Controllable Preparation and Catalytic Performance of Heterogeneous

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Controllable Preparation and Catalytic Performance of Heterogeneous Fenton-like #-Fe2O3/ crystalline glass microsphere Catalysts Hongbao Yao, Yu Xie, Yu Jing, Yujun Wang, and Guangsheng Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03440 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017

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Industrial & Engineering Chemistry Research

Controllable Heterogeneous

Preparation

and

Fenton-like

Catalytic α-Fe2O3/

Performance crystalline

of

glass

microsphere Catalysts

Hongbao Yao1, Yu Xie1, Yu Jing1, Yujun Wang1*, Guangsheng Luo1

1

State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua

University, Beijing 100084, China

Corresponding author: Tel: 86-10-62798447, Fax: 86-10-62770304 Email address: [email protected]

1

ACS Paragon Plus Environment


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Abstract Highly dispersed α-Fe2O3 nanoparticles were immobilized onto crystalline glass microspheres for the first time to be heterogeneous Fenton catalysts (FeCG) with favorable activity. The average particle size of the supported iron oxides ranges from 2.6 nm to 6.5 nm and can be achieved controllably with great ease. Typically, through hydrothermal treatment, it was found that amorphous glass support develops into mixed crystals of primitive SiO2, CaSi2O5 and Ca2MgSi2O7, with ~180% improvement in specific surface areas. Most importantly, after Fe Loading, not only OH· but also HO2· radical species with high intensity were generated for FeCG, while OH· alone was produced for commercial α-Fe2O3 in the presence of H2O2, thus accelerating the redox recycling between Fe(II) and Fe(III) and presenting much superiority in the azo dye AO7 decoloration. The pseudo-zero order reaction constant was determined to be 0.384 mg/(L·min), an ~65.5% improvement over that of commercial α-Fe2O3 under the same experimental conditions. Only 1.26 mg/L Fe leaching was detected under optimum conditions in addition to simple catalyst recovery by gravity. No remarkable decrease in the AO7 decoloration efficiency was observed after six cycles, indicating the favorable stability.

Key word Hematite

crystalline glass

Fenton-like

degradation

2


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1. Introduction Wastewater treatment, especially the disposal of the colored effluents coming from various industries, is of prime importance all over the world1, 2. As a typical advanced oxidation process, the Fenton reaction (Fe2+/Fe3+ + H2O2) has been widely employed for the elimination of recalcitrant organic compounds due to its high reactivity, low toxicity and low commercial cost. Nevertheless, there are strict limitations of strong acidic conditions (normally with optimum pH value at 2~3) considering the formation of ferric hydroxide sludge at pH > 4, which adversely affects the catalytic performance3-5. Accordingly, heterogeneous Fenton catalysts, mainly various Fe (hydr)oxides such as Fe3O4, Fe2O3, FeOOH, appear to be interesting alternatives1, 5-7. Among them, α-Fe2O3 is one of the promising due to its small band gap (2.1 eV), chemical and thermal stability with low iron release8-10. For example, α-Fe2O3 nanoparticles were synthesized to activate peroxymonosulfate and effectively degrade dichlorophenoxyacetic acid in the presence of UV light in the work of Nematollah11. Hager et.al.12 proposed a green procedure of α-Fe2O3 preparation using citrus reticulum peels extract and compare the reactivity on different type of organic pollutants like Maxilon blue, Neolan Blue 2G and 2,6- dichlorophenol. However, the high electron-hole recombination rate and easy aggregation for pure α-Fe2O3 nanoparticles restrict its further practical application7, 10. Accordingly, different supports were hybridized with α-Fe2O3 nanoparticles, such as zeolite13, clay14, SiO215 and Al2O316. For instance, Lim et.al.17 prepared highly active composites by impregnating iron oxide nanoparticles in alumina coated mesoporous SBA-15 silica, which increases the dispersion of the iron oxide nanoparticles and facilitates the redox cycle of iron species. In addition, Ce 16, S18, N19, and Ti20 – doped α-Fe2O3 nanocomposites are also developed with superior catalytic performance. These kinds of catalysts generally face the drawbacks of relatively harsh synthetic conditions, complicated synthesis routes, and high cost 21. Much other attention has also been focused on the carbon-based heterogeneous Fenton catalyst. Fan et al.

