Carbon-Coated Ni-Fe Nanocatalysts: Bridging the Gap in Cinnamaldehyde Hydrogenation Performance and Durability

Abstract

This study focuses on the synthesis and evaluation of carbon−coated Ni−Fe alloy catalysts (NiFe_x@C, x = 0, 0.3, 0.7, 1.1) for the hydrogenation of cinnamaldehyde. The catalysts were characterized using Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), Raman spectroscopy, and X-ray Photoelectron Spectroscopy (XPS). The introduction of Fe was found to increase the average particle size of the bimetallic catalysts compared to the monometallic Ni@C. Ni existed in both metallic and oxide states, while Fe exhibited multiple oxidation states in the bimetallic catalysts. The carbon layer, approximately 2–3 nm thick, was confirmed to envelop the alloy particles. The catalytic performance of carbon−coated Ni−Fe alloy catalysts indicated that the addition of Fe to Ni@C enhanced the selectivity towards hydrocinnamaldehyde (HCAL), with NiFe_0.7@C showing the highest selectivity (~88.6%) but at a reduced conversion rate. The carbon layer played a pivotal role in the stability and reusability of the catalysts. NiFe_0.7@C maintained consistent performance over multiple reaction cycles, while NiFe_0.7 NPs (without a carbon layer) exhibited significant deactivation. Both catalysts displayed strong magnetism, facilitating easy separation from the reaction mixture. This study sheds light on the significance of the carbon layer in bimetallic catalysts and provides valuable insights for designing efficient catalysts for hydrogenation processes.

1. Introduction

The selective hydrogenation of α, β−unsaturated aldehydes and ketones, compounds often derived from biomass, remains a pivotal research area in catalysis. This interest is primarily due to the competitive hydrogenation between the internal olefinic (C=C) and carbonyl (C=O) groups, posing significant challenges in achieving high selectivity for a single functional group [1,2]. Notably, selective hydrogenation products of α, β−unsaturated aldehydes, including furfural, crotonaldehyde, acrolein, and cinnamaldehyde (CAL), have garnered attention owing to their broad applications in biomedicine, food, flavor, and petrochemical industries. This surge in interest aligns with the growing global demand for natural resources [3,4,5]. Cinnamaldehyde, a prototypical α, β−unsaturated aldehyde, and its primary hydrogenation product, hydrocinnamaldehyde (HCAL), are of particular commercial significance [6]. HCAL’s utility extends beyond its use in sunscreens and herbicides; it serves as an organic intermediate in synthesizing various drug precursors, including cinnamic acid and HIV protease inhibitors [7,8].

Although the hydrogenation of the C=C bond in cinnamaldehyde is thermodynamically and kinetically more favorable, the conjugated system formed by the interaction of the C=O bond with the C=C bond, along with the steric hindrance effect of the benzene ring, also renders the C=O bond prone to reduction [4,9,10]. Consequently, developing catalytic systems that exhibit both high activity and selectivity, specifically promoting the generation of hydrocinnamaldehyde in single and high yields, represents a considerable challenge.

Currently, a diverse array of catalysts, including noble metals (e.g., Pt, Ru, Ir, Pd, and Rh) and non−noble metals (e.g., Ni, Co, and Cu), has been rigorously studied. Notably, palladium (Pd) and rhodium (Rh) are known to favor the hydrogenation of the C=C bond [11,12,13,14,15]. Despite their effectiveness, the use of precious metals in catalysis is hindered by their limited availability and high cost. Consequently, there is increasing interest in non-precious metal−based catalysts such as Co, Cu, Ni. For Co−based catalysts, hydrogenation of C=O is preferred to obtain more cinnamyl alcohol products [16]. Although Cu-based catalysts can also be applied to the cinnamaldehyde hydrogenation reaction, the reaction conditions are harsh and the activity and selectivity are poor [17,18]. Nickel (Ni), a non−precious metal, stands out due to its abundance, low cost, and sustainability. Moreover, Ni is more favorable for the activated hydrogenation of C=C because of its narrower D−band width [19,20]. In addition, Ni is effective in dissociating H₂, and its hydrogenation activity is superior to that of other base metals, which has better potential for application [21,22]. However, the high hydrogenation activity of Ni results in decreased selectivity, and the high specific surface energy and agglomeration−prone nature of Ni nanoparticles reduces their catalytic stability. Therefore, effective solutions to modulate the catalytic performance of Ni−based catalysts are urgently needed [23,24,25,26].

