Recent Progress on Low-Temperature Selective Catalytic Reduction of NO_x with Ammonia

Abstract

Selective catalytic reduction of nitrogen oxides (NO_x) with ammonia (NH₃-SCR) has been implemented in response to the regulation of NO_x emissions from stationary and mobile sources above 300 °C. However, the development of NH₃-SCR catalysts active at low temperatures below 200 °C is still needed to improve the energy efficiency and to cope with various fuels. In this review article, recent reports on low-temperature NH₃-SCR catalysts are systematically summarized. The redox property as well as the surface acidity are two main factors that affect the catalytic activity. The strong redox property is beneficial for the low-temperature NH₃-SCR activity but is responsible for N₂O formation. The multiple electron transfer system is more plausible for controlling redox properties. H₂O and SO_x, which are often found with NO_x in flue gas, have a detrimental effect on NH₃-SCR activity, especially at low temperatures. The competitive adsorption of H₂O can be minimized by enhancing the hydrophobic property of the catalyst. Various strategies to improve the resistance to SO_x poisoning are also discussed.

1. Introduction

Anthropogenic pollutant emissions increased with human activity until a few decades ago, posing a threat to human well-being. However, over the past few decades, regulations on the emission of these pollutants and the development of technologies to control emissions have had some success in preventing significant pollution. Sulfur oxides (SO_x), nitrogen oxides (NO_x and N₂O), CO, and volatile organic compounds (VOCs) are representative gaseous pollutants that are now strictly regulated [1]. In addition, greenhouse gases, including CO₂ and methane, are currently or will be regulated in the near future, depending on the country.

NO_x, such as nitrogen monoxide (NO) and nitrogen dioxide (NO₂), is formed from a variety of stationary and mobile sources. Fossil fuel-based power plants and internal combustion engine-based transportation vehicles are prime examples of stationary and mobile sources of NO_x emissions, respectively. In any case, thermal NO_x, formed at high temperatures when nitrogen oxidizes with oxygen in the air, is the primary pathway for NO_x emissions. As conventional power plants and internal combustion engine-based vehicles are replaced with renewable energy and electric vehicles, respectively, to cope with the CO₂ emissions problem, the NO_x emissions are expected to continue to decline. Meanwhile, with ammonia gaining much attention as a non-carbon fuel, fuel NO_x, which is formed through the partial oxidation of ammonia, can be another pathway for NO_x emissions in the stationary sources [2,3,4].

The selective catalytic reduction of NO_x with ammonia (NH₃-SCR) is the most widely adopted method for controlling NO_x emissions from stationary sources among the various methods developed to date [5,6]. The following reactions occur during NH₃-SCR in the absence of oxygen.

6NO + 4NH₃ → 5N₂ + 6H₂O

(1)

6NO₂ + 8NH₃ → 7N₂ + 12H₂O

(2)

NO + NO₂ + 2NH₃ → 2N₂ + 3H₂O

(3)

In practice, because most flue gases contain varying concentrations of oxygen, the following reactions take place.

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

(4)

2NO + 4NH₃ + 2O₂ →3N₂ + 6H₂O

(5)

2NO₂ + 4NH₃ + O₂ → 3N₂ + 6H₂O

(6)

In addition, ammonia oxidations, shown below, can occur in the presence of oxygen, which is not favorable for NH₃-SCR.

4NH₃ + 3O₂ → 2N₂ + 6H₂O

(7)

4NH₃ + 5O₂ → 4NO + 6H₂O

(8)

4NH₃ + 7O₂ → 4NO₂ + 6H₂O

(9)

2NH₃ + 2O₂ → N₂O + 3H₂O

(10)

N₂O can be additionally formed via the following reaction in the presence of the catalyst.

4NO + 4NH₃ + 3O₂ → 4N₂O + 6H₂O

(11)

Metal (W and/or Mo) oxides-promoted V₂O₅/TiO₂ is a representative commercial catalyst for NH₃-SCR, achieving high NO_x conversion over a wide operating temperature range of 300–400 °C [7,8,9]. Because flue gas composition varies depending on the fuel (e.g., coal, biomass, organic wastes, oil, natural gas, etc.), the NH₃-SCR unit is placed in different locations in the flue gas treatment process depending on the process characteristics [10]. Generally, since the flue gas contains fly ash, impurities, and SO₂, the dust removal device (e.g., electrostatic precipitator) and the flue gas desulfurization device can be installed to remove the fly ash and SO₂ in the flue gas, respectively [11]. After these units, additional preheaters are required to raise the flue gas temperature to the proper operating temperature of vanadia-based commercial catalysts because the flue gas temperature is reduced below 200 °C. Therefore, low-temperature NH₃-SCR catalysts need to be developed to eliminate the increased operating cost and an extra capital cost due to these additional preheating units.

To date, a variety of NH₃-SCR catalysts [12,13,14,15,16,17,18,19,20,21,22,23], including Cu-based [24], Ce-based [25,26,27,28], Mn-based [29,30,31], Ba-based [32], and carbon materials-supported catalysts [33,34], have been studied that are active below 200 °C. Compared with NH₃-SCR catalysts operating at medium and high temperatures, low-temperature NH₃-SCR catalysts are susceptible to poisoning by water vapor and SO₂ in flue gas [19,21]. In particular, SO₂ in flue gas can be catalytically oxidized to form SO₃, which reacts with ammonia and then converts to ammonium salts (NH₄HSO₄ (ABS) and (NH₄)₂SO₄ (AS)), which can block the active sites of NH₃-SCR [11,35]. Furthermore, additional side reactions forming N₂O have been observed over these catalysts. Low-temperature NH₃-SCR catalysts capable of solving the above problems have not yet been reported. This review article summarizes recent progress on low-temperature NH₃-SCR catalysis and briefly discusses research directions to address the current obstacles.

2. Low-Temperature NH₃-SCR Catalysts

Vanadia-based catalysts active at moderate temperatures can provide meaningful information about low-temperature NH₃-SCR catalysis. Arnarson et al. [36] proposed a reaction mechanism of ‘Standard NH₃-SCR’ combining with ‘Fast NH₃-SCR’ on a VO_x/TiO₂(001) catalyst model (Figure 1). The two cycles shared the same reduction part (A $\to$ B $\to$ D $\to$ E $\to$ F $\to$ P $\to$ R $\to$ A in Figure 1) but used NO + O₂ (F $\to$ I $\to$ M $\to$ N $\to$ P in Figure 1) and NO₂ (F $\to$ G′ $\to$ H′ $\to$ P in Figure 1) for the re-oxidation process, respectively. They noted that the rate of formation and desorption of H₂O is a decisive factor in the ‘Standard NH₃-SCR’ reaction at low temperatures and that the reaction of NO₂ with the reduction sites is responsible for accelerating the ‘Fast NH₃-SCR’ reaction at low temperatures [36].

The reaction mechanisms [36,37,38] over V₂O₅/TiO₂ catalyst reveal that NH₃-SCR activity depends on a number of factors, including the redox property of vanadium species between V⁺⁵ and V⁺⁴, Brønsted and Lewis acid sites on the catalyst surface, dispersion of vanadium species, acidic and basic properties of the support, contribution of a support to the redox property of vanadium species, the surface hydrophobicity of the support, the hydrothermal stability of the support, and adsorption property of SO_x. Therefore, these factors should also be considered when designing low-temperature NH₃-SCR catalysts. First of all, the redox properties of the active metal oxides are crucial for this reaction, so Mn, Fe, Co, Cu, and Ce can be selected as promising candidates for the active metal because they have a variety of metal oxides with different oxidation states and are interconvertible under reaction conditions. However, cobalt oxide can be excluded as a promising candidate for this reaction because of its very high activity towards complete oxidation [39]. Therefore, Cu-, Fe-, Mn-, and Ce-based low-temperature NH₃-SCR catalysts are covered in the following sections.

2.1. Cu-Based Catalysts

Cu-containing metal oxides and Cu-based small-pore zeolites have been reported to be active for low- and medium-temperature NH₃-SCR. Various factors, including support, Cu precursor, promoter, crystal structure, preparation method, and interface engineering between Cu and support, have been considered [24]. Research on Cu-based catalysts has been motivated by their high NH₃-SCR activity, especially at low temperatures, but their low SO₂ tolerance is recognized as a significant barrier to field application. Moreover, due to the low hydrothermal stability of Cu-containing metal oxides, recent research has focused on Cu-based small pore zeolites, as shown in

Table 1. NH₃-SCR activity over some Cu-based catalysts.

Catalysts 1	Reaction Conditions						NOx Conversion (%)	Ref.
	NO (ppm)	NH3 (ppm)	O2 (vol%)	H2O (vol%)	Space Velocity	T (°C)
Cu-LTA	500	500	5	10	100,000 h−1	230–500	>90%	[40]
Cu-LTA	500	500	5	10	100,000 h−1	165–470	>90%	[41]
Cu-ZSM-5	500	575	4	5	30,000 h−1	175–375	>90%	[42]
Cu-ZSM-5	1000	1000	3	-	50,000 h−1	200–375	~100%	[43]
Cu-ZSM-5	500	500	5	10	100,000 h−1	200–400	>90%	[44]
Cu-Beta	1000	1000	6	-	300,000 h−1	250–325	>90%	[45]
Cu/BEA	400	400	8	5	30,300 h−1	225–475	>90%	[46]
Cu-SSZ-13	500	500	10	5	80,000 h−1	160–500	>90%	[47]
Cu-SSZ-13	500	500	5	5	400,000 h−1	210–520	>90%	[48]
Cu-SSZ-16	500	500	10	-	42,500 mL·h−1·g−1	190–440	>90%	[49]
Cu-SSZ-39	500	500	5	5	250,000 h−1	225–500	>90%	[50]
Cu-SSZ-52	500	500	5	5	240,000 h−1	200–550	>90%	[51]
Cu-SAPO-34	500	500	6.1	6.4	300,000 h−1	190~500	>90%	[52]
Cu-RTH	500	500	5	10	100,000 h−1	470–750	>90%	[53]
Cu-ERI	300	300	5	3	50,000 h−1	240–500	>90%	[54]
Cu-UZM-9	500	500	5	10	100,000 h−1	250–650	>90%	[55]
Cu-UZM-35	500	500	5	10	100,000 h−1	200–420	>90%	[56]
Cu-ZJM-7	500	500	5	5	80,000 h−1	190~550	>90%	[57]
Cu-SAPO-18	1000	1100	5	10	30,000 h−1	250–500	>90%	[58]
Cu-SAPO-18	500	500	14	5	130,000 h−1	210–540	>80%	[59]
Cu/SSZ-13@SiC	500	500	10	5	80,000 h−1	200–360	>90%	[60]
CuFe/BEA	200	200	10	5	40,000 h−1	225–375	>90%	[61]
Fe/Cu-SSZ-13	500	500	5	-	50,000 h−1	150–500	>90%	[62]
CuY-SAPO-34	350	350	8	5	30,000 h−1	200–310	>90%	[63]
CuNd/SAPO-34	200	200	10	5	40,000 h−1	200–400	>90%	[64]
Cu-Ce-La-SSZ-13	500	500	5	-	150,000 h−1	175–400	>90%	[65]
Ce-Cu-SAPO-18	500	500	14	5	130,000 h−1	250–500	>90%	[66]
La-Cu-SAPO-18	500	500	5	-	20,000 mL·h−1·g−1	250~450	>90%	[67]
La-Cu-SAPO-18	500	500	14	5	130,000 h−1	210–580	>80%	[59]
Ce-Cu-SAPO-18	500	500	14	5	130,000 h−1	200–600	>90%	[59]
Nd-Cu-SAPO-18	500	500	14	5	130,000 h−1	250–550	>90%	[59]
Gd-Cu-SAPO-18	500	500	14	5	130,000 h−1	300–460	>90%	[59]
Tb-Cu-SAPO-18	500	500	14	5	130,000 h−1	220–570	>90%	[59]
Ho-Cu-SAPO-18	500	500	14	5	130,000 h−1	250–550	>90%	[59]
Lu-Cu-SAPO-18	500	500	14	5	130,000 h−1	250–520	>90%	[59]
Cu-Ce-USY	500	500	5	-	48,000 h−1	180~250	>90%	[68]
CuO(111)/TiO2	500	500	5	-	45,000 h−1	235–285	>90%	[69]
Cu/ZrO2	600	600	10	5	58,333 h−1	200–360	>90%	[70]
LaCuO3-x/meso-Al2O3	500	500	3	10	100,000 h−1	220–275	>50%	[71]
CuAl-(LDO)/(CNTs)	600	600	5	-	45,000 h−1	176–275	>90%	[72]
meso-Cu-SSZ-13@meso-Al-Si	500	500	5	5	400,000 h−1	250–520	>90%	[73]
Cu-SAPO-34@Fe-MOR	500	500	5	-	50,000 h−1	375–525	>90%	[74]
Cu-SSZ13@Ce0.75Zr0.25O2	500	500	5	-	60,000 h−1	220–480	>90%	[75]
Cu-SSZ-13 HN	500	500	3	-	120,000 h−1	200~550	>90%	[76]

¹ LDO: layered double oxide; USY: ultrastable Y; CNTs: carbon nanotubes; meso: mesoporous; Al-Si: aluminosilicate; HN: hollow nanocube.

[40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76]. Among them, low-temperature NH₃-SCR activity was reported over Cu-zeolites such as Cu-LTA [41], Cu-ZSM-5 [42,43,44], Cu-SSZ-13 [47], Cu-SSZ-16 [49], Cu-SSZ-52 [51], Cu-SAPO-34 [52], Cu-UZM-35 [56], and Cu-ZJM-7 [57], metal-promoted Cu-zeolites such as Ce-Cu-SAPO-18 [59], Fe/Cu-SSZ-13 [62], CuY-SAPO-34 [63], CuNd/SAPO-34, Cu-Ce-La-SSZ-13 [65], and Cu-Ce-USY [68], and other Cu-based oxides such as Cu/ZrO₂ [70] and CuAl layered double oxide (LDO) supported on carbon nanotubes (CNTs) [72].

Only a few reports can be found on Cu-containing metal oxides in the last few years. Chen et al. [69] examined the crystal-plane effects of CuO over CuO/TiO₂ catalysts on NH₃-SCR and reported that the proportion of Cu⁺ and surface-adsorbed oxygen (O_α) in CuO(111)/TiO₂ catalyst was higher than that of CuO(001)/TiO₂ catalyst, which could facilitate the NH₃-SCR reactions. Liu et al. [71] reported that LaCuO_3-x/meso-Al₂O₃ enriched with Cu³⁺ species exhibited significantly higher catalytic activity for NH₃-SCR than CuO/meso-Al₂O₃ counterpart in the low temperature range (100–280 °C), which was attributed to the unique nature of Cu³⁺ species, which had more acid sites and higher redox properties to promote the adsorption and activation of ammonia and NO_x.

Cu-containing zeolites have been intensively studied for NH₃-SCR and/or urea-SCR for their applications to stationary and mobile sources [57,77,78]. The NH₃-SCR performance of Cu-exchanged zeolite catalysts depends on the amount of isolated copper ion sites, SiO₂/Al₂O₃ ratios, and topological structures. The isolated Cu²⁺ species serve as redox sources, the zeolite supplies acidic sites, and the channel structure affects the diffusion of reactants, intermediates, and products. A variety of zeolite catalysts for Fe or Cu ion exchange have been developed, all of which exhibit a wide temperature window, including ZSM-5, SAPO-18, SAPO-34, SSZ-13, SSZ-16, SSZ-39, AFX, BEA, ERI, KFI, LTA, Nu-3, PST-7, RHO, RTH, Sigma-1, UZM-35, etc. [78,79,80,81,82,83]. In terms of the hydrothermal stability, ion-exchanged small-pore zeolites such as Cu-SSZ-13, Cu-SSZ 16, Cu-SSZ-39, Cu-SAPO-18, Cu-SAPO-34, and Cu-KFI are superior to the medium-pore ZSM-5 zeolite catalysts, and finally, Cu-SSZ-13 catalysts have been commercialized for NO_x control in diesel-powered vehicles owing to their outstanding NO_x removal efficiency and hydrothermal stability [77,79,84]. A comparison of the NH₃-SCR activity of Cu- and Fe-zeolites showed that at low temperatures (below 350 °C), the former was more active than the latter, but above 350 °C, the latter was more active than the former, which was ascribed to the fact that Cu-zeolites had higher adsorption sites for NH₃ than Fe-zeolites [85].

The reaction mechanism over Cu-SSZ-13 is shown in Figure 2 [86]. A mobile NH₃-complex, monomeric Cu ion was identified to be active in the NO_x reduction cycle in the redox mechanism. The Cu⁺ ion complexes thus formed subsequently migrate between the zeolite cages to form dimeric species that are important for O₂ activation, which is the rate-limiting step essential for the re-oxidation of Cu⁺ to complete the catalytic cycle [87]. Density functional theory (DFT) calculations on a reaction mechanism for low-temperature NH₃-SCR over Cu-CHA reveal that ammonia-solvated Cu cations, Cu(NH₃)₂⁺, are responsible for O₂ activation as well as the formation of the key intermediates HO-NO and H₂N-NO and that Brønsted acid site is related to decomposition of HO-NO and H₂N-NO to N₂ and H₂O [88]. Oxygen activation requires pairs of Cu(NH₃)₂⁺ complexes, but HO-NO and H₃N-NO coupling may occur on single complexes [88].

Catalyst deactivation in the presence of SO_x in the flue gas can be classified into two categories. One is irreversible deactivation due to the formation of inactive Cu sulfate species through interaction between SO_x and active Cu species [89,90,91,92,93,94,95]. The other is reversible deactivation resulting from the deposition of sulfate species (e.g., ABS and AS) [96]. SO₂ poisoning inhibits the oxidation of NO to NO₂, resulting in a lower NH₃-SCR activity below 350 °C [79]. Above this temperature, sulfate compounds are unstable so that the available Cu sites can be sufficient for NH₃-SCR activity [79]. Therefore, sulfur poisoning beyond 350 °C is insignificant.

Table 2. NH₃-SCR activity in the presence of H₂O and/or SO₂ over some Cu-based catalysts.

