Preparation of copper doped magnetic porous carbon for removal of methylene blue by a heterogeneous Fenton-like reaction

Abstract

High-specific-surface-area copper doped magnetic porous carbon (CuFe₂O₄/Cu@C) was fabricated by annealing iron, copper and 1,3,5-benzenetricarboxylic ([Cu/Fe]-BTC) metal–organic coordination polymers, which were prepared via a one-pot solvothermal method. The novel CuFe₂O₄/Cu@C catalyst consists of Cu (3.80%), CuFe₂O₄ (64.84%), and C (31.36%). Scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, elemental analysis, inductively coupled plasma, Brunauer–Emmett–Teller surface area measurement, and vibrating sample magnetometer analysis were used to characterize the materials. The as-prepared materials were employed as a heterogeneous Fenton’s reagent with the addition of H₂O₂ for degradation of methylene blue (MB). The results showed that the materials effectively catalyzed H₂O₂ to generate hydroxyl radicals (˙OH). And due to their magnetism, the materials can be easily separated from wastewater to achieve repeatability. It also turned out that CuFe₂O₄/Cu@C had a higher catalytic activity than Fe₃O₄@C, which proved the importance of copper doped into the catalyst. This work indicated that porous carbon composites provide good support for the development of a highly efficient heterogeneous Fenton catalyst, which is useful for environmental pollution cleanup.

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1. Introduction

Treatment and control of organic pollutants have become priority tasks in the drinking water industry. However, the removal of the most organic pollutants from drinking water is a difficult task, because their odor threshold is extremely low. Only a few water treatment technologies have been successfully applied to remove organic pollutants from water, including the use of physical methods,^1,2 biotechnology,³ and chemical technology.⁴ However, physical methods, such as liquid–liquid extraction, ion-exchange, adsorption, and air or steam stripping, are ineffective on pollutants that are not readily adsorbed or volatile; these approaches have disadvantages because they simply transfer the pollutants to another phase rather than destroying them.⁵ The adsorption process of activated carbon to remove organic pollutants is a recognized practice; however, the treatment may be costly.⁶ Ozone and hypochlorite oxidations are efficient decolorizing methods, but they are undesirable because of the high cost of equipment, operating costs, and secondary pollution.⁷

Recently, chemical treatment methods based on the generation of hydroxyl radicals, known as advanced oxidation processes (AOPs), have been recently applied for pollutant degradation because of the high oxidative power of the OH radical. These techniques generate a free hydroxyl radical (˙OH) that degrades most of the organic pollutants rapidly and non-selectively. The Fenton reaction is a well-studied AOP that uses hydrogen peroxide as an oxidant in the presence of a catalyst.^8–10 The nonselective attack of organic compounds by the hydroxyl radicals results in mineral end-products. The classical Fenton reagent consists of a homogeneous solution of iron ions and hydrogen peroxide. Catalysis is effective only within a low and narrow pH range. This process also produces iron sludge as a treatment byproduct, which is an additional environmental contaminant. Therefore, heterogeneous catalysts have been developed to overcome these problems.^11–14 The Fe²⁺ ion in Fe₃O₄ makes this compound a very efficient catalyst for the degradation of organic pollutants and allows it to be easily separated from the reaction medium using an external magnetic field. Therefore, magnetite is suitable as a heterogeneous Fenton catalyst. However, the aggregation of Fe₃O₄ nanoparticles, which is the major challenge in using catalytic nanoparticles in practical application, substantially reduces catalytic efficiency.¹⁵ Hence, significant efforts have been made to overcome these disadvantages.^10,16,17 Iron oxides can be immobilized in organic or inorganic supports to form the novel heterogeneous Fenton catalysts. Among the porous solids used as supports for iron phases, carbon materials, such as graphene¹⁸ and activated carbon,¹⁹ gained popularity because of their excellent chemical stability, mechanical strength, and large surface area. Porous carbon possesses high specific surface area that can improve the activity of iron oxide by preventing the aggregation and improving the dispersibility of the catalyst.

