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Article

Au Nanoparticles Supported on Hydrotalcite-Based MMgAlOx (M=Cu, Ni, and Co) Composite: Influence of Dopants on the Catalytic Activity for Semi-Hydrogenation of C2H2

by
Xun Sun
1,
Wenrui Lv
1,
Yanan Cheng
1,
Huijuan Su
1,
Libo Sun
1,
Lijun Zhao
1,
Zifan Wang
2,3,* and
Caixia Qi
1,*
1
Shandong Applied Research Center of Gold Nanotechnology (Au-SDARC), School of Chemistry & Chemical Engineering, Yantai University, Yantai 264005, China
2
Key Laboratory of Biotechnology and Bioengineering of State Ethnic Affairs Commission, Biomedical Research Center, Northwest Minzu University, Lanzhou 730030, China
3
Engineering Research Center of Key Technology and Industrialization of Cell-Based Vaccine, Ministry of Education, Lanzhou 730030, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(5), 315; https://doi.org/10.3390/catal14050315
Submission received: 31 March 2024 / Revised: 24 April 2024 / Accepted: 7 May 2024 / Published: 10 May 2024
(This article belongs to the Special Issue Nanomaterials in Catalysis: Design, Characterization and Applications)
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Abstract

:
Semi-hydrogenation of acetylene to ethylene over metal oxide-supported Au nanoparticles is an interesting topic. Here, a hydrotalcite-based MMgAlOx (M=Cu, Ni, and Co) composite oxide was exploited by introducing different Cu, Ni, and Co dopants with unique properties, and then used as support to obtain Au/MMgAlOx catalysts via a modified deposition–precipitation method. XRD, BET, ICP-OES, TEM, Raman, XPS, and TPD were employed to investigate their physic-chemical properties and catalytic performances for the semi-hydrogenation of acetylene to ethylene. Generally, the catalytic activity of the Cu-modified Au/CuMgAlOx catalyst was higher than that of the other modified catalysts. The TOR for Au/CuMgAlOx was 0.0598 h−1, which was 30 times higher than that of Au/MgAl2O4. The SEM and XRD results showed no significant difference in structure or morphology after introducing the dopants. These dopants had an unfavorable effect on the Au particle size, as confirmed by the TEM studies. Accordingly, the effects on catalytic performance of the M dopant of the obtained Au/MMgAlOx catalyst were improved. Results of Raman, NH3-TPD, and CO2-TPD confirmed that the Au/CuMgAlOx catalyst had more basic sites, which is beneficial for less coking on the catalyst surface after the reaction. XPS analysis showed that gold nanoparticles exhibited a partially oxidized state at the edges and surfaces of CuMgAlOx. Besides an increased proportion of basic sites on Au/CuMgAlOx catalysts, the charge transfer from nanogold to the Cu-doped matrix support probably played a positive role in the selective hydrogenation of acetylene. The stability and deactivation of Au/CuMgAlOx catalysts were also discussed and a possible reaction mechanism was proposed.

1. Introduction

Selective hydrogenation of C2H2 is essential because petrochemicals involve many downstream products made from ethylene [1]. Small amounts of acetylene by-products from naphtha cracking to prepare ethylene exceed the 5 ppm limit for downstream products [2,3]. Semi-hydrogenation of acetylene to produce ethylene is recognized as the most efficient solution to this problem [4,5,6,7,8]. Therefore, developing a highly selective catalyst for acetylene hydrogenation is highly demanded [9].
Conventional palladium-based catalysts are commonly used due to their high efficiency in the ethylene conversion process [10]. The high costs of palladium-based catalysts, their low selectivity for ethylene, and the formation of carbon deposits from over-hydrogenation products, which seriously affect the lifetime of the catalysts, are the main problems in current industrial applications [11,12,13,14,15]. Excessive hydrogenation of ethylene over palladium-based catalysts is due to the presence of consecutive palladium ensembles [16]. Researchers typically add a second metal to the catalyst to form disordered alloys or ordered metal structures to improve the catalytic performance of palladium-based catalysts [17,18,19,20]. Unfortunately, while this approach improves the selectivity of the palladium-based catalysts, the acetylene conversion decreases. This is due to the lower amount of palladium used in this method and the poor ability of isolated palladium atoms to activate H2 [21,22,23]. Han’s Pd@Ag/TiO2 catalysts exhibited enhanced ethylene selectivity because the Ag shell layer blocks the high coordination sites on the palladium core [24]. Also, metal single-atom catalysts (SACs) exhibited approximately one-tenth the catalytic activity of nanoparticle catalysts [25]. In addition to this, the modification of supports has been heavily investigated [26,27]. Although previous methods improved ethylene selectivity, they had a negative impact on the catalytic activity of palladium-based catalysts. Therefore, developing new catalysts with high selectivity and stability is urgently needed.
Supported Au nanocatalysts have been extensively studied since the pioneering work of Haruta [28]. In addition to typical low-temperature CO oxidation, Au catalysts have shown great potential for various other reactions such as propylene epoxidation, glucose oxidation, acetylene hydrochlorination, and selective hydrogenation of nitroaromatics [29,30,31,32,33]. These studies have revealed that the Au nanocatalysts exhibit more significant potential than the palladium catalysts. Additionally, Au nanocatalysts are found to be considerably selective for ethylene in acetylene hydrogenation. This selectivity is attributed to the different desorption temperatures of C2H2 and olefins on the surface of Au nanoparticles [34,35,36,37,38]. Research has shown that smaller-sized Au particles have higher catalytic activity in hydrogenation. [35,39]. There is a debate about the active site of hydrogenation, with some researchers assuming that the metallic Au species are responsible, while others support the oxidized Au species as the active site [40,41,42]. The activation of H2 usually limits the hydrogenation process. Studies suggest the H2 activation site is at the interface between the Au and the support [43,44]. Therefore, the nature of the support is recognized as an essential factor affecting the catalytic activity of Au catalysts [45,46,47,48].
We conducted extensive research on Au catalysts that exhibit high selectivity towards ethylene. Y. Zhang et al. reported strong interactions between the N species on the support and Au nanoparticles, and the TOF value of Au/CNA was five times higher than that of Au/Al2O3 [49]. L. Sun et al. added CeO2 to Au/Al2O3 catalysts to change the acid–base properties of the support surface and electronic states of the Au species to improve catalytic activity [50]. Also, they prepared Au/MgOx–Al2O3 with different crystal structures ranging from mixed-phase to pure spinel. This opened promising possibilities for engineering the crystal phase of the support for highly efficient heterogeneous Au catalysts [51]. These studies have demonstrated that modification of the support is an effective means of improving catalyst activity [52]. All of the supporting studies mentioned above involved adding some dopants to the surface of the support through post-modification. In this work, we utilized the properties of hydrotalcite to prepare composite oxide supports by introducing dopants to study the effect of different M dopants (M=Cu, Ni, and Co) on Au/MMgAlOx catalysts and their catalytic performance. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and temperature-programmed desorption (TPD) techniques were systematically used to characterize the catalysts. The catalytic performance of a fixed-bed microreactor for C2H2 semi-hydrogenation was investigated.

