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Low-temperature catalytic decomposition of 130 tetrato octa- PCDD/Fs congeners over CuOX and MnOX modified V2O5/TiO2-CNTs with the assistance of O3 Rixiao Zhao, Dongdong Jin, Hangsheng Yang, Shengyong Lu, Phillip M Potter, Cuicui Du, Yaqi Peng, Xiaodong Li, and Jianhua Yan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02977 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on September 27, 2016

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Environmental Science & Technology

1

Low-temperature catalytic decomposition of 130 tetra- to octa- PCDD/Fs

2

congeners over CuOX and MnOX modified V2O5/TiO2-CNTs with the assistance

3

of O3

4 †

‡

‡

†

5

Rixiao Zhao, *, Dongdong Jin, *, Hangsheng Yang, *, Shengyong Lu, *, Phillip M.

6

Potter,§ Cuicui Du, Yaqi Peng, Xiaodong Li, Jianhua Yan †

†

†

†

7 8

†

9

Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, China.

State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power

10

‡

11

Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, China

12

§

13

70803, American

State Key Laboratory of Silicon Materials, School of Materials Science and

Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana

14

ACS Paragon Plus Environment


15

Abstract: In this study, a reliable and steady PCDD/F generation system was utilized

16

to investigate the performance of catalysts, in which 130 congeners of tetra- to

17

octa-polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) vapors were

18

studied under simulated flue gas with/without O3. TiO2 and carbon nanotubes (CNTs)

19

supported vanadium oxides (VOX/TiO2-CNTs) modified with MnOX and CuOX,

20

which were reported to be beneficial to the decomposition of model molecules, were

21

found to have a negative effect on the removal of real PCDD/Fs in the simulated flue

22

gas without O3. Moreover, the addition of MnOX presented different effects depending

23

on whether CuOX existed in catalysts or not, which was also contrary to its effects on

24

the degradation of model molecules. In an O3-containing atmosphere, low

25

chlorination level PCDD/Fs congeners were removed well over VOX-MnOX/TiO2

26

-CNTs, while high chlorination level PCDD/Fs congeners were removed well over

27

VOX-CuOX/TiO2-CNTs. Fortunately, all PCDD/Fs congeners decomposed well over

28

VOX-MnOX-CuOX/ TiO2-CNTs. Finally, the effects of tetra- to octa- chlorination level

29

for the adsorption and degradation behaviors of PCDD/Fs congeners were also

30

investigated.

31 32

Keyword: Dioxins; congener; catalytic formation; catalytic decomposition; ozone;

33


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34

I. Introduction

35

Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), in brief

36

dioxins or PCDD/Fs, are a group of stable, persistent, and highly toxic chlorinated

37

organic compounds. These compounds share similar structure with three-ring

38

polycyclic aromatics, in which the central ring contains one or two oxygen atoms and

39

the external rings are substituted with chlorine atoms at various positions. According

40

to the number and substitution sites of chlorine atoms, PCDDs and PCDFs have 75

41

and 135 homologues, respectively, among which 2,3,7,8 -tetrachlorodibenzo-p-dioxin

42

(2,3,7,8 -TeCDD) is the most toxic congener. Given that toxic PCDD/Fs are severely

43

hazardous to the environment and public health,1 stringent laws and regulations have

44

been established to control dioxin emissions. In China, for example, the latest

45

standard for pollution control on municipal solid waste incineration (MSWI), released

46

in July 2014, improved the dioxin emission limitation from 1.0 to 0.1 ng I-TEQ/Nm-3

47

for newly-built incinerators and requested previously-built incinerators to meet the

48

new limitation in January 2016.

49

A variety of techniques have been developed for dioxin removal including

50

adsorption, thermal combustion, and catalytic oxidation.2 Among them, catalytic

51

oxidation is the most promising technology which can completely oxidize dioxins into

52

CO2, H2O, and HCl (or Cl2)3 and thus development of proper catalysts remains as the

53

research focus for many years.

