supplementary information (50 ml, 66 mm, ethanol/water=1/4, v/v) and then stirred at room...

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Supporting Information

Catalytically Highly Active Top Gold Atom on Palladium Nanocluster

Haijun Zhang1, 2, Tatsuya Watanabe1, Mitsutaka Okumura2,3, Masatake Haruta2, 4, and Naoki

Toshima1, 2*

1. Department of Applied Chemistry, Tokyo University of Science Yamaguchi, SanyoOnoda,

Yamaguchi 756-0884, Japan

2. Core Research for Environmental Science and Technology (CREST), Japan Science and

Technology Agency, Kawaguchi, Saitama 332-0012, Japan

3. Graduate School of Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560-0043,

Japan

4. Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo

Metropolitan University, Minami-osawa, Hachioji, Tokyo 192-0397, Japan

*Corresponding author; E-mail of the corresponding author: [email protected]

SUPPLEMENTARY INFORMATIONDOI: 10.1038/NMAT3143

NATURE MATERIALS | www.nature.com/naturematerials 1

© 2011 Macmillan Publishers Limited. All rights reserved.

http://www.nature.com/doifinder/10.1038/nmat3143

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EXPERIMENTAL SECTION

Materials

Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4⋅3H2O, 99.9%) purchased from Tokyo Kasei

Kogyo, Ltd., and palladium chloride (PdCl2, 99.9%) and PVP (poly(N-vinyl-2-pyrrolidone, K35,

molecular weight about 40,000) purchased from Wako Pure Chemical Industries, Ltd., were used

without further purification. All glassware and Teflon-coated magnetic stirring bars were cleaned

with aqua regia, followed by copious rinsing with purified water. The water was purified using a

Millipore Milli-RX 12 plus water system.

Preparation of ‘Mother Clusters’ (PVP-Protected Pd NCs)

Preparation of dispersions of PVP-protected Pd NCs was carried out by an alcohol reduction

method [S1]. A solution of PdCl2 (50 mL, 0.66 mM, ethanol/water=1/4, v/v) was added to a PVP

solution (50 mL, 66 mM, ethanol/water=1/4, v/v) and then stirred at room temperature for 15 min.

The mixed solutions were stirred and heated to reflux at 100°C for 2 h. The color of the mixed

solutions slowly changed from dark yellow to transparent brown at the beginning of the refluxing.

The colloidal dispersions were filtered through an ultrafilter membrane with a cutoff

molecular-weight of 10,000 (Toyo Roshi Kaisha, Ltd.) and washed twice with water and then once

with ethanol under nitrogen to remove any extra agents and byproducts. The residual ethanol from

the PVP-protected Pd NCs was removed using a rotary evaporator at 40 °C. The PVP-protected Pd

NCs were finally obtained as powders by vacuum drying at 40 °C for 48 h. The distribution of the

particle diameters of the Pd NCs is nearly symmetric, and the population of the NCs has a peak at

the diameter of about 1.8 nm. The average diameter (1.8±0.6 nm) indicates that the PVP-protected

Pd NCs consist of about 147 atoms in a particle on average. We demonstrate this for size-selected

PdN (where N=147) NCs, where Pd147 is known to be a possible ‘magic number’ cluster. The Pd147 2 NATURE MATERIALS | www.nature.com/naturematerials

SUPPLEMENTARY INFORMATION DOI: 10.1038/NMAT3143



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NCs with the ideal size of about 1.8 nm can be characterized by a cuboctahedron with eight

triangular {111} faces and six square {100} faces. The mean particle diameters (1.8 nm) were used

for the approximate calculation and preparation of the Pd/Au catalyst with a crown-jewel structure

in the following step.

Preparation of Pd/Au Catalysts with a ‘Crown-Jewel’ Structure

Based on the galvanic replacement process described by Xia’s group [S2-S3], an aqueous

solution of HAuCl4 was added to the colloidal dispersion of the as-synthesized Pd147 mother

clusters to obtain the CJ-Au/Pd NCs. A series of CJ-Au/Pd NCs with various Au contents were then

prepared. The detail compositions and preparation conditions are shown in Table S1. For example,

the CJ-2 NCs (all the top Pd atoms were replaced by Au atoms, and the atomic ratio of Pd147 to the

Au3+ in the synthetic solution is 147/12) were prepared as follows. An aqueous solution of

HAuCl4⋅4H2O (20 mL, 0.135 mM) was dropwise added into an as-prepared Pd147 colloidal

dispersion (50 mL, 0.66 mM) with continuous stirring at room temperature in a N2 atmosphere. The

mixtures were then further treated by heat for 30 min in a bath of 100 °C. Transparent and brownish

dispersions were then obtained. The PVP-protected CJ-Au/Pd NCs catalysts were finally obtained

after the washing, rotary evaporating, and vacuum drying.

