Selective Electrocatalytic Degradation of Odorous Mercaptans Derived

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Selective Electrocatalytic Degradation of Odorous Mercaptans Derived from S-Au Bond Recognition on a Dendritic Gold/Boron-doped Diamond Composite Electrode Shouning Chai, Yujing Wang, Ya-nan Zhang, Meichuan Liu, Yanbin Wang, and Guohua Zhao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00393 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on July 4, 2017

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Environmental Science & Technology

1

Selective

Electrocatalytic

Degradation

of

Odorous

2

Mercaptans Derived from S-Au Bond Recongnition on a

3

Dendritic Gold/Boron-doped Diamond Composite Electrode

4 5

Shouning Chai,†,‡ Yujing Wang,§ Ya-nan Zhang,† Meichuan Liu,† Yanbin Wang, †

6

and Guohua Zhao*†

7 8

†

9

Assessment and Sustainability, Tongji University, Shanghai 200092, China.

School of Chemical Science and Engineering, Shanghai Key Lab of Chemical

10

‡

11

Shaanxi 710055, China

12

§

13

Xi’an, Shaanxi 710032, China

College of Science, Xi’an University of Architecture and Technology, Xi’an,

School of Materials and Chemical Engineering, Xi’an Technological University,

14 15 16 17 18 19 20 21 22 1

ACS Paragon Plus Environment


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23

ABSTRACT:

24

To improve selectivity of electrocatalytic degrading to toxic, odorous mercaptans,

25

the fractal-structured dendritic Au/BDD (boron doped diamond) anode with

26

molecular recognition is fabricated through facile replacement method. The

27

characterizations of SEM and TEM show that the gold dendrites compose of

28

single crystalline and have high population of the Au (111) facet. The distinctive

29

structure endows the electrode with advantages of low resistivity, high active

30

surface area, and prominent electrocatalytic activity. To evaluate selectivity, the

31

dendritic Au/BDD is applied in degrading two groups of synthetic wastewater

32

containing thiophenol/2-mercaptobenzimidazole (targets) and phenol/2-Hydroxyb

33

-enzimidazole (interferences), respectively. Results show that targets removals

34

reach 91%/94%, while interferences removals are only 58%/48% in a short time.

35

The corresponding degradation kinetic constants of targets are 3.25 times and 4.1

36

times that of interferences in the same group, demonstrating modification of

37

dendritic

38

target-selectivity.

39

electrocatalytic degradation derives from preferential recognition and fast

40

adsorption to thiophenol depending on strong Au-S bond. The efficient, selective

41

degradation is attributed to the synergetic effects between accumulative behaviour

42

and outstanding electrochemical performances. This work provides a new strategy

43

for selective electrochemical degradation of contaminants for actual wastewater

44

treatment.

gold

on XPS

BDD and

could

effectively

EXAFS

further

enhance reveal

2


that

electrocatalytic the

selective

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45

Table of Contents (TOC)

46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 3



64

INTRODUCTION

65

Mercaptans, as raw materials for synthesis of pharmaceuticals, pesticides and fine

66

chemicals, are a kind of highly toxic, offensively odorous and corrosive pollutants.1

67

Thiophenol and 2-mercaptobenzimidazole (2-MBI) are two representative members

68

that more toxic than common aliphatic mercaptans.2,3 It is reported that, thiophenols

69

could target central nervous system, liver and kidney, which prolonged exposure in

70

water can cause a series of serious health problems in living body, for example, their

71

median lethal dose (LC50) values is at a low range of 0.01−0.4 mM in fish.4 Previous

72

studies also reported that the use of 2-MBI may induce tumors, cause allergic

73

reactions and be toxic to aquatic organisms.5 Considering their serious harmfulness

74

and the continuing environmental concerns, the disposal of wastewater containing

75

mercaptans has become an urgent problem at present. In the past decades, several

76

techniques have been studied for removing organosulfur pollutants such as

77

adsorption, photocatalysis, biodegradation, and so on.1,6-9 Nevertheless, their practical

78

applications are also limited by some defects including hard reutilization, low

79

efficiency, and easy sulfur-poisoning. Besides that, electrocatalytic oxidation with

80

anode is known as a promising approach for recalcitrant organic pollutants

81

decomposing since its strong oxidation performance, mild treatment condition and

82

environmental compatibility,10-12 which should also be suitable for degrading

83

mercaptans efficiently. However, toxic mercaptans with low level are always

84

coexisting with a great deal of less toxic, biodegradable contaminants in many effluent

85

streams. Regarding to these wastewaters, the application of electrocatalytic 4


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degradation is not optimal if it wants to achieve complete detoxification and

87

purification due to poor reaction selectivity to target contaminants and high electric

88

energy consumption. Previous studies have indicated that the poor selectivity is

89

resulted from some oxidative free radical with a strong redox potential.13 The poor

90

selectivity may be ascribed to nonselective hydroxyl radicals dominated reaction and

91

approximate oxidation potential of various pollutants with similar chemical

92

structure.14 To overcome these drawbacks, the combination electrocatalytic oxidation

93

with conventional biological treatment would be a more effective and energy-saving

94

process for actual multi-pollutants complex wastewater, i.e. preferentially degrade

95

toxic, odorous mercaptans to harmfulless, inodorous substances highly selective

96

electrocatalytic oxidation firstly, and then biological treatment is used to decompose

97

residuals.15,16 Thus, the problem how to realize selective electrocatalytic degradation

98

to target mercaptans becomes a great challenge and arduous task in the environmental

99

catalysis field.

