Introducing MXenes into the Heterogeneous Catalyst: Synthesizing Mo 2 CT x @Fe 3 O 4 with Excellent Recoverability to Degrade Methylisothiazolinone in the Electro-Fenton System

Abstract

Methylisothiazolinone (MIT) is a commonly used bactericide in wastewater treatment. Residual MIT in wastewater can lead to high environmental risks and toxicity. In this work, an emerging material MXenes has been introduced into the heterogeneous electro-Fenton catalysts to degrade MIT. Ti₃C₂T_x@Fe₃O₄, V₂CT_x@Fe₃O₄, and Mo₂CT_x@Fe₃O₄ were assessed as catalysts for MIT removal.

The reasons for the differences among the three catalyst effects were analyzed according to different characterization results. Mo₂CT_x@Fe₃O₄ exhibited the best catalytic activity for MIT degradation. At pH = 3, the removal rate of MIT and corresponding chemical oxygen demand of catalyst Mo₂CT_x@Fe₃O₄ were 93.41% and 62.46% after 120 min.

Among the three catalysts, Mo₂CT_x@Fe₃O₄ had larger surface area and porosity. Mo₂CT_x@Fe₃O₄ had the highest surface iron content, which meant that Fe₃O₄ was more easily loaded on the surface of Mo₂CT_X. What is more, Mo₂CT_X had the strongest ability to accelerate the regeneration of Fe²⁺. The durability of Mo₂CT_x@Fe₃O₄ was also evaluated. After four cycles, the removal efficiency of MIT only decreased from 92.51% to 89%. This work supports the development of heterogeneous electro-Fenton catalysts and the degradation of MIT.

Introduction

Methylisothiazolinone (2-methyl-4-isothiazolin-3-one; MIT) is a commonly used non-oxidative biocide and preservative in many industries, and in cosmetic, personal care, for example, wet wipe (Latheef and Wilkinson, 2015). Due to its strong allergenic properties, Herman et al. (2019) summarized the maximum concentration of MIT in cosmetics under European and international legislation.

In the field of water and wastewater treatment, biocides are increasingly used to prevent biofilm buildup on the membranes of reverse osmosis (RO) systems (Li et al., 2016). And MIT was found at amounts of up to 160 mg/L in RO concentrates from wastewater reclamation plants (Tang et al., 2012). MIT was highly bio-toxic, which would affect the nitrification process (Amat et al., 2015). If released directly into the environment, the concentrated MIT can pose a higher environmental danger, as well as present challenges for bio-treatment if recycled in wastewater treatment plants (Wang et al., 2020b). As a result, effective MIT degradation methods need to be discovered.

Some investigations on MIT degradation have been conducted, including biological processes, electrochemical oxidation, photocatalytic degradation, and ozonation. Advanced oxidation processes (AOPs) have attracted extensive attention from many researchers because of their high mineralization efficiency and fast oxidation reaction, which have been applied to eliminate organic contaminants (Ma et al., 2021).

The Fenton process is the most widely used for wastewater treatment among the AOPs (Lin et al., 2022). The classical Fenton reaction refers to the activation of hydrogen peroxide (H₂O₂) by iron salts (Neyens and Baeyens, 2003). Instead, in the electro-Fenton system, H₂O₂ can be produced in situ and reacts with Fenton's catalyst, which produces .OH indirectly (Casado, 2019).

Many researchers have gradually begun to pay attention to heterogeneous Fenton catalysis to avoid the development of ferric hydroxide sludge and to avoid the effect of a limited pH range on homogeneous Fenton reactions (Zhu et al., 2019). The heterogeneous electro-Fenton reaction is constituted by Fenton chemistry and electrochemical synthesis of H₂O₂.

In heterogeneous catalysis, iron is stabilized within the catalyst's structure instead of being added directly. Therefore, heterogeneous Fenton catalysts can retain their stability to decompose H₂O₂ into ·OH, preventing the obvious leaching of iron ions and the generation of iron hydroxide precipitation (Rao et al., 2022; Zhu et al., 2019). There are a number of contentious issues or concerns in the development of heterogeneous Fenton catalysis, and one of the most important concerns is how to improve the catalytic activity (Zhu et al., 2019).

MXenes are an emerging family of two-dimensional (2D) transition metal carbides/nitride materials. MXenes are commonly made by etching out a layer (A) from a three-dimensional structure consisting of MAX, with a formula of M_n ₊ ₁X_nT_x, in which T signifies the functional group such as hydroxyl, oxygen, or fluorine and n is the integer (Kumar et al., 2022).

