catalysts MnO x Supported on Hierarchical SAPO-34 for the Low-Temperature Selective Catalytic Reduction of NO with NH 3 : Catalytic Activity and SO 2 Resistance

: The ethanol dispersion method was employed to synthesize a series of MnO x /SAPO-34 catalysts using SAPO-34 with the hierarchical pore structure as the zeolite carrier, which were prepared by facile acid treatment with citric acid. Physicochemical properties of catalysts were characterized by XRD, XPS, BET, TEM, NH 3 -TPD, SEM, FT-IR, Py-IR, H 2 -TRP and TG/DTG. NH 3 SCR performances of the hierarchical MnO x /SAPO-34 catalysts were evaluated at low temperatures. Results show that citric acid etching solution at a concentration of 0.1 mol/L yielded a hierarchical MnO x /SAPO-34-0.1 catalyst with ca .15 wt.% Mn loading, exhibiting optimal catalytic activity and SO 2 tolerance at low temperatures. Almost 100% NO conversion and over 90% N 2 selectivity at 120 °C under a gas hourly space velocity (GHSV) of 40,000 h − 1 could be obtained over this sample. Furthermore, the NO conversion was still higher than 65% when 100 ppm SO 2 was introduced to the reaction gas for 4 h. These could be primarily attributed to the large speciﬁc surface area, high surface acidity concentration and abundant chemisorbed oxygen species provided by the hierarchical pore structure, which could also increase the mass transfer of the reaction gas. This ﬁnding suggests that the NH 3 -SCR activity and SO 2 poisoning tolerance of hierarchical MnO x /SAPO-34 catalysts at low temperatures can be improved by controlling the morphology of the catalysts, which might supply a rational strategy for the design and synthesis of Mn-based SCR catalysts.


Introduction
Nitrogen oxides (NO x ), which are released from stationary and automobile exhausts, are major atmospheric pollutants that result in many environmental issues, such as haze, acid rain and photochemical smog [1]. To date, the best developed and most efficient flue gas cleaning technology for NO x abatement from stationary sources is the selective catalytic reduction (SCR) of NO x with ammonia [2][3][4]. Owing to their high efficiency in eliminating NO x , V 2 O 5 -WO 3 /TiO 2 catalysts have been used commercially as an NH 3 -SCR catalyst. However, there are some drawbacks to these commercial catalysts, including biological toxicity and low SCR performance below 300 • C, which limit their application in NH 3 -SCR systems [1,4]. In recent years, the low-temperature NH 3 -SCR technology, working downstream after the electrostatic precipitator, where most of the flue gas is cooled down and the SO 2 /dusts are removed, has gained increasing attention [5,6].
Manganese-containing catalysts exhibit desirable catalytic activities at low temperatures, which received much attention in recent years [7][8][9][10][11]. However, Mn-based catalysts are easily deactivated when the treated flue gas contains SO 2 . For this reason, the sulfurpoisoning mechanism of catalysts caused by SO 2 and SO 3 has been widely explored. The

SEM and TEM
SEM was used to observe the morphological features of both the synthesized and reference zeolites ( Figure 2). Cubic morphology of the characteristic CHA structure was observed in all samples. As the molar concentration of citric acid increased, the surface roughness of the SAPO-34 molecular sieve gradually increased. When the molar concentration of citric acid etching solution was 0.125 mol/L, the structure of the catalyst collapsed, which caused its external specific areas to decrease. Compared with SAPO-34, which displayed well-developed crystal faces, hierarchical SAPO-34 crystals showed a rough surface because of the surface etching with citric acid solution. As expected, the MnOx/H-SAPO-34-0.1 zeolite retained its original structure well after loading manganese species, compared with the MnOx/SAPO-34 sample. The fact that the MnOx/H-SAPO-34-0.1 sample had much higher external specific surface areas than those of MnOx/SAPO-34 prepared by conventional methods might be due to the special structure of the former.  Figure 5b corresponded to the (110) plane of MnO2 and the (222) plane of Mn2O3, respectively. The most active crystal plane in the NH3-SCR reaction was considered to be the (110) plane of MnO2, which was highly dispersed in MnOx/H-SAPO-34-0.1 [25]. This was one of the reasons for the high SCR activity of the MnOx/H-SAPO-34-0.1 catalyst at low temperatures.

