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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NCHEM.1721

Exceptional oxidation activity with size-controlled supported gold

clusters of low atomicity

Avelino Corma,1,* Patricia Concepción,1 Mercedes Boronat,1 Maria J. Sabater,1 Javier

Navas,1 Miguel José Yacaman,2 Eduardo Larios,2,3 Alvaro Posadas,2 M. Arturo López-

Quintela,4 David Buceta,4 Ernest Mendoza,5 Gemma Guilera,6 Alvaro Mayoral7

1 Instituto de Tecnología Química, Universidad Politécnica de Valencia-Consejo

Superior de Investigaciones Científicas (UPV-CSIC), Av. de los Naranjos s/n,

46022 Valencia, Spain.

2 Department of Physics University of Texas at San Antonio One 1604 circle San

Antonio Texas 78249 Texas, USA.

3 On Leave from the University of Sonora.

4 Dept. Physical Chemistry, Faculty of Chemistry, Lab of Nanotechnology and

Magnetism (NANOMAG) Research Technological Institute, University of Santiago de

Compostela, E-15782 Santiago de Compostela, Spain.

5 Grup de Nanomaterials Aplicats, Centre de Recerca en Nanoenginyeria, Universitat

Politècnica de Catalunya, c/ Pascual i Vila 15, 08028 Barcelona, Spain.

6 ALBA Synchrotron, Experiments Division, Crta. BP 1413, de Cerdanyola del Vallès a

Sant Cugat del Vallès, 08290 Cerdanyola del Vallès, Barcelona, Spain.

7 Laboratorio de Microscopias Avanzadas, Instituto de Nanociencia de Aragon,

Universidad de Zaragoza, Mariano Esquillor Edificio I+D, 50018, Zaragoza, Spain.

© 2013 Macmillan Publishers Limited. All rights reserved.



Supplementary information

This supplementary information includes:

1. Catalyst synthesis.

2. HAADF-STEM characterization of the as-prepared catalyst

3. XPS characterization of the as-prepared catalyst

4. Raman and IR spectroscopic studies of the evolution of the gold species under

synthesis conditions.

5. HAADF-STEM characterization of the evolution of the catalyst under reaction

conditions

6. X-ray Absorption Spectroscopy (XAS) measurements

7. UV/Vis spectroscopy

8. Synthesis and characterization of size controlled gold clusters

9. Catalytic activity of 0.8 and 1.2 nm gold nanoparticles supported on MWCNTs

10. DFT results

11. Analysis of H2O2 and H2O formed, and O2 consumed during the reaction.

12. References




1. Catalyst synthesis.

The synthetic route to obtain isolated gold atoms supported on Thin Multiwalled

Carbon Nanotubes (MWCNT) starts by wrapping the CNTs with the polyelectrolyte

polyallylamine hydrochloride (PAH). For this purpose, multiwalled CNTs are

dispersed in a 1 wt.% aqueous solution of PAH (Sigma-Aldrich) at pH = 9 to a

concentration of 1 mg/mL. A combination of rapid stirring and ultrasonication is used to

ensure the presence of dispersed individual nanotubes. Excess PAH is removed by

vacuum filtration and successive washing with ultra pure water. Then, the CNTs are

resuspended in water at pH = 9. The reaction to synthesize isolated gold atoms consists

of adding to 50 ml of CNT – PAH solution the adequate amount of HAuCl4 aqueous

solution to yield a 0,1wt% metallic gold content. Finally, sodium citrate aqueous

solution (citrate:Au molar ratio 1700) is added. After 3 days aging at room temperature,

citrate ions are removed by centrifugation at the following conditions: 1h at 6000 rpm

10°C followed by 1 h at 8000 rpm 10°C. Finally the sample is dried by lyophylization.

The removal of the citrate ions has been followed by IR spectroscopy. Fig. S1 shows the

IR spectra of sample A prior to centrifugation (with excess of citrate ions, spectra a) and

after centrifugation (spectra b). IR bands at 1675, 1553 and 1396 observed on the

sample prior to centrifugation are associated to citrate ions. The weak intensity of the IR

bands is due to the low amount of citrate ions (due to the low gold loading of the

sample). After centrifugation of sample A no IR bands are observed in the 1690-1350

cm-1 IR range, in accordance with the effective removal of non interacting citrate ions.




