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Article

Regulating Crystal Facets of MnO2 for Enhancing Peroxymonosulfate Activation to Degrade Pollutants: Performance and Mechanism

by
Juncong Fu
1,†,
Peng Gao
2,†,
Lu Wang
1,
Yongqing Zhang
1,3,4,*,
Yuhui Deng
1,
Renfeng Huang
1,
Shuaifei Zhao
5,
Zebin Yu
6,
Yuancheng Wei
7,
Guangzhao Wang
8 and
Shaoqi Zhou
9
1
School of Environment and Energy, Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, South China University of Technology, Guangzhou 510640, China
2
School of Chemistry and Molecular Bioscience, University of Wollongong, Wollongong, NSW 2500, Australia
3
The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, Guangzhou 510006, China
4
State Key Laboratory of Pulp and Paper, South China University of Technology, Guangzhou 510006, China
5
Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia
6
School of Resources, Environment and Materials, Guangxi University, Nanning 530004, China
7
Preliminary Work Management Center for Government Investment Project of Shenzhen Longhua District, Shenzhen 518110, China
8
Key Laboratory of Extraordinary Bond Engineering and Advanced Materials Technology of Chongqing, School of Electronic Information Engineering, Yangtze Normal University, Chongqing 408100, China
9
Guizhou Academy of Sciences, Shanxi Road 1, Guiyang 550001, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2022, 12(3), 342; https://0-doi-org.brum.beds.ac.uk/10.3390/catal12030342
Submission received: 4 March 2022 / Revised: 14 March 2022 / Accepted: 15 March 2022 / Published: 17 March 2022
(This article belongs to the Special Issue Advanced Catalysts for Persulfate Activation)
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Abstract

:
On the catalyst surface, crystal facets with different surface atom arrangements and diverse physicochemical properties lead to distinct catalytic activity. Acquiring a highly reactive facet through surface regulation is an efficient strategy to promote the oxidative decomposition of wastewater organic pollutants via peroxymonosulfate (PMS) activation. However, the mechanism through which crystal facets affect PMS activation is still unclear. In this study, three facet-engineered α-MnO2 with different exposed facets were prepared via a facile hydrothermal route. The prepared 310-MnO2 exhibited superior PMS activation performance to 100-MnO2 and 110-MnO2. Moreover, the 310-MnO2/PMS oxidative system was active over a wide pH range and highly resistant to interfering substances from wastewater. These advantages of the 310-MnO2/PMS system make it highly promising for practical wastewater treatment. Based on quenching experiments, electron paramagnetic resonance (EPR) analysis, solvent exchange, and electrochemical measurements, mediated electron transfer was found to be the dominant nonradical pathway for p-chloroaniline (PCA) degradation. A sulfhydryl group (-SH) masking experiment showed that the highly exposed Mn atoms on the 310-MnO2 surface were sites of PMS activation. In addition, density functional theory (DFT) calculations confirmed that the dominant {310} facet promoted adsorption/activation of PMS, which favored the formation of more metastable complexes on the α-MnO2 surface. The reaction mechanism obtained here clarifies the relationship between PMS activation and crystal facets. This study provides significant insights into the rational design of high-performance catalysts for efficient water remediation.

Graphical Abstract

1. Introduction

In recent years, the environmental and health problems caused by non-biodegradable persistent organic pollutants (POPs) discharged into the aquatic environment have sounded an alarm, especially due to their fatal toxicity [1,2]. A multitude of treatment technologies, such as adsorption [3], membrane separation [4], biodegradation [5], and advanced oxidation processes (AOPs) [1], has been developed to protect aquatic systems against these refractory pollutants. Among these treatment technologies, AOPs are the most effective against organic pollutant removal owing to the abundance of powerful reactive oxygen species (ROSs) they produce [6,7]. Currently, many AOPs, such as the traditional Fenton system (Fe2+/H2O2) [8], photocatalysis [9], ozonation [10], electrochemical oxidation [11], and their combinations [12], are commonly used for POP removal. However, high energy input and potential secondary pollution limit their applications.
Persulfate activation, an emerging AOP-like pollutant control approach, is of great interest since it overcomes the shortcomings above. Persulfate activation not only removes pollutants by generating highly active radicals (such as E(•OH/OH−) = 1.8–2.7 V and E(SO4•−/SO42−) = 2.5–3.1 V), but also oxidatively decomposes pollutants by nonradical pathways (such as singlet oxygen (1O2), surface-activated complex, and direct electron transfer) [13]. Solid persulfates, including peroxydisulfate (PDS) and peroxymonosulfate (PMS), are more stable and easier to store and transport than H2O2 and O3 [14]. Given its asymmetric structure, PMS is more easily activated by various catalysts than PDS [15]. Moreover, aqueous organic pollutants can be effectively removed by activated PMS.
Since PMS has a relatively low reactivity toward POPs, effective PMS activation is a prerequisite for its environmental remediation application. Compared to traditional energy-based PMS activation methods (e.g., heat, UV, and ultrasonic), transition metal (Co, Ni, Ag, Cu, Ru, V) catalyst-based PMS activation is more cost-effective and uses a simpler reactor/system configuration [16]. Although cobalt-based materials are among the best options for activating PMS [17], the secondary pollution from the leaching of cobalt ions would lead to adverse effects on the environment. Therefore, it is imperative to develop environmentally friendly transition metal oxide catalysts. MnO2 is a promising alternative PMS activation catalyst due to its low cost, multiple Mn valence states, abundance, and environmental friendliness [18]. By tuning MnO2’s surface properties, such as crystal facets [19], crystalline phase [16], material dimension [20], and the number of defects [21], the catalytic performance of MnO2 towards PMS can be effectively enhanced. Unfortunately, the mechanism of this enhancement is still ambiguous. Thus, to design more efficient catalysts for water remediation, it is crucial to understand the relationship between the PMS activation process and the surface properties.
As an inherent surface property, the crystal facet induces specific physical and chemical surface activity due to the differences in its atomic distribution and the distorted electronic structure of its surface [22]. The adsorption and activation of reaction molecules on the catalyst surface are the key steps during the persulfate-catalyzation process. In addition, the microstructure of the material surface significantly affects the persulfate activation pathway, leading to enhanced catalytic efficiency [23]. Previous work [24] demonstrated that CuO crystal facets played an important role in PS activation. Similarly, it was reported that the δ-MnO2 {−111} facet exhibited superior persulfate activation performance compared to the {001} facet [19]. Based on the cases above, we reason that MnO2 with different exposed crystal facets affects PMS activation efficiency. However, how crystal facets affect PMS activation is still unknown. Understanding PMS activation on the MnO2 surface with different exposed facets is of great significance in designing high-performance PMS activation catalysts for organic pollutant removal.
In this study, three highly active nano-MnO2 catalysts with different exposed facets were synthesized via the facile hydrothermal route. Taking PCA as the model contaminant, the catalytic performance of α-MnO2 towards PMS was proven to be effectively enhanced by selectively exposing the high-index {310} facet. The factors affecting PCA degradation were systematically investigated. The resistance of the system to interfering substances from wastewater was also evaluated by introducing humic acid and inorganic anions. Cyclic experiments and manganese ion leaching experiments were carried out to test the stability and reusability of 310-MnO2. Different pollutants were degraded in the 310-MnO2/PMS oxidative system to explore its broad applicability. Based on an intermediate analysis, three pathways of PCA degradation in the 310-MnO2/PMS system were proposed. Combining experimental studies and DFT calculations, the mechanism through which the crystal facets affected PMS activation was attributed to the different surface atom arrangements and enhanced adsorption of PMS molecules on the 310-MnO2 surface. Unfortunately, there are few studies on the enhancement of persulfate activation by regulating crystal facets of manganese oxide, and how crystal facets affect PMS activation is still unclear. Unlike previous studies of persulfate activation by MnO2, which were mainly focused on morphology, crystal phase, defect regulation, and manganese-based composites, this work focuses on regulating the crystal surface of MnO2 to enhance PMS activation. The effect of crystal facets on PMS activation was comprehensively and deeply studied. This work provides atomic-scale insights into the relationship between PMS activation performance and crystal facets, as well as theoretical guidance for the rational design of high-performance PMS catalysts for practical wastewater treatment.

