In Situ Spectroscopic Investigation of the Molecular and Electronic Structures of SiO₂ Supported Surface Metal Oxides

Abstract

Groups 5−7 transition metal oxides (V2O5, Nb2O5, Ta2O5, CrO3, MoO3, WO3, Re2O7) were anchored on a SiO2 support via incipient wetness impregnation and calcination. The molecular and electronic structures of the dehydrated supported metal oxides and the SiO2 support were determined by combined in situ Raman, IR, and UV−vis spectroscopy under dehydrated conditions. In situ Raman characterization reveals that the supported metal oxides are only present as surface species below the maximum dispersion limit (where crystalline metal oxide nanoparticles are absent). In situ IR analysis shows that the surface metal oxides anchor to the SiO2 support at Si−OH and adjacent Si−O−Si sites. The corresponding in situ UV−vis diffuse reflectance spectroscopy indicates that the dehydrated surface metal oxide species are present as isolated structures. Isotopic D2O−H2O exchange demonstrates that the dehydrated surface MOx species possess the MO oxo functionality but no MOH bonds. The number of MO oxo bonds was found to be related to the metal oxide oxidation state or group number. The group 5 surface metal oxides are present as surface monoxo OM(OSi)3 species. The group 6 surface metal oxides primarily contain the surface dioxo (O)2M(OSi)2 structures with some surface monoxo OMO4 species also present. In situ Raman under reduction environments allowed for the discrimination between these multiple MOx surface species as well as between vibrations from the metal oxides and the silica support. Only rhenia from the group 7 surface metal oxides was examined and found to possess the surface trioxo (O)3ReOSi structure. These molecular and electronic structural insights for dehydrated surface metal oxides on SiO2 catalysts will facilitate the establishment of fundamental structure−activity relationships for future catalytic reaction studies.