Exceptional oxidation activity with size-controlled supported gold clusters of low atomicity

TL;DR: An easy method is reported to prepare isolated gold atoms supported on functionalized carbon nanotubes and their performance in the oxidation of thiophenol with O2, showing that single gold atoms are not active, but they aggregate under reaction conditions into gold clusters of low atomicity that exhibit a catalytic activity comparable to that of sulfhydryl oxidase enzymes.

Abstract: The catalytic activity of gold depends on particle size, with the reactivity increasing as the particle diameter decreases. However, investigations into behaviour in the subnanometre regime (where gold exists as small clusters of a few atoms) began only recently with advances in synthesis and characterization techniques. Here we report an easy method to prepare isolated gold atoms supported on functionalized carbon nanotubes and their performance in the oxidation of thiophenol with O2. We show that single gold atoms are not active, but they aggregate under reaction conditions into gold clusters of low atomicity that exhibit a catalytic activity comparable to that of sulfhydryl oxidase enzymes. When clusters grow into larger nanoparticles, catalyst activity drops to zero. Theoretical calculations show that gold clusters are able to activate thiophenol and O2 simultaneously, and larger nanoparticles are passivated by strongly adsorbed thiolates. The combination of both reactants activation and facile product desorption makes gold clusters excellent catalysts.

Summary

Summary

• The catalyticic activity of gold depends on particle size, with reactivity increasing as particle diameter decreases.
• An easy method to prepare isolated gold atoms supported on functionalized carbon nanotubes and their performance in the oxidation of thiophenol with O2 are described.
• Single gold atoms are not active and they aggregate under reaction conditions into gold clusters of low atomicity, which show a catalytic activity comparable to that sulfhydryl oxidase enzymes.
• When clusters grow into larger nanoparticles, catalyst activity drops to zero.

Main Text

• Gold has attracted wide interest as catalyst in the last years due to its unexpected activity and, specially, to its high selectivity in organic reactions.
• The influence of particle size has been extensively investigated, and a volcano type curve with a maximum in activity at an optimum diameter has been reported for CO oxidation, 7 alkane oxidation, 9 or propene epoxidation with O2 and H2, 10 while in other cases an exponential increase in activity with decreasing particle size has been observed.
• Recent advances in aberration corrected scanning transmission electron microscopy (STEM) have allowed imaging with atomic resolution.
• The isolated gold atoms initially present in the as-prepared catalyst aggregate into gold clusters of different atomicity under reaction conditions.
• 34,35 These clusters finally aggregate into larger and not active nanoparticles.

Results and discussion

• Isolated gold atoms supported on functionalized multiwalled carbon nanotubes have been synthesized by controlling the pH value of the medium, the concentration of the HAuCl4 precursor, and by introducing sodium citrate that acts as reducing agent and stabilizer, as described in detail in Supplementary Section 1.
• The induction period observed with sample A would indicate that the active species are not the isolated gold atoms present in the as-prepared catalyst, but other metal species that are being formed during the reaction.
• Atomic gold cluster distributions calculated on samples A, B and C by analyzing more than 200 gold species are comparatively depicted in Fig. 3e, in which aggregation of the initially isolated gold atoms into clusters and finally nanoparticles is clearly observed.
• The atomic orbital distribution shows that these extra electrons are localized on the S lone pair orbitals leading to strong repulsion between them and to incapacity to form the S-S bond.
• Thiophenol adsorbs on all these clusters forming a stable Au--S bond and transferring ~0.2 e from the S lone pair to the metal LUMO (Supplementary Fig. S21).

Conclusions

• The authors have succeeded in preparing single isolated gold atoms on MWCNTs, and have studied their performance in the oxidation of thiophenol to disulfide by O2.
• Following the evolution of the catalyst during the reaction the authors have shown that isolated atoms are not active and they aggregate under reaction conditions into gold clusters of low atomicity.
• Finally, when clusters grow into nanoparticles of diameter ≥ 1 nm, the catalyst activity drops to zero.
• Theoretical calculations show that only gold clusters of low atomicity are able to simultaneously adsorb and activate thiophenol and O2, while the strong Au-S interaction in 1 nm gold nanoparticles leads to formation of very stable RS-Au-SR units that impede the reaction to proceed.
• The combination of reactant activation and desorption facilities makes the cluster successful for this oxidation reaction.

Figures

Figure 4. Oxidation of thiophenol to disulfide in the presence of O2 catalysed by size selected gold clusters. Gold clusters of Au3, Au5-7 and Au7-10 were synthesised by an electrochemical method and deposited on functionalized MWCNT. Their activity was tested in the oxidation of thiophenol. Yield to disulfide with reaction time using Au3, Au5-7 and Au7-10 gold clusters supported on MWCNs as catalyst.

