The Latest Research Progress of NH 3 -SCR in the SO 2 Resistance of the Catalyst in Low Temperatures for Selective Catalytic Reduction of NO x

: The selective catalytic reduction (SCR) has been widely used in industrial denitriﬁcation owing to its high denitriﬁcation e ﬃ ciency, low operating costs, and simple operating procedures. However, coal containing a large amount of sulfur will produce SO 2 during combustion, which makes the catalyst easy to be deactivated, thus limiting the application of this technology. This review summarizes the latest NH 3 -SCR reaction mechanisms and the deactivation mechanism of catalyst in SO 2 -containing ﬂue gas. Some strategies are summarized for enhancing the poison-resistance through modiﬁcation, improvement of support, the preparation of complex oxide catalyst, optimizing the preparation methods, and acidiﬁcation. The mechanism of improving sulfur resistance of catalysts at low temperatures is summarized, and the further development of the catalyst is also prospected. This paper could provide a reference and guidance for the development of SO 2 resistance of the catalyst at low temperatures.


Introduction
Nitrogen oxide (NO x ) is a general term composed of nitrogen, oxygen, and other compounds. It is one of the major pollutants from the exhaust gas of thermal power plants, industrial furnaces, motor vehicles, ship exhaust emissions, and includes N 2 O, NO, NO 2 , etc.-among which NO and NO 2 account for the largest proportion [1]. A large amount of NO x emitted into the air will cause a series of environmental concerns. Coal matters a lot in the energy consumption of China, which is the largest source of NO x , accounting for 67% of the total NO x emissions in China. Among all coal-fired industries, thermal power plants have the largest NO x emission, the NO x emission standard of which is 100 mg/Nm 3 [2]. Therefore, exploring and developing efficient exhaust gas deNO x technology has been an area of intense investigation.
Among all flue gas denitrification technologies, selective catalytic reduction (SCR) is an extensively applied technology due to its low reaction temperature and high denitrification efficiency [3,4]. Selective catalytic reduction (SCR) mainly refers to the reaction of NO x using NH 3 as a reducing agent in the presence of O 2 to produce pollution-free N 2 and H 2 O, whose core is the catalyst. The main reaction equations are (1)-(3) [5]. 4 V 2 O 5 /TiO 2 catalyst is a mature and typical catalyst which has a high denitrification efficiency and has been commercialized for a long time. However, this catalyst still has some problems, such as low selectivity of N 2 at high temperatures and a narrow reaction temperature window 300 • C-400 • C. SO 2 can be effortlessly converted to SO 3 in the smoke, resulting in catalyst deactivation [6]. Additionally, vanadium oxide is toxic and easy to cause secondary pollution and other environmental problems. On the basis of original catalyst, the new V 2 O 5 /TiO 2 catalyst-adding WO 3 and MoO 2 as the promoter-has solved some problems, but there are still some pivotal challenges such as low catalytic activity at low temperatures and poor performance for the improved catalyst [7]. Besides, there is no room for a denitration device between the air preheater and economizer in the built thermal power plants in China, which makes the flue gas temperature lower after treatment. In order to meet the use conditions of conventional catalysts, the flue gas must be heated, which increases the cost of flue gas treatment. As another main source of flue gas emission, industrial combustion boilers and industrial furnaces have lower flue gas emission temperatures, which makes the existing SCR catalyst difficult to meet the use requirements. The working temperature of the low-temperature SCR catalyst is 150 • C-300 • C or lower. The position of the reactor is behind the dust removal and desulfurization device, which can effectively avoid the toxic effect of dust and high concentration SO 2 on the catalyst. Therefore, the research and development of SCR catalysts at low temperatures have been brought to the fore by data as the rapid rise of increasingly severe NO x emission standards.

