Peroxymonosulfate-Activation-Induced Phase Transition of Mn3O4 Nanospheres on Nickel Foam with Enhanced Catalytic Performance

The transformations of physicochemical properties on manganese oxides during peroxymonosulfate (PMS) activation are vital factors to be concerned. In this work, Mn3O4 nanospheres homogeneously loaded on nickel foam are prepared, and the catalytic performance for PMS activation is evaluated by degrading a target pollutant, Acid Orange 7, in aqueous solution. The factors including catalyst loading, nickel foam substrate, and degradation conditions have been investigated. Additionally, the transformations of crystal structure, surface chemistry, and morphology on the catalyst have been explored. The results show that sufficient catalyst loading and the support of nickel foam play significant roles in the catalytic reactivity. A phase transition from spinel Mn3O4 to layered birnessite, accompanied by a morphological change from nanospheres to laminae, is clarified during the PMS activation. The electrochemical analysis reveals that more favorable electronic transfer and ionic diffusion occur after the phase transition so as to enhance catalytic performance. The generated SO4•− and •OH radicals through redox reactions of Mn are demonstrated to account for the pollutant degradation. This work will provide new understandings of PMS activation by manganese oxides with high catalytic activity and reusability.


Introduction
Advanced oxidation processes (AOPs) with free radicals have received much attention for the various applications in environmental remediation, including degradation and mineralization of organic molecules [1], detoxification [2], and disintegration of sludge [3]. The sulfate radical (SO 4 •− ) provided tremendous potential as an advanced alternative to hydroxyl radical (•OH) due to its advantages including higher standard redox potential (E SO •− from the FT-IR spectrum shown in Figure S1. This result agrees with the pre ported Mn3O4, which is attributed to Mn-O stretching vibrations on octahedra hedral sites [22]. The broad absorption band at around 3400 cm −1 could be assig stretching vibration of the adsorbed hydroxyl group. These spectroscopic resu confirm the Mn3O4 phase.

Effect of Catalyst Loading
The samples with different catalyst loading were prepared using different dosages of manganese salt (Mn(CH3COO)2•4H2O). The prepared samples were denoted as Mn3O4/NF-M (M means the dosage of manganese salt). The mass loadings of Mn3O4/NF-0.12, Mn3O4/NF-0.24, and Mn3O4/NF-0.48 are about 0.3, 0.6, and 0.9 mg cm −2 , respectively. The SEM images of Mn3O4/NF-0.24 and Mn3O4/NF-0.12 are present in Figure S2. It can be observed that the Mn3O4 nanospheres aggregated by nanoparticles were uniformly distributed on the nickel foam surface. In contrast with the morphology of Mn3O4/NF-0.48, the Mn3O4 nanospheres exhibited decreasingly smaller dimensions with lower dosages of manganese salt. Figure 3a shows the AO7 degradation of different samples under the same conditions. The Mn3O4/NF-0.48 presented a better PMS activation performance than Mn3O4/NF-0.24 and Mn3O4/NF-0.12. A degradation efficiency of 94% could be achieved for Mn3O4/NF-0.48 within 30 min, which was 83.5% and 63% for Mn3O4/NF-0.24 and Mn3O4/NF-0.12, respectively. The results of AO7 degradation with the Mn3O4/NF system could be fitted to pseudo-first-order kinetics, as shown in Figure 3b. The degradation rate constant (k) of Mn3O4/NF-0.48 was estimated to be 0.095 min −1 , higher than those of Mn3O4/NF-0.24 (0.060 min −1 ) and Mn3O4/NF-0.12 (0.033 min −1 ). The better degradation performance of Mn3O4/NF-0.48 could be attributed to its raised catalyst loading.

Effect of Nickel Foam
The effect of nickel foam was evaluated by the comparison of the performance using blank nickel foam, Mn3O4 powder, and Mn3O4/NF-0.48. Figure 3c displays AO7 degrada-  Figure 3d shows the corresponding UV-vis spectra of the reaction system as a function of degradation time. The consecutive reduction in the absorbance at around 484 nm proved the destruction of azo chromophore.

