Impact of crystal types of AgFeO2 nanoparticles on the

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Impact of crystal types of AgFeO2 nanoparticles on the peroxymonosulfate activation in the water Ying Zhao, Hongze An, Jing Feng, Yueming Ren, and Jun Ma Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00658 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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Environmental Science & Technology

Impact of crystal types of AgFeO2 nanoparticles on the peroxymonosulfate activation in the water

Ying Zhao,† Hongze An,† Jing Feng,† Yueming Ren,*,† and Jun Ma*,‡ †College

of Material Science and Chemical Engineering, Harbin Engineering

University, Harbin, 150001, China ‡State

Key Laboratory of Urban Water Resource and Environment, Harbin Institute of

Technology, Harbin, 150090, China

*Corresponding

author. Tel./Fax: +86-451-82569890; +86-451-82368074 E-mail address: [email protected] (Y.M Ren); [email protected] (J. Ma) 1 ACS Paragon Plus Environment


1

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ABSTRACT

2

A simple co-precipitation method was developed to synthesize AgFeO2

3

nanoparticles (NPs) with hexagonal 2H and 3R polytypes coexistence. The ratio of

4

2H and 3R types in AgFeO2 NPs were regulated by controlling the calcination

5

temperature (300 oC, 400 oC, 500 oC). Such AgFeO2 NPs were used as heterogeneous

6

catalysts to activate peroxymonosulfate (PMS) for removal of Orange I (OI) in the

7

water. External water conditions effects and the stability of AgFeO2 NPs were

8

investigated. The catalytic performance of AgFeO2 NPs was found to be significantly

9

enhanced with the increasing content of 2H-AgFeO2. 1O2, O2•-, SO4•- and •OH were

10

identified as the dominating reactive oxygen species (ROSs) participated in the

11

catalytic process. The electron transfer of Ag0/Ag+ and Fe2+/Fe3+ cycles facilitated the

12

decomposition of PMS to generate ROSs. The surface hydroxyl groups (-OH) were

13

regarded as the catalytic active sites. The higher 2H-AgFeO2 content in AgFeO2 NPs

14

promoted the concentration of surface hydroxyl groups (C-OH) and the reactivity of

15

AgFeO2 NPs for PMS activation. Based on theoretical calculations, the 2H-AgFeO2

16

(004) plane with more Fe sites was more conducive to bind with the -OH compared to

17

3R-AgFeO2 (012) plane, ascribing to the stronger adsorption energy and shorter Fe-O

18

bond

length

between

2H-AgFeO2

2 ACS Paragon Plus Environment

and

-OH.

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19

INTRODUCTION

20

Orange I (OI) is typical one of azo dyes which are widely used in textile industry with

21

70% of total world production of dyes.1-3 They are very stable and resistant

22

biodegrade with the complex, aromatic molecular structure and electron-deficient azo

23

linkage.4 Some unfixed azo dyes will inevitably be discharged to cause serious

24

damage for the aquatic environment.5 Several treatment approaches including

25

biodegradation6, adsorption,7 filtration,8 coagulation9 and oxidative degradation10

26

have been applied, but these processes are proved to be limited in terms of cost and

27

effective application.

28

Peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs) have been

29

extensively used to degrade stubborn organic compounds due to the formation of

30

nonselective reactive oxygen species (ROSs), such as hydroxyl (•OH) and sulfate

31

(SO4•-) radicals, with high redox potential, wide pH range, and strong oxidizing

32

ability.11-13 Among PMS activation strategies, transition-metal based heterogeneous

33

catalysts including metal oxides,14 spinel ferrite,15 perovskite,16 and metal-organic

34

frameworks (MOFs)17 have a great advantage owing to the low energy consumption

35

and high efficiency. So far, cobalt-based catalysts have been considered as the best for

36

PMS activation.18 However, Co ions leaching and potential environmental toxicity

37

remarkably restrict its practical application. Therefore, other low-toxicity catalysts

38

have been prepared to improve the efficiency and steady. Wide strategies for

39

modifying metal-based catalysts are reported, including surface modification (-COOH,

40

C=O, etc.), doping (metal ions, C, S, N, etc.) or compositing (activated carbon,



metal

oxide,

etc.).19,20

But

some

synthesis

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41

graphene,

procedure

is

still

42

energy-intensive and complex. Recently, in situ modification to tune the crystal

43

microstructure (e.g. exposed special surfaces or crystallite phases) has received an

44

increasing attention.21 TiO2 is typically used as a microstructure regulation catalyst.

45

The surface of rutile TiO2 is more propitious to form surface hydroxyl groups with

46

respect to anatase TiO2, leading to the divergence of catalytic performance.22

47

Additionally, in comparison with {110} facet, the exposed {012} facet on CuFeO2

48

properly lengthens the O-O bond in H2O2, which favors the H2O2 activation and •OH

49

radical formation due to easy electron transfer.23 Therefore, it is highly desirable to

50

develop a high-performance and stable heterogeneous catalyst of PMS activation by

51

effective microstructure control.

52

Silver ferrite (AgFeO2) is a kind of nontoxic delafossite semiconductor, and its

53

common two crystal types include the hexagonal 2H (P63/mmc) and 3R (R-3m)

54

depending on different stacking of the alternating layers along the c-axis (Figure

55

S1).24

56

co-precipitation27 and solid-state reaction28 are reported. Most prepared silver ferrites

57

are formed as a mixture of 3R and 2H polytypes due to the limited thermal stability of

58

AgFeO2. Exclusively, pure 3R-AgFeO2 and 2H-AgFeO2 are prepared separately

59

under special conditions (high-pressure of 3 GPa and 6 GPa) by Terada et al.29 Thus,

60

a facile co-precipitation method is selected to obtain biphasic AgFeO2 without any

61

special conditions in our work. Besides, the calcination temperature as a significant

62

factor has been reported to influence morphology, crystal structure or other

Various

synthesis

methods

including

metathetical,25


hydrothermal,26

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63

physiochemical properties of catalysts.30

64

Usually, AgFeO2 has been used in other fields, such as electrical, biology and

65

optical, etc. Recently, it is considered as a promising photocatalyst to remove methyl

66

orange due to its narrow band gap.31 Later, some researchers further report its

67

heterostructure composites, including AgFeO2/g-C3N4,32 Ag/AgFeO2,33 AgFeO2-G,34

68

AgI/AgFeO2,35 AgCl/Ag/AgFeO2,36 all of whose exhibit an outstanding photocatalytic

69

performance. However, note that there is no report of AgFeO2 for PMS activation in

70

water purification, especially the impact mechanism of different crystal types on PMS

71

catalytic performance.

