Facile synthesis of a novel CeO₂/glass bead catalyst with enhanced catalytic oxidation performance

Abstract

Nano-ceria with a cubic lattice was successfully coated onto porous glass bead supports for the first time, especially without the use of NH₃·H₂O. Scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Brunauer–Emmett–Teller methods were employed to characterize the prepared novel catalysts. Accordingly, the role of the glass bead support was discussed in detail. Most importantly, the AO7 degradation rate in the presence of H₂O₂ reached 85% using the prepared catalyst but just 32.9% for pure ceria under the same conditions. This represents an approximately 2.6-fold improvement, which is an excellent enhancement of catalytic performance in a slurry reactor. The apparent reaction constant was calculated as 0.876 L mmol⁻¹ min⁻¹. In addition, the AO7 degradation rate could reach as high as 99% within 50 s even with a high initial AO7 concentration of 150 ppm in a fixed bed reactor. Notably, the supported catalyst also achieves the goal of easy recovery from the reactor liquid and exhibits superior catalytic stability.

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1. Introduction

One of the major threats to water quality is chemical pollution from heavy metals, solvents, dyes, pesticides, etc. in direct or indirect ways.¹ In particular, azo dyes, characterized by the presence of azo bonds (–N=N–), pose a threat to the surrounding ecosystems due to their nonbiodegradability, toxicity and potential carcinogenic nature.^2,3 Advanced oxidation processes (AOPs) are emerging as effective alternative technologies in pre- or post-biological treatment processes and have captured extensive attention for the removal of organic wastewater pollutants.⁴

Notably, ceria (CeO₂), one of the most reactive rare earth metal oxides, has been widely employed in wastewater treatment processes. Owing to its unique fluorite type structure and oxygen vacancy properties, the availability of surface Ce³⁺ at such defect sites presents enhanced catalytic activity.⁵ Moreover, oxygen vacancy defects on the surface of ceria can also be rapidly formed and eliminated resulting in repeated redox cycling between Ce⁴⁺ and Ce³⁺. For example, Heckert et al.⁶ first confirmed the generation of HO˙ via eqn (1) and (2) in the presence of H₂O₂, indicating ceria can be used as a Fenton-like catalyst for the oxidation of organic pollutants.

Ce³⁺ + H₂O₂ → Ce⁴⁺ + HO˙ + OH⁻ (1)

Ce⁴⁺ + H₂O₂ → Ce³⁺ + HO₂˙ + H⁺ (2)

Similarly, Ji and co-workers⁷ also found that 95% of acid orange 7 could be degraded in CeO₂/H₂O₂ systems under visible light irradiation, which indicates excellent catalytic oxidation activity. Furthermore, Fe,⁸ Au,⁹ Pd¹⁰ or SO₄ ^2–11 etc. doped or modified CeO₂ was found to present an enhanced catalytic performance in organic pollutant degradation compared to that of pure CeO₂ in the presence of H₂O₂.

However, there is little literature available concerning ceria immobilization, which may be due to the potentially lower reactivity than pure nanoparticles that is observed after introduction of supports.¹² On the other hand, it should be noted that catalyst immobilization, such as coating nanoparticles over silica, alumina or zeolite, has been widely employed as an effective alternative to reduce the high cost of filtration steps.^12–15 As far as we know, the examination of supported CeO₂ catalysts applied to organic pollutant degradation in the presence of H₂O₂ is rare. Although, as mentioned previously, approaches to improving catalytic performance have been studied intensively.

Therefore, it is of paramount importance and essential to prepare supported CeO₂ catalysts with features that allow easy recovery as well as a comparable or even higher catalytic performance than pure CeO₂ in the presence of H₂O₂ for organic pollutant degradation. Therefore, to address the abovementioned problem, porous glass beads were chosen as the support in this work. Importantly, to the best of our knowledge, it is the first time the preparation of a CeO₂/glass bead catalyst and an evaluation of its catalytic oxidation activity in the presence of H₂O₂ have been reported.

