Biogenic Synthesis of Pd-Based Nanoparticles with Enhanced

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Biogenic Synthesis of Pd-Based Nanoparticles with Enhanced Catalytic Activity Lu Xiong, Xing Zhang, Yu-Xi Huang, Wu-Jun Liu, Yali Chen, Shengsong Yu, Xiao Hu, Lei Cheng, Dong-Feng Liu, and Han-Qing Yu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00322 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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ACS Applied Nano Materials

Biogenic Synthesis of Pd-Based Nanoparticles with Enhanced Catalytic Activity

Lu Xiong†, Xing Zhang†, Yu-Xi Huang, Wu-Jun Liu*, Ya-Li Chen, Sheng-Song Yu, Xiao Hu, Lei Cheng, Dong-Feng Liu, Han-Qing Yu* CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science & Technology of China, Hefei, 230026, China

† These authors contributed equally. *Corresponding authors: Dr. Wu-Jun Liu, E-mail: [email protected] Prof. Han-Qing Yu, E-mail: [email protected]

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ABSTRACT: Searching efficient sustainable approaches for the metal nanoparticles

2

(NPs) synthesis has become a research focus. Bio-reduction with metal-reducing

3

bacterium at ambient temperature provides a green route for metal NPs synthesis,

4

especially for precious metals. In this work, activated Pd NPs were synthesized by

5

contacting Pd2+ solution with an efficient metal-reducing Shewanella oneidensis

6

bacterium, in which the bacterium acted as reducing, capping and stabilizing agents,

7

and the shape and composition of the Pd NPs could be tuned with the activation of

8

KOH at elevated temperatures. The as-activated Pd NPs showed a favorable

9

performance toward catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol

10

(4-AP), with a remarkable apparent kinetic constant of 5.0 × 10-3 s-1, which was 12

11

times greater than that of the raw biogenic Pd NPs, even comparable to that of the

12

commercial 5.0 wt.% Pd/C. Changes of the Pd NPs aggregates, ratio of Pd to S,

13

surface area of the support at different temperatures and with different activation

14

reactants were tested to explore the improvement associated with the KOH-activation

15

at elevated temperatures. Such an activation approach was also successfully applied

16

for improving the catalytic activity of biogenic Au NPs. This work may offer a

17

sustainable, cost-effective and efficient approach to prepare biogenic metal

18

nanomaterials for catalytic organic reaction or polutants removal.

19

Biogenic

synthesis;

Pd

20

KEYWORDS:

21

4-nitrophenol (4-NP); Shewanella oneidensis

nanoparticles;

2


catalytic

reduction;

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Table of Content (TOC) Arts

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3



24

INTRODUCTION

25 26

Compared to the bulk materials with the same chemical composition, the

27

nanoparticles (NPs) with a large surface to volume ratio and small size have great

28

different physical, chemical, and biomedical properties, such as mechanical

29

properties, melting point, electrical and thermal conductivity, optical absorption,

30

catalytic activity, as well as biological and sterical properties.1-3 Conventional

31

chemical synthesis approaches have been widely used for the NPs synthesis, and the

32

size, shape, and chemical composition of the nanoparticles can be easily tuned.4-5

33

However, in the chemical synthesis process, the use of flammable or toxic chemicals

34

(e.g., hydrogen gas, hydrazine hydrate, or sodium borohydride) is usually

35

unavoidable, which may introduce the harsh chemicals on the NPs surfaces, and thus

36

raising the toxicity or other harmful issues.6-7

37

As an alternative, biosynthesis develops quickly because of its growing success

38

in the synthesis of metal NPs, and more importantly, biosynthesis of NPs can be

39

regarded as an environmentally friendly process since it avoids the use of flammable

40

or toxic chemicals.8-9 Among all the biosynthesis methods, the bio-reductive

41

deposition using the metal-reducing bacterium offers a highly tunable green method

42

to synthesize various metal (e.g., Pd, Au, and Pt) NPs,10-11 which can be used as

43

catalysts, biomedical agents, and electronic materials, etc. Bio-reduction Pd, as an

