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Efficient Destruction of Pollutants in Water by a Dual-Reaction-Center FentonLike Process over Carbon Nitride Compounds (CN)-Complexed Cu(II)-CuAlO2 Lai Lyu, Dengbiao Yan, Guangfei Yu, Wenrui Cao, and Chun Hu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06545 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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Environmental Science & Technology

1

Efficient Destruction of Pollutants in Water by a Dual-Reaction-Center

2

Fenton-Like Process over Carbon Nitride Compounds (CN)-Complexed

3

Cu(II)-CuAlO2

4

Lai Lyuab, Dengbiao Yanc, Guangfei Yub, Wenrui Caoa and Chun Hu*,ab

5 6

a

7

Water Quality and Conservation of the Pearl River Delta, Ministry of Education,

8

School of Environmental Science and Engineering, Guangzhou University,

9

Guangzhou 510006, China b

10 11 12 13

Research Institute of Environmental Studies at Greater Bay, Key Laboratory for

Key Laboratory of Drinking Water Science and Technology, Research Center for

Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China c

School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China

14 15

*Corresponding author Tel: +86-10-62849628; fax: +86-10-62923541;

16

e-mail: [email protected] / [email protected]

17

1 ACS Paragon Plus Environment


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ABSTRACT

19

Carbon nitride compounds (CN) complexed with the in-situ produced Cu(II) on

20

the surface of CuAlO2 substrate (CN-Cu(II)-CuAlO2) is prepared via a surface growth

21

process for the first time, which exhibits exceptionally high activity and efficiency for

22

the degradation of the refractory pollutants in water through a Fenton-like process in a

23

wide pH range. The reaction rate for BPA removal is ~25 times higher than that of the

24

CuAlO2. According to the characterization, Cu(II) generation on the surface of

25

CuAlO2 during the surface growth process result in the marked decrease of the

26

surface oxygen vacancies and the formation of the C-O-Cu bridges between CN and

27

Cu(II)-CuAlO2 in the catalyst. The electron paramagnetic resonance (EPR) analysis

28

and density functional theory (DFT) calculations demonstrate that the dual reaction

29

centers are produced around the Cu and C sites due to the cation–π interactions

30

through the C-O-Cu bridges in CN-Cu(II)-CuAlO2. During the Fenton-like reactions,

31

the electron-rich center around Cu is responsible for the efficient reduction of H2O2

32

to •OH, and the electron-poor center around C captures electrons from H2O2 or

33

pollutants and diverts them to the electron-rich area via the C-O-Cu bridge. Thus, the

34

catalyst exhibits excellent catalytic performance for the refractory pollutant

35

degradation. This study can deepen our understanding on the enhanced Fenton

36

reactivity for water purification through functionalizing with organic solid-phase

37

ligands on the catalyst surface.

38


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INTRODUCTION

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Growing discharges of nonbiodegradable and persistent organic pollutants

41

adversely affect the environment and potentially threaten human health.1, 2 Advanced

42

oxidation processes (AOPs) have been applied for eliminating the recalcitrant organic

43

pollutants in water due to the generation of the highly reactive free radicals.3, 4 Among

44

various AOPs, Fenton reaction (Fe2+/H2O2) is an especially powerful and

45

environmentally friendly method,5, 6 since it does not require any special energy input

46

and can rapidly destruct the pollutant structure with the generated highly aggressive

47

free hydroxyl radical (•OH).7-9 Unfortunately, the classical Fenton reaction suffers

48

from its drawbacks such as the poor recyclability,10 the narrow working pH range

49

and the accumulation of Fe-containing sludge,12, 13 which restrict its wide applications.

50

To avoid or minimize these drawbacks, many researchers focus on looking for

51

efficient heterogeneous Fenton-like catalysts as alternatives to the homogeneous

52

process.14-18

11

53

However, few of the developed heterogeneous Fenton catalysts exhibit good

54

activity and high catalytic efficiency under neutral conditions, which is due to the

55

rate-limiting step upon the reduction of the stationary M(n+m)+ to Mn+ (M represent

56

metal species) by oxidizing H2O2 on the solid-liquid interface.9, 19-21 In addition, in

57

this step, H2O2 was finally decomposed into O2•− or O2, leading to invalid

58

consumption of H2O2.8, 12 Moreover, surface oxygen vacancies (Vo) were easy to be

59

produced in the synthesis process of the metal-containing catalyst, which often

60

promote the rapid decomposition of H2O2 to H2O and O2,19, 22, 23 leading low H2O2

61

utilization. The problems above are contributed to the dependency for the redox of the

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metal ions which are difficult to solve by conventional means.24

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Recently, our research20 has revealed that constructing dual reaction centers with 3 ACS Paragon Plus Environment


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the non-uniform distribution of the electrons in a catalyst by lattice-doping method is

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essential for overcoming the limitations of the classical Fenton reaction, which gives

66

us inspirations for in-depth exploration of more efficient dual-center Fenton catalysts

67

using other means for tuning the electron distribution. Cation–π interactions are

68

important intermolecular binding forces, which are relevant for the aromatic

69

recognition in the chemical and biological processes.25,

70

particularly crucial process in the cation–π interactions for transition-metal ions.26 It is

71

reasonable to believe that creating a special bonding bridge enhancing the cation–π

72

interaction through the charge transfer is the key for inducing the non-uniform

73

distribution of the electrons on the catalyst surface.

26

Charge transfer is a

74

Herein, a novel Fenton catalyst consisting of aromatic carbon nitride compounds

75

(CN) complexed with in-situ produced Cu(II) on the surface of CuAlO2 substrate

76

(CN-Cu(II)-CuAlO2)

77

CN-Cu(II)-CuAlO2 exhibits excellent Fenton-like activity and efficiency in a wide pH

78

range for the degradation of the refractory pollutants, as demonstrated with phenol,

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2-chlorphenol, ibuprofen, phenytoin and bisphenol A (BPA). The generation of Cu(II)

80

on the surface of CuAlO2 and the formation of the key C-O-Cu bridges between CN

81

and Cu(II)-CuAlO2 in the catalyst during the surface complexation process were

82

verified and characterized by several characterization techniques. The electron-rich

83

Cu and electron-poor C centers (i.e., dual reaction centers) are produced due to the

84

cation–π interactions through the C-O-Cu bridges in CN-Cu(II)-CuAlO2, as

85

confirmed by the electron paramagnetic resonance (EPR) analysis and density

86

functional theory (DFT) calculations. A preliminary effort to identify a correlation

87

between the surface electron properties of CN-Cu(II)-CuAlO2 and catalytic

88

performance has been undertaken, and a dual-reaction-center mechanism for the

is

prepared

via

a

surface


complexation

process.