72

Herein, this work aims at regulating various ratio of 2H and 3R type in AgFeO2

73

NPs, obtaining abundant active sites by controlling calcination temperature using a

74

simple co-precipitation. An efficient heterogeneous catalytic PMS system was

75

established. The characters, catalytic activity and reusability of different crystal type

76

ratios of AgFeO2 NPs were carried out. The active site on the surface of AgFeO2 NPs

77

via the quantitative correlation of C-OH and OI removal efficiency is determined. The

78

mechanism of PMS activation occurring on the 400-AFO surface is elucidated by

79

ESR, quenching tests and XPS analysis. Finally, the density functional theory

80

calculation is applied to clarify the impact mechanism of crystal types on PMS

81

activation.

82

MATERIALS AND METHODS

83 84

Chemicals. All chemicals (Text S1) were used directly without any purification and the deionized (DI) water was employed for preparing all the solutions.



85

Catalysts preparation and characterization. AgFeO2 nanoparticles (NPs) were

86

prepared by a modified co-precipitation method as reference.27 First, 10 mmol

87

Fe(NO3)3·9H2O and 10 mmol AgNO3 were dissolved into 50 mL DI water with

88

continuous stir to form the homogeneous solution. Next, 4 M NaOH solution acting as

89

a precipitation agent was slowly added into the obtained solution under magnetic

90

stirring, then the mixture was heated in a water bath at 80 oC for 5 h. The precipitates

91

were collected by centrifuge and washed several times by the DI water, then they

92

were dried at 70 oC for 12 h. After that, the products were calcined at 300 oC, 400 oC,

93

500 oC for 2 h in the air atmosphere. The samples were abbreviated to 300-AFO,

94

400-AFO and 500-AFO and AFO (no calcination), respectively. Characterization

95

details were given in Text S2.

96

Oxidation procedure and analysis. An oxidation reaction was conducted in a

97

beaker containing 500 mL of 4 mg·L-1 OI at 25 ± 2 oC. First, pH value of the OI

98

solution was adjusted by 0.1 M NaOH solution. Then, 0.05 g catalyst was put into the

99

reactor under a sequential stir, later, 10 μmol PMS was added to initiate the oxidation

100

reaction to for OI degradation. Water samples were taken by a glass syringe at a

101

certain time-point and immediately quenched by sodium nitrite. Then it was filtrated

102

through 0.45 μm cellulose acetate membrane for OI analysis in the filter liquor. For

103

recycle, the used catalysts after several test were washed with deionized water and

104

ethanol by the centrifuge, then they were dried at 70 oC for 12 h and calcinated at 400

105

oC

106

for 2 h.

The concentration of OI was analyzed using TU-1810 UV-vis spectrophotometer


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107

(China) at a wavelength of 480.5 nm. The total organic carbon (TOC) was detected by

108

a total organic carbon analyzer (Multi N/C 3100, Jena, Germany). The concentration

109

of surface hydroxyl groups (C-OH, mmol·g-1) was analyzed by the saturated

110

deprotonation method,37 and the details were presented in Text S3. The reactive

111

species were detected by electron spin resonance (ESR, Bruker, Germany) using

112

5,5-dimethyl-1-pyrrolidine N-oxide (DMPO, 100 mM) and 2,2,6,6-tetramethyl-4-

113

piperidinol (TEMP, 100 mM) as the spin-trapping agents. The EPR spectra was

114

obtained at room temperature with a center field of 3480 G, a sweep width of 100 G, a

115

sweep time of 40 s, a modulation frequency of 100 kHz, a modulation amplitude of 1

116

G, a microwave frequency of 9.76 GHz, and a microwave power of 20-30 mW. The

117

pHzpc of the samples was determined by mass titration.38 A pH meter (FE20, Mettler

118

Toledo, China) was used to measure pH values in the solution. Each experiment was

119

repeated for three times.

120

Theoretical calculation methods. The periodic density functional theory (DFT)

121

calculations were carried out using Cambridge Serial Total Energy Package (CASTEP)

122

codes.39 The generalized gradient approximation (GGA) with Perdew-Burke-

123

Ernzerhof (PBE) was utilized as the exchange-correlation function. The DFT-D

124

method was used for all calculations to consider the van der Waals forces.40 To avoid

125

the interaction of surfaces between periodic images, a 20 Å vacuum above the surface

126

was taken. For geometrical optimization, the convergence tolerance of energy was 2.0

127

e−5 eV·Atom-1, the max stress on the atoms was 0.1 GPa, and the maximum-allowed

128

force and displacement were 0.05 eV·Å-1 and 0.002 Å, respectively. All atoms were



129

allowed to be relaxed fully. The adsorption energy of -OH on the 2H-AgFeO2 (004)

130

plane (Eads(-OH)) was defined as: Eads(-OH) = E-OH on 2H-AgFeO2 (004) - E2H-AgFeO2 (004) - E-OH

131 132

where E-OH

133

E2H-AgFeO2 (004) and E-OH are the energy of 2H-AgFeO2 (004) and -OH, respectively.

134

The adsorption energy of -OH on the 3R-AgFeO2 (012) plane is obtained using the

135

same method.

