Investigation of Sulfated Iron-Based Catalysts with Different Sulfate Position for Selective Catalytic Reduction of NOx with NH3

The Fe/(SZr) and S(Fe/Zr) sulfated iron-based catalysts, prepared by impregnation methods through changing the loading order of Fe2O3 and SO42− on ZrO2, were investigated on selective catalytic reduction (SCR) of NOx by ammonia. It was studied that the existent forms of Fe2O3 and SO42− on the surface of catalysts were affected by the loading order. The Fe/(SZr) catalyst surface had isolated Fe2O3 and SO42− species and followed both the L-H mechanism and the E-R mechanism, whereas the S(Fe/Zr) catalyst contained SO42− specie and sulfate only and mainly followed the E-R pathway. These factors affected the redox ability and NH3 adsorption, which might be key to the SCR reaction.


Introduction
Nitrogen oxides usually come from stationary sources, such as coal-fired power plants, industrial boilers, and mobile sources, like diesel engines and marine engines. They have caused lots of harmful problems to the ecosystem health. For example, the acid rain would dissolve toxic metals into the water, resulting in reducing species. The photochemical smog could harm plants and reduce atmospheric visibility. Besides, nitrogen oxides made people face existential threats. The resulting photochemical fumes could irritate the eyes, invade the lungs, and even cause death. Therefore, it is important to remove nitrogen oxides. Currently, selective catalytic reduction of NO x by ammonia (NH 3 -SCR) is an efficient technology to abate NO x in the flue gas from stationary sources [1]. The core of this method is catalysts. Typically, the V 2 O 5 -WO 3 /TiO 2 is the most commonly used catalysts [2]. However, there are still some shortcomings of vanadium-based catalysts that need to be addressed, including the narrow temperature window, the low N 2 selectivity at high temperatures, losing SCR activity by SO 2 poisoned, and the secondary pollution to the environment. Due to these disadvantages, the development of other metals for vanadium and the modification of the support have been investigated over the past several decades. Iron-based catalysts have received considerable attention for its well NH 3 -SCR activity and N 2 selectivity, such as α-Fe 2 O 3 [3], γ-Fe 2 O 3 [3], MgFe 2 O 4 [4], Fe 2 O 3 /SiO 2 [5], Fe 2 O 3 /TiO 2 [6], Fe 2 O 3 /TiO 2 -pillared clays [1], Fe-ZSM5 [7], and Fe-MOR [8]. Besides iron-based catalysts, other sulfated catalysts were reported to be active for SCR reaction. Grange et al. [9,10] reported that V 2 O 5 -WO 3 catalyst supported on TiO 2 -SO 4 2− extended high activity up to 450 • C, and for sulfate, increased the acid sites and improved the interaction between V 2 O 5 and WO 3 . Ke et al. [11] reported that cobalt sulfate exhibited high NO x conversion and well resistance to H 2 O and SO 2 poisoning, because the existence of sulfate reduced the number of bulk Co 3 O 4 and cooperated with cobalt oxides to improve the activity of SCR reaction. Gu et al. [12] reported that sulfated CeO 2 possessed high NO x conversion and N 2 selectivity in the temperature range of 200-570 • C, and suggested that sulfate enhanced the active oxygen species and NH 3 adsorption on surface, which were responsible for the higher reactivity of sulfated CeO 2 .
The iron-based oxide and sulfate catalysts for SCR of NO x by NH 3 have been investigated in recent years. However, few studies have been performed on the preparation process about sulfating. In this study, a series of sulfated iron-based catalysts were prepared through different sulfate-loaded order and characterized by XRD, BET, XPS, Raman, and NH 3 -TPD. The high activities of sulfated iron-base catalysts were also explained on the basis of the characterization results.

