Enhanced Electroreductive Removal of Bromate by a Supported PdâIn

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Enhanced electroreductive removal of bromate by a supported Pd-In bimetallic catalyst: Kinetics and mechanism investigation Huachun Lan, Ran Mao, Yating Tong, Yanzhen Liu, Huijuan Liu, Xiaoqiang An, and Ruiping Liu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02822 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on September 30, 2016

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Environmental Science & Technology

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Enhanced electroreductive removal of bromate by a

2

supported Pd-In bimetallic catalyst: Kinetics and

3

mechanism investigation

4 5 6 7 8

Huachun Lan, Ran Mao, Yating Tong, Yanzhen Liu, Huijuan Liu, Xiaoqiang An,

9

Ruiping Liu*

10 11 12 13 14 15

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

16

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

17 18 19

*Corresponding author

20

Tel.: +86 10 62849160; fax: +86 10 62849160

21

E-mail address: [email protected] (R P. Liu)

22

ACS Paragon Plus Environment


23

ABSTRACT

24

In this work, the electroreductive removal of bromate by a Pd1-In4/Al2O3 catalyst in

25

a three-dimensional electrochemical reactor was investigated. 96.4% of bromate could

26

be efficiently reduced and completely converted into bromide within 30 min under

27

optimized conditions. Based on the characterization results and kinetics analysis, a

28

synergistic effect of Pd and In was observed, and Pd1-In4/Al2O3 had the highest

29

reaction rate constant of 0.1275 min-1 (vs. 0.0413, 0.0328, and 0.0253 min-1 for

30

In/Al2O3, Pd/Al2O3 and Al2O3). The results of electron spin resonance and scavenger

31

experiments confirmed that both direct electron transfer and indirect reduction by

32

atomic H* were involved in the bromate removal process, while the direct reduction

33

played a more important role. Moreover, the introduction of In could increase the zeta

34

potential of Pd1-In4/Al2O3, facilitating bromate adsorption and its subsequent

35

reduction on the catalyst. Finally, a reaction mechanism for bromate reduction by

36

Pd1-In4/Al2O3 was proposed based on all the experimental results.


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TOC/ABSTRACT ART

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39 40



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INTRODUCTION

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Bromate is an oxyhalide disinfection byproduct frequently detected in drinking

43

water from ozonation of bromide-containing source waters.1 Because of its

44

carcinogenic and genotoxic properties, the World Health Organization (WHO) has

45

promulgated a 10 µg/L standard for bromate in drinking water.2 To meet this strict

46

limitation, numerous treatment methods have been explored, including filtration,

47

chemical

48

photocatalysis.3-8 Among these technologies, liquid phase catalytic hydrogenation has

49

proven to be an efficient and clean method to eliminate bromate.9, 10 Wang et al.

50

reported the hydrogenation reduction of bromate using a core-shell-structured catalyst

51

with encapsulated Pd nanoparticles.11 However, the catalytic hydrogenation needs an

52

external supply of H2, while the storage and transportation of H2 may be a hidden

53

danger. Alternatively, the electrochemical reduction of bromate is found to be a

54

promising approach due to its low maintenance requirements and efficient

55

minimization of toxic chemicals and secondary pollution.12-14

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Matsuda reported an efficient electrochemical method to reduce bromate to bromide

57

using a two-compartment electrolytic flow cell with activated carbon felt as

58

electrodes.15 The results showed that bromate contamination can be removed within a

59

few minutes, but this process required acidic conditions. Zhao et al. studied the

60

effectiveness of a boron-doped diamond electrode in bromate removal by

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electrochemical reduction, and nearly 90% of bromate could be removed with the bias

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potential of -1.0 V within 2 h.14 However, the application of these cathodes for water

reduction,

biodegradation,

activated

carbon


techniques,

and

Kishimoto and

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treatment would be limited due to the low A/V ratio (ratio of the electrode area and

64

solution volume). Hence, it is necessary to design a new electrochemical reactor and

65

electrode with high efficiency. Recently, a three-dimensional electrochemical reactor

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using granular activated carbon (GAC) or modified GAC as particle electrodes

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exhibited higher efficiency in treatment of heavy oil refinery wastewater compared

68

with a two-dimensional electrode reactor (without particles).16 Moreover, the removal

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rate of the target pollutants could also be increased by using three-dimensional

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electrodes with more available reactive sites and electrons compared to the

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conventional two-dimensional electrodes.17, 18

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It is widely reported that Pd has the unique property of activating H2 to produce

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continuously adsorbed atomic H* and trapping electrons, which could serve as a

