NaBr-Enhanced CaO-Based Sorbents with a Macropore-Stabilized

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Environmental and Carbon Dioxide Issues

NaBr enhanced CaO-based sorbents with a macropore-stabilized microstructure for CO2 capture Yongqing Xu, Haoran Ding, Cong Luo, Qi Zhang, Ying Zheng, Xiaoshan Li, Yingchao Hu, and Liqi Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01327 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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Energy & Fuels

NaBr enhanced CaO-based sorbents with a macropore-stabilized microstructure for

1 2

CO2 capture

3

Yongqing Xu†; Haoran Ding†; Cong Luo∗,†; Qi Zhang‡; Ying Zheng†; Xiaoshan Li†;

4 5

Yingchao Hu†; Liqi Zhang†

6 7

† State Key Laboratory of Coal Combustion, School of Energy and Power

8

Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R.

9

China ‡ School of Energy and Power Engineering, Huazhong University of Science and

10 11

Technology, Wuhan 430074, P. R. China

12

Abstract

13 14 15

Calcium looping process(CaLP) is well considered as an cost-effective scenario for

16

trapping CO2 from flue gas. However, the CO2 capture capacity of natural CaO-based

17

sorbents spoiled rapidly over the long-term cycles. In this work, NaBr was introduced

18

to enhance the cyclic CO2 capture capacity of CaO sorbents. The NaBr modified CaO

19

showed an improved activity and durability for carbonation. After 100 cycles, the

20

“NaBr/CaO-10/100” maintained a capacity of 0.202 g-CO2/g-sorbent, which was

21

about 185% higher than that of unmodified CaCO3 precursor. The mechanism of ∗

C. L: tel,+86-27-87542417-301; fax,+86-27-87545526; e-mail, [email protected].

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Page 2 of 33

22

enhancement was analyzed by Simultaneous Thermal Analyzer(STA), In-situ X-Ray

23

Powder Diffraction(In Situ XRD), Inductively Coupled Plasma Optical Emission

24

Spectrometer(ICP-OES), Field Emission Scanning Electron Microscope Coupled

25

Energy Dispersive X-ray Spectrometer (FSEM-EDS) and N2 physical absorption, and

26

the results showed that the modified sorbent formed a well-linked macro-pore

27

structure, which was relatively stable at high temperature reactions; besides, NaBr

28

incorporated inside the CaO crystal lattice promoted the durability of pore structures

29

and cyclic CO2 capture capacity. NaBr is an effective promoter that has the ability of

30

enhancing the cyclic CO2 capture capacity of CaO-based sorbents.

31 32

Keywords: carbon dioxide, calcium looping process, NaBr, doping

33 34

Introduction

35 36

The global climate is outlining unequivocal signals of warming, resulting in

37

sensible ascent in average ocean and air temperatures, as well as the alarming growth

38

in the average sea level.1 The enormous anthropogenic emissions of greenhouse gases

39

are the major contributory elements to the global warming and CO2 is accepted as the

40

largest contributor.2 CO2 capture and storage(CCS)3,

41

technologies that have the potential to isolating CO2 from power plants and some

42

industrials. Among them, calcium looping process(CaLP)5 is considered as one of the

43

most promising scenarios for CO2 capture, as show in Fig. 1 (schematic diagram).


4

is serials of developing

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This process has several intrinsic advantages such as the high theoretical absorption

45

capacity, abundance of raw materials (limestone6, carbide slag7, steel slag8 and so on)

46

and wide applicability of the calcium looping process (no need to scale-up existing

47

power station boilers) and so on. It is based on the reversible carbonation and

48

decarbonation reactions between CaO(g) with CO2(s) as show in Eq. (1).

49

CaO( s) + CO2 ( g ) ↔ CaCO3 ( s)

∆H r ,298 K = −178 kJ/mol

(1)

50 51

Fig. 1 schematic diagram of calcium looping process(CaLP).

52 53

The carbonation process is exothermic and the counterreaction process, calcination

54

process, is endothermic, which means that calcination is favored by higher

55

temperatures9. The calcination process will occurred only if the partial pressure of

56

ambient CO2 is below the decomposition pressure of carbonate product, which is

57

controlled by equilibrium thermodynamics10. Based on enough experiments, a thermal

58

equilibrium diagram of the reaction temperature versus CO2 partial pressure was

59

recorded and then a typical expression for equilibrium decomposition pressure Peq

60

with ambient temperature is proposed11, 12, as shown in Eq(2).