2. Results and Discussion

2.1. Characterization of Catalyst

Three α-MnO2 with different exposed facets were prepared via the facile hydrothermal method. The phase structure of the prepared materials was identified by XRD (Figure 1a). All the samples were consistent with pure α-MnO2 (PDF #44-0141), which has a well-defined 2 × 2 tunnel structure. However, the XRD peak intensities of the samples differed. For the 310-MnO2, the diffraction peak corresponding to the {310} facet was sharper than for the {100} and {110} facets. Therefore, the {310} facet was the dominant exposed crystal facet of 310-MnO2. This result is consistent with previous reports [25]. Similarly, the MnO2 catalysts with exposed {110} (green line Figure 1a) and {100} (cyan line Figure 1a) facets were identified.
Electron paramagnetic resonance (EPR) spectroscopy was employed to analyze the differences in oxygen vacancy content among the three MnO2 catalysts (Figure 1b). However, unlike typical EPR spectra of oxygen vacancy [21,23,26], no obvious oxygen vacancy signal was observed in this work, which means that the oxygen vacancy content was negligible.
To obtain the data from the BET surface area and pore properties, the N2 adsorption/desorption isotherms of the three MnO2 catalysts were measured (Figure S1). The relevant data are provided in Table S1. The results demonstrate that the 310-MnO2 had the largest surface area (116.0 m2/g), followed by 110-MnO2 (104.9 m2/g) and 100-MnO2 (67.8 m2/g). However, the order of the surface area sizes on the three MnO2 catalysts was inconsistent with the catalytic activity observed in the organic pollutant degradation experiments, indicating that the specific surface area was not the decisive factor in the catalytic activity.
The morphologies of the obtained MnO2 samples with different exposed facets were determined by FE-SEM and HR-TEM analysis. The FE-SEM images (Figure 1c–h) showed that all the MnO2 samples exhibited a nanowire morphology. The 110-MnO2 nanowire had a diameter of 60–70 nm and a length of several micrometers, while the 310-MnO2 and 100-MnO2 were thinner, with diameters of 40–50 nm. The energy-dispersive spectrometer (EDS) results (Figure S2) demonstrated that the surface of the 310-MnO2 had the highest Mn/O ratio (1.41), followed by 100-MnO2 (0.75) and 110-MnO2 (0.51). The order of the Mn/O ratio on different exposed crystal facets was consistent with the theoretical atomic arrangement analysis (Figure 2(a4–c4)).
HR-TEM further confirmed that the MnO2 with different exposed crystal facets was synthesized successfully. The observed lattice distances of the {100}, {110}, and {310} facets were 0.49 nm, 0.69 nm, and 0.31 nm, respectively (Figure 2). These results agree with the calculated lattice distances based on XRD characterization, where the lattice distances for the facets were 4.90 Å {100}, 6.92 Å {110}, and 3.09 Å {310}, respectively. Since the α-MnO2 grew along the (001) direction and had four exposed sides as the main exposed facet, it was easy to find that the 310-MnO2 possessed four {310} facets exposed on the sides of the nanowires [25]. Similar phenomena were also found in the 100-MnO2 and 110-MnO2. Combined with the XRD results, the α-MnO2 with different exposed crystal facets was successfully synthesized. By analyzing the atomic occupancy (Figure 2(a4–c4)) on the different facets, it was found that the exposed atoms on the {100} and {110} facets were mainly O, while Mn atoms were significantly enriched on the {310} facet. The MnO2 with different crystal facets exhibited different PMS activation effects, which might have been due to different surface active sites, i.e., different Mn atom distributions on the surface [24]. Highly exposed surface Mn atoms might have facilitated electron transfer between the pollutants and the catalyst [27,28].
The surface chemical states and element composition of the obtained MnO2 were investigated by XPS. The Mn 2p3/2 peak can be divided into three peaks, located at 642.9, 641.6, and 640.6 eV, corresponding to the binding energy of Mn4+, Mn3+, and Mn2+, respectively (Figure 3b) [29]. The ratio of the different types of Mn atom to the total Mn is summarized in Table S2. Meanwhile, the average oxidation state (AOS) of Mn (Figure 3c) was calculated by the equation AOS = 8.956–1.126∆E (ΔE is the binding energy difference of Mn 3s) [30]. The AOS of different MnO2 was essentially the same, which means a negligible difference in oxygen vacancy content. In other words, the enhancement of PMS activation on the MnO2 with different facets was not related to the oxygen vacancy content. Moreover, the O 1s spectrum (Figure 3d) was divided into two peaks, located at 531.5 eV (surface-adsorbed oxygen (Oads, such as O−, O2− and –OH)) and 529.8 eV (lattice oxygen (Olatt)) [22]. The relevant data are provided in Table S2. The chemical state of the Mn and the content of the oxygen species did not change significantly before or after the reaction, suggesting that the PMS activation process over the MnO2 surface was a catalytic reaction.

