DOI: 10.1002/chem.201304221
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& Confined Nanoreactors
Assembly of a Nanoreactor System with Confined Magnetite Core and Shell for Enhanced Fenton-Like Catalysis Tao Zeng,[a] Xiaole Zhang,[a, b] Saihua Wang,[a] Yurong Ma,[a] Hongyun Niu,[a] and Yaqi Cai*[a]
Abstract: Conventional solid catalysts for heterogeneous Fenton-like reactions in bulk solution usually suffer from aggregation and vulnerability, which greatly lower the catalytic efficiency and hamper their practical application. Herein, we demonstrate a promising yolk–shell nanostructure with both the core and the shell composed of magnetite (designated as yolk-like Fe3O4@Fe3O4/C) as a nanoreactor capable of accommodating the Fenton-like reaction into its void space. Benefiting from the mesoporous shell and perfect interior cavity of this composite, reactants can access and be abundantly confined within the microenvironment where Fe3O4 sites are dispersed on the entire cavity surfaces, thus leading to a higher catalytic efficiency compared with the conventional solid catalysts in bulk solution. The chosen model re-
Introduction Heterogeneous Fenton-like reactions on solid catalysts are increasingly replacing iron salt-based homogeneous Fenton reactions because of the advantages associated with avoiding the precipitation of soluble iron salts and the difficulties in recycling.[1] Recently, zero-valent iron,[2] iron-based clays,[1b, 3] ironcontaining materials,[4] and iron oxide minerals[5] as heterogeneous Fenton-like catalysts have been the subject of extensive research and the highly reactive and non-selective hydroxyl radicals (COH) generated can decompose recalcitrant organic pollutants, thus offering a promising alternative for environmental remediation. Among these catalysts, magnetite (Fe3O4) and maghemite (g-Fe2O3) have attracted more interest owing to their good catalytic activity and fantastic magnetically separable properties, which are of significant economic and environmental benefit owing to the reduced consumption of additional substances, energy, and time used for catalyst recy-
[a] Dr. T. Zeng, Dr. X. Zhang, S. Wang, Y. Ma, H. Niu, Prof. Y. Cai State Key Laboratory of Environmental Chemistry and Ecotoxicology Research Center for Eco-Environmental Sciences Chinese Academy of Sciences, Beijing 100085 (China) Fax: (+ 86) 10-62849182 E-mail: [email protected] [b] Dr. X. Zhang College of Life Science Hebei United University, Tangshan 063000 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304221. Chem. Eur. J. 2014, 20, 6474 – 6481
action of chlorophenols degradation in the presence of the as-prepared materials as well as hydrogen peroxide (H2O2) confirms this assumption. Under the optimal reaction conditions, more than 97 % 4-chlorophenol (4-CP) can be degraded in the Fe3O4@Fe3O4/C nanoreactor, whereas only 28 % can be achieved by using bare Fe3O4 particles within 60 min. Furthermore, owing to the existence of the outermost carbon layer and high-magnetization properties, the nanoreactor can be re-used for several runs. The synthesized nanoreactor displays superior catalytic activity toward the Fenton-like reaction compared with the bare solid catalysts, and thereby holds significant potential for practical application in environmental remediation.
cling.[6] However, most Fenton-like iron oxide-based catalysts reported in previous studies have tended to agglomerate during operation, resulting in a significant deterioration of the catalytic activity.[6b] Another issue of concern is that these catalysts are unstable and the catalytic sites are contaminated readily when directly exposed in bulk reaction solution.[7] Accordingly, it was considered that transferring the Fenton-like reaction into a nanoreactor might be an ideal way to address the above difficulties. In addition, several recent studies have proved that catalysts confined in the inner space of nanoreactors display enhanced catalytic performance.[8] In this regard, the employment of nanoreactors for Fenton-like catalysis would help to protect the active sites and achieve improved catalytic efficiency, however, little work has been reported in this respect.[9] As a novel class of complex nanoarchitectures, yolk–shell structures with a void space between the core and the shell can provide a microenvironment for a variety of fields such as chemical storage,[10] compartmentation,[11] and drug loading.[12] In particular, this nanostructure shows promise for applications as a nanoreactor for several important reactions because of its unique feature that the permeable shell can protect the core from aggregation while allowing the diffusion of small active molecules in and out of the interior microenviroment.[13, 14] In the case of yolk–shell nanostructures utilized in Fenton-like catalysis, most existing materials that encapsulate a metal core inside the inert shell are unable to efficiently catalyze the reaction due to the lack of sufficient Fenton-like catalytically active sites within the nanoreactor, although some of them contain
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Full Paper a single magnetite core which is commonly used for either convenient separation or catalyst support.[14b, 15] To increase the potential applications of such nanoarchitectures as nanoreactors in Fenton-like catalysis, it is essential to develop new methods for functionalizing the internal cavity. A catalytic shell instead of the conventional inert shell together with a catalytic core that surrounds the interior space would be more desirable for providing a large catalytic surface area in the cavity and thus producing nanoreactors with enhanced activity.[13a, 16] To the best of our knowledge, no study so far has succeeded in designing and utilizing yolk–shell nanoreactors with both the core and shell composed of active components for enhanced Fenton-like catalysis. In contrast to previous studies in which Fenton-like reactions directly took place in bulk solution, of interest to us in this work is the arrangement of this reaction into a special confined microenvironment to achieve a higher catalytic performance. Novel yolk–shell nanoreactors with a Fe3O4 core, a mesoporous Fe3O4/C double-layered shell, and a void space between the core and the shell (see the structural model in Figure 1 a) were therefore prepared in which each part of the
Figure 1. a) Schematic illustration for the preparation of a yolk–shell Fe3O4@Fe3O4/C nanoreactor. TEM images: b) Fe3O4, c) Fe3O4@SiO2, d, e) Fe3O4@SiO2@Fe3O4/C. f) Structural model of Fe3O4@SiO2@Fe3O4/C. g) STEM image and EDX mapping of Fe3O4@SiO2@Fe3O4/C.
nanoreactor has its crucial role in the functioning. The cavity of the nanoreactor provides a perfect microenvironment where the Fenton-like reaction can be facilitated due to the confinement effect toward the reactants and the abundant active sites found on the entire cavity surface. The Fe3O4 constituents derived from the core together with the inner shell can also enable the magnetic isolation from the suspension once the catalytic reaction is complete, instead of just serving as catalytic sites. The outer carbon layer of the shell may help to enrich Chem. Eur. J. 2014, 20, 6474 – 6481
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the organic reactants into the cavity, while acting as a buffer to abate the erosion of interior Fe3O4 components by external harsh conditions. The Fenton oxidation of chlorophenols, a group of highly toxic environmental contaminants, was then chosen to check the catalytic performance of the yolk-like Fe3O4@Fe3O4/C microsphere as a nanoreactor. As expected, the catalytic ability of the nanoreactor strikingly exceeded that of the bare Fe3O4. Finally, the applicability of this composite in heterogeneous Fenton-like reactions was systematically assessed in terms of the influence of the main factors, extent of mineralization, degradation intermediates, as well as the reusability of the material. From the point of view of fundamental research and practical application, such a novel yolk–shell catalytic system may open new opportunities for improved Fenton-like catalysis used in environmental remediation.
Results and Discussion The typical synthesis of the yolk–shell Fe3O4@Fe3O4/C nanoreactor is illustrated in Figure 1 a. The pre-synthesized magnetic Fe3O4 particles (Figure 1 b) were first coated with a silica shell through the hydrolysis of tetraethyl orthosilicate (TEOS) to form a core–shell structured Fe3O4@SiO2 microsphere (Figure 1 c). A double-layer coating of the inner Fe3O4 layer and the outer carbon layer was then deposited simultaneously on the surface of Fe3O4@SiO2 by a hydrothermal reaction with ferrocene and hydrogen peroxide (H2O2). From the TEM images of Fe3O4@SiO2@Fe3O4/C (Figure 1 d and e), it can be seen that the microspheres exhibit a defined multilayered structure. The Fe3O4 cores with a mean diameter of 200 nm are wrapped in a 40 nm thick silica shell, which is successively covered by another shell consisting of a deep contrast inner layer and a light contrast outer layer. The bright parts in the STEM image (Figure 1 g) suggest the distribution of metal elements, which is further confirmed by the detailed EDX mapping analysis. It was found that the Fe signal was dispersed in both the shell and the core, the Si signal was only present in the middle layer, whereas the O signal was distributed throughout the composites. These results indicate the successful integration of different components in the Fe3O4@SiO2@Fe3O4/C hybrid. The yolk-like Fe3O4@Fe3O4/C nanoreactor was finally obtained by the selective etching of the silica shell with aqueous ammonia.[12] The typical TEM and SEM images in Figure 2 a show that the as-prepared nanoreactors exhibit a regular spherical shape with very rough surfaces, and a void space between the core and the shell is distinctly formed, which is further evidenced by a purposely selected broken sphere (see the inset in Figure 2 b). The structure of the double-layered shell is not destroyed after the corrosion of the SiO2 layer, although the outermost carbon layer seems to be somewhat intolerable during the hydrothermal reaction of aqueous ammonia and shows a slight decrease in thickness (Figure 2 b). Moreover, the obtained nanoreactors exhibit good dispersibility, thus avoiding the decrease in catalytic performance caused by aggregation. The Fe and O signals in the EDX mapping results (see Figure S2 in the Supporting Information) present a similar yolklike distribution, thus implying the successful preparation of
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Figure 2. a) TEM image of yolk–shell Fe3O4@Fe3O4/C nanoreactor. b) Magnified TEM image of Fe3O4@Fe3O4/C nanoreactor. c, d) HRTEM images of the shell and the core, respectively. The insets in (a) and (b) are the SEM images of the Fe3O4@Fe3O4/C nanoreactor.
