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Silica-Encapsulated Gold Nanoclusters for Efficient Acetylene Hydrogenation to Ethylene Haijun Chen, Zhimin Li, Zhaoxian Qin, Hyung J Kim, Hadi Abroshan, and Gao Li ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00384 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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ACS Applied Nano Materials

Silica-Encapsulated Gold Nanoclusters for Efficient Acetylene Hydrogenation to Ethylene Haijun Chen$,†,&, Zhimin Li†,&, Zhaoxian Qin†, Hyung J. Kim#, Hadi Abroshan‡,§,*, and Gao Li†,* $

College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China

†

State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China #

Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States

‡SUNCAT

Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, 443 Via Ortega, Stanford, California 94305, United States §School

of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, United States

ABSTRACT: Selective conversion of acetylene impurities to ethylene is a major step in processing chemical feedstock in industry. Currently, this step is widely performed through catalytic hydrogenation of acetylene over palladium-based materials. Due to a lack of selectivity, however, this method suffers from the formation of ethane to a considerable extent. Herein, we present gold nanoclusters encapsulated in microporous silica as robust catalysts for hydrogenation of acetylene with 98% selectivity towards ethylene formation. These encapsulated catalysts are designed to avoid aggregation of nanoclusters during the thermal pretreatment at ~500 °C by maintaining the average nanocluster size around 1.6 nm. This approach yields the acetylene conversion efficiency of ~95% at 280 °C. The catalysts are characterized by UV-vis, FT-IR, NMR and X-ray spectroscopies as well as density functional theory (DFT). Analysis shows that the preservation of the nanoscale size of the gold particles at operating temperature coupled with the maintenance of positive charges on the surface active sites induced by the presence of electronegative species of microporous silica can be utilized as an efficient strategy to significantly improve the acetylene activation for the semi-hydrogenation reaction. KEYWORDS: gold cluster, Au25, semi-hydrogenation, acetylene, microporous silica

■ INTRODUCTION Semi-hydrogenation of acetylene (HC≡CH) to ethylene (H2C=CH2) is a key catalytic reaction to eliminate acetylene impurities of chemical feedstock, which is often catalyzed over traditional palladium-based materials.1,2 Ethane is a byproduct of the acetylene semi-hydrogenation process due to the high tendency of Pd-based catalysts to adsorb ethylene. This poses a serious challenge for the direct application of the resulting product in ensuing chemical processes, e.g., polymerization. In this regard, several strategies have been explored to improve the efficiency for the selective formation of ethylene, including the modifications of electronic and geometric properties of palladium-based catalysts through the use of various supports, promoters and additives. 2-6 For example, Pei et al employed Pd single-atom embedded on Cu and Ag particles to achieve ~100% acetylene conversion with 85% and 81% selectivity for the formation of ethylene.4,5 Nevertheless, most of these strategies show moderate improvement in the catalytic activity, lack stability for a prolonged reaction time, and/or require complex pretreatment prior to the reaction. Therefore, design and development of catalysts for efficient semi-hydrogenation of acetylene continues to be a subject of importance worldwide. Gold nanoparticles have received significant attention as catalysts over the past few decades for various organic reactions, thanks to the robust and green nature of gold-based materials.7-14 A previous study by Domen, Tamaru and their coworkers has demonstrated that gold nanoparticles have con-

siderable potential for selective formation of ethylene through the semi-hydrogenation of acetylene.12 Following studies have indicated the catalytic performance of gold-based materials for the semi-hydrogenation depends strongly on the catalysts’ size. In particular, gold particles at nanoscale, e.g., 2-3 nm, are capable of selectively converting acetylene to ethylene with ~ 100% conversion rate.15 An increase in the particle size of the gold catalyst is found to decrease the conversion rate of acetylene drastically, while it has a minimal effect on the reaction selectivity.16-21 Therefore, a precise control of the size of catalysts is critical to harvest the maximum potential of gold-based materials for the semi-hydrogenation reaction. Ultra-small gold clusters (2-3 nm) with atomic precision are among the first candidates for the hydrogenation reaction. However, the gold nanoclusters aggregate to form larger clusters (> 5 nm) during thermal pretreatment that is usually required to prime the catalysts for the reaction. This in turn leads to a significantly lower activity for the hydrogenation reaction, which is attributed to the transition of small non-metallic nanoparticles to metallic form of Au as the particles size increases.22,23 Therefore, it is both worthwhile and desirable to devise strategies that prevent the nanoparticle aggregation during the thermal pretreatment. Herein, we present a protocol for synthesis of gold nanoclusters encapsulated in microporous silica that efficiently blocks the cluster aggregation even at temperature as high as 500 °C. The thermally pretreated gold clusters are found to catalyze the semi-hydrogenation reaction with an over 95% acetylene conversion efficiency that is much higher

