Promoted performance of a MnO_x/PG catalyst for low-temperature SCR against SO₂ poisoning by addition of cerium oxide

Abstract

Manganese oxide supported palygorskite (MnO_x/PG) catalysts are considered highly efficient for lower-temperature selective catalytic reduction (SCR) of flue gas NO by ammonia. However, during the SCR reaction, the surface of the MnO_x/PG catalyst tends to get poisoned by SO₂. In this study, we have doped the MnO_x/PG catalyst with cerium oxide to improve their resistance against SO₂ poisoning. The SCR activity of the CeO₂-doped catalyst (Mn–Ce/PG) is found to be remarkably higher than that of the undoped MnO_x/PG catalyst in the temperature range of 100–200 °C in the presence of 400 ppm SO₂. Of all the different samples with different concentrations of Ce doping, the catalyst Mn₈Ce₅/PG exhibits the highest SCR activity in the presence of SO₂-containing gases. Systematic analyses of the sample performed by using XRD, XPS, SEM-EDS, temperature programmed desorption (TPD) and TGA suggest that the cerium oxide doping can obviously inhibit the SO₂-poisoning of the active species manganese oxides, which is widely believed to be one of the major reasons for the deactivation of manganese-based catalysts. However, the deposition of ammonium sulfates on the surface of catalysts, which is yet another important cause of SO₂ poisoning, can hardly be avoided via the addition of cerium oxide. On the basis of systematic studies, it can be assumed that the addition of cerium oxide can enhance the stability of the active species via the reduction of manganese sulfate rather than ammonium sulfates, thereby improving its resistance toward SO₂ poisoning.

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1. Introduction

NO_x gas emitted from stationary and mobile combustion sources leads to a variety of undesirable and environmentally harmful effects. Selective catalytic reduction (SCR) has been proven to be the most effective technology for the abatement of nitrogen oxides from industrial flue gases.¹ Commercially, V₂O₅–WO₃/TiO₂ catalysts are being widely used for medium–high-temperature SCR. Moreover, manganese-based catalysts that are often prepared by supporting manganese oxides on different carriers such as MnO_x/TiO₂,^2,3 MnO_x/Al₂O₃ (ref. 4) and MnO_x/ZSM-5 (ref. 5) have been reported to be potential candidates for low-temperature SCR of NO with NH₃. In our previous study,⁶ we demonstrated the use of the clay mineral, palygorskite (PG), as a support for the preparation of a novel MnO_x/PG catalyst. The catalyst thus obtained is found to be very active for SCR at low temperatures.

However, these manganese-based catalysts are sensitive to the residual SO₂ in the flue gas and tent to get deactivated dramatically at low temperatures. Recently, ceria (CeO₂) has received considerable attention for the SCR of NO, primarily owing to its exceptional oxidation and reduction abilities. Some studies have analyzed the functionalities of CeO₂ mixed with MnO_x catalyst to form mixed oxides catalysts such as MnO_x–CeO_x/CNTs,^7,8 Mn–Ce/TiO₂,⁹ and MnO_x–CeO_x.¹⁰ The addition of CeO₂ to the Mn-based catalysts enhances the catalytic activity as well as improves the sulfur resistance to some extent. Nevertheless, the addition of CeO₂ to the Mn-based catalysts does not completely solve the issues related to deactivation, especially at low temperatures. Therefore, it is highly imperative to explore the reason underlying the deactivation phenomenon occurring in low-temperature SCR catalysts to effectively circumvent the deactivation issue of catalyst during SCR of SO₂-containing gases. To this end, some authors have suggested that the formation and accumulation of excess ammonium sulfate is the key reason underlying SO₂ poisoning during low-temperature NH₃-SCR reaction.^11–13 Moreover, all these studies have unanimously concluded that the gas-phase SO₂ combines with NH₃ to form ammonium sulfates. Similarly, another study suggests that the manganese oxides catalyst is consumed by SO₂, thereby reducing the stability of the active species. Wu et al.¹⁴ had reported that the doping of Ce on Mn/TiO₂ catalyst could prevent the catalyst from being sulfated and inhibit the formation of ammonium sulfate on the surface of the catalyst. However, the exact mechanism underlying the phenomenon of SO₂-resistance observed in Ce-doped catalysts is still unclear. Thus, in this study we have elucidated the role of ceria doping on the Mn–Ce/PG catalysts for the NH₃-SCR reaction. Based on the results, we have proposed the plausible mechanism underlying the phenomenon of SO₂-resistance exhibited by the Ce-doped Mn₈Ce_x/PG catalysts.