61

 20474  Peq = 4.137 × 107 exp  −  atm T  

(2)

62

Nevertheless, the primary demerit of this process is sintering13, 14 occurred at the

63

repeated high temperature reactions, accordingly, the CO2 capture capacity of the

64

CaO-based sorbents decayed drastically during the long-term cycles. Grasa and

65

Abanades et al.15 have presented a semi-empirical formula to analysis the attenuation


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of carbonation conversion of CaO in long-term cycles, as show in Eq. (3).

XN =

67

1 1 + kN 1− Xr

+ Xr

(3)

68

Where, X N is the carbonation conversion achieved after Nth carbonation, k is

69

the de-activation constant and X r represents the residual carbonation conversion

70

after infinite number of cycles.

71

Sintering is the main reason for the drastic decay of carbonation conversion of CaO.

72

Specifically, the pore structure and specific surface area of CaO are destroyed within

73

the duration of high temperature reaction process.

74

On the basis of a large amount of experimental data, German and Munir et al.16

75

have modeled the decay rate of specific surface area at high temperatures, as

76

illustrated in Eq. (4)

(

77

S0 − S γ ) = K st S0

(4)

78

Where γ is a mechanism-derived parameter and K s represents a sintering

79

constant which grows-up exponentially with temperature ( min −1 ), t shows the

80

sintering time (min), S0 and S represent specific surface areas before and after

81

sintering.

82 83

84

85

Coble et al.17 put forward a model to estimate the decay of porosity of CaO at long-term high temperature sintering as illustrated in Eq. (5).

ε 0 − ε = k p ln(

t+a ) t0 + a

(5)

Where k p represents a constant on diffusion, ε is the porosity at time t , ε 0


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and t0 represent initial porosity and time at which grain shrinkage begins, a is a

87

constant which is associated with the property of materials.

88

The carbonation process is generally initiated with a rapid chemical reaction

89

controlled stage which was followed by a precipitate transition to a product layer

90

diffusion controlled stage with a very slow reaction rate18, 19. There is a critical

91

thickness of product layer20, which is well regarded as 49nm (±19% of standard

92

deviation), when the reaction kinetics of carbonation shifted to a slow product

93

controlled stage from the fast chemical reaction controlled stage.

94

During a typical calcium looing process(CaLP), the CaO sorbent was reutilized in

95

long-term cycles. However, due to the drastic decay of CO2 capture capacity of CaO

96

sorbents21, a mass of fresh CaO stocks were still needed, hence increasing the

97

operating cost of the system22. Whereupon, recycling the waste calcium precursor23, 24

98

has been well presented; besides, a great number of technologies have also been

99

proposed for enhancing the CO2 capture capacity of CaO-based sorbents, including activation25,

hydration26,

organometallic

calcium

precursor27,

28

100

steam

,

101

acid-pretreatment29, templating with pore creating materials30,

102

supporting33-35 with other oxides, improving the reaction conditions36, 37, employing

103

some advanced preparation methods such as vapor-phase deposition38, 39, combustion

104

synthesis40 and so on41, 42.

31

, doping32 or

105

Among these techniques, doping43 has been considered as a cost-effective method

106

to enhance the CO2 capture capacity of CaO. However, alkali salt is well considered

107

as low melting material which cannot enhance the cyclic CO2 capture behavior of


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108

CaO sorbents. There still a few researchers44-46 suggested that HBr can effectively

109

enhance the cyclic CO2 capture capacity of the sorbents, but the HBr is an strong

110

corrosive acid that not well suited for industrial CaL process. In our previous work,

111

we have found that sea water hydrated lime sorbents kept a CO2 capture capacity of

112

0.31 g of CO2/g of sorbent after 40 cycles, which was about 130% higher than that of

113

original limestone precursor. However, that work failed to explore the enhancing

114

mechanism due to the fact that limestone and sea salt were all mixtures. By means of

115

DFT calculation, Wang et al.