MXene sheets possess good electrical conductivity that are comparable to multilayer graphene, and the presence of -OH surface groups following the hydrogen fluoride (HF) treatment caused their high hydrophilicity with modest contact angles (Naguib et al., 2012). MXenes have high surface area and abundant surface-active sites. Importantly, the unique 2D-layered nanostructure and sheet surface electronegativity of MXenes can immobilize Fenton reagents inside the layer spaces and improve the distribution of functional nano-particles (Ihsanullah, 2020).

However, research on the application of MXenes in AOP is very limited, especially about the MXene-mediated heterogeneous Fenton-like reaction system for the rapid removal of MIT from water.

Herein, we synthesized three kinds of MXene (Ti₃C₂T_x, V₂CT_x, and Mo₂CT_x) by HF etching, and finally compounded the MXene@Fe₃O₄ hybrids by the coprecipitation method, so as to enhance the heterogeneous electro-Fenton efficiency. The influences of the three MXenes as Fe₃O₄ supporting materials on the removal efficiency of MIT and corresponding chemical oxygen demand (COD) were compared.

One of the desirable properties of the iron-based catalysts' supporting materials is the ability to perform multiple cycles and less leaching of Fe ions. Therefore, we also investigated the cycling of Mo₂CT_x@Fe₃O₄.

Material and Methods

Chemicals

Mo₂Ga₂C and V₂AlC were purchased from Beijing Forsman Technology Co., Ltd. HF, FeCl₃·6H₂O, FeSO₄·7H₂O, H₂SO₄, NaOH, coumarin (C₉H₆O₂), phenanthroline (C₁₂H₈N₂), Na₂S₂O₃, acetonitrile (CH₃CN), and C₄K₂O₉Ti∙2H₂O were obtained from Chengdu Chron Chemicals Co., Ltd. 2-methyl-4-isothiazolin-3-one, Ti₃AlC₂, Na₂SO₄ was purchased from Aladdin reagent (Shanghai) Co., Ltd. All the chemicals used were analytical grade. All chemicals were used without any further purification.

Synthetic procedures

MXenes

In a typical synthesis, 2.0 g Ti₃AlC₂ powders were slowly added into 20 mL 40 wt% HF solution and stirred at 40℃ for 40 h to remove Al layer in a Teflon container. After HF treatment, the mixture was washed copiously by adding deionized water and centrifugation at 3000 rpm several times for 3 min per cycle until the pH of the supernatant of the last centrifugation was around 7.0. Subsequently, the sample was dried in a vacuum oven at 70°C for 24 h to yield Ti₃C₂T_x (Xi and Li, 2021). In addition, V₂CT_x and Mo₂CT_x were etched by this method.

MXene@Fe₃O₄

MXene@Fe₃O₄ was prepared by the coprecipitation method. First, stock solutions of 0.215 g FeCl₂•4H₂O, 0.583 g FeCl₃•6H₂O, and 1.7 mL HCl (6 mol/L) were prepared as a source of iron by dissolving the respective chemicals in 25 mL nitrogen-saturated deionized water. The mixed solution cited earlier was denoted as solution A. Similarly, stock solutions of 15 g NaOH and 0.4 g MXene (prepared Ti₃C₂T_x/V₂CT_x/Mo₂CT_x) were dispersed in 250 mL nitrogen-saturated deionized water as the alkali sources, denoted as solution B.

Solution A was slowly dropped into solution B, and then the precipitation was obtained by gentle magnetic stirring at 80℃ for 1 h. The precipitate was washed to neutral with deionized water and vacuum dried at 80℃ for 12 h to obtain MXene@Fe₃O₄ (Ti₃C₂T_x@Fe₃O₄, V₂CT_x@Fe₃O₄, Mo₂CT_x@Fe₃O₄).

Catalytic experiments

The relevant experimental parameters and steps are as follows: using a 100 mg/L MIT solution as a simulated wastewater treatment reaction, adding 50 mM of Na₂SO₄ as an electrolyte, setting the flow rate of the aeration pump at 0.2 L/min, and the speed of the mechanical stirrer was at 80 rpm to ensure that the catalyst was fully mixed in the solution system, and controlling the current to be constant at 300 mA.

Before each heterogeneous electro-Fenton experiment, adsorption saturation experiments were conducted on the catalyst under neutral conditions. The initial concentrations of MIT and Na₂SO₄, as well as other related parameters in the adsorption equilibrium experiment, were the same as those in the electro-Fenton experiment.