SO 2 Poisoning Mechanism of Low-Temperature Catalyst
At present, some power plants adopt wet desulfurization to remove SO 2 with lower flue gas temperature, failing to meet the reaction requirements of V 2 O 5 /TiO 2 catalyst. Therefore, there are many drawbacks such as low denitrification efficiency and catalyst waste. After desulfurization, tiny amounts of SO 2 still exist in the exhaust gas, bringing about the deactivation of SCR catalyst. Therefore, developing a vanadium-free catalyst with great denitrification performance and sulfur and water resistance at low temperatures is extremely necessary [8]. To solve the sulfur poisoning of catalysts, many scholars have done a lot of research and elaborated on the poisoning mechanism in detail. Pan et al. [9] studied the deactivation mechanism of MnO x /MWCNTs catalyst at low temperatures. The MnSO 4 was formed by the reaction between the active components Mn and SO 2 under the condition with O 2 , which not only decreased the content of the active site but also slowed down the reaction process. On the other hand, the deposition such as (NH 4 ) 2 SO 4 and NH 4 HSO 4 formed by the reaction between SO 2 and NH 3 would block the active sites and reduce the specific surface area. In addition, the intermediate substances such as N 2 O 4 and nitrosyl (NO -) would be produced by the adsorbed NO species and further participate in the reaction. However, SO 2 could compete for adsorption sites with NO x on the catalyst surface sharply and had the momentous inhibition influence on the formation of N 2 O 4 and nitroso, leading to the reduction of NO x conversion rate. These two substances were the intermediate medium and there was a major role to play in the SCR reaction. Jiang et al. [10] synthesized Fe-Mn/TiO 2 catalyst and explored its SCR performance systematically. NO complexes, monodentate nitrate, and bidentate nitrate would be found after NO and O 2 reacted for 30 min without SO 2 , but these substances were replaced by sulfate when SO 2 was introduced, which also indicated that SO 2 could compete for adsorption sites with NO on the catalyst surface, leading to the disappearance of intermediates and reducing reaction efficiency. A similar SO 2 poisoning mechanism was also obtained in the study of Xu et al. [11] on Ce/TiO 2 catalyst and Zhang et al. [12] on Cu-SAPO-34 catalyst.
From the above studies, the SO 2 deactivation mechanism on the catalyst at low temperatures can be observed mainly in the following three aspects. (1) The ammonium sulfate and ammonium bisulfate are formed by the reaction of SO 2 and NH 3 in the presence of O 2 and attach to the catalyst surface, which can decrease the surface area, pore volume, and pore size of the catalyst, and then reduce the reaction rate. However, ammonium sulfate and ammonium bicarbonate will self-decompose when the NH 3 -SCR reaction is carried out above 280 • C and 350 • C, respectively, so the catalytic activity is able to be restored by the washing method at low temperatures [13]. (2) In the presence of O 2 , SO 2 will react with the active component (mainly transition metal) on the catalyst surface to generate metal sulfate salt, which will cause irreversible deactivation of the catalyst. (3) SO 2 will compete with NO at the adsorption sites on the catalyst surface when these acidic gases are present in the reaction system, which would reduce the formation of SCR intermediate products and the catalytic efficiency of catalyst. Figures 1-3 show the mechanism of catalyst sulfur poisoning.

Research Progress of SO 2 Resistance Catalyst at Low Temperatures
Catalyst is usually composed of active component and support, and there are other forms of catalysts such as composite oxide catalysts. To enhance the low-temperature sulfur resistance of catalysts, many scholars have focused their attention on the improvement of active components and supports. In addition, some scholars have found that the preparation method-the handling catalyst by acidification and reaction conditions-can make a difference in the SO 2 resistance of catalyst.

Effects of Active Components
The active component, which is composed of one or more substances, is the main unit of catalyst and affects the NH 3 -SCR reaction significantly. Using rare earth metals as well as transition metal oxides to improve active components is one of the most effective methods to improve sulfur resistance at low temperatures.

