Sulfur and Water Resistance of Mn-Based Catalysts for Low-Temperature Selective Catalytic Reduction of NO x : A Review

Selective catalytic reduction (SCR) with NH3 is the most efficient and economic flue gas denitrification technology developed to date. Due to its high low-temperature catalytic activity, Mn-based catalysts present a great prospect for application in SCR de-NOx at low temperatures. However, overcoming the poor resistance of Mn-based catalysts to H2O and SO2 poison is still a challenge. This paper reviews the recent progress on the H2O and SO2 resistance of Mn-based catalysts for the low-temperature SCR of NOx. Firstly, the poison mechanisms of H2O and SO2 are introduced in detail, respectively. Secondly, Mn-based catalysts are divided into three categories—single MnOx catalysts, Mn-based multi-metal oxide catalysts, and Mn-based supported catalysts—to review the research progress of Mn-based catalysts for H2O and SO2 resistance. Thirdly, several strategies to reduce the poisonous effects of H2O and SO2, such as metal modification, proper support, the combination of metal modification and support, the rational design of structure and morphology, are summarized. Finally, perspectives and future directions of Mn-based catalysts for the low-temperature SCR of NOx are proposed.


Introduction
Nitrogen oxides (NO x , x = 1,2) emitted from power plants and diesel engines are major air pollutants that can cause acid rain, photochemical smog, ozone depletion, and other severe environmental problems [1][2][3][4][5].Selective catalytic reduction (SCR) with NH 3 is the most efficient and economic method for post-NO x abatement, and V 2 O 5 -WO 3 (MoO 3 )/TiO 2 has been the most popular commercial SCR catalyst since the 1970s [6,7].However, V 2 O 5 -based catalysts have drawbacks, such as the toxicity of vanadium, SO 2 oxidation to SO 3 , over-oxidation of NH 3 to N 2 O, and a high working temperature [8].Because of the high working temperature window (300-400 • C), V 2 O 5 -based catalysts have to be placed upstream of the dust removal system and desulfurization units to avoid costly heating of the flue gas, where the catalysts are susceptible to deactivation by dust accumulation and SO 2 poison.Therefore, SCR catalysts that are environmentally friendly and can work at low temperatures (around 250 • C or even lower) urgently need to be developed [9][10][11].
Due to its high low-temperature catalytic activity, manganese oxide (MnO x ) has been intensively studied in recent decades [4, [12][13][14].Recently, our research group has also made a series of progress in the low-temperature SCR of NO with NH 3 over Mn-based catalysts [15][16][17][18][19][20][21].However, several problems, including thermal instability, narrow operation window, and poor resistance to H 2 O and SO 2 poison, remain.Among these drawbacks, the poor tolerance to H 2 O and X NO , X NO -U, and X NO -A represent NO x conversion of regular SCR reaction, NO x conversion under tolerance test and after tolerance test, respectively.

The Poisoning Mechanism of H 2 O
Water vapor has a negative effect on SCR reaction mainly because of the loss of available active sites on the surface of catalysts [43, [56][57][58].Even under dry conditions, the catalysts can be affected by the water vapor produced in the SCR reaction [56,59].It is believed that the poisonous effects of H 2 O can be generally divided into two aspects: reversible and irreversible deactivation.As shown in Figure 1, the competitive adsorption between H 2 O and NH 3 (or NO) is generally considered as the cause of reversible deactivation.Less adsorption of reacting agents on the surface leads to a decrease in NO x conversion.Fortunately, this effect generally disappears if H 2 O vapors are removed [60].The formation of additional surface hydroxyls (-OH) caused by dissociative adsorption and decomposition of H 2 O on the catalyst surface is likely to be the reason of irreversible deactivation, and this effect can occur at a relatively low temperature (below 200 • C) [61].Because of the good thermal stability of hydroxyls (in the 250-500 • C range), the NO x conversion cannot be recovered even shutting the H 2 O stream down at such a low temperature, thus resulting in an irreversible deactivation [62].

The Poisoning Mechanism of H2O
Water vapor has a negative effect on SCR reaction mainly because of the loss of available active sites on the surface of catalysts [43, [56][57][58].Even under dry conditions, the catalysts can be affected by the water vapor produced in the SCR reaction [56,59].It is believed that the poisonous effects of H2O can be generally divided into two aspects: reversible and irreversible deactivation.As shown in Figure 1, the competitive adsorption between H2O and NH3 (or NO) is generally considered as the cause of reversible deactivation.Less adsorption of reacting agents on the surface leads to a decrease in NOx conversion.Fortunately, this effect generally disappears if H2O vapors are removed [60].The formation of additional surface hydroxyls (-OH) caused by dissociative adsorption and decomposition of H2O on the catalyst surface is likely to be the reason of irreversible deactivation, and this effect can occur at a relatively low temperature (below 200 °C) [61].Because of the good thermal stability of hydroxyls (in the 250-500°C range), the NOx conversion cannot be recovered even shutting the H2O stream down at such a low temperature, thus resulting in an irreversible deactivation [62].

The Poisoning Mechanism of SO2
The presence of a significant amount of SO2 in flue gas has a critical influence on the catalyst for SCR reaction at low temperatures.The poisonous effects of SO2 can be generally classified in two categories: reversible and irreversible deactivation.For reversible deactivation, as displayed in Figure 1, SO2 is easily oxidized to SO3, which will easily react with NH3 to generate ammonia sulfate.The ammonia sulfates (NH4HSO4 and (NH4)2SO4) could cover on the active sites of catalysts and lead to a decrease in NOx conversion [63][64][65].In addition, the competitive adsorption between SO2 and NO on the active sites of the catalysts also contributes to the poisoning effect of SO2 on the SCR reaction [66].However, the reversible effect can be eliminated by washing with water or acid solution, or high temperature treatment of catalyst.For the irreversible case, as illustrated in Figure 1, SO2 (or SO3) can directly react with active components and form metallic sulfate, which leads to surface active site loss.Hence, the conversion decreases.Due to the high thermal stability of metallic sulfate, washing with water or high temperature treatment cannot bring much recovery of NOx conversion.When H2O and SO2 are introduced simultaneously, water will make the poisoning effect of SO2 severer, leading to a great decrease in the NOx conversion.Mn-based catalysts can work at low temperatures, which means that the SCR unit can be installed downstream of the dust removal system and desulfurization units.According to the Chinese Standard (GB13233-2011) and the EU Standard (BREF), residual SO2 in flue gas after desulfurization (35-150 ppm depending on different fuels and desulfurization methods) is allowed.The remaining SO2 and H2O in flue gas still have inevitable effects [59,67].Thus, developing Mn-based catalysts with good tolerance to water and sulfur is crucial for commercial applications.