116

capacity of CaO, but the doping effect has not been detected by experiments yet. In

117

this work, NaBr was doped to study the mechanism of Na+ on CO2 capture capacity

118

enhancement of CaO sorbents.

47

stated that Na+ doping can boost the CO2 capture

119 120

2. Experimental section

121

2.1 Materials and sorbent preparation

122

The CaCO3 precursor and NaBr used in this paper were analytical grade purchased

123

from Sinopharm Chemical Reagent Co., Ltd, P. R. China. The preparation process is

124

based on the method which has been reported in our previous work48. 10 g of CaCO3

125

precursor was calcined at 850 oC, then it was poured into 50 ml of salt solution which

126

had a given mass of NaBr dissolved. After that, the milk-like mixture was agitated

127

continuously at 80 oC by a magnetic stirrer for 45 min, and then the mud-like mixture

128

was dried at 100 oC for two days. At last, the lumpy product was ground and sieved to

129

grains in 0.2-0.3 mm. For conveniently describing, the obtained doped sorbents were


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named as “NaBr/CaO-xx/100” (“xx/100” is the mole ratio of NaBr to CaO in the

131

obtained sorbents).

132 133

2.1 Experimental methods

134

The cyclic carbonation/decarbonation performance of these sorbents were

135

examined by a simultaneous thermal analyzer (STA 2500 Regulus, Netzsch), and the

136

testing principle was illustrated in Fig. 2. Loading of about 8mg of samples, a

137

measure crucible was held on the end of the measure holder, while another empty

138

crucible was placed on the end of reference holder. A thermal analysis unit coupled

139

with high sensitive analytical balance system was linked with these two holders. Prior

140

to the calcium looping process, the samples were heated to 850 oC at a rate of 20

141

o

142

complete precalcination at 850 oC for 10min, 50 carbonation/decarbonation repeated

143

tests were conducted continuously under the alternating of atmospheres as show in

144

Table 1.

C/min under an N2 flow(high pure, 99.999%) of 170 ml/min in the STA. After

145 146

Fig. 2. Schematic diagram of the simultaneous thermal analysis system.

147 148

Table 1. Reaction conditions for calcium looping.

149 150 151

The Nth and accumulative CO2 capture capacity are calculated as Eq. (6) and Eq. (7).


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CN =

152

mN − m0 m0

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(6)

n

C = ∑ CN

153

(7)

k =1

154

Where C N and mN are the CO2 capture capacity and the mass of CaO sorbent

155

after Nth carbonation, m0 is the mass of CaO sorbent after completely calcination, C

156

is the accumulative CO2 capture capacity.

157 158

3 Results and discussion

159

3.1 cyclic CO2 capture behavior of NaBr enhanced CaO-based sorbents.

160

The cyclic CO2 capture performance of these modified samples was examined by

161

the STA for 100 cycles, and the CO2 capture capacity C N of the samples were

162

calculated by mass change, assuming that the mass change were only resulted from

163

the carbonation and decarbonation of CaO. As the results shown in Fig. 3, the CO2

164

capture capacity of CaCO3 precursor decayed dramatically over 100 cycles, from 0.59

165

g-CO2/g-sorbent during the 1st carbonation to 0.071 g-CO2/g-sorbent during the 100th

166

carbonation. But all of the NaBr modified sorbents showed continuously increase of

167

capture capacity in the initial 4 cycles, after that the rate of loss-in-capacity of this

168

modified sorbents were much slower than that of CaCO3 precursor with the increase

169

of cyclic numbers. Over the long-term cycles, the stability of CO2 capture activity of

170

the modified sorbent was enhanced with the increase of doping ratios. After 100

171

cycles, the “NaBr/CaO-10/100” held a capacity of 0.202 g-CO2/g-sorbent(about 184%

172

higher than that of CaCO3 precursor), which was tightly followed by the


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“NaBr/CaO-5/100”, “NaBr/CaO-1/100” and “NaBr/CaO-0.5/100”, with roughly

174

178%, 147%, 101% higher than that of CaCO3 precursor. Hence, it can be speculated

175

that the cyclic CO2 capture capacity became more stable as the NaBr doping ratio

176

increase.

177 178

Fig. 3. Cyclic CO2 capture behavior of the NaBr enhanced sorbents.