22

compared the removal efficiencies of azo dye Acid Black 24 in several

treatment processes and found that FeGAC/H2O2 (catalytic oxidation with iron oxide-coated granular activated carbon) exhibited the best performance, where activated carbon function as both excellent adsorbents and effective electron transfer catalysts. Similarly, graphene oxide23, 24, 3



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carbon nanotubes25, and carbon fibers26 are also claimed to present a synergic effect with iron catalysis and show enhanced catalytic activity in the heterogeneous Fenton reaction. However, carbon materials are usually in form of granules or powders and are difficult to recover from the reaction media1, 27. Therefore, it remains challenging to design heterogeneous Fenton catalysts with higher activity, simple synthesis routes, and better reusability. In our previous work, porous glass beads with different morphology were controllably prepared and employed as the carrier for supported Pd and TS-1 catalysts based on the excellent ion-exchange properties28-30. Accordingly, α-Fe2O3/crystalline glass microsphere catalysts were synthesized for the first time and systematically examined from the point of view of preparation and application. Crystalline glass microspheres with a nanoporous surface were selected as the support, and there appear to be three benefits: 1) easier to obtain highly dispersed α-Fe2O3 nanoparticles with small diameters ranging from 2.6 nm to 6.5 nm; 2) accelerate the redox recycling between Fe(II) and Fe(III) and cause much higher catalytic activity in Fenton-like reactions; and 3) easy recovery through gravity as well as excellent stability. This success may lead to a new pathway for future heterogeneous Fenton reaction technology development.

2. Experimental 2.1. Materials and Chemicals Glass microspheres with diameters of 100 ± 5 µm were supplied by Hebei Chiye Corporation. Analytical

grade

FeCl3,

azo

dye

acid

orange

7

(AO7),

H2O2

(28%),

5,5-dimethyl-1-pyridine-N-oxide (DMPO) and ethanol were purchased from Beijing Chemical Plant and used without further purification. Commercial α-Fe2O3 nanopowder with a diameter of 20 ± 5 nm was purchased from Alfa Aesar (China) Chemicals Co., Ltd. Double distilled water was employed throughout all the experiments. 2.2. Catalyst Preparation Glass microspheres were first processed using a subcritical water treatment method based on our previous work28, 31 with a slight modification. Typically, 10 g of glass beads and 400 mL of deionized water were put together into a tank reactor and then maintained for 1 h with a temperature of 573 K and pressure of 8.3 MPa under a stirring rate of 600 rpm. The crystalline glass beads were subsequently separated by natural settling, washed with deionized water and 4


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dried at 60 °C overnight for further use. The glass microspheres before and after the subcritical water treatment process are denoted as G and CG, respectively. To prepare the α-Fe2O3/crystalline glass microsphere (FeCG) catalyst, a certain amount of CG and FeCl3 aqueous solution with a solid/liquid mass ratio of 1:100 was mixed together. The mixture was put into a water bath shaker under the temperature of 298 K and a rotating speed of 120 rpm. Afterwards, the resulting products were separated by natural settling and washed with deionized water and ethanol several times. Then, the solids were dried at 80 °C overnight, following a calcination procedure at 600 °C for 2 h with a heating rate of 2 °C /min. The as-prepared FeCG composites with different initial FeCl3 concentrations (ranging from 50 ppm, 100 ppm, and 200 ppm to 400 ppm) are hereafter labeled FeCG50, FeCG100, FeCG200 and FeCG400, respectively. 2.3. Characterization of the catalyst The surface morphology of the prepared catalyst was investigated using scanning electron microscopy (SEM, JEOL JSM 7401F, JEOL Ltd., Japan). The surface chemical composition of the samples was detected by X-ray photoelectron spectroscopy (XPS, PHI Quantera SXM ， ULVAC-PHI, Japan), and the binding energy was calibrated with C1s at 284.8 eV. The crystal structure of the supported iron oxide was determined by X-ray diffraction (XRD, Model D8 ADVANCE, Bruker). Additionally, the corresponding nanoparticle size was investigated using a transmission electron microscope (TEM, EOL JSM 2010, JEOL Ltd, Japan). The Brunauer-Emmett-Teller (BET) surface areas and pore size distributions of samples were measured at 77 K on a Quantachrome Autosorb- 1-C chemisorption–physisorption analyzer. The specific surface area was calculated from the adsorption branches in the relative pressure range of 0.05–0.25. The pore diameter was calculated from the desorption branches using the Barrett– Joyner–Halenda (BJH) method, and pore volumes were estimated from the adsorbed amount at a relative pressure of 0.99. An inductively coupled plasma atomic emission spectrometer (ICP, IRIS Intrepid II XSP from Thermo Fisher Corp., America) was employed to measure the Fe loading amounts of the prepared catalyst. DMPO spin trapping Electron Paramagnetic Resonance (JES-FA200 EPR Spectrometer, Japan) measurements for the catalyst powders were recorded at room temperature by adding DMPO (50 mM) into the reaction suspension with an average of three scans. 5