Metal–organic frameworks (MOFs) are crystalline materials composed of metal ions and organic ligands. The pore size and functional groups of MOFs can be adjusted given the variety of metal ions and organic ligands.²⁰ These distinct characteristics make MOFs excellent candidates for various applications, such as gas storage,^21,22 chemical separation,²³ drug delivery,²⁴ and catalysis.²⁵ In recent years, the utilization of MOFs as heterogeneous catalysts has received significant attention. However, the application of MOFs in special environments, such as acidic, alkali, and high-temperature environments, is limited because of their low stability. Moreover, the recycling of MOFs is a complicated task after the reaction. The utilization of MOFs as templates or precursor materials to produce porous carbonaceous materials has been demonstrated because MOFs have a large surface area and possess a significant amount of carbon.^26,27

In this work, we present a facile method to synthesize CuFe₂O₄/Cu@C composite materials derived from iron, copper, and 1,3,5-benzenetricarboxylic ([Cu/Fe]-BTC) metal–organic coordination polymers prepared by using a one-pot solvothermal method. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) spectra, vibrating sample magnetometry (VSM), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and Brunauer–Emmett–Teller (BET) analyses were used to characterize the magnetic porous composite material. The ability of the particles to facilitate the Fenton oxidation of methylene blue (MB) was tested under varying conditions of initial pH, H₂O₂ concentration, NH₂OH, catalyst dosage concentration, MB concentration, and temperature. The stability and reusability of the magnetic porous carbon spheres were also tested. Moreover, the catalytic activity of CuFe₂O₄/Cu@C and Fe₃O₄@C were compared.

2. Experimental

2.1 Materials

Cu(NO₃)₂ and FeCl₃ were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). Benzene-1,3,5-tricarboxylicacid (H₃BTC) and hydroxylamine hydrochloride (HONH₃Cl) were purchased from Aldrich. Sodium hydroxide and hydrogen peroxide were purchased from Shanghai Chemical Reagents Company. MB was supplied by Tianjin Tiantai Fine Chemicals Co., Ltd. All the chemicals were of analytical grade.

2.2 Synthesis of CuFe₂O₄/Cu@C composite

CuFe₂O₄/Cu@C composites were prepared by a solvothermal method under autogenous pressure. In a typical experiment, 0.3244 g FeCl₃ (2 mmol), 0.3624 g Cu(NO₃)₂ (1.5 mmol) and 0.6305 g H₃BTC (3 mmol) were firstly dissolved in 20 mL ethanol under vigorous stirring for 30 min. Then the mixture was sealed in a 60 mL Teflon-lined stainless-steel autoclave and heated at 78 °C for 8 h. After that, the autoclave was carefully removed and allowed to cool to room temperature. The as-synthesized polymer was washed with ethanol several times and centrifuged and then dried at 60 °C in vacuum for 12 h. The dried polymer powder was obtained. Finally, the resulting powder was annealed at 500 °C in nitrogen atmosphere for 3 h. After cooling to room temperature, the as-prepared CuFe₂O₄/Cu@C composite were collected for use as follows.

2.3 Synthesis of Fe₃O₄@C composite

For comparison, Fe₃O₄@C composites were also prepared by a solvothermal method. In a typical experiment, 1.6220 g FeCl₃ and 1.4010 g H₃BTC were dissolved in 30 mL ethanol under vigorous stirring for 30 min. The following process was just the same as the synthesis of the CuFe₂O₄/Cu@C composite.

2.4 Dye decoloration through a heterogeneous Fenton reaction

The catalytic property of CuFe₂O₄/Cu@C materials were explored by studying the change of the absorbance intensity at the maximum absorbance wavelength of the MB dye. Typically, 5 mg resulting catalysts were added to 10 mL of a 20 mg L⁻¹ MB solution in the presence of H₂O₂ and NH₂OH, and the suspension was shaken in a thermostated shaker at 130 rpm at 30 °C. After the reaction, the composites were separated using an external magnetic field, then the concentration of MB was determined using a UV-Vis spectrometer, with a maximum absorbance wavelength for MB at 664 nm. Total organic carbon (TOC) was determined by a TOC analyzer after filtration through a 0.45 μm membrane filter.

