Highly Advanced Degradation of Thiamethoxam by Synergistic

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Energy, Environmental, and Catalysis Applications

Highly Advanced Degradation of Thiamethoxam by Synergistic Chemisorption-Catalysis Strategy Using MIL(Fe)/Fe-SPC Composite with Ultrasonic Irradiation Yanan Wei, Bingfeng Wang, Xinfang Cui, Yaseen Muhammad, Yanjuan Zhang, Zuqiang Huang, Xuesheng Li, Zhenxia Zhao, and Zhongxing Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12908 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Highly Advanced Degradation of Thiamethoxam by Synergistic Chemisorption-Catalysis Strategy Using MIL(Fe)/Fe-SPC Composite with Ultrasonic Irradiation Yanan Weia&, Bingfeng Wangc&, Xinfang Cuia, Yaseen Muhammada, d, Yanjuan Zhanga, Zuqiang Huanga, Xuesheng Lib, Zhenxia Zhaoa*, and Zhongxing Zhaoa, b*

a

School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China

b

Guangxi Key Laboratory for Agro-Environment and Agro-Product Safety, Guangxi University,

Nanning 530004, China c

Department of Applied Chemistry, College of Materials and Energy, South China Agricultural

University, Guangzhou 510642, China d

Institute of Chemical Sciences, University of Peshawar, 25120, KP, Pakistan

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: MIL(Fe)/Fe-SPC composite was fabricated from MIL-100(Fe) via in-situ growth on a unique Fe-doped nano-spongy porous biocarbon (Fe-SPC) and was used as Fenton-like catalyst for advanced degradation of thiamethoxam (THIA). Fe was loaded on silkworm excrement and calcined to Fe-SPC with nano-spongy and high sp2 C structure. The in-situ growth strategy embedded the Fe-SPC into MIL-100(Fe) crystals and formed conductive heterojunctions with intensified interface by Fe-bridging effect which was confirmed by negative shift of Fe3+ binding energy in XPS. MIL(Fe)/Fe-SPC composites exhibited high degree of crystallinity and surface area (BET: 1730 m2/g). LC-MS and DFT simulations demonstrated that THIA was converted to a relatively stable compound (C4H5N2SCl), which could be captured by MIL-100(Fe) with strong chemical bonding energy (Fe-N, -587 kJ/mol), followed by a significant geometric distortion, resulting in a thorough degradation. Efficient charge separation and synergistic chemisorption-catalysis strategy resulted in the high catalytic activity of MIL(Fe)/Fe-SPC. The composite catalyst concurrently exhibited high mineralization ratio with 95.4% total organic carbon removal (at 25 ºC and 180 min) and good recycling ability under wider neutral/alkaline conditions. Endorsing to these intriguing properties, MIL(Fe)/Fe-SPC can be deemed an efficient contender for removal of hard-degradable pesticides and other environmental pollutants in practical applications. KEYWORDS:

MIL(Fe)/Fe-SPC

thiamethoxam

advanced

catalyst;

degradation;

efficient DFT

chemisorption-catalysis strategy


charge

separation;

simulation;

synergistic

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1. INTRODUCTION Thiamethoxam (THIA), a typical neonicotinoid insecticide, has been widely used in sustainable agriculture for the effective control of numerous sucking and biting insect pests.1 Inevitably, it’s uncontrolled use leads to the contamination of soil and water resources due to its low soil sorption, high leaching and solubility in water.2 It has been reported with severe environmental problems and health risks to biota due to its highly toxic3 and bio-accumulative nature4. Recently, the degradation and abatement measures of THIA have received great attentions,5-7 especially for its advanced degradation. Among these degradation approaches, catalysis has earned a vital position credited to the ease of process operation, high efficiency and fast degradation kinetics.5 Many studies have been devoted to the designing of efficient heterogeneous catalysts for the degradation of THIA. De Urzedo6 reported the UV light assisted photocatalytic degradation of THIA with 96% degradation efficiency within 60 min. Zhao Q.H.7 used ozonation approach for the catalytic degradation of THIA in aqueous solution (200 mg/L), and 76% of chemical oxygen demand was removed after 180 min. Similarly, Romina Ž.8 applied immobilized TiO2 as photocatalysts and reported 14.4 ± 2.9% THIA degradation rate within 120 min. Meijide J.9 reported 95% THIA degradation within 5 min at pH=2.8 and 50 mA using Fenton and electro-Fenton synergistic treatments, though the total organic carbon (TOC) contents still remained high after 1-2 h reaction.6,9 This suggested that THIA was only transformed to other organic intermediates without being completely degraded 3



to hazard-free species. Here it is noteworthy to mention that some resulting intermediates may be even more toxic and harmful than parent THIA.7 These deficiencies in the reported studies urge for an urgent need and challenge of designing a series of catalysts with concurrently high activity and high TOC removal ratio for THIA degradation from waste water. Currently, metal-organic frameworks (MOFs) have been extensively applied in hazardous gas adsorption and separation10-13 owing to their ultra-high surface area and tunable chemical functionality. MOFs have also been selected as porous catalysts for organic compound degradation and other redox reactions14-18 attributed to the existence of many unsaturated open metal sites causing high catalytic activity.19,20 Li Z.H.21 used MIL-100(Fe) and MIL-68(Fe) to hydroxylate benzene to phenol under visible light irradiation in the presence of H2O2 with 30.6% conversion. Zaltariov M.F.22 reported fast photodegradation rate of Congo red over Co/Cu based MOFs as catalysts. Thus, these unsaturated open metal sites in MOFs can be motivated to generate electrons and holes that can take part in the redox reactions in their vicinity.23,24 Moreover, catalytic activity can be further improved owing to their unique selective adsorption, marking them superior to some traditionally prepared metal supported catalysts.23,24 However, pure MOFs usually exhibit low quantum efficiency and poor electronic conductivity,25 which can cause a fast recombination of the electron-hole (e-/h+). Overcome this shortcoming, many researchers proposed the combination of MOFs with carbon substrates with good conductivity.26,27 The tight interface between 4


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MOFs and carbons is of crucial importance for the catalytic activity, which will affect the formation of hetero-junction, uniform distribution of catalytic active sites and charge transfer. There are rare reports on intensifying binding interface of MOFs and porous carbon substrate to enhance interfacial conductivity and synergistic catalysis for advanced degradation of THIA so far. Herein, a unique Fe-doped nano-spongy porous carbon (Fe-SPC) derived from silkworm excrement (SE) (from agricultural waste) was composited with MIL-100(Fe) as Fenton-like catalysts and applied for the advanced degradation of THIA in aqueous phase. Fe ions acted as “bridge” linker to strengthen interface between MIL-100(Fe) and Fe-SPC nano-sponge, which efficiently improved charges separation/transfer of MIL-100(Fe). This Fenton-like catalyst showed ultra-high TOC removal ratio of 95% within 180 min at 25 ºC, and good recycling stability under neutral and wider alkaline media. The synergistic chemisorption/catalysis degradation strategy was investigated and verified by liquid chromatography coupled with mass spectrometry (LC-MS) and quantum chemical computation. The prepared samples were characterized by advanced characterization techniques.

