Review of Researches on SCR Catalyst with Low Temperature and high Sulfur Tolerance and Theoretical Design

. Selective catalytic reduction (SCR) of nitrogen oxides (NOx) using ammonia (NH 3 ) is currently the main technology for flue gas denitration. However, the currently widely used commercial catalysts (such as V 2 O 5 -WO 3 / TiO 2 , V 2 O 5 -MoO 3 / TiO 2 , etc.) have the disadvantages of high operating temperature, narrow active temperature window, and high catalytic cost. Therefore, in recent years, researchers have devoted themselves to the development of low-cost and efficient low-temperature SCR catalytic materials. This paper summarizes the research progress of low-temperature (less than 250 ℃ ) selective catalytic reduction of NOx by unsupported metal oxide catalysts, supported metal oxide catalysts, precious metals, and molecular sieve catalysts. Among them, manganese-based catalysts show good low-temperature selectivity and stability, and have good application prospects. Finally, the research directions of manganese low temperature SCR catalysts are prospected and theoretically designed based on the existing problems.


Introduction
Nitric oxide (NO X ) refers to compounds composed of nitrogen and oxygen.Common NO X include N 2 O, NO, NO 2 , N 2 O 3 and N 2 O 5 .NO and NO X are common atmospheric pollutants.NO X has the following three aspects of harms, which are harms to physical health, damages of acid rains to crops, soils and forest, and atmospheric damages.
90~95% of NO X in atmosphere comes from emission of industrial factories.In the following text, statistics on the proportions of NO 2 cities with different concentrations in the recent 6 years (Figure 1) [1], data of national NO X emission in the recent 7 years (Figure 2) [2], annual average concentrations of NO 2 in key cities in the recent 10 years (Figure 3) [3] and NOx emission of national key industries (Figure4) [3] were made.According to the variation trend, we found that China has made great efforts to reduce NO X emission and achieved significant effect.However, how to decrease NO X emission from industrial factors is still the key research hotspot in future.

Conclusions and reviews of researches on low-temperature SCR catalysts
Selective catalytic reduction (SCR) reaction is the denitration reaction which is applied relatively widely.At present, vanadium tungsten titanium catalyst (V2O5-WO3(MoO3)/TiO2) is the SCR catalyst which is applied the mostly.The activity window of vanadium tungsten titanium is 300~400℃ and it is put at the tail gas.Existing studies on SCR catalyst are shown in Figure 5, where precious metals include Pt, Pb and Ag [4][5][6][7] and transition metal oxide catalysts refer to Mn, Fe, V, Cu, Cr and Co [8][9][10][11][12][13][14].

2.1.Carrier-free metallic oxide catalysts
Carrier-free metallic oxide catalysts refer to Mn, Fe, V, Cu, Cr, Co, etc. [8][9][10][11][12][13][14].Take Mn for example.It can be divided into two types.One is the single metallic oxide catalyst MnO2 which has good sulfur resistance under low temperature.However, selectivity of N2 is relatively poor.The other is the multi-component metallic oxide catalyst CeO2-MnO2, where Ce and Mn have synergic effect.Following reactions can be occurred: Therefore, the supported SCR catalysts which use transition metallic oxide as the activity center are the key development objects in future.