Ce-Modified Catalysts
Cerium has an excellent ability to store and release O 2 . The electron transfer between Ce 4+ and Ce 3+ is favorable for catalyst to form reactive oxygen species, promoting the conversion of NO into NO 2 . Besides, the catalyst modified with Ce can increase the content of acidic sites and reduce the oxidizability of catalyst in some extent [14,15]. SO 2 would preferentially react with CeO 2 on the surface of catalyst to form Ce 2 (SO 4 ) 3 in the presence of O 2 and H 2 O, which can reduce the reaction between NH 3 and SO 2 to produce ammonium sulfate and ammonium bisulfate, and inhibit the catalyst deactivation [16].
Wei et al. [17] studied the Mn/TiO 2 catalyst modified by Ce and the sulfur resistance was tested. The results demonstrated that the surface of Ce-doped catalyst retained the Lewis acid sites and produced new Brønsted acid sites. After doping with Ce, SO 2 was preferentially adsorbed on Ce in the form of sulfate instead of the Lewis acid sites and Brønsted acid sites on MnO x , which were the active sites of Mn-Ce/TiO 2 catalyst. Ammonium sulfate, ammonium hydrogen sulfate, and titanium sulfate on the surface of the catalyst were also inhibited to form, which improved the sulfur resistance of catalyst. Jin et al. [18] also explored the CeO 2 -modified Mn-Ce/TiO 2 catalyst. It was found that SO 2 would react with O 2 to produce SO 3 , which reacted with CeO 2 preferentially and reduced the sulfation of active component MnO x . It meant that CeO 2 can act as a catalyst SO 2 collector, which can limit the sulfation of the active component ( Figure 4). The V 2 O 5 -WO 3 /CeO 2 -TiO 2 catalyst prepared by coprecipitation method showed the best low-temperature catalytic efficiency and sulfur tolerance when the ratio of Ce/Ti was 0.1. Ce modification could not only increase the surface area and adsorbed oxygen but also strengthen the interaction between Ce and Ti [19]. The research of Lee et al. [20] revealed that the Sb-V 2 O 5 /TiO 2 catalyst with 10% CeO 2 could markedly strengthen the catalytic performance and sulfur resistance of catalyst at 220 • C-550 • C. XRD test showed that active components were more evenly distributed on Sb-V 2 O 5 /TiO 2 catalyst than those on CeO 2 /TiO 2 catalyst. Adding CeO 2 not only enhanced the acidity of the catalyst but also reduced SO 2 adsorption due to the rejection to SO 2 , further slowing down the effect of SO 2 poisoning. The research of France et al. [15] indicated that compared with the original catalyst, the catalyst modified by the CeO 2 could increase the amount of the chemical adsorption of oxygen on the surface, which could increase the rate of NO oxidation to NO 2 and facilitate the rapid response of NH 3 -SCR.
In the presence of SO 2 , the catalytic efficiency of catalyst decreased slightly, while the catalyst could nearly return to the initial catalytic state after SO 2 was stopped ( Figure 5). Based on the V 2 O 5 /TiO 2 catalyst, Ma et al. [21] added WO 3 and CeO 2 to it and considered the SO 2 poisoning of the catalysts at low temperatures. CeO 2 doping improved the redox ability while inhibiting the formation and deposition of (NH 4 ) 2 SO 4 on the catalyst surface. Figure 6 showed the low-temperature deactivation mechanism of the catalyst under the condition with SO 2 .

Fe-Modified Catalysts
Fe is also a common and efficient modifier for low-temperature SCR catalyst [22]. Due to its low price, excellent reduction performance, and various chemical values, Fe can be used to improve the active components and support of catalysts, which has also been widely concerned by scholars.
Cao et al. [23] used sol-gel method to synthesize the Fe-doped Mn-Ce/γ-Al 2 O 3 catalyst. Compared with the original catalyst, Fe-modified catalyst could reach 95% conversion efficiency of NO X at 250 • C-350 • C and had excellent sulfur and water resistance. The experiment results demonstrated that the surface area, acid sites, and adsorption capacity of NO, as well as the pore size of the catalyst were increased after Fe was doped, which could enhance the catalytic efficiency and sulfur resistance of catalyst at low temperatures. Lewis acid sites accounted for more than Brønsted acid sites, although the number of Brønsted sites increased with the addition of Fe. According to some research [24], the SCR reaction of the catalyst mainly follows the Eley-Rideal (E-R) mechanism at high temperatures. It means that the adsorbed NH 3 and NH 4 + species can react with NO to generate N 2 and H 2 O finally.
Among them, the NH 4 + species is formed by NH 3 adsorbed on Brønsted acid sites. Therefore, Brønsted acid sites are favorable for the process of E-R mechanism and further promote the SCR reaction. Figure 7 was a flow chart of the reaction of the Fe-modified enhanced catalyst. Based on Mn-Ce/TiO 2 catalyst, Shen et al. [25] synthesized Fe-Mn-Ce/TiO 2 catalyst modified with Fe. It was found that when Fe/Ti = 0.1, Fe-Mn-Ce/TiO 2 catalyst could reach 96.8% NO x conversion efficiency at 180 • C as well as wonderful sulfur resistance over the Mn-Ce/TiO 2 catalyst on account of the increase of acid sites. Wu et al. [26] studied the V 2 O 5 /TiO 2 catalyst doped with Fe and found that the Fe-V 2 O 5 /TiO 2 catalyst could achieve almost 100% conversion efficiency at 270 • C. The addition of Fe promoted the dispersion of active components, which is related to the special surface area, the synergistic effect, etc., of catalyst. With the increase of Fe doping, the BET (Brunauer-Emmett-Teller) surface area also increased. Besides, the synergistic effects between Fe, V 2 O 5 , and TiO 2 are beneficial to improve the dispersion of active components. Meanwhile, the content of adsorbed oxygen and acid sites increased, improving the activity and sulfur resistance at low temperatures.

