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Interface Engineering of Gold Nanoclusters for CO Oxidation Catalysis Yingwei Li, Yuxiang Chen, Stephen D. House, Shuo Zhao, Zahid Wahab, Judith C. Yang, and Rongchao Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07552 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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ACS Applied Materials & Interfaces

Interface Engineering of Gold Nanoclusters for CO Oxidation Catalysis Yingwei Li,† Yuxiang Chen,† Stephen D. House,‡ Shuo Zhao,† Zahid Wahab,†,§ Judith C. Yang,‡ Rongchao Jin†* †

Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States Chemical and Petroleum Engineering, and Physics, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States § Department of Chemistry, Kohat University of Science and Technology, Kohat District 26000, Khyber Pakhtunkhwa, Pakistan (*Email: [email protected])

‡

Abstract Catalysts based on atomically precise gold nanoclusters serve as an ideal model to relate the catalytic activity to the geometrical and electronic structures as well as the ligand effect. Herein, we investigate three series of ligand (thiolate) protected gold nanoclusters, including Au38(SR)24, Au36(SR’)24, and Au25(SR”)18, with a focus on their interface effects using carbon monoxide (CO) oxidation as a probe reaction. The first comparison is within each series, which reveals the same trend for the three series that, rather than the bulkiness of carbon tails as commonly thought, the steric hindrance of ligands at the interface between the thiolate, Au, and CeO2 inhibits CO adsorption onto Au sites and hence adversely affects the activity of CO oxidation. The second comparison is between the sets Au38(SR)24 and Au36(SR’)24 of nearly the same size, which reveals that the Au36(SR’)24 nanoclusters (with FCC structure) is not sensitive to thermal pretreatment conditions, while the Au38(SR)24 catalysts (icosahedral structure) is and an optimum activity is observed at a pretreatment temperature of 150 oC. Overall, the atomically precise Aun(SR)m nanoclusters have revealed unprecedented details on the catalytic interface and atomic structure effects. It is hoped that such insights will benefit the ultimate goal of catalysis in future design of enzyme‐like catalysts for environmentally friendly green catalysis. Keywords: gold, nanocluster, CO oxidation, interface effect, cluster size

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1. Introduction Gold, due to its chemically inert nature, had long been regarded as being catalytically inactive compared to other noble metals. However, small gold nanoparticles are found to be indeed quite active in certain reactions in which those traditional noble metal catalysts (e.g. Pd, Ru, and Rh) instead cannot catalyze, such as hydrochlorination of alkynes and epoxidation of propylene.1 Gold nanoparticles (NPs) are also effective in selective hydrogenation and oxidation reactions (e.g. low temperature carbon monoxide (CO) oxidation),2,3 carbon‐carbon coupling reactions,1,4 as well as carbon‐heteroatom bond‐ forming reactions.5 The catalytic activity of gold NPs greatly depends on size,6 and the range of 1 to 10 nm is of critical importance,7 owing to the high surface‐to‐volume ratio, surface geometric effect (e.g., the peculiar arrangement or low coordination number of surface atoms), electronic properties, and quantum size effect.8 Quantization of electronic structure in nanoclusters leads to a distinct energy gap (HOMO‐LUMO) so that reactants can be activated via charge transfer.9 However, as nanoparticles usually have non‐uniform sizes and morphologies, it is difficult to establish unambiguous relationships between the atomic‐level structures and the catalytic properties of catalysts based on conventional nanoparticles.10 Atomically precise, ligand‐protected gold nanoclusters (NCs) have recently emerged as a new class of nanoparticles,11,12 and the total structures of various gold nanoclusters protected by thiolates (e.g. Au25(SCH2CH2Ph)18,13 Au38(SCH2CH2Ph)24,14 Au36(SPh‐tBu)24,15 and other sizes16–18), as well as structures of nanoclusters protected by selenides19,20, phosphines,21–25 and alkynyls26–28 have been revealed by X‐ray crystallography. Such NCs offer a great opportunity to precisely correlate geometric and/or electronic structures with their catalytic activity, so as to gain fundamental insights into the intrinsic mechanism.3,8 Size control of NPs is usually achieved by surface ligand management, but the enhancement in stability provided by ligands is often accompanied by drop in catalytic activity for conventional NPs (e.g. Ir, Pd),29–31. Nevertheless, in certain cases the protecting ligands can be beneficial in activity improvement; for example, high activity for aerobic oxidation of alcohol was observed for poly(N‐vinyl‐ 2‐pyrrolidone) (PVP) stabilized Au clusters,32 which was explained by increasing electron density on Au core due to PVP donation. Au25(SR)18 with strong quantum confinement effects shows remarkable activity in selective oxidation of styrene,33 and the electron‐rich kernel as well as low‐coordinate surface Au atoms are identified to be responsible for the complete selectivity in hydrogenation of α,β‐ unsaturated ketones to unsaturated alcohols.34 The ligand‐protected Au25(SR)18 NCs were also found to be effective and recyclable in other hydrogenation reactions.35,36 The composite of glutathione‐ protected Au clusters and TiO2 due to effective adsorption of carboxylic group onto TiO2 surface gives rise to outstanding photocatalytic and photoelectrochemical applications.37,38 Zheng et. al also demonstrated that ligand‐capped Au/Ag alloy NCs work much better in hydrolytic oxidation of organosilanes than partially or completely ligand removed counterparts.39 Catalysts based on “ligand‐on” NCs are promising in fundamental research of catalysis, such as mapping out the active sites and understanding the interfacial effects. It is of importance to find a balance on optimizing the catalytic performance while preserving the size/structure of the NCs;40,41 the latter is critical in mapping out the precise size dependent catalytic reactivity and mechanism. While recent research has explored the catalytic reaction scopes of ligand‐protected gold NCs, details on how different ligands potentially affect the catalytic activity of NCs are still scarce and thus deserve major efforts. Understanding the ligand effects may promote future design of enzyme‐like catalysts for environmentally friendly green catalysis. 2


