Enhanced Catalytic Performance of Hierarchical MnO_x/ZSM-5 Catalyst for the Low-Temperature NH₃-SCR

Abstract

A ZSM-5 zeolite with a hierarchical pore structure was synthesized by the desilication-recrystallization method using tetraethyl ammonium hydroxide (TEAOH) and cetyltrimethylammonium bromide (CTAB) as the desilication and structure-directing agents, respectively. The MnO_x/ZSM-5 catalyst was synthesized by the ethanol dispersion method and applied for the low-temperature selective catalytic reduction of NO_x with NH₃. The results showed that NO_x conversion of the hierarchical MnO_x/ZSM-5 catalyst could reach 100% at about 120 °C and could be maintained in the temperature range of 120–240 °C with N₂ selectivity over 90%. Furthermore, the hierarchical MnO_x/ZSM-5catalyst presented better SO₂ resistance performance than the traditional catalyst in the presence of 100 ppm SO₂ at 120 °C. XRD, SEM, TEM, XPS, BET, NH₃-TPD, and TG were applied to characterize the structural properties of the MnO_x/ZSM-5 catalysts. These results showed that the MnO_x/ZSM-5 catalyst had micropores (0.78 nm) and mesopores (3.2 nm) leading to a larger specific surface area, which improved the mass transfer of reactants and products while reducing the formation of sulfates. The better catalytic performance over hierarchical MnO_x/ZSM-5 catalyst could be attributed to the higher concentration of Mn⁴⁺ and chemisorbed oxygen species and higher surface acidity. The improved SO₂ resistance was related to the catalyst’s hierarchical pore structure.

1. Introduction

Selective catalytic reduction of NO_x by NH₃ (NH₃-SCR) is the most widely applied technology to remove NO_x from stationary sources. The catalyst is the key factor that can determine the efficiency of the selective catalytic reduction system [1]. The temperature window of traditional commercial catalyst (V₂O₅-WO₃(MoO₃)/TiO₂) is 300–400 °C, and low-temperature NH₃-SCR catalysts, which can be placed downstream of the desulfurizer and electrostatic precipitator, have attracted increasing attention in recent years [2]. Research on the low-temperature NH₃-SCR reaction using transition metal oxides (MnO_x [3], FeO_x [4], CuO_x [5], CeO_x [6]) as the active components has become widespread. Among these metal oxides, MnO_x exhibits excellent low-temperature catalytic activity, which can be attributed to the variable valence states of manganese and oxygen species [7]. However, there still remains a challenge of SO₂ poisoning for low-temperature NH₃-SCR catalysts. The deposition of sulfite and sulfate species and sulfate of manganese oxide on the catalysts’ surface lead to the inhibition of the catalysts. Therefore, improving the SO₂ tolerance of catalysts is required. Many efforts have been made to promote SO₂ tolerance of NH₃-SCR catalysts. Yu et al. [8] demonstrated that Pr modification could enhance the sulfur dioxide resistance of MnO_x/SAPO-34 catalyst. Compared with manganese oxide, SO₂ preferentially combines with PrO_x to form praseodymium sulfate and protecting the active sites of Mn. Fan et al. [9] demonstrated that when the cerium oxides were incorporated in the framework and the surface of the Mn/SAPO-34 catalyst, the sulfate of the manganese oxide and the formation of NH₄HSO₄ could be inhibited. In recent years, catalysts with hierarchical pore structures have attracted much attention for environmental catalytic applications. Due to their adjustable pore sizes and unique structural properties, Gao et al. [10] demonstrated manganese oxide nanoparticles with a hierarchical mesopore structure in the walls of the macroporous skeleton supported by three-dimensionally ordered macroporous carbon, which exhibited better low-temperature NH₃-SCR reaction activity, stability, as well as H₂O and SO₂ resistance. Fang et al. [11] prepared foam-like Fe₂O₃@CuO_x monolith catalysts with a three-dimensional hierarchical structure and displayed a higher activity, stability, as well as SO₂ and H₂O resistance than the catalyst without Fe₂O₃. In addition, some research studies have also focused on the hierarchical zeolite supported metal oxides. Pt nanoparticles were encapsulated in a hollow ZSM-5 through a “dissolution-recrystallization” process [12]. The Pt/hollow ZSM-5 catalyst showed excellent SO₂ and H₂O tolerance, but the active temperature range of the catalyst was very narrow. When the catalyst showed a maximum NO conversion at 90 °C, the activity then decreased with increasing temperature. Other catalysts like hierarchical Fe/ZSM-5 [13], hierarchical Fe-Beta [14], and hierarchical Fe-ZSM-5@CeO₂ [15] were synthesized and applied in the SCR reaction.