the yolk–shell nanoreactor. High-resolution TEM images of the shell and the core (Figure 2 c and d) display regular lattice fringes with an interplanar spacing of 0.299 nm, which is highly consistent with the d value of the (220) planes of magnetite structure.[17] The XRD patterns of samples at each step are displayed in Figure 3 a. Six characteristic signals at 2q = 30.1 8, 35.4 8, 43.1 8, 53.4 8, 57.5 8, and 62.9 8 correspondingly indexed as (220), (311), (400), (422), (511), and (440), can be clearly distinguished, which matches well with the database of the face-center-cubic phase of Fe3O4 (JCPDS Card No 19-0629).[18] In the XRD patterns of other samples, similar features to those of pure Fe3O4 particles are observed, thus confirming the presence of the magnetite component in all of these composites. For Fe3O4@ SiO2 and Fe3O4@SiO2@Fe3O4/C, the broad band at 2q = 22 8 can be assigned to the amorphous silica shell (JCPDS Card No. 290085),[19] which disappeared in the pattern of yolk-like Fe3O4@ Fe3O4/C owing to the elimination of silica. The synthesis of yolk–shell Fe3O4@Fe3O4/C can also be monitored by FTIR analysis as shown in Figure 3 b. The absorption band observed at 560 cm 1 in all the spectra is related to
Figure 3. a) XRD patterns of various samples. b) FTIR spectra of various samples. c) N2 adsorption–desorption isotherms of yolk-like Fe3O4@Fe3O4/C nanoreactor, the inset shows the distribution of pore diameter. d) VSM curves of various samples. Chem. Eur. J. 2014, 20, 6474 – 6481
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Full Paper Fe O groups. After coating the silica shell onto the Fe3O4 surface, some new signals emerged around 810–950 cm 1 and 1080 cm 1, and several corresponding to the Si-O-Si and Si OH stretching bands, whereas the Fe O band was weakened because of the shielding effect. In comparison with the spectrum of Fe3O4@SiO2, a mild decrease in the silica bands and an increase in the Fe O band seen in the spectrum of Fe3O4@SiO2@ Fe3O4/C indicate the successful integration of the Fe3O4/C layer. The presence of signals at 1600–1800 cm 1 assigned to C=O stretching vibrations means that abundant carboxyl groups exist in the outer shell, which can help to increase the dispersivity of the materials in aqueous solution.[20] In regard to the spectrum of yolk-like Fe3O4@Fe3O4/C, the Fe O band further strengthened and the Si-O-Si bands almost vanished because of the removal of the middle silica shell and partial etching of the outermost carbon layer. Nitrogen adsorption–desorption isotherms of the core shell (see Figure S3 in the Supporting Information) and yolk shell (Figure 3 c) Fe3O4@Fe3O4/C exhibit typical Langmuir I-V behavior, indicating the presence of well-defined mesopores in the structure. As compared to the curve of solid Fe3O4@SiO2@ Fe3O4/C, the larger hysteresis loop appearing in yolk-like Fe3O4@Fe3O4/C represents unique ink-bottle-shaped pores, in which large cavities are connected through narrow channels.[12b] This is consistent with the nanostructure observed in the TEM images. Notably, the total pore volume increases significantly from 0.18 to 0.31 cm3 g 1 after the treatment of the solid Fe3O4@SiO2@Fe3O4/C with aqueous ammonia. Evidently, the interior large cavity left after the removal of silica is responsible for the pore volume increase. Calculated from the desorption branch of the nitrogen isotherm with the BJH method, the pore diameter of yolk-like Fe3O4@Fe3O4/C is measured to be 3.84 nm, which is larger than that of core-shell Fe3O4@SiO2@Fe3O4/C (3.41 nm). The enlargement of pore diameter can also be attributed to the etching of aqueous of ammonia. Thanks to the existence of these mesopores in the shell, the small reactive molecules are permitted to enter and leave the interior cavity readily. The magnetic properties of the as-prepared samples were evaluated by using a vibrating sample magnetometer at room temperature. Figure 3 d illustrates the M–H hysteresis loops of these composites measured by sweeping the external field between 1 and 1 T. All the four magnetization curves show no remanence and coercivity, suggesting the superparamagnetic character of each sample. The saturation magnetization value of Fe3O4@SiO2 (41.8 emu g 1) is lower than that of Fe3O4 (63.7 emu g 1) due to the coating of the non-magnetic silica shell. After covering the surface of Fe3O4@SiO2 with another magnetic Fe3O4/C layer, the magnetization of Fe3O4@SiO2@Fe3O4/C (33.5 emu g 1) exhibits a slight decrease rather than a growth, which is possibly attributed to the simultaneous introduction of a thick layer of mesoporous carbon together with the Fe3O4 layer. As for the final product yolk–shell Fe3O4@Fe3O4/C, the magnetization remarkably increases to 53.1 emu g 1 owing to the full etching of the non-magnetic silica component. Consequently, the strong magnetic Fe3O4 components in the Fe3O4@Fe3O4/C product provide an easy Chem. Eur. J. 2014, 20, 6474 – 6481
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and convenient way to separate them from the reaction system in addition to offering an efficient Fenton-like catalytic active site. The catalytic performance of a yolk-like Fe3O4@Fe3O4/C microsphere as nanoreactor was evaluated by using the model reaction of degradation of 4-CP under the assistance of H2O2. The initial concentration of 4-CP was 1.56 mm, and the weight of solid catalysts in the suspension (pH 4.0) was 0.5 g L 1. Aliquots were taken out after regular intervals of reaction time, and the degradation level was determined by liquid chromatography after removing the solid materials. Various control experiments were performed to compare the efficiencies of 4-CP removal, and all reactions were conducted in the dark to avoid the influence of light. As shown in Figure 4 a, use of H2O2 alone leads to a slight removal of 4-CP within 210 min, which is negligible compared with the fast degradation of 4-CP by heterogeneous Fenton-like reaction. When bare Fe3O4 nanoparticles were applied as a Fenton-like catalyst, about 97 % removal of 4-CP was observed after 210 min in the presence of
Figure 4. a) Fenton-like degradation of 4-CP under different conditions: I) 0.5 g L 1 as-prepared yolk-like Fe3O4@Fe3O4/C nanoreactor and 20 mm H2O2, II) 0.5 g L 1 bare Fe3O4 and 20 mm H2O2, III) 0.5 g L 1 solid SiO2@Fe3O4/C without cavity and 20 mm H2O2, IV) 0.5 g L 1 yolk-like SiO2@Fe3O4/C and 20 mm H2O2, V) 20 mm H2O2, and VI) 0.5 g L 1 yolk-like Fe3O4@Fe3O4/C. Other reaction conditions were set as follows: pH 4.0, T = 298 K, initial 4-CP concentration = 1.56 mm. C0 and Ct denote the initial concentration and instantaneous concentration at various reaction times of 4-CP, respectively. b) Schematic illustration showing the Fenton-like degradation of 4-CP in the yolk-like Fe3O4@Fe3O4/C nanoreactor.
