Recent Progress on Improving Low-Temperature Activity of Vanadia-Based Catalysts for the Selective Catalytic Reduction of NO x with Ammonia

: Selective catalytic reduction of NO x with NH 3 (NH 3 -SCR) has been successfully applied to abate NO x from diesel engines and coal-ﬁred industries on a large scale. Although V 2 O 5 -WO 3 (MoO 3 ) / TiO 2 catalysts have been utilized in commercial applications, novel vanadia-based catalysts have been recently developed to meet the increasing requirements for low-temperature catalytic activity. In this article, recent progress on the improvement of the low-temperature activity of vanadia-based catalysts is reviewed, including modiﬁcation with metal oxides and nonmetal elements and the use of novel supports, di ﬀ erent synthesis methods, metal vanadates and speciﬁc structures. Investigation of the NH 3 -SCR reaction mechanism, especially at low temperatures, is also emphasized. Finally, for low-temperature NH 3 -SCR, some suggestions are given regarding the opportunities and challenges of vanadia-based catalysts in future research.


Introduction
Nitrogen oxides (NO x , including NO and NO 2 ), primarily emitted from fossil fuel combustion in both stationary and mobile sources, are major pollutants in the atmosphere. NO x is harmful to human health and can lead to the greenhouse effect, ozone depletion, acid rain, photochemical smog, and haze. Therefore, reducing the emission of NO x has become one of the most urgent atmospheric environment problems [1][2][3]. Increasingly stringent policies and legislation to control the emission of nitrogen oxides have been enacted all over the world.
In the 1970s, selective catalytic reduction of NO x with NH 3 (NH 3 -SCR) was first applied to NO x removal from stationary sources in Japan [4], and has subsequently been widely used all over the world for NO x control from both stationary and mobile sources [5]. The NH 3 -SCR process mainly comprises the following reactions: 4NO(g) + 4NH 3 (g) + O 2 (g) → 4N 2 (g) + 6H 2 O(g) 2NO 2 (g) + 4NH 3 (g) + O 2 (g) → 3N 2 (g) + 6H 2 O(g) 2NO(g) + 4NH 3 (g) + 2NO 2 (g) → 4N 2 (g) + 6H 2 O(g) Catalysts 2020, 10 6NO(g) + 4NH 3 (g) → 5N 2 (g) + 6H 2 O(g) 6NO 2 (g) + 8NH 3 (g) → 7N 2 (g) + 12H 2 O(g) Catalysts are the key components in NH 3 -SCR technology for abating NO x emission. Currently, V 2 O 5 -WO 3 /TiO 2 is commercially applied because of its excellent catalytic performance at 300-400 • C and strong SO 2 resistance [6]. Due to the high operating temperature, the precipitator and desulfurizer units should be installed downstream of the SCR unit in power plants [7]. However, this results in deactivation of the catalysts by poisoning with heavy metals, phosphorus and alkali/alkaline earth metals existing in the stack gases. Furthermore, the flue gas temperature from steel, glass and cement plants is lower than the working temperature of traditional V-based catalysts [8]. In order to avoid rapid deactivation and the need for additional energy consumption and to satisfy increasingly stringent NO x emission limits, the study of novel low-temperature vanadia-based SCR catalysts with high performance is highly desirable and has attracted broad attention to the deNO x process. Figure 1 shows the numbers of papers published each year from 1991 to 2020 containing "vanadium" and "SCR" in their contents using the "web of science" search engine. The increasing trend in the number of publications reflects the popularity, importance and enthusiasm for research of the vanadia-based catalysts for NH 3

-SCR.
Catalysts 2020, 10, x FOR PEER REVIEW 2 of 19 6NO(g) + 4NH (g) → 5N (g) + 6H O(g) 6NO (g) + 8NH (g) → 7N (g) + 12H O(g) Catalysts are the key components in NH3-SCR technology for abating NOx emission. Currently, V2O5-WO3/TiO2 is commercially applied because of its excellent catalytic performance at 300-400 °C and strong SO2 resistance [6]. Due to the high operating temperature, the precipitator and desulfurizer units should be installed downstream of the SCR unit in power plants [7]. However, this results in deactivation of the catalysts by poisoning with heavy metals, phosphorus and alkali/alkaline earth metals existing in the stack gases. Furthermore, the flue gas temperature from steel, glass and cement plants is lower than the working temperature of traditional V-based catalysts [8]. In order to avoid rapid deactivation and the need for additional energy consumption and to satisfy increasingly stringent NOx emission limits, the study of novel low-temperature vanadia-based SCR catalysts with high performance is highly desirable and has attracted broad attention to the deNOx process. Figure 1 shows the numbers of papers published each year from 1991 to 2020 containing "vanadium" and "SCR" in their contents using the "web of science" search engine. The increasing trend in the number of publications reflects the popularity, importance and enthusiasm for research of the vanadia-based catalysts for NH3-SCR. Herein, this work reviews the latest progress, especially the progress in the last five years, on vanadia-based catalysts for low-temperature NH3-SCR, including a summary of methods for improving the performance at low temperature over vanadia-based catalysts and a discussion of the reaction mechanism of low-temperature NH3-SCR, and finally proposes suggestions for the development of vanadia-based catalysts with superior low-temperature SCR activity.

