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Catalysts 2018, 8(1), 11; https://doi.org/10.3390/catal8010011

Review
Sulfur and Water Resistance of Mn-Based Catalysts for Low-Temperature Selective Catalytic Reduction of NOx: A Review
Center of Nanomaterials for Renewable Energy, State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710049, China
*
Author to whom correspondence should be addressed.
Received: 6 December 2017 / Accepted: 3 January 2018 / Published: 7 January 2018

Abstract

:
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.
Keywords:
selective catalytic reduction; Mn-based catalysts; H2O and SO2 resistance; low-temperature; de-NOx

1. Introduction

Nitrogen oxides (NOx, 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 NH3 is the most efficient and economic method for post-NOx abatement, and V2O5–WO3(MoO3)/TiO2 has been the most popular commercial SCR catalyst since the 1970s [6,7]. However, V2O5-based catalysts have drawbacks, such as the toxicity of vanadium, SO2 oxidation to SO3, over-oxidation of NH3 to N2O, and a high working temperature [8]. Because of the high working temperature window (300–400 °C), V2O5-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 SO2 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 (MnOx) 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 NH3 over Mn-based catalysts [15,16,17,18,19,20,21]. However, several problems, including thermal instability, narrow operation window, and poor resistance to H2O and SO2 poison, remain. Among these drawbacks, the poor tolerance to H2O and SO2 is one of the most significant disadvantages, which limits the practical application of Mn-based catalysts [22]. Researchers have done numerous studies to develop Mn-based catalysts with good tolerance to water and sulfur and to uncover the deactivation mechanism of Mn-based catalysts in the presence of water and sulfur. In this review, we focused on the recent progress on the water and sulfur resistance of Mn-based catalysts. To make the organization clear, catalysts were introduced by the following three categories, single MnOx catalysts, Mn-based multi-metal oxide catalysts and Mn-based supported catalysts. Table 1 summarizes Mn-based catalysts reported in the literature that have exhibited good performance in the presence of water and sulfur.

2. The Poisoning Mechanism of Mn-Based Catalysts

2.1. 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].

2.2. 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.

2.3. The Effect on N2 Selectivity

The N2 selectivity is another indicator for the evaluation of SCR catalysts, which is closely related to the yield of N2O. During NH3–SCR reaction, N2O can be produced together with N2, especially at high temperatures. However, for Mn-based catalysts, some N2O can also be formed even at low temperatures due to side reactions resulting from the oxidative properties of manganese oxides, whether the NH3–SCR reaction follows the Eley–Rideal (E–R) mechanism or the Langmuir–Hinshelwood (L–H) mechanism. It has been reported that H2O presents a positive effect on N2 selectivity. Xiong et al. found that the formation of N2O over the Mn–Fe spinel and MnOx–CeO2 catalysts following the E-R mechanism was notably restrained by H2O due to the decrease in the oxidation ability of MnOx, the suppression of NH3 adsorption and the inhibition of the interface reaction. Furthermore, the generation of N2O through the L–H mechanism was completely suppressed by H2O due to the fact that the formation of NH4NO3 was inhibited or the decomposition of generated NH4NO3 was promoted [68,69]. As regards the effect of SO2 on N2 selectivity, there is a lack of research in this area at the moment.

3. Research Progress of Mn-Based Catalysts for Water and Sulfur Resistance

3.1. Single MnOx Catalysts

It has been proven that pure MnOx has terrific catalytic activity for the SCR of NOx with NH3 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 MnOx. Tang et al. prepared a series of amorphous MnOx 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 MnOx (CP) exhibited the best sulfur and water resistance, but the MnOx (SP) presented a larger surface area (150 m2g−1 for MnOx (SP) and 96 m2g−1 for MnOx (CP)). As shown in Figure 2, the NOx conversion at 80 °C over MnOx (CP) decreased from 98 to 73% in 3 h. After turning off SO2 and H2O, the NOx conversion was quickly restored to 90%. After the MnOx (CP) was heated for 1–2 h in N2 at 280 °C, its activity was restored to the initial level. Kang et al. prepared two MnOx catalysts using sodium carbonate (SC) and ammonia (AH) as precipitants [23]. They found that an MnOx-SC catalyst showed better SCR activity and great sulfur and water tolerance, and they ascribed this to the larger surface area (173.3 m2/g for MnOx-SC and 18.7 m2/g for MnOx-AH). As displayed in Figure 3, the NOx conversion over the MnOx-SC catalyst was decreased from 100 to 94% after both SO2 (100 ppm) and H2O (11 vol %) were fed into the reaction system with aspace velocity of 50,000 h−1, which is still very high de-NOx activity at 120 °C. Moreover, its activity was rapidly recovered to 100% after the supply of SO2 and H2O was cut off.

3.2. 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.