Catalysts 1	Reaction Conditions						NOx Conversion (%)	Effects of H2O/SO2	Ref
	NO (ppm)	NH3 (ppm)	O2 (vol%)	H2O (vol%)	Space Velocity	T (°C)
Cu-SAPO-18	500	500	14	5	130,000 h−1	200–550	>80%	Reversible inhibition with 5% H2O and 100 ppm SO2 at 300 °C	[97]
Cu-Ce-SAPO-18	500	500	14	5	130,000 h−1	200–600	>90%	Reversible inhibition with 5% H2O and 100 ppm SO2 at 300 °C	[97]
Cu/TNU-9	500	500	5	10	10,000 h−1	237–400	>90%	Stable NOx conversion with 10% H2O and 100 ppm SO2 at 250 °C	[98]
Cu–Ce/TNU-9	500	500	5	10	10,000 h−1	225–400	>90%	Stable NOx conversion with 10% H2O and 100 ppm SO2 at 250 °C	[98]
Cu–Ce–La/TNU-9	500	500	5	10	10,000 h−1	200–425	>90%	Stable NOx conversion with 10% H2O and 100 ppm SO2 at 250 °C	[98]
Mn-Ce/Cu-SSZ-13	500	500	3	3	50,000 h−1	125–450	>90%	Reversible inhibition with 3% H2O and 100 ppm SO2 at 300 °C	[99]
CuSbTiOx	700	700	4	4	60,000 h−1	250–300	>85%	Slowly deactivated with 5% H2O and 150 ppm SO2 at 250 °C	[100]
CuO@Cu3(BTC)2	600	600	4	-	60,000 h−1	180–240	>80%	Stable NOx conversion with 4% H2O and 150 ppm SO2 at 200 °C	[101]
Cu0.5Ce0.5W5Ox	500	500	5	-	36,000 h−1	270–390	~100%	Stable NOx conversion with 5% H2O and 50 ppm SO2 at 240 °C	[102]
WOx/Cu-CeO2	500	500	5	-	60,000 h−1	220–400	>90%	Stable NOx conversion with 10% H2O and 100 ppm SO2 at 250 °C	[103]
Nb0.05CuCeTi	600	600	3	5	40,000 h−1	160–360	>90%	Reversible inhibition with 5% H2O and 50 ppm SO2 at 250 °C	[104]
CuCeNbOx	600	600	5	-	108,000 h−1	185–360	>90%	Stable NOx conversion with 5% H2O and 100 ppm SO2 at 250 °C	[105]
Cu–HPMo/TiO2	500	500	8	-	15,000 h−1	150–350	>80%	100% NOx conversion with 4% H2O and 200 ppm SO2 at 200 °C	[106]
Cu–Ce–La/SSZ-13	500	500	5	3	150,000 h−1	210–450	>90%	Stable NOx conversion with 10% H2O and 100 ppm SO2 at 300 °C	[107]
Cu/(ZSM-5@CeO2)	1000	1000	8	5	50,000 h−1	225–550	>95%	Stable NOx conversion with 5% H2O and 200 ppm SO2 at 275 °C	[108]
Cu–Ce–La/SSZ-13@ZSM-5	500	500	5	3	150,000 h−1	200–450	>80%	Stable NOx conversion with 10% H2O and 100 ppm SO2 at 300 °C	[107]
Cu-SSZ-13@Ce-MnOx/MS	500	500	5	3	150,000 h−1	175–475	>90%	Relatively stable NOx conversion with 10% H2O and 100 ppm SO2 at 250 °C	[109]

¹ BTC: 1,3,5-Benzenetricarboxylic acid; MS: mesoporous silica.

summarizes the effects of H₂O and SO₂ on the NH₃-SCR over some Cu-based catalysts [97,98,99,100,101,102,103,104,105,106,107,108,109]. Among them, CuO@Cu-metal organic frameworks (MOFs) core-shell catalyst [101] and Cu-doped phosphomolybdic acid catalyst [106] showed a relatively stable catalytic performance even in the presence of H₂O and SO₂ at low temperatures.

Hammershøi et al. [89] reported that the NH₃-SCR activity over Cu-SSZ-13 catalysts was significantly inhibited at 160–350 °C after exposure to SO₂, but preserved above 350 °C. Jangjou et al. [110] compared SO₂-induced catalyst deactivation at [Cu^IIOH]⁺ and Cu²⁺ sites in Cu-SSZ-13. For Cu²⁺ sites, the NH₃-SCR activity was inhibited due to the formation of ammonium sulfate, which could be fully recovered by regeneration at >380 °C. This implies that Cu²⁺ sites do not adsorb sulfur during the exposure to SO₂ [110]. On the other hand, [Cu^IIOH]⁺ sites could adsorb sulfur directly to form Cu bisulfite, causing the irreversible deactivation [110]. Therefore, the sulfur poisoning of [Cu^IIOH]⁺ was much more severe than that of Cu²⁺. H₂SO₄ formed in the presence of SO₂ and H₂O plays an important role in the formation of ammonium sulfate [111,112]. Another route to the formation of Cu bisulfate is the interaction between SO₂ and CuO_x nanoclusters [113].

The effect of SO₂ on Cu-SAPO-34 deactivation was also investigated [91,114]. The NH₃-SCR activity of sulfur-poisoned Cu-SAPO-34 was decreased significantly at 100–500 °C, though sulfur accumulation and zeolite structure collapse were not observed [114]. The deactivation was attributed to the decreased amount of isolated Cu²⁺ ions [91,114]. It was speculated that sulfur poisoning could hinder the copper redox transformation (between Cu^II and Cu^I) in Cu-SAPO-34, which led to the sulfation of Cu sites [115]. In addition, Cu-SAPO-34 favored the formation of stable Al₂(SO₄)₃ species compared with Cu-SSZ-13, resulting in an irrecoverable loss in NH₃-SCR activity [111].

When screening Cu-containing zeolite catalysts, hydrothermal stability has become an important selection criterion in addition to high NO_x conversion over a wide range of reaction temperatures for application in transport vehicles, but this is not as important for low-temperature NH₃-SCR catalysts for application in stationary sources. Rather, resistance to water and SO₂ at low temperatures is an additional factor to be considered in the development of NH₃-SCR catalysts. In order to improve the low-temperature NH₃-SCR activity of Cu-containing zeolite catalysts, the Cu active sites can be modified by introducing the heteroatoms to facilitate the redox property of Cu and metal oxides to accelerate NO oxidation for ‘Fast NH₃-SCR’ [116]. Other zeolites with different structures, including AEI and LTA, the small-pore intergrown zeolites, including AFX/CHA and CHA/AEI, and the zeolite morphology can be further examined even though they were excluded because of their poor hydrothermal stability at high temperatures [116]. Additional doping of Y [63] and lanthanides [59,60,61,62,63,64] to Cu-zeolites was reported to be effective for NH₃-SCR. The composite catalysts such as MnO₂-CeO₂/Cu/SSZ-13 showed excellent NH₃-SCR activity and N₂ selectivity in the temperature ranges of 125 to 450 °C because more active monodentate nitrate was formed on the surface of the composite catalyst compared with Cu/SSZ-13 alone [99].

Chen et al. [107] synthesized a core–shell structure Cu–Ce–La/SSZ-13@ZSM-5 catalyst by a self-assembly method and applied it to NH₃-SCR. They observed that Cu–Ce–La/SSZ-13@ZSM-5 with an appropriate shell thickness presented better NH₃-SCR activity and hydrothermal stability than Cu–Ce–La/SSZ-13 because some metal ions were transferred and redistributed during the assembly of the ZSM-5 shell, resulting in the conversion of [Cu(OH)]⁺–Z to Cu²⁺–2Z species and the functionalization of the shell phase, which was beneficial for the adsorption and activation of NH₃. Chen et al. [109] synthesized a multifunctional core-shell catalyst with Cu-SSZ-13 as the core phase and Ce-MnO_x supported mesoporous silica as the shell phase via self-assembly and impregnation. The core-shell catalyst exhibited excellent low-temperature activity, SO₂ tolerance, and hydrothermal stability compared with the Cu-SSZ-13 [109]. The Ce-MnO_x species dispersed in the shell can rapidly activate NO and oxidize it to NO₂, which allows the NH₃-SCR reaction on the core-shell catalyst to be initiated in the shell phase [109]. Meanwhile, Ce-MnO_x species can react preferentially with SO₂ as sacrifice components, effectively avoiding the sulfur inactivation of the copper active sites [109]. This catalyst showed relatively stable NO conversion in the presence of 10% H₂O and 100 ppm SO₂ at 250 °C [109]. It is noteworthy that few low-temperature Cu-based catalysts with high resistance to H₂O and SO₂ poisoning have been reported.

2.2. Fe-Based Catalysts

Various Fe-based catalysts, including single iron oxides, Fe-containing mixed metal oxides, supported iron oxides, supported Fe-containing multicomponent metal oxides, and Fe-containing zeolites, have been reported for NH₃-SCR due to their excellent redox properties and low cost. Another advantage of Fe-based catalysts is their relatively good resistance to water and SO₂, although this has only been reported at temperatures above 200 °C [11].

γ-Fe₂O₃ has been mainly reported for NH₃-SCR among various single-phase Fe₂O₃ with different crystalline structures (e.g., α, β, γ, and ε) [11]. Yu et al. [117] proposed two different reaction mechanisms over γ-Fe₂O₃ (Figure 3). One is the Langmuir–Hinshelwood (L–H) mechanism in which NH₃ and NO are adsorbed on the active sites, Fe³⁺-OH and Fe³⁺=O, respectively, and reacted to form N₂ and H₂O, which is prevalent at temperatures below 300 °C [117]. The other is the Eley–Rideal (E–R) mechanism in which NH₃ is first chemisorbed on Fe³⁺-OH to form Fe³⁺-O^−$\cdots$H-N⁺H₃, which can be further reacted with gaseous NO to produce N₂ and H₂O, which is the main pathway at high temperatures above 300 °C [117]. In any case, this reaction requires the cooperation of acidic sites and redox properties. They also confirmed that the NH₃-SCR activity decreased firstly and then increased slowly after the introduction of SO₂ at temperatures ranging from 225 to 275 °C [117]. This was ascribed to the formation of iron sulfate species inhibiting the adsorption of NO_x, thus interfering with the L–H reaction pathway [117]. On the other hand, the iron sulfate species formed enhanced the surface acidity, which promoted the E–R reaction pathway and further promoted NH₃-SCR activity [117].

Recent work on single Fe oxides has focused on how to increase the low-temperature NH₃-SCR activity. Qin et al. [118] prepared various Fe₃O₄ nanostructures exposed with different crystal planes from MIL-100(Fe) as the Fe precursor and found that the catalysts with more (1 1 1) had better NH₃-SCR performance than those with (1 0 0) exposure, which they attributed to the preferential exposure of the Fe₃O₄ (1 1 1) crystal faces leading to higher adsorbed oxygen concentration and surface acidity. Yang et al. [119] found through DFT calculations that the ‘Fast NH₃-SCR’ reaction was the dominant pathway for the NH₃-SCR reaction on Fe₂-N₆ catalysts, with the energy barrier of the rate-determining step (HONO formation) being 1.00 eV, much lower than that of other NH₃-SCR catalysts, enabling excellent low-temperature activity in the temperature window of 300–500 K. Zhang et al. [120] prepared a highly defective α-Fe₂O₃ with enhanced acid (Lewis and Brønsted) and redox properties on homoatomic dinuclear sites comprising more positively charged Fe³⁺ and oxygen vacancy-coupled Fe²⁺ ions. The catalyst showed enhanced NH₃-SCR activity at low temperatures without the addition of other acid transition metals and showed resistance to poisoning of H₂O and SO₂ due to the large amount of Brønsted acid [120].

Various metal oxide-promoted iron oxides have been investigated to enhance the cooperation of surface acidity and redox property to facilitate the low-temperature NH₃-SCR. The positive effects of Mn [121], Nb [122], Mo [123], Ce [124], Sm [125], and W [123,126,127,128] in promoted iron oxides on the catalytic activity have been reported.

Table 3. NH₃-SCR activity in the presence of H₂O and/or SO₂ over some Fe-based catalysts.

Catalysts	Reaction Conditions						NOx Conversion (%)	Effects of H2O/SO2	Ref.
	NO (ppm)	NH3 (ppm)	O2 (vol%)	H2O (vol%)	Space Velocity	T (°C)
Defective α-Fe2O3	500	500	5.3	-	50,000 h−1	250–300	~100%	100% NOx conversion with 5% H2O and 50 ppm SO2 at 300 °C	[120]
MnFeOx	500	500	5	5	75,000 h−1	75–275	~100%	A rather stable NOx conversion with 5% H2O and 50 ppm SO2 at 100 °C	[121]
Ce/α-Fe2O3	500	500	5	5	90,000 h−1	175–325	>95%	Stable NOx conversion wit 5% H2O and 200 ppm SO2 at 250 °C	[124]
Fe–Ce/TiO2	1000	1000	3	-	30,000 h−1	175–350	>90%	Slowly deactivated with 500 ppm SO2 at 250 °C	[129]
Mo0.4Ce0.3FeOx	2000	2000	8	-	32,000 h−1	200–350	>95%	Stable NOx conversion with 10% H2O and 200 ppm SO2 at 250 °C	[130]
Fe–Ce–W oxides	450	450	2.5	5	20,000 h−1	250–500	>90%	Relatively stable NOx conversion with 5% H2O and 200 ppm SO2 at 350 °C	[131]
Sm/Fe2O3	500	500	5	5	14,400 h−1	175–350	~100%	Stable NOx conversion with 5% H2O and 100 ppm SO2 at 275 °C	[125]
WOx/Fe2O3	500	500	5	-	50,000 h−1	300–425	>90%	Stable NOx conversion with 100 ppm SO2 at 300 °C	[128]
H3PW12O40-Fe2O3	1000	1100	6	-	13,200 h−1	300–500	>90%	Stable NOx conversion with 10% H2O and 200 ppm SO2 at 280 °C	[132]
Mn-W-Sb modified siderite	500	500	5	-	30,000 h−1	175–375	>90%	Irreversible deactivation with 5% H2O and 100 ppm SO2 at 210 °C	[133]
Fe2O3-promoted halloysite-supported CeO2-WO3	500	500	5	-	40,000 h−1	275–420	>95%	Relatively stable NOx conversion with 8% H2O and 100 ppm SO2 at 300 °C	[134]
Fe2O3-CeO2@Al2O3	500	500	5	-	20,000 mL·g−1·h−1	250–430	>90%	Stable NOx conversion with 10% H2O and 500 ppm SO2 at 270 °C	[135]
Fe-ZSM-5@CeO2	500	500	5	-	177,000 h−1	250–425	>90%	Stable NOx conversion with 10% H2O and 100 ppm SO2 at 350 °C	[136]
Fe-Beta@CeO2	500	500	3	5	50,000 h−1	225–575	>90%	Stable NOx conversion with 5% H2O and 100 ppm SO2 at 300 °C	[137]

summarizes the effects of H₂O and SO₂ on the NH₃-SCR over some Fe-based catalysts [120,121,124,125,128,129,130,131,132,133,134,135,136,137]. Among them, a relatively stable catalytic performance was observed over Mn-Fe oxides [121] even in the presence of H₂O and SO₂ at low temperatures.

It is noteworthy that, except for catalysts containing additional Mn, few Fe-based catalysts are active for NH₃-SCR at low temperatures (<200 °C) while being resistant to H₂O and SO₂ poisoning. Jiang et al. [138] prepared a phosphotungstic acid (HPW)-promoted Fe-based catalyst from MIL-100(Fe) as the Fe precursor by hydrothermal method and reported that active oxygen species-rich γ-Fe₂O₃ was the main Fe phase, which enhanced NO adsorption and activation, leading to faster NH₃-SCR. The role of HPW was to increase the total acidic sites along with promoting the reactivity of NH₃ adsorbed on Lewis acidic sites at low temperatures. Notably, WO₃-promoted Fe₂O₃ was reported to have a wide temperature window and excellent water and sulfur resistance, showing relatively stable NO conversion in the presence of 100 ppm SO₂ at 300 °C [128]. The roles of WO₃ are known to inhibit the crystallization of the Fe₂O₃ phase and formation of inactive nitrate, instead increasing Lewis acid sites and appropriate redox properties [127,128]. A kind of composite catalyst, HPW-decorated ring-like Fe₂O₃ synthesized via mechanical-chemistry grinding of HPW and Fe₂O₃ nanorings prepared by a microwave-assisted hydrothermal method, showed good NH₃-SCR performance over a wide temperature window of 250–500 °C [139] and an outstanding resistance against SO₂, showing relatively stable NO conversion in the presence of 10% H₂O and 200 ppm SO₂ at 280 °C [132]. Sun et al. [125] reported that Sm modification could weaken the sulfation of active Fe sites in Sm-doped Fe₂O₃, which was also supported by the DFT calculation results that SO₂ could be more easily adsorbed on the Sm/Fe₂O₃ catalyst with the adsorption sites located at the Sm atom and its neighboring Fe atoms. They observed that more reactive nitrate species were formed on the sulfated Sm/Fe₂O₃ catalyst due to the presence of more un-sulfated Fe sites, which they explained as making the Sm/Fe₂O₃ catalyst resistant to SO₂ poisoning, showing relatively stable NO conversion in the presence of 5% H₂O and 100 ppm SO₂ at 275 °C [125]. Tan et al. [140] reported that the low-temperature (<250 °C) NH₃-SCR activity of FeTiO_x catalyst could be dramatically enhanced by CeO₂ doping, which can be attributed to the presence of a unique Ce-O-Fe structure that contributes to the improvement of redox properties. Chen et al. [124] prepared a single-atom Ce-modified α-Fe₂O₃ catalyst by a citric acid-assisted sol–gel method and reported that a high NO conversion was maintained in the presence of 5% H₂O and 200 ppm SO₂ at 250 °C. They claimed that the atomic dispersion of the Ce species to maximize the amounts of Fe–O–Ce sites in the Ce-doped FeO_x catalyst was critical because the formation of oxygen vacancies in the Fe–O–Ce sites, which could promote the oxidation of NO to NO₂ and decomposition of ABS, was more favorable than that in the Fe–O–Fe sites in the Ce-free α-Fe₂O₃ catalyst [124]. Ma et al. [141] compared a serial of Cu_0.02Fe_0.2Ce_yTi_1-yO_x catalysts prepared by the sol–gel method and found that Cu_0.02Fe_0.2Ce_0.2Ti_0.8O_x exhibited superior low-temperature NH₃-SCR performance in the presence and absence of water, which they attributed to the optimal distribution of surface acidity, enhanced surface oxygen content, and surface redox cycle (Ce⁴⁺ + Fe²⁺ $\to$ Ce³⁺ + Fe³⁺). Yao et al. [133] prepared Mn-W-Sb modified siderite catalysts by impregnation method and found that the Mn-doping enhanced adsorbed NO₂ formation by synergistic catalysis with Fe³⁺ and that the addition of Sb inhibited sulfate formation on the surface of the catalyst in the presence of SO₂ and H₂O, showing relatively stable NO conversion in the presence of 5% H₂O and 100 ppm SO₂ at 210 °C. Xu et al. [121] prepared various MnFeO_x catalysts with different molar ratios and observed high low-temperature NH₃-SCR activity. The Mn–Fe-0.2 (Mn/Fe = 0.2) catalyst presented excellent SO₂/H₂O tolerance, showing a rather stable NO_x conversion even in the presence of 5% H₂O and 50 ppm SO₂ at 100 °C [121]. Doping Mn not only inhibited the phase transformation of iron oxide (Fe₂O₃) but also strengthened the interaction between MnO_x and Fe₂O₃ due to the electron transfer between them, which led to the formation of Mn–O–Fe [121]. Bai et al. [142] prepared an amorphous metal oxide (FeO_x-Mn_0.1O_y) with a large surface area, sufficient oxygen vacancies, and excellent redox properties and observed high adsorption and activation capacities for O₂ and NO, which further enhanced the catalytic activity at low temperatures (90–240 °C). The incorporation of Mn into the FeO_x species suppressed the crystallization of hematite, further increasing the surface area and surface acid sites [142]. In addition, the incorporation of manganese increased the number of oxygen vacancies, which decreased the apparent activation energy of hematite and enhanced the redox properties of the amorphous FeO_x-MnO_y catalyst [142].