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2.4. Measurements of Catalytic performance The catalytic performance was evaluated by investigating the AO7 degradation in the presence of H2O2 with and without visible light irradiation. The reaction was conducted in a slurry tank reactor. Typically, 0.1 g of prepared catalyst was added into 25 mL AO7 aqueous solution with a concentration of 60 ppm. Then H2O2 was dropped to above mixture to initiate the catalytic oxidation reaction. After the reaction, the AO7 solution sample was withdrawn immediately and analyzed using a UV–vis spectrophotometer at 480 nm. A series of parallel experiments were examined to study the effect of different H2O2 concentration (20~160 mmol) and different reaction times (0~2 h). Visible light irradiation was carried out using a 500 W Xenon lamp. The decoloration efficiency in this work was defined as the percentage reduction in the AO7 concentration relative to the initial value. The concentration of iron elements in AO7 solution was measured on the Atomic Absorption Spectroscopy (AAS, Z5000, Hitachi) with an air-acetylene flame.

3. Results and Discussion 3.1 Characteristics of the Prepared Catalysts Fig. 1 (a) and (b) show the obvious color changes of CG supports from white to reddish brown after iron oxide loading, indicating that the iron oxide was successfully coated on the glass support. SEM was further employed to investigate the surface morphology change of several samples. Compared with Fig. 1 (c), (d) and (e), all three samples exhibit spherical shapes, indicating that the glass microspheres present excellent structural stability even after hydrothermal treatment at 573 K. On the other hand, after this hydrothermal etching, a layer of uniform flakes appeared on the surface of CG, as depicted in Fig. 2 (a) and (b), while there was a smooth surface for untreated G. In addition, it can be clearly observed that the nanopores formed by these flakes were coated with iron oxides in the FeCG sample, as shown in Fig. 2 (c). Furthermore, the presence of Fe was also approved using the Energy Dispersive X-ray Spectroscopy (EDX) elemental mapping method, as shown in Fig. 3.

6


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Fig. 1 Color change of CG supports before (a) and after (b) α-Fe2O3 loading; SEM images of (c) G, (d) CG and (e) FeCG400 XRD patterns were also employed to determine the crystal structure of the supported FeCG sample. It was found that the G sample is amorphous32 based on the presence of a broad peak at 2θ of approximately 22 degrees. Interestingly, the CG sample presents several noteworthy peaks at 2θ of approximately 21.2, 28.6, 30.8 and 50.0 degrees, which seems to be the mixed crystals of primitive SiO2 (JCPDS 85-0621), CaSi2O5 (JCPDS 15-0130) and Ca2MgSi2O7 (JCPDS 88-0778). Similar phenomena have also been reported before where Hench et.al.33 found that amorphous calcium phosphate glass film develops into a crystalline hydroxyl-apatite layer, but the specific crystal structure was unknown.

Fig. 2 High Resolution SEM images of (a) G, (b) CG and (c) FeCG400

7



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Fig. 3 EDX elemental mapping of the FeCG400 sample Moreover, the abovementioned peaks disappeared while some characteristic peaks with relatively weak intensity at 2θ of 33.1, 35.6, 49.6 and 54.1 degrees were observed for the FeCG400 sample, which are typical signals of α-Fe2O3 crystals (JCPDS 89-0599). The weak intensity may be due to the shield effect of CG supports considering the ultralow Fe loading amounts in the as-prepared novel catalyst. ICP was accordingly employed to determine the Fe content in the prepared catalyst. The result (Section 3.2) shows that the Fe loading dosage was ~10.2 mg/g in the FeCG400 sample under our experimental conditions. In addition, XPS spectroscopy was further used to analyze the chemical valence states of the supported Fe element. As shown in Fig. 5, the binding energy values of Fe 2p1/2 and Fe2p3/2 are 724.4 eV and 710.8 eV, respectively, which are attributed to α-Fe2O3 34, 35 and validate the XRD results.

