Highly Efficient NO decomposition via dual-functional catalytic

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Highly Efficient NO decomposition via dual-functional catalytic perovskite hollow fiber membrane reactor coupled with partial oxidation of methane at medium-low temperature Zhigang Wang, Ziwei Li, Yifan Cui, Tianjia Chen, Jiawei Hu, and Sibudjing Kawi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b02530 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on August 7, 2019

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Environmental Science & Technology

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Highly Efficient NO decomposition via dual-functional catalytic perovskite hollow fiber membrane reactor coupled with partial oxidation of methane at medium-low temperature Zhigang Wang a, Ziwei Li b, Yifan Cui a, Tianjia Chen a, Jiawei Hu a and Sibudjing Kawi* a a

Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore, 117576

b

School of Chemical Engineering, Guizhou Institute of Technology, Guiyang, China 550003

*To whom correspondence should be addressed

Telephone: (65)65166312; Fax: (65) 6779 1936 Email: [email protected]

(S. Kawi)

Manuscript submitted to Environmental Science & Technology on 26 April 2019

1


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1

Abstract

2

A novel dual-functional catalytic perovskite hollow fiber membrane reactor was

3

fabricated by integrating BaBi0.05Co0.8Nb0.15O3-δ (BBCN) perovskite hollow fiber

4

membrane with Ni-phyllosilicate hollow sphere catalysts for simultaneous NO

5

decomposition and partial oxidation of methane (POM) reaction. With this novel

6

catalytic membrane reactor, NO could be completely converted to N2 at a

7

medium-low temperature (675oC) owing to instantaneous oxygen removal from the

8

NO decomposition reaction system. Coupled POM reaction on the other side of

9

BBCN hollow fiber membrane not only increased the driving force for oxygen

10

permeation but also produced valuable products (syngas). This novel membrane

11

reactor showed high NO removal capacity at comparatively low temperatures

12

(675~700oC), which is 100~200oC lower than other membrane reactors reported in

13

literature. In addition, even with the presence of a 2~5% oxygen concentration in NO

14

stream, NO could still be completely decomposed to N2 via this catalytic BBCN

15

membrane reactor. Evidently, the application of this novel catalytic membrane reactor

16

could overcome the inhibition of oxygen present atmosphere for NO decomposition

17

and achieve a remarkably high efficiency for NO removal.

18 19 20 21

Key words

22

NO decomposition; BBCN perovskites; POM reaction; hollow fiber membrane

23

reactor; dual-functional 2



24

1. Introduction

25

Nitrogen oxides (NOx) are detrimental pollutants that greatly contribute to the

26

formation of acid rain and photo-chemical smog. At the same time, NOx are extremely

27

toxic for human body. NOx are mainly produced by automobiles and stationary power

28

plants 1-3. Currently, several methods have been widely applied for NOx removal, such

29

as selective catalytic reduction (SCR)

30

non-selective catalytic reduction (NSCR) 9 and direct NOx decomposition 1, 10-12. Both

31

SCR and SNCR use NH3 or urea as a reductant to reduce NOx to N2 and water. For

32

SCR, V2O5–WO3 (MoO3)/TiO2 has been widely employed as the catalyst to control

33

the emission of NO from stationary coal fired power plants or diesel engine at around

34

200–400°C 2, 5, 8, 13-17. In contrast, the catalyst absent SNCR process incurs low capital

35

cost, yet the NOx removal efficiency is lower than SCR and the operating temperature

36

is as high as 800~1100°C 4. However, both SCR and SNCR have to bear high

37

operating costs due to the consumption of reductants. As of NSCR, three-way

38

catalytic converter is the most common apparatus to abate NOx emission from petrol

39

engine. CO, NOx and hydrocarbons are converted into CO2 and N2 via this catalytic

40

converter with noble-metal based catalysts

41

additional reductants as the unburnt hydrocarbons and CO are used as reductants.

42

However, a stoichiometric air/fuel ratio is required to efficiently convert CO,

43

hydrocarbons and NOx to harmless products, and the presence of oxygen in the flue

44

gas could undermine NOx conversion. Hence, the requirement of the stoichiometric

45

air/fuel ratio imposes constrain on the highly efficient lean-burn engine 18.

4-8,

selective non-catalytic reduction (SNCR)4,

9, 18.

