Ordered mesoporous hematite promoted by magnesium selective leaching as a highly efficient heterogeneous Fenton-like catalyst

Abstract

Ordered mesoporous hematite with an ultrahigh surface area (up to 200 m² g⁻¹) was prepared through a hard templating method of Mg and Fe in mesoporous silica KIT-6 (meso-Mg/Fe₂O₃) and used for highly efficient wet peroxide oxidation of methylene blue. The obtained results showed that approximately two thirds of Mg cations were removed in the leaching process, resulting in a highly porous hematite with a significant amount of defects in the structure. The activated mesoporous iron oxide exhibited excellent catalytic activity for the degradation of methylene blue, achieving above 95% removal of 60 mg L⁻¹ methylene blue after 3 h at reaction conditions of initial pH, 0.6 mg L⁻¹ catalyst and 2600 mg L⁻¹ H₂O₂ dosage. The apparent rate constant of used meso-Mg/Fe₂O₃ is 1.972 h⁻¹, which is 1.22, 3.02 and 4.53 times those of meso-Fe₂O₃, con-Mg/Fe₂O₃ and α-Fe₂O₃, respectively. With the increase of reusability of the meso-Mg/Fe₂O₃ catalyst, both the leaching concentration of Mg and the catalytic activity of the catalyst increased, which is quite different with the catalytic mechanism of composite components in heterogeneous Fenton-like processes, such as Fe–Cu and Fe–Zn composites. The leaching concentrations of iron in the catalysts were found to be low (<5 mg L⁻¹) in consecutive runs. Hence, the meso-Mg/Fe₂O₃ catalyst has proved to be an attractive alternative in the treatment of environmental refractory organic pollutants and has a unique and superb catalytic activity in the heterogeneous Fenton-like system.

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Introduction

The conventional Fenton process requires strong acidic conditions (pH < 3) to prevent the hydrolysis of ferrous and ferric ions and to maintain acceptable conversion rates. In addition, non-recyclable soluble iron salts also yield large amounts of iron oxide sludge, which is regarded as a secondary pollutant and the loss of catalyst.^1,2 In order to overcome the abovementioned disadvantages, heterogeneous Fenton-like catalysts have been investigated widely. In the past decade, several iron-based catalysts coupled with H₂O₂ for heterogeneous Fenton-like catalysis have been investigated,³ i.e. iron oxides and hydroxides⁴ iron composites mixed with suitable transition metal such as Co, Mn, Cu, Cr, and Zn,⁵ supported iron catalysts, including oxide supported iron catalysts,⁶ carbon material supported iron catalysts,^7–10 Fe exchanged zeolites, iron exchanged resin, industrial solid waste supported Fe catalysts, and mesoporous material supported Fe catalysts.¹¹ However, the heterogeneous Fenton-like activities of these catalysts were relatively slow if they were operated at high pH values or without external power supplies such as UV, ultrasound or microwave. Moreover, the leaching of active metals (such as Cu, Co and Cr) also leads to the depletion of their catalytic activity and environmental pollution problems. To overcome these drawbacks of heterogeneous Fenton-like systems, the newly developed heterogeneous catalysts should be recyclable in a broad pH range (pH = 3.0–9.0) and stable enough to avoid the serious leaching of active metals during operation.¹² Therefore, the key problem is how to design and developed heterogeneous Fenton-like catalysts, which can effectively generate ˙OH by H₂O₂ decomposition with broad usability and high durability.

Recently, various strategies have been proposed to enhance the performance of heterogeneous Fenton-like catalysts,¹³ e.g. loading the catalysts on carriers with high surface area to improve their dispersion¹⁴ and reducing the size of the catalyst to nano-scales to increase surface energy.¹⁵ Among these methods, mesoporous iron oxides are of particular interest because they combine large internal surface area, nanosized walls, uniform pore size distributions, large pore volume, controllable compositions and surface functionalities.¹⁶ For example, Cornu C. et al. found that the loading of Fe to mesoporous SBA-15 could greatly enhance the degradation performance for methylene blue and studied the formation and location of iron species (including Fe²⁺ and Fe³⁺) in Fe/SBA-15 catalysts.¹⁷ Xia et al. introduced alumina into Fe–Cu bimetallic oxide supported MCM-41 catalysts to obtain high degradation activities for phenol.¹⁸ However, these methods still need to be improved to increase the dispersion of metals active sites on the support and overcome the vulnerable leakage problems of these metals.^19,20 In addition, a wide range of iron-based metal composites with highly ordered mesostructures have been successfully synthesized with a hard templating (nanocasting) method. Some of them exhibited unique heterogeneous Fenton-like catalytic properties.^21,22 Wang et al. prepared ordered mesoporous copper ferrite (meso-CuFe₂O₄) with a KIT-6 hard templating method, which can efficiently increase the degradation activity for imidacloprid.²³ Su et al. reported that the substitution of Fe²⁺ by Zn²⁺ can remarkably increase the activity of mesoporous Fe₃O₄ nanoparticles.²⁴ However, the leaching of active metals (Fe, Cu, Zn et al.) from these mesoporous catalysts was also inevitable, which undoubtedly leads to the reduction of their catalytic activity in the long term.

