Multiwalled carbon nanotube-supported CuCo₂S₄ as a heterogeneous Fenton-like catalyst with enhanced performance

Abstract

In this paper, we prepared CuCo₂S₄/MWCNTs as a heterogeneous iron-free bimetal catalyst for the degradation of methylene blue (MB) dye by uniformly synthesizing CuCo₂S₄ nanoparticles on the surfaces of multiwalled carbon nanotubes (MWCNTs). The homogeneous nanostructure, tuned by MWCNTs, and the efficient electron transfer from CuCo₂S₄ to MWCNTs resulted in a quite decent catalytic activity, which was superior to that of pristine CuCo₂S₄ and monometal sulfides. Moreover, CuCo₂S₄/MWCNTs maintained a good catalytic performance over a broad pH range of 3 to 12, and exhibited better MB degradation rates under alkaline conditions. Meanwhile, the catalyst was reusable for up to three cycles due to the strong interaction between the nanoparticles and MWCNTs. The results demonstrate that CuCo₂S₄/MWCNTs is an improved Fenton catalyst and a promising candidate to substitute iron-based catalysts for the treatment of wastewater containing dyes.

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1. Introduction

Wastewater containing organic dyes and nitrogen-containing compounds has been an environmental problem in the past decades. In order to address this urgent issue, numerous wastewater treatment techniques have been proposed, such as adsorption, membrane separation, photocatalysis and biodegradation.^1–4 Among these physicochemical methods, the Fenton reaction, which uses Fe²⁺/H₂O₂ to generate hydroxyl radicals (·OH), attracts extensive attention due to its cost-saving and environmentally friendly nature.⁵ In addition to commonly used iron salts, copper, cobalt, cerium and manganese salts were also found to be effective in Fenton-like reactions.^6–9 However, these homogeneous catalysts cause secondary pollution, because they are difficult to recover. Moreover, a lot of sludge is produced with the use of iron salts during the catalytic process.¹⁰ To overcome these negative effects, recyclable heterogeneous catalysts like metal oxides, hydroxides and sulfides have been used to replace homogeneous catalysts.^11–16 Since then, enormous progress has been made in heterogeneous catalysis; however, most catalysts still show stability issues at certain values of pH, particularly under strong alkaline conditions. Hence, it is necessary to prepare other types of catalysts with a high catalytic activity and a wide applicable pH range.

One method is the preparation of an iron-free bimetal heterogeneous catalyst. The iron-based bimetal catalysts such as CoFe₂O₄ and CuFe₂O₄ showed high catalytic efficiencies with an increased activity of Fe₂O₃ but were limited by the narrow applicable pH range of Fe²⁺.^17,18 Moreover, copper and cobalt catalysts have been studied separately, but the combined effect of copper and cobalt in the Fenton reaction has not been studied.^19,20 Thus, we drew our attention on their bimetal sulfides. CuCo₂S₄ has been demonstrated to be effective in providing multiple redox states, which is beneficial for reaction activity.²¹ Furthermore, the higher electrical conductivity of CuCo₂S₄ compared to that of CuCo₂O₄ can accelerate the electron transfer. In addition, the respective workable pH ranges for the Cu⁺/H₂O₂ and Co²⁺/H₂O₂ systems are broader than that for Fe²⁺/H₂O₂, and the synergistic effect between copper and cobalt may further broaden the applicable pH range for the reaction. The other method involves the use as substrates of carbon nanomaterials, such as multiwalled carbon nanotubes (MWCNTs) and graphene. Owing to their large surface area and good conductivity, MWCNTs effectively enhance the activity of catalysts and introduce additional functional properties as matrix materials.²² For the Fenton reaction, an optimum addition of MWCNTs can enhance the degradation performance of the catalysts.²³ The surface functionalities and structural defects of MWCNTs can act as active sites to catalyze H₂O₂ involved reactions.²⁴ Moreover, the robust matrix composed of MWCNTs can improve the reusability of the catalysts and broaden the practicable pH range.²⁵ Fe₃O₄–MWCNT nanocomposite can be used in the pH range from 1 to 10, which indicates the better stability of the catalyst after the incorporation of MWCNTs.²⁶