SCR Catalytic Activity and Influence of SO 2
The activity curves of the sulfated iron-based catalysts are shown in Figure 1a. The Fe/Zr catalyst exhibited the lowest activity in the temperature range of 150-500 • C, and its NO x conversion reached the highest 46.6% at 350 • C and the NO x conversion decreased sharply to negative value in the temperature range of 400-500 • C, which was attributed to the NH 3 oxidation to NO x at the high temperature. Notably, the activities of sulfated iron-based catalysts improved remarkably in the temperature range of 250-500 • C compared with Fe/Zr catalyst. The NO x conversion of Fe/(SZr) catalyst reached above 90% in 300-450 • C, amazingly, the NO x conversions increased to 98%, but which decreased to 50% at 500 • C. The temperature window of NO x conversion of Fe/(SZr) catalysts became narrow and moved to high temperature. The NO x conversion of Fe/(SZr) catalyst was reached 90% at 400-450 • C. The SCR reaction of the catalysts mainly follows the Eley−Rideal (E-R) mechanism and/or Langmuir-Hinshelwood mechanism (L-H) [13,14]. It is the critical reaction step to adsorb NH 3 to form activated ammonia species on the catalyst and then react with NO or adsorbed oxynitride species to generate N 2 and H 2 O. Therefore, the acidic sites are key for the SCR reaction. The sulfated iron-based catalysts were prepared by adding SO 4 2− , which could remarkably increase the acidic sites. Therefore, the activity improves obviously with the sulfated iron-based catalyst. The N 2 selectivity is one of the important indicators for the activities of NO x selective catalytic reduction by NH 3 . The N 2 selectivity of Fe/Zr, Fe/(SZr), and S(Fe/Zr) is showed in Figure 1b. S(Fe/Zr) showed excellent N 2 selectivity in 150-500 • C. The N 2 selectivity of Fe/Zr catalyst decreased from 250 • C, and the N 2 selectivity of Fe/Zr catalyst dropped greatly at 500 • C to 88%. The slight decrease in N 2 selectivity in the high temperature range might be due to the higher ability of oxidization of NH 3 to NO [15], which resulted in less production of N 2 . Besides, according to He et al. [16], the intermediate specie -HNO would combine with -NH to produce N 2 O after the oxidization of NH 3 , which led to less N 2 selectivity.
As shown in Figure 1c, this work investigated the effect of SO 2 on the activity of Fe/(SZr) and S(Fe/Zr) catalysts. In the figure, the activity of the S(Fe/Zr) catalyst remained stable for 840 min in the presence of SO 2 , but the NO x conversion rate was relatively low. For the Fe/(SZr) catalyst, the de-NO x reactivity slightly decreased and the NO x conversion rate kept stable between 770 and 840 min when there was SO 2 . This demonstrated that the Fe/(SZr) catalyst had superior SO 2 tolerance and exhibited higher activity. Typically, it was recognized that the Langmuir-Hinshelwood (L-H) mechanism or/and Eley-Rideal (E-R) mechanism were followed by catalysts during the reaction [17]. Meanwhile, it could be assumed that there was competitive adsorption between SO 2 and NO, in which SO 2 was easier to adsorb than the other. Therefore, the Langmuir-Hinshelwood (L-H) mechanism was suppressed, but the Eley-Rideal (E-R) mechanism still existed on the Fe/(SZr) catalyst. It was the reason that Fe/(SZr) had stronger SO 2 resistance. These would be analyzed in more detail in Discussion section.

XRD
XRD patterns of the ZrO 2 , α-Fe 2 O 3 , Fe/Zr, Fe/(SZr), and S(Fe/Zr) catalysts are shown in the Figure 2. Both the ZrO 2 and α-Fe 2 O 3 showed obvious characteristic peaks. However, the patterns of Fe/Zr, Fe/(SZr), and S(Fe/Zr) exhibited only one phase, which corresponded to the cubic crystal structure of ZrO 2 . In other words, the X-ray diffraction peaks of α-Fe 2 O 3 or Fe 2 (SO 4 ) 3 did not appear in the patterns, which demonstrated that Fe 2 O 3 and SO 4 2− dispersed well on the surface of ZrO 2 .
According to references [15,18], the shape of peaks would be changed or new peak would appear if Fe 2 O 3 or/and SO 4 2− entered into the crystal ZrO 2 . These proved that the sulfating process did not affect the crystal form of Fe/Zr, Fe/(SZr), and S(Fe/Zr) catalysts. In fact, the change of crystal structure and high dispersion had significant influences on the catalytic performance [15], which might affect the existent forms of active species and pathways followed on catalysts.

