Effect of Ce / Y Addition on Low-Temperature SCR Activity and SO 2 and H 2 O Resistance of MnO x / ZrO 2 / MWCNTs Catalysts

The effects of SO2 and H2O on the low-temperature selective catalytic reduction (SCR) activity over MnOx/ZrO2/MWCNTs and MnOx/ZrO2/MWCNTs catalysts modified by Ce or Y was studied. MnCeZr and MnYZr catalysts reached nearly 100% and 93.9% NOx conversions at 200 ◦C and 240 ◦C, respectively. They displayed a better SO2 tolerance, and the effect of H2O was negligible. The structural properties of the catalysts were characterized by XRD, H2-TPR, XPS, and FTIR before and after the reaction. The results showed that Ce could increase the mobility of the oxygen and improve the valence and the oxidizability of manganese, while the effect of Y was the opposite. This might be the main reason why the catalytic activity of MnCeZr was better than MnYZr in the presence or absence of SO2 and H2O. Doping Ce or Y broadened the active temperature window. Ce or Y, which existed in the catalysts with a high dispersion or at the amorphous state, preferred to react with SO2 to form sulfate species, and protected the manganese active sites from combing with SO2 to some extent, which coincided with the theory of ionic polarization.


Introduction
Selective catalytic reduction (SCR) technology is widely used to remove nitrogen oxides from flue gas and has the advantages of stability and efficiency [1,2].Catalysts are a crucial factor in SCR technology and directly determine the efficiency of the system.However, conventional commercial catalysts (V 2 O 5 -WO 3 (MoO 3 )/TiO 2 ) have a relatively narrow temperature window of 300-400 • C, and they are not active at the typical temperature of flue gases (≤250 • C) in the catalytic bed located downstream of the desulfurizer and particulate removal device, and will be deactivated by SO 2 and H 2 O [1,3,4].Therefore, there is great interests in developing new catalysts which are active at relatively low temperatures and have better resistance to SO 2 and H 2 O.Some transition metal oxides (Fe, V, Cr, Cu, Mn) supported on catalysts have been found with high activities at low temperatures [5][6][7][8][9].Among them, Mn-based catalysts have been investigated in depth via several means because manganese oxides contain various kinds of labile oxygen, which is beneficial for the fulfillment of the catalytic cycle [9][10][11][12][13][14][15][16].MnO 2 is the most active species among MnO 2 , Mn 5 O 8 , Mn 2 O 3 , and Mn 3 O 4 [17].
Carbon nanotubes (CNTs) have been extensively studied in low temperature SCR reactions based on their unusual physical and chemical proprieties, such as their unique electronic properties, special structure, higher surface areas, and so on [18,19].In our previous study, Zr and Mn supported on multi-wall carbon nanotubes (MWCNTs) for NH 3 -SCR reactions showed high activity at low temperatures.The modification of zirconium could increase the dispersion of MnO x on the MWCNTs, the specific surface area, the total pore volume and the average pore size of the catalysts.The modification of zirconium could also enhance the atomic concentration of chemisorbed oxygen on the catalyst surface and promote the conversion of Mn 3+ to Mn 4+ .Therefore, the amounts of the active sites on the surface and the redox ability of the catalysts were improved.Additionally, the amounts and strength of acid on the surface of the catalysts were increased.However, their SCR activity was reduced in the presence of SO 2 and H 2 O. Catalysts were deactivated by the formation of (NH 4 ) 2 SO 3 and NH 4 HSO 4 which did not decompose at low temperatures and blocked the pore channels of the catalysts or the competitive adsorption with NO [20][21][22][23][24][25][26].H 2 O inhibited the SCR reaction due to its competitive adsorption with NH 3 on the Lewis sites [27,28].
According to the theory of ionic polarization [29], the formation of a metal sulfate can be regarded as the inverse process of metal sulfate decomposition (Equation ( 1)).If MeSO 4 is harder to decompose, MeSO 4 is more stable and it is easier for Me and SO 3 to form MeSO 4 .If the decomposition temperature of MeSO 4 is higher than that of manganese sulfate, Me prefers to combine with SO 3 over Mn.The decomposition temperature of ceric sulfate (973 • C) and yttrium sulfate (976 • C) were higher than manganese sulfate (814-928 • C), which indicates that ceric sulfate and yttrium sulfate are more easily formed than manganese sulfate, and manganese sites over these catalysts can be protected.Therefore, Ce and Y were selected to improve the tolerance of catalysts caused by SO 2 and H 2 O. MeSO