In this study, we prepared CuCo₂S₄/MWCNTs by a mild two-step hydrothermal method, in which CuCo₂S₄ nanoparticles were deposited on MWCNTs. Compared to monometal catalysts (CuS and CoS) and pristine CuCo₂S₄, the CuCo₂S₄/MWCNT composite exhibited a higher Fenton-like catalytic efficiency. Furthermore, CuCo₂S₄/MWCNTs tolerated a broad range of pH and exhibited a good reusability. The achieved high catalytic performance demonstrated that the synthesized CuCo₂S₄/MWCNT composite was an efficient Fenton-like catalyst for potential future applications. The mechanism for catalytic enhancement in CuCo₂S₄/MWCNTs is also discussed at the end of the paper.

2. Experimental

2.1. Materials

Copper acetate monohydrate (Cu(CH₃COO)₂·H₂O, 99.0%), cobalt acetate tetrahydrate (Co(CH₃COO)₂·4H₂O, 99.5%) and L-cysteine (C₃H₇NO₂S, 99%) were purchased from Aladdin, China. Sodium carbonate (Na₂CO₃, ≥99%) and methylene blue (MB, ≥98%) were obtained from Adamas Reagent Co., Ltd. Triethylamine (C₆H₁₅N, 99.0%), hydrogen peroxide (H₂O₂, 30% w/w), nitric acid (HNO₃, 65.0–68.0%) and sulfuric acid (H₂SO₄, 95–98%) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All the abovementioned chemicals were used directly without any further purification. MWCNT powder (purity > 95%, diameter: 30–50 nm) was provided by Chengdu Organic Chemicals Co., Ltd and the Chinese Academy of Sciences.

2.2. Preparation of CuCo₂S₄/MWCNTs

MWCNTs were first acidified with a HNO₃/H₂SO₄ mixture at 60 °C, as reported in the literature.²⁷ The functionalization process was effective in removing impurities.²⁸ Then, 20.0 mg of functionalized MWCNTs were added to 10.0 ml of distilled water and sonicated for 3 hours. The copper and cobalt solution was prepared by adding 64.5 mg of Cu(CH₃COO)₂·H₂O and 161.0 mg of Co(CH₃COO)₂·4H₂O into 10 ml of distilled water and stirred for 30 minutes. Subsequently, the functionalized MWCNT suspension was added to the abovementioned solution and further stirred for 30 minutes to form a uniform mixture. Then an aqueous solution of 32.0 mg of Na₂CO₃ was added to the mixture, followed by the addition of 0.5 ml of triethylamine. The mixture was then transferred to a 50.0 ml Teflon-lined stainless steel autoclave, heated at 180 °C for 12 h and allowed to cool to room temperature. The precipitate was washed with distilled water and ethanol several times and dried at 70 °C to obtain copper and cobalt hydroxide/MWCNTs.

CuCo₂S₄/MWCNTs was subsequently prepared. First, 50.0 mg of hydroxide/MWCNTs was dispersed in 10.0 ml of distilled water. An L-cysteine solution was prepared by dissolving 75.0 mg of L-cysteine in 10.0 ml of distilled water. Then, this solution was added to the hydroxide/MWCNT suspension, followed by stirring and heating at 180 °C for 6 h in a Teflon-lined stainless steel autoclave. Finally, the sample was washed and dried at 70 °C. CuCo₂S₄, CuS and CoS were prepared without MWCNTs by the same experimental conditions.