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than that for the large Au particles (5 nm) supported over SiO2 (ca. 15%). In this paper, we employ a combined approach of theory and experiments to study the origin of the high activity of the encapsulated gold nanoclusters in microporous silica. Results indicate that the adsorption and activation of acetylene are significantly improved due to the nanoscale dimensions of the catalyst as well as the presence of positively charged gold atoms, i.e., Au+, on surface active sites.

■ EXPERIMENTAL METHODS Chemicals. All chemicals were purchased with analytical grade and used without purification. Au(PPh3)Cl and NaBH4 were purchased from Adamas. Ammonia was purchased from Kermel Chemical Reagent Co., Ltd., (3-mercaptopropy)triethoxysiliane was purchased from WD Silicone Co., Ltd. Tetraethyl orthosilicate was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. A Barnstead NANO pure Diwater TM system was used to prepare Nanopure water (resistance 18.2 MΩ cm). Aqua regia (HCl : HNO3 = 3 : 1, v/v) and copious nanopure water were used to wash and rinse all glassware. We dried all glassware using an oven prior to use. Synthesis of Au25(PPh3)10[SC3H6Si(OC2H5)3]5Cl2 clusters. 44 mg Au(PPh3)Cl was dissolved in a mixture solution containing 5 mL dichloromethane and 5 mL ethanol and stirred for 30 min at 600 rpm. Then, 49 µL (3mercaptopropy)trimethoxysiliane (MPTS) was added to the solution. After 30 min, 3.4 mg sodium borohydride (dispersed in 2 mL ethanol) was added dropwise and the solution color turned to deep red in a few minutes. After stirring for 30 min, another 24 µL MPTS was added. The UV-vis spectrum of the solution thus prepared was monitored to ensure the Au25 clusters were successfully synthesized before terminating the reaction. Next, the solvent was removed using rotary evaporation. Finally, the gold clusters (noted as [Au25]) were extracted with methanol. Preparation of Au 25@SiO2-500 catalyst. 0.9 mL tetraethyl orthosilicate (TEOS, dissolved in 17 mL ethanol) was added to a mixed solution (containing 2.7 mL ammonia, 98 mL ethanol, and 31 mL water) in a dropwise fashion under strong stirring. Next, 48 mL of an ethanol solution containing 2.55 mL TEOS and 4.1 mg Au25(PPh3)10Cl2(SC3H6Si(OC2H5)3)5 cluster was slowly added to the mixed solution. After stirring for 12 h, the solid was collected by filtration and washed with ethanol and acetone several times. The as obtained solid was then dried at 60 °C under vacuum. Further, the sample was calcined at 500 °C in air for 1 h. Characterization. The UV-Vis spectrum of the free gold nanoclusters (dissolved in ethanol) was obtained using a Hewlett-Packard (HP) Agilent 8453 diode array spectrophotometer at room temperature. A PerkinElmer Lamada 750 UV/VIS/NIR Spectrometer was used to record diffuse reflectance UV-vis spectra of the gold nanoclusters. A JEM-2100 microscope was used to carry out the transmission electron microscopy (TEM) with an accelerating voltage of 200 kV. One drop of an ethanolic solution of the samples was placed on a carbon thin-film coated TEM grid, and allowed to dry in air at room temperature before the measurement. A PerkinElmer ICP-MS NexION 300D was employed to perform inductively coupled plasma–mass spectrometry (ICP-AES). Before the measurement, we dissolved the samples in an aqua regia

