Study On Low-temperature SCR Denitration Performance And Reaction Mechanism Over MnFeO_x/SiO₂/SiO₂ Catalyst

The ammonia selective catalytic reduction(NH₃-SCR)denitration technology has the characteristics of high selectivity,high efficiency and economy,and has become the most widely used stationary source denitration technology in the world.However,most of the commercial denitrification catalysts are V₂O₅-WO₃/TiO₂,and the temperature window is in the middle and high temperature range of 300-400°C.The catalyst is prone to be clogged,sintered,and poisoned.Besides,the catalyst is expensive and the active component vanadium is toxic and easy to pollute the environment.Therefore,manganese iron oxide with high selectivity,low cost,and pollution-free was selected as the active component of the catalyst for experimental research in this paper.Firstly,mesoporous silica was used as the carrier and the MnO_x/SiO₂ catalyst were prepared with two different manganese precursors(manganese nitrate and manganese acetate)by the impregnation method.The NH₃-SCR activity of the catalysts with different manganese precursors and different Mn/Si ratio was compared through the activity experiments,and the relationships between manganese precursor and NH₃-SCR activity were studied and analyized by XRD,BET,Raman spectroscopy,XPS and H₂-TPR.Based on the DFT calculation,using the DMol3 calculation module in the Material Studio(MS,version 17.1.0.48)software,the adsorption behaviors of the reactants and the activation energy barriers of the NH₃-SCR reaction pathways on the MnO₂/SiO₂ catalyst and the Mn₂O₃/SiO₂catalyst surface were compared,and the influence on the mechanism of NH₃-SCR over the two catalysts was analyzed.Then,on the basis of 0.4MN catalyst,iron oxide was added to prepare MnFeO_x/SiO₂ catalyst,which improved the NO conversion rate and N₂ selectivity of the catalyst at a reaction temperature greater than 240°C and was suitable for wider applications.The adsorption behaviors of the reactants and reaction pathways of NH₃-SCR on the MnFeO_x/SiO₂ catalyst surface were also studied by DFT calculations,and the synergy between manganese oxide and iron oxide in NH₃-SCR process was analyzed.Finally,the MnFeO_x/SiO₂ catalyst was doped with Zr to improve the resistance of the catalyst to H₂O and SO₂ components,which laid the foundation for the research and development of low-temperature and high-efficiency NH₃-SCR denitration catalyst.The main conclusions obtained in this thesis are as follows:1.The activity tests of aMN and aMA catalysts which were prepared with different precursors(manganese nitrate and manganese acetate)showed that when the Mn/Si ratio reached 0.4,both the0.4MA catalyst and the 0.4MN catalyst obtained the widest active temperature windows(150-330°C and 120-270°C,respectively).The low-temperature activity of the 0.4MN catalyst was higher.As the reaction temperature increased,the N₂ selectivity of the catalysts decreased and the N₂ selectivity of the 0.4MN catalyst was lower than that of the 0.4MA catalyst.The characterization results of the0.4MN and 0.4MA catalysts show that MnO₂ species was mainly formed on the surface of 0.4MN catalysts and a little MnO₂ crystals were formed.Mn₂O₃ species was also exist and dispersed in an amorphous state.The surface of 0.4MA catalyst was mainly composed of a small part of crystalline Mn₂O₃ species and amorphous MnO₂ species.Combined with the results of various characterizations,the difference in NH₃-SCR activity between MnO₂ species and Mn₂O₃ species was the main factor leading to the difference in NH₃-SCR performance of 0.4MN and 0.4MA catalysts.2.Based on DFT calculations,the adsorption behaviors of NH₃,NO or O₂ molecules on the surface of MnO₂/SiO₂ and Mn₂O₃/SiO₂ catalysts were studied.The results showed that the surface of MnO₂/SiO₂ was beneficial to NH₃ adsorption while the Mn₂O₃/SiO₂ surface was easier to adsorb NO and O₂ molecules and the adsorbed NO could produce various of configurations.Compared with the difference of adsorption energy,the excellent adsorption performance of MnO₂/SiO₂ for NH₃ and to exert the role of the reaction mechanism based on NH₃ adsorption and dehydrogenation more effectively