The mesoporous Fe-doped CeO₂ catalyst after modifying organic sulfate functional groups showed excellent activity in a temperature range of 250–450 °C, which was ascribed to the strong electron interaction between Fe³⁺-O-Ce⁴⁺ species and sulfate groups, which modifies the acidity and redox properties of the catalyst [143]. Wang et al. [144] prepared a series of sulfated modified Fe–Ce composite oxide Fe_1–xCe_xO_δ-S catalysts and reported that the Fe_0.79Ce_0.21O_δ-S catalyst achieved the low-temperature NH₃-SCR activity at temperatures of 175–375 °C. They claimed that sulfation formed a large amount of sulfate on the catalyst surface and provided abundant Brønsted acid sites, which enhances NH₃ adsorption capacity and improves overall NO_x conversion efficiency [144]. The introduction of Ce was the main determinant to control the low-temperature activity of the catalyst by modulating its redox ability, and they found that there was a strong interaction between Fe and Ce in the Fe_0.79Ce_0.21O_δ-S catalyst, which changed the electron density around the Fe ions, which weakened the strength of the Fe–O bond and improved the lattice oxygen mobility of the catalyst [144]. In addition, during the reaction, the Fe-Ce composite oxide catalyst showed higher surface lattice oxygen activity and a faster bulk lattice oxygen replenishment rate [144].

The support of the supported iron oxide can increase the dispersion of the iron oxide, providing high surface active sites per mass of iron oxide, provide additional surface acid sites on the support itself and at the interface between the support and the iron oxide, and improve the redox properties of the iron oxide. Different crystal planes of the support (e.g., TiO₂ [145] and CeO₂ [146]) of supported iron oxides were compared. Monolayer Fe₂O₃ supported on TiO₂ nanosheets exhibited a better low-temperature NH₃-SCR activity than that supported on TiO₂ nanospindles because the former had more acidic sites, oxygen defects, and reactive oxygen species [145]. The iron oxides supported on CeO₂ nanorods achieved higher catalytic activity for NH₃-SCR than those supported on CeO₂ nanopolyhedra, which was explained by the DFT calculation results that the Fe₂O₃/CeO₂ {110} catalyst was more reactive to NO and NH₃ than the Fe₂O₃/CeO₂ {110} [146]. The Fe₂O₃{1 1 3}-TiO₂ exhibited superior NO_x removal capacity and a broader temperature operating range than Fe₂O₃{0 1 2}-TiO₂ and Fe₂O₃{0 1 4}-TiO₂, which can be attributed to the improved redox properties, as well as the presence of additional active oxygen species, surface acid sites, and adsorbed nitrate species on the Fe₂O₃{1 1 3}-TiO₂ catalyst [147].

2.3. Mn-Based Catalysts

Mn-based catalysts, including MnO_x, Mn-containing mixed metal oxides, supported MnO_x, and supported Mn-containing multicomponent metal oxides, have been regarded as the promising low-temperature NH₃-SCR catalysts [30,148,149,150]. A comparison of the low-temperature NH₃-SCR activity between various transition metal oxides supported on TiO₂ showed that it decreased in the following order: Mn > Cu ≥ Cr >> Co. > Fe >> V >> Ni [151]. First-principles calculations also revealed that the superior oxidative dehydrogenation performance of Mn-based catalysts to NH₃ lowered the energy barrier for the activation of NH₃ and reduced the formation of the key intermediate NH₂NO, the rate-determining step in NH₃-SCR, over Mn-, Fe-, and Ce-based oxide catalysts [152].

Among various single-phase manganese oxides, including MnO₂, Mn₅O_8, Mn₂O₃, Mn₃O₄, and MnO, MnO₂, and Mn₂O₃ were reported to have the highest low-temperature activity per unit surface area and N₂ selectivity, respectively [153]. The crystallinity and valence state of MnO_x affected by the preparation method and pretreatment conditions have been reported to influence the NH₃-SCR performance [154]. Higher low-temperature NH₃-SCR activity was obtained for the less crystalline MnO_x [155,156]. A comparison of β-MnO₂ and α-Mn₂O₃ showed that the former had a higher NO_x conversion and N₂O production rate per unit surface area than the latter, which was explained by the lower Mn–O bond energy of β-MnO₂, which promoted the activation of NH₃ [157]. The birnessite-type MnO₂ catalyst, prepared from Mn(NO₃)₂·4H₂O, having the fewest OH groups, was reported to exhibit the best catalytic activity and excellent SO₂ resistance among the same-type catalysts prepared from different Mn precursors [158].

A novel synthetic method was applied to prepare high-surface-area manganese oxide with low crystallinity for this reaction. Mesoporous α-MnO₂ nanosheets prepared by a solvent-free synthetic method had a high surface area and a mesopore size of 4 nm with large oxygen vacancies and showed 100% NO_x conversion under a gas hourly space velocity (GHSV) of 700,000 h⁻¹ at 100 °C [159]. Xu et al. [160] prepared MnO_x catalysts with 3D structure by the hard-template method using KIT-6 as a template possessing high reducibility with abundant surface oxygen species and Mn⁴⁺ species and reported that more Lewis acid sites and Brønsted acid sites on the surface were beneficial for the adsorption and activation of NH₃, leading to the higher NH₃-SCR activity. Chen et al. [161] synthesized a novel MnO_x catalyst with a large surface area, small particle size, and more crystalline defects from MOF-Mn₃(BTC)₂(H₂O)₆ using different amounts of polyvinyl pyrrolidone (PVP) and found that this catalyst had abundant acid sites, Mn⁴⁺, and surface chemical oxygen, promoting the NH₃-SCR reaction. Moreover, the high SO₂ tolerance was also observed because an irreversible sulfurization rate was reduced and an adsorption of active bidentate nitrates and NH₄⁺ was promoted even in the coexistence of sulfates, showing relatively stable NO conversion in the presence of 50 ppm SO₂ at 175 °C. Zhang et al. [162] also prepared various MnO_x catalysts from MOF-74 under different pretreatment conditions and reported that their redox capability could be controlled by changing the ratio of Mn⁴⁺/Mnⁿ⁺ and O_α/(O_α + O_β), resulting in the promotion of the adsorption and oxidation of NO to facilitate the ‘Fast NH₃-SCR’ reaction. Zhou et al. [163] prepared hydrothermally stable MnO_x/Al₂O₃ catalysts with highly dispersed low-coordinated Mn active sites that were originally created with triethanolamine and observed excellent low-temperature NH₃-SCR activity because of active low-coordinated Mn species possessing reactive redox sites and Lewis acid sites.

Li et al. [164] employed DFT calculations for the reaction mechanism of NH₃-SCR over Mn/γ-Al₂O₃ catalyst. NH₃ is mainly adsorbed on the Lewis acid sites and forms coordinated NH₃. Subsequently, an N–H bond in the adsorbed NH₃ can be dissociated to form NH₂*, which can react with the gaseous NO and generate NH₂NO*. NH₂NO* was finally decomposed into N₂* and H₂O* (Figure 4). On the surface of the Mn/γ-Al₂O₃ catalyst, the adsorbed O₂ was decomposed into the active oxygen atoms, which could oxidize NO into NO₂. Among three possible routes for N₂O formation, such as NO decomposition, deep dehydrogenation of NH₃, and two-step dehydrogenation of NH₂NO, the deep dehydrogenation of NH₃ appears to be mainly responsible over the Mn/γ-Al₂O₃ catalyst in both the L–H and E–R reaction mechanisms.

The formation of N₂O over Mn-based catalysts was systematically examined over MnO_x-TiO₂ catalysts with different Mn/Ti ratios [165]. As the Mn/Ti ratio increased, the MnO_x species layer expanded on TiO₂ support, leading to an increase of redox sites but a decrease of surface acid sites, contributing to the non-selective oxidation of NH₃ on MnO_x species and the over-activation of NH₃ at the Mn-Ti interface and resulting in the formation of N₂O. Yang et al. [166] proposed the scheme for N₂O formation according to the L–H and E–R mechanisms, in which monodentate –NO₃⁻ (and NH₄NO₃*) and –NH are key intermediates for N₂O formation, respectively (Figure 5). DFT calculations and thermodynamic/kinetic analysis of NH₃-SCR over α-MnO₂ with specific (100), (110) and (310) exposure planes showed that the α-MnO₂ catalyst exposed with the (310) plane exhibited the best NH₃-SCR catalytic performance and the highest N₂ selectivity due to the low energy barrier of NH₃ dehydrogenation and NH₂NO generation and the difficulty of NH₂ dissociation [167].

Although manganese oxides exhibit high NH₃-SCR activity at low temperatures, their application in practical processes is limited due to their sensitivity to SO₂ and low N₂ selectivity [21,168]. Therefore, Mn-containing mixed metal oxides and supported Mn-containing metal oxides have been adopted to address these issues. For Mn-containing mixed metal oxides, various second metals including Al [169], Si [170], Ti [171], V [172], Cr [173], Fe [174,175,176,177,178], Co [179,180,181,182], Ni [183], Cu [184,185], Y [186], Zr [187], Nb [188], Mo [189,190,191], Ag [192,193], Sn [194], Sb [195], La [196,197], Ce [197,198,199,200,201,202,203,204], Pr [205], Nd [206], Sm [207,208,209], Eu [210], Gd [211], Dy [212,213], Ho [214], Er [215], Tm [216], Ta [217], W [218,219], and Bi [220] have been examined. The effects of H₂O and SO₂ on the NH₃-SCR activity over unsupported Mn-based catalysts, supported Mn-based catalysts, and core-shell Mn-based catalysts are summarized in

Table 4. NH₃-SCR activity in the presence of H₂O and/or SO₂ over some unsupported Mn-based catalysts.

Catalysts 1	Reaction Conditions						NOx Conversion (%)	Effects of H2O/SO2	Ref.
	NO (ppm)	NH3 (ppm)	O2 (vol%)	H2O (vol%)	Space Velocity	T (°C)
MnOx	500	500	5	-	140,000 h−1	125–200	>90%	Relatively stable NOx conversion with 50 ppm SO2 at 175 °C	[161]
MnOx-SiO2	1000	1000	10	-	24,000 h−1	150–225	>90%	Relatively stable NOx conversion with 100 ppm SO2 at 225 °C	[221]
Mn-TiOx	500	500	5	-	100,000 h−1	160–370	>90%	Relatively stable NOx conversion with 5% H2O at 140 °C. Relatively stable NOx conversion with 100 ppm SO2 at 260 °C.	[222]
Cr-MnOx	1000	1000	3	-	30,000 h−1	115–220	>90%	Reversible inhibition with 100 ppm SO2 at 120 °C	[223]
CrMn2O4 spinel	500	500	5	-	32,000 h−1	80–225	>90%	Relatively stable NO conversion with 10% H2O and 150 ppm SO2 at 200 °C	[173]
MnOx-Fe	1000	1000	3	5	30,000 h−1	93–220	>90%	Relatively stable NO conversion with 5% H2O and 100 ppm SO2 at 200 °C	[224]
MnFeCo-LDO	550	550	5	-	30,600 h−1	63–400	>90%	Stable NO conversion with 5% H2O and 100 ppm SO2 at 120 °C	[225]
Mn-Fe-Mg oxides	1000	1000	4	-	30,000 h−1	125–200	>90%	Stable NOx conversions with 3% H2O and 100 ppm SO2 at 150 °C	[226]
Mn-Fe-Al oxides	500	500	5	-	60,000 h−1	110–250	>90%	Stable NOx conversions with 100 ppm SO2 at 150 °C	[227]
Mn-Fe-Ce oxides	500	500	11	-	36,000 h−1	180–277	>90%	Stable NOx conversions with 100 ppm SO2 at 225 °C	[228]
Mesoporous Mn-Fe-Ce-Ti oxides	600	480	2	-	24,000 h−1	210–395	>90%	Reversible inhibition with 300 ppm SO2 at 240 °C	[229]
Mn2Co1Ox	1000	1000	5	-	30,000 h−1	150–325	>90%	Reversible inhibition with 10% H2O and 100 ppm SO2 at 200 °C	[230]
Mn-Co oxides	500	500	5	-	23,000 h−1	100–300	>90%	Reversible inhibition with 8% H2O at 120 °C	[231]
Mn-Co oxides	500	500	3	-	30,000 h−1	100–280	>90%	Stable NOx conversions with 5% H2O and 100 ppm SO2 at 160 °C	[232]
Mn-Co oxides	500	500	5	-	140,000 h−1	80–200	>90%	Stable NOx conversions with 5% H2O and 50 ppm SO2 at 125 °C	[233]
Mn-Co oxides	500	500	5	-	50,000 h−1	60–300	>90%	Reversible inhibition with 5% H2O and 100 ppm SO2 at 200 °C	[234]
Co1Mn4Ce5Ox	500	500	5	-	48,000 h−1	80–175	>90%	Relatively stable NOx conversion with 10% H2O and 150 ppm SO2 and at 175 °C	[235]
MnCoVOx	500	500	5	-	60,000 mL·g−1·h−1	175–425	>90%	Relatively stable NO conversion with 5% H2O and 100 ppm SO2 at 200 °C	[236]
Mn-Ni oxides	550	550	5	-	64,000 h−1	105–275	>90%	Stable NOx conversion with 100 ppm SO2 at 230 °C	[237]
NiMn2O4	500	500	5	-	32,000 h−1	73–250	>98%	Stable NOx conversions in the presence of 150 ppm SO2 at 175 °C	[238]
Ni1Mn4O5	500	500	5	-	48,000 h−1	125–200	>90%	Relatively stable NOx conversion with 10% H2O and 150 ppm SO2 at 175 °C	[235]
Ni1Mn2O4-S	500	500	5	-	68,000 h−1	90–230	>90%	Relatively stable NOx conversion with 5% H2O and 100 ppm SO2 at 150 °C	[239]
Ni1Mn0.5Al0.5Ox	500	500	5	5	60,000 h−1	100–250	>90%	Relatively stable NO conversion with 5% H2O and 100 ppm SO2 at 200 °C	[240]
Nia-xMnxAlOy	500	500	6.5	5	45,000 h−1	120–225	>90%	Deactivation with 5% H2O and 100 ppm SO2 at 210 °C	[241]
Mn-Ni-Ti oxides	500	500	5	-	40,000 h−1	320–460	>90%	Relatively stable NOx conversion with 10% H2O and 100 ppm SO2 at 400 °C	[242]
Mn-Ni-Ti oxides	1000	1000	3	-	40,000 h−1	190–360	~100%	Relatively stable NOx conversion with 15% H2O and 100 ppm SO2 at 240 °C	[243]
Ni0.5Mn0.5Fe0.5Ox	500	500	5	-	60,000 h−1	100–300	>80%	Relatively stable NOx conversion with 5% H2O and 100 ppm SO2 at 200 °C	[244]
Mn-Zr oxides	1000	1000	3	-	30,000 h−1	100–200	~100%	Reversible deactivation with 5% H2O and 100ppm SO2 at 150 °C	[245]
Mn-Zr-Ti oxides	650	650	5	-	36,000 h−1	160–300	>90%	Relatively stable NOx conversion with 3% H2O and 50 ppm SO2 at 180 °C	[246]
Mn2Nb1Ox	500	500	5	-	50,000 h−1	120–200	>90%	Irreversible deactivation with 100 ppm SO2 at 200 °C	[247]
Mn-Ce oxides	1000	1000	2	-	35,000 h−1	100–300	>80%	Relatively stable NOx conversion with 12% H2O and 100 ppm SO2 at 110 °C	[248]
Mn-Ce oxides	3000	3000	15	-	60,000 mL·g−1·h−1	100–250	>80%	Deactivation with 5% H2O and 100 ppm SO2 at 175 °C	[206]
Mn-Ce nanowire aerogel	500	500	5	-	32,000 h−1	100–400	>90%	Stable NOx conversion with 10% H2O and 250 ppm SO2 at 150 °C	[249]
Ce-Ti/MnO2	600	600	6	-	40,000 h−1	100–225	>95%	Relatively stable NOx conversion with 10 vol% H2O and 50 ppm SO2 at 150 °C	[250]
Mn–Ce–Ti oxides	500	500	5	-	14,400 h−1	170–320	>90%	Stable NO conversion with 5 vol% H2O and 100 ppm SO2 at 200 °C	[251]
Mn-Ce-Ti oxides	500	500	5	-	64,000 h−1	150–350	>90%	Stable NO conversion with 5 vol% H2O and 50 ppm SO2 at 200 °C	[252]
Mn-Sn-Ce oxides	1000	1000	2	-	35,000 h−1	75–250	>90%	Stable NO conversion with 9% H2O and 100 ppm SO2 at 220 °C	[194]
Mn-Sn-Ce oxides	1000	1000	2	-	35,000 h−1	75–225	~100%	Relatively stable NOx conversion with 12% H2O and 100 ppm SO2 at 110 °C	[248]
Mn-Sn−Ce oxides	500	500	5	-	60,000 mL·g−1·h−1	175–275	>60%	Stable NO conversion with 5% H2O and 100 ppm SO2 at 250 °C	[253]
Mn-Pr-Ce oxides	600	600	5	-	108,000 h−1	150–400	>80%	Stable NO conversion with 5% H2O and 100 ppm SO2 at 250 °C	[205]
Mn-Sm oxides	500	500	5	-	48,600 h−1	50–250	>90%	Stable NO conversion with 2% H2O and 100 ppm SO2 at 250 °C	[254]
Mn-Sm oxides	500	500	5	-	60,000 mL·g−1·h−1	200–325	>80%	Deactivation with 5% H2O and 100 ppm SO2 at 175 °C	[206]
Mn-Sm-Ti oxides	500	500	5	-	36,000 h−1	150–300	>90%	Stable NO conversion with 5% H2O and 100 ppm SO2 at 200 °C	[255]
Mn-Sm-Ti oxides	500	500	5	-	50,000 h−1	75–230	>90%	Stable NOx conversion with 5% H2O and 100 ppm SO2 at 100 °C	[256]
Mn-Sm-Fe oxides	500	500	5	-	60,000 h−1	75–200	~100%	Relatively stable NOx conversion with 5% H2O and 100 ppm SO2 at 200 °C	[257]
Mn-Sm-Zr-Ti oxides	500	500	5	-	30,000 h−1	125–275	~100%	Relatively stable NOx conversion with 2.5% H2O and 100 ppm SO2 at 200 °C	[258]
Mn-Sm-Ce-Ti oxides	500	500	5	-	80,000 h−1	150–400	>90%	Relatively stable NO conversion with 5% H2O and 200 ppm SO2 at 200 °C	[259]
Mn-Nd oxides	500	500	5	-	60,000 mL·g−1·h−1	125–230	>90%	Relatively stable NO conversion 5% H2O and 100 ppm SO2 at 175 °C	[206]
Mn-Eu oxides	600	600	5	-	108,000 h−1	130–400	~100%	Relatively stable NO conversion with 5% H2O and 100 ppm SO2 at 350 °C	[260]
Mn-Eu-Ce oxides	500	500	10	10	60,000 h−1	100–250	>90%	Stable NOx conversions with 10% H2O and 50 ppm SO2 at 230 °C	[210]
Mn-Gd oxides	500	500	5	-	100,000 h−1	120–330	~100%	Stable NOx conversions with 100 ppm SO2 at 200 °C	[261]
Mn-W-Ce oxides (W0.1Mn0.4Ce0.5)	500	500	5	-	300,000 h−1	150–270	>90%	Stable NOx conversions with 60 ppm SO2 at 175 °C	[262]
BiMnO3	1000	1000	5	-	10,000 h−1	160–250	>80%	Relatively stable NOx conversions with 5% H2O and 100 ppm SO2 at 140 °C	[220]