Characterization of NCs

The UV–Vis (ultraviolet and visible light) absorption spectra were measured over the range of

200-800 nm with a Shimadzu UV-2500PC recording spectrophotometer using a quartz cell with a

10 mm optical path length.

Transmission electron microscopy (TEM) images were observed using a JEOL TEM 1230 at the

accelerated voltage of 80 kV. The specimens were obtained by placing one or two drops of the

colloidal ethanol solution of CJ-Au/Pd NCs onto a thin amorphous carbon film-covered copper NATURE MATERIALS | www.nature.com/naturematerials 3




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microgrid and dried it in air at room temperature. Prior to the specimen preparation, the colloidal

ethanol solutions were sonicated for 10 min to obtain a better particle dispersion on the copper grid.

An image analysis was performed using iTEM software (Olympus Soft Imaging Solution GmbH).

For each sample, at least 200 particles from different parts of the grid were generally used to

estimate the mean diameter and size distribution of the particles. The high-angle annular dark-field

scanning TEM (HAADF-STEM) were observed using a JEOL TEM 2010F microscope equipped

with CEOS spherical aberration correctors at the accelerating voltage of 120 kV in the UBE

Scientific Analysis Laboratory (Japan). High-resolution electron energy loss spectroscopy (EELS)

measurements were carried out using an ENFINA1000 (Gatan, Inc.) detector with about a 0.2 nm

beam diameter attached to the HAADF-STEM equipment.

XPS measurement was performed by a Quantum 2000 scanning X-ray photoelectron spectrometer

(PHILIPS) with Al Ka radiation (E = 1486.6 eV) using an electron neutralizing gun, the analyzing

area for the XPS sample on a SUS substrate is about 100 µm in diameter. Binding energies (BE) are

obtained from the peak position of an asymmetric peak-fitting curve and referenced to the C(1s)

binding energy of the adventitious carbon contamination taken at 284.6 eV. The measurements

were carried out at least three times for the same samples and the average values are shown in the

Table S3. The error was within ±0.03 eV. The analyses of Au and Pd were based on the Au 4f7/2

and Pd 3d5/2 photopeaks, respectively.

The metal content of the PVP-protected CJ-Au/Pd NCs was determined by optical emission

spectroscopy with inductive coupled plasma (ICP-OES, Varian 720-ES). For this purpose, 2 mg of

the samples was solubilized in 2 mL aqua regia (HCl/HNO3=3/1, vol) for 24 h, and then the

solubilized solution was diluted to 20 mL using pure water for the measurement. ICP results

showed that metal compositions in the present CJ-Au/Pd NCs were almost the same as those in the 4 NATURE MATERIALS | www.nature.com/naturematerials




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starting solution.

Catalytic Properties

1) Glucose Oxidation at Controlled pH

The catalytic performance of all the catalysts was evaluated using glucose oxidation as the model

reaction. The reactions were carried out at 60 °C in a 50-mL glass beaker placed in a thermostat

(about 2000 mL). During the experiment, the pH of the reaction suspension was kept constant at 9.5

by the addition of a 1 mol L-1 NaOH solution using an automatic potentiometric titrator (Kyoto

Electronics MFG. Co. Ltd., Japan). Oxygen was bubbled through the suspension at the flow rate of

100 mL min-1 at atmospheric pressure. The suspension was vigorously stirred by a magnetic stirrer.

The starting concentration and volume of the glucose solution was 0.264 mol L-1 and 30 mL,

respectively, and the charged weight of the catalyst was about 2 mg. The catalytic reactions were

automatically carried out for 2 h. The turnover frequency (TOF) was calculated from the slope of a

straight line fitted to the NaOH amount vs. reaction time curve. The initial specific activity related

to the metal content of the catalysts was calculated for comparison. A typical NaOH amount vs.

time diagram with the fit line is shown in Figure S2. The slope of the fit line reflects the initial

activity of the catalyst. The catalytic activities of all the samples were measured at least twice under

the same conditions and the mean value of the measuring results was used as the TOF value.

The TOF values of the top Au atoms were approximately calculated by the following equation:

GPd × XPd + GAu × XAu = GPd/Au

where GPd and GPd/Au are the catalytic activity of the Pd mother clusters and CJ-Au/Pd NCs,

respectively, which were evaluated by the above-mentioned method for glucose oxidation. XPd and

XAu are the atomic ratios of Pd and Au in the CJ-Au/Pd NCs, respectively, which was measured by

ICP-OES. GAu is the calculated catalytic activity of the Au atoms. The detail activity calculations of NATURE MATERIALS | www.nature.com/naturematerials 5




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the top Au atoms of CJ-Au/Pd NCs are listed in Table S2.