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Full Paper H2O2. For comparison, solid SiO2@Fe3O4/C without cavities and yolk–shell SiO2@Fe3O4/C with cavities (see TEM images in Figure S4 in the Supporting Information) were fabricated and employed to catalyze the degradation of 4-CP. It can be found that the solid SiO2@Fe3O4/C is also able to degrade 97 % of 4CP within 180 min. These means that both the Fe3O4 core and Fe3O4/C shell can catalyze the reaction solely. As for the yolk– shell SiO2@Fe3O4/C composite, it exhibits a higher catalytic activity than those of solid SiO2@Fe3O4/C and bare Fe3O4, and only 120 min was required to remove almost all 4-CP. We hypothesized that the superior catalytic performance of yolk– shell SiO2@Fe3O4/C may be ascribed to its unique yolk-like structure. Most notably, 4-CP could be almost completely degraded within 60 min using the yolk-like Fe3O4@Fe3O4/C composite as a nanoreactor, implying a higher catalytic efficiency of it than that of the yolk–shell SiO2@Fe3O4/C composite. Since the shell and cavity size of these two samples are very similar (see Figure S4 in the Supporting Information), the superior activity of Fe3O4@Fe3O4/C can be only attributed to the presence of the Fe3O4 core. The negligible decrease of 4-CP (less than 10 %) with the yolk-like Fe3O4@Fe3O4/C nanoreactor in the absence of H2O2 may be attributed to the surface adsorption ability of the carbon layer. The excellent catalytic performance of yolk-like Fe3O4@ Fe3O4/C nanoreactors for the degradation of 4-CP is explained as illustrated in Figure 4 b. First, the mesoporous Fe3O4/C shells favor the enrichment of reactants from bulk solution and the fast diffusion of them toward Fe3O4 active sites, which is beneficial for the heterogeneous reaction. Second, the cavity of the yolk-like nanoreactor provides a microenvironment to confine the reactants, leading to a higher instantaneous concentration of reactants in the interior of the nanoreactor and thus affording a driving force to increase the degradation rate.[21] Third, the inner Fe3O4 layer of the shell together with the Fe3O4 cores surrounding the void space are both bestrewn with available active sites, which greatly increases the accessibility of reactants to catalytic sites so as to accelerate the degradation reaction. Since the solution pH has a remarkable effect on the Fenton reaction, the influence of the pH of the initial solution on the degradation of 4-CP in the yolk-like Fe3O4@Fe3O4/C nanoreactor was estimated. As shown in Figure S5 a in the Supporting Information, the decrease of 4-CP is over 95 % within 60 min at pH 3.0 and 4.0, although the difference in degradation rate between them exists. The degradation efficiency at pH 5.1 is not as high as those at low pH. At pH 6.5, the degradation rate further reduced and the conversion of 4-CP is less than 20 % over a period of 150 min. The results show a similar tendency to previous reports that the reaction rate of the Fenton process decreased with the increase of solution pH.[21] The degradation of 4-CP with different catalyst dosage and different H2O2 concentrations was also investigated and the results are shown in Figure S5 b and Figure S5 c in the Supporting Information. The efficiency of 4-CP removal after a reaction time of 60 min increased from 53 % to 96 % with the Fe3O4@Fe3O4/C dosage increasing from 0.1 g L 1 to 1 g L 1. This originates from the fact that there is an increasing number of active sites available for Chem. Eur. J. 2014, 20, 6474 – 6481
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the H2O2 decomposition.[22b] The degradation percentage of 4CP within 150 min increased from 33 % to 99 % as the H2O2 concentration increased from 1.2 to 20 mm. The faster degradation rate of 4-CP under higher H2O2 concentration must be attributed to the fact that the 4-CP removal is directly related to the concentration of the reactive oxygen species (ROS) produced from the reaction between H2O2 and Fe3O4. However, as the H2O2 concentration further increased, an inhibition instead of an improvement of 4-CP removal was observed. A possible reason for this is the scavenging effect of excessive H2O2 toward COH, which leads to a competitive reaction with COH between 4-CP and H2O2.[4b, 23] A control experiment was first carried out to verify the possible contributions of a homogeneous reaction catalyzed by the leached metal ions. Yolk-like Fe3O4@ Fe3O4/C was suspended in 20 mm H2O2 at pH 4.0 for 45 min and then isolated by an external magnet. The filtrate was tested for the degradation of 4CP under the same conditions as the solid catalyst and a conversion of only about 4 % was detected after a reaction time of 60 min, which is much lower than that observed for solid Fe3O4@ Fe3O4/C. Therefore, it can be concluded that the degradation of 4-CP mainly occurs in a heterogeneous reaction rather than a homogeneous reaction. To identify the primary ROS for the degradation of 4-CP in the heterogeneous catalytic process, tert-butyl alcohol, which has high reactivity with COH, was adopted as scavenger for the reaction. The results (Figure 5 a) show that the degradation of 4-CP was significantly inhibited by the addition of tert-butyl alcohol. The efficiency of 4-CP removal decreased with an increasing concentration of tert-butyl alcohol and was almost completely restrained in the presence of 25 mm tert-butyl alcohol. This indicates that 4-CP removal is primarily attributed to the oxidation ability of COH in the Fenton-like system.[24] The degree of mineralization of 4-CP was analyzed by the TOC content of the reaction solution. Figure 5 b shows that the final TOC value of the reaction solution was reduced to 50.8 % of the original solution, implying that some organic residues derived from 4-CP decomposition remained in solution. In addition, the possible derivatives generated from the eroded carbon layer may contribute to the residual TOC as well. Since the toxicity of CPs is associated with chloride substituents, the dechlorination level was also determined. A dechlorination efficiency of 86.4 % indicated that most of the chlorines bound to the aromatic ring were released. The main intermediate products generated during different degradation periods were identified by HPLC and IC analysis (Figure 6 a). The concentration of hydroquinone initially increased to a peak value at 30 min, and then slowly decreased till complete elimination within 180 min. Some smaller molecular organic acids including formic acid, fumaric acid, malonic acid, and acetic acid were gradually generated and eventually left in the reaction solution, which could account for the residual TOC in solution. The identified intermediates were in agreement with the results of some work on the degradation of CPs by other kinds of catalysts. Based on these intermediates and some reported information,[22a, 25] a possible degradation pathway of 4-CP catalyzed by yolk-like Fe3O4@Fe3O4/C in the presence of H2O2 was
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Figure 5. a) Effect of the tert-butyl alcohol scavenger on the degradation of 4-CP. b) The efficiency of TOC removal and dechlorination in the degradation of 4-CP. Reaction conditions: T = 298 K, pH 4.0, initial 4-CP concentration = 3.12 mm, and catalyst dosage = 0.5 g L 1.
proposed. The chlorine atom at the para-site of 4-CP was first detached by the attack of COH to form a hydroxyphenyl radical, which could then undergo another attack of COH to yield hydroquinone. Hydroquinone is capable of being rapidly oxidized to benzoquinone under such oxidation conditions. Subsequently, the ring of benzoquinone was further cleaved by COH to form short chain carboxylic acids such as fumaric acid and malonic acid, which were subsequently degraded to acetic acid and formic acid or thoroughly oxidized to CO2 and H2O. The catalytic activity for the Fenton-like oxidation of several chlorophenol analogues including 2-CP, 3-CP, 2,4-DCP, and 2,4,6-TCP was also investigated to gain insight into the universality of the yolk-like Fe3O4@Fe3O4/C nanoreactor. All the reactions for these compounds were carried out under the same conditions as that for 4-CP and the progress of each of these reactions was then monitored by HPLC. As shown in Table S1 in the Supporting Information, similar conversion efficiencies were obtained for the one-chlorine-atom-containing CPs (2-CP, 3-CP, and 4-CP) regardless of the position of the chlorine substituent, and they could be almost completely removed within 60 min. With regard to 2,4-DCP and 2,4,6-TCP, however, only 85.5 % and 50.1 % were degraded within a reaction time of Chem. Eur. J. 2014, 20, 6474 – 6481
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Figure 6. a) Evolution of the intermediates with reaction time. b) Reusability of the nanoreactors in the degradation of 4-CP. Reaction conditions: T = 298 K, pH 4.0, initial 4-CP concentration = 1.56 mm, and catalyst dosage = 0.5 g L 1.
