Hydrophilic modification of ordered mesoporous carbon supported Fe nanoparticles with enhanced adsorption and heterogeneous Fenton-like oxidation performance

Abstract

In this study, an ordered mesoporous carbon catalyst containing uniform iron oxide nanoparticles (Fe/meso-C) has been synthesized and underwent hydrophilic surface modification with hydrogen peroxide, which shows excellent adsorption and heterogeneous Fenton degradation performance for methylene blue (MB). Characterization using XRD, TEM, SEM, TG and N₂ sorption–desorption isotherms showed that Fe/meso-C treated with hydrogen peroxide (H-Fe/meso-C) maintained a hexagonally arranged mesostructure, uniform mesopore size (∼2.3 nm), high surface area (up to 530 m² g⁻¹) and moderate pore volume (0.29 cm³ g⁻¹) as an untreated catalyst. The Fe₂O₃ nanoparticles were highly dispersed in the carbon framework and mesopore channels. The hydrophilicity of the catalyst surface also improved after H₂O₂ modification. As a milder oxidizing agent, hydrogen peroxide was used to introduce the oxygen-containing group on the carbon surface. Due to the hydrophilic surface and retaining mesoporous structure, the H-Fe/meso-C catalyst presents a better Fenton-like catalytic performance than Fe/meso-C. The adsorption and heterogeneous Fenton-like degradation of MB reached 96% in 220 min with optimal oxidation conditions of 30 mg L⁻¹ MB solution, 0.7 g L⁻¹ catalyst, 50 mmol L⁻¹ H₂O₂ and the initial pH value.

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1. Introduction

Nowadays, protection and preservation of the quality of water resources is one of the most critical tasks to maintain the quality of our lives and ensure sustainable development in many regions around the world.^1,2 The nature of the refractory pollutants present in water is quite variable. Particular attention should be made to the pollution of water by synthetic dyes from the textile industry.³ Actually, the removal of synthetic dyes from wastewater is really challenging to the related industries, since the synthetic dyes used are stable compounds, difficult to destroy by common wastewater treatments. Among the different approaches for dye elimination, advanced oxidation processes (AOPs) are becoming important technologies for dye wastewater treatment.⁴ And the Fenton reaction is the most intensively investigated AOP. Compared to the homogeneous Fenton, heterogeneous Fenton has attracted more attention due to a broader working pH and less iron sludge generation.^5,6

In heterogeneous Fenton reactions, Fe-based catalysts have recently attracted ever growing interest because of their combined simplicity, efficiency and low investment cost due to reusability.³ Different iron oxides and iron hydroxides as heterogeneous Fenton-like catalysts have been studied, such as hematite, goethite and magnetite.⁷ However, many of these catalytic systems do not exhibit favorable degradation activities, which is due to Fe³⁺ being unable to effectively catalyze the generation of ˙OH from H₂O₂. In this regard, the impregnation method has been used to deposit iron species onto different types of solid supports (e.g., clays, zeolites and carbon materials).^1,8 In all of these supports, carbon material has been intensively investigated due to the combination of its low price, high porosity and surface area, stability in acidic and basic media, and easily tunable surface chemistry, which could be difficult to replace by other kinds of inorganic supports and efficiently enhance the performance of heterogeneous Fenton reactions.⁹ Furthermore, carbon material could also adsorb the pollutant or even be a catalyst by itself, once the surface of the carbon material has unpaired π electrons, which can react with H₂O₂ leading to the generation of hydroxyl radicals.^4,10 Castro et al. prepared activated carbon supported iron oxide, which showed intense removal of the methylene blue through combined adsorption and oxidation processes.¹¹ Wang et al. synthesized high surface area mesoporous copper ferrite (meso-CuFe₂O₄) and discussed the superiority of the bimetal catalyst since the redox properties of dissolved transition metal cations (Fe²⁺, Cu²⁺) allow for the generation of highly active hydroxyl radicals in the presence of hydrogen peroxide.¹² Duarte et al. investigated the influence of carbon particle size in Fe supports for removal of the azo dye Orange II.¹³ With the decrease in carbon particle size, the dispersion of iron in the catalysts was improved, which also favored the catalytic activity. Then they studied the textural and chemical surface properties of fresh and spent activated carbon during heterogeneous Fenton processes. And the dye removal efficiencies of these catalysts were varied with different interactions of the oxidized products with the carbon support.¹⁴ Karumuri’s research showed that the hydrophilic properties of carbon supports could have a significant influence in catalytic oxidation.¹⁵ To the best of our knowledge, the surface properties of carbon based heterogeneous Fenton catalysts, especially the hydrophilic nature of carbon supports, have not been thoroughly investigated. The carbon supported catalysts still face obvious problems like instability for reuse, low amount of iron loading and inconvenience to recycle.^16,17