Figure S1. IR spectra of sample A before (spectra a) and after (spectra b)

centrifugation.

2. HAADF-STEM characterization of the as-prepared catalyst

STEM images give a contrast which depends on the atomic number (Z) of the atom

imaged, and adding aberration correction allows visualizing single atoms. The high-

angle annular dark field STEM (HAADF-STEM) image shown in Figure S2 clearly

evidences the presence of isolated atoms (monomers and dimers), that appear as white

dots which correspond to a high Z element and not to defects or carbon atoms.




Figure S2. HAADF-STEM image of sample A showing the presence of gold atoms and

dimers.

Since the intensity of atoms depends on the depth of the atoms in the sample, this effect

has been used to distinguish between dimers and atoms. As shown in Figure S3, profile

a) represents a dimer because intensities are similar, while profile b) could correspond

to two isolated atoms at different depth in the sample, because intensities are different.




a

ba

1

2

a

ba

1

2

Figure S3. Intensities of gold atoms and dimers in HAADF-STEM images.

3. XPS characterization of the as-prepared catalyst

X-ray photoelectron spectra were collected using a SPECS spectrometer with a

150 MCD-9 detector and using a non monochromatic MglKα (1253.6eV) X-Ray source.

In order to avoid photo reduction or sintering of the gold atoms induced by the X-Ray

source, spectra acquisition has been done at -170 ºC using 50 Watt of the X-Ray source.

Binding energy (BE) values were referenced to C1s peak (284.5 eV). A Au4f7/2 binding

energy of 85.4 eV has been obtained on the 0,1wt% gold sample (sample A), which

corresponds to gold in an oxidation state (+1).1




Figure S4. XPS spectra of the Au4f peak of sample A.

4. Raman and IR spectroscopic studies of the evolution of the gold species under

synthesis conditions.

Raman spectra were acquired with a Renishaw Raman In Via spectrometer

equipped with a Leika DM LM microscope and a 514-nm HPNIR diode laser as an

excitation source. The laser power at the sample was 20 mW. A 50× objective of 8 mm

optical length was used to focus the depolarized laser beam onto a 3–5 µm spot on the

sample and collect the backscattered light. The Raman scattering was collected in a

static-scan mode in the 100–2500 cm−1 spectral region with a resolution > 4 cm-1.

Twenty scans were accumulated for each spectrum. The sample solution were placed

inside Raman adapted cells for liquid analysis.

Infrared spectroscopic (FTIR) experiments were performed in a Bio-Rad FTS-

40A spectrometer with a DTGS detector. The sample solution was deposited on a

germanium disc and lead to evaporation before measurement.




Figure S5. Raman spectra of 0.07 M HAuCl4 aqueous solution a) at pH 2.2; b) at pH

7.3; c) at pH 9.1; d) after addition of sodium citrate to solution c (Au:citrate molar ratio

1.5:1).

Figure S5 shows the room temperature Raman spectra of the HAuCl4 aqueous

solution at different pH values. The Raman spectra of the HAuCl4 0.07 M aqueous

solution at pH 2.2 shows two bands at 325 and 348 cm-1 corresponding of the AuCl4-

complex.2 When the pH increases to 7.3, a new peak at 577 cm-1 starts to appear

associated to the Au-OH stretching vibration. At pH 9.1 no bands associated to the Au-

Cl stretching vibrations are observed, and only the band at 577 cm-1 due to Au-OH is

observed, which is consistent with the hydrolysis of the AuCl4- complex. Since the

synthesis of the Au/MWCNTs catalyst is carried out at pH 9.1, we studied the addition

of citrate ions into the above solution. No change is observed in the Raman spectra,




where the band at 577 cm-1 still remains. The bands associated to the COO- stretching

vibration were poor resolved by Raman spectroscopy, therefore IR spectra were

acquired on the same sample solution. In this case new bands at 1536 and 1323 cm-1

associated to the interaction of the carboxylate groups with the gold atoms have been

clearly identified. (Figure S6).