Cu-Modified Catalysts
Cu has many advantages such as good thermal conductivity, corrosion resistance, strong chemical stability, and low toxicity. In the field of NH 3 -SCR, Cu has been studied for its high catalytic activity, multiple recycling, mild temperature, and simple ligand [27].
Zhao et al. [28] added Cu, Fe, Mn, and Co to the V 2 O 5 /TiO 2 catalyst to prepare the modified catalysts by impregnation method. Among all the modified catalysts, the Cu-V 2 O 5 /TiO 2 catalyst exactly exhibited the best catalytic performance and can reach 90% conversion efficiency of NO x at 225 • C-375 • C. After introducing SO 2 and H 2 O, the catalytic efficiency of catalyst decreased slightly for a long time. Xu et al. [29] explored the low-temperature catalytic performance and sulfur resistance of Cu-Fe/Beta catalyst. Compared with Cu/Beta catalyst and Fe/Beta catalyst, Cu-Fe/Beta catalyst broadened the temperature window and could achieve more than 80% catalytic efficiency at 125 • C-500 • C. Besides the interaction between Cu and Fe, the increasing of Cu 2+ /Cu + and Fe 3+ /Fe 2+ was also the main reason for the improvement of activity and sulfur resistance. Li et al. [30] doped light CuO into TiO 2 /CeO 2 catalyst and found the catalytic performance of the TiO 2 -CuO/TiO 2 catalyst was increased obviously. When Cu/Ce = 0.005, TiO 2 -CuO/TiO 2 catalyst exhibited the most excellent catalytic performance and sulfur tolerance, as shown in Figure 8. The addition of CuO produced more adsorbed oxygen and Ce 3+ species, which enhanced the reduction performance and surface acidity of TiO 2 -CuO/TiO 2 catalyst.

W-Modified Catalysts
W has been widely used in conventional commercial catalysts. The addition of WO 3 can widen the reaction temperature window of catalyst, increase the content of active substances and acid sites, and effectively promote the SCR activity and SO 2 tolerance of catalyst at low temperatures [31].
Chen et al. successfully used impregnation method to synthesize W-modified CeO 2 /TiO 2 catalyst. The experimental results showed that when Ce and W were impregnated together and W content is 6%, CeO 2 -WO 3 /TiO 2 catalyst can achieve over 90% of the catalytic efficiency at 200 • C-450 • C with excellent sulfur resistance. The addition of W significantly increased the content of Ce 3+ and was propitious to the conversion of NO to NO 2 , thus promoting the rapid reaction of SCR and greatly improving the activity and sulfur resistance at low temperatures [32]. Shan et al. [33] prepared Ce-W-TiO 2 catalyst by coprecipitation method. The experiment showed that Ce 0.2 W 0.2 TiO x catalyst could achieve over 90% catalytic efficiency at 275 • C-450 • C and had excellent sulfur and water resistance. Besides, Ce 0.2 W 0.2 TiO x catalyst could still maintain considerable catalytic activity at high space velocity. The addition of W in Ce 0.2 W 0.2 TiO x catalyst produced more acid sites, oxygen vacancies, and active substances, which were conducive to improving the catalytic activity and sulfur resistance at low temperatures. On the basis of Ce/TiO 2 catalyst, Jiang et al. [34] used sol-gel method to prepare CeO 2 -MoO 3 -WO 3 /TiO 2 catalyst by adding WO 3 and MoO 3 . The results demonstrated that adding W and Mo greatly would produce more active sites and acid sites, which made CMWT catalyst achieve 93.8% catalytic efficiency and showed excellent sulfur and water resistance at 275 • C-450 • C under large space velocity.