2.2.The supported metallic oxide catalysts
Common supported metallic oxide catalysts include Al2O3, TiO2, carbon base, molecular sieve, MnO2, etc. Singoredjo et al. [15] prepared M nitric oxides/ Al2O3 catalyst by using manganese acetate and manganous nitrate as the precursors for the first time, and found that the M nitric oxide prepared by using manganese acetate as the precursor had better dispersity and stronger denitration activity.Cao et al. [16][17][18] modified the highly dispersed Zr, Ag and Fe in Mn-Ce/γ-Al2O3, finding that specific surface area of γ-Al2O3 is the key to improve denitration activity.As a good carrier, Al2O3 can promote denitration of SCR, but it has poor resistance to SO2, thus restricting its applications in coal-fired boiler.Although the acidity of TiO2 is lower than that of Al2O3, TiO2 can be dispersed in metallic active components well, promote decomposition of surface sulfates and provide good SO2 resistance.Therefore, TiO2 is one of carriers of the NH3-SCR catalyst.At present, V2O5-WO3/TiO2 is the most mature commercial catalyst and it has relatively high activity and SO2 resistance.Jin et al. [19] prepared Mn-Ce supported catalyst by using Al2O3 and TiO2 as the carrier.They found that TiO2 carrier presented better activity under 150℃, but it was the Al2O3 carrier presented better activity after 220℃.Combining with TPD results, the surface of Mn-Ce/TiO2 is more occupied by Lewis acid sites and reactions are implemented through the E-R mechanism.On contrary, the surface of Mn-Ce/Al2O3 is mainly occupied by Brønsted acid sites.These two catalysts follow different reaction paths.However, specific surface area of ordinary TiO2 is relatively low and active components are difficult to be used fully.Besides, SCR catalyst based on TiO2 shows moderate performance under low temperature.Gálvez et al. [20] introduced in few vanadium compounds to the surface of active carbon and the acid site on the loaded active carbon surface is increased.The conversion rate of NOx was 40% at 125℃ and it was further increased to as high as 80% at 200℃, which was far higher than the adsorption efficiency of pure active carbon (43%).Guo et al. [21] studied properties of oxygen-containing functions on coal-based active carbon surface in SCR reaction and found that carboxyls, anhydrides and phenolic groups on active carbon surface enhanced adsorption of NH3, thus promoting conversion rate of NOx.Moreover, carbon nanotubes (CNTs), a kind of special ordered material, are also tried to be used as the carrier of SCR catalyst because of their unique nanostructure and electrical properties.Su et al. [22] found that M nitric oxide (Mn-in-CNTs) which was restricted in carbon nanotube channels had higher capacity of oxygen supply and NO adsorption.The Mn-in-CNTs have stronger NH3-SCR activity than the catalyst where M nitric oxide is anchored onto external surface of CNTs (Mn-out-CNTs).
Molecular sieve is a type of porous material formed by silicon and aluminum tetrahedron.It has very high specific surface, thermal stability and surface acidity, and it is one of high-quality carriers for supported SCR catalyst.Currently, ZSM-5, SPAO-34 and SSZ-13 are common molecular sieves.In 1989, IwamotoM et al. [23] discovered for the first time that Cu2+ exchanged ZSM-5 molecular sieve could catalyze decomposition of NO and nitric oxide conversion efficiency of Cu-ZSM-5 with 73% exchange could reach 85% at 550℃.Choong-KilSeo et al. [24] studied influences of ZrO2 adulteration on denitration activity of Cu-ZSM-5, finding that the NOx conversion rate of Cu-ZSM-5-ZrO2(2wt%) was 10%~20% higher than that of Cu-ZSM-5.Ford Company [25] compared thermal stability of metal molecular sieve catalyst and vanadium-based catalyst.After keeping under 670℃ for 64h, it was found that thermal stability of the metallic molecular sieve catalyst was attributed to the vanadium base.Meanwhile, the reaction temperature of Fe molecular sieve was lower compared to the Cu molecular sieve.Previous studies all focus on Fe and Cu.The Fe-Mn/ZSM-5 prepared by Mu et al. [26] showed relatively high NH3-SCR activity during 100~300℃.Through a in-situ infrared left, it found that appropriate Mn4+/Mn3+ and Fe3+/Fe2+ promoted generation of linear nitrite and monodentate nitrate which were in favor of reactions, thus improving low-temperature activity of NH3-SCR.Du et al. [27] prepared Fe-ZSM-5@silicalite-1 with core-shell structure through the secondary hydrothermal technique and the Fe-ZSM-5 after modification of silicalite-1 shell improved the SCR activity and water resistance.These were mainly attributed to NOx adsorption and the extremely good hydrophobicity of the silicalite-1 shell.
To sum up, traditional Meta carrier have small specific area, easy damping and easy toxicity due to limitations of their physical and chemical properties.Al2O3 catalyst is easy to react with water due to the existence of Al atoms, which restricts its applications in humid environment.Carbon-based material has developed diameters and rich functional groups, but it has poor thermal stability.Therefore, these materials are difficult to realize industrialized applications.Other nanomaterials, such as active carbon fiber, CNTs and graphene, have specific structures, specific pore diameters and good performance.However, they are still in the laboratory level for the high preparation cost.Molecular sieve not only is equipped with rich specific surface areas, pore diameter structures, good thermal stability and high mechanical strength, but also shows outstanding adsorption effect to micromolecules.Hence, molecular sieve has a promising application prospects as a supported catalyst.

Introduction to the MnO2 structure
MnO2 is the sum of a type of compounds.It is embedded with potassium ions and has more than 10 crystalline phase [28].
The microstructures of β-MnO2 which are observed along the Z axis are shown in Figure 6.In Figure 6(a), a manganese atom is connected with 6 oxygen atoms, forming a octahedron with group symmetry.The sp2 hybridization of β-MnO2 is shown in Figure 6(b), which makes the pore passages dispersed.Hence, β-MnO2 cannot be used as catalyst.The microstructure of α-MnO2(OMS-2) is shown in Figure 7, which include skeleton, sp2 hybridization and sp3 hybridization.The stacking modes of α-MnO2 and β-MnO2 atoms are different.The former one combines pore passages to be quadruple of the original structure.α-MnO2 and β-MnO2 are tetrahedron and octahedral structures, which are stacked along the Z axis.They grow along one direction during synthesis and nanowires are generated [29].In a word, we have to determine where are active sites of α-MnO2.Therefore, we have to accomplish the surface analysis of α-MnO2 to disclose the catalysis principle.