3.2.1. 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 MnOx. CeO2 has been studied extensively due to its good characteristics, such as increasing surface acidity after SO2 poisoning [78,79], high surface area [80,81], good dispersion of MnOx on the surface [45], and the redox shift between Ce4+ and Ce3+. It should be noted that the shift between Ce4+ and Ce3+ 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 SO2 and 2.5% H2O were added to the reaction gas at 120 °C. Moreover, the NO conversion was restored after SO2 + H2O 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 Mn5Ce5(ST) catalyst 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 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.
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], and found that a proper Sm modification (Sm/Mn = 1:10) enhanced the sulfur and water tolerance of MnOx. As shown in Figure 11, the NOx conversion over the Sm–Mn-0.1 catalyst could be maintained at about 91% at 100 °C when 2% H2O and 100 ppm SO2 were added into the feed gas, and the NOx conversion was recovered to 97% after both H2O and SO2 were removed from the feed gas. Sun et al. prepared a Eu-modified MnOx 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 SO2 and H2O on MnEuOx-0.1 was weak, and the NOx conversion over MnEuOx-0.1 kept over 90% in the presence of 100 ppm SO2 and 5% H2O. Furthermore, the NOx conversion nearly recovered its original level after the supply of SO2 and H2O 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 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 H2O and SO2 was cut off. Chang et al. found that Sn doping could enhance the sulfur resistance of Mn–Ce catalysts because SO2 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 SO2 and H2O and recovered to almost the original level within less than 3 hafter SO2 and H2O were removed. Gao et al. [39] found that the SCR pathways over MnOx–CeO2 catalyst are based on the adsorption, activation, and reaction of monodentate nitrite species and coordinated NH3 species, and these species are significantly inhibited by SO2 through competitive adsorption. In contrast, 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 SO2 and 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.

3.3. 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.

3.3.1. 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 H2O 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 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 addition [9]. As displayed in Figure 20, SO2 presented an obvious poisonous effect on SCR activity of Mn/TiO2 at low temperatures because the NO conversion over the MnTi catalyst decreased from 93 to 30% in the presence of SO2 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 SO2 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 MnO2–Fe2O3–CeO2/TiO2 catalyst. The NO conversion over this catalyst was stable at 80% under astream of SO2. Shen et al. [43] found that the addition of proper iron enhanced the tolerance of TiO2-supported Mn–Ce catalyst to water and sulfur. As exhibited in Figure 22, Fe(0.15)–Mn–Ce/TiO2 showed higher resistance under 3 vol % H2O and 0.01 vol % SO2 and still provided 83.8% NO conversion over afurther 5 h, an improvement over the Mn–Ce/TiO2 catalyst. Shen et al. found that titanium-pillared clays (Ti–PILCs) presented advantages in sulfur tolerance over traditional TiO2 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 % H2O and 0.01 vol % SO2, suggesting that Mn–CeOx/Ti–PILC(S) 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 Mn0.2Ce0.1Ti0.7Ox catalyst 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.

3.3.2. 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 [email protected] 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].

3.3.3. Other Supported Mn-Based Catalysts

Mixed metal oxides and SiO2 have also been studied as substrates for supporting SCR catalysts. Yao et al. prepared MnOx/SiO2, MnOx/Al2O3, MnOx/TiO2, and MnOx/CeO2 catalysts and found that the catalytic activity in the presence of H2O and SO2 was in the order of MnOx/SiO2 < MnOx/TiO2 < MnOx/CeO2 < MnOx/Al2O3 (Figure 29) [87]. Shen et al. also compared the tolerance to H2O and SO2 of MnOx-supported on various substrates including Al2O3, TiO2, CeO2, ZrO2, and Ce0.5Zr0.5O2. Their 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].

4. 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 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 H2O and SO2. To date, many strategies have been taken to reduce the poisoning effect on Mn-based catalysts.

4.1. 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 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 Mnn+, SO42−, 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.

4.2. 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 good options. It has been widely reported that TiO2 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 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].

4.3. 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.

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.

4.5. 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 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 MnCoxOy nanocubes on a Ti mesh as a novel monolith de-NOx catalyst for SCR [109]. They found that this monolithic catalyst exhibited better SCR activity than MnCoxOy@honeycomb ceramics. Meanwhile, the water resistance test results of this novel monolithic catalyst were promising. Xu et al. successfully synthesized a series of MnOx–CeO2/WO3-ZrO2 monolithic catalysts that showed good tolerance to water and sulfur [110].

5. Conclusions and Perspectives

Recent progress on the sulfur and water resistance of Mn-based catalysts for the low-temperature selective catalytic reduction of NOx 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 SO2 and H2O is still worthwhile. Resistance to SO2 and H2O directly decides whether this catalyst can be commercialized. Up to now, mixing (or doping) MnOx 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 H2O and SO2. However, the reasons why the tolerance of Mn-based catalysts to H2O and SO2 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 H2O and SO2 poison.
(5)
N2 selectivity is an important indicator for the commercialization of SCR catalysts, which is closely related to the yield of N2O. However, there is currently a lack of research on the effect of SO2 on N2 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.

Acknowledgments

This work was sponsored by the National Natural Science Fund Committee-Baosteel Group Corporation Steel Joint Research Fund, China (U1460105), the National Science Foundation of China (51521065), and the Natural Science Foundation of Shaanxi Province, China (2015JM2055).

Conflicts of Interest

There are no conflict of interest.

