Solvothermal synthesis of MnxFe3 − xO4 nanoparticles with interesting physicochemical characteristics and good catalytic degradation activity

Materials and Design 97 (2016) 341–348

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Solvothermal synthesis of MnxFe3 − xO4 nanoparticles with interesting physicochemical characteristics and good catalytic degradation activity Meng Li a, Qiang Gao a,⁎, Teng Wang a, Yan-Sheng Gong a, Bo Han b, Kai-Sheng Xia b, Cheng-Gang Zhou b,⁎ a b

Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China Sustainable Energy Laboratory, China University of Geosciences, Wuhan 430074, PR China

a r t i c l e

i n f o

Article history: Received 3 January 2016 Received in revised form 22 February 2016 Accepted 23 February 2016 Available online 24 February 2016 Keywords: Manganese ferrite Interesting physicochemical characteristics Good catalytic activity

a b s t r a c t In this work, composition-adjusted manganese ferrites (MnxFe3 − xO4) were synthesized by a simple one-step solvothermal route and characterized by ICP-AES, XRD, SEM, N2 adsorption/desorption, and VSM. It was identified that the chemical composition had a significant influence on the physicochemical characteristics of MnxFe3 − xO4 samples. The specimen with value of x = 1.07 (Mn1.07Fe1.93O4) exhibited a unique combination of good monodispersity, regular morphology (sphere-like), abundant porosity, uniform size, distinct crystalline structure, satisfactory water dispersability, and high magnetic responsivity. Activities of these magnetically recoverable MnxFe3 − xO4 samples were evaluated by employing them as Fenton catalysts for the degradation of highly concentrated methylene blue (MB, 400 mg·L−1). Exceptionally high activity for the catalytic degradation of MB without any external energy input was achieved, being 1–2 orders of magnitudes greater than that over other Fe-based or Mn-based heterogeneous catalysts. We demonstrated that the large surface area of MnxFe3 − xO4 as well as the high activity and favorable regeneration of the Mn2+ sites were responsible for the excellent performance. Overall, this study provides not only a practical strategy for rational design of targeted spinel-type ferrite nanomaterials, but also a new physical insight on catalytic activity of manganese ferrite and its application to organic pollutant removal. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Manganese ferrite, generally formulated as MnFe2O4, is of considerable current interest owing to its broad applications in many fields, such as battery, magnetic materials, and diverse catalytic processes [1,2]. As the physicochemical characteristics (e.g., morphology, size, and structure) of MnFe2O4 have great influences on its properties when the dimension is reduced to the nanometer scale, nanostructured MnFe2O4 can show unique electrical, magnetic, and catalytic properties, which are quite different from those of the bulk counterpart [3]. Therefore, controlled synthesis of nanometer-sized MnFe2O4 is of great importance for investigating its structure–performance relationship and thus improving its application performances. So far, a variety of techniques including co-precipitation, nanocasting, and sol-gel have been widely used for preparation of nanometer-sized MnFe2O4 [4–6]. But aggregation of nanoparticles is a critical obstacle in majority of these synthesis techniques. Recently, solvothermal synthetic methodology has been used to prepare MnFe2O4 nanoparticles. The enormous advantage, always reported, of the solvothermal method in comparison with other methods is that by ⁎ Corresponding authors. E-mail addresses: [email protected] (Q. Gao), [email protected] (C.-G. Zhou).

http://dx.doi.org/10.1016/j.matdes.2016.02.103 0264-1275/© 2016 Elsevier Ltd. All rights reserved.

this method metal oxide nanocrystals with good monodispersity can be easily obtained [7–10]. To date, a number of studies on the preparation of MnFe2O4 via solvothermal process have been published, but it still remains a highly sophisticated challenge to prepare high-quality MnFe2O4 nanoparticles that simultaneously possess good monodispersity, regular morphology, high porosity, uniform size, distinct crystalline structure, high magnetic responsivity, and other interesting physicochemical characteristics. On the other hand, the ever-increasing pollution of water resources is currently one of the greatest concerns for science and the general public [11]. Especially, the colored dye effluents are considered to be highly toxic to the aquatic biota and affect the symbiotic process by disturbing the natural equilibrium [12]. Therefore, numerous attempts are being made to decontaminate dyes-polluted waters, including physical, chemical, and biological methods [13]. However, most of these methods are not sufficiently effective due to the essentially chemical inertness and stability of organic dyes [14]. In recent years, advanced oxidation processes (AOPs) based on the generation of free hydroxyl radical (•OH) which can degrade most organic pollutants quickly and non-selectively, are reported as preferred alternatives for removal of dyes from effluents [15]. Among various AOPs, Fenton process is an especially powerful one due to its simplicity, efficiency, and environmental benignity [16,17]. However, the application of classical Fenton

