Catalytic CO Oxidation Using Bimetallic MxAu25–x Clusters: A

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Catalytic CO Oxidation Using Bimetallic MxAu25−x Clusters: A Combined Experimental and Computational Study on Doping Effects Weili Li,†,∥,⊥ Chao Liu,‡,⊥ Hadi Abroshan,§ Qingjie Ge,*,† Xiujuan Yang,‡ Hengyong Xu,† and Gao Li*,‡

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Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ‡ Gold Catalysis Research Center, State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China § Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ∥ University of Chinese Academy of Sciences, Beijing 100049, China ABSTRACT: Heteroatom-doped gold clusters have been of great interest to modify the catalytic performance of the clusters, although the structure−activity correlation is rarely discussed. Herein, we explore the catalytic activity of bimetallic gold-based MxAu25−x(SR)18 (M = Cu and Ag, and SR = SC2H4Ph) clusters and compare with that of homogold Au25(SR)18 in the CO oxidation reaction. It is found that the CeO2supported clusters show catalytic activity for the reaction in the order CuxAu25−x(SR)18 > Au25(SR)18 > AgxAu25−x(SR)18. The supported clusters exhibit excellent catalytic activity and durability for prolonged conversion of CO into CO2 (98% in the case of CuxAu25−x(SR)18 at 120 °C for 110 h). Fourier-transform infrared (FT−IR) analyses as well as the density functional theory (DFT) calculations suggest that the thiolate ligands are partially removed under reaction conditions (T > 120 °C). The metal atoms thus exposed (Au, Ag, and Cu) are deemed as the catalytic active sites. DFT calculations suggest that metal exchange between the icosahedral core and the staple motif of the clusters at elevated temperature plays an important role on the conversion rate of CO oxidation. The adsorption of CO on the clusters would be more preferable to occur in the order Cu2Au23(SCH3)15 > Au25(SCH3)15 > Ag2Au23(SCH3)15, which is in line with the experimental results on the CO oxidation catalyzed by the bimetallic clusters.

1. INTRODUCTION Gold nanoparticles have been presented as a new robust and green class of catalysts for different organic synthesis over the past decade or so.1−6 However, issues, such as the polydispersity of the nanoparticles can pose a serious challenge to establish definitive structure−activity relationships, which is a key importance for the development of new catalysts for specific reactions. Fortunately, recent synthesis of atomically precise gold nanoclusters protected by thiolate ligands (e.g., Au25(SR)18 cluster, “−SR” represents thiolate ligands) provides molecularlevel understanding of surface structure of the metal catalysts to pursue the fundamental mechanistic understanding of heterogeneous catalysis.7−10 The Au25(SR)18 nanocluster is composed of a 13-atom icosahedral core (Au13) and six staple shell motifs Au2(SR)3, that is, Au13@Au12(SR)18.11 The catalytic efficiency and selectivity of such an ultrasmall particle have been investigated in several reactions including carbon−carbon coupling reactions and selective hydrogenation and oxidation, just to name a few.12,13 It is commonly acknowledged that CO is a serious poison to the fuel cell platinum catalyst; therefore, the conversion of CO into CO2 is highly desirable for the removal of CO from the H2 feed gas of the fuel cells.14 Ever since Haruta and co-workers reported on the potential applications of supported gold nanoparticles for the catalytic CO oxidation,15 different gold © 2016 American Chemical Society

nanoclusters (e.g., Au25(SR)18 and Au25(PPh3)10(SR)5Cl2) have been extensively investigated to improve the catalytic efficiency of the reaction under different conditions.8,16−18 Despite extensive studies on the catalytic CO oxidation using supported Au nanoclusters, the reaction mechanism is still debatable. Studies suggest that gold nanoparticles may undergo structural changes in the interfacial regions on metal oxide supports (e.g., CeO2), and the metal sites at the interface of the gold particles/ support are responsible for the catalytic reactions.8 According to recent studies, partial removal of the protecting ligands (SR) of the Au25(SR)18 at high temperatures (e.g., >130 °C) can expose some bare gold atoms which are deemed as the catalytic active sites of the reactions.5,8,19,20 Doping of the Au25(SR)18 nanocluster can largely modify the electronic and physical properties of the cluster, which may exert a strong influence on the catalytic properties of the Au nanoclusters.21−25 The recent synthesis of the bimetallic goldbased MxAu25−x(SR)18 nanoclusters (e.g., M = Ag, Pt, and Cu, x = 1, 2, 3, etc.) indicates that the clusters have a similar framework as Au25(SR)18.21−26 The Ag and Cu atoms are preferentially located at the 13-atom icosahedral core of the nanoclusters (i.e., Received: January 25, 2016 Revised: March 26, 2016 Published: April 25, 2016 10261

