NiAl2O4 in chemical looping

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The supplied oxygen properties of NiO/NiAl2O4 in chemical looping reforming of biomass pyrolysis gas: the influence of synthesis method Yan Sun, EnChen Jiang, Xiwei Xu, Jiamin Wang, and Zhiyu Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03234 • Publication Date (Web): 21 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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ACS Sustainable Chemistry & Engineering

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The Supplied Oxygen Properties Of NiO/NiAl2O4 in Chemical Looping

2

Reforming of Biomass Pyrolysis Gas: The Influence of Synthesis Method

3

Yan Sun, Enchen Jiang*, Xiwei Xu*, Jiamin Wang, Zhiyu Li

4

College of Materials and Energy in South China Agricultural University, Guangzhou 510640

5

Corresponding author：Jiang Enchen, Xu Xiwei

6

Email: [email protected];

7

Mailing address: NO.483, Wushan road, Tianhe distribution, Guangzhou City, Guangdong

8

Province, China.

[email protected]

9 10

Abstract: In the manuscript, the oxygen supplied properties of NiO/NiAl2O4

11

synthesized via different methods were investigated. Imp OCs showed highest

12

effective oxygen amount of 14.08% (9.32 %( NiO) and 4.76 %( NiAl2O4)) and

13

medium oxygen releasing rate via CO-TGA. The O transfer rate of OCs synthesized

14

with Pcp method is slow due to the big particle size. The supplied oxygen content is

15

much less at the first stage for OCs, because NiO is inside the NiAl2O4, and the

16

strong interaction between NiO and NiAl2O4. The supplied O rate of Imp OCs is

17

proper (not too fast or too slow) due to the uniform distribution and moderate particle

18

size of NiO, and proper interaction between NiO and NiAl2O4. The CH4 chemical

19

looping reforming varied from the full oxidation (CH4→CO2) to partial oxidation

20

(CH4→CO) followed by cleavage reaction(CH4→H2+C), depending on the content of

21

effective supplied oxygen amount in OCs. Moreover, it also proved that CO-TGA and

22

CH4 chemical looping reforming in fixed bed are two effective methods for analyze 1

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

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the supplied oxygen properties of OCs.

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Keywords: Supplied oxygen properties; NiO/NiAl2O4 oxygen carriers; Synthesis

25

method; Chemical looping reforming (CLR)

26

Introduction

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Biomass pyrolysis gas(BPG), including H2, CH4, CO, CO2 and a small amount

28

of C2H4 and C2H6, is the by-products of the fast pyrolysis biomass for bio-oil or the

29

convenient pyrolysis for bio-char. The yield is 1.5*105 m3/year. However, the

30

application in industry of BPG is restricted due to the low heat value and C/H ratio,

31

which is not beneficial for supplying heat or synthesizing chemical products.

32

Therefore, the high value utilization of biomass pyrolysis gas is promising and urgent.

33

At present, several technologies were considered as potential option for improving the

34

value of biomass pyrolysis gas, such as chemical looping reforming (CLR), steam

35

reforming and so on1,2. CLR is a competitive option to produce H2 as well as

36

capturing CO2 3.

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Moreover, oxygen carriers (OCs) is one of the most important factors in the CLR,

38

especially, the supplied oxygen properties of OCs such as supplied oxygen content,

39

supplied oxygen rate and supplied oxygen temperature were the key point for

40

improving the effectivity during the CLR4. However, the research about the supplied

41

oxygen properties is rare. Moreover, the methods for analysis the supplied oxygen

42

properties of OCs was rare. As usual, it was analyzed indirectly via the reactive

43

activity or target product yield. Moreover, no effective and easy methods or 2


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technology was found to directly test the supplied oxygen properties of OCs.

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It was widely accepted that the physical and chemical properties such as BET

46

surfaces, pore size and volume, the particle size and distribution, the interaction of

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active metal with the supports, played an important role in the supplied oxygen

48

properties of OCs5. For example, Akbari-Emadabadi found adding zirconium to the

49

OCs could improve the supplied oxygen properties and stability of OCs6.