54

V2O5/TiO2 catalysts (VTi) commonly applied to control NOX emission were found

55

to be effective in the decomposition of PCDD/Fs.4 However, the optimal active

56

temperature of VTi, normally below 200 °C, was always higher than that of the flue

57

gas where the catalyst reactors were installed. So, low-temperature active catalyst

58

development has become a hot topic recently. MnOX had been reported to be among

59

the most efficient transition metal oxide catalysts for catalytic destruction of

60

pollutants.5,6 Utilization of MnOX and CuOX oxides as catalysts or as components of



61

catalysts is an effective way to remove volatile organic compounds (VOCs) and NOX

62

at relatively low temperatures.7-10

63

Carbon nanotubes (CNTs) were recently regarded as important catalytic supports

64

due to their unique electronic transportation properties and facile flowing bond.11,12 In

65

fact, CNTs are superior adsorbents for dioxins and aromatics.13,14 The dominant

66

aromatic adsorption mechanism on CNTs studied by experiments and simulations15-17

67

indicates that aromatic rings are adsorbed parallel to the surface of CNTs by π-π

68

interaction between the benzene ring and the CNTs, which benefited the VOCs

69

catalytic decomposition.13-20

70

Applying ozone in the catalytic reactions was reported to be able to lower the

71

reaction temperature and activation energy for VOCs (benzene, cyclohexane, and

72

toluene) oxidation over various transition metal oxides,21-23 such as MnOX, NiOX,

73

CoOX, FeOX, and CuOX.24-27 A previous study pointed out that ozone decomposition

74

on catalysts played an important role in VOCs oxidation process.28 When O3

75

decomposed on the catalyst surface, new active oxygen species, such as O* (* denotes

76

active sites on the catalyst), superoxide (O-), and peroxide (O22-) formed,29-31 which

77

could promote VOCs oxidation;32,33 on the other hand, ozone supplied strong

78

oxidation groups which would accelerate the conversion rate from V4+Ox to V5+Ox,

79

thus improved the activity of the catalyst.34

80

Due to the high cost and complexity of dioxins measurement, many researchers use

81

the model molecules like chlorobenzene and furan to find a simple method to design

82

catalysts.35,36 However, it is necessary to verify whether the catalyst designed using

83

model molecules is applicable to decompose PCDD/Fs. Previous research has

84

revealed that adding WO3 and MoO3 into V2O5/TiO2 catalyst can efficiently promote

85

the decomposition of benzene and chlorobenzene, but has a negative effect on the

86

oxidation of real dioxins in practice.37 In catalytic decomposition of PCDD/Fs over

87

metal oxide catalysts,38-40 primarily 17 congeners of toxic dioxins were focused on,

88

while the other congeners (as potentially toxic) were not carefully considered.


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In this paper, the decomposition and adsorption of 130 congeners of tetra- to octa-

90

PCDD/Fs are investigated under conditions of simulated air with/without O3 over

91

TiO2 and carbon nanotubes (CNTs) supported VOX, CuOX, and MnOX catalysts and

92

their relative mechanism are discussed.

93

II. Experimental Section

94

Catalyst preparation and characterization. In this study, 4 kinds of TiO2 and

95

CNTs supported catalysts, namely VOx/TiO2-CNTs (V/T-C), VOx-MnOx/TiO2-CNTs

96

(VMn/T-C), VOx-CuOx/ TiO2-CNTs (VCu/T-C), and VOx-MnOx-CuOx/TiO2-CNTs

97

(VMnCu/T-C), were prepared by combination of mechanical mixing, sol-gel and

98

impregnation methods using tetrabutyl titanate, ammonium metavanadate, manganese

99

acetate, and copper nitrate as the precursors. Details of the catalyst preparation

100

process and characterization are provided in Supporting Information (Text S1-S2,

101

Table S1 and Figure S1-S2). The results of the characterization suggested that all of

102

the catalysts contained similar contents of VOX, TiO2, and CNTs, and had similar

103

specific surface areas, which minimized the influence of these variables. We will

104

focus on the effects of MnOX and CuOX addition on PCDD/Fs decomposition in the

105

following discussion.

106

Experimental setup. A laboratory-scale reaction setup was used to investigate the

107

catalytic oxidation of gas phase PCDD/Fs, as shown in Figure 1. The reaction setup

108

included three parts: A stable home-made dioxin generator which could continuously

109

supply gas-phase PCDD/Fs; a catalytic reactor system including a quartz tube placed

110

in a horizontal furnace with a temperature controller; and an offgas collection system

111

with two toluene bottles in ice bath. The dioxins of the generator came from the stock

112

solution, which was extracted from fly ash of a hazardous waste rotary kiln

113

incinerator in China and the purification process was described in Ref. 38. The

114

catalyst unit put in the reactor consisted of 5 aluminum plates (2.0 cm × 8.0 cm)

115

uniformly coated with 2.0 g catalyst. The geometry of the reactor is shown in the

116

previous study.41 During experiments, the stock solution was injected in the carrier



