Highly Efficient Iron Nanocatalyst Stabilized by Double-Walled Carbon

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Highly Efficient Iron Nanocatalyst Stabilized by Double-Walled Carbon Nanotubes and Mixed Metal Oxides for Degradation of Cationic and Anionic Dyes by a Fenton-like Process Wei Li, Tingting Sun, and Feng Li* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, P.O. Box 98, Beijing, 100029, P.R. China S Supporting Information *

ABSTRACT: In the present work, a three-dimensional iron nanocatalyst stabilized by double-walled carbon nanotubes and Mg−Al mixed metal oxides (Fe/DWCNTs−MMO) was directly generated by a facile catalytic chemical vapor deposition process, where Fe(CN)63−-intercalated Mg−Al layered double hydroxide as catalyst precursor was utilized to catalyze the growth of DWCNTs. This preparation route did not require complex pretreatment of DWCNTs and subsequent immobilization of metal nanoparticles. The resulting Fe/DWCNTs−MMO was applied as a Fenton-like catalyst for the degradation of methylene blue and methyl orange dyes. The materials were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, N2 adsorption−desorption experiments, and Raman spectra. The results revealed that Fe/ DWCNTs−MMO as Fenton-like catalyst showed superior catalytic activity in the degradation of model dyes to a traditional catalyst system derived from Fe(CN)63−-intercalated Mg−Al layered double hydroxide, where iron nanoparticles were only deposited onto Mg−Al MMO flakes formed. The possible mechanism for the degradation of dyes was discussed. Moreover, asformed Fe/DWCNTs−MMO possessed high structural stability under the reaction conditions and could be reused five times without remarkable activity loss.

1. INTRODUCTION Over the past few decades, Fenton technology has been explored extensively for the chemical decomposition of various organic contaminants originating from industrial discharged wastewaters because of the decrease in the quality of drinking water available in the world.1,2 By utilizing Fenton reagents composed of Fe2+/Fe3+ and hydrogen peroxide, hydroxyl radicals generated in aqueous solution may rapidly oxidize organic pollutants.3−6 Recently, zerovalent iron-based heterogeneous Fenton-like systems are gaining increasing attention in the treatment of pollutants in aqueous solutions.7−10 Especially, the development of nanoscale iron particles has attracted considerable interest because of their high surface area and excellent surface reactivity.11,12 The aggregation of metal nanoparticles (NPs), however, may result in the decrease in the surface area and thus their efficiency. Therefore, it has always been a hot issue to develop highly dispersed Fe-based Fenton-like nanocatalysts. As one of the most important carbon materials, carbon nanotubes (CNTs) have been used widely as support materials in the field of heterogeneous catalysis,13,14 because of their fascinating physicochemical properties. To date, the most effective method for large-scale production of CNTs is catalytic chemical vapor deposition (CCVD).15−17 In CCVD, carbon atoms from decomposition of carbon-containing gases deposit on the surface of catalytically active metal NPs supported on high-surface-area supports to form various CNTs. On the other hand, a variety of metals and metal oxides have been immobilized onto CNTs to achieve enhanced performance.18−23 The adhesion of guest components on CNTs, however, usually is not strong enough to survive the mechanical © XXXX American Chemical Society

shaking involved in reactions, owing to the drawback of conventional impregnation. Commonly, the applied strategy for effective immobilization of active species is surface modification of CNTs with functional groups or organic binders,24 which can enhance the interactions between CNTs and the desired components. In addition, combining one-dimensional CNTs with two-dimensional materials can construct three-dimensional nanocomposites with unexpected properties.25−27 Therefore, high dispersion of uniform CNTs in nanocomposites always is a challenge in order to achieve extraordinary performance. Layered double hydroxides (LDHs), which have a formula of [M1−x2+Mx3+(OH)2]x+[Ax/n]n−·mH2O, are well-known as a class of highly ordered two-dimensional materials.28−30 Their structure may be flexibly designed by varying the types of metal cations located within the layers and the anions intercalated into the interlayer domains. Thus, well-dispersed mixed metal oxide (MMO) supported metal NPs can be obtained by calcination of LDH precursors containing desired metal species either in the form of cations or in the form of complexes, followed by reduction process.31−33 Recently, we found that cobalt and nickel nanocatalysts derived from Coand Ni-containing LDH precursors could catalyze the growth of multiwalled CNTs (MWCNTs) in CCVD.34,35 A few works were reported on the fabrication of few-walled CNTs using LDHs containing Fe cations in the brucite-like layers.36,37 Received: August 19, 2014 Revised: October 22, 2014 Accepted: November 4, 2014