SEM and TEM
SEM was used to observe the morphological features of both the synthesized and reference zeolites ( Figure 2). Cubic morphology of the characteristic CHA structure was observed in all samples. As the molar concentration of citric acid increased, the surface roughness of the SAPO-34 molecular sieve gradually increased. When the molar concentration of citric acid etching solution was 0.125 mol/L, the structure of the catalyst collapsed, which caused its external specific areas to decrease. Compared with SAPO-34, which displayed well-developed crystal faces, hierarchical SAPO-34 crystals showed a rough surface because of the surface etching with citric acid solution. As expected, the MnO x /H-SAPO-34-0.1 zeolite retained its original structure well after loading manganese species, compared with the MnO x /SAPO-34 sample. The fact that the MnO x /H-SAPO-34-0.1 sample had much higher external specific surface areas than those of MnO x /SAPO-34 prepared by conventional methods might be due to the special structure of the former.  Figure 5b corresponded to the (110) plane of MnO 2 and the (222) plane of Mn 2 O 3 , respectively. The most active crystal plane in the NH 3 -SCR reaction was considered to be the (110) plane of MnO 2 , which was highly dispersed in MnO x /H-SAPO-34-0.1 [25]. This was one of the reasons for the high SCR activity of the MnO x /H-SAPO-34-0.1 catalyst at low temperatures.

BET
The N2 adsorption-desorption isotherms and pore size distribution of all samples shown in Figure 6. As seen in Figure 6a, MnOx/SAPO-34 presented typical type I adso tion-desorption isotherms in the low-pressure regions (P/P0 < 0.01). MnOx/H-SAPO-34exhibited a representative characteristic type IV isotherm and a well-defined hystere loop at the pressure regions of 0.4 < P/P0 < 0.9 [26]. Hysteresis loops could be observed the region of 0.4 < P/P0 < 0.9 for sample MnOx/H-SAPO-34-0.1, suggesting that there w secondary larger pores in the microporous structure of the SAPO-34 crystal [27]. The p size distribution of samples in Figure 6b,c illustrates the existence of a mesopore struct with a pore size of approximately 5 nm. The results of the structural properties of catalysts are listed in Table 1

BET
The N2 adsorption-desorption isotherms and pore size distribution of all samples shown in Figure 6. As seen in Figure 6a, MnOx/SAPO-34 presented typical type I adso tion-desorption isotherms in the low-pressure regions (P/P0 < 0.01). MnOx/H-SAPO-34exhibited a representative characteristic type IV isotherm and a well-defined hystere loop at the pressure regions of 0.4 < P/P0 < 0.9 [26]. Hysteresis loops could be observed the region of 0.4 < P/P0 < 0.9 for sample MnOx/H-SAPO-34-0.1, suggesting that there w secondary larger pores in the microporous structure of the SAPO-34 crystal [27]. The p size distribution of samples in Figure 6b,c illustrates the existence of a mesopore struct with a pore size of approximately 5 nm. The results of the structural properties of catalysts are listed in Table 1.

BET
The N 2 adsorption-desorption isotherms and pore size distribution of all samples are shown in Figure 6. As seen in Figure 6a, MnO x /SAPO-34 presented typical type I adsorption-desorption isotherms in the low-pressure regions (P/P 0 < 0.01). MnO x /H-SAPO-34-0.1 exhibited a representative characteristic type IV isotherm and a well-defined hysteresis loop at the pressure regions of 0.4 < P/P 0 < 0.9 [26]. Hysteresis loops could be observed in the region of 0.4 < P/P 0 < 0.9 for sample MnO x /H-SAPO-34-0.1, suggesting that there were secondary larger pores in the microporous structure of the SAPO-34 crystal [27]. The pore size distribution of samples in Figure 6b,c illustrates the existence of a mesopore structure with a pore size of approximately 5 nm. The results of the structural properties of the catalysts are listed in Table 1

