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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 Sb 2 O 3 -V 2 O 5 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 15 V solidstate NMR. The catalytic performance of the TiO 2 -supported materials was tested for o-xylene oxidation. After calcination of the Sb 2 O 3 -V 2 O 5 binary mixture at 673 K, Sb 3+ is almost quantitatively oxidized to Sb 5+ , while both V 3+ and V 4+ are detected. V 3+ and some V 4+ are most likely located in a nonstoichiometric VSbO 4 -like structure, while the majority of V 4+ preferentially concentrates within shear domains in oxygendeficient V 2 O 5-x particles. In the titania-supported catalyst system, both Sb 2 O 3 and V 2 O 5 spread on the anatase surface. Sb 3+ is oxidized to Sb 5+ , and V 3+ , V 4+ , and V 5+ are detected. VSbO 4 -like structures are not observed. The presence of antimony leads to the formation of presumably V 3+ -O-V 5+ redox couples. The paramagnetic centerssin contrast to the binary mixturesare 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.

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