There appears to be three potential benefits to employing porous glass beads as the support: (1) it is much easier to obtain highly dispersed CeO₂ nanoparticles without the addition of NH₃·H₂O (2) the alkalinity resulting from silicates (Na₂SiO₃, MgSiO₃ and CaSiO₃) on the surface of the glass bead supports improves the reactivity of H₂O₂ (3) it is possible to separate the catalyst from the liquid phase efficiently just through natural settling. Furthermore, the catalytic performance reported here was conducted both in a slurry reactor and a fixed bed reactor.

2. Experimental

2.1 Materials and chemicals

Glass beads with diameters of 100 μm, consisting of 59.7 wt% SiO₂, 9.8 wt% MgO, 25.1 wt% Na₂O, and 4.9 wt% CaO, were purchased from Hebei Chiye Corporation. Analytical grade Ce(NO₃)₃·6H₂O, azodye acid orange 7 (AO7), H₂O₂ (28%) and ethanol were purchased from Beijing Chemical Plant and used without further purification. Double distilled water was employed throughout the experiments.

2.2 Catalyst preparation

Glass beads were first pre-processed using a subcritical water treatment method based on our previous work, with a slight modification.^16–18 Typically, 10 g of glass beads and 400 mL of deionized water were put together into a tank reactor. The reactor was then gradually heated to 573 K with a pressure of 8.3 MPa and maintained for 1 h under a stirring rate of 600 rpm. Afterwards, the glass beads were separated by natural settling and washed with deionized water several times. Then, the processed glass beads were dried at 60 °C overnight for further use.

The pure CeO₂ nanoparticles were prepared following a widely employed precipitation method.^3,7 Specifically, 4 mL NH₄OH (28 wt% NH₃) was added into 100 mL Ce(NO₃)₃ solution with a concentration of 0.1 M under vigorous stirring. After 2 h, the resulting purple precipitate was collected by centrifugation and washed 3 times with water and ethanol. Consequently, the precipitate was dried at 80 °C for 12 h before it was calcined at 550 °C for 4 h with a heating rate of 2 °C min⁻¹. To synthesize the supported CeO₂/glass bead catalyst, pretreated glass beads (3 g) and Ce(NO₃)₃ aqueous solution (0.04 mol L⁻¹, 120 mL) were first mixed together. This mixture was then stirred for 2 h at room temperature. After that, the solid phase was separated and washed with water and ethanol, before being dried at a temperature of 80 °C for 12 h. The supported CeO₂/glass bead catalyst was finally obtained after a calcination procedure at 550 °C for 4 h with a heating rate of 2 °C min⁻¹.

2.3 Characterization of the catalyst

The Brunauer–Emmett–Teller (BET) surface areas and pore size distribution of samples were measured at 77 K on a Quantachrome Autosorb-1-C chemisorption–physisorption analyzer. Specific surface area was calculated from the adsorption branches in the relative pressure range of 0.05–0.25. Pore diameter was calculated from the desorption branches using the Barrett–Joyner–Halenda (BJH) method, and pore volumes were estimated from the adsorbed amount at a relative pressure of 0.99. The surface morphology of the prepared catalyst was investigated using scanning electron microscopy (SEM, JEOL JSM 7401F, JEOL Ltd., Japan). The supported CeO₂ nanoparticles on porous glass beads were investigated using a transmission electron microscope (TEM, EOL JSM 2010, JEOL Ltd, Japan). X-ray diffraction (XRD, Model D8 ADVANCE, Bruker) was used to determine the crystal structure of the supported ceria. An inductively coupled plasma atomic emission spectrometer (ICP, IRIS Intrepid II XSP from ThermoFisher Corp., America) was employed to measure the ceria loading amounts of the prepared catalyst. The surface chemical composition of the samples was detected by X-ray photoelectron spectroscopy (XPSPHI Quantera SXM, ULVAC-PHI, Japan) and binding energy was calibrated with C 1s at 284.8 eV. CO₂-TPD experiments were performed on Chemisorb-2720 with a thermal conductivity detector made by Micromeritics Company.