Catalysts Modified with Other Metal Elements
Xu et al. [35] explored the NH 3 -SCR performance of V 2 O 5 -Sb 2 O 3 /TiO 2 catalyst systematically, which widened the temperature window and showed better sulfur tolerance in comparison to the conventional catalyst. The results demonstrated that adding Sb could slow down the SO 2 oxidation in exhaust gas, making the catalyst produce less ammonium sulfate and ammonium bisulfate on the surface when the temperature was low. Yang et al. [36] demonstrated that Cr-V/TiO 2 catalyst doped with Cr not only improved the catalytic performance but also improved the low-temperature sulfur tolerance of catalyst. Cr doping could increase the number of acid sites, which inhibited the adsorption of SO 2 and promoted the NH 3 adsorbed on the catalyst surface. In addition, the surfactant dispersion and the ratio of V 4+ /V 5+ were also improved. Figure 9 was the mechanism of Cr doping improving sulfur resistance of Cr-V/TiO 2 catalyst. Tian et al. [37] studied the Ca-doped Mn/TiO 2 catalyst. It was found that after SO 2 was introduced into the catalyst, CaSO 4 was preferentially generated by reaction with Ca, which could avoid the sulfation of active components and improve the catalytic activity and sulfur resistance at low temperatures. Figure 10 shows the typical SO 2 -tolerant modified catalyst at low temperatures for selective catalytic reduction (SCR) reaction.
Above all, the content of oxygen adsorbed on the active material and catalyst surface-and even the number of acid sites-could be increased by adding Ce, Fe, Cu, W, and others to the active components, which would accelerate the rapid reaction of SCR and increase the catalytic performance of catalyst. Under the reaction conditions containing SO 2 , these additives can preferentially react with SO 2 as the SO 2 catchers to avoid the sulfation of active substances. Besides, the stability of NH 4 HSO 4 and (NH4) 2 SO 4 on the surface was reduced, and the low-temperature sulfur tolerance of catalyst was effectively improved. It is believed that transition metals and rare earth elements will have promising applications in improving sulfur resistance of catalyst at low temperatures.

Effects of Supports
The support of catalyst is of vital importance in the catalytic activity. Loading the active components onto the support contributes to improving the specific surface area, thermal resistance, and mechanical strength of catalyst. Especially, the supports can slow down SO 2 poisoning on the catalytic activity of catalyst. As of now, there are TiO 2 , Al 2 O 3 , activated carbon and zeolites, and other conventional supports in practical application. However, different supports show different catalytic activity and sulfur resistance, which makes the research and improvement of the support become a part of the emphasis of research to improve the low-temperature SO 2 resistance of catalyst.  [15,20,23,25,28,29,33,35].