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reagent, i.e., H2O2/Fe2 + or Fe3 +, limited by the narrow working pH range (~3) and production of a large amount of iron sludge at the end of process [18]. To overcome these bottlenecks, considerable efforts have recently been devoted to developing heterogeneous Fenton systems, which can offer some advantages for the degradation of organic pollutants, such as a broad working pH range, no sludge generation, and the possibility of recycling the catalysts [19]. Regarding heterogeneous Fenton catalysts, two main approaches are considered: i) immobilization of iron catalysts on porous materials such as activated carbon, silica, alumina, or clays; and ii) direct use of iron oxides and hydroxides, such as Fe2O3, Fe3O4 (magnetite), and FeOOH [19]. However, almost all of them showed low activities compared to the conventional homogeneous Fenton reagents [20]. Introduction of external energies including ultrasound, UV light, microwave, and electricity can greatly promote Fenton reaction, but this also inevitably results in the need for specific equipment at additional cost [20]. Therefore, it is highly desirable to explore alternative heterogeneous Fenton catalysts which can realize efficient catalytic degradation without the need of any external energy input. In this paper, composition-adjusted manganese ferrites (MnxFe3 − xO4) with varying Mn contents (x = 0.73, 1.03, and 1.19) were synthesized by using a simple one-step solvothermal method through rational design. To our knowledge till now there have been relatively few studies dealing with composition-adjusted manganese ferrites especially via solvothermal route, although the variation of the chemical composition might have a profound effect on its physicochemical characteristics and properties (e.g., textural parameters, magnetic property, and catalytic activity) [21,22]. Interestingly, it was found that the specimen with value of x = 1.07 (Mn1.07Fe1.93O4) exhibited a unique combination of good monodispersity, regular morphology (sphere-like), abundant porosity, uniform size, distinct crystalline structure, satisfactory water dispersability, and high magnetic responsivity. Through multitechnique characterization analyses, a possible mechanism for the evolution of MnxFe3 − xO4 nanostructures was proposed. Furthermore, the MnxFe3 − xO4 samples were employed as Fenton catalysts towards catalytic degradation of highly concentrated methylene blue (MB, 400 mg·L−1) under mild conditions. Exceptionally high activity for the catalytic degradation of MB without any external energy input was achieved. The reason for promoting the Fenton chemistry was discussed and elucidated.

2. Experimental 2.1. Preparation of MnxFe3 − xO4 samples The information on chemicals used in this work can be seen in Text S1. Three MnxFe3 − xO4 samples (denoted as MFO-1, MFO-2, and MFO3) were all synthesized via a one-step solvothermal route. A detailed procedure was as follows: FeCl3·6H2O (18.5 mmol) and a varied amount of MnCl2·4H2O were dissolved in 100 mL of ethylene glycol (EG), and then 15.0 g of sodium acetate (NaAc) and 60 mL of ethylenediamine (EDA) were added. After vigorous stirring for 30 min, the homogeneous mixture was sealed in a Teflon-lined stainless-steel autoclave (200 mL). The autoclave was then heated to 200 °C, maintained at this temperature for 8 h, and allowed to be cooled to room temperature. The magnetic product was collected with the help of magnet, followed by washing with ethanol and water several times, then the product was dried in vacuum at 60 °C for 8 h. The added amounts of MnCl2·4H2O for MFO-1, MFO-2, MFO-3 were 9.3, 18.5, and 37 mmol, respectively. The preparation procedure of pure Fe3O4 was essentially similar to that of MnxFe3 − xO4, except without the addition of manganese salt.

