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• Ethylene is one of the fundamental raw materials and serves as the precursor for chemical intermediates such as polyethylene, ethylbenzene, ethylene oxide, and dichloroethane [1]. Generally, ethylene is obtained through the cracking of naphtha, which typically co-produces a trace amount of acetylene (0.5% to 3.0%) [2]. Acetylene can react with the Ziegler-Natta catalyst in the subsequent polymerization process, which is the primary use of ethylene, leading to catalyst deactivation and product degradation [3]. The acetylene content in ethylene feed should be reduced to below 5 ppm prior to polymerization [4,5]. Typically, the trace acetylene is removed from ethylene through selective hydrogenation using supported metal catalysts in the industry [6–9]. Acetylene hydrogenation catalysts are required to efficiently remove acetylene in the presence of excess ethylene. Therefore, the catalyst must exhibit both high activity and high selectivity to prevent the over-hydrogenation of ethylene, which could lead to unnecessary ethylene consumption and potentially reaction runaway due to this highly exothermic process. Additionally, the formation of carbon deposition, which could result in catalyst deactivation, should be avoided during the reaction.

  The catalytic performance in acetylene hydrogenation is primarily determined by the catalyst structure. Under reaction conditions, light elements can incorporate into the metal interstitial sites and form various subsurface structures. These distinct interstitial atoms can influence the structure of active sites or directly participate in the reactions, thereby affecting the reaction mechanism and catalytic performance [10–14]. Subsurface interstitial hydrogen structures can be generated and can affect the catalysis during hydrogenation reactions. For instance, Maeland et al. identified two types of Pd hydrides with face-centered cubic structures: α-PdH_x and β-PdH_x, under low and high hydrogen content atmospheres, respectively [15]. Subsequent studies revealed that the β-PdH_x structure exhibited higher catalytic activity but lower selectivity compared to α-PdH_x in acetylene hydrogenation reactions [16,17]. It was found that carbon atom can also be introduced into the subsurface interstitial sites and modulate the catalytic performance of Pd during catalysis [16]. Teschner et al. demonstrated that subsurface carbon can inhibit the hydrogen atom introduction and enhance the selectivity during alkyne hydrogenation reactions [18]. Theoretical calculations have shown that subsurface carbon atoms can influence the adsorption strength of acetylene and ethylene on the Pd surface, thereby improving ethylene selectivity [19].

  Based on the demand for high activity and selectivity in acetylene hydrogenation reaction, supported palladium-based catalysts were generally adopted in industry. However, due to the limited reserves and high cost of palladium, replacing it with non-precious metals has drawn much attention. The non-precious metal nickel is highly active and has widely served as hydrogenation catalysts [20–23]. However, Ni catalysts often result in the over-hydrogenation of ethylene and significant deactivation in the acetylene hydrogenation reaction. This is attributed to the strong bonding with intermediates and the formation of subsurface hydrogen structures during catalysis [24–26]. Therefore, modulating the surface/subsurface structure of Ni has been investigated to improve the catalytic performance in these reactions [27,28]. Ni shares a similar electronic structure and exhibits dynamic subsurface behavior comparable to Pd under reaction conditions [18,29]. Controlling subsurface atoms can serve as an effective approach to improving its selectivity. However, introducing carbon into the interstitial sites of Ni is quite challenging and can lead to phase transitions, surface encapsulation, and even carbon layer growth. In our previous studies, we addressed this issue by introducing Zn to form the Ni₃Zn alloy, thereby expanding its octahedral interstitial site and successfully incorporating carbon atoms under an acetylene atmosphere at low temperatures. The obtained Ni₃ZnC_0.7 catalyst exhibits superior selectivity and stability in selective hydrogenation reactions [30,31].

  Herein, our research seeks to further understand and enhance the selectivity of Ni-based catalysts for acetylene hydrogenation by manipulating the subsurface structure. Through alloying nickel with gallium, we prepared the supported Ni₃Ga structure, which facilitates the incorporation of carbon atoms into its octahedral interstitial sites, resulting in the formation of Ni₃GaC_0.5 catalysts. The successful alloying and carbon introduction were confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The catalytic performance of the obtained Ni-based catalysts was further evaluated under various hydrogenation conditions, focusing on their selectivity and stability. Moreover, the changes in the nickel electronic structure after carbon incorporation were investigated, aiming to establish a correlation between structural evolution and catalytic performance.

