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Article

Novel Fenton-like Catalyst HKUST-1(Cu)/MoS2-3-C with Non-Equilibrium-State Surface for Selective Degradation of Phenolic Contaminants: Synergistic Effects of σ-Cu-Ligand and ≡Mo–OOSO3− Complex

by
Xiaoze Yin
1,2,†,
Huaqin Yin
1,2,†,
Renjie Wang
1,2,
Jinnan Wang
1,2,* and
Aimin Li
1,2
1
State Key Laboratory of Pollution Control and Resource Reuse, Nanjing 210023, China
2
School of the Environment, Nanjing University, Nanjing 210023, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Water 2024, 16(1), 121; https://doi.org/10.3390/w16010121
Submission received: 6 December 2023 / Revised: 19 December 2023 / Accepted: 21 December 2023 / Published: 28 December 2023
(This article belongs to the Topic Advanced Oxidation Processes for Wastewater Purification)
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Abstract

:
Novel Fenton-like catalyst HKUST-1(Cu)/MoS2-3-C with a non-equilibrium-state surface was constructed for selective degradation of phenolic contaminants. Electron-polarized distribution facilitated the formation of σ-Cu-ligand between electron-poor Cu centre and phenolic compounds, which not only enhanced radicals generation but also accelerated the Cu(I)/Cu(II) redox. Meanwhile, ≡Mo–OOSO3− complexes formed by the electron-rich Mo centre and peroxymonosulfate (PMS), could directly oxidize phenolic contaminants with the generation of SO4•−. The radical quenching experiments and EPR tests indicated that both SO4•− and •OH played a dominant role in the reaction. Additionally, O2 could be reduced to O2•− by OVs and subsequently converted into 1O2 over the Mo centre. DFT calculation, FT-IR, and in situ Raman spectra analysis results demonstrated that phenolic compounds and PMS were respectively adsorbed by electron-poor Cu centre and electron-rich Mo centre, favouring the electrons transfer from phenolic contaminants to Mo centre for PMS activation. With synergistic effects of σ-Cu-ligand and ≡Mo–OOSO3− complexes, HKUST-1(Cu)/MoS2-3-C achieved a high degradation rate of phenolic contaminants and utilization efficiency of PMS.

Graphical Abstract

1. Introduction

Cu-based catalysts attracted great interest in the selective degradation of phenolic contaminants due to the formation of σ-Cu-ligand between phenolic hydroxyl groups and Cu(II) [1]. Different from traditional Fenton-like reaction, σ-Cu-ligand could be ruptured by persulfate (PMS) with the generation of SO4•− and HO-adduct radicals, which were further oxidized to hydroxylation products by reduction of Cu(II) to Cu(I) [2]. However, the formation of σ-Cu-ligand in the system would be gradually decreased with the phenolic compounds degradation, which caused the reaction to follow the traditional Fenton-like reaction mechanism (Equations (1) and (2)). In such case, although PMS could act as the electron acceptor and electron donor with the redox of Cu(I)/Cu(II) [1], reduction of Cu(II) (E0Cu(II)/Cu(I) = 0.15 V) by HSO5− (E0HSO5−/HSO4− = 1.8 V) is thermodynamically unfavourable [3]. Thus, the reduction of Cu(II) to Cu(I) was very slow in the absence of reductive agents, which significantly restricted the Cu(I)/Cu(II) circulation [4]. Therefore, most Cu-based catalysts still faced the problems of the rate limitation step (Equation (2)) and low utilization of PMS.
Cu(I) + HSO5− → Cu(II) + SO4•− + OH−
Cu(II) + HSO5− → Cu(I) + SO5•− + H+
To resolve the above problems, Cu-based catalysts with dual-reaction centres were constructed for the enhancement of catalytic activity and selective conversion of oxidants to radicals [5]. By doping different electro-negative metals, the non-uniform distribution of electron density of lattice oxygen was formed, which induced the formation of an electron-rich centre and an electron-poor centre. Due to the galvanic-like cell effect, electrons of organics could be captured by the electron-poor centre and subsequently transferred to the electron-rich centre for oxidant activation [6]. However, owing to high electronegativity, Cu (1.9 eV) generally acted as an electron-rich centre in dual reaction systems, which prevented the formation of σ-Cu-ligand with phenolic compounds because organics were preferentially accumulated around the electron-poor centre [7,8]. Previous work demonstrated that new ligands with strong electron-donating ability could act on MOFs to increase the charge density on the adjacent original ligand sites. This inspires us to construct an electron-poor Cu centre by doping higher electronegative metal into MOFs, which might facilitate the formation of σ-Cu-ligand for enhancement of PMS utilization and selective degradation of phenolic contaminants.
On the other hand, interface interaction between oxidant and catalyst also played an important role in the PMS activation process. Generally, as PMS was bounded on the catalyst surface, electrons were immediately transferred from transition metal oxides to PMS accompanied by O–O bond broken and generation of ROS (e.g., SO4•−, •OH and O2•−). Thus, a strong adsorption force between the catalyst and PMS could promote electron migration and ROS generation. As a commonly used co-catalyst, molybdenum disulfide (MoS2) with a 2D layered structure could not only act as the electron bridge for electron transfer [9] but also form an intermediate metal–peroxy complex (≡Mo–OOSO3−) with PMS due to high adsorption energy. Notably, such ≡Mo–OOSO3− complex would decompose to other reactive species with the generation of radicals (SO4•−, •OH) [10,11]. More importantly, Mo (2.16 eV) with higher electronegativity introduced into Cu-MOFs might induce the formation of a non-equilibrium-state surface, which can simultaneously enhance the phenolic compounds degradation around electron-poor Cu centre and PMS activation around the electron-rich Mo centre.
Herein, novel Fenton-like catalyst HKUST-1(Cu)/MoS2-3-C with a non-equilibrium-state surface derived from Cu-MOFs was constructed for the highly efficient degradation of phenolic contaminants. MoS2 with strong electron-accepting ability introduced into Cu-MOFs could act on the MOFs to decrease charge density on the adjacent original ligand sites, which induced the formation of electron-poor Cu centre and electron-rich Mo centre. In the catalytic reaction system, the electron-poor Cu centre facilitated the formation of σ-Cu-ligand complexes with phenolic compounds, while the electron-rich Mo centre favoured the formation of ≡Mo–OOSO3− complex with PMS. Meanwhile, electrons of organics could be captured by the electron-poor Cu centre and subsequently transferred to the electron-rich Mo centre for PMS activation, avoiding the accumulation of Cu(II)/Mo(VI) and PMS invalid decomposition. Combining the advantages mentioned above, HKUST-1(Cu)/MoS2-3-C achieved a high degradation rate of phenolic contaminants and utilization efficiency of PMS.