2.3. Characterization

The structural phase of the samples was investigated by X-ray diffraction (XRD) with a Miniflex II (Rigaku) using a monochromatized X-ray beam from Cu Kα (λ = 0.154050 nm) radiation. The morphology of the samples was studied via scanning electron microscopy (SEM, JSM-6700, JEOL) and transmission electron microscopy (TEM, TecnaiF2, Philips) with energy dispersive spectrometry (EDS). To study the surface area and pore structure of the samples, the nitrogen adsorption–desorption isotherms were recorded using ASAP2020 (Micromeritics) at 77 K. Raman spectra were recorded on a Labram Hr800 Evolution (Horiba). Furthermore, the surface electronic structure was analyzed by X-ray photoelectron spectroscopy (XPS) using an ESCALAB 250Xi (Thermo Fisher).

For the catalytic activity tests, 10.0 mg of catalyst was added to an MB aqueous solution and stirred for 3 hours to reach adsorption equilibrium. Subsequently, 1.0 ml of H₂O₂ was added and the concentration of MB solution was determined using Spectrometer UV-1100 (Mapada). To study the stability of the catalysts under different pH values, the pH in the mixtures of MB solution and catalysts was adjusted with HCl or NaOH solution before the catalytic reaction. The degradation experiments were then conducted and the MB concentration in solution was determined after 20 minutes. To test the reusability of the catalyst, 10.0 mg of catalyst was used to degrade MB during 20 minutes for three cycles. The catalyst was collected after each reaction by filtration and washed with ethanol and water several times. Afterwards, the catalyst was used again in the adsorption and degradation experiments under the same conditions as described above.

3. Results and discussion

XRD experiments were taken to investigate the chemical structures of CuCo₂S₄ and CuCo₂S₄/MWCNTs. As shown in Fig. S1,† the diffraction peaks at 2θ = 26.6°, 31.2°, 37.9°, 46.9°, 49.9° and 54.8° can be indexed to the diffraction of the (022), (113), (004), (224), (115) and (044) planes of the CuCo₂S₄ standard structure (JCPDS file no. 42-1450), respectively. Compared to pristine CuCo₂S₄, the peaks of CuCo₂S₄ in CuCo₂S₄/MWCNTs are sharper and no apparent peaks of secondary phases are observed. The higher purity and crystallinity of CuCo₂S₄ in CuCo₂S₄/MWCNTs demonstrate that MWCNTs can assist the nucleation of CuCo₂S₄. Typical diffraction peaks of MWCNTs were not detected, which indicates the uniform dispersion of MWCNTs in the CuCo₂S₄/MWCNT composite.

The SEM and TEM images of samples are shown in Fig. 1 and S2–S5.† As the scanning electron microscopy (SEM) image shows in Fig. 1a, the MWCNTs were decorated and separated by CuCo₂S₄ nanoparticles without apparent agglomeration, exactly in agreement with the XRD results. The transmission electron microscopy (TEM) image was also recorded as shown in Fig. 1b, which demonstrates that CuCo₂S₄ nanoparticles had a diameter of approximately 20 nm and were dispersed uniformly on the surfaces of MWCNTs or their interlayers. Such uniform dispersion of the CuCo₂S₄ nanoparticles in an interlaced structure inhibited their aggregation, which prevented the efficiency loss of the catalyst after use. A typical HRTEM image, shown in Fig. 1c, demonstrates that the CuCo₂S₄ nanoparticles were attached to the outer walls of MWCNTs, and exhibited clear lattice fringes. The interplanar space was calculated from the inverse FFT image (Fig. 1c, inset), and it was determined to be 0.33 nm, which belongs to the (022) plane of CuCo₂S₄ and is in good agreement with the XRD results.

Fig. 1 SEM image (a), TEM image (b) and HRTEM image (c) of CuCo₂S₄/MWCNTs. The image inset in (c) is the inverse Fast Fourier Transform (FFT) image of the square marked region in (c).