As we know, the leaching of Mg from mesoporous catalysts was unavoidable and even more serious than active metals such as Fe, Cu, Zn.²⁵ Hu et al. reported that the leaching concentration of Mg from mesoporous CoMg/SBA-15 was much higher than that of Co. Interestingly, BET surface area and pore volume also increased after Mg selective leaching in this catalyst system.²⁶ Jiao et al. reported the fabrication of mesoporous Co₃O₄ with Mg substitution, which exhibited high oxygen evolution activities in a visible-light-driven water oxidation system. Approximately one third of Mg cations were removed in the leaching process, resulting in a highly porous cobalt oxide with a significant amount of defects in the spinel structure.²⁷ In this report, the Mg components in iron-based mesoporous structure are not used as the active metals for the heterogeneous Fenton-like catalysts, but implied to facilitate the leaching process of mesoporous α-Fe₂O₃ substituted by Mg ions. During the selective leaching of Mg ions, the porous structure of α-Fe₂O₃ still remained. Moreover, the surface area of the catalyst was also increased in favor of the adsorption of the macromolecules (scheme shown in Fig. 1).^16,27

Fig. 1 Schematic illustration of mesoporous Mg–Fe₂O₃ through Mg ions leaching. Arrows show the Mg leaching sites (i.e., defect sites).

In this study, highly ordered mesoporous magnesium-substituted α-Fe₂O₃ (meso-Mg/Fe₂O₃) was successfully synthesized through the nanocasting strategy and used as a heterogeneous Fenton catalyst with broad usability and enhanced durability. In addition, methylene blue (MB) is a typical synthetic cationic dye, which is non-biodegradable, extensively used in textile industry and usually chosen as a model contaminant in the relative studies, whose presence in wastewater may cause the risk of nausea, vomiting and burning of the eye.¹¹ Therefore, MB was selected as a model compound to monitor the Fenton catalytic activity of meso-Mg/Fe₂O₃. The purpose of this study was to elucidate the effect of ordered mesoporous structure and the role of magnesium on the catalytic activity of meso-Mg/Fe₂O₃. For comparison, conventional α-Fe₂O₃, ordered α-Fe₂O₃ and conventional Mg–Fe mixed metal oxides were synthesized as reference catalysts. According to the detected ˙OH radical and surface reaction revealed by physico-chemical characterizations, the possible activation mechanism of meso-Mg/Fe₂O₃ was proposed. Finally, the stability and reusability of the catalyst was also investigated.

Experimental

1. Materials

All chemicals required to prepare the mesoporous KIT-6 silica and derived Mg substituted Fe₂O₃ catalysts were used as purchased. Tetraethylorthosilicate (TEOS) and triblock copolymer Pluronic P123 (M_w = 5800, EO₂₀PO₇₀EO₂₀) was purchased from Sigma-Aldrich. Ferric nitrate (Fe(NO₃)₃·9H₂O), magnesium nitrate (Mg(NO₃)₂·6H₂O), butyl alcohol, hydrochloric acid (HCl), sodium hydroxide (NaOH), and n-hexane were obtained from Tianjin Guangfu Chemical Reagent Co., Ltd., China. H₂O₂ (30%, v/v). Ethanol, isopropanol and methylene blue (MB) were supplied by Shanghai Aladdin Chemical Reagent Co., Ltd, China. Deionized water was used throughout the experiments.

2. Synthesis of ordered mesoporous Mg-substituted α-Fe₂O₃

Ordered mesoporous Mg-substituted α-Fe₂O₃ was synthesized by a nanocasting strategy with mesoporous silica KIT-6.²⁷ Preparation of mesoporous silica KIT-6 was synthesized according to the previous literature.²⁸ In a typical synthesis, 0.6 g as-prepared mesoporous silica KIT-6 was dispersed in 5 mL toluene and stirred for 30 min, then 1.139 g Fe(NO₃)₃·9H₂O and 0.362 g Mg(NO₃)₂·6H₂O at an Fe/Mg molar ratio of 2 were added and refluxed for 6 h at 343 K. After refluxing, the resulting mixture was dried in an oven at 333 K overnight and calcined at 873 K for 3 h (heating rate 1 K min⁻¹). Finally, the silica template KIT-6 was dissolved by 2 M NaOH in 333 K under continuously stirring for 24 h, and the red product was produced by washing with distilled water and ethanol and drying in a vacuum oven. Pure α-Fe₂O₃ mesoporous sample was also prepared by the same synthetic approach. The conventional Mg–Fe mixed metal oxides without mesoporous structure were prepared using the traditional solid-phase method, the specific steps are as follows: the corresponding mixture in stoichiometric proportions of Fe(NO₃)₃·9H₂O and Mg(NO₃)₂·6H₂O were calcined at 873 K for 3 h with a heating rate of 1 K min⁻¹, and the product was denoted as con-Mg/Fe₂O₃.

3. Characterization

Powder X-ray diffraction (XRD) patterns were measured on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (λ = 0.154 nm) as the X-ray source. High resolution transmission electron microscopy (HRTEM) images was collected on a Hitachi H-7650 transmission electron microscope with field emission gun at 100 kV. The surface morphology of the solid samples were investigated on a Hitachi S-4800 Scanning Electron Microscope (SEM). N₂ adsorption–desorption isotherms were measured at 77 K using a Micromeritics Tristar 3000 sorptometer outgassed at 423 K for a minimum of 12 h. The specific surface area and the pore size distribution curves are obtained using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods from the adsorption data. Fourier transform infrared (FT-IR) spectroscopy were performed to assess different functional groups of the materials using KBr pressed disks with a Bruker TENSOR37 FT-IR spectrometer transmission analyzer. X-ray photoelectron spectroscopy (XPS) was recorded on a Kratos ASIS-HS X-ray photoelectronspectroscope with an Al Kα source at 150 W (15 kV, 10 mA).