DOI: 10.1021/acs.jpcc.6b00793 J. Phys. Chem. C 2016, 120, 10261−10267

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The Journal of Physical Chemistry C MxAu13−x@Au12(SR)18).21−26 Experimental and theoretical investigations have revealed that doping of the Au25(SR)18 cluster with a single platinum atom to form Pt1Au24(SR)18 increases the stability of the cluster.26 The electronic structure of Au25(SR)18 doped by silver atoms also show notable changes, and doping with chromium, manganese, or iron results in formation of paramagnetic clusters.27−29 Tsukuda and co-workers showed that catalytic activity of Au25 cluster can be significantly enhanced by single Pd atom doping (i.e., PdAu24) for the aerobic alcohol oxidation.20 In our previous work, Pt1Au24(SR)18/TiO2 catalyst is studied for the styrene epoxidation, which also showed a higher activity and selectivity than Au25(SR)18/TiO2 catalyst.26 These results obviously suggest that the catalytic activity of the clusters can be tailored by doping of the particles to improve their potential applications in heterogeneous catalysis. Herein, we explore the activity of bimetallic MxAu25−x(SR)18 nanoclusters (M = Au, Cu, and Ag, and SR = SC2H4Ph, hereafter) for the catalyzed CO oxidation. Results show that the CeO 2 -supported CuxAu25−x(SR)18 catalyst performs better than Au25(SR)18 catalyst, while the AgxAu25−x(SR)18 cluster is less active than Au25(SR)18. Further, we demonstrate the significant role of dopant atoms in the catalytic oxidation through a combined approach of experimental and computational methods.

5.3 and 3.4 for the AgxAu25−x(SR)18 and CuxAu25−x(SR)18 clusters, respectively. 2.3. Synthesis of CeO2 Support. CeO2 oxide is synthesized via the coprecipitation of (NH4)2Ce(NO3)6 in the presence of (NH4)2CO3. Typically, (NH4)2CO3 (1.63 mol/L, dissolved in water) was slowly added to a solution of (NH4)2Ce(NO3)6 in a dropwise fashion at 55 °C until pH of the solution reached 9−10. After stirring for 2 h, the precipitates were collected by centrifugation (2000 rpm, 4 min), and washed with water for three times. The as-obtained solids were dried at 120 °C overnight (ca. 14 h). The solid sample was grinded into powder, and was calcined in a muffle at 500 °C under atmospheric condition for 4 h with a heating rate of 5 °C/min. 2.4. Immobilization of MxAu25−x(SR)18 Clusters on CeO2 Support. Typically, 2 mg MxAu25−x(SR)18 clusters were dissolved in 5 mL CH2Cl2, and 200 mg CeO2 oxide was added. After stirring for 6 h at room temperature, the supernatant became colorless. The MxAu25−x(SR)18/CeO2 catalysts were collected by centrifugation (3000 rpm, 1 min), and was air-dried at room temperature. ICP−MS analysis indicates that the loading amount of the Au 2 5 (SR) 1 8 , Ag x Au 2 5 − x (SR) 1 8 , and CuxAu25−x(SR)18 clusters on the oxide support is ca. 0.91%, 0.94%, and 1.02%, respectively. 2.5. Catalytic Activity Test for CO Oxidation. The catalytic activity of the MxAu25−x(SR)18/CeO2 (M = Au, Ag, and Cu) was tested in a continuous flow fixed bed quartz reactor (8 mm inside diameter) under ambient pressure and with gas hourly space velocity (GHSV) ranging from 15 000 and 30 000 mL g−1 h−1. In a typical experiment, 50 mg of MxAu25−x(SR)18/CeO2 catalysts were heated to a reaction temperature at a heating rate of 5 °C/min in an O2 flow (30 mL/min). The catalysts were kept at the temperature for 2 h and then spontaneously cooled to ambient temperature before switching to the reactant gas mixture consisting of 1.67% CO, 3.33% O2, and 95% He (v/v). The flows of inlet gases were controlled by mass-flow controllers. The catalyst was conditioned for 30 min under this mixture at ambient temperature before the products were analyzed by an online gas chromatograph (Shimadzu, GC-8A), which was equipped with a carbon molecular sieve column (TDX-01, Dalian Zhonghuida Scientific Instrument Co. Ltd.) and a thermal conductivity detector. Analogous measurements were performed between ca. 20 and 120 °C with an interval of 20 °C. The reaction temperature was controlled by a programmable temperature controller and detected by a movable thermocouple inside the catalyst bed. 2.6. Computational Details. Density Functional Theory (DFT) calculations are performed to investigate CO adsorption on partially dethiolated Au25(SR)18, AgxAu25−x(SR)18, and CuxAu25−x(SR)18 nanoclusters (NCs). All ground state geometry optimizations were carried out with the PBE0 hybrid density functional and def2-SV basis sets.31−33 For gold atoms, we applied effective core potentials with the scalar relativistic corrections.34 The adsorption energy (ΔEad) is defined as [E(NCs/CO) − E(NCs) − E(CO)], where E(NCs/CO), E(NCs), and E(CO) are the energies of the adsorbed system, isolated gold clusters, and the isolated CO molecule, respectively. All calculations were carried out with the Gaussian09 package.35 The structure of Au25(SR)18 was employed as a starting point for DFT calculations for MxAu25−x(SR)18 nanoclusters.11