136

RESULTS AND DISCUSSION

on 2H-AgFeO2 (004)

is the total energy of -OH on 2H-AgFeO2 (004), the

137

Characterization of different catalysts. The typical XRD patterns of catalysts

138

calcined at different temperature were displayed in Figure 1. Obviously, AgFeO2 NPs

139

were successfully synthesized without any impurity peaks such as Ag2O and FeOOH

140

existing. The peaks at 2θ of 34-42o and 49-55o in the diffraction pattern were broader

141

and asymmetry, indicating that all catalysts were consisted of hexagonal 2H (JCPDS:

142

25-0765) and 3R (JCPDS: 75-2147) phases.27 However, peaks belonging to (00l)

143

family were sharp and well separated. They were common to both 3R and 2H types

144

due to the equal distance between the layers arranging along the z-axis, this also made

145

the separation of the peaks belonging to each of the crystal types difficult. Hence,

146

phase fractions for all AgFeO2 NPs were determined by Rietveld refinement of XRD

147

patterns (Table 1, Figure S2).41 Clearly, 60.38% of diffraction peaks in AFO could

148

be indexed to the hexagonal delafossite structure of 3R-AgFeO2 with space group

149

R-3m. The remaining 39.62% was in good agreement with 2H-AgFeO2 with pace

150

group P63/mmc. After the calcination treatment, part of 3R-AgFeO2 in AgFeO2 NPs


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151

underwent crystal type transition to 2H-AgFeO2, and 400-AFO had a maximum

152

2H-AgFeO2 content of 60.44%. Such transformation could result in a change in the

153

surface properties of the AgFeO2 NPs, consequently affecting its catalytic

154

performance for PMS activation, which would be discussed later. Meanwhile, the

155

lattice parameters of 3R and 2H polytypes of different catalysts were also defined, it

156

could be found that the calcination strategy had no apparent effect on the crystalline

157

structure. The Debye-Scherrer formula was applied to estimate average crystallite size

158

of each catalyst according to the 2θ position of the three diffraction peaks (Table 1).

159

It indicated that the particles of all AgFeO2 NPs were constructed by small

160

nanocrystals with an average crystal size of about 20 nm. The slight increase in

161

crystal size after calcination was due to recrystallization.

162

The TG-DSC profiles of AFO under air atmosphere were mainly divided into four

163

stages (Figure S3). An initial weight loss of 0.4% below 100 °C was owing to the

164

evaporation of physisorbed water. The exothermic peak at about 235 °C was due to

165

the decomposition of remaining nitrate ions.42 Additionally, the wide exothermal peak

166

in the temperature region of 263-680 °C was related to the oxygen release from metal

167

oxides and the recrystallization.43 The crystal type transformed from 3R-AgFeO2 to

168

2H-AgFeO2 in this temperature region. Over 680 °C, the decomposition of AgFeO2

169

was observed to release oxygen leading to the sharp mass decrease (3.6%).44

170

Furthermore, the O 1s spectra (Figure S4) could be deconvoluted into two

171

characteristic peaks, including lattice oxygen species (Olatt) at about 529.5 eV and

172

surface hydroxyl groups or adsorbed oxygen (Oads) at about 531.1 eV, respectively.45



173

The amount of Olatt was found to be reduced by 20.9% after calcination at 400 °C.

174

The release of Olatt led to the formation of more oxygen defects in the catalyst, which

175

caused the partial structural skeletal collapse of AgFeO2. The arrangement of layer

176

stacking transformed from repeating the same atomic position in every third layer for

177

3R-AgFeO2 to every second layer 2H-AgFeO2, which increased the content of

178

2H-AgFeO2. It was in accordance with results revealed by TG-DSC.

179

TEM images showed that all AgFeO2 NPs owned homogeneous porous

180

nanoparticles with an approximate diameter of 25 nm (Figure 2a, d, Figure S5a, b).

181

It was in good agreement with the size estimated from the XRD results. Moreover, the

182

morphology of AgFeO2 NPs had barely been altered after calcination under the air

183

atmosphere. Figure 2b, c indicated that the lattice intervals of 0.310 nm and 0.254 nm

184

for AFO were conformity with (004) plane of 2H-AgFeO2 and (012) plane of

185

3R-AgFeO2, respectively. For 400-AFO, the (004) plane of 2H-AgFeO2 with lattice

186

spacing of 0.311 nm was also found in Figure 2e. And the lattice spacing of 0.252 nm

187

was corresponding to (012) plane of 3R-AgFeO2 (Figure 2f). The above results meant

188

that both AFO and 400-AFO were mixed with 2H and 3R polytypes. The insert of

189

relevant selected area electron diffraction (SAED) patterns (Figure 2a, d) clearly

190

showed a polycrystalline structure with diffraction rings, which further confirmed the

191

coexistence of 2H and 3R polytypes in the obtained AgFeO2 before and after

192

calcination. The EDX mappings of AFO and 400-AFO (Figure S5c, Figure 2g)

193

illustrated that Ag, Fe, and O species were uniformly distributed throughout the

194

prepared particles.


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195

The N2 adsorption-desorption isotherms of the four catalysts (Figure S6) were all

196

of type IV with obvious hysteresis hoops of type H3, indicating the mesoporisity of

197

the samples. The specific surface areas of AFO, 300-AFO, 400-AFO, and 500-AFO

198

were 39.65 m2·g-1, 29.96 m2·g-1, 33.71 m2·g-1 and 29.48 m2·g-1, respectively (Table

199

S1).

200

Catalytic PMS performance and reusability of catalysts. Figure 3a indicated

201

the degradation efficiency of OI in various oxidation systems. Obviously, less than

202

2% of OI could be wiped off in PMS alone system, explaining that PMS itself could

203

hardly attack OI molecular without the addition of catalyst. A controllable test using

204

only 400-AFO catalyst proved that less than 5% of OI was removed, revealing its

205

negligible adsorption capacity for OI removal, and the natural light also exhibited a

206

very limited effect on the OI degradation (Figure S7). Only 37% of OI was got rid of

207

within 30 min in the PMS/AFO oxidation system. However, the calcined catalysts

208

exhibited a higher catalytic performance than that of AFO for PMS activation, and the

209

removal rate of OI could achieve 65%, 88% and 71% after 30 min catalyzed by

210

300-AFO, 400-AFO, 500-AFO, respectively. In addition, pseudo-first-order kinetics

211

were also established to further evaluate the catalytic reactivity (Figure S8). The

212

value of k in PMS/400-AFO system (0.068 min-1) was more than 4 times higher than

213

that of in PMS/AFO system (0.015 min-1), which implied PMS decomposition to

214

ROSs was improved in different levels by adjusting calcination temperature (Figure

215

3b). 400-AFO showed the highest efficiency for PMS activation in heterogeneous

216

catalytic system. This might be related to the more ROSs producing in such an



217

oxidation system.