Effects of Degradation Conditions
The effects of PMS concentration, initial pH value, and reaction temperature have been investigated in the Mn 3 O 4 /NF-0.48 system. As shown in Figure 4a, PMS concentration promotes the degradation efficiency significantly in the range of 0.2 to 1 mM oxone, which could be ascribed to the involvement of more reactive species activated from PMS. Once the concentration of PMS is higher than 1 mM, the positive effect on degradation tends to be slight. In the current reaction system, the PMS dosage is in plenty relative to the catalyst loaded on nickel foam. In consideration of the degradation efficiency and operation cost, 1 mM of PMS concentration can be more adequate for AO7 degradation.  The influence of the initial pH value on the degradation of AO7 was examined. As shown in Figure 4b, a weakening trend of degradation performance with an increasing initial pH value can be observed. It is reported that manganese oxides are generally negatively charged under neutral and basic conditions [30,32]. The electrostatic repulsion between the catalyst and PMS anion or anionic dye AO7 might be one of the reasons. In addition, the deprotonation of HSO5 − into SO4 2− and O2 under basic conditions would cause invalid PMS consumption [33]. Nonetheless, the system of Mn3O4/NF-0.48 with PMS shows efficiency in the pH range from 3 to 9, since the degradation efficiencies remain above 90%. The influence of the initial pH value on the degradation of AO7 was examined. As shown in Figure 4b, a weakening trend of degradation performance with an increasing initial pH value can be observed. It is reported that manganese oxides are generally negatively charged under neutral and basic conditions [30,32]. The electrostatic repulsion between the catalyst and PMS anion or anionic dye AO7 might be one of the reasons. In addition, the deprotonation of HSO 5 − into SO 4 2− and O 2 under basic conditions would cause invalid PMS consumption [33]. Nonetheless, the system of Mn 3 O 4 /NF-0.48 with PMS shows efficiency in the pH range from 3 to 9, since the degradation efficiencies remain above 90%.
The effect of the reaction temperature on AO7 degradation is displayed in Figure 4c, showing an accelerated degradation at higher temperature. The degradation rate constant at 30, 40, 50, and 60 • C can be deduced to be 0.085, 0.112, 0.139, and 0.178 min −1 . The relationship between the rate constants and reaction temperatures obeys the Arrhenius equation closely as shown in Figure 4d. The activation energy (E a ) is estimated to be 20.52 kJ/mol, which is at a lower level than those of AO7 degradation over some reported catalysts, for example, Mn 3 O 4 -rGO (49.5 kJ/mol) [25], MnFe 2 O 4 (31.7 kJ/mol) [34], Co/Bi 25 FeO 40 (51.3 kJ/mol) [35], and Co 3 O 4 /N-doped graphene (41.6 kJ/mol) [36]. It suggests that the Mn 3 O 4 /NF exhibits a low chemical reaction energy barrier as a promising catalyst for PMS activation. The influence of the stirring rate on AO7 degradation was explored. As shown in Figure S4, a degradation rate constant (k) of 0.094 min −1 could be achieved under the stirring rate of 200 rpm, which was 0.024 and 0.04 min −1 under the stirring rates of 50 and 400 rpm, respectively. Thus, a stirring rate of 200 rpm is suitable for mass transfer.  The catalyst after four cycles of repetitive usages was further investigated to make clear the reason for enhanced degradation performance. The XRD pattern in Figure 6a demonstrates a set of characteristic peaks that can be indexed to the standard contour of birnessite MnO2 (JCPDS no. 18-0802). The diffraction peaks at 2θ values of 18.7°, 36.8°, and 65.7° can be attributed to the (101), (006), and (119) planes of the layered structure. These weak diffraction peaks indicate the low crystallinity of the catalyst. The broad peak at the range of 20° to 30° may be ascribed to the adsorbed organic intermediates on the catalyst surface [37]. Figure 6b displays deconvoluted Raman bands at 508, 578, 652, and 720 cm −1 , which could be assigned to the vibrational features of birnessite manganese oxide [38]. Additionally, the band at 620 cm −1 may be raised due to the presence of MnOOH on the catalyst surface [39]. These results attest to a phase transition from spinel Mn3O4 to birnessite manganese oxide after PMS activation on AO7 degradation. The catalyst after four cycles of repetitive usages was further investigated to make clear the reason for enhanced degradation performance. The XRD pattern in Figure 6a demonstrates a set of characteristic peaks that can be indexed to the standard contour of birnessite MnO 2 (JCPDS no. 18-0802). The diffraction peaks at 2θ values of 18.7 • , 36.8 • , and 65.7 • can be attributed to the (101), (006), and (119) planes of the layered structure. These weak diffraction peaks indicate the low crystallinity of the catalyst. The broad peak at the range of 20 • to 30 • may be ascribed to the adsorbed organic intermediates on the catalyst surface [37]. Figure 6b displays deconvoluted Raman bands at 508, 578, 652, and 720 cm −1 , which could be assigned to the vibrational features of birnessite manganese oxide [38]. Additionally, the band at 620 cm −1 may be raised due to the presence of MnOOH on the catalyst surface [39]. These results attest to a phase transition from spinel Mn 3 O 4 to birnessite manganese oxide after PMS activation on AO7 degradation. catalyst surface [37]. Figure 6b displays deconvoluted Raman bands at 508, 578, 652, and 720 cm −1 , which could be assigned to the vibrational features of birnessite manganese oxide [38]. Additionally, the band at 620 cm −1 may be raised due to the presence of MnOOH on the catalyst surface [39]. These results attest to a phase transition from spinel Mn3O4 to birnessite manganese oxide after PMS activation on AO7 degradation. XPS was employed to analyze the evolution of the surface chemistry on the catalyst sample. From the Mn 3s core-level XPS spectra (Figure 7a), the energy separation of the two peaks was measured to be 5.70, 5.06, and 4.58 eV for pristine Mn3O4/NF-0.48 and the samples after one cycle and four cycles, respectively. This value has been proved to be linearly corelated to the mean valence state of Mn [40], which then can be estimated to be +2.50, +3.23, and +3.77 for the three samples, respectively. This verifies the increasing trend of the mixed Mn valence states during the repeated PMS activation on AO7 degradation. The shifts of the broad Mn 2p3/2 peak toward higher binding energy of the samples after one cycle and four cycles (Figure 7b) further confirm the oxidation of Mn. The O 1s spectra (Figure 7c) are deconvoluted into three bands which, respectively, correspond to the Mn- XPS was employed to analyze the evolution of the surface chemistry on the catalyst sample. From the Mn 3s core-level XPS spectra (Figure 7a), the energy separation of the two peaks was measured to be 5.70, 5.06, and 4.58 eV for pristine Mn 3 O 4 /NF-0.48 and the samples after one cycle and four cycles, respectively. This value has been proved to be linearly corelated to the mean valence state of Mn [40], which then can be estimated to be +2.50, +3.23, and +3.77 for the three samples, respectively. This verifies the increasing trend of the mixed Mn valence states during the repeated PMS activation on AO7 degradation. The shifts of the broad Mn 2p 3/2 peak toward higher binding energy of the samples after one cycle and four cycles (Figure 7b In particular, more than twice the content of the structure water on the surface was obtained after four cycles of degradation. The surface morphologies of the catalyst samples collected after the first and fourth cycles were observed using SEM. As shown in Figure 8a-c, the pristine Mn3O4 nanospheres in situ grew into a lamella structure. With an increasing cycle number, the lamellae exhibited a larger lateral diameter, and interconnected with each other assembling porous architecture (Figure 8d-f). The corresponding TEM image (Figure 8g)   ion diffusion [22]. This allows more active surface sites for the adsorption and reactions of the reactants, improving the catalyst activity.

