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Effect of SO2 on Activated Carbon Honeycomb Supported CeO2-MnOx Catalyst for NO Removal at Low Temperature Yanli Wang, Xiaoxiao Li, Liang Zhan, Cui Li, Wenming Qiao, and Licheng Ling Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie504074h • Publication Date (Web): 11 Feb 2015 Downloaded from http://pubs.acs.org on February 14, 2015

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Industrial & Engineering Chemistry Research

Effect of SO2 on Activated Carbon Honeycomb Supported CeO2-MnOx Catalyst for NO Removal at Low Temperature Yanli Wang, Xiaoxiao Li, Liang Zhan*, Cui Li, Wenming Qiao, Licheng Ling State Key Laboratory of Chemical Engineering, Key Laboratory for Specially Functional Polymers and Related Technology of Ministry of Education, East China University of Science and Technology, Shanghai 200237, PR China

* Corresponding author. Tel: +86 21 64252924; Fax: +86 21 64252914. E-mail address: [email protected] (Liang Zhan) 1

ACS Paragon Plus Environment


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Abstract: The influence mechanisms of SO2 on selective catalytic reduction (SCR) of NO over activated carbon honeycomb supported MnOx (Mn/ACH) and CeO2-MnOx (CeMn/ACH) catalysts were investigated at low temperature. The catalysts under different conditions were characterized by transient response experiment, nitrogen adsorption, XRD and XPS techniques. Results indicate that SO2 deactivates the SCR activities of Mn/ACH and CeMn/ACH catalysts at 160 oC. The deactivation is mainly due to the deposition of ammonium sulphite and ammonium sulfate on the catalysts’ surface during the NO removal process in the presence of SO2. Compared with Mn/ACH, CeMn/ACH catalyst exhibits a higher resistance to SO2 poisoning, because the addition of CeO2 inhibits the formation of ammonium salts. These ammonium salts decreases the SCR activities through pore blocking, however, it can be completely removed by thermal regeneration at 350 oC. Furthermore, the competitive adsorption between SO2 and NH3 also decreases the SCR activities. Keywords: Activated carbon honeycomb; Selective catalytic reduction; CeO2-MnOx; Nitric oxide; SO2

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1. Introduction Selective catalytic reduction (SCR) of nitrogen oxides (NOx) with NH3 has been proven to be the most effective technology for the removal of NOx emission from flue gases.

1

Honeycomb catalysts have received increasing attention, due to their low

resistance to flow and low plugging caused by particles.

2,3

Currently, commercial

honeycomb V2O5/TiO2, V2O5-WO3/TiO2 catalysts and honeycomb cordierite-based CuO/Al2O3 catalysts have shown high activity for NO removal, but these catalysts have to be used at temperatures above 350 °C to avoid the SO2 deactivation of the catalysts. 4-6 For this reason, the SCR process must be placed in the upstream of boiler air preheater and electrostatic precipitators. However, the high concentrations of particulates and SO2 in the flue gases shorten the life of the catalyst. Therefore, low-temperature SCR catalysts have been widely studied due to energy consumption reducing and operation cost saving. Since the flue gas still contains some residual SO2 even after desulfurization, the residual SO2 should be considered. Most researchers agreed that SO2 deactivated seriously SCR activity at low temperature,

7-11

several poisoning mechanisms of SO2

have been reported for NO removal. Kijlstra et al. 7 proved that the transformation of MnO to MnSO4 on MnOx/Al2O3 catalyst significantly deactivated the catalyst’s SCR activity. Xie et al. 8 proposed that the main reason for the deactivation effect of SO2 over CuO/Al2O3 catalyst was that the formation of ammonium sulfate caused a serious pore plugging at temperatures of 200-300 oC. Wu et al. 9,10 investigated that the effects of SO2 on NO conversions of Mn/TiO2 and Mn-Ce/TiO2 at temperatures below 200 oC and found that NO conversion of Mn/TiO2 significantly decreased in the presence of SO2 and the addition of CeO2 into Mn/TiO2 greatly improved the resistance to SO2 poisoning, because CeO2 can prevent the formation of Mn(SO4)x, Ti(SO4)2 and 3



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greatly inhibit the depositions of NH4HSO4 and (NH4)2SO3. Xu et al.