2.5 Characterization

The morphology of the sample was examined by scanning electron microscope (SEM, JSM-6701F, JEOL, Japan). FT-IR spectras were obtained in transmission mode on a FT-IR spectrometer (American Nicolet Corp., Model 170-SX) using the KBr pellet technique. XRD (Rigaku D/MAX-2400 X-ray diffractometer with Ni-filtered Cu Kα radiation (λ = 1.54056)) was used to investigate the crystal structure of the nanoparticles. The Fe and Cu content of the prepared nanocatalysts were obtained by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Magnetization measurements at room temperature were obtained using a vibrating sample magnetometer (LAKESHORE-7304, USA) at room temperature. The X-ray photoelectron spectroscopy (XPS) spectra were obtained with an ESCALab220i-XL electron spectrometer (VG Scientific) using 300 W Al-Kα radiation. The N₂ adsorption–desorption isotherm was measured at liquid nitrogen temperature (77 K) using a Micromeritics ASAP 2020 analyzer. The specific surface area was calculated by the BET method. The pore size distribution was obtained using the Barret–Joyner–Halenda (BJH) method. UV-Vis detection was carried out on a TU-1810PC UV-Vis spectrophotometer (Purkinje General, China). TOC was determined using a TOC analyzer (Elementar vario TOC cube, Hanau, Germany).

3. Results and discussion

3.1 Morphology and structure characterization

A solvothermal method was used to prepare CuFe₂O₄/Cu@C and Fe₃O₄@C composite materials under autogenous pressure. Fig. 1 shows representative SEM images of CuFe₂O₄/Cu@C and Fe₃O₄@C composites. The SEM image (Fig. 1a) shows that the Fe₃O₄@C composite is in an amorphous state and the Fe₃O₄ crystal disperses well in the substrate (Fig. 1b and c). Based on Fig. 1d–f, the CuFe₂O₄/Cu@C composites have the same amorphous states.

Fig. 1 SEM images of Fe₃O₄@C (a–c) and CuFe₂O₄/Cu@C (d–f).

PXRD analysis also provided information on the composition of the as-synthesized samples. To verify the formation of CuFe₂O₄/Cu@C and Fe₃O₄@C composites, we obtained XRD patterns of [Cu/Fe]-BTC and Fe-BTC samples, as shown in Fig. 2. The position of the two groups of diffraction peaks is consistent with the standard XRD data for the MIL-100 (Fe) in Fig. 2a and b. The results are also consistent with other findings in the literature.^28,29 The finding demonstrates the formation of [Cu/Fe]-BTC and Fe-BTC composites. After the carbonization process, some diffraction peaks disappeared. As shown in Fig. 2c, the diffraction peaks of Fe₃O₄@C at 2θ = 30.26°, 35.66°, 43.22°, 53.62°, 57.21°, and 62.74° are ascribed to the (220), (311), (400), (422), (511), and (440) planes, which agree well with the standard XRD data for the cubic phase Fe₃O₄ (JCPDS card, file no. 89-4319), with a face-centered cubic structure.³⁰ This result indicates the formation of the crystal Fe₃O₄. Meanwhile, the diffraction peaks of CuFe₂O₄/Cu@C at 2θ = 30.17°, 35.58°, 43.36°, 53.52°, 57.21°, 62.67°, and 74.22° were ascribed to the (220), (311), (400), (422), (511), (440), and (533) planes, which agree with the standard XRD data for CuFe₂O₄ (JCPDS card, file no. 77-0010). However, the diffraction peaks at 2θ = 43.36° and 74.22° were particularly strong, and an additional diffraction peak appeared at 2θ = 50.54°. As stated in a previous report,³¹ the diffraction peaks at 2θ = 43.36°, 74.22°, and 50.54° belong to (111), (200), and (220) planes, which agree well with Cu⁰ (JCPDS card, file no. 70-3039). This result indicates that the CuFe₂O₄/Cu@C composite was formed.

Fig. 2 XRD patterns of (a) Fe-BTC, (b) [Cu/Fe]-BTC, (c) Fe₃O₄@C and (d) CuFe₂O₄/Cu@C.

EA and ICP-AES analyses determined the chemical composition of the composite. Fe₃O₄@C consists of Fe (30.61%) and C (37.37%), and CuFe₂O₄/Cu@C consists of Cu (19.31%), Fe (21.84%), and C (31.36%). The relative molar ratio of Cu to Fe was 0.88 based on the quantitative analyses. This result is consistent with that of XRD and confirms the existence of Cu⁰.