As a new kind of carbon nanomaterial, ordered mesoporous carbons have been proven to be excellent catalytic supports due to their well-controlled pore structures, high surface areas, uniform and tunable pore sizes and good electrical conductivity and thermal stability.^18,19 Undoubtedly, ordered mesoporous carbons incorporated with iron nanoparticles could hold great promise in developing high performance iron-based heterogeneous Fenton catalysts. Ordered mesoporous carbons with incorporated functional nanoparticles can be synthesized through either a hard-templating approach or soft-templating approach.²⁰ For example, Kim et al. synthesized magnetic ordered mesoporous carbon using the impregnation method.²¹ However, the Fe nanoparticles in the above-reported mesoporous carbon suffered severe aggregation with broad size distributions. Additionally, a variety of mesoporous carbons embedded with metal oxide nanoparticles have been reported using previously synthesized inorganic nanoparticles as a metallic precursor. The synthesis processes could be less controllable as the capped nanoparticles cannot be well dispersed in the solution of carbon precursors and templates.^8,22,23 Recently, Zhao and his co-workers fabricated ordered mesoporous carbon with acetylacetone chelate-assisted co-assembly to iron nanoparticles.²⁴ The iron-containing nanoparticles are partially embedded with the remaining part exposed in the mesopore channels. Nevertheless, this multi-step approach is rather low in efficiency and high in cost. Meanwhile, the surface of the carbon support was not hydrophilically modified as the carbon support was made from phenolic resin. The degradation performances of all these kinds of catalysts were limited since the heterogeneous Fenton reaction was carried out in aqueous conditions. So, some special functional groups are introduced on the carbon matrix surface to increase the hydrophilic and acidic properties, such as hydroxyl, phenol, lactone and other groups, by oxidation treatment. In order to retain the mesoporous structure of such catalysts, mild oxidation agents, such as hydrogen peroxide, should be used in the modification processes.²⁵

Herein, we demonstrate a surface hydrophilic modification route for the synthesis of high-quality ordered mesoporous carbons incorporated with highly dispersed uniform Fe nanoparticles as a heterogeneous Fenton-like oxidation catalyst, which shows an enhanced adsorption and degradation performance of methylene blue (MB). The synthesis is accomplished by slow evaporation of an ethanol solution with resol and ferric nitrate as the precursors and triblock copolymer F127 as a template. Then the catalysts were hydrophilically modified by hydrogen peroxide with different concentrations. During the heterogeneous Fenton reaction with MB, the hydrogen peroxide modified Fe-based mesoporous carbon catalyst shows higher activity and stability than the untreated catalyst in the adsorption and degradation processes, which is due to the unique nanostructure and high porosity of the catalysts. The effects of further H₂O₂ oxidation treatment of the nanocomposites for the removal of dye molecules were studied in detail.

2. Experimental

2.1. Materials

The poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymer Pluronic F127 (M_w = 12 600, PEO₁₀₆PPO₇₀PEO₁₀₆) purchased from Sigma Aldrich was used as a template while phenol, formalin solution (37 wt%), sodium hydroxide, hydrochloric acid, ethanol, acetylacetone received from Tianjin Chemical Corp. and ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O) were used as the salt precursor. Methylene blue, sodium hydroxide (NaOH), H₂O₂ (30 wt%) and other chemicals used in this study were of analytical grade and used without further purification.

2.2. Synthesis and modification of H-Fe/meso-C catalysts

The mesoporous carbon materials were synthesized following the procedure reported by Zhao and Wang et al.²⁶ The mesoporous Fe/meso-C composites were synthesized using the multicomponent co-assembly method with Pluronic F127 as the template in an ethanol solution. In a typical synthesis, 5.0 g of phenol was melted at 40–42 °C, then 1.06 g of a NaOH (20 wt%) aqueous solution was added under stirring for 10 min. Then 8.8 g of a formalin solution (37 wt% formaldehyde) was added slowly and heated at 70 °C for 60 min. After cooling to room temperature, the pH value of the solution was adjusted to about 6.0 with 2.0 mol L⁻¹ HCl solution. The resol precursors were re-dissolved in ethanol (20 wt%) and sodium chloride was separated as a precipitate. The Fe/meso-C was obtained using ferric nitrate following the chosen weight ratio of F127 : resol : Fe₂O₃ : ethanol = 1 : 1 : 0.04 : 17. 0.2 g of Fe(NO₃)₃·9H₂O was added into 5.0 g of the resol precursor solution. After further stirring at 50 °C for 30 min with a condenser tube, the mixture was transferred into Petri dishes, followed by evaporation of ethanol for 8 h at room temperature and 100 °C for 24 h. The obtained composite films were scraped off and calcined at 600 °C in N₂ for 3 h. The temperature rate was 1 °C min⁻¹. The carbonized samples were denoted as Fe/meso-C.

The ordered mesoporous carbon co-assembled with Fe nanoparticles was further treated by H₂O₂ to enhance their hydrophilic properties. In detail, 0.5 g of Fe/meso-C was added into 8 g of 5–15 wt% H₂O₂ solution at room temperature, then the reaction mixture was stirred at 60 °C for 30 min. After separation with a magnet and washing with ethanol, the magnetic nanocomposites were dried at 100 °C for 4 h. The resulting catalysts were ground thoroughly and denoted as H-Fe/meso-C. Fig. 1 illustrates the fabrication and H₂O₂ modification processes of ordered mesoporous carbon supported Fe nanoparticles.