2. Results and Discussion

2.1. Characterization of the Catalysts

2.1.1. Crystal and Structural Properties

Figure 1 shows the results of the nitrogen adsorption–desorption isotherm to characterize the surface area and pore size of the obtained catalysts. All catalysts exhibited typical type IV nitrogen adsorption–desorption isotherms as shown in Figure 1a. In addition, hysteresis loops coinciding with the structure of the mesoporous material were observed between 0.4 and 1.0 relative pressure [50]. The pore size distribution of the Au/MgAl2O4, Au/NiMgAlOx, and Au/CoMgAlOx catalysts shown in Figure 1b was found to be relatively narrow, with an average pore size of about 4.0 nm. In contrast, the Au/CuMgAlOx catalysts had a much broader pore size distribution, with an average pore size of approximately 11.0 nm. Table 1 summarizes the physical properties of each catalyst. The BET surface area of the Au/CuMgAlOx and Au/CoMgAlOx catalysts was found to be much higher than that of the Au/MgAl2O4 catalyst. Among all, Au/CuMgAlOx had the highest BET surface area of 233 m2 g−1. In contrast, Au/NiMgAlOx had a significantly lower BET surface area of 69 m2 g−1. As shown in Table 1, the pore volumes of the Au/MgAl2O4 and Au/NiMgAlOx catalysts were relatively small: 0.21 and 0.13 cm3 g−1, respectively. However, the Au/CuMgAlOx catalyst had a larger pore volume of 0.85 cm3 g−1. These results indicate that doping with Cu and Co improved the catalysts’ pore structure, which led to an increase in both the specific surface area and pore volume.
XRD characterized the crystal structure of the Au/MMgAlOx catalysts. As shown in Figure 2a, the characteristic diffraction peaks at 2θ values of 11.9°, 60.9°, and 66.1° correspond to the crystalline planes of hydrotalcite flakes (003), (113), and (116) (JCPDS 35-0964). This indicates that the lamellar structure of the hydrotalcite in the catalysts remained intact and was not destroyed [53]. The diffraction peaks related to the (311) and (400) crystal planes of the MgAl2O4 phase (JCPDS 21-1152) were observed at 36.9° and 45.1° for each catalyst. These peaks indicate that the layered structure of the hydrotalcite precursor partially transformed into the MgAl2O4 structure after calcination and reduction [54]. As shown in Figure 2b, the characteristic diffraction peak of the MMgAlOx (M=Cu, Co) support prepared by thermal decomposition tends to disappear at 11.9°, indicating a high degree of decomposition of the Mg-Al hydrotalcite, which may be the reason for its larger specific surface area. However, after loading gold, the crystallinity of the (003) plane of the Mg-Al hydrotalcite increases significantly. The relatively low ratio of Al: Mg (mol: mol) in the catalyst preparation raw materials, approximately 1:1 (mol:mol), results in partial substitution of Mg2+ by Al3+ in the hydrotalcite framework, leading to the formation of Al-O-Al. This could be one of the reasons why the samples retain a partial hydrotalcite structure even after a high-temperature treatment at 500 °C [55]. On the other hand, the alkaline treatment process for preparing the gold catalyst may trigger the “memory effect” of the hydrotalcite, transforming some metal oxides into the Mg-Al hydrotalcite structure [53]. No diffraction peak related to CuO, NiO, and CoO was observed in the XRD, which suggests that CuO, NiO, and CoO were well dispersed in the catalyst bulk phase [56]. Similarly, Au was well dispersed in the catalyst.
The results of the ICP-OES analysis are listed in Table 2. The content of metal elements in the catalysts was similar to the dose used in the preparation process, which indicates that the dopant was successfully introduced into the hydrotalcite precursor.

2.1.2. Catalyst Morphology and Particle Size Distribution of Au Nanoparticles

SEM and HRTEM images of the Au/MMgAlOx series catalysts are displayed in Figure 3. The SEM images (Figure 3a,c,e,g) all showed a hydroaluminate lamellar structure in morphology. It was found that there was no significant difference in morphology between catalysts doped with different metals and undoped catalysts.
Numerous studies have shown that the particle size of Au has an important effect on the performance of catalysts. In general, the smaller the Au particle size of the nano-Au catalyst, the higher its catalytic activity [35,39,57]. HRTEM images of each catalyst and the size distribution of Au particles are shown in Figure 3b,d,f,h. The Au nanoparticles for all four catalysts were spherical and uniformly dispersed on the support. Among them, the sizes of the Au nanoparticles of the Au/MgAl2O4 and Au/NiMgAlOx catalysts were smaller, with average particle sizes of 3.0 and 3.8 nm, respectively. In contrast, Au particle sizes of the Au/CuMgAlOx and Au/CoMgAlOx catalysts were larger, with average sizes of 18.5 nm and 14.7 nm, respectively. At lower pH values, Au NPs tend to increase in size through agglomeration and growth, while at higher pH values, they tend to decrease in size only through surface growth [58]. During the gold loading process, the transformation of metal oxides to a Mg-Al hydrotalcite structure in the Au/MMgAlOx (M=Cu, Co) catalyst consumes a certain amount of OH−. This may be the reason for the larger particle size of its Au NPs. Interestingly, as described below, the Au/CuMgAlOx catalyst with larger Au nanoparticles showed the highest catalytic activity at 250 °C compared to the other catalysts. Therefore, catalyst performance in this experiment is less affected by the size of the Au nanoparticles.

