Manganese oxides (MnOx) exhibit excellent low-temperature activities in SCR reaction of NO with NH3, and they are considered as a promising catalyst for future application. However, MnOx still suffer from poor resistance to SO2 and H2O. In this work, Pr was used to modify MnOx catalysts via co-precipitation process, and MnPrOx catalyst exhibited superior SO2 resistance over pure MnOx catalyst. A series of characterization techniques, such as XRD, BET, H2-TPR, XPS, TG and in-situ DRIFTS, were adopted to explore the promoting effect of Pr modification on SO2 resistance over MnOx catalyst in detail. The results indicated that, in the presence of SO2 in feed gas, it seemed that PrOx rather than MnOx were apt to react with SO2, which was favorable to protect Mn active sites on the catalyst surface to some extent. It was known that there was strong competitive adsorption between SO2 and NH3/NO species on SCR catalyst surface. In-situ DRIFTS results revealed that the adsorption of NH3 and NO on MnOx catalyst had been severely suppressed after the introduction of SO2, resulting in the relevant SCR reactions following either L-H or E-R mechanisms would be inhibited obviously. But for Pr-modified MnOx catalyst, the introduction of SO2 almost had no effect on the adsorption of NH3 on catalyst surface, while it exerted a relatively noticeable impact on the adsorption of NO. As a result, SCR reactions occurred on Pr-modified MnOx catalyst surface could still proceed in a near-normal way through E-R rather than L-H mechanisms. Therefore, Pr modification on MnOx catalyst exhibited a distinctively promoting effect on SO2 resistance performance in SCR process.
Research Article
An Investigation on the Promoting Effect of Pr Modification on SO2 Resistance Over MnOx Catalysts for Selective Reduction of NO with NH3
https://doi.org/10.21203/rs.3.rs-629463/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 18 Oct, 2021
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Pr-modified MnOx catalyst exhibited superior SO2 resistance over MnOx catalyst.
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Pr modification could inhibit the sulfation of Mn active sites on the surface of MnOx catalyst.
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For Pr-modified MnOx catalyst, the adsorption of NH3 on Lewis acid sites was almost not affected by SO2, and the inhibition degree of SO2 for NO adsorption was lower than that of MnOx catalyst.
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The SCR reaction on Pr-modified MnOx catalyst might follow E-R mechanism in the presence of SO2.
Nitrogen oxides (NOx) emitted from high-temperature combustion processes in diesel engines, power stations and industrial heaters are important atmospheric pollutants, which can lead to several environmental problems, for example, acid rain, ozone depletion, photochemical smog and greenhouse effects (Du et al. 2020; Wang et al. 2019; Zhu et al. 2020; Kim et al. 2020). Over the past few decades, selective catalytic reduction (SCR) with NH3 has become the most effective technology for abating NOx emissions from mobile and stationary sources (Gong et al. 2020; Ma et al. 2020). Catalysts play a crucial role in controlling the construction cost and creating efficient reactions in SCR systems (Fan et al. 2017). V2O5-WO3(MoO3)/TiO2 catalysts are often used in power plants due to their excellent catalytic performance in the temperature range of 300-400 ºC. However, for some plants with lower temperature flue gas, flue gas should be reheated by spare heater unit to meet the working temperature of commercial V-based catalysts. Undoubtedly, such heating process would cost more energy for industrial application. Therefore, it is of great significance to develop non-toxic low-temperature SCR catalysts at desired working temperatures. So far, numerous low-temperature catalysts have been developed, including zeolites, transitional metal oxides, and rare-earth metal oxides. Among of them, MnOx catalysts have attracted increasing attention due to their low-temperature catalytic activities and low cost (Yang et al. 2020; Xie et al. 2020; Lee et al. 2019; Zhang et al. 2019; Liu et al. 2020). Nevertheless, MnOx catalysts still suffer from some challenging problems, such as narrow operation window, low N2 selectivity, and poor resistance to H2O and SO2, which restricts their commercial application. Especially, the poor SO2 and H2O tolerance of MnOx catalysts pose the greatest challenge for practical applications (Khan et al. 2020; Kang et al. 2020).