XPS was employed to investigate the chemical state of the surface of the obtained Fe₃O₄@C and CuFe₂O₄/Cu@C. Fig. S1a† exhibits the overall survey of the as-synthesized Fe₃O₄@C composite and clearly shows the signals of elemental C, O, and Fe. Fig. S1a† clearly illustrates that the signal of C is significantly stronger than those of other elements (i.e., O and Fe) on the catalyst surface. This phenomenon results from the carbonization process. In addition, Fig. S1b† shows the XPS spectra of the Fe 2p spectrum. The Fe 2p spectrum can be deconvoluted into two peaks centered at 725.2 and 711.1 eV, which correspond to the peaks of Fe 2p_1/2 and Fe 2p_3/2. Meanwhile, the Fe 2p_3/2 spectrum was resolved into three peaks at 710.0, 711.1, and 712.4 eV, which compared well with Fe²⁺ octahedral species, Fe³⁺ octahedral species, and Fe³⁺ tetrahedral species. In addition, the Fe 2p_1/2 spectrum has the same regulation. The position of the Fe 2p peaks corresponds to that of Fe₃O₄.³² Fig. S1c† shows the XPS spectra of O 1s, which can be deconvoluted into three peaks centered at 529.9, 531.6, and 533.1 eV that correspond to C–O, C=O, and O–C=O species. Fig. S1d† shows the XPS spectra of C 1s, which also proves the existence of the C–O, C=O, and O–C=O species. Fig. S2† shows the overall survey of the CuFe₂O₄/Cu@C composite, which clearly indicates the signals of elemental C, O, Cu, and Fe. Fig. S2b and S2c† show the XPS spectra of C 1s and O 1s that verify the existence of C–O, C=O, and O–C=O species. The Fe 2p spectrum (Fig. S2d†) can be deconvoluted into two peaks centered at 725 and 710 eV, which correspond to the peaks of Fe 2p_1/2 and Fe 2p_3/2. Meanwhile, the Fe 2p_3/2 (Fe 2p_1/2) spectrum was resolved into two peaks at 710.9 and 712.9 eV (724.5 and 727.4 eV), which compared well with the Fe³⁺ octahedral species and the Fe³⁺ tetrahedral species. Fig. S1e† shows the XPS spectra of the Cu 2p spectrum. The Cu 2p_3/2 (Cu 2p_1/2) can be deconvoluted into three peaks centered at 933.9, 935.2, and 931.9 eV (953.6, 955.1, and 951.6 eV), which compared well with the Cu²⁺ octahedral species, the Cu²⁺ tetrahedral species, and Cu⁰, respectively.^33,34 This result also confirms the existence of Cu⁰.

The BET surface area was measured using the N₂ adsorption/desorption isotherms at −197.2 °C. Prior to measurements, the samples were evacuated at 100 °C for 10 h. The BET surface areas (Fig. 3) of Fe₃O₄@C and CuFe₂O₄/Cu@C were 168.95 and 160.74 m² g⁻¹, respectively. The high BET surface areas of the catalyst supports should be useful in pollutant removal by increasing the contact frequency between the pollutants and catalyst. The inset figures show the pore size distribution curve of Fe₃O₄@C and CuFe₂O₄/Cu@C. The adsorption average pore widths were 4.31 and 2.11 nm, respectively. The two composites exhibited a significant difference in the pore size distribution because of the Cu⁰ block part of the channel pore.

Fig. 3 N₂ adsorption–desorption isotherms of (a) Fe₃O₄@C and (b) CuFe₂O₄/Cu@C. The pore-size-distribution curve of (inset a) Fe₃O₄@C and (inset b) CuFe₂O₄/Cu@C obtained from the desorption data through the BJH method.

The magnetic properties of the resultant core–shell microspheres were investigated at room temperature with vibrating sample magnetometry in the field range −10 kOe to 10 kOe (Fig. 4). The magnetization saturation values of Fe₃O₄@C and CuFe₂O₄/Cu@C were 41.84 and 33.36 emu g⁻¹, respectively. The measured values indicated the strong magnetic properties of the prepared catalysts. All of the curves present a magnetic hysteresis loop that confirms the strong magnetic response to a varying magnetic field. Furthermore, Fe₃O₄@C and CuFe₂O₄/Cu@C indicated ferromagnetic-type curves that show a hysteresis loop. The values of coercivity and remanence are summarized in Table 1. The rapid aggregation of the catalysts can be observed from their homogeneous dispersion in the presence of an external magnetic field applied for 20 s. This observation directly demonstrates the convenient separation of catalysts through an external magnetic field, which is important in practical manipulation.