FT-IR
The chemical states of fresh and deactivated catalysts were studied by FT-IR spectroscopy and the results are illustrated in Figure

FT-IR
The chemical states of fresh and deactivated catalysts were studied by FT-IR spectroscopy and the results are illustrated in Figure [28]. When the MnO x /SAPO-34 and MnO x /H-SAPO-34-0.1 were exposed to SO 2 atmosphere, both of them showed a spectrum roughly similar to that of the fresh catalysts. However, one new peak appeared at 1420 cm −1 , which could be assigned to NH 4 + species chemisorbed on Brønsted acid sites. This implied that after the sulfur resistance test, ammonium species were formed on the surface of the catalyst. Concurrently, a new weak band appeared at 525 cm −1 ,which might be assigned to the characteristic frequencies of the SO 4 2− [29]. These findings indicate that sulfate species may form during the SCR reaction in the presence of SO 2 by binding to metal oxides or adsorbed NH 3 species. From the above results, the bands at 1420 and 525 cm −1 were clearly visible for MnO x /SAPO-34 after the sulfur resistance test. However, these significant peaks were not obvious in the spectrum of MnO x /H-SAPO-34-0.1, indicating that sulfate species were only slightly deposited on this sample.

R PEER REVIEW
7 of 17 assigned to the characteristic frequencies of the SO4 2− [29]. These findings indicate that sulfate species may form during the SCR reaction in the presence of SO2 by binding to metal oxides or adsorbed NH3 species. From the above results, the bands at 1420 and 525 cm −1 were clearly visible for MnOx/SAPO-34 after the sulfur resistance test. However, these significant peaks were not obvious in the spectrum of MnOx/H-SAPO-34-0.1, indicating that sulfate species were only slightly deposited on this sample.

