Low-Temperature Catalytic Decomposition of 130 Tetra- to Octa

Sep 26, 2016 - †State Key Laboratory of Clean Energy Utilization, Institute for Thermal ... Zhejiang University, Zheda Road 38, Hangzhou 310027, Chi...
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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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Low-temperature catalytic decomposition of 130 tetra- to octa- PCDD/Fs

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

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of O3

4 †







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

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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

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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

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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

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

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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

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

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

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

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

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

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investigated.

31 32

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

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I. Introduction

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

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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

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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

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

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

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

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

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

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

48

new limitation in January 2016.

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A variety of techniques have been developed for dioxin removal including

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

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

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

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research focus for many years.

54

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

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

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

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

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

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the most efficient transition metal oxide catalysts for catalytic destruction of

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

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

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at relatively low temperatures.7-10

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

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

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

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

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

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

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catalytic decomposition.13-20

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

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

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

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

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

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

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

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

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

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thus improved the activity of the catalyst.34

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

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

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

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

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

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

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

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

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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-

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

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

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their relative mechanism are discussed.

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II. Experimental Section

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

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

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

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

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impregnation methods using tetrabutyl titanate, ammonium metavanadate, manganese

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

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

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

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

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

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

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following discussion.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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

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

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

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All isotope standards were purchased from Cambridge Isotope Laboratories. The

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

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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,

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

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

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

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

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III. Results and Discussion

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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relevant data is shown in Table 1.

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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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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,

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

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

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

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

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

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

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

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

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

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

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atoms was necessary.

198 199 200

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

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

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

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

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

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

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

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

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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

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temperatures.

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Considering that TiO2 supported V2O5 was reported to efficiently reduce the

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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chlorine.46

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

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

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

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

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

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

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

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

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PCDD/Fs

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

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

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

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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

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

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

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

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also the concentrations of 130 dioxin congeners.

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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

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

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

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

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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

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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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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

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

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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

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

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

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

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

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

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Actually, our previous study found that O3 enhanced the chlorobenzene

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

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was also reported that the optimal temperature for chlorobenzene decomposition on

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

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

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

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

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

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

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

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

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degradation of the dioxins.

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

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

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

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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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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

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chlorination increasing.

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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

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

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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

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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

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

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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

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

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

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

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

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

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

379

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

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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

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

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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

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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

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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

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

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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.

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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.

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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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