BET and ICP
It is well known that the surface area is crucial to catalytic activity because large surface area could facilitate to well dispersion. However, the surface areas of sulfated iron-based catalysts were so low ( Table 1) that the distinction among them could be neglected. Meanwhile, integra mass concentrations of Fe, S, and Zr measured by the ICP equipment are also summarized in Table 1. The integra mass concentrations of Fe, S, and Zr were close to their calculated values.

Raman
To investigate the species on the surface of sulfated iron-based catalysts, the Raman spectra were collected at room temperature. Figure 3 showed the Raman spectra of different sulfated iron-based catalysts and pure Fe 2 O 3 . The Fe/(SZr), S(Fe/Zr), and ZrO 2 catalysts all showed similar peak shapes, but the strength was different in the wavenumber range of 0-650 cm −1 , which meant that the sulfate and Fe 2 O 3 species on catalyst surfaces did not have effects on ZrO 2 . Besides, several bands in the range of 800-1500 cm −1 were detected, in which the peak centered at 1278 cm −1 on Fe/(SZr) was assigned the fingerprint peak of Fe 2 O 3 [19]. There was no characteristic peak in the range of 800-1500 cm −1 on ZrO 2 . Only one peak appeared at 992 cm −1 on the surface of S(Fe/Zr) catalyst, which was assigned to the S=O bands of isolated sulfate. For Fe/(SZr) catalyst, three peaks located at 992, 1082, and 1278 cm −1 could be detected, respectively. Among them, the band at 1082 cm −1 was induced by the S=O bond of the polynuclear sulfate type [10]. These demonstrated that species such as Fe 2 O 3 , isolated SO 4 2− , and sulfate existed on Fe/(SZr) catalyst. In short, it could be concluded that the S(Fe/Zr) catalyst surface had isolated SO 4 2− and sulfate because of the different sulfation preparation processes, which led to different species on the surface of catalysts. These affected the mechanisms followed by catalysts.

XPS
X-ray photoelectron spectroscopy (XPS) was used to characterize chemical states and surface atomic concentrations of Fe, O, S, and Zr elements on Fe/(SZr) and S(Fe/Zr) catalysts. Peaks were fitted by Gaussian-Lorentz curves [20]. These results are shown in Figure 4. Meanwhile, the surface atomic concentrations of Fe, S, and Zr are summarized in Table 2.  The curves of Fe2p are displayed in the Figure 4a. The peak located at 727.0 eV was corresponded to the Fe2p 1/2 of Fe 3+ , and peaks referred to the Fe2p 3/2 of Fe 3+ were at the banding energy of 710.9 and 711.7 eV. Besides, the banding energy at 724.0 and 725.0 eV were corresponded to the Fe2p 1/2 of Fe 2+ . According to the previous studies [18], the peaks at 715.0 and 714.6 eV were identified as the fingerprint peak influenced by SO 4 2− . The ratio of Fe 3+ /(Fe 2+ + Fe 3+ ) on Fe/(SZr) was 69.6%, which was larger than that of S(Fe/Zr) (58.1%). It was known that the catalyst surface containing more Fe 3+ species would exhibit better performances during the redox reaction because the transformation between Fe 2+ and Fe 3+ could irritate the redox circle, which agreed with the results of SCR catalytic activity. Besides, these results also indicated that the main specie on the surface of Fe/(SZr) was Fe 2 O 3 , corresponding to the Raman results. Additionally, Figure 4b showed the XPS results of O 1s. The profiles of O 1s exhibited large differences between Fe/(SZr) and S(Fe/Zr) catalysts. For the Fe/(SZr) catalyst, the two peaks appeared at 530.1 and 529.7 eV. The banding energy at 530.1 eV was attributed to the surface-chemisorbed labile oxygen (denoted as O α ), and the 529.7 eV peak was corresponded to the lattice oxygen (denoted as O β ) [21]. However, three peaks located at 531.9, 530.7, and 530.1 eV could be observed on the S(Fe/Zr) catalyst, which demonstrated that there were three forms of oxygen on this catalyst. In addition to the surface-chemisorbed oxygen and the lattice oxygen, the O 2 2− contained in the SO 4 2− species led to the appearance of the peak located at 531.9 eV, which was in accordance with our previous research [2]. Typically, the surface-chemisorbed labile oxygen played important roles in the SCR reaction. More the content of O α , the higher catalytic activity of catalysts because the surface oxygen (Oα) could promote the activation of NH 2 − formed by NH 3 due to the more efficient mobility of electrons than lattice oxygen [21]. Therefore, the Fe/(SZr) catalyst had superior redox activity. For Fe/(SZr) catalyst,  Figure 4d. Each catalyst was fitted with two characteristic peaks. For the Fe/(SZr) catalyst, the banding energy located at 184.8 eV was corresponded with the Zr 3d 3/2 . Another peak at 182.4 eV was referred to the Zr 4+ species. The positions of peaks were similar, which certified that the existence of Zr species was stable and not affected by the sulfating process.
The ICP results demonstrated that the integral mass concentrations of Fe, S, and Zr were all extremely similar for sulfated iron-based catalysts. Combined with the XPS results of concentrations of virous elements, it could be concluded that the preparation method of Fe/(SZr) and S(Fe/Zr) merely affected the surface atomic concentrations, rather than the integral atomic concentrations.  [23], and another peak at 417 • C could be assigned to the peak produced by the machine. However, for the other three catalysts, the intensity of this peak was too slight too be observed. The pattern of SZr showed only one peak centered at 585 • C, which corresponded to the reduction of SO 4 2− [24]. The H 2 -TPR curves of Fe/(SZr) and S(Fe/Zr) showed only one peak, which centered at 403 and 486 • C, respectively. Their reduction peak temperature (T red ) were all higher than that of Fe/Zr catalyst and lower than that of SZr catalyst, which were referred to the overlapping of the stepwise reduction peaks of Fe 2 O 3 and SO 4 2− . According to reference [25], lower the T red is, the stronger is the redox ability of the catalyst. Among Fe/(SZr) and S(Fe/Zr) catalysts, the redox abilities were as following: Fe/(SZr) > S(Fe/Zr). By combining the Raman results, it was concluded that the surface species were different because of the different sulfated order. In other words, the existent forms of Fe 2 O 3 and SO 4 2− on the surface of ZrO 2 resulted in the different interaction of Fe 2 O 3 and SO 4 2− , which impacted the redox ability.