218

Figure 3c illustrated UV-vis absorption spectra of OI variation at different time

219

intervals in different catalytic oxidation systems. The main absorbance peaks of OI

220

located at λ of 477 nm (-N=N-) originating from the n–π* transition of the azo, and

221

characteristic peaks in the UV region locating at 240 nm (benzene ring), 290 nm

222

(naphthalene ring) and 330 nm were related to the π–π* transitions of aromatic rings,

223

respectively.46 Obviously, the peaks at λ of 477 nm became weaker along with the

224

reaction time, suggesting that destruction of azo groups by oxidation in these four

225

catalytic processes. In addition to this rapid decolorization affect, the decay of the

226

absorbance at 290 nm or 330 nm was considered as evidence of aromatic parts

227

degradation in the dye molecule and its intermediates.47 Furthermore, OI was more

228

readily mineralized or broken down into small molecule intermediates in

229

400-AFO/PMS system than others (Figure S9).

230

Cycling tests were conducted to evaluate the reusability of 400-AFO in catalytic

231

PMS system. As shown in Figure 3d, the OI degradation decreased markedly after

232

three times use with simple reprocess of centrifugation and drying. However, the

233

catalytic performance would fully be recovered and remained well after three-cycle

234

runs accompanying with a thermal treatment at 400 °C for 2 h. Only a slight

235

decrement of catalytic activity might be due to the loss of few functional

236

surface-active sites after oxidation reaction. The visible Raman spectra (532 nm) of

237

400-AFO before and after reaction was used to characterize the changes on the crystal

238

structure. Figure S10a showed that two Raman active modes at 342 cm-1 and 626


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239

cm-1 could be assigned to Eg and A1g, respectively.48 Notably, after using, the intensity

240

of Raman spectra increased and A1g peak moved toward the lower Raman shifts, as

241

compared with the fresh 400-AFO. It indicated the increment of lattice oxygen with

242

the oxidation of metal ions, which coincided with XPS results below that the lattice

243

oxygen content of 400-AFO increased by 17.32% after used.49,50 Furthermore, the

244

XRD pattern of 400-AFO after the reaction (Figure S10b) observed that three new

245

diffraction peaks appeared at 2θ around 27.94o, 32.40o and 46.34o, respectively. This

246

suggested that a small amount of Ag was oxidized to form silver oxides during the

247

process of PMS activation, which was consistent with Raman and XPS results. And

248

they could be recovered effectively by thermal treatment. Moreover, there was no

249

obvious discrepancy on the morphology and surface area of 400-AFO after reaction

250

(Figure S11). These results demonstrated the high recyclability of 400-AFO for OI

251

degradation by activation of PMS.

252

Influential factors on PMS catalytic activity of 400-AFO. To exhibit the best

253

performance of tested catalysts and its high potential for practical application, the

254

influential factors including catalyst dose (0.02-0.2 g·L−1), PMS concentration (5-40

255

μM), initial OI concentration (2-6 mg·L-1) and solution pH (4-11) were further

256

examined in PMS/400-AFO system (Figure S12). The optimized 400-AFO dosage,

257

PMS concentration and initial OI concentration were 0.1 g·L-1, 20 μM and 4 mg·L-1,

258

respectively. The PMS/400-AFO system showed a wide range of pH application

259

(approximately 5-10). Detail discussions could be seen in Text S4.

260

Identification of active site on catalyst surface. The observed distinctions in



261

catalytic performance may be attributed to the differences of surface active sites of the

262

catalysts. Therefore, a serious of experiments were carried out to determine the

263

number of active sites on the surface of AgFeO2 NPs. Firstly, the surface acidity of

264

catalysts was tested by IR spectra using pyridine as probe molecule at room

265

temperature. In Figure S13, the peaks at 1215 cm-1 and 1596 cm-1 were assigned to

266

physisorption pyridine and the interaction with hydrogen bonds, respectively. The

267

peaks at 1438 cm-1 were due to chemisorption pyridine on Lewis acidic sites, as well

268

as the peaks at 1480 cm-1 belonged to both Lewis and Bronsted acid sites.51 It

269

suggested that sufficient Lewis acid sites existed on the surface of all catalysts at

270

different calcination temperature. They could act as the electron acceptor to react with

271

water molecular and could then transform to Bronsted acidic sites, leading to the

272

dissociation of water.52 Such conversion promoted the formation of effective hydroxyl

273

groups (-OH) on catalysts surface, combining with HSO5- for the next decomposition.

274

More suitable sites of -OH brought the high catalytic reactivity of AgFeO2 NPs.

275

Secondly, phosphate substitution experiments were conducted to check the vital

276

function of -OH as active sites on the surface of AgFeO2 NPs. In Figure S14a,

277

evidently, the degradation efficiency of OI was inhibited gradually as the

278

concentration of the phosphate increased from 0 to 10 mg·L-1. Less than 8% OI was

279

removed with the presence of 10 mg·L-1 phosphate in different catalytic PMS systems,

280

which was considered as the results of catalysts adsorption (Figure S14b). Phosphate

281

could react with SO4•- to produce less reactive radicals, while the reaction rate

282

constant was ~105 L·mol-1·s-1, which was much lower than that for SO4•- with organic 14 ACS Paragon Plus Environment

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pollutants (107-109 L·mol-1·s-1).53,54 Such that radical scavenging effects phosphate

284

was not obvious in the PMS/AgFeO2 system. Therefore, almost complete inhibition

285

on OI removal suggested that phosphate formed an inner-sphere complex with the

286

surface Lewis sites, and the maximum substituted for surface hydroxyl groups on the

287

catalyst in water.55 Thus, it was proved qualitatively that -OH were the key active

288

sites on the AgFeO2 NPs surface to control the cleavage of the peroxide bond in the

289

process of PMS oxidation.