solution, diluted with Nanopure water (resistance 18.2 MX cm, and purified using a NANOpure Di-water TM system. X-ray photoelectron spectroscopy (XPS) was performed using a Thermofisher ESCALAB 250 Xi electron spectrometer (300W AlKα radiation). The pressure was ~ 3×10-8 Pa. The C1s line at 284.6 eV from adventitious carbon was used as reference to calculate the binding energies. The gold content of the [Au25]@SiO2-500 and [Au25]/SiO2-500 were 1.0 and 0.79 wt%, determined by the ICP analysis. An ASAP 2420 (Micromeritics) was used to examine specific surface area and pore size distribution of the catalysts via N2 adsorption-desorption isotherms (BET and BJH methods) at -196°C. The samples were degassed for 6 h at 100°C to desorb surface impurities. Semi-hydrogenation investigation. The semihydrogenation of acetylene was performed with gas hourly space velocity (GHSV) of 18,000 mL h-1 g-1 at ambient pressure. We used a continuous flow fixed bed quartz reactor (8 mm inside diameter). 150 mg Au25@SiO2 catalysts were used. A gas mixture consisting of 2.0% C2H2, 20.0% H2 and balanced with He was introduced in the reactor. Mass-flow controllers were employed to control the flows of inlet gases. To examine the temperature dependency of the catalytic reaction in the range of 50 to 320 °C, we kept the reaction temperature (TR) constant for 25 min at a given TR. An on-line gas chromatograph (Agilent Technologies 6890N that is equipped with a FID detector) was used to analyze the gas mixture from the microreactor outlet. To maintain the TR at a given point, a programmable temperature controller was coupled with a moveable temperature detector placed inside the catalyst bed. DFT calculation. All calculations were spin polarized and carried out using the Quantum Espresso package. 24 The Projector Augmented-Wave (PAW)25 method and the Perdew-Burke-Ernzerhof functional (PBE) 26,27 were used for the calculations. A kinetic energy cutoff of 500 eV was used and integration was carried out in the reciprocal. Different k-point samplings were examined to check where energy is converged. A 3 × 3 slab of Au(111) containing 36 gold atoms in four layers was used. This model was large enough to avoid lateral interactions of adsorbent (i.e., C 2H2). During the geometry optimizations, adsorbates and the top two layers of the slabs were allowed to relax, while the two bottom layers were fixed at their bulk-optimized positions. For the case of Au25S5, we used structural framework of the Au25(PPh3)10Cl2(SC3H6Si(OC 2H5)3)5 cluster, and removed all organic ligands. All atoms of the later cluster were allowed to move during the optimization.

■ RESULTS AND DISCUSSION Synthesis and characterization of Au25@SiO2-500 catalysts. Au25(PPh3)10Cl2(SC3H6Si(OC2H5)3)5 clusters (hereafter, abbreviated as Au25) were prepared using the previously reported method.28 Next, the Au25 nanoclusters were cohydrolyzed with tetraethyl orthosilicate (TEOS) in ammonia solution, resulting in the encapsulation of Au 25 by microporous silica (Au25@SiO2).29,30 The Au25@SiO2 sample thus prepared was annealed at 500 oC in the presence of oxygen to remove organic ligands of its gold nanoclusters (hereafter, this ligand-


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ACS Applied Nano Materials free sample will be denoted as Au25@SiO2-500). For comparison, a sample of Au25 supported over SiO2 (Au25/SiO2) was annealed similarly at 500 oC to generate its ligand-free counterpart (Au25/SiO2-500). For details on the preparation of these catalyst samples, the reader is referred to the Experimental section. The samples were analyzed using UV-vis and solidstate 31P MAS NMR spectroscopies, and scanning transmission electron microscopy (STEM). We consider these results next.

Figure 1. (a) UV-vis spectra of an ethanol solution containing the Au25(PPh3)10(SC3H6Si(OC2H5)3)5Cl2 clusters. For the Au25@SiO2 and Au25/SiO2, diffuse reflectance UV-vis spectra are shown. (b) Solid-state 31P MAS NMR spectrum of Au25@SiO2. The Au25 nanoclusters dissolved in an ethanol solution exhibit two UV-vis peaks at 413 and 690 nm (Figure 1a, black line), in good agreement with previous studies. 25,26 The Au25@SiO2 and Au25/SiO2 samples also show two main but diffuse reflectance UV-vis peaks centered at ca. 415 and 695 nm (Figure 1a, red and blue lines). These results indicate that the structure of Au25 nanoclusters in the Au25@SiO2 and Au25/SiO2 samples is similar to that of Au25 in the ethanol solution. The Au25@SiO2 sample was analyzed further using TEM and solid-state 31P MAS NMR spectrometry. According to the TEM images, the particle size of the gold nanoclusters is ca. 1.3±0.5 nm (Figure S1 in the Supporting Information), consonant with the result for the intact Au25 nanoclusters. The chemical shift found at 33.5 ppm in the 31P NMR study of the Au25@SiO2 sample is assigned to phosphines ligated on the nanoclusters (Figure 1b). The other peak at  = 51.4 ppm can