This technique does not require

3


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46

Recently, direct NOx decomposition (2NO = N2 + O2) has become more

47

attractive because the process is simple and exempts the requirement of additional

48

reductants. Several catalysts have shown good performance for NOx decomposition,

49

such as Cu-ZSM-5 zeolites, Pd/Al2O3, metal-doped Co3O4 and perovskite oxides 11, 12.

50

However, several problems constrain the further application of the above catalysts.

51

Cu-ZSM-5 and Pd/Al2O3 are easily deactivated by strongly adsorbed oxygen on the

52

catalyst surface when oxygen presents in the flue gas. Metal-doped Co3O4 is lack of

53

thermal stability due to the thermal reduction of Co3O4 to CoO at high temperature.

54

Perovskite oxides with typical ABO3 or A2BO4 structure have a large number of

55

oxygen vacancies19,

56

600oC and are sustainable for long-term operation at higher temperatures

57

(700~800°C) for NOx decomposition

58

for NO decomposition based on perovskite oxides catalyst, it exhibits a better stability

59

in the presence of oxygen. Furthermore, as the temperature of the exhaust gas at the

60

outlet of engines is higher than 800oC, the operating temperature for perovskite oxides

61

catalyst can be provided. In addition, increasing the reaction temperature is expected

62

to decrease the negative effects of water and sulfur compounds. Thus far, several

63

perovskite catalysts showed good performance for direct NOx decomposition, such as

64

La0.7Ba0.3Mn0.8In0.2O3 11, La0.7Ce0.3SrNiO4 21, La0.8Sr0.2CoO3 21, (Gd0.7Y0.26Ba0.04)2O2.96

65

12

66

despite the promising results.

67

20,

which show a good activity for NOx decomposition above

11, 12.

Although the required temperature is high

etc. However, the NO decomposition is still inhibited by the presence of oxygen

To overcome this drawback, simultaneous removal of oxygen from the reaction 4



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68

system is an efficient way to enhance the NO decomposition. The application of

69

oxygen permeable perovskite membrane can fulfill this requirement. This perovskite

70

membrane functions as both the catalyst for NO decomposition and the membrane for

71

oxygen separation. The instantaneous removal of oxygen from the reaction system via

72

coupled oxygen-consuming reaction on the other side of the membrane, i.e.

73

constructing the so-called dual-functional membrane reactor, can increase the driving

74

force

75

BaCoxFeyZr1-x-yO3-σ (BCFZ) hollow fiber membrane reactor coupled partial oxidation

76

of methane (POM) for NO and N2O decomposition and achieved remarkable results

77

22, 23.

78

coupled dry reforming of methane (DRM) for N2O decomposition to improve the

79

performance of the novel membrane reactor 24. In the novel dual-functional membrane

80

reactor, two separate reactions simultaneously operated in one membrane reactor, and

81

the products from two reactions are individually exported. This economical and

82

efficient method allows NOx decomposition to be enhanced while producing syngas

83

simultaneously by POM or DRM reaction. However, the operating temperature for the

84

above-mentioned membrane reactors to completely convert NOx is 850oC and above.

85

In order to decrease energy consumption and capital cost, it is required to decrease the

86

operating temperature of this novel dual-functional membrane reactor to decompose

87

NOx. In addition, the capacity of NOx removal should be further improved to increase

88

the efficiency. Recently, our group reported BaBi0.05Co0.8Nb0.15O3-σ (BBCN) hollow

89

fiber membrane with excellent oxygen permeability even at low temperatures

for

oxygen

permeation.

For

example,

Jiang

and

Caro

reported

In their later work, they reported BaFe0.9Zr0.05Al0.05O3-σ membrane reactor

5


25, 26.

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90

The membrane with the hollow fiber configuration has several advantages, such as

91

high packing capacity, thin membrane wall and easy integration with catalysts 27-39. In

92

this work, high oxygen permeable BBCN hollow fiber membrane was integrated with

93

high sintering resistant Ni-phyllosilicate hollow sphere catalyst to form a catalytic

94

BBCN hollow fiber membrane reactor for simultaneous NO decomposition and POM

95

reaction. The operating temperature was successfully decreased to 675oC and a

96

comparable NO removal capacity was achieved.

