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), gyy= 2.0059 and gxx= 2.0004. Using oxygen isotopically enriched in 17O, it was possible to show that the oxygen nuclei were equivalent with a hyperfine constant Axx= 76 G, while the Ayy and Azz 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 O2– ions and the adsorption sites the Ca2+ surface cations.