The Poisoning Mechanism of SO 2
The presence of a significant amount of SO 2 in flue gas has a critical influence on the catalyst for SCR reaction at low temperatures.The poisonous effects of SO 2 can be generally classified in two categories: reversible and irreversible deactivation.For reversible deactivation, as displayed in Figure 1, SO 2 is easily oxidized to SO 3 , which will easily react with NH 3 to generate ammonia sulfate.The ammonia sulfates (NH 4 HSO 4 and (NH 4 ) 2 SO 4 ) could cover on the active sites of catalysts and lead to a decrease in NO x conversion [63][64][65].In addition, the competitive adsorption between SO 2 and NO on the active sites of the catalysts also contributes to the poisoning effect of SO 2 on the SCR reaction [66].However, the reversible effect can be eliminated by washing with water or acid solution, or high temperature treatment of catalyst.For the irreversible case, as illustrated in Figure 1, SO 2 (or SO 3 ) can directly react with active components and form metallic sulfate, which leads to surface active site loss.Hence, the conversion decreases.Due to the high thermal stability of metallic sulfate, washing with water or high temperature treatment cannot bring much recovery of NO x conversion.When H 2 O and SO 2 are introduced simultaneously, water will make the poisoning effect of SO 2 severer, leading to a great decrease in the NO x conversion.Mn-based catalysts can work at low temperatures, which means that the SCR unit can be installed downstream of the dust removal system and desulfurization units.According to the Chinese Standard (GB13233-2011) and the EU Standard (BREF), residual SO 2 in flue gas after desulfurization (35-150 ppm depending on different fuels and desulfurization methods) is allowed.The remaining SO 2 and H 2 O in flue gas still have inevitable effects [59,67].Thus, developing Mn-based catalysts with good tolerance to water and sulfur is crucial for commercial applications.

The Effect on N 2 Selectivity
The N 2 selectivity is another indicator for the evaluation of SCR catalysts, which is closely related to the yield of N 2 O.During NH 3 -SCR reaction, N 2 O can be produced together with N 2 , especially at high temperatures.However, for Mn-based catalysts, some N 2 O can also be formed even at low temperatures due to side reactions resulting from the oxidative properties of manganese oxides, whether the NH 3 -SCR reaction follows the Eley-Rideal (E-R) mechanism or the Langmuir-Hinshelwood (L-H) mechanism.It has been reported that H 2 O presents a positive effect on N 2 selectivity.Xiong et al. found that the formation of N 2 O over the Mn-Fe spinel and MnO x -CeO 2 catalysts following the E-R mechanism was notably restrained by H 2 O due to the decrease in the oxidation ability of MnO x , the suppression of NH 3 adsorption and the inhibition of the interface reaction.Furthermore, the generation of N 2 O through the L-H mechanism was completely suppressed by H 2 O due to the fact that the formation of NH 4 NO 3 was inhibited or the decomposition of generated NH 4 NO 3 was promoted [68,69].As regards the effect of SO 2 on N 2 selectivity, there is a lack of research in this area at the moment.

Single MnO x Catalysts
It has been proven that pure MnO x has terrific catalytic activity for the SCR of NO x with NH 3 but poor resistance to the poison of water and sulfur at low temperatures [4, [70][71][72].It has been reported that several factors, such as the preparation method and the specific surface area, have a great influence on the tolerance of MnO x .Tang et al. prepared a series of amorphous MnO x catalysts using three methods, the solid phase reaction method (SP), the co-precipitation method (CP), and the rheological phase reaction method (RP) [13], and they found that the MnO x (CP) exhibited the best sulfur and water resistance, but the MnO x (SP) presented a larger surface area (150 m 2 g −1 for MnO x (SP) and 96 m 2 g −1 for MnO x (CP)).As shown in Figure 2, the NO x conversion at 80 • C over MnO x (CP) decreased from 98 to 73% in 3 h.After turning off SO 2 and H 2 O, the NO x conversion was quickly restored to 90%.After the MnO x (CP) was heated for 1-2 h in N 2 at 280 • C, its activity was restored to the initial level.Kang et al. prepared two MnO x catalysts using sodium carbonate (SC) and ammonia (AH) as precipitants [23].They found that an MnO x -SC catalyst showed better SCR activity and great sulfur and water tolerance, and they ascribed this to the larger surface area (173.3 m 2 /g for MnO x -SC and 18.7 m 2 /g for MnO x -AH).As displayed in Figure 3, the NO x conversion over the MnO x -SC catalyst was decreased from 100 to 94% after both SO 2 (100 ppm) and H 2 O (11 vol %) were fed into the reaction system with aspace velocity of 50,000 h −1 , which is still very high de-NO x activity at 120 • C.Moreover, its activity was rapidly recovered to 100% after the supply of SO 2 and H 2 O was cut off.

Mn-Based Multi-Metal Oxide Catalysts
It has been widely demonstrated that mixing or doping MnOx with other metal oxides can greatly improve the water and sulfur resistance of single MnOx catalysts because of the synergistic effect between them [73].For Mn-based binary metal oxide catalysts, it has been reported that different dopants have different effects on the improvement of the tolerance to water and sulfur [74,75].For Mn-based ternary metal oxide catalysts, the modification of a small amount of a third element can enhance the synergistic effect resulted from the changes in both electronic and structural properties.

Mn-Based Multi-Metal Oxide Catalysts
It has been widely demonstrated that mixing or doping MnO x with other metal oxides can greatly improve the water and sulfur resistance of single MnO x catalysts because of the synergistic effect between them [73].For Mn-based binary metal oxide catalysts, it has been reported that different dopants have different effects on the improvement of the tolerance to water and sulfur [74,75].For Mn-based ternary metal oxide catalysts, the modification of a small amount of a third element can enhance the synergistic effect resulted from the changes in both electronic and structural properties.