2. EXPERIMENTAL SECTION 2.1. Materials. All the starting materials used in this study were of analytical reagent grade and used without further purification. Raw SE was purchased from farmer’s market (Yizhou, China). THIA (99.0%), Fe(NO3)3·9H2O (99.0%), 1,3,5-Benzenetricarboxylic

acid

(H3BTC,

99.0%),

5


FeCl2·4H2O

(99.0%),


5,5-Dimethylpyrroline-1-oxide (DMPO) and ZnCl2 (99.0%) were purchased from Aladdin Industrial Co. Ltd (Shanghai, China).

2.2. Synthesis procedure. Synthesis of SE based Fe-SPC: SE was washed 3-4 times with deionized water, filtered and dried at 110 ºC overnight and was crushed to 1-2 mm uniform particle size. After drying, 1.00 g SE was mixed with ZnCl2 and FeCl2 (weight ratio of 5:100:2 for SE:ZnCl2:FeCl2) in 5 mL HCl solution (1.0 mol/L), followed by thermal treatment at 160 ºC for 24 h in a Teflon-lined stainless-steel autoclave. After cooling down to ambient temperature, the sample was freeze-dried under vacuum for 24 h. The frozen sample was activated at 800 ºC with a heating rate of 2.5 ºC/min for 1.0 h under N2 atmosphere. The activated sample was sequentially washed with 1.0 mol/L HCl and 10 wt.% HF at ambient temperature under continuous stirring (150 rpm) for 12 h to remove residual ZnCl2, FeCl2, and silicon dioxide. This was followed by another washing with distilled water until the pH was 7.0. The dried sample was named as Fe-SPC.28,29 For comparison, SE based SPC without Fe loading was also prepared. SPC was prepared via similar procedure to that of Fe-SPC wihtout the addition of FeCl2 during the activation process.28-32 Synthesis of MIL(Fe)/Fe-SPC composite: MIL-100(Fe) was prepared following previously published procedure33 with slight modifications. Typically, MIL-100(Fe) was prepared by mixing Fe(NO3)3 (2.5 mmol), H3BTC (1.68 mmol) and deionized water (12 mL) followed by the addition of an appropriate amount of Fe-SPC with vigorous stirring (250 rpm) till a homogeneous suspension is obtained. This suspension was heated at 150 ºC for 12 h in a Teflon-lined stainless-steel autoclave. 6


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The autoclave was cooled to room temperature and a brown precipitate was recovered by filtration which was washed several times with deionized water and methanol. The corresponding MIL(Fe)/Fe-SPC composites were designated as MIL(Fe)/Fe-SPCx, where x represents the amount of Fe-SPC (wt.%) in MIL(Fe) setting as = 3, 6 and 10.

2.3. Physical characterization. The surface morphology of the samples was observed using Hitachi S-3400N type scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS, Tokyo, Japan). Spherical aberration corrected environmental transmission electron microscopy (Cs-corrected ETEM)

was

carried

out

using

TITAN ETEM G2 80-300

(FEI,

USA)

microscope at 300 kV. To avoid the damage to Cs-corrected ETEM equipment, Fe-SPC was immersed into HCl solution to etch Fe element prior to imaging. The crystal structure of the samples was analyzed by X-ray diffraction (XRD, RIGAKU, Japan) at a scan rate of 2 º/min and a monochromatic X-ray beam with nickel-filtered Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) (PHI 5000C ESCA, USA) was used to analyze the surface element components of the samples at 14.0 kV, 300 W and 93.9 eV. Nitrogen physisorption analysis was performed using a porosity analyzer (Micromeritics ASAP 2460) at -196 ºC. Prior to adsorption measurements, the samples were degassed at 150 ºC for 12 h under 200 mm Hg vacuum. The specific Langmuir and BET surface area, pore size and pore volume distribution were calculated from N2 adsorption/desorption isotherms and validated by density functional theory (DFT). 7



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2.4. THIA degradation. THIA degradation was carried out using 50 mL THIA solution (60 mg/L, pH=7.5) in the presence of H2O2 (40 mmol/L), to which 50 mg catalyst was added and was ultrasonically treated with an ultrasonic probe (Ultrasonic Cell Crusher, XO-1000D, 600 W) for 100 min at 25 ºC. At certain intervals of time, 1 mL suspension was filtered for analysis. The influence of pH was studied using HCl or NaOH solutions. Mineralization was monitored by a Multi N/C 3100 TOC analyzer (JENA, Germany), calibrated with potassium hydrogen phthalate. Electron spin resonance (ESR) spectra were measured on a JES FA200 spectrometer.

2.5. Analysis of THIA during degradation process. The degradation of THIA in aqueous medium was monitored by reverse phase high pressure liquid chromatograph (Agilent 1260, USA) equipped with a ZORBAX SB C18 column and a diode array detector at 250 nm. The mobile phase used was composed of water: methanol (75:25 v/v) with a flow rate of 1.00 mL/min for 20 min at 30 °C. Linear regression of THIA concentration (y, µg) and absorbance (x, mAU) was performed to obtain a standard curve (Figure S1), and THIA degradation ratio was calculated using Eq. (1): Tr (%) =

C × 100 % C0

Eq. (1)

Where C0 and C (µg/mL) are concentration of THIA at 0 min and after 100 min of reaction, respectively.

2.6.

Identification

of

intermediate

products

during

THIA

degradation. Intermediate products during THIA degradation were detected using 8


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liquid chromatography coupled with mass spectrometry (LC-MS, Thermo Scientific, USA), with Hypersil C18 column (250 mm 4.6 mm i.d, particle size 5 µm, 40 ºC). The flow rate of mobile phase was 1.0 mL/min, including methanol (gradient, 0 min-10 min: 30-100% methanol, post-time 3 min) and 0.05 wt.% aqueous formic acid4. Electrospray ion source ionized samples with spray voltage of 3.5 kV at 320 °C in a full scan range m/z of 50-500 Da.34,35 Structure of aqueous THIA and degradation intermediates were examined and elucidated using isotopic peak distribution. 1,10-phenanthroline was used to measure ion leaching, and absorbance measured by ultraviolet spectrophotometry (UV-2550, SHIMADZU).