The ZSM-5 zeolite has adjustable surface acidity, shape selectivity, and a stable framework structure, which has been widely applied as a carrier in the SCR reaction. In this work, a hierarchical ZSM-5 zeolite was successfully prepared via a desilication-recrystallization method, and cetyltrimethylammonium bromide (CTAB) was used as the structure-directing agent. The hierarchical MnO_x/ZSM-5 catalyst was synthesized by the ethanol dispersion method and applied in the NH₃-SCR reaction. We studied the effect of the hierarchical pore structure on the catalytic performance through comparison with a traditional MnO_x/ZSM-5 catalyst. The hierarchical MnO_x/ZSM-5 catalyst showed better low-temperature NH₃-SCR catalytic activity and higher SO₂ tolerance. Furthermore, XRD, SEM, TEM, XPS, BET, NH₃-TPD, and TG were employed to explore the structural properties of the MnO_x/ZSM-5 catalyst. The results of comparable MnO_x/ZSM-5 catalysts demonstrated that a larger specific surface area and pore size, higher surface acidity, and higher concentration of Mn⁴⁺ and chemisorbed oxygen species could be provided by the hierarchical pore structure, thus promoting the low-temperature NH₃-SCR catalytic performance.

2. Results and Discussion

2.1. XRD Results

The XRD patterns of MnO_x/HZ-ET (hierarchical MnO_x/ZSM-5, the treated ZSM-5 was denoted as HZ-ET) and MnO_x/Z-P (conventional MnO_x/ZSM-5, ZSM-5 zeolite without treatment was denoted as Z-P) are shown in Figure 1. Two diffraction peaks between 2θ = 7 and 10° and three diffraction peaks between 2θ = 22 and 25° were observed, and these peaks were attributed to the characteristic diffraction peaks of the MFI zeolite, indicating that the MFI characteristic structure of ZSM-5 was not destroyed after desilication-recrystallization. No peaks of manganese oxide could be seen in the XRD spectra, indicating that manganese oxide in the catalyst existed either in an amorphous or highly dispersed state. TEM images would be used to further elucidate the structure of the MnO_x. The relative crystallinity results of MnO_x/HZ-ET and MnO_x/Z-P are shown in

Table 1. Relative crystallinity (RC) of the catalysts.

Samples	RC (%)
Z-P	100
MnOx/HZ-ET	81.4
MnOx/Z-P	79.3

. Compared with the parent ZSM-5 (Z-P), the lower relative crystallinity (R_C) of the MnO_x/HZ-ET was attributed to the desilication, which destroyed the crystal structure [16] and the interaction between the MnO_x and the zeolite [17]. The R_C of MnO_x/HZ-ET was higher than MnO_x/Z-P because of recrystallization, which indicated that the framework structure of MnO_x/HZ-ET was more complete.

2.2. SEM and TEM Results

Figure 2 shows the SEM images of MnO_x/Z-P and MnO_x/HZ-ET catalysts. The two catalysts showed a hexagonal morphology of the characteristic MFI structure. Compared with the smooth surface of the MnO_x/Z-P catalyst (Figure 2a), the surface of the MnO_x/HZ-ET catalyst (Figure 2b) was rougher, and the pore structure could be seen on the surface.

Figure 3 shows TEM images of MnO_x/HZ-ET catalysts. Regular mesopores are observed in Figure 3a,b. At the same time, some irregular, larger pore size mesopores compared with the above regular mesopores can be observed in Figure 3c,d. The micropores and regular/irregular mesopores were exhibited in MnO_x/HZ-ET with different size ranges. HRTEM images of MnO_x/HZ-ET catalyst are shown in Figure 3e,f. The lattice fringe spacing of 0.437 nm in Figure 3f corresponds to the (021) plane of Mn₃O₄, while the lattice fringe spacings of 0.690 nm and 0.272 nm in Figure 3e correspond to the (110) plane of MnO₂ and the (222) plane of Mn₂O₃, respectively. Combined with the XRD analysis, the results of TEM images showed that manganese oxide was not amorphous, but existed in various forms of crystallization in highly dispersed states. The (110) planes of MnO₂ in Figure 3e was considered as the most active crystal plane in the NH₃-SCR reaction [18] and provided a large number of active sites and oxygen vacancies. This was one of the reasons why the MnO_x/HZ-ET catalyst showed better low-temperature SCR activity than MnO_x/Z-P.