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Fenton-like reaction has been proposed.

90 91

EXPERIMENTAL SECTION

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Preparation of catalysts. CN-Cu(II)-CuAlO2 is prepared by two steps. The first one

93

is to synthesize CuAlO2. In a typical procedure, 4.0 g of Cu(CH3COO)2•H2O was

94

dissolved in 30 mL of ethylene glycol to form solution A. Then, 7.5 g of

95

Al(NO3)3•9H2O was dissolved in 15 mL of ethylene glycol to form solution B. Next,

96

solution B was added to solution A and stirred at room temperature for 2 h. After the

97

mixture is kept at 150 ºC, vaporizing the ethylene glycol and volatile substances and

98

getting the dry gel precursor. Then, the dry gel precursor was calcined in a muffle

99

furnace at 1200 ºC in air for 2 h. The resulting solid was washed with deionized water

100

several times. After drying at 60 ºC, CuAlO2 was obtained. Next, the prepared

101

CuAlO2 and urea were mixed in deionized water with the mass ratio of 1:5. After that,

102

the mixture was calcined for 4 h at 550 ºC in a muffle furnace. After natural cooling,

103

the composite was obtained, which was designated as CN-Cu(II)-CuAlO2. The

104

pristine g-C3N4 was prepared using the same procedure without the addition of

105

CuAlO2.

106

Procedures and analysis. The Fenton-like catalytic experiments were carried out

107

under ambient conditions by using BPA, phenol, 2-chlorphenol, ibuprofen and

108

phenytoin as model contaminants. Their structures are shown in Figure S1

109

(Supporting Information, SI). In a typical experiment, 1.0 g L-1 catalysts were

110

dispersed in 25 mg L-1 pollutant solutions and the whole process is keep 35 ºC

111

(summer room temperature). The mixture was magnetic stirred for 30 min to establish

112

the adsorption/desorption equilibrium. Then, H2O2 was added to the pollutant

113

suspensions under stirring throughout the experiment. At given time intervals, 4 mL 5 ACS Paragon Plus Environment


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aliquots were collected and filtered using a Millipore filter (pore size 0.22 µm) for

115

analysis. Then, the enzyme catalase was added to destroy the residual H2O2.

116

The pollutant concentrations were measured by a high performance liquid

117

chromatography (HPLC, 1200 series; Agilent) with an auto-sampler, a Zorbax SB-Aq

118

column (4.6 × 250 mm, 5 µm; Agilent) and an UV detector at the wavelength of 225

119

nm. The mobile phase was a mixture of methanol/water and was operated at a

120

flow-rate of 1.0 mL min-1. The total organic carbon (TOC) was determined by a

121

TOC-VCPH analyzer (Shimadzu) using high-temperature combustion. The H2O2

122

concentration was determined using the reported DPD method.27 The amount of

123

metallic ions releasing from the catalysts during the reaction were measured using the

124

inductively coupled plasma optical emission spectrometry (ICP-OES) on an Optima

125

2000 (Perkin Elmer, U.S.A.). To test the catalyst stability and recyclability, the used

126

CN-Cu(II)-CuAlO2 sample was filtered, washed with water, and dried at 60 ◦C. The

127

catalyst was continued to be used in the second cycle.

128

BMPO-trapped

EPR

signals

were

detected

in

different

air-saturated

129

methanol/aqueous dispersions of the corresponding samples. Typically, in the absence

130

of H2O2, 0.05 g of the prepared powder sample was added to 1 mL of water (for

131

detecting •OH) or water/methanol (10%/90%, V/V, for detecting O2•−). 20 µL of

132

BMPO (250 mM) was added and held for 5 min. Then, the solution was sucked into

133

the capillary. The EPR spectra were recorded on a Bruker A300-10/12 EPR

134

spectrometer at room temperature. In the presence of H2O2, 0.01 g of the prepared

135

powder sample was added to 1 mL of water (for detecting •OH) or methanol (for

136

detecting O2•−). Then, 100 µL of the above suspension, 10 µL of BMPO (250 mM)

137

and 10 µL of H2O2 (30%, w/w) were mixed and held for 5 min. The solution was

138

sucked into the capillary to carry out EPR detection. 6 ACS Paragon Plus Environment

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The attenuated total reflection Fourier-transform infrared spectroscopy

140

(ATR-FTIR) was recorded using a Nicolet 8700 FTIR spectrophotometer (Thermo

141

Fisher Scientific Inc., USA) equipped with a Universal ATR accessory. To prepare an

142

ATR sample, 0.1 g catalyst and 2 mL D2O were added to a 5 mL centrifuge tube and

143

sonicated for 5 min. Subsequently, the solid in the stock was separated by

144

centrifugation and treated again with another aliquot of D2O in order to eliminate

145

residual H2O. The sample was sealed and centrifuged immediately. Half of the

146

supernatant was used as the reference, and the solid resuspended in the other half was

147

used as the sample. The BPA solution (100 ppm) was also prepared by D2O and the

148

prepared process of the sample with BPA was the same as that of the pure D2O. The

149

chemicals and reagents, characterization and density functional theory (DFT)

150

calculations are presented in the SI.

151 152

RESULTS AND DISCUSSION

153

Characterization of catalysts. Figure 1a shows the powder X-ray diffraction (XRD)

154

patterns of the as-synthesized samples. The XRD peaks in the samples of CuAlO2 and

155

CuAlO2 (550 ◦C, 4 h) were indexed to the standard CuAlO2 (e.g., 006, 101, 012, 009

156

and 018 planes at 2θ = 31.6°, 36.5°, 37.8°, 48.2° and 56.9°, respectively; JCPDS No.

157

73-9485),28 indicating that the second calcination procedure did not affect the

158

structural properties of the catalyst without the addition of urea. However, in addition

159

to the typical CuAlO2 diffraction pattern, two evident peaks indexed to CuO (200) at

160

2θ = 35.8°29 and CuO (111) at 2θ = 39.1°30 were observed for CN-Cu(II)-CuAlO2,

161

indicating that the second calcination procedure significantly influenced the crystal

162

structure of catalyst in the presence of urea, which led to the formation of Cu(II).