2.2. Investigation into catalytic activities of MnxFe3 − xO4 The catalytic activities of MnxFe3 − xO4 samples were evaluated by the oxidative degradation of methylene blue (MB) in the dark. Typically, 10 mL of MB dye solution (400 mg·L−1) containing 1.5 mL of 30 wt% H2O2 was added into a vial that included 5 mg of catalysts, and the mixture was shaken in the dark with a speed of 170 rpm at 30 °C. At predetermined time intervals, the catalyst was separated immediately from the solution by addition of an external magnet. The concentrations of MB in the course of degradation were measured at the maximum absorption wavelength (665 nm) by using the UV–vis spectrophotometer. 3. Results and discussion 3.1. Characterization of materials The MnxFe3 − xO4 samples were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), N2 adsorption/desorption, and vibrating sample magnetometer (VSM) (Text S2). According to these results, the possible formation process of MnxFe3 − xO4 samples was proposed. 3.1.1. Composition analysis Chemical compositions of the MnxFe3 − xO4 samples, analyzed by ICP-AES, shows that the formula of MFO-1, MFO-2, and MFO-3 are Mn0.73Fe2.27O4, Mn1.07Fe1.93O4, and Mn1.19Fe1.81O4, respectively (Table 1). As well known, Fe3O4 has an cubic spinel structure with formula of [Fe3+]tetrahedral[Fe3+ Fe2+]octahedralO4: half of Fe3+ ions take up the tetrahedral sites (site A); half of Fe3+ ions and all Fe2+ ions occupy the octahedral sites (site B) [23]. Previous studies revealed that, Fe2+ or Fe3+ ions in the site B were much more easily substituted by several divalent transition-metal ions (M2+: Mn2+, Ni2+, Co2+, Zn2+, Cu2+, etc.) than Fe3 + ions in the site A [22]. When the M2 +/Fetotal mole ratio is b0.5, the M2 +-doped magnetite can be roughly depicted as [Fe3 +]3+ 2+ Fe2+ tetrahedral[Fe 1 − x Mx ]octahedralO4 (0 b x b 1) [22]. Furthermore, if the M2+ content further increase (e.g., 1 b x b 2), all the Fe2+ ions and some Fe3 + ions in the site B would be substituted by M2 + ions to form non-stoichiometric [Fe3 +]tetrahedral[Fe32 +− x M2x +]octahedralO4 − δ, where δ is the degree of oxygen vacancies [24]. Accordingly, it might be assumed that both MFO-2 (i.e., Mn1.07Fe1.93O4) and MFO-3 (i.e., Mn1.19Fe1.81O4) contain some oxygen vacancies in their structures, and the MFO-3 has a larger amount of oxygen deficiencies than MFO-2. 3.1.2. XRD patterns The crystalline natures of Fe3O4 and MnxFe3 − xO4 were examined by wide-angle XRD (Fig. 1). Obviously, all the diffraction peaks of the Fe3O4 sample agree well with the cubic spinel structure of magnetite (JCPDS card no. 19-0629). The MnxFe3 − xO4 with the similar structure can be identified as MnFe2O4 (JCPDS card no. 01-075-0035). The diffraction peaks at 2θ values of 18.2°, 30.2°, 35.4°, 43.1°, 53.3°, 56.8°, and 62.4° can be indexed to the (111), (220), (311), (400), (422), (511), and (440) planes of magnetite, respectively [10]. Moreover, as expected, all four samples show approximately identical peaks regardless of Mn2 + content, and there is no evidence of peaks corresponding to Table 1 Chemical compositions of samples. Sample

Formulaa

BET area (m2·g−1)

Pore volume (cm3·g−1)

Particle size (nm)

Fe3O4 MFO-1 MFO-2 MFO-3

– Mn0.73Fe2.27O4 Mn1.07Fe1.93O4 Mn1.19Fe1.81O4

18.7 44.3 86.3 69.2

0.18 0.31 0.53 0.47

120 25 80 50

a

Calculated according to ICP-AES data (see Table S1).

M. Li et al. / Materials and Design 97 (2016) 341–348

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3.1.3. FESEM/TEM images and EDS mapping The sizes and morphologies of Fe3O4 and MnxFe3 − xO4 samples were investigated by FESEM and TEM (Fig. 2). Obviously, all the samples consist of well-dispersed nanospheres with narrow size distributions. The nanosphere sizes are about 120, 25, 80, and 50 nm for Fe3O4, MFO-1, MFO-2, and MFO-3, respectively. When comparing the difference between grain sizes (from XRD) and nanosphere sizes (from TEM and SEM), we can conclude that the average nanosphere of each sample consist of grain aggregates. Noticeably, the nanospheres of Fe3O4 and MFO-1 are relatively dense (Fig. 2a and b), whereas those of MFO-2 and MFO-3 exhibit a loose structure (Fig. 2c and d). Certainly, our findings emphasize that the Mn content has a significant influence on the nanosphere size, morphology, and porosity of MnxFe3 − xO4 products. Additionally, Fig. S1 gives a typical EDS elemental mapping analysis of the MFO-2 sample, revealing that Fe and Mn elements are homogeneously distributed in the whole MFO-2 sample. Fig. 1. XRD patterns of Fe3O4 and MnxFe3 − xO4 samples.