Forming a metal alloy by the addition of a second metal component is an effective approach to modify the catalytic performance. The incorporation of the second metal greatly changes the hydrogenation selectivity and activity of a metal by geometric and electronic effects [12,13,20]. For instance, Yang et al. [27] synthesized a bimetallic PdAu/MSN catalyst using an organic impregnation−hydrogen reduction method. The synergistic effect of the PdAu alloy was found to significantly influence the catalyst’s behavior, facilitating the hydrogenation of cinnamaldehyde to hydrocinnamaldehyde while suppressing over−hydrogenation to hydrocinnamyl alcohol. Similarly, Konuspayeva et al. [13] created AuRh nanoparticles via a colloidal method. The inclusion of gold was observed to promote the stable presence of Rh in its metallic state, thereby enhancing the catalyst’s overall performance. Such a methodology has also been proven effective for Ni catalysts. For instance, Putro et al. [28] prepared a Ni−Fe/CeO₂ catalyst with exceptional conversion and cyclohexanone selectivity. Vilar et al. [29] demonstrated that Fe incorporation into Ni−based catalysts can effectively improve C=C selectivity.

In addition to selectivity, the stability of Ni−based catalysts is compromised by sintering and agglomeration during practical use. Efforts to mitigate this issue have focused on dispersing metals across various supports to maintain stability and prevent deactivation. For instance, Xin et al. [30] leveraged the unique structure of LaNiO₃ to synthesize Ni/La₂O₃ catalysts, achieving a high dispersion of Ni nanoparticles and enhanced catalytic performance. Similarly, Lv et al. [31] employed oxidized carbon cloth to support NiFe alloy nanoparticles, utilizing the anchoring effect of oxygen−containing functional groups to ensure uniform dispersion and prevent agglomeration.

Nevertheless, support−based methods for metal dispersion are not without limitations and often fall short in preventing the complete oxidation of metal nanoparticles. An alternative and promising strategy is the application of a carbon layer to encapsulate the catalyst, which not only bolsters resistance to oxidation but also mitigates sintering and volume expansion during use [32,33,34,35,36,37,38]. Yoo et al. [32], for example, highlighted the protective role of a carbon shell layer in their study of carbon−encapsulated FeP nanoparticle catalysts. K−edge EXAFS analysis revealed that while the bare FeP surfaces were prone to oxidation in acidic conditions, those enveloped in carbon exhibited negligible chemical coordination changes, underlining the carbon layer’s efficacy in oxidation prevention. Qi et al. [33] demonstrated the structural benefits of carbon coating through chemical vapor deposition on core−shell nanostructures. Even after 100 cycles, Si@C−2 electrodes retained their integrity with minimal expansion compared to their uncoated counterparts. Similarly, Zhang et al. [34] reported the successful application of a carbon coating on T-MXene@C, which showed no signs of TiO₂ formation and minimal degradation over 3000 cycles, attesting to the role of carbon coating in enhancing both the oxidation resistance and long−term structural stability of the material.