163

Differently, although the surface growth process of CN-Cu(II)-CuAlO2 was prepared 7 ACS Paragon Plus Environment


164

using the g-C3N4 synthesis process, there was no significant XRD peaks were indexed

165

to g-C3N4, suggesting that the produced surface species was not the standard g-C3N4

166

structure with the characteristic (100) and (002) planes due to the influence of the

167

surface Cu species of CuAlO2 during this surface growth process.

168

The field emission scanning electron microscopy (FESEM) images of Figure

169

S2a,b (SI) show that CuAlO2 consists of loosely packed grains with irregular shapes.

170

The size of the fine particles is less than 100 nm and that of coarse particles is about 2

171

µm. CN-Cu(II)-CuAlO2 (Figure S2c,d, SI) shows a larger particle size (0.4−3 µm)

172

and a smoother surface generally. This is due to the in-situ growth of urea and

173

covering the carbon nitride compounds on the CuAlO2 surface. Figure S3a (SI)

174

shows the transmission electron microscopy (TEM) of CN-Cu(II)-CuAlO2. As seen

175

from the micrographs, the CN-Cu(II)-CuAlO2 grain exhibits a plate like elongated

176

morphology, which may form due to anisotropic crystal growth observed in CuAlO2.

177

The HR-TEM image of CN-Cu(II)-CuAlO2 (Figure S3b, SI) exhibits clear lattice

178

fringes. The measured distances of 0.21 and 0.24 nm can be attributed to the dhkl of

179

(104) and (012) planes of the CuAlO2 substrate structure. The EDS linescan (Figure

180

1b) and TEM elemental mappings (Figure 1c) of CN-Cu(II)-CuAlO2 show that Cu,

181

Al and O were the main elements and uniformly distributed in the particle bulk

182

horizontally and vertically. C was mainly distributed on the surface and edge of the

183

particles. The top and bottom parts of C mapping were influenced by the C-containing

184

support grid for TEM measurements. Differently, the N signals (may affected by the

185

O element) were relatively weak on the whole scale. These results revealed the in-situ

186

growth and formation of CN on the catalyst surface during the synthesis process.

187

The X-ray photoelectron spectroscopy (XPS) spectra on Cu 2p3/2 of CuAlO2 and

188

CN-Cu(II)-CuAlO2 samples were exhibited in Figure 2a,b. For CuAlO2 (Figure 2a), 8 ACS Paragon Plus Environment

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the obvious binding energy (BE) peak at 932.1 eV was attributed by the reduction

190

state copper species.31 The auger parameters obtained at 1848.8 eV confirmed the

191

reduction state copper species belonging to Cu(I). This result suggested that Cu

192

species mainly exist in the form of cuprous state in CuAlO2. The weak peak located at

193

934.6 eV accompanied by the appearance of shake-up satellite lines were attributed by

194

the small amount of Cu(II)

195

revealed that both Cu(I) and Cu(II) existed on the surface of CuAlO2 with the Cu(I) to

196

Cu(II) atomic ratio of 1:0.61. After the surface growth process, the surface atomic

197

ratio of Cu(I) to Cu(II) was significantly changed (Figure 2b), which obtained with

198

1:1.79 for CN-Cu(II)-CuAlO2. The results indicated that the surface growth of the

199

carbon nitride compounds (CN) on the CuAlO2 substrate greatly increased the

200

proportion of Cu(II) on the surface of CN-Cu(II)-CuAlO2, suggesting the surface CN

201

compounds had a strong interaction with the Cu species during the synthesis process.

20

on the surface of CuAlO2. The XPS curve fittings

202

CuAlO2 showed mixed bands in the BE range of 72 to 80 eV (Figure S4a, SI),

203

the deconvolution of which indicated the co-presence of Al 2p (73.3 eV), Cu 3p3/2

204

(74.4) and Cu 3p1/2 (76.8 eV).32 However, Al 2p did not appear, and only Cu 3p3/2

205

(74.2) and Cu 3p1/2 (77.0 eV) were observed in the same BE range of

206

CN-Cu(II)-CuAlO2 (Figure S4b, SI). This result revealed that the surface growth of

207

CN obscured the Al species in the substrate.

208

In the C 1s XPS spectra of CN-Cu(II)-CuAlO2 (Figure 2c), the distinct bands at

209

284.7 and 288.7 eV corresponded to the sp2 C-C bonds and the sp2-hybridized C

210

(N-C=N) in the aromatic tri-s-triazine rings,33 respectively, which were very close to

211

the characteristic peaks of the pristine g-C3N4 (Figure S5a, SI). This result suggested

212

that the produced surface species on the CN-Cu(II)-CuAlO2 sample formed the

213

g-C3N4-like configuration although they were not the standard g-C3N4 due to the 9 ACS Paragon Plus Environment


214

influence of the surface Cu species of CuAlO2 during this surface growth process.

215

Differently, the intensity proportion of the peak corresponding to C-C at 284.7 eV

216

markedly increased, and the intensity proportion corresponding to N-C=N at 288.7 eV

217

significantly decreased in CN-Cu(II)-CuAlO2, which revealed that the excess C atoms

218

were incorporated into the tri-s-triazine of g-C3N4, and then occupied and replaced the

219

N atoms, resulting in the great increase of C-C and decrease of N-C=N. This result

220

could also be confirmed by the atomic ratio of C to N. The value of

221

CN-Cu(II)-CuAlO2 was ~63.9, which was much higher than that of the pristine

222

g-C3N4 (~0.79).

223

The above results suggested that the CN compounds on the surface of

224

CN-Cu(II)-CuAlO2 was actually the g-C3N4 containing the excess C (i. e. C-g-C3N4).

225

In addition, a new peak at 286.5 eV for the C 1s spectra emerged in

226

CN-Cu(II)-CuAlO2 (Figure 2c), which is ascribed to the C atoms of the aromatic ring

227

bonding to the deprotonated hydroxyl groups (C-O-M).34, 35 In combination with the

228

Cu XPS analysis of CuAlO2 and CN-Cu(II)-CuAlO2, the binding energy in Cu 2p3/2,

229

Cu 3p3/2 and Cu 3p1/2 were all shifted obviously after the surface growth of CN

230

compounds, indicating that the CuAlO2 substrate had a strong interaction with

231

C-g-C3N4, and the C-O-M bond occurs at the Cu sites. Thus, the evidence above

232

revealed that the connection between the surface CN compounds and the CuAlO2

233

substrate was achieved by the C-O-Cu bridge.

