Synthesis of magnetic CuFe2O4/Fe2O3 core-shell materials and their application in photo-Fenton-like process with oxalic acid as a radical-producing source

ABSTRACT In this work, we proposed to synthesize CuFe2O4/Fe2O3 core-shell materials with different Fe2O3 contents in order to create new and efficient photo-Fenton-like catalysts for the degradation of methylene blue with oxalic acid as a radical-producing source. The catalysts were prepared through two stages: first, CuFe2O4 was prepared by a hydroxide coprecipitation – annealing method and then, Fe2O3 was immobilized on CuFe2O4 surface by a simple impregnation – annealing procedure. According to the experimental results, our CuFe2O4/Fe2O3 core-shell materials exhibit high photo-Fenton-like catalytic activity for the degradation of methylene blue under both UVA light and visible light, as well as good ferromagnetic properties, which allows them to be easily separated from the solution by a magnet. Among them, the catalyst prepared with the molar CuFe2O4/Fe2O3 ratio of 1:2 showed the best catalytic performance with the rate constant of 2.103 h–1 under UVA light and 0.542 h–1 under visible light, which were 2 times higher than CuFe2O4 sample. The enhanced catalytic activities of our core-shell materials can be attributed to the high content of surface Fe3+ species, high specific surface area and the presence of rod-like Fe2O3 particles on their surface.


Introduction
The existence of various dye molecules in textile wastewater has been widely considered as one of major environmental problems that our world is facing today. Due to their toxicity, organic dyes have many negative effects not only on human health but also on aquatic life [Berradi et al. 2019, Hassan et al. 2018, Ito et al. 2016, Lellis et al. 2019. Therefore, treatment of textile wastewater containing organic dyes has become an urgent need for the environment protection. Unfortunately, the decomposition of organic dyes is usually ineffective since these organic molecules are remarkably resistant to conventional biological methods such as activated sludge or anaerobic digestion [Katheresan et al. 2018]. Over the past decades, homogeneous Fenton processes based on the reactions of Fe 3+ /Fe 2+ ions and H 2 O 2 were commonly known as a potentially better oxidation approach owing to the generation of highly reactive oxygen species like hydroxyl radicals which can completely mineralize most of organic species [Fenton 1894;Gligorovski et al. 2015, Neamtu et al. 2003, Wang and Xu 2012. More specially, it was reported that the production of hydroxyl radicals was greatly improved under UV light [Zepp et al. 1992]. However, the homogeneous Fenton and photo-Fenton technologies still present some shortcomings for practical applications. Firstly, Fe 3 + /Fe 2+ ions as homogeneous catalysts are completely dissolved in reaction solution, leading to enormous challenges in catalyst recovery after wastewater treatment. Secondly, for the recovery, these homogeneous catalysts usually require a post-neutralization process, which possibly causes the increase in the treatment cost as well as the formation of ferric sludge as a secondary pollution source [Catrinescu et al. 2003, Hanna et al. 2008.
In order to overcome these obstacles, several magnetic photo-Fenton-like catalysts were developed in the literature [Heidari et al. 2019, Liu et al. 2012, Sharma et al. 2015, Sharma and Singhal 2018 (in general, the photo-Fenton-like reactions are the advanced oxidation processes using oxidants other than H 2 O 2 and transition metals as catalysts other than iron under illumination [Rodríguez-Narváez et al. 2019]). These heterogeneous magnetic catalysts relies on the ferrite materials which belong to the spinel structure with the common formula M 2+ Fe 3+ 2 O 4 (M = Fe, Mn, Cu, Zn, Ni). Owing to the tunability of their structure and composition, they do not only display promising photo-Fenton catalytic activities but also exhibit highly ferromagnetic properties which help them to be easily separated from the solution after the treatment [Sharma et al. 2015].
Nevertheless, the activity of ferrite catalysts is still limited and needs to be further improved for the practical applications. Recently, Guo et al. proved that the addition of tartaric acid into the H 2 O 2 -ferrite-photo system could enhance the decolorization of methylene blue from 52% to 92.1% within 80 minutes [Guo et al. 2019]. Unfortunately, it was widely recognized that H 2 O 2 is hard to be stocked for a long period because this compound is unstable and easily decomposes to form oxygen gas and water during the storage.
According to our previous reports, the replacement of H 2 O 2 by oxalic acid can extend the storage time and notably ameliorate the photo-Fenton performance of these catalysts [Dinh et al. 2017, Ngo TPH andLe TK 2018]. In fact, the ferric species on the surface of ferrite catalysts and oxalic acid dissolved in solution seem to be able to create ferrioxalate complexes which can enhance the photoabsorption of UV-visible light to produce more hydroxyl radicals and thus improve the photo-Fenton efficiency [Liu et al. 2012]. Beside, our previous works also proved that the amounts of different ions on the surface of ferrite catalysts greatly affect their photo-Fenton performance [Ngo TPH and Le TK 2018]. It was observed that the photo-Fenton catalytic activity tends to rise when the surface Fe content of our catalysts gradually increases [Ngo TPH and Le TK 2018]. With that in mind, this work aims to prepare new magnetic photo-Fenton catalysts based on CuFe 2 O 4 /Fe 2 O 3 core-shell materials with different Fe 2 O 3 contents on their surface in order to enhance the photo-Fenton catalytic activity. Actually, before our study, the combination of Fe 2 O 3 and CuFe 2 O 4 was carried out by Silva et al. who used the modified Pechini method to form new heterogeneous α-Fe 2 O 3 /CuFe 2 O 4 catalysts [Silva et al 2020]. These new mixed oxides exhibited both magnetic behavior and high photo-Fenton catalytic activity for the degradation of methylene blue under visible light. However, since the modified Pechini method is based on the polyesterification between citrate salts and ethylene glycol followed by a pyrolysis at high temperatures, this technique is not only inappropriate to synthesize the core-shell materials but also complicated, requiring a careful control of polymerization and a long reaction time (24 hours). Therefore, in this study, we proposed to apply a facile impregnationannealing method to synthesize our CuFe 2 O 4 /Fe 2 O 3 core-shell materials with enhanced catalytic activities. Moreover, oxalic acid was used as a stable and effective radical-producing source instead of H 2 O 2 to further improve the photo-Fenton performance. The influences of Fe 2 O 3 contents on the crystal structure, morphology, surface composition and magnetic properties of our catalysts were also discussed in details.