Performance Improvement at Low Temperatures
The narrow operating temperature window of vanadia-based catalysts restricts their broader applications in the deNOx process. Though catalysts with high loadings of V2O5 exhibit high catalytic activity, high vanadia content leads to a decrease in N2 selectivity and thermal stability and an increase in the catalytic oxidation of sulfur dioxide to sulfur trioxide, which induces severe corrosion problems for equipment [9]. Therefore, researchers have devoted much effort to improving the lowtemperature catalytic activity of vanadia-based catalysts, including modification of the active presented superior catalytic performance [16]. Hu et al. reported that V-Ce(SO4)2/Ti catalysts with abundant reactive acid sites exhibited better activity than the commercial V-W/Ti catalyst and showed a strong tolerance to SO2 and H2O [17]. The catalysts 3Ce-V-W/TiO2 [18], V-5W/Ce/Ti [19], V0.8WTiCe0.25 [20] and V1Ce10Ti [21] all exhibited excellent SCR activity.
Chi et al. adopted an ultrasonic-assisted impregnation method to prepare Ce-Cu modified V2O5/TiO2 catalysts, exhibiting high NO conversion (>97%) at 200-400 °C [22]. Xu et al. found that among TiO2-supported vanadium oxide catalysts modified with different amounts of Ce and Sb, V5Ce35Sb2/Ti presented the best SCR performance, over which 90% NO conversion was obtained at 210 °C [23]. The doping of V and Sb into Ce/Ti increased the concentration of Ce 4+ , and V/Sb/Ce/Ti exhibited superior catalytic performance and N2 selectivity (as shown in Figure 2) [24].   [24]. Reproduced with permission from [24], copyright 2015, Elsevier.
Besides modification with metal oxides, doping with nonmetal elements can also enhance the SCR performance of the catalyst. Marberger et al. found that the doping of 2-4 wt.% SiO 2 into VW/Ti can hamper the growth of anatase crystallites at 600 and 650 • C, resulting in preservation of the low-temperature catalytic activity [31]. Zhao et al. found that the addition of N and S to V 2 O 5 /TiO 2 can significantly influence NH 3 -SCR performance, and a catalyst with 3:1:100 (S:N:Ti) molar ratio exhibited superior performance attributed to the large pore volume and surface area, strong reducibility and increased surface acidity, electronic interactions between TiO 2 and the dispersed vanadia species, and inhibition of the anatase-to-rutile phase transformation [32,33]. The SCR activity at low temperatures was promoted evidently by the doping of F into V 2 O 5 /TiO 2 because modification with F promoted the interaction of vanadium species and TiO 2 by means of oxygen vacancies with electrons [34]. Liang et al. also found that F-V 2 O 5 -WO 3 /TiO 2 could enhance the SCR activity and the resistance to sulfur and water. The surface morphology of the catalyst was eroded and the grain size was reduced by the introduction of an appropriate amount of F [35]. The introduction of a certain amount of phosphorus into V 2 O 5 -WO 3 /TiO 2 increased the intrinsic activity for the enhancement of Lewis and Brønsted acid sites on V 2 O 5 -WO 3 /TiO 2 , and induced the formation of more polymeric surface vanadyl species through spatial effects [36]. Nam et al. added a low content of P into V/Sb/Ce/Ti, and Sb-phosphate and Ce-phosphate instead of VOPO 4 were formed, contributing to the enhancement of the redox properties while keeping the acidity almost unchanged, resulting in the promotion of the SCR activity [37]. A heteropoly acid (HPA)-promoted V 2 O 5 /TiO 2 catalyst presented better catalytic performance and stronger potassium tolerance than WO 3 -promoted catalysts [38]. The modification of vanadia-based catalysts and their catalytic activity are shown in Table 1. The improvements in low-temperature performance accomplished via modification with metal oxides or nonmetal elements were mainly related to modulation of the redox capability and acidity of the vanadia-based catalysts. Besides doping with active components, the catalyst supports were also modified to further improve the deNO x activity.

The Effect of Different Supports
TiO 2 has been applied as a support for commercial vanadia-based catalysts for decades due to its favorable properties, including good surface dispersion of V species on TiO 2 in comparison with other supports (Al 2 O 3 , SiO 2 , et al.) as well as weak and reversible sulfation under SCR reaction conditions [8,39,40]. However, at high reaction temperatures, anatase TiO 2 will transform to rutile TiO 2 , which leads to catalyst sintering accompanied by decreases in the surface area and activity [41]. Recently, many attempts have been made to improve vanadia-based catalysts by modifying the TiO 2 support or using diverse support materials.
Vanadium oxides dispersed on microporous TiO 2 supports produced much less N 2 O than commercial TiO 2 -supported catalysts during the process of NH 3 -SCR reaction, because the formation of bulk-like V 2 O 5 species, which resulted in the formation of N 2 O, was suppressed (as shown in Figure 3) [42]. The pore structure of TiO 2 determined the types of vanadium species present, which affected the sulfur resistance during the SCR reaction. A 5 wt.%V/Ti (microporous TiO 2 ) showed stronger resistance to sulfur poisoning and better activity than a 5 wt.%V/Ti (mesopore DT-51) having bulk-like VO x species [43]. A catalyst with 5 wt.% vanadia ultrasonically impregnated on a TiO 2 support with a large surface area (380.5 m 2 /g) had a 100 • C wider operating temperature window and higher N 2 selectivity than the traditional vanadia-based catalyst due to enhanced redox capability and total acidity [44]. Ti-bearing blast furnace slag was used to prepare the TiO 2 -SiO 2 support, which was then used to support V 2 O 5 -WO 3 and the catalyst presented enhanced acidity and redox ability for active species and promoted the SCR activity compared with a catalyst using commercial TiO 2 as support [45]. Mixed tungsten−titanium-pillared clays were used to support vanadium oxides, and the synthesized catalysts showed a uniform distribution of WTi-pillars between the clay layers and showed good catalytic performance in NH 3 -SCR [46]. Catalysts on other novel supports were also investigated, such as V 2 O 5 supported on reduced TiO 2 [47], vanadium impregnated on silica-pillared layered titanate (SiO 2 -Ti 4 O 9 ) [48], supported vanadium-substituted Keggin polyoxometalates (POM) [49], V 2 O 5 /H 2 Ti 3 O 7 -nanotubes and V 2 O 5 -WO 3 /H 2 Ti 3 O 7 -nanotubes [50], 1V4Ce/Ti-PILC (1 wt.% V and 4 wt.% Ce) [51], and V/Ce 1−x Ti x (x = 0.3, 0.5) [52].

