Surface Rhenium Oxide-Support Interaction for Supported Re2O7 Catalysts

The molecular structures of the two-dimensional rhenium oxide overlayers on different oxide supports (Al2O3, TiO2, ZrO2, SiO2, and MgO) under ambient and dehydrated conditions were determined by Raman spectroscopy. The surface rhenium oxide species under ambient conditions are hydrated by adsorbed moisture and are essentially in an aqueous medium. Consequently, the surface rhenium oxide species under ambient conditions resemble the ReO−4 ion in an aqueous solution independent of the specific oxide support. Upon dehydration at elevated temperatures, the surface rhenium oxide species interact with the oxide support by a bridged oxygen (with exception of the Re2O7/MgO system). The molecular structures of the dehydrated surface rhenium oxide species possess an isolated, four-coordinated rhenium oxide species with three terminal ReO bonds and one bridging ReOSupport bond. For the Re2O7/MgO systems, the surface rhenium oxide species is primarily present as a compound(s) formed by a strong acid-base interaction between the deposited rhenium oxide and the magnesium oxide support. Methanol oxidation over the supported rhenium oxide catalysts revealed that the activity is strongly dependent on the specific oxide support and increases with increasing reducibility of the surface rhenium oxide species. The activity and reducibility of the supported rhenium oxide catalysts are attributed to the difference in the R7+Osupport bond strength.