SCR Performance
NH 3 -SCR activity experiments over the MnZr, MnCeZr, and MnYZr catalysts were carried out, as depicted in Figure 1.The results showed that the activities of the catalysts were significantly improved and had obvious differences in NO x conversion with the increasing temperature.Nearly 100% of NO x conversion over the MnCeZr catalyst occurred at 200 • C, while the maximum NO x conversions over MnZr and MnYZr were 93.6% at 200 • C and 99.3% at 240 • C, respectively.The NO x conversions remained above 90% over MnZr, MnCeZr, and MnYZr catalysts in the temperature range of 190-220 • C, 170-270 • C, and 220-270 • C, respectively.According to the experimental data, the addition of Ce on MnZr catalysts improved the catalytic activity significantly while Y gave the opposite effect.However, they both enlarged the active temperature windows of the catalysts.

Effect of SO2 on SCR Catalytic Activity
Figure 2 shows the effect of SO2 on the NOx conversion over MnZr, MnCeZr and MnYZr catalysts.The NOx conversion decreased as SO2 was introduced into the system.The NOx conversion of MnZr showed a relatively faster deactivation, and decreased from 88.6% to 37.4% after 20 min in the presence of SO2.In contrast, the NOx conversions of MnCeZr and MnYZr decreased to 85% and 74.8%, respectively.Their NOx conversion declined slowly and were maintained above 60% for the next 105 min, respectively.It can be deduced from Figure 2 that the poisoning of SO2 on the SCR activity of the catalysts was most likely caused by the sulfation of the active sites.This coincides with the work of Pan [24].The addition of Ce or Y on MnZr slowed the sulfation progress and resulted in the improvement of SO2 resistance.Furthermore, it was evident that the catalytic activity of MnCeZr was better than that of MnYZr in the reactions.When SO2 was absent from the feed gas, the NOx conversion could not be recovered.This indicated that the deactivation caused by SO2 was irreversible.

Effect of SO 2 on SCR Catalytic Activity
Figure 2 shows the effect of SO 2 on the NO x conversion over MnZr, MnCeZr and MnYZr catalysts.The NO x conversion decreased as SO 2 was introduced into the system.The NO x conversion of MnZr showed a relatively faster deactivation, and decreased from 88.6% to 37.4% after 20 min in the presence of SO 2 .In contrast, the NO x conversions of MnCeZr and MnYZr decreased to 85% and 74.8%, respectively.Their NO x conversion declined slowly and were maintained above 60% for the next 105 min, respectively.It can be deduced from Figure 2 that the poisoning of SO 2 on the SCR activity of the catalysts was most likely caused by the sulfation of the active sites.This coincides with the work of Pan [24].The addition of Ce or Y on MnZr slowed the sulfation progress and resulted in the improvement of SO 2 resistance.Furthermore, it was evident that the catalytic activity of MnCeZr was better than that of MnYZr in the reactions.When SO 2 was absent from the feed gas, the NO x conversion could not be recovered.This indicated that the deactivation caused by SO 2 was irreversible.

Effect of H2O on SCR Catalytic Activity
Figure 2 shows that when 10 vol.% of H2O was continuously introduced into the system, the NOx conversions on MnZr, MnCeZr, and MnYZr had a significant drop in a short time and then remained stable at this level.The NOx conversions on MnCeZr and MnYZr decreased to about 70% and 60%, respectively.When H2O was completely removed from the feed gas, the activity could almost completely recover to its original level.H2O caused a decline in activity and the inhibiting effect of H2O on the SCR activity of the catalysts was reversible.

Effect of H2O and SO2 on SCR Catalytic Activity
Figure 2 further shows that when 0.01 vol.% of SO2 and 10 vol.% of H2O were introduced into the system, a sharp decline of the NOx conversion from 98.91% to 50.24% on MnCeZr was observed, and then the NOx conversion dropped slowly in the rest time.According to the experimental data, the coexistence of SO2 and H2O deactivated the catalysts more seriously than either in the presence of SO2 or H2O alone.Moreover, the activity was not recovered after cutting off H2O + SO2, implying that the coexistence of H2O and SO2 had a synergistic poisoning effect on the SCR activity.