44

appealing catalyst, has been used in catalyzing various chemical reactions and shown

45

favourable performance.12-23 4


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46

Shewanella oneidensis belongs to a class of bacteria known as "dissimilatory

47

metal-reducing bacteria (DMRB)" due to their ability to couple metal reduction with

48

their metabolism. Its tolerance and capacity to use heavy metals are attributed to its

49

metabolism pathway. Multidrug efflux transporters, detoxification proteins, extra

50

cytoplasmic sigma factors and Per-Arnt-Sim (PAS) domain regulators are shown to

51

have higher expression activity in presence of heavy metals. Cytochrome c class

52

protein SO3300 also has an elevated transcription. Due to these characteristics, S.

53

oneidensis is recognized as a robust metal reducing bacterium and can be used for

54

biogenic Pd(II) reduction.24 Besides its robust metal reducing ability, S. oneidensis

55

can act as an engineering strain in various bioelectrochemical systems like microbial

56

fuel cell, and thus is widely used for wastewater treatment and resource recovery.

57

In this work, activated Pd NPs were synthesized by contacting the Pd2+ solution

58

with the S. oneidensis at ambient temperature. In this synthetic process, the bacterium

59

acted as reducing, capping and stabilizing agents. Many compounds with high

60

reducing capacities are present in bacterial cells, e.g., proteins, polypeptides, and

61

extracellular polymeric substances.25-26 They can act as a reductant for the reduction

62

of

63

polysaccharides, proteins and peptides can also act as capping and stabilizing agents,

64

in which the complicated hydrogen-bond network induced by the polysaccharide and

65

proteins framework hinders the aggregation of as-synthesized NPs. Use of nontoxic

66

polysaccharides and proteins can minimize negative environmental impacts by

67

avoiding using of hazardous capping agents (e.g., oleic acid, oleylamine,

high-valent

metal

species

into

metal

nanoparticles.

5


Meanwhile,

the


68

trioctylphosphine).7 The shape and composition of nanoparticles could be tuned with

69

the activation of KOH or NaOH at elevated temperatures. KOH and NaOH are two

70

typical activation agents and have been widely used in the activation of bioinorganic

71

catalysts. Compared to other activation methods such as ZnCl2 and H3PO4 activation,

72

the alkali treatment is more environmentally friendly as no hazardous materials like

73

heavy metals (Zn) or low-valent phosphorus species (PH3) are released in the process.

74

Meanwhile, the porous structure formed from alkali treatment is usually more

75

abundant than those formed from ZnCl2 and H3PO4 activation.

76

In this work, activated Pd NPs were synthesized through reduction of Pd(II)

77

solution with an efficient metal-reducing S. oneidensis bacterium at ambient

78

temperature. The shape and composition of the bio-reduced Pd NPs were tuned with

79

via KOH activation at elevated temperatures. The performance of the as-synthesized

80

bio-Pd NPs was evaluated through aqueous catalytic 4-NP reduction to 4-AP, a model

81

reaction widely used in the evaluation of the catalytic activity of the metal NPs. Such

82

an activation approach was also successfully applied for improving the catalytic

83

activity of biogenic Au NPs.

84 85

EXPERIMENTAL SECTION

86 87

Synthesis of Biogenic-Pd. The Pd NPs were synthesized through an anaerobic

88

bio-reductive deposition using the S. oneidensis as a bio-reductant. Generally, S.

89

oneidensis was cultured in Luria-Bertani medium (Yeast extract 5 g, NaCl 5 g, 6


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tryptone 10 g/L) overnight at 30 oC and harvested by centrifuging at 5000 rpm

91

for 5 min before transferring to the mineral medium. The mineral medium

92

contained (per liter): sodium 4-(2-hydroxyethyl)-1-piperazineethane-sulphonic

93

acid 2.38 g, NH4Cl 0.46 g, K2HPO4 0.224 g, KH2PO4 0.224 g, MgSO4 0.059 g,

94

(NH4)2SO4 0.224 g, sodium lactate (60 % w/w) 2 mL and trace elements

95

solution 10 mL. The trace elements solution contained (per liter): nitrilotriacetic