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dx.doi.org/10.1021/ie503300r | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Figure 1. XRD patterns (A) of MgAl-LDH (a) and MgAlFe-LDH (b) and SEM image (B) of MgAlFe-LDH.

loaded in a ceramic boat was heated to 900 °C at a rate of 3 °C/min under a flow of N2 [as protecting gas, flow rate 300 cm3 (STP)/min]. Second, 10% H2/N2 (v/v) with a flow rate of 600 cm3 (STP)/min was switched into the furnace, and the temperature was maintained at 900 °C for 10 min. Third, N2 with a flow rate of 300 cm3 (STP)/min and CH4 with a flow rate of 200 cm3 (STP)/min were switched into the furnace, and the temperature was maintained for 30 min. After the reaction, CH4 was switched off and N2 was continued until the furnace was cooled down to room temperature. At last, the resultant Fe/DWCNTs−MMO black powder was collected from the ceramic boat. To obtain purified DWCNTs, Fe/DWCNTs− MMO was dissolved in dilute hydrochloric acid (6.0 M), and then the suspension was filtered, washed with deionized water, and dried at 70 °C in air for 12 h. In addition, Fe/MMO sample was synthesized by calcination of MgAlFe-LDH precursor at 900 °C, followed by reduction in 10% H2/N2 atmosphere at 900 °C for 10 min. For comparison, Fe/ MWCNTs catalyst with an Fe content of 1.4 wt % also was synthesized by liquid-phase reduction. FeCl3·6H2O (0.2 g) was dissolved in 50 mL of deionized water, and commercial MWCNTs (2.0 g) were added to this solution and the mixture stirred for 30 min. Subsequently, 1.0 M NaBH4 solution (5.0 mL) was added dropwise into this mixture under nitrogen atmosphere and maintained for 30 min. Finally, the formed sample was collected by filtration. 2.2. Characterization. Powder X-ray diffraction (XRD) patterns were collected using a Shimadzu XRD-6000 diffractometer under the following conditions: 40 kV, 30 mA, graphite-filtered Cu Kα source (λ = 0.154 18 nm). Elemental analysis for metal ions in samples was carried out on a Shimadzu ICPS-75000 inductively coupled plasma emission spectrometer (ICP-ES). Analytical solutions were prepared by dissolving the samples in chloronitrous acid. Scanning electron microscopy (SEM) observations were performed on a Hitachi S-4700 apparatus with an applied voltage of 20 kV. Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) observations were investigated on a JEOL JEM-2100 electron microscope operated at 120 and 200 kV, respectively. N2 adsorption−desorption isotherms of samples were obtained using a Micromeritics ASAP 2020 sorptometer apparatus. The specific surface areas were calculated from the multipoint Brunauer−Emmett−Teller (BET) method, while

Compared with multiwalled CNTs, few-walled CNTs possess better mechanical, electrical, and thermal properties and thus exhibit wider potential as high-performance catalyst supports. In this work, iron nanocatalyst stabilized by double-walled CNTs (DWCNTs) and MgAl−MMO flakes (Fe/DWCNTs− MMO) was directly fabricated in situ by CCVD, where Fe(CN)63−-intercalated MgAl-LDH as catalyst precursor was utilized to catalyze the growth of DWCNTs, and simultaneously, metallic Fe NPs were generated with DWCNTs. The main objective of the current study was to test the applicability of Fe/DWCNTs−MMO as Fenton-like catalyst for the removal of cationic (methylene blue, MB) and anionic (methyl orange, MO) model dyes. It was found that as-formed Fe/DWCNTs− MMO nanocatalyst exhibited superior catalytic degradation ability to a traditional catalyst system derived from Fe(CN)63−intercalated MgAl-LDH, where Fe NPs were only deposited onto MgAl−MMO flakes. The more efficient Fenton-like process was associated with the unique heterostructure of the Fe/DWCNTs−MMO nanocomposite system, which possessed strong interactions between Fe NPs and carbon materials formed and highly dispersed catalytically active metallic Fe species. To the best of our knowledge, no endeavor has been made to explore the promotion of the catalytic performance of Fe-based Fenton-like catalyst systems through the introduction of double DWCNTs and MgAl−MMO matrices.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Materials. 2.1.1. Synthesis of Catalyst Precursor. Mg(NO3)2·6H2O, Al(NO3)3·9H2O, and K3[Fe(CN)6] with a [Mg2+]/[Al3+] molar ratio of 2.0 and a [Fe(CN)63−]/[Al3+] molar ratio of 0.3 were dissolved in 100 mL of CO2-free deionized water with the total cation concentration of 0.05 M. Subsequently, the above salt solution was titrated with a solution of CO(NH2)2 {[CO(NH2)2] = 3.3 ([Mg2+] + [Al3+])} under stirring. Then the aqueous solution was aged at 100 °C for 12 h under N2 atmosphere. The resulting suspension was centrifuged and washed with deionized water and then dried under vacuum at 70 °C for 12 h. The obtained sample was denoted as MgAlFe-LDH. For comparison, a reference MgAl-LDH sample also was prepared in the absence of K3[Fe(CN)6] according to the above procedure. 2.1.2. Synthesis of Fe-Based Catalysts. Fe/DWCNTs− MMO was directly synthesized in a horizontal quartz tube furnace by CCVD of methane. First, MgAlFe-LDH (100 mg) B



