The ethylene used in the production of polymers is obtained by the cracking of naphtha and typically contains on the order of 1% acetylene [1]. The presence of acetylene is poisonous to the catalyst in ethylene polymerization [2, 3]. The selective hydrogenation of acetylene to ethylene is the most widely used method by industry to remove acetylene from ethylene feed gas [4]. Thus, catalysts for selective hydrogenation of acetylene should have both high activity for acetylene conversion and high selectivity for ethylene [5, 6]. Pd-based catalysts are typically used to eliminate the traces of acetylene from ethylene [7-10]. However, they show poor selectivity for ethylene because the over-hydrogenation of acetylene and ethylene leads to production of oligomers ('green oil') and wastes ethylene in the feed gas [2, 3, 11].

Since the pioneering work of Haruta et al. [12] in 1987, supported Au nanoparticles have been found to exhibit promising catalytic performance in a variety of reactions, including the water-gas shift reaction [13], selective oxidation of alcohols [14], and selective hydrogenation of nitroarenes [15, 16]. Au catalysts have proven to be highly selective in the hydrogenation of acetylene to ethylene; however, the conversion of acetylene is very low [17-20]. One of the most promising approaches to improve the conversion of acetylene over Au catalysts is to synthesize Au-based bimetallic catalysts; for example, Au-Pd [21], Au-Fe [22], Au-Ag [23], and Au-Cu [24]. Among these catalysts, the Au-Pd bimetallic system is the most popular. The catalytic conversion of acetylene can be efficiently improved by the addition of Pd compared with that of the monometallic Au catalyst. Au-Pd core-shell nanoparticles have been constructed by various methods to tune the selectivity for ethylene in acetylene hydrogenation [25, 26]. However, it has been proved that the contiguous Pd can lead to the over-hydrogenation of acetylene and/or ethylene [27]. In our previous work [28], we tried to isolate Pd atoms in Au-Pd bimetallic catalysts by synthesizing Au-Pd alloy via a two-step method by tuning the ratio of Pd to Au. The alloyed Pd single-atom catalyst showed both high conversion and selectivity in acetylene hydrogenation with excess ethylene. We proposed that the alloyed Pd single atoms drove the conversion of acetylene, while Au played an important role in preventing the over-hydrogenation of acetylene.

Ni is from the same group as Pd in the periodic table. Supported Ni catalysts are important in hydrogenation reactions [29, 30]. Thus, adding Ni to Au might improve its catalytic performance in hydrogenation reactions. Although Ni does not form alloys with Au as easily as Pd does, Au-Ni bimetallic catalysts have been reported to show synergistic effects in various reactions, such as the hydrogenolysis of benzylic alcohols [31], the water-gas shift reaction [32], CO oxidation [32], chemoselective hydrogenation of nitroarenes [15], ethanol conversion into linear primary alcohols [33], and selective hydrogenation of 1, 3-butadiene [34].

In this work, we synthesize a series of Au-Ni bimetallic nanoparticles supported on silica (SiO₂) with different atomic ratios of Ni to Au to investigate their synergistic effect in acetylene hydrogenation. The catalytic performance of the Au-Ni bimetallic catalysts in acetylene hydrogenation is investigated. The energy-dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) are used to characterize the interaction between Au and Ni in the bimetallic catalysts. This work could provide new insights into the intrinsic origin of the high catalytic performance of Au-Ni bimetallic catalysts and guide the design of new catalysts for hydrogenation reactions.