It is well known that there are mainly two causes for the deactivation of MnOx catalysts at low temperature when SO2 existed in flue gas. On one hand, NH3 reductant would react with SO2 to form ammonium sulfate, further covering the catalyst active sites. On the other hand, Mn active sites would also react with SO2 to form metal sulfates (Chang et al. 2013; Jin et al. 2014; Gao et al. 2017; Shao et al. 2020; Chen et al. 2021). Some previous works reported that the doping of rare-earth element could improve the SO2 resistance of Mn-based catalysts. It was found that Gd modification could inhibit the transformation of MnO2 to Mn2O3 and the generation of MnSO4, hinder the decrease of Lewis acid sites and the increase of Brønsted acid sites, and alleviate the competitive adsorption between NO and SO2 (Fan et al. 2018). Similarly, Eu was used to restrain the formation of sulfate species over Mn/TiO2 catalyst under SCR conditions (Liu et al. 2018). Zhang et al. reported that Ho modification was beneficial to suppress the competitive adsorption of SO2 with NH3 and/or NO on the surface of Mn/TiO2 catalyst, thus improving the SO2 resistance (Zhang et al. 2020). The previous studies demonstrated that the introduction of rare-earth elements is a promising option to enhance the SO2 resistance of Mn-based catalysts.
Due to incompletely occupied 4f and empty 5d orbitals, rare-earth element praseodymium (Pr) has been widely applied as an additive in various fields (Li et al. 2021; Qi et al. 2020; Yan et al. 2020; Mamidi et al. 2018). As a promoter, Pr has also been used to enhance the catalytic activity, resistance to SO2 and H2O of Mn-based catalysts. The introduction of Ce/Pr to MnOx/SAPO-34 catalysts had enhanced the SCR performance and SO2 resistance (Yu et al. 2017). It was reported that the SCR performance and resistance to SO2 poisoning of Fe-Mn/TiO2 catalysts could be improved with the addition of a suitable amount of Pr (Hou et al. 2020). It was reported that MnOx@PrOx catalysts with a hollow urchin-like core–shell structure exhibited excellent SCR activity and resistance to SO2 and H2O (Cheng et al. 2020). Though some researchers have reported that the introduction of Pr could improve SO2 resistance of Mn-based catalysts, few studies have investigated systematically the promoting effect and reaction mechanism of Pr on SO2 tolerance over MnOx catalysts.
In this work, pure MnOx catalyst and Pr-modified MnOx catalyst with Pr/Mn molar ratio of 0.1 were synthesized via co-precipitation method, and the SCR test results showed that Pr modification on MnOx catalyst had obviously improved the catalytic performance and resistance to SO2 and H2O. Based on the prepared catalysts, the promoting effect and reaction mechanism of Pr modification on SO2 resistance was explored in detail with various characterization techniques, such as XRD, BET, H2-TRP, XPS, TG and in-situ DRIFTS. And a possible mechanism was also proposed for the excellent SO2 tolerance of Pr-modified MnOx catalysts.
2.1. Catalyst preparation
All reagents were analytical grade and used without further purification. Pr-modified MnOx catalyst with Pr/Mn molar ratio of 0.1 was prepared using co-precipitation method with Mn(NO3)2·4H2O and Pr(NO3)3·6H2O as precursors. Firstly, 9.036 g Mn(NO3)2·4H2O and 1.566 g Pr(NO3)3·6H2O were dissolved in 50 mL deionized water successively, and stirred at room temperature for 30 min. Then ammonia solution (25 wt.%) was added dropwise to the resultant solution to initiate the co-precipitation reaction until the solution pH reached 10. Next, the solution was continuously stirred for 2 h, followed by aging at room temperature for 3h. Finally, the precipitates were separated by filtrate, dried at 120 °C overnight, and calcined at 450 °C for 3 h in air. For comparison, MnOx catalyst without Pr modification was also prepared by using the same method without the addition of Pr element. In this work, Pr-modified MnOx catalyst with Pr/Mn molar ratio 0.1 was designated as MnPrOx. The MnOx and MnPrOx catalysts after SO2 resistance tests were denoted as MnOx-S and MnPrOx-S, respectively.
2.2. Catalyst characterization
The X-ray diffraction (XRD) patterns of the prepared catalysts were obtained by an X-ray diffractometer (Empyrean, PANalytical) with Cu Kα as radiation source in a 2θ range of 5-70° at a scanning step size of 0.02°.