2.3. BET Results

Figure 4 shows the nitrogen adsorption-desorption isotherms and the pore size distribution curves of MnO_x/Z-P and MnO_x/HZ-ET catalysts. Figure 4a shows that MnO_x/Z-P exhibited type I isotherms with no distinct hysteresis loop, which indicated that MnO_x/Z-P was a typical microporous material, where MnO_x/HZ-ET displayed typical type I isotherms at low relative pressure (P/P₀ < 0.01) and type IV isotherms with a clear hysteresis loop at higher relative pressure (P/P₀ > 0.4), demonstrating the co-existence of micropores and mesopores created through the desilication-recrystallization processes. Figure 4b shows that the micropore size distributions of MnO_x/HZ-ET catalyst centered around 0.78 nm, which was larger than that of MnO_x/Z-P (around 0.73 nm). The enlargement of the micropores in MnO_x/HZ-ET may be due to the destruction of pore walls by treatment with TEAOH [19]. The pore size distribution of MnO_x/Z-P (Figure 4b) showed no obvious peaks in the range of 2–20 nm, and MnO_x/HZ-ET showed a narrow and intense peak at 3.2 nm. In addition, MnO_x/HZ-ET showed broad peaks between 4 and 15 nm, which possibly indicated that the intracrystalline mesopores were caused by desilication and existed in an irregular state [16]; this result was consistent with the TEM images.

Table 2. Structural properties of the catalysts.

Sample	SBET/(m2·g−1)	Sext/(m2·g−1)	Vtotal/(cm3·g−1)	Vmicro/(cm3·g−1)	Vmeso/(cm3·g−1)
MnOx/HZ-ET	328	124	0.191	0.079	0.112
MnOx/Z-P	247	12	0.157.	0.098	0.059

shows the corresponding results of structural properties of MnO_x/Z-P and MnO_x/HZ-ET catalysts. The specific surface areas and the external surface areas of MnO_x/HZ-ET were 328 m²/g and 124 m²/g, which was higher than MnO_x/Z-P (247 m²/g and 12 m²/g, respectively). MnO_x/HZ-ET contained more mesopores and less micropores than MnO_x/Z-P, which was attributed to the transformation of part of the microporous phase into the mesoporous phase. The larger specific surface area and pore size within hierarchical MnO_x/HZ-ET improved the mass transfer of reactants and products during the NH₃-SCR reaction, thus improving its catalytic performance compared to MnO_x/Z-P, which only had microporous structures.

2.4. XPS Results

The surface atomic composition was analyzed by XPS. Figure 5 shows the XPS spectra of Mn 2p and O 1s for the MnO_x/Z-P and MnO_x/HZ-ET catalysts. The XPS spectra of Mn 2p_3/2 could be fit into three characteristic peaks ascribed to Mn²⁺ at 640.4 eV, Mn³⁺ at 642.0 eV, and Mn⁴⁺ at 644.1 eV. The XPS spectra of O 1s could be divided into two characteristic peaks ascribed to lattice oxygen species (O_α), around 529.7 eV, and chemisorbed oxygen species (O_β), around 531.5 eV [20]. The corresponding results are summarized in

Table 3. Surface atomic concentrations and relative concentration ratios of Mn and O.