2.7. Parameters and process simulation. The representative clusters were extracted from the MOF crystal structures around the active sites. For MIL-100, three Fe atoms, one O atom and six surrounding linkers were chosen as the model. Smaller benzoic acid (BA) linkers were used instead of the H3BTC linkers to save computational cost, as shown in Figure S2. DFT calculations were performed using Gaussian 09, revision C.01, software package36 with functional B3LYP. The 6-311g (d, p) basis set was employed for C, H and O atoms,37 while the LANL2DZ basis set was employed for Fe atom with an effective core potential.38,39 The geometry optimization calculations for THIA-Fe3O(BA)4 and P3-Fe3O(BA)4 were performed with a frozen optimized Fe3O(BA)4 fragment. The self-consistent field (SCF) was set with an extra step to quadratically convergent if the first order SCF would not converge. Frequency calculations were subsequently employed to examine the stationary point. Chemical structures of intermediate product from THIA degradation 9



was also optimized with B3LYP density function using the polarized continuum model and dielectric constant of water condition.31,40 Finally, the net charge, molecular electrostatic potential (MEP), 3-D bond length/angle on the atoms were assigned through the analysis of natural bonding orbitals (NBO).

3. RESULTS AND DISCUSSION 3.1. Textural characterization. SEM and TEM images of the synthesized Fe-SPC shown in Figure 1A-1B suggest the presence of coral-like structure with abundant connected channels on the surface. The magnified image (inset of Figure 1A) clearly indicates a wrinkle micro-structure. TEM image of Fe-SPC (after Fe being etched, Figure 1B) suggested a unique 3D nano-spongy network packed with amount of nano-sheets. This structure possessed abundant porous channels which will be beneficial for the formation of multiple interfaces between Fe-SPC and MOFs. Moreover, the corresponding EDS (Figure S3A) results indicated well dispersed Fe on the Fe-SPC surface without obvious agglomerations with a content of about 0.65 at.%. This could be beneficial for homogeneous growth of MIL-100(Fe) with high coverage on Fe-SPC. In order to ensure the subsequent uniform growth of MIL-100(Fe), Fe-SPC was ball milled into tiny carbon sheets before adding to the MIL-100(Fe) precursors solution. SEM and TEM images of MIL(Fe)/Fe-SPC6 composite were shown in Figure 1C and Figure 1D. The edges of these intergrowth crystals in Figure 1C were not clear compared to the intact octahedral structure of pristine MIL-100(Fe) (Figure S3B). Moreover, many crystals crowded together and 10


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formed a large particle with an average crystal size of 100 nm. The crystal size was reduced to half of the parent one (200 nm), which could be attributed to the restricted growth of MOFs crystals on Fe-SPC sponge. The in-situ growth of MIL-100(Fe) crystals on Fe-SPC sponge can be more clearly seen in TEM image (Figure 1D) with diamond shadow and a typical octahedral geometry confirming the successful growth of MIL-100(Fe) from Fe-SPC with good crystallinity.41 At the crystal edge, the nano-spongy Fe-SPC was partly embedded in MIL-100(Fe), and thus formed close intergrowth between MIL-100(Fe) and Fe-SPC as shown in enlarged image of Figure 1D.

Figure 1. SEM image of (A) Fe-SPC, (B) TEM image of Fe-SPC, (C) SEM image of MIL(Fe)/Fe-SPC6 and (D) TEM image of MIL(Fe)/Fe-SPC6.

PXRD patterns of Fe-SPC and MIL(Fe)/Fe-SPCX composites compared with the pristine MIL-100(Fe) are shown in Figure 2. A sharp diffraction peak in Figure 2A at 26º and a weak peak at 43º in the carbon Fe-SPC were indexed to (002) and (100) 11



crystal planes of a typical graphitic structure (JCPDS No. 05-0661).42 The relatively sharp peak at 26º was indicative of well-crystallized graphite fringes originating from amorphous SE source.43 Moreover, three peaks in Fe-SPC at 29.9, 35.3 and 44.5º were ascribed to iron carbide (Fe3C) (JCPDS No. 35-0772)44 formed from Fe doped in Fe-SPC during calcination. Furthermore, compared to non-Fe SPC sample, abundant amorphous carbon in Fe-SPC was catalytically decomposed to sp2 graphitic carbon by Fe.45,46 •

(A)

• Graphite

(B) MIL(Fe)/Fe-SPC10

♦ Fe3C

♦ ♦

MIL(Fe)/Fe-SPC6

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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•♦

MIL(Fe)/Fe-SPC3

MIL-100(Fe)

10

20

30

40

50

60

70

80

5

10

15

2θ (degree)

20 25 2θ (degree)

30

35

40

Figure 2. PXRD patterns of: (A) Fe-SPC and (B) MIL(Fe)/Fe-SPCX composites and parent

MIL-100(Fe)

For comparison, effect of Fe doping in SPC on the crystal growth of MIL-100(Fe) was investigated by PXRD and the results are compiled in Figure S4. Pristine MIL-100(Fe) in both composites exhibited similar PXRD pattern (CCDC 640536),33 indicating its successful in-situ growth on SPC and Fe-SPC surface. However, degree of crystallinity of MIL-100(Fe) in MIL(Fe)/Fe-SPC6 composites was clearly higher than that of MIL-100(Fe) in MIL(Fe)/SPC6. This discrepancy was attributed to the presence of Fe3C species on Fe-SPC which provided the growth conjunction as the metallic centers with MIL-100(Fe) and Fe-SPC sponge in MIL(Fe)/Fe-SPC6.47 12


Page 13 of 44

Furthermore, influence of Fe-SPC amount on the crystallinity of MIL-100(Fe) was also investigated as shown in Figure 2B. The similar patterns exhibited by parent and 3-10 wt.% of Fe-SPC incorporated MIL-100(Fe) suggested the well maintained crystal structure of MIL-100(Fe) upon the introduction of Fe-SPC into MOFs growth system. The intensity of MIL-100(Fe) characteristic peak shows a volcano trend with increasing Fe-SPC content in MIL(Fe)/Fe-SPCX composites, where MIL(Fe)/Fe-SPC6 possessed the highest crystallinity. These results suggested that an optimism amount of Fe-SPC is required to efficiently enhance the degree of crystallinity of MIL-100(Fe). Similar results have been reported earlier for (GO)/MIL composites,48 where the crystallinity of MIL-101 was destroyed by beyond 4 wt.% GO content. Attributed to the results in this report, we proposed to dope Fe species in porous carbon prior to MOFs growth. 700

