The mechanism of the catalytic oxidation of CO with 0, on an Mo catalyst supported on silica has been studied by a tracer technique using 1 8 0 2 . The supported Mo catalyst was prepared by using the ready reaction between Mo(q3c3H5)4 and the OH groups on silica. Extended X-ray absorption fine structure (EXAFS) spectroscopy showed that the Mo species attached to silica are dispersed atomically and have a dioxostructure. At steady state during oxidation with CO/O, = 2/1, 32% of the attached Mo species were estimated to be present as Mo6+ and 68% as Mo4+. The catalytic oxidation was accompanied by oxygen isotope exchange between 0, molecules. The oxidation of the oxostructure (Mo4+) to the dioxostructure (Mo6') by 0, molecules during the catalytic oxidation produces 0 atoms on the silica surface. Recombination of two migrating 0 atoms leads to oxygen isotope exchange between 0, molecules. The mechanism of the catalytic oxidation of CO with 0, on bulk MOO, was re-examined with reference to that elucidated on the Si0,-supported Mo catalyst.

Oxygen isotope exchange between C 1802 and MOO, has been examined in connection with the catalytic oxidation of C l60 with "02 on MOO,. Oxide ions at mobile and immobile sites on MOO,, which oxidize CO in the catalytic oxidation, were found to work also as active sites in the oxygen isotope exchange between C 1802 and Mo "03. The observed time dependences of the yields of C 1802, C '*O ' 6O and C ' 602 in the exchange agreed satisfactorily with those calculated from the mobile and immobile site model. In the calculation, the number of immobile sites was estimated to be 1.8% of the total surface lattice oxide ions. The exchange rate at immobile sites was 13% of the total exchange rate.

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