179 180

The CaO sorbents would be reused to cyclic carry off CO2 from flue gases. Thus

181

the accumulative CO2 carry capacity over the long-term cycles should be evaluated

182

and the results were listed in Table 2. After 100 repeated cycles, the

183

“NaBr/CaO-5/100” captured 24.370 g CO2(about 108% higher than that of CaCO3

184

precusor),

185

“NaBr/CaO-1/100” and “NaBr/CaO-0.5/100” , with about 24.370, 22.460 and 21.320

186

g of CO2 carrying capacity.

which

was

closely

followed

by

the

“NaBr/CaO-10/100”,

187 188

Table 2. Accumulative CO2 capture capacity over 100 calcium looping process.

189 190

The mechanism of cyclic CO2 capture capacity enhancement of doped CaO should

191

be further investigated. The “NaBr/CaO-50/100” was examined by the STA to analyze

192

the effect of NaBr on doped CaO, and the result as shown in Fig. 4 revealed that the

193

sample underwent a severe weight loss over 750 oC, which illustrated that NaBr

194

would sublimate rapidly when the temperature of the sample higher than 750 oC. A


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195

marked endothermic peak appeared at 750 oC, which suggested a drastic sublimation

196

rate at this high temperature.

197 198

Fig. 4 Thermoanalysis of NaBr doped CaO by a STA at high temperature.

199 200

To analyze the ratio of NaBr remained inside the sorbent, the “NaBr/CaO-10/100”

201

was tested by an Inductively Coupled Plasma Optical Emission Spectrometer

202

(ICP-OES). Before the test, the samples were calcined at 850 oC, and then dissolved

203

by dilute nitric acid. The result was listed in Table 3 that the atom ratios of Na to Ca

204

inside the sample decreased from 10:100 to 3.7:100 after calcination at 850 oC, which

205

illustrated that there was still NaBr remained inside the doped sample.

206 207

Table 3. Mole ratio of Na:Ca (tested by ICP-OES)

208 209

To characterize the crystal structure change of the doped samples, the

210

“NaBr/CaO-10/100” was examined by an In-situ XRD apparatus(D8 Advance,

211

Bruker), and the interplanar spacing was calculated by Braggs law as shown in Eq. 8.

212

As the result illustrated in Fig. 5, the interplanar spacing of (200), (311) and (222)

213

became larger when the temperature gone up, while the spacing changed back to its

214

original state when the temperature reduced. The lattice expansion was the results of

215

the high temperature, accordingly trapped sodium atoms inside the grains. Besides,

216

the (311) and (222) revealed convincing additional diffraction peaks, which


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Energy & Fuels

217

represented some potential doping atoms inside the crystal structures. The enhanced

218

cyclic CO2 capture capacity of the CaO may be resulted from the Na+ inside the CaO

219

crystal lattice.

220

2d sin θ = nλ , n=1,2......

221

(8)

222 223

Fig. 5 In situ XRD patterns of “NaBr/CaO-10/100” at different temperatures (step

224

size= 0.020481) (1#, room temperature ; 2#, heating up to 650 oC; 3#, heating up to

225

850 oC; 4#, cooling back down to 650 oC; 5#, cooling back down to room

226

temperature)

227 228

To analyze morphology of the calcined sorbents after long-term cycles, a Field

229

Emission Scanning Electron Microscopic Coupled Energy Dispersive X-Ray

230

Spectroscopy (FSEM-EDXS) was employed and the topographies of the various

231

calcined CaO were displayed in Fig. 6. After 50 cycles, the topography of calcined

232

“CaCO3 precursor” seemed coarse, as illustrated in Fig. 6(a) and (b), which were

233

slightly fluffier than that of the “NaBr/CaO-1/100” as shown in Fig. 6(c) and (d).

234

While the “NaBr/CaO-10/100” showed much more porous than “NaBr/CaO-1/100”

235

after the same 50 cycles, as represented in Fig. 6(e) and (f).

236

It is well stated that porous structure49 is beneficial to the diffusion of CO2 into the

237

CaO pore walls, accordingly enhance the CO2 capture capacity of the sorbents. The

238

“NaBr/CaO-10/100” held a host of uniform macro-pores (about 1 um) inside the CaO


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239

particle, and the well inter-connected micro-scale pores should be good depressor that

240

inhibited the agglomeration of CaO grains.

