Research Status and Prospect on Vanadium-Based Catalysts for NH3-SCR Denitration

Selective catalytic reduction of NOx with NH3 is one of the most widely used technologies in denitration. Vanadium-based catalysts have been extensively studied for the deNOx process. V2O5/WO3(MoO3)TiO2 as a commercial catalyst has excellent catalytic activity in the medium temperature range. However, it has usually faced several problems in practical industrial applications, including narrow windows of operation temperatures, and the deactivation of catalysts. The modification of vanadium-based catalysts will be the focus in future research. In this paper, the chemical composition of vanadium-based catalysts, catalytic mechanism, the broadening of the temperature range, and the improvement of erosion resistance are reviewed. Furthermore, the effects of four major systems of copper, iron, cerium and manganese on the modification of vanadium-based catalysts are introduced and analyzed. It is worth noting that the addition of modified elements as promoters has greatly improved the catalytic performance. They can enhance the surface acidity, which leads to the increasing adsorption capacity of NH3. Surface defects and oxygen vacancies have also been increased, resulting in more active sites. Finally, the future development of vanadium-based catalysts for denitration is prospected. It is indicated that the main purpose for the research of vanadium-based modification will help to obtain safe, environmentally friendly, efficient, and economical catalysts.


Introduction
Nitrogen oxides (NO x ) are one of major air pollutants that could cause great harm to the ecological environment and human health. How to remove NO x effectively is a significant issue of environmental protection. Currently, there are three kinds of reductant for the selective catalytic reduction of NO x : NH 3 , Urea and HC. NH 3 -SCR (selective catalytic reduction with ammonia) is the most mature industrial technology at present, and its main reaction process [1][2][3] is: The technology has been used in hundreds of large power plants in the United States, Germany, and Japan. Environmental conditions have a great impact on this technology. For example, it is important to control oxygen content of the exhaust appropriately for NH 3 -SCR. In the absence of O 2 , the reducing agent cannot reduce NO. When the oxygen content of the exhaust is below 5%, the catalytic reduction reaction of NH 3 can be carried out to convert NO x into N 2 and H 2 O. When the oxygen concentration is too high, the activity of the catalyst will be reduced, and NH 3 will be oxidized to NO 2 , resulting in new pollution.
The catalyst is the key to determining the reactivity of NH 3 -SCR under the proper external conditions. In the 1960s, a study on exhaust gas-purifying catalysts was started. A three-way catalyst (TWC) consisting of precious metals was researched in the initial stage. Although the catalytic effect is ideal, considering the cost and the source of precious metals, the direction has shifted to transition metal oxide catalysts. Vanadium-based catalysts are used due to the high activity of SCR reactions. With the continuous improvement of environmental protection and energy awareness and the increasing demand for low NO x emissions, the traditional vanadium-based catalysts are no longer satisfactory. The performance improvement of vanadium-based catalysts has become the focus of research.
In this paper, the characteristics and performance improvement of vanadium-based catalysts is the main line. The research progress of vanadium-based catalysts at home and abroad is reviewed. The advantages and disadvantages, mechanism, and improvement methods are summarized. The development of vanadium-based catalysts for denitrification is prospected.

Composition of Vanadium Based Catalyst
The catalyst is obtained in such a way that the active component and promoter are loaded on catalyst support. The catalyst support is believed to be important for SCR reaction because of its high surface areas, good thermal stability, and high active substance dispersion on surfaces [4]. In general, TiO 2 -anatase is widely used as a support for vanadium-based catalyst because of its great SO 2 resistance and excellent dispersion of V 2 O 5 . A small amount of sulfate species loaded on the support enhances the surface acidity of the V 2 O 5 /TiO 2 catalyst, increases the amount of adsorbed NH 3 , and improves the SCR activity [5,6]. However, pure TiO 2 is poorly active during the SCR reaction due to its weak acidity and poor redox ability. Additionally, TiO 2 will transform from anatase to rutile at high temperatures, which will affect the stability of the support. Therefore, some modifications have been investigated to overcome these defects. Adrian, M. et al. [7] chose TiO 2 -WO 3 -SiO 2 as a catalyst support for optimization, using rare metal to inhibit the phase transition of titanium dioxide, to avoid high temperature sintering and surface area loss, and improve the structural strength of the support. At the same time, SiO 2 is beneficial to SO 2 resistance. Cheng, J. et al. [8] synthesized TiO 2 -PILC as a catalyst support for the SCR of NO x with NH 3 due to pillared interlayered clays' large specific surface areas, acidity, and high thermal stability. Other catalyst support is also helpful for the performance of the catalyst, such as TiO 2 -pal [4], and activated carbon (AC) [9].
The active component is the dominant factor affecting vanadium-based catalyst performance, and it can be divided into single vanadium oxide VO x [10,11] and multi-metal oxide compounds such as VO x -WO 3 [12], VO x -CeO x [8]. V 2 O 5 catalyst is a kind of structure-sensitive catalyst, whose activity is affected by the structure and coverage of V 2 O 5 , the crystal form of catalyst support, and the modification element. Increasing the amount of V 2 O 5 added can help to improve the catalytic activity. However, V 2 O 5 will promote the transformation of anatase to rutile, which is not conducive to the thermal stability of the catalyst, resulting in the shortened life of the catalyst and the decrease of selectivity [13]. In view of this, the content of vanadium in the commercial catalyst VO x -WO x /TiO 2 is generally limited to less than 2.6%. The coverage rate is also an important parameter, which can be adjusted by the V loading or the calcination temperature [14]. The coverage rate for ideal V 2 O 5 -WO 3 /TiO 2 catalysts is 25-50%. Below 25% indicates that the vanadium content is too low or the surface area is too high and the catalyst activity is low. With coverage of more than 50%, the stability of the catalyst and the conversion of NO x at high temperatures cannot be guaranteed [7,14]. The vanadium coverage of conventional catalysts is below the dispersion limit that monolayer 6-7 atom/nm 2 to avoid the crystalline V 2 O 5 [15]. It is generally believed that in the dry anatase surface at low loading V 2 O 5 , the dehydrated surface vanadium generally tend to possess isolated four-fold coordination. The isolated VO 4 units consist of one terminal V=O bond and three bridging V-O-Ti bonds, as shown in Figure 1a. As the coverage increases, the number of V-O-V bridges per V increases. In practical application, the vanadium species mainly exists in the form of polyvanadate, as shown in Figure 1b. This species has higher SCR reactivity than the monomer (Ti-O) 3 V=O [3,15,16]. At high loading V 2 O 5 , crystalline vanadia is generated on sulfated TiO 2 (Figure 1c). In view of the volatilization of vanadium at high temperatures, the use of metal vanadate (MeVO 4 ) in the active phase can increase the melting point of the catalyst [17,18]. Moreover, Zhou, X. et al. [19] synthesize the V@Mn-Fe/ATP catalyst with a similar core-shell structure successfully. It was found that the V 2 O 5 layer as a coating might protect other active metals of the catalyst. phase can increase the melting point of the catalyst [17,18]. Moreover, Zhou, X. et al. [19] synthesize the V@Mn-Fe/ATP catalyst with a similar core-shell structure successfully. It was found that the V2O5 layer as a coating might protect other active metals of the catalyst.

