Spatial Distribution of the Vanadium Atomic Species in MoVTeO and MoVTeNbO Oxide Catalysts as Revealed by High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy

STEM-HAADF has been used to characterize a light alkanes mild oxidation catalysts corresponding to two different MoVTe(Nb)O M1 phases. For that purpose a semi-automatic integration routine allowing the analysis of several hundreds atomic columns with a normalization procedure has been developed to determine quantitatively the vanadium content of mixed (V, Mo) crystallographic sites with a low statistical variability. The results obtained have been compared to previous data obtained on other M1 phases with also different compositions. The comparison showed that the V distribution over the sites was not strongly influenced by the presence or not of niobium, the method of synthesis used or the nature of the Te or Sb cations in the hexagonal channels but depended mainly of the total V content of the phases. Extrapolation of low V content allowed discussing the possible location of the vanadium active sites