Iron–copper bimetallic nanoparticles supported on hollow mesoporous silica spheres: the effect of Fe/Cu ratio on heterogeneous Fenton degradation of a dye

Abstract

A series of iron–copper bimetallic nanoparticles supported on hollow mesoporous silica spheres with different Fe/Cu ratios were prepared using a simple post-impregnation and sodium borohydride reduction strategy. Different Fe/Cu ratios were obtained by addition of various amounts of ferric and copper salts. The catalysts were characterized by XRD, XPS, nitrogen physisorption, SEM and TEM. To explore the difference of catalytic reactivity among the various FeCu/HMS composites, Fenton-like catalytic oxidation of orange II was chosen as the model catalysis reaction. The catalytic results showed that the catalytic activity depended highly upon the Fe/Cu ratio. Particularly, 1/3 (2Fe6Cu/HMS) is the optimum Fe/Cu mass ratio to achieve the best catalytic activity. Based on the results of detection of ˙OH radicals and cyclic voltammograms, the important origin of the synergetic effect in 2Fe6Cu/HMS is considered as the combination of iron and copper species, which can accelerate the interfacial electron transfer in redox cycles of Fe³⁺/Fe²⁺ and Cu²⁺/Cu⁺ pairs, followed by the increase in the ˙OH radical generation. The 2Fe6Cu/HMS also possesses less pH dependence and maintains its high activity even under alkaline conditions. The optimal parameters in the degradation of 100 mg L⁻¹ orange II are 27.4 mM of H₂O₂ with 1.0 g L⁻¹ of catalyst, performed at a pH of 7.0 and 30 degree celsius. It is worth noting that 2Fe6Cu/HMS also possesses potential to treat a high concentration of dye pollutants. 77.7% of orange II was removed when conducted at 1000 mg L⁻¹ initial dye concentration. The stability and recoverability of the composite catalyst were assessed it exhibited a good performance after 5 consecutive runs. The as-synthesized composite catalyst proved to be an attractive alternative for dye wastewater treatment applications.

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1. Introduction

Water resources have become a significant issue in recent decades due to restricted supply and severe water pollution. Dyeing wastewater is one of the major sources of water pollution which has received wide attention.^1,2 Due to the existence of toxic, non-biodegradable and refractory pollutants, dyeing wastewater is extremely difficult to treat by the traditional methods, such as biological treatment and physical adsorption, which only concentrate the pollutants and require a further secondary process for complete destruction of dyes.^1,3,4 Advanced oxidation technologies (AOTs) involving the production of highly reactive hydroxyl radicals (˙OH) have shown great potential for the treatment of organic pollutants. Among them, Fenton processes are used as a powerful source of hydroxyl radicals from H₂O₂ in the presence of Fe²⁺/Fe³⁺ to decompose many organic compounds including dyes.⁵ However, the usage of homogeneous Fenton processes exist certain disadvantages such as strong acid condition demanded, impossible regeneration of catalyst and treatment of iron-containing sludge presented in discharges.^6,7 Theses drawbacks stimulate the development of heterogeneous Fenton processes by using solid catalysts such as zero-valent iron and iron oxide minerals.^8–11 However, these catalysts show relatively inert activity if operated at high pH values or without external power supplies, which will increase input energy and extra cost.^12,13

Recently, another metal species, copper, which possesses less pH dependent property and a similar role as iron reacting with hydrogen peroxide to produce hydroxyl radicals, has attracted great attention. The synergistic effects of two-metal redox couples between iron and copper could enhance the catalytic activity of catalyst. Combination of iron with copper to fabricate iron–copper bimetallic nanoparticles as heterogeneous catalyst has become a hotspot in Fenton chemistry, especially for supporting iron–copper bimetallic nanoparticles on matrixes as the composite catalysts. Various supports, such as zeolite,¹⁴ clay,¹⁵ PAN fiber,¹⁶ MCM-41,¹⁷ ZSM-5 (ref. 18) and ordered mesoporous carbon materials¹³ have been devoted to fabricated iron–copper composite catalysts. Nevertheless, the nanoparticles in these supports are basically in a two-dimensionally confined space, which would limit the efficiency of reaction between the contact interfaces for reactants.¹⁹ Hollow mesoporous silica spheres (HMS), which have an interior cavity transfixed by mesoporous silica shell, have aroused striking attention due to their unusual structures.^20–23 Specifically, the “open” shell morphology and hollow interior allows the substrates more easy access to active sites, and their inner-outer shell surfaces facilitate contact with reactant molecules.^24–26 In previous work,²⁷ we have prepared a heterogeneous Fenton composite catalyst (FeCu/HMS) by using the hollow mesoporous silica spheres (HMS) as support for the iron–copper bimetallic nanoparticles. The efficient removal of orange II dye was achieved and the catalytic activity of FeCu/HMS was higher than the solid-core without hollow structured mesoporous silica spheres as supports (FeCu/MS). According to the previous reports,^14–16,28 it was confirmed that various ratio of iron to copper significantly influence their catalytic activity on degradation of organic compounds, which is due to the synergistic effects of two-metal redox couples between iron and copper. However, the optimal ratio of iron to copper was various in different catalytic systems, which inspired us to explore the effect of different Fe/Cu ratio on their catalytic activity in FeCu/HMS heterogeneous Fenton system.