300 min, demonstrating a relative low reaction rate compared with those of the three CPs containing one chlorine atom. This is in good accordance with the findings in other work that the chlorine content of different chlorophenol compounds often had a significant effect on the oxidation rate because some favorable sites of the aromatic ring susceptible to COH attack could be blocked by chlorine atoms, thus inhibiting the oxidation of CPs.[26] An important issue with regard to the potential application of these materials is the reusability and stability in the operating process. This was investigated by repeated usage of the Fe3O4@Fe3O4/C nanoreactor for the catalytic oxidation of 4-CP. With the superparamagnetism character, the Fe3O4@Fe3O4/C nanoreactor can be easily and quickly separated from the reaction system by applying an external magnetic field after being used. The result in Figure 6 b shows that it can be successfully reused for four successive cycles with a removal efficiency of > 91 % within a reaction time of 90 min. Additionally, TEM, FTIR, XRD, and XPS measurements were conducted to check the possible change in morphology and composition of the Fe3O4@Fe3O4/C nanoreactor before and after use for four runs. The FTIR spectra (see Figure S6 in the Supporting Information)
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Full Paper of the fresh and used nanoreactor showed no significant change, indicating that almost no organic residue was absorbed onto the material. XRD patterns (see Figure S7 in the Supporting Information) of the reclaimed nanoreactors also reveal the unaltered crystal structure of Fe3O4 in the composites as compared with the fresh material. The TEM image (Figure S8 in the Supporting Information) of the used nanoreactor indicates that the clear yolk-like structure of the composite is well maintained in spite of the slightly decreased thickness of the outer carbon layer. This proves that the outer carbon layer of the composite may serve as a cushion to limit the damage to interior components during the longterm experiment under harsh conditions. In the XPS analysis (see Figure S9 in the Supporting Information), the increased intensity of Fe2p3 of the recovered nanoreactor is also ascribed to the decrease of the surface carbon component. These results all support the good stability of the presented Fe3O4@Fe3O4/C nanoreactor.
Conclusion In summary, novel hierarchical yolk–shell structured nanoreactors with Fe3O4 cores and Fe3O4/C shells have been assembled for the first time by a facile strategy and used to accommodate the Fenton-like reaction into its void space. The performance of this reaction occurring inside the nanoreactor strikingly exceeded that on pure Fe3O4 particles in bulk solution when applied to the degradation of CPs. The inclusion of the interior cavity is crucial for it provides a Fe3O4-walled microenvironment to confine high-concentration reactants and thus promote the degradation reaction. The degree of mineralization and the nature of the intermediates were determined and the possible degradation pathway based on COH attack was proposed. Furthermore, owing to the high magnetization and good stability, the nanoreactors can be conveniently isolated from the reaction solution and reused for four successive cycles with high removal efficiency. Such studies highlight the potential of novel magnetite-based Fenton-like catalysts as a realizable tool for efficient environmental remediation and we believe that the obtained materials can be extended for various advanced applications.
Experimental Section Chemicals HPLC-grade acetone and acetonitrile were supplied by Fisher Scientific (Fair Lawn, NJ). 2-Chlorophenol (2-CP), 3-chlorophenol (3CP), 4-chlorophenol (4-CP), 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), and tetraethyl orthosilicate (TEOS) were obtained from Acros Organics (Morris Plains, NJ), and aqueous ammonia (25 %, w/w) from Alfa Aesar (Ward Hill, MA). Ferric chloride hexahydrate (FeCl3·6 H2O), sodium acetate (NaAc), ethylene glycol (EG), trisodium citrate dihydrate, ferrocene, hydrogen peroxide (H2O2), tert-butyl alcohol, sulfuric acid (H2SO4), and anhydrous ethanol were guarantee-grade reagents from Beijing Chemicals Co. Ltd. (Beijing, China). All chemicals were used as received without any further purification. Ultrapure water was prepared in the laboratory Chem. Eur. J. 2014, 20, 6474 – 6481
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using a Milli-Q SP reagent water system from Millipore (Milford, MA).
Preparation of yolk–shell Fe3O4@Fe3O4/C nanoreactor Core-shell Fe3O4@SiO2 particles were first synthesized according to the previous reports.[27] Briefly, FeCl3·6H2O (2.60 g), trisodium citrate (1.35 g), and NaAc (4.8 g) were dissolved in ethylene glycol (80 mL) with magnetic stirring at room temperature for 0.5 h. The homogeneous yellow solution was subsequently transferred to a Teflonlined stainless-steel autoclave and sealed to heat at 200 8C for 10 h, and then cooled to room temperature. The black product was washed with ethanol and deionized water three times, respectively. An aqueous dispersion of the above magnetite particles (35 mL, 0.02 g mL 1) was mixed with anhydrous ethanol (140 mL) and aqueous ammonia solution (3.0 mL, 28 wt %) under stirring and then stirred for a further 15 min at room temperature. After the addition of 2.0 mL of TEOS, the reaction proceeded for 10 h under continuous stirring. The resultant product was collected with a magnet and washed with ethanol four times, followed by drying under vacuum. To synthesize the yolk–shell Fe3O4@Fe3O4/C nanoreactor, Fe3O4@ SiO2 particles (100 mg) and ferrocene (200 mg) were dispersed in acetone (65 mL) with vigorous stirring for 0.5 h.[28] Next, H2O2 (2 mL) was added dropwise to the mixture, which was stirred for another 2 h. After that, the solution was transferred into a Teflonlined stainless-steel autoclave and sealed to heat at 210 8C for 48 h. When the autoclave was cooled to room temperature, the obtained product was collected and washed with ethanol three times, which was then redispersed in ultrapure water (60 mL) containing aqueous ammonia (3 mL). After a 30 min stirring period, the mixture was transferred into a Teflon-lined stainless-steel autoclave again and heated to 150 8C for 6 h. Finally, the product was collected with the help of a magnet, and then washed with ultrapure water several times.
Degradation of CPs by Fenton-like experiments All the experiments were conducted in a conical flask (50 mL) in the dark with a rotate speed of 300 rpm. The Fenton-like reaction was initiated by adding a known concentration of H2O2 to a pH-adjusted solution (by H2SO4) containing Fe3O4@Fe3O4/C composites and target CP. Samples were taken out at given time intervals and centrifuged for the subsequent determination. Control experiments were carried out as above and the effects of pH, H2O2 concentration, and catalyst loading on the reaction were investigated. The reusability of the nanoreactors was examined by recovering with a magnet, washing with deionized water, drying and using in the next run under similar experimental conditions. Each experiment was repeated in triplicate and all results are presented as a mean value.
Sample determination Dionex ultimate 3000 HPLC (Dionex, Sunyvale, CA) with a PDA-100 photodiode array detector and an Acclaim 120 C18 column (5 um, 4.6 250 mm) was applied to analyze the sample concentration. The mobile phase was composed of acetonitrile and water (70:30, v/v) at a flow rate of 1.0 mL min 1 with a column temperature of 30 8C. A multiple wavelength setting was applied for quantification of CPs: 276 nm for 2-CP and 3-CP, 280 nm for 4-CP, 286 nm for 2,4DCP, and 290 nm for 2,4,6-TCP. For the hydroquinone analysis, the UV absorbance wavelength was set to 254 nm. The chloride ion (Cl ) and carboxylic acids were determined by using a Dionex ICS-
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[7]
[8]
Characterization The morphologies of the synthesized materials were surveyed by using a Tecnai G2 F20 HRTEM with an energy dispersive X-ray spectrometry (HRTEM-EDX, FEI, Netherlands). X-ray diffraction studies (XRD, PANalytical X’ Pert diffractometer, Almelo, Netherlands) were performed by using a monochromatized X-ray beam with nickel-filtered CuKa radiation at a scan rate of 0.4 8 min 1 . X-ray photoelectron spectroscopy (XPS) measurements were conducted by applying a Thermo Scientific ESCA-Lab-200i-XL spectrometer (Waltham, MA) with monochromatic AlKa radiation (1486.6 eV). FTIR spectra were taken in KBr pressed pellets on a Nicolet Thermo NEXUS 670 Infrared Fourier Transform Spectrometer (Waltham, MA). The magnetization curves of the products were measured with a LDJ9600 vibrating sample magnetometer (VSM, Troy, MI). Nitrogen sorption isotherms were measured at 77 K with a Quadrasorb SI Four Station Surface Area Analyzer and Pore Size Analyzer (Quantachrome Instruments, Boynton Beach, FL). Before measurements, the samples were degassed in a vacuum at 300 8C for at least 6 h. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas using adsorption data in a relative pressure range from 0.05 to 1.0. The pore volumes and pore size distributions were calculated by using the Barrett–Joyner–Halenda (BJH) model and the total pore volumes were estimated from the adsorbed amount at a relative pressure P/P0 of 0.993.
Acknowledgements
[9] [10]
[11] [12]
[13]
[14]
[15]
[16]
This work was jointly supported by the Special Fund of National Key Scientific Instrument and Equipment Development (2012YQ09022907), the National Basic Research Program of China (2011CB936001), and the National Natural Science Foundation of China (21277152, 21277002, 21321004).