NH 3 -TPD
The NH 3 -TPD patterns of Fe/(SZr) and S(Fe/Zr) catalysts are shown in Figure 6. One wide NH 3 desorption peak of these three catalysts was displayed in the temperature range of 30-500 • C in the figure. The amounts of NH 3 desorption were different greatly among these two catalysts, and the sequence was following: Fe/(SZr) (38.1 µmol·g −1 ) > S(Fe/Zr)(6.3 µmol·g −1 ). The NH 3 species desorbed in high temperature range on Fe/(SZr) catalyst reduced a little, but the NH 3 species desorbed in low and medium temperature range increased more greatly than that on the surface of S(Fe/Zr) catalyst, which might be one of the important reason for that Fe/(SZr) catalyst exhibited the higher SCR activity in low and medium temperature range than other catalysts.

In Situ DRIFTS Studying of the NH 3 -Adsorption on Different Catalysts
The in situ DRIFTS spectra of Fe/(SZr) and S(Fe/Zr) catalysts, which were adsorbed and saturated with NH 3 at 30 • C and then purged with N 2 , is displayed in Figure 7. Several absorbing vibration peaks at 3202, 3043, 2840, 1820, 1685, 1600, 1440, and 1257 cm −1 appeared over both Fe/(SZr) and S(Fe/Zr) catalysts. Among them, the absorbing vibration peaks at 3202, 3043, and 2840 cm −1 were assigned to the N-H bond. For Fe/(SZr) catalyst, the absorbing vibration peaks at 1820 and 1257 cm −1 were detected. The absorbing vibration peaks at 1440 and 1257 cm −1 of S(Fe/Zr) catalysts were observed. The peaks at 1685 and 1440 cm −1 were attributed to the symmetric and asymmetric absorbing vibration adsorbed to Brønsted acid sites [26], while the peaks at 1600 and 1257 cm −1 were ascribed to absorbing vibration adsorbed on Lewis acid sites [27,28]. Compared with S(Fe/Zr) catalyst, the peak intensities of absorbed NH 3 species bounded to both Brønsted and Lewis acidic sites on the Fe/(SZr) catalyst surface were stronger in the figure. These mainly related to various forms of species on the Fe/(SZr) catalyst, such as isolated Fe 2 O 3 , SO 4 2− , and sulfate species. Fe 2 O 3 could provide Lewis acid sites, whereas SO 4 2− supplied Brønsted acid sites, which could adsorb more NH 3 species and were in accordance with the results of NH 3 -TPD.