2.2. PCA Degradation in the Facet-Engineered MnO2/PMS System

2.2.1. PCA Degradation in Different α-MnO2/PMS Systems

To investigate the PMS activation over MnO2 with different facets, PCA was chosen as the target pollutant, and the PCA degradation experiments are displayed in Figure 4a. It was arduous to remove the PCA with α-MnO2 alone, and only a 5% removal efficiency was achieved through adsorption. In the non-catalytic PMS system, the removal efficiency of the PCA over 5 min was about 10%, which indicated that the unactivated PMS had a limited ability to oxidize PCA. By contrast, 100% of the PCA was degraded within 5 min in the facet-engineered α-MnO2/PMS systems, indicating that the facet-engineered α-MnO2 catalyzed the production of more powerful oxidizing substances from the PMS for PCA degradation. Interestingly, among the three facet-engineered α-MnO2 catalysts, the 310-MnO2 exhibited the best PMS activation performance, in which nearly 100% of the PCA was degraded in 3 min, while 67% and 84% of PCA were degraded in the 110-MnO2/PMS and the 100-MnO2/PMS systems, respectively. The reaction rate constant k of the 310-MnO2/PMS system was three times that of the 110-MnO2/PMS system (Figure 4b). Consequently, in subsequent optimization experiments, the 310-MnO2 was chosen as the model catalyst for PMS activation.

2.2.2. Effect of PMS Concentration

The impact of the PMS concentration (0.5–3 mM) on the PCA degradation was evaluated (Figure 4c). Previous studies pointed out that in some heterogeneous PMS activation systems, excessive PMS had a negative impact on contaminant degradation [15,31,32]. However, in the 310-MnO2/PMS system, increasing the PMS concentration increased the PCA degradation. With the PMS concentration increasing from 0.5 mM to 3 mM, the corresponding k value also increased, from 0.2003 min−1 to 1.3916 min−1 (Figure 4d). A similar phenomenon was observed in the edge-nitrogenated biochar/PDS system, in which pollutant degradation occurred through the electron transfer process (nonradical pathway) [33]. However, only a slight improvement was observed when the PMS exceeded 2 mM, suggesting insufficient active sites on the surface of 310-MnO2 [34].

2.2.3. Effect of Catalyst Dosage

The catalyst dosage had a significant impact on the PCA degradation (Figure 4e). As the catalyst dosage increased from 0.03 g·L−1 to 0.3 g·L−1, the PCA degradation rate increased markedly from 0.2321 min−1 to 6.5713 min−1 (Figure 4f). In addition, the PCA degradation with different catalyst dosages fitted the pseudo-first-order kinetic model (Figure S3). The presence of more catalyst resulted in more active sites, facilitating PMS adsorption and activation on the MnO2 surface [35]. Moreover, compared with the PMS concentration, the catalyst dosage played a more significant role in the PCA degradation. A similar phenomenon was also observed by Chen et al. [36], who found the sulfacetamide degradation rate was impacted more by the catalyst dosage than the PMS concentration.

2.2.4. Effect of Initial pH

The solution pH is a significant factor affecting the generation of ROSs in the chemical oxidation process [37]. Hence, the PCA removal efficiency under different initial pH conditions was systematically investigated. At an initial pH ranging from 3–9, the PCA was completely degraded within 5 min, indicating that the 310-MnO2 was effective at activating the PMS over a wide initial pH range (Figure 5a). Moreover, the PMS was more easily activated at pH = 9 because the pKa of PMS was 9.4 [18]; therefore, the 310-MnO2/PMS system at pH = 9.0 exhibited a slightly higher PCA degradation rate than for pH = 3–7. The isoelectric point (pHpzc) of 310-MnO2 was 3.3 (Figure S4) and the pKa of the PCA was 4.2 [38]. When the solution pH = 3, the catalyst surface was positively charged, which favored the adsorption of PMS anions on the catalyst surface, thereby achieving a marginally faster PCA degradation rate than at pH = 5–7.

