Preparation And Mechanism Study Of Mn-based Low-temperature NH₃-SCR Catalyst With High SO₂ Resistance

Recent years,the total emission of nitrogen oxides in China is still huge.High efficient denitrification technologies must be used to meet the requirement of“ultra-clean emissions”.The commercial vanadium-based SCR catalyst shows a narrow operation temperature window of 300-400°C,as a result,it needs to be located in the high-temperature and high-ash area upstream of the electrostatic precipitator.During long-term operation the V-based catalyst is prone to be worn and poisoned,which seriously affects the life of the SCR equipment.In the future,locating the SCR system in the low-ash area downstream of electrostatic precipitator and desulfurizer will be the development trend of the current denitration technology,the core of which is to develop low-temperature SCR catalysts with high efficiency and resistance.This paper focused on Mn-based low-temperature SCR catalysts.By using experimental and quantum chemical calculation methods,the effects of support characteristics,active metal kinds and preparation methods on the denitrification performance of the catalysts and the SCR reaction mechanism were studied.Moreover,a novel method was proposed that using the sieving characteristics of zeolites to improve the SO₂ resistance of the catalyst.The main contents of this paper are as follows:Firstly,the effect of the support characteristics of anatase TiO₂ on the performance of the catalyst was investigated.The results showed that the denitrification activity of Mn-Cu/TiO₂{001}catalyst was 9.29%higher than that of Mn-Cu/TiO₂{101}catalyst and it showed a wider active temperature window.Preferential exposure of{001}faces was favorable to increase the dispersion of the active metal and the atomic ratio of Mn⁴⁺and O?,also,it was favorable to enhance the Lewis acidity on the catalyst and the synergistic effect between the active metals.The catalyst models of MnO₂/TiO₂?001?and MnO₂/TiO₂?101?was constructed using the quantum chemical calculation method.The adsorption characteristics,oxidizability and oxygen defect formation ability of the two catalyst surfaces were investigated.The calculation results showed that NH₃,NO,NO₂ and O₂tended to adsorb on the TiO₂?001?surface rather than on the TiO₂?101?surface,and the TiO₂?001?surface was more oxidized and more prone to generate oxygen vacancies.The loading of MnO₂ significantly promoted the performance of the TiO₂?001?surface.And the adsorption performance,oxidizability and oxygen defect formation ability of the Mn?001?surface were obviously better than that of the Mn?101?surface.Secondly,by using the anatase TiO₂ with{001}faces preferentially exposed as the support,the effects of Ce,Fe and Cu doping to the activity of the Mn/TiO₂{001}catalysts were studied.The MnCe001 catalyst prepared by Ce doping showed the best dispersion of the active components,the strongest synergy and the richest Lewis acid sites.The activity of MnCe001 was 3.35%higher than that of MnCu001 and was 8.74%higher than that of MnFe001.Also,the MnCe001 catalyst showed the strongest resistance to H₂O and SO₂,the activity of which only decreased by 11.84%after adding H₂O and SO₂ for 165 min.And the poisoning effect of H₂O and SO₂ to the MnCe001 catalyst was reversible.Thirdly,the effect of preparation methods on the denitrification performance of monolithic Mn-Ce/Al₂O₃/cordierite catalysts was studied.A new preparation method,modified deposition precipitation?MDP?,was proposed.The experimental results showed that in the preparation process of the catalyst using MDP method,Mn²⁺and Al³⁺formed a small cluster of?Mn-OH-Al?as a result of the co-attraction of OH^-,which reduced the difference between the solubility product of these three metal hydroxides and avoided the occurrence of successive precipitation.MDP method effectively enhanced the interaction between the active component and the washcoat and increased the atomic ratio of Ce³⁺and O_b on the catalyst surface.Therefore,the activity of the MDP3 catalyst prepared by MDP method was 10.14%and 7.04%higher than of the 1IMP1 and DP2 catalysts which were prepared by conventional impregnation method and the deposition precipitation method.The MDP3 catalyst showed the strongest resistance to H₂O and SO₂,the activity of which only decreased by 13.41%after adding 10 vol.%H₂O and 400 ppm SO₂ for 4 h.Fourthly,a simplified Mn/Al₂O₃ catalyst model was constructed and the reaction mechanism on the catalyst was revealed using quantum chemical calculation method.The results showed that NH₃ was mainly adsorbed on the Lewis acid sites of the Mn/Al₂O₃catalyst and participated in the SCR reaction as coordinated NH₃.NO₂ was formed through the oxidation of gaseous NO by active oxygens or lattice oxygens,wherein active oxygens was more reactive.N₂O formation on the Mn/Al₂O₃ catalyst surface was mainly caused by deep dehydrogenation of NH₃,while the energy barrier of which was high indicating that it was difficult to generate N₂O on the surface of the Mn/Al₂O₃ catalyst.Finally,in view of the problem that Mn-based SCR catalysts are susceptible to the poison of SO₂ at low temperature,a novel method was proposed,that is,using a zeolite with suitable pore size to sieve the SCR reaction gases.Hence,SO₂ of large molecular dynamic diameter was prevented from contacting with the low temperature SCR catalyst,which was expected to fundamentally solve the problem.The experimental results showed that after loading RHO molecular sieve seeds by rubbing method an uniform and continuous RHO zeolite membrane could be grown on the stainless steel tubes through aging at 25°C for 4 d and crystallizing at 60°C for 6 h.RHO zeolite membrane exhibited a great NO/SO₂ separation selectivity.A very high NO/SO₂ separation selectivity was obtained when the SO₂ inlet concentration was low.The NO/SO₂ separation selectivity was significantly increased from 5.45 to 36.14 when using NO/SO₂/N₂ mixtures.Water greatly inhibited the permeation of SO₂,thus improving the separation performance of NO/SO₂.Surface diffusion and activated diffusion were the main transport mechanisms of the RHO zeolite membrane.