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In this work, we design several series of Au NCs with well defined structures of gold kernels as well as staple motifs, for their performance in catalytic CO oxidation. These NCs can be divided into three categories: 1) Au38(SR)24 with different R groups; 2) Au36(SR’)24 with 4 different R’ groups; and 3) Au25(SR”)18 with 3 different R” groups. All these NCs are in non‐metallic states.43 We choose CO oxidation reaction as a probe reaction because its mechanism has been well studied for heterogeneous catalysis based on gold NPs42–48 and NCs.49–51 CO oxidation performed on our designed NCs leads to an unambiguous conclusion that even a slight increment in the steric hindrance at the interface formed by thiolate ligands, Au, and CeO2 can result in a dramatic decrease in the reactivity of Au sites. This is different from the commonly thought steric hindrance from the bulkiness of protecting ligands. Gold NCs protected by ligands that are less bulky at the bonding position with Au atoms will certainly be more effective in CO oxidation reaction. Other factors such as the gold core structure and electron delocalization and configuration are also mapped out.

2. Experimental 2.1. Chemicals Tetrachloroauric (III) acid (HAuCl4∙3H2O, 99.999% metal basis, Aldrich), glutathione (G‐SH, 98%, Acros Organics), 2‐phenylethanethiol (HSCH2CH2Ph, 99%, Aldrich), thiophenol (HSPh, 97%, Aldrich), o‐ toluenethiol (o‐MBT, 97% Acros Organics), cyclopentanethiol (HS‐c‐C5H9, 97%, Aldrich), 4‐tert‐ butylbenzenethiol (HSPh‐tBu, 97%, Alfa Aesar), 2‐thionaphthol (2HSNap, 98%, Alfa Aesar), 1‐ thionaphthol (1HSNap, 99%, Alfa Aesar), 1‐hexanethiol (96%, Acros Organics), tetra‐n‐octylammonium bromide (TOABr, 98%, Alfa, Aesar), sodium borohydride (NaBH4, 99.99% metal basis, Aldrich), cerium nitrate hexahydrate (99.99%, Aldrich), oxalic acid (>99.0%, Aldrich), acetone (HPLC grade, 99.9%, Aldrich), toluene (HPLC grade, 99.9%, Aldrich), tetrahydrofuran (THF, HPLC grade, 99.9%, Aldrich), ethanol (HPLC grade, Aldrich), methanol (HPLC grade, 99.9%, Aldrich), and dichloromethane (DCM, HPLC grade, 99.9%, Aldrich), were used as received. 2.2. Synthesis and characterization of Aun(SR)m nanoclusters Most of the preparations followed previous work (see supporting information for details). UV‐ vis and UV‐vis‐NIR spectra of as‐synthesized clusters were measured on a Hewlett‐Packard Agilent 8453 diode array spectrophotometer, and a Shimadzu UV‐3600 plus spectrometer, respectively, at room temperature. MALDI mass spectrometry (MALDI‐MS) was performed on a PerSeptive Biosystems Voyager DE super‐STR time‐of‐flight (TOF) mass spectrometer. Scanning transmission electron microscope (STEM) images were obtained on JEOL JEM2100F S/TEM with high‐angle annular dark‐filed (HAADF) STEM mode and operating voltage at 200 kV. 2.3. Preparation of Aun(SR)m/CeO2 catalysts CeO2 is prepared by coprecipitation of oxalic acid and cerium nitrate hexahydrate followed by the procedure reported in previous work.52 The as‐prepared CeO2 was calcinated by heating to 580 oC at a rate of 10 oC/min and keeping this temperature for 1 h before loading. A powder of CeO2 (100 mg) was impregnated by soaking in a CH2Cl2 solution (2 mL) of 1 mg Aun(SR)m nanoclusters in a sealed vial for 24 h, followed by drying at room temperature with slow N2 blowing into the open vail for 30 min until CH2Cl2 was completely evaporated. The loading of net Au (%) in the supported catalysts is given in Table S1. For 3