NH3-SCR Reaction Mechanism of Vanadium-Based Catalyst
Zhu, M. et al. [2] used elemental tagging to study the SCR reaction pathway of vanadium-based catalysis and found that the SCR reaction occurs at the surface V 5+ O4 site. This is shown in Figure 2, which exhibits a schematic diagram of the standard SCR reaction and indicates that SCR reaction is divided into three steps. First, NH3 adsorption is observed on the surface of the acid center. Next, the adsorbed ammonia reacts with NO to form an intermedium. The intermedium can decompose to N2 and H2O and reduce surface V 5+ O4 sites. Finally, the reduced V 4+ O3 is reoxidized by O2. Through the kinetic study, the rate-determining step of the SCR reaction is that the V 5+ on the surface is reduced to V 4+ by NO and NH3, particularly the breaking of N-H bonds during the process of formation or decomposition of the intermedium. The vanadium-based catalyst V2O5-WO3/TiO2 has been commercialized and applied in some thermal power plants. In V2O5-WO3/TiO2 catalysts, the O=VO3 site is involved in the redox cycle, which is the most important for the SCR reaction. The O=WO4 site enhances the surface acidity of the catalyst and promotes the adsorption of NH3 [12]. Tungsten oxide as a promoter has two primary roles in catalysis: it restrains the generation of bulk polymeric V aggregates and retains Brønsted acid sites at high temperatures [21].

NH 3 -SCR Reaction Mechanism of Vanadium-Based Catalyst
Zhu, M. et al. [2] used elemental tagging to study the SCR reaction pathway of vanadium-based catalysis and found that the SCR reaction occurs at the surface V 5+ O 4 site. This is shown in Figure 2, which exhibits a schematic diagram of the standard SCR reaction and indicates that SCR reaction is divided into three steps. First, NH 3 adsorption is observed on the surface of the acid center. Next, the adsorbed ammonia reacts with NO to form an intermedium. The intermedium can decompose to N 2 and H 2 O and reduce surface V 5+ O 4 sites. Finally, the reduced V 4+ O 3 is reoxidized by O 2 . Through the kinetic study, the rate-determining step of the SCR reaction is that the V 5+ on the surface is reduced to V 4+ by NO and NH 3 , particularly the breaking of N-H bonds during the process of formation or decomposition of the intermedium. The vanadium-based catalyst V 2 O 5 -WO 3 /TiO 2 has been commercialized and applied in some thermal power plants. In V 2 O 5 -WO 3 /TiO 2 catalysts, the O=VO 3 site is involved in the redox cycle, which is the most important for the SCR reaction. The O=WO 4 site enhances the surface acidity of the catalyst and promotes the adsorption of NH 3 [12]. Tungsten oxide as a promoter has two primary roles in catalysis: it restrains the generation of bulk polymeric V aggregates and retains Brønsted acid sites at high temperatures [21]. phase can increase the melting point of the catalyst [17,18]. Moreover, Zhou, X. et al. [19] synthesize the V@Mn-Fe/ATP catalyst with a similar core-shell structure successfully. It was found that the V2O5 layer as a coating might protect other active metals of the catalyst.

NH3-SCR Reaction Mechanism of Vanadium-Based Catalyst
Zhu, M. et al. [2] used elemental tagging to study the SCR reaction pathway of vanadium-based catalysis and found that the SCR reaction occurs at the surface V 5+ O4 site. This is shown in Figure 2, which exhibits a schematic diagram of the standard SCR reaction and indicates that SCR reaction is divided into three steps. First, NH3 adsorption is observed on the surface of the acid center. Next, the adsorbed ammonia reacts with NO to form an intermedium. The intermedium can decompose to N2 and H2O and reduce surface V 5+ O4 sites. Finally, the reduced V 4+ O3 is reoxidized by O2. Through the kinetic study, the rate-determining step of the SCR reaction is that the V 5+ on the surface is reduced to V 4+ by NO and NH3, particularly the breaking of N-H bonds during the process of formation or decomposition of the intermedium. The vanadium-based catalyst V2O5-WO3/TiO2 has been commercialized and applied in some thermal power plants. In V2O5-WO3/TiO2 catalysts, the O=VO3 site is involved in the redox cycle, which is the most important for the SCR reaction. The O=WO4 site enhances the surface acidity of the catalyst and promotes the adsorption of NH3 [12]. Tungsten oxide as a promoter has two primary roles in catalysis: it restrains the generation of bulk polymeric V aggregates and retains Brønsted acid sites at high temperatures [21].  Although vanadium-based catalysts have been used in industrial applications, the reaction mechanism for vanadium-based catalysts remains controversial. It is mainly divided into two theories: the Langmuri-Hinshelwood theory (L-H theory) and the Eley-Rideal theory (E-R theory). It is widely believed that the vanadium-based catalyst selectively reduces NO x via the E-R mechanism at medium-high temperature. A schematic diagram of the NH 3 -SCR reaction mechanism is shown in Figure 3. Although ammonia is adsorbed on both Lewis acid sites and Brønsted acid sites, NH 3 is more prone to be adsorbed at the Brønsted acid sites than at the Lewis acid sites. NH 3 forms NH 4+ on the Brønsted acid site (V-OH) and reacts with the V=O group to generate intermediate products, and then decomposes into N 2 and H 2 O. However, NH 3 adsorption on Lewis acid sites will produce N 2 O [3]. Although vanadium-based catalysts have been used in industrial applications, the reaction mechanism for vanadium-based catalysts remains controversial. It is mainly divided into two theories: the Langmuri-Hinshelwood theory (L-H theory) and the Eley-Rideal theory (E-R theory). It is widely believed that the vanadium-based catalyst selectively reduces NOx via the E-R mechanism at medium-high temperature. A schematic diagram of the NH3-SCR reaction mechanism is shown in Figure 3. Although ammonia is adsorbed on both Lewis acid sites and Brønsted acid sites, NH3 is more prone to be adsorbed at the Brønsted acid sites than at the Lewis acid sites. NH3 forms NH 4+ on the Brønsted acid site (V-OH) and reacts with the V=O group to generate intermediate products, and then decomposes into N2 and H2O. However, NH3 adsorption on Lewis acid sites will produce N2O [3]. Aside from the Langmuri-Hinshelwood model and the Eley-Rideal model, a surface Mar-van Krevelen reaction mechanism has been proposed for the SCR reaction. According to the Mar-Van Krevelen mechanism, SCR reaction divides into three steps: adsorption, migration, and reoxidation. There are two different types of centers that prevail on the SCR catalysts: S1 sites, related to vanadium and its redox cycle, and S2 sites, used to store ammonia and nitrates [23]. However, there is controversy around the step of vanadium reoxidation. According to the sites of detached gaseous oxygen, two different possibilities for vanadium reoxidation steps were proposed: in one case that the vacancy in the vanadia lattice is filled using molecular oxygen, and the other through the creation of a vacancy in the titania lattice [24].
In fact, the SCR reaction mechanism is closely related to the reaction temperature. With different reaction temperatures, the number and distribution of the active sites of the catalysts are different, and the types, quantities and distributions of the acids are not the same. Therefore, the reaction mechanism of vanadium-based catalysts may also change with different reaction temperatures. It is worth noting that current studies only focus on the formation of acid centers and the activity of acids in acid-base mechanism. It also has a certain significance to study the effect of alkaline position on the adsorption and the activity [3].

