Kinetic study of heterogeneous oxygen-exchange reactions and bulk self-diffusion of oxygen

Apparent activation energies are determined for oxygen-isotope exchange reactions between O-2, CO, or NO and preoxidized or prereduced CaO surfaces. Oxygen exchange between (NO)-O-16 or (CO)-O-16 and isotope labeled (CaO)-O-18 surfaces produced an apparent activation energy of 2 kcal/mol. Similar values are obtained for single-isotope-exchange between (OO)-O-18-O-18 and prereduced (CaO)-O-16 surfaces. Apparent activation energies of 15-18 kcal/mol are found for single and double exchange between (OO)-O-18-O-18 and preoxidized (CaO)-O-16 surfaces, as well as for double exchange between (OO)-O-18-O-18 and prereduced (CaO)-O-16 surfaces. The low apparent activation energies are believed to result from adsorbed intermediates, whereas the high values may involve the formation of a singlet O-2 transient. It is shown that eventually the self-diffusion of oxygen ions in the bulk becomes the rate-determining step in the isotope-exchange reaction. Apparent activation energies are determined, and the values are found to depend on surface treatment, (i.e., 44 kcal/mol under reducing and 78 kcal/mol under oxidizing conditions). The involvement of oxygen vacancies under reducing conditions is discussed.