4. Catalytic tests

Catalytic tests were carried out under mild conditions (atmospheric pressure and ∼298 K) in a thermostatic reactor of 250 mL with continuous stirring. The reaction was performed in the kinetic regime and no external and internal diffusional resistances occurred. The initial reaction pH was 7.2 and could be adjusted by adding 0.1 mol L⁻¹ H₂SO₄ to the desired pH during the degradation processes. In a typical experiment, 50 mL of MB solution was prepared with an initial concentration of 60 mg L⁻¹, and given amounts of hydrogen peroxide and catalyst were added at reaction time = 0 min. At regular intervals, 1.0 mL sample was collected in 1.5 mL centrifuge tubes and immediately mixed with 0.1 mL isopropanol to quench ˙OH. It was centrifuged three times (5 min each) at 13 400 rpm to separate the catalysts, and the supernatant liquid was collected for analysis.

5. Sample analysis

The concentration of MB in the supernatant liquid was determined at an absorbance peak of 664 nm using a UV-Vis spectrophotometer (UV-752, Shanghai Rex Instrument Co., Ltd., China). ˙OH concentration was determined by the terephthalic acid (C₈H₆O₄, TA) fluorescence method.²⁹ The experimental procedure was similar to the measurement of Fenton catalytic activity except that the MB aqueous solution was replaced by 5 × 10⁻⁴ mol L⁻¹ terephthalic acid aqueous solution with a concentration of 2 × 10⁻³ mol L⁻¹ NaOH. The photoluminescence spectra (PL) of TA samples were obtained using a fluorescence spectrophotometer (F-380, Tianjin Gangdong Instrument Co., Ltd., China) at room temperature, and a wavelength excitation of 325 nm. The concentration of Fe(II) ions was determined by reacting the recovered solution after testing with 1,10-phenanthroline (maximum of absorption at 510 nm) by a Varian Cary 50 UV-Vis spectrophotometer. The Fe(III) concentration was determined by reaction with thiocyanate (maximum of absorption at 480 nm) under acidic conditions.³⁰ The variation of H₂O₂ concentration during the reaction was analyzed colorimetrically by the same UV-Vis spectrophotometer after complexation with titanium salt.³¹

Result and discussion

1. Characterization of as-prepared samples

In order to investigate the structure–performance relationship of meso-Mg/Fe₂O₃, the morphology of as-prepared catalyst samples was characterized by SEM and the images are shown in Fig. 2. It can be seen that the size of these catalyst samples is homogeneous. By comparison between meso-Mg/Fe₂O₃ (Fig. 2C and E) and the KIT-6 templates (Fig. 2A), an increased roughness of catalyst surface in meso-Mg/Fe₂O₃ could be observed. The morphology of meso-Mg/Fe₂O₃ is also very different from meso-Fe₂O₃ and con-Mg/Fe₂O₃ (Fig. 2B–D). The former still maintained a similar surface morphology of KIT-6, and the latter was composed of particles with 200–1000 μm in diameter. After the recycling of meso-Mg/Fe₂O₃ and con-Mg/Fe₂O₃ (Fig. 2E and F), the catalyst particles aggregated and became larger. To further investigate the mesostructure of meso-Mg/Fe₂O₃, representative TEM images of as-synthesized catalyst samples are depicted in Fig. 3. From Fig. 3A–C, the mesoporous structure of meso-Mg/Fe₂O₃ is clearly visible even after the replication of KIT-6.^32,33 Fig. 3D shows clear crystal lattice fringes, which indicate that meso-Mg/Fe₂O₃ was synthesized with a highly crystalline nature. After the recycling of the meso-Mg/Fe₂O₃ (Fig. 3E and F), the crystalline nature of the catalyst still remained.²¹

Fig. 2 SEM images of KIT-6 (A), meso-Fe₂O₃ (B), meso-Mg/Fe₂O₃ (C), con-Mg/Fe₂O₃ (D), postreaction meso-Mg/Fe₂O₃ (E) and postreaction con-Mg/Fe₂O₃ (F).

Fig. 3 TEM images of KIT-6 (A), meso-Fe₂O₃ (B), meso-Mg/Fe₂O₃ (C), high resolution meso-Mg/Fe₂O₃ (D), postreaction meso-Mg/Fe₂O₃ (E) and high resolution postreaction meso-Mg/Fe₂O₃ (F).

The successful preparation of meso-Mg/Fe₂O₃ is further confirmed by low-angle powder XRD analysis, as shown in Fig. 4A. The prepared meso-Mg/Fe₂O₃ catalyst indicated the excellent replication of KIT-6 templates. Mesoporous α-Fe₂O₃ was also in good agreement with the values reported in the literature.³⁴ Compared with as-made meso-Mg/Fe₂O₃, there was a decrease in intensity of the diffraction peaks for the post-reaction meso-Mg/Fe₂O₃, suggesting the increased hematite particle dispersion, which may be attributed to the decrease in electron density contrast due to the increased Mg leaching during the reaction process.²⁶ Wide-angle XRD results (including meso-Fe₂O₃, meso-Mg/Fe₂O₃, con-Mg/Fe₂O₃ and post-reaction meso-Mg/Fe₂O₃) are shown in Fig. 4B. The 2θ values of meso-Mg/Fe₂O₃ at 24.09°, 33.07°, 35.55°, 40.76°, 49.34°, 53.93°, 57.31°, 62.28° and 63.85° could be indexed to the (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (1 2 2), (2 1 4) and (3 0 0) planes of hexagonal hematite (α-Fe₂O₃, JCPDS no. 33-0664), respectively. The diffraction pattern for meso-Mg/Fe₂O₃ closely matches the standard α-Fe₂O₃, confirming the success of magnesium substitution to iron in the spinel. The peaks of meso-Mg/Fe₂O₃ are relatively broader than the peaks of con-Mg/Fe₂O₃, which is possibly due to the pore walls that were composed of small particles. The average crystallite size of meso-Mg/Fe₂O₃ and post-reaction meso-Mg/Fe₂O₃ was determined and calculated to be 7.3 and 5.1 nm, respectively, using the Scherrer equation:

where D_hkl, λ, β_hkl, and θ_hkl were the volume-averaged particle diameter, X-ray wavelength, full width at half maximum (FWHM) and diffraction angle, respectively.