Performance Improvement of Vanadium Based Catalysts
Commercial vanadium-based catalysts are widely used in industry due to their high activity and excellent SO2 resistance. However, they have a narrow activity window, and weak ability for resisting VOX species to thermal sintering. The metal oxide is also over-oxidized, which not only oxidizes NH3 to N2O and NO, but also oxidizes SO2 to SO3. The generated SO3 continues to react with NH3 to generate ammonium sulfate species, thereby plugging the pores and masking the active sites, thus decreasing the activity of the SCR reaction [25]. The inevitable soot and some alkali metals in factory waste can also affect the activity of catalysts and even lead to deactivation of catalysts [4,5,26]. The evaluation of vanadium-based catalytic activity is shown in Figure 4. It can be seen that the NOx conversion rate of conventional catalysts is higher than 90% at 300-400 °C, and has little effect in the Aside from the Langmuri-Hinshelwood model and the Eley-Rideal model, a surface Mar-van Krevelen reaction mechanism has been proposed for the SCR reaction. According to the Mar-Van Krevelen mechanism, SCR reaction divides into three steps: adsorption, migration, and reoxidation. There are two different types of centers that prevail on the SCR catalysts: S1 sites, related to vanadium and its redox cycle, and S2 sites, used to store ammonia and nitrates [23]. However, there is controversy around the step of vanadium reoxidation. According to the sites of detached gaseous oxygen, two different possibilities for vanadium reoxidation steps were proposed: in one case that the vacancy in the vanadia lattice is filled using molecular oxygen, and the other through the creation of a vacancy in the titania lattice [24].
In fact, the SCR reaction mechanism is closely related to the reaction temperature. With different reaction temperatures, the number and distribution of the active sites of the catalysts are different, and the types, quantities and distributions of the acids are not the same. Therefore, the reaction mechanism of vanadium-based catalysts may also change with different reaction temperatures. It is worth noting that current studies only focus on the formation of acid centers and the activity of acids in acid-base mechanism. It also has a certain significance to study the effect of alkaline position on the adsorption and the activity [3].

Performance Improvement of Vanadium Based Catalysts
Commercial vanadium-based catalysts are widely used in industry due to their high activity and excellent SO 2 resistance. However, they have a narrow activity window, and weak ability for resisting VO X species to thermal sintering. The metal oxide is also over-oxidized, which not only oxidizes NH 3 to N 2 O and NO, but also oxidizes SO 2 to SO 3 . The generated SO 3 continues to react with NH 3 to generate ammonium sulfate species, thereby plugging the pores and masking the active sites, thus decreasing the activity of the SCR reaction [25]. The inevitable soot and some alkali metals in factory waste can also affect the activity of catalysts and even lead to deactivation of catalysts [4,5,26]. The evaluation of vanadium-based catalytic activity is shown in Figure 4. It can be seen that the NO x conversion rate of conventional catalysts is higher than 90% at 300-400 • C, and has little effect in the presence of SO 2 . However, through experiments, we found that the longer the SO 2 exists, the greater the decrease of catalyst activity as shown in Figure 4b. In this case, the performance improvement of vanadium-based catalysts has been the focus of research. In order to improve the activity of vanadium-based catalysts, the method usually has been adopted by changing the catalyst preparation, improving the performance of the support, adding modification elements and so on. presence of SO2. However, through experiments, we found that the longer the SO2 exists, the greater the decrease of catalyst activity as shown in Figure 4b. In this case, the performance improvement of vanadium-based catalysts has been the focus of research. In order to improve the activity of vanadium-based catalysts, the method usually has been adopted by changing the catalyst preparation, improving the performance of the support, adding modification elements and so on. Usually the preparation of catalysts requires drying and calcining, and other processes. Some catalysts are activated by calcination. The calcination temperature affects the crystal structure of the catalyst. The adsorption sites and the adsorption numbers of different crystal planes are different. The surface sites' variation lie with the nature of the oxide, preparative method used, conditions of activation, and impurities [27]. At the same time, both lattice defects and inhomogeneous surfaces have an effect on the catalytic properties.
The crystal form of catalyst support has a significant influence on SCR reactions, for example, TiO2-anatase has higher dispersibility to active components than TiO2-rutile and leads to greater performance. The methods of preparing catalyst supports include impregnation, coprecipitation, hydrothermal, and sol-gel. The impregnation method is the most widely used method because of its simple and easily-operated process. These methods mainly control the particle size and morphology of the support, as well as the binding strength of active components to the support, thus affecting the dispersion of active components and the activity of the catalyst. Pappas, D.K. et al. [28] have used the alkaline hydrothermal method to prepare titanium nanotubes and co-precipitation to prepare spherical nanoparticles and nanotubes for comparison. It was found that different forms of supports have certain effects on the SCR reaction. Using titanium nanotubes as support has a better effect than spherical titanium nanoparticles. This is because the multilayer structure of the titanium nanotube can provide greater surface area to facilitate the dispersion of active components, so as to provide more active sites. Vuong, T.H. et al. prepared CeO2 supports by a citrate or a precipitation method and then got loading V/CeO2 catalysts by wet impregnation. Comparing with the citrate sol-gel protocol, the mesoporous CeO2 support prepared by the precipitation method is much more active due to markedly higher surface area, higher concentration of Ce 3+ bulk species and oxygen vacancies, which are beneficial for oxygen transport through the lattice [10].
Adding a modified element to modify the catalyst is beneficial to improve the performance of vanadium-based catalysts. Different modified elements may cause different effects on the VOx active sites because of different physical and chemical properties of the surface. The presence of surface acid sites depends on both the specific oxide-supported ligand and the surface density of the metal oxide overlayer [29]. At ambient conditions, vanadium species on titania has distorted the octahedral oxygen environment with one short (V=O) and one long V-O axial bond at low loading V2O5 [30]. The theoretical value for monolayer coverage of dehydrated VOx/TiO2 catalysts is 7.9 atom/nm 2 [29]. Due to stronger interactions between vanadium and ceria, vanadium oxide species disperse on ceria up to 9 V atom/nm 2 of support determined by Raman Spectroscopy [31]. Vanadia reacts with cerium Usually the preparation of catalysts requires drying and calcining, and other processes. Some catalysts are activated by calcination. The calcination temperature affects the crystal structure of the catalyst. The adsorption sites and the adsorption numbers of different crystal planes are different. The surface sites' variation lie with the nature of the oxide, preparative method used, conditions of activation, and impurities [27]. At the same time, both lattice defects and inhomogeneous surfaces have an effect on the catalytic properties.
The crystal form of catalyst support has a significant influence on SCR reactions, for example, TiO 2 -anatase has higher dispersibility to active components than TiO 2 -rutile and leads to greater performance. The methods of preparing catalyst supports include impregnation, coprecipitation, hydrothermal, and sol-gel. The impregnation method is the most widely used method because of its simple and easily-operated process. These methods mainly control the particle size and morphology of the support, as well as the binding strength of active components to the support, thus affecting the dispersion of active components and the activity of the catalyst. Pappas, D.K. et al. [28] have used the alkaline hydrothermal method to prepare titanium nanotubes and co-precipitation to prepare spherical nanoparticles and nanotubes for comparison. It was found that different forms of supports have certain effects on the SCR reaction. Using titanium nanotubes as support has a better effect than spherical titanium nanoparticles. This is because the multilayer structure of the titanium nanotube can provide greater surface area to facilitate the dispersion of active components, so as to provide more active sites. Vuong, T.H. et al. prepared CeO 2 supports by a citrate or a precipitation method and then got loading V/CeO 2 catalysts by wet impregnation. Comparing with the citrate sol-gel protocol, the mesoporous CeO 2 support prepared by the precipitation method is much more active due to markedly higher surface area, higher concentration of Ce 3+ bulk species and oxygen vacancies, which are beneficial for oxygen transport through the lattice [10].