The selectivity to gluconic acid of the CJ-Au/Pd NCs is about 100%. The particle size did not

increase at all after one cycle of the reaction. However, there was a tendency that the NCs form

agglomerates after the glucose oxidation reaction.

2) H2O2 Decomposition

The reaction was initiated by simultaneously injecting 0.06mL of the 0.001M CJ-Au/Pd colloidal

dispersion into 2.94 mL of an H2O2 solution (0.3 vol%) in a quartz cell, and then the mixed solution

was stirred for 2 min at room temperature. A Shimadzu UV-Vis spectrophotometer was used to

follow the kinetics of the catalyzed reactions by monitoring the decreasing rate of the H2O2

absorption band at 235 nm. The reaction rate was calculated by comparing the change in the H2O2

absorbance intensity at 235 nm before and after the catalyzed reaction.

Density functional theory (DFT) calculation

The structure and properties of the M55 clusters are perfomed using the DMol3 DFT package. In

these calculations, an all-electron relativistic core treatment and a doubled numerical basis set with

polarization functions were employed. The rPBE functional is used for the DFT calculations. All

the calculations spin restricted SCF calculations were carried out with a convergence criterion of

10-5 a.u. on the total energy and the electron density. We used the convergence criteria of 0.004

hartree/A on the force parameters, 0.005A on the displacement parameter, and 2×10-5 hartreee on

the total energy in the geometry optimization. The Mulliken population analysis was used for the

investigation of the atomic charges of the investigated clusters. For these calculations, �PBE

functional and DNP basis sets were used.

6 NATURE MATERIALS | www.nature.com/naturematerials




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Table S1 Preparation conditions of CJ-Au/Pd NCs catalyst.

Theoretical compositions, mol Preparation conditions

Code Feeding ratios of Pd/Au3+

Pd/Au and RPVP after replacement reaction

Final Au, mol%

Temperature, °C

Time, min

Remarks

CJ-1 147/6 138/6, RPVP=102 4.2 100 30

CJ-2 147/12 129/12, RPVP=104 8.5 100 30

Pd atoms at top are replaced by Au atoms

CJ-3 147/24 111/24, RPVP=109 17.8 100 30 Pd atoms at top and edge are replaced by

Au atoms

(RPVP: the molar ratio of PVP to the total metal in the NCs)

Table S2 Calculation of activity of CJ-Pd/Au NCs

Theoretical, mol ICP result, mol Activity for glucose oxidation

Sample Starting ratios

Final ratios

Au/% Pd/Au Au/% mol-glucose⋅h-1⋅mol-M -1 mol-glucose⋅h-1⋅mol-Au -1

Au Pure Au - 100 - 100 6230* -

Pd Mother cluster - 0 - 0 5190* -

CJ-1 Pd/Au3+= 147/6

Pd/Au= 138/6 4.2 20.6 4.6 13920* 194980**

CJ-2 Pd/Au3+= 147/12

Pd/Au= 129/12 8.5 12.2 7.6 19540* 194000**

CJ-3 Pd/Au3+= 147/24

Pd/Au= 111/24 17.8 5.53 15.3 18230* 90420**

(*: Obtaining from titration curves and normalized to the total metal; **: The activity of present Pd was subtracted from the CJ NCs; Replacement reaction: 3Pd + 2Au3+ = 2Au + 3Pd2+)

Table S3 Preparation conditions, average sizes and XPS results of CJ-4 NCs catalysts prepared

with low contents of PVP

Code Theoretical compositions,

atomic Average sizes

nm Binding Energy Au(4f7/2), eve





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CJ-4 Pd/Au3+=147/12,

RPVP=5.5 3.4±1.2 82.45

Au Bulk Au - 84.00

Au-1 Tyrosine-capped gold

nanoparticles 6 84.0

Ref. [S4]

Au-2 PVP-protected gold

nanoparticles 2.6±0.6 82.7

Ref. [S5]



Ref. [S5]



Ref. [S5]

Au-5 Ceria-supported gold

catalyst 2-5 83.9

Ref. [S6]

Table S4 Mulliken charges of Pd55 and crown jewel-structured Pd43Au12 model clusters

Pd43Au12 Sites Pd55,

Mulliken charges Atom kind Mulliken charges Top -0.006 Au -0.116 Edge 0.012 Pd 0.056 Face 0.017 Pd 0.030

(a) (b)

Figure S1. Schematic illustration of (a) crown-jewel structured Au/Pd NCs (The Pd147 cluster decorated with a top Au atom) and (b) the image of crown jewel (The red sphere in Figure S1a

shows the presence of top Au atom.).





