PREPARATION OF THREE-DIMENSIONAL MAGNETIC GAMMA MANGANESE DIOXIDE/ZINC IRON OXIDE NANOHYBRID ON GRAPHENE, AND USE THEREOF AS CATALYST FOR DECOMPOSING HARMFUL ORGANIC WASTE

Abstract

A nanohybrid includes: reduced graphene oxide (rGO); zinc ferrite (ZnFe.sub.2O.sub.4) nanoparticles dispersed in the rGO; and manganese dioxide (MnO.sub.2) nanoflakes three-dimensionally attached on the rGO. The nanohybrid reduces recombination of graphene through the synergistic effects of MnO.sub.2 nanoflakes, ZnFe.sub.2O.sub.4 nanoparticles, and graphene, and increases the surface area of the catalyst, thus being capable of exhibiting higher catalytic activity than the conventional δ-MnO.sub.2@ZnFe.sub.2O.sub.4, γ-MnO.sub.2@rGO, and ZnFe.sub.2O.sub.4@rGO composites in the decomposition of harmful organic waste.

Claims

1. A three-dimensional (3D) MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid catalyst comprising: reduced graphene oxide (rGO); zinc ferrite (ZnFe.sub.2O.sub.4) nanoparticles dispersed in the rGO; and manganese dioxide (MnO.sub.2) nanoflakes attached three-dimensionally on the rGO.

2. The 3D MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid catalyst according to claim 1, wherein the manganese dioxide (MnO.sub.2) is in the gamma (γ) form.

3. The 3D MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid catalyst according to claim 1, wherein an average thickness of the manganese dioxide (MnO.sub.2) nanoflakes is 2 to 5 nm.

4. The 3D MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid catalyst according to claim 1, wherein the MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid has a Brunauer-Emmett-Teller (BET) specific surface area of 200 to 500 m.sup.2/g and includes pores with an average diameter of 2 to 15 nm.

5. A catalyst for decomposing harmful organic waste comprising the 3D MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid catalyst according to claim 1 and peroxymonosulfate (PMS).

6. A method for preparing a three-dimensional (3D) MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid catalyst, comprising: dispersing zinc ferrite (ZnFe.sub.2O.sub.4) nanoparticles in a graphene oxide (GO) solution to prepare a ZnFe.sub.2O.sub.4/GO solution; adding a manganese precursor and an acid to the ZnFe.sub.2O.sub.4/GO solution to prepare a suspension; and performing heat treatment of the suspension to obtain a nanohybrid (MnO.sub.2@ZnFe.sub.2O.sub.4/rGO) with manganese dioxide (MnO.sub.2) nanoflakes attached three-dimensionally on reduced graphene oxide (rGO) in which the zinc ferrite (ZnFe.sub.2O.sub.4) nanoparticles are dispersed.

7. The method according to claim 6, wherein the manganese precursor is any one selected from the group consisting of potassium permanganate (KMnO.sub.4), manganese nitrate (Mn(NO.sub.3).sub.2), manganese hydrochloride (MnCl.sub.2), manganese sulfate (MnSO.sub.4), and manganese acetate (Mn(CH.sub.3COO).sub.2).

8. The method according to claim 6, wherein the acid is any one selected from the group consisting of hydrochloric acid (HCl), sulfuric acid (H.sub.2SO.sub.4), and nitric acid (HNO.sub.3).

9. The method according to claim 6, wherein 0.1 to 0.7 g of the manganese precursor is included, and 0.3 to 2.0 mL of the acid is included.

10. The method according to claim 6, wherein the heat treatment of the suspension is carried out at 50 to 150° C. for 5 to 20 hours.

11. The method according to claim 6, wherein an average thickness of the manganese dioxide (MnO.sub.2) nanoflakes is 2 to 5 nm.

12. The method according to claim 6, wherein the MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid has a BET specific surface area of 200 to 500 m.sup.2/g and includes pores with an average diameter of 2 to 15 nm.

Description

BRIEF DESCRIPTIONS OF DRAWINGS

[0013] FIG. 1 is a diagram showing a preparation process of a 3D γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid by a hydrothermal synthesis method.

[0014] FIG. 2 shows XRD patterns of ZnFe.sub.2O.sub.4, ZnFe.sub.2O.sub.4@rGO, γ-MnO.sub.2@rGO, δ-MnO.sub.2@ZnFe.sub.2O.sub.4, and γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO.

