Mesoporous silica coating on hierarchical flowerlike Fe₂O₃ with enhanced catalytic activity for Fenton-like reaction

Abstract

Hierarchical flowerlike Fe₂O₃ was coated with mesoporous silica to form a Fe₂O₃@mesoporous silica composite with a core/shell structure through a simple solution-based method with the assistance of Fe₂O₃ as a precursor. When used as a catalyst for a Fenton-like reaction for the degradation of methylene blue, the Fe₂O₃ mesoporous silica composite was much more active than the bare flowerlike Fe₂O₃, which suggests that the catalytic activity of the flowerlike Fe₂O₃ in the Fenton-like reaction was dramatically enhanced by the mesoporous silica coating.

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The Fenton reaction based on the generation of hydroxyl radicals from the decomposition of hydrogen peroxide in the presence of ferrous ions (Fe²⁺) in an acidic condition is one of the most cost-effective methodologies to treat waste water.^1–5 Most of the organic pollutants are degraded to low-molecular-mass carboxylic acids and CO₂ in the Fenton reaction.¹ Homogeneous catalysts are efficient towards the Fenton reaction; however, there are some drawbacks of homogeneously catalyzed Fenton reactions such as the need for large amounts of transition metal salts, the production of large amounts of sludge, and the need for pH value adjustments before and after Fenton reactions.³ Heterogeneous catalysts for Fenton-like reaction are attracting increasing research attention due to the advantages of allowing easier separation from the effluent and reuse without activity loss.³ In the past few years, numerous types of materials, such as clays, silicas, zeolites, metals and metal oxides, have been developed as active heterogeneous catalysts for Fenton-like reactions.^6–9 Among them, iron-based materials are promising catalysts for practical application as they are low cost and storable.^10,11 However, the heterogeneous Fenton-like activities of these catalysts are relatively low in normal reaction conditions without any external energy input such as light irradiation.¹² Therefore, designing and developing heterogeneous catalysts with high activity and high durability are most important for Fenton-like reactions.

Recently, various strategies have been developed to improve the performance of iron-based heterogeneous Fenton-like catalysts. Reducing the size of the catalyst to the nano-scale is an effective way to increase the surface area and surface energy for a reaction.^3,13,14 Loading the catalysts with small sized particles onto carriers with high surface areas can improve the dispersion of the catalysts and further improve the catalytic activity.^15–17 Constructing a specific nanostructure is another way to build active Fe-based Fenton-like catalysts.^18–21 Panda et al. reported a mesoporous Fe₂O₃–SiO₂ composite catalyst that showed rapid decolourization of methyl orange by a Fenton-like reaction.²² Zheng et al. prepared an ordered mesoporous hematite by a magnesium selective leaching method and then found that the mesoporous iron oxide is an effective Fenton-like catalyst for the degradation of methylene blue (MB).¹² Very recently, a yolk/shell structured Fe₂O₃@mesoporous silica nanoreactor was developed by our group.²³ Compared with bare Fe₂O₃ nanoparticles, this nanoreactor exhibited enhanced catalytic activity for the Fenton-like reaction of the degradation of MB without adjusting the pH values. Liu et al. also reported Fe⁰@SiO₂ nanoparticles with a yolk/shell structure as a nanoreactor and they demonstrated enhanced catalytic activity of the nanoreactor for the Fenton-like catalytic oxidation of phenol.²⁴ However, developing Fe-based nanostructures with high catalytic activity and reusability for Fenton-like reactions is still desirable.

Hierarchical structured Fe₂O₃ is a promising catalyst for Fenton-like reactions as its nano-sized building blocks could provide a large specific surface area, more catalytically active sites and facile mass transportation pathways while their entire micro-metered size was favourable for catalyst recovery.²⁵ However, the application of hierarchical structured Fe₂O₃ in Fenton-like reactions is rarely reported. In this report, a flowerlike Fe₂O₃ with a hierarchical structure was prepared through a simple hydrothermal method. Then, the flowerlike Fe₂O₃ was coated with mesoporous silica to form an Fe₂O₃@mesoporous silica (Fe₂O₃@meso-SiO₂) composite with a core/shell structure. The flowerlike Fe₂O₃ and the Fe₂O₃@meso-SiO₂ composite were both used as catalysts for Fenton-like reactions in the degradation of MB. It was found that the catalytic activity of the flowerlike Fe₂O₃ to Fenton-like reactions was dramatically enhanced by the mesoporous silica coating.