¹ LDO: layered double oxide.

[161,173,194,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262],

Table 5. NH₃-SCR activity in the presence of H₂O and/or SO₂ over some supported Mn-based catalysts.

Catalysts 1	Reaction Conditions					NOx Conversion (%)	Effects of H2O and SO2	Ref.
	NO (ppm)	NH3 (ppm)	O2 (vol%)	Space Velocity	T (°C)
Mn/Fe-Ti spinel	500	500	2	24,000 mL·g−1·h−1	150–250	>95%	Stable NOx conversion with 8% H2O and 60 ppm SO2 at 200 °C	[263]
Mn/ZrO2-TiO2	500	500	4	35,000 h−1	175–350	>97%	Irreversible deactivation with 10% H2O and 200 ppm SO2 at 200 °C	[264]
Mn/CeO2-TiO2	200	220	8	60,000 h−1	180–250	>90%	Deactivation with 6% H2O and 100 ppm SO2 at 180 °C	[265]
Mn/CeO2-ZrO2	600	660	6	45,000 h−1	120–220	>90%	Stable NOx conversion with 3% H2O and 100 ppm SO2 at 180 °C	[266]
Mn/CeO2-ZrO2-Al2O3	1000	1000	5	10,000 h−1	150–300	>90%	Relatively stable NOx conversion with 10% H2O and 100 ppm SO2 at 200 °C	[267]
MnOx(0.25)/CoSn3Al-LDO	500	500	5	30,000 h−1	100–350	>95%	Irreversible deactivation with 5% H2O and 150 ppm SO2 at 200 °C	[268]
MnCrOx/Sepiolite	1000	1000	5	45,000 h−1	125–340	>90%	Relatively stable NOx conversion with 200 ppm SO2 at 220 °C	[269]
TiO2-MnOx/CeO2-ZrO2	500	500	5	60,000 mL·g−1·h−1	175–225	>90%	Relatively stable NOx conversion with 5% H2O and 100 ppm SO2 at 200 °C	[171]
Fe-Mn/TiO2	1000	1000	10	30,000 h−1	100–330	>90%	Stable NOx conversion with 100 ppm SO2 at 125 °C	[270]
Fe-Mn/Ti-Zr	1000	1000	4	30,000 h−1	80–180	>90%	Deactivation with 6% H2O and 100 ppm SO2 at 150 °C	[187]
Fe-Mn-Ce/TiO2	600	600	3	50,000 h−1	155–260	>90%	Stable NOx conversion with 3% H2O at 140/180 °C	[271]
Ni-Mn/TiO2	400	400	2	50,000 h−1	187–240	>90%	Superior H2O durability at 200 °C with 10% H2O	[272]
Zr-Mn/attapulgite	600	600	3	40,000 h−1	153–250	>90%	Stable NOx conversion with 300 ppm SO2 at 200 °C	[273]
xSb-4Ce-10Mn/TiO2	500	500	3	75,000 mL·g−1·h−1	135–325	>95%	Deactivation with 5% H2O and 100 ppm SO2 at 200 °C	[195]
Ce-Mn-TNTs	500	500	5	30,000 h−1	150–425	>90%	Stable NOx conversion with 10% H2O and 100 ppm SO2 at 280 °C	[274]
Ce-Mn/TiO2	800	800	3	40,000 h−1			Deactivation with 3% H2O and 100 ppm SO2 at 150 °C	[275]
Ce-Mn/TiO2	720	800	3	30,000 h−1	120–200	>90%	Deactivation with 100 ppm SO2 at 180 °C	[276]
Ce-Mn-V-W/TiO2	1500	1500	3	40,000 h−1	150–400	>90%	Stable NOx conversion with 5% H2O and 100 ppm SO2 at 250 °C	[277]
Nd-Mn/TiO2	600	600	3	40,000 h−1	100–320	100%	Stable NOx conversion with 3% H2O and 50 ppm SO2 at 120 °C	[278]
Eu-Mn/TiO2	600	600	5	108,000 h−1	175–400	>90%	Deactivation with 100 ppm SO2 at 150 °C	[279]
Ho-Mn-Ce/TiO2	800	800	5	10,000 h−1	140–220	>90%	Deactivation with 10% H2O and 300 ppm SO2 at 180 °C	[280]
Ho-Fe-Mn/TiO2	800	800	5	20,000 h−1			Stable NOx conversion with 15% H2O and 200 ppm SO2 at 120 °C	[215]
Gd-MnOx/ZSM-5	800	800	5	40,000 h−1	110–240	>90%	Relatively stable NO conversion with 100 ppm SO2 at 180 °C	[212]
Ce0.5/Co1Mn0.5Al0.5Ox	500	500	5		100–250	>90%	Deactivation with 100 ppm SO2 at 200 °C	[281]
Mn-Ce-V/AC	500	500	5	18,000 h−1	125–300	>95%	Deactivation with 10% H2O and 100 ppm SO2 at 200 °C	[282]
Nb2O5-Zn-Ce-Mn/AC	500	500	11	14,500 h−1	150–250	>90%	Deactivation with 100 ppm SO2 at 200 °C	[189]
NiMnOx/activated coke	500	500	10	30,000 h−1	125–250	>80%	Stable NOx conversion with 200 ppm SO2 at 200 °C	[283]
La-Mn-Fe/activated coke	500	500	5	6000 h−1	150–300	>90%	Reversible deactivation with 5% H2O and 200 ppm SO2 at 150 °C	[284]
Mn/BC	600	600	11	12,000 h−1	160–240	>80%	Reversible deactivation with 100 ppm SO2 at 180 °C	[285]
Zr-Mn/BC	500	500	5	36,000 h−1	125–250	>75%	Stable NOx conversion with 5% H2O and 100 ppm SO2 at 200 °C	[286]
MnOx@CNTs	500	500	3	30,000 h−1	165–325	>90%	100% NOx conversion with 4% H2O at 225 °C	[287]
MnOx/Functionalized multi-walled CNTs	900	900	5	30,000 h−1	150–300	>80%	Deactivation with 2.5% H2O and 100 ppm SO2 at 200 °C	[288]
MnOx-CeOx/CNTs	400	400	3	12,000 h−1	200–260	>90%	Stable NOx conversion with 100 ppm SO2 at 220 °C	[289]
MnOx-CeOx@CNTs	500	500	3	10,000 h−1	200–350	>90%	Stable NOx conversion with 4% H2O and 100 ppm SO2 at 300 °C	[290]
MnCe/granular AC-CNTs	500	550	5	10,000 h−1	125–200	>80%	Stable NOx conversion with 5% H2O and 50 ppm SO2 at 150 °C	[291]
MnOx-CeO2(8:1)/graphene	500	500	5	24,000 h−1	80–140	>99%	Reversible deactivation with 10% H2O and 200 ppm SO2 at 140 °C	[292]

¹ LDO: layered double oxide; TNTs: titanium nanotubes; AC: active carbon; BC: biochar; CNTs: carbon nanotubes.

[171,187,189,195,212,215,263,264,265,266,267,268,269,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292], and

Table 6. NH₃-SCR activity in the presence of H₂O and/or SO₂ over some Mn-based core-shell catalysts.

Catalysts 1	Reaction Conditions					NOx Conversion (%)	Effects of H2O/SO2/	Ref.
	NO (ppm)	NH3 (ppm)	O2 (vol%)	Space Velocity	T (°C)
MnOx@TiO2	500	500	5	24,000 h−1	110–260	>90%	Deactivation with 10% H2O and 200 ppm SO2 at 160 °C	[293]
MnOx@TiO2	500	500	5	30,000 h−1	130–375	>90%	Reversible inhibition with 5% H2O at 180 °C	[294]
MnOx–CeO2@TiO2	500	500	5	24,000 h−1	150–210	>90%	Reversible inhibition with 200 ppm SO2 at 180 °C	[295]
MnOx@PrOx	800	800	5	40,000 h−1	93–240	>90%	Irreversible inhibition with 10% H2O and 100 ppm SO2 at 160 °C	[296]
MnOx@CeO2	800	800	5	40,000 h−1	114–220	>90%	Deactivation with 100 ppm SO2 at 220 °C	[297]
α-MnO2@CeO2	500	500	3	100,000 mL·g−1·h−1	125–250	>90%	Deactivation with 200 ppm SO2 at 220 °C	[298]
MnFe@CeOx	500	500	5	30,000 h−1	160–240	>90%	Reversible inhibition with 5% H2O at 160 °C	[299]
MnOx@Eu-CeOx	600	600	2.5	90,000 h−1	100–210	>90%	Reversible inhibition with 100 ppm SO2 at 200 °C	[300]
MnOx@Fe2O3/TNT	500	500	5	40,000 h−1	180–360	~100%	Deactivation with 150 ppm SO2 at 240 °C	[301]
Ho-TNT@Mn	500	500	5	30,000 h−1	110–300	>90%	Slowly deactivated with 100 ppm SO2 at 180 °C	[302]
Mn-TNTs@Ce	500	500	5	30,000 h−1	130–375	>90%	Relatively stable NO conversion with 5 % H2O and 100 ppm SO2 at 200 °C	[303]
H-MnO2@TiO2@HL	500	500	5	30,000 h−1	175–260	>90%	Reversible inhibition with 5% H2O at 180 °C	[294]
Ce@Mn@TiOx	1000	1000	5	30,000 h−1	140–230	>90%	Inhibition with 5% H2O at low temperature	[304]
MnCeOx@ZSM-5	500	500	5	960,000 mL·g−1·h−1	220–380	>90%	Relatively stable NO conversion with 5% H2O and 100 ppm SO2 at 300 °C	[305]
Fe2O3@MnOx@CNT	550	550	5	20,000 h−1	120–300	>90%	Reversible inhibition with 10% H2O and 100 ppm SO2 at 180 °C	[306]
mesoTiO2@MnCe/CNTs	500	500	3	10,000 h−1	225–400	>90%	Relatively stable NO conversion with 200 ppm SO2 at 300 °C	[307]
Co(3-x)MnxO4@TiO2	500	500	5	24,000 h−1	125–275	>80%	Relatively stable NO conversion with 10% H2O and 100 ppm SO2 at 225 °C	[308]

¹ TNT: titanium nanotube; TNTs: titanium nanotubes; layered double oxide; HL: hydrophobic layer; CNT: carbon nanotube; CNTs: carbon nanotubes.

[293,294,295,296,297,298,299,300,301,302,303,304,305,306,307,308], respectively. Among them, a relatively stable catalytic performance was observed over unsupported Mn-based oxides such as Mn oxides [161], Mn-Cr oxides [173,223], Mn-Fe oxides [224], Mn-Fe-Co oxides [225], Mn-Fe-Mg oxides [226], Mn-Fe-Al oxides [227], Mn-Co oxides [230,232,233,234], Mn-Co-Ce oxides [235], Mn-Co-V oxides [236], Mn-Ni oxides [235,238,239], Mn-Ni-Al oxides [240], Mn-Ni-Fe oxides [244], Mn-Ni-Ce oxides [235], Mn-Zr-Ti oxides [246], Mn-Ce oxides [248,249], Mn-Ce-Ti oxides [250,251,252], Mn-Ce-Sn oxides [248], Mn-Sm-Ti oxides [255,256], Mn-Sm-Fe oxides [257], Mn-Sm-Zr-Ti oxides [258], Mn-Sm-Ce-Ti oxides [259], Mn-Nd oxides [206], Mn-Gd oxides [261], Mn-W-Ce oxides [262], and Bi-Mn oxides [220], and supported Mn-based catalysts such as Mn/Fe-Ti spinel [263], Mn/CeO₂-ZrO₂ [266], Mn/CeO₂-ZrO₂-Al₂O₃ [267], TiO₂-MnO_x/CeO₂-ZrO₂ [171], Fe-Mn/TiO₂ [270], Zr-Mn/attapulgite [273], Ce-Mn-V-W/TiO₂ [277], Nd-Mn/TiO₂ [278], Ho-Fe-Mn/TiO₂ [215], Gd-MnO_x/ZSM-5 [212], NiMnO_x/activated coke [283], La-Mn-Fe/activated coke [284], Mn/biochar (BC) [285], Zr-Mn/BC [286], MnCe/Granular activated carbon (AC)-carbon nanotubes (CNTs) [291], and MnO_x-CeO₂/graphene [292], and Mn-based core-shell catalysts such as MnO_x@Eu-CeO_x [300], Mn-titanium nanotubes@Ce [303], and Fe₂O₃@MnO_x@carbon nanotube (CNT) [306] even in the presence of H₂O and SO₂ at low temperatures.

Liang et al. [221] reported that the MnO_x-SiO₂ mixed oxide catalyst synthesized by the co-precipitation method showed high NH₃-SCR activity with high N₂ selectivity and sulfur resistance because doping SiO₂ increased the specific surface area of the catalyst, surface acidity, and NH₃ adsorption. Doping with SiO₂ inhibited the adsorption of SO₂ for surface deposition of sulfate, effectively protecting the redox properties of the catalyst so that the MnO_x-SiO₂ catalyst showed relatively stable NO conversion in the presence of 100 ppm SO₂ at 225 °C [221]. Zhu et al. [222] synthesized mesoporous Mn-TiO_x with a three-dimensional structure by a solvent-free self-assembling strategy and observed an outstanding sulfur poisoning tolerance. The hierarchical cross-linked structure with a large specific surface area appeared to be favorable for enhancing the synergy between Mn and Ti, exposing more active sites, and dispersing the poisoning species [222]. Wang et al. [309] synthesized a series of Mn_xCe_yTi_z catalysts with three-dimensionally ordered macroporous (3DOM) structure by a soft template method and reported that after adding Ti, the redox capacity of the 3DOM-Mn₃Ce₁Ti₁ catalyst decreased and the N₂ selectivity was enhanced due to the strong adsorption of NH₃ on the acid sites, widening the temperature window. Yao et al. [171] found that the TiO₂-modified MnO_x/CeO₂-ZrO₂ nanorod exhibited higher catalytic activity than the MnO_x/CeO₂-ZrO₂ nanorod due to the larger amount of oxygen vacancy, the higher ratio of Mn⁴⁺ among Mn species, the higher proportion of surface adsorbed oxygen species among oxygen species, and the improvement of surface acidity. Furthermore, the TiO₂ modification appropriately weakened the redox properties of the MnO_x/CeO₂-ZrO₂ nanorods, effectively inhibiting the nonselective oxidation of NH₃ to N₂O [171]. Finally, TiO₂-modified MnO_x/CeO₂-ZrO₂ nanorod exhibited excellent H₂O + SO₂ tolerance, showing relatively stable NO conversion in the presence of 5% H₂O and 100 ppm SO₂ at 200 °C [171]. Xu et al. [256] reported that over a wide operating temperature range (60–225 °C), Ti-doped Sm–Mn mixed oxide showed a superior NH₃-SCR performance, especially excellent SO₂/H₂O resistance. Stable NO_x conversion was achieved even in the presence of 5% H₂O and 100 ppm SO₂ at 100 °C [256]. The inclusion of Ti can inhibit MnO_x crystallization to increase the high specific surface area and the amount of acid sites, while Sm doping can modulate the redox properties to promote NO oxidation and inhibit NH₃ nonselective oxidation [256].

