Influence of the impregnation order on the synergy between Ag and V2O5/TiO2 catalysts in the total oxidation of Cl-aromatic VOC

Silver is added to V2O5/TiO2 catalysts by a wet impregnation method and the influence of the impregnation order is studied. Silver and vanadia were introduced by co-impregnation or by impregnation in 2 steps (V first or Ag first). Vanadia loading always corresponds to 2.49 wt% while 9 different loadings of silver were explored (0.02–12.5 wt%). Whatever the way of synthesis, the addition of silver to V2O5/TiO2 formulation induces: (i) a decrease of the specific surface; (ii) a higher vanadium oxidation state and (iii) a decrease of the Brønsted acidity. A synergy can be evidenced for the co-impregnated Ag–V2O5/TiO2 catalysts in the total oxidation of chlorobenzene. This synergy is observed for several loadings of Ag with a maximum at a loading of 0.05 wt%. This synergy can be related to the higher oxidation state of vanadium induced by silver, to the minimum decrease of the specific surface and to the absence of AgCl at the surface of the catalysts. This last aspect is confirmed via the investigation of the catalytic performances in the total oxidation of benzene. In this system, a synergy is observed for the co-impregnated catalysts and when the vanadium is impregnated first. On the contrary, no synergy was observed when silver is impregnated first. This result suggests that the synergy can be obtained only if the vanadium keeps a direct interaction with the support and if a good “degree of accessibility” of the gaseous phase to the silver species is maintained.