Adding a modified element to modify the catalyst is beneficial to improve the performance of vanadium-based catalysts. Different modified elements may cause different effects on the VO x active sites because of different physical and chemical properties of the surface. The presence of surface acid sites depends on both the specific oxide-supported ligand and the surface density of the metal oxide overlayer [29]. At ambient conditions, vanadium species on titania has distorted the octahedral oxygen environment with one short (V=O) and one long V-O axial bond at low loading V 2 O 5 [30]. The theoretical value for monolayer coverage of dehydrated VO x /TiO 2 catalysts is 7.9 atom/nm 2 [29]. Due to stronger interactions between vanadium and ceria, vanadium oxide species disperse on ceria up to 9 V atom/nm 2 of support determined by Raman Spectroscopy [31]. Vanadia reacts with cerium oxide to form CeVO 4 phase, so two types of vanadium species (VO x and CeVO 4 ) are dispersed on the surface of cerium dioxide. CeVO 4 promotes the formation of Brønsted acid sites while VO x and CeO 2 provide Lewis acid sites on the surface [32]. However, the formation of Brønsted acid sites is related to vanadium species VO x in bulk CeVO 4 after aging at 500 • C [18]. Therefore, the Lewis acid sites may be converted to the Brønsted acid sites at high temperature SCR reaction. In bulk CeVO 4 catalysts, the isolated vanadium species in the tetrahedral coordination are basically stable after the zircon-type structure CeVO 4 aging at 500 • C [18]. In the Ce-doped V 2 O 5 -WO 3 /TiO 2 catalyst, the presence of Ce 3+ creates a charge imbalance, vacancies, and unsaturated chemical bonds, thus increasing the chemisorbed oxygen on the surface. In addition, the coexistence of Ce 4+ was found to play an important role in the synergic effect on the SCR reaction [33]. At ambient conditions for fresh Fe-V/TiO 2 catalysts, the structures of the vanadium species exist in octahedral (70%) and tetrahedral (30%) coordination [34]. The acid sites on V 2 O 5 -Fe 2 O 3 /TiO 2 are mainly derived from Fe 2 O 3 -TiO 2 , so the adsorbed NH 3 prefer to be activated by Fe 3+ , and then V 5+ accelerate the regeneration of Fe 3+ [35]. In FeVO 4 /TiO 2 catalysts, the FeVO 4 loading amount should be less than 9 wt% to avoid exceeding the monolayer dispersion limit. The Fe 3+ -O-V 5+ linkages with a low coordination number of vanadium atoms are associated with structural defects on the surface. The presence of an electronic inductive effect between Fe 3+ and V 5+ was beneficial for high deNO x efficiency and N 2 selectivity [36]. At low temperatures, FeVO 4 shows low activity for SCR in FeVO 4 /TiO 2 -WO 3 -SiO 2 catalysts. However, in the decomposition of FeVO 4 , the VO x species exhibits higher SCR activity. So, the active species for SCR is not FeVO 4 but VO x species [37]. The presence of iron can be useful in altering the electron density of the VO x surface species and inducing more Lewis acid sites and Brønsted acid sites for SCR reaction [38]. Currently, in the catalytic denitrification reaction of manganese-based bimetallic oxide catalysts, Mn-OH as a Brønsted acid site adsorbs NH 3 [39]. The addition of Mn shows a promoting effect on the activity of the V 2 O 5 /TiO 2 catalyst because manganese oxides contain various valence states, facilitating the completing of the catalytic cycle. As promoter, the ideal loading of Mn is about 2% for V 2 O 5 /TiO 2 catalysts to avoid the oxidation of NH 3 at high temperatures. The presence of the synergetic effect between V and Mn contributes the improvement of SCR activity. The redox cycle (V 4+ + Mn 4+ ↔ V 5+ + Mn 3+ ) reduces the energy required for electron transfer between Mn and V active sites, and promotes the adsorption and activation of NH 3 and NO. Mn 3+ is conducive to NH 3 -SCR reactions due to its ability to induce the formation of more surface chemisorbed oxygen [40]. In Cu-V/TiO 2 catalysts, the copper species exist mainly as Cu 2+ [21,41]. The performance improvement of vanadium-based catalysts is mainly in the following two directions. The first is to widen the temperature window of vanadium-based catalysis, so that it can guarantee the activity of the medium-temperature section and have very good low-temperature activity at the same time. The second is to improve the resistance to sulfate poisoning of vanadium-based catalysts and the erosion ability of water and alkaline metals. Many researchers adopted different methods of improvement and achieved varying results.

The Broadening of the Temperature Ranges
In practice, the temperature of exhaust gas is about 200 • C. Therefore, the broadening of the temperature range is mainly important for the low temperature section. Adding modified elements (Cu, Ce, Fe, Mn, etc.) is a more effective method to solve it.
