The Latest Research Progress of NH₃-SCR in the SO₂ Resistance of the Catalyst in Low Temperatures for Selective Catalytic Reduction of NO_x

Abstract

The selective catalytic reduction (SCR) has been widely used in industrial denitrification owing to its high denitrification efficiency, low operating costs, and simple operating procedures. However, coal containing a large amount of sulfur will produce SO₂ during combustion, which makes the catalyst easy to be deactivated, thus limiting the application of this technology. This review summarizes the latest NH₃-SCR reaction mechanisms and the deactivation mechanism of catalyst in SO₂-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 SO₂ resistance of the catalyst at low temperatures.

1. 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 N₂O, NO, NO₂, etc.—among which NO and NO₂ 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/Nm³ [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 NH₃ as a reducing agent in the presence of O₂ to produce pollution-free N₂ and H₂O, whose core is the catalyst. The main reaction equations are (1)–(3) [5].

$${4 \mathrm{NO} + 4 \mathrm{NH}}_{3 }{+ \mathrm{O}}_{2 }\to { 4 \mathrm{N}}_{2 }{+ 6 \mathrm{H}}_2\mathrm{O}$$

(1)

$$2{ \mathrm{NO}}_2{ + 4 \mathrm{NH}}_3{ + \mathrm{O}}_{2 }\to { 3 \mathrm{N}}_2{ + 6 \mathrm{H}}_2\mathrm{O}$$

(2)

$${\mathrm{NO} + \mathrm{NO}}_2{ + 2 \mathrm{NH}}_3 \to { 2 \mathrm{N}}_2{ + 3 \mathrm{H}}_2\mathrm{O}.$$

(3)

V₂O₅/TiO₂ 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 N₂ at high temperatures and a narrow reaction temperature window 300 °C–400 °C. SO₂ can be effortlessly converted to SO₃ 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 V₂O₅/TiO₂ catalyst—adding WO₃ and MoO₂ 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 SO₂ 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.

2. SO₂ Poisoning Mechanism of Low-Temperature Catalyst

At present, some power plants adopt wet desulfurization to remove SO₂ with lower flue gas temperature, failing to meet the reaction requirements of V₂O₅/TiO₂ catalyst. Therefore, there are many drawbacks such as low denitrification efficiency and catalyst waste. After desulfurization, tiny amounts of SO₂ 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 MnSO₄ was formed by the reaction between the active components Mn and SO₂ under the condition with O₂, 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 (NH₄)₂SO₄ and NH₄HSO₄ formed by the reaction between SO₂ and NH₃ would block the active sites and reduce the specific surface area. In addition, the intermediate substances such as N₂O₄ and nitrosyl (NO^–) would be produced by the adsorbed NO species and further participate in the reaction. However, SO₂ could compete for adsorption sites with NO_x on the catalyst surface sharply and had the momentous inhibition influence on the formation of N₂O₄ 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/TiO₂ catalyst and explored its SCR performance systematically. NO complexes, monodentate nitrate, and bidentate nitrate would be found after NO and O₂ reacted for 30 min without SO₂, but these substances were replaced by sulfate when SO₂ was introduced, which also indicated that SO₂ could compete for adsorption sites with NO on the catalyst surface, leading to the disappearance of intermediates and reducing reaction efficiency. A similar SO₂ poisoning mechanism was also obtained in the study of Xu et al. [11] on Ce/TiO₂ catalyst and Zhang et al. [12] on Cu-SAPO-34 catalyst.

From the above studies, the SO₂ 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 SO₂ and NH₃ in the presence of O₂ 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 NH₃-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 O₂, SO₂ 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) SO₂ 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. Figure 1, Figure 2 and Figure 3 show the mechanism of catalyst sulfur poisoning.

3. Research Progress of SO₂ 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 SO₂ resistance of catalyst.

3.1. Effects of Active Components

The active component, which is composed of one or more substances, is the main unit of catalyst and affects the NH₃-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.