be attributed to free PPh3 ligands, which are detached from Au25 nanoclusters under the alkaline condition (presence of ammonia), in good agreement with previous studies. 31,32 To obtain a quantitative understanding of the size distribution of gold clusters encapuslated in microporous silica, the Au25@SiO2 sample was analyzed using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The average size of the gold nanoclusters was found to be 1.5±0.4 nm (Figure 2a), consistent with that of the parent-clusters.29-32 Energy-dispersive X-ray spectroscopy analysis of the sample shows that gold, sulfur, and phosphine elements are uniformly distributed inside the silica framework (Figures 2b and 2c). This indicates that the Au25 nanoclusters do not aggregate, but are distributed throughout the microporous silica SiO2. The annealing process of Au25@SiO2 was monitored using the temperature programmed oxidation (TPO) method. Figure S2 shows that the organic tailors of the clusters became detached at 400-490 °C, and further oxidized to release H2O and CO2. In light of this finding, we chose 500 °C as the calcination temperature to remove organic ligands and produce Au25@SiO2-500 and Au25/SiO2-500 catalysts. Results of inductively coupled plasma (ICP) mass spectrometry indicate the gold content of the Au25@SiO2-500 and Au25/SiO2-500 is ~ 1.0 and 0.89 wt%, respectively. Analysis of 31P solid state NMR spectrometry of the samples shows a 31P chemical shift at 0.73 ppm, implying that all PPh3 ligands of the clusters are oxidized to form phosphate (Figure S3 in the Supporting Information). The pore size distribution of the samples is estimated by N2 adsorption/desorption isotherms analysis. The specific area of Au25@SiO2 and Au25@SiO2-500 is found to be 67 and 216 m2g-1 (Figure S4 in the Supporting Information). A size analysis of Au25@SiO2-500 via the BJH method reveals the microporosity of SiO2 in the range of 0.5-1.4 nm (Figure S5 in the Supporting Information). Of note, the un-calcined precursor Au25@SiO2 shows no obvious characteristics of porosity. Therefore, the microporosity of SiO2 was generated during the thermal pretreatment for the elimination of the organic ligands of Au25 nanoclusters. The HAADF-STEM analysis of the Au25@SiO2-500 samples shows the size of gold nanoclusters is ~ 1.6 ± 0.5 nm (Figure 3a), close to that of their parent nanoparticles.29-32 We also note the gold particles of the Au25@SiO2-500 samples are distributed throughout the SiO2 framework (Figures 3b-c),

Figure 2. (a) HAADF-STEM and the corresponding size distribution histogram of the Au25@SiO2. (b and c) Energy-dispersive

X-ray spectroscopy of Au25@SiO2.



similar to the un-calcined Au25@SiO2 (Figures 2b-c). These results indicate that the Au25 nanoclusters were encapsulated in the microporous silica, and did not aggregate during the calcination at 500 oC. However, the nanoclusters on the silica surface were found to aggregate and form bigger nanoparticles of size 4.2±1.1 nm during the thermal calcination (Figure 3d). With these results in mind, we turn to the catalytic performance of Au25@SiO2-500 and Au25/SiO2-500 for semihydrogenation of acetylene next.

Figure 4. (a) Catalytic performance of Au25@SiO2-500 and Figure 3. (a) HAADF-STEM and the corresponding size distribution histogram of Au25@SiO2-500. (b and c) Energydispersive X-ray spectroscopy element of Au25@SiO2-500. (d) TEM image of Au25/SiO2-500 and the corresponding size distribution histogram.

Catalytic test for semi-hydrogenation reaction of acetylene. The Au25@SiO2-500 and Au25/SiO2-500 catalysts were both found to be highly selective for ethylene production via the hydrogenation of acetylene (Figure 4a, blue points). The selectivity towards the desired product, ethylene, slightly decreases as the reaction temperature increases. The Au25/SiO2500 catalyst exhibits a very low catalytic efficiency for C2H2 conversion in the entire temperature range we studied; for example, it is ~ 0.0 and 14.6% at 50 and 300 °C, respectively. In contrast, the C2H2 conversion over Au25@SiO2-500 reaches to a converged rate of 95% at 280 °C. From the kinetics data, we estimated the apparent activation energy (Ea) for C2H2 hydrogenation with the Au25@SiO2-500 and the Au25/SiO2-500 catalysts. The results for Ea obtained using the Arrhenius relation are 38.8 and 54.4 kJ·mol⁻1 for Au25@SiO2-500 and Au25/SiO2-500, respectively (Figure 4b). The activation energy over the Au25@SiO2-500 is ~15.6 kJ·mol⁻1 lower than that over the Au25/SiO2-500 catalyst, another indication that the Au25@SiO2-500 catalyst is superior to the Au25/SiO2-500 in catalytic activity.