Drawing from these foundations, we hypothesized that encapsulating alloy particles within a carbon layer could enhance both selectivity and maintain dispersion, thereby rendering the catalyst more suitable for real−world applications. In the current study, we detail the synthesis of carbon−coated Ni-Fe alloy catalysts via co-precipitation and hydrothermal methods. We meticulously characterized the NiFe alloy structure and probed the composition and dimensions of the carbon shell using XRD, Raman spectroscopy, and TEM analyses. Our investigation extends to the catalytic performance, particularly assessing the influence of Fe doping on the selectivity towards HCAL during cinnamaldehyde hydrogenation. Furthermore, we delved into the antioxidative capabilities of the carbon layer by comparing the stability of catalytic cycles between bare NiFe_0.7 nanoparticles and carbon−encased NiFe_0.7@C, elucidating the protective efficacy of carbon coatings on bimetallic catalysts.

2. Results and Discussion

2.1. Characterizations of the Catalysts

Four types of catalysts with various Fe/Ni molar ratios were prepared by hydrothermal reaction of glucose and NiFe materials to make glucose self−aggregate on the surface of the materials, and then carbonized at high temperature in Ar atmosphere. The obtained catalysts are denoted NiFe_x@C, where x = 0, 0.3, 0.7, and 1.1, respectively. The Fe/Ni molar ratios of the catalysts were determined by an Inductively Coupled Plasma−Optical Emission Spectrometer (ICP−OES), which matched the designated values (Table S1).

The XRD spectra were obtained to study the crystalline structure of these catalysts (Figure 1A), which reveal interesting insights. For the monometallic Ni@C, diffraction peaks at 2θ = 52.3°, 61.1°, and 91.9° correspond to the Ni (111), (200), and (220) planes (PDF#04−0850) [39,40,41]. With the addition of Fe in NiFe_x@C, a notable shift of the Ni (111) peak to a lower angle was observed. The peaks at 2θ = 50.3°, 58.7°, and 87.8° correspond to the (111), (200), and (220) planes of pure Fe (PDF#89−4185). The appearance of XRD peaks between pure Ni and pure Fe suggests the formation of a Ni−Fe alloy [42,43,44]. Moreover, the carbothermal reduction process under an argon atmosphere, utilizing glucose as both a carbon source and reducing agent, allows for the co−reduction of Fe and Ni metal ions, resulting in Ni−Fe alloy nanoparticles in a metallic state [45,46].

The crystallite sizes were calculated using Scherrer equation, as shown in Table S1, indicate that the addition of Fe increased the average crystal size from 10.8 nm to 36.2 nm. This increase is attributed to lattice expansion due to the substitution of larger Fe atoms for smaller Ni atoms [47,48].

The Raman scattering spectra were acquired to investigate the carbon layer structure of these nanocomposites. Figure 1B demonstrates the collected Raman signals of NiFe_x@C (x = 0, 0.3, 0.7, and 1.1). Characteristic peaks in the range of 800 cm⁻¹ to 2000 cm⁻¹ were observed. Specifically, the D band at 1345 cm⁻¹ indicates disordered carbon with structural defects, while the G band at 1590 cm⁻¹ corresponds to highly ordered graphitic carbon [49].

The intensity ratio I_D/I_G is commonly used to assess the degree of defects in the carbon matrix [50,51]. The calculated I_D/I_G values for NiFe_x@C (x = 0, 0.3, 0.7, and 1.1) were 1.06, 0.93, 0.97, and 0.99, respectively. These values suggest that the outer carbon shell of NiFe_x@C consists of defective graphene−like carbon layers [52]. This specific arrangement of the carbon matrix is pivotal in preventing the agglomeration of NiFe crystals during the growth phase of the alloys. It facilitates the formation of uniformly dispersed Ni−Fe nanoparticles, which is a key factor in enhancing their catalytic performance [53].