Fig. 4 Magnetic hysteresis loops of Fe₃O₄@C (a), and CuFe₂O₄/Cu@C (b). The inset is a magnified view of the magnetization versus field curves.

Table 1 Magnetic properties of Fe₃O₄@C and CuFe₂O₄/Cu@C

Samples	Magnetic properties
	MS (emu g^−1)	HC (Oe)	MR (emu g^−1)
Fe3O4@C	41.84	103	4.77
CuFe2O4/Cu@C	33.36	117	3.69

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3.2 Catalytic properties of magnetic porous carbon composite

To prove that doping with copper can improve catalytic activity, we subsequently selected CuFe₂O₄/Cu@C for further analysis as a catalyst for MB degradation. The ability of CuFe₂O₄/Cu@C to remove MB was tested in different systems (Fig. 5). Adding the CuFe₂O₄/Cu@C composite to a MB solution resulted in almost 18.2% MB removal, which was likely due to adsorption only. Using the CuFe₂O₄/Cu@C composite in the presence of both NH₂OH and H₂O₂ led to the removal of nearly all of the MB within 30 min. The degradation of MB was compared using pristine Fe₃O₄,³⁵ prepared by traditional coprecipitation. The CuFe₂O₄/Cu@C composite performed more efficiently than the unmodified Fe₃O₄ using similar dosages of the catalyst. Conversely, when NH₂OH and H₂O₂ were added into the MB solution without the CuFe₂O₄/Cu@C composite, only 2% of the MB was removed because of the free radicals formed by H₂O₂ in the presence of the reducing reagent NH₂OH.³⁶ This result suggests that the CuFe₂O₄/Cu@C composite helped degrade MB through adsorption, thus improving the reaction rate.

Fig. 5 Removal efficiency of MB under different conditions within 30 min. Reaction conditions: initial MB concentration, 20 mg L⁻¹; initial H₂O₂ concentration, 16 mmol L⁻¹; NH₂OH concentration, 4 mmol L⁻¹; catalyst load, 0.5 g L⁻¹; initial solution pH, 7.0; T = 303 K.

The progress of MB degradation can be easily followed by the decrease in absorbance at λ_max (664 nm) of MB with time. Fig. 6 shows the successive UV-Vis spectra of MB in the presence of the CuFe₂O₄/Cu@C composite, H₂O₂, and NH₂OH in aqueous solution. The adsorption bead decreased rapidly and disappeared in 30 min without the appearance of new adsorption bands in the visible or ultraviolet regions. This observation indicates that the benzene ring and heteropolyaromatic linkages of MB were likely destroyed and MB was almost completely degraded. Therefore, pseudo-first order kinetics on the degradation of MB, described as ln(C/C₀) = −kt, can be applied to better understand the MB degradation reaction kinetics. C is the concentration of MB at time t, C₀ is the initial concentration of MB, and k is the rate constant that can be calculated from the slope of the straight line. The inset in Fig. 6 reveals the linear relationship between ln(C/C₀) and the reaction time t, which matches well with the first-order reaction kinetics. The rate constant k of MB degradation was calculated to be 0.1094 min⁻¹ at 30 °C with CuFe₂O₄/Cu@C composite. The calculated value is significantly higher than the literature reported value of 2.35 × 10⁻³ min⁻¹ using titanomagnetite as the heterogeneous catalyst³⁷ and 5.73 × 10⁻² min⁻¹ using FeS as the heterogeneous catalyst.³⁸

Fig. 6 UV-Vis spectral changes of the 20 mg L⁻¹ MB solution in the degradation process as a function of the reaction time in the presence of 0.5 g L⁻¹ CuFe₂O₄/Cu@C, 6.67 mmol L⁻¹ NH₂OH, and 5.33 mmol L⁻¹ H₂O₂ at T = 303 K and pH 5.0. The inset figure is a first-order linear relationship.

The as-synthesized CuFe₂O₄/Cu@C composite showed excellent degradation compared with Fe₃O₄@C (Fig. 7). The dynamic degradation curve shows that the removal efficiency reached 100% within 15 min in the presence of the CuFe₂O₄/Cu@C composite. However, when Fe₃O₄@C worked as a catalyst, MB degradation was only 56% after 15 min. To Fe₃O₄, the value was only 2%. Fig. 7b shows that the homogeneous solution generated more ˙OH in the presence of the CuFe₂O₄/Cu@C composite. The result indicates the synergistic effect of Cu⁰ in the generation of ˙OH.