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the pore size distribution and most frequent pore diameter were determined according to the Barrett−Joyner−Halenda (BJH) method applied to the desorption isotherms. Raman spectra at room temperature were collected on a microscopic confocal Raman spectrometer (Jobin Yvon Horiba HR800) using an Ar+ laser of 532 nm wavelength as excitation source. The decrease in the total organic carbon (TOC) was evaluated using an Apollo 9000 TOC analyzer (Terkmar Dohrmann Co.). 2.3. Degradation Experiments of Dyes. The catalyst (25.0 mg) was added to a solution containing 45.0 mL of dye solution (50.0 mg/L) and agitated for 60 min to reach adsorption−desorption equilibrium. The experiments were initiated by adding 5.0 mL of 10.0% H2O2 to the solution. This mixture was shaken in a water bath adjusted to 25 °C under atmospheric pressure. Samples were taken at a given time interval during the dye degradation, and then 10 μL of tertbutyl alcohol was immediately added into sample as reaction inhibitor. Samples were filtered to remove catalyst particles before analyses of the solution. The solution concentration was determined by absorbance at 665 nm for MB and 465 nm for MO in UV−vis absorption spectrum. In addition, blank experiments were carried out under the same conditions, with just H2O2 solution and without catalyst. In the cycling tests, the mixture solution was centrifuged to remove the catalyst after the reaction. The catalyst was washed with ethanol and deionized water and finally dried at 70 °C for 12 h. Then, fresh dye and H2O2, as well as the recovered catalyst, were employed in the repeated reactions.

Figure 2. TEM images of calcined MgAlFe-LDH before (a) and after (b) a reduction of 10 min.

note that a large amount of one-dimensional carbon nanostructures extend perpendicularly from both sides of the flakes and further distribute densely over the flakes (Figure 3a),

Figure 3. SEM images (a, b) showing one-dimensional carbon nanostructures distributing over both sides of the MMO flakes.

due to the high density of small metal catalyst particles. Furthermore, the two-dimensional flake morphology is well preserved after the growth of carbon nanostructures. The closeup SEM image reveals that these carbon nanostructures interlinked with two-dimensional MMO flakes have uniform diameters (Figure 3b). A high yield of 2.45 g carbon materials/ g catalyst is achieved over these MMO flakes. As shown in Figure 4, the XRD patterns of carbon materials grown over calcined MgAlFe-LDH further confirm characteristic (111) and (110) diffractions of metallic Fe phase and (002) diffraction of graphite carbon appearing at 42.8°, 44.7°,