234

The N 1s XPS of CN-Cu(II)-CuAlO2 was fitted into three main peaks at 398.8,

235

400.1 and 402.3 eV (Figure S5b, SI), corresponding to the C=N-C in triazine rings,

236

the tertiary N in the form of N-(C)3 and the amino functional groups (C-N-H),33

237

respectively. The relative intensity proportion of C=N-C in triazine rings was greatly

238

decreased in CN-Cu(II)-CuAlO2 compared with the pristine g-C3N4 (Figure S5c, SI), 10 ACS Paragon Plus Environment

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239

which further confirmed that the N atoms were substantially replaced by excess C

240

atoms during the surface growth process.

241

The O1s XPS of CuAlO2 (Figure S5d, SI) shows strong signals in the range of

242

528−534 eV, which can be fitted into 3 deconvolutions around 530.2 eV (lattice

243

O-Cu(I) of CuAlO2),36, 37 530.9 eV (lattice O-Al(III) of CuAlO2)36 and 532.2 eV

244

(surface –OH on CuAlO2).38-40 For the O1s XPS of CN-Cu(II)-CuAlO2 (Figure 2d),

245

the three peaks were still retained. The peak at 530.2 eV obviously weakened

246

compared with that of CuAlO2, which was due to the conversion of Cu (I) to Cu (II)

247

after the surface growth process according to the foregoing analysis. The emergence

248

of a new peak at 532.5 eV futher confirmed the produced Cu(II) connecting with the

249

deprotonated hydroxyl group through C-O-Cu bonding bridge,40,

250

consistent with the result of C 1s XPS. In addition, the oxygen vacancy concentration

251

could be reflected from the variation of area ratio R of the lattice O 1s peaks to the

252

surface O 1s peaks (Plattice/Psurface).41 The value of R(CN-Cu(II)-CuAlO2)=2.68 was

253

much higher than that of R(CuAlO2)=1.89, indicating that the surface oxygen vacancy

254

(Vo) concentration of CN-Cu(II)-CuAlO2 was significantly lower than that of CuAlO2.

255

Formation of active dual reaction centers. The EPR technique was used to monitor

256

the electronic band structure information and investigate the presence of the single

257

electrons in the prepared samples. As shown in Figure 3a, g-C3N4 showed a sharp

258

EPR signal at g=1.996, which originated from the uncoupled electron of the skeleton

259

CN aromatic rings.42, 43 CuAlO2 showed no detectable EPR signals, indicating no

260

single electrons existing in CuAlO2. However, the as-prepared CN-Cu(II)-CuAlO2

261

displayed a very strong EPR signal at g=2.084, which was significantly enhanced

262

compared with the pure g-C3N4, revealing the conspicuous increase of the electron

263

density around Cu due to the Cu(II)-π interactions of the formed C-O-Cu bonding 11 ACS Paragon Plus Environment

41

which was


264

bridge on CN-Cu(II)-CuAlO2.44 These results indicated that the C-O-Cu produced by

265

the Cu(II) connecting with the deprotonated hydroxyl group on C-g-C3N4 gathered a

266

large amount of single electrons around the Cu, forming the electron-rich reaction

267

centers.

268

The distributions of the valence-electron density can provide useful information

269

for the reaction center analysis. The negative and positive areas are expected to be the

270

promising reactive sites for the reduction and oxidation reaction,45 respectively.

271

Therefore, we investigated the distribution of the valence-electron density on

272

CN-Cu(II)-CuAlO2 through DFT calculations. Figure 3b,c shows the optimized

273

geometry structure (left) and the corresponding two-dimensional valence-electron

274

density color-filled maps (right) of the CN-Cu(II)-CuAlO2 model fragments. The

275

construction and optimization of the geometry structure model were achieved by

276

using the B3LYP functional and Gaussian 09 package. The dangling bonds were

277

terminated with H atoms to obtain a neutral cluster. The valence-electron density was

278

analyzed using the Multiwfn package. As shown in Figure 3b, the largest electron

279

distribution area appears around the Cu atom, and its maximum valence-electron

280

density is as high as 4.0 e/Å3 compared with the O, C and N atoms in the

281

CN-Cu(II)-CuAlO2 model fragment, which theoretically confirms that the

282

electron-rich center is formed around the Cu atom, being consistent with the

283

experimental results. The valence-electron densities around C and N on the aromatic

284

rings of CN are remarkably lower than those around Cu and O atoms, revealing that

285

the π electrons on the surface of CN-Cu(II)-CuAlO2 shift to the Cu center (π→Cu, σ

286

donation) through the special Cu-O-C bonding bridge. Thus, the electron-poor centers

287

must be formed on the aromatic rings of CN. To accurately determine the electron

288

poor sites, the valence-electron density map of the CN fragments on the 12 ACS Paragon Plus Environment

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CN-Cu(II)-CuAlO2 model was magnified and presented on Figure 3c. Obviously, the

290

narrowest electron distribution area appears around the C atoms, and its maximum

291

valence-electron density is only ~0.2 e/Å3, which is much smaller than that of the N

292

atoms, indicating that the electron-poor sites is around the C atoms. In the aromatic

293

rings of CN, the π electrons tends to be concentrated around the N atoms, which

294

suggests that in addition to the π→Cu (σ donation) electron transfer process, C

295

replacing N in the CN-rings can also promote the formation of the electron-poor C

296

centers.

297

The highest occupied molecular orbital (HOMO) and lowest unoccupied

298

molecular orbital (LUMO) distribution information of the dual reaction center

299

fragment of CN-Cu(II)-CuAlO2 was obtained via DFT calculations (Figure S6, SI).

300

The energy separation of the HOMO and LUMO is often used as an indicator of

301

chemical reactivity.46 The HOMO–LUMO gap for pure g-C3N4 was calculated as 5.17

302

(α electron gap = β electron gap), representing a high kinetic stability and a low

303

chemical reactivity. However, the gaps of the α and β electrons for the dual reaction

304

center fragment of CN-Cu(II)-CuAlO2 evidently reduce to 3.62 eV and 2.24 eV,

305

respectively, which is energetically favorable to extract the electrons from a low-lying

306

HOMO and to add the electrons to a high-lying LUMO, forming an activated complex

307

of the potential reaction.47 This result theoretically demonstrates that the chemical

308

reactivity of CN-Cu(II)-CuAlO2 is significantly enhanced by the formation of the dual

309

reaction centers (electron-rich and electron-poor centers).