97

2. Experimental Section

98

2.1 Synthesis of BBCN hollow fiber membranes and catalysts

99

BBCN hollow fiber membranes were fabricated via phase inversion and sintering

100

techniques which can be found in our previous work 25, 40. To increase the membrane

101

mechanical strength, the sintering temperature for ceramic BBCN hollow fiber

102

membranes was increased to 1200oC with a 5 hours duration compared with the

103

previous report. Ni-phyllosilicate hollow sphere catalysts were synthesized using

104

nickel nitrate hexahydrate (97%) as the precursor via a hydrothermal and H2 reduction

105

method using SiO2 as the silica source and chemical template as described elsewhere.

106

41, 42.

107

S1. The other detail of Ni-phyllosilicate catalyst was also described in the supporting

108

information (SI).

109

2.2 Integration of BBCN membrane with catalyst for NO decomposition and POM

110

reaction

111

The morphology of synthesized Ni-phyllosilicate sphere can be seen from Fig.

The schematic of catalytic BBCN perovskite hollow fiber membrane reactor for 6



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112

NO decomposition and POM reaction is shown in Fig. 1. 50 mg Ni-phyllosilicate

113

hollow sphere was diluted with 450mg quartz silica powders, and then was reduced at

114

750oC for 1 hour under 50 vol% H2 stream. After cooling down, the catalyst was

115

packed around the middle part outside of BBCN hollow fiber membrane, and was

116

fixed by quartz wool. The integrated catalytic membrane reactor was placed in a

117

tubular furnace with an effective membrane area of ~1.8 cm2. 10 vol% NO balanced

118

with He was introduced into the lumen side of the hollow fiber membrane at varying

119

flow rates. To evaluate the effect of oxygen on the performance for NO

120

decomposition, air was mixed with 10 vol% NO to obtain 2 vol% to 5 vol% O2

121

concentration in the NO stream. On the shell side of hollow fiber membrane, CH4 was

122

fed with different flow rates. All reactant gases were started to introduce into the

123

membrane reactor at 600oC, before which the reactor was kept in He during heating

124

up. The membrane reactor was first activated at 750oC and subsequently cooled down

125

to 700 oC followed by 675 oC. All products firstly passed through a condenser at 5 oC

126

to condense any moisture before injection into Gas Chromatography (GC) equipped

127

with a TCD detector. The injection was periodically switched from the lumen side to

128

the shell side of the hollow fiber membrane to measure the concentration of gas

129

products from NO decomposition and POM reaction produced from lumen side and

130

shell side of the membrane reactor, respectively.

131

The NO conversion (𝑋𝑁𝑂), CH4 conversion (𝑋𝐶𝐻4), N2 selectivity (𝑆𝑁2) and CO

132

selectivity (𝑆𝐶𝑂) are defined as follows:

133

𝑋𝑁𝑂 =

𝑜𝑢𝑡 𝐹𝑖𝑛 𝑁𝑂 – 𝐹𝑁𝑂

𝐹𝑖𝑛 𝑁𝑂

(1)

× 100% 7


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𝑜𝑢𝑡 𝐹𝑖𝑛 𝐶𝐻4 – 𝐹𝐶𝐻4

134

𝑋𝐶𝐻4 =

135

𝑆𝐶𝑂 = 𝐹𝐶𝑂 + 𝐹𝐶𝑂 × 100%

𝐹𝑖𝑛 𝐶𝐻4

(2)

× 100%

𝐹𝐶𝑂

(3)

2

2𝐹𝑁2

(4)

136

𝑆𝑁2 = 𝐹𝑖𝑛

137

Where 𝐹𝑖 is the flow rate of species i with unit as ml/min.

138

2.3 Characterizations

139

The morphology of the BBCN hollow fiber membrane was detected using a Scanning

140

Electron Microscope (SEM, JEOL, JSM-6701F). High resolution Transmission

141

electron microscopy (HR-TEM) was employed to detect the morphology of the

142

catalyst before and after reduction. Specific surface area, pore volume and average

143

pore diameter of the catalyst were determined via nitrogen physical adsorption at 77 K

144

with a Micromeritics ASAP 2020 system, using the Brunauer–Emmett–Teller (BET)

145

method. Carbon residue on the spent catalyst after POM reaction in this

146

dual-functional membrane reactor was measured via thermo-gravimetric analysis

147

(TGA). Around 20 mg spent catalyst was used for TGA analysis and heated in static

148

air while the temperature was increased to 900 oC at a ramping rate of 10 oC /min,

149

before which the catalysts were dried in 100oC for 2 hours to remove the moisture.