manganese oxides (e.g., MnO2 or Mn3O4) in anyone of the MnxFe3 − xO4 samples (Fig. 1), indicating that the Mn2+ is introduced into the magnetite structure rather than precipitating as a manganese oxide on the surface of the magnetite [25]. With the increase of Mn2 + content, the diffraction peaks slightly but continuously shift towards lower angles (Fig. 1), indicating a gradual expansion of magnetite lattice [26]. This can be explained by the fact that the radius of Mn2+ is 0.82 Å, whereas those of Fe2+ and Fe3+ are 0.77 and 0.65 Å, respectively [26]. Also, it is found that the diffraction peaks of the MnxFe3 − xO4 samples are somewhat weaker and broader than those of Fe3O4, indicating a slightly lower overall structural order. Nevertheless, all the MnxFe3 − xO4 samples still exhibit well-defined XRD patterns. Furthermore, according to the Scherrer formula, the grain sizes of Fe3O4, MFO-1, MFO-2, and MFO-3 were found to be 9.6, 9.2, 4.5, and 5.9 nm, respectively.

3.1.4. N2 adsorption/desorption N2 adsorption/desorption isotherms and BJH pore size distributions of Fe3O4, MFO-1, MFO-2, and MFO-3 are shown in Fig. 3. It can be found that all these samples show a type IV isotherm with a big H3 hysteresis loop (Fig. 3a). This type of hysteresis loop should result from the inter-particles pores [14]. Pore size distributions, calculated via BJH method, are presented in Fig. 3b. The Fe3O4, MFO-1, MFO-2, and MFO3 show peaks centered at ca. 72.6, 29.1, 46.1, and 30.0 nm, respectively, which should correspond to the inter-nanospheres pores due to nanosphere packing. Understandably, the larger the nanosphere size is, the larger the pore becomes. Besides, the MFO-2 has another peak at ca. 4.5 nm (Fig. 3b), which should correspond to the inner mesopores of MFO-2 nanospheres. The detailed texture parameters are summarized in Table 1. Obviously, the textural parameters of MnxFe3 − xO4 are found to be closely related to the Mn content. When the x value was 0.73, the corresponding sample (MFO-2) shows the largest surface area (86.3 m2·g−1) and highest porosity (0.53 cm3·g−1), which should

Fig. 2. FESEM and TEM images of Fe3O4 (a), MFO-1 (b), MFO-2 (c), and MFO-3 (d).

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Fig. 3. N2 adsorption/desorption isotherms (a) and pore size distributions (b) of four samples.

be ascribed to the presence of abundant mesopores inside MFO-2 nanospheres (Fig. 2c).

3.1.5. Possible Formation Process of MnxFe3 − xO4 On the basis of the above-obtained results, it is possible to shed light on the formation process of MnxFe3 − xO4 nanostructures. As depicted in the Experimental section, the Fe3O4 and MnxFe3 − xO4 samples were synthesized via one-step thermal treatment of FeCl3·6H2O (18.5 mmol), MnCl2·4H2O (9.3, 18.5, or 36.9 mmol), EG, NaAc, and EDA. It is usually supposed that EG acts as a high-boiling-point solvent and a reducing agent during a solvothermal process, while NaAc provides a basic medium and EDA plays a role in slowing down the nucleation and crystal growth rates because of its strong chelating ability towards transition-metal ions [27]. In the synthesis of pure Fe3O4, a portion of Fe3+ ions were reduced by EG and converted into Fe2+ ions. At the same time, NaAc in the solution hydrolyzed and released OH− ions. Subsequently, the Fe3O4 nanospheres were formed via the following reaction: 2Fe3+ + Fe2+ + 8OH− → F3O4 ↓ + 4H2O [27]. When the Mn2 + ions were introduced into the synthesis system, the Fe3 + and Mn2+ could directly react with OH− ions to form Mn2+-doped magnetite [9]. Accordingly, it is conceivable that the number of crystal nuclei would be much larger than that in the case of Fe3O4 formation and the final crystallites would be smaller in size. So, it was understandable that the nanosphere sizes of MFO-1 (25 nm), MFO-2 (80 nm), and MFO-3 (50 nm) were much smaller than that of Fe3O4 (120 nm) (Fig. 2). Nevertheless, the complex nature of the particle growth process made it difficult to obtain a straightforward correlation between particle size and changes in manganese precursor concentration [28]. Generally, the particle growth is believed to be a combination of nuclei growth and primary particle aggregation, and the main driving force for holding particles together in aggregates is the inter-particles cohesion [9]. In the present study, the left metal ions (e.g. Mn2+ and Fe3+) in solution might be adsorbed onto the surface of nucleus and form a positively charged layer [27], which would weaken the inter-particles cohesion and further prevent the aggregation of seeds. In cases of MFO-1, the left amount of metal ions might be considerably high, so the primary particles were well stabilized without significant aggregation. On the contrary, for MFO-3, the left amount of metal ions might be relatively low, so the secondary particles with a relatively dense packing could be formed from primary particles. Between the two conditions, the left amount of metal ions in the case of MFO-2 might be relatively moderate. As a result, the primary particles could form aggregate structures, but the structure could exhibit abundant mesopores (Fig. 3b).