Figure 5. Structures involved in the mechanism of disulfide formation catalyzed by Au I species. Gold, Sulfur, Carbon, Oxygen and Hydrogen atoms are depicted in golden, blue, orange, red and white, respectively. The italic numbers that appear beside the arrows are the reaction and activation (denoted with #) energies in kJ/mol calculated for each elementary step. The numbers in blue that appear next to various selected atoms are net atomic charges, in units of e. In structure 6, qT is the total charge on each of the two fragments framed by dotted lines. TS8 is a transition state structure. Atomic distribution of the highest occupied molecular orbitals (HOMOs) of selected structures is also depicted.

Figure 1. Identification of isolated gold atoms. Isolated gold atoms supported on functionalized multiwalled carbon nanotubes have been synthesized (sample A) and their presence confirmed by Cs-corrected high resolution STEM analysis and XAS analysis. a, BF-STEM image of sample A, which does not allow to clearly visualize gold atoms supported on the MWCNTs. b, HAADF-STEM image of sample A clearly showing the presence of isolated atoms (monomers and some dimers), that appear as white dots on the carbon nanotube. c, d, XAS at the Au L3-edge (c) and EXAFS Fourier transforms (d) of supported Au NP of 4 nm (red line), of the reference (PPh3)AuCH3 (blue line), and of sample A (black line). The similarity between the blue and black

Figure 2. Oxidation of thiophenol to disulfide in the presence of O2. Single isolated gold atoms (sample A) are not active in the oxidation of thiophenol showing an induction period of 6min, which indicate that other metal species are being formed during reaction. The evolution of the gold species was followed by stopping the reaction at different times. The catalyst samples (sample B, C, D taken at 6, 12, 120 min of reaction of sample A) were used to carry out a new reaction under the same conditions. The yield to disulfide with reaction time using as catalyst samples A, B, C, and D is shown. Inset: zoom of sample A in the beginning of the reaction showing the induction period.

Figure 6. Reactivity of gold clusters of low atomicity. a, Atomic distribution of the highest occupied molecular orbitals (HOMOs) of Au3, Au5, Au6 and Au7 clusters, and optimized structures of the complexes formed by interaction of these clusters with O2. The numbers in black are the optimized O-O bond lengths in Å, and the numbers in blue are the net charge on O2 in units of e. b, Structures involved in the mechanism of

Figure 3. Evolution of the gold species present on the catalyst. The evolution of gold species present on the catalyst was followed by stopping the reaction at different times, isolating the catalyst and characterising it by means of HAADF-STEM. a, Isolated gold atoms present on the as-prepared catalyst, sample A. b, Image of sample B taken at 6 min reaction, showing the presence of small clusters with 4–13 atoms. c, d, Two different images of sample C taken at 12 min reaction, in which both small clusters and some large nanoparticles can be observed. e, In sample D, taken after 120 min of reaction, most gold has aggregated into nanoparticles with diameter ≥ 2 nm. Additional HAADF-STEM images at different magnifications are included as Supplementary Figures S9-S12. f, Comparison of the atomic gold cluster distributions calculated on samples A, B and C by analyzing more than 200 gold species.

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Additional Information
https://dx.doi.org/10.1038/nchem.1721
http://hdl.handle.net/10251/67246
Nature Publishing Group
Corma Canós, A.; Concepción Heydorn, P.; Boronat Zaragoza, M.; Sabater Picot, MJ.;
Navas Escrig, J.; Yacaman, MJ.; Larios, E.... (2013). Exceptional oxidation activity with size-
controlled supported gold clusters of low atomicity. Nature Chemistry. 5(9):775-781.
doi:10.1038/NCHEM.1721.

1
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 Mayoral
7
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.

2
Summary
The catalyticic activity of gold depends on particle size, with reactivity increasing as
particle diameter decreases. Investigation of the trends in the subnanometer regime,
where gold exists as small clusters of a few atoms, is now starting thanks to recent
advances in synthesis and characterization techniques. An easy method to prepare
isolated gold atoms supported on functionalized carbon nanotubes and their
performance in the oxidation of thiophenol with O
2
are described. Single gold atoms are
not active and they aggregate under reaction conditions into gold clusters of low
atomicity, which show a catalytic activity comparable to that sulfhydryl oxidase
enzymes. When clusters grow into larger nanoparticles, catalyst activity drops to zero.
Theoretical calculations show that gold clusters are able to simultaneously activate
thiophenol and O
2
, while larger nanoparticles become passivated by strongly adsorbed
thiolates. The combination of an optimum for reactants activation and product
desorption makes gold clusters excellent catalysts.
Main Text
Gold has attracted wide interest as catalyst in the last years due to its unexpected
activity and, specially, to its high selectivity in organic reactions.
1-3
The catalytic
properties of gold depend on several factors that in some cases are intimately related:
gold particle size and morphology, metal-oxide support interaction, oxidation state of
the active sites, etc.
4-8
The influence of particle size has been extensively investigated,
and a volcano type curve with a maximum in activity at an optimum diameter has been
reported for CO oxidation,
7
alkane oxidation,
9
or propene epoxidation with O
2
and H
2
,
10
while in other cases an exponential increase in activity with decreasing particle size has
been observed.
5,11,12
However, the trends in catalytic activity when the particle diameter