2. EXPERIMENTAL METHODS 2.1. Synthesis of MxAu25−x(SR)18 Nanoclusters. The Au25(SR)18, CuxAu25−x(SR)18, and AgxAu25−x(SR)18 nanoclusters were synthesized according to a previously reported method.30 For the case of the CuxAu25−x(SR)18 cluster, HAuCl4·3H2O (0.12 mmol), Cu(OAc)2 (0.015 mmol), and tetraoctylammonium bromide (0.16 mmol) were dissolved in THF (10 mL) in a three-necked flask. After stirring for 10 min, 90 mL PhCH2CH2SH was added to the reactor. The solution gradually turned light yellow within 20 min. Next, an aqueous solution of NaBH4 (51 mg, freshly dissolved in 3.0 mL ice-cold nanopure water) was added to the reactor at once under slow stirring (100 rpm). The reaction was stopped after 3 h, and THF was removed by rotary evaporation. The remaining black solids were collected and washed three times with methanol. Finally, the CuxAu25−x(SR)18 nanocluster was extracted using acetone. A similar procedure using AgOAc is followed for synthesis of AgxAu25−x(SR)18 clusters. 2.2. Characterization. The UV−vis spectra of the MxAu25−x(SR)18 nanoclusters (dissolved in CH2Cl2) were recorded on a Hewlett-Packard (HP) Agilent 8453 diode array spectrophotometer. MALDI mass spectrometry was performed with a PerSeptive Biosystems Voyager DE super-STR time-offlight (TOF) mass spectrometer. Trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenyldidene]malononitrile was used as the matrix in MALDI−MS analysis. Typically, 0.1 mg matrix and 10 μL analyte stock solution were mixed in 10 μL CH2Cl2. Ten μL solution was applied to the steel plate and then dried under vacuum (at room temperature) prior to MALDI mass spectrometry analysis. Fourier-transform infrared (FT−IR) measurements were recorded on a Bruker VERTEX 70 instrument with Bruker HYPERION 3000 (resolution, 1 cm−1; scans, 12; range, 2000−3600 cm−1). Inductively coupled plasma−mass spectrometry (ICP−MS) was recorded on a PerkinElmer ICP-MS NexION 300D. The samples were dissolved in aqua regia solution (HCl/HNO3 = 3:1 v/v) and was diluted with Nanopure water (resistance 18.2 MX·cm,) before the test. The ICP−MS analysis shows that on average x =

3. RESULTS AND DISCUSSION The synthesis of the homogold Au25(SR)18 and two bimetallic CuxAu25−x(SR)18 and AgxAu25−x(SR)18 nanoclusters followed 10262

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Figure 1. (A) UV−vis spectra of Au25(SR)18, AgxAu25−x(SR)18, and CuxAu25−x(SR)18 clusters. Positive-mode MALDI mass spectra of the (B) Au25(SR)18, (C) AgxAu25−x(SR)18 (x = 3 to 6), and (D) CuxAu25−x(SR)18 (x = 0 to 4). The insets in (C) and (D) are the zoom-in spectra.