2.1.3. Acid–Base Property

The property of the acid–base is one of the influencing factors of catalyst performance, and characterization of the acid–base of the modified catalysts was carried out [59,60]. CO2-TPD determined the alkalinity of different catalysts with CO2 as the probe molecule. Figure 4a shows that all four catalysts exhibited CO2 desorption peaks around 100 °C and 380 °C. Among them, the desorption peak at around 100 °C was of low intensity due to weak primary sites on the catalyst surface, corresponding to CO2 adsorbed on the −OH groups on the surface. The more substantial desorption peak around 380 °C is due to the medium to robust primary site on the catalyst, corresponding to the adsorption of CO2 by the M-O- in the catalyst [61]. The Au/CuMgAlOx catalyst had the highest CO2 desorption peak temperature of 390 °C. The temperatures of the carbon dioxide desorption peaks of the other catalysts were almost identical at 375 °C (Au/MgAl2O4), 374 °C (Au/NiMgAlOx), and 373 °C (Au/CoMgAlOx). Figure 4b represents the amount of CO2 desorption calculated from the CO2 desorption curve (at 380 °C), which reflects the amount of alkaline site on the catalyst’s surface. The Au/MgAl2O4 catalyst had the lowest amount of CO2 desorption at 2.97 mmol g−1. The CO2 desorption amounts of Au/NiMgAlOx and Au/CoMgAlOx were 3.90 mmol g−1 and 3.84 mmol g−1, respectively. The Au/CuMgAlOx catalyst had the highest CO2 desorption amount of 5.57 mmol g−1, indicating that the amount of alkaline site on the surface of the Au/CuMgAlOx catalyst was higher compared with the other catalysts. The above results show that some dopants (Ni and Co) had little effect on the fundamental strength of the catalyst surface but increased the base content of the catalyst. However, the fundamental strength of the catalyst was significantly increased, and the alkali content increased, with the introduction of the Cu species. The significant increase in the alkaline sites of the Au/CuMgAlOx catalyst may be related to its larger specific surface area and excellent pore properties. In contrast, the relatively low crystallization degree of the Au/CoMgAlOx catalyst may account for its lower increase in alkaline site content.
Subsequently, the acidity of the catalyst surface was investigated, using NH3 molecules as probes, and the results are shown in Figure 5. Each catalyst had two peaks of NH3 desorption: a weaker peak at around 110 °C and a more substantial peak at around 375 °C. The broad peak at 110 °C is attributed to the desorption from the weak acidic sites on the catalyst surface or physically adsorbed NH3. The strong peak near 375 °C is attributed to the medium–strong acidic site on the catalyst surface due to the adsorption of NH3 on the Lewis acidic site on the catalyst, according to previous reports [62]. The temperatures of the NH3 desorption peaks did not differ much: they were 378 °C (Au/MgAl2O4), 380 °C (Au/CuMgAlOx), 372 °C (Au/NiMgAlOx), and 371 °C (Au/CoMgAlOx), suggesting that the doping with different metals did not have much effect on the acidity of the catalyst surface. Based on the NH3 desorption curve, the NH3 desorption amount was calculated to compare each catalyst’s acidic site on the surface. The amount of NH3 desorption of Au/MgAl2O4 was the highest at 4.91 mmol g−1, followed by Au/CuMgAlOx at 3.76 mmol g−1. The Au/NiMgAlOx and Au/CoMgAlOx catalysts had similar desorption amounts of around 3.0 mmol g−1, which suggests that doping with metal reduces the amount of acid on the surface of the catalysts. According to reports, the Lewis acidic sites of Mg-Al mixed oxides are usually caused by Al3+ in the Al3+-O2−-Mg2+ structure. The possible introduction of metal oxides covers the Lewis acidic sites, resulting in the weakening of the Lewis acidity of Mg-Al mixed oxides [63].
The base amount, acid amount, and base amount/acid amount data for each catalyst are summarized in Table 3. As shown in Table 3, Au/MgAl2O4 has the smallest base/acid amount of 0.60, Au/CuMgAlOx has the most significant base/acid amount of 1.48, and Au/NiMgAlOx and Au/CoMgAlOx have base/acid amounts of 1.32 and 1.29, respectively. X. Sun et al. have previously reported that, since acetylene belongs to the acidic molecules, the alkaline nature of the catalyst surface has a positive effect on the adsorption of acetylene on the Au surface, which can lead to a better catalytic performance of the catalyst [49]. Therefore, it was hypothesized that a higher base/acid ratio has an auxiliary effect on the adsorption of acetylene at the perimeter of Au nanoparticles, which is a potential reason for the higher acetylene conversion of the Au/CuMgAlOx catalyst.