2. Experimental

2.1 Catalyst preparation

The catalysts were prepared by the wetness co-impregnation method loading of active species and additive species.¹⁵ Palygorskite as support was pore volume impregnated by mixed aqueous solution of manganese nitrate and cerium nitrate as precursors. The mass ratios of manganese and cerium to palygorskite were 8 wt% and 1–5 wt%, respectively. The catalysts were dried initially at 50 °C for 6 h, followed by drying at 110 °C for 12 h. The dried catalysts were ground and sieved to 20–40 mesh, and subsequently calcined in air at 300 °C for 3 h. Catalysts thus obtained were designated as Mn₈/PG and Mn₈Ce_x/PG (x = 1–5), wherein “8” and “x” refer to the mass percentage of each element.

2.2 Determination of catalytic activity

SCR activity tests were performed in a quartz fixed-bed continuous flow reactor at a temperature of 100–300 °C. Typically, 2 g of the catalyst was used in the SCR tests. The flow rates of all the different reactant gases including NO–Ar, NH₃–Ar, SO₂–Ar and pure O₂ were controlled via respective mass flow meters. The reactant gas comprised 600 ppm NO, 600 ppm NH₃, 400 ppm or 600 ppm SO₂ (when used), 3 vol% O₂ and Ar as balance. The concentrations of NO and SO₂ in the inlet and outlet streams of the reactor were on-line monitored by a NO analyzer (Testo 350XL) and a SO₂ (Testo 300) analyzer, respectively. The conversion of NO and oxidation of SO₂ were calculated using the formulae (1)–(3)

2.3 Catalyst characterization

The microstructure and elementary composition of the catalyst after SCR reaction in SO₂-containing gas were analyzed by using scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The surface atomic state of the catalyst was determined by using X-ray photoelectron spectroscopy (XPS, ESCALAB250 spectrometer) using Al Kα radiation. Moreover, the Mn 2p XPS spectrum was recorded from the SO₂-poisoned catalyst. Besides, X-ray diffraction (XRD) was performed on a D/MAx2500V diffractometer (Rigaku, Japan) equipped with a monochromated Cu Kα radiation source. The catalysts were scanned at 2θ ranging from 10–70°.

Furthermore, temperature programmed desorption (TPD) experiments were performed on 1 g of the SO₂-poisoned catalyst. In the typical process, the samples were initially reacted with 400 ppm of SO₂-containing gases at the reaction temperature for 2 h. Subsequently, the TPD experiments were performed by raising the temperature from 60 to 800 °C at a heating rate of 5 °C min⁻¹ in Ar (350 mL min⁻¹) atmosphere.

Moreover, thermogravimetric analysis (TGA, DTG-60H) of the samples was performed in a static oxygen atmosphere in the temperature range from 20 to 800 °C at a heating rate of 10 °C min⁻¹. Approximately 5–9 mg of the sample was used for the TGA.

3. Results and discussion

3.1 Catalytic activity

Fig. 1 shows the NO conversion of the different Mn₈Ce_x/PG catalysts in the studied temperature range. Among all samples, the catalyst (Mn₈/PG) without Ce-doped exhibits the lowest catalytic activity in the temperature range of 200–250 °C in the presence of 400 ppm SO₂. After a certain amount of cerium oxide doped, the NO conversion is obviously enhanced especially at temperatures higher than 200 °C, in spite of that the increase in catalytic activity is not absolutely synchronized with the increase in Ce content. In case of less than 5 wt% Ce added, there is no a regular trend on the performance of catalysts upon Ce doping at temperatures lower than 200 °C. It implies that there may be a competition between the increase deposition of ammonium sulfates and the decrease formation of manganese sulfates caused by the doping of Ce. However, all the Ce-doped catalysts demonstrate higher catalytic activities in the temperature range of 200–300 °C, suggesting that the deposition of ammonium sulfates becomes less important due to thermal decomposition. Notably, the best performance of SCR was obtained on Mn₈Ce₅/PG catalyst in the whole temperature range with the NO conversion of 85% and 95% at 200 °C and 250 °C, respectively. For further study the influence of cerium doping on the catalysts, Mn₈Ce₅/PG catalyst was also evaluated for N₂ selectivity. The selectivity of manganese based catalysts was widely considered as an important index for SCR. In this work, for Mn₈Ce₅/PG, the N₂ selectivity was observed to be about 96% as calculated from the results shown in Fig. 1b. In the outlet flow stream, less than 20 ppm of NO₂ is detected besides NO. These results indicate that the Ce-doped catalysts exhibit efficient resistance toward SO₂-poisoning in the medium–high-temperature range with a high N₂ selectivity, which is in agreement with the founding by other researchers.^16,17