Figure S6. IR spectra of a) aqueous solution of sodium citrate (dotted line) b) Au-citrate

solution (red line) (Au:citrate molar ratio 1.5:1).

5. HAADF-STEM characterization of the evolution of the catalyst under reaction

conditions

STEM characterization of metal atoms and clusters is a challenging task, due to the

relative unstability of the system under the electron beam. In this work, to avoid or

minimize the changes in the sample caused by the electron beam, images of each

sample area were acquired only once, using low-dose imaging techniques, with the

microscope operated at 80 kV and an electron dose of 1.4x103 e-/angstrom2. This results

in lower resolution images, and requires HAADF-STEM image simulations to identify

gold clusters of different atomicity. Configurations of Aun nanoclusters (n = 2, 3, 4, ...)

were optimized through genetic-algorithms. The Genetic Algorithms are sophisticated




strategies of global search that have proven to be efficient in the search of global

minima with regard to other methods involving derivatives. This is a search technique

based on the principles of natural evolution; the operators used in the search resemble

the evolutionary processes of genetic crossover, natural selection and mutation to

explore potential energy hyper surfaces. We used the BCGA code3 which incorporates

the semi-empiric Gupta potential to model interatomic interaction of transition and

noble metal atoms. The set of parameters this model potential requires for gold atoms

were taken from Cleri et al.4 and the initial configurations for all cluster sizes were

generated randomly. Simulated STEM Images were obtained by QSTEM software

using experimental parameters in STEM experimental images. An example for Au7 is

shown in Figure S7.

a ba b

Figure S7. a) Au7 cluster configuration optimized through genetic-algorithms and b)

HAADF-STEM simulation image by QSTEM for configuration showed in a).

The HAADF-STEM image of sample B taken at 6 min reaction and the simulated

STEM images of the gold clusters is shown in Figure S8. Small clusters with 4 to 13

atoms can be observed, and isolated gold species and dimers are minoritary.




Au 3 Au 7

Au 4

Au 2

a

Figure S8. HAADF-STEM image of sample B taken at 6 min reaction.

Additional HAADF-STEM images of samples A, B, C and D at different

magnifications are shown in Figures S9-S12.




Figure S9. HAADF-STEM images of sample A at different magnifications.




2 nm1 nm

1 nm0.5 nm

2 nm1 nm

1 nm0.5 nm

Figure S10. HAADF-STEM images of sample B at different magnifications.




Figure S11. HAADF-STEM images of sample C at different magnifications.




Figure S12. HAADF-STEM images of sample D at different magnifications.

6. X-ray Absorption Spectroscopy (XAS) measurements

XAS data at the Au L3-edge (11919 eV) was collected at the ELETTRA

Synchrotron (Trieste, Italy) on the XAFS beam line. The storage ring operated at 2 GeV

with a current of 300 mA. A Si(111) crystal was used in the double crystal

monochromator, and that was detuned by 30% to reject higher energy harmonics. The

X-ray absorption was measured at RT in fluorescence mode using a Ketek Silicon Drift

Detector to record the fluorescence signal. The I0 was recorded using an ion chamber

filled with the optimal gas. All samples were prepared as pressed pellets. Reference




samples of Au2O3, Au(OH)3 AuCl, (PPh3)AuCH3, Au foil and 3wt% of 4 nm Au NPs

were also measured in transmission mode. Fluorescence measurements of Au/MWCNT

samples took about 9 h each. Despite the long measurements the data quality of these

very demanding samples was not of the quality required for EXAFS data quantification.

For this reason, the analysis was addressed from a qualitative point of view and the

EXAFS signal was inspected within a wave number range of k=3.6-9.6 Å-1. Data

reduction was performed using the VIPER program and following the standard

procedures.5,6

The FT of the EXAFS spectrum of sample C, taken at 12 min of reaction, is

shown in Figure S13. Besides the peak at 1.7 Å assigned to Au atoms directly attached

to thiolate and/or citrate ligands, a peak at 2.5 Å is also observed associated to

formation of Au-Au bonds.

Figure S13. EXAFS FT of 4 nm Au NPs (red), sample A before reaction (black) and

sample C taken after 12 min reaction (green). L = Ligand.