[0015] FIG. 3 shows SEM and TEM images of ZnFe.sub.2O.sub.4@rGO (a, b), γ-MnO.sub.2@rGO (c, d), δ-MnO.sub.2@ZnFe.sub.2O.sub.4 (e, f), and γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO (g, h).

[0016] FIG. 4 shows (a) a HRTEM image, (b) EDS spectrum, and (c) elemental mapping images of γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO.

[0017] FIG. 5 shows SEM images of γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrids, which were synthesized using different amounts of KMnO.sub.4 and HCl:(a) 0.15 g KMnO.sub.4+0.33 ml HCl, (b) 0.225 g KMnO.sub.4+0.5 ml HCl, (c) 0.45 g KMnO.sub.4+1 ml HCl, and (d) 0.6 g KMnO.sub.4+1.33 ml HCl.

[0018] FIG. 6 shows TEM images of γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrids, which were synthesized using different amounts of KMnO.sub.4 and HCl:(a) 0.15 g KMnO.sub.4+0.33 ml HCl, (b) 0.225 g KMnO.sub.4+0.5 ml HCl, (c) 0.45 g KMnO.sub.4+1 ml HCl, and (d) 0.6 g KMnO.sub.4+1.33 ml HCl.

[0019] FIG. 7 shows XPS spectra of a γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid:(a) survey scan, (b) Mn 2p, (c) Fe 2p, (e) C is, and (f) O is energy regions.

[0020] FIG. 8 shows (a) Raman spectra of GO and the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid, (b) FT-IR spectra of GO and the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid, and (c) N.sub.2 adsorption-desorption isotherm of the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid (the inset presents pore-size distribution (PSD)).

[0021] FIG. 9 shows (a) degradation of phenol (20 mg/L) using ZnFe.sub.2O.sub.4@rGO, γ-MnO.sub.2@rGO, δ-MnO.sub.2@ZnFe.sub.2O.sub.4, and γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO, (b) effects of catalyst loading in the presence of 20 ppm phenol and 2 g/L PMS, and (c) effects of PMS loading in the presence of 20 ppm phenol and 0.2 g L.sup.−1 catalyst.

[0022] FIG. 10 shows (a) an M-H hysteresis loop for the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid at 300 K, and (b) a reusability test of the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid for degradation of 20 ppm phenol using 0.2 g L.sup.−1 catalyst and 2 g L.sup.−1 PMS for five successive cycles.

[0023] FIG. 11 is a diagram showing phenol degradation mechanism of the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid.

MODES FOR CARRYING OUT INVENTION

[0024] Hereinafter, the present invention will be described in detail.

[0025] The present inventors prepared a 3D γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid catalyst using a simple hydrothermal method and have completed the present invention by determining that it can exhibit high catalytic activity in degrading harmful wastewater compared to the existing δ-MnO.sub.2@ZnFe.sub.2O.sub.4, γ-MnO.sub.2@rGO, and ZnFe.sub.2O.sub.4@rGO composites by reducing aggregation of graphene through the synergetic effects of MnO.sub.2 nanoflakes, ZnFe.sub.2O.sub.4 nanoparticles, and graphene, and increasing activity of a radical source by increasing the surface area of the catalyst.

[0026] The present invention provides a 3D MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid catalyst containing reduced graphene oxide (rGO); zinc ferrite (ZnFe.sub.2O.sub.4) nanoparticles dispersed in the rGO; and manganese dioxide (MnO.sub.2) nanoflakes attached three-dimensionally on the rGO.

[0027] Here, the manganese dioxide (MnO.sub.2) may have the gamma (γ) form, and an average thickness of the prepared manganese dioxide (MnO.sub.2) nanoflakes may be 2 to 5 nm.

[0028] In addition, the MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid can increase the Brunauer-Emmett-Teller (BET) specific surface area to 200 to 500 m.sup.2 g.sup.−1 by including pores with an average diameter of 2 to 15 nm, thus increasing the catalytic activity for degrading harmful organic waste.

[0029] Further, the present invention provides a catalyst for decomposing harmful organic waste, containing the 3D MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid catalyst and peroxymonosulfate (PMS).