Jiang et al. [282] reported that doping V₂O₅ to Mn-Ce/AC enhanced the NO conversion, N₂ selectivity, and SO₂ tolerance than Mn-Ce/AC because doping with V₂O₅ enhanced surface acidity and enriched surface chemisorbed oxygen. The stronger surface acidity of Mn-Ce-V/AC inhibited the competitive adsorption of SO₂ and suppressed the reaction between adsorbed SO₂ and NH₃ species [282]. In addition, the cluster of vanadium oxide species partially prevented the dispersed Mn-Ce solid solution from being sulfated by SO₂ [282]. This catalyst showed relatively stable NO conversion in the presence of 10% H₂O and 100 ppm SO₂ at 200 °C [282]. Li et al. [236] reported that the Mn_0.50Co_0.49V_0.01O_x catalyst exhibited superior catalytic performance over a wide temperature window (162–508 °C) among ternary MnCoVO_x catalysts and also displayed enhanced SO₂ tolerance, showing relatively stable NO conversion in the presence of 5% H₂O and 100 ppm SO₂ at 200 °C.

Gao et al. [173] prepared Cr-Mn mixed-oxide catalysts by citric acid method and observed good low-temperature NH₃-SCR performance at 100–225 °C owing to their high specific surface area, more active sites (Mn³⁺ and Mn⁴⁺), and effective electron transfer (Cr⁵⁺ + 2Mn³⁺ $\to$ Cr³⁺ + 2Mn⁴⁺). The stronger Mn–O binding energy and weaker dehydrogenation ability in CrMn₂O₄ spinel were the main reasons for the lower N₂O by-product than Mn₃O₄ [173]. The CrMn₂O₄ spinel showed relatively stable NO conversion in the presence of 10% H₂O and 150 ppm SO₂ at 200 °C [173]. The formation of Cr(III) sulfate could protect Mn active sites from sulfating. Furthermore, the transformation of -HSO₃ and SO₄²⁻ to (H⋯SO₄²⁻) can provide new Brønsted acid sites for ionic NH₄⁺, enhancing the NH₃-SCR activity via ‘Fast-NH₃-SCR’ (NO₂ + NH₄⁺ $\to$ NH₄NO₂ $\to$ N₂ + 2H₂O) [173].

The DFT calculation results indicated that Fe doping promoted the adsorption of NH₃ on the Mn sites over the β-MnO₂ (110) surface, weakened SO₂ adsorption capacity, and limited the oxidation of SO₂, which promoted the decomposition of ammonium sulfate and relieved the catalytic site blockage on the catalyst surface [310]. DFT calculation demonstrated that the thermodynamically most favored structure after Fe doping was MnO_2-x, and the structure remained stable as the doping amount increased [311]. The adsorption behavior of gas molecules on MnO_2-x showed that NH₃ and H₂O preferred to adsorb on the Mn Lewis acid sites, while NO and SO₂ preferred to adsorb on the O sites. MnO_2-x with enhanced acidity after Fe doping showed good SO₂ tolerance [311]. Wu et al. [312] prepared various Mn/TiO₂ catalysts promoted with different transition metals, including Fe, Cu, Ni, and Cr, by the sol–gel method and found that more NO could be oxidized to NO₂ and nitrate and then reacted with NH₃, thus leading to the enhanced NH₃-SCR activity at low temperatures. The promoting effect of Fe on Mn-Fe composite oxide catalysts was explained by the fact that Fe promoted NO oxidation and generated more monodentate and bidentate nitrate adsorption species on the catalyst surface [312]. It was also reported that the addition of Fe enhanced the amount and intensity of Brønsted and Lewis acid sites, which promoted the absorption of NH₃ to form active intermediates, thereby improving the low-temperature NH₃-SCR performance [313].

Jiang et al. [232] prepared MnO₂–CoO_x catalysts with different Mn/Co ratios using PVP as an assistant to control the morphology and exposed crystal faces of composite oxides. The best Mn–Co oxide showed stable NO_x conversions in the presence of 5% H₂O and 100 ppm SO₂ at 160 °C [232]. Feng et al. [314] prepared a series of Mn-Co mixed metal oxides, Mn_yCo_3–yO_x, by an ultrasonic technology followed by a hydrothermal treatment and found that they exhibited much better NH₃-SCR performance than single metal oxides (MnO_x or CoO_x). This can be attributed to the strong interaction between MnO_x and CoO_x, which reduces the crystallinity of the Mn_yCo_3–yO_x catalyst, increases the specific surface area, establishes the redox cycle of Co³⁺ + Mn³⁺ $\to$ Co²⁺ + Mn⁴⁺, improves the reducibility, enhances the surface acid sites, and accelerates the reaction between adsorbed NO_x species and coordinated NH₃ bound to Lewis acid sites [314]. Chen et al. [233] prepared a MnCoO_x catalyst using glucose as the forming pore agent and urea as the structural direction reagent and found that it exhibited higher low-temperature activity and SO₂ resistance than pure MnO_x catalysts. They revealed that the high surface area, abundant chemisorbed oxygen species, and the redox cycle (Co³⁺ + Mn³⁺ $\to$ Mn⁴⁺ + Co²⁺) collectively account for the enhanced catalytic activity of MnCoO_x catalysts [233]. The Mn(5)Co(5)O_x catalyst showed stable NO_x conversions even in the presence of 5% H₂O and 50 ppm SO₂ at 125 °C [233]. Zhang et al. [281] prepared a highly active Ce/CoMnAl from layered double hydroxides (LDHs), which showed low-temperature NH₃-SCR with good thermal stability and SO₂/H₂O resistance. The Ce_0.5/Co₁Mn_0.5Al_0.5O_x-layered double oxide (LDO) showed relatively stable NO_x conversion at 200 °C for 10 h with 100 ppm SO₂ in the feed stream [281].

Gao et al. [238] reported that the Mn-Ni spinel nanosheet (NiMn₂O₄) prepared by the urea hydrolysis hydrothermal synthesis exhibited excellent resistance to H₂O and SO₂ in the low temperature range at 150–300 °C. This catalyst showed stable NO_x conversions even in the presence of 150 ppm SO₂ at 175 °C [238]. Ni-doped MnO_x catalysts prepared by the solvent-free doping method maintained excellent low temperature (100–200 °C) NH₃-SCR activity, which was attributed to the fact that the appropriate nickel loading could effectively adjust the surface lattice oxygen activity and acidity of the catalyst, ensuring its ability to activate NH₃ at low temperatures [315]. Liu et al. [316] reported that NiMn₂O₄ catalysts prepared via the solvothermal method exhibited excellent NH₃-SCR performance in the temperature range of 85 to 285 °C due to their large specific surface area, high Mn⁴⁺/Mnⁿ⁺ ratio, sufficient O_α, appropriate acidity, and redox ability. Yan et al. [240] reported that the Ni₁Mn_0.5Al_0.5O_x showed the highest NH₃-SCR activity at 100–250 °C among LDHs-derived NiMnAlO_x catalysts because of more active species, abundant surface oxygen, moderate acidic sites, and redox properties. This catalyst also exhibited better resistance to SO₂ with fewer sulfate species deposited on the surface, showing relatively stable NO conversion in the presence of 5% H₂O and 100 ppm SO₂ at 200 °C [240]. Hou et al. [241] prepared a series of highly dispersed Ni_4-xMn_xAlO_y catalysts derived from LDHs and reported that the optimal Ni_4-xMn_xAlO_y catalyst exhibited high NH₃-SCR performance at a low temperature range of 120~210 °C but was deactivated in the presence of 5% H₂O and 100 ppm SO₂ at 210 °C. Yan et al. [244] prepared NiMnFeO_x catalysts by the LDH-derived oxide method and reported that the optimized Ni_0.5Mn_0.5Fe_0.5O_x catalyst had the best NH₃-SCR activity, excellent N₂ selectivity, a wider active temperature range (100–250 °C), higher thermal stability, and better H₂O and/or SO₂ resistance. This catalyst showed relatively stable NO conversion even in the presence of 5% H₂O and 100 ppm SO₂ at 200 °C [244]. Wang et al. [283] reported that NiMnO_x supported on active coke prepared by deposition-precipitation method showed the high NH₃-SCR activity because of the higher content of (Mn³⁺ + Mn⁴⁺), the high ratio of Mn³⁺/Mn⁴⁺, and Ni³⁺/Ni²⁺. This catalyst showed a stable NO conversion even in the presence of 200 ppm SO₂ at 200 °C [283].

Doping MnO_x and CeMnO_x with Y did not improve NO conversion, but it broadened the range of high N₂ selectivity compared with the undoped case [186]. The addition of Zr to Fe-Mn-Ti oxide catalysts was reported to decrease the deactivation rate in the presence of 100 ppm SO₂ at 150 °C [187]. Chen et al. [285] prepared several metal (Zr, Ni, and Co) oxide-doped BC-supported Mn oxide (MnO_x) catalysts by the impregnation method and found that the Zr-Mn/BC catalyst exhibited the best NH₃-SCR activity among them because of the high concentration of Mn⁴⁺, more surface oxygen (O_α), excellent redox property, and more Lewis acid sites and Brønsted acid sites. Furthermore, the Zr-Mn/BC catalyst showed stable NO conversion in the presence of 5 vol% H₂O and 100 ppm SO₂ at 200 °C [285]. Che et al. [317] reported that the highly dispersed active MnO_x species supported on the amorphous structure of ZrTiO_x had a uniquely bridged Mn³⁺ bonded with the support through oxygen linked to Ti⁴⁺ and Zr⁴⁺, respectively, which regulated the optimal oxidizability of the MnO_x species, leading to high NH₃-SCR activity and N₂ selectivity.

Zhou et al. [188] found that doping Nb₂O₅ could effectively enhance the NH₃-SCR performance and N₂ selectivity of the Zn-Mn-Ce/AC catalyst. Meanwhile, Nb₂O₅ could significantly improve the SO₂ poisoning resistance of the Zn-Mn-Ce/AC catalyst, showing relatively stable NO conversion in the presence of 100 ppm SO₂ at 225 °C because Nb₂O₅ could react with SO₂ in a preferential way to restrain the sulfuration of manganese and cerium oxides on the catalyst [188]. More importantly, Nb₂O₅ reacted with SO₂ to form Nb sulfate and then formed a new acidic site on the Zn-Mn-Ce/AC catalyst surface, which promoted the adsorption of NH₃ and inhibited the adsorption of SO₂, then restricted the reaction between NH₃ and SO₂ and hindered the formation of ammonium sulfate channels [188]. The promotional effect of Mo in Mo-MnO_x catalyst was reported to be due to its higher Mn⁴⁺ content, abundant surface adsorbed oxygen (O_α), and more acid sites [189]_. Li et al. [318] employed Mo and V to simultaneously fine-tune the acid and redox sites of MnO_x and found that the adjusted Mn_0.90Mo_0.09V_0.01O_x catalyst demonstrated excellent low-temperature activity and a significantly broader active temperature window.

Chang et al. [194] reported that SnO₂–MnO_x–CeO₂ catalysts prepared by a co-precipitation method showed remarkably high activity, N₂ selectivity, and SO₂ resistance at temperatures in the range of 200–500 °C, which can be due to the significant enhancement of Lewis acid sites generated by surface sulfation during the SO₂-containing NH₃-SCR reaction. They also explained that the high SO₂ tolerance can be ascribed to the inhibited formation of MnSO₄ [194]. This catalyst showed stable NO_x conversions in the presence of 12% H₂O and 100 ppm SO₂ at 110 °C [194]. Wang et al. [268] synthesized Sn-doped modified CoAl-LDH by a hydrothermal method and used it as a support for the preparation of the active hexagonal sheet structure of the MnO_x(0.25)/CoSn₃Al-LDO catalyst, which also showed relatively stable NO conversion at 200 °C after the introduction of 150 ppm SO₂. Sn doping increased the ratios of Mn⁴⁺/Mn³⁺ and Co³⁺/Co²⁺ as well as the adsorbed oxygen amount on the catalyst surface, improved the redox capacity of the catalyst, and increased the number of acid sites, resulting in the excellent low-temperature NH₃-SCR performance [268].

Xie et al. [195] prepared a series of xSb-4Ce-10Mn/TiO₂ (x = 0, 2, 3, 4, 5, 6) catalysts by a reverse co-precipitation method and observed that the addition of Sb⁵⁺/Sb³⁺ slightly decreased the catalytic activity but helped improve the H₂O/SO₂ resistance. They found that the Sb⁵⁺/Sb³⁺ addition decreased the Mn⁴⁺ and Ti⁴⁺ content but increased the Ce³⁺, surface adsorbed oxygen content, and the surface area, which promoted the decomposition of AS and inhibited the formation of MnSO₄ [195]. However, stable NOx conversion was not achieved in the presence of 5% H₂O and 100 ppm SO₂ at 200 °C [195].

Li et al. [319] reported that the addition of Mn to ceria greatly promotes the formation of surface oxygen vacancies that readily capture O₂ from the air and form surface reactive oxygen species (superoxide and peroxide), which efficiently oxidize NO to NO₂ and then promote the ‘Fast NH₃-SCR’ reaction. Rong et al. [206] prepared the MnXO_x catalysts (i.e., MnSmO_x, MnNdO_x, and MnCeO_x) by the reverse co-precipitation method and found that the MnCeO_x catalyst showed the best low-temperature catalytic activity and excellent H₂O + SO₂ resistance because of the largest amount of acid sites and the best reducibility among these MnXO_x catalysts. However, stable NO_x conversion was not achieved over these catalysts in the presence of 5% H₂O and 100 ppm SO₂ at 175 °C [206]. Fang et al. [320] reported enhanced NH₃-SCR activity and SO₂ tolerance of Ce-modified birnessite-MnO₂ and attributed it to the preferential adsorption and oxidation of SO₂ onto Ce species to form a new adsorption site for NH₄⁺, Ce₂(SO₄)₃, which protects the Mn active site from sulfidation and deactivation.

It is generally accepted that homogeneous metal doping into the MnO_x matrix is beneficial for NH₃-SCR activity. However, Cheng et al. [321] claimed that the presence of crystalline Mn₃O₄ and its interfacial interaction with CeO₂ played a significant role in boosting the NH₃-SCR performance, suggesting the great synergy between CeO₂ and the crystalline phase of Mn₃O₄, which were responsible for NO_x adsorption and the formation of NH₂ species through the activation of NH₃, respectively. Zhu et al. [322] prepared a series of Ce_x-Mn-Ti_y catalysts by the co-precipitation method and found that the best NH₃-SCR activity was obtained over the Ce_0.1-Mn-Ti_0.1 catalyst with the proper redox ability, abundant acid sites, high content of Mn⁴⁺ and Ce³⁺, and surface-adsorbed oxygen. Wei et al. [251] prepared Mn–Ce–Ti–O composite aerogels with large mesopore size and reported that the Mn–Ce–Ti–O catalyst calcined at 600 °C showed stable NO conversion in the presence of 5 vol% H₂O and 100 ppm SO₂ at 200 °C, which they attributed to its large pore size, averaging 32 nm, and abundant Lewis acid sites, as the former promoted the decomposition of ABS and the latter reduced the vapor pressure of NH₃. Teng et al. [323] prepared a series of rare-earth (Ce, La, Nd, and Y) oxide-doped Mn/Al₂O₃ catalysts by the impregnation method and found that rare earth oxide addition significantly enhanced the NH₃-SCR catalytic activity of Mn/Al₂O₃, with CeMn/Al₂O₃ obtaining the highest activity at 125–300 °C. Except for Y₂O₃, the rare earth oxide additives promoted the surface distribution of Mn elements and enhanced the ratio of chemisorbed oxygen and Mn⁴⁺ on the surface of Mn/Al₂O₃ [323]. In addition, the introduction of rare earth oxides enhanced the reducibility of MnO₂ species and obtained a larger amount of weak acid sites [323]. Contrary to some reports on the promotional effect of Ce in Ce-MnO_x catalysts, Gevers et al. [324] reported that the intrinsic NH₃-SCR activity of the Mn active sites is not positively affected by Ce species at low temperatures based on the fact that the surface-area normalized activity did not increase by Ce addition. They concluded that Ce decreased the average oxidation state and activity of Mn active species and was just a structural promotor, increasing catalyst surface area, and that addition of Ce suppressed second-step oxidation reactions and thus N₂O formation by structurally diluting MnO_x [324]. The diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis showed that N₂O was mainly generated by the reaction between NO and excessive hydrogen abstraction of NH₃ (i.e., forming NH species) and that Ca addition inhibited the formation of NH species on the catalyst surface, thereby reducing the formation of N₂O [325]. Ca addition could also lead to a decreased formation of NO₂, which could react with NH₃ to form N₂O, contributing to the promoted selectivity of N₂ over Ca-modified Ce-Mn/TiO₂ [325].

Wu et al. [205] reported that the Pr modification could boost the MnCeO_x catalysts’ NH₃-SCR activity and SO₂ resistance because of the expanded specific surface area, the improved dispersion of active components, the enhanced surface acidity and redox ability, and the generated more chemisorbed oxygen (O_ads) species. The MnCePrO_x-0.3 catalyst exhibited stable NO conversion even in the presence of 5 vol% H₂O and 100 ppm SO₂ at 250 °C [205]. Wang et al. [259] reported that the MnCeSmTiO_x catalyst preserved higher catalytic performance after introducing H₂O and SO₂ compared with the catalysts without adding Sm. They claimed that the synergistic effect of the Lewis acid sites and oxidation catalytic sites of mixed oxides was responsible for NH₃-SCR by following the L–H mechanism [261]. Doping Sm into MnCeSmTiO_x was reported to be able to increase oxygen vacancies and transfer electrons to Mn⁴⁺ and Ce⁴⁺, promoting the formation of active adsorbed NO₂, bidentate nitrate, and bridging nitrate intermediates and suppressing SO₂ poisoning by inhibiting the oxidation of SO₂ by Mn⁴⁺ and Ce⁴⁺ [259]. The MnCeSmTiO_x catalyst exhibited a rather stable NO conversion even in the presence of 5% H₂O and 200 ppm SO₂ at 200 °C [259]. Kim et al. [326] reported that the addition of Sm to MnO_x/TiO₂ increased the redox properties and the proportion of Mn⁴⁺ in MnO_x catalysts, and the addition of Ce further improved the redox properties of Sm-MnO_x/TiO₂ catalysts. They also reported that when exposed to SO₂/O₂, Sm and Ce-modified MnO_x/TiO₂ not only inhibited the degradation but also increased the N₂ selectivity by inhibiting the formation of N₂O₄ intermediate species, thereby inhibiting the formation of N₂O [326].