2.1.4. Chemical States of Au, Co, and Cu in Catalysts

The XPS for Au 4f of each catalyst is shown in Figure 6. A strong peak around 89 eV in the spectrum is due to the overlap of the binding energies of Mg 2s (89.0 eV) and Au 4f5/2 [64,65]. Therefore, the binding energy of Au 4f7/2 (Au0 = 84.0 eV) was focused on. For the Au/MgAl2O4 catalyst with a smaller average Au particle size (3.0 nm), the peak at 84.1 eV indicates that the state of the gold on its surface should be the Au0 species [66,67]. The surface of the Au/NiMgAlOx catalyst exhibits smaller Au NPs, and the characteristic peak of its Au 4f7/2 is observed at 84.3 eV. The binding energy of Au 4f is affected by the final state effect, and the larger Au particle size in some samples can influence its binding energy [68]. The binding energies of Au 4f7/2 for the Au/CoMgAlOx and Au/CuMgAlOx catalysts are 83.8 and 84.6 eV, respectively. The decrease in binding energy of Au 4f7/2 for the Au/CoMgAlOx catalyst may be related to the final state effect, while the positive shift in binding energy of Au 4f7/2 for the Au/CuMgAlOx catalyst with a larger average Au particle size may be attributed to the strong electronic interaction between Au and Cu species in the catalyst. On the XPS curve, a shift towards higher binding energy is generally associated with a decrease in electron density or an increase in the oxidation state [69]. Therefore, we speculate that some Au on the surface of the Au/CuMgAlOx catalyst is in an electron-deficient state [70].
As shown in Table 4, the concentration of heteroatoms (M=Cu, Co, and Ni) introduced on the surface of the Au/MMgAlOx catalyst is significantly lower than that in the bulk phase. This suggests that the heteroatoms are not simply adsorbed on the surface of the catalyst, but rather form a composite oxide. The relatively high concentrations of C and O on the surface of the catalyst may be attributed to the Mg-Al hydrotalcite phase, which is consistent with the XRD results. The Au atoms are uniformly dispersed on the surface of the Au/MMgAlOx catalyst. After introducing heteroatoms, the Mg/Al atomic ratio on the surface of the Au/CoMgAlOx catalyst increases, which may be caused by the partial substitution of Mg2+ by Co2+. This observation is consistent with the results of the Co 2p XPS analysis. The decrease in the Mg/Al atomic ratio on the surface of the Au/CuMgAlOx and Au/NiMgAlOx catalysts compared to that of the Au/CoMgAlOx catalyst may be related to the higher crystallization degree of the hydrotalcite phase.
In order to verify the electron transfer between Au and the supports, we analyzed the chemical valence states of Cu in Au/CuMgAlOx and CuMgAlOx, and the results are shown in Figure 7. As shown in Figure 7, Cu possibly exists in the catalyst in two forms. The appearance of satellite peaks at around 943.2 eV indicates the presence of Cu2+, but it is difficult to determine the presence of metallic Cu or Cu+ because both Cu1+ and Cu0 contribute to the Cu 2p3/2 peaks [51]. In CuMgAlOx, the characteristic peak of the Cu species is found near 934.4 eV, whereas the characteristic peak of the Cu2+ species in Au/CuMgAlOx near 934.0 eV is shifted by 0.4 eV in the direction of low binding energy compared to CuMgAlOx, indicating that the Cu2+ gained electrons [55]. Combined with the results of Au 4f spectroscopy, it was concluded that there was a strong electronic interaction between Au and Cu in the Au/CuMgAlOx catalyst, and the electrons would be transferred from Au to Cu. L. Sun [50] et al. suggested that the positively charged Au (Auδ+) promoted the adsorption of acetylene and the dissociation of H2 in the selective hydrogenation reaction of acetylene.
As shown in Figure 8, the Co 2p spectrum consists of two satellite peaks (S1 and S2) and two sets of spin-orbit peaks, indicating the existence of two valence states of Co species on the catalyst surface. The characteristic peaks at 780.7 eV and 795.7 eV belong to Co3+, while the set of spin-orbit peaks with higher binding energy is attributed to Co2+ [71]. On the surface of the CoMgAlOx sample, Co species mainly exist in the form of Co3+, while a large amount of Co2+ is found on the surface of the Au/CoMgAlOx sample. This may be related to the reformation of the hydrotalcite phase. Through peak area integration, the area ratios of Co3+/Co2+ for the two catalysts are 2.55 (CoMgAlOx) and 0.55 (Au/CoMgAlOx), respectively. Therefore, we speculate that the decrease in the binding energy of the Au 4f7/2 orbit on the surface of the Au/CoMgAlOx sample is mainly influenced by the final state effect.

2.2. Catalytic Activity Tests

The catalytic performance of each catalyst in the acetylene selective hydrogenation reaction was tested, and the results are shown in Figure 9a. All catalysts showed catalytic activity above 150 °C From the acetylene conversion curves in the figures, the acetylene conversions of Au/MgAl2O4 and Au/CoMgAlOx catalysts increased slowly with the increase in reaction temperature, and the performance of the catalysts did not improve after doping with Co, but rather decreased. The acetylene conversions of Au/CuMgAlOx and Au/NiMgAlOx showed a trend of increasing up to 250 °C and then decreasing with the increase in the reaction temperature. At 250 °C the highest acetylene conversions of 32% for Au/CuMgAlOx and 11% for Au/NiMgAlOx were obtained, respectively. This phenomenon is consistent with the XPS results of Au 4f.
A comparison of the catalytic performance of the Au/CuMgAlOx catalyst and its support CuMgAlOx is also represented in Figure 9a. It can be seen that the acetylene conversion of CuMgAlOx also tended to increase and then decrease as the reaction temperature increased, reaching a maximum value of 8% at 200 °C. CuMgAlOx was rapidly deactivated at 250 °C, while Au/CuMgAlOx achieved the highest acetylene conversion. Comparison of CuMgAlOx and Au/CuMgAlOx activity evaluation data reveals that the Au species play a crucial role in the C2H2 selective hydrogenation reaction. The conversion of Au/CuMgAlOx is around four times higher than that of CuMgAlOx while maintaining high selectivity. It was found that the performance of the catalyst was significantly improved after loading Au. The turnover rates (TORs) were calculated based on the total Au content and compared with the results of related studies. The TOR of Au/MgAl2O4 was only 0.016 h−1 at 250 °C. However, the TOR value of Au/CuMgAlOx was 0.059 h−1. Its TOR was greater by a factor of about 37, while that of the Au/3%MgOx–Al2O3–550 catalyst for the selective hydrogenation of C2H2 at 300 °C was 0.041 h−1 [51]. This behavior can be plausibly explained by the exceptional physicochemical characteristics of the Au/CuMgAlOx catalysts and the chemical valence states of the Au and Cu species on the surface. Overall, Au/CuMgAlOx had a higher TOR at 250 °C, which implies that Au/CuMgAlOx is a potential catalyst for C2H2 selective hydrogenation.