Identification of Radicals
It has been reported that the organic pollutant degradation with PMS usually follows two different mechanisms, including radical and nonradical pathways. EPR was employed to identify the active species for catalysis. The radical signals captured by DMPO are shown in Figure 10. Compared with the weak signals without catalyst addition, high intensities of DMPO-•OH and DMPO-SO4 •− signals could be observed in the Mn3O4/NF-0.48/PMS system. Both signals exhibit enhanced intensities with the increase in reaction time from 5 min to 10 min. It suggests that both •OH and SO4 •− could be derived via catalysis during PMS activation and would be accumulated within the reaction time. In Based on the abovementioned results, it can be seen that Mn 3 O 4 catalyst samples suffer a chemical oxidation process after AO7 degradation with PMS activation, leading to a solid-phase transition from spinel to birnessite and morphological evolution from nanospheres to a lamella structure. The enhanced degradation performance of the catalyst with repeated use is most probably associated with the phase and morphology changes. The properties of electron transport and ion diffusion resistivity of the catalysts are analyzed using electrochemical impedance spectroscopy (EIS). Nyquist plots and the fitting lines of the Mn 3 O 4 /NF-0.48 and the sample after four cycles of AO7 degradation are compared in Figure 9a. The approximate semicircle at high frequency represents a charge-transfercontrolled process. The series resistance (R s ) and charge-transfer resistance (R ct ) are fitted to be 1.906 Ω and 9.858 Ω for Mn 3 O 4 /NF-0.48, while they change to be 1.797 Ω and 0.01 Ω after four cycles. The phase transition introduces minimal change to the intrinsic resistance of the catalyst but a great extent of reduction on the R ct . This indicates better efficient electron transfer at the interface, which could promote the reaction between the catalyst and PMS. The straight line at low frequency represents an ion-diffusion-controlled process. The higher slop of the line implies a more favorable ionic diffusion of the catalyst after four cycles. The real part of impedance (Z') is plotted versus the reciprocal of the square root of frequency (ω −0.5 ) in the intermediate frequency range, which can derive the ion diffusion resistance (σ) through the slope of linear fitting (Figure 9b). The σ value is found to decrease considerably from 9.418 Ω/s −0.5 to 0.253 Ω/s −0.5 after four cycles. The decrease in σ may be ascribed to the lamella structure, which offers open channels for facile ion diffusion [22]. This allows more active surface sites for the adsorption and reactions of the reactants, improving the catalyst activity.