Herein, a series of iron–copper bimetallic nanoparticles supported on HMS with different Fe/Cu ratio were prepared using a simple post-impregnation and sodium borohydride reduction strategy. The different Fe/Cu ratios were obtained by addition of various amounts of ferric and cupric salts. The obtained catalysts were used as the heterogeneous Fenton catalyst in degradation of orange II, a typical azo dye which is widely used in textile industry. The primary objective of this study is to examine the catalytic activity of different Fe/Cu ratios on dye degradation. It is very important to explore the role of iron and copper in the Fe–Cu bimetal hollow mesoporous silica spheres composite catalyst for effective treatment of organic pollutants. Furthermore, a possible mechanism was proposed based on the detection of ˙OH and changes of redox potentials in cyclic voltammograms. High-performance liquid chromatograph-mass spectrometer (HPLC-MS) was applied to determine the intermediates in order to propose the possible degradation pathway of orange II. Finally, stability and reusability for degradation of the catalyst was evaluated.

2. Experimental

2.1. Chemicals

Analytical reagents of tetraethylorthosilicate (TEOS), concentrated ammonia aqueous solution (NH₃·H₂O 25 wt%), absolute ethanol, hydrogen peroxide (H₂O₂, 30 wt%), nitric acid (HNO₃, 65 wt%), cetyltrimethylammonium bromide (CTAB), ferrous sulfate (FeSO₄·7H₂O), copper nitrate (Cu(NO₃)₂·3H₂O), sodiumhydroxide (NaOH), sodium borohydride (NaBH₄) and acid orange II were purchased from Sinopharm Chemical Reagent Co., Ltd. Benzoic acid (BA) and P-hydroxy benzoic acid (PBA) were purchased from ANPEL Scientific Instruments (Shanghai) Co., Ltd. Titanyl sulphate and ammonium acetate were purchased from Aladdin Industrial Corporation. All chemicals were used as received without any further purification. Deionized water (Millipore) was used in all experiments.

2.2. Synthesis of hollow mesoporous silica sphere

The hollow mesoporous silica spheres were prepared according to the previous literature.²⁹ In a typical procedure, CTAB was dissolved in ethanol aqueous solution containing concentrated ammonia aqueous solution (1 mL, 25 wt%). Then, the mixture was heated to 35 °C, and TEOS (1 mL) was rapidly added under vigorous stirring. The molar ratio of the reaction mixture was 1.00 TEOS : 0.0922 CTAB : 2.96 NH₃ : 621 H₂O : 115 C₂H₅OH. After stirring at 35 °C for 24 h, the white product was collected by centrifugation at 4000 rpm for 10 min and washed three times with ethanol. To prepare the mesoporous silica hollow spheres, the as-made Stöber silica spheres were incubated in pure water (160 mL) at 70 °C for 12 h and collected by centrifugation, then washed three times with ethanol. The products were filtered and dried at 100 °C. Finally, the template removal was carried out in a muffle furnace at a heating rate of 1 °C min⁻¹ to 550 °C, and holding it for 6 h. The calcined samples were denoted as HMS.

2.3. Synthesis of iron–copper bimetallic composite catalysts with different Fe/Cu ratio

The FeCu/HMS bimetallic composites were prepared by a direct impregnation and chemical reduction method. The mass ratio of Fe : Cu were 8 : 0, 6 : 2, 4 : 4, 2 : 6, 0 : 8, respectively. And the mass ratio of (Fe + Cu) : SiO₂ were 0.08 : 1. In a typical procedure for mass ratio of Fe : Cu 2 : 6, 0.1 g of HMS was added into 5 mL aqueous solution containing 0.01 g FeSO₄·7H₂O and 0.023 g Cu(NO₃)₂·3H₂O with stirring for 1 h under N₂ atmosphere. Then the suspension was dried under vacuum at 50 °C, followed by the addition of 1 mL of fresh NaBH₄ aqueous solution with stirring for 3 h under N₂ atmosphere. The molar ratio of NaBH₄ to the total metal (iron and copper) (B/(Fe + Cu)) was 6/1 with adequate NaBH₄ for the formation of iron and copper nanoparticles. The products were immersed in methanol for 12 h. The obtained catalysts were collected by filtration and washed with methanol for three times. The final products were denoted as 2Fe6Cu/HMS. The other synthesized composites with Fe : Cu mass ratio of 8 : 0, 6 : 2, 4 : 4, 0 : 8, were denoted as 8Fe/HMS, 6Fe2Cu/HMS, 4Fe4Cu/HMS, 8Cu/HMS, respectively.

2.4. Characterization

The crystalline phase of the samples was identified by X-ray diffraction (XRD) patterns on a Bruker AXS D8 advance powder diffraction system using Cu Kα (λ = 1.5418 Å), operating at 40 kV and 40 mA. The mesoporous phases of the samples were identified at the range of 0.6–8° with the scanning speed of 0.01° s⁻¹. Wide diffraction angle analysis was conducted at the range of 10–95° by the same XRD instrument. The X-ray photoelectron spectra (XPS) spectra were obtained by using a PHI Quantera II ESCA System with Al Kα radiation at 1486.8 V. Nitrogen adsorption–desorption isotherms were collected at 77 K using a Micromeritics 2020 analyzer (USA). The surface morphology of various samples were examined by Quanta 250F scanning electron microscope (SEM) at an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) images were collected conducted on a TECNAI G2 20 LaB₆ electron microscope (200 kV). STEM-EDS images were performed by FEI Tecnai F30 field emission electron microscope (equipped with an energy-dispersive spectrometer, EDS) at 300 kV. To determine the metal loading contents in samples, all samples were digested by dilute nitric acid solution, then the mixture were filtered through 0.22 μm membrane filter and the metal ion containing filtrates were analyzed by inductive coupled plasma atomic emission spectroscopy (ICP-AES) (Optima 7000DV, PerkinElmer, USA). The iron and copper ions leaching of samples were determined by atomic absorption spectrometer (AAS) (PinAAcle 900T, PerkinElmer, USA). Cyclic voltammetric measurement of materials was performed on electrochemical workstation (CHI660E, CH Instrument, Shanghai) with a three-electrode electrochemical cell at 25 °C using a 1 M KOH electrolyte. The standard three-electrode electrochemical cell was fabricated using glassy carbon with deposited sample as the working electrode, platinum wire as the counter electrode, and Ag/AgCl as the reference electrode. The potential window for cycling was confined between −1.2 and 1.0 V.