(A)

N2 Adsorbed Amount (cm /g, STP)

600

3

3

N2 Adsorbed Amount (cm /g, STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500 400 300 200

MIL-100(Fe) MIL(Fe)/Fe-SPC6 MIL(Fe)/SPC6

100 0 0.0

0.2

0.4 0.6 0.8 Relative Pressure (P/P0)

1.0

700

(B)

600 500 400 300 200 MIL(Fe)/Fe-SPC3 MIL(Fe)/Fe-SPC6 MIL(Fe)/Fe-SPC10

100 0 0.0

0.2

0.4 0.6 0.8 Relative Pressure (P/P0)

1.0

Figure 3. Nitrogen adsorption isotherms of: (A) MIL-100(Fe), MIL(Fe)/Fe-SPC6 and MIL(Fe)/SPC6, and

(B) MIL(Fe)/Fe-SPCX composites.

13



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Table 1. Textural properties of SPC, Fe-SPC, MIL-100(Fe) and MIL(Fe)/Fe-SPCX composites SBET*

Smic*

Smeso*

Vt*

Vmic*

(m2/g)

(m2/g)

(m2/g)

(cm3/g)

(cm3/g)

SPC

836

691

145

1.37

0.06

Fe-SPC

265

239

26

0.66

0.01

MIL-100(Fe)

1405

899

507

0.83

0.34

MIL(Fe)/Fe-SPC3

1439

933

506

0.79

0.33

MIL(Fe)/Fe-SPC6

1739

921

810

1.11

0.32

MIL(Fe)/Fe-SPC10

781

516

265

0.53

0.23

MIL(Fe)/SPC6

1119

677

442

0.69

0.25

Sample

*SBET - BET surface area; Smic - Microporous surface area; Smeso - Mesoporous surface area; Vt total volume and Vmic is the microporous volume.

Figure 3 and Figure S5 show N2 isotherms and the corresponding pore size distribution of SPC, Fe-SPC, pristine MIL-100(Fe) and MIL(Fe)/Fe-SPCX composites, while results of their textural parameters are listed in Table 1. Parent SPC was observed to possess a type-II profile characterized by hierarchical mesoporous structures.49 After doping Fe species, the surface area and pore volume dramatically decreased (Table 1), and large amount of micropores disappeared in Fe-SPC sample (Figure S5A), which was assumed to have been blocked by the resulting Fe3C species. From Figure 3A, it can be seen that both MIL(Fe)/SPC6 and MIL(Fe)/Fe-SPC6 composites show very similar isotherm to parent MIL-100(Fe), suggesting their similar pore size distribution. As expected, MIL-100(Fe) grown from Fe-doped carbon sheets possessed much higher surface area than its parent sample, while its 14


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surface area dramatically decreased without Fe sites on SPC sheets. This suggested the mutually independent nature of MIL-100(Fe) crystals from porous carbon substrate in the absence of any conjunction or bridge sites i.e. Fe-binding sites. These Fe-binding sites could be beneficial to the formation of MOF crystal nucleus, facilitating the growth of MIL-100(Fe) particles.50 Furthermore, we monitored the effect of number of metal binding sites on the crystal growth of MIL-100(Fe). With increasing Fe site on SPC, the surface area of MIL-100(Fe) first increased, and then gradually decreased suggesting that an optimum amount of Fe-SPC is beneficial for the crystal growth of MIL-100(Fe), and enhancement in its surface area and pore volume. As depicted from Figure 3B and Table 1, MIL(Fe)/Fe-SPC6 possessed the largest SBET (1730.9 m2/g) among these composites with a total volume of 1.11 cm3/g corresponding to an increase of 33.7% compared to original MIL-100(Fe). However, excessive amount of Fe-SPC would lead to structural collapse or crystallinity loss as observed in 54.9% decrease in SBET of MIL(Fe)/Fe-SPC10 as that of original MIL-100(Fe), which were consistent with their corresponding PXRD results in Figure 2B. Furthermore, pore size distribution of MIL(Fe)/Fe-SPC10 composites determined by DFT were very similar as shown in Figure S5B, which is indicative of high accuracy in operation during synthesis of these composites.

15



(B) C1s C1s

O1s

Fe 2p

O1s

Fe 2p

O1s

Fe 2p

Intensity (a.u.)

Fe-SPC

C1s (79.3%) MIL(Fe)/Fe-SPC6

C1s (66.1%)

Fe3C Fe5C2

MIL-100(Fe)

200

(C) Fe2p

400 600 Binding Energy (eV)

712.6

712.2

705

710

284

O-C=O

286 288 290 Binding Energy (eV)

292

294

Raw Simulated

726.0

MIL(Fe)/Fe-SPC6 717.0

282

Sp3 C

(D) O1s

MIL-100(Fe) 717.4

700

280

800

725.6


730

COOH

Intensity (a.u.)

0

Raw Simulated

2

Sp C

Intensity (a.u.)

(A)

Relative Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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735

540

Fe-O-C

538


530

528

Figure 4. (A) XPS survey spectra of SPC, MIL(Fe)/Fe-SPC6 and MIL-100(Fe); (B) C1s core energy

levels for Fe-SPC sample; (C) Fe2p core energy levels for MIL(Fe)/Fe-SPC6 and MIL-100(Fe) and (D) O1s core energy levels for MIL(Fe)/Fe-SPC6

XPS measurements (Figure 4) were employed to investigate the surface elemental composition of Fe-SPC, MIL(Fe)/Fe-SPC6 and MIL-100(Fe) samples. The full survey spectrum shown in Figure 4A verified the presence of C, O, and Fe in these samples. For Fe-doped SPC, the contents of C and Fe were 79.3 and 0.79 at.%, respectively. The XPS spectrum of C1s for Fe-SPC shown in Figure 4B can be deconvoluted into five peaks centered at 282.3, 283.1, 284.6, 285.3 and 288.7 eV. The peaks at 282.3 and 283.1 eV corresponded to the FeC-like structures (Fe3C and Fe5C2),51,52 which was consistent with PXRD results (Figure 2). Moreover, the peaks 16