Transient Reaction Studies over the Fe/(SZr) Catalyst
In Figure 8a, after the sample exposed to NO + O 2 reached saturation and was purged with N 2 at 250 • C, the nitrate species were stabilized on the surface of Fe/(SZr) catalyst (1525, 1442, 1380, and 1324 cm −1 ) [29,30]. After NH 3 was introduced into the reaction cell, the absorbed NH 3 species bounded to different acid sites on catalysts surface were observed (such as the peak at 1608 and 1300 cm −1 , which could be attributed to the symmetric and asymmetric stretching vibrational mode of NH 3 species on Lewis surface acid sites [31][32][33]). The peak at 1442 cm −1 could be assigned to the symmetric stretching vibration of NH 4 + species bound to Brønsted acid sites [14].) After NH 3 was introduced sustainably for 60 min, the adsorbed peaks did not change on the surface of catalysts. These suggested that the nitrate species adsorbed on catalyst surface were active in the SCR reaction, which could react with NH 3 existed in air quickly for the redox reaction. As shown in Figure 8b, the stable absorbing peaks attributed to NH 3 species were formed over Fe/(SZr) catalyst after introduction of NH 3 and purging at 250 • C. (The peaks located at 1608 and 1300 cm −1 can be assigned to the symmetric and asymmetric stretching vibrational peaks of NH 3 species on Lewis surface acid sites [21,31,33]. The peak at 1430 cm −1 can be attributed to the symmetric stretching vibration of NH 4 + species bounded to Brønsted acid sites [34,35].) With the introduction of NO + O 2 for 1 min, the NH 3 species on catalyst surface became weaker, and the peak at 1631 cm −1 related to stretching vibration of NO 2 appeared [36,37]. That was to say that the NH 3 and oxynitride species can coexist on the surface of catalysts. After introducing NO + O 2 for approximately 3 min, the NH 3 species almost disappeared and the peaks of stretching vibration of NO 2 enhanced. Meanwhile, some steady nitrate species (1324 cm −1 ) formed on catalyst surface, indicating that NH 3 species adsorbed on the surface of catalysts were active species in the reaction. Overall, both adsorbed nitrate and NH 3 species were active species to proceed SCR reaction.

Transient Reaction Studies over the S (Fe/Zr) Catalyst
When the S (Fe/Zr) catalyst was saturated with NO + O 2 and purged with N 2 at 250 • C, a small amount of nitrate species was formed on the surface (1592 and 1334 cm −1 ). Several absorbing peaks, which were assigned to weakly adsorbed NH 3 species, appeared immediately on catalyst surface after introducing NH 3 . (The peaks at 1608 and 1324 cm −1 were ascribed to the symmetric and asymmetric stretching vibrational peaks of NH 3 species bounded to Lewis surface acid sites [38]. The absorbing peak at 1429 cm −1 was due to the symmetric stretching vibration of NH 4 + species on Brønsted acid sites [39].) As shown in Figure 9b, steady peaks representing absorbed NH 4 + species located at 1431 and 1321 cm −1 appeared after saturation with NH 3 and purging at 250 • C, which could be attributed to symmetric stretching vibration of NH 4 + species bounded to Brønsted acid sites [40].  [42][43][44]. After introducing NO + O 2 for 10 min, the NO 2 stretching vibration peaks almost disappeared, but NH 4 + species remained adsorbed on Brønsted acid sites (1431 and 1321 cm −1 ) [44]. The NH 4 + stretching vibration peaks still existed at 60 min, suggesting that NH 4 + species adsorbed on the catalyst surface were extremely stable and only a fraction of them participated in the reaction. Therefore, although the NH 3 species on the surface of catalysts could participate in the reaction, the steady NH 4 + species affected the catalytic activity of SCR reaction.
It could be deduced that the SCR reaction on S(Fe/Zr) catalyst surface also followed the E-R pathway: First, NH 3 adsorbed on SO 4 2− existed on the catalyst surface to form the NH 4 + species, which could react with NO x conducting the oxidation reaction, and further generated N 2 and H 2 O. In conclusion, considering the reaction mechanisms of Fe/(SZr) and S(Fe/Zr) catalysts, it could be inferred that Fe 3+ and SO 4 2− afforded crucial active sites of sulfated iron-based catalysts, and the active species on sulfated iron-based catalyst surface were Fe 2 O 3 , SO 4 2− , and Fe 2 (SO 4 ) 3 .

