Determination of the oxidation state of antimony, iron and vanadium in mixed vanadium and iron antimonate oxide catalysts

Vanadium and mixed vanadium and iron antimonates with rutile-type structures have been studied by XANES at Sb L1 edge, 57Fe Mössbauer spectroscopy and ESR spectroscopy at 77 K. The results showed that both antimony and iron remained at their highest oxidation state, i.e. SbV and FeIII, whereas vanadium was present as VIII and VIV. Two types of VIV species were distinguished corresponding to well-isolated vanadyl species in distorted octahedral coordination and vanadyl species in the same coordination but close to each other and in a dipole–dipole interaction. Both VIII and total VIV concentrations decreased when the iron content increased, whereas isolated VIV concentration increased first and then decreased, with a maximum for x = 0.2 in FexV1-xSbO4. The observed variations in cationic composition are discussed in relation with the catalytic properties of the compounds in the ammoxidation of propane. Isolated VIV-O moieties appeared to be the most active and selective catalytic sites.