NH3-TPD and Py-IR
NH3-TPD experiments were conducted to probe the surface acid properties of both hierarchical and conventional MnOx/SAPO-34 catalysts. As shown in Figure 8a and Table  2, the catalysts exhibited two desorption peaks in the whole temperature range. The peak from 170 to 220 °C was likely due to NH3 adsorbed on physical and weak acid sites. The desorption peak at 380-530 °C was assigned to NH3 adsorbed on the strong acid sites [30]. The strong acid amount of the hierarchical MnOx/H-SAPO-34-0.1 was the highest among the samples, which suggests that the larger pore structure and higher surface area enhanced the concentration and acidity of strong acid sites. Therefore, the hierarchical MnOx/SAPO-34 catalysts with larger specific surface areas, which could provide more strong acid sites on the surface of the catalysts for the adsorption and activation of NH3, exhibited the optimal NH3-SCR performance at low temperatures. Pyridine is larger than the eight-ring diameter of the CHA structure. The IR spectra of adsorbed pyridine was therefore related to the acid site in the mesopore channels [31]. The Py-IR measurement defined and established the types of acid sites in Figure 8b. Furthermore, the amounts of total and medium strong acid over the MnOx/SAPO-34 catalysts were acquired with the number of desorbed pyridine molecules at 200 °C. Brønsted acid (B) and Lewis acid (L) were found to be present on both MnOx/SAPO-34 and MnOx/H-SAPO-34-x, corresponding to the bands at around 1535 and 1445 cm −1 , respectively. In addition, the peak at around 1490 cm −1 was assigned to both Brønsted and Lewis acid sites. As shown in Figure  8b, the Lewis acid sites were the main acid sites of the as-obtained catalysts, and there was also a small number of Brønsted acid sites. With the increase in the concentration of citric 2.1.5. NH 3 -TPD and Py-IR NH 3 -TPD experiments were conducted to probe the surface acid properties of both hierarchical and conventional MnO x /SAPO-34 catalysts. As shown in Figure 8a and Table 2, the catalysts exhibited two desorption peaks in the whole temperature range. The peak from 170 to 220 • C was likely due to NH 3 adsorbed on physical and weak acid sites. The desorption peak at 380-530 • C was assigned to NH 3 adsorbed on the strong acid sites [30]. The strong acid amount of the hierarchical MnO x /H-SAPO-34-0.1 was the highest among the samples, which suggests that the larger pore structure and higher surface area enhanced the concentration and acidity of strong acid sites. Therefore, the hierarchical MnO x /SAPO-34 catalysts with larger specific surface areas, which could provide more strong acid sites on the surface of the catalysts for the adsorption and activation of NH 3 , exhibited the optimal NH 3 -SCR performance at low temperatures. Pyridine is larger than the eight-ring diameter of the CHA structure. The IR spectra of adsorbed pyridine was therefore related to the acid site in the mesopore channels [31]. The Py-IR measurement defined and established the types of acid sites in Figure 8b. Furthermore, the amounts of total and medium strong acid over the MnO x /SAPO-34 catalysts were acquired with the number of desorbed pyridine molecules at 200 • C. Brønsted acid (B) and Lewis acid (L) were found to be present on both MnO x /SAPO-34 and MnO x /H-SAPO-34-x, corresponding to the bands at around 1535 and 1445 cm −1 , respectively. In addition, the peak at around 1490 cm −1 was assigned to both Brønsted and Lewis acid sites. As shown in Figure 8b, the Lewis acid sites were the main acid sites of the as-obtained catalysts, and there was also a small number of Brønsted acid sites. With the increase in the concentration of citric acid etching solution, the number of Brønsted acid sites increased gradually. In addition, the MnO x /H-SAPO-34-0.1 catalyst had the highest B/L ratio (0.65) among all of the catalysts, which was favorable for the oxidation of NO to NO 2 for the reaction of NO +NH 3 + O 2 . sites for the adsorption of NH3 on the surface of the catalyst, which was in accordance with the results of the SO2 resistance test.  2.1.6. H2-TPR H2-TPR characterization was performed to investigate the redox properties of the catalysts, and the corresponding H2-TPR profiles are shown in Figure 9. Both MnOx/H-SAPO-34-0.1 and MnOx/SAPO-34 catalysts presented two reduction peaks in the range of 200-800 °C. For the profile of MnOx/SAPO-34, the peaks at 416 and 513 °C could be assigned to MnO2/Mn2O3→Mn3O4 and Mn3O4→MnO, respectively [32]. Compared with the conventional MnOx/SAPO-34 catalyst, it is clear that the reduction peaks shifted to a lower temperature centering at 351 and 453 °C, suggesting that the reducibility of MnOx/H-SAPO-34-0.1 highly increased. Based on previous studies, the reduction peak area had a direct relationship with the consumed content of H2 [33]. It can be seen that the reduction peak area of MnOx/H-SAPO-34-0.1 that appeared at relatively low temperature was much larger than that of MnOx/SAPO-34, suggesting that the manganese species on the MnOx/H-SAPO-34-0.1 surface is highly dispersed, which is consistent with the results of the EDS mapping [34]. In addition, the Mn species with high valence states such as Mn 4+ and Mn 3+ , which were conductive to the adsorption of NH3 and NO to form Mn 4+ -NH3 and Mn 3 + -NO3, could enhance the catalytic activity in the NH3-SCR reaction [35]. Therefore, the MnOx/H-SAPO-34-0.1 catalyst with higher reducibility exhibited optimal lowtemperature performance.  SO 2 produced sulfur ammonium salt, which could cover and occupy some active sites and had a serious influence on the strong acidity of the catalysts. As seen in Figure 8, the MnO x /H-SAPO-34-0.1 sample had a higher amount of strong acid sites and Brønsted acid sites than the MnO x /SAPO-34 sample. This discovery indicates that the hierarchical pore structure could effectively inhibit the sulfation of active sites and provide more acid sites for the adsorption of NH 3 on the surface of the catalyst, which was in accordance with the results of the SO 2 resistance test.  [32]. Compared with the conventional MnO x /SAPO-34 catalyst, it is clear that the reduction peaks shifted to a lower temperature centering at 351 and 453 • C, suggesting that the reducibility of MnO x /H-SAPO-34-0.1 highly increased. Based on previous studies, the reduction peak area had a direct relationship with the consumed content of H 2 [33]. It can be seen that the reduction peak area of MnO x /H-SAPO-34-0.1 that appeared at relatively low temperature was much larger than that of MnO x /SAPO-34, suggesting that the manganese species on the MnO x /H-SAPO-34-0.1 surface is highly dispersed, which is consistent with the results of the EDS mapping [34]. In addition, the Mn species with high valence states such as Mn 4+ and Mn 3+ , which were conductive to the adsorption of NH 3 and NO to form Mn 4+ -NH 3 and Mn 3+ -NO 3 , could enhance the catalytic activity in the NH 3 -SCR reaction [35]. Therefore, the MnO x /H-SAPO-34-0.1 catalyst with higher reducibility exhibited optimal low-temperature performance.
main causes of catalyst deactivation [47]. Therefore, the hierarchical pore structure can attain a dynamic balance between the formation and decomposition of ammonium sulfate, which could provide an increased surface area for the reaction process and prolong the retention of reactants on the catalyst surface. Moreover, the special structure decreased the possibility of surface active sites being taken up by SO2 and prevented the formation of sulfates from blocking the active sites, leading to a high SO2 resistance [48].     [46]. Therefore, the morphological features of MnO x /H-SAPO-34-0.1 indicate that the hierarchical pore structure could produce extra surface vacancies to activate oxygen [33]. The hierarchical pore structure appeared to facilitate the transformation of NO to NO 2 , which improved the SCR performance at low temperatures. It is worth noting that the atomic ratio of S in the deactivated MnO x /SAPO-34 catalyst was slightly higher than that in the deactivated MnO x /H-SAPO-34-0.1 sample, indicating that the sulfates did not accumulate in large quantities on the surface of the MnO x /H-SAPO-34-0.1-S sample, which is in keeping with the results of FT-IR. It has been reported that the deposition of ABS and the sulfurating of active component Mn were the main causes of catalyst deactivation [47]. Therefore, the hierarchical pore structure can attain a dynamic balance between the formation and decomposition of ammonium sulfate, which could provide an increased surface area for the reaction process and prolong the retention of reactants on the catalyst surface. Moreover, the special structure decreased the possibility of surface active sites being taken up by SO 2 and prevented the formation of sulfates from blocking the active sites, leading to a high SO 2 resistance [48].

