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Article

One-Pot Synthesis of Magnetic Core-Shell Fe3O4@C Nanospheres with Pt Nanoparticle Immobilization for Catalytic Hydrogenation of Nitroarenes

by
Jun Qiao
1,2,
Yang Gao
1,
Kai Zheng
3,
Chao Shen
2,
Aiquan Jia
1 and
Qianfeng Zhang
1,*
1
School of Materials Science and Engineering, Institute of Molecular Engineering and Applied Chemistry, Anhui University of Technology, Ma’anshan 243002, China
2
College of Biology and Environmental Engineering, Zhejiang Shuren University, Hangzhou 310015, China
3
College of Petroleum Chemical Industry, Changzhou University, Changzhou 213164, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(10), 5773; https://doi.org/10.3390/app15105773
Submission received: 25 April 2025 / Revised: 18 May 2025 / Accepted: 20 May 2025 / Published: 21 May 2025
Browse Figures
Versions Notes

Abstract

:
Magnetic materials with intriguing structural and functional modifications demonstrate broad application potential in various fields, including drug delivery, absorption, extraction, separation, and catalysis. In particular, the catalytic hydrogenation of functionalized organic nitro compounds represents a significant research focus in contemporary catalysis studies. A facile synthesis of Fe3O4@C–Pt core-shell nanocatalysts was developed in this work through a sequential process involving (1) one-pot hydrothermal synthesis followed by N2-annealing to obtain the Fe3O4@C core and (2) subsequent solvothermal deposition of platinum nanoparticles. Comprehensive characterization was performed using FT-IR, XRD, Raman spectroscopy, TEM, XPS, BET surface area analysis, TGA, and VSM techniques. The resulting magnetic nanocatalysts exhibited uniformly dispersed Pt nanoparticles and demonstrated exceptional catalytic performance in nitroarene hydrogenation reactions. Remarkably, the system showed excellent functional group tolerance across all 20 substituted nitroarenes, consistently yielding corresponding aromatic amine products with >93% conversion efficiency. Furthermore, the magnetic responsiveness of Fe3O4@C–Pt enabled convenient catalyst recovery through simple magnetic separation, with maintained catalytic activity over 10 consecutive reuse cycles without significant performance degradation.

1. Introduction

Metallic nanoparticles such as platinum (Pt), silver (Ag), gold (Au), and palladium (Pd) have garnered significant attention due to their exceptional catalytic activity in diverse reactions [1,2,3,4]. Nevertheless, the propensity of metal nanoparticles to undergo aggregation, thereby reducing their catalytic activity, remains a critical challenge for their practical applications in heterogeneous catalysis [5,6]. The state-of-the-art strategy centers on immobilizing metal nanoparticles onto porous carbon matrices via covalent anchoring, which effectively mitigates aggregation while preserving accessible active sites for heterogeneous catalytic applications [7,8,9]. While magnetic supports exhibit unparalleled advantages in catalyst recovery via magnetic separation compared with conventional solid carriers [10], their propensity for oxidative dissolution under harsh reaction conditions remains a critical limitation that necessitates surface stabilization strategies.
In recent years, carbon has emerged as a promising shell material for encapsulating magnetic cores, with its hierarchical porosity and high specific surface area enabling enhanced catalytic performance and structural stability [11,12,13,14,15,16]. The integration of magnetic cores (e.g., Fe3O4) within carbon-encapsulated architectures not only imparts ferromagnetic responsiveness to the composite but also mitigates oxidative dissolution of the core through carbon shielding, while simultaneously offering abundant anchoring sites for nanocatalyst immobilization. In order to effectively stabilize metal nanoparticles, the shell material must act as a substrate with a hierarchically porous structure and superior dispersion properties. These structural features are essential for strengthening metal support electronic coupling and optimizing the catalytic activity of noble metal catalysts [17]. At present, there are two methods to synthesize core-shell Fe3O4@C. Firstly, magnetic Fe3O4 nanospheres are synthesized and then wrapped in a heat treatment using carbon sources such as glucose, dopamine, and polymers. Although Fe3O4@C supports show good dispersion, the preparation method is time-consuming, and the steps are cumbersome [18,19,20,21,22,23]. Therefore, one-step preparation of Fe3O4@C supports was investigated in detail, and the Fe3O4@C composite was further prepared through a solvothermal approach employing ferrocene as the iron and carbon source. However, the synthesis was conducted at a relatively elevated temperature of 220 °C and involved the use of acetone, a volatile solvent with a low boiling point, which poses safety concerns [24]. Meanwhile, loading precious metals into the carbon shell layer is a highly challenging research endeavor. Most current studies utilize the impregnation method. However, the catalysts obtained through this method exhibit rather poor dispersion of active components [25,26]. It is still a great challenge to safely prepare the shell material with excellent dispersion of metal nanoparticles.
Fe3O4@C core-shell nanospheres were synthesized via a one-pot hydrothermal method in this paper, followed by annealing under a nitrogen atmosphere. Subsequently, platinum (Pt) nanoparticles were uniformly deposited onto the carbon shell of Fe3O4@C through a solvothermal approach, using ethylene glycol as a reducing agent. The as-prepared Fe3O4@C–Pt composite was systematically characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) analysis, thermogravimetric analysis (TGA), and vibrating sample magnetometry (VSM). The current results indicate excellent dispersion of the Fe3O4@C–Pt composite, with Pt nanoparticles uniformly anchored on the carbon shell surface. Nitroarenes possess extensive application value, exemplified by their use in explosives manufacturing (e.g., trinitrotoluene, TNT). In particular, aromatic amines synthesized via catalytic reduction of nitroarenes exhibit substantial market potential in pharmaceutical synthesis, agrochemical production, and dye and fragrance preparation, as well as serving as versatile intermediates in organic synthesis. The catalyst exhibited remarkable catalytic activity in the hydrogenation of nitroarenes to corresponding aromatic amines. Furthermore, Fe3O4@C–Pt demonstrated a rapid magnetic separation capability and further retained stable catalytic efficiency over at least 10 consecutive cycles.