Sample preparation
In our work, all chemicals were commercially available and used without further purification. The preparation of CuFe 2 O 4 /Fe 2 O 3 core-shell materials with different Fe 2 O 3 contents was effectuated through two stages. In the first stage, CuFe 2 O 4 nanoparticles were synthesized by a simple hydroxide coprecipitation -annealing method which used NaOH as the precipitator. Briefly, Cu(NO 3 ) 2 .3H 2 O and Fe(NO 3 ) 3 .9H 2 O ((>98%, purchased from Sigma Aldrich) in a molar ratio of 1:2 were dissolved in 200 mL distilled water to form a solution containing 0.10 mol.L -1 Cu 2+ and 0.20 mol.L -1 Fe 3+ . Next, 400 mL solution of 0.40 mol.L -1 NaOH was slowly dropped into the above solution under constant stirring to obtain a brown co-precipitate. This coprecipitate was washed with distilled water, dried at 150°C during 2 hours and then annealed in an electric furnace at 800°C for 2 hours to produce magnetic CuFe 2 O 4 nanoparticles.
In the second stage, Fe 2 O 3 was immobilized on the surface of CuFe 2 O 4 nanoparticles via a facile impregnation -annealing process. Firstly, 1.20 g synthesized CuFe 2 O 4 powder was dispersed in 400 mL solution of NaOH (0.225 mol.L -1 ). The suspension was constantly stirred by a mechanic agitator. In the other hand, a series of aqueous Fe(NO 3 ) 3 solutions were prepared with different concentrations (0.05, 0.10, 0.15 mol.L -1 ). These concentrations were calculated following the desired molar CuFe 2 O 4 /Fe 2 O 3 ratios (1:1, 1:2, 1:3, respectively). Then, the aqueous Fe 3+ solutions were quickly poured into the suspension containing NaOH and CuFe 2 O 4 to form a slurry solution. This slurry solution was regularly stirred for 30 minutes. After that, the magnetic powders were separated from the solution by a magnet, dried at 150°C for 1 hour and annealed at 500°C for 2 hours. Finally, the products were washed with distilled water, collected by a magnet and dried again at 150°C for 1 hour. All the samples were named as CuFe 2 O 4 /Fe 2 O 3 -X (X = 0, 1, 2 and 3 corresponding to the molar Fe 2 O 3 /CuFe 2 O 4 ratios). Besides, Fe 2 O 3 nanoparticles (without magnetic CuFe 2 O 4 cores) were also prepared from aqueous Fe(NO 3 ) 3 and NaOH solutions in the same conditions (annealing at 500°C for 2 hours) in order to compare their photo-Fenton activity with that of our CuFe 2 O 4 /Fe 2 O 3 samples.