TG/DTG
In order to explore the SO 2 resistance of MnO x /H-SAPO-34-0.1 and MnO x /SAPO-34 catalysts, TG experiments were performed. The results are shown in Figure 11. As seen in Figure 11a, three weight losses are shown in the TG curves in the tested range. The weight loss below 200 • C was likely due to the evaporation of absorbed water, whereas the weight loss (A) between 200 and 450 • C was caused by the decomposition of ABS, and the latter weight loss (B) at 700-850 • C corresponded to the decomposition of MnSO 4 [44]. To investigate the possible pore size effect, a decomposition experiment was also performed on the MnO x /H-SAPO-34-0.1 sample. The results are shown in Figure 11b. It was found that the TG signals of MnO x /H-SAPO-34-0.1 displayed similar trends to those of MnO x /SAPO-34. However, the weight loss (A) of MnO x /SAPO-34 and MnO x /H-SAPO-34-0.1 was measured at 4.05% and 3.39%, respectively. Smaller weight loss (A) was observed in MnO x /H-SAPO-34-0.1, indicating that less ABS formed on the sample, which was consistent with the XPS results. Meanwhile, the weight loss (B) of MnO x /SAPO-34 and MnO x /H-SAPO-34-0.1 was measured at 2.12% and 1.73%, respectively. The results of BET and NH 3 -TPD suggest that the MnO x /H-SAPO-34-0.1 catalyst with a larger specific surface area along with abundant acid sites, which could offer a high-efficiency place to trigger the SCR reaction, resulted in improved SO 2 tolerance [49]. The TG/DTG analysis indicated that fewer sulfates were deposited on the surface of the MnO x /H-SAPO-34-0.1 catalyst, which provided evidence for the anti-SO 2 capability of the hierarchical pore structure. Consistent with the results of NH 3 -TPD and Py-IR, the effect of the hierarchical pore structure was to make a balance between effective acid sites and the formation/decomposition of ABS, thus enhancing the deNO x properties and suppressing the blocking effect of SO 2 . Consequently, these phenomena suggest that the hierarchical pore structure promoted the diffusion of the reaction gas and minimized the sulfur poisoning of active Mn sites. This finding is consistent with the results obtained with regard to the sulfur poisoning resistance of the MnO x /H-SAPO-34-0.1 catalyst.
with abundant acid sites, which could offer a high-efficiency place to trigger the SCR reaction, resulted in improved SO2 tolerance [49]. The TG/DTG analysis indicated that fewer sulfates were deposited on the surface of the MnOx/H-SAPO-34-0.1 catalyst, which provided evidence for the anti-SO2 capability of the hierarchical pore structure. Consistent with the results of NH3-TPD and Py-IR, the effect of the hierarchical pore structure was to make a balance between effective acid sites and the formation/decomposition of ABS, thus enhancing the deNOx properties and suppressing the blocking effect of SO2. Consequently, these phenomena suggest that the hierarchical pore structure promoted the diffusion of the reaction gas and minimized the sulfur poisoning of active Mn sites. This finding is consistent with the results obtained with regard to the sulfur poisoning resistance of the MnOx/H-SAPO-34-0.1 catalyst.

