Investigation on the mechanism of Nb and Si co-doping on low SO3 generation from V-based catalyst during NH3-SCR process

Abstract

Generally, SO3 tends to be formed in the process of selective catalytic reduction (SCR), and SO3 has negative effects on atmospheric environmental production and the safe operation of coal-fired power plants. In this study, a series of Nb and Si codoped V-based catalysts were prepared via the impregnation method, and Nb and Si species were applied to reduce the SO3 generation ratio from the catalyst. Moreover, the controlled condensation method was used to collect the SO3, which formed during the NH3-SCR reaction process. The results indicated that the Nb and Si codoped catalysts exhibited the best NO reduction performance with the lowest SO3 generation ratio. The SO3 generation ratio and the NO conversion ratio for the VW2NbTi-10Si catalyst were 0.39% and 92%, respectively. The physicochemical properties of the catalysts were analysed by a series of characterization techniques. The X-ray diffraction (XRD) results revealed that the Nb and Si species could inhibit the crystallization of TiO2, and the X-ray photoelectron spectroscopy (XPS) results indicated that Nb and Si could decrease the ratio of Oα and V5+ on the surface of the catalyst. From the CO2 and NH3 temperature programmed desorption (CO2 and NH3-TPD) results, the Nb and Si species could reduce the number of alkaline sites on the catalyst surface, and the Si species could significantly enhance the acidity of weak and strong acid sites over the catalyst. Combined with the results of N2-adsorption and desorption, CO2-TPD, H2 temperature programmed reduction (H2-TPR) and XPS experiments, the addition of Nb and Si species was beneficial to inhibiting the adsorption and oxidation of SO2 on the surface of catalysts, resulting in a significant decrease in SO3 emissions. Therefore, in-situ diffuse reflective infrared Fourier transform spectroscopy (DRIFTS) analysis demonstrated that the Nb and Si species could restrict the reaction between SO2 and V species, decreasing the formation of the intermediate product VOSO4. Furthermore, the enhancement of reaction temperature and NH3 volumetric fraction would significantly promote the formation of SO3. However, the variations in the volumetric fraction of O2 and NO expressed ignorable effects on the SO3 generation ratio.