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Article

Enhanced Electrocatalytic Degradation of Phenol by Mn-MIL-100-Derived Carbon Materials

by
Xueping Sun
1,†,
Haitao Liu
1,†,
Dan Chen
1,2,*,
Ya Zhang
2,
Xinbai Jiang
1,* and
Jinyou Shen
1,2
1
Key Laboratory of Environmental Remediation and Ecological Health, Ministry of Industry and Information Technology, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
2
Engineering Research Centre of Chemical Pollution Control, Ministry of Education, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Water 2025, 17(7), 1103; https://doi.org/10.3390/w17071103
Submission received: 12 March 2025 / Revised: 3 April 2025 / Accepted: 5 April 2025 / Published: 7 April 2025
(This article belongs to the Special Issue Interfacial Engineering Solutions for Enhanced Degradation of Toxic Contaminants in Water Systems)
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Abstract

:
To achieve high electrooxidation efficiency for phenol, this study explored the fabrication of Mn-MIL-100 catalysts at various calcination temperatures, loaded onto a carbon paper (CP) anode. The materials were characterized using scanning electron micros-copy, X-ray photoelectron spectroscopy, thermogravimetric analysis, and X-ray diffraction. Their electrocatalytic activities under various calcination temperatures were evaluated through cyclic voltammetry (CV) tests, while the effect of pH in the Mn-MOF modified CP electrodes on phenol degradation performance was investigated using the potentiostatic discharge method. Mn-MOF@CP calcined at 400 °C and 500 °C (denoted as Mn400@CP and Mn500@CP, respectively) exhibited significantly enhanced cyclic voltammetry current responses in phenol solution, attributed to an increase in oxygen vacancy concentration. A phenol degradation efficiency of 96.00 ± 1.53% was achieved by Mn400@CP within 16 h, while it was only 60.12 ± 2.03% for Mn500@CP and 8.01 ± 2.00% for the blank CP at pH 4. Additionally, Mn400@CP consistently demonstrated superior phenol degradation efficiency over Mn500@CP across various pH values. The outstanding electrocatalytic activity of Mn400@CP for phenol oxidation could be attributed to its lower charge transfer resistance. A radical-mediated oxidation pathway was proposed for the Mn400@CP electrocatalytic system, elucidating its phenol degradation mechanism. These findings highlighted the potential of Mn-MOF-derived carbon-based materials for the degradation of organic contaminants.