Catalytic Activity Tests
As shown in Figure 12, performances of the hierarchical MnOx/SAPO-34 and MnOx/SAPO-34 catalysts during the NH3-SCR reaction were evaluated in the temperature

Catalytic Performance of the Low-Temperature NH 3 -SCR 2.2.1. Catalytic Activity Tests
As shown in Figure 12, performances of the hierarchical MnO x /SAPO-34 and MnO x / SAPO-34 catalysts during the NH 3 -SCR reaction were evaluated in the temperature range of 80-240 • C. To explore the influence of the hierarchical pore structure on the NH 3 -SCR reaction, the NO conversion over MnO x /SAPO-34 was measured for comparison. The MnO x /SAPO-34 showed little activity until the temperature reached 120 • C, and the NO conversion gradually increased to the maximum value (about 90%) at 180 • C. At low temperatures, hierarchical MnO x /SAPO-34 catalysts performed better than MnO x /SAPO-34. The NO conversion with different hierarchical MnO x /H-SAPO-34 catalysts as a function of the molar concentration of the citric acid etching solution in the NH 3 -SCR reaction is shown in Figure 12. Among the hierarchical MnO x /SAPO-34 catalysts, the MnO x /H-SAPO-34-0.1 catalyst exhibited the optimal NO conversion of 95% with an N 2 selectivity over 90% in the temperature range of 80-240 • C. The superior NH 3 -SCR performance obtained with MnO x /H-SAPO-34-0.1 may due to the intercalation of mesoporous structures, which provided more channels and could substantially promote the mass transfer of reactants or products at low temperatures.  The effect of SO2 on the catalytic performance of MnOx/SAPO-34 and MnOx/H-SAPO-34-0.1 catalysts was investigated at 120 °C. As shown in Figure 13, when 100 ppm SO2 was added into the feed gas, a drop in NO conversion of approximately 80% occurred over MnOx/SAPO-34. After removing the SO2, the NO conversion did not fully recover. Nev-