2. Materials and Methods

2.1. Chemicals

The following agents were analytical grade, acquired from a commercial source:1,3,5-benzenetricarboxylic acid (H3BTC, 98%, Aladdin Biochemical Technology Co., Ltd., Shanghai, China), copper(II) nitrate trihydrate (Cu(NO3)2•3H2O, 99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), ethanol(≥99.5%, Sinopharm Chemical Reagent Co., Ltd.), dimethylformamide (DMF, ≥99.5%, Sinopharm Chemical Reagent Co., Ltd.), sodium molybdatedihydrate (Na2MoO4•2H2O, 99%, Aladdin Biochemical Technology Co., Ltd.), thiuronium (CH5N2S, ≥98.0%, Aladdin Biochemical Technology Co., Ltd.), potassium peroxymonosulfate (2KHSO5•KHSO4•K2SO4, 99%, Macklin Biochemical Technology Co., Ltd., Shanghai, China), Bisphenol A (BPA, >99%, Aladdin Biochemical Technology Co., Ltd.), Phenol (≥99.5%, Sinopharm Chemical Reagent Co., Ltd.), Nitrobenzene (≥99.5%, Aladdin Biochemical Technology Co., Ltd.), Benzoic acid (≥99.5%, Aladdin Biochemical Technology Co., Ltd.), Aniline (≥99.5%, Sinopharm Chemical Reagent Co., Ltd.), L-Histidine (C6H9N3O2, ≥99%, Aladdin Biochemical Technology Co., Ltd.), tertbutylalcohol (TBA, ≥99.5%, Sinopharm Chemical Reagent Co., Ltd.), methanol (≥99.5%, Sinopharm Chemical Reagent Co., Ltd.). 5,5-Dimethyl-1-pyrroline N-oxide (DMPO, 98%) was obtained from Sigma-Aldrich Co., Ltd., Shanghai, China. 2,2,6,6-Tetramethylpiperidine (TEMP, 98%) was purchased from Tokyo Chemical Industry Co., Ltd., Tokyo, Japan. Ultrapure water was used for all the experiments.

2.2. Preparation of Catalysts

The synthesis process of HKUST-1(Cu)/MoS2-x-C is shown in Scheme 1. In addition, MoS2 and MoS2-C are also prepared for comparative studies (Text S1).
Synthesis of HKUST-1: In flask A, 5.0 g of Cu(NO3)2•3H2O was dissolved in 60 mL of ultrapure water. In flask B, 1.36 g of H3BTC was dissolved in 60 mL of ethanol. After mixing with 4 mL of DMF, Cu(NO3)2 solution was subsequently added to an ethanol solution of H3BTC under continuous stirring for 2 h. Then the mixture was transferred to a stainless steel lined Teflon autoclave reactor. After being heated at 80 °C for 20 h, the obtained blue crystal HKUST-1(Cu) was washed with ultrapure water and ethanol successively and dried in a vacuum oven at 60 °C for 12 h before use.
Synthesis of HKUST-1(Cu)/MoS2-x-C: In flask C, a certain amount of as-synthesized HKUST-1(Cu) was dispersed in 40 mL of DMF solution. In flask D, a certain amount of Na2MoO4•2H2O and CH5N2S (the molar ratio of Na2MoO4•2H2O to CH5N2S was 1:2) were dissolved in 40 mL of ultrapure water with continuously stirring for 30 min. Then the DMF solution of HKUST-1(Cu) was slowly added into the solution of Na2MoO4•2H2O and CH5N2S with continuous stirring. After ultrasonic treatment for 2 h, the mixture was transferred to a stainless steel lined Teflon autoclave reactor and heated at 200 °C for 24 h. The black precipitate was collected by centrifugation and washed with ultrapure water and ethanol successively. After being dried in a vacuum oven at 60 °C for 12 h, the product was heated in a tube furnace at 800 °C for 2 h under an argon atmosphere. Finally, the black powder of HKUST-1(Cu)/MoS2-x-C (x = 1, 2, 3 represented the molar ratio of Mo to Cu; C denoted calcination) was obtained.

2.3. Characterization

Surface morphology and lattice fringe of HKUST-1(Cu)/MoS2-3-C were characterized by scanning electron microscopy (SEM, ZEISS Gemini 300, Tokyo, Japan) and transmission electron microscopy (TEM, FEI Talos F200X G2, Tokyo, Japan), respectively. To observe the crystal structure of the catalysts, X-ray diffraction (XRD) spectra were collected using an XRD-6000 X-ray diffractometer (Shimadzu, Kyoto, Japan) with a Cu K radiation (1.5406 Å) over the 2θ range of 10–90°. To identify the valence state of an element, X-ray photoelectron spectroscopy (XPS) of samples was analyzed by a PHI 5000 Versa Probe Instrument (Chigasaki, Japan) using monochromatic Al Kα radiation (225 W, 15 mA, 15 kV). During the deconvolution process of XPS curves, all binding energies were referenced to the C 1s peak at 284.8 eV.
FT-IR spectra (Thermo Scientific Nicolet iS20, Waltham, MA, USA) and ATR-FT-IR spectra were applied to analyze the formation of σ-Cu-ligand and ≡Mo–OOSO3− complex, respectively. Furthermore, in situ Raman spectra were conducted on Horiba LabRAM HR Evolution (Gloucestershire, London, UK) with a 532 nm excitation laser, which could demonstrate the interaction between PMS molecules and Mo. In addition, to evaluate the electron transfer ability of catalysts, electrochemical measurements were conducted by the electrochemical workstation (CHI760E, Shanghai, China) with a three-electrode system (80 mL 0.5 M Na2SO4 as the electrolyte). In addition, the reactive species produced in the catalytic oxidation system were captured by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and 2,2,6,6-tetramethyl-4-piperidone (TEMP). The formed adducts were analyzed using an EMX-10/12 Electron paramagnetic resonance spectrometer (Karlsruhe, Germany).