100

For the electrocatalytic degradation process, the selection of anode is crucial since

101

the removal efficiency and energy consumption is directly determined by anode’s

102

properties.13 Boron-doped diamond film (BDD), as a new-style carbonaceous material,

103

has been widely considered as an outstanding electrode for applications in wastewater

104

treatment, in virtue of its remarkable chemical inertness, electrochemical stability,

105

low background current, and wide window potential.17,18 In operation, abundant

106

weakly physisorbed •OH are generated on the BDD surface when a certain positive

107

potential applied, which presents strong oxidation ability and plays a decisive role in 5



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decomposing the organic pollutants.19,20 Compared with Pt, CeO2, SnO2, and PbO2

109

etc. anodes, BDD as a non-active anode has a weak interaction between the anode

110

surface and electrogenerated •OH, leading to a larger overpotential for O2

111

evolution.12,13,21 Thus, BDD exhibits outstanding mineralization ability and great

112

current efficiency for recalcitrant organic pollutants decomposing. In spite of these

113

advantages, as-grown BDD also shows the disadvantage of lack of selectivity to target

114

contaminants in

115

target-selective oxidation on BDD can be achieved via in situ ingenious modification.

heterogeneous catalysis.11,22,23 Herein,

our concern

is if

116

It is noteworthy that many approaches have been proposed to improve selectivity in

117

photocatalysis field recently, including control of electrostatic attraction or repulsion

118

between pollutants and catalyst surface,24 construction of shape/size-selective

119

titanosilicate molecular sieves,25,26 graft a layer of precognition functional polymer or

120

specific organic molecules that might physisorb target molecules on photocatalyst,27-29

121

and even design of all inorganic molecular imprinted photocatalyst using target

122

template.14,30 Obviously, it could be concluded from previous studies that directed

123

diffusion of target pollutants and even preferentially enriched on the surface of

124

catalyst is prerequisite to achieve selective degradation through increasing their

125

chance of catalytic reaction. Nevertheless, aforementioned several approaches are not

126

suitable for enhancing electrocatalytic selectivity of BDD because of its distinctive

127

inertness, rigid microstructure, and high oxidative decomposing ability to modified

128

polymeric host matrix in molecular imprinted technology.

6


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Besides, others specific adsorption and recognition of catalysts to target molecules

130

could also use to improving electrocatalytic target-selectivity and efficiency. It is

131

well-known that the strong covalent S-Au bond (bond energy about 30~40 kJ mol-1)

132

between surfur and gold atoms could be easily formed, which has been widely

133

application in self-assembly and molecule detection.31-34 Therefore, we assume that

134

the selective electrocatalytic oxidation to low-level mercaptans wastewater would be

135

achieved by virtue of strong affinity of the −SH group toward gold surface. In view of

136

this, we can devote to design a novel anode via constructing nanostructured gold on

137

BDD to reach our goal. In general, the catalytic activity of nanostructured gold

138

depends on its particular architecture and configuration show different generally.35,36

139

Recently, various fractal nano-dendritic gold catalysts were prepared in a solution of

140

HAuCl4 in the ionic liquid [BMIM][PF6] or by means of dodecyl trimethyl

141

ammonium bromide, cyclodextrin or cysteine as assistant additions in aqueous

142

solution, and which was found possessing unusual performances due to its unique

143

nano architecture and high surface energy.29,37 Moreover, their dispersive,

144

unconsolidated, hyperbranched construction and exposed Au (111) facet are

145

appropriate to selectively adsorb sulfur-containing contaminants. In this work, a

146

dendritic Au/BDD composite electrode was constructed by a displacement grown

147

method with electrodeposited Zn as template in aqueous solution without any

148

structure-director and special medium. The physicochemical and electrochemical

149

characterizations of dendritic Au/BDD are investigated systematically. To determine

150

the catalytic selectivity of sample, thiophenol and phenol mixture solution, and 2-MBI 7



151

and 2-hydroxybenzimidazole (2-HBI) mixture solution are used in degradation

152

experiment. This work would provide a distinctive technique for selective removal of

153

sulfur-containing organic pollutants in wastewater treatment.

154

EXPERIMENTAL

155

Materials. BDD (10 × 60 mm) was purchased from Centre Suisse d'Electronique et

156

de Microtechnique SA (CSEM, Switzerland) and synthesized on a single crystal p-Si

157

(100) wafer by microwave-assisted chemical vapor deposition (MP-CVD) technique.