Impact of SO 2 on Catalytic Activity
The effect of SO 2 on the catalytic performance of MnO x /SAPO-34 and MnO x /H-SAPO-34-0.1 catalysts was investigated at 120 • C. As shown in Figure 13, when 100 ppm SO 2 was added into the feed gas, a drop in NO conversion of approximately 80% occurred over MnO x /SAPO-34. After removing the SO 2 , the NO conversion did not fully recover. Nevertheless, the NO conversion of MnO x /H-SAPO-34-0.1 was still maintained at 100% after the 40-min test. The MnO x /SAPO-34 catalyst showed a relatively faster deactivation in a short period of time, indicating a notable difference in reaction efficiency. The SCR performance of MnO x /H-SAPO-34-0.1 decreased slowly and remained above 65% for the next 200 min. Pan et al. concluded that competitive adsorption between reactant molecules and toxicants on the surface of the catalyst may promote the deactivation of the catalyst [50]. Compared to traditional MnO x /SAPO-34 catalysts, the MnO x /H-SAPO-34-0.1 catalyst's hierarchical pore structure was therefore found to be a key factor contributing to its high SO 2 poisoning resistance. The hierarchical pore structure might inhibit ammonium bisulfate aggregation and facilitate dispersion of the active phase. Reaction conditions: 800 ppm NH3, 800 ppm NO, 100 ppm SO2 (when ne vol.% O2, Ar to balance, T = 120 °C, gas hourly space velocity (GHSV) = 40,000 h −1

Preparation of Hierarchical SAPO-34
The SAPO-34 zeolite with a hierarchical pore structure (H-SAPO-34) was o by efficient post-synthesis via citric acid etching based on previous studies [31]. rier catalyst in this study was a commercial SAPO-34 with a Si/Al mass ratio (XFNANO Company, Nanjing, China). Typically, a certain amount of citric acid (A Shanghai, China) was dissolved in 100 mL of ethanol and stirred for 30 min at ro perature. An amount of 3 g SAPO-34 was then added to the solution with vigorous for 30 min. Then, the solution was transferred into an oil bath at 90 °C for 6 h.  Reaction conditions: 800 ppm NH 3 , 800 ppm NO, 100 ppm SO 2 (when needed), 5 vol.% O 2 , Ar to balance, T = 120 • C, gas hourly space velocity (GHSV) = 40,000 h −1 .

Preparation of Hierarchical SAPO-34
The SAPO-34 zeolite with a hierarchical pore structure (H-SAPO-34) was obtained by efficient post-synthesis via citric acid etching based on previous studies [31]. The carrier catalyst in this study was a commercial SAPO-34 with a Si/Al mass ratio of 0.27 (XFNANO Company, Nanjing, China). Typically, a certain amount of citric acid (Aladdin, Shanghai, China) was dissolved in 100 mL of ethanol and stirred for 30 min at room temperature. An amount of 3 g SAPO-34 was then added to the solution with vigorous stirring for 30 min. Then, the solution was transferred into an oil bath at 90 • C for 6 h. The product was washed with deionized water, filtered and dried overnight at 110 • C. The as-synthesized products were calcined at 550

Preparation of the Catalysts
The ethanol dispersion method was used to prepare MnO x /SAPO-34 catalysts with a hierarchical pore structure. Manganese nitrate (50% by weight in H 2 O, Aladdin, Shanghai, China) was used as a precursor. The mass fraction of manganese was 15% and 2.72 mL of 50 wt.% Mn (NO 3 ) 2 was dissolved in 50 mL ethanol and stirred at ambient temperature. An amount of 3g H-SAPO-34-x was then added while being stirred. The solution was treated with ultrasound for 30 min and stirred continuously at 80 • C until the solvent evaporated completely. The products were dried at 100 • C and calcined at 400 • C for 4 h. The synthesized catalysts were denoted as MnO x /H-SAPO-34-y (y = 0.01, 0.1 and 0.125, where y is the molar concentration of the citric acid etching solution). The ordinary MnO x /SAPO-34 catalyst was prepared following the same method for comparison.