310

BMPO-trapped EPR spectra were used to detect •OH and HO2•/O2•− radicals in

311

the CN-Cu(II)-CuAlO2 air-saturated aqueous and methanol-aqueous dispersions

312

without adding H2O2, respectively. As shown in Figure S7a (SI), six distinct and

313

sharp

characteristic

peaks

of

BMPO-HO2•/O2•− 13

ACS Paragon Plus Environment

were

observed

in

the


methanol-aqueous

dispersion,

which

Page 14 of 33

314

CN-Cu(II)-CuAlO2

315

electron-rich reaction centers on CN-Cu(II)-CuAlO2 with copious single electrons

316

could efficiently reduce the dissolved O2 to HO2•/O2•−. By using the method of

317

detecting •OH radicals in aqueous dispersion, a series of distinct EPR signals were

318

also detected in the CN-Cu(II)-CuAlO2 suspension without adding H2O2 (Figure S7b,

319

SI). Although the EPR signals are complex due to the effect of the unpaired electrons

320

on CN-Cu(II)-CuAlO2, they also can split into four single lines with a spacing of ~14

321

G in the magnetic field due to the hyperfine interaction between the electron spin

322

of •OH and the orbital spin of N atom in BMPO. This result suggested that

323

CN-Cu(II)-CuAlO2 donated the electrons to O2 in the electron-rich reaction centers

324

and simultaneously accepted the electrons from H2O to produce •OH in the

325

electron-poor reaction centers, forming an electron transfer cycle. This effect was

326

predominantly due to the orbital interactions involving the electron transfer of π→Cu

327

(σ donation) and Cu→π* (π back-donation). All of the above evidence shows that the

328

activated dual reaction centers have been successfully built on CN-Cu(II)-CuAlO2.

329

Efficient degradation of pollutants over CN-Cu(II)-CuAlO2/H2O2 suspension.

330

The catalytic activities of the prepared samples were initially evaluated through the

331

degradation of BPA in the presence of H2O2 under the mild conditions. As shown in

332

Figure 4a, no significant BPA degradation was observed in the g-C3N4/H2O2

333

suspension. The degradation of BPA in the CuAlO2/H2O2 suspension was only 15.8%

334

within 120 min. Astonishingly, in the CN-Cu(II)-CuAlO2/H2O2 suspension, the

335

degradation degree of BPA could reach 95.5% within 120 min under the neutral and

336

mild conditions, which was 25 and 45 times higher than that in the suspensions of

337

CuAlO2/H2O2 and g-C3N4/H2O2, respectively. In addition, the TOC removal rate

338

reached 41.5% in CN-Cu(II)-CuAlO2/H2O2 suspension within 180 min (Figure 4b), 14 ACS Paragon Plus Environment

indicated

that

the

Page 15 of 33


339

which was greatly higher than that in the suspensions of g-C3N4 or CuAlO2 (almost no

340

any removal of TOC).

341

To further investigate the activity and adaptability of CN-Cu(II)-CuAlO2 for

342

different organic pollutants, four other refractory organic pollutants including phenol,

343

2-chlorphenol, ibuprofen and phenytoin were used to be degraded in the

344

CN-Cu(II)-CuAlO2 suspensions under natural conditions. As shown in Figure 4c, all

345

of the refractory compounds were substantially degraded and the degradation rate

346

could reached 88.3%-97.5% within 120 min. In addition, the degradation of higher

347

initial concentrations for BPA and 2-chlorphenol by CN-Cu(II)-CuAlO2 were also

348

carried out as shown in Figure S8 (SI). The degradation rates of both BPA (100 mg

349

L-1) and 2-CP (100 mg L-1) could reach ~90% within 120 min.

350

During the Fenton-like reactions, the maximum concentration of dissolved Cu

351

was ~0.8 mg L-1 (including the surface free Cu ions), which was much lower than the

352

limitations of the EU directives (< 2.0 mg L-1) and USA regulations (< 1.3 mg L-1).

353

No Al ions was detected in the CN-Cu(II)-CuAlO2 suspension. In addition, only 3.3%

354

TOC was removed for BPA degradation by the homogeneous Fenton reaction (Cu2+

355

concentration 0.8 mg L−1) within 120 min, which was much lower than that in the

356

CN-Cu(II)-CuAlO2 suspension, suggesting that the contribution for the pullutant

357

degradation of the released ions was negligible. The durability of CN-Cu(II)-CuAlO2

358

was tested by recovering the solid catalyst through filtration, washing and drying. As

359

shown in Figure S9 (SI), the degradation of BPA in the CN-Cu(II)-CuAlO2

360

suspension was not significantly decreased within the margin of tolerance even after 6

361

successive cycles of degradation testing. Cu2+ leaching from the catalyst significantly

362

decreased over the reuse cycles. These results suggested that CN-Cu(II)-CuAlO2 was

363

an efficient Fenton catalyst with a good repeatability and stability. 15 ACS Paragon Plus Environment


364

The pH is often regarded as a sensitive factor for Fenton reaction, so the effect of

365

the initial pH value on BPA degradation in CN-Cu(II)-CuAlO2 suspension was

366

determined as presented in Figure 4d. Under the neutral conditions, the catalyst

367

showed the highest activity. In the range of pH 7-9, the degradation of BPA slightly

368

decreased with the increase of the initial pH, but it still could reach 70% within 120

369

min at pH 9. In the range of pH 5-7, the degradation rate of BPA was almost

370

unchanged with the change of the initial solution pH, which indicated that the activity

371

of the catalyst was not obviously affected by the pH values. In aqueous solution, the

372

metal-containing catalyst often forms surface hydroxyl groups by dissociative

373

chemisorptions of water molecules,48 leading to the activity of the metal-containing

374

catalyst strongly dependent on the solution pH, while CN-Cu(II)-CuAlO2 avoid this

375

effect due to the formation of the dual reaction centers. After the reaction, the final pH

376

values of the solutions (initial pH 6, 7, 8 and 9) generally declined (Figure S10, SI)

377

due to the production of the acidic intermediates, such as some low-molecular weight

378

organic acids, from the BPA degradation. For the solution of initial pH 5, the final pH

379

value did not change, indicating that the acidic intermediates had been degraded under

380

this condition. We also carried out the activity evaluation of CN-Cu(II)-CuAlO2 at

381

room temperature (Figure S11, SI). Within 120 min, the BPA degradation rates in the

382

CN-Cu(II)-CuAlO2/H2O2 suspensions at room temperature (~25°C), 30°C and 35°C

383

were ~90%, ~93% and ~96%, respectively. The results suggested that the activity of

384

the catalyst for BPA degradation did not change obviously at the three different

385

temperatures and the reactions were not highly temperature-dependent.