241

Fig. 6 FSEM images of calcined sorbents after 50 cycles: (a) and (b) CaCO3

242 243

precursor; (c) and (d) “NaBr/CaO-1/100”; (e) and (f) “NaBr/CaO-10/100”

244 245

To investigate the uniformity of NaBr doping inside the CaO particles, the

246

“NaBr/CaO-10/100” was further tested by EDXS, and the elemental mapping

247

(calcined at 750 oC) in Fig. 7 proved the well-disperse Ca, O, Na and Br inside the

248

particles, although the peak intensity of Na and Br were relatively weak. After higher

249

temperature calcination process and 50 cycles, the signal of Na and Br were much

250

weaker as shown in Fig. S1 and S2 (Supporting Information).

251

Fig. 7 Elemental mapping of images (Na, Br, Ca, O).

252 253 254

It is regarded that the sintering of CaO sorbents are linked with the specific surface

255

area and porosity of the calcined sorbents. The BET surface areas of the calcined

256

sorbents after different cycles were examined by nitrogen physical absorption at -196

257

o

258

samples were degassed under vacuum condition for 180 min, and the specific surface

259

areas were analyzed by Brunauer Emmett Teller (BET) model. As the results listed in

260

Table 4, the fresh CaO derived from “CaCO3 precursor” held the largest BET surface

C (3H-2000PS, Beishide Instrument Technology Co., Ltd). Before the test, the


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area, with 10.719 m2/g, which was about 3 times that of fresh CaO from

262

“NaBr/CaO-10/100”. After 30 cycles, the BET surface area of calcined CaO derived

263

from “CaCO3 precursor” dropped to 6.368 m2/g, but the CaO derived from

264

“NaBr/CaO-10/100” soared to 7.844 m2/g.

265

The isothermal N2 adsorption/desorption profiles of the two CaO sorbents after

266

various cycles were plotted in Fig. 8. The fresh CaO derived from “CaCO3 precursor”

267

displayed Ⅳ isotherm sorption curves with a marked H1 type hysteresis loop in high

268

relative pressure zone, which were related to the capillary condensation when filling

269

and emptying of nitrogen molecular on the well-interlinked macro-pore walls. After

270

30 cycles, the isotherm sorption/desorption curves of “CaCO3 precursor” was so

271

smooth

272

“NaBr/CaO-10/100” held a Ⅲ isotherm sorption curves, which was similar with that

273

of “CaCO3 precursor” after 30 cycles with low porosity; however, after 30 cycles, a

274

noteworthy H1 type hysteresis loop was observed in high relative pressure zone,

275

which was extremely similar with the fresh CaO of “CaCO3 precursor”. The porosity

276

of

277

“NaBr/CaO-10/100” was low in porosity after the first calcination, but over the

278

long-term cycles, it formed well-linked macro-pore structures, which was relatively

279

stable at high temperature reactions. Hence it can be speculated that the NaBr may

280

enhance the stability of microstructure of CaO sorbents.

that

“CaCO3

illustrated

precursor”

low

sank

porosity.

drastically

The

fresh

during

30

CaO

originated

cycles;

by

281 282

Table 4. BET specific surface area of calcined samples after various cycles


from

contrast,

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283 284 285

Fig. 8. N2 isotherm adsorption/desorption curves of the calcined samples after various cycles.

286 287

Conclusion

288 289

In this study, NaBr was introduced to enhance the cyclic CO2 capture capacity of

290

CaO sorbents. Results indicated that after 100 cycles, the “NaBr/CaO-10/100”

291

achieved a capacity of 0.202 g-CO2/g-sorbent, which was about 185% higher than that

292

of unmodified CaCO3 precursor, and the modified CaO sorbent captured about

293

24.370g of CO2 over the repeated 100 cycles, which was about 107% higher than that

294

of unmodified CaCO3 precursor; besides, the NaBr enhanced sorbent formed a

295

well-linked macro-pore structure, which illustrated relatively stable activity at high