Fig. 4 (A) Low-angle XRD patterns of meso-Fe₂O₃, meso-Mg/Fe₂O₃ and postreaction meso-Mg/Fe₂O₃; (B) high-angle XRD patterns of α-Fe₂O₃, con-Mg/Fe₂O₃, meso-Fe₂O₃, meso-Mg/Fe₂O₃ and postreaction meso-Mg/Fe₂O₃.

Textural properties of mesoporous catalysts were investigated by N₂ adsorption–desorption measurements, as shown in Fig. 5. The corresponding parameters, including BET surface area, pore volume and average pore size, are listed in Table 1. As shown in Fig. 5A, the isotherm of meso-Mg/Fe₂O₃ showed a type IV isotherm at P/P₀ about 0.5, indicating a typical shape of mesoporous materials (Fig. 5A). The hysteresis shaping started from about 0.65 and extended almost to 0.95, indicating their large porosity.³⁵ The specific surface area (90.7 m² g⁻¹) and the pore diameter (9.1 nm) are consistent with previous reports on others mesoporous metal oxides, which indicates the successful fabrication of mesoporous Mg/Fe₂O₃.

Fig. 5 (A) N₂ adsorption–desorption isotherms for meso-Mg/Fe₂O₃, acid washed meso-Mg/Fe₂O₃ and postreaction meso-Mg/Fe₂O₃; (B) pore size distributions from the desorption branches through the BJH method for meso-Mg/Fe₂O₃, acid washed meso-Mg/Fe₂O₃ and postreaction meso-Mg/Fe₂O₃.

Table 1 The structural and chemical properties of meso-Mg/Fe₂O₃ and selected iron-based catalysts^a

Catalysts	SBET (m^2 g^−1)	Vpore (cm^3 g^−1)	Vmicro (cm^3 g^−1)	Dp (nm)	Mg wt%	Fe wt%	Mg to Fe ratio
a Fe content in wt%: as determined by ICP-AES analysis; Vpore: mesopore volume measured at P/P0 = 0.97; Vmicro: micropore volume calculated from t-plot; Dp: BJH average pore size calculated on the adsorption branch.
KIT-6	575	1.08	0.23	6.8	N/A	N/A	N/A
Meso-Fe2O3	96.3	0.32	0.043	8.6	N/A	76.9	N/A
Meso-Mg/Fe2O3	90.7	0.37	0.056	9.1	11.0	43.3	0.59
Postreaction meso-Mg/Fe2O3	190.4	0.44	0.064	12.7	3.67	61.2	0.14
Acid washed meso-Fe2O3	103.7	0.35	0.049	9.4	N/A	72.3	N/A
Acid washed meso-Mg/Fe2O3	203.9	0.41	0.061	10.2	3.98	58.7	0.16
Con-Mg/Fe2O3	9.3	0.06	0.007	—	16.3	68.7	0.54

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The surface physico-chemical characteristics of the as-prepared catalysts were further explored by FT-IR analysis. According to the infrared active modes polarized to the c-axis of the hexagonal crystal system of α-Fe₂O₃, the typical adsorption peaks of α-Fe₂O₃ had a broad absorption at around 560 cm⁻¹ and a narrower one at around 480 cm⁻¹.^36,37 In Fig. 6A, the dominant bands of the meso-Mg/Fe₂O₃ (at about 537 cm⁻¹ and 471 cm⁻¹) are typical characteristic of crystalline α-Fe₂O₃. Interestingly, the dominant absorption peak at 568 cm⁻¹ of α-Fe₂O₃ shifts to somewhat lower frequencies at 551 cm⁻¹ of meso-Fe₂O₃ and 537 cm⁻¹ of meso-Mg/Fe₂O₃. Moreover, the above absorption peak also obviously becomes broader than α-Fe₂O₃ from Fig. 6A. This might be due to the changes in the structure of the catalyst, since the catalyst consists of large numbers of small subparticles in the mesoporous structure.³⁸ Thus, when the average crystallite diameter of subparticles is reduced, the absorption peaks become broader. These results are consistent with XRD and TEM. In Fig. 6B, the absorption peak at 540 cm⁻¹ of post-reaction meso-Mg/Fe₂O₃ hardly changes compared with the one of as-made meso-Mg/Fe₂O₃, suggesting that the catalyst has not been affected by our experimental conditions.

Fig. 6 FT-IR spectra of (A) bulk Fe₂O₃, meso-Fe₂O₃, meso-Mg/Fe₂O₃, con-Mg/Fe₂O₃, and (B) con-Mg/Fe₂O₃, meso-Mg/Fe₂O₃ and postreaction meso-Mg/Fe₂O₃ (for interpretation of the references in color for this figure legend, the reader is referred to the web version of this article).