This experiment used a disposable syringe for water sample collection, with a sampling interval set at 30 min. Each sampling required a 0.45 μm needle filter for further collection and measurement. After each reaction, the surface of the electrode plate was cleaned with deionized water for 30 min to ensure cleanliness. Three parallel experiments were conducted for each group of experiments.

Analysis

UV–visible absorption spectra and fluorescence spectra were recorded on 752 UV–visible spectrophotometer (Jinghua, Shanghai) and F-7000 fluorescence spectrophotometer (Hitachi, Japan), respectively. All the samples were immediately filtered through a Millipore filter (pore size 0.45 μm). The concentration of MIT was measured by UV–Vis spectrophotometer (UV-2450, Shimadzu, Japan) at 273 nm.

The degradation efficiency was represented with (C₀-C_t)/C₀*100%, where C_t and C₀ represent the instant and initial MIT concentrations, respectively. We used the standard dichromate method to measure COD. The concentration of H₂O₂ was determined by the potassium titanyl oxalate colorimetric method. Using coumarin as a quantitative probe to detect hydroxyl radical, the generated 7-hydroxycoumarin with strong fluorescence was detected at 456 nm (excited at 346 nm). 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) as a probe was used for the semi-quantitative determination of superoxide radicals. The concentration of ferrous ions was quantified by UV-vis spectrophotometry and 1,10-phenanthroline.

Results and Discussion

Synthesis and characterization

Scanning electron microscopy (SEM) was carried out to probe the morphology and structural feature of the three MAX, three MXenes (Supplementary Fig. S1d–f), and three MXene@Fe₃O₄ samples (Fig. 1a–c). Similar to those previous reports by Yi et al., (2020) and Matthews (Matthews et al., 2022), the layered Ti₃C₂T_x (Supplementary Fig. S1e) and V₂CT_x (Supplementary Fig. S1f) with a typical accordion-like architecture differed significantly from the pristine Ti₃AlC₂ and V₂AlC MAX particles (Supplementary Fig. S1e, f insert), exposing much more basal plane and edge sites, which acted to support catalysts.

FIG. 1.

SEM patterns and EDS spectra of Mo₂CT_x@Fe₃O₄ (a, d), Ti₃C₂T_x@Fe₃O₄ (b, e), and V₂CT_x@Fe₃O₄ (c, f). EDS, Energy Dispersive Spectrometer; SEM, scanning electron microscopic.

The conversions to Ti₃C₂T_x and V₂CT_x were further confirmed by Energy Dispersive Spectrometer (EDS) elemental composition analysis and X-ray diffraction (XRD) results. As shown in Supplementary Fig. S1b, c, the signals of the Al element on Ti₃C₂T_x and V₂CT_x were very faint. EDS results in Fig. 1d–f also confirmed the presence of –F termination groups of MXenes.

The diffraction peak of Ti₃C₂T_{x (}Supplementary Fig. S2b) at 2θ = 39° was very weak after the HF exfoliation process, and it showed that the Al layer in Ti₃AlC₂ was almost eliminated by HF solution (Yi et al., 2020). The observed sharp XRD peaks of V₂CT_x in Supplementary Fig. S2c most probably originate from partially etched MXene particles that still contain Al (Thakur et al., 2019). Almost all MXene synthesized by HF etching would have a small amount of unreacted MAX phase (Naguib et al., 2012).

Supplementary Figure S1d showed the typical SEM image of Mo₂CT_X, which maintained the features of the MAX precursor. It could be seen from the figure that Mo₂CT_X had a layered structure. The EDS results in Supplementary Fig. S1a showed the presence of a large amount of Au in Mo₂CT_X. To better observe the morphology of the material, gold spraying treatment was carried out during the testing process, and the sample itself did not contain Au.

In the XRD patterns of Mo₂CT_X, Mo₂CT_X showed a wide peak at 7.3° after HF etching. It might be caused by the removal of Ga (Liu et al., 2021a). The yields of Mo₂CT_X, Ti₃C₂T_X, and V₂CT_X were 43.39%, 77.13%, and 71.84%, respectively. The yields of Ti₃C₂T_X and V₂CT_X were close to the respective theoretical yield. Due to the strong bonding of M-Ga in the precursor, it was more difficult to prepare Mo₂CT_x than Ti₃C₂T_x and V₂CT_x under the same conditions, so the yield of Mo₂CT_X was slightly lower than the theoretical yield.

