Structure of molybdenum oxide supported on silica SBA-15 studied by Raman, UV–Vis and X-ray absorption spectroscopy

[Display omitted] ► Dispersed molybdenum oxide consists of monomeric and connected species. ► For the loadings studied no dependence on preparation or loading is observed. ► Raman, UV–Vis and XA spectroscopy provide complementary structural information. The structure of molybdenum oxide supported by silica SBA-15 has been studied by visible Raman spectroscopy, diffuse reflectance UV–Vis spectroscopy and X-ray absorption spectroscopy in the dehydrated state obtained after thermal treatment at elevated temperatures (≥350 °C). No dependence of the molybdenum oxide structure on preparation procedure or loading has been observed within the range of loadings studied in detail (0.2–0.8 Mo/nm 2). X-ray absorption spectroscopy (XAS) reveals that the dehydrated state consists of a mixture of monomeric and connected molybdenum oxide centres. While the presence of crystalline MoO 3 can be excluded by Raman spectroscopy, tetrahedrally and octahedrally coordinated MoO 4 and MoO 6 units are identified by XAS. The MoO 6 units possess connectivity similar to that of MoO 3 building blocks, whereas the MoO 4 units are isolated or connected to other Mo x O y units. These results are supported by UV–Vis spectra showing intensity at wavelengths (>300 nm) typical for dimeric and/or oligomeric species.