2.4 Measurements of catalytic performance

The catalytic performance was evaluated by investigating the decomposition of AO7 aqueous solution in the presence of H₂O₂. For the slurry rector mode, typically, 0.2 g of prepared catalyst was added into the AO7 solution with a concentration of 60 ppm, followed by 1 h of vigorous stirring in the dark for the establishment of an adsorption/desorption equilibrium. Hydrogen peroxide was then added into the above mixture to initiate the catalytic oxidation reaction. For the fixed bed reaction mode, the AO7 solution mixed with H₂O₂ flowed through a fixed tubular reactor with a length of 10 cm and an inner diameter of 0.84 cm using an advection pump. Net wires on both sides of the reactor were presented to prevent the catalyst from being washed away. After the reaction, the AO7 solution sample was withdrawn immediately and analyzed using a UV-vis spectrophotometer at 480 nm to determine the AO7 concentration. Also, the degradation rate in this work was defined as the ratio of AO7 concentration after reaction to initial AO7 concentration.

3. Results and discussion

3.1 Characteristics of the supported CeO₂/glass bead catalyst

Fig. 1(a) and (b) show the obvious color change of porous glass bead supports from white to yellow before and after ceria loading, indicating that the ceria was successfully coated on the glass bead support. Scanning electron microscopy (SEM) was used to investigate the surface morphology of the prepared catalyst. Fig. 1(c) and (e) show that glass bead supports keep their spherical shapes after ceria loading. Furthermore, as presented in Fig. 1(d), a layer of uniform flakes was clearly observed on the surface of pure glass bead support.¹⁸ Notably, the micro-pores formed by these flakes were blocked after ceria was loaded on the glass bead supports as shown in Fig. 1(f). HRTEM images in Fig. 2(a) show that the particle size of CeO₂ is about 10 nm with clear lattice fringes. The corresponding SAED pattern shown in Fig. 2(b) further confirms that as-prepared ceria nanoparticles has fluorite structure with high crystallinity.

Fig. 1 Changes in color of (a) glass bead support (b) glass bead after ceria loading (c and d) SEM images of glass bead support before ceria loading (e and f) SEM images of CeO₂/glass bead catalyst.

Fig. 2 (a) HRTEM images of supported CeO₂ nanoparticles (b) SEAD pattern of the as-prepared sample.

In addition, the specific surface area, pore volume and pore diameter of the three samples was measured through the N₂ isothermal adsorption–desorption curves and the corresponding results are listed in Table 1. The specific surface area and pore volume of the porous glass beads decreased from 186.61 m² g⁻¹ and 0.31 cm³ g⁻¹ to 94.49 m² g⁻¹ and 0.24 cm³ g⁻¹, respectively, after ceria component loading, which is consistent with what was observed in the SEM images. Furthermore, inductively coupled plasma atomic emission spectrometry (ICP) was employed to determine the ceria content in the prepared catalyst, revealing an approximate Ce loading amount of 1.97 wt%.

Table 1 BET results of the three samples

	Specific surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore diameter (nm)
Glass bead support	186.61	0.31	3.84
CeO2	54.58	0.14	8.88
CeO2/glass bead	94.49	0.24	3.77

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It is well known that ceria nanoparticles are generally prepared through adding alkaline substances (normally NH₃·H₂O) into a precursor solution containing Ce³⁺ followed by a calcination process.^3,7 Interestingly, supported CeO₂/glass bead catalyst in this work are synthesized just through mixing the glass beads with Ce(NO₃)₃ aqueous solution directly before a calcination procedure. Consequently, glass bead supports are assumed to exhibit alkalinity, which avoids the addition of NH₃·H₂O. CO₂-TPD experiments were accordingly conducted to detect the surface alkalinity of glass bead support before and after ceria loading. As shown in Fig. 3, a noteworthy CO₂ desorption peak at 100–200 °C was observed for both kinds of samples. The peak was attributed to medium basic sites¹⁹ on the surface of glass bead supports, thus confirming the abovementioned assumption. Meanwhile, it can be seen that the area of CO₂ desorption peaks for glass beads are much larger, i.e., a higher density of basic sites, than that of CeO₂/glass bead, which means that CeO₂ nanoparticles formed at the basic sites of the support and resulted in the reduction of surficial basic sites. To conclude, ceria nanoparticles with cubic lattice were successfully loaded onto porous glass bead supports, importantly, without the use of alkaline agents, such as NH₃·H₂O.

Fig. 3 CO₂-TPD analysis of glass bead support and CeO₂/glass bead catalyst.