Effect of H 2 O on SCR Catalytic Activity
Figure 2 shows that when 10 vol.% of H 2 O was continuously introduced into the system, the NO x conversions on MnZr, MnCeZr, and MnYZr had a significant drop in a short time and then remained stable at this level.The NO x conversions on MnCeZr and MnYZr decreased to about 70% and 60%, respectively.When H 2 O was completely removed from the feed gas, the activity could almost completely recover to its original level.H 2 O caused a decline in activity and the inhibiting effect of H 2 O on the SCR activity of the catalysts was reversible.

Effect of H 2 O and SO 2 on SCR Catalytic Activity
Figure 2 further shows that when 0.01 vol.% of SO 2 and 10 vol.% of H 2 O were introduced into the system, a sharp decline of the NO x conversion from 98.91% to 50.24% on MnCeZr was observed, and then the NO x conversion dropped slowly in the rest time.According to the experimental data, the coexistence of SO 2 and H 2 O deactivated the catalysts more seriously than either in the presence of SO 2 or H 2 O alone.Moreover, the activity was not recovered after cutting off H 2 O + SO 2 , implying that the coexistence of H 2 O and SO 2 had a synergistic poisoning effect on the SCR activity.2), the newly formed crystal phases MnSO4 on MnZr decreased the SCR activity.However, there were no detectable peaks of MnSO4 for the used MnCeZr and MnYZr, indicating that these sulfate species may exist as amorphous species or do not reach the detection limit.

H2-TPR Results
Figure 4 shows the H2-TPR results of various fresh catalysts.The three catalysts all present three redox peaks in the temperature ranges of 100-200 °C, 250-400 °C, and 450-700 °C, which correspond to the reduction of the surface oxygen, MnO2 to Mn3O4 and Mn3O4 to MnO, respectively [9,30].The peak at 325 °C which corresponded to the reduction of MnO2 to Mn3O4 played an important role in the reaction over MnZr [17,31].It shifted to a lower temperature at 299 °C and a higher temperature at 344 °C over MnCeZr and MnYZr, respectively.Considering the SCR activity (Figure 1), this may have been due to the addition of Ce to MnZr, which improved the mobility of the oxygen as well as the valence and oxidization properties of manganese, while the addition of Y gave the opposite effect [32,33].   2), the newly formed crystal phases MnSO 4 on MnZr decreased the SCR activity.However, there were no detectable peaks of MnSO 4 for the used MnCeZr and MnYZr, indicating that these sulfate species may exist as amorphous species or do not reach the detection limit.

H 2 -TPR Results
Figure 4 shows the H 2 -TPR results of various fresh catalysts.The three catalysts all present three redox peaks in the temperature ranges of 100-200 • C, 250-400 • C, and 450-700 • C, which correspond to the reduction of the surface oxygen, MnO 2 to Mn 3 O 4 and Mn 3 O 4 to MnO, respectively [9,30].The peak at 325 • C which corresponded to the reduction of MnO 2 to Mn 3 O 4 played an important role in the reaction over MnZr [17,31].It shifted to a lower temperature at 299 • C and a higher temperature at 344 • C over MnCeZr and MnYZr, respectively.Considering the SCR activity (Figure 1), this may have been due to the addition of Ce to MnZr, which improved the mobility of the oxygen as well as the valence and oxidization properties of manganese, while the addition of Y gave the opposite effect [32,33]. Figure 4 displays the H2-TPR results of the fresh and used catalysts when SO2 was introduced to the system.In the case of the MnZr catalysts, the peak of MnO2 to Mn3O4 at 325 °C disappeared while two new peaks at 416 °C and 453 °C appeared after the reaction under SO2 flow.Based on the activity test and characterization results, the poisoning effect of SO2 on the catalysts was directly related to the newly formed sulfate species, which decreased the mobility of the oxygen that led to a decrease in SCR activities [34].With the addition of Ce or Y to MnZr, the reduction peak of MnO2 to Mn3O4 did not disappear, but became weaker after the reaction under SO2 flow.It may be deduced that the addition of Ce or Y inhibited the formation of sulfate species and protected the active sites to some extent.

