New Insight into SO2 Poisoning and Regeneration of CeO2âWO3

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New Insight into SO2 Poisoning and Regeneration of CeO2-WO3/ TiO2 and V2O5-WO3/TiO2 Catalysts for Low-temperature NH3-SCR Liwen Xu, Chizhong Wang, Huazhen Chang, Qingru Wu, Tao Zhang, and Junhua Li Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01990 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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Environmental Science & Technology

1 2 3 4

New Insight into SO2 Poisoning and Regeneration of CeO2-WO3/TiO2 and V2O5-WO3/TiO2 Catalysts for Low-temperature NH3-SCR

5 6

Liwen Xua#, Chizhong Wangb#, Huazhen Changa#∗, Qingru Wub, Tao Zhanga, Junhua Lib

7

a

8

China

9

b

10

School of Environment and Natural Resources, Renmin University of China, Beijing 100872,

State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC),

School of Environment, Tsinghua University, Beijing 100084, China

11

∗

Corresponding author. Tel.: +86-10-62512572; E-mail address: [email protected] (H. Chang). 1

ACS Paragon Plus Environment


12

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ABSTRACT

13

In this study, the poisoning effects of SO2 on the V2O5-WO3/TiO2 (1%VWTi) and

14

CeO2-WO3/TiO2 (5%CeWTi) selective catalytic reduction (SCR) catalysts were

15

investigated in the presence of steam, and also the the regeneration of deactivated

16

catalysts was studied.. After pretreating the catalysts in a flow of NH3 + SO2 + H2O + O2

17

at 200 °C for 24 h, it was observed that the low-temperature SCR (LT-SCR) activity

18

decreased significantly for the 1%VWTi and 5%CeWTi catalysts. For 1%VWTi,

19

NH4HSO4 (ABS) was the main product detected after the poisoning process. Both of

20

NH4HSO4 and cerium sulfate species were formed on the poisoned 5%CeWTi catalyst,

21

indicating that SO2 reacted with Ce3+ /Ce4+, even in the presence of high concentration of

22

NH3. The decrease of BET specific surface area, NOx adsorption capacity, the ratio of

23

chemisorbed oxygen, and reducibility were responsible for the irreversible deactivation

24

of the poisoned 5%CeWTi catalyst. Meanwhile, the LT-SCR activity could be recovered

25

for the poisoned 1%VWTi after regeneration at 400 °C, but not for the 5%CeWTi catalyst.

26

For industrial application, it is suggested that the regeneration process can be utilized for

27

1%VWTi catalysts after a period of time after NH4HSO4 accumulated on the catalysts.

28

Keywords: SO2 poisoning, low-temperature SCR, V2O5-WO3/TiO2, CeO2-WO3/TiO2,

29

regeneration

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1. INTRODUCTION

31

Selective catalytic reduction of NOx by NH3 at low temperature (LT-SCR) attracted

32

great interest from industry and researchers over the past two decades. LT-SCR is a

33

potential technology for eliminating NOx from the emissions of power plants and other

34

stationary sources. In addition to V2O5-WO3 (MoO3) /TiO2, large quantities of catalysts

35

have been developed for LT-SCR, such as MnOx-CeO2,

36

(or Mo) mixed oxides,

37

capacity and remarkable redox properties, is a promising additive and active component

38

for LT-SCR.10 It has been reported that CeW/Ti, CeMo/Ti and even Ce/Ti catalysts

39

showed excellent SCR activity over a broad temperature window. 10-15 Ce-M/Ti (M=W, or

40

Mo) catalysts are assumed to be potential candidates to replace the poisonous commercial

41

V2O5-WO3 (or MoO3)/TiO2 catalysts.

5-7

and Cu-SAPO-34.

8, 9

1, 2

Fe-Mn-Ti, 3 Fe-W-O, 4 Ce-W

CeO2, exhibiting a high oxygen storage

42

A challenge for the industrial application of LT-SCR is SO2 poisoning on catalysts. In

43

the presence of SO2, the SCR performance decreases continuously for most catalysts at

44

temperatures below 300 °C.

45

reasons. First, SO2 can react with NH3 and H2O to produce NH4HSO4 (ABS), which

46

easily obstructs the active sites. Li et al.