N2 adsorption-desorption isotherms of the prepared catalysts were tested by a physisorption meter (ASAP 2020 PLUS, Micromeritics). The catalyst samples were degased under vacuum at 200 °C for 3 h before each test. The N2 adsorption-desorption data were obtained at liquid nitrogen temperature (-196 °C).
H2-temperature programmed reduction (H2-TPR) tests were carried out on a chemisorption analyzer (Autochem II 2920, Micromeritics). Before each test, the catalyst samples were pretreated in He stream (50 mL/min) at 200 °C for 1 h, and then cooled down to 50 °C. Finally, the catalyst samples were heated from 50 to 800 °C at a heating rate of 10 °C/min in 10 vol% H2/Ar stream.
X-ray photoelectron spectroscopy (XPS) measurements were performed on a surface analysis photoelectron spectrometer (AXIS-ULTRA DLD-600W, Shimadzu) using Al Kα as radiation source at 300 W. Binging energy was referred to the C 1s peak at 284.6 eV as standard.
Thermogravimetric (TG) analysis tests were carried out on a thermogravimetric analyzer (TGA-50, Shimadzu) with the heating rate of 10 °C/min from 20 to 1000 °C under N2 atmosphere.
In-situ DRIFTS measurements were carried out on a FTIR spectrometer (Nicolet iS50, Thermo Fisher Scientific). The DRIFT spectra were recorded by accumulating 64 scans and 4 cm−1 resolution. Before each test, the catalyst samples were pretreated at 350 °C for 30 min under N2 flow, and then cooled down to target reaction temperature to collect the background.
2.3. Catalyst activity
NH3-SCR catalytic performance measurements were performed three times for each kind of catalysts, and the averaged values were used to plot the curves. For a typical test, the catalyst sample (1 mL) was filled into a fixed-bed quartz reactor of 6 mm internal diameter. Firstly, the catalyst sample was pretreated at 200 °C for 1 h in N2 stream, and then cooled down to 60 °C. Then a reactant gas mixture including 500 ppm NO, 500 ppm NH3, 5 vol% O2, 100 ppm SO2 (when used), 10 vol% H2O (when used) and N2 as balance gas passed the catalyst sample. The total flow rate of the reaction mixture gas was 500 mL/min, and the gas hourly space velocity (GHSV) was 30,000 h−1. The reaction temperature increased from 60 to 300 °C. The concentrations of mixture gases (NO, NO2, N2O, SO2, NH3 and H2O) were recorded continuously by a FTIR spectrometer (Antaris IGS, Thermo Fisher Scientific) equipped with a MCT detector. NO conversion efficiencies of the catalysts were calculated according to the following equation:
3.1. NH3-SCR performance
Fig. 1a shows the activities of MnOx and MnPrOx catalysts for NH3-SCR reaction in the temperature range of 60-300 ºC. NO conversion efficiency of pure MnOx catalyst increased gradually from 60 to 180 ºC with the increase of reaction temperature, and it reached ~ 100% at 180 ºC. When further increasing reaction temperature from 180 to 300 ºC, NO conversion efficiency decreased slowly. The addition of Pr element to MnOx catalyst led to an obvious enhancement on NO conversion performance. For MnPrOx catalyst, NO conversion efficiency was ~ 100 % in the temperature window of 120-220 ºC. It indicated that Pr modification could greatly improve low-temperature activity and widen operation temperature window of pure MnOx catalyst.
In practical circumstance, there are a small amount of H2O and SO2 in feed gas, which can lead to the deactivation of the catalysts (Lee et al. 2013). Therefore, the tests of H2O and SO2 resistance were performed at 200 ºC over MnOx and MnPrOx catalysts, respectively.
Fig. 1b shows the influence of H2O on NO conversion performance of MnOx and MnPrOx catalysts. After 5 vol% H2O was introduced into feed gas, NO conversion efficiency of pure MnOx catalyst decreased quickly down to ~ 60 %, and it remained at ~ 60 % with the increase of reaction time. NO conversion efficiency recovered to ~ 97 % within half an hour after stopping H2O injection. It suggested that the deactivation of MnOx catalyst by H2O poisoning was reversible. However, NO conversion efficiency of MnPrOx catalyst could be hardly influenced by H2O during the whole reaction process, showing excellent resistance to H2O poisoning.