We prepared the catalysts by a two-step method previously developed by our group [15]. In this method, commercial SiO₂ (Qingdao Ocean Chemical Plant, A_BET = 463 m²/g) was first functionalized with 3-aminopropyltriethoxysilane (APTES; Acros Organics, 99%) by heating under reflux in alcohol at 80 ℃ for 24 h. After suction filtration, washing with ethanol, and drying at 60 ℃ for 1 d, APTES-functionalized SiO₂ was obtained (denoted as SiO₂-APTES). In the first step, SiO₂-APTES (1.0 g) was dispersed in HAuCl₄ solution (20 mL, 2.0 g_Au/L) and stirred for 30 min. After being filtered and washed with deionized water, the recovered solid was dispersed in deionized water (10 mL) and then NaBH₄ solution (10 mL, 0.2 mol/L) was added dropwise to reduce Au³⁺. After stirring for 15 min, suction filtration, and washing with deionized water, the Au-loaded SiO₂ support was obtained. In the second step, the Au-loaded SiO₂ support was dispersed in Ni(NO₃)₂ solution (10 mL) with a certain concentration and stirred for 30 min. Then, tert-butylamine borane solution (10 mL, 0.1 mol/L) was added dropwise to reduce Ni²⁺ under continuous stirring for 30 min. After suction filtration, washing with deionized water, drying at 60 ℃ for 1 d, and calcination at 500 ℃ for 3 h, the AuNi_x/SiO₂ catalysts were obtained (x refers to the atomic ratio of Ni to Au). According to the corresponding step of the above method, monometallic Au/SiO₂ and Ni_0.5/SiO₂ (Ni_0.5/SiO₂ has the same Ni loading as AuNi_0.5/SiO₂) catalysts were prepared after being calcined at 500 ℃ for 3 h. The catalysts (e.g., AuNi₁/SiO₂) reduced at a certain temperature (e.g., 450 ℃) under H₂/He flow are denoted by the suffix "-H₂-temperature" (e.g., AuNi₁/SiO₂-H₂-450).

The performance of the as-prepared catalysts in the selective hydrogenation of acetylene was tested using a fixed-bed reactor. All the gases used in the reaction were controlled by mass flow controllers. Before the reaction, catalyst (30 mg) was loaded into the reactor and reduced in an H₂/He (20 mL/min; volume ratio of 4:1; H₂: UHP, 99.999%; He: UHP, 99.999%) gas stream at a certain temperature for 70 min. After cooling to room temperature in He (20 mL/min), feed gas consisting of C₂H₂/H₂/He (30 mL/min; volume ratio of 1:20:79) stabilized at room temperature for more than 50 min was introduced into the reactor. As the temperature was increased from 100 to 300 ℃ at a heating rate of 8 ℃/min, the catalyst's activity was tested every 40 ℃ and each reaction temperature was kept constant for 25 min before being increased to the next temperature. The analysis of the outlet gas components was performed by an online gas chromatograph (GC; Agilent Technologies 6890N) equipped with a flame ionization detector (FID).

In this study, the C₂ products detected by GC/FID were only C₂H₄ and C₂H₆. Therefore, the acetylene conversion and ethylene selectivity were calculated as follows:

(1)

(2)

The metal loading of the as-prepared catalysts was measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES; Thermo IRIS Intrepid Ⅱ XSP, Thermo Electron Co.). X-ray diffraction (XRD) patterns were obtained on an X-ray diffractometer (PW3040/60, X'pert Pro Super, PANalytical) equipped with a Cu K_α radiation source (λ = 0.15432 nm) operating at 40 kV and 40 mA. The data were collected in continuous mode in the 2θ range from 10 ° to 80 ° with a scan rate of 10 °/min.

Transmission electron microscopy (TEM), HRTEM, and EDS measurements were conducted with an electron microscope (JEM-2100F, JEOL) operating at 200 kV. In situ DRIFTS data were recorded by an infrared spectrometer (Vertex 70, Bruker) with a mercury-cadmium-telluride detector and operated at room temperature and atmospheric pressure. The spectra were obtained by collecting 64 scans with 4 cm^-1 resolution. Before CO adsorption, each as-prepared sample was loaded into the DRIFTS cell (HC-500, Pike technologies) and reduced in an H₂/He gas stream (20 mL/min; volume ratio of 4:1) at 450 ℃ for 30 min, followed by purging with pure He (20 mL/min) for 10 min and then cooling to room temperature. After a background spectrum was collected for the sample, a CO/He gas stream (20 mL/min; volume ratio of 1:99) was introduced into the DRIFTS cell for about 30 min, followed by purging with pure He (20 mL/min) for 30 min. Spectra were collected during this process.

The reduction pretreatment of the catalyst could influence the states of Au and Ni and thus the interaction between them. To investigate whether a synergistic effect exists between Au and Ni, we first measured the catalytic performance of AuNi₁/SiO₂ reduced at different temperatures. The acetylene conversion and ethylene selectivity at reaction temperatures from 100 to 300 ℃ are given in Fig. 1. With increasing reduction temperature of the catalyst, its activity in acetylene hydrogenation rises considerably (Fig. 1(a)). Meanwhile, the selectivity for ethylene slightly improved as the reduction temperature rose from 300 to 450 ℃ and then declined dramatically when the reduction temperature exceeded 450 ℃ (Fig. 1(b)). For the catalyst reduced at 550 ℃, the conversion of acetylene was 100% over the whole reaction temperature range but its highest selectivity was only 18%.