Fig. 4 XRD patterns of G, CG, FeCG50, FeCG100, FeCG200 and FeCG400 samples

8


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Fig. 5 XPS spectroscopy of FeCG400 Nitrogen adsorption and desorption curves and the relative pore size distribution are presented in Fig. 6. The values of specific surface areas, pore volumes, and mean pore sizes are listed in Table 1. The results indicate that there is no pore structure on the surface of the initial glass microspheres. However, after the hydrothermal treatment process, the specific surface area of glass microspheres was significantly enlarged from 0.76 m2/g to 135.83 m2/g, which is consistent with what was observed from SEM results and, most importantly, makes it much easier to be supported with iron components. After the Fe coating, the specific surface area decreases compared with pure CG support, but it increases with an increasing Fe loading amount. This may be due to α-Fe2O3 nanoparticles tend to be located on the outside of surficial channel at high loading amounts

36, 37

. In addition, with the increase of Fe loading, the bimodal pore size

distribution at the range of 3-5 nm and 7-10 nm trends toward a single pore size distribution at the range of 3-5 nm. Additionally, the average pore diameter seems to be almost unchanged for all samples. 180

0.4

60

0 0.0

0.5

1.0

Relative Pressure (P/P0)

0.5

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(b)

dV(logd) (mg/L)

1.0

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Adsorption Desorption 120

Volume (cm3/g)

(a) Volume (cm3/g)

dV(logd) (mg/L)

1.5

Adsorption Desorption

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0 0.0

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d (nm)

(c)

0.0 1.51

100

100


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0.0 0.61

10

(d)

100

150

d (nm)


25

0

0.0

0.5

1.0


9

0.2

Volume (cm3/g)

0.4

50

dV(logd) (mg/L)

Volume (mg/L)

75

dV(logd) (mg/L)

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


1.0

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50

0 0.0

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ACS Paragon Plus Environment 0.0

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10

d (nm)

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d (nm)

100


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(e) Volume (cm3/g)

dV(logd) (mg/L)

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1.0

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0 0.0

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0.5

1.0


1

10

100

d (nm)

Fig. 6 Pore size distribution and relative N2 adsorption-desorption curves of sample CG, FeCG50, FeCG100, FeCG200 and FeCG400, respectively.

Table 1 Summary of BET results and corresponding Fe loading amounts of different samples Sample

Specific surface area

Pore volume

Pore diameter

Fe contents

(m2/g)

(cm3/L)

(nm)

(mg/g)

G

0.76

-

-

-

CG

135.83

0.24

3.70

-

FeCG50

44.95

0.16

3.86

0.26

FeCG100

62.95

0.17

3.79

0.68

FeCG200

95.38

0.21

3.80

0.91

FeCG400

106.22

0.24

3.82

1.02

3.2 Particle Size Control The particle size of prepared α-Fe2O3 nanoparticles can be easily controlled just by changing the initial FeCl3 concentration in our synthesis procedure. First, the effect of different initial FeCl3 concentrations on the Fe adsorption capacity over CG support was conducted in detail. The result in Fig. 7 (a) shows that the Fe loading amounts increase from 1.6 mg/g to 10.2 mg/g with an increasing initial FeCl3 concentration from 30 ppm to 400 ppm. Furthermore, the experimental 10


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data correlated well with the Freundlich Model:

log = log + log (1) where qe is the amount of Fe species adsorbed at equilibrium in mg/g, c is the solute residual concentration in ppm, and k and n are Freundlich constants related to the adsorption capacity and intensity of adsorption, respectively. The fitting results give an n value of 1.7, with a strong linear relationship supported by an R2 value of 0.9796. 1.2

12

(a)

(b) Experimental Data Linear Fit

Log qe (mg/g)

Loading amounts (mg/g)

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


0.8

8

0.4

4

0

0

90

180

270

Concentration (ppm)

0.0

360

1.2

1.4

Log c (ppm)

1.6

Fig. 7 (a) Adsorption capacities under different initial FeCl3 concentrations at room temperature (b) Adsorption isotherm fitting using Freundlich model Fig. 8 shows the TEM images of several FeCG samples and the corresponding primary particle size distribution. The mean primary particle size here was quantified based on a statistical number-weighted method (̅ = ∑ / ∑ , ni is the number of counted α-Fe2O3 nanoparticles with a diameter of di) by surveying more than 200 particles on the TEM images through Digital Micrograph Software. It shows that the average diameter changes from 2.6 nm to 6.5 nm, increasing 150%, when the actual Fe loading amounts increase from 0.16 wt% to 1.02 wt%, based on the ICP data from Fig. 7. Additionally, the prepared α-Fe2O3 nanoparticles exhibit excellent dispersity when the initial FeCl3 concentration is below than 400 ppm, as shown in Fig. 8. Comparisons of the mean diameters of α-Fe2O3 nanoparticles with other works are shown in Table 2. Obviously, the α-Fe2O3 nanoparticles synthesized in this work present much superiority.