2. Experimental

2.1. General Characterization

FTIR spectra were obtained using a Bruker TENSOR 27 spectrometer. Samples were mixed with KBr at a 1:100 mass ratio and pressed into pellets under 10 MPa of pressure. Spectra were collected in the range of 4000~400 cm⁻1 with 32 scans per measurement. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku Ultima IV diffractometer with Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 40 mA. Data acquisition covered the 2θ range of 20~70° with a step size of 0.02°. Raman spectra were acquired using a Renishaw inVia confocal Raman spectrometer with 514.5 nm of Ar⁺ laser excitation. Transmission electron microscopy (TEM) images were acquired using a JEOL JEM-2100F microscope operating at 200 kV. XPS measurements were performed on a Thermo Scientific K-Alpha spectrometer using monochromatic Al Kα radiation (1486.6 eV) under an ultrahigh vacuum (<5 × 10⁻9 mbar), with pass energies of 20 eV for high-resolution scans and 100 eV for survey spectra. All binding energies were referenced to the adventitious carbon C1s peak at 284.8 eV. The surface area and pore distribution were measured using a Micromeritics ASAP 2020 via BET method with N₂ adsorption at 77 K. The samples were degassed at 200 °C for 12 h before analysis. The BET calculations used a P/P₀ range of 0.05~0.30, while the pore size distribution was determined using the non-localized density functional theory (NLDFT) method. Thermogravimetric analysis with differential thermogravimetry (TGA/DTG) was performed using a Shimadzu DTG-60H simultaneous thermal analyzer under flowing air (30 mL/min). The specimens (4.225 mg) loaded in alumina crucibles underwent heating from ambient temperature to 1000 °C at a constant rate of 10 °C/min. Both mass variation profiles and derivative thermogravimetric signals were synchronously recorded with a mass resolution of ±0.1 μg. The magnetic measurements were assessed by a Lake Shore 7404 in the range from −30 kOe to + 30 kOe at room temperature. Platinum was analyzed using ICP-OES (PerkinElmer Optima 2100 DV, radial mode) with 0.7 L/min of Ar flow, 1.3 kW of RF power, and a 5 s integration time for Pt lines at 214.423 nm and 265.945 nm.

2.2. Synthesis of Fe3O4@C Nanospheres

All reagents were of analytical grade (AR), purchased from Macklin Biochemical Technology, and used without further purification. Fe3O4@C was synthesized following a reported method [27,28]. Briefly, 3.600 g (0.020 mol) of C6H12O6 was dissolved in 130 mL of deionized water, while 4.848 g (0.012 mol) of Fe(NO3)3·9H2O was dissolved in 30 mL of deionized water. The two solutions were rapidly mixed and stirred for 0.5 h. The resulting yellow solution was transferred to a 250 mL Teflon-lined autoclave and heated at 190 °C for 9 h. After cooling to room temperature, the brown solid product was collected, washed thoroughly with deionized water and ethanol (three times each), and dried at 60 °C for 12 h under vacuum (denoted as Precursor). The Fe3O4@C composite was then obtained by annealing the precursor at 550 °C for 2 h under a N2 atmosphere (heating rate: 5 °C/min), labeled as Fe3O4@C nanospheres. To study the temperature-dependent phase and porosity, the precursors were calcined at 450, 500, 550, and 600 °C, with samples designated as Fe3O4@C-X (X = calcination temperature).

