Study On Preparation And Properties Of F, And S Doped V₂O₅/TiO₂Catalysts For Selective Catalytic Reduction Of NO_x

Nitrogen oxide (NOx) is one of the main atmospheric pollutants, and the selective catalytic reduction (SCR) of NOx by NH3as reductant is an effective method for elimination of NOx in flue gases from stationary sources. Recently, due to NOx emissions are becoming more and more severe, the clear cut goal for the NOx control during the "twelfth five-year" has been proposed by our government. According to the situation of our country, the development of wide active temperature window SCR catalysts is the direction of research to develop the flue gas denitrification technology with independent intellectual property right in our country.In this study, several modified V2O5/TiO2catalysts were investigated by non-metal-doping in TiO2support to achieve wide temperature window and better catalytic performances under different flue gas conditions. The structure and catalytic properties of modified V2O5/TiO2catalyst as well as the effects of promotional activities by doping have been systematically studied by using X-ray diffraction (XRD), Raman spectra (Raman), X-ray photoelectron spectroscopy (XPS), H2temperature-programmed reduction (H2-TPR), NH3temperature-programmed desorption (NH3-TPD), Electron paramagnetic resonance (EPR) and in situ infrared spectroscopy (DRIFTS), and the mechanisms that the modification of doped-elements on the VTi-based catalysts were also studied in depth.The modification of TiO2support by F-doping was undertaken by sol-gel method and V2O5was introduced into the TiO2support by wet impregnation. It was found that the F-doping could increase the concentration of active oxygen and improve the adsorption and activation of NH3on the catalyst surface. As a result, the V2O5/F-doped TiO2catalyst exhibited enhanced activity for NOx removal than the V2O5/TiO2catalyst did below513K. Theoretical studies displayed that oxygen vacancies could be easily created on the TiO2surface, resulting in the higher redox activity of V2O5.It was observed that F-doping could introduce the extra and light-independent fixed oxygen vacancies which were beneficial to form superoxide radicals by molecular simulation calculation. This paper focused on the efforts to elucidate the morphology and stability of oxygen adsorbed on the catalyst surface using the simple and sensitive EPR techniques and XPS spectra. And the study made a further discussion on the role of O2-for promoting effects and characteristic changes during the adsorption and reaction process of reactants at the early stage of SCR reaction, with the aim of revealing the relevance of superoxide radical and catalytic properties.The enhancement of reactant gas adsorption and activation induced by doping F ions over V2O5/TiO2catalyst was observed in the experiments. The effects of S-doping on the SCR activity over V2O5/TiO2catalyst were investigated detailedly. It was observed that substitutional S was successful inserted into the TiO2lattice in the form of anion-and cation-doping. It has been found that the strong interactions of the introduced S with V and Ti resulted in better dispersibity and high concentration of superoxide radicals and Ⅴ(Ⅳ) species, leading to the enhancement of SCR activity and the activity window was further broadened to low-temperature area.Based on above results, F and S co-doping of V2O5/TiO2catalyst was prepared to improve its activity, and the effect of co-doping of F and S on the SCR performance was studied. The higher improved effect was observed for co-doped V2O5/TiO2catalyst compared to the single doping of F or S. This could be attributed to the fact that more amounts of surface oxygen vacancies and greater extent surface distortion could be responsible for the more NH3adsorption and activation. Meanwhile, the presence of superoxide radicals made more NO oxidized to NO2and improved SCR reaction activity below513K.Taken together, the nonmetal modification of V2O5/TiO2catalysts were developed, which possessed the advantages that they were easy to be prepared, and had a wide active temperature window, and high SCR activity. In particular, it was found that NOX conversion was proportional to the concentration of superoxide radicals. In addition, this paper investigated the role of superoxide radical in the catalytic reaction, which provided useful theoretical and experimental basis for SCR catalysts development with wide temperature window.