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Figure 1. Scheme of the regular selective catalytic reduction (SCR) reaction, the H2O poisoning effect, and the SO2 poisoning effect.
Figure 1. Scheme of the regular selective catalytic reduction (SCR) reaction, the H2O poisoning effect, and the SO2 poisoning effect.
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Figure 2. The effect of SO2 and H2O on NOx conversion over MnOx (CP) and MnOx (SP) (dotted line: only added 10% H2O; solid line: added 10% H2O + 100 ppm SO2). (Reproduced with permission from Reference [13], Copyright 2007, Elsevier).
Figure 2. The effect of SO2 and H2O on NOx conversion over MnOx (CP) and MnOx (SP) (dotted line: only added 10% H2O; solid line: added 10% H2O + 100 ppm SO2). (Reproduced with permission from Reference [13], Copyright 2007, Elsevier).
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Figure 3. The effects of H2O and SO2 on NOx conversions over MnOx-SC catalyst at 120 °C. Reactants: 500 ppm NO, 500 ppm NH3, 5 vol % O2 in N2. The gas hourly space velocity (GHSV) was 50,000 h−1. (Reproduced with permission from Reference [23], Copyright 2006, Springer).
Figure 3. The effects of H2O and SO2 on NOx conversions over MnOx-SC catalyst at 120 °C. Reactants: 500 ppm NO, 500 ppm NH3, 5 vol % O2 in N2. The gas hourly space velocity (GHSV) was 50,000 h−1. (Reproduced with permission from Reference [23], Copyright 2006, Springer).
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Figure 4. The effect of on-stream time on SCR activity with H2O + SO2 and without H2O + SO2 (Reaction conditions: 120 °C, [NH3] = [NO] = 1000 ppm, [O2] = 2%, GHSV = 42,000 h−1. Catalyst: MnOx(0.3)–CeO2). (Reproduced with permission from Reference [24], Copyright 2003, The Royal Society of Chemistry).
Figure 4. The effect of on-stream time on SCR activity with H2O + SO2 and without H2O + SO2 (Reaction conditions: 120 °C, [NH3] = [NO] = 1000 ppm, [O2] = 2%, GHSV = 42,000 h−1. Catalyst: MnOx(0.3)–CeO2). (Reproduced with permission from Reference [24], Copyright 2003, The Royal Society of Chemistry).
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Figure 5. Catalytic performance of Mn5Ce5(ST) catalysts in the presence of H2O and SO2 (Reaction conditions: NO = 500 ppm, NH3 = 500 ppm, O2 = 5%, H2O = 5%, SO2 = 50 ppm, GHSV = 64,000 h−1). (Reproduced with permission from Reference [25], Copyright 2013, Elsevier).
Figure 5. Catalytic performance of Mn5Ce5(ST) catalysts in the presence of H2O and SO2 (Reaction conditions: NO = 500 ppm, NH3 = 500 ppm, O2 = 5%, H2O = 5%, SO2 = 50 ppm, GHSV = 64,000 h−1). (Reproduced with permission from Reference [25], Copyright 2013, Elsevier).
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Figure 6. The effect of H2O and SO2 on NO conversion for Mn/CeO2 catalysts (Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol %, [SO2] = 100 ppm, [H2O] = 5 vol %, N2 balance, T = 200 °C, 60,000 mL g−1 h−1). (Reproduced with permission from Reference [26], Copyright 2017, Elsevier).
Figure 6. The effect of H2O and SO2 on NO conversion for Mn/CeO2 catalysts (Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol %, [SO2] = 100 ppm, [H2O] = 5 vol %, N2 balance, T = 200 °C, 60,000 mL g−1 h−1). (Reproduced with permission from Reference [26], Copyright 2017, Elsevier).
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Figure 7. SCR activities on the Fe–Mn-based transition metal oxides in the presence of SO2 + H2O (Reaction conditions: 0.5 g catalyst, [NO] = [NH3] = 1000 ppm, [O2] = 2%, [SO2] = 37.5 ppm and [H2O] = 2.5% (when used), He = balance, total flow rate = 100 mL/min, and GHSV = 15,000 h−1). (Reproduced with permission from Reference [27], Copyright 2002, The Royal Society of Chemistry).
Figure 7. SCR activities on the Fe–Mn-based transition metal oxides in the presence of SO2 + H2O (Reaction conditions: 0.5 g catalyst, [NO] = [NH3] = 1000 ppm, [O2] = 2%, [SO2] = 37.5 ppm and [H2O] = 2.5% (when used), He = balance, total flow rate = 100 mL/min, and GHSV = 15,000 h−1). (Reproduced with permission from Reference [27], Copyright 2002, The Royal Society of Chemistry).