The Cu based catalyst has good low temperature activity. It has been found that the combination of copper oxide and other metal oxides can obtain better activity. The NO x conversion rate of the catalyst for the Cu-V system is described in Figure 5. It can be seen from diagram that at the optimum Cu-V ratio, the conversion of NO x reaches 90% at 180 • C and nearly 100% at 220 • C [6]. The synergistic effect of Cu and V alleviated the influence of VO x aggregation on TiO 2 crystal form, enhanced the adsorption capacity of the catalyst surface, and facilitated to the SCR reaction at low temperatures. However, Cu is conducive to forming CuSO 4 with SO 2 , and the ability of resisting SO 2 poisoning is poor [5,26]. adsorption capacity of the catalyst surface, and facilitated to the SCR reaction at low temperatures. However, Cu is conducive to forming CuSO4 with SO2, and the ability of resisting SO2 poisoning is poor [5,26]. Ce, as a rare earth element, has been studied more in recent years. It has some kind of role of "backup chemical bond" due to the variability of the coordination number. As a modified element, it could facilitate the valence of vanadium species rapidly changing by a redox cycle of V 5+ + Ce 3+ ↔ V 4+ + Ce 4+ . There are experiments using CeVO4 as a precursor loaded onto the support and compared with the performance of V2O5/TiO2 [7,18]. It shows that the catalyst has a high catalytic activity and a wide temperature window with the best Ce content and a suitable calcination temperature (vanadate decomposition temperature). The activity evaluation of the catalyst with different calcining temperatures in fast conditions is shown in Figure 6. It exhibits that the NOx conversion rate reaches over 90% at the optimum heat treatment temperature, from 200-450 °C. The calcined CeVO4 exhibited better NO conversion and N2 selectivity. The redox ability of Ce can increase the surface acidity of the catalyst and improve the storage capacity of oxygen. Ce 4+ /Ce 3+ can form solid solutions with other oxides to increase the active sites. Meanwhile, Ce can effectively reduce the sintering tendency [7,42]. In addition, the activity distribution of V2O5/CeO2 mixed oxides is somewhat complex. The existence of CeVO4 and the correlation of the V-O-Ce subsequently constitute two systems: VOx/CeO2 and CeVO4/CeO2. The coexistence of low concentration of Ce 4+ species assists to enhance the rate NO conversion to nitrogen [18]. And the synergistic effect of two oxides of CeO2 and V2O5 generated by the decomposition of cerium vanadate promotes redox cycle and improves the activity of the catalyst.  Ce, as a rare earth element, has been studied more in recent years. It has some kind of role of "backup chemical bond" due to the variability of the coordination number. As a modified element, it could facilitate the valence of vanadium species rapidly changing by a redox cycle of V 5+ + Ce 3+ ↔ V 4+ + Ce 4+ . There are experiments using CeVO 4 as a precursor loaded onto the support and compared with the performance of V 2 O 5 /TiO 2 [7,18]. It shows that the catalyst has a high catalytic activity and a wide temperature window with the best Ce content and a suitable calcination temperature (vanadate decomposition temperature). The activity evaluation of the catalyst with different calcining temperatures in fast conditions is shown in Figure 6. It exhibits that the NO x conversion rate reaches over 90% at the optimum heat treatment temperature, from 200-450 • C. The calcined CeVO 4 exhibited better NO conversion and N 2 selectivity. The redox ability of Ce can increase the surface acidity of the catalyst and improve the storage capacity of oxygen. Ce 4+ /Ce 3+ can form solid solutions with other oxides to increase the active sites. Meanwhile, Ce can effectively reduce the sintering tendency [7,42]. In addition, the activity distribution of V 2 O 5 /CeO 2 mixed oxides is somewhat complex. The existence of CeVO 4 and the correlation of the V-O-Ce subsequently constitute two systems: VO x /CeO 2 and CeVO 4 /CeO 2 . The coexistence of low concentration of Ce 4+ species assists to enhance the rate NO conversion to nitrogen [18]. And the synergistic effect of two oxides of CeO 2 and V 2 O 5 generated by the decomposition of cerium vanadate promotes redox cycle and improves the activity of the catalyst. adsorption capacity of the catalyst surface, and facilitated to the SCR reaction at low temperatures. However, Cu is conducive to forming CuSO4 with SO2, and the ability of resisting SO2 poisoning is poor [5,26]. Ce, as a rare earth element, has been studied more in recent years. It has some kind of role of "backup chemical bond" due to the variability of the coordination number. As a modified element, it could facilitate the valence of vanadium species rapidly changing by a redox cycle of V 5+ + Ce 3+ ↔ V 4+ + Ce 4+ . There are experiments using CeVO4 as a precursor loaded onto the support and compared with the performance of V2O5/TiO2 [7,18]. It shows that the catalyst has a high catalytic activity and a wide temperature window with the best Ce content and a suitable calcination temperature (vanadate decomposition temperature). The activity evaluation of the catalyst with different calcining temperatures in fast conditions is shown in Figure 6. It exhibits that the NOx conversion rate reaches over 90% at the optimum heat treatment temperature, from 200-450 °C. The calcined CeVO4 exhibited better NO conversion and N2 selectivity. The redox ability of Ce can increase the surface acidity of the catalyst and improve the storage capacity of oxygen. Ce 4+ /Ce 3+ can form solid solutions with other oxides to increase the active sites. Meanwhile, Ce can effectively reduce the sintering tendency [7,42]. In addition, the activity distribution of V2O5/CeO2 mixed oxides is somewhat complex. The existence of CeVO4 and the correlation of the V-O-Ce subsequently constitute two systems: VOx/CeO2 and CeVO4/CeO2. The coexistence of low concentration of Ce 4+ species assists to enhance the rate NO conversion to nitrogen [18]. And the synergistic effect of two oxides of CeO2 and V2O5 generated by the decomposition of cerium vanadate promotes redox cycle and improves the activity of the catalyst.  Iron-based catalysts have attracted much attention because of their cheap, nontoxic, and high hydrothermal stability and N 2 selectivity. But at low temperature, they exhibits a poor conversion rate of NO x . In SCR reactions temperatures below 300 • C, the active site of iron is mainly in the form of isolated Fe 3+ . When the temperature rises, oligomeric iron oxide clusters, and iron oxide particles become dominant. This contributes more to the SCR reaction and makes it more active. The introduction of Fe can increase the weak acid sites of the catalyst and the NH 4+ formed by NH 3 in the weak acid sites is more easily attached and activated in the SCR reaction [17]. Nickel added as a modified element could change the electronic environment of the metal surface through the formation of heteroatom bonds, thereby modifying its electronic structure with the ligand effect, improving the surface properties [43]. Wu, G. et al. [44] have conducted experiments by adding nickel to the FeV/Ti catalyst to investigate the effect on the SCR reaction. Figure 7 shows the evaluation of the catalytic activity of different catalysts in the Fe-Ni-V system. It can be seen that, with the optimum proportion, the catalytic activity of NO x reaches over 90% at 250-550 • C, and the NO x conversion rate at 280-500 • C reaches 100%. The results show that Ni promotes the interaction between FeVO 4 nanoparticles and TiO 2 , and improves the acidity and reducing activity. When the content of Ni is 0-0.4, Ni is highly dispersed on the surface of FeVO 4 . When the content is 0.6-1, Ni reacts with FeVO 4 to form nickel-iron spinel, so that more defects and oxygen vacancies are formed on the surface of the catalyst. The addition of Ni affects the grain size, changes the surface properties of the catalyst, has better activity at the test temperature, and can widen the temperature window. Iron-based catalysts have attracted much attention because of their cheap, nontoxic, and high hydrothermal stability and N2 selectivity. But at low temperature, they exhibits a poor conversion rate of NOx. In SCR reactions temperatures below 300 °C, the active site of iron is mainly in the form of isolated Fe 3+ . When the temperature rises, oligomeric iron oxide clusters, and iron oxide particles become dominant. This contributes more to the SCR reaction and makes it more active. The introduction of Fe can increase the weak acid sites of the catalyst and the NH 4+ formed by NH3 in the weak acid sites is more easily attached and activated in the SCR reaction [17]. Nickel added as a modified element could change the electronic environment of the metal surface through the formation of heteroatom bonds, thereby modifying