50

Moreover, the physical and chemical properties of OCs were decided by the

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synthesis methods. For example, the synthesis method played an important role in the

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structures and active metals distribution of OCs. Table.1 shows the BET results of

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NiO/Al2O3 OCs synthesized with different method. It is obvious that BET surface

54

varied with the synthesized methods, and both of the applied route and performance

55

were different as well. However, there are rare publications focusing on the effect of

56

synthesis method on structure and supplied oxygen properties of OCs.

57 58

Table 1. Results of structure properties of OCs from different research theses Name

Synthesis method

Surface area

Pore size

Pore volume

/m2·g-1

/nm

/m3·g-1

Reference

NiO/Al2O3

Co-precipitation

142.2

9.5

0.34

7

NiO/Al2O3

Co-precipitation

126.0

9.4

0.29

8

NiO/Al2O3

Impregnation

49.1

15.4

0.22

9

NiO/γ-Al2O3

Impregnation

193.4

10.2

0.50

10

59 60

Ni-based OCs is one of the most extensive materials during CLR, due to high

61

supplied oxygen reactivity and stability.11. At current, typical support such as Al2O3, 3



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SiO2, TiO2, ZrO2 and zeolite was widely investigated. Among those supports, Al2O3

63

attracts extensive attention due to high thermos-stability, good porous structure and

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low cost. Both of the lab scale experiment and the magnify instrument test illustrated

65

that NiO/Al2O3 was potential option of CLC

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NiAl2O4 , which would decrease the activity of OCs5,12-15, in the high

67

temperature(1100-1400 ℃ ) or after cycles redox. Therefore, NiO/NiAl2O4 was

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investigated as the one of promising oxygen carrier during CLR.

12

. However, NiO/Al2O3 would form

69

In this manuscript, the supplied oxygen properties of NiO/NiAl2O4 synthesized

70

via different methods were investigated. Furthermore, the supplied oxygen properties

71

were analyzed via TGA and fixed bed reactor.

72

Method and Experiment

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OCs preparation

74

OCs were prepared via impregnation (Imp), precipitation (Pcp) and one pot

75

synthesis (Cop) methods. Imp and Pcp methods were based on the NiAl2O4

76

preparation. All of the methods were expressed briefly in Fig.1.

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(1) The synthesis of NiAl2O4

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Ni(NO3)2·6H2O (8.72g)and Al(NO3)3·9H2O(22.50g)

were dissolved in 300ml

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deionized water and the molar ratio of Ni/Al is 1:2. (NH4)2·CO3 (0.5mol/L) was

80

added in the mixture solution of nickel nitrate and aluminium nitrate till the pH

81

reached 4~5. Meanwhile, the nitrate solution became colloidal, and colloidal was

82

dried at 85℃ for 6h till it became solid. And the solid was calcined at 400℃ for 2h,

83

and then at 900℃ for 6h. Finally, the NiAl2O4 supporter was obtained. 4


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(2) The preparation of NiO/NiAl2O4 via imp method

85

NiAl2O4 supporters was impregnated in the nickel nitrate solution at 65℃ and

86

stirred for 12h. And then it was dried at 105℃ for 6h. Finally, calcined the dried solid

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powders at 400℃ for 2h and at 900℃ for 6h, then the Imp OC was obtained.

88

(3) The preparation of NiO/NiAl2O4 via Pcp method

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In the Pcp method, NiAl2O4 supporters was impregnated in the nickel nitrate

90

solution. And salvolatile solution(0.5mol/L) was added into the nitrate solution till the

91

pH is 8. After filtration, the residue was calcined at 400℃for 2h and at 900℃ for 6h.

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(4) The preparation of NiO/NiAl2O4 via one pot synthesis (Cop) methods

93 94

Cop method is the same with the procedure of the NiAl2O4 preparation, but the moler ratio of Ni/Al is 4.1:1.

95 96 97

Fig.1 Sketch of the process in different synthesis method OCs characterizations 5



98

Specific surface areas and pore volumes were determined from N2

99

adsorption/desorption isotherms obtained at 77 K using a Micromeritics Gemini VII

100

2390 gas-adsorption analyzer.

101

STA449C Jupiter® thermogravimetric analyzer was used to detect the OC

102

oxidation ability and the reaction temperature. Approximately 15 mg sample was

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placed in a Al2O3 crucible and heated at a rate of 10℃/min. N2 (30 ml/min) was used

104

as the protective gas. N2/CO (95%/5%) (30 ml/min) was used as purging gas.