The Effect of Different Supports
TiO2 has been applied as a support for commercial vanadia-based catalysts for decades due to its favorable properties, including good surface dispersion of V species on TiO2 in comparison with other supports (Al2O3, SiO2, et al.) as well as weak and reversible sulfation under SCR reaction conditions [8,39,40]. However, at high reaction temperatures, anatase TiO2 will transform to rutile TiO2, which leads to catalyst sintering accompanied by decreases in the surface area and activity [41]. Recently, many attempts have been made to improve vanadia-based catalysts by modifying the TiO2 support or using diverse support materials.
Vanadium oxides dispersed on microporous TiO2 supports produced much less N2O than commercial TiO2-supported catalysts during the process of NH3-SCR reaction, because the formation of bulk-like V2O5 species, which resulted in the formation of N2O, was suppressed (as shown in Figure  3) [42]. The pore structure of TiO2 determined the types of vanadium species present, which affected the sulfur resistance during the SCR reaction. A 5 wt.%V/Ti (microporous TiO2) showed stronger resistance to sulfur poisoning and better activity than a 5 wt.%V/Ti (mesopore DT-51) having bulklike VOx species [43]. A catalyst with 5 wt.% vanadia ultrasonically impregnated on a TiO2 support with a large surface area (380.5 m 2 /g) had a 100 °C wider operating temperature window and higher N2 selectivity than the traditional vanadia-based catalyst due to enhanced redox capability and total acidity [44]. Ti-bearing blast furnace slag was used to prepare the TiO2-SiO2 support, which was then used to support V2O5-WO3 and the catalyst presented enhanced acidity and redox ability for active species and promoted the SCR activity compared with a catalyst using commercial TiO2 as support [45]. Mixed tungsten−titanium-pillared clays were used to support vanadium oxides, and the synthesized catalysts showed a uniform distribution of WTi-pillars between the clay layers and showed good catalytic performance in NH3-SCR [46]. Catalysts on other novel supports were also investigated, such as V2O5 supported on reduced TiO2 [47], vanadium impregnated on silica-pillared layered titanate (SiO2-Ti4O9) [48], supported vanadium-substituted Keggin polyoxometalates (POM) [49], V2O5/H2Ti3O7-nanotubes and V2O5-WO3/H2Ti3O7-nanotubes [50], 1V4Ce/Ti-PILC (1 wt.% V and 4 wt.% Ce) [51], and V/Ce1-xTix (x = 0.3, 0.5) [52]. . NOx conversion and N2O formation during SCR of NO with NH3 over VOx/DT-51 and VOx/MP-TiO2 calcined at various temperatures. Measurement was performed after reaching steady state in wet conditions at 400 °C [42]. Reproduced with permission from [42], copyright 2018, Elsevier.
Ce-based supports for vanadium oxide NH3-SCR catalysts have also attracted interest. In our previous study, the homogeneous precipitation method was used to prepare vanadium oxides/CeO2 catalysts, and the catalyst exhibited high NOx conversion and strong tolerance to SO2 and H2O [53]. Incorporating Ti into V2O5/CeO2 improved the NOx conversion, N2 selectivity and resistance to SO2 and H2O due to the lower crystallinity, more abundant acid sites, better dispersion of surface V species and greater number of reduced species [54]. Modification of VOx/CeO2 with NbOx promoted the redox capability and acidity, leading to better NH3-SCR activity and stronger tolerance to . NO x conversion and N 2 O formation during SCR of NO with NH 3 over VO x /DT-51 and VO x /MP-TiO 2 calcined at various temperatures. Measurement was performed after reaching steady state in wet conditions at 400 • C [42]. Reproduced with permission from [42], copyright 2018, Elsevier.
Ce-based supports for vanadium oxide NH 3 -SCR catalysts have also attracted interest. In our previous study, the homogeneous precipitation method was used to prepare vanadium oxides/CeO 2 catalysts, and the catalyst exhibited high NO x conversion and strong tolerance to SO 2 and H 2 O [53]. Incorporating Ti into V 2 O 5 /CeO 2 improved the NO x conversion, N 2 selectivity and resistance to SO 2 and H 2 O due to the lower crystallinity, more abundant acid sites, better dispersion of surface V species and greater number of reduced species [54]. Modification of VO x /CeO 2 with NbO x promoted the redox capability and acidity, leading to better NH 3 -SCR activity and stronger tolerance to SO 2 /H 2 O than the unmodified VO x /CeO 2 catalyst; and 30Nb-1VO x /CeO 2 exhibited better NH 3 -SCR performance than 3V 2 O 5-10WO 3 /TiO 2 (as shown in Figure 4) [55,56].
Carbon materials, including carbon nanotubes (CNT), carbon fibers (ACF) and activated carbon (AC), can also be used to support vanadia-based catalysts for NH3-SCR for large specific surface area and pore volume. Carbon nanotube-supported vanadium oxide (V2O5/CNT) exhibited high NO reduction activity and high stability at low-temperatures, and the activity in the presence of sulfur dioxides was significantly promoted [63]. Sulfur dioxide could enhance the catalytic activity of a V2O5/SiC catalyst at 250 °C because pre-adsorption with SO2 + O2 enhanced NH3 adsorption and at low temperatures, NO could effectively react with the ammonium bisulfate that was formed on the surface of the catalyst [64]. Excellent SCR performance and SO2 resistance were also achieved by 3 wt.%Fe-0.5 wt.%V/AC [65], and Mn-Ce-V/AC [66].
Improving the properties of the support could thus improve the dispersion of vanadium species and the interaction between the support and active components, enhance electron transfer between active components, enlarge the specific surface area, and strengthen the redox capability and acidity, all of which are beneficial to accelerating the SCR reaction.

The Effect of Preparation Method
The preparation methods utilized for catalysts mainly include the precipitation method, sol−gel method, hydrothermal method, impregnation method, and other specialized synthesis methods. Different synthesis methods and synthesis parameters influence the nanostructures, morphologies, and surface physicochemical properties, finally affecting the NH3-SCR catalytic activity. Hence, it is ZrO 2 supports with various morphologies (mesoporous, rod, star, sphere, hollow) were loaded with vanadium oxide, and 3 wt.% V/Zr (mesoporous) exhibited a wide operating temperature window and excellent N 2 selectivity. The content of tetravalent vanadium ions on the surface of the catalyst and catalytic performance decreased in the order of mesoporous > hollow spheres > stars > rods [57]. In addition, there are several reports on the use of Zr-modified supports such as in 5wt.%V/Zr 0.3 Ce 0.7 [58], V/ZrCe 0.6 [59], Co-V/Zr-Ce [60], which enhanced the SCR performance of the vanadia-based catalysts. A catalyst with vanadium−tungsten oxides supported on CuCeO y microflowers showed excellent deNO x performance at low temperatures, attributed to the facile electron transfer among V, Cu and Ce ions, decreasing the apparent activation energy of the NH 3 -SCR reaction (E a = 16.59 kJ/mol) [61]. A V/Ce 0.9 Fe 0.1 solid solution also presented relatively high SCR activity at low temperature (below 300 • C) [62].
Carbon materials, including carbon nanotubes (CNT), carbon fibers (ACF) and activated carbon (AC), can also be used to support vanadia-based catalysts for NH 3 -SCR for large specific surface area and pore volume. Carbon nanotube-supported vanadium oxide (V 2 O 5 /CNT) exhibited high NO reduction activity and high stability at low-temperatures, and the activity in the presence of sulfur dioxides was significantly promoted [63]. Sulfur dioxide could enhance the catalytic activity of a V 2 O 5 /SiC catalyst at 250 • C because pre-adsorption with SO 2 + O 2 enhanced NH 3 adsorption and at low temperatures, NO could effectively react with the ammonium bisulfate that was formed on the surface of the catalyst [64]. Excellent SCR performance and SO 2 resistance were also achieved by 3 wt.%Fe-0.5 wt.%V/AC [65], and Mn-Ce-V/AC [66].
Improving the properties of the support could thus improve the dispersion of vanadium species and the interaction between the support and active components, enhance electron transfer between active components, enlarge the specific surface area, and strengthen the redox capability and acidity, all of which are beneficial to accelerating the SCR reaction.