1. Introduction

Phenol is a significant substance and a main pollutant in industrial production [1]. Phenols can be found in a variety of industries, such as resin and plastic manufacturing, petroleum refining, steel production, textiles, tannery, paper and pulp, pharmaceuticals, as well as food processing industries [2]. Phenol is an organic substance that is refractory to degradation and can cause serious harm to the ecosystem even at low concentrations [3], and it is also designated as a priority pollutant by the U.S. Environmental Protection Agency (EPA) [4]. Prolonged or excessive exposure to phenol can lead to adverse health effects, including skin irritation, stomach upset, and headaches [2]. Moreover, studies have found that phenol, even at low concentrations, has many adverse effects on fish and aquatic ecosystems [5]. Because of the toxicity of phenol, the EPA has established stringent water purification standards for surface water containing less than one part per billion of phenol [6]. Therefore, to better promote the sustainable development of society, it is of critical importance to develop an effective and environment-friendly method to degrade phenol.
Traditional methods for removing phenolic contaminants from aqueous solutions can be categorized into these primary types: biological, physical, and chemical approaches. The biodegradation of phenolic compounds is very restricted due to their chemical stability, high toxicity, and refractory nature, which can inhibit microbial activity [7]. Therefore, physical and chemical methods are usually needed to reduce the biological toxicity of phenol. In the process of physically removing toxic pollutants, adsorption is widely used. Currently, activated carbon is the most widely used adsorbent. However, the excessive cost and challenges associated with regeneration limit the economic viability of activated carbon as an effective adsorbent [8]. The effects of flocculating, precipitating, and adding drugs to wastewater via conventional chemical process treatment are very limited [9]. At present, the electrochemical process is considered one of the most effective low-cost technologies [10]. The electrooxidation process is well-suitable for the degradation of aromatic pollutants, which can be utilized either for pretreatment to promote biodegradability or even for complete mineralization of organic pollutants into water and carbon dioxide [11]. In the process of wastewater treatment by an electrochemical method, electrode materials play a very important role [12]. By brushing graphene ink on a carbon fiber substrate, a modified electrode has been developed and utilized for the electrode in the Fenton process, and the degradation efficiency of phenol has more than doubled [13]. Therefore, the selection of anode materials with high efficiency and fast kinetic properties is very important.
Due to their extensive surface area, exceptional porosity, and versatile structures and properties, crystalline porous materials such as MOFs can be widely used in catalysis, gas separation and retention, supercapacitors, and lithium-ion batteries [14,15]. MOFs had explicit structure, a large number of unsaturated metal sites and functional groups, as well as semi-flexible and orient facet characteristics, which could provide more active sites for the electrochemical redox process [16,17]. Among them, MIL-100 (MIL represents for Materiaux de l′Institute Lavoisier) series [M3O(OH)(H2O)2(C6H3(CO2)3)2] MOFs are well known owing to their excellent chemical stability, high thermal stability, and outstanding performance with an extensive specific area (SBET reaching up to 2000 m2 g−1) [18]. However, due to the poor electrical conductivity of MOFs, the stability in long-term application process is difficult to guarantee. Therefore, few studies have been conducted on their use as electrocatalysts for pollutant removal [19,20].
Derivatives of metal oxides and carbon composite materials from MOFs offered structural advantages, as periodic MOF materials with ordered structures ensured the uniform distribution of derivative components. The incorporation of transition metals (such as iron, manganese, and zinc) further modified the physicochemical and chemical properties of biochar [21]. Previous studies have demonstrated the versatility of Mn-MIL-100 frameworks in advanced oxidation processes. It was reported that Mn-MIL-100 heterogeneous catalyst could be used for the selective oxidative cleavage of alkenes [22]. Bi et al. [23] prepared an Mn-based catalyst using a non-thermal method with Mn-MIL-100 as a precursor, which effectively oxidized and degraded toluene. Its excellent catalytic performance was attributed to its high surface area, low-temperature reduction characteristics, abundant surface-adsorbed oxygen, and the presence of Mn3+. These studies demonstrated the great potential of Mn-MIL-100 for the oxidative degradation of organic pollutants. Xiong et al. [24] successfully fabricated CuxO/C composites using a two-step calcination strategy employing Cu-MOFs as both template and precursor, and the prepared composites exhibited excellent Fenton-like activity, which was mainly ascribed to their excellent specific surface area, adsorption performance, and electron transfer performance in the crystallized microstructure. Zhang et al. [25] reported the in situ synthesis of porous Mn2O3 material through the calcination of Mn-MIL-100 metal-organic frameworks at 400 °C, which exhibited high capacity and long-term cycling stability. Besides, oxygen vacancies were generated in a type of perovskite oxide calcined at 1100 °C for peroxymonosulfate activation in the advanced oxidation of phenol, facilitating a free radical degradation pathway [26]. Furthermore, Zhang et al. [27] synthesized a MnCeOx catalyst at different calcination temperatures and demonstrated that the calcination temperature positively influenced the surface chemical composition, adsorbed oxygen, oxygen vacancy concentration, and low-temperature reduction ability of the catalyst. These findings indicated that metal oxide-derived materials obtained through the pyrolysis of Mn-MIL-100 at high temperatures tend to possess high stability and catalytic activity. However, limited studies have focused on Mn-MIL-100-derived materials for phenol electrooxidation, particularly regarding the influence of calcination temperature on catalytic performance.
This study focused on the preparation of Nafion-coated carbon paper (CP) modified with Mn-MIL-100 calcined at temperatures of 300 °C, 400 °C, and 500 °C, and its utilization as an electrochemical anode for the catalytic degradation of phenol. Various physical-chemical analytical techniques were used to analyze the structure and components of Mn-MOF electrocatalysts. Additionally, the influence of environmental factors, such as pH levels, on the stability of Mn-MOF@CP was evaluated. The electrocatalytic activity of CP, Mn400@CP, and Mn500@CP in phenol oxidation was evaluated and systematically compared. Moreover, the feasible free radicals and electron transfer pathway of the Mn-MOF-modified CP electrode, which contributed to the enhanced electrocatalytic degradation of phenol, was initially investigated.

2. Materials and Methods

2.1. Fabrication of Mn-MIL-100 Materials

The solvothermal synthesis method was used to synthesize a series of Mn-MIL-100 after reasonable adjustment and modification [28]. The detailed steps include: (1) manganese nitrate (Mn(NO3)2·4H2O) (300 mg) and 1,3,5-benzenetricarboxylic acid (400 mg) were added to methanol (25 mL) with continuous magnetic stirring until complete dissolution was achieved. (2) transferring the dissolved mixture into a stainless steel autoclave with a Teflon liner, and heating at 160 °C for 90 min; (3) after heating, the mixture was naturally cooled to room temperature, and the precipitate was collected by centrifugation at 9000 rpm for 5 min. The precipitate was then washed three times with deionized water or ethanol to remove residual solvents. (4) the obtained brown product was dried overnight in a vacuum oven at 80 °C, yielding the precursor of Mn-MIL-100 MOF; (5) the dried precursor samples divided into several portions and heated in air at a rate of 5 °C min−1 from ambient temperature to 300 °C, 400 °C, 500 °C, respectively, with a 2-h holding period at each target temperature to synthesize Mn300, Mn400, Mn500. Finally, the samples were allowed to cool naturally in the furnace before being subjected to further testing [29]. It should be noted that solvents and reagents used were analytically graded and could be used directly without further purification.