105

Temperature increased from 30 to 900 °C, and kept at 900℃ for 10min.

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The Scanning electron microscopy (SEM) (Hitachi-S4800 FESEM) was used to

107

investigate the surface morphology difference between the fresh OCs and the spend

108

OCs.

109

The crystallographic phases of the samples were confirmed by XRD

110

measurements (D8 VENTURE Bruker, CuKα radiation). Powder patterns were

111

collected within a 2θ range from 5 to 80 with a step of 10°.

112

The analysis of supplied oxygen properties of OCs

113

The analysis of supplied oxygen properties of OCs for CH4 chemical looping

114

consumption in the fixed bed

115

The analysis of supplied oxygen properties of OCs for CH4 chemical looping

116

consumption in the fixed bed at 800℃. 5 g oxygen carrier was placed in the reactor,

117

which was used in our previous research16. After reaching the setting temperature, 6


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118

CH4 (20 vol% CH4/80vol%N2) was introduced into the fixed reactor. The total gas

119

flow rate was controlled by two mass flow controller at 60 mL/min (room conditions).

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The gas products were collected and analyzed by gas chromatograph (Agilent 6842A

121

GC). The analysis method of GC is similar with our previous research17. Nitrogen was

122

employed as a carrier gas. CH4 conversion and CO2 selectivity were calculated based

123

on the analysis results of GC according to the following equation:

124

125

126

CH4 conversion= 1 −

CO2 selectivity =

×

（）×

(1)

(2)

Nomenclature: V1x: Gas concentration of x in the Outlet gas;

127

V0x: Gas concentration of x in the inlet gas;

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Fv0: the gas flow rate of Inlet gas;

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Fv1: the gas flow rate of Outlet gas;

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The analysis of supplied oxygen properties of OCs for CO chemical looping

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consumption in TGA

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The analysis of supplied oxygen properties of OCs for CO chemical looping

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consumptions were carried out in a TGA reactor (NETZSCH, STA449C/PC). An

134

amount of 15 mg of oxygen carrier was placed in TG reactor. The oxygen carrier was

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heated to 800 ℃ under atmosphere of pure N2 at a rate of 10 ℃/min. CO (5 vol%

136

CO in N2) were introduced into TG reactor. The total gas flow rate of the reaction was

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controlled by a mass flow controller at specific flow rate of 20 mL/min (room 7



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138

conditions). The relative amount of NiO and NiAl2O4 were calculated based on the

139

analysis results of TGA according to the following equation:

140 141 142

NiO relative amount/% =

× ×

× 100%

NiAl2O4 relative amount/%= 1- NiO relative amount/%

(3) (4)

Nomenclature: MNiO: Relative molecular mass of NiO;

143

mNiO: The mass losing at first reduction peak;

144

Mo: Relative atomic mass of Oxygen;

145

m: The mass of sample;

146

Results and Discussion

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OCs characterization

148

XRD analysis of OCs prepared with three different methods

149

NiAl2O4 support was the control sample. The XRD was carried to analyze the

150

composition and crystal structure of the OCs, and the result was showed in Fig.2. It

151

observed that the main diffraction peaks were matched with the standard

152

JCPDS73-0239 (NiAl2O4) and JCPDS04-0835(NiO). The peaks at 37.00°, 44.99°,

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59.66°, 65.54° were assigned to the (311), (400), (511), (440) diffraction planes of the

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NiAl2O4 spinel, respectively. While the peaks at 37.25°,43.28° and 62.88°were

155

assigned to the (111), (200), (220) diffraction planes of the NiO. Moreover, NiAl2O4

156

strong diffraction peaks and NiO weak diffraction peaks were found in the XRD

157

pattern of support NiAl2O4, which induced that most of the NiO was converted into 8


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158

NiAl2O4 or the particle size of NiO is so small that it can’t be detected via XRD.

159

Noticeable, there are no obvious peaks for Al2O3 in all OCs. It is possible that the NiO

160

disperses on the surface of the OCs while the Al2O3 stays inside the OCs. The results

161

were similar with the research of Gil-Calvo et.al18.