XPS Results
The surface atomic compositions by XPS analysis are given in Table 1.It was noted that surface oxygen on the fresh MnZr catalysts was 7.96% and increased to 9.22% with the addition of Ce while the surface oxygen decreased to 7.23% with Y addition.This is in accordance with H2-TPR results, which indicated that Ce improves the oxygen storage capacity of the catalysts while the effect of Y was opposite.Compared to the fresh catalysts, the manganese concentration in the catalysts declined by 31.3% (MnZr), 18.2% (MnCeZr), and 5.3% (MnYZr) after the reaction under SO2 flow, respectively.The formation of sulfate species on the active sites reduced the amount of surface manganese, which was in accordance with the results of Luo's study [34].The results also confirmed that the sulfation of the active sites was inhibited to some extent after loading Ce or Y. Naturally, some sulfur and nitrogen atoms were detected under the XPS measurement on the surface of the used catalysts, which confirmed that sulfate species were formed on the surface of the catalysts.Figure 4 displays the H 2 -TPR results of the fresh and used catalysts when SO 2 was introduced to the system.In the case of the MnZr catalysts, the peak of MnO 2 to Mn 3 O 4 at 325 • C disappeared while two new peaks at 416 • C and 453 • C appeared after the reaction under SO 2 flow.Based on the activity test and characterization results, the poisoning effect of SO 2 on the catalysts was directly related to the newly formed sulfate species, which decreased the mobility of the oxygen that led to a decrease in SCR activities [34].With the addition of Ce or Y to MnZr, the reduction peak of MnO 2 to Mn 3 O 4 did not disappear, but became weaker after the reaction under SO 2 flow.It may be deduced that the addition of Ce or Y inhibited the formation of sulfate species and protected the active sites to some extent.

