Remarkable Enhancement in Aerobic Alcohol Oxidation Catalysis of Au25 Clusters by Single Pd Atom Doping

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1 Supporting Information Remarkable Enhancement in Aerobic Alcohol Oxidation Catalysis of Au25 Clusters by Single Pd Atom Doping Songhai Xie, Hironori Tsunoyama, Wataru Kurashige, Yuichi Negishi, Tatsuya Tsukuda*, Catalysis Research Center, Hokkaido University, Nishi10, Kita21, Sapporo , Japan, Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo , Japan, Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo , Japan. 1. Experimental A. Chemicals All reagents are commercially available and were used without further purification. Hydrogen tetrachloroaurate tetrahydrate (HAuCl4 4H2O), palladium sodium chloride trihydrate (Na2PdCl4 3H2O), tetraoctylammonium bromide ((C8H17)4NBr), sodium tetrahydroborate (NaBH4), methanol, toluene, acetone, cerium(iv) sulphate, dichloromethane (CH2Cl2), benzyl alcohol, potassium carbonate and sodium sulphate were obtained from Wako Pure Chemical Industries. 1- Dodecanethiol (1- C12H25SH) was purchased from Sigma- Aldrich. Tetrabutylammonium perchlorate ((C4H9)4NClO4) was purchased from Tokyo Kasei. Trans- 2- [3- (4- tert- butylphenyl)- 2- methyl- 2- propenylidene] malononitrile (DCTB) was purchased from Fluka. Deionized water with a resistivity of >18 MΩ cm was used in the present study. B. Synthesis of Au25(SC12H25)18 and PdAu24(SC12H25)18 i) Au25(SC12H25)18 (Au25:SC12) The procedure is described in detail in Ref. 1. Crude samples of Au:SC12H25 clusters were prepared by the Brust method 2 with a slight modification. First, 150 ml of TOABr (5.1 mm) in toluene was added to an aqueous solution (150 ml) of HAuCl4 (5 mm). After stirring for 30 min, the organic phase was separated by removing the aqueous layer and C12H25SH (9 mmol) was added to the toluene solution. The resulting mixture was then stirred for 30 min. The mixture was cooled to c.a. 273 K for 30 min in a cooling bath. An aqueous solution of NaBH4 (50 mm, 150 ml) was cooled to c.a. 273 K S1

2 and then injected rapidly into the mixture under vigorous stirring. After 3 h, the organic phase was washed thoroughly with water and evaporated to dryness. The product was centrifuged twice at 2500 rpm with methanol to remove excess C12H25SH, TOABr, and other by- products. Au25:SC12 clusters were extracted from the dry samples using pure acetone. The purity of Au25:SC12 was further enhanced by washing the extract with methanol and performing repeated crystallization from acetone. ii) PdAu24(SC12H25)18 (Pd1Au24:SC12) The procedure is given in Ref. 3. Dodecanethiolate- protected Pd Au binary clusters were prepared by the Brust method 2 in a similar manner to the phenylethanethiolate- protected Pd Au binary clusters reported by the Murray group. 4 First, a toluene solution (150 ml) of (C8H17)4NBr (0.85 mmol) was added to an aqueous solution (150 ml) containing 0.54 mmol of HAuCl4 and 0.21 mmol of Na2PdCl4. After stirring for 30 min, the organic phase was separated by removing the aqueous layer, and C12H25SH (9 mmol) was added to the separated toluene solution and the resulting solution was stirred for 30 min. The mixture was then cooled to 0 C for 30 min in an ice bath. An aqueous solution of NaBH4 (7.5 mmol, 150 ml) was cooled to 0 C and then injected rapidly into the mixture under vigorous stirring. After reduction for 3 h, the organic phase was evaporated and the product was washed five times with methanol to remove excess C12H25SH, (C8H17)4NBr, and byproducts. A mixture of Au25:SC12 and Pd1Au24:SC12 was obtained by extracting from the dried products with acetone. The mixture was then separated into two fractions by reverse- phase high- performance liquid chromatography. MALDI mass spectra of fractions confirmed that each fraction contained only Au25:SC12 and Pd1Au24:SC12. 3 C. Adsorption of Au25:SC12 and Pd1Au24:SC12 onto CNT 300 mg CNT (Nanocyl, NC- 7000, Belgium; surface area: m 2 /g) were dispersed in toluene and sonicated for c.a. 1 h. A toluene solution containing a calculated amount ( wt.%) of Au25:SC12 or Pd1Au24:SC12 was injected into a suspension of CNT under vigorous stirring. After 1 h, the composites (Au25:SC12/CNT and Pd1Au24:SC12/CNT) were collected by filtration and dried in vacuum (< 20 Pa) for 12 h. D. Calcination of Au25:SC12/CNT and Pd1Au24:SC12/CNT S2