74

strong reducing agent in catalytic reductive reactions.19 Moreover, Pd-based

75

bimetallic catalysts were found to exhibit better activity and selectivity in many

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important chemical reactions compared with monometallic Pd catalysts.20-22 It has

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been reported that the enhanced catalytic activity could be attributed to the promotion

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effect of the second metal element such as Cu, Au, Bi, Sn, Fe, etc.23-27 For instance,

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the catalytic hydrogenation efficiency of Pd/SnO2 and Pd/Al2O3 towards

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N-nitrosodimethylamine and nitrate reduction was greatly enhanced by the addition of

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Sn and In.28, 29 The efficiency of Cu- and Sn-doped carbon nanofiber/sintered metal

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fiber filters was also evaluated by bromate and nitrate reduction in continuously

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operated reactors.30 However, studies on the electroreduction of bromate in a

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three-dimensional electrochemical reactor by Pd-based three-dimensional suspended



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electrodes are still limited. In addition, the effect of the dosage of other

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transition-metal (Cu, Sn, Bi and In) dopants on the catalytic activity of Pd also needs

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to be investigated.

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Hence, in this study, different Pd-based supported catalysts (Pd-M/Al2O3, M = Cu,

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Sn, Bi and In) were synthesized for the first time, and evaluated based on their

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bromate reduction efficiency. Pd1-In4/Al2O3 was proven to have the highest efficiency

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as particle electrodes in the electrochemical reductive removal of bromate in a

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three-dimensional electrochemical reactor. Moreover, the bromate reduction followed

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pseudo-first-order kinetics. The catalyst was further characterized by X-ray powder

94

diffraction

95

Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS) and Zeta

96

potential analysis to study the synergistic effect of Pd and In within the catalyst. The

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effect of catalyst dosage, solution pH and applied current on the bromate reduction

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over Pd1-In4/Al2O3 was also studied in detail. Based on electron spin resonance (ESR)

99

scavenger experiments, both direct electron transfer and indirect reduction by atomic

100

H* were confirmed to be involved in the bromate removal process. Finally, an

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electrochemical reduction mechanism of bromate over Pd1-In4/Al2O3 was proposed

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according to all the experimental results. The findings in this study demonstrate the

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potential of the three-dimensional electrochemical reactor as an effective system for

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eliminating bromate contamination in water.

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MATERIALS AND METHODS

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(XRD),

Transmission

Electron

Microscope

(TEM),

The catalysts were prepared by a conventional impregnation method. Briefly,


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γ-Al2O3 was immersed in PdCl2 solution containing 0.1 M hydrochloric acid,

108

followed by ultrasonic treating for 360 min. After standing for 24 hours, the obtained

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samples were dried at 120 oC, calcined at 300 oC for 120 min, and finally reduced at

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200 oC in a H2 atmosphere for 300 min (flow rate: 100 mL/min). The resulting

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supported Pd catalysts were denoted as x% Pd/Al2O3, where x is the Pd loading

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amount (wt. %). Bimetallic Pd–M/Al2O3 catalysts (M = In, Cu, Sn, Bi) were obtained

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by the same impregnation method with the theoretical Pd:M ratio of 1:4. Pd-In/Al2O3

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with different Pd:In ratios was denoted as Pdx-Iny/Al2O3, where x:y is the theoretical

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Pd:In ratio.

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A three-dimensional electrochemical reactor was used for the electrochemical

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experiments, which was separated into a cathode cell and anode cell by a

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proton-exchange membrane (Nafion117, Dupont). For each batch experiment, the

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cathode cell was filled with 50 mL aqueous solution of bromate using Na2SO4 as the

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supporting electrolyte. RuO2/Ti was used as the cathode and anode with an effective

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geometric surface area of 100 cm2, and the distance between cathode and anode was 4

122

cm. The reactor was controlled by a DC power supply source AMERLLPS302A

123

(Dahua instrument corporation of Beijing). The initial concentration of bromate was

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100 µg/L unless otherwise specified, and the experiments were performed at room

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temperature. The system was magnetically stirred at a rate of 800 rpm.

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The concentrations of bromate and bromide were measured using a Dionex 2000

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ion chromatograph (IC), equipped with an IonPac AS-19 anion-exchange analytical

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column, an IonPac AG19 guard column, and KOH solution as mobile phase eluent.