The addition of Eu to MnO_x/TiO₂ was reported to be effective against the resistance of SO₂ (i.e., NO_x conversion gradually decreased but reached a steady-state value in the presence of 100 ppm SO₂ at 150 °C) [279]. Wang et al. [210] prepared a series of MEuMnO_x ternary oxides (M = Ce, Ni, Co, Sb, Sn, Mo) by one-pot co-precipitation method and observed that CeEuMnO_x ternary oxide catalysts showed a high NH₃-SCR activity at 100–250 °C and stable NO_x conversions in the presence of 10 vol% H₂O and 50 ppm SO₂ at 230 °C. The facile electron transfer through the redox cycle of Mn³⁺ + Ce⁴⁺ $\to$ Mn⁴⁺ + Ce³⁺ and enhanced oxygen mobility can promote the formation of more Mn species in high oxidation states and chemisorbed oxygen, accelerating oxidation of NO, and the adsorbed NO₂ formed can facilitate the ‘Fast NH₃-SCR’ reaction to improve low-temperature activity [210]. The addition of Ce to the EuMnO_x catalyst boosts adsorption of NH₃ and NO_x species. NH₃ species are activated as crucial intermediates (NH₂) to promote the NH₃-SCR reaction [210]. Guan et al. [211] synthesized a series of Gd-modified MnO_x/ZSM-5 catalysts via a citric acid–ethanol dispersion method and found that the Gd-modified MnO_x/ZSM-5 catalyst presented the higher catalytic activity and better SO₂ resistance than MnO_x/ZSM-5 in the presence of 100 ppm SO₂. The high catalytic performance was mainly owing to the large surface area, enriched Mn⁴⁺ and surface chemisorbed oxygen species, strong redox properties, and the proper acidity of the catalyst [211]. The addition of Gd inhibited the reaction between the SO₂ and MnO_x active sites to form inactive bulk manganese sulfate, resulting in high SO₂ resistance [211]. The GdMn/Z-0.3 catalyst showed a rather stable NO conversion in the presence of 10% H₂O and 100 ppm SO₂ at 180 °C [211]. Gao et al. [212] synthesized Dy-doped MnFe oxides with the morphology of one-dimensional nanowire by using the electrospinning method and found that the doping of Dy with an appropriate amount enhanced the surface Mn⁴⁺ concentration and surface chemical adsorption of oxygen and increased adsorption capacity for NH₃ and NO. Zhuang et al. [214] investigated the effect of SO₂ on the low-temperature NH₃-SCR activity for Ho-modified Fe-Mn/TiO₂ catalyst. The Fe_0.3Ho_0.1Mn_0.4/TiO₂ catalyst showed excellent SO₂ durability at 120 °C when the concentration of SO₂ was lower than 400 ppm, and the catalytic activity could recover considerably after the SO₂ supply was interrupted [214]. When the concentration of SO₂ was increased to 1000 ppm, the deactivation behavior was irreversible, but the deactivated catalyst could be regenerated after thermal reduction (350 °C) with 5% NH₃ [214]. Zhao et al. [304] synthesized the Ho-modified titanium nanotubes (TNTs)@MnO_x, where the manganese oxide active species was confined in a Ho-modified TNTs, and observed the improved SO₂ resistance and N₂ selectivity during NH₃-SCR at low temperatures, showing NO conversion, which decreased slowly over time in the presence of 100 ppm SO₂ at 180 °C. They claimed that the pore confinement effect of Ho-TNTs on Mn increased the dispersion of Mn, thereby promoting the interfacial effect between Mn and Ho, and the electron synergy between Mn and Ho inhibited the electron transfer between SO₂ and Mn, thus preventing the poisoning of SO₂ [302]. They also found that the interaction between Ho and Mn contributed to the proper redox ability to induce electron transfer to inhibit the generation of Mn⁴⁺, thereby achieving high N₂ selectivity [302].

Zha et al. [218] found that the addition of tungsten created more Brønsted acid sites and reduced the energy barrier for NO_x species adsorbed on the surface. They also claimed that the formed NH₃ species and NO_x species of MnCeW/m-TiO₂ were more reactive due to the promotional effects of W than those of MnCe/m-TiO₂ [218]. Liu et al. [327] reported the promotional effect of W-doping on the CoMn₂O₄/TiO₂ catalyst on the resistance to H₂O at low temperature. The superior performance of the CoMn₂O₄/W-TiO₂ catalyst was ascribed to its unique spinel structure, mesoporous structure, highly dispersed tungsten species, and larger surface acidity [327]. The electron cycle among Mn, Co, W, and Ti helped to keep the Mn³⁺/Mn⁴⁺ and Co³⁺ at high concentrations and improve lattice oxygen mobility [327]. Both Lewis and Brønsted acid sites were generated by the incorporation of tungsten in the presence of H₂O, which enhanced NH₃ adsorption and thus promoted the E–R reaction pathway [327]. In addition, the N₂ selectivity was significantly enhanced by H₂O, which could be attributed to the decreased redox activity [327].

Yan et al. [305] assembled a confined MnCeO_x catalyst using a mesoporous zeolite (ZSM-5) as the shell and Mn-Ce oxides as the active core (MnCeO_x@ ZSM-5) with a simple one-pot method and found that the novel MnCeO_x@ ZSM-5 catalyst displayed enhanced water and SO₂ resistance as compared with the MnCeO_x supported on ZSM-5 (MnCeO_x/ZSM-5) and its precursor (MnCeO_x@Al-SiO₂), which was ascribed to the zeolite shell’s shielding effect and the synergy between the alumina-silica zeolite shell’s acidic properties and the mixed oxide cores’ redox properties. This catalyst showed relatively stable NO conversion in the presence of 5% H₂O and 100 ppm SO₂ at 300 °C [305]. Cai et al. [328] fabricated MnFe@CeO_x@TiO_x nanocage with a yolk-shell structure, where the CeO₂ shell could effectively increase the oxygen vacancy defect sites and the TiO₂ shell could remarkably improve the surface acid sites, which exhibited excellent NH₃-SCR performance in the temperature range of 120–240 °C, which was ascribed to the increased proportion of active species (Mn⁴⁺, Fe³⁺, Ce³⁺, and O_ads) and enhanced interaction between metal oxides. Huang et al. [304] prepared a series of yolk-shell-structured Ce@Mn@TiO_x, where the TiO₂ shell could provide more surface acid sites to promote the adsorption and activation of ammonia (Figure 6). The Ce@Mn@TiO_x exhibited excellent NH₃-SCR performance at low temperature and H₂O and SO₂ tolerance, showing relatively stable NO conversion in the presence of 100 ppm SO₂ at 200 °C [304]. Qiao et al. [329] prepared various MnO_x-ZSM-5 catalysts with different MnO_x locations and found that MnO_x clusters dispersed on the outside of ZSM-5 exhibited higher catalytic activity at low temperature than MnO_x nanoparticles encapsulated in the channels of the support because the excellent redox ability and the dominant Mn³⁺ species over the former one improved the oxidation of NO to NO₂ and the formation of unstable NO_x intermediates, enhancing the low-temperature NH₃-SCR activity. Ran et al. [330] synthesized a series of MnO_x-CeO_x confined in the mesopores of SBA-15 and found that they displayed enhanced resistance to SO₂. Guo et al. [331] reported that the catalyst with larger mesopores exhibited much improved sulfur resistance. Zhang et al. [307] synthesized a core-shell structural catalyst, carbon nanotube (CNT)-supported MnO_x and CeO_x nanoparticles coated with mesoporous TiO₂ sheaths (mesoTiO₂@MnCe/CNTs), in which the meso–TiO₂ sheaths could prevent the generation of ammonium/manganese sulfate species from blocking the active sites, resulting in a higher SO₂-tolerance during NH₃-SCR. The reversible deactivation was observed over this catalyst due to the presence of 200 ppm SO₂ at 300 °C [307]. Yan et al. [305] prepared the core-shell MnCeO_x@mesorporus ZSM-5 catalysts, in which a mesoporous zeolite (ZSM-5) covers the active core Mn-Ce oxides as the shell, and observed enhanced water and SO₂ resistance during NH₃-SCR owing to the zeolite shell’s shielding effect, which hinders the formation of sulfate species, and the synergy between the alumina-silica zeolite shell’s acidic properties and the mixed oxide cores’ redox properties. Zhao et al. [303] constructed a Mn-TNTs@Ce catalyst, where MnO_x was confined in TNT via in situ introduction following Ce ion exchange into the skeleton of TNTs. The nanotube-confined structure improved the surface acidity to restrain SO₂ adsorption, and Ce species acted as a protective site protecting Mn from SO₂ attacking [303]. Furthermore, the short-range structure Ce-O-Ti promoted the electron transformation between Mn and Ce to suppress the formation of -NH and NH₄NO₃, resulting in inhibiting the generation of N₂O [303]. It is remarkably that the Mn-TNTs@Ce catalyst showed a relatively stable NO conversion even in the presence of 5 vol% H₂O and 100 ppm SO₂ at 200 °C [303].

It is noteworthy that several promising Mn-based catalysts capable of performing NH₃-SCR at low temperatures, even in the presence of H₂O and SO₂, have been reported; however, most of them have only been shown to be stable for short reaction times, and catalyst analysis of post-reaction samples has been limited. For previously reported promising catalysts, long-term lifetime tests should be performed at low temperatures using flue gases containing H₂O and SO₂ to observe deactivation phenomena, and real-time analysis should be performed to identify and validate detailed mechanisms of resistance to H₂O and SO₂.

2.4. Ce-Based Catalysts

Since cerium-exchanged sodium-type mordenite (CeNa-MOR) was first reported to be active for NH₃-SCR over a wide temperature range of 250–560 °C [332], unsupported and supported ceria, metal-doped ceria, and multicomponent Ce-containing mixed metal oxides [333,334,335] have been applied to NH₃-SCR [336]. While NH₃-SCR activity was reported to be dependent on ceria morphology (i.e., nanosphere is better than nanocube), unsupported ceria showed relatively low catalytic activity at low temperatures, even after sulfation to provide surface acidic sites [337]. Therefore, metal-doped ceria and multicomponent Ce-containing mixed metal oxide catalysts have received much attention for application in low-temperature NH₃-SCR. Since Cu-, Fe-, and Mn-containing Ce-based catalysts have already been covered in the previous sections, this section mainly introduces Ce-based catalysts with other metals. The effects of H₂O and SO₂ on the NH₃-SCR activity over Ce-based catalysts are summarized in

Table 7. NH₃-SCR activity in the presence of H₂O and/or SO₂ over some Ce-based catalysts.

Catalysts 1	Reaction Conditions						NOx Conversion (%)	Effects of H2O/SO2	Ref.
	NO (ppm)	NH3 (ppm)	O2 (vol%)	H2O (vol%)	GHSV	T (°C)
CO-pretreated CeO2	600	600	5	-	108,000 h−1	250–350	>90%	Stable NO conversion with 5% H2O and 100 ppm SO2 at 200 °C	[338]
H2SO4-pretreated CeO2	500	500	5	-	60,000 mL·g−1·h−1	350–475	>90%	Stable NO conversion with 5% H2O and 100 ppm SO2 at 400 °C	[339]
CeO2/HAT	500	500	5	-	177,000 h−1	250–400	>90%	Stable NO conversion with 5% H2O and 50 ppm SO2 at 300 °C	[340]
TiOx/CeO2	500	500	5	-	90,000 h−1	250–450	>80%	Irreversible deactivation with 5% H2O and 200 ppm SO2 at 300 °C	[341]
CeOx/TiO2	500	500	5	-	90,000 h−1	250–450	>80%	Irreversible deactivation with 5% H2O and 200 ppm SO2 at 300 °C	[341]
CeTiOx Hollow nanotube	1000	1000	3	-	40,000 h−1	180–390	>98%	Stable NO conversion with 6% H2O and 100 ppm SO2 at 240 °C	[342]
CeO2-TiO2/P25	500	500	5	5	60,000 mL·g−1·h−1	300–450	>80%	Stable NO conversion with 5% H2O and 100 ppm SO2 at 400 °C	[343]
CeTiOx	600	600	3	-	150,000 h−1	180–300	>80%	Slowly deactivated with 5% H2O and 100 ppm SO2 at 225 °C	[344]
F-Ce-Ti oxides	500	600	5	-	41,000 h−1	180–240	>90%	Reversible deactivation with 100 ppm SO2 at 150 °C	[345]
P-CeO2/TiO2	500	500	5	-	60,000 h−1	200–430	>80%	Relatively stable NO conversion with 5% H2O and 500 ppm SO2 at 200 °C	[346]
Ce/Ti-Si-Al oxides	500	500	5	-	65,000 h−1	215–465	>80%	Reversible deactivation with 10% H2O and 100 ppm SO2 at 320 °C	[347]
Ce/TiO2-SiO2	500	500	3	-	28,000 h−1	250–450	>90%	100% NOx conversion for 24 h with 10% H2O and 200 ppm SO2 at 300 °C	[348]
Ce/Mo-TiO2	500	500	5	-	60,000 mL·g−1·h−1	200–350	~100%	Stable NO conversion with 2% H2O and 100 ppm SO2 at 250 °C	[349]
WO3/CeO2	500	500	3.5	-	60,000 mL·g−1·h−1	250–450	~100%	Slowly deactivated with 4% H2O and 300 ppm SO2 at 200 °C	[350]
Ce-W/UiO-66	300	300	3	-	10,000 h−1	200–350	>90%	Stable NO conversion with 5% H2O and 200 ppm SO2 at 250 °C	[351]
Ti-Sn-Ce-Ox	600	600	6	-	20,000 h−1	200–375	>90%	Stable NO conversion with 10% H2O and 300 ppm SO2 at 200 °C	[352]
CeBi/TiO2	600	600	5	-	108,000 h−1	250–400	>95%	Relatively stable NO conversion with 5% H2O and 100 ppm SO2 at 150 °C	[353]
CeSnOx/TiO2	500	500	5	-	30,000 h−1	200–420	>90%	Relatively stable NO conversion with 5% H2O and 100 ppm SO2 at 220 °C	[354]
Sn-Ce-Ti oxides	500	500	5	-	30,000 h−1	180–460	>90%	Stable NO conversion with 300 ppm SO2 at 240 °C	[355]
CuSO4-CeSnTiOx	500	500	5	-	30,000 h−1	240–340	>90%	Relatively stable NO conversion with of 5% H2O and 100 ppm SO2 at 240 °C	[356]
Co-Ce-Ti oxides	500	500	5	-	30,000 h−1	200–440	~100%	Slowly deactivated with 5% H2O and 100 ppm SO2 at 180 °C	[357]
Ge-Ce-W oxides	1000	1000	2	-	50,000 h−1	200–470	>95%	Deactivation with 100 ppm SO2 at 220 °C	[358]
Ce–Nb oxides	650	650	5	-	120,000 mL·g−1·h−1	220–400	~100%	Stable NO conversion with 5% H2O and 50 ppm SO2 at 280 °C	[359]
Ce0.4Nb0.6 nanospheres	500	500	5	-	30,000 h−1	250–450	>98%	Reversible deactivation with 200 ppm SO2 at 300 °C	[360]
Ce20Nb20Ti oxides	1000	1000	3	-	90,000 h−1	250–460	>95%	Deactivated with 10% H2O and 200 ppm SO2 at 350 °C	[361]
Ce-Nb-Ti oxides	1000	1000	3	-	90,000 h−1	250–500	>90%	Irreversible deactivation with 500 ppm SO2	[362]
Nb/CeSi2	500	500	4	-	60,000 h−1	215–450	>80%	Relatively stable NO conversion with 5% H2O and 200 ppm SO2 at 250 °C	[363]
Ni-Ce-La oxides	1000	1000	5	-	20,000 h−1	270–390	>90%	Stable NO conversion with 10% H2O and 500 ppm SO2 at 300 °C	[364]
Mo-CeO2/TiO2	500	500	5	-	128,000 h−1	275–400	>90%	Relatively stable NO conversion with 5% H2O and 50 ppm SO2 at 300 °C	[365]
Mo0.1CeSi2	500	500	5	-	90,000 h−1	215–400	>90%	100% NOx conversion for10 h with 5% H2O and 200 ppm SO2 at 250 °C	[366]
Cr-Ce-Ti oxides	500	500	5	-	40,000 h−1	182–405	>90%	Slowly deactivated with 5% H2O and 100 ppm SO2 at 250 °C	[367]
Cr1CeZr2Ox	600	600	5	5	108,000 h−1	200–350	>90%	Stable NO conversion with 5% H2O and 100 ppm SO2 at 250 °C	[368]
Zr-CeTiOx	500	500	5	-	60,000 h−1	200–375	>95%	Stable NO conversion with 5% H2O and 50 ppm SO2 at 225 °C	[369]
P-Ce-Zr-Ti oxides	800	720	3	-	30,000 h−1	180–400	>80%	Relatively stable NO conversion with 5% H2O and 200 ppm SO2 at 300 °C	[370]
Ce-Sn-W-Ba oxides/TiO2	930	930	10	-	8000 h−1	235–470	>90%	Relatively stable NO conversion with 5% H2O and 200 ppm SO2 at 350 °C	[371]
CeO2-WO3-palygorskite/TiO2	500	500	5	-	30,000 h−1	240–400	>80%	Relatively stable NOx conversion with 100 ppm SO2 at 360 °C	[372]
Sulfated CeO2-Rod	500	500	3	-	150,000 mL·g−1·h−1	300–450	~100%	Stable NOx conversion with 5% H2O and 100 ppm SO2 at 250 °C	[373]
Sulfated CeO2-ZrO2	250	250	2.5	-	90,000 h−1	250	90%	Relatively stable NOx conversion with 200 ppm SO2 at 250 °C	[374]
CeO2@TiO2 Core-shell	500	500	5	-	24,000 h−1	200–450	>80%	Irreversible deactivation with 200 ppm SO2 at 280 °C	[375]
CeO2@TiO2	500	500	5	-	36,000 h−1	225–400	>95%	Relatively stable with 5% H2O and 100 ppm SO2 resistance at 350 °C	[376]

¹ HAT: halloysite.