Fig. 1 (a) The SCR activity of different Mn₈Ce_x/PG catalysts in the presence of SO₂. (b) Concentrations of NO and NO₂ for Mn₈Ce₅/PG. Reaction conditions: [NH₃] = [NO] = 600 ppm, [SO₂] = 400 ppm, [O₂] = 3 vol%, Ar balance, GHSV = 5000 h⁻¹, total flow rate 350 mL min⁻¹.

3.2 SO₂ oxidation activity

As mentioned earlier, the manganese-based catalysts are prone to poison by SO₂ during the SCR of NO. Therefore, researchers have long sought a suitable material that can act as a bridge and remove SO₂ to avoid the depositions of ammonium and manganese sulfates (SO₂-poisoning). Previous studies have demonstrated that the cerium oxide doping of Mn/TiO₂ catalyst can prevented the oxidation of SO₂ to SO₃, which in turn would be beneficial for improving their catalytic activity in the presence of SO₂-containing gases.^14,18

Therefore, in this study, we investigate the effect of Ce addition on the SO₂ oxidation on the surface of catalyst. Thus, we compare the changes in SO₂ concentration with reaction time on the surface of the undoped (Mn₈/PG) catalysts both in the absence and presence of oxygen (Fig. 2). For Mn/PG catalyst, the SO₂ oxidation depends on the original lattice oxygen companied with manganese specie, which cannot be recovered by gaseous O₂ after the formation of MnSO₄. In other words, the presence of gaseous O₂ does not change the degree of SO₂ oxidation for Mn/PG catalyst due to the steady formation of MnSO₄. Intriguingly, the outlet SO₂ concentration of Mn₈Ce₅/PG is slightly lower than that of Mn₈/PG in the absence of oxygen. Peralta¹⁹ reported that the SO₂ gas molecules would tend to adsorb first onto Ce due to the presence of vacancies in Ce. In the case of Mn₈Ce₅/PG, the outlet concentration of SO₂ is virtually zero during the first 30 min, which is far lower than that of the Mn₈/PG catalyst in the presence of oxygen. It suggests that unlike previous study, the addition of Ce accelerates the oxidation of SO₂ to SO₃ in the presence of gaseous O₂, which is the main reason for the formation of sulfates.

Fig. 2 The SO₂ oxidative behavior over Mn₈/PG and Mn₈Ce₅/PG in the absence and presence of O₂ at 200 °C. Reaction conditions: [SO₂] = 400 ppm, [O₂] = 3 vol%, Ar balance, total flow rate 350 mL min⁻¹.

3.3 Characterizations of the catalysts

Fig. 3 shows the XRD patterns of Ce-modified catalysts. All the samples calcined at 300 °C were observed to maintain the primary structure of palygorskite. The strong diffraction peak at 8.436° ascribed to palygorskite was found to be too high for distinguishing other weak peaks. Thus, XRD patterns were only shown in the diffraction range of 10° to 70°. The diffraction peaks observed at 37.1°, 42.3° and 56.5° matching MnO₂ well [PDF# 30-0820]. However, all these patterns do not show intense peaks of Mn₂O₃ because that Mn₂O₃ is more prone to form amorphous state than MnO₂.^20,21 Obviously, the peaks corresponding to MnO₂ weakened with the addition of Ce, indicating that the addition of Ce enhances active component dispersion, which is in accordance with the founding by Wang.²²

Fig. 3 XRD patterns of catalysts: (a) Mn₈/PG; (b) Mn₈Ce₁/PG; (c) Mn₈Ce₂/PG; (d) Mn₈Ce₃/PG; (e) Mn₈Ce₄/PG; (f) Mn₈Ce₅/PG.