its electronic structure with the ligand effect, improving the surface properties [43]. Wu, G. et al. [44] have conducted experiments by adding nickel to the FeV/Ti catalyst to investigate the effect on the SCR reaction. Figure 7 shows the evaluation of the catalytic activity of different catalysts in the Fe-Ni-V system. It can be seen that, with the optimum proportion, the catalytic activity of NOx reaches over 90% at 250-550 °C, and the NOx conversion rate at 280-500 °C reaches 100%. The results show that Ni promotes the interaction between FeVO4 nanoparticles and TiO2, and improves the acidity and reducing activity. When the content of Ni is 0-0.4, Ni is highly dispersed on the surface of FeVO4. When the content is 0.6-1, Ni reacts with FeVO4 to form nickel-iron spinel, so that more defects and oxygen vacancies are formed on the surface of the catalyst. The addition of Ni affects the grain size, changes the surface properties of the catalyst, has better activity at the test temperature, and can widen the temperature window. Manganese bases have attracted attention due to their high activity at low temperatures. The crystalline phase structure of the manganese-based catalysts prepared by different methods can affect the reactivity of SCR, and the irregular surface structure causes more defects and active sites, thus increasing the reactivity of SCR. Liu, J. et al. [45] have studied the SCR reaction mechanism of manganese-based catalysts and believe that the highly active manganese reacts mainly according to the L-H reaction mechanism at low temperature. Pure MnOx is unsuitable as a catalyst because of its low surface area and poor thermal stability [46]. However, manganese-based catalysts have lower N2 selectivity, and are easily deactivated under SO2 and H2O conditions, causing irreversible loss of oxidation ability, resulting in the blockage of pores and active sites by ammonium sulfate salts, limiting the further popularization. Therefore, Zheng, Z. et al. [47] utilize the excellent lowtemperature performance of MnOx to make up for the shortcoming of the narrow temperature window of V2O5/TiO2 and prepare an Mn-V-Ce/TiO2 catalyst. This catalyst uses Mn to provide lowtemperature active sites, and Ce and V provide sufficient oxygen vacancies while enhancing the surface acidity, thereby significantly improving the low-temperature activity. As shown in Figure 8, the activity of Mn-V-Ce/TiO2 catalyst at different temperatures is evaluated. It is found that the NOx conversion rate is as high as 99.2% at 150 °C, indicating excellent low-temperature activity of the catalyst. Manganese bases have attracted attention due to their high activity at low temperatures. The crystalline phase structure of the manganese-based catalysts prepared by different methods can affect the reactivity of SCR, and the irregular surface structure causes more defects and active sites, thus increasing the reactivity of SCR. Liu, J. et al. [45] have studied the SCR reaction mechanism of manganese-based catalysts and believe that the highly active manganese reacts mainly according to the L-H reaction mechanism at low temperature. Pure MnO x is unsuitable as a catalyst because of its low surface area and poor thermal stability [46]. However, manganese-based catalysts have lower N 2 selectivity, and are easily deactivated under SO 2 and H 2 O conditions, causing irreversible loss of oxidation ability, resulting in the blockage of pores and active sites by ammonium sulfate salts, limiting the further popularization. Therefore, Zheng, Z. et al. [47] utilize the excellent low-temperature performance of MnO x to make up for the shortcoming of the narrow temperature window of V 2 O 5 /TiO 2 and prepare an Mn-V-Ce/TiO 2 catalyst. This catalyst uses Mn to provide low-temperature active sites, and Ce and V provide sufficient oxygen vacancies while enhancing the surface acidity, thereby significantly improving the low-temperature activity. As shown in Figure 8, the activity of Mn-V-Ce/TiO 2 catalyst at different temperatures is evaluated. It is found that the NO x conversion rate is as high as 99.2% at 150 • C, indicating excellent low-temperature activity of the catalyst.

The Improvement of Erosion Resistance
V2O5-WO3(MoO3)/TiO2, which has been put into industrial use, is subjected to a lot of erosion in the actual industry environment, one of which is high concentrations of SO2. V2O5 possesses SO2 durability, but SO3 is generated due to peroxidation, and the resulting sulfuric acid (hydrogen) salt will affect the catalyst activity. The addition of WO3 is helpful to increase the Brønsted acid sites, enhance the reaction activity by improving the redox capacity at low temperatures, and to restrain the phase transition, but it still faces erosion of hydrogen sulfate. Li, C. et al. [48] studied the formation and decomposition of ammonium bisulfate over V2O5-WO3/TiO2 catalysts at different temperatures. With the combination of ammonium hydrogen sulfate (ABS) and metal oxides (WO3 and TiO2), electrons deviate from the sulfate, thereby weakening the stability of the ABS and reducing the decomposition temperature. However, the vanadium-generated intermediate VOSO4 makes the sulphate electrons saturated. The presence of NO and O2 could break the bonds inside the ABS, and react with the ammonia species produced in it, allowing the NH3 to be separated from the ABS, accelerating the decomposition of the ABS and competing to inhibit its formation. It was found that the trace ABS on TiO2 had little effect on the NH3-SCR reaction at 300 °C. Under 300 °C, sulfuric acid (hydrogen) ammonia species cover the active sites, masking VOSO4 intermediate products, resulting in a poor NH3-SCR reaction performance. Therefore, it is necessary to study the SO2 corrosion resistance of catalysts applied to low temperature.
Compared with WO3, MoO3 is less affected by SO2 and the chemical properties of MoO3 are similar to those of WO3. Kwon, D.W. et al. [49] have used ammonium molybdate (NH4)2MoO4 and ammonium heptamolybdate (NH4)2Mo2O7 as precursors of molybdenum sources, and ammonium metatungstate hydrates as precursors for tungsten sources, compared with WO3 and MoO3 properties, to study MoO3's ability of resisting SO2. The catalyst was characterized by X-ray photoelectron spectroscopy (XPS) analysis, and the results showed that the binding energy of ammonium heptamolybdate was higher than that of ammonium molybdate. High binding energy can promote the interaction of metal atoms and oxygen, which is beneficial to the redox and SCR reactions of NOx. Furthermore, the Mo 6+ /Mo 5+ of ammonium heptamolybdate is higher than that of ammonium molybdate. The presence of Mo 6+ on the surface of the catalyst can inhibit the reaction between SO2 and V=O, reduce the absorption of SO2, and thus play the role of resisting SO2. Therefore, high Mo 6+ /Mo 5+ exhibits excellent resistance to SO2. Figure 9 shows relative activity in the presence of SO2 for the SCR of NO by NH3 over different catalysts at 250 °C. It can be seen that the SO2 has a certain effect on the catalyst. However, compared to the V2O5/TiO2 and the V2O5/WO3 system, the time for V2O5/MoO3 to remain active is more prolonged. That is, V2O5/MoO3 is more resistant to SO2.

The Improvement of Erosion Resistance
V 2 O 5 -WO 3 (MoO 3 )/TiO 2 , which has been put into industrial use, is subjected to a lot of erosion in the actual industry environment, one of which is high concentrations of SO 2 . V 2 O 5 possesses SO 2 durability, but SO 3 is generated due to peroxidation, and the resulting sulfuric acid (hydrogen) salt will affect the catalyst activity. The addition of WO 3 is helpful to increase the Brønsted acid sites, enhance the reaction activity by improving the redox capacity at low temperatures, and to restrain the phase transition, but it still faces erosion of hydrogen sulfate. Li, C. et al. [48] studied the formation and decomposition of ammonium bisulfate over V 2 O 5 -WO 3 /TiO 2 catalysts at different temperatures. With the combination of ammonium hydrogen sulfate (ABS) and metal oxides (WO 3 and TiO 2 ), electrons deviate from the sulfate, thereby weakening the stability of the ABS and reducing the decomposition temperature. However, the vanadium-generated intermediate VOSO 4 makes the sulphate electrons saturated. The presence of NO and O 2 could break the bonds inside the ABS, and react with the ammonia species produced in it, allowing the NH 3 to be separated from the ABS, accelerating the decomposition of the ABS and competing to inhibit its formation. It was found that the trace ABS on TiO 2 had little effect on the NH 3 -SCR reaction at 300 • C. Under 300 • C, sulfuric acid (hydrogen) ammonia species cover the active sites, masking VOSO 4 intermediate products, resulting in a poor NH 3 -SCR reaction performance. Therefore, it is necessary to study the SO 2 corrosion resistance of catalysts applied to low temperature.