158

The final boron content was of the order of 8000 ppm, and the average resistivity was

159

~0.1 Ω cm.

160

Methanol was of HPLC grade, and ZnCl2, KCl, H3BO3, HAuCl4, thiophenol,

161

phenol, 2-MBI, 2-HBI and other chemicals used in this study were analytical grade,

162

purchased from Sigma-Aldrich Company. All these chemicals were used as received

163

without further purification. For solution preparation and chromatographic purposes,

164

ultrapure water (Milli-Q water, Millipore) was used.

165

Fabrication and Characterization of Dendritic Au/BDD. BDD was first immersed

166

in boiling aqua regia for 30 min to completely remove the contaminants on its surface,

167

then for -OH to terminate the sample surface, BDD underwent an anodic treatment at

168

+3 V in 2 M H2SO4, and finally washed ultrasonically by water, ethanol and water in

169

turn.38,39 The electro-deposition on the oxygen-terminated BDD was carried out in the

170

static 0.05 M ZnCl2/0.1 M KCl/0.05 M H3BO3 mixed solution with a constant applied

171

voltage -3.5 V for 120 s. And then, it was placed in the aqueous solution of 1 mM

172

HAuCl4 for a certain period and ensured that the replacement reaction of Zn particles 8


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was finished completely. Finally, the treated BDD electrode was kept in 0.1 M H2SO4

174

to remove residual Zn before the calcined treatment at 450 °C for 1 h and the

175

dendritic Au/BDD obtained.

176

The morphology of electrode samples was characterized by a scanning electron

177

microscopy (EFEG-SEM, Model Quanta 200 FEG, FEI) and a transmission electron

178

microscopy (TEM, JEM-2100, JEOL) with an accelerating voltage of 200kV. X-ray

179

diffraction (XRD, Model D/max2550VB3+/PC, Rigaku) analysis was performed

180

using a diffractometer with Cu K⍺ radiation, with an accelerating voltage of 40 kV

181

and current of 30mA. The static state contact angle of pure water on the electrode

182

surface was determined by drop shape analysis system DSA100 (Krüss, Germany) at

183

room temperature. The XPS spectra were determined on a X-ray photoelectron

184

spectrometer (AXIS-ULTRA DLD, Shimadzu, Japan) using monochromatized Al Kα

185

radiation (1486.6 eV). The binding energy measurements were corrected for charging

186

effects with reference to the C1s peak of the adventitious carbon (284.6 eV). X-ray

187

absorption measurements were performed at the beam line BL14W1 of the Shanghai

188

Synchrotron Radiation Facility (SSRF) of China. X-ray absorption near-edge structure

189

(XANES) spectra and Au L3-edge extended X-ray absorption fine structure (EXAFS)

190

were recorded in a transmission mode by using ion chambers to measure the radiation

191

intensity. The station was operated with a Si (111) double-crystal monochromator.

192

All electrochemical measurements were carried out in a three-electrode cell system

193

of CHI 660c (CHI Co., USA) electrochemical workstation. A saturated calomel

194

electrode (SCE) served as the reference electrode and Pt foil as the counter electrode. 9



195

The electrochemical impedance spectroscopy (EIS) was used to determine the

196

conductivity of catalysts at the open circuit potential, with the frequency range from

197

1×105 to 1×10−3 Hz and amplitude 5 mV, and the electrolyte was 0.5 mM [Fe(CN)6]3-/

198

[Fe(CN)6]4- solution.

199

Selective Elecrocatalytic Oxidation Experiment and Analysis. The elecrocatalytic

200

oxidation was carried out in a cylindrical single compartment cell equipped with a

201

magnetic stirrer and a jacketed cooler to maintain a constant temperature (25 ± 2 °C).

202

The dendritic Au/BDD and BDD electrodes with working area of 4 cm2 worked as

203

anode, respectively, and a Pt foil with the same area was used as cathode with the

204

electrode gap of 1 cm. Two groups of 50 mL mixed binary pollutants solution with

205

0.05 M Na2SO4 electrolyte were degraded in the cell. One was composed of 50 mg

206

L−1 thiophenol and 50 mg L−1 phenol, the other was composed of 50 mg L−1 2-MBI

207

and 50 mg L−1 2-HBI. The current density was controlled to be constant at 20 mA

208

cm-2 by a direct current potentiostat. The stirring rate was about 600 rpm.

209

The concentration of four pollutants during the degradation was measured by

210

high-performance liquid chromatography (HPLC, Agilent HP1100) with AQ-C18

211

column (4.6 mm × 250 mm, particles size 5 µm) and UV detector, the detective

212

wavelength were at λ = 270 nm, 236 nm, 300 nm, and 278 nm for thiophenol and

213

phenol, 2-MBI, and 2-HBI, respectively. Methanol/1 wt% acetum mixtures (70:30,

214

V/V) were employed as the mobile phase at the flow rate of 1 mL min−1. The injection

215

volume was 20 µL.