7. UV/Vis spectroscopy

The DR UV/Vis spectra were recorded at room temperature in a Cary 5

spectrometer using BaSO4 as reflectance standard.

The evolution of the UV/Vis spectrum of sample A taken at different reaction

times is shown in Figure S14. The adsorption spectrum of sample A before reaction

(spectrum A) shows a maximum adsorption at 210 nm, which agrees to those reported

for isolated gold atoms,7,8 and less intense bands at 307 and 412 nm which are related to

adsorption bands of the amine polyelectrolyte used in the synthesis of the Au/MWCNT

sample. When sample A is exposed to reaction conditions new bands appear. Thus,

bands located at 230, 270 and 307 nm are observed on the sample taken during the

induction period (spectrum B), followed by the growth of a new band at 357 nm once

the induction period finishes and the reaction starts (spectrum C). At the end of the

reaction a more continuous decrease of the adsorption band is observed (spectrum D).

The UV/Vis spectrum of thiophenol shows two well defined adsorption bands at 230

and 270 nm (see inset in Figure S14). Thus the bands at 230 and 270 nm observed on

sample A under reaction conditions should be related to adsorbed thiophenol species,

while only the band at 357 nm can unambiguously be associated to gold species.

Assignation of the UV/Vis bands to a specific gold cluster atomicity is highly

controversial in the literature. Most of the literature data rely on theoretical calculations

and on the optical properties of mass selected gold clusters in a matrix,7,8 while few

studies have been done on gold clusters deposited on inorganic supports. In these cases

a strong effect of the cluster geometry on the optical properties has been reported.9,10

Accordingly, a detailed picture of the cluster atomicity cannot be deduced from the

UV/Vis spectra. However, the appearance of an adsorption band at 357 nm red shifted

with respect to the adsorption spectrum of the fresh sample at 210 nm suggests an

increase in the gold cluster atomicity. Moreover, the appearance of a continuous

adsorption spectrum at the final of the reaction agrees well with formation of gold

clusters of higher atomicity.11




Figure S14. UV/Vis spectra of sample A a) prior to catalytic test, b) during the

induction period, c) once finished the induction period and when the reaction starts, e)

at the end of the reaction. Inset: UV/Vis spectra of thiophenol.

8. Synthesis and characterization of size controlled gold clusters

Isolated gold clusters with 3, 5-7 and 7-10 atoms were synthesized using modifications

of the electrochemical method previously reported for nanoparticles.12 All syntheses

were carried out under nitrogen atmosphere in a 3 electrode electrochemical cell, with

silver/silver chloride as a reference, gold foil as a counter electrode and platinum foil as

the working electrode. MiliQ quality water was used in all the process. Prior to any

synthesis the electrodes were polished with alumina, washed thoroughly with water and

sonicated. In addition, Pt electrode was electrochemically cleaned by cyclic

voltammetry in 1M MeOH/1M NaOH solution followed by cyclic voltammetry in 1M




H2SO4. Sample Au3 was synthesized in MiliQ water at 35ºC fixing the potential at -8V

during 6200 s. Sample Au5-7 was synthesized in MiliQ water at 25ºC fixing the current

density in four steps: 1420 s at -20mA, 6200 s at -25 mA, 3600 s at -35 mA and 10000 s

at -45 mA. Sample Au7-10 was synthesized in 0.1M KNO3 at 25ºC fixing the current

density in two steps: 10000s at -30 mA and 10000 s at -40 mA.

The atomicity of size controlled gold clusters in solution was characterized by

combining several spectroscopic techniques. Figure S15 shows the UV/Vis absorption

spectra of the gold clusters synthesized electrochemically. The absence of Plasmon band

at ~520 nm, indicates that particles are smaller than ~ 1.5-2.0 nm. Moreover, the

presence of only discrete, molecule-like bands in the UV region indicates that clusters

are very small, in the size range: Aun, n < ~ 20 atoms.13




Figure S15. UV/Vis spectra of Au3, Au5-7 and Au7-10 gold clusters.