Catalysts Characterization
X-ray powder diffraction (XRD) measurement in the 2θ range of 0 • -50 • was obtained with the SmartLab (3KW) Japan Rigaku (Tokyo, Japan) X-ray diffractometer D8 using Cu Ka radiation (λ = 1.5418 Å). The scanning step size was 0.02 • ·s −1 . The distribution of Mn, Si, Al, P and O species was observed using a field emission SEM in JEOL JSM-6700F (Tokyo, Japan) with X-ray energy-dispersive spectrometry (EDS). The micro-structural characterization by transmission electron microscope (TEM) images was determined by JEM-2010 (JEOL, Tokyo, Japan) with a working voltage of 200 KV. BET specific surface area and pore characterization were tested using the N 2 adsorption-desorption on an ASAP 2020 (Drive Norcross, GA, USA) analyzer at −195 • C. X-ray photoelectron spectroscopy (XPS) was performed through the spectrum (K-Alpha + ULTRA DLD) equipped with an Al Kα (1487 eV) radiation source. Thermogravimetry (TG) was conducted using a STA449C-QMS403 thermal analyzer (Netisch, Germany) at a temperature range of 200-900 • C, with a heating rate of 10 • C min −1 in 5% O 2 /Ar. A Thermo Nicolet iS50 Spectrometer with a resolution of 4 cm −1 was used to measure the FT-IR spectra of catalysts. NH 3 temperature-programmed desorption (NH 3 -TPD) experiments were conducted on the Tp-5080 (Xianquan Industrial and Trading Co., Ltd., Tianjin, China). To explore pyridine adsorption, the sample was dehydrated at 450 • C under a dynamic vacuum (1.5 × 10 −3 Pa), followed by saturated adsorption of pyridine at room temperature. Py-IR spectra were then evacuated at 200 • C. H 2 -TPR was conducted using a Tp-5080 (Xianqua, China) at a temperature range of 0-800 • C.

Catalysts Evaluation
A fixed-bed quartz flow reactor at atmospheric pressure was used to carry out SCR activity tests for the catalysts. The reaction temperature was increased from 25 to 240 • C at a rate of 5 • C/min, with an isotherm step of 20 • C. An amount of 800 mg of 40-60 mesh catalysts was used in each test. The simulated gas was composed of 800 ppm NH 3 , 800 ppm NO, 5.0% O 2 and 100 ppm SO 2 (when needed) and balanced by Ar. All of the tests were performed with a total flow rate of 600 mL/min and a gas hourly space velocity (GHSV) of 40,000 h −1 . A NO-NO 2 -NO x analyzer (Thermal Scientific, model 42i-HL, Waltham, MA, USA) was used to measure the concentrations of NO and NO 2 .

Conclusions
In this work, citric acid treatment was used to synthesize a series of hierarchical MnO x /SAPO-34 catalysts. It was found that the hierarchical pore structure improved the low-temperature activity and SO 2 resistance of the MnO x /SAPO-34 catalyst. Among them, MnO x /H-SAPO-34-0.1 presented the optimal SCR performance with more than 90% NO conversion at 110-240 • C. Moreover, the impact of introducing SO 2 to MnO x /H-SAPO-34-0.1 on NO conversion was lesser than that observed in the MnO x /SAPO-34 catalyst. Numerous characterizations demonstrated that the hierarchical pore structure significantly increased the BET surface area and Mn 4+ percentage as well as the acid site quantity of the MnO x /SAPO-34 zeolite, all of which were responsible for improving the SCR activity at low temperatures. The results of TG/DTG showed that fewer manganese sulfate species and ABS formed on the surface of the MnO x /H-SAPO-34-0.1-S catalyst. The XPS results indicated that the MnO x /H-SAPO-34-0.1 catalyst retained a high ratio of Mn oxides with a high valence state after sulfation. These phenomena could be ascribed to the fact that the hierarchical pore structure facilitated the decomposition of surface sulphates deposited on the catalyst during the SCR reaction, thus effectively reducing the SO 2 poisoning of active Mn sites. Overall, the low-temperature SCR activity and SO 2 tolerance of the MnO x /SAPO-34 zeolite were significantly improved by the hierarchical pore structure, which could supply a rational strategy for the further design of Mn-based SCR catalysts.

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