386

Fenton-like reaction mechanism on active dual reaction centers. The H2O2

387

decomposition experiment was performed in the presence of the catalysts and BPA

388

(Figure 5a). The decomposition rate of H2O2 in the CN-Cu(II)-CuAlO2 system was 16 ACS Paragon Plus Environment

Page 16 of 33

Page 17 of 33


389

significantly slower than that in the CuAlO2 system, suggesting that CuAlO2 and

390

CN-Cu(II)-CuAlO2 exhibited the opposite reactivity for the decomposition of H2O2

391

compared to the degradation of BPA. CN-Cu(II)-CuAlO2 exhibited the highest

392

catalytic activity toward to the BPA degradation but consumed the least amount of

393

H2O2, revealing a high utilization efficiency of H2O2 in CN-Cu(II)-CuAlO2 system,

394

which indicated that the formation of the dual reaction centers were extremely

395

selective for promoting the pollutant degradation with minor spurious decomposition

396

of H2O2 to O2. Generally, oxygen vacancies often take part in the fast decomposition

397

of H2O2 into O2.19, 22 According to the previous XPS results, the oxygen vacancy

398

concentration of CuAlO2 was significantly more than that of CN-Cu(II)-CuAlO2.

399

Thus H2O2 was rapidly and invalidly decomposed in the CuAlO2 system with very

400

little pollutant degradation.

401

Figure 5b shows the in-situ ATR-FTIR spectra for the CuAlO2 and

402

CN-Cu(II)-CuAlO2 suspensions. In order to clearly distinguish the hydroxyl groups

403

from different sources, we carried out the experiments with D2O instead of H2O. The

404

bands in the range of 2200-2700 cm−1 are assigned to the -OD stretching of D2O ν(-OD)

405

on the catalyst surface, and the band at ~1190 cm−1 is assigned to the molecular

406

bending modes of D2O δ(D-O-D).49 For CuAlO2/D2O suspension, the ν(-OD) bands

407

centered at 2302 and 2560 cm-1. However, the bands toward adsorbing BPA on

408

CuAlO2 surface were shifted to low frequency (2550 and 2290 cm-1), which was due

409

to the deprotonation of the phenolic OH group of BPA complexing with the surface

410

Cu via σ bonding to the lone pairs of the oxygen atom.12 For CN-Cu(II)-CuAlO2/D2O,

411

the intensities of all the peaks corresponding to D2O greatly weakened, and the ν(-OD)

412

shifted to lower frequency (2285 cm-1) compared to the CuAlO2 suspension (2302

413

cm-1). The results indicated that the CN compounds had complexed with the surface 17 ACS Paragon Plus Environment


414

Cu via σ bonding on CN-Cu(II)-CuAlO2, forming the dual reaction centers, which

415

greatly impedes the direct adsorption of D2O on the Cu sites. In the presence of BPA,

416

the ν(-OD) band did not continuously shift to low frequency but to a higher frequency

417

(2304 cm-1), which is the evidence that the pollutants do not directly adsorb to the

418

electron-rich Cu centers, but are mainly degraded in the electron-poor centers during

419

the Fenton-like reaction.

420

The conversion paths of H2O2 on different surfaces of the catalysts during the

421

Fenton-like reaction process were studied by the EPR spin-trap technique with BMPO.

422

As shown in Figure 5c, after adding H2O2, very weak BMPO-•OH signals were

423

detected in the CuAlO2/H2O2 system, suggesting the low conversion rate of H2O2 to

424

•

425

very strong BMPO-•OH signals, which was ~4 times higher than that in the

426

CuAlO2/H2O2 system. This result indicated that H2O2 were efficiently reduced to •OH

427

by the free electrons on the electron-rich centers of CN-Cu(II)-CuAlO2. Similarly, no

428

significant BMPO-HO2•/O2•− signals were observed in the CuAlO2/H2O2 system

429

(Figure 5d) due to the direct oxidation of H2O2 by the surface oxygen vacancies on

430

CuAlO2, whereas four distinct characteristic signals of BMPO-HO2•/O2•− were

431

observed in the CN-Cu(II)-CuAlO2/H2O2 system. The strong BMPO-HO2•/O2•−

432

signals might originate from three reactions: the reduction of the dissolved O2 on the

433

electron-rich reaction centers, the oxidation of H2O2 on the electron-poor reaction

434

centers and the activation of H2O2 in the small amount of surface copper ions. These

435

phenomena suggested that the creation of the dual reaction centers could efficiently

436

and rapidly activate H2O2 and produce the reactive free radicals, which was extremely

437

selective for promoting the pollutant degradation with minor spurious decomposition

438

of H2O2 to O2. However, CuAlO2 with a great amount of oxygen vacancies only

OH on the surface of CuAlO2. In contrast, CN-Cu(II)-CuAlO2/H2O2 system exhibited


Page 18 of 33

Page 19 of 33


439

promoted the rapid and inefficient decomposition of H2O2 and did not result in the

440

generation of the reactive species in the solutions. This is the reason that

441

CN-Cu(II)-CuAlO2 exhibited the highest catalytic activity toward the pollutant

442

degradation but consumed the least amount of H2O2, whereas CuAlO2 exhibited low

443

activity for pollutant degradation but invalidly consumed a large amount of H2O2

444

during the Fenton-like reactions. These results confirmed that the formation of the

445

dual reaction centers on CN-Cu(II)-CuAlO2 was responsible for the high utilization

446

efficiency of H2O2 and the high Fenton-like reactivity. Moreover, the •OH signal

447

intensities for the CN-Cu(II)-CuAlO2/H2O2 system are not changed after adding BPA

448

(Figure 5c), indicating that the presence of the organic pollutants does not affect the

449

reduction of H2O2. However, the HO2•/O2•− signals are evidently weakened in the

450

CN-Cu(II)-CuAlO2/H2O2 system after adding BPA (Figure 5d), which is because the

451

electron-poor C sites tend to be complexed with the electron-rich organic compounds.

452

Thus, the organic pollutants can also act as electron donors at the electron-poor sites,

453

avoiding the oxidation of partial H2O2 into HO2•/O2•−.