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Figure 8. Lifetime, SO2 tolerance, and water resistance of Fe-(0.4)MnOx(CA-500) catalyst (Reaction conditions: 120 °C, [NO] = [NH3] = 1000 ppm, [O2] = 3%, [SO2] = 100 ppm, [H2O] = 5%, N2 as balance, and GHSV = 30,000 h−1; Plasma treatment conditions: 10 MHz, 25 °C, pure oxygen with 50 mL/min under 2.4 s of residence time, and duration of 6 h): (a) Lifetime testing; (b) regeneration property; (c) SO2 tolerance; (d) water resistance; (e) the combined effect of SO2 and H2O. (Reproduced with permission from Reference [28], Copyright 2012, American Chemical Society).
Figure 8. Lifetime, SO2 tolerance, and water resistance of Fe-(0.4)MnOx(CA-500) catalyst (Reaction conditions: 120 °C, [NO] = [NH3] = 1000 ppm, [O2] = 3%, [SO2] = 100 ppm, [H2O] = 5%, N2 as balance, and GHSV = 30,000 h−1; Plasma treatment conditions: 10 MHz, 25 °C, pure oxygen with 50 mL/min under 2.4 s of residence time, and duration of 6 h): (a) Lifetime testing; (b) regeneration property; (c) SO2 tolerance; (d) water resistance; (e) the combined effect of SO2 and H2O. (Reproduced with permission from Reference [28], Copyright 2012, American Chemical Society).
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Figure 9. (a) H2O resistance, (b) SO2 tolerance, and (c) H2O and SO2 synergetic effect study of the catalysts at 175 °C (Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 3 vol %, [H2O] = 8 vol % (when used), [SO2] = 200 ppm (when used), N2 balance, and GHSV = 38,000 h−1). (Reproduced with permission from Reference [29], Copyright 2014, American Chemical Society).
Figure 9. (a) H2O resistance, (b) SO2 tolerance, and (c) H2O and SO2 synergetic effect study of the catalysts at 175 °C (Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 3 vol %, [H2O] = 8 vol % (when used), [SO2] = 200 ppm (when used), N2 balance, and GHSV = 38,000 h−1). (Reproduced with permission from Reference [29], Copyright 2014, American Chemical Society).
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Figure 10. H2O resistance, SO2 tolerance, and H2O and SO2 synergetic effect study of the MnCo2O4 catalyst at 200 °C (Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = [H2O] = 5 vol %, [SO2] = 100 ppm, N2 balance, and GHSV = 50,000 h−1). (Reproduced with permission from Reference [30], Copyright 2015, Elsevier).
Figure 10. H2O resistance, SO2 tolerance, and H2O and SO2 synergetic effect study of the MnCo2O4 catalyst at 200 °C (Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = [H2O] = 5 vol %, [SO2] = 100 ppm, N2 balance, and GHSV = 50,000 h−1). (Reproduced with permission from Reference [30], Copyright 2015, Elsevier).
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Figure 11. The effect of H2O and SO2 on the catalytic activities of the MnOx and Sm–Mn-0.1 catalysts for the SCR reaction at 100 °C (Reaction conditions: 0.3 g catalyst, 500 ppm NO, 500 ppm NH3, 5% O2, Ar to balance, GHSV = 49,000 h−1). (Reproduced with permission from Reference [33], Copyright 2016, American Chemical Society).
Figure 11. The effect of H2O and SO2 on the catalytic activities of the MnOx and Sm–Mn-0.1 catalysts for the SCR reaction at 100 °C (Reaction conditions: 0.3 g catalyst, 500 ppm NO, 500 ppm NH3, 5% O2, Ar to balance, GHSV = 49,000 h−1). (Reproduced with permission from Reference [33], Copyright 2016, American Chemical Society).
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Figure 12. Effect of SO2 and H2O on the SCR activities of MnOx and MnEuOx-0.1 catalysts (Reaction conditions: [NO] = [NH3] = 600 ppm, [SO2] = 100 ppm, [H2O] = [O2] = 5%, balance Ar, GHSV = 108,000 h−1, reaction temperature = 350 °C). (Reproduced with permission from Reference [34], Copyright 2017, Elsevier).
Figure 12. Effect of SO2 and H2O on the SCR activities of MnOx and MnEuOx-0.1 catalysts (Reaction conditions: [NO] = [NH3] = 600 ppm, [SO2] = 100 ppm, [H2O] = [O2] = 5%, balance Ar, GHSV = 108,000 h−1, reaction temperature = 350 °C). (Reproduced with permission from Reference [34], Copyright 2017, Elsevier).
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Figure 13. The effects of SO2 and H2O on the SCR activities of MnOx(0.4)–CeO2(500)-based catalysts (Reaction conditions: 0.2 g catalyst, [NH3] = [NO] = 1000 ppm, [O2] = 2%, He = balance and total flow rate = 100 mL/min, T = 150 °C). (Reproduced with permission from Reference [35], Copyright 2004, Elsevier).
Figure 13. The effects of SO2 and H2O on the SCR activities of MnOx(0.4)–CeO2(500)-based catalysts (Reaction conditions: 0.2 g catalyst, [NH3] = [NO] = 1000 ppm, [O2] = 2%, He = balance and total flow rate = 100 mL/min, T = 150 °C). (Reproduced with permission from Reference [35], Copyright 2004, Elsevier).