2.2. Fabrication of Mn-MOF-Modified CP Electrodes

In order to remove impurities, CP electrode (CeTech, Taichung, Taiwan), cut into small pieces (0.19 mm × 3.0 cm× 3.0 cm), were ultrasonically cleaned in acetone for 1 h, followed by ultrasonication for 2 h in deionized water. The pretreated samples were stored in ethanol to maintain sterility. Prior to experimental use, the CP substrates were oven-dried at 60 °C for 12 h under atmospheric conditions.
The CP electrode modified with Mn-MOF was fabricated following the method described by Song et al. [18]. Briefly, Mn300/400/500-MOF (12 mg) was mixed with Nafion (50 μL, 5%, DuPont, Wilmington, DE, USA) and ethanol (950 μL) to prepare a homogeneous catalyst ink. A total of 100 μL of the catalyst ink was drop-coated uniformly onto the pretreated CP electrode. The Mn300/400/500-MOF-modified electrodes were then dried in an oven for 12 h at 55 °C. The CP electrode loaded with the Mn300/400/500-MOF catalyst was labeled Mn300/400/500-MOF@CP. For comparison, CP electrodes modified with Nafion alone were constructed as well and marked as Nafion@CP.

2.3. Characterization and Analysis

Thermogravimetric analysis (TGA) of the Mn-MIL-100 samples was obtained from a Thermo-Gravimetric analyzer (SDT Q600 Simultaneous DSC-TGA, TA, New Castle, DE, USA) to determine the appropriate calcination temperature and the stability of the samples. Crystalline phases were analyzed via X-ray diffraction (XRD, D8 Advance, Bruker, Germany) with Cu-Kα radiation (λ = 1.5418 Å, 40 kV, 30 mA) over a 2θ range of 5–80°, and surface chemistry was characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA). Field-emission scanning electron microscopy (FE-SEM, Quanta 250 FEG, FEI Company, Hillsboro, OR, USA) revealed the surface morphology of as-prepared materials, while SEM-EDS mapping (JSM-IT500HR, JEOL, Tokyo, Japan) confirmed the homogeneous distribution of catalysts. A VMP3 potentiostat (Bio-Logic Science Instruments, Grenoble, France) with a three-electrode system: working electrode, Pt counter, and Ag/AgCl (3 M KCl) reference was employed for electrochemical evaluations. Cyclic voltammetry (CV) was performed in solutions containing 0.25 M NaCl and 100 mg L−1 phenol at a 5 mV s−1 scan rate. Cathodic adsorption (CA) tests maintained 1.0 V vs. Ag/AgCl for 16 h in 0.25 M NaCl under N2. Electrolyte pH (3–7) was modified using 0.1 M NaOH/HCl. The initial concentration of phenol was 100 mg L−1, and the volume of solution used for degradation was 100 mL. Phenol concentrations were quantified via high-performance liquid chromatography (HPLC; Waters 2695 with 2998 PDA detector and XBridge C18 column) using an isocratic mobile phase (40:60 v/v methanol: water) at 1.0 mL min−1. All experiments were performed in triplicate and the standard deviation was calculated.

3. Results and Discussion

3.1. Microstructure Characterization

TGA was conducted on Mn-MIL-100 to evaluate its thermal stability and determine the appropriate calcination temperature. As shown in Figure 1a, the material exhibited a first-stage weight loss of 30% from 30 °C which can be explained by the escape of water molecules. The second stage of weight loss started at 390 °C, with a significant reduction in sample weight of up to 40%. Attributed to the decomposition of organic ligands in air, Mn-MIL-100 experienced significant weight loss when the temperature was increased to just below 400 °C [25]. As a result, calcination temperatures of 300 °C, 400 °C, and 500 °C were selected for further processing of the Mn-MIL-100 material.
The XRD patterns of Mn-MIL-100, Mn300, Mn400, and Mn500 were shown in Figure 1b. The diffraction peaks of the as-synthesized Mn-MIL-100 closely matched the simulated pattern. The residual solvent molecules on the material are one of the reasons for the variation in the relative intensity of the reflection [15]. The narrow and sharp XRD characteristic peaks suggest that the prepared Mn-MIL-100 exhibits high crystallinity and purity. The results demonstrated that the solvothermal method was easy to control and could be used to synthesize high-crystalline and high-purity Mn-MIL-100. The characteristic peak of the Mn-MIL-100 crystal (θ = 10.3°) still appeared in the Mn300 carbon-based material, indicating that the Mn-MIL-100 organic ligand was not fully carbonized and partially retained the topology of the metal-organic ligand when the pyrolysis temperature was 300 °C, which was consistent with the TGA curve data. The disappearance of Mn-MIL-100’s characteristic XRD peaks at 400 °C pyrolysis temperature conclusively demonstrated the substantial completion of carbonization processes. The calcination of the Mn-MIL-100 material at a relatively high heating rate of 5 °C min−1 revealed that mixtures of Mn3O4 and Mn5O8 were obtained at pyrolysis temperatures of 400 °C and 500 °C instead of the pure phases that typically occur sequentially [29]. As the pyrolysis temperature increased, a phase transition occurred, leading to crystal lattice defects and a reduction in surface amorphous oxygen. Consequently, the crystallinity of manganese oxide improved, accompanied by an increase in the relative intensity of the diffraction peaks [28].
To investigate the surface chemical composition and valence state of fresh and calcined catalysts at different temperatures, an XPS survey scan was presented in Figure 1c. The catalyst indicated the presence of Mn, O, and C. In addition, the XPS spectra of Mn 2p for the synthetic sample were illustrated in Figure 1d. The spectrum exhibited two primary peaks at ∼653.87 eV and ∼641.08 eV, which correspond to the Mn 2p1/2 and 2p3/2 symmetric peaks with spinorbital splitting, respectively [30]. In addition, the significant satellite feature located at 646.4 eV was ∼5.4 eV on the higher binding energy side, indicating that the Mn2+ state was dominant in Mn [31]. It has been observed that the XPS spectra of Mn300 and Mn-MIl-100 were similar in shape and peak position, indicating that surface Mn mainly occurred in the Mn2+ state. Considering that the three oxidation states of Mn (II, III, IV) have multiple splits, the asymmetric Mn2p3/2 XPS signal can be divided into three parts: BE = 641 eV, BE = 642.2 eV, and BE = 643.38 eV, which belong to Mn2+, Mn3+ and Mn4+ on the material surface, respectively [32] When the pyrolysis temperature increased to 400 °C, compared with the XPS spectrum of Mn300, the satellite peak of Mn400 disappeared, the characteristics of Mn-MIL-100 basically disappeared, and the proportion of Mn3+ increased. Interestingly, as the temperature increased further to 500 °C, the proportion of Mn3+ decreased, while the proportion of Mn4+ increased. In addition, the O 1s spectra in Figure S3 exhibited three distinct subpeaks, representing different forms of oxygen bonding. The peak at 531.5 eV represented the oxygen chemisorbed at the oxygen vacancy, while the peaks at 529.4 eV and 530.1 eV correspond to the metal oxide and oxygen in O-H respectively [33,34]. The enhanced intensity of peak at 531.5 eV in Mn400 indicated a higher oxygen vacancy concentration compared to Mn500, which would facilitate the electrocatalytic oxidation and degradation of phenol [26].