2.3. Synthesis of Fe3O4@C–Pt Nanospheres

The Fe3O4@C–Pt catalyst was synthesized via a modified literature method [29]. Briefly, 400 mg of Fe3O4@C nanospheres were dispersed in 40 mL of ethylene glycol (EG) under vigorous stirring. A solution of 14 mg H2PtCl6·6H2O in 11 mL of deionized water was then injected into the mixture. After 0.5 h of sonication, the homogeneous dispersion was transferred to an autoclave and heated at 120 °C for 12 h. The Fe3O4@C–Pt composite was collected via vacuum filtration, thoroughly washed with ethanol to eliminate chloride residues, and vacuum-dried at 60 °C for 12 h.

2.4. Catalytic Hydrogenation of Nitroarene Reactions Evaluation

In the typical procedure, 1 mmol of nitro compound, 5 mL of solvent, 2 mmol of reducing agent, and 20 mg of Fe3O4@C–Pt catalyst were added to a glass reactor. The reaction mixture was stirred at 70 °C, and progress was monitored with thin-layer chromatography (TLC) at specified time intervals (1 h). The isolation and purification procedures for the hydrogenation products are outlined below. Firstly, the mixture was filtered through celite to remove solid impurities and obtain a clear filtrate. Subsequently, the clarified filtrate was evaporated to dryness using a rotary evaporator under reduced pressure. Finally, the residue was purified via column chromatography with a petroleum ether/ethyl acetate solvent system (5:1, v/v). The resulting products were further identified and analyzed using a 1H NMR spectrum.

2.5. Catalyst Reused Evaluation

After each hydrogenation of nitroarenes, the catalyst was magnetically separated, washed three times with 2 mL of water and ethanol each, and then employed in subsequent reactions after being dried under vacuum.