2.3. Coke Analysis and Stability

The stability of a catalyst is an important indicator for evaluating its performance. The time flow analysis of the two catalysts in the selective hydrogenation of acetylene at 200 °C under typical reaction conditions is shown in Figure 10. The initial activities of both catalysts agreed with the catalytic experiments’ results. As shown in Figure 10, the Au/CuMgAlOx catalyst decreased from an initial conversion of 30% to 24% in the first 420 min. Then, the Au/MgAl2O4 catalyst without dopant was deactivated after 180 min, followed by 500 min of a slow decay period. In summary, the catalytic performance of the Au/CuMgAlOx catalyst was relatively stable. With the Cu introduction, the Au-based catalyst’s initial activity reached 7 times that before modification, and compared to the Au/MgAl2O4, the catalytic activity was sustained for 2.3 times longer. Although the catalytic performance of the Au catalysts improved, the catalytic activity of the catalysts needs to be discussed in more detail in our subsequent work, especially in practical applications.
In the acetylene selective hydrogenation reaction, its by-products (“green oil”) can quickly form carbonaceous deposits on the active metal surface of the catalyst, and the carbon buildup is an essential reason for the reduction in catalyst activity [72]. Raman spectroscopy is a powerful technique for detecting carbonaceous deposits on catalyst surfaces. Therefore, we performed Raman spectroscopic characterization of two catalysts after stability testing. As shown in Figure 11, the characteristic peaks near 1112 cm−1 and 1501 cm−1 of the Au/MgAl2O4 catalyst are attributed to the C species with amorphous and ordered or graphitic carbon structures, respectively, which suggests that the “green oil” produced on the surface of the Au/MgAl2O4 catalyst was partially converted to carbonaceous deposits during the stability test [50,51,73]. Therefore, we assume that the coverage of active sites on the surface of the Au/MgAl2O4 catalyst by carbonaceous deposits is an essential reason for its rapid deactivation in the stability test.
However, the characteristic peaks of the Au/CuMgAlOx catalyst near 1112 cm−1 were significantly weaker in intensity, with a flattened peak shape and a broader peak top. Its characteristic peak near 1501 cm−1 was displaced to 1420 cm−1, and the peak intensity and shape were similar to those near 1112 cm−1 [74]. This characteristic peak indicates the presence of a small amount of unsaturated aliphatic hydrocarbons on the catalyst surface instead of the ordered or graphitic carbon structure of the C species, which may be caused by the introduction of Cu species inducing a significant decrease in the percentage of acidic sites on the surface of the Au/CuMgAlOx catalyst. The ordered or graphitic carbon structure is more inactive than the unstructured carbon, so we consider this an important reason for the better stability of the Au/CuMgAlOx catalyst [74,75].
A comparison of TEM characterization and activity data revealed that Au particle size is not the main factor affecting the catalytic activity of Au/CuMgAlOx and that oxidized Au species play an active role in the reaction. In order to investigate the main reason for the high-temperature deactivation of the catalyst, we analyzed the chemical valence states of the Au species on the catalyst surface after the high-temperature reaction by XPS. The Au 4f XPS patterns of the Au/CuMgAlOx catalysts before and after the reaction are shown in Figure 12, and it can be clearly seen that the characteristic peaks of the Au species on the surface of Au/CuMgAlOx were negatively displaced after the reaction. After the high-temperature reaction, the catalyst rapidly deactivated, and the Au species on its surface mainly existed in the metallic state. Therefore, we speculate that the strong electronic interaction between Au and Cu is the main reason for the high catalytic activity of the Au/CuMgAlOx catalyst.
The surface atomic ratio of the samples after stability testing is shown in Table 5. The presence of a larger amount of C species on the catalyst’s surface before the reaction may be caused by the incomplete decomposition of the hydrotalcite. After the reaction, no loss of Au species was observed for both catalysts. However, the surface C/Al ratio of the Au/MgAl2O4 catalyst increased from 1.13 to 2.12, while the surface C/Al ratio of the Au/CuMgAl2O4 catalyst only increased by 0.54. The Au/CuMgAlOx catalyst exhibited better resistance to carbon deposition.

2.4. Study on the Reduction Behavior of C2H2 on the Surface of the Au/CuMgAlOx Catalyst

XPS analysis and activity data indicated that the electronic synergy between Au and copper is more favorable to the catalytic activity of the catalyst. Therefore, we performed TEM and EDS (HAADF) tests on the catalyst surface to study the elemental distribution on the catalyst surface. As shown in Figure 13, the Cu and Au species are uniformly distributed on the surface of the Au/CuMgAlOx catalyst, and a small amount of aggregation of the Au species exists. This result is consistent with the larger average particle size of Au nanoparticles obtained from the TEM analysis. Interestingly, we find that the distribution of the Au species is similar to the dispersion trajectory of the Cu species, a phenomenon that the presence of solid electron transfer between the Au and Cu species may cause.
We have tried to describe the reaction mechanism of the Au/MgAl2O4 catalyst in the C2H2 selective hydrogenation reaction. The interface between the Au and copper species would be the active site, and Scheme 1 illustrates a possible mechanism. Introducing the Cu species increased the proportion of Auδ+ in the catalyst and enhanced the C2H2 adsorption capacity. The partially positively charged Au present at the Au-Cu interface was the site for the adsorption activation of H2 molecules, and a small portion of H2 molecules were adsorbed and activated on the surface of the Cu species. The hydrogen overflow between Au and Cu facilitated the hydrogenation of acetylene, which improved the catalytic performance. C2H4 is more readily desorbed from the Au surface than C2H2, making it difficult for the reduced C2H2 molecules to over-hydrogenate into other by-products.

