Effect of titanium substitution on the structure of VSbO 4 catalysts for propane ammoxidation

Abstract

The effect of titanium substitution on vanadium antimony oxide propane ammoxidation catalysts was investigated by UV Raman spectroscopy and X-ray diffraction. UV Raman spectra exhibit bands in the region of 450–1000 cm −1, which are associated with the rutile structure, a cation vacancy site, and V 2O 5. Calculations of the vibrational frequencies made with the use of CASTEP optimized slab models provide good agreement with experimental results. With titanium substitution, the rutile bands shift and broaden. Comparison of freshly prepared and ground samples shows that crystalline V 2O 5 forms upon grinding in air. Titanium stabilizes vanadium in the vanadium antimonate rutile structure and thus prevents the formation of crystalline V 2O 5. An in situ Raman characterization of titanium-containing vanadium antimony oxide catalysts was made in reducing and oxidizing atmospheres. The band associated with two-coordinate oxygen at cation vacancy sites disappears under reduction conditions, then returns after reoxidation in air. This indicates that this lattice oxygen moiety is key to ammonia activation. The titanium-substituted sample shows additional characteristics that indicate titanium improves the reduction and oxidation capability of the vanadium antimonate catalyst.