47

VW/Ti catalyst after SO2 sulfation at low temperature. The effect of NH4HSO4 gradually

48

attracted attention, and some studies have reported its poisoning effects on LT-SCR

16, 17

This deactivation effect could be attributed to two

18, 19

reported that NH4HSO4 could form over a

3



19, 20

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49

catalysts.

50

investigate the effect of NH4HSO4 on SCR catalysts. However, the deactivation

51

mechanism of SO2 in the SCR reactor from the actual flue gas in the SCR reactor might

52

be much different from that of NH4HSO4 impregnation. Second, SO2 can react directly

53

with active sites to produce thermally stable sulfates, which interrupts the redox property

54

of SCR catalysts, leading to irreversible deactivation of the catalysts. 21 Although SO2 has

55

a promotion effect on SCR activity over CeO2 because of the initial sulfation of CeO2, 22

56

it is believed that Ce-based catalysts can be deactivated by SO2 at low temperature. It has

57

been proposed that surface Ce(SO4)2 forms after SO2 poisoning, and it is the most stable

58

poisoning species on CeO2.

59

determined by the length of time exposed to SO2, the concentration of SO2, co-existence

60

of O2 and H2O, and reaction temperatures.

Moreover, many researchers have used NH4HSO4 impregnation to

23

The extent of the deactivation of LT-SCR catalysts is

61

To the best of our knowledge, there is a trade-off relations between the sulfur

62

poisoning problem of catalysts and stable operation of LT-SCR. Deposition of ABS on

63

active sites and generation of metal sulfates result in a decrease of activity over LT-SCR

64

catalysts, and it remains a problem how to regenerate deactivated catalysts. Jin et al.

65

reported that the deposited ammonium (bi)sulfates on Mn-Ce/TiO2 catalyst can be

66

completely removed by a water-washing treatment. In our previous study, it was

67

proposed that heat treating with H2 is an effective way to regenerate a SO2-poisoned

24

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25

68

GeO2-CeO2-WO3 catalyst.

69

SO2 and H2O poisoned catalysts at low temperature.

There are limited systematic studies on the regeneration of

70

In this study, the effects of SO2 and H2O in situ poisoning on V2O5-WO3/TiO2 and

71

CeO2-WO3/TiO2 catalysts were investigated. The flue gas conditions in stationary sources

72

(such as power plant) were simulated. Various characterizations were employed to

73

explore the poisoning mechanism of SO2 over the two kinds of catalysts. Furthermore, a

74

simple regeneration method was applied for poisoned V2O5-WO3/TiO2 catalysts.

75

2. EXPERIMENTAL SECTION

76

2.1. Catalyst synthesis

77

CeO2-WO3/TiO2 and V2O5-WO3/TiO2 catalysts were prepared by the impregnation

78

method, using Degussa AEROSIL TiO2 P25 as the support. The 5 wt% CeO2-WO3/TiO2

79

(denoted as 5%CeWTi) catalyst (the mass ratio of WO3 is 5 wt%) was prepared by

80

impregnating TiO2 powder with an aqueous solution of cerium nitrate (Ce(NO3)3•6H2O),

81

ammonium tungstate hydrate ((NH4)10W12O41•xH2O) and oxalic acid (H2C2O4•2H2O).

82

The combined solution was stirred for 2 h, then dried at 110 °C and calcined at 500 °C

83

for 4 h in air. The 1 wt% V2O5-WO3/TiO2 (denoted as 1%VWTi) catalyst (the mass ratio

84

of WO3 is 5 wt%) was prepared by the same method using ammonium vanadate

85

(NH4VO3), ammonium tungstate hydrate ((NH4)10W12O41•xH2O) and oxalic acid

86

(H2C2O4•2H2O) as precursors. 5



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In situ poisoning of the catalysts was performed by using a fixed bed quartz reactor

88

for 24 h at 200 °C. The inlet gas was 500 ppm NH3, 500 ppm SO2, 5% H2O, 5% O2, and

89

N2 as the balance gas. The samples could be denoted as –p, for example, 1%VWTi-p.

90

Regeneration of poisoned catalysts was performed with the same fixed bed quartz

91

reactor. The catalysts were heated to and kept at 400 °C for 30 min. The inlet gas was N2.

92

The samples are denoted as –r, for example, 1%VWTi-r.