Characterization
The structural and phase analyses of CuFe 2 O 4 and CuFe 2 O 4 /Fe 2 O 3 samples were performed by powder X-ray diffraction (XRD) using a BRUKER-Binary V3 X-ray diffractometer with monochromatic Cu Kα source (λ = 1.5406 Å) operated at 40 kV and 40 mA. The phase identification and the Rietveld refinement were carried out using Joint Committee on Powder Diffraction Standards database (JCPDS cards) and Fullprof 2009 software, respectively. The surface morphology of our samples were characterized via field emission scanning electron microscopy (FE-SEM) images taken on a HITACHI SU8000 with an accelerating voltage of 5 kV. The specific surface area (S BET ) of samples was measured through nitrogen adsorptiondesorption isotherms recorded at 77 K using a NOVA 1000e analyzer (Quantachrome Instruments).
The Fourier transform infrared studies (FT-IR) for all our samples were carried out in KBr matrix using Bruker VERTEX 70 spectrometer. The FTIR spectra were recorded at the wavenumber resolution of 4 cm -1 and in the wavenumber range of 4000-400 cm −1 . The surface atomic composition of our samples was also investigated by low voltage energy-dispersive X-ray spectroscopy (EDX) on a HITACHI SU8000 instrument operated at 5 kV (corresponding to the penetration depth of 50 nm [Takano 2011]).
The magnetic properties of CuFe 2 O 4 and CuFe 2 O 4 /Fe 2 O 3 -2 samples were measured at room temperature using a vibrating sample magnetometer PPMS6000 (Quantum Design) in the magnetic field range varying from −11 kOe to 11 kOe. The saturation magnetization (M S ), the remanent magnetization (M R ) and the coercivity (H c ) of each sample were determined from the obtained corresponding hysteresis loops.

Catalytic tests
The photo-Fenton-like catalytic activities of CuFe 2 O 4 , Fe 2 O 3 and CuFe 2 O 4 /Fe 2 O 3 samples with H 2 C 2 O 4 as a radical-producing source were evaluated through the degradation of methylene blue (MB, purchased from Merck) under UVA light and visible light illumination. The typical experiments were conducted in a batch-type reactor based on a glass beaker containing a solution of MB (2 × 10 −5 mol.L −1 ) and H 2 C 2 O 4 (10 −3 mol.L −1 ). Firstly, 0.125 g of catalytic powder was dispersed into this solution, which was then constantly stirred by a mechanic agitator in the dark for 60 minutes until the MB adsorption-desorption equilibrium was established. The solution temperature was maintained at about 30°C by a circulation system of water. Next, for photo-Fenton-like reactions, the suspension was irradiated by a visible light (VIS) lamp (9 W Osram Dulux S with the visible-light intensity of 12.5 W.m -2 ) or an ultraviolet A (UVA) light lamp (9 W Radium 78 with the UVA-light intensity of 33.0 W.m -2 ) fixed 10 cm above the solution surface. The light intensity of both lamps was measured by an Ocean Optics USB4000 spectrometer. It should be noted that MB has the maximum absorption at 664 nm whereas the light spectra of both our UVA and visible lamps (Figure 1, measured by a StellarNet spectrometer USB4C00211) does not show any peak around this wavelength. Furthermore, the capacity of our lamps is only 9 W, much lower than that of other works [Liu et al. 2012, Sharma et al. 2018, which allows us to avoid the selfsensitization of MB molecules. At given time intervals, aliquots (5 mL) of solution were collected, followed by the separation of catalytic powder from the solution by a magnet. Finally, the concentrations of remaining dye were analyzed using a Helios Omega UV -VIS spectrophotometer (Thermo Fisher Scientific, USA) at 664 nm. The total organic carbon (TOC) of the solution was also determined by Shimadzu TOC-VCPH analyzed. The calibration curve was prepared using potassium hydrogen phthalate (99.99%, purchased from Merck).