Fig. 1c shows the influence of SO2 on NO conversion performance of MnOx and MnPrOx catalysts. After 100 ppm SO2 was introduced into feed gas, NO conversion efficiency of pure MnOx catalyst decreased quickly down to 17.8 % within 1 h, then further decreasing slowly to 2.7 % after 4 h. After shutting off SO2 gas, there was no sign of recovery in NO conversion efficiency, suggesting that irreversible deactivation occurred over pure MnOx catalyst. Normally, SO2 resistance performance of pure MnOx catalysts was poor, and this result was consistent with some previous studies (Fan et al. 2018, 2020). Compared with pure MnOx catalyst, MnPrOx catalyst displayed much better SO2 resistance performance. A high NO conversion efficiency of 96.2 % was maintained over MnPrOx catalyst after feeding SO2 for 2 h. Then it decreased quickly down to 20.5 % after introduction of SO2 for 4 h, which was still much higher than that of pure MnOx catalyst. The results indicated that the introduction of Pr element could greatly enhance the SO2 resistance of pure MnOx catalyst.
Fig. 1d shows the influence of H2O and SO2 simultaneous on NO conversion performance of MnOx and MnPrOx catalysts. For pure MnOx catalyst, the introduction of H2O accelerated the poisoning effect of SO2. However, the introduction of H2O alleviated the poisoning effect of SO2 over MnPrOx catalyst. NO conversion efficiency of MnPrOx catalyst was still 87.6 % after introducing SO2 for 4h. In addition, after H2O and SO2 were shut off, NO conversion efficiency was not changed over pure MnOx catalyst. However, NO conversion efficiency was recovered to 95.2 % over MnPrOx catalyst. It was concluded that Pr modification could obviously improve the resistance to SO2 and H2O of pure MnOx catalyst.
The results showed that the introduction of Pr element could greatly improve SO2 resistance of pure MnOx catalyst. Therefore, the promoting effect for Pr modification on SO2 resistance were further analyzed through the following characterizations techniques.
3.2. XRD
Fig. 2 displays the XRD patterns of fresh and deactivated catalysts. Several diffraction peaks attributed to Mn3O4 appeared in the XRD spectrum of MnOx catalyst, indicating the main crystal phase of MnOx catalyst was Mn3O4. After doping Pr element in MnOx catalyst, there was no obvious diffraction peaks appeared in MnPrOx catalyst. It suggested that Pr modification promoted the transformation of MnOx from crystalline to highly dispersed amorphous state, and PrOx was highly dispersed on the catalyst surface. Compared to fresh catalysts, there was no new peaks appeared in the XRD spectra of deactivated catalysts. But the intensities of observed peaks decreased, which possibly due to the coverage of sulfate species formed on the catalyst surface, as confirmed by the TG analysis results.
3.3. N2 adsorption/desorption
Fig. 3 shows N2 adsorption-desorption isotherms of fresh and deactivated catalysts. The adsorption-desorption isotherms of these catalysts were type IV, implying that the mesoporous structures of the catalysts were not changed after SO2 resistance tests (Gao et al. 2018). The specific surface areas, pore diameters, and pore volumes of these catalysts were summarized in Table.1. The results showed that the pore diameters of deactivated catalysts became larger, which might be caused by the generated sulfate species partially blocking the pore of the catalysts (Zhang et al. 2020). In addition, the specific surface area of MnOx catalyst decreased from 14.03 to 11.88 m2/g, and that of MnPrOx catalyst decreased from 64.76 to 40.76 m2/g after SO2 resistance tests. Noticeably, MnPrOx-S catalyst still exhibited a much larger specific surface area than MnOx-S catalyst. Hence, higher specific surface area might be one reason for superior SO2 resistance of Pr-modified MnOx catalyst.