2.4. Catalytic Degradation Experiments

The catalytic performance of samples was evaluated by the degradation of phenolic compounds (BPA, phenol) and other aromatic compounds (Nitrobenzene, Benzoic acid, and Aniline). Typically, 50 mL aqueous solutions of a certain concentration of phenolic compound and catalyst were mixed in 100 mL beaker flasks. Before 1 mM PMS was added in, the suspensions were magnetically stirred for 30 min to reach absorption-desorption equilibrium between the catalyst and organics. Then, 1 mL aliquots were sampled at given time intervals and filtered through a Mixed Cellulose Ester (MCE) filter (0.22 μm) before analysis. Notably, to quench the residual oxidant, 50 μL MeOH was immediately added to the sample for termination reaction. The residual concentration of BPA in solution was measured by a 1200 series HPLC equipped with a UV–visible detector and C-18 column (Eclipse XBD-C18, Agilent Technologies, Santa Clara, CA, USA, 4.5 × 150 mm, 5 μm). The degradation intermediates of BPA were analyzed by HPLC-MS (Ultimate 3000 UHPLC-Q Exactive, Thermo Fisher, Waltham, MA, USA) with a scan range of m/z 50–750 in a negative mode. The capillary voltage and operating temperature were 2500 V and 300 °C, respectively. The leached metal ions in suspensions were measured by Inductively coupled plasma optical emission spectrometer (ICP-OES, Thermo Fisher iCAP PRO, Waltham, MA, USA). In addition, considering that inorganic anions and natural organic matter (NOM) widely existed both in natural water and wastewater, influences of anions (Cl−, SO42−, NO3−and CO32−) and humic acid (HA) on catalytic degradation of BPA were also conducted.

3. Results and Discussion

3.1. Morphology and Structure of Catalysts

HKUST-1(Cu)/MoS2-3-C shows a loosely amorphous structure with MoS2 nanosheets embedded in HKUST-1(Cu) (Figure 1a). This structure makes the catalyst have an appropriate BET surface area (208.9509 m2/g, Figure S5). The resolved lattice fringes of 0.21 nm and 0.25 nm are respectively ascribed to Cu (111) and Cu2O (111) (Figure 1b), while the lattice fringe of 0.62 nm and 0.27 nm belong to the (002) and (100) plane of MoS2. In this heterostructure, bonding among the interfaces of different components can enhance the rate of electron transfer [12]. EDS images indicate that Cu, Mo, O, S, and C are uniformly dispersed on the catalyst surface, which might favour the contact of active sites and reactants (Figure 1c). In addition, XRD spectra of HKUST-1(Cu)/MoS2-3-C and other as-synthesized materials are shown in Figure 1d. For HKUST-1(Cu)-C, characteristic peaks at 38° and 45° respectively represent Cu2O (PDF#78-2076) crystalline phases of (111) and (200), while the diffraction peaks at 46° and 79° are respectively ascribed to Cu (PDF#85-1326) crystalline phases of (111) and (220). Thus, both Cu2O and metallic Cu phases were formed after carbonizing the Cu-MOF [13,14]. As for MoS2-C, peaks at 14, 33, 39, and 58° are ascribed to MoS2 (PDF#73-1508) phases of (002), (100), (103), and (110), respectively. Since both characteristic peaks of Cu2O, Cu, and MoS2 can be observed in the spectra of HKUST-1(Cu)/MoS2-3-C, it can be concluded that MoS2 is doped on HKUST-1(Cu) successfully [13,14].
XPS spectra of HKUST-1(Cu)/MoS2-3-C (Figure 2a) confirm the existence of C, O, Cu, Mo, and S, which agrees with the EDS analysis. In the spectra of C 1s (Figure 2b), the binding energy of 284.8, 285.6, and 286.4 eV are respectively ascribed to C–C, C–O, and C–O–C species [14], which indicates that organic ligands of MOF (1,3,5-benzenetricarboxylate) are successfully transformed into a carbon-based skeleton. As for the spectra of Cu 2p (Figure 2c), two main peaks of Cu 2p3/2 (932.5 eV) and Cu 2p1/2 (952.5 eV) are observed, together with their satellite peaks (943 eV and 963 eV). Due to the similar binding energy of Cu and Cu+ (Cu2O), these two characteristic peaks overlap. In order to identify the Cu element more accurately, a high-resolution Auger Cu LMM spectrum was recorded. In the spectrum of Cu LMM (Figure 2d), characteristic peaks located at 569.8 eV and 568.1 eV are respectively assigned to Cu(I) (Cu2O) and Cu [15], while the peak at 572.7 eV corresponds to Cu(II) (CuO) [14,16].
The proportion of Cu species is calculated by integration of the peak areas, which follows the order of Cu(II) (57.31%) > Cu(I) (35.09%) > Cu (7.60%). Herein, Cu(II) plays the dominant role in the formation of σ-Cu(II)-ligand, while Cu(I) can directly reduce oxidants with the generation of radicals [17]. Notably, binding energies of Cu 2p3/2 (932.9 eV) and Cu 2p1/2 (952.8 eV) in HKUST-1(Cu)/MoS2-3-C are higher than those in HKUST-1(Cu)-C (932.6 eV and 952.4 eV) (Figure S2), suggesting the strong interaction between Mo and Cu in HKUST-1(Cu)/MoS2-3-C [13,14]. It can be explained as follows: owing to higher electronegativity, Mo (2.16 eV) could capture the electrons from the Cu ligand, which induces the formation of a non-equilibrium-state surface with electron-poor Cu centre and electron-rich Mo centre in HKUST-1(Cu)/MoS2-3-C. In addition, the peak at 236.1 eV corresponded to Mo 3d3/2 of Mo6+ might be ascribed to the partial oxidation of Mo4+ on the MoS2 surface [18]. Furthermore, a slight shift to the higher binding energy of S 2p from 161.6 eV to 161.8 eV for S 2p3/2 and from 162.8 eV to 162.9 eV for S 2p1/2 (Figure 2e), suggesting the formation of S-C bonding in HKUST-1(Cu)/MoS2-3-C [14,19]. Such S-C bonding not only strengthened the structural stability of catalysts but also accelerated the electron transfer between MoS2 and HKUST-1(Cu)-C [20,21].