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Figure S2. The typical NaOH amount-time curve for glucose oxidation over CJ-Au/Pd NCs

catalyst (CJ-2) and a straight trend line for determining TOF (Glucose/Au=44,400(mol ratio), O2, 100ml/min, 2 hours, 60°C.).

(a) UV-Vis spectra

Pd147 mother NCs, prepared by alcohol reduction method

Au NCs, prepared by rapid injection of NaBH4

50nm





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Catalysts Theoretically compositions

Concentrations for UV-Vis

Size/nm Shape Remark

Pd Pd 0.66 mM 1.8±0.6 spherical CJ-1 Pd138Au6 0.47 mM 1.8±0.5 spherical CJ-2 Pd129Au12 0.45 mM 1.7±0.5 spherical CJ-3 Pd111Au24 0.43 mM 1.6±0.5 spherical

Au Au 0.66 mM 1.4±0.5 spherical

The theoretically compositions of the

catalysts were calculated using

following reaction, 3Pd + 2Au3+ = 2Au +

3Pd2+ (b) Sizes, shapes, theoretically compositions and concentrations

Figure S3. (a) UV-Vis spectra and (b) the sizes, shapes and theoretically compositions and

concentrations of aqueous colloidal dispersions of Pd, Au, and CJ-Au/Pd NCs series catalysts.

(a) CJ-1 NCs

(b) CJ-2 NCs

50nm

50nm

Dav = 1.8 nm σ = 0.5 nm

Dav = 1.7 nm σ = 0.5 nm





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(c) CJ-3 NCs

Figure S4 TEM micrographs and size distribution histograms of (a) CJ-1, (b) CJ-2, and (c) CJ-3

Au/Pd NCs prepared by replacement reaction method.

Figure S5. Typical HADDF-STEM images of CJ-Au/Pd NCs recorded along the [110] zone axis.

(The red cycles indicate vacancies formed during the replacement reaction.)

0 S 14 S

50nm

Dav = 1.6 nm σ = 0.5 nm





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28 S 42 S

Figure S6. HADDF-STEM images of irradiation time-dependent shape evolution of CJ-Au/Pd NCs recorded along the [110] zone axis (The red arrow shows the gradually disappearance of a top atom

after several tens of seconds of irradiation by the electron beam.).

Figure S7. Schematic illustration of the activity of top, edge, and face Au atoms decorated on the Pd147 NCs (The yellow spheres: Pd atoms.).

Figure S8 Schematic illustration of electronic charge transfer effects in CJ-Au/Pd NCs catalysts (Ei: Ionization energy.).

Charge transfer e-

> Ei / eV: 9.22 8.34

Au Pd

Au Pd e-

δ- e-

Au

δ- δ-

δ- δ-

δ-

Top Au atom

Edge Au atom

Face Au atom

Activity: > >

{100}

{111}





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Figure S9. Catalytic activities for H2O2 decomposition of CJ-Au/Pd NCs series catalysts with

various compositions (The activity was normalized to the total metal in the clusters.).

References

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bimetallic nanoparticles: a calorimetric study. J. Phys. Chem. B 109, 16326-16331(2005).

[S2] Sun, Y. G. & Xia, Y. N. Shape-controlled synthesis of gold and silver nanoparticles. Science

298, 2176-2179(2002).

[S3] Sun, Y. G., Mayers, B. T. & Xia, Y. N. Template-Engaged Replacement Reaction: A one-step

approach to the large-scale synthesis of metal nanostructures with hollow interiors. Nano Lett. 2,

481-485(2002).

[S4] Selvakannan, P. R., Swami, A., Srisathiyanarayanan, D., Shirude, P. S., Pasricha, R., Mandale,

A. B. & Sastry, M. Synthesis of aqueous AuCore-AgShell nanoparticles using tyrosine as a

pH-dependent reducing agent and assembling phase-transferred silver nanoparticles at the air-water

interface. Langmuir, 20, 7825-7836(2004).

[S5] Tsunoyama, H., Ichikuni, N., Sakurai, H. & Tsukuda, T. Effect of electronic structures of Au

clusters stabilized by poly(n-vinyl-2-pyrrolidone) on aerobic oxidation catalysis. J. Am. Chem. Soc.

131, 7086-7093(2009).

[S6] Chang, L. H., Sasirekha, N., Rajesh, B. & Chen, Y. W. CO oxidation on ceria- and manganese

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