The marked change of the catalytic performance of AuNi₁/SiO₂ with reduction temperature may be attributed to the reduction temperature influencing the interaction between Au and Ni in the catalyst. With increasing reduction temperature, more Au-Ni alloy will form, which might promote acetylene conversion. Therefore, the activities of the catalysts are correlated with their content of Au-Ni alloy.

In addition to the reduction temperature, the loading of Ni is another factor that strongly affects the composition of the Au-Ni alloy nanoparticles. Therefore, the effect of the Ni:Au atomic ratio on catalytic performance was studied. According to Fig. 1, we found that 450 ℃ was the optimal reduction temperature for relatively high acetylene conversion and the highest ethylene selectivity. Therefore, we reduced all the catalysts with different Ni:Au atomic ratios at 450 ℃. The acetylene conversion and ethylene selectivity of AuNi_x/SiO₂-H₂-450 with different Ni:Au atomic ratios are given in Fig. 2. The monometallic Au/SiO₂-H₂-450 catalyst exhibited the highest selectivity for ethylene (~90%) over the whole reaction temperature range, while its conversion of acetylene increased with temperature and the highest conversion was only ~30%. For Ni_0.5/SiO₂-H₂-450, which had similar Ni loading to that of AuNi_0.5/SiO₂-H₂-450, almost no acetylene was converted until 220 ℃ and the highest conversion was 5% with selectivity below 40%. The addition of Ni to the monometallic Au catalyst greatly improved its catalytic performance. The conversion of acetylene by each AuNi_x/SiO₂-H₂-450 catalyst was higher than the sum of the conversions by the Au/SiO₂-H₂-450 and Ni_0.5/SiO₂-H₂-450 catalysts. As the Ni:Au atomic ratio increased from 0.125 to 0.5, there was an obvious improvement of catalytic activity. The catalyst with a Ni:Au atomic ratio of 0.5 exhibited the highest acetylene conversion and ethylene selectivity of the catalysts. When the Ni:Au atomic ratio was increased to 1, the catalyst displayed similar catalytic performance to that of AuNi_0.5/SiO₂-H₂-450. Compared with those reported in the literature for monometallic Au catalysts [18, 19, 28, 35], the activities of the optimized Au-Ni bimetallic catalysts were higher. The ethylene selectivity of the bimetallic catalysts was lower compared with that of Au catalysts and higher compared with those of monometallic Pd and Ni catalysts [2, 3, 11, 36, 37]. These results imply that the enhancement of activity caused by adding Ni can be ascribed to a synergistic effect between Au and Ni in the hydrogenation of acetylene.

We determined the actual metal loadings of all the as-prepared catalysts by ICP-AES; the results are shown in Table 1. The actual loadings of Ni and Au were close to the theoretical values. By using tert-butylamine borane as the reduction agent, Ni can be successfully loaded into the bimetallic catalysts. The actual atomic ratios of Ni to Au were slightly higher than the corresponding nominal values.

The XRD patterns of AuNi_x/SiO₂ (x = 0, 0.25, 0.5, 1) and Ni_0.5/SiO₂ after reduction at 450 ℃ are depicted in Fig. 3. All the AuNi_x/SiO₂-H₂-450 catalysts displayed four broad peaks at 2θ = 38.2°, 44.4°, 64.6°, and 77.5°, which corresponded to the reflections of the (111), (200), (220), and (311) lattice planes of metallic Au, respectively. The broad peaks indicate the small size of the AuNi_x/SiO₂-H₂-450 catalyst particles. According to the Scherrer equation, the estimated average particle size of Au/SiO₂ is about 2.7 nm. With increasing Ni loading, the peaks of Au remained broad, and there was no marked variation of particle size. The estimated average crystallite sizes of Au in the Au/SiO₂-H₂-450, AuNi_0.25/SiO₂-H₂-450, AuNi_0.5/SiO₂-H₂-450, and AuNi₁/SiO₂-H₂-450 catalysts are 2.7, 2.8, 2.7, and 3.0 nm, respectively. Meanwhile, there was no obvious peak of metallic Ni observed from the XRD patterns of the AuNi_x/SiO₂-H₂-450 and Ni_0.5/SiO₂-H₂-450 catalysts. The absence of a Ni phase in the XRD patterns suggests that Ni is highly dispersed in the catalysts or the content of Ni is below the detection limit. We also did not identify any peaks consistent with the Au-Ni alloy phase. This result implies that the content of Au-Ni alloy was too low to be detected by XRD or no Au-Ni alloy formed.