The morphology and elemental composition of the surface of Mn₈Ce₅/PG catalyst is investigated using SEM and EDS after NH₃-SCR reaction in the presence of SO₂-containing gases. As illustrated in Fig. 4A, the SEM image of the Mn₈Ce₅/PG catalyst after NH₃-SCR reaction in the presence of 600 ppm of SO₂ for 2 h at 150 °C exhibits a clear boundary between the part I and part II regions, while the size of part I is much smaller than that of part II. Fig. 4B presents the magnified image of the selected area in part I in Fig. 4A, which shows a type of granular material. In the actual experiment, there was a kind of white layer on the surface of the SO₂-poisoned catalyst.

Fig. 4 (A): SEM images for Mn₈Ce₅/PG catalyst after 600 ppm SO₂ poisoning for 2 h at 150 °C; (B) the magnified images of the selected area in the picture A in part I.

Table 1 shows the elemental compositions of the noted regions of part I and part II. As can be seen, the mass percentage of S in part I is much lesser than that in part II, whereas the mass percentage of Mn and Ce are higher than that in part II. Moreover, a certain quantity of N is detected in part II with the N/S stoichiometric ratio of 2.53 which is greater than that of (NH₄)₂SO₄, (NH₄)₂SO₃ and NH₄HSO₄. This suggests that the granular material (white layer) in the part II is mainly composed of ammonium sulfates and there may be other compounds containing the N element existed (Mn-nitrate or adsorbed ammonia) apart from the ammonium sulfates. Thus, it can be deduced that the main reason for the deactivation of Ce-added catalysts at 150 °C is the formation of ammonium sulfates.

Table 1 The surface elemental composition of part I and part II from EDS analysis

Element	Mass percentage (%)		Element	Mass percentage (%)
	Part I	Part II		Part I	Part II
C	1.15	—	Ce	7.92	3.22
O	36.44	37.20	Mg	3.70	1.47
N	—	17.52	Al	2.50	—
S	2.65	15.80	Si	23.72	13.50
Mn	15.34	5.95	K	0.83	0.64
Fe	3.48	1.49	Ca	2.07	0.31

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Furthermore, we have analyzed the oxidation state of manganese on the surface of the Mn₈/PG and Mn₈Ce₅/PG catalysts before and after SO₂-poisoning at 150 °C as determined using XPS spectroscopy (Fig. 5). The typical Mn 2p XPS spectra for all of the catalysts are almost similar. The broad Mn 2p_3/2 peak in the spectra is the characteristic of a mixed-valence manganese system.¹⁴ 2p_3/2 binding energies at 640.3, 641.3 and 642.1 are attributed to the presence of surface Mn²⁺, Mn³⁺ and Mn⁴⁺ species in the catalysts, respectively.^23,24 The estimated surface concentration of Mn and the relative percentage of Mn^R+ (R = 2, 3, 4) in Mn 2p_3/2 are listed in Table 2. On surface of each catalyst, manganese specie exists in various oxidation states such as Mn²⁺, Mn³⁺ and Mn⁴⁺. The Mn⁴⁺ and Mn³⁺ atomic ratio increased after Ce-added, indicating that the addition of Ce enhances the transition of Mn²⁺ to Mn³⁺ and Mn⁴⁺, which is accordance with the found by Kwon.²⁵ In case of Ce-added catalysts, the atomic ratio of surface Mn²⁺ is found to be much less than that of undoped catalysts, especially for SO₂-poisoning catalysts. According to the results in Table 2, atomic ratio of surface Mn²⁺ for SO₂-poisoning Mn₈Ce₅/PG is only 9.4%, while for SO₂-poisoning Mn₈/PG the percentage is 36.8. It indicates that lesser amount of MnSO₄ is deposited onto the surface of Mn₈Ce₅/PG catalyst during the SCR reaction in the presence of SO₂, which could be attributed to the addition of cerium oxide.

Fig. 5 XPS spectra of Mn 2p for the catalysts before and after 400 ppm SO₂ poisoning for 2 h at 150 °C. (A): Mn₈/PG-SCR, (B): SO₂-poisoned Mn₈/PG, (C): Mn₈ Ce₅/PG-SCR, (D): SO₂-poisoned Mn₈Ce₅/PG.