As for the MXene@Fe₃O₄ composites, it could be observed that the MXene nanosheets were mixed with Fe₃O₄ nanoparticles (Fig. 1a–c). The EDS results of those three MXene@Fe₃O₄ composites are shown in Fig. 1d–f. It confirmed the presence of –F termination groups of MXene and the successful synthesis of ferroferric oxide. The atom percent of Fe in Mo₂CT_x@Fe₃O₄ was higher than that of Ti₃C₂T_x@Fe₃O₄ and V₂CT_x@Fe₃O₄. However, EDS only detected the surface atoms with a depth of a few nanometers.

The results detected by EDS only represented local element content and needed to be comprehensively analyzed in combination with other results. The crystal phase of the as-prepared materials was characterized using XRD. The XRD patterns of Ti₃C₂T_x showed typical peaks at 8.5°, 34.2°, 41.6°, and 60.4° attributed to (002), (101), (105), and (110) planes (PDF#52-0875), respectively.

Substantial reductions in the high index (101) and (105) planes of Ti₃AlC₂ confirmed the conversion of approximately all the Ti₃AlC₂ to Ti₃C₂T_X (Rethinasabapathy et al., 2022). The XRD result of Ti₃C₂T_x@Fe₃O₄ (Fig. 2b) showed typical diffraction peaks of Fe₃O₄ at 30.2°, 35.6°, 37.1°, 42.8°, and 62.8° corresponding to the (220), (311), (222), (400), and (440) planes (PDF#19-0629), respectively.

FIG. 2.

XRD patterns of Ti₃C₂T_x(a), Ti₃C₂T_x and Ti₃C₂T_x@Fe₃O₄ (b), Mo₂CT_x and Mo₂CT_x@Fe₃O₄ (c), V₂CT_x and V₂CT_x@Fe₃O₄ (d). XRD, X-ray diffraction.

In the XRD pattern of Ti₃C₂T_x, it was recognized that the peaks at 17.8° and 27° represented Ti₃C₂(OH)₂ (Qing et al., 2016). However, they disappeared as Fe₃O₄ nanoparticles were embedded in Ti₃C₂T_x (Li et al., 2021). The changes cited earlier proved the successful synthesis of Ti₃C₂T_X@Fe₃O₄. The XRD patterns of Mo₂CT_X@Fe₃O₄ and V₂CT_X@Fe₃O₄ composites were consistent with the XRD patterns of Mo₂CT_X and V₂CT_X, respectively. No characteristic peak signal of Fe₃O₄ was detected in the XRD patterns of Mo₂CT_X@Fe₃O₄ and V₂CT_X@Fe₃O₄, but abundant Fe content could be detected in the EDS energy spectra.

X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical composition and the surface chemical state of MXene@Fe₃O₄ further. The high-resolution XPS survey spectrum of Mo₂CT_x@Fe₃O₄ revealed C 1s, O 1s, Mo 3d, Fe 2p, and Ga 2p peaks (Fig. 3A(a)). In the high-resolution XPS spectrum of Ti₃C₂T_x@Fe₃O₄ (Fig. 3A[b]), the distinct signals of C, O, Ti, F, and Fe elements were observed.

FIG. 3.

XPS spectra (A) and Fe 2p XPS spectra (B) of Mo₂CT_x@Fe₃O₄ (a), Ti₃C₂T_x@Fe₃O₄ (b), and V₂CT_x@Fe₃O₄ (c). XPS, X-ray photoelectron spectroscopy.

The full spectrum in Fig. 3A(c) showed the coexistence of Fe, F, O, V, and C elements, consistent with the result of EDS. We further collected the data on chemical valence by fitting the high-resolution spectra of each element. Three pairs of peaks were identified in Mo 3d XPS spectrum (Supplementary Fig. S5e). The binding energies (BEs) of the Mo 3d_5/2 peaks located at 228.0 eV corresponded to Mo-C of molybdenum carbides.

The Mo-C bond indicated that Mo₂CT_x was successfully synthesized. The BEs located at 229.2/232.2ev and 232.6/235.7ev corresponded to Mo⁴⁺ and Mo⁶⁺ species, respectively. The peaks of high-valence-state Mo were attributed to the inevitable partial oxidation of the active surface atoms of Mo₂CT_x (Mei et al., 2020). The peaks (Supplementary Fig. S5b) located at 283.54 eV and 286.05 eV were ascribed to the C–Mo bond and C–O/C = O (Cheng et al., 2018).