3.1.6. Magnetic measurements As shown in Fig. 4, the saturation magnetization (Ms) values of Fe3O4, MFO-1, MFO-2, and MFO-3 samples are up to 68.2, 62.7, 51.6, and 47.5 emu·g−1, respectively. Previous studies have confirmed that the Ms values of magnetic particles are affected by their crystallinity and particle size [29]. As revealed by XRD results, the grain sizes of Fe3O4, MFO-1, MFO-2, and MFO-3 samples are 9.6, 9.2, 4.5, and 5.9 nm, respectively. This means an approximate correlation between the decrease of the saturation magnetization and the decrease of the grain size. The lower Ms value of MFO-3 than that of MFO-2 may be due to the poorer crystallinity of MFO-3 (Fig. 1). Interestingly, all four samples are found to be superparamagnetic (Fig. 4). This result might be due to the following reason: The primary nanoparticles of Fe3O4 and MnxFe3 − xO4 samples, below a certain critical size, possess single domain instead of a usual multidomain structure of bulk magnetic materials [29]. The rapid magnetic response ability and satisfactory water dispersability of MnxFe3 − xO4 samples were verified by a preliminary experiment. As shown in the insert of Fig. 4, for example, the magnetic MFO-2 nanoparticles can be completely separated from the aqueous solution when the solution is subjected to an external magnetic field within 1 min. Conversely, when the magnet is removed, slight agitation can also bring the MFO-2 nanoparticles to back into the aqueous solution

Fig. 4. Hysteresis curves of four samples with photos about the magnetic separability of MFO-2 (inset).

M. Li et al. / Materials and Design 97 (2016) 341–348

(Fig. 4). The result indicates that the MFO-2 sample has a potential advantage for recovering, recycling, and reusing it in the heterogeneous catalysis process. 3.2. Catalytic activity of MnxFe3 − xO4 Towards MB degradation The availability of MnxFe3 − xO4 samples as heterogeneous Fenton catalysts for degradation of organic pollutants was investigated in a neutral solution with the addition of H2O2. Methylene blue (MB), as a typical industrial pollutant, was chosen as a model with a high initial concentration (400 mg·L−1) to examine the catalytic efficiency of MnxFe3 − xO4 samples. For the sake of comparison, the catalytic degradation of MB on pure Fe3O4 was also studied. 3.2.1. Comparison of catalytic activity Fig. 5 displays the temporal evolution of the absorption spectra of four identical MB solutions degraded by Fe3O4, MFO-1, MFO-2, and MFO-3, respectively. It can be found that the pure Fe3O4 sample can contribute little to this catalytic process at room temperature. This is well consistent with the reported results [22,30]. In contrast, the MnxFe3 − xO4 samples (i.e., MFO-1, MFO-2, and MFO-3) show high catalytic efficiencies for the degradation of MB. To figure out the reason of MB degradation, two control experiments, without addition of MnxFe3 − xO4 or H2O2, were also conducted, it was found that no obvious decolorization could be observed in both cases (Fig. S2). These results indicated that the degradation of MB was caused by H2O2-induced oxidation and catalyzed by MnxFe3 − xO4 nanoparticles. Furthermore, the presence of Mn2+ in the oxides did play a vital role for the oxidative degradation of MB. As shown in Fig. 5, the catalytic efficiencies of the Mn2+-doped follow the order: MFO-1 b MFO-2 ≈ MFO-3. For example, the degradation degrees of aqueous MB reach 63.6%, 73.2%, and 71.1% within the initial 30 min for MFO-1, MFO-2, and MFO-3, respectively; and after 120 min, the degradation degrees are up to 85.8%, 91.4%, and 92.5%; the total degradation degrees achieve 97.5%, 98.9%, and 98.2% within 360 min.