117

gas (N2:O2 = 9:1, 1.0 L min-1), creating small droplets of stock solution easily to be

118

carried on by the gas flow. The gas went through a preheater, in which the effect of

119

solvent was eliminated as much as possible and the PCDD/Fs volatilize was made

120

more uniform, and then mixed with the standard air (0.5 L min-1). The resulted gas

121

with a flowrate of 1.5 L min-1 was introduced into the reactor immediately to react

122

with the catalyst, the corresponding Gas Hourly Space Velocity (GHSV) was 45000

123

h-1. When O3 was needed in the system, a laboratory O3 generator (CF-G-3)

124

connected to the standard air line was opened and it produced 200 ppm O3 into the

125

reactor. The concentration of the ozone was detected by an UV ozone detector

126

(BEYOK ozone Inc., Zhejiang, China). Reaction time of each run was 1.0 h. After

127

each run, the reactor was cleaned (flushing the reactor three times with toluene and

128

blowing dry by blower), the cleaning fluid was mixed with the XAD-2 powder and

129

the solution in the tail toluene bottles, the mixture and the used catalyst were then put

130

into two Soxhlet extractors, respectively, for the further analysis.38 Finally fresh

131

catalyst was loaded in the reactor for a new run.

132 133

Figure 1. Schematic diagram of experimental setup.

134 135

PCDD/Fs analysis. Pretreatment of PCDD/Fs was conducted according to the EPA

136

method 1613 (United States Environmental Protection Agency, 1994). All solvents

137

(pesticide residue analysis grade) were purchased from Mallinckrodt Baker Inc., USA.

138

All isotope standards were purchased from Cambridge Isotope Laboratories. The

139

target compounds were all Tetra- to Octa-CDD/Fs (in principle, 136 congeners). The

140

pretreated samples were analyzed by HRGC/HRMS consisting of a 6890 Series gas


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chromatograph (Agilent, USA) and coupled to a JMS-800D mass spectrometer (JEOL,

142

Japan). A DB-5 ms capillary column (60 m × 0.25 mm inside diameter, 0.25µm film

143

thickness) was used to separate the congeners of PCDD/Fs. The temperature program

144

and mass spectrometer were operated as described in Ref. 42. Toxic equivalents were

145

calculated by using the international toxicity equivalency factor (I-TEF).

146

III. Results and Discussion

147

Initial concentration of PCDD/Fs. The concentrations of 136 PCDD/F congeners

148

at the outlet of the generating system, which were averaged from three repeated

149

experiments, were determined as the initial PCDD/F concentrations of the catalytic

150

reactor. The Relative Standard Deviation (RSD) values of 136 tetra- to octa- dioxin

151

concentrations were used to evaluate the stability of the dioxin generator. As shown in

152

Table S2, the RSD of 6 congeners (1469-TCDD, 1478-TCDD, 1236-TCDD,

153

1269-TCDD, 1289-TCDD and 13479-TCDD) were higher than 30%, and were thus

154

excluded to evaluate the catalyst performance in this study. Only 130 PCDD/F

155

congeners were analyzed in this paper, among them, 40 RSD values were below 10%,

156

83 RSD values were ranged from 10% to 20%, and 7 RSD values were ranged from

157

20% to 30%. The sum inlet concentrations of PCDDs and PCDFs were 132.4 ng Nm-3

158

and 514.3 ng Nm-3, respectively. I-TEQ value of the dioxin vapor was 7.9 ng I-TEQ

159

Nm-3. Considering the initial concentrations of these 130 congeners were trace level,

160

the dioxin generator used in this study was very stable. However, the study for 130

161

congeners one by one was not recommended because the analysis could be really

162

complex, in this study, 130 tetra- to octa-polychlorinated PCDD and PCDF congeners

163

were classified to 10 groups based on chlorination level for simplification, the

164

relevant data is shown in Table 1.

165

Table 1. Inlet concentrations of PCDD/F isomers. Isomer

Average concentration (ng/Nm3)

RSD (%)

T4CDD

35.2

10.0

P5CDD

40.0

9.8



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H6CDD

33.9

10.5

H7CDD

15.8

8.8

O8CDD

7.5

8.5

T4CDF

240.2

14.6

P5CDF

163.9

10.4

H6CDF

77.7

8.3

H7CDF

26.8

7.7

O8CDF

5.8

5.3

166

Catalytic activity in the simulated air atmosphere without O3. Between the

167

V/T-C, VMn/T-C, VCu/T-C, and VMnCu/T-C catalysts, the efficiency of PCDD/F

168

removal in the study was defined as “(concentration after reaction - inlet

169

concentration)/inlet concentration*100%”. Compared to the inlet PCDD/Fs as shown

170

in Figure 2, it could be found that the emission of PCDD/Fs (in offgas and on reactor)