Discussion
It could be concluded from the results of transient studies that the NH 3 -SCR reaction on Fe/(SZr) catalyst followed Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms. It was not only that the NH 3 species adsorbed on the surface could react with gaseous NO + O 2 but also, at the same time, NO x on the catalyst surface could form large amount of highly active NO 2 species, which could react with adsorbed NH 3 for SCR reaction. The E-R mechanism was executed on S(Fe/Zr) surface in the reaction, which meant that the gaseous NH 3 absorbed first on the surface of catalysts, and then reacted with NO x in air for redox reaction. Besides, the adsorbed NH 4 + on catalyst surface was very stabilized and only partly activated to participate in the reaction over the S(Fe/Zr) catalyst.
Combined results of Raman and XPS spectra showed that there were different forms of species on Fe/(SZr) and S(Fe/Zr) catalysts. The existent forms of Fe 2 O 3 and SO 4 2− on catalysts influenced the mutually coordinated impact on these two catalysts, which determined large differences in the redox ability and adsorption performance of catalysts, and also affected the nitrogen oxide and NH 3 adsorption species participating in the reaction on the catalyst surface. There were more Fe 3+ and surface-chemisorbed labile oxygen on the surface of Fe/(SZr) catalyst, which facilitated the redox cycle and led to better performance during the reaction. The differences caused the SCR reaction on the surface of Fe/(SZr) and S(Fe/Zr) catalysts to follow different mechanisms, which resulted in the different SCR reaction path on the catalyst surface, and the SCR reaction activity of the catalyst were influenced. Not only the sulfate species but also isolated Fe 2 O 3 and SO 4 2− ions existed on the Fe/(SZr) catalyst.

Catalyst Preparation
All the catalysts were synthesized through the method of impregnation. The specific experimental steps were as follows: First, the weighed ZrO 2 power was dissolved in distilled water, and some quantities of Fe(NO 3 ) 3 ·9H 2 O and (NH 4 ) 2 SO 4 were added subsequently. Then, the solution was heated to 70 • C. Meanwhile, it was kept stirring till the paste was formed. After that, the paste was dried overnight at 120 • C and calcined under air at 500 • C for 4 h. Finally, several iron-based catalysts with different molar ratios were obtained, signed as Fe x S y Zr (x = 3, y = 0; x = 0, y = 5; x is the wt.% of Fe 2 O 3 and y is the wt.% of SO 4 2− ). In this paper, Fe3Zr and S5Zr were abbreviated as FeZr and SZr, respectively. Some SZr power was dissolved and a certain amount of Fe(NO 3 ) 3 ·9H 2 O was added. Meanwhile, some FeZr catalyst was dissolved and some quantities of (NH 4 ) 2 SO 4 were added. These two solutions were heated to 70 • C. Similarly, these solutions were kept stirred until they formed paste and then were dried overnight at 120 • C. Afterwards, they were calcined under air at 500 • C for 4 h. Finally, Fe/(SZr) and S/(FeZr) catalysts were obtained.

Catalytic Activity Measurement
Measurements of catalytic activity were performed in a fixed-bed quartz reactor with an inner diameter of 9 mm. Precisely, 0.5 g of 40-60 mesh catalysts were used in the reactor. Specific experimental mixture gas conditions were as follows: 500 ppm NO, 500 ppm NH 3 , 3 vol.% O 2 , and N 2 were used as the balance gas. The total flow rate was 300 mL/min, whereas corresponding small space-time velocity of the gas was approximately 47,000 h −1 . The Fourier-transform infrared spectrometer (FT-IR) gas analyzer (Gasmet Dx-4000) was used to measure NO x , N 2 O, and NH 3 concentrations in both the inlet and outlet. When the catalytic reaction reached a stable state of half an hour at each temperature, the activity data were collected.

