18O tracer studies of CO oxidation with O2 on MoO3. Part 3.—Reaction mechanism of oxygen isotope exchange between CO2 and MoO3

Abstract

The kinetics and mechanism of the oxygen exchange between C¹⁸O₂ and MoO₃ have been examined in the temperature range 673–793 K and in the pressure range 24–146 Pa. The rate of oxygen exchange was proportional 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 catalytic oxidation of CO with O₂ on MoO₃, k_co2, at 673–793 K; both had similar frequency factors and fractions of effective collisions were estimated 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 proposed in the oxygen exchange mechanism.