The detailed particle size distribution of AuNi_x/SiO₂-H₂-450 (x = 0, 0.25, 0.5, 1) was further characterized by TEM, as illustrated in Fig. 4. The metal particles in all catalysts were uniformly dispersed on the support. No very big particles were observed in arbitrarily selected regions of each catalyst. We calculated the average particle sizes of all the catalysts by arbitrarily selecting more than 200 particles from different areas. The histograms in Fig. 4 reveal that the average particle sizes of Au, AuNi_0.25, AuNi_0.5, and AuNi₁ are 2.8, 2.9, 3.1, and 3.1 nm, respectively, which is in agreement with the XRD results. The Ni_0.5/SiO₂-H₂-450 catalyst was difficult to characterize clearly by TEM, so a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of this sample was captured (Fig. 4(e)). As shown in Fig. 4(e), the Ni species were highly dispersed on the silica support in Ni_0.5/SiO₂-H₂-450, with a particle size of 2.4 nm.

To observe the composition of the metal nanoparticles in the AuNi_x/SiO₂-H₂-450 catalysts, EDS was used to characterize the individual particles in the AuNi_0.25/SiO₂-H₂-450 and AuNi_0.5/SiO₂-H₂-450 catalysts. As shown in Fig. 5(a) and (b), from the HAADF-STEM images, several randomly selected individual particles were analyzed by EDS. The results revealed that the particles of the two catalysts contained both Au and Ni, although their chemical composition was different. The metal particles containing both Au and Ni suggest the formation of Au-Ni alloy was possible under the catalyst preparation conditions.

To further understand the geometric structure of the Au-Ni bimetallic particles, we used HRTEM to characterize the lattice fringes of individual particles of the AuNi_0.25/SiO₂-H₂-450 and AuNi_0.5/SiO₂-H₂-450 catalysts. Both catalysts consisted of metal particles with a lattice spacing between those of Au(111) (0.235 nm) and Ni(111) (0.202 nm), as shown in Fig. 5(c) and (d). Therefore, the Au-Ni bimetallic catalysts contained Au-Ni alloy.

Although it is not easy for bulk Au and Ni to form an alloy, Labat et al. [38] reported that the segregation of Au to the surface results in the interfacial mixing of Au and Ni to decrease the elastic stress of Ni during its growth process in Au@Ni core-shell nanocrystals. Recently, Tsuji and colleagues [39] investigated the epitaxial growth of Ni shells on polyhedral Au cores. When the particle size decreased to the nanometer level, the formation of Au-Ni alloy became possible. Zhou et al. [40] synthesized Au-Ni alloy in oleylamine solution using trioctylphosphine and butyllithium as protecting and reducing agents, respectively. Meanwhile, Liu's group developed a general two-step method to synthesize Au-Cu [41], Au-Ag [23] and Au-Pd [42, 43] alloy nanoparticles dispersed on SiO₂ and/or alumina supports using NaBH₄ as the reducing agent. Soon after, Wei et al. [15] proved that NaBH₄ was not appropriate for the formation of Au-Ni alloy nanoparticles. However, when tert-butylamine borane was used as the reducing agent, highly dispersed Au-Ni alloy nanoparticles were successfully synthesized on an SiO₂ support. Their extended X-ray absorption fine structure results demonstrated that a substantial fraction of Ni was alloyed with Au in their AuNi₃/SiO₂ catalyst. In this work, all of the AuNi_x/SiO₂ bimetallic catalysts were prepared by the same two-step method and tert-butylamine borane was used as a relatively weak reducing agent for Ni²⁺ in the second step. Based on the noble metal-induced reduction (NMIR) mechanism [44], Au particles will be able to induce the preferential deposition of Ni on their surfaces. This allows Au and Ni to interact intimately and thus, Au-Ni alloy could form on the AuNi_x/SiO₂-H₂-450 catalysts, as indicated by the EDS and HRTEM results.