2.5. Catalytic performance

A series of batch experiments were carried out to measure the catalytic activity of the samples by degrading orange II in aqueous solutions. In a typical experiment, 0.01 g of catalyst was added into 10 mL of orange II solution in a flask (50 mL). The pH of the reaction medium was adjusted by using an appropriate amount of 0.1 M NaOH or 0.1 M HNO₃ to a given value. The covered flasks were kept at a mild temperature (30 °C) in a thermostatic air bath oscillator with a constant speed of 200 rpm. Then, a certain concentration of H₂O₂ (30 wt%) was added to the suspension and initiated the degradation reaction. Each flask was taken out from the water bath at different time intervals and the supernatant solution was collected by filtration for UV-vis absorbance analysis using a universal microplate spectrophotometer (PowerWave XS). The used catalysts were collected by centrifugation then washing with methanol for stability tests. The quantitation of hydroxyl radicals and H₂O₂ concentrations were determined by a previous reported method in literature.^27,30 The degradation products of orange II were detected by high-performance liquid chromatograph-mass spectrometer (HPLC-MS) (Agilent 6410, Agilent Technologies Incorporation, USA), using Agilent Eclipse Plus-C18 columns (3.5 μm, 2.1 × 150 mm).

3. Results and discussion

3.1. Morphology and physicochemical properties of catalysts

Fig. 1 shows the low-angle (A) and wide-angle XRD (B) patterns of the obtained samples. In Fig. 1A, the low angle range of XRD pattern for pure HMS (a) exhibits an intense reflection at low 2θ, which is a typical worm-like ordered pore structure similar to the previous works.³¹ After incorporation with metal species, the peak intensities of the modified samples (b–f) decrease, indicated that a less ordered structure was formed and the ordered mesoporous framework of the materials still remained.^32,33 In Fig. 1B, all samples have the broad peak at about 22.8° which are refer to amorphous silica. Moreover, an obvious diffraction peak at 44.6° in Fe/HMS and a peak at 43.2° in Cu/HMS can be observed, which are corresponding to the (110) diffraction of α-Fe (JCPDS no. 06-0696) and (111) diffraction of copper (JCPDS no. 99-0034), respectively. Differently, only weaker and broader diffractions at 43.2° and 44.6° can be found in iron–copper containing samples (c–e). It may be attributed to low metal loading and small metal particles deposited on mesoporous silica spheres, which is consistent with the previous reports.^17,33

Fig. 1 (A) Small-angle and (B) wide-angle XRD patterns of (a) blank HMS, (b) 8Fe/HMS, (c) 6Fe2Cu/HMS, (d) 4Fe4Cu/HMS, (e) 2Fe6Cu/HMS and (f) 8Cu/HMS.

To further confirm the existence of metals in composite catalysts, iron and copper elements were verified by XPS results. In four iron-containing samples (Fig. 2Ab–e), the photoelectron peaks at 706.3 eV representing the binding energy of zero-valent iron (Fe 2p_3/2).³⁴ Moreover, the other two main bands centered at round 711.0 and 724.6 eV representing the binding energies of Fe 2p_3/2 and 2p_1/2, respectively. This result indicated that the nanoparticles were enveloped by a layer of iron oxides, which is due to the easily oxidized of zero-valent iron when exposed in air.^35,36 In Fig. 2B, two main photoelectron peaks at 932.3 eV (Cu 2p_3/2), 952.1 eV (Cu 2p_1/2) can be observed, which are in accordance with binding energy of zero-valent copper in four copper-containing samples (c–f).³⁷ It is worthy to note that the peak intensities of iron or copper in all composite catalysts enhanced with the increase of iron or copper loading mass, demonstrating that the different ratios of iron to copper in characterized samples. The certain amount of iron and copper in FeCu/HMS can be verified by ICP-AES (Table 1). The results showed that the metal loading contents of all the samples were approximate to the theoretical value with different Fe/Cu ratios, further confirming that the different components of iron–copper species were successfully supported on blank silica spheres.

Fig. 2 (A) Fe and (B) Cu XPS of (a) blank HMS, (b) 8Fe/HMS, (c) 6Fe2Cu/HMS, (d) 4Fe4Cu/HMS, (e) 2Fe6Cu/HMS and (f) 8Cu/HMS.