Crystal structure and phase composition
XRD characterization was used to determine the crystal structure of our magnetic CuFe 2 O 4 powder and CuFe 2 O 4 /Fe 2 O 3 core-shell materials ( Figure 2). From their patterns, the quantitative analysis of phase composition was carried out by using Fullprof 2009 program and the results are given in Table 1

Morphology and surface specific area
The FE-SEM images of CuFe 2 O 4 and CuFe 2 O 4 /Fe 2 O 3 core-shell samples are represented in Figure 3. It can be seen that the CuFe 2 O 4 sample is composed of agglomerated cubic particles with their size range between 80 and 200 nm ( Figure 3a). core during the synthesis process. Interestingly, the FE-SEM observation (with magnification of 100 K) also showed the evolution of particle size when the content of Fe 2 O 3 increased. For CuFe 2 O 4 /Fe 2 O 3 -1 sample, the rod-like particles are about 100 nm in length and 40 nm in diameter (Figure 3b). This length tends to increase to 150 nm whereas the diameter tends to decrease to 25 nm for the core-shell material prepared with the molar CuFe 2 O 4 /Fe 2 O 3 ratio of 1:2 ( Figure 3c). However, when the molar CuFe 2 O 4 /Fe 2 O 3 ratio was up to 1:3 (Figure 3d), the rod-like particles were strongly shortened (60-70 nm in length). More specially, in this sample, some rod-like particles seems to be transformed to spherical particles with the size of about 60 nm. This result suggests that loading a very large quantity of Fe 3+ ions on the surface of CuFe 2 O 4 may reduce the available surface area for the development of Fe 2 O 3 seeds, as a consequence, hindering the growth of rod-like Fe 2 O 3 nanoparticles.
Besides, we also noticed the difference in surface texture between CuFe 2 O 4 and CuFe 2 O 4 /Fe 2 O 3 -2 samples via FE-SEM images with higher magnification (×200 K). Owing to the presence of randomly distributed rod-like particles, the surface of CuFe 2 O 4 /Fe 2 O 3 -2 sample (Figure 3f) becomes roughness with more porosity than CuFe 2 O 4 surface (Figure 3e), likely resulting in higher surface area for core-shell materials. In order to better investigate the effect of loading Fe 2 O 3 on the surface morphology of CuFe 2 O 4 , the nitrogen adsorption-desorption isotherms analysis was carried out. For CuFe 2 O 4 /Fe 2 O 3 -2 sample, the BET specific surface area was found to be 11.498 m 2 .g -1 , which is about ten  times higher than that of CuFe 2 O 4 sample (only 1.159 m 2 .g -1 ). The enhanced surface area was also observed for ZnO/Fe 2 O 4 hollow nanospheres [Li et al. 2014] and MgFe 2 O 4 @SiO 2 core shell nanocomposites [Tiwari and Kaur 2020]. These FE-SEM and BET results reinforce the fact that Fe 2 O 3 nanoparticles were successfully bound to the surface of CuFe 2 O 4 , making the material surface rougher, thereby increasing the specific surface area.

Magnetic properties
The magnetic hysteresis curves of our samples are depicted in Figure 5a. From these curves, the magnetic parameters such as saturation magnetization, remanent magnetization and coercivity are determined and shown in  [Bhowmik and Saravanan 2010] on the surface of CuFe 2 O 4 . However, the magnetic properties of our core-shell materials are still good enough to make them easily be separated from the solution by a magnet (Figure 5b).