[17] [18] [19] [20] [21]
Keywords: chlorophenol · Fenton-like reactions heterogeneous catalysis · magnetite · nanostructures
· [22]
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Received: October 29, 2013 Revised: November 26, 2013 Published online on April 15, 2014
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& Confined Nanoreactors
Assembly of a Nanoreactor System with Confined Magnetite Core and Shell for Enhanced Fenton-Like Catalysis Tao Zeng,[a] Xiaole Zhang,[a, b] Saihua Wang,[a] Yurong Ma,[a] Hongyun Niu,[a] and Yaqi Cai*[a]
Abstract: Conventional solid catalysts for heterogeneous Fenton-like reactions in bulk solution usually suffer from aggregation and vulnerability, which greatly lower the catalytic efficiency and hamper their practical application. Herein, we demonstrate a promising yolk–shell nanostructure with both the core and the shell composed of magnetite (designated as yolk-like Fe3O4@Fe3O4/C) as a nanoreactor capable of accommodating the Fenton-like reaction into its void space. Benefiting from the mesoporous shell and perfect interior cavity of this composite, reactants can access and be abundantly confined within the microenvironment where Fe3O4 sites are dispersed on the entire cavity surfaces, thus leading to a higher catalytic efficiency compared with the conventional solid catalysts in bulk solution. The chosen model re-
Introduction Heterogeneous Fenton-like reactions on solid catalysts are increasingly replacing iron salt-based homogeneous Fenton reactions because of the advantages associated with avoiding the precipitation of soluble iron salts and the difficulties in recycling.[1] Recently, zero-valent iron,[2] iron-based clays,[1b, 3] ironcontaining materials,[4] and iron oxide minerals[5] as heterogeneous Fenton-like catalysts have been the subject of extensive research and the highly reactive and non-selective hydroxyl radicals (COH) generated can decompose recalcitrant organic pollutants, thus offering a promising alternative for environmental remediation. Among these catalysts, magnetite (Fe3O4) and maghemite (g-Fe2O3) have attracted more interest owing to their good catalytic activity and fantastic magnetically separable properties, which are of significant economic and environmental benefit owing to the reduced consumption of additional substances, energy, and time used for catalyst recy-
[a] Dr. T. Zeng, Dr. X. Zhang, S. Wang, Y. Ma, H. Niu, Prof. Y. Cai State Key Laboratory of Environmental Chemistry and Ecotoxicology Research Center for Eco-Environmental Sciences Chinese Academy of Sciences, Beijing 100085 (China) Fax: (+ 86) 10-62849182 E-mail: [email protected] [b] Dr. X. Zhang College of Life Science Hebei United University, Tangshan 063000 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304221. Chem. Eur. J. 2014, 20, 6474 – 6481
action of chlorophenols degradation in the presence of the as-prepared materials as well as hydrogen peroxide (H2O2) confirms this assumption. Under the optimal reaction conditions, more than 97 % 4-chlorophenol (4-CP) can be degraded in the Fe3O4@Fe3O4/C nanoreactor, whereas only 28 % can be achieved by using bare Fe3O4 particles within 60 min. Furthermore, owing to the existence of the outermost carbon layer and high-magnetization properties, the nanoreactor can be re-used for several runs. The synthesized nanoreactor displays superior catalytic activity toward the Fenton-like reaction compared with the bare solid catalysts, and thereby holds significant potential for practical application in environmental remediation.
cling.[6] However, most Fenton-like iron oxide-based catalysts reported in previous studies have tended to agglomerate during operation, resulting in a significant deterioration of the catalytic activity.[6b] Another issue of concern is that these catalysts are unstable and the catalytic sites are contaminated readily when directly exposed in bulk reaction solution.[7] Accordingly, it was considered that transferring the Fenton-like reaction into a nanoreactor might be an ideal way to address the above difficulties. In addition, several recent studies have proved that catalysts confined in the inner space of nanoreactors display enhanced catalytic performance.[8] In this regard, the employment of nanoreactors for Fenton-like catalysis would help to protect the active sites and achieve improved catalytic efficiency, however, little work has been reported in this respect.[9] As a novel class of complex nanoarchitectures, yolk–shell structures with a void space between the core and the shell can provide a microenvironment for a variety of fields such as chemical storage,[10] compartmentation,[11] and drug loading.[12] In particular, this nanostructure shows promise for applications as a nanoreactor for several important reactions because of its unique feature that the permeable shell can protect the core from aggregation while allowing the diffusion of small active molecules in and out of the interior microenviroment.[13, 14] In the case of yolk–shell nanostructures utilized in Fenton-like catalysis, most existing materials that encapsulate a metal core inside the inert shell are unable to efficiently catalyze the reaction due to the lack of sufficient Fenton-like catalytically active sites within the nanoreactor, although some of them contain
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Full Paper a single magnetite core which is commonly used for either convenient separation or catalyst support.[14b, 15] To increase the potential applications of such nanoarchitectures as nanoreactors in Fenton-like catalysis, it is essential to develop new methods for functionalizing the internal cavity. A catalytic shell instead of the conventional inert shell together with a catalytic core that surrounds the interior space would be more desirable for providing a large catalytic surface area in the cavity and thus producing nanoreactors with enhanced activity.[13a, 16] To the best of our knowledge, no study so far has succeeded in designing and utilizing yolk–shell nanoreactors with both the core and shell composed of active components for enhanced Fenton-like catalysis. In contrast to previous studies in which Fenton-like reactions directly took place in bulk solution, of interest to us in this work is the arrangement of this reaction into a special confined microenvironment to achieve a higher catalytic performance. Novel yolk–shell nanoreactors with a Fe3O4 core, a mesoporous Fe3O4/C double-layered shell, and a void space between the core and the shell (see the structural model in Figure 1 a) were therefore prepared in which each part of the
Figure 1. a) Schematic illustration for the preparation of a yolk–shell Fe3O4@Fe3O4/C nanoreactor. TEM images: b) Fe3O4, c) Fe3O4@SiO2, d, e) Fe3O4@SiO2@Fe3O4/C. f) Structural model of Fe3O4@SiO2@Fe3O4/C. g) STEM image and EDX mapping of Fe3O4@SiO2@Fe3O4/C.
nanoreactor has its crucial role in the functioning. The cavity of the nanoreactor provides a perfect microenvironment where the Fenton-like reaction can be facilitated due to the confinement effect toward the reactants and the abundant active sites found on the entire cavity surface. The Fe3O4 constituents derived from the core together with the inner shell can also enable the magnetic isolation from the suspension once the catalytic reaction is complete, instead of just serving as catalytic sites. The outer carbon layer of the shell may help to enrich Chem. Eur. J. 2014, 20, 6474 – 6481
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the organic reactants into the cavity, while acting as a buffer to abate the erosion of interior Fe3O4 components by external harsh conditions. The Fenton oxidation of chlorophenols, a group of highly toxic environmental contaminants, was then chosen to check the catalytic performance of the yolk-like Fe3O4@Fe3O4/C microsphere as a nanoreactor. As expected, the catalytic ability of the nanoreactor strikingly exceeded that of the bare Fe3O4. Finally, the applicability of this composite in heterogeneous Fenton-like reactions was systematically assessed in terms of the influence of the main factors, extent of mineralization, degradation intermediates, as well as the reusability of the material. From the point of view of fundamental research and practical application, such a novel yolk–shell catalytic system may open new opportunities for improved Fenton-like catalysis used in environmental remediation.
Results and Discussion The typical synthesis of the yolk–shell Fe3O4@Fe3O4/C nanoreactor is illustrated in Figure 1 a. The pre-synthesized magnetic Fe3O4 particles (Figure 1 b) were first coated with a silica shell through the hydrolysis of tetraethyl orthosilicate (TEOS) to form a core–shell structured Fe3O4@SiO2 microsphere (Figure 1 c). A double-layer coating of the inner Fe3O4 layer and the outer carbon layer was then deposited simultaneously on the surface of Fe3O4@SiO2 by a hydrothermal reaction with ferrocene and hydrogen peroxide (H2O2). From the TEM images of Fe3O4@SiO2@Fe3O4/C (Figure 1 d and e), it can be seen that the microspheres exhibit a defined multilayered structure. The Fe3O4 cores with a mean diameter of 200 nm are wrapped in a 40 nm thick silica shell, which is successively covered by another shell consisting of a deep contrast inner layer and a light contrast outer layer. The bright parts in the STEM image (Figure 1 g) suggest the distribution of metal elements, which is further confirmed by the detailed EDX mapping analysis. It was found that the Fe signal was dispersed in both the shell and the core, the Si signal was only present in the middle layer, whereas the O signal was distributed throughout the composites. These results indicate the successful integration of different components in the Fe3O4@SiO2@Fe3O4/C hybrid. The yolk-like Fe3O4@Fe3O4/C nanoreactor was finally obtained by the selective etching of the silica shell with aqueous ammonia.[12] The typical TEM and SEM images in Figure 2 a show that the as-prepared nanoreactors exhibit a regular spherical shape with very rough surfaces, and a void space between the core and the shell is distinctly formed, which is further evidenced by a purposely selected broken sphere (see the inset in Figure 2 b). The structure of the double-layered shell is not destroyed after the corrosion of the SiO2 layer, although the outermost carbon layer seems to be somewhat intolerable during the hydrothermal reaction of aqueous ammonia and shows a slight decrease in thickness (Figure 2 b). Moreover, the obtained nanoreactors exhibit good dispersibility, thus avoiding the decrease in catalytic performance caused by aggregation. The Fe and O signals in the EDX mapping results (see Figure S2 in the Supporting Information) present a similar yolklike distribution, thus implying the successful preparation of
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Figure 2. a) TEM image of yolk–shell Fe3O4@Fe3O4/C nanoreactor. b) Magnified TEM image of Fe3O4@Fe3O4/C nanoreactor. c, d) HRTEM images of the shell and the core, respectively. The insets in (a) and (b) are the SEM images of the Fe3O4@Fe3O4/C nanoreactor.