Fig. 1 Schematic illustration of the fabrication of the ordered mesoporous carbon catalyst containing uniform iron oxide nanoparticles (Fe/meso-C) and modification by 5 wt% H₂O₂ solution (H-Fe/meso-C).

2.3. Characterization

The crystalline phases in the material were studied using X-ray diffraction techniques on a Philips X’pert instrument with a Cu Kα radiation source in the 2θ range of 0.5–10°. Metal dispersion on the carbon surface and the nature of the particles were analyzed using HRTEM and SEM on a Hitachi H-7650 transmission electron microscope and Hitachi S-4800 scanning electron microscope. Nitrogen sorption–desorption isotherms were measured at 77 K with a Micromeritics Tristar 3000 analyzer after being outgassed at 423 K for a minimum 12 h. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface area (S_BET) in a relative pressure range from 0.05 to 0.25. The Barrett–Joyner–Halenda (BJH) model was used to study the pore volume and size distribution, and the total pore volume (V_t) was estimated at a relative pressure P/P₀ of 0.995. X-ray photoelectron spectroscopy was performed on a Kratos ASIS-HS X-ray photoelectron spectroscope equipped with a standard and monochromatic source (Al Kα) operated at 150 W (15 kV, 10 mA). Thermogravimetric analysis (TGA) curves were measured using a TA SDT-Q600 analyzer from 25 to 900 °C at a heating rate of 5 °C min⁻¹ in airflow of 80 mL min⁻¹. FT-IR was performed with a Bruker TENSOR37 FT-IR spectrometer transmission analyzer in KBr pressed pellets. The concentration of MB in the supernatant liquid was determined at the absorbance peak of 664 nm with a UV-vis spectrophotometer (UV-752, Shanghai Rex Instrument Co., Ltd., China). ˙OH concentration was determined by the benzoic acid (C₇H₆O₂) fluorescence method.²⁷ The photoluminescence spectra (PL) of benzoic acid samples were obtained using a Fluorescence Spectrophotometer (F-380, Tianjin Gangdong Instrument Co., Ltd., China.) at the excitation wavelength of 325 nm.

2.4. Catalytic tests

The catalytic experiments were carried out in a batch reactor at ambient atmosphere at 30 °C. All the experiments were carried out in a stirred slurry batch reactor with the temperature controlled by a thermostatic bath. In a typical experiment, certain amounts of catalysts were dispersed in 50 mL of the MB solution (30 mg L⁻¹). After stabilization of temperature and pH (adjusted with 0.1 mol L⁻¹ HCl solution), the suspensions were magnetically stirred for about 30 min to achieve adsorption/desorption equilibrium between the dye and catalyst. Then, a certain amount of H₂O₂ was added to the above suspensions. At given intervals of degradation, 2 mL of the reaction solution was sampled and immediately centrifuged to remove the catalyst for analysis. Mineralization was evaluated by TOC analyses using Shimadzu TOC-VWP equipment and iron leaching was quantified using Varian Vista inductively coupled plasma atomic emission spectroscopy (ICP-AES, America) with a wavelength range of 167–785 nm and a limit detection of 0.01 μg L⁻¹.

3. Results and discussion

3.1. Characterization of as-prepared carbon-based catalysts

Small-angle and wide-angle X-ray diffraction (SAXRD and WAXRD) patterns are illustrated in Fig. 2. As is shown in Fig. 2A, the samples of mesoporous carbon and Fe/meso-C show a wide peak at about 0.70°, which is related to the reflections of the 2-D hexagonal meso-structured carbon. However, compared with the above catalysts, the H-Fe/meso-C presents a broader diffraction peak, which indicates that the mesoporous structures are slightly deteriorated but not destroyed with modification by hydrogen peroxide. In the wide-angle XRD patterns shown in Fig. 2B, five resolved diffraction peaks (30.1°, 35.5°, 43.1°, 57.3°, and 62.7°) were observed in the Fe/meso-C, which can be assigned to 220, 311, 400, 511 and 440 reflections of crystalline γ-Fe₂O₃ phases (JCPDS card no.89-5892). These results indicate that the iron-oxide nanoparticles are well crystallized and uniformly dispersed in the carbon matrix.²⁶ The H-Fe/meso-C catalysts reveal similar diffraction patterns as the Fe/meso-C sample, suggesting that the hydrogen peroxide treatment has no significant influence on the well-crystallized mesoporous structure of these catalysts. To further investigate the crystal nature and influence of temperature on the type of iron-based particles, the H-Fe/meso-C catalyst was calcined at 800 °C for 3 h under N₂ atmosphere. The WAXRD patterns are shown in Fig. 3. In Fig. 3, the catalysts show the same six resolved diffraction peaks of crystalline γ-Fe₂O₃. And another five diffraction peaks also emerged in the calcined H-Fe/meso-C patterns. Among them, the diffraction peaks at 31.4°, 39.7° and 49.7° may be attributed to α-Fe₂O₃, 43.0° contributes to Fe₃C and 45.2° contributes to iron nanoparticles formed by in situ reduction during the calcination.