3 Calcination of the Au25:SC12/CNT and Pd1Au24:SC12/CNT composites was performed in a quartz- tube oven in vacuum (<0.003 Torr): the oven temperature was raised to 50 C at a rate of 2 C/min and then maintained for 0.5 h. Subsequently, the oven temperature was elevated to 300 or 450 C at a rate of 2 C/min and was kept for 2 h. E. Characterization i) MALDI mass spectrometry Matrix- assisted laser desorption ionization (MALDI) mass spectra were recorded on a linear- mode TOF mass spectrometer (Applied Biosystem, Voyager Linear RD VDA 500) using a nitrogen laser (337 nm). DCTB was used as the MALDI matrix. The laser fluence and cluster- to- matrix ratio (1:1000) were optimized to supress fragmentation. ii) Optical absorption spectroscopy UV visible spectra of Au25:SC12 and Pd1Au24:SC12 in toluene were performed on a V- 670 (JASCO, Japan) spectrophotometer. iii) Transmission electron microscopy (TEM) and energy-dispersive X-ray analysis spectroscopy (EDS) TEM observations were performed on a JEM 2100F (JEOL, Japan) microscope operated at 200 kv. The magnifications were calibrated by Au standard samples. In order to reduce damage by the e- beam radiation, all TEM images were recorded by a CCD camera ( pixels, KeenviewG2, Olympus, Japan) under a low- dose mode (< 1 pa/cm 2 ). Toluene dispersions of Au25:SC12 and Pd1Au24:SC12 were dropcasted onto a hydrophobic carbon coated copper grid and dried in air. Au25:SC12/CNT, Pd1Au24:SC12/CNT, Au25/CNT and PdAu24/CNT were dispersed in methanol and sonicated for 1 min; one drop of the suspension was transferred to a holey grid and dried in air. EDS was preformed using a Si(Li) detector (EDAX, USA). iv) Mass analysis of desorbed species during in calcination Desorbed species during calcination were identified in situ by mass spectrometry using a gas chromatography mass spectrometry (GC- MS) instrument (Shimadzu, GCMS- QP2010 Plus, Japan) with a direct insertion (DI) function. Approximately 1 mg of catalyst was placed in the heating apparatus and the temperature was increased from RT to 500 C at a rate of 40 C/min. Desorbed species S3

4 were ionized by electron impact (70 ev, 60 μa) and their mass spectra were measured in the m/z range The ion source temperature was set to 200 C. v) Inductive coupled plasma (ICP) Content of gold in the catalysts was determined on a plasma atomic emission spectrometer (Shimadzu, ICPE- 9000, Japan). Measured amount of supported catalysts was placed into a capped crucible and calcined at 600 C for 2 h. The residues were dissolved in a fresh aqua regia solution (4 ml) for 24 h and diluted to 50 ml for analysis. vi) Thermogravimetric (TG) analysis TG analysis of Au25:SC12H25/CNT was performed on a TG/DTA analyzer (Bruker, MTC1000SA, Germany) in vacuo. The temperature was increased from RT to 50 C at a rate of 2 C/min and maintained for 30 min. The temperature was subsequently elevated to 500 C at a rate of 2 C/min and maintained at 500 C for 30 min. Approximately 10 mg of Au25:SC12H25/CNT (5 wt.%) was used for the measurement. S4

5 2. Results A. Characterization of Au25:SC12 and Pd1Au24:SC12 i) MALDI mass spectrometry Figure S1. Negative- ion MALDI mass spectra of (a) Au25:SC12 and (b) Pd1Au24:SC12 used in the present study. The Au21(SC12H25)14 ion in (a) was produced by laser- induced fragmentation. 5 ii) TEM Figure S2. Typical TEM images of (a) Au25:SC12 and (b) Pd1Au24:SC12 with the corresponding size distributions (insets). S5

6 iii) UV-vis spectroscopy Figure S3. UV- vis spectra of (a) Au25:SC12 and (b) Pd1Au24:SC12 before loading (red) and the filtrate after loading onto CNT (black). The profiles of the filtrates did not show any absorption peaks, indicating that the clusters were completely adsorbed onto CNT. B. Calcination of Au25:SC12/CNT i) TG Analysis Figure S4. TG profiles of Au25:SC12H25/CNT (5 Au wt.%) in vacuo. The weight lost was 3.78 wt.%, indicating that the weight ratio between ligands and Au was This ratio agrees well with that of Au25(SC12H25)18 (0.74). S6

7 ii) Mass spectrometric analysis of desorbed species S7

8 Figure S5. MS analysis results for Au25:SC12/CNT (5 Au wt.%) obtained using a DI function. The sample temperature was increased upto 500 C at a rate of 40 C/min. The main species desorbed in the range 300 to 450 C were assigned to didodecyl disulfide, 1- dodecanthiol, and 1- dodecene. No species with m/z > 200 were detected. C. Characterization of Au25/CNT and Pd1Au24/CNT i) EDS analysis of Pd1Au24/CNT Figure S6. Typical EDS spectrum of a single cluster in Pd1Au24/CNT(450). The average ratio of Au:Pd was 22:1. ii) TEM analysis of Au25/CNT before and after catalytic usage Figure S7. Typical TEM images and size distributions of Au25/CNT(450) (a, c) before and (b, d) after catalytic usage. S8

9 References 1. Chaki, N. K.; Negishi, Y.; Tsunoyama, H.; Shichibu, Y.; Tsukuda, T. J. Am. Chem. Soc. 2008, 130, Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, Negishi, Y.; Kurashige, W.; Niihori, Y.; Iwasa, T.; Nobusada, K. Phys. Chem. Chem. Phys. 2010, 12, Fields- Zinna, C. A.; Crowe, M. C.; Dass, A.; Weaver, J. E. F.; Murray, R. W. Langmuir 2009, 25, Dass, A.; Stevenson, A.; Dubay, G. R.; Tracy, J. B.; Murray, R. W. J. Am. Chem. Soc. 2008, 130, S9