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Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 2396−2403

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Fabrication of Tubelike Co3O4 with Superior Peroxidase-Like Activity and Activation of PMS by a Facile Electrospinning Technique Guoshuai Liu,†,‡ Hongshui Lv,*,† Haiyan Sun,*,† and Xiangzhu Zhou§ †

School of Paper-making and Botanical Resources Engineering, Key Lab of Pulp and Paper Science & Technology, Ministry of Education (Shandong Province), and §School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, Shandong 250353, P. R. China ‡ State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, P. R. China S Supporting Information *

ABSTRACT: Tubelike Co3O4 was successfully prepared by a facile electrospinning route combined with a subsequent calcining treatment. The as-prepared tubelike Co3O4 was exploited as peroxidase mimics and to activate peroxymonosulfate (PMS) to produce sulfate radicals. The tubelike Co3O4 exhibited intrinsic peroxidase-like activity and high ability of activating PMS to degrade 2,4-dichlorophenol compared with the powders for the high specific surface area and surface Co2+ and adsorbed oxygen contents. Furthermore, the electronic properties of Co3O4 and the catalytic mechanism on H2O2 and PMS were investigated by electron spin resonance test and density functional theory calculation. This study details the insights into Co3O4 nanomaterials for activation of peroxides and application in sustainable remediation. procedures need to be applied to remove the templates.17,18 Therefore, a route to fabricate tubelike Co3O4 structures with low cost and simple procedure is a great challenge. Among the synthesis methods of nanomaterials, electrospinning is a convenient method to perform the controllable fabrication of different oxide semiconductors with uniform characteristics, high reproducibility, and low cost.21,22 Meanwhile, for the catalysts, the as-prepared long and continuous one-dimensional architectures will make them easier to be recycled. Furthermore, the fibers generally own high specific surface areas and pores, which will greatly increase the utilization rate of nanomaterials and then enhance the catalytic performances.23 Herein, we present a simple procedure to prepare tubelike Co3O4 structures just by annealing Co(NO3)2/PVP fiber which was simply fabricated by an electrospinning technique. The fiber exhibited beltlike structure. During the annealing process, the fiber transformed to tubelike structures from microbelts. The crystalline structure and morphology of as-prepared sample were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Brunauer−Emmett−Teller (BET) specific surface area was also calculated by the measured nitrogen adsorption−desorption isotherms. The tubelike Co3O4 prepared by electrospinning were implemented in

1. INTRODUCTION In recent years, reactive radicals, such as ·OH, SO4•−, and O2•−, have been applied to degrade organic pollutants. These radicals are usually generated from scission of peroxide bond by special catalysts.1 Nowadays, the activation of peroxide (H2O2, peroxymonosulfate (PMS)) coupled with transition metals by homogeneous or heterogeneous catalytic oxidation to produce reactive radicals has attracted numerous research and analysis studies.1−7 However, the scalable application of homogeneous catalytic reaction was limited because of the toxicity of metal ion which would pollute the water.1 To avoid this disadvantage, many heterogeneous reagents have been proposed in PMS activation. Most of the heterogeneous catalysts for activation of PMS are cobalt-based compounds, such as Co3O4, CoO, NiCo2O4, and CoFe2O4.2−7 Co3O4, with a structure of Co2+ and Co3+ ions, was reported to have a superior performance in activating PMS.8 When being used to activate PMS to degrade organic pollutants, it exhibited pronounced activation at neutral pH for its fast degradation rate, low leaching Co ion concentration, and good stability for multiple degradation runs.2,9 In particular, it can be easily recovered with only a magnet. Furthermore, the catalysts based on Co3O4 nanomaterials exhibit size- and structure-dependent properties,10,11 which impels researchers to explore facile routes to prepare Co3O4 nanomaterials with novel morphologies and unique structures. Up to now, diverse Co3O4 nanomaterials have been reported, such as nanocrystals,12 nanosheets,13−15 nanobelts,16 nanotubes,17,18 and hierarchical structures.19,20 As to the synthesis of tubelike Co3O4 structures in the previous report, sacrificial templates were always involved, so necessary © 2018 American Chemical Society

Received: Revised: Accepted: Published: 2396

October 8, 2017 January 13, 2018 January 29, 2018 January 29, 2018 DOI: 10.1021/acs.iecr.7b04180 Ind. Eng. Chem. Res. 2018, 57, 2396−2403