3. RESULTS AND DISCUSSION 3.1. Characterization of Materials. Figure 1A presents the XRD patterns of LDH precursors. It is worthwhile to note that both MgAlFe-LDH and MgAl-LDH samples exhibit the typical characteristic diffractions for (003), (006), (012), (015), and (018) crystal planes of carbonate-type hydrotalcite-like materials, which correspond to basal spacing and higher-order diffractions.38 With the introduction of Fe(CN)63− complex anions, the (003) diffraction at 2θ of around 11.5° decreases in intensity, while a series of new (003), (006), and (009) diffractions corresponding to another crystalline LDH phase appear at 2θ of around 8.2°, 16.4°, and 24.7°, respectively. The diffractions at 11.5° and 8.3° arise from the basal spacings of approximately 0.76 and 1.06 nm, respectively, in agreement with the reported values for carbonate-intercalated LDH and Fe(CN)63−-intercalated LDH.39 As a result, the expanded interlayer structure of MgAlFe-LDH suggests the intercalation of Fe(CN)63− anions into the interlameller space in the competition of CO32−, and both of the two anions coexist in the interlayer domains. Further, a SEM image of MgAlFe-LDH (Figure 1B) depicts the plateletlike morphology of large crystallites with a lateral size of ca. 2.0 μm and a thickness of tens of nanometers, indicative of a uniform and well-crystalline structure. The large plateletlike MgAlFe-LDH crystallites can be first calcined into Mg−Al−Fe mixed metal oxide (MgAlFe−MMO) flakes by subsequent dehydration and decomposition of layers during the heating process (Figure 2 a). After reduction of MgAlFe−MMO flakes at 900 °C for 10 min, some black Fe NPs with a size ranging from 2 to 8 nm were found to be highly dispersed on the flakes (Figure 2b). Subsequently, after the introduction of methane as a carbon source, it is interesting to

Figure 4. XRD patterns of carbon products over calcined MgAlFeLDH. C



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and ∼26°, respectively. In addition, the characteristic diffractions related to Al2O3, MgO, and MgAl2O4 phases can be observed in the XRD patterns. HRTEM studies give further information about the morphology and microstructure of carbon products obtained by CCVD over Fe-based catalyst at 900 °C. As shown in Figure 5a−c, carbon products clearly present the typical structure of

1.42 nm, in good agreement with the above HRTEM observations. Further, as shown in Figure 6a, some small NPs with a diameter of about 2−5 nm are colocated with nanotubes on the surface of MMO matrix. Besides, graphitic carbon materials are found to surround or encapsulate NPs in the Fe/DWCNTs− MMO (Figure 6b). The NPs are metallic Fe, based on the chemical phases identified by XRD (Figure 4). A typical HRTEM image depicts the lattice fringes with the interplanar spacing of about 2.03 Å for a single nanoparticle (Figure 6c), corresponding to the (110) plane of metallic Fe phase. The result suggests that the synthesis of DWCNTs occurs through a base-growth mode;43 i.e., the Fe nanocrystal remains anchored to the substrate during CNT synthesis. It has been proposed that the weak interactions between the catalyst particle and the support usually result in tip growth mode, whereas the strong interactions lead to base growth.44 In our case, the interactions between the Fe NPs and the MMO matrix are strong, thus leading to a base-growth mechanism. Therefore, with the introduction of methane at 900 °C, reduced MgAlFe−MMO offers two functions: the lamellar flakes act as matrix to deposit carbon materials as large graphitic shells on the Fe NPs, while the small metallic Fe NPs generated by the reduction of MMO flakes catalyze the growth of DWCNTs. On the other hand, it is well-known that the external diameter of carbon nanostructures produced by the CCVD method usually is dependent on the size of catalytically active metal NPs.45−47 Correspondingly, the diameters of CNTs formed may allow an estimate of the size distribution of metal NPs. In our present system, the actual average size of Fe NPs in the Fe/DWCNTs−MMO should be consistent with the external diameter of DWCNTs (about 2.2 nm), as well as the size of Fe NPs in the Fe/MMO. Figure 7 shows the nitrogen adsorption−desorption isotherms and pore size distribution curves of Fe/MMO and Fe/DWCNTs−MMO. Fe/DWCNTs−MMO shows type IV isotherms with an H3-type hysteresis loop that does not exhibit any limiting adsorption at high P/P0, indicating that the capillary condensation in the mesopores with three-dimensional interconnected geometry takes place on the as-calcined MMO flakes, due to aggregates of particles resulting in cylindrical pores from irregular voids between particles.48 In addition, Fe/ DWCNTs−MMO displays a broad pore size distribution in the range of 2−100 nm centered at about 5.0 nm. In comparison, Fe/MMO possesses the same type IV isotherms and, however, exhibits a narrower pore size distribution and smaller pore volume than Fe/DWCNTs−MMO (Table 1), which is associated with the more severe aggregates of particles. Furthermore, the BET specific surface area of Fe/DWCNTs−

Figure 5. HRTEM images (a−c) of carbon products showing the structure of DWCNTs and the Raman spectrum (d) of as-grown DWCNTs. The inset in panel d shows the RBM peak.