Table. 1. Specific surface areas, pore volumes and pore sizes of fresh and deactivated catalysts.
|
Catalysts |
SBET (m2/g) |
Pore Diameter (nm) |
Pore Volume (cm3/g) |
|
MnOx |
14.03 |
27.86 |
0.10 |
|
MnOx-S |
11.88 |
36.29 |
0.11 |
|
MnPrOx |
64.76 |
11.30 |
0.22 |
|
MnPrOx-S |
40.76 |
18.67 |
0.21 |
3.4. H2-TPR
The reduction properties of fresh and deactivated catalysts were researched via H2-TPR, and the results were shown in Fig. 4. H2 consumption values and the reduction peak temperatures were listed in Table. 2. There were two reduction peaks in the curves of fresh catalysts. The low-temperature reduction peak could be assigned to the reduction of MnO2 to Mn2O3 and then to Mn3O4 (Sun et al. 2017; Liu et al. 2020), while the high-temperature reduction peak could be assigned to the reduction of Mn3O4 to MnO (Shu et al. 2019; Niu et al. 2021). The curves of deactivated catalysts displayed similar reduction peaks as that of fresh catalysts. However, the reduction peak temperatures of deactivated catalysts shifted to higher temperature range, suggesting that redox properties of the catalysts decreased after SO2 resistance tests. It's worth noting that there was a relatively lower temperature of low-temperature reduction peak and higher H2 consumption value for MnPrOx–S catalyst compared to MnOx-S catalyst. It indicated that Pr modification was conduce to keep the redox ability of MnOx catalyst when SO2 existed in feed gas. Therefore, higher redox property might be one reason for better SO2 resistance of Pr-modified MnOx catalyst.
Table. 2. H2-TPR results of fresh and deactivated catalysts.
|
Catalysts |
Peak temperature (ºC) |
H2 consumption (cm³/g STP) |
|
|
Peak 1 |
Peak 2 |
||
|
MnOx |
319 |
449 |
588.15 |
|
MnOx-S |
419 |
558 |
611.45 |
|
MnPrOx |
304 |
429 |
542.35 |
|
MnPrOx-S |
402 |
569 |
625.65 |
3.5. XPS
X-ray photoelectron spectra (XPS) were performed to obtain the information about the valence states of active species and atomic concentrations on the catalyst surface. The photoelectron spectra of Mn 2p, O 1s and S 2p were shown in Fig. 5 and the analysis results were shown in Table. 3.
Fig. 5a shows the Mn 2p spectra of fresh and deactivated catalysts. There were two main peaks ascribed to Mn 2p3/2 and Mn 2p1/3 for these catalysts. The Mn 2p2/3 peaks could be fitted into three sub-peaks. The obtained peaks could be attributed to Mn2+(639.7~640.5 eV), Mn3+(641.1~641.4 eV) and Mn4+(642.8~643.6 eV), respectively (Meng et al. 2015; Guo et al. 2017). Here Mnn+ referred to the sum of Mn2+, Mn3+ and Mn4+. As shown in Table. 3, the relative percentages of Mn4+/Mnn+ on deactivated catalysts all decreased comparing with fresh catalysts, which was unfavorable to the low-temperature SCR reaction. In addition, Mn atomic concentration on the surface of MnOx catalyst decreased by 4.28 %, while that of MnPrOx catalyst decreased by 3.48 % after SO2 resistance tests. It demonstrated that Pr modification could obviously inhibit the sulfation of Mn active sites on the catalyst surface, thus improving the SO2 resistance of MnOx catalyst.
Fig. 5b shows the O 1s spectra of fresh and deactivated catalysts, which could be split into two peaks. One peak at 528.2~528.6 eV could be assigned to the lattice oxygen species O2- (denoted as Oβ) at low binding energy. The other peak at 530.1~530.2 eV could be attributed to the chemisorbed oxygen species at high binding energy (denoted as Oα), including O- (defect-oxide) and OH- (hydroxyl-like group) (Wu et al. 2020; Boningari et al. 2015). As shown in Table. 3, the relative percentages of Oα/(Oα+Oβ) on deactivated catalysts were higher than that of fresh catalysts, which might be reason of the existence of hydroxyl-like groups offered by S=O (Tang et al. 2018). The hydroxyl-like groups were favorable to form more acid sites on the catalyst surface and further affecting the SCR reaction process (Tang et al. 2018).
Fig. 5c shows the S 2p spectra of MnOx-S and MnPrOx-S catalysts, which could be split into two peaks. The two peaks could be assigned to S6+ in the form of SO42- (168.8 eV) and S4+ in the form of SO32- (167.3 eV), respectively (Sheng et al. 2018; Arfaoui et al. 2018). It suggested that sulfate and sulfite formed on the catalyst surface after SO2 resistance tests. As shown in Table. 3, the S content of on the surface of MnPrOx-S catalyst was 3.48 %, which was slightly more than that of MnOx-S catalyst (2.58 %), indicating that there was more sulfate species on the surface of MnPrOx catalyst.
