Systematic investigation of supported transition metal oxide based formulations for the catalytic oxidative elimination of (chloro)-aromatics -: Part II:: Influence of the nature and addition protocol of secondary phases to VOx/TiO2

This contribution reports, for the first time, a systematic investigation of the effect of the adding secondary phases on the activity of VOx/TiO2 based catalysts in the course of the total oxidation of benzene and chlorobenzene as model molecules for dioxins. The catalysts consisted in binary formulations VOx-secondary phases (CrOx, MnOx, SnOx, WOx, NbOx, TaOx, MoOx, ZrOx or BiOx). Three synthesis pathways differing by the order of synthesis of both phases were investigated (1-2, 2-1 and 1,2 both together). Catalysts synthesized in a unique step demonstrated the best activities for the oxidation of benzene. Moreover, WO, and MOO, phases induce a synergetic effect with VOx. XPS demonstrated that this effect resides in the absence of mutual spreading between both phases preventing the VOx covering by the secondary phases and promoting the stabilization of the links VOx-TiO2. The investigation of different ratios WOx/VOx, and MoOx/VOx on the total oxidation of chlorobenzene was performed on binary formulations supported on classical TiO2 or on sulfated TiO2. For both supports and both binary formulations, the optimum ratio is equal to 1. This ratio is the highest that prevents the presence of crystallites of MoOx or WOx, which seems to be detrimental for the catalyst activity. Moreover, the resistance of the formulations in front of the chlorinated volatile organic compounds (Cl-VOCs) and the better activity of the formulations supported on sulfated TiO2 were proven. The activation effects brought by WOx, MoOx and by sulfated TiO2 are linked to the increase of the number of Bronsted acid sites as proven by FTIR with adsorbed pyridine. Moreover, the strong Lewis sites present on the sulfated TiO2 promote the dispersion of the active and secondary phases. Furthermore, these investigations pointed out the importance of the presence of Bronsted sites for the adsorption of VOCs and Cl-VOCs aromatics. (c) 2006 Elsevier B.V. All rights reserved.