The flowerlike Fe₂O₃ precursor was synthesized in ethylene alcohol with a simple hydrothermal method, according to the reference.²⁵ The X-ray diffraction (XRD) pattern of the Fe₂O₃ precursor (Fig. S1, ESI†) shows the emergence of diffraction peaks in striking similarity to those of other metal oxide precursors reported in the literature.^26,27 The as-synthesized Fe₂O₃ precursor was calcined in a muffle at 500 °C to produce crystalline Fe₂O₃. The XRD pattern shows that the crystal structure of the obtained flowerlike Fe₂O₃ agrees well with α-Fe₂O₃ (Fig. 1a). The room-temperature hysteresis curves showed that the flowerlike Fe₂O₃ was weakly magnetic (Fig. S2†). Mesoporous silica was coated on the flowerlike Fe₂O₃ through the reaction of the Fe₂O₃ precursor, the base (ammonia solution), the template (cetyltrimethyl ammonium bromide, CTAB) and silica source (tetraethyl orthosilicate, TEOS) in a mixture solution of water and ethanol (4 : 3, v/v).^28,29 The precipitate was collected by centrifugation and then calcined in a muffle at 500 °C to produce the final Fe₂O₃@meso-SiO₂ composite. The Fe₂O₃@meso-SiO₂ composite has the same XRD pattern as the flowerlike Fe₂O₃ (Fig. 1b), indicating that the crystal structure of the flowerlike Fe₂O₃ was not changed after the mesoporous silica coating.

Fig. 1 XRD pattern of the flowerlike Fe₂O₃ (a) and the flowerlike Fe₂O₃@meso-SiO₂ composite (b); both the XRD patterns agree well with α-Fe₂O₃.

The obtained flowerlike Fe₂O₃ and Fe₂O₃@meso-SiO₂ composite were further characterized by SEM and TEM. The SEM image shows that the crystalline Fe₂O₃ has the morphology of a flower (Fig. 2a). The TEM image shows that the flowerlike structure of Fe₂O₃ consists of petals as primary building units, which are composed of numerous inter-connected nanoparticles as secondary building units (Fig. 2b). After the mesoporous silica coating, the Fe₂O₃@meso-SiO₂ composite has almost the same morphology as the flowerlike Fe₂O₃ except that the width of the petals of the flowers became a little thicker due to the mesoporous silica coating (Fig. 2c). The TEM image of the Fe₂O₃@meso-SiO₂ composite further proves that the mesoporous silica coating did not change the morphology of the flowerlike Fe₂O₃ (Fig. 2d). The core/shell structure of the Fe₂O₃@meso-SiO₂ composite was also demonstrated by the TEM image. The TEM image with higher magnification shows that mesoporous silica was coated uniformly on the surface of the crystalline Fe₂O₃. The dividing line between the Fe₂O₃ core and the mesoporous silica shell is visible (Fig. 3e). The thickness of the mesoporous shell is about 30 nm. The Fe₂O₃@meso-SiO₂ composites were carefully treated with 2 M HCl to remove the metal oxide cores. Mesoporous silica hollow flowers with replica morphologies were then obtained when the metal oxide cores were removed (Fig. 2f), which suggests that the mesoporous silica coating on the flowerlike Fe₂O₃ had integrity and fully covered the surface of the crystalline Fe₂O₃. The mesoporous pores are visible in Fig. 2e and the inset of Fig. 2f, proving that the silica coating is mesoporous.

Fig. 2 SEM (a) and TEM (b) images of the flowerlike Fe₂O₃; inset in (a) is the schematic of the flowerlike Fe₂O₃; (c) and (d) are the SEM and TEM images of the flowerlike Fe₂O₃@meso-SiO₂ composite, respectively; inset in (c) is the schematic of Fe₂O₃@meso-SiO₂ composite; (e) is the enlarged TEM image of the selected area of (d) in red circle; (f) is the TEM image of the outer meso-SiO₂ coating of the flowerlike Fe₂O₃@meso-SiO₂ composite after the removal of the Fe₂O₃ core; inset is the enlarged TEM image of selected area in the red circle.

Fig. 3 The N₂ adsorption–desorption isotherms of the Fe₂O₃@meso-SiO₂ composite; the inset is the NLDFT pore size distribution of the Fe₂O₃@meso-SiO₂ composite.