Au25/SiO2-500 for the semi-hydrogenation of acetylene at different reaction temperatures. Reaction conditions: gas mixture consists of 2.0% C2H2, 20.0% H2, and 78.0% He (v/v), GHSV = 18,000 mL g−1 h−1, 150 mg gold nanocluster catalysts. (b) Arrhenius plots for C2H2 conversion rate over the Au25@SiO2500 and the Au25/SiO2-500 catalysts. DFT calculation. To understand high catalytic activity of gold nanoclusters, we performed DFT calculations and examined the interactions of C2H2 with the catalysts’ surface. We first considered a slab of Au(111) to investigate the lowering of catalytic activity caused by the particle size growth as the HAADF-STEM results above indicated the formation of big gold particles during the thermal pretreatment. It is worthy of note that Au(111) possesses the lowest surface energy among all Au facets. Therefore, an increase in the particle size leads to a higher ratio of Au(111) facet. DFT predicts that acetylene is located at ~ 3.90 Å above the slab model of Au(111) with the interaction energy of -0.03 eV and C—C atomic distance of 1.21 Å (Figure 5A). Using the nudged elastic band (NEB) approach,35 we estimated that the activation energy of acetylene over the slab model is ~ 0.34 eV (Figure 5B). This value is considerably higher than the thermal energy (~ 0.03 eV), and thus a slow rate of acetylene adsorption and activation is expected as the size of gold particles grows, in good accord with the experimental results. In the final state, acetylene chemisorbs over two surface gold atoms of the model with the C—Au and C—C atomic distances of 2.09 and 1.32 Å, respectively (Figure 5C). The adsorption energy of the final adsorbed state is ~ +0.02 eV (Figure 5C). Here and hereafter, the adsorption energy is calculated as , where and


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ACS Applied Nano Materials are the energy of the system under consideration with adsorbed acetylene, and total energy of its isolated reactants, respectively. We extended our Au(111) model by adding Aun species (n =1, 2, and 3) on top of the slab’s surface to study the effect of the presence of low coordinated surface gold atoms on the adsorption of acetylene. DFT predicts the surface gold atoms thus generated are capable of interacting with acetylene considerably more exothermically than the Au(111) slab. The adsorption energy of acetylene on the surface Au n species is in the range of -0.46 to -1.10 eV (Figure S6 in the Supporting Information). These results indicate that the presence of low coordinated gold atoms at the surface of the particle can facilitate the acetylene activation. Since the relative population of such gold atoms in the particles of smaller size is considerably higher than that in bigger particles, it is expected that Au25@SiO2-500 exposes a higher density of active gold atoms and thus exhibits higher activity for the adsorption of acetylene. For further insight, we studied acetylene adsorption on the activated catalyst by employing Au25S5 as a model for the gold nanocluster after the thermal calcination where the organic ligands are detached from the nanocluster, i.e., Au25@SiO2500. Acetylene is found to interact exothermically with a surface gold atom of Au25S5 with the adsorption energy of -0.70 eV and the C—Au and C—C atomic distances of ~ 2.22 and 1.24 Å, respectively (Figure 5D). The adsorbed acetylene may change to a different adsorption mode by forming bonds with