3.2. Surface Morphology

Morphological characterization of Mn-MIL-100, Mn300, Mn400, and Mn500, as shown in Figure 2, was investigated via FE-SEM. Mn-MIL-100 exhibited an octahedral morphology with a uniform size of ~900 nm (Figure 2a), consistent with the XRD pattern [25]. It was observed in Figure 2b–d that the Mn-MIL-100 derived carbon-based material did not resemble the regular octahedral crystal form of the Mn-MIL-100 template; instead, the crystal edges were passivated and composed of irregular porous cubes [35]. With the increased pyrolysis temperature, the surface of the Mn-MIL-100-derived carbon-based materials became rougher, the agglomeration phenomenon was more obvious, the crystals showed varying degrees of shrinkage, and a large number of holes formed. This was attributable to the shrinkage of the crystal structure and the formation of mesopores caused by the loss of organic ligands at high temperatures [28]. When the pyrolysis temperature reached 500 °C, it was found that some crystals of the Mn500 carbon-based materials agglomerated into large pieces compared with those of the other carbon-based materials.
Morphological characterization of Mn-MOF@CP, Mn300@CP, Mn400@CP, and Mn500@CP was conducted via FE-SEM, and the results were shown in Figure S1. Similar to the electron micrographs of Mn300@CP and Mn-MOF@CP in Figure S1b, both had cubic crystals uniformly covering the carbon fiber surface of the CP substrate. In contrast, it can be seen in Figure S1c that irregular Mn400 cubic crystals are uniformly attached to most areas of the surface of the Mn400@CP-modified electrode. It may be that Mn400 was a mixture of manganese oxides, accompanied by the collapse of the metal-organic skeleton structure and crystal shrinkage, leading to irregular crystal formation. The crystals began to agglomerate slightly, accumulating in large parts of the blank carbon paper. As the pyrolysis temperature increased to 500 °C, a significant number of Mn500 crystals were found to be longitudinally aggregated into blocks, with only a small portion of the carbon fibers coated by the Mn500 surface. As a result, the effective active area of the Mn500-modified CP electrode was reduced due to the agglomeration of crystals (Figure S1d). The results showed that the Mn-MOF@CP, Mn300@CP, and Mn400@CP modified electrodes had a large active surface area, which was conducive to promoting the electrochemical activity of the electrodes. In particular, the oxygen-calcined carbon-based derivatives Mn300 and Mn400 had numerous surface pores that provided additional surface active sites, thereby promoting their electrochemical activity.