3. Materials and Methods

3.1. Catalyst Preparation

MMgAlOx (M:Mg:Al = 5:45:50 metal oxide molar ratio) mixed oxide supports were prepared by the co-precipitation method from metal salt solutions. All the materials used were of analytical purity. In a typical procedure, to prepare the CoMgAlOx support, the desired amount of metal precursor was dissolved in double-distilled water and mixed under mild stirring conditions (Solution A). After the dropwise addition of liquid A was completed, the stirring was continued in a water bath for 12 h. The filtrate was pumped and washed until the filtrate was neutral, and the filter cake was dried in a drying oven for 12 h, ground, and then the powder was roasted in a muffle furnace at 500 °C (5 °C/min) for 4 h to obtain the CoMgAlOx support. Similarly, CuMgAlOx, NiMgAlOx, and MgAl2O4 were prepared under the same conditions and by the same method. Au was loaded by a modified deposition–precipitation method [28]. First, the amount of chloroauric acid solution required was calculated based on the amount of Au loaded. Subsequently, the required chloroauric acid solution was pipetted into a beaker with a pipette gun, an appropriate amount of deionized water (depending on the amount of water absorbed by the support) was added, shaken well, and then the pH of the mixture was adjusted to 7~8 with KOH (1.0 M). The support was added to the above solution and ultrasonically dispersed homogeneously. It was left to stand for 4 h, poured into dilute ammonia to soak and treat for 24 h, filtered and washed until the presence of Cl− could not be detected, and then dried in an oven at 60 °C for 12 h to obtain the Au/MMgAlOx catalyst. For comparison, Au/MgAl2O4 samples were also synthesized under the same conditions. ICP measurements showed that the corresponding Au loadings for Au/MMgAlOx (Cu, Co, and Ni) were 0.38, 0.46, and 0.51 wt.%, respectively, and that those of Au/CuMgAlOx were lower than those of the Au/MgAl2O4 catalyst (0.42 wt.%).

3.2. Catalyst Characterization

X-ray powder diffraction (XRD): Tests were carried out on a Shimadzu (Kyoto, Japan) XRD-6100 X-ray diffractometer using a Cu Kα diffractive light source with a wavelength of 1.54 Å, an operating voltage of 40 kV, an operating current of 100 mA, a test range of 5 to 80°, and a scanning speed of 20°/min. Nitrogen physical adsorption and aesorption (BET): The samples were physically adsorbed and desorbed in nitrogen using an ASAP 2020 analyzer (Micromeritics, Norcross, GA, USA). The samples were first degassed under vacuum at 200 °C, and then tested in liquid nitrogen for adsorption and desorption. The test results were calculated using the BET method to obtain the samples’ specific surface area, and the samples’ pore size distribution was obtained using the BJH method.
Inductively coupled plasma emission spectrometry (ICP-OES): The samples were weighed and treated with freshly prepared aqua regia for 24 h. All samples were then transferred to volumetric flasks and filtered to obtain a clear solution, which was tested for the actual content of the elements in the samples on a PE optima 8000 instrument (Waltham, MA, USA). Scanning electron microscope (SEM): The catalyst morphology was observed on a JSM-7900F (JEOL, Akishima, Tokyo, Japan) scanning electron microscope. High-resolution transmission electron microscopy (TEM): TEM images of the samples were taken on an FEI (Lausanne, Switzerland) TF20 transmission electron microscope with the energy spectrum model Super-X.
NH3 programmed temperature rise desorption (NH3–TPD): This was carried out on a TP-5080 fully automated multiple adsorption instrument. After the support gas flow (He gas stream, 30 mL/min) was stabilized, the sample was pretreated at 250 °C for 2 h. When the temperature was lowered to 50 °C, the sample was switched to 10% NH3/He adsorption for 30 min, followed by blowing the He gas stream. After the baseline was stabilized, the test was carried out in the range of 50~800 °C at a heating rate of 10 °C/min, and a TCD detector detected the recordings. CO2 programmed thermal desorption (CO2–TPD): similar to NH3-TPD, but the test gas was changed to CO2.
X-ray photoelectron spectroscopy (XPS): The XPS was conducted using a Thermo-Fisher Escalab 250Xi electron spectrometer (Waltham, MA, USA) with Al Ka (1486.6 eV). The surface elemental composition and chemical valence states of the Au/MMgAlOx (M=Cu, Co, and Ni) catalysts were measured using X-ray photoelectron spectroscopy (XPS) with a Thermo-Fisher Escalab 250Xi electron spectrometer (Waltham, MA, USA). The spectrum excitation energy was set at 20 eV. The results were corrected using the C 1s peak with a binding energy of 284.8 eV. Spectral analysis was conducted using CasaXPS (Casa2318PR1-0) software, and the W Tougaard background type was selected for fitting the XPS curves.