11



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Fig. 8 TEM images of different FeCG samples and relative primary particle size distribution: (a) and (b) FeCG 50; (c) and (d) FeCG 100; (e) and (f) FeCG 200; (g) and (h) FeCG 400 Table 2 Comparisons in average diameter of α-Fe2O3 nanoparticles with other works. Mean Crystal Size (nm)

Reference

2.6-6.5

This work

5-10

38

2-20

39

8.4-14.2

40

< 10

41

10-40

42

3.3 Comparison of Catalytic Reactivity under Different Conditions 12


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The decoloration performance of azo dye AO7 using FeCG catalysts with different Fe loading amounts was also accordingly discussed in detail. As shown in Fig. 9, the decoloration efficiency improves by ~25.7% when the Fe loading amounts increase from 0.16% wt% to 0.68 wt%. Apparently, the increase in the active components will inevitably promote the catalytic oxidation reactions. However, the decoloration performance deteriorates with a much higher Fe concentration. This is mainly due to the formation of larger particles of α-Fe2O3 nanoparticles under this scenario, as observed from TEM results, thus hindering effective diffusion and contact between the reactants towards the active sites, leading to poor catalytic activity. A similar trend was also claimed in other kinds of heterogeneous Fenton catalysts. For example, Zubir et. al.

24

found that coated Fe3O4 nanoparticles with a higher loading than 15 wt% on graphene oxide will be against the AO7 decomposition.

Decolouration Efficiency (%)

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40

35

30

25

20

0

4

8

12

Fe Loading Amounts (mg/g)

Fig. 9 Effect of Fe loading amounts on the decoloration of AO7 in the presence of H2O2 [AO7 initial concentration 60 ppm, AO7 volume 25 mL, H2O2 concentration 80 mmol/L, catalyst dosage 0.1 g, reaction time 1 h] Fig. 10 (a) shows the examination of AO7 decoloration with the addition of H2O2, CG/H2O2, α-Fe2O3/H2O2 and FeCG/H2O2 under different reaction times with or without sunlight irradiation. The result shows that AO7 aqueous solution is notably stable and can barely decompose, even in the presence of H2O2 or α-Fe2O3/H2O2 alone in the dark. Nevertheless, AO7 starts to degrade with sunlight irradiation in both cases, and the decoloration efficiency for the α-Fe2O3/H2O2 system improves by ~ 53.6% over that of H2O2 alone. Obviously, the introduction of light irradiation accelerates the heterogeneous Fenton reaction catalyzed by α-Fe2O3 particles. A similar phenomenon has also been reported in other work, mainly because the regeneration of Fe(II), with 13



the production of new HO· radicals, follows a photoreduction process in this case 43:

Fe(III) + → + (2)

!" Fe(II) + · (3) Interestingly, AO7 can also degrade in the presence of CG and H2O2 at same time. The AO7 decoloration efficiency for the CG/H2O2 system was 29.2% without light irradiation and 50.2% with it, improving ~71.9%. Notably, for the FeCG/H2O2 system, the AO7 decoloration efficiency could reach 49.8% even in the dark, while as discussed before, no reaction occur for α-Fe2O3/H2O2 under the same experimental conditions. Furthermore, with the introduction of visible light irradiation, the decoloration efficiency reaches 73.9% for FeCG/H2O2, still ~80.2% higher than that of the α-Fe2O3/H2O2 system. Furthermore, the AO7 decoloration rate can be expressed as follows:

−

&'7) = & )+ &'7), (4) *

where k is the intrinsic reaction rate coefficient, and m and n are the reaction orders towards hydrogen peroxide and AO7, respectively. Considering that H2O2 has an extremely strong chemical interaction with α-Fe2O3, in addition to the excessive dosages, the change in the amount of surface peroxide species here is limited 44. Approximately,