3.2. Catalytic Performance

Within 30 min, BPA degradation rates follow the order of HKUST-1(Cu)/MoS2-3-C/PMS (100%) > HKUST-1(Cu)-C/PMS (59.7%) > MoS2-C/PMS(57.6%) > PMS(5%) (Figure 3a), which suggests the highest catalytic activity of HKUST-1(Cu)/MoS2-3-C. The pseudo-first-order equation is used to fit the BPA degradation curves (Figure 3c), with the rate constant (k) following the order of HKUST-1(Cu)/MoS2-3-C (0.1404 min−1) > HKUST-1(Cu)-C (0.0247 min−1) > MoS2-C (0.0184 min−1). Additionally, the BPA degradation experiment over different catalysts (HKUST-1(Cu)/MoS2-x-C) indicates that the optimal ratio (x) of Mo:Cu is 3 (Figure 3b). Such extraordinary performance might be explained as follows: high adsorption energies (Eads) between Mo and PMS molecules [22] facilitated the formation of ≡Mo–OOSO3− complex, which favoured the BPA degradation and radicals generation [19,23,24]. On the other hand, Cu(II) induced the formation of σ-Cu-ligand with BPA, which promoted the BPA degradation and redox of Cu(II)/Cu(I) [1,25,26].
To evaluate the utilization of PMS (η) in different reaction systems, the ABTS method (detailed information provided in Text S4) is used to determine PMS concentration. The PMS utilization can be evaluated by using the stoichiometric efficiency method [27,28], which indicates that the introduction of MoS2 into HKUST-1(Cu)-C obviously enhances PMS utilization. The corresponding PMS stoichiometry efficiency over different catalysts followed the order of HKUST-1(Cu)/MoS2-3-C (0.14) > MoS2-C (0.11) > HKUST-1(Cu)-C (0.09) (Table S1). Furthermore, catalytic degradations of various aromatic organics (Nitrobenzene, Benzoic acid, Phenol, and Aniline) over HKUST-1(Cu)/MoS2-3-C demonstrate that functional groups show great influence on organics degradation. Being attributed to σ-Cu-ligand formed between HKUST-1(Cu)/MoS2-3-C and phenolic compounds, the TOC removal rate of BPA and phenol are much higher than those aromatic organics without phenolic hydroxyl groups (Nitrobenzene, Benzoic acid, and Aniline) (Figure 3f). Thus, HKUST-1(Cu)/MoS2-3-C can selectively degrade phenolic contaminants.
The influence of inorganic anions (Cl−, SO42−, NO3−and CO32−) and humic acid (HA) on BPA degradation are conducted to evaluate the anti-inference ability. Previous work reported that inorganic anions could act as radicals quencher with the second-order kinetic rate constants following the order of k(•OH/Cl−: 3~4.3 × 109 M−1s−1) > k(•OH/CO32−: 3.2~4.2 × 108 M−1s−1) > k(SO4•−/Cl−: 2.3~660 × 106 M−1s−1) > k(SO4•−/CO32−: 1~6.1 × 106 M−1s−1) > k(SO4•−/NO3−: 5~210 × 104 M−1s−1) [19,29,30]. Herein, BPA removal is not decreased in the presence of three inorganic anions (Cl−, SO42−, NO3−) (Figure 3g), which might be explained by that σ-Cu-ligand can directly contribute to BPA degradation even without radicals. However, an obvious inhibition is observed as CO32− is added in the reaction system, which can be explained by that σ-Cu-ligand can be hardly formed between ionized BPA and Cu centre in alkaline CO32− solution (pH = 10.92). In addition, although HA shows a slight adverse effect on BPA degradation due to competition for radicals (Figure 3h) [31,32], the supramolecular structure of HA prevents the formation of σ-Cu-ligand with catalyst. Thus HKUST-1(Cu)-C/MoS2-3-C exhibits strong anti-inference ability for co-existence of NOM. Addionally, the RhB degradation experiment also be implemented to verify the suitability of the catalyst for the degradation of dye (Figure S6), which proves the catalyst has the potential to be applied to various industrial wastewater treatment.
The reusability and stability of the catalyst are important factors related to the practical application. As expected, HKUST-1(Cu)-C/MoS2-3-C exhibits satisfied stability. Even after five cycles, HKUST-1(Cu)-C/MoS2-3-C could still maintain more than 80% degradation rate of BPA within 60 min (Figure 4a). The concentrations of Mo and Cu leached after the reaction are detected, which are much lower than the limitation of the standard proposed by EPA (Table S6). Additionally, the morphology and structure of HKUST-1(Cu)/MoS2-3-C are not destroyed after successive experiments (Figure 4b), exhibiting superior stability. In addition, an obvious increase proportion of Cu(I) from 35.09% to 55.53% and Mo(IV) from 93.22% to 95.40% are observed after the reaction (Figure 4c–f), which suggests that PMS activation over HKUST-1(Cu)/MoS2-3-C did not follow the electron transfer behavior of the classic Fenton reaction involving transition metal oxidized by oxidants. Thus, the catalytic process is dominated by the synergistic effect of non-equilibrium-state surface, σ-Cu-ligand, and ≡Mo–OOSO3− that directly capture electrons from phenolic compounds for PMS activation.