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at 284.6, 285.3, and 288.7 eV were attributed to sp2 C, sp3 C and O=C-O species respectively,53 while very small amount of sp3 C in the Fe-doped carbon matrix was observed. In contrast, traditional porous carbon retained a higher amount of sp2 C after low temperature carbonization, attributed to the Fe catalysis, which had transformed to sp2 graphite carbon structure. This enriched sp2 carbon can enhance the electrical conductivity of MIL(Fe)/Fe-SPCX composites.24 Moreover, the XPS spectrum of MIL(Fe)/Fe-SPC6 composite shows a higher intensity of C confirming the presence of additional carbon content from Fe-SPC. For comparison, Fe 2p XPS spectra of parent MIL-100(Fe) and MIL(Fe)/Fe-SPC6 composite are shown in Figure 4C. Fe 2p peaks of MIL-100(Fe) at binding energy of 712.6, 717.4 and 726.0 eV are assigned to Fe 2p3/2 and Fe 2p1/2 for Fe(III) oxide.54 In case of MIL(Fe)/Fe-SPC6, the binding energy of Fe 2p peaks was shifted to lower value by 0.4 eV i.e. 712.2, 717.0 and 725.6 eV, which could be attributed to the formation

of

heterojunction

between MIL-100(Fe) and Fe-SPC,

resulting

in

increased electron density on Fe3+.55 The O 1s high-resolution XPS spectra of MIL(Fe)/Fe-SPC6 composite is displayed in Figure 4D suggesting two peaks at binding energy of 533.5 eV and 532 eV. The peak at 533.5 eV is ascribed to -COOH, the oxygen components on the carboxylate groups53 while at 532 eV is assigned to the Fe-O-C bond in MIL(Fe)/Fe-SPC6. 56-60

3.2. THIA degradation by MIL(Fe)/Fe-SPCX composites. Ultrasonic Fenton-like catalytic degradation of THIA was carried out by MIL-100(Fe) and its Fe doped MIL(Fe)/Fe-SPCX composites. As illustrated in Figure 5A, THIA 17



Page 18 of 44

degradation rate was significantly increased after adding Fe-SPC into MIL-100(Fe), following

the

order

of:

MIL(Fe)/Fe-SPC6

>

MIL(Fe)/Fe-SPC10

>

MIL(Fe)/Fe-SPC3 > MIL-100(Fe). Different from MIL(Fe)/Fe-SPCX composite, 15.0% and 96.3% of THIA were removed using MIL-100(Fe) and Fe-SPC respectively within 20 min in ultrasound and H2O2 environment till equilibrium was attained. In order to investigate THIA adsorption on these two porous materials, samples from reaction medium were extracted and filtered using methanol as solvent. The calculated and those extracted from reaction medium content of THIA were almost identical, suggesting the incapable nature of both MIL-100(Fe) and Fe-SPC for THIA degradation, while the marginal conversion was due to adsorption of THIA. On the contrary, no THIA was detected in the extract from MIL(Fe)/Fe-SPCX composite reaction medium indicating their highly efficient degradation nature. Excluding the adsorption effect in THIA conversion by the prepared samples, the degradation kinetic rate for THIA on MIL(Fe)/Fe-SPCX composites was calculated as shown in Figure 5C applying a pseudo-first order kinetic model using Eq. (2): ln(

C ) = kt C0

Eq. (2)

Where C/Co is the normalized concentration of THIA, k is the apparent reaction rate constant (min−1), and t is reaction time (min). A volcano type trend of THIA degradation with increasing Fe-SPC content in MIL(Fe)/Fe-SPCX composites was observed, where MIL(Fe)/Fe-SPC6 possessed the highest degradation ability among all the samples. Reaction rate reached to 1.158 min−1 on MIL(Fe)/Fe-SPC6, about 3.3 times higher than that on MIL(Fe)/Fe-SPC3. 18


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The fast reaction kinetics could be attributed to the synergistic effect of the hetero-junction of Fe-SPC and MIL-100(Fe) resulting in an enhanced degradation. It is believed that MIL-100(Fe) possesses a porous framework with low electron conductivity and hence poor electron transfer from Fe to O in Fenton-like catalytic reaction. After doping Fe-SPC into porous framework by Fe bridging to MOFs, Fe-SPC facilitates effective charge transfer through MIL(Fe)/Fe-SPCX composites.61 Besides, porous MIL(Fe)/Fe-SPCX can enrich both THIA and H2O2 on its surface due to its higher adsorption properties, and thus further enhance catalytic activity. This represents a synergistic effect of chem-sorption and catalysis for THIA degradation. However, overuse of Fe-SPC can cause significant loss in porosity (Figure 2) and crystallinity of MIL-100(Fe) (Figure 3), which in turn decreases the net THIA degradation. From these facts, one can conclude that an appropriate loading of Fe-SPC on MIL-100(Fe) is the key to achieve optimal THIA degradation by MIL(Fe)/Fe-SPCX composites. Control and contrast experiments shown in Figure 5C suggest no decomposition of THIA in the absence of catalyst within 100 min upon exposure to only H2O2 and ultrasound irradiation. Similar was the case for using MIL(Fe)/Fe-SPC6 without adding H2O2 or exposure to ultrasound irradiation due to poor efficiency of strong oxidizing radicals’ generation. Moreover, MIL(Fe)/SPC6 shows merely 51.1% THIA degradation ratio as compared to that of MIL(Fe)/Fe-SPC6, confirming that Fe species on the porous carbon are critical component for the catalytic THIA degradation. This could be due to the fact that FeC species benefit crystal growth of 19



MIL-100(Fe) from heterogeneous carbon support, and hence enhanced the synergistic

effect

between

MIL(Fe)/Fe-SPC6

and

H2O2

under

ultrasonic

environment. 100

100

(A)

80

2

ka = 0.348 R = 0.987 80

MIL-100(Fe)

ln(Ct/Co)

THIA degradation rate (%)

MIL(Fe)/Fe-SPC3

60

MIL(Fe)/Fe-SPC10 40

(B) 2

ka = 0.792 R = 0.977 60 40

MIL(Fe)/Fe-SPC6

20

MIL(Fe)/Fe-SPC3 MIL(Fe)/Fe-SPC6 MIL(Fe)/Fe-SPC10

20

Fitting linear

Fe-SPC

0 0

20

40

60

80

2

ka = 1.158 R = 0.982

0

100

0

10

20

Time (min)

30

40

50

60

70

80

Time (min)

100 THIA degradation rate (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 44

80

(C) 60 40

H2O2 (US*) MIL(Fe)/Fe-SPC6 (US*) MIL(Fe)/Fe-SPC6+H2O2 (US*) MIL(Fe)/SPC6+H2O2 (US*) MIL(Fe)/Fe-SPC6+H2O2 (US*)