XPS Results
The surface atomic compositions by XPS analysis are given in Table 1.It was noted that surface oxygen on the fresh MnZr catalysts was 7.96% and increased to 9.22% with the addition of Ce while the surface oxygen decreased to 7.23% with Y addition.This is in accordance with H 2 -TPR results, which indicated that Ce improves the oxygen storage capacity of the catalysts while the effect of Y was opposite.Compared to the fresh catalysts, the manganese concentration in the catalysts declined by 31.3% (MnZr), 18.2% (MnCeZr), and 5.3% (MnYZr) after the reaction under SO 2 flow, respectively.The formation of sulfate species on the active sites reduced the amount of surface manganese, which was in accordance with the results of Luo's study [34].The results also confirmed that the sulfation of the active sites was inhibited to some extent after loading Ce or Y. Naturally, some sulfur and nitrogen atoms were detected under the XPS measurement on the surface of the used catalysts, which confirmed that sulfate species were formed on the surface of the catalysts.The XPS spectra for the O 1s species is shown in Figure 5.The peaks at 529.8 and 531.8 eV correspond to the lattice oxygen (O β ) and the surface oxygen (O α ), respectively [35].From the calculations, the ratio of (O α )/(O α + O β ) in MnZr (46.1%) was lower than that in MnCeZr (52.4%) and MnYZr (72.6%), respectively.This is owing to the fact that loading Ce or Y increased the defects of metal oxides and oxygen vacancies.[36].Figure 6 shows the XPS spectra for Mn 2p of the fresh and used catalysts after reacting with SO 2 .Compared to the fresh catalysts, the Mn 2p 1/2 and 2p 3/2 peaks of the used MnZr catalysts (Figure 6a) moved from 653.4 eV and 642.2 eV to 654.1 eV and 642.7 eV, respectively.This was due to the sulfation of the manganese, which made the Mn 2p 3/2 peak of the Mn/TiO 2 catalysts shift towards a higher binding energy (approximately 0.7 eV after SCR with SO 2 ), in accordance with the study of Jin [37].When loading Ce or Y, the peaks were not significantly varied.The results also further confirmed that the sulfation of manganese was inhibited to some extent after loading Ce or Y.The Ce 3d XPS pattern (Figure S1) indicated that Ce existed as Ce 4+ and Ce 3+ .The characteristic peaks u, u'', u''', v, v'', and v''' corresponded to Ce 4+ , and peaks u' and v' were attributed to Ce 3+ [31].From the calculations, the ratio of Ce 4+ /Ce 3+ in the used MnCeZr catalysts was lower (from 6.87% to 3.28% after SCR with SO 2 ), which indicated that Ce 4+ was partially reduced to Ce 3+ .The Y 3d XPS pattern (Figure S2) of MnYZr remained almost unchanged after reacting with SO 2 , which may be interpreted as an indication that the valence of Y was not varied.
extent after loading Ce or Y.The Ce 3d XPS pattern (Figure S1) indicated that Ce existed as Ce 4+ and Ce 3+ .The characteristic peaks u, u'', u''', v, v'', and v''' corresponded to Ce 4+ , and peaks u' and v' were attributed to Ce 3+ [31].From the calculations, the ratio of Ce 4+ /Ce 3+ in the used MnCeZr catalysts was lower (from 6.87% to 3.28% after SCR with SO2), which indicated that Ce 4+ was partially reduced to Ce 3+ .The Y 3d XPS pattern (Figure S2) of MnYZr remained almost unchanged after reacting with SO2, which may be interpreted as an indication that the valence of Y was not varied.The sulfation of the active sites on the catalysts was the process of SO2 combining with the surface oxygen, which was accompanied by changes in the surface oxygen.According to the results of Dupin's study [38], the binding energy of 527.7-530.6 eV corresponded to the lattice oxygen; 530.6-531.1 eV was ascribed to OH − ; and 531.1-532 eV was denoted as O − which could supply the defects of lattice oxygen, such as the oxygen in Mn-O-Ce, Ce=O, and Mn-O-Y species.More than 532 eV was attributed to the weakly adsorbed oxygen in the surface.Surface oxygen of the fresh and used catalysts by XPS analysis is shown in Figure 7.According to the results of XPS, it was observed that the binding energy of 529.8 eV corresponding to the lattice oxygen exhibited the largest peak areas for all samples.Combined with the results of XRD indicating that there were many crystalline phases of ZrO2, it was implied that the peak was attributed to the lattice oxygen of ZrO2.The peaks at 531.2 eV and 532.5 eV of the MnZr catalysts were ascribed to O − and the surface weakly adsorbed The sulfation of the active sites on the catalysts was the process of SO 2 combining with the surface oxygen, which was accompanied by changes in the surface oxygen.According to the results of Dupin's study [38], the binding energy of 527.7-530.6 eV corresponded to the lattice oxygen; 530.6-531.1 eV was ascribed to OH − ; and 531.1-532 eV was denoted as O − which could supply the defects of lattice oxygen, such as the oxygen in Mn-O-Ce, Ce=O, and Mn-O-Y species.More than 532 eV was attributed to the weakly adsorbed oxygen in the surface.Surface oxygen of the fresh and used catalysts by XPS analysis is shown in Figure 7.According to the results of XPS, it was observed that the binding energy of 529.8 eV corresponding to the lattice oxygen exhibited the largest peak areas for all samples.Combined with the results of XRD indicating that there were many crystalline phases of ZrO 2 , it was implied that the peak was attributed to the lattice oxygen of ZrO 2 .The peaks at 531.2 eV and 532.5 eV of the MnZr catalysts were ascribed to O − and the surface weakly adsorbed oxygen, respectively.Due to the sulfate ions formed after reacting with SO 2 , these two peaks moved to 531.6 eV and 532.8 eV, respectively, while a new peak appeared at 530.6 eV, corresponding to OH − at the same time.This was caused by the decomposition of the formed sulfate ions.This phenomenon was also observed on the catalysts after loading Ce or Y.The peak at 529.2 eV, which was ascribed to the disappearance of the oxygen of CeO 2 over the MnCeZr catalysts, indicated that a large number of CeO 2 was consumed after SCR with SO 2 .Table 2 shows the proportions of different O species obtained by the peak fitting of the O 1s curves.The O 1s peaks were numbered as O 1s 1, O 1s 2, O 1s 3, and O 1s 4, from low to high binding energy.According to the experimental data, the lattice oxygen and the surface oxygen showed a significant decrease after reacting with SO 2 , while OH − and O − increased.This may be caused by the hydrolyzation of the sulfate ions.According to the results of XRD, H2-TPR, and XPS, it was concluded that SO2 combined with Ce  According to the results of XRD, H 2 -TPR, and XPS, it was concluded that SO 2 combined with Ce or Y easily compared with Mn on the catalysts.Ce or Y inhibited the formation of manganese sulfate and protected the manganese active sites to some extent.