DWCNTs with the hollow core and an inner diameter of about 1.6 nm in most cases. Fe NPs are not readily observed at the tips of DWCNTs formed. The typical Raman spectrum of asgrown DWCNTs (Figure 5d) shows a sharp E2g in-plane vibration of sp2-bonded carbon atoms at 1570 cm−1 (G-band) and a low A1g breathing vibration of carbon atoms with dangling bonds in plane terminations of disordered graphite at 1332 cm−1 (D-band).40,41 A low value of ID/IG intensity of 0.34 confirms the high crystallinity of DWCNTs formed. In addition, the main radial breathing mode (RBM) in a lowfrequency range centered at about 175 cm−1 demonstrates the structure of DWCNTs. According to the formula d = 234/ (ωRBM − 10),42 the estimated diameter of DWCNTs is about

Figure 6. HRTEM images (a−c) of Fe/DWCNTs−MMO sample. D



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the degradation experiments of cationic MB and anionic MO dyes were carried out as probe reactions. No base or acid was used to adjust the pH value of reaction solutions, and all reactions were performed in the dark to avoid the influence of light. To demonstrate the activity of Fe/DWCNTs−MMO nanocatalyst for the removal of MB and MO, the absorption changes of dyes during the degradation are showed in Figure 8. It is seen that the intensity of the strong absorption band at 665 nm for MB solution decreases gradually with the reaction time, indicating that the conjugated π system of the MB molecule has been destroyed.49 Meanwhile, the other two absorptions at 292 and 245 nm associated with the phenothiazine species are reduced with the reaction time.50,51 As for MO solution, two strong absorption bands at 465 and 270 nm, which are assigned to the azo band under the strong influence of the electrondonating dimethylamino group and the π−π* transition related to aromatic rings, respectively,52 are reduced gradually with the reaction time, indicative of the destruction of the azo groups and aromatic rings. After 6 h, the absorbance of MB and MO solutions is close to zero with the Fe/DWCNTs−MMO catalyst, confirming the almost complete oxidative decomposition of MB and MO. The dependence of MB and MO degradation over different catalysts was investigated under the same operating conditions. As shown in Figure 9, both MB and MO exhibit a little selfdegradation in the absence of catalyst with the addition of H2O2. Fe/MMO shows a relatively low activity and only 37% of MB and 56% of MO are decolored after 2 h. In contrast, Fe/ DWCNTs−MMO is very active in Fenton-like reactions. The degradation percentages for MB and MO dyes reach about 54% and 80% after 1 h, respectively. After 6 h, the catalyst can convert the blue MB and orange MO solutions into colorless ones. In addition, it is found that the removal efficiency of TOC for Fe/DWCNTs−MMO is approximately 99% after 6 h. Further, according to the TOC removal curves with the reaction time (Figure S1, Supporting Information), one can note that the degradation rates of MB and MO dyes are higher than the corresponding TOC reduction rates, especially at the initial reaction stages, due to the formation and accumulation of incompletely degraded intermediates. Fourier transform infrared spectroscopy (FT-IR) of the fresh and used catalysts (not shown) also demonstrates that there is almost no organic residue in the used catalysts. It confirms that the MB and MO

Figure 7. Nitrogen adsorption−desorption isotherms of Fe/MMO (a) and Fe/DWCNTs−MMO (b). The inset shows the pore size distributions.

Table 1. Analytical and Structural Data Obtained for Different Catalysts sample

Fe (wt %)a

SBETb (m2 g−1)

Vpc (cm3 g−1)

dBJHd (nm)

Fe/MMO Fe/DWCNTs−MMO

3.2 1.3

65 32

0.19 0.11

2.4, 3.7 3.9, 5.6

a

Determined by ICP-ES. bBET surface area. cTotal pore volume. Most frequent pore diameter obtained from the desorption branch of the isotherm using the BJH method.