20 0 0

20

40 60 Time (min)

80

100

Figure 5. Effects of Fe-SPC adding amounts on (A) catalysis degradation of THIA and (B) degradation

kinetics under similar condition with H2O2 (40 mmol/L) and ultrasound; (C) different conditions of control, MIL(Fe)/Fe-SPC6 and MIL(Fe)/SPC6 (CTHIA=60 mg/L, VTHIA=50 mL, catalyst=50 mg, T=25 ºC, US* represents ultrasound)

Solution pH is an influential parameter for catalytic THIA degradation as reported earlier,3 and hence effect of pH was studied with the results compiled in Figure 6A. Catalytic degradation rate decreases (gray column) with increasing initial pH followed by an increase up to 100% degradation at pH = 7.5 (the natural pH of THIA solution) using MIL(Fe)/Fe-SPC6. Further increase in pH from weak to strong basic medium leads to decrease in THIA degradation. This was further investigated 20


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in terms of Fe3+ content in different pH solution to test the stability of MIL-100(Fe) and its carbon composites. Figure 6B shows the leak content of Fe3+ from MOFs as a function of pH. Clearly, both of these samples were partly destroyed in acidic medium (pH < 7.5), which leaked more Fe3+ from their frameworks. Interestingly, less Fe3+ was leaked from MIL(Fe)/Fe-SPC6 as compared to MIL-100(Fe), indicating the enhanced resistance of the former to acidic medium, which could be of great practical application in water purification under acidic environment. Subsequently, the contribution provided from leaking Fe3+ to THIA degradation was also tested as shown in Figure 6A. An equivalent mass of leaking Fe3+ salt (FeCl3) was added to THIA solution under similar reaction conditions with MIL(Fe)/Fe-SPC6, and the resulting THIA degradation is shown as purple column in Figure 6A. It was found that in acidic medium, degradation ratio was mostly controlled by Fe3+ as structural collapse of MIL(Fe)/Fe-SPC6 under such condition occurs predominantly45. Apart from the effective role of Fe3+ under acidic medium, the newly synthesized MIL(Fe)/Fe-SPC6 also possessed high THIA degradation activity in neutral media. With increasing basicity, THIA degradation ratio gradually decreased, which is in accordance to previously published literature.62 Typical Fenton-like reaction ideally occurs within pH range of 3-5,9 and highly alkaline medium inhibits the generation of hydroxyl radicals, which in turn leads to decreased THIA degradation.63 After being composited with Fe-loaded porous carbon, the stability of MIL-100(Fe) crystal structure was dramatically strengthened. The coordinated Fe3+ in the framework exhibited high resistance to alkalinity of the 21



medium during THIA degradation. From the stability and high activity, the synthesized MIL(Fe)/Fe-SPCx therefore merit further consideration for an array of catalytic degradation applications with wider neutral and alkaline pH.64 Meanwhile, for reusability performance evaluation of MIL(Fe)/Fe-SPCx, catalyst was separated from reaction products by simple filtration, and was tested in a fresh THIA degradation reaction under similar conditions. The results shown in Figure 6C revealed that MIL(Fe)/Fe-SPC6 remained highly active with similar degradation kinetics after five consecutive cycles suggesting its excellent recyclability for THIA degradation. The PXRD patterns and composite morphology of pre and post reaction MIL(Fe)/Fe-SPC6 samples remained highly identical as shown in Figure 6D and Figure S6. These results conclusively suggested that MIL(Fe)/Fe-SPC6 still maintained its original morphology and crystal structure after five consecutive reuses, which could be beneficial for its industrial applications.

60

60

40

40

20

20

Acidic solution Basic solution Fe3+ leak rate (%)

80

2

3

4

5

6 7 8 pH value

9

30 MIL-100(Fe)

pH = 7.5

20 10 MIL(Fe)/Fe-SPC6 3+

Fe Leak from framework

0

0

0

(B)

3+

80

40

100 Fe leak rate from MOFs (%)

MIL(Fe)/Fe-SPC6 3+ Fe leaching rate 3+ Free Fe

100 (A) THIA degradation rate (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 44

10 11

2

3

22


4

5

6

pH

7

8

9

10

11

Page 23 of 44

100

(C)

80

st

1

(B) 2

nd

rd

3

th

4

5

th

Intensity (a.u.)

THIA degradation rate (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40

Before catalytic reaction

After catalytic reaction

20 0 0

100

100 100 Time (min)

100

5

100

10

15

20 25 2θ (degree)

30

3+

Figure 6. (A) Effect of pH on THIA degradation ratio using MIL(Fe)/Fe-SPC6, free Fe

35

40

content as

catalyst, and Fe3+ leaching ratio. (B) Fe3+ leaching ratio from MIL(Fe)/Fe-SPC6 in acid and basic solution. (C) Degradation of THIA over MIL(Fe)/Fe-SPC6 after five consecutive cycles, (D) PXRD patterns of MIL(Fe)/Fe-SPC6 before and after catalytic reaction.

Though, complete degradation of THIA has been ubiquitously reported over porous materials,9 still its conversion into non-toxic fractions is a matter of greater concern.7 Besides degradation kinetics, the nature of the resulting degraded fractions in terms of toxicity is in fact crucial to evaluate the catalyst performance and overall applicability of the process.63 In this regard, TOC removal ratio related to mineralization extent was further investigated using MIL(Fe)/Fe-SPC6 under ultrasonic-assisted Fenton-like catalytic degradation of THIA at neutral pH. The results compiled in Figure 7A suggest that with increasing reaction time, TOC removal ratio increased gradually suggestive of onward decomposition of intermediates in the proceeding steps of the reaction. Noteworthily, TOC removal ratio of MIL(Fe)/Fe-SPC6 reached up to 95.4% within 3 h, which was 1.3 times higher than that of MIL(Fe)/SPC6. It could be attributed to the formation of hetero-junction between Fe-SPC and MIL-100(Fe), and hence the resulting conductive interface (Figure 1D, TEM) and high contact surface (Table 1). 23



In order to get deeper insight into the efficiency of electron transport and the reaction pathway, we confirmed the major active oxygen species involved in the degradation of THIA using ESR technique.63 The ESR spectra in Figure 7B exhibited a characteristic 4-fold peak with an intensity ratio of 1:2:2:1, which was accorded to typical DMPO-•OH complex adduct.24 In this system, •OH radical from H2O2 excitation accounts for the effectiveness of Fenton-like catalyst in THIA degradation. The intensity of 4-fold peak directly determines the amount of •OH species. Clearly, MIL(Fe)/Fe-SPC6 exhibited much higher characteristic quartet peaks of DMPO-•OH than MIL-100(Fe), indicating higher activity of the former to concomitantly excite •OH and transport electron. These results further confirmed that Fe-SPC composited MIL-100(Fe) turned out to be an excellent heterogeneous catalyst for the advanced oxidative degradation of THIA at a wide neutral-alkaline range of pH. 100

3000

(A)

(B)

MIL(Fe)/Fe-SPC6

MIL(Fe)/Fe-SPC6 80

1500

60

MIL(Fe)/SPC6 40

Intensity (a.u.)

