Antimony oxide-modified vanadia-based catalysts-physical characterization and catalytic properties
Antimony-modified vanadia-on-titania catalysts were prepared for the selective oxidation of o-xylene to phthalic anhydride by ball milling of powder mixtures followed by calcination. A binary Sb2O3-V2O5 system was also prepared for comparison purposes. The resulting materials were physically characterized by surface area measurements, X-ray diffraction analysis (XRD), laser Raman spectroscopy, X-ray absorption fine structure (XAFS) spectroscopy, electron spin resonance (ESR), magnetic susceptibility determination, and 15V solid-state NMR. The catalytic performance of the TiO2-supported materials was tested for o-xylene oxidation. After calcination of the Sb2O3-V2O5 binary mixture at 673 K, Sb3+ is almost quantitatively oxidized to S5+, while both V3+ and V4+ are detected. V3+ and some V4+ are most likely located in a nonstoichiometric VSbO4-1ike structure, while the majority of V4+ preferentially concentrates within shear domains in oxygen-deficient V2O5-x particles. In the titania-supported catalyst system, both Sb2O3 and V2O5 spread on the anatase surface. Sb3+ is oxidized to Sb5+, and V3+, V4+, and V5+ are detected. VSbO4-like structures are not observed. The presence of antimony leads to the formation of presumably V3+-O-V5+ redox couples. The paramagnetic centers-in contrast to the binary mixture-are largely isolated. Antimony preferentially migrates to the surface and appears to exhibit a dual function catalytically. It is inferred from the experimental data that the addition of antimony leads to site isolation and to a reduction of surface acidity. We suggest that V-O-V-O-V domains or clusters are interrupted by incorporation of Sb to form V-O-Sb-O-V species. As a consequence of this site isolation and a reduction of surface acidity, overoxidation of o-xylene is reduced. These two effects are therefore most probably responsible for the improved selectivity of the ternary catalyst system over the binary one toward phthalic anhydride.
Spengler, J., F. Anderle, E. Bosch, R. K. Grasselli, B. Pillep, P. Behrens, O. B. Lapina, A. A. Shubin, H-J. Eberle, and H. Knözinger. "Antimony oxide-modified vanadia-based catalysts physical characterization and catalytic properties." The Journal of Physical Chemistry B 105, no. 44 (2001): 10772-10783.
Journal of Physical Chemistry B