216

RESULTS AND DISCUSSION 10


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The BDD film is cleaned after pre-treatment, which is a polycrystalline thin film

218

grown on the Si wafer. Figure 1A shows that the continuous BDD surface is smooth,

219

crack free, and dense. The average crystallite grain size is in micrometers. Zn was

220

chosen as a bridge to construct dendritic Au on the BDD surface since Zn was one of

221

the active metals with higher hydrogen over potential compared with BDD. After

222

electro-deposition, the Zn nanoparticles dispersed grow on hollow position among

223

several grains preferentially in virtue of innumerable micro hydrogen bubbles in-situ

224

generated as template (Figure 1B). Then, the Zn/BDD was immersed in the HAuCl4

225

solution, the galvanic displacement reaction (4Zn + 2HAuCl4→2Au + 4ZnCl2 + H2↑)

226

occurred immediately, and it was observed that the irregular stellate Zn/Au alloy

227

nanoparticles demonstrating anisotropic crystal growth after treated 15 min, as shown

228

in Figure 1C. After 180 min, the displacement reaction was complete sufficiently,

229

Figure 1D displays the morphology of dendritic Au/BDD, the well-defined fractal

230

structured dendritic gold with micron stem length grow outward on the BDD film,

231

which performs three-fold symmetry (inset) and every stems grow out the similar

232

secondary structured branches and tertiary leaves. Meanwhile, partial BDD surface is

233

still exposed avoiding totally covered. A possible mechanism is proposed for the

234

formation of dendritic Au/BDD in Figure 1E. As the elemental Au generated initially,

235

innumerable primary cell are also formed, which consist of Zn cathode, Au anode,

236

BDD as the conducting line, and [AuCl4]- solution as electrolyte. The half-cell

237

reaction as follows: 1/2Zn - e-=1/2Zn2+ (cathode); 1/3[AuCl4]- + e-=1/3Au + 4/3Cl-.

238

This cell reaction take place on the Au crystal nucleus, instead of the direct surface 11



239

replacement reaction, owing to relative high activation energy for the latter.40 The

240

peculiar hierarchical Au dendrites are seldom obtained via common directly

241

electrodeposition method because of the same deposition rate on different gold crystal

242

facets.

243

Figure 2 displays the XRD patterns of the BDD (curve a), Zn/BDD (curve b),

244

Zn@Au/BDD (curve c), and dendritic Au/BDD (curve d), which indicates that the Au

245

dendrites have face-centered cubic (fcc) crystal structure, and the characteristic peaks

246

of Au are stronger while those peaks assigned to Zn become weaker and disappear

247

eventually after 180 min along with replacement reaction proceeding. The sharp

248

diffraction peak at 38° corresponding to (111) crystal facet also demonstrates that the

249

crystallinity of Au dendrites is better. The ratios of the peak intensities assigned to

250

(111) facet relative to those (200) and (220) facet could effectively reflect preferential

251

growth tendency of different Au facets. Those values are 3.7 and 8.8 respectively,

252

which are notably greater than common poly-Au (1.9 and 3.1, respectively).36 The

253

results confirm the higher population of the Au (111) facet, and the Au dendrites grow

254

along the direction preferentially. 41,42

255

Further TEM observations were carried out to study the morphology and

256

crystallographic orientation of the Au dendrites. Figure 3A shows a typical TEM

257

image with low resolution of two main stems on an Au dendrite. The planar

258

projection angels between the branches groups and the stems ac. 70°, approximately

259

equal to the theoretical angle 67.8° between two direction of cubic structure in

260

the (112) projection plane, which is consistent with the observations reported in

—

12


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previous literatures.37 The corresponding selected area electron-diffraction (SAED)

262

pattern (Figure 3B) shows clear diffraction spots, confirming that the dendrites are

263

composed of single crystalline Au. In addition, the high-resolution TEM images

264

(Figure 3C and 3D) display the equivalent lattice fringe spacing of 0.24 nm at the tips

265

of the branch and the trunk, which are all in good agreement with the d-spacing value

266

between the (111) planes of fcc structure Au. 41,43

267

Nyquist diagram of the electrochemical impedance spectroscopy is always

268

employed to estimate the electron transfer resistance (Ret). The size of arc diameter

269

on the Nyquist plot is closely relevant to the resistance of electron transfer on the

270

electrode surface. Nyquist plots at the dendritic Au/BDD and BDD are shown in

271

Figure 4A. At the same time, the equivalent circuit, often used to depict the

272

electrochemical behaviour of electrode, was employed for fitting the impedance

273

spectra. The fitted equivalent circuit for the dendritic Au/BDD agrees with Randles

274

model. Rs, Rct, Cdl, and Zw represent the solution resistance, charge transfer

275

resistance, double layer capacitance, and Warburg resistance, respectively. The fitted