171

was reduced in each test at 220 °C. However, a considerable number of dioxins

172

remained on the catalyst surface, resulting in an increase of total PCDD/Fs. After

173

passing through V/T-C catalyst, the total concentration and toxicity (based on

174

concentrations and I-TEQ) of PCDD/Fs increased 45% and 23%, respectively,

175

indicating that V/T-C was active in removing model molecules,43 but not active in

176

decomposing dioxins. Compared with V/T-C, the doping of MnOX in VMn/T-C still

177

induced a slight increase of PCDD/Fs, which was also contrary to the effect of

178

manganese oxide on the removal of model molecules.44 Over VCu/T-C in the

179

presence of CuOX, a large number of PCDD/Fs were synthesized with the

180

concentration and toxicity increasing as high as 263% and 295%, respectively. It was

181

interesting to find that the addition of MnOX in the VCu/T-C catalyst would efficiently

182

reduce the activity of VCu/T-C to generate PCDD/Fs. PCDD/Fs concentration and

183

toxicity after reaction increased 237% and 245% over VMnCu/T-C, respectively, this

184

could be ascribed to the decrease of surface CuOX concentration by MnOX, thus

185

reducing the opportunity of PCDD/Fs synthesis catalyzed by CuOX. Figure 2 also


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showed that most of the PCDD/Fs were adsorbed on the VCu/T-C and VMnCu/T-C,

187

while on the V/T-C and VMn/T-C, most of them were flushed into the offgas and

188

adsorbed on the reactor, indicating the excellent adsorption of the formed PCDD/Fs

189

on these two Cu-containing catalysts. The investigation of the effect of chlorination

190

degree of dioxins on the catalytic activity was shown in Figure S3. It was found that

191

over all these catalysts the synthesis efficiencies of the congeners would initially

192

increase and then decrease with the increasing of the chlorination levels. In most cases,

193

HpCDD/Fs were the easiest congeners to be synthesized (the exceptions were the

194

PCDFs on the V/T-C and VMn/T-C catalysts). A general conclusion was that the

195

congeners with higher chlorinated levels were easier to be synthesized, but OCDD/Fs

196

were not the easiest congeners to be synthesized as a supply of additional chlorine

197

atoms was necessary.

198 199 200

Figure 2. The formation efficiencies of PCDD/Fs over this series of catalysts at 220 °C.

201

The temperature effect on the catalytic reaction over VMn/T-C as shown in Figure

202

3 was also investigated and PCDD/Fs synthesis was found at each reaction

203

temperature. Even at temperatures as low as 120 °C, the total concentration and

204

I-TEQ of PCDD/Fs increased about 52.2% and 27.3%, respectively. The optimal

205

temperature for PCDD/Fs synthesis was obtained at 170 °C, which was lower than the

206

reported critical PCDD/Fs synthesis temperature in waste incineration (200-400 °C),



207

further indicating that VMn/T-C, which was reported to be a good catalyst for model

208

molecules destruction, might facilitate the synthesis of PCDD/Fs.

209 210

Figure 3. The formation efficiencies of PCDD/Fs on VMn/T-C catalyst at different

211

temperatures.

212

Considering that TiO2 supported V2O5 was reported to efficiently reduce the

213

emission of real dioxins with a remarkable efficiency of >90% at 200 °C,37 the

214

synthesis of PCDD/Fs on V/T-C might mainly be attributed to the existence of CNTs.

215

In the study of the emission reduction using model molecules, the adsorption capacity

216

of CNTs was beneficial to both the direct reduction of emission caused by the

217

stagnation of contaminants on the surface of CNTs, and the decomposition reaction,

218

because CNTs prolonged the residence time of model molecules on the catalyst

219

surface thus prolonged the reaction time to destroy them. For example, the addition of

220

CNTs into titania-supported manganese oxide was reported to enhance the

221

performance in chlorobenzene catalytic oxidation.45 The PCDD/Fs emission is a

222

dynamic balance of its synthesis and destruction, in this study, the synthesis rate

223

surpassed the destruction rate. Although CNTs would promote the PCDD/Fs

224

destructions, they promoted the PCDD/Fs synthesis as follows:

225

(i) CNTs themselves were a carbon source for PCDD/Fs. Previous studies indicated

226

that dioxins could be synthesized from macromolecular carbon matrix (so called

227

residual carbon) via the de novo process in the presence of organic or inorganic


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chlorine.46

229

(ii) The absorptivity of CNTs was also beneficial to PCDD/Fs synthesis. In this study,

230

in addition to a variety of dioxins congeners, there were many kinds of polycyclic

231

aromatic hydrocarbons (PAHs) in the PCDD/Fs stock solution, such as anthracene,

232

pyrene, 2-phenylnaphthalene and so on (shown in Figure S4). Intermediate products

233

like polychlorinated phenol (PCPh) and polychlorinated benzene (PCBz) could be

234

produced during the catalytic reaction. All of them could serve as the precursors for

235

PCDD/Fs synthesis via catalytic condensation reactions,47,48 which was defined as

236

precursors synthesis pathway. In this pathway, some researchers had proposed that

237

PCDD/Fs

238

Langmuir-Hinshelwood (L-H) mechanism,49-51 meaning that the adsorption of

239

precursors on the catalyst surface was a crucial step. Therefore, the role of CNTs

240

adsorption capacity in the catalytic reaction system was not only to prolong the

241

reaction time for the destruction of PCDD/Fs, but also to provide more opportunities

242

for PCDD/Fs precursor synthesis.

were

formed

by

Eley-Rideal

(E-R)

mechanism

and/or

243

The catalytic activity with O3. As the catalysts prepared in this study would

244

enhance the synthesis of the PCDD/Fs, rather than decomposition, ozone was added

245

in the reaction system to try to promote PCDD/Fs decomposition. For convenience, in

246

this part, the PCDD/Fs decomposition efficiency, adsorption efficiency, and removal

247

efficiency, referred as ‘‘DE’’, ‘‘AE’’ and ‘‘RE’’, respectively, were defined based on

248

the international toxic equivalent quantity (I-TEQ) of 17 toxic dioxin congeners and

249

also the concentrations of 130 dioxin congeners.

250

DE (%) = (PCDD/Fs inlet – PCDD/Fs off gas – PCDD/Fs on catalyst) ÷ PCDD/Fs inlet

251

AE (%) = (PCDD/Fs on catalyst) ÷ PCDDF/s inlet

252

RE (%) = (PCDD/Fs inlet – PCDD/Fs off gas) ÷ PCDD/Fs inlet

253

Effects of MnOX and CuOX on the PCDD/Fs decomposition with O3. The

254

overall analysis of DE and AE values at 220 °C were shown in Figure 4. The

255

calculations based on the I-TEQ of 17 toxic dioxins and the concentrations of 130



256

dioxin congeners also brought different DE and AE values on same catalysts.

257

Fortunately, the trends of data were similar among different experimental conditions.

258

The DE values were negative over V/T-C and VMn/T-C, indicating the synthesis of

259

PCDD/Fs. For V/T-C, the DE was -46% based on the concentration, that is, the

260

synthesis efficiency was 46%, which was very similar to that shown in Section 3.3,

261

indicating that O3 addition almost showed no influence on the decomposition of

262

PCDD/Fs at 220 °C. While for VMn/T-C, the synthesis of PCDD/Fs based on

263

concentration decreased, showing the positive effect of the MnOX adding on

264

PCDD/Fs decomposition, though it enhanced the synthesis of the toxic congeners,

265

making the production more harmful to the environment based on I-TEQ. Over

266

VCu/T-C and VMnCu/T-C, the DE values were positive, namely PCDD/Fs were

267

decomposed. The AE over VCu/T-C were high, while over VMnCu/T-C, not only the

268

AE values, but also the DE values were high, achieved 84% and 86%, respectively,

269

based on their concentrations and I-TEQ. In summary, the co-adding of MnOX and

270

CuOX in the V/T-C catalyst showed the best performance in PCDD/Fs degradation

271

with O3. From literature, copper (CuOx or CuCl2) was always regarded as the most

272

effective catalyst for PCDD/Fs synthesis,52 we have also demonstrated that adding of

273

CuOX in the catalyst enhanced the PCDD/Fs synthesis. However, when O3 was

274

introduced into the reaction system, CuOX component promoted the PCDD/Fs

275

decomposition at 220 °C.


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276 277 278 279

Figure 4. Overall DE, AE for the different experimental conditions based on concentrations of 130 dioxins congeners and I-TEQ of 17 toxic dioxins congeners; ozone concentration = 200 ppm, GHSV = 45000 h-1 .