96

acid 1.5 g, MnCl2·4H2O 0.1 g, FeSO4·7H2O 0.3 g, CoCl2·6H2O 0.17 g, ZnCl2

97

0.1 g, NiCl2·6H2O 0.219 g, Na2SeO3 0.109 g, CuSO4·5H2O 0.04 g,

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AlK(SO4)2·12H2O 0.005 g, H3BO3 0.005 g, Na2MoO4·2H2O 0.106 g, and

99

NaWO4·2H2O 0.02 g. The washed cells were re-suspended in 500 mL mineral

100

medium in serum bottles to a final concentration of about OD600 = 2.0 ± 0.1.

101

Subsequently, aqueous Na2PdCl4 was added to a final concentration of 400 µM

102

and the serum bottles were incubated for 3 h at 30 oC. For HAuCl4, the final

103

concentration was 600 µM and the serum bottles were incubated under the

104

same conditions. The incubated cells were centrifuged again at 8000 rpm for 3

105

min, and then freeze dried to obtain dried cells. Thereafter, the dried cells were

106

mixed with KOH in a mass ratio of 2:1 and the resulting mixture was annealed

107

in a tube furnace at a heating rate of 5 oC/min to the set value for 3 h at 30 mL

108

min-1 of argon flow. The heated samples were cooled down to room

109

temperature and washed with 5% HCl and afterwards distilled water for three

110

times.

7



111

Characterizations. XRD (X-ray diffraction) (Rigaku TTR-III, Japan)

112

analysis was performed with Cu Kα radiation in a scan rate of 0.05 o/s. N2

113

adsorption-desorption isotherm analysis was carried out on a ASAP 2020 M+C

114

Micromeritics Gemini apparatus (Micromeritics Co., USA) at 77 K, and the

115

surface area of the materials was calculated with the Barrett-Emmett-Teller

116

(BET) method. X-ray photoelectron spectroscopy (XPS) of the materials was

117

investigated with an ESCALAB 250 instrument (Thermo-VG Scientific, UK).

118

Scanning electron microscopy (SEM) images were investigated through a field

119

emission SEM (Supra 40, Zeiss Co., Germany). Transmission electron

120

microscopy (TEM) investigations of the materials were conducted with a

121

JEM-2100F instrument (JEOL, Japan). Inductively coupled plasma-atomic

122

emission spectrometer (ICP-AES, Optima 7300 DV, Perkin Elmer Co., USA)

123

was used to determine the Pd or Au content. The chemical reactions occurring

124

in the alkali treatments was analyzed using a thermogravimetric analyzer

125

(DTG-60H/DSC-60, Shimadzu Co., Japan) from room temperature to 600 oC

126

under N2 flow with a heating rate of 10 oC/min.

127

Catalytic Performance Evaluation. The catalytic performance of the

128

activated bio-Pd NPs was evaluated in aqueous reduction of the 4-NP to 4-AP.

129

Briefly, 1 mg of the catalyst was ultrasonically dispersed in 10 mL of water.

130

NaBH4 solution was prepared by dissolving 50 mg of NaBH4 in 50 mL of water

131

in ice bath. 2 mL of the NaBH4 solution were mixed with 0.5 mL of 1 mM

132

4-NP in a molar ratio of 1:100, and the mixture was further diluted by adding 6 8


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mL of deionized water. 3 mL of the mixed solution and 1 mL of catalyst ink

134

(containing 0.1 mg of catalyst) were used for the measurement. The reaction

135

was performed in a standard quartz cell with a path length of 1 cm. The UV-vis

136

spectra were scanned in a wavelength of 200-500 nm at 20 oC. Deionized water

137

(18.2 MΩ·cm) was used throughout the experiments. All the glassware were

138

immersed in aqua regia (VHNO3: VHCl=1:3) to remove inorganic residuals before

139

use.

