.0004. Using oxygen isotopically enriched in ¹⁷O, it was possible to show that the oxygen nuclei were equivalent with a hyperfine constant A_xx= 76 G, while the A_yy and A_zz values of the tensor were too small to be resolved. The molecular ion O^–₂ was adsorbed with its internuclear axis (z) parallel to the surface plane and was shown to exchange readily with gas phase oxygen. O^–₂ could be eliminated by further pyridine adsorption and restored again by a consecutive oxygen adsorption. Several such cycles could be produced easily. For the O^–₂ molecular ions, the electron donor sites are thought to be the surface coordinatively unsaturated O^2– ions and the adsorption sites the Ca²⁺ surface cations.