STEM, commercial TiO2 is used instead of CeO2 as the supporter so as to increase the Z‐contrast with the Au clusters. 2.4. Catalytic activity test for CO oxidation 100 mg CeO2 supported Aun(SR)m catalysts were mixed with glass wool and tested for CO oxidation in a fixed‐bed, continuous flow reactor (8 mm inside diameter) under ambient pressure. Prior to CO oxidation reaction, the as‐prepared catalysts were pretreated in the presence of O2 by slowly heating up to 150 oC within 20 min and keeping this temperature for 1.5 h, and then cooled to room temperature in an O2/He atmosphere. The reaction gas mixture which is composed of 3% CO, 10% O2 and 87% He passed through the catalyst at a flow rate of 40 mL∙min‐1. The products were analyzed by an online gas chromatograph (HP 6890 series GC) equipped with a thermal conductivity detector. Analogous measurements were performed between room temperature and 150 oC and were detected by a movable thermocouple inside the catalyst bed. The area of the signals was integrated and scaled by their corresponding conversion coefficients for detector response (2.38 for CO and 2.08 for CO2) to get total quantities to determine the percent of CO conversion.

3. Results and discussion The designed NCs include three series: 1) Au38(SR)24, where SR = SCH2CH2Ph, SPh, or o‐MBT (2‐ methylbenzenethiol); 2) Au36(SR’)24, where SR’ = S‐c‐C5H9, SPh, SPh‐tBu, or 2SNap (2‐naphthalenethiol); and 3) Au25(SR”)18, where SR” = SCH2CH2Ph, 2SNap, or 1SNap (1‐Naphthalenethiol). All these NCs are non‐metallic (HOMO‐LUMO gaps ¥ 1 eV).43 For the preparation of catalysts for CO oxidation reaction, we choose CeO2 as a support due to its excellency in studying oxygen‐ion insertion reactions53 and capability to afford higher activity in CO oxidation.54 CeO2 can efficiently supply lattice oxygen at reaction sites for oxygen vacancy formation,55,56 and higher concentration of oxygen vacancy defects results in higher turnover numbers toward CO oxidation.57 3.1. Au38(SR)24 nanoclusters and their CO oxidation performance Fig. 1a shows the UV‐vis‐NIR spectra of three different Au38(SR)24 with different R groups. For Au38(SCH2CH2Ph)24 (black line), a series of peaks are observed, and these distinct features are characteristic of Au38(SR)24 nanoclusters58 and thus can serve as spectroscopic fingerprints. MALDI‐MS in (Fig. 1b black line) also shows an intense peak at 10778.71 Da corresponds to Au38(SCH2CH2Ph)24 (calculated MW = 10778.27 Da); of note, small peaks at ~9300 to 9400 Da are fragments caused by laser irradiation during the MALDI process (laser wavelength = 337 nm). We also prepared the Au38(SPh)24 nanoclusters by ligand exchange of Au38(SCH2CH2Ph)24 with HSPh under a mild condition.59 The absorption peaks in the UV‐vis region are at 470, 560, and 655 nm, exactly the same as reported,59 while in the NIR region, another peak centered at ~1105 nm is also observed (Fig. 1a, blue line). MALDI‐MS (Fig. 1b) shows an intense peak corresponding to Au38(SPh)23 (one ligand lost) and the weaker peak to Au34(SPh)19, with their interval being a characteristic fragmentation signature of Au4(SR)4 commonly observed in other nanoclusters, such as Au25(SR)18,60,61 Au36(SR)24,15 as well as Au38(SR)24,58 due to high laser fluence in MALDI analysis.