TOC Removal Ratio (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 44

0 MIL-100(Fe)

-1500

20 0 0

20

40

60 80 100 120 140 160 180 Degradation time (min)

-3000 3460

3461

3462 3463 Magnetic field (G)

3464

3465

Figure 7. (A) Rate of TOC removal in ultrasonic Fenton-like catalytic degradation of THIA. (B)

DMPO spin trapping ESR spectra over different catalysts with 50 mg catalyst, 50 mL of THIA at 60 mg/L, 0.25 mL 30% wt. H2O2, pH of 7.5.

Based on the above observations, the mechanism for ultrasonic-assisted degradation of THIA over MIL(Fe)/Fe-SPCX was proposed (Figure 8). First, Fe(III) 24


Page 25 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


on the surface of MIL(Fe)/Fe-SPCX decomposes H2O2 to HO2• radicals accompanied with the generation of Fe(II) (Eq. 3). The as-formed Fe(II) generated charge carriers are exposed to ultrasonic irradiation in water which oxidizes Fe(II) back to Fe(III) (Eqs. 4-5), and the released electrons are subsequently transferred to the surface of Fe-SPC, and then to H2O2 acting as an efficient scavenger. H2O2 is then attacked by electron and forms •OH radicals (surface found or in bulk solution) (Eq. 6), which eventually attacks and oxidizes THIA. Fe(III) + H2O2 → Fe(II) + HO2• + H+

Eq. (3)

Fe(II) + US → h+ +e-SPC

Eq. (4)

Fe(II) + h+ → Fe(III)

Eq. (5)

e-SPC + H2O2 → •OH + OH-

Eq. (6)

Figure 8. Proposed reaction mechanism for ultrasonic-assisted degradation of THIA over MIL(Fe)/Fe-SPCX

3.3. Identification of intermediates and proposed THIA degradation pathways. Intermediates of THIA degradation reaction were monitored by LC-MS. 25



The HPLC chromatogram of THIA samples were collected at different reaction times (0~180 min), and five intermediates (P1 to P5) were identified (Figure 9). The shorter retention times of degradation intermediates from P2 to P5 than that of THIA was indicative of their increased polarity, while P1 exhibited comparatively longer retention time than THIA. Additionally, THIA peak completely disappeared within 100 min of reaction, indicative of its complete degradation. The peak areas of all degraded intermediates increased dramatically during the initial 20 min of reaction, and then decreased gradually with reaction time. Among all the intermediates, the highest peak area representing P3 after 100 min of reaction suggested its highly stable nature under the current experimental conditions. However, this peak gradually decreased with time and finally disappeared at 180 min, which was once again consistent with the rate of TOC removal in THIA degradation. The determined corresponding molecular masses of these five intermediates are shown in Figure S7, while their retention time, molecular masses and mass fragments are summarized in Table S1. As shown, the eluted intermediates, i.e., P1, P2, and P3 all contained heteroaromatic ring. It can be seen that the oxadiazinehydro ring in THIA molecule is more active than heteroarmatic ring, which became a prime degradation target. Figure 10 shows the proposed structure of intermediates P1 to P5, and possible fragmentation pathway based on LC-MS results. First, THIA was decomposed to P1 and P2 under the attack of •OH radicals. P2 was then converted into P3, which further decomposed to P4 and P5. The presence of P1 to P5 as intermediates coincided with previously reported literatures about THIA 26


Page 26 of 44

Page 27 of 44

degradation.5,7,65

0 min P3 P2

P5

20 min

Intensity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


P4

P1

60 min 100 min 140 min 180 min 0

1

2

3 4 5 Time (min)

6

7

8

Figure 9. LC-MS analysis of intermediates from THIA degradation.

Table 2. Changes of bond length and angle of P3 intermediate before and after being adsorbed on Fe cluster of MIL-100(Fe)

Bond length (Å)

Penta-heteroaromatic ring

Branched Chain

Position

1C-13N

13N-2C

2C-3C

3C-4S

1C-4S

1C-6Cl

3C-7C

7C-10N

B. A.*

1.28

1.41

1.36

1.83

1.83

1.79

1.51

1.45

A. A.*

1.34

1.42

1.37

1.75

1.77

1.68

1.49

1.49

∆ change*

0.06

0.01

0.01

-0.08

-0.06

-0.11

-0.02

0.04

Bond Angel of Penta-heteroaromatic ring (º) Position

1C-4S-3C

3C-2C-13N

1C-13N-2C

4S-3C-2C

4S-1C-13N

B. A.*

88.5

110.0

116.7

116.0

108.8

A. A.*

89.7

116.3

110.0

110.0

114.2

∆ degree*

0.8

6.3

-6.7

-6.0

5.4

* B. A. is before adsorption; A. A. is after adsorption; ∆ change and the ∆ degree are the value of bond length and bond angle in P3 after and before adsorption on MIL-100(Fe). 27



Compared

to

other

reported

Fenton-like

catalysts,

Page 28 of 44

MIL(Fe)/Fe-SPCX

concomitantly shows fast THIA degradation and high TOC removal efficiency. From LC-MS results, one can conclude that THIA and P3 species are crucial for THIA advanced oxidative degradation in Fenton-like catalytic system. The plausible mechanism of THIA degradation on MIL(Fe)/Fe-SPCX was further validated by analyzing the reactive sites of THIA and P3 in terms of electro-philicity and nucleo-philicity of the attacking species using MEP.

Figure 10. Proposed structure and fragmentation pathway of intermediates P1, P2, P3, P4 and P5.