Compared with WO 3 , MoO 3 is less affected by SO 2 and the chemical properties of MoO 3 are similar to those of WO 3 . Kwon, D.W. et al. [49] have used ammonium molybdate (NH 4 ) 2 MoO 4 and ammonium heptamolybdate (NH 4 ) 2 Mo 2 O 7 as precursors of molybdenum sources, and ammonium metatungstate hydrates as precursors for tungsten sources, compared with WO 3 and MoO 3 properties, to study MoO 3 's ability of resisting SO 2 . The catalyst was characterized by X-ray photoelectron spectroscopy (XPS) analysis, and the results showed that the binding energy of ammonium heptamolybdate was higher than that of ammonium molybdate. High binding energy can promote the interaction of metal atoms and oxygen, which is beneficial to the redox and SCR reactions of NO x . Furthermore, the Mo 6+ /Mo 5+ of ammonium heptamolybdate is higher than that of ammonium molybdate. The presence of Mo 6+ on the surface of the catalyst can inhibit the reaction between SO 2 and V=O, reduce the absorption of SO 2 , and thus play the role of resisting SO 2 . Therefore, high Mo 6+ /Mo 5+ exhibits excellent resistance to SO 2 . Figure 9 shows relative activity in the presence of SO 2 for the SCR of NO by NH 3 over different catalysts at 250 • C. It can be seen that the SO 2 has a certain effect on the catalyst. However, compared to the V 2 O 5 /TiO 2 and the V 2 O 5 /WO 3 system, the time for V 2 O 5 /MoO 3 to remain active is more prolonged. That is, V 2 O 5 /MoO 3 is more resistant to SO 2 .
It is important to note that, when testing against SO2 toxicity, the controlled erosion conditions are only SO2. However, in practical applications, the emitted gas may contain a certain amount of water vapor. At this point, SO2 and H2O, NH3 more easily form ammonium sulfate on the surface of the catalyst to deactivate the catalyst. Generally, at temperatures between 200 and 300 °C, vanadiumbased catalyst SCR reactions are strongly affected by coordination of water. The adsorbed ammonia on oxo vanadia Lewis acid sites and ammonium ions associated with Brønsted acid sites selective reduce weakly adsorbed NO via the E-R mechanism, and the water appears to shift the equilibrium between NH3 and NH4 + . At this time, the ratio between Lewis acid, Brønsted acid sites and V=O sites depend on the content of water vapor [15]. Because of the low SO2 resistance of V2O5-WO3/TiO2, there are experiments showing that selecting Ce to replace V and the prepared Ce0.2W0.2TiOx has good NOx conversion activity and N2 selectivity [42]. The activity of the catalyst was stable under the condition of SO2 only, while the decrease of the activity of the catalyst was observed under the condition of both SO2 and H2O. In addition, the performance of the Fe1V1/TiO2 catalyst was tested with an aqueous test and anhydrous test, as shown in Figure 10, and the results showed that the activity in the anhydrous test was obviously better than that of the water test [17]. Based on this, it is necessary to discharge the prior drying or improve the resistance of the catalyst to water erosion performance.  It is important to note that, when testing against SO 2 toxicity, the controlled erosion conditions are only SO 2 . However, in practical applications, the emitted gas may contain a certain amount of water vapor. At this point, SO 2 and H 2 O, NH 3 more easily form ammonium sulfate on the surface of the catalyst to deactivate the catalyst. Generally, at temperatures between 200 and 300 • C, vanadium-based catalyst SCR reactions are strongly affected by coordination of water. The adsorbed ammonia on oxo vanadia Lewis acid sites and ammonium ions associated with Brønsted acid sites selective reduce weakly adsorbed NO via the E-R mechanism, and the water appears to shift the equilibrium between NH 3 and NH 4 + . At this time, the ratio between Lewis acid, Brønsted acid sites and V=O sites depend on the content of water vapor [15]. Because of the low SO 2 resistance of V 2 O 5 -WO 3 /TiO 2 , there are experiments showing that selecting Ce to replace V and the prepared Ce 0.2 W 0.2 TiOx has good NOx conversion activity and N 2 selectivity [42]. The activity of the catalyst was stable under the condition of SO 2 only, while the decrease of the activity of the catalyst was observed under the condition of both SO 2 and H 2 O. In addition, the performance of the Fe 1 V 1 /TiO 2 catalyst was tested with an aqueous test and anhydrous test, as shown in Figure 10, and the results showed that the activity in the anhydrous test was obviously better than that of the water test [17]. Based on this, it is necessary to discharge the prior drying or improve the resistance of the catalyst to water erosion performance. It is important to note that, when testing against SO2 toxicity, the controlled erosion conditions are only SO2. However, in practical applications, the emitted gas may contain a certain amount of water vapor. At this point, SO2 and H2O, NH3 more easily form ammonium sulfate on the surface of the catalyst to deactivate the catalyst. Generally, at temperatures between 200 and 300 °C, vanadiumbased catalyst SCR reactions are strongly affected by coordination of water. The adsorbed ammonia on oxo vanadia Lewis acid sites and ammonium ions associated with Brønsted acid sites selective reduce weakly adsorbed NO via the E-R mechanism, and the water appears to shift the equilibrium between NH3 and NH4 + . At this time, the ratio between Lewis acid, Brønsted acid sites and V=O sites depend on the content of water vapor [15]. Because of the low SO2 resistance of V2O5-WO3/TiO2, there are experiments showing that selecting Ce to replace V and the prepared Ce0.2W0.2TiOx has good NOx conversion activity and N2 selectivity [42]. The activity of the catalyst was stable under the condition of SO2 only, while the decrease of the activity of the catalyst was observed under the condition of both SO2 and H2O. In addition, the performance of the Fe1V1/TiO2 catalyst was tested with an aqueous test and anhydrous test, as shown in Figure 10, and the results showed that the activity in the anhydrous test was obviously better than that of the water test [17]. Based on this, it is necessary to discharge the prior drying or improve the resistance of the catalyst to water erosion performance. Figure 10. The effect of water vapor on NOx conversion of Fe1V1/TiO2 catalyst [17]. Figure 10. The effect of water vapor on NO x conversion of Fe 1 V 1 /TiO 2 catalyst [17].