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Atomic H* signals were measured on a Bruker model ESR 300E spectrometer, with a

130

Quanta-Ray

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5,5’-dimethyl-1-pirroline-N-oxide (DMPO) as the spin-trap reagent. XRD patterns of

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the catalysts were recorded with an X’Pert PRO Powder diffractometer (PANalytical

133

Co.). TEM images and Energy Dispersive Spectrometer (EDS) images of the samples

134

were obtained on a Transmission Electron Microscope JEM-2100F Field Emission

135

Electron Microscope (JEOL Ltd.). XPS analysis was carried out with a Kratos AXIS

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Ultra X-ray photoelectron spectrometer. The specific surface area measurement was

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performed by the BET method using a surface area analyzer, ASAP 2020 HD88

138

(Micromeritics Co.). The surface potential of the samples was characterized by a

139

Nano Particle Sizing & Zeta Potential Analyzer (DelsaNano C, Beckman Coulter

140

Ltd.).

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RESULTS AND DISCUSSION

Nd:YAG

laser

system

as

the

irradiation

source,

and

142

Efficient electroreduction of bromate by Pd1-In4/Al2O3 catalyst. The

143

effect of Pd loading on Al2O3 on the bromate reduction was optimized first. As shown

144

in Figure 1A, the removal efficiency of bromate increased to a maximum value, and

145

then decreased with increasing Pd content (the initial activity of Pd/Al2O3 was 27.74,

146

39.48, 48.36, and 29.4 µg·gCat-1·h-1 when Pd content was 1.5%, 2.0%, 3.0% and

147

4.0%, Figure 1D). Higher amounts of Pd will aggregate on the surface of Al2O3,

148

which may result in locally excessive H2 evolution31 and then inhibit the H2 efficiency

149

by restricting the access of bromate to the interior H2 generation sites. Cu, Sn, Bi, and

150

In as the promoting metal for Pd-M/Al2O3 were chosen, and their efficiency was also


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evaluated based on bromate reduction (Figure 1B). Compared with Pd/Al2O3, all the

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bimetal catalysts exhibited an improved electroreduction activity with the order of

153

Pd-Cu/Al2O3 < Pd-Sn/Al2O3 < Pd-Bi/Al2O3 < Pd-In/Al2O3 (the initial activity was

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49.12, 68.86, 80.24 and 145.58 µg·gCat-1·h-1, respectively, Figure 1D). Pd-In/Al2O3

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had the highest activity toward bromate reduction, and the optimal atomic ratio of Pd

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to In was further evaluated. As depicted in Figure 1C, the electroreduction rate of

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bromate increased dramatically with In content, and the initial activity of Pd-In/Al2O3

158

was 34.26, 44.9, 55.24, 100.42 and 145.58 µg·gCat-1·h-1 with a Pd/In ratio of 4:1, 2:1,

159

1:1, 1:2 and 1:4 (Figure 1D). Hence, Pd1-In4/Al2O3 with a Pd loading amount of 3.0%

160

was chosen in the subsequent experiments.

161

In addition, the bromate electroreduction and the generation of bromide were

162

further investigated for different particle electrode materials, and the obtained results

163

were found to follow pseudo-first order kinetics. As shown in Figure 2, the bromate

164

concentration decreased by 54.0%, 64.2% and 71.4% in the presence of Al2O3,

165

In/Al2O3 and Pd/Al2O3, while about 96.4% of bromate was reduced at 30 min with

166

Pd1-In4/Al2O3. That is to say, when the initial concentration of bromate was 100 µg/L,

167

the remaining bromate in solution was only 3.6 µg/L, which was much lower than the

168

standard of 10 µg/L. Moreover, the reaction rate constant of Pd1-In4/Al2O3 was 0.1275

169

min-1, which was also much higher than that of other catalysts (0.0413, 0.0328, and

170

0.0253 min-1 for Pd/Al2O3, In/Al2O3, and Al2O3). It was worthy to note that the

171

reaction constants for bromide formation were a little lower than that for bromate

172

reduction (0.099, 0.0371, 0.0278, and 0.0215 min−1). The bromine balance will be



173

analyzed subsequently. The above results indicated that a synergistic effect of Pd and

174

In existed within the catalyst and that it could enhance the current efficiency (as

175

shown in Figure S1, the electric energy consumption to remove 1 mg bromate was

176

0.0065, 0.0105, 0.0127 and 0.0166 KWh for Pd1-In4/Al2O3, Pd/Al2O3, In/Al2O3, and

177

Al2O3, respectively). Hence, Pd1-In4/Al2O3 was proven to be an ideal particle

178

electrode material for the removal of bromate in water by electroreduction.

