KINETIC-STUDY OF HETEROGENEOUS OXYGEN-EXCHANGE REACTIONS AND BULK SELF-DIFFUSION OF OXYGEN

Apparent activation energies are determined for oxygen-isotope exchang e reactions between O-2, CO, or NO and preoxidized or prereduced CaO s urfaces. Oxygen exchange between (NO)-O-16 or (CO)-O-16 and isotope la beled (CaO)-O-18 surfaces produced an apparent activation energy of 2 kcal/mol. Similar values are obtained for single-isotope-exchange betw een (OO)-O-18-O-18 and prereduced (CaO)-O-16 surfaces. Apparent activa tion energies of 15-18 kcal/mol are found for single and double exchan ge 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 resu lt from adsorbed intermediates, whereas the high values may involve th e formation of a singlet O-2 transient. It is shown that eventually th e self-diffusion of oxygen ions in the bulk becomes the rate-determini ng step in the isotope-exchange reaction. Apparent activation energies are determined, and the values are found to depend on surface treatme nt, (i.e., 44 kcal/mol under reducing and 78 kcal/mol under oxidizing conditions). The involvement of oxygen vacancies under reducing condit ions is discussed.