140

It should be noted that since the catalytic reduction reaction is very rapid, we

141

carried out the reduction reaction in-situ in a standard quartz cell with 1 cm of path

142

length. After the reaction, such a small amount of the catalyst could not be recycled

143

effectively. Because of these reasons, the recyclability of the as-activated Pd NPs

144

could not be evaluated as usual. In fact, due to the low abundance and high cost, the

145

dosage of noble metal-based catalysts in the reactions is always very low, but their

146

catalytic activity is remarkably higher than those of megadosed non-noble

147

metal-based catalysts. Because of this reason, in many studies using the noble

148

metal-based catalysts the recycling performance of the catalysts could not be

149

evaluated as well.27, 28

150 151

RESULTS

152 153

Characteristics of the Bio-Pd. The synthesis process of the bio-Pd NPs includes two

154

stages: i.e., (1) bio-reductive convention of PdCl42- into the Pd NPs by S. oneidensis; 9



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and (2) activation of the Pd NPs with alkali thermal treatment. At first stage, S.

156

oneidensis acted as reducing, capping and stabilizing agents. As shown in Figure S1,

157

the injection of NaPdCl4 to the culture medium could induce a rapid colour change

158

from pink to brownish black in the serum bottles, confirming the reduction of Pd.

159

After the freeze-drying dehydration of the cells, and small Pd nanoparticles with a

160

size of around 10 nm dispersed separately on the S. oneidensis were formed (Figure

161

S2). At the second stage, the activation was performed at 300 oC (Bio-Pd-300), 400

162

o

163

which S. oneidensis cell was carbonized and activated to produce porous structure.

164

The main reactions happening in this stage can be illustrated as follows: First, the

165

functional groups of microbial cells are decomposed at a high activated temperature to

166

release some volatiles (e.g., H2O, CO2, and CO), which can be captured by KOH to

167

form K2CO3 (Reaction 1). The formation of H2O and CO2 in the activation system

168

also positively contributes to the development of porosity via physical activation by

169

CO2 and steam; at high temperatures, KOH itself reacts with carbon to release CO and

170

H2. The escape of the gaseous species produces many pore structure (Reaction 2).29

171

With a further increase in temperature, the newly formed K2CO3 reacts with carbon to

172

release more gaseous species (e.g., CO and K),30 resulting in the formation of larger

173

pores (Reactions 3 and 4). After the removal of the intercalated K compounds by acid

174

washing, activated carbon with a high porosity and large surface area is obtained.

C (Bio-Pd-400) and 500 oC (Bio-Pd-500), respectively, with KOH under N2 flow, in

2KOH + CO2→ K2CO3 + H2O↑

(1)

2C + 2KOH→2 CO↑+ 2K+ H2↑

(2) 10


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K2CO3 + C → K2O + 2CO↑

(3)

K2O+ C→ 2K + CO↑

(4)

175

After activation at different temperatures, the microstructure of the biomass

176

support and Pd NPs changed significantly. For Bio-Pd-300, single cells could not be

177

identified but rough sketch of cells survived (Figure 1a), and the size and dispersion

178

of the Pd NPs after the heat treatments remains unchanged (Figure 1b). When the

179

annealing temperature increases to 400 oC, the cell structure was totally destroyed and

180

a sponge with diverse size of pores appears (Figure 1c). The TEM image of the

181

Bio-Pd-400 shown in Figure 1d reveals that Pd NPs dispersed separately on the

182

carbon-sheet support. As the annealing temperature further increases to 500 oC,

183

sponge structure is the main morphology accompanied by wrinkled carbon sheets

184

(Figure 1e), and drastic aggregation of Pd particles was observed (Figure 1f). The

185

reason for the morphology change of the biomass support at a certain temperature

186

might be because carbon oxidation and transformation of KOH to K2O started at

187

about 400 oC and the formed K2CO3 favoured the formation of porous structure.30-31

188

The HADDF image and element mapping of bio-Pd-400 (Figure 1g) show that the

189

elements C, O, N, Pd, and S were evenly distributed in the sample. We also calculated

190

the particle size distribution of the Pd NPs in the bio-Pd-400 sample (Figure 1h),

191

which shows that most of the Pd NPs had a particle size range from 5 to 25 nm.