276

values are given in Table S1. The Rct value of dendritic Au/BDD is 62 ohm, much

277

less than that of BDD (3850 ohm). The results indicate that the growth of gold

278

dendrites with remarkable electrical conductivity is beneficial to the charge transfer

279

directionally and decrease the electrical resistance of BDD. Dendritic Au growth

280

facilitates mass and electron transports, accordingly improves the elecctocatalytic

281

ability and reduces the corresponding energy consumption. Furthermore, the

282

probability of lattice distortion at interfacial regions and the number of crystal 13



283

boundaries greatly increase in dendritic gold comparing with common gold

284

nanoparticles. Consequently, abundant formed surface defect sites would be the active

285

centres responsible for the high catalytic activity.44 To investigate electrocatalytic

286

behaviour of dendritic Au/BDD, the cyclic voltammograms (CVs) in the

287

[Fe(CN)6]3-/[Fe(CN)6]4- solution are performed. As illustrated in Figure 4B, the redox

288

peak current density on BDD is small, which equal to 0.79 mA cm-2 and -0.50 mA

289

cm-2, respectively, due to its high electrochemical impedance and poor electrocatalytic

290

activity. However, the corresponding peak values increased to 2.05 mA cm-2 and

291

-1.73 mA cm-2 on the dendritic Au/BDD. Compared with BDD, the potential

292

difference of the dendritic Au/BDD between the anodic and the cathodic peaks, ∆Ep,

293

reduces from 1.70 to 0.57 V, which means that the reversibility of the electrode is

294

better. The negative shift of the anodic peak potential and positive shift of the

295

cathodic peak potential of the CVs at dendritic Au/BDD indicates that the redox

296

reaction of the [Fe(CN)6]3-/[Fe(CN)6]4- redox couple needs less energy. Therefore, it

297

can be expected that the modification of dendritic Au is beneficial to BDD

298

electrochemical performances including enhanced conductivity and electrocatalytic

299

activity. Figure 4C displays CVs recorded in deoxygenated 0.5 M H2SO4 for dendritic

300

Au/BDD. Distinct three oxidation peaks of Au surface monolayer corresponding to

301

different crystal facets were observed in the potential range of 1.0 to 1.4 V,

302

respectively, while the background current is very low in the same condition. The

303

construction of dendritic Au could significantly enhancee the conductivity and

304

electrocatalytic activity. The sharp peak at ca. 1.4 V is often indicative of Au (111) 14


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facet, which further demonstrating the (111) facet is predominant crystallographic

306

plane on the dendritic Au/BDD.45 Moreover, the electrochemical active surface area

307

of dendritic Au is calculated to be 0.014 cm2 (SA=Qe/390 µC cm2, Qe is equal to 5.4

308

µC, which is integral electrical quantity of reduction peak) from the charge consumed

309

during the reduction of AuOx corresponding to the peak at 0.87 V (in Figure 4C)

310

using the reported value of 390 µC cm2 for a clean Au electrode.44 The active surface

311

area is much less than the real surface area of BDD substrate, indicating that only a

312

small part of BDD surface is covered by dendritic Au and most is exposed.

313

The selectivity of dendritic Au/BDD was evaluated in two groups of synthetic

314

wastewater composed of thiophenol/2-MBI (target pollutant) and phenol/2-HBI

315

(interference pollutant) with equal mass concentration due to their similar chemical

316

structure (Supporting Information (SI) Figure S1). Inset of Figure 5 shows the

317

removal of two pollutants. After 100 min, 91% thiophenol and 58% phenol are

318

removed on the dendritic Au/BDD, while the corresponding removal rate is 62% and

319

75% on bare BDD, respectively. The phenol is more easily oxidized than thiophenol

320

on nonselective BDD, which maybe attribute to the stronger electron donating

321

conjugation effect of –OH than that of –SH, leading to higher electron density of

322

benzene ring in phenol and easily be attacked by •OH. Meanwhile, it is easily noted

323

that the efficiency of electrocatalytic oxidation to thiophenol increase significantly

324

after dendritic Au assembled on the BDD. With regard to mixed solution of 2-MBI

325

and 2-HBI, the time dependence of pollutants removal is presented in inset of Figure

326

S2. The removal rates of 2-MBI and 2-HBI reach to 94% and 48% on the dendritic 15



327

Au/BDD, respectively at 75 min, while the corresponding values are 54% and 82% at

328

bare BDD. The above results not only reveal that the dendritic Au/BDD has excellent

329

electrocatalytic performance, but also exhibit high selectivity to target pollutants.

330

Previous studies have revealed that there exists some correlation among

331

hydrophobicity, oxygen evolution potential (OEP), and mineralization capability for

332

BDD. As-grown BDD via CVD is largely hydrogen terminated and hydrophobic in

333

nature.46 The wettability test of water at BDD and dendritic Au/BDD were determined,

334

and the contact angle is about 95º at BDD. After chemically cleaned with acid and

335

modified with hydrophilic dendritic Au, a distinct decrease of hydrophobicity was

336

observed (Figure S3). Although the hydrophobic property was change, the good

337

situation is that a relatively high OEP of ~2.1 V still maintained for dendritic

338

Au/BDD. The OEP is also much higher than that of common DSA anode (Figure S4),

339

ensuring that it still exhibits strong mineralization ability to recalcitrant organic

340

pollutants. Furthermore, the decrease of OEP is maybe related to the wettability of

341

electrode surface.

