Total oxidation of chlorobenzene on Ag or Au-VOx/TiO2 catalysts

(eng) The catalytic total oxidation is a very promising solution for the dioxins abatement directly at their sources of emission. In this work, an innovative approach is explored to improve the efficiency of VOx/TiO2 catalysts: the addition of noble metals. The effect of the interaction between vanadium oxide and a noble metal, Ag and Au, was studied in the total oxidation of chlorobenzene, a model molecule of dioxins. The properties of the Ag and Au-VOx/TiO2 systems were correlated with their catalytic performances in order to identify the mechanism driving these metal-oxide interactions. With Au-containing catalysts, a competition for the gaseous oxygen present in the gas flow was revealed between Au and VOx supported species. The most efficient system is obtained with a deposition-precipitation method in which Au is deposited first. In this system, gold and vanadium have an influence on the properties of each other. Nevertheless, the determining influence of its oxidation state and of its nanoparticle size indicates clearly that Au is the main active phase. On the opposite, a synergy is observed between Ag and VOx. At an optimal loading, Ag indeed induces positive changes of the vanadium properties (reducibility, oxidation state, nature of the species). The synergy extent increases with the size of the Ag nanoparticles decreasing. However, the positive role played by silver is only possible if three conditions are satisfied: the formation of AgCl must be avoided, a good “degree of accessibility” of the gaseous phase to the silver species and a direct interaction between the support and vanadium. The synergy between silver and vanadium is thus the result of a complex compromise between several parameters.