18O tracer studies of CO oxidation with O2 on MoO3. Part 1.—Diffusion of 18O atoms from active sites during the catalysis and the determination of the number of active sites

Abstract

The catalytic oxidation of CO with ¹⁸O₂ has been carried out on MoO₃ at 753 K. The isotopic composition of the product CO₂ was examined as a function of time. The amount of C ¹⁸O₂ was always negligible. The percentage of C ¹⁸O ¹⁶O increased rapidly at the beginning of the reaction then gradually reached a plateau value. Interruption of the oxidation did not significantly affect the percentage of C ¹⁸O ¹⁶O in the CO₂ produced. Oxygen vacancies produced by the reduction of MoO₃ with CO apparently remained unchanged on the surface after evacuation for 120 min at 718 K. The incorporation of O₂ into oxygen vacancies took place rapidly. A small fraction of ¹⁸O atoms taken up into oxygen vacancies were recovered in subsequent CO reduction processes, while the greater part of the ¹⁸O diffused into the bulk of the MoO₃. The catalytic oxidation of CO with ¹⁸O₂ on MoO₃ could be explained in terms of mobile and immobile sites with regard to ¹⁸O atoms taken up into vacancies of active oxygen. This model enables calculation of the time dependence of the yield of C ¹⁸O ¹⁶O in the product CO₂ and estimation of the number of immobile sites.