Photo-Fenton catalytic activity
The photo-Fenton catalytic performance of CuFe 2 O 4 , Fe 2 O 3 and CuFe 2 O 4 /Fe 2 O 3 core-shell materials was evaluated through the MB degradation under UVA light and visible light (Figures 6a and 6b, respectively). The apparent rate constant (k) for each sample is calculated using the pseudo-first-order Langmuir-Hinshelwood kinetic model (Figures 6c and 6d However, when the Fe 2 O 3 content was further enhanced, the rate of MB degradation tended to decrease. These results prove that the photo-Fentonlike catalytic activity of our samples is able to be controlled by the CuFe 2 O 4 /Fe 2 O 3 ratios used in catalyst preparation. Moreover, the UV-light-induced catalytic activity is always higher than the visible-light-induced catalytic activity for all catalysts, indicating that the light energy is also a factor affecting their catalytic performance. In order to verify the stability and the reproducibility of our catalysts, the leaching test and the reuse tests for the CuFe 2 O 4 /Fe 2 O 3 -2 sample were carried out as follows: after the first photo-Fenton catalytic run with our CuFe 2 O 4 /Fe 2 O 3 -2 sample and oxalic acid under UVA light or visible light, the catalyst was removed from the solution by a magnet, washed with distilled water and dried at 150°C for 1 hour. Then, the catalyst was reused for four next consecutive runs in the same conditions. Besides, the iron concentration in the solution of the first run was evaluated by atomic absorption spectrometry (AAS) using Shimadzu AA-6300 spectrometer. Within 0.5 hour under UVA light or 3 hours under visible light, the catalytic performance of the CuFe 2 O 4 /Fe 2 O 3 -2 sample was only slightly reduced after four-time reuses (Figure 7a). The FE-SEM image also displays that rod-like and spherical Fe 2 O 3 nanoparticles still cover nearly the entire surface of this sample after 5 consecutive catalytic tests (Figure 7b), indicating the reproducibility and the potential of this catalyst for practical applications. Furthermore, the AAS result shows a very low concentration of leached Fe 3+ ions (1.56 mg.L -1 ), suggesting that our heterogeneous catalyst is stable in the experimental conditions and the dissolution of iron species by oxalic acid can be neglected.   Finally, Figure 8 compares the TOC evolution versus irradiation time of CuFe 2 O 4 and our coreshell CuFe 2 O 4 /Fe 2 O 3 -2 sample. It was observed that CuFe 2 O 4 /Fe 2 O 3 -2 sample always showed the better TOC removal than CuFe 2 O 4 under UVA light and visible light. Nevertheless, for both samples, the TOC removal is slower than the discoloration of MB solution, which indicates that most MB molecules were effective broken by the highly reactive oxygen species but small organic intermediates produced from the MB degradation are difficult to be removed from the solution.

Discussion
As shown in Figure 6 and Table 3, Fe 2 O 3 nanoparticles (without magnetic CuFe 2 O 4 cores) exhibited great photo-Fenton catalytic activities for MB degradation (k = 3.285 h -1 under UVA light and k = 0.655 h -1 under visible light). However, since these nanoparticles are antiferromagnetic and well dispersed in the solution, they are difficult to be recovered and reused. In contrast, although CuFe 2 O 4 and other ferrite materials can be easily recovered owing to their ferromagnetic properties, their catalytic performance was not high enough for practical applications due to the limited iron species on their surface. Therefore, the combination of Fe 2 O 3 and CuFe 2 O 4 can be a promising solution for the improvement of magnetic photo-Fenton catalysts. On the other hand, it should be reminded that our catalysts did not display photocatalytic activities in the experimental conditions (using a 9 W Osram Dulux S lamp as visible light source and a 9 W Radium 78 lamp as UVA light source) although in some studies, the authors reported the photocatalytic activity of CuFe 2 O 4 materials. For these studies, the authors usually used a 500 W xenon lamp [Zhu et al. 2013] or a 150 W xenon arc lamp [Ismael et al. 2020]. Their   capacity is much higher than that of our lamps (only 9 W). There are also some works on CuFe 2 O 4 photocatalysts using a 8 W lamp, but this lamp emits the light in the UVC region  whereas our study only uses the UVA lamp and visible lamp. Hence, the photocatalytic activity of our samples can be excluded from this study. According to the experimental results, via the immobilization of Fe 2 O 3 onto CuFe 2 O 4 particles, we really improved the photo-Fenton-like catalytic activity of CuFe 2 O 4 for the degradation of MB under both UVA light and visible light. This enhancement of activity should be explained by various factors including the change in phase composition, the evolution of morphology and the variations of functional groups on the surface of our catalysts. In fact, the growth of Fe 2 O 3 nanoparticles on the surface of magnetic CuFe 2 O 4 cores did not only increase the hematite phase in the structure of samples but also modified the distribution of metallic ions on their surface. As displayed in the FTIR spectra, the magnetic CuFe 2 O 4 powder shows an extremely weak M octa -O peak, indicating a very limited presence of surface octahedral metal ions (Cu 2+ and Fe 3+ ). In contrast, when the surface of CuFe 2 O 4 was coated by Fe 2 O 3 nanoparticles, the intensity of this peak remarkably increased, which can be associated with the fact that our Fe 2 O 3 nanoparticles crystallize in the corundum structure and contain all Fe 3+ ions in octahedral sites [Kraushofer et al. 2018, Li et al. 2016]. Interestingly, the M tetra -O peak of these CuFe 2 O 4 /Fe 2 O 3 core-shell materials is still intense, even more intense than that of CuFe 2 O 4 sample. These results demonstrate that our CuFe 2 O 4 /Fe 2 O 3 core-shell catalysts contain the high amounts of both tetrahedral and octahedral Fe 3+ ions on their surface, which can be considered as the main reason for the improvement of photo-Fenton-like catalytic performance. Among all our catalysts, the CuFe 2 O 4 /Fe 2 O 3 -2 sample seems to show the highest content of surface Fe 3+ ions owing to the most intense M octa -O and M tetra -O peaks in its FTIR spectrum. The enhanced Fe content on the surface of our core-shell catalysts is also supported by the EDX study. In fact,  [Jeong and Yoon 2005, Liu et al. 2012, Ngo TPH and Le TK 2018. Then, due to light irradiation, these ferrioxalate complexes will be excited to produce numerous radicals such as C 2 O 4