Table 1 Metal composition in synthesized samples

Sample	Fe content (wt%)	Theoretical value of Fe content (wt%)	Cu content (wt%)	Theoretical value of Cu content (wt%)
HMS	0	0	0	0
8Fe/HMS	7.2	7.4	0	0
6Fe2Cu/HMS	5.4	5.6	1.7	1.9
4Fe4Cu/HMS	3.3	3.7	3.4	3.7
2Fe6Cu/HMS	1.8	1.9	5.5	5.6
8Cu/HMS	0	0	7.4	7.4

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SEM images of all samples are shown in Fig. 3. The pure HMS possesses a spherical morphology with smooth surface and uniform diameter of about 560 nm (Fig. 3A). After incorporation of metal nanoparticles, the composite catalysts maintain the spherical morphology with some metal nanoparticles sticking on the outer shells. To further investigate the dispersion of metal nanoparticles and interior structure of as-synthesized materials, TEM images have been shown in Fig. 4. It can be observed that a noticeable contrast between the cavity and shell existed in all samples. Differently from blank HMS, the uniformly dispersed dark spots can be observed in metal supported samples, which corresponded to iron–copper bimetallic nanoparticles. Furthermore, the STEM and energy-dispersive X-ray spectroscopy (EDS) elemental mapping of 2Fe6Cu/HMS have been performed (Fig. 4G). It can also be seen that iron and copper are uniformly distributed on the silica matrix. The results of SEM, TEM, STEM and elemental mapping further demonstrated that the metal nanoparticles have been loaded in hollow mesoporous spheres.

Fig. 3 SEM of (A) blank HMS, (B) 8Fe/HMS, (C) 6Fe2Cu/HMS, (D) 4Fe4Cu/HMS, (E) 2Fe6Cu/HMS and (F) 8Cu/HMS.

Fig. 4 TEM of (A) blank HMS, (B) 8Fe/HMS, (C) 6Fe2Cu/HMS, (D) 4Fe4Cu/HMS, (E) 2Fe6Cu/HMS and (F) 8Cu/HMS; (G) STEM and elemental mapping images of 2Fe6Cu/HMS for Si, O, Fe and Cu with color superposition.

N₂ adsorption–desorption isotherms of the samples are illustrated in Fig. 5A. All of samples exhibit typical IV N₂ adsorption isotherms. Specifically, the inflections at P/P₀ = 0.2–0.3, revealing the narrow pore size distribution of mesoporous materials.²⁹ Nevertheless, the inflection become weaker after incorporating with metal, which reflects the decreasing of order mesoporous structures. This phenomenon is consistent with the XRD results. In addition, the hysteresis loops at higher relative pressure (P/P₀ > 0.8) of isotherms reveal the existence of interior cavity in hollow mesoporous spheres,³⁸ which are in agreement with the TEM observation. The BET surface area and total pore volume of 8Fe/HMS (592 m² g⁻¹, 0.41 cm³ g⁻¹), 6Fe2Cu/HMS (599 m² g⁻¹, 0.41 cm³ g⁻¹), 4Fe4Cu/HMS (578 m² g⁻¹, 0.41 cm³ g⁻¹), 2Fe6Cu/HMS (582 m² g⁻¹, 0.41 cm³ g⁻¹) and 8Cu/HMS (576 m² g⁻¹, 0.41 cm³ g⁻¹) are lower than those of blank HMS (765 m² g⁻¹, 0.56 cm³ g⁻¹) (Table 2), which may be attributed to the increment of sample density after loading metals. The BJH average pore size distribution derived from the adsorption branch for all samples are shown in Fig. 5B. The average pore size of 8Fe/HMS (2.31 nm), 6Fe2Cu/HMS (2.31 nm), 4Fe4Cu/HMS (2.31 nm), 2Fe6Cu/HMS (2.31 nm) and 8Cu/HMS (2.31 nm) are also smaller than those of blank HMS (2.51 nm), which may be due to the supported metals in mesopore channels.²⁷

Fig. 5 N₂ adsorption/desorption isotherms (A) and pore size distributions (B) of (a) blank HMS, (b) 8Fe/HMS, (c) 6Fe2Cu/HMS, (d) 4Fe4Cu/HMS, (e) 2Fe6Cu/HMS and (f) 8Cu/HMS.

Table 2 Textural properties of synthesized samples

Sample	SBET^a (m^2 g^−1)	VTotal^b (cm^3 g^−1)	DBJH^c (nm)
a The specific surface areas.b Total pore volume.c Pore diameter.
HMS	765	0.56	2.51
8Fe/HMS	592	0.41	2.31
6Fe2Cu/HMS	599	0.41	2.31
4Fe4Cu/HMS	578	0.41	2.31
2Fe6Cu/HMS	582	0.41	2.31
8Cu/HMS	576	0.41	2.31

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3.2. Catalytic tests of different Fe/Cu ratio catalysts

To evaluate the catalytic activity of catalysts with different Fe/Cu ratio on orange II degradation, the conditions of experiments were set at catalyst dosage 1 g L⁻¹, H₂O₂ 27.4 mM, pH at 7.0, 30 degree celsius and 100 mg L⁻¹ orange II. Fig. 6 shows the evolution of orange II removal within 2 h for these experiments. It can be seen that catalyst free (only using H₂O₂ as the oxidant) only got 18.7% orange II abatement after 2 h reaction. With addition of 8Fe/HMS, 6Fe2Cu/HMS, 4Fe4Cu/HMS, 2Fe6Cu/HMS, 8Cu/HMS as the catalysts, the orange II removal efficiency increased to 23.8%, 84.0%, 87.9%, 92.8%, 85.0%, respectively. The results showed that different Fe/Cu ratios of the iron–copper component in catalysts manifest diversity orange II removal efficiency. It is noteworthy that the orange II removal efficiency by iron–copper bimetallic catalysts were superior than monometallic iron catalyst (8Fe/HMS). Moreover, the orange II removal efficiency was enhanced by using a bimetal catalyst of iron–copper which copper contents increase. Whereas, with increasing the copper content to a hundred percent (8Cu/HMS), the degradation efficiency of orange II reduced, which is lower than that of 4Fe4Cu/HMS and 2Fe6Cu/HMS. The above results confirm that the catalytic activity of FeCu/HMS composite catalyst depends highly upon the Fe/Cu ratio. The 2Fe6Cu/HMS with a Fe/Cu mass ratio of 1/3 showed the best catalytic activity.