[338,339,340,341,342,343,344,345,346,347,348,349,350,351,352,353,354,355,356,357,358,359,360,361,362,363,364,365,366,367,368,369,370,371,372,373,374,375,376]. Among them, a relatively stable catalytic performance was observed over CO-pretreated CeO₂ [338], F-Ce-Ti oxides [345], P-CeO₂/TiO₂ [346], Ti-Sn-Ce-O_x [352], and CeBi/TiO₂ [353] even in the presence of H₂O and SO₂ at low temperatures.

Zhou et al. [344] utilized bimetallic MOFs to prepare CeMO_x (M=Ti, Cu) catalysts, allowing the homogeneous distribution of promoters. The CeTiO_x catalyst with high acidity and good redox properties with abundant Ce³⁺ and Ti⁴⁺ showed high NH₃-SCR activity from 180 to 300 °C and maintained stable performance in SO₂/H₂O at 225 °C [344]. Meanwhile, the aliovalent substitution of ceria by Cu²⁺ in CeCuO_x formed oxygen vacancies and enhanced its redox properties but led to poor N₂ selectivity due to NH₃ over-oxidation [344]. Guo et al. [353] used Bi as the modifier to boost the performance of the Ce/TiO₂ catalyst for NH₃-SCR. The CeBi/TiO₂ catalyst with a proper Bi content showed a rather stable NO conversion in the presence of 5% H₂O and 100 ppm SO₂ at 150 °C because the addition of Bi could generate more Ce³⁺ and chemisorbed surface oxygen species, along with enhanced redox capability and surface acidity [353].

Jin et al. [371] investigated the role of each metal component in Ce-Sn-W-Ba-O_x/TiO₂ and found that TiO₂ provided sufficient acid sites, CeO₂ enhanced redox properties, weakened acid strength, and increased chemisorbed oxygen concentration, which helped to promote the activation and desorption of NH₃. They also reported that SnO₂ increased chemisorbed oxygen concentration, enhanced redox properties, and improved the activation rate of NH₃, WO₃ increased the amount of acid, and BaO enhanced resistance to water vapor and SO₂ [371]. A rather stable NO conversion was achieved at 350 °C in the presence of 5% H₂O and 200 ppm SO₂ [371].

Zeng et al. [346] prepared a P-doped CeO₂/TiO₂ catalyst by dispersing CeO₂ on strongly acidic and highly stable mesoporous P-doped TiO₂. The P-doped CeO₂/TiO₂ catalyst exhibited much higher catalytic activity than the CeO₂/TiO₂ catalyst at temperatures of 200–450 °C and a rather stable NO conversion even in the presence of 5% H₂O and 500 ppm SO₂ at 200 °C [346]. Mu et al. [354] synthesized an ordered mesoporous CeSnO_x/TiO₂ catalyst through a classical evaporation-induced self-assembly (EISA) strategy and ascribed the low-temperature NH₃-SCR activity to abundant reactive oxygen species (O_α), improving the surface acidity and redox capacity of the catalyst. This catalyst also showed a rather stable NO conversion even in the presence of 5% H₂O and 100 ppm SO₂ at 220 °C [354]. Mu et al. [356] prepared an ordered mesoporous catalyst, CeSnTiO_x, modified with copper sulfate and revealed that the copper sulfate-modified one increased the reaction sites (redox site and acid site). They reported that Cu species can interfere with the electron cycle of the catalyst and reduce the strong redox performance of the Ce active site, which can effectively inhibit the adsorption of SO₂, while S-species (from SO₄²⁻) can change the distribution of acid sites on the catalyst surface and the total acid content to realize a ‘Fast NH₃-SCR’ reaction [356]. This catalyst also showed a rather stable NO conversion even in the presence of 5% H₂O and 100 ppm SO₂ at 240 °C [356]. Liu et al. [377] prepared a series of W-modified Ce-Sn catalysts by the co-precipitation method and reported that the W species regulated the structure by promoting the formation of the Ce-doped tetragonal SnO₂ (t-Sn(Ce)O₂) active phase while preventing the generation of the Sn-doped cubic CeO₂ (c-Ce(Sn)O₂) phase. In addition, the highly dispersed W species on the surface of the Ce₁W_0.24Sn₂O_x catalyst also coupled with Ce species to form new Ce-O-W active sites [377]. The W modification also promoted the ability of the catalysts to oxidize NO to NO₂ at 150–300 °C [377]. Liu et al. [378] reported the promotional effect of Ti in Ti-doped Ce– Sn mixed oxide (Ce–Sn–Ti) catalysts on the low-temperature NH₃-SCR. The appropriate doping Ti formed Sn–O-Ti, Sn–O-Ce and Ti–O-Ce structures, which could increase the content of Ce³⁺ through electron transfer from Sn or Ti to Ce (Ce³⁺ + Ti⁴⁺ $\to$ Ce⁴⁺ + Ti³⁺ and 2Ce⁴⁺ + Sn²⁺ $\to$ 2Ce³⁺ + Sn⁴⁺) [378]. The solid solution structure increased specific surface areas, active sites (Ce³⁺), and Lewis acid sites over the Ce_0.6Sn_2.4Ti₂ catalysts [378].

Yang et al. [379] reported that the emergence of Ce³⁺–O–Ce³⁺ structural units induced by Mo doping in Mo-doped CeO₂ catalysts led to much better NH₃-SCR performance by achieving low-energy barrier activation of NH₃ molecules, thereby changing the dominant reaction mechanism in the catalytic reaction. They also observed the same trend in (W)-doped CeO₂ catalysts, further confirming the pivotal role of Ce³⁺–O–Ce³⁺ structural units [379]. The addition of MoO₃ improved the activity of the CeO₂/TiO₂ catalyst for NH₃-SCR irrespective of the presence of H₂O and SO₂ because the introduction of Mo to the CeO₂/TiO₂ catalyst can suppress the adsorption of H₂O and SO₂ and the formation of surface sulfate species [365]. The MoO₃-promoted CeO₂/TiO₂ exhibited higher activity than CeO₂/TiO₂ even in the co-presence of H₂O and SO₂, showing relatively stable NO conversion in the presence of 5% H₂O and 50 ppm SO₂ at 300 °C [365]. Liu et al. [380] reported the enhanced NH₃-SCR performance of MoO₃/CeO₂ catalyst by introducing Cu to induce the generation of active Mo=O structure because the adding of Cu into MoO₃/CeO₂ catalyst could create unsaturated sites on CeO₂ for Mo anchor, and the enhanced electron transfer from Mo to Cu would cause the formation of a new terminal Mo=O with highly distorted octahedral geometry, which is a new Lewis acid site for coordinated NH₃ production. Meanwhile, the added Cu creates the adsorbed site for gaseous NO, and the formed Mo-O-Cu pair center facilitates the transformation of ionic NO₂^– generated from NO adsorption to NO₂, which is conducive to the ‘Fast NH₃-SCR reaction’ process [380]. Mo doping to CeO₂-SiO₂ mixed-oxide NH₃-SCR catalyst exhibited high low-temperature NH₃-SCR activity and superior N₂ selectivity and resistance to SO₂/H₂O poisoning [366]. The Mo-doped CeO₂-SiO₂ catalyst showed a rather stable NO conversion even in the presence of 5% H₂O and 200 ppm SO₂ at 250 °C [366]. Two elements (Mo and Cu) could be selected for doping into CeO₂ to promote the NH₃ adsorption of CeO₂-based materials by means of DFT calculations [381].

A comparison of NH₃-SCR activity among CeO_x supported on WO₃ nanorods (WO₃-NR) and WO₃ microspheres self-assembled by nanorods (WO₃-MP), and WO₃ nanoparticles (WO₃-NP) revealed that WO₃-NP enclosed with (0 0 1) facets of the hexagonal WO₃ showed the best redox ability due to the largest molar ratio of W⁵⁺/(W⁵⁺ + W⁶⁺), the highest concentration of Ce⁴⁺ on the surface of Ce/WO₃-NP, the strongest surface redox properties, and the largest molar ratio of O_α/(O_α + O_β), which was beneficial for NH₃-SCR activity in the temperature range of 250–400 °C [382]. Hao et al. [103] designed a novel WO_x/Cu-CeO₂ oxide catalyst in which the WO_x species are highly dispersed on the {111}/{100}-terminated surface of Cu-doped CeO₂ nanospheres, which exhibits excellent NH₃-SCR performance over a wide operating temperature window (200–400 °C), as well as good sulfur resistance and N₂ selectivity. The stable NO conversion was obtained in the presence of 5% H₂O and 100 ppm SO₂ at 250 °C [103]. They found that the Cu introduction into the lattice of CeO₂ not only increased the surface Ce³⁺ and oxygen vacancy concentration but also provided more sites for capture and dispersion of WO_x species, thus mediating and improving the Lewis and Brønsted acid sites and reducibility of catalyst [103]. They also found that electronic interaction between WO_x and Cu-doped CeO₂ was enhanced due to the existence of interactions of short-range Ce-O-Cu and Ce-O-W [103]. Fang et al. [383] designed a Ce₁–W₁/TiO₂ model catalyst by anchoring Ce₁–W₁ atom pairs on anatase TiO₂(001) to investigate the synergy between Ce and W in NH₃-SCR and found that the Ce₁–W₁ synergy not only shifted down the lowest unoccupied states of Ce₁ near the Fermi level, thus enhancing the abilities in adsorbing and oxidizing NH₃ but also makes the frontier orbital electrons of W₁ delocalized, thus accelerating the activation of O₂.

The promotional effect of Nb doping to CeO₂/TiO₂ in terms of the resistance to high GHSV, H₂O, and SO₂ was ascribed to the better dispersion of Ce, more chemisorbed oxygen species, and Ce³⁺/Ce⁴⁺ redox pairs on the catalyst surface [361]. The Ce₂₀Nb₂₀Ti catalyst was slowly deactivated in the presence of 10% H₂O and 200 ppm SO₂ at 300 °C [361]. Zhu et al. [360] prepared a series of porous Ce_xNb_1-x (x = 0, 0.2, 0.4, 0.6, 1) hollow nanospheres by a modified seed-mediated growth approach and found that a strong interaction between Nb₂O₅ and CeO₂ affected the oxygen defects, valence, reducibility, and the number of acid sites. The porous Ce_0.4Nb_0.6 nanospheres with more acid sites and excellent reducibility exhibited superior catalytic activity in the temperature range of 250–450 °C and high catalytic stability for H₂O and SO₂, showing reversible deactivation in the presence of 200 ppm SO₂ at 300 °C, which can be attributed to the porous double-shell structure and the strong interaction of Nb₂O₅ and CeO₂ species [360]. Jiang et al. [362] synthesized Ce-Nb-Ti metal oxide catalysts (CeNbTi) and found that CeNbTi prepared with the impregnation method possessed superior NH₃-SCR performance in the temperature range of 250–450 °C and better resistance to SO₂ and potassium, showing irreversible deactivation in the presence of 500 ppm SO₂. The best catalyst had relatively high concentrations of Ce³⁺ and chemisorbed oxygen, a strong synergistic effect among different components, and well-dispersed active species [362]. More importantly, the catalyst had stronger reduction capacity and a large number of Lewis acid sites, which contributed to its excellent catalytic performance [362]. Zhang et al. [359] synthesized CeO₂, Ce–Nb–O_x and Nb₂O₅ catalysts by the citric acid method and found that the mixed oxide Ce–Nb–O_x presented a higher NH₃-SCR activity than the single oxide CeO₂ or Nb₂O₅ catalyst. In addition, the Ce–Nb–O_x catalyst showed high resistance towards H₂O and SO₂ at 280 °C [359]. The incorporation of Nb provides abundant oxygen vacancies for capturing more surface adsorbed oxygen, which provides a superior redox capability and accelerates the renewal of active sites. Niobium pentoxide shows high surface acidity, which is partly retained in the Ce–Nb–O_x catalyst possessing a high content of Lewis and Brønsted acid sites [359]. Ding et al. [363] synthesized a set of catalysts for the Nb/CeSi₂ with different loadings of niobium and found that a catalyst for 20Nb/CeSi₂ exhibited the best low-temperature NH₃-SCR performance while maintaining excellent SO₂/H₂O resistance, showing relatively stable NO conversion in the presence of 5% H₂O and 200 ppm SO₂ at 250 °C.

Since the redox properties of ceria can operate at medium and high temperatures, in order to achieve low-temperature NH₃-SCR activity, catalysts containing ceria should include Cu, Fe, and Mn together in the catalyst composition. Among them, Mn is the most promising metal component for this purpose according to the literature.

2.5. Other Catalysts

Generally, the surface concentration of vanadium is an important factor to determine the surface vanadium species such as monomeric vanadyl without V–O–V bonds, oligomeric vanadyl with V–O–V bridge, and crystallized V₂O₅ nanoparticles, which can be found at a low surface concentration (<2 V atoms/nm²), moderate surface concentration (2–8 V atoms/nm²), and high surface density (>8 V atoms/nm²), respectively [7]. The polymeric vanadyl species have been reported to have a higher NH₃-SCR activity than the monomeric vanadyl species because the coupling effect of the polymeric structure shortens the reaction pathway for the regeneration of redox sites [38]. Inomata et al. [384] reported that defective bulk vanadium oxide (V(V) + V(IV)) catalysts, synthesized by the calcination of vanadium(IV)-oxalate at 270 °C, showed NH₃-SCR activity at a low temperature below 150 °C. The transformation of crystalline V₂O₅ on low-vanadium-loading catalysts into an active polymeric vanadyl species under NH₃-SCR reaction conditions was reported to explain a remarkably enhanced catalytic performance of the vanadia-based catalyst at low temperatures [385]. Highly active supported oligomeric surface vanadia sites, which can provide exclusive centers for adsorbed bridging and bidentate nitrates and assist in NH₃ activation to generate amide intermediates, could be fabricated by transformation from monomeric surface VO_x sites from a classic supported V₂O₅–WO₃/TiO₂ catalyst via a H₂ plasma treatment [386]. Lin et al. [387] prepared vanadia supported on defective TiO₂ nanosheets, which demonstrated excellent NH₃-SCR performance over a wide temperature range of 140–380 °C. This catalyst was also resistant to H₂O and SO₂ poisoning, maintaining an NO conversion at 180 °C [387]. Hwang et al. [388] mechanochemically localized vanadia on the surface of WO₃-TiO₂ by physically grinding high-vanadia-loading V₂O₅/WO₃-TiO₂ with WO₃-TiO₂. They found that clustered vanadia sites exhibited enhanced activity for low-temperature (<250 °C) NH₃-SCR [388]. This catalyst also exhibited superior sulfur resistance at 220 °C because the exposed TiO₂ sites absorbed ABS from the clustered vanadia sites, preventing the blockage of the catalytic active sites [388].

A comparison of NH₃-SCR activity among V₂O₅, Na_0.33V₂O₅, and Na_1.2V₃O₈ synthesized by the oxalate method revealed that the reaction rate of Na_0.33V₂O₅ was 6.1-times higher than that of V₂O₅, indicating that Na intercalation accelerates NH₃-SCR cycles [389]. Bulk W-substituted vanadium oxide catalysts with the high redox ability and reactivity of Brønsted acid sites were reported to be active for NH₃-SCR at a low temperature (100–150 °C) and in the presence of water (~20 %). Lewis acid sites of W-substituted vanadium oxide are converted to Brønsted acid sites in the presence of water vapor, and NH₄⁺ species adsorbed on Brønsted acid sites react with NO with the reduction of V⁵⁺ sites at 150 °C [390]. Zr-doped CeVO₄ solid solution catalysts showed excellent NH₃-SCR performance from 150 to 375 °C because of the increased Brønsted/Lewis acid sites and facilitated electron transfer among Ce, Zr, and V [391]. The Ce_0.85Zr_0.15VO₄ catalyst was slowly deactivated at 190 °C in the presence of 8% H₂O and 200 ppm SO₂ [391]. Furthermore, TiO₂ nanosheets promoted the activity and H₂O/SO₂ tolerance compared with TiO₂ nanoparticles for Zr–CeVO₄/TiO₂ catalysts [392]. However, TiO₂ nanosheets were slowly deactivated at 225 °C in the presence of 8% H₂O and 200 ppm SO₂ [392]. FeVO₄/TiO₂ was reported to show stable NO conversion in the presence of 200 ppm SO₂ at 190 °C and reversible deactivation due to 5% H₂O at 240 °C [393]. Kim et al. [394] prepared various Ce_0.5RM_0.5V₁O₄ catalysts in which half of Ce in TiO₂-supported Ce₁V₁O₄ was replaced by RM (Tb, Er, or Yb) and observed the promotive effect anticipated by RM substitution for Ce_0.5RM_0.5V₁O₄ only at high temperatures. Ce_0.5Er_0.5V₁O₄ possessed the greatest Lewis acidity/redox feature, thus revealing the best performance at elevated temperatures [394].

Liu et al. [395] prepared a serial of W_aCo_0.4TiO_x catalysts (a = 0.04, 0.06, 0.08, 0.10) by the sol–gel method, and the W_0.08Co_0.4TiO_x catalyst achieved the best NH₃-SCR activity at 190–430 °C and good resistance to H₂O/SO₂. However, the W_0.08Co_0.4TiO_x catalyst was slowly deactivated at 280 °C in the presence of 100 ppm SO₂ [395]. The doping of W enhanced the acidity and weakened the oxidation capacity of W_aCo_0.4TiO_x, but the high surface acidity, especially the large number of Brønsted acid sites, can compensate for the effect of the reduced oxidation capacity on the low-temperature activity [395].

Phosphates are used in a variety of catalytic reactions due to their excellent thermal stability, proton conductivity, ion exchange, and acidity, and in P-doped CeO₂/TiO₂ catalysts, the amorphous CePO₄ species has been demonstrated to promote NH₃-SCR activity by activating the NH₄⁺ species at the Brønsted acid site of the phosphate radical while generating NH₂ species that react with gaseous NO via the E–R mechanism [12]. The phosphorus is the acid site for NH₃ adsorption, and CeO₂ is the redox site. The NH₃-SCR proceeded according to the L–H mechanism, and the excellent redox properties promoted the ‘Fast NH₃-SCR’ reaction after the introduction of 3% H₂O + 100 ppm SO₂ at 240 °C [396]. The Ce-O-P catalysts prepared with a sol–gel method exhibited enhanced NH₃-SCR activity due to increased surface acidity and redox capacity and improved SO₂ tolerance in the presence of 5% H₂O and 100 ppm SO₂ at 250 °C due to the effect of SO₂ capture by abundant reactive oxygen species [397]. Zhao et al. [398] synthesized a series of cerium phosphate catalysts with different crystal phases by hydrothermal method and co-precipitation method. Hexagonal cerium phosphate (CePO₄-H) showed better NH₃-SCR activity in the temperature range of 300–500 °C than monoclinic cerium phosphate (CePO₄-M) and mixed phases of CePO₄-H and CePO₄-M because CePO₄-H had much stronger surface acidity and more surface adsorbed oxygen species [398].