3.3. Analysis of Electrochemical Activity

CV tests shown in Figure 3a were conducted on different electrodes, including the CP, Mn-MOF@CP, Mn300@CP, Mn400@CP, and Mn500@CP electrodes, in a 0.25 M sodium chloride solution with 100 mg L−1 phenol. No distinct redox peaks were observed within the potential range from −0.4 V to 0.8 V for the blank CP electrode, the Mn-MOF@CP electrode, or the Mn300@CP-modified electrode, indicating that these three electrodes have no electrooxidative activity for phenol. In contrast, the oxidation peak of the Mn400@CP- and Mn500@CP-modified electrodes was located at 0.39 V, with no reduction peak. This may be attributed to the increase in the degree of carbonization of the organic ligand of the Mn-MIL-100 material with increasing pyrolysis temperature, leading to an increase in the conductivity of the modified electrode. Furthermore, the XRD results of the derived carbon-based material could also be attributed to manganese oxide as a transition metal oxide, which has good catalytic and electrochemical properties. The oxidation peak in the CV test was primarily because of the oxidation of phenol to benzoquinone [36]. Besides, the Mn400@CP exhibited a slightly higher oxidation peak current than the Mn500@CP electrode, along with a greater number of electrochemically active sites, demonstrating its superior electrochemical activity. It was probably due to the higher oxygen vacancy concentration in Mn400 compared to Mn500 indicated by the O 1s spectra in Figure S3 [37,38].
pH was closely related to redox potential, and the degree of acidity and alkalinity directly affected electron transfer in the redox process [39]. The acid-base dissociation constant (pKa) of phenol was 9.98, indicating that phenol existed as a weak acid in aqueous solutions with pH < 9.98 [40]. The oxidation-reduction process was more intense in an acidic environment and less pronounced in an alkaline environment [41]. Therefore, the electrochemical activity of the modified electrode was investigated within the pH range of 3 to 7. Figure 3b,c demonstrated the cyclic voltammetry curves of the Mn400@CP- and Mn500@CP-modified electrodes, respectively, measured over a range of pH values (3, 4, 5, 6, and 7). Even with the change in pH, the voltammetry curves of the two modified electrodes remained relatively stable. The results indicated that the modified electrode retained its electrochemical activity over a wide range of pH conditions and that the catalytic oxidation activity of phenol was relatively unaffected by pH changes. It was observed in Figure 4b that Mn400@CP presented a maximum redox current of 0.42 mA and a maximum active region area at a lower spike potential of 0.39 V at pH = 4. Moreover, Mn500@CP exhibited a maximum redox current of 0.35 mA at a peak potential of 0.42 V at pH = 4. Under the condition of the pH value of the corresponding Mn400@CP, the peak potentials of the values always Mn500@CP were lower, implying that the electric catalytic oxidation of phenol on Mn400@CP was more favorable than that on Mn500@CP. The Mn400@CP- and Mn500@CP-modified electrodes showed excellent electrocatalytic activity for phenol at pH = 4, which was consistent with the literature described by Wu et al. [36].
The Electrochemical impedance (EIS) technique held significant importance among electrochemical analysis techniques as it was considered one of the most interpretative methods for assessing electrochemical processes occurring at the electrode/electrolyte interface [42]. The Nyquist plots of these modified electrodes are depicted in Figure 3d. This figure was derived from the combination of a semicircle in the high-frequency region and a straight line in the low-frequency region, which was consistent with the characteristics of the impedance spectrum of the film electrode or powder microelectrode. The associated equivalent circuit consists of an ohmic resistor (Rohm) combined in parallel with a charge transfer resistor (Rct) and a diffusion resistor. The relationship between the electrocatalytic behavior and the charge transfer resistance could be expressed as the lower the Rct value was, the higher the catalytic degradation activity of the modified electrode [42]. The diameter of the semicircle in the curve represented the charge transfer resistance of the electrode. The χ2 values for CP, Mn300@CP, Mn400@CP, and Mn500@CP were 0.0017, 0.0023, 0.0015, and 0.0011, respectively, demonstrating a good fitting quality of the EIS data. It can be seen from Figure 4d that the modified electrode exhibited a smaller semicircle diameter than the blank CP electrode, indicating a significantly lower Rct fitting value of 46.95 Ω than the 65.1 Ω value of the blank CP electrode. Compared with that of the blank CP electrode, the Rct of the modified electrode was significantly lower, indicating that the electron transfer efficiency was improved after CP electrode modification. The Rct fit value of Mn400@CP was 39.65 Ω, which was lower than that of Mn300@CP; this may be attributed to the improved affinity of the manganese oxide electrode material for ion diffusion and electron transfer at the electrochemical interface [43]. The Rct fitting value of Mn500@CP was 58.26 Ω, which was similar to that of the blank CP electrode, likely due to only a slight amount of Mn500 crystals being present on the surface of the Mn500@CP electrode. The results show that the Mn400@CP-modified electrode was more conducive to the electrocatalytic oxidation degradation of phenol, which was consistent with the CV results.