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However, it should be noted that the catalytic performance described only by the degradation degree towards MB might be unconvincing and improper because the weight of catalyst, the volume of H2O2 solution used, the initial concentration of MB, and the volume of MB solution were all responsible for the degradation of MB. Recently, the catalytic performance of catalyst has been proposed to be estimated by the equation as follows [31]:

Q¼

ðC 0 −C ÞV V 0m

ð1Þ

where Q (g·L−1·g−1) is the consumption of MB caused by 1 g of catalyst and 1 L of 30 wt% H2O2 solution; V (mL) and V′ (mL) are the volume of the MB solution and the volume of 30 wt% H2O2 solution, respectively; m (g) is the mass of the catalyst; C0 (g·L−1) and C (g·L−1) are the concentrations of MB at the beginning and ending of degradation, respectively. By calculating and comparing the Q values of Fe3O4 (19.61 g·L− 1·g− 1), MFO-1 (518.24 g·L− 1·g−1), MFO-2 (528.48 g·L−1·g−1), and MFO-3 (522.84 g·L−1·g−1), it is found that the MnxFe3 − xO4 samples have much higher catalytic activities than pure Fe3O4 (Table 2). Moreover, the Q values of MnxFe3 − xO4 samples (especially MFO-2) are 1–2 orders of magnitudes greater than that over other Fe-based or Mn-based heterogeneous catalysts (Table 2) [8, 31–41], indicating the remarkable activities of these catalysts. 3.2.2. Kinetic analysis To describe MB-decolorizing processes more clearly, the kinetic experiments were conducted over more time points with MnxFe3 − xO4 as the catalysts (Fig. 6), and the kinetic data were fitted to the following kinetic equation [42]: −

dC t ¼ kC t : dt

Fig. 5. Normalized UV–vis spectra of MB vs. reaction time in the presence of Fe3O4 (a), MFO-1 (b), MFO-2 (c), and MFO-3 (d).

ð2Þ

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Table 2 Comparison of catalytic efficiencies between various catalysts. Catalyst

Q (g·L−1·g−1)

Temperature (°C)

Ref.

Hierarchical mesoporous MnO2 Fe3 − xTixO4 ACK/Fe β-MnO2 nanorods β-MnO2 nanorods β-MnO2 hollow octahedral Graphene oxide with organo-building blocks of Fe-aminoclay Fe-species-loaded mesoporous MnO2 Fe3O4/FeMnOx core/shell nanoparticles Mesoporous manganese ferrite nanocomposites Magnetic Fe2MnO4 activated carbons Fe2.77Mn0.23O4 Fe3O4 MFO-1 MFO-2 MFO-3

12.26 3.00 12.00 2.46 8.31 3.20 2.16 18.96 3.00 15.00 36.67 26.81 19.61 518.24 528.48 522.84

30 30 At room temperature At room temperature At room temperature 17 20 ± 0.2 25 25 ± 1 27 29 ± 1 25 30 30 30 30

[31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] This work This work This work This work

Then,  ln

C0 Ct

 ¼ kt

ð3Þ

where Ct and k are the MB concentration at time t and the rate constant, respectively. The fitting results are shown in Fig. 6. It can found that, the initial rate constants (k, min− 1) of the reaction were calculated as 1.05 × 10−4, 1.244 × 10−2, 1.594 × 10−2, and 1.541 × 10−2 for Fe3O4, MFO-1, MFO-2, and MFO-3, respectively, indicating that the degradation rates of MB on MnxFe3 − xO4 samples are much more rapid than that of MB on Fe3O4. Compared with the reported homogeneous and heterogeneous Fenton systems [31,43], these MnxFe3 − xO4-based systems also show a higher initial rate constant, indicating that the catalytic oxidation process progresses more quickly. It can be inferred that the Mn2+ in the oxides plays an important role in enhancing catalytic activity; further discussion is presented in Section 3.2.4. In the second set of experiments, the reaction temperature was varied with the purpose of both investigating its effects on the degradation kinetics and allowing the determination of the Arrhenius-type dependence of the rate constant k on the temperature (MFO-2 was used as model catalyst due to its best catalytic performance) [44]: ð−Ea =RT Þ