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Figure 1. a) UV‐vis‐NIR spectrum and b) MALDI‐MS of Au38(SCH2CH2Ph)24, Au38(SPh)24, and Au38(o‐MBT)24 nanoclusters. Inset in a): Structure of Au38 (‐R groups omitted for clarity, color code: Magenta = Au, and yellow = S).

Au38(o‐MBT)24 nanoclusters were prepared by ligand exchange of Au38(SCH2CH2Ph)24 at 80 oC, and to the best of our knowledge, this is the first time that o‐MBT protected Au38 is reported. From Fig. 1a (red line), the new Au38 also shows a series of peaks at 423, 536, 595, 647, 780, and 1003 nm, similar to its Au38(SCH2CH2Ph)24 parent cluster, but with some peaks blue‐shifted and others red‐shifted. Its MALDI‐MS spectrum, however, is more similar to that of Au38(SPh)24, probably because both of them are protected by aromatic ligands. Strong fragmentation (Fig. 1b, red line) is observed, in which the two most intense mass peaks correspond to Au34(o‐MBT)19, and Au34(o‐MBT)20, whereas the Au38(o‐MBT)23 peak is relatively weaker. Nonetheless, the combination of UV‐vis‐NIR and MS spectrum confirm Au38(o‐ MBT)24. From the UV‐vis‐NIR and MALDI MS spectra, all three samples are composed of 38 gold atoms and their structures should be the same as the X‐ray structure of Au38(SCH2CH2Ph)24.14 Slight distortion could occur due to different electronic structures of the ligands. STEM images of the free and oxide‐ supported Au38(SCH2CH2Ph)24 nanoclusters are shown in Fig. 2. The measured particle size is consistent with the X‐ray crystallographic measurements.14

Figure 2. STEM images of (A) free Au38(SCH2CH2Ph)24; (B) the supported Au38(SCH2CH2Ph)24. The scale bars in both (A) and (B) are 10 nm.

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The CO oxidation performance of Au38(SCH2CH2Ph)24‐based catalyst (Fig. 3a) is similar to the earlier work.52 The CO conversion is over 90% when the reaction temperature is above 100 oC, and its 50% conversion temperature is just over 60 oC which is remarkable. As Nie et al. reported that O2 pretreatment of Aun(SCH2CH2Ph)m/CeO2 at 150 oC for 1 to 2 h significantly enhanced the CO oxidation activity,50,52 we applied this process to all of our samples as a standard procedure. STEM images of supported Au38(SCH2CH2Ph)24 pretreated for 1.5 h at 150 oC and 180 oC, respectively, are given in Fig. S1 to illustrate the stability of Au38 catalyst at performing temperature of 150 oC, while at 180 oC, NCs begin to aggregate to form larger particles.

Figure 3. a) CO oxidation activity of Au38(SCH2CH2Ph)24, Au38(SPh)24, and Au38(o‐MBT)24 nanoclusters supported on CeO2; b) molecular structures of ligands on Au38(SR)24, where SR = SCH2CH2Ph, SPh, or o‐MBT.