Figure 2. Catalytic activity of (A) Au25(SR)18/CeO2, (B) AgxAu25−x(SR)18/CeO2, and (C) CuxAu25−x(SR)18/CeO2 catalysts for the CO oxidation. (D) Blank experiments only using plain CeO2 as catalyst and the control reactions using Au(I)-SR polymers and Ag(I)-SR and Cu(II)-(SR)2 analogs as catalyst. All catalysts were pretreatmented under a N2 atmosphere at room temperature for 30 min. Reaction conditions: GHSV = 15000 mL g−1 h−1, 50 mg catalyst (ca. 1 wt % MxAu25−x(SR)18 cluster or Au(I)-SR polymers and Ag(I)-SR and Cu(II)-(SR)2 analogs loading). The red dotted lines show induction periods of the MxAu25−x(SR)18/CeO2 catalysts.

the protocols reported in literature.30 These nanoclusters were then characterized by UV−vis and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. As shown

in Figure 1A (black line), the UV−vis spectrum of Au25(SR)18 cluster has three distinct peaks at 670, 450, and 400 nm, in consistent with the previous study.9 The MALDI mass spectrum 10263

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Figure 3. (A) Catalytic activity of the Au25(SR)18/CeO2, CuxAu25−x(SR)18/CeO2, and AgxAu25−x(SR)18/CeO2 catalysts after thermal treatment (120 °C under the reaction gas (1.67% CO, 3.33% O2, and 95% He, v/v) atmosphere for 1 h). Reaction conditions are the same as noted in Figure 2. (B) Recyclability test of the CuxAu25−x(SR)18/CeO2 catalyst, up to 4th cycle. (C) CO conversion (%) as a function of reaction time for the CuxAu25−x(SR)18/CeO2 catalyst at 120 °C. Reaction conditions: GHSV = 30 000 mL g−1 h−1, 50 mg catalyst (ca. 1 wt % CuxAu25−x(SR)18 cluster loading).

of the Au25(SR)18 cluster shows an intense peak at m/z = 7394 Da, which corresponds to the theoretical molecular weight of the cluster. The UV−vis of the CuxAu25−x(SR)18 nanocluster shows a weak and broad peak centered at ca. 690 nm (Figure 1A, blue line). The AgxAu25−x(SR)18 cluster exhibits several weak peaks at 470, 520, and 670 nm (Figure 1A, red line). Of note, the UV−vis profile of the bimetallic CuxAu25−x(SR)18 and AgxAu25−x(SR)18 nanoclusters is affected by the number of the dopants (i.e., x) present in the structure of the clusters.8 The molecular mass and chemical formula of the bimetallic nanoclusters are further characterized using MALDI mass spectrometry. A serial of peaks are observed in the mass spectra of the AgxAu25−x(SR)18 and CuxAu25−x(SR)18 nanoclusters (Figure 1C and D). The average spacing of peaks is found to be 89.0 Da for the case of AgxAu25−x(SR)18 cluster (Figure 1C, inset), which equals to the average atomic mass (AM) difference of gold and silver (AMAu − AMAg = 197.0 − 107.9 = 89.1 Da). This indicates that, in comparison to the homogold cluster, some of the gold atoms were replaced with silver atoms. The peak assignments are as follows: peak I at m/z = 7126 is assigned to Ag3Au22(SR)18, peak II at 7037 to Ag4Au21(SR)18, peak III at 6948 to Ag5Au20(SR)18, and peak IV at 6859 to Ag6Au19(SR)18 (Figure 1C, inset). As shown in Figure 1D, the MALDI mass spectrum of the CuxAu25−x(SR)18 shows that the average of peak spacing is 133.5 Da, which implies the substitution of gold by copper atoms (AMAu − AMCu = 197.0−63.5 = 133.5 Da). The mass peaks are assigned as follows (Figure 1D, inset): peak V at m/z = 7394 to Au25(SR)18, peak VI at 7261 to Cu1Au24(SR)18, peak VII at 7126 to Cu2Au23(SR)18, peak VIII at 6995 to Cu3Au22(SR)18, and peak IX at 6860 to Cu4Au21(SR)18. Of note, the number of metal atoms in all clusters is preserved to be 25. The maximum replacement of gold by dopant atoms for CuxAu25−x(SR)18 and AgxAu25−x(SR)18 are found to be 4 and 6, respectively (Figure 1C and D). The catalytic activity of the nanoclusters is examined for the oxidation of CO into CO2. The MxAu25−x(SR)18/CeO2 (M = Au, Ag, and Cu) catalysts are prepared by impregnation of CeO2 powders in a dichloromethane solution of the clusters. Of note, the CeO2 is chosen as the support since no catalytic activity is observed using other oxides (e.g., SiO2, ZnO, and TiO2), consistent with the previous study.16 The higher activity of the clusters over CeO2 in comparison with other oxides is usually referred to the surface oxygen vacancies due to the quick and reversible redox between Ce4+ and Ce3+.36 These vacancies are