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Figure 14. Influence of H2O and combined H2O and SO2 on NOx conversion of FeMnOx and Ce(12.5) (Reaction conditions: [NO] = [NH3] = 0.1%, [H2O] = 5% or 10%, [SO2] = 100 ppm, [O2] = 3%, N2 balance, GHSV = 30,000 h−1; reaction temperature = 120 °C). (Reproduced with permission from Reference [36], Copyright 2017, Elsevier).
Figure 14. Influence of H2O and combined H2O and SO2 on NOx conversion of FeMnOx and Ce(12.5) (Reaction conditions: [NO] = [NH3] = 0.1%, [H2O] = 5% or 10%, [SO2] = 100 ppm, [O2] = 3%, N2 balance, GHSV = 30,000 h−1; reaction temperature = 120 °C). (Reproduced with permission from Reference [36], Copyright 2017, Elsevier).
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Figure 15. (a) NO conversion in the presence of 100 ppm SO2 and 12% H2O at 110 °C; (b) NO conversion in the presence of 100 ppm SO2 at 250 °C (Reaction conditions: 0.2 g samples, 1000 ppm NO, 1000 ppm NH3, 2% O2, N2 balance, GHSV = 35,000 h−1). (Reproduced with permission from Reference [37], Copyright 2012, Elsevier).
Figure 15. (a) NO conversion in the presence of 100 ppm SO2 and 12% H2O at 110 °C; (b) NO conversion in the presence of 100 ppm SO2 at 250 °C (Reaction conditions: 0.2 g samples, 1000 ppm NO, 1000 ppm NH3, 2% O2, N2 balance, GHSV = 35,000 h−1). (Reproduced with permission from Reference [37], Copyright 2012, Elsevier).
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Figure 16. The resistance to H2O and/or SO2 over Co1Mn4Ce5Ox and Ni1Mn4Ce5Ox (Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol %, [SO2] = 150 ppm, [H2O] = 10 vol %, N2 to balance, total flow rate = 200 mL/min, GHSV = 48,000 h−1). (Reproduced with permission from Reference [39], Copyright 2017, Elsevier).
Figure 16. The resistance to H2O and/or SO2 over Co1Mn4Ce5Ox and Ni1Mn4Ce5Ox (Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol %, [SO2] = 150 ppm, [H2O] = 10 vol %, N2 to balance, total flow rate = 200 mL/min, GHSV = 48,000 h−1). (Reproduced with permission from Reference [39], Copyright 2017, Elsevier).
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Figure 17. Response of NOx conversion over MnZr and 15WMnZr catalysts at 300 °C to intermittent feed of H2O and SO2 (Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5%, [H2O] = 5%, [SO2] = 50 ppm, GHSV = 128,000 h−1). (Reproduced with permission from Reference [40], Copyright 2016, Elsevier).
Figure 17. Response of NOx conversion over MnZr and 15WMnZr catalysts at 300 °C to intermittent feed of H2O and SO2 (Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5%, [H2O] = 5%, [SO2] = 50 ppm, GHSV = 128,000 h−1). (Reproduced with permission from Reference [40], Copyright 2016, Elsevier).
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Figure 18. NO conversion on the various Fe–Mn-based catalyst in the presence of SO2 + H2O (Reaction conditions: temperature = 150 °C, [NO] = [NH3] = 1000 ppm, [O2] = 2%, [SO2] = 100 ppm, [H2O] = 2.5%, balance He, total flow rate 100 mL/min, catalyst 0.5 g). (Reproduced with permission from Reference [41], Copyright 2003, Elsevier).
Figure 18. NO conversion on the various Fe–Mn-based catalyst in the presence of SO2 + H2O (Reaction conditions: temperature = 150 °C, [NO] = [NH3] = 1000 ppm, [O2] = 2%, [SO2] = 100 ppm, [H2O] = 2.5%, balance He, total flow rate 100 mL/min, catalyst 0.5 g). (Reproduced with permission from Reference [41], Copyright 2003, Elsevier).
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Figure 19. Stability of NO reduction over 10% Mn/Fe–Ti spinel in the presence of H2O and SO2 (Reaction conditions: [NH3] = [NO] = 500 ppm, [SO2] = 60 ppm, [H2O] = 8%, catalyst mass = 500 mg, the total flow rate = 100 mL and GHSV = 12000 cm3 g−1 h−1). (Reproduced with permission from Reference [42], Copyright 2016, Elsevier).
Figure 19. Stability of NO reduction over 10% Mn/Fe–Ti spinel in the presence of H2O and SO2 (Reaction conditions: [NH3] = [NO] = 500 ppm, [SO2] = 60 ppm, [H2O] = 8%, catalyst mass = 500 mg, the total flow rate = 100 mL and GHSV = 12000 cm3 g−1 h−1). (Reproduced with permission from Reference [42], Copyright 2016, Elsevier).