150

X-ray photoelectron spectroscopy (XPS, KRATOS AXIS spectrometer equipped with

151

mono Al Kasource hv = 1486 eV) was used to measure the binding energies of Ba4d,

152

O1s and Co3p elements on the fresh and spent membrane surface. The results were

153

referenced to the standard calibrated value of the adventitious carbon, C 1s

154

hydrocarbon peak at 284.5 eV prior to fitting the spectra of samples. The crystal phase

155

structures of fresh BBCN membrane, spent BBCN membrane and spent ground

𝑁𝑂

― 𝐹𝑜𝑢𝑡 𝑁𝑂

× 100%

8



156

BBCN powers were determined by X-ray diffraction (XRD, Shimadzu XRD-6000

157

power diffract meter) using Cu K-α radiation (λ=1.5406 Å). Continuous scan mode

158

was used to collect 2θ data from 20o to 80o at room temperature.

159 160

Fig.1 The schematic diagram of dual-functional membrane reactor for NO

161

decomposition and POM reaction

162

3. Results and discussion

163

3.1 NO decomposition and POM reaction via the catalytic BBCN hollow membrane

164

reactor

165

The result of NO decomposition and POM reaction via the catalytic BBCN

166

hollow fiber membrane reactor at different temperature is shown in Fig. 2. It can be

167

seen that both NO and CH4 conversion increased with time at 750oC. The increase of

168

CH4 conversion is mainly attributed to two factors. Firstly, oxygen permeability of 9


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169

BBCN perovskite membranes could be activated at 750 oC to provide more oxygen

170

for CH4 conversion. Additionally, the catalyst was further reduced at 750 oC to

171

increase the catalytic activity for POM reaction. The pre-reduced catalyst could have

172

been partially oxidized when exposed in atmospheric condition prior to the

173

assembling of the catalytic membrane reactor. This result also corresponds to our

174

previous study

175

oxygen consumption on the shell side of BBCN hollow fiber membrane, thus

176

significantly increased the driving force for oxygen permeation through the

177

membrane. Hence, NO conversion was also increased correspondingly until finally

178

reached a complete NO conversion with 60 ml/min 10% NO feeding rate at 750oC.

179

When temperature was decreased, NO conversion and CH4 conversion decreased

180

concomitantly. With considering the fact that oxygen permeability of perovskite

181

membranes decreases with decreasing temperature, there will be insufficient oxygen

182

for POM reaction and thus methane conversion could decrease

183

methane conversion also leads to the decrease of NO decomposition. From these

184

phenomena, it can be concluded that NO conversion is significantly affected by the

185

driving force and permeability for oxygen permeation through the BBCN membrane.

186

With decreasing the temperature, NO conversion could reach around 90%, 50% and

187

30% at 725, 700 and 675oC, respectively with a fixed NO (10 vol%) feed rate of 60

188

ml/min. It can also be observed from Fig. 2 (b) that the Ni-phyllosilicate catalyst

189

showed a good selectivity during POM reaction with a CO selectivity of ~95%. In

190

short, the conversion of NO and CH4 in this catalytic BBCN membrane reactor could

43.

Meanwhile, the increase in methane conversion accelerated the

10


43.

The reduced


191

affect each other. Methane conversion increased the driving force for oxygen

192

permeation which leads to increase in NO decomposition. Meanwhile, the increase in

193

NO conversion provides more oxygen to react with methane, therefore increasing the

194

methane conversion.

195 196

Fig. 2 (a) NO conversion & (b) CH4 conversion and CO selectivity via BBCN

197

perovskite hollow fiber membrane reactor as a function of temperature; Core side: 60

198

ml/min (FNO= 6ml/min, FHe=54ml/min); Shell side: FCH4=6ml/min

11


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199 200

Fig. 3 NO conversion capacity via catalytic BBCN hollow fiber membrane reactor at

201

675oC and 700 oC; Core side: F10%NO=10~30ml/min; Shell side: FCH4=3ml/min,

202

FHe=3ml/min

203

To investigate the capacity of NO removal of this catalytic BBCN hollow fiber

204

membrane reactor at low temperatures (675 and 700 oC), NO (10 vol %) feeding rate

205

was varied to achieve an almost complete NO conversion (> 99%) as shown in Fig. 3.