An excellent dye and H₂O₂ adsorption is an important factor for the efficiency of the Fenton reaction in the degradation of dyes. Nitrogen adsorption was measured to evaluate the adsorptive properties and verify the surmise of surface area increase based on the CuCo₂S₄/MWCNT morphology. Fig. 2 shows the nitrogen adsorption/desorption isotherms of CuCo₂S₄ and CuCo₂S₄/MWCNTs, and the nitrogen adsorption/desorption isotherms of MWCNTs are shown in Fig. S6.† The BET surface area of CuCo₂S₄ is 29 m² g⁻¹, while that of CuCo₂S₄/MWCNTs is 38 m² g⁻¹. The pore volume of CuCo₂S₄/MWCNTs is 0.21 cm³ g⁻¹, which is higher than that of CuCo₂S₄ (0.14 cm³ g⁻¹). As a result, CuCo₂S₄/MWCNTs provides more active sites and can adsorb a larger amount of dye and H₂O₂ molecules.

Fig. 2 Nitrogen adsorption/desorption isotherms of CuCo₂S₄ and CuCo₂S₄/MWCNTs at 77 K.

Fig. S7† shows the Raman spectra. The peaks of the MWCNTs and CuCo₂S₄/MWCNTs spectra can be grouped into seven bands, depending on the model proposed by Rebelo et al.²⁹ The relative areas of satellite bands (S, Dl, Dr, Gl and Gr bands) and those of graphitic-type domain bands (D and G bands) are summed respectively and their ratio is used to reflect the chemical functionalization degree. The ratio for MWCNTs was 0.5, while for CuCo₂S₄/MWCNTs it was 0.7. The results show an increased structural complexity in MWCNTs which can be ascribed to the aromaticity disruption and vacancies formation.³⁰

X-ray photoelectron spectroscopy (XPS) spectra, shown in Fig. 3, were recorded to identify the elemental chemical states and to help elucidate the mechanism for the Fenton reaction of CuCo₂S₄ and CuCo₂S₄/MWCNTs. All results were calibrated according to the peak position of C 1s, which was assumed to be at 284.8 eV. Fig. 3a is the full scan survey XPS spectrum of CuCo₂S₄/MWCNTs. Fig. 3b shows the C 1s spectrum of CuCo₂S₄/MWCNTs, which can be divided into five types of peaks, attributed to different types of carbon in MWCNTs: C–C/C=C, C–O, C=O, COO and O–COO.^31–33 The presence of oxygen-containing functional groups confirms the functionalization of MWCNTs, which facilitated the deposition of CuCo₂S₄ nanoparticles on MWCNTs. CuCo₂S₄ was considered to have a valence distribution of Cu^1.2+(Co^2.4+)₂(S^1.5−)₄, with coexistence of Cu⁺, Co²⁺ and Co³⁺.³⁴ Fig. 3c shows the Cu 2p spectra of CuCo₂S₄ and CuCo₂S₄/MWCNTs. In CuCo₂S₄ the doublet peaks at 932.2 eV and 952.1 eV are attributed to Cu⁺ 2p_3/2 and 2p_1/2, respectively, and in CuCo₂S₄/MWCNTs, the doublet peaks at 932.6 eV and 952.6 eV are attributed to Cu⁺ 2p_3/2 and 2p_1/2. Cu²⁺ is generally considered as nonexistent in pure CuCo₂S₄. However, the doublets at 934.5 eV and 954.4 eV in CuCo₂S₄ and the doublets at 935.1 eV and 955.1 eV in CuCo₂S₄/MWCNTs demonstrate the presence of Cu²⁺, which was caused by exposure to air.³⁵ The peak at 944.2 eV in CuCo₂S₄ and the peak at 944.9 eV in CuCo₂S₄/MWCNTs are shake-up satellites. The Co 2p spectra of CuCo₂S₄ and CuCo₂S₄/MWCNTs are presented in Fig. 3d and can be fitted into two spin–orbit doublets and two shake-up satellites. In the Co 2p_3/2 spectrum of CuCo₂S₄, the peaks at 778.6 eV and 780.7 eV were attributed to Co²⁺ and Co³⁺, while in the Co 2p_3/2 spectrum of CuCo₂S₄/MWCNTs the peaks attributed to Co²⁺ and Co³⁺ appeared at 779.1 eV and 781.2 eV, respectively. Thus, all results suggest the coexistence of Co²⁺ and Co³⁺. We can see that upon incorporation of MWCNTs, both Cu 2p and Co 2p peaks shifted to higher binding energies. The shift may be attributed to the excellent dispersion of the CuCo₂S₄ nanoparticles on the MWCNTs and the great conductivity of MWCNTs. Therefore, the CuCo₂S₄ nanoparticles serve as electron donors and the electrons can be transferred to the MWCNTs.³⁶ These cause a decrease in the density of the outer electron cloud of Cu and Co, and the increase in the binding energies of Cu 2p and Co 2p. Moreover, the surface ionic ratios of Co²⁺/Co³⁺ were 1.1 : 1 and 2.5 : 1 in CuCo₂S₄ and CuCo₂S₄/MWCNTs, respectively. The relative increase in Co²⁺ content may be ascribed to electron transfer during the synthesis.³⁷ Finally, the S 2p spectrum of CuCo₂S₄/MWCNTs can be divided into four peaks, indicating the complex valence state of S, as shown in Fig. S8.†