As previously reported for silver, gold and cupper clusters without strong binding

ligands,14,15,16 photoluminescence emission can provide a relatively accurate

approximation of the number of atoms in a cluster, just by applying the spherical

Jellium model. According to this model, the band gap is given by the following simple

relation: Eg = EFermi/N1/3, where EFermi is the Fermi energy of bulk metal (gold in our

case; EFermi = 5.4eV), and Eg the band gap energy of the cluster, assumed to be identical




to the photoluminescence emission energy. According to this, the cluster samples can be

identified as follows: sample with emission energy = 3.67 eV: Au3; sample with

emission energies = 3.24 and 2.97 eV: Au5-Au6; sample with emission energies = 2.83,

2.73 and 2.55 eV: Au7, Au8 and Au10 (see Figure S16).

Figure S16. Photoluminescence emission spectra of Au3, Au5-7 and Au7-10 gold clusters.




In order to be more confident about cluster atomicity, sample Au5-7 has also been

characterized by mass spectroscopy which, up to now, is the most suitable technique for

its unambiguous characterization. Mass spectra analysis of sample Au5-7 was carried out

by ESI-TOF. Samples were analyzed using a Bruker Microtof mass-spectrometer

operating in negative ionization mode. Temperature control and nitrogen drying gas (1µ

L/min) in ESI source were employed to assist the ionization process. All spectra were

acquired in reflectron mode of the TOF mass spectrometer equipped with multi step

detection for obtain maximum sensitivity, 15000 (FWHM).; isotopic resolution was

observed throughout the entire mass range detected. For full scan MS analysis, the

spectra were recorded in the range of m/z 200 to 1400. Calibration was carried out

employing reserpine (609m/z) in FIA 10 pg (100:1 S/N 200µ L/min). For the detection

of clusters 10 µL of sample were employed at flow rate of 0.1 ml/min in 0.1% formic

acid in MeOH/H2O (1:1).

Figure S17. Set of peaks of Aun from ESI-Ms spectrum acquired using clusters of

sample Au5-7 containing 0.1% formic acid. Detected peaks show that the predominant

species are Au5, Au6 and Au7 clusters (which nicely agrees with previous estimations)

interacting with different species in solution as to get stable close shell structures, as it

is described in following table.




m/z (exp) formula m/z (theor)

402.80 [Au7 (OH)5 (H2O)8 ]-4 402.72

418.77 [Au7 (O2)2 (OH)5 (H2O)8 ]-4 418.71

434.75 [Au7 (O2)2 (OH)5 (H2O)8 (CH3OH)2]-4 434.73

544.73 [Au5 (OH)5 (H2O) ]-2 544.43

560.71 [Au5 (OH)5 (H2O) (CH3OH)]-2 560.44

576.68 [Au5 O2 (OH)5 (H2O) (CH3OH)]-2 576.44

702.64 [Au6 (O) (OH)2 (H2O)6 (CH3OH)2]-2 702.46

718.61 [Au6 (O) (OH)2 (H2O)6 (CH3OH)3]-2 718.47

Previous studies done with these and other clusters of different metals and sizes have

shown that the clusters remain stable after deposition onto a support.17,18

9. Catalytic activity of 0.8 and 1.2 nm gold nanoparticles supported on MWCNTs.

Two samples containing gold nanoparticles of different sizes (∼0.8 nm and 1.2

nm, corresponding to Au15-25 and Au∼50), have been prepared. The 0.8 nm gold

nanoparticle sample has been prepared according to the method described in the

experimental section, adjusting the gold amount in the synthesis to yield a final 3wt%

gold loading. HRTEM microscopy analysis shows the presence of 0.8 nm gold

nanoparticles (Figure S18a) while in HAADF-STEM images some gold atoms and

clusters are also observed. (Fig.S18b). In order to ensure the absence of gold atoms

and/or small gold clusters, gold nanoparticles were synthesized “ex situ” according to

Xu et al.19 and then supported on the functionalized MWCNTs. In this way, gold

nanoparticles of ~1.2 nm diameter supported on the functionalized MWCNTs were

prepared. HRTEM microscopy analysis shows the presence of 1.2 nm gold

nanoparticles (Figure S18c). The catalytic behavior of both samples in the oxidation of

thiophenol in presence of O2 is shown in Figure S19. Very low or no catalytic activity is

observed in both cases, confirming that gold nanoparticles of 0.8 and 1.2 nm are not

active in this reaction. The slight activity observed on the 0.8 nm sample could be

related to the presence of some gold atoms and clusters, which have been shown to be

extremely active in this reaction.




a) b)

c)

a) b)

c)

Figure S18. HRTEM (a) and HAADF-STEM (b) image of 0.8 nm Au NP supported on

functionalized MWCNTs. (c) HRTEM image of 1.2 nm Au NP supported on

functionalized MWCNTs.