Table 2 XPS results for surface Mn atom percentage

Catalyst	Mn (%)	Mn^2+ (%)	Mn^3+ (%)	Mn^4+ (%)
Mn8/PG-SCR	1.06	11.1	35.2	53.7
Mn8/PG 400 ppm SO2	0.89	36.8	28.1	35.1
Mn8Ce5/PG-SCR	1.38	4.4	38.5	57.1
Mn8Ce5/PG 400 ppm SO2	1.35	9.4	30.0	60.6

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3.4 Thermal analysis

Fig. 6 shows the TG/DTG curves of the Mn₈/PG and Mn₈Ce₅/PG catalysts after NH₃-SCR reaction for 2 h in the presence of 400 ppm SO₂ at 150 °C. The typical TG curves show five weight losses and the DTG shows the corresponding peaks. The weight loss peak I appearing at around 100 °C can be attributed to the evaporation of surface adsorbed water molecules in the catalysts.²⁶ The second weight loss peak II at around 200 °C is close to the decomposition temperature of ammonium sulfate.²⁷ The broad peak III arising in the temperature range between 350 and 500 °C can be ascribed to the decomposition of NH₄HSO₄ (350 °C)^28,29 and the removal of the residual physisorbed water and hydroxyl groups on the surface of palygorskite.³⁰ Moreover, there are two other smaller peaks in the temperature range of 550–750 °C. The peak IV should be attributed to the morphological change of manganese oxides,³¹ while the peak V is associated with the decomposition of MnSO₄.^11,32,33 Most importantly, the MnSO₄ decomposition (peak V) is not observed in Fig. 5B, indicating that the addition of Ce inhibits the formation of MnSO₄. These results are found to be in good agreement with the XPS results.

Fig. 6 TG/DTG curves of samples after SCR reaction in the presence of 400 ppm SO₂ at 150 °C. (A) Mn₈/PG; (B) Mn₈Ce₅/PG.

3.5 Temperature programmed desorption of SO₂

In principle, the amount of desorbed SO₂ reflects the degree of poisoning of the catalysts. Therefore, in this study, we have performed the SO₂-TPD to investigate the poisoning effect of SO₂ on the Mn₈/PG and Mn₈Ce₅/PG catalysts. Prior to the analysis, NH₃-SCR reaction was performed for 2 h in the presence of 400 ppm SO₂. The corresponding SO₂-poisoned catalysts were denoted as Mn₈/PG/T/U and Mn₈Ce₅/PG/T/U series, wherein T indicates the reaction temperature. During the SCR reaction, some white ammonium sulfates were deposited on the surface of catalysts and at the end part of the reactor. Therefore, the catalysts that underwent the SCR reaction were further heated at 300 °C for 2 h in Ar atmosphere and the reactor was washed in distilled water to remove the deposited ammonium sulfates. The corresponding catalysts were denoted as Mn₈/PG/T/H and Mn₈Ce₅/PG/T/H series.

Fig. 7 shows the SO₂-TPD profiles of the catalysts. As can be seen, all of the samples present similar SO₂-TPD profile and have a strong desorption peak (T = 930 °C) in the temperature range of 750–1000 °C. However, for Mn₈Ce₅/PG/150 °C/U catalyst, there is an additional small desorption peak at around 300 °C, which can be assigned to the accumulation of ammonium sulfates on the surface of the catalysts, indicating that the addition of Ce increases the deposition of ammonium sulfates (T < 150 °C) by converting SO₂ into SO₃ upon the injection of SO₂ in the selective catalytic reduction (SCR) reaction. Compared with Mn₈Ce₅/PG/150 °C/U catalyst, the peak at 930 °C for Mn₈Ce₅/PG/150 °C/H becomes obviously weaker, which suggests the formation of some substance on the surface of the catalyst that almost decomposes at 300 °C. In conjunction with the SEM-EDS and XPS results, this substance is speculated to be ammonium sulfates. Interestingly, there is no change in the concentration of desorbed SO₂ after heating the Mn₈Ce₅/PG/200 °C/U catalyst. In contrast, the strong desorption starting at 800 °C is attributed to the decomposition of manganese sulfate.³⁴ Comparing Fig. 7A and B, it is found that the concentration of SO₂ desorbed from the Mn₈Ce₅/PG/200 °C/H is higher than that from Mn₈Ce₅/PG/150 °C/H, while that from Mn₈Ce₅/PG/200 °C/U is lower than that from Mn₈Ce₅/PG/150 °C/U. This means that on the Mn₈Ce₅/PG catalyst the ammonium sulfates tend to deposit at lower temperature, while manganese sulfate tends to deposit at higher temperature.

Fig. 7 TPD profiles of different catalysts after SCR reaction for 2 h at 150 °C and 200 °C in the presence of 400 ppm SO₂.