Article

Industrial & Engineering Chemistry Research

mL). The reaction was performed for 120 min. At a fixed time interval, 0.5 mL of solution sample was taken from the mixture using a syringe with a filter of 0.45 μm, and the concentration of 2,4-DCP was determined by using high-performance liquid chromatography (HPLC) consisting of UV−vis detector (L2420) and a reversed-phase column of Luna C18 according to the methods reported in the literature.24 Calculation Details and Models. All the calculations were performed with the DMol3 program package in Materials Studio of Accelrys Inc. In the computation, exchange correlation effects were described by the generalized gradient approximation (GGA) developed by Perdew, Burke, and Ernzerhof (PBE).24 The molecular orbitals were expanded into a double-numerical basis with polarization functions (DNP). For O atoms, all-electron basis sets were used. Effective core potentials were used for Co atoms; that is, the outer electrons (3d74s2) were treated as valence electrons, and the remaining electrons were replaced by effective core potentials. A Fermi smearing of 0.01 Hartree and a cutoff energy of 4.5 Å were used. The Brillouin zone integrations were performed using a 5 × 5 × 5 Monkhorst-pack grids, and the self-consistent field convergence criterion was set to be an energy change of 10−6 Hartree. The convergence criteria of optimal geometry based on the energy, force, and displacement convergence were 1 × 10−5 Hartree, 2 × 10−3 Hartree/Å, and 5 × 10−3 Å, respectively.

peroxidase-like activity and activating PMS for the degradation of 2,4-dichlorophenol (2,4-DCP), showing higher peroxidaselike and PMS activation activity than the control powders. Furthermore, the catalytic activity of tubelike Co3O4 was analyzed by electron spin resonance (ESR) test and the density functional theory (DFT) calculation, and the possible catalytic mechanism was also proposed.

2. MATERIALS AND METHODS All the chemicals used were of analytical reagent grade. Synthesis. In the synthesis of the spinnable solution, 1.0 g poly(vinylpyrrolidone) (PVP, Mw = 1 300 000) was dissolved in 10 mL of ethanol with magnetic stirring. When PVP was dissolved completely, 2 mL of aqueous solution containing 0.23 g of Co(NO3)2·6H2O was added. The mixture was magnetically stirred overnight to make it homogeneous. Then the mixture was transferred to a needle tube with the inner diameter of 0.5 mm. The electrospinning voltage was set as 20 kV, and the receiving distance between needle and collector was 20 cm while the pumping speed was 0.003 mm/s. After the electrospinning process, the matlike product was collected and heated at 80 °C overnight. Finally, the dried matlike product was treated in a muffle furnace to 300 °C with heating rate of 1 °C/min and kept at 300 °C for 1 h and then heated to 450 °C with heating rate of 0.5 °C/min, being kept at that temperature for 2 h. In addition, part of the spinnable solution was dried at 80 °C for a few days to make the solvent volatilize completely. Then, the control powders were obtained by being calcined in a muffle furnace with the same heat procedures to prepare tubelike Co3O4 structures. Characterization. The X-ray diffraction (XRD) patterns of the samples were collected at room temperature on X-ray diffractometer (Bruker D8 Advance) with a graphite monochromator and CuKα (λ = 0.15418 nm) radiation. The morphology and microstructure of the products were characterized using a transmission electron microscope (TEM, JEOL JEM-2100) and scanning electron microscope (SEM, FEI Quanta 200). X-ray photoelectron spectroscopy (XPS) was conducted on PHI Quantera SXM (UlVAC-PHI) with an Al Kα as the X-ray source and corrected with the C 1s line at 284.8 eV. Electron spin resonance (ESR) measurement was performed on an ESR spectrometer (Bruker EMX) at room temperature with 5,5′-dimethyl-1-pyrrolline N-oxide (DMPO) as a spin trap. The nitrogen adsorption−desorption isotherms were measured on an apparatus (V-Sorb2800p) at liquid nitrogen temperature (T = −196 °C). Peroxidase-Like Activity Measurement. The reaction was performed in 3 mL of acetate buffer (0.2 M, pH = 4.0). First, 36 μg of catalyst was dispersed in the above solution and sonicated for 5 min. Then, 150 μL (6.2 μmol) of 3,3′,5,5′tetramethylbenzidine (TMB, 10 mg/mL, with dimethyl sulfoxide as solvent) and 160 μL (5.2 μmol) of H2O2 (30 wt %) were added as substrates. After reactiion for a certain time, the absorbance of TMB-derived oxidation product was examined at 652 nm on a UV−vis spectrometer (Perkin Elmer Lambda 35). The peroxidase-like activity was evaluated by absorbance data. Activation of PMS and the Degradation Experiment. In a typical experimental procedure, the degradation of 2,4DCP was studied and compared at pH-neutral condition (pH 7.2 ± 0.2) to reflect the behavior of Co3O4 tubes (200 mg·L−1) for activation of PMS (150 mg·L−1). It was carried out in a 1 L glass beaker containing 25 mg·L−1 of 2,4-DCP solutions (500