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Figure 20. SCR activities of Mn/TiO2 and Ce-doped Mn/TiO2 in the presence of SO2 (Reaction conditions: [NH3] = [NO] = 1000 ppm, [O2] = 3%, [SO2] = 100 ppm, [H2O] = 3 vol %, N2 balance, GHSV = 40,000 h−1, reaction temperature = 150 °C; hollow symbols for MnTi and solid symbols for MnCeTi).
Figure 20. SCR activities of Mn/TiO2 and Ce-doped Mn/TiO2 in the presence of SO2 (Reaction conditions: [NH3] = [NO] = 1000 ppm, [O2] = 3%, [SO2] = 100 ppm, [H2O] = 3 vol %, N2 balance, GHSV = 40,000 h−1, reaction temperature = 150 °C; hollow symbols for MnTi and solid symbols for MnCeTi).
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Figure 21. Scanning electron microscope (SEM) micrographs of fresh and SO2-poisoned catalysts. (A) MnTi, (B) MnTi-S, (C) MnCeTi, and (D) MnCeTi-S. (Reproduced with permission from Reference [9], Copyright 2009, Elsevier).
Figure 21. Scanning electron microscope (SEM) micrographs of fresh and SO2-poisoned catalysts. (A) MnTi, (B) MnTi-S, (C) MnCeTi, and (D) MnCeTi-S. (Reproduced with permission from Reference [9], Copyright 2009, Elsevier).
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Figure 22. Effect of H2O and SO2 on NO conversion over Mn–Ce/TiO2 and Fe–Mn–Ce/TiO2 catalysts (Reaction conditions: 0.06 vol % NO, 0.06 vol % NH3, 3 vol % O2, 3 vol % H2O (when used), 0.01 vol % SO2 (when used), balance N2, GHSV 50,000 h−1, total flow rate 300 mL/min, tested at 180 °C). (Reproduced with permission from Reference [43], Copyright 2010, Science Direct).
Figure 22. Effect of H2O and SO2 on NO conversion over Mn–Ce/TiO2 and Fe–Mn–Ce/TiO2 catalysts (Reaction conditions: 0.06 vol % NO, 0.06 vol % NH3, 3 vol % O2, 3 vol % H2O (when used), 0.01 vol % SO2 (when used), balance N2, GHSV 50,000 h−1, total flow rate 300 mL/min, tested at 180 °C). (Reproduced with permission from Reference [43], Copyright 2010, Science Direct).
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Figure 23. Effect of H2O and SO2 on NO conversion over Mn–CeOx/TiPILC(S) at 200 °C (Reaction conditions: 0.06 vol % NO, 0.06 vol % NH3, 3 vol % O2, 3 vol % H2O (when used), 0.01 vol % SO2 (when used), balance N2, GHSV 50,000 h−1, total flow rate 300 mL/min). (Reproduced with permission from Reference [44], Copyright 2012, Science Direct).
Figure 23. Effect of H2O and SO2 on NO conversion over Mn–CeOx/TiPILC(S) at 200 °C (Reaction conditions: 0.06 vol % NO, 0.06 vol % NH3, 3 vol % O2, 3 vol % H2O (when used), 0.01 vol % SO2 (when used), balance N2, GHSV 50,000 h−1, total flow rate 300 mL/min). (Reproduced with permission from Reference [44], Copyright 2012, Science Direct).
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Figure 24. The effects of the H2O and SO2 in the Mn/Ce(4)–TiO2 catalysts with different ratio of Ce/Ti (Reaction condition: 200 ppm NO, 8% O2, 6% H2O, 100 ppm SO2, 0.28 g of sample and 500 cc/min total flow rate, tested at 180 °C). (Reproduced with permission from Reference [45], Copyright 2012, Elsevier).
Figure 24. The effects of the H2O and SO2 in the Mn/Ce(4)–TiO2 catalysts with different ratio of Ce/Ti (Reaction condition: 200 ppm NO, 8% O2, 6% H2O, 100 ppm SO2, 0.28 g of sample and 500 cc/min total flow rate, tested at 180 °C). (Reproduced with permission from Reference [45], Copyright 2012, Elsevier).
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Figure 25. Response of the NOx conversion over Mn0.2Ce0.1Ti0.7Ox catalyst at 200 °C to the intermittent feed of H2O and SO2 (Reaction condition: 500 ppm of NO, 500 ppm of NH3, 5% O2, 5% H2O, 50 ppm of SO2, balance He, GHSV = 64,000 h−1). (Reproduced with permission from Reference [46], Copyright 2014, American Chemical Society).
Figure 25. Response of the NOx conversion over Mn0.2Ce0.1Ti0.7Ox catalyst at 200 °C to the intermittent feed of H2O and SO2 (Reaction condition: 500 ppm of NO, 500 ppm of NH3, 5% O2, 5% H2O, 50 ppm of SO2, balance He, GHSV = 64,000 h−1). (Reproduced with permission from Reference [46], Copyright 2014, American Chemical Society).
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Figure 26. SO2 + H2O tolerance test at 240 °C (Reaction conditions: [NH3] = [NO] = 550 ppm, [O2] = 5 vol %, [SO2] = 100 ppm, [H2O] = 10 vol % (when used), N2 as balance gas, GHSV = 20,000 h−1). (Reproduced with permission from Reference [49], Copyright 2016, The Royal Society of Chemistry).