3.1.1. Ce-Modified Catalysts

Cerium has an excellent ability to store and release O₂. The electron transfer between Ce⁴⁺ and Ce³⁺ is favorable for catalyst to form reactive oxygen species, promoting the conversion of NO into NO₂. Besides, the catalyst modified with Ce can increase the content of acidic sites and reduce the oxidizability of catalyst in some extent [14,15]. SO₂ would preferentially react with CeO₂ on the surface of catalyst to form Ce₂(SO₄)₃ in the presence of O₂ and H₂O, which can reduce the reaction between NH₃ and SO₂ to produce ammonium sulfate and ammonium bisulfate, and inhibit the catalyst deactivation [16].

Wei et al. [17] studied the Mn/TiO₂ 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, SO₂ 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/TiO₂ 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 CeO₂-modified Mn–Ce/TiO₂ catalyst. It was found that SO₂ would react with O₂ to produce SO₃, which reacted with CeO₂ preferentially and reduced the sulfation of active component MnO_x. It meant that CeO₂ can act as a catalyst SO₂ collector, which can limit the sulfation of the active component (Figure 4).

The V₂O₅–WO₃/CeO₂–TiO₂ 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–V₂O₅/TiO₂ catalyst with 10% CeO₂ 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-V₂O₅/TiO₂ catalyst than those on CeO₂/TiO₂ catalyst. Adding CeO₂ not only enhanced the acidity of the catalyst but also reduced SO₂ adsorption due to the rejection to SO₂, further slowing down the effect of SO₂ poisoning. The research of France et al. [15] indicated that compared with the original catalyst, the catalyst modified by the CeO₂ could increase the amount of the chemical adsorption of oxygen on the surface, which could increase the rate of NO oxidation to NO₂ and facilitate the rapid response of NH₃-SCR. In the presence of SO₂, the catalytic efficiency of catalyst decreased slightly, while the catalyst could nearly return to the initial catalytic state after SO₂ was stopped (Figure 5). Based on the V₂O₅/TiO₂ catalyst, Ma et al. [21] added WO₃ and CeO₂ to it and considered the SO₂ poisoning of the catalysts at low temperatures. CeO₂ doping improved the redox ability while inhibiting the formation and deposition of (NH₄)₂SO₄ on the catalyst surface. Figure 6 showed the low-temperature deactivation mechanism of the catalyst under the condition with SO₂.

3.1.2. 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-Al₂O₃ 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 NH₃ and NH₄⁺ species can react with NO to generate N₂ and H₂O finally. Among them, the NH₄⁺ species is formed by NH₃ 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/TiO₂ catalyst, Shen et al. [25] synthesized Fe–Mn–Ce/TiO₂ catalyst modified with Fe. It was found that when Fe/Ti = 0.1, Fe–Mn–Ce/TiO₂ catalyst could reach 96.8% NO_x conversion efficiency at 180 °C as well as wonderful sulfur resistance over the Mn–Ce/TiO₂ catalyst on account of the increase of acid sites. Wu et al. [26] studied the V₂O₅/TiO₂ catalyst doped with Fe and found that the Fe–V₂O₅/TiO₂ 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, V₂O₅, and TiO₂ 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.

3.1.3. Cu-Modified Catalysts

Cu has many advantages such as good thermal conductivity, corrosion resistance, strong chemical stability, and low toxicity. In the field of NH₃-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 V₂O₅/TiO₂ catalyst to prepare the modified catalysts by impregnation method. Among all the modified catalysts, the Cu–V₂O₅/TiO₂ catalyst exactly exhibited the best catalytic performance and can reach 90% conversion efficiency of NO_x at 225 °C–375 °C. After introducing SO₂ and H₂O, 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 Cu²⁺/Cu⁺ and Fe³⁺/Fe²⁺ was also the main reason for the improvement of activity and sulfur resistance. Li et al. [30] doped light CuO into TiO₂/CeO₂ catalyst and found the catalytic performance of the TiO₂–CuO/TiO₂ catalyst was increased obviously. When Cu/Ce = 0.005, TiO₂–CuO/TiO₂ catalyst exhibited the most excellent catalytic performance and sulfur tolerance, as shown in Figure 8. The addition of CuO produced more adsorbed oxygen and Ce³⁺ species, which enhanced the reduction performance and surface acidity of TiO₂–CuO/TiO₂ catalyst.

3.1.4. W-Modified Catalysts

W has been widely used in conventional commercial catalysts. The addition of WO₃ can widen the reaction temperature window of catalyst, increase the content of active substances and acid sites, and effectively promote the SCR activity and SO₂ tolerance of catalyst at low temperatures [31].