290

Finally, the concentration of -OH (C-OH) on catalysts surface was quantitatively

291

measured by a saturated deprotonation method. From Table S1, the obtained C-OH of

292

raw AFO was relatively low of 2.00 mmol·g-1 and it attained a high concentration of

293

4.83 mmol·g-1 for 400-AFO. And C-OH presented a positive linear relationship (R2 =

294

0.951) with the OI removal rates in PMS catalytic oxidation systems (Figure S15).

295

The result conclusively showed the importance for surface -OH served as the active

296

sites for PMS decomposition and ROSs generation. It was consistent with our

297

previous work.15

298

PMS activation behavior by 400-AFO. In situ ESR test was conducted to verify

299

the main ROSs working in the 400-AFO/PMS catalytic process. As depicted in

300

Figure 4a, no characteristic peaks were identified in DMPO and PMS alone system,

301

indicating that no radical could be produced without catalyst. However, after

302

400-AFO addition, strong ESR signals of DMPO-OH adducts (with αN = αH= 14.8 G)

303

and DMPO-SO4 (with αN = 13.2 G, αH = 9.6 G, αH = 1.48 G and αH = 0.78 G) adducts

304

were detected as the coexistence of both •OH and SO4•-.56 Simultaneously, a relatively 15 ACS Paragon Plus Environment


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305

weak sextet peaks of DMPO-O2 (αN = 14.2 G, αH = 11.2 G, and αH = 1.3 G) adducts

306

were also observed.57 Moreover, Figure 4b witnessed the characteristic 1:1:1 triplet

307

signals for TEMP-1O2 in PMS alone systems, revealing 1O2 could be produced from

308

the PMS self-decomposition process.58 Notably, the addition of 400-AFO

309

significantly enhanced the relative intensity of the EPR signals, suggesting its

310

excellent catalytic performance to activate PMS. These results provided a direct

311

evidence for the generation of •OH, SO4•-, O2•- and 1O2 during the activation of PMS

312

by 400-AFO.

313

Quenching experiments were carried out to further confirm the contributions of

314

these ROSs (Figure 4c). In comparison with the control experiment (no scavenger),

315

the degradation rates of OI were reduced by 6% and 17% with the presence of

316

concentration of 100 mmol·L-1 of tert-butanol (TBA, •OH scavenger) or methanol

317

(MeOH, •OH and SO4•- scavenger), respectively.59 It indicated that SO4•- and minor

318

•OH

319

para-benzoquinone (p-BQ) was responsible for O2•- quenching, the addition of p-BQ

320

exhibited remarkable decrement of 54% on OI removal and the k constant was

321

reduced from 0.068 min-1 to 0.011 min-1 (Figure 4b).60 Additionally, sodium azide

322

(NaN3) was an effective scavenger for 1O2 (1 × 109 M-1·s-1), •OH (1.2 × 1010 M-1·s-1)

323

and SO4•- (2.52 × 109 M-1·s-1).61 The inhibition of OI degradation was obvious and

324

about 40% OI was removed when 10 mmol·L-1 NaN3 was added. NaN3 showed a

325

much higher inhibition effect on OI degradation comparing with methanol and TBA,

326

which can be an evidence for presence of 1O2 in solution to OI. Consequently, it had

participated

in

the

degradation

process.


Then,

10

mmol·L-1

of

Page 17 of 43


327

been validated that both O2•- and 1O2 were the dominant reactive species in the

328

400-AFO/PMS degradation process.

329

Moreover, N2 bubbling exerted insignificant variation on OI degradation (Figure

330

S16), revealing that the origin of O2•- and 1O2 generation from lattice oxygen in

331

AgFeO2 NPs or PMS rather than from the conversion of dissolved O2 in the reaction

332

solution. After adding p-BQ or NaN3, there was also no visible discrepancy on

333

inhibition effect under air and N2 atmosphere. It suggested that O2•- and 1O2 were still

334

produced when there was no dissolved O2 in the 400-AFO/PMS system.

335

XPS analysis was applied for better understanding the role of Ag, Fe, and O species

336

during heterogeneous oxidation process. There was almost no change on the element

337

composition in the fresh and used 400-AFO catalysts (Figure 5a). Figure 5b showed

338

the XPS spectra of Fe 2p for 400-AFO catalyst before and after reaction. The Fe 2p3/2

339

peaks at 710.6 eV, 712.3 eV and Fe 2p1/2 peak at 724.1 eV, 725.8 eV could

340

correspond to Fe2+ and Fe3+, with relative ratios to the overall Fe intensity of 37.13%

341

and 62.87%, respectively. The two shake-up satellite peaks at 719.3 eV and 732.5 eV

342

suggested that the presence of Fe3+ in fresh 400-AFO.62 After oxidation, the binding

343

energy of Fe 2p3/2 shifted from 711.5 eV to 711.2 eV, such transformation to a lower

344

binding energy proved that an amount of Fe3+ turned to Fe2+ on the surface of

345

catalysts,63 and about 3.55% Fe3+ was reduced to Fe2+ during the catalytic process.

346

In Figure 5c, the peaks located at 367.8 eV and 373.8 eV were assigned to Ag 3d5/2

347

and Ag 3d3/2 of Ag+ on the surface, while the rest two peaks at 368.3 eV and 374.2 eV

348

could be ascribed as Ag0, respectively.36 Based on the deconvolution of Ag 3d



349

envelop, the relative contents of Ag+ and Ag0 accounted for 43.82% and 56.18%,

350

respectively. After oxidation, they changed to 46.28% and 53.72%, respectively. This

351

signified that Ag0 would provide the electrons, leading to the increase of Ag+ during

352

the catalytic oxidation reaction, at the same time, the Ag+ would accept the electrons

353

from the system to form Ag0, which indicated the involvement of Ag0-Ag+-Ag0 in the

354

catalytic cycling.