Figure 26. SO2 + H2O tolerance test at 240 °C (Reaction conditions: [NH3] = [NO] = 550 ppm, [O2] = 5 vol %, [SO2] = 100 ppm, [H2O] = 10 vol % (when used), N2 as balance gas, GHSV = 20,000 h−1). (Reproduced with permission from Reference [49], Copyright 2016, The Royal Society of Chemistry).
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Figure 27. SO2 + H2O tolerance test at 180 °C (Reaction conditions: [NH3] = [NO] = 500 ppm, [O2] = 7 vol %, [SO2] = 200 ppm, [H2O] = 10 vol % (when used), N2 as balance gas, GHSV = 67,000 h−1). (Reproduced with permission from Reference [51], Copyright 2015, Elsevier).
Figure 27. SO2 + H2O tolerance test at 180 °C (Reaction conditions: [NH3] = [NO] = 500 ppm, [O2] = 7 vol %, [SO2] = 200 ppm, [H2O] = 10 vol % (when used), N2 as balance gas, GHSV = 67,000 h−1). (Reproduced with permission from Reference [51], Copyright 2015, Elsevier).
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Figure 28. X-ray photoelectron spectroscopy (XPS) spectra of S 2p for poisoned Mn/ACH and CeMn/ACH catalysts. (Reproduced with permission from Reference [81], Copyright 2015, American Chemical Society).
Figure 28. X-ray photoelectron spectroscopy (XPS) spectra of S 2p for poisoned Mn/ACH and CeMn/ACH catalysts. (Reproduced with permission from Reference [81], Copyright 2015, American Chemical Society).
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Figure 29. The results of H2O + SO2 resistance at 200 °C of these supported Mn-based catalysts with different supports. (Reproduced with permission from Reference [87], Copyright 2017, Elsevier).
Figure 29. The results of H2O + SO2 resistance at 200 °C of these supported Mn-based catalysts with different supports. (Reproduced with permission from Reference [87], Copyright 2017, Elsevier).
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Figure 30. The effect of H2O and SO2 on NO conversion for MnOx(0.6)/Ce0.5Zr0.5O2 (Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 3 vol %, N2 balance, T = 180°C, catalyst 0.5 g, GHSV 30,000 h−1). (Reproduced with permission from Reference [52], Copyright 2014, Elsevier).
Figure 30. The effect of H2O and SO2 on NO conversion for MnOx(0.6)/Ce0.5Zr0.5O2 (Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 3 vol %, N2 balance, T = 180°C, catalyst 0.5 g, GHSV 30,000 h−1). (Reproduced with permission from Reference [52], Copyright 2014, Elsevier).
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Figure 31. NH3–SCR activity over WySnMnCeOx catalysts in the presence of SO2/H2O at 200 °C (Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5%, [SO2] = 100 ppm (when used), [H2O] = 5% (when used), N2 balance, total flow rate 100 mL min−1 and GHSV = 60,000 mLg−1h−1). (Reproduced with permission from Reference [53], Copyright 2017, Elsevier).
Figure 31. NH3–SCR activity over WySnMnCeOx catalysts in the presence of SO2/H2O at 200 °C (Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5%, [SO2] = 100 ppm (when used), [H2O] = 5% (when used), N2 balance, total flow rate 100 mL min−1 and GHSV = 60,000 mLg−1h−1). (Reproduced with permission from Reference [53], Copyright 2017, Elsevier).
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Figure 32. Effect of water vapor and SO2 on NO conversion over the MnOx/3DOMC (black), MnOx/NAC (red), and MnOx/TiO2 (blue) catalysts (Reaction conditions: 190 °C, 1000 ppm of NO, 1000 ppm of NH3, 5% of O2, 5% of water vapor, and/or 200 ppm of SO2, He balance). (Reproduced with permission from Reference [54] Copyright 2015, The Royal Society of Chemistry).
Figure 32. Effect of water vapor and SO2 on NO conversion over the MnOx/3DOMC (black), MnOx/NAC (red), and MnOx/TiO2 (blue) catalysts (Reaction conditions: 190 °C, 1000 ppm of NO, 1000 ppm of NH3, 5% of O2, 5% of water vapor, and/or 200 ppm of SO2, He balance). (Reproduced with permission from Reference [54] Copyright 2015, The Royal Society of Chemistry).
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Figure 33. Effects of H2O and SO2 on NOx conversion over the Ni0.4Mn0.6Ti10 catalyst. (Reproduced with permission from Reference [47], Copyright 2017,The Royal Society of Chemistry).
Figure 33. Effects of H2O and SO2 on NOx conversion over the Ni0.4Mn0.6Ti10 catalyst. (Reproduced with permission from Reference [47], Copyright 2017,The Royal Society of Chemistry).