Similarly, comparing Fig. 7A with 7C, it can be observed that the SO₂ concentration desorbed from Mn₈Ce₅/PG is lower than that from Mn₈/PG catalyst after heating treatment at 300 °C. Moreover, the SO₂-TPD curves of the Mn₈/PG catalyst before and after heating at 300 °C are almost similar, which implies that the amount of MnSO₄ formed on the surface of Mn₈/PG catalyst is relatively higher than that on the Mn₈Ce₅/PG surface. Similarly, the concentration of SO₂ desorbed from Mn₈/PG is higher than that from Mn₈Ce₅/PG at 200 °C. The SO₂-TPD results demonstrate that the reaction temperature has a great influence on the desorption of SO₂ behavior. Higher reaction temperature favors the formation of MnSO₄, while lower reaction temperature enhances the accumulation of ammonium sulfates.

Fig. 8 shows the schematic of the mechanism underlying the NH₃-SCR reaction over MnO_x/PG catalyst in the presence of SO₂. Unlikely previous study, the consumption of SO₂ do not result in the formation of Ce₂(SO₄)₃ but the formation of ammonium sulfates and manganese sulfates.^18,25,35 On the basis of the above results, the SO₂ deactivation of the catalyst can be attributed to the following two reasons. During the SCR, SO₂ is oxidized to SO₃ by the manganese oxides, which produces (NH₄)_xSO₄ and MnSO₄ combined with NH₃ and MnO₂.³⁶ However, with the addition of CeO₂, the active species (manganese oxides) transform between MnO₂ and Mn₂O₃. According to study on MnO_x–CeO₂ catalyst reported by Qi et al., there exists a synergistic mechanism between the manganese and cerium oxides as shown in the following reactions (4)–(6):^37,38

2MnO₂ → Mn₂O₃ + O (4)

Mn₂O₃ + 2CeO₂ → 2MnO₂ + Ce₂O₃ (5)

Ce₂O₃ + 1/2O₂ → 2CeO₂ (6)

Fig. 8 The mechanism on SCR of NO_x with NH₃ over MnO_x supported palygorskite catalyst in the presence of SO₂. (A) T < 150 °C; (B) T > 150 °C.

In case of the MnO_x/PG catalyst, the accumulation of (NH₄)_xSO₄ is more favored at lower temperatures (T < 150 °C). With the addition of cerium oxide to the catalyst, the degree of SO₂ oxidation is improved, which in turn increases the deposition of ammonium sulfates. Meanwhile, the amount of MnSO₄ gets decreased so as to slightly improve the catalytic activity at lower temperatures. Some studies have reported that the deactivation of SO₂ tends to decrease with increase in temperature.^39,40 For reaction temperatures higher than 150 °C, the formation of ammonium sulfates is decreased while that of MnSO₄ is increased for the MnO_x/PG catalyst. The major reason for SO₂ deactivation is not the deposition of ammonium sulfates, but is the deposition of MnSO₄ at medium temperature (150–250 °C). Therefore, the decreased accumulation of MnSO₄ plays a more important role in the higher catalytic activity of the Mn–Ce/PG catalyst.

4. Conclusion

In summary, CeO₂-doped MnO_x/PG catalysts (Mn–Ce/PG) effectively resisted SO₂ poisoning during the low-temperature SCR of NO with NH₃. Different sulfates are formed at different temperatures during SCR on the surface of the Mn₈/PG and Mn₈Ce₅/PG catalyst. This implies that different sulfates are responsible for the SO₂ poisoning of the catalysts under different conditions. Results indicate that the accumulation of ammonium sulfate is favored at 150 °C, while manganese sulfate is predominantly formed at 200 °C. It was found that the cerium oxide addition did not inhibit the formation of ammonium sulfates. Rather, it aggravated the oxidation of SO₂ to some extent and increased the deposition of ammonium sulfates, thereby restraining the formation of manganese sulfate on the surface of catalyst. Accordingly, it is the reduction of manganese sulfate rather than the ammonium sulfates, which is responsible for the observed resistance of the Ce-doped catalysts toward SO₂ poisoning during SCR of NO with NH₃.

Acknowledgements

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (40902020, 51002042), Ph. D. Programs Foundation of Ministry of Education of China (20090111120019), Programs Foundation of Provincial Education Department of Anhui (no. KJ2009B027, 2009SQRZ015, 2009AJZR0148, 2011AJZR0080).

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