3. Results and Discussion

To obtain information on the functional groups in the samples, Fourier transform infrared spectroscopy (FT-IR) was performed on the precursor, Fe3O4@C, and Fe3O4@C–Pt. As shown in Figure 1, all samples exhibited a broad infrared absorption peak at 3440 cm−1, which is attributed to the stretching vibrations of surface hydroxyl groups [30]. Distinct absorption peaks at 2924 and 2852 cm−1 correspond to the asymmetric and symmetric stretching vibrations of methyl or methylene groups, respectively, likely resulting from incomplete carbonization of glucose [31]. A broad absorption peak observed at 1620 cm−1 is associated with the stretching vibrations of C=C bonds [32,33], while a sharp and intense peak at 1384 cm−1 arose from C–H bending vibrations [34]. In the range of 1000~1300 cm−1, broad absorption peaks (particularly at 1105 and 1026 cm−1) are ascribed to C–OH stretching vibrations and O–H bending vibrations, indicating the presence of hydrophilic groups and partially carbonized glucose derivatives [35]. Notably, Fe3O4@C and Fe3O4@C–Pt displayed strong infrared absorption peaks at 638 and 590 cm−1, which are characteristic of the intrinsic stretching vibrations of Fe–O bonds. In the infrared spectrum of the precursor sample, a distinct absorption band observed at 590 cm−1 can be ascribed to the Fe–O stretching vibration [36,37,38]. A weak absorption peak at 451 cm−1, ascribed to Fe–O bending vibrations [39], was also observed but absent in the hydrothermal precursor. The presence of Fe3O4 in both the Fe3O4@C and Fe3O4@C–Pt samples is preliminarily confirmed by these findings.
To investigate the crystallinity and phase composition of the catalyst samples, X-ray powder diffraction (XRD) was employed. The XRD patterns of the hydrothermal precursor and samples calcined at different temperatures are shown in Figure 2a. The hydrothermal precursor exhibited five characteristic peaks at 2θ = 35.1°, 40.5°, 53.9°, 60.8°, and 63.9°, which aligned well with the reference diffraction peaks of FeOOH (JCPDS No. 77-0247) [40], confirming its FeOOH-dominated composition. To evaluate the effect of the calcination temperature on the phase composition of Fe3O4@C, the hydrothermal precursor was calcined under a nitrogen atmosphere to obtain Fe3O4@C–X samples (where X denotes the calcination temperature). As shown in Figure 2a, all calcined samples retained the Fe3O4@C nanosphere structure, with characteristic diffraction peaks at 2θ = 30.4°, 35.8°, 37.4°, 43.5°, 54.0°, 57.5°, and 63.2°, corresponding to cubic magnetite (Fe3O4, JCPDS No. 75-0449) [41]. However, a weak diffraction peak at 2θ = 33.3°, attributed to hematite (Fe2O3, JCPDS No. 84-0309), was observed in all samples, indicating unavoidable partial oxidation of Fe3O4 to Fe2O3 during high-temperature calcination (As marked by the dashed circle in Figure 2a). Notably, the Fe2O3 peak intensity was minimized for the sample calcined at 550 °C (Fe3O4@C–550), suggesting optimal phase purity under this condition. Therefore, Fe3O4@C–550 was selected as the representative sample (denoted as Fe3O4@C in subsequent discussions unless specified). The XRD patterns of Fe3O4@C and Fe3O4@C–Pt are comparatively analyzed in Figure 2b. The observed diffraction peaks at 2θ = 39.9°, 46.4°, and 67.7° in the Fe3O4@C–Pt sample correspond to the (111), (200), and (220) crystallographic planes of face-centered cubic (FCC) Pt, respectively, which match well with metallic platinum (Pt, JCPDS No. 87-0640) [42]. The remaining characteristic XRD peaks of the Fe3O4@C–Pt sample aligned perfectly with those of Fe3O4@C, and no impurity-related diffraction signals were detected, confirming the successful incorporation of Pt nanoparticles into the Fe3O4@C matrix.
The absence of carbon-related diffraction peaks in the XRD patterns suggests low crystallinity of carbon in the samples, consistent with amorphous or poorly graphitized carbon structures. Raman spectroscopy, a powerful tool for probing carbonaceous materials, was employed to further investigate the crystallinity and carbon speciation. As shown in Figure 3, both Fe3O4@C and Fe3O4@C–Pt exhibited two prominent Raman bands at 1365 cm−1 (D-band) and 1592 cm−1 (G-band). The broad D band at 1365 cm−1 corresponds to disordered carbon structures at the edges of graphite sheets of a carbonaceous material, which is related to the existence of amorphous carbon. (e.g., edge defects or amorphous domains), while the sharp G band at 1592 cm−1 arose from the in-plane stretching vibrations of sp2-hybridized ordered graphitic carbon. The intensity ratio of the D band to the G band (ID/IG) quantifies the degree of graphitization. For Fe3O4@C, the ID/IG ratio was 0.77, whereas Fe3O4@C–Pt exhibited a slightly higher value of 0.82. Notably, the G band dominated over the D band in both samples, indicating the presence of partially graphitized carbon layers on the surface of Fe3O4@C–Pt and Fe3O4@C [43]. This enhanced graphitic ordering facilitates electron transfer between carbon and platinum nanoparticles, which is critical for improving catalytic activity in nitroarene hydrogenation [44]. Therefore, not only was the existence of carbon in the samples confirmed in the Raman spectroscopy, but direct evidence for the structural evolution from disordered carbon to graphitic domains was also provided by the Raman spectroscopy during synthetic experiment. To determine the chemical composition of the Fe3O4@C nanospheres, TGA of Fe3O4@C in air (ambient to 1000 °C at 10 °C/min; Figure S1) was performed to reveal 47.2% Fe3O4 and 46.6% carbon contents.