355

For O 1s spectrum in Figure 5d, after catalytic reaction, it could be found that the

356

relative content of Oads reduced by 17.32%, which indicated that surface hydroxyl

357

groups were involved in the catalytic reaction accompanying the redox of metal ions.

358

The relative content of Olatt accordingly increased from 20.86% to 38.19%, implying

359

some Oads could convert into Olatt by obtaining electrons from the system, concomitant

360

with the oxidation of Ag0 and Fe2+.

361

Based on above results, possible process of the PMS activation by AgFeO2 NPs

362

was proposed (Eqs. 1-10) and also schematically shown in Figure 6. Water molecules

363

were adsorbed on part of metal ions which acted as Lewis sites to generate

364

Ag+/Fe3+--OH on the catalyst surface. After addition of PMS, the HSO5- species

365

would bond with Ag+/Fe3+--OH by the hydrogen bond to form Ag+/Fe3+-OH-HSO5-.

366

O-H stretching band of HSO5- became weaker due to the strong electron gravitation of

367

Ag+/Fe3+, resulting in the reduction of Ag+/Fe3+ to Ag0/Fe2+, respectively (Eq. 1).

368

Then, Ag0/Fe2+ could react with HSO5- adsorbed on the catalyst surface to generate

369

SO4•- and themselves were oxidized to Ag+/Fe3+, respectively (Eq. 2). Meanwhile,

370

SO4•- could partially react with H2O or OH- to generate •OH (Eq. 3). 1O2 and O2•18 ACS Paragon Plus Environment

Page 18 of 43

Page 19 of 43


371

would be generated through Eqs. 4-8.61,64 The redox cycles of Fe3+/Fe2+ and Ag+/Ag0

372

facilitated the electrons transfer and formation of various ROSs during AgFeO2

373

NPs/PMS oxidation. Meanwhile, some adsorbed O2 could acquire electrons to be O2-

374

along with the oxidation of metal ions (Eq. 9). Finally, the generating ROSs rapidly

375

interacted with the OI molecules and degraded them to small organic intermediates or

376

even CO2 (Eq. 10).

377

Ag  / Fe3  OH   HSO5   Ag 0 / Fe 2  SO5   H 2O

(1)

378

Ag 0 / Fe 2  HSO5   Ag  / Fe3  SO 4   OH 

(2)

379

SO 4   OH  / H 2O  SO 4 2  OH

(3)

380

HSO5  (SO5 2 )  H 2O  H 2O 2  HSO 4  (SO 4 2 )

(4)

381

 OH  H 2O 2  HO 2   H 2O

(5)

382

HO2   H   O 2 

(6)

383

2O2   2H 2O  H 2O 2  2OH  1 O 2

(7)

384

HSO5   SO5 2-  SO 4 2  HSO 4  1 O 2

(8)

385

O 2  4e   2O 2

(9)

386

SO 4  / O 2  /1O 2 /  OH  OrangeI  int ermediates  CO 2

387

Enhancement mechanism of calcination modification. Crystal phase of TiO2

388

could influence the numbers of surface hydroxyl groups which were considered to be

389

a decisive parameter contributing to the catalytic performance.22 It was wondering if

390

there existed a relationship between crystal type and the decomposition of PMS. To

391

test it, we correlated the values of k constants and C-OH to the ratio of 2H-AgFeO2 for

392

different catalysts. Interestingly, it was observed that the k constants in different


(10)


393

catalytic systems increased linearly with the raise of the relative contents for

394

2H-AgFeO2 (Figure 7, R2 = 0.921). Furthermore, a positive correlation between the

395

2H-AgFeO2 contents and C-OH existing for different calcination temperature catalysts

396

(R2 = 0.976). The C-OH enhanced with the 2H-AgFeO2 contents for different catalysts

397

and 400-AFO possessed a maximum 2H-AgFeO2 content of 60.44% with the most

398

amount of -OH on its surface.

399

Further, to explore why it is more propitious for the formation of -OH on the

400

surface of 2H-AgFeO2 in comparison with 3R-AgFeO2, DFT calculations were

401

performed to evaluate the adsorption energy of -OH onto the planes of 2H-AgFeO2

402

(004) and 3R-AgFeO2 (012), respectively. Based on XRD analysis, the (004) and (012)

403

planes were selected for target calculation of 2H-AgFeO2 and 3R-AgFeO2 crystal

404

owing to the maximum peak exposure.42 The optimized surface structure in Figure

405

S17 witnessed that only Fe atoms exposed on the 2H-AgFeO2 (004) plane, while both

406

Fe and Ag atoms appeared on the surface of 3R-AgFeO2 (012). Hence, DFT models

407

of favorable adsorption structures of -OH on different sites of 2H-AgFeO2 (004) and

408

3R-AgFeO2 (012) plane were established, respectively (Figure 8), and the

409

corresponding adsorption energies (Eads(-OH)) and bond length (lM-O) listed in Table

410

S2. Among them, compared with Ag site on 3R-AgFeO2 (012), the adsorption

411

enhancement of -OH on Fe atom on 2H-AgFeO2 (004) or 3R-AgFeO2 (012) was

412

found due to the higher Eads(-OH) and shorter lFe-O. It suggested the Fe atom was the

413

more active adsorption site of -OH on AgFeO2 surface. Moreover, there were more Fe

414

atoms to be exposed on the 2H-AgFeO2 (004) surface than 3R-AgFeO2 (012), and the


Page 20 of 43

Page 21 of 43


415

strongest Eads(-OH) (-4.085 eV) and shortest lFe-O (1.784 Å) indicated that it was more

416

favorable for -OH binding onto 2H-AgFeO2 (004). The theoretical prediction was in

417

good agreement with the experiments that 400-AFO with the highest 2H-AgFeO2

418

ratio exhibited the best catalytic reactivity for PMS activation.

419

The 2H-AgFeO2 content on the surface of AgFeO2 NPs could be effectively

420

tailored by suitable thermal treatment, and the stronger surface interactions between

421

-OH and 2H-AgFeO2 (004) favored to form the higher density of -OH on the surface

422

of 2H-AgFeO2, providing more active sites to bond with PMS and facilitating the

423

transport of electrons to attack PMS into ROS. Therefore, it was concluded that the

424

crystal type determined the density of -OH on the surface of catalyst, which affected

425

PMS activation, and the 2H-AgFeO2 was more appropriate for the oxidation system.