the yolk–shell nanoreactor. High-resolution TEM images of the shell and the core (Figure 2 c and d) display regular lattice fringes with an interplanar spacing of 0.299 nm, which is highly consistent with the d value of the (220) planes of magnetite structure.[17] The XRD patterns of samples at each step are displayed in Figure 3 a. Six characteristic signals at 2q = 30.1 8, 35.4 8, 43.1 8, 53.4 8, 57.5 8, and 62.9 8 correspondingly indexed as (220), (311), (400), (422), (511), and (440), can be clearly distinguished, which matches well with the database of the face-center-cubic phase of Fe3O4 (JCPDS Card No 19-0629).[18] In the XRD patterns of other samples, similar features to those of pure Fe3O4 particles are observed, thus confirming the presence of the magnetite component in all of these composites. For Fe3O4@ SiO2 and Fe3O4@SiO2@Fe3O4/C, the broad band at 2q = 22 8 can be assigned to the amorphous silica shell (JCPDS Card No. 290085),[19] which disappeared in the pattern of yolk-like Fe3O4@ Fe3O4/C owing to the elimination of silica. The synthesis of yolk–shell Fe3O4@Fe3O4/C can also be monitored by FTIR analysis as shown in Figure 3 b. The absorption band observed at 560 cm 1 in all the spectra is related to
Figure 3. a) XRD patterns of various samples. b) FTIR spectra of various samples. c) N2 adsorption–desorption isotherms of yolk-like Fe3O4@Fe3O4/C nanoreactor, the inset shows the distribution of pore diameter. d) VSM curves of various samples. Chem. Eur. J. 2014, 20, 6474 – 6481
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Full Paper Fe O groups. After coating the silica shell onto the Fe3O4 surface, some new signals emerged around 810–950 cm 1 and 1080 cm 1, and several corresponding to the Si-O-Si and Si OH stretching bands, whereas the Fe O band was weakened because of the shielding effect. In comparison with the spectrum of Fe3O4@SiO2, a mild decrease in the silica bands and an increase in the Fe O band seen in the spectrum of Fe3O4@SiO2@ Fe3O4/C indicate the successful integration of the Fe3O4/C layer. The presence of signals at 1600–1800 cm 1 assigned to C=O stretching vibrations means that abundant carboxyl groups exist in the outer shell, which can help to increase the dispersivity of the materials in aqueous solution.[20] In regard to the spectrum of yolk-like Fe3O4@Fe3O4/C, the Fe O band further strengthened and the Si-O-Si bands almost vanished because of the removal of the middle silica shell and partial etching of the outermost carbon layer. Nitrogen adsorption–desorption isotherms of the core shell (see Figure S3 in the Supporting Information) and yolk shell (Figure 3 c) Fe3O4@Fe3O4/C exhibit typical Langmuir I-V behavior, indicating the presence of well-defined mesopores in the structure. As compared to the curve of solid Fe3O4@SiO2@ Fe3O4/C, the larger hysteresis loop appearing in yolk-like Fe3O4@Fe3O4/C represents unique ink-bottle-shaped pores, in which large cavities are connected through narrow channels.[12b] This is consistent with the nanostructure observed in the TEM images. Notably, the total pore volume increases significantly from 0.18 to 0.31 cm3 g 1 after the treatment of the solid Fe3O4@SiO2@Fe3O4/C with aqueous ammonia. Evidently, the interior large cavity left after the removal of silica is responsible for the pore volume increase. Calculated from the desorption branch of the nitrogen isotherm with the BJH method, the pore diameter of yolk-like Fe3O4@Fe3O4/C is measured to be 3.84 nm, which is larger than that of core-shell Fe3O4@SiO2@Fe3O4/C (3.41 nm). The enlargement of pore diameter can also be attributed to the etching of aqueous of ammonia. Thanks to the existence of these mesopores in the shell, the small reactive molecules are permitted to enter and leave the interior cavity readily. The magnetic properties of the as-prepared samples were evaluated by using a vibrating sample magnetometer at room temperature. Figure 3 d illustrates the M–H hysteresis loops of these composites measured by sweeping the external field between 1 and 1 T. All the four magnetization curves show no remanence and coercivity, suggesting the superparamagnetic character of each sample. The saturation magnetization value of Fe3O4@SiO2 (41.8 emu g 1) is lower than that of Fe3O4 (63.7 emu g 1) due to the coating of the non-magnetic silica shell. After covering the surface of Fe3O4@SiO2 with another magnetic Fe3O4/C layer, the magnetization of Fe3O4@SiO2@Fe3O4/C (33.5 emu g 1) exhibits a slight decrease rather than a growth, which is possibly attributed to the simultaneous introduction of a thick layer of mesoporous carbon together with the Fe3O4 layer. As for the final product yolk–shell Fe3O4@Fe3O4/C, the magnetization remarkably increases to 53.1 emu g 1 owing to the full etching of the non-magnetic silica component. Consequently, the strong magnetic Fe3O4 components in the Fe3O4@Fe3O4/C product provide an easy Chem. Eur. J. 2014, 20, 6474 – 6481
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and convenient way to separate them from the reaction system in addition to offering an efficient Fenton-like catalytic active site. The catalytic performance of a yolk-like Fe3O4@Fe3O4/C microsphere as nanoreactor was evaluated by using the model reaction of degradation of 4-CP under the assistance of H2O2. The initial concentration of 4-CP was 1.56 mm, and the weight of solid catalysts in the suspension (pH 4.0) was 0.5 g L 1. Aliquots were taken out after regular intervals of reaction time, and the degradation level was determined by liquid chromatography after removing the solid materials. Various control experiments were performed to compare the efficiencies of 4-CP removal, and all reactions were conducted in the dark to avoid the influence of light. As shown in Figure 4 a, use of H2O2 alone leads to a slight removal of 4-CP within 210 min, which is negligible compared with the fast degradation of 4-CP by heterogeneous Fenton-like reaction. When bare Fe3O4 nanoparticles were applied as a Fenton-like catalyst, about 97 % removal of 4-CP was observed after 210 min in the presence of
Figure 4. a) Fenton-like degradation of 4-CP under different conditions: I) 0.5 g L 1 as-prepared yolk-like Fe3O4@Fe3O4/C nanoreactor and 20 mm H2O2, II) 0.5 g L 1 bare Fe3O4 and 20 mm H2O2, III) 0.5 g L 1 solid SiO2@Fe3O4/C without cavity and 20 mm H2O2, IV) 0.5 g L 1 yolk-like SiO2@Fe3O4/C and 20 mm H2O2, V) 20 mm H2O2, and VI) 0.5 g L 1 yolk-like Fe3O4@Fe3O4/C. Other reaction conditions were set as follows: pH 4.0, T = 298 K, initial 4-CP concentration = 1.56 mm. C0 and Ct denote the initial concentration and instantaneous concentration at various reaction times of 4-CP, respectively. b) Schematic illustration showing the Fenton-like degradation of 4-CP in the yolk-like Fe3O4@Fe3O4/C nanoreactor.