Table. 3. XPS results of fresh and deactivated catalysts.
|
Catalysts |
O |
Mn |
Pr |
S |
Area ratio |
|||
|
Mn4+/Mnn+ |
Mn3+/Mnn+ |
Mn2+/Mnn+ |
Oα/(Oα+Oβ) |
|||||
|
MnOx |
49.05 |
28.39 |
-- |
-- |
21.0 |
33.2 |
45.8 |
27.8 |
|
MnOx-S |
50.18 |
21.50 |
-- |
2.58 |
18.9 |
31.9 |
49.2 |
35.6 |
|
MnPrOx |
50.18 |
21.50 |
4.31 |
-- |
24.2 |
29.8 |
46.0 |
35.1 |
|
MnPrOx-S |
49.34 |
18.02 |
3.80 |
3.48 |
23.8 |
27.9 |
48.3 |
44.5 |
3.6. TG
In order to further distinguish the sulfate species on the surface of deactivated catalysts poisoned by SO2, thermogravimetric (TG) analysis of MnOx-S and MnPrOx-S catalysts were performed, and the TG curves were shown in Fig. 6. In the temperature range from room temperature to 200 °C (stage 1), the weight loss could be attributed to the evaporation of physically absorbed water. In the temperature range of 200-400 °C (stage 2), the weight loss could be ascribed to the decomposition of (NH4)2SO4 or NH4HSO4. The weight loss above 400 °C (stage 3) could be assigned to the decomposition of metal sulfates (Tang et al. 2018; Sun et al. 2018). For MnOx-S catalyst, there was no obvious weight loss in the temperature range of 200-400 °C (stage 2), indicating that no (NH4)2SO4 or NH4HSO4 was formed on the surface of MnOx catalyst. The sharp declines in weight loss at 550 °C and 760 °C were due to the thermal transformation of MnO2 to Mn2O3 and the decomposition of MnSO4, respectively (Fan et al. 2018). It indicated that the rapid deactivation of pure MnOx catalyst was due to the formation of MnSO4, as illustrated in Fig. 1c. For MnPrOx-S catalyst, the distinct weight loss could be observed in the temperature range of 200-400 °C (stage 2), suggesting that Pr modification could promote the formation of (NH4)2SO4 or NH4HSO4 on the catalyst surface. At higher temperature range (above 400°C), compared with MnPrOx-S catalyst, MnOx-S catalyst showed more obvious thermal transformation of MnO2 to Mn2O3 and decomposition of MnSO4. It indicated that Pr modification could inhibit the thermal transformation of MnO2 to Mn2O3 and the generation of MnSO4 to some extent, thus enhancing the SO2 resistance of MnOx catalyst.
3.7. In-situ DRIFTS
In order to further investigate the adsorption behavior and the SCR reaction mechanism in the presence of SO2 at low temperature, in-situ DRIFTS tests were performed at relatively low temperature (200 °C).
3.7.1. SO2 + O2 adsorption on the catalysts
Fig. 7 shows in-situ DRIFT spectra of SO2 + O2 adsorption over MnOx and MnPrOx catalysts. For a typical test, the catalyst sample was pretreated for 30 min, then 100 ppm SO2 and 5 vol% O2 (100 mL/min) were introduced for 30min, IR spectra were recorded continuously. As shown in Fig. 7a, several bands at 1617, 1550, 1423, 1336, 1273, 1251, 1148 and 1052 cm-1 were observed over MnOx catalyst. The band at 1617 cm−1 was attributed to adsorbed HSO4-. The band at 1423 cm−1 was assigned to the symmetrical stretching vibration of gaseous SO2. The bands at 1336, 1273 and 1251 cm−1 were ascribed to the asymmetric stretching vibration of O=S=O (Tang et al. 2020) The band at 1148 cm−1 was correspond to bulk metal sulfate, and the band at 1052 cm−1 was assigned to the S–O stretching vibrations of bidentate sulfates located on the catalyst surface (Gao et al. 2019). As shown in Fig. 7b, several bands at 1641, 1535, 1146 and 1043 cm-1 were observed over MnPrOx catalyst. The band at 1636 cm−1 was assigned to adsorbed H2O (Wang et al. 2019). The band at 1146 cm−1 was attributed to bulk metal sulfate, and the band at 1043 cm−1 was ascribed to the S–O stretching vibrations of bidentate sulfates located on the catalyst surface (Gao et al. 2019). Comparing the adsorption spectra of these two catalysts, it could be found that the absorbance intensities of the bands increased with the increase of reaction time. However, fewer sulfate species were observed over MnPrOx catalyst, indicating that the adsorption capacity of SO2 on the surface of MnPrOx catalyst was weaker than that of MnOx catalyst. Therefore, Pr modification could improve the SO2 resistance of MnOx catalyst.