93

2.2. Catalytic performance

94

Measurements of the steady-state NH3-SCR activity were carried out in a fixed bed

95

quartz reactor with the outlet gas monitored by a NOx analyzer (Eco Physics, CLD

96

822Mh). The inlet gas was 500 ppm NO, 500 ppm NH3, 5% O2 and N2 as the balance gas.

97

The gas hourly space velocity (GHSV) was approximately 30,000 h-1. All of the tubings

98

in the reactor system heated to 120 °C to prevent water condensation and ammonium

99

nitrate deposition. The NOx conversion rate was calculated according to Eqs. (1)

100

conversion % = 1 −

× 100%

(1)

101 102

2.3. Catalyst characterization

103

Thermal gravimetric analysis (TGA) was carried on a TGA/DSC 1 STARe. The TGA

104

furnace was first preheated to 120 °C and kept at 120 °C for 20 min; then, the TGA

105

furnace was heated to 1000 °C at a heating rate of 20 °C/min in a flow of air. 6


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106

H2-TPR was carried out with a Micromeritics AutoChem II 2720 instrument. The

107

sample was first preheated to 200 °C in a He flow for 1 h. Then the TCD signal stabilized

108

in a H2 flow and the sample was heated to 1000 °C with a heating rate of 10 °C/min. The

109

amount of H2 consumed as a function of temperature was monitored by the signal of the

110

TCD.

111

Temperature programmed desorption of NO+O2 (NO+O2-TPD) was carried out with

112

a NOx analyzer (Eco Physics, CLD 822Mh). The sample was first preheated to 200 °C in

113

a N2 flow for 1 h. Then the sample adsorbed NO+O2 ([NO] = 500 ppm, [O2] = 5%) until

114

saturated in room temperature. After swept with N2 for 1 h, the sample was heated to

115

400 °C with a heating rate of 10 oC/min. The NO and NO2 concentrations were

116

monitored by the testing instrument as a function of temperature.

117

The experiment details of XRD, NH3-TPD, and XPS could be found in SI.

118

3. RESULTS

119

3.1. Effects of In Situ SO2 + H2O Poisoning and Regeneration on SCR Performance.

120

Figure 1 and Figure S1 shows the NH3-SCR activities over 1%VWTi and 5%CeWTi

121

catalysts. Fresh 1%VWTi and 5%CeWTi catalysts exhibited excellent NOx conversion

122

(higher than 85%) over the temperature range of 250-400 °C, which is in agreement with

123

previous studies.11,

124

significantly over both of the catalysts, especially at low temperature. It was observed

26

After in situ poisoning for 24 h, NOx conversion decreased

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that the decreasing extent of NOx conversion over the 5%CeWTi-p catalyst was less than

126

that over the 1%VWTi-p catalyst at 200-300 °C, suggesting that the poisoning effect was

127

slower on the 5%CeWTi-p catalyst. A simple regeneration treatment was performed by

128

heating the poisoned catalysts in N2 atmosphere at 400 °C. The low-temperature SCR

129

(LT-SCR) activity of the 1%VWTi-r catalyst was well recovered compared with the

130

poisoned sample. NOx conversion was slightly higher than the fresh sample at

131

temperatures lower than 300 °C. By contrast, the regeneration process had little impact

132

on the activity of the 5%CeWTi-r catalyst, indicating that the in situ poisoning effects are

133

much different between the 1%VWTi and 5%CeWTi catalysts.

134

3.2. Characterization of Poisoned and Regenerated SCR Catalysts.

135

3.2.1 BET and XRD.

136

The BET specific surface areas of different samples are displayed in Table 1. It was

137

observed that the surface area decreased to 46.2 m2/g for 1%VWTi-p and recovered to

138

49.6 m2/g after regeneration. By contrast, in situ poisoning had a significant impact on

139

the surface area of the 5%CeWTi catalyst, which decreased from 54.0 m2/g to 34.9 m2/g.

140

Meanwhile, the surface area of 5%CeWTi-r couldn’t be recovered after the regeneration

141

process. XRD analysis was performed to examine the crystal structures of poisoned and

142

regenerated catalysts. The XRD patterns shown in Figure S2 reveal that no diffractions

143

could be assigned to NH4HSO4, VOSO4, Ce(SO4)2 or Ce2(SO4)3 were observed in the 8


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1%VWTi and 5%CeWTi catalysts after in situ poisoning and the subsequent regeneration

145

process. Therefore, it is suggested that in situ poisoning results in the formation of

146

amorphous surface species rather than crystallized phases.