3.2 Catalytic reactivity examination in a slurry reactor

Effect of H₂O₂ addition amounts on AO7 degradation was firstly tested under the condition of different H₂O₂ concentration varying from 0 to 80 mmol L⁻¹. The result (Fig. 4(a)) shows that the AO7 degradation rate increases with an increasing H₂O₂ concentration, but with a final steady value of 95.6%. This is mainly due to the easier formation of highly reactive HO˙ radical under higher H₂O₂ concentrations. Additionally, it should be noted that excess H₂O₂ can inhibit the catalytic reaction through the overcomplexation of H₂O₂ and surface Ce³⁺ ions.^7,9 Notably, the concentration of AO7 decreased by approximately 17.5% even in the absence of hydrogen peroxide, indicating that the equilibrium absorption capacity of AO7 on the prepared catalyst is 1.4 mg g⁻¹.

Fig. 4 Effect of (a) H₂O₂ concentration and [AO7 concentration 60 ppm, AO7 volume 25 mL, catalyst dosages 0.2 g, 25 °C] (b) catalyst dosages on AO7 degradation in CeO₂/glass bead-H₂O₂ system [AO7 concentration 60 ppm, AO7 volume 25 mL, H₂O₂ concentration 80 mmol L⁻¹, 25 °C].

Fig. 4(b) shows the effect of catalyst dosages on AO7 degradation in the presence of H₂O₂. The concentration of AO7 and H₂O₂ was fixed at 60 ppm and 80 mmol L⁻¹, respectively, while added catalyst amounts varied from zero to 0.5 g. The result shows that the stable AO7 dye cannot be mineralized in the presence of hydrogen peroxide alone in our experiments. Furthermore, the AO7 degradation rate increases overwhelmingly following the addition of the prepared catalyst, improving by 34.4%, 56.8%, 72.2% and 84.6% as the catalyst dosage respectively changed from 0.1 g, 0.2 g and 0.35 g to 0.5 g.

To gain deep insight into the catalytic reactivity of the prepared catalyst, two groups of AO7 degradation experiments were conducted with the addition of supported ceria and pure ceria catalyst under a series of reaction times. The active ceria amounts were all kept identical with a fixed H₂O₂ concentration of 80 mmol L⁻¹ and an AO7 concentration of 60 ppm. The corresponding results are shown in Fig. 5(a). Notably, at the initial reaction time, the AO7 concentration in both of the groups decreased, resulting from the absorption equilibrium of AO7 onto the surface of the catalysts. In addition, with increasing reaction time, AO7 degradation rates in the two groups both increased accordingly. However, it should be noted that the AO7 degradation rate could reach as high as 85% at 0.5 h for the supported-CeO₂/H₂O₂ system, but just 32.9% for the pure CeO₂/H₂O₂ system under the same conditions (AO7 degradation rate was improved approximately 2.6 fold). Therefore, the catalytic oxidation performance was intensively improved with the introduction of the novel CeO₂/glass bead catalyst.

Fig. 5 (a) Comparison of AO7 degradation in the CeO₂–H₂O₂ system and CeO₂/glass bead-H₂O₂ system under a series of reaction times (b) linear fitting of concentration versus reaction time [AO7 concentration 60 ppm, AO7 volume 25 mL, H₂O₂ concentration 80 mmol L⁻¹, 25 °C].

Furthermore, the AO7 degradation rate constant with addition of pure ceria and the prepared novel catalyst was obtained. Typically, the reaction rate can be expressed as follows:

where k is the intrinsic reaction rate coefficient; m and n are the reaction orders towards hydrogen peroxide and AO7, respectively.

Considering that H₂O₂ has an extremely strong chemical interaction with CeO₂, in addition to the excessive H₂O₂ amounts, change in the amount of surface peroxide species here is limited.²⁰ Approximately,

where k_a is the apparent reaction rate constant.

Therefore, the k_a was determined to be 0.0025 min⁻¹ for the CeO₂–H₂O₂ system with an n value of 1, as shown in Fig. 5(b). A similar phenomenon has also been reported previously,²⁰ where AO7 concentration decrement can be well-fitted in apparent first order kinetics. Interestingly, the n value was calculated as 2 for the CeO₂/glass bead-H₂O₂ system and the corresponding k_a was 0.876 L mmol⁻¹ min⁻¹. Therefore, the introduction of the glass bead support plays a key role in enhanced catalytic oxidation performance.

