Low temperature NH3-SCR performance and mechanism of Mn and Fe supported CeCO3F-monazite catalysts

Abstract

The group has shown that Baiyun Ebo rare earth concentrate has excellent performance in NH₃-SCR denitrification when used as a carrier, where rare earth elements are mainly present in cerium fluorocarbon ore (CeCO₃F) and monazite (CePO₄) mineral phases. In this paper, a new low-temperature NH₃-SCR catalyst of Mn–Fe/CeCO₃F-monazite was prepared by an impregnation method, using synthetic CeCO₃F and purified monazite as carriers. By exploring its denitrification performance and mechanistic analysis, it provides theoretical guidance for the use of rare earth concentrates as low-temperature NH₃-SCR catalysts. Our previous studies have determined the optimum loading of Fe, so this paper needs to be investigated for the optimum doping ratio of the active substance Mn. The results of the activity tests, XRD and BET have determined that the best denitrification rate and catalytic performance was achieved at a ratio of Mn : Ce of 1 : 5. The denitrification activity of the different catalysts was investigated by loading Fe, Mn and Fe and Mn together. The results obtained by means of experimental analyses such as XRD, SEM, BET and activity tests showed that the composite catalyst loaded with Fe and Mn at the same time, had the highest activity and its denitrification rate could reach 94.8% at 250 °C. This is mainly attributed to the fact that the interaction of Fe, Mn can promote the dispersion of each other on the carrier surface, which greatly improves the specific surface area of the catalyst. The introduction of Fe and Mn increases the acidic sites and the amount of acid on the catalyst surface, which results in the formation of a large number of oxygen vacancies and the presence of more oxygen species on the catalyst surface, which facilitate the migration of oxygen. The new catalyst was investigated by Fourier transform infrared (FTIR) spectroscopy to characterise the adsorption and transformation behaviour of the reactive species on the surface of the catalyst, and to investigate the reaction mechanism. The results showed that the entire reaction process followed the L–H mechanism, with the gaseous NO adsorption and activation on the catalyst surface generating bidentate nitrate, bridging nitrate species and NH₃/NH₄⁺ species as the main intermediate species involved in the reaction, both of which underwent redox reactions on the catalyst surface to produce N₂ and H₂O. The above results indicated that the CeCO₃F-monazite carrier has excellent performance, and provided a theoretical basis for the high-value utilization of rare earth concentrates.