Fig. 6 The remove efficiency of orange II with different catalysts during the reaction in 2 h, at pH 7.0 with 27.4 mM H₂O₂, 1 g L⁻¹ catalyst dosage, 100 mg L⁻¹ orange II, 30 °C.

Moreover, the evolution of the pH during the degradation of orange II with different catalysts have also been determined. As shown in Fig. S1,† the final pH of the solution by using 8Fe/HMS, 6Fe2Cu/HMS, 4Fe4Cu/HMS, 2Fe6Cu/HMS, 8Cu/HMS as the catalysts and catalyst free were 6.83, 5.93, 5.70, 5.66, 5.77 and 6.90, respectively. The pH of the degraded solution with different catalysts became lower during the orange II degradation compared with initial pH of the solution (pH = 7). Interestingly, from the tendency of the solution pH, it can be seen that the reacted solutions were more acid when using more active catalysts. It may be attributed to that, during the degradation of organic pollutants, the compound structure would be broken and further degraded into small molecules such as carboxylic acids,³⁹ which was also consistent with the intermediates detected by HPLC-MS (Table S1†). Meanwhile, the leaching of metallic ions from the catalyst is an important problem in the Fenton process. The ferric and cupric ions leaching were detected and exhibited in Fig. S2.† The results show that the concentration of iron ions leaching were 0.071, 0.05, 0.032 and 0.012 mg L⁻¹, respectively, when using 8Fe/HMS, 6Fe2Cu/HMS, 4Fe4Cu/HMS and 2Fe6Cu/HMS as the catalysts for the first run (Fig. S2A†). In the case of copper ions leaching, the concentration of copper ions leaching were 0.087, 0.145, 0.198 and 0.302 mg L⁻¹, respectively, when using 6Fe2Cu/HMS, 4Fe4Cu/HMS, 2Fe6Cu/HMS and 8Cu/HMS as the catalysts for the first run (Fig. S2B†). All of them are lower than 2 mg L⁻¹, which is the legal limit imposed by the European Union.¹²

Previous researches have reported that the activity of the heterogeneous Fenton catalysts depends on the amount of hydroxyl radicals produced from H₂O₂ decomposition during the degradation of organic pollutants.^16,40–42 To investigate the origin of catalytic activity in FeCu/HMS complexes, catalyst free and different Fe/Cu ratio of catalysts were used to determine the performance on H₂O₂ decomposition and hydroxyl radical generation. Fig. 7A shows the results of H₂O₂ decomposition with different catalysts. The lowest activity for H₂O₂ self decomposition (9.9%) was got in catalyst free system. Moreover, the decomposition rate of H₂O₂ on 8Fe/HMS, 6Fe2Cu/HMS, 4Fe4Cu/HMS, 2Fe6Cu/HMS, 8Cu/HMS were 20.7%, 25.0%, 56.8%, 76.0%, 41.0%, respectively. Meanwhile, the amount of generated ˙OH radicals with various catalysts were presented in Fig. 7B. During the analysis, the cumulative amount of ˙OH radicals were performed by using benzoic acid as a probe and further determined through the HPLC. The amount of ˙OH radicals on catalyst free, 8Fe/HMS, 6Fe2Cu/HMS, 4Fe4Cu/HMS, 2Fe6Cu/HMS, 8Cu/HMS were 24.7 μmol L⁻¹, 49.5 μmol L⁻¹, 61.9 μmol L⁻¹, 127.2 μmol L⁻¹, 197.7 μmol L⁻¹, 94.3 μmol L⁻¹, respectively. The results showed that the capacity of H₂O₂ decomposition and hydroxyl radical generation are agreed with the removal efficiency tendency of orange II for catalysts with different Fe/Cu ratios. It is duo to that at the initial stage, the H₂O₂ molecules were adsorbed on the catalysts. After that, the catalysts reacted with H₂O₂ molecules and catalyze H₂O₂ decomposition to produce ˙OH radicals. Meanwhile, the more active catalyst could catalyze more H₂O₂ decomposition and produce more ˙OH radicals. On this basis, more ˙OH radicals could attack more dye molecules and exhibit higher removal efficiency of orange II. Therefore, the most powerful ability of orange II removal was obtained by 2Fe6Cu/HMS derived from the excellent catalytic ability, which could catalyze more H₂O₂ decomposition and generate higher hydroxyl radicals.

Fig. 7 Decomposition of H₂O₂ (A) and generation of hydroxyl radicals (B) with different catalysts during the reaction in 2 h, at pH 7.0 with 27.4 mM H₂O₂, 1 g L⁻¹ catalyst dosage, 30 °C.

3.3. Effect of the initial pH of the orange II aqueous solution

Generally, the catalytic activity of iron based heterogeneous Fenton catalysts is seriously influenced by the pH of solution. Therefore, it is greatly demanded to develop a novel catalyst to be applied in heterogeneous Fenton in a wide pH range. To estimate the effect of the initial pH on degradation of orange II in 2Fe6Cu/HMS system and 8Fe/HMS system, different initial pH values (5.0, 7.0 and 9.0) were set up, and the other operating conditions were invariant (27.4 mM H₂O₂, 1 g L⁻¹ catalyst dosage, 30 degree celsius, 100 mg L⁻¹ orange II). As shown in Fig. 8A and B, the removal efficiencies of orange II treated by 2Fe6Cu/HMS after 2 h were proximate at different initial pH values. About 93.0%, 92.8% and 92.6% of orange II removal can be achieved by 2Fe6Cu/HMS under the initial pH values of 5.0, 7.0 and 9.0, respectively. Whereas, in 8Fe/HMS system, lower orange II removal efficiencies were obtained after 2 h of treatment, and their removal rates dropped rapidly from 29.1% to 21.2% with an increase of the initial pH value from 5.0 to 9.0.