The Effect of Preparation Method
The preparation methods utilized for catalysts mainly include the precipitation method, sol−gel method, hydrothermal method, impregnation method, and other specialized synthesis methods. Different synthesis methods and synthesis parameters influence the nanostructures, morphologies, and surface physicochemical properties, finally affecting the NH 3 -SCR catalytic activity. Hence, it is important to optimize the synthesis method to improve the NH 3 -SCR activity over vanadia-based catalysts.
Compared to materials prepared by incipient wetness impregnation, coprecipitated V 2 O 5 -WO 3 /TiO 2 catalysts presented superior SCR performance derived from their increased ammonia adsorption capacity due to the existence of new surface WO x site-associated surface defects on the TiO 2 support [67]. A V/Ce/WTi-DP (deposition precipitation) catalyst exhibited more O α , stronger reducibility and more surface Ce species, thus showing less N 2 O formation and better medium-temperature NH 3 -SCR performance than similar catalysts prepared by impregnation [68]. CeO 2 -modified V 2 O 5 /TiO 2 synthesized via chemical vapor condensation exhibited a higher ratio of Ce 3+ and showed higher reducibility and acidity than that synthesized by the impregnation method [69]. In our previous work, VO x /CeO 2 synthesized via the homogeneous precipitation method presented better catalytic performance and stronger tolerance to H 2 O and SO 2 than that synthesized via sol−gel method, incipient wetness impregnation and rotary evaporation impregnation, which can be ascribed to the lower crystallinity of CeO 2 on the surface, greater abundance of acid sites and vanadium species, and better dispersion of V species [53]. The support synthesis methods had an effect on structure and performance of VO x /CeO 2 catalysts, and the catalyst from precipitation method prepared-support was more active than that from citrate method due to higher surface area and a more effective incorporation of V sites into the support surface [70]. A series of VWTi nanoparticle catalysts were directly synthesized via the sol−gel method, and V 0.02 W 0.04 Ti showed the best SCR activity and the lowest apparent activation energy due to a high concentration of distorted and reducible vanadium species [71].
Besides these conventional synthesis methods, several novel preparation methods were developed to enhance catalytic activity of vanadia-based NH 3 -SCR catalysts. Chen et al. prepared V 0.1 Ti 0.9 O 2-δ catalysts by flame synthesis with promoted low-temperature activity due to strong redox properties, and different synthesis conditions also effected the properties of the catalysts and their catalytic activity [72]. Arfaoui prepared a V 2 O 5 -CeO 2 -TiO 2 -SO 4 2− nanostructured aerogel catalyst via a one-step sol−gel method accompanied by the supercritical drying process, which presented a large surface area (66 m 2 /g), large porosity and good thermal stability [16]. Tuning the interaction degree supplied by external forces can regulate the morphology. Rotation-assisted hydrothermal synthesis can be used to prepare thick multiwalled titanate nanotubes and when used as the support for vanadium−tungsten-oxide, the resulting catalysts presented stable catalytic activity because the sintering of VO x was suppressed by the multiwalled nanotubes [73]. Doronkin et al. found that V/Ti and V-W/Ti catalysts prepared via incipient wetness impregnation and grafting with highly dispersed, isolated and polymeric V-oxo species exhibited high SCR activity [74].
In the synthesis of catalysts, different preparation parameters can also affect the physicochemical properties and the catalytic activity. For example, different vanadium precursors have an influence on the dispersion of V species and catalytic performance [75]. V 2 O 5 /TNTs catalysts prepared using VOSO 4 as the V precursor exhibited stronger synergistic effects with the TNTs than those made with a NH 4 VO 3 precursor, and active metal precursors containing cation groups were superior to those with anion groups (as shown in Figure 5) [76]. During the preparation process of iron vanadate, the pH value determined the stoichiometry of the FeVO 4 /TiO 2 -WO 3 -SiO 2 product: vanadium-rich samples were prepared at pH lower than 6, stoichiometric FeVO 4 was obtained in the pH range of 4-6, while samples prepared at pH > 6 were rich in Fe 2 O 3 [77]. The NH 3 adsorption capacity of Ce-doped V 2 O 5 -WO 3 /TiO 2 was affected by the sequence of impregnation by Ce and W, and 0.2V-5W-5Ce/Ti exhibited stronger SO 2 and H 2 O tolerance than 0.2V-(5Ce5W)/Ti and 0.2V-5Ce-5W/Ti [78]. Compared to the untreated Catalysts 2020, 10, 1421 9 of 19 sample, V-Ce-Ni/TiO 2 catalysts treated by nonthermal plasma possessed smaller particle size, better dispersion and more uniform distribution of active sites, as well as larger number of oxygen vacancy and acid sites. Therefore, the plasma-treated catalysts showed improved catalytic performance at low temperatures with a wider operating temperature window [79]. Moreover, the treatment atmosphere can also have an influence on catalytic performance. The ratios of active polymeric V-O-V species and further NH 3 -SCR activity decreased in the following order: V/Ti > V/Ti-O 2 > V/Ti-N 2 > V/Ti-NH 3 [80].

