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Abstract

Spectroscopic analysis was used to gain new insight into the molecular structures occurring during the synthesis of highly dispersed silica SBA-15 supported vanadia (VOx/SBA-15). VOx/SBA-15 was prepared by a grafting/anion-exchange procedure. As a first step of the procedure, the inner pores of SBA-15 are functionalized via grafting of 3-aminopropyltrimethoxysilane. After formation of the corresponding ammonium salt, decavanadate (V10O286-) is incorporated into the pores by anion exchange. In the final step, calcination of the decavanadate precursor yields the chemically bonded vanadia species. Using this approach, vanadium loadings of up to 22 wt % V on SBA-15 were obtained. As followed by Raman spectroscopy, upon dehydration, the structure of the supported vanadia changes dramatically. Raman and diffuse reflectance UV-VIS spectroscopy under dehydrated conditions reveal the presence of different vanadia structures (monomers, polymers and crystals) as a function of vanadium loading (0 – 22 wt % V). The maximum coverage of vanadia species on SBA-15 is achieved at ~7.2 wt % V (2.3 V/nm2). At loadings up to 7.2 wt % V, the vanadia species are mainly present as isolated tetrahedral species, whereas at higher loadings V2O5 crystallites are formed, in addition to monomeric and polymeric vanadia species.