As presented in Supplementary Fig. S5, three samples all contained C–F corresponding peak (around 288.7eV). The indistinctive C–F might be derived by HF etching during the preparation process of MXenes (Wang et al., 2020a). The Ti 2p XPS spectrum (Supplementary Fig. S5d) was divided into three pairs of peaks. The two peaks at 454.9 eV (Ti 2p_3/2) and 460.9 eV (Ti 2p_1/2) could be ascribed to the Ti-C bond, respectively, indicating the presence of Ti₃C₂T_x (Peng et al., 2016).

The peaks (Supplementary Fig. S5a) located at 281.7 eV and 284.8 eV were ascribed to C-Ti-T_x and C-C (Yan et al., 2019). The V 2p spectrum (Supplementary Fig. S5f) showed that the surface of V₂CT_x was dominated by V⁴⁺ and V⁵⁺ (Liu et al., 2021b). The O 1s XPS spectrum was deconvoluted into tree component peaks at about 530.3 eV and 531.7 eV corresponding to V–O in oxide and O–H bond, respectively (Narayanasamy et al., 2020).

In addition, the peak at the binding energy value of around 715 − 735 eV was assigned to Fe 2p Fig. 3B showed that the binding energies of Fe 2p_1/2 and Fe 2p_3/2 were located at 724.4 and 710.6 eV, respectively, which suggested the successful synthesis of Fe₃O₄ on the MXenes (Yamashita and Hayes, 2008). For Mo₂CT_x@Fe₃O₄, Ti₃C₂T_x@Fe₃O₄, and V₂CT_x@Fe₃O₄, the XPS spectra displayed no major shift in the Fe 2p3/2 and Fe 2p1/2 peak values (Fig. 3B).

However, there were apparent changes in the peak intensity for Fe species in Fig. 3B. A low quantity of Fe²⁺ was also observed for Ti₃C₂T_x@Fe₃O₄ (Fig. 3B (b)) and still lower for V₂CT_x@Fe₃O₄ (Fig. 3B (c)) in comparison to Mo₂CT_x@Fe₃O₄ (Fig. 3B (a)).

Further, as the surface area and porosity characteristics are critical to the electrocatalytic performance, we also conducted Barrett–Emmett–Teller (BET) test. The N₂ adsorption-desorption isotherms of Mo₂CT_x@Fe₃O₄, Ti₃C₂T_x@Fe₃O₄, and V₂CT_x@Fe₃O₄ are shown in Fig. 4. The detailed values of specific surface area, average pore diameter, and pore volume are presented in Table 1.

FIG. 4.

The N₂ adsorption–desorption isotherms of Mo₂CT_x@Fe₃O₄, Ti₃C₂T_x@Fe₃O₄, and V₂CT_x@Fe₃O₄ (d).

Table 1.

Specific Barrett–Emmett–Teller Surface Area, Pore Volume, and Average Pore Diameter of the Ti₃C₂T_X@Fe₃O₄, V₂CT_X@Fe₃O₄, and Mo₂CT_X@Fe₃O₄ Samples

Sample	Surface area (m²/g)	Pore volume (cm³/g)	Barrett–Joyner-Halenda desorption average pore diameter (nm)
Ti₃C₂T_x@Fe₃O₄	13.54	0.045	5.33
V₂CT_x@Fe₃O₄	11.43	0.048	4.23
Mo₂CT_x@Fe₃O₄	47.66	0.072	6.22

According to the IUPAC classification (Thommes et al., 2015), three curves showed type IV isotherm (in) with one clear H2(b)-type hysteresis loop from P/Po ∼0.4 to 1.0, which was characteristic of mesoporous materials. The isotherm of samples possessed both mesoporous and microporous structures (Chandran et al., 2020). Different from the type H3 loop commonly found in plate-like particles, the H2 loop might be caused by the load of Fe₃O₄.

Moreover, the surface area of Mo₂CT_x@Fe₃O₄, Ti₃C₂T_x@Fe₃O₄, and V₂CT_x@Fe₃O₄ determined by BET was 47.66, 13.54, and 11.43 m²/g, respectively. Compared with Ti₃C₂T_x@Fe₃O₄ and V₂CT_x@Fe₃O₄, Mo₂CT_x@Fe₃O₄ had higher porosity and a larger surface area. A specific larger surface area provided a wider contact area of Fe₃O₄ and MIT, and afforded more exposed catalytic active sites, contributing to better catalytic performance. Higher porosity with bigger interconnected pores, on the other hand, would make it easier for reactants/products to access or escape the active areas (Wang and Tang, 2021).

