STRUCTURAL DETERMINATION OF SURFACE RHENIUM OXIDE ON VARIOUS OXIDE SUPPORTS (AL2O3, ZRO2, TIO2 AND SIO2)

In the present investigation Raman and IR spectroscopy were used to determine the structure of Re2O7/Al2O3, Re2O7/ZrO2, Re2O7/TiO2 and Re2O7/SiO2 as a function of loading under ambient and in situ dehydrated conditions. Under ambient conditions, the surface rhenium oxide is hydrated and possesses the structure of the ReO4 ion in aqueous solution, independent of coverage or support type. Under dehydrated conditions, the in situ Raman and IR spectra show that only one surface rhenium oxide species is present on the silica support, whereas two surface rhenium oxide species are present on the alumina, zirconia and titania supports. The concentration ratio of the two species is a function of the coverage, and their structures are similar, possessing three terminal Re=O bonds and one bridging Re-0-support bond (C3v symmetry). Differences in properties between the surface rhenium oxide species were determined by TPR as a function of coverage and support type. The in situ Raman, IR and TPR measurements suggest that the bridging Re-0-support bond strength decreases with increasing coverage, while the TPR data further indicate that the Re-O-support bond strength decreases in the order Al2O3 > SiO2 almost-equal-to ZrO2 > TiO2 for a given rhenium oxide loading.