k ¼ Ae

The results obtained for the MB degradation at four different temperatures (20, 30, 40, and 50 °C) using MFO-2 as catalyst are shown in Fig. 7a. Clearly, the reaction rate increases with the increase of temperature, which is expected due to the exponential dependency of the kinetic constants with the reaction temperature (Arrhenius law) [45]. The higher catalytic activity of MFO-2 at higher temperatures implies an accelerated decomposition of H2O2 into hydroxyl radicals. According to the Eq. (3), the initial rate constants of the reaction were calculated as 0.575 × 10−2, 1.594 × 10−2, 3.791 × 10−2, and 6.525 × 10−2 at 20, 30, 40, and 50 °C, respectively. Furthermore, an Arrhenius plot (lnk vs. 1/T) is shown in Fig. 7b, from which the apparent activation energy Ea for MB degradation on MFO-2 is calculated to be 64.63 kJ·mol−1. Generally, the Ea values of ordinary chemical reactions are usually between 60 and 250 kJ·mol−1 [45]. The results presented here imply that the catalytic oxidation of MB in aqueous solution by this oxidation process requires a lower activation energy and can be easily achieved. 3.2.3. Plausible catalytic mechanism Fenton oxidation is a quite complex process, and its mechanism is still far from fully understood [46]. Notwithstanding, it seems to be a general agreement that the limiting step is the formation of the hydroxyl radicals through the following reaction [46]: Fe2þ þ H2 O2 →Fe3þ þ HO  þHO– :

ð5Þ

ð4Þ

where A is the pre-exponential (or frequency) factor and Ea is the activation energy, respectively.

Fe2+ is slowly regenerated through the reaction between Fe3+ and H2O2: Fe3þ þ H2 O2 →Fe2þ þ HOO  þHþ :

ð6Þ

The HO• species formed through Eq. (5) will attack the organic pollutants present in the wastewater [46]. In contrast to the Fe2+ ions free in solution, the Fe2+ centers on a spinel are not sufficiently efficacious for the decomposition of H2O2 [47]. So, it is not surprising that pure Fe3O4 showed low catalytic activity towards MB degradation. Previous studies have indicated that the presence of Mn in the spinel structure could produce a remarkable increase in the H2O2 decomposition according to the highly active reaction [22,48]: Mnsurf 2þ þ H2 O2 →Mnsurf 3þ þ HO  þHO– :

Fig. 6. Normalized concentration of MB vs. reaction time and fitting results of kinetic data.

ð7Þ

The Mn3+ sruf species could be readily reduced to regenerate the active Mn2+ species by two possible pathways: i) via electron transfer; and ii) via oxygen vacancies [22]. When the Mn content in MnxFe3 − xO4 is relatively low (i.e., 0 b x b 1, like MFO-1), the single electron transfer reduction of Mn3+ by the Fe2+ could occur via the following thermodynamically favorable reaction

M. Li et al. / Materials and Design 97 (2016) 341–348

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Fig. 7. Normalized concentration of MB vs. reaction time in the presence of MFO-2 (a), and the plot of lnk vs. 1/T (b).

[22]: Fe2þ þ Mn3þ →Fe3þ þ Mn2þ



 E0 ¼ 0:73 V :

ð8Þ

Considering the fact that spinel Fe3O4 is a semiconductor with a narrow band gap (0.1 eV) and shows a very high conductivity with almost metallic character (ca. 102–103 Ω−1 cm−1) [22], it is possible that the fast electron transport in MFO-1 results in a relatively high activity in decomposing H2O2 into free radicals, leading to relatively high activity towards MB degradation. If the Mn content in MnxFe3 − xO4 is sufficiently high (i.e., 1 b x b 2, like MFO-2 and MFO-3), that is to say, all Fe2 + ions and some Fe3 + ions in the spinel structure have been replaced with Mn2 + ions, and the oxygen vacancies (V) will be generated [22]. Further, the oxygen vacancies will react with Mn3+ as a consequence of the requirement for the global electroneutrality of MnxFe3 − xO4, which leads to a transformation of Mn3+ into Mn2+ [49]. A simplified view of this mechanism is given in Eq. (9). The oxygen vacancies can be efficiently regenerated according to Eq. (10) [22]. 2Mnsurf 3þ þ Vsurf þ H2 O→2Mnsurf 2þ þ Vsurf −O þ 2Hþ Vsurf −O þ 2H2 O2 →Vsurf þ H2 O þ O2

