The Selective Catalytic Reduction Of NO_x By NH₃ On Manganese-based Catalysts

Nitrogen oxide（NO_x）emissions are very harmful to the environment and human health.Selective catalytic removal of NO_xby NH₃（NH₃-SCR）is considered to be one of the most effective technologies.Manganese-based catalysts are deeply investigated in NH₃-SCR reaction due to their variable valence and excellent redox ability.However,pure Mn O_xcatalyst has the problems of narrow operating temperature window,low N₂selectivity at high temperature and poor sulfur resistance,which limits its practical application.In this thesis,manganese-based catalysts were fabricated by using metal-organic frameworks（MOFs）as the template,doping transition metals,forming composite metal oxides,and pretreatment by SO₂.The catalysts with higher activity,wider operating temperature window and strong SO₂tolerance were synthesized.The physical and chemical properties of the catalysts were characterized by various characterization techniques,and the NH₃-SCR reaction mechanism and sulfur resistance mechanism on the catalyst have been proposed.The main research and the results are as follows:（1）A porous Mn O_xcatalyst（Mn-MU）was designed and synthesized using MOF-Mn as a precursor.The results of NH₃-TPD and NH₃/pyridine infrared adsorption experiments show that MOF as a precursor is not only beneficial to the increase of acid amount on Mn O_x,but also can introduce hydroxyl groups-Br（?）nsted（B）acid sites.In the presence of SO₂,the-OH can preferentially adsorb SO₂to form HSO₄^-that can participate in the reaction,reducing the chance of the redox site being attacked by SO₂.Therefore,compared with the catalyst obtained by direct pyrolysis of manganese acetate,Mn-MU catalyst not only showed higher low-temperature activity,but also improved sulfur resistance.The NO_xconversion on Mn-MU can still be maintained above 90%after introduction of flue gases containing 50 ppm SO₂for 6 h at 175 ℃（GHSV=140,000）,and the activity can be restored after removal of SO₂.（2）Fe was doped into MOF-derived Mn-MU（Fe/Mn-IC）catalyst.Compared with Mn-MU,the active temperature window(T₈₀)of Fe/Mn-IC catalyst was broadened from 100-250 ℃ to 75-275 ℃,and the N₂selectivity in the whole temperature window was also improved.On one hand,the doping of Fe in the Fe/Mn-IC catalyst increases the acidity of Mn O_x,which promotes the adsorption of NH₃and improves the NH₃-SCR activity.On the other hand,the doping of Fe appropriately weakens the oxidation ability of Mn O_x,which is conducive to the improvement of N₂selectivity.In-situ DRIFTS experiments show that the NH₃-SCR reaction on Fe/Mn-IC catalyst can follow both the Eley-Rideal（E-R）mechanism and Langmuir-Hinshelwood（L-H）mechanism.In the former one,the coordinated NH₃and NH₄⁺are the active species of NH₃-SCR reaction.For the L-H mechanism,the bridge nitrate and bidentate nitrate can react with the adsorbed NH₃species to form active intermediates NH₂NO₃and NH₄NO₃,and finally generate N₂and H₂O.（3）Mn Co O_xcatalyst was synthesized by one-pot hydrothermal method using cheap glucose and urea as pore-forming agent and structure-directing agent,respectively.The catalyst not only shows good low-temperature activity for NH₃-SCR reaction,but also exhibits significantly enhanced sulfur resistance.The activity of Mn（5）Co（5）O_xcatalyst is significantly higher than that of single Mn O_xand Co O_x.The NO_xconversion of the catalyst reaches85%at 75 ℃,and the NO_xconversion is close to 100%at 100-200 ℃.Compared with Mn O_x,Mn（5）Co（5）O_xcatalyst shows stronger tolerance to SO₂.The NO_xconversion can still remain above 85%after reaction in the presence of SO₂for 10 h at 175 ℃,and the activity can be restored after removing SO₂.Through H₂-TPR,XPS,TG-MS and in-situ DRIFTS characterizations,it can be seen that a large amount of manganese sulfate is deposited on Mn O_xcatalyst in the presence of SO₂,which poisons the redox sites and inhibits the adsorption and activation of NO,so that the mechanism is completely transformed from L-H to E-R,resulting in obvious deactivation of the catalyst.For Mn（5）Co（5）O_xcatalyst,the strong interaction between Mn and Co forms the stronger L acid centers,which can reduce the adsorption of SO₂and make SO₂in a weak position in the competitive adsorption with NO,so that the reaction proceeds by L-H mechanism,accompanied by E-R mechanism.（4）By sulfurization pretreatment to regulate the oxidation ability and the acidity of the Mn-Co oxides sample,and further improved the catalytic performance.Below 200 ℃,the NH₃-SCR activity of Mn-Co oxides decreased with the increase of pretreatment SO₂concentration,while above250 ℃,high pretreatment SO₂concentration led to enhanced activity.This phenomenon contributes to the fact that the redox sites and L acid sites relating to oxidation ability of catalysts were passivated,while the B acid sites were promoted with the increase of pretreatment SO₂concentration.Mn Co-S30 obtained by pretreated using 30 ppm SO₂possesed a considerable number of redox sites,L acid and B acid sites,thus showing the widest temperature window.The NO_xconversion can reach more than 80%in the temperature range of 80-350 ℃.NH₃-L,NH₄⁺-B,gaseous NO₂,bidentate nitrate and cis-nitrite are the active species of NH₃-SCR reaction on Mn Co-S30.This study provides a variety of ideas for the design and modification of efficient Mn-based de NO_xcatalysts,and reveals the mechanisms of improvement of low-temperature activity and sulfur resistance of manganese-based catalysts,which could contribute to the development of the low-temperature de NO_xcatalysts.