Fig. 2 Small angle XRD patterns (A) of meso-Fe₂O₃, meso-C, Fe/meso-C and H-Fe/meso-C; wide angle XRD patterns (B) of bulk Fe₂O₃, meso-Fe₂O₃, Fe/meso-C and H-Fe/meso-C.

Fig. 3 The wide angle XRD patterns of the H-Fe/meso-C nanoparticles composites carbonized at 800 °C in N₂.

To further investigate the porous structure and distribution state of iron species in Fe/meso-C and H-Fe/meso-C catalysts, SEM (EDS elemental mapping characterization included) and TEM characterizations were carried out and the related images of the above catalysts are shown in Fig. 4 and Table 1. There were more than ten images acquired for each sample for characterization using SEM and TEM. Although several of the acquired images showed great differences, which were taken for the catalyst samples, most of the pictures still show similar morphologies of these catalysts. By comparing the SEM images of Fe/meso-C (Fig. 4A) and H-Fe/meso-C (Fig. 4B) catalysts, characterization of the surface roughness of these catalysts is obtained by evaluating the brightness differences between the neighbouring regions of these catalysts. Evidence shows that an increased roughness of the catalyst surface after hydrogen peroxide treatment could be observed indicating a correlation between the texture and the fractal dimension of the featured images. After the 5 wt% hydrogen peroxide treatment of Fe/meso-C, the particle size of the catalyst also remains nearly unchanged. The elemental distribution mapping results of the above catalysts obtained by EDS characterization are shown in Fig. 4C and D. The EDS elemental maps confirm that the Fe element is highly dispersed in Fe/meso-C and H-Fe/meso-C.²⁸ Hydrogen peroxide treatment did not change the uniform distribution of Fe in the carbon based catalysts. From the relative atomic content of existing elements shown in Table 1, the oxygen content on the mesoporous carbon supports were greatly improved after hydrogen peroxide treatment of the H-Fe/meso-C, indicating the increase of surface oxygen-containing groups caused by the oxidation of hydrogen peroxide. The H-Fe/meso-C surface behaved more hydrophilic due to the increased numbers of oxygen-containing groups.¹⁵ The uniform distribution of Fe element on the catalyst surface was also confirmed by TEM characterization. The TEM images of Fe/meso-C (Fig. 4E) and H-Fe/meso-C (Fig. 4F) show a stripe-like and hexagonally arranged pore morphology.²⁹ The as-made H-Fe/meso-C possesses a highly uniform mesostructure with 2-D hexagonal pore symmetry and a mean pore size of about 5.4 nm, just as indicated in Fig. 4F. In Fig. 4E and F, the Fe/meso-C preserves the long ordered arrangement of the lattice fringes and the iron oxide nanoparticles are well incorporated in the carbon framework, which are in accordance with the XRD measurements. The size distribution curves of Fe₂O₃ nanoparticles display a mean diameter centered at ∼12 nm with a standard deviation of about 5% for 100 nanoparticles using a Nano Measurer (Fig. 4E). After 5 wt% hydrogen peroxide treatment of Fe/meso-C, Fe₂O₃ nanoparticles of ∼20 nm can be obtained without aggregation (Fig. 4F). Meanwhile, the porous structure of H-Fe/meso-C which originates from the structure of Fe/meso-C still remained.

Fig. 4 SEM images of Fe/meso-C (A) and H-Fe/meso-C (B); EDS images of C (C) and Fe (D) of H-Fe/meso-C; TEM images of Fe/meso-C (E) and H-Fe/meso-C (F).

Table 1 The EDX comparison of the Fe nanoparticle incorporated ordered mesoporous carbon and selected iron-based catalysts

Element	Meso-C	Bulk-Fe2O3	Fe/meso-C	H-Fe/meso-C
C	92.73%	—	86.31%	83.00%
O	6.81%	14.12%	5.94%	10.75%
Fe	0.46%	85.88%	7.75%	6.31%

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The thermo-gravimetric analysis (TGA) is commonly performed under air flow to estimate the thermal stability and phase purity of Fe₂O₃ nanoparticle incorporated mesoporous carbon. The detailed results after charring the catalysts are shown in Fig. 5. The weight change of bulk Fe₂O₃ might come from some organic impurities. A slight weight loss of Fe/meso-C and H-Fe/meso-C exits below 150 °C, which is attributed to adsorbed water. The sharp weight loss in the temperature range of 400–600 °C was due to the charring of carbon. The synthesized Fe/meso-C exhibits a narrow temperature range, indicating the highly analogous pore structure of the catalysts. The sharp weight loss of H-Fe/meso-C happens at a lower temperature and this process has a wider range than that of pure Fe/meso-C, which also confirmed that the number of organic groups on the catalyst surface increased after hydrogen peroxide treatment.^30,31 By TG characterization of these catalysts, the weight percentage of the residues is about 6.2%, corresponding to the received Fe₂O₃ from the oxidation of iron oxides in the Fe/meso-C catalysts.

Fig. 5 Thermogravimetric (TG) analysis curves recorded in air for bulk Fe₂O₃, Fe/meso-C and H-Fe/meso-C.