206

It can be seen that even at temperatures as low as 675 and 700 oC, this catalytic

207

membrane reactor was capable of completely decomposing NO (10 vol %) with flow

208

rate of 12 and 25 ml/min, respectively. Thus, the BBCN catalytic membrane reactor is

209

proven to be able to carry out NO decomposition at significantly lower temperatures

210

comparing with conventional membrane reactors reported in literature, which require

211

an operating temperature of 850˚C and above for complete NO conversion 22-24.

212

3.2 The advantages of catalytic BBCN hollow fiber membrane reactor for 12


NO


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213

decomposition

214 215 216

Fig. 4 NO conversion via BBCN perovskite powders at different temperatures

217

Table 1 NO conversion via BBCN hollow fiber membrane without reaction (Helium

218

as sweep gas on the shell side) Temperature (oC)

NO conversion (%)

800

0.9

750

0

700

0

219

To further study the rate-controlling factor for NO decomposition via BBCN

220

catalytic membrane reactor, BBCN perovskite powders were tested for direct NO

221

decomposition with a NO (10 vol %) feeding rate of 60 ml/min. Low NO conversion

222

was observed when using BBCN perovskite powders in a fixed bed reactor and the

223

results are shown in Fig 4. Even when the temperature was as high as 800 oC, NO 13


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224

conversion only reached 3.6%. As the temperature decreases to 700 oC and below,

225

almost no NO conversion was observed. It demonstrated that simultaneous removal of

226

oxygen from NO decomposition drastically increased NO conversion. In direct NO

227

decomposition on perovskite catalysts, the oxygen vacancy in perovskites played a

228

crucial role for the reaction. Oxygen in NO was adsorbed into the oxygen vacancy of

229

the perovskite catalyst surface, and then NO decomposed into N and O radicals,

230

subsequently formed N2 and O2

231

oxygen from oxygen vacancy was necessary for the regeneration of active sites. As

232

such, if the oxygen produced from NO decomposition was strongly adsorbed on the

233

perovskite catalysts, NO conversion would be inhibited. On the other hand,

234

simultaneous oxygen removal from the reaction (2NO↔N2+O2) could enhance the

235

forward reaction and prevent the side reaction (NO+O2↔NO2). Hence, the application

236

of perovskite membrane for NO decomposition showed great advantages.

21.

Thereafter, the desorption of surface adsorbed

237

To efficiently remove oxygen by the BBCN membrane, the driving force for

238

oxygen permeation plays an important role. To study the effect of oxygen permeation

239

driving force on NO decomposition, a blank BBCN hollow fiber membrane was

240

tested for NO decomposition. On the shell side of hollow fiber membrane, helium gas

241

(30 ml/min) was used as sweeping gas to create oxygen partial pressure gradient

242

through the BBCN membrane. The driving force was much weaker than the one

243

coupled with POM reaction

244

observed at 750oC and merely a 0.9 % conversion could be found at 800oC via this

245

blank BBCN hollow fiber membrane as shown in Table 2, which is much lower than

43.

The results showed that no NO conversion was

14



246

the value obtained from the catalytic BBCN hollow fiber membrane reactor with

247

coupled POM reaction. This result demonstrated the importance of driving force for

248

NO decomposition via the catalytic BBCN hollow fiber membrane reactor. The

249

performance of NO decomposition via the blank BBCN hollow fiber membrane was

250

even worse than the one via the fixed bed reactor with BBCN perovskite powders as

251

catalysts. Such observation is contradictory to the previous conclusion that

252

simultaneous removal oxygen from NO decomposition could increase NO conversion.

253

It could be explained by when NO (10 vol %) feeding rate is fixed at 60 ml/min,

254

BBCN powders have much more surface area than BBCN hollow fiber membrane and

255

therefore result in longer contact time. As a result, a lower NO conversion for the

256

blank BBCN membrane reactor was observed. Based on the comparison of

257

performance among the three reactors -catalytic BBCN membrane reactor, blank

258

BBCN membrane reactor without coupled with POM reaction and fixed bed reactor

259

loaded with BBCN perovskite powders, it can be conclude that NO and CH4

260

conversion in the catalytic BBCN membrane reactor was mainly controlled by the

261

oxygen permeation driving force through the membrane and oxygen permeability of

262

the membrane at various temperatures.

