Necessary conditions for a synergy between Ag and V 2O 5 in the total oxidation of chlorobenzene

Abstract

V 2O 5/TiO 2 catalysts are widely developed for the total oxidation of dioxins. However, this study presents an innovative way to improve the performances of V 2O 5/TiO 2 catalysts by the addition of silver. Three kinds of catalysts were synthesized: Ag/TiO 2, V 2O 5/TiO 2 and Ag–V 2O 5/TiO 2. The vanadia loading always corresponds to a 0.75 theoretical monolayer while 9 different loadings of silver were explored (0.02–12.5%). This new kind of catalysts is tested in the total oxidation of chlorobenzene which is a classical model molecule for dioxins. The samples were characterized by BET, XPS, XRD, ICP-AES and TEM before and after catalytic tests. The specific surface decreases with the increase of the silver loading for Ag/TiO 2 and Ag–V 2O 5/TiO 2 catalysts. XPS results show that the addition of silver to a V 2O 5/TiO 2 formulation induces a higher oxidation state of the vanadium which should be beneficial on the performances. A synergy is indeed observed for several loadings of silver with a maximum at a loading of 0.05 wt.%. This synergistic effect between silver and vanadia can only be achieved in the total oxidation of chlorobenzene by choosing appropriate operating conditions, namely conditions avoiding AgCl formation. This synergy is directly linked to an optimal silver loading which offers the best compromise between a high oxidation state of vanadia without a significant decrease of the specific surface of the catalyst.