3.4. Application of the Modified Eelectrodes in Phenol Degradation

To verify the performance of the Mn400@CP- and Mn500@CP-modified electrodes as electrochemical anodes for phenol oxidation at different pH values, a constant anode potential of 1.0 V was applied, at which the current density was 0.167 mA/cm2. As illustrated in Figure 4a, the phenol removal efficiency of the bare CP electrode was significantly lower than that of the modified electrodes, indicating its limited degradation capability at pH 4. The blank CP electrode exhibited a phenol removal efficiency of only 8.01 ± 2.00%, which further ascent after 16 h. Remarkably, while the bare CP electrode showed limited efficacy, the Mn400@CP electrode achieved near-total phenol degradation (96.00 ± 1.53%) after 12 h. Although the final degradation efficiency of phenol at the Mn500@CP modified electrode was only 60.12 ± 2.03%, it continued to degrade, which was still higher than the 8.01 ± 2.00% degradation efficiency of the blank CP electrode.
In addition, to examine the impact of the initial pH of the solution on phenol removal, five control groups with various pH levels (3, 4, 5, 6, and 7) were subjected to Mn400@CP and Mn500@CP. Mn400@CP had the lowest phenol degradation efficiency of 63.07 ± 3.05% at pH 3 but reached its maximum removal efficiency of 96.00 ± 1.53% at pH 4 (Figure 4b). As the pH increased to 5 and 6, the removal efficiency decreased slightly to 80.11 ± 2.37% and 75.23 ± 1.89%, respectively. However, the removal efficiency at pH = 7 increased slightly to 78.14 ± 2.03% compared with 75.23 ± 1.89% at pH = 6. The variation trend of the phenol removal efficiency of Mn500@CP was similar to that of Mn400@CP. At pH 3, 4, 5, 6, and 7, the degradation efficiency was 46.33 ± 3.12%, 60.12 ± 2.03%, 57.09 ± 2.52%, 52.27 ± 2.77%, and 54.10 ± 3.89%, respectively (Figure 4c). With increasing pH, the degradation efficiency of phenol initially increased, then decreased, and subsequently increased slightly, showing a trend similar to the previous finding [43]. The observed performance aligns with the CV analysis, confirming Mn400@CP’s exceptional electrocatalytic activity toward refractory organic pollutants. Furthermore, the involvement of active radicals was assessed using various quenching agents, including p-benzoquinone (p-BQ) for O2− and tert-butanol (TBA) for ·OH [44]. As illustrated in Figure 4d, the electrocatalytic degradation process was significantly suppressed by TBA, suggesting that ·OH played a crucial role. Besides, the recycled CP, Mn400@CP, and Mn500@CP were subjected to multi-cycle tests to verify the long-term service performance of the electrodes. As presented in Figure S4, Mn400@CP retained a high phenol degradation efficiency of over 90% after four cycles of reuse. As shown in Table S1, the maximum phenol removal efficiency of Mn400@CP reached 96.00 ± 1.53%, which was comparable to that of conventional electrodes, highlighting its excellent electrocatalytic degradation performance. In addition, Mn400@CP exhibited relatively low energy consumption (2.5 kWh g−1 phenol−1), significantly reducing operating costs compared to other electrodes with energy consumptions of 4.81, 2.74, 22.22, 24.94, and 31.92 kWh g−1 phenol−1.

3.5. Assessment of Stability

The time-dependent current responses of both Mn400@CP and Mn500@CP during phenol degradation were illustrated in Figure S2. Both curves exhibited a similar trend, with the polarization current decreasing as time progressed. Specific analysis found that the initial polarization current was approximately 1.65 mA for Mn400@CP and 0.8 mA for Mn500@CP. After 16 h, both the oxidation currents of the Mn400@CP and Mn500@CP electrodes equalized. As shown in Figure S2, a current retention of 95.00% for Mn400@CP was recorded after 16 h of continuous operation at 1.0 V, while only 86.20% was observed for Mn500@CP, demonstrating the superior stability of Mn400@CP. However, the higher reduction current for Mn400@CP than for M500@CP indicates its enhanced catalytic activity in phenol degradation.
To further assess the stability of the MOF-derived carbon-based material-modified electrodes after phenol degradation, FE-SEM analysis was conducted. This stability was a critical factor for practical applications. As shown in Figure S5a, irregular porous cubic Mn400 crystals were evenly distributed on the Mn400@CP electrode surface following the electrooxidation degradation. However, as presented in Figure S5e, the Mn500 carbon-based material still exhibited crystal agglomeration into large blocks, and part of the CP electrode was exposed. For the purpose of further determining the composition of the substances on the Mn400@CP and Mn500@CP surfaces, elemental mapping was performed using EDS analysis. To characterize the surface-bound species distribution, elemental mapping was performed using EDS analysis. For the Mn400@CP electrode shown in Figure S5b, Mn was evenly dispersed across the surface of the carbon fibers, aligning with the position of Mn400 (Figure S5a). In addition, as shown in Figure S5c,d, the Cl/Na element was observed across the electrode surface. As illustrated in Figure S5f–h, Mn was uniformly dispersed in the Mn500@CP sample, while Cl/Na species preferentially accumulated along carbon fiber interfaces. It suggested that the material attached to Mn500@CP consisted of sodium chloride crystals, likely precipitated from the electrolyte solution. The presence of Mn400 crystals on the CP electrode (Figure S5a) further confirmed the excellent stability of Mn400 composites. This stability was likely attributed to the carbonization of organic ligands during high-temperature pyrolysis, which increased the stability of the system. Besides, the concentration of Mn ions was below the detection limit after phenol degradation, indicating the exceptional stability of Mn400@CP for contaminant degradation. Given its superior electrochemical activity and stability, Mn400@CP emerged as a promising anode material for contaminant degradation.