263

15


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264 265

Fig. 5 The effect of co-feed oxygen concentration (2%~5%) on NO decomposition at

266

700 oC; Core side: F10%NO=7.5~15ml/min; Shell side: FCH4=3ml/min, FHe=3ml/min

267 268

3.3 The effect of the co-feed oxygen on NO decomposition

269

In the real industrial practice, there is a certain amount of gaseous phase oxygen

270

in the present of the flue gas. The performance of NO decomposition with a co-feed

271

2%~5% oxygen was studied via the catalytic membrane reactor at 700 oC and results

272

are shown in Fig. 5. NO feeding rate was varied to reach a complete NO conversion in

273

the presence of gaseous phase oxygen. It can be seen from Fig. 5 that the capacity of

274

NO decomposition was inhibited by the presence of oxygen and decreased with

275

increasing oxygen concentration. It can be understood that there is a competition

276

between the gaseous phase oxygen and the oxygen dissociated from NO to be

277

adsorbed into the oxygen vacancy on the membrane inner surface, which hinders NO 16



278

decomposition. However, owing to the high oxygen permeation driving force and

279

high oxygen permeability through BBCN hollow fiber membrane coupled with POM

280

reaction, a complete NO conversion can still be reached in the presence of 2% to 5%

281

oxygen under a high concentration feed NO (10 vol%) of 15 to 7.5 ml/min,

282

respectively. It is reported that the co-feeding O2 may bring the side reaction to

283

produce NO2 owing to the reaction of NO and O2 23. However, no NO2 can be found

284

in our study, which may be attributed to the fast removal of oxygen owning to the

285

high oxygen permeability of the membrane and the low reaction temperature (700oC).

286

3.4 The effect of CH4 feeding rate on NO decomposition

287 288

Fig. 6 The effect of methane feeding rate on NO conversion via catalytic BBCN

289

hollow fiber membrane reactor with co-feed O2 concentration of 3% at 700 oC; Core

290

side: F10%NO=10 ml/min, F air=1.7 ml/min; Shell side: (a): FCH4=1.7 ml/min, FHe=4.3

291

ml/min; (b): FCH4=3ml/min, FHe=3ml/min

292 17


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293

During the long-term NO decomposition test in the catalytic membrane reactor,

294

it was found that the ratio of CH4 to NO and O2 feeding rate could affect the stability

295

of the membrane reactor. As shown in Fig. 6, the stability of NO conversion with a

296

CH4 feeding rate of 1.7 ml/min ( condition (a)) is better than the one with a CH4

297

feeding rate of 3ml/min (condition (b)) when NO (10 vol%) and air feeding rate was

298

fixed as 10 and 1.7 ml/min, respectively. The molar ratio of CH4:O (O from NO and

299

O2) is 3:1.714 and 1.7:1.714 when CH4 feeding rate was set at 3 ml/min and 1.7

300

ml/min, respectively. If all oxygen (O from NO and O2) can react with methane based

301

on the POM reaction stoichiometry (CH4+0.5O2 →CO+H2), oxygen is insufficient for

302

a CH4 feeding rate of 3 ml/min but just enough for a CH4 feeding rate of 1.7 ml/min.

303

However, more oxygen was required than the theoretical value due to formation of

304

trace amount of side product-CO2. Thus, under limited oxygen supply, excess CH4

305

could decompose into carbon and hydrogen. Formation of carbon could coke the

306

catalyst and decrease the catalytic activity for POM reaction. The amount and nature

307

of carbon residue on the spent catalyst was measured by TGA/DTA analysis. The

308

weight loss from TGA profiles and exothermic peak (positive peaks) from DTA

309

profiles are mainly attributed to carbon combustion in air with increasing temperature.

310

DTA results showed the exothermic peaks located at around 500 ~ 650 oC during the

311

TGA/DTA analysis for the spent catalyst, indicates that α-type carbon (Cα) residue

312

(can be oxidized below 650 oC) was formed on the spent catalyst44. It can also be seen

313

from Fig. 7 that the carbon residue under condition (a) is much lower than that under

314

condition (b), even with a 5 hours longer duration than the condition in (b). In short, 18



315

the excess methane had high tendency for methane decomposition to form carbon

316

which caused catalyst deactivation after long-term reaction, thereby resulting in the

317

decrease of the activity for POM. Thus, the oxygen permeation driving force

318

enhanced by the parallel POM reaction will decrease, leading to the decline of NO

319

decomposition. Meanwhile, the TGA/DTA analysis for the spent BBCN hollow fiber

320

membrane (Fig. 7) showed that there is no weight loss and exothermic peaks,

321

indicating that there was no carbon residue on the membrane surface.