FTIR Analysis
To understand the role of Ce or Y better, FTIR experiments were performed.As shown in Figure 8, the spectrum of all fresh catalysts exhibited three characteristic peaks at 3440 cm −1 , corresponding to the vibration of -OH units [39][40][41]; at 1645 cm −1 , which was ascribed to the stretching vibration of C=C on the surface of MWCNTs [42]; and a broad peak in the 1000-400 cm −1 range, which was attributed to the stretching vibration of metallic oxide groups (Me-O-Me) [43,44].Compared to the fresh catalysts, a peak around 1102 cm −1 , assigned to the SO 4 2− species, appeared after reacting with SO 2 [6,45].From the partial magnification of corresponding FTIR spectra, the peak became weaker after loading Ce or Y, indicating that the addition of Ce or Y may prevent the manganese active sites from being sulfated to some extent [46].

Pretreatment of MWCNTs
The MWCNTs were purchased from Shenzhen Nanotech Port Co. Ltd. (Shenzhen, China) According to Wang et al. [9], the MWCNTs were pretreated in HNO3 solution by ultrasound for 30 min and heated at 100 °C for 4 h in a water-bath.Subsequently, MWCNTs were treated with filtration and washed by deionized water until the pH was neutral.The last sample was dried at 100 °C overnight for further use.

Preparation of MnOx/ZrO2/MWCNTs
The catalysts were prepared by step impregnation methods.Zirconium was supported on multi-wall carbon nanotubes (MWCNTs) to obtain zirconium-modified MWCNTs.The precursor was zirconium nitrate solution and zirconium loading was 30 wt. %.The sample were calcinated at 400 °C in an N2 atmosphere for 2 h and denoted as ZrO2/MWCNTs.Subsequently, to obtain MnOx/ZrO2/MWCNTs, 10 wt.% Mn was loaded on ZrO2/MWCNTs by using the same method, for which the precursor was manganese nitrate.The detailed process has been previously described in Reference [47].

Preparation of Mn-CeOx(YOx)/ZrO2/MWCNTS
MnOx/ZrO2/MWCNTs were introduced into a beaker containing 20 mL ethanol, and then stirred magnetically for 2 h.Cerium nitrate solution, used as the Ce precursor and controlled at 6 wt.% Ce loading with MWCNTS, was gradually added to the abovementioned mixture at room

Pretreatment of MWCNTs
The MWCNTs were purchased from Shenzhen Nanotech Port Co. Ltd. (Shenzhen, China) According to Wang et al. [9], the MWCNTs were pretreated in HNO 3 solution by ultrasound for 30 min and heated at 100 • C for 4 h in a water-bath.Subsequently, MWCNTs were treated with filtration and washed by deionized water until the pH was neutral.The last sample was dried at 100 • C overnight for further use.

Preparation of MnO x /ZrO 2 /MWCNTs
The catalysts were prepared by step impregnation methods.Zirconium was supported on multi-wall carbon nanotubes (MWCNTs) to obtain zirconium-modified MWCNTs.The precursor was zirconium nitrate solution and zirconium loading was 30 wt. %.The sample were calcinated at 400 • C in an N 2 atmosphere for 2 h and denoted as ZrO 2 /MWCNTs.Subsequently, to obtain MnO x /ZrO 2 /MWCNTs, 10 wt.% Mn was loaded on ZrO 2 /MWCNTs by using the same method, for which the precursor was manganese nitrate.The detailed process has been previously described in Reference [47].
3.3.Preparation of Mn-CeO x (YO x )/ZrO 2 /MWCNTS MnO x /ZrO 2 /MWCNTs were introduced into a beaker containing 20 mL ethanol, and then stirred magnetically for 2 h.Cerium nitrate solution, used as the Ce precursor and controlled at 6 wt.% Ce loading with MWCNTS, was gradually added to the abovementioned mixture at room temperature and stirred for 3 h.The mixture was held under ultrasound for 30 min, then dried in the oven at 60 • C for 12 h.The powder was transferred to a tube furnace and calcined at 400 • C under N 2 for 2 h to obtain a Mn-CeO x /ZrO 2 /MWCNTS composite (denoted as MnCeZr).For comparison, Mn-YO x /ZrO 2 /MWCNTS (denoted as MnYZr) was prepared using yttrium nitrate as the Y precursor and controlling to 2 wt.% Y loading with MWCNTS by a similar method.