3.3. Catalytic Activity Measurement

The catalytic performance of acetylene selective hydrogenation was tested by loading 0.1 g of catalyst into a U-shaped quartz tube reactor at a temperature of 100~350 °C, with a test interval of 50 °C and an injection interval of 25 min. The gas composition was 0.6% C2H2 + 3.0% H2 + 96.4% Ar (v/v), and the reaction airspeed was 17,200 mL·gcat−1·h−1. The gas composition was analyzed online by Shimadzu GC-2014 gas chromatography (FID (HP-ALUMINA), TCD). Based on the analytical results, the acetylene conversion (X) and ethylene selectivity (S) were calculated using the following equations:
X = ( C 2 H 2 ) in − ( C 2 H 2 ) out ( C 2 H 2 ) in × 100 %
S = ( C 2 H 4 ) out ( C 2 H 2 ) in − ( C 2 H 2 ) out × 100 %
where (C2H2)in, (C2H2)out, and (C2H4)out are the concentrations of inlet acetylene, outlet acetylene, and outlet ethylene, respectively.
We evaluated the activity of the catalyst in terms of turnover rates (TORs) [76]:
TORs = A/B
A = V[C2H2] × X
B = Vm × m[Au]
where V[C2H2] is the flow rate of acetylene, X is acetylene conversion, Vm is the molar volume of gas, and m[Au] is actual loading of Au.

4. Conclusions

This study demonstrates that the Au/CuMgAlOx catalyst has a superior catalytic performance to Au/MMgAlOx (M=Ni/Co) and Au/MgAl2O4. The catalytic activity of its CuMgAlOx support demonstrates that the Au component plays a significant role in the C2H2 reduction reaction. At lower temperatures (250 °C), the Au/CuMgAlOx catalyst exhibits nearly 100% selectivity towards C2H4 and achieves a conversion rate of up to 38% for C2H2, with a TOR value of 0.0598 h−1. However, the TOR of Au/MgAl2O4 is only 0.0016 h−1 at 250 °C. The introduction of dopants effectively enhances the catalytic activity and stability of the Au catalyst. The BET results indicate that the Au/CuMgAlOx catalyst possesses a large specific surface area and excellent pore properties, which are important reasons for its high catalytic activity. According to the results of CO2–TPD and NH3–TPD, the introduction of the dopant significantly alters the acid–base sites of the parent support. The Au/CuMgAlOx catalysts have a high base-to-acid ratio. The XPS results show that some of the Au NPs on the surface of the Au/CuMgAlOx catalyst are deficient in electrons. Based on the XPS results of CuMgAlOx, we believe that there might be a strong electronic synergy between Au and Cu. Introducing dopants reduces the electron density of some Au species and changes the acidity of the support. The catalyst stability is improved by enhancing the catalytic activity and inhibiting the formation of many "hard" carbon deposits. Valuable information for the rational design of Au-based catalysts for the selective hydrogenation of acetylene can be gleaned from these results.

Author Contributions

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

Funding

The authors are grateful for financial support from the Shandong Natural Science Foundation (ZR 2022MB082, ZR 2022QB120), the Natural Science Foundation of Gansu Province (No. 23JRRA718), the National Natural Science Foundation of China (21603181, 21802117), and the Science and Technology Innovation Development Plan of Yantai (2023JCYJ075).

Data Availability Statement

The data presented in this study are available on request from the corresponding author via e-mail: [email protected] (C.Q.), [email protected] (Z.W.).