3.6. Possible Degradation Mechanism Involved in Phenol Oxidation

To determine the degradation pathway of phenol, the Gibbs free energy of the transition state was calculated based on density functional theory, and the position of hydroxyl radical attack was investigated first. The results are shown in Figure S8. Due to the initial state being the same, comparing the Gibbs free energy of the transition state could be dependent on the dynamic selection of the product structure. When the Gibbs free energy was low, the reaction was more likely to occur [45]. Among the formed transition states, phloroglucinol had the lowest Gibbs free energy (−457.817614 kJ mol−1), indicating that the reaction tended to produce phloroglucinol.
Furthermore, the composition of the electrolyte solution changed during the electrooxidation of phenol, as indicated by the HPLC analysis shown in Figure S6. The intermediates generated during phenol degradation were further identified via high-performance liquid chromatography (HPLC)/mass spectrometry (MS), which revealed the presence of phloroglucinol, pyruvate, adipic acid, malonic acid, acrolein, 2,4-hexadienedial, pyrocatechol, and ω-aldehyde hexanoic acid, the mass spectra of which were shown in Figure S7. However, certain predicted intermediates involved in the degradation of phenol, such as acetone, propanal, and 2-hydroxypropanal, were not detected in this study. A possible pathway involved in phenol oxidation on the Mn400@CP anode was proposed and exhibited in Figure 5. First, hydroxyl radicals were generated on the surface of mixed manganese oxides, which attacked benzene rings and produced intermediates such as pyrocatechol and phloroglucinol. In degradation pathway I, phloroglucinol was then broken down to produce malonic acid and acetone. In degradation pathway II, the C-C bond between the two hydroxyl functional groups was broken along with the ring opening of the intermediate pyrocatechol, and the two hydroxyl groups at each end of the carbon chain were oxidized to two aldehyde groups. The subsequent step involved the breaking of the C-C bond between the two carbon-carbon double bonds, leading to the formation of 2-acrolei. After this, the carbon-carbon double bond was directly added to the water under acidic electrolyte conditions, resulting in the formation of a 2-hydroxypropanal. Finally, the hydroxyl group was oxidized to the keto group, and the aldehyde was oxidized to the carboxylic acid. In pathway III, hydroreduction of pyrocatechol was performed to form o-cyclohexanediol, and then the C-C bond between the two hydroxyl groups was broken to obtain hexanediol. The hydroxyl group was oxidized to an aldehyde group, which was then oxidized to a carboxyl group, in turn forming ω-aldehyde hexanoic acid and adipic acid.
Based on the aforementioned analysis, a potential mechanism for phenol oxidation on the Mn400@CP cathode was proposed, as illustrated in Figure 6. Briefly, phenol molecules were enriched at the interface between the electrode and electrolyte. According to the results of the XPS analysis, a large number of Mn2+, Mn3+, and Mn4+ existed in Mn400. In the act of electrical oxidation, Mn ions of different valence states could be converted to each other, and some of the electrons could be stored and released on the electrode surface during this process [46,47]. H2O decomposed to produce ·OH under the induction of electrons on the electrode surface, and the free radicals attacked the phenol to cause the ring opening of the phenol. Finally, the phenol was almost completely removed from the electrochemical system.

4. Conclusions

In this study, carbon-based materials derived from Mn-MIL-100 were synthesized through pyrolysis at various high temperatures, resulting in the successful fabrication of Mn300@CP, Mn400@CP, and Mn500@CP-modified electrodes. Among them, Mn400@CP exhibited lower resistance and a stronger cyclic voltammetry response to phenol, consistently demonstrating superior electrochemical activity in phenol oxidation. The exceptional electrocatalytic performance of Mn400@CP can be ascribed to its calcination at 400 °C, which retained the MOF’s ordered porous structure while creating a high density of active surface sites. Furthermore, the synergistic transformation of polyvalent Mn further boosted the material’s catalytic activity. Consequently, the promising electrocatalytic efficiency demonstrated by Mn400@CP establishes it as a leading anode material for wastewater treatment systems targeting persistent organic pollutants.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w17071103/s1. Table S1 Summary of the electrode degradation performance; Figure S1 SEM images of Mn-MIL-100@CP (a), Mn300@CP (b), Mn400@CP (c), Mn500@CP (d); Figure S2 I-t curves of the Mn400@CP, and 500@CP in 0.25M NaCl with a constant anode potential of 1.0 V; Figure S3 XPS spectra of O 1s of Mn400 and Mn500; Figure S4 Reusability experiment of CP, Mn400@CP and Mn500@CP; Figure S5 SEM images and corresponding EDS mapping images of Mn400@CP (a–d) and 500@CP (e–h) after electrooxidation of phenol; Figure S6 HPLC analysis of samples before and after phenol degradation; Figure S7 The spectra of important MS fragmentation patterns in phenol degradation; Figure S8 Gibbs free energy of the transition states. Refs. [48,49,50,51,52,53] have been included in the Supplementary Materials.

Author Contributions

Conceptualization, X.S., H.L., Y.Z. and D.C.; methodology, X.S., H.L. and Y.Z.; validation, X.S., H.L. and Y.Z.; formal analysis, Y.Z.; investigation, X.S., H.L., Y.Z. and D.C.; resources, X.S. and H.L.; data curation, X.S., H.L. and Y.Z.; writing—original draft preparation, D.C.; writing—review and editing, X.S., H.L. and D.C.; visualization, D.C., X.J. and J.S.; supervision, D.C., X.J. and J.S.; project administration, X.S., H.L. and D.C.; funding acquisition, D.C. and J.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financed by National Natural Science Foundation of China (No. 52470081 and 52100092), and the Open Fund of Key Laboratory of Environmental Remediation and Ecological Health, Ministry of Industry and Information Technology (JSEP2024003).