3. RESULTS AND DISCUSSION The Crystalline Structure and Morphology of Co3O4 Tubes. The crystal structure of the obtained product was determined by XRD. As shown in Figure 1a, all the diffraction peaks are matched well with those of cubic Co3O4 (JCPDS no.43-1003). The diffraction peaks at 31.3°, 36.9°, 38.6°,44.9°, 55.7°, 59.4°, and 65.3° are indexed to (220), (311), (222), (400), (422), (511), and (440) reflections. There are no diffraction peaks of other impurities, indicating the pure phase of the product. According to Scherrer line width analysis on (311) reflection, the crystalline size of the sample is estimated ca. 38.9 ± 2.8 nm. The morphologies of PVP/cobalt nitrate and those after calcination at 400 °C were characterized by SEM and TEM. As shown in Figure 1b, after electrospinning process, the product which was named precursor presents beltlike structure with width of ca. 1.7 ± 0.5 μm, and the length of an individual belt is up to hundreds of micrometers. The surface of the belt is smooth for the amorphous nature of PVP/cobalt nitrate. In order to remove the organics completely in the precursor, it was treated in a muffle furnace to 300 °C with heating rate of 1 °C/min and kept at 300 °C for 1 h and then heated to 450 °C with heating rate of 0.5 °C/min, being kept at that temperature for 2 h. As presented in Figure 1c, the calcination did not destroy the one-dimensional structure of the fibers. The cross section in the SEM image (inset in Figure 1c) and the obvious contrast between the edge and center in the TEM image (Figure 1d) demonstrate the hollow structure of the fibers. The diameter of the tubes is ca. 210 nm after the removal of PVP, and the thickness of the wall of the tubes is ca. 25 nm. From the highly magnified SEM image (inset in Figure 1c), the nanoparticles comprising the tubes are irregular, and their size is in the range of 20−30 nm. The particle size seems to be a little smaller than that calculated by the Scherrer equation from the XRD patterns, which may be due to the assembly of several nanoparticles.25 Therefore, it is speculated that the wall of the 2397

DOI: 10.1021/acs.iecr.7b04180 Ind. Eng. Chem. Res. 2018, 57, 2396−2403

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Industrial & Engineering Chemistry Research