Sample	Atomic Fraction/(%)			XO/(%)		XMn/(%)
	Mn	O	Others	Oα	Oβ	Mn2+	Mn3+	Mn4+
MnOx/HZ-ET	9.62	58.51	31.87	12.91	87.09	9.78	41.12	49.10
MnOx/Z-P	8.47	58.85	32.68	24.59	75.41	15.35	48.29	36.36

. It was clear that the molar concentration of manganese on the surface of MnO_x/HZ-ET (9.62%) was higher than MnO_x/Z-P (8.47%), and the relative surface concentration ratios of Mn⁴⁺ (Mn⁴⁺/Mn) of MnO_x/HZ-ET(49.09%) were much higher than MnO_x/Z-P (36.36%). It was reported that Mn⁴⁺ could promote the oxidation of NO to NO₂, and higher concentration ratios of Mn⁴⁺ species were beneficial for the improvement of their redox cycle, thus promoting the activity in the NH₃-SCR reaction at low temperatures [21]. In addition, the percentage of chemisorbed oxygen species O_β in MnO_x/HZ-ET (87.09%) was much higher than MnO_x/Z-P (75.41%). The chemisorbed oxygen species O_β was reported to be more active than O_α because of their higher mobility. Higher percentage of O_β was beneficial for the oxidation of NO to NO₂, enhancing the low-temperature activity through the “fast SCR” reaction [22]. Therefore, the excellent low-temperature SCR activity of MnO_x/HZ-ET was attributed to the higher percentage of active Mn⁴⁺ and chemisorbed oxygen species.

2.5. NH₃-TPD Results

The surface acidity properties of the catalysts were analyzed by the NH₃-TPD technique. Figure 6 shows that the NH₃ desorption profiles of MnO_x/HZ-ET and MnO_x/Z-P exhibited two distinct desorption peaks. The desorption peaks below 350 °C were associated with the weakly acidic sites, while the desorption peaks ranging from 350 to 550 °C were associated with the strongly acidic sites [23]. The quantified results of NH₃-TPD are summarized in

Table 4. The NH₃ desorption of the catalysts.

Sample	Area of Peak		Total Area of Peaks
	Peak 1	Peak 2
MnOx/HZ-ET	1415	1671	3086
MnOx/Z-P	1878	1042	2920

, and the area of the peak was proportional to the amount of acid. It could be found that MnO_x/HZ-ET showed more total acid amount than MnO_x/Z-P. Although the weak acid amount of MnO_x/Z-P was higher than MnO_x/HZ-ET, the strong acid and total acid amount of MnO_x/HZ-ET were both higher than MnO_x/Z-P, which may be ascribed to the higher specific surface area of the MnO_x/HZ-ET catalyst. It could be concluded that MnO_x/HZ-ET with a hierarchical pore structure could provide more strong surface acidity sites, which would be beneficial for the adsorption and activation of NH₃, resulting in increased low-temperature NH₃-SCR performance.

2.6. SCR Performance

Low-temperature NH₃-SCR activities and N₂ selectivities of MnO_x/HZ-ET and MnO_x/Z-P were tested over the temperature range of 80–240 °C, and the results of the NO_x conversion are shown in Figure 7. The MnO_x/Z-P catalyst showed low NO_x conversion in the temperature range of 80–180 °C, and 99% NO_x conversion was obtained around 180 °C. Meanwhile, the N₂ selectivity of MnO_x/Z-P was lower than 90% over the whole temperature range. Compared with MnO_x/Z-P, MnO_x/HZ-ET showed significantly higher NO_x conversion over the temperature range of 80–120 °C. When the temperature reached 120 °C, nearly 100% NO_x conversion could be obtained with a broader operating temperature window (120–240 °C). Over 90% N₂, selectivity could be maintained for the MnO_x/HZ-ET catalyst throughout the entire temperature range. Conventional microporous catalysts were reported to have several drawbacks in the SCR reaction, such as diffusion limitations of the reactants and products [24]. Therefore, it was reasonable to deduce that the mass transfer of reactants and products could be enhanced with the existence of mesopores at low temperatures, and as a result of this, better low-temperature NH₃-SCR performance could be obtained using the hierarchical MnO_x/HZ-ET catalyst.

2.7. Effect of SO₂ on SCR Catalytic Activity

Figure 8 shows the NO_x conversion for the MnO_x/HZ-ET and MnO_x/Z-P catalysts in the presence of 100 ppm SO₂ at 120 °C and 180 °C, respectively. The results showed that NO_x conversion noticeably decreased with the addition of SO₂ into the feed gas. The SCR activity of the MnO_x/Z-P catalyst decreased from 100% to 15% after 60 min of SO₂ addition. The NO_x conversion of MnO_x/HZ-ET decreased from 100% to 60% after the SO₂ was added for 1 h. The NO_x conversion could not be recovered over the two catalysts when SO₂ was absent from the reaction mixture, which indicated that the deactivation was irreversible. (NH₄)₂SO₄ or NH₄HSO₄ may be formed during the reaction when SO₂ was added into the reaction atmospheres. These species could only decompose over 300 °C and may block the zeolite channels, leading to reduced catalytic activities. At the same time, catalysts with a larger pore size could reduce the formation of sulfate species [25]. Comparing the SO₂ tolerance of MnO_x/HZ-ET and MnO_x/Z-P, the hierarchical pore structure MnO_x/HZ-ET catalyst could provide larger specific surface area and larger pore size, which was beneficial for its SO₂ resistance.