Fe- and Cu-based catalysts have been demonstrated to have enhanced high-temperature NH₃-SCR activity in the presence of SO₂ due to the increased acidity derived from the sulfate formed. However, supported sulfate catalysts have the disadvantage of exhibiting low-temperature NH₃-SCR activity due to their poor redox properties. Therefore, it is necessary to enhance the redox ability of the catalyst to improve its low-temperature NH₃-SCR activity. The active ingredient, sulfate, can prevent the formation of metal sulfates, but the deposition of AS/ABS with decreasing temperature is the main cause of catalyst deactivation. Therefore, future research on sulfate-based catalysts should focus on improving their redox capacity while promoting the degradation of AS/ABS [399]. NO_x reduction conversions and N₂ selectivity of sulfated CeO₂ cubes prepared by the impregnation of CeO₂ cubes by ammonium sulfates could be significantly improved compared with pure CeO₂ cubes. Sulfation treatment could create and strengthen Brønsted acid sites originated from the protons on surface sulfates, further facilitating ammonia adsorption and activation [400]. Yu et al. [401] prepared CuSO₄/TiO₂, Fe₂(SO₄)₃/TiO₂, MnSO₄/TiO₂, Ce(SO₄)₂/TiO₂, and CoSO₄/TiO₂ catalysts via a sol–gel protocol. The presence of SO₂ had little influence on the activity of all metal sulfate catalyst samples [401]. CuSO₄/TiO₂ showed the highest NH₃-SCR activity and apparent activation energy among the metal sulfate catalysts following the sequence of CuSO₄/TiO₂ < Fe₂(SO₄)₃/TiO₂ < MnSO₄/TiO₂ < Ce(SO₄)₂/TiO₂ < CoSO₄/TiO₂ [401]. The amount of acid sites was the main factor that influenced the catalytic activity of the metal sulfate catalysts [401].

3. Strategies to Improve the Low-Temperature NH₃-SCR Activity

The reaction mechanism for NH₃-SCR indicates that both surface acid site and redox property are required to complete the catalytic cycle. Two representative reaction mechanisms have been proposed for this reaction: The L–H and E–R mechanisms, which predominate at low and medium/high temperatures, respectively. In any case, adsorption of NH₃ on the surface acid sites followed by oxidative dehydrogenation is an important step in this reaction. Therefore, the NH₃-SCR catalyst surfaces must have specific acid sites that are required for the adsorption of NH₃ onto the catalyst surface. Lewis and Brønsted acidic sites enable the adsorption of NH₃ and conversion of NH₃ to NH₄⁺, respectively, which is a prerequisite for the generation of the -NH₂ active species, and its subsequent interaction with NO₂ and NO species is an essential component of this reaction. The key to broadening the active temperature window of NH₃-SCR catalysts, including the low-temperature region, is to activate the N-H bond in the adsorbed NH₃. The oxidative activation of adsorbed NH₃ is very important. Dehydrogenation of NH₃ produces the active NH₂ species, and -NH₂ can react with gaseous NO or adsorbed NO/NO₂/nitrate, nitrite, and other species to form N₂. The oxidation capability of the catalyst surface determines the efficiency of NH₃ dehydrogenation; therefore, the surface redox sites of the catalyst play an important role. At low temperatures, the adsorption of NH₃ is not a significant obstacle; rather, the oxidation capacity of the catalyst itself becomes particularly important at low temperatures. However, it is important to note that the surface oxidation of the catalyst should not be too strong, as too much surface oxidation will result in the over-oxidation of NH₃ to NH or -N, resulting in the consumption of NH₃ and the production of significant amounts of N₂O (Figure 7). The over-oxidation of NO₂ to NO₃^– also adversely affects NH₃-SCR activity and N₂ selectivity, since the formation of N₂O is usually due to the reaction of adsorbed -NH with gas-phase NO or surface-bound NH₄⁺ with NO₃^– (Figure 7). Therefore, the oxidizing power of the catalyst should be ‘just right,’ meaning that it should have enough oxidizing power to remove the first H from NH₃, but not over-oxidize NH₃ to -NH.

Many researchers have observed that low-temperature NH₃-SCR reactions occur primarily through the L–H mechanism. NO₂, nitrite, and nitrate species are important intermediates in the L–H mechanism, and the activation of NO to NO₂ (gas phase or adsorption) is very important. The importance of NO₂ generation by NO oxidation can also be seen from the fact that NO₂ contributes to the occurrence of the ‘Fast NH₃-SCR’ pathway. It is widely recognized that when a certain amount of NO₂ coexists with NO, the reaction rate increases significantly, resulting in ‘Fast NH₃-SCR’, which significantly improves NH₃-SCR performance. Koebel et al. [402,403] first discovered that partial conversion of NO to NO₂ with the help of an oxidizing catalyst greatly improved the catalytic performance of the NH₃-SCR, especially at low temperatures. The optimal NO₂:NO ratio is 1:1, which allows all NO_x to react through a ‘Fast NH₃-SCR’ reaction. The evolution of NO₂ into nitrate and nitrite after adsorption is also important. Nitrite/nitrate bonding can occur to form monodentate nitrate, linear nitrite, bridging nitrate, and bidentate nitrate. These nitrates/nitrites have specific reactivity at certain temperatures; the level of reactivity depends on the catalyst system. In general, low-stability nitrates/nitrites react with NH₄⁺ at low temperatures and contribute to enhanced low-temperature activity, whereas high-stability nitrates/nitrites are detrimental to low-temperature activity because they are too stable to react with NH_x species and thus poison the active site.

Some promising strategies to improve the acidity and redox properties of metal oxide catalysts have been proposed, such as modification or doping of transition and rare earth metals, optimization of preparation methods, generation of new nanostructures, morphology modification, exposure of specific crystal faces, etc. [12]. As shown in the previous sections, MnO_x-based catalysts exhibit very good low-temperature NH₃-SCR activity but always produce a large amount of by-product N₂O. In order to regulate the redox property and surface acidity of MnO_x catalysts, multicomponent Mn-based composite oxides have been reported. They generally contain VO_x, Fe₂O₃, or CuO as the main active metal oxides and Al₂O₃, SiO₂, TiO₂, CrO_x, CoO_x, NiO, Y₂O₃, ZrO₂, NbO_x, MoO_x, AgO_x, SnO₂, SbO_x, LaO_x, CeO_x, PrO_x, NdO_x, SmO_x, EuO_x, GdO_x, DyO_x, HoO_x, ErO_x, TmO_x, TaO_x, and WO₃ as promoters. The creation of a multi redox cycle over a multicomponent oxide catalyst effectively promotes electron transfer and facilitates the adsorption/activation of NO/NH₃ (Figure 7).

Highly dispersed metal oxide composites can be prepared by utilizing MOF [375,404,405,406] or LDH [216,407,408,409] as precursors and constructing solid solution catalysts. Designing specific nanostructures such as nanoneedles, nanospheres, nanotubes, or core-shell structures can increase the amount of acid sites/reactive oxygen species and promote the formation of active species. Selective exposure of certain facets of the active ingredient, such as MnO_x and Fe₂O₃, as well as TiO₂ and CeO₂ support, can improve redox/acidic properties and metal-support interactions. Additionally, acidification or functionalization of the support with specific acids or oxygen/nitrogen-containing groups can significantly improve acidity and the interaction between the active ingredient and support.

4. Strategies to Improve the H₂O/SO₂ Tolerance at Low Temperatures

Since water vapor is an unavoidable part of real flue gas, its effect on NH₃-SCR activity has been an important issue. Positive or negative effects have been reported depending on the reaction conditions (e.g., reaction temperature), catalyst, and reducing agent [410,411]. Although the negative effect of water vapor on NH₃-SCR activity over low-temperature catalysts has been frequently reported due to its competitive adsorption of NH₃ and/or NO onto the active sites, regardless of the L–H and E–R mechanisms, irreversible deactivation was seldom reported and reversible deactivation was commonly observed, where initial catalytic activity could be recovered after stopping the introduction of water vapor into the feed [412]. Controlling the surface hydrophobicity of the catalyst by incorporating hydrophobic metal oxide shells into the active site [293] or using hydrophobic supports [33,34] can be effective in minimizing the adverse effects of water at low temperatures.

In contrast to the effect of water vapor, SO₂ has been reported to have a negative effect on NH₃-SCR catalytic activity and is known to cause irreversible deactivation, especially for low-temperature catalysts [19,31,412,413,414,415], which limits the practical application of low-temperature catalysts.

There are several strategies to improve the SO₂ tolerance of catalysts: (1) reducing the adsorption of SO₂ and its subsequent oxidation; (2) improving the adsorption of NH₃ and NO to form active intermediates in the presence of surface sulfate species; (3) building sacrificial sites to protect active sites; (4) promoting the decomposition of sulfate; and (5) searching for SO₂-tolerant compounds with good NH₃-SCR activity [12].

(1)

Reducing the adsorption of SO₂ and its subsequent oxidation. Inhibiting the adsorption and oxidation of SO₂ can significantly reduce the deposition of sulfates. Increasing the acidity of the catalyst can reduce SO₂ adsorption to some extent. However, on the other hand, more NH₃ can also be adsorbed on the surface acidic sites, promoting the formation of ammonium sulfate. Therefore, it is important to fine-tune the surface acidity to reduce the adsorption of SO₂ and NH₃ simultaneously. The introduction of SiO₂, Al₂O₃, and TiO₂ to the low-temperature NH₃-SCR catalysts might be effective for this purpose. Inactive metal sulfates and ammonium sulfates (i.e., AS and ABS) are derived from the reaction of SO₃ with metal oxides and NH₃, respectively, so it is necessary to inhibit the oxidation of SO₂ to SO₃ by decreasing the oxidation ability of active sites via adding electron-donating promoters [282,310]. Since NO₂, which can form under ‘Fast NH₃-SCR’ conditions at low temperatures, can promote the oxidation of SO₂ to SO₃, designing a catalyst that can follow the ‘Fast NH₃-SCR’ while inhibiting the oxidation of SO₂ to SO₃ is challenging but necessary to achieve high low-temperature NH₃-SCR activity as well as SO₂ tolerance (Figure 8).

(2)

Improving the adsorption of NH₃ and NO to form active intermediates in the presence of surface sulfate species. When the active sites begin to be sulfated by SO_x, the redox capacity of the active sites is reduced, and this leads to a decrease in NH₃-SCR activity, especially at low temperatures. Active NO_x species are adsorbed competitively with SO_x, but the adsorption of NH₄⁺ species is generally enhanced by new Brønsted acid sites derived from the deposited sulfate. Therefore, improving the adsorption of active nitrite/nitrate and gaseous NO₂ species in the presence of sulfate species is important to achieve SO₂ durability in NH₃-SCR reactions that follow the L–H mechanism. Compared with un-doped catalysts, more NH₄⁺, nitrates, and NO₂ were formed on Eu-modified Mn/TiO₂ [278] and Zr-modified Fe–Mn/TiO₂ [187] catalysts following the L–H mechanism, leading to improved SO₂ tolerance. If the catalyst follows the E–R mechanism, where the active NH₃ and NH₄⁺ species react directly with gaseous NO, this can lead to high SO₂ tolerance as NO does not need to be competitively adsorbed onto the catalyst surface as nitrate or nitrite. However, the E–R mechanism is not prevalent at low temperatures.

(3)

Building sacrificial sites to protect active sites. A rather simple strategy in which H-Y zeolite was physically mixed with V₂O₅/TiO₂ to trap ABS was reported to be effective to protect vanadium active sites, maintaining stable performance in the presence of 10% H₂O and 30 ppm SO₂ at a low temperature of 220 °C [416]. Ceria is well known to interact with SO₂ to form CeSO₄ or Ce₂(SO₄)₃, which can be used as a sacrificial site to protect the main active phase from sulfation, thereby improving the SO₂ tolerance of the catalyst [417]. Cr [173] and Nb [188] were also reported to react with SO₂ preferentially to prevent the sulfuration of active metal oxides (Figure 8). Designing a core–shell structure with outer sacrificial sites for SO_x adsorption and ammonium sulfate deposition and inner active sites for NH₃-SCR can effectively protect active sites from sulfate deposition [17,22,23,216,418,419].

(4)

Promoting the decomposition of sulfates. Ammonium sulfate and metal sulfate are two common sulfates observed during NH₃-SCR in the presence of SO₂. The former can be deposited on the catalyst and block surface active sites, but can be removed by water washing and heat treatment. However, the thermally stable metal sulfates formed through sulfation of active sites irreversibly deactivate the catalysts. Compared with metal sulfates, ammonium sulfates (AS and ABS) can decompose at lower temperatures. Therefore, reducing the decomposition temperature of ammonium sulfates can improve the SO₂ tolerance at lower temperatures. It was found that ABS decomposed more easily on SBA-15 with larger pores [330]. Therefore, it can be said that the decomposition of ammonia sulfate can be facilitated via constructing some mesoporous structure. Since metal sulfates are very difficult to decompose at low temperatures, it is desirable to interfere with their formation by adding suitable electrophilic or nucleophilic additives that can interact strongly with SO₄^2– and metal cations [12]. Chen et al. [420] proposed to place a single Mo atom on TiO₂ to form a Mo-Ti acid-base double site on Mo and Ti that can adsorb NH₄⁺ and HSO₄⁻ from ABS, respectively. When NH₄⁺ is oxidized by surface lattice oxygen on the Mo site, the electrons left on the dual site will transfer to the adsorbed HSO₄⁻ on the Ti site, releasing SO₂ at low temperatures. Fe doping [310] and Sb addition [195] were also reported to promote the decomposition of ammonium sulfate and inhibit the formation of MnSO₄.

(5)

Searching for SO₂-resistant compounds. Some metal sulfates, including CuSO₄ and FeSO₄, are known to have improved SO₂ tolerance, but due to their poor redox properties, they can exhibit NH₃-SCR activity only at moderate and/or high temperatures. Therefore, it is necessary to improve the redox properties of the catalyst to enhance its low-temperature activity. Nevertheless, the removal of deposited ammonia sulfate is not easy at low temperatures, and the resulting catalyst deactivation is inevitable. Therefore, future research should focus on preventing the deposition of ammonia sulfate at low temperatures or promoting its decomposition even if it is deposited while enhancing its redox ability.

The strategies mentioned above may seem different, but they are closely related. Considering all the above strategies, a promising low-temperature NH₃-SCR catalyst could have the following properties:

(1)

The active site must be composed of a multi-component metal oxide with moderate oxidation capacity not to oxidize SO₂ to SO₃, but still retain the oxidation capacity to oxidize NO to NO₂ and follow the ‘Fast NH₃-SCR’ pathway. Different catalyst compositions can be screened by DFT calculations that allow for the comparison of competing oxidation reactions, including SO₂ oxidation and NO oxidation, on the proposed catalyst surface [421,422,423]. The other type of active site is the promoted metal sulfate, which has both a high oxidation capacity to activate adsorbed NO species to follow the L–H reaction mechanism and the ability to decompose ammonium sulfate at low temperatures.

(2)

The active site mentioned above should be protected from the adsorption of SO_x and/or the ammonium sulfate produced by introducing some kind of sacrificial and/or protective component on or near the surface of the active site. The surface acidity, pore structure, and surface composition of the protective layer must be fine-tuned to take into account the adsorption property of SO_x and the decomposition of ammonium sulfate.

5. Summary

The redox properties and surface acidity are important factors affecting the catalytic activity of NH₃-SCR. Ammonia is adsorbed on the Lewis and Brønsted acid sites in the form of NH₃ and NH₄⁺, respectively, and is oxidatively dehydrogenated to form active intermediates. NO can react directly with adsorbed intermediates derived from NH₃ in the E–R mechanism, or in the L–H mechanism prevalent at low temperatures, NO can be adsorbed as is or oxidized to NO₂ and then adsorbed and react with adsorbed ammonia or ammonia derivatives. Moderate redox properties are very important for low-temperature NH₃-SCR catalysts, as strong redox properties can reduce N₂ selectivity due to the formation of unwanted N₂O, and weak redox properties can reduce the overall denitrification rate.

Water vapor and SO_x are often found in flue gas along with NO_x, and their effect on NH₃-SCR activity is particularly important at low temperatures. Water vapor in flue gas can cause reversible deactivation because it adsorbs competitively with NO_x and ammonia for active sites. SO₂ in flue gas can adsorb strongly to the active site or oxidize to SO₃ and further react with ammonia in the presence of water vapor to form ammonium bisulfate and/or ammonium sulfate, which irreversibly blocks the active site. SO₃ can react directly with active metal oxides to form metal sulfates with weak redox properties, resulting in low NH₃-SCR activity at low temperatures.

Among Cu-, Fe-, Mn-, and Ce-based catalysts, Mn-containing multivalent metal oxides can be promising candidates for low-temperature NH₃-SCR. The redox properties of MnO_x can be tuned by the incorporation of other multivalent metal oxides. The electronic properties of the active metal can be fine-tuned by adding appropriate promoters. The reduction of NH₃-SCR activity by water vapor can be overcome by increasing the surface hydrophobicity, which can be accomplished by using a hydrophobic support or by applying a hydrophobic sheath to the outside of the active metal oxide. When selecting the active metal oxide, DFT calculations should be performed to compare SO₂ adsorption, NO oxidation, and SO₂ oxidation to ensure that the oxidation of NO occurs without the adsorption of SO₂ and the oxidation of SO₂, allowing ‘Fast NH₃-SCR’ to proceed. The mesoporous shell will not only help to remove ammonium sulfate at the reaction temperature but also prevent SO₂ poisoning. Metal sulfate catalysts with strong redox properties and V-based catalysts containing oligomeric vanadyl species with sulfur-resistant properties still need to be further investigated for application in NH₃-SCR at low temperatures.