two surface gold atoms of Au25S5 with adsorption energy of ~ -0.82 eV, and Au—C and C—C atomic distances of 2.05 and 1.33 Å, respectively (Figures 5E and 5F). The activation energy for the adsorption mode change was found to be ~ 0.45 eV. The energy of this amount can be easily supplied from the previous exothermic step. The results on adsorption energetics presented here indicate the gold particles of smaller size tend to chemisorb acetylene better, in good agreement with the spillover phenomenon observed for Au 25@SiO2-500 at low temperature. Specifically, acetylene conversion over Au25@SiO2-500 is higher at 50 °C than at 100 °C (red triangles in Figure 4a). This counterintuitive result is mainly due to the strong chemisorption of acetylene over the gold nanoclusters at 50 °C, which in turn lowers the concentration of acetylene in the reaction medium that can be accessed by the reaction detector. In other words, it is not the full conversion to ethylene but the strong chemisorption on gold nanoclusters that reduces the concentration of acetylene at 50 °C. To obtain experimental evidence for DFT predictions that C2H2 interacts more strongly with the Au25@SiO2-500 catalyst than the Au25/SiO2-500, we studied their IR spectra (Figure 6a). We first exposed the catalysts to C2H2, followed by vacuuming the systems to remove all non-interacting C2H2. Comparison of IR spectra of Au25/SiO2-500 prior and subsequent to vacuuming shows the disappearance of adsorbed C 2H2 after vacuuming. The Au25@SiO2-500 sample after vacuuming, on the other hand, exhibits a peak at 3238 cm1, corresponding to C—H stretching vibrations of acetylenes, strongly adsorbed over the catalyst. For completeness, the adsorption intensity of

Figure 5. Optimized Structure of acetylene, (A) physisorbed on, (C) bonded to two surface gold atoms of the Au(111) surface,

and (B) in the transition state between (A) and (C). Optimized structure of acetylene adsorbed to (D) one and (F) two gold atoms of the Au25S5 model, and (E) their transition state. Color code: Au, red; S, yellow; C, cyan; H, white.


ACS Applied Nano Materials acetylene over the Au25/SiO2-500 and Au25@SiO2-500 samples were investigated by measuring the differential heat as a function of adsorbate uptake. Results in Figure 6b show that while the acetylene adsorption capacity of the Au 25/SiO2-500 is very low (4.9 μmol·g-1 adsorbate), Au25@SiO2-500 can take up a relative high amount of C2H2 (ca. 20 μmol·g-1). These results along with the IR analysis are in concert with our theoretical finding, viz., acetylene interacts with Au25@SiO2-500 more strongly than with Au25/SiO2-500. While the focus of our study is the catalyst activity for the acetylene adsorption and activation, a considerable promotion of, e.g., H2 activation is also expected. According to previous studies by Corma et al., H2 tends to dissociate better on low coordinated Au atoms than on well-defined planes of big gold particles.33,34 Since an ultra-small gold cluster possesses a high proportion of corner and edge sites, we anticipate that H2 activation would show a significant acceleration over the Au25@SiO2-500 catalyst, compared to Au25/SiO2-500.

for the Au25@SiO2-500, corresponding to Au 4f7/2 (89.7 and 87.8 eV) and Au 4f5/2 (84.3 and 83.5 eV), indicating the presence of the Au+ species and metallic gold,37 in good accord with CO–FTIR results. For the case of Au25/SiO2-500 catalysts, the gold atoms were found to be mainly metallic (Au0) as only one prominent set of XPS peaks at 87.1 (Au 4f 7/2) and 83.4 eV (Au 4f5/2) was observed. To reveal the origin of the different oxidation states of gold in the catalyst samples, the S and P XPS spectra of Au25@SiO2-500 and Au25/SiO2-500 were investigated. Results indicate the existence of sulfide and high valent phosphorus in the Au25@SiO2-500 sample (Figures S7 and S8 in the Supporting Information). We also note that gold nanoparticle is encapsulated by microporous silica which is an oxygen rich materials. The abundant presence of the electronegative species (oxygen, sulfide and phosphorus) is deemed to induce positive charges on nearby surface gold atoms of the nanoclusters. For the case of the Au 25/SiO2-500 catalyst, no sulfur and phosphorus species were found, implying that all the thiolate and phosphine ligands of the Au25/SiO2-500 catalyst were removed during the thermal annealing (Figures S7 and S8 in the Supporting Information). These results along with the experimental analysis presented above suggest that the presence of Au+ species due to the electronegative nature of the surrounding elements in microporous silica may assist the acetylene activation for the semihydrogenation reaction.

Figure 6. (a) IR spectra of acetylene adsorption and (b) differential heat versus adsorbate uptake for C 2H2 adsorption over Au25/SiO2-500 and Au25@SiO2-500 catalysts. The chemical state of the surface gold atoms was investigated by the infrared spectra of carbon monoxide adsorption (CO–FTIR) on the catalysts. Figure 7 shows that for the Au25@SiO2-500 catalyst, there are two CO adsorption bands centered at 2160 and 2110 cm1, assigned to CO adsorbed over Au+ and Au0 species, respectively.36 By contrast, the Au25/SiO2-500 catalyst exhibits a weak CO adsorption band at 2133 cm1, indicating the catalyst is composed of big nanoparticles with metallic characteristics, i.e. Au0.