N₂ adsorption–desorption isotherms (Fig. 6) of these carbon-based catalysts show typical type-IV curves, which reflect a high uniformity of the mesopore size and similar pore structures.^32,33 The BET surface areas of the mesoporous Fe/meso-C and H-Fe/meso-C are 532 and 561 m² g⁻¹ and the pore volumes show no obvious change. When iron was doped into ordered mesoporous carbon, the BET surface area and mean pore size of the Fe/meso-C increased, which confirms that metal doping of the carbon support may increase BET surface area and pore size of the mesoporous carbon. The reason might be that the shrinkage of the carbon matrix could be hindered by the iron entrapped into the support during the carbonization process.²² After hydrogen peroxide treatment, the BET surface area and pore size distribution of the H-Fe/meso-C further increased, indicating etching of the carbon surface with H₂O₂. Some new mesopores can be generated or adjacent mesopores become united during the etching processes.²⁴

Fig. 6 N₂ adsorption–desorption isotherms at −195 °C (A) and the corresponding pore size distributions (B) of the meso-Fe₂O₃, meso-C, Fe/meso-C and H-Fe/meso-C.

XPS characterization was also used to analyze the chemical environment and oxidation state of the carbon-based catalysts. As shown in Fig. 7A, Fe/meso-C exhibited a wider peak of the O1s element than meso-C samples, which was attributed to the generated γ-Fe₂O₃, C–O or other phases in the Fe/meso-C during the carbonization processes.³⁴ After hydrogen peroxide treatment, H-Fe/meso-C showed an abnormal O1s peak due to the introduction of the oxygen-containing groups on the catalyst surface. In Fig. 7B, the spectrum peaks of Fe2p_3/2 and Fe2p_1/2 in Fe/meso-C were about 711.0 eV and 724.8 eV, respectively.³⁵ After hydrogen peroxide modification, the spectrum peaks of Fe2p_3/2 and Fe2p_1/2 in H-Fe/meso-C changed to 710.5 eV and 724.1 eV, which might be due to the bond generated by the iron and oxygen-containing group.³⁶

Fig. 7 XPS spectra for O1s regions (A) and Fe2p regions (B) of meso-C, Fe/meso-C and H-Fe/meso-C.

In the FT-IR spectrum of the carbon-based mesoporous catalysts (Fig. 8), the band at 3425 cm⁻¹ arising from vibrational stretching of the –OH groups was stronger and broader after hydrogen peroxide treatment. It indicates that more –OH functional groups might be generated in the carbon framework. The two bands at 1612 and 1200 cm⁻¹ assigned to the C–C and C–O stretching vibration broadened, which also confirmed the above results.^37,38 With hydrogen peroxide treatment, H₂O₂ oxidized active sites such as –CH₂ and –CH groups of the carbon framework into –C–OH and –C=O groups with the elimination of the CO₂ species. It derives from etching of the carbon walls, and the BET surface area and mean pore size sequentially increased from this etching effect.²⁴

Fig. 8 The FT-IR spectra of Fe/meso-C (a), 5% H-Fe/meso-C (b), 10% H-Fe/meso-C (c) and 15% H-Fe/meso-C (d).

The hydrophilic test results of the mesoporous carbon-based catalysts are listed in Table 2. Fe/meso-C showed a surface hydrophobic property due to its static water contact angle of 113.49°. After modification with hydrogen peroxide, the H-Fe/meso-C catalyst contact angle decreased, which suggested that the surface hydrophilic nature was enhanced.¹⁵ With the increase of the H₂O₂ concentration, the H-Fe/meso-C surface became more hydrophilic, which was due to the increased oxygen-containing group numbers during hydrogen peroxide treatment of the catalysts.

Table 2 Structural and textural properties of the Fe nanoparticle incorporated ordered mesoporous carbon and selected iron-based catalysts

Catalyst	SBET (m^2 g^−1)	Vpore (cm^3 g^−1)	Vmicro (cm^3 g^−1)	Dp (nm)	a0 (nm)	h (nm)	Contact angle
Meso-C	516	0.31	0.14	2.1	25.02	22.72	—
Fe/meso-C	532	0.29	0.14	2.1	20.53	18.43	113.49 ± 0.18
5% H-Fe/meso-C	561	0.29	0.13	2.3	21.07	18.97	116.45 ± 2.66
10% H-Fe/meso-C	589	0.30	0.13	2.3	—	—	108.05 ± 6.63
15% H-Fe/meso-C	613	0.31	0.12	2.4	—	—	96.47 ± 5.53

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3.2. Comparison and efficient elimination of MB with Fe/meso-C and H-Fe/meso-C

Since there is a co-existence of complex adsorption and oxidation during heterogeneous Fenton processes, the adsorption processes of MB were firstly studied with Fe/meso-C and H-Fe/meso-C. A batch reactor of 50 mL was employed to carry out the adsorption kinetic study of MB. According to the results, 50 mg of carbon-based catalysts were added to a certain concentration of the MB solution with continuous magnetic stirring and pH adjustment. Prior to the adding of H₂O₂, the adsorption processes of MB could be investigated on all the samples in the first 30 min, then a certain amount of hydrogen peroxide was added (Fig. 9). The heterogeneous Fenton-like oxidation began and the elimination efficiencies of MB were measured at certain intervals.³⁰ All these degradation experiments have been carried out with triplicate samples.

