SURFACE-STRUCTURE AND REACTIVITY OF V-TI-O CATALYSTS PREPARED BY SOLID-STATE REACTION .2. NATURE OF THE ACTIVE PHASE FORMED DURING ORTHO-XYLENE OXIDATION

The modifications occurring to V2O5 crystallites supported on TiO2 (anatase or rutile) in contact with an o-xylene/air flow at about 600 K are characterized by determining the change in the valence state of vanadium through chemical analyses, the variation in the vanadium coordination environment through FT-IR and solid-state 51V NMR spectroscopies, and the modification in the nature and distribution of the vanadium-oxide phase by means of XRD and XPS analyses. Results are analyzed with reference to the time-on-stream modifications in the catalytic behavior in o-xylene oxidation to phthalic anhydride of these catalysts and to the catalytic behavior and physicochemical characteristics of V3O7 supported on TiO2 and of an unsupported partially reduced vanadium-oxide phase. On anatase samples, the TiO2 surface is covered homogeneously by a VIVVV partially hydrated mono- or bilayer. Overlaying this phase a hydrated V3O7-like phase (VV: VIV ratio of about 2 : 1) is also present as amorphous multilayer patches. In rutile samples, the former phase is no longer present, but instead islands of partially oxidized V2O4 are found together with the same partially reduced V-oxide phase as in the anatase samples. Catalytic results indicate that both phases have roughly similar catalytic behaviors and that the role of the TiO2 support both in the anatase and rutile forms is to increase the number of surface sites, but not to modify their nature which depends only on the modifications occurring during the catalytic tests. © 1991.