[0030] Here, a high-efficiency catalytic effect can be exhibited through the high surface area of the 3D MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid catalyst and sulfate radicals generated by activation of PMS by MnO.sub.2 and ZnFe.sub.2O.sub.4. Particularly, in the case that 0.2 g L.sup.−1 δ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid and 2.0 g L.sup.−1 PMS are added, harmful organic waste can be completely degraded within a short time.

[0031] Further, the present invention provides a 3D MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid catalyst preparation method including dispersing zinc ferrite (ZnFe.sub.2O.sub.4) nanoparticles in a graphene oxide (GO) solution to prepare a ZnFe.sub.2O.sub.4/GO solution; adding a manganese precursor and an acid to the ZnFe.sub.2O.sub.4/GO solution to prepare a suspension; and performing heat treatment of the suspension to obtain a nanohybrid (MnO.sub.2@ZnFe.sub.2O.sub.4/rGO) with manganese dioxide (MnO.sub.2) nanoflakes attached three-dimensionally on reduced graphene oxide (rGO) in which zinc ferrite (ZnFe.sub.2O.sub.4) nanoparticles are dispersed.

[0032] Here, the manganese precursor may be any one selected from the group consisting of potassium permanganate (KMnO.sub.4), manganese nitrate (Mn(NO.sub.3).sub.2), manganese hydrochloride (MnCl.sub.2), manganese sulfate (MnSO.sub.4), and manganese acetate (Mn(CH.sub.3COO).sub.2), but is not limited thereto.

[0033] In addition, the acid may be any one selected from the group consisting of hydrochloric acid (HCl), sulfuric acid (H.sub.2SO.sub.4), and nitric acid (HNO.sub.3), but is not limited thereto.

[0034] In addition, 0.1 to 0.7 g of the manganese precursor may be included, and 0.3 to 2.0 mL of the acid may be included. Preferably, 0.45 g of the manganese precursor and 1 mL of the acid may be included.

[0035] In addition, the heat treatment of the suspension may be performed at 50 to 150° C. for 5 to 20 hours, and preferably, at 100° C. for 12 hours, but is not limited thereto.

[0036] Here, in the case of exceeding the conditions of the preparation method, the nanohybrid (MnO.sub.2@ZnFe.sub.2O.sub.4/rGO) with manganese dioxide (MnO.sub.2) nanoflakes attached three dimensionally on reduced graphene oxide (rGO) in which zinc ferrite (ZnFe.sub.2O.sub.4) nanoparticles are dispersed according to the present invention is not formed properly, so the catalytic activity is not excellent, which may cause a problem that it cannot be usefully used as a catalyst for decomposing harmful organic waste.

[0037] In addition, an average thickness of the prepared manganese dioxide (MnO.sub.2) nanoflakes may be 2 to 5 nm, and the MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid may include pores having an average diameter of 2 to 15 nm to increase the BET specific surface area to 200 to 500 m.sup.2/g, thereby increasing the catalytic activity for decomposing harmful organic waste.

[0038] According to the present invention, the nanohybrid (MnO.sub.2@ZnFe.sub.2O.sub.4/rGO) with manganese dioxide (MnO.sub.2) nanoflakes attached three-dimensionally on reduced graphene oxide (rGO) in which zinc ferrite (ZnFe.sub.2O.sub.4) nanoparticles are dispersed was prepared simply using the hydrothermal self-assembly synthesis method. This nanohybrid exhibited a high BET specific surface area which is an important property for excellent catalytic activity, through the synergistic effects of MnO.sub.2, ZnFe.sub.2O.sub.4, and graphene. Further, it exhibited a high-efficiency catalytic effect through the sulfate radicals (SO.sub.4*) generated by the activation of PMS by MnO.sub.2 and ZnFe.sub.2O.sub.4, could be recovered easily using a magnet, and could be reused more than 5 times. Therefore, since the nanohybrid catalyst according to the present invention has excellent catalytic activity and reusability, it may be usefully applied to removal of hard-to-degrade waste materials.

[0039] Hereinafter, the present invention will be described in detail through examples. It would be clear to a person skilled in the art that these examples are merely for illustrating the present invention specifically and that the scope of the present invention is not limited by the examples.