Performance of catalysts in reactions

Catalytic degradation of MIT

The catalytic activity of the catalysts was evaluated through an electro-Fenton system. For comparison, the various catalysts of Ti₃C₂T_x@Fe₃O₄, V₂CT_x@Fe₃O₄, and Mo₂CT_x@Fe₃O₄ were investigated under the same conditions. And the results are shown in Fig. 5. The removal rate of MIT was calculated by measuring the concentration of MIT before and after the reaction, representing the removal of the compound.

FIG. 5.

Removal of MIT by three catalysts: Ti₃C₂T_x@Fe₃O₄, V₂CT_x@Fe₃O₄, and Mo₂CT_x@Fe₃O₄ (a), and its COD removal (b) under same conditions (initial pH = 3, applied current = 0.3A, aeration volume = 0.2 L·min⁻¹, 0.05 M Na₂SO₄). MIT, methylisothiazolinone.

Accordingly, the COD removal rate tended to represent the complete mineralization degree of MIT. It was obvious that Mo₂CT_x@Fe₃O₄ had the best catalytic degradation effect of MIT, followed by Ti₃C₂T_x@Fe₃O₄, and V₂CT_x@Fe₃O₄ was the weakest. The COD removal rate (Fig. 5b) also had the same trend. We also conducted a control experiment using Fe₃O₄ as a catalyst.

Under the same experimental conditions, the removal rate of MIT after 2 h was 60.13%, confirming the role of MXene in the catalytic reaction. In the presence of Mo₂CT_x@Fe₃O₄, the COD removal efficiency at 120 min was 62.46% and the removal rate of MIT was 93.41% (supporting electrolyte = 0.05 M Na₂SO₄, initial pH = 3). In addition, when Mo₂CT_X was used as a catalyst to degrade MIT, the degradation rate of MIT was 73.01%, further indicating the synergistic effect of Mo₂CT_X and Fe₃O₄.

Wang et al. (2019) used a Ti/SnO₂-Ab₂O₃/α, β-PbO₂ electrode to degrade MIT, whereas the degradation rate of 100 mg/L MIT was 56.8% after 210 min of oxidation (supporting electrolyte = 0.1 M Na₂SO₄, initial pH = 5.35). At present, when the initial pH = 5 (Fig. 5a), using Mo₂CT_x@Fe₃O₄ as the catalyst, the removal rate of MIT and COD still reached 83% and 58.23%, respectively.

The electro-Fenton process combines an oxygen reduction reaction (ORR) and Fenton-like reaction for application in water treatment. In this work, hydrogen peroxide was not added, but 2-electron ORR occurred on the cathode plate [Eq. (1)]. In Supplementary Fig. S3a, V₂CT_x@Fe₃O₄ had the highest hydrogen peroxide concentration. Excessive H₂O₂ would eliminate free radicals [Eqs. (2) and (3)]. In addition, the rapid regeneration of Fe²⁺ was essential for the continuous production of ·OH.

Among Mo₂CT_X, Ti₃C₂T_X, and V₂CT_X, Mo₂CT_X could accelerate the Fe(III)/Fe(II) cycle most quickly, which was mentioned later. When Mo₂CT_x@Fe₃O₄ was used as a catalyst, the formation rate of H₂O₂ was moderate and Fe²⁺ could be rapidly regenerated, so the amount of hydroxyl was the largest. Generally, active oxygen radicals (∙OH and ∙O₂^-) would be generated during H₂O₂ activation, which was responsible for the contaminant degradation. The trends of ∙OH (Supplementary Fig. S3b) and ∙O₂^- concentrations (Supplementary Fig. S3c) were consistent with the removal efficiency of MIT and COD. $O_{2} + 2 e^{-} + 2 H^{+} \to H_{2} O_{2}$ (1)

H_{2} O_{2} + \cdot O H \to H O_{2} \cdot + H_{2} O

(2)

H O_{2} \cdot + H O_{2} \cdot \to H_{2} O_{2} + O_{2}

(3)

H O_{2} \cdot \to H^{+} + {O_{2}}^{-}

(4)

As presented in Supplementary Fig. S1, three Mxenes had different lamellar structures, which affected not only the load of Fe₃O₄ but also the specific surface area. According to the EDS result, Mo₂CT_x@Fe₃O₄ had the highest surface iron content among three catalysts, which meant that Fe₃O₄ was more easily loaded on the surface of Mo₂CT_x.