To elucidate the microstructure and morphology of each of the samples, transmission electron microscopy (TEM) was performed on Fe3O4@C and Fe3O4@C–Pt. Figure 4a–c displays TEM images of Fe3O4@C, while Figure 4e–g corresponds to Fe3O4@C–Pt. Both samples exhibited a uniform spherical morphology with a thin carbon shell (~2 nm for Fe3O4@C and ~3 nm for Fe3O4@C–Pt) encapsulating the surface (Figure 4c,g). The particle size distributions (Figure 4d,h) revealed average diameters of 40–50 nm for Fe3O4@C and 50–60 nm for Fe3O4@C–Pt (The curve represents the frequency distribution curve). Notably, the Fe3O4@C–Pt sample showed distinct Pt nanoparticles (~2 nm) on its carbon surface, marked by red circles in Figure 4f. The measured lattice spacing of 0.226 nm matched the (111) crystallographic plane of metallic platinum (Figure 4g), confirming the successful loading of Pt nanoparticles [45]. Additionally, lattice fringes with a spacing of 0.208 nm (Figure 4c,g) were attributed to the (400) plane of magnetite (Fe3O4) [46], corroborating the coexistence of Fe3O4, carbon, or Pt in both samples.
XPS is an effective technique for evaluating the surface composition and valence states of catalysts. As shown in Figure 5a, the survey spectrum of Fe3O4@C–Pt confirmed the presence of carbon (C), oxygen (O), iron (Fe), and platinum (Pt). This result is consistent with the aforementioned XRD data (Figure 2b). The high-resolution C1s spectrum (Figure 5b) can be deconvoluted into three peaks centered at 284.8, 286.1, and 287.9 eV, corresponding to C=C (red line), C–C/C–O (blue line), and C=O (green line) bonds, respectively [47]. This demonstrates abundant carbon species within the material, showing consistency with the Raman spectroscopy results (Figure 3) and conclusively verifying carbon’s presence. Similarly, the O1s spectrum (Figure 5c) was fitted with three peaks at 530.1, 531.6, and 533.3 eV, assigned to Fe–O–Fe (red line), O=C (blue line), and C–O (green line) species, respectively [48]. This finding not only corroborates the FTIR spectra in Figure 1 but also definitively verifies the material’s abundance of oxygen-containing functional groups, specifically carboxyl (–COOH) and carbonyl (C=O) moieties. The Fe 2p high-resolution spectrum (Figure 5d) was deconvoluted into five characteristic peaks at binding energies of 710.6, 712.4, 719.3, 723.7, and 725.3 eV. The peaks at 710.6 and 723.7 eV correspond to the 2p3/2 (red line) and 2p1/2 (blue line) states of Fe2+, respectively, while the peaks at 712.4 and 725.3 eV are attributed to the 2p3/2 (green line) and 2p1/2 (purple line) states of Fe3+, respectively. A distinct satellite peak observed at 719.3 eV (yellow line) confirmed the coexistence of Fe2+ and Fe3+ species [49]. This observation is in good agreement with the characteristic spectral signatures displayed in Figure 4c, unequivocally confirming the coexistence of Fe3⁺ and Fe2⁺ oxidation states that are diagnostic of magnetite (Fe3O4) formation. In the Pt4f spectrum (Figure 5e), four distinct peaks were identified at Pt4f7/2 (71.1 eV, purple line), Pt4f5/2 (74.3 eV, blue line), Pt4f7/2 (72.5 eV, green line), and Pt4f5/2 (76.6 eV, red line), which were assigned to metallic Pt⁰ and oxidized PtII species [50]. In Figure 5b–e, the gray curve represents the raw data, while the light cyan curve corresponds to the fitted results.
The porosity of the samples was further investigated through N2 adsorption-desorption isotherm measurements [51]. As shown in Figure 6a, all samples exhibited Type I isotherms with a minor H3-type hysteresis loop at P/P₀ > 0.90, suggesting the coexistence of micropores and mesopores. The pore size distribution (PSD) curves of the three samples (Figure 6b) were analyzed using the non-localized density functional theory (NLDFT) method [52]. Specifically, Fe3O4@C showed pore diameters ranging from 0.6 to 70 nm, while the other two samples (Fe3O4@C–Pt and Precursor) exhibited narrower distributions of 1.3~70 nm. Notably, after supporting Pt on Fe3O4@C, the resulting Fe3O4@C–Pt demonstrated enlarged pore sizes while retaining a relatively high specific surface area (54 m2/g) and abundant mesopores (Table S1). Remarkably, the specific surface area of Fe3O4@C–Pt in this work was significantly larger than that reported in our previous study [53], providing favorable sites for catalytic processes. According to Table S1, the precursor exhibited a low specific surface area (24 m2/g). However, after calcination at 550 °C under N2, Fe3O4@C underwent phase transformation, resulting in a sharp increase in the specific surface area accompanied by the generation of micropores. In summary, the hierarchical micro-mesoporous structure of Fe3O4@C–Pt maximized the active site exposure, facilitated substrate adsorption and transport, and thereby enhanced catalytic performance. Detailed PSD data, BET surface areas, and NLDFT-calculated pore volumes are summarized in Table S1.
The Fe3O4@C–Pt composite demonstrated dual functionality through its catalytic activity and magnetic retrievability, enabling efficient recycling in reaction cycles. The magnetic properties of Fe3O4@C and Fe3O4@C–Pt were analyzed via vibrating sample magnetometry (VSM). As shown in Figure 7, a saturation magnetization (Ms) of ~45 emu/g for Fe3O4@C was revealed. Upon supporting Pt, Fe3O4@C–Pt exhibited a slightly reduced Ms of 44 emu/g, which may have arisen from low Pt loading (0.57 wt. %, confirmed by ICP) or uniform Pt dispersion on the carbon surface. This observation aligns with the XRD patterns (Figure 2b) and TEM imaging (Figure 4f,g).
The hydrogenation of nitrobenzene (1a) catalyzed by Fe3O4@C–Pt was systematically evaluated. Two critical parameters—catalyst selection and reducing agents—were investigated. The initial control experiments employed toluene as the solvent and hydrazine hydrate (N2H4•H₂O) as the reductant. Notably, no reaction occurred in the absence of a catalyst or with Fe3O4@C alone (Table S2, entries 1–2). Fe3O4@C–Pt exhibited significant catalytic activity, achieving >99% conversion (Table S2, entry 3). This confirms that Pt nanoparticles on Fe3O4@C serve as active sites for nitrobenzene hydrogenation. Furthermore, the reducing agent plays a crucial role. Hydrazine hydrate demonstrated superior reduction efficiency compared with H2, NaBH4, and ammonium formate (Table S2, entries 4–7), likely due to its moderate reducing power and environmental benignity.