162 163

Fig.2 the XRD patterns of different synthesis OCs

164

After adding NiO, the NiO diffraction peaks are obvious in the Pcp OCs and Imp

165

OCs, while the NiAl2O4 diffraction peaks turns weak relatively. Those proved that

166

NiO particle dispersed on the surface of the NiAl2O4 support and the crystal structure

167

was stable. However, for Cop OCs, showed NiO diffraction peaks is slightly weaker

168

and NiAl2O4 diffraction peaks is stronger than the other samples. Basing on the

169

difference between these syntheses methods, it is possible that nickel nitrate

170

concentrates is too high to disperse equably, or part of NiO particle is surrounded by

171

the NiAl2O4 particle. 9



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172

Moreover, the crystal size and relative crystallinity were list in Table 2. It can be

173

seen that the relative crystallinity increased when adding the NiO. Especially, the

174

relative crystallinity for Pcp-NiO/NiAl2O4 reached 100%. The results are consistent

175

with the peak intensity in XRD (Fig.2) and the peaks are much sharper than other OCs.

176

It is possible that the second calcination promoted the formation of crystal structure

177

for Imp-NiO/NiAl2O4 and Pcp-NiO/NiAl2O4. And it is obvious the particle size is

178

much bigger for the Imp-NiO/NiAl2O4 (37.6nm) and Pcp-NiO/NiAl2O4 (45.9nm) than

179

Cop-NiO/NiAl2O4 (18.5nm). It is possible that the second calcination also promotes

180

the increase of crystal size. Especially, the crystal size of NiAl2O4 increased from 7nm

181

to 19.4nm for Imp-NiO/NiAl2O4 and 31.4nm for Pcp-NiO/NiAl2O4 after two times

182

calcination.

183

Table.2 the crystal size and relative crystallinity of different synthesis OCs Catalysts

Crystal size/nm

Relative crystallinity/%

NiAl2O4

NiO

NiAl2O4

7±0.2

-

96.71

Cop-NiO/NiAl2O4

5.74±4.1

18.5±1.2

97.71

Imp-NiO/NiAl2O4

19.4±8.9

37.6±1.0

98.64

Pcp-NiO/NiAl2O4

31.4±12.1

45.9±17.6

100

184 185

XPS analysis of OCs prepared with three different methods

186

The XPS analysis result was showed in Fig.3.The entirety pattern and Ni2p3,

187

O1s, Al2p high resolution photoelectron spectroscopy of different OCs were

188

displayed in the Fig.3(a-d) respectively, while further peak-differentiating and

189

imitating results of Ni2p3 were showed as attached figure in each patterns. The 10


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190

element analysis indicated that the atoms ratio of Ni/Al in the surface of NiAl2O4

191

was approximate 1/2 which meets the stoichiometry of NiAl2O4. When adding NiO,

192

the theoretical calculation value for Ni/Al would increase to1.28. While, actually,

193

the Ni/Al ratio on the surface is 0.82, 1.10 and 1.20 for Cop、Imp and Pcp OCs,

194

respectively. It induced that the distribution of NiO on the Imp and Pcp OCs is more

195

uniform at the surface. while much of the NiO may be in the core part for Cop OCs.

196

Those also induced that the interaction between NiO and NiAl2O4 is varied with the

197

synthesis methods. Therefore, XPS was used to further evaluate the binding state of

198

NiO and NiAl2O4. It is obvious that there are two kinds of peak around 870~850eV.

199

Binding energy for one centered around 860eV was the satellite peak. And the other

200

one which is the main Ni2p3 peak is with binding energy around 856eV. After

201

peak-differentiating and imitating, there are two fitting peaks could be find in the

202

patterns. Based on the research of Gil-Calvo19, the standard signal of NiAl2O4 stays

203

at 856.0eV while NiO stays at 854.0eV. For support NiAl2O4 in the Fig.3(a)，there is

204

a weak Ni2+(NiO) peak，inducing that most of the element Ni stay as NiAl2O4 and

205

only a bit of NiO exist in the OCs which exhibit the same results of XRD. Moreover,

206

the binding energy of NiAl2O4 is matched with the standard substance, indicating

207

that NiAl2O4 owns better crystal structure. Compared the OCs synthesized with

208

three different methods, the binding energy of the spinel support NiAl2O4 in Cop

209

OC is different due to the effect of SMSI(Strong-Metal-Support-Interaction). There

