Flame-made vs. wet-impregnated vanadia/titania in the total oxidation of chlorobenzene: Possible role of VOx species

Vanadia/titania particles with a specific surface area (SSA) around 50 m2 g−1 and a V2O5 content up to 30 wt.% (corresponding to a V surface density up to 33 V nm−2) were prepared by flame spray pyrolysis as well as by classic wet-impregnation. The catalysts were characterized by nitrogen adsorption, X-ray diffraction, temperature programmed reduction, Raman spectroscopy, X-ray photoelectron spectroscopy and tested in the total oxidation of chlorobenzene. Depending on vanadia content, monomeric, polymeric and crystalline vanadia species were formed. The dispersion of the VOx species was in general higher for flame-made catalysts. While the classic wet-impregnated catalysts already showed crystalline V2O5 when the V surface density reaches 8 V nm−2, the flame-made ones exhibited only amorphous VOx species up to 16 V nm−2. The activity of flame-made and wet-impregnated catalysts increased with increasing V2O5 loading and therefore depended on the VOx species structure: catalysts exceeding a V surface density of 8 V nm−2 containing high amounts of amorphous polymeric and/or crystalline VOx species showed significantly higher activity than catalysts with lower V surface density. Wet-impregnated catalysts with numerous V–O–V bonds as involved in polymeric and crystalline VOx species showed superior activity than FSP-made ones of similar composition. This contribution proposes a discussion aiming at understanding the role played by the different types of VOx species in the total oxidation of volatile organic compounds on the example of chlorobenzene.