11



192

193 194 195 196 197 198

Figure 1. SEM and TEM images: (a, b) Bio-Pd-300, (c, d) Bio-Pd-400, and (e, f) Bio-Pd-500, (g) The HADDF image and elemental mapping of C, O, N, Pd, and S for the Bio-Pd-400 sample, (h) particle size distribution of the Pd nanoparticles for the Bio-Pd-400 sample, (measured and calculated from Figure 1d).

199

The changes of Pd associated with the increase of annealing temperature were

200

characterized by XRD (Figure 2a). Obviously, higher temperatures lead to a greater

201

crystallinity, which has a substantial effect on the catalytic performance. The Bio-Pd

202

prepared under the freeze-drying conditions has a low crystallinity of Pd with broad

203

peaks (JCPDS 87-0639). Under heating circumstances, sulfur is released from the 12


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disintegrated bacterial cells and poisoned the metal surface due to structural and

205

electronic effects.22 Diffraction peaks of the Bio-Pd-300 are consistent with Pd4S

206

(JCPDS 73-1387). For the Bio-Pd-400, mixed compounds of Pd and Pd4S were

207

observed and Pd accounted for a relatively low ratio. Crystallinity and percentage of

208

Pd in the mixture distinctly increases in the Bio-Pd-500 sample. To identify whether

209

KOH influenced the loss of sulfur, the bio-supported Pd heated at 400 oC without

210

KOH for 3 h (its TEM image and low catalytic activity are shown in Figure S3) was

211

also tested by XRD and only Pd4S was found. Since KOH is decomposed to K2O at

212

about 400 oC, soluble sulfate might be formed through a series of complicated

213

reactions among K2O, H2O and S, which caused a decline of sulfur.

214

The pore structure and surface areas of the bio-Pd samples were analysed with N2

215

adsorption-desorption isotherms. As shown in Figure 2b, the N2 adsorption-desorption

216

isotherms of the samples including the freezing-dried Bio-Pd, Bio-Pd-300,

217

Bio-Pd-400, and Bio-Pd-500 could be categorized as type IV with hysteresis loops,

218

suggesting that the bio-Pd samples have mesopore structure. This was also confirmed

219

by their pore size distribution profiles (Figure S4). The BET surface-area values of the

220

samples were calculated as 71.4, 231.8, 621.1 and 995.1 m2 g-1, respectively.

221

Obviously, a much higher surface area of the biomass-support sample was obtained

222

after the activation process.

223

The chemical reactions in the alkali treatments of the bio-Pd samples were probed

224

by TG-DTG. As shown in Figure 2c, there were two main stages in the alkali

225

treatment of bio-Pd. The first stage began at about 30 oC, and finishes at about 200 oC, 13



226

with the peak was found in temperature around 100 oC, which could be attributed to

227

the release of free water and bound water in the bio-Pd sample. The second stage

228

began after 200 oC, and finished at about 400 oC with a peak found at 310 oC. At this

229

stage, about 48% of the total weight was lost, which might be attributed to the release

230

of volatiles produced in the decomposition of the bacterial biomass.

231

XPS measurements were carried out to investigate the chemical states of Pd

232

before and after the KOH treatments. As shown in the Pd 3d spectrum before the

233

KOH treatments (Figure 2c), the two main peaks at around 340.8 and 335.0 eV should

234

be assigned to the metallic Pd (Pd(0)) 3d3/2 and 3d5/2, respectively, confirming the

235

formation of Pd(0) by the bioreduction of S. oneidensis. After the KOH treatments,

236

besides the Pd(0) and Pd(II) species, a new species of Pd4S with the binding energy of

237

336.9 and 342.3 eV was found, which is consistent with the XRD results. It should be

238

noted that the peak intensity of Pd after the KOH treatments was much higher than

239

that without treatments, suggesting that more Pd species exposed on the surface after

240

the KOH treatments. This could greatly benefit for the catalytic performance of the

241

materials.