optimal response, the effects of reaction pH, temperature, and concentration of the catalyst on the peroxidase-like activity of the tubes were studied. As shown in Figure S2, the tubes present catalytic activity at a wide range of pH and temperature, demonstrating their high temperature resistance and alkali resistance as a peoxidase mimic. From Figure S2, tubelike Co3O4 presents the best peroxidase-like activity at 4 and 35 °C for pH and temperature. The concentration of the catalyst was set to be 12 mg·L−1, for the reaction velocity increased slowly with additional catalyst (Figure S2c). At the optimized condition, the peroxidase-like activity of the tubes was measured. For the contrast, Co3O4 powder was obtained by drying the spinnalbe solution followed by heat treatment with the same procedure as that to prepare the tubes. XRD patterns of the powder are presented in Figure S3.Their catalytic activities can be compared by the absorption at 652 nm. As shown in Figure 3a, in the absence of catalyst, TMB can nearly not be oxidized by H2O2, while the catalytic activity of tubes is 2.4 times that of powders, which may be due to its higher specific surface area and more active sites than the powders. Additionally, stability is also important to evaluate a catalyst. In the experiment, Co3O4 tubes were used 10 times to test their stability, and the peroxidase-like activity was recorded in each cycle. From Figure 3b, the peroxidase-like activity of the tubes decreased to 84.2% of the initial, demonstrating their superior stability. Co3O4 was reported to catalyze the decomposition of H2O2 into reactive hydroxyl radical (·OH),30,31 so ESR spin-trapping technique is employed to detect ·OH. In the measurement, DMPO was used as a spin trap. From the ESR spectra shown in Figure 4, without any catalyst, nearly no signals are detected, while in the presence of the catalyst, there are four lines with intensity ratio of 1:2:2:1, which is the characteristic ESR signal of nitroxide radical generated from the reaction between ·OH and DMPO.32,33 From the above analysis, it is speculated that in the catalytic reaction, Co3O4 first break down H2O2 to ·OH, and then the highly reactive ·OH oxidizes TMB to its oxidation form. Therefore, Co3O4 presents intrinsic peroxidase-like activity. Additionally, the line intensity of the tubes is stronger than that of the powders, demonstrating that the tubes own the higher ability to generate ·OH, which is coincident with their peroxidase-like activities. Activation of PMS and Catalytic Oxidation of 2,4-DCP. The catalytic performance of Co3O4 tubes was studied by activation of PMS for removal of 2,4-DCP. Figure 5a presents the removal profiles of 2,4-DCP under various reaction systems. A control test using only Co3O4 tubes or the powders shows that less than 8% of 2,4-DCP was adsorbed in 50 min. The adsorption amount of 2,4-DCP by Co3O4 tubes is almost coincident with that by the Co3O4 powders, revealing that their adsorption capacity is not responsible for the degradation of 2,4-DCP and only Co3O4 nanomaterials cannot degrade 2,4DCP directly. In addition, only PMS also could not induce significant 2,4-DCP degradation, for less than 12% of 2,4-DCP was degraded, indicating that thermal activation of PMS for production of reactive radicals was negligible at the current reaction conditions. The simultaneous presence of Co3O4 nanomaterials and PMS led to significant enhancement of 2,4-DCP removal efficiency compared to the adsorption and oxidation with PMS alone. For Co3O4 powder sample, the removal efficiency of 2,4-DCP is 72% at the reaction time of 50 min, whereas the removal efficiency by the tubes can reach as high as 97% within 50 min in the same period. Furthermore,

Figure 1. SEM (b) image of PVP/cobalt nitrate, XRD pattern (a), SEM (c), TEM (d), and HR-TEM (e) images of the obtained tubes after calcination. The inset in (c) is the high magnification of a single tube.

tubes may be composed of a nearly single layer of nanoparticles. The HR-TEM image (Figure 1e) shows the clear continuous parallel lattice fringes, indicating the good crystallization of the fibers. The lattice spacings, ca. 0.24 and 0.29 nm, are consistent with the (311) and (220) interplanar spacings of Co3O4. Besides, XPS analysis was also performed to characterize the surface composition and element species of the tubelike Co3O4. As shown in Figure S1a, there are Co, O, and C elements in the sample. Figure 2a is the typical Co 2p spectrum with two sharp peaks with spin−orbit splitting of 15.1 eV and additional satellite peaks, which is well consistent with that reported.26,27 The peak with binding energy at 794.7 eV corresponded to Co 2p1/2, whereas the other with binding energy at 779.6 eV is attributed to Co 2p3/2. Two kinds of Co species were fitted into two doublets by a Gaussian method. The peaks located at 795.5 and 780.6 eV are ascribed to tetrahedral Co2+, while those at 794.3 and 779.2 eV are assigned to octahedral Co3+.28 Figure 2b present the high-resolution O 1s spectrum, which was deconvoluted into two components, O1 (529.6 eV) and O2 (531.6 eV). The O1 peak belongs to the lattice oxygen, whereas the O2 peak is attributed to the surface adsorbed oxygen.29 The XPS results further confirm the formation of Co3O4 with spinel structure. Peroxidase-Like Activity of Co3O4 Tubes. The peroxidase-like activity of the as-prepared tubelike structure was investigated with TMB and H2O2 as substrates. To acquire an 2398

DOI: 10.1021/acs.iecr.7b04180 Ind. Eng. Chem. Res. 2018, 57, 2396−2403

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Industrial & Engineering Chemistry Research

Figure 2. XPS spectra of Co3O4 tubes before and after the activation reactions of H2O2 (a, b) and PMS (c, d).