Mn-Based Binary Metal Oxide Catalysts
Among the metal elements, cerium [24,25], chromium [10], iron [27,28,76], cobalt [29,30], copper [52], nickel [77], and several other elements have drawn the most attention as the mixture or dopant to construct binary metal oxide catalysts with MnO x .CeO 2 has been studied extensively due to its good characteristics, such as increasing surface acidity after SO 2 poisoning [78,79], high surface area [80,81], good dispersion of MnO x on the surface [45], and the redox shift between Ce 4+ and Ce 3+ .It should be noted that the shift between Ce 4+ and Ce 3+ can result in the formation of oxygen vacancies and anincrease in the chemisorbed oxygen on the surface of Mn-Ce binary metal oxide catalysts, which are helpful for the enhancement of water and sulfur resistance [25,82].Qi and Yang [24] reported that the Mn-Ce catalyst with a proper mole ratio (Mn/(Mn+Ce) = 0.3) showed great tolerance to water and sulfur.As illustrated in Figure 4, the NO conversion over an Mn-Ce catalyst gradually decreased from 100 to 95% within 4 h after 100 ppm SO 2 and 2.5% H 2 O were added to the reaction gas at 120 • C.Moreover, the NO conversion was restored after SO 2 + H 2 O was stopped.Liu et al. prepared an Mn-Ce catalyst by the surfactant-template method using hexadecyltrimethyl ammonium bromide (CTAB) as the template.The obtained Mn 5 Ce 5 (ST) catalyst presented a noticeable decrease in the catalytic activity for the NO x conversion at 100 • C in the presence of H 2 O and SO 2 (Figure 5), a slight inhibiting effect was observed from 150 to 200 • C, and the promoting effect was exhibited above 200 • C [25].Yao et al. successfully prepared a series of Mn/CeO 2 catalysts via impregnation using deionized water, anhydrous ethanol, acetic acid, and oxalic acid as a solvent and found that Mn/Ce-OA (oxalic acid) exhibited the best water and sulfur tolerance among all catalysts (Figure 6) [26].Chen et al. [10] found that the SO 2 tolerance of MnO x was dramatically enhanced by the introduction of Cr due to the formation of CrMn 1.5 O 4 .
presented a noticeable decrease in the catalytic activity for the NOx conversion at 100 °C in the presence of H2O and SO2 (Figure 5), a slight inhibiting effect was observed from 150 to 200 °C, and the promoting effect was exhibited above 200 °C [25].Yao et al. successfully prepared a series of Mn/CeO2 catalysts via impregnation using deionized water, anhydrous ethanol, acetic acid, and oxalic acid as a solvent and found that Mn/Ce-OA (oxalic acid) exhibited the best water and sulfur tolerance among all catalysts (Figure 6) [26].Chen et al. [10] found that the SO2 tolerance of MnOx was dramatically enhanced by the introduction of Cr due to the formation of CrMn1.5O4.presented a noticeable decrease in the catalytic activity for the NOx conversion at 100 °C in the presence of H2O and SO2 (Figure 5), a slight inhibiting effect was observed from 150 to 200 °C, and the promoting effect was exhibited above 200 °C [25].Yao et al. successfully prepared a series of Mn/CeO2 catalysts via impregnation using deionized water, anhydrous ethanol, acetic acid, and oxalic acid as a solvent and found that Mn/Ce-OA (oxalic acid) exhibited the best water and sulfur tolerance among all catalysts (Figure 6) [26].Chen et al. [10] found that the SO2 tolerance of MnOx was dramatically enhanced by the introduction of Cr due to the formation of CrMn1.5O4.Iron is another potential element that has been demonstrated to play a positive role in the sulfur and water tolerance of Mn-based catalysts.Long et al. [27] observed that Fe-Mn-based transition metal oxides were resistant to H2O and SO2 at 140-180 °C (Figure 7).Chen et al. [28] found that the NO conversion over an Fe-Mn mixed oxide catalyst decreased slightly from 100 to 87% in 4 h at 120 °C in the presence of 5% H2O and 100 ppm SO2, which could be restored to 93% after the stopping of both SO2 and H2O (Figure 8).They attributed the enhanced resistance to the Iron is another potential element that has been demonstrated to play a positive role in the sulfur and water tolerance of Mn-based catalysts.Long et al. [27] observed that Fe-Mn-based transition metal oxides were resistant to H 2 O and SO 2 at 140-180 • C (Figure 7).Chen et al. [28] found that the NO conversion over an Fe-Mn mixed oxide catalyst decreased slightly from 100 to 87% in 4 h at 120 • C in the presence of 5% H 2 O and 100 ppm SO 2 , which could be restored to 93% after the stopping of both SO 2 and H 2 O (Figure 8).They attributed the enhanced resistance to the formed Fe 3 Mn 3 O 8 phase in Fe-Mn mixed oxides.Yang et al. [76] prepared an Mn-Fe spinel catalyst and found that the NO conversion over Mn-Fe spinel catalyst decreased from 100 to about 60% after the addition of H 2 O and SO 2 for 100 min, and the NO conversion could be recovered to the original level after washing catalyst with water.Iron is another potential element that has been demonstrated to play a positive role in the sulfur and water tolerance of Mn-based catalysts.Long et al. [27] observed that Fe-Mn-based transition metal oxides were resistant to H2O and SO2 at 140-180 °C (Figure 7).Chen et al. [28] found that the NO conversion over an Fe-Mn mixed oxide catalyst decreased slightly from 100 to 87% in 4 h at 120 °C in the presence of 5% H2O and 100 ppm SO2, which could be restored to 93% after the stopping of both SO2 and H2O (Figure 8).They attributed the enhanced resistance to the formed Fe3Mn3O8 phase in Fe-Mn mixed oxides.Yang et al. [76] prepared an Mn-Fe spinel catalyst and found that the NO conversion over Mn-Fe spinel catalyst decreased from 100 to about 60% after the addition of H2O and SO2 for 100 min, and the NO conversion could be recovered to the original level after washing catalyst with water.It has been reported that cobalt also presents a positive role on the tolerance of Mn-based catalysts to sulfur and water.Zhang et al. [29] found that the MnxCo3-xO4 nanocage catalyst exhibited decent SO2 tolerance due to its hierarchically porous structure, abundant active sites, and strong interaction between Mn and Co oxides (Figure 9).Qiu et al. prepared a mesoporous 3D-MnCo2O4 catalyst, which exhibited great SCR activity and good tolerance to sulfur and water [30,31].As illustrated in Figure 10, the NO conversion over MnCo2O4 was maintained at 86% in the presence of 5 vol% H2O and 100 ppm SO2.Futhermore, the NO conversion could be recovered to 93% after the supply of H2O and SO2 was cut off.It has been reported that cobalt also presents a positive role on the tolerance of Mn-based catalysts to sulfur and water.Zhang et al. [29] found that the Mn x Co 3−x O 4 nanocage catalyst exhibited decent SO 2 tolerance due to its hierarchically porous structure, abundant active sites, and strong interaction between Mn and Co oxides (Figure 9).Qiu et al. prepared a mesoporous 3D-MnCo 2 O 4 catalyst, which exhibited great SCR activity and good tolerance to sulfur and water [30,31].As illustrated in Figure 10, the NO conversion over MnCo 2 O 4 was maintained at 86% in the presence of 5 vol % H 2 O and 100 ppm SO 2 .Futhermore, the NO conversion could be recovered to 93% after the supply of H 2 O and SO 2 was cut off.It has been reported that cobalt also presents a positive role on the tolerance of Mn-based catalysts to sulfur and water.Zhang et al. [29] found that the MnxCo3-xO4 nanocage catalyst exhibited decent SO2 tolerance due to its hierarchically porous structure, abundant active sites, and strong interaction between Mn and Co oxides (Figure 9).Qiu et al. prepared a mesoporous 3D-MnCo2O4 catalyst, which exhibited great SCR activity and good tolerance to sulfur and water [30,31].As illustrated in Figure 10, the NO conversion over MnCo2O4 was maintained at 86% in the presence of 5 vol% H2O and 100 ppm SO2.Futhermore, the NO conversion could be recovered to 93% after the supply of H2O and SO2 was cut off.Copper also presents a positive effect on the tolerance of MnOx-based catalysts to water and sulfur.Kang et al. reported a Cu-Mn mixed oxide catalyst, which exhibited good tolerance to water and sulfur [32].When 100 ppm SO2 and 11 vol % H2O were added to the reaction gas, the NOx conversion over Cu-Mn oxides decreased from 95 to 64% at 125 °C after 4 h, and the NOx conversion was gradually recovered after stopping the supply of SO2 and H2O.
Recently, several rare earth elements have been demonstrated to promote the enhanced tolerance to sulfur and water.For instance, Meng et al. developed a Sm-modified MnOx catalyst [33],  Copper also presents a positive effect on the tolerance of MnO x -based catalysts to water and sulfur.Kang et al. reported a Cu-Mn mixed oxide catalyst, which exhibited good tolerance to water and sulfur [32].When 100 ppm SO 2 and 11 vol % H 2 O were added to the reaction gas, the NO x conversion over Cu-Mn oxides decreased from 95 to 64% at 125 • C after 4 h, and the NO x conversion was gradually recovered after stopping the supply of SO 2 and H 2 O.
Recently, several rare earth elements have been demonstrated to promote the enhanced tolerance to sulfur and water.For instance, Meng et al. developed a Sm-modified MnO x catalyst [33], and found that a proper Sm modification (Sm/Mn = 1:10) enhanced the sulfur and water tolerance of MnO x .As shown in Figure 11, the NO x conversion over the Sm-Mn-0.1 catalyst could be maintained at about 91% at 100 • C when 2% H 2 O and 100 ppm SO 2 were added into the feed gas, and the NO x conversion was recovered to 97% after both H 2 O and SO 2 were removed from the feed gas.Sun et al. prepared a Eu-modified MnO x catalyst [34].They tested the sulfur and water resistance of this catalyst at a higher temperature (350 • C), instead of a low temperature, such as 100 • C. The deactivation effect of SO 2 and H 2 O on MnEuO x -0.1 was weak, and the NO x conversion over MnEuO x -0.1 kept over 90% in the presence of 100 ppm SO 2 and 5% H 2 O. Furthermore, the NO x conversion nearly recovered its original level after the supply of SO 2 and H 2 O was cut off (Figure 12).