The N₂ adsorption–desorption isotherms of the Fe₂O₃@mesoporous SiO₂ composite are of type II and the hysteresis is of type H3. The remarkably sharp capillary condensation step between 0.2 and 0.4 P/P₀ indicates that the sample possessed a narrow size distribution (Fig. 3).³⁰ The pore size distribution, calculated by the Non-Local Density Functional Theory (NLDFT) method, showed that the pore size was 1.5 nm (inset of Fig. 3), which agrees well with the mesoporous pores. The mesoporous properties of the Fe₂O₃@meso-SiO₂ composite were attributed to the mesoporous silica coating. Moreover, the Fe₂O₃@meso-SiO₂ composite had a large BET surface area of 482.4 m² g⁻¹, which is much bigger than the bare flowerlike Fe₂O₃ (BET surface area of bare flowerlike Fe₂O₃ is 94.5 m² g⁻¹, whereas the BET surface area of mesoporous silica hollow flower is 792.4 m² g⁻¹). The larger BET surface area would result in enhanced adsorption ability when the Fe₂O₃@meso-SiO₂ composite was used as a catalyst in a solution. Moreover, the sizes of the organic molecules such as methylene blue (MB, 0.7 × 0.16 nm)³¹ are much smaller than the size of the mesoporous pores. The mesoporous pores were big enough for the reagents to diffuse and pass through so that the reagents could arrive and react on the active sites of the flowerlike Fe₂O₃ core without hindrance. We envision that the Fe₂O₃@meso-SiO₂ composite would have significantly different catalytic activity than the bare flowerlike Fe₂O₃ as a Fenton-like catalyst.

Fenton-like reactions were carried out for MB degradation to test the activities of the flowerlike Fe₂O₃ and the flowerlike Fe₂O₃@meso-SiO₂ composites (Fig. 4). No acid or base was used to adjust the pH value of the reaction solution. At −60 min, the catalyst was added to the MB solution, and the mixture was kept in dark for 60 min to establish an adsorption–desorption equilibrium. The Fenton-like reaction was started at the 0 min mark when a definite amount of H₂O₂ was added to the solution. As shown in Fig. 4, the bare flowerlike Fe₂O₃ showed relatively low activity and less than 15% of MB was decolourized after 120 min. The catalytic activity of the flowerlike Fe₂O₃ is much lower than the mesoporous hematite reported in the literaure.¹² The mesoporous shell itself (the inner Fe₂O₃ core was removed by acid etching) also showed a certain ability to decrease the MB concentration during the first 60 min and approximately 30% decrease in concentration was observed. However, the concentration of MB remained almost unchanged after the addition of H₂O₂, indicating that the concentration decrease was due to the adsorption of MB molecules by the mesoporous SiO₂ shell.

Fig. 4 Fenton-like degradation of MB as a function of time using (a) bare flowerlike Fe₂O₃ as the catalyst; (b) pure mesoporous SiO₂ as the catalyst; and (c) the flowerlike Fe₂O₃@mesoporous SiO₂ composite as the catalyst; reaction conditions: 0.5 g L⁻¹ equivalent Fe₂O₃, 50 mg L⁻¹ MB, 9.0 g L⁻¹ H₂O₂, room temperature, the pure mesoporous SiO₂ is the mesoporous SiO₂ shell of the Fe₂O₃@mesoporous SiO₂ composite whose inner Fe₂O₃ core was removed by acid etching, C_i denotes the initial concentration of MB, the catalyst was mixed with the MB solution in the dark in the first 60 min to reach an adsorption–desorption equilibrium and the Fenton-like reaction was started at the 0 min mark when H₂O₂ was added to the solution.