242

14


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244 245 246 247 248 249

Figure 2. (a) XRD results of the Bio-Pd, Bio-Pd-400 without KOH, Bio-Pd-300, Bio-Pd-400 and Bio-Pd-500; (b) Nitrogen adsorption-desorption isotherms of the Bio-Pd, Bio-Pd-300, Bio-Pd-400 and Bio-Pd-500; (c) The TG and DTG curves of the bio-Pd sample from room temperature and 600 oC under N2 flow; (d) and (e) XPS Pd 3d spectra of the Bio-Pd and Bio-Pd-400.

250 251

Catalytic Performance of the Activated Bio-Pd. The catalytic performance of

252

the bio-Pd was first evaluated through catalytic reduction of the 4-NP to 4-AP with

253

NaBH4, and the reaction process was monitored with UV-vis spectroscopy. As shown

254

in Figure 3a, The absorption peak at wavelength of 400 nm kept almost unchanged

255

within 60 min in the absence of catalysts, suggesting that no reaction occurred in the

256

mixed solution due to the kinetic barrier, even though the 4-NP reduction to 4-AP by

257

NaBH4 is thermodynamically feasible. After the catalyst was introduced in the

258

system, the reaction can proceed quickly. After 14 min, the absorption at 400 nm 15



259

vanished with the catalysis of benchmark 5.0 wt.% Pd/C catalyst (Figure 3b), while

260

only half of the 4-NP can be reduced over the raw bio-Pd catalyst in 20 min (Figure

261

3c). As shown in Figure 3d-f, the fading and bleaching of the yellow colour were

262

found to be much faster on the activated catalysts compared to the Bio-Pd. Notably,

263

more than 95% of the 4-NP can be reduced within 10 min when using Bio-Pd-400 as

264

a catalyst, even better than the benchmark 5.0 wt.% Pd/C.

265 266 267 268 269

Figure 3. Time-dependent UV-vis absorption spectra of the 4-NP and NaBH4 mixed solution with (a) no catalyst, (b) commercial Pd/C, (c) freeze-drying Bio-Pd, (d) Bio-Pd-300, (e) Bio-Pd-400, and (f) Bio-Pd-500.

270

It should be noted that the dosage of NaBH4 was excess to ensure that the

271

reaction occurred in a pseudo-first-order, which was confirmed by the linear

272

relationship of ln(At/A0) with reaction time (Figure S5).32 In order to give a better

273

comparison, activities of these catalysts were quantified. The reaction apparent rate

274

constants kapp are listed in Table S1. Obviously, all the activated catalysts had a larger

275

kapp value than the Bio-Pd in an order of Bio-Pd-400 > benchmark 5.0 wt.% Pd/C >

276

Bio-Pd-300 > Bio-Pd-500 > Bio-Pd. The Bio-Pd-400 exhibited the highest catalytic 16


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277

reactivity with a kapp value of 5.0 × 10-3 s-1, which was 12 times larger than that for

278

the raw Bio-Pd and comparable to that of the benchmark 5.0 wt.% Pd/C. Furthermore,

279

compared to other related Pd-based catalysts with a kapp value of 1.5 × 10

280

microgel-Pd), 4.41 × 10 -3 s-1 (for spherical polyelectrolyte brush-Pd)，3.59 × 10 -3 s-1

281

(poly(amidoamine)-Pd) and 65.8 × 10

282

polystyrenesulfonate),33 the Bio-Pd-400 had a comparable kapp value. Such good

283

catalytic performance also makes the biogenic Pd NPs be comparable to those of

284

conventional synthesized materials (Table S2).

-3

-3

s-1 (for

s-1 (Pdnano-poly-(3,4)ethylenedioxythiophene/

285

Considering the different amounts of Pd in catalysts, an activity parameter,

286

defined as the ratio of kapp to the Pd mass, was used to evaluate the catalytic

287

performance. According to the ICP-AES results, the mass ratio of Pd was 1.4% for

288

the freeze-drying Bio-Pd, 3.4% for the commercial Pd/C, 2.3% for the Bio-Pd-300,

289

3.2% for the Bio-Pd-400 and 9.6% for the Bio-Pd-500. The order of the activity

290

parameter was as: benchmark 5.0 wt.% Pd/C ≈ Bio-Pd-400 > Bio-Pd-300 > Bio-Pd >

291

Bio-Pd-500. These results clearly indicate that the Bio-Pd-400 is an efficient catalyst