Fig. 8 The remove efficiency of orange II by 2Fe6Cu/HMS, 8Fe/HMS with different initial pH values (A) during the reaction in 2 h (B) at 2 h, with 27.4 mM H₂O₂, 1 g L⁻¹ catalyst dosage, 30 °C, 100 mg L⁻¹ orange II.

The results revealed that the catalytic activity of 2Fe6Cu/HMS was slightly affected by the initial pH compared with 8Fe/HMS. In 2Fe6Cu/HMS system, the co-operation of copper and iron could improve the reactivity of iron because of the high standard reduction potential difference (0.78 V) between Cu and Fe.⁴³ Furthermore, the formation of the galvanic coupling between Cu and Fe could promote the corrosion rate of Fe, which could facilitate the generation of hydroxyl radical (OH˙) under oxic conditions.^43–45 Thus, orange II in an aqueous solution also could be degraded effectively by 2Fe6Cu/HMS even if the initial pH is 9.0.

3.4. Effect of parameters on catalytic activity of 2Fe6Cu/HMS for degradation of orange II

3.4.1. Effect of H₂O₂ dosage. The effect of H₂O₂ dosage on the catalytic activity of 2Fe6Cu/HMS for orange II removal was investigated. The experiments were executed at five H₂O₂ concentrations of 6.8, 13.7, 27.4, 54.8, 109.6 mM, respectively. As shown in Fig. 9A and B, the degradation of orange II was remarkably enhanced with increasing concentration of H₂O₂ from 6.8 to 27.4 mM, especially in the first 15 min (inset in Fig. 9A). Because H₂O₂ is a source of ˙OH in the system, more reactive radicals would be produced with higher H₂O₂ concentration.¹⁷ However, the removal rate of orange II was gradually decreased when H₂O₂ dosage was from 27.4 to 109.6 mM. This phenomenon is probably due to unavailing consumption of H₂O₂ which derived from the scavenging effect of H₂O₂ (described in eqn (1) and (2)).¹³ Based on the above results, 27.4 mM H₂O₂ is the optimal concentration for 2Fe6Cu/HMS in degradation of orange II.

H₂O₂ + ˙OH → H₂O + ˙O₂H (1)

˙O₂H + ˙OH → H₂O + O₂ (2)

Fig. 9 Effect of parameters on catalytic activity of 2Fe6Cu/HMS for degradation of orange II: effect of H₂O₂ dosage (A) during the reaction in 2 h (B) at 2 h; effect of catalyst dosage (C) during the reaction in 2 h (D) at 2 h; effect of initial dye concentration (E) during the reaction in 2 h (F) at 2 h. Except for the investigated parameters, other parameters fixed on pH 7.0 with 27.4 mM H₂O₂, 1 g L⁻¹ catalyst dosage, 100 mg L⁻¹ orange II, 30 °C.

3.4.2. Effect of catalyst dosage. The effect of catalyst dosage on the catalytic activity of 2Fe6Cu/HMS for orange II removal was investigated with five different catalyst dosages (0.25, 0.5, 1.0, 2.0, 4.0 g L⁻¹). As depicted in Fig. 9C and D, the removal efficiency of orange II was improved from 74.2 to 92.8% with catalyst dosage increased from 0.25 to 1.0 g L⁻¹ during the reactions. As it is known to all, catalyst is the major source to activate H₂O₂ to generate ˙OH. More amount of catalyst provides more active sites and produce higher quantity of ˙OH that enhanced orange II removal efficiency.⁴⁶ Nevertheless, further increasing catalyst dosage (from 2.0 to 4.0 g L⁻¹), the final orange II removal efficiency was decreased from 92.9 to 92.5%, although the removal efficiency slightly increased during the first 15 min (inset in Fig. 9C). The subdued removal efficiency may be due to the excessive catalysts, which would reduce the unit surface adsorption of H₂O₂ and decrease the density of surface adsorbed H₂O₂ on the catalyst surface.⁴⁷ Moreover, the ˙OH radicals may be scavenged by the superfluous catalysts.⁴⁸ Based on these results, 1.0 g L⁻¹ of catalyst is the optimal dosage in degradation of orange II.

3.4.3. Effect of initial dye concentration. The effect of the initial concentration of orange II on the catalytic activity of 2Fe6Cu/HMS was explored. In Fig. 9E and F, it can be observed that the final dye removal rate were 98.5%, 92.8%, 88.1%, 83.8%, 77.7% when conducted at dye concentrations of 50, 100, 300, 500, 1000 mg L⁻¹, respectively. The results indicated that the removal efficiency was limited by a higher initial concentration of orange II. It may be attributed to competitive adsorption among dye molecules on the limited reaction area of catalyst when conducted in higher concentration dye.⁴⁶ Moreover, the more intermediates could be generated by the higher concentration of orange II, which would reduce the active surface sites available for H₂O₂ and orange II.⁴³ Despite of this, it is worthy to note that relative high removal efficiency was obtained by 2Fe6Cu/HMS even operating at 1000 mg L⁻¹ orange II initial concentration, and the capital cost analysis based on our experimental condition has been calculated (Table S2†). In addition, the total amount of removed orange II was improved with the increasing initial dye concentration. Specifically, 49.2, 92.8, 264.3, 419.2, 777.1 mg L⁻¹ of orange II were removed when initial dye concentration were 50, 100, 300, 500, 1000 mg L⁻¹, respectively.