To obtain information about the interaction between Au and Ni on the surface of the catalysts, we employed in situ DRIFTS using CO as the probe molecule to study the surface properties of the AuNi_x/SiO₂-H₂-450 (x = 0, 0.25, 0.5, 1) and Ni_0.5/SiO₂-H₂-450 catalysts. The variation of in situ DRIFTS results for the catalysts in CO atmosphere is shown in Fig. 6. As illustrated in Fig. 6(a), a strong absorption band at 2113 cm^-1 originating from CO appeared for the Au/SiO₂-H₂-450 sample, which was ascribed to CO adsorbed on metallic Au [45, 46]. The in situ DRIFTS plot for the Ni_0.5/SiO₂-H₂-450 catalyst (Fig. 6(b)) contained only bands from gas-phase CO; thus, no CO absorbed on the monometallic Ni catalyst. The reason for this may be that there were too few metallic Ni particles existing in the monometallic Ni catalyst to be detected.

The DRIFTS results for the AuNi_x/SiO₂-H₂-450 catalysts in Fig. 6(c)-(e) contain strong absorption bands at 2113 cm^-1 from CO accompanied with a weak band at 2063 cm^-1, which can be assigned to CO adsorbed on metallic Au and metallic Ni [45-47]. With the increasing Ni content, the intensity of the absorption band at 2063 cm^-1 increased. For the AuNi_x/SiO₂-H₂-450 catalysts, the appearance of the an absorption band consistent with CO on a metallic Ni surface means that the Au nanoparticles induced the reduction of Ni through the NMIR. This phenomenon further confirms that there was an intimate interaction between Au and Ni in the bimetallic nanoparticles. These results are in good agreement with those of EDS (Fig. 5(a) and (b)) and HRTEM (Fig. 5(c) and (d)). Nikolaev et al. [48] found that their Au-NiO/Al₂O₃ catalyst contained Au^δ+ because of the electron transfer from the electron-rich Au⁰ particles to the electron-deficient NiO. However, no positively charged Au was detected in our work, indicating that there was no obvious electron transfer between Au and Ni.

The DRIFTS analysis of Au/SiO₂-H₂-450 in Fig. 6(a) revealed that there was only metallic Au in this catalyst. Thus, the activity of Au/SiO₂-H₂-450 in acetylene hydrogenation (Fig. 2(a)) can be attributed to the metallic Au, as reported in the literature [49, 50]. The DRIFTS results for the AuNi_x/SiO₂-H₂-450 catalysts in Fig. 6(c)-(e) show that Au exists in its metallic state accompanied with metallic Ni. Yang et al. [4] proposed that acetylene adsorbed strongly on Ni, which leads to its low activity in acetylene hydrogenation, while the doping a Ni catalyst with Au can weaken the acetylene adsorption and give rise to increased activity. Thus, the separated Ni plays an important role in catalyzing acetylene hydrogenation [51]. The HRTEM results in Fig. 5(c) and (d) demonstrated that the Au-Ni bimetallic nanoparticles contained Au-Ni alloy. Therefore, we presume that the formation of Au-Ni alloy converts the continuous Ni into separated Ni particles and weakens the acetylene adsorption on Ni. Thus, the separated metallic Ni gives rise to the activity of the Au-Ni bimetallic catalysts in acetylene hydrogenation. Meanwhile, the ethylene selectivity of the Au-Ni bimetallic catalysts was much higher than that of the monometallic Ni catalyst. According to the density functional theory calculations of Yang et al. [4], the adsorption energies of ethylene on Ni(111) and Au/Ni(111) surfaces are -0.58 and -0.27 eV, respectively. The adsorption of ethylene on Ni was weakened by Au, making it easier to desorb, which will contribute to the improved ethylene selectivity in acetylene hydrogenation upon addition of Ni to Au. Thus, the Au in the Au-Ni bimetallic catalysts might play an important role in promoting the selectivity for ethylene. Therefore, the synergistic effect between Au and Ni in the bimetallic catalysts in hydrogenation of acetylene can be ascribed to the formation of Au-Ni alloy.

We synthesized silica-supported Au-Ni bimetallic catalysts with different Ni:Au ratios and studied the synergistic effect between Au and Ni in the selective hydrogenation of acetylene. The Au-Ni bimetallic catalysts exhibited higher acetylene conversion than the corresponding monometallic Au and Ni catalysts. The results of HRTEM, EDS and in situ DRIFTS measurements revealed the formation of Au-Ni alloy. The dilution of Ni by Au contributed to not only the enhancement of acetylene conversion but also ethylene selectivity. These results provide a reference for the design of gold-based bimetallic catalysts for selective hydrogenation reactions.