3.2.2.Mn-Based Ternary Metal Oxide Catalysts
It has been reported that the introduction of a small amount of a third element can enhance the tolerance of Mn-based binary metal oxide catalysts to H2O and SO2.Qi et al. successfully prepared Mn-Fe-Ce mixed oxides that performed well under 100 ppm SO2 and 2.5% H2O condition (Figure 13) [35].The NO conversion over Mn-Fe-Ce decreased from 98 to 95% in 3 h in the presence of SO2 and H2O and then restored quickly to its original level after the supply of SO2 and H2O was cut off.France et al. developed a CeFeMnOx catalyst that exhibited excellent sulfur and water resistance at a

3.2.2.Mn-Based Ternary Metal Oxide Catalysts
It has been reported that the introduction of a small amount of a third element can enhance the tolerance of Mn-based binary metal oxide catalysts to H2O and SO2.Qi et al. successfully prepared Mn-Fe-Ce mixed oxides that performed well under 100 ppm SO2 and 2.5% H2O condition (Figure 13) [35].The NO conversion over Mn-Fe-Ce decreased from 98 to 95% in 3 h in the presence of SO2 and H2O and then restored quickly to its original level after the supply of SO2 and H2O was cut off.

Mn-Based Ternary Metal Oxide Catalysts
It has been reported that the introduction of a small amount of a third element can enhance the tolerance of Mn-based binary metal oxide catalysts to H 2 O and SO 2 .Qi et al. successfully prepared Mn-Fe-Ce mixed oxides that performed well under 100 ppm SO 2 and 2.5% H 2 O condition (Figure 13) [35].The NO conversion over Mn-Fe-Ce decreased from 98 to 95% in 3 h in the presence of SO 2 and H 2 O and then restored quickly to its original level after the supply of SO 2 and H 2 O was cut off.France et al. developed a CeFeMnO x catalyst that exhibited excellent sulfur and water resistance at a low temperature [36].As presented in Figure 14, the NO conversion over this catalyst only decreased from 100 to 75% when water and sulfur were introduced, and then recovered to 95% after the supply of H 2 O and SO 2 was cut off.Chang et al. found that Sn doping could enhance the sulfur resistance of Mn-Ce catalysts because SO 2 was easier to react with Ce on the surface instead of forming ammonia sulfate; meanwhile, more surface acid sites were introduced due to Sn doping [37,38].As shown in Figure 15, the NO conversion was kept at around 70% in the presence of SO 2 and H 2 O and recovered to almost the original level within less than 3 hafter SO 2 and H 2 O were removed.Gao et al. [39] found that the SCR pathways over MnO x -CeO 2 catalyst are based on the adsorption, activation, and reaction of monodentate nitrite species and coordinated NH 3 species, and these species are significantly inhibited by SO 2 through competitive adsorption.In contrast, over Co-and Ni-doped MnO x -CeO 2 catalysts, the primary NO x adsorbed species are in the form of bidentate nitrate without the influence by SO 2 .The NO conversion over Co-and Ni-doped MnO x -CeO 2 catalysts decreased 20% after 150 ppm SO 2 and 10% H 2 O were introduced, and recovered after the supply of SO 2 and H 2 O was cut off (Figure 16).Liu et al. successfully prepared WO 3 promoted Mn-Zr mixed oxide catalyst [40].As shown in Figure 17, the NO x conversion over MnWZr catalyst was maintained above 90% in the presence of 50 ppm SO 2 and 5% H 2 O, and the conversion quickly recovered after the supply of SO 2 and H 2 O was cut off.over Co-and Ni-doped MnOx-CeO2 catalysts, the primary NOx adsorbed species are in the form of bidentate nitrate without the influence by SO2.The NO conversion over Co-and Ni-doped MnOx-CeO2 catalysts decreased 20% after 150 ppm SO2and 10% H2O were introduced, and recovered after the supply of SO2 and H2O was cut off (Figure 16).Liu et al. successfully prepared WO3 promoted Mn-Zr mixed oxide catalyst [40].As shown in Figure 17, the NOx conversion over MnWZr catalyst was maintained above 90% in the presence of 50 ppm SO2 and 5% H2O, and the conversion quickly recovered after the supply of SO2 and H2O was cut off.

Supported Mn-Based Catalysts
Supports play an important role in NH3-SCR reaction.Proper supports not only can provide a huge surface to disperse the active components and prevent the formation of large crystalline particles but can also affect the sulfur and water tolerance.To date, various materials, such as TiO2, carbon materials, and Al2O3, have been explored as supports to load Mn-based catalysts.

TiO2 Supported Mn-Based Catalysts
TiO2 is known to be more resistant to sulfur poisoning because of the stability of sulfates on the TiO2 surface is weaker than that on other oxides [59], which made TiO2 an ideal support for the loading of Mn-based catalysts.
Qi and Yang [41] prepared a series of MnTi and FeMnTi catalysts.As shown in Figure 18, the NO conversion over Fe-Mn/TiO2 was decreased from 100 to 90% within 5 h at 150 °C after 100 ppm SO2 and 2.5% H2O were added.After the supply of SO2 and H2Owas cut off, the NO conversion recovered to 100% again quickly.Yang et al. investigated the sulfur and water tolerance of Fe-Ti spinel supported MnOx catalyst [42].As shown in Figure 19, the NOx conversion at 200 °C gradually decreased from 100 to 83% and then kept unchanged after 8% of H2O and 60 ppm of SO2 were introduced.After the supply H2O and SO2 was shut off, the NOx conversion rapidly recovered to 100%.Wu et al. found that the sulfur resistance of Mn/TiO2 can be greatly improved by Ce

Supported Mn-Based Catalysts
Supports play an important role in NH 3 -SCR reaction.Proper supports not only can provide a huge surface to disperse the active components and prevent the formation of large crystalline particles but can also affect the sulfur and water tolerance.To date, various materials, such as TiO 2 , carbon materials, and Al 2 O 3 , have been explored as supports to load Mn-based catalysts.