454

The remained single electrons of CN-Cu(II)-CuAlO2 after reacting with the

455

dissolved O2/H2O and H2O2 were measured through EPR. As shown in Figure S12

456

(SI), the EPR signal intensity almost did not change after reaction with the dissolved

457

O2/H2O and H2O2. This result further confirmed the dual center reaction process and

458

revealed the electron compensation effect on the dual reaction centers that O2 or H2O2

459

captured electrons from the electron-rich centers, and simultaneously H2O or H2O2

460

donated electrons to the electron-poor centers. These donated electrons of H2O or

461

H2O2 were quickly diverted to the electron-rich centers from the electron-poor centers

462

due to the special connecting mode of Cu-O-C by the produced Cu(II) connecting

463

with the deprotonated hydroxyl group on C-g-C3N4. The EPR signal that did not 19 ACS Paragon Plus Environment


464

weaken after reaction revealed the excellent electron cycle capability of the dual

465

reaction centers on CN-Cu(II)-CuAlO2.

466 467

ASSOCIATED CONTENT

468

Supporting Information

469

Supplementary Methods and Figures S1-S12. This material is available free of charge

470

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

471

AUTHOR INFORMATION

472

Corresponding Author

473

*[email protected] / [email protected] (Hu C.)

474

Notes

475

The authors declare no competing financial interest.

476

ACKNOWLEDGMENTS

477

This work was supported by the National Key Research and Development Plan

478

(2016YFA0203200), the National Natural Science Foundation of China (51538013)

479

and the Science Starting Foundation of Guangzhou University (27000503151 and

480

2700050302).

481 482

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(44) Boukhvalov, D. W.; Osipov, V. Y.; Shames, A. I.; Takai, K.; Hayashi, T.; Enoki,

622

T., Charge transfer and weak bonding between molecular oxygen and graphene zigzag

623

edges at low temperatures. Carbon 2016, 107, 800-810.

624

(45) Chu, S.; Wang, Y.; Guo, Y.; Feng, J. Y.; Wang, C. C.; Luo, W. J.; Fan, X. X.; Zou,

625

Z. G., Band structure engineering of carbon nitride: In search of a polymer

626

photocatalyst with high photooxidation property. ACS Catal. 2013, 3, (5), 912-919..

627

(46) Zhang, L. P.; Xia, Z. H., Mechanisms of oxygen reduction reaction on

628

nitrogen-doped graphene for fuel cells. J. Phys. Chem. C 2011, 115, (22),

629

11170-11176.

630

(47) Aihara, J., Reduced HOMO-LUMO gap as an index of kinetic stability for

631

polycyclic aromatic hydrocarbons. J. Phys. Chem. A 1999, 103, (37), 7487-7495.

632

(48) Tamura, H.; Mita, K.; Tanaka, A.; Ito, M., Mechanism of hydroxylation of metal

633

oxide surfaces. J. Colloid. Interf. Sci. 2001, 243, (1), 202-207.

634

(49) Belhadj, H.; Melchers, S.; Robertson, P. K. J.; Bahnemann, D. W., Pathways of

635

the photocatalytic reaction of acetate in H2O and D2O: A combined EPR and

636

ATR-FTIR study. J. Catal. 2016, 344, 831-840.

637 638


Page 26 of 33

Page 27 of 33


639

Figure Captions

640

Figure 1. (a) XRD patterns of the prepared samples. (b) EDS linescan (from TEM)

641

and (c) elemental mappings (from TEM) of CN-Cu(II)-CuAlO2.

642

Figure 2. XPS spectra in Cu 2p for (a) CuAlO2 and (b) CN-Cu(II)-CuAlO2, (c) C 1s

643

and (d) O 1s for CN-Cu(II)-CuAlO2.

644

Figure 3. (a) EPR spectra of the fresh g-C3N4, CuAlO2 and CN-Cu(II)-CuAlO2

645

samples. (b) and (c) DFT calculations for the optimized structure (left) and the

646

corresponding two-dimensional valence-electron density color-filled maps (right) of

647

the CN-Cu(II)-CuAlO2 model in -Cu(II)-CN vision fragment (b) and -CN vision

648

fragment (c). Orange, red, gray, blue and white circles denote Cu, O, C, N, and H

649

atoms, respectively. The valence-electron density is given in e/Å3.

650

Figure 4. (a) BPA degradation curves in various suspensions with H2O2 (Insert shows

651

the corresponding kinetic curves). (b) TOC removal curves during BPA degradation.

652

(c) Decomposition curves of different pollutants in CN-Cu(II)-CuAlO2 suspensions

653

with H2O2. (d) Effect of initial pH values for BPA degradation. Reaction conditions:

654

Natural initial pH (pH 6-7, except d), initial pollutants 25 mg/L, initial H2O2 10 mM,

655

catalyst 1.0 g L−1.

656

Figure 5. (a) H2O2 decomposition curves during BPA degradation in CuAlO2 and

657

CN-Cu(II)-CuAlO2 suspensions. Reaction conditions: Natural initial pH, initial

658

pollutants 25 mg/L, initial H2O2 10 mM, catalyst 1.0 g L−1. (b) ATR-FTIR spectra of

659

CuAlO2 and CN-Cu(II)-CuAlO2 in D2O environment with/without BPA. BMPO

660

spin-trapping EPR spectra for (c) •OH and (d) HO2•/O2•− in various suspensions in the

661

presence of H2O2 with/without pollutants.

662



Page 28 of 33

663 ∗ g-C3N4

018

♥

♥ ♥ ♥

10

♥

♥

1200

♥

♥ ♥

♥

♥

♥

♥

♥

♥

CuAlO2 (550°C,4h)

♥

♥

♥

♥

C N O Al Cu

CPS

♥

CN-Cu(II)-CuAlO2

♥

110 1010

♥

009

♥

107

006

002 101 111 012 104

♥

♦ ♦

CuAlO2 ∗100 g-C3N4

664

1800

(b)

♥

♥ CuAlO 2 ♦ CuO

003

Intensity (a.u.)

(a)

600

♥ ♥

♥

∗ 002

20

30

40 50 2θ (degree)

60

0 0.0

70

0.2

0.4

0.6 0.8 Distance (nm)

1.0

1.2

665 666

Figure 1. (a) XRD patterns of the prepared samples. (b) EDS linescan (from TEM)

667

and (c) elemental mappings (from TEM) of CN-Cu(II)-CuAlO2.