2. Efficient MB degradation performance in the presence of meso-Mg/Fe₂O₃

In the previous studies, α-Fe₂O₃ has been proven to be easily recyclable and an efficient catalyst for the degradation of MB, therefore control experiments for the meso-Mg/Fe₂O₃–H₂O₂–MB system were evaluated.^39,40 The external and internal diffusional resistances of the degradation processes were excluded, and the reaction was performed in the kinetic regime before the analysis. Prior to the addition of H₂O₂, the mixed solution of MB and the catalysts was stirred in the dark for 30 min to establish the equilibrium adsorption state.²⁶

A series of experiments were performed by varying the catalyst loading of meso-Mg/Fe₂O₃ from 0.2 g L⁻¹ to 0.6 g L⁻¹ and the result is depicted in Fig. 7A. From Fig. 7A, the degradation efficiency increased with the meso-Mg/Fe₂O₃ catalyst loading. At 0.6 g L⁻¹ of meso-Mg/Fe₂O₃, MB degradation reached almost 98% in 180 min.⁴¹ Moreover, the data for MB concentration decay were further analyzed by the first order kinetic equation, which could be expressed as

where k′ is the apparent reaction constant and C₀ and C are the initial concentration and the concentration at time t, of MB, respectively. The degradation of MB follows pseudo-first order reaction kinetics. The apparent rate constant of used meso-Mg/Fe₂O₃ is 1.972 h⁻¹, which is 1.22, 3.02 and 4.53 times than those of meso-Fe₂O₃, con-Mg/Fe₂O₃ and α-Fe₂O₃. The rate constant k′ increased linearly with increasing catalyst dosage (see Fig. 7B and C).

Fig. 7 (A) Removal efficiency of methylene blue in the presence of meso-Mg/Fe₂O₃ 0.4 g L⁻¹ (a), H₂O₂ (2600 mg L⁻¹) (b), meso-Mg/Fe₂O₃ (0.2 g L⁻¹) + H₂O₂ (c), meso-Mg/Fe₂O₃ (0.4 g L⁻¹) + H₂O₂ (d) and meso-Mg/Fe₂O₃ (0.6 g L⁻¹) + H₂O₂ (e); (B) kinetic curves of methylene blue degradation on the dependence of ln(C/C₀) versus time; (C) kinetic analysis of methylene blue degradation on the dependence of meso-Mg/Fe₂O₃ catalyst loading (initial pH, H₂O₂ dosage = 2600 mg L⁻¹ and initial MB concentration = 60 mg L⁻¹).

During the heterogeneous Fenton degradation of MB, it should be noted that the adsorption of MB by meso-Mg/Fe₂O₃ could achieve about 9% (Fig. 7A, curve (a)), which is due to the high specific surface area of meso-Mg/Fe₂O₃ (90.7 m² g⁻¹). The experiment in homogeneous systems was also performed. After vigorous agitation for 4 h, the meso-Mg/Fe₂O₃ catalyst was removed and H₂O₂ was added into the clear filtrate (380 μL, 30 wt%). As shown in Fig. 7A and 8A, the removal of MB by adsorption was 9%, the removal of MB by pure H₂O₂ at 6 h was 12%, while the removal of MB catalyzed by leaching Fe ions (concentration ∼5 mg L⁻¹) was 25%. Therefore, the contribution of homogeneous Fenton reaction was 3%. The results of this experiment clearly indicate that the production of hydroxyl radicals observed primarily occurs at the nanoparticle surface rather than being catalyzed by the Fe ions leached into the filtrate. To accurately estimate the catalytic activity of meso-Mg/Fe₂O₃, comparable amounts of FeSO₄ (Fe ions concentration 60 mg L⁻¹) were used to catalyze the degradation of MB. The results show that 60 mg L⁻¹ MB could be completely degraded in 30 min (Fig. 8A). These results indicated that the contribution of the Fenton-like reaction was mainly dominated by the meso-Mg/Fe₂O₃ catalysts. From Fig. 8B, four types of catalysts, with the same Fe loading, display distinctly different catalytic activities (meso-Mg/Fe₂O₃ > meso-Fe₂O₃ > con-Mg/Fe₂O₃ > α-Fe₂O₃). The excellent catalytic activity of mesoporous catalysts strongly originates from the ordered mesoporous network structure and large pore size, which contributes to the raised numbers of accessible active sites of the catalysts and increased mass transfer of MB in the catalysts porosity.⁴²

Fig. 8 (A) The degradation efficiency of methylene blue with different catalyst during the reaction. Homogeneous Mg²⁺ (5 mg L⁻¹) (a), homogeneous Fe²⁺ (5 mg L⁻¹) (b), α-Fe₂O₃ (c), con-Mg/Fe₂O₃ (d), meso-Fe₂O₃ (e), meso-Mg/Fe₂O₃ (0.6 g L⁻¹) (f) and homogeneous Fe²⁺ (60 mg L⁻¹) (g); (B) the kinetic curves of methylene blue degradation on the dependence of In(C/C₀) versus time (initial pH, H₂O₂ dosage = 2600 mg L⁻¹ and initial MB concentration = 60 mg L⁻¹).

It is well known that pH value is one of most significant factors in the Fenton reaction system.^43,44 Investigation and extension of pH range on the performance of the heterogeneous Fenton-like catalyst is of crucial importance for the development of new heterogeneous Fenton-like processes.¹⁶ As shown in Fig. 9, the initial rate was relatively slow at pH 3.5, and then it followed a fast degradation rate. In case of pH 7.5, the rate of reaction was very fast and the conversion of MB was 98% over a period of 180 min. The results indicated that the reaction performance is highest at around pH 7.5 and decreased with increasing pH. In general, the homogeneous Fenton system shows best performance at pH ≈ 3.⁴⁵ At neutral and even higher pH, formation of Fe(OH)₃ precipitate (pK_sp = 37.4) prevents the catalysis reaction proceeding. However, the addition of Mg into heterogeneous Fenton catalysts could expand its degradation pH range during the reaction processes. The substitution of Mg on heterogeneous Fenton catalysts makes the slightly acidic surface of catalysts become slightly basic.²⁶ These basic sites on the catalyst surface can promote the formation of the surface Fe–OH complex, which is crucial in activating H₂O₂.^46,47 Therefore, compared with the conventional Fenton reaction, meso-Mg/Fe₂O₃ could be used over a wide pH range.