Metal Vanadates
Although vanadia-based catalysts show superior catalytic performance at medium temperatures and have been widely applied in the deNOx process, the volatility and the toxicity of vanadium oxide at high temperature is a crucial problem. However, metal vanadates present better thermal stability than V2O5 due to their higher melting points (e.g., 780, 850, 1030 and 1100 °C for copper vanadate, iron vanadate, manganese vanadate and cerium vanadate, respectively). Therefore, metal vanadates can be candidates for vanadia-based catalysts with great potential [81].
Iron vanadates (FeVO4) have recently been extensively studied as active NH3-SCR catalysts. Abundant surface defects existed on a FeVO4/TiO2 catalyst for the adsorption and activation of reactants, and the true active sites were FeVO4 phase surface-enriched with VOx species. Thus, similar to V2O5/TiO2, FeVO4/TiO2 showed superior catalytic performance and H2O/SO2 tolerance [1]. Fe0.75V0.25Oδ exhibited excellent SCR performance in the temperature range of 175-400 °C. It was found that the formation of amorphous FeVO4 resulted from the incorporation of V into Fe2O3, and the apparent activation energy decreased due to the synergistic effect of FeVO4 and Fe2O3 improving the catalytic activity at low temperatures [82]. Due to the greater electronic inductive effect, Fe2V4O13/TiO2 showed stronger redox capability and more sites accessible to NOx/NH3 than FeVO4/TiO2 and thus presented higher activity in the presence of H2O [83].
Marberger et al. found that compared to the 2.3 wt.% V2O5/TiO2-WO3-SiO2 catalyst, 4.5 wt.% FeVO4/TiO2-WO3-SiO2 showed enhanced catalytic performance, and the decomposition of FeVO4 led to an activation effect due to the dispersal and migration of VOx species to the surface of the support material, which were the active species responsible for NH3-SCR [84]. Wu et al. proposed that the performance of FeVO4/TiO2-WO3-SiO2 was structure-sensitive, and that the pH values adopted during the preparation process had an influence on the nanostructure and morphology. The optimal catalyst showed 90% NOx conversion in the temperature range of 246-476 °C and strong tolerance to SO2 and H2O [85]. The doping of Er into FeVO4/TiWSi could block the transformation to rutile, inhibiting the deactivation of FeVO4 due to thermal aging and thus improving the activity after thermal treatment, and the Fe loading determined the medium/low-temperature catalytic activity. Fe0.5Er0.5VO4 was found to exhibit superior catalytic performance and stability [86].  [76]. Reproduced with permission from [76], copyright 2018, Elsevier.

Metal Vanadates
Although vanadia-based catalysts show superior catalytic performance at medium temperatures and have been widely applied in the deNO x process, the volatility and the toxicity of vanadium oxide at high temperature is a crucial problem. However, metal vanadates present better thermal stability than V 2 O 5 due to their higher melting points (e.g., 780, 850, 1030 and 1100 • C for copper vanadate, iron vanadate, manganese vanadate and cerium vanadate, respectively). Therefore, metal vanadates can be candidates for vanadia-based catalysts with great potential [81].
Iron vanadates (FeVO 4 ) have recently been extensively studied as active NH 3 -SCR catalysts. Abundant surface defects existed on a FeVO 4 /TiO 2 catalyst for the adsorption and activation of reactants, and the true active sites were FeVO 4 phase surface-enriched with VO x species. Thus, similar to V 2 O 5 /TiO 2 , FeVO 4 /TiO 2 showed superior catalytic performance and H 2 O/SO 2 tolerance [1]. Fe 0.75 V 0.25 O δ exhibited excellent SCR performance in the temperature range of 175-400 • C. It was found that the formation of amorphous FeVO 4 resulted from the incorporation of V into Fe 2 O 3 , and the apparent activation energy decreased due to the synergistic effect of FeVO 4 and Fe 2 O 3 improving the catalytic activity at low temperatures [82]. Due to the greater electronic inductive effect, Fe 2 V 4 O 13 /TiO 2 showed stronger redox capability and more sites accessible to NO x /NH 3 than FeVO 4 /TiO 2 and thus presented higher activity in the presence of H 2 O [83].
Marberger et al. found that compared to the 2.3 wt.% V 2 O 5 /TiO 2 -WO 3 -SiO 2 catalyst, 4.5 wt.% FeVO 4 /TiO 2 -WO 3 -SiO 2 showed enhanced catalytic performance, and the decomposition of FeVO 4 led to an activation effect due to the dispersal and migration of VO x species to the surface of the support material, which were the active species responsible for NH 3 -SCR [84]. Wu et al. proposed that the performance of FeVO 4 /TiO 2 -WO 3 -SiO 2 was structure-sensitive, and that the pH values adopted during the preparation process had an influence on the nanostructure and morphology. The optimal catalyst showed 90% NO x conversion in the temperature range of 246-476 • C and strong tolerance to SO 2 and H 2 O [85]. The doping of Er into FeVO 4 /TiWSi could block the transformation to rutile, inhibiting the deactivation of FeVO 4 due to thermal aging and thus improving the activity after thermal treatment, and the Fe loading determined the medium/low-temperature catalytic activity. Fe 0.5 Er 0.5 VO 4 was found to exhibit superior catalytic performance and stability [86].
The oxidation of NO to NO 2 was enhanced by the coexistence of Ce 4+ species stabilized as CeO 2 with bulk CeVO 4 . Aging treatments under wet atmosphere at 500 and 600 • C did not result in the sublimation and loss of vanadium [87]. The introduction of Zr into CeVO 4 to form a Ce 1−x Zr x VO 4 solid solution led to high activity, with a 125 • C light-off temperature and a wide temperature window of 150-375 • C by reason of the increased surface area, surface active oxygen species and acid sites of the catalysts [81]. Zr-CeVO 4 /TiO 2 -nanosheets showed better SCR performance, stability and tolerance to H 2 O/SO 2 than nanoparticles due to their more abundant Brønsted acid sites and active oxygen species [88]. A catalyst with Ce:V = 1:1 supported on sulfated zirconia showed high activity in the presence of SO 2 and potassium. The incorporation of vanadium led to the formation of CeVO 4 , preventing reaction between SO 2 and CeO 2 and maintaining the reactivity of active sites, thus enhancing the tolerance toward SO 2 [89]. The doping of Sn into CeVO 4 catalysts can broaden the temperature window and improve the resistance to SO 2 and H 2 O, mainly resulting from the large specific surface area (from 40.75 to 49.05 m 2 /g), strong interactions among vanadium, cerium and tin, and large numbers of oxygen vacancies and acid sites [90].
Compared to CuV 2 O 6 (Cu 1 /Ti), Cu 2 V 2 O 7 (Cu 2 /Ti), and Cu 3 V 2 O 8 (Cu 3 /Ti), Cu 5 V 2 O 10 (Cu 5 /Ti) presented optimal redox behavior and the largest number of acid sites, and therefore presented the greatest SCR performance at low temperatures [91]. The addition of Sb as a promoter can further improve the SCR activity over copper vanadate catalysts due to the improved redox properties [91]. SO 2 and O 2 can be used to modify Sb-promoted Cu 3 V 2 O 8 on TiO 2 , and 400 • C was an adequate functionalization temperature to promote the redox behavior and increase the number of Brønsted acid sites, resulting from the more abundant surface Cu(SO 4 ) or from a proper combination of the metal-bound SO Y 2− species with monodentate and bidentate binding configurations, to enhance the catalytic performance (as shown in Figure 6) [92].
Catalysts 2020, 10, x FOR PEER REVIEW 10 of 19 solution led to high activity, with a 125 °C light-off temperature and a wide temperature window of 150-375 °C by reason of the increased surface area, surface active oxygen species and acid sites of the catalysts [81]. Zr-CeVO4/TiO2-nanosheets showed better SCR performance, stability and tolerance to H2O/SO2 than nanoparticles due to their more abundant Brønsted acid sites and active oxygen species [88]. A catalyst with Ce:V = 1:1 supported on sulfated zirconia showed high activity in the presence of SO2 and potassium. The incorporation of vanadium led to the formation of CeVO4, preventing reaction between SO2 and CeO2 and maintaining the reactivity of active sites, thus enhancing the tolerance toward SO2 [89]. The doping of Sn into CeVO4 catalysts can broaden the temperature window and improve the resistance to SO2 and H2O, mainly resulting from the large specific surface area (from 40.75 to 49.05 m 2 /g), strong interactions among vanadium, cerium and tin, and large numbers of oxygen vacancies and acid sites [90]. Compared to CuV2O6 (Cu1/Ti), Cu2V2O7 (Cu2/Ti), and Cu3V2O8 (Cu3/Ti), Cu5V2O10 (Cu5/Ti) presented optimal redox behavior and the largest number of acid sites, and therefore presented the greatest SCR performance at low temperatures [91]. The addition of Sb as a promoter can further improve the SCR activity over copper vanadate catalysts due to the improved redox properties [91]. SO2 and O2 can be used to modify Sb-promoted Cu3V2O8 on TiO2, and 400 °C was an adequate functionalization temperature to promote the redox behavior and increase the number of Brønsted acid sites, resulting from the more abundant surface Cu(SO4) or from a proper combination of the metal-bound SOY 2− species with monodentate and bidentate binding configurations, to enhance the catalytic performance (as shown in Figure 6) [92].  [92]. Reproduced with permission from [92], copyright 2019, Elsevier.
In summary, metal vanadates can not only increase the low/medium-temperature NH3-SCR activity but also enhance the thermal stability [93,94]. The metal vanadates have aroused the interest of researchers due to their excellent chemical and physical properties. In future studies, the metal vanadate can obtain higher SCR activity through doping different elements or changing the In summary, metal vanadates can not only increase the low/medium-temperature NH 3 -SCR activity but also enhance the thermal stability [93,94]. The metal vanadates have aroused the interest of researchers due to their excellent chemical and physical properties. In future studies, the metal vanadate can obtain higher SCR activity through doping different elements or changing the preparation methods, etc.