Fig. 3 Full survey scan XPS spectrum (a) and XPS spectrum of C (b) of CuCo₂S₄/MWCNTs. XPS spectra of Cu (c) and Co (d) of CuCo₂S₄ and CuCo₂S₄/MWCNTs.

The Fenton-like catalytic rates of CuS, CoS, CuCo₂S₄ and CuCo₂S₄/MWCNTs are shown in Fig. 4a. CuCo₂S₄ and CuCo₂S₄/MWCNTs showed a competitive catalytic efficiency when used in small amounts. Among these catalysts, CuCo₂S₄/MWCNTs showed the highest catalytic performance and completed the degradation within 6 minutes. However, when pristine CuCo₂S₄ was used as catalyst, the degradation reaction took up to 12 min, while their corresponding monometal sulfides exhibited a poor efficiency at the specified time. Thus, it was evident that bimetal sulfide CuCo₂S₄ exhibits a higher Fenton-like catalytic activity than their corresponding monometal sulfides. Futhermore, the addition of MWCNTs further enhanced the catalytic rate. The high catalytic rate can be mainly attributed to the increased surface area and improved charge transfer in the composites.

Fig. 4 (a) MB degradation rate using CuS, CoS, CuCo₂S₄ and CuCo₂S₄/MWCNTs as catalysts. Conditions: original concentration of MB = 20.0 mg l⁻¹, volume of MB solution = 30.0 ml, additional H₂O₂ = 1.0 ml, catalyst amount = 10.0 mg, room temperature, initial pH = 6. (b) MB degradation rate at high concentration using CuS, CoS, CuCo₂S₄ and CuCo₂S₄/MWCNTs as catalysts. Conditions: original concentration of MB = 50.0 mg l⁻¹, volume of MB solution = 30.0 ml, additional H₂O₂ = 1.0 ml, catalyst amount = 10.0 mg, room temperature, initial pH = 6. (c) MB degradation efficiency using CuCo₂S₄ and CuCo₂S₄/MWCNTs at different initial pHs during 20 minutes and (d) MB degradation rate using CuCo₂S₄/MWCNTs at different initial pHs. Conditions: original concentration of MB = 20.0 mg l⁻¹, volume of MB solution = 30.0 ml, additional H₂O₂ = 1.0 ml, catalyst amount = 10.0 mg, room temperature.