3.2.Research review on MnO2 low-temperature SCR catalyst
MnO2 has been widely applied due to the excellent catalysis performance.According to experimental results, the MnO2/CeO2 catalyst has extremely high NO catalytic performances and even 100% conversion rate can be achieved at 50℃ [30].This almost realizes NOx removal under room temperature.Moreover, performances of the MnO2/CeO2 catalyst are improved significantly by adjusting proportion of Mn and Ce.This study can be viewed as a typical representative for MnO2 catalytic denitration.
MnO2 has following two disadvantages.One is excessive strong oxidability.The selectivity of N2 is low and some by-products are N2O.Under normal conditions, N2O mainly come from the reaction between NH3 and NO.Combining with previous studies, there's might be a decomposition route of NH4NO3 [31,32].
The second is poor sulfur resistance of MnO2.It can be seen from Figure 8 that the catalyst loses activity completely after about 4h accession under the atmospheric environment of 600ppm SO2 and 5% H2O.Even when SO2 and H2O are cut off, the catalyst still cannot return to the previous conversion rate.According to experimental results, the Mn-based catalysts have extremely poor sulfur resistance and they cause irreversible toxicity rather than simple absorption toxicity.This is one disadvantage of Mn-based catalysts.Therefore, current studies shall focus on improvement of sulfur resistance and selectivity, and providing a new idea to improve sulfur resistance.

Research conclusions on sulfur resistance improvement
The mechanism of toxication of SO2 under high temperature is completely different from those of ordinary molecules.Different from ordinary materials, SO2 doesn't generate activity inhibition of catalyst due to competitive adsorption.This is mainly because SO2 has reaction activity and it often react with the catalyst or its reactants.For SCR, SO2 serves for different roles in the high-temperature interval and low-temperature interval.SO2 might promote the reaction in the high-temperature interval [34].This can be explained as follows.NH3 adsorption under high temperature is very weak and it often has desorption, making it difficult to participate in the reaction [35].Therefore, SCR catalyst requires certain acidity under high temperature to stabilize the adsorbed NH3.B acid sites will be generated on the catalyst surface under the existence of water vapor and sulfur, thus facilitating NH3 adsorption and increase activity of the catalyst.According to the study of Liu et al. [36], SO42-can appear as strong B acid.As a result, we conclude that appropriate access of SO2 or sulfating of catalyst is beneficial.Therefore, existence of SO2 under high temperature is generally not a significant problem.Under low temperature, SO2 presents toxic properties of catalyst.This has different explanations at present.When the catalyst contains SO2, NH4HSO4 is generated on the catalyst surface, accompanied with catalyst toxicity caused by metal sulfate.NH4HSO4 is covered on surface of the catalyst, thus separating activity centers on catalyst surface from reactants.Hence, the catalyst looses activity.Metal sulfate often can stabilize valence-changing elements of the catalyst into the low-valence state to separate them from SCR reduction and thereby trigger inactivation.In a word, it takes a very long period to observe significant inactivation of catalyst under low SO2.This property brings a great challenge to study sulfur resistance of catalyst and it is easy to cause wrong understanding and wrong explanation of experimental data.As a result, improving sulfur resistance of catalyst is a very challenging study.
On this basis, the following schemes to improve sulfur resistance are proposed through literature review: (1) increasing acid sites.Since SO2 is a typical acid molecule, attentions can be paid to acidizing of catalyst surface to improve the sulfur resistance.Besides, SCR catalysis requires participation of certain acid sites in the reaction, the performance of catalyst can be improved by introducing in the acid sites.(2) Introducing in sacrificial agent.Catalyst can be protected by using some materials as the sacrificial agent to react with SO2 firstly.Take vanadium tungsten titanium for example, Ti is very easy to be the center of sulfate.We think Ce can be used as the sacrificial agent.There are three associated studies.One is to add Ce into Mn-based and Ti-based catalysts, which can increase sulfur resistance significantly.The second is to add Ce into FeM nitric oxide to increase sulfur resistance significantly.The third one is add Ce to protect active sites and the sulfur resistance is increased significantly.As a result, adding sacrificial agent, especially Ce, into catalysts under low temperature might be the most feasible method to improve sulfur resistance.Future studies will focus on dose of Ce and anti-toxicity mechanism.

Conclusions
In this study, harms of NOx are introduced and a statistics on NOx emission in China in recent years is made.Subsequently, controlling technologies and generation mechanism of NOx are described.At present, SCR is a mature technology.Several catalysts for SCR reaction are analyzed and the promising application prospects of molecular sieve as a supported catalyst are recognized.Based on analysis of the MnO2 structure, we believe that analysis of α-MnO2 surface structure is the only that has to be done at present.Attentions shall be paid to disclose the catalysis principle and active sites and why potassium ion can stabilize pores.By analyzing catalytic performance of MnO2, this study finds two disadvantages of MnO2 catalyst, which are excessive strong oxidization and poor sulfur resistance.Finally, we suggested to increasing acid sites and introducing in sacrificial agent to improve the sulfur resistance.
Moreover, future research keys are proposed, which are the dosage of Ce and anti-toxicity mechanism.

Figure 8 .
Figure 8. NO conversion rate of different Fe-Ho-Mn/TiO2under the atmospheric contents of SO2 and H2O[33]