  The supported alloy and interstitial carbide catalysts were prepared through an impregnation method followed by hydrogen reduction and acetylene treatment. XRD was employed to identify the phase structure of Ni₃Ga and Ni₃GaC_0.5. As shown in Fig. 1 and Fig. S1 (Supporting information), a prominent diffraction peak at 51.7° was observed in Ni₃Ga/oCNT, Ni₃Ga/Al₂O₃, and Ni₃Ga/SiN catalysts, corresponding to the (200) crystal plane of the Ni₃Ga intermetallic compound with a face-centered cubic (FCC) structure. The XRD results confirm the successful fabrication of the Ni₃Ga alloy structure through impregnation and reduction on different supports. After the acetylene treatment, the obtained catalysts demonstrated a slight shift of the diffraction peaks relative to the Ni₃Ga phase, specifically observed at 43.5°, 50.7°, and 74.6°. These peaks, corresponding to the (111), (200), and (220) crystal planes of Ni₃GaC_0.5, suggest an expanded lattice spacing. These results indicate the successful incorporation of carbon atoms into the octahedral interstitial sites of Ni₃Ga following acetylene treatment. It is worth noting that the same method was successfully employed to prepare Ni₃GaC_0.5 catalysts with interstitial carbide structure on different supports.

  Figure 1

  

  Figure 1.  XRD patterns of Ni₃Ga and Ni₃GaC_0.5 supported on (a) oCNT and (b) Al₂O₃, respectively.

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  Subsequently, the microstructure of the obtained catalysts was systematically identified using TEM techniques, which are a powerful tool for characterizing the structure of supported catalysts [32,33]. Figs. 2a and b display the typical TEM images of the Ni₃Ga/oCNT and Ni₃GaC_0.5/oCNT catalysts, accompanied by histograms that illustrate the particle size distribution (PSD). Both Ni₃Ga and Ni₃GaC_0.5 nanoparticles (NPs) exhibit a uniform distribution on the oCNT support. The PSD analysis indicated a size range of 1.0–11.0 nm for the Ni₃Ga NPs, with an average size of 5.4 ± 1.7 nm. After acetylene treatment, the Ni₃Ga NPs underwent a structural transformation; carbon atoms occupied the octahedral interstitial sites within the Ni₃Ga alloy, resulting in the formation of the Ni₃GaC_0.5 structure. This process leads to a significant increase in particle size, ranging from 2.0 nm to 15.0 nm with an average of 8.0 ± 2.0 nm. Moreover, a comprehensive microstructural analysis of both catalysts was conducted using high-resolution (HR) TEM. As shown in Fig. 2c and Fig. S2 (Supporting information), HRTEM results reveal that the lattice spacings of the NP are 0.204 nm and 0.177 nm, with an angle of 54.7°. These values correspond to the (111) and (200) crystal planes of Ni₃Ga, respectively. After the catalyst was treated with an acetylene atmosphere to introduce carbon atoms, lattices with the same angle of 54.7° was observed in the HRTEM image, showing increased spacings of 0.208 nm and 0.180 nm (Fig. 2d and Fig. S2), corresponding to the (111) and (200) crystal planes of Ni₃GaC_0.5, respectively. These results, consistent with the crystal structure of Ni₃Ga and Ni₃GaC_0.5, confirm that carbon atoms were introduced into the Ni₃Ga interstitial sites through acetylene treatment.

  Figure 2

  

  Figure 2.  TEM and HRTEM images of (a, c) Ni₃Ga/oCNT and (b, d) Ni₃GaC_0.5/oCNT. The top-right insets are the corresponding PSD histograms and FFTs. HAADF-STEM images of (e) Ni₃Ga/oCNT and (f) Ni₃GaC_0.5/oCNT with the corresponding EDX elemental maps of C (yellow), Ni (green), and Ga (blue).

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  Further characterization was carried out using energy-dispersive X-ray spectroscopy (EDX) elemental mapping to ascertain the composition and distribution of elements within the catalysts. Fig. 2e and Fig. S3 (Supporting information) display the high-angle annular dark-field scanning TEM (HAADF-STEM) images of the Ni₃Ga/oCNT catalyst, where the K-edge signals of Ni and Ga are homogeneously distributed on the oCNT support, and no carbon signals are detected within the NPs, indicating a bimetallic alloy structure. Following acetylene treatment, as shown in Fig. 2f and Fig. S4 (Supporting information), the Ni₃GaC_0.5/oCNT catalyst exhibits the presence of Ni, Ga, and C within the NPs, demonstrating a homogeneous distribution of all three elements and the successful formation of the interstitial carbide structure.

