TiO2-ZrO2 based catalysts for the total decomposition of pollutants”

Volatile organics compounds and chlorinated organics molecules are important class of air pollutants, emitted from industrial process and urban incinerators. The polyclorodibenzo-p dioxins (PCDD) and polychlorodibenzofurans (PCDF), frequently denoted as “dioxins” are chlorided organics molecules belonging to the category of persistent organic pollutants (POPs) and once formed, these molecules persist in the ecosystem for several years. Although they may be formed naturally [1], the mainly originate from anthropogenic sources and the potential exposure to dioxins from diet, environment and occupation should be evaluated, to limit their total intake below the recommended levels [2]. Catalytic combustion is one of the most promising technologies for their destruction, due to its definitive character and save of energy, but also for the relatively dilute steams of VOC to treat. VOCs and Cl-VOCs can be, in fact, oxidised over catalyst at temperatures much lower than those used for the thermal oxidation. In the case of stationary sources, the most important applications consist in VOC abatement and and DeNOx process [3] and commercial catalysts for the latter are based on V2O5/WO3 mixed oxides supported on a high surface area TiO2 (anatase). In a previous work [4] we demonstrated that isolated and well dispersed vanadium sites resulted beneficial for o-DCB conversion. Nevertheless, the presence of high amount of tungsten and/or vanadium on the support were proven to lead to the formation of partial oxidation products. Thus, the identification of new formulations with high catalytic activity, able to increase the oxidation potential of classical SCR catalysts, is an interesting goal for commercial applications. In this work, we have investigated the effect of additives such as La2O3,WO3, SiO2, Fe2O3 and P2O5 on the activity of V2O5-ZrO2-TiO2 catalysts on the course of the total decomposition of o-dichlorobenzene. A series of catalysts prepared by incipient wetness impregnation of vanadium on doped TiO2-ZrO2 supports were characterized by XRD, TEM, H2-TPR, Raman spectroscopy and surface area measurements. All studied catalysts were active in the decomposition of o-DCB, but the kind of additive present on the support was demonstrated to be important in controlling the vanadium dispersion on the support and, thus, the material activity. While La2O3 and P2O5 have a negative impact, WO3, and Fe2O3 have a positive influence on catalyst performances. This beneficial effect seems to resides in the absence of mutual spreading between VOx and these two oxides, preventing the coverage of VOx by the secondary phase and promoting the links between vanadium and TiO2 based support. On the contrary La2O3 and P2O5 species tend to react and spread on vanadium sites, leading to a decrease of the number of active sites at the surface of the catalyst.