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further

reported the deactivation of SO2 over Ce/TiO2 catalyst is mainly caused the formation of high thermally stable Ce2(SO4)3 and Ce(SO4)2, which disrupts the Ce4+/Ce3+ redox cycle. Recently, we developed CeO2 and MnOx supported on an activated carbon honeycomb catalyst (CeMn/ACH) for NO removal at low temperatures.

12

CeMn/ACH catalyst combines the high SCR activities of MnOx-based catalyst at low temperatures, the unique redox properties of CeO2 and honeycomb structures of ACH support. More than 98% NO conversion is obtained over the CeMn/ACH catalyst at temperatures of 80-200 oC, when the loadings of Ce and Mn are 7.6 wt% and 3.0 wt%, respectively. Considering industrial application, it is necessary to investigate the SO2 resistance of CeMn/ACH catalyst during the NO removal process at the temperatures of 80-200 oC. In this work, we further mainly study the effects of SO2 on SCR activities of CeMn/ACH catalyst at low temperatures. The information obtained is important for providing the knowledge of the mechanism of SO2 deactivation over activated carbon supported SCR catalysts.

2. Experimental 2.1 Catalyst preparation The ACH used is a homemade product with a cell density of 10 cells per square inch (cpsi) and a wall thickness of 1.0 mm. The ACH monoliths were subjected to carbonization at 700 oC for 3 h. The CeMn/ACH catalyst was prepared by pore volume impregnation using a mixed solution of manganese acetate and cerium nitrate as described elsewhere. 12 The weight percentage of Ce and Mn of CeMn/ACH is 7.6 wt % and 3.0 wt%, respectively, which is denoted CeMn/ACH. For convenience, the 4


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fresh catalyst is denoted CeMn/ACH-F, the catalyst poisoned by SO2 is denoted CeMn/ACH-S, and the sample of CeMn/ACH-S thermally regenerated at 350 oC in N2 for 2 h is denoted CeMn/ACH-S-T. For comparison, Mn/ACH catalyst was also prepared using the same method. 2.2 Activity test SCR activity measurement was carried out in a fixed-bed reactor with an internal diameter of 20 mm. The feed gas contained 500 ppm NO, 500 ppm NH3, 5.0 vol% O2, 300 ppm SO2 (when used) and balance N2. The total flow rate was maintained at 500 ml/min, corresponding to a gas hourly space velocity (GHSV) of 1910 h−1. The concentrations of NO, SO2 and O2 were continually measured online by a flue gas analyzer (MRU VARIO PLUS, Germany). 2.3 Characterization Pore structures were characterized by nitrogen adsorption at 77K using a Micromeritics ASAP 2020 M analyzer. X-ray diffraction (XRD) patterns were measured with a Riguku D/Max 2550 powder diffractometer using Cu Kα radiation operated at 40 kV and 100 mA. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI 5000 VersaProbe system with Al Kα radiation (1486.6 eV) at a pressure lower than 10-8 Torr.

3. Results and Discussion 3.1. Effect of SO2 on NO removal activity To evaluate the effects of SO2 on NO removal activities of Mn/ACH and CeMn/ACH catalysts, transient response experiments were performed on these catalysts at 160 oC. The results are shown in Figure 1. In the absence of SO2 (stage A), the steady-state NO conversions of Mn/ACH and CeMn/ACH are about 63% and 84%, respectively. After the introduction of SO2 into the reactants (stage B), NO 5