Catalytic performance was also tested under a different initial dye concentration (50 mg l⁻¹) with the same amount of catalyst. As shown in Fig. 4b, CuCo₂S₄/MWCNTs degraded 97.0% of MB within 12 min, whereas after the same time, CuCo₂S₄ degraded only 74.4%. Photographs of the MB solution are shown in Fig. S9.† The results demonstrate that the addition of MWCNTs improved the degradation rate to a larger extent. The catalytic rates of our samples (no. 1–4) and relevant Fenton-like catalysts reported in the literature, namely, CuS (no. 5–7), Co₃O₄ (no. 6), iron-based bimetal composites (no. 9–16) and CNTs-based composites (no. 17–19) are listed in Table S1.† It is noted that the as-prepared CuCo₂S₄/MWCNT composite stands out as an efficient catalyst owing to the combination of low catalytic dosage, convenient application method, high catalytic rate and highly efficient degradation of MB. The catalytic performance of CuCo₂S₄/MWCNTs is superior to that of monometal sulfides CuS and Co₃O₄, and is comparable to that of iron-based bimetal composites. Additionally, compared with CNT-based composites, our samples can be prepared by a simple synthetic procedure, and exhibit an uniform morphology and high catalytic activity.

Furthermore, a series of degradation experiments were conducted to investigate the catalytic performance of CuCo₂S₄ and CuCo₂S₄/MWCNTs at different initial pHs. Before adding H₂O₂, the mixtures of MB solution and catalysts were adjusted to a pH value of 3–12. By contrast to iron-based catalysts, which are restricted to a certain pH range, the iron-free CuCo₂S₄ and CuCo₂S₄/MWCNTs catalysts worked over a broad pH range and their MB degradation efficiency exceeded 75.0% within 20 minutes, as shown in Fig. 4c. For CuCo₂S₄, Cu⁺/H₂O₂ system is stable in acidic and neutral conditions, whereas the Fe²⁺/H₂O₂ system is stable only at pH ≤ 4; moreover, the Co²⁺/H₂O₂ system exhibits the similar or even higher catalytic activity under alkaline conditions.³⁸ Therefore, the combination of copper and cobalt results in a high tolerance of CuCo₂S₄ towards a broad pH range. In addition, CuCo₂S₄ showed better performance in a neutral pH range. The declining catalytic activity under acidic/alkaline conditions was ascribed to the relatively inferior stability of CuCo₂S₄ at extreme pHs. The improved performance of CuCo₂S₄/MWCNTs compared to pristine CuCo₂S₄ at extreme pHs was attributed to the additional active sites of the functionalized MWCNTs and their stable tailored structure. Fig. S10† shows the catalytic rate of CuCo₂S₄ and Fig. 4d exhibits the MB degradation rate using CuCo₂S₄/MWCNTs at different pHs. It can be seen that CuCo₂S₄/MWCNTs performed better at alkaline pHs than at acidic pHs. The pH-dependent activity may be due to the cationic character of the MB molecules and the catalytic properties of CuCo₂S₄.³⁹ Firstly, the pH of the solution can change the surface charge in CuCo₂S₄. When the pH value exceeds pH_pzc (the point of zero charge), the surface charge of CuCo₂S₄ becomes negative and larger amounts of cationic MB molecules can be adsorbed on the catalysts. Besides, in acidic and near-neutral conditions a portion of Cu⁺ ions are oxidized to Cu²⁺ by molecular oxygen and cannot generate ·OH radicals. It is worth mentioning that the degradation efficiency of CuS/graphene under acidic conditions is much lower than under neutral conditions, as described in the literature.⁴⁰ Hence, the high efficiency of CuCo₂S₄ and CuCo₂S₄/MWCNTs under acidic conditions can be attributed to the synergistic effect of copper and cobalt. In conclusion, the results demonstrate that, unlike iron-based catalysts, the iron-free bimetal CuCo₂S₄ and CuCo₂S₄/MWCNTs catalysts can be directly used in wastewater treatment without any pH limitations.