d

MMO is about 2 times larger than that of Fe/MMO, owing to the existence of a heteronanostructure of Fe/DWCNTs− MMO containing a large amount of DWCNTs and graphitic carbon materials. 3.2. Degradation of MB and MO Model Dyes. It is wellknown that the Fenton oxidation process is based on the action of the hydroxyl radical generated by Fenton or Fenton-like reagents in aqueous solution. Therefore, it is likely that due to the unique dispersion nature and local environment of Fe NPs in the Fe/DWCNTs−MMO, it can be utilized as a highly efficient Fenton-like catalyst for decolorization of aqueous dye solutions. In this study, to demonstrate the activity of Fe/ DWCNTs−MMO for the degradation of organic pollutants,

Figure 8. Absorption changes of MB (A) and MO (B) during the degradation process over the Fe/DWCNTs−MMO catalyst. E



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Figure 9. Degradation of MB (A) and MO (B) solutions monitored as the normalized concentration change vs reaction time using different catalysts: (a) without catalyst, (b) Fe/MMO, and (c) Fe/DWCNTs−MMO.

Figure 10. Effects of initial dye concentration (A, B) and catalyst dosage (C, D) on MB and MO degradation over the Fe/DWCNTs−MMO nanocatalyst.

F



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On the other hand, the effect of the initial concentrations of MB and MO dyes on the degradation over the Fe/DWCNTs− MMO nanocatalyst was studied by varying the initial concentration of dyes under otherwise the same conditions. As shown in Figure 10A,B, with the initial concentration of MB and MO increasing to 75 and 100 mg/L, the degradation percentage of MB decreases to about 96.0 and 89.0% after 6 h, while the degradation percentage of MO decreases to about 93 and 87%. It implies that the reaction time for complete degradation of MB and MO should be extended. The increase in the initial concentration of dye may lead to the increase in the enrichment of dye molecules on the Fe/DWCNTs−MMO surface. Consequently, a larger number of surface active sites can be occupied by dye molecules, which may inhibit the generation of more hydroxyl radicals on the surface. Further, the catalytic performance of Fe/DWCNTs−MMO with different dosages was investigated under the same experimental conditions (Figure 10C,D). When the dosage of Fe/ DWCNTs−MMO is 0.75 g/L, the initial degradation rates of MB and MO are increased significantly, in comparison to those using the dosage of 0.5 g/L (Figure 9). This is because an increasing amount of catalyst inevitably produces more hydroxyl radicals through H2O2 decomposition, thus leading to the greatly enhanced degradation activity.58,59 However, compared to those using the catalyst dosage of 0.5 g/L, the degradation rates of MB and MO are only slightly enhanced when the catalyst dosage reaches 1.0 g/L. This is because the excess amount of Fe2+ species formed on the catalyst surface during the course of oxidation may react with hydroxyl radicals to form inactive OH−,60 thus in turn resulting in the rapid decrease of hydroxyl radicals. Moreover, the recyclability of Fe/DWCNTs−MMO nanocatalyst as efficient Fenton-like catalyst for the removal of MB and MO was also investigated, and results show that Fe/ DWCNTs−MMO maintains excellent activity with nearly 95% degradation percentage of MB and MO after five consecutive cycles (Figure 11). It is found that Fe concentration in solution for the MB degradation determined by ICP-ES after five consecutive cycles is about 0.03, 0.02, 0.02, 0.01, and 0.02 ppm, respectively, indicative of a negligible Fe leaching from Fe/ DWCNTs−MMO. Therefore, the slight decrease in the

molecules are completely degraded into water and carbon dioxide. As a result, Fe/DWCNTs−MMO appears to be much more effective than Fe/MMO as a Fenton-like catalyst in terms of the speed and extent of dye removal. Furthermore, the degradation of MB and MO over Fe/MWCNTs catalyst was investigated in Fenton-like reactions for comparison. Observation of TEM images of Fe/MWCNTs sample (Figure S2, Supporting Information) reveals that few small and irregular NPs are well-dispersed on the surface of MWCNTs due to a very low Fe content of 1.4 wt %. As shown in Figure S3 (Supporting Information), 28% of MB and 40% of MO are degraded over Fe/MWCNTs after 6 h, indicative of a much lower activity than that of Fe/DWCNTs−MMO. According to the experimental results, a possible mechanism is discussed. In the present system, an oxidation of organic contaminants by the generated hydroxyl radicals through the Fe0/H2O2 system functions should take place.53−55 Initially, Fe0 NPs are oxidized into Fe2+ species via a two-electron transfer from the particle surface to H2O2 (eq 1), then the hydroxyl radicals responsible for the oxidation of organic pollutants, as well as Fe3+ species, are further generated by the Fenton reaction (eq 2).11 In addition, the generated Fe3+ species also are reduced to Fe2+ species by Fe0 (eq 3), which further promotes the production of more hydroxyl radicals.8 Fe0 + H 2O2 + 2H+ → Fe2 + + 2H 2O