342

The selectivity of dendritic Au/BDD maybe attribute to the preferential adsorption

343

of thiophenol molecules on the exposed surfaces with single-crystalline gold dendrites

344

crystal planes as a result of formation of Au-S bonds. This hypothesis could be proved

345

by equilibrium adsorption test, the adsorptive capability of thiophenol on dendritic

346

Au/BDD is much higher than BDD, and the equilibrium adsorption removal to phenol

347

is only one-third to thiophenol on the dendritic Au/BDD after 60 min. Meanwhile,

348

XPS with high sensitivity was used to study fast adsorption of thiophenol on the 16


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electrode and the combination of Au and S atoms. As shown in the survey spectra

350

(Figure S5), the peaks of Au 4f and S 2p indicate that the grown dendritic Au and

351

adsorbed thiophenol. The Au 4f spectra (Figure 6A) display a doublet characteristic of

352

Au 4f7/2 and Au 4f5/2 centered at 84.0 eV and 87.7 eV, respectively, and peak

353

separation equal to 3.7 eV, which is in agreement with the spectral values for single

354

crystal Au.47 Figure 6B shows that the S 2p peak is broad and complicated because of

355

the overlapping contribution of sulfur with different chemical state, which would be

356

fitted by four peaks. The domain peak of S 2p3/2 at 162.1 eV is characteristic of

357

sulfur in Au-S bond, illustrating that plenty of thiophenol molecules assembled on the

358

single crystal Au surface. The peak of S 2p3/2 at 163.9 eV is assigned to the

359

sulfhydryl groups for noncovalently conjugated thiophenol enriched on the electrode

360

surface.47,48 In order to investigate the interaction between thiophenol and the

361

dendritic Au/BDD during degradation, the sample used for degrading mixed solution

362

of thiophenol and phenol for 30 min was selected to analyse. Figure 6C shows Au

363

L3-edge XANES spectra for the dendritic Au/BDD sample together with the Au foil as

364

a reference. The characteristic three-peak pattern following the edge jump at ~11919

365

eV is an indication of the existence of an (fcc) gold structure. The first resonance at

366

the edge is often known as the white line, arising from electronic transition from the

367

2p3/2 to the 5d5/2, 3/2 states.49,50 Although the 5d orbitals in Au atoms are nominally full

368

because of s-p-d hybridization, a small white line still could be detected in the

369

XANES of bulk Au. The white-line intensity for the Au foil is rather low, while it is

370

intensified for the dendritic Au/BDD, suggesting a growing number of charges 17



Page 18 of 40

371

transferred from Au to S atoms due to the preferential adsorption of thiophenol and

372

formation of Au-S bond during degradation.51 In the post-edge region, the resonance

373

peak of the dendritic Au/BDD is significantly broadened and attenuated relative to

374

that of the Au foil, which is attributed to the nanosize effect of the former. The

375

nanosize effect can be understood as that, namely, when the nanoparticle size

376

decreases, the increase in the percentage of surface atoms results in a decrease in the

377

average number of neighboring Au atoms. More explicit information for the

378

thiophenol adsorption behavior could be observed from the Fourier transforms (FT) of

379

k2χ(k) curves of EXAFS spectra (Figure 6D). In terms of dendritic Au/BDD, the FTs

380

curve demonstrate an apparent peak at 1.9 Å ascribed to the Au–S shell, while the

381

intensity of the peak in the range of 2.0–3.0 Å assigned to the first Au–Au shell shows

382

weaker comparing with that of gold foil, which is imply the lower coordination

383

number of Au surface atoms and increasing disorder due to the emergence of the

384

surface Au-S contribution near 2 Å. These results reveal that the strong thiophenol

385

adsorption via Au-S bond truly modulate the dendritic Au surface states, which are

386

consistent

387

chronocoulometric method (Text in SI, Figure S6),14,53

388

of target thiophenol and nontarget phenol on dendritic Au/BDD surface are calculated

389

as 1.1×10-10 and 4.1×10-11 mol cm-2, respectively, which is in accordance with the sole

390

equilibrium adsorption result.

with

the

previous

reports.51,52

Additionally,

according

to

a

the adsorption capacity (Γ0)

391

To further study the selectivity of electrocatalytic oxidation, the kinetics of

392

thiophenol and phenol decay is analysed in detail. Figure 5 shows the relationships 18


Page 19 of 40


393

between ln (C0/C) and time on two electrodes, in which all processes follow

394

pseudo-first-order kinetic. The values of apparent rate constant k are listed in Table 1.