Fig. 9 Effect of initial pH on degradation of methylene blue with Mg/Fe₂O₃ suspensions: pH 3.5 (a), pH 5.5 (b), pH 7.5 (c), pH 8.5 (d) and pH 9.5 (e) (H₂O₂ dosage = 2600 mg L⁻¹, initial MB concentration = 60 mg L⁻¹ and catalyst dosage = 0.6 g L⁻¹).

The remained TOC after total MB degradation with different catalyst could be attributed to organic acids accumulated in the solution (Fig. 10). It can be observed that pure H₂O₂ and α-Fe₂O₃ did not produce a significant TOC reduction. Con-Mg/Fe₂O₃ and meso-Fe₂O₃ showed 22% and 31% TOC reduction, respectively. In addition, meso-Mg/Fe₂O₃ produced the highest TOC reduction of about 54% after 280 min reaction. In order to identify the end products, degraded samples of MB were analyzed by ion exclusion chromatography (results not reported). Oxalic acid and formic acid were identified as byproducts of the aromatic ring cleavage and are often detected after degradation of MB.⁴³

Fig. 10 Temporal changes of TOC during the degradation of methylene blue with different catalyst. H₂O₂ (2600 mg L⁻¹) (a), α-Fe₂O₃ (b), con-Mg/Fe₂O₃ (c), meso-Fe₂O₃ (d) and meso-Mg/Fe₂O₃ (0.6 g L⁻¹) (e) (initial pH, H₂O₂ dosage = 2600 mg L⁻¹ and initial MB concentration = 60 mg L⁻¹).

In order to compare the effect of the meso-Mg/Fe₂O₃–H₂O₂ Fenton system and H₂O₂ on the ˙OH radicals concentration, experiments using different heterogeneous Fenton catalysts are researched under the same conditions. The generated ˙OH radicals in the catalytic process were investigated according to the reaction ˙OH and terephthalic acid producing 2-hydroxyterephthalic acid. The PL intensity at 425 nm gradually rises with increasing irradiation time in the presence of different catalysts (Fig. 11). From Fig. 11D, the ˙OH intensity of meso-Mg/Fe₂O₃ is higher than that of the individual meso-Fe₂O₃ and con-Mg/Fe₂O₃. Therefore, the degradation activity of the meso-Mg/Fe₂O₃ could also be expected to be higher than that of meso-Fe₂O₃ and con-Mg/Fe₂O₃. The abovementioned results also account for the higher catalytic activity of meso-Mg/Fe₂O₃ in comparison with that of α-Fe₂O₃. From Fig. 11C, the generation rates of ˙OH radicals in con-Mg/Fe₂O₃ and meso-Mg/Fe₂O₃ were slower than that of α-Fe₂O₃ and meso-Fe₂O₃ at the beginning of the reaction. After 60 min, in sharp contrast, the abovementioned Mg-substituted samples (especially meso-Mg/Fe₂O₃) showed much higher generation rates compared to the pure α-Fe₂O₃ and meso-Fe₂O₃. This indicates that the initial surface structure of the as-made catalyst plays a critical role at the beginning of the reaction. However, if the reaction time is long enough, the meso-Mg/Fe₂O₃ catalyst experienced the Mg leaching under catalytic environment. The departure of Mg cations from the catalysts could create defects or vacancies in the meso-Mg/Fe₂O₃ structure, and therefore activate the catalyst.²⁷ A more stable surface of the catalyst is formed for prolonged catalytic oxidation, leading to the increased catalyst activity (shown in Table 1).²¹ The leaching process is slow and could not be observed in a short reaction time (30 min). The leaching/activation mechanism could be summarized in Fig. 1.

Fig. 11 Total concentrations of ˙OH formed as a function of time. α-Fe₂O₃ (A), meso-Fe₂O₃ (B), con-Mg/Fe₂O₃ (C) and meso-Mg/Fe₂O₃ (D) (pH = 3.00, H₂O₂ dosage = 1300 mg L⁻¹, catalyst dosage = 0.3 g L⁻¹ and benzoic acid used as a probe at 1200 mg L⁻¹).

In order to clarify the origin of the difference in activities, detailed structural characterizations of meso-Mg/Fe₂O₃ were studied before and after the degradation of MB. The chemical composition of meso-Mg/Fe₂O₃ and related catalysts was confirmed by ICP-AES, and the Mg/Fe molar ratio is shown in Table 1. From Table 1, the Mg/Fe molar ratio was reduced to 0.14 compared to the initial Mg/Fe ratio of 0.59. The post-reaction meso-Mg/Fe₂O₃ exhibited typical isotherms for traditional ordered mesoporous materials and a much higher BET surface area (190.4 m² g⁻¹) than the as-made catalyst (90.7 m² g⁻¹). Thus, Mg leaching during the reaction processes increases the surface area of meso-Mg/Fe₂O₃ by approximately 100 m² g⁻¹. To verify the contributions of Mg cations to the increased surface area, the as-made meso-Mg/Fe₂O₃ and meso-Fe₂O₃ were treated in a diluted aqueous HNO₃ solution (pH = 3) for 30 min and investigated by N₂ adsorption measurements. The BET surface areas for acid-treated meso-Mg/Fe₂O₃ and meso-Fe₂O₃ are 203.9 m² g⁻¹ and 103.7 m² g⁻¹, respectively. These results are consistent with our conclusion that the Mg leaching contributes ∼100 m² g⁻¹ surface area. The departure of Mg cations from mesoporous Fe₂O₃ surface may create defects or vacancies in the iron oxide surface and leads to a highly porous structure.²⁷