−

&'7) = . &'7), (5) *

where ka is the apparent reaction rate constant. The result shows that the AO7 concentration decrement can be well-fitted in apparent zero order kinetics. Additionally, the ka was determined to be 0.384 mg/(L·min) for FeCG/H2O2, an ~65.5% improvement over α-Fe2O3/H2O2 under visible light irradiation, as shown in Fig. 10 (b). Notably, the ka value was still calculated as 0.197 mg/(L·min) for FeCG/H2O2 even in the dark, while it was zero for α-Fe2O3/H2O2 and presented much superiority over the novel FeCG catalyst. 54

60 (a)

45 H2O2 a -Fe2O3/H2O2

30

CG/H2O2 FeCG100/H2O2

36

FeCG100 + visible light

18

H2O2+ visible light

15

a -Fe2O3/H2O2+ visible light CG/H2O2 + visible light

0

(b) FeCG100 a -Fe2O3+ visible light

C0-C (mg/L)

Concentration (mg/L)

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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Fig. 10 (a) Comparison of AO7 decoloration under different conditions with a series of reaction times (b) linear fitting of concentration versus reaction time [AO7 initial concentration 60 ppm, AO7 volume 25 mL, H2O2 concentration 160 mmol/L, catalyst dosage 0.1 g] 3.4 DMPO spin trapping EPR observation and possible mechanism discussion To gain a deep insight into the possible formation of active oxygen species for the prepared catalyst in the presence of H2O2, a DMPO spin trapping EPR spectroscopy measurement was accordingly carried out. As shown in Fig. 11, a typical four-peak signal of the DMPO–•OH adducts with an intensity ratio of 1:2:2:1 was observed for the α-Fe2O3/H2O2 system at 5, 8 and 11 min, which means that the hydroxyl radical is the major active intermediate throughout the reaction. This is well known to be explained by the reaction of Fe(II) with hydrogen peroxide, giving rise to hydroxyl radicals 9, 26 . Interestingly, identical DMPO–•OH adducts were also detected for the CG/H2O2 system at the initial 5 min. Moreover, different signals arose beginning at 8 min, which seem to be DMPO-·HO2 adducts

45

. This result implies that the CG support itself can also promote the

decomposition of H2O2, resulting in the formation of reactive radicals. Most importantly, after the loading of α-Fe2O3 onto the CG support, the DMPO–•OH signals with the highest intensity were observed at 5 min. In addition, the DMPO–•OH signal decay and DMPO-·HO2 signals accordingly grow strong after 11 min. From Fig. 11 (a), it can also be clearly seen that the intensity of the signals of the hydroxyl radicals follow the sequence FeCG100/H2O2 > CG/H2O2 > α-Fe2O3/H2O2, which is consistent with the abovementioned trend in the catalytic activity under the experimental conditions.

15



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Fig. 11 DMPO spin trapping EPR spectra of α-Fe2O3/H2O2, CG/H2O2 and FeCG100/H2O2 system at (a) 5 min; (b) 8 min; and (c) 11 min. (*) DMPO-·OH; (◊) DMPO-·HO2 Accordingly, the possible reaction pathways for FeCG/H2O2 system can be described below: 01

2 !" · + · + (6) Fe(III) + ·→ Fe(II) + + (7) Fe(II) + → Fe(III) + · + 3 (8) Specifically, in the presence of CG support, the H2O2 can decompose quickly into ·OH and ·HO2 radicals and the decoloration of AO7 can thus be initiated. Most importantly, the Fe(III) on the surface of α-Fe2O3 can also react with ·HO2 radicals to produce Fe(II), which accelerates the formation of ·OH radicals through the followed reaction between Fe(II) and H2O2. Under light irradiation, not only a photoreduction process occurs as described in Equations (2) and (3), and the 16


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reaction described in Equation (6) will also be enhanced, as observed from ~71.9% improvement in abovementioned experiments, resulting in higher catalytic activity. In addition, Feng et.al.10 employed UV irradiation to accelerate the AO7 degradation in α-Fe2O3/H2O2 system, where a decoloration rate (equal to the decoloration efficiency divided by reaction time and catalyst concentration) of 0.05 L h-1g-1 can be achieved. In this work, a value of 0.097 L h-1g-1 was obtained for FeCG100/ H2O2, improving by ~94%, in the degradation of AO7. To conclude, the introduction of CG support speeds up the redox recycling between Fe(II) and Fe(III) on the surface of α-Fe2O3, as shown in Fig. 12, which is the rate controlling step of the Fenton reaction 9. Therefore, this novel FeCG heterogeneous Fenton-like catalyst present superior catalytic activity.