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conversions decrease with time on stream, NO conversion of Mn/ACH quickly decreases to a steady-state value of 33% in 120 min, while NO conversion on CeMn/ACH gradually decreases to about 44% in 420 min. After SO2 is shut off (stage C), NO conversions cannot be recovered, indicating the deactivation occurs in the presence of SO2. The removal of NO+NH3+O2 from the feed stream in stage D leaves pure N2 to purge the catalyst surface, and then the used catalysts are thermally regenerated in N2 at 350 oC for 2 h. Afterwards, the catalysts are cooled to 160 oC, and the feed was then changed to the reactants containing NO, NH3 and O2 (stage E) under the same conditions in stage A. During this process, NO conversion of Mn/ACH and CeMn/ACH catalysts restore to 58% and 74%, respectively, which are slightly lower than those in stage A. NO conversions on Mn/ACH and CeMn/ACH catalysts decrease with time on stream after the reintroduction of SO2 in the feed (stage F), and the results are similar to those in stage B. The above results clearly indicate that SO2 has poisoning effect on the SCR activities of Mn/ACH and CeMn(1)/ACH at 160 oC. It should be pointed out that the addition of CeO2 into Mn/ACH enhances the resistance to SO2 poisoning. Furthermore, NO conversions can be effectively recovered by the thermal regeneration at 350 oC. As shown in Figure 1, NO conversions of the catalysts after thermal regeneration are mostly restored, suggesting that a certain amount of sulfur compounds formed during the NO removal process in the presence of SO2 cause the deactivation of the catalysts. 3.2. SO2 deactivation mechanisms of CeMn/ACH catalyst To illuminate the mechanism of SO2 deactivation, N2 adsorption, XRD and XPS were performed on Mn/ACH and CeMn/ACH catalysts under different conditions. Figure 2 shows the N2 adsorption-desorption isotherms of the fresh (Mn/ACH-F, CeMn/ACH-F) and poisoned (Mn/ACH-S, CeMn/ACH-S) samples. Their pore 6


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structure parameters are listed in Table 1. Compared with Mn/ACH-F catalyst, Smic and Vmic of Mn/ACH-S catalyst are decrease by 26.5% and 30.6%, respectively. Smic and Vmic of CeMn/ACH-S are decrease by 18.7% and 18.4%, respectively, as compared to that of CeMn/ACH-F. These data indicate that NO removal in the presence of SO2 leads to the decrease of Smic and Vmic. This suggests the sulfur compounds formed on the catalyst surface cause micropore plugging. Additionally, the decrease range of Smic and Vmic over CeMn/ACH-S are much lower than those of Mn/ACH-S, suggesting that the addition of CeO2 can inhibit the micropore plugging. After thermal regenerated at 350 oC for 2 h, both Mn/ACH-S-T and CeMn/ACH-S-T have the similar Smic and Vmic valves with those of Mn/ACH-F and CeMn/ACH-F samples. This implies the sulfur compounds which block the catalyst’s micropores can be fully removed by thermal regeneration, giving a long cycle life. To understand the surface composition of the poisoned Mn/ACH and CeMn/ACH catalysts, XRD analyses were performed. Figure 3 exhibits the XRD patterns of Mn/ACH-S and CeMn/ACH-S, along with those of fresh samples for comparison. Mn/ACH-F appears the crystallized Mn3O4 phase. With the addition of CeO2, the diffraction peaks of cubic CeO2 can be observed on CeMn/ACH-F, however, manganese oxide phase cannot be observed. 12 Compared to XRD patterns of the fresh samples (Mn/ACH-F, CeMn/ACH-F), no visible diffraction peaks of sulfate can be observed on Mn/ACH-S and CeMn/ACH-S, which suggests that no crystalline sulfate phase is formed or the surface sulfate amount exceeds the detection limit of XRD. To further determine the form of surface species, these samples were then subjected to XPS analysis. Figure 4 reveals the Mn 2p spectra of Mn/ACH and CeMn/ACH before and after poisoned by SO2. Compared with Mn/ACH-F, the Mn 2p3/2 peak for the poisoned Mn/ACH-S does not markedly shift to higher binding 7



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energy (from 641.7 eV to 642.1 eV). No obvious change can be observed from the binding energy of Mn 2p3/2 for the fresh and poisoned CeMn/ACH (CeMn/ACH-S). According to the literatures reported, when manganese sulfate was formed on Mn/TiO2 and Mn-Ce/TiO2 catalysts,

9,10

the binding energy of Mn 2p3/2 would

increase about 0.7~0.8 eV. 14,15 Such results clearly indicate that no manganese sulfate is formed on Mn/ACH-S and CeMn/ACH-S, which should be attributed to different interactions between metal oxides and ACH support. Figure 5 exhibits the Ce 3d spectra of the fresh and poisoned CeMn/ACH (CeMn/ACH-S) catalysts. The u‴, u″, u, v‴, v″, and v peaks are attributed to Ce4+, while the peaks labeled as u′ and v′ are assigned to Ce3+.