, O 2
•and • OH (eq. 1-4) [Liu et al. 2012, Mulazzani et al. 1986], which are both highly reactive and thus able to degrade effectively MB molecules in solution. Therefore, the best performance of CuFe 2 O 4 /Fe 2 O 3 -2 catalyst is likely attributed to the highest surface Fe 3+ content of this sample.
Secondly, when Fe 2 O 3 nanoparticles were combined with CuFe 2 O 4 , the specific surface area of our catalysts was strongly enhanced (about ten times higher than that of CuFe 2 O 4 ). This can increase the reactive sites on their surface, leading to the enhancement of their photo-Fenton-like activity. Moreover, we also noticed a sound correlation between the shape of Fe 2 O 3 particles and the catalytic activity. Depending on the molar CuFe 2 O 4 /Fe 2 O 3 ratios, the Fe 2 O 3 particles could be transformed between the spherical shape and the rod-like morphology. It seems that the rod-like Fe 2 O 3 particles in CuFe 2 O 4 /Fe 2 O 3 -1 and CuFe 2 O 4 /Fe 2 O 3 -2 samples showed the better performances than the spherical Fe 2 O 3 particles in CuFe 2 O 4 /Fe 2 O 3 -3 catalyst. Although the reason for the enhanced activities of rod-like particles is still unclear and needs to be further studied, this phenomena was also observed in some previous works [Chaudhari et al. 2012, Liang et al. 2015]. Chaudhari et al. compared the peroxidase mimic activity of hematite iron oxides with different nanostructures, including hexagonal prism, cube-like and rod-like particles, and found that the rod-like particles showed the best activity [Chaudhari et al. 2012]. Likewise, Liang et al. reported that the photocatalytic degradation of rhodamine B can be improved when using X-shaped α-Fe 2 O 3 nanocrystals which are composed of two rod- like particles crossing each other [Liang et al. 2015].
These results indicate that the particle shape also plays an important role in catalytic performance. As a result, by managing the molar CuFe 2 O 4 /Fe 2 O 3 ratios, we can control the particle shape of Fe 2 O 3 and consequently the photo-Fenton-like activity of our materials.

Conclusion
In summary, we successfully developed a new and effective magnetic photo-Fenton-like catalysts based on the immobilization of Fe 2 O 3 nanoparticles on the surface of CuFe 2 O 4 particles. These core-shell materials exhibited excellent activities for the degradation of methylene blue in the presence of oxalic acid under both UVA light and visible light. Among them, the CuFe 2 O 4 /Fe 2 O 3 -2 catalyst show the best catalytic performance, which can be assigned not only to the highest surface Fe 3+ content of this sample but also to the high specific surface area and the presence of Fe 2 O 3 rod-like particles immobilized on the surface of its magnetic CuFe 2 O 4 cores. Moreover, owing to the presence of magnetic CuFe 2 O 4 cores, our coreshell catalysts can be easily separated from the solution and recovered by using a magnet, making them suitable for practical applications. Declarations

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