322

In addition, the excess methane increases the reducibility of the atmosphere on

323

the shell side of the BBCN hollow fiber membrane, which eventually broke the

324

BBCN membrane under condition (b) after 19 hours. Even under condition (a), the

325

outer surface of the BBCN hollow fiber membrane was also slightly reduced. It can be

326

seen from Fig. 8 (a) that the BBCN hollow fiber membrane as a whole still

327

maintained the intactness, but a thin porous layer could be found on the outer side of

328

the membrane as shown in Fig. 8 (b). It can also be seen from Fig. 8 (c) that the

329

ceramic grains were isolated rather than closely joined together like the bulk part of

330

the membrane as shown on Fig 8 (d). This phenomenon could be attributed to the

331

reduction of the membrane external surface by reducing gas.

332 19


Page 20 of 30

Page 21 of 30


333

Fig. 7 TGA/DTA analysis for spent Ni-phyllosilicate hollow sphere catalyst under

334

condition (a) and (b); spent BBCN hollow fiber membrane under condition (a) after

335

the long-term reaction

336

337 338

Fig. 8 SEM images of spent BBCN hollow fiber membrane after the reaction under

339

condition (a), (a): cross section; (b): membrane wall; (c): external surface and (d):

340

bulk area

341

To further analyze the change of the spent hollow fiber membrane external

342

surface after 24 hours of reaction under condition (a), fresh and spent BBCN

343

membrane external surface were detected and compared by XPS characterization. The

344

XPS spectra of Co 3p, Ba 4d and O 1s for fresh and spent membrane surface were

345

shown in Fig. 9. The details of binding energy and relative areas ratio are summarized

346

in Table 2. As the binding energy of Co 2p and Ba 3d overlaps

347

were chosen to analyze the change of valent for cobalt and barium. It can be seen 20


45,

Co3p and Ba 4d


348

from Fig. 9 (a) that Co 3p can be deconvoluted into two peaks, the one with higher

349

binding energy (62.5 ~ 62.8 eV) is attributed to Co2+ while the one with lower binding

350

energy (60.4 ~ 60.5 eV) is attributed to Co3+ 46, 47. It can be seen from Table 2 that the

351

area ratio of Co2+ for spent membrane is higher than the one for fresh membrane, with

352

an increase from ~ 26% to ~32%. In addition, it can be seen from Fig. 9 (c) that O1s

353

was deconvoluted into two peaks, the one with higher binding energy (~ 531.1 eV) is

354

attributed to adsorbed oxygen while the one with lower binding energy (~529.0 eV) is

355

attributed to lattice oxygen

356

(7.2%) was much lower than the one for fresh membrane (22.5%). All results showed

357

above demonstrate that the external surface of the BBCN membrane was partially

358

reduced after the long-term reaction. In addition, it also can be found from Fig. 9 (b)

359

and Table 2 that the binding energy of Ba 4d for the spent membrane was shifted to a

360

higher value with a 0.8 eV increment. The lower binding energy of 88.3 and 90.8 eV

361

could be attributed to Ba-O in the perovskite structure while the higher binding energy

362

of 89.1 and 91.6 eV could be assigned to Ba4d of BaCO3 49. Barium oxides segregated

363

from perovskite structure due to the reduction of perovskites, and easily reacted with

364

CO2 produced by the side reaction during POM reaction to form barium carbonate.

365

Hence, it can be concluded that the external surface of the BBCN hollow fiber

366

membrane under condition (a) was partially reduced after long-term reaction and

367

formed barium carbonate. This layer formed on the external surface could affect the

368

oxygen permeability of the membrane, thereby decreasing the activity for NO

369

decomposition and POM reaction.

48.

The lattice oxygen area ratio for spent membrane

21


Page 22 of 30

Page 23 of 30


370

371 372

Fig. 9 XPS binding energy of Co3p (a), Ba4d (b) and O1S (c) for fresh membrane and

373

spent membrane surface

374 375 376 377 378 379 380 381 382

Table 2 XPS binding energy summary of Co3p, Ba4d and O1s for the fresh and spent

383

membrane Fresh Membrane surface Spectral region

Spent membrane surface

BE (eV)

Area (%)

BE (eV)

Area (%)