147

3.2.2 TGA analysis.

148

TGA was carried out to study the formation of the surface species after in situ

149

poisoning. Figure 2 shows the mass loss of the poisoned 1%VWTi and 5%CeWTi

150

catalysts via TGA analysis. The 1%VWTi-p catalyst exhibited a dramatic weight loss at

151

350-500 °C and remained stable at higher temperatures. This weight loss is in accordance

152

with the decomposition temperature of pure NH4HSO4, implying the formation of

153

NH4HSO4 on the 1%VWTi-p catalyst. Decomposition of NH4HSO4 has also been

154

investigated by other researchers.

155

loss occurred at 300-500 °C and 650-800 °C, which resembles the TGA profile of pure

156

Ce2(SO4)3 with rising temperature. In addition, Figure 2 shows no evident weight loss for

157

the TiO2-p sample, ruling out the formation of titanium sulfate species. It is therefore

158

suggested that both NH4HSO4 and cerium sulfates species were likely to form on the

159

5%CeWTi-p catalyst. The decomposition temperature of cerium sulfate was lower on the

160

5%CeWTi-p catalyst than pure Ce2(SO4)3 and Ce(SO4)2 used in our previous study. 2 One

161

possible explanation for this result is that amorphous surface sulfate species formed on

162

the 5%CeWTi-p catalyst.

27

For the 5%CeWTi-p catalyst, two stages of weight

9



163

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3.2.3 XPS.

164

To examine the chemical status of the surface species after poisoning and

165

regeneration, XPS analysis was performed on fresh, poisoned and regenerated catalysts,

166

and the results are shown in Figure 3 and Table 1. In Figure 3(a) and (b), the peaks

167

located at 169.1 eV are attributed to the S 2p signal of SO42-, 22 indicating the presence of

168

sulfur species on both the 1%VWTi-p and 5%CeWTi-p catalysts. SO32- species likely did

169

not exist on the catalyst surface according to from the S 2p spectra,

170

presence of O2 during the in situ poisoning process. Table 1 reveals that the surface

171

atomic ratio of sulfur on the 5%CeWTi-p catalyst (3.15%) was much larger than that on

172

the 1%VWTi-p catalyst (1.72%). In accordance with the higher weight loss of the

173

5%CeWTi-p catalyst in the aforementioned TGA result, larger amounts of sulfur species

174

formed on the 5%CeWTi-p catalyst in comparison to the 1%VWTi-p catalyst. After the

175

regeneration process, the atomic ratio of surface sulfur over the 1%VWTi-r (0.95 %) and

176

5%CeWTi-r (1.88 %) catalysts decreased, suggesting that the unstable surface sulfates

177

(i.e., NH4HSO4) decomposed. The bulk S content of samples was determined by ICP.

178

The content of sulfur on the 5%CeWTi-p catalyst (1.35 wt.%) was also much larger than

179

that on the 1%VWTi-p catalyst (0.562 wt.%). After regeneration, the S content was still

180

higher on the 5%CeWTi-r catalyst (0.548 wt.%) than that of the 1%VWTi-r catalyst

181

(0.0985 wt.%). Note that the sulfur species were still found over the 5%CeWTi-r catalyst

28

owing to the

10


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182

and that sulfur species with strong metal-sulfur bonding were likely to be highly

183

preserved after the regeneration process.

184

Figure 3(c) shows overlapping V 2p signals from the V3+, V4+ and V5+ species on the

185

1%VWTi catalysts. The V 2p peak of the 1%VWTi catalyst shifted slightly to higher

186

bonding energy after the in situ poisoning and shifted back after regeneration. This is

187

inconsistent with the peaks shifts of W 4f and Ti 2p in the fresh, poisoned and

188

regenerated 1%VWTi catalysts (Figure S3). These changes might be due to the formation

189

of surface sulfate species on the 1%VWTi-p catalyst. Despite the possible formation of

190

VOSO4 on the 1%VWTi-p catalysts as suggested in the previous studies,

191

strength between surface V (or W, Ti) and S was so weak that they were broken in the

192

regeneration process. Thus, the SCR activity was well recovered after the regeneration of

193

the 1%VWTi catalyst.