The morphology of the catalysts was studied by Transmission Electron Microscopy (TEM). The TEM images and corresponding particle size distribution of the samples are depicted in Figure 2 and Figures S1–S3. Notably, the bimetallic catalyst NiFe_0.7@C exhibited an average particle size of 29.2 nm (Figure 2A), with nanoparticles well−dispersed. In Figure 2B, the TEM image reveals a darker core representing the alloy that can be confirmed by the line scan results (Figure 2C), surrounded by a bright outer layer indicative of the carbon shell. This suggests that the NiFe_0.7@C particles consist of alloys encapsulated within a thin carbon layer with a thickness of approximately 2 to 3 nm.

Different morphology was observed with different chemical compositions of the catalysts. The average particle sizes of the monometallic Ni@C and bimetallic NiFe_0.3@C and NiFe_1.1@C were determined to be 7.3 nm, 17.3 nm, and 41.5 nm, respectively, as shown in Figures S1–S3. These observations suggest that the introduction of Fe, which possesses a larger atomic radius compared to Ni, promotes the growth of the alloy nanoparticles [47,48]. Additionally, the surfaces of Ni@C, NiFe_0.3@C, and NiFe_1.1@C were found to be coated with a carbon layer approximately 2 to 3 nm thick.

Besides the catalyst morphology, critical information regarding the element distribution was also revealed by TEM analysis. A line scan analysis (Figure 2C) was conducted to examine the elemental distribution within the NiFe_0.7@C catalyst. The results show a highly uniform distribution of nickel and iron, further substantiating the composition of the catalyst. These results further confirm the formation of carbon−coated Ni−Fe alloy catalysts. The findings from the TEM analysis align well with the results obtained from XRD and Raman spectroscopy, providing a consistent and comprehensive understanding of the catalyst structure.

To elucidate the chemical states of the metal atoms in NiFe_x@C (x = 0, 0.3, 0.7, and 1.1), X-ray Photoelectron Spectroscopy (XPS) was employed, with results displayed in Figure 3A,B. In Ni@C and NiFe_0.3@C (Figure 3A), Ni was predominantly present in the metallic state, as indicated by the characteristic peaks of Ni⁰ (2p_3/2: 852.9 eV and 2p_1/2: 870.3 eV). However, for the NiFe_0.7@C and NiFe_1.1@C catalysts, two distinct chemical states of Ni were observed: Ni⁰ (2p_3/2: 852.9 eV and 2p_1/2: 870.3 eV) and NiO (2p_3/2: 855.3 eV and 2p_1/2: 873.1 eV) [54,55,56]. The presence of metal oxides on the sample surface is likely attributed to exposure to air during the testing process.

In Figure 3B, the Fe 2p XPS spectrum of NiFe_x@C (x = 0.3, 0.7, and 1.1) reveals four peaks at 707.3 eV, 710.3 eV, 711.1 eV, and 713.4 eV, corresponding to Fe⁰, Fe²⁺, Fe³⁺, and Ni LMM, respectively [57,58]. Notably, the diffraction peak of FeO_X was absent in the XRD pattern, suggesting the possible amorphous structure of FeO_X in the bimetallic samples [59].

Taken together, the XRD and XPS results indicate that the NiFe_x@C catalysts predominantly exist in a metal alloy state. Furthermore, the presence of a carbon layer on the surface appears to effectively prevent deep oxidation of the metal components, underscoring the protective role of carbon encapsulation.

2.2. Catalytic Performance of the Catalysts

The hydrogenation performance of the catalysts in converting cinnamaldehyde is illustrated in Figure 4A. While the monometallic Ni@C catalyst exhibited excellent catalytic activity, its selectivity towards HCAL was notably limited. This can be attributed to the robust hydrogenation capability of Ni, which under the given reaction conditions, tends to produce a greater proportion of the deeper hydrogenation product, hydrocinnamyl alcohol (HCOL), thereby compromising selectivity [60].