  The catalytic performance of these catalysts in the selective hydrogenation of acetylene was subsequently investigated. Fig. S5 (Supporting information) displays the performance of the Ni₃GaC_0.5/oCNT and Ni₃Ga/oCNT catalysts, highlighting their acetylene conversion and ethylene selectivity across a temperature range of 50–180 ℃. The Ni₃GaC_0.5/oCNT catalyst displayed an enhancement in acetylene conversion from 3% to 100% as the temperature was raised, while the ethylene selectivity impressively remained above 70% throughout this range. In contrast, the Ni₃Ga/oCNT catalyst demonstrated a sharp decrease in ethylene selectivity, falling to approximately −220%, as the acetylene conversion increased. The negative value of ethylene selectivity is due to the simultaneous hydrogenation of the large amount of ethylene to ethane in the feed gas.

  The activity and selectivity of the catalysts under various hydrogen to acetylene ratios are depicted in Figs. 3a and b. The results demonstrated that the activity of both catalysts increased as the hydrogen ratio increased. The Ni₃GaC_0.5/oCNT catalyst exhibited high selectivity across a wide range of H₂/C₂H₂ ratios, indicating its stability under fluctuating hydrogen conditions. Conversely, the Ni₃Ga/oCNT catalyst displayed a significant decrease in ethylene selectivity with increasing hydrogen concentration, accompanied by a rapid increase in selectivity towards the over-hydrogenation product ethane. The results indicate that variations in H₂ concentration significantly influence the hydrogenation reaction on the surface of bimetallic alloy catalysts.

  Figure 3

  

  Figure 3.  Conversion and selectivity of (a) Ni₃GaC_0.5/oCNT and (b) Ni₃Ga/oCNT catalysts under various H₂ to C₂H₂ ratios in acetylene hydrogenation reaction. Stability test of (c) Ni₃GaC_0.5/oCNT and (d) Ni₃Ga/oCNT, respectively. Reaction conditions: 0.5 vol% C₂H₂, 20.0 vol% C₂H₄, 1.5–5.0 vol% H₂, helium as balance.

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  Furthermore, the Ni₃GaC_0.5/oCNT catalyst illustrates the remarkable ethylene selectivity, reaching up to 82% with nearly complete acetylene conversion (Fig. S6 in Supporting informaiton). The catalyst also showed selectivity of 7% for the full hydrogenation product ethane and 11% for the polymerization products C₄. On the other hand, the Ni₃Ga/oCNT catalyst exhibited extremely poor ethylene selectivity, reflected by a value of −497% at 92% acetylene conversion, primarily due to the over-hydrogenation of ethylene to ethane, which resulted in an ethane selectivity as high as 563%. Additionally, the interstitial carbide catalyst displayed approximately 23% lower selectivity for C₄ compared to the bimetallic alloy catalyst, indicating an inhibition of the reaction pathway for polymerization products. The long-term stability test of the Ni₃GaC_0.5/oCNT catalyst, as illustrated in Fig. 3c, underscored its exceptional stability with acetylene conversion (98%) and ethylene selectivity (83%) remaining virtually unchanged over a 23-h test period, indicating no significant deactivation. In contrast, the Ni₃Ga/oCNT catalyst, maintained consistent acetylene conversion but showed an increase in ethylene selectivity from −154% to approximately −30% (Fig. 3d). This suggests a potential decrease in the conversion rate of feed ethylene, hinting at gradual catalyst deactivation over time. Such improved stability of the Ni₃GaC_0.5/oCNT catalyst is attributed to the interstitial carbon structure, which inhibits the formation of carbonaceous deposits on the catalyst surface, in accordance with previous studies [30].

  The electronic structure of the catalytic active sites is crucial for the selectivity and efficiency of hydrogenation reactions, as it governs the adsorption thermodynamics and kinetic barriers for reactants and products [34]. X-ray photoelectron spectroscopy (XPS) was employed to probe the electronic structure of Ni₃Ga/oCNT and Ni₃GaC_0.5/oCNT catalysts (Fig. 4). The high-resolution XPS spectra (Fig. 4b) demonstrate Ni²⁺ and Ni⁰ states with binding energies for Ni²⁺ 2p_1/2 and Ni²⁺ 2p_3/2 at 873.1 and 855.4 eV, respectively, alongside Ni⁰ 2p_1/2 and Ni⁰ 2p_3/2 peaks at 869.8 and 852.7 eV. The satellite peaks associated with Ni, manifested in the green region of the spectrum, suggest a significant oxidation of Ni within the alloy under the ex-situ characterizations, primarily leading to the observation of Ni²⁺ species [35].