XPS analysis of meso-Mg/Fe₂O₃ was carried out to better understand the roles of iron and magnesium in Fenton-like catalytic oxidation (as shown in Fig. 12). The O1s core level spectrum shows the dominant oxide peaks at around 530.1 eV, which are in good agreement with the literature values of α-Fe₂O₃.⁴⁸ In the meso-Mg/Fe₂O₃ and post-reaction meso-Mg/Fe₂O₃ catalysts, this peak practically disappears and shift to 531.7 eV and 530.8 eV. During the heterogeneous Fenton oxidation processes, the Mg ions were removed from the meso-Mg/Fe₂O₃ structure. The departure of Mg cations from catalyst structure may prompt the O1s peak of meso-Mg/Fe₂O₃ gradually close to the peak of pure α-Fe₂O₃ due to strong interaction between Fe³⁺ and related metal oxides. A very complex line-shape is observed in the Fe2p spectra (Fig. 12B) of the same catalysts. As expected, the typical photoelectron peaks at 711 and 725 eV are the characteristic doublets of Fe³⁺2p_3/2 and Fe³⁺2p_1/2 core-level spectra of α-Fe₂O₃. In addition, the spectra of meso-Mg/Fe₂O₃ and post-reaction meso-Mg/Fe₂O₃ were fitted with six contributions.⁴ The peaks at 724.5 eV and 711.1 eV in the meso-Mg/Fe₂O₃ are assigned to Fe³⁺2p_3/2 and Fe³⁺2p_1/2, suggesting the presence of Fe³⁺ cations. The other peaks are due to shake-up satellites of the 2p_3/2 and 2p_1/2 peaks. In post-reaction meso-Mg/Fe₂O₃, the Fe³⁺2p_3/2 and Fe³⁺2p_1/2 peaks are shifted to 724.1 eV and 710.7 eV, respectively, due to the interaction between the clusters of iron oxides and related oxides. Furthermore, the Fe2p peaks were gradually close to the peaks of pure α-Fe₂O₃, indicating the valence of Fe was not changed and its impact on Fe was gradually reduced with the leaching of Mg cations.

Fig. 12 XPS spectra of the α-Fe₂O₃, meso-Mg/Fe₂O₃ and postreaction meso-Mg/Fe₂O₃ samples: O1s (A) and Fe2p (B).

Once mesoporous α-Fe₂O₃ is fabricated through substitution and selective leaching of magnesium, a highly porous mesoporous Mg modified Fe₂O₃ with a significant amount of defects was made. The catalyst showed an enhanced activity in the MB degradation under mild conditions. Based on the experimental results of the XPS and the measured amount of ˙OH radicals, a plausible mechanism related to the active ˙OH radicals in the meso-Mg/Fe₂O₃ system is proposed. Firstly, MB was adsorbed on the surface of meso-Mg/Fe₂O₃. Once H₂O₂ was added, Fe(III) initiated H₂O₂ decomposition started from the reduction of Fe(III) and the formation of HO₂˙ (O₂˙⁻). Then, OH radicals and reduced Fe(II) species were generated according to the Haber–Weiss mechanism.^49,50 Next, the ˙OH radicals reacted with MB both adsorbed on the catalyst surface and dissolved in the bulk solution, leading to the further degradation and mineralization of pollutants. Nevertheless, the decomposition of H₂O₂ to generate ˙OH radicals can be described in the following reactions:¹⁸

≡Fe(III) + H₂O₂ → ≡Fe(III)(H₂O₂)_ads (1)

≡Fe(III)(H₂O₂)_ads → ≡Fe(II) + HO₂˙ +H⁺ (2)

≡Fe(II) + H₂O₂ → ≡Fe(III) + ˙OH + OH⁻ (3)

It can be noted that at pH ≥ 4 in a heterogeneous system, the catalytic reaction mainly occurs on the catalyst surface. Therefore, the adsorption of H₂O₂ onto the metal centers plays an important role in this heterogeneous Fenton-like reaction. In the case of meso-Mg/Fe₂O₃, the surface nature of Mg is also highly hydroxylated, which can facilitate the formation of the surface ˙OH complexes.^46,51 This is crucial for the adsorption of H₂O₂ and consequently contributes to a more rapid performance of the oxidation processes.⁴⁷ Moreover, MB hardly degraded in the presence of dissolved Mg²⁺ ions and H₂O₂, indicating that Mg²⁺ ions is not responsible for the MB degradation. Hence, a much better catalytic activity of meso-Mg/Fe₂O₃ compared with meso-Fe₂O₃ was ascribed to the synergetic effects of the desirable basic sites on the surface of the Mg and increased BET surface area and broader pore size distributions.