Data Availability Statement

The data supporting the findings of this study are included in the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) TGA of the Mn-MIL-100 materials; (b) XRD patterns of the prepared Mn-MIL-100 and Mn-MIL-100 at 300, 400 and 500 °C; (c) high-resolution XPS spectra of Mn-MIL-100, Mn300, Mn400 and Mn500; (d) high-resolution XPS spectra of Mn 2p of Mn-MIL-100, Mn300, Mn400 and Mn500.
Figure 1. (a) TGA of the Mn-MIL-100 materials; (b) XRD patterns of the prepared Mn-MIL-100 and Mn-MIL-100 at 300, 400 and 500 °C; (c) high-resolution XPS spectra of Mn-MIL-100, Mn300, Mn400 and Mn500; (d) high-resolution XPS spectra of Mn 2p of Mn-MIL-100, Mn300, Mn400 and Mn500.
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Figure 2. SEM images of Mn-MIL-100 (a), Mn300 (b), Mn400 (c), and Mn500 (d).
Figure 2. SEM images of Mn-MIL-100 (a), Mn300 (b), Mn400 (c), and Mn500 (d).
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Figure 3. Electrochemical characterization of Mn-MOF@CP derivatives: (a) Comparative CVs of CP, Mn-MOF@CP, Mn300@CP, Mn400@CP, and Mn500@CP in 0.25 M NaCl with 100 mg L−1 phenol; (b) pH-dependent CV responses of Mn400@CP (pH 3–7); (c) pH-dependent CV responses of Mn500@CP (pH 3–7); (d) Nyquist plots of the thermally modified electrodes. The inset showed the equivalent circuit.
Figure 3. Electrochemical characterization of Mn-MOF@CP derivatives: (a) Comparative CVs of CP, Mn-MOF@CP, Mn300@CP, Mn400@CP, and Mn500@CP in 0.25 M NaCl with 100 mg L−1 phenol; (b) pH-dependent CV responses of Mn400@CP (pH 3–7); (c) pH-dependent CV responses of Mn500@CP (pH 3–7); (d) Nyquist plots of the thermally modified electrodes. The inset showed the equivalent circuit.
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Figure 4. (a) Removal efficiencies for phenol of the Mn400@CP, Mn500@CP, and blank CP electrodes at pH = 4; removal efficiencies for phenol of Mn500@CP (b) and Mn400@CP (c) in solutions with various pH values; (d) effect of radical scavengers on phenol degradation (TBA: 100 mM, p-BQ: 0.2 mM).
Figure 4. (a) Removal efficiencies for phenol of the Mn400@CP, Mn500@CP, and blank CP electrodes at pH = 4; removal efficiencies for phenol of Mn500@CP (b) and Mn400@CP (c) in solutions with various pH values; (d) effect of radical scavengers on phenol degradation (TBA: 100 mM, p-BQ: 0.2 mM).
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Figure 5. Speculated pathways involved in the electrooxidation of phenol with Mn400@CP. The red arrows (→) indicate the path I, the blue arrows (→) indicate the path II, and the green arrows (→) indicate the path III.
Figure 5. Speculated pathways involved in the electrooxidation of phenol with Mn400@CP. The red arrows (→) indicate the path I, the blue arrows (→) indicate the path II, and the green arrows (→) indicate the path III.
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Figure 6. Proposed mechanism for the enhanced electrocatalytic degradation of phenol.
Figure 6. Proposed mechanism for the enhanced electrocatalytic degradation of phenol.
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Sun, X.; Liu, H.; Chen, D.; Zhang, Y.; Jiang, X.; Shen, J. Enhanced Electrocatalytic Degradation of Phenol by Mn-MIL-100-Derived Carbon Materials. Water 2025, 17, 1103. https://doi.org/10.3390/w17071103

AMA Style

Sun X, Liu H, Chen D, Zhang Y, Jiang X, Shen J. Enhanced Electrocatalytic Degradation of Phenol by Mn-MIL-100-Derived Carbon Materials. Water. 2025; 17(7):1103. https://doi.org/10.3390/w17071103

Chicago/Turabian Style

Sun, Xueping, Haitao Liu, Dan Chen, Ya Zhang, Xinbai Jiang, and Jinyou Shen. 2025. "Enhanced Electrocatalytic Degradation of Phenol by Mn-MIL-100-Derived Carbon Materials" Water 17, no. 7: 1103. https://doi.org/10.3390/w17071103

APA Style

Sun, X., Liu, H., Chen, D., Zhang, Y., Jiang, X., & Shen, J. (2025). Enhanced Electrocatalytic Degradation of Phenol by Mn-MIL-100-Derived Carbon Materials. Water, 17(7), 1103. https://doi.org/10.3390/w17071103

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