<Example 1>Preparation of 3D δ-MnO.SUB.2.@ZnFe.SUB.2.O.SUB.4./rGO Nanohybrid Catalyst

[0040] All materials used below were of high purity grade, purchased from Sigma-Aldrich, and used as received without further purification. Graphene oxide (GO) was generated using Tour's method (ACS Nano, 4 (2010) 4806-4814; 12 (2018) 2078). FIG. 1 describes synthesis of a 3D δ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid including dispersion by sonication of 0.24 g of ZnFe.sub.2O.sub.4, which was synthesized previously via a hydrothermal method (Mater. Res. Bull. 45 (2010) 755-760), in an aqueous solution of 80 mL GO (1 mg ml.sup.−1) for 1 hour. Subsequently, 0.45 g of KMnO.sub.4 and 1 mL of HCl (37%) were added to the ZnFe.sub.2O.sub.4 dispersed GO aqueous solution with stirring for 30 minutes. After the above process, the resulting suspension was transferred to a 120 mL autoclave and heat-treated at 100° C. for 12 hours. After the reaction, the autoclave was cooled down naturally to room temperature. Then, samples were collected by centrifugation, washed with deionized water, and dried in a vacuum oven at room temperature for 30 minutes to prepare a nanohybrid catalyst (hereinafter, referred to as ‘3D δ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO’).

<Comparative Example 1>Preparation of δ-MnO.SUB.2.@ZnFe.SUB.2.O.SUB.4 .Catalyst

[0041] A catalyst was prepared using the same process described above for Example 1 except that GO was not included (hereinafter, referred to as ‘δ-MnO.sub.2@ZnFe.sub.2O.sub.4’).

<Comparative Example 2>Preparation of γ-MnO.SUB.2.@rGO Catalyst

[0042] A catalyst was prepared using the same process described above for Example 1 except that ZnFe.sub.2O.sub.4 was not included (hereinafter, referred to as ‘γ-MnO.sub.2@rGO’).

<Comparative Example 3>Preparation of ZnFe.SUB.2.O.SUB.4.@rGO Catalyst

[0043] A catalyst was prepared using the same process described above for Example 1 except that MnO.sub.2 was not included (hereinafter, referred to as ‘ZnFe.sub.2O.sub.4@rGO’).

<Example 2>Analysis

[0044] Powder X-ray diffraction (XRD; PANalytical, X′Pert-PRO MPD) was carried out using Cu Kα radiation. The structural information of the samples was analyzed using Fourier-transform infrared (FT-IR; Bio-Rad, Excalibur Series FTS 3000) spectroscopy and Raman spectroscopy (Horiba, XploRA plus). X-ray photoelectron spectroscopy (XPS; Kratos Analytical, AXIS Nova) was used to examine the surface components of the samples. The Brunauer-Emmett-Teller (BET) specific surface area (S.sub.BET) and pore size distribution (PSD) of the samples were investigated using an N.sub.2 adsorption-desorption apparatus (Micromeritics 3Flex Surface Characterization Analyzer). Field-emission scanning electron microscopy (FE-SEM; Hitachi, S-4800) and transmission electron microscopy (TEM; Philips, CM 200) were used to determine the morphology and structure of the samples. Magnetization measurements were carried out at room temperature using a vibrating sample magnetometer (VSM; Dexing, Model 250). The metal content of the composite was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES; PerkinElmer, Optima 8300).

<Example 3>Catalytic Activity Measurement

[0045] In a catalytic activity measurement test, a 10 mg of catalyst was added to 50 mL of 20 ppm phenol solution, which was then stirred for 30 minutes to achieve adsorption-desorption equilibrium. Then, to start reaction tests, 0.3 mM peroxymonosulfate (PMS) was added to the reaction solution. After a certain period of time, 1.5 mL of the aqueous sample was withdrawn using a syringe and filtered into a vial. The concentration of phenol was analyzed by high-performance liquid chromatography (HPLC; Young Lin, Series YL9100) equipped with a YL9120 UV/Vis detector; the UV wavelength was adjusted to 275 nm. A C-18 column (Sun Fire) was used to separate the organic solution from a mobile solution at a flow rate of 1 mL min.sup.−1. The eluent was prepared by mixing water, 1 vol. % acetic acid solution, and methanol in the ratio of 50:40:10. To examine catalytic stability and reusability, the nanohybrid was used 5 times. After each experiment, it was collected using a magnet, washed with deionized water, and dried in a vacuum oven at room temperature.