3. Excellent chemical reusability and stability of meso-Mg/Fe₂O₃ catalyst

Fig. 13 shows the MB degradation on meso-Mg/Fe₂O₃ in three different batch runs. After each run, the catalyst was recovered by filtration and washed with deionized water and dried at 80 °C overnight. It can be noted that the activity of meso-Mg/Fe₂O₃ increased after regeneration. As mesoporous α-Fe₂O₃ is the active component for heterogeneous Fenton-like reactions, substitution of Fe with alkaline metals will significantly decrease the activity at first batch run. Then, with the leaching of Mg and surface restructuring under a catalytic environment, the Mg-substituted catalysts (especially meso-Mg/Fe₂O₃) were activated (the enlarged part of Fig. 13).²⁷ This indicates that meso-Mg/Fe₂O₃ possesses enhanced chemical and catalytic stability. Notably, with the increase of reusability of the meso-Mg/Fe₂O₃ catalyst, both the leaching concentration of Mg and the catalytic activity of the catalyst increased, which is quite different with the catalytic mechanism of composite components, such as Fe–Cu and Fe–Zn composites, in heterogeneous Fenton processes.^23,24 The addition of Cu²⁺ is thermodynamically favorable for the reduction of Fe³⁺, which is beneficial for the redox cycles of Fe²⁺/Fe³⁺ and even for the regeneration of the active species Fe²⁺. In the meso-Mg/Fe₂O₃–H₂O₂–MB system, approximately two thirds of Mg cations were removed in the leaching/activating process. Especially, the meso-Mg/Fe₂O₃ with unique mesoporous structure possesses ultrahigh surface area and enlarged pore size, which effectively reduced the mass transfer resistance and enhance the number of active sites of the catalyst.

Fig. 13 Degradation of methylene blue in different batch runs in the H₂O₂–meso-Mg/Fe₂O₃ system. First run (a), second run (b) and third run (c) (initial pH, H₂O₂ dosage = 2600 mg L⁻¹, initial MB concentration = 60 mg L⁻¹ and catalyst dosage = 0.6 g L⁻¹).

The stability of the catalyst was also evaluated under the reaction conditions of the meso-Mg/Fe₂O₃–H₂O₂–MB system. Table 2 shows the influence of pH, H₂O₂ concentration, catalyst dosage and reusage on dissolved iron concentrations during the reactions. It can be seen that the dissolved iron species increased from 3.45 mg L⁻¹ to 6.72 mg L⁻¹ after 3 h when the pH decreased from 7.5 to 2.0. Table 2 also shows that dissolved iron species increased from 3.17 mg L⁻¹ to 4.53 mg L⁻¹ after 3 h reaction when the H₂O₂ concentration increased from 1600 mg L⁻¹ to 4600 mg L⁻¹ at an initial pH of 7.5.^52,53 The leaching concentration of Mg decreased after the second and third cycles, which indicates that the Mg cations in the meso-Mg/Fe₂O₃ also reduced. Magnesium is an essential element required by the human body. It is not regulated in aqueous environments, which implies that the Mg ions leached from the meso-Mg/Fe₂O₃ catalyst will not result in any extra environmental and health problems. The treated waste water could be directly discharged into the sewage system.

Table 2 Effect of pH values on the methylene blue degradation efficiency (%) and iron leaching (mg L⁻¹)^a

Run no.	pH0	[H2O2] (mg L^−1)	Catalyst dosage (g L^−1)	Degradation efficiency (%)	Fe leaching (mg L^−1)	Mg leaching (mg L^−1)
a pH0: initial pH adjusted at the beginning of the degradation of methylene blue, [H2O2]: initial H2O2 dosage, catalyst dosage: meso-Mg/Fe2O3 used in the reaction, Fe leaching and Mg leaching in mg L^−1: as determined by ICP-AES analysis.
1	2	2600	0.6	2.3	6.72	—
2	4	2600	0.6	9.5	5.36	—
3	6	2600	0.6	28.2	4.31	—
4	7.5	1600	0.6	91.7	3.17	—
5	7.5	4600	0.6	96.1	4.53	—
6	7.5	2600	0.9	98.7	5.41	—
7	7.5	2600	0.6	93.2	3.45	217.6
First reused meso-Mg/Fe2O3	7.5	2600	0.6	96.5	5.13	47.3
Second reused meso-Mg/Fe2O3	7.5	2600	0.6	97.8	6.84	36.4

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Conclusions

The ordered mesoporous magnesium-substituted α-Fe₂O₃ (meso-Mg/Fe₂O₃) was successfully synthesized using KIT-6 as a template through a hard templating strategy. The texture, morphology and structure of the meso-Mg/Fe₂O₃ catalyst before and after degradation of MB were characterized by SEM, HRTEM, XRD, N₂ adsorption–desorption, FT-IR spectroscopy and XPS analysis. The pseudo-first order reaction kinetics fitted well to all experiments. With the increase of reusability of the meso-Mg/Fe₂O₃ catalyst, both the leaching concentration of Mg and the catalytic activity of the catalyst increased, which is quite different with the catalytic mechanism of composite components, such as Fe–Cu and Fe–Zn composites, in heterogeneous Fenton-like processes. Post-reaction meso-Mg/Fe₂O₃ showed a much higher BET surface area and broader pore size distribution than the as-made sample, which is likely due to Mg leaching during the Fenton-like catalysis process. The structural framework of as-prepared catalyst was not changed during the reaction. In addition, the released amounts of Fe were found to be low. Hence, the meso-Mg/Fe₂O₃ catalyst has proven to have a unique and superb catalytic activity in the heterogeneous Fenton-like system and it will be an attractive alternative in the treatment of environmental problems.

Acknowledgements

This study was supported by the National Natural Science Foundation of China, NSFC (21101113, 51172157, 51202159, 51208357, 51472179), the Doctoral Program of Higher Education, the Ministry of Education (20120032120017), and the Municipal Natural Science Foundation of Tianjin (13JCYBJC16900, 13JCQNJC08200).

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