3.3. Catalytic Mechanism

The degradation intermediates of BPA over HKUST-1(Cu)/MoS2-3-C are identified by ESI-MS analysis (Tables S2 and S3, Figures S7–S9), and the possible degradation pathways are proposed (Figure 5). On one hand, BPA was transformed into monohydroxylated BPA (m/z = 243.9) and 2,2-diphenylpropane (m/z = 195.1). Followed by the attack on alkyl carbon, these intermediates were decomposed to 4-hydroxyacetophenone (m/z = 135.0) and acetophenone (m/z = 117.9) respectively. On the other hand, the aromatic ring which was at the para-position of the phenolic hydroxyl group was initially hydroxylated to form 4-isopropylphenol (m/z = 135.1), hydroquinone (m/z = 109.0), and phenol (m/z = 93.0). With the proceeding of the ring-rupturing reaction, the above phenolic compounds were decomposed to maleic acid (m/z = 116.9), glycerol (m/z = 92.0), glycolic acid (m/z = 77.0), and carbonic acid (m/z = 62.0), and finally mineralized into CO2 and H2O.
Aromatic organics preferred to be adsorbed and degraded on the surface of copper-based catalysts [1,6,26] due to the formation of complexes between Cu and phenolic hydroxyl groups [33,34]. In spectra of HKUST-1(Cu)/MoS2-3-C (Figure 6a), the peak of the phenolic hydroxyl group shifts from 3331.7 cm−1 to 3450.1 cm−1 after BPA adsorption, which can be explained by that coordination sphere formed between phenolic hydroxyl groups of BPA and Cu(II) [35] causes the deprotonation of the phenolic hydroxyl group and the difference in surroundings. As PMS is added in, the characteristic peak of phenolic hydroxyl groups further shifts from 3450.1 cm−1 to 3451 cm−1, suggesting the formation of new complexes between Cu(II) and degradation intermediates of BPA [1]. Meanwhile, vibration bands of aromatic rings are disappeared due to the destruction of aromatic rings. After the reaction for 60 min, the spectra of HKUST-1(Cu)/MoS2-3-C were the same as a fresh catalyst, demonstrating that BPA and its intermediates were mineralized completely.
In situ Raman spectra (Figure 6b) can demonstrate the adsorption affinity between PMS and catalytic sites. The band centred at 980 cm−1 and 1060 cm−1 are respectively ascribed to SO42− and HSO5− [3,36,37], thus changes in the ratio of I1060/I980 can represent PMS consumption rate which is related to the adsorption affinity for PMS on catalyst [22]. I1060/I980 of HKUST-1(Cu)/MoS2-3-C+PMS (0.32) is much lower than that of HKUST-1(Cu)-C+PMS (0.78), suggesting a higher utilization rate of PMS over HKUST-1(Cu)/MoS2-3-C. In the composite of HKUST-1(Cu)/MoS2-3-C, MoS2 with superior adsorption affinity of PMS facilitated the HSO5− conversion to SO42−, which can be further proved and discussed by DFT calculation. In addition, the ATR-FTIR test indicates that bands of S–O stretching vibration of HSO5− and SO42− shift from 1101.86 cm−1 to 1107.90 cm−1 over HKUST-1(Cu)/MoS2-3-C (Figure 6c), suggesting the formation of metastable ≡Mo–OOSO3− complex [38]. In addition, the characteristic peak at 3163.04 cm−1 of HKUST-1(Cu)/MoS2-3-C in solution was ascribed to the surface hydroxyl groups [3]. As PMS is added to the reaction system, the characteristic peak of hydroxyl groups on the catalyst surface shifts from 3163.04 to 3187.84 cm−1, which suggests the electron transfer from hydroxyl groups to Mo [39]. Such phenomenon could be explained as follows: partial substitution of −OH by HSO5− on Mo strengthened the electron-withdrawing ability of Mo, which also confirms the formation of ≡Mo–OOSO3− complex [3,10]. Additionally, the addition of MoS2 into the catalyst enhances the conductivity for charge transfer during the reaction, which is shown in electrochemical impedance spectroscopy (EIS) (Figure 6d). The Nyquist plots semicircle radius of HKUST-1(Cu)/MoS2-3-C was much smaller than that of HKUST-1(Cu)-C, demonstrating that MoS2 effectively decreased electrical impedance.
Various radial scavengers are separately added in the reaction system to identify the major ROS (Figure 7a). As reported, methanol was commonly used to quench SO4•− (k = 2.5 × 107 M−1 s−1) and •OH (k = 9.7 × 108 M−1 s−1) [19]. TBA, L-histidine (L-his), and p-BQ were used to quench •OH (k = 3.8 ~ 7.6 × 108 M−1•s−1) [24], 1O2 and O2•− [40], respectively. Considering that SO4•− and •OH were generally major ROS in the system of PMS activation [41], sufficient dosages of MeOH and TBA were added to the reaction system. Consequently, the BPA degradations are seriously inhibited in the presence of either p-BQ or L-his, which can be inferred that both 1O2 and O2•− played the dominant role in the PMS activation process (Equations (3) and (4)) [42,43]. In addition, obvious decreases in BPA removal are observed in the presence of MeOH and TBA, suggesting the main ROS of SO4•− during the BPA degradation process. To directly identify the ROS, EPR tests are conducted using DMPO and TEMP as spin-trapping agents (Figure 7b–d). DMPO-•OH and DMPO-SO4•− signals observed in the system of (HKUST-1(Cu)/MoS2-3-C+PMS+DMPO) indicate the generation of SO4•− and •OH. The signal intensity of DMPO-•OH is strengthened after the addition of BPA, suggesting that oxidation of σ-Cu-ligand contributed to radical generation. Additionally, the detected 1:1:1 triplet signal confirms the presence of 1O2, which is consistent with the quenching experiment results (Figure 7a). Furthermore, the triplet signal is significantly strengthened as BPA is added, indicating that electrons transfer from BPA to PMS accelerates the 1O2 production (Equations (9) and (10)) [44,45,46,47].