3.5. Cyclic voltammetry of catalysts

As we know, the catalytic activity of catalysts is related to their redox potentials. To explore the origin of outstanding catalytic activity in 2Fe6Cu/HMS, the cyclic voltammetry was used to investigate the changes of its redox potentials. For contrast, the cyclic voltammetries of blank HMS, 8Fe/HMS and 8Cu/HMS were also investigated. As can be seen in Fig. 10, no obvious peak was observed in cyclic voltammogram of blank HMS, indicating that no redox couple existed in blank sample. However, for metal supported samples, 8Fe/HMS displays two peaks at about −0.54 V and −0.82 V, which may be assigned to the Fe³⁺/Fe²⁺ (eqn (3)) and Fe²⁺/Fe⁰ (eqn (4)) redox couples of catalyst, respectively. In addition, three significant peaks at around −0.48 V, −0.25 V and 0.87 V were observed in 8Cu/HMS may be produced by Cu⁺/Cu⁰ (eqn (5)), Cu²⁺/Cu⁰ (eqn (6)) and Cu²⁺/Cu⁺ (eqn (7)) redox couples, respectively. Differently, in the cyclic voltammetry curve of 2Fe6Cu/HMS, the corresponding peaks of Fe³⁺/Fe²⁺ and Fe²⁺/Fe⁰ redox couples shift to −0.66 V and −0.98 V. Meanwhile, the corresponding peaks of Cu⁺/Cu⁰, Cu²⁺/Cu⁰ and Cu²⁺/Cu⁺ redox couples also observed a shift to −0.45 V, −0.22 V and 0.88 V, which is similar to previous reports.^16,49 It should be noted that the potential values shown in eqn (3)–(7) correspond to the standard reduction electrode potential of species. Therefore, some deviations in the potential of the cyclic voltammetric waves are expected to occur due to the current practical experimental conditions.⁵⁰ Additionally, according to eqn (8), the Fe³⁺ reduction by Cu⁺ is thermodynamically feasible, which is conductive to the redox cycles of Fe³⁺/Fe²⁺ and Cu²⁺/Cu⁺.¹³ Moreover, the redox couples of Fe³⁺/Fe²⁺ and Cu²⁺/Cu⁺ were obviously intensified in the case of 2Fe6Cu/HMS compared with Fe³⁺/Fe²⁺ redox couple in 8Fe/HMS and Cu²⁺/Cu⁺ redox couple in 8Cu/HMS. The increase intensity of redox peaks indicated that there are more electrochemically accessible sites on the surface of catalyst^14,16 and the redox reaction kinetics of Fe³⁺/Fe²⁺ and Cu²⁺/Cu⁺ was enhanced in 2Fe6Cu/HMS.¹³

Fe³⁺ + e⁻ ↔ Fe²⁺, E^θ = 0.77 V (3)

Fe²⁺ + 2e⁻ ↔ Fe, E^θ = −0.41 V (4)

Cu⁺ + e⁻ ↔ Cu, E^θ = 0.52 V (5)

Cu²⁺ + 2e⁻ ↔ Cu, E^θ = 0.34 V (6)

Cu²⁺ + e⁻ ↔ Cu⁺, E^θ = 0.16 V (7)

Fe³⁺ + Cu⁺ → Fe²⁺ + Cu²⁺, ΔE = 0.71 V (8)

Fig. 10 Cyclic voltammograms of 8Fe/HMS, 2Fe6Cu/HMS, 8Cu/HMS and HMS.

3.6. Possible mechanisms

On the basis of the above experimental results, we proposed a possible catalytic mechanism of 2Fe6Cu/HMS during the Fenton reaction. In the initial step, the H₂O₂ molecules adsorbed on 2Fe6Cu/HMS can oxidize the Fe⁰ to Fe²⁺ via a two electron transfer and Cu⁰ to Cu⁺ via a one electron transfer (eqn (9) and (10)). Then, the generated Fe²⁺ and Cu⁺ were oxidized immediately by H₂O₂ to produce ˙OH radicals (eqn (11)–(14)), respectively. Meanwhile, the generated Fe³⁺ can also react with Cu⁺ to form Fe²⁺ and Cu²⁺ (eqn (15)) due to the potential difference between the Fe³⁺/Fe²⁺ and Cu²⁺/Cu⁺. This process can accelerate the interfacial electron transfer in redox cycles of Fe³⁺/Fe²⁺ and Cu²⁺/Cu⁺ pair existed in 2Fe6Cu/HMS, which was in accordance with the changes of redox potentials observed in cyclic voltammetry curve. On this basis, more ˙OH radicals can be generated (consistent with the results of ˙OH radicals generating amount), which can possess more powerful aggressivity to dye molecules during the Fenton catalytic reaction.