Fig. 7 Removal efficiency of MB under different catalysts (a), the dosage of HO˙ generated under different catalysts (b). Conditions: (a) MB, 20 mg L⁻¹; H₂O₂, 5.33 mmol L⁻¹; NH₂OH, 6.67 mmol L⁻¹; catalysts, 0.5 g L⁻¹; T = 303 K; pH 5.

3.3 Exploration of the degradation conditions

3.3.1 Effect of pH. An assessment of the effects of pH was initially investigated in the presence of H₂O₂ and NH₂OH (Fig. 8a). Tests were conducted at pH 3.02, 5.12, 8.14, and 10.11. MB degradation was nearly 100% within 15 min at pH 3.02, 5.12, and 8.14. However, the removal efficiency of the composite decreased to 80% at pH 10.11. Generally, pH 3.0 is optimal for organic pollutant degradation by Fenton systems.³⁹ The best pH value for the degradation of MB in this Fenton-like system was approximately 3, as seen in Fig. 8a. This observation is consistent with the classical Fenton reaction. The classical Fenton reagent is effective only within a low and narrow pH range, but the CuFe₂O₄/Cu@C composite resulted in a broader range for the Fenton reaction. The degradation of MB is less efficient at a higher pH because of the instability of H₂O₂ in an alkaline solution; the production of ˙OH on the surface of CuFe₂O₄/Cu@C composite was gradually restricted with increasing pH, which also resulted in a slower degradation rate of MB.⁴⁰ The process of H₂O₂ activation by CuFe₂O₄/Cu@C composite is speculated to involve the following steps:^41–43

HO˙ + Cu → Cu⁺ + OH⁻ (1)

Cu²⁺ + NH₂OH → Cu⁺ + ˙ONH₂ + H⁺ (2)

5Fe³⁺ + ˙ONH₂ + 2H₂O → 5Fe²⁺ + NO₃⁻ + 6H⁺ (3)

Fe³⁺ + Cu⁺ → Fe²⁺ + Cu²⁺ (4)

Fe²⁺·H₂O + H₂O₂ → Fe²⁺·H₂O₂ (5)

Fe²⁺·H₂O₂ → Fe³⁺ + ˙OH + OH (6)

˙OH + MB → CO₂ + H₂O (7)

Fig. 8 Effect of the (a) pH, (b) H₂O₂ concentration, (c) NH₂OH concentration, (d) dosage of the catalyst, (e) MB concentration, and (f) temperature conditions: (a) MB, 20 mg L⁻¹; H₂O₂, 5.33 mmol L⁻¹; NH₂OH, 6.67 mmol L⁻¹; CuFe₂O₄/Cu@C, 0.5 g L⁻¹; T = 303 K; (b) MB, 20 mg L⁻¹; pH, 5.0; NH₂OH, 6.67 mmol L⁻¹; CuFe₂O₄/Cu@C, 0.5 g L⁻¹; T = 303 K; (c) MB, 20 mg L⁻¹; H₂O₂, 5.33 mmol L⁻¹; pH, 5.0; CuFe₂O₄/Cu@C, 0.5 g L⁻¹; T = 303 K; (d) MB, 20 mg L⁻¹; H₂O₂, 5.33 mmol L⁻¹; NH₂OH, 6.67 mmol L⁻¹; pH, 5.0; T = 303 K; (e) pH, 5.0; H₂O₂, 5.33 mmol L⁻¹; NH₂OH, 6.67 mmol L⁻¹; CuFe₂O₄/Cu@C, 0.5 g L⁻¹; T = 303 K; (f) MB, 20 mg L⁻¹; H₂O₂, 5.33 mmol L⁻¹; NH₂OH, 6.67 mmol L⁻¹; CuFe₂O₄/Cu@C, 0.5 g L⁻¹; pH, 5.0.