In addition, the actual application of high-temperature exhaust gas may be in some alkaline environment, and the catalyst may suffer alkali poisoning. Chen, L. et al. [50] have investigated the effects that alkali metals and alkaline earth metals may have on catalysts. It was found that the degree of poisoning of the catalyst increased with the increase of alkalinity, and the N 2 O conversion rate decreased with the increase of alkaline content. The toxic element occupies the non-atomic pore site of the V 2 O 5 [010] plane, so that the Brønsted acid and V 5+ =O sites are blocked. In alkali metals such as Na, K will reduce the stability of Brønsted acid sites, reduce the absorption of ammonia, and reduce the amount of surface chemically adsorbed oxygen. By contrast, the effect of alkaline earth metals such as Ca and Mg on the reducibility of V is not obvious. Hu, W. et al. [51] have prepared V-Ce(SO 4 ) 2 /Ti catalysts, utilizing the high oxygen storage ability and strong redox ability of Ce, and sulfate SO 4 2− that can increase surface acidity to obtain better performance by synergizing Ce with SO 4 2− . Ce introduces a high proportion of surface-active oxygen to increase the redox capacity of the catalyst. The abundant NO + and NO 3on the surface form a redox cycle. The catalyst has better resistance to SO 2 , H 2 O, and alkali metal than the vanadium-based commercial catalyst. Gao et al. [52] used sulfided zirconia as a catalyst support and added Ce:V = 1:1 to obtain a CeVSZ(x) catalyst. The support provides too many acidic sites to react with the alkali metal and delay the acidity reduction. V partially replaces Ce to form CeVO 4 , avoiding CeO 2 from being converted into Ce 2 (SO 4 ) 3 to make it permanently inactivated. The CeVSZ(x) catalyst can maintain activity higher than 97% in the atmosphere of SO 2 and K coexistence, and there is no significant decrease in activity within 400 min. Huang, Z. et al. [53] prepared a hexagonal structure of WO 3 (HWO) as a support to load V 2 O 5 . The corner-sharing WO 6 octahedra form hexagonal tunnels oriented along the C-axis with space group P6/mmm, and the external surface of HWO allows V 2 O 5 to be highly dispersed. The tunnels have relatively smooth surfaces and suitable sizes for alkalis' trapping specifically. Alkalis are readily inserted into tunnels by ion exchange reaction, so HWO has excellent resistance to alkali metal erosion. Figure 11 shows the catalytic activity evaluation of the In addition, the actual application of high-temperature exhaust gas may be in some alkaline environment, and the catalyst may suffer alkali poisoning. Chen, L. et al. [50] have investigated the effects that alkali metals and alkaline earth metals may have on catalysts. It was found that the degree of poisoning of the catalyst increased with the increase of alkalinity, and the N2O conversion rate decreased with the increase of alkaline content. The toxic element occupies the non-atomic pore site of the V2O5 [010] plane, so that the Brønsted acid and V 5+ =O sites are blocked. In alkali metals such as Na, K will reduce the stability of Brønsted acid sites, reduce the absorption of ammonia, and reduce the amount of surface chemically adsorbed oxygen. By contrast, the effect of alkaline earth metals such as Ca and Mg on the reducibility of V is not obvious. Hu, W. et al. [51] have prepared V-Ce(SO4)2/Ti catalysts, utilizing the high oxygen storage ability and strong redox ability of Ce, and sulfate SO4 2− that can increase surface acidity to obtain better performance by synergizing Ce with SO4 2− . Ce introduces a high proportion of surface-active oxygen to increase the redox capacity of the catalyst. The abundant NO + and NO 3-on the surface form a redox cycle. The catalyst has better resistance to SO2, H2O, and alkali metal than the vanadium-based commercial catalyst. Gao et al. [52] used sulfided zirconia as a catalyst support and added Ce:V = 1:1 to obtain a CeVSZ(x) catalyst. The support provides too many acidic sites to react with the alkali metal and delay the acidity reduction. V partially replaces Ce to form CeVO4, avoiding CeO2 from being converted into Ce2(SO4)3 to make it permanently inactivated. The CeVSZ(x) catalyst can maintain activity higher than 97% in the atmosphere of SO2 and K coexistence, and there is no significant decrease in activity within 400 min. Huang, Z. et al. [53] prepared a hexagonal structure of WO3 (HWO) as a support to load V2O5. The corner-sharing WO6 octahedra form hexagonal tunnels oriented along the C-axis with space group P6/mmm, and the external surface of HWO allows V2O5 to be highly dispersed. The tunnels have relatively smooth surfaces and suitable sizes for alkalis' trapping specifically. Alkalis are readily inserted into tunnels by ion exchange reaction, so HWO has excellent resistance to alkali metal erosion. Figure 11 shows the catalytic activity evaluation of the V2O5/HWO catalyst and the V2O5/WO3-TiO2 catalyst at 350 °C under a gas containing NO, NH3, SO2 and alkali metal particles. It can be seen that the activity of V2O5/HWO is much higher than that of V2O5/WO3-TiO2 catalysts under the coexistence of SO2 and alkali metal.

Conclusions and Perspective
The vanadium-based catalyst is the most mature catalyst, and has a high catalytic activity at medium temperatures. However, considering the thermal instability of vanadium, the lowtemperature catalytic demand, and severe working environment in industrial production, the improvement of vanadium-based catalysts has become a research hotspot. The most common

Conclusions and Perspective
The vanadium-based catalyst is the most mature catalyst, and has a high catalytic activity at medium temperatures. However, considering the thermal instability of vanadium, the low-temperature catalytic demand, and severe working environment in industrial production, the improvement of vanadium-based catalysts has become a research hotspot. The most common improvement method is in the modification of vanadium-based catalysts. Four major systems of cerium, iron, copper and manganese are widely used in this field. Cerium has excellent ability to store and release oxygen. The redox cycle is realized by Ce 3+ /Ce 4+ electron pairs, which provide oxygen vacancy and active sites and enhance the activity of catalysts. CeVO 4 stabilizes the surface V 5+ species, accelerates redox cycle and NO 2 production, and helps to spur fast SCR. The cooperative effect between Ce and V leads to the enhancement of SCR activity. The presence of Ce 3+ /Ce 4+ increases surface chemisorbed oxygen and provide more acid sites. CeVO 4 promotes the formation of Brønsted acid sites while VO x and CeO 2 provide Lewis acid sites on the surface. In addition, the Lewis acid sites may be converted to the Brønsted acid sites at high temperature SCR reactions. However, Ce is prone to generate Ce 2 (SO 4 ) 3 with sulfate to cause permanent deactivation of the catalyst. At low temperatures, the unrivaled, high-activity of manganese-based oxides and copper-based oxides can significantly broaden the temperature window of vanadium-based catalysts, though they have poor low-temperature erosion resistance. The redox cycle (V 4+ + Mn 4+ ↔ V 5+ + Mn 3+ ) reduces the energy required for electron transfer between Mn and V active sites, and promotes the adsorption and activation of NH 3 and NO. Mn 3+ is conducive to NH 3 -SCR reactions because it induces the formation of more surface chemisorbed oxygen. By introducing copper species, strong acid sites are increased. It is worth noting that Lewis acid sites are the strong acid sites in Cu-V/TiO 2 catalysts. Iron is cheap and nontoxic material with high hydrothermal stability. Due to the presence of Fe 3+ -O-V 5+ linkages, FeVO 4 incorporate a lot of surface defects which can adsorb or activate reactants. The adsorbed NH 3 prefers to be activated by Fe 3+ due to the rapid electron transfer. The presence of electronic inductive effect between Fe 3+ and V 5+ was beneficial for high deNO x efficiency and N 2 selectivity. The VO x species, the decomposition product of FeVO 4 , exhibits higher SCR activity as active component. The presence of iron can be useful by altering the electron density of the VO x surface species and inducing more Lewis acid sites and Brønsted acid sites for SCR reactions.
Although the research on denitrification catalysts has attracted widespread attention, the mechanism of SCR reaction is not yet clear. Research on the resistance of SO 2 , H 2 O, and alkali metals has made some progress, but there is still a lack of research on its inhibition mechanism. With the increasing importance of environmental protection, the development of vanadium-based catalysts is a general trend. Further study is expected to realize on the catalysis mechanism of synergistic effect and the performance optimization.