Catalytic Evaluations
The activity evaluation was carried out in a fixed-bed quartz continuous flow reactor under atmospheric pressure.150 mg of the MnCeZr and MnYZr catalysts were used in the tests, respectively.The feed gas was composed of 0.06% NO, 0.06% NH3, 0.01% SO 2 (when added), 10% H 2 O (when added) and 3.0% O 2 balanced by Ar.The total flow rate was 600 mL/min, corresponding to a gas hourly space velocity (GHSV) of 40,000 h −1 .The inlet and outlet concentrations of NO/NO 2 were analyzed by an online chemiluminescence NO-NO 2 -NO x analyzer (Thermal Scientific, model 42i-HL).The activity data were recorded when the reactions reached the steady state condition at each temperature.The reaction temperature was controlled from 80 to 300 • C with an isotherm step of 20 • C. The heating rate was about 5 • C/min.NO x conversions were obtained by the following equation: where δ(NO x ) represents the NO x conversion; and φ (NO x ) in and φ (NO x ) out correspond to the inlet and outlet volume fraction of NO x , respectively.

Catalyst Characterization
X-ray powder diffraction (XRD) patterns were recorded on a D8 Advace diffractometer (Bruker, Karlsruhe, Germany) with Cu Kα radiation (40 kV, 40 mA).Data were collected between 2θ = 10 • -90 • with 0.02 • steps, and the XRD phases were identified by comparison with the reference data files from the Joint Committee on Power Diffraction Standards (JCPDS).
Hydrogen temperature-programmed reduction (H 2 -TPR) of the catalyst was carried out on an Auto Chem II (Micromeritics, Gwinnett County, GA, USA).The samples were first flushed with Ar (30 mL/min) at 350 • C for 1 h.After that, the experiments were carried out at a heating rate of 10 • C/min from 60-700 • C in 10% H 2 /Ar (30 mL/min).The H 2 consumption was recorded by a Thermal Conductivity Detector (TCD).
The chemical state and surface composition of the catalysts were analyzed by X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos, UK).All spectra were acquired at a basic pressure 3.33 × 10 −6 Torr with Al Kα radiation (hv = 1253.6eV) at 15 kV.The binding energy calibration was performed using the C 1s peak in the background as the reference energy (284.6 eV).
A Fourier transform infrared (FTIR) spectroscope (Nicolet 6700, Thermo Fisher Scientific, Waltham, MA, USA) was used to analyze the functional groups and determine the composition of the samples.Samples were achieved by pressing the mixture of catalysts and KBr.The weight ratio of catalysts : KBr was about 200.The FTIR measurements were performed at 4 cm −1 resolution with an accumulation of 64 scans.

Conclusions
The addition of Ce or Y loading on the MnO x /ZrO 2 /MWCNTs catalysts were prepared using the step impregnation method.Under simulated SCR reaction conditions, Ce improved the activity of the catalysts while the effect of Y was the opposite, however, they both enlarged the active temperature windows.The inhibition effect of H 2 O on SCR activity was reversible, while the poisoning of SO 2 was more serious and irreversible when Ce or Y existed in the catalysts.This study found that the catalytic activity of MnCeZr was better than MnYZr in the abovementioned reaction conditions.Furthermore, Ce could improve the H 2 O resistance of the catalysts to some extent, while the effect of Y was not obvious.For the MnCeZr catalysts, the coexistence of SO 2 and H 2 O deactivated the catalysts more seriously than that in the presence of SO 2 or H 2 O alone, implying that the co-existence of H 2 O and SO 2 had a synergistic inhibition effect on SCR activity.Based on the results of XRD, H 2 -TPR, XPS, and FTIR, Ce or Y existed in the catalysts in high dispersion or amorphous form.Ce could increase the mobility of the oxygen as well as the valence and oxidization of manganese while the effect of Y was the opposite, which may have been the main reason for the better catalytic activity of MnCeZr compared with MnYZr.Ce or Y combined with SO 2 easily to form sulfate species on the catalysts compared with Mn, then inhibited the formation of sulfate species and protected the manganese active sites to some extent, which was in accordance with the theory of ionic polarization.

