Investigation of surface structures of supported vanadium oxide catalysts by UV-vis-NIR diffuse reflectance spectroscopy

UV-vis-NIR diffuse reflectance spectroscopy (DRS) was applied to study the local structures of V(V) cations on various oxide supports (Al2O3, ZrO2 TiO2, Nb2O5, CeO2, and SiO2) under hydrated and dehydrated conditions. The edge energy (E-g) of the LMCT transitions of V(V) cations was used to elucidate the local structures of V(V) cations, and a correlation between the edge energy and the number of the covalent V-O-V bonds (CVB) around the central V(V) cations was established based on some V(V) reference compounds/oxides. For TiO2 Nb2O5, and CeO2 supported vanadia catalysts, the strong support absorption in the same region as the V(V) cations prevents a reliable determination of the local structure of the surface vanadium oxide species by either the LMCT band position or the edge energy. For Al2O3, ZrO2, and SiO2 supported vanadia catalysts, the average CVB number derived from the edge energy allows the assignment of the possible structure of the surface vanadium oxide species, which is a strong function of the support, environmental conditions, and vanadia surface density. The DRS results provide reliable information and new insights into the structural characteristics of the surface vanadium oxide species on these oxide supports under different environmental conditions.