Figure 3. Images of the relative activity (a) and cycling test result (b) of Co3O4 samples.

The degradation efficiency of 2,4-DCP increased from 79% to 97% to 100% with the catalyst concentration from 100 mg·L−1 to 250 mg·L−1 (Figure S4), and the corresponding k values were 0.031, 0.067, and 0.074 min−1, respectively. These results confirmed that (i) the tubelike Co3O4 catalyst played a key role in activating PMS for enhanced degradation of 2,4-DCP, and (ii) such enhancement should originate from the catalytic activity of tubelike Co 3O4 catalyst. For a degradation experiment, the efficient mineralization of organic dyes and pollutants is very important for preventing secondary pollution. In order to study the mineralization, TOC (total organic carbon) for 2,4-DCP solution with the catalytic oxidation by Co3O4 tubes activating PMS was also investigated. As shown in Figure S5, the TOC removal can reach ca. 62% after 50 min of irradiation. The high TOC removal indicates that the degradation of 2,4-DCP is mainly caused by the reactive free radical rather than the simple decolorization.

the degradation of 2,4-DCP follows a pseudo-first-order reaction, and its kinetics can be described as ln(C0/C) = kt, where C (mg·L−1) is the concentration of 2,4-DCP at a special reaction time (t, min), C0 (25 mg·L−1) is the initial concentration of 2,4-DCP, and k (min−1) is the apparent kinetic constant. The k values derived from Figure 5a follow the order: the tubes (0.067 min−1) > the control powders (0.030 min−1), as shown in Figure 5b. The result of the catalytic oxidation of 2,4-DCP shows that Co3O4 tubes owned higher activation efficiency toward PMS compared with the powders. The catalytic activation of PMS and the degradation performance were better than those reported in the literature, and the data are summarized in Table 1. As it shows, the facile prepared tubelike Co3O4 catalyst owned a relatively high degradation performance toward 2,4-DCP organic pollutant. We also examined the dependence of 2,4-DCP degradation on the dosage of catalyst based on the reaction time of 50 min. 2399

DOI: 10.1021/acs.iecr.7b04180 Ind. Eng. Chem. Res. 2018, 57, 2396−2403

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Industrial & Engineering Chemistry Research

Table 1. Catalytic Activation Comparison List of the CobaltBased Catalysts pollutants and concentration

system PMS/Co3O4 PMS/Co3O4 PMS/Co3O4− rGO/PMS PMS/Co3O4− KIT6 PMS/Co3O4− SBA15 PMS/Co3O4 tubes PMS/Co3O4 powders

orange G, 32 mg/L methylene blue, 32 mg/L phenol, 20 mg/L

kinetic constant (k, min−1)

ref

0.0367 0.0306

5 5

0.042

4

0.079

3

0.045

3

chloramphenicol, 100 mg/L chloramphenicol, 100 mg/L 2,4-DCP, 25 mg/L

0.067

2,4-DCP, 25 mg/L

0.030

this work this work

Figure 4. ESR spectra of hydroxyl radical for different samples.

The ESR measurements were performed using DMPO as trapping agent. As shown in Figure 6, there is no observation of definable ESR peaks for blank experiments where Co3O4 and PMS were not added. When Co3O4 tubes were added in the PMS solution, the DMPO−SO4•− adduct appeared in accordance with the hyperfine splitting constants of αN = 13.2 G, αH = 9.6 G, αH = 1.48 G, and αH = 0.78 G. The quartet lines with peak strength of 1:2:2:1 and hyperfine coupling constant of αN = 1.49 mT and αH = 1.49 mT were ascribed to the typical signals of DMPO−·OH adduct. The appearance of · OH radicals might result from the oxidation of H2O by SO4•− radicals.34−36 This provides a direct evidence for the formation of SO4•− radicals, indicating that Co3O4 tubes can activate PMS effectively to produce sulfate radical which is responsible for the degradation of 2,4-DCP. Based on the catalytic results of H2O2 and PMS by Co3O4 tubes and powders, the peroxidase-like activity and the PMS activation ability of Co3O4 tubes were higher than those of the powder samples, which might be due to their unique features. As characterized by nitrogen adsorption−desorption tests (Figure S6), the specific surface for the tubes was around 35.0 m2·g−1, while that for the control powder sample was about 20.0 m2·g−1. That is, the specific surface area of the tubes was 1.75 times that for the powder. The kinetic content of tubelike CO3O4 is 2.2 times that for the powder. Therefore, the