210

is electron transfer between Ni2+ (NiO) and Ni2+(NiAl2O4), which enhances the

211

combination between NiO and NiAl2O4, resulting in the increasing of Ni2+(NiO) 11



212

B.E and decreasing of Ni2+(NiAl2O4) B.E. Nevertheless, B.E. of Ni2+(NiO) for both

213

Pcp and Imp OCs was 854.07eV, which was matched with the standard one. It

214

proved that the NiO was more decentralized and standalone in both of the Pcp and

215

Imp OCs. The B.E of Ni2+( NiAl2O4) in the Pcp and Imp OCs was a little lower than

216

the standard one. It is possible that NiO plays a role in the electric distribution of

217

NiAl2O4, resulting the B.E. of Ni2+( NiAl2O4) in Pcp and Imp OCs decreasing due to

218

the increasing of electric amount of NiAl2O4 on the surface. The decrease of B.E. is

219

beneficial for the O release in NiAl2O4 due to the weak absorption for O. Moreover,

220

the results of XRD and XPS show that the composition is similar for OCs

221

synthesized with different methods.

222

Fig.3 Survey XPS patterns of different OCs (a) NiAl2O4, (b)Cop OCs, (c)Pcp OCs

223

(d)Imp OCs

224

Structure properties 12


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225

The N2 adsorption–desorption isotherms of OCs were analyzed and showed in

226

Fig.4. All of the four fresh samples displayed typical Ⅳ type adsorption isotherm

227

and H1 type hysteresis loop. It is obvious that all OCs are mesoporous materials with

228

straight-through channel, meanwhile the mesoporous of the OCs are disorder. The

229

results are similar with the researches of Kim 20 and Jayasree21 that common synthesis

230

method caused the particle stacking and forming the disorder porous. It is clear that

231

the textural properties regulated by the synthesis methods.

232

structural parameters were listed in Table.3. The results showed that the BET surface

233

areas of NiAl2O4 support is 70.302 m2·g-1, while it was 55.560 m2·g-1, 48.849

234

m2·g-1and 40.317 m2·g-1 for Cop, Pcp and Imp OCs respectively. It is noticeable that

235

the Cop OCs obtained from one-pot synthesis method showed high surface and pore

236

volume, because NiO and NiAl2O4 are uniformly distribution in NiO/NiAl2O4 (Cop)

237

and stalk pore size is big due to the gap between different particles. However, the NiO

238

adhered on the surface of NiAl2O4 for NiO/NiAl2O4 (Pcp) and NiO/NiAl2O4 (Imp).

239

Therefore, the piled pore for pure NiAl2O4 or NiO is small. Moreover, the adding of

240

NiO also blocked the pores of NiAl2O4 during the impregnation, causing the

241

significant decrease of the surface area and pore volume.

13


The details about


Page 14 of 29

242 243

Fig.4 N2 adsorption/desorption isotherms and pore size distributions of fresh samples

244 245

Table.3 N2-absorption of OCs synthesized with different methods Pore

Pore volume

size/nm

/cm³·g-1

70.302

12.550

0.279

NiO/NiAl2O4(Cop)

55.560

16.770

0.216

NiO/NiAl2O4(Pcp)

48.849

11.555

0.178

NiO/NiAl2O4(Imp)

40.317

12.140

0.136

OCs

Surface area/m²·g-1

NiAl2O4

246

The SEM in Fig.5 was used to observe the difference of the surface

247

characterization in OCs synthesized with different methods. The NiAl2O4 support was

248

composed of regular sharps particle with the size of 50nm. And the SEM image was

249

the same with the one in the research of Jayasree21 which showed homogeneous and

250

uniformly grains. It is noticeable that NiO and Al2O3 particles, which showed

251

diffraction peaks in the XRD patterns, were not mixed in the particles, inducing that

252

there were rare NiO and Al2O3 existing in the OCs. The particle size is around 15nm 14


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253

in the Cop OCs and the small particle stacked together. Those revealed that there is

254

extra NiO which connected with NiAl2O4 particles as a bridge. And the connection of

255

NiO with NiAl2O4 prevented the agglomeration and growth of NiO particles.

