Mechanism of the catalytic oxidation of CO with O2 on an Mo catalyst supported on silica and on bulk MoO3

Abstract

The mechanism of the catalytic oxidation of CO with O₂ on an Mo catalyst supported on silica has been studied by a tracer technique using ¹⁸O₂. The supported Mo catalyst was prepared by using the ready reaction between Mo(η³-C₃H₅)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/O₂= 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 O₂ molecules. The oxidation of the oxostructure (Mo⁴⁺) to the dioxostructure (Mo⁶⁺) by O₂ molecules during the catalytic oxidation produces O atoms on the silica surface. Recombination of two migrating O atoms leads to oxygen isotope exchange between O₂ molecules. The mechanism of the catalytic oxidation of CO with O₂ on bulk MoO₃ was re-examined with reference to that elucidated on the SiO₂-supported Mo catalyst.