Acknowledgments

Thank you for the sample characterization equipment provided by the sample testing platform of Yantai University.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Nitrogen adsorption and desorption isotherms; and (b) pore size distribution of catalysts.
Figure 1. (a) Nitrogen adsorption and desorption isotherms; and (b) pore size distribution of catalysts.
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Figure 2. XRD patterns of Au/MgAl2O4 and Au/MMgAlOx (M=Cu/Ni/Co) catalysts. (a: Au/MgAl2O4, b: Au/CuMgAlOx, c: Au/NiMgAlOx, d: Au/CoMgAlOx, e: MgAl2O4, f: CuMgAlOx, g: Au/NiMgAlOx, and h: CoMgAlOx).
Figure 2. XRD patterns of Au/MgAl2O4 and Au/MMgAlOx (M=Cu/Ni/Co) catalysts. (a: Au/MgAl2O4, b: Au/CuMgAlOx, c: Au/NiMgAlOx, d: Au/CoMgAlOx, e: MgAl2O4, f: CuMgAlOx, g: Au/NiMgAlOx, and h: CoMgAlOx).
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Figure 3. SEM and HRTEM images and particle size distribution of catalysts. (a,b) Au/MgAl2O4, (c,d) Au/CuMgAlOx, (e,f) Au/NiMgAlOx, and (g,h) Au/CoMgAlOx.
Figure 3. SEM and HRTEM images and particle size distribution of catalysts. (a,b) Au/MgAl2O4, (c,d) Au/CuMgAlOx, (e,f) Au/NiMgAlOx, and (g,h) Au/CoMgAlOx.
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Figure 4. (a) CO2-TPD curves; and (b) CO2 desorption amount of Au/MgAl2O4 and Au/MMgAlOx (M=Cu/Ni/Co) catalysts.
Figure 4. (a) CO2-TPD curves; and (b) CO2 desorption amount of Au/MgAl2O4 and Au/MMgAlOx (M=Cu/Ni/Co) catalysts.
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Figure 5. NH3-TPD curves of Au/MgAl2O4 and Au/MMgAlOx (M=Cu/Ni/Co) catalysts.
Figure 5. NH3-TPD curves of Au/MgAl2O4 and Au/MMgAlOx (M=Cu/Ni/Co) catalysts.
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Figure 6. XPS profiles of Au 4f of Au/MgAl2O4 and Au/MMgAlOx (M=Cu/Ni/Co) catalysts. (The right figure is the enlarged area of Au 4f7/2).
Figure 6. XPS profiles of Au 4f of Au/MgAl2O4 and Au/MMgAlOx (M=Cu/Ni/Co) catalysts. (The right figure is the enlarged area of Au 4f7/2).
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Figure 7. XPS profiles of Cu 2p of Au/CuMgAlOx and CuMgAlOx. (a) CuMgAlOx; (b) Au/CuMgAlOx).
Figure 7. XPS profiles of Cu 2p of Au/CuMgAlOx and CuMgAlOx. (a) CuMgAlOx; (b) Au/CuMgAlOx).
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Figure 8. XPS profiles of Co 2p of Au/CoMgAlOx and CoMgAlOx. (a) CoMgAlOx; (b) Au/CoMgAlOx).
Figure 8. XPS profiles of Co 2p of Au/CoMgAlOx and CoMgAlOx. (a) CoMgAlOx; (b) Au/CoMgAlOx).
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Figure 9. (a) Catalytic performance of catalysts; and (b) data error analysis (250 °C).
Figure 9. (a) Catalytic performance of catalysts; and (b) data error analysis (250 °C).
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Figure 10. Evolution of acetylene conversion comparing the Au/CuMgAlOx to the Au/MgAl2O4 catalysts during the hydrogenation of acetylene at 200 °C.
Figure 10. Evolution of acetylene conversion comparing the Au/CuMgAlOx to the Au/MgAl2O4 catalysts during the hydrogenation of acetylene at 200 °C.
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Figure 11. Raman spectra of the used Au/CuMgAlOx and Au/MgAl2O4 catalysts (after 12 h continuous hydrogenation reaction; gas condition: 0.6 vol% C2H2/3.0 vol% H2/balance Ar).
Figure 11. Raman spectra of the used Au/CuMgAlOx and Au/MgAl2O4 catalysts (after 12 h continuous hydrogenation reaction; gas condition: 0.6 vol% C2H2/3.0 vol% H2/balance Ar).
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Figure 12. XPS profiles of Au 4f of Au/CuMgAlOx. (a) Au/CuMgAlOx−350; (b) Au/CuMgAlOx.
Figure 12. XPS profiles of Au 4f of Au/CuMgAlOx. (a) Au/CuMgAlOx−350; (b) Au/CuMgAlOx.
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Figure 13. TEM micrographs of the Au/CuMgAlOx catalyst, and the corresponding elemental mappings.
Figure 13. TEM micrographs of the Au/CuMgAlOx catalyst, and the corresponding elemental mappings.
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Scheme 1. The preparation processes and the possible catalytic mechanism of Au/CuMgAlOx.
Scheme 1. The preparation processes and the possible catalytic mechanism of Au/CuMgAlOx.
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Table 1. BET surface area and porous property of the studied catalysts.
Table 1. BET surface area and porous property of the studied catalysts.
CatalystSBET/m2 g−1Dpore/nmVpore/cm3 g−1
Au/MgAl2O41364.10.21
Au/CuMgAlOx23311.00.85
Au/NiMgAlOx694.30.13
Au/CoMgAlOx2123.90.31
Table 2. Element composition of bulk phase in catalysts.
Table 2. Element composition of bulk phase in catalysts.
CatalystIngredient Ratio (mol)Physical Composition (mol)
MMgAlMMgAl
Au/MgAl2O4—4550—47.552.4
Au/CuMgAlOx545505.347.447.2
Au/NiMgAlOx545505.747.646.6
Au/CoMgAlOx545505.147.547.3
Table 3. Acid and base amount of Au/MgAl2O4 catalyst and Au/MMgAlOx (M=Cu/Ni/Co) catalysts.
Table 3. Acid and base amount of Au/MgAl2O4 catalyst and Au/MMgAlOx (M=Cu/Ni/Co) catalysts.
CatalystBase/mmol g−1Acid/mmol g−1Base/Acid
Au/MgAl2O42.974.910.60
Au/CuMgAlOx5.573.761.48
Au/NiMgAlOx3.902.961.32
Au/CoMgAlOx3.842.971.29
Table 4. The surface atomic composition of the Au/MgAlOx catalyst was determined through XPS analysis.
Table 4. The surface atomic composition of the Au/MgAlOx catalyst was determined through XPS analysis.
CatalystAtomic Content (%)
MAuAlMgCO
Au/MgAl2O4-0.5314.1915.4416.0653.70
Au/CuMgAlOx0.750.4116.5713.3415.1953.10
Au/NiMgAlOx0.990.4817.5513.6815.8251.49
Au/CoMgAlOx0.640.501315.4616.4852.53
Table 5. Surface atomic ratio of the catalysts before and after the reaction determined by XPS analysis.
Table 5. Surface atomic ratio of the catalysts before and after the reaction determined by XPS analysis.
CatalystAtomic Ratio
Al/AuC/Al
Au/MgAl2O426.771.13
Au/CuMgAlOx40.410.92
Au/MgAlOx−12 h37.222.12
Au/CuMgAlOx−12 h27.311.46
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Sun, X.; Lv, W.; Cheng, Y.; Su, H.; Sun, L.; Zhao, L.; Wang, Z.; Qi, C. Au Nanoparticles Supported on Hydrotalcite-Based MMgAlOx (M=Cu, Ni, and Co) Composite: Influence of Dopants on the Catalytic Activity for Semi-Hydrogenation of C2H2. Catalysts 2024, 14, 315. https://doi.org/10.3390/catal14050315

AMA Style

Sun X, Lv W, Cheng Y, Su H, Sun L, Zhao L, Wang Z, Qi C. Au Nanoparticles Supported on Hydrotalcite-Based MMgAlOx (M=Cu, Ni, and Co) Composite: Influence of Dopants on the Catalytic Activity for Semi-Hydrogenation of C2H2. Catalysts. 2024; 14(5):315. https://doi.org/10.3390/catal14050315

Chicago/Turabian Style

Sun, Xun, Wenrui Lv, Yanan Cheng, Huijuan Su, Libo Sun, Lijun Zhao, Zifan Wang, and Caixia Qi. 2024. "Au Nanoparticles Supported on Hydrotalcite-Based MMgAlOx (M=Cu, Ni, and Co) Composite: Influence of Dopants on the Catalytic Activity for Semi-Hydrogenation of C2H2" Catalysts 14, no. 5: 315. https://doi.org/10.3390/catal14050315

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