296

temperature reactions. The mechanism of NaBr enhancement on doped CaO was

297

analyzed by STA, In-situ XRD, BET, ICP-OES and SEM. It revealed that NaBr inside

298

the CaO grains underwent drastically sublimate under high temperature (higher than

299

750 oC), but the interplanar spacing of CaO crystal cell became larger when the

300

temperature gone up, accordingly trapped sodium and bromide ion inside the grains,

301

and the sodium and bromide ion doped inside the CaO crystal lattice boosted the

302

durability of micropore structures and cyclic CO2 capture capacity. In conclusion,

303

NaBr is an effective promoter that has the ability of enhancing the cyclic CO2 capture

304

capacity of CaO-based sorbents.


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Acknowledgements

307

The financial supports from National Natural Science Foundation of China (No.

308

51606076) and the Foundation of State Key Laboratory of Coal Combustion

309

(FSKLCCB1705) are sincerely acknowledged. The authors are also grateful for the

310

support from the “Analytical and Testing Center” at Huazhong University of Science

311

& Technology.

312 313

References:

314 315

(1) Pachauri, R. K.; Allen, M. R.; Barros, V. R.; Broome, J.; Cramer, W.; Christ, R.;

316

Church, J. A.; Clarke, L.; Dahe, Q.; Dasgupta, P., Climate change 2014: synthesis

317

report. Contribution of Working Groups I, II and III to the fifth assessment report of

318

the Intergovernmental Panel on Climate Change. IPCC: 2014.

319

(2) Lackner, K. S., A guide to CO2 sequestration. Science 2003, 300, (5626),

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456 457


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458 459 460

Table 1. Reaction conditions for calcium looping. Temperature(°C)

N2(ml/min)

CO2(ml/min)

Duration(min)

carbonation

850

50

120

5

calcination

850

170

0

5

461 462


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463 464 465

Table 2. Accumulative CO2 capture capacity over 100 calcium looping process. Samples

Accumulative CO2 capture capacity(g)

CaCO3 precusor

11.768

NaBr/CaO-0.5/100

21.320

NaBr/CaO-1/100

22.460

NaBr/CaO-5/100

24.522

NaBr/CaO-10/100

24.370

466 467


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468 469 470

Table 3. Mole ratio of Na:Ca (tested by ICP-OES) Samples

mole ratio of Na/Ca

NaBr/CaO-10/100 before calcination

10:100

NaBr/CaO-10/100 after calcination

3.7:100

471 472


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473 474 475

Table 4. BET specific surface area of calcined samples after various cycles Sample

BET specific surface area(m2/g)

CaCO3, after 0 cycles

10.719

NaBr/CaO-10/100, after 0 cycles

3.724

CaCO3, after 30 cycles

6.368

NaBr/CaO-10/100, after 30 cycles

7.844

476 477 478


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479

480 481 482

Fig. 1 schematic diagram of calcium looping process(CaLP).

483


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484 485

486 487 488

Fig. 2. Schematic diagram of the simultaneous thermal analysis system.

489


Energy & Fuels 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

490 491

492 493 494

Fig. 3. Cyclic CO2 capture behavior of the NaBr enhanced sorbents.

495


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496 497

498 499 500

Fig. 4 Thermoanalysis of NaBr doped CaO by a STA at high temperature.

501


Energy & Fuels 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

502 503

504 505 506

Fig. 5 In situ XRD patterns of “NaBr/CaO-10/100” at different temperatures (step

507

size= 0.020481) (1#, room temperature; 2#, heating up to 650 oC; 3#, heating up to

508

850 oC; 4#, cooling back down to 650 oC; 5#, cooling back down to room

509

temperature)

510


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512 513 514

Fig. 6 FSEM images of calcined sorbents after 50 cycles: (a) and (b) CaCO3 precursor; (c) and (d) “NaBr/CaO-1/100”; (e) and (f) “NaBr/CaO-10/100”

515


Energy & Fuels 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

516

517 518

Fig. 7 Elemental mapping of images (Na, Br, Ca, O).

519


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520

521 522 523 524

Fig. 8. N2 isotherm adsorption/desorption curves of the calcined samples after various cycles.

525 526 527


NaBr-Enhanced CaO-Based Sorbents with a Macropore-Stabilized

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