Comparing to Au38(SCH2CH2Ph)24/CeO2, the Au38(SPh)24/CeO2 catalyst shows lower activity, and Au38(o‐MBT)24 is even less. The results indicate a distinct ligand effect on the catalytic performance of Au38 nanoclusters. Previous work suggested that the perimeter sites of the interface between gold and support should be the active centers for CO oxidation,50,62 and further evidence makes the hypothesis more specific, i.e., the interface between the ligand, Au, and CeO2 three parts.63 But how critical is the role of the interface in catalytic activity? The literature gives no clear evidence. Here, our study on three Au38(SR)24/CeO2 catalysts with the same Au38 core but different thiolate ligands shows an explicit answer ⎯ when the sulfur atom is directly connected to the aromatic ring, Au38(SPh)24/CeO2 becomes less active in CO oxidation, while an aliphatic chain between S and benzene ring in S‐CH2CH2‐Ph makes the interface between the three parts less crowded, hence higher catalytic activity. Furthermore, the methyl group which sits at the ortho position to sulfur atom in o‐MBT ligand results in even higher hindrance (Fig. 3b), hence, even lower activity for CO oxidation. Given the mechanism that O2 is activated on CeO2 whereas CO is activated on gold sites,63 the steric hindrance of ligand would prevent CO molecules from attaching onto the Au sites. 3.2. Au36(SR’)24 nanoclusters and their CO oxidation performance To further investigate the ligand effect, we prepared four kinds of Au36(SR’)24 NCs (where, R’ is to be differentiated from R in the case of Au38(SR)24). The UV‐vis absorbance of Au36(S‐c‐C5H9)24 is shown in Fig. 4a (black line), and the two peaks at 345 and 555 nm are its characteristic.64 The spectrum of Au36(SPh)24 (Fig. 4a, blue line) also agrees well with previous work65 with two peaks at 370, and 575 nm, and one shoulder at 410 nm. The Au36(SPh‐tBu)24 nanocluster (Fig. 4a, red line) shows two absorption peaks at 375, and 560 nm which are similar to the previous report,64 and one can also identify a shoulder 6


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at 425 nm. As the spectra of Au36(SPh)24 and Au36(SPh‐tBu)24 are close, we also performed MALDI‐MS on these two clusters. As expected for 4‐tert‐butylbenzenethiolate‐protected clusters (marked in red in Fig. 4b), the most intense peak belongs to Au36(SPh‐tBu)23, and fragments are also observed, which can be attributed to Au32(SPh‐tBu)19 and Au32(SPh‐tBu)20.15 The MALDI‐MS peak of Au36(SPh)23, although not as intensive as the Au32(SPh)19 and Au32(SPh)20 fragments (Fig. 4b, blue line), proves the purity of the Au36(SPh)24 sample.59 Finally, the purity of Au36(2SNap)24 is evidenced by peaks at 380, 440 nm, and 580 nm (Fig. 4a, dark cyan line), and its MALDI‐MS spectrum (Fig. S2) gives a single mass peak for Au36(2SNap)23 and one‐ligand loss is quite common in the Au36 series.

Figure 4. a) UV‐vis spectrum of Au36(S‐c‐C5H9)24, Au36(SPh)24, Au36(SPh‐tBu)24, and Au36(2SNap)24 nanoclusters and b) t MALDI‐MS of Au36(SPh)24 and Au36(SPh‐ Bu)24. Inset: Structure of Au36 (‐R groups omitted for clarity, color labels: Magenta = Au, and yellow = S).

The catalytic activity of the Au36 series is shown in Fig. 5. An interesting finding is that, although the ‐SPh‐tBu ligand is bulkier than ‐SPh, the activity of Au36(SPh)24 and Au36(SPh‐tBu)24 in CO oxidation is about the same. This further emphasizes the critical role of the interface, rather than the overall R group, because the bulky tert‐butyl group on the benzene ring of HSPh‐tBu is far on the end of the carbon tail,66 not being close to the Au‐S interface, thus it turns out not to affect the CO absorption onto the Au sites. This result again indicates the importance of the interface sites. Other than that, the catalytic activities of Au36(S‐c‐C5H9)24/CeO2 and Au36(2SNap)24/CeO2 catalysts are quite consistent with the Au38 series, in which Au36 with the smaller cyclopentanethiol ligand exhibits the highest CO conversion, ‐SPh and ‐SPh‐ t Bu cases are intermediate, and the bulkiest 2‐thionaphthol gives the lowest performance (Fig. 5a/b). The adverse effect of thionaphthol ligand will be discussed further in the Au25(SR”)18 series (vide infra).

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Figure 5. a) CO oxidation activity of Au36(S‐c‐C5H9)24, Au36(SPh)24, Au36(SPh‐ Bu)24, and Au36(2SNap)24 nanoclusters supported on CeO2; b) molecular structures of ligands on Au36(SR’)24, where in SR’ = S‐c‐C5H9, SPh, SPh‐tBu, or 2SNap.