Chen et al. successfully used impregnation method to synthesize W-modified CeO₂/TiO₂ catalyst. The experimental results showed that when Ce and W were impregnated together and W content is 6%, CeO₂–WO₃/TiO₂ 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 Ce³⁺ and was propitious to the conversion of NO to NO₂, 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–TiO₂ catalyst by coprecipitation method. The experiment showed that Ce_0.2W_0.2TiO_x catalyst could achieve over 90% catalytic efficiency at 275 °C–450 °C and had excellent sulfur and water resistance. Besides, Ce_0.2W_0.2TiO_x catalyst could still maintain considerable catalytic activity at high space velocity. The addition of W in Ce_0.2W_0.2TiO_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/TiO₂ catalyst, Jiang et al. [34] used sol-gel method to prepare CeO₂–MoO₃–WO₃/TiO₂ catalyst by adding WO₃ and MoO₃. 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.

3.1.5. Catalysts Modified with Other Metal Elements

Xu et al. [35] explored the NH₃-SCR performance of V₂O₅–Sb₂O₃/TiO₂ 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 SO₂ 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/TiO₂ 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 SO₂ and promoted the NH₃ adsorbed on the catalyst surface. In addition, the surfactant dispersion and the ratio of V⁴⁺/V⁵⁺ were also improved. Figure 9 was the mechanism of Cr doping improving sulfur resistance of Cr–V/TiO₂ catalyst.

Tian et al. [37] studied the Ca-doped Mn/TiO₂ catalyst. It was found that after SO₂ was introduced into the catalyst, CaSO₄ 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 SO₂-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 SO₂, these additives can preferentially react with SO₂ as the SO₂ catchers to avoid the sulfation of active substances. Besides, the stability of NH₄HSO₄ and (NH4)₂SO₄ 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.

3.2. 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 SO₂ poisoning on the catalytic activity of catalyst. As of now, there are TiO₂, Al₂O₃, 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 SO₂ resistance of catalyst.

3.2.1. Effects of TiO₂ Support

TiO₂ is one of the most common supports and has great catalytic efficiency. Compared with other supports, TiO₂ 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 TiO₂ support on active components and the interaction between them also play significant roles in SO₂ tolerance.

Zhang et al. [39] used impregnation method to synthesize the TiO₂/CeO₂ catalyst by reversing the support and active components of the most common CeO₂/TiO₂ catalyst. They found that the TiO₂/CeO₂ catalyst with changed structure had better catalytic activity and sulfur resistance due to the protection of TiO₂, which could maintain the 90% conversion efficiency of NO_x at 300 °C after introducing SO₂. Although the active sites of TiO₂/CeO₂ catalyst were also covered by sulfate, the NH₃-SCR reaction could still be carried out by synergistic action between surface sulfate and CeO₂. Figure 11 showed the sulfur resistance mechanism of Ti/Ce catalyst.

Peng et al. [40] used impregnation to synthesize the SiO₂–CeO₂–WO₃/TiO₂ catalyst. It was reported that when Ti/Si = 3, CeO₂–WO₃/TiO₂–SiO₂ catalyst could achieve more than 80% catalytic efficiency at 250 °C–500 °C and maintain good sulfur and water resistance. SiO₂ 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/TiO₂ catalyst by adding Al₂O₃ and honeycomb wire into the support, and synthesized the Ce–Fe/WMH catalyst by immersion method. It was reported that in the presence of SO₂, 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 NH₃ in the form of NH₄⁺ instead of SO₂. Excellent SO₂ durability of Ce–Fe/WMH depended on these two key factors. Figure 12 was the reaction mechanism of Ce–Fe/WMH catalyst.

3.2.2. Effects of Al₂O₃ Support

The high-thermal-stabilized Al₂O₃ support exists plenty of hydroxyl groups and acidic sites on the surface, which matter a lot in good SO₂ tolerance. Additionally, Al₂O₃ can promote NO oxidation, NH₃ absorption, and the rapid reaction of SCR, attracting a lot of attention and application in the field of the NH₃-SCR [42].