16-18

It can be observed that

the intensities of Ce3+ peaks slightly increase after SCR reaction in the presence of SO2, indicating increased Ce3+ concentration. The results agree with the results reported by Smirnov et al. 19 on a CeO2 film surface, which showed a portion of Ce4+ transformed into Ce3+ after the sulfation, owing to the following reaction: 2CeO2+3SO2+O2→Ce2(SO4)3. Similar result was also reported by Xu et al. 11 Because Ce2(SO4)3 on CeMn/ACH does not decompose or be reduced at 350 oC in N2,

20

the

presence of Ce2(SO4)3 on CeMn/ACH-S-T may results in a slightly lower NO conversion, as compared to CeMn/ACH-F (Figure 1). Additionally, the surface atomic concentration of oxygen on CeMn/ACH-S is slightly lower than that on CeMn/ACH-F (see Table 2). It can be inferred that the slightly increased Ce3+ concentration on CeMn/ACH-S surface leads to the decrease in surface oxygen concentration. Figure 6 reveals the S 2p spectra of Mn/ACH-S and CeMn/ACH-S. It can be seen that two peaks at 168.5eV and 169.7eV appear. The peak at 168.5eV is assigned to SO32- species, and the peak at 169.7eV is assigned to SO42- species. 8


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This

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implies that SO32- and SO42- species coexist. Considering the binding energy of Mn 2p3/2 for Mn/ACH-S and CeMn/ACH-S does not change much compared to the fresh samples, it can be concluded that the ammonium sulphite and ammonium sulfate salts are deposited on Mn/ACH-S and CeMn/ACH-S. As shown in Table 2, a certain amount of sulfur and nitrogen are detected besides manganese and cerium elementals on the poisoned samples, it further confirms the formation of the ammonium sulphite and ammonium sulfate salts on the surface of catalyst in the presence of SO2, which causes a decreased SCR activity. Additionally, the sulfur contents of Mn/ACH-S-T and CeMn/ACH-S-T (Table 2) are relatively higher than those of their fresh samples, which explains a slightly decreased NO conversions after thermal regeneration (Figure 1). Furthermore, it is also observed from Table 2 that the ammonium salts concentrations on Mn/ACH-S surface is much higher than those on CeMn/ACH-S, indicating that the addition of CeO2 inhibit the deposition of these ammonium salts to some extent. The deactivation of SO2 should be attributed to two main reasons. On one hand, the formed ammonium sulphite and ammonium sulfate salts on the catalyst surface would block the catalyst’s micropores. On the other hand, there exists the competitive adsorption between SO2 and NH3. In the above transient experiments the outlet SO2 concentration was also monitored, and the results are presented in Figure 1. From Figure 1 stage C，the release of the small amount of SO2 can be still observed after the removal of SO2 from the feed. Since these ammonium sulfate salts do not decompose or be reduced at 160 oC, the release of SO2 may result from SO2 adsorbed on the catalyst. That is to say, SO2 is adsorbed on the catalyst’s surface or pores during the NO removal process in the presence of SO2. It is generally agreed that NH3 adsorption is a key step and NO adsorbed hardly participates in the SCR reaction. 9


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These


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information suggests that the competitive adsorption between SO2 and NH3 reduces the number of active sites for the adsorption of NH3,