The reusability of CuCo₂S₄ and CuCo₂S₄/MWCNTs was evaluated and the results are shown in Fig. 5. After 3 cycles of reactions, the catalytic activity of CuCo₂S₄ decreased and 60.5% of MB was degraded after 20 minutes. An improvement in the reusability could be observed when MWCNTs were introduced. The results show that CuCo₂S₄/MWCNTs maintained a high removal efficiency better than CuCo₂S₄, and 98.0% of MB could be degraded by CuCo₂S₄/MWCNTs in the second cycle, and 84.8% in the third cycle. The results further verify the strong anchoring of CuCo₂S₄ in CuCo₂S₄/MWCNTs, which avoids the aggregation of CuCo₂S₄ upon reuse, hence strengthening the stability of the CuCo₂S₄/MWCNTs catalyst in the Fenton reaction. XRD experiments of CuCo₂S₄/MWCNTs after 1 and 2 cycles were conducted to study the predominant effects of the catalytic reaction after different cycles, and the results are shown in Fig. S11.† We can see that the XRD pattern of CuCo₂S₄/MWCNTs after 1 cycle displays both the peaks of CuCo₂S₄ and weak peaks of MWCNTs. Thus, the XRD spectrum reveals that the CuCo₂S₄/MWCNTs catalyst is stable, and only a small part of CuCo₂S₄ is lost after the first cycle. After the second cycle, some peaks attributed to impurities appeared; this was due to the dissolution of cobalt in the reaction media. The losing of cobalt ions leads to the decrease in catalytic activity in the third cycle. The results are confirmed by energy dispersive spectrometry (EDS, shown in Fig. S12 and Table S2†).

Fig. 5 Reusability study of CuCo₂S₄ (A) and CuCo₂S₄/MWCNTs (B) for the MB degradation. Conditions: initial concentration of MB = 20.0 mg l⁻¹, volume of MB solution = 30.0 ml, added H₂O₂ = 1.0 ml, catalyst amount = 10.0 mg, room temperature, initial pH = 6, degradation time = 20 min.

The catalytic process of CuCo₂S₄/MWCNTs can be described as follows: before the Fenton reaction begins, large amounts of MB molecules are adsorbed by CuCo₂S₄/MWCNTs; upon addition of H₂O₂, H₂O₂ molecules are readily adsorbed, and the catalyzed reaction occurs on the CuCo₂S₄ nanoparticle surface and MWCNTs to generate ·OH, which will attack MB molecules, either adsorbed or in solution; as the adsorbed MB molecules are degraded, the free MB molecules in solution are adsorbed and then decomposed until the degradation process is completed.

To further understand the reasons for the enhanced catalytic activity, we first considered the catalytic mechanism of CuCo₂S₄ individually. The CuCo₂S₄/H₂O₂ system, formed on the surface of CuCo₂S₄, is a combination of the Cu⁺/H₂O₂, Co²⁺/H₂O₂ and Cu⁺/Co³⁺ redox systems. The Cu⁺/H₂O₂ reaction systems are similar to Fe²⁺/H₂O₂ (eqn (1) and (2)):⁴⁰

Cu⁺ + H₂O₂ → Cu²⁺ + OH⁻ + ·OH (1)

Cu²⁺ + H₂O₂ → H⁺ + ·O₂H + Cu⁺ (2)

The proposed mechanisms for Co²⁺/H₂O₂ are shown below (eqn (3) and (4)).^41–43

Co²⁺ + H₂O₂ → Co³⁺ + OH⁻ + ·OH (3)

Co³⁺ + H₂O₂ → Co²⁺ + ·O₂H + H⁺ (4)

In combination with the experimental results, the better performance of CuCo₂S₄ compared to monometal sulfides can be ascribed to the following reasons:

(i) Co³⁺ ions are thermodynamically susceptible to be reduced by Cu⁺ ions (eqn (5)). The redox reaction of Cu⁺/Co³⁺ will accelerate the redox reactions of both Cu⁺/Cu²⁺ and Co²⁺/Co³⁺, thus promoting the overall Fenton reaction cycle, and consequently, improving the catalytic rate.

Cu⁺ + Co³⁺ → Cu²⁺ + Co²⁺ (5)

(ii) The operating pH range of the iron-free Fenton-like Cu⁺/H₂O₂ and Co²⁺/H₂O₂ systems are both broader than that of Fe²⁺/H₂O₂. The stability of CuCo₂S₄ at extreme pHs and the combination of the Cu⁺/H₂O₂ and Co²⁺/H₂O₂ systems result in a broader workable pH range.