Fig. 9 The degradation efficiency of methylene blue with different catalysts during the reaction (A) and the kinetic curves of methylene blue degradation showing the dependence of ln(C/C₀) versus time (B). Except for the investigated parameter, all other parameters were fixed (initial pH, H₂O₂ concentration of 50 mmol, catalyst loading of 0.7 g L⁻¹, methylene blue concentration of 30 mg L⁻¹ and temperature of 25 °C).

The adsorption and degradation of several carbon-based catalysts used in the MB elimination processes are shown in Fig. 9. There was little elimination in the procedures of meso-C and α-Fe₂O₃, which indicated that the saturated extent of adsorption of Fe/meso-C was limited and the mesoporous structure of the catalysts offered great advantages in the heterogeneous Fenton-like oxidation reaction. With the modification of introducing oxygen-containing groups, the H-Fe/meso-C catalyst exhibited a higher constant for the degradation rate than that of meso-C and α-Fe₂O₃ (Fig. 9B), which is a result of the catalyst after hydrogen peroxide treatment being more hydrophilic and able to be completely dispersed in solution. However, the differences of the degradation rates between H-Fe/meso-C and the unmodified Fe/meso-C catalyst are not very significant, especially the first run of H-Fe/meso-C and unmodified Fe/meso-C. The data of batch reuse performances for H-Fe/meso-C and unmodified Fe/meso-C are also shown in Fig. 12A and B. The degradation rate constants for H-Fe/meso-C (0.562 h⁻¹) are even lower than that of Fe/meso-C (0.617 h⁻¹). In particular, in the first hour, the degradation rate for H-Fe/meso-C is even much slower than that of Fe/meso-C. In the second and third run of these catalysts, the degradation rates of H-Fe/meso-C obviously increased (0.608 h⁻¹ and 0.397 h⁻¹), and they are obviously higher than those of Fe/meso-C (0.271 h⁻¹ and 0.112 h⁻¹). In order to clarify this problem, H-Fe/meso-C was treated with distilled water again (no hydrogen peroxide was added), which was labeled as W-Fe/meso-C. After the degradation tests of MB for W-Fe/meso-C, the degradation rate of W-Fe/meso-C (0.953 h⁻¹) was much higher than that of H-Fe/meso-C and Fe/meso-C (0.562 h⁻¹ and 0.617 h⁻¹). This might be due to the iron species adsorbed on Fe/meso-C also involved in the degradation processes, which similar results were also reported by Lin and Rodriguez et al.^39,40 For Fe/meso-C, the degradation rate increased since there are iron species adsorbed on the catalyst surface at the beginning of the processes. These iron species contribute to the degradation processes. With the removal of these iron species from the Fe/meso-C catalyst surface, the degradation rates slightly decreased. Meanwhile, after H₂O₂ treatment and washing of the H-Fe/meso-C, there are few iron species left on the H-Fe/meso-C catalyst surface. Therefore, in the first hour of degradation, the rates for H-Fe/meso-C and W-Fe/meso-C were slower than that of the Fe/meso-C. And the improvement of the total degradation rate for H-Fe/meso-C and W-Fe/meso-C might be due to H₂O₂ and water washing treatments enlarging the mesoporous size during the processes. Therefore, more Fe₂O₃ nanoparticles were embedded in the carbon framework exposed in the mesopore channel, which provided more active sites to participate in the Fenton-like catalytic reaction.²² For the increased hydrophilic surface properties of H-Fe/meso-C, hydrogen peroxide could be more easily accessed at the catalyst surface, hence the catalyst dosage could be reduced with the same degradation effect as the original Fe/meso-C.³⁸ In Fig. 12B and D, the degradation rate for repeated H-Fe/meso-C catalysts showed similar trends, which also confirmed the above deduction. This made the difference in the increase of the catalytic performance result from adsorbed iron species and porous changes coming from H₂O₂ surface modification. Therefore, treatment with H₂O₂ can improve the catalytic degradation by changing the porous and surface property. However, further research will be needed to clarify a detailed mechanism that explains the performance of this material as a heterogeneous Fenton-like catalyst.