<Experimental Example 1>Structural Analysis of 3D γ-MnO.SUB.2.@ZnFe.SUB.2.O.SUB.4./rGO Nanohybrid

[0046] 1. XRD Analysis

[0047] XRD analysis was performed on ZnFe.sub.2O.sub.4 and the composites of Example 1 and Comparative Examples 1 to 3 to determine the crystalline structure of the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid (FIG. 2). As a result, for ZnFe.sub.2O.sub.4, the XRD peaks at 30.1, 35.26, 42.8, 53.2, 56.6, and 62.2° 2θ were indexed to the (220), (311), (400), (422), (511), and (440) crystal planes of spinel ZnFe.sub.2O.sub.4 (JCPDS 22-1012), respectively, which confirmed formation of a cubic structure. In addition, a weak broad peak was observed at 22° 2θ, which was indexed to the (002) plane of reduced graphene oxide in the ZnFe.sub.2O.sub.4@rGO nanocomposite. The characteristic XRD peaks of γ-MnO.sub.2 were observed in both γ-MnO.sub.2@rGO and the nanohybrid γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO, at 22.2, 36.1, 42.2, 55.6, and 67.9° 2θ, which were indexed to the (101), (201), (211), (212), and (610) planes, respectively, of Ramsdellite γ-MnO.sub.2 (JCPDS 44-0142). Furthermore, no XRD peaks of S—MnO.sub.2 were observed in the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid, confirming that the MnO.sub.2 is present only in the gamma (γ) form. The XRD peak of rGO was not detected in the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid due to the anchoring of ZnFe.sub.2O.sub.4 nanoparticles (NPs) as spacers and 3D γ-MnO.sub.2 nanoflakes on the rGO surface, which can minimize re-stacking of rGO sheets.

[0048] 2. SEM, TEM Analysis

[0049] FIG. 3 shows SEM and TEM images of the samples. FIGS. 3a and 3b show SEM and TEM images of ZnFe.sub.2O.sub.4@rGO, respectively, which show that all ZnFe.sub.2O.sub.4 NPs were intercalated between the graphene nanosheets. Further, the SEM (FIG. 3c) and TEM (FIG. 3d) images of γ-MnO.sub.2@rGO showed aggregated nanoflakes of γ-MnO.sub.2 deposited on the rGO nanosheets. The SEM (FIG. 3e) and TEM (FIG. 3f) images of δ-MnO.sub.2@ZnFe.sub.2O.sub.4 showed a microspherical structure with a uniform hydrangea-like structure with a diameter of 3-5 m. FIGS. 3g and 3h are SEM and TEM images of the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid, which show an intimate combination of 3D γ-MnO.sub.2 nanoflakes, 2-5 nm in thickness, anchored on the rGO nanosheets intercalated with ZnFe.sub.2O.sub.4. Such a combination showed high chemical performance due to rapid transportation of electrons between the γ-MnO.sub.2 nanoflakes and the rGO nanosheets intercalated with ZnFe.sub.2O.sub.4 NPs.

[0050] The HRTEM image in FIG. 4a shows well-resolved lattice fringes with typical d-spacings of 0.38 and 0.25 nm, which correspond to the (201) plane of MnO.sub.2 and the (311) plane of ZnFe.sub.2O.sub.4, respectively. EDS of the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid (FIG. 4b) indicated the presence of Mn, O, C, Fe, and trace amounts of Zn, which was due to partial leaching of Zn from zinc ferrite after the addition of HCl during the preparation process. The elemental mapping images (FIG. 4c) showed that Mn, Fe, and Zn were dispersed uniformly on the rGO nanosheets, which further confirmed the successful synthesis of the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid.