60.5

73.8

60.4

68.2

62.5

26.2

62.8

31.8

Co3p Ba4d

88.3

89.1 22



90.8 O1s

Page 24 of 30

91.6

529.0

22.5

529.0

7.2

531.2

77.5

531.1

92.8

384

It should be noted that the BBCN hollow fiber membrane as a whole still

385

remained intact which can also be confirmed by the XRD patterns of the spent

386

membrane as shown in Fig. 10 (c). No obvious impurity phases were observed on the

387

XRD patterns of the spent membrane while the XRD patterns matched the cubic

388

perovskite phase structure

389

observed in the XRD patterns of the membrane after long-term reaction. The tiny

390

peak located at 24 degree could correspond to BaCO3 (#41-0373). The inference was

391

also consistent with the result from XPS analysis. However, the intensity of this peak

392

is quite weak, which means the amount of barium carbonate is quite less. Hence, it

393

can be concluded that the reduced layer was negligible compared with the entire

394

hollow fiber membrane. In addition, the BBCN perovskite material was also stable

395

during NO decomposition, which can be proved by the XRD patterns of the spent

396

BBCN perovskite powders after NO decomposition. It can be seen from Fig. 10 (b)

397

that the spent BBCN powders maintained a pure cubic perovskite structure.

25.

Only a tiny peak located at around 24 degree could be

398

It should be noted that the reduction of the membrane can be suppressed via

399

controlling the NO and CH4 feeding rate, thereby improving the stability of the

400

catalytic membrane reactor. As the reduction of the membrane surface was essentially

401

due to insufficient permeated oxygen for POM reaction (including side reaction), the

402

decrease in CH4 feeding rate could increase the O to CH4 ratio and protect the

403

membrane. However, further decrease in CH4 feeding rate could decrease the driving 23


Page 25 of 30


404

force for oxygen permeation through the membrane, resulting in the sacrifice of the

405

capacity for NO decomposition. In conclusion, NO decomposition and POM reaction

406

in this catalytic membrane reactor required mutual matching at an optimum reaction

407

rate to obtain the desired performance.

408 409

Fig. 10 XRD patterns for (a): fresh BBCN membrane; (b): Spent BBCN powders after

410

DeNOx reaction and (c): Spent BBCN membrane after NO decomposition and POM

411

reaction under condition (a); (“p” annotated as cubic perovskite phase)

412

3.5 Comparison with the performance from other literatures reported

413

There are several pioneer works reported dual-functional catalytic membrane

414

reactor for NOx decomposition. Comparison between the performance of catalytic

415

BBCN hollow fiber membrane reactor in this work and the ones from other literatures

416

is shown in Table 3. It can be seen that most works reported an operating temperature

417

above 850oC to fulfill a complete NO conversion. In contrast, the catalytic BBCN 24



Page 26 of 30

418

hollow fiber membrane reactor can be operated at 750 oC to treat more NO.

419

Moreover, even when the temperature decreases to 675 oC, a comparable amount of

420

NO can still be fully decomposed. This remarkable performance is mainly attributed

421

to the good permeability of BBCN hollow fiber membrane and high catalytic activity

422

of Ni-phyllosilicate hollow sphere catalysts.

423

Table 3 Comparison with the performance from other literatures reported Membrane

NOx (%) &

effective

Feeding rate

Membrane

Temperature

Conversion

Coupled

(oC)

(%)

reaction

875

100

850

60

850

100

Ref.

reactor area (cm2)

(ml/min)

BCFZ hollow fiber

0.86

NO (10%) & 30

BCFZ hollow fiber

0.86

BFZ-Al disk

2.0

BBCN hollow fiber

N2O (20%) & 30

N2O (10%) & 30

800

75

900

100

POM

23

POM

22

DRM

24

810

70

NO (10%) & 60

750

100

POM

NO (10%) & 25

700

100

POM

NO (10%) & 12

675

100

POM

This 1.8

work

424 425 426

Associated Content

427

Supporting Information

428

Experimental details, Figure S1. This material is available free of charge via the

429

Internet at http://pubs.acs.org.

430 431

Acknowledgement 25


Page 27 of 30


432

The authors generously thank financial support from Ministry of Education in

433

Singapore (MOE) Tier 2 grant (WBS: R279-000-544-112), Singapore Agency for

434

Science, Technology and Research (A*STAR) AME IRG grant (No. A1783c0016)

435

and National Environment Agency (NEA) in Singapore (WTE-CRP 1501-103)

436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471

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Highly Efficient NO decomposition via dual-functional catalytic

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