TiO 2 Supported Mn-Based Catalysts
TiO 2 is known to be more resistant to sulfur poisoning because of the stability of sulfates on the TiO 2 surface is weaker than that on other oxides [59], which made TiO 2 an ideal support for the loading of Mn-based catalysts.
Qi and Yang [41] prepared a series of MnTi and FeMnTi catalysts.As shown in Figure 18, the NO conversion over Fe-Mn/TiO 2 was decreased from 100 to 90% within 5 h at 150 • C after 100 ppm SO 2 and 2.5% H 2 O were added.After the supply of SO 2 and H 2 O was cut off, the NO conversion recovered to 100% again quickly.Yang et al. investigated the sulfur and water tolerance of Fe-Ti spinel supported MnO x catalyst [42].As shown in Figure 19, the NO x conversion at 200 • C gradually decreased from 100 to 83% and then kept unchanged after 8% of H 2 O and 60 ppm of SO 2 were introduced.After the supply H 2 O and SO 2 was shut off, the NO x conversion rapidly recovered to 100%.Wu et al. found that the sulfur resistance of Mn/TiO 2 can be greatly improved by Ce addition [9].As displayed in Figure 20, SO 2 presented an obvious poisonous effect on SCR activity of Mn/TiO 2 at low temperatures because the NO conversion over the MnTi catalyst decreased from 93 to 30% in the presence of SO 2 within 6.5 h, while the NO conversion over MnCeTi still maintained at about 84% under the same conditions.As shown in Figure 21, the surface of fresh catalysts was smooth and uniform (Figure 21A,C).After the catalyst was poisoned with 100 ppm SO 2 for 24 h, the significant agglomeration and deposition could be observed from the surface of MnTi-S (Figure 21B), while only a few deposited particles (no agglomeration) appeared on the surface of MnCeTi-S (Figure 21D).Yu et al. [80] developed a mesoporous MnO 2 -Fe 2 O 3 -CeO 2 /TiO 2 catalyst.The NO conversion over this catalyst was stable at 80% under astream of SO 2 .Shen et al. [43] found that the addition of proper iron enhanced the tolerance of TiO 2 -supported Mn-Ce catalyst to water and sulfur.As exhibited in Figure 22, Fe(0.15)-Mn-Ce/TiO 2 showed higher resistance under 3 vol % H 2 O and 0.01 vol % SO 2 and still provided 83.8% NO conversion over afurther 5 h, an improvement over the Mn-Ce/TiO 2 catalyst.Shen et al. found that titanium-pillared clays (Ti-PILCs) presented advantages in sulfur tolerance over traditional TiO 2 supports [44].It can be seen from Figure 23 that the NO conversion was stable at around 90% without any obvious decrease in the presence of 3 vol % H 2 O and 0.01 vol % SO 2 , suggesting that Mn-CeO x /Ti-PILC(S) possessed good resistance to H 2 O and SO 2 .Lee et al. prepared a series of Mn/Ce-TiO 2 catalysts and found that Mn(20)/Ce(4)-TiO 2 showed good H 2 O and SO 2 tolerance [45].As shown in Figure 24, the NO conversion decreased to 60% and it recovered to almost the original level when the SO 2 supply was shut off.Park et al. [83] prepared Mn/Ti catalysts via chemical vapor condensation (CVC) method and claimed that this Mn/Ti not only showed higher activity at low temperature but also exhibited better tolerance to water and sulfur.Only a small NO conversion decrease from 70 to 58% was found under 200 ppm of SO 2 in 250 min at 100 • C. Liu et al. [46] prepared an Mn-Ce-Ti catalyst using the hydrothermal method, and the NO x conversion over the Mn 0. possessed good resistance to H2O and SO2. Lee et al. prepared a series of Mn/Ce-TiO2 catalysts and found that Mn(20)/Ce(4)-TiO2 showed good H2O and SO2 tolerance [45].As shown in Figure 24, the NO conversion decreased to 60% and it recovered to almost the original level when the SO2 supply was shut off.Park et al. [83] prepared Mn/Ti catalysts via chemical vapor condensation (CVC) method and claimed that this Mn/Ti not only showed higher activity at low temperature but also exhibited better tolerance to water and sulfur.Only a small NO conversion decrease from 70 to 58% was found under 200 ppm of SO2 in 250 min at 100 °C.Liu et al. [46] prepared an Mn-Ce-Ti catalyst using the hydrothermal method, and the NOx conversion over the Mn0.2Ce0.1Ti0.7Oxcatalyst under H2O and SO2 was further investigated at 200 °C.As shown in Figure 25, the introduction of H2O and SO2 induced a slight decrease in NOx conversion.After H2O and SO2 were excluded from the reactant feed, the NOx conversion completely recovered.

Carbon Materials Supported Mn-Based Catalysts
Carbon materials, such as activated carbon (AC), activated carbon fiber (ACF), carbon nanotube (CNT), and graphene (GE), have been widely studied as substrates for supporting low-temperature SCR catalysts due to their high specific surface area, unique pore structure, excellent dispersion of active components, and chemical stability [70,[84][85][86].Among these carbon materials, CNT and GE have been considered as good supports that can enhance the tolerance of H2O and SO2.
Zhang et al. prepared a novel MnCe@CNTs-R catalyst, which exhibited great tolerance to 100 ppm SO2 and 4 vol % H2O due to the good dispersion degree of the active components on the surface of CNTs [48].The coexistence of SO2 and H2O induced a 13% NO conversion decrease and the NO conversion was recovered to 90% after the supply of SO2 and H2O was cut off.Cai et al. designed a multi-shell Fe2O3@MnOx@CNTs catalyst and found that the Fe2O3 shell effectively suppressed the formation of the surface sulfate species, which led to a good tolerance to H2O and SO2 (Figure 26) [49].Lu et al. successfully synthesized a series of TiO2-graphene-supported Mn and Mn-Ce catalysts with good tolerance to H2O and SO2 due to the well dispersed Mn component (Figure 27) [50,51].Wang et al. investigated the effect of SO2 on activated carbon honeycomb (ACH)-supported MnOx and CeO2-MnOx catalysts, and the S 2p XPS results are displayed in Figure 28.The peak intensity of Mn/ACH was much higher than that of CeMn/ACH, indicating that Ce doping on ACH had an inhibition of sulfates loading [81].

Carbon Materials Supported Mn-Based Catalysts
Carbon materials, such as activated carbon (AC), activated carbon fiber (ACF), carbon nanotube (CNT), and graphene (GE), have been widely studied as substrates for supporting low-temperature SCR catalysts due to their high specific surface area, unique pore structure, excellent dispersion of active components, and chemical stability [70,[84][85][86].Among these carbon materials, CNT and GE have been considered as good supports that can enhance the tolerance of H 2 O and SO 2 .
Zhang et al. prepared a novel MnCe@CNTs-R catalyst, which exhibited great tolerance to 100 ppm SO 2 and 4 vol % H 2 O due to the good dispersion degree of the active components on the surface of CNTs [48].The coexistence of SO 2 and H 2 O induced a 13% NO conversion decrease and the NO conversion was recovered to 90% after the supply of SO 2 and H 2 O was cut off.Cai et al. designed a multi-shell Fe 2 O 3 @MnO x @CNTs catalyst and found that the Fe 2 O 3 shell effectively suppressed the formation of the surface sulfate species, which led to a good tolerance to H 2 O and SO 2 (Figure 26) [49].Lu et al. successfully synthesized a series of TiO 2 -graphene-supported Mn and Mn-Ce catalysts with good tolerance to H 2 O and SO 2 due to the well dispersed Mn component (Figure 27) [50,51].Wang et al. investigated the effect of SO 2 on activated carbon honeycomb (ACH)-supported MnO x and CeO 2 -MnO x catalysts, and the S 2p XPS results are displayed in Figure 28.The peak intensity of Mn/ACH was much higher than that of CeMn/ACH, indicating that Ce doping on ACH had an inhibition of sulfates loading [81].(Reproduced with permission from Reference [49], Copyright 2016, The Royal Society of Chemistry.).Mn-Fe catalyst and found that its SCR activity was suppressed gradually in the presence of SO 2 and H 2 O, and the inhibitory effect was relieved after heating treatment [88].
results showed that the resistance ability was decreased in the following order: MnOx/Ce0.5Zr0.5O2>MnOx/Al2O3 >MnOx/CeO2 >MnOx/TiO2 >MnOx/ZrO2, and they ascribed the excellent toleranceof MnOx/Ce0.5Zr0.5O2 to the combination of the advantages of the two supports (ZrO2 and CeO2) (Figure 30) [52].Huang et al. prepared a mesoporous silica-supported Mn-Fe catalyst and found that its SCR activity was suppressed gradually in the presence of SO2 and H2O, and the inhibitory effect was relieved after heating treatment [88].

