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 N 16 O or C 16 O and isotope labeled Ca 18 O surfaces produced an apparent activation energy of 2 kcal/mol. Similar values are obtained for single- isotope-exchange between 18 O 18 O and prereduced Ca 16 O surfaces. Apparent activation energies of 15−18 kcal/mol are found for single and double exchange between 18 O 18 O and preoxidized Ca 16 O surfaces, as well as for double exchange between 18 O 18 O and prereduced Ca 16 O 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.
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Acke et al. (1998) studied this question.