| Abstract
| - A sample of MoOx/SiO2, in which all of the Mo cations are present as isolated mono-oxo molybdate moieties,was prepared and investigated to understand the redox chemistry of such molybdate species and their abilityto exchange oxygen with O2 and H2O. Raman spectroscopy was used to monitor the exchange of 18O for 16Oin the MoO bond of isolated molybdate species, whereas mass spectrometry was used to follow the isotopiccomposition of the gaseous species, i.e., O2 and H2O. Reduction in H2 at 920 K results in the loss of one Oatom per Mo atom, and consistent with this, all of the MoVI cations are reduced to MoIV cations. Ramanspectroscopy shows that virtually all MoO bonds of the original molybdate species are lost upon reduction.While reoxidation of MoIV cations by O2 is quantitative, studies using 18O2 reveal that only a small part of thenewly formed MoO bonds are 18O labeled, and that the balance are 16O labeled, indicating that O-atomexchange between the support, SiO2, and the supported MoOx species occurs during reoxidation. Rapid exchangeof O atoms was observed upon exposure of both bare SiO2 and MoOx/SiO2 to H218O at 920 K, and thepresence of MoOx species was found to enhance the rate of exchange. By contrast, very slow exchange of Oatoms was observed when the oxidized catalyst was exposed to 18O2 at 920 K. In situ observations of thecatalyst during exposure to a mixture of H2 and 18O2 at 920 K showed that all of the Mo cations remained inthe VI oxidation state and that O atom exchange occurred at a rate comparable to that observed upon exposureto H218O. The results of this investigation suggest that reoxidation of MoIV cations following H2 reductioninvolves the formation of a Mo−peroxide species and subsequent O atom migration from such a species tothe SiO2 support. It is proposed that the steady-state oxidation of H2 also involves the formation of Mo−peroxide species by interaction of O2 with a small number of MoIV centers. The Mo−peroxide species arethen rapidly reduced by H2 to form H2O and a MoO bond. The rapid exchange of O atoms between the gasphase and the catalyst observed during steady-state oxidation of H2 is attributed to interactions of the productH2O with the catalyst, rather than to O atom migration originating from the Mo−peroxide species formed onthe catalyst surface.
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