426

Environmental implications. A series of AgFeO2 NPs mixing with 2H and 3R

427

polytypes were synthesized via a facile co-precipitation method at different

428

calcination temperatures (300 °C, 400 °C, 500 °C). The well-defined AgFeO2 NPs

429

with different 2H-AgFeO2 content exhibited excellent catalytic activity in activation

430

of PMS for OI removal over relatively wide pH range, low PMS concentration and

431

catalyst dosage. Such a simple, low toxicity, low energy input, and comparatively

432

efficient PMS-based oxidation process was desirable for the degradation of

433

micropollutants in case of small-scale water treatment. Unlike from conventional

434

oxidation systems, the major ROSs in AgFeO2 NPs/PMS systems were revealed to be

435

1O

436

enhance C-OH and the reactivity of AgFeO2 NPs for PMS activation, as confirmed by

2

and O2•-. Importantly, the higher 2H-AgFeO2 content on AgFeO2 NPs could



Page 22 of 43

437

experimental results and DFT calculations. This work demonstrated that the crystal

438

type control was a critical parameter to design high-performance metal-based

439

catalysts with abundant active sites in the advanced oxidation water purification.

440 441 442

ASSOCIATED CONTENT Supporting

Information.

Text

S1-S4:

details

regarding

chemicals,

443

characterization, the description of saturated deprotonation method, influential factors

444

on PMS catalytic activity of 400-AFO; Table S1-S2: major characteristic parameters,

445

the calculation results of DFT; Figure S1-S17: different crystal structure of AgFeO2,

446

Rietveld refinement of XRD patterns for different catalysts, TG-DSC curve of AFO,

447

O1s XPS spectra of AFO and 400-AFO, TEM images of 300-AFO and 500-AFO, and

448

EDX mappings of AFO, N2 adsorption-desorption isotherms of AgFeO2 NPs, OI

449

removal in 400-AFO adsorption alone and 400-AFO/PMS without light, the

450

pseudo-first-order kinetic rate plot of OI degradation, TOC removal efficiency,

451

Raman spectra and XRD patterns of 400-AFO before and after reaction, TEM image

452

and N2 adsorption-desorption isotherms of 400-AFO after reaction, effect of various

453

external water conditions on OI degradation, infrared spectra of pyridine molecules

454

adsorbed on different catalysts, effect of phosphate on OI degradation in different

455

PMS oxidation systems, relationship between C-OH and OI removal efficiency in

456

various systems, catalytic elimination of OI under N2 bubbling in different reaction

457

systems, optimized surface structures of 2H-AgFeO2 (004) plane and 3R-AgFeO2

458

(012) plane.


Page 23 of 43


459

AUTHOR INFORMATION

460

Corresponding author

461

*E-mail: [email protected] (Y.M Ren), [email protected] (J. Ma).

462

Notes

463

The authors declare no competing ﬁnancial interest.

464

ACKNOWLEDGEMENTS

465

The authors appreciate the financial support of the National Natural Science

466

Foundation of China (No.51378141), Fundamental Research Funds for the Central

467

Universities

468

(LC2017020).

(HEUCFG201802),

Heilongjiang

Natural

Science

Foundation

469 470 471 472

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Page 33 of 43


Table 1. Crystallite size, lattice parameters and fraction of two phases 2H-AgFeO2 and 3R-AgFeO2 from Rietveld refinement of catalysts. 2H-AgFeO2

Crystallite size

(nm)a

a

[Å]b

c

[Å]b

3R-AgFeO2

Fraction (%)b

a

[Å]b

c

[Å]b

Fraction (%)b

AFO

19.40

3.038

12.404

39.62

3.039

18.663

60.38

300-AFO

22.73

3.039

12.394

51.04

3.040

18.669

48.96

400-AFO

19.33

3.033

12.402

60.44

3.037

18.672

39.56

500-AFO

23.33

3.039

12.405

53.25

3.040

18.675

46.75

a

Rwpb, Rpb 0.1178 0.0765 0.1051 0.0723 0.1196 0.0776 0.1220 0.0813

Calculated by the Debye-Scherrer equation: DXRD=Rλ/Bcosθ, where DXRD is the average crystallite size (nm), R

is 0.89 (Scherrer’s constant), λ means the incident wavelength, which is 0.154 nm in this study, and B and θ are the width at half maximum and diffraction angle of (002), (004) and (110), respectively. b

Determined using Rietveld refinement of XRD patterns.



Figure captions Figure 1. XRD patterns of different catalysts. Figure 2. TEM images of (a) AFO and (d) 400-AFO, the insets are SAED patterns, HRTEM images of (b, c) AFO and (e, f) 400-AFO, and EDX mappings of (g) 400-AFO. Figure 3. (a) Removal efficiency, (b) k constants, (c) time-dependent UV-vis absorption spectra of OI in different PMS oxidation systems, and (d) consecutive use of the catalytic activity of 400-AFO. Catalysts: 0.1 g·L-1, C0[OI]: 4 mg·L-1, C[PMS]: 20 μmol·L-1, Initial pH 7.0, T: 25±2 °C. Figure 4. (a) DMPO and (b) TEMP trapped EPR spectra of PMS activation over 400-AFO catalyst. (c) Effect of different scavengers on OI degradation, and (d) corresponding k constants in the 400-AFO/PMS oxidation system. Catalysts: 0.1 g·L-1, C0[OI]: 4 mg·L-1, C[PMS]: 20 μmol·L-1, C[MeOH] = C[TBA]: 100 mmol·L-1, C[p-BQ] = C[NaN3]: 10 mmol·L-1, Initial pH 7.0, T: 25±2 °C. Figure 5. XPS spectra of (a) the overall survey, (b) Ag 3d, (c) Fe 2p, and (d) O 1s on 400-AFO before and after the catalytic PMS of OI degradation. Figure 6. Proposed process of PMS activation on the surface of AgFeO2 NPs in the water. Figure 7. Relationship between k constants, C-OH and ratio of 2H-AgFeO2. Figure 8. DFT calculation of -OH binding on (a) Fe atom on the surface of 2H-AgFeO2 (004), (b) Fe and (c) Ag atoms on the 3R-AgFeO2 (012), respectively.