Yao et al. [43] systematically explored the NH₃-SCR performance of Mn/Al₂O₃, Mn/TiO₂, Mn/CeO₂, and Mn/SiO₂ catalysts. The active components were found to have better dispersion on the surface of Al₂O₃. Mn/Al₂O₃ 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 ZrO₂-modified MnO_x–CeO_y/Al₂O₃–ZrO₂ catalyst increased the ratio of Ce⁴⁺/Ce³⁺ and acid sites, and SCR intermediates. After introducing SO₂, the formed bidentate sulfates increased the Lewis acid sites, further promoting the adsorption of NH₃ and improving the SO₂ tolerance of catalyst.

3.2.3. 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 V₂O₅/TiO₂–CNTs catalysts. Among all the modified catalysts, the content of chemically adsorbed oxygen could be increased with the addition of Ce and TiO₂. The special support structure reduced the oxidizability of catalyst and strengthened acidity, which would relatively cut off the process of SO₂ to SO₃ and contribute to NH₃ adsorption, leading to the excellent low-temperature catalytic performance and sulfur tolerance of V₂O₅/TiO₂ carbon nanotube catalyst.

3.2.4. 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 SO₂ 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 SO₂, which resulted from the synergistic effect between the Cu-SSZ-13, MnO_x, and CeO₂ species. Figure 13 shows the typical SO₂-tolerant catalysts with different supports at low temperatures for SCR reaction.

3.3. 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 MnO₂ 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 Mn₂O₃ on the surface of Fe₂O₃ nanocrystals and prepared MnFeO_x catalysts. The experiment indicated that the MnFeO_x catalyst reached 98% catalytic efficiency at 200 °C. The doping of Mn₂O₃ improved the content of active components and the adsorption of NH₃, 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 SO₂. 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.

3.4. Other Strategies to Improve the SO₂ Resistance

3.4.1. 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/TiO₂ catalyst by different methods. It was reported that Mn/TiO₂ 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 CeO₂/TiO₂ 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/TiO₂ catalyst by the water-phase method. Compared with the CeWTiO_x catalyst, the Ce–W/TiO₂ 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 SO₂. Vuong et al. [60] found that compared with the V/CeO₂ 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 Ce³⁺ concentration, leading to a greater catalytic performance and sulfur resistance under low temperatures.

3.4.2. 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 et al. [63] explored the catalytic performance of the SO₄^2–-modified Mn–Co–Ce/TiO₂–SiO₂ catalyst. The results illustrated that the catalyst treated with 0.1 mol/l of sulfuric acid exhibited the 99.5% conversion efficiency of NO_x at 250 °C when introducing 50 of ppm SO₂. The catalytic efficiency failed to decrease in a certain temperature range. Zhang et al. [64] used ammonium sulfate as the source of sulfate ion to prepare the SO₄^2–/CeO₂ catalyst. It was reported that the conversion efficiency 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 SO₂ 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 SO₂ 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 SO₂ and indirectly reducing the reduction of SCR intermediate.

3.4.3. Effects of Preparation and Reaction Conditions

In addition to the above factors, the preparation conditions of the catalyst and the reaction conditions for NH₃-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 Mn⁴⁺ 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 SO₂ tolerance of catalyst. Liu et al. [67] used coprecipitation to prepare Ce₃W₂SbO_x catalyst. The results evidenced that Ce₃W₂SbO_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/TiO₂ catalysts which were calcined under N₂, O₂, 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 N₂, O₂, and air atmosphere. It was reported that the catalyst calcined under the N₂ 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/TiO₂ catalyst.

4. Conclusions and Perspectives

Facing progressively strict legislation and policies to control NO_x emission, the research and design of low-temperature catalysts for NH₃-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 SO₂-resistance-poisoning ability of catalyst to achieve practical application is still an urgent problem.

In the presence of O₂, SO₃ is easily formed on the catalyst due to the oxidation reaction of SO₂, and further combined with NH₃ to produce NH₄HSO₄ and/or (NH₄)₂SO₄, 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 SO₂ tolerance. Firstly, it is to reduce the adsorption of SO₂ on the catalyst. The highly acidic catalysts are effective to prevent the SO₂ adsorbing. Secondly, preventing the oxidation of SO₂ to SO₃ plays a significant role in high SO₂ tolerance by reducing the redox ability of catalyst, which can cut off the oxidation of SO₂ 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 SO₂ tolerance of SCR catalysts.