395

The k value of thiophenol decay on BDD is 0.010 min−1, approximately equal to that

396

of phenol, 0.014 min−1, demonstrating that no selective electrocatalytic oxidation

397

occurred. In contrast, the k value of thiophenol on dendritic Au/BDD increases to

398

0.026 min−1, which is 3.25 (αDendritic

399

revealing its distinguished electrocatalytic selectivity to target thiophenol. For the

400

other mixed solution of 2-MBI and 2-HBI degradation, the similar selectivity to

401

2-MBI was demonstrated, and the α2-MBI/2-HBI value reached to 4.10 (Figure S2 and

402

Table 1). Moreover, high selectivity factor R (defined as the ratio αDendritic

403

to αBDD) of 4.58 and 8.20 further confirms that the modification of single crystal

404

dendritic Au with dominant (111) facet on BDD enhances the electrocatalytic

405

selectivity toward the sulfur-containing target contaminant.14,29

Au/BDD=kthiophenol/kphenol)

times for phenol,

Au/BDD

406

The selective electrocatalytic degradation mechanism of mercaptans on the

407

dendritic Au/BDD is investigated. Cyclic voltammetry was performed in 0.05 M

408

Na2SO4 blank solution and 0.05 M Na2SO4 + 50 mg L-1 thiophenol solution (Figure

409

S7), respectively. The cyclic voltammograms in the two solutions almost completely

410

overlap each other in the potential range of -0.2 to 2.0 V. There is no obvious

411

oxidative peak of thiophenol appearing in the latter solution, indicating that

412

electrocatalytic oxidation of thiophenol mainly triggered by•OH-induced indirect

413

electrocatalysis rather than direct electrocatalysis on dendritic Au/BDD.19,20 At the

414

same time, the electrocatalytic oxidation behavior of binary mixed pollutants (2-MBI 19



415

and 2-HBI) on dendritic Au/BDD were also determined as presented in Figure S8.

416

There are three oxidation peaks appearance at the potential range of 0~1.5 V. With

417

respect to the strongest oxidation peak, the corresponding potential of pollutants

418

oxidation at dendritic Au/BDD (about 1.05 V) (vs. SCE) is lower than that on BDD

419

(about 1.2 V) (vs. SCE). The enhanced electrocatalytic activity should be attributed to

420

construction of dendritic Au on BDD. The peak current density of pollutants

421

electro-oxidation in a forward scan of dendritic Au/BDD is higher that of BDD due to

422

a good deal of pollutant molecules participate in the electrochemical reaction at

423

dendritic Au surface. The results reveal that, as a highly reactive electrocatalyst, gold

424

also plays a significant contribution to pollutants degradation of via direct elecatalytic

425

oxidation way simultaneously. Herein, it is believed that the electrocatalytic

426

selectivity of mercaptans is achieved depending on specific adsorption and

427

accumulation effect. When the analogous substances coexisting, mercaptans

428

molecules are able to surpass interference pollutants molecules in the adsorption

429

competition and occupy mostly active adsorption sites on Au crystal, and adsorbed

430

mercaptans degradation occurs directly over the anode trough the adsorbed •OH. On

431

the other side, most of micron-sized dendritic Au grows outward from the BDD plane

432

rather than tiling horizontal as shown in Figure 1D, which would form an

433

“adsorption-controlled layer” with tens of micron thickness in the vicinity of BDD

434

surface during degradation. The micron magnitude of this layer just is just the order

435

that the distribution distance of free •OH on BDD.54 Under the driving effect of strong

436

Au-S interaction, the continuous directed diffusion and accumulation of mercaptans 20


Page 20 of 40

Page 21 of 40


437

toward gold interface results in a relative higher concentration than interferences in

438

this layer. In this case, more mercaptans also would be rapidly oxidized by abundant

439

of free •OH electro-generated on BDD before •OH quenching.18,55

440

Thus, dendritic Au/BDD, as a novel fractal nanostructured electrode, not only has

441

prominent electrochemical performance, but also presents highly selective capability.

442

These distinctive properties are ascribed to synergistic effect of two factors. First,

443

BDD is selected as the base electrode, which has higher oxygen evolution potential

444

than common DSA electrodes, so it exhibits strong electrochemical degradation

445

ability to organic pollutants. Second, compared with the bare BDD, the modified

446

dendritic Au single crystal with numerous physisorption and chemisorption sites can

447

preferentially adsorb and accumulate a large number of mercaptans molecules and are

448

decomposed subsequently, which plays dominant role for selective electrocatalytic

449

degradation to the target pollutant in a complex wastewater. From the perspective of

450

actual applications, the stability of electrode is an important issue that must be

451

considered. The reusability of dendritic Au/BDD was successfully carried out by

452

degrading two groups of pollutants over the reused sample (Figure S9). The

453

electrocatalytic degradation efficiencies were nearly maintained at the level of fresh

454

sample after five consecutive runs. To determine the structural stability, the

455

morphology and crystal structure of the used dendritic Au/BDD after five degradation