(iii) CuCo₂S₄ has good electrical conductivity and hence electrons can be readily transfer to the MWCNTs.⁴⁴

Thus, the function of MWCNTs in CuCo₂S₄/MWCNTs is concluded as follows:

(i) MWCNTs significantly tuned the nanostructure of CuCo₂S₄/MWCNTs. Firstly, as the XRD results showed, the MWCNTs provide nucleation sites such that the CuCo₂S₄ nanoparticles on MWCNTs exhibit a higher purity and crystallinity than pristine CuCo₂S₄. Secondly, the CuCo₂S₄ nanoparticles are loaded on the MWCNTs uniformly so aggregation of CuCo₂S₄ nanoparticles can be inhibited. Furthermore, the size of the CuCo₂S₄ nanoparticles was controlled by MWCNTs. Thirdly, the larger surface area of CuCo₂S₄/MWCNTs compared to pristine CuCo₂S₄ helps to adsorb a larger amount of MB and H₂O₂ molecules. The adsorbed H₂O₂ can be catalyzed quickly by the active sites on the surface of CuCo₂S₄/MWCNTs, and the generated ·OH radicals can immediately attack the nearby adsorbed MB molecules.⁴⁵ Fourthly, CuCo₂S₄/MWCNTs forms a more stable structure than CuCo₂S₄. MWCNTs act as a matrix such that CuCo₂S₄ nanoparticles are not easily leaked. The stable structure helps CuCo₂S₄/MWCNTs to operate well at extreme pHs or after several reaction cycles.⁴⁶

(ii) Electrons from the CuCo₂S₄ nanoparticles can move to MWCNTs, due to the good conductivity of MWCNTs, which will accelerate electron transfer, promote the redox rate of the Fenton reaction cycle and further enhance the catalytic rate compared to CuCo₂S₄.⁴⁷ It is notable that a larger ratio of Co²⁺/Co³⁺ in CuCo₂S₄/MWCNTs than in pure CuCo₂S₄ is conducive to the initiation of the catalytic reaction, and meanwhile, it helps to economize H₂O₂ for fewer Co³⁺ are needed to be reduced at the beginning.

(iii) The defects in MWCNTs are deemed as active sites to produce ·OH, which further enhance catalytic efficiency. The diffused MB and H₂O₂ molecules can be trapped by the incomplete dangling bonds and the strongly distorted π-electron system around the defects.⁴⁸ Additionally, defects with an increased electron density can serve as reducing active sites to reduce H₂O₂ and produce ·OH.²⁴

A scheme with the mechanism of the enhanced catalytic process is shown in Fig. 6.

Fig. 6 Scheme illustrating the mechanism of the enhanced catalytic process of CuCo₂S₄/MWCNTs.

4. Conclusions

In summary, a CuCo₂S₄/MWCNT composite was synthesized as a heterogeneous Fenton-like catalyst by a mild two-step hydrothermal method. The catalyst CuCo₂S₄ exhibited a higher catalytic rate than CuS and CoS. Moreover, the iron-free catalysts CuCo₂S₄ exhibited good performance over a broader pH range of 3–12. MWCNTs provide stable frameworks where CuCo₂S₄ nanoparticles could be anchored and strongly bonded, favoring the uniform growth of the nanoparticles. As a result, the addition of MWCNTs enhanced the catalytic rate and the catalytic performance at extreme pHs, and improved the reusability. Furthermore, the mechanism for the enhanced catalytic process with CuCo₂S₄/MWCNTs was discussed. In conclusion, the CuCo₂S₄/MWCNT composite is a promising Fenton-like catalyst for wastewater containing dyes.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (21273236) and the Science and Technology Planning Projects of the Fujian Province, China (2014H2008 and 2015I0008) and STS Project of Fujian-CAS (2016T3036).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra01269a

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