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which leads to a decrease in

the adsorption amount of NH3, and hence decrease in SCR activity. The further experiments are underway to elucidate the competitive adsorption between SO2 and NH3. 4. Conclusions (1) SO2 significantly deactivates the SCR activity of Mn/ACH at low temperature, but the resistance to SO2 poisoning can be enhanced for CeO2 modified Mn/ACH catalyst. At 160 oC, in the presence of SO2, NO conversion decreases quickly from 63% to about 33% for 120 min over Mn/ACH, while NO conversion decreases from 84% to about 44% for 420 min over CeMn/ACH. (2) The ammonium sulphite and ammonium sulfate salts are deposited on the surface of Mn/ACH and CeMn/ACH during the NO removal process in the presence of SO2. And the concentrations of these salts on the poisoned CeMn/ACH surface are greatly lower than those on the poisoned Mn/ACH. (3) The formed ammonium sulphite and ammonium sulfate block the micropores of the catalyst, causing a decreased micropore surface area and volume and thus a decreased SCR activity. Because the addition of CeO2 inhibits the deposition of these salts, CeMn/ACH exhibits higher resistance to SO2 poisoning. Furthermore, the competitive adsorption between SO2 and NH3 also decreases the SCR activities. Therefore, it can be concluded that CeMn/ACH catalyst is suitable for practical application in the feed stream containing no or low SO2 concentration. (4) The SCR activities of Mn/ACH and CeMn/ACH can be effectively recovered after the thermal regeneration at 350 oC. This is because the ammonium sulphite and ammonium sulfate salts can be fully removed by the thermal regeneration. 10


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Acknowledgments The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (51472086, 20806024, 51002051), the Natural Science

Foundation

of

Shanghai

City

(12ZR1407200)

program of Shanghai Subject Chief Scientist (B type, 13XD1424900).

11


and


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Table 1 Pore parameters of Mn/ACH and CeMn/ACH catalysts before and after poisoned by SO2 Samples Mn/ACH-F Mn/ACH-S Mn/ACH-S-T CeMn/ACH-F CeMn/ACH-S CeMn/ACH-S-T

SBET

(m2/g) ) 601 412 574 519 442 506

Smic

(m2/g) 415 305 414 381 309 380

Vtotal

(cm3/g) 0.321 0.261 0.299 0.272 0.233 0.261

Vmic

(cm3/g) 0.219 0.152 0.219 0.202 0.164 0.201

SBET: BET surface; Smic: Micropore surface area; Vtotal: Total pore volume; Vmic: Micropore volume F: fresh catalyst; S: catalyst poisoned by SO2; T: catalyst regenerated at 350 oC in N2

Table 2 Surface atomic concentrations of Mn/ACH and CeMn/ACH before and after poisoned by SO2 Samples Mn/ACH-F Mn/ACH-S Mn/ACH-S-T CeMn/ACH-F CeMn/ACH-S CeMn/ACH-S-T

Ce 0 0 0 1.62 1.38 0.55

Surface atomic concentration (%) Mn C O S 12.58 69.58 17.83 0 8.63 68.47 19.89 1.47 1.63 88.49 8.89 0.79 5.87 51.30 41.21 0 5.19 52.55 39.63 0.36 0.86 82.27 16.06 0.15

F: fresh catalyst; S: catalyst poisoned by SO2; T: catalyst regenerated at 350 oC in N2

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N 0 1.55 0.2 0 0.88 0.11

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Lists of figures Figure 1 Effect of SO2 on NO removal activities of Mn/ACH and CeMn/ACH catalysts at 160 oC. Figure 2 Nitrogen adsorption-desorption isotherms of Mn/ACH and CeMn/ACH before (F) and after (S) poisoned by SO2. Figure 3 XRD patterns of Mn/ACH and CeMn/ACH before (F) and after (S) poisoned by SO2. Figure 4 XPS spectra of Mn2p for the fresh and poisoned catalysts (a) Mn/ACH and (b) CeMn/ACH. Figure 5 XPS spectra of Ce3d for the fresh and poisoned CeMn/ACH catalysts. Figure 6 XPS spectra of S2p for the poisoned Mn/ACH and CeMn/ACH catalysts.

13



200

NO conversion (%)

100

-SO2

80 CeMn/ACH

60 40

Mn/ACH

20 A

0 0

C

B 240

480

+300ppm SO2

o 350 C regeneration in N2 for 2 hour

+300ppm SO2

D 720

SO2 concentration (ppm)

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150

100

50

E

F 960

0 1200

Time on stream (min) Figure 1 Effect of SO2 on NO removal activities of Mn/ACH and CeMn/ACH catalysts at 160 oC.