280

The effect of temperature on PCDD/Fs catalytic decomposition over VMn/T-C with

281

O3 was studied and shown in Figure 5. Though at 220 °C PCDD/Fs were synthesized,

282

at 120 °C and 170 °C, the DE of PCDDF/s were all positive based on concentration

283

and I-TEQ. In detail, PCDD/Fs were mainly decomposed (62% DE and 26% AE) at

284

170 °C, while half of PCDD/Fs were adsorbed at 120 °C (46% and 40% based on

285

concentrations).

286 287 288 289

Figure 5. Overall DE, AE for the different experimental conditions based on concentrations of 130 dioxins congeners and concentrations of 17 toxic dioxin congeners over VMn/T-C, ozone concentration = 200 ppm, GHSV = 45000 h-1 .

290

The rate of ozone decomposing on the transitional metal oxides (TMOs) was

291

different. The difference may be attributed to the conductivity of the TMOs, p-type



292

oxides were active while n-type oxides were not.28 during the ozone decomposition on

293

TMOs, the oxygen atom (atom O) was regarded as one of the most important

294

production.28 The strong oxidizing property of atom O promoted the decomposition of

295

dioxins significately.53 However, the recombination of the atom O, which was

296

regarded as a side reaction, was inevitable. When the O3 decomposed too fast, excess

297

atomic oxygen in the system would accelerate its recombination, blocking dioxin

298

oxidation to some extent. In this study, for V/T-C, both V2O5 and TiO2 were not active

299

to activate ozone decomposition, thus little O was produced from the surface and the

300

PCDD/Fs congeners could not be degraded. At the same time, the PCDD/Fs were

301

synthesized inevitably through the way described previously, resulting in an increase

302

of PCDD/Fs at the outlet of the reactor. For VMn/T-C, MnOX was an excellent

303

ozone-decomposing catalyst, 28,33,52 even when the reaction temperature was as low as

304

120 °C, it decomposed the ozone and produced atomic oxygen. The produced O

305

suppressed PCDD/Fs emission through two ways. Firstly, O as a strong oxidant

306

decomposed the PCDD/F molecules directly; secondly, it oxidized the precursors of

307

the PCDD/Fs contained in the stock solution or produced from the degrading some

308

PAHs, thus suppressing PCDD/Fs synthesis. With increasing temperature, more

309

atomic oxygen was produced, the above two aspects became more prominent,

310

explaining the high DE of PCDD/Fs at 170 °C. However, further increasing the

311

temperature to 220 °C promoted the O3 decomposition and atomic oxygen

312

recombination, which inhibited the PCDD/Fs oxidation. At this temperature, the

313

unavoidable synthesis of dioxins occurred. As a result, PCDD/Fs concentration

314

increased as shown in Figure 5.

315

For VCu/T-C, CuOX showed the worst activity for O3 decomposition. 28,53,54 The

316

oxygen atoms were produced at a relatively slow rate, so the catalyst system could

317

only degrade PCDD/F congeners at high temperatures. When MnOX and CuOX were

318

combined in VMnCu/T-C, CuOX could restrain the over-quick O3 decomposition on

319

the MnOX surface, so the cooperation of CuOX and MnOX extended the lifetime of


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320

free oxygen atoms produced from O3. Even at 220 °C, sufficient atomic oxygen was

321

still on the catalyst surface, which efficiently improved the overall activity of the

322

catalyst by inhibiting the PCDD/Fs synthesis and accelerating their destruction.

323

Actually, our previous study found that O3 enhanced the chlorobenzene

324

decomposition at high temperature of 300 °C over CuOX/CNTs.55 Similar phenomena

325

was also reported that the optimal temperature for chlorobenzene decomposition on

326

manganese oxide was lower than that on iron oxide in the presence of ozone, as ozone

327

decomposed faster on manganese oxide than on iron oxide.53,56

328

Based on the above results and discussion, it could be concluded that the worst

329

scenarios for dioxin degradations was on the VCu/T-C catalyst without the ozone at

330

220 oC. Under this condition, a lot of dioxins were formed rather than degraded.

331

While on the VMnCu/T-C catalyst with ozone at 220 oC, the dioxins were degraded

332

with highest efficiencies as the lifetime of the oxygen atom produced from the ozone

333

decomposition was extended effectively, it was the most promising scenarios for the

334

degradation of the dioxins.