179

Characterization of Pd1-In4/Al2O3. The changes of physicochemical properties

180

responsible to the higher efficiency of Pd1-In4/Al2O3 were characterized by TEM,

181

XRD, BET and XPS analysis. As depicted in Figure 3A and B, the lattice spacing of

182

0.228 nm belonged to the Pd(111)-In(111) bimetal. The average metal particle size

183

was found to be ca. 7.7 nm. The measured ratio of Pd:In at different sites of the

184

composite was determined to be from 1:2.78 to 1:4.96. Besides, Pd particles

185

aggregated to some extent, while In was highly dispersed on the surface of Al2O3. The

186

diffraction peaks assigned to the (111), (200) and (220) reflections of metallic Pd were

187

observed for the Pd/Al2O3 catalyst (Figure 3C), reflecting the typical face centered

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cubic crystallographic structure,10 while the new peak at 39.6° can be attributed to the

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Pd-In alloy.32 Table 1 shows the BET results for different catalysts. The average pore

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size of the mesoporous Al2O3 was 15.65 nm, which was larger than the size of the

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metal nanoparticles (7.7 nm). The BET area and pore volume were similar for Al2O3,

192

Pd/Al2O3 and Pd4-In1/Al2O3. However, a significant decrease of BET area and pore

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volume was observed for Pd1-In4/Al2O3 (42 m2/g and 0.22 cm3/g vs. 64-74 m2/g and

194

0.34-0.35 cm3/g), which meant that some of the loaded metals might occupied the


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volume in the mesopores. This result indicates that the specific surface area was not a

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critical factor for the high activity of Pd1-In4/Al2O3. In addition, the impact of In

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introduction on the valence state of Pd was studied by XPS (Figure 4). With the

198

increase of In loading (Pd, Pd4-In1 and Pd1-In4), the binding energy of Pd3d shifted

199

towards a higher value from 334.4 eV (3d5/2), 339.7 eV (3d3/2) to 334.8 eV, 340.0 eV

200

and 335.8 eV, 341.1 eV. Meanwhile, the position of the peak assigned to In (0), with

201

binding energy of 444.79 eV (3d5/2) and 452.39 eV (3d3/2), did not change with In

202

content. This proved that a Pd-In alloy was formed and that In could change the

203

chemical environment of Pd.33

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Effect of different parameters on Pd1-In4/Al2O3 efficiency. Figure S2

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explores the influence of different parameters such as catalyst dosage, solution pH

206

and current density on the electroreduction performance of Pd1-In4/Al2O3. The

207

bromate removal rate increased with the dosage of Pd1-In4/Al2O3 (0.2-1.0 g/L), and

208

then maintained a steady level when the catalyst concentration was further increased

209

to 1.5 g/L; while the effect of initial solution pH was not significant, and almost 100%

210

of bromate was degraded after reaction for 25 min. The weak pH effect reflects that

211

the electrocatalytic reduction of bromate with the Pd1-In4/Al2O3 can be performed

212

under a wide pH range. Since the generation of atomic H* through the Volmer

213

reaction is favored in acid solutions,34 the high efficiency of bromate removal at

214

neutral pH indicated that the contribution of atomic H* to bromate reduction is likely

215

to be insignificant. In addition, the influence of current density on the Pd1-In4/Al2O3

216

efficiency was also examined at 0.3 to 1.2 mA/cm2. The reduction rate markedly



217

increased with current density (0.3 to 0.9 mA/cm2), and then remained constant at

218

higher current of 1.2 mA/cm2. Higher current density will produce more available

219

electrons, 35 leading to higher efficiency in bromate reduction. The similar efficiency

220

of Pd1-In4/Al2O3 at 1.2 and 0.9 mA/cm2 should be attributed to the limitation of active

221

sites in the catalyst, interfering with the electron transfer to bromate.

222

Proposed mechanism of bromate reduction on Pd1-In4/Al2O3. Figure 5A

223

depicts bromate removal by adsorption over different materials without the addition

224

of applied current. Compared with Al2O3 without bromate adsorption, only 7% and 11%

225

of bromate was adsorbed on Pd/Al2O3 and In/Al2O3. Pd1-In4/Al2O3 exhibited the

226

highest adsorption capacity and 48.6% of bromate could be removed under the same

227

conditions. However, there were no bromide species detected during the adsorption

228

process. This was because of the much higher zeta potential of Pd1-In4/Al2O3 than

229

other three catalysts at neutral pH (inset of Figure 5A). Hence, the more positive

230

surface charge facilitated the adsorption of the negatively charged bromate on

231

Pd1-In4/Al2O3. After the addition of applied current, the removed bromate was almost

232

completely reduced to bromide for all the tested catalysts (inset of Figure 5B). For

233

example, the sum of bromate and bromide at 30 min accounted for 99.1% of the total

234

bromine added into the reactor for Pd1-In4/Al2O3, which indicated that the decreased

235

bromate was completely converted into bromide. Regarding the slight decrease in the

236

sum of bromate and bromide during the reaction process (5-20 min), this was due to

237

the formation of other bromine species such as BrO2-, HBrO2 and HOBr/OBr-.36

238

However, they were finally converted into bromide, since the mass balance of total


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bromine was recovered at 30 min.