The Fenton oxidation kinetics would be significantly affected by different initial parameters, such as pH, catalyst dosage, concentration of H₂O₂ and so on. Therefore, various comparative experiments were carried out to investigate the experimental conditions of the Fenton-like oxidation reaction, as shown in Fig. 10. In Fig. 10A, the influence of catalyst dosage on MB degradation performance was studied. With the increase of the catalyst amount, the degradation rate of MB improved, which suggests that a larger amount of catalyst supplies more active sites. Therefore, more active radicals could be achieved during the heterogeneous Fenton degradation process. Meanwhile, adsorption of methylene blue also greatly increased with the catalyst dosage changing from 0.7 to 1.2 g L⁻¹. When the catalyst dosage increased, more mesoporous and microporous pores and channels in the catalysts were exposed during the degradation process, which could lead to an increased adsorption of organic molecules. Meanwhile, the increased amount of Fe species could react with ˙OH to produce hydroperoxyl radicals, which had little effect on degradation.^40,41 Therefore, an excessive amount of catalyst decreased the degradation rate of MB. The accumulation of the catalyst particles was thought to be another reason for this phenomenon.⁴² Hence, the high catalytic activity and elimination rate of MB could be gained using a limited catalyst dose and 0.7 g L⁻¹ H-Fe/meso-C might be used for this Fenton-like oxidation system. The ultimate degradation rate could reach up to 96%.

Fig. 10 The degradation of methylene blue in heterogeneous Fenton processes catalyzed by H-Fe/meso-C with different catalyst loading (A), pH value (B) or initial H₂O₂ concentration (C). Except for the investigated parameter, all other parameters were fixed (initial pH, H₂O₂ concentration of 50 mmol, catalyst loading of 0.7 g L⁻¹, methylene blue concentration of 30 mg L⁻¹ and temperature of 25 °C).

The initial pH value of the catalytic system also has a decisive influence on the elimination of MB in the Fenton-like reaction system, which is due to the significant effect of the number of active species ˙OH playing a core role in this oxidation reaction. As shown in Fig. 10B, the study was conducted at a pH range from 4.0 to 8.0 in 0.7 g L⁻¹ H-Fe/meso-C and 50 mmol H₂O₂. Among them, a pH value of 7.33 was the original pH of the catalytic system without adjustment with acid. From Fig. 10B, the appropriate pH for the degradation of MB was also at this initial pH, which might be resulting from that, in weak alkaline solutions, instead of O₂ and H₂O, more hydroxyl radicals could be obtained in the decomposition of H₂O₂.⁴³ Therefore, the following H-Fe/meso-C Fenton-like reactions will be carried out at the initial pH value.

Fig. 10C shows the influence of H₂O₂ concentration of carbon-based catalysts on MB degradation from 10 to 200 mmol L⁻¹. It is clearly seen that the system containing 150 mmol L⁻¹ H₂O₂ achieves 96% degradation at first, while elimination of MB slightly decreases in the same period of time with 50 mmol L⁻¹ H₂O₂. When H₂O₂ concentration was higher than 150 mmol L⁻¹, MB degradation slowed down and decreased. This is probably due to excess H₂O₂ acting as a competitor, reacting with hydroxyl radicals (˙OH) in advance and producing superoxide (O₂˙⁻) and hydroperoxyl (HOO˙) radicals that show a slower reaction rate than ˙OH, which results in the low MB degradation.^36,44 Combined with the MB discoloration rate and economical utilization of the oxidant, 50 mmol L⁻¹ was chosen to be the optimal H₂O₂ concentration.

3.3. Mineralization and a plausible degradation mechanism for H-Fe/meso-C

In order to investigate the plausible mechanism of the H-Fe/meso-C Fenton oxidation system, benzoic acid was used as a fluorescent probe to measure the generated ˙OH radicals with a Fluorescence Spectrophotometer (F-380, Tianjin Gangdong Instrument Co., Ltd., China) at an excitation wavelength of 325 nm.²⁷ In a typical experiment, the procedures were carried out under the conditions of 10 mmol benzoic acid, 50 mmol L⁻¹ H₂O₂ and 1 g L⁻¹ catalyst before and after modification of hydrogen peroxide. In this experiment, benzoic acid could easily react with ˙OH radicals producing highly fluorescent p-hydroxybenzoic acid.

As can be seen in Fig. 11, the H-Fe/meso-C Fenton-like oxidation system generated more ˙OH radicals at the early period of the experiment, which could cause a higher MB degradation rate in the heterogeneous Fenton processes. After 30 minutes, the fluorescence intensity increased slowly both in the Fe/meso-C and H-Fe/meso-C catalytic systems. Based on the results of XRD and determination of ˙OH concentrations by fluorescence spectra, a plausible mechanism for the heterogeneous Fenton-like reaction was proposed. According to the Haber–Weiss mechanism, the decomposition of H₂O₂ and degradation of MB mainly occurred on the iron oxide surface.⁴⁶ Fe²⁺ also activates H₂O₂ to generate ˙OH on the catalyst surface.²³ Hence, the catalytic performance varied remarkably with the changes of the surface properties of the solid Fenton-like catalysts. For the H-Fe/meso-C catalyst, the modification of H₂O₂ made the surface of the catalyst more hydrophilic, which made it easy to expose more active sites to make contact with H₂O₂ and the organic pollutant. Thus more Fe species attacked H₂O₂ molecules and more ˙OH could be created to react with the organic dye during the degradation processes. Therefore, the H-Fe/meso-C exhibited a higher degradation rate in the Fenton-like catalytic oxidation system.³⁰

Fig. 11 Total concentration of ˙OH formed as a function of time for Fe/meso-C (A) and H-Fe/meso-C (B). Except for the investigated parameter, all other parameters were fixed (initial pH, H₂O₂ concentration of 50 mmol, catalyst loading of 0.7 g L⁻¹, benzoic acid was used as a probe at 10 mmol and a temperature 25 °C).