2.3. Stability and Universal Adaptability

2.3.1. Universal Adaptability of 310-MnO2

The natural organic matters (NOMs) and inorganic anions from actual water bodies tend to affect contaminant degradation during the PMS-based oxidation process by quenching radicals, generating other reactive species, or adsorbing on the catalyst surface [39,40,41,42]. For example, humic acid (HA) inhibits pollutant degradation by competing for reactive oxidizing substances [33]. Besides, another quintessential example is that Cl− reacts with SO4•− and •OH to produce fewer reactive Cl• radicals, resulting in a significant inhibition of pollutant degradation [43]. Therefore, it is necessary to explore their influence on pollutant degradation in the 310-MnO2/PMS system. In this work, the impact of HA and different co-existing anions (ClO4−, Cl−, SO42−, NO3−) on PCA degradation were investigated. As shown in Figure 5b, HA had little impact on PCA degradation. Furthermore, the presence of anions only slightly reduced the PCA degradation rate (Figure 5c–f), which is attributed to the negative impact of the increased ionic strength [44]. Due to fact that the solution pH (pH ≈ 3) was lower than the isoelectric point of 310-MnO2 (pHpzc = 3.3), the catalyst surface was positively charged and could adsorb PMS anions. However, increased ionic strength reduces the zeta potential of catalyst particles in solution [43], which is unfavorable for the electrostatic attraction between the catalyst and the PMS. The above results suggested that the 310-MnO2/PMS system was strongly resistant to interference.
Different pollutants, including PCA, Rhodamine B (RhB), phenol, tetracycline (TC), and imidacloprid (IMI), were degraded in the 310-MnO2/PMS oxidative system to explore its broad applicability. As shown in Figure 6a, the 310-MnO2/PMS system had an excellent removal effect on RhB and PCA: both were completely degraded within 5 min. Moreover, 100% of the phenol was removed in 15 min, while 80% of the TC was decomposed within 30 min. Interestingly, only 28% of the IMI was degraded in 30 min. Previous studies showed that radical systems degrade IMI effectively, while nonradical oxidation systems show low reactivity toward IMI [45]. Similarly, the selective removal of contaminants was also reported in other nonradical systems [34,46,47,48]. Thus, we speculated that the 310-MnO2/PMS did not depend on free radicals.
To further assess the potential of 310-MnO2 in practical applications, PCA degradation experiments were carried out in deionized water (DW), tap water (TW), lake water (LW), and river water (RW) (Figure 6c). Compared with the rate in DW, the degradation rate of the PCA in actual water decreased slightly. However, after 5 min of reaction, the PCA removal rate still reached 97.5%, 94.4%, and 95.5% in TW, LW, and RW, respectively. The above results suggest that the 310-MnO2/PMS system is highly promising for practical wastewater treatment.

2.3.2. Stability of 310-MnO2

The stability of the catalytic materials is one of the core issues that needs to be considered before practical application. Materials with low stability increase the cost of operation and cause secondary pollution. Therefore, we carried out a cycle test (Figure 6d) to evaluate the stability of the 310-MnO2. The results indicated that the 310-MnO2 had high stability, with the PCA degradation efficiency remained at 80% after the 10th cycle. As the number of cycles increased, the PCA degradation rate constant (k) on the 310-MnO2 slightly decreased, but the k value after the 10th cycle was still equivalent to that of the fresh 110-MnO2 (Figure 6e). In addition, nearly no manganese ions were dissolved during the reaction process (Figure 6f). This result might have been caused by the material oxalate ions serving as a capping agent, both the stability and percentages of the {310} facet were improved [14,25]. Accordingly, the reason for the decrease in reaction rate might be related to the generated intermediate products adsorbing on the catalyst surface, thus inhibiting the exposure of the active sites [49]. Due to the 310-MnO2’s outstanding stability and universal adaptability, it will have broad applications in actual wastewater treatment.

2.4. Mechanism of PMS Activation by 310-MnO2

2.4.1. Quenching Experiments and PMS Decomposition Experiments

To unveil the PMS activation mechanism on the 310-MnO2 catalyst, a series of quenching experiments were performed by adding different scavengers to the 310-MnO2/PMS system. EtOH is commonly used as a scavenger for •OH (k•OH /ethanol = 1.2 × 109–2.8 × 109 M−1 S−1) and SO4•− (kSO4•−/ethanol = 1.6 × 107–7.7 × 107 M−1 S−1) due to its high reactivity [50]. TBA is an effective scavenger that distinguishes between SO4•− and •OH, since its reaction rate with •OH (k•OH /TBA=3.8 × 108–7.6 × 108 M−1 S−1) is nearly 1000 times higher than with SO4•− (kSO4•−/TBA = 4 × 105–9.1 × 105 M−1 S−1) [51,52]. The addition of 750 mM EtOH or 750 mM TBA showed a negligible impact on PCA oxidation (Figure 7a), suggesting that neither •OH nor SO4•− were the dominant active oxidizing species for PCA degradation. The p-BQ, with a high reaction rate constant with O2•− (kO2•−/ p-BQ = 0.9–1.0 × 109 M−1 S−1), was selected to explore the possible role of O2•− in the reaction system [53]. However, the PCA degradation was not inhibited at all by the introduction of 1.5 mM or 15 mM p-BQ (Figure 7a), thus excluding the presence of O2•− in the 310-MnO2/PMS system. The experimental results suggested that nonradical species rather than radicals played the dominant role in the PCA degradation. Based on previous reports, three major nonradical oxidation pathways were proposed, including singlet oxygen (1O2) oxidation, direct electron transfer, and surface-bound active species [13,54,55,56,57]. 1O2 is a potential active oxidizing species that can be produced during PMS activation by transition metal oxide or carbon-based material [58,59,60]. To verify whether 1O2 took part in the PCA degradation in 310-MnO2/PMS system, FFA, a typical scavenger for 1O2 (k1O2/FFA = 1.2 × 108 M−1 S−1), was chosen [61]. The addition of 2.5 mM FFA only slightly inhibited the PCA degradation (Figure 7b). However, increasing the FFA to 75 mM inhibited nearly 40% of the PCA degradation.
Further experiments were performed to investigate the removal of FFA with PCA present in the 310-MnO2/PMS system. Considering that FFA has a high reaction rate constant with 1O2 (k1O2/FFA = 1.2 × 108 M−1 S−1), if 1O2 is the dominant active oxidizing substance in the PCA degradation system, FFA is decomposed preferentially. However, it was found that the 310-MnO2/PMS system preferentially decomposed the PCA oxidatively, and the FFA only began to degrade after the PCA was completely decomposed (Figure 7c). We also investigated the FFA degradation with different PCA concentrations (0.5 mM and 1 mM). Interestingly, the time when the FFA degradation began was delayed with increasing PCA concentration due to the preferential degradation of the increased PCA. These results indirectly indicate that 1O2 is not the main active oxidizing substance. The inhibition of PCA degradation by FFA is likely to have been due to the high concentration of FFA altering the surface properties of the catalyst, eventually resulting in the inhibition of pollutant adsorption and degradation [62].
The corresponding PMS consumption was also determined to reveal the possible PMS activation mechanism (Figure 7d). A slight decomposition of PMS was observed in the PMS-only, PMS/PCA, and 310-MnO2/PMS systems. However, in the presence of PCA, notable PMS decomposition occurred in the 310-MnO2/PMS system, suggesting that both 310-MnO2 and PCA were necessary for efficient PMS decomposition. These results further excluded the existence of radicals.