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The electroreduction mechanism of bromate by Pd1-In4/Al2O3 was further

241

investigated because direct electron transfer and atomic H* may be both involved in

242

the reaction process.37,

243

DMPO-H39 with strong intensity was detected by ESR in the presence of catalyst

244

compared with the control experiment (no catalyst). This confirmed that atomic H*

245

was indeed produced and involved in the bromate reduction process. However, the

246

peak intensity of DMPO-H was similar for Pd/Al2O3, In/Al2O3 and Pd1-In4/Al2O3,

247

which indicated that the introduction of In had no positive effect on the atomic H*

248

generation for Pd1-In4/Al2O3. Tert-BuOH is a quenching reagent towards atomic H*,

249

therefore its influence on bromate electroreduction was also studied. As shown in

250

Figure 6B and 6C, a significant inhibition of bromate removal in the presence of

251

Tert-BuOH (10 mM) was observed for Pd/Al2O3 and Pd1-In4/Al2O3 catalysts with

252

efficiency decreases of 27.5% and 19.2%, while the negative effect of Tert-BuOH was

253

not significant for the control experiment, with only 5% inhibition. It can be

254

speculated that the 37% bromate removal in the control should come from direct

255

electron transfer, and both direct electron transfer and indirect atomic H* reduction

256

contributed to the bromate removal in the presence of catalysts. The higher

257

contribution of direct electron reduction to bromate removal in the presence of

258

catalysts (especially for Pd1-In4/Al2O3) was attributed to the increase of electron

259

transfer to the adsorbed bromate due to the unique property of Pd metal in trapping

260

electrons and the promoting effect of In within the catalyst. Since dissolved oxygen

38

As shown in Figure 6A, nine characteristic peaks of



261

(DO) could compete with target pollutants for the reaction sites and then suppress the

262

removal efficiency toward pollutants,38 DO in the reaction system was removed by N2

263

sparging, and its concentration was reduced from 7.42 to 0.80 mg/L. As shown in

264

Figure 7, the bromate reduction efficiency was greatly enhanced when DO was

265

removed, and the reaction constant was 3 times higher than that in the presence of DO

266

(0.3709 min-1 vs. 0.1275 min-1). This indicated the competition effects with oxygen

267

for reactive sites on the catalyst by consuming both the electrons and atomic H*. As

268

shown in Figure S3, a reaction mechanism for bromate reduction over Pd1-In4/Al2O3

269

was proposed based on all the experimental results. Bromate was first adsorbed on

270

Pd1-In4/Al2O3, and after the addition of applied current, electrons and atomic H* were

271

both generated in the reaction system. The direct reduction of bromate by electrons

272

played a more important role in bromate removal in the presence of Pd1-In4/Al2O3

273

catalyst, probably due to the enhancement of electron transfer by Pd-In within the

274

catalyst. Compared to the other catalysts such as Pd-Cu, Pd-Sn, and Pd-Bi, the higher

275

activity of Pd1-In4/Al2O3 towards bromate reduction may be due to two aspects: (i) the

276

improved adsorption of bromate on the catalyst, and (ii) the enhanced transfer of

277

electrons and atomic H* from catalyst to bromate.

278

ASSOCIATED CONTENT

279

Supporting Information

280

Additional experimental details and data are presented in the Supporting

281

Information section.

282

Author information


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283

Corresponding author

284

Tel.: +86 10 62849160; fax: +86 10 62849160

285

E-mail address: [email protected] (R P. Liu)

286 287 288

Acknowledgement This work was supported by the National Natural Science Foundation of China (Nos. 51478455, 51438011, 51422813).

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(13) Mao, R.; Zhao, X.; Lan, H.; Liu, H.; Qu, J. Graphene-modified Pd/C cathode

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and Pd/GAC particles for enhanced electrocatalytic removal of bromate in a

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continuous three-dimensional electrochemical reactor. Water Res. 2015, 77,



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1-12.

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(14) Zhao, X.; Liu, H.; Li, A.; Shen, Y.; Qu, J. Bromate removal by

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electrochemical reduction at boron-doped diamond electrode. Electrochim.

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Acta 2012, 62, 181-184.

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reduction using an activated carbon felt electrode. Environ. Sci. Technol. 2009,

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43, 2054-2059.