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Full Paper H2O2. For comparison, solid SiO2@Fe3O4/C without cavities and yolk–shell SiO2@Fe3O4/C with cavities (see TEM images in Figure S4 in the Supporting Information) were fabricated and employed to catalyze the degradation of 4-CP. It can be found that the solid SiO2@Fe3O4/C is also able to degrade 97 % of 4CP within 180 min. These means that both the Fe3O4 core and Fe3O4/C shell can catalyze the reaction solely. As for the yolk– shell SiO2@Fe3O4/C composite, it exhibits a higher catalytic activity than those of solid SiO2@Fe3O4/C and bare Fe3O4, and only 120 min was required to remove almost all 4-CP. We hypothesized that the superior catalytic performance of yolk– shell SiO2@Fe3O4/C may be ascribed to its unique yolk-like structure. Most notably, 4-CP could be almost completely degraded within 60 min using the yolk-like Fe3O4@Fe3O4/C composite as a nanoreactor, implying a higher catalytic efficiency of it than that of the yolk–shell SiO2@Fe3O4/C composite. Since the shell and cavity size of these two samples are very similar (see Figure S4 in the Supporting Information), the superior activity of Fe3O4@Fe3O4/C can be only attributed to the presence of the Fe3O4 core. The negligible decrease of 4-CP (less than 10 %) with the yolk-like Fe3O4@Fe3O4/C nanoreactor in the absence of H2O2 may be attributed to the surface adsorption ability of the carbon layer. The excellent catalytic performance of yolk-like Fe3O4@ Fe3O4/C nanoreactors for the degradation of 4-CP is explained as illustrated in Figure 4 b. First, the mesoporous Fe3O4/C shells favor the enrichment of reactants from bulk solution and the fast diffusion of them toward Fe3O4 active sites, which is beneficial for the heterogeneous reaction. Second, the cavity of the yolk-like nanoreactor provides a microenvironment to confine the reactants, leading to a higher instantaneous concentration of reactants in the interior of the nanoreactor and thus affording a driving force to increase the degradation rate.[21] Third, the inner Fe3O4 layer of the shell together with the Fe3O4 cores surrounding the void space are both bestrewn with available active sites, which greatly increases the accessibility of reactants to catalytic sites so as to accelerate the degradation reaction. Since the solution pH has a remarkable effect on the Fenton reaction, the influence of the pH of the initial solution on the degradation of 4-CP in the yolk-like Fe3O4@Fe3O4/C nanoreactor was estimated. As shown in Figure S5 a in the Supporting Information, the decrease of 4-CP is over 95 % within 60 min at pH 3.0 and 4.0, although the difference in degradation rate between them exists. The degradation efficiency at pH 5.1 is not as high as those at low pH. At pH 6.5, the degradation rate further reduced and the conversion of 4-CP is less than 20 % over a period of 150 min. The results show a similar tendency to previous reports that the reaction rate of the Fenton process decreased with the increase of solution pH.[21] The degradation of 4-CP with different catalyst dosage and different H2O2 concentrations was also investigated and the results are shown in Figure S5 b and Figure S5 c in the Supporting Information. The efficiency of 4-CP removal after a reaction time of 60 min increased from 53 % to 96 % with the Fe3O4@Fe3O4/C dosage increasing from 0.1 g L 1 to 1 g L 1. This originates from the fact that there is an increasing number of active sites available for Chem. Eur. J. 2014, 20, 6474 – 6481
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the H2O2 decomposition.[22b] The degradation percentage of 4CP within 150 min increased from 33 % to 99 % as the H2O2 concentration increased from 1.2 to 20 mm. The faster degradation rate of 4-CP under higher H2O2 concentration must be attributed to the fact that the 4-CP removal is directly related to the concentration of the reactive oxygen species (ROS) produced from the reaction between H2O2 and Fe3O4. However, as the H2O2 concentration further increased, an inhibition instead of an improvement of 4-CP removal was observed. A possible reason for this is the scavenging effect of excessive H2O2 toward COH, which leads to a competitive reaction with COH between 4-CP and H2O2.[4b, 23] A control experiment was first carried out to verify the possible contributions of a homogeneous reaction catalyzed by the leached metal ions. Yolk-like Fe3O4@ Fe3O4/C was suspended in 20 mm H2O2 at pH 4.0 for 45 min and then isolated by an external magnet. The filtrate was tested for the degradation of 4CP under the same conditions as the solid catalyst and a conversion of only about 4 % was detected after a reaction time of 60 min, which is much lower than that observed for solid Fe3O4@ Fe3O4/C. Therefore, it can be concluded that the degradation of 4-CP mainly occurs in a heterogeneous reaction rather than a homogeneous reaction. To identify the primary ROS for the degradation of 4-CP in the heterogeneous catalytic process, tert-butyl alcohol, which has high reactivity with COH, was adopted as scavenger for the reaction. The results (Figure 5 a) show that the degradation of 4-CP was significantly inhibited by the addition of tert-butyl alcohol. The efficiency of 4-CP removal decreased with an increasing concentration of tert-butyl alcohol and was almost completely restrained in the presence of 25 mm tert-butyl alcohol. This indicates that 4-CP removal is primarily attributed to the oxidation ability of COH in the Fenton-like system.[24] The degree of mineralization of 4-CP was analyzed by the TOC content of the reaction solution. Figure 5 b shows that the final TOC value of the reaction solution was reduced to 50.8 % of the original solution, implying that some organic residues derived from 4-CP decomposition remained in solution. In addition, the possible derivatives generated from the eroded carbon layer may contribute to the residual TOC as well. Since the toxicity of CPs is associated with chloride substituents, the dechlorination level was also determined. A dechlorination efficiency of 86.4 % indicated that most of the chlorines bound to the aromatic ring were released. The main intermediate products generated during different degradation periods were identified by HPLC and IC analysis (Figure 6 a). The concentration of hydroquinone initially increased to a peak value at 30 min, and then slowly decreased till complete elimination within 180 min. Some smaller molecular organic acids including formic acid, fumaric acid, malonic acid, and acetic acid were gradually generated and eventually left in the reaction solution, which could account for the residual TOC in solution. The identified intermediates were in agreement with the results of some work on the degradation of CPs by other kinds of catalysts. Based on these intermediates and some reported information,[22a, 25] a possible degradation pathway of 4-CP catalyzed by yolk-like Fe3O4@Fe3O4/C in the presence of H2O2 was
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Figure 5. a) Effect of the tert-butyl alcohol scavenger on the degradation of 4-CP. b) The efficiency of TOC removal and dechlorination in the degradation of 4-CP. Reaction conditions: T = 298 K, pH 4.0, initial 4-CP concentration = 3.12 mm, and catalyst dosage = 0.5 g L 1.
proposed. The chlorine atom at the para-site of 4-CP was first detached by the attack of COH to form a hydroxyphenyl radical, which could then undergo another attack of COH to yield hydroquinone. Hydroquinone is capable of being rapidly oxidized to benzoquinone under such oxidation conditions. Subsequently, the ring of benzoquinone was further cleaved by COH to form short chain carboxylic acids such as fumaric acid and malonic acid, which were subsequently degraded to acetic acid and formic acid or thoroughly oxidized to CO2 and H2O. The catalytic activity for the Fenton-like oxidation of several chlorophenol analogues including 2-CP, 3-CP, 2,4-DCP, and 2,4,6-TCP was also investigated to gain insight into the universality of the yolk-like Fe3O4@Fe3O4/C nanoreactor. All the reactions for these compounds were carried out under the same conditions as that for 4-CP and the progress of each of these reactions was then monitored by HPLC. As shown in Table S1 in the Supporting Information, similar conversion efficiencies were obtained for the one-chlorine-atom-containing CPs (2-CP, 3-CP, and 4-CP) regardless of the position of the chlorine substituent, and they could be almost completely removed within 60 min. With regard to 2,4-DCP and 2,4,6-TCP, however, only 85.5 % and 50.1 % were degraded within a reaction time of Chem. Eur. J. 2014, 20, 6474 – 6481
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Figure 6. a) Evolution of the intermediates with reaction time. b) Reusability of the nanoreactors in the degradation of 4-CP. Reaction conditions: T = 298 K, pH 4.0, initial 4-CP concentration = 1.56 mm, and catalyst dosage = 0.5 g L 1.
300 min, demonstrating a relative low reaction rate compared with those of the three CPs containing one chlorine atom. This is in good accordance with the findings in other work that the chlorine content of different chlorophenol compounds often had a significant effect on the oxidation rate because some favorable sites of the aromatic ring susceptible to COH attack could be blocked by chlorine atoms, thus inhibiting the oxidation of CPs.[26] An important issue with regard to the potential application of these materials is the reusability and stability in the operating process. This was investigated by repeated usage of the Fe3O4@Fe3O4/C nanoreactor for the catalytic oxidation of 4-CP. With the superparamagnetism character, the Fe3O4@Fe3O4/C nanoreactor can be easily and quickly separated from the reaction system by applying an external magnetic field after being used. The result in Figure 6 b shows that it can be successfully reused for four successive cycles with a removal efficiency of > 91 % within a reaction time of 90 min. Additionally, TEM, FTIR, XRD, and XPS measurements were conducted to check the possible change in morphology and composition of the Fe3O4@Fe3O4/C nanoreactor before and after use for four runs. The FTIR spectra (see Figure S6 in the Supporting Information)
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Full Paper of the fresh and used nanoreactor showed no significant change, indicating that almost no organic residue was absorbed onto the material. XRD patterns (see Figure S7 in the Supporting Information) of the reclaimed nanoreactors also reveal the unaltered crystal structure of Fe3O4 in the composites as compared with the fresh material. The TEM image (Figure S8 in the Supporting Information) of the used nanoreactor indicates that the clear yolk-like structure of the composite is well maintained in spite of the slightly decreased thickness of the outer carbon layer. This proves that the outer carbon layer of the composite may serve as a cushion to limit the damage to interior components during the longterm experiment under harsh conditions. In the XPS analysis (see Figure S9 in the Supporting Information), the increased intensity of Fe2p3 of the recovered nanoreactor is also ascribed to the decrease of the surface carbon component. These results all support the good stability of the presented Fe3O4@Fe3O4/C nanoreactor.