Fig. 12. Possible reaction pathways of FeCG catalyst in the presence of H2O2 3.5 Cyclic Stability of the Prepared Catalyst It is well known that the cyclic stability of a catalyst plays a key role in its practical application, and thus this was also examined in detail in this work. It should be noted that Azo dye AO7 was acidic in nature and seemingly in favor of dissolution of the supported α-Fe2O3 nanoparticles. Therefore, the leaching of iron during the reaction process was studied in the first place. As shown in Fig. 13, the total dissolved iron was found to increase with an increasing initial Fe loading amounts of the FeCG catalysts. However, in the case of FeCG100/AO7 with an optimized Fe loading, and the detected concentration of dissolved Fe in the reaction process was only 1.26 mg/L, which was much lower than the legal limit of 2 mg/L 46 imposed by the directives of the European Union, indicating 4.6% Fe leaching and good stability of the supported nano-Fe2O3 in acidic solution.

17



Concentration (mg/L)

2.5 2.0 1.5 1.0 0.5 0.0

FeCG50

FeCG100

FeCG200

FeCG400

Sample

Fig. 13 Fe leaching detection of different FeCG samples during AO7 decoloration process [AO7 initial concentration 60 ppm, AO7 volume 25 mL, H2O2 concentration 80 mmol/L, catalyst dosage 0.1 g, reaction time 10 h] Furthermore, the experiment was also conducted for six successive cycles each lasting 2 h, where each run was performed with a fixed AO7 concentration of 60 ppm, an H2O2 concentration of 160 mmol/L and 0.1 g of catalyst. The recovery of the FeCG catalyst was through natural settling, followed by deionized water washing and drying procedures. As shown in Fig. 14, no remarkable decrease in the AO7 decoloration efficiency was observed even after six cycles compared to the fresh catalyst, indicating the favorable stability of the prepared FeCG catalyst.

0.9

Decolouration Efficiency

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

0.6

0.3

0.0

1

2

3

4

Run times

5

6

Fig. 14 Cyclic performance of AO7 decoloration for FeCG100/H2O2 system [AO7 initial concentration 60 ppm, AO7 volume 25 mL, H2O2 concentration 160 mmol/L, catalyst dosage 0.1 g, reaction time 2 h, light irradiation]

4. Conclusion In this work, novel heterogeneous Fenton-like α-Fe2O3/ crystalline glass microsphere catalysts were prepared, characterized and applied in the AO7 decolorization in the presence of 18


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H2O2 for the first time. After hydrothermal treatment, glass microspheres were chosen as the support, which not only improved the catalytic oxidation performance of the α-Fe2O3 nanoparticles but also achieved the goal of recovery with great ease. Moreover, Fe loading amounts varying from 2.8 mg/g to 10.2 mg/g with mean particle sizes of α-Fe2O3 nanoparticles ranging from 2.6 nm to 6.5 nm can be controllably obtained. In addition, the bimodal pore size distribution tends to be a single pore size distribution with increased Fe loading for the as-prepared catalyst. Notably, the AO7 decoloration efficiency could reach 49.8% for FeCG100/H2O2 in the dark, while it is inactive for α-Fe2O3/H2O2 under the same experimental conditions. Furthermore, the pseudo-zero order reaction constant was determined to be 0.384 mg/(L·min) for FeCG100/H2O2 under visible light irradiation, an ~65.5% improvement over α-Fe2O3/H2O2. The introduction of CG support brings active HO2· radical species, speeding up the redox recycling between Fe(II) and Fe(III) on the surface of α-Fe2O3 and resulting in much higher catalytic activity. Additionally, only 1.26 mg/L Fe leaching was observed under optimum conditions, and no remarkable decrease in the AO7 decoloration efficiency was observed after six cycles, indicating the favorable stability and feasibility of practical application.

Acknowledgements We gratefully acknowledge the support of the National Basic Research Program of China (2013CB733600) and the National Natural Science Foundation of China (21276140, 20976069 and 21036002).

Supporting Information Comparisons in specific surface areas of different supported α-Fe2O3 catalysts, AO7 decoloration under different H2O2 concentrations and evolved UV-vis spectra during AO7 decoloraiton in the presence of FeCG100/H2O2.

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Graphic Abstract: 23



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Controllable Preparation and Catalytic Performance of Heterogeneous

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