Intriguingly, the introduction of the second metal, Fe, into the Ni@C catalyst resulted in a nuanced change in performance. Specifically, the conversion rate declined from 100% to 36.2% with increasing Fe content. This decrease in catalytic activity might be linked to the substitution of some active Ni sites by Fe, which possesses relatively inferior hydrogenation performance [61]. However, despite the reduced conversion rate, the addition of Fe consistently favored the formation of HCAL. This observation aligns with existing literature, which suggests that Fe modulates Ni-based catalysts, enhancing the hydrogenation of the C=C bond [29]. Notably, the NiFe_0.7@C catalyst demonstrated a substantial increase in HCAL selectivity, reaching approximately 88.6%.

These results underscore the delicate balance between catalytic activity and selectivity. While the monometallic Ni@C catalyst is highly active, its selectivity is suboptimal. Conversely, the bimetallic NiFe_0.7@C catalyst, though less active, offers significantly enhanced selectivity towards the desired product, HCAL.

2.3. Cycle Stability of the Catalyst

The long−term stability and reusability of catalysts are crucial for their practical application. Among the catalysts tested, NiFe_0.7@C displayed the best catalytic performance. Hence, we selected NiFe_0.7@C and NiFe_0.7 NPs (nanoparticles without a carbon layer) to evaluate their catalytic cycle results, as presented in Figure 4B. The NiFe_0.7 NPs were obtained by burning off the carbon layer from NiFe_0.7@C. Raman spectroscopy results (Figure S4) confirmed the effective removal of the carbon layer after burning in air and subsequent reduction, as evidenced by the absence of characteristic D−band and G−band peaks at 1340 cm⁻¹ and 1585 cm⁻¹. Post−reduction, the NiFe_0.7 NPs maintained their alloy structure, with no metal oxides detected in the XRD spectrum (Figure S5). The XPS results (Figure S6) revealed two chemical states for Ni and three for Fe in the NiFe_0.7 NPs, suggesting the presence of metal oxides on the surface, likely due to oxidation in the absence of carbon layer protection.

TEM imaging (Figure S7) of NiFe_0.7 NPs further corroborated the lack of a carbon layer, with line scans confirming the even distribution of metal species even after the carbon layer’s removal. This makes NiFe_0.7 NPs an appropriate contrast sample for testing cinnamaldehyde hydrogenation.

Both NiFe_0.7@C and NiFe_0.7 NPs exhibit strong magnetism, facilitating their easy separation from the reaction solution for catalytic cycle reactions. Figure 4B showcases the reusability of these catalysts. Notably, NiFe_0.7@C maintained its cinnamaldehyde conversion efficiency and HCAL selectivity over multiple cycles. In contrast, while NiFe_0.7 NPs showed slightly higher HCAL selectivity due to the absence of a carbon layer barrier, they suffered a significant decrease in cinnamaldehyde conversion.

Post−cycling tests revealed that the used NiFe_0.7@C catalyst still maintained good particle morphology and alloy structure, with a slight increase in crystal size (Figures S8 and S9). However, NiFe_0.7 NPs exhibited particle agglomeration and the formation of Ni_0.4Fe_2.6O₄ (Figures S5 and S10). XPS results indicated that the nickel in the used NiFe_0.7 NPs underwent significant oxidation (Figure S11), confirming that unprotected metal nanoparticles are prone to oxidation and deactivation during catalytic recycling [62,63].

In summary, these results demonstrate that the carbon layer plays a crucial role in preventing the oxidation of metal nanoparticles during reuse, thereby endowing the catalyst with superior catalytic cycle stability.

3. Experimental Section

3.1. Materials

Nickel acetate tetrahydrate (AR grade), ferrous chloride tetrahydrate (99.95%), anhydrous sodium carbonate (GR grade), glucose (AR grade), nonane (standard for GC), isopropyl alcohol (AR grade), cinnamaldehyde (≥95% (GC), cinnamyl alcohol (≥99% (GC)), hydrocinnamaldehyde (95%) and hydrocinnamyl alcohol (99%) were supplied by Aladdin Scientific Corp. Ethylene glycol (99%) and ethanol (99.5%) were supplied by Beijing InnoChem Science & Technology Co., Ltd. (Beijing, China).