3.4. Chemical reusability and stability of as-prepared carbon-based catalysts

The reusability and stability of the carbon-based catalysts were evaluated by repetitive reaction in three consecutive runs under identical conditions of 0.7 g L⁻¹ catalyst, 50 mmol L⁻¹ H₂O₂ and 30 mg L⁻¹ MB solution. After each run, the catalyst was separated by filtration, washed with ethanol and deionized water, then dried at 100 °C for 6 h to carry out the next experiment. As shown in Fig. 12A and C, the kinetic analysis of the Fe/meso-C heterogeneous Fenton-like oxidation procedures shows that the degradation of MB follows the pseudo-first order. The apparent rate constants of Fe/meso-C for the three different reuse batches are 0.617, 0.271 and 0.112 h⁻¹ (Fig. 12C). The initial catalytic activity of Fe/meso-C weakened gradually over the three cycles. This might be caused by the residual by-products and reactants adhered on the active sites of catalyst and a small amount of iron leaching in the oxidation reaction and washing processes.⁴⁷ It could also be further confirmed in the XRD and TEM patterns of the Fe/meso-C observed in Fig. 2 and 4E, respectively.

Fig. 12 Degradation of methylene blue with reuse performance of Fe/meso-C (A) and H-Fe/meso-C (B); the kinetic curves of methylene blue degradation showing the dependence of ln(C/C₀) versus time of Fe/meso-C (C) and H-Fe/meso-C (D). Except for the investigated parameter, all other parameters were fixed (initial pH, H₂O₂ concentration of 50 mmol, catalyst loading of 0.7 g L⁻¹, methylene blue concentration of 30 mg L⁻¹ and temperature of 25 °C).

In regard to hydrophilic H-Fe/meso-C (Fig. 12B and D), the reusable heterogeneous Fenton-like reactions were carried out under the same conditions. Compared to primary Fe/meso-C, the modified H-Fe/meso-C catalyst exhibited better stability at a higher degradation rate during the three reuse cycles. Among these processes, the catalytic activity slightly increased in the second run (0.562 h⁻¹ versus 0.608 h⁻¹), which was quite different from the catalytic mechanism of ordered mesoporous composite components in heterogeneous Fenton-like processes, such as Fe–Cu and Fe–Zn composites.^48,49 The Fe–Cu and Fe–Zn composites influenced the catalytic activity by introducing the second metal, which increased the number of active sites and the synergistic effect of Cu or Zn to the Fe species on the surface of the catalyst, and then the components were more conducive to promote interfacial electron transfer.²³ However, the degradation rate of H-Fe/meso-C changed by the modification of H₂O₂, which might be due to the more hydrophilic surface property and the enlarged mesoporous size. Therefore, more Fe₂O₃ nanoparticles embedded in the carbon framework could be exposed in the mesopore channel, which provides more active sites to participate in the Fenton-like catalytic reaction.⁴⁵ The peroxide oxidation treatment also enhanced the reuse stability of the H-Fe/meso-C catalyst. In this study, the final degradation could reach up to 96% in 10 h and the leaching of iron could be neglected (0.73 mg L⁻¹), measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES), which is much lower than the legal limit imposed by the directives of the European Union (2 mg L⁻¹).⁵⁰ Hence, the H-Fe/meso-C catalyst has been proven to be an attractive alternative in the treatment of environmental refractory organic pollutants and have a unique and superb catalytic activity in the heterogeneous Fenton-like system.

4. Conclusions

In this study, ordered mesoporous carbon supported Fe₂O₃ nanoparticles underwent hydrophilic surface modification with hydrogen peroxide, which shows enhanced adsorption and heterogeneous Fenton oxidation performance for methylene blue. The morphology, crystallinity and structure of the catalysts before and after Fenton-like reaction were characterized by XRD, TEM, TGA and N₂ sorption–desorption. The mesoporous structure of the carbon support remains and the highly dispersed iron particles of the catalyst increase the degradation performance of MB after hydrogen peroxide treatment. With the optimal oxidation conditions of 30 mg L⁻¹ MB solution, 0.7 g L⁻¹ catalyst, 50 mmol L⁻¹ H₂O₂ and the initial pH value, the adsorption and heterogeneous degradation of MB could achieve about 96% in 220 min. After the modification with hydrogen peroxide, the hydrophilic H-Fe/meso-C catalyst exhibits a higher degradation rate in the Fenton-like catalytic system. Based on the small amount of iron leaching and the stability of the 2-D hexagonally arranged pore structure, the ordered mesoporous carbon supported Fe nanoparticles treated with hydrogen peroxide could be used as a promising cyclically heterogeneous Fenton-like catalyst.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21101113, 51172157, 51202159, 51208357, 51472179, 51572192), Doctoral Program of Higher Education, Ministry of Education (20120032120017), General Program of Municipal Natural Science Foundation of Tianjin (13JCYBJC16900, 13JCQNJC08200).

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