29

the bonding

194

As shown in Figure 3(d) and Table 1, the surface ratio of Ce3+/(Ce3+ + Ce4+) of the

195

5%CeWTi catalyst (23.6%) was increased dramatically after in situ poisoning (33.8%).

196

Since SO42- (formed from the oxidation of SO2) can be deposited on CeO2 rather than on

197

WO3 and TiO2, the increased ratio of Ce3+ demonstrated that formation of Ce2(SO4)3 was

198

possible on the 5%CeWTi-p catalyst. Despite the high ratio of surface Ce3+, formation of

199

Ce4+ sulfate species, i.e., Ce(SO4)2, was also possible during in situ poisoning. After the

200

regeneration process, the ratio of Ce3+ / (Ce3+ + Ce4+) (see Table 1) and the peak positions 11



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201

of W 4f and Ti 2p (Figure S3) remained unchanged, indicating the high stability of the

202

surface cerium sulfates. Therefore, unlike the 1%VWTi catalyst, the degradation of the

203

SCR activity might be related to the formation of both NH4HSO4 and cerium sulfates

204

over the 5%CeWTi-p catalyst. The LT-SCR activity could not be recovered with the

205

retained cerium sulfates on the catalyst surface after regeneration at 400 °C.

206

The O 1s spectra are shown in Figure 3 (e and f). The results revealed that both of

207

chemical adsorbed oxygen (Oα) and lattice oxygen (Oβ) were presented on the 1%VWTi

208

and 5%CeWTi catalysts. 2 After in situ poisoning, the ratios of Oα / (Oα + Oβ) decreased

209

and the O 1s peaks shifted to higher binding energies on the 1%VWTi-p and 5%CeWTi-p

210

catalysts. It is proposed that chemisorbed oxygen is an important active species in the

211

SCR reaction.

212

restrained SCR activity after in situ poisoning. The ratio of Oα / (Oα + Oβ) increased again

213

on 1%VWTi-r catalyst after heating regeneration. It was also observed that the decrease

214

of Oα / (Oα + Oβ) could not be recovered for the 5%CeWTi-r catalyst due to the formation

215

of cerium sulfates after in situ poisoning.

216

3.2.4 H2-TPR.

30

The decrease of the ratio of Oα should be an important factor for the

217

H2-TPR was performed to investigate the redox properties of the 1%VWTi and

218

5%CeWTi catalysts. Figure 4 shows the H2-TPR profiles of fresh, poisoned and

219

regenerated samples. In Figure 4(a), the reduction peak

at the temperatures below 12


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220

600 °C was ascribed to the coupled reduction of V5+ to V3+ and W6+ to W4+ in the fresh

221

1%VWTi catalyst, while the peak at a temperature of approximately 765 °C was caused

222

by the reduction of W4+ to metallic W. 26 After in situ poisoning, a strong peak appeared

223

at 497 °C and the H2 consumption of the first reduction peak increased significantly due

224

to the coupled reduction of V5+, W6+ and SO42- species. The peak positions of the V5+→

225

V3+ reduction remained nearly unchanged after poisoning and regeneration, indicating

226

that in situ poisoning had little effect on the reducibility of the 1%VWTi-p catalysts. The

227

intensity of the first peak of 1%VWTi-r decreased after regeneration, implying that the

228

sulfate species decomposed during the heating treatment. It is envisioned that

229

redispersion of vanadium species occurred after the regeneration with a H2 consumption

230

at temperature lower than 400 C in the 1%VWTi-r. The peak at high temperature shifted

231

to a lower temperature after poisoning and was restored after regeneration, confirming

232

that the effect of poisoning disappeared on the 1%VWTi-r catalyst.

233

For the fresh 5%CeWTi catalyst (Figure 4(b)), a faint peak at 487 °C appeared, which

234

could be attributed to reduction of surface Ce4+ and W6+, and a strong peak at 788 °C

235

ascribing to bulk Ce4+ to Ce3+ and W4+ to metallic W. 6, 31 In the profiles of 5%CeWTi-p

236

and 5%CeWTi-r samples, a sharp peak appeared at ca. 565 °C was caused by the coupled

237

reduction of Ce4+, W6+ and SO42-.