Through the BET analysis, Mo₂CT_x@Fe₃O₄ had a high specific surface area and exposed more active sites. The porosity was also the largest, which would be conducive to the transport of Fe²⁺ in the electro-Fenton process. Mo₂CT_x, with a ternary precursor of Mo₂Ga₂C containing double A layers, was theoretically predicted to have high electrochemical and thermoelectric properties among all available MXenes (Wan et al., 2021). Ti₃C₂T_X had excellent conductivity (Feng et al., 2020), which even exceeded the conductivity of Mo₂CT_X. Even so, the degradation effect of Mo₂CT_X@Fe₃O₄ on MIT was still stronger than that of Ti₃C₂T_X@Fe₃O₄, indicating that conductivity did not dominate the reaction process.

There are two main reaction routes between heterogeneous catalysts and H₂O₂ (Thomas et al., 2021). The reaction of unavoidably leached Fe from catalysts with H₂O₂ under acidic condition homogeneous Fenton reaction [Eqs. (5) and (6)], and the reaction of surface Fe (≡Fe(II) and ≡Fe(III)) with H₂O₂ [Eqs. (7) and (8)].

It was generally believed that in the heterogeneous Fenton reaction, as long as the catalyst was stable under acidic conditions, the organic pollutants were mainly oxidized by the ROS produced by ≡Fe(III) or ≡Fe(II) (Zhu et al., 2019). Fe²⁺ played an important role in the electro-Fenton reaction. Of the three catalysts, Mo₂CT_X@Fe₃O₄ had the largest surface area.

The more catalytic active sites were exposed, the more Fe²⁺ could be absorbed on its surface to greatly promote the activation of H₂O₂. It has been reported that MXene could accelerate the conversion of Fe³⁺ to Fe²⁺ [Eq. (9)] (Song et al., 2021). The Fe 2p XPS spectrum (Fig. 3B) showed that the iron content of Mo₂CT_x@Fe₃O₄ was the largest in the synthesized MXene@Fe₃O₄, especially Fe²⁺.

The change of the content of different Fe species further illustrated a phenomenon that the addition of MXene could promote the conversion of Fe³⁺ to Fe²⁺ through the oxidation-reduction reaction between low valence atoms and Fe³⁺ (Chen et al., 2022). From Supplementary Fig. S5d–f, we could also see that the Mo–C bond was very strong.

The Ti-C peaks for Ti₃C₂T_x@Fe₃O₄ were weak, and the V–C bond in the V 2p XPS spectrum of V₂CT_x@Fe₃O₄ (Supplementary Fig. S5f) was almost absent. Among the three catalysts, Mo in Mo₂CT_X@Fe₃O₄ existed in a more reduced form. In other words, Mo₂CT_x had the strongest ability to accelerate the conversion of Fe³⁺ to Fe²⁺, which was also consistent with the degradation of MIT. $F e^{2 +} + H_{2} O_{2} \to F e^{3 +} + \cdot O H + O H^{-}$ (5)

F e^{3 +} + H_{2} O_{2} \to F e^{2 +} + H O_{2} \cdot + H^{+}

(6)

\equiv F e (I I) + H_{2} O_{2} \to \equiv F e (I I I) + \cdot O H + O H^{-}

(7)

\equiv F e (I I I) + H_{2} O_{2} \to \equiv F e (I I) + H O_{2} \cdot + H^{+}

(8)

F e^{3 +} + e^{-} \to F e^{2 +}

(9)

Based on the results cited earlier, we proposed a possible mechanism for the degradation of MIT in the heterogeneous electro-Fenton system. First, O₂ underwent a 2-electron ORR at the cathode to generate H₂O₂, which reacted with Fe²⁺ loaded on the surface of MXene to generate ·OH.

At the same time, a small amount of ·O^2- was generated during the electro-Fenton reaction process. Second, the reducibility of MXene could accelerate the transformation of Fe³⁺ to Fe²⁺, thereby producing more ·OH. Finally, active free radicals attacked MIT, causing it to decompose into intermediates or CO₂ and H₂O.

Catalytic performance of Mo₂CT_x@Fe₃O₄ at different pH

Initial pH had a significant effect on Fenton reaction. We controlled the initial pH in the range of 3–9 (Supplementary Fig. S6a, b). The removal efficiency of MIT decreased with the increase of initial pH, which accorded with the electro-Fenton reaction. The removal efficiency of COD also decreased. When pH >3, on the one hand, the homogeneous Fenton reaction of partially dissolved iron ions was disturbed.