668


Page 29 of 33


669

2.0x10

4

1.5x10

4

+

CuAlO2 Cu 2p +

1.0x10 5.0x10

3

(b) 1.8x10

4

1.5x10

4

1.2x10

4

9.0x10

3

6.0x10

3

3.0x10

3

Cu LMM-Auger

(Cu ) 932.1

+

Cu (1848.8)

1830

4

2+

Cu :Cu =1:0.61

CPS

CPS

(a) 2.5x104

1840 1850 1860 Auger Parameter (eV)

+

2+

Cu :Cu =1:1.79

CN-Cu(II)-CuAlO2 Cu 2p

Cu LMM-Auger

+

Cu (932.8)

+

Cu (1849.9) 2+

Cu (934.7)

1830

1840 1850 1860 Auger Parameter (eV) 2+

Cu satellite (941.6) (944.0)

2+

(Cu ) 934.6 2+

(Cu satellite)

0.0 925

0.0 930

670

935 940 945 Binding energy (eV)

950

4

(c) 1.2x10

925

950

CN-Cu(II)-CuAlO2 O 1s

Plattice/Psurface=2.68

4

4x10

284.7 C-C

9.0x10

CPS

4

CPS

935 940 945 Binding energy (eV)

4

(d) 5x10

CN-Cu(II)-CuAlO2 C 1s

3

3

6.0x10

3x10

530.2 O-Cu(I)

531.1 O-Al(III)

532.0 surface -OH

4

2x10

286.5 C-O-M

3

3.0x10

284

286

288

532.5 C-O-Cu

4

288.7 N-C=N

1x10

0.0

671

930

0 528

290

Binding energy (eV)

530

532 534 Binding energy (eV)

536

672

Figure 2. XPS spectra in Cu 2p for (a) CuAlO2 and (b) CN-Cu(II)-CuAlO2, (c) C 1s

673

and (d) O 1s for CN-Cu(II)-CuAlO2.

674



(a) 1.5x10

5

g-C3N4

g=1.996

Intensity (a.u.)

Page 30 of 33

1.0x10

5

5.0x10

4

CuAlO2

g=2.084

0.0 2.6

2.4

2.2

675

CN-Cu(II)-CuAlO2

2.0 1.8 g Value

1.6

1.4

(b)

(c)

676 677

Figure 3. (a) EPR spectra of the fresh g-C3N4, CuAlO2 and CN-Cu(II)-CuAlO2

678

samples. (b) and (c) DFT calculations for the optimized structure (left) and the

679

corresponding two-dimensional valence-electron density color-filled maps (right) of

680

the CN-Cu(II)-CuAlO2 model in -Cu(II)-CN vision fragment (b) and -CN vision

681

fragment (c). Orange, red, gray, blue and white circles denote Cu, O, C, N, and H

682

atoms, respectively. The valence-electron density is given in e/Å3.

683


Page 31 of 33


684

(b) 1.0

(a) 1.0

0.4

-1.5 -2.0 -2.5

g-C3N4

-3.0

CuAlO2

-3.5

g-C3N4

0.2

k=0.027

CN-Cu(II)-CuAlO2

0

30 60 90 Reaction time(min)

CuAlO2

120

0

g-C3N4

120

Phenol 2-Chlorphenol Ibuprofen Phenytoin

0.6 0.4

CuAlO2 CN-Cu(II)-CuAlO2

0

30

60 90 120 150 Reaction time (min)

(d) 1.0

180

pH=5 pH=6 pH=7 pH=8 pH=9

0.8

BPA C/C0

0.8

0.6 0.4 0.2

0.2

686

0.7

0.5

30 60 90 Reaction time (min)

(c) 1.0

0.0

0.8

0.6

CN-Cu(II)-CuAlO2

685

C/C0

TOC/TOC0

k=0.0011

-1.0

0.6

0.0

0.9

k=0.0006

0.0 -0.5

ln(C/C0)

BPA (C/C0)

0.8

0

30

60

90

0.0

120

0

30

Reaction time (min)

60 90 Reaction time (min)

120

687

Figure 4. (a) BPA degradation curves in various suspensions with H2O2 (Insert shows

688

the corresponding kinetic curves). (b) TOC removal curves during BPA degradation.

689

(c) Decomposition curves of different pollutants in CN-Cu(II)-CuAlO2 suspensions

690

with H2O2. (d) Effect of initial pH values for BPA degradation. Reaction conditions:

691

Natural initial pH (pH 6-7, except d), initial pollutants 25 mg/L, initial H2O2 10 mM,

692

catalyst 1.0 g L−1.

693



Page 32 of 33

694

(a)

1.0

(b)

Decomposition of H2O2

2302

CN-Cu(II)-CuAlO2

Absorbance (a. u.)

C/C0

CuAlO2/D2O

2290

0.8 0.6 0.4 0.2

1193

2560 CuAlO2/BPA/D2O

1190 2285

2550

CN-Cu(II)-CuAlO2/D2O

2304 CN-Cu(II)-CuAlO2/BPA/D2O

1190

2560

0.0

CuAlO2

0

30

695

(d)

•

BMPO- OH (with H2O2)

0

CuAlO2/H2O2

2400

•

2100 1800 1500 -1 Wavenumber (cm )

•−

0

CuAlO2/H2O2 CN-Cu(II)-CuAlO2/H2O2

CN-Cu(II)-CuAlO2/BPA/H2O2

CN-Cu(II)-CuAlO2/BPA/H2O2

3500 3520 Magnetic field (G)

3540

1200

BMPO-HO2/O2 (with H2O2)

CN-Cu(II)-CuAlO2/H2O2

3480

696

2700

180

Intensity (a.u.)

Intensity (a.u.)

(c)

60 90 120 150 Reaction time (min)

3480

3500 3520 Magnetic field (G)

3540

697

Figure 5. (a) H2O2 decomposition curves during BPA degradation in CuAlO2 and

698

CN-Cu(II)-CuAlO2 suspensions. Reaction conditions: Natural initial pH, initial

699

pollutants 25 mg/L, initial H2O2 10 mM, catalyst 1.0 g L−1. (b) ATR-FTIR spectra of

700

CuAlO2 and CN-Cu(II)-CuAlO2 in D2O environment with/without BPA. BMPO

701

spin-trapping EPR spectra for (c) •OH and (d) HO2•/O2•− in various suspensions in the

702

presence of H2O2 with/without pollutants.

703


Page 33 of 33


704

Table of Contents Art

705


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