  Figure 4

  

  Figure 4.  (a) XPS survey and (b) Ni 2p spectra of Ni₃Ga/oCNT and Ni₃GaC_0.5/oCNT catalyst, respectively.

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  In the case of the Ni₃GaC_0.5/oCNT catalyst, the binding energies for Ni⁰ 2p_1/2 and Ni⁰ 2p_3/2 are observed at 870.2 and 853.1 eV, respectively. These values represent a notable shift by approximately 0.4 eV to higher binding energies relative to the Ni₃Ga/oCNT catalyst. Such a shift can be rationalized by the increased electronegativity of carbon (2.5) in comparison to nickel (1.9), which suggests the formation of an interstitial carbide that alters the electronic structure of Ni, leading to the electron deficiency of Ni. In addition, a marginal elevation in the binding energy of the Ga⁰ 2p_3/2 peak for the Ni₃GaC_0.5 catalyst, as detailed in Fig. S7 (Supporting information), provides further evidence of charge transfer between Ga and C atoms, indicative of the formed interstitial carbide structure.

  Indeed, the XPS results coupled with the electronic structure analysis suggest that the electron transfer from Ni active sites to the interstitial carbons is the underlying reason for the observed performance difference between the Ni₃Ga/oCNT and Ni₃GaC_0.5/oCNT catalysts. This electron transfer leads to a more electron-deficient state of Ni, which weakens the adsorption strength of reaction intermediates on the catalyst surface, thus enhancing the selectivity [36]. Furthermore, it has been shown that subsurface hydrogen in Ni-based catalysts is more reactive compared to surface hydrogen during selective hydrogenation processes. The formation of interstitial carbide structures can effectively block the formation of subsurface hydrogen and its participation in the reaction, preventing over-hydrogenation [18].

  In this study, the Ni₃Ga and Ni₃GaC_0.5 catalysts on different supports were successfully prepared using the impregnation method. TEM analysis confirmed the uniform distribution of single-crystal NPs on the oCNT support for both catalysts. EDX elemental mapping further validated the homogeneous distribution of elements within the catalysts. The XPS results showed that the incorporation of interstitial carbon atoms into the Ni₃GaC_0.5 catalysts imparts a distinctive electronic structure to the Ni active site, which is conducive to the desorption of intermediates in selective hydrogenation reactions. The formation of subsurface interstitial carbon and the resultant electron-deficient state of Ni are crucial for improving the performance of the acetylene selective hydrogenation reaction, leading to enhanced ethylene selectivity and stability. The findings of this study offer valuable insights into the structure-performance relationship of transition metal catalysts with interstitial modifications, highlighting the importance of the electronic structure modulation of the Ni active site by the presence of carbon atoms in the Ni₃GaC_0.5/oCNT catalyst in achieving efficient selective hydrogenation of acetylene. These insights contribute to the design of novel non-precious metal catalysts with superior catalytic properties.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Nos. 22002173, 22072164, 52161145403), Natural Science Foundation of Liaoning Province (No. 2022-MS-004), China Postdoctoral Science Foundation (No. 2020M680999), LiaoNing Revitalization Talents Program (No. XLYC1807175), and the foundations of Shenyang National Laboratory for Materials Science.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109525.

  
  
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• 

  Figure 1  XRD patterns of Ni₃Ga and Ni₃GaC_0.5 supported on (a) oCNT and (b) Al₂O₃, respectively.

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  Figure 2  TEM and HRTEM images of (a, c) Ni₃Ga/oCNT and (b, d) Ni₃GaC_0.5/oCNT. The top-right insets are the corresponding PSD histograms and FFTs. HAADF-STEM images of (e) Ni₃Ga/oCNT and (f) Ni₃GaC_0.5/oCNT with the corresponding EDX elemental maps of C (yellow), Ni (green), and Ga (blue).

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  Figure 3  Conversion and selectivity of (a) Ni₃GaC_0.5/oCNT and (b) Ni₃Ga/oCNT catalysts under various H₂ to C₂H₂ ratios in acetylene hydrogenation reaction. Stability test of (c) Ni₃GaC_0.5/oCNT and (d) Ni₃Ga/oCNT, respectively. Reaction conditions: 0.5 vol% C₂H₂, 20.0 vol% C₂H₄, 1.5–5.0 vol% H₂, helium as balance.

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  Figure 4  (a) XPS survey and (b) Ni 2p spectra of Ni₃Ga/oCNT and Ni₃GaC_0.5/oCNT catalyst, respectively.

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