3.3.2 Effect of H₂O₂ concentration. Another important parameter in removing MB is the concentration of H₂O₂. The added amount of H₂O₂ varied from 2.67 mmol to 10.67 mmol (Fig. 8b). The removal efficiency of MB improved as the H₂O₂ concentration increased from 2.67 mmol L⁻¹ to 5.33 mmol L⁻¹, but the removal efficiency did not change significantly from 5.33 mmol L⁻¹ to 10.67 mmol L⁻¹. This observation may have occurred because H₂O₂ could not generate a sufficient number of ˙OH radicals, which slowed the oxidation rate and reduced the removal efficiency at low concentrations. Higher concentrations of H₂O₂ could generate enough ˙OH radicals. However, the use of excess H₂O₂ is wasteful and costly, and this process can also reduce the efficiency because of the hydroxyl radical scavenging effect⁴⁴ (eqn (8) and (9)). Hence, 5.33 mmol L⁻¹ is optimal for MB degradation by the Fenton-like system.

H₂O₂ + ˙OH → H₂O + ˙OOH (8)

˙OOH + ˙OH → H₂O + O₂ (9)

3.3.3 Effect of NH₂OH concentration. NH₂OH is a common reducing agent that is used to eliminate problems associated with the traditional homogenous Fenton system, such as the accumulation of Fe³⁺ and the narrow pH range limits.⁴⁵ Research indicates that adding NH₂OH to a Fenton-like system could accelerate the redox cycles of Fe³⁺ to Fe²⁺. Hence, compared with no NH₂OH, the new system notably alleviated the Fe³⁺ accumulation, enhanced the reaction rates, and expanded the effective pH range. The generation of ˙OH was significantly faster because of the participation of NH₂OH. In addition, the NO₃⁻ and N₂O were proposed as the dominant end-products of NH₂OH. Considering the excellent effects of NH₂OH in a homogenous Fenton system, we added it into a heterogeneous Fenton-like system to evaluate its synergistic effect. The added amount of NH₂OH varied from 1.67 to 6.67 mmol L⁻¹ (Fig. 8c). The degradation percentage of MB increased with NH₂OH concentration. When the NH₂OH concentration reached 6.67 mmol L⁻¹, the removal efficiency was above 100%. These results indicate that NH₂OH also produced a positive effect in the heterogeneous Fenton-like system. NH₂OH accelerated the redox cycles of Fe³⁺ to Fe²⁺ involving the following steps:

Fe³⁺ + NH₂OH → Fe²⁺ + ˙ONH₂ + H⁺ (10)

Fe³⁺ + ˙ONH₂ → Fe²⁺ + NHO + H⁺ (11)

2NHO → N₂O + H₂O (12)

3.3.4 Effect of catalyst dosage. The use of excessive catalyst is wasteful and costly; hence, the dosage of the catalyst was also evaluated using the CuFe₂O₄/Cu@C composite of 0.2, 0.5, 1.0, and 1.5 g L⁻¹ in degradation reactions. The initial rates increased with the amount of CuFe₂O₄/Cu@C composite. The improved efficiency caused by increased catalyst loading introduced more active sites, which generated more free-radical species that promote the degradation reaction. In Fig. 8d, the removal efficiency reached 100% within 20 min when the catalyst concentration was 0.5 g L⁻¹.

3.3.5 Effect of MB dosage. The influence of MB dosage was investigated using the MB at 20, 60, and 100 mg L⁻¹ (Fig. 8e). The efficiency decreased because of the increased concentration of MB within 30 min. This result indicated that the extension of the reaction time is necessary when dealing with a large amount of organic pollutants.

3.3.6 Effect of the temperature. Determining the reaction rates under different temperatures is often very useful in providing insights on reaction mechanisms. Increasing temperature usually causes a significant increase in the reaction rate depending on the reaction processes. Experiments on the effect of temperature were conducted to evaluate the activation energy (Fig. 8f). An Arrhenius equation describing the relationship between rate constants and temperature was applied:
where k is the measured first-order rate constant, A₀ is a frequency factor, E_a is the activation energy, R is the ideal gas constant, and T is the temperature. The activation energy for the degradation of MB was measured in a closed batch system, and the temperature varied from 293 K to 313 K. The value of the activation energy was 38.16 kJ mol⁻¹, which indicates that the reaction is a surface-controlled reaction.⁴⁶ The rate constants at different temperatures are summarized in Table 2.