3.3 Catalytic reactivity examination in a fixed-bed reactor mode

It is well known that supported-type catalysts are generally employed in the fixed bed reactor considering its practical application. Thus, AO7 degradation using the prepared novel catalyst was also conducted in a fixed bed reactor. Particularly, different residence times were obtained by changing the liquid phase flow rate from 1 mL min⁻¹ to 9 mL min⁻¹. The effect of H₂O₂ addition amounts on AO7 degradation was first conducted. As shown in Fig. 6(a), the AO7 degradation rate improves with an increasing residence time, which is likely due to the more adequate contact among catalysts and reactants as well as the longer reaction time. In addition, the AO7 degradation rate improves with an increasing H₂O₂ concentration, which is consistent with what was observed in the slurry reactor and could be assigned to the easier formation of HO˙ radical with high H₂O₂ concentration.

Fig. 6 AO7 degradation in a fixed bed reactor under (a) different H₂O₂ concentration [catalyst dosages 0.35 g, AO7 concentration 50 ppm, 25 °C] and (b) different AO7 initial concentration [catalyst dosages 0.35 g, H₂O₂ concentration 160 mM, 50 °C].

Fig. 6(b) shows the effect of different AO7 initial concentration on the catalytic performance with a fixed H₂O₂ concentration of 160 mM. Interestingly, the higher AO7 concentration seems to incur a lower AO7 degradation rate at the same reaction time. Other work^7,20 revealed that there is competitive adsorption between AO7 and H₂O₂ on the surface of CeO₂, while surface peroxide species resulting from H₂O₂ play a key role in catalytic performance. Therefore, it is understandable that the AO7 degradation rate decreases with a higher AO7 concentration under a fixed concentration of H₂O₂. In addition, it should be noted that the AO7 degradation rate could reach as high as 99% within 50 s even with an initial AO7 concentration of 150 ppm.

3.4 Mechanism of enhanced catalytic performance

Typically, the Fenton-like activity of the CeO₂/system is closely related to the interfacial peroxide-like species via the interaction of H₂O₂ with Ce³⁺. Previous studies reported that the free Ce³⁺ ions are believed to activate the reactivity of adjacent surface peroxides species, and thus significantly accelerate the catalytic oxidation process in a CeO₂/H₂O₂ system.^7,9,20,21 Therefore, to reveal the role of the glass bead support and enhanced catalytic performance, XPS spectra were employed to trace the surface chemical states of the as-prepared catalyst as well as pure ceria, as shown in Fig. 7.

Fig. 7 Fine XPS spectra of Ce 3d for (a) pure CeO₂ and (b) CeO₂/glass bead catalyst.

Importantly, the Ce_3d fine XPS spectra showed that the introduction of the glass bead support results in some changes in the chemical state of cerium ions. Furthermore, the deconvolution of the Ce 3d fine XPS spectrum could be labeled as two pairs of doublets (ν₀/u₀ and ν′/u′) and three pairs of doublets (ν/u, ν′′/u′′ and ν′′′/u′′′), in which νⁿ and uⁿ refer to the 3d_5/2 and 3d_3/2 spin–orbit component of cerium ions, respectively.^22–25 Accordingly, the peaks at ν₀, ν′, u₀ and u′ (880.4, 885.5, 898.8, 903.7 ± 0.7 eV) represent the presence of Ce³⁺, while the characteristic peaks of Ce⁴⁺ are located at ν, ν′′, ν′′′, u, u′′ and u′′′ (882.7, 888.96, 898.2, 901.3, 907, 916.7 ± 0.7 eV).^26–28 Thus, the relative Ce³⁺ concentration in the catalysts can be determined by calculating the relative integrated area ratios based on eqn (5):

where A_i is the integration area of peak “i”.

The XPS spectra of Ce_3d with its corresponding deconvoluted peaks for pure CeO₂ and the prepared CeO₂/glass bead catalyst are presented in Fig. 7(a) and (b). The result shows that the concentration of Ce³⁺ in the novel CeO₂/glass bead catalyst is 46.8%, higher than that of bare CeO₂ (the value is 33.5%), which seems to account for the improved catalytic oxidation activity with the introduction of the glass bead support. Similar phenomena have been reported in the literature,⁸ where Fe³⁺ doped into bare CeO₂ results in higher surface Ce³⁺ concentration, confirmed by an XPS spectrum, as well as enhanced catalytic performance.

