A study of the redox properties of MoOx/SiO2

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 ability to exchange oxygen with O2 and H2O. Raman spectroscopy was used to monitor the exchange of 18O for 16O in the Mo=O bond of isolated molybdate species, whereas mass spectrometry was used to follow the isotopic composition of the gaseous species, i.e., O2 and H2O. Reduction in H2 at 920 K results in the loss of one O atom per Mo atom, and consistent with this, all of the Mo(VI) cations are reduced to Mo(IV) cations. Raman spectroscopy shows that virtually all Mo=O bonds of the original molybdate species are lost upon reduction. While reoxidation of Mo(IV) cations by O2 is quantitative, studies using 18O2 reveal that only a small part of the newly formed Mo=O bonds are 18O labeled, and that the balance are 16O labeled, indicating that O-atom exchange between the support, SiO2, and the supported MoOx species occurs during reoxidation. Rapid exchange of O atoms was observed upon exposure of both bare SiO2 and MoOx/SiO2 to H2(18)O at 920 K, and the presence of MoOx species was found to enhance the rate of exchange. By contrast, very slow exchange of O atoms was observed when the oxidized catalyst was exposed to 18O2 at 920 K. In situ observations of the catalyst during exposure to a mixture of H2 and 18O2 at 920 K showed that all of the Mo cations remained in the VI oxidation state and that O atom exchange occurred at a rate comparable to that observed upon exposure to H2(18)O. The results of this investigation suggest that reoxidation of Mo(IV) cations following H2 reduction involves the formation of a Mo-peroxide species and subsequent O atom migration from such a species to the 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 Mo(IV) centers. The Mo-peroxide species are then rapidly reduced by H2 to form H2O and a Mo=O bond. The rapid exchange of O atoms between the gas phase and the catalyst observed during steady-state oxidation of H2 is attributed to interactions of the product H2O with the catalyst, rather than to O atom migration originating from the Mo-peroxide species formed on the catalyst surface.