believed to not only increase the dispersion and stability of clusters but also enhance CO oxidation rate.36−38 Although the mechanism of CO oxidation catalyzed by Aun(SR)m nanoclusters supported on CeO2 remains as a mystery, it is proposed that CeO2 may strongly interact with the Aun(SR)m clusters and help removal of thiolate ligands in the presence of O2 gas.8 The detailed interactions between gold clusters and supports deserve more research in the future studies. The fresh Au25(SR)18/CeO2 catalysts are examined for CO oxidation prior to any thermal treatments, which gives rise to a very low catalytic activity if T < 80 °C (0.0−2.3% CO conversion, Figure 2A). However, the CO conversion dramatically increases to 75.1% when the reaction temperature is risen to 100 °C and maintained for an induction period of 10 h (Figure 2A, dash red line). The catalytic activity of Au25(SR)18/CeO2 gradually increases up to 90.5% at 120 °C (Figure 2A), consistent with the previous studies.39,40 The AgxAu25−x(SR)18/CeO2 catalysts show no catalytic activity at 80 °C and only 3.0% CO conversion was determined at 100 °C (Figure 2B). The induction period of the AgxAu25−x(SR)18/CeO2 catalyst occurred at 120 °C, where the CO conversion increases from 5.3% to 86.3% (Figure 2B). The CuxAu25−x(SR)18/CeO2 showed 5.9% and 10.4% CO coversion at 80 and 100 °C, respectively. The induction period of the later catalyst appears at 120 °C that CO conversion increases from 21.7% to 95.9% (Figure 2C). Also notable is CO conversion is not detected for the case of blank experiment in which the clusters are absent and only CeO2 oxide is present (Figure 2D, black line). This confirms that catalytic activity arises from the nanoclusters. For completeness, catalytic activity of CeO2-supported Au(I)SR, Ag(I)-SR, and Cu(II)-(SR)2 polymers was investigated under the same conditions as for the MxAu25−x(SR)18/CeO2 (M = Au, Ag, and Cu). We note that the Au(I)-SR/CeO2 and Ag(I)SR/CeO2 show no activity for CO conversion into CO2 even at T = 120 °C (Figure 2D, red and blue lines). The Cu(II)-(SR)2/ CeO2 gives rise to 36.2% conversion of CO at 120 °C (Figure 2D, green line), which is considerably lower than that for CuxAu25−x(SR)18 clusters (95.9% CO conversion at 120 °C). The control reactions indicate that catalytic active sites are located on the clusters. We further examined the catalytic activity of the nanoclusters for the second cycle. Compared with the first cycle, there is no induction period for the three catalysts during the CO oxidation reactions, as shown in Figure 3A. Results indicate that CO conversion catalyzed by the CeO2-supported Au25(SR)18, 10264

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conversion in the second cycle is higher with no induction period in comparison to that in the first cycle. To rationalize the effect of doping agents (e.g., Ag and Cu) on the CO oxidation, we used similar model for DFT calculations as applied in the previous study by Jiang et al.8 It is hypothesized that a small number of thiolate ligands are removed to expose open metal sites to catalyze the reaction.8 Three thiolate ligands from the same dimeric staple motif of Au25(SCH3)18 are removed to result in Au13@Au12(SR)15, and the cluster was optimized (Figure 5A). Next, the geometry optimization was performed for

