Performance Optimization And Mechanism Study Of Mn-based Medium-low Temperature SCR Catalyst Derived From Layered Double Hydroxides

In recent years,nitrogen oxides?NO_x?released by power industries?coal-fired power plants?and non-power industries?waste incineration plants?,which are one of the fixed sources of pollution,are one of the main pollutants in the atmosphere,which can cause serious environmental problems,such as acid rain,photochemical smog,and ozone depletion.Selective catalytic reduction technology with catalyst as the core part can effectively reduce the level of nitrogen oxides in the flue gas of coal-fired power plants.Considering the potential application prospect of the design of the flue gas de NO_x treatment system in China's coal-fired power plants and the treatment of low-temperature flue gas in non-power industries,it is necessary to develop an SCR catalyst with high de NO_x performance,high resistance to poisoning and high N₂ selectivity at medium-low temperatures.Manganese oxide can exhibit excellent de NO_x performance at medium-low temperatures for its good redox ability during NH₃-SCR process.However,the catalyst prepared by Mn element can be influenced easily by SO₂,H₂O and alkali metals in the flue gas,leading to the significant deactivation of the catalyst.Moreover,due to the strong oxidizing ability of Mn O_x,a large amount of by-product N₂O can be produced during the NH₃-SCR reaction over this kind of catalysts,which will lead to a decrease in N₂ selectivity.Because of that,the pure Mn O_x-based SCR catalysts can be limited for its practical industrial application.Therefore,in this paper,a new manganese-based SCR catalyst derived from corresponding LDH precursor is prepared by combining the high dispersibility and element controllability of LDH structure,and the high redox performance of Mn O_x.Meanwhile,Fe and Co can be introduced to improve the anti-poisoning performance and N₂ selectivity of manganese-based SCR catalysts,respectively.And the structures of catalysts and their LDH precursors and corresponding catalytic mechanisms are also investigated.Mn Al-NO₃ LDHs is prepared as a precursor of SCR catalyst by co-precipitation method,and Mn Al O_x catalyst is obtained after calcination.The optimized Mn₁Al₁O_x catalyst shows poor N₂selectivity and anti-poisoning performance during the SCR reaction.After that,the Fe-modified Mn₁Al₁-NO₃ is prepared by the same co-precipitation.The results show that the optimized Mn₁Fe_0.25Al_0.75O_x catalyst has a significantly improved resistance to SO₂,H₂O and alkali metal poisoning compared to Mn₁Al₁O_x,and the poisoning effect of SO₂and H₂O in the flue gas on the catalyst within 10h is reversible.However,the N₂ selectivity of the catalyst during the SCR reaction is similar to Mn₁Al₁O_x,which still has not been improved.Therefore,Co modification of the catalyst is performed to improve its N₂selectivity.The preferred Co_0.5Mn₁Fe_0.25Al_0.75O_x catalyst has the best de NO_x performance,and the NO_x conversion is almost 100%in the range of 100-200 ^oC.It is worth noting that the N₂ selectivity of Co_0.5Mn₁Fe_0.25Al_0.75O_x catalyst can be significantly improved after the introduction of Co,and the anti-poisoning ability still maintains the high level.In the range of 80-300 ^oC its N₂ selectivity slowly decreases with increasing temperature and is always higher than 75%.After that,the mechanism of the catalytic performance,anti-poisoning performance and N₂selectivity of Mn₁Al₁O_x,Mn₁Fe_0.25Al_0.75O_x and Co_0.5Mn₁Fe_0.25Al_0.75O_x catalysts are mainly studied in this paper.BET,XRD,NH₃-TPD,H₂-TPR and in-situ DRIFTS experiments are carried out on each catalyst.The results show that the high surface acidity,high dispersion performance,and high redox performance of the catalysts are the main reasons for the excellent low-temperature de NO_xactivity of each catalyst.The lower sulfate formation rate on the surface of the Fe-modified catalyst suppresses the SO₂,H₂O and alkali metals poisoning of the catalysts,and thermogravimetric analysis shows that the Mn SO₄ component formation process is difficult to take place during the SCR reaction,which makes Mn₁Fe_0.25Al_0.75O_x still have high catalytic activity within a long texting process.The introduction of Co element can significantly change the N₂O formation mechanism on the catalyst surface,and a stable Co/Mn redox process is formed during the reaction.Therefore,the continuous adsorption of NO and the formation of nitrate/nitrite components are promoted,so that N₂O follows L-H mechanism is produced.The results above are also demonstrated by transient reaction kinetics experiments and DFT theoretical calculations.