456

cycles were examined by XRD and SEM. As depicted in Figure S10, the main

457

diffraction peaks corresponding to (111) crystal facet of dendritic Au is still sharp and

458

strong, and no new diffraction peaks appears. Moreover, the dendritic structure of Au 21



Page 22 of 40

459

was remained the same as fresh sample (Figure S11). All of these results suggested

460

the dendritic Au/BDD shows good stability during utilization. In addition, the

461

passivated one can be recycled after activating with cyclic voltammetry in dilute

462

sulfuric acid solution. Despite two groups of binary pollutant mixtures we selected

463

cannot represent the complex actual wastewater, however, this study provides a new

464

way of thinking and approach for functional electrode design. Meanwhile, the ideas

465

can also be widely applied for removing, detecting or analysing for other

466

sulfur-containing organic contaminants with high toxicity and low concentration.

467

ASSOCIATED CONTENT

468

Supporting Information

469

The Supporting Information is available free of charge via the Internet at

470

http://pubs.acs.org. Figures S1-S6 (PDF)

471

AUTHOR INFORMATION

472

Corresponding Authors

473

*Phone: 86-21-65988570-8244; fax: 86-21-65982287;

474

E-mail: [email protected]

475

Notes

476

The authors declare no competing financial interest.

477

ACKNOWLEDGMENT

478 479

This

work

Foundation

of

was China

financially supported

by

National

(Project No. 21537003, 21507103).

480 22


Natural

Science

Page 23 of 40


481 482 483

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658 659 660 661 662 663 664 665 666 667 668 669 31



670

Figure Captions

671

Figure 1. SEM images of BDD (A), Zn/BDD (B), Zn@Au/BDD of replaced for 15

672

min in HAuCl4 (C), and dendritic Au/BDD (D) with high magnification SEM (inset);

673

Schematic illustration of gold dendrites growth process at BDD (E).

674

Figure 2. XRD patterns of BDD (a), Zn/BDD (b), Zn@Au/BDD of displaced for 15

675

min in HAuCl4 (c), and dendritic Au/BDD (d).

676

Figure 3. (A) TEM image of Au dendrites; (B) SEAD pattern of Au dendrites; (C, D)

677

HRTEM images of the branch tip denoted as C) and stem edge denoted as D) in (A),

678

respectively.

679

Figure 4. (A) Nyquist plot of dendritic Au/BDD in 5 mM [Fe(CN)6]3-/[Fe(CN)6]4-

680

solution, inset: the corresponding equivalent circuit model and Nyquist plot of bare

681

BDD in the same solution; Cyclic voltammograms of dendritic Au/BDD and bare

682

BDD in 5 mM [Fe(CN)6]3-/[Fe(CN)6]4- solution (B) and in 0.5 M H2SO4 solution at

683

scan rate of 50 mV s-1 (C), respectively.

684

Figure 5. The removal kinetics of thiophenol (A) and phenol (B) using dendritic

685

Au/BDD (a, a’) and BDD anode (b, b’), respectively. Inset: the removal of thiophenol

686

(A) and phenol (B) with electrocatalytic oxidation time using dendritic Au/BDD (a, a’)

687

and BDD anode (b, b’), respectively; (a, b) obtained from the equilibrium adsorption

688

test; (a’, b’) obtained from electrocatalytic oxidation process.

689

Figure 6. Au 4f (A) and S2p (B) XPS spectra of dendritic Au/BDD performing

690

equilibrium adsorption test in thiophenol and phenol mixed binary solution for 60 min;

691

ex situ Au L3-edge XANES spectra of dendritic Au/BDD used for degrading mixed 32


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692

solution of thiophenol and phenol for 30 min and a gold foil reference (C);

693

Fourier-transformed k2χ(k) EXAFS spectra of the sample and a gold foil reference

694

(D).

695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 33



714 715

Figure 1

716 717 718 719 720 721

34


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Page 35 of 40


722 723

Figure 2

724 725 726 727 728

35



729 730

Figure 3

731 732 733 734 735 736 737 738 739 740 741 742 743 744 36


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745

746

747 748

Figure 4 37



749

750 751 752 753 754 755 756 757 758 759 760

Figure 5

38


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761

762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784

Figure 6

39



Page 40 of 40

785

Table 1. Rate constants and selectivity rate for electrocatalytic degradation of target pollutant (thiophenol or 2-MBI) in the presence of interference (phenol or 2-HBI) using two electrodes

k

Dendritic Au/BDD

BDD

kthiophenol (min -1)

0.026

0.010

kphenol (min-1)

0.008

0.014

k2-MBI (min -1)

0.037

0.012

k2-HBI (min -1)

0.009

0.024

αthiophenol/phenol

3.25

0.71

α2-MBI/2-HBI

4.10

0.50

Rthiophenol/phenol

4.58

R2-MBI/2-HBI

8.20

786

40


Selective Electrocatalytic Degradation of Odorous Mercaptans Derived

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