238

peak of the surface SO42- revealed that the sulfate species that bonded to Ce were very

32

After regeneration, the existence of the reduction

13



239

stable on 5%CeWTi-r catalyst.

240

3.2.5 Adsorption of NH3 and NOx.

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Adsorption of NH3 and NOx on the catalysts plays an important role in the NH3-SCR

242

activity. NH3-TPD was performed to titratethe surface acidity of catalysts before and

243

after poisoning. As shown in Figure S4 and Table 1, the amounts of NH3 adsorption

244

increased slightly in the 1%VWTi-p and 5%CeWTi-p catalysts, which might be due to

245

the abundant Brønsted acid sites resulted from the formation of SO42- after poisoning.

246

Figure 5 shows the NO+O2-TPD profiles of fresh, poisoned and regenerated catalysts.

247

For the fresh 1%VWTi and 5%CeWTi catalysts, the desorption peaks were attributed to

248

weakly adsorbed NOx (NO and NO2), nitrite and nitrate species. The desorption peaks

249

decreased significantly after in situ poisoning, revealing that the adsorption of NOx was

250

obviously affected by poisoning. Especially for the 1%VWTi-p catalyst, nearly no

251

desorbed NOx was detected over the test temperature range, which might be responsible

252

for the inhibited SCR performance at low temperature. The degradations of NOx

253

adsorption were primarily caused by the formation of surface sulfate species, which

254

suppressed the adsorption of NO and oxidation of NO to NO2. After regeneration in N2,

255

the NOx adsorption capacity was restored to a certain extent on 1%VWTi-r. However, the

256

amount of desorbed NOx decreased further on 5%CeWTi-r.

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257

4. DISCUSSION

258

4.1 Deficiency of CeWTi and other metal oxides catalysts for LT-SCR.

259

SO2 poisoning is possibly the most important obstacle for LT-SCR. Generally, it has

260

been proposed that sulfur poisoning of LT-SCR catalysts is attributed to the formation of

261

NH4HSO4 and metal sulfates in the presence of O2 and H2O. 33, 34

262

The formation of NH4HSO4 and cerium sulfate species on the poisoned 5%CeWTi

263

catalyst was confirmed by the TGA, XPS and H2-TPR results, which indicated that SO2

264

inevitably reacted with Ce3+ /Ce4+ even in the presence of abundant NH3. It is notable that

265

ABS decomposed at approximately 400 °C, while the cerium sulfate species did not

266

decompose until 600 °C (Figure S5). In H2-TPR, a strong reduction peak at

267

approximately 579 °C was attributed to the cerium sulfates species. This was also

268

confirmed in the sulfation of CeO2 in our previous studies.

269

XPS spectra further demonstrated the formation of the SO42- on the 5%CeWTi-p catalyst

270

(see Figure 3). The ratio of Ce3+ / Ce3+ + Ce4+ was not affected by the regeneration

271

process, demonstrating that these sulfates were stable at 400 °C.

272

22, 32

The S 2p signal in the

For cerium-based catalysts, some debate still remains on the resistance to SO2. It has 12

273

been reported that CeW catalyst has a superior resistance to sulfur poisoning.

274

found that CeO2 can promote the oxidation of SO2 to SO3 on the VTi catalyst, resulting in

275

the formation of cerium sulfates.

29

Others

After exposure to SO2 and O2 at 300 °C for 8 h, the 15



Page 16 of 33

276

formation of cerium sulfate species on CeO2 and their promotional effect on SCR activity

277

were investigated extensively in our previous study. 22 In the present work, the amount of

278

adsorbed NH3 per surface area increased after poisoning, indicating that the formation of

279

sulfate species increased the acidity of the 5%CeWTi-p catalyst.

280

However, after pretreatment under a more substantial flue gas condition of NH3 +

281

SO2 + H2O + O2 (which was equivalent to actual flue gas) at 200 °C for 24 h, it was

282

found that the poisoning effects on the LT-SCR performance over the 5%CeWTi catalysts

283

could not be ignored. The surface area obviously decreased after poisoning for both the

284

5%CeWTi-p and 1%VWTi-p catalyst, and could not be recovered by regeneration for the

285

5%CeWTi-r sample. The amount of NOx adsorption degraded dramatically due to the

286

decreased surface area and, more importantly, due to the coverage of NOx adsorption sites

287

by the formed sulfates species (see Figure S6). The adsorption capacity could not be

288

restored on the 5%CeWTi-r catalyst since the sulfate species were still present after

289

regeneration. Moreover, the ratio of Oα/Oα + Oβ decreased after in situ poisoning. It is

290

believed that Oα is crucial for the oxidation of NO into NO2, a major step of the

291

“fast-SCR” process.