[0051] Based on the experimental results, a possible mechanism was proposed to understand growth mechanism of MnO.sub.2 nanoflakes on rGO sheets (FIGS. 5 and 6). In the first step, ZnFe.sub.2O.sub.4 was dispersed in the graphene oxide solution by sonication for 1 hour to intercalate the ZnFe.sub.2O.sub.4 nanoparticles between GO sheets. Subsequently, different amounts of KMnO.sub.4 and HCl were used to induce the growth of MnO.sub.2 nanoflakes. As a result, using a small amount of KMnO.sub.4 (0.15 g), the Mn.sup.2+ions in the solution bound with the negatively charged oxygen-containing functional groups of GO sheets via electrostatic force, the addition of 0.33 ml HCl to the solution of ZnFe.sub.2O.sub.4/GO and KMnO.sub.4 resulted in oxidation of Mn.sup.2+ ions to MnO.sub.2, and nucleated MnO.sub.2 combined on the surface of the GO sheets and started growing into nanoflakes (FIGS. 5a and 6a). Then, increasing the amounts of KMnO.sub.4 and HCl to 0.45 g and 1 ml, respectively, resulted in the formation of MnO.sub.2 nanoflakes on the rGO sheets without aggregation (FIGS. 5c and 6c). However, increases in the amounts of KMnO.sub.4 and HCl to 0.6 g and 1.33 ml, respectively, produced aggregated MnO.sub.2 nanoflakes on the surface of rGO (FIGS. 5d and 6d), enabling preparation of the 3D γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid catalyst.

[0052] 3. XPS Analysis

[0053] The chemical bonding state of the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid was analyzed by XPS (FIG. 7). As shown in FIG. 7a, the survey spectrum revealed the existence of Mn2p, Fe 2p, Zn 2p, O 1 s, and C 1 s energy regions. The Mn 2p spectrum of FIG. 7b revealed two main peaks at approximately 642.5 and 654.2 eV, corresponding to the binding energies of Mn2p.sub.3/2 and Mn 2p.sub.1/2, respectively. The spin energy separation between Mn2p.sub.3/2 and Mn 2p.sub.1/2 was 11.7 eV, showing good agreement with the known spectra of MnO.sub.2 and also revealing the formation of MnO.sub.2 on the rGO/ZnFe.sub.2O.sub.4 hybrid. The Fe 2p spectrum in FIG. 7c shows the binding energies of Fe 2p.sub.3/2 at 711.4 and 713.1 eV, which correspond to the octahedral and tetrahedral sites, respectively. The peak at 725.6 eV for Fe 2p.sub.1/2 and the two satellite peaks at 719.3 and 732.5 eV suggest that only Fe.sup.3+exists in the ZnFe.sub.2O.sub.4 particles in the nanohybrid. The Zn 2p spectrum in FIG. 7d shows two major peaks centered at 1044.8 and 1021.7 eV, which were assigned to Zn 2p.sub.1/2 and Zn 2p.sub.3/2, respectively. The low intensity of these two Zn 2p peaks was attributed to the leaching of zinc from the zinc ferrite sample during preparation of the hybrid, which was also confirmed by the EDS analysis. The O is region in FIG. 7f showed three peaks centered at 530.0, 531.5, and 533.2 eV, which were assigned to Mn—O—Mn, Mn—O—H, and C—O/C═O bonds, respectively. For the C is spectrum in FIG. 7e, the main peak is located at 284.7 eV, which was attributed to C—C and C═C bonds, whereas the two weak peaks centered at 285.6 and 289.0 eV correspond to C—O and C═O bonds, respectively.

[0054] 4. Raman Spectrum Analysis

[0055] FIG. 8a shows the Raman spectra of both GO and γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid. As a result, two characteristic peaks corresponding to the D-band (1360 cm.sup.−1), which originated from disorder and defects of carbon materials, and the G-band (1589 cm.sup.−1), which was attributed to the vibration of sp.sup.2-hybridized carbon, were present in both samples. After the hydrothermal reduction treatment, the ID/IG intensity ratio for GO increased from 0.906 to 1.308 in the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid due to an increase in the degree of defects and disorder and a decrease in the mean size of the sp.sup.2 domains. The increase in the ID/IG ratio confirmed that GO had been deoxygenated and reduced to rGO. The peak at 646 cm.sup.−1 corresponds to the A.sub.g mode of MnO.sub.2, which originates from the MnO.sub.6 octahedral breathing vibrations. This shows that MnO.sub.2 had been anchored successfully on the rGO sheets.