Figure 3
Figure 3 displays the XRD patterns of different catalysts at wide diffraction angles.The XRD profiles of MnCeZr and MnYZr remained almost unchanged.ZrO2 and MnOx were the major species after the introduction of Ce or Y, respectively, which indicated that the loading of Ce or Y did not change the structure of MnZr.The only exception was that the XRD pattern of MnCeZr showed a weak peak at 47.5°, representing CeO2 (PDF#34-0394).No other characteristic peaks of Ce and Y compounds over MnCeZr and MnYZr were observed, indicating that Ce and Y were well incorporated and dispersed in the catalysts, or that the formed compounds were not large enough to be detected.

Figure 3
Figure 3 displays the XRD patterns of different catalysts at wide diffraction angles.The XRD profiles of MnCeZr and MnYZr remained almost unchanged.ZrO 2 and MnO x were the major species after the introduction of Ce or Y, respectively, which indicated that the loading of Ce or Y did not change the structure of MnZr.The only exception was that the XRD pattern of MnCeZr showed a weak peak at 47.5 • , representing CeO 2 (PDF#34-0394).No other characteristic peaks of Ce and Y compounds over MnCeZr and MnYZr were observed, indicating that Ce and Y were well incorporated and dispersed in the catalysts, or that the formed compounds were not large enough to be detected.

Figure 3 .
Figure 3. XRD patterns of catalysts before and after the reaction in the presence of SO2.

Figure 3
Figure3displays the XRD patterns of the fresh and used catalysts with/without loading Ce or Y on MnZr.The peak that appeared at 42.7° can be assigned to the phases of MnSO4 (PDF#35-0751) formed.With the SCR activity results (Figure2), the newly formed crystal phases MnSO4 on MnZr decreased the SCR activity.However, there were no detectable peaks of MnSO4 for the used MnCeZr and MnYZr, indicating that these sulfate species may exist as amorphous species or do not reach the detection limit.

Figure 3 .
Figure 3. XRD patterns of catalysts before and after the reaction in the presence of SO 2 .

Figure 3
Figure 3 displays the XRD patterns of the fresh and used catalysts with/without loading Ce or Y on MnZr.The peak that appeared at 42.7 • can be assigned to the phases of MnSO 4 (PDF#35-0751) formed.With the SCR activity results (Figure2), the newly formed crystal phases MnSO 4 on MnZr decreased the SCR activity.However, there were no detectable peaks of MnSO 4 for the used MnCeZr and MnYZr, indicating that these sulfate species may exist as amorphous species or do not reach the detection limit.

Figure 4 .
Figure 4. H2-TPR profiles of catalysts before and after the reaction in the presence of SO2.

Figure 4 .
Figure 4. H 2 -TPR profiles of catalysts before and after the reaction in the presence of SO 2 .

Figure 5 .
Figure 5. O 1s XPS spectra of catalysts before and after doping of Ce and Y.Figure 5. O 1s XPS spectra of catalysts before and after doping of Ce and Y.

Figure 5 .
Figure 5. O 1s XPS spectra of catalysts before and after doping of Ce and Y.Figure 5. O 1s XPS spectra of catalysts before and after doping of Ce and Y. Catalysts 2017, 7, 181 8 of 14

Figure 7 .
Figure 7. O 1s XPS spectra of catalysts before and after the reaction in the presence of SO2.(A) MnZr; (B) MnCeZr; and (C) MnYZr.M: Surface metal or sulfur.

Figure 7 .
Figure 7. O 1s XPS spectra of catalysts before and after the reaction in the presence of SO 2 .(A) MnZr; (B) MnCeZr; and (C) MnYZr.M: Surface metal or sulfur.

Table 1 .
Surface atomic concentration of catalysts before and after the reaction in the presence of SO 2 .

Table 2 .
Proportions of different O species obtained by the peak fitting of the O 1s curves.

Table 2 .
Proportions of different O species obtained by the peak fitting of the O 1s curves.