335

Effects of the chlorination level. The effects of the chlorination level on PCDD/F

336

catalytic oxidation were very complex. Generally, the different chlorination level of

337

PCDD/Fs influenced both their adsorptions and reactions on the catalyst surface.57

338

With the chlorination increasing, the molecular weights of PCDD/F congeners

339

increased, and their volatilities decreased, leading to their good absorptivity.40,58 At

340

the same time, their redox potential increased, which made the high chlorinated

341

congeners more difficult to be degraded.40,58 Moreover, the absorptivity of the

342

congeners deeply influenced their reaction with the catalyst, as the congeners

343

adsorption on the catalyst surface was the prerequisite of their reaction with catalyst.36

344

Good absorptivity of the congener always implied a longer retention time on the

345

catalyst, making it easier to be decomposed. In this study, the DE, AE, and RE values

346

of PCDD/F isomers with tetra- to octa- chlorination on different catalysts were sorted

347

out in Figure 6 to study their effects in detail. Over V/T-C at 220 °C as shown in



348

Figure 6a, AE values increased with chlorination level increasing evidently, which

349

could be attributed to the low volatility of high chlorination congeners. All the DE of

350

PCDD/F isomers from tetra- to octa-chlorination were negative, indicating the

351

synthesis of PCDD/Fs instead of destruction. With the increasing of the chlorination,

352

PCDD/Fs formation promoted, for example, DE of T4CDD, H7CDD, O8CDD were

353

-34%, -71% and -68%, respectively. The increasing of synthesis efficiencies with

354

chlorination level up to H7CDD/Fs indicated that high chlorination level congeners

355

were easy to be synthesized. The cause might be the stable chemical properties of the

356

high chlorinated congeners.59 A slight decrease of O8CDD/Fs synthesis compared to

357

H7CDD/Fs was observed, possibly because of the insufficient Cl supply. It was

358

evident that high AE values were the major factor which made the RE values, i.e.,

359

sum of the AE and DE values, positive and show the increasing tendency with the

360

chlorination increasing.

361

On VMn/T-C at 220 °C as shown in Figure 6b, the DE values of the congeners

362

were also negative with similar trend on V/T-C. But the PCDD/Fs synthesis was

363

suppressed slightly; in fact, the DE of TCDD was 1% (positive), indicating a

364

degradation but not synthesis. The activity of VMn/T-C for degrading dioxins

365

emerged, though it was weak. The weak activity of the catalytic surface led to the

366

result that the AE decreased abnormally with chlorination increasing. As indicated in

367

the previous analysis, the oxygen atom produced from the ozone decomposition on

368

VMn/T-C catalyst at 220 °C recombined very quickly, suggesting the relative low

369

activity of the catalytic surface. As was known, to break the phenyl ring was much

370

harder than to break the C-Cl bond.60 So the low activity of the VMn/T-C surface may

371

be not efficient to prevent the formation of dioxins from PAHs or precursors

372

condensation, but active to degrade the formed PCDD/F congeners with higher

373

chlorination to those with lower chlorination.61 Thus more low chlorinated congeners

374

were adsorbed on the catalyst surface. The decreasing trendies of the DE and AE

375

values with the chlorination increasing made the decreasing trendies of the RE values.


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Page 17 of 25


376

Over the same catalyst of VMn/T-C, when the reaction temperature decreased to

377

170 °C and 120 °C as shown in Figure 6c and Figure 6d, respectively, almost all the

378

DE values were positive, and they decreased as the chlorination level increased,

379

indicating the harder decomposition of highly chlorinated PCDD/F congeners.

380

Normally, the congeners with higher chlorination were stable in chemical properties,59

381

though their lower vapor pressure could benefit their adsorption on the catalyst, if

382

they could not be decomposed efficiently, they were ready to be adsorbed on the

383

catalyst surface, which covered the active sites, leading to the activity decline of the

384

catalyst.57 In addition, as mentioned previously, the PCDD/Fs at higher chlorination

385

level were easier to be formed, which would also contribute to the decreasing of their

386

DE values. This was in good agreement with the results shown in Figure 6a. The AE

387

values were almost unchanged in Figure 6c and Figure 6d, which were different from

388

those shown in Figure 6b, the possible reason was that the lower chlorination

389

congeners adsorbing on the catalyst surface were degraded as the activity of the

390

catalytic surface was enhanced by the longer lifetime oxygen atom. The RE values of

391

PCDD/F congeners shown in Figure 6 (c), and (d) decreased with the chlorination

392

increasing, mainly because of the DE values decreasing, suggesting that the VMn/T-C

393

catalyst was not active enough and was only effective to remove low chlorinated

394

PCDD/F congeners.