292

temperature after poisoning in H2-TPR, demonstrating that the reducibility decreased

293

after poisoning and remained after regeneration for the 5%CeWTi catalyst (see Figure 4).

294

All of the above results indicate that the 5%CeWTi catalyst can be deactivated

35

The position of the first reduction peak shifted to a high

16


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295

irreversibly after exposure to a gas flow of NH3 + SO2 + H2O + O2 at 200 °C. The

296

poisoning effect of a gas flow of NO + NH3 + SO2 + H2O + O2 on SCR activity was also

297

irreversibe over the 5%CeWTi catalyst (see Figure S7). In this work, it is important to

298

find that cerium sulfate species could form even in the presence of high concentration of

299

NH3. The possible mechanism of sulfate poisoning and regeneration is shown in Scheme

300

1.

301

SO2 poisoning is one of the most important problems for LT-SCR catalysts, which is

302

the major obstacle for their industrial application. It has been reported that the formation

303

of MnSO4 is the main reason for the deactivation of MnOx/Al2O3 in the presence of SO2

304

and that MnSO4 only decomposes at temperatures higher than 1290 °C.

305

have identified the formation of both ammonium sulfate and Cu sulfate species in

306

Cu-SAPO-34.

307

decomposition of Cu sulfate species. Despite the introduction of additives that can react

308

more easily with SO2, which can prolong the poisoning of active sites, 2, 36 the formation

309

of metal sulfates species ultimately leads to the irreversible deactivation of LT-SCR

310

catalysts.

311

4.2 Regeneration of VWTi catalysts (Recycling of VWTi for LT-SCR?).

312 313

8

34

Researchers

It was found that a temperature higher than 480 °C is needed for the

Although the addition of W to V2O5/TiO2 catalysts is believed to improve the sulfur resistance in NH3-SCR,

33, 37, 38

the effect of sulfur poisoning could not be neglected for 17



19, 29

Page 18 of 33

314

the LT-SCR activity of VWTi.

315

was also significantly affected by poisoning in a stream of NH3 + SO2 + H2O + O2 at

316

200 °C for 24 h. The deactivation of 1%VWTi was primarily due to the deposition of

317

NH4HSO4 on the catalyst surface. The BET surface area obviously decreased and the

318

NOx adsorption capacity notably diminished. After poisoning, 1%VWTi-p showed almost

319

no capacity for NOx adsorption, which might be the most important reason for the

320

decrease of LT-SCR activity. Meanwhile, sulfur poisoning had little effect on the crystal

321

structure, surface V species and reducibility of the 1%VWTi catalyst.

322

In the present work, the SCR activity of 1%VWTi

It has been reported that the LT-SCR activity can be enhanced as the increase of 39

323

vanadium loading on VWTi catalysts,

324

the catalysts is intensified at high vanadium loading.

325

effect of SO2 could not be avoided in LT-SCR over high V2O5 loading catalysts. In the

326

present work, after regeneration at 400 °C, the NH4HSO4 species decomposed on the

327

1%VWTi-r catalyst. The BET surface area and NOx adsorption capacity recovered and a

328

certain amount of sulfates (possibly VOSO4) remained on the catalyst surface, resulting

329

in an even better SCR activity at low temperature. For industrial application, it is

330

suggested that the direct application of V2O5-WO3/TiO2 catalyst in LT-SCR is also

331

difficult, only if regenerating after a period of time as NH4HSO4 accumulates on the

332

catalyst surface in the flue gas.

whereas, SO2 oxidation and sulfur poisoning of 40

It seems that the deactivation

18


Page 19 of 33


333 334

AUTHOR CONTRIBUTIONS

335

#

336

ACKNOWLEDGEMENTS

L.X., C.W. and H.C. have equal contributions.

337

This work was financially supported by the National Key R&D Program of China

338

(No. 2016YFC0203900, 2016YFC0203901), National Natural Science Foundation of

339

China (Grant No. 51778619, 21577173), Postdoc Grant (No. 043206019) and the Sino

340

Japanese Cooperation Project (Grant No. 2016YFE0126600).