3.7.2. Effect of SO2 on NH3 adsorption on the catalysts
Aiming to insight into the effect of SO2 on NH3 adsorption on the surface of MnOx and MnPrOx catalysts, in-situ DRIFTS measurement of the adsorption of NH3 + SO2 at 200 °C were performed. After pretreatment, the catalysts were exposed to 500 ppm NH3 (100 mL/min) for 30 min, following by the addition of SO2 for 30 min, and the IR spectra were recorded with the increase of reaction time.
As shown in Fig. 8a, several bands were observed in the spectra of MnOx catalyst after the adsorption of NH3 for 30 min. The bands at 1192 and 1603 cm-1 could be assigned to coordinated NH3 on Lewis acid sites, and the band at 1427 cm-1 could be ascribed to NH4+ species on Brønsted acid sites (Fan et al. 2018). The absorbance intensity of the band (1427 cm-1) attributed to NH4+ species on Brønsted acid sites was enhanced gradually with the introduction of SO2. In addition, a new band (1678 cm-1) ascribed to NH4+ species on Brønsted acid sites appeared in the spectra of MnOx catalyst. Zhang et al. also found that SO2 could facilitate the generation of NH4+ species on Brønsted acid sites (Zhang et al. 2020). However, the band (1192 cm-1) assigned to coordinated NH3 species on Lewis acid sites disappeared within 1 min after the introduction of SO2. In the meantime, the bands (1268, 1144 and 1029 cm-1) assigned to sulfate species appeared, and the intensities of these bands also enhanced gradually with the increase of SO2 injection duration. It suggested that the adsorption of NH3 on Lewis acid sites was restrained by the introduction of SO2, thus reducing the SCR activity at low temperature. As shown in Fig. 8b, the change in absorbed bands of MnPrOx catalyst was similar to that of MnOx catalyst. However, the absorbance intensity of the band (1181 cm-1) assigned to coordinated NH3 species on Lewis acid sites increased gradually after the introduction of SO2. According to the previous study, the adsorbed NH3 species on Lewis acid sites played a key role in the process of SCR reaction at low temperature (Zuo et al. 2014). The results showed that Pr modification could inhibit the competitive adsorption of SO2 and NH3 on Lewis acid sites, which might be an important factor for high SO2 tolerance of MnPrOx catalyst.
3.7.3. Effect of SO2 on NO + O2 adsorption on the catalysts
In order to investigate the effect of SO2 on NO adsorption on the surface of MnOx and MnPrOx catalysts. After pretreatment, the catalysts were pre-adsorbed with 500 ppm NO and 5% O2 for 30 min, then 100 ppm SO2 was introduced. As shown in Fig. 9a, several bands of adsorbed NO species appeared at 1627, 1545, 1345, and 1266 cm-1 in the spectra of MnOx catalyst after the introduction of NO + O2 for 30 min, which could be assigned to adsorbed NO2 species (1627 cm-1), bidentate nitrate (1545 cm-1), free NO3- (1345 cm-1), monodentate nitrate (1266 cm-1), respectively (Fan et al. 2018; Gao et al. 2017). With the introduction of SO2, the absorbance intensities of these bands in the spectra of MnOx catalyst decreased clearly. In addition, several new bands (1616, 1337, 1274, 1152 and 1057 cm-1) assigned to sulfate species appeared with the injection of SO2, and the absorbance intensities of these bands increased gradually. It indicated that there was obvious competitive adsorption between SO2 and NO on the surface of MnOx catalyst. In other words, SO2 could restrain the adsorption of NO on the catalyst surface, which obviously inhibited the SCR reaction following L-H mechanism on MnOx catalyst. This might be one important reason for the decline of NO conversion efficiency in the presence of SO2 (Jin et al. 2014). For MnPrOx catalyst, the inhibiting effect of SO2 on NO adsorption on the catalyst surface was similar to that of MnOx catalyst. It's worth noting that the band (1557 cm-1) attributed to bidentate nitrate was still present after the introduction of SO2 for 30 min, while this band (1545 cm-1) of MnOx catalyst disappeared within 5 min. It indicated that the adsorption capacity of MnPrOx catalyst for NO was slightly stronger than that of MnOx catalyst in the presence of SO2. This might be one reason for better SO2 resistance of Pr-modified MnOx catalyst.