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Figure 34. Effects of SO2 and H2O on the NOxconversion of W(0.25)–Mn(0.25)–Ti(0.5) at GHSV of 25,000 h−1 (Reaction conditions: [NO] = [NH3] = 1000 ppm, [O2] = 5 vol %, [SO2] = 100 ppm, [H2O] = 10 vol %, in He as balance). (Reproduced with permission from Reference [55], Copyright 2016, Elsevier).
Figure 34. Effects of SO2 and H2O on the NOxconversion of W(0.25)–Mn(0.25)–Ti(0.5) at GHSV of 25,000 h−1 (Reaction conditions: [NO] = [NH3] = 1000 ppm, [O2] = 5 vol %, [SO2] = 100 ppm, [H2O] = 10 vol %, in He as balance). (Reproduced with permission from Reference [55], Copyright 2016, Elsevier).
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Table 1. Summary of the current status of H2O and SO2 tolerance study on Mn-based catalysts in the literature.
Table 1. Summary of the current status of H2O and SO2 tolerance study on Mn-based catalysts in the literature.
CatalystReaction ConditionsT/℃XNOXNO-UXNO-AReferences
MnOx500 ppm NH3, 500 ppm NO, 3% O2, 10% H2O, 100 ppm SO2GHSV at 47,000 h−18098%70%90%[13]
MnOx500 ppm NH3, 500 ppm NO, 5% O2, 11% H2O, 100 ppm SO2GHSV at 50,000 h−1120100%94%100%[23]
Mn–Ce1000 ppm NH3, 1000 ppm NO, 2% O2, 2.5% H2O, 100 ppm SO2GHSV at 42,000 h−1120100%95%100%[24]
Mn–Ce500 ppm NH3, 500 ppm NO, 5% O2, 5% H2O, 50 ppm SO2GHSV at 64,000 h−1150~98%~95%/[25]
Mn–Ce500 ppm NH3, 500 ppm NO, 5% O2, 5% H2O, 100 ppm SO2 60,000 mL g−1 h−1200~97%~70%~85%[26]
Mn–Fe1000 ppm NH3, 1000 ppm NO, 2% O2, 2.5% H2O, 37.5 ppm SO2GHSV at 15,000 h−1160100%~98%/[27]
Mn–Fe1000 ppm NH3, 1000 ppm NO, 3% O2, 5% H2O, 100 ppm SO2GHSV at 30,000 h−1120100%87%93%[28]
Mn–Co500 ppm NH3, 500 ppm NO, 3% O2, 8% H2O, 200 ppm SO2GHSV at 38,000 h−1175100%90%100%[29]
Mn–Co500 ppm NH3, 500 ppm NO, 5% O2, 5% H2O, 100 ppm SO2GHSV at 50,000 h−1200100%80%90%[30,31]
Mn–Cu500 ppm NH3, 500 ppm NO, 5% O2, 11% H2O, 100 ppm SO2GHSV at 50,000 h−112595%64%~90%[32]
Mn–Sm500 ppm NH3, 500 ppm NO, 5% O2, 2% H2O, 100 ppm SO2GHSV at 49,000 h−1100100%91%97%[33]
Mn–Eu600 ppm NH3, 600 ppm NO, 5% O2, 5% H2O, 100 ppm SO2GHSV at 108,000 h−1350100%90%95%[34]
Mn–Fe–Ce1000 ppm NH3, 1000 ppm NO, 2% O2, 2.5% H2O, 100 ppm SO2GHSV at 42,000 h−115098%95%98%[35]
Mn–Ce–Fe1000 ppm NH3, 1000 ppm NO, 3% O2, 10% H2O, 100 ppm SO2GHSV at 30,000 h−1120100%75%95%[36]
Mn–Sn–Ce1000 ppm NH3, 1000 ppm NO, 2% O2, 12% H2O, 100 ppm SO2GHSV at 35,000 h−1110100%70%90%[37,38]
Mn–Ce–Ni500 ppm NH3, 500 ppm NO, 5% O2, 10% H2O, 150 ppm SO2GHSV at 48,000 h−1175~90%~78%~90%[39]
Mn–Ce–Co500 ppm NH3, 500 ppm NO, 5% O2, 10% H2O, 150 ppm SO2GHSV at 48,000 h−1175~90%~72%~90%[39]
Mn–W–Zr500 ppm NH3, 500 ppm NO 5% O2, 5% H2O, 50 ppm SO2 GHSV at 128,000 h−1300100%~90%100%[40]
Mn–Fe/TiO21000 ppm NH3, 1000 ppm NO, 2% O2, 2.5% H2O, 100 ppm SO2GHSV at 15,000 h−1150100%90%100%[41]
Mn/Fe–TiO2500 ppm NH3, 500 ppm NO, 2% O2, 8% H2O, 60 ppm SO2GHSV at 12,000 h−1200100%83%100%[42]
Mn–Ce/TiO21000 ppm NH3, 1000 ppm NO, 3% O2, 3% H2O, 100 ppm SO2GHSV at 30,000 h−1150100%84%/[9]
Mn–Fe–Ce/TiO2600 ppm NH3, 600 ppm NO, 3% O2, 3% H2O, 100 ppm SO2GHSV at 50,000 h−1180100%84%90%[43]
Mn–Ce/Ti–PILC600 ppm NH3, 600 ppm NO, 3% O2, 3% H2O, 100 ppm SO2GHSV at 50,000 h−1200~95%~90%~90%[44]
Mn–Ce/TiO2220 ppm NH3, 200 ppm NO, 8% O2, 6% H2O, 100 ppm SO2GHSV at 60,000 h−1180100%62%70%[45]
Mn–Ce/TiO2500 ppm NH3, 500 ppm NO, 5% O2, 5% H2O, 50 ppm SO2GHSV at 64,000 h−1200~95%~90%~93%[46]
Ni–Mn/TiO21000 ppm NH3, 1000 ppm NO, 3% O2, 15% H2O, 100 ppm SO2GHSV at 40,000 h−1240100%~95%100%[47]
[email protected]500 ppm NH3, 500 ppm NO, 3% O2, 4% H2O, 100 ppm SO2GHSV at 10,000 h−1300100%87%90%[48]
Fe2O3@MnOx@CNTs550 ppm NH3, 550 ppm NO, 5% O2, 10% H2O, 100 ppm SO2GHSV at 20,000 h−124097%91%95%[49]
Mn–Ce/TiO2–graphene500 ppm NH3, 500 ppm NO, 7% O2, 10% H2O, 200 ppm SO2GHSV at 67,000 h−118095%95%100%[50,51]
MnOx(0.6)/Ce0.5Zr0.5O600 ppm NH3, 600 ppm NO, 3% O2, 3% H2O, 200 ppm SO2GHSV at 30,000 h−1180100%~92%~98%[52]
WySnMnCeOx500 ppm NH3, 500 ppm NO, 5% O2, 5% H2O, 100 ppm SO2 60,000 mL g−1 h−1200~97%~90%~95%[53]
MnOx/3DOMC1000 ppm NH3, 1000 ppm NO, 5% O2, 5% H2O, 200 ppm SO2GHSV at 36,000 h−1190100%~87%~95%[54]
W0.25–Mn0.25–Ti0.51000 ppm NH3, 1000 ppm NO, 5% O2, 10% H2O, 100 ppm SO2GHSV at 25,000 h−1/~100%~100%/[55]
XNO, XNO-U, and XNO-A represent NOx conversion of regular SCR reaction, NOx conversion under tolerance test and after tolerance test, respectively.

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