256

A kind of strip particle which was thin and long was found in Pcp OCs. It is

257

possible that agglomeration NiAl2O4 particles was formed during the second

258

calcination. And some particles with 10 nm, which may derive from the loading NiO,

259

were stacking on the surface of the strip particles, inducing that that the particles

260

distribution were not homogeneous.

261

The particles in Imp OCs could be divided into regular big particle (50-100nm)

262

and anomalous small particle (10-20nm). It is possible that the big particle grows

263

from NiAl2O4 particles, arranging closely while the small one dispersed uniformly.

264

Jiang18 also found that NiO mainly covered the surface of the carrier in the formation

265

of particle stack in impregnation methods. Base on the results of SEM, it is clear that

266

the second loading results in bigger particle, and the different loading routes make the

267

formation of NiAl2O4 and NiO crystal in different way.

268

15



269 270

Fig.5 SEM images of fresh OCs with different synthesis method

271

(a) NiAl2O4 support, (b) Cop OCs, (c) Pcp OCs, (d) Imp OCs

272

On the basis of the above results, the OCs synthesized with different methods

273

showed different pore structure and particle distribution. It is obvious that the particle

274

size of Cop OCs is smallest with the grains mixing uniformity. The Pcp OCs showed

275

biggest average particle size and the particle size is inhomogeneous distribution. For

276

the Imp OCs the structure of the NiO is dispersing homogeneous on the surface of

277

NiAl2O4 particle, and particle size was not big as NiAl2O4 particle and or small as

278

NiO particle.

279

The oxygen supplied properties of OCs synthesized with different methods

280

The supplied oxygen properties of OCs for chemical looping reforming of CO on

281

the TGA

282

TGA was used to analyze the properties of supplied oxygen properties such as

283

the effective content of oxygen and rate of supplying oxygen. And CO is one of the

284

main and important composition in bio-gas, which could direct reflect the supplied

285

oxygen properties of OCs 22. The results were showed in Fig.6, based on the data from

286

TGA, the relative amount of effective O and supplied O temperature for NiO and

287

NiAl2O4 as well as supplied O rate could be calculated via the equation (4).

288

Generally, there were two mass losing peaks in the curve of TGA. One is in the 16


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289

range of 400℃-700℃, representing the O2- losing in NiO. The other one around 850℃

290

was the O mass loss peak of NiAl2O4. The conclusion is consistent with Jerndal23 who

291

also found that , the reduction peaks of NiO and NiAl2O4were 500-550℃ and

292

800-1050℃, respectively.

293 294 295

Fig.6 TGA results in the CO reduction reaction between different OCs Moreover, it is obvious that the supplied O for NiO was divided three different

296

stages in the TGA curves. Hou et.al

297

temperature of NiO due to the different interaction degree between NiO and NiAl2O4

298

support. The similar results also were obtained in several researches

299

reduction comprised two peaks. They represented the supplied O of NiO which

300

possessed weak and strong interaction with support.

24

found that the difference of reduction

24,25

that NiO

301

In this experiment, the supplied oxygen around 450℃, corresponded to the NiO

302

possessed weak interaction with support NiAl2O4 . While the NiO possessed strong

303

interaction peaks were divided into two parts around 480-700℃ due to the difference

304

of NiO particle size12. The amount, temperature and rate of supplied O were showed 17



305

in the Table.4.

306

It is accepted that the more NiO reduced in low temperature zone, the easier the

307

OCs to supply oxygen. For OCs synthesized with different methods, the supplied O

308

amount is lowest (1.22%) for Pcp OCs in the range of weak peaks. However, strong

309

peak of NiO in Pcp OCs is the highest (4.64%) and the supplied O temperature is

310

widest from 430-750℃. Those revealed that NiO particles dispersed asymmetrically

311

and NiO particle size was too big to inhibit the release of lattice oxygen. In contrast,

312

Imp OCs showed equal supplied O amount of NiO at each stage. When the interaction

313

of NiO with support is proper, the supplied O amount in low temperature can be

314

improved and the sintering can be hindered 25. The reduction peaks in Cop OCs was

315

concentrated in the first two peaks, due to the homogeneous particles size of Cop OCs.

316

However, the total effective supplied oxygen amount of NiO is in the order: Cop

317

NiAl2O4 in chemical looping

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