Identification of Radicals
It has been reported that the organic pollutant degradation with PMS usually follows two different mechanisms, including radical and nonradical pathways. EPR was employed to identify the active species for catalysis. The radical signals captured by DMPO are shown in Figure 10. Compared with the weak signals without catalyst addition, high intensities of DMPO-•OH and DMPO-SO4 •− signals could be observed in the Mn3O4/NF-0.48/PMS system. Both signals exhibit enhanced intensities with the increase in reaction time from 5 min to 10 min. It suggests that both •OH and SO4 •− could be derived via catalysis during PMS activation and would be accumulated within the reaction time. In addition, the nonradical 1 O2 signal captured using TEMP is present in Figure S5. The TEMP-1 O2 signal in the Mn3O4/NF-0.48/PMS system showed roughly the same intensity

Identification of Radicals
It has been reported that the organic pollutant degradation with PMS usually follows two different mechanisms, including radical and nonradical pathways. EPR was employed to identify the active species for catalysis. The radical signals captured by DMPO are shown in Figure 10. Compared with the weak signals without catalyst addition, high intensities of DMPO-•OH and DMPO-SO 4 •− signals could be observed in the Mn 3 O 4 /NF-0.48/PMS system. Both signals exhibit enhanced intensities with the increase in reaction time from 5 min to 10 min. It suggests that both •OH and SO 4 •− could be derived via catalysis during PMS activation and would be accumulated within the reaction time. In addition, the nonradical 1 O 2 signal captured using TEMP is present in Figure S5. The TEMP-1 O 2 signal in the Mn 3 O 4 /NF-0.48/PMS system showed roughly the same intensity as that of the raw PMS system without catalyst addition. It indicates that nonradical 1 O 2 formed through spontaneous PMS decomposition, which presented little contribution to the AO7 degradation based on the above results under the raw PMS condition. Therefore, radical •OH and SO 4 •− were the contributing species for the AO7 degradation in the Mn 3 O 4 /NF-0.48/PMS system. as that of the raw PMS system without catalyst addition. It indicates that nonradical 1 O2 formed through spontaneous PMS decomposition, which presented little contribution to the AO7 degradation based on the above results under the raw PMS condition. Therefore, radical •OH and SO4 •− were the contributing species for the AO7 degradation in the Mn3O4/NF-0.48/PMS system.