CuxAu25−x(SR)18, and AgxAu25−x(SR)18 nanoclusters at room temperature is 7.4%, 7.4%, and 9.6%, and increased at 100 °C to 92.3%, 96.6%, and 84.5%, respectively (Figure 3A). According to above results, the conversion rate is in the order of CuxAu25−x(SR)18 > Au25(SR)18 > AgxAu25−x(SR)18. We also examined the recyclability of CuxAu25−x(SR)18/CeO2 catalyst under the identical conditions as indicated in Figure 2. As shown in Figure 3B, no appreciable loss of catalytic activity is observed after four cycles (higher cycles are not tested). These results indicate the clusters can be characterized as good recyclable catalysts for CO oxidation reaction. Further, we investigated the durability of the CuxAu25−x(SR)18/CeO2 catalyst, which is found to yield the highest conversion of CO (Figure 3B). The reaction conditions are the same as that noted in Figure 2, except that the GHSV increased to 30 000 mL g−1 h−1. The CO conversion at 120 °C as a function of reaction time is monitored for 125 h. As shown in Figure 3B, the fresh CuxAu25−x(SR)18/CeO2 gave rise to 15.5% CO conversion at the beginning of the reaction, then increased to 86.3% after 12.5 h (induction period), which is consistent with the catalytic results of the first cycle of the cluster (Figure 2C). Finally, the rate of conversion converged to ∼98% with no appreciable loss of activity to the end of experiment (Figure 3C), indicating that the catalyst is robust and holds promise for prolonged periods of time. Of note, the turnover frequency (TOF = [reacted mol of CO]/[(mol of cluster) × (reaction time)]) of the reaction is found to be ca. 4 s−1. Previous studies suggest that the surface thiolate groups of the gold nanoclusters can be partially removed (e.g., in the presence of oxidants such as O2 and O3).17,39−44 To investigate this, we performed FT−IR measurements of the CeO2-supported Au25(SR)18, AgxAu25−x(SR)18, and CuxAu25−x(SR)18 catalysts before and after the thermal treatment in the presence of a gas mixture consisting of O2 and CO. The fresh catalysts show IR absorption bands at 2860, 2930, and 2960 cm−1, which correspond to CH stretching vibrations of the thiolate ligands (SCH2CH2Ph), Figure 4, black lines. After thermal treatment

Figure 5. (A) Structure of the Au13@M2Au10(SCH3)15 clusters, where M = Au, Ag, and Cu. (B) CO adsorption on the open metal site (M) of clusters. Color code: Au, green; S, yellow; C, gray; and O, red. Of note, two dethiolated metal atoms (M) are shown in pink.

M2Au23(SCH3)15 systems, in which the exposed gold atoms are replaced with either Ag or Cu. Results show the two open metal atoms (M sites, Figure 5A) form a bond with a distance of 2.894, 2.868, and 2.514 Å for AuAu, AgAg, and CuCu, respectively. DFT also indicates CO molecule can adsorb on one of the dethiolated metal atoms (Figure 5B) via an exothermic step of 1.19, 0.75, and 1.30 eV and with CM atomic bond length of 1.950, 2.071, and 1.876 Å for the cases of Au13@ Au 1 2 (SCH 3 ) 1 8 , Au 1 3 @Ag 2 Au 1 0 (SCH 3 ) 1 8 , and Au 1 3 @ Cu2Au10(SCH3)18, respectively (Table 1). The chemisorption Table 1. Interatomic Distances and Adsorption Energy of CO on M2Au23(SR)15 Models, Where M = Au, Ag, and Cu and R = CH3a MM (Å) ΔEad (eV) MC (Å) CO (Å)

Au25(SR)15a

Ag2Au23(SR)15

Cu2Au23(SR)15

2.894 (2.9) −1.19 (−1.12) 1.950 (1.95) 1.133 (1.134)

2.868 −0.75 2.071 1.128

2.514 −1.30 1.876 1.131

a

Results reported in ref 8 are given in parentheses to compare with those obtained in our study. Negative value in ΔEad indicates a favorable interaction.

Figure 4. FT−IR spectra of the CeO2-supported (A) Au25(SR)18 and (B) AgxAu25−x(SR)18 catalysts before and after thermal treatment (at 120 and 150 °C) in the presence of a mixture of gases (1.67% CO, 3.33% O2, and 95% He, v/v).

of CO molecule on Au and Cu sites results in slightly increment of the CO bond length (1.133 and 1.131 Å) in compression to that for an isolated CO which is 1.128 Å (Table 1). Interestingly, the CO bond length of adsorbed CO molecule on Ag atoms remains as before adsorption (isolated CO). DFT calculations show the adsorption of CO on the different doped systems would be more preferable to occur in order of Au13@Cu2Au10(SCH3)15 > Au13@Au12(SCH3)15 > Au13@Ag2Au10(SCH3)15, in good agreement with the experimental results of catalytic performance of the clusters. One potential explanation is that the CO adsorption and activation can be a key step of the reaction. It is worthwhile to mention that the trend of the CO adsorption

at 120 °C, the intensity of the absorption bands became slightly weaker (Figure 4, red lines). These bands considerably lose their intensity after thermal treatment at 150 °C (Figure 4, blue lines). Similar behavior is found for the case of CuxAu25−x(SR)18 and thus is not shown. Consistent with thermogravimetric analysis in the previous study,10 these results strongly indicate that the  SCH2CH2Ph ligands were gradually removed from the surface of the clusters at high temperatures to expose metal active sites to reactants. This speculation is in good agreement with the catalytic performance of the clusters. We note that the CO 10265