ð9Þ

vacancies in its structure than MFO-2, the later showed a slightly higher catalytic activity (Table 2). This phenomenon might be explained by the fact that the MFO-2 consists of nanospheres containing abundant mesopores, as shown by TEM (Fig. 2).

3.2.4. Stability of catalyst The stability of catalyst is an important factor related to its application potential. As an example, the recyclability of MFO-2 catalyst was evaluated for the repeated degradation of MB under identical conditions. An external magnet was used to separate the MFO-2 from the solution after each treatment. Then the separated MFO-2 were added again to fresh MB solution. This process was repeated for five times. Results indicate a slight decrease of catalytic ability of the MFO-2 with the increase of the recycle times (Fig. 8), which may result from the loss of catalyst and adsorption of organic reactants on the catalyst. Degradation of MB exceeds 90% after five consecutive cycles, indicating that the asprepared magnetic porous MFO-2 has a satisfactory catalytic stability during the degradation reaction. Although five times is not long enough to evaluate the long-term performance, we can preliminarily deduce that the MFO-2 catalyst has a certain potential for sustainable use in the Fenton process.

ð10Þ 4. Conclusion

Because of the favorable Mn3 + reduction by oxygen vacancies regenerating the active species Mn2+, both MFO-2 and MFO-3 showed remarkable catalytic activities towards MB degradation. In addition, although MFO-3 contained a larger amount of Mn2 + and oxygen

Fig. 8. The degradation degree (%) vs. cycle number with MFO-2 as catalyst.

In the present study, a series of interesting MnxFe3 − xO4 samples (Mn0.73Fe2.27O4, Mn1.07Fe1.93O4, and Mn1.19Fe1.81O4) were prepared by a simple one-step solvothermal route and tested in catalyzing the degradation of MB. The physicochemical characteristics of the MnxFe3 − xO4 nanomaterials were carefully measured, and the results showed that the Mn1.07Fe1.93O4 exhibited a unique combination of good monodispersity, regular morphology (sphere-like), abundant porosity, uniform size, distinct crystalline structure, satisfactory water dispersability, and high magnetic responsivity. Activities of these MnxFe3 − xO4 samples were evaluated for catalytic degradation of MB under mild conditions, and the results showed that the MnxFe3 − xO4 samples, especially the Mn1.07Fe1.93O4, had a much higher activity than other Fe-based or Mn-based heterogeneous catalysts reported in the literatures. It was demonstrated that the large surface area of MnxFe3 − xO4 as well as the high activity and favorable regeneration of the Mn2+ sites were responsible for the excellent performance. Taking advantage of high magnetic responsivity, the MnxFe3 − xO4 could be easily separated from reaction medium by addition of an external magnetic field, greatly facilitating the recovery and reuse of catalysts. The results obtained indicate the MnxFe3 − xO4 catalysts to be suitable candidates for the removal of organic pollutants in wastewaters by means of the heterogeneous Fenton reaction without the need of any external energy input.

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Acknowledgements The authors acknowledge the research grant provided by the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUG120115), Special Fund for Basic Scientific Research of Central Colleges, China University of Geosciences (Wuhan) (No. CUGL090305), Land Resources Geology Survey Projects of China (Grant No. 12120113015300), and National Natural Science Foundation of China (Grant No. 21303170). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.matdes.2016.02.103. References [1] S.H. Sun, H. Zeng, D.B. Robinson, S. Raoux, P.M. Rice, S.X. Wang, G.X. Li, Monodisperse MFe2O4 (M = Fe, Co, Mn) nanoparticles, J. Am. Chem. Soc. 126 (2004) 273–279. [2] H. Yang, C.X. Zhang, X.Y. Shi, H. Hu, X.X. Du, Y. Fang, Y.B. Ma, H.X. Wu, S.P. Yang, Water-soluble superparamagnetic manganese ferrite nanoparticles for magnetic resonance imaging, Biomaterials 31 (2010) 3667–3673. 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