O-18 TRACER STUDIES OF CO OXIDATION WITH O2 ON MOO3 .3. REACTION-MECHANISM OF OXYGEN-ISOTOPE EXCHANGE BETWEEN CO2 AND MOO3

The kinetics and mechanism of the oxygen exchange between C O-18(2) an d MoO3 have been examined in the temperature range 673-793 K and in th e pressure range 24-146 Pa. The rate of oxygen exchange was proportion al to the pressure of CO2 with an activation energy of 94.5 kJ mol-1, which was smaller than that for the catalytic oxidation of CO with O2 on MoO3 by 22.1 kJ mol-1. The rate constant of oxygen exchange between CO2 and MoO3, R(CO2), was 51-25 times larger than that for the cataly tic oxidation of CO with O2 on MoO3, k(CO2), at 673-793 K; both had si milar frequency factors and fractions of effective collisions were est imated to be almost the same. The difference between R(CO2) and k(CO2) originated from the difference in the activation energies between the two reactions. An active complex with a unidentate carbonate was prop osed in the oxygen exchange mechanism.