Effects of TiO 2 Support
TiO 2 is one of the most common supports and has great catalytic efficiency. Compared with other supports, TiO 2 can provide more meaningful acid sites with the characteristics of large surface area, high dispersion of active components, and strong chemical stability [24,38]. Besides, the protection by TiO 2 support on active components and the interaction between them also play significant roles in SO 2 tolerance.
Zhang et al. [39] used impregnation method to synthesize the TiO 2 /CeO 2 catalyst by reversing the support and active components of the most common CeO 2 /TiO 2 catalyst. They found that the TiO 2 /CeO 2 catalyst with changed structure had better catalytic activity and sulfur resistance due to the protection of TiO 2 , which could maintain the 90% conversion efficiency of NO x at 300 • C after introducing SO 2 . Although the active sites of TiO 2 /CeO 2 catalyst were also covered by sulfate, the NH 3 -SCR reaction could still be carried out by synergistic action between surface sulfate and CeO 2 . Figure 11 showed the sulfur resistance mechanism of Ti/Ce catalyst.
Peng et al. [40] used impregnation to synthesize the SiO 2 -CeO 2 -WO 3 /TiO 2 catalyst. It was reported that when Ti/Si = 3, CeO 2 -WO 3 /TiO 2 -SiO 2 catalyst could achieve more than 80% catalytic efficiency at 250 • C-500 • C and maintain good sulfur and water resistance. SiO 2 doping significantly improved the specific surface area and showed more Brønsted acid sites, which would play a key role in excellent SCR performance of catalyst at low temperatures. Shu et al. [41] improved the Ce-Fe/TiO 2 catalyst by adding Al 2 O 3 and honeycomb wire into the support, and synthesized the Ce-Fe/WMH catalyst by immersion method. It was reported that in the presence of SO 2 , the sulfation of Ce was prior to Ce-Fe/WMH in SCR reaction. At the same time, more strong acid sites provided by surface hydroxyls adsorbed more NH 3 in the form of NH 4 + instead of SO 2 . Excellent SO 2 durability of Ce-Fe/WMH depended on these two key factors. Figure 12 was the reaction mechanism of Ce-Fe/WMH catalyst.

Effects of Al 2 O 3 Support
The high-thermal-stabilized Al 2 O 3 support exists plenty of hydroxyl groups and acidic sites on the surface, which matter a lot in good SO 2 tolerance. Additionally, Al 2 O 3 can promote NO oxidation, NH 3 absorption, and the rapid reaction of SCR, attracting a lot of attention and application in the field of the NH 3 -SCR [42].
Yao et al. [43] systematically explored the NH 3 -SCR performance of Mn/Al 2 O 3 , Mn/TiO 2 , Mn/CeO 2 , and Mn/SiO 2 catalysts. The active components were found to have better dispersion on the surface of Al 2 O 3 . Mn/Al 2 O 3 catalyst had more acidic sites and stronger adsorption capacity for NO x , which made its catalytic performance and sulfur tolerance superior to the other three supports in the whole temperature range. Qu et al. [44] found that the ZrO 2 -modified MnO x -CeO y /Al 2 O 3 -ZrO 2 catalyst increased the ratio of Ce 4+ /Ce 3+ and acid sites, and SCR intermediates. After introducing SO 2 , the formed bidentate sulfates increased the Lewis acid sites, further promoting the adsorption of NH 3 and improving the SO 2 tolerance of catalyst.

Effects of Activated Carbon Support
Activated carbon material is amorphous carbon obtained by processing, and includes activated carbon and activated carbon fiber. It has a larger specific surface area, prominent pore structure, strong adsorption capacity, and low price, therefore receiving plenty of attention [45]. Besides, the special structure of activated carbon material is responsible for good sulfur tolerance. Li et al. [46] prepared Ce-, Co-, Cr-, Mo-, and Ni-modified V 2 O 5 /TiO 2 -CNTs catalysts. Among all the modified catalysts, the content of chemically adsorbed oxygen could be increased with the addition of Ce and TiO 2 .
The special support structure reduced the oxidizability of catalyst and strengthened acidity, which would relatively cut off the process of SO 2 to SO 3 and contribute to NH 3 adsorption, leading to the excellent low-temperature catalytic performance and sulfur tolerance of V 2 O 5 /TiO 2 carbon nanotube catalyst.

Effects of Zeolite Support
Zeolite is generally composed of crystalline porous aluminosilicates, which refer to microporous materials with skeletal structures. It has attracted much attention due to its high surface area, strong thermal stability, and nontoxicity. However, it is easy to be poisoned in the poor reaction environment and lead to the deactivation of catalyst [47,48], while some kinds of zeolite show good SO 2 tolerance. Liu et al. [49] found that compared with Mn-Ce and Cu-SSZ-13 catalyst, the Mn-Ce/Cu-SSZ-13 catalyst could maintain 90% NO x conversion at 300 • C after introducing SO 2 , which resulted from the synergistic effect between the Cu-SSZ-13, MnO x , and CeO 2 species. Figure 13 shows the typical SO 2 -tolerant catalysts with different supports at low temperatures for SCR reaction.