Strategies to Reduce the Poisoning Effect
Although there are many factors that affect the water and sulfur tolerance of Mn-based catalysts, such as the preparation method, the reaction temperature, the gas hourly space velocity (GHSV), and the morphology, structure, and surface area of the catalyst, deactivation can be attributedto three main causes: (1) the competitive adsorption between SO2 and NO, H2O, and NH3 on the active sites, (2) the blocking effect of the NH4HSO4 and (NH4)2SO4 formed on the surface results showed that the resistance ability was decreased in the following order: MnOx/Ce0.5Zr0.5O2>MnOx/Al2O3 >MnOx/CeO2 >MnOx/TiO2 >MnOx/ZrO2, and they ascribed the excellent toleranceof MnOx/Ce0.5Zr0.5O2 to the combination of the advantages of the two supports (ZrO2 and CeO2) (Figure 30) [52].Huang et al. prepared a mesoporous silica-supported Mn-Fe catalyst and found that its SCR activity was suppressed gradually in the presence of SO2 and H2O, and the inhibitory effect was relieved after heating treatment [88].

Strategies to Reduce the Poisoning Effect
Although there are many factors that affect the water and sulfur tolerance of Mn-based catalysts, such as the preparation method, the reaction temperature, the gas hourly space velocity (GHSV), and the morphology, structure, and surface area of the catalyst, deactivation can be attributedto three main causes: (1) the competitive adsorption between SO2 and NO, H2O, and NH3 on the active sites, (2) the blocking effect of the NH4HSO4 and (NH4)2SO4 formed on the surface

Strategies to Reduce the Poisoning Effect
Although there are many factors that affect the water and sulfur tolerance of Mn-based catalysts, such as the preparation method, the reaction temperature, the gas hourly space velocity (GHSV), and the morphology, structure, and surface area of the catalyst, deactivation can be attributedto three main causes: (1) the competitive adsorption between SO 2 and NO, H 2 O, and NH 3 on the active sites, (2) the blocking effect of the NH 4 HSO 4 and (NH 4 ) 2 SO 4 formed on the surface active sites, and (3) the formation of metallic sulfate, which reduces the active sites on the surface.Hence, suppressing the three negative effects is the key to enhancing resistance against H 2 O and SO 2 .To date, many strategies have been taken to reduce the poisoning effect on Mn-based catalysts.

Metal Modification
Metal modification or doping is a common solution to the problem.Most transition metals have been used as dopants to modify Mn-based catalysts for good resistance to SO 2 and H 2 O. Cerium has been fully studied, and the mechanism has been uncovered.Cerium reacts more sensitively with SO 2 , so the formation of NH 4 HSO 4 and (NH 4 ) 2 SO 4 is reduced on the surface of Ce-modified Mn-based catalysts [36,79,89].Furthermore, metallic sulfates formed by cerium and SO 2 are relatively stable and can provide surface acid sites to enhance the adsorption of NH 3 and to inhibit the catalytic oxidization of NH 3 at the same time, thus promoting SCR reactions in the presence of SO 2 and H 2 O [68,78,90].Liu et al. confirmed, using density functional theory, that Ce isable to inhibit the formation of ammonia sulfate on the surface of catalysts, which is believed to be a key factor in improving tolerance [79].It was also reported that iron is capable of decreasing the formation rate of sulfate species, thus promoting tolerance [41,91].Furthermore, several reports have shown that the doping of a third metal, such as Sn and W, into Mn-based catalysts can further improve resistance to SO 2 and H 2 O [38,92,93].Zhang et al. found that resistance to SO 2 and H 2 O over the W-modified SnMnCeO x catalysts, in comparison with unmodified SnMnCeO x , was further improved (Figure 31) [53].They attributed this improvement to the introduction of WO x species, which prevented the formation of (NH 4 ) 2 SO 4 on the catalyst and blocked the interactions between Mn n+ , SO 4 2− , and gaseous SO 3 [37].Rare earths have drawn an increasing amount of attention recently, and Sm and Eu doping have been shown to have a positive influence on the tolerance of SO 2 and H 2 O [33,34].However, the mechanism of SO 2 and H 2 O resistance still needs to be uncovered.
Metal modification or doping is a common solution to the problem.Most transition metals have been used as dopants to modify Mn-based catalysts for good resistance to SO2 and H2O.Cerium has been fully studied, and the mechanism has been uncovered.Cerium reacts more sensitively with SO2, so the formation of NH4HSO4 and (NH4)2SO4 is reduced on the surface of Ce-modified Mn-based catalysts [36,79,89].Furthermore, metallic sulfates formed by cerium and SO2 are relatively stable and can provide surface acid sites to enhance the adsorption of NH3 and to inhibit the catalytic oxidization of NH3 at the same time, thus promoting SCR reactions in the presence of SO2 and H2O [68,78,90].Liu et al. confirmed, using density functional theory, that Ce isable to inhibit the formation of ammonia sulfate on the surface of catalysts, which is believed to be a key factor in improving tolerance [79].It was also reported that iron is capable of decreasing the formation rate of sulfate species, thus promoting tolerance [41,91].Furthermore, several reports have shown that the doping of a third metal, such as Sn and W, into Mn-based catalysts can further improve resistance to SO2 and H2O [38,92,93].Zhang et al. found that resistance to SO2 and H2O over the W-modified SnMnCeOx catalysts, in comparison with unmodified SnMnCeOx, was further improved (Figure 31) [53].They attributed this improvement to the introduction of WOx species, which prevented the formation of (NH4)2SO4 on the catalyst and blocked the interactions between Mn n+ , SO4 2− , and gaseous SO3 [37].Rare earths have drawn an increasing amount of attention recently, and Sm and Eu doping have been shown to have a positive influence on the tolerance of SO2 and H2O [33,34].However, the mechanism of SO2 and H2O resistance still needs to be uncovered.

Proper Support
It is well believed that loading Mn-based SCR catalysts on a suitable support is an effective measure to enhance the tolerance to SO2 and H2O [60,94] because of the high thermal and mechanical stability, large surface area, and highly dispersed active sites.Furthermore, the interaction between support and active components exhibits positive effects on the tolerance to SO2 and H2O [95].Therefore, it is very important for Mn-based catalysts to choose an appropriate support.Among several supports, TiO2, porous carbon material, and CNTs are considered to be

Proper Support
It is well believed that loading Mn-based SCR catalysts on a suitable support is an effective measure to enhance the tolerance to SO 2 and H 2 O [60,94] because of the high thermal and mechanical stability, large surface area, and highly dispersed active sites.Furthermore, the interaction between support and active components exhibits positive effects on the tolerance to SO 2 and H 2 O [95].Therefore, it is very important for Mn-based catalysts to choose an appropriate support.Among several supports, TiO 2 , porous carbon material, and CNTs are considered to be good options.It has been widely reported that TiO 2 can provide a higher specific surface area [58], a higher surface acidity [96], and a good dispersion of active components, all of which effectively enhance SO 2 and H 2 O resistance [97].Gao et al. reported a novel nanocomposite of MnO x nanoparticles supported on three-dimensionally ordered macroporous carbon (MnO x /3DOMC).They found that this novel catalyst exhibited good water and sulfur tolerance (Figure 32) [54].As a special ordered carbon material with unique nanostructure and electronic properties, carbon nanotubes (CNTs) have been reported to be an interesting support for SCR catalysts [98,99].Zhang et al. proved that active components were well dispersed on the surface of the support such that the blocking effect caused by NH 4 HSO 4 and (NH 4 ) 2 SO 4 was reduced [48].
enhance SO2 and H2O resistance [97].Gao et al. reported a novel nanocomposite of MnOx nanoparticles supported on three-dimensionally ordered macroporous carbon (MnOx/3DOMC).They found that this novel catalyst exhibited good water and sulfur tolerance (Figure 32) [54].As a special ordered carbon material with unique nanostructure and electronic properties, carbon nanotubes (CNTs) have been reported to be an interesting support for SCR catalysts [98,99].Zhang et al. proved that active components were well dispersed on the surface of the support such that the blocking effect caused by NH4HSO4 and (NH4)2SO4 was reduced [48].