2.4.2. EPR Detection and Solvent Exchange Experiment

To shed light on the PMS activation mechanism, EPR tests were conducted using TEMP to capture 1O2 and DMPO to capture short-lifetime radicals. The absence of classical radical adduct signals (Figure 7e) further confirmed that •OH, SO4•−, and O2•− were not generated during the reaction. However, a clear six-line spectrum with a large span was observed in the EPR spectrum, which was attributed to Mn [63]. The ~3500 G signal was due to the direct oxidation of DMPO to DMPOX instead of a radical attack [32]. The PMS led to weak triplet signals (1:1:1) due to its small amount of self-decomposition (Figure 7f) [7]. As the catalysts were introduced into the PMS solution, a remarkable TEMPO triplet signal was obtained; this is usually employed as evidence for the existence of 1O2 [21,48]. However, solvent exchange (replacing H2O with D2O) (Figure 8a) did not accelerate the PCA degradation. As we know, the degradation rate of the oxidative system based on 1O2 should be faster in D2O than in H2O, since the lifetime of 1O2 in D2O (20–32 µs) is significantly longer than in H2O (2 µs) [64]. These contradictory results suggested that the TEMPO signal obtained in the 310-MnO2/PMS system did not prove the formation of 1O2 [33]. Nardi et al. proposed that it may be misleading to detect 1O2 using the TEMPO/EPR method, since the excited intermediate also can extract one electron from TEMP to form TEMP+·, which combines with the dissolved oxygen to generate TEMPO [65]. A similar phenomenon was also observed in the edge-nitrogenated biochar/PDS system, in which pollutant degradation occurred through the electron transfer process [33]. In addition, a significant increase in dissolved oxygen (DO) concentration was observed (Figure 8b) in a typical singlet oxygen system (p-BQ/PMS), which is consistent with the research of Zhou et al. [66]. However, no significant DO increase occurred in the 310-MnO2/PMS system. Therefore, 1O2 did not contribute to PCA degradation in the 310-MnO2/PMS system.

2.4.3. Electrochemical Analysis and Sulfhydryl Modification Experiments

According to the above results, other nonradical pathways, instead of 1O2, may be involved in the 310-MnO2/PMS/PCA system. The electron transfer pathway was confirmed in some single-atom catalysts, transition metal oxide catalysts, and carbon-based materials [18,32,33,49]. To further identify the oxidative mechanism in our system, cyclic voltammetry (CV) experiments were carried out. The current density of the 310-MnO2 changed slightly after adding PMS (Figure 8c). The slight charge rearrangement between the catalyst and PMS to form a metastable complex might have been responsible for this [67]. However, an apparent increase in the current density appeared once the PCA was introduced, indicating that a current flow formed from PCA to the metastable complex [32]. The interaction between the 310-MnO2 and PMS was confirmed by ATR-FTIR. A red-shift of about 8 cm−1 was observed at the wavelength of 1095 cm−1 (S-O stretching) when the 310-MnO2 was added into the PMS solution (Figure S5a), suggesting the formation of complexes on the 310-MnO2 surface [16,18]. Moreover, both a strong reduction and oxidation peaks were observed in the CV curves, suggesting the reversibility of the process, which is essential for continuous PMS activation.
As mentioned above, the exposed atoms on the {100} and {110} facet were mainly O, while many Mn atoms were enriched in the surface of the {310} facet. To determine whether the surface Mn atoms were the sites of PMS activation, the sulfhydrylization reagent 4-mercaptobenzoic acid was employed to block surface Mn atoms through complexation between the Mn and the sulfhydryl group [24]. After modification with 4-mercaptobenzoic acid, a characteristic peak of the thiophenol S-H bond appeared at 2561 cm−1 in the 310-MnO2 FTIR (Figure S5b), indicating that the modification was successful. In addition, PCA degradation was almost completely inhibited in the 310-MnO2/PMS system after surface modification with the -SH group (Figure 8d), confirming that Mn atoms on the catalyst surface were sites of PMS activation. Thus, in the 310-MnO2/PMS/PCA system, PMS was adsorbed on the surface Mn sites, forming a metastable complex. Next, the PCA was adsorbed on the metastable complex surface through a “donor-acceptor complex” mechanism. There, the 310-MnO2 served as a bridge for electron transfer between the PCA and the PMS.

2.4.4. DFT Calculations

Crystal facet engineering can modify surface atomic distribution and decrease adsorption energy [19]. To further explore why different crystal facets of α-MnO2 produce distinct catalytic performances, the adsorption energies of the PMS molecule on the α-MnO2 surface were calculated using density functional theory (DFT).The DFT calculation method is provided in Text S1. The adsorption energies (Eads) of the PMS molecules over the α-MnO2 with {100}, {110}, and {310} facets were −1.50 eV, −0.41 eV, and −2.11 eV, respectively (Figure 8e–g). Due to the relatively negative adsorption energy and the highly exposed surface Mn atoms of the {310} facet, the PMS molecules were easily adsorbed on the {310} facet to form more metastable complexes, thereby promoting PMS activation and pollutant degradation.