341

Supporting Information Available.

342

Characterization method, XPS, XRD, TPD and additional results are shown. This

343

information is available free of charge via the Internet at http://pubs.acs.org/.

344 345

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346 347

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464

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465

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Page 26 of 33

466

Table 1. The BET specific surface area, NH3 adsorption amount, bulk S content and

467

surface atomic ratios of the fresh, poisoned and regenerated catalysts. Samples

BET

NH3

NH3

bulk

specifi

adsorpti

adsorption

S

c

on

amount per

conte

surface

amounta

surface area

nt b

area

(µmol/g)

(µmol/m2)

(%)

Surface atomic ratios c (%) S

Oα /

Ce3+/

V5+/

(Oα+Oβ)

(Ce3++Ce4

(V3++V4+

+

+V5+)

)

2

(m /g)

1%VWTi

53.0

184.2

3.48

-

25.2

-

-

44.6

1%VWTi-p

46.2

218.1

4.72

0.56

17.5

1.72

-

37.5

23.3

0.95

-

42.7

2 1%VWTi-r

49.6

119.8

2.41

0.09 85

5%CeWTi

54.0

237.1

4.39

-

32.2

-

23.6

-

5%CeWTi-

34.9

269.3

7.71

1.35

28.2

3.15

33.8

-

44.9

128.6

2.88

0.54

28.1

1.88

34.1

-

p 5%CeWTir

468

8

a. calculated from NH3-TPD; b. from ICP; c. calculated from XPS.

26


Page 27 of 33


469

Figure captions

470

Figure 1. NOx conversions of the fresh, poisoned, and regenerated samples: (a) 1%VWTi

471

catalysts, (b) 5%CeWTi catalysts. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] =

472

5 % and N2 as balance, GHSV = 30,000 h-1.

473

Figure 2. TGA profiles of pure NH4HSO4, Ce2(SO4)3, TiO2-p, 1%VWTi-p and

474

5%CeWTi-p samples. All the samples were tested in a flow of air.

475

Figure 3. XPS profiles of S 2p (a), V 2p (c), O 2p (e) in 1%VWTi samples and S 2p (b),

476

V 2p (d), O 2p (f) in 5%CeWTi samples.

477

Figure 4. H2-TPR curves of fresh, poisoned and regenerated samples. (a) 1%VWTi, (b)

478

5%CeWTi.

479

Figure 5. NO+O2-TPD results of fresh, poisoned and regenerated samples. (a) 1%VWTi,

480

(b) 5%CeWTi.

481

Scheme 1. The proposed mechanisms of low-temperature SCR over in situ poisoned and

482

regenerated 1%VWTi and 5%CeWTi catalysts.

27



Page 28 of 33

483 484

Figure 1. NOx conversions of the fresh, poisoned, and regenerated samples: (a) 1%VWTi

485

catalysts, (b) 5%CeWTi catalysts. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] =

486

5 % and N2 as balance, GHSV = 30,000 h-1.

487

488 489

Figure 2. TGA profiles of pure NH4HSO4, Ce2(SO4)3, TiO2-p, 1%VWTi-p and

490

5%CeWTi-p samples. All the samples were tested in a flow of air.

491 28


Page 29 of 33


492

493

494

495 496

Figure 3. XPS profiles of S 2p (a), V 2p (c), O 2p (e) in 1%VWTi samples and S 2p (b),

497

V 2p (d), O 2p (f) in 5%CeWTi samples.

29



Page 30 of 33

498 499

Figure 4. H2-TPR curves of fresh, poisoned and regenerated samples. (a) 1%VWTi, (b)

500

5%CeWTi.

501

30


Page 31 of 33


502

503 504

Figure 5. NO+O2-TPD results of fresh, poisoned and regenerated samples. (a) 1%VWTi,

505

(b) 5%CeWTi.

506

31



Page 32 of 33

507

508 509

Scheme 1. The proposed mechanisms of low-temperature SCR over in situ poisoned and

510

regenerated 1%VWTi and 5%CeWTi catalysts.

511

32


Page 33 of 33


512 513

Table of Contents

514

515

33


New Insight into SO2 Poisoning and Regeneration of CeO2âWO3

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