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250

3 Quantity adsorbed (cm /g)

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200 150 100 Mn/ACH-F Mn/ACH-S CeMn/ACH-F CeMn/ACH-S

50 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0) Figure 2 Nitrogen adsorption-desorption isotherms of Mn/ACH and CeMn/ACH before (F) and after (S) poisoned by SO2.

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*:Cubic CeO2 #: Mn3O4

Intensity (a.u.)

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* CeMn/ACH-S *

#

*

*

CeMn/ACH-F

#

Mn/ACH-S

# #

# ##

0

10

20

30

40

50

60

Mn/ACH-F

70

80

90

o

2theta( ) Figure 3 XRD patterns of Mn/ACH and CeMn/ACH before (F) and after (S) poisoned by SO2.

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(a)

Mn 2p3/2

Mn 2p1/2

641.7

Intensity (a.u.)

653.7

Mn/ACH-F

642.1

Mn/ACH-S

660

655

650

645

640

635

Binding Energy (eV)

(b)

Mn 2p3/2

Mn 2p1/2

641.9 653.6

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CeMn/ACH-F

CeMn/ACH-S

660

655

650

645

640

635

Binding Energy (eV) Figure 4 XPS spectra of Mn 2p for the fresh and poisoned catalysts (a) Mn/ACH and (b) CeMn/ACH.

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u

'''

''

u

u

'

u

v

'''

v

''

v

'

v

CeMn/ACH-S

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ce 3d '''

u

''

u

' u u

v

'''

'' v v

'

v

CeMn/ACH-F

Ce 3d 920

910

900

890

880

870

Binding Energy (eV) Figure 5 XPS spectra of Ce 3d for the fresh and poisoned CeMn/ACH catalysts.

18


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S 2p

168.5eV 169.7eV

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mn/ACH-S 168.5eV

169.7eV

CeMn/ACH-S 174

172

170

168

166

164

Binding Energy (eV) Figure 6 XPS spectra of S 2p for the poisoned Mn/ACH and CeMn/ACH catalysts.

19



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References (1) Bosch, H.; Janssen, F. Catalytic reduction of nitrogen oxides: a review on the fundamentals and technology. Catal. Today 1988, 2, 369. (2) Williams, J. L. Monolith structures, materials, properties and uses. Catal. Today 2001, 69, 3. (3) Koebel, M.; Elsener, M. Selective Catalytic reduction of NO over commercial DeNOx Catalysts: comparison of the measured and calculated performance. Ind. Eng. Chem. Res. 1998, 37, 327. (4) Nova, I.; Lietti, L.; Tronconi, E.; Forzatti, P. Dynamics of SCR reaction over a TiO2-supported vanadia-tungsta commercial catalyst. Catal. Today 2000, 60, 73. (5) Su, J.; Liu, Q.; Liu, Z.; Huang, Z. Honeycomb CuO/Al2O3/Cordierite catalyst for selective catalytic reduction of NO by NH3-effect of Al2O3 coating. Ind. Eng. Chem. Res. 2008, 47, 4295. (6) Liu, Q.; Liu, Z.; Zhu, Z.; Xie, G.; Wang, Y. Al2O3-coated honeycomb cordierite-supported CuO for simultaneous SO2 and NO removal from flue gas: effect of Na2O additive. Ind. Eng. Chem. Res. 2004, 43, 4031. (7) Kijlstra,W. S.; Biervliet, M.; Poels, E. K.; Bliek, A. Deactivation by SO2 of MnOx/Al2O3 catalysts used for the selective catalytic reduction of NO with NH3 at low temperatures. Appl. Catal., B 1998, 16, 327. (8) Xie, G.; Liu, Z.; Zhu, Z.; Liu, Q.; Ge, J.; Huang, Z. Simultaneous removal of SO2 and NOx from flue gas using a CuO/Al2O3catalyst sorbent––I. Deactivation of SCR activity by SO2 at low temperatures. J. Catal. 2004, 224, 36. (9) Wu, Z.; Jin, R.; Wang, H.; Liu, Y. Effect of ceria doping on SO2 resistance of Mn/TiO2 for selective catalytic reduction of NO with NH3 at low temperature. Catal. Commun. 2009, 10, 935. 20


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