2.5. Removal of TOC and Proposed Pathway of PCA Degradation

To investigate the mineralization of PCA degradation, the degradation of the total organic carbon (TOC) in the PCA by the 310-MnO2/PMS system was tested (Figure S6). In 120 min, 67.6% of the TOC was removed, indicating the incomplete mineralization of the PCA and the generation of intermediates.
Interestingly, the solution changed from colorless to wine-red during the reaction, indicating the presence of chromophore in the intermediate products, possibly due to the formation of –N=N–. GC-MS was further used to understand the degradation pathway of the PCA in the 310-MnO2/PMS system. Possible intermediate products are provided in Figure S7, and three pathways of PCA degradation are proposed (Figure 9). In the first pathway (I), one molecule of PCA loses the Cl atom, the other molecule loses the amino group (–NH2), and they further dimerize to form N-(4-chloro-phenyl)-benzene-1,4-diamine (a). Next, hydroquinone (b) was produced from the oxidation of (a) [68]. In the second pathway (II), the PCA molecule was first dechlorinated to produce aniline (c) and then reacted to form azobenzene (d) [34]. In the third pathway (III), two PCA molecules dimerized to generate 4,4′-dichloroazobenzene (e). Although further intermediates were not detected, 4-chloro-phenyl-diazen (f) was reported as an oxidation by-product of (e) [68]. Finally, the PCA and its intermediates were mineralized into H2O and CO2.

3. Materials and Methods

3.1. Chemicals

All chemicals used in the experiments were analytical (AR)- or HPLC-grade. Detailed information is shown in the Supporting Information (Text S2).

3.2. Preparation of α-MnO2 with Different Facets

Synthesis of 310-MnO2: In a typical procedure, KMnO4 (3.161 g, 20 mmol) and (NH4)2C2O4·H2O (1.421 g, 10 mmol) were mixed in deionized (DI) water (70 mL) under magnetic stirring for 30 min. The solution was then transferred to a Teflon-lined stainless-steel autoclave and kept at 180 °C for 24 h. After the reaction was finished, the obtained precipitates were further washed several times with DI water and ethanol, and then dried at 105 °C for 24 h.
Synthesis of 110-MnO2: 110-MnO2 was carried out by the same route as for 310-MnO2, but (NH4)2C2O4 was replaced by (NH4)2SO4.
Synthesis of 100-MnO2: In a typical procedure, MnSO4·H2O (1.352 g, 8 mmol), (NH4)2S2O8 (1.826 g, 8 mmol), (NH4)2SO4 (1.982 g, 15 mmol), and KNO3 (0.809 g, 8 mmol) were mixed in DI water (40 mL) under magnetic stirring for 30 min. Next, the mixture was transferred to a Teflon-lined stainless-steel autoclave and kept at 120 °C for 20 h. The obtained precipitates were further washed several times with DI water and ethanol, and then dried at 105 °C for 24 h.

3.3. Experimental Procedures

All solutions were prepared with DI water. Batch experiments were carried out in 150-millilitre conical flasks containing 100 mL reaction solution with magnetic stirring (650 rpm). To completely disperse the catalyst into water solution, 10 mg catalyst was added to 90 mL of DI water and sonicated for 5 min before the catalytic reaction. Next, 10 mL of 10 mM PCA and 46 mg of PMS were added in sequence. At the given time intervals, 0.5 mL of the mixture was removed and filtered (0.45 um), and the reaction was terminated by adding Na2S2O3 (1 M, 0.5 mL).
To assess the effect of solution pH on PMS activation, the initial solution pH of the reaction system was adjusted to the desired value with NaOH (0.1 mol/L) or H2SO4 (0.1 mol/L). Experiments with varying PMS concentrations, amounts of catalyst and anion species were conducted to investigate the influence of these parameters. Quenching experiments were carried out utilizing EtOH, TBA, p-BQ, and FFA as quenchers.

3.4. Analytical Techniques

A scanning electron microscope (SEM, SU8010, Hitachi Limited, Tokyo, Japan) was used to observe the surface morphology of the materials. A transmission electron microscope (TEM, Talos F200X, FEI, Hillsboro, OR, USA) was used to observe the morphology and exposed crystal facet of the materials. The phase structure of the catalysts was characterized by an X-ray diffractometer (XRD, D8 Advance, Bruker, Karlsruhe, Germany). X-ray photoelectron spectrometry (XPS, PHI X-tool, ULVAC–PHI, Chigasaki, Kanagawa, Japan) was used to investigate the chemical valence and composition of the catalysts. The surface interaction between the 310-MnO2 and PMS was confirmed by Fourier-transform infrared spectroscopy (ATR-FTIR, Thermo Scientific iD5 and Thermo Scientific Nicolet iS5, Shanghai Linglu Instrument Equipment Co., Ltd., Shanghai, China).
Pollutant concentrations were determined by high-performance liquid chromatography (HPLC) equipped with a reversed-phase C18 column (250 mm × 4.6 mm). Analytical details for the organic compounds are provided in the Supporting Information (Table S3). The PMS concentration was determined by the iodide ion color spectrometry [69]. The manganese ion leaching during the reaction process was detected by ICP-OES (Optima 8300, PerkinElmer, Waltham, MA, USA). The dissolved oxygen concentration in the reaction solution was determined by a portable dissolved oxygen analyzer (ST300D, Ohaus, Changzhou, China). Total organic carbon (TOC) was measured by an Elementar vario TOC (Elementar Analysensysteme GmbH, Hanau, Germany). Radicals in solution were analyzed by electron paramagnetic resonance (Bruker A300, Bruker, Karlsruhe, Germany). The parameters were set as follows: microwave frequency 9.8752 GHz, sweep width 100 G, center field 2504.12 G, modulation amplitude 1 G, time constant 327.68 ms, and sweep time 40 s.