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120 °C), which in turn expose open metal sites (M) for the catalytic reaction. DFT calculation suggests that the CO adsorption on the clusters would be more preferable to occur as Cu2Au23(SCH3)15 > Au25(SCH3)15 > Ag2Au23(SCH3)15, in good agreement with the experimental results. While our study reported here contributes novel insights on CO adsorption and oxidation, some aspects were not properly accounted for in our analysis. These include molecular mechanism of CO oxidation and possible roles of CO bond activation in the ratedetermining step of the reaction. Also, we used x = 1 or 2 for MxAu25−x(SC2H4Ph)18 in DFT calculation, but the MALDI−MS results suggest x = 0 to 4 for CuxAu25−x(SR)18 and x = 3 to 6 for AgxAu25−x(SR)18. Therefore, it would be worthwhile in the future to extend the current study and investigate details of CO oxidation pathways catalyzed by the nanoparticles and how doping amount may influence the catalytic activity.

(ΔEad) results are in well consistent with those previously reported on the pure metals (ΔEad,Cu > ΔEad,Au > ΔEad,Ag).45,46 A possible reason includes smaller size of Cu and Au in comparison with that of Ag, thereby facilitating a better interaction with CO. We further investigated the CO adsorption energy when dopant atoms located at the 13-atom icosahedral core instead of the staple motif (i.e., MxAu13−x@Au12(SCH3)18, where x = 1 or 2). We considered two possible configurations: (i) a single Ag or Cu atom is located at the center of the Au13 core to form M1Au12@Au12(SCH3)15 (Figure 6A) and (ii) two atoms (Ag or

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86 8437 9229. E-mail: [email protected] (Q.G.). *Tel: +86 8246 3017. E-mail: [email protected] (G.L.).

Figure 6. Structure of (A) M1Au12@Au12(SCH3)15 and (B) M2Au11@ Au12(SCH3)15 clusters, where M = Au, Ag, and Cu. The dopant atoms are located at the center and surface of the 13-atom icosahedral core in (A) and (B), respectively. The same color code is used as that in Figure 5.

Author Contributions ⊥

W.L. and C.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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Cu) are placed in close proximity at the surface of the icosahedral core to result in M2Au11@Au12(SCH3)15 (Figure 6B). Of note, the active metal sites for CO adsorption are gold atoms on the staple motif in both configurations. DFT calculations show adsorption energy of CO on M1Au12@Au12(SCH3)15 is −1.17 and −1.12 eV for silver and copper doped systems, respectively. These results are notably close to that obtained for the Au25(SCH3)15 cluster (−1.19 eV, Table 1). However, the CO adsorption energy on Ag2Au11@Au12(SCH3)15 and Cu2Au11@ Au12(SCH3)15 are found to be −1.06 and −1.21 eV, respectively. These findings may indicate the effects of dopants are more pronounced if they are located at the staple motif of the clusters to interact directly with CO. It is worth mentioning that previous studies indicate that Ag and Cu atoms are preferentially located at icosahedral core instead of the staple motif.21−25 Therefore, on the basis of our calculations one may expect similar catalytic activity of MxAu25−x(SCH2CH2Ph)15 (M = Ag and Cu) clusters at low temperatures. However, at elevated temperature metal exchanges between the icosahedral core and the staple motif is prudent to consider. Experimental results also indicate that the clusters show similar activity at lower temperatures ( Au25(SC2H4Ph)18 > AgxAu25−x(SC2H4Ph)18 (based on the conversion of CO). Fourier-transform infrared analysis indicated that the surface ligands are capable to leave the clusters at high temperature (T > 10266

DOI: 10.1021/acs.jpcc.6b00793 J. Phys. Chem. C 2016, 120, 10261−10267

Article

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DOI: 10.1021/acs.jpcc.6b00793 J. Phys. Chem. C 2016, 120, 10261−10267