Table 2 The measured rate constants of MB degradation at different temperatures

T (K)	CuFe2O4/Cu@C
	k (min^−1)	R^2
293	0.06562	0.987
303	0.10942	0.998
313	0.17850	0.989

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3.4 Stability and reuse of the catalyst

The stability of the CuFe₂O₄/Cu@C composite is a critical factor for its practical applications. To evaluate the catalyst stability of the catalysts, we carried out recycling reactions for the degradation of MB. The solid catalyst was magnetically separated from the reactant mixture, washed with ethanol, and vacuum-dried at 60 °C after each catalytic cycle. The MB concentrations were detected to evaluate the catalytic activity in the repeated process, and the initial MB concentration was 20 mg L⁻¹. As shown in Fig. 9, the catalyst retained its original activity after 10 repeated reuses. Both the MB and TOC concentrations were detected to evaluate the catalytic activity in the repeated process. This result indicates that the efficiency of TOC removal was lower than the MB removal efficiency with each repetition; the maximum efficiency of TOC removal was 70%, whereas it was nearly 100% in MB removal. The eventual decrease of the catalytic activity may have been caused by the incomplete removal of byproducts during washing. The stability of the CuFe₂O₄/Cu@C composite was better compared with other heterogeneous Fenton catalysts.^47,48 To investigate the structure of the catalyst after the Fenton reaction, XPS was used after 10 repeated reuses (Fig. 10). The details of the Fe 2p peaks and Cu 2p peaks of the CuFe₂O₄/Cu@C composite before and after use during MB degradation are presented in Table 3. This result indicates the presence of one oxidation state for the surface iron species before being used. The Fe 2p_3/2 (Fe 2p_1/2) spectrum was resolved into two peaks at 710.9 and 712.9 eV (724.5 and 727.4 eV), which compared well with the Fe³⁺ octahedral species and Fe³⁺ tetrahedral species. For the sample after MB degradation, Fe³⁺ was 68.6% and Fe²⁺ was 31.4%. These values indicate that part of Fe³⁺ in the outermost layer of the catalyst was deoxidized into Fe²⁺ during the Fenton reaction. For the copper species, Cu 2p_3/2 (Cu 2p_1/2) can be deconvoluted into three peaks centered at 933.9, 935.2, and 931.9 eV (953.6, 955.1, and 951.6 eV), which compared well with Cu²⁺ octahedral species, Cu²⁺ tetrahedral species, and Cu⁰, respectively. However, the percentage of Cu²⁺ decreased from 80.3% to 60.6% after MB degradation. Copper was involved in the heterogeneous Fenton-like reaction. All of these results further indicate that the catalyst exhibited excellent durability because of the protective effects of the porous carbon support and the presence of NH₂OH.

Fig. 9 Reuse of the catalyst. Reaction conditions: MB, 20 mg L⁻¹; H₂O₂, 5.33 mmol L⁻¹; NH₂OH, 6.67 mmol L⁻¹; CuFe₂O₄/Cu@C, 0.5 g L⁻¹; T = 303 K; pH, 5.

Fig. 10 XPS spectrum of iron before (a) and after (c) and copper before (b) and after (d) degradation of MB.

Table 3 XPS analysis of iron and copper on CuFe₂O₄/Cu@C before and after degradation of MB

		Fe^2+	Fe^3+	Cu	Cu^+	Cu^2+
Binding energy (eV)	Fresh	—	711.4	931.9	—	934.1
	Used	710.3	711.9	931.9		934.3
The ratio of element valence state (%)	Fresh	0	100	19.7	—	80.3
	Used	31.4	68.6	39.4		60.6

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4. Conclusion

In conclusion, the CuFe₂O₄/Cu@C composite was fabricated by annealing [Cu/Fe]-BTC at 500 °C in an inert atmosphere. The resultant composite material showed excellent catalytic performance in MB degradation and superparamagnetic behavior that enabled magnetic separation and convenient recovery from the reaction mixture. Moreover, the composite was stable and reusable, indicating that it could overcome the drawbacks of homogeneous catalysts. For comparison, the Fe₃O₄@C composite was fabricated using the same strategy. Results showed that CuFe₂O₄/Cu@C had a higher catalytic activity than Fe₃O₄@C, which proved the importance of copper doped into the catalyst. Therefore, the composite catalysts show great potential for industrial applications.

Acknowledgements

The authors would like to express their appreciation for research funding provided by the National Natural Science Foundation of China (No. 21374045, 21074049) and the National Science Foundation for Fostering Talents in Basic Research of the National Natural Science Foundation of China (Grant No. J1103307).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12621e

‡ These authors contributed equally to this work.

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