3.8. Discussion
In this paper, Pr-modified MnOx catalyst exhibited superior SO2 resistance at low temperature over pure MnOx catalyst. The previous study reported that the deactivation of Mn-based catalysts at low temperature was mainly due to the sulfation of the Mn active sites (Yu et al. 2017). The characterization analysis results demonstrated that PrOx rather than MnOx were apt to react with SO2, which was favorable to protect Mn active sites on the catalyst surface to some extent, and Pr modification promoted the formation of (NH4)2SO4 or NH4HSO4 on the catalyst surface. In-situ DRIFTS results showed that there was strong competitive adsorption between SO2 and NH3/NO on the surface of MnOx catalyst. The adsorption of NH3 and NO on MnOx catalyst had been severely suppressed after the introduction of SO2, resulting in that the relevant SCR reactions following either L-H or E-R mechanisms would be inhibited obviously. However, for Pr-modified MnOx catalyst, the introduction of SO2 almost had no effect on the adsorption of NH3 on catalyst surface. The competitive adsorption between SO2 and NO was also observed on the surface of Pr-modified MnOx catalyst, while the degree of inhibition for NO adsorption was lower than that of MnOx catalyst. Therefore, only the SCR reaction through L-H mechanism was suppressed. As a result, SCR reaction could still proceed in a near-normal way through E-R mechanism on Pr-modified MnOx catalyst. In the end, the promoting effect and reaction mechanism of Pr modification on SO2 resistance for NH3-SCR reaction over MnOx catalyst were proposed, which was shown in Fig. 10.
In this work, the promoting effect and reaction mechanism of Pr modification on SO2 resistance over MnOx catalyst were investigated systematically. The results showed that Pr modification on MnOx catalyst could effectively improve SO2 tolerance at low temperature. After 100 ppm SO2 was introduced into feed gas, NO conversion efficiency of pure MnOx catalyst decreased quickly down to ~ 20 % within 1 h. However, NO conversion efficiency of MnPrOx catalyst decreased down to ~ 20 % after the introduction of SO2 for 4 h. The characterization analysis results showed that the deactivation of pure MnOx catalyst was mainly due to the sulfation of the Mn active sites. After Pr modification on MnOx catalyst, it seemed that PrOx rather than MnOx were apt to react with SO2, which was favorable to protect the Mn active sites on catalyst surface to some extent. In-situ DRIFTS results revealed that there was strong competitive adsorption between SO2 and NH3/NO on the surface of MnOx catalyst. Therefore, the SCR reactions following E-R or L-H mechanisms were both inhibited obviously. However, for Pr-modified MnOx catalyst, the adsorption of NH3 was almost not affected by SO2, and the inhibition degree of SO2 for NO adsorption was lower than that of MnOx catalyst. The SCR reaction on Pr-modified MnOx catalyst might still proceed in a near-normal way E-R mechanism in the presence of SO2. Therefore, Pr-modified MnOx catalyst exhibited superior SO2 resistance over MnOx catalyst.
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Authors' contributions
ZTH and GPZ designed the experiment, GPZ performed the experiment, analyzed and processed the data. HD and YG also assisted in the experiment. The first draft of the manuscript was written by GPZ. ZTH and XXP reviewed and revised the manuscript. All authors read and approved the final manuscript.
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Competing interests
The authors declare that they have no competing interest.
Funding
This work was supported by National Natural Science Foundation of China (51779024 and 51979045), Natural Science Foundation of Liaoning Province of China (2020MS130), and Fundamental Research Funds for the Central Universities (3132019330).
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