Notably, although the addition of a high concentration of MeOH can completely block the O2•− production from the chain reactions of SO4•− and •OH (Equations (5)–(7)) [48], the removal rate of BPA can be still maintained at 21.15% (Figure 7a). In the present work, asymmetric signals at g = 2.003 ascribed to OVs unpaired electrons are trapped in Solid EPR (Figure 7e) [49,50], which suggested that O2 might be directly reduced to O2•− by OVs over HKUST-1(Cu)/MoS2-3-C (Equation (8)) [51,52]. By analyzing O 1s in XPS spectra before and after the reaction, three characteristic peaks at 530.7, 531.8, and 533.2 eV are identified, which are respectively assigned to lattice oxygen of metal oxides (OLatt), absorbed oxygen (OAds) on OVs and surface absorbed water (OSurf) (Figure 7f) [53,54]. The proportion of OAds decreased from 48.08% to 34.80% after the reaction, suggesting the consumption of OVs on HKUST-1(Cu)/MoS2-3-C during the process of O2 reduction to O2•−. Meanwhile, BPA degradation kinetic constant under O2 purging is higher than that under N2 purging (Figure S4), which further proves that O2 reduction to O2•− over OVs contributes to BPA degradation [42,49,55]
O2•− + ≡Mo(VI) → + 1O2 + ≡Mo(IV)
2O2•− + 2H2O → 1O2 + H2O2 + 2OH−
SO4•− + H2O → •OH + H+ + SO42−
HSO5− + H2O → H2O2 + HSO4−
H2O2 + •OH → H2O + O2•− + H+
O2 + OVs ⟶ O2•−
HSO5− → SO5•− + H+ + e−
2SO5•− → 1O2 + 2SO42−
Adsorption energy is calculated using the DFT analysis, which can demonstrate the interface interaction (Figure 8a–e). The adsorption energy of PMS on HKUST-1(Cu)/MoS2-3-C (Eads = −2.728 eV) is higher than that on HKUST-1(Cu)-C (Eads = −2.614 eV) (Figure 8b,d), suggesting that PMS prefers to be adsorbed on MoS2. Additionally, the O-O bond of PMS adsorbed on HKUST-1(Cu)/MoS2-3-C is lengthened from 1.351 Å to 1.394 Å (Figure 8e), which induces the O-O bond is broken for the generation of ROS [22]. Meanwhile, due to the formation of a non-equilibrium-state surface, BPA prefers to be adsorbed on electron electron-poor Cu centre (Eads = −2.194 eV) rather than electron-rich Mo centre (Eads = −0.869 eV) to compose σ-Cu-ligand (Figure 8a,c). Thus, both PMS and BPA could be accumulated on HKUST-1(Cu)/MoS2-3-C, which not only facilitated the PMS conversion to ROS but also promoted the BPA degradation. Furthermore, Bader charges can directly display the charge density between the interface of PMS, BPA, and catalytic sites during the reaction. Charge density between BPA and Cu(II) (−0.451|e|) is higher than that between BPA and MoS2 (−0.259|e|), which indicates that the formation of σ-Cu-ligand strengthens the adsorption affinity of BPA. Furthermore, charge density between PMS and MoS2 (−0.787|e|) is higher than that between PMS and HKUST-1(Cu)-C (−0.622|e|), also proves that PMS prefers to be adsorbed on the electron-rich Mo centre for activation.
Based on the above results, a possible catalytic mechanism is proposed (Figure 9). In the composite of HKUST-1(Cu)/MoS2-3-C, a new ligand of MoS2 with strong electron-accepting ability decreased the charge density of Cu, inducing the formation of a non-equilibrium-state surface with electron-poor Cu centre and electron-rich Mo centre in MOFs. During the initial reaction process, the electron-poor Cu centre facilitated the formation of σ-Cu-ligand with phenolic compounds (Equation (6)) [1]. Such σ-Cu-ligand would be ruptured under the attack of PMS, accompanied by the generation of •OH/SO4•− and Cu(II) reduction to Cu(I) (Equation (7)). Notably, the non-equilibrium-state surface was broken by the formation of σ-Cu-ligand, which not only promoted the electrons (captured by electron-poor Cu centre) transfer to electron-rich Mo centre but also facilitated the formation of ≡Mo–OOSO3− complex (Equation (8)). Herein, ≡Mo–OOSO3− complex could oxidize organics with the generation of SO4•− (Equation (11)) [10,56,57]. Additionally, O2 could be reduced to O2•− by OVs over HKUST-1(Cu)/MoS2-3-C (Equation (10)) [49], also contributing to 1O2 generation (Equation (11)) [43]. Thus, ROS were generated via four-electron transfer routes during the degradation process of phenolic compounds: (1) the first electron transfer route was from σ-Cu-ligand to PMS, with the generation of SO4•−/•OH and Cu(II) reduction to Cu(I); (2) the second transfer route was from electron-rich Mo centre to PMS with the generation of SO4•−/•OH; (3) the third transfer route was from organics to ≡Mo–OOSO3− complexes, accompanying with organics oxidation and SO4•− generation; and (4) the fourth transfer route was from OVs to O2 with the generation of O2•−. Finally, BPA was degraded and mineralized by ROS (Equation (12)).
Water 16 00121 i001
Water 16 00121 i002
≡Mo(IV) + HSO5− → ≡Mo–OOSO3− + H+
≡Mo–OOSO3− + H+ → ≡Mo (IV) + SO4•− + OH−
O2 + e− (OVs) → O2•−
≡Mo(VI) + O2•− → ≡Mo(IV) + 1O2
•OH/SO4•−/1O2/O2•− + BPA → CO2 + H2O

4. Conclusions

In the present work, the PMS activation over novel catalyst HKUST-1(Cu)/MoS2-3-C was investigated for selective degradation of phenolic compounds. A new ligand of MoS2 with a strong electron-accepting ability decreased the charge density of Cu, resulting in the formation of non-equilibrium-state surface potential. σ-Cu-ligand formed between the electron-poor Cu centre and phenolic compounds not only contributed to radicals generation but also accelerated the redox of Cu(I)/Cu(II). Meanwhile, PMS preferred to be adsorbed on electron-rich Mo centre with the formation of ≡Mo–OOSO3− complex, which also enhanced phenolic compounds degradation and radicals production. In addition, the phenolic compounds and PMS were respectively adsorbed onto the electron-poor Cu centre and electron-rich Mo centre, promoting the electron transfer from phenolic compounds to PMS for PMS activation. Being attributed to the synergistic effect of σ-Cu-ligand and ≡Mo–OOSO3− complex, HKUST-1(Cu)/MoS2-3-C achieved the high degradation rate of phenolic contaminants and utilization efficiency of PMS.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w16010121/s1, Figure S1: TEM imagine of HKUST/MoS2-3-C; Figure S2: High-resolution XPS spectra of Cu 2p for HKUST-1(Cu)-C; Figure S3: Degradation efficiency of BPA under different quenching conditions (p-BQ concentration = 5, 10, 15, 20 mM); Figure S4: Degradation efficiency and kinetics of BPA under nitrogen/oxygen purging; Figure S5: N2 adsorption/desorption isotherms and pore-size distribution of HKUST-1(Cu)-C; Figure S6: Degradation of RhB in HKUST-1(Cu)/MoS2-3-C system. Reaction conditions: [RhB] = 20 mg/L, [PMS] = 1 mM, [Catalysts] = 0.3 g/L; Figure S7: ESI-MS spectra of BPA (sampling time: 10 min); Figure S8: ESI-MS spectra of BPA (sampling time: 30 min); Figure S9: ESI-MS spectra of BPA (sampling time: 60 min); Table S1: The effectiveness of PMS utilization for BPA (20 mg/L) degradation during the Fenton-like reaction; Table S2: The chemical formulas and fragments (m/z) of intermediate products during BPA; Table S3: The molecular structure and fragments (m/z) of intermediate products during BPA degradation; Table S4: Operation parameters of ESI-MS; Table S5: HPLC operating conditions for the detection of BPA; Table S6: The leached concentration of Mo and Cu ions after BPA degradation; Text S1: Synthesis of comparative catalysts; Text S2: Preparation of working electrode for EIS; Text S3: Raman analysis procedures for in situ characterization of the catalyst surface during catalytic PMS decomposition; Text S4: ABTS method for measuring PMS; Text S5: EPR analysis procedures for detecting radical signals.

Author Contributions

Conceptualization, X.Y. and J.W.; methodology, H.Y.; software, H.Y. and R.W.; validation, X.Y.; formal analysis, R.W.; investigation, X.Y. and H.Y.; data curation, H.Y.; writing—original draft preparation, X.Y. and H.Y.; writing—review and editing, J.W.; visualization, A.L.; supervision, J.W.; project administration, J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 52070095), the Natural Science Foundation of Jiangsu Province (BK 20231407), and the social development project of Jiangsu Province (BE 2022771).

Data Availability Statement

The data are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Scheme 1. Preparation process of HKUST-1(Cu)/MoS2-x-C.
Scheme 1. Preparation process of HKUST-1(Cu)/MoS2-x-C.
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Figure 1. (a) TEM imagine of HKUST-1(Cu)/MoS2-3-C; (b) HRTEM imagine of HKUST-1(Cu)/MoS2-3-C; (c) SEM imagine of HKUST-1(Cu)/MoS2-3-C and corresponding elemental mapping of Cu, O, Mo, S and C; (d) XRD spectra of several catalysts.
Figure 1. (a) TEM imagine of HKUST-1(Cu)/MoS2-3-C; (b) HRTEM imagine of HKUST-1(Cu)/MoS2-3-C; (c) SEM imagine of HKUST-1(Cu)/MoS2-3-C and corresponding elemental mapping of Cu, O, Mo, S and C; (d) XRD spectra of several catalysts.
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Figure 2. XPS spectra of (a) survey; (b) C 1s; (c) Cu 2p; (d) Cu LMM; (e) Mo 3d; (f) S 2p of HKUST-1(Cu)/MoS2-3-C.
Figure 2. XPS spectra of (a) survey; (b) C 1s; (c) Cu 2p; (d) Cu LMM; (e) Mo 3d; (f) S 2p of HKUST-1(Cu)/MoS2-3-C.
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Figure 3. (a) Degradation of BPA in different catalysts systems; (b) Degradation of BPA in different HKUST-1(Cu)/MoS2-x-C systems; (c) The kinetic degradation curves in different catalysts systems; (d) Pseudo-first-order rate constants k in different reaction systems; (e) Degradation of 20 mg/L BPA, Nitrobenzene, Benzoic Acid, Phenol and Aniline in HKUST-1(Cu)/MoS2-3-C system; (f) TOC removal rate of 20 mg/L BPA, Nitrobenzene, Benzoic Acid, Phenol and Aniline in HKUST-1(Cu)/MoS2-3-C system; (g) Effects of inorganic anions (SO42−, NO3−, Cl−, CO32−) with the initial anion concentration of 1 mM on BPA degradation; (h) Effects of HA (1, 5, 10 mg/L) on BPA degradation. Reaction conditions: [BPA] = 20 mg/L, [PMS] = 1 mM, [Catalysts] = 0.3 g/L.
Figure 3. (a) Degradation of BPA in different catalysts systems; (b) Degradation of BPA in different HKUST-1(Cu)/MoS2-x-C systems; (c) The kinetic degradation curves in different catalysts systems; (d) Pseudo-first-order rate constants k in different reaction systems; (e) Degradation of 20 mg/L BPA, Nitrobenzene, Benzoic Acid, Phenol and Aniline in HKUST-1(Cu)/MoS2-3-C system; (f) TOC removal rate of 20 mg/L BPA, Nitrobenzene, Benzoic Acid, Phenol and Aniline in HKUST-1(Cu)/MoS2-3-C system; (g) Effects of inorganic anions (SO42−, NO3−, Cl−, CO32−) with the initial anion concentration of 1 mM on BPA degradation; (h) Effects of HA (1, 5, 10 mg/L) on BPA degradation. Reaction conditions: [BPA] = 20 mg/L, [PMS] = 1 mM, [Catalysts] = 0.3 g/L.
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Figure 4. (a) Catalytic stability tests in five consecutive runs of HKUST-1(Cu)/MoS2-3-C for BPA degradation; (b) SEM imagine of HKUST-1(Cu)/MoS2-3-C before and after reaction; High-resolution XPS spectra of (c,d) Cu LMM; (e,f) Mo 3d for before and after reaction.
Figure 4. (a) Catalytic stability tests in five consecutive runs of HKUST-1(Cu)/MoS2-3-C for BPA degradation; (b) SEM imagine of HKUST-1(Cu)/MoS2-3-C before and after reaction; High-resolution XPS spectra of (c,d) Cu LMM; (e,f) Mo 3d for before and after reaction.
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Figure 5. Possible degradation pathways of BPA in HKUST-1(Cu)/MoS2-3-C+PMS system.
Figure 5. Possible degradation pathways of BPA in HKUST-1(Cu)/MoS2-3-C+PMS system.
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Figure 6. (a) FT-IR spectra for various samples [(I) fresh catalyst, (II) BPA, (III) catalyst absorbed BPA, (IV) catalyst in the reaction with BPA in the presence of PMS, (V) used catalyst]. Reaction conditions: [BPA] = 20 mg/L, [PMS] = 1 mM, [Catalysts] = 0.3 g/L; (b) In situ Raman spectra of PMS, HKUST-1(Cu)-C+PMS, HKUST-1(Cu)/MoS2-3-C+PMS; (c) ATR-FTIR spectra for PMS solution alone, HKUST-1(Cu)/MoS2-3-C in water, and HKUST-1(Cu)/MoS2-3-C in PMS solution; (d) Nyquist plots of HKUST-1(Cu)-C and HKUST-1(Cu)/MoS2-3-C electrodes.
Figure 6. (a) FT-IR spectra for various samples [(I) fresh catalyst, (II) BPA, (III) catalyst absorbed BPA, (IV) catalyst in the reaction with BPA in the presence of PMS, (V) used catalyst]. Reaction conditions: [BPA] = 20 mg/L, [PMS] = 1 mM, [Catalysts] = 0.3 g/L; (b) In situ Raman spectra of PMS, HKUST-1(Cu)-C+PMS, HKUST-1(Cu)/MoS2-3-C+PMS; (c) ATR-FTIR spectra for PMS solution alone, HKUST-1(Cu)/MoS2-3-C in water, and HKUST-1(Cu)/MoS2-3-C in PMS solution; (d) Nyquist plots of HKUST-1(Cu)-C and HKUST-1(Cu)/MoS2-3-C electrodes.
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Figure 7. (a) Degradation efficiency of BPA under different quenching conditions. Quenching conditions: [MeOH] = 1 M, [TBA] = 1 M, [L-his] = 20 mM, [p-BQ] = 20 mM, [BPA] = 20 mg/L, [PMS] = 1 mM, [Catalysts] = 0.3 g/L. EPR detection for (b) •OH/SO4•−; (c) 1O2 and (d) O2•− in different reaction systems; (e) Solid EPR spectra of HKUST-1(Cu)/MoS2-3-C; (f) High-resolution XPS spectra of O 1s before and after reaction.
Figure 7. (a) Degradation efficiency of BPA under different quenching conditions. Quenching conditions: [MeOH] = 1 M, [TBA] = 1 M, [L-his] = 20 mM, [p-BQ] = 20 mM, [BPA] = 20 mg/L, [PMS] = 1 mM, [Catalysts] = 0.3 g/L. EPR detection for (b) •OH/SO4•−; (c) 1O2 and (d) O2•− in different reaction systems; (e) Solid EPR spectra of HKUST-1(Cu)/MoS2-3-C; (f) High-resolution XPS spectra of O 1s before and after reaction.
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Figure 8. Optimized configurations of BPA/PMS absorbed on (a,b) HKUST-1(Cu)-C, (c,d) MoS2; The Eads and Q referred to the adsorption energy and Bader charge of PMS; (e) The optimized configuration of BPA/PMS absorbed on HKUST-1(Cu)/MoS2-3-C.
Figure 8. Optimized configurations of BPA/PMS absorbed on (a,b) HKUST-1(Cu)-C, (c,d) MoS2; The Eads and Q referred to the adsorption energy and Bader charge of PMS; (e) The optimized configuration of BPA/PMS absorbed on HKUST-1(Cu)/MoS2-3-C.
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Figure 9. The possible mechanism of BPA degradation in HKUST-1(Cu)/MoS2-3-C/PMS system.
Figure 9. The possible mechanism of BPA degradation in HKUST-1(Cu)/MoS2-3-C/PMS system.
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MDPI and ACS Style

Yin, X.; Yin, H.; Wang, R.; Wang, J.; Li, A. Novel Fenton-like Catalyst HKUST-1(Cu)/MoS2-3-C with Non-Equilibrium-State Surface for Selective Degradation of Phenolic Contaminants: Synergistic Effects of σ-Cu-Ligand and ≡Mo–OOSO3− Complex. Water 2024, 16, 121. https://doi.org/10.3390/w16010121

AMA Style

Yin X, Yin H, Wang R, Wang J, Li A. Novel Fenton-like Catalyst HKUST-1(Cu)/MoS2-3-C with Non-Equilibrium-State Surface for Selective Degradation of Phenolic Contaminants: Synergistic Effects of σ-Cu-Ligand and ≡Mo–OOSO3− Complex. Water. 2024; 16(1):121. https://doi.org/10.3390/w16010121

Chicago/Turabian Style

Yin, Xiaoze, Huaqin Yin, Renjie Wang, Jinnan Wang, and Aimin Li. 2024. "Novel Fenton-like Catalyst HKUST-1(Cu)/MoS2-3-C with Non-Equilibrium-State Surface for Selective Degradation of Phenolic Contaminants: Synergistic Effects of σ-Cu-Ligand and ≡Mo–OOSO3− Complex" Water 16, no. 1: 121. https://doi.org/10.3390/w16010121

APA Style

Yin, X., Yin, H., Wang, R., Wang, J., & Li, A. (2024). Novel Fenton-like Catalyst HKUST-1(Cu)/MoS2-3-C with Non-Equilibrium-State Surface for Selective Degradation of Phenolic Contaminants: Synergistic Effects of σ-Cu-Ligand and ≡Mo–OOSO3− Complex. Water, 16(1), 121. https://doi.org/10.3390/w16010121

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