Specific Structures
Tuning the structure and morphology of nanoparticle catalysts can enhance the NH 3 -SCR activity. Thus, researchers have made great efforts to regulate the structure and morphology of vanadia-based catalysts. A FeVO 4 nanorod/TiO 2 monolith catalyst exhibited a remarkably higher catalytic activity than a traditional FeVO 4 nanoparticle/TiO 2 catalyst, due to the predominantly exposed reactive planes (−210) contributing to the stronger redox capability and more abundant surface active oxygen species [95]. Multichannel TiO 2 nanotubes can provide abundant surface-adsorbed oxygen species and anchor active components efficiently, and a CeVTi-nanotube catalyst presented satisfactory NH 3 -SCR activity in the temperature range of 220-460 • C [96]. Bulk TiO 2 was treated by a hydrothermal reaction to obtain zeolitic microporous TiO 2 to support vanadia-based catalysts, and compared to conventional V 2 O 5 /TiO 2 , the catalyst not only maintained excellent SCR performance but also suppressed N 2 O emission significantly [97]. V/CeO 2 nanopolyhedrons showed better SCR activity than nanocubes and nanorods for their abundant surface acid sites and appropriate redox ability (as shown in Figure 7) [98]. TiO 2 with different crystal types was used to support 3 wt.%V 2 O 5 , and the catalytic activity was found to depend on the dominant crystal facets of the TiO 2 nanoparticles. V 2 O 5 loaded onto sheet-like TiO 2 , on which anatase (001) facets were preferentially exposed, presented a better catalytic performance than that loaded onto commercial TiO 2 (TiO 2 -P25) or octahedral TiO 2 with preferentially exposed anatase (101) facets, as the catalyst presented larger amounts of chemisorbed oxygen and better dispersion and stronger reducibility of V species [99]. A titania nanotube-encapsulated vanadium oxide catalyst exhibited a monolayer of isolated species where V 5+ was the dominant oxidation state and demonstrated a wide operating temperature window, which was due to the active species being well dispersed on the support surface [100].