Figure 6. ESR test of sulfate radical for different samples.

higher specific surface area of the tubelike Co3O4 increased the adsorption of 2,4-DCP and provided more reaction sites than the powder sample, which favored improving its catalytic activity. Besides, it is reported that the adsorbed oxygen combined with Co2+ to form CoOH+ is the rate-limiting step in activating PMS reactions.37−39 It could be concluded that the contents of Co2+ and adsorbed oxygen played key roles to activate PMS. As to XPS results shown in Figure 2 and Figure

Figure 5. Degradation of 2,4-DCP by Co3O4 activation of PMS (a) and its kinetic constant (b). 2400

DOI: 10.1021/acs.iecr.7b04180 Ind. Eng. Chem. Res. 2018, 57, 2396−2403

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Industrial & Engineering Chemistry Research S7, the content of surface Co2+ and the relative adsorbed oxygen contents in the as-prepared Co3O4 tubes are higher than those of the control powders, which might be another factor to result in high ability to activate PMS. From this view of point, although the specific surface area of Co3O4 is lower than that reported by Deng et al. and both Co2+ constants were comparable, the ability to activate PMS of the tubelike Co3O4 was a little superior, which might be due to the relatively high adsorbed oxygen constant. From the above analysis, it is concluded that the synergistic effects of surface Co2+ and adsorbed oxygen constants and specific surface area make 1D electrospun Co3O4 tubes exhibit enhanced catalytic activity. Furthermore, the stability of the catalyst is important for its practical application, so the stability performance of the tubes in the activation of PMS was investigated, as shown in Table S1. During the five-cycle reaction period, the tubelike Co3O4 was found to decompose 2,4-DCP stably, achieving the maximum removal efficiency of 96% within each circle. Meanwhile, as measured by ICP-OES, there was no observation of metal leaching throughout the experiments, and the cobalt ions were undetectable in the aqueous solution, which might be due to the high crystallinity of the tubelike CO3O4.4 Additionally, XPS was implemented again to illustrate the structure of the samples after the catalytic reaction. By quantitatively analysis of Co 2p and O 1s spectra, the surface cobalt and oxygen compositions of Co3O4 before and after reaction are presented in Figure 2. The surface atomic ratio of Co2+ and Co3+changed from 2.09 to 2.03 and 2.02 after Co3O4 tubes were performed in the activation of H2O2 and PMS cycling reactions, respectively. The very slight change of Co3+/ Co2+ ratio indicated that most Co2+ was regenerated from Co3+ reduction, implying that the tubelike Co3O4 catalyst maintained the excellent activation potential after the cycling test.23 The fitting of O 1s spectra of Co3O4 tubes after reaction reveals that the relative molar ratio of O1 and O2 is a little lower than that before. Thus, the balance of Co3+/Co2+ and O1/O2 couples made the outstanding stability of tubelike Co3O4 catalyst during the cycling tests. Therefore, the good stability of the tubelike Co3O4 was mainly due its high crystallinity and special surface chemical state. In addition, the unique magnetic property of Co3O4 is favorable for recycling, which also ensures the stability of the catalyst. The mechanism between the high catalytic efficiency and magnetic property will be discussed below. Mechanism of the Peroxidase-Like Activity and Activating PMS. In order to clarify the catalytic mechanism of Co3O4 on H2O2 and PMS in detail, the DFT calculation was taken. Co3O4 has two distinct Co sites in a two-formula-unit face-centered-cubic (fcc) primitive cell. One site is occupied by a Co2+ ion, which is surrounded by a tetrahedral O2− coordination sphere (CoT), while another site is occupied by Co3+ ions and is in an octahedral environment (CoO) surrounded by six O2− ions (Figure 7a). The tetrahedrally coordinated Co2+ ion has a magnetic moment of 3.20 μB, slightly more than the expected spin-only value of 3 μB. In contrast, Co3+ is diamagnetic in nature. Therefore, the magnetic properties of Co3O4 are governed by the Co2+ ions, which are located in the [111] plane of the unit cell (Figure 7b),40,41 and the magnetic property of Co3O4 is beneficial for the catalyst to be recycled, which guarantees the stability of Co3O4. Figure S8a and S8b shows band structure and density state of Co3O4, which is consistent with the previous report.42 We further investigate the intricate interactions between the peroxides (H2O2 and PMS) and Co3O4 catalysts. As Table 2 shows, the