Composite Oxide Catalysts
In recent years, extensive work has been done in the area of composite oxide catalyst. Compared with the supported catalysts, these catalysts all use metal oxides and have no clear support or active components. Many achievements have been made in the research of the catalytic performance and sulfur tolerance of catalyst.
Fan et al. [50] modified MnO 2 with a proper amount of Gd. It was reported that adding Gd could have a positive impact on the surface area of catalyst, acid sites, and chemisorption oxygen, which inhibited the sulfation of active substance. The catalyst would reach 100% conversion efficiency within 120 • C-330 • C and had excellent sulfur resistance. Li et al. [51] doped Mn 2 O 3 on the surface of Fe 2 O 3 nanocrystals and prepared MnFeO x catalysts. The experiment indicated that the MnFeO x catalyst reached 98% catalytic efficiency at 200 • C. The doping of Mn 2 O 3 improved the content of active components and the adsorption of NH 3 , which made MnFeO x catalyst have a good resistance to sulfur and water. Furthermore, they also studied the mechanism of the catalyst reaction ( Figure 14). Sun et al. [52] prepared FeCeO x catalyst by impregnation method and found the FeCeO x catalyst can reach more than 94% catalytic efficiency at 200 • C-300 • C when Fe:Ce = 1:6. The catalytic efficiency was only slightly decreased when adding 100 ppm SO 2 . The NiCeLaO x catalyst studied by Zhang et al. [53], the Cu modified CeCuTiO x catalyst studied by Yang et al. [54], the Co-modified and Ni-modified CoMnCeO x , and NiMnCeO x catalysts studied by Gao et al. [55] all exhibited good SCR catalytic efficiency and sulfur resistance at low temperatures.

Effects of Preparation Methods
So far, the impregnation method, precipitation method, citric acid method, ion exchange method, and sol-gel method have been the main preparation methods, which have been used for a long time. Besides, many scholars have done a lot of research on how the preparation methods affect the activity and sulfur tolerance of catalyst. Preparation methods of catalyst mainly have an influence on the physical aspect, including the surface area, active components, supports, and surface oxygen vacancy [56]. However, due to the diversity of catalysts, the influence of preparation methods on low-temperature catalytic performance and sulfur tolerance of catalyst will have a difference.
Jiang et al. [57] synthesized Mn/TiO 2 catalyst by different methods. It was reported that Mn/TiO 2 catalyst synthesized by sol-gel method exhibited the highest content of Mn, the largest specific surface area, and the lowest crystallinity; its catalytic performance and sulfur resistance were more outstanding than the other two methods (Figure 15). After preparing the CeO 2 /TiO 2 catalyst by different methods, Gao et al. [58] explored its catalytic efficiency for SCR reaction and obtained similar conclusions with Jiang.
Liu et al. [59] prepared CeWTiO x catalyst by coprecipitation method and the Ce-W/TiO 2 catalyst by the water-phase method. Compared with the CeWTiO x catalyst, the Ce-W/TiO 2 catalyst prepared could reach 90% conversion efficiency at 205 • C-515 • C and could keep a higher degree of catalytic efficiency after the introduction of SO 2 . Vuong et al. [60] found that compared with the V/CeO 2 catalyst synthesized by citric acid method, the catalyst synthesized by precipitation method showed the characteristics of larger specific surface area, increased oxygen vacancy, and higher Ce 3+ concentration, leading to a greater catalytic performance and sulfur resistance under low temperatures.

