Description of active sites on molybdenum oxide as detected by isotope exchange between (CO2)-O-18 and (MoO3)-O-16

The oxygen exchange reaction between (CO2)-O-18 and (MoO3)-O-16 has been studied on well characterized MoO3 crystallites. Temperature programmed desorption of adsorbed (CO2)-O-18 has shown that the extent of surface oxygen exchange is higher on partially reduced samples exposing the (010) face. This has been related with the easier reducibility of the (010) face. Moreover, experiments in a recycle reactor at moderate reaction temperature (623K) suggest that surface oxygen exchange is more rapid on the basal (010) face. On the other hand, when the experiment is run at higher reaction temperature (773K), using a higher O-18((CO2)-O-18)/O-16(MoO3) ratio and for a longer period, a continuous oxygen exchange reaction occurs, specially with samples presenting lateral faces. In this latter conditions, two contributions to the exchange reaction can be distinguished. The first corresponds to a fast surface oxygen exchange. The second contribution which is more prominent over samples with (100) faces can be assigned to migration of oxygen atoms from lateral towards basal faces and/or to a slow migration of oxygen atoms from the bulk structure of MoO3 towards the lateral faces. From the results of this study it is speculated on the importance of a preferential oxygen migration from the bulk MoO3 towards the lateral faces, which is related with the selective partial oxidation of olefins observed on these faces.