[0056] 5. FT-IR Analysis

[0057] Evidence for the reduction of oxygen functional groups on the GO surface was obtained from the FT-IR spectra of GO and γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid, as shown in FIG. 8b. The characteristic peaks of GO can be observed at 1056.8 cm.sup.−1 (stretching vibrations of epoxy groups, C—O), 1398.5 cm.sup.−1 (O—H deformation vibrations of tertiary C—OH), 1625.3 cm.sup.−1 (0—H deformation vibrations of COOH groups), and 1721.0 cm.sup.−1 (C═O stretching vibrations of COOH groups). The broad absorption peak from 3100 to 3700 cm.sup.−1 is assigned to the O—H stretching vibration. In contrast, after the hydrothermal reduction treatment, most of the oxygen-containing functional groups in the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid had been minimized. The C═O band disappeared, and the band intensities of O—H and C—O decreased, indicating the partial reduction of GO. Finally, the two strong absorption peaks in the spectrum of γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO at wavelengths lower than 722.1 and 550 cm.sup.−1 were assigned to the stretching vibrations of Mn—O—C and Fe—O bonds, respectively, and the weak absorption peak at 433.9 cm.sup.−1 was attributed to the Zn—O bond due to the leaching of zinc, as mentioned above.

[0058] 6. S.sub.BET, PSD Analysis

[0059] As shown in FIG. 8c, S.sub.BET and PSD of the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO were analyzed. According to the IUPAC classification, the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid showed a type IV isotherm with an H3 hysteresis loop, indicating its mesoporous nature. The S.sub.BET of γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO was calculated to be 376.9 m.sup.2 g.sup.−1. The inset in FIG. 8c presents the PSD of γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO, showing a sharp maximum at approximately 3.5 nm and a broad peak at 5.5 nm with a resulting average pore diameter of 8.15 nm, which exhibited a superior S.sub.BET value compared to the catalysts of Comparative Examples 1 to 3 as shown in Table 1 below. The high surface area allows for efficient transportation of pollutants to the active sites of the catalyst and provides more adsorption/reaction sites for peroxymonosulfate (PMS) activation during the catalytic reaction, which results in higher catalytic activity.

TABLE-US-00001 TABLE 1 Pore Average pore First order S.sub.BET Volume diameter rate constant Sample (m.sup.2 g.sup.−1) (cm.sup.3 g.sup.−1) (nm) (min.sup.−1) R.sup.2 γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO 376.9 1.055 8.15 0.094 0.9912 δ-MnO.sub.2@ZnFe.sub.2O.sub.4 223.0 0.942 16.66 0.033 0.972 γ-MnO.sub.2@rGO 46.0 0.246 24.98 0.0235 0.988 ZnFe.sub.2O.sub.4@rGO 153.9 0.339 8.13 0.0114 0.983

<Experimental Example 2>Catalytic Activity

[0060] 1. Catalytic Activity of γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO Nanohybrid

[0061] The catalytic performance of the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid and the other composites for phenol degradation via peroxymonosulfate (PMS) activation was analyzed (FIG. 9a). Control experiments for catalytic activity using PMS only and γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid without PMS were conducted. As a result, the sample using the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid and PMS showed the highest catalytic efficiency among all the samples tested. Further, in the absence of a catalyst, PMS could not be activated and the amount of sulfate radicals (SO.sub.4′.sup.−) generated was insufficient to degrade the 20 ppm phenol solution. Furthermore, the use of the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid without the addition of PMS resulted in the adsorption of approximately 20% of phenol after 3 hours. By combining PMS with the composites, PMS was activated on the active sites of the metal oxides, and the catalytic activity of the composites was observed in the following order: γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO >δ-MnO.sub.2@ZnFe.sub.2O.sub.4 >γ-MnO.sub.2@rGO >ZnFe.sub.2O.sub.4@rGO.