Combination of Metal Modification and Support
The combination of metal modification and support is considered to be a good way of enhancing the water and sulfur tolerance of Mn-based catalysts due to the advantages of both strategies.Compared with non-supported mixed metal oxides, supported catalysts often possess a larger specific surface area and a better dispersion of active components, which facilitates tolerance to water and sulfur.On the other hand, compared with supported single MnOx catalysts, the synergistic effect introduced by one or more modifiers can reduce the poisonous effect and protect the surface active components.Thus, combining two measures, mixing (or doping) MnOx with suitable metal oxides and loading active components on a suitable support, is the best way to enhance the tolerance to water and sulfur.Chen et al. prepared an NiMn/Ti catalyst and investigated the effects of H2O and SO2 on its SCR performance (Figure 33) [47].They found that the coexistence of 100 ppm SO2 and 15 vol % H2O led to an apparent decrease in NOx conversion, and the NOx conversion recovered quickly to 100% after stopping the addition of H2O.Chen et al. found that W-modified MnOx/TiO2 exhibited better tolerance to SO2 than MnOx/TiO2 catalyst due to the fact that W addition inhibited the formation of sulfate species [93].Wang et al. reported a series of W-modified MnOx/TiO2 and found that a W(0.25)-Mn(0.25)-Ti(0.5)catalyst showed the best SCR activity and good tolerance to water and sulfur.As illustrated in Figure 34, the W(0.25)-Mn(0.25)-Ti(0.5)catalyst presented a 100% NOx conversion from 140 to 260°C [55].Our group successfully prepared a europium-modified TiO2-supported Mn-based catalyst and found that this catalyst showed better tolerance than the Mn-TiO2 catalyst due to the highly dispersed MnOx and Eu2O3 on the surface of TiO2 [18].Zhao et al. synthesized an Nb-modified Mn/Ce/Ti catalyst and tested the water and sulfur tolerance at a high GHSV of 180,000 h −1 at 175 °C [100].They found that the catalyst was deactivated with a decreased NO conversion from 100 to 10% within 4 h, which recovered to almost the original level after regeneration by washing.

Combination of Metal Modification and Support
The combination of metal modification and support is considered to be a good way of enhancing the water and sulfur tolerance of Mn-based catalysts due to the advantages of both strategies.Compared with non-supported mixed metal oxides, supported catalysts often possess a larger specific surface area and a better dispersion of active components, which facilitates tolerance to water and sulfur.On the other hand, compared with supported single MnO x catalysts, the synergistic effect introduced by one or more modifiers can reduce the poisonous effect and protect the surface active components.Thus, combining two measures, mixing (or doping) MnO x with suitable metal oxides and loading active components on a suitable support, is the best way to enhance the tolerance to water and sulfur.Chen et al. prepared an NiMn/Ti catalyst and investigated the effects of H 2 O and SO 2 on its SCR performance (Figure 33) [47].They found that the coexistence of 100 ppm SO 2 and 15 vol % H 2 O led to an apparent decrease in NO x conversion, and the NO x conversion recovered quickly to 100% after stopping the addition of H 2 O. Chen et al. found that W-modified MnO x /TiO 2 exhibited better tolerance to SO 2 than MnO x /TiO 2 catalyst due to the fact that W addition inhibited the formation of sulfate species [93].Wang et al. reported a series of W-modified MnO x /TiO 2 and found that a W(0.25)-Mn(0.25)-Ti(0.5)catalyst showed the best SCR activity and good tolerance to water and sulfur.As illustrated in Figure 34, the W(0.25)-Mn(0.25)-Ti(0.5)catalyst presented a 100% NO x conversion from 140 to 260 • C [55].Our group successfully prepared a europium-modified TiO 2 -supported Mn-based catalyst and found that this catalyst showed better tolerance than the Mn-TiO 2 catalyst due to the highly dispersed MnO x and Eu 2 O 3 on the surface of TiO 2 [18].Zhao et al. synthesized an Nb-modified Mn/Ce/Ti catalyst and tested the water and sulfur tolerance at a high GHSV of 180,000 h −1 at 175 • C [100].They found that the catalyst was deactivated with a decreased NO conversion from 100 to 10% within 4 h, which recovered to almost the original level after regeneration by washing.

4.4.Rational Design of Structure and Morphology
The rational design of catalyst structure and morphology is another method of reducing the poisoning effect of SO2 and H2O.Shen et al. developed a hollow MnOx-CeO2 mixed oxide catalyst, which exhibited good SCR performance under water and sulfur poison at a high GHSV of 120,000 h −1 due to the hollow structure [101].Zhang et al. [28] found that an MnxCo3-xO4 catalyst with a nanocage structure exhibited much SO2 and H2O tolerance than MnxCo3-xO4 without a nanocage structure.Qiu et al. synthesized a mesoporous 3D-MnCo2O4 catalyst that exhibited great SCR activity and good tolerance to sulfur and water [30,31], and the mesoporous structure enabled a dynamic balance between the formation and decomposition of ammonium sulfate, and thus suppressed the blocking effect during the SCR reaction.Li et al. prepared Mn2O3-doped Fe2O3 hexagonal microsheet catalyst [102] and found the single H2O resistance (15%) and the single SO2 resistance (100 ppm) over this catalyst were good and stable with the NO conversion at around 92% and 85% for 100 h, respectively, because of this special structure.

Monolithic Catalysts
Preparing monolithic catalysts may be an option to promote the tolerance of Mn-based SCR catalysts to SO2 and H2O.As well known, the commercial catalysts (V2O5-WO3(or MoO3)/TiO2) used

4.4.Rational Design of Structure and Morphology
The rational design of catalyst structure and morphology is another method of reducing the poisoning effect of SO2 and H2O.Shen et al. developed a hollow MnOx-CeO2 mixed oxide catalyst, which exhibited good SCR performance under water and sulfur poison at a high GHSV of 120,000 h −1 due to the hollow structure [101].Zhang et al. [28] found that an MnxCo3-xO4 catalyst with a nanocage structure exhibited much better SO2 and H2O tolerance than MnxCo3-xO4 without a nanocage structure.Qiu et al. synthesized a mesoporous 3D-MnCo2O4 catalyst that exhibited great SCR activity and good tolerance to sulfur and water [30,31], and the mesoporous structure enabled a dynamic balance between the formation and decomposition of ammonium sulfate, and thus suppressed the blocking effect during the SCR reaction.Li et al. prepared Mn2O3-doped Fe2O3 hexagonal microsheet catalyst [102] and found the single H2O resistance (15%) and the single SO2 resistance (100 ppm) over this catalyst were good and stable with the NO conversion at around 92% and 85% for 100 h, respectively, because of this special structure.