The solvent plays a pivotal role in organic reactions. To evaluate its impact, eight solvents were screened (Table S3, entries 1–8). Notably, water achieved a yield of >99% aniline, highlighting its advantages as a green solvent—non-toxic, sustainable, and environmentally benign—for organic transformations. Regarding the impact of temperature on the reaction, when the reaction was carried out at a low temperature, including ambient conditions, the resulting yield was low (Table S3, entries 9–10). Another notable factor that impacted the product yield was the duration of the reaction. It is evident that as the reaction time decreased, the yield also decreased; the yield of aniline was only 75% after 2 h compared with that at 3 h (Table S3, entries 1 and 11). As corroborated by previous studies (Table S3, entries 12–14), heterogeneous noble-metal Pt catalysts similarly demonstrate exceptional nitrobenzene hydrogenation performance [54,55,56]. It can be inferred that the optimal reaction conditions were determined: hydrazine hydrate as the reducing agent, water as the solvent, maintaining a temperature of 70 °C, and a reaction time of 3 h.
Based on the aforementioned results, the Fe3O4@C–Pt catalyst was further employed to expand the substrate scope to 20 substituted nitroarenes. As shown in Table 1, all substrates were transformed into the corresponding aromatic amines with high activity, achieving yields exceeding 93%. The experimental findings revealed that the product yields were significantly influenced by the electronic effects of functional groups on the nitroarenes. Specifically, the nitrobenzene derivatives bearing electron-donating groups (–CH3, –OCH3, –NH2, and –SO3H) as well as two biphenyl derivatives were efficiently reduced to the target anilines with high yields (Table 1, entries 1–9, 2a–2i, and 18–19, 2r–2s) [57]. Furthermore, the nitroarenes bearing electron-withdrawing carbonyl groups (–CHO, –COCH3, and –COOH) demonstrated consistently high yields (≥94%) (Table 1, entries 10–12, 2j–2l) [58].
Halogenated nitroarenes (–F, –Cl, –Br, and –I) were successfully reduced to halo-substituted anilines with minimal by-products (Table 1, entries 13–17, 2m–2q) [59,60]. However, 4–iodonitrobenzene (Table 1, entry 15, 2o) displayed slower reaction kinetics and a reduced yield compared with other halogenated substrates, likely due to dehalogenation during hydrogenation. Finally, the reduction of a heterocyclic nitro compound, 3–nitropyridine, under Fe3O4@C–Pt catalysis afforded 3–aminopyridine with a yield of 94% after 9 h (Table 1, entry 20, 2t) [61]. The 1H NMR spectra of all products (2a–2t) are provided in the Supplementary Materials (Figures S1–S20). To further elucidate the 1H NMR spectra of the catalytic hydrogenation products in the Supplementary Materials, this section employs the 1H NMR data of aniline as an illustrative example. The aromatic proton signals in the 1H NMR spectrum of aniline (generated via catalytic hydrogenation of nitrobenzene) were observed in the δ 6.61–7.12 ppm range, consistent with the deshielding effect induced by the strong electron-donating nature of the amino group (–NH2) on the benzene ring. Specifically, the doublet at δ 6.61 (J = 7.5 Hz, 2H) corresponded to the para-hydrogens of the benzene ring, arising from coupling with the two adjacent ortho-hydrogens. The triplet at δ 6.73 (J = 7.7 Hz, 1H) was attributed to the meta-hydrogen, which was split by coupling with two neighboring ortho-hydrogens. The triplet at δ 7.12 (J = 7.9 Hz, 2H) was assigned to the ortho-hydrogens, which were split by coupling with one meta-hydrogen and one para-hydrogen. Additionally, the singlet at δ 3.51 (integrating to 2H) corresponded to the exchangeable protons of the amino group (–NH₂). This sharp singlet, rather than a broad peak, may reflect rapid proton exchange under the measurement conditions (e.g., in chloroform-d), which suppresses the peak broadening typically associated with amine protons. The observed splitting patterns, chemical shifts, and integration ratios collectively confirmed the structure of aniline, with the electron-donating amino group modulating both the shielding effects and spin-spin coupling interactions across the aromatic system.
Activity and stability are critical attributes of heterogeneous catalysts. To evaluate these properties, a hot filtration test was performed. Following a 1 h reaction period, the Fe3O4@C–Pt catalyst was efficiently isolated from the reaction mixture via magnetic separation. Notably, when the nitrobenzene hydrogenation reaction was subsequently continued under catalyst-free conditions, the aniline yield remained constant (maintained at 47% for 2.5 h), confirming the absence of significant leaching of active components from the catalyst (Figure S2). To investigate the dispersion state of the catalyst in the solvent and its post-magnetic separation behavior, a simple experimental study was conducted with photographic documentation. The dispersion state of Fe3O4@C–Pt in aqueous solution and their rapid magnetic separation are demonstrated in Figure 8a,b, respectively. Fe3O4@C–Pt exhibited excellent recyclability in the catalytic hydrogenation of nitrobenzene, as shown in Figure 8c. Although a slight decline in catalytic efficiency was observed during the sixth cycle, the aniline yield remained stable at 96%. The final results demonstrate that Fe3O4@C–Pt retained significant catalytic activity for nitrobenzene hydrogenation even after 10 consecutive cycles.