[0062] In particular, the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid showed complete degradation of 20 mg L.sup._(20 ppm) phenol after 30 min in the presence of PMS. A comparison with the results of the other composites revealed 22, 39, and 41% phenol degradation on ZnFe.sub.2O.sub.4@rGO, γ-MnO.sub.2@rGO, and δ-MnO.sub.2@ZnFe.sub.2O.sub.4, respectively, indicating the synergetic effects on catalytic activity of combining MnO.sub.2, ZnFe.sub.2O.sub.4, and rGO into a nanohybrid. This high-efficiency catalytic effect is attributed to the high surface area of the nanohybrid compared to the other composites (Table 1) and the ability of both MnO.sub.2 and ZnFe.sub.2O.sub.4 to activate PMS through an electron transfer mechanism to produce sulfate radicals (Eqs. (1) to (4) below). As a result, the composites containing both MnO.sub.2 and ZnFe.sub.2O.sub.4 showed high catalytic efficiency compared to other composites lacking one of them (FIG. 9a). In addition, the rGO sheets increased the adsorption property of the nanohybrids through 7L-7L stacking interactions between phenol and the aromatic region of the graphene sheets. Moreover, the uniform dispersion of MnO.sub.2 nanoflakes over the rGO surface (FIG. 4c) can provide more active sites for phenol degradation. The high efficiency of the nanohybrid is also due to the strong bonding between MnO.sub.2 and graphene (Mn—O—C), which was confirmed by the O is deconvolution of the XPS data (FIG. 7f).

[0063] Further analysis of the performance of the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid under different reaction conditions was carried out and the effects of catalyst loading and PMS loading were analyzed (FIGS. 9b and 9c). The efficiency of phenol degradation increased with increasing amounts of nanohybrid and PMS due to increase in the number of active sites and active sulfate radicals SO.sub.4′.sup.−, respectively. In particular, 0.2 g L.sup.−1γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid loading and 2.0 g L.sup.−1 PMS loading showed complete degradation of 20 mg L.sup.−1 (20 ppm) phenol after 30 minutes.

[0064] 2. Magnetism and Reusability of γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO Nanohybrid Catalyst

[0065] The catalyst magnetism and reusability of the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid were evaluated from successive catalytic experiments by taking advantage of the magnetic property of the nanohybrid, which exhibited paramagnetic behavior and a slim hysteresis loop with a saturation magnetization value of approximately 7 emu g.sup.−1 at 2 θ,000 Oe, magnetic coercivity (Hc) of 293.67 Oe, and remanence (M.sub.R) of 0.266 emu g.sup.−1 (FIG. 10a and inset). In addition, the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid showed a similar rate of phenol degradation after 5 cycles as shown in FIG. 10b, indicating the potential reusability of the nanohybrid.

[0066] 3. Catalytic Mechanism of γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO Nanohybrids

[0067] Based on the above results, an activation mechanism of peroxymonosulfate (PMS) on the active sites of the γ-MnO.sub.2@ZnFe.sub.2O.sub.4/rGO nanohybrid for phenol degradation is as follows (Eqs. (1) to (7) and FIG. 11).

[0068] [Equations]

Mn(IV)+HSO.sub.5.sup.−.fwdarw.Mn(III)+HO*+SO.sub.4*.sup.−  (1)

Mn(III)+HSO.sub.5.sup.−.fwdarw.Mn(IV)+SO.sub.5*.sup.−+H.sup.+  (2)

Fe(II)+HSO.sub.5.sup.−.fwdarw.Fe(III)+SO.sub.4*.sup.−+OH.sup.−  (3)

Fe(III)+HSO.sub.5.sup.−.fwdarw.Fe(II)+SO.sub.5*.sup.−−+H.sup.+  (4)

2SO.sub.5*.sup.−+2OH.sup.−.fwdarw.2SO.sub.4.sup.2−+2HO*+O.sub.2  (5)

Fe(III)+Mn(III).fwdarw.Fe(II)+Mn(IV)  (6)

SO.sub.4*.sup.−+HO*+C.sub.6H.sub.6OH.fwdarw.Several steps.fwdarw.CO.sub.2+H.sub.2O+SO.sub.4.sup.2−  (7)

[0069] The Mn(IV)/Mn(III) and Fe(II)/Fe(III) transitions involve electron transfer, which is responsible for the catalytic reaction. In the first stage, the active sites of both MnO.sub.2 and ZnFe.sub.2O.sub.4 on the nanohybrid can activate PMS to generate active radicals (Eqs. (1) to (4)), which can contribute to phenol degradation (Eq. (7)). Hydroxyl radicals (HO) are generated further after the depletion of SO.sub.4*.sup.− in a rapid reaction with phenol in the first stage (Eq. (5)), and HO* becomes the only radical that reacts with phenol in the last stage of the reaction. The return to the original oxidation states of the metals (Mn(IV) and Fe(II)) is due to the recovery reactions on the reduced hybrid (Eq. (6)).