DRIFTS and transient-response study of vanadia/titania catalysts during toluene partial oxidation

Abstract

The work was aimed on the determination of the role of bulk V₂O₅ present in vanadia/titania catalysts for toluene partial oxidation. Two catalysts with 1.8 and 11.1 wt% of V were studied by in situ DRIFTS and transient-response methods. Bulk V₂O₅ was found by FT-Raman spectroscopy only in the 11.1 wt% V/TiO₂ catalyst while monolayer vanadia species (monomeric and polymeric) in both samples. Toluene interaction in the presence of gaseous oxygen showed that the selectivity to benzaldehyde and benzoic acid was similar for the two catalysts. The nature of adsorbed species was also similar. Toluene interaction with the oxidised samples in the absence of gaseous oxygen showed rapid formation of coordinatively adsorbed benzaldehyde (∼1625 cm⁻¹) and benzoate/carboxylate species (1600–1400 cm⁻¹), accompanied by the disappearance of the monomeric vanadia species (∼2037 cm⁻¹). The catalyst containing V₂O₅ could convert toluene more deeply than the catalyst without V₂O₅, namely to adsorbed benzoic acid (∼1670 cm⁻¹). Additionally, large quantities of gaseous products (CO_x, benzaldehyde, H₂O) were obtained with the former catalyst, while almost none with the latter. Thus, bulk V₂O₅ could supply oxygen for the monolayer vanadia species easily reducible by toluene. This oxygen is required for the product formation and desorption.