3.3. Au25(SR”)18 nanoclusters and their CO oxidation performance All the Au25 nanoclusters show a characteristic optical spectrum with absorbance at 400, 445, 680, and 800 nm (Fig. 6a, black line) is in agreement with the standard spectrum of Au25(SCH2CH2Ph)18.13 The product purity is further verified by MALDI‐MS (Fig. 6b). As for the Au25(1SNap)18 cluster, we observed peaks at 390, 468, 683, and 798 nm in UV‐vis spectrum (Fig. 6a red line), which are consistent with the previous report.67 The negative‐mode MALDI‐MS spectrum (Fig 6a, red) also shows a significant peak attributed to Au25(1SNap)18, and its fragments at lower range of m/z. Finally, Au25(2SNap)18 also exhibits absorbance at 480 and 700 nm (Fig. 6a, blue line) and its MALDI‐MS shows evidence of its high purity.

Figure 6 a) UV‐vis spectra and b) MALDI‐MS of Au25(SCH2CH2Ph)18, Au25(1SNap)18, and Au25(2SNap)18 nanoclusters. Inset in a): Structure of Au25 (‐R groups omitted for clarity, color labels: Magenta = Au, and yellow = S).

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Figure 7. a) CO oxidation activity of Au25(SCH2CH2Ph)18, Au25(2SNap)18, and Au25(1SNap)18 nanoclusters supported on CeO2; b) molecular structures of ligands on Au25(SR”)24, where in SR” = SCH2CH2Ph, 2SNap, or 1SNap.

The Au25(SCH2CH2Ph)18/CeO2 catalyst pretreated at 150 oC shows relatively high CO oxidation conversion above 100 oC (Fig. 7a, black line), and its 50% CO conversion with dry feed gas is at ~82 oC which is comparable to Nie’s report.50 However, with increasing bulkiness from ‐SCH2CH2Ph to ‐SNap, the catalytic activities of Au25(2SNap)18/CeO2 and Au25(1SNap)18/CeO2 (Fig. 7a, blue and red lines, respectively) are significantly reduced. Besides, we also found Au25(1SNap)18/CeO2 catalyst is slightly worse than Au25(2SNap)18/CeO2, which can also be explained by ligand hinderance because sulfur at 1o position leads to even more coverage of the perimeter interface. The results of the Au25 series further consolidates the conclusion from the Au38 and Au36 series that the steric hindrance of ligands at the cluster/CeO2 interface deteriorates CO conversion by hampering the way for CO to be adsorbed as illustrated in Scheme 1.

Scheme 1. Steric hinderance of ligand at the interface for Aun(SR)m Nanoclusters in catalyzing CO oxidation.

It should be noted that it is also possible Au clusters with sterically crowded interface might decompose upon heating and result in poorer activity. So, we provided the thermalgravimetric analysis (TGA) data on some NCs (Fig. S3) and compared them with those revealed in literatures.58,68,69 It is obvious that 150 oC is a safe temperature for all NCs to sustain without decomposition. Partial detachment of thiolate ligands at more sterically crowded interface is also possible. But in this way, it would be easier for CO or O2 to be attached onto the exposed active sites and subsequently, the 9



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catalytic activity would increase. However, this is in contrast to our experiment results. Therefore, the poor catalytic activity is related to the sterically crowded interface, not due to decomposition or detachment. 3.4. Pretreatment‐Temperature Dependence of Au38(SPh)24/CeO2 and Au36(SPh)24/CeO2 catalysts The above results clearly show a large influence of protecting ligands (i.e. the interface hindrance) on the catalytic performance of gold nanoclusters. Furthermore, when the ligand is the same, Au38(SPh)24/CeO2 and Au36(SPh)24/CeO2 (both pretreated at 150 oC) show a gap in their catalytic performance (Fig. S4). Thus, some other factors also influence the activity. Herein, the thermal sensitivity of the clusters is identified to the factor. Au38(SPh)24 exhibits a sensitive dependence on the pretreatment temperature (Fig. 8a), with 150 oC being the best, and increasing pretreatment temperature to 180 oC (or 225 oC) results in a decreased catalytic activity. Similar observation is also found for Au38(SCH2CH2Ph)24 (Fig. 8b). We further treated the two Au38(SR)24/CeO2 catalysts at 100 oC (far below the ligand removal temperature) for a prolonged time (10 h), and found that, although CO conversion of Au38(SCH2CH2Ph)24/CeO2 approaches the value of 150 oC‐pretreated sample (Fig. 8b), lower temperature is not enough to sufficiently activate Au38(SPh)24/CeO2 (Fig. 8a). In general, the Au38(SR)24 catalysts show a large difference in catalytic activity at different pretreatment temperatures.