The Mechanism of PMS Activation and Phase Transition
Based on above results, the phase transition induced by PMS activation was schematically proposed in Figure 11. Firstly, Mn(II) can produce SO4 •− by reacting with HSO5 − (Equation (1)). The oxidation reaction of Mn(II) may induce its extraction from tetrahedral sites, which is similar to the dissolution of Mn(II) form Mn3O4 matrix under electrochem-

The Mechanism of PMS Activation and Phase Transition
Based on above results, the phase transition induced by PMS activation was schematically proposed in Figure 11. Firstly, Mn(II) can produce SO 4 •− by reacting with HSO 5 − (Equation (1)). The oxidation reaction of Mn(II) may induce its extraction from tetrahedral sites, which is similar to the dissolution of Mn(II) form Mn 3 O 4 matrix under electrochemical oxidation conditions in aqueous solution [41,42]. Meanwhile, Mn(III) at the octahedral sites can be oxidized to Mn(IV), while one fourth of octahedron Mn cations migrate to the (101) plane of spinel, leading to the in situ formation of a layered structure [43]. Additionally, Mn at the higher valence states can then be reduced by HSO 5 − to generate SO 4 •− (Equations (2) and (3)). The generated SO 4 •− can be readily converted to •OH via the oxidation of water (Equation (4)). Based on the surface chemistry of the catalyst after repeated cycles, the extracted Mn(II) and free H 3 O + may fill into the MnO 6 layer gap to recover the charge balance after the ion extraction and stabilize the structure [44]. These atomic movements during the solid-phase transition produce a large strain between the phase edges, which could cause the grown birnessite lamella to peel off from the Mn 3 O 4 surface [45], leading to morphological change.
Molecules 2023, 28, x FOR PEER REVIEW 11 of 14 Figure 11. Scheme for the phase transition induced by PMS activation.

Synthesis of Mn3O4 Nanospheres on Nickel Foam
Mn3O4 nanospheres were fabricated on nickel foam via a facile hydrothermal route, which was modified based on a reported recipe [46]. In brief, a precursor solution was firstly prepared through the dissolution of 0.48 g Mn(CH3COO)2•4H2O into 3 g ethanol with 1 g deionized water under vigorous stirring for 15 min, followed by the addition of 13 mL ethylene glycol with continuous stirring for 30 min. A piece of pretreated nickel foam (~320 g m −2 , 4 cm × 3.5 cm × 0.1 cm) was dipped into the above precursor solution. The pretreatment of nickel foam was conducted via drying after successive ultrasonic cleaning in acetone, ethanol, and deionized water, respectively. The solution and the nickel foam were statically aged for 3 days and then transferred into an autoclave for a hydrothermal reaction at 170 °C for 5 h. Finally, the nickel foam sample was removed and rinsed with deionized water and ethanol, and dried in air at 50 °C. For comparison, blank nickel foam and Mn3O4 powder were prepared using the same route but without the ad-