Figure S19. Yield to disulfide versus reaction time in the oxidation of thiophenol

catalyzed by a) 0.8 nm Au NP and b) 1.2 nm Au NP supported on functionalized

MWCNTs.




10. DFT results

The optimized geometries of the most stable structures obtained by interaction of

thiophenol and O2 with isolated AuI species and with small Au3, Au5, Au6 and Au7

clusters are depicted in Figures S20 and S21, respectively.

In order to determine the influence of the the gold-support interface on the activation of

reactants, the interaction of thiophenol and O2 with a AuI species and a Au3 cluster

directly attached to an amino group of the PAH polyelectrolyte coating the carbon

nanotubes have been calculated and compared with those discussed in the manuscript.

The optimized geometries, calculated energies and net atomic charges shown in Figure

S22 indicate that the amino groups do not participate directly in the activation of

reactants. Moreover, their presence does not modify significatively the charge

distribution of the gold atoms and clusters, and does not influence the interaction of

gold with O2 and thiophenol, demonstrating the validity the reaction mechanism

described in the manuscript.




Figure S20. Optimized structures of the adsorption complexes, reaction intermediates

and transition states involved in the mechanism of disulfide formation catalyzed by AuI

species. Distances in Å. Au, S, C, O, Na and H atoms are depicted as golden, blue,

orange, red, large white and small white balls, respectively.




Figure S21. Optimized structures of thiophenol and O2 adsorbed on Au3, Au5, Au6 and

Au7 clusters. Distances in Å, atomic charges in e, and interaction energies in kJ/mol.

Interaction energies are calculated as the difference between the total energy of the

adsorption complex and the sum of the energies of isolated Aun cluster and thiophenol

and O2 molecules. Au, S, C, O and H atoms are depicted as golden, blue, orange, red

and white balls, respectively.




Figure S22. Optimized structures of thiophenol and O2 interacting with AuI species and

Au3 cluster directly attached to an amino group of the PAH polyellectrolyte coating the

carbon nanotubes. Distances in Å, atomic charges in e, and interaction energies in

kJ/mol. Au, S, N, C, O and H atoms are depicted as golden, blue, green, orange, red and

white balls, respectively.




11. Analysis of H2O2 and H2O formed, and O2 consumed during the reaction.

H2O2 formed as a reactive intermediate species has been determined by

introducing thioanisol (Ph-S-CH3) in the reaction media. Thioanisol reacts specifically

with H2O2 yielding a mixture of sulphone and sulphoxide in methanol as solvent, but

does not react with molecular O2 in the presence of the Au/MWCNT catalyst.

Therefore, the detection of sulphoxide (~ 1.1%) and sulfone (0,5% yield) when

thioanisol (0.98mmol, 125.4mg) is incorporated to a methanolic solution (1ml)

containing equimolecular amounts of thiophenol (0.99mol, 110.6mg) in the presence of

the Au/MWCNT catalyst (2x10-5mmol), confirms that H2O2 is formed as reaction

intermediate in the oxidation of thiophenol.

The amounts of oxygen consumed and water formed during the oxidation of

thiophenol were analyzed under the following reaction conditions: 2 mmol thiophenol,

1.5 x 10-5 mmol Au (Au/MWCNT, 0.1 % wt, sample A), room temperature, PO2 = 5 bar.

The amount of moisture was analyzed in a Karl Fischer tritator (Mettler-Toledo, DL-31,

1.1). It was found that 0.75 mmol water was formed and 0.4 mmol O2 were consumed

after 92% conversion of thiophenol. These amounts perfectly match with a chemical

balance where the thiophenol/O2 ratio is approx. 4, whereas the thiophenol/H2O ratio is

approx. 2, according to the following equation:

4 R-SH + O2 2 R-S-S-R + 2 H2O

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