In sharp contrast, the Fe₂O₃@meso-SiO₂ composite was very active in these Fenton-like reactions. The Fe₂O₃@meso-SiO₂ composite showed a similar adsorption ability to the mesoporous silica shell in the first 60 min before the addition of H₂O₂, but the concentration of MB decreased rapidly when the Fenton-like reaction was started by the addition of H₂O₂. The MB solution was fully decolourized in 120 min by the Fe₂O₃@meso-SiO₂ composite catalyst. The extent of degradation was further analyzed by the total organic carbon (TOC) of the reaction solution. TOC analysis showed that the TOC value was reduced by 85%, relative to the original solution catalyzed by the Fe₂O₃@meso-SiO₂ composite. Such a TOC value decrease, as well as the full MB concentration decrease, offered strong evidence for our conclusion that the MB molecules were completely degraded into water and CO₂. Compared with mesoporous hematite in the literature,¹² the Fe₂O₃@meso-SiO₂ composite showed a similar catalytic activity. In addition, the reuse of the Fe₂O₃@meso-SiO₂ composite catalyst was also tested. Although the weak magnetism of the Fe₂O₃@meso-SiO₂ composites made them hardly available to be recovered in magnetic field (Fig. S3†), they can be easily recovered from the reaction mixture by centrifugation and reused in subsequent reaction cycles due to their micro-metered size. The reaction activity of the Fe₂O₃@meso-SiO₂ composite to the degradation of MB was almost unchanged in the next 5 runs compared to the first cycle (Fig. S4†).

The outstanding activity of the catalyst was attributed to the specific hierarchy of the flowerlike Fe₂O₃ core and the mesoporous silica shell of the Fe₂O₃@meso-SiO₂ composite. The generation of ˙OH radicals in the catalytic process was investigated according to the reaction of ˙OH radicals with terephthalic acid, producing 2-hydroxyterephthalic acid. The photoluminescence spectra intensity at 425 nm gradually increases with increasing irradiation time in the presence of different catalysts (Fig. S5†). The ˙OH radical intensity of Fe₂O₃@meso-SiO₂ composite was notably higher than that of the bare flowerlike Fe₂O₃. Therefore, the degradation activity of the Fe₂O₃@meso-SiO₂ composite could be expected to be higher than that of bare flowerlike Fe₂O₃. In addition, the Fenton-like catalytic reaction mainly occurs on the catalyst surface, near a neutral environment.¹² Therefore, the adsorption of reagents onto the catalyst plays an important role in this heterogeneous Fenton-like reaction. In case of the flowerlike Fe₂O₃, the nano-sized building blocks of the flowerlike Fe₂O₃ provide a large surface area for contact and sufficient active sites for the decomposition of H₂O₂ to produce ˙OH radicals in the Fenton-like reaction. The easy escape of the ˙OH radicals and low mass transfer of MB molecules to the surface of the catalyst limited the activity of the bare flowerlike Fe₂O₃. In the Fe₂O₃@meso-SiO₂ composite, the mesoporous shell of the Fe₂O₃@meso-SiO₂ composite can adsorb the MB molecules from the bulk solution and enrich them on the surface of the flowerlike Fe₂O₃ core, whereas ˙OH radicals generated on the surface of the flowerlike Fe₂O₃ core were also trapped by the mesoporous silica shell, resulting in a higher reactant concentration on the surface of the flowerlike Fe₂O₃. The degrading reactions were confined within the mesoporous pores, where the Fe₂O₃ core was more accessible than for the bare flowerlike Fe₂O₃ in the bulk solution. Moreover, contact opportunities between MB molecules and hydroxyl radicals were also increased in the Fe₂O₃@meso-SiO₂ composite compared with the bare flowerlike Fe₂O₃ due to the mesoporous silica coating. Thus, the catalytic activity of the flowerlike Fe₂O₃ was enhanced by the mesoporous silica coating.

Conclusions

To summarize, a flowerlike Fe₂O₃ with hierarchical structure was synthesized through a simple hydrothermal method. The flowerlike Fe₂O₃ was coated with mesoporous silica to form an Fe₂O₃@meso-SiO₂ composite in aqueous media. SEM and TEM images demonstrated that in the Fe₂O₃@meso-SiO₂ composite, mesoporous silica was coated uniformly and completely on the surface of the flowerlike Fe₂O₃ core. When used as a catalyst for the Fenton-like reaction in the degradation of MB, the Fe₂O₃@meso-SiO₂ composite is much more active than the bare flowerlike Fe₂O₃, which suggests that the mesoporous silica coating can enhance the catalytic activity of the hierarchical Fe₂O₃.

Acknowledgements

We are thankful for the financial support from the National Basic Research Program of China (MOST 2013CB933004, 2014CB931802), the National Natural Science Foundation of China (NSFC 51302008, 51502089), the Fundamental Research Funds for the Central Universities (2016MS03) and the Chinese Academy of Sciences.

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Footnote

† Electronic supplementary information (ESI) available: Details of experiment. See DOI: 10.1039/c6ra15092f

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