395

Over VCu/T-C at 220 °C as shown in Figure 6e, 9 kinds of the congeners were

396

degraded (except O8CDF), though the efficiencies were not high. The trends of DE

397

and AE were generally opposite, that is, when DE decreased, the AE increased, and

398

vice versa. The trends indicated that the adsorption of the dioxins on the catalyst was

399

the prerequisite for their degradation.38 Considering the adsorption of ozone on the

400

catalyst,53 it could be speculated that the dioxins were degraded on the VCu/T-C

401

catalyst with ozone via the L-H mechanism, which involves the reaction of the

402

adsorbed surface species.62 It was interesting that over VCu/T-C, with the increase of

403

chlorination level, the DE values of congeners did not drop so obviously as that over



404

VMn/T-C with the chlorination increasing, showing its better activity to decompose

405

the high chlorinated level congeners. The possible reason was that, (1) as pointed

406

previously, the lifetime of the produced O extended, making the activity of the surface

407

stronger; (2) CuOX rapidly removed the adsorbed inorganic chlorine species, or

408

dissociative Cl produced from the catalytic process on the surface or active sites of the

409

catalyst via the synthesis and re-oxidization of Cl-containing intermediate,63

410

suppressing the synthesis of PCDD/Fs, especially the PCDD/Fs with higher

411

chlorination level which needed more Cl and made the decomposition more

412

prominent.

413

Over VMnCu/T-C at 220 °C as shown in Figure 6f, the trend of AE and DE values

414

was similar with that shown in Figure 6e, indicating the degradation of dioxins on this

415

catalyst with ozone also followed the L-H mechanism. The main difference was that

416

all of the 10 kinds of congeners were degraded with high efficiencies on VMnCu/T-C

417

catalyst. VMnCu/T-C may combine the advantages of the VMn/T-C and VCu/T-C

418

catalyst, that is, the lifetime of produced O from the ozone decomposition was

419

prolonged with the joint action of MnOx and CuOx, the activity of catalytic surface

420

was then enhanced enough to degrade all kinds of the dioxins, in defiance of their

421

chlorination levels. It was also found that over VMnCu/T-C catalyst, only one of 130

422

dioxin congeners (1267-TCDD), was synthesized at 220 °C (Figure S5), indicating

423

that VMnCu/T-C efficiently decomposed almost all of the 130 PCDD/F congeners

424

with the assistance of O3, and on which there was the lowest risk of PCDD/F

425

formation.


Page 18 of 25

Page 19 of 25


426 427 428 429 430

Figure 6. AE, DE and RE of 10 groups of PCDD/Fs based on concentrations of 130 dioxin congeners. (a) V/T-C at 220 °C, (b) VMn/T-C at 220 °C , (c) VMn/T-C at 170 °C, (d) VMn/T-C at 120 °C, (e) VCu/T-C at 220 °C, and (f) VMnCu/T-C at 220 °C, the ozone concentration = 200 ppm.

431

Associated Content

432

Supporting Information

433

This material is available free of charge via the Internet at http://pubs.acs.org.



434

The detailed information of the preparation of the catalysts. The formulas of V/T-C,

435

VMn/T-C, VCu/T-C and VMnCu/T-C catalysts. The characterization results of the

436

prepared catalysts including the XRD, SEM, XPS and BET. The table of 136 tetra- to

437

octa- PCDD/F congener concentrations and the RSD data. The Figure showing the

438

generation efficiencies of PCDD/Fs after catalytic reaction at 220 °C over the

439

different catalysts. The Figure showing the result of GC-MS detection for the stock

440

dioxin solution. The Figure showing the DE and RE values of 130 PCDD/F congeners

441

in different reaction modes.

442

Author Information

443

*

444

*

445

Tel/Fax: +86-571-87951404; E-mail: [email protected] (Yang H.); Tel/Fax:

446

+86-571- 87952834; [email protected] (Lu S.).

447

Notes

448

The authors declare no competing financial interest.

449

Acknowledgements

450

This work was supported by the Environmentally Sustainable Management of

451

Medical Wastes in China (Contract No. C/V/S/10/251), the Zhejiang Provincial

452

Natural Science Foundation of China (Grant No. Z4080070). The National Natural

453

Science Foundation of China (51276162), the Zhejiang Provincial Natural Science

454

Foundation of China (R14E060001), the Doctoral Program of Higher Education

455

(20130101110097) and the Program of Introducing Talents of Discipline to University

456

(B08026).

Co-first author

Co-corresponding author.


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