Monolithic Catalysts
Preparing monolithic catalysts may be an option to promote the tolerance of Mn-based SCR catalysts to SO2 and H2O.As well known, the commercial catalysts (V2O5-WO3(or MoO3)/TiO2) used

Rational Design of Structure and Morphology
The rational design of catalyst structure and morphology is another method of reducing the poisoning effect of SO 2 and H 2 O. Shen et al. developed a hollow MnO x -CeO 2 mixed oxide catalyst, which exhibited good SCR performance under water and sulfur poison at a high GHSV of 120,000 h −1 due to the hollow structure [101].Zhang et al. [28] found that an Mn x Co 3−x O 4 catalyst with a nanocage structure exhibited much better SO 2 and H 2 O tolerance than Mn x Co 3−x O 4 without a nanocage structure.Qiu et al. synthesized a mesoporous 3D-MnCo 2 O 4 catalyst that exhibited great SCR activity and good tolerance to sulfur and water [30,31], and the mesoporous structure enabled a dynamic balance between the formation and decomposition of ammonium sulfate, and thus suppressed the blocking effect during the SCR reaction.Li et al. prepared Mn 2 O 3 -doped Fe 2 O 3 hexagonal microsheet catalyst [102] and found the single H 2 O resistance (15%) and the single SO 2 resistance (100 ppm) over this catalyst were good and stable with the NO conversion at around 92% and 85% for 100 h, respectively, because of this special structure.

Monolithic Catalysts
Preparing monolithic catalysts may be an option to promote the tolerance of Mn-based SCR catalysts to SO 2 and H 2 O.As well known, the commercial catalysts (V 2 O 5 -WO 3 (or MoO 3 )/TiO 2 ) used in thermal power plant are in the monolithic form because the honeycomb monoliths are suitable for a high gas flow rate, reduce pressure drop problems, exhibit high tolerance to dust and attrition, and are easy to regenerate [103][104][105][106]. Recently, metal foam and wire mesh as novel monolithic support for Mn-based and other vanadium-free SCR catalysts are drawing an increasing amount of attention due to their high porosity, stability, thermal conductivity, and mass transfer ability [107,108].Xu et al. prepared porous MnCo x O y nanocubes on a Ti mesh as a novel monolith de-NO x catalyst for SCR [109].They found that this monolithic catalyst exhibited better SCR activity than MnCo x O y @honeycomb ceramics.Meanwhile, the water resistance test results of this novel monolithic catalyst were promising.Xu et al. successfully synthesized a series of MnO x -CeO 2 /WO 3 -ZrO 2 monolithic catalysts that showed good tolerance to water and sulfur [110].

Conclusions and Perspectives
Recent progress on the sulfur and water resistance of Mn-based catalysts for the low-temperature selective catalytic reduction of NO x has been reviewed comprehensively in this work.Although much progress has been made, many questions still need to be answered, and many problems need to be solved: (1) The exploration of novel Mn-based catalysts with excellent resistance to SO 2 and H 2 O is still worthwhile.Resistance to SO 2 and H 2 O directly decides whether this catalyst can be commercialized.Up to now, mixing (or doping) MnO x with suitable metal oxides and loading Mn-based active components on a suitable support are considered an efficient strategy.
Discovering new doping elements and novel supports may be promising research directions.(2) The actual effect of every specific doping element on tolerance promotion needs to be explained.
To date, many works have been done to test the tolerance of Mn-based catalysts to H 2 O and SO 2 .However, the reasons why the tolerance of Mn-based catalysts to H 2 O and SO 2 can be enhanced by mixing (or doping) them with other suitable elements need to be further explored in detail.(3) The role of support ought to be further analyzed.Does support only provide a higher specific surface area and a good dispersion of Mn?Is the support involved in SCR reaction?Such questions need to be answered.(4) Long-term tolerance tests need to be conducted.Most tests only last for several hours, and it is hard to predict the long-term performance of the catalyst under H 2 O and SO 2 poison.(5) N 2 selectivity is an important indicator for the commercialization of SCR catalysts, which is closely related to the yield of N 2 O.However, there is currently a lack of research on the effect of SO 2 on N 2 selectivity over Mn-based catalysts.Therefore, it is necessary to carry out this research in the near future.( 6) Most studies focus on powder catalysts.From a commercial perspective, monolithic catalysts should be given more consideration.

Figure 1 .
Figure 1.Scheme of the regular selective catalytic reduction (SCR) reaction, the H2O poisoning effect, and the SO2 poisoning effect.

Figure 1 .
Figure 1.Scheme of the regular selective catalytic reduction (SCR) reaction, the H 2 O poisoning effect, and the SO 2 poisoning effect.

Figure 2 .
Figure 2. The effect of SO 2 and H 2 O on NO x conversion over MnO x (CP) and MnO x (SP) (dotted line: only added 10% H 2 O; solid line: added 10% H 2 O + 100 ppm SO 2 ).(Reproduced with permission from Reference [13], Copyright 2007, Elsevier).

2
Ce 0.1 Ti 0.7 O x catalyst under H 2 O and SO 2 was further investigated at 200 • C. As shown in Figure 25, the introduction of H 2 O and SO 2 induced a slight decrease in NO x conversion.After H 2 O and SO 2 were excluded from the reactant feed, the NO x conversion completely recovered.Catalysts 2018, 8, 11 16 of 29

Figure 25 .
Figure 25.Response of the NO x conversion over Mn 0.2 Ce 0.1 Ti 0.7 O x catalyst at 200 • C to the intermittent feed of H 2 O and SO 2 (Reaction condition: 500 ppm of NO, 500 ppm of NH 3 , 5% O 2 , 5% H 2 O, 50 ppm of SO 2 , balance He, GHSV = 64,000 h −1 ).(Reproduced with permission from Reference [46], Copyright 2014, American Chemical Society).

3. 3 . 3 .
Other Supported Mn-Based Catalysts Mixed metal oxides and SiO 2 have also been studied as substrates for supporting SCR catalysts.Yao et al. prepared MnO x /SiO 2 , MnO x /Al 2 O 3 , MnO x /TiO 2 , and MnO x /CeO 2 catalysts and found that the catalytic activity in the presence of H 2 O and SO 2 was in the order of MnO x /SiO 2 < MnO x /TiO 2 < MnO x /CeO 2 < MnO x /Al 2 O 3 (Figure 29) [87].Shen et al. also compared the tolerance to H 2 O and SO 2 of MnO x -supported on various substrates including Al 2 O 3 , TiO 2 , CeO 2 , ZrO 2 , and Ce 0.5 Zr 0.5 O 2 .Their results showed that the resistance ability was decreased in the following order: MnO x /Ce 0.5 Zr 0.5 O 2 > MnO x /Al 2 O 3 > MnO x /CeO 2 > MnO x /TiO 2 > MnO x /ZrO 2 , and they ascribed the excellent toleranceof MnO x /Ce 0.5 Zr 0.5 O 2 to the combination of the advantages of the two supports (ZrO 2 and CeO 2 ) (Figure 30) [52].Huang et al. prepared a mesoporous silica-supported

Figure 29 .
Figure 29.The results of H 2 O + SO 2 resistance at 200 • C of these supported Mn-based catalysts with different supports.(Reproduced with permission from Reference [87], Copyright 2017, Elsevier).

Figure 33 .
Figure 33.Effects of H 2 O and SO 2 on NO x conversion over the Ni 0.4 Mn 0.6 Ti 10 catalyst.(Reproduced with permission from Reference [47], Copyright 2017,The Royal Society of Chemistry).

Table 1 .
Summary of the current status of H 2 O and SO 2 tolerance study on Mn-based catalysts in the literature.TiO 2 -graphene 500 ppm NH 3 , 500 ppm NO, 7% O 2 , 10% H 2 O, 200 ppm SO 2 GHSV at 67,000 h −1