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(16) Wei, L. Y.; Guo, S. H.; Yan, G. X.; Chen, C. M.; Jiang, X. Y. Electrochemical

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pretreatment of heavy oil refinery wastewater using a three-dimensional

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electrode reactor. Electrochim. Acta 2010, 55, 8615-8620.

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(17) Kong, W. P.; Wang, B.; Ma, H. Z.; Gu, L. Electrochemical treatment of

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anionic surfactants in synthetic wastewater with three-dimensional electrodes.

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J. Hazard. Mater. 2006, 137, 1532-1537.

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(18) Mascia, M.; Vacca, A.; Palmas, S. Fixed bed reactors with three dimensional

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electrodes for electrochemical treatment of waters for disinfection. Chem. Eng.

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J. 2012, 211-212, 479-487.

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(19) Chaplin, B. P.; Reinhard, M.; Schneider, W. F.; Schüth, C.; Shapley, J. R.;

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Strathmann, T. J.; Werth, C. J. Critical review of Pd-based catalytic treatment

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of priority contaminants in water. Environ. Sci. Technol. 2012, 46,

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11469-11470.

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(20) Ksar, F.; Ramos, L.; Keita, B.; Nadjo, L.; Beaunier, P.; Remita, H. Bimetallic

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palladium-gold nanostructures: application in ethanol oxidation. Chem. Mater.


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(21) Spee, M. P.; Boersma, J.; Meijer, M. D.; Slagt, M. Q.; van Koten, G.; Geus, J.

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W. Selective liquid-phase semihydrogenation of functionalized acetylenes and

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propargylic alcohols with silica-supported bimetallic palladium-copper

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catalysts. J. Org. Chem. 2001, 66, 1647-1656.

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(22) Son, S. U.; Jang, Y.; Park, J.; Na, H. B.; Park, H. M.; Yun, H. J.; Hyeon, T.

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Designed synthesis of atom-economical Pd/Ni bimetallic nanoparticle-based

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catalysts for sonogashira coupling reactions. J. Am. Chem. Soc. 2004, 126,

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5026-5027.

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(23) Yoshinaga, Y.; Akita, T.; Mikami, I.; Okuhara, T. Hydrogenation of nitrate in

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water to nitrogen over Pd–Cu supported on active carbon. J. Catal. 2002, 207,

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(24) Hou, W.; Dehm, N. A.; Scott, R. W. Alcohol oxidations in aqueous solutions

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using Au, Pd, and bimetallic AuPd nanoparticle catalysts. J. Catal. 2008, 253,

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(25) Wenkin, M.; Ruiz, P.; Delmon, B.; Devillers, M. The role of bismuth as

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promoter in Pd–Bi catalysts for the selective oxidation of glucose to

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gluconate. Journal of molecular catalysis A: chemical, 2002, 180, 141-159.

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(26) Wang, Y.; Qu, J.; Wu, R.; Lei, P. The electrocatalytic reduction of nitrate in

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(27) Shao, M. H.; Sasaki, K.; Adzic, R. R. Pd-Fe nanoparticles as electrocatalysts



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for oxygen reduction. J. Am. Chem. Soc. 2006, 128, 3526-3527.

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waters: the effect of tin oxides in the catalytic hydrogenation of nitrate by

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Pd/SnO2 catalysts. Appl. Catal., B 2002, 38, 91-99.

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Palladium−indium catalyzed reduction of N-nitrosodimethylamine: indium as

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a promoter metal. Environ. Sci. Technol. 2008, 42, 3040-3046.

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García-Bordejé, E. Nanostructured catalysts for the continuous reduction of

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hydrogenation in a solid polymer electrolyte reactor. J. Appl. Electrochem.

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platinum-indium and platinum-tin alloys. Journal of the Less Common

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Metals, 1968, 16, 427-440.

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characterization of Pd–In and Pt–In supported on Al2O3 and SiO2, active

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catalysts for nitrate hydrogenation. Appl. Catal., A 2008, 348, 60-70.

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(34) Gennero de Chialvo, M. R.; Chialvo, A. C. Kinetics of hydrogen evolution

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reaction with Frumkin adsorption: re-examination of the Volmer-Heyrovsky

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and Volmer-Tafel routes. Electrochim. Acta 1998, 44, 841-851.


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(35) Mao, X. H.; Ciblak, A.; Amiri, M.; Alshawabkeh, A. N. Redox control for

401

electrochemical dechlorination of trichloroethylene in bicarbonate aqueous

402

media. Environ. Sci. Technol. 2011, 45, 6517-6523.