3.2. Catalyst Preparation

Anhydrous sodium carbonate (6.0 g) was dissolved in 160 mL of ultrapure water. Ni(CH₃COO)₂·4H₂O (7.0 g) and FeCl₂·4H₂O (1.68 g) were dissolved in 100 mL of ethylene glycol at 160 °C under stirring. When the solid particles were completely dissolved, the aqueous sodium carbonate solution was added dropwise to the ethylene glycol solution at a rate of 3–5 s/drop. When all drops of the aqueous sodium carbonate solution were added, the suspension formed continued to be stirred at 160 °C for one hour. After cooling, it was washed and filtered three times with ultrapure water and ethanol and the resulting filter cake was divided into five equal mass portions. The filter cake was homogeneously dispersed in a polytetrafluoroethylene liner containing 40 mL of glucose (1.06 g) aqueous solution and subjected to a hydrothermal reaction at 175 °C for 18 h. After cooling to room temperature and filtering by washing three times with ultrapure water and ethanol, it was dried in an oven at 120 °C for 12 h. The dried product was calcined under argon at 600 °C (ramp rate 10 °C/min) for 4 h to obtain NiFe_0.3@C (0.3 is the molar ratio of Fe to Ni). Ni@C, NiFe_0.7@C and NiFe_1.1@C were prepared according to the same experimental method. NiFe_0.7 NPs were obtained by calcining the prepared NiFe_0.7@C in the air for 3 h to burn off the carbon layer encapsulated on the surface of the catalyst and reducing it for 2 h in a hydrogen atmosphere.

3.3. Catalyst Characterization

Inductively coupled plasma optical emission spectrometry (ICP−OES) analyses were conducted on a PerkinElmer Optima 2100DV instrument to acquire the composition of the catalysts.

Elemental composition and valence states near the catalyst surface were recorded using X-ray Photoelectron Spectroscopy (XPS) (ESCALAB 250Xi system, Thermo Scientific, Waltham, MA, USA) with Al Kα as the X-ray radiation source. The data were processed with Avantage software and calibrated concerning the C 1 s peak of adventitious carbon (284.8 eV).

The powder X-ray Diffraction (XRD) patterns of the catalysts were recorded using a Bruker AXS D8 Advance equipped with a cobalt−palladium (Kα−radiation source, λ = 1.79021 Å, Bruker, Saarbrücken, Germany). The XRD was operated at a voltage of 35 kV, a current of 40 mA, a scanning range of 25° to 95°, and a scanning rate of 0.2 s/step. And the particle size of the catalyst was calculated using the Scherrer equation. The Scherrer equation is as follows:

$$D_{hkl}=\frac{\mathrm{K}\mathsf{\lambda }}{B_{hkl}\cos \mathsf{\theta }}$$

where D_hkl is the crystallite size in the direction perpendicular to the lattice planes. (hkl) are the Miller indices of the planes being analyzed, i.e., the (111) plane in the present work. K is a numerical factor frequently referred to as the crystallite−shape factor, and a K factor of 0.89 was employed in the calculation. λ is the wavelength of the X-rays, B_hkl is the width (full width at half−maximum) of the X-ray diffraction peak (51~53°), corresponding to the (111) plane. θ is the Bragg angle.

Raman spectra were recorded at room temperature using a 532 nm laser excitation line on a LabRAM HR800 Raman microscope. The test power was 10 W and the spectral resolution of all measurements was 4 cm⁻¹.

A Talos 200A (FEI, Hillsboro, OR, USA)−type electron microscope was used to record Transmission Electron Microscopy (TEM) images. In addition, 1–2 mg of a sample was dispersed in 2 mL of ethanol, ultrasonically, for 30 min, using anhydrous ethanol as a solvent, then the suspension was dropped onto a porous carbon−covered copper mesh and dried under an infrared lamp to prepare the samples for TEM studies. The operating voltage was 200 kV, and both transmission (TEM) and scanning transmission (STEM) modes were used. STEM images were acquired using a high−angle annular dark field detector (HAADF) (JEOL-2100F microscope, Kyoto, Japan).