Specific Structures
Tuning the structure and morphology of nanoparticle catalysts can enhance the NH3-SCR activity. Thus, researchers have made great efforts to regulate the structure and morphology of vanadia-based catalysts. A FeVO4 nanorod/TiO2 monolith catalyst exhibited a remarkably higher catalytic activity than a traditional FeVO4 nanoparticle/TiO2 catalyst, due to the predominantly exposed reactive planes (−210) contributing to the stronger redox capability and more abundant surface active oxygen species [95]. Multichannel TiO2 nanotubes can provide abundant surfaceadsorbed oxygen species and anchor active components efficiently, and a CeVTi-nanotube catalyst presented satisfactory NH3-SCR activity in the temperature range of 220-460 °C [96]. Bulk TiO2 was treated by a hydrothermal reaction to obtain zeolitic microporous TiO2 to support vanadia-based catalysts, and compared to conventional V2O5/TiO2, the catalyst not only maintained excellent SCR performance but also suppressed N2O emission significantly [97]. V/CeO2 nanopolyhedrons showed better SCR activity than nanocubes and nanorods for their abundant surface acid sites and appropriate redox ability (as shown in Figure 7) [98]. TiO2 with different crystal types was used to support 3 wt.%V2O5, and the catalytic activity was found to depend on the dominant crystal facets of the TiO2 nanoparticles. V2O5 loaded onto sheet-like TiO2, on which anatase (001) facets were preferentially exposed, presented a better catalytic performance than that loaded onto commercial TiO2 (TiO2-P25) or octahedral TiO2 with preferentially exposed anatase (101) facets, as the catalyst presented larger amounts of chemisorbed oxygen and better dispersion and stronger reducibility of V species [99]. A titania nanotube-encapsulated vanadium oxide catalyst exhibited a monolayer of isolated species where V 5+ was the dominant oxidation state and demonstrated a wide operating temperature window, which was due to the active species being well dispersed on the support surface [100]. Figure 7. NO conversion as a function of temperature in the NH3-SCR reaction [98]. Reproduced with permission from [98], copyright 2018, Elsevier.

Reaction Mechanism at Low Temperatures
Although vanadia-based NH3-SCR catalysts have been commercialized for decades, the reaction mechanism on the catalysts has still been a focus of research in recent years, including the identification of active sites and intermediates and elucidation of the interaction of the active sites with the reactants. The NH3-SCR reaction pathway proceeds as follows: the adsorbed NH3 species react either with adsorbed nitrites/nitrates (Langmuir−Hinshelwood mechanism, i.e., L-H) or directly with gaseous NO (Eley−Rideal mechanism, i.e., E-R) to generate intermediates that subsequently Figure 7. NO conversion as a function of temperature in the NH 3 -SCR reaction [98]. Reproduced with permission from [98], copyright 2018, Elsevier.

Reaction Mechanism at Low Temperatures
Although vanadia-based NH 3 -SCR catalysts have been commercialized for decades, the reaction mechanism on the catalysts has still been a focus of research in recent years, including the identification of active sites and intermediates and elucidation of the interaction of the active sites with the reactants. The NH 3 -SCR reaction pathway proceeds as follows: the adsorbed NH 3 species react either with adsorbed nitrites/nitrates (Langmuir−Hinshelwood mechanism, i.e., L-H) or directly with gaseous NO (Eley−Rideal mechanism, i.e., E-R) to generate intermediates that subsequently decompose to N 2 and H 2 O [5,101,102]. The acid sites and redox sites work together in the NH 3 -SCR reaction on the catalysts, and the close coupling of acid and redox sites is a design principle for SCR catalysts with excellent NO x purification efficiency [103].
In a debating issue about the active sites of NH 3 -SCR on VWTi catalysts, it has been reported before that both surface NH 4 + ,ads and NH 3,ads participated in the NH 3 -SCR reaction and the coverage of surface tungsten and vanadia oxide species, moisture, and temperature determined their relative population [104]. The large amount of surface NH 4 + ,ads intermediates present a lower specific SCR activity (TOF) than the minority surface NH 3,ads intermediates. The SCR reaction rate does not depend on the exposed Ti 4+ sites on the support. Marberger et al. [105] proposed that NO reacts predominantly with coordinated NH 3 adsorbed on Lewis acid sites consisting of isolated V 5+ which were reduced only in the coexistence of NH 3 and NO, and the reduction of V 5+ was accompanied by the formation of a nitrosamide intermediate at low temperature. Brønsted acid sites, serving as an NH 3 pool and hardly contributing to the NH 3 -SCR reaction, can replenish the Lewis acid sites. The rate-determining step is re-oxidation and regeneration of active Lewis acid sites.
The presence of H 2 O and changes in the redox state of vanadia-based catalysts led to a reversible change between Brønsted and Lewis acid sites. Under reducing conditions, the surface was enriched with Lewis acid sites because that ammonia could be coordinated on the reduced vanadium sites as supplementary of Lewis acid sites, while more abundant Brønsted acid sites on the V/Ti catalyst surface were generated under an oxidizing environment [106]. Brønsted acid sites are important at low temperatures, while Lewis acid sites dominate the overall reaction at high temperatures. Lewis acid sites can transform to Brønsted acid sites at temperatures above 300 • C through hydrogen migration [107]. The number of Lewis acid sites decreased and the amount of Brønsted acid sites increased with changes in the vanadium content. The increase of vanadium pentoxide loading resulted in the increase of the amount of V-OH in polymeric vanadyl and the decrease of the amount of isolated vanadyl (V=O) species [108].
Most current investigations of the NH 3 -SCR reaction process over V-based catalysts have mainly concentrated on isolated monomeric vanadyl species, while the polymerization of vanadium species and the coupling effects were not taken into consideration. In our previous study [109], DFT calculations were carried out to elucidate the differences between the NH 3 -SCR mechanisms taking place on polymeric and monomeric vanadyl species at atomic scale. The results showed that NH 3 adsorbed on surface Ti sites transfered a hydrogen atom to the vanadyl species firstly and then reacted with gaseous NO according to the Eley−Rideal mechanism. An NH 2 NO intermediate and a V-OH or V-OH 2 group were formed and then N 2 and H 2 O were generated from the decomposition of NH 2 NO. The consumed surface oxygen on the vanadyl species was replenished by gas-phase O 2 . The catalytic cycle was completed when V=O groups were regenerated (as shown in Figure 8).
For the monomeric vanadyl species, a VOOH intermediate was formed after the transfer of an H atom of the adsorbed NH 3 to the adsorbed O 2 on vanadyl species and rapidly transformed into O=V-OH. However, for polymeric vanadyl species, the thermal stability and lifetime of the VOOH intermediate were enhanced because a hydrogen bond was formed between VOOH and the adjacent V=O group. Therefore, on polymeric vanadyl species, the VOOH intermediate can easily react with NO. The overall reaction barrier of the catalytic cycle decreases and the reaction pathway for the regeneration of redox sites is shortened due to the coupling effect of polymeric vanadyl species. Therefore, the NH 3 -SCR reaction rate is significantly enhanced on polymeric species. Jaegers et al. clarified that the formation of oligomeric vanadia structures on supported V2O5-WO3/TiO2 was promoted by the unreactive surface tungsten oxide (WO3), revealing a 2-site mechanism due to the presence of a proportional relationship of SCR reaction rate to the square of surface VOx concentration. The enhancement of the NH3-SCR reaction from the incorporation of a promoter occurs via a structural effect generated by adjacent surface sites rather than an electronic effect [110].
The presence of H2O can have an influence on the reaction pathway for NH3-SCR at low temperatures. In the standard and fast SCR reactions, adsorbed NH3 reacts with nitrite species to generate N2 and H2O (nitrite path), while the reaction between gaseous NO and NH4NO3 formed from adsorbed ammonia species and surface nitrates to emit NO2, H2O and N2 (NH4NO3 path) does not occur at low temperatures without H2O, and in the presence of H2O both the "nitrite path" and the "NH4NO3 path" contribute to the NH3-SCR reaction [111].
For vanadia-based catalysts, different NH3-SCR reaction pathways occur on various vanadium species. The reaction mechanism can be more complicated over modified vanadia-based catalysts and is also related to the concentration of vanadium oxide and the reaction temperature. Therefore, a variety of modern characterization and computational techniques should be applied in order to have a better view of the NH3-SCR reaction mechanism, guiding the design of excellent deNOx catalysts.