On the other hand, hydrogen peroxide seems to decompose into active HO˙ radicals much easier in an alkaline environment. It is known that HO˙ radicals play a key role in the catalytic oxidation reaction for the CeO₂/H₂O₂ system. Interestingly, the glass bead supports contain Na₂SiO₃, CaSiO₃ and MgSiO₃ (ref. 17 and 29) resulting in its alkaline nature. Therefore, the glass bead support itself was assumed to be beneficial to the AO7 degradation in the presence of hydrogen peroxide alone.

Accordingly, a series of experiments were conducted to validate the above hypothesis. Specifically, AO7 at a concentration of 60 ppm was added with the different ingredients listed in Fig. 8 and the mixture was allowed to react for 2 h. The result shows that the AO7 is extremely stable and coexisted with hydrogen peroxide alone or glass bead supports. However, the AO7 was degraded by approximately 29% in the presence of both glass bead supports and hydrogen peroxide, which confirms the above proposed assumption. Most importantly, AO7 was degraded by approximately 96% in the CeO₂/glass beads-H₂O₂ system but just 35% in the CeO₂–H₂O₂ system. Undoubtedly, the introduction of CeO₂ onto the glass bead support substantially improved the performance of AO7 degradation in the presence of hydrogen peroxide.

Fig. 8 Comparison of AO7 degradation under different conditions [AO7 concentration 60 ppm, AO7 volume 25 mL, H₂O₂ concentration 80 mmol L⁻¹, reaction time 2 h, 25 °C].

3.5 Cyclic performance of the supported CeO₂/glass bead catalyst

It is well known that the recycling of a catalyst plays a key role in every catalytic reaction and consideration of its practical application cannot be neglected, therefore, it was examined in detail in this work. The experiment was conducted for six successive cycles each lasting 2 h. Specifically, each run was performed with a fixed AO7 concentration of 60 ppm, an H₂O₂ concentration of 80 mmol L⁻¹ and 0.5 g of catalyst. Additionally, it should be noted that the recovery of the catalyst was through natural settling, followed by a deionized water and ethanol washing procedure. Compared to the fresh catalyst, no remarkable decrease in the AO7 degradation rate was observed even after six cycles, as shown in Fig. 9, indicating the favorable stability of the prepared CeO₂/glass bead catalyst.

Fig. 9 Cyclic performance of AO7 degradation in the CeO₂/glass bead-H₂O₂ system [AO7 concentration 60 ppm, AO7 volume 25 mL, H₂O₂ concentration 80 mmol L⁻¹, catalyst dosages 0.5 g, 25 °C].

4. Conclusions

In this work, a novel CeO₂/glass bead catalyst was successfully prepared without the addition of NH₃·H₂O and employed for the degradation of acid orange 7 in the presence of H₂O₂ for the first time. Accordingly, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), inductively coupled plasma atomic emission spectrometry (ICP) and Brunauer–Emmett–Teller (BET) methods were also employed for the catalyst characterization. In addition, degradation of acid orange 7 was used as the probe reaction and the effect of different H₂O₂ concentrations, catalyst dosages and reaction times were investigated in a slurry reactor. The AO7 degradation was also performed in a fixed bed reactor considering its practical application and presented a superior catalytic performance.

Furthermore, a series of catalytic runs, as well as X-ray photoelectron spectroscopy (XPS) characterization, were conducted to reveal the role of the glass bead support. A mechanism for the enhanced catalytic oxidation performance was thus proposed. Most importantly, the introduction of the glass bead support not only improved the catalytic oxidation performance in the CeO₂/H₂O₂ system but also achieved the goal of ceria recovery with great ease. Last but not least, the prepared catalyst presented excellent recycling stability, suggesting the feasibility of practical application.

Acknowledgements

We gratefully acknowledge the support of the National Basic Research Program of China (2013CB733600) and the National Natural Science Foundation of China (21276140, 20976069 and 21036002).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21657a

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