3
is smaller than 1 nm have been little investigated due to the difficulty in achieving a
very narrow distribution of cluster sizes and to the detection limit of traditional
spectroscopic and microscopic techniques. Recent advances in aberration corrected
scanning transmission electron microscopy (STEM) have allowed imaging with atomic
resolution. Then, gold atoms in combination with gold clusters and nanoparticles have
been detected on gold samples prepared by conventional synthesis procedures,
13-15
while direct evidence of isolated gold atoms in the absence of clusters or nanoparticles
has been recently provided by Gates et al.
16,17
A pronounced sensitivity to cluster size
and charge state has been found when studying the interaction of small molecules with
gold atoms and/or clusters,
18-22
as well as in chemical processes like CO oxidation,
13
propene epoxidation,
23
ethylene hydrogenation,
24
and iodobenzene dissociation.
25
While it appears that in order to control reactivity, the atomicity control of the
gold clusters is crucial, the synthesis of size-selected metal clusters and their deposition
over a solid support is a challenging task.
26
The wet-chemistry methods for preparing
supported metal clusters involve the anchoring of well defined precursors to an adequate
support,
27,28
followed by removal of the ligands by post-synthesis treatments, trying to
prevent cluster agglomeration during these steps.
9,29-31,32
Soft landing of monodisperse
metal clusters grown in the gas phase and with precise size selection by mass
spectrometry is a more straightforward method, but it requires sophisticated equipment,
and the scaling up of the process is a major drawback.
20,23,33
In this work, isolated gold atoms in the absence of gold clusters and/or
nanoparticles have been synthesized by a very easy, reproducible and environmental
friendly process, and their performance in the aerobic oxidation of thiophenol with O
2
has been investigated. The isolated gold atoms initially present in the as-prepared
catalyst aggregate into gold clusters of different atomicity under reaction conditions. It

4
is clearly shown that, while the activity of single isolated gold atoms and of gold
nanoparticles is negligible, atomic aggregates with 5 to 10 atoms either generated under
reaction conditions or synthesized separately and deposited on functionalized MWCNTs
give turnover frequencies (TOF) as high as 7*10
5
h
-1
, which are of the same order of
magnitude than those found with sulfhydryl oxidase enzymes.
34,35
These clusters finally
aggregate into larger and not active nanoparticles. The reaction mechanism has been
theoretically studied and it is shown that isolated gold atoms cannot activate O
2
, while
small gold clusters are excellent catalysts for O
2
activation and formation of disulfide.
Results and discussion
Catalyst synthesis and characterization. Isolated gold atoms supported on
functionalized multiwalled carbon nanotubes (MWCNTs) have been synthesized by
controlling the pH value of the medium, the concentration of the HAuCl
4
precursor, and
by introducing sodium citrate that acts as reducing agent and stabilizer, as described in
detail in Supplementary Section 1. Cs-corrected high resolution STEM analysis
confirms the presence of isolated gold atoms. Figure 1a shows a bright field (BF-
STEM) image of sample A containing 0.1wt% gold which does not allow to clearly
visualize gold atoms supported on the MWCNTs. However, the high-angle annular dark
field STEM (HAADF-STEM) image shown in Fig. 1b clearly evidences the presence of
isolated atoms (monomers and some dimers), that appear as white dots which
correspond to a high Z element and not to defects or carbon atoms (see Supplementary
Fig. S2 and S3).
The chemical nature of these isolated atoms has been investigated by X-ray
absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS).
According to XAS measurements, isolated gold atoms are mostly present as cationic

Frequently asked questions

Q1. What are the contributions in this paper?

In this paper, the performance of single isolated gold atoms on MWCNTs was investigated in the oxidation of thiophenol to disulfide by O2. 

Q2. What is the atomicity of the clusters?

Single gold atoms are not active and they aggregate under reaction conditions into gold clusters of low atomicity, which show a catalytic activity comparable to that sulfhydryl oxidase enzymes. 

Q3. What is the name of the paper?

Investigation of the trends in the subnanometer regime, where gold exists as small clusters of a few atoms, is now starting thanks to recent advances in synthesis and characterization techniques.