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(36) Qu, J.; Zou, X.; Liu, B.; Dong, S. Assembly of polyoxometalates on carbon

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nanotubes paste electrode and its catalytic behaviors. Anal. Chim. Acta 2007,

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599, 51-57.

406

(37) Li, T.; Farrell, J. Reductive dechlorination of trichloroethene and carbon

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tetrachloride using iron and palladized-iron cathodes. Environ. Sci. Technol.

408

2000, 34, 173-179.

409

(38) Xie, W. J.; Yuan, S. H.; Mao, X. H.; Hu, W.; Liao, P.; Tong, M.; Alshawabkeh,

410

A. N. Electrocatalytic activity of Pd-loaded Ti/TiO2 nanotubes cathode for

411

TCE reduction in groundwater. Water Res. 2013, 47, 3573-3582.

412

(39) Mao, R.; Li, N.; Lan, H.; Zhao, X.; Liu, H.; Qu, J.; Sun, M. Dechlorination of

413

trichloroacetic acid using a noble metal-free graphene-Cu foam electrode via

414

direct cathodic reduction and atomic H*. Environ. Sci. Technol. 2016, 50,

415

3829-3837.



416

Table and figure captions

417

Table 1. BET results for different catalysts.

418

Figure 1. (A) Effect of Pd loading, (B) co-metals, and (C) Pd: In ratio on the

419

electrochemical reduction of bromate; (D) the initial activity of the catalyst under

420

varying conditions (catalyst dosage = 1.0 g/L, Pd content of the catalysts in Figure 1B

421

and C was 3.0%, initial bromate concentration = 100 µg/L, 2 mM Na2SO4, pH = 7.0,

422

current density = 0.9 mA/cm2).

423

Figure 2. The electrochemical reduction of bromate and the generation of bromide

424

with different catalysts (catalyst dosage = 1.0 g/L, initial bromate concentration = 100

425

µg/L, 2mM Na2SO4, pH = 7.0, current density = 0.9 mA/cm2).

426

Figure 3. (A) TEM image of Pd1-In4/Al2O3, (B) EDX images for Pd1-In4/Al2O3

427

element mapping, and (C) XRD patterns of (a) Al2O3, (b) Pd/Al2O3 and (c)

428

Pd1-In4/Al2O3.

429

Figure 4. XPS spectra of Pd in Pd/Al2O3, Pd4-In1/Al2O3, and Pd1-In4/Al2O3 catalysts;

430

and XPS spectra of In for Pd4-In1/Al2O3 and Pd1-In4/Al2O3 catalysts.

431

Figure 5. (A) The adsorption of bromate using different catalysts without applying

432

any electrochemical potential (catalyst dosage = 1.0 g/L, initial bromate concentration

433

= 100 µg/L, 2mM Na2SO4, pH = 7.0); inset shows the surface zeta potentials of

434

different catalysts at pH 7.0; (B) the bromine mass balance during the electrochemical

435

reduction process (catalyst dosage = 1.0 g/L, initial bromate concentration = 10 mg/L,

436

2mM Na2SO4, pH = 7.0, current density = 2 mA/cm2), inset shows the bromine

437

balance with different catalysts under the same other conditions.


Page 22 of 31

Page 23 of 31


438

Figure 6. (A) The DMPO spin-trapping ESR spectra with different catalysts at the

439

current density of 0.9 mA/cm2; (B) electrochemical reduction of bromate in different

440

systems with various C4H10O concentrations (catalyst dosage = 1.0 g/L, initial

441

bromate concentration = 100 µg/L, 2 mM Na2SO4, pH = 7.0, current density = 0.9

442

mA/cm2); (C) bromate removal with and without C4H10O under different conditions at

443

the reaction time of 20 min.

444

Figure 7. Effect of DO on electrochemical reduction of bromate in the reactor with

445

Pd1-In4/Al2O3 catalyst (catalyst dosage = 1.0 g/L, initial bromate concentration = 100

446

µg/L, 2 mM Na2SO4, pH = 7.0, current density = 0.9 mA/cm2).

447



Page 24 of 31

448 449 450

BET area (m2/g)

Pore volume

Pore size (nm)

(cm3/g)

451

Al2O3

67.97

0.27

15.65

Pd/Al2O3

63.70

0.34

21.27

Pd-In/Al2O3 (4:1)

74.62

0.35

19.13

Pd-In/Al2O3 (1:4)

41.90

0.22

21.31

Table 1.


