The Performance And Reaction Mechanism Of Mn-based Catalysts For The Low-temperature Removal Of NO_x And Toluene From Flue Gas

The massive emissions of NO_x caused by overusing or misusing of nitrogen fertilizers and combustion of fossil fuels has exacerbated atmospheric nitrogen deposition thus leads to a series of frequent ambient pollution problems.NO_x emissions from stationary sources are the main contributor to atmospheric NO_x,therefore,the effective control of NO_x in stationary sources is the key to improving ambient air quality.Currently,Ammonia selective catalytic reduction（NH₃-SCR）is one of the most effective and mature technologies for controlling NO_x from stationary sources,and the core of NH₃-SCR technology is the development of excellent performance catalyst,and V₂O₅-WO₃/Ti O₂ is a typical commercial catalyst that has many inevitable disadvantages still exist in its practical application,such as the toxicity of VO_x and a high working temperature window.Hence,it is imperative to develop green and effective low-temperature SCR catalysts with tolerance to SO₂ and H₂O.Moreover,the emission of NO_x in industrial fossil combustion process is accompanied by the emission of VOCs.Although the emissions concentration of VOCs in the flue gas is not that high,the total absolute quantity of VOCs emissions is huge thus causes a serious threat to the environment and human health.Those reported catalyst,are characterized by poor anti-deactivation in complex flue gases,can only simply achieve efficient removal of separate pollutants（NO_x or toluene）and find it difficult to implement effective synergistic removal of multiple pollutants.Therefore,it is of great significance to develop and design catalysts with high activity and stability for simultaneous low-temperature NO_x and VOCs removal to achieve efficient synergistic control of NO_x and VOCs to further improve the ambient air quality in China.（1）To address the problems of VW/Ti de NO_x catalysts for relatively poor low-temperature activity and the toxicity of vanadium species,a series of Mn-based lotus leaves-derived biochar de NO_x catalysts were prepared,the effects of biochar carriers and modification methods（Na OH,HNO₃,and CTAB modification）on the SCR activity of the catalysts were investigated,and the structure–activity relationship was studied by BET,H₂-TPR,NH₃-TPD,XRD,XPS,FESEM and TEM,and so forth.the results as followed:1)Na OH modified biochar is an excellent carrier for the preparation of low-temperature efficient biochar denitrification catalysts,and the 25%Mn/LBC-OH catalyst showed over 95%NO_xconversion at 225℃and 37,500 h^-1.2)the presence of a large number of high valent Mn species,suitable surface acidity,excellent reduction capacity and high dispersion of MnO_xon the catalyst surface are the important reasons for the excellent SCR performance of25%Mn/LBC-OH catalyst.In addition,the anti-poisoning experiments showed highly dispersed 25%Mn/LBC-OH catalyst exhibited improved tolerance to SO₂ and H₂O than others.（2）Based on the background of the synergistic abatement of NO_x and toluene in coal-fired flue gas,the scientific problem of developing catalysts with high activity and stability for the low-temperature simultaneous removal of NO_x and toluene was addressed.The two different morphologies MnO_x catalysts（nanorod and nanocubid）were prepared,and the structure–activity relationship for the low temperature synergistic removal of NO_x and toluene was investigated.Moreover,the MnO_x nanorod was selected as a model catalyst,and modern characterization methods,including TPD,GC-MS,and in situ DRIFTs experiments,were used to systematically explore the interaction effects between the catalytic oxidation of toluene and the reduction of NO_x over the model catalyst（MnO_x nanorod）.MnO_x nanorod and MnO_x nanocuboid were synthesized by traditional hydrothermal method,and the MnO_x nanorod catalyst exhibited a greater than 85%catalytic activity in the synergistic removal of NO_x and toluene at 275℃.The MnO_x nanorod catalyst had a high proportion of Mn⁴⁺,and an excellent redox ability,compared with the MnO_x nanocuboid catalyst,and these characteristics resulted in the excellent catalytic activity in the simultaneous abatement of NO_x and toluene.However,the mutual inhibitory effects between the SCR reactants and toluene over the model catalyst（MnO_x nanorod）were investigated using TPD,In situ DRIFTs and GC-MS analyses.The impacts of toluene on the SCR reaction included the following:1)the formation of intermediate products in the oxidation process of toluene covered the active sites;2)the inhibition of“fast SCR”occurred by suppressing NO oxidation to NO₂;and 3)toluene affected the adsorption and activation of NH₃,which affected the SCR activity of the catalyst.The impacts of the SCR component on toluene oxidation reaction included the following:1)NH₃ also limited the adsorption and activation of toluene on the catalyst;2)the SCR component（NO and NH₃）competed for reactive oxygen species with the oxidation reaction of toluene,thereby affecting the deep oxidation of toluene and resulting in the incomplete oxidation of toluene at low temperatures;and 3)the intermediate products of the incomplete oxidation of toluene easily aggregated on the surface of the catalyst at low temperatures,resulting in the decline in its catalytic activity.（3）Based on the above problem of MnO_x catalysts with the formation of by-products deposited on the catalyst surface leading to poor catalytic activity and anti-deactivation.The pore size effect of the catalysts on performance for the low-temperature simultaneous removal of NO_x and toluene were further studied.A series of Mn Ce/zeolite catalysts with different pore size structures were prepared by ethanol dispersion method,the structure–activity relationship for the low temperature synergistic removal of NO_x and toluene over Mn Ce/zeolite was investigated,and the effects of different pore sizes on the catalytic activity and the anti-deactivation were thoroughly studied.The conclusions were as follows:four Mn Ce/zeolite catalysts（Mn Ce/ZSM-5,Mn Ce/Al MCM-41,and Mn Ce/ZSM-5@MCM-41）were prepared.Among them,the Mn Ce/ZSM-5@MCM-41 catalyst had the widest temperature range of 260-350℃with both NO_x conversion>90%and toluene conversion>70%,which showed the excellent performance for the low-temperature simultaneous removal of NO_x and toluene and exhibited a relatively good anti-deactivation.These systematic characterization results demonstrated that the Mn Ce/ZSM-5@MCM-41 catalyst with micro-mesopore structure had excellent catalytic activity,which was associated with the suitable surface acidity,high proportion of Mn⁴⁺and Ce³⁺species,and abundant surface chemisorbed oxygen species.However,the mutual inhibitory effects between SCR and toluene oxidation over different catalyst were investigated using TPD,and GC-MS analyses.The results showed that Mn Ce/ZSM-5@MCM-41 catalyst after MCM-41 shell wrapping,which could appropriately moderate the inhibitory effect of toluene and intermediate species on the adsorption of NH₃/NO_x on the catalyst surface,and could reduce to form benzonitrile and benzamides during toluene intermediate species reacted with NH₃/NO_x process.This is an important reason why Mn Ce/ZSM-5@MCM-41 catalyst has relatively better anti-deactivation performance than Mn Ce/ZSM-5 catalyst.