Mechanism Study And Designed Synthesis Of Mn-Based Catalysts For NH₃-SCR Of NO_x At Low-Temperatures

Nitrogen oxides,as one of the main air pollutants,has attracted much attention for its harm on humans and the ecological environment.In the face of the increasingly stringent emission standards and the national emission reduction task of nitrogen oxides,it is of great significance to study on the efficient denitrification（de NO_x）technologies.Ammonia selective catalytic reduction（NH₃–SCR）of NO_x is considered to be the most mature and efficient denitration technology.The high working temperature of the traditional V₂O₅–WO₃（Mo O₃）/Ti O₂ catalyst forces the denitration device to place upstream of the electrostatic precipitator and the desulfurizer,which brings many problems,such as:the dust with high velocity would lead to the loss of active component and environmental pollution;the SO₃formed by SO₂ oxidation can react with the overflowed NH₃ and water vapour to form NH₄HSO₄,leading to the blockage of downstream piping.Meanwhile,the traditional NH₃–SCR de NO_x process is not applicable for the denitration of low–temperature flue gas in steel,cement,glass,and other industries.The development of high–efficiency NH₃–SCR de NO_x catalyst used at low–temperatures is an effective means to solve the above problems.Mn–based catalysts,as one class of the most promising candidates for low–temperature NH₃–SCR de NO_x catalysts,have attracted much attention for their excellent activity.However,there are still many problems,such as:the denitration efficiency of low–temperature NH₃–SCR de NO_x catalysts is lower than that used at high–temperatures;the SO₂ resistance of Mn–based catalysts is poor,which is also a bottleneck that limits their industrial application;and the mechanism and the key steps of the low–temperature NH₃–SCR de NO_x process are still controversial,which also makes it difficult to the targeted design and the precise modification of catalysts.Given the problems above,this work in–deep studied the low–temperature NH₃–SCR de NO_x reaction mechanism over pristine Mn–based catalysts,and reveals the key step in the NH₃–SCR de NO_x process,on the basis of which,the metal（Fe）doped and non–metal（graphitic carbon）modified Mn–based catalysts were design synthesized.The effect of metal（Fe）and non–metal（graphitic carbon）on the low–temperature NH₃–SCR de NO_x performance were studied,the structure–function relationship between catalytic active sites and the key steps over NH₃–SCR de NO_x process were constructed,and the active site with high intrinsic activity and the active site with high SO₂resistance were revealed.This work studied the low–temperature NH₃–SCR de NO_x mechanism and SO₂ poisoning mechanism of the supported Mn（x）–Ti O₂ catalysts.The de NO_x mechanism study indicated that the low–temperature NH₃–SCR de NO_x reaction is the coupling of the NO–to–NO₂ oxidation and the"Fast SCR"de NO_x reaction.Manganese oxides provided the redox sites for NO–to–NO₂ oxidation,while the Lewis acid sites can promote the adsorption of NH₃and"Fast SCR"de NO_x reaction.The low–temperature NH₃–SCR de NO_x reaction rate was determined by the NO–to–NO₂oxidation rate.Combined with in–situ DRIFT experiments and density functional theory（DFT）calculations,an integrated NO–to–NO₂ oxidation mechanism was proposed,and the primary reason why the"fast SCR"reaction can occur rapidly at low temperatures was revealed.From the SO₂ poisoning mechanism study,it was found that the reason that lead to the deactivation of catalysts was the formation of（NH₄）₂SO₄ and NH₄HSO₄ species,which covers the active sites of catalysts.The formation of（NH₄）₂SO₄ and NH₄HSO₄ indicated that SO₂ was oxidized,which could be attributed to the strong oxidizing ability of Mn O_x species.Regulating the oxidation ability of Mn O_x could be the key point to improve the SO₂ resistance.Based on the de NO_x mechanism study over pristine Mn–based catalysts,this work investigates the effect of metal element（Fe）doping on the low–temperature NH₃–SCR de NO_xperformance of Mn₃O₄.In order to construct the structure–activity relationship,a series of Fe doped Mn₃O₄ spinel catalysts(Fe_xMn_3-xO₄ NPs)with adjustable Fe/Mn molar ratio,uniform morphology and homogeneous structure were targeted synthesized.The optimal Fe_0.35Mn_2.65O₄NPs catalyst can achieve 90%NO_x conversion in 140～250°C in the high GHSV of 200000 h^-1.It was found that the excellent low–temperature NH₃–SCR activity of Fe_0.35Mn_2.65O₄ NPs was attributed to its high NO–to–NO₂ oxidation activity.The NO–to–NO₂oxidation mechanism over Fe_xMn_3-xO₄ NPs was Mv K mechanism,and the formation of oxygen vacancy（NO+O_L→NO₂+O_V）was the key step.The doping of Fe into the central of octahedral sites(Fe_oct)in the spinel structure can form the Fe_oct–O–Mn_tetsites(Mn_tetrepresents the Mn in the central of tetrahedron site in the spinel structure)which have the lower formation energy of O_V,and can promote the formation of O_V.Fe_0.35Mn_2.65O₄ NPs catalyst is rich in the Fe_oct–O–Mn_tetsites,which attributes to its high intrinsic activity of NO–to–NO₂ oxidation at low–temperatures.Subsequently,this work investigates the effect of non–metal（graphitic carbon）modification on the low–temperature NH₃–SCR de NO_x performance of Mn₃O₄.A novel graphitic carbon modified Mn₃O₄ catalyst（Mn₃O₄@C）was developed.By using a confinement–carbonization–oxidation strategy derived from Mn–BTC MOFs,the small–sized Mn₃O₄ nanoparticles can be embedded into the graphitic carbon frame,and the strong interaction between graphitic carbon and Mn₃O₄ can be realized.Different from that Fe element doping can enhance the oxidizing ability of Mn₃O₄,the modification of graphitic carbon can weaken the oxidizing ability of Mn₃O₄,which aims to inhibit SO₂ oxidation and to improve the SO₂ resistance.The evaluation results show that Mn₃O₄@C exhibits excellent SO₂resistance（maintain>80%NO_x conversion in the stability test（8 h）of SO₂ resistance）with a good low–temperature NH₃–SCR de NO_xactivity.Combined with the results of XPS,XAS,Raman and DFT calculations,it was found that electrons can transfer from graphite carbon to Mn₃O₄surface through Mn–O–C_DG bond,leading to the strengthen of Mn–O bonds and the increase of O_V formation energy.The increased O_V formation energy led to the weakened oxidizing ability of Mn₃O₄@C.There were no sulfate species on the surface of the reacted Mn₃O₄@C after stability test of SO₂ resistance,indicating the regulation of graphitic carbon on the oxidizing ability Mn₃O₄ effectively inhibited SO₂ oxidation,which is the key point for the excellent SO₂resistance of Mn₃O₄@C catalyst.