292

for the 4-NP reduction to 4-AP. Though the Bio-Pd-500 had the most Pd-loading,

293

lowest Pd/S ratio and highest surface area of the support, its activity was relatively

294

poor because of the severe aggregation of Pd.34 It should be noted that the Bio-Pd

295

without any heat treatments still showed a low activity even though no Pd4S species

296

was found. The main reason for this observation can be explained as follows: The

297

Bio-Pd had a low Pd content and small surface area compared to those catalysts with

298

thermochemical treatments. Pd could be the main active catalytic site, while high 17



299

surface area would not only stabilize Pd nanoparticles, but also facilitate diffusions of

300

the reactants and products to the active catalyst sites.

301

Expanding of the Bio-Reductive Deposition Method. In order to demonstrate

302

the wide applicability of the bio-reductive deposition method proposed in this work,

303

the same method was expanded to the synthesis of biogenic Au nanoparticles, and the

304

performance of the bio-Au was also evaluated in the aqueous catalytic 4-NP reduction

305

to 4-AP.35-37 The Bio-Au-400 prepared with the same approach exhibited the highest

306

activity and achieved the complete conversion of 4-NP to 4-AP in 5 min. Activities of

307

these Au-based catalysts followed the order of Bio-Au-400 > Bio-Au-500 >

308

Bio-Au-300 ≈ Bio-Au (shown in Figure 4). This result clearly demonstrates that our

309

activation approach could be used as a general way to improve the catalytic activity of

310

the biogenic metal particles.

311 18


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312 313 314

Figure 4. Time-dependent UV-vis adsorption spectra of the 4-NP and NaBH4 mixed solution with: (a) freezing-dry Bio-Au, (b) Bio-Au-300, (c) Bio-Au-400, and (d) Bio-Au-500

315 316

DISCUSSION

317 318

The mechanism for the catalytic 4-NP reduction is very complex, and has been

319

investigated extensively. A mechanistic investigation reported by Layek et al.38 in the

320

Au NPs catalyzing 4-NP reduction suggests that the reduction proceeds usually in two

321

procedures: (1) the adsorption and diffusion of 4-NP on the metal NPs surface, and

322

(2) electron transfer from the BH4- to the 4-NP with the medium of metal NPs. As a

323

strong nucleophile, the high electron injection capability of BH4- can facilitate

324

electrons transfer from BH4- to the 4-NP via Pd NPs, which is favourable for the

325

overcoming of the reaction kinetic barrier.39 Similar mechanism for 4-NP reduction

326

was also proposed by Bendi and co-workers40 when cellulose nanofiber film

327

embedding Cu NPs were employed as a catalyst, and by Zhang et al.41 when the

328

carbon nanofibers support Ag NPs were employed as a catalyst. As for the catalytic

329

reduction by the Bio-Pd, it may undergo a very similar mechanism: as shown in

330

Figure 5, BH4– is first adsorbed on the Pd NPs surface to produce a Pd hydride

331

complex, and 4-NP is also adsorbed reversibly on the surface of Pd NPs. Thereafter,

332

the hydrogen atoms and electrons was transferred from the Pd hydride complex to the

333

nitro group of 4-NP, further going through several hydrodeoxygenation reactions to

334

produce the 4-AP (Eq. 5).42

19



(5)

335

336 337 338 339 340

Page 20 of 30

Figure 5. Possible reaction mechanism for the reduction of 4-NP by the NaBH4 with the catalysis of the Pd NPs in the bio-Pd materials.

341

As a green material, Biogenic metal NPs have attracted great interests in recent

342

years.43-49 However, the use of bacteria for the synthesis of metal NPs as catalysts has

343

been restricted owing to the toxicity nature of metals to most of bacteria. In this work,

344

this barrier is successfully overcome by a robust Pd-reducing bacterium, S.