(1)

Fe 2 + + H 2O2 → Fe3 + + OH− + •OH

(2)

2Fe3 + + Fe0 → 3Fe 2 +

(3)

The above experimental results have demonstrate that catalytically active Fe NPs in the Fe/MMO can be further isolated and coated by the DMCNTs formed through a basegrowth process during the CCVD. Correspondingly, compared with those in the Fe/MMO, Fe NPs in the Fe/DWCNTs− MMO present a different and unique local environment. Surprisingly, as for Fe/DWCNTs−MMO, physically isolated Fe NPs are active for a Fenton-like process, even though H2O2 probably is not in direct contact with active Fe sites. Therefore, Fe NPs must have interacted with DWCNTs or graphitic carbons shells and affect the properties of the outer walls, where hydroxyl radical is generated. In a control experiment, Fe/ MMO mixed physically with purified DWCNTs was used and it showed slightly lower activity than Fe/MMO catalyst. Purified DWCNTs also do not show a discernible catalytical activity in the degradation of dyes. As a result, the outstanding activity of Fe/DWCNTs−MMO should be related to the interactions between metallic Fe NPs formed and DWCNTs or graphitic carbon shells, which may change their local work function,56,57 thus leading to a more efficient Fenton-like process for the degradation of organic dyes. In addition, the enrichment of MB and MO onto the surface of Fe/DWCNTs−MMO, resulting in a higher reactant concentration on the catalyst, can promote the degradation of dye molecules in situ, because Fe/ DWCNTs−MMO possesses a higher specific surface area than Fe/MMO. In general, the much enhanced catalytic activity should be due to the unique heterostructure of Fe/DWCNTs− MMO that benefits interactions between Fe NPs and carbon materials, as well as the enrichment of reactants. A further detailed investigation of how carbon materials (DWCNTs or graphitic carbon shells) contribute to the enhanced catalytic activity of Fe/DWCNTs−MMO will be the focus of future work.

Figure 11. Reproducibility of Fe/DWCNTs−MMO in a Fenton-like process. G



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degradation percentage of MB after five cycles should be ascribed to a trace loss of Fe/DWCNTs−MMO during the operating process. The above result proves the excellent reusability and high structural stability of Fe/DWCNTs−MMO catalyst.

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4. CONCLUSIONS In summary, the Fe complexes combined with MgAl-LDH to form a kind of catalyst precursor for the growth of DWCNTs. Combining materials with one-dimensional DWCNTs and two-dimensional lamellar MgAl−MMO flakes led to a threedimensional Fe/DWCNTs−MMO heterostructure. As-formed DWCNTs- and MMO-stabilized iron nanocatalyst showed enhanced catalytic degradation activity toward MB and MO decolorization via a heterogeneous Fenton-like mechanism, in comparison with a traditional Fe/MMO system. The Fe speciation information was found to be not identical in both Fe/DWCNTs−MMO and Fe/MMO, indicating that the enhanced activity was directly associated with the unique heterostructure of Fe/DWCNTs−MMO, which possessed the strong interactions between Fe NPs and DWCNTs or graphitic carbon shells formed, as well as highly dispersed catalytically active Fe NPs. Therefore, this work demonstrates that Fe(CN)63−-intercalated LDH is an extraordinary precursor for the fabrication of three-dimensional composites with DWCNTs and two-dimensional MMO flakes, and as-formed Fe/ DWCNTs−MMO Fenton-like catalyst with rather high catalytic activity can be potentially applied in the field of the removal of dyes.

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ASSOCIATED CONTENT

* Supporting Information S

The TOC removal curves during the degradation of dyes, TEM images of Fe/MWCNTs sample, and degradation curves of MB and MO solutions with the reaction time in the presence of Fe/ MWCNTs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 10 64451226. Fax: +86 10 64425385. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Basic Research Program of China (No. 2011CBA00506), National Natural Science Foundation of China (No. 21325624, 20171016) and Program for Beijing Engineering Center for Hierarchical Catalysts.

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REFERENCES

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Highly Efficient Iron Nanocatalyst Stabilized by Double-Walled Carbon

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