Oxygen exchange between C18O2 and basic metal oxides (CaO, MgO, ZrO2, ZnO)

A comparative Temperature Programmed Desorption study of the interaction of C(18)O(2) with the surface of different basic metal oxides (MgO, CaO, ZrO(2), ZnO) has been carried out. Under our experimental conditions, C(18)O(2), C(18)O(16)O and C(16)O(2) have been detected as desorption products by mass spectrometry. The isotopic distribution depends on the desorption temperature and at higher temperatures the oxygen exchange between chemisorbed C(18)O(2) and the solid surfaces is nearly total for all the studied oxides. Complementary information has been obtained from the infrared spectra of CO(2) adsorbed at room temperature and after outgassing at increasing temperatures. It is found that the strength of the adsorption sites for carbon dioxide follows the order CaO > MgO > ZrO(2) > ZnO. The extension of the oxygen exchange can be related with the surface structure of the metal oxide and with the type of CO(2) adsorbed species. The mechanism of oxygen exchange seems to be connected with the presence of bidentate or polydentate carbonates. The oxygen exchange reaction results from the migration of carbon dioxide on the oxide surface, which occurs at higher desorption temperatures. CaO shows a minimum oxygen exchange capacity.