Influence of Zn in the iron ore sintering flue gas on the removal of NO_x and dioxins by V₂O₅–WO₃/TiO₂ catalyst

Iron ore sintering is a process in which fuel, flux, and iron ore powders are mixed and sintered into a block under incomplete melting conditions. The flue gas from iron ore sintering process is one of the largest sources of nitrogen oxide (NO_x) and dioxin emissions in industries. The V₂O₅–WO₃/TiO₂ (VWTi) catalyst can simultaneously remove NO_x and dioxins, but the presence of the complex flue gas results in the deactivation of the catalysts. In response to this challenge, this study carried out experiments for ZnCl₂, ZnO, and ZnSO₄ poisoning over the VWTi catalyst via wet impregnation method. The effects of the different Zn species on the simultaneous removal of NO_x and dioxins (chlorobenzene was used as the simulant for dioxins) by the VWTi catalyst were studied under simulated conditions of the iron ore sintering flue gas. The surface physicochemical properties of the fresh and poisoned catalysts were characterized to reveal the deactivation mechanism, and the regeneration experiments of the poisoned catalysts were investigated. Results showed that deactivation through catalytic denitrification and chlorobenzene (CB) catalytic degradation processes could be observed in different Zn-containing catalysts. The poisoning effect was more obvious with the increase of Zn content, and the effects of deactivation were as follows: ZnCl₂>ZnO>ZnSO₄. Results from physical and chemical analyses indicated that Zn species had a significant influence on the chemical environment of the active substances on the surface of the catalysts. Zn species caused a slight agglomeration of particles on the surface of the catalysts, a decrease in the number of surface acid sites, a reduction in the reducibility of surface V species, and a decrease in the chemisorbed oxygen ratio and the molar ratio of n(V⁵⁺)/n(V⁴⁺). The regeneration experiments confirmed that employing the dilute sulfuric acid solution washing method was effective for recovering the catalytic activity, whereas the water washing method failed to restore the catalytic activity. The mechanism of Zn salt poisoning is as follows: Zn²⁺ reacts with the acid sites V=O and V−OH on the surface of the catalyst to form V−O−Zn, which adversely affects the adsorption of NH₃ and CB, resulting in the catalyst poisoning and deactivation. The \rmSO_4^2- in ZnSO₄ provides a new acidic site for the adsorption and transformation of NH₃ and CB alleviating the poisoning effect. The Cl⁻ in ZnCl₂ produces HCl as a by-product after the reaction, resulting in more active sites poisoning on the surface of the catalyst and deepening the poisoning effect.