3.4. Catalyst Activity Evaluation

The liquid phase hydrogenation of cinnamaldehyde was carried out in a stainless steel high−pressure reactor. To the reactor, 20 mg of catalyst, 1 mmol of cinnamaldehyde, 0.1 mL of n−nonane (internal standard), and 5 mL of isopropanol (solvent) were added. The reactor was first purged three times with hydrogen gas (2 MPa), and then hydrogen gas (1 MPa) was introduced into the autoclave. The reaction was carried out at 100 °C for 3 h with stirring at 800 rpm. After the pressure was relieved from the autoclave at room temperature, the reaction products were separated by filtration and analyzed using Gas Chromatography (GC) (Agilent, Santa Clara, CA, USA). Notably, the NiFe_0.7 NPs were reduced by hydrogen prior to the reaction and were transferred and added to the reactants via a glove box to complete the reaction. The used catalyst was washed three times with isopropanol and dried at 60 °C for 12 h for the next catalytic reaction.

The conversion rate of CAL and the selectivity of the hydrogenation products were calculated using the following equation:

$$\mathrm{CAL}\mathrm{conversion}\left(\%\right)=\frac{\mathrm{moles}\mathrm{of}\mathrm{CAL}\mathrm{before}\mathrm{reaction}-\mathrm{moles}\mathrm{of}\mathrm{CAL}\mathrm{after}\mathrm{reaction}}{\mathrm{moles}\mathrm{of}\mathrm{CAL}\mathrm{before}\mathrm{reaction}}\times 100\%$$

$$\mathrm{Product}\mathrm{selectivity}\left(\%\right)=\frac{\mathrm{moles}\mathrm{of}\mathrm{product}}{\mathrm{moles}\mathrm{of}\mathrm{CAL}\mathrm{consumed}}\times 100\%$$

4. Conclusions

This study successfully synthesized carbon−coated Ni−Fe alloy catalysts (NiFe_x@C) with varying Fe contents and evaluated their performance in the hydrogenation of cinnamaldehyde. The NiFe_x@C catalysts were successfully synthesized with a uniform carbon layer of approximately 2–3 nm thickness, as confirmed by TEM, XRD, and Raman spectroscopy. The incorporation of Fe led to an increase in the average particle size of the bimetallic catalysts compared to the monometallic Ni@C. XPS analysis revealed that Ni existed in both metallic and oxide states in the bimetallic catalysts, while Fe presented multiple oxidation states. The carbon layer on the surface of the catalysts effectively prevented deep oxidation of the metal. The addition of Fe to Ni@C improved the selectivity towards hydrocinnamaldehyde (HCAL) while reducing the conversion rate of cinnamaldehyde. Among the synthesized catalysts, NiFe_0.7@C demonstrated the highest selectivity for HCAL (~88.6%). The carbon layer played a crucial role in the stability and reusability of the catalysts. NiFe_0.7@C maintained consistent catalytic performance and HCAL selectivity over multiple reaction cycles. In contrast, NiFe_0.7 NPs (without a carbon layer) showed significant deactivation and oxidation over repeated cycles. Both NiFe_0.7@C and NiFe_0.7 NPs exhibited strong magnetism, allowing for easy separation from the reaction mixture, which is advantageous for practical applications.

In conclusion, the carbon−coated Ni−Fe alloy catalysts demonstrated promising potential for selective hydrogenation reactions. The carbon layer not only enhanced the selectivity towards the desired products but also significantly improved the stability and reusability of the catalysts. This study provides valuable insights into the design of bimetallic catalysts with enhanced performance for hydrogenation processes.