Page 34 of 43

Page 35 of 43


(114) (116)

(008) (0012) (110) (110)

(105) (108)

(101) (012) (101) (102) (104) (103) (006) (009)

(004) (006)

(002) (003)

Intensity (a.u.)

Figure 1. XRD patterns of different catalysts.

500-AFO 400-AFO 300-AFO

AFO 2H-AgFeO2 JCPDS： 25-0765 3R-AgFeO2 JCPDS： 75-2147

10

20

30

40

50

2 degree

60


70

80


Page 36 of 43

Figure 2. TEM images of (a) AFO and (d) 400-AFO, the insets are SAED patterns, HRTEM images of (b, c) AFO and (e, f) 400-AFO, and EDX mappings of (g) 400-AFO.

(a)

(b)

2H (004)

(c)

3R (012) 2 1/nm

0.254 nm 3R (012)

0.310 nm

2 1/nm

2H (004)

2 nm

2 nm

20 nm 20 nm

(d)

2H (004) 2H (102)

3R (012)

(f)

(e) )

2 1/nm

2

0.252 nm

nm

3R (012)

0.311 nm

2 1/nm

2H (004)

20 nm 2 20

2 nm

nm

(g)

2 nm

2 nm

nm

Fe

Ag


O

Page 37 of 43


Figure 3. (a) Removal efficiency, (b) k constants, (c) time-dependent UV-vis absorption spectra of OI in different PMS oxidation systems, and (d) consecutive use of the catalytic activity of 400-AFO. Catalysts: 0.1 g·L-1, C0[OI]: 4 mg·L-1, C[PMS]: 20 μmol·L-1, Initial pH 7.0, T: 25±2 °C.

1.0

(a)

0.06 -1

k (min )

0.6 0.4

PMS 400-AFO 500-AFO 300-AFO AFO

0.2

0

5

0.04

0.02

10

15 20 Time (min)

25

30

0.00

AFO

300-AFO 400-AFO 500-AFO

1st run

1.0

2nd run

3rd run

0.8

C/Co

C/Co

0.8

0.0

(b)

0.08

0.6 0.4 0.2 0.0

uncalcination calcination

(d) 0


30

Time (min)

60

90


Page 38 of 43

Figure 4. (a) DMPO and (b) TEMP trapped EPR spectra of PMS activation over 400-AFO catalyst. (c) Effect of different scavengers on OI degradation, and (d) corresponding k constants in the 400-AFO/PMS oxidation system. Catalysts: 0.1 g·L-1, C0[OI]: 4 mg·L-1, C[PMS]: 20 μmol·L-1, C[MeOH] = C[TBA]: 100 mmol·L-1, C[p-BQ] = C[NaN3]: 10 mmol·L-1, Initial pH 7.0, T: 25±2 °C.

 



Intensity (a.u.)



 DMPO-OH

       

 

(b)

 DMPO-SO4  DMPO-O2



Intensity (a.u.)



(a)

PMS+400-AFO

PMS+400-AFO

PMS alone

PMS alone

3440

3460

3480

3500

3440

3520

(c)

0.08

0.8 k (min-1)

C/C0

3500

3520

(d)

0.06

0.6 Control TBA MeOH NaN3

0.4 0.2 0.0

3480

Magnetic Field (G)

Magnetic Field (G) 1.0

3460

0.04

0.02

p-BQ 0

5

10

15 20 Time (min)

25

30

0.00

Control


TBA

MeOH

NaN3

p-BQ

Page 39 of 43


Figure 5. XPS spectra of (a) the overall survey, (b) Ag 3d, (c) Fe 2p, and (d) O 1s on 400-AFO before and after the catalytic PMS of OI degradation.

(a)

Survey

Fe 2p O 1s

Fe 2p

Fe2+ (37.13%)

(b)

Ag 3d C 1s

Fe3+ (62.87%)

Fresh

Fresh

Intensity (a.u.)

Fe2+ (40.68%)

Used

Used 1000

800

600

400

0 750

200

Ag 3d

Fresh

740

Ag Ag (56.18%)

Used

Olatt

Oabs (79.13%)

(20.86%)

Used

Ag+ Ag0 (53.72%)

700

O 1s

(43.82%)

0

710

Fresh

+

Olatt

Oabs (61.81%)

(46.28%)

(38.19%)

(d)

(c) 376

Fe3+ (59.32%) 730 720

374

372

370

368

536

366

534

Binding Energy (eV)


532

530

528

526


Page 40 of 43

Figure 6. Proposed process of PMS activation on the surface of AgFeO2 NPs in the water.

AgFeO2 NPs

Na SO5•-

H2O

Mn+

O2

Mn+1

O2-

e

-OH

HSO5-

e SO4•-

S O C N H

H2O

OHHSO5-

•OH

Degradation

1O 2

Orange I

•-

M = Ag, Fe

O2


Page 41 of 43


Figure 7. Relationship between k constants, C-OH and ratio of 2H-AgFeO2. 5

k (min-1)

0.08

400-AFO

R2=0.976 500-AFO

0.06 300-AFO

4 3 2

0.04 AFO

R2=0.921

0.02

1 0

36

40

44 48 52 56 60 Ratio of 2H-AgFeO2 (%)


64

Surface hydroxyl concentration (mmol g-1)

0.10


Page 42 of 43

Figure 8. DFT calculation of -OH binding on (a) Fe atom on the surface of 2H-AgFeO2 (004), (b) Fe and (c) Ag atoms on the 3R-AgFeO2 (012), respectively.

(a)

(b)

(c)

1.784 Å

2.028 Å 1.794 Å

Ag

O

Fe


H

Page 43 of 43


Table of Contents (TOC) Art


Impact of crystal types of AgFeO2 nanoparticles on the

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