Roles of Acidification
Among all the measures to enhance the sulfur resistance of the catalyst, some researchers have tried to form sulfate on the surface of catalysts by acidification or modification with sulfate, promoting the acidity and catalytic efficiency of catalyst [61,62]. Qiu  of NO x was improved obviously after sulfation, whose catalytic efficiency could reach more than 90% at 200 • C-480 • C when doping 2.5% sulfate ion. The increase of acid sites was also the key to promote the SO 2 resistance of catalyst. Du et al. [65] found that Ce-TiO x catalyst modified with ferric sulfate and copper sulfate had the highest catalytic activity and sulfur resistance than V-W-Ti catalyst at 150 • C-350 • C. From these studies, the major factors for the improvement of low-temperature catalytic efficiency and SO 2 resistance of the acidification catalyst can be summarized as follows: (1) It can react with active components and promote the conversion between active component ions, increasing the reaction rate. (2) The improvement of catalytic performance and sulfur tolerance of sulfated catalyst is mainly affected by the increase of acid sites. (3) It reacts with other substances of catalyst to produce sulfate, making sulfate ions occupy the adsorption position of SO 2 and indirectly reducing the reduction of SCR intermediate.

Effects of Preparation and Reaction Conditions
In addition to the above factors, the preparation conditions of the catalyst and the reaction conditions for NH 3 -SCR have a bearing on the catalytic performance and sulfur tolerance of catalyst.
Meng et al. [66] synthesized a series of SmMnO x catalysts under different calcination temperatures. The data demonstrated that the low-temperature catalytic activity of SmMnO x catalyst improved obviously from 350 • C to 450 • C, while the specific surface area decreased above 550 • C. Besides, the reduction of Mn 4+ species and adsorbed oxygen resulted in a decrease of NO oxidation rate and intermediate content, which further brought down the low-temperature catalytic performance and SO 2 tolerance of catalyst. Liu et al. [67] used coprecipitation to prepare Ce 3 W 2 SbO x catalyst. The results evidenced that Ce 3 W 2 SbO x catalyst showed great catalytic performance, sulfur, and water resistance when the temperature was low. At the same time, the higher the space velocity was, the lower the catalytic activity was, while the space velocity failed to greatly affect the catalytic efficiency of the catalyst at high temperatures. Qi et al. [68] explored the catalytic performance and sulfur tolerance of the Mn-Ce/TiO 2 catalysts which were calcined under N 2 , O 2 , and air atmosphere. The research data indicated that the NO x conversion efficiencies of the catalysts were 94%, 75.6%, and 85.6%, respectively, when the catalysts were calcined under N 2 , O 2 , and air atmosphere. It was reported that the catalyst calcined under the N 2 atmosphere could reduce the oxidation of MnO x and the formation of crystals, and increase dispersion of active components and acidic sites, having a positive impact on catalytic performance and sulfur tolerance of Mn-Ce/TiO 2 catalyst.

Conclusions and Perspectives
Facing progressively strict legislation and policies to control NO x emission, the research and design of low-temperature catalysts for NH 3 -SCR have received a great deal of attention. Although the poisoning mechanism of catalyst at low temperatures has been studied thoroughly, how to maintain the high catalytic efficiency of catalyst at low temperatures and promote the SO 2 -resistance-poisoning ability of catalyst to achieve practical application is still an urgent problem.
In the presence of O 2 , SO 3 is easily formed on the catalyst due to the oxidation reaction of SO 2 , and further combined with NH 3 to produce NH 4 HSO 4 and/or (NH 4 ) 2 SO 4 , which can deposit on the surface of catalyst and inhibit the reaction gas to be adsorbed on the catalyst to participate in the SCR reaction. Besides, sulfate of active components can be formed and cause irreversible deactivation of catalyst. Therefore, it is effective to adopt several measurements to improve the SO 2 tolerance. Firstly, it is to reduce the adsorption of SO 2 on the catalyst. The highly acidic catalysts are effective to prevent the SO 2 adsorbing. Secondly, preventing the oxidation of SO 2 to SO 3 plays a significant role in high SO 2 tolerance by reducing the redox ability of catalyst, which can cut off the oxidation of SO 2 to some extent. Furthermore, the synergistic effect between catalyst components can also improve the sulfur resistance of catalyst, such as the construction of sacrificial sites, which is responsible for the reduction of active components sulfation. Along with these existing excellent sulfur resistant catalysts, it is expected that future studies will focus on optimizing the supports and preparation methods and concentrating on the application of new structures and technology, which are effective strategies to improve the low-temperature SO 2 tolerance of SCR catalysts.