Figure 8. CO oxidation activity of a) Au38(SPh)24 and b) Au38(SCH2CH2Ph)24 pretreated at different temperatures.

On the other hand, catalytic activity of Au36(SPh)24/CeO2 is not very much affected by pretreatment temperature. Instead, pretreatment at 225 oC (and 180 oC as well) only slightly improves the CO oxidation activity (Fig. 9a), similar to the case of Au28(SPh‐tBu)20 (both Au36 and Au28 clusters have kernels of FCC structures).15 The disadvantage of low temperature pretreatment is not obvious, either (Fig. 9a); as for Au36(SPh‐tBu)24, pretreatment at 100 oC (10 h), 150 oC or 225 oC (1.5 h) also shows almost overlapped data (Fig. 9b), i.e. the CO conversion activity of Au36(SPh‐tBu)24‐based catalysts do not change with the pretreatment temperature.

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Figure 9. CO oxidation activity of a) Au36(SPh)24 and b) Au36(SPh‐ Bu)24 pretreated at different temperatures.

In order to obtain insight into the influence of pretreatment‐temperature dependence of CO oxidation, we performed pretreatment at different temperatures on Au38(SPh)24 and Au36(SPh)24 nanoclusters, followed by MALDI‐MS analysis. The (unsupported) nanoclusters were directly coated onto the inner wall of reactor (quartz tube) and heated to the pretreatment temperature in the O2/He mixture for 1.5 h so as to simulate the pretreatment process. As shown in Fig. S5a, the optical absorbance of Au38(SPh)24 nanoclusters after heating at 150 oC is almost the same as that of the as‐ synthesized sample, and MALDI‐MS (Fig. S5b) also shows similar spectra, indicating that the kernel and staple structures of Au38(SPh)24 do not change after pretreatment. But the Au38(SPh)24 could not sustain 180 oC‐pretreatment or above. By contrast, as shown in Fig. S6a/S6b, Au36(SPh)24 could sustain higher pretreatment temperature (180 oC) than its counterpart Au38(SPh)24, as UV‐vis and MALDI‐MS spectrum are barely changed, although 225 oC is too high to maintain the intact Au36(SPh)24. By combining such observations with the catalysis results above, we conclude that the geometric structure of the nanoclusters is also responsible for the different behaviors in CO oxidation activity. Higher dependency on pretreatment temperature is observed for Au38(SPh)24 with a bi‐ icosahedral kernel structure, which might convert to FCC when ligands are removed, and its performance correspondingly drops to be close to that of Au36(SPh)24 with FCC kernel (Fig. S7).

4. Conclusions In summary, we have obtained valuable insights into the catalytic behavior of ligand‐protected nanoclusters in CO oxidation by investigating three series of catalysts, including Au38(SR)24 with three types of ligands, Au36(SR’)24 with four types of ligands, and Au25(SR”)18 with three types of ligands. First of all, by comparing the activity within each series (Au38, Au36, or Au25), we have identified that the steric hindrance of ligand at the interface between the thiolate, Au, and CeO2, rather than the bulkiness of carbon tails as commonly thought, is a critical factor for the catalytic activity because it blocks the CO adsorption onto Au sites and thus adversely affects the activation of CO. Second, comparison between Au38 and Au36 clusters shows that Au38(SR)24/CeO2 has an optimum activity in CO oxidation when pretreated at ~150 oC, while the Au36(SR’)24/CeO2 performance does not change much with 11



pretreatment temperatures (100 to 225 oC). This observation indicates that the detailed core structure also plays a role in the catalytic reaction; specifically, the icosahedral structure of Au38(SR)24 shows better activity than FCC‐structured Au36(SR’)24. Understanding how ligand‐protected NCs behave in catalytic reactions may promote future design of enzyme‐like catalysts for environmentally friendly green catalysis.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx. Synthetic details are given in supporting information, and STEM of catalysts after pretreatment, TGA, UV‐vis and MALDI‐MS for clusters pretreated at different temperature are given in Figures S1‐S7. The authors declare no competing financial interest.

Acknowledgements R.J. acknowledges support from the Air Force Office of Scientific Research under AFOSR Award No. FA9550‐15‐1‐9999 (FA9550‐15‐1‐0154). J.Y. and S.H. acknowledge the financial support of DOE BES through grant DE FG02‐03ER15476 and NSF DMREF under contract No. CHE‐1534630.

Notes and references (1)

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