Conclusion In summary, novel hierarchical yolk–shell structured nanoreactors with Fe3O4 cores and Fe3O4/C shells have been assembled for the first time by a facile strategy and used to accommodate the Fenton-like reaction into its void space. The performance of this reaction occurring inside the nanoreactor strikingly exceeded that on pure Fe3O4 particles in bulk solution when applied to the degradation of CPs. The inclusion of the interior cavity is crucial for it provides a Fe3O4-walled microenvironment to confine high-concentration reactants and thus promote the degradation reaction. The degree of mineralization and the nature of the intermediates were determined and the possible degradation pathway based on COH attack was proposed. Furthermore, owing to the high magnetization and good stability, the nanoreactors can be conveniently isolated from the reaction solution and reused for four successive cycles with high removal efficiency. Such studies highlight the potential of novel magnetite-based Fenton-like catalysts as a realizable tool for efficient environmental remediation and we believe that the obtained materials can be extended for various advanced applications.
Experimental Section Chemicals HPLC-grade acetone and acetonitrile were supplied by Fisher Scientific (Fair Lawn, NJ). 2-Chlorophenol (2-CP), 3-chlorophenol (3CP), 4-chlorophenol (4-CP), 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), and tetraethyl orthosilicate (TEOS) were obtained from Acros Organics (Morris Plains, NJ), and aqueous ammonia (25 %, w/w) from Alfa Aesar (Ward Hill, MA). Ferric chloride hexahydrate (FeCl3·6 H2O), sodium acetate (NaAc), ethylene glycol (EG), trisodium citrate dihydrate, ferrocene, hydrogen peroxide (H2O2), tert-butyl alcohol, sulfuric acid (H2SO4), and anhydrous ethanol were guarantee-grade reagents from Beijing Chemicals Co. Ltd. (Beijing, China). All chemicals were used as received without any further purification. Ultrapure water was prepared in the laboratory Chem. Eur. J. 2014, 20, 6474 – 6481
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using a Milli-Q SP reagent water system from Millipore (Milford, MA).
Preparation of yolk–shell Fe3O4@Fe3O4/C nanoreactor Core-shell Fe3O4@SiO2 particles were first synthesized according to the previous reports.[27] Briefly, FeCl3·6H2O (2.60 g), trisodium citrate (1.35 g), and NaAc (4.8 g) were dissolved in ethylene glycol (80 mL) with magnetic stirring at room temperature for 0.5 h. The homogeneous yellow solution was subsequently transferred to a Teflonlined stainless-steel autoclave and sealed to heat at 200 8C for 10 h, and then cooled to room temperature. The black product was washed with ethanol and deionized water three times, respectively. An aqueous dispersion of the above magnetite particles (35 mL, 0.02 g mL 1) was mixed with anhydrous ethanol (140 mL) and aqueous ammonia solution (3.0 mL, 28 wt %) under stirring and then stirred for a further 15 min at room temperature. After the addition of 2.0 mL of TEOS, the reaction proceeded for 10 h under continuous stirring. The resultant product was collected with a magnet and washed with ethanol four times, followed by drying under vacuum. To synthesize the yolk–shell Fe3O4@Fe3O4/C nanoreactor, Fe3O4@ SiO2 particles (100 mg) and ferrocene (200 mg) were dispersed in acetone (65 mL) with vigorous stirring for 0.5 h.[28] Next, H2O2 (2 mL) was added dropwise to the mixture, which was stirred for another 2 h. After that, the solution was transferred into a Teflonlined stainless-steel autoclave and sealed to heat at 210 8C for 48 h. When the autoclave was cooled to room temperature, the obtained product was collected and washed with ethanol three times, which was then redispersed in ultrapure water (60 mL) containing aqueous ammonia (3 mL). After a 30 min stirring period, the mixture was transferred into a Teflon-lined stainless-steel autoclave again and heated to 150 8C for 6 h. Finally, the product was collected with the help of a magnet, and then washed with ultrapure water several times.
Degradation of CPs by Fenton-like experiments All the experiments were conducted in a conical flask (50 mL) in the dark with a rotate speed of 300 rpm. The Fenton-like reaction was initiated by adding a known concentration of H2O2 to a pH-adjusted solution (by H2SO4) containing Fe3O4@Fe3O4/C composites and target CP. Samples were taken out at given time intervals and centrifuged for the subsequent determination. Control experiments were carried out as above and the effects of pH, H2O2 concentration, and catalyst loading on the reaction were investigated. The reusability of the nanoreactors was examined by recovering with a magnet, washing with deionized water, drying and using in the next run under similar experimental conditions. Each experiment was repeated in triplicate and all results are presented as a mean value.
Sample determination Dionex ultimate 3000 HPLC (Dionex, Sunyvale, CA) with a PDA-100 photodiode array detector and an Acclaim 120 C18 column (5 um, 4.6 250 mm) was applied to analyze the sample concentration. The mobile phase was composed of acetonitrile and water (70:30, v/v) at a flow rate of 1.0 mL min 1 with a column temperature of 30 8C. A multiple wavelength setting was applied for quantification of CPs: 276 nm for 2-CP and 3-CP, 280 nm for 4-CP, 286 nm for 2,4DCP, and 290 nm for 2,4,6-TCP. For the hydroquinone analysis, the UV absorbance wavelength was set to 254 nm. The chloride ion (Cl ) and carboxylic acids were determined by using a Dionex ICS-
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Full Paper 2000 system equipped with a DS6 conductivity detector, a Dionex IonPac AS11-HC analytical column (4 250 nm), an IonPac AG11-HC guard column (4 50 mm), and a Dionex ASRS UrtraIIsuppressor (4 mm). The mobile phase was 1 mm KOH at a flow rate of 1.0 mL min 1. TOC was measured by using a TOC/TN analyzer (liquic TOC II) (Elementar Corporation, Germany) with deionized water and 0.8 % HCl as mobile phase.
[7]
[8]
Characterization The morphologies of the synthesized materials were surveyed by using a Tecnai G2 F20 HRTEM with an energy dispersive X-ray spectrometry (HRTEM-EDX, FEI, Netherlands). X-ray diffraction studies (XRD, PANalytical X’ Pert diffractometer, Almelo, Netherlands) were performed by using a monochromatized X-ray beam with nickel-filtered CuKa radiation at a scan rate of 0.4 8 min 1 . X-ray photoelectron spectroscopy (XPS) measurements were conducted by applying a Thermo Scientific ESCA-Lab-200i-XL spectrometer (Waltham, MA) with monochromatic AlKa radiation (1486.6 eV). FTIR spectra were taken in KBr pressed pellets on a Nicolet Thermo NEXUS 670 Infrared Fourier Transform Spectrometer (Waltham, MA). The magnetization curves of the products were measured with a LDJ9600 vibrating sample magnetometer (VSM, Troy, MI). Nitrogen sorption isotherms were measured at 77 K with a Quadrasorb SI Four Station Surface Area Analyzer and Pore Size Analyzer (Quantachrome Instruments, Boynton Beach, FL). Before measurements, the samples were degassed in a vacuum at 300 8C for at least 6 h. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas using adsorption data in a relative pressure range from 0.05 to 1.0. The pore volumes and pore size distributions were calculated by using the Barrett–Joyner–Halenda (BJH) model and the total pore volumes were estimated from the adsorbed amount at a relative pressure P/P0 of 0.993.
Acknowledgements
[9] [10]
[11] [12]
[13]
[14]
[15]
[16]
This work was jointly supported by the Special Fund of National Key Scientific Instrument and Equipment Development (2012YQ09022907), the National Basic Research Program of China (2011CB936001), and the National Natural Science Foundation of China (21277152, 21277002, 21321004).
[17] [18] [19] [20] [21]
Keywords: chlorophenol · Fenton-like reactions heterogeneous catalysis · magnetite · nanostructures
· [22]
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Received: October 29, 2013 Revised: November 26, 2013 Published online on April 15, 2014
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