Redox dynamics and structure/activity relationships in vanadium-oxide on titania catalyst

The in-situ evolution of crystallites of V2O5 on the TiO2 surface during interaction with the o-xylene/air reagent mixt. involves, together with their spreading on the TiO2 surface, partial redn. and the formation of an amorphous phase characterized by a VV:VIV ratio of 2:1 and an IR band centered at 995 cm-1. The phase does not form directly by redn., but involves a preliminary redn. to a phase with a lower mean oxidn. state. This reduced V oxide phase is then partially reoxidized to the active phase with a decrease in the formation of the intermediate phthalide and an increase in the selectivity to phthalic anhydride. A similar evolution in the catalytic behavior and in the mean oxidn. state is obsd. in unsupported V2O5 which after ∼200 h, transforms into V3O7. This phase has a VV:VIV ratio similar to that present in the active phase of V oxide supported on TiO2, but does not show its characteristic IR band centered at 995 cm-1. Using a prereduced unsupported V oxide, it is possible to decrease the activation time considerably and to obtain a final catalyst, after in-situ treatment, whose catalytic behavior is very comparable to that of V/Ti/O, but again characterized by the presence of a phase (V3O7) different from that present in the V/Ti/O system. The catalytic behavior in o-xylene conversion to phthalic anhydride is not related to the presence of a unique special surface structure of V oxide on the TiO2 surface, but rather to a suitable VV:VIV ratio and surface distribution. These features can also be realized in unsupported V oxide by using a suitable prepn. and activation procedure. [on SciFinder(R)]