4. Conclusions

In summary, a facile one-pot synthesis strategy was successfully developed for fabricating magnetically separable core-shell Fe3O4@C–Pt nanocatalysts. Pt nanoparticles (~2 nm) were uniformly dispersed on the Fe3O4@C support surface. Systematic characterization techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption-desorption analysis (BET), confirmed the well-defined core-shell architecture and chemical composition of the Fe3O4@C–Pt catalyst. A yield of ≥93% was achieved in the catalytic hydrogenation of 20 substituted nitroarenes to aromatic amines using this catalyst, with water employed as a green solvent and hydrazine hydrate utilized as the reductant. Rapid magnetic separation from the reaction mixture was enabled by the embedded Fe3O4 core catalyst (Fe3O4@C–Pt). Robust stability was confirmed through recycling experiments, with catalytic activity retained over 10 consecutive cycles.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/app15105773/s1. Figure S1: TGA curve of the Fe3O4@C in air. Figure S2: Hot filtration experiment of hydrogenation of nitrobenzene. Table S1: BET results (surface area and pore volume) of Precursor, Fe3O4@C, and Fe3O4@C-Pt. Table S2: Optimization of catalyst and reducing agent. Table S3: Optimization of reaction conditions. Figures S3–S22: 1H NMR spectrum of 2a–2t.

Author Contributions

Conceptualization, A.J. and Q.Z.; methodology, J.Q.; formal analysis, Y.G. and K.Z.; investigation, J.Q. and Y.G.; resources, J.Q.; writing—original draft preparation, J.Q.; writing—review and editing, J.Q., C.S., and A.J.; supervision, C.S., A.J., and Q.Z.; project administration, C.S. and Q.Z.; funding acquisition, C.S. and Q.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Zhejiang Provincial National Natural Science Foundation of China (No. LQ21E020003) and the Zhejiang Shuren University Basic Scientific Research Special Funds (2020XZ011).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

We thank Yang Gao and Kai Zheng for the TGA, NMR spectra, and XRD analysis. The authors would like to thank Shiyanjia Lab (www.shiyanjia.com) for the XPS, TEM, and VSM analysis. https://www.shiyanjia.com/all.html (accessed on 19 May 2025).

