A new approach to the characterization of V species in doped-V/Ti/O catalysts by means of TPR and TPO measurements: a study of the effect of promoters in the oxidation of o-xylene

摘要

Titania-supported vanadium oxide systems, catalysts for the oxidation of o-xylene to phthalic anhydride, were characterized by means of Raman spectroscopy, Thermal-Programmed Reduction and Oxidation, and adsorption/TP Desorption of methanol, with the aim of defining a method for the quantification of the different V species. It was found that vanadium oxide, either as polyvanadate dispersed over titania, or in the form of bulk vanadia, spontaneously releases molecular oxygen at 600-650°C, whereas isolated V species, chemically interacting with the support, is not susceptible of self-reduction. The latter species is that predominant in samples having low vanadium oxide loading (≤ 2 wt.% V_2O_5, with TiO_2 surface area 22.5 m~2/g), and possesses the highest intrinsic activity in o-xylene conversion. In samples having higher vanadia loading, instead, the activity is determined by the amount of dispersed polyvanadate and of bulk vanadia. The effect of Sb, promoter of activity for V/Ti/O catalysts, was explained in terms of an increase of the dispersion of the most active species, and of stabilization of the latter towards segregation. These promotional effects are more pronounced in the co-presence of Cs and Sb.