Catalyst Characterization
X-ray diffraction (XRD) was carried out on a D/MAX-RB system with Cu Ka radiation. The diffraction patterns were recorded in the range of 2u from 10 • to 90 • in steps of 0.018 and 1 s/step.
The Quantachrome Autosorb AS-1 System was used to measure the BET specific surface area, pore size, and pore volume through N 2 adsorption at 77 K.
X-ray photoelectron spectroscopy (XPS) was used to observe chemical valences of catalysts, atomic concentration, and surface species. These experiments were conducted on ESCALab 220i-XL electron spectrometer with radiations of 300 W Mg Ka.
The results of Raman spectra were obtained by using the Raman microscope (InVia reflection, Renishaw), which was equipped with a deep depletion thermoelectric cooled charge-coupled device (CCD) array detector and an advanced Leica microscope (long working distance objective lens 50×).
H 2 -temperature-programmed reduction (TPR) curves were recorded through the chemisorption analyzer (Micromeritics ChemiSorb 2720, Micromeritics Norcross, America) using 40 mg of samples. First, samples were pretreated in N 2 at 300 • C for 1 h and then cooled down to the room temperature. Subsequently, using 10% H 2 /Ar reducing gas with a flow rate of 50 mL/min, the catalyst was reduced from room temperature to 1000 • C with a temperature gradient of 10 • C/min. The consumed H 2 was calculated by integrating the corresponding TCD signal strength.
NH 3 temperature-programmed desorption (TPD) was performed in a fixed-bed quartz reactor. First, each sample was degassed under the atmosphere of N 2 at 500 • C for 1 h. Second, 500 ppm NH 3 was adsorbed for 1 h after cooling to room temperature. Third, the desorption was carried out in N 2 at the temperature of the last-step response experiment until no NH 3 was detected. Finally, temperature-programmed desorption (TPD stage) was performed at 10 • C/min up to 500 • C. During these experiments, typically, the sample mass of 0.5 g and a gas flow rate of 300 mL/min were needed.
The Nicolet NEXUS 870 FT-IR (Nicolet Madison, America) spectrometer was used to characterize the in situ DRIFTS spectra. Before the experiments, each sample was pretreated at 300 • C for 1 h in N 2 with the flow rate of 100 cm 3 /min.

Conclusions
The Fe/(SZr) and S(Fe/Zr) sulfated iron-based catalysts synthesized by impregnation methods were investigated in this research. The activities of sulfated iron-based catalysts improved remarkably compared with Fe/Zr catalyst in the temperature range of 250-500 • C. The NO x conversion and N 2 selectivity exhibited large distinction between Fe/(SZr) and S(Fe/Zr) sulfated iron-based catalysts. The study results revealed that the existence of sulfate, isolated Fe 2 O 3 , and SO 4 2− species on Fe/(SZr) catalyst supplied more acid sites, which could adsorb more NH 3 species and reacted with gaseous NO + O 2 . Besides, the reaction between highly active NO 2 species and NO x on the catalyst surface was beneficial to the catalytic activity. Thus, the SCR reaction on Fe/(SZr) catalyst surface followed both the L-H mechanism and the E-R mechanism. For S(Fe/Zr) catalyst, the SO 4 2− and sulfate were existed on the surface. The Fe 3+ was covered by the independent SO 4 2− , so the Fe 3+ of the catalyst could not participate in the redox reaction well, which affected the progress of the SCR reaction on the catalyst. The absorbed NH 4 + reacted with NO x in air and made the S(Fe/Zr) catalyst mainly follow the E-R pathway. In other words, different loading positions of sulfate caused distinctive active components on Fe/(SZr) and S(Fe/Zr) surface, leading to the difference of reaction mechanisms and affected the redox ability and NH 3 adsorption. Therefore, the catalytic activity was affected.
Author Contributions: Writing-original draft preparation, B.F.; writing-review and editing, Z.Z. and Q.L.; supervision, C.L. All authors have read and agree to the published version of the manuscript.