Page 25 of 31


452 453 454

Bromate (C/Co)

No Pd Pd-1.5% Pd-2.0% Pd-3.0% Pd-4.0%

0.8 0.6

1.0 0.8 0.6 0.4

0.4

0.2

A

0.0

0.2 0.0

C 0

5

10

15

20

25

30

30

D

120 90 60 30 0

Reaction time (min)

455 456

25

Figure 1


Pd:In=1:4

0.4

20

Pd:In=1:1 Pd:In=1:2

0.6

15

Pd:In=2:1

0.8

10

Pd:In=4:1

Bromate (C/Co)

Pd/In=4:1 Pd/In=2:1 Pd/In=1:1 Pd/In=1:2 Pd/In=1:4

5

-1

1.0

0

150

Pd-In

30

Pd-Bi

25

Pd-Cu Pd-Sn

20

Pd-3.0% Pd-4.0%

15

Pd-1.5%

10

B

Pd-2.0%

5

-1

0

Initial activity (µg •gCat h )

0.2

Pd-In Pd-Cu Pd-Sn Pd-Bi

Bromate (C/Co)

1.0


Page 26 of 31

457 458 459 460 1.0 1.0

Bromate

Bromide

0.8

C/Co

0.8

0.6

0.6

0.4

0.4

Al2O3

Al2O3

Pd/Al2O3

0.2

In/Al2O3

0

5

10

15

20

25

0.0

Pd-In/Al2O3

30

3.5

0

10

2.4

k = 0.1275

Bromate

k = 0.0413 k = 0.0328 k = 0.0253

2.0

20

25

30

k = 0.099

Bromide

1.5

k = 0.0371 k = 0.0278 k = 0.0215

1.6 1.2

1.0

0.8

0.5

0.4 0.0 0

5

10

15

20

25

Reaction time (min)

0

5

10

15

20

Reaction time (min)

461 462 463

15

2.0

2.5

0.0

5

2.8

3.0

Ln (C/Co)

Pd/Al2O3 In/Al2O3

Pd-In/Al2O3

0.0

0.2

Figure 2


25

Page 27 of 31


464 465 466 467

468 ∆

C Intensity (a.u.)

∆

∗

∆ ∆∗

∆

∆∆

∆

∗: Pd-In alloy ♦: Pd ∆: Al2O3 ∗ ∆ (c)

♦ ♦

♦

(b) (a)

20 469 470

30

40

50 2θ (deg)

60

Figure 3

471


70

80


Page 28 of 31

472 473 474

Pd

335.8 eV

445.9 eV

In 341.1 eV

453.5 eV

Pd:In=1:4 334.8 eV 340.0 eV Pd:In=4:1

Pd:In=1:4

334.4 eV 339.7 eV Pd

330 333 336 339 342 345 348

Pd:In=4:1

440

Binding Energy (eV)

444

448

475 476

452

Binding Energy (eV)

Figure 4


456

Page 29 of 31


477 478

1.0

A

48

Zeta potential (m V)

Bromate (C/Co)

0.9 0.8 Al2O3

0.7

Pd-In/Al2O3

40

In/Al2O3

32 24

Al2O3

16

Pd/Al2O3

8 0

Different samples

In/Al2O3

0.6

Pd/Al2O3

Pd-In/Al2O3

0.5 0

5

10

15

20

25

30

Reaction time (min)

0.8

Reaction time = 20 min

B

Other bromine

Bromide

Bromate

100

0.6 Total bromine Bromate Bromide

0.4 0.2

B rom ine ratios (% )

Bromide (C/Ctheory)

1.0

80 60 40 20 0

Control Al2O3 Pd/Al2O3 In/Al2O3Pd-In/Al2O3

0.0 0

5

10 15 20 Reaction time (min)

25

479 480

Figure 5


30


Page 30 of 31

481 482 483 484 100 1.0

A

Control with TBA Control

Bromate (C/Co)

Intensity (a.u.)

0.8 In

0.6

B

0.4

Pd

Reaction time = 25 min

Bromate removal (%)

Pd-In

Pd-In Pd-In with TBA Pd Pd with TBA

0.2 Control

3475

3500

3525

3550

Magnetic Field (G)

3575

60 19.2 %

C 40 5%

0 0

5

10

15

20

25

Reaction time (min)

485 486

without TBA with 10 mM TBA

20

0.0 3450

80

27.5 %

Figure 6


Pd-In

Pd

Control

Different conditions

Page 31 of 31


487 488 489 490

6

Control Deoxygen

0.8 0.6

Ln ( C/C o)

Bromate (C/Co)

1.0

5

k=0.3709

4 k=0.1275

3 2 1 0

0.4

0

5

10

15

20

25

Reaction time (min)

0.2 0.0 0

5

10

15

20

25

Reaction time (min) 491 492

Figure 7


30

Enhanced Electroreductive Removal of Bromate by a Supported PdâIn

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