2160 Abs. (a.u.)

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2112

Au25@SiO2-500

2131

Au25/SiO2-500 2300

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Figure 7. FT-IR spectra of CO adsorbed on Au25/SiO2-500 and Au25@SiO2-500. The oxidation state of Au atoms of the catalysts was analyzed further by XPS. Figure 8a shows two sets of XPS peaks

Figure 8. Au 4f XPS spectra of the (a) Au25@SiO2-500 and (b) Au25/SiO2-500. The Au25/SiO2-500 only contains Au0 species, while the Au25@SiO2-500 presents two gold species, i.e., Au0 and Au+. Recyclability. We finally examine the Au25@SiO2-500 catalyst durability for prolonged conversion of acetylene to ethylene at 280 °C. The acetylene conversion with the catalyst was found to decrease gradually, it reduces to ~50% after 15 hrs of operation (Figure S9 in the Supporting Information). Though speculative, there are two possible reasons for the deactivation of the catalyst over operation time: (i) cluster growth to form bigger gold particles, and hence loss of the active surface area, and (ii) carbonaceous deposition formation, which can potentially cover the active sites and therefore block the hydrogenation reaction. To examine these possibilities, we first collected the used catalysts, and then calcined it at 400 °C for 1 h to remove any organic deposits. This thermal treatment was found to recover the activity of the catalyst (Figure S9). We next studied the particle size distribution of the recovered catalyst using transmission electron microscopy (TEM). Analysis of TEM images of the calcined catalyst indicates the average size of the gold clusters is about 2.2 nm (Figure S10 in the Supporting Information), demonstrating minimal particle growth occurred during the operation of Au25@SiO2-500 catalyst. Carbon deposition over the cata-


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ACS Applied Nano Materials lyst during the acetylene conversion was found to be ~ 8% at 280 oC (Figure S11 in the Supporting Information). These results suggest that the decrease in catalyst’s activity over time is likely due to the carbon deposition that arises from the ethylene polymerization, blocking the active sites at the surface of the gold clusters. Nevertheless, the fact that recycled catalyst is characterized by nearly the same activity and selectivity as the fresh catalyst shows the good recyclability of the Au25@SiO2-500 catalyst in the semi-hydrogenation reaction. ■ CONCLUSIONS In this paper, we presented a protocol to encapsulate gold nanoclusters in microporous silica. The encapsulated nanoclusters were thermally annealed at 500 °C to remove their organic ligands. Detailed experimental analysis shows the annealed clusters do not aggregate; they maintain the average size of ~ 1.6 nm. The as obtained nanoclusters exhibits robust catalytic activity for semi-hydrogenation of acetylene with acetylene conversion efficiency of ~99.5% and selectivity of 98% towards ethylene formation at 280 °C. Analysis based on a combined approach of theory and experiments indicates that the small size of the gold clusters and the presence of positively-charged surface gold atoms induced by the electronegative species in the silica pores play an important role in facilitating the activation of acetylene, compared to the big gold nanoparticles. The results of this work may open a new avenue for the synthesis of confined gold nanocluster catalysts for robust catalytic performance of a variety of reactions.

■ ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Additional characterization data, including TEM image, TPO, solid-state 31P MAS NMR spectrum, N2-sorption isotherm curves, P 2p and S 2p XPS spectra, durability test, optimized Structure of acetylene adsorbed over low coordinated gold atoms (PDF).

■ AUTHOR INFORMATION Corresponding Author *Email: *[email protected] (H.A.), [email protected] (G.L.) &Z.L.

and H.C. contributed equally to this work.

ORCID Hadi Abroshan: 0000-0003-1046-5170 Hyung J. Kim: 0000-0003-4334-1879 Gao Li: 0000-0001-6649-5796

Notes The authors declare no competing ﬁnancial interest.

■ ACKNOWLEDGMENT G. L. acknowledges financial supports by the fund of the National Natural Science Foundation of China (No. 21601178). BL14B and BL17B beamline of National Facility for Protein Science (NFPS), Shanghai Synchrotron Radiation Facility (SSRF) for providing the beam time.

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