Conclusions and Perspective
Commercial V2O5-WO3/TiO2 catalysts have been widely applied for abating nitrogen oxides from mobile and stationary sources. However, the poor low-temperature activity of vanadia-based catalysts restricts their broader application in the process of NOx purification. Therefore, this review summarizes the latest research progress on the improvement of deNOx performance over vanadiabased catalysts at low temperatures and the NH3-SCR reaction mechanism, and outlines opportunities and challenges for vanadia-based catalysts in the near future.
The NH3-SCR catalytic performance over vanadia-based catalysts at low temperatures is mainly determined by polymeric vanadyl species, while monomeric species presented lower catalytic activity. The NH3-SCR reaction needs acid sites and redox sites to work together. The good dispersion of sites with the same function and close coupling of sites with different functions are crucial for the design of excellent NH3-SCR catalysts. However, considering the complexity of the SCR reaction, the NH3-SCR reaction mechanisms and the structure−activity relationships still need to be investigated in-depth to gain an enhanced understanding and thus improve catalyst design theory. Furthermore, Jaegers et al. clarified that the formation of oligomeric vanadia structures on supported V 2 O 5 -WO 3 /TiO 2 was promoted by the unreactive surface tungsten oxide (WO 3 ), revealing a 2-site mechanism due to the presence of a proportional relationship of SCR reaction rate to the square of surface VO x concentration. The enhancement of the NH 3 -SCR reaction from the incorporation of a promoter occurs via a structural effect generated by adjacent surface sites rather than an electronic effect [110].
The presence of H 2 O can have an influence on the reaction pathway for NH 3 -SCR at low temperatures. In the standard and fast SCR reactions, adsorbed NH 3 reacts with nitrite species to generate N 2 and H 2 O (nitrite path), while the reaction between gaseous NO and NH 4 NO 3 formed from adsorbed ammonia species and surface nitrates to emit NO 2 , H 2 O and N 2 (NH 4 NO 3 path) does not occur at low temperatures without H 2 O, and in the presence of H 2 O both the "nitrite path" and the "NH 4 NO 3 path" contribute to the NH 3 -SCR reaction [111].
For vanadia-based catalysts, different NH 3 -SCR reaction pathways occur on various vanadium species. The reaction mechanism can be more complicated over modified vanadia-based catalysts and is also related to the concentration of vanadium oxide and the reaction temperature. Therefore, a variety of modern characterization and computational techniques should be applied in order to have a better view of the NH 3 -SCR reaction mechanism, guiding the design of excellent deNO x catalysts.

Conclusions and Perspective
Commercial V 2 O 5 -WO 3 /TiO 2 catalysts have been widely applied for abating nitrogen oxides from mobile and stationary sources. However, the poor low-temperature activity of vanadia-based catalysts restricts their broader application in the process of NO x purification. Therefore, this review summarizes the latest research progress on the improvement of deNO x performance over vanadia-based catalysts at low temperatures and the NH 3 -SCR reaction mechanism, and outlines opportunities and challenges for vanadia-based catalysts in the near future.
The NH 3 -SCR catalytic performance over vanadia-based catalysts at low temperatures is mainly determined by polymeric vanadyl species, while monomeric species presented lower catalytic activity. The NH 3 -SCR reaction needs acid sites and redox sites to work together. The good dispersion of sites with the same function and close coupling of sites with different functions are crucial for the design of excellent NH 3 -SCR catalysts. However, considering the complexity of the SCR reaction, the NH 3 -SCR reaction mechanisms and the structure−activity relationships still need to be investigated in-depth to gain an enhanced understanding and thus improve catalyst design theory. Furthermore, due to the multielement composition of the developed catalysts, the synergistic reaction mechanisms between different active components, additive and diverse supports need in-depth exploration.
The modification of vanadia-based catalysts with metal oxides or nonmetal elements can improve the low-temperature SCR activity effectively. Diverse supports have been used to improve the dispersion of vanadium oxide and the interaction between the supports and the active components. The utilization of different preparation methods and use of metal vanadates and specific structures can also promote the catalytic performance of vanadia-based catalysts at low temperatures. Though great progress has been made recently, the catalysts still face many challenges in practical application, such as alkali poisoning and halogen poisoning. We need to develop catalysts with better performance to deal with the practical problems with superior low-temperature activity, wide operation windows, strong mechanical and thermal stability, and excellent resistance to alkali, heavy-metal, SO 2 , and P/HCl poisoning. However, due to the volatility and toxicity of vanadium oxide, in the future catalysts with a low loading of vanadium or even no vanadium may be the main development trend in the field of NO x removal.

Conflicts of Interest:
The authors declare no conflict of interest.