Figure 7. (a) Co3O4 face-centered-cubic unit cell of Co3O4 with two nonequivalent Co ions: Co2+ with tetrahedrally coordinated oxygens (CoT) and Co3+ with octahedrally coordinated oxygens (CoO). (b) Model of Co3O4 magnetic structure of Co3O4, where CoT (blue and yellow spheres) located in the [111] plane interact antiferromagnetically. (bottom) Schematic diagram showing the expected crystal field splittings of (left) a Co2+ ion in a tetrahedral field (left) and (right) a Co3+ ion in an octahedral field (right).

Table 2. Adsorption Energy Eads, Electron Transfer between Bulk Co3O4 and the Adsorbed Molecule Q, and the O−O Bond Length (lO−O) of H2O2 and PMS free H2O2 free PMS adsorbed on Co3O4 (H2O2) adsorbed on Co3O4 (PMS)

Eads (KJ·mol−1)

Q (e)

lO−O (Å)

− − 83.4 153.2

− − 0.36 0.59

1.471 1.326 1.487 1.354

corresponding O−O bond lengths (lO−O) in free H2O2 and PMS are 1.471 and 1.326 Å, respectively. For the adsorption of H2O2 and PMS on the bulk Co3O4, the lO−O increased remarkably compared with that of free H2O2 and PMS molecules. The prolonged bond length suggests the activated cleavage of the peroxo bond to generate SO4•− radicals.43,44 Thus, Co3O4 is theoretically proved to have excellent performance to activate H2O2 and PMS molecules. The noticeable electron transfer tendency (Q) also indicates the generation of free radicals, which appears to be in good accordance with the experimental results (ESR measurement). Besides, it has been explained that the catalytic activity of the peroxides originates from the different oxidation states of metal oxide.45,46 As depicted in Figure 7, there are two different oxidation states (Co2+ and Co3+) in Co3O4, which favors the catalytic process. From the analysis above, it can be concluded that the high catalytic efficiency and stability of Co3O4 for H2O2 and PMS activation are mainly dependent on its unique electronic and crystal structure.

4. CONCLUSION Tubelike Co3O4 was successfully fabricated by a facile electrospinning technique followed by a heat treatment. Co3O4 tubes exhibited excellent peroxidase-like activity within a wide range of pH and temperature and also can activate PMS to produce sulfate radical for the organic pollutant degradation. Furthermore, the activation mechanism of Co3O4 for H2O2 and PMS was investigated by the DFT method. The structural stability, electronic properties of Co3O4, and adsorption properties of H2O2 and PMS were also studied. This investigation will promote the development of 1-D metal 2401

DOI: 10.1021/acs.iecr.7b04180 Ind. Eng. Chem. Res. 2018, 57, 2396−2403

Article

Industrial & Engineering Chemistry Research

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oxides and concludes new insights for the activation of peroxides.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b04180. XPS spectra of Co3O4 tubes in the activation of H2O2 and PMS, the effect of reaction conditions on the peroxidase-like activity of Co3O4 tubes, XRD patterns of the control powder sample, the effect of the catalyst concentration on the activation of PMS, TOC removal trend of Co3O4 tubes with the reaction time, the nitrogen adsorption−desorption isotherms of Co3O4 tubes and powders, XPS spectra of Co3O4 powders, the band structure and density of state (DOS) of Co3O4, and the cycling test result of Co3O4 tubes in the activation of PMS (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-0531-89631632. Fax: +86-0531-89631632. E-mail: [email protected] (H.L.). *Tel.: +86-0531-89631632. Fax: +86-0531-89631632. E-mail: [email protected] (H.S.). ORCID

Haiyan Sun: 0000-0002-2261-7031 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 21401114) and Project of University Innovation of Jinan (Grant No. J14LC02).

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REFERENCES

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