XRD and TG were applied to identity the formation of (NH₄)₂SO₄ or NH₄HSO₄. Figure 9 shows the XRD patterns of poisoned MnO_x/HZ-ET and MnO_x/Z-P catalysts. The peak at 2θ = 33° observed over the used MnO_x/Z-P could be attributed to the phase of formed NH₄HSO₄. No obvious peaks of NH₄HSO₄ could be observed in the XRD pattern of used MnO_x/HZ-ET, which may be because the NH₄HSO₄ formed existed in amorphous species or was below the detection limit. Figure 10 presents the TG curves of the poisoned MnO_x/HZ-ET and MnO_x/Z-P catalysts. The first weight loss emerging at 80–110 °C could be due to the evaporation of water in the catalysts. Two other weight losses at about 250 °C and 350 °C were close to the decomposition temperature of (NH₄)₂SO₄ or NH₄HSO₄ [8]. For MnO_x/HZ-ET, the intensities of two weight losses was lower than that of MnO_x/Z-P. The XRD and TG results showed that the MnO_x/HZ-ET with a larger pore size could reduce the deposition of (NH₄)₂SO₄ or NH₄HSO₄ on the catalyst surface during the NH₃-SCR reaction with SO₂, which was one of the reasons for the better SO₂ resistance of the MnO_x/HZ-ET catalyst.

3. Materials and Methods

3.1. Catalyst Preparation

The hierarchical ZSM-5 was prepared from commercial MFI-type zeolites by sequential desilication-recrystallization based on previous studies [16,19,26]. The parent zeolite in this study was a commercial H form ZSM-5 with a Si/Al mass ratio = 38 (XFNANO Company, Nanjing, China). Typically, three grams of ZSM-5 zeolite and 1 g cetyltrimethylammonium bromide (CTAB, Aladdin, Shanghai, China) were dispersed in 30 mL of 1 M tetraethyl ammonium hydroxide (TEAOH, Aladdin, Shanghai, China), and the mixture was stirred at ambient temperature. The solution was transferred into a Teflon-lined autoclave and treated at 150 °C for 24 h. The product was washed with distilled water, filtered, dried, and calcined at 550 °C for 4 h to remove the templates. The treated sample was denoted as HZ-ET, and for comparison, the ZSM-5 zeolite without treatment was denoted as Z-P.

The catalysts were synthesized by the ethanol dispersion method, and manganese nitrate (50 wt.% in H₂O, Aladdin, Shanghai, China) was used as a precursor. Typically, one-point-nine-five grams of 50 wt.% Mn(NO₃)₂ (Mn loading = 15 wt.%) were dissolved in 75 mL ethanol under stirring at ambient temperature. Subsequently, two grams of HZ-ET or Z-P powder were added and stirred. Then, the solution was treated with ultrasound for 0.5 h and stirred continuously at 85 °C until the solvent was completely evaporated. The products were dried at 80 °C and calcined at 400 °C for 3 h. The catalyst was denoted as MnO_x/HZ-ET. For comparison, MnO_x/Z-P was also prepared according to the same method.

3.2. Low-Temperature NH₃-SCR Activity Measurements

The NH₃-SCR activity tests were carried out in a fixed-bed quartz glass reactor. Five-hundred milligrams of 40–60 mesh catalysts were used in the test. The reaction temperature was increased from room temperature to 240 °C with a heating rate of 5 °C/min with an isotherm step of 20 °C. The gas was composed of 800 ppm NH₃, 800 ppm NO, 5.0 vol% O₂, and 100 ppm SO₂ (when added), balanced by Ar. The flow rate was 600 mL/min with a gas hourly space velocity (GHSV) of 40,000 h⁻¹. The concentrations of NO/NO₂ were measured by a NO–NO₂–NO_x analyzer (Thermal Scientific, model 42i-HL, Waltham, USA), and N₂ gas chromatography was used to analyze N₂-selectivity. The NO_x removal efficiency and the N₂ selectivity are calculated as follows:

$${\mathrm{NO}}_{\mathrm{x}}\mathrm{conversion}=\frac{{\left[{\mathrm{NO}}_{\mathrm{x}}\right]}_{\mathrm{inlet}}-{\left[{\mathrm{NO}}_{\mathrm{x}}\right]}_{\mathrm{outlet}}}{{\left[{\mathrm{NO}}_{\mathrm{x}}\right]}_{\mathrm{inlet}}}\times 100\%$$

(1)

$${\mathrm{N}}_2\mathrm{selectivity}=(1-\frac{2{\left[{\mathrm{N}}_2\mathrm{O}\right]}_{\mathrm{outlet}}}{{\left[{\mathrm{NO}}_{\mathrm{x}}\right]}_{\mathrm{inlet}}+{\left[{\mathrm{NH}}_3\right]}_{\mathrm{inlet}}-{\left[{\mathrm{NO}}_{\mathrm{x}}\right]}_{\mathrm{outlet}}-{\left[{\mathrm{NH}}_3\right]}_{\mathrm{outlet}}})\times 100\%$$

(2)

where ${\left[{\mathrm{NO}}_{\mathrm{x}}\right]}_{\mathrm{inlet}}$ and ${\left[{\mathrm{NH}}_3\right]}_{\mathrm{inlet}}$ represent the inlet concentration (ppm) of NO_x and NH₃, respectively. ${\left[{\mathrm{NO}}_{\mathrm{x}}\right]}_{\mathrm{outlet}}$, ${\left[{\mathrm{NH}}_3\right]}_{\mathrm{outlet}}$, and ${\left[{\mathrm{N}}_2\mathrm{O}\right]}_{\mathrm{outlet}}$ represent the outlet concentration (ppm) of NO_x and NH₃ and N₂O, respectively.

3.3. Characterization

X-ray diffraction (XRD) patterns were recorded on a D8 Advance diffractometer (Bruker, Karlsruhe, Germany) with Cu Kα radiation. Chemical states of all elements were analyzed by X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos, U.K.) with Al Kα radiation (hυ = 1253.6 eV). NH₃-TPD analysis was performed on Tp 5080 (Xianquan Industrial and Trading Co., Ltd, Tianjin, China). A scanning electron microscope (SEM) was carried out on ZEISS Merlin instrument (Carl Zeiss AG, Jena, Germany). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed on TECNAI G2 F20 (FEI, Hillsboro, OR, USA). N₂ physisorption was performed at −196 °C using a surface analyzer (Micomeritics, ASAP 2020, Norcross, USA), and all samples were degassed in a vacuum at 30 °C for 6 h prior to measurements. The Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area. The micropore surface area and micropore volume were evaluated using a t-plot. The Barrett–Joyner–Halenda (BJH) method was used to determine the pore size distributions from desorption branches. Thermogravimetric analysis (TG) was carried out in a static N2 atmosphere using a TGA/DSC 3+instrument (Mettler, Zurich, Switzerland). Fifteen milligrams of each sample were analyzed between 30 and 800 °C at a rate of 10 °C/min.

4. Conclusions

A MnO_x/HZ-ET catalyst, which possessed a hierarchical pore structure, was successfully prepared through desilication-recrystallization and ethanol dispersion methods. Compared with MnO_x/Z-P prepared using traditional microporous ZSM-5 as a support, MnO_x/HZ-ET showed better low-temperature NH₃-SCR activity with nearly 100% NO_x conversion over a broad temperature window from 120 °C to 240 °C and above 90% N₂ selectivity throughout the entire temperature range. The low temperature NH₃-SCR performance of MnO_x/HZ-ET could be attributed to the hierarchical pore structure, higher concentration of chemisorbed oxygen and Mn⁴⁺ species, as well as appropriate acid strengths and amounts. The MnO_x/HZ-ET catalyst also showed better SO₂ tolerance than the MnO_x/Z-P, and 60% NO_x conversion could be maintained after the SO₂ was added for 1 h, which could be related to its larger specific surface area and larger pore size, which may reduce the deposition of ammonium sulfate.