MEP of THIA and P3 calculated at B3LYP/ 6-311++G (d,p) level available in Gaussian 09 are shown in Figure 11. The electrostatic potential of the molecule is shown in colors with an order of: blue (positive) > green > yellow > orange > red (negative). As shown, the negative and positive regions of THIA molecule located around nitro group and oxadiazinehydro ring (Figure 11A), respectively. Credited to the vulnerable nature of these sites to electrophilic and nucleophilic attacks respectively, they are immediately attacked by •OH radicals and are converted into P1, P2 and P3 intermediates. However, P3 displayed most area of very light yellow color as shown in Figure 11B suggesting its highly stable structure towards degradation and hence is the main reason for low TOC removal ratio in THIA 28


Page 29 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


degradation as reported previously.3,7,9

Figure 11. Molecular electrostatic potential (MEP) for THIA (A) and P3 (B).

From our LC-MS results, it is worth exploring the cause of high TOC removal ratio by as-designed MIL(Fe)/Fe-SPCX composites. The formation of mutual interaction between THIA/P3 and composite catalysts can be alleged as the key point for enhancing THIA advanced degradation. Therefore, the frontier orbital alignment, bond length/angle, and binding energy of P3 were further systematically calculated before and after being adsorbed on MIL-100(Fe).

Figure 12. The optimized intermediate structure of P3 and its corresponding (A) HOMO of P3 and (B) HOMO of P3 adsorbed on Fe cluster of MIL-100(Fe). Carbon, oxygen, chlorine, sulphur and

iron atoms are depicted in gray, red, green, yellow, and purple, respectively.

The electron densities of HOMO and LUMO of P3 before and after adsorption on Fe cluster of MIL-100(Fe) are shown in Figure 12. Based on DFT simulation results, 29



Fe sites in MIL-100(Fe) could not bond with N in THIA due to its steric hindrance. The possible degradation pathway may be the pore/cage adsorption and •OH radical attack produced from H2O2. P3 intermediate is believed highly stable (Figure 12A), however, in our system, the HOMO of the penta-heteroaromatic ring on P3 and the unoccupied orbital of Fe in MIL-100(Fe) overlapped after P3 being adsorbed on MIL-100(Fe) (Figure 12B). From its Mulliken charge in Figure S8, electrons were transferred from heteroaromatic ring in P3 to MIL-100(Fe). These results indicated a unique formation of a stronger Fe-N bond between P3 and MIL-100(Fe) compared with other reported catalysts.63 By calculation, the binding energy of P3 adsorbed on MIL-100(Fe) was -586.9 kJ/mol, which probably led to the obvious structural change in P3. These changes, such as bond lengths and bond angles, in P3 were monitored by DFT calculations, and the key parameters are listed in Table 2. The significant changes of bond length and angle appeared in the penta-heteroaromatic ring of adsorbed P3 (Table 2). Among these chemical bonds, the bond length of 1C-13N double bond increased from 1.28 to 1.34 Å, which subsequently induced the decomposition of P3 to P4. Meanwhile, the chemical bonds connected to the heteroatoms, such as 3C-4S, 1C-4S and 1C-6Cl, are considered more vulnerable to breakage due to their relatively longer bond lengths.7,66 Apart from this, the bond angels in Table 2 also indicated the deformation of the penta-heteroaromatic ring in adsorbed P3 which led to a rapid rise in inner ring tension in its ring. These changes and disequilibrium facilitated the decomposition of P3 into smaller molecules (P4 or/and P5). As a result, 30


Page 30 of 44

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the stable P3 become unstable after being adsorbed on MIL-100(Fe), and it definitely accelerated its advanced decomposition in the currently proposed system.

4. CONCLUSIONS In summary, MIL-100(Fe) was in-situ grown on silkworm excrement derived Fe-doped nano-spongy porous carbon and successfully formed the MIL(Fe)/Fe-SPC composite for advanced degradation of THIA. Results showed that MIL(Fe)/Fe-SPC exhibited better crystallinity and higher surface area (BET: 1730 m2/g) compared to parent MIL-100(Fe). The formation of hetero-junction between MIL-100(Fe) and Fe-SPC can be proved from negative shift of Fe3+ binding energy in XPS. Catalytic degradation performance showed that MIL(Fe)/Fe-SPC composite concurrently exhibited fast degradation of THIA with ultrahigh TOC removal ratio (95.4% at 25 ºC and 180 min). The enhanced catalytic ability of MIL(Fe)/Fe-SPC composite was attributed to the intensifying interface between MIL-100(Fe) and Fe-SPC with Fe as bridging component. Based on LC-MS analysis and DFT calculations, THIA was initially degraded to a relatively stable intermediate (C4H5N2SCl), and subsequently adsorbed by MIL-100(Fe) with strong chemical bonding energy (Fe-N, -586.94 kJ/mol). Chemisorption induced a significant charge transfer and geometric distortion, and caused its continual degradation to inorganic small molecules. Thus, synergistic effect of efficient charge separation, strong chemisorption and good Fenton-like catalytic activity enabled MIL(Fe)/Fe-SPC composite to significantly enhance advanced degradation performance for THIA. The newly designed MIL(Fe)/Fe-SPC Fenton-like catalyst can be deemed of great assistance in similar applications involving abatement of hardly degradable pesticides and other environmental pollutants.

31



Page 32 of 44

■ ASSOCIATED CONTENT Corresponding Authors *E-mail: [email protected] (Zhongxing Zhao) *E-mail: [email protected] (Zhenxia Zhao)

Author Contributions #

Yanan Wei and Bingfeng Wang contributed equally to this work.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (No. 21666004 and 21676059), Natural Science Foundation of Guangxi Zhuang

Autonomous

Region,

China

(No.

2016GXNSFAA380229,

2017GXNSFFA198009 and 2017GXNSFEA198001), Scientific Research Foundation of Guangxi University (No. XJPZ160713), and Guangxi Distinguished Experts Special Foundation of China. We thank Dr. Zhiqun Tian and all members of Guangxi Key Laboratory for Electrochemical Energy Materials for characterization facilitation.

Supporting Information Details of the standard curve of THIA concentration in aqueous medium, metal clusters (Fe3O(BA)4) from MIL-100(Fe), EDS elemental mapping image of Fe-SPC, SEM image of MIL-100(Fe), PXRD patterns of MIL(Fe)/ SPC6, Nitrogen adsorption Isotherms and DFT pore size distribution of MIL(Fe)/Fe-SPCX, TEM image of post catalytic reaction MIL(Fe)/Fe-SPC6, LC-MS molecular mass profile of four intermediates, Mulliken charges of intermediate P3 before and after being adsorbed on 32


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Fe cluster of MIL-100(Fe).

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Highly Advanced Degradation of Thiamethoxam by Synergistic

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