was one of the main reasons for the higher NO catalytic removal efficiency of MnO₂/SiO₂catalyst than that of Mn₂O₃/SiO₂ catalyst.For the MnO₂/SiO₂ catalyst and the Mn₂O₃/SiO₂ catalyst,the decomposition reactions of*-NH₂NO-*was the rate determining step of the main N₂ formation pathway.Moreover,the activation energy barrier of the decomposition reaction of*-NH₂NO-*over MnO₂/SiO₂ catalyst was higher than that over the Mn₂O₃/SiO₂ catalyst,which was another main reason for the higher NO conversion efficiency of the MnO₂/SiO₂ catalyst at the reaction temperature below 150°C.However,the Mn₂O₃/SiO₂ catalyst had higher N₂ selectivity.This was because the activation energy barrier of*-NH dehydrogenation reaction over the Mn₂O₃/SiO₂ catalyst surface,which was extremely difficult to produce*-N,so the formation of N₂O was more difficult.In addition,the rapid SCR reaction was relatively easy to proceed for both MnO₂/SiO₂ and Mn₂O₃/SiO₂ catalysts,and it was not the main factor which brought the difference in the NH₃-SCR performance of the two catalysts.3.Iron oxide was added to 0.4MN catalyst to prepare MnFeO_x/SiO₂ catalyst.The activity tests found that 0.4MN-0.3FN catalyst had the best NO conversion efficiency.The active temperature window of 0.4MN(120-270°C)catalyst was widened to 120-330°C,and the N₂ selectivity was also significantly improved.Iron oxide promoted the decline of the crystallization of SiO₂ and MnO₂ in the catalyst,especially greatly improved the dispersion of MnO₂,so that the MnO₂ and Fe₂O₃ species on the catalyst surface were mainly in a highly dispersed state.Based on the DFT calculations,iron oxide had a certain inhibitory effect on the NH₃ dehydrogenation process,but it promoted the adsorption of NO and O₂.Especially under the synergy of manganese-iron oxide,the O₂ molecule was much easier to dissociate.The reaction process to generate N₂ through N-NO was a new reaction pathway,which greatly improved the conversion efficiency of NO at medium and high temperatures.On the other hand,iron oxide also strongly improved the N₂ selectivity which was also the reason for the increased NO conversion efficiency and N₂ selectivity over MnFeO_x/SiO₂ catalyst at high temperature.4.The addition of iron oxide improved the resistance of the catalyst to water and sulfur.The0.4MN-0.3FN catalyst was further modified with Zr and the resistance to water and sulfur was further improved.The results showed that the 0.4MN-0.3FN-0.006Zr catalyst still achieved the NO conversion above 90%after the sulfur resistance test and had the best sulfur resistance.From the TG experiments,the sulfur resistance of 0.4MN-0.3FN-0.006Zr catalyst mainly resulted from inhibiting the deposition of ammonium bisulfate on the catalyst surface,and the deactivation of 0.4MN-0.3FN-0.006Zr catalyst was mainly caused by the sulfation of Mn and Fe active sites.The DFT calculation results also showed that the addition of iron oxide significantly weakened the adsorption capacity of H₂O molecule and SO₂ molecule on the catalyst surface,which could improve the water and sulfur resistance of the catalyst.Zr doping had little effect on the adsorption capacity of H₂O molecule on the catalyst surface,which meant Zr had little effect on the water resistance.SO₂ molecules tended to be preferentially adsorbed over Zr site,which could reduce the sulfation effect of SO₂ on the active components.At the same time,the adsorption energys of SO₂ at Zr sites were not too high,which could avoid the difficulty of desorption of adsorbed SO₂ to gradually accumulate on the surface.Besides,after Zr doping,ammonium bisulfate will preferentially form over the Zr site and was more prone to decomposition,which makes it difficult for ammonium bisulfate to deposit.However,hydrogen sulfate was also more difficult to desorb from the Zr site,indicating that the Zr site was easier to be sulfated.The decomposition and desorption processes of ammonium bisulfate molecule on the catalyst surface explained the results of the TG experiments very well.It can be seen that Zr element effectively protected the Mn and Fe active sites,which was the main mechanism for Zr doping to improve the sulfur resistance of the catalyst.