345

oneidensis. Generally, to fabricate NPs/carbon hybrid materials with good catalytic

346

properties, three strategies are usually adopted: the first one is to increase the

347

dispersion of small-sized metal nanoparticles on carbon support; the second one is to

348

provide surface functional groups of carbon support to create more binding sites and 20


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349

surface anchoring groups; and the last one is to dope some heteroatoms into carbon

350

support to improve the hydrophily of nanostructure for better catalytic activity in

351

aqueous system. Nevertheless, most of these approaches would raise extra costs and

352

environmental concerns.

353

Shewanella oneidensis could secrete high hydrogenases, which are enzymes with

354

plenty of amine groups that could act as an efficient bio-reductant. In this Pd synthesis

355

process, S. oneidensis acted as reducing, capping and stabilizing agents. Many

356

compounds with high reducing capacities, e.g., proteins, polypeptides, and

357

extracellular polymeric substances, are present in S. oneidensis cells. They can act as

358

a reductant for the reduction of Pd(II) into metal nanoparticles. Meanwhile, the

359

polysaccharides, proteins and peptides in the cells can act as capping and stabilizing

360

agents, in which the complicated hydrogen-bond network induced by the

361

polysaccharide and proteins framework hinders the aggregation of the as-synthesized

362

NPs. Compared to the conventional chemical methods, Shewanella oneidensis as a

363

carbon support precursor for the bio-Pd NPs has several attractive advantages: (1)

364

Shewanella

365

functionalization and heteroatom doping; (2) it exhibits a similar level of Pd(II)

366

reduction rate to chemical reduction, making it superior to other bacteria; and (3) its

367

high tolerance to Pd(II) enables the design of a wide range of Pd-loading catalysts.

368

Therefore, for the microbe-mediated synthesis process of high-concentration Pd NPs,

369

Shewanella oneidensis is an ideal candidate and the bio-Pd could be obtained in a

370

cost-effective and environmentally friendly manner. Such direct fabrication with

oneidensis

has

multiple

roles

for

21


metal

reduction,

surface


371

biogenic material and bacterium itself truly paves the way towards green synthesis of

372

metal-based hybrid catalysts. More importantly, in light of the powerful reducing

373

capability of Shewanella towards a wide range of metal cations (e.g., Ag+, Pt2+, and

374

Au3+), the strategy reported in this work may also be used to fabricate other noble

375

metal based materials and be potentially employed as efficient and advanced catalysts.

376

It should be pointed that the evaluation of energy consumption of the

377

high-temperature KOH and NaOH treatment depends on various factors, and the

378

energy efficiency is a key parameter. In this work, it is difficult to get the energy

379

efficiency of the high-temperature alkali treatment. Thus, we cannot evaluate the

380

energy consumption of the high-temperature alkali treatment at the present stage.

381 382

CONCLUSIONS

383 384

In summary, we have synthesized the activated Pd NPs using a sustainable

385

bio-reduction approach by a robust metal-reducing bacterium at ambient temperature.

386

The as-synthesized activated Bio-Pd NPs exhibits an excellent catalytic activity

387

toward aqueous 4-NP reduction to 4-AP with a high kinetic constant, which is

388

12-times larger than that of raw Bio-Pd, and even comparable to the benchmark 5.0

389

wt.% Pd/C. The proper size distribution of Pd nanoparticles, low ratio of Pd/S and

390

high surface area of the biomass support are found to be responsible for such excellent

391

performance. This work may provide a new way for the sustainable synthesis of metal

22


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392

NPs with high catalytic performance toward organic synthesis or pollutants

393

elimination.

394 395

ASSOCIATED CONTENT

396

Supporting Information

397

Figures S1-S5 and Tables S1-S2 This material is available free of charge via the

398

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

399 400

ACKONWLEDGMENTS

401

This work is supported by the National Natural Science Foundation of China

402

(21590812, 21607147), and the Collaborative Innovation Center of Suzhou

403

Nano Science and Technology of the Ministry of Education of China.

404 405

NOTES

406

The authors declare no competing financial interest.

407 408

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Biogenic Synthesis of Pd-Based Nanoparticles with Enhanced

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