On the other hand, H₂O₂ could become unstable and break up into O₂ and H₂O (Akerdi et al., 2017). The removal efficiency of MIT and COD still reached 76.6% and 49.1% after 120 min even at neutral pH. Nevertheless, in general, the Mo₂CT_X@Fe₃O₄ catalyst exhibited enhanced electro-Fenton performance over a wide range of pH values.

Recycling test

Actually, the stability of catalyst was of great significance for the catalytic reaction. Thus, cyclic experiments were carried out. Mo₂CT_x@Fe₃O₄ was collected with ultrapure water washing after each cycle, and then a new cycle was started after drying. The removal rates of MIT during the four consecutive runs were 92.51%, 92%, 91%, and 89%. There was almost no visible change in the MIT removal rate (Fig. 6) before and after four cycles.

FIG. 6.

The reusability of Mo₂CT_x@Fe₃O₄.

These results confirmed that Mo₂CT_x@Fe₃O₄ could be reused at least four times without significantly reducing its efficiency, which would help reduce the cost of producing water treatment catalysts. The declining trend of the COD removal rate was also slight. Closely related to the stability and reusability of the catalyst was the absence of leaching of metal species from solid catalyst (Ganiyu et al., 2018). Only Fe²⁺ was involved in the catalytic process, so we measured its concentration, and the result is shown in Supplementary Fig. S4.

The content of Fe²⁺ leached by V₂CT_X@Fe₃O₄ was the highest, whereas the content of Fe²⁺ leached by Mo₂CT_X@Fe₃O₄ was initially high, but gradually decreased over time, whereas the content of Fe²⁺ leached by Ti₃C₂T_X@Fe₃O₄ remained at a relatively low level. This indicated that Mo₂CT_X@Fe₃O₄ and Ti₃C₂T_X@Fe₃O₄ had good reusability as catalysts.

Conclusion

Three types of MXene (Ti₃C₂T_x, V₂CT_x, and Mo₂CT_x) were synthesized by the commonly used HF acid etching method, used as heterogeneous Fenton catalyst-Fe₃O₄ supporting. MXenes had excellent ORR activity and stability, and MXene's multilayer structure could avoid aggregation of Fe₃O₄ to expose abundant active sites. Mo₂CT_x had relatively high electrochemical, provided a better place for in situ coprecipitate synthesis of Fe₃O₄, and also increased the specific surface area of the composites.

EDS and XPS results showed that Fe²⁺ content in Mo₂CT_X@Fe₃O₄ was higher than that of other catalysts. The rate of H₂O₂ generation by Mo₂CT_X was moderate, and Mo₂CT_X could accelerate Fe(III)/Fe(II) cycle faster. So Mo₂CT_X@Fe₃O₄ generated the most hydroxyl groups, which was consistent with the degradation rate of MIT.

In addition, Mo₂CT_X@Fe₃O₄ could be reused at least four times without significantly reducing its efficiency. Our research suggested that Mo₂CT_x@Fe₃O₄ is a promising catalyst for MIT degradation, and applying MXenes to modify iron-based materials is a feasible strategy for constructing heterogeneous Fenton catalysts.

Footnotes

Authors' Contributions

L.W.: Writing—original draft(equal); Formal analysis(equal). K.Z.: Writing—original draft(equal); Formal analysis(equal). Q.R.: Conceptualization(lead); Investigation(lead). H.-q.L.: Resources(lead); Writing—review and editing(lead). P.Y.: Supervision(lead).

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This work was supported by the International Scientific and Technological Innovation and Cooperation Project of Sichuan (Grant No. 2019YFH0170).

Supplementary Material

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Supplementary Material

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Introducing MXenes into the Heterogeneous Catalyst: Synthesizing Mo 2 CT x @Fe 3 O 4 with Excellent Recoverability to Degrade Methylisothiazolinone in the Electro-Fenton System

Abstract

Introduction

Material and Methods

Chemicals

Synthetic procedures

MXenes

MXene@Fe3O4

Catalytic experiments

Analysis

Results and Discussion

Synthesis and characterization

Performance of catalysts in reactions

Catalytic degradation of MIT

Catalytic performance of Mo2CTx@Fe3O4 at different pH

Recycling test

Conclusion

Footnotes

Authors' Contributions

Author Disclosure Statement

Funding Information

Supplementary Material

References

Supplementary Material

MXene@Fe₃O₄

Catalytic performance of Mo₂CT_x@Fe₃O₄ at different pH