Synthesis of Mn 3 O 4 Nanospheres on Nickel Foam
Mn 3 O 4 nanospheres were fabricated on nickel foam via a facile hydrothermal route, which was modified based on a reported recipe [46]. In brief, a precursor solution was firstly prepared through the dissolution of 0.48 g Mn(CH 3 COO) 2 ·4H 2 O into 3 g ethanol with 1 g deionized water under vigorous stirring for 15 min, followed by the addition of 13 mL ethylene glycol with continuous stirring for 30 min. A piece of pretreated nickel foam (~320 g m −2 , 4 cm × 3.5 cm × 0.1 cm) was dipped into the above precursor solution. The pretreatment of nickel foam was conducted via drying after successive ultrasonic cleaning in acetone, ethanol, and deionized water, respectively. The solution and the nickel foam were statically aged for 3 days and then transferred into an autoclave for a hydrothermal reaction at 170 • C for 5 h. Finally, the nickel foam sample was removed and rinsed with deionized water and ethanol, and dried in air at 50 • C. For comparison, blank nickel foam and Mn 3 O 4 powder were prepared using the same route but without the addition of manganese salt or nickel foam, respectively.

Characterization
The X-ray diffraction (XRD, SmartLab, Rigaku, Tokyo, Japan) was conducted using Cu-Kα radiation (λ = 1.5406 Å). Raman scattering was recorded using a LabRAM HR Evolution Raman spectrometer with a 532 nm laser (Horiba Jobin-Yvon, Villeneuve d'Ascq, France). Fourier transform infrared (FT-IR) was performed using a Fourier transform infrared spectrometer (Shimadzu IRAffinity-1S, Kyoto, Japan). The morphology was observed via field emission scanning electron microscopy (FESEM, Hitachi S4800, Kyoto, Japan) and field-emission transmission electron microscopy (TEM, FEI Tecnai TF20, Hillsboro, OR, USA). The elemental energy dispersive spectroscopy (EDS) analysis was conducted using an Oxford EDS detector. X-ray photoelectron spectroscopy (XPS) was performed using a monochromatic Al Kα X-ray source with photon energy of 1486.6 eV (Thermo Scientific K-Alpha, ThermoFisher, Waltham, MA, USA). Electrochemical impedance spectroscopy (EIS) was performed on a CHI760E electrochemical workstation (Chenhua, China) with a Pt wire counter electrode and an Ag/AgCl (3 M KCl) reference electrode. An electron paramagnetic resonance spectrometer (EPR, JEOL JES FA200, Tokyo, Japan) was used to identify the radical species with DMPO and TEMP spin-trapping agents.

Evaluation of Catalytic Activity
The catalytic activity was evaluated using the degradation of AO7 under a constant stirring of 200 rpm with the nickel foam sample at room temperature. In a typical procedure, the nickel foam sample was firstly immersed into 100 mL 20 mg L −1 of AO7 solution in a 250 mL beaker. After constant stirring for 30 min, 1 mmol/L oxone (2KHSO 5 •KHSO 4 •K 2 SO 4 ) was added to initiate the reaction. At given time intervals, 2 mL solution samples were collected, followed by the addition of 2 mL ethanol for quenching of the reaction. The concentration of AO7 was detected using a UV-vis spectrophotometer (Shimadzu UV-1800, Japan) at 485 nm. For comparison, the catalytic activity of blank nickel foam and Mn 3 O 4 powder was evaluated through the same route. The effect of pH was investigated using 1 M HCl or 1 M NaOH. The stability of the catalyst was evaluated through reutilization of the nickel foam sample after washing and drying within four successive degradation cycles.

Conclusions
In summary, Mn 3 O 4 nanospheres homogeneously loaded on nickel foam are prepared using a simple hydrothermal route for PMS activation to degrade AO7. The Mn 3 O 4 /NF/PMS system displays favorable catalytic activity and an enhanced degradation rate with repeated usage. A transition from spinel nanospheres to birnessite laminae driven by PMS is revealed, which can lead to more favorable electronic transfer and ionic diffusion. The phase transition is proposed to be comprised of Mn extraction, rearrangement, and the insertion of cations between layers, accompanied by redox reactions of Mn to generate active radicals of SO 4 •− and •OH. This work will provide new insights into PMS activation by manganese oxides.