≡Fe⁰ + H₂O₂ + 2H⁺ → ≡Fe²⁺ + 2H₂O (9)

2≡Cu⁰ + H₂O₂ + 2H⁺ → ≡2Cu⁺ + 2H₂O (10)

≡Fe²⁺ + H₂O₂ → ≡Fe³⁺ + ˙OH + OH⁻ (11)

≡Fe³⁺ + H₂O₂ → ≡Fe²⁺ + ˙O₂H + H⁺ (12)

≡Cu⁺ + H₂O₂ → ≡Cu²⁺ + ˙OH + OH⁻ (13)

≡Cu²⁺ + H₂O₂ → ≡Cu⁺ + ˙O₂H + H⁺ (14)

≡Fe³⁺ + ≡Cu⁺ → ≡Fe²⁺ + ≡Cu²⁺ (15)

3.7. The degradation pathway of orange II

In order to investigate the decolorization mechanism of orange II in 2Fe6Cu/HMS system, HPLC-MS analysis was employed and the possible intermediate products identified are shown in Table S1.† Based on the results and previous studies,^{39,46,51–54} a plausible degradation pathway of orange II is proposed in Scheme 1. The ˙OH radicals played the major role in the heterogeneous Fenton oxidation of orange II. Generally, ˙OH radicals attack organic compounds by electrophilic addition or hydrogen abstraction mechanism.^46,51,55 In the primary degradation of orange II, the ˙OH radicals attacked the orange II to generate 4-[2-(2-hydroxy-1-naphthalenyl)hydrazinyl]-benzenesulfonic acid (A) through electrophilic addition reaction. Then the cleavage of N–N leads to the formation of 4-amino-benzenesulfonic (B)³⁹ and 1-amino-2-naphthol.⁵² However, 1-amino-2-naphthol was not be detected in the LC-MS analysis. This may be due to oxygen sensitive of 1-amino-2-naphthol, which can be decomposed under aerobic conditions.^52–54 Further oxidation of 1-amino-2-naphthol led to the production of quinine structural intermediate, such as 2-hydroxy-1,4-naphthalenedione (C),⁵¹ then reversible form 1,2,4-naphthalenetriol.⁵² Moreover, the napthoquinone rings and hydroxylated aromatic groups derivatives would be attacked by ˙OH radicals, resulting in the formation of 1,2-naphthalenediol (D),³⁹ then opening of the benzene ring, leading to the generation of 1,2-benzenedicarboxylic acid (E).^39,52 These compounds later to further oxidation resulted in aromatic ring cleavage and generation of aliphatic acids.^39,52

Scheme 1 The proposed degradation pathway of orange II in the 2Fe6Cu/HMS system.

3.8. The stability and recyclability of catalysts

The stability of 2Fe6Cu/HMS was investigated by five consecution experiments at the same conditions. The used catalysts were collected by centrifugation after each turn, and then washed with methanol by ultrasonic and dried for repeated use. As shown in Fig. 11, the removal rates of orange II after 2 h at each turn are 92.8%, 87.6%, 82.1%, 77.8%, 75.1%, respectively. The results suggested that the efficiency of catalyst slightly subdued in consecution experiments, which would be a normal phenomenon in catalytic reaction.^56,57 The stability tests of other catalysts (8Fe/HMS, 6Fe2Cu/HMS, 4Fe4Cu/HMS, 8Cu/HMS) were also evaluated and similar results were obtained (Fig. S3†). Therefore, the decrease efficiency during the degradation may be caused by the active constituent leaching from the catalyst and catalyst poisoning caused by intermediate products.²⁷ Significantly, it was observed that the 2Fe6Cu/HMS remains relative high catalytic activity for orange II removal after five consecutive runs. The results demonstrated that the fabricated 2Fe6Cu/HMS possesses favorable stability and recyclability performance.

Fig. 11 Degradation of orange II in different batch runs in the 2Fe6Cu/HMS system.

4. Conclusions

In the present work, a series of iron–copper bimetallic nanoparticles supported on HMS with different Fe/Cu ratios were prepared using a simple post-impregnation and sodium borohydride reduction strategy. The different Fe/Cu ratios were obtained by addition of various amounts of ferric and copper salts. The catalysts possessed a typical hollow mesoporous structure with interior cavity transfixed by mesoporous silica shell. Metal nanoparticles are highly dispersed in the matrix of hollow mesoporous silica spheres, illustrated by XRD, XPS, nitrogen physisorption, SEM, TEM. The catalytic test results of various FeCu/HMS composite catalysts showed that the catalytic activity depends highly upon the Fe/Cu ratio. Moreover, 1/3 (2Fe6Cu/HMS) is the optimum Fe/Cu mass ratio to achieve the best catalytic activity. Based on the results of ˙OH radicals detecting and cyclic voltammograms, the important origin of synergetic effect in 2Fe6Cu/HMS is considered as the combination of iron and copper species, which can accelerate the interfacial electron transfer in redox cycles of Fe³⁺/Fe²⁺ and Cu²⁺/Cu⁺ pair, followed by the increase in the ˙OH radicals generation. The 2Fe6Cu/HMS also possesses less pH dependent and keeps the high activity even at alkaline circumstance. The optimal parameters in degradation of 100 mg L⁻¹ orange II is 27.4 mM H₂O₂ with 1.0 g L⁻¹ of catalyst which performed at pH of 7.0 and 30 degree celsius. It is worthy to note that the 2Fe6Cu/HMS also possesses potential to treat high concentration dye pollutants and 77.7% of orange II were removed when conducted at 1000 mg L⁻¹ dye initial concentration. The possible intermediates were identified and a plausible degradation pathway of orange II was proposed. The stability and recoverability of composite catalyst were assessed and exhibited a good performance after 5 consecutive runs. The as synthesized composite catalyst was proved to be applied as an attractive alternative in the dye wastewater treatment.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant no. 51478224), the priority academic program development of Jiangsu higher education institutions and Research Innovation Grant for Graduate of Jiangsu Common High School (Grant no. KYZZ15_0115).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08501f

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