Conflicts of Interest

The authors declare no conflicts of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Applsci 15 05773 g001
Figure 1. FT-IR spectra of Precursor, Fe3O4@C, and Fe3O4@C–Pt.
Figure 1. FT-IR spectra of Precursor, Fe3O4@C, and Fe3O4@C–Pt.
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Figure 2. (a) XRD patterns of the precursors and samples obtained at different calcination temperatures. (b) XRD patterns of Fe3O4@Cand Fe3O4@C–Pt.
Figure 2. (a) XRD patterns of the precursors and samples obtained at different calcination temperatures. (b) XRD patterns of Fe3O4@Cand Fe3O4@C–Pt.
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Figure 3. Raman spectra of the Fe3O4@C and Fe3O4@C–Pt nanospheres. (Green-Cumulative Fit Peak, Red-Fit Peak D, Blue-Fit Peak G, Black-Initial Peak Data).
Figure 3. Raman spectra of the Fe3O4@C and Fe3O4@C–Pt nanospheres. (Green-Cumulative Fit Peak, Red-Fit Peak D, Blue-Fit Peak G, Black-Initial Peak Data).
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Figure 4. (a–c) TEM images of Fe3O4@C. (d) Particle-size distribution of Fe3O4@C. (e–g) TEM images of Fe3O4@C–Pt (Pt nanoparticles circled red in (f)). (h) Particle size distribution of Fe3O4@C–Pt.
Figure 4. (a–c) TEM images of Fe3O4@C. (d) Particle-size distribution of Fe3O4@C. (e–g) TEM images of Fe3O4@C–Pt (Pt nanoparticles circled red in (f)). (h) Particle size distribution of Fe3O4@C–Pt.
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Figure 5. XPS spectra of Fe3O4@C–Pt: (a) survey spectrum, (b) C1s, (c) O1s, (d) Fe2p, and (e) Pt4f.
Figure 5. XPS spectra of Fe3O4@C–Pt: (a) survey spectrum, (b) C1s, (c) O1s, (d) Fe2p, and (e) Pt4f.
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Figure 6. (a) N2 adsorption-desorption isotherms. (b) Pore diameter distribution curves of Precursor, Fe3O4@C, and Fe3O4@C–Pt, with inset in (b) showing pore diameter distribution from 1 to 100 nm.
Figure 6. (a) N2 adsorption-desorption isotherms. (b) Pore diameter distribution curves of Precursor, Fe3O4@C, and Fe3O4@C–Pt, with inset in (b) showing pore diameter distribution from 1 to 100 nm.
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Figure 7. Magnetic hysteresis curves of Fe3O4@C–Pt (red line) and Fe3O4@C (blue line).
Figure 7. Magnetic hysteresis curves of Fe3O4@C–Pt (red line) and Fe3O4@C (blue line).
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Figure 8. Photographs of the magnetically separable Fe3O4@C–Pt (a) dispersed in water and (b) subjected to an external magnetic field. (c) Recyclability of the Fe3O4@C–Pt catalyst.
Figure 8. Photographs of the magnetically separable Fe3O4@C–Pt (a) dispersed in water and (b) subjected to an external magnetic field. (c) Recyclability of the Fe3O4@C–Pt catalyst.
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Table 1. Fe3O4@C–Pt catalyzed hydrogenation of different nitroarenes a.
Table 1. Fe3O4@C–Pt catalyzed hydrogenation of different nitroarenes a.
Applsci 15 05773 i001
EntrySubstrateProduct (2a–2s)Time (h)Yield b (%)
1Applsci 15 05773 i002Applsci 15 05773 i0032a3 h99
2Applsci 15 05773 i004Applsci 15 05773 i0052b3 h96
3Applsci 15 05773 i006Applsci 15 05773 i0072c3 h99
4Applsci 15 05773 i008Applsci 15 05773 i0092d6 h94
5Applsci 15 05773 i010Applsci 15 05773 i0112e3 h97
6Applsci 15 05773 i012Applsci 15 05773 i0132f3 h99
7Applsci 15 05773 i014Applsci 15 05773 i0152g3 h99
8Applsci 15 05773 i016Applsci 15 05773 i0172h3 h99
9Applsci 15 05773 i018Applsci 15 05773 i0192i3 h98
10Applsci 15 05773 i020Applsci 15 05773 i0212j6 h94
11Applsci 15 05773 i022Applsci 15 05773 i0232k6 h95
12Applsci 15 05773 i024Applsci 15 05773 i0252l9 h95
13Applsci 15 05773 i026Applsci 15 05773 i0272m3 h99
14Applsci 15 05773 i028Applsci 15 05773 i0292n6 h95
15Applsci 15 05773 i030Applsci 15 05773 i0312o9 h93
16Applsci 15 05773 i032Applsci 15 05773 i0332p6 h95
17Applsci 15 05773 i034Applsci 15 05773 i0352q6 h99
18Applsci 15 05773 i036Applsci 15 05773 i0372r3 h98
19Applsci 15 05773 i038Applsci 15 05773 i0392s6 h95
20Applsci 15 05773 i040Applsci 15 05773 i0412t9 h94
a Reaction conditions: nitroarenes 1a (1 mmol), N2H4·H2O (4 mmol), 0.5 mol% catalysts, and H2O (3 mL) under air. b Isolated yield.
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Qiao, J.; Gao, Y.; Zheng, K.; Shen, C.; Jia, A.; Zhang, Q. One-Pot Synthesis of Magnetic Core-Shell Fe3O4@C Nanospheres with Pt Nanoparticle Immobilization for Catalytic Hydrogenation of Nitroarenes. Appl. Sci. 2025, 15, 5773. https://doi.org/10.3390/app15105773

AMA Style

Qiao J, Gao Y, Zheng K, Shen C, Jia A, Zhang Q. One-Pot Synthesis of Magnetic Core-Shell Fe3O4@C Nanospheres with Pt Nanoparticle Immobilization for Catalytic Hydrogenation of Nitroarenes. Applied Sciences. 2025; 15(10):5773. https://doi.org/10.3390/app15105773

Chicago/Turabian Style

Qiao, Jun, Yang Gao, Kai Zheng, Chao Shen, Aiquan Jia, and Qianfeng Zhang. 2025. "One-Pot Synthesis of Magnetic Core-Shell Fe3O4@C Nanospheres with Pt Nanoparticle Immobilization for Catalytic Hydrogenation of Nitroarenes" Applied Sciences 15, no. 10: 5773. https://doi.org/10.3390/app15105773

APA Style

Qiao, J., Gao, Y., Zheng, K., Shen, C., Jia, A., & Zhang, Q. (2025). One-Pot Synthesis of Magnetic Core-Shell Fe3O4@C Nanospheres with Pt Nanoparticle Immobilization for Catalytic Hydrogenation of Nitroarenes. Applied Sciences, 15(10), 5773. https://doi.org/10.3390/app15105773

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