4. Conclusions

In summary, three α-MnO2 with different dominant exposed crystal facets were successfully prepared via the facile hydrothermal route. The prepared 310-MnO2 exhibited excellent PMS activation performance and extremely high stability. In addition, the 310-MnO2/PMS system could degrade a wide range of pollutants and remove the target pollutants effectively in an actual water body. More importantly, the 310-MnO2/PMS oxidative system exhibited a wide pH adaptation range and showed good selectivity in the presence of natural organic matter and inorganic anions. The quenching experiments, EPR analysis, solvent exchange, and CV curves demonstrated that mediated electron transfer was the dominant mechanism in the PCA degradation. During the reaction, the electron was transferred from the adsorbed PCA (as an electron donor) to the adsorbed PMS (as an electron acceptor) on the catalyst’s surface with the 310-MnO2 acting as an electron transporter. The Mn atoms on the catalyst surface were confirmed to be sites of PMS activation by modifying MnO2 with -SH group. The DFT calculations verified that PMS molecules can be strongly adsorbed on the {310} facet with low adsorption energy, thereby promoting PMS activation and pollutant degradation. In this study, we demonstrated that for transition metal oxides, the dominant exposed facets significantly affect PMS activation. The reaction mechanism we discovered provides guidance for the design of novel and efficient wastewater treatment catalysts.

Supplementary Materials

The following supporting information can be downloaded at: https://0-www-mdpi-com.brum.beds.ac.uk/article/10.3390/catal12030342/s1, Text S1: DFT Calculation Method; Text S2: Reagents used in the experiment; Text S3: Experimental sample preparation; Table S1: Properties of the three MnO2 catalysts; Table S2: Chemical and surface compositions of facet engineered MnO2 before and after the reaction; Table S3: Analytical details for organic compounds by HPLC; Figure S1: The N2 adsorption/desorption isotherms of the three MnO2 catalysts; Figure S2: EDS-mapping images of (a) 100-MnO2, (b) 110-MnO2, and (c) 310-MnO2; Figure S3: (a) Effect of MnO2 dosage in 310-MnO2/PMS system and (b) correlation of the rate constants to the MnO2 dosage; Figure S4: The zeta potential curves of the 310-MnO2; Figure S5: (a) ATR-FTIR spectra demonstrating the PMS complexation on the catalyst surface and (b) ATR-FTIR spectra of 310-MnO2 solid before and after modifying with -SH group; Figure S6: TOC removal of PCA degradation with 310-MnO2/PMS system; Figure S7: Mass spectra and proposed structure of possible intermediate products.

Author Contributions

Conceptualization, J.F.; Funding acquisition, Y.Z.; Investigation, J.F.; Methodology, J.F., P.G., L.W. and G.W.; Resources, Y.W.; Software, J.F., P.G., L.W. and G.W.; Supervision, Y.Z., Z.Y. and S.Z. (Shaoqi Zhou); Validation, Y.Z.; Writing–original draft, J.F.; Writing–review & editing, Y.Z., Y.D., R.H., S.Z. (Shuaifei Zhao), Z.Y. and S.Z. (Shaoqi Zhao). All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Key Research and Development Program of China (2016YFC0400708), the National Natural Science Foundation of China (51572089), the Natural Science Foundation of Guangdong Province, China (2019A1515011761) and the Foundation of Science and Technology Planning Project of Guangzhou, China (201804010398).

Data Availability Statement

Data is contained within the article or Supplementary Materials.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Catalysts 12 00342 g001 550
Figure 1. (a) XRD patterns of 100-MnO2, 110-MnO2, and 310-MnO2; (b) solid EPR spectra of three MnO2 catalysts; SEM images of (c,f) 100-MnO2, (d,g) 110-MnO2, and (e,h) 310-MnO2.
Figure 1. (a) XRD patterns of 100-MnO2, 110-MnO2, and 310-MnO2; (b) solid EPR spectra of three MnO2 catalysts; SEM images of (c,f) 100-MnO2, (d,g) 110-MnO2, and (e,h) 310-MnO2.
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Figure 2. HRTEM images of (a1–a3) 100-MnO2, (b1–b3) 110-MnO2, and (c1–c3) 310-MnO2; profile analysis and corresponding atom arrangement on the surface of (a4) 100-MnO2, (b4) 110-MnO2, and (c4) 310-MnO2 in a 2 × 2 unit cell. Small (gold) spheres are oxygen and large (red and green) are manganese.
Figure 2. HRTEM images of (a1–a3) 100-MnO2, (b1–b3) 110-MnO2, and (c1–c3) 310-MnO2; profile analysis and corresponding atom arrangement on the surface of (a4) 100-MnO2, (b4) 110-MnO2, and (c4) 310-MnO2 in a 2 × 2 unit cell. Small (gold) spheres are oxygen and large (red and green) are manganese.
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Figure 3. XPS spectra of α-MnO2 with different exposed crystal facets: (a) survey spectra, (b) Mn 2p, (c) Mn 3s, and (d) O 1s.
Figure 3. XPS spectra of α-MnO2 with different exposed crystal facets: (a) survey spectra, (b) Mn 2p, (c) Mn 3s, and (d) O 1s.
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Figure 4. (a) Removal of PCA by different systems and (b) corresponding reaction rate constant k, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; (c) effect of PMS concentration in 310-MnO2/PMS system and (d) corresponding linear fitting curve, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 0.5–3 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; (e) effect of MnO2 dosage in the 310-MnO2/PMS system and (f) corresponding linear fitting curve, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.03–0.3 g/L, without pH adjustment.
Figure 4. (a) Removal of PCA by different systems and (b) corresponding reaction rate constant k, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; (c) effect of PMS concentration in 310-MnO2/PMS system and (d) corresponding linear fitting curve, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 0.5–3 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; (e) effect of MnO2 dosage in the 310-MnO2/PMS system and (f) corresponding linear fitting curve, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.03–0.3 g/L, without pH adjustment.
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Figure 5. (a) Effect of initial pH, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, initial pH = 3–9; the influences of (b) humic acid, (c) ClO4−, (d) Cl−, (e) SO42−, and (f) NO3−, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment.
Figure 5. (a) Effect of initial pH, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, initial pH = 3–9; the influences of (b) humic acid, (c) ClO4−, (d) Cl−, (e) SO42−, and (f) NO3−, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment.
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Figure 6. (a) Removal of various organic pollutants by 310-MnO2/PMS system and (b) corresponding reaction rate constant k, reaction conditions: (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, (PCA)0 = 1.0 mM, (RhB)0 = 20 mg/L, (TC)0 = 25 mg/L, (Phenol)0 = 20 mg/L, (IMI)0 = 10 mg/L, without pH adjustment; (c) degradation of PCA in actual water by 310-MnO2/PMS system; (d) cycle tests of PCA degradation and (e) corresponding reaction rate constant k, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; (f) the dissolution of manganese ions during the reaction, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment.
Figure 6. (a) Removal of various organic pollutants by 310-MnO2/PMS system and (b) corresponding reaction rate constant k, reaction conditions: (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, (PCA)0 = 1.0 mM, (RhB)0 = 20 mg/L, (TC)0 = 25 mg/L, (Phenol)0 = 20 mg/L, (IMI)0 = 10 mg/L, without pH adjustment; (c) degradation of PCA in actual water by 310-MnO2/PMS system; (d) cycle tests of PCA degradation and (e) corresponding reaction rate constant k, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; (f) the dissolution of manganese ions during the reaction, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment.
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Figure 7. (a) PCA degradation in the presence of various free radical scavengers and (b) singlet oxygen scavenger, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; (c) FFA degradation by the 310-MnO2/PMS system in the presence of PCA, reaction conditions: (PCA)0 = 0.5–1.0 mM, (FFA)0 = 0.1 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; (d) decomposition of PMS in PMS alone, 310-MnO2/PMS, PMS+PCA, 310-MnO2/PMS/PCA systems, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; EPR signals of different PMS activation system with the existence of (e) DMPO and (f) TEMP, reaction conditions: (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, (DMPO)0 = 0.05 M, (TEMP)0 = 0.1 M, without pH adjustment.
Figure 7. (a) PCA degradation in the presence of various free radical scavengers and (b) singlet oxygen scavenger, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; (c) FFA degradation by the 310-MnO2/PMS system in the presence of PCA, reaction conditions: (PCA)0 = 0.5–1.0 mM, (FFA)0 = 0.1 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; (d) decomposition of PMS in PMS alone, 310-MnO2/PMS, PMS+PCA, 310-MnO2/PMS/PCA systems, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; EPR signals of different PMS activation system with the existence of (e) DMPO and (f) TEMP, reaction conditions: (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, (DMPO)0 = 0.05 M, (TEMP)0 = 0.1 M, without pH adjustment.
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Figure 8. (a) Effect of reaction solvents (H2O and D2O) and corresponding reaction rate constant k, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; (b) dissolved oxygen changes in different systems, (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, (p-BQ)0 = 20 μM, without pH adjustment; (c) CV curves obtained under different conditions; (d) PCA removal in 310-MnO2/PMS system after surface manganese atom block and corresponding reaction rate constant k, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; adsorption energy of PMS over α-MnO2 with different exposed crystal facets: (e) 100-MnO2, (f) 110-MnO2, and (g) 310-MnO2.
Figure 8. (a) Effect of reaction solvents (H2O and D2O) and corresponding reaction rate constant k, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; (b) dissolved oxygen changes in different systems, (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, (p-BQ)0 = 20 μM, without pH adjustment; (c) CV curves obtained under different conditions; (d) PCA removal in 310-MnO2/PMS system after surface manganese atom block and corresponding reaction rate constant k, reaction conditions: (PCA)0 = 1.0 mM, (PMS)0 = 1.5 mM, (MnO2)0 = 0.1 g/L, without pH adjustment; adsorption energy of PMS over α-MnO2 with different exposed crystal facets: (e) 100-MnO2, (f) 110-MnO2, and (g) 310-MnO2.
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Figure 9. Three proposed pathways of PCA degradation in the 310-MnO2/PMS system.
Figure 9. Three proposed pathways of PCA degradation in the 310-MnO2/PMS system.
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Fu, J.; Gao, P.; Wang, L.; Zhang, Y.; Deng, Y.; Huang, R.; Zhao, S.; Yu, Z.; Wei, Y.; Wang, G.; et al. Regulating Crystal Facets of MnO2 for Enhancing Peroxymonosulfate Activation to Degrade Pollutants: Performance and Mechanism. Catalysts 2022, 12, 342. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12030342

AMA Style

Fu J, Gao P, Wang L, Zhang Y, Deng Y, Huang R, Zhao S, Yu Z, Wei Y, Wang G, et al. Regulating Crystal Facets of MnO2 for Enhancing Peroxymonosulfate Activation to Degrade Pollutants: Performance and Mechanism. Catalysts. 2022; 12(3):342. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12030342

Chicago/Turabian Style

Fu, Juncong, Peng Gao, Lu Wang, Yongqing Zhang, Yuhui Deng, Renfeng Huang, Shuaifei Zhao, Zebin Yu, Yuancheng Wei, Guangzhao